Scientific Bases for the Preparation of Heterogeneous Catalysts, Volume 175: Proceedings of the 10th International Symposium, Louvain-la-Neuve, Belgium, July 11-15, 2010 (Studies in Surface Science and Catalysis)

Scientific Bases for the Preparation of Heterogeneous Catalysts

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Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedings of the 10th International Symposium Louvain-la-Neuve, Belgium, July 11-15, 2010

Vol. 175

Edited by E.M. Gaigneaux*, M.Devillers*, S.Hermans*, P.A. Jacobs**, J.A. Martens**, P.Ruiz* * Université Catholique de Louvain, Louvain-la-Neuve, Belgium **Katholieke Universiteit Leuven, Heverlee (Leuven), Belgium

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Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands Linacre House, Jordan Hill, Oxford OX2 8DP, UK First edition 2010

Copyright © 2010 Elsevier B.V. All rights reserved

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Contents The nanoscale integration of heterostructures in chemo- and bio-catalysis G. D. Stucky How the manufacturing technology of industrial catalysts can influence their mechanical strength N. Pernicone, T. Fantinel, V. Trevisan, F. Pinna Coating metallic foams and structured reactors by VOx/TiO2 oxidation catalyst: application of RPECVD A. Essakhi, A. Löfberg, Ph. Supiot, B. Mutel, S. Paul, V. Le Courtois, E. Bordes-Richard

1

9

17

Washcoating of metallic monoliths and microchannel reactors L.C. Almeida, F.J. Echave, O. Sanz, M.A. Centeno, J.A. Odriozola, M. Montes

25

Monolithic catalysts for the decomposition of energetic compounds D. Amariei, R. Amrousse, Y. Batonneau, R. Brahmi, Ch. Kappenstein, B. Cartoixa

35

Glass fiber materials as a new generation of structured catalysts B.S. Bal’zhinimaev, E.A. Paukshtis, O.B. Lapina, A.P. Suknev, V.L. Kirillov, P.E. Mikenin, A.N. Zagoriuko

43

A novel electrochemical route for the catalytic coating of metallic supports F. Basile, P. Benito, G. Fornasari, M. Monti, E. Scavetta, D. Tonelli, A. Vaccari

51

Solution combustion synthesis as intriguing technique to quickly produce performing catalysts for specific applications S. Specchia, C. Galletti, V. Specchia Impact of NO on the decomposition of supported metal nitrate catalyst precursors and the final metal oxide dispersion M. Wolters, I.C.A. Contreras Andrade, Peter Munnik, J.H. Bitter P.E. de Jongh, K.P. de Jong A novel approach to synthesize highly selective nickel silicide catalysts for phenylacetylene semihydrogenation X. Chen, A. Zhao, Z. Shao, Z. Ma, C. Liang

59

69

77

Preparation of calcium titanate photocatalysts for hydrogen production K. Shimura, H. Miyanaga and H. Yoshida

85

A new procedure to produce carbon-supported metal catalysts J. Hoekstra, P.H. Berben, J.W. Geus, L.W. Jenneskens

93

Use of zeta potential measurements in catalyst preparation S. Soled, W. Wachter, H. Wo

101

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Contents

The superior activity of the CoMo hydrotreating catalysts, prepared using citric acid: what’s the reason? 109 A.V. Pashigreva, O.V. Klimov, G.A. Bukhtiyarova, M.A. Fedotov, D.I. Kochubey, Yu.A. Chesalov, V.I. Zaikovskii, I.P. Prosvirin, A.S. Noskov Elucidation of the surface configuration of the Co(II) and Ni(II) aqua complexes and of the Cr(VI), Mo(VI) and W(VI) monomer and polymer oxo-species deposited on the titania surface during impregnation 117 G.D. Panagiotou, Th. Petsi, J. Stavropoulos, Ch.S. Garoufalis, K. Bourikas, C. Kordulis, A. Lycourghiotis Innovative characterizations and morphology control of γ-AlOOH boehmite nanoparticles: towards advanced tuning of γ-Al2O3 catalyst properties M. Digne, R. Revel, M. Boualleg, D. Chiche, B. Rebours, M. Moreaud, B. Celse, C. Chanéac, J.-P. Jolivet Highly active and selective precious metal catalysts by use of the reductiondeposition method P.T. Witte, M. de Groen, R.M. de Rooij, P. Bakermans, H.G. Donkervoort, P. H. Berben, J.W. Geus Investigation of the role of stabilizing agent molecules in the heterogeneous nucleation of rhodium(0) nanoparticles onto Al-SBA-15 supports R. Sassine, E. Bilé-Guyonnet, T. Onfroy, A. Denicourt, A. Roucoux, F. Launay Preparation of the polymer-stabilized and supported nanostructured catalysts E. Sulman, V. Matveeva, V. Doluda, L. Nikoshvili, A. Bykov, G. Demidenko, L. Bronstein Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction C. Jin, W. Xia, J. Guo, T.C. Nagaiah, M. Bron, W. Schuhmann, M. Muhler Synthesis and characterization of highly loaded Pt/carbon xerogel catalysts prepared by the Strong Electrostatic Adsorption method N. Job, F. Maillard, M. Chatenet, C.J. Gommes, S. Lambert, S. Hermans, J.R. Regalbuto, J.-P. Pirard Catalytic wet air oxidation of succinic acid over monometallic and bimetallic gold based catalysts: influence of the preparation method R. Nedyalkova, M. Besson, Cl. Descorme Design of hierarchical functional porous mixed oxides from single precursors A. Lemaire, B.-L. Su Hierarchical porous catalyst support: shaping, mechanical strength and catalytic performances S. Ould-Chikh, S. Pavan, A. Fecant, E. Trela, C. Verdon, A. Gallard, N. Crozet, J.L. Loubet, M. Hemati, L. Rouleau

127

135

145 153

161

169

177 185

193

Contents Catalytic property of carbon-supported Pt catalysts covered with organosilica layers on dehydrogenation of organic hydride K. Nakagawa, Y. Tanimoto, T. Okayama, K.-I. Sotowa, S. Sugiyama, T. Moriga Molecular aspects of solid silica formation I. Halasz, M. Agarwal, R.E. Patterson

vii

201 209

A novel continuous approach for the synthesis and characterization of pure and mixed metal oxide systems applied in heterogeneous catalysis S. Kaluza, M. Muhler

217

Innovative preparation of Au/C by replication of gold-containing mesoporous silica catalysts F. Kerdi, V. Caps, A. Tuel

221

TiO2 photocatalysts prepared by thermohydrolysis of TiCl4 in aqueous solutions A. Di Paola, M. Bellardita, L. Palmisano Metal complex-assisted polymerization of thermosetting resins: a convenient one-step procedure for the preparation of heterogeneous catalysts U. Arnold, M. Döring

225

229

Synthesis and study of mesoporous WO3-ZrO2-SiO2 solid acid S.V. Prudius, O.V. Melezhyk, V.V. Brei

233

Citral hydrogenation over Pt-M/CeO2 catalysts (M= Zn, Zr) M. Aoun, M. Chater, P. Marecot, C. Especel, G. Lafaye

237

Foam-supported catalysts tailored for industrial steam reforming processes R. Faure, F. Basile, I. Bersani, Th. Chartier, A. Cuni, M. Cornillac, P. Del Gallo, G. Etchegoyen, D. Gary, F. Rossignol, A. Vaccari

241

Synthesis of ordered nanostructured CuO-CeO2 catalysts by hard template method 245 P. Djinovic, J. Batista, J. Levec, A. Pintar Fine-tuning of vanadium oxide nanotubes J. Emmerich, M. Dillen, C.E.A. Kirschhock, J.A. Martens

249

Plasma-assisted design of supported cobalt catalysts for Fischer-Tropsch synthesis J. Hong, W. Chu, Y. Ying, Petr A. Chernavskii, A. Khodakov

253

Chemical vapor deposition of Fe(CO)4(SiCl3)2 for the synthesis of hydrogenation catalyst made of highly dispersed iron silicide particles on silica J. Guan, A. Zhao, X. Chen, M. Zhang, C. Liang

259

Laser electrodispersion technique for the preparation of self-assembled metal catalysts T.N. Rostovshchikova, S.A. Nikolaev, E.S. Lokteva, S.A. Gurevich, V.M. Kozhevin, D.A. Yavsin, A.V. Ankudinov

263

viii

Contents

Nitrogen doped TiO2 photocatalyst prepared by low energy N + implantation technique T. Yoshida and E. Kuda

267

Preparation and characterization of shape-selective ZSM-5 catalyst for para-methyl ethylbenzene production with toluene and ethylene B. Liu, Z. Yu, Y. Meng, L. Cui, Z. Zhu

271

Microwave-assisted preparation of Mo2C/CNTs nanocomposites as an efficient support for electrocatalysts towards oxygen reduction reaction M. Pang, L. Ding, C. Li, C. Liang

275

Laser-induced photocatalytic inactivation of coliform bacteria from water using Pd-loaded nano-WO3 A. Bagabas, M. Gondal, A. Khalil, A. Dastageer, Z. Yamani, M. Ashameri

279

Effect of carbon nanotube basicity in Pd/N-CNT catalysts on the synthesis of R-1-phenyl ethyl acetate S. Sahin, P. Mäki-Arvela, J.-Ph. Tessonnier, A. Villa, L. Shao, D.S. Su, R. Schlögl, T. Salmi, D. Yu. Murzin Metal-carbon nanocomposite systems as stable and active catalysts for chlorobenzene transformations E. Lokteva, A. Erokhin, S. Kachevsky, A. Yermakov, M.Uimin, A. Mysik, E. Golubina, K. Zanaveskin, A. Turakulova, V. Lunin

283

289

Development and design of Pd-containing supported catalysts for hydrodechlorination E.V. Golubina, E.S. Lokteva, S.A. Kachevsky, A.O. Turakulova, V.V. Lunin

293

Role of deposition technique and support nature on the catalytic activity of supported gold clusters: experimental and theoretical study E.V. Golubina, D.A. Pichugina, A.G. Majouga, S.A. Aytekenov

297

Nanosized nickel ferrite catalysts for CO2 reforming of methane at low temperature: effect of preparation method and acid-base properties R. Benrabaa, H. Boukhlouf, E. Bordes-Richard, R.N. Vannier, A. Barama

301

Hierarchical porous Ce-Zr materials for oxidation of diesel soot particulate N.V. Zaletova, A.O. Turakulova, V.V. Lunin The role of organic additives in the synthesis of mesoporous aluminas and Ni/mesoporous alumina catalysts F. Bentaleb and E. Marceau Inverse replica of porous glass as catalyst support S. Wohlrab, A. Janz, M.-M. Pohl, S. Kreft, D. Enke, A. Koeckritz, A. Martin, B. Luecke

305

311 315

Contents The use of small volume TOC analysis as complementary, indispensable tool in the evaluation of photocatalysts at lab-scale S. Ribbens, V. Meynen, K. Steert, K. Augustyns, P. Cool Enzymatic oxidation of phenols by immobilized oxidoreductases B. Tikhonov, A. Sidorov, E. Sulman, V. Matveeva

ix

321 325

A coordinative saturated vanadium containing Metal Organic Framework that shows a remarkable catalytic activity K. Leus, I. Muylaert, V. Van Speybroeck, G.B. Marin, P. Van Der Voort

329

Influence of the preparation conditions on properties of gold loaded on the supports containing group five elements I. Sobczak , J. Florek, K. Jagodzinska, M. Ziolek

333

High loaded Ni/SiO2 catalyst for producing ultra-pure inert gas J.W. Son, S. Yoon, H.G. Oh, D.Y. Shin, C.W. Lee

339

The effect of 3d-cation modification on properties of cordierite-like catalysts E.F. Sutormina, L.A. Isupova, N.A. Kulikovskaya, A.V. Kuznetsova, E.I. Vovk

343

Large-scale synthesis of porous magnetic composites for catalytic applications H. Falcon, P. Tartaj, A.F. Rebolledo, J.M. Campos-Martin, J.L.G. Fierro, S.M. Al-Zahrani

347

Preparation of gallium oxide photocatalysts for reduction of carbon dioxide H. Yoshida and K. Maeda

351

Catalytic combusion of methane on ferrites M.V. Bukhtiyarova, A.S. Ivanova, E.M. Slavinskaya, L.M. Plyasova, V.A. Rogov, V.V. Kaichev

355

Polymer-based nanocatalysts for phenol CWAO E. Sulman, V. Doluda, N. Lakina, A. Bykov, V. Matveeva, L. Bronstein

361

A new sulphonic acid functionalized periodic mesoporous organosilica as a suitable catalyst E. De Canck, C. Vercaemst, F. Verpoort, P. Van Der Voort

365

Effect of the preparation procedure on the structural peculiarities and catalytic properties of Pt/(CeO2-TiO2) catalysts in CO oxidation A.A. Shutilov and G.A. Zenkovets

369

Study of the sorption of Cu(II) species on the “TiO2/KNO3” interface A. Georgaka and N. Spanos

373

Hydrogenation/hydrogenolysis of benzaldehyde over CaTiO3 based catalysts N. Sayad, A. Saadi, S. Nemouchi, A. Taibi-Benziada, C. Rabia

377

x

Contents

VSbOx phases formed on MCM-41 supports H. Golinska and M. Ziolek

381

Influence of the preparation conditions of Ca doped Ni/olivine catalysts on the improvement of gas quality produced by biomass gasification D.C. Cárdenas-Espinosa and J.C. Vargas

385

Effect of ethylenediamine as chelating agent of cobalt species upon the cobalt-support interactions: application to the VOC catalytic removal F. Wyrwalski, J.-M. Giraudon, J.-F. Lamonier

389

Influence of support on the ammoxidation activity of VPO catalysts V.N. Kalevaru, B. Luecke, A. Martin

393

Rationalization of the aqueous impregnation of molybdenum heteropolyanions on γ-alumina support J. Moreau, O. Delpoux, K. Marchand, M. Digne, S. Loridant

397

Mesoporous SBA-15 silica modified with cerium oxide: Effect of ceria loading on support modification L.F. Liotta, G. Di Carlo, F. Puleo, G. Pantaleo, G. Deganello

401

Synthesis and characterization of catalysts obtained by trifluoromethanesulfonic acid immobilization on zirconia M. Gorsd, M. Blanco, L. Pizzio

405

Influence of precursor on the particle size and stability of colloidal gold nanoparticles A. Alshammari, A. Köckritz, V.N. Kalevaru, A. Martin

409

V-Mo-Nb-W-containing hydrotalcite-like materials as precursors of catalysts for oxidative dehydrogenation of hydrocarbons and alcohols I.P. Belomestnykh, G.V. Isaguliants, S.P. Kolesnikov, V.P. Danilov, O.N. Krasnobaeva, T.A. Nosova, T.A. Elisarova

413

Synthesis of high-surface area CeO2 through silica xerogel template: influence of cerium salt precursor L.F. Liotta, G. Di Carlo, F. Puleo, G. Marci, G. Deganello

417

Iron based catalyst for hydrocarbons catalytic reforming: A metal-support interaction study to interpret reactivity data L. Di Felice, C. Courson, P.U. Foscolo, A. Kiennemann

421

Ecofriendly catalysts based on mixed xerogels for liquid phase oxidations by hydrogen peroxide M. Palacio, P. Villabrille, G. Romanelli, P. Vázquez, C. Cáceres

425

Preparation of MgF2-MgO supports with specified acid-base properties, and their influence on nickel catalyst activity in toluene hydrogenation M. Zieliński, M. Wojciechowska

429

Contents Pd supported catalysts: Evolution of support during Pd deposition and K doping R. Pellegrini, G. Leofanti, G. Agostini, E. Groppo, M.R. Chierotti, R. Gobetto, C. Lamberti Investigation of carbon and alumina supported Pd catalysts during catalyst preparation R. Pellegrini, G. Leofanti, G. Agostini, E. Groppo, C. Lamberti

xi 433

437

Advanced photocatalytic activity using TiO2/ceramic fiber-based honeycomb S.M. Jung, J.H. Lee, M.S. Han, J.S. Choi, S.J. Kim, J.H. Seo, H.Y. Lim

441

Incorporation of group five elements into the faujasite structure M. Trejda, A. Wojtaszek, A. Floch, R. Wojcieszak, E.M. Gaigneaux , M. Ziolek

445

Glycerol conversion into H2 by steam reforming over Ni and PtNi catalysts supported on MgO modified γ-Al2O3 A. Iriondo, M.B. Güemez, V.L. Barrio, J.F. Cambra, P.L. Arias, M.C. Sánchez-Sánchez, R.M. Navarro, J.L.G. Fierro Butyraldehyde production by butanol oxidation over Ru and Cu catalysts supported on ZrO2, TiO2 and CeO2 A. Iriondo, M.B. Guemez, J. Requies, V.L. Barrio, J.F. Cambra, P.L. Arias, J.L.G. Fierro Preparation of Au nanoparticles on Ce-Ti-O supports S.A.C. Carabineiro, A.M.T. Silva, G. Dražić, J.L. Figueiredo Preparation, active component and catalytic properties of supported vanadium catalysts in the reaction of formaldehyde oxidation to formic acid E.V. Danilevich, G. Ya. Popova, T.V. Andrushkevich, Yu.A. Chesalov, V.V. Kaichev, A.A. Saraev, L.M. Plyasova

449

453

457

463

Investigation of different preparation methods of PtIr, PtIrSn and PtIrGe catalysts 467 Chr. Poupin, C. La Fontaine, L. Pirault-Roy Perovskite-type catalysts for the water-gas-shift reaction F. Basile, G. Brenna, G. Fornasari, P. Del Gallo, D. Gary and A. Vaccari Evaluation of different methods to prepare the Fe2O3/MoO3 catalyst used for selective oxidation of methanol to formaldehyde Karim H. Hassan and P.C.H. Mitchell Formation of active component of MoVTeNb oxide catalyst for selective oxidation and ammoxidation of propane and ethane E.V. Ischenko, T.V. Andrushkevich, G.Ya. Popova, V.M. Bondareva, Y.A. Chesalov, T.Yu. Kardash, L.M. Plyasova, L.S. Dovlitova, A.V. Ischenko Functionalization of carbon nanofibers coated on cordierite monoliths by oxidative treatment S. Armenise, M. Nebra, E. Garcia-Bordejé, A. Monzón

471

475

479

483

xii

Contents

Synthesis of mesoporous silicas functionalized with trans (1R, 2R) - diaminocyclohexane by sol-gel method F. Fakhfakh, L. Baraket, A. Ghorbel, J.M. Fraile, J.A. Mayoral

487

Physico-chemical and catalytic properties of effective nanostructured MnCeOx systems for environmental applications F. Arena, G. Trunfio, J. Negro, C. Saja, A. Raneri, L. Spadaro

493

Novel method for doping of nano TiO2 photocatalysts by chemical vapor deposition T.M. Cuong, Vu.A. Tuan, B.H. Linh, D.T. Phuong, T.T.K. Hoa, N.D. Tuyen, N.Q. Tuan, H. Kosslick Study on the preparation of active support and multi-porous supported catalyst V.A. Tuan, B.H. Linh, D.T. Phuong, T.T.K. Hoa, N.T. Kien, N.H. Hao, H. Kosslick , A. Schulz The influence of preparation procedure on structural and surface properties of magnesium fluoride support and on the activity of ruthenium catalysts for selective hydrogenation of chloronitrobenzene M. Pietrowski and M. Wojciechowska

497

501

505

Bimetallic Co-Mo-complexes with optimal localization on the support surface: A way for highly active hydrodesulfurization catalysts preparation for different petroleum distillates 509 O.V. Klimov, A.V. Pashigreva, K.A. Leonova, G.A. Bukhtiyarova, S.V. Budukva, A.S. Noskov Mn, Mn-Cu and Mn-Co mixed oxides as catalysts synthesized from hydrotalcite type precursors for the total oxidation of ethanol D. Aguilera, A. Perez, R. Molina, S. Moreno

513

Mesoporous manganese oxide catalysts for formaldehyde removal: Influence of the cerium incorporation J. Quiroz-Torres, R. Averlant, J.-M. Giraudon, J.-F. Lamonier

517

Nickel nanoparticles with controlled morphologies application in selective hydrogenation catalysis J. Aguilhon, C. Boissière, O. Durupthy, C. Thomazeau, C. Sanchez

521

Behavior of NiMo(W)/Zr-SBA-15 deep hydrodesulfurization catalysts in presence of aromatic and nitrogen-containing compounds A. Soriano, P. Roquero, T. Klimova

525

Effect of citrate addition in NiMo/SBA-15 catalysts on selectivity of DBT hydrodesulfurization D. Valencia, I. García-Cruz, T. Klimova

529

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xiii

Investigation of the microwave heating techniques for the synthesis of LaMnO3+δ : influence of the starting materials 533 R. Kahia, C. Menu, J.-M. Giraudon, J.-F. Lamonier The novel route of preparation of the supported gold catalysts by deposition-precipitation O.A. Kirichenko, G.I. Kapustin, V.D. Nissenbaum, O.P. Tkachenko, V.A. Poluboyarov, A.L. Tarasov, A.V. Kucherov, L.M. Kustov A new approach for the dispersion of VOPO4.2H2O through exfoliation and its catalytic activity for the selective oxidation of cyclohexane P. Borah, C. Pendem, A. Datta Mesoporous CuO-Fe2O3 composite catalysts for complete n-hexane oxidation S. Todorova, J.-L. Cao, D. Paneva, K. Tenchev, I. Mitov, G. Kadinov, Z.-Y. Yuan, V. Idakiev Preparation of PtRu/C electrocatalysts by hydrothermal carbonization using different carbon sources M.M. Tusi, M. Brandalise, R.W.R. Verjúlio-Silva, O.V. Correa, J.C. Villalba, F.J. Anaissi, A.O. Neto, M. Linardi, E.V. Spinacé

537

541 547

551

Preparation of PtSn/C electrocatalysts using electron beam irradiation D. F. Silva, A. O. Neto, E.S. Pino, M. Linardi, E.V. Spinacé

555

Preparation of PtSn/C skeletal-type electrocatalyst for ethanol oxidation R. Crisafulli, A. O. Neto, M. Linardi, E.V. Spinacé

559

Preparation of binary M/Mn (M=Co, Cu, Zn) oxide catalysts by thermal degradation of heterobimetallic complexes V.G. Makhankova, O.V. Khavryuchenko, V.V. Lisnyak, V.N. Kokozay Preparation of highly active gas oil HDS catalyst by modification of conventional oxidic precursor with 1,5-pentanediol S. Herry, O. Chassard, P. Blanchard, N. Frizi, P. Baranek, C. Lancelot, E. Payen, S. van Donk, J.P. Dath, M. Rebeilleau

563

567

Hierarchical meso-/macroporous phosphated and phosphonated titania nanocomposite materials with high photocatalytic activity T.-Y. Ma, X.-Z. Lin. Z.-Y. Yuan

571

Gold and CuO nanocatalysts supported on hierarchical structured Ce-doped titanias for low temperature CO oxidation T.-Y. Ma and Z.-Y. Yuan

575

Facile preparation of MoO3/SiO2-Al2O3 olefin metathesis catalysts by thermal spreading D.P. Debecker, M. Stoyanova, U. Rodemerck, E.M. Gaigneaux

581

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Mesoporous TiO2-SBA15 composites used as supports for molybdenum-based hydrotreating catalysts M.T. N. Dinh, C. Lancelot, P. Blanchard, C. Lamonier, M. Bonne, S. Royer, P. Marécot, F. Dumeignil, E. Payen p-Hydroxybenzoic acid degradation by Fe/Pd-HNT catalysts with in situ generated hydrogen peroxide A.Turki, H. Kochkar, G. Berhault, A. Ghorbel

587

593

Synthesis of ionic liquid templated zeolite like structures A. Martín, S. Ivanova, F.R. Sarria, M.Á. Centeno, J.A. Odriozola

597

New class of acid catalysts for methanol dehydration S. Ivanova, X. Nitsch, F. R. Sarria, B. Louis, M.Á. Centeno, A.C. Roger, J.A. Odriozola

601

One-Pot deposition of palladium on hybrid TiO2 nanoparticles: application for the hydrogenation of cinnamaldehyde A. Mehri, H. Kochkar, S. Daniele, V. Mendez, G. Berhault, A. Ghorbel

605

Catalytic activity of nanostructured Pd catalysts supported on hydrogenotitanate nanotubes K. Jabou, H. Kochkar, G. Berhault, A. Ghorbel

609

Temperature - dependent evolution of molecular configurations of oxomolybdenum species on MoO3/TiO2 catalysts monitored by in situ Raman spectroscopy 613 G. Tsilomelekis, A. Tribalis, A.G. Kalampounias, S. Boghosian, G.D. Panagiotou, K. Bourikas, C. Kordulis, A. Lycourghiotis Preparation of nanosized bimetallic Ni-Sn and Ni-Au/SiO2 catalysts by SOMC/M. Correlation between structure and catalytic properties in styrene hydrogenation L. Deghedi, J.-M. Basset, G. Bergeret, J.-P. Candy, M. C. Valero, J.-A. Dalmon, A. D. Mallmann, A.-C. Dubreuil, L. Fischer Microwave-assisted synthesis of Au, Ag and Au-Ag nanoparticles and their catalytic activities for the reduction of nitrophenol S. Albonetti, M. Blosi, F. Gatti, A. Migliori, L. Ortolani, V. Morandi, G. Baldi, A. Barzanti, M. Dondi A new composite micro/meso porous material used as the support of catalyst for polyaromatic compound hydrogenation J. Yu, Y. Tian, X. Ma, Y. Li Photodeposition of Au and Pt on ZnO and TiO2 S.A.C. Carabineiro, B.F. Machado, G. Dražić, R.R. Bacsa, P. Serp, J.L. Figueiredo, J.L. Faria Cellulose-templated materials for partial oxidation of methane: effect of template and calcination parameters on catalytic performance C. Berger-Karin, E.V. Kondratenko

617

621

625 629

635

Contents Highly porous hydrotalcite-like film growth on anodised aluminium monoliths F.J. Echave, O. Sanz, L.C. Almeida, J.A. Odriozola, M. Montes The influence of impregnation temperature on the pzc of titania and the loading of Ni upon preparation of Ni/TiO2 catalysts J. Kyriakopoulos, G. Panagiotou, T. Petsi, K. Bourikas, C. Kordulis, A. Lycourghiotis

xv 639

643

Immobilization of homogeneous catalysts in nanostructured carbon xerogels C.C. Gheorghiu, M. Pérez-Cadenas, M.C. Román-Martínez, C. S.-M. de Lecea, N. Job

647

Coating method for Ni/MgAl2O4 deposition on metallic foams C. Cristiani, C.G. Visconti, S. Latorrata, E. Bianchi, E. Tronconi, G. Groppi, P. Pollesel

653

Use of commercial carbons as template for the preparation of high specific surface area perovskites R.K.C. de Lima, E.D. da Silva, E.A. Urquieta-González Ethyl acetate combustion catalyzed by oxidized brass micromonoliths O. Sanz, S.A. Cruz, J.C. Millán, M. Montes, J.A. Odriozola

657 661

Preparation of CMI-1 supported H3+xPMo12-xVxO40 for the selective oxidation of propylene S. Benadji, P. Eloy, A. Léonard, B.-L. Su, C. Rabia, E.M. Gaigneaux

665

Direct addition of the precursor salts of Mo, Co or Ni oxides during the sol formation of γ-Al2O3 and ZrO2 - The effect on metal dispersion E.P. Baston, E.A. Urquieta-Gonzalez

671

Glycothermal synthesis as a method of obtaining high surface area supports for noble metal catalysts W. Walerczyk, M. Zawadzki, J. Okal

675

Synthesis and characterization of cok-12 ordered mesoporous silica at room temperature under buffered quasi neutral pH J. Jammaer A. Aerts, J. D’Haen, J.W. Seo, J.A. Martens

681

Spray drying of porous alumina support for Fischer-Tropsch catalysis A. Lind, R. Myrstad, S. Eri, T. H. Skagseth, E. Rytter A. Holmen

685

Ni/SiO2 fiber catalysts prepared by electrospinning technique for glycerol reforming to synthesis gas P. Reubroycharoen, N. Tangkanaporn, C. Chaiya

689

Selective preparation of β-MoO3 and silicomolybdic acid(SMA) on MCM-41 from molybdic acid precursor and their partial oxidation performances T.M. Huong, N.H.H. Phuc, H. Ohkita, T. Mizushima, N. Kakuta

695

xvi

Contents

Functionalization of carbon xerogels for the preparation of Pd/C catalysts by grafting of Pd complex C. Diverchy, S. Hermans, N. Job, J.-P. Pirard, M. Devillers

699

Preparation of Pd-Bi catalysts by grafting of coordination compounds onto functionalized carbon supports C. Diverchy, S. Hermans, M. Devillers

703

Novel dicarboxylate heteroaromatic metal organic frameworks as the catalyst supports for the hydrogenation reaction V.I. Isaeva, O.P. Tkachenko, I.V. Mishin, E.V. Afonina, G.I. Kapustin, L.M. Kozlova, W. Grünert, L.M. Kustov Monitoring of the state of silver in porous oxides during catalyst preparation E. Sayah, D. Brouri, A. Davidson, P. Massiani Strong electrostatic adsorption for the preparation of Pt/Co/C and Pd/Co/C bimetallic electrocatalysts L. D’Souza, and J.R. Regalbuto

707

711

715

Preparation of gold catalysts supported on SiO2-TiO2 for the CO PROX reaction L. Gonzalo-Chacón, B. Bachiller-Baeza, A. Guerrero-Ruiz, I. Rodríguez-Ramos

719

A method of preparation of active TiO2-SiO2 photocatalysts for water purification M.P. Fedotova, G.A. Voronova, E.Yu. Emelyanova, O.V. Vodyankina

723

n-Heptane hydroconversion on bifunctional hierarchical catalyst derived from zeolite MCM-22 M. Kollár, M.R. Mihályi, J. Valyon

727

Preparation and characterization of nanocrytallines Mn-Ce-Zr mixed oxide catalysts by sol-gel method: application to the complete oxidation of n-butanol S. Azalim, R. Brahmi, M. Bensitel, J.-M. Giraudon, J.-F. Lamonier

731

SCR activity of conformed CuOx/ZrO2-SO4 catalysts S.B. Rasmussen, J. Due-Hansen, M. Yates, P. Ávila, R. Fehrmann

735

Pore design of pelletised VOx/ZrO2-SO4/Sepiolite composite catalysts S.B. Rasmussen, J. Due-Hansen, M. Yates, M. Villaroel, F.J. G. Llambías, R. Fehrmann, P. Ávila

739

Titanium oxide nanotubes as supports of Au or Pd nano-sized catalysts for total oxidation of VOCs H.L. Tidahy, T. Barakat, R. Cousin, C. Gennequin, V. Idakiev, T. Tabakova, Z.-Y. Yuan, B.L. Su, S. Siffert Preparation of Alkali-M/ZrO2 (M= Co or Cu) for VOCs oxidation in the presence of NOx or carbonaceous particles A. Aissat, S. Siffert, D. Courcot

743

747

Contents Design of appropriate surface sites for ruthenium-ceria catalysts supported on graphite by controlled preparation method J. Álvarez-Rodríguez, A. Maroto-Valiente, M. Soria-Sánchez, V. Muñoz-Andres, A. Guerrero-Ruiz Preparation of monolithic catalysts for space propulsion applications R. Amrousse, R. Brahmi, Y. Batonneau, C. Kappenstein, M. Théron, P. Bravais Synthesis of mixed zirconium-silver phosphates and formation of active catalyst surface for the ethylene glycol oxidation process N.V. Dorofeeva, O.V. Vodyankina, O.S. Pavlova, G.V. Mamontov Characterization of cobalt nanoparticles on different supports for Fischer-Tropsch synthesis M.C. Rangel, A. Khodakov, F.J.C.S. Aires, M.O. de Souza, J.-G. Eon, L.M. dos Santos, A.O. de Souza, A.G. Constant Enhanced dibenzothiophene desulfurization over NiMo catalysts simultaneously impregnated with saccharose J. Escobar, J.A. Toledo, A.W. Gutiérrez, M.C. Barrera, M.A. Cortés, C. Angeles, L. Díaz

xvii

751

755

759

763

767

Preparation of Pt on Nay zeolite catalysts for conversion of glycerol into 1,2-propanediol S.V. de Vyver, E.D. Hondt, B.F. Sels, P.A. Jacobs

771

Alkali metal supported on mesoporous alumina as basic catalysts for fatty acid methyl esters preparation R.M. Bota, K. Houthoofd, P.J. Grobet, P.A. Jacobs

775

Modifications of porous stainless steel previous to the synthesis of Pd membranes C. Mateos-Pedrero, M.A. Soria, I. Rodríguez-Ramos, A. Guerrero-Ruiz

779

Design of nano-sized FeOx and Au/FeOx catalysts for total oxidation of VOC and preferential oxidation of CO 785 S. Albonetti, R. Bonelli, R. Delaigle, E.M. Gaigneaux, C. Femoni, P.M. Riccobene, S. Scirè, C. Tiozzo, S. Zacchini, F. Trifiró Supported Pd nanoparticles prepared by a modified water-in-oil microemulsion method R. Wojcieszak, M.J. Genet, P. Eloy, E.M. Gaigneaux, P. Ruiz

789

Preparation of silica-coated Pt-Ni alloy nanoparticles using microemulsions and formation of carbon nanofibers by ethylene decomposition K. Nakagawa, S. Takenaka, H. Matsune, M. Kishida

793

Sol-gel synthesis combined with solid exchange method, a new alternative process to prepare improved Pd/ZrO2-Al2O3-SiO2 catalysts S. Fessi, A. Ghorbel, A. Rives

797

xviii

Contents

Sol-gel synthesis of micro- and mesoporous silica in strong mineral acid A. Depla, C. Kirschhock, J. Martens Ag-V2O5/TiO2 total oxidation catalyst: autocatalytic removal of the surfactant and synergy between silver and vanadia D.P. Debecker, R. Delaigle, M.M.F. Joseph, C. Faure, E.M. Gaigneaux Controlled synthesis of porous heteropolysalts used as catalysts supports S. Paul, A. Miňo, B. Katryniok, E. Bordes-Richard, F. Dumeignil

801

805 811

Influence of the sodium-based precipitants on the properties of aluminum-doped hematite catalysts for ethylbenzene dehydrogenation A.S.R. Medeiros, M. do C. Rangel

815

Effect of the preparation method on the properties of hematite-based catalysts with lanthanum for styrene production M. de S. Santos , S.G. Marchetti, A. Albornoz, M. do C. Rangel

819

Low-organics method to synthesize silver nanoparticles in an aqueous medium N. Ballarini, F. Cavani, E. D. Esposti, Z. Sobalik, J. Dedecek

823

Clusters as precursors of nanoparticles supported on carbon nanofibers D. Vidick, S. Hermans, M. Devillers

827

X-ray photoelectron spectroscopy study of nitrided zeolites M. Srasra, S. Delsarte, E.M. Gaigneaux

831

Development of a modified co-precipitation route for thermally resistant, high surface area ceria-zirconia based solid solutions A. Pappacena, K. Schermanz, A. Sagar, E. Aneggi, A. Trovarelli Deposition of gold clusters onto porous coordination polymers by solid grinding T. Ishida, N. Kawakita, T. Akita, M. Haruta

835 839

Influence of the preparation methods for Pt/CeO2 and Au/CeO2 catalysts in CO oxidation S. Shimada, T. Takei, T. Akita, S. Takeda, M. Haruta

843

Author index

849

Foreword This issue of Studies in Surface Science and Catalysis contains the Proceedings of the 10th International Symposium on the Scientific Bases for the Preparation of Heterogeneous Catalysts, held on the campus of the “Université catholique de Louvain” (UCL) in Louvain-la-Neuve, Belgium, on July 11-15, 2010. This series of symposia was initiated in 1975 on a regular 4-year interval basis. As for previous editions, this 10th Symposium was made possible thanks to the organizational skills of the members of the “Unité de catalyse et chimie des matériaux divisés” of UCL, benefiting from the assistance of the “Unité de chimie des matériaux inorganiques et organiques” [now merged together – with others- into the “Institute of Condensed Matter and Nanosciences” (IMCN Institute), UCL] and the “Centrum voor oppervlaktechemie en katalyse” of the Katholieke Universiteit Leuven (KULeuven). This symposium being the 10th in 2010, and 35 years after its foundation, was very special for the catalysis community: an occasion to reminisce on the progress in the field of heterogeneous catalysts’ preparation. With the years, the level of complexity attained by solid catalysts has been growing. The solids used as supports have welldefined textural characteristics, redox or acido-basic properties. The porosity can be tuned according to the application or even be bimodal. Diffusion challenges have been tackled. The active phase is now deposited in controlled ways in order to obtain the desired structure at the nanoscale. Progresses in nanotechnology have benefitted the subject. However, some old concepts have also been revisited, sometimes with new insight, sometimes giving old things a new name. Complex architectures are built on surfaces, with often a source of inspiration coming from Nature: bio-inspired or enzymes mimicks. Hybrid materials try to take the best from two worlds, for instance polymer science and inorganic chemistry or metal/organic in Metal Organic Frameworks (MOFs). This leads naturally to supported homogeneous catalysts, which is an active area of research since several years now. While some might have sought the ‘universal’ catalyst that would have been efficient for a multitude of applications, the inverse trend seems to have taken over, and specific materials are designed for specific target reactions or processes. Not only does the heavy chemical industry make use of catalysts but also fine chemicals syntheses for food additives, vitamins or drugs production. New areas have emerged where heterogeneous catalysts are important, and very often in relation with environmental issues: destruction of pollutants, photo-catalysis to use light as alternative source of energy, etc., with automotive car exhausts being now mostly equipped with catalytic converters, which was not the case 35 years ago. Catalysis has thus a bright future, at the forefront of science in various areas. Most research is now conducted by multidisciplinary teams, composed of engineers, chemists, materials scientists, physicists, or even bio-chemists, biologists or bio-engineers. Indeed, expertise in these fields is complementary and leads to real breakthroughs. Research in heterogeneous catalysis has also benefitted from recent developments in characterization methods. Tremendous advances in surface characterization and solids analysis has allowed a more precise picture of the active sites to be gained. Time resolved spectroscopy, pulse studies, and highly sophisticated methods once limited to ultra high vacuum operation that have ‘bridged the pressure gap’ have permitted to study catalysts at work. Important principles underlining the formation of the active phase, mechanistic aspects of the activity and mechanisms of deactivation have been

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unraveled, sometimes even visualized. This has confirmed statements made in the past based on indirect evidence, or brought new light on old technologies. This opens the door to tailor-made catalysts, and gives plenty of work in the area of catalysts preparation, with new challenges appearing every day. In this context, a symposium on the preparation of heterogeneous catalysts finds an easy justification, especially when bringing together delegates from academia and the industry. For this jubilee edition, the industrial and academic communities have more than ever shown a sustained interest in the event. More than 350 abstracts were submitted, constituting a record among all previous editions. In handling this huge success, the Organizing and Scientific Committees have preferred to maintain a human-sized Symposium with, in particular, a strong wish not to plan parallel sessions for oral communications. Therefore, a severe evaluation procedure was applied to select approximately 240 contributions. The criteria favored by the local Organizing Committee and the international Scientific Committee, exclusively constituted of delegates with an industrial appointment, were strongly focused on catalysts preparation aspects, privileging novelty and innovative procedures in the field, with the discussion of physico-chemical characteristics and catalytic properties being limited to the identification of the influence and control of the preparation parameters. The Symposium covered the following topics: scaling up, shaping and macrostructured catalysts, basic understanding and innovations in unit operations, nanostructured catalysts, hierarchical porous supports and hybrid catalysts and in situ spectroscopic follow-up of catalysts preparation. These topics served as guidelines for the sessions in the program of oral communications. Out of the selected papers, 40 contributions were presented orally, including 5 extended communications (one for each topic). The 193 other contributions were presented during two poster sessions. In addition, the opening invited lecture given by Professor Galen Stucky (University of California, Santa Barbara) addressed the question of the nanoscale integration of heterostructures in chemo- and bio-catalysis. The organizers are deeply indebted to the members of the Scientific Committee for their efforts in selecting the abstracts in order to maintain the high scientific level of the Symposium. In addition, all the papers included in this issue of Studies in Surface Science and Catalysis have been submitted to a systematic peer-reviewing procedure carried out by the Scientific Committee and senior members of the Organizing Committee’s laboratories, as well as of other Belgian laboratories involved in heterogeneous catalysis. The organizers thus wish to express their sincere gratitude to all of those who have participated in this outstanding work, guaranteeing, hopefully, a high quality level for the present volume. A special thought towards the post-docs of the Unité de catalyse: Victor Baldovino, M. Nawfal Ghazzal, Raquel Mateos, Robert Wojcieszak, who greatly helped the board in its editing work. The crisis has made that the sponsors were less numerous, but the organizers are indebted to some public and private sponsors who wished to perpetuate their financial support, without whom the organization of this Symposium would have been more difficult. The first companies to answer to our request this time were ExxonMobil, Johnson Matthey, Praxair and Micromeritics. The organizers are grateful to all of those who have contributed to the practical success of the event: secretaries and technical staff, trainees, Ph.D. students and postdocs of the co-organizing laboratories. The organizers wish to express their sincere gratitude to Professor B. Delvaux, Rector of UCL, Professor V. Yzerbyt, Prorector for research, Professor P. Bertrand, Vice rector for the Science and Technology sector, the

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“Service des auditoires” and the “Direction du développement institutionnel et culturel (DIC)”, for allowing the event to be patronized again by the University. Warm thanks are specifically due to Ms. Françoise Somers, Ms. Nathalie Blangenois and Ms. Jacqueline Boniver, for their constructive cooperation and decisive contribution to the conference organisation and editorial work of this volume.

The Editors.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Organizing committee Prof. M. DEVILLERS, Université catholique de Louvain Prof. E. GAIGNEAUX, Université catholique de Louvain Prof. S. HERMANS, Université catholique de Louvain Prof. P.A. JACOBS, Katholieke Universiteit Leuven Prof. J. MARTENS, Katholieke Universiteit Leuven Prof. P. RUIZ, Université catholique de Louvain

Honorary members Prof. B. DELMON, Université catholique de Louvain Dr G. PONCELET, Université catholique de Louvain

Scientific committee Dr S. ABDO, UOP, USA Dr M.P. ATKINS, Petronas Research, Malaysia Dr M. CLAREMBEAU, Ineos Services, Belgium Dr A. DE ANGELIS, ENI, Italy Dr M.P. DE FRUTOS, Repsol, Spain Prof. M. DEVILLERS, Université catholique de Louvain Prof. E. GAIGNEAUX, Université catholique de Louvain Dr J.J. HEISZWOLF, Albemarle, The Netherlands Prof. S. HERMANS, Université catholique de Louvain Prof. P.A. JACOBS, Katholieke Universiteit Leuven Dr K. JOHANSEN, Haldor Topsøe, Denmark Dr D. JOHNSON, Lucite Intl, U.K. Dr S. KASZTELAN, Institut Français du Pétrole, France Dr E. KRUISSINK, DSM Research, The Netherlands Dr A. LIEBENS, Solvay, Belgium Dr M. LOK, Avantium Technologies, The Netherlands Dr E. LOX, Umicore, Belgium Prof. J.A. MARTENS, Katholieke Universiteit Leuven Dr M. MERTENS, ExxonMobil Chemical Europe, Belgium Dr K. MÖBUS, Evonik/Degussa, Germany Dr B. REESINK, BASF, The Netherlands Dr M. RIGUTTO, Shell Global Solutions, The Netherlands Dr M. RUITENBEEK, DOW Benelux, The Netherlands Prof. P. RUIZ, Université catholique de Louvain Dr E. RYTTER, Statoil Hydro, Norway Dr F. SCHMIDT, Germany Dr M. TWIGG, Johnson Matthey, U.K. Dr W. VERMEIREN, Total Petrochemicals, Belgium

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The nanoscale integration of heterostructures in chemo- and bio-catalysis Galen D. Stucky Department of Chemistry & Biochemistry and Materials Department, University of California, Santa Barbara, California 93106 USA

Abstract During the past twenty years improvements in synthesis and characterization capabilities have made possible the designed molecular assembly of complex materials with spatially distinct, multifunctional features that are hierarchically structured. These materials are systems in their own right, with property variables that can built in or used in a dynamic mode. This offers a challenging, but very real opportunity to control chemo- and bioprocesss systems. An example is given of the use of high-surface-area inorganic interfaces to control the catalytically driven bioprocesses of a biosystem of some complexity, followed by a selected overview of some recent strategies for the synthesis of multicompositional functional units and their use in controlling processes in chemo catalysis. Keywords: biosystems, catalytic systems, interfaces, bioprocess control, heterostructured materials

1. Introduction Living biosystems provide a sophisticated catalysis model that is currently well beyond the best bench-top and commercial efforts (Barnes 1986; Rich 2003; Litvin 2009; O’Driscoll 2007). In biogenesis, the components of the organized, integrated, multicomponent system are created and function via non-linear, frequently autocatalytic processing. The bioprocess is typically directed by entropy change and interface interactions in confined spaces with high fidelity selectivity at the atomic scale. The biosystem space/time assembly definition of structure and function makes possible the closely coupled organization and processing of integrated organic and inorganic domains. In their Introductory Perspective to a Special Feature of the Proceedings of the National Academy of Sciences on “Complex systems: From Chemistry to Systems Biology”, John Ross and Adam Arkin (Ross 2009) summarize it succinctly and well. “There is great interest in complex systems in chemistry, biology, engineering, physics, and gene networks, among others. The complexity comes from the fact that in many systems there are a large number of variables, many connections among the variables including feedback loops, and many, usually nonlinear, equations of motion, or kinetic and transport equations. ‘‘Many’’ is a relative term; a properly interacting system of just three variables can show deterministic chaos, a complex behavior indeed. For the natural scientist and the engineer, nearly all their systems are complex [emphasis added]. Many problems still resist the arguments of symmetry, averaging, time-scale separation, and covariation that often underlie complexity reductions.” Experimentally, from a combinatorial perspective of finding the “right catalyst” (e.g., Gobin 2008; Polshettiwar 2009) there is great flexibility for catalyst design but a major challenge in predicting the performance consequences of the catalysts that are

2

G.D. Stucky

created. The systems approach to composite materials and device assembly and design is an intriguing potential route for the control of bio and catalytic processes. Whether or not it will be a commercially successful approach remains to be determined, but there is little doubt but that it will provide a new perspective of complex system design and function. In the first part of the presentation, an example will be given of the use of highsurface-area inorganic interfaces to control the catalytically driven bioprocesses of a biosystem of some complexity. The latter part of the presentation is a selected overview of some recent strategies for the synthesis of multicompositional functional units and their use in controlling processes in chemo catalysis.

2. Inorganic interface control of bioprocesses This issue is of increasing interest because of the use of silica mesoporous agents as cardiovascular drug delivery agents for the treatment of cancer (Klichko 2009; Park 2009; Slowing 2006) and as gene transfection agents (Radu 2004). In the example presented here, the high-surface-area inorganic phase, which can include a zeolite, mesoporous structure or layered clay structure, is considered as a system in its own right, and is most effective as a porous heterostructure that is capable of acting as a delivery or uptake agent for heat, electrolytes, or large enzymatic biomolecules. The biosystem response is considered in the context of the new total system that is created by its interface with the original unperturbed biosystem. The inorganic interface is also used to probe the network nodes of the total biosystem by monitoring the bioprocess activity response to the interface upon selective depletion of the normal biosystem proteins. The biosystem of interest is the blood clotting cascade, which consists of 122 proteins, including enzymes, that form a system network with 278 known interactions that can be further complicated by anticoagulation agents such as Warfarin and heparin. The system is autocatalytic with the formation of thrombin, which then further catalyzes the activation of the clotting part of the cascade. It is also self-regulating with uncontrolled anti-coagulation catalysis resulting in coagulopathy and bleeding diathesis. Our in vitro studies (Ostomel 2006abc, 2007; Baker 2007, 2008) and in vivo studies carried out by UHUHS (Ahuja 2006; McCarron 2008) have shown that arterial hemorrhaging can be very effectively controlled to give close to 100% survivability (Kheirabadi 2009) by interfacing an appropriate inorganic material with the blood coagulation system and catalytically accelerating the blood coagulation process. The highest efficacy, at low therapeutic material dosage levels, is obtained when a pure silica mesoporous material is used with pore sizes above 24 nm. If the mesoporous material is also used as a delivery agent for thrombin, which can be readily loaded into its 3D cage structure, an even greater efficiency for blood clotting is obtained. The inorganic system variables evaluated in this research with different inorganic agents were composition, heterostructure, time to initiate coagulation, rate of coagulation, strength of the resulting fibrin network, heat transfer, local dehydration of the blood, electrolyte delivery or uptake, zeta potential in simulated body fluid, dissolution or exfoliation of the inorganic phase, accessible protein surface area, and biocompatibility. Studies of the blood clotting cascade system with and without the inorganic are still in progress, but inorganic variables that are highly correlated with the coagulation response of the biosystem, and most importantly from a mechanistic point of view, specific blood clotting factors, have been identified. The use of the inorganic interface to probe and better map out the processes of this biosystem is expected to continue for some time

Nanoscale integration of heterostructures in chemo- and bio-catalysis

3

in the future, and will include microfluidic real time studies and system modeling. The in vivo validation of the in vitro studies carried out in our laboratories ultimately resulted in its adoption for military and civilian use (products commerically available from Z-Medica Inc.). From a pragmatic perspective, approaching this problem as a system problem was very effective. However, one important point must be made in this connection. The tie lines that connected benchtop research, scale-up (including in vitro to in vivo evaluation), formulating a commercial product, and even receiving critical evaluations from medical personnel regarding the pros and cons in field application were exceptional in terms of the short response time and openness of communication. This greatly expedited and facilitated the practical design of the most effective materials for this application. The most important characteristic of any complex system is the function for which it was intended to deliver. This determines the synthesis strategies that are used, the characterization techniques that are applied, and the on-going guidance of the direction of the research. The most serious limitation in systems analysis is the generation of sufficient experimental data to develop a meaningful understanding of the network interactions so that a predictive model can be generated.

3. Interfaces and the multicompositional, hierarchical assembly of functional units A desirable way to introduce coupled multiple subsystem functionalities is by the integrated, but spatially distinct, organization of domains with different composition. The size, composition and morphology of these heterostructure domains are dependent on the application. If 3D interconnected porous domains are introduced to control residence times in catalytic processes, pore or cage diameters on the order of 100 nm may be in order. For phonon Rayleigh scattering, smaller domain sizes may be appropriate. For oxidation-reduction, electron transport processes such as photo-catalysis (Yates 2009) and photovoltaic applications, two key challenges are electron-hole recombination and the existence of electron trap states at the domain interfaces as well as in the bulk. For photovoltaics, we have shown that the use of semimetal nanoscale heterostructures epitaxially implanted at the charge transfer interface of p-n semiconductor junctions resolves the problem of interface voltage bias and electron-hole recombination, giving ballistic electron transport and very close to the theoretical open circuit voltage (Zide 2006). This was a proof-of-principle exercise in that the synthesis was carried out using molecular beam epitaxial growth. The efficient use of electronhole states is equally important in catalytic redox processes. The use of heterostructured composite materials was also an exercise that grew out of earlier research (Yang 1998) in which single process chemical reactions were used to synthesize nanostructured composite materials made up of binary or ternary crystalline nanoparticles 4-8 nm in diameter that were organized into 3D periodic mesoporous structures. This was extended to the multicompositional, hierarchical assembly of functional units consisting of quantum dot chalcogenides with titanium dioxide nanoparticles into 3D mesostructures for visible light electron photoexcitation into the TiO2 conduction band (Bartl 2004). For example, PbS nanoparticles, which have an absorption spectrum profile that closely matches that of the solar spectrum and a conduction band at higher energy than that of anatase (TiO2), can be assembled in parallel and homogeneously integrated in situ with anatase nanocrystals into a highly ordered 3D mesostructured, flat thin film of macroscale dimensionality (Bartl 2004; Cha 2003). Thermal stabilization of the nanoscale anatase heterostructures in these 3D mesostructures is achieved by

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G.D. Stucky

carbonization of the pores, which prevents formation of the rutile phase until 750°C (Tang 2004) and facilitates electron transport. We have recently extended this approach to a simple and widely applicable methodology for the synthesis of homogenous multicomponent nanoparticle arrays that are mesostructured (Fan 2006). This ability to use molecular assembly to synthesize high surface area porous 3D material systems with built-in multi-component functionalities and substructures has the potential of offering an exceptionally rich generic platform for catalyst fabrication. The synthesis procedure uses simple molecular precursors to generate a colloidal dispersion of metal oxo-acetate nanoparticles for an extensive list of metal species. Different amounts of nanoparticles made up of different compositions can be simultaneously generated with nanoparticle sizes in the range of 3 to 6 nm. This offers a mix-and-match opportunity to make variable composition nanoscale heterostructures by co-condensation of multicomponent nanoparticles and other ionic species in the reaction solution. The nanoparticles are quite stable and their growth can be readily controlled by the slow introduction of water from the ambient environment and esterfication of the acetic and acetate groups used in the preparation. This is in contrast to the diverse condensation and metal oxide particle growth behavior observed for metal alkoxides in ethanolic solutions. The multicomposition collection of nanoparticles that are not preformed, as is usually done for supported nanoparticle catalysts, but rather synthesized along with the support can be easily organized into a high surface area 3D mesoporous configuration, e.g. a cubic (Im3m) cage structure. As an example of a ternary phase, we have studied the phase segregation and crystallization behavior of the mesoporous NiO-2SiO2-2ZrO2 system in detail. A sample was made that had a solid phase composition determined to by EDX to be 1.19:1.96:2.00, which is close to the starting synthesis composition of 1:2:2. Between room temperature and 800°C the sample remains amorphous but with a well-ordered mesoporous structure. After heating to 900°C, the zirconia component in the mesophase crystallizes into a tetragonal phase; the other components, however, remain amorphous. The crystal size of tetragonal zirconia is ~5.3 nm based on WAXRD peak broadening by the Scherer formula. A further increase in the temperature to 1000°C does not cause additional crystal growth. SAXRD and TEM investigation confirm the preservation of mesoscopic ordering (p6m) even after heating to 1000°C. The zirconia nanoparticles retain a size of ~5 nm that are uniformly embedded in the one-dimensional channel walls, and in fact, the entire framework structure shows a homogeneous distribution of the zirconia nanoparticles and amorphous NiO and SiO2 phases. This material has a pore size of ~4.2 nm and a surface area of 102 m2/g. Both NiO and ZrO2 are particularly interesting for catalytic use in chemical and petrochemical processes because of their acidic, basic, and redox properties. (Mango 1996; Postula 1994) We believe that mesoporous multicomponent materials with other specifically chosen formulations, NiO-xSiO2-yZrO2, heterostructures are promising catalytic system candidates because of the homogeneous dispersion of each component, existence of tetragonal ZrO2 nanocrystals, and silica-stabilized mesoporous framework. We have demonstrated the ability to easily process the diverse multicomponent mesoporous metal oxides (in addition to that described above) that we have made in the form of thin films, free-standing membranes, and monoliths This presents substantial advantages over previously reported methods for commercial applications for heterogeneous catalysts, energy storage, photocatalysis, or nanostructured photovoltaics, where large quantities of material are required. The “single pot” approach that combines synthesis and processing is potentially environmentally friendly and cost-effective.

Nanoscale integration of heterostructures in chemo- and bio-catalysis

5

The mesostructured wall thicknesses are of the order of 4 to 8 nm, depending on the synthesis methodology that is used so that active nanoparticle catalysts embedded in the wall matrix are easily accessible for gas or liquid phase reactions. To test this hypothesis the selective oxidation of benzyl alcohol to benzaldehyde was examined (Fan 2009). Using anatase as the primary nanoparticle matrix, nanoparticles of the first row transition metal element oxides integrated into the mesostructure walls were systematically examined using the one pot synthesis strategy described above, starting with molecular precursors. Copper and iron oxide nanoparticles embedded in the titania mesostructure walls were the most promising for the catalytic reaction carried out between 150 and 250°C. The concentration of copper oxide nanoparticles was then varied between 0 and 10 wt%. The best catalytic composition was determined to be 3K-Cu-50TiO2 which displays a stable benzaldehyde yield of >99% over a period of 50 hours for the benzyl alcohol-to-benzaldehyde selective oxidation transformation at 203°C. The best calcination temperature for this heterostructure catalyst composition before use was 350°C. The role of the host matrix, commonly called the support, is of considerable importance, and we believe that the nanostructured integration of the active catalytic species with the support is definitely a positive attribute for catalyst design and function. Several other strategies have been extensively considered for making use of nanoscale heterostructure properties. John Meurig Thomas has recently promoted the “sprinkle” methodology (Thomas 2009): ‘‘To design new solid catalysts take high-area nanoporous solids of the appropriate kind and ‘‘sprinkle’’ spatially isolated active centers over the entire (internal) surface area.’’ Tai and co-workers (Tai 2001) have functionalized Au and other metal nanoparticles with coordinating organothiol groups and have found that a homogeneous dispersion of metal nanoparticle catalyst sites is best obtained in a milder way. The approach is based on the general concept of utilizing relatively weak interactions between metal nanoparticles and the substrates in an aprotic solvent, which creates a homogeneous loading of the nanoparticles. The dispersion is then locked in by temperature treatment up to 400°C, depending on the metal and the support. Little aggregation or change in the size of metal nanoparticles is observed for a variety of metal oxide supports, e.g. in the catalytic selective hydrogenation of nitroaromatics (Shimizu 2009), oxidation of CO (Tai 2009), or oxidation of the polar ethanol molecule using supported gold nanoparticles at 200°C (Zheng 2006). In spite of its successes, the grafting, or “sprinkling”, of nanoparticles onto a substrate can have limitations with respect to long-term thermal and chemical stability under many catalytic reaction conditions. The integration of hetero/nano structures into the catalyst support matrix described above, or the covalent attachment (Margolese 2000; Nakazawa 2008) of the catalytic functionality to the support is a more robust solution, and has been shown to be effective for improving the yield and catalytic performance/stability of homogeneous catalysts (Terry 2007; Nakazawa 2008) and enzymes (Han 2002; Hartman 2010). The “ship in a bottle” (Herron 1985, 1986) heterostructure concept for packaging active catalyst species in a confined space has been extended to core/shell particle structures with porous walls containing nanoparticles with high chemical reactivity (Kustova 2008). The ability to encapsulate individual crystalline nanoparticles with chemically active crystal faces or edges and to prevent their aggregation or loss at temperatures under reaction conditions to 1000K make them attractive candidates (Huang 2009; Joo 2009). By appropriately mixing or otherwise spatially organizing core shell particles with different catalytic capabilities, independent catalytic processes could in principal be coupled together in a Kreb’s cycle or other contiguous system

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configuration. Some thoughts on this hierarchical heterostructure approach will be presented in the oral presentation as time permits.

References N. Ahuja, T. A. Ostomel, P. Rhee, G. D. Stucky, R. Conran, Z. Chen, G. Al-Bubarak, G. Velmahos, M. deMoya, and H. B. Alam, 2006, Testing of modified zeolite hemostatic dressings in a large animal model of lethal groin injury, J. Trauma 61, 1312-1320 M. Auffan, J. Rose, J-Y. Bottero, G. V. Lowry, J-P. Jolivet, and M. R. Wiesner, 2009, Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective, Nature Nanotechnology 4, 641-641 S. E. Baker, A. M. Sawvel, N. Zheng, and G. D. Stucky, 2007, Controlling bioprocesses with inorganic surfaces: layered clay hemostatic agents, Chem. Mater. 19, 4390-4392 S. E. Baker, A. M. Sawvel, J. Fan, Q. Shi, N. Strandwitz, and G. D. Stucky, 2008, Blood clot initiation by mesocellular foams: dependence on nanopore size and enzyme immobilization, Langmuir 24, 14254-14260 S. J. Barnes and P. D. Weitzman, 1986, Organization of citric acid cycle enzymes into a multienzyme cluster, FEBS Lett. 201, 267-270 M. H. Bartl, S. P. Puls, J. Tang, H. C. Lichtenegger, and G. D. Stucky, 2004, Cubic mesoporous frameworks with a mixed semiconductor nanocrystalline wall structure and enhanced sensitivity to visible light, Angew. Chemie Intl Ed. 43, 3037-3040 J. N. Cha, M. H. Bartl, M. S. Wong, A. Popitsch, T. J. Deming, and G. D. Stucky, 2003, Microcavity lasing from block peptide hierarchically assembled quantum dot spherical resonators, Nano Letters 3, 907-911 J. Fan, S. W. Boettcher, and G. D. Stucky, 2006, Nanoparticle assembly of ordered multicomponent mesostructured metal oxides via a versatile sol-gel process, Chem. Mater. 18, 6391-6396 J. Fan, Y. Li, Y. Dai, N. Zheng, J. Guo, and G. D. Stucky, 2009, Low-temperature, highly selective, gas-phase oxidation of benzyl alcohol over mesoporous K-Cu-TiO2 with stable copper(I) oxidation state, J. Am. Chem. Soc. 131, 15568-15569 O. C. Gobin and F. Schüth, 2008, On the suitability of different representations of solid catalysts for combinatorial library design by genetic algorithms, J. Comb. Chem. 10, 835-846 Y-J. Han, J. T. Watson, G. D. Stucky and A. Butler, 2002, Catalytic activity of mesoporous silicate-immobilized chloroperoxidase, J. Molecular Catalysis B 17, 1-8 M. Hartmann and D. Jung, 2010, Biocatalysis with enzymes immobilized on mesoporous hosts: the status quo and future trends, J. Materials Chemistry 20, 844-852 N. Herron, G. D. Stucky, and C. A. Tolman, 1985, The reactivity of tetracarbonylnickel encapsulated in zeolite X. A case history of intrazeolite coordination chemistry, Inorganica Chimica Acta 100,135-40 N. Herron, 1986, A cobalt oxygen carrier in zeolite Y. A molecular “ship in a bottle”, Inorg. Chem. 25, 4714-4717 X. Huang, C. Guo, J. Zuo, N. Zheng, and G. D. Stucky, 2009, An assembly route to inorganic catalytic nanoreactors containing sub-10-nm gold nanoparticles with anti-aggregation properties, Small 5, 361-365 S. H. Joo, J. Y. Park, C-K. Tsung, Y. Yamada, P. Yang, and G. A. Somorjai, 2009, Thermally stable Pt/mesoporous silica core-shell nanocatalysts for high-temperature reactions, Nature Materials 8, 126-131 B. S. Kheirabadi, M. R. Scherer, J. S. Estep, M. A. Dubick, and J. B. Holcomb. 2009, Determination of efficacy of new hemostatic dressings in a model of extremity arterial hemorrhage in swine, J. Trauma 67, 450-460 Y. Klichko, M. Liong, E. Choi, S. Angelos, A. E. Nel, J. F. Stoddart, F. Tamanoi, and J. I. Zink, 2009, Mesostructured silica for optical functionality, nanomachines, and drug delivery, J. Am. Ceram. Soc. 92, S2-S10

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M. Kustova, M. S. Holm, C. H. Christensen, Y-H. Pan, P. Beato, T. V. W. Janssens, F. Joensen, and J. Jesper, 2008, Synthesis and characterization of mesoporous ZSM-5 core-shell particles for improved catalytic properties, in Studies in Surface Science and Catalysis 174A (Zeolites and Related Materials), 117-122 O. Litvin, H. C. Causto, B-J. Chen and D. Pe’er, 2009, Modularity and Interactions in the genetics of gene expression, Proc. Natl Acad. Sciences USA 106, 6441-6446 F. D. Mango, 1996, Transition metal catalysis in the generation of natural gas, Org. Geochem. 24, 977-984 D. I. Margolese, J. Melero, S. C. Christiansen, B. F. Chmelka, and G. D. Stucky, 2000, Direct syntheses of ordered SBA-15 mesoporous silica containing sulfonic acid groups, Chem. Mater. 12, 2448-2459 R. McCarron, 2008, Comparative testing of hemostatic dressings in a severe groin hemorrhage, presentation at Advanced Technology Applications for Combat Casualty Care (ATACCC) conference, St. Pete FL 10-13 Aug 2008 A. Miller, H. Reuter, and S. Dillinger, 1995, Supramolecular inorganic chemistry: Small guests in small and large hosts, Ang. Chem. Int. Ed. Engl. 34, 2328-2364 B. S. Moore and J. Piel, 2000, Engineering biodiversity with type II polyketide synthase genes, Antonie van Leeuwenhoek 78, 391-398 J. Nakazawa and T. D. P. Stack, 2008, Controlled loadings in a mesoporous material: click-on silica, J. Am. Chem. Soc. 130, 14360-14361 C. O’Driscoll, 2007, Chiral synthesis : reflective work, Chemistry & Industry 9, 22-25 T. A. Ostomel, P. K. Stoimenov, P. A. Holden, H. B. Alam, and G. D. Stucky, 2006a, Host-guest composites for induced hemostasis and therapeutic wound healing in traumatic injuries, J. Thrombosis and Thrombolysis 22, 55-67 T. A. Ostomel, Q. Shi, and G. D. Stucky, 2006b, Oxide hemostatic activity, J. Am. Chem. Soc. 128, 8384-8385 T. A. Ostomel, Q. Shi, C-K. Tsung, H. Liang, and G. D. Stucky, 2006c, Spherical bioactive glass with enhanced rates of hydroxyapatite deposition and hemostatic activity, Small 2, 12611265 T. A. Ostomel, Q. Shi, P. K. Stoimenov, and G. D. Stucky, 2007, Metal oxide surface charge mediated hemostasis, Langmuir 23, 11233-11238 J-H. Park, L. Gu, G. von Maltzahn, E. Ruoslahti, S. N. Bhatia, and M. J. Sailor, 2009, Biodegradable luminescent porous silicon nanoparticles for in vivo applications, Nature Materials 8, 331-336 V. Polshettiwar, B. Baruwati, and R. S. Varma. 2009, Self-assembly of metal oxides into threedimensional nanostructures: synthesis and application in catalysis, ACS Nano 3, 728-734 W. S. Postula, Z. T. Feng, C. V. Philip, A. Akgerman, and R. G. Anthony, 1994, Conversion of synthesis gas to isobutylene over zirconium dioxide based catalysts, J. Catal. 145, 126-131 D. R. Radu, C. Y. Lai, K. Jeftinija, E.W. Rowe, S. Jeftinija, and V. S. Y. Lin, 2004, A polyamidoamine dendrimer-capped mesoporous silica nanosphere-based gene transfection reagent, J. Am. Chem. Soc. 126, 13216-13217 P. R. Rich, 2003, The molecular machinery of Keilin's respiratory chain, Biochem. Soc. Trans. 31 (Pt 6), 1095-2105 J. Ross and A. P. Arkin, 2009, Complex systems: From chemistry to systems biology, Proc. Natl Acad. Sciences USA 106 (16), 6433-6434 J. C. Schon and M. Jansen, 1996, First step towards planning of syntheses in solid-state chemistry: determination of promising structure candidates by global optimization, Ang. Chem. Int. Ed. Engl. 35, 1286-1304 K. Shimizu, Y. Miyamoto, T. Kawasaki, T. Tanji, Y. Tai, and A. Satsuma, 2009, Chemoselective hydrogenation of nitroaromatics by supported gold catalysts: mechanistic reasons of sizeand support-dependent activity and selectivity, J. Phys. Chem. C 113, 17803-17810 I. Slowing, B. G. Trewyn, and V. S.-Y. Lin, 2006, Effect of surface functionalization of MCM41-type mesoporous silica nanoparticles on the endocytosis by human cancer cells, J. Am. Chem. Soc. 128, 14792-14793

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Y. Tai, M. Watanabe, K. Kaneko, S. Tanemura, T. Miki, J. Murakami, and K. Tajiri, 2001, Preparation of gold cluster/silica nanocomposite aerogel via spontaneous wet-gel formation, Adv. Mater. 13, 1611-1614 Y. Tai, W. Yamaguchi, K. Tajiri, and H. Kageyama, 2009, Structures and CO oxidation activities of size-selected Au nanoparticles in mesoporous titania-coated silica aerogels, Appl. Catal. A 364, 143-149 J. Tang, Y. Wu, E. W. McFarland, and G. D. Stucky, 2004, Synthesis and photocatalytic properties of highly crystalline and ordered mesoporous TiO2 thin films, Chem. Commun., 1670-1671 T. J. Terry, G. Dubois, A. Murphy, and T. D. P. Stack, 2007, Site isolation and epoxidation reactivity of a templated ferrous bis(phenanthroline) site in porous silica, Angew. Chem. Int. Ed. 46, 945-947 J. M. Thomas, 1994, Turning points in catalysis, Ang. Chem. Int. Ed. Engl. 33, 913-937 J. M. Thomas, J. C. Hernandez-Garrido, and R. G. Bell, 2009, A general strategy for the design of new solid catalysts for environmentally benign conversions”, Top. Catal. 52, 1630-1639 P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, and G. D. Stucky, 1998, Generalized syntheses of large-pore mesoporous metal oxides with semicrystalline frameworks, Nature 396, 152-154 J. T. Yates Jr., 2009, Photochemistry on TiO2: Mechanisms behind the surface chemistry, Surface Science 603, 1605-1612 N. Zheng and G. D. Stucky, 2006, A general synthetic strategy for oxide-supported metal nanoparticle catalysts, J. Am. Chem. Soc. 128, 14278-14280 J. M. O. Zide, A. Kleiman-Shwarsctein, N. C. Strandwitz, J. D. Zimmerman, T. T. SteenblockSmith, A. C. Gossard, A. Forman, A. Ivanovskaya, and G. D. Stucky, 2006, Increased efficiency in multijunction solar cells through the incorporation of semimetallic ErAs nanoparticles into the tunnel junction, Appl. Phys. Lett. 88, 162103

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

How the manufacturing technology of industrial catalysts can influence their mechanical strength Nicola Perniconea, Tania Fantinelb, Valentina Trevisanb, Francesco Pinnab a

Consultant, Via Pansa 7, 28100 Novara, Italy. University of Venice, Dept. of Chemistry and Consorzio INSTM, 30123 Venice, Italy. Email address: [email protected]

b

Abstract The various physical properties characterizing the mechanical strength of catalysts are discussed: abrasion resistance, crush strength, attrition resistance. The related measurements have been performed mainly using ASTM standard methods with some improvements. It is shown how modifications of the manufacturing technology can improve the abrasion resistance of the traditional ammonia synthesis catalyst (oxidepromoted magnetite) and of the PTA catalyst (Pd on active carbon). As to crush strength, it is discussed how some preparation variables can be changed to improve the performance of catalysts for the production of styrene (Fe-K-Ca-Ce-Mo oxides), formaldehyde (Fe-Mo oxides), methanol (Cu-Zn-Al oxides). As to powders, it is shown that the attrition resistance of aluminas to be used for fluid bed catalysts can be improved by suitable fluorination. Finally, it is stressed that mechanical strength is a property not less important than activity and selectivity for industrial catalysts and that no catalyst must be loaded in an industrial reactor without previous check of its mechanical properties. Keywords: catalyst strength, attrition, abrasion, catalyst manufacture

1. Introduction Mechanical strength is a property of utmost importance for the industrial use of heterogeneous catalysts, consisting of either powders or pellets (cylinders, spheres, rings, granules, with size of few millimeters). For an effective industrial exploitation the pellets may not break nor abrade, while the powders may not generate fines by attrition. In other words, industrial catalysts must have a high mechanical strength. There are many examples of laboratory catalysts very successful as to activity and selectivity, but unsuitable to industrial development due to lack of mechanical strength, whose measurement is a key part of every quality control for any industrial catalyst. In practice, no catalyst should be loaded in an industrial reactor without having tested its mechanical properties. The practical consequences of the lack of mechanical strength are as follows: a) for fluid bed reactors Increase of catalyst consumption b) for axial fixed bed reactors Increase of pressure drop c) for radial fixed bed reactors Bypass formation with decrease of conversion d) for slurry reactors Problems in catalyst separation

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e) for trickle bed reactors Contamination of reaction product In all these cases a large increase of production costs is to be expected. The following three different physical properties characterize the mechanical strength of catalysts: 1) abrasion resistance (also called abrasion loss), for pellets 2) crush strength, also for pellets 3) attrition resistance, for powders As there is no straightforward procedure for the measurement of such properties, many arbitrary standard methods have been developed in different laboratories, with consequent problems in the comparison of data. Starting from the Seventies, ASTM was successful in the development of standard unified methods, which are widely used worldwide. Our work has been performed using such methods, sometimes with some modification. The literature relating the mechanical properties of catalysts with their manufacturing technology is very scanty. An old paper by one of the authors can be mentioned [1]. Some data about crush strength can be found in a recent paper [2]. It should be remarked that mechanical strength of catalysts is not a property amenable to be studied on the lab scale, because at least pilot-scale machines for catalyst forming are required. That means that such studies are usually performed during the scale-up of catalyst manufacture [3]. During this step of catalyst development mechanical strength is even a priority, as activity and selectivity have been already studied very deeply on the lab scale. In this report we will show how some variables of catalyst preparation can influence the final mechanical properties, starting in some cases from tests on different commercial catalysts.

2. Experimental techniques 2.1. Abrasion resistance For the measurement of the abrasion resistance the ASTM standard method D4058 has been used [4]. In practice it consists of determining the amount of fines formed after having rounded the catalyst in a strictly defined drum under specified conditions. The drum is equipped with a radial baffle so that in each revolution the catalyst falls from a height of about 20 cm. This test well simulates the abrasion suffered by the catalyst during reactor loading, much less that due to small displacements during reactor running. Nevertheless it is widely used worldwide. We have adopted the following experimental conditions: Catalyst mass 100 g Rotation rate 60 rpm Test duration 30 min Sieve size 0.85 mm The reproducibility of the test was within 5%.

2.2. Single-pellet crush strength First of all it should be remarked that the axial crush strength of cylindrical pellets and rings is completely useless for catalytic purposes as it is much higher than the radial one, therefore not relevant to catalyst behavior. No problem for spheres and extrudates, where only one crush strength exists. For extrudates, mostly not having constant length, the crush strength is frequently expressed in kg/mm, though making the measurement on pieces with the same length gives more reliable data.

How the manufacturing technology can influence their mechanical strength

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This measurement is usually performed by a dynamometer. However most commercial dynamometers have been designed for high loads (hundreds and even thousands of kg), while most catalysts crush under few kg. Small-load dynamometers with efficient calibration system are required for reliable measurements on catalysts. In our machine the pellet is placed on a steel dish and the piston approaches it with a speed of 10 mm/min. The breaking force is monitored on a digital display and/or on the PC screen. It is convenient to make the measurement on dry samples. For small-size extrudates it is strongly recommended to follow the crush curve on the PC, as the detection of the breaking point is not easy with the display only. There are usually large differences in crush strength from pellet to pellet, so that a distribution curve must be obtained (see Fig. 1). The number of tests to be performed ranges from 20 to even 100 in the most adverse cases. The crush strength is expressed as the average value. Standard conditions for the measurement of single-pellet crush strength can be found in the ASTM methods D4179 [5] and D6175 [6]. 12

Count, step 0.1Kg

10 8 6 4 2 0

0

0.4

0.8

1.2

1.6

2

F, Kg

Fig.1. Crush strength distribution curve of an activated methanol synthesis catalyst.

2.3. Bulk crush strength For catalysts having shape of granules and also of irregular spheres and extrudates single-pellet measurements are unreliable. So bulk measurements have been developed consisting of applying a pressure on a catalyst volume and measuring the amount of fines formed during this treatment. There are two alternatives: a) measuring the pressure required to obtain a prefixed amount of fines (1% for ASTM D7084 [7]) b) measuring the amount of fines obtained under a prefixed pressure. We have used the latter procedure with the following operating conditions: Catalyst volume 12.5 ml Pressure 23 kg/cm2 Sieve size 0.85 mm It should be remarked that bulk crush strength simulates well the mechanical stress the catalyst may withstand in an industrial reactor.

2.4. Attrition resistance The attrition resistance of a fluid-bed catalyst can be evaluated by measuring the amount of very fine powder formed after an intensive attrition stress caused by several

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high-speed air jets. This procedure simulates in a few hours what happens in an industrial reactor during months. An example is given by the ASTM method D5757 [8]. We have adopted the same instrumentation and procedure, but with the following improvements: a) no obsolete dry or wet test meter, but mass flow controller b) no fines collection assembly, but exit gas to vent c) powder humidification only when indispensable d) previous elimination of fines below 45 micrometers e) duration of the air jet treatment: 1 hour f) after that the whole remaining powder is sieved again at 45 micrometers and the coarser fraction weighed to determine the amount of fines produced. Such modifications make the test faster and more reproducible.

3. Results and discussion 3.1. Abrasion resistance As a general rule, insufficient abrasion resistance sometimes occurs for granules, less for extrudates, more rarely for pellets. An undesired general problem is the formation of fines during reactor loading with obvious safety consequences. Furthermore, radial reactors can show bypass formation, while in trickle bed reactors the reaction product may be contaminated by catalyst fines. Two examples are discussed here, showing how to resolve the problem by modifying the preparation technology. The traditional ammonia synthesis catalyst (oxide-promoted magnetite) is frequently employed in its reduced-passivated form [9] to decrease the activation time in the reactor. As it consists of small irregular granules, large formation of fines frequently occurs during reactor loading. Furthermore also bypass formation sometimes occurs in the widely used radial reactors. The problem can be resolved by introducing a rounding step at the end of the preparation process, preferably before, but also after, prereduction. In fact, our experiments have shown that the abrasion loss of prereduced ammonia catalysts strongly decreases when the rounding time increases (Fig. 2).

Abrasion Loss, %

4

3

2

1

0

0

50

100

150

200

Rounding Time, min

Fig. 2. Prereduced ammonia synthesis catalyst. Increase of abrasion resistance with rounding time.

Of course other preparation variables can influence the abrasion resistance of this catalyst, but it is remarkable that, whichever level of abrasion resistance has been reached, the catalyst can be brought back to the specs by a final rounding treatment.

How the manufacturing technology can influence their mechanical strength

13

The catalyst employed for the purification of terephthalic acid (0.5% Pd on 2-4 mm active carbon granules, trickle bed reactor, [10]) can release black powder impairing the whiteness of the resulting PTA. This is due to small displacements of the catalyst granules in the running trickle bed. The problem is serious also for the connected loss of Pd (egg-shell catalyst). It cannot be easily resolved in the not unusual case of incorrect supporting of Pd. However, when the problem is caused by the low abrasion resistance of the active carbon, rounding the support before impregnation can be a good solution. Like for the ammonia synthesis catalyst, our experiments have shown that the abrasion loss of some poor active carbons decreases when the rounding time increases (Fig. 3). It can be immediately deduced that the catalyst so obtained, if properly manufactured, not only will give whiter PTA, but also will lose less Pd and have longer life. 3.0

Abrasion Loss, %

2.5 2.0 1.5 1.0 0.5

0

50

100

150

200

Rounding Time, min

Fig. 3. Active carbon granules 2-4 mm. Increase of abrasion resistance with rounding time.

3.2. Crush strength The crush strength is of special interest for the bottom catalyst layers and, for ringshaped catalysts, also during catalyst loading in the reactor. Average values of radial crush strength hardly exceed 10 kg in most industrial catalysts. Much lower values are usually found for ring-shaped catalysts. There are a lot of preparation variables influencing the crush strength of the final catalyst, depending on the formation procedure (tableting, extrusion, beading [11]). We will discuss here some examples. Styrene catalysts are formed by extrusion. A preferred composition is Fe-K-Ca-CeMo oxides [12,13]. The crush strength can be increased by increasing the calcination temperature, but this operation is very critical as to activity (Fig. 4). It results that a calcination temperature very close to 970°C should be adopted. It is interesting that small amounts of Mg compounds can strongly impair the crush strength (Fig. 4). Therefore the purity of the raw materials must be carefully checked.

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8

90

6

80

Radial Crush Strength, Kg

100

4 2 0 800

70

Mg 0.5%

60

Mg 1.0%

850

900

950

Conversion, %

10

1000

50 1050

Temperature, °C

Fig. 4. Styrene catalysts. Fe-K-Ca-Ce-Mo oxides. Extrudates 5x3 mm. Influence of calcination temperature on activity and crush strength.

Formaldehyde catalysts are ring-shaped (for instance 4x2x4 mm) and consist of FeMo oxides [10]. Their radial crush strength is very low (about 0.3 kg) and there is a definite need to increase it, in order to avoid extensive breaking during reactor loading. In fact the crush strength increases with the calcination temperature, but unfortunately there is at the same time a sudden decrease of surface area (Fig. 5), therefore of activity. A suitable compromise can be looked for. It should be remarked that temperatures higher than 700°C cannot be used, due to extensive shrinking of the rings. 5 4

0.45

3 0.40 2

2

Surface Area, m /g

Radial Crush Strength, Kg

0.50

0.35

0.30 450

1

500

550

600

650

700

0 750

Temperature,°C

Fig. 5. Ring-shaped Fe-Mo oxide catalysts. Influence of calcination temperature on surface area and radial crush strength.

Methanol synthesis catalysts consist of Cu-Zn-Al oxides and are activated in situ by reduction of CuO to Cu [14]. They are cylindrical pellets made by tableting. During activation, if the pellets have been completely dried, the catalyst loses about 10% of its mass as water coming from reduction of CuO by hydrogen. This makes the pellets more brittle, therefore decreasing the crush strength, which of course must be measured not on the fresh, but on the activated catalyst. It can be thought that, if the fresh catalyst contains additional water, its crush strength will be lower. This has been fully confirmed by our results (Fig. 6). Therefore the powder to be tableted must be brought to dryness, though this can make tableting more problematic.

How the manufacturing technology can influence their mechanical strength

15

Radial Crush Strength, Kg

14 12 10 8 6 4 2 0

0

2

4

6

8

10

Water Content, wt%

Fig. 6. Methanol synthesis catalysts. Influence of water content on crush strength of activated catalysts.

3.3. Attrition resistance Increasing the attrition resistance of the few fluid-bed catalysts brings to an appreciable reduction of catalyst costs due to a decreased consumption. High surface area alumina in its various forms (boehmite, bayerite, gamma, delta) and silica-alumina are the preferred supports for fluid bed catalysts. However in many cases their attrition resistance is not satisfactory, like, for instance, in the case of pure and silica containing boehmites. A remarkable improvement of the attrition resistance of these supports can be obtained by fluorination with HF (Table 1). Table 1. Attrition resistance of alumina supports for fluid bed catalysts. Sample

% fines formed

% fines formed after fluorination

C 1.5*

15.6

11.3

C 5*

15.0

11.2

CP

18.9

10.5

D

25.4

14.0

*SiO2 content

Of course fluorination can bring to modification of other properties, like surface area and acidity, which should be tested.

4. Conclusions Mechanical strength is a property of utmost importance for the industrial use of heterogeneous catalysts. Abrasion resistance and radial crush strength (for pellets) and attrition resistance (for powders) should be routinely measured for quality control of industrial catalysts before reactor loading or better before catalyst purchasing. Such measurements can be conveniently performed using the respective ASTM standard methods, whose possible improvements are suggested. Many variables of the catalyst preparation procedure can influence the mechanical properties of the final catalyst. It is remarked that this R-D step must be performed during the scale-up of catalyst production at the pilot scale. It is shown that the mechanical strength of the following catalysts can be improved in the following ways:

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Ammonia (oxide-promoted magnetite) Prerounding PTA (Pd on active carbon) Support prerounding Styrene (Fe-K-Ca-Ce-Mo oxides) Optimizing calcination temperature Formaldehyde (Fe-Mo oxides) Optimizing calcination temperature Methanol (Cu-Zn-Al oxides) Dryness before tableting Fluid bed aluminas Fluorination Finally, it is stressed that the mechanical properties of the fresh catalyst are completely useless when the catalyst has to be activated in the industrial reactor [15]. The activation conditions must of course be optimized to get a high mechanical strength of the working catalyst, which should be tested after activation and, if necessary, suitable passivation.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

F. Traina and N. Pernicone, Preparation techniques and their influence on the properties of the solid catalysts, Chim. Ind. (Milan), 52 (1970) 1. D. Wu, J. Zhou and Y. Li, Mechanical strength of solid catalysts. Recent developments and future prospects, AIChE J., 53 (2007) 2618. N. Pernicone, Scale-up of catalyst production, Catal. Today, 34 (1997) 535. Annual Book of ASTM Standards, ASTM International Publ., West Conshohocken, Pa., USA, (2009) p. 329. Annual Book of ASTM Standards, ASTM International Publ., West Conshohocken, Pa., USA, (2009) p. 334. Annual Book of ASTM Standards, ASTM International Publ., West Conshohocken, Pa., USA, (2009) p. 430. Annual Book of ASTM Standards, ASTM International Publ., West Conshohocken, Pa., USA, (2009) p. 438. Annual Book of ASTM Standards, ASTM International Publ., West Conshohocken, Pa., USA, (2009) p. 423. F. Pinna, T. Fantinel, G. Strukul, A. Benedetti and N. Pernicone, TPR and XRD study of ammonia synthesis catalysts, Appl. Catal A., 149 (1997) 341. N. Pernicone, Catalysis at the nanoscale level, CATTECH, 7 (2003) 196. N. Pernicone and F. Traina, Commercial catalyst preparation, in Applied Industrial Catalysis, Vol.3, Academic Press, (1984) p. 1. J.L. Smith, B.S. Masters and D.J. Smith, Dehydrogenation catalyst, U.S.Pat. 4,467,046 (1984) L.Forni and N.Pernicone, Catalyst for the dehydrogenation of ethylbenzene to styrene, Eur. Pat. Appl. 03714907.7-1211 (2003). C. Baltes, S. Vukoievic and F. Schuth, Correlations between synthesis precursor and catalyst structure and activity of a large set of CuO/ZnO/Al2O3 catalysts for methanol synthesis, J. Catal., 258 (2008) 334. N. Pernicone and F. Traina, Catalyst activation by reduction, in Preparation of Catalysts II, Elsevier (1978) p. 321.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Coating metallic foams and structured reactors by VOx/TiO2 oxidation catalyst: Application of RPECVD Adil Essakhia, Axel Löfberga, Philippe Supiotb, Brigitte Mutelb, Sébastien Paula, Véronique Le Courtoisa, Elisabeth Bordes-Richarda a

Université Lille Nord de France - Unité de Catalyse et de Chimie du Solide, UMR CNRS 8181, 59655 Villeneuve d’Ascq, France – [email protected] b Institut d’Electronique, Microelectronique et Nanotechnologie (IEMN) - UMR CNRS 8520, 59655 Villeneuve d’Ascq, France

Abstract VOx/TiO2, catalyst of oxidative dehydrogenation of propane, was immobilised on a SiO2 film coating stainless-steel (SS) plates and foams figuring out structured reactors. SiO2 is expected to act as a primer and a barrier against poisoning of VOx/TiO2 catalyst by elements of SS. The adhesive SiO2 layer was first coated on 2D- (plates) and 3D-SS substrates (foams) A good adhesion was obtained after polymerisation by RPECVD (Remote Plasma Enhanced Chemical Vapor Deposition) of a 6 µm-thick tetramethyldisiloxane polymer layer, followed by calcination (650°C) to obtain the SiO2 layer. A post-treatment in N2/1.5%O2 plasma afterglow was necessary to eliminate remaining carbon traces after calcination. The resulting SiO2/SS objects were dip-coated in TiO2–anatase aqueous suspension. Vanadium isopropoxide was grafted on calcined TiO2/SiO2/SS, yielding VOx polyvanadates after calcination at 450°C. The mechanical stability of the VOx/TiO2 catalyst immobilized onto SiO2/SS was examined by scratch test and ultrasonic bath experiment. The successive coated layers were studied by Raman spectroscopy, SEM-EDX, electron probe microanalysis and XPS. A special RPECVD reactor was designed to coat foams instead of plates. For the first time, a thin and homogeneous layer of silica could be deposited through the whole foam. The other steps were applied to obtain VOx/TiO2/SiO2/SS foams. XPS and Raman characteristics of deposits were the same than for coated plates and VOx/TiO2 powders. Keywords: RPECVD, catalytic coatings, SiO2, TiO2, metallic foam, structured reactor

1. Introduction Thin films of oxides as coating materials have several applications in the field of sensors, electronic and photonics devices, environmental purification, sterilization and deodorization, self-cleaning surfaces (textiles, windows…), biosensors, orthopaedics, etc. Although it was at first to disperse metals and oxides as aggregates on an oxidic support, another large application is heterogeneous catalysis, as far as shaped 3Dcarriers like monoliths, foams, or structured walls of reactors are concerned. Most reactions being strongly exothermic, it is important to get rid of hot spots which are present in the catalytic pellets in fixed bed reactors. Indeed, hot spots are responsible for structural damages and early deactivation of the catalyst, and generally they promote reactions leading to by-products, like carbon oxides in selective oxidation reactions. An alternative which was proposed for automotive pollution control more than 30 years ago is the use of 3D structures like monoliths. These opened structures which are covered by

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a thin layer of catalyst favour efficient heat and mass transfers between the gaseous reactants, the catalytic active phase and the wall of the reactors in which they are inserted. A larger and larger use of monoliths and foams varying by the substrates and shapes is now made. As in other applications the deposit must be mechanically, chemically and thermally stable. However in catalytic applications its surface has also to be strongly reactive and to be able to suffer high flowrates of reactants/products without attrition, erosion or collapsing. As far as reactive thin films of oxides are concerned, a large challenge is to succeed in coating metallic substrates of reactors or inserted carriers with thick and high surface area layers of active phase. For most applications, the oxidic coating may bear no chemical resemblance with the metallic (e.g. stainless steel) substrate. This makes more difficult to get a good holding of the coating and to avoid poisoning of the active phase by its elements. In former studies of a two-layers catalyst (VOx/TiO2) deposited on stainless steel (SS) plates figuring out the reactor walls, iron was shown to diffuse outwards and to make surface iron vanadates that decreased the selectivity to propene in the oxidative dehydrogenation of propane [1-3]. In another case, Co/SiO2 for Fisher-Tropsch reaction, a primer layer consisting in a thin layer of silica could be deposited by remote plasma enhanced chemical vapor deposition (RPECVD) [2-5]. This method was successful for coating cobalt on porous silica support onto silica-coated SS plates which were used in a specially designed reactor [2,3,6-8]. The methods of coating based on powder suspension and sol-gel preparation are well documented [9,10], and several others are known (magnetron sputtering, atomic layer deposition, chemical vapor deposition, reactive plasma, etc.) to make thin films. The use of PECVD is an alternative method worthwhile to be studied as shown by our previous results. In the preparation of thin films, PECVD has not received as high attention as methods using evaporation, though a wide range of experimental parameters can be varied to control the microstructure of the films [5,11-13]. The process we used is based on cold plasma assisted polymerization of tetramethyldisiloxane (TMDSO), the precursor of silica. The silica-like layer coated on stainless steel substrate is intended to ensure the mechanical steadiness [6-8], to act as a bonding layer for the deposition of the support of the active phase which is VOx/TiO2, and also to hinder Fe diffusion during the reaction. Successful results obtained on this catalyst/structured wall reactor or catalyst/3D-carriers system, the active phase of which is well-known for its applications in chemical industry (o-xylene oxidation to phthalic anhydride), as well as in pollution abatement (deNOx with ammonia, COV, etc.), could help to promote the use of new reactors with enhanced heat and mass transfers.

2. Experimental 2.1. Preparation of VOx/TiO2/SiO2/SS plates and foams

Three main steps are required to prepare the catalytic system VOx/TiO2/SiO2/SS. The stainless steel (AISI 316L) plates (50 mm × 20 mm × 0.5 mm) and foam (Porvair®, 40 ppi, density 5.4%) substrates were first sonicated with ethanol (30 min) to eliminate organic pollutants, and then twice in deionized water (30 min) before drying at 110°C for 3 h. 2.1.1. Coating of SS by SiO2 by RPECVD The new experimental set-up of the cold remote nitrogen plasma assisted polymerization reactor was adapted from [5,14]. Details can be found in [15]. Summarising, the nitrogen flow was excited by a microwave discharge (2450 MHz–200 W) in a fused silica tube. By continuous pumping (roots pump Pfeiffer), the reactive

Coating metallic foams and structured reactors by VOx/TiO2 oxidation catalyst

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species, mainly atomic nitrogen in the ground electronic state N(4S), flowed from the discharge zone to the deposition zone located 1 m downstream. The TMDSO monomer (Sigma Aldrich, grade 97%), premixed with O2, was introduced in the remote nitrogen plasma through a coaxial injector. Flowrates of N2, O2 and TMDSO at 1800, 25, 5 sccm, respectively, were controlled at 550 Pa by means of MKS mass-flow controllers. The deposition rate was in situ measured by interferometry using He-Ne laser (λ = 632.8 nm) and a photodetector. The substrate was first pre-treated by remote nitrogen afterglow for 5 min for cleaning and then the monomer/O2 mixture was added without air exposure for the deposition step. Details about the reaction mechanism of decomposition of the monomer can be found in [14]. The TMDSO plasma polymer (ppTMDSO) coated substrate was then post-treated in a N2-1.5% O2 remote plasma during 5 min and the film was mineralized by thermal treatment in air to obtain the SiO2 layer. To remove carbon remaining traces shown by laser Raman spectroscopy (3000 cm-1 for CH3 and 1500 cm-1 for Si-C bonds), a N2/1.5%O2 remote plasma treatment was finally applied during 5 min. A special sample holder was designed to coat metallic foams which were cut in 1 cm × 1 cm × 0.7 cm blocks [15]. The same procedure than for plates was repeated. 2.1.2. Coating of SiO2/SS by TiO2 SiO2/SS plates or foams were dipped under stirring in an aqueous suspension containing 60 wt% TiO2 (Sigma-Aldrich) particles during 5 min, and withdrawn at 6 mm.s-1 [1,2]. The TiO2/SiO2/SS carriers were calcined in air flow at 110°C during 1 hour, and then at 700°C during 2 h (heating rate 80°C/min). 2.1.3. Coating of TiO2/SiO2/SS by polyvanadates The TiO2/SiO2/SS carriers were dip-coated in various amounts of VO(OPr)3 in dry ethanol, yielding VOx/TiO2/SiO2/SS coated plates and foams after calcination in air at 450°C for 4 h.

2.2. Methods of analysis Most analyses were performed at every stage of preparation. The silica films were analyzed using a Fourier Transform Infrared (FTIR) Perkin-Elmer spectrometer. Spectra were recorded in specular reflexion mode in the spectral range 4000- 400 cm-1 (resolution 4 cm-1). Laser Raman spectra (LRS) of TiO2/SiO2/SS carriers were recorded on LabRAM Infinity spectrometer (Jobin Yvon) equipped with a liquid nitrogen detector and a frequency-doubled Nd:YAG laser supplying the excitation line at 532 nm. The power applied on the sample was less than 5 mW. X-ray photoelectron spectroscopy (XPS) experiments were carried out on pieces of covered substrates using VG-Escalab 220 XL spectrometer. A monochromatic Al Kα X-ray source was used and electron energies were measured in the constant analyzer energy mode. The pass energy was 100 eV and 40 eV for the survey and single element spectra, respectively. XPS binding energies were referred to C 1s core level at 285 eV. Electron Probe Micro Analysis (EPMA) was performed on samples embedded into epoxy resin and polished with abrasive discs (2400 to 3 µm granulometry). A Bal-Tec SCD005 sputter coated allowed depositing a thin carbon film. Elemental analysis were made using a wavelength dispersive X-ray spectrometer (Cameca SX-100 microprobe analyser) working at 15 kV and 15 nA for back scattered electron (BSE) images and at 15 kV and 49 nA for Si, Ti and Fe Kα X-ray profiles and mapping using TAP, PET and LiF crystals respectively. The Scanning Electron Microscope (SEM) Hitachi 4100 S was equipped with micro-analyzed (EDS) and a Field Emission Gun (FEG). The working voltage was 15 KeV. The analyzed volume was estimated to be approximately 1 µm3.

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The wettability of the deposited silica layers was evaluated from contact angle with deionized water using a Krüss computer-controlled goniometer (± 1° accuracy). BET method and porosimetry was applied to TiO2 coatings on silica after scratching of the layer, confirming that the initial characteristics (10 m2/g, 99.8% anatase, average particle size 10 µm) of TiO2 remained the same after these operations.

3. Results and discussion 3.1. Coating of SS plate and foam by silica The rate of deposition of the ppTMDSO polymer film onto SS plates was evaluated by two methods depending on the thickness (t). In situ measurements by interferometry were made for t < 12 µm. For t > 5µm, EPMA-BSE images were obtained ex situ on samples after various deposition times. In the chosen experimental conditions the deposit rate was 1.0 ± 0.1 µm.min-1 with a very good linearity for thicknesses ranging from 1 to 35 µm (Figure 1). Figure 2 (a-c) shows SEM pictures of the SS plate (after corrosion by 30% sulfuric acid, 1 h). After coating, the thickness of the layer (t ≈ 30 μm) is quite regular on the whole plate (Figure 2 b). FTIR spectroscopy of ppTMDSO/SS confirmed the polymerisation process by, e.g., the presence of asymmetric Si-O-Si stretching at 1000 and 1200 cm-1. The intensity of the double band at 600 cm-1, which is characteristic of the asymmetric elongation of Si-O-Si bond in a polymeric conformation, was observed to increase. However at this stage, it showed also that the organic component of the siloxane was not removed completely during the decomposition of the precursor – and thus that polymerisation was not completed – since small bands assigned to the asymmetric Si-CH3 and symmetric Si-H stretchings modes were found at 2145 cm-1 [6-8,14], as well as the symmetric and asymmetric stretchings ones of Si-CH3 at ca. 2910 and 2960 cm-1, respectively. The mineralisation to SiO2 being required for our applications, the variation of four parameters (post-treatment or not in N2/O2 remote plasma, heating rate from 1 to 5°C/min, temperature of plateau 450 and 650°C, thickness of polymer layer from 5 to 15 µm) was examined using a matrix experiments. As shown by BES images in the example of a 15 µm-thick film/SS, post-treated (5 min) in N2/1.5%O2 remote plasma and calcined at 5°C/min up to 650°C (1 h), the film burst in pieces (Figure 2 c). XPS confirmed the presence of surface iron together with Si, C and O photopeaks. The peak at 710.7 eV (Fe2p3/2 binding energy) accounts for the formation of Fe2O3 which could be expected owing to the oxidising post-treatment. Two oxygen photopeaks are due to Si-O and Fe-O bonds at 532.9 and 529.7 eV [16]. Most carbon comes from contamination but a peak at 288.6 eV could correspond to remaining polymeric carbon. As the cross-cut-tape test also revealed that the coating was not adherent enough, it was decided to decrease the thickness and to modify the post-treatment. The best results were obtained for a 5 μm thick film/plate without N2/O2 plasma afterglow treatment and calcined at 1°C/min heating rate up to 650°C (1 h). XPS analysis indicated a low carbon level with surface composition SiC0.05O1.8. After the treatment the layer shrinked by 68% down to a 1.6 μm thickness. SEM (Figure 2 d,e) and EPMA (Figure 3) showed that the plate was covered homogeneously by the SiOx layer, all Fe being hidden even in the case of a stripe. The mechanical stability examined by both cross-cut-tape test and ultrasonic n-heptane bath was good. FTIR and Raman spectroscopies showed that the remaining carbon (νs and νas of Si-CH3 at 2910 and 2960 cm-1) was greatly reduced by a N2/O2 plasma afterglow treatment performed during 5 min [15], since the final surface composition obtained by XPS was SiC0.01O1.82.

Coating metallic foams and structured reactors by VOx/TiO2 oxidation catalyst

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Figure 1. Linear relationship between the thickness of ppTDMSO layer and the time of deposition.

a

b

Resin

SS

c

d

e

SS

SiO2 Resin

Figure 2. Pictures obtained by SEM or EPMA of SS plates, before (a), after coating by ppDMSO polymer (ppDMSO/SS) (b); and of SiO2/SS after mineralisation (c-d) and optimised treatment (e).

a

b

Figure 3. EPMA of Si and Fe for well-coated SiO2 plate along (a), line “a”; (b), line “b” drawn on Figure (2e).

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The whole optimised coating process was applied to cover SS foam, using the sample holder specially designed for the plasma to convectively flow through the whole volume of the foam [15]. No acidic treatment was applied to the foam before coating as its microstructure favoured the anchoring of ppDMSO film (Figure 4 a,b). The foam was cut in several pieces and examined by EPMA. The homogeneity of the coating for an initial thickness t ≈ 5 μm was found to be good throughout the foam. A picture of a branch is shown Figure 4 c.

a

b

c

Figure 4. Pictures obtained by SEM or EPMA of foams before (a), after coating by ppDMSO polymer (ppDMSO) (b); SiO2/SS is obtained after mineralisation (c).

3.2. Coating of TiO2 on SiO2/SS

SS plates coated with 1.6 μm SiC0.01O1.82 (further called SiO2/SS) film were dip-coated in an aqueous suspension of 60 wt% TiO2 which had first been stabilised by stirring during one hour at room temperature. The optimal composition of the aqueous TiO2 suspension was determined after measuring the zeta potential. After being withdrawn at constant rate (6 mm.s-1) the TiO2/SiO2/SS plates were calcined at 700°C (2 h). However the titania coating was not homogeneous, as shown by EPMA on Figure 5.

Figure 5. Aggregates of TiO2 coating SiO2/SS plates without pretreatment and profiles of Si, Ti Fe (from top to bottom) along the arrow drawn on picture.

The thickness of TiO2 is ≈ 6 µm in grey areas (zone 1) and higher in aggregates (≈ 12 µm, zone 2) while the black colour (zone 3) accounts for uncovered SiO2. Moreover the mechanical stability was very poor. The contact angle was measured for SS (87°), ppTMDSO/SS (104°), and SiO1.8/SS (34°) (Figure 6 a-c). Several pretreatments of SiO2/SS were performed to decrease the hydrophobicity of the surface and favour further adhesion of titania [15]. The best contact angle was obtained by combined pouring in Brown solution (1 g NaOH in 4 mL ethanol and 3 mL H2O) followed by drying (1 h) at 110°C, and CNRP N2/O2 55 min treatement (Figure 6 d). The resulting coatings (ca. 6 μm) of titania onto SiO2/SS were reproducible, homogeneous and stable after the calcination step (Figure 7).

Coating metallic foams and structured reactors by VOx/TiO2 oxidation catalyst

a

b

c

23

d

Figure 6. Contact angles measured: (a) 87° for bare SS plate; (b) 104° for ppTMDSO/SS; (c) 34° for SiO1.8/SS; (d) 29° for Brown solution treated SiO1.8/SS.

SS (316L)

TiO2 SiOx

Resin

Figure 7. EPMA picture of TiO2 coating SiO2/SS plate and profiles of Si, Ti and Fe (from up to bottom) along arrow drawn.

a

b

c

d

Figure 8. Coating of TiO2 on SiO2/SS foams observed by EPMA before (a), (b), and after calcination (c),(d).

The same optimized treatment was applied to SiO2/SS foams, resulting on the covering by a uniform 15 μm thick layer of TiO2. EPMA profile image and X-ray mapping give evidence for the two successive TiO2 and SiO2 layers (Figure 8). After calcination at 700°C for 2 h, the layer did not resist well, it shrinked and cracked (Figure 8). Tries were done with thinner layers, and finally the highest thickness compatible with mechanical stability of both TiO2 layer and TiO2/SiO2/SS foam itself is

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6 μm (Figure 8). Finally, both TiO2/SiO2/SS plates and foams were impregnated by various amounts of vanadium, using vanadium isopropoxide to be grafted [1,2]. Amounts corresponding to a theoretical monolayer (ca. 3.5 wt% V2O5) and above (up to 20 wt% V2O5) were grafted. XPS analyses showed that the surface vanadium concentration was directly related to the initial concentration of VO(OPr)3 in ethanol solutions [2]. The vanadium concentration being very small, only XPS analysis could reveal its presence. The V2p3/2 photopeak at 517.3 eV means that the oxidation state is mostly V5+. These figures are similar to those found for VOx/TiO2 powders.

4. Conclusion RPECVD technique could be used with success to coat stainless-steel plates in a uniform way by a silica-like layer which is expected to protect the active phases from poisoning. After a proper treatment to enhance its wettability, this layer serves also as a carrier to TiO2-anatase, which is required to enhance the catalytic properties of the active polyvanadate phase. The optimised procedure could be applied to cover stainlesssteel foams for the first time. The catalytic properties of both plates and foams in the oxidative dehydrogenation of propane in specially designed reactors is in progress.

Acknowledgements The Agence Nationale de la Recherche (ANR-France) is aknowledged for its support to project Millicat (ANR-06-BLAN-0126).

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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T. Giornelli, A. Löfberg, E. Bordes-Richard, 2005, Thin Solid Films, 479, 64-72. T. Giornelli, A. Löfberg, E. Bordes-Richard, 2006, Appl. Catal. A: General, 305, 197-203. T. Giornelli, A. Löfberg, L. Guillou, S. Paul, V. Le Courtois, E. Bordes-Richard, 2007, Catal. Today, 128, 201-209. A. Borrás, A. Yanguas-Gil, A. Barranco, J. Cotrino, A. R. González-Elipe, 2007, Phys. Rev. B 76 5303. P. Supiot, C. Vivien, A. Granier, A. Bousquet, A. Mackova, F. Boufayed, D. Escaich, P. Raynaud, Z. Stryhal, J. Pavlik, 2006, Plasma Proc. Polym., 3, 100. L. Guillou, V. Le Courtois, P. Supiot, 2005, Materiaux & Techniques, 93, 335-345. L. Guillou, D. Balloy, P. Supiot, V. Le Courtois, 2007, Appl. Catal. A: General, 324, 42-51. L. Guillou, P. Supiot, V. Le Courtois, Surf. Coat. Technol. 202 (2008) 4233. L.L.P. Lim, R.J. Lynch, S.-I. In, 2009, Appl. Catal. A: General, 365, 214. V. Meille, 2006, Appl. Catal. A: General 315, 1. A. Amassian, P. Desjardins, L. Martinu, 2004, Thin Solid Films 447, 40. S. Pongratz, A. Zoller, 1992, Annu. Rev. Mater. Sci., 22, 279. L. Martinu, D. Poitras, 2000, J. Vac. Sci. Technol. A 18, 2619. F. Callebert, P. Supiot, K. Asfardjani, O. Dessaux, P. Dhamelincourt, J. Laureyns, 1994, J. App. Polym. Sci., 52, 1595. A. Essakhi, A. Löfberg, P. Supiot, B. Mutel, S. Paul, V. Le Courtois, E. Bordes-Richard, Proc. 6th Int. Symp. Polyimides and Other High Temperature/High Performance Polymers: Synthesis, Characterization and Applications, Melbourne, Florida, USA, November 9-11, 2009. Submitted. http://www.lasurface.com

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Washcoating of metallic monoliths and microchannel reactors L. C. Almeida1, F. J. Echave1, O. Sanz1, M. A. Centeno2, J. A. Odriozola2, M. Montes1 1

Department of Applied Chemistry, University of the Basque Country, Paseo Manuel de Lardizabal, 3, ES-20018 San Sebastián, Spain. 2 Inorganic Chemistry Department and Institute of Material Sciences of Seville, University of Sevilla - CSIC, Avenida Américo Vespucio 49, 41092 Sevilla, Spain

Abstract The most important parameters controlling the washcoating of metallic structures from catalytic slurries are reviewed. The slurry must be stable with adequate rheological properties controlled by the solid content, particle size and additives. The metallic substrate must be pre-treated to obtain an adherent surface scale compatible with the coating and presenting appropriate surface roughness. The quality of the produced coating (homogeneity, specific load and adherence) depends essentially on slurry properties (viscosity and solid content) and on the technique used to remove slurry excess. Keywords: washcoating, catalyst preparation, structured catalysts, microchannel reactor

1. Introduction During the last decade, catalysts coated on metallic surfaces are reaching a prime importance [1]. Two types of structures justify this interest: metallic honeycombs and microchannel reactors. When pressure drop and thermal and mechanical resistance are key issues in catalytic processes, metallic honeycombs (monoliths) are the best option [2], while microchannel reactors are in the nucleus of the new tendency to process intensification [3]. The main step for manufacturing metal-based structured catalysts and reactors is the obtention of coatings with the required characteristics: amount loaded, homogeneity and adhesion. For this purpose, different techniques have been proposed [2,4-7], among them, in situ growing, electro-deposition, anodization, CVD, PVD…etc. However, the most popular and versatile is washcoating or dip coating using slurries or sols. It is possible to find in the literature works describing two steps procedures, which start with the coating of the catalytic support and finish with the impregnation of the active phase. In this communication, however, we will consider only washcoating procedures using slurries prepared from previously synthesized catalysts. The washcoating technique allows obtaining coatings on most metallic surfaces with the required homogeneity, amount loaded and adhesion provided that the metallic surface properties (alloy composition and pre-treatment conditions), the coating procedure including excess elimination and, in particular, the properties of the catalyst slurries are well controlled. This communication reviews the main parameters controlling the coating of metallic monoliths and microchannel reactors on the basis of our experience studying different systems: catalysts for Fischer-Tropsch synthesis [8], methane combustion

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[9,10], VOC elimination [11-14], CO [15-17] and phenol [18] oxidation, hydrogenation of sunflower oil [19], methanol and ethanol reforming, WGS and PROX. In these studies, we have tested different alloys, Al-alloyed ferritic steels, AISI 304, brass and aluminium, and different catalysts.

2. Results and discussion The first step to washcoat a metallic substrate is to prepare a stable slurry of the catalyst to be deposited. Next, nature and roughness of the substrate surface must be adequate in order to fix the catalyst coating. Finally, the metallic monolith is washcoated by immersing and withdrawing in the slurry followed by the elimination of the excess. In the following, we will discuss some of the most important variables controlling every step.

2.1. Slurry Stable slurries (non-settling) are obtained when the terminal velocity of the particles is very small. Small velocities are the result of cancelling the gravity force by the drag force. Particle settling is well described in the creeping flow regime that states for Newtonian fluids in which, the terminal velocity depends directly on the square of the particle size and the difference in density between the solid and the fluid, and inversely on the viscosity. Therefore, for a particular catalyst and liquid (usually water) to increase the stability it is convenient to reduce the particle size and to increase the viscosity of the medium. Let analyze these two parameters: particle size and viscosity. 2.1.1. Catalyst particle size Milling can reduce particle size, being ball milling the technique most frequently used in several studies about this parameter [20-23]. To obtain stable slurries of different solids particle size distributions below 10 µm have been proposed [24,25]. Nevertheless, the positive effect of decreasing particle size has a limit since very small particles induce flocculation. Indeed, the smaller the particles size the higher the surface to volume ratio, therefore, the interaction between particles that produces gelling is favoured. In a comparative study two different aluminas were used. Ball milled alumina, d4/3 = 2.8 µm, allowed preparing a slurry with 25% content in solids adequate to washcoat Fecralloy monoliths with 1 mg/cm2 load and 70% adherence. By contrast, Alumina Cabot FA 100, d4/3 = 0.34 µm, produce slurries with the same rheological properties having a maximum of 10% solid content resulting in washcoated monoliths with only 0.3 mg/cm2 load with even lower adherence, 38%. 2.1.2. Viscosity Assuming the Einstein model for the dilute dispersion of hard spheres the viscosity of ideal water slurries (non-interacting spheres) only depends on the solid content, being higher as the solid content increases [26]. Nevertheless, when considering surface interactions additives will play a fundamental role. First, we will consider pH modifiers since polarization of the surface of the oxide particles controls inter-particle interactions. In a first approach, pH values far enough from the pH value of the iso-electric point of the solid will assure the particles mutually repel each other and cannot aggregate, since they will bear electric charges of the same sign. However, the pH not only controls the stability of the slurry via peptization-flocculation, but also the viscosity because it controls the aggregate size. Other additives may be used in order to promote slurry stability, long-chain surfactants containing hydrophilic and hydrophobic groups, for instance, adsorbs on the catalyst surface leading to steric stabilization of the slurry [26]. Thickeners that increase the viscosity can be also used [2,27], inorganic colloids (alumina, silica...) or organic

Washcoating of metallic monoliths and microchannel reactors

27

compounds (polyvinyl alcohol, polyvinylpirrolidone, ammonium methacrylate...) are examples of these additives. However, it has to be considered that surfactants, thickeners and other additives may present competitive and synergic mechanisms making it difficult to a priori predict the behavior of such complex mixtures.

2.2. Washcoating procedure In these procedures, monoliths or, in general, metallic structures are dipped into suitable slurries, kept in the particle dispersion for a certain period of time and finally withdrawn. Once the metallic monolith is withdrawn it must be drained and the excess slurry eliminated. To form a thin oxide layer on the metal surface the metallic structure has to be dried and calcined to suitable temperatures. The procedure ends by evaluating the produced washcoated material by measuring three characteristics: the specific obtained load (mg/cm2), the homogeneity of the coating (by optical or electron microscopy) and the adherence (by the ultrasonic bath test). Relevant parameters controlling this procedure are discussed below. 2.2.1. Dipping and withdrawing velocity For the washcoating of ceramic monoliths this is a key factor in order to ensure the filter cake formation proposed by Kolb et al. [28]. However, for metallic substrates where the porosity of the oxide scale formed on the alloy surface is negligible no special care must be taken to control this velocity. Importantly, the dipping velocity and the time the monolith is kept in the dispersion must be long enough to allow the slurry filling completely the channels, these variables depending on the slurry viscosity. 2.2.2. Substrate surface Passive layer, protective oxide scale produced by the spontaneous oxidation in air or by a specific pre-treatment, covers all the non-noble metals and alloys. The chemical nature and physical appearance of this scale influences relevant aspects of the washcoating process. The first one is the contact angle with the slurry that is also affected by the surface roughness. For low contact angles, the capillary rise will allow a quick channel filling and the substrate is well coated by the slurry. On the contrary, if the contact angle is high slow channel filling must be expected, and at the limit, for contact angles higher than 90º, the substrate became hydrophobic, making it extremely difficult to fill the channels by dipping. The contact angle depends on the surface energies of both solid and liquid and hence, the surface nature and slurry additives modify surface energy of either the solid or the liquid. For instance surfactants, used as slurry additives, modify the liquid surface tension and therefore the contact angle with the metallic substrate. The chemical nature of the oxide scale, in particular the chemical composition and hydroxylation degree, determines the oxide scale surface energy and controls the contact angle with the slurry. These surface energy modifications may result in wetting (hydrophilic) or non-wetting character (hydrophobic) of the slurry. The metal substrate surface is of paramount importance in determining the adhesion of the coating. The formation of chemical bonds (chemical compatibility) between the oxide scale and the slurry may enhance the adhesion of the coating; however, as pointed out by Agrafiotis et al., the washcoating adhesion to the support takes primarily place through mechanical mechanisms such as “anchoring” and interlocking of the washcoat particles to the surface irregularities of the support and to a much lesser extent via chemical or affinity mechanisms [24]. Depending on the alloy nature and treatment of metallic substrates surface irregularities or roughness can be tailored [2]. Figure 1 shows two examples of proper ratio between the dimension of surface roughness and catalyst particle size: the catalyst particle must be smaller than the surface roughness to fit inside.

28

L.C. Almeida et al.

a)

b)

Figure 1. a) 20%Co/Al2O3 on Fecralloy calcined in air 22 h at 900ºC. b) Pt/ZSM5 on aluminum anodized under cracking conditions [29].

2.2.3. Size and shape of the channels Capillary forces acting during coating and drying produce a certain accumulation of catalyst at the channel corners. As Figure 2 shows, the relative importance of these accumulations depending on size and shape (corner angle) of the channels.

Figure 2. Micrographs showing catalyst accumulation in metallic substrates (Fecralloy). Left side: 20%Co/Al2O3 on square (700 x 700 µm) microchannels. Right side: ZSM-5 deposited on micromonoliths (222 cells/cm2) [30].

The influence of the size of the channel on the coating adhesion was studied by means of different metallic substrates. Homemade Fecralloy monoliths of 350 cpsi (M sample) and 1180 cpsi (µM sample), Duocel aluminum foam of 10 ppi (E10 sample) and Fecralloy plates with square microchannels of 0.7 x 0.7 mm2 (µP sample) were coated with a steam reforming catalyst. The non-settling slurry contained 12.3% of catalyst (7.5%Ni/La-Al2O3), 3.8% polyvinyl alcohol and 8.5% colloidal alumina in water Nyacol AL 20); this formulation resulted in a viscosity of 9.6 mPa·s (measured at 3240 s-1). After coating, the excess of slurry was eliminated by centrifugation at 500 rpm for 2.5 min in the case of monoliths and foams, and by air blowing for the microchannels plates. Several successive coatings were carried out until the desired load was reached; a drying step (120ºC for 30 min) is always performed between successive coatings. Figure 3 presents the obtained specific load, and Table 1 the adhesion and the textural properties.

Washcoating of metallic monoliths and microchannel reactors

Specific load (mg/cm2)

4

3

μM - 6 μM - 7 M-5 M-6

29

E-5 E-6 μP - 3 μPS - 3

2

1

0 1

2

3

4

5

6

7

Number of coatings

Figure 3. Specific load vs. coating number obtained on different metallic substrates with a Ni/LaAl2O3 slurry.

The results in figure 3 clearly indicate that the coating procedure is very reproducible obtaining additive loads by repeated coatings. The specific load is similar for different geometries, and only slightly higher load were obtained on microchannel plates as a result of the different method used to eliminate the excess. The textural properties of the catalyst were preserved on the coating. The adhesion was very high for all the coating inside channels, increasing on decreasing the hydraulic diameter as a consequence of geometrical constraints. In the case of foams, the adhesion was significantly lower due to the external character of the coating around the foam struts (no geometrical constraints). Table 1. Textural properties and adherence of Ni/La-Al2O3 coatings on different substrates. The textural characteristics of the powder obtained after drying the slurry are shown for comparison. 2

SBET(m /g) Vp (cm3/g) Dp (nm) Hydraulic diameter (µm) Adherence (%)

Powder 51 0.26 16.9 -

μM-7 56 0.26 15.0

M-7 52 0.30 17.3

373

835

98

89

E10-5 -

μPS-3 700

51

93

2.2.4. Viscosity and additives The key parameter that controls the coating results is viscosity. Low viscosities allow to obtain highly adherent and homogeneous coatings but with low specific loads. Thus for obtaining the target loading numerous coating are required. On the contrary, high viscosity will allow high specific load per coating although the homogeneity is lower (accumulations and, at the limit, channels blocking) resulting in lesser adherent coatings. The optimal viscosity ranges between 5–30 mPa·s as proposed by several authors [3135]. However, the non-Newtonian character of the slurries with high solid contents makes difficult to compare the viscosity values obtained at different shear rates. The rheological properties are mainly controlled by the solid content of the slurry and the peptization step (pH and additives) [33]. But, as mentioned above, the role of additives (organics or inorganics) is complex producing synergic or competitive effects in the process variables. Table 2 resumes studies of the influence of these variables on

30

L.C. Almeida et al.

the washcoat of Co catalysts on different supports over Fecralloy monoliths (350 cpsi, pretreated at 900ºC for 22 h). Polyvinyl alcohol, PVOH, and colloidal alumina, CA (Nyacol S20, 20% solid content) were used as additives with three different supports presenting different particle size: Titania Millenium G5, Alumina Cabot FA-100 and Alumina Spheralite SCS 505 milled to different particle size distributions. Slurries were prepared with the maximum allowable solid content reaching a viscosity that ranges between 5 and 15 mPa·s. The slurry excess was removed by centrifuging at 500 rpm for 5 min, except for TiO2 + CA slurry that presented high viscosity and required 1200 rpm to prevent channel blocking. The coating procedure was repeated five times with drying at 120ºC between coatings. Finally, coated monoliths were calcined at 500ºC for 4 h. Table 2. Effect of slurry composition on viscosity, specific load and adherence of washcoated Fecralloy monoliths. Slurry TiO2 (27.3%) TiO2 + PVOH (27.3 + 6.4%) TiO2 + CA (27.3 + 13.2%) TiO2 + PVOH+CA (27.3 + 6.6 + 6.4%) Al2O3CABOT (11.7%) Al2O3CABOT + PVOH (11.7 +1.3%) Al2O3CABOT + CA (11.7 +1.3%) Al2O3CABOT + PVOH+A.C. (11.7 +1.3 +1.3%) Al2O3SPHERALITE(1) + CA (17.7 + 4.3%) Al2O3SPHERALITE(2) + CA (17.7 + 4.3%) Al2O3SPHERALITE(3) + CA (17.7 + 4.3%)

D4,3 (µm) 1.6 1.6 1.6 1.6 0.34 0.34 0.34 0.34 2.8 6.6 13.3

Viscosity (mPa·2) 5.2 7.6 21.8 12.3 6.6 5.6 11.8 7.3 10.8 10.5 11.8

Specific load Adherence (mg/cm2) (%) 0.41 24 0.41 27 1.14 73 1.30 76 0.32 40 0.22 100 0.23 96 0.22 95 1.05 97 1.40 93 1.55 91

As discussed in section 3.1.1, the small particle size of the alumina Cabot did not allow solid contents higher than 11.7% without jellification. Therefore, low specific load were always obtained. On the other hand, alumina Spheralite shows a wide particle size distribution (Figure 4) allowing high specific load with moderate solid contents. Nijhuis et al. proposed a model in which the smaller particles are allocated between the bigger ones increasing the adherence [32].

10 8 6

Co/Al2O3 Spheralite (1) Co/Al2O3 Spheralite (2) Co/Al2O3 Spheralite (3) dpd4,3 = 6.6 μm

Volume (%)

12

dpd4,3 = 0.34 μm

14

Co/Al2O3 Cabot FA100 dpd4,3 = 2.8 μm

4

dpd4,3 = 13.3 μm

2 0 0.01

0.1

1

10

100

1000

Particle size (mm)

Figure 4. Particle size distributions (left) and schematic representation of the coating with alumina Spheralite presenting a wide particle size distribution.

Washcoating of metallic monoliths and microchannel reactors

31

PVOH slightly increased the viscosity of the titania slurry without changing either the specific load or the adherence. However, in the case of the alumina Cabot the viscosity decreased resulting in low specific loads, although the adherence was excellent which is coherent with the low load. Nevertheless, adherence change is very interesting taking into account that PVOH disappears during calcination. Colloidal alumina (CA) had a very positive effect in the titania slurry, increasing viscosity and thus load, but specially significantly increasing adherence. This is probably due to the increase in chemical compatibility between the alumina scale covering Fecralloy and the titania particles. The simultaneous use of PVOH and CA with titania allowed reducing the viscosity while increasing both load and adherence. With alumina Cabot the use of both additives, simultaneously or separately, resulted in lower loads, but the adherence increased dramatically. 2.2.5. Elimination of the slurry excess Usually, excess slurry is removed either by air blowing [2,4,24,28,32] or centrifuging [29,36]. In general, by gravitational draining or by applying some form of pressure or vacuum to clear the channels of the excess but the adhered catalyst layer. Air blowing is easier, in particular when big structured devices are considered, but should be carefully executed to prevent heterogeneity [37]. Figure 5 presents a comparison of the specific load obtained coating microchannel plates, µP (Fecralloy plates with 10 microchannels of 0.7 x 0.7 x 20 mm3) and Fecralloy micromonoliths, µM (1180 cpsi) using a slurry adjusted to pH 3 using HNO3 and containing 20% catalyst (20%Co-0.5%Re/Al2O3) and 6% colloidal alumina (Nyacol AL20). The loading obtained when the excess is removed by centrifugation is half the one obtained after eliminating the excess by air blowing, whatever the structure, microchannel or micromonoliths, considered. Menon et al. [36] has previously reported a similar trend.

Figure 5. Effect of procedure used to eliminate the slurry excess on the specific load.

2.2.6. Drying During drying of slurries and colloids, strong capillary forces are generated contracting the solid coating. This can produce cracks and detachments from the substrate surface. When this phenomenon reduces the coating adherence, two strategies may be used. The first one is the use of additives, like PVOH or surfactants, to reduce surface tension. The second approach eliminates the capillary forces in using freeze-drying, but special care must be taken during freezing to prevent movement of the liquid phase, the freeze-dryer may allow keeping the monolith horizontal while continuously rotating it around its axis [32].

32

L.C. Almeida et al.

3. Conclusions To washcoat metallic substrates slurries must be stable and present moderate viscosities (5–30 mPa·s). Particle size (usually around 1 to 10 µm), solid content (as high as possible) and additives control these properties. The protecting scale covering the metallic substrate must be adherent, compatible with the catalytic coating and presenting a surface roughness that allow the catalyst particles to fit inside surface grooves favoring mechanical anchoring of the catalyst to the scale. On washcoating, the obtained specific load depends on the viscosity, the solid content and the method used to remove the slurry excess. In general, to obtain the target load repeated coatings that generate thin layers resulted in more homogeneous layers. The coating adherence depends on the compatibility with the substrate surface (chemical and physical) and the use of additives like binders remaining after calcination or compounds allowing a soft drying to prevent cracks and detachments. Mechanical constraints increase with decreasing channel size improving adherence.

Acknowledgements Financial support by MEC (MAT2006-12386-C05), UPV/EHU (GUI 07/63) and Junta de Andalucía (P06-TEP-01965) is gratefully acknowledged.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

A. Cybulski, J. A. Moulijn (Eds.), 2006, Structured Catalysts and Reactors, 2nd Edition, Marcel Dekker, Inc., New York. P. Avila, M. Montes, E. E. Miro, (2005) Chem Eng J 109, 11-36. K. Jahnisch, V. Hessel, H. Lowe, M. Baerns, (2004) Angewandte Chemie-International Edition 43, 406-446. V. Meille, (2006) Applied Catalysis A-General 315, 1-17. N. Burgos, M. A. Paulis, M. Montes, 2003, Journal of Materials Chemistry, 13, 6, 14581467. I. Barrio, I. Legorburu, M. Montes, M. I. Domínguez, M. A. Centeno, J. A. Odriozola, 2005, Catalysis Letters, 101, 3-4, 151-157. D. M. Frías, S. Nousir, I. Barrio, M. Montes, L. M. Martínez, M. A. Centeno, J. A. Odriozola, 2007, Applied Catalysis A-General, 325, 2, 205-212. L. C. Almeida, O. González, O. Sanz, A. Paúl, M. A. Centeno, J. A. Odriozola, M. Montes, 2007, Natural Gas Conversion VIII, 167, 79-84 E. Arendt, A. Maione, A. Klisinska, O. Sanz, M. Montes, S. Suarez, J. Blanco, P. Ruiz, 2008, Applied Catalysis A-General, 339, 1, 1-14. E. Arendt, A. Maione, A. Klisinska, O. Sanz, M. Montes, S. Suarez, J. Blanco, P. Ruiz, 2009, Journal of Physical Chemistry C, 113, 37, 16503-16516. N. Burgos, M. Paulis, J. Sambeth, J. A. Odriozola, M. Montes, 1998, Preparation of Catalysts VII, 118, 157-166. N. Burgos, M. Paulis, M. M. Antxustegi, M. Montes, 2002, Applied Catalysis BEnvironmental, 38, 4, 251-258. B. P. Barbero, L. Costa-Almeida, O. Sanz, M. R. Morales, L. E. Cadus, M. Montes, 2008, Chemical Engineering Journal, 139, 2, 430-435. O. Sanz, F. J. Echave, M. Sanchez, A. Monzon, M. Montes, 2008, Applied Catalysis AGeneral, 340, 1, 125-132. L. M. Martínez, D. M. Frías, M. A. Centeno, A. Paúl, M. Montes, J. A. Odriozola, 2008, Chemical Engineering Journal, 136, 2-3, 390-397. O. Sanz, L. M. Martínez, F. J. Echave, M. I. Domínguez, M. A. Centeno, J. A. Odriozola, M. Montes, 2009, Chemical Engineering Journal 151, 324-332.

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17. L. M. Martínez., O. Sanz, M. I. Domínguez, M. A. Centeno, J. A. Odriozola, 2009, Chemical Engineering Journal, 148, 1, 191-200. 18. L. M. Martínez , M. I. Domínguez, N. Sanabria, W. Y. Hernández, S. Moreno, R. Molina, J. A. Odriozola, M. A. Centeno, 2009, Applied Catalysis A-General, 364, 1-2, 166-173. 19. J. F. Sanchez, O. J. Gonzalez Bello, M. Montes, G.M. Tonetto, D.E. Damiani, 2009, Catalysis Communications, 10, 10, 1446-1449. 20. M. Z. He, Y. M. Wang, E. Forssberg, 2004, Powder Technology, 147, 1-3, 94-112. 21. Shi, F. N.; Napier-Munn, T. J., 2002, International Journal of Mineral Processing, 65, 3-4, 125-140. 22. He, M. Z.; Wang, Y. M.; Forssberg, 2006, E., Powder Technology, 161, 1, 10-21. 23. Tanaka, S.; Kato, Z.; Uchida, N.; Uematsu, 2003, K., American Ceramic Society Bulletin, 82, 8, 9301-9303. 24. C. Agrafiotis, A. Tsetsekou, A. Akonomakou, 1999, J. Mater Sci Lett, 18, 1421-1424. 25. J. R. González-Velasco, M. A. Gutiérrez-Ortiz, J. L. Marc, J. A. Botas, M. P. GonzálezMarcos, G. Blanchard, 2003, Ing. Eng. Chem. Res., 42, 311-317. 26. T. C. Patton, 1979, Paint Flow and Pigment Dispersions, 2nd ed. John Wiley, New York. 27. C. Agrafiotis, A. Tsetsekou, 2000, J Mater Sci, 35, 951-960. 28. W. B. Kolb, A. A. Papadimitriou, R. L. Cerro, D. D. Leavitt, J. C. Summers, 1993, Chemical Engineering Progress, 89, 61-67. 29. O. Sanz, L. C. Almeida, J. M. Zamaro, M. A. Ulla, E. E. Miro, M. Montes, 2008, Applied Catalysis B-Environmental, 78, 1-2, 166-175. 30. A. Eleta, P. Navarro, L. Costa, M. Montes, Microporous and Mesoporous Materials, 2009, 123, 113-122. 31. X. D. Xu, J. A. Moulijn, 1998, Preparation of Catalysts VII, 118, 845-854. 32. T. A. Nijhuis, A. E. W. Beers, T. Vergunst, I. Hoek, F. Kapteijn, J.A. Moulijn, 2001, Catal Rev, 43, 345-380. 33. D. -J. Liu, D. R. Winstead, N. Van Den Bussche, 2003, US Patent 6,540,843 B1. 34. V. Meille, S. Pallier, G.V.S.C. Bustamante, M. Roumanie, J.P. Reymond, 2005, Applied Catalysis A – General, 286, 232-238. 35. L. Giani, C. Cristiani, G. Groppi, E. Tronconi, 2006, Applied Catalysis B-Environmental 62, 121-131. 36. P. G. Menon, M. F. M. Zweinkels, E. M. Johansson, S. G. Jaras, 1998, Kinetics and Catalysis, 39[5], 670. 37. A. L. Tonkovich, B. L. Yang, T. Mazanec, F. P. Daly, S. P. Fitzgerald, R. Arora, D. Qiu, B. Yang, S. T. Perry, K. Jarosh, P. W. Neagle, D. J. Hesse, R. Taha, R. Long, J. Marco, T.D. Yuschak, J. J. Ramler, M. Marchiando, 2005, Tailored and uniform coatings in microchannel apparatus, US Patent 2005/0244304 A1.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Monolithic catalysts for the decomposition of energetic compounds Dan Amariei,a Rachid Amrousse,a Yann Batonneau,a Rachid Brahmi,a Charles Kappensteina and Bruno Cartoixab a

LACCO (Laboratoire de Catalyse en Chimie Organique), UMR CNRS 6503, University of Poitiers, Faculty of Sciences, 40 Avenue de Recteur Pineau, Poitiers 86022, France b CTI (Céramiques Techniques et Industrielles), F-30340 Salindres France.

Abstract Pellet-based catalysts have been developed more than 60 years ago for the decomposition of hydrogen peroxide and hydrazine for propulsion applications. Cellular ceramic supports are now proposed to replace such catalyst support for monopropellant decomposition or bipropellant ignition. Different honeycomb supports have been manufactured by CTI Company and used as catalyst support for lab-scale reactor as well as for full-scale application. The support parameters are the chemical nature (cordierite, mullite, mullite–zircone, SiC…), the channel shape and density. For fullscale reactors, dedicated apparatus have been developed to control the key parameters during the preparation of the catalysts: quality and homogeneity of the wash-coating layer, impregnation conditions to reach a high loading level of active phase. Keywords: cellular ceramics, honeycomb monoliths, propulsion, energetic compounds

1. Introduction The catalytic decomposition of energetic compounds is used for propulsion application (launcher, satellites and missiles) and gas generator (e.g. rescue systems) [1,2]. The role of the catalyst is to trigger the decomposition or the ignition of the propellant and the resulting hot gases are expelled through a converging-diverging nozzle generating thrust. Figure 1 shows the scheme of a catalytic engine for monopropellant and Figure 2 displays the 5 N engine used on the Pioneer 10 and 11 mission to the outer planets and beyond the solar system (1972-1997). The propellant is hydrazine N2H4 decomposed on Shell 405, an Al2O3-supported iridium catalyst. catalyst

thrust pressurized liquid monopropellant

valve

hot gases injector

nozzle

Figure 1. Scheme of a remotely controlled monopropellant catalytic engine.

The catalytic bed must be able to start the decomposition of the energetic propellant (H2O2, N2H4, N2O…) at low temperatures (to avoid preheating of the engine) with a short ignition delay (10 to 20 ms). It must present very good thermal and mechanical properties to resist and survive very severe conditions (frequent thermal and overpressure shocks at high flow rates) for long term use (up to 15 years or more) without performance degradation. The catalyst porosity must be adapted to very high reaction rates with good heat and mass transfer during the transformation into hot gases.

D. Amariei et al.

36

The most important catalyst is iridium supported on alumina [2]; it was developed for hydrazine decomposition about 60 years ago at the beginning of space exploration. Silver gauzes have been used too for the decomposition of hydrogen peroxide for bipropellant rockets using kerosene as a fuel.

Figure 2. Dual thruster MRE-1 (TRW). The ellipses show the decomposition chamber containing the catalyst.

The catalysts developed during the fifties and sixties were prepared on pellet substrates, displaying sphere or more irregular grain shape, and made of porous transition alumina; this alumina is generally manufactured by a dedicated preparation procedure to resist thermal shocks. The development of car exhaust catalysts leads to the use of monoliths as catalyst supports [3,4] and cellular ceramics are today easily commercially available. Recently, we proposed the use of cellular ceramic as catalyst support for the decomposition of hydrogen peroxide [5,6]. Compared to extrudates or pellets, honeycomb monoliths show many advantages: (i) lower pressure drop, (ii) better thermal shock and attrition resistance with limited fine formation, (iii) uniform flow distribution and mass/heat transfer conditions, (iv) shorter diffusion length, and (v) large heritage from cleaning of car exhaust gases. Therefore, the development of monolithic reactors for propulsion applications represent a very attractive alternative for macro- or micro-propulsion systems and the following propellants are currently under study: (i) H2O2 for monopropellant or hybrid engine; (ii) nitrous oxide; (iii) energetic ionic liquids like HAN (hydroxylammonium nitrate NH3OH+NO3-); and more recently (iv) cryogenic H2-O2 mixtures [7]. We present in this paper an overview of the current development of monolith-based catalysts at the lab scale (decomposition of hydrogen peroxide) as well as at full-scale (ignition of cold H2-O2 mixtures).

2. Geometric parameters of cellular ceramics

2.1. Honeycomb monoliths

Honeycomb monoliths are described by simple geometric parameters defined in Figure 3, and determined from channel shape (square, circle, hexagon or triangle), channel dimensions and wall thickness. The mathematical relations are presented below and examples of typical values are given in Table 1. Channel density [mm-2]:

n = 1 / dch2

Channel density [cpsi]:

n(cpsi) = 25.42 / dch2

Geometric surface area [mm-1]:

GSA = 4n (dch – lch)

Open fraction area:

OFA = n (dch – lch)2

Monolithic catalysts for the decomposition of energetic compounds Hydraulic diameter [mm]:

Dh = 4 x OFA / GSA

Thermal integrity factor:

TIF = dch / lch

Mechanical integrity factor:

MIF = lch2 / dch (dch – lch)

Channel size dch /mm

37

Open fraction area (OFA)

Wall thickness

Geometric surface area (GSA) /mm -1

lch /mm

Figure 3. Geometric parameters for a honeycomb monolith support with square channels. Table 1. Values of the geometric parameters for cordierite monoliths with square channels; cpsi = channels per square inch. (Samples supplied by CTI Company). Channel density

dch /mm

Ich /mm

n /mm-2

n /cpsi

GSA /mm-1

OFA

Dh /mm

MIF

TIF

400 cpsi

1.30

0.35

0.592

382

2.25

0.534

0.95

0.099

3.71

100 cpsi

2.40

0.40

0.174

112

1.39

0.690

2.00

0.033

6.00

2.2. Foams

The cell density in foams is defined in ppi (pores per inch) and the calculation of GSA is more complicated. The free volume of the foam can be described as a network of interconnected cells of complicated geometry. To overcome sophisticated calculations, the cells can be modelized as a set of partially open spheres with constant diameter; the open part of the sphere realizes thus the interconnection of the foam. From Figure 4-a, the average cell size (i.e. sphere diameter) is estimated about 1 mm for 20 ppi and 0.6 mm for 30 ppi, whereas the interconnection can be roughly estimated to represent about half the surface of the spheres. With these assumptions, it is possible to obtain a rough value of the GSA and to make a useful comparison with honeycomb monoliths. The calculated values are 2.33 mm-1 for 20 ppi foam and 3.24 mm-1 for 30 ppi foam; the first value compares well with the corresponding value for 400 cpsi monolith (2.25 mm-1).

3. Preparation of lab-scale cellular ceramic catalysts Honeycombs made of different materials (cordierite, mullite, mullite–zirconia, alumina, silicon carbide, yttrium-stabilized zirconia) have been supplied by CTI Company [8] as supports to prepare catalysts for lab-scale reactors. They present square channels and a total volume between 1 and 2 cm3. The external shape can be parallelepiped or cylindrical. The preparation of square-shaped monoliths for H2O2 decomposition has been presented at the previous Catalyst Preparation Symposium [5], focusing on the influence of the washcoat procedure and the nature of the active phase on the catalytic activity. Figure 4-b displays one example of such catalysts, as received and after washcoating, impregnation and reduction steps.

38

D. Amariei et al.

Figure 4. 20 ppi (left) and 30 ppi (right) alumina foams (a); lab-scale monolith before (b) and after (c) wash-coat, impregnation and reduction procedures (Ag-based catalyst).

Cylindrical monoliths are presented in Figure 5. They have been obtained by cutting a honeycomb block but suffer damaged channels (Figure 5-a) or directly manufactured; in this case, the external distorted channels are clogged to preserve only the square channels (Figure 5-b). For cylindrical foams with external irregularities (Figure 5-c) leading to preferential pathways, they can by wrapped with refractory fibers and blocked inside the decomposition chamber.

Figure 5. Cylindrical supports: cut in a honeycomb block (a); directly manufactured (b); cut in a foam block (c).

An important part of the current work is carried out in the field of the FP7-European project GRASP (Green Advanced Space Propulsion) [9]. As a GRASP partner, CTI Company manufactures tailored ceramic samples that differ in nature, channel shape (squares or triangles) and channel density (400 and 600 cpsi). One example of procedure to prepare the catalysts is as follows: Step 1, acidic treatment to clean the surface. The as-received monoliths are put for 1 h in a concentrated solution of nitric acid, then heated in a muffle furnace up to 300°C. Step 2, preparation of colloidal suspensions. Two suspensions (sol) have been prepared depending on the nature of the active phase: (i) Sol-A was obtained by mixing diluted nitric acid, pseudo-boehmite (AlOOH, Disperal P2, 200 m2.g-1, Sasol Compagny) and urea; (ii) Sol-B was made from aluminum chloride, aluminum powder and urea. They were mixed using a high-shear mixer (23 000 rpm). After drying and heating at 500°C in air, the resulting powders display the characteristic XRD profile of -alumina and the specific surface area is 100 m2 g-1 for Sol-A and 255 m2 g-1 for Sol-B. Step 3, monolith washcoating. The monoliths are put into the sol for 1 h with careful control of the temperature and viscosity. Then, they are retrieved and the channels are flushed with a smooth argon flow before drying and thermal treatment. Up to three successive washcoat stages can be carried out to get a given porous wash-coat content. Step 4, impregnation with the active phase precursor. The objective is to reach 10-20 wt.-% active phase based on the washcoat layer. For platinum-based catalysts, the washcoated samples were immersed in an aqueous solution of chloroplatinic acid under mechanical stirring, and then heated in a sand bath at 50-60°C until complete dryness. Finally, the samples were thermally treated in air and reduced under H2/He flow. The SEM view of the wash-coat layer deposited on cordierite from both colloidal suspensions shows that Sol-A leads to a more homogeneous layer than Sol-B (Figure 6).

Monolithic catalysts for the decomposition of energetic compounds

39

Figure 7 displays the XRD profile of Pt-based catalysts prepared on cordierite or silicon carbide monoliths. The formation of metallic platinum is clearly evidenced; from the profile of Pt diffraction peaks; the average size is estimated about 13 nm. Sol B

Sol A

Figure 6. SEM view of the wash-coat layer deposited on cordierite. 1000

Intensity /counts

1400

800

Intensity /counts

1200

GC013 (+300)

600

Pt

1000

Pt

400

800

Pt Pt

600 GS045 (+300) 400

200

0

GC023

30

200

35

40

45

50 2 theta

0

GS056 30

35

40

45

50 2 theta

Figure 7. XRD profile of Pt-based catalyst on cordierite (left) and SiC (right) monoliths. The platinum metal diffraction peaks are indicated. The other peaks origin from the support.

4. Preparation of full-scale catalysts

4.1. Objectives

The catalytic ignition of cold hydrogen-oxygen mixtures is of current interest for the French Space Agency (CNES, Centre National d'Etudes Spatiales) [7]. The first application of this ignition system is a small upper stage thruster (class 10 to 100 N) using gaseous O2 and H2 in order to settle the liquid in the tanks during ballistic coast phases. A second, more prospective application could be the thrust chamber of a larger liquid O2 - liquid H2 cryogenic engine such as the VINCI engine developed by European companies. In both cases, catalytic ignition allows combustion initiation at low temperatures, 180 to 300 K, without the need of a spark delivered by high voltage electrical discharges. As the stoichiometric O2-H2 mixture leads to temperatures higher than 3000 K, both applications need to work in hydrogen excess (fuel rich) or oxygen excess (fuel lean), to control the maximum temperature and thus to avoid any thermal deterioration of the catalytic bed.

4.2. Supports

The supports were specially manufactured by CTI Company; they are cylindrical honeycomb-type ceramic monoliths 50 mm diameter and 100 mm length, i.e. two orders of magnitude larger than the lab-scale catalysts (Figure 8-a, b) [10]. For all samples, the active phase is deposited after specific wash-coating procedures to increase the specific surface area of these supports from 0.5 m2 g-1 for the as-received monolith to 22 m2 g-1

D. Amariei et al.

40

for the wash-coated monoliths. The catalyst samples have been prepared by LACCO, which was in charge of optimizing the catalyst preparation method with regards to the functional objectives aimed at Air Liquide dedicated test bench.

100 mm

50 mm

a

b

c

d

Figure 8. View of a mullite 400 cpsi (a) and cordierite 100 cpsi (b) monoliths; furnace (c) and quartz reactor (d) for thermal treatments; the red ellipse shows the HCl trap.

4.3. Catalyst preparation

Different active phases have been deposited by impregnation method onto the washcoated monoliths. Several preparation parameters have been tested: (i) the nature of the ceramic used for the base monolith supports (mullite: 3Al2O3·2SiO2 or cordierite: 2MgO·2Al2O3·5SiO2); (ii) the channel density (100 or 400 cpsi, i.e. channels per square inch); (iii) the nature of the active phase (Ir, Pt, Pd or Rh); (iv) the wash-coating procedure; and (iv) the content of the active phase (15 to 40 wt.-% of the washcoat mass or 10 to 40 g L-1). Table 2 gathers typical values of different representative catalyst samples. Table 2. Typical values of iridium catalysts supported on different monolithic carriers. support

Cordierite, 400 cpsi

Cordierite, 100 cpsi

Mullite, 400 cpsi

134.30

106.07

154.34

196

196

196

Wash-coat mass /g

18.35

9.37

13.10

Iridium mass /g

4.43

2.57

4.25

Initial mass /g Volume /cm

3

For the wash-coating step, the monoliths are dipped into the colloidal suspension, in a specially lab-made double-wall beaker for a fine control of the temperature. The monoliths are periodically removed and turned over. The final unclogging of the channels is performed under weak flow of argon to remove excess colloidal solution. Beside the composition of the suspension, key parameters are the viscosity, the temperature and the duration of the wash-coating process which must be carefully

Monolithic catalysts for the decomposition of energetic compounds

41

controlled. The wash-coated monoliths are dried in a dedicated system allowing horizontal rotation to ensure a homogeneous distribution of the coating layer. Finally, thermal treatment of the coated monoliths at higher temperature (between 400°C and 700 °C) was carried out under air in a muffle furnace. The impregnation is performed generally from an aqueous solution of known concentration of the metal precursor. The procedure is to immerse overnight the coated monoliths into the precursor solution under mechanical agitation. The excess of the solution is then evaporated. When the precursor solution is completely evaporated, the impregnated monoliths are carefully dried before thermal treatment. This is carried out in a lab-made quartz reactor adapted to the size of the monolithic catalysts (Figure 8-c and d). For platinum, rhodium and iridium active phases, this treatment corresponds to a reduction under hydrogen flow diluted in helium. During the reduction of metal chloride precursors, the reaction produces quantitatively gaseous hydrogen chloride, e.g. in the case of platinum: H2PtCl6(s) + 2 H2(g) Pt(s) + 6 HCl(g) Therefore, the reduction extent can be easily controlled by determining the amount of produced HCl; this is done by a down-stream trap containing a basic solution; after the reduction, the rest of basic solution is titrated; key parameters are flow rate and composition of hydrogen/helium mixture to avoid redox decomposition of the precursor and formation of chlorine as it could be the case for platinum or iridium precursors H2PtCl6 or H2IrCl6.

4.4. Catalyst characterization

Pre-tests and post-tests chemical analyses and characterizations have also been performed to verify the characteristics of catalysts using different methods [10]: - X-ray diffraction discloses the presence of metallic Pt, Rh or Ir particles; - metallic accessibility from hydrogen chemisorption leads to values in the range 30–33 % for iridium dispersion, (2.7 to 3.0 nm average crystallite size) which reduces to about 20% after tests; - transmission electron microscopy associated with EDX analysis to verify the presence of iridium in the wash-coat layer and the absence of impurities as chlorine; - specific surface area determination in the range 20 to 25 m2 g-catalyst-1; - elemental analyses of active phase by ICP-OES technique which permit to control the active phase variation along the monolith axis.

5. Conclusion One of the major drawbacks of honeycomb monolith for monopropellant decomposition is the initial laminar flow in the straight channels which delay the decomposition reaction. One way to overcome this difficulty is to increase the monolith length or to use foams instead of honeycombs to create turbulence but at the expense of higher pressure drop and lower mechanical strength. Thus, a comparison of catalytic activity between both monolith types would be very useful. Another way proposed by Robocasting Enterprises Company [11] is to create turbulence and preserve a periodical structure; this is done by manufacturing the monoliths from alternating rods as it can be seen in Figure 9.

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Figure 9. Monolith made of alternating rods (Robocasting Enterprises).

Acknowledgements We aim to thank CNES, Air-Liquide Company, and European Community (FP7 GRASP project) for funding different parts of this study. CTI Company is acknowledged for the supply of the different monoliths.

References 1.

R. W. Humble, G. N. Henry, W. J. Larson, Space propulsion analysis and design, MacGraw-Hill, New-York, 1995. 2. Y. Batonneau, C. Kappenstein, and W. Keim, “Catalytic decomposition of energetic compounds: gas generator, propulsion”, in Handbook of Heterogeneous Catalysis, G. Ertl, H. Knözinger, F. Schüth, and J. Weitkamp Eds, Vol. 5, Chapter 12.7, VCh-Wiley, Weinheim, Germany, 2008, pp. 2647-2676. 3. A. Cybulski, J. A. Mouljin, “Structured Catalysts and Reactors”, Marcel Dekker, 1997. 4. T. A. Nijhuis, A. E. W. Beers, T. Vergunst, I. Hoek, F. Kapteijn, J. A. Moulijn, “Preparation of monolithic catalysts”, Catalysis Reviews - Science and Engineering, 2001, 43 (4), 345-80. 5. R. Brahmi, Y. Batonneau, C. Kappenstein, P. Miotti, M. Tajmar, C. Scharlemann and M. Lang, “Ceramic catalysts for the decomposition of H2 O2”, Studies in Surface Science and Catalysis, 2006, 162, 649-656. 6. C. Kappenstein, R. Brahmi, D. Amariei, Y. Batonneau, S. Rossignol, J. P. Joulin, “Catalytic decomposition of energetic compounds-Influence of catalyst shape and ceramic substrate”, AIAA Papers 2006-4546. 7. P. Bravais, Y. Batonneau, D. Amariei, C. Kappenstein and M. Théron, “Experimental investigation of catalytic ignition of cold O2/H2 mixtures,” Space Propulsion 2008, Heraclion, Greece, May 2008, 3AF Publisher, Paper S51. 8. Website: http://www.ctisa.fr/ 9. Website: http://www.grasp-fp7.eu/grasp/ 10. R. Amrousse, R. Brahmi, Y. Batonneau, C. Kappenstein, M. Theron, and P. Bravais, “Catalytic Ignition of Cold H2/O2 Bipropellant Mixtures”, AIAA Paper 2009-5473. 11. Website: http://www.robocasting.net/

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Glass fiber materials as a new generation of structured catalysts Bair S. Bal’zhinimaev, Evgenii A. Paukshtis, Olga B. Lapina, Alexey P. Suknev, Viktor L. Kirillov, Pavel E. Mikenin, Andrey N. Zagoriuko Boreskov Institute of Catalysis SB RAS, prospekt Akademika Lavrentieva, 5, 630090, Novosibirsk, Russia

Abstract Molecular structure of Zr-silicate glass fiber materials was studied to evaluate their potentiality in catalysis. Basing on NMR and IRS data the framework structure where Zr(IV) cations serve as a connectors linked with a few SiO4 tetrahedra was proposed. The effective ways of transition ions (Pt, Pd, Co) incorporation into the glassmatrix and their stabilization in highly dispersed state (clusters) were found. The obtained glass fiber based catalysts showed high activity and selectivity in oxidation of hydrocarbons and selective hydrogenation of acetylene-ethylene feedstock. The example of successful design of structured bed and commercialization of VOC removal process is presented.

Keywords: glass fiber catalysts, structure, clusters, oxidation, selective hydrogenation 1. Introduction The glass fiber materials of silicate origin are for a long time produced in industry and are widely used as perfect heat and electric insulators. At the same time these materials are much less known as catalyst supports despite their obvious advantages such as high thermal stability, high mechanical strength, improved hydrodynamic properties as well as the possibility to create the new types of structured catalyst beds and the catalytic reactors with new flexible designs. The glass fiber catalysts (GFCs) reveal unique catalytic performance in many oxidation reactions due to the ability of the glass to stabilize small nanoclusters or separate ions of transient metals in the bulk of fibers [1-3]. It results in high catalytic activity and high catalyst resistance to poisoning and deactivation in aggressive reaction media. Notably, excellent catalyst performance is achieved at very low noble metal content (0.01-0.02% wt.) thus providing quite reasonable pricing for the catalyst. The present paper devoted to study of molecular structure of promising Zr-silicate glass fiber materials, features of active component introduction into the bulk of glass matrix, as well as testing of obtained catalysts in deep oxidation of hydrocarbons and selective hydrogenation of acetylene.

2. Zr-silicate glass fiber materials The industrial production of these materials includes making of glass melt at high temperatures, manufacturing of separate fibers with typical diameter of 7-10 microns, twisting of these fibers into threads (0.3-1.0 mm in diameter) with final manufacturing of glass fiber fabric. The important stage is a leaching with inorganic acids resulting in complete removal of sodium. As follows from ICP data the leached Zr-silicate glass material used for preparation of GFCs contains 80.7% wt. SiO2, 16.5% wt. ZrO2, 2% wt. Al2O3 and the rest are oxides of Fe, Ca, Mg etc. It corresponds to atomic ratio of

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main glass forming elements Zr/Si=0.1 wtat was confirmed with XPS technique. Specific surface area measured by BET technique is ca. 1 m2/g and corresponds well to geometrical surface of glass fibers.

3. Molecular structure of zirconium-silicate glass fiber materials The molecular structure of silicate glasses is not clear yet though some models were proposed [4,5]. In particular, it was found that SiO4 tetrahedra in the network are interconnected randomly with each other via bridge oxygens. However, this networks is not continuous, it is alternated by protons bound with non-bridging oxygens. In other words, more dense silicate layers alternate with less dense interspaces with H-bonded hydroxyl groups. In the present work, IR and NMR spectroscopies were used to reveal the structure of zirconium-containing glass fiber. Solid-state 29Si MAS NMR studies were performed using a Bruker AVANCE-400 (9.4 T) spectrometer at resonance frequencies 79.46 MHz for 29Si with rotation frequency 10-15 kHz, pulse duration of 7 μs (π/2) and pulse delay of 10-20 s. The chemical shift values were referenced to external reference tetramethylsilane (TMS). It is known that silicate glass may have five types of distinct silicon microstructures (denoted as Si(n) or Q(n), where n is the number of silicon atoms in the second coordination sphere, n = 0, 1, 2, 3, 4). The introduction of zirconia into silicon lattice leads to the formation of some possible silicon environments and can be considered within different Sin (mZr) units, where m (m≤n) is the number of zirconium atoms bound with the central silicon atom via bridging oxygen. Identification of different types of the oxygen environment of silicon Si(n) in 29Si MAS NMR spectra is based on the value of isotropic shift. The 29Si MAS NMR spectra of the leached zirconium-silicate fiber glass materials used for catalyst preparation show the lines with chemical shifts –93, –102, –109 and – 113 ppm. According to [6] and 1H-29Si CP MAS NMR data (present work), these lines are assigned to Q3 (1Zr, OH), Q3 (0Zr,OH), Q4 (1Zr) and Q4 (0Zr), respectively. (1H-29Si CP MAS NMR data allow to reveal the spectral lines corresponding to the Q3 type silicon bound with OH group). Table 1 presents NMR data and relative content for each type of silicon atoms in zirconium-silicate fiber glasses both as received (initial) and calcined at 700oC. Table 1. The ratio of lines in the 29Si MAS NMR spectra of zirconium-silicate material. Sample

Chem.shift, pm

Rel.content, %

Width, Hz

Type

Initial sample

-93

7

510

Q3(1Zr, OH)

-102

36

760

Q3(0Zr, OH)

-109

40

883

Q4(1Zr)

-113

17

713

Q4 (0Zr)

-109

57

1170

Q4 (1Zr)

-113

43

909

Q4 (0Zr)

Calcined at 700оС

It seen the fraction of silicon atoms bound with zirconium Q4 (1Zr) and Q3 (1Zr, OH) is rather high (above 47%). This ratio keeps after thermal treatment: 43% Q4 (0Zr) and 57% Q4 (1Zr). Taking into account that coordination number of zirconium with respect to oxygen in zirconium-silicate glasses is equal to six [7] and Zr-O-Zr bonds are

Glass fiber materials as a new generation of structured catalysts

45

absent we conclude that each zirconium atom is surrounded by six -O-Si bonds. Thus, taking into account the total ratio Zr/Si = 0.1, approximately up to 40% of Si atoms are not linked with Zr4+ cations, e.g. zirconium atoms are linked with each other via 3-4 silicon-oxygen tetrahedra. To satisfy the ratio between silicon atoms bound and not bound with zirconium (the ratio between Q4 (1Zr) + Q3 (1Zr, ОН) and Q4 (0Zr) + Q3 (0Zr, ОН) is close to 1), it is necessary to add a link of silicon-oxygen tetrahedra not bound with zirconium of type Q4 (0Zr) or Q3 (0Zr, ОН). The noncalcined sample comprises a large amount of hydroxyl groups (nearly 40% Q3 (0Zr, ОН)), i.e. approx. 2/3 of silicon-oxygen tetrahedra not bound with zirconium have a hydroxyl group, whereas the fraction of tetrahedra bound with zirconium and hydroxyl group is low (1/7 of all silicon atoms bound with zirconium), i.e. Q3 (1Zr, ОН) is present not in each structural unit. The formation of Zr-O-Si bonds is confirmed by IR studies in the region of SiO vibrations, as measured on an Alpha spectrometer (Bruker) by the ATR technique. The spectra show absorption bands at 1070 and 1170 cm–1 corresponding to SiO4 stretching vibrations, which are shifted to lower frequencies in comparison with the spectra of silica gel (1132 and 1250 cm–1). The low-frequency shift may result from an appearance of more heavy atom in the silicate network; in our case, it is obviously zirconium. Besides, a new band at 1030 cm–1, which is likely assigned to stretching vibrations of a Zr-O-Si fragment. Thus, the data of 29Si MAS NMR and IR spectroscopy unambiguously prove the incorporation of zirconium ions into the silicate network. The following facts: i) the coordination number of zirconium with respect to oxygen is equal to 6, ii) silicon atoms in the second coordination sphere have only one zirconium atom, hence all zirconium atoms are separated by at least two silicon-oxygen tetrahedra (see Table 1), iii) silicate species tend to form the cyclic structures (it is known from the chemistry of silicates), and iv) zirconium-silicate bonds are stable and do not break even at high temperatures (see Table 1) allows to conclude that zirconiumsilicon structure can be presented as a framework with six-coordinated zirconium atoms in the vertices (Fig. 1).

Fig. 1. The scheme of zirconium-silicate framework. Dashed line showes framework unit.

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This gives a framework resembling the structure of metal-organic frameworks (MOF); the framework is formed by zirconium atoms, and the ligand cross-linking (three-dimensional in our case) is made by silicon-oxygen tetrahedra. It is clear that the structure is not regular and the length of ligands can be varied between 2 and 4. Besides, to maintain electroneutrality of the structure, protons should reside on two of six Zr-O-Si bridges.

4. Preparation of glass fiber catalysts The presence of bridge protons (Zr-OH-Si) is very important because the introduction of different cations of transition metals into the bulk of glass takes place namely via ion exchange with these protons. Introduction of transition metals, f.e. platinum, and its stabilization in the highly dispersed state is performed in two stages. First stage is a ion exchange of Pt amine complexes with protons of glass: 2 ZrOHSiglass + [Pt(NH3)4]2+solution↔ (ZrOSi)2 [Pt(NH3)4]glass + 2H+solution The second stage is a calcination of impregnated and dried sample at elevated temperatures to reduce Pt(II) cations into Pt0 with ammonia ligands: (ZrOSi)2 [Pt(NH3)4]+5/2O2→2(ZrOSi)H + Pt0 +2N2 + 5H2O Indeed, as follows from UV-Vis DRS studies the Pt amine complexes incorporate into glass matrix. UV-Vis spectra were recorded using Shimadzu 2501 spectrometer equipped with diffusion reflection attachment ISF-240. As seen from Fig. 2 (curves 1 and 2) the spectrum of impregnated and washed out sample is very close to that of Pt amine complex in solution (see characteristic absorption bands at 44600, 42000 and 35000 cm-1). It means, the environment of Pt(II) located inside glassmatrix is similar to that of in the tetra-ammonia complex in water solution.

Fig. 2. UV-Vis spectra of 0.02% Pt/FG samples: 1 – impregnated with further washing out and drying at 120oC; 2 – impregnating solution of [Pt(NH3)4]Cl2 in water; 3 – calcined in air at 300oC; 4 – calcined at 350oC but without previous washing and drying.

Glass fiber materials as a new generation of structured catalysts

47

In course of further sample calcination at 300oC the intensity of the band at 44600 cm sharply decrease and new band at 39800 cm-1 is simultaneously appeared. This band may be attributed to d-d transitions in small charged platinum clusters due to the following reasons. First, the electron microscopy data of GFC samples [1] showed the presence of Pt species with size not exceeding 1 nm and the absence of metal particles at the outer fiber surface. According to XPS data obtained in combination with ion etching the Pt clusters are localized in the upper layers of glass fibers at the depth up to 10 nm. Moreover, the UV-Vis DRS study of the sample prepared without washing and drying procedures (curve 4) shows the appearance of intensive band at 47800 cm-1, corresponding to metal particles of 5-8 nm in size located at the fiber surface (according to electron microscopy data [1]). Second, the position of absorbance bands, corresponding to d-d transitions is sensitive to ability of surrounding ligands to donate electron density towards cations. For example, for Pt(II) the replacement of oxygen ligands for ammonia ones results in band shift from 20000-25000 to 32000-36000 cm-1. Platinum atoms have higher electron donating ability than ammonia, therefore, the bands of these transitions must shift to higher frequency region. Therefore, the band at 38000-40000 cm-1 are quite probable caused by ions of two-valent platinum surrounded number of metal atoms. In other words, the band at 39800 cm-1 can be attributed to positively charged Pt clusters formed due to reduction of Pt(II) with ammonia. Indeed, as follows from TPO (temperature programmed oxidation) studies the formation of molecular nitrogen near 300oC takes place (Fig. 3). TPO profile was measured with continuous MS monitoring of gas phase composition using quadrupole mass-spectrometer VG Sensorlab 200D. The absorbed H2O and NH3 during impregnation (ability of glass fibers to absorb significant amounts of polar molecules was shown in [2]) are desorbed at T 1, leads to an increase of not only the heat release but also in the gas-phase production, which is an important factor in controlling the product specific surface area and the spongy morphology of the primary obtained solid. It is possible to control the process by changing the Φ value, or using complex fuels and/or adding inert easily gasified precursors: combustion conditions depend in fact, on the chemical nature of the reactive solution formed [8-10]. For example, even if the systems have comparable energy for product formation, the activity of NH2 groups appears to be higher as compared to the OH group, which in turn is more active than COOH. This explains why, glycine, which contains the NH2 group, is a more reactive fuel than citric acid, which contains only OH and COOH groups [11]. What makes this process interesting for a potential application on a larger scale is that the energy (heat) necessary is basically provided by the exothermic reaction itself and hardly any external supply is required. Metal nitrates, in fact, can simply decompose upon calcination into metal oxides, by mere heating to or above their decomposition temperature. Subsequently the so-formed oxides may take part in other reactions to form other compounds. With such procedure, though, a continuous supply of heat from the outside should be necessary to maintain the system at the appropriate temperature, whereas the mixture of nitrates and organic molecule, suitable to serve as fuel, can be ignited at a relatively low temperature and the following reaction provides the heat required to complete the process. Notwithstanding the high temperature reached in the reacting mixture, the occurrence of sintering is reduced due to the very short residence time. Last but not least, SCS results a very interesting technique considering its simple adaptability for in situ catalysts deposition on structured supports, as ceramic or metallic monoliths, foams, tissues, mattresses, etc., as outcome of engineering industrialized or semi-industrialized processes. In fact, once prepared, the precursors solution can be deposited onto the structured supports by infusion, immersion, or spraying. The catalytic layer strictly anchored to the support can be easily obtained by placing the infused/immersed/sprayed support into an oven to start up the exothermic synthesis reactions. A series of continuous conveyor belts, ovens and infusion spraying nozzles can be, in fact, designed to realize a continuous industrial process. In view of the speediness of in situ SCS method for structured catalysts preparation and of its relatively low cost, in terms of starting materials and energetic expense, such a technique represents a very promising and cost-effective alternative to more traditional processes for catalytic systems preparation proposed in the recent past, as deep coating or wash-coating [13,14]. As a drawback of this technique, during the synthesis the formation of NOx is possible [12]: metal nitrates can undergo a partial thermal oxidation just before the main reaction is ignited, thus releasing nitrogen oxides, as well organic fuels containing nitrogen atom, like glycine and β-alanine, can decompose generating NOx. Apparently, being the process very fast, little time is given for thermal decomposition prior to ignition, and only small amounts of NOx are expected to be released. Doubtless, NOx emissions can anyhow become an issue when scaling up the process to the industrial level, where larger production is necessary. In that case a small NOx abatement reactor by selective catalytic reduction with ammonia might be envisaged.

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2.2. Catalyst synthesis and characterization The synthesis of LaMnO3·2ZrO2 catalyst, starting from metal nitrates of La, Mn and Zyrconyl and using glycine as fuel is reported as an example. The overall reactions in the general form can be written as: La(NO3)3 + Mn(NO3)3 + 2ZrO(NO3)2 + 44/9COOH-CH2-NH2 → LaMnO3·2ZrO2 + 88/9CO2 + 110/9H2O + 125/18N2 The precursors and fuel (Φ = 1), dosed in the stoichiometric amount, were dissolved in distilled water and the resulting solution, thoroughly stirred to ensure complete dissolution of all reagents, was then transferred in a ceramic dish and placed into an electric oven set at 450°C. After water evaporation and a significant increase in the system viscosity, the mixture frothed and swelled, until a fast and explosive reaction took off, and large amounts of gases evolved. The heat released in the fast reaction allowed the formation of the LaMnO3·2ZrO2 powder of a foamy structure, easy to be crumbled. The whole process was over after 5–6 min, but the time between the actual ignition and the end of the reaction was less than 10 s. Figure 1 shows a sequence of the SCS of LaMnO3·2ZrO2 powder: the foamy and spongy structure of the catalyst is visible.

Figure 1. Sequence of the SCS of LaMnO3·2ZrO2 powder.

Deeper investigations by FESEM enlightened such a structure, as shown in Fig. 2: perforated waffles with nanometric pores and complex network configuration were present. The BET specific surface area of the as-prepared sample was 132 m2 g-1 [15].

Figure 2. FESEM images of LaMnO3·2ZrO2 powder prepared by SCS.

The XRD spectrum for as-synthesized sample is presented in Fig. 3: weak diffraction peaks of tetragonal ZrO2 and orthorhombic LaMnO3 were detected [15], enlightening the simultaneous growth of both phases and the purity of the as-prepared sample. The quality of the ceramic oxide is very high. Moreover, the grain sizes of the two detected phases were calculated via the Scherrer equation: 45 nm for LaMnO3 and 10 nm for ZrO2 [15], pointing out how the SCS allowed reaching real nanoscale dimensions.

Solution Combustion Synthesis as intriguing technique

63

Figure 3. XRD spectrum of LaMnO3·2ZrO2 powder prepared by SCS.

SCS allows the preparation of excellent materials also of true catalysts such as e.g. noble metal containing complex catalysts. Very high dispersion of the metal can be obtained. For example, the addition of Pd nitrate to the starting solution allows obtaining very fine Pd clusters on the support (Φ = 1): La(NO3)3 + Mn(NO3)2 + 2ZrO(NO3)2 + Pd(NO3)2 + 56/9C2H5O2N → Pd(LaMnO3·2ZrO2) + 112/9CO2 + 140/9H2O + 155/18N2 FT-IR spectroscopy analysis of a pure powder pressed disk of Pd(LaMnO3·2ZrO2) catalyst prepared by SCS, after outgassing at 500°C, revealed the small particle size of the powder, thanks to the very high transmission of the IR light. The low temperature adsorption of CO allowed enlightening a highly dispersed zerovalent Pd metal centers [16]: sign that a good dispersion of the noble metal can be obtained by SCS method. The as-prepared Pd(LaMnO3·2ZrO2) catalyst is very suitable for natural gas (NG) combustion [12,15]. The catalytic activity towards CH4 total oxidation was tested in a lab-scale fixed-bed reactor: 0.1 g of catalyst was mixed with 0.9 g of SiO2 (0.2–0.5 mm in size, to prevent the catalytic bed clogging), sandwiched between two quartz wool layers, and inserted in a quartz tube (4 mm ID). The obtained reactor was placed into a PID regulated electrical oven and fed with 50 Ncm3 min-1 of a gaseous mixture containing 2% CH4 and 16% O2 in He [12], which corresponded to a gas hourly space velocity (GHSV) of 6,000 h-1. The reactor temperature was measured by a Kthermocouple placed inside the catalytic bed. By feeding the reactive gaseous mixture, the catalytic bed was first heated up to 800°C at 50°C min-1, then the oven temperature was decreased at 2°C min-1 rate by monitoring the outlet CO2, CO, CH4 and O2 concentrations with a continuous analyzer (ABB Company), thus allowing the evaluation of CH4 conversion. The catalyst presented a T50 (50% CH4 conversion temperature) of 570°C, compared to T50 equal to 720°C of a pure SiO2 fixed bed tested in the same conditions [15].

3. Practical application In the light of the numerous stringent European regulations being proposed or adopted as regards NOx and CO emissions from domestic appliances fuelled with NG, a greater penetration of low-NOx burners into the global boilers market is expected in the next future. In the last decade pre-mixed combustion within porous media has been the object of extensive experimental and theoretical research [17,18], especially in the light of its remarkable potential in enhancing the efficiency of the heat transfer and reducing the impact on the environment related to pollutant emissions: CO2 and unburned hydrocarbons (HC) are well known as greenhouse gases, while CO and NOx are toxic, even in very small concentrations. A major goal and challenge for modern NG burners for domestic boiler applications is a wide power modulation range, which can satisfy the demands of

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different uses, ranging from hot sanitary water production, which requires on average 25 kW per apartment, down to 2-3 kW, that represent the necessary heating requirement of medium size apartments built with well efficient thermal insulation. At the same time, this would entail a decrease in the number of start-up and switch-off cycles, which can cause high energy loss, significant CO emissions and material stresses due to thermal shock. In this context, notwithstanding the great advantage coming with the low NOx emissions over the large modulation range, non catalytic premixed burners generally suffer from high and almost unacceptable CO and HC emissions at low Qs values (e.g. 200-400 kW m-2, corresponding to the lowest power values of the modulation range); the comparatively low flame temperatures occurring in the ‘weak’ radiant regime significantly affect the completeness of NG combustion. An improvement in the performance of the radiant premixed burners could be obtained by adopting perovskite-based catalysts, attractive because of their low cost, thermo-chemical stability at comparatively high temperature (900-1100 °C) and catalytic activity [19]: such catalysts increase the fuel flow rate fraction burnt within or just downstream the burner deck, thus maximizing the heat fraction transferred by radiation, cooling the flame temperature and improving the combustion completeness with lower CO, unburned HC and NOx levels. For such a purpose, a deposition technique based on in situ SCS was developed for the application of the catalyst on metal fiber burners. Based on the results previously obtained on powders [18,20,21], the most promising Pd(LaMnO3·2ZrO2) catalyst was employed. For its deposition on FeCrAlloy® fiber burner, optimum operating procedure to guarantee both good adherence of the catalytic layer to the metallic surface and high enough specific surface area of the deposited catalyst layer, was determined; the rapidity and low cost characteristics of the SCS route were preserved, too. Firstly, the FeCrAlloy® supports were kept at 1200°C for 10 min under O2 flow (0.5 vol % in N2) so as to favor the regular growth on the fiber surface of α-Al2O3 grains into a uniform protective layer, moreover characterized by an external surface morphology able to ensure a good adherence of the catalytic phase to be deposited on the metallic mat [22]. Subsequently, starting from a solution containing all the catalysts precursors (see reaction 2), an in situ spray-pyrolysis SCS was adopted to develop the catalyst on the metal fiber panels made of FeCrAlloy®, and therefore produce catalytic burners. The aqueous solution of the precursors was sprayed over the surface of the FeCrAlloy® panels, previously heated at 400°C. Due to in situ pyrolysis SCS occurring on the hot panel’s surface, catalyst formation was obtained. The panels were then placed back into the hot oven to stabilize the coating. The spray deposition cycle was repeated several times in order to achieve the desired catalyst load (namely, 2% w/w). For a further stabilization and complete crystallization of the catalytic phase, the burners were finally calcined at 900°C for 2 h in still air. SEM analyses were carried out on the as-prepared burners to evaluate the adherence quality of the catalytic layer and to verify whether the highly porous morphology observed on catalytic powders was maintained after the deposition on the metal fibers. As enlightened on Fig. 4, a highly corrugated and porous catalyst layer was formed, assuring an optimum gas-solid interaction for the heterogeneous catalysis; the thickness of the catalytic layer was about 2-3 μm.

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Figure 4. SEM images of FeCrAlloy® fibers catalyzed with Pd(LaMnO3·2ZrO2) by in situ SCS.

Tests under realistic operating conditions were performed on a partially modified commercial condensing boiler test rig for domestic application (Giannoni France), mounting a round flat catalytic burner. A bare burner was also tested as a reference counterpart to assess the effectiveness of catalyzed burner. CH4 was fed to a modulating electrovalve, able to vary its volumetric flow rate (max power output: 30 kW). Air coming from a blower was mixed with CH4 in a Venturi positioned so that a proper mixing was achieved before entering the burner. The cylindrical fiber-mat burner, fitted vertically in the combustion chamber, fired through the heat exchanger coils. The burner diameter was approximately 10 cm. Figures of the catalytic burner firing in two different combustion regimes are shown in Fig. 5.

Figure 5. Pictures of the FeCrAlloy® catalyzed burners at high (left) and low (right) power.

Tests were carried out over a wide range of operating conditions by varying the nominal power (Q) from 12 to 28 kW and the air excess (Ea) from 5 to 45% (i.e. λ from 1.05 to 1.45). The flue gases composition (O2, CO2, CO, and NO) was monitored by means of a multiple gas continuous analyzer (ABB Company). Figure 6 shows the CO and NO concentrations attained in the flue gases for both burners. When the λ approached stoichiometric condition, the non-catalytically assisted combustion was strongly penalized, given that the reduced O2 partial pressure can be a limiting factor for the conversion of CO into CO2, while in the presence of the catalyst those unacceptable CO emissions were lowered significantly. The beneficial effect of the catalyst was slightly less evident at higher both Ea and Q values. Considering the NO emissions, the contributions of the catalyst to the combustion was less evident, independently of Ea and Q: the NO emissions from the catalytic burner were only slightly lower compared to those of the bare counterpart.

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Figure 6. CO and NO emission vs Ea of air as a function Q.

4. Conclusions Solution Combustion Synthesis (SCS) is becoming one of the most important ways to produce a wide range of advanced porous ceramic, metallic materials and nanostructured catalysts, compared to the more conventional and expensive processes. SCS process is, in fact, characterized by exothermic, fast and self-sustaining reactions, formation of high purity products with a variety of size and shape, relatively easy procedures, use of relatively simple equipment and cheap reactants. Thanks to these main characteristics, SCS is easily tunable to complex systems to produce directly in situ structured catalysts. A successful example of in situ SCS was reported: a series of experimental tests on ad-hoc prepared catalytic premixed burner for household applications displayed the lower environmental impact mainly in terms of CO, compared to the bare counterpart, when a Pd(LaMnO3·2ZrO2) catalyst was properly deposited over the burner. The catalytic burner was able, in fact, to stabilize the combustion process within the porous medium in a greater extent, thus maximizing the heat fraction transferred by radiation (higher thermal efficiency), cooling the flame temperature (slightly lower NO emissions) and enhancing the degree of completeness of NG combustion (lower CO emissions).

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

A. Varma, A.S. Rogachev, A.S. Mukasyan, S. Hwang, Adv. Chem. Eng. 24 (1998) 79. K.C. Patil, S.T. Aruna, S. Ekambaran, Curr. Op. Solid State Mat. Sci. 2 (1997) 158. K.C. Patil, S.T. Aruna, T. Mimani, Curr. Op. Solid State Mat. Sci. 6 (2002) 507. M.A. Keane, J. Mat. Sci. 38 (2003) 4661. M.A. Pena, J.G.L. Fierro, Chem. Rev. 101 (2001) 1981). J.J. Moore, H.J. Feng, Prog. Mat. Sci. 39 (1995) 243. J.J. Moore, H.J. Feng, Prog. Mat. Sci. 39 (1995) 275. A.S. Mukasyan, C. Costello, K.P. Sherlock, A. Varma, Sep. Pur. Tech. 25 (2001) 117. K. Deshpande, A.S. Mukasyan, A. Varma, Chem. Nanomat. 16 (2004) 4896. A. Mukasyan, P. Epstein, P. Dinka, Proc. Comb. Inst. 31 (2007) 1789. A.S. Mukasyan, P. Dinka, Int. J. Self-Prop. High-Temp. Synth. 16 (2007) 23. A. Civera, G. Negro, S. Specchia, G. Saracco, V. Specchia, Catal. Today 100 (2005) 275. S. Cimino, R. Pirone, L. Lisi, Appl. Catal. B: Environ. 35 (2002) 243. M. Valentini, G. Groppi, C. Cristiani, M. Levi, E. Tronconi, P. Forzatti, Catal. Today 69 (2001) 307. [15] S. Specchia, E. Finocchio, G. Busca, P. Palmisano, V. Specchia, J. Catal. 263 (2009) 134. [16] M. Daturi, G. Busca, R.J. Willey, Chem. Mat. 7 (1995) 2115. [17] S. Specchia, A. Civera, G. Saracco, Chem. Eng. Sci. 59 (2004) 5091.

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[18] S. Specchia, M.A. Ahumada Irribarra, P. Palmisano, G. Saracco, V. Specchia, Ind. Eng. Chem. Res. 46 (2007) 6666. [19] M.F.M. Zwinkels, S.G. Järâs, P. Govin Menon, T.A. Griffin, Catal. Rev. Sci. Eng. 35 (1993) 319. [20] P. Forzatti, G. Groppi, Catal. Today 54 (1999) 165. [21] S. Specchia, A. Civera, G. Saracco, V. Specchia, Catal. Today 117 (2006) 427. [22] D. Ugues, S. Specchia, G. Saracco, Ind. Eng. Chem. Res. 43 (2004) 1990.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Impact of NO on the decomposition of supported metal nitrate catalyst precursors and the final metal oxide dispersion Mariska Wolters, Ignacio C. A. Contreras Andrade, Peter Munnik, Johannes H. Bitter, Petra E. de Jongh, Krijn P. de Jong Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands

Abstract In previous communications we have shown that the decomposition of nickel and cobalt nitrate in a flow of NO/He prevented agglomeration, yielding high nickel and cobalt oxide dispersions. We now report on the impact of NO on the decomposition of first row transition metal nitrates, i.e. Sc, Mn, Fe, Co, Ni, Cu, Zn, using thermal gravimetric analysis and mass spectrometry. It was found that NO decreased the temperature of decomposition significantly for all investigated metal nitrates. For cobalt, nickel and copper nitrate it was verified that decomposition in the presence of NO yielded high dispersions and narrow particle size distributions, whereas in Ar agglomeration resulted in broad particle size distributions. The beneficial effect of NO on the dispersion of Co, Ni and Cu coincided with a large difference in the decomposition profiles of these metal nitrates compared to that in Ar. It was found that NO induced fast and complete hydrolysis to highly dispersed cobalt, nickel and copper hydroxynitrates which decomposed to yield highly dispersed metal oxides. This is in contrast to literature reports that ascribe loss in dispersion to the formation of metal hydroxynitrate intermediates. Keywords: nitric oxide, calcination, copper, nickel, cobalt

1. Introduction Transition metal nitrate hydrates are industrially favored precursors for the preparation of supported metal (oxide) catalysts because of their high solubility and facile nitrate removal. The final phase and particle size depend on the experimental conditions, as reported for both supported and unsupported metal nitrates [1-3]. Several authors report that decreasing the water partial pressure during the decomposition of unsupported nickel nitrate hexahydrate, via vacuum or a high gas flow, increases the final NiO surface area [3, 4]. The low water partial pressure results in dehydration of the nickel nitrate hydrate to anhydrous nickel nitrate followed by decomposition to NiO. Decomposition at higher particle pressures, however, occurred through the formation of intermediate nickel hydroxynitrates prior to decomposition to NiO. Thus, NiO obtained via intermediate nickel hydroxynitrate species showed a poorer surface area (1 m2/g) compared to NiO obtained via anhydrous nickel nitrate species (10 m2/g) [4]. For supported zinc and copper nitrate a similar observation was reported. Louis et al. investigated the impact of the drying temperature on the copper and zinc oxide dispersion and found that drying at elevated temperatures (90-200°C) resulted in the formation of wide particle size distributions [5-7]. The loss in dispersion was ascribed to the formation of large copper and zinc hydroxynitate crystals during drying, which decompose to form large CuO and ZnO agglomerates. Highest dispersions were

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obtained by drying at room temperature, which prevents hydrolysis to the hydroxynitrate, followed by thermal decomposition in the presence of hydrogen. The latter was essential as decomposition in air yielded wide particle size distributions, irrespective of the drying treatment. In the case of supported cobalt and nickel nitrate agglomeration during drying is limited, but extensive during high temperature decomposition (240-350°C) [8, 9]. Again efficient decomposition gas removal positively affects the dispersion [8], however, agglomeration cannot fully be prevented in this way. Our group recently reported a facile method to prevent agglomeration via thermal decomposition of silica supported nickel or cobalt nitrate in a NO/He flow, reducing the average metal oxide particle size from 10-35 nm in air to 2-7 nm in 1%NO/He [10, 11]. We now report on the impact of NO on the low and high temperature decomposition steps of silica-supported first row d-metal nitrates, and the resulting metal oxide dispersions. It was found that NO significantly lowered the decomposition temperatures of all investigated metal nitrates and changed the decomposition pathways of cobalt, nickel and copper nitrate. For the latter metal nitrates it was verified that an improved dispersion was obtained after thermal treatment in the presence of NO as compared to Ar.

2. Experimental 2.1. Sample preparation Silica supported samples were typically prepared by impregnation, Davicat 1404 silica gel supplied by Grace Davidson (SA = 470 m2 g-1, PV = 0.9 ml g-1, PD = 7 nm), to incipient wetness with a 3 M solution of the appropriate metal nitrate. The metal nitrate solutions were made by dissolving the respective metal nitrate hydrate in a 0.1 M HNO3 solution. In view of the solubility of scandium nitrate a 2 M solution was prepared. After an equilibration period of 15 minutes the impregnates were dried for 48 h in a dessicator to remove most of the solvent water, and stored in closed containers in a dessicator. This approach was preferred over drying at elevated temperatures or no drying because it leaves the nitrate intact and facilitates handling and storage, as compared to wet samples. SBA-15 (SA = 600 m2 g-1, PV = 0.7 ml g-1, PD = 8 nm) samples were prepared similarly to the silica gel samples. Table 1 lists the used metal nitrates, intended metal loading and sample codes, where “SG” stands for silica gel and “SBA” for SBA-15. Table 1. Sample codes and metal loadings. Sample code

Metal nitrate

Metal loading (wt% )

SG-Sc / SBA-Sc

Sc(NO3)3 •4H2O

8/6

SG-Mn / SBA-Mn

Mn(NO3)2 •4H2O

14 / 10

SG-Fe / SBA-Fe

Fe(NO3)3 •9H2O

14 / 10

SG-Co / SBA-Co

Co(NO3)2 •6H2O

15 / 11

SG-Ni / SBA-Ni

Ni(NO3)2 •6H2O

15 /11

SG-Cu / SBA-Cu

Cu(NO3)2 •3H2O

16 /12

SG-Zn / SBA-Zn

Zn(NO3)2 •6H2O

16 /12

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2.2. Thermal analysis Thermal gravimetric analysis (TGA) was performed with a Perkin-Elmer Pyris 1 apparatus. Typically 15 mg of impregnated silica gel was heated with a ramp of 5°C min-1 to 750°C in a 10 ml min-1 flow of Ar or 10% NO/Ar. In parallel evolved gas analysis was performed with a quadrupole Pfeiffer Omnistar mass spectrometer, which was connected to the outlet of the TGA apparatus. Ion currents were recorded for m/z values (m = molar mass of Xz+ ion, z = charge of the ion) of 14, 15, 16, 17, 18, 28, 30, 32, 44, 46, 62 and 63.

2.3. Ex situ thermal treatment and characterization

Typically 100 mg of SBA-15 impregnate was heated with a ramp of 1°C min-1 to 300500°C (depending on nitrate) in a 100 ml min-1 flow of air or 1%NO/Ar. Both air and Ar thermal treatment lead to significant agglomeration [9]. The resulting SBA-15 supported oxides were analyzed with powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD patterns ranging from 10 to 80°2θ were obtained at room temperature with a Bruker-AXS D8 Advance X-ray Diffractometer setup using Co-Kα1,2 radiation. The average metal oxide particle size was calculated using the Debye-Scherrer equation on the most intense diffraction lines. TEM images were obtained on a Tecnai 20 operating at 200 keV.

3. Results and discussion 3.1. Impact of NO on the temperature of decomposition In a previous communication we have shown that NO affects the decomposition temperature of nickel nitrate [11]. However, in this case a drying treatment at 120°C had been applied prior to the measurements, and hence the first hydrolysis step to nickel hydroxynitrate had already occurred. Here, we investigate the impact of NO on the temperature of decomposition of the metal nitrate hydrate, using TGA and MS. Figure 1 shows the TGA and MS results for the thermal treatment of Ni/SG in NO and Ar. The DTA plot, calculated from the TGA signal by taking the derivative, combines the weight-loss steps with the nature of the evolved gasses detected with MS. The temperature of decomposition (Td) was determined from the MS results by measuring the onset of the first NO/NO2 evolution peak. The NO (m/z=30) signal was used for the Ar thermal treatment because of its high intensity, but for the NO thermal treatment NO2 (m/z=46) was used because of the high NO background signal. In Figure 2 an overview is given of the Tds of the investigated nitrates. In general the Td values found in this study are lower than reported previously, which may be ascribed to the presence of the support [12]. In the Ar treatment a clear trend in the Td is observed. With the exception of scandium and iron nitrate, the Td gradually decreases going from the left to right in the periodic table, which can be explained by the decreasing radius of the divalent metal cations. Scandium and iron are trivalent cations and therefore have a smaller cation radius compared to the divalent cations, which decreases the pKa and thus facilitates hydrolysis to the metal hydroxynitrate. The hydrolysis reaction is depicted for copper nitrate in Equation 1. The presence of NO decreases the Td to below 100°C for all investigated metal nitrates. We propose the decrease in Td results from the facilitated decomposition of HNO3 formed during hydrolysis (Equation 1). HSC chemistry for windows 4.1 was used to calculate the reaction enthalpy for the decomposition of HNO3 in the absence or presence of NO (Equations 2 and 3, respectively) and showed that NO lowered the ΔHr0 from +65 kJ/mol in Ar to +36.5 kJ/mol in NO. This is also a possible explanation for the

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smaller differences in Td between different metal nitrates, as hydrolysis is in most cases feasible at such a low temperature that the dehydration rate becomes rate determining. 2Cu(NO3)2•3H2O Æ Cu2(OH)3NO3 + 3HNO3 + 3H2O 2HNO3 Æ 2NO2 + H2O + O2 2HNO3 + NO Æ 3NO2 + H2O

(1) (2) (3)

Figure 1. TGA (left, bottom), DTA (left, top) and MS (right) traces of nickel nitrate Ni/SG during thermal treatment in Ar (…) and NO (―).Gas evolution indicated as; I: H2O, II: H2O + NOx, III: NOx.

Figure 2. Temperature of the low temperature decomposition of SG supported metal nitrates during thermal treatment in 10% v/v NO/Ar (■) or Ar (●). Derived from the onset of the first NO/NO2 evolution peak.

On the impact of NO on the metal nitrate decomposition

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3.2. Impact of the gas atmosphere on the dispersion The ex situ thermally treated samples were characterized using XRD and TEM. SBA-15 was chosen as a support because in this case TEM analysis yields more information. Of all investigated metal oxides only cobalt, nickel and copper oxide showed diffraction lines, indicating the other oxides were either amorphous or the particles too small to detect. For SBA-Co, Ni and Cu large differences between the air and NO thermal treatments were observed both with XRD and TEM. Air calcination resulted in broad particle size distributions and large average crystallite sizes, whereas NO thermal treatment yielded small particles with narrow size distributions (Table 2). The positive effect of NO on the dispersion of cobalt and nickel oxide has been reported previously [10, 11], but the effect on copper we report here for the first time. Table 2. Metal oxide crystallite sizes as obtained from XRD. Sample

Particle size (nm)

x

Air

NO

SBA-Co

10

5

SBA-Ni

12

4

SBA-Cu

23

7

3.3. Impact of the gas atmosphere on the decomposition pattern 3.3.1. Scandium, manganese, iron and zinc nitrate The DTA curves of the decomposition of SG-Sc, SG-Mn, SG-Fe and SG-Zn are shown in Figure 3. The gas evolution, as determined from MS traces is indicated in the curve as follows; I: H2O, II: H2O + NOx and III: NOx. Manganese and iron nitrate decompose well below 200°C in Ar, where the first decomposition step involves only dehydration (I) and the second both dehydration and nitrate decomposition (II), in agreement with literature [2, 13]. The limited stability of iron nitrate may be ascribed to the small radius and high valency, resulting in destabilization of the nitrate anion (vide supra). However, manganese is comparable to cobalt and copper, but still decomposes at a significantly lower temperature. A possible explanation could be a difference in the extend of back donation. It is reported that transition metal nitrates decompose at lower temperatures than alkali metal nitrates with a similar charge density, because of back-donation of delectrons to empty π*-orbitals of the nitrate anion [14]. The presence of NO shifts the second decomposition step (II) to lower temperatures, now almost coinciding with the dehydraton step (I). In contrast, scandium and zinc nitrate show a three step decomposition. After partial dehydration (I), both dehydration and nitrate decomposition occurs in the 100200°C temperature range (II). The last decomposition step occurs much high temperatures (200-500°C) with only NOx evolution and no significant water evolution (III). All steps are broad and continuous such that no intermediates can be identified. For zinc nitrate hydrolysis to Zn(NO3)2·2Zn(OH)2 has been proposed as the first decomposition step [15], but the absence of water evolution at higher temperatures suggests anhydrous zinc nitrate was also formed. Moreover the significant evolution at higher temperatures suggests these species to be more stable than reported previously [12, 15, 16]. A likely explanation is the factor ten higher flow rate used in literature. A higher flow rate, i.e. faster removal of decomposition gasses, might accelerate decomposition.

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The decomposition of supported scandium nitrate hydrate has not been reported before, therefore no comparison to literature can be made. In general the decomposition patterns of scandium nitrate and zinc nitrate are more similar to that of main group metal nitrates such as potassium and calcium nitrate, which typically display decomposition over a very broad temperature range [14, 17]. The resemblance in behavior between scandium and zinc, and non d-metal nitrates is possibly be explained by their empty and full d-shell, respectively, resulting in a lower extent of back bonding between the metal ion and the nitrate. An additional explanation involves the fact that molecular oxygen has to evolve to complete the decomposition, and that the combination of two oxygen radicals to O2 is more efficiently catalyzed by oxides such as CuO, Mn2O3, Fe2O3 and NiO than ZnO and likely Sc2O3 [18]. As a consequence the decomposition is not accelerated by the increasing oxide formation. For both scandium and zinc nitrate the presence of NO during decomposition shifts the decomposition steps involving NOx evolution to lower temperatures, causing the first two decomposition steps in the 25-100°C temperature range to overlap. The high temperature (III) decomposition step is also shifted to lower temperatures, but stretches over a larger temperature range.

Figure 3. DTA curves of the decomposition of iron, manganese, zinc and scandium nitrate in Ar (…) and 10%NO/Ar (―). Gas evolution indicated as; I: H2O, II: H2O + NOx, III: NOx.

3.3.2. Cobalt, nickel and copper nitrate For this group of metal nitrates the difference between NO and Ar thermal treatment is much more apparent than for the other investigated metal nitrates (Figure 4). Decomposition in Ar results in all cases in a broad multi-step NO2 evolution ranging between 100-400°C. Several intermediates have been reported, including several dehydration stages and metal hydroxynitrates [4, 19, 20]. However, the broad pattern suggests multiple phases are present. Heat treatment in NO on the other hand results in rapid hydrolysis of the metal nitrate to its respective metal hydroxynitrate. This was confirmed with in situ XRD and IR (Figure 5), which show the presence of metal hydroxynitrates (a hexagonal layered structure, and a strong νOH band around 3600 cm-1). This phase is then stable over a certain temperature range before decomposing to the metal oxide in a single sharp step (with the exception of the two step decomposition of copper hydroxynitrate to CuO). Even though the decomposition is much faster in the presence of NO, in all cases the agglomeration is significantly reduced (Table 2). Metal hydroxynitrates bear close resemblance to metal hydroxides, which are generally less

On the impact of NO on the metal nitrate decomposition

75

prone to agglomeration than metal nitrate hydrates. It is therefore postulated that the formation of highly dispersed hydroxynitrates is the key to the high dispersion obtained in NO. In several previous reports hydroxynitrate formation resulted in the formation of very large copper oxide particles. An explanation for the apparent discrepancy is the high dispersion of the metal hydroxynitrate phase formed in NO (25 nm) copper hydroxynitrate crystal domains that are typically observed after drying at 120°C in stagnant air [5].

Figure 4. DTA curves of the decomposition of cobalt, nickel and copper nitrate in Ar (…) and 10%NO/Ar (―). Gas evolution indicated as; I: H2O, II: H2O + NOx, III: NOx.

Figure 5. XRD patterns (left) and IR spectra (right) of copper, nickel and cobalt hydroxynitrate formed at 150°C in 10%NO/Ar.

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4. Conclusions The impact of NO on the decomposition pattern and dispersion of first row transition metal nitrates was investigated using TGA-MS, XRD and TEM. The presence of nitric oxide lowered the temperature of decomposition significantly for all investigated metal nitrates. For cobalt, nickel and copper nitrate it was verified that the presence of NO during decomposition led to an improved metal oxide dispersion, where decomposition in Ar resulted in agglomeration. It was found that NO, in contrast to Ar, induced rapid hydrolysis of cobalt, nickel, and copper nitrate to highly dispersed metal hydroxynitrates, which was ascribed to a decrease in the decomposition temperature of nitric acid, the product of the hydrolysis. Hence, we show that NO affects the decomposition over the whole temperature range.

Acknowledgements The authors kindly thank Marjan Versluijs and Fred Broersma for their help with the TGA-MS experiments and and Cor van der Spek for the TEM analysis. Johnson Matthey Catalysts are acknowledged for the financial contribution to this work and Steve Pollington and John Casci for their scientific contributions.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

F. Paulik, J. Paulik and M. Arnold, Thermochimica Acta, 121, 1987, 137 M.A.A. Elmasry, A. Gaber and E.M.H. Khater, J. Therm. Anal. Calorim., 52, 1998, 489 J. Estelle, P. Salagre, Y. Cesteros, M. Serra, F. Medina and J.E. Sueiras, Solid State Ionics, 156, 2003, 233 P.L. Llewellyn, V. Chevrota, J. Ragaib, O. Cerclier, J. Estienne and F. Rouquerol, Solid State lonics, 101-103 1997, 1293 T. Toupance, M. Kermarec and C. Louis, J. Phys. Chem. B, 104, 2000, 965 C. Chouillet, F. Villain, M. Kermarec, H. Lauron-Pernot and C. Louis, J. Phys. Chem. B, 107, 2003, 3565 S. Catillon-Mucherie, F. Ammari, J.M. Krafft, H. Lauron-Pernot, R. Touroude and C. Louis, J. Phys. Chem. C, 111, 2007, 11619 J. van de Loosdrecht, S. Barradas, E.A. Caricato, N.G. Ngwenya, P.S. Nkwanyana, M.A.S. Rawat, B.H. Sigwebela, P.J. van Berge and J.L. Visagie, Top. Catal., 26, 2003, 121 J.R.A. Sietsma, J.D. Meeldijk, M. Versluijs-Helder, A. Broersma, A.J. van Dillen, P.E. de Jongh and K.P. de Jong, Chem. Mater., 20, 2008, 2921 J.R.A. Sietsma, J.D. Meeldijk, J.P. den Breejen, M. Versluijs-Helder, A.J. van Dillen, P.E. de Jongh and K.P. de Jong, Angew. Chem. Int. Ed., 46, 2007, 4547 J.R.A. Sietsma, H. Friedrich, A. Broersma, M. Versluijs-Helder, A. Jos van Dillen, P.E. de Jongh and K.P. de Jong, J. Catal., 260, 2008, 227 T. Nissinena, M. Leskel, M. Gasika and J. Lamminen, Thermochimica Acta, 427, 2005, 155 K. Wieczorek-Ciurowa and A.J. Kozak, J. Therm. Anal. Calorim., 58, 1999, 647 S. Yuvaraj, L. Fan-Yuan, C. Tsong-Huei and Y. Chuin-Tih, J. Phys. Chem. B, 107, 2003, 1044 B. Malecka, R. Gajerski, A. Malecki, M. Wierzbicka and P. Olszewski, Thermochimica Acta, 404, 2003, 125 T. Cseri, Békássy, G. Kenessey, G. Liptay and F. Figueras, Thermochimica Acta, 288, 1996, 137 C. Ettarh and A.K. Galwey, Thermochim. Acta, 288, 1996, 203 P.G. Dickens and M.B. Sutcliffe, Trans. Faraday. Soc., 60, 1964, 1272 W. Brockner, C. Ehrhardt and M. Gjikaj, Thermochim. Acta, 456, 2007, 64 A. Malecki, R. Gajerski, S. Labus, B. Prochowska-Klisch and K.T. Wojciechowski, J. Therm. Anal., 60, 2000, 17

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A novel approach to synthesize highly selective nickel silicide catalysts for phenylacetylene semihydrogenation Xiao Chen, Anqi Zhao, Zhengfeng Shao, Zhiqiang Ma, Changhai Liang* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China

Abstract Nickel silicides (NiSix) have been prepared by carbon template for nanostructured NiO and further silicidization with SiH4/H2 at relatively low temperature and atmospheric pressure. The results showed that the formation of nickel silicides involves the following sequence, Ni (cubic) → Ni2Si (orthorhombic) → NiSi (orthorhombic) → NiSi2 (cubic), with increasing temperatures. The as-prepared nickel silicides showed above 92% selectivity to styrene in the semihydrogenation of phenylacetylene due to the electronic and geometrical effects derived from the addition of Si into Ni particles. Keywords: nickel silicide, carbon template, silicidization, phenylacetylene semihydrogenation

1. Introduction The removal of phenylacetylene from styrene to low ppm range by semihydrogenation has become an important industrial process, because catalysts currently used for polystyrene production are extremely sensitive to phenylacetylene in styrene feed stocks. A number of catalysts including supported Pd, Ni, Pt, Pb, Ru, Rh, and Cu [1-6], have been developed and improved in the past tens of years. Palladium catalysts moderated by transition metals and /or by gaseous carbon monoxide in the feed, are now used to remove phenylacetylene from styrene [1, 2]. Supported Ni catalysts showed low activity and selectivity for phenylacetylene hydrogenation in the presence of styrene [3]. Novel catalytic materials for phenylacetylene semihydrogenation with high selectivity to styrene are highly desired under mild conditions. Transition metal silicides with unique physical and chemical properties, such as unusual structural, electronic, magnetic, and catalytic properties, have shown high selectivity in some hydrogenation reactions [7, 8]. Recently, Baiker et al. [7] indicated that amorphous Pd81Si19 catalyst in supercritical CO2 afforded high selectivity to styrene, which was usually necessary for high selectivity in “Lindlar-type” hydrogenations. Supported Ni silicides exhibited highly selectivity for the competitive dehydrogenation and hydrogenolysis of cyclohexane [9]. Supported Co silicides showed high activity and selectivity in selective hydrogenation of naphthalene [10]. However, conventional preparation methods, such as molten salt method, co-reduction route, and chemical vapor deposition inherited from the microelectronic industry resulted in a low surface area and a relatively low catalytic activity. Herein, we firstly demonstrate that carbon template method for nickel oxide and further SiH4/H2 silicidization to nickel silicides are of great potential in controlled synthesis of transition metal silicides. The as-prepared nickel silicides showed high styrene selectivity in the semihydrogenation of

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phenylacetylene. Formation mechanism of nickel silicides and effect of Si on catalytic properties of nickel silicides were studied in detail.

2. Experimental 2.1. Catalyst preparation Ultrahigh surface area carbon material [11] was prepared by a direct chemical activation route in which petroleum coke was reacted with excess KOH at 900oC to produce carbon materials containing potassium salts. These salts were removed by successive water washings. The surface area of the carbon materials measured by the BET method is about 3234 m2/g, the pore volume is about 1.78 cm3/g and the average pore size is about 2.2 nm. The carbon materials were impregnated at room temperature with a saturated aqueous solution of nickel nitrate. The slurry was then filtered and squeezed to remove the liquid on the carbon surface [12]. After drying in air at 60oC for 8 h, the resulting sample was transferred to a quartz reactor inside a tubular resistance furnace. The carbon template was removed by combustion at 500oC under a mixture gas of 20% O2 in Ar. Prior to reduction and silicidization, the reactor was purged with Ar to eliminate residual gases. NiO was reduced in a flow rate of 30 sccm H2 from room temperature to 450 oC, where it was held for 4 h. The reduced samples were cooled to the silicidization temperature under H2 atmosphere, and were silicidized with a 10% SiH4/H2 mixture for 15 min [9]. Then SiH4 was first stopped and the silicidized sample was cooled down to room temperature in H2 (30 sccm), passivated in 1% O2/Ar overnight. The obtained solids will be designated as T-NiSix, where T refers to the silicidization temperature. The exhaust gas was treated with water or alkali liquor during the silification. The reactions are as follows: SiH4 + 2H2O → SiO2 + 4H2 SiH4 + 2KOH + H2O → K2SiO3 + 4H2

2.2. Characterization TG/DTG experiments were performed in Mettler Toledo TGA/SDTA851e thermogravimetry to understand decomposition process of nickel nitrate and the removal temperature of carbon template. The nickel nitrate impregnated carbon was placed in the atmosphere of 80% Ar and 20% O2 and heated at 5 oC/min to the final temperature of 750oC. X-ray diffraction analysis of the samples was carried out using a Rigaku D/MaxRB diffractometer with Cu Kα monochromatized radiation source, operated at 40 KV and 100 mA. The average size of nickel silicide particles was evaluated by the Scherrer formula. The molar fraction Yi of NiSix in a Ni-Ni2Si-NiSi-NiSi2 mixture was calculated from XRD patterns as:

Yi =

Si S1 + S 2 + S 3 + S 4

Where S1, S2, S3, and S4 are the peak areas of the most intense reflections [13] of Ni, Ni2Si, NiSi, and NiSi2, respectively. Magnetic measurements were preformed on a JDM-13 vibrating sample magnetometer. M/H measurements were made with applied fields up to 4000 Oe at room temperature, and the complete hysteresis loops were recorded.

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The particle size and distribution of the samples were analyzed by transmission electron microscopy (TEM) (Tecnai G220 S-Twin, 200 kV). Powder samples were ultrasonicated in ethanol and dispersed on copper grids covered with a porous carbon film. Energy dispersive X-ray spectroscopy was also performed in the same microscopy.

2.3. Hydrogenation activity measurements

Liquid-phase semihydrogenation of phenylacetylene was carried out in a 50 cm3 closed vessel at controlled temperature. The catalyst was always activated in an ultrapure hydrogen stream at 300oC for 1 h, followed by cooling to room temperature. Approximately 0.2 g of the catalyst was placed in the reactor with 10 mL of 1 M phenylacetylene-ethanol solution. The vessel was filled with H2 to 0.27 MPa and vented it three times so as to remove the air in the vessel. Then the reactor was filled with H2 to 0.41 MPa pressure. The reaction was carried out at 50 oC for 5 h with the stirring condition. The products were analyzed by 7890 gas chromatograph with FID detector.

3. Results and discussion 3.1. Synthesis of nanostructured NiO by the carbon template method 100

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111 220

311 222

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20 100

200

300

400

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500

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Fig.1. a TG/DTG curves of Ni(NO3)2-impregnated carbon; b XRD pattern of NiO prepared by the carbon template method.

Figure 1a shows TG/DTG curves of the Ni(NO3)2-impregnated carbon under 80% Ar and 20% O2. As can be seen in Fig. 1a, mass loss of about 24% from 60 to 160oC can be attributed to the removal of surface physisorbed water and partial dehydration of the precursor. The maximum rate of mass loss occurs at 260oC due to the decomposition of Ni(NO3)2 and the partial combustion of carbon template. A small peak at 330 oC may be due to the further loss of carbon template. The carbon combustion is completed at temperature higher than 400oC. It had been found that the presence of the metal nitrate can catalyze the combustion of the activated carbon [14], which results to sintering of the synthesized inorganic particles. Therefore, Ar was firstly passed over the Ni(NO3)2impregnated carbon before the oven was heated to 300oC in order to avoid severe combustion reaction, then Ar was switched to the 20% O2/Ar and heated to 500 oC for 200 min to remove carbon template completely in the preparation process. XRD pattern of nanostructured NiO by the carbon template method is shown in Fig. 1b. The (111), (200), (220), (311), and (222) reflections due to cubic phase NiO (JCPDS No. 47-1049) were clearly observed. The average size of the particles estimated by the Scherrer equation is about 20 nm. The peaks due to the carbon template cannot be observed, indicating the carbon template was completely removed in the process.

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3.2. Synthesis of nickel silicides by reduction and silicidization 100

400oC

•− −NiO

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♣♦

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400 - NiSix ♣

44

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♥ ♥ ♣♥ ♥

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NiO

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Intensity(a.u.)

NiSi NiSi2

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Ni Ni2Si

a

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♠−Ni ♥− −Ni2Si ♣− −NiSi ♦− −NiSi2

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•

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Fig. 2. a XRD patterns of Ni and NiSix; b The molar fraction Yi of NiSix in a Ni-Ni2Si-NiSi-NiSi2 mixture calculated from XRD patterns of NiSix.

Figure 2a shows XRD patterns of Ni and NiSix obtained by reduction with H2 and silicidization with SiH4/H2. The crystalline phases of the samples are identified according to the JCPDS files (Ni, cubic, No. 04-0850; Ni2Si, orthorhombic, No. 481339; NiSi, orthorhombic, No. 38-0844; NiSi2, cubic, No. 43-0989). XRD patterns of all samples have intensive diffraction peaks at 44.51, 51.85, and 76.37°, which are due to metallic nickel. The diffraction peaks due to Ni2Si at 32.48, 39.46, 42.38, 43.49, 45.50, 48.76, 53.34, and 69.00° are observed on the 300 - NiSix sample indicates that Ni2Si is formed by the silicidization of metallic nickel with SiH4. Meanwhile, weak peaks at 45.84 and 47.28° are also observed, and can be assigned to NiSi. When the temperature increases to 350°C, the diffraction peaks due to NiSi become sharper while the peaks of Ni become weaker. Further increasing silicidization temperature to 400 °C, new diffraction peaks at 28.60, 47.41, 56.33, and 76.59°, which can be attributed to NiSi2, are observed. When the silicidization temperature increases to 450°C, the peaks due to NiSi2 become sharper while the peaks of NiSi become weaker, indicating the transformation from NiSi to NiSi2. As shown in the inset in Fig. 2a, there is a slight diffraction peak shift from 45.50 to 45.84o with the increasing of silicidization temperature, which is due to the transformation of Ni2Si (121) to NiSi (112). In addition, the diffraction peak at 47.28o shifted to 47.41o with increasing of temperature, which is due to the transformation of NiSi (211) to NiSi2 (220). Fig. 2b shows that the molar fraction Yi of NiSix calculated from XRD patterns. It is clear that the formation of nickel silicides involves the following sequence, Ni→Ni2Si→NiSi→NiSi2, with the increase of silicidization temperatures. The phase transformation among metallic nickel and its silicides with the elevated silicidization temperatures was also observed by Foggiato et al. [15]. Figure 3 gives the crystal structures of Ni2Si, NiSi, and NiSi2. NiSi2 with a cubic cell belongs to the Fm3m, No. 225 space group. Surface on NiSi2 is characterized by alternating Si and Ni atoms, and has no direct Ni-Ni or Si-Si bonds. Its bonding type is different from metallic Ni and bulk Si [16]. Ni2Si with a orthorhombic cell belongs to the Pbnm, No. 62 space group, while NiSi with a simple orthorhombic primitive cell belongs to the Pnma, No. 62 (oP8 Pearson symbol) space group. The orthorhombic unit cell contains four nickel and four silicon atoms. Nickel atoms have six first silicon

A novel approach to synthesize highly selective nickel silicide catalysts

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neighbors forming a strongly distorted octahedron, and silicon atoms have six first neighbors (nickel atoms), which form a distorted trigonal prism [17].

Fig. 3. The crystal structures of Ni2Si, NiSi, and NiSi2.

M (emu/g)

The formation of nickel silicides was also confirmed by magnetic measurements [18]. Fig. 4 shows the magnetization curves of the as-prepared Ni nanoparticles and the NiSix samples. All of the NiSix samples show certain coercivity at room temperature, which is characteristic for the ferromagnetic behavior of nanoparticles. The variability of coercivity is attributed to the formation of the NiSix phases. Their saturation magnetization values (Ms) at 4000 Oe drastically decrease after an amount of Si atoms adding into Ni. With increasing the silicidization temperatures, the metallic nickel was transformed into NiSi2 via Ni2Si and NiSi, while Ms values decrease firstly, and then increase. This may be due to different electronic and crystal structures of Ni2Si and NiSi2. Jarrige et al. [19] reported that the d states in Ni2Si had a strong Ni metal-like character, while NiSi was found to be non-ferromagnetic [17]. As shown in the inset in Fig. 4, the nickel silicide samples have magnetic character, indicating that the materials can easily be separated from reaction mixture in a magnetic field. 60

A

Ni

40 300 - NiSix 450 - NiSix 400 - NiSix 350 - NiSix

20 0 -20 -40 -60

-4000

-2000

0

2000

4000 Hext(Oe)

Fig. 4. Hysteresis M/H loops at room temperatures corresponding to Ni and NiSix.

In order to investigate the structural properties and the composition distribution, TEM, HRTEM, and EDX were carried out. It can be seen that the particles of NiSix are aggregated to a certain extent. HRTEM image of the 350 - NiSix sample demonstrated that outer surface of nickel particles was covered with a thin layer of NiSix, i.e. Ni2Si

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[20] (the lattice spacing of 0.25 nm corresponds to (020) plane of Ni2Si) and NiSi [21] (the lattice spacing of 0.21 nm corresponds to (210) planes of NiSi). The EDX spectrum further confirmed the existence of Si element in Ni particles in the samples.

Fig. 5. A representative TEM image (a), HRTEM image (b), and EDX spectrum (c) of the 350NiSix catalyst.

3.3. Phenylacetylene semihydrogenation reaction Figure 6 shows the conversion and selectivity of the NiSix samples in the phenylacetylene semihydrogenation reaction. Phenylacetylene conversions are 100, 56.4, 14.6, 33.8, and 79.9%, respectively, over metallic Ni, 300 - NiSix, 350 - NiSix, 400 - NiSix, and 450 NiSix catalysts. The corresponding selectivities to styrene are 86.8, 90.5, 92.3, 89.4, and 87.7%, respectively. The conversions for the samples NiSix are lower than Ni while the selectivities are improved, which may be due to Si atoms residing in the interstitial sites between Ni atoms, changing the nickel unit cell lattice thereby influencing adsorption of styrene. This means that formation of NiSix prevents styrene overhydrogenation to ethylbenzene. The phenylacetylene conversion decreases initially and then increases, while the selectivity to styrene follows a parabolic curve with elevated silicidization temperatures. The 350 - NiSix sample shows the lowest conversion but the highest styrene selectivity. The reason is that the Ni2Si (3d8.74sp1.2) [22] with a stronger Ni (3d94s1) metal-like character is firstly formed with the elevated silicidized temperatures, and has similar chemical properties to metallic Ni. Subsequently, the NiSi (3d8.64sp1.3) is formed, and both its electronic structure and crystal structure are significantly changed, such that catalytic activity falls off. When the silicidization temperature reached 450oC, the catalyst particles are partly agglomerated, which further reduces the number of active sites, but the NiSi2 phase with similar crystal structure to metallic Ni appears, which may have similar chemical properties to metal Ni from the perspective of the geometrical effect, so that the phenylacetylene conversion increases again. It had been

A novel approach to synthesize highly selective nickel silicide catalysts

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reported that Raney Ni catalysts were not suitable for selective hydrogenation of phenylacetylene due to their low activity and selectivity. However, the as-prepared NiSix catalysts show high activity and selectivity, which is attributed to the formation of metallic Ni particles modified by Si atoms using SiH4/H2 silicidization. These results indicate that the carbon template method for oxides and further SiH4/H2 silicidization to nickel silicides is a promising approach in the selective hydrogenation of phenylacetylene. 100

C% S%

80

%

60

40

20

0

Ni

300 - NiSix

350 - NiSix

400 - NiSix

450 - NiSix

Fig. 6. Results of the phenylacetylene semihydrogenation over the as-prepared NiSix catalysts.

4. Conclusions Nanoscale nickel silicides show excellent catalytic activity and high selectivity for phenylacetylene semihydrogenation and have been successfully synthesized using a carbon template method for oxides and further SiH4/H2 silicidization to silicides. XRD, magnetic measurements, HRTEM, and EDX confirm the formation of NiSix involved the following sequence, Ni→Ni2Si→NiSi→NiSi2, with the increasing silicidization temperatures. Si atoms reside into the interstitial sites between Ni atoms and change the nickel unit cell lattice, influencing catalytic activity and selectivity.

Acknowledgments We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 20973029), the Program for New Century Excellent Talents in Universities of China (No. NCET-07-0133) and Doctoral Fund of Ministry of Education of China (No. 20070141048).

References 1. 2. 3.

S. Domínguez-Domínguez, Á. Berenguer-Murcia, Á. Linares-Solano, and D. CazorlaAmorós, 2008, Inorganic materials as supports for palladium nanoparticles: Application in the semi-hydrogenation of phenylacetylene, J. Catal., 257, 87-95 S. Domínguez-Domínguez, Á. Berenguer-Murcia, B. K. Pradhan, Á. Linares-Solano, and D. Cazorla-Amorós, 2008, Semihydrogenation of Phenylacetylene catalyzed by palladium nanoparticles supported on carbon material, J. Phys. Chem. C, 112, 3827-3834 F. M. Bautista, J. M. Campelo, A. Garcia, D. Luna, J. M. Marinas, R. A. Quiros, and A. A. Romero, 1998, Influence of surface support properties on the liquid-phase selective hydrogenation of phenylacetylene on supported nickel catalysts, Catal. Lett., 52, 205-213

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X. Chen et al. B. A. Wilhite, M. J. McCready, and A. Varma, 2002, Kinetics of phenylacetylene hydrogenation over Pt/γ - Al2O3 Catalyst, Ind. Eng. Chem. Res., 41, 3345-3350 R. D. Adams, B. Captain, and L. Zhu, 2006, The importance of cluster fragmentation in the catalytic hydrogenation of phenylacetylene by PtRu5 carbonyl cluster complexes, J. Organomet. Chem., 691, 3122-3128 J. Pellegatta, C. Blandy, V. Collière, R. Choukroun, B. Chaudret, and P. Cheng, 2002, Karine philippot catalytic investigation of rhodium nanoparticles in hydrogenation of benzene and phenylacetylene, J. Mol. Cata. A: Chem., 178, 55-61 R. Tschan, R. Wandeler, M. S. Schneider, M. M. Schubert, and A. Baiker, 2001, Continuous semihydrogenation of phenylacetylene over amorphous Pd81Si19 alloy in “supercritical” carbon dioxide: relation between catalytic performance and phase behavior, J. Catal., 204, 219-229 H. Thomas and II Maugh, 1984, A new route to intermetallics metal silicides and related intermetallic compounds with unusual properties are formed by exposing supported metals to volatile organometallics, Science, 225, 403 R. G. Nuzzo, L. H. Dubois, N. E. Bowles, and M. A. Trecoske, 1984, Derivatized, high surface area, supported nickel catalysts, J. Catal., 85, 267-271 C. Liang, A. Zhao, X. Zhang, Z. Ma, and R. Prins, 2009, CoSi particles on silica support as a highly active and selective catalyst for naphthalene hydrogenation, Chem. Commun., 20472049 G. C. Grunewald and R. S. Drago, 1991, Carbon molecular sieves as catalysts and catalyst supports, J. Am. Chem. Soc., 113, 1636-1639 C. Liang, Z. Ma, H. Lin, L. Ding, J. Qiu, W. Frandsen, and D. Su, 2009, Template preparation of nanoscale Ce xFe1-xO2 solid solutions and their catalytic properties for ethanol steam reforming, J. Mater. Chem., 19, 1417-1424 C. Liang, F.Tian, Z. Li, Z. Feng, Z. Wei, and C. Li, 2003, Preparation and adsorption properties for thiophene of nanostructured W2C on ultrahigh-surface-area carbon materials, Chem. Mater., 15, 4846-4853 F. Schüth, 2003, Endo- and exotemplating to create high-surface-area inorganic materials, Angew. Chem. Int. Ed., 42, 3604-3622 J. Foggiato, W. S. Yoo, M. Ouaknine, T. Murakami, and T. Fukada, 2004, Optimizing the formation of nickel silicide, Mater. Sci. Eng. B, 114-115, 56-60 R. G. Nuzzo and L. H. Dubois, 1984, The chemisorption and catalytic properties of nickel intermetallic compounds: studies of single crystalline and high surface area, suppourted materials, Appl. Surf. Sci., 19, 407-413 D. Connétable1 and O. Thomas, 2009, First-principles study of the structural, electronic, vibrational, and elastic properties of orthorhombic NiSi, Phys. Rev. B: Condens. Matter, 79, 094101 H. Praliaud and G. A. Martin, 1981, Evidence of a strong metal-support interaction and of Ni-Si alloy formation in silica-supported nickel catalysts, J. Catal., 72, 394-396 I. Jarrige, N. Capron, P. Jonnard, 2009, Electronic structure of Ni and Mo silicides investigated by x-ray emission spectroscopy and density functional theory, Phys. Rev. B: Condens. Matter, 79, 035117 X. Q. Yan, H. J. Yuan, J. X. Wang, D. F. Liu, Z. P. Zhou, Y. Gao, L. Song, L. F. Liu, W. Y. Zhou, G. Wang, and S. S. Xie, 2004, Synthesis and characterization of a large amount of branched Ni 2Si nanowires, Appl. Phys. A, 79, 1853-1856 C. J. Kim, K. Kang, Y. S. Woo, K. G. Ryu, H. Moon, J. M. Kim, D. S. Zang, and M. H. Jo, 2007, Spontaneous chemical vapor growth of NiSi nanowires and their metallic properties, Adv. Mater., 19, 3637-3642 A. Franciosi and J. H. Weaver, 1982, Electronic structure of nickel silicides Ni2Si, NiSi, and NiSi2, Phys. Rev. B: Condens. Matter, 26, 546-553

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of calcium titanate photocatalysts for hydrogen production Katsuya Shimura, Hiroyo Miyanaga and Hisao Yoshida* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, *[email protected]

Abstract Various CaTiO3 samples having different particle sizes, shapes, crystal defects and impurity phases were prepared by three methods, i.e., co-precipitation, homogeneous precipitation and solid-state reaction methods. The CaTiO3 samples were loaded with Pt co-catalyst (0.1 wt%) and examined for both the photocatalytic water decomposition (WD) and the photocatalytic steam reforming of methane (PSRM). The highest activities for the WD and the PSRM were obtained over the samples prepared by the solid-state reaction method from rutile and anatase TiO2, respectively. The controlling factors in their activity were discussed. Keywords: photocatalysis, hydrogen, water, methane, calcium titanate

1. Introduction The development of a hydrogen production method from renewable resources and natural energy would be important to realize a sustainable society. Since the 0 photocatalytic water decomposition (referred to as WD; H2O → H2 + 1/2O2, ΔG298 K =237 kJ/mol) would be one of the most desirable systems, various photocatalysts for it has been developed so far [1]. On the other hand, the photocatalytic hydrogen production from water and biomass such as ethanol [2], saccharides [3] and methane [4] is also valuable. In these systems, hydrogen could be obtained more efficiently than the WD due to having a low ∆G value, and the carbon dioxide formed from them would not influence the global warming in the carbon neutral concept. We found that some kinds of Pt-loaded semiconductors could efficiently produce hydrogen from water vapor and 0 methane ( CH4 + 2H2O → 4H2 + CO2, ΔG 298 K =113 kJ/mol) [4-7], which can be also interpreted as photocatalytic steam reforming of methane, thus referred to as PSRM. CaTiO3 has been known to show the photocatalytic activities. Pt-loaded CaTiO3 was reported to show photocatalytic activities for both the WD [8] and the PSRM [7] upon the UV light irradiation. Generally, important factors of the semiconductor photocatalyst in these reactions are considered to be some structural ones such as the crystallite size, the specific surface area and the band structure. In the present study, we prepared various CaTiO3 samples by some methods and examined their photocatalytic activities for both the WD and the PSRM. And, we discussed the important factors of CaTiO3 structure in the photocatalytic activity for the WD and the PSRM.

2. Experimental 2.1. Catalyst preparation CaTiO3 samples were prepared by three methods, i.e., co-precipitation method (CP), homogeneous precipitation method (HP) and solid-state reaction method (S).

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In the co-precipitation method, CaCl2·2H2O (Wako, 99.9%, 5.4 g) and TiCl4 HCl solution (Wako, 16-17% as Ti, 11.0 g) were dissolved in distilled water (100 ml). The whole solution was added at one time to an aqueous solution (300 ml) of (NH4)2C2O4·H2O (Kishida, 99.5%, 10.4 g), where white precipitation was soon obtained. After neutralization of the solution by an aqueous NH3 (Wako, 10%) the white precipitation was recovered by centrifugal separation and washed with distilled water several times. The obtained powder was dried at 353 K, followed by calcination in air at various temperatures (973-1273 K) for 10 h. In the homogeneous precipitation method, (COOH)2·2H2O (9.3 g), CaCl2·2H2O (5.4 g), and TiCl4 HCl solution (11.0 g) were added to distilled water (300 ml), followed by heating to 363 K. After adding (NH2)2CO (Kishida, 99.0%, 33.1 g) the solution was kept at 363 K until pH of the solution reached to 7 (it took about 5.5 h), where white precipitate was gradually obtained as hydrolysis of the urea proceeded. The precipitates were recovered and washed with distilled water. The obtained powder was dried at 353 K, followed by calcination in air at various temperatures (973-1273 K) for 10 h. In the solid-state reaction method, the starting materials were mixed by a wet ballmilling method. As the starting TiO2 material, employed were TiO2 (Kojundo, 99.9%, rutile, 2.1 m2/g), JRC-TiO-6 (Catalysis Society of Japan, rutile, 100 m2/g), JRC-TIO-7 (ibid, anatase, 270 m2/g) and JRC-TIO-7 pre-calcined in air at 673 K for 5 h (anatase, 130 m2/g), which referred to as R2, R100, A270 and A130, respectively. CaCO3 (Kojundo, 99.99%, 22.1 g), TiO2 (17.6 g), alumina balls (150 g, 1 cm in diameter) and acetone (80 ml) were put into a plastic bottle (300 ml) and they were mixed at room temperature (120 rpm, 24 h), followed by drying in an oven (343 K) overnight. The mixed powder was dried at 343 K overnight and calcined in air at various temperatures (1073-1473 K) for 10 h. The prepared CaTiO3 sample was referred to as CaTiO3(method, TiO2 source (if necessary), calcination temperature) such as CaTiO3(CP, 973) and CaTiO3(S, R2, 1073). Pt co-catalyst was loaded by photodeposition method, as the similar way to the previous study [8]. Loading amount was 0.1 wt%. The CaTiO3 sample (2.0 g) was dispersed in methanol aqueous solution (10 vol%, 400 ml) containing H2PtCl6 (Wako, 99.9%), followed by photoirradiation for 1 h using a 300 W xenon lamp. 2.2. Characterization Powder X-ray diffraction (XRD) patterns were recorded on a MiniFlex II/AP (Rigaku) using Ni-filtered Cu Kα radiation by using Si powder as an internal standard. Mean crystallite size of the samples was estimated from the diffraction line at 33.2 degree. Diffuse reflectance (DR) UV-visible spectra were recorded on a V-670 (JASCO) equipped with integrating sphere. The Brunauer–Emmett–Teller (BET) specific surface area was calculated from the amount of N2 adsorption at 77 K which was measured on a Monosorb (Quantachrome). SEM images were recorded by S-5200 (Hitachi). 2.3. Photocatalytic reaction test The reaction tests were carried out with a fixed-bed flow type reactor [4-7]. The catalysts were granulated to the size of 400-600 μm. The quartz cell (60 × 20 × 1 mm3) was filled with a mixture of the catalyst (0.8 g) and quartz granules (0-0.7 g). The reaction gas, water vapor (1.5%) or a mixture of water vapor (1.5%) and methane (50%), was introduced into the reactor at the flow rate of 40 ml/min and the reaction was carried out upon photoirradiation with the 300 W xenon lamp. The light intensity measured in the range of 230-280 nm and 310-400 nm were 42 mW/cm2 and 84 mW/cm2, respectively. The outlet gas was analyzed by on-line gas chromatography with a thermal conductivity detector.

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3. Results and discussion 3.1. Characterization of CaTiO3 samples prepared by the CP and HP methods Fig. 1 shows the SEM images of the representative CaTiO3 samples prepared by the CP and HP methods. For CaTiO3(CP, 1073), various sized particles (ca. 50-500 nm) were observed (Fig. 1a). When the calcination temperature increased to 1273 K, the size increased to ca. 0.1-5 μm and the shape was still irregular (Fig. 1b). For CaTiO3(HP, 1073), very small particles (ca. 50-200 nm) were observed and the size and shape of the particles were uniform in comparison with the CaTiO3(CP) samples (Fig. 1c). When the calcination temperature increased to 1273 K, the size slightly increased to ca. 200-500 nm (Fig. 1d).

(a)

(b)

1.00μm

(c)

2.00μm

(d)

1.00μm

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Fig. 1 SEM images of (a) CaTiO3(CP 1073), (b) CaTiO3(CP, 1273), (c) CaTiO3(HP, 1073) and (d) CaTiO3(HP, 1273).

The different morphology of these two series of CaTiO3 samples would be originated from the generation mechanism of the precursors. In the preparation of the CaTiO3(CP) samples, the white precipitation, i.e. CaTiO(C2O4)2·H2O, was produced quickly when the (NH2)2C2O4 aqueous solution was added to another solution containing Ca2+ and Ti4+ ions. Since the concentration of C2O42- ion in the solution was not homogeneous, the size and the shape of the particles in the precipitation and the calcined sample would not become homogeneous. On the other hand, for the CaTiO3(HP) samples, as the urea hydrolyzed, C2O42- ion was gradually produced in the solution and reacted with Ca2+ and Ti4+ ions to form CaTiO(C2O4)2·H2O. Since C2O42- ion was homogeneously produced in the solution, the size and the shape of the particles in the precursor and the calcined sample would be uniform. Fig. 2 shows the XRD patterns of the representative CaTiO3 samples. In the CaTiO3(CP) samples, single phase of CaTiO3 was obtained by calcination at 1073 K and higher temperatures (Fig. 2b). In the CaTiO3(HP) samples, CaTiO3 could be prepared at 1073 K and higher calcination temperatures, but impurities such as Ca(OH)2 were also existed (Fig 2d). In the HP method, the hydrolysis of urea forms both NH3 and CO2. Produced CO2 dissolved in water to form CO32- ion, which may precipitate Ca2+ ion as CaCO3. This may change the ratio of Ca2+ to Ti4+ in the calcined sample, which would provide the excess Ca specie as Ca(OH)2 in air.

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Fig. 3 shows the DR UV-vis spectra of the CaTiO3(CP) and CaTiO3(HP) samples Fig. 2 XRD patterns of (a) CaTiO3(CP, 973), calcined at 1073 and 1273 K. The (b) CaTiO3(CP, 1073), (c) CaTiO3(HP, 973) absorption edge of the CaTiO3(HP) samples and (d) CaTiO3(HP 1073). ○: CaTiO3, ▲: was slightly at the shorter wavelength than Ca(OH)2, ●: unknown and ■: Si for the that of CaTiO3(CP) samples calcined at angle correction. each temperature, and the edges shifted to longer wavelength when calcined at higher temperatures for each sample. The shift would be concerned with the variation of the crystallite size in the CaTiO3 samples. The band gap energy estimated from the adsorption edge of the spectra was 3.5-3.6 eV. The color of the CaTiO3(CP) sample calcined at 973 K was white, while that of the sample calcined at 1073 K was light pink and it became dark with increasing the calcination temperature. In the CaTiO3(HP) samples, even the sample calcined at 973 K looked pink and the color became dark with increasing the calcination temperature. These result consisted with the absorption at visible light region in the UV-vis spectra (Figs. 3b, 3c and 3d). The maximum of the band was around 500 nm. The formation of crystal defects such as Ti3+ sites and oxygen vacancies would be the reason for the coloration. The deeper color of the CaTiO3(HP) samples than that of the CaTiO3(CP) samples showed the existence of larger amount of the crystal defects. In (A) (B) addition, when Ti(SO4)2 was (a) (a) 30 20 used instead of TiCl4 in the CP method, the color of the CaTiO3 20 samples became much deeper. (b) From these results, it is 10 suggested that unreacted (b) 10 additives such as urea and the counter anions of the starting 0 0 materials such as Cl- would 973 1073 1173 1273 973 1073 1173 1273 adsorb on the surface of the Calcination temperature / K precursor and cause the formation of defects during the Fig. 4 (a) Crystallite size and (b) specific surface area of calcination. the CaTiO3 samples calcined at various temperatures prepared by (A) the CP method and (B) the HP method.

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Fig. 4 shows the crystallite size and the BET specific surface area of the CaTiO3(CP) and CaTiO3(HP) samples. As the calcination temperature increased, the crystallite size tended to increase though the CaTiO3(HP) samples calcined at high temperature (10731273 K) were exceptional (Fig. 4a). The BET specific surface areas of CaTiO3(CP, 973) and CaTiO3(HP, 973) samples were 18 and 24 m2/g, respectively, and they drastically decreased to less than 10 m2/g with increasing the calcination temperature (Fig. 4b). The crystallite size and the specific surface area of the samples calcined at the same temperature were almost the same values, which implied that they would be mainly influenced by not the preparation method but the calcination temperature. As a conclusion of this section, the CP method provided the single phase of CaTiO3 having irregular morphology, and the HP method gave the small CaTiO3 particles of a uniform shape, although the crystal defects and the impurity phase of Ca(OH)2 existed. 3.2. Characterization of CaTiO3 samples prepared by the S method Influence of calcination temperature was examined for the CaTiO3(S, R2) samples. In their SEM images, aggregated particles were observed, as the representative one was shown in Fig. 5a. The particle size (ca. 1 μm) did not depend on the calcination temperature and was almost the same as that of R2 used as the starting TiO2 material. In their XRD patterns, large diffraction lines assignable to unreacted TiO2 (rutile) and Ca(OH)2 were observed when the sample was calcined at 1073 K (Fig. 6a) and they were hardly observed for the sample calcined at 1273 K and higher temperatures (Fig. 6b). For CaTiO3(S, R2, 1073), a large absorption band assigned to TiO2 (rutile) was observed (Fig. 7a), while it quite decreased but still remained in CaTiO3(S, R2, 1273) sample (Fig. 7b). Result of XRD and UV-vis showed that higher temperature than 1273 K was required to obtain a pure CaTiO3 by the S method from this TiO2 samples. The color of the samples calcined at 1173 K and lower temperatures was white, while that of CaTiO3(S, R2, 1273) was light pink. In other words, these CaTiO3(S) samples had a smaller amount of crystal defects than CaTiO3(CP) and CaTiO3(HP) sample did, although high temperatures were required to obtain a single phase.

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Other three kinds of TiO2 powders, R100, A130 and A270, were also examined as the starting materials for the preparation of CaTiO3 samples by the S method, followed by calcination at 1273 K. When TiO2 powders of higher surface area were used, the particle size of CaTiO3 samples became smaller as shown in SEM images (Figs. 5b-5d). From XRD, the formation of CaTiO3 was confirmed and no impurity phases were observed (not shown). From UV-vis spectra, the small absorption in the range of 350400 nm disappeared in the three CaTiO3(S, 1273) samples (Fig 7c and 7d). These results showed that no impurities were existed in the three CaTiO3(S, 1273) samples prepared from TiO2 particles of high surface area. The colors of all the CaTiO3(S, 1273) catalysts including CaTiO3(S, R2, 1273) were pale pink, showing that amount of crystal defects would be similarly small. The crystallite size did not depend on the kind of TiO2 (Fig. 8a), while the BET specific surface area of three CaTiO3(S, 1273) samples prepared from TiO2 particles of high surface area was larger than that of CaTiO3(S, R2, 1273) (Fig. 8Bb). The specific surface area of the two CaTiO3 samples prepared from anatase was smaller than that of CaTiO3(S, R100, 1273), suggesting that the specific surface area of 20 (A) (B) A130 and A270 would be (a) 30 reduced by sintering to be less than 100 m2/g (a) when they transformed 20 10 to rutile during the (b) calcination. 10 As a conclusion of this (b) section, it was found 0 0 that the CaTiO3 samples 1073 1173 1273 1373 1473 R R A A 2 100 130 270 of high surface area Calcination temperature / K Starting TiO2 material without impurities could be obtained by using TiO2 of high Fig. 8 (a) Crystallite size and (b) BET specific surface area of specific surface area as (A) CaTiO3(S, R2) samples calcined at various temperatures and (B) CaTiO3(S, 1273) prepared from various TiO2 raw materials. a starting material.

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3.3. Photocatalytic activities of the prepared CaTiO3 samples Fig. 9 shows the hydrogen production rate in the WD and the PSRM over Pt(0.1 wt%)/CaTiO3 samples. On almost all the samples, the hydrogen production rate in the PSRM was much higher than that in the WD on the present photocatalysts, where the loading amount of 0.1 wt% was employed to be suitable for the PSRM. This might be excess for the WD since Pt co-catalyst could promote the reverse reaction of the WD (H2 +1/2O2 → H2O) [9]. The hydrogen production rate in the WD on the Pt/CaTiO3(CP) and Pt/CaTiO3(S) samples tended to increase with decreasing the calcination temperature (Figs. 9Aa and 9Ca). In other words, it increased with increasing the specific surface area of the catalyst or decreasing the defects. Thus, one possibility is that the number of the surface Pt nano-particles per unit area decreased with the increase of the surface area and the reaction probability between the oxygen produced on the surface of CaTiO3 and the hydrogen produced over Pt might decrease. However, the activities of the Pt/CaTiO3(CP, 973) and Pt/CaTiO3(HP, 973) samples were low, although the specific surface area of them was high. This showed that a large crystallite size would be also important to promote the WD. The highest activity was obtained over Pt/CaTiO3(S, R2, 1073) having a high surface area and a relatively large crystallite size. Among the four Pt/CaTiO3(S, 1273) samples prepared from various TiO2 samples, Pt/CaTiO3(S, R100, 1273) with the largest surface area showed the highest activity for the WD (Fig. 9Da). 0.8

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Fig. 9 Hydrogen production rate on the Pt(0.1 wt%)/CaTiO3 samples in (a) the WD and (b) the PSRM; (A) the Pt/CaTiO3(CP) samples, (B) the Pt/CaTiO3(HP) samples, (C) the Pt/CaTiO3(S, R2) samples and (D) the Pt/CaTiO3(S, 1273) samples prepared from various TiO2 starting materials.

In the CaTiO3(CP) and CaTiO3(S, R2) samples, the activity for the PSRM first increased with increasing the calcination temperature (Figs. 9Ab and 9Cb), and the highest activity was obtained over CaTiO3(CP, 1073) and CaTiO3(S, R2, 1273) samples, respectively. The increase of the activity would be related to the increase of the crystallite size (Figs. 4 and 8), which would promote the smooth migration of photogenerated carriers in the conduction and valence bands to the surface. However, the activity decreased with further increasing the calcination temperature though the crystallite size increased. The color of CaTiO3(CP, 1073) and CaTiO3(S, R2, 1273) samples was pale pink, while that of CaTiO3(CP, 1173-1273) and CaTiO3(S, R2, 1473) samples was grayish pink. Therefore, these defects would decrease the activity. On the other hand, in the CaTiO3(HP) samples, the activity monotonously decreased with increasing the calcination temperature (Fig. 9Bb). As mentioned, in these samples, the color was pink even when calcined at 973 K and became dark with increasing the

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calcination temperature. Thus, the negative effect of the defects formation would become superior to the positive effect of the crystallite growth, resulting the decrease of the activity. These facts in Figs. 9A-9C show that the CaTiO3 samples with large crystallite sizes and few crystal defects would be effective for promoting the PSRM. Among the four CaTiO3(S, 1273) samples (Fig. 9D), the CaTiO3 samples prepared from anatase showed higher activity than the CaTiO3 samples prepared from rutile. The highest activity was obtained over CaTiO3(S, A130, 1273). Though the color of the CaTiO3(S, A130, 1273) and CaTiO3(S, A130, 1273) samples were similar to each other, the specific surface area and the crystallite size of the former were a little larger than those of the latter as shown in Fig. 8. These factors would improve the PSRM activity.

4. Conclusions We prepared CaTiO3 samples by three methods, i.e., co-precipitation (CP), homogeneous precipitation (HP) and solid-state reaction (S) methods, and examined their photocatalytic activities for the water decomposition (WD) and the steam reforming of methane (PSRM). By the CP method, large CaTiO3 particles without impurity phases could be obtained at 1073 K and higher temperatures though the particle size and shape were not homogeneous. By the HP method, small CaTiO3 particles with the regular shape could be obtained although the crystal defects formed even at low temperatures like 973 K and impurities such as Ca(OH)2 coexisted. By the S method, the crystallites with few defects could be produced, though high temperatures such as 1273 K or higher were required to obtain pure CaTiO3. When rutile TiO2 of large surface area was used as the start material for the S method, CaTiO3 of high specific surface area was obtained. CaTiO3(S, R2, 1073) and CaTiO3(S, R100, 1273) showed the highest activity for the WD and CaTiO3(S, A130, 1273) was the best for the PSRM. For the WD, the high surface area of CaTiO3 was more important than the large crystallite size of them, while, for the PSRM, the large crystallite size was more important. CaTiO3 samples with few defects showed high activities for both reactions.

References [1] A. Kudo and Y. Miseki, 2009, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev., 38, 253–278. [2] T.Sakata and T. Kawai, 1981, Heterogeneous photocatalytic production of hydrogen and methane from ethanol and water, Chem. Phys. Lett., 80, 341–344. [3] T. Kawai and T. Sakata, 1980, Conversion of carbohydrate into hydrogen fuel by a photocatalytic process, Nature, 286, 474–476. [4] H. Yoshida, S. Kato, K. Hirao, J. Nishimoto and T. Hattori, 2007, Photocatalytic steam reforming of methane over platinum-loaded smiconductors for hydrogen production, Chem. Lett., 36, 430–431. [5] H. Yoshida, K. Hirao, J. Nishimoto, K. Shimura, S. Kato, H. Itoh and T. Hattori, 2008, Hydrogen production from methane and water on platinum loaded titanium oxide photocatalysts, J. Phys. Chem. C, 112, 5542–5551. [6] K. Shimura, S. Kato, T. Yoshida, H. Itoh, T. Hattori and H. Yoshida, Photocatalytic steam reforming of methane over sodium tantalate, J. Phys. Chem. C, accepted. [7] K. Shimura and H. Yoshida, Energy Environ. Sci. in revision. [8] H. Mizoguchi, K. Ueda, M. Orita, S-C. Moon, K. Kajihara, M. Hirano and H. Hosono, 2002, Decomposition of water by a CaTiO3 photocatalyst under UV light irradiation, Mater. Res. Bull., 37, 2401–2406. [9] S. Sato and J. M. White, 1980, Photodecomposition of water over Pt/TiO2 catalysts, Chem. Phys. Lett., 72, 83–86.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A new procedure to produce carbon-supported metal catalysts Jacco Hoekstra,a,b Peter H. Berben,b John W. Geus,a Leonardus W. Jenneskens*a a

Organic Chemistry & Catalysis, Debye Institute For Nanomaterials Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands b BASF Catalysts, Strijkviertel 67, 3454 PK De Meern, The Netherlands

Abstract Mechanically strong carbon support bodies of a narrow size distribution are produced from the renewable biomass resources cellulose and table sugar. To this end Micro Crystalline Cellulose (MCC) spheres or partially Carbonized Sucrose (CS) spheres prepared by hydrothermal treatment of a sucrose solution were used. Via wet impregnation these spheres are easily loaded with various base metal salt precursors. By keeping the loaded spheres in a stagnant inert nitrogen atmosphere at elevated temperatures (500°C-800°C) the metal salt precursors are reduced to the corresponding metals without an external hydrogen gas source. During pyrolysis the MCC/CS spheres provide the required reducing environment. Keywords: activated carbon, carbohydrates, hydrothermal, pyrolysis, reduction

1. Introduction Activated Carbon (AC) is frequently used as a support in heterogeneous catalysis. It is attractive due to inertness of carbon in acidic and basic environments, and because precious metals, such as platinum, can easily be reclaimed by combustion of the carbon.1 AC is generally produced from naturally occurring carbon sources, such as nutshells, wood or peat. By pyrolysis in the absence of air or treatment with steam a carbon support of a high specific surface area is obtained. However, with AC it is difficult to produce bodies of a desired size distribution that are mechanically strong and thus attrition resistant. Also the pore structure and the surface characteristics (hydrophilic vs. hydrophobic) are difficult to control. The reduction of base metal salt precursors to supported metal particles is often a challenge. Copper oxide, for instance, is difficult to reduce since its reduction with hydrogen gas is highly exothermic. To prevent sintering copper catalysts have to be reduced with a highly diluted hydrogen gas flow, the inert gas deals with the heat of reduction. Consequently, the reduction of copper catalysts involves a large period of time. With less noble metals, such as cobalt and especially iron, the reduction is also problematic. Water vapor strongly affects the rate of reduction (cobalt) or even prevents the reduction thermodynamically (iron). With the usual hydrophilic, highly porous supports, such as alumina and silica, it is difficult to decrease the water vapor pressure within the support bodies to a level where reduction to the metal can proceed smoothly. Usually, the reduction to metallic iron particles only proceeds at temperatures where the iron particles sinter appreciably. Hydrophobic supports, such as (graphitic) carbon facilitate the reduction of catalysts due to a more rapid transport of water vapor. Though hydrogen reduction of supported catalysts is common practice, operations with hydrogen

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are dangerous due to the extensive explosive limits. With carbon supports hydrogen reduction can lead to reaction of the carbon to methane. The objective of our work is to provide mechanically strong carbon support bodies of a narrow size distribution from renewable biomass resources. A second objective is to provide supported base metal catalysts without using hydrogen gas as a reducing agent. We start from hydrophilic bodies of carbohydrates, such as Micro Crystalline Cellulose (MCC) spheres or partially Carbonized Sucrose (CS) spheres prepared by hydrothermal treatment of an aqueous sucrose (common table sugar) solution. Impregnation of the MCC/CS spheres with aqueous solutions of base metal salt precursors can be readily executed. It has been found that during pyrolysis of the loaded MCC/CS spheres in a stagnant inert nitrogen gas atmosphere at elevated temperatures (500°C-800°C) the base metal salt precursors are rapidly reduced to the corresponding metals in the absence of an external hydrogen gas source. During pyrolysis the MCC/CS spheres provide the required reducing environment. A mechanically strong carbonaceous catalyst support of a narrow size distribution loaded with various base metal catalysts results.

2. Experimental 2.1. MCC-spheres Commercially available MCC spheres (Cellets, neutral pellets of Syntapharm GmbH, Mülheim an der Ruhr, Germany), employed for the slow release of drugs, with a size range of 100 µm-200 µm were used as received.

2.2. CS-spheres The CS spheres were obtained from a hydrothermal treatment of sucrose (table sugar) according to a modified literature procedure.2 A 1M aqueous solution of sucrose in demineralised water was placed in a Teflon-lined autoclave. The solution was kept at 160°C for 4h. Afterwards the solid product was separated by centrifugation and was washed with a mixture of ethanol, acetone and demineralised water until a colorless solution was obtained. The resulting black powder was dried at room temperature in vacuo to constant weight.

2.3. Wet impregnation with metal salt precursors The hydrophilic MCC and CS spheres were loaded via wet impregnation. The spheres were immersed in a 2M solution of Fe(NO3)3·9H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O and Cu(NO3)2·2.5H2O, respectively. The mixtures were left for 24h. under occasional stirring. Next, the impregnated spheres were filtered using a Büchner funnel with glass filter. The isolated spheres were dried at room temperature in vacuo to constant weight.

2.4. Pyrolysis of loaded MCC/CS spheres The impregnated spheres were pyrolyzed under a stagnant inert nitrogen gas atmosphere in a quartz tube reactor (Thermolyne 21100 furnace). The heating rate was 5°C/min and the samples were treated for 3 h. at temperatures between 500°C and 800°C with consecutive 100°C increments.

2.5. Characterization of (pyrolyzed) MCC/CS spheres The morphology of the (pyrolyzed) MCC/CS spheres was examined with a Philips XL30 SFEG Scanning Electron Microscope (SEM). The samples were placed onto an aluminum stub coated with carbon tape.

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Both Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) were used to analyze the formed metal particles and the carbonized support. Samples were prepared by grinding and subsequently suspended in ethanol under ultrasonic treatment. One or two drops of the thus prepared sample were placed onto a holey carbon film on a copper (or nickel) grid. The samples were analyzed with a FEG-Technai-20 TEM apparatus operated at 200 KeV. Energy Dispersive X-ray (EDX) elemental analysis was performed with the installed TIA software. A High-Angle Annular Dark-Field detector (HAADF) provided images of the electron scattered over large angles, which are thus dominated by the heavier elements. Raman spectra were recorded using a Kaiser RXN spectrometer equipped with a 70 mW, 532 nm diode laser for excitation (data point resolution 2 cm-1).

3. Results and discussion 3.1. Structural properties of carbonized MCC 3.1.1. MCC loaded with Fe(NO3)3·9H2O SEM-images (Back-Scattered Electrons (BSE), indicative for heavier elements) show the typical morphology of the carbonized MCC-spheres in Figure 1. The surface of the spheres is quite rough. The color of the spheres has changed from white to black due to pyrolysis of the cellulose. As can be inferred from Figure 1 the resulting carbonized MCC spheres have a narrow size range (diameter ca. 100 μm).

Figure 1. SEM-images (BSE) of carbonized MCC spheres loaded with Fe(NO3)3·9H2O, pyrolyzed at 500°C (left) and 800°C (right). Scale bars 100 μm.

Whereas the distribution of iron is uniform after thermal treatment at 500°C, pyrolysis at 800°C brings about segregation and sintering of iron at the external edge of the carbon bodies. An important observation is that both the 500°C and 800°C samples are ferromagnetic, which indicates that reaction to either magnetite (Fe3O4) or metallic iron has proceeded. Figure 2 represents a HAADF STEM-image with EDX elemental analysis from the sample pyrolyzed at 500°C. The oxygen K-signal follows the iron K- and L-signals which indicate that the iron particles are oxidized. The STEM-image indicates that a high density of iron oxide particles is formed. The size of the iron oxide particles is 3 nm-10 nm. In Figure 3 a TEM lattice image of such a particle is shown.

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Figure 2. STEM-image (HAADF) with EDX elemental analysis along the line indicated of an MCC sphere loaded with Fe(NO3)3·9H2O and pyrolyzed at 500°C. The bright spots represent iron oxide particles. Scale bar 20 nm.

Figure 3. HR-TEM-image of an iron oxide particle with a size of ca. 6 nm (circle). Scale bar 2 nm.

When pyrolyzed at 800°C metallic iron particles are formed in larger clusters (20 nm – 20 μm, see also Figure 1). In the temperature regime between 500°C and 800°C the iron oxide particles are reduced to metallic iron,3 which sinters severely above 700°C. This is substantiated by TEM-analysis (Figure 4).

Figure 4. TEM-image (HAADF) of an MCC-sphere loaded with Fe(NO3)3·9H2O and pyrolyzed at 800°C with reduced particles of a larger size. Scale bar 500 nm.

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It is important to note that no external hydrogen gas source has been employed to reduce the base metal salt precursor. Clearly, MCC spheres during pyrolysis under a stagnant inert nitrogen atmosphere provide the reducing environment. Literature data indicate that during the pyrolysis of cellulose three kinds of species are formed, viz. char, tar (mainly levoglucosan) and light gases (mainly CO, H2 etc. released between 400°C and 800°C).4 It is anticipated here that CO and/or amorphous (carbonaceous) material acts as the reducing agent(s) of the iron precursors. 3.1.2. MCC loaded with Cu(NO3)2·2.5H2O Figure 5 represents SEM-images (BSE) of MCC spheres loaded with Cu(NO3)2·2.5H2O and subsequently pyrolyzed at 800°C under a stagnant nitrogen atmosphere. At the higher magnification (right) small supported copper particles can be observed. The primary difference between iron and copper is that the copper particles sinter appreciably less above 700°C. The copper particles have a size of about 100 nm.

Figure 5. SEM-images (BSE) of MCC-spheres loaded with Cu(NO3)2·2.5H2O and pyrolyzed at 800°C at different magnifications. Scale bar left 50 μm and right 5 μm.

In Figure 6 a STEM-image (HAADF) with EDX-analysis is given of the MCC supported copper particles after pyrolysis at 500°C. The copper particle size is 5-20 nm. From elemental analysis it is apparent that the copper catalysts are already in their reduced state, since the oxygen K-signal is very low in comparison to the copper L-signal. If the material was copper oxide, we expect a considerably more intense oxygen K-signal (compare Figure 2 for iron oxide). Therefore, it is concluded that copper particles are passivated. 4000,00

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Figure 6. STEM-image (HAADF) with EDX elemental analysis along the line indicated. The bright spots represent copper particles. Scale bar 50 nm.

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In Figure 7 a STEM-image (HAADF) is given for a sample heated at 800°C. Also here the oxygen K-signal is very low indicating that the copper catalysts have been reduced. Interestingly, the copper particle size is similar both at 500°C and 800 °C. The copper particles are thus much less prone to sinter at elevated temperatures than the iron particles. 600,00

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Figure 7. STEM-image (HAADF) with EDX elemental analysis along the line indicated. The bright spots represent copper particles. Scale bar 200 nm.

3.1.3. MCC loaded with Co(NO3)2·6H2O and Ni(NO3)2·6H2O The MCC-spheres loaded with cobalt- and nickel-salts behave more or less the same as the iron- and copper-loaded spheres. At 500°C nickel is already in its reduced state, whereas cobalt is still in an oxidic state. At 600°C cobalt is also fully reduced. The sintering characteristics of nickel and cobalt follow the behavior of copper. Some sintering occurs, but not as much as displayed by iron. The formation of the catalysts is believed to be as follows,5 at first the metal nitrates will be decomposed to the corresponding metal oxides (T < 240°C), then the carbohydrate bodies become pyrolyzed and transform into carbon (T = 300°C-600°C). At the lower temperatures the metal oxides are most likely reduced by the carbon monoxide that is released during the pyrolysis of the cellulose.

3.2. Raman spectroscopy To substantiate that the supports consist of (graphitic) carbon, Raman-spectra were recorded for various samples. An illustrative spectrum is shown for the iron loaded sample pyrolyzed at 800°C (Figure 8, see also Figure 1). The spectrum contains two strong peaks at 1344 cm–1 and 1584 cm–1. The peak at 1584 cm–1, called the G band, is due to in-plane bond stretching of pairs of sp2 hybridized C-atoms in graphitic planes. The D band at 1344 cm–1 stems from ring-breathing vibrations in benzene or condensed benzene rings in amorphous G band D band carbon. 5 Raman spectroscopy gives evidence that in the case of iron, cobalt or nickel loaded MCC spheres extensive graphitization occurs at temperatures above 700°C. With copper the extent of graphitization is substantially less which is apparent from the broadening of the G-band (not shown). 800 700 600

Intensity (a.u.)

500 400 300 200 100

0 1200

1300

1400

1500

1600

1700

1800

-1

Raman shift (cm )

Figure 8. Raman spectrum of MCC loaded with Fe(NO3)3·9H2O, pyrolyzed at 800°C.

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99

3.2. Structural properties of CS spheres Since the MCC spheres are used as an excipient in medicines, and thus have to be very pure, their price is quite high. To find an alternative for the MCC-spheres we considered hydrothermally treated sucrose spheres, which are much cheaper. Figure 9 (SEM + STEM) shows the morphology of the CS spheres after isolation.

Figure 9. SEM-image (left, BSE, scale bar 10 μm) and STEM-image (right, HAADF, scale bar 1 μm) showing the morphology of CS spheres hydrothermally treated at 160°C.

The size of the CS sphere is 2 μm-8 μm. Conglomerates of these spheres are also formed. From literature it is known that it is possible to tune the size of the spheres by adjusting the experimental conditions time, temperature and concentration of the sucrose solution. The CS sphere consist of a graphitic core with functional (-OH, -COOH) groups on the periphery, i.e. under hydrothermal conditions not all functional groups are lost. Due to these functional groups the CS spheres are hydrophilic and can easily be loaded with base metal salt precursors. 3.2.1. CS loaded with Ni(NO3)2·6H2O Figure 10 displays a SEM-image (BSE) of CS impregnated with Ni(NO3)2·6H2O pyrolyzed at 800°C. Finely divided nickel particles have formed onto the surface of the CS spheres.

Figure 10. SEM-image (BSE) of finely divided Nickel particles onto the surface of the CS spheres. Scale bar 1 μm.

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From the STEM-image (HAADF) and EDX-analysis (Figure 11) it is apparent that the nickel particles are in their reduced state. The oxygen (K-signal) level is very low. The results for the CS spheres were similar to those of the MCC spheres (vide supra). The metal nitrates were first converted to the corresponding oxides, and subsequently the metal oxide particles became reduced by the pyrolysis gases released from the further decomposing CS spheres. Thus, the CS spheres are a good and cheap alternative for the MCC spheres. 14000,00 12000,00

Intensity (a.u.)

10000,00 8000,00

OK Ni L

6000,00

Ni K

4000,00 2000,00

29,46

27,00

24,55

22,09

19,64

17,18

14,73

9,82

12,27

7,36

4,91

2,45

0,00

0,00

Location (10 nm)

Figure 11. STEM-image (HAADF) with EDX elemental analysis along the line indicated. The bright spots indicating the nickel particles. Scale bar 100 nm.

4. Conclusions Highly uniform carbonaceous catalyst supports have been synthesized with two carbohydrate precursors, Micro Crystalline Cellulose (MCC) spheres and partially carbonized sucrose (CS) spheres. Both hydrophilic carbon support precursors can be easily impregnated with a range of base metal catalyst precursors. A simple pyrolysis procedure reduces the metal salt precursors first to the corresponding metal oxides and ultimately to the corresponding metals. The thermally decomposing carbohydrate bodies provide the reducing environement (CO and/or amorphous (carbonaceous) material) for the reduction of the base metal precursors. With Raman, SEM and (S)TEM it is shown this procedure results in (graphitic) carbon-supported base metal catalysts.

References [1] F. Rodriguez-Reinos, 1998, The Role Of Carbon Materials In Heterogeneous Catalysis, Carbon, 36, 3, 159-175. [2] Q. Wang, H. Li, L. Chen, X. Huang, 2001, Monodispersed Hard Carbon Spherules With Uniform Nanopores, Angew. Chem. Int. Ed., 39, 2211-2214. [3] F. Gong, T. Ye, T. kan, Y. Torimoto, M. Yamamoto, Q. Li, 2009, Direct Reduction Of Iron Oxides Based On Steam Reforming Of Bio-Oil: A Highly Efficient Approach For Production Of DRI From Bio-oil And Iron Ores, Green Chem., 11, 2001-2012. [4] D.K. Shen, S. Gu, 2009, The Mechanism For Thermal Decomposition Of Cellulose And Its Main Products, Bioresource Technology, 100, 6496-6504. [5] M. Sevilla, C. Sanchis, T. Valdes-Solis, E. Morallon, A.B. Fuertes, 2008, Direct Synthesis Of Graphitic Carbon Nanostructures From Saccharides And Their Use As Electrocatalytic Supports, Carbon, 46, 6, 931-939.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Use of zeta potential measurements in catalyst preparation Stuart Soled,a William Wachter,b Hyung Woa a

ExxonMobil Research and Engineering Company, Corporate Strategic Research, Annandale, NJ 08801, USA b ExxonMobil Research and Engineering Company, ExxonMobil Process Research, Annandale, NJ 08801, USA

Abstract We illustrate two applications where zeta potential measurements have provided useful information for catalyst preparations. In the first case, we decribe how maximizing the electrostatic attraction between the active complex in an impregnating solution and the support leads to smaller metal particles upon reduction. This approach allowed synthesis of small platinum crystallites on a yttria-modifed amorphous silica-alumina support. The yttrium oxide not only titrates some acid sites but it provides more positively charged surface regions (at a given solution pH) on the support that better disperse the anionic chloroplatinate anion and the subsequently formed Pt crystallites. In the second application, we studied the attrition resistance of FCC (fluid cat cracking) catalysts, ~70 micron spray dried particles formed from micron-sized USY (ultrastable Y) zeolite crystals and submicron sol particles. The attrition is minimized when the larger zeolite particles are uncharged while the submicron-sized sol particles are highly charged. The results suggest that the stable colloid formed from nanocrystalline haloing provides an optimized dried and calcined agglomerate. Keywords: zeta potential, fluid cat cracking

1. Introduction Surface charging of small oxide particles can provide a useful tool to enhance catalyst syntheses. In this paper we describe two applications where surface charges are used advantageously- in the first, matching complementary charges on the support surface and metal impregnate complex are used to optimize Pt metal dispersion. This approach was described many years ago by Brunelle [1], and more recently expanded and refined by Regalbuto [2]. To determine the zero point of charge of a support, they measure the buffering action of the surface as it is exposed to solutions with different amounts of acid or base. Zeta potential measurements use a different approach to measure (near) surface charge but the concepts of optimizing support and impregnate electrostatic interactions remain the same. In the bifunctional catalyst briefly described here, a surface yttrium oxide partial monolayer is added to an amorphous silica-alumina support to temper its acidic properties and Pt is added to this modified support to allow bifunctional catalysis. In the second application, we discuss the use of surface charging concepts and zeta potential measurements to optimize the attrition resistance of fluid cat cracking (FCC) catalyst composite particles. The FCC particles consist of micron size zeolite particles held together by submicron sol particles to form 50-70μ composites. In this study, we start with a suite of USY zeolites of variable bulk Si-Al ratios and first determine their relative surface compositions using isoelectric points (IEP). We established an excellent

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correlation between the measured isoelectric points and the XPS-determined relative surface concentrations of Si and Al. We then show how appropriately matching the IEP of the zeolites with the properties of the sol particles can create an attrition-resistant composite. Small particle metal oxides lower their free energies by terminating their surfaces with the lowest possible charge, which normally means with oxygen. Such an atomic arrangement would create a stoichiometry richer in oxygen than the (crystallographically imposed) bulk stoichiometry and would generate a net negative charge on small oxide particles- this of course, does not occur. Instead, the surfaces normally terminate in hydroxyl groups rather than oxide anions. The hydroxyls effectively lower the surface anion charge from –2 to –1 and allow the particles to maintain a neutral total charge. Some of the hydroxyls may, on calcination, condense with release of water. This condensation creates bridged oxygen anions or coordinatively unsaturated metal cations but it maintains the neutral charge of the metal oxide (or more correctly, metal oxyhydroxide) particles. When small oxide (i.e. oxyhydroxide) particles contact liquid water they no longer have to remain electrically neutral; surface charges can develop because ions in solution are available to neutralize these charges by forming a classical double layer surrounding the particle. Consequently, surface hydroxyls on small oxide particles in aqueous suspension will ionize and their surfaces develop a net positive or negative charge, as a function of the pH (the protonation or deprotonation driving force) in the contacting solution. Zeta potential refers to the charge at the interface of the shear plane separating the tightly held compact layer and the more loosely held diffuse layer of the classical double layer. The isoelectric point represents the pH at which the zeta potential equals zero, and it reveals information about the near surface cation composition of the oxide support. At a specific pH for each solid, small but equal amounts of M(OH2)+ and MO– co-exist, with M(OH) being the most abundant surface species; this represents the isoelectric point. Consequently, the zeta potential is positive at pH values below the isoelectric point, where M(OH2)+ become the majority surface species as protons are donated from the hydronium ions, and negative at pH values above the isoelectric point. There is a difference between the zero point of charge and the IEP, but for most cases considered here (i.e. weak specific anion adsorptions), they are closely related. The IEP gives information about the chemical nature of the metal oxide support because oxides containing cations with high charge to radius ratio (for ex. Si+4) have low IEPs and oxides with lower charge-density cations have higher IEPs. This occurs because at high charge densities (e.g. for Si+4) a large driving force (low pH) is required to protonate the OH group as it is close to the small and already highly charged Si+4 cation. In other words, a protonated OH2+ on silica (if it were even to exist) would donate its proton to a solution at low pH. This sounds strange since silica classically acts as a non-acidic inert support in gaseous environments typical of most catalytic reactions, but in aqueous suspension it is very acidic. In contrast, Al+3 cations, because they have a lower charge and are larger than Si+4, are more easily protonated and therefore have a higher isoelectric point (~9). Ti+4 like Si+4 is tetravalent, but it has a larger cationic radius and thereby is more easily protonated: its isoelectric point is between 6 and 7 [3]. For mixed oxide supports the average surface population of the cations at the measured IEP reflects a molar average of the individual metal oxide IEP’s. Historically, electrophoresis measurements were used to measure isoelectric points, but fortunately, during the last couple of decades, simple and inexpensive laboratory instrumentation has become available to measure zeta potential even in relatively high

Use of zeta potential measurements in catalyst preparation

103

concentration suspensions. These instruments measure acoustic signals created when the double layer around small particles distorts when these particles are placed in a megahertz ac electric field.

2. Experimental Table 1 lists the suite of ultrastable Y zeolites zeolites investigated in this study. Five grams of each zeolite was added to 200 cc of water and dispersed for 5 minutes using an ultrasonic dispersion probe and then measured with a Matec 8050 electrokinetic instrument. Zeta potentials were monitored during titrations using 1N HCl or 1N NaOH; titration with HCl if the initial zeta potential was negative and with NaOH if the initial zeta potential was positive. The crystallinity of the samples was determined by x-ray diffraction following ASTM Procedure D-3906-91. XPS measurements were performed on a Leybold-Heraeus ultra high vacuum system equipped with an Al Kα x-ray source (hυ=1486.6 eV) and a hemispherical energy analyzer. Photoemission spectra were obtained normal to the analyzed surface of pressed wafer samples with the electron analyzer operating at 50 eV pass energy. Surface areas were determined by a multipoint BET measurement after outgassing at 300C. The Davison Attrition Index (DI) uses 7.0 cc of sample catalyst which is screened to remove particles in the 0 to 20 micron range. The remaining particles are then contacted in a hardened steel jet cup having a precision bored orifice through which an air jet of humidified (60%) air is passed at 21 liter/minute for 1 hour. The DI is defined as the percent of 0-20 micron fines generated during the test relative to the amount of >20 micron material initially present, i.e., the DI = 100 x (wt% of 0 – 20 micron material formed during test)/ (wt% of original 20 microns or greater material before the test). The lower the DI, the more attrition resistant is the catalyst. The zeolites were obtained from an external source; consequently, the details regarding their modification are not known. Table 1. Properties of USY zeolites. Sample Label

% wt. Na

A B

BET Surface Area (m2/g)

% Crystallinity

Bulk Si/Al

XPS Si(2p)/Al(2p)

0.86

4.62

8.82

543

93

1.5

3.54

4.6

590

98

C

0.43

2.99

3.76

569

79

D

0.15

2.66

1.73

560

67

E

0.03

6.55

3.56

642

88

F

0.14

2.74

1.58

543

66

G

1.5

2.38

1.65

490

67

H

0.11

2.57

0.93

498

83

I

0.15

2.69

0.86

593

91

J

0.65

2.71

0.86

667

98

K

2.8

2.61

1.45

605

99

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S. Soled et al.

For the supported SiO2-Al2O3 catalysts, an amorphous silica alumina with a bulk concentration of 55%SiO2 and 45%Al2O3 was used. Aqueous yttrium nitrate was impregnated via incipient wetness impregnation onto the supports, and the samples were then dried and calcined at 450C. 0.3% wt. Pt was impregnated using a hexachlorplatinate precursor.

3. Results and discussion

7.4

60%

7

48%

6.6

36%

6.2

24%

5.8

12%

5.4 0

2

4

6

8

10

12

14

16

% Pt dispersion (H/M)

isoelectric point

Professor Brunelle in 1978 authored a masterful review describing a strategy for optimizing supported metal catalyst preparations by maximizing the electrostatic attraction between the precursor and support [1]. Others have followed and expanded on this protocol over the years, with many of the newer studies described by Regalbuto [2]. Our work is also based on enhancing electrostatic interactions but uses zeta potential measurements to chose the optimum preparation method for synthesizing Pt clusters on modified silica-alumina supports. In this study we were interested in tempering the acidity of amorphous silica-alumina by titrating with partial monolayers of yttrium oxide so that the residual acid sites would have strengths similar to chlorided or fluorided alumina [4]. The application involved bifunctional catalysis so optimizing platinum dispersion on the modified silica-alumina supports was important. Yttrium oxide is mildly basic and disperses readily onto the silica-alumina surface [5]. Since the isoelectic point of an oxide is related to the charge to radius ratio of its surface cations, the large trivalent yttrium cations have high isoelectric points- above 11 [3]. We determined that as the yttria surface population increases, the isoelectric point of the modified silica-alumina increases as does the hydrogen chemisorption uptakes (dual isotherm measurements at 40oC) of the reduced hexachloroplatinate anion (see Figure 1). The IEP represents an average surface state, so that below monolayer coverage we measure a contribution from regions of silica-alumina and regions of yttrium oxide. At a given pH, the regions of the support with a higher positive charge more strongly attract the dicholorplatinate anion and produce more dispersed Pt. It is reasonable to assume (although not proven here) that the Pt is preferentially located on the yttrium oxide.

0%

18

% Y2O3 in Y2O3/SiO2-Al2O3 Figure 1. Isoelectric Point and Strong Hydrogen Chemisorption of .3%Pt/Yttria Modified-Silica Alumina.

Use of zeta potential measurements in catalyst preparation

105

The second problem we addressed concerns a common issue in catalyst synthesis, namely how to insure that larger particles formed from assembling smaller components maintain physical integrity. In the case of FCC catalysts, 50-70μ spray dried agglomerates were created from micron sized zeolites held together with submicron sol particles. The question we addressed is how to minimize the attrition of the composite. The suite of zeolites chosen consisted of ultrastable Y zeolites of 1-3 micron size with the properties shown in Table 1. The IEP of each of the USY samples was measured by titration with either HCl or NaOH. The fractional surface aluminum concentration for the suite of USY samples was also measured using XPS, and Figure 2 compares the two measurements.

isoelectric point

10 8 6 4 2 0

0

0.1

0.2

0.3

0.4

0.5

0.6

XPS surface Al/(Si + Al) Figure 2. Isoelectric point vs. XPS fractional aluminum content for suite of USY zeolites.

Although we expect XPS to sample to a penetration depth of ~40Å, the fractional aluminum content correlates well with the measured isoelectric points. The variation of isoelectric points from 2 to 9 suggests that for some of the USY samples the surface is silica-rich whereas other samples have predominantly alumina-rich surfaces. Because these samples were obtained from an outside source, the exact methodology used to surface enrich the USY samples is not know, although it would interesting to understand how to do this. Note that although the XPS ratios do correlate with the isoeletric points, the XPS ratios do not extend from aluminum contents of 0 to 1, suggesting that the subsurface layers are not as enriched in Si or Al as the surface. We did have concerns about the stability of the zeolites in acidic media, so we checked the time dependence of our measurements. When we changed the “soak” time in the acid from 15 to 150 seconds between measuring each data point, the zeta potential measurements did not change substantively. This probably results from the low concentration of acid present during titration (~5ml of 1 N HCl in 200 cc water) and the low reactivity at room temperature. Two different sols were used to bind the USY particles together. One was a SuperD silica sol prepared by reacting a sodium silicate solution with an aluminum sulfate/sulfuric acid solution under high shear to a pH 3.0 and the other was an aluminum chlorohydrol sol stable at pH 4.3. In Figure 3 we show schematically what we are attempting to achieve in the spray drying process, with the relative sizes of the USY zeolites and sol particles depicted.

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composite catalyst

+

(70μ)

submicron sol particles

zeolite, clay or alumina binder 1μ

Figure 3. Binding of components of an FCC catalyst.

The attrition of the composite FCC particles was measured using the Davison Attrition Test. Higher values of the index indicate weaker particles that attrit easier. A series of catalysts were prepared by spray drying the USY zeolites with one of the two sols and the attrition of the composite was measured. The results are shown in Figure 4.

Davison Attrition Index (increasing weakness) ==>

60 50 40 30 20 10 0

0

1

2

3

4

5

⏐sol pH - zeolite isoelectric point⏐

6

Figure 4. Correlation of FCC Agglomerate Strength and the difference between the absolute valuer of the sol pH and the zeolite IEP.

It is intriguing that the composite is strongest when the larger zeolite crystals have their IEP near to or at the pH of the sol. In the pH stability range of the small sol particles (2-4 for Super-D and 4-5 for alumina chlorohydrol) the sol nanoparticles are electrotatically stabilized with a high surface charge. The results in Figure 4 suggest that at the pH where the sol particles are highly charged, the zeolite should have no surface charge. This result may seem counterintuitive as one might think that the sol particles and the zeolites should be oppositely charged. However, this is not what this data suggests, so another phenomena is operating to create the stable agglomerate. A publication by Jennifer Lewis may help explain this phenomena [6]. She introduced the concept of nanoparticle haloing as a self-organizing process that imparts stability to naturally attractive particles by decorating their superficial areas with highly charged nanoparticles present at critical concentrations. She observed and calculated this effect for the case of micron sized silica particles decorated with a zirconia sol and found that the most stable arrangement occurred when the larger silica particles were

Use of zeta potential measurements in catalyst preparation

107

uncharged and the smaller zirconia nanoparticles had a high charge. Figure 5 represents this phenomena [7].

Figure 5. Representation of Nanoparticle Haloing (from 7).

We are hypothesizing that the FCC particles we have studied behave in an analogous way, with the zeolite crystallites replacing the silica spheres and the zirconia sol being replaced by either Super-D or alumina sol. This stable collodial suspension so formed by nanoparticle haloing has an optimized mixing of the sol and zeolite crystals. On drying and calcination, a porous but strong network forms to hold the agglomerates together. This approach has allowed formation of strong catalysts, even when using the Al chlorohydrol sol. The latter sol was not known to provide stable agglomerates.

4. Conclusion Zeta potential measurements have been used to successfully develop optimized metal dispersion on support oxides. They have also provided a useful tool for the design of attrition resistant FCC catalyst particles.

Acknowledgments The authors wish to thank Joe Baumgartner for help with some of these measurements.

References 1. 2. 3. 4. 5. 6. 7.

J. P. Brunelle, 1978, Pure Appl. Chem., 50, 1211. J. R. Regalbuto, 2009, in “Synthesis of Solid Catalysts”, (ed. K.P. de Jong) Wiley-VCH, p. 33. G. A. Parks and P.L. De Brnyn, 1962, J. Phys. Chem., 66, 967. S. Soled, G. B. McVicker, W.E. Gates and S. Miseo, 1995, US Pat. 5,457,253. S. L. Soled, G. McVicker, S. Miseo, W. Gates, and J. Baumgartner, 1996, Stud. Surf. Sci. Catal., 101A, 563-572 J.A. Lewis, 2001, Langmuir, 17, 8414. M. Jacoby, Jan. 7, 2002, Chemical and Engineering News, p. 11.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The superior activity of the CoMo hydrotreating catalysts, prepared using citric acid: what’s the reason? A.V. Pashigreva, O.V. Klimov, G.A. Bukhtiyarova, M.A. Fedotov, D.I. Kochubey, Yu.A. Chesalov, V.I. Zaikovskii, I.P. Prosvirin, A.S. Noskov Boreskov Institute of Catalysis SB RAS, Novosibirsk,630090, Russia

Abstract It was demonstrated, that the main positive role of citric acid during the hydrotreating catalysts preparation is consist in the formation of bimetallic complex Co2[Mo4(C6H5O7)2O11]•nH2O, that is a good precursor for selective formation of catalyst active phase, so called Co-Mo-S phase type II. The preparation method for this bimetallic complex using different precursor is described. The catalysts prepared by the complex deposition onto alumina support were studied during the different stages of the catalyst genesis. Applicability of these catalysts for ultra low sulfur diesel production was shown. Keywords: hydrotreating catalysts, citric acid, bimetallic CoMo complex

1. Introduction The impregnated solutions containing cobalt and molybdenum compounds and citric acid (CA) is widely used for high active hydrotreating catalysts preparation, including industrial catalysts. An obvious positive effect of citric acid on the activity of catalysts is explained in variety of ways by different authors. Thus, Fujikawa et al. [1] prescribes the superior catalyst activity to the formation of cobalt citrate complexes, which are thermally stable at temperatures up to 200ºC and contributes the selective formation of the catalyst active component, so called Co-Mo-S phase of type II, described in [2]. Bergwerff et al. [3] reported the formation in a solution of molybdenum citrate complexes. Deposition of these Mo complexes onto Al2O3 support provides uniform molybdenum distribution along the carrier granule and ensures a high dispersion of MoS2 particles, that increases the relative activity of catalyst. In the cited and other papers of mentioned authors, the complex formation between CA and the individual components of the impregnating solution - cobalt or molybdenum was detected, while the formation of bimetallic Co-Mo-CA complexes was not revealed. Earlier Van Veen [4] reported that the positive role of chelating organic ligands consists in the formation of CoMoL compounds in the impregnating solution, whose structure remains unchanged after the drying of the catalyst; moreover the ligands screen metals from the interaction with the carrier. During the sulfidation stage of the CoMoL containing catalysts the Co-Mo-S phase type II is mainly formed, whereas in the catalysts prepared without using of chelating agents the low active Co-Mo-S phase type I is formed. Concerning our experimental results, the reasonable Van Veen’s conclusions should be supplemented by the followed statements: 1. Chelating ligands promotes the coordination of Co towards molybdenum containing anion forming a bimetallic compound with a close proximity of

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2.

cobalt and molybdenum, that further leads to the selective formation of bimetallic Co-Mo-S phase. Bimetallic complexes stabilized by chelating ligands, have relatively large size, that does not allow it to penetrate into the narrow pores of the carrier not available for heteroatomic organic molecules of the hydrotreated feedstock.

In the current paper it was demonstrated, that the use of labile bimetallic Co-Mo complexes with a close proximity between Co and Mo in combination with vacuum impregnation of Al2O3, low-temperature drying (3 Å contains Mo-Mo distance (3.36 Å) and Mo-Co one (3.41Å).

Raman intensity, arb. units

940

210 344

900

380

863

H2MoO4 + CoAcet MoO3 + CoAcet AHM + CoCarb MoO3 + CoCarb CoMo4CA

200

400

600

Raman shift, cm

800

1000

-1

Fig. 1 Laser Raman spectra of (CoMo 4CA) complex derived from the different precursors.

FT amplitude

The superior activity of CoMo hydrotreating catalysts

113

H2MoO4 + CoAcet MoO3 + CoAcet PMA + CoCarb MoO3 + CoCarb CoMo4CA 0

2

4

6

R-δ, A Fig: 2. Fourier transform of molybdenum K-edge EXAFS spectra for the (CoMo4CA) complex derived from the different precursors.

Thus, it can be concluded that bimetallic complex (CoMo4CA) with structure described in [6,13] can be obtained using any of the above specified compounds of Mo and Co in aqueous solutions containing components with the molar ratio of Mo/CA/ Co=2/1.2/1 and pH laying in the range of 2.0-3.5. The catalyst prepared by vacuum impregnation of Al2O3 with (CoMo4CA) solution and dried at 120 and 400ºC was studied by XPS. The Mo3d spectrum of Co-MoCA/ Al2O3-120 sample (Table 2) contains a peak of Mo3d5/2 with a binding energy of 232.3 eV. Although, this value corresponds to Mo6+ it differs significantly from the value of 232.6-232.9 eV characteristic of calcined Co-Mo catalysts [14]. The obtained values of the binding energy Mo3d5/2 agree well with 232.2 eV, corresponding to molybdenum complexes with polybasic organic acids in the nonsulfided hydrotreating catalysts [15]. In the C1s spectrum of the Co-MoCA/Al2O3-120 sample the peak of Eb = 284.8 eV corresponding to the surface hydrocarbon and the peak of 288.7 eV from carboxyl groups of CA was elucidated. The second peak disappears after drying of the catalyst at 400ºC, apparently, due to removal of citrate ligands. In the spectrum of Co 2p the increase of binding energy on 0,2 eV with increasing of drying temperature from 120 to 400ºC can be considered as experimental error or may be a consequence of removal of the citric ligands bonded with cobalt. Table 2. Binding energies (eV) measured by XPS. Sample

Mo 3d

Co 2p

C 1s

Al 2p

O1s

S2p

Co-MoCA/Al2O3-120

232.3

781.8

284.8; 288.7

74.7

531.6

—

Co-MoCA/Al2O3-400

232.8

782.0

284.8

74.7

531.6

—

Co-MoCA-S/Al2O3

228.6

778.8

284.8

74.7

531.6

161.8

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According to FTIR, Raman, XAS-spectroscopy data obtained for catalyst surface species the coordination of Co2+ cations towards the molybdenum containing anion through the carboxyl groups of citrate ligands and terminal oxygen atoms bonded with molybdenum, as well as the distance Co-Mo = 3.41 Å typical for (CoMo4CA) [6,13,16] remain unaltered. After the drying at temperatures up to 220ºC the citrate ligands (C6H5O7)2– turn into itaconate (C5H4O4)2–, but at the same time, although the distance Co-Mo is increased on 0,1 Å, the coordination of cobalt towards molybdenum anion is saved [16]. The treatment of the catalysts at temperature higher than 220ºC results in decomposition of the supported complex with the formation of undesirable surface compounds. Thus, the citrate ligands composing of (CoMo4CA) complex provide the stability of its structure in the solution and on the alumina surface and allow preservation of the coordination of cobalt to the molybdenum containing anion and close arrangement of Co and Mo atoms, in the case that catalysts were dried at temperature up to 220ºC. The detailed study of sulfidation behavior of the Co-MoCA/Al2O3 catalysts during the liquid phase sulfidation using straight-run gas oil spiked with dimethyldisulfide (DMDS) showed that the complete or partial conservation of initial structure of the complexes on the support surface after thermal treatment favor the formation of an active sulfide CoMo catalysts [8]. It was stated that the conservation of the complexes results in delaying the interaction of sulfiding agent with the surface Co-Mo compound due to stabilization by carboxylated ligands (citrate or itaconic). The appreciable sulfidation of the catalysts at the stage of low-temperature activation-sulfidation at 230°C began after the carboxylate ligands of the initial complex start decomposing to generate surface Co and Mo compounds which are capable of interacting with the sulfiding agent. As a result the longer period of time is needed to saturate the CoMo compound by sulfur during the sulfidation at 230-240°C. In this case the sulfidation of catalysts proceeds simultaneously with the decomposition of carboxilated ligands providing favorable conditions for the formation of the active Co-Mo-S phase type II, while Mo and Co atoms are situated in close proximity during sulfidation. According to XPS data for Co-MoCA-S/Al2O3 sample the sulfide components are at the atomic ratio S/Mo = 2.0, that indicates that prepared catalyst is fully sulfided. The XPS spectra (Table 2) contains narrow and intense peaks corresponded to CoMoS phase (778.8 eV Co2p3/2 and 228.6 eV Mo3d5/2) and do not contain any significant peaks that could be assigned to Co9S8 (778.1 eV) or to oxygen-containing compounds of Co2+ (781.7 ± 0.3 eV), as well as the presence of Mo5+ compounds (230.0 ± 0.1 eV) and Mo 6 + (232.5 ± 0.3 eV) is fully excluded [14,17]. Processing of the Mo and Co K-edge EXAFS data allows peaks related to distances Mo-S = 2.40 Å (c.n.=5.0), Mo-Mo = 2.60 Å (c.n.= 3.1), Co-S = 2.22 Å (c.n.= 4.0) and Co-Mo = 2.78 Å (c.n.= 0.8) to be identified in the curves. According to HRTEM, the prepared catalyst characterized with 11% Mo loading contains MoS2 particles with an average size of 31 Å, the mean stacking number of 1.72, and there are ca. 50 layers of MoS2 per 1000 nm2 of the catalyst surface. The set of data obtained with the Co-MoCA-S/Al2O3 catalysts lead us to conclude that the most all cobalt and molybdenum atoms here are constituents of Co-Mo-S phase of type II. The absence of appreciable quantities of oxygen-containing compounds in the catalyst sulfided at temperature not exceededing 340ºC allows the conclusion that the citrate ligands favor the avoidance of the strong interaction between the supported metals and carrier surface and the formation of compounds included Mo-O-Al fragments that can be fully sulfided only at the temperatures higher than 600ºC [2].

The superior activity of CoMo hydrotreating catalysts

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Table 3. The results of SRGO hydrotreating activity of the Co-MoCA/Al2O3 catalysts.

Testing conditions: 3.5 MPa, liquid hourly space velocity of 1.0 h–1, H2/feed volume ratio of 300. Initial compounds for the synthesis of (CoMo4CA) complex

Temperature for 10 ppm S fuel production (оC)

(AHM), CA, (CoAcet)

351

(AHM), CA, (CoNitr)

353

MoO3, CA, (CoCarb)

348

MoO3, CA, (CoAcet)

350

The metals content are equal 11.0±0.2% Mo and 3.5±0.2% Сo based on sample calcined at 550оС. The Co-MoCA-S/Al2O3 catalysts obtained via bimetallic (CoMo4CA) complex that can be prepared from different initial compounds, after the drying at 120ºC and sulfidation with DMDS have shown the comparable activity in the hydrotreatment of diesel fuel (Table 3). All the prepared catalysts allow ultra low sulfur diesel fuel to be obtained.

4. Conclusions Thus, it is proved that much higher activity of the catalysts prepared using citric acid is due to: 1. CA provides the formation of stable tetrameric anion [Mo4O11(C6H5O7)2]4– in the studied intervals of the concentrations and pH. 2. CA enables the bimetallic CoMo complex to be obtained through the coordination of Co2+ cations towards Mo-containing anions. The CoMo-complex could be described by Co2[Mo4O11(C6H5O7)2]×xH2O formula. The existence of this complex within the impregnated solution ensures the formation of the oxide precursors with close proximity between Co and Mo during its deposition at an alumina surface and provides the formation of highly active disperse sulfide particles during the sulfiding step. So, CA assures the stabilisation of the vicinal arrangement of Co and Mo during the genesis of catalysts. 3. Citric ligands screen the metals from the strong interaction with support. 4. CA provides simultaneous sulfidation of Co and Mo favoring to the selective formation of the hydrotreating catalysts active sites.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

T. Fujikawa, M. Kato, T. Ebihara et al., J. Jpn. Petrol. Inst., 48(2) (2005) 114. H. Topsoe, Applied Catalysis A: General 322 (2007) 3. J.A. Bergwerff, M. Jansen, R.G. Leliveld et al., J. Catal. 243 (2006) 292. J.A.R. Van Veen, E. Gerkema, A.M. Van der Kraan, A. Knoestera, J.Chem.Soc., Chem. Commun., (1987) 1684. O.V. Klimov, A.V. Pashigreva, G.A. Bukhtiyarova et al., Catal. Today (2009), doi:10.1016/ j.cattod.2009.07.095 O.V. Klimov, A.V. Pashigreva, M.A. Fedotov et al., J.Mol.Catal. A, 2010, in press. A.V. Pashigreva, O.V. Klimov, G.A. Bukhtiyarova et al., Catal. Today (2009), doi:10.1016/ j.cattod.2009.07.096 A.V. Pashigreva, G.A. Bukhtiyarova, O.V. Klimov et al., Catal. Today 149 (2010) 19. D.I. Kochubey, EXAFS-Spectroscopy of the Catalysts, Science, Novosibirsk, 1992, p. 144.

116 10. 11. 12. 13. 14. 15. 16. 17.

A. Pashigreva et al. K.V. Klementev, J. Phys. D: Appl. Phys. 34 (2001) 209. J.J. Rehr, A.L. Ankudinov, Radiat. Phys. Chem. 70 (2004) 453. N.W. Alcock, M. Dudek, R. Grybos et al., J.Chem.Soc.,Dalton Trans. (1990) 707. O.V. Klimov, A.V. Pashigreva, D.I. Kochubey et al., Doklady Physical Chemistry, 424 (2009) 35. Y. Okamoto, T. Imanaka, S. Teranishi, J.Catal. 65 (1980) 448. L. Coulier, V.H.J. De Beer, J.A.R. Van Veen, J.W. Niemantsverdriet, J. Catal. 197 (2001) 26. G.A. Bukhtiyarova, O.V. Klimov, D.I. Kochubey et al., NIMA, 603 (2009) 119. A.D. Gandubert, E. Krebs, C. Legens et al., Catal. Today. 130 (2008) 149.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Elucidation of the surface configuration of the Co(II) and Ni(II) aqua complexes and of the Cr(VI), Mo(VI) and W(VI) monomer and polymer oxo–species deposited on the titania surface during impregnation George D. Panagiotou,a Theano Petsi,a John Stavropoulos,a Christos S. Garoufalis,b Kyriakos Bourikas,c Christos Kordulis,a,d Alexis Lycourghiotis*a a

Department of Chemistry, University of Patras, GR–265 00 Patras, Greece Department of Physics, University of Patras, GR–265 00 Patras, Greece c School of Science and Technology, Hellenic Open University, 18 Parodos Aristotelous St., GR–26335, Patras, Greece d Institute of Chemical Engineering and High Temperature Chemical Processes, FORTH/ICE–HT, P.O. Box 1414, GR–265 00 Patras, Greece *corresponding author (e–mail: [email protected]) b

Abstract The structure of the precursor Co(II) and N(II) aqua complexes and the Cr(VI), Mo(VI) and W(VI) monomer and polymer oxo–species formed upon impregnation at the interface developed between the surface of the titania grains and the impregnating solution was thoroughly elucidated. Moreover, the interfacial speciation was determined for various surface concentrations of the precursor species regulated by adjusting the corresponding solution concentrations and the pH of the impregnating solution. Keywords: titania, catalysts preparation, interfacial chemistry, adsorption

1. Introduction In the preparation of supported catalysts we disperse “bi–dimensional” species or nano– particles of a catalytically active metal, oxide or sulphide on the surface of a rather limited number of supports with high specific surface area. Among these supports TiO2 is important. The impregnation step is critical for controlling the physicochemical characteristics of the precursor species and thus the characteristics and the catalytic behavior of the aforementioned “bi–dimensional” species or nano–particles. In order to obtain this control it is necessary to understand at molecular level the impregnation step [1,2]. This mainly concerns the “Equilibrium Deposition Filtration” and the “Homogeneous Deposition–Precipitation” techniques which favour deposition, upon the equilibration of the suspension, at the interface developed between the surface of the titania grains and the solution (interfacial deposition) and to a lesser extent the incipient wetness or wet impregnation techniques. In this communication we report on the structure and interfacial speciation of the precursor Co(II) and Ni(II) aqua complexes and the Cr(VI), Mo(VI) and W(VI) oxo–species formed at the interface developed between the surface of the TiO2 grains and the solution. The work is based on a recently development concise picture concerning the acid–base behavior of the TiO2 (hydr)oxo–groups, considered as the receptor sites for the deposition of the aforementioned species, and the structure of the “TiO2/electrolytic solution” interface as well [3].

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2. Experimental Several methodologies based on potentiometric titrations, microelectrophoresis and macroscopic adsorption measurements have been used in conjunction with Diffuse Reflectance, Raman and Electron Paramagnetic Resonance spectroscopy. Semi– empirical quantum mechanical calculations, stereochemical considerations and quantum mechanical calculations in the frame of the DFT are followed. The above are then used for developing a quantitative model for the interfacial deposition studied. Details concerning the combined application of these methodologies to obtain the interfacial structure and speciation of the aforementioned species have been reported elsewhere [4– 10]. The majority crystal terminations (1 0 1) and (1 0 0) of the anatase nanocrystals, comprised in the titania grains, were chosen to exemplify the interfacial structures. A titania rich in anatase (Degussa P25) has been used in all cases.

3. Results and discussion 3.1. Structure and interfacial speciation of the Cr(VI) oxo–species

A schematic representation of the local structures of the deposited species (CrO42–, HCrO4– and Cr2O72–) is illustrated in Figure 1 [8]. These species are electrostatically retained above the bridging hydroxyls forming ion pairs. Each CrO42– or HCrO4– ion is located above one bridging hydroxyl whereas each Cr2O72– ion is located above two bridging hydroxyls. The deposited species are located between plane 1 and plane 2 of the compact layer of the interface, with an equal distribution of their charge. Plane 2 Plane 1

Plane 0

Figure 1. Structures of the deposited CrO42–, HCrO4– and Cr2O72– ions on the anatase (1 0 1) crystal termination. Ti: gray; H: blue; O in TiO2 cluster: red; O in chromates: yellow; Cr: cyan. The dotted line indicates electrostatic bond between chromates and surface bridging hydroxyls.

At too low Cr(VI) surface concentrations (0–0.3 μmol Cr(VI) m–2) only the CrO42– and HCrO4– ions are deposited, with a preference of the titania surface for the first. At higher surface concentrations the Cr2O72– ions are, in addition, deposited. In the pH range 7.0–8.0 the electrostatic adsorption of the CrO42– ions on the bridging surface hydroxyls is the predominant deposition process. In contrast, at pH=4.5 the Cr(VI) deposition occurs mainly through the electrostatic adsorption of the HCrO4– ions on the bridging surface hydroxyls. In the intermediate pH range both electrostatic adsorptions contribute to the whole deposition process. The electrostatic adsorption of the Cr2O72– ions over two neighboring bridging surface hydroxyls becomes important at pH 450°C), from which are inherited particle morphology (topotactic transition [4]) and relevant surface properties. The control of boehmite synthesis is thus of paramount interest for catalytic properties enhancement. Given the importance of particle morphology as a key parameter in designing and controlling material properties, size and shape accurate characterization attracts a great deal of attention. In this goal, an approach using the analysis of transmission electron microscopy (TEM) images is developed to estimate the size of the boehmite nanoparticles. Complementary, for very small particles, a novel method is proposed for

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three–dimensional size and shape determination of nanoparticles based on calculations of X–Ray Diffraction (XRD) powder patterns with respect to nanoparticle crystal structure, size, and shape. Additionally, an automated method using XRD is developed to characterize alumina, allowing the detection of additional phases and the determination of the structural evolution of the gamma alumina phase. In term of boehmite synthesis, sol–gel routes, and more specifically precipitation in aqueous medium of metallic salts, are favored techniques for syntheses of inorganic particles with accurate size and shape control. Indeed, by adjusting a set of parameters, nanomaterials with finely tuned characteristics and properties can be obtained. We show that the addition of complexing species (polycarboxylates and polyols) in the reacting medium may play an important role in the shape and size of oxide nanoparticles.

2. Particles size and shape characterizations 2.1. Transmission electron microscopy TEM is a key technique to determine particles morphology. For boehmite, the direct image observation is often difficult because of high aggregation and overlapping between particles. To overcome such difficulties, an advanced TEM images analysis, efficient for high particles size and based on dilution model, is developed [5]. This method is illustrated on boehmite sample Disperal 40 (Sasol Germany GmbH). High resolution transmission electron micrographs (HR–TEM) are performed on a JEOL 2100F, at an acceleration voltage of 200.0 kV. The nanoparticles are ultrasonicated in ethanol and dispersed on carbon covered Cu–grids. Images are acquired with a resolution of 0.41nm/pixel. TEM images contain electronic noise and white diffraction artefacts localized on the edges of the boehmite nanoparticles. Image filters (median filter, bilateral filter [6] with Tukey’s biweight function and morphological opening by reconstruction [7]) are performed in order to improve without damage the image quality of the edge transitions and the grey level intensities corresponding to the nanoparticles (Figure 1). The observations are then modelized by means of a dilution model [5,8]. A dilution model is used to simulate situations with thick slices with mass accumulation over the thickness. A dilution random function (DRF) is constructed from primary function Z t' ( x ) and from a Poisson point process P with intensity μ n (dx ) ⊗ θ (dt ) . The DRF is given by : Z i (x ) =

∑ {Z it' (x − xk )} (1). This model is characterized by its centered

( t k , x k )∈P

k

covariance function: C (h) = θ g(h) (2), with θ and g(h) denoting respectively the induced intensity in two dimensions of the 3D Poisson point process (θ = θ 3 e, θ 3 being the 3D intensity and e the thickness of the slice), and the transitive covariogram of the primary grains ie in our case the boehmite nanoparticles. For a primary grain function Z t' (x ) , the

{

}

transitive covariogram gt(h) is given by: g t (h) = ∫ E Z 't (x − y )Z 't (x + h − y ) dy (3). Raw R

information on the nanoparticles can be obtained by analysis of the covariance function. The range of this curve corresponds to the average size of the boehmite nanoparticles: a size of 35 nm is also deduced.

Innovative characterizations and morphology control

129

Figure 1. From left to right: initial experimental image; filtered image; realization of dilution model with fitted parameters.

More precise results can be obtained with an original approach by numerical calculation of transitive covariogram of a 3D geometric model of the boehmite nanoparticles. A geometrical model based on “thick parallelogram” is parameterized with three parameters L, l and e (Figure 2). The angle value, measured on TEM images, is fixed to 104°, indicating that the sample particles expose (101) planes. The 3D orientation of the nanoparticules follows a uniform law of distribution. For a set of fixed parameters, the covariogram is obtained by this way : 3D simulation of a nanoparticle with the three parameters (L, l,e,), 3D rotation following a uniform law, additive projection on a plane, calculation of the logarithm image, and measure of the covariance. The covariogram is obtained by taking the average covariance curve for typically 500 realisations. The size of the boehmite nanoparticles is obtained by estimation of L, l and e, which give the best fit between the numerical covariogram and the experimental centered covariance according to equation (2). The weakest mean square error is obtained for L = 35.5 nm, l = 36.0 nm and e = 5.5 nm (Figure 2). The values of L and l are very close, indicating a diamond–shaped morphology. Figure 1 presents an achievement of a dilution model with these parameters, for visual comparison with experimental images.

Figure 2. Left : 3D geometrical model of boehmite nanoparticules. Right : experimental centered covariance and numerical transitive covariogram for L=35.5 nm, l=36.0 nm and e=5.5 nm. Range of covariance curve is reached for a size of 35 nm.

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2.2. Powder x–ray diffraction 2.2.1. Principles and processing When the boehmite particle size becomes smaller than 10 nm, the morphology determination from TEM images becomes more difficult. Indeed, the identification of the primary particles shape is ambiguous, even using advanced image analysis. On the contrary, for such particles size, the XRD powder analysis becomes very sensitive to morphological effects. An efficient way to extract quantitative information about morphology from the XRD pattern is the use of the Debye formula. The Debye formula is a general law, allowing for any atoms cluster, the full calculation of diffraction pattern (ie the calculation of the peaks position, intensity and shape without further assumption). The principle of the developed method is presented elsewhere in detail [9]: it is assumed that the boehmite particles expose four different faces, namely the (100), (101), (001) and (101). These crystallographic orientations correspond to the four low surface energies planes [10]. As a consequence, a boehmite particle can be fully described by four distances, for instance the distances from the surface to the center of mass of the particle. Starting from the boehmite unit cell, a set of particles is generated by varying the particle edges lengths. For each particle, the simulated XRD pattern is calculated applying the Debye formula and stored. A database of morphology–dependent XRD patterns is also obtained. Once the sample experimental pattern is recorded, the particles size and shape are determined as follows: the experimental pattern is compared to all the simulated patterns of the database, by calculating the weighted profile R–factor, Rwp (as usually done for Rietveld refinements, for instance). The Rwp value is calculated between 21 and 80° (2θ) and three parameters are optimized to minimize the Rwp value: the scaling factor between the experimental and simulated patterns and two other parameters for the pattern background (linear). The particle leading to the lowest Rwp value corresponds to the most representative particle morphology. Important work is done to automate the method and improve its accuracy. An exhaustive database is generated in order to cover all the possible morphologies: first the particles volume is fixed to 4 nm3 (below this volume, the particles do not exhibit exploitable patterns). All the different shapes are generated by varying the percentage of the exposed surfaces, with an incremental step of 5% (the sum of the surface percentages remaining equal to 100%). This leads to 1326 shape configurations. Next, for each of these configurations, the particle is increased in volume by multiplying the initial volume by a √2 factor, in an iterative way. The iterations are stopped when the volume reaches about 1024 nm3: beyond this value, the XRD pattern becomes less sensitive to particles morphology, compared to the electronic microscopy methods. A sum of 22542 configurations is also generated and covers all the possible particles size and shape. With such a database, the determination of the optimized morphology takes less than five minutes, allowing a fast screening of boehmite samples. 2.2.2. Application examples The method is applied to three commercial samples, namely Disperal, Pural SB3 and Disperal P2 (Sasol Germany GmbH) and two home–made samples (Nano–A and Nano– B), which exhibit very small particles (see section 3 for the synthesis concept). Comparisons between the experimental and simulated patterns are given for the Pural SB3 and Nano–B samples on Figure 3. Whatever the sample, a rather good agreement is observed, except for the (020) and the (200) peaks. The (020) peak position is well simulated, but not its intensity. This is due to two main effects: first, the hydrogen

131

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atoms are not taken into account in the simulation and they significantly modify the intensity of this low–angle peak. Next, the linear model of the background is not satisfactory in this region (2θ < 21°). The high experimental intensity of the (200) peak is due to oriented aggregation effects that are not taken into this single particle model. a) (020)

(150) (002)

(021) (130)

(200) (151)

b)

2θ (°)

Figure 3. Experimental (gray line) and optimized simulated (black line) powder diffraction patterns of two boehmite samples : a) Pural SB3 and b) Nano–B.

Table 1 summarizes the size and shape parameters obtained for the five samples. The method allows an accurate determination of the nanoparticles sizes, ranking from 5.6 for the Disperal sample to 2.5 nm for the Nano–B sample. The particles also exhibit different shapes, arising from different synthesis conditions. The (100) surface appears as difficult to expose (A(100) = 0–5%), whereas the ratio between the (010) and the (101) surface area can be finely adjusted. Several samples exhibit a significant portion of exposed (001) surface (A(001)=20% for Nano–B sample). Table 1. Size and shape of boehmite nanoparticles determined by the XRD method. Rwp (%)

Area (nm2)

A(100) (%)

A(010)

A(001)

A(101)

Sample

(%)

(%)

(%)

Diameter (nm)*

Disperal

21.2

296

0

30

10

60

5.6

Pural SB3

15.2

133

5

55

0

40

3.5

Disperal P2

13.4

98

5

50

5

40

3.1

Nano–A

14.1

96

5

35

0

60

3.1

Nano–B

13.6

58

0

30

20

50

2.5

* mean diameter of an hypothetical sphere exhibiting the same volume as the sample particles

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Some comments should be done about the Rwp values. They stand between 13 and 21%, which is remarkable, taking into account that only seven parameters are optimized (4 for the particle morphology and 3 for the Rwp calculation). The XRD patterns variability mainly arises from the differences between particles morphology. The model can be improved by adding other degrees of freedom. Preliminary results show that the introduction of a particle size distribution, instead of a single particle model, significantly decreases the Rwp values.

3. Synthesis of boehmite particles with tunable size and shape Boehmite nanoparticles with finely tuned morphologies can be obtained via soft chemistry routes, through precipitation of aluminum precursor in an aqueous solution. Previous studies have shown how the pH value of the reaction medium and the temperature may influence boehmite particle size and shape, whose variations were shown to be related to surface charges values (directly depending on pH) and thus to surface energies [11]. Complexing species such as polycarboxylates, polyols, sulfates, phosphates may be also used to tune particle properties. Indeed, adsorption of such species also modify oxide–solution interfaces, and thus affect the crystal growth and the particle morphology. A recent work focusing on boehmite syntheses performed in the presence of xylitol exclusively [12] shows how xylitol may influence particle size and shape. These effects are related to xylitol–surface interactions studied through adsorption isotherms and DFT calculations of surface energies. More generally, new boehmite morphologies and textures can be obtained through the use of low amounts of polyols [12,13] or polycarboxylates used as complexing agents during material synthesis. In comparison to usual particle morphologies, the important observed changes are strongly related to the complexing strength of the additive used. In the case of polyols, significant particle size decreases may occur, depending on polyol nature, and particularly on the carbon chain length and the number of OH groups. Some unusual morphologies obtained are presented here. Boehmite nanoparticles are synthesized in aqueous medium through precipitation of aluminum nitrate Al(NO3)3 in presence of polyols or polycarboxylates at pH=11.5. The pH is adjusted by addition of sodium hydroxide, NaOH and the resulting suspensions are then aged in a stove at 95°C for one week. Reference syntheses are also achieved without complexing species, according to the procedures described in [11]. Final pH values of the suspensions are equal to 11.5 in every case. Final aluminum and complexing species concentrations are respectively 0.07 mol.L–1 and 0.007 mol.L–1. Figure 4 shows TEM micrographs obtained for particles synthesized without complexing agent, in the presence of xylitol and of tartrate ions. The presence of xylitol in the synthesis medium strongly affects particle size and specific surface area (from 180 m2.g–1 without polyol to 270 m2.g–1 in the presence of xylitol). However particle morphology determination is complex starting from only TEM observations only. However, characterizations performed from XRD simulation evidenced a diamond– shaped morphology, similar to the one obtained without polyol but with higher (101) surfaces ratio increased to 63% of the total surface (vs. 47% for particle synthesized without polyols). Such a phenomenon is also observed using tartrate ions as complexing species. TEM picture shows diamond–shaped particles with intermediate size between those obtained without additive and in the presence of xylitol. Particles exhibit a 104° typical angle between lateral surfaces suggesting lateral faces to be (101) planes. Particle

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133

dimensions obtained from TEM and XRD show that (101) surface represent 64% of the total particle surface. Since γ–alumina is obtained from boehmite by a topotactic transformation [4], properties of the resulting materials are inherited from the boehmite precursor. Therefore, these methods will provide a promising way to control surface properties of γ–alumina.

Figure 4. TEM micrographs of boehmite nanoparticles synthesized (a) in standard conditions at pH=11.5 (without complexing agents), (b) in presence of xylitol, and (c) in presence of tartrate ions.

4. Monitoring of γ–alumina formation Regarding the phase of industrial interest, a calcination step is required to obtain γ– alumina from boehmite. The recovery of γ–alumina with respect to calcination conditions is often qualitatively checked by TGA and XRD measurements. Nevertheless, the XRD patterns of γ–alumina samples are often complex and unambiguous to interpret, because of the possible presence of several polymorphs and the fine evolution of diffraction peaks. In the same spirit as the boehmite case, an automated method is developed and validated, allowing the detection of additional phases and the determination of the structural evolution of γ–alumina phase in the analyzed sample. This method is based on the decomposition of the diffraction peaks of the DRX pattern and the automatic attribution of these peaks to a determined alumina phase. This attribution begins with the detection of the 3 characteristic peaks of α–alumina. For each signal, the position, the middle–height width and the intensity are analyzed. After these analyses (or if no α– alumina phase presence is detected), the characteristic peaks of the θ–alumina phase are then considered. Following the same methodology, the quantity of θ–alumina is characterized. Then the γ–alumina and δ–alumina phase amounts are determined. This method allows fast and systematic analysis of the presence of all alumina crystallographic phases. The quantification of their contribution to the XRD pattern is properly analyzed in an unequivocal and reproducible way (Figure 5).

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Figure 5. Principle of the automatic analyses of alumina XRD pattern.

5. Conclusions and perspectives A full range of characterization techniques has been developed to reveal the morphology of boehmite particles and to follow the formation of γ–alumina during calcination. These tools allow deep analysis of novel preparations of boehmite and γ–alumina: they offer a promising way for a better design of γ–alumina catalyst supports.

References [1] Schüth, F., Sing, K. and Weitkamp, J. (eds.), Handbook of Porous Solids, Wiley–VCH Verlag GmbH, Weinheim, Germany, 2002, vol. 3, pp. 1591. [2] Ertl, G., Knözinger H. and Weitkamp, J. (eds.), Handbook of Heterogenous Catalysis, VCH Verlag Gesellchaft, Weinheim, Germany, 1997, pp. 1802. [3] M. Digne, P. Sautet, P. Raybaud, P. Euzen and H. Toulhoat. J. Catal., 226 (2004) 54. [4] B. C. Lippens and J. H. de Boer, Acta Crystallogr., 17 (1964) 1312. [5] M. Moreaud, R. Revel, D. Jeulin and V. Morard, Image Anal. Stereol., 28 (2009) 187. [6] C. Tomasi, R. Manduchi, Proceddings IEEE , Bombay, India, (1998) 839. [7] J. Serra, Image analysis and mathematical morphology, Academic Press, London, United Kingdom, 1982. [8] D. Jeulin, Sci. Terre, 30 (1991) 225. [9] D. Chiche, M. Digne, R. Revel, C. Chanéac and J.–P. Jolivet, J. Phys. Chem. C, 112 (2008) 8524. [10] P. Raybaud, M. Digne, R. Iftimie, W. Wellens, P. Euzen and H. Toulhoat, J. Catal., 201 (2001) 236. [11] J.–P. Jolivet, C. Froidefond, A. Pottier, C. Chanéac, S. Cassaignon, E. Tronc and P. Euzen, J. Mater. Chem., 14 (2004) 3281. [12] D. Chiche, C. Chizallet, O. Durupthy, C. Chanéac, R. Revel, P. Raybaud and J.–P. Jolivet, Phy. Chem. Chem. Phys., 11 (2009) 11310. [13] D. Chiche, C. Chanéac, R. Revel and J.–P. Jolivet, Stud. Surf. Sci. Catal., 162 (2006) 393.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Highly active and selective precious metal catalysts by use of the reduction-deposition method Peter T. Witte,a Mariëtte de Groen,a Ralph M. de Rooij,a Pablo Bakermans,a Hans G. Donkervoort,a Peter H. Berben,a John W. Geus.b a b

BASF Nederland B.V., Strijkviertel 67, 3454 ZG De Meern, the Netherlands Utrecht University, Padaulaan 8, 3584 CH Utrecht, the Netherlands

Abstract New mono- and bimetallic precious metal catalysts are prepared by reduction-deposition, in a way that is suitable for large scale production. The preparation is done in water, without the use of any organic solvents and makes use of commercially available starting materials. The supported Pd catalysts are highly active lead-free alternatives for the well-known Lindlar catalyst in the semi-hydrogenation of substituted acetylenes. Impurities in the substrate cause deactivation after recycling, but this does not affect the catalysts selectivity. Keywords: heterogeneous catalysis, nanocatalyst, palladium, semi-hydrogenation.

1. Introduction Catalysts based on colloidal suspensions attracted much attention in recent years, both as supported[1] and as quasi-homogeneous[2] catalysts. These catalysts are prepared by the so-called reduction-deposition method, where a metal is first reduced in solution in the presence of a stabiliser before it is deposited on a heterogeneous support. By using the appropriate reaction conditions, metal crystallites of < 10 nm are available.[3] These catalysts are called nanocatalysts, although the metal crystallite size is not much different from commercial catalysts prepared by traditional methods. However, the metal crystallite size distribution of the nanocatalysts is narrower and they do not contain large metal crystallites that are sometimes observed in traditional catalysts (see right-hand side of Figure 1).

Figure 1. TEM image of 5%Pd on C prepared by standard methods.

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Large metal crystallites as observed in Figure 1 will only contribute marginally to the catalyst activity, while they contain a significant fraction of the total amount of metal. The catalyst activity can thus be boosted by preparing catalysts that do not contain these larger metal crystallites. For industrial applications the use of nanocatalysts prepared by reduction-deposition is hampered by their cumbersome preparation. In general low metal concentrations are employed,[4a] but also the use of low-boiling organic solvents,[4b] high temperatures,[4c] very fast addition of reagents,[4d] or the use of reagents that are expensive or not commercially available[4e] makes their production on an industrial scale difficult. Bönnemann et al. used ammonium borohydride salts, such as [NBu4][BEt3H], as combined reducing and stabilising agents.[4e] According to the authors[5] they can use much higher metal concentrations, because the reducing and stabilising functionalities are combined into one reagent. So when a metal is reduced by the hydride, there is always a quaternary ammonium moiety nearby to immediately stabilise the formed metal crystallite. However, these ammonium borohydrides are very sensitive towards water and can only be handled in a specialised organometallic lab.[6] Therefore their use in industrial applications is limited. In our work, we use the commercial hexadecyl(2hydroxyethyl)dimethyl ammonium dihydrogenphosphate (HHDMA, Figure 2) as a water-soluble stabiliser/reductant for the preparation of metal nanoparticles. We report here the preparation of precious metal catalysts by reduction-deposition, which meets all requirements for production on large scale. New Pd catalysts were tested in the semi-hydrogenation of substituted acetylenes. HO H2PO4 N

Figure 2. Hexadecyl(2-hydroxyethyl)dimethyl ammonium dihydrogenphosphate.

2. Results and discussion 2.1. Preparation of colloidal suspensions of Pd, Pt and bimetallic Pd-Pt Upon mixing HHDMA and Na2PdCl4 a colour change from yellow to red is observed. When a mixture of HHDMA and Na2PdCl4 is kept at room temperature, orange crystals form over a period of hours. A crystal structure determination[7] shows the hydrogen bonding of two units of the cationic moiety of HHDMA to a PdCl4 anion (Figure 3, Table 1). The alcohol functionality acts as a hydrogen bond donor, the metal-bound chlorine as acceptor. The Pd is located on an inversion center and the PdCl4 moiety is therefore exactly planar.

Figure 3. Molecular structure of (HHDMA)2PdCl4 in the crystal.

Highly active and selective PM catalysts by use of the reduction-deposition method 137 Table 1. Selected bond distances and angles in (HHDMA)2PdCl4. Bond (Ǻ) Pd-Cl(1) 2.3069(4) Pd-Cl(2) 2.3017(4)

Angle (°) Cl(1)-Pd-Cl(2) 90.13(2) Cl(1)-Pd-Cl(1a) 89.87(2)

Hydrogen bond distance (Ǻ) Cl(1)···H(1o) 2.47(3) Cl(1)···O(1) 3.1988(16)

At higher temperatures HHDMA is able to reduce Na2PdCl4, which is indicated by a colour change from red to dark brown. Figure 4 shows a TEM image of a Pd colloidal suspension (from hereon referred to as c-Pd) formed in the presence of 10 eq. HHDMA. The metal crystallite size of the formed nanoparticles is 4-8 nm. When c-Pd is formed in the presence of 2 eq. HHDMA, TEM analysis shows that large agglomerates of Pd(0) particles are formed. STEM (Scanning Transmission Electron Microscopy) measurements using a HAADF (High Angle Annular Dark Field) detector show that the agglomerates consist of Pd crystallites of the same size as those formed in the reaction with 10 eq. HHDMA. Apparently, 2 eq. HHDMA is sufficient for full reduction of Pd(II) to Pd(0), but not for stabilisation of the formed Pd nanoparticles. Attempts to determine the oxidation product of HHDMA in the reaction mixture by 13C NMR were unsuccessful, because of the low concentration of the product.

Figure 4. TEM image of c-Pd formed with 10 eq. HHDMA.

The metal concentration used for the preparation of c-Pd (0.75 g/L Pd in H2O) approaches the concentration used by Bönnemann et al. (3.5 g/L Pd in THF).[8] The concentration is more than 10 times higher than in the alcohol reduction method described by Toshima et al. (72 mg/L Pd in MeOH/H2O)[4b] and the citrate reduction method described by Turkevich et al. (33 mg/L Pd in H2O).[4a] These last two methods use a stabiliser that is not chemically bound to the reductor. Turkevich states that doubling the metal concentration in his preparation leads to a significant increase of the Pd crystallite size. Using the same metal concentrations as for Pd and only a higher reduction temperature, we were able to prepare colloidal suspensions of Pt(0), and by mixing the Pd and Pt starting materials bimetallic colloidal particles could be prepared (metal crystallite size in c-Pt ~2 nm; c-PdPt 4-8 nm). Agglomeration of metal crystallites is prevented by the steric bulk of the C16 alkyl chain of the organic moiety of HHDMA.[1,2,3] This is best illustrated by using choline (Me3N+CH2CH2OH) to reduce Na2PdCl4. Although the normal colour changes are observed upon addition and heating to 80°C (yellow to red to dark brown), a black precipitate is formed within minutes at 80°C. The much smaller organic moiety of choline does not have the ability to stabilise the Pd crystallites at this temperature, so agglomeration takes place resulting in the precipitation of Pd-black. The traditional way of describing a metal particle in a colloidal suspension, in which the polar ammonium functionality is directed towards the metal centre while the apolar alkyl chain is sticking out, does not explain why the metal particles formed with

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HHDMA are highly soluble in water and cannot be extracted into an organic solvent. It is more likely that these metal particles are stabilised by a double layer of HHDMA, where the outer shell of the colloidal particle exists of ammonium functionalities of the second HHDMA layer. An inverted monolayer, in which the polar functionalities stick out into the aqueous solution while the apolar alkyl chains interact with the metal surface, seems unlikely to us. Although a fully reduced metal surface in itself is not ionic, polarisation by ionic species is possible. In organic solvents, XANES studies on colloidal suspensions indicate the interaction of the reduced metal surface with the ionic moiety of alkyl ammonium stabilisers.[9]

2.2. Deposition of metal nanoparticles on a heterogeneous support Mixing c-Pd, c-Pt or c-PdPt with a heterogeneous support, like activated carbon (C) or titanium silicate (TiS), yields the supported metal particles (Figure 5).

Figure 5. TEM images: top left) c-Pd/TiS; top right) c-PdPt/TiS; bottom) c-Pt/TiS.

TEM imaging shows that at high metal loadings (~1%) the metal crystallites cluster to form large agglomerates. In these agglomerates not all colloidal particles are directly bound to the support and unsupported particles are often observed, probably formed during the ultrasonic pre-treatment of the TEM samples. This is never observed in TEM pictures of catalysts of low metal loadings, in which all colloidal particles are directly bound to the support. Apparently the colloid-support interaction is stronger than the colloid-colloid interaction. When free HHDMA is added to the support before metal deposition, the maximum metal loading is drastically decreased. This shows that HHDMA competes with c-Pd for active sites on the support, so it is likely that both bind through the same mechanism.

Highly active and selective PM catalysts by use of the reduction-deposition method 139 The bimetallic c-PdPt/TiS was analysed by STEM-EDX (Energy Dispersive X-ray analysis). The very high resolution of this analysis is unique (an electron beam with a diameter of 0.8 nm is used) and allows for the elemental analysis of individual metal crystallites. A scan over the surface of the material showed that all peaks of Pt and Pd coincide, so the metal crystallites are truly bimetallic (Figure 6). No monometallic metal crystallites, indicated by a peak of only Pt or only Pd, were observed. The IR spectrum of TiS support shows peaks at 3700 cm–1 from Ti- and Si-OH groups and at 1640 cm–1 from absorbed water. The water peak is also observed in the spectrum of c-Pd/TiS, but here the peak at 3700 cm–1 is not found. The Ti- and Si-OH groups are clearly involved in the bonding of the Pd colloids, probably by exchange with phosphate anions. IR spectroscopy shows that water is removed after heating c-Pd/TiS to 350°C (HHDMA remains bound to the catalyst), while heating a TiS sample to 250°C is sufficient to remove all water. This indicates that water is more strongly bound to c-Pd/TiS than to TiS, probably because of interaction with the hydrophilic HHDMA. This is consistent with TGA measurements that show that all water is removed from TiS at 250°C, while c-Pd/TiS needs to be heated to 400°C to remove all water.

Figure 6. STEM-EDX analysis of c-PdPt/TiS; Pt (---); Pd (___); distance A-B 110 nm.

2.3. Semi-hydrogenation of 3-hexyn-1-ol by c-Pd/TiS The catalytic semi-hydrogenation of substituted acetylenes is a well-known method to obtain cis olefins. However, overhydrogenation to the fully hydrogenated product and isomerisation to the trans olefin are known to occur. The hydrogenation of 3-hexyn-1-ol (Scheme 1) is an industrially relevant application, since cis-3-hexen-1-ol (leaf alcohol) is an important compound for the fragrance industry. OH

H2 OH

H2

OH

H2

OH

Scheme 1. Hydrogenation of 3-hexyn-1-ol to the cis-3-hexen-1-ol and formation of byproducts.

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We tested several catalysts for the hydrogenation of 3-hexyn-1-ol using the same amount of supported catalyst, although the metal loading differed significantly. In our test set up the hydrogen uptake was measured. Full hydrogenation to hexanol is achieved by 2.0 litre of H2. When a Pd/C catalyst prepared by standard methods is used, a fast uptake of 2 L of H2 is observed (Figure 7a). GC analysis shows full conversion to hexanol and the additional formation of hexane by C-O hydrogenolysis, which explains why the H2 uptake stops only after 2.1 L H2. When this Pd/C is treated with HHDMA, the H2 uptake curve shows that the catalyst activity is lower in the second part of the reaction, although this reaction is still taking place at a considerable rate. The H2 uptake stops after 2.0 L, and GC measurements show only formation of hexanol. Clearly, addition of HHDMA to Pd/C, makes this catalyst somewhat more selective.

Figure 7. H2 uptake curves for 3-hexyn-1-ol hydrogenation: a) Pd/C; b) Pd/C treated with HHDMA; c) c-Pd/TiS; d) Lindlar.

The H2 uptake curve of c-Pd/TiS shows a slower H2 uptake, which stops after 1.0 L H2 is consumed (Figure 7c), indicating a selective semi-hydrogenation to the olefin. When the reaction is stopped immediately after the consumption of 1.0 L H2, GC analysis shows a high selectivity towards the cis olefin. Since the unsupported c-Pd also turned out to be active and selective in this test reaction, hot filtration experiments were performed to see if the hydrogenation was quasi-homogeneous or purely heterogeneous. It was found that Pd leaching takes place at high metal loading, which is consistent with the weak bonding of metal agglomerates mentioned above. Table 2. Catalytic hydrogenation of acetylenic substrates: R1-C≡C-R2 Æ R1-CH=CH-R2. R1 R2 catalyst conversion (%) olefin (%) cis olefin (%) CH2CH3 CH2CH2OH c-Pd/TiS 97 99% 97% CH2CH3 CH2CH2OH Lindlar >99 99 97 c-Pd/TiS 97 95 n.a. H CMe2OH H CMe2OH Lindlar >99 96 n.a. H Ph c-Pd/TiS 93 94 n.a. H Ph Lindlar a) 95 97 n.a. 90 89 97 Me Ph c-Pd/TiS a) Me Ph Lindlar a) 48 b) 96 98 a) 5 times more catalyst used; b) reaction stops before full conversion is reached.

Highly active and selective PM catalysts by use of the reduction-deposition method 141 The catalyst most often used for the semi-hydrogenation of substituted acetylenes is the Lindlar catalyst (5%Pd + 2-3%Pb on CaCO3). Because of environmental reasons the use of Pb is not desirable. When the Lindlar catalyst is used in our test reaction, the H2 uptake curve shows that this hydrogenation also stops after 1.0 L H2 is consumed (Figure 7d). However, the exact end point of the reaction is more difficult to determine, since the reaction slows down significantly before full conversion is reached. Although the H2 consumption does not exceed 1.0 L, prolonged reaction times should be avoided since this leads to cis-trans isomerisation. GC analysis shows that the Lindlar catalyst has a high cis-selectivity (>99% after 40 min reaction time) at 95% conversion, while this has lowered to 97% at full conversion (100 min reaction time). When the hydrogenation using c-Pd/TiS is continued after uptake of 1.0 L H2, we observe very slow overhydrogenation, cis-trans isomerisation and isomerisation to 2- and 4-hexenol.

2.4. Recycling of c-Pd/TiS The stability of c-Pd/TiS in the 3-hexyn-1-ol hydrogenation was tested by recycling the catalyst 7 times. Although the selectivity remains constant, a linear decrease of the activity is observed (Figure 8). Before the start of run 8 a prehydrogenation step is performed to test if the deactivation is due to partial oxidation of the Pd. However, the drop in the reaction rate observed in run 8 is the same as in all other runs. 60

100 99

selectivity (%)

97

40

96 30

95 94

20

93 92

reaction rate (mL H2 per min)

50

98

10

91 90

0 run 1

2

3

4

5

6

7

8

Figure 8. Catalytic hydrogenation of 3-hexyn-1-ol: reaction rate (grey bars); olefin selectivity (___); cis selectivity (---); conversion ~96% for all runs.

Carbon analysis of the spend catalyst shows that the amount of HHDMA on the catalyst drops dramatically during the hydrogenations. The amount of catalyst used in the experiments was too low to also measure the metal content, but in a separate experiment it was shown that washing c-Pd/TiS with EtOH removes only the HHDMA and not the metal. This washed c-Pd/TiS has an identical activity and selectivity in a hydrogenation as the fresh catalyst Therefore it can be concluded that the catalyst deactivation observed in the recycling experiments is not caused by leaching of the metal or of the HHDMA. Most likely the deactivation is caused by impurities in the substrate. The high selectivity of c-Pd/TiS cannot be an effect of the presence of HHDMA (as in run b of Figure 5), since washed c-Pd/TiS contains almost no HHDMA and still has the same high selectivity. The selectivity of c-Pd/TiS is much higher than that of Pd/C prepared by standard methods, so it clearly is a direct effect of the reduction/deposition preparation method.

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3. Conclusions We developed a new method for preparing mono- and bimetallic precious metal catalysts through reduction-deposition that is suitable for large-scale production, since it does not use organic solvents or compounds that are not available on production scale. The newly developed Pd-catalysts are highly active and selective in the semihydrogenation of substituted acetylenes to the corresponding cis-olefins and are suitable as lead-free alternatives for the Lindlar catalyst. The catalyst remains highly selective after recycling, but the activity decreases because of impurities in the substrate.

4. Experimental section 4.1. Preparation of catalysts[10] c-Pd: A solution of 15 g HHDMA in 1 L water was heated to 60°C. A solution of 0.75 g Pd (as Na2PdCl4) in 10 mL water was added. The mixture was heated to 80°C and stirred at this temperature for 2 hours. c-Pt: As Pd, but using H2PtCl6 and a reaction temperature of 95°C. c-PdPt: As Pt, but using a mixture of equal amounts of Pd and Pt. c-Pd/TiS: A slurry of 75 g TiS powder in 750 mL water was stirred for 30 minutes at room temperature, after which c-Pd was added. After an additional 45 minutes of stirring, the catalyst was filtered off and washed. Analysis: 0.47% Pd, 6.1% C, 18% water. IR (cm-1): 1200 (TiS support), 1640 (water), 1410, 1470, 2855, 2925 + shoulder (HHDMA). TGA (weight loss): 6% 25-100°C, 4% 100-300°C, 6% 300-400°C, 0% 400650°C. PSD: d(0.1) 5.0 d(0.5) 22.4 d(0.9) 47.8. Catalyst used for recycling experiments: PSD: d(0.1) 13.7 d(0.5) 23.1 d(0.9) 53.9. Fresh: 4.6% C; spend: 0.6% C. c-Pd/C: As cPd/TiS, but using carbon powder. Analysis: 0.60% Pd, 63% water.

4.2. Hydrogenation of 3-hexyn-1-ol[10] Pd/C, c-Pd/TiS: A 250 mL stainless steel autoclave was charged with 50 mg catalyst (dry weight), 100 mL 96% ethanol, and 5 mL 3-hexyn-1-ol and the mixture was heated to 30°C. Without stirring the autoclave was flushed with hydrogen and pressurised with 3 bars of hydrogen. The reaction was started by starting the stirring (1500 rpm). Lindlar: As Pd/C and c-Pd/TiS, but with a 15 minute prehydrogenation step.

Acknowledgement We gratefully acknowledge Dr Guido Mul and Ana Rita Almeida of TU Delft for their help with IR and TGA measurements.

References [1] A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 2002, 102, 3757-3778. [2] D. Astruc, F. Lu, J.R. Aranzaes, Angew. Chem. Int. Ed. 2005, 44, 7852-7872. [3] H. Bönnemann, K.S. Nagabhushana in Metal nanoclusters in catalysis and materials science: the issue of size control, Part 1, Chapter 2 (Eds.: B. Corain, G. Schmid, N. Toshima), Elsevier, 2007; and references therein. [4] a) J. Turkevich, G. Kim, Science 1970, 169, 873-879; b) H. Hirai, H. Chawanya, N. Toshima, React. Polym. 1985, 3, 127-141; c) F. Bonet, V. Delmas, S. Grugeon, R. Herrera Urbina, P-Y. Silvert, K. Tekaia-Elhsissen, Nanostruc. Mat. 1999, 11, 1277-1284; d) W. Lu, B. Wang, K. Wang, X. Wang, J.G Hou, Langmuir 2003, 19, 5887-5891: e) H. Bönnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Joussen, B. Korall, Angew. Chem. Int. Ed. 1991, 30, 1312-1314. [5] H. Bönnemann, personal communication. [6] P.T. Witte, unpublished results.

Highly active and selective PM catalysts by use of the reduction-deposition method 143 [7] CCDC 754963 contains the supplementary crystallogrpahic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. [8] H. Bönnemann, R. Brinkmann, Appl. Organometal. Chem. 1994, 8, 361-378. [9] S. Bucher, J. Hormes, H. Modrow, R. Brinkmann, N. Waldöfner, H. Bönnemann, L. Beuermann, S. Krischok, W. Maus-Friedrichs, V. Kempter, Sur. Sci. 2002, 497, 321-332. [10] P.T. Witte, WO patent, 2009, 096783.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Investigation of the role of stabilizing agent molecules in the heterogeneous nucleation of rhodium(0) nanoparticles onto Al-SBA-15 supports R. Sassinea, E. Bilé-Guyonnetb, T. Onfroyc, A. Denicourtb, A. Roucouxb, F. Launaya* a

Laboratoire de Réactivité de Surface (LRS), UPMC Paris 06, UMR 7197 CNRS, 4 place Jussieu, 75252 Paris Cedex 05, France. [email protected] b Equipe “Chimie Organique et Supramoléculaire”, Ecole Nationale Supérieure de Chimie de Rennes, UMR 6226 CNRS, avenue du Général Leclerc, 35708 Rennes Cedex 7, France. c Laboratoire RMN des Matériaux Nanoporeux (RMN), UPMC Paris 06, FRE 3230 CNRS, 4 place Jussieu, 75252 Paris Cedex 05, France.

Abstract Interactions of stabilizing agent molecules (HEA16Cl) with Rh(0) particles and mesoporous aluminosilic supports were studied in rhodium(0) catalysts obtained by heterogeneous nucleation. HEA16Cl/Rh(0) proximity was indirectly demonstrated by monitoring the CO coverage of metal surface using FTIR spectroscopy as well as by the examination of the thermal decomposition profiles of HEA16Cl (TGA). Complementary work performed in the absence of Rh led us to identify electrostatic interactions between HEA+ ions and the support for surfactant concentrations in the range used for catalyst preparation. Keywords: stabilizing agent, mesoporous support, nanoparticles, interaction

1. Introduction Stabilizing agent assisted reduction of metal salts in solution leads to very reproducible particle size distributions of nanometer scale. Metal colloids thus obtained present specific physico-chemical properties particularly useful for various catalysis applications [1]. However, continuous flow processes may require the deposition of such active phase onto supports. Mesoporous materials are good candidates due to their large specific surface area, porosity and opportunities for particles confinement. Impregnation or one-pot [2,3] strategies are usually implemented in order to insert pre-formed colloidal particles. We recently developed another procedure based on the heterogeneous nucleation of rhodium colloids onto Na-Al-SBA-15 by the reduction of rhodium (III) chloride in the presence of N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium chloride (HEA16Cl), used as a stabilizing agent. Particles@Na-Al-SBA-15 were shown to be particularly active (R.T., atmospheric of pressure of H2) in the liquid phase hydrogenation of various aromatic derivatives [4]. A more thorough characterization of prepared materials is proposed in this work. The present study aims in particular to better understand the role of stabilizing agent molecules. Adsorption tests were carried out by contacting various amounts of stabilizing agent with the Na form of the support until equilibration. Isotherms have been built from the analysis of the recovered solids

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by thermogravimetric analyses (TGA) and filtrates by total organic carbon (TOC) determination as well as conductivity.

2. Experimental Chemicals in this study i.e. tetramethyl orthosilicate (TMOS, 99%, Fluka), Pluronic P123 triblock copolymer (EO20PO70EO20, Mw = 5800, Aldrich), aluminium isopropoxide (Al(OiPr)3, 98%, Aldrich), rhodium(III) chloride (RhCl3,xH2O, 38-40%, Strem) and sodium borohydride (NaBH4, 99%, Aldrich) were used as received.

2.1. Materials synthesis Al-SBA-15 mesoporous silica with a nominal Si/Al ratio = 10 was prepared by cohydrolysis and co-condensation of a mixture of Al(OiPr)3 and TMOS [5]. The resulting solid was characterized by a surface area (SBET) = 810 ± 50 m2 g-1, a total pore volume (VP) = 1.06 ± 0.04 cm3 g-1, an average pore diameter (DP) = 7.6 ± 0.2 nm and an experimental Si/Al ratio = 12. Na-Al-SBA-15 was obtained after stirring 3 g of AlSBA-15 solid in 300 mL of 4 M NaCl for 48 h at 80°C. The solid was then filtered out and washed with hot distilled water. Na-AlSBA-15 thus prepared was characterized by SBET = 750 ± 60 m2 g-1, VP = 1.06 ± 0.06 cm3 g-1, DP = 7.8 ± 0.1 nm.

2.2. Rhodium catalyst synthesis Rh(0) particles were obtained by reducing Rh(III) ions in the presence of Na-Al-SBA15 and a quaternary ammonium salt, HEA16Cl [6]. The reductant used was sodium borohydride. In a typical synthesis (Rh/Na-Al-SBA-15 sample), a weighted amount (0.5 g) of support was added to 8.5 mL of a HEA16Cl aqueous solution. The resulting suspension was stirred for 24 h at room temperature before addition of RhCl3.xH2O (0.012 g, 4.8 10-5 mol). After 2 h, NaBH4 (0.005 g, 1.3 10-4 mol) was introduced. Final volume was 12.5 mL. The solid was filtered 2 h later, washed with 50 mL of distilled water and dried at 60ºC for 24 h. Two other samples (Rh/Na-Al-SBA-15 (1/2) and Rh/Na-Al-SBA-15 (0)) were prepared similarly (Table 1). Table 1. Summary of samples codes. Samples

HEA16Cl (g)

Rh/Na-Al-SBA-15

0.0315

Rh/Na-Al-SBA-15 (1/2)

0.0158

Rh/Na-Al-SBA-15 (0)

0

2.3. Adsorption tests Na-Al-SBA-15 (0.2 g) was dispersed in 20 mL of water in the presence of varying amounts of HEA16Cl (from 0.0063 to 0.378 g). After 24 h, the suspension was centrifuged. The residual surfactant was determined in the supernatant using TOC analysis. Meanwhile, HEA16Cl adsorbed on the support was quantified by TGA after drying.

2.4. Characterization The physico-chemical properties of the solids were studied by different techniques. Nitrogen adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP-2000 physisorption analyzer. SBET was determined according to the BET equation, whereas the pore size distribution in the mesopore region was obtained applying the BJH method to the desorption branch of the isotherms. VP was estimated from the nitrogen adsorption at P/P0 = 0.99. X-ray diffraction analyses (XRD) were

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carried out at low angles on a Bruker D8 diffractometer using Cu Kα radiation (1.5418 Å). Transmission Electron Micrographs (TEM) were collected on a 200 kV JEOL JEM 2011 UHR (LaB6) microscope equipped with an Orius Gatan camera. This instrument was equipped with a system for EDX (PGT detector) with an analysis domain of 100 nm. The local distribution of Al, Si, Na or Rh was assessed by comparing surface areas of Si Kα, Al Kα, Na Kα and Rh Lα peaks. Transmission FTIR spectra of adsorbed CO were collected on a Bruker Vector 22 spectrometer using a DTGS detector (resolution 2 cm-1, 64 scans per spectrum). TGA of the samples were carried out on a TA Instruments – Waters LLC, SDT Q600 analyzer with a heating rate of 10°C min-1 in the presence of air flow (100 mL min-1). The amount of total organic carbon was determined on a Shimadzu TOC-VCSH analyzer.

3. Results and discussion Interactions of stabilizing agent molecules with Rh(0) particles and the mesoporous aluminosilic support were studied in rhodium(0) catalysts and alternately by sorption isotherms of the surfactant alone. The support used in the whole study is an aluminosilicic mesoporous material of the Al-SBA-15 type with Si/Al ≈ 12 treated with NaCl. Apart from a slight decrease in specific surface area (about 8%), textural properties of Na-AlSBA-15 are quite similar to those of Al-SBA-15 (Table 2). Na dispersion was shown to be homogeneous (Na/Al molar ratio = 0.7) throughout the whole samples. Assuming that all Al3+ cations are in tetrahedral environments (c.a. 1.3 mmol g-1 of Bronsted acid sites), it appears that H+/Na+ exchange is only partial.

3.1. Supported nanoparticles Two types of materials were obtained from Na-Al-SBA-15 either in the presence or in the absence of stabilizing agent molecules. They are denoted Rh/Na-Al-SBA-15, Rh/Na-Al-SBA-15(1/2) or Rh/Na-Al-SBA-15 (0), respectively. In the case of Rh/NaAl-SBA-15 and Rh/Na-Al-SBA-15(1/2), the aluminosilic solid was contacted with HEA16Cl for 24 h. Then, rhodium chloride (III) was introduced in order to obtain a 1 wt% Rh(0) catalyst after reduction. The amount of stabilizing agent molecules was halved in Rh/Na-Al-SBA-15(1/2). A reference material, Rh/Na-Al-SBA-15(0), was prepared in the same way but using H2O instead of an aqueous solution of HEA16Cl in the first step. In both cases, recovered filtrates were colorless which is consistent with a near-complete Rh incorporation. Nominal stabilizing agent molecules and rhodium quantities in Rh/Na-Al-SBA-15 sample (HEA16Cl/Rh ≈ 2 and NaBH4/Rh ≈ 2.7) were chosen in order to be approximately the same as for the preparation of aqueous colloidal dispersions [6]. Moreover, it has to be noted that HEA16Cl and Rh quantities introduced were well below the estimated ion-exchange capacity of the supports. Rh(0) nanoparticles were characterized by TEM (Figure 1). In the presence of stabilizing agent molecules (Rh(0)/Na-Al-SBA-15 and Rh/Na-Al-SBA-15(1/2) samples), particles are rather small. Counts conducted on 1700 and 508 particles in Rh(0)/Na-Al-SBA-15 and Rh/Na-Al-SBA-15(1/2) samples indicate that their average diameters are 2.5 ± 0.5 nm and 2.8 ± 0.7 nm, respectively. Halving of the amount of HEA16Cl has little noticeable effect on the size distribution. Rhodium particles are fairly well-dispersed across the grains of the support and, for some (Rh(0)/Na-Al-SBA-15 sample), partly located on the outer surface. Few agglomerates are visible on transmission electron micrographs. The magnitude of the particle size has been validated by high angle X-ray diffraction. In addition to poorly defined signals assigned to the support for 10 < 2θ < 90°, the diffractogram of Rh/Na-Al-SBA-15 sample is characterized by a broad 111 FCC peak centered ca. 41° [7]. In the absence of stabilizing agent molecules, particles

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are much less dispersed. Many rhodium aggregates are located outside the grains. Clearly, the size of individual particles is much larger than the diameter of the pores. The effect of HEA16Cl is obvious. In the presence of the stabilizing agent, the size distribution of particles is almost superimposable to that obtained in solution [4].

A

B D

C F

E Figure 1. TEM pictures of Rh/Na-Al-SBA-15(0) (A,B), Rh/Na-Al-SBA-15 (C) and Rh/Na-AlSBA-15(1/2) (E). Particle size histograms of Rh/Na-Al-SBA-15 (D) and Rh/Na-Al-SBA-15(1/2) (508 particles, E).

In situ formation of the particles more strongly affects the textural parameters than the H+/Na+ ion exchange step (Table 2). After reduction, the specific surface area has decreased by about 20%. Values of pore volume and average diameter are almost unchanged due to the low amount of rhodium incorporated (around 1 wt.%).

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Table 2. Evolution of the textural properties through the different steps of the catalyst preparation. SBET (m2 g-1)

Vp (cm3 g-1)

Dp (nm)

Al-SBA-15

827

1.1

7.7

Na-Al-SBA-15

759

1.05

7.8

Rh/Na-Al-SBA-15

595

0.95

7.45

Like in solution, above characterizations are in agreement with the involvement of stabilizing agent molecules in the formation of particles with a monodisperse size as the result of ammonium cations/particles interactions [6]. The amount of HEA16Cl embedded in Rh/Na-Al-SBA-15 sample was estimated by thermogravimetric analysis (Figure 2). Hence, Rh/Na-Al-SBA-15 is characterized by an initial weight loss due to physisorbed water (≈ 10-12 wt.%, maximum of the derivative weight at 54°C) and a second one, between 270 and 550°C (< 10 wt.%, maxima at 352°C (A) and 490°C (B)), due to the degradation of stabilizing agent molecules. Integration of the second signal allowed an estimation of the amount of residual HEA16Cl in Rh/Na-Al-SBA-15. This corresponds to almost all stabilizing agent molecules contacted with the support before rhodium incorporation. In addition to physisorbed water, it can be noted that the weight derivative of the thermogravimogram of pure HEA16Cl (not shown here) involves only one maximum (A) at 248°C under the same analysis conditions, i.e. 100°C lower than for HEA16Cl in Rh/Na-Al-SBA-15 sample. In the absence of rhodium (not shown here), three signals were observed at 303°C (A), 550°C (B) with shoulders at 200°C and 245°C. Shift of the (A) and (B) maximums clearly establish that stabilizing agent molecules interact with the solid in different ways depending on the presence of rhodium. 0,25 100 95 90 Loss weight (%)

Derivative weight (%/°C)

0,2

0,15

85 80 75

0,1 70 20

120

220

320

420

520

620

720

820

Temperature (°C)

0,05

0 20

120

220

320

420

520

620

720

820

Temperature (°C)

Figure 2. First derivatives and TGA curves (see insert) of Rh/Na-Al-SBA-15.

A more effective demonstration of the proximity between the surfactant and supported particles could be obtained by monitoring CO adsorption on Rh(0) by infrared spectroscopy. This technique is used to characterize the oxidation state of Rh and the nature of the probed atoms. Recently, in the case of Rh(0)-MCM-41 materials devoid of organic compounds [8], dispersion of Rh(0) was traced by comparing the number of metal atoms available for CO adsorption with their total number obtained by elemental analysis. The average size of particles could even be determined by this way assuming they are cubic objects. Results agreed with TEM. A similar analysis has been performed on Rh/Na-Al-SBA-15(1/2). Indeed, the two bands previously assigned to geminal CO species at ~ 2100 and 2040 cm-1 and the one characteristics of linear CO at

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2060 cm-1 are observed at equilibrium pressure [9,10] (Figure 3). However the intensity of these three bands are extremely low in comparison to the Rh loading (0.63 wt.%). As a result, completely erroneous estimations of the average diameter and Rh dispersion were obtained. Undoubtedly the strong difference between transmission electron microscopy and CO adsorption results can be considered as an indirect evidence of the interaction of the metal particle with stabilizing agent molecules. Moreover, problems of particles surfaces accessibility have been highlighted by the relatively slow kinetics of the establishment of equilibria (results not shown here).

Absorbance (a.u.)

0.001

2200

2150

2100 2050 Wavenumber (cm-1)

2000

1950

Figure 3. IR spectrum of Rh/Na-Al-SBA-15 (1/2) after adsorption of 5 Torr of CO at equilibrium pressure.

Further experiments, not detailed here, have been carried out in order to better understand the influence of synthesis parameters on the final dispersion of particles. They allowed to show the importance of the addition rank of the reagents. Thus, it was clear that the support must be contacted first and long enough with the stabilizing agent. Given the cationic nature of the surfactant and the ion exchange capacity of Na-AlSBA-15, HEA16Cl/Na-Al-SBA-15 interactions have been thoroughly studied (prior to Rh introduction). Results are detailed in the next part.

3.2. Surfactant adsorption The behaviour of stabilizing agent molecules toward the Na-Al-SBA-15 support has been studied from analyses of filtrates and solids recovered in several impregnation tests carried out with different amounts of surfactant. The concentration range, [HEA16Cl]0, tested was between 1 and 50 mmol L-1. In addition to thermogravimetric analyses of the centrifuged solids, supernatants were controlled by potentiometric, conductivity measurements and total organic carbon determination. For each stabilizing agent concentration studied, particular attention was paid to the extent of changes in conductivity (Δσ) between a HEA16Cl solution contacted with the support for 24 h and a blank solution (no support). These tests were performed routinely after a period of 24 h which is more than enough to reach a steady state as indicated by monitoring Δσ values vs time (not shown here). Largely positive and increasing Δσ values observed for 0.9 ≤ [HEA16Cl]0 ≤ 10.8 mmol L-1 have been attributed to the release of H+ (as evidenced by a pH decrease) and Na+ cations in solution. It can be noted that Δσ values of the supernatant measured under the same conditions are significantly smaller in the presence of SBA-15 silica. Some representatives graphs of the evolution of the weight loss (and its first derivative) for different solids recovered after 24 h adsorption tests are shown in Figure 4. Thermogravimetric analyses show that weight losses between 100 and 900°C increase

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with [HEA16Cl]0. Parallely, the amount of physisorbed water become less and less significant. A

B 100 NaAlSBA_1 NaAlSBA_4 NaAlSBA_6 NaAlSBA_20

90

Loss weight (%)

85 80 75

0,3

III

0,25 Derivative weight (%/°C)

95

0,2

NaAlSBA_1 NaAlSBA_4 NaAlSBA_6 NaAlSBA_20

I

0,15

70

0,1

65

0,05

II

60 0

55

20

20

120

220

320

420

520

620

720

120

220

820

320

420

520

620

720

820

Temperature (°C)

o

Temperature ( C)

Figure 4. TGA (A) and first derivatives curves (B) of Na-Al-SBA-15 samples recovered from adsorption tests in the presence of 1.8 (NaAlSBA_1), 7.2 (NaAlSBA_4), 10.8 (NaAlSBA_6) and 36 (NaAlSBA_20) mmol L-1 of HEA16Cl.

The adsorption isotherm of stabilizing agent molecules over Na-Al-SBA-15 could be plot directly from TGA measurements (weight losses between 100 and 900°C) or indirectly from TOC analyses. Both curves are displayed in Figure 5. For 0.9 ≤ [HEA16Cl]0 ≤ 10.8 mmol L-1, (i.e. 0.2 ≤ [HEA16Cl]éq ≤ 1.2 mmol L-1), the two approaches lead to superimposed curves. However, more significant differences could be observed for the “plateau” which is either at 1.4 or 1.2 mmol of HEA16Cl g-1 according to TOC and TGA analyses, respectively.

HEACl adsorbed (mmol g-1 support)

1,6 1,4 1,2 1,2

1

1

0,8

0,8

0,6

0,6 0,4

0,4

0,2

0,2

0 0

0,1

0,2

0,3

0 0

5

10

15

20

25

30

35

40

[HEACl] at equilibrium (mmol L-1)

Figure 5. HEA16Cl adsorption isotherms from TOC (S) and TGA (¡) analyses.

Thermal degradation of HEA16Cl is different from that obtained starting with pure HEA16Cl (vide supra). Moreover, the curve profile become more complex with increasing HEA16Cl concentrations. Besides the loss of H2O, three types of maxima (I, II and III) were detected on the first derivative curve. Only I (circa 340°C) and II (c.a. 500°C) are observed for 0.9 ≤ [HEA16Cl]0 ≤ 1.8 mmol L-1. For [HEA16Cl]0 ≥ 7.2 mmol L-1, another signal (III, c.a. 235°C) starts to emerge. The area of the latter is the main one for [HEA16Cl]0 ≥ 10.8 mmol L-1 (NaAlSBA_6). Signals I and II were shown to saturate for 10.8 ≤ [HEA16Cl]0 ≤ 30 mmol L-1. I and III are believed to be related to two forms of the stabilizing agent molecules differing in their interaction with the support. Peak I would correspond to molecules involved in a stronger association with the surface. Since I appears at the lowest HEA16Cl concentrations, that is to say, for

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tests leading to higher conductivity variations in solution, it is conceivable that the corresponding interaction is of electrostatic nature. This latter interpretation would mean that a certain amount of chloride ions corresponding to the portion of HEA+ linked to the surface by electrostatic interaction (peak I) is leached into solution. Analyses of similar materials obtained by deposition of HEA16Cl on H-Al-SBA-15 (instead of Na-Al-SBA-15) are not in agreement with this hypothesis. Indeed, values of the Cl/N molar ratio of the solid after adsorption are approximately equal to 1 and irrespective of the concentration of stabilizing agent molecules tested. Retention of chloride ions could be explained by the presence of ≡SiOH2+ groups on the surface. Indeed pH values measured after 24 h are between 3.85 and 4.41, i.e. below the PZC of the support.

4. Conclusion Like in aqueous solution, HEA16Cl molecules appear to be involved in the stabilization of the particles formed by the heterogeneous nucleation pathway. Besides similarities with aqueous Rh(0) colloids in terms of particle sizes distribution, HEA16Cl interaction with the metal surface was indirectly highlighted through adsorption measurements of CO followed by infrared spectroscopy and by some changes in the thermal decomposition profiles of HEA16Cl. The influence of the stabilizing agent was related to prior adsorption of the molecules to the support in the first stage of the Rh/Na-AlSBA-15 synthesis. Clearly, it was determined that a Na+ (or H+)/HEA+ exchange takes place. Further control of particle size and distribution could be the result of admicellization processes already described for different supports [11].

Acknowledgments The authors would like to thank Dr F. Lequeux and J. Marchal (Laboratoire Physicochimie des Polymères et des Milieux Dispersés, ESPCI ParisTech) for the use of the TOC analyzer. We gratefully acknowledge financial support from the ANR in the “Chimie pour le Développement Durable” Program (ANR-08-CP2D-14).

References [1] A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 102 (2002) 3757. [2] R.M. Rioux, H. Song, J.D. Hoefelmeyer, P. Yang, G.A. Somorjai, J. Phys. Chem. B 109 (2005) 2192. [3] J.P.M. Niederer, A.B.J. Arnold, W.F. Hölderich, B. Spliethof, B. Tesche, M. Reetz, H. Bönnemann, Top. Catal. 18 (2002) 265. [4] M. Boutros, A. Denicourt-Nowicki, A. Roucoux, L. Gengembre, P. Beaunier, A. Gédéon, F. Launay, Chem. Commun. (2008) 2920. [5] B. Jarry, F. Launay, J.-P. Nogier, J.-L. Bonardet, Stud. Surface Sci. Catal. 158B (Molecular Sieves: From Basic Research to Industrial Applications) (2005) 1581. [6] J. Schulz, A. Roucoux, H. Patin, Adv. Synth. Catal. 345 (2003) 222. [7] S. Alayoglu, B. Eichhorn, J. Am. Chem. Soc. 130 (2008) 17479. [8] M. Boutros, F. Launay, A. Nowicki, T. Onfroy, V. Herlédan-Semmer, A. Roucoux, A. Gédéon, J. Mol. Catal. A : Chem. 259 (2006) 91. [9] C.A. Rice, S.D. Worley, C.W. Curtis, J.A. Guin, A.R. Tarrer, J. Chem. Phys. 74 (1981) 6487. [10] A.C. Yang, C.W. Garland, J. Phys. Chem. 61 (1957) 1504. [11] Z. Li, L. Gallus, Colloids Surfaces A 264 (2005) 61.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of the polymer-stabilized and supported nanostructured catalysts E. Sulmana, V. Matveevaa, V. Doludaa , L. Nikoshvilia, A. Bykova, G. Demidenkoa, L. Bronsteinb a b

Tver Technical University, A.Nikitina str., 22, Tver, 170026, Russia Indiana University, Bloomington, IN 47405, USA

Abstract In this work we report a comparative study of two types of nanoparticulate catalytic systems based on two amphiphilic block copolymers and a nanoporous polymer, hypercrosslinked polystyrene (HPS). Nanostructures in polymers (block copolymer micelle cores or nanopores) control nanoparticle (NP) formation and location while polymeric environment (functional groups) influences the catalytic performance. Catalytic properties of these nanocomposites were studied in selective hydrogenation of triple bond of dehydrolinalool and in direct selective oxidation of two monosaccharides: L-sorbose and D-glucose. In hydrogenation, the highest selectivity of 99% was achieved for Pd, PdZn, PdAu, and PdPt catalytic NPs in polystyrene-b-poly(4-vynyl pyridine) micelles due to a modifying influence of pyridine units, while the highest activity of 49.2 mol LN/(mol Pd . s) was observed for the PdPt NPs due to synergy of catalytic activity of both metals in hydrogenation. In oxidation of L-sorbose and D-glucose, the highest activities were observed for the Pt and Ru catalysts, respectively, based on HPS due to better access of catalytic sites of NPs. Keywords: metal nanoparticles, nanocomposites, hydrogenation, oxidation, monosaccharides

1. Introduction The demand for high quality vitamins, pharmaceuticals, and food supplements has been increasing throughout the world. With considerable medical evidence linking a diet and diet supplements to human health and with an attempt to facilitate a non-drug approach to combat common ailments, vitamins A, E, K, and C are increasingly being incorporated into functional foods [1,2]. Catalysis is a key methodology for the efficient industrial production of important biologically active compounds and pure optically active substances [3]. Thus the application of new methods and technologies in catalysis is very important [4]. Metal NPs have unique catalytic properties [5-7] due to high number of surface atoms, leading to a high number of reactive sites. Catalytic properties of NPs depend on their size, size distribution and environment [8]. The NP surface plays an important role in catalysis and determines the NP selectivity and activity. The studies of the last decade demonstrate that formation of NPs in nanostructured polymeric environment allows control over NP size and size distribution; in so doing, the stabilizing polymer may strongly influence the surface of NPs [9-12]. NPs formed in different types of nanostructured polymers (dendrimers, block copolymers, layer-by-layer polyelectrolyte structures, etc) were studied in various catalytic reactions [9-11]. Amphiphilic block copolymers are largely studied for NP

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formation. Controlled growth of metal NPs in a polymer matrix is also possible if it occurs in cavities or pores. In this case, the size of the growing particles can be limited to the cavity size. HPS is the first representative of a new class of cross-linked polymers characterized by unique topology and unusual properties. Due to its high crosslinking density, which can exceed 100%, HPS consists of nanosized rigid cavities of about 4 nm in size. HPS has a large inner surface area (usually nearly 1000 m2/g) and the ability to swell in any liquid medium including precipitating agents of the starting polymer. All of that makes it a promising matrix for nanoparticle formation. In this paper we present a comparative study of two types of nanoparticulate catalysts based on amphiphilic block copolymers (polystyrene-b-poly(4-vinyl pyridine) (PS-b-P4VP) and poly(ethylene oxide)-b-poly(2-vinyl pyridine) (PEO-b-P2VP)) and HPS. The catalytic properties of these nanocomposites were studied in selective hydrogenation of a triple bond of dehydrolinalool (DHL) and direct selective oxidation of monosaccharides: L-sorpbose and D-glucose.

2. Experimental 2.1. Materials PS-b-P4VP (Mn D19,400; Mw D22,500, and relative 4-VP content of 0.340) was prepared via living anionic polymerization and was a gift from Max Plank Institute of Colloids and Interfaces, Potsdam/Golm, Germany). PEO350-b-P2VP135 (MPEOn = 15,400, MP2VPn = 14,100, Mw/Mn = 1.04) was purchased from Polymer Source Inc., Canada, and used as received. The HPS was purchased from Purolite Int. (UK), as Macronet MN270/38600 type 2/100 (designated as HPS). HAuCl4×3H2O, Pd(CH3COO)2, Na2PdCl4, Zn(CH3COO)2, K[Pt(C2H4)Cl3]×H2O (Zeise salt), Ru(OH)Cl3, LiB(C2H5)3H (SH, 1M solution of LiB(C2H5)3H in THF)), NaBH4 were obtained from Aldrich and used as received. H2PtCl6×6H2O was obtained from Reakhim (Moscow, Russia). Dehydrolinalool (99% purity) was supplied by pharmaceutical company OAO “Belgorodvitaminy” (Belgorod, Russia) and distilled under vacuum (40–45°C at 50–60 kPa). D-glucose and L-sorbose were provided by Fluka. Reagent-grade THF was purchased from Aldrich. Isopropanol (i-PrOH) and toluene were obtained from Aldrich and distilled before use. KOH, NaOH, HCl, NaHCO3 and hydrogen (KhimMedService, Tver, Russia) were used as received.

2.2. Catalyst synthesis The synthesis of the catalysts was based on the formation of metal compound NPs or metal NPs after reduction of metal compounds in the cores of amphiphilic block copolymer micelles or in the pores of HPS [13-17]. 2.2.1. Catalysts on the base of amphiphilic block copolymers The synthesis of these catalytic systems is based on incorporation of metal compounds into the micelle core of amphiphilic block copolymer micelles followed by reduction with a formation of NPs. The core block contains functional groups which coordinate with metal compounds and the core serves as a nanoreactor for NP formation, while the corona block provides solubility in a selective solvent. Micellar catalysts in this work were prepared using PEO-b-P2VP and PS-b-P4VP. Mono- (Pd and Pt) and bimetallic (PdAu, PdPt and PdZn) catalysts were synthesized by solubilization of appropriate metal salts into the P4(2)VP micelle cores followed by reduction.

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2.2.2. Catalysts on the base of amphiphilic block copolymers HPS can control formation of Pt NPs within the pores. In this case HPS plays two roles: of nanostructured matrix for NP growth and the heterogeneous catalyst support. Incorporation of Pt, Pd, and Ru species in HPS was carried out via impregnation of the H2PtCl6 or Pd(CH3COO)2 solutions in THF and the Ru(OH)Cl3 solution in a complex solvent (THF-methanol-water) followed by reduction (for Pd, using NaBH4) or without any further treatment (for Pt and Ru). In this work we discuss catalytic samples HPS-Ru-1, HPS-Ru-2, HPS-Ru-3 containing 0.5, 0.9, and 2.9 wt.% Ru, respectively.

2.3. Catalytic testing Hydrogenation of a triple bond of dehydrolinalool was carried out at ambient pressure in a glass batch isothermal reactor installed in a shaker and connected to a gasometric burette. In the case of micellar catalysts based on amphiphilic block copolymers, different solvents providing the better swelling of micelle corona and access of the substrates to catalytic sites were used. For PS-b-P4VP based catalyst, toluene was used, while for PEO-b-P2VP based catalyst, a mixture of 30 vol. % water and 70 vol. % of isopropyl alcohol (i-PrOH) was emploied. The oxidation of monosaccharides was conducted batchwise at ambient pressure in PARR 4592 apparatus in water. To maintain pH of 6.0 – 7.5, NaHCO3 was added using automated feeder. The samples of the reaction mixtures were periodically removed and analyzed using gas chromatography (in hydrogenation) and HPLC (in oxidation).

2.4. Physicochemical characterization The catalysts studied were characterized using X-ray powder diffraction (XRD), X-ray fluorescence analysis (XFA), transmission electron microscopy (TEM), X-ray adsorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and liquid nitrogen physisorption methods.

3. Results and discussion 3.1. Catalyst characterization 3.1.1. Catalytic NPs formed in the micelle cores of amphiphilic block copolymers XRD and TEM data for monometallic Pd NPs (PS-b-P4VP-Pd, PEO-b-P2VP-Pd) showed that in all the cases small particles with a diameter of 1.5-2.0 nm are formed, the diffraction patterns of which are typical for Pd(0) [15,16]. In the case of Pt NPs (PSb-P4VP-Pt, PEO-b-P2VP-Pt), the NP diameters are similar. The electronic properties of bimetallic (PdAu, PdPt, and PdZn) NPs were studied using XRD, TEM, XPS, and FTIR of the adsorbed CO. Bimetallic based catalysts contained 1.5-2 nm NPs with a narrow particle size distribution, but with different NP morphology: cluster-in-cluster for PdPt and PdZn and core-shell for PdAu [15]. Addition of a modifying metal (Au, Pt and Zn) leads to a change of the NP electronic properties as well. 3.1.2. Catalytic NPs stabilized in HPS By XPS Pt-containing HPS contains Pt in three forms: Pt(0), Pt(II), Pt(IV), thus the compound nanoparticles formed are expected to have a mixed composition. In this paper we compare three Pt catalysts: HPS-Pt-1, HPS-Pt-2, and HPS-Pt-3 containing 0.9, 2.9, and 4.9 wt.% Pt, respectively. Independently of the Pt loading, Pt NP size (by TEM) is in the range 1.8.-2.0 nm. HPS-Pd contains 0.05 wt.% Pd while NPs measure about 2 nm. Ru-containing HPS samples, studied by TEM and diffuse reflectance

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infrared Fourier transform spectroscopy of adsorbed CO, showed the presence of NPs of mixed composition (metal/metal oxide) with a mean diameter of 1.0–1.2 nm.

3.2. Catalytic testing 3.2.1. Selective hydrogenation of a triple bond of dehydrolinalool Palladium is well known to be the best metal for the selective hydrogenation of alkynes to alkenes. However, traditional catalysts often require modification to increase selectivity and prevent complete hydrogenation. Historically, Pd/CaCO3 and Pd/Al2O3 catalysts poisoned by lead acetate, quinoline and pyridine as additional modifiers are used [18]. Though the selectivity of these catalysts is rather high (about 95%), the use of modifiers leads to the decrease of the product quality [19]. Table 1 shows catalytic properties of the catalysts in dehydrolinalool DHL hydrogenation to linalool (LN) (Fig. 1). OH

OH

AcO

[H] Catalyst IP [H]

DHL

LN (target product) Catalyst

OH 3

O Vitamin E

OH

3

O

3 DiHL(side product)

O

Vtamin K 1

Figure 1. Scheme of DHL hydrogenation (DiHL – dihydrolinalool).

All micellar catalysts based on PS-b-P4VP (# 1-4, Table 1) show outstanding selectivity (99.8% at 100% conversion) and high activity compared to other catalysts (# 5-6, Table 1). Comparison of the data presented in Table 2 (# 1-4) shows that catalytic activity is higher for bimetallic NP based samples than for the samples based on Pd NP. This can be explained by the modifying influence of gold, platinum, and zinc towards palladium. Moreover, the highest activity is observed for the PdPt-containing catalyst due to synergy of catalytic activity of both metals in hydrogenation, while Zn and Au are not catalytically active in this reaction. Thus the second metal can change the electronic properties of the catalyst, this in turn change can influence the energy of metal–hydrogen and metal–substrate bonds and the amount of hydrogen adsorbed. We believe that combination of these factors determines the change of catalytic activity of bimetallic catalysts, which is reflected in TOF values [20]. Formation of Pd NPs in the micelle cores of PS-b-P4VP and PEO-b-P2VP should be analogous and the particles sizes are alike. Moreover, the pyridine modifying groups are present in both cases. However, the activity of PEO-b-P2VP-Pd is only half of that of PS-b-P4VP-Pd. It is noteworthy that the latter form micelles in toluene, while the former form micelles in polar media [21], thus рН can strongly influence the micelle structure, NP formation and catalytic properties. For catalytic reactions with PEO-b-P2VP-Pd, we used a mixed solvent including water and i-PrOH. i-PrOH is a good solvent for DHL, while presence of 30 vol.% water keeps the PEO-b-P2VP-Pd micelles intact. It is known that alkaline medium (addition of KOH) modifies Pd catalytic systems [22] and leads to a selectivity increase in hydrogenation of alkyne alcohols. In our case, the optimal conditions for DHL hydrogenation with the PEO-b-P2VP-Pd catalyst are achieved at рН 13.0. However,

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even in these conditions the selectivity of PEO-b-P2VP-Pd (Table 1 (#5)) is slightly lower than that of PS-b-P4VP-Pd. We ascribe this decrease of selectivity to less efficient modification of the NP surface with 2VP units (nitrogen is located near the polymer chain) than with 4VP ones. As for the decrease of the catalytic activity of PEOb-P2VP-Pd compared with that of PS-b-P4VP-Pd, we tentatively ascribe it to the amphiphilic nature of DHL: presence of hydroxyl group at the third carbon atom and a different coordination on Pd NPs stabilized in different block copolymer micelles. We believe that in non-polar solvent (toluene) the DHL molecule tends to move inside the polar P4VP core due to OH group, then the terminal triple bond is likely to be situated near the Pd NP surface and hydrogenation is highly probable. In polar solvents (for PEO-b-P2VP-Pd), the amphiphilic substrate stays in the reaction solution and less actively penetrates the micelle cores containing Pd NPs, thus reaction rate decreases. In DHL hydrogenation with HPS-Pd, where Pd NPs are located in the pores of HPS, the catalytic activity is remarkably high, while selectivity is upsettingly low due to the absence of proper NP particle modification. Indeed, HPS contains no functional groups which might adsorb on the NP surface and modify the catalytic site formation. Table 1. The catalytic properties in DHL hydrogenation. #

Catalyst

Selectivitya, %

Pd loading, %

TOF, mol LN/

(wt.)

(mol Pd . s)b

0.04

18.5

99.0

1.

PS-b-P4VP-Pdc

2.

c

PS-b-P4VP-PdA u

0.04

36.9

99.0

3.

c

PS-b-P4VP-PdZn

0.04

34.4

98.5

4.

c

PS-b-P4VP-PdPt

0.04

49.2

98.5

5.

PEO-b-P2VP-Pdd

0.06

9.6

98.0

0.05

65.7

94.2

6.

f

HPS-Pd

a)

Selectivity is measured at 100% of DHL conversion Activity was calculated as the amount of moles of LN formed per second per Pd mole c) Reaction conditions: 90°C, 960 shaking/min (regime without diffusion limitations), toluene (30 ml), Co (substrate concentration) 0.44 mol/l, Cc (catalyst concentration) 2.3⋅10-5 mol Pd/l; d) Reaction conditions: 70°C, 960 shaking/min (regime without diffusion limitations), and solvent: ‘i-PrOH + water’ (30 ml), Co 0.4 mol/l, Cc 1.72⋅10-5 mol Pd/l; f) Reaction conditions: 90°C, 960 shaking/min (regime without diffusion limitations), toluene (30 ml), Co 0.4 mol/l, Cc 6.8⋅10-5 mol Pd/l. b)

3.2.2. Selective oxidation of monosaccharides Catalytic properties of NPs stabilized in polymeric matrices discussed above, were also studied in selective oxidation of monosaccharides. For L-sorbose, only the OH group situated at the 1st carbon atom should be oxidized, while for D-glucose, only the aldehyde group. There are several approaches to oxidation of monosaccharides: chemical, electrochemical, biotechnological, and catalytic. Advantages of direct catalytic oxidation include robustness, no need of group protection and deprotection and higher purity of the final products due to high selectivity of the metals used. Pt catalysts are generally used for catalytic L-sorbose oxidation. In our preceding work we developed highly efficient Pt-containing nanocatalysts based on HPS for L-sorbose oxidation by molecular oxygen in mild conditions [23]. According to literature, for catalytic oxidation of

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D-glucose Ru catalysts are mostly efficient. Earlier we demonstrated that Ru-containing catalysts based on HPS are the active catalysts in D-glucose oxidation [24]. Here we discuss Pt and Ru compound NPs (no reduction) formed in block copolymers and HPS in catalytic oxidation of L-sorbose and D-glucose (Fig. 2). Table 2 shows catalytic properties of these nanocomposites. CH2OH

COOH

O

H HOH2C H

OH

OH

C

O

HO

C

H

H

C

OH

HO

C

H

[O]

HO CH2OH

Catalyst

H

L - sorbose

H

H OH OH H

COOH

O H H

OH

[O] Catalyst

OH

D - glucose

CH2OH

2-keto-L-gulonic acid (target product) (a)

H

C

OH

HO

C

H

H

C

OH

H

C

OH

CH2OH

D-gluconic acid (target product) (b)

Figure 2. Scheme of L-sorbose (a) and D-glucose (b) oxidation.

The data presented in Table 2 show the lowest activity and selectivity for PS-b-P4VP-Pt (Table 2, # 1) due to the two factors. First, catalytic NPs are buried under the PS corona which is insoluble in water. Second, the substrate is too polar to penetrate the P4VP core. Indeed, for PEO-b-P2VP-Pt containing water soluble corona, the catalytic activity is higher (Table 2, # 2) by a factor of 2.0-2.5, but the selectivity of L-sorbose is still low. Ru-containing catalysts based on both amphiphilic block copolymers behave in a similar way (Table 2, # 3-4). On the other hand, Pt compound NPs formed in HPS pores (Table 2, #5-7), are remarkably active and selective in L-sorbose oxidation. We demonstrated that 2-ketogulonic acid (the target product) modifies the NP surface leading to its excellent stability and most likely high selectivity, while accessibility of the NPs in the pores allows exceptional activity. It is noteworthy that increase of the Pt content leads to the decrease of catalytic activity. According to nitrogen sorption experiments the increase of metal loading leads to a noticeable decrease of the BET surface area (by 30%), while the pore volume decreases only slightly, revealing blocking of a fraction of micropores. This can results in poor accessibility of some NPs and lower activity. As might be expected, Ru NPs formed in HPS showed highest catalytic activity in D-glucose oxidation (Table 2, # 8-10) while those formed in block copolymers demonstrate mediocre performance. In the HPS-Ru series, the best catalyst (highest activity and selectivity) also contains about 0.9 wt.% of active metal similar to the HPSPt series. It is noteworthy that HPS based catalysts show very high stability in oxidation of monosaccharides. Many catalysts are known to lose their activity after 2-3 repeated uses due to a loss of active metal or depositing the reaction products on the catalyst surface. We demonstrated that activity and selectivity of Pt- and Ru-containing catalysts based on HPS do not decrease even after 15 repeated uses, revealing the exceptional stability of these catalysts. We believe this stability is due to formation of NPs in the pores of a comparable size (micropores) and stabilization of the NP surface with the target molecules, preventing the loss of catalytic species, while macropores provide excellent mass transfer of reactants and products.

Preparation of the polymer-stabilized and supported nanostructured catalysts

159

Table 2. Testing of the catalysts in L-sorbosea and D-glucoseb oxidation. #

Catalyst

Active

TOF, mol S/

metal

(mol Me . s) × 103

Selectivity, %

loading, % (wt.)

1.

PS-b-

L-sorbose

D-glucose

L-sorbose*

D-glucose**

0.9

1.7

0.8

85

97

0.9

4.7

1.5

88

98

1.0

0.5

1.2

67

97

1.0

0.8

2.4

71

98

P4VP-Pt 2.

PEO-bP2VP-Pt

3.

PS-bP4VP-Ru

4.

PEO-bP2VP-Ru

5.

HPS-Pt-1

0.9

8.1

1.0

95

98

6.

HPS-Pt-2

2.9

7.5

2.8

98

98

7.

HPS-Pt-3

4.9

6.8

5.0

92

98

8.

HPS-Ru-1

0.5

0.6

3.0

64

99

9.

HPS-Ru-2

0.9

1.4

8.0

65

99

10.

HPS-Ru-3

2.9

2.1

7.0

63

99

a)

-6

3

Reaction conditions: 70°C, water as a solvent, V(02) 14 · 10 m /s; stirring rate 1000 rpm; NaHCO3 is added in the equivalent amount to L-sorbose; C0 0.106 M; (C0/Cc ) 53.6 mol/mol Pt. b) Reaction conditions: 70°C, solvent water, V(02) 14 · 10-6 m3/s; stirring rate 1000 rpm; NaHCO3 is added in the equivalent amount D-glucose; C0 0.03 M; Cc 1.5⋅10-3 mol Ru/l. * Selectivity measured at 80% of L-sorbose conversion ** Selectivity measured at 95% of D-glucose conversion

4. Conclusions A comparative study of the catalytic properties of the systems based on two types of nanostructured polymers, amphiphilic block copolymers and nanoporous HPS, in selective hydrogenation of DHL and selective oxidation of monosaccharides demonstrated the following general trends. The highest selectivity in hydrogenation was achieved for NPs formed in the PS-b-P4VP micelles due to modification of the NP surface with 4VP units. Use of the PdPt NPs instead of Pd monometallic ones leads to a significant increase of activity due to synergy of activity of both catalytic metals. HPS-Pd shows high activity in the DHL hydrogenation due to high accessibility of NPs, but poor selectivity. In oxidation processes, where modification of Pt or Ru species with pyridine units is not needed, the highest activity and selectivity are observed for the HPS based catalysts. Here the successful modification (for enhanced selectivity and stability) occurs due to L-sorbose or D-glucose oxidation products.

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Acknowledgements This work has been supported by Federal Education Agency of Russian Federation (contract P 344), Federal Science and Innovations Agency of Russian Federation (02.552.11.7075), the NATO Science for Peace Program (grant SfP-981438) and by the 6th Framework Program project “NANOCAT” (contract number 506621-1). We also thank Prof. Dr. M. Antonietti for the PS-b-P4VP.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Weber, W., Nutrition 17 (2001) 880-887. Goss-Sampson, M.A., D.P.R. Muller, J.K. Lloyd, Journal of Human Nutrition and Dietetics 2 (2008) 145-150. Bonrath, W., T. Netscher, Appl. Catal. A 280 (2005) 55-73. Bonrath, W., M. Eggersdorfer, T. Netscher, Catal.Tod. 121 (2007) 45-57. Fendler, J.H., in, Nanoparticles and Nanostructured Films: Preparation, Characterization and Applications, Wiley-VCH, New-York, 1998. Wieckowski, A., E.R. Savinova, C.G. Vayenas, Catalysis and Electrocatalysis at Nanoparticle Surfaces, Inc., New York, N. Y., 2003. Schmid, G., Nanoparticles: From Theory to Application, Wiley-VCH Verlag GmbH & Co., Weinheim, 2004. Somorjai, G.A., A.M. Contreras, M. Montano, R.M. Rioux, Proc. Nat. Acad. Sci. 103 (2006) 10577-10583. Astruc, D., F. Lu, J.R. Aranzaes, Angew. Chem. Int. Ed. 44 (2005) 7852-7872. Mueller, C., M.G. Nijkamp, D. Vogt, European Journal of Inorganic Chemistry 20 (2005) 4011-4021. Bronstein, L.M., in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, APS, Stevenson Ranch, CA, 2004, pp. 193-206. Bronstein, L.M., in: J.A. Schwarz, C.I. Contescu, K. Putyera (Eds.), Dekker Encyclopedia of Nanoscience and Nanotechnology, Marcel Dekker, Inc., New York, 2004, pp. 2903-2916. Bates, F.S., G.H. Fredrickson, Phys. Today 52 (1999) 32. Foerster, S., M. Antonietti, Adv. Mater. 10 (1998) 195-217. Mayer, A.B.R., J.E. Mark, Colloid Polym. Sci. 275 (1997) 333-340. Seregina, M.V., L.M. Bronstein, O.A. Platonova, D.M. Chernyshov, P.M. Valetsky, J. Hartmann, E. Wenz, M. Antonietti, Chem. Mater. 9 (1997) 923-931. Klingelhoefer, S., W. Heitz, A. Greiner, S. Oestreich, S. Förster, M. Antonietti, J. Am. Chem. Soc. 119 (1997) 10116. Lindlar, H., Helv. Chim. Acta 35 (1952) 446. Sulman, E.M., Russ. Chem. Rev. 63 (1994) 923-936. Bronstein, L.M., D.M. Chernyshov, I.O. Volkov, M.G. Ezernitskaya, P.M. Valetsky, V.G. Matveeva, E.M. Sulman, J. Catal. 196 (2000) 302-314. Semagina, N.V., A.V. Bykov, E.M. Sulman, V.G. Matveeva, S.N. Sidorov, L.V. Dubrovina, P.M. Valetsky, O.I. Kiselyova, A.R. Khokhlov, B. Stein, L.M. Bronstein, J. of Mol.Catal. A 208 (2004) 273-284. Sulman, E.M., Russ. Chem. Rev. 63 (1994) 923-936. Bronstein, L., G. Goerigk, M. Kostylev, M. Pink, I.A. Khotina, P.M. Valetsky, V.G. Matveeva, E.M. Sulman, M.G. Sulman, A.V. Bykov, N.V. Lakina, R.J. Spontak, J. Phys. Chem. B 108 (2004) 18234-18242. Sulman, E., V. Doluda, S. Dzwigaj, E. Marceau, L. Kustov, O. Tkachenko, A. Bykov, V. Matveeva, M. Sulman, N. Lakina, Journal of Molecular Catalysis A: 278 (2007) 112–119.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction Chen Jina, Wei Xiaa, Junsong Guoab, Tharamani Chikka Nagaiahb, Michael Bronb, Wolfgang Schuhmannb, Martin Muhlera* a

Laboratory of Industrial Chemistry, Ruhr-University Bochum, D-44780 Bochum Elektroanalytik & Sensorik, Ruhr-Universität Bochum, D-44780 Bochum * Fax: 0049-234-32-14115 Email: [email protected]

b

Abstract Carbon nanotube (CNT) supported sulfided Rh catalysts were prepared applying three different routes: deposition-precipitation (DP), grafting of colloidal Rh nanoparticles, and polythiophene-assisted synthesis. The catalysts (1.4-1.8 wt%) prepared by DP were synthesized on CNTs from RhCl3 using hydrogen peroxide and subsequent exposure to on-line generated H2S followed by heat treatment. The Rh particles were found to be highly dispersed on the CNT surface. Alternatively, RhSx/Rh nanoparticles with four different loadings (4.3-21.9 wt%) grafted on carbon nanotubes were prepared through a functionalization of CNTs with short chain thiols and subsequent binding of colloidal Rh nanoparticles onto the thiolated CNTs. All steps of the synthesis were monitored by XPS. Finally, polythiophene/CNT composites were prepared and employed in the preparation of Rh17S15/Rh nanoparticles supported on CNTs. The CNTs with the highest polythiophene loading yielded the highest amount of Rh17S15 after Rh deposition and thermal treatment. The activity and stability of the prepared catalysts were studied towards the oxygen reduction reaction. Keywords: carbon nanotubes, RhSx/Rh, oxygen reduction reaction, hydrochloric acid electrolysis

1. Introduction In chlorine industry, the recovery of chlorine from electrolysis of aqueous HCl can be achieved by replacing the traditional hydrogen-evolving cathode by an oxygenconsuming cathode, a so called “oxygen-depolarized cathode” (ODC). The evolution of H2 at the cathode involves high energy consumption and safety problems, while the ODC process makes a lowering of the cell voltage by as much as 1 V possible, corresponding to a theoretical energy saving of ca. 700 kWh per ton of Cl2 (g) [1]. The oxygen reduction reaction (ORR) remains a major challenge in basic as well as applied electrochemistry. Platinum is reported to be the most active catalyst for the ORR in acidic media either for proton exchange membrane fuel cells or for electrolysers [2]. However, under the highly corrosive conditions of HCl electrolysis, Pt has serious durability problems. Adsorbed Cl− leads to the blockage of active sites, and even dissolution of Pt may occur. The voltage shift during an uncontrolled cell shut-down results in a significant loss of Pt [3], which poses significantly operating risks. Because of the intrinsic stability problems of Pt and Pt-based alloys in HCl electrolysis, tremendous efforts have been made to develop alternative catalysts. It was recently reported that transition metal chalcogenides of the general type RhSx exhibit a comparable catalytic activity and better stability as compared to Pt under the same

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corrosive conditions as mentioned above [4]. In spite of the fact that the modification of Rh-based catalysts with sulfur can reduce their ORR activity, for example, in aqueous H2SO4 [6], the Cl− contamination in the HCl electrolyte has a more negative effect than S on metallic Rh as site blocking agents [5]. Meanwhile, RhSx-type catalysts are not severely depolarized by the presence of Cl− and remain uninfluenced by a possible uncontrolled shut-down of the electrolysis cell. Therefore, RhSx-type catalysts are considered the most promising systems for ODC electrolysis of HCl. Different methods have been reported for the synthesis of transition metal chalcogenide catalysts of the RhSx type, including reaction of metals [5] and carbonyl precursors [6] with sulfur or selenium in different solvents, deposition-precipitation from solution with H2S [7], high temperature treatment with gas H2S [8] and post treatment with selenic acid [9]. However, the preparation of RhSx with well-defined properties still remains as a challenge. Furthermore, Rh is one of the most expensive noble metals. Consequently, optimizing the dispersion and loading is of vital importance. In this contribution, sulfided Rh catalysts supported on CNTs were prepared by different methods including deposition-precipitation (DP), grafting, and polymerassisted synthesis. The aim is to improve the dispersion of the active phase on CNTs, and enhance the catalytic activity and stability of the catalysts under highly corrosive conditions.

2. Experimental Multi-walled CNTs with inner diameters of 20-50 nm and outer diameters of 70-200 nm were obtained from Applied Sciences Inc. (Ohio, USA). Chemicals used in the syntheses include rhodium chloride (98%, Aldrich), hydrogen peroxide (30%, J. T. Baker), sulfur (99.5%, Riedel-de Haёn), MoS2 (99%, Aldrich), thionyl chloride (99%; Aldrich), 4-aminothiophenol (97%, Aldrich), sodium borohydride (98%, powder, ACROS), sodium citrate dehydrate (99% J. T. Baker), ferrocene (Merck, 98%), thiophene (Fluka, 98%), hydrochloric acid (J. T. Baker, 37-38%), nitric acid (65%, J. T. Baker) and toluene (J. T. Baker). Gases used in the synthesis include helium (99.9999%) and hydrogen (99.9999%)

2.1. Preparation of RhSx by deposition-precipitation (?) employing H2O2 and post-treatment with on-line generated H2S

The as-received CNTs were first treated at 800°C in helium to remove surface polyaromatics. The CNTs were subsequently treated with concentrated HNO3 at 120°C for 90 min to introduce oxygen-containing functional groups and to remove the residual Fe growth catalysts. For the synthesis of the RhSx catalysts, 100 mg of HNO3-treated CNTs were added to 10 ml of aqueous solution containing 87.2 mg of RhCl3 and the mixture was sonicated for 120 min. 20 mL of hydrogen peroxide were subsequently added dropwise to the suspension at a rate of 0.2 ml min-1. The resulting mixture was refluxed for 15 h at 75°C, filtrated after cooling, washed with deionized water, and dried at 60°C overnight. A parallel experiment was performed with Vulcan XC 72 carbon (Cabot) as support. The obtained CNT-supported rhodium oxide was treated at 200°C with H2S in a vertical quartz tube reactor (inner diameter 20 mm). H2S was generated online by passing a flow of H2 in He (20 sccm H2 in total 100 sccm) through 60 mg of molten sulfur and then through a MoS2 catalyst bed (50 mg of MoS2 in 500 mg of quartz powder) at 400°C. After the H2S treatment, the samples were treated in helium for 120 min at 400°C, 650°C, and 900°C.

Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction 163

2.2. Preparation of Rh-RhSx nanoparticles grafted on carbon nanotubes 2.2.1. Preparation of Rh colloid 10 mL of a sodium citrate solution (0.25 mM) were first added to 90 mL of aqueous solutions of RhCl3 with the following concentrations: 0.015 mM, 0.0324 mM, 0.0729 mM and 0.125 mM. The mixtures were stirred for 3 min. Concurrently, 100 mL of 0.05 M NaBH4 solution were prepared by adding NaBH4 to 100 mL of ice-cold sodium citrate solution (0.025 M). A volume of 3 mL of the obtained solution was added dropwise to each RhCl3/sodium citrate solution and the mixtures were stirred for 10 min to obtain a dark colloid suspension. 2.2.2. Thiolation of CNTs The thiolation of the HNO3-treated CNTs was achieved by reaction with SOCl2 to form acyl chloride-functionalized CNTs, and then with 4-aminothiophenol (NH2C6H4SH). The detailed procedure is described elsewhere [10]. 2.2.3. Grafting of the Rh colloid onto the thiolated CNTs 30 mg of thiolated CNTs were added to the colloidal suspensions with different amounts of rhodium colloids. The mixtures were sonicated for 1 min, and then stirred for 30 min at room temperature. The suspensions were filtered, washed with deionized water, and dried at 60°C.

2.3. Polythiophene-assisted preparation of RhSx on CNTs 2.3.1. Deposition of iron oxide on CNTs Iron oxide was deposited on HNO3-treated CNTs by chemical vapor deposition (CVD) under oxidizing conditions using ferrocene as precursor [11]. The CVD was performed in a fixed-bed reactor to achieve a theoretical Fe loading of 10 wt%. The sample was reduced at 400°C for 2 h with a gas mixture of He (50 sccm) and H2 (50 sccm). 2.3.2. Coating of polythiophene on CNTs (polythiophene/CNT) A thin layer of iron oxides forms after exposing the reduced metallic Fe nanoparticles to air. These oxides, together with Fe, can function as oxidative initiator for the polymerization of thiophene. Different amounts of polythiophene on CNT were obtained by exposing the iron/iron oxide-coated CNTs to a vapor of HCl and thiophene, which was obtained by passing He through HCl (30 sccm He) and thiophene (20 sccm He) solution. The reaction was carried out for 5, 30, and 90 min. 2.3.3. Impregnation of polythiophene/CNTs with rhodium and subsequent thermal treatment (Rh-S/CNT). The three different polythiophene/CNT samples where impregnated with Rh employing an aqueous solution of RhCl3 to yield a theoretical Rh-loading of 30 wt%. Water was evaporated at 100 mbar and 60°C for 1 h with a BÜCHI Rotavapor R-114. The samples were subsequently dried at 50 mbar and 60°C for 2 h. The dried samples were treated at 650°C for 2 h in He. The catalyst samples obtained from the CNTs coated with polythiophene for 5, 30 and 90 min are labeled Rh-S/CNT-5, Rh-S/CNT-30 and Rh-S/CNT-90.

2.4. Characterization The morphology of the catalysts was studied by scanning electron microscopy (LEO Gemini 1530). Transmission electron microscopy (TEM) measurements were carried out with a Hitachi-H-8100 instrument. The mean particle size was obtained by analyzing ca. 100-220 particles with the iTEM software. Elemental analysis was performed using an Elementar Vario III atomic absorption spectrometer (AAS) by dissolving samples with aqua regia and H2SO4 at 300°C. X-ray diffraction (XRD) was carried out using a Philips

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X-Pert MPD system with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ultra-high vacuum set-up equipped with a Gammadata-Scienta SES 2002 analyzer. A flood gun was used to compensate for the charging effects. The binding energies were calibrated with the C 1s peak (284.5 eV). Thermogravimetry was performed with a Cahn TG-2131 thermobalance in pure O2 with a heating rate of 2 K/min.

2.5. Electrocatalytic ORR activity and stability test The ORR activity of the RhSx/CNT catalysts was characterized by rotating disc electrode (RDE) measurements at a scan rate of 5 mV s-1 in 0.4 M HCl at rotation rates of 100, 400 and 900 rpm. A single compartment electrochemical cell was used equipped with a Ag/AgCl/3 M KCl as reference electrode (RE), a Pt foil as counter electrode (CE), and a catalyst-coated glassy carbon electrode (GCE) as working electrode. Electrocatalytic stability tests were conducted in a flow cell applying a pulsed interruption mode with an operation time of 10 min at 0.3 V vs. Ag/AgCl/3 M KCl and an interruption time of 2 min, which is intended to simulate conditions occurring during cell shutdown in industrial HCl electrolysis. The currents were recorded, and the performance of the catalysts before and after a 24 h test period was studied by linear sweep voltammetry (LSV) at a scan rate of 5 mVs-1 from 0.6 V to –0.2 V. Further experimental details are described elsewhere [12].

3. Results and discussion The first experimental approach to prepare RhSx catalysts supported on CNTs consists in the deposition precipitation of RhCl3 using hydrogen peroxide and subsequent exposure to on-line generated H2S [13]. Vulcan XC72 carbon black was also employed as the support during the same preparation procedure, but a weight loss of more than 50 % was recorded after refluxing with H2O2 for 15 h. Thus, further experiments have not been carried out with this material. In the case of CNTs, the weight loss during refluxing was negligible. Rh oxide immobilized on CNTs was obtained after this DP process. A subsequent treatment with H2S at 200°C was carried out to initiate the formation of RhSx nanoparticles. The H2S was prepared online from S and H2 at 400°C catalyzed by MoS2. A continuous H2S supply monitored by online mass spectrometry of about 70 min was achieved using 60 mg S. The resulting sample was further treated at 400°C, 650°C, and 900°C in He.

Fig. 1 TEM images of CNT-supported sulfided rhodium catalysts treated in helium at (a) 400°C, (b) 650°C, and (c) 900°C.

Figure 1 displays TEM images of catalysts prepared via DP with H2O2, H2S treatment and subsequent heat treatment, as described in the experimental part. The average diameters of the samples treated at 400°C, 650°C, and 900°C are 1.0 ± 0.5 and 2.5 ± 1.0 nm, and 8.7 ± 4.7 nm, respectively. The Rh loadings determined by AAS were 1.4 wt% (400°C), 1.5 wt% (650°C), and 1.9 wt% (900°C). The slight increase of the

Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction 165 loading can be due to the weight loss during the heat treatment from the decomposition of oxygen-containing functional groups on the CNT surfaces. The amount of anchoring sites (surface functionalities) and surface defects is crucial to achieve thermally stable, small particles at high metal loadings. On the specific type of CNTs employed by us, a significant improvement of loading based on the present H2O2 method can hardly be achieved without increasing the particle size. The XRD patterns of the samples heat treated at 400°C and 650°C (not shown) displayed only the characteristic peaks of hexagonal graphite from the CNTs. No contribution from Rh or any RhSx phase was detected, in agreement with the small particle size observed with TEM. The peaks corresponding to the fcc metallic Rh appeared after treatment at 900°C. Rh sulfide was not detected by XRD in all three samples. However, peaks located at 162 eV in XPS 2p spectra (not shown) confirm the presence of RhSx in the samples treated at 400°C and 650°C. The activity for electrocatalytic oxygen reduction has been evaluated with RDE measurements in 0.4 M HCl at room temperature (not shown). The activities of the investigated catalyst samples were found to be 650°C > 400°C > 900°C > Rh oxide. As it could be expected, the post-treatment with H2S significantly enhances the activity of the catalyst suggesting the formation of a RhSx surface layer. Among them, the sample treated at 650°C shows the best performance, which can be tentatively assigned to the formation of a stable RhSx layer.

Fig. 2 Linear sweep voltammograms of sulfided Rh catalysts before and after 24 h stability test in an O2 saturated flow cell at a scan rate of 5 mVs-1 from 0.6 V to –0.2 V at room temperature. 0.4 M HCl was used as electrolyte at a flow rate of 0.33 ml min-1.

The stability of the RhSx catalyst supported on CNTs under conditions relevant to industrial electrolysis was studied in a flow cell. Linear sweep voltammograms (LSV) of the samples recorded before and after operation for 24 h (including periodic interruptions as describe in the experimental part) are shown in Fig. 2. The performance before and after 24 h of operation shows the same activity sequence as the RDE results. The half wave potentials of the oxygen reduction shifted from 0.041 V in Rh-400, to 0.028 V in Rh-650, and to 0.019 V in Rh-900. The potential shift decreases with increasing treatment temperature which is attributed to the increasing particle size. In summary, synthesis of RhSx on CNTs via H2O2 precipitation and post treatment with H2S gives highly dispersed low loading catalysts. The catalyst sample with ca. 9 nm nanoparticles shows the best stability. In order to enhance the RhSx loading and stability, RhSx/Rh nanoparticles grafted on carbon nanotubes were prepared through the functionalization of CNTs with short chain thiols and subsequent binding of colloidal Rh nanoparticles onto the thiolated CNTs [14]. The carboxyl group is one of the major groups generated on CNTs by HNO3 treatment [14]. In the first step of the catalyst preparation, these carboxyl groups react

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with thionyl chloride to generate acyl chloride groups (Eqn. (1)). The thiolation of the CNTs was achieved by the reaction of the acyl chloride-functionalized CNTs with 4-aminothiophenol. The short spacer groups between the CNTs and the –SH groups were supposed to enhance the stability of the grafted RhSx/Rh nanoparticles while maintaining the electron transfer between the CNT support and the grafted colloidal Rh during the electrocatalysis. Colloidal rhodium nanoparticles were prepared from RhCl3 as a metal precursor, sodium citrate as a stabilizer, and NaBH4 as a reducing agent. The subsequent grafting of colloidal Rh onto the thiolated CNTs was achieved by adding the thiolated CNTs to four different concentrations of colloidal suspensions. The adsorption process was complete as indicated by a colorless filtrate. The Rh loadings determined by AAS were 4.3, 6.4, 16.1 and 21.9 wt%, corresponding to theoretical loadings of 5, 10, 20 and 30 wt%, respectively. The loss of Rh can be due to incomplete anchoring. (Eqn. 1.) All described steps of the modifications of the CNTs and grafting of colloidal Rh were monitored by XPS as shown in Fig. 3. In the XPS S 2p spectra, the three peaks located at binding energies of 168.6 eV, 163.7 eV, and 162.5 eV correspond to S=O, S−H, and Sδ- species, respectively (Fig. 3a) [15]. S=O species as indicated by the peak at 168.6 eV originate from the thionyl chloride treatment of the HNO3-treated CNTs in Fig. 3a. This contribution in trace (1) and (2) decreases upon grafting with the rhodium colloid in traces 3-6. The thiolation of the CNTs upon modification of the acyl chlorideterminated CNTs with 4-aminothiophenol is indicated by a strong increase of the peak at 163.7 eV, which is characteristic of thiol groups [16]. The signal in traces 3-6 at 162.5 eV appears after grafting with colloidal Rh, demonstrating the presence of Rh-S species [17].

Fig. 3 XPS S 2p (a) and N 1s (b) spectra of acyl chloride-functionalized CNTs (1), thiolated CNTs (2), and CNT-supported rhodium sulfide catalysts with Rh loadings of 4.3% (3), 6.4% (4), 16.1% (5), 21.9% (6).

The HNO3 treatment can form NOx species on the CNT surface, and the interaction between the NOx and SOCl2 can possibly account for the N 1s peak at 401.6 eV in Fig. 3b. The thiolation of the acyl chloride-terminated CNTs with 4-aminothiophenol (trace (2) in Fig. 3b) gave rise to the nitrogen peak typical for amides at 399.4 eV [18]. A shift

Carbon nanotube-supported sulfided Rh catalysts for the oxygen reduction reaction 167 of the 399.4 eV peak to lower binding energies is not observed after grafting with rhodium colloid (Fig. 3b, traces 3-6), thus excluding the formation of rhodium nitride. The TEM particle sizes (rtem) obtained for the different samples are quite similar (ca. 7 nm). No increase of the particle size was found in the highly loaded samples. The XRD particle sizes (rxrd) estimated from the Scherrer’s equation for all the samples were about 3 nm. The larger rtem than rxrd could be due to agglomeration and a sulfide/oxide layer outside a metallic Rh core. Based on the XRD and XPS results, the structure of the nanoparticles is assumed to consist of a metallic Rh core covered by a sulfide layer. RDE measurements disclosed that the 16.1% Rh catalyst was more active than the 4.3% and 6.4% samples. However, an even higher loading of 21.9% Rh did not lead to a further increase in performance presumably due to the lowered accessibility of the active sites at the aggregated RhSx/Rh nanoparticles. The stability for ORR of the 16.1% Rh catalyst was investigated under the same conditions as the sulfided Rh catalysts prepared by DP followed by H2S treatment. The 16.1% Rh catalyst showed a smaller half wave potential shift (0.012 V) than the Rh-900 sample (0.019 V) after a stability test for 24 h (Fig. 4) while with a more positive onset potential than the Rh-900. The results indicate that the grafted sample is more stable than the Rh-900 (rtem = 8.7 nm, rxrd = 8.0 nm) even with a much smaller particle size (rtem = 7nm, rxrd = 3 nm). Hence, we can conclude that grafting enhances the stability of the Rh catalysts.

Fig. 4 Linear sweep voltammograms of 16.1% Rh-S/CNTs and Rh-900 before and after 24 h stability test in an O2-saturated flow cell at a scan rate of 5 mVs-1 from 0.6 V to –0.2 V at room temperature. 0.4 M HCl was used as electrolyte at a flow rate of 0.33 ml min-1.

The sulfide species, although proven by XPS, were not detected by XRD in the above-mentioned samples. The following section focuses on enhancing both the amount and the dispersion of Rh sulfides on CNTs using polythiophene as sulfur source [19]. First, CNTs were coated by polythiophene synthesized by iron-catalyzed gas-phase polymerization. The iron catalysts were deposited on the CNTs by chemical vapor decomposition. Samples with three different polymerization times (5, 30, and 90 min) were prepared and the composition and loading of the polythiophene coating were analyzed by XPS and thermogravimetry (TGA). Increasing the polymerization time to 90 min led to the formation of a network of CNTs with pyrolysed polythiophene as linker. CNT-supported rhodium catalysts were obtained by impregnation of the polythiophene-coated CNT substrate with rhodium chloride, and subsequent thermal treatment in helium at 650°C. It was found that the degree of dispersion, the loading of the catalyst particles and the amount of Rh17S15 (determined by XRD) were enhanced by increasing the amount of polythiophene. A higher amount of polythiophene also led to a

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lower charge transfer resistance of the obtained catalysts, as evidenced by electrochemical impedance spectroscopy.

4. Conclusions Highly dispersed sulfided Rh catalysts supported on CNTs were prepared through deposition-precipitation using H2O2 resulting in low loadings. The sample treated at 650°C showed the highest activity, while treatment at 900°C led to the highest electrochemical stability. Grafting enhanced the loading and the stability. Finally, polythiophene-assisted syntheses were used to prepare Rh17S15/Rh catalysts supported on CNTs, and the dispersion, loading, and the amount of Rh17S15 increased with increasing amount of polythiophene.

Acknowledgements Chen Jin thanks the International Max Planck Research School Surface and Interface Engineering in Advanced Materials (SurMat) for a research grant. Dr. Tharamani Chikka Nagaiah is grateful to the Alexander von Humboldt Foundation for a postdoctoral fellowship.

References [1] F. Federico, G.N. Martelli, D. Pinter, in: J. Moorhouse (Eds.), Modern chlorine-alkali technology, Wiley, 2001. [2] J.M. Ziegelbauer, A.F. Gulla, C. O’Laoire, C. Urgeghe, R.J. Allen and S. Mukerjee, Electrochim. Acta, 52 (2007) 6282. [3] T.J. Schmidt, U.A. Paulus, J.A. Gasteiger and R.J. Behm, J. Electroanal. Chem., 508 (2001) 41. [4] A.F. Gulla, L. Gancs, R.J. Allen and S. Mukerjee, Appl. Catal. A:General, 326 (2007) 227. [5] D. Cao, A. Wieckowski, J. Inukai and N. Alonso-Vante, J. Electrochem. Soc., 153 (2006) A869. [6] M. Bron, P. Bogdanoff, S. Fiechter, M. Hilgendorff, J. Radnik, I. Dorbandt, H.Schulenburg and H. Tributsch, J. Electroanal. Chem., 517 (2001) 85. [7] A.V. Mashkina and T.S. Sukhareva, React. Kinet. Catal. Lett., 67 (1999) 103. [8] M. Lacroix, N. Boutarfa, C. Guillard, M. Vrinat and M. Breysse, J. Catal., 120 (1989) 473. [9] H. Schulenburg, M. Hilgendorff, J. Radnik, I. Dorbandt, P. Bogdanoff, S. Fiechter, M. Bron and H. Tributsch, J. Power Sources, 155 (2006) 47. [10] Y. Kim and T. Mitani, J. Catal., 238 (2006) 394. [11] W. Xia, D. Su, A. Birkner, L. Ruppel, Y. Wang, C. Wöll, J. Qian, C. Liang, G. Marginean, W. Brandl and M. Muhler, Chem. Mater., 17 (2005) 5737. [12] C. Jin, W. Xia, T.C. Nagaiah, J. Guo, X. Chen, N. Li, M. Bron, W. Schuhmann and M. Muhler, J. Mater. Chem., 2009, doi: 10.1039/b916192a. [13] C. Jin, W. Xia, T.C. Nagaiah, J. Guo, X. Chen, M. Bron, W. Schuhmann and M. Muhler, Electrochim. Acta, 54 (2009) 7186. [14] S. Kundu, Y. Wang, W. Xia and M. Muhler, J. Phys. Chem. C, 112 (2008) 16869. [15] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data, J. Chastain (eds.), Perkin-Elmer corporation, Minnesota, 1992. [16] N. Kocharova, T. Aaritalo, J. Leiro, J. Kankare and J. Lukkari, Langmuir, 23 (2007) 3363. [17] D. Grumelli, C. Vercat, G. Benitez, M. E. Vela and R. C. Salvarezza, J. Phys. Chem. C, 111 (2007) 7179. [18] D.N. Hendrickson, J.M. Hollander and W.L. Jolly, Inorg. Chem., 8 (1969) 2642. [19] C. Jin, T.C. Nagaiah, W. Xia, M. Bron, W. Schuhmann and M. Muhler, ChemCatChem, submitted.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis and characterization of highly loaded Pt/carbon xerogel catalysts prepared by the Strong Electrostatic Adsorption method Nathalie Job,a Frédéric Maillard,b Marian Chatenet,b Cédric J. Gommes,a Stéphanie Lambert,a Sophie Hermans,c John R. Regalbuto,d Jean-Paul Pirarda a

Laboratoire de Génie Chimique, Université de Liège, B6a, B-4000 Liège, Belgium LEPMI, UMR 5631 CNRS/Grenoble-INP/UJF, BP75, F-38402 St Martin d’Hères Cedex, France c Unité de Chimie des Matériaux Inorganiques et Organiques, Université Catholique de Louvain, Place Louis Pasteur 1/3, B-1348 Louvain-la-Neuve, Belgium d Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton Street, Chicago, IL 60607, USA b

Abstract In order to decrease the mass transport limitations reported in classical PEMFC electrodes, Pt/carbon xerogel catalysts have great potential to replace Pt/carbon black catalysts. These nanostructured materials with well defined pore texture allow for better gas/water diffusion and better contact between the platinum particles and the ionomer (Nafion®). Pt/carbon xerogel catalysts with high metal content (~ 25 wt.%) and high metal dispersion (nanoparticles ca. 2 nm in size) were prepared via the ‘Strong Electrostatic Adsorption’ method; the impregnation-drying-reduction step with H2PtCl6 was repeated until the desired metal loading was achieved. However, both physicochemical and electrochemical characterization show that the use of H2PtCl6 leads to Pt catalysts poisoned with chlorine, especially if the reduction temperature is lower than 450°C. This induces a dramatic decrease of the Pt utilization ratio in the final PEMFC catalytic layer. Keywords: carbon xerogels, Pt/C catalysts, PEM fuel cells, electrochemistry

1. Introduction Pt/C catalysts with high metal weight percentage are classically used in Proton Exchange Membrane Fuel Cells (PEMFCs) [1]. Indeed, minimization of ohmic and transport losses within the electrode, and compensation for both the sluggish oxygen reduction reaction rate and the usual lack of contact between Pt and the ionomer result from a compromise: (i) the thickness of the catalytic layer must be low, and (ii) the metal loading of the electrode must be high. In order to decrease the mass transport limitations encountered in PEMFC electrodes, which are prepared with Pt/carbon black catalysts, it was recently proposed to replace the classical carbon black support by carbon xerogels [2], i.e. nanostructured materials with well defined pore texture prepared by evaporative drying and pyrolysis of organic gels. Carbon xerogels allow for better gas/water diffusion within the pore texture of the electrode and better contact between the platinum particles and the ionomer (Nafion®).

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In this study, a carbon xerogel with selected pore size in the macropore range was used as Pt catalyst support. The catalyst was designed for use as oxygen reduction catalyst at the cathode of a PEM fuel cell. So as to obtain Pt/carbon xerogel catalysts with high metal content, the ‘Strong Electrostatic Adsorption’ (SEA) method [3, 4] was applied; H2PtCl6 aqueous solution was used as metal precursor. This method consists of maximizing the electrostatic interactions between the metal precursor and the support by adjusting the pH of the slurry. The latter depends on the surface chemistry of the carbon and on the chemical nature of the metal precursor. The goal was to obtain highly loaded Pt/carbon xerogel catalysts while preserving high metal dispersion: indeed, following the literature, the optimal Pt particle size for efficient oxygen reduction is ca. 2-3 nm [5]. The obtained catalyst was reduced under flowing hydrogen at various temperature conditions and characterized using both physico-chemical (TEM, XPS, CO chemisorption) and electrochemical (CO stripping voltammograms, ORR measurements) techniques. The samples were used as Pt/C catalyst at the cathode side of an air/H2 PEM fuel cell. The goal of the study was to demonstrate the effect of Pt chlorine poisoning, originating from the decomposition of H2PtCl6, and the importance of the catalyst reduction treatment on the cell performance. Finally, physico-chemical and electrochemical characterization allowed us to study the effect of poisoning on the metal availability and on the oxygen reduction kinetics.

2. Experimental 2.1. Catalyst preparation The carbon support chosen for this study was a micro-macroporous carbon xerogel with a macropore size ranging from 50 to 85 nm, a specific surface area of 640 m² g-1 and a total pore volume of about 2.1 cm³ g-1 (including 0.26 cm³ g-1 of micropores, i.e. pores smaller than 2 nm). Carbon xerogels are materials composed of interconnected spherical-like microporous nodules. So, they classically display a bimodal pore size distribution: micropores inside the carbon nodules, and larger pores identified as the voids located between the nodules [6]. The size of the nodules and that of the voids inbetween is controlled by the synthesis conditions of the material [6, 7]. The carbon support was synthesized by the drying and pyrolysis of a resorcinolformaldehyde aqueous gel, following a procedure developed in a previous study [8]: the R/C (resorcinol/sodium carbonate) molar ratio of the gel precursor solution was chosen equal to 1000, while all other synthesis variables (from gel preparation to pyrolysis conditions) were kept identical as in the above-mentioned study. In brief, resorcinol and sodium carbonate were solubilized in water and then formaldehyde was added. Gelation and ageing were performed at 85°C (72 h), and were followed by evaporative drying (60-150°C, 1 day) and pyrolysis (800°C, 2 h) under nitrogen flow. Pt/carbon xerogel catalysts were obtained via the ‘Strong Electrostatic adsorption’ (SEA) method. This consists of maximizing the electrostatic interactions between the metal precursor and the support by adjusting the pH of the slurry to the adequate value, which depends on the surface chemistry of the carbon and on the precursor chosen. Indeed, interaction between the support and the metal precursor depends on both the precursor nature (anion or cation, size, etc.) and on the carbon surface chemistry. At pH values lower than the Point of Zero Charge (PZC), i.e. the pH at which the surface is neutral in terms of charge, the surface is positively charged, and the adsorption of anions is favoured (Fig. 1a). At pH higher than the PZC, the surface is negatively charged, and the adsorption of cations is enhanced. The PZC of carbon xerogels after

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pyrolysis is about 9.5 [4], which leaves a large pH range for the adsorption of both Pt anions and cations. However, the Pt uptake is limited by steric effects: indeed, in the case of impregnation with H2PtCl6, for instance, the maximum surface density can be calculated as a close packed arrangement of chloroplatinic acid complexes which retain one hydration sheath [3]. As a result, the Pt uptake vs. final pH of the impregnation solution at equilibrium passes through a maximum (Fig. 1b): indeed, in the case of the impregnation of carbon xerogels with H2PtCl6 aqueous solutions (1000 ppmPt), the initial pH leading to the highest Pt uptake was found to be 2.5 (final pH at equilibrium ~ 3.0), and the corresponding maximum Pt surface density was found equal to 0.8-0.9 µmol m-2, which corresponds to a weight percentage ranging from 8 to 10 wt.% [4].

pH < PZC

OH2

Kads,an +

Adsorption of anions e.g. [Pt(Cl6)]2-

K2

PZC

OH

H+ (pH shift) « Point of Zero Charge »

K1

pH > PZC

O-

Kads,cat

Adsorption of cations e.g. [Pt(NH3)]2+

Pt surface density (µmol m-2)

1

(a)

(b)

0.75

0.5

0.25

0 0

4

8

12

Final pH

Fig. 1. (a) Principles of the Strong Electrostatic Adsorption (SEA) method: depending on the PZC of the support and on the impregnation pH, the adsorption of ositively or negatively charged species is favoured; (b) due to steric effects, the Pt uptake is limited and the Pt surface density vs. pH curve presents a maximum (results from [4]).

The procedure used for preparing Pt/carbon xerogels via the SEA method is fully described in reference [9]. Briefly, the carbon support was contacted with the impregnation solution at the optimal pH value of 2.5 until the equilibrium was reached. The impregnated catalyst was then filtered, dried and reduced under flowing hydrogen. In order to increase the Pt weight percentage up to acceptable values for electrochemical applications, this impregnation-drying-reduction cycle was repeated up to three times using the same original catalyst batch. Note also that various final reduction temperatures (200, 350 and 450°C) were tested to evaluate the effect of Pt poisoning by chlorine.

2.2. Catalyst characterization 2.2.1. Physico-chemical characterization Catalysts were characterized using several complementary techniques. The Pt content was evaluated by ICP-AES after elimination of the carbon and solubilisation of the metal [10]. In order to measure the size of the metal particles, the catalysts were investigated by transmission electron microscopy, with a Jeol 2010 (200 kV) device (LaB6 filament). The samples were crushed and dispersed in ethanol and subsequently deposited onto a copper grid. Particle size distributions were obtained by image analysis performed on a set of at least 1000 particles: the procedure is described in [9]. The samples were analyzed by X-ray photoelectron spectroscopy (XPS), performed on an SSI-X-probe (SSX-100/206) spectrometer from Fisons. The samples were stuck onto troughs with double-sided adhesive tape then placed on an insulating home-made ceramic carrousel with a nickel grid 3 mm above the samples, to avoid differential

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charging effects. A floodgun set at 8 eV was used for charge stabilisation. The energy scale was calibrated by taking the Au 4f7/2 binding energy at 84 eV. The C1s binding energy of contamination carbon set at 284.8 eV was used as internal standard value. Data treatment was performed with the CasaXPS program (Casa Software Ltd). Finally, CO chemisorption was used to determine the accessible Pt surface, SPt-Chem. Isotherms were measured with a Fisons Sorptomatic 1990 equipped with a turbomolecular vacuum pump that allows vacuum of 10-3 Pa. The entire procedure, from the sample preparation to the adsorption measurement, is fully described elsewhere [10]. Briefly, a first CO adsorption isotherm was achieved so as to measure the total amount of adsorbed carbon monoxide (chemisorbed + physisorbed). The catalyst was then outgassed, and a second CO adsorption isotherm (physisorbed CO) was measured. The amount of CO forming the chemisorbed monolayer on surface Pt atoms, deduced by extrapolating the nearly horizontal difference curve to the uptake axis, was used to calculate SPt-Chem. 2.2.2. Electrochemical characterization All electrochemical measurements were carried out in sulphuric acid (1 M) at 25°C. The voltammetric experiments were performed using an Autolab-PGSTAT20 potentiostat with a three-electrode cell and a saturated calomel electrode (SCE) as a reference electrode (+0.245 V vs. normal hydrogen electrode, NHE). The catalyst sample was deposited on a rotating disk electrode (EDT 101, Tacussel), used as working electrode. All details, from sample preparation to experimental conditions, are extensively described in reference [9]. The electrochemically active Pt surface area of the catalysts, SPt-Strip, was determined by CO stripping. This electrochemical technique consists in the electrooxidation of a CO monolayer previously adsorbed on the Pt surface. It allows estimating the real Pt surface area, assuming that the electrooxidation of a COads monolayer requires 420μC per cm2 of Pt. Since CO stripping is performed in aqueous electrolyte, it implies 100% utilization of the Pt surface atoms and is influenced neither by contact problems between the metal and the electrolyte nor by mass-transport limitations. In addition, the electrooxidation of a COads monolayer is a structure-sensitive reaction and provides a wealth of information on the particle size distribution and the presence/absence of particle agglomeration [11, 12]. The CO stripping voltammograms were recorded at 0.02 V s-1 between +0.045 and +1.245 V vs. NHE, after saturation of the electrolyte by CO (6 min bubbling) and removal of the non-adsorbed CO from the cell by purging with Ar (39 min). The electrocatalytic activity for the ORR of the elaborated Pt/C nanoparticles was measured in O2-saturated liquid electrolyte. The quasi-steady-state voltammograms were recorded at 10-3 V s-1 from +1.095 to +0.245 V vs. NHE. To account for the reactants diffusion-convection in the liquid layer, the experiment was repeated at four RDE rotation speeds (42, 94, 168 and 262 rad s-1) [13]. 2.2.3. Fuel cell test The catalysts were tested as PEM fuel cell cathodic catalytic layers on a unit cell-test bench: 50 cm² Membrane-Electrode Assemblies (MEAs) were prepared by the decal method as described in reference [2]. The electrolyte was a Nafion® membrane, and the anode a commercial anode made from Pt-doped carbon black (40 wt.%, TKK) deposited by Paxitech onto a carbon felt (0.6 mgPt cm-2 mixed with Nafion®). The thickness of the cathode was kept constant by keeping constant the carbon mass in the catalytic layer. The Nafion®/carbon mass ratio of the ink used to prepare the MEAs was fixed at 0.5. After a standardized start-up procedure, polarization curves, i.e. the Ucell = f(jm) curves, were measured by setting the cell voltage at each desired value for 15 min, which

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assured the stabilization of the current. The current, jm, was normalized to the Pt metal loading of the cathode.

3. Results and discussion Figure 2 shows examples of TEM micrographs and the corresponding Pt particle size distributions obtained by image analysis of two Pt/carbon xerogel catalysts: the first one (Fig. 2a) was prepared by single impregnation of the support, and the second one (Fig. 2b) was obtained by three consecutive impregnation-drying-reduction cycles. Results show that repeating the impregnation with H2PtCl6 as Pt precursor yields an increase of the catalyst metal content up to 22.3 wt.% while keeping homogeneously dispersed Pt nanoparticles ca. 2 nm in size.

(a)

(b)

Fig. 2. TEM micrographs of Pt/carbon xerogel catalysts and particle size distributions obtained by image analysis. (a) Single impregnation, 7.5 wt.% Pt and (b) triple impregnation, 22.3 wt.%.

Table 1 regroups the characterization data of three catalysts, prepared from the same batch of double-impregnated sample (metal loading: 15.0 wt.%); the only difference is the temperature (200-450°C) and duration (1-5 h) of the reduction treatment, performed under flowing H2. The samples are denoted as follows: the letter ‘C’ is followed by the reduction temperature (in °C) and the reduction time (h). The average particle size, dTEM, obtained by image analysis, does not change when increasing the reduction temperature from 200°C (C-200-1) to 450°C (C-450-5). This agrees with a set of additional experiments performed under N2, which showed that Pt nanoparticles begin to sinter only at T ≥ 600°C. Interestingly, the Pt surface area detected by CO chemisorption, SCO-chem, increases from 92 (C-200-1) to 124 m² gPt-1 (C-450-5). In addition, chlorine is detected at the surface of every sample by XPS measurements. It seems thus that, when the reduction temperature is too low, the catalyst remains poisoned by chlorine coming from the metal precursor (H2PtCl6). The calculated Cl/Pt ratio decreases with increasing the reduction temperature, from 0.33 (C-200-1) to 0.07 (C-450-5): so, removing chlorine species completely from the catalyst proves difficult. One also notices that the CO chemisorption and XPS data do not match well. Indeed, the Cl/Pt ratio decreases by a factor 5 between C-200-1 and C-450-5 while the detected Pt surface shows an increase of 30% only; in addition, from sample C-350-3 to sample

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C-450-5, Cl/Pt is multiplied by 3 while SCO-chem remains almost constant (118 and 124 m² gPt-1, respectively). Table 1. Physico-chemical and electrochemical characterization of catalysts reduced under various conditions. Sample

C-200-1 C-350-3 C-450-5

TEM dTEM (nm) ± 0.2 1.8 1.8 1.8

CO chemisorption SCO-chem (m² gPt-1) ± 10% 92 118 124

XPS Cl/Pt (-) ± 10% 0.33 0.23 0.07

CO stripping SCO-strip (1) SCO-strip (7) (m² gPt-1) (m² gPt-1) ± 10% ± 10% 37 69 61 87 129 142

ORR b (V dec-1) ± 5% -0.074 -0.070 -0.066

SA90 (µA cm²Pt-1) ± 5% 10 11 12

CO stripping measurements allowed us to obtain complementary data about catalyst poisoning. Figure 3a shows the CO stripping voltammograms of two catalysts: C-200-1 and C-450-5, obtained by two different reduction treatments of the same initial batch (double impregnation, 15.0 wt.% metal). Globally, the surface of the peak detected between 0.7 and 1.0 V vs. ENH represents the current exchanged to electrooxidize the CO monolayer adsorbed on the Pt surface, and can be regarded as the electrochemically accessible Pt surface [9]. The curves in grey correspond to the voltammograms obtained on the fresh catalysts while the black curves are those measured after 7 consecutive CO strippings. One can observe that (i) the charge under the peak increases with the reduction temperature, (ii) the charge under the peak increases after several consecutive CO strippings, whatever the reduction temperature, and (iii) the peak is shifted toward lower potentials when the reduction temperature increases or when the number of consecutive CO strippings increases. These observations are in agreement with the hypothesis of catalyst poisoning. First, as already observed by CO chemisorption, the accessible Pt surface measured on the fresh catalyst increases with the reduction temperature: SCO-strip (1), increases from 37 (C-200-1) to 129 m² gPt-1 (C-450-5). Note that, contrary to CO chemisorption measurements, CO stripping data match perfectly the Cl/Pt ratios obtained by XPS: indeed, the relationship between Cl/Pt and SCO-strip (1) is close to linearity (not shown). Second, the detected surface increases after several CO strippings: this is due to the fact that chlorine species are progressively removed by repetitive adsorption/desorption processes. Indeed, Holscher and Sachtler [14] showed that CO is one of the strongest poisons adsorbed onto platinum: in the presence of CO, poisons originally adsorbed onto the Pt particle surface should be displaced. However, the kinetics of displacement may be too slow to be completed within a few minutes: this would explain why CO chemisorption in gaseous phase, during which equilibrium is reached prior to any further gas injection, leads to larger Pt surfaces than COads stripping in liquid phase; the differences in surface detected by CO chemisorption and CO stripping could then be due to slow Cl displacement kinetics. Finally, the positive shift of the onset of the CO electrooxidation peak may be ascribed to the competition between water and chloride species for the Pt adsorption sites. Indeed, previous studies [15, 16] have suggested that only a fixed number of active sites which are able to form OH species and to initiate the CO electrooxidation exist on the Pt surface. Competitive adsorption by Cl- species thus decreases artificially the number of active sites and shifts both the onset and the main CO electrooxidation peak towards positive potentials.

Synthesis and characterization of highly loaded Pt/carbon xerogel 75

1

(a)

(b)

50

0.75 U cell (V)

-1

I (A gPt )

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25

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0.4

0.8

1.2 E (V vs. ENH)

0.5

0.25

0 0

-50

2

4 -1 j m (kA gPt )

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Fig. 3. Electrochemical measurements on 15.0 wt.% Pt/C catalysts (double impregnation) reduced under H2 during 1h at 200°C, sample C-200-1 ({) and 5h at 450°C, sample C-450-5 (‘). (a) COads stripping voltammograms, measurements in H2SO4 1 M at 25°C, sweep rate of 0.02 V s-1. Curves in grey: first CO stripping; curves in black: voltammogram after 7 consecutive CO strippings. (b) Polarization curves at 70°C for Membrane-Electrode Assemblies with cathode processed with the same catalysts.

Oxygen Reduction Reaction (ORR) performed on the rotating disk electrode shows that the presence of chlorine has little effect on the intrinsic reactivity of the Pt sites. To evaluate the catalyst activity towards oxygen reduction, two parameters were evaluated: (i) the Tafel slope, b, and (ii) the specific activity at 0.90 V vs. NHE, SA90. This potential corresponds to 0.34 V ORR overpotential in 1 M sulphuric acid, a value classically monitored in a PEMFC cathode at low current densities, i.e. under kinetic control. Regarding samples C-200-1, C-350-3 and C-450-5, neither b nor SA90 changes significantly with the reduction treatment (Table 1), indicating that the reaction mechanism and kinetics remain the same. The only difference between the three samples is the accessible Pt surface, which decreases due to Cl poisoning when the reduction temperature is too low. Finally, Fig. 3b shows the impact of Cl poisoning on the functioning of an air/H2 fuel cell. The cathode of the Membrane-Electrode Assembly (MEA) was processed either with sample C-200-1, or with catalyst C-450-5. The catalyst poisoning dramatically decreases the current produced at a fixed potential. However, further calculation shows that the decrease of accessible Pt surface due to Cl coverage is not sufficient to explain the poor performance of catalyst C-200-1. Much probably, the presence of chlorine also hampers the contact between the Pt particles and the ionomer (i.e., Nafion®), and decreases thus further the amount of Pt atoms that are truly available for the oxygen reduction in the monocell. This was checked by in situ cyclic voltammetry: the detected Pt surface per mass unit of metal was lower in the processed MEA than in the initial catalyst powder.

4. Conclusions So as to obtain Pt/carbon xerogel catalysts with high metal content, the ‘Strong Electrostatic Adsorption’ method was applied. By repeating the impregnation-dryingreduction step with H2PtCl6 as Pt precursor, it was possible to increase the catalyst metal content up to 22.3 wt.% while keeping homogeneously dispersed Pt nanoparticles ca. 2 nm in size. However, both physico-chemical and electrochemical characterization

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show that the use of H2PtCl6, which is a very classical impregnation precursor, yields Pt catalysts poisoned with chlorine species. Chlorine coming from the metal precursor decomposition appears difficult to remove completely: even after reduction under H2 at 450°C for 5 h, Cl poisoning is still detected via electrochemical methods. The presence of chlorine on the Pt particles leads obviously to decreasing the active Pt surface, which can dramatically reduce the Pt utilization ratio in the PEMFC catalytic layer. Further work is in progress to extend the SEA method to Cl-free Pt precursors.

References [1] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, 2005, Appl. Catal. B, 56 (1-2), 9-35. [2] N. Job, J. Marie, S. Lambert, S. Berthon-Fabry, P. Achard, 2008, Energ. Convers. Manage., 49 (9), 2461-2470. [3] J.R. Regalbuto, in: Catalyst Preparation: Science and Engineering, J.R. Regalbuto (ed.), CRC Press, Taylor & Francis Group, Boca Raton, 2007, p. 297. [4] S. Lambert, N. Job, L. D’Souza, M.F.R. Pereira, R. Pirard, J.L. Figueiredo, B. Heinrichs, J.P. Pirard, J.R. Regalbuto, 2009, J. Catal., 261 (1), 23-33. [5] K. Kinoshita, Electrochemical Oxygen Technology, Wiley, New York, 1992, p. 48. [6] N. Job, R. Pirard, J. Marien, J.-P. Pirard, 2004, Carbon, 42 (3), 619-628. [7] S.A. Al-Muhtaseb, J.A. Ritter, 2003, Adv. Mater., 15 (2) 101-114. [8] N. Job, A. Théry, R. Pirard, J. Marien, L. Kocon, J.-N. Rouzaud, F. Béguin, J.-P. Pirard, 2005, Carbon, 43 (12) 2481-2494. [9] N. Job , S. Lambert, M. Chatenet, C.J. Gommes, F. Maillard, S. Berthon-Fabry, J.R. Regalbuto, J.-P. Pirard, 2010, Catal. Today, in press. [10] N. Job, M.F.R. Pereira, S. Lambert, A. Cabiac, G. Delahay, J.-F. Colomer, J. Marien, J.L. Figueiredo, J.-P. Pirard, 2006, J. Catal., 240 (2), 160-171. [11] F. Maillard, M. Eikerling, O.V. Cherstiouk, S. Schreier, E. Savinova, U. Stimming, 2004, Faraday Discuss., 125, 357-377. [12] F. Maillard, S. Schreier, M. Hanzlik, E.R. Savinova, S. Weinkauf, U. Stimming, 2005, Phys. Chem. Chem. Phys., 7 (2) 385-393. [13] A.J. Bard, L.R. Faulkner, Electrochemical methods: fundamentals and applications, Wiley, New-York, 1992, p. 283. [14] H.H. Holscher, W.M.H. Sachtler, 1966, Discuss. Faraday Soc., 41, 29-42. [15] F. Maillard, E.R. Savinova, U. Stimming, 2007, J. Electroanalytical Chem., 599 (2), 221-232. [16] B. Andreaus, F. Maillard, J. Kocylo, E. R. Savinova, M. Eikerling, 2006, J. Phys. Chem. B, 110 (42), 21028-21040.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Catalytic wet air oxidation of succinic acid over monometallic and bimetallic gold based catalysts: Influence of the preparation method Radka Nedyalkova, Michèle Besson and Claude Descorme* Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON), UMR 5256 CNRS – Université de Lyon, 2 avenue A. Einstein, 69626 Villeurbanne, France *E-mail : [email protected]

Abstract Different methods for the preparation of gold catalysts (mono and bimetallic) were used – the modified deposition-precipitation (MDP), the deposition-precipitation by ammonia (DPA) and the colloidal method (CM). The catalytic performances of all samples were evaluated in the catalytic wet air oxidation (CWAO) of succinic acid under mild conditions (190°C, 50 bar total pressure). The results showed that the preparation procedure and the addition of a second metal (Pt or Ru) clearly influence the catalytic activity and selectivity, depending on the size of the gold particles and the nature of the second metal. Keywords: Au-Pt(Ru) bimetallic catalysts, catalytic wet air oxidation (CWAO), organic compounds

1. Introduction Gold has long been disregarded for catalytic applications, due to its inert nature in the bulk state. Since Haruta’s [1,2] discovery of the remarkable activity of supported gold nanoparticles in oxidation reactions, different methods to prepare highly active gold catalysts have been developed. The same group has developed/adapted four different techniques that allow the deposition of gold nanoparticles on certain metal oxides: the co-precipitation (CP), the co-sputtering, the deposition-precipitation (DP) and the gasphase grafting [3]. Between all techniques developed so far, it appears that the deposition-precipitation is the most successful for preparing highly dispersed Au catalysts. The DP method has numerous variations, depending on the pH, the temperature of deposition, the precipitation agent and the state of the support (oxide or hydroxide). However, the DP method still cannot completely avoid the adsorption of gold hydroxyl chlorides species onto the support or the wrapping of chlorides in the precipitate, which may cause the deactivation of the Au catalysts. Grunwaldt et al. [4] first developed a two-stage method, based on Au colloids, for the preparation of Au/TiO2 and Au/ZrO2 catalysts employing tetrakis(hydroxymethyl)-phosphonium chloride (THPC) as the reducing and capping agent. Later, Porta et al. [5] employed poly(vinylalcohol) (PVA) as the protective agent and prepared Au/C and Au/TiO2 catalysts. The simplicity of the colloidal method and the possibility to prepare chlorine free catalysts are the main advantages, motivating his large application not only for the preparation of mono but also bimetallic catalysts. The interest towards bimetallic heterogeneous catalysts is increasing since the presence of a second metal can influence the catalytic properties, improving their activity, stability and/or selectivity. The presence of one less reducible component, which strongly interacts with the support,

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may stabilise the second, more noble metal, in the highly dispersed state. Recently, Pd and Pt were used for the preparation of colloidal bimetallic gold catalysts and apply in different oxidation reactions [6]. Furthermore, the catalytic properties of metal-oxidesupported gold catalysts strongly depend on the nature, the texture and the structure of the support. The support must present a defective surface available for strong interactions with the gold precursor. It is well known that reducible metal oxide supports (TiO2, Fe2O3, Co3O4), supplying reactive oxygen to the active gold sites, are more active in oxidation reactions than non reducible supports. Ceria has been regarded as one of the most important component in many catalytic systems due to its remarkable redox properties [7]. Recently, it has been shown that gold catalysts supported on ceria exhibit higher activity in the succinic acid wet air oxidation than Au/TiO2 [8]. Evidence was provided that the activity depended on the gold particle size. From the environmental point of view, the removal of toxic organic compounds from aqueous wastewaters is drawing a lot of attention and the wet air oxidation (WAO) is a suitable technology for that. The main disadvantage is that WAO requires high temperature and pressure (200-350°C, 70-230 bar), conditions that severely affect the economics of this technology. Using a catalyst, the operating conditions can be made significantly milder (120-220°C, 5-50bar). Since heterogeneous catalysts might easily be removed, their development and optimization has been the subject of several works in the recent decades. For the first time, Besson et al. [9] have reported that the Au/TiO2 catalyst is a promising candidate in the CWAO of succinic acid. The main disadvantage is that gold catalysts are not stable upon long term reactions and recycling. The important challenge is then to get stable gold catalysts in the CWAO. Deactivation may occur by sintering, poisoning of the active sites or “fouling” of the catalyst surface by adsorption of intermediate reaction products. Also, in hot acidic environments, the active components might be dissolved into the liquid phase (leaching). In order to reduce leaching and prevent the gold particles from sintering, the active phase might be incorporated into a catalyst support. On the other hand, the presence of a second metal may induce significant changes in both activity and stability. In our study, Pt and Ru were chosen as the second metal. Both concepts are the basis of the present study aiming to achieve an active and stable gold catalyst in the CWAO of succinic acid.

2. Experimental 2.1. Catalyst preparation A total of six gold catalysts were prepared by different methods. The total metal loading was fixed at 3wt.%. Two monometallic gold on ceria catalysts were prepared by the modified deposition precipitation method (MDP) and the deposition-precipitation by ammonia (DPA). First of all, the Ce(NO3)3.6H2O aqueous solution was precipitated with K2CO3 at 60°C and pH=9. In the case of the MDP, just after aging of the Ce(OH)4 precipitate for 1h at 45°C, the deposition of HAuCl4 was performed at pH=7. The resulting precipitates were aged for 1 h at 45°C, filtered and washed until no Cl - and NO3- could be detected. For the DPA method, the as prepared Ce(OH)4 precipitate was carefully washed to eliminate the K+ and NO3- ions and dried at 80°C. Noteworthy, to achieve a 3wt.% metal loading, a test was performed to determine the weight lost upon the transformation from Ce(OH)4 to CeO2 at 400°C for 2h. After that, the deposition of gold was performed as follows: the support was suspended in deionizer water, the gold precursor (HAuCl4.3H2O 6 10-4 M) was added and pH was adjusted to 11 with ammonia and maintained for 1 h. After washing and drying at 80°C under vacuum, the solid was

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calcined under flowing air (6 L h-1) at 400°C for 2h. Three bimetallic gold catalysts were synthesized using the MDP method: 2wt.%Au-1wt. %Pt/CeO2 MDP I, 2wt.%Au1wt.%Pt/CeO2 MDP II and 2wt.%Au-1wt.%Ru/CeO2 MDP II. I indicates that the two salts (HAuCl4 and Pt(NH3)4(NO3)2) were introduced simultaneously, while II indicates that Pt or Ru, respectively, was introduced first. As a precursor ruthenium nitrosil nitrate was used. After aging for 1h at 45°C, the received precipitate was washed carefully, until no Cl- and NO3- could be detected, and dried at 80°C under vacuum. The solids were calcined at 400°C for 2h under flowing air (6 L h-1). One bimetallic catalyst noted 2wt.%Au-1wt.%Ru/CeO2 CM was prepared via the colloidal method as follows: the gold precursor was dissolve in 400 mL deionised water in the presence of polyvinyl alcohol (PVA 2wt.% solution) under vigorously stirring. After that, the ruthenium precursor was added and the slurry was kept under stirring for 3 min. Then, the 0,1M NaBH4 solution, freshly prepared, was added to the solution to obtain a colloidal sol. Once the sol was obtained, the immobilisation on the Ce(OH)4 support was carried out for 2h. The resulting solid was centrifuged, carefully washed, dried at 80°C under vacuum and finally calcined at 400°C for 2h under flowing air. For the bimetallic catalysts, the thermal treatment is an important step, not only as far as the transformation of Ce(OH)4 into CeO2 is concerned but considering the interaction between the metal particles and the support. Before reaction all samples were reduced at 300°C for 2h under flowing H2 (12L h-1).

2.2. Characterization of the catalysts XRD patterns were obtained on a Siemens D5005 diffractometer (Cu Kα, 0.15406 nm). The metal concentration in the liquid phase after 8 h reaction was repeatedly measured by ICP-OES. The metal concentration in the solution was systematically lower than 0.5 ppm (detection limit), indicating that no leaching occurred.

2.3. Catalytic activity Experiments were carried out in a 300 mL autoclave made of Hastelloy C22 (model 4836, Parr Instrument Inc.). In a typical run, the autoclave was loaded with 150 mL succinic acid aqueous solution (5 g L-1, i.e. initial total organic carbon (TOC) = 2032 mg L-1) and 0.5 g catalyst. After the reactor was out gassed under argon, the mixture was heated to the reaction temperature (190°C) under stirring. Then, the stirrer was stopped and air was admitted into the reactor until the predefined pressure was reached (50 bar total). The reaction finally started when the stirrer was switched on again. This point was taken as ‘‘zero time’’ and one sample was withdrawn to measure the exact initial concentration of succinic acid. The liquid samples were periodically withdrawn from the reactor, centrifuged to remove any catalyst particle in the liquid sample and further analyzed. The substrate and the reaction intermediates (acetic acid and acrylic acid) were analyzed by HPLC (Shimadzu) using an ICSep Coregel 107H column. The mobile phase was a 0.005N H2SO4 aqueous solution (0.5 mL min-1). The HPLC system was equipped with a UV–vis detector set at 210 nm. The TOC in the liquid samples was measured with a Shimadzu 5050 TOC analyzer after subtraction of the inorganic carbon (IC) contribution from the total carbon (TC). Furthermore, the carbon mass balance in the liquid phase could be checked by comparing the TOC values with the total carbon concentrations in the liquid phase derived from the HPLC analysis.

3. Results and discussion Table 1 summarizes the metal loadings as measured by ICP. These results show a good agreement between the theoretical and experimental values for Au and Ru, especially

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for the catalysts prepared by DPA and CM, keeping in mind that the support was in the form of Ce(OH)4 and transformed into the oxide form during the calcination in air at 400°C. In the case of Pt, the experimental values are almost twice lower than the expected values. This unusually low platinum loading could indeed be connected with the nature of the precursor, whom precipitation at pH 7 and 45°C was not complete. Andreeva et al. [10] have applied the same method for the preparation of 3wt.%Au/CeO2 catalysts, but at higher temperature (60°C). They found that Ce3+ ion act as a reducing agent, converting Au3+ to Au0 during the preparation. In turn, Ce3+ ions are oxidized to Ce4+. Using XRD [10], the average gold particle size was estimated to be about 15 nm. In our study we decreased the temperature to 45°C in order to slow down the reduction process and decrease the gold particle size. At such low temperature and pH 7, the gold loading decreased slightly and part of the Pt was again lost upon washing since the Pt complex was not fully hydrolyzed. However, lowering the precipitation temperature, the average gold particle size decreased to 8 nm (Table 1). Table 1. Catalysts chemical composition measured by ICP-OES and average gold particle size derived from XRD measurements. Samples

Au , wt%

Pt, wt%

Ru, wt%

DAu a, nm

3wt.%Au/CeO2 MDP

2.7

-

-

8.0

3wt.%Au/CeO2 DPA

2.9

-

-

6.0

2wt.%Au-1wt.%Pt/CeO2 MDP I

1.74

0.45

-

25.0

2wt.%Au-1wt.%Pt/CeO2 MDP II

1.78

0.40

-

10.0

2wt.%Au-1wt.%Ru/CeO2 MDP II

1.82

-

0.85

10.0

2.0

-

0.8

6.0

2wt.%Au-1wt.%Ru/CeO2 CM a

derived from XRD diffractograms using Debye-Scherrer equation

Figure 1 compares the XRD patterns of mono and bimetallic catalysts. For all samples, the diffraction lines for CeO2 are typical of the cubic structure of fluorite type oxides. In the case of the monometallic catalysts, the main line characteristic for Au (2θ=38.2°) is more intense for the catalyst prepared by MDP than DPA, indicating that gold was better dispersed on the catalyst synthesized by DPA. In the case of bimetallic catalysts, although the diffraction lines characteristic for Au, Pt and Ru are very close in position, we could clearly see the difference between the MDP I and MDP II preparation routes. Bimetallics prepared by mixing the two salts led to catalysts with a lower dispersion (DAu=25 nm). On the opposite, when the salts were precipitated one after the other, the gold particles dispersion was improved (DAu=10 nm). Finally, gold particles of approximately 6 nm were obtained by the colloidal method, probably because of the presence of the protecting agent (PVA) that could stabilized the gold colloids at a higher dispersion state. It is noteworthy that, as far as the amount of Ru and Pt was 1wt.% and 0.5wt.%, respectively, that is below the detection limit for XRD, the dispersion of the second metal could not accurately be estimated from the XRD patterns.

3 2

CeO2

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CeO2

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CeO2

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Figure 1. XRD patterns for: A – (1) - pure support, (2) - 3Au/CeO2 MDP, (3) - 3Au/CeO2 DPA; B - (1) - 2Au-1Pt/CeO2 MDP I, (2) - 2Au-1Pt/CeO2 MDP II, (3) – 2Au-1Ru/CeO2 MDP II, (4) – 2Au-1Ru/CeO2 CM.

The catalytic performances of the monometallic gold catalysts in the catalytic wet air oxidation of succinic acid are presented on Fig. 2 and on Fig. 3 for the bimetallic Au-Pt and Au-Ru catalysts. As a preliminary test, a blank was performed to confirm that succinic acid is stable under the applied reaction conditions in the absence of catalyst. Furthermore, it is known that succinic acid might be intermediately degraded to acetic and acrylic acids or directly mineralized into CO2 and H2O [11]. In the presence of the ceria support 100% succinic acid conversion was achieved after 6h. Acrylic acid concentration was systematically very low and was not reported in the figures. 30

20

-1

without catalyst CeO2 3Au/CeO2 MDP 3Au/CeO2 DPA

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Acetic acid, mmol L

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Figure 2. Evolution of the succinic and acetic acid concentrations as a function of time upon the CWAO of succinic acid over pure ceria and monometallic catalysts.

For the monometallic Au catalysts, 100% conversion was reached after only 3h for the catalyst prepared by DPA and 4h for the corresponding one prepared by MDP.

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Acetic acid, mmol L-1

Succinic acid, mmol L-1

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without catalyst 2Au-1Pt/CeO2 MDP I 2Au-1Pt/CeO2 MDP II 2Au-1Ru/CeO2 MDP 2Au-1Ru/CeO2 CM

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Figure 3. Evolution of the succinic and acetic acid concentrations as a function of time upon the CWAO of succinic acid over bimetallic catalysts.

The acetic acid concentration reached 26.8 and 29.0 mmol L-1 in the case of the DPA and MDP catalysts, respectively. In the case of the bimetallic catalysts, the highest activity was observed for the 2wt.%Au-1wt.%Ru/CeO2 MDP II catalyst. 1.0

0.6 0.4

CeO2

0.2 0.0 0.0

0.2

0.4

acetic acid acrylic acid mineralization

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acetic acid acrylic acid mineralization

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3Au/CeO2MDP

0.0 0.0

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0.6 0.4 0.2 0.0 0.0

3Au/CeO2DPA 0.2

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1.0

Overall conversion Figure 4. Distribution of the reaction products as a function of the overall conversion.

Comparing the curves for the reaction product distribution as a function of the overall conversion (Fig. 4) for the ceria support and the monometallic gold catalysts, the direct mineralization pathway becomes predominant in the case of the catalysts. The faster direct mineralization for the DPA catalyst is certainly to be connected with the higher dispersion of the gold particles. These results are in good agreement with the results reported by Bond et al. [12] who showed that the catalytic activity rapidly increases as the gold particle size decreases. Another phenomenon concerns the reaction mechanism: acrylic acid was not formed as an intermediate product in the presence of highly dispersed gold particles. For the bimetallic catalysts, the reaction pathway

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depends of a nature of the second metal (Fig. 5). In the presence of Pt, no acrylic acid was detected, while for Ru the production of acrylic acid was much more significant. 1.0

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2Au-1Ru/CeO2 MDP II

2Au-1Pt/CeO2 MDP II 0.0 0.0

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0.8

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Overall conversion

Figure 5. Distribution of the reaction products as a function of the overall conversion.

The most active 2wt.%Au-1wt.%Ru/CeO2 MDP II catalyst was then submitted to a stability test (Fig. 6). Noteworthy, as the experiments were carried out in a batch reactor, the stability was studied by recycling the catalyst. For that reason, the first run was repeated three times in order to recover enough catalyst to perform a second run using the same amount of catalyst (0.5g). After every run, the catalyst was washed with cold water and dried overnight in air at 80°C. The results obtained upon three independent runs performed on the fresh catalyst showed a perfect reproducibility. Furthermore, deactivation was observed upon recycling. The total organic carbon (TOC) at the end of the 8 h run increased from 125 to 450 ppm. A similar deactivation was observed by Besson et al. [9] on a 2.2wt.%Au/TiO2 catalyst prepared by DP using NaOH and tested under the same reaction conditions. To get a better idea about the possible reasons for this deactivation, the catalyst was reduced again at 300°C for 2h under flowing H2 (12 L h-1) in between the two runs. The results showed that deactivation is partially reversible and might be connected with the metal particle surface re-oxidation. In that case, the TOC in the liquid phase after 8 h reaction reached 280 ppm. No Au or Ru leaching could be detected (< 0.1 ppm).

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Time, h Figure 6. Stability test of the 2Au-1Ru/CeO2 MDP II catalyst in the CWAO of succinic acid. Full symbols : first run (three independent tests); … dried only; 7 re-reduced

CeO2

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COT, mg L-1

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only dried after direct reduced after direct direct direct direct

Intensity, [a.u.]

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184

2 1

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2θ, deg

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Figure 7. Comparison of the XRD patterns of the fresh (1) and re-reduced (2) 2wt.%Au1wt.%Ru/CeO2 MDP II catalyst

In Fig. 7 the XRD patterns of the fresh and used catalysts are compared. A slight increase in the gold particle size might be evidenced. As a conclusion, gold sintering is mainly responsible for the observed deactivation. This phenomenon is certainly related to the intrinsic instability of gold at elevated temperature which might somehow be related, in combination with particle size effects, to the lower melting point of bulk gold compared to other noble metals.

4. Conclusions The CWAO of succinic acid under mild reaction conditions over monometallic and bimetallic gold catalysts strongly depends on the applied preparation method. High dispersion of gold resulted in higher performances. The presence of a second metal has a beneficial effect on the catalytic activity and stability. The reaction product distribution was also affected by the nature of the second metal.

Acknowledgments We gratefully acknowledge the financial support from the Agence Nationale de la Recherche (Project ANR Blanc CatOxOr).

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. M. Haruta et al., J. Catal. 144 (1993) 175 M. Haruta, Cattech 6 (2002) 102 J.D. Grunwaldt, C. Kiener, C. Wögerbauer, A. Baiker, J. Catal. 181 (1999) 223 F. Porta, L. Prati, M. Rossi, G. Scari, J. Catal. 211 (2002) 464 N. Dimitratos, C. Messi, F. Porta, L. Prati, A. Villa, J. Mol. Catal A: Chem 256 (2006) 21 A. Trovarelli, Catal. Rew. Sci. Eng. 38 (1996) 439 N.D. Tran - PhD Thesis, IRCELYON (2008) M. Besson, A. Kallel, P. Gallezot, R. Zanella, C. Louis, Catal. Commun 4 (2003) 471 D. Andreeva et al., Appl. Catal. A: General 246 (2003) 29 J.C. Béziat, M. Besson, P. Gallezot, S. Durécu, Ind. Eng. Chem. Res. 38 (1999) 1310 G.C. Bond, C. Louis, D.T. Thompson, Catalysis by Gold, I.C. Press, London, 2006

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Design of hierarchical functional porous mixed oxides from single precursors Arnaud Lemaire a and Bao-Lian Su a,b a

Laboratory of Inorganic Materials Chemistry (CMI), University of Namur (FUNDP), 61 rue de Bruxelles, 5000 Namur, Belgium. b State key Laboratory of Advanced technology for Materials Synthesisand Processing, 122 Luoshi Road, Wuhan, China.

Abstract Porous materials have been prepared via a single-source pathway. A one-step synthesis pathway has been developed for the design of hierarchically structured macromesoporous aluminosilicates with high tetrahedral aluminium content from a single molecular alkoxide precursor already containing Si-O-Al bonds (sec-BuO)2-Al-O-Si(OEt)3. The compensation of the cleavage of the intrinsic Al-O-Si linkage is successfully achieved by using highly alkaline media and the employment of reactive silica co-reactants, or aluminium selective chelating agents, leading to aluminosilicate materials with Si/Al ratios close to one and very high proportion of tetrahedral aluminium species. The macro-mesoporosity was spontaneously generated by the hydrodynamic flow of solvents released during the rapid hydrolysis and condensation processes of this double alkoxide. Secondly, recent advances in the conception of mesoporous zirconosilicate with homogeneous repartition of zirconium into the silicate structure (Si/Zr ~ 4), achieved by the use of Zr[OSi(OsBu)3]4 molecular precursor, are presented. Keywords: homogeneous mixed-oxides, self-formation phenomenon, aluminosilicates, zirconosilicates

1. Introduction Zeolites are successfully used in many industrial processes (such as the cracking of petroleum feedstocks), due to their various compositions and excellent textural characteristics. Nevertheless, present zeolites have a limited pore size (below 2 nm) which restricts their usefulness to processes involving small molecules. Mesoporous structures with broader pore size and high surface areas were then developed. However, this class of molecular sieves, mainly based on silica, exhibits a lack of active sites and is often limited to the use as catalyst supports. Consequently, many efforts are now devoted to the homogeneous insertion of the higher number of active sites inside the silica matrix, such as trivalent aluminium atoms, leading to acido-basic properties. This work reports first the synthesis of aluminosilicate materials almost constituted of Al-O-Si linkages and featuring a double sized-porosity, by the controlled polymerisation of a single molecular precursor. This part is followed by a brief report about the design of a mesoporous and homogeneous zirconosilicate material with an important amount Zr-O-Si bonds, by the similar use of a single molecular precursor.

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2. Macro-mesoporous aluminosilicate materials Integrating macroporosity in catalysis would permit better mass transfer to the active sites situated in the micro- and mesopores that are contained within the material’s walls, especially when large molecules are used. It is envisaged that these catalysts, with multiple porosity hierarchy, will allow in the near future the diffusion of the heavier oil fractions across the solids because of their macroporosity. Recently, a self-formation phenomenon of porous hierarchy which yields hierarchical macro-mesoporosity within oxide materials without the need of any physical templating agent, has been described [1]. The multiple porosity structure can be auto-generated by the brutal release of a porogen during the fast reactions of hydrolysis and polycondensation. The simultaneous polymerisation of two independent alkoxide precursors (alkoxisilane and aluminium alkoxide) leads to hierarchical macro-mesoporous aluminosilicate materials with interesting textural properties for catalytical applications [2]. Nevertheless, due to the very important differences of polymerisation rate between the two alkoxide precursors, inhomogeneous materials, constituted of separate oxides (Al2O3 and Si-O-Al in poor proportion) are obtained. The idea was to conserve the alkoxide ability to spontaneously generate macro-mesoporous structure, but to improve the incorporation of aluminium into the silicate framework. This was achieved by the use of the single molecular precursor (sec-BuO)2-Al-O-Si(OEt)3, which is containing both a pre-formed Al-O-Si linkage and alkoxy functions. This pre-formed Al-O-Si bond is very sensitive to hydrolysis, resulting in some ruptures and the preferential polymerisation through Al-O-Al linkages. Control of synthesis conditions is necessary to ensure the achievement of an only Al-O-Si linkages constituted material. Consequently, several strategies were employed such as the use chelating agents or inorganic silica co-reactant in high alkaline media.

2.1. Macro-mesoporous aluminosilicate materials from chelating agents An alkaline media, known to favours the conversion of aluminium precursors into monomeric (AlO4)- species, was combined with carboxylate anions as they are ideal chelating agents for aluminium atoms. The use of such anions as controlling agent of the polymerisation rate of aluminium in water has been well documented in the literature [3]. Upon hydrolysis, most of alkoxy groups are quickly substituted by water, while stronger complexing ligands will be less easily removed during hydrolysis steps. Thus, the polymerisation rate of the aluminium functionalities is reduced and prevents the cleavage of the Al-O-Si linkage. Some different carboxylate molecules were investigated (sodium acetate, sodium l-lactate, sodium oxalate, sodium citrate, sodium ethylenediaminetetraacetate, and a long alkyl chained carboxylate molecule; sodium caprylate). In these carboxylates, the number of carboxylate functions and the length of the hydrocarbon chains differ, which is impacting the aluminium incorporation into the silica framework. Among those synthesis, best results in terms of aluminium incorporation have been obtained with the sample prepared from a 1:1 (BusO)2-Al-O-Si(OEt)3/sodium citrate molar ratio. For sake of clearness, only this material, named CaCi13-1, was presented here. Ca-Ci13-1 material was prepared and characterised as follow : 4.3 g of sodium citrate dihydrate (99%) (chelating agent/Al = 1) were added in 60.0 ml of pH 13.0 sodium hydroxide solution (NaOH, 98%). After dissolution, 5.0 g of commercially available (BusO)2-Al-O-Si-(OEt)3 (Gelest, 95%) was slowly added dropwise into the above solution under very slow stirring. After 1 hour, the mixture was transferred into a Teflon-lined autoclave and heated at 80°C for 24 hours. The solid product was then filtered, washed, dried (40°C) and calcined (650°C, 10 hours, ambient atmosphere). Transmission electron microscopy (TEM) experiments were performed on a Philips

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Tecnaï-10 microscope at an acceleration voltage of 80 kV with powder samples embedded in an epoxy resin and ultramicrotomed. The N2 adsorption-desorption isotherms were measured at –196°C with a volumetric adsorption analyzer Micromeretics Tristar 3000. The macroporous array was studied using a JEOL FESEM scanning electronic microscope (SEM) with conventional sample preparation and imaging techniques. Mercury intrusion-extrusion curves and corresponding pore size distributions were collected with a Micromeritics Autopore IV. Finally, the environments of the Al and Si atoms were studied by means of 27Al and 29Si MAS NMR spectroscopies with a Bruker Avance 500 spectrometer and the Si/Al ratios were investigated using a Philips PU9200X atomic absorption spectrometer. SEM images in Fig. 1 (a and b) show Ca-Ci13-1 particles constituted of an irregular and open array of 3D interconnected macrochannels with openings ranging from 1 to 3 µm and separated by thick walls of about 3 µm large. Note that this macrostructure morphology is completely independent of the carboxylate nature of the added chelating ligand. The presence of this macrostructure inside the particles as well as the observation of a disordered mesoporosity (5–10 nm) inside macroporous walls is confirmed by cross-sectional TEM images presented in Fig. 1 (c and d).

3 µm

Fig. 1. SEM images (a and b) and TEM images (c and d) of Ca-Ci13-1 material.

As regards the development of macroporous structure, work previously carried out in our laboratory suggests a mechanism based on the synergy between the polymerisation kinetics of the inorganic precursors and the hydrodynamic flow of the solvent. The drop of (BusO)2-Al-O-Si-(OEt)3 added to the aqueous media polymerises quickly, which releases alcohol molecules. As the reaction progresses, more and more solvent molecules are generated, which in turn can produce microphase-separated domains of aluminosilicate-based nanoparticles and water/alcohol microdrops which will be converted in macrochannels. The solid structure would then grow around these channels until the single molecular precursor is depleted, resulting in a macroporous particle. The disordered mesoporosity arises from voids developing between the aluminosilicate nanoparticles as they begin to aggregate. These macropores are very different from those encountered for materials prepared from of two separate aluminium and silica alkoxide precursors (straight and parallel macrochannels) [2]. This observation could be explained by the presence of an alkoxysilane function that slows down the hydrolysis and polycondensation steps of the single molecular source. The inorganic phase surrounding water/alcohol channels harden more slowly, allowing a greater isotropy within the progression of solvents macrochannels. This may explain the non linear direction, the larger void volume and the thin partitions of these macrochannels.

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The coordination environments of Al atoms in the calcined material CaCi13-1 was characterised by 27Al MAS NMR. The spectrum showed at the Fig. 2(a) is presenting a very homogeneous aluminosilicate material only constituted of intra-framework aluminium species (59 ppm). As the Si/Al ratio is equal to 1.1, this material could be considered as an almost pure aluminosilicate material, only constituted of Al-O-Si linkages. For more accurate confirmations about the Al-O-Si repartition into the CaCi13-1 material, the chemical environment of Si was studied by 29Si MAS-NMR technique. The spectrum shows at the Fig. 2(b), in contrast to pure silica materials, evidence of a significant shift toward the lower field, such as very low silica content zeolites do, and assigned to silicate species surrounded by aluminium atoms [4]. This 29 Si MAS-NMR spectrum consist in a large peak, ranging from -75 ppm to -105 ppm, which could be decomposed into 3 peaks (Si(OAl)4 at -85 ppm, -Si(OAl)3 at -90 ppm and =Si(OAl)2 at -95 ppm), meaning that the aluminosilicate framework is constituted of a major part of Si(OAl)4 species. (a)

(b)

Fig. 2. (a) 27Al MAS NMR and (b) 29Si MAS NMR spectra of the Ca-Ci13-1 materials.

The textural property of the prepared CaCi13-1 material was assessed by N2 adsorption-desorption measurements. Fig. 3(a) is showing an isotherm which is relatively well matching to a type IV isotherm (according to the IUPAC classification), characteristic of mesoporous compounds. This one exhibits a capillary condensation step that can be seen at relative pressures (p/p0) of about 0.55-0.80 indicating the presence of mesopores. The specific surface areas have been calculated by the BET method giving value of 142 m²/g. The analysis of the pore size distributions (Fig. 3(a), inset), calculated by the BJH method from the adsorption branch of the isotherm, reveals a fairly broad distribution ranging from 5 to 10 nm, which is in good agreement with TEM observations. Log Differential Intrusion (ml/g)

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Some more quantitative study of the macrostructure was performed via mercury intrusion-extrusion porosimetry. The pore size distribution obtained from this analysis, exposed at the Fig. 3(b), is clearly showing one peak located around 3 µm which could correspond to the macrostructure. This hypothesis is in good agreement with the previous SEM and TEM characterizations. The mercury extrusion curve appearance (data not shown) is the same than the intrusion curve, testifying of an interconnected macrostructure and a high pressure-resistant matrix.

2.2. Macro-mesoporous aluminosilicate material from inorganic silica co-reactants When a mixture of di-s-butoxyaluminoxytriethoxysilane and silica co-reactant is added to an aqueous media, the likelihood that polymerisation occurs between highly reactive, but less numerous aluminium sites and most widespread and less reactive silicon sites increase, favouring heterocondensation reactions (Al-O-Si) instead of Al-O-Al linkages. This was again combined with highly alkaline solutions. The added source of silica was the reactive tetramethoxysilane (TMOS). This co-precursor was intimately mixed in a 1:1 molar ratio with 5.0 g of (BusO)2-Al-O-Si-(OEt)3, and the mixture was added dropwise in 60.0 ml alkaline solutions (pH = 13.0 and 13.5). After 1 hour, mixtures were transferred into a Teflon-lined autoclave and heated at 80°C for 24 hours. The solid products were filtered, washed and dried (40°C). Materials were analyzed in a similar way that solids prepared at the point 2.1. These two materials, prepared from a molar ratio of (BusO)2-Al-O-Si-(OEt)3/TMOS = 1 at pH = 13.0 and pH = 13.5, are named TM13 and TM13.5 respectively. The comparison of those two materials prepared in quite identical synthetic conditions, but with a slight pH difference, illustrates the pH-dependence of both the macroporous array development and the aluminium incorporation into the tetrahedral silica network. Nevertheless, this point is not fully yet well understood and remains under investigations. The morphological structure of those materials was directly visualised by SEM analysis. The sample synthesised at pH 13.0 (TM13, SEM images at Fig. 4(a)), display material only constituted of highly spongy particles of ca. 10-20 µm which are fully comprised of very regular micrometre-sized macrovoids. These numerous regular 1 to 2 µm spherical voids are separated by thin walls and are found over the entire surface of the particle as well as within the particle. Fig. 4(b) represent SEM image of the material TM13.5 which shows a “reverse macrostructure”, consisting in the stacking of microsized (1-2 µm of diameter) hollow spheres. Image taken at higher resolution (Fig. 4(b), inset) exhibits fully independent hollow spheres, among plausible debris of destroyed hollow bubbles. The presence of those macrovoids within the particles is confirmed by TEM images (Fig. 5). The TEM images at Fig. 5(a) of the TM13 material highlight circular openings of ~ 2 µm large, surrounded by very thin walls of about 100 to 400 nm of thickness. A deeper look into the structure (Fig. 5(a), inset) reveals a vermicular mesoporosity contained into the walls separating macrovoids. Fig. 5(b) present the case of the TM13.5 material, prepared at a higher pH (13.5) value. The TEM images, taken at different level of magnifications, confirm that the macroporous material seems to be constructed by the stacking of independent and hollow microspheres of about 1 µm large, as it can be visualized by SEM at Fig. 4(b). Moreover, material prepared at pH 13.5 shows cavities between the stacked mesoporous aluminosilicate nanoparticles of 50 nm long, which could suggest a further interparticular porosity (Fig. 5(b) inset). Again, the morphology of the macrostructure is different from the one obtained by the use of two independent precursors or by chelating agents. An explanation could arise from the fact that, when pure metal alkoxides are carried into an aqueous solution

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by a dropwise addition, the very fast transformation of hydrophobic metal alkoxide into metal oxide is permitting the freezing of the droplet during the polymerisation process.

Small droplet shape generates important curved surface, which is applying some pressure on the water/alcohol microdroplets contained into the near surface of the metal alkoxide drop. This pressure should be the driving force responsible of the straight shaped macrostructure. Whereas, in the case of the mixture of di-s-butoxyaluminoxytriethoxysilane and silica inorganic co-reactant, even in the case of a very low dropwise addition, all the droplets of this mixture are gathering to form one unique homogeneous gelatinous cloud. In this “unique drop”, the surface/volume ratio is less important and no driving forces are applied onto the microdroplets of solvents, which are staying in static spherical shaped configuration during all the polymerisation process. Some investigations were realized by optical microscopy (data not shown), showing the apparition of those microbubbles of solvent during the polymerisation process. 27 Al MAS NMR spectra of the TM13 and TM13.5 materials are presented at Fig. 6(a and b). In presence of added TMOS, the final material possesses more important amount of tetrahedral aluminium relative to the octahedral proportion. When the pH of the solution increases to 13.5, the addition of TMOS to the single precursor allow the achievement of a very homogeneous material (Fig. 6(b)), only constituted of intraframework aluminium. 29Si MAS-NMR investigation of TM13 and TM13.5 is again showing evidence of the significant shift toward the lower field corresponding to very well mixed and enriched aluminosilicate materials. The 29Si NMR spectrum of the TM13.5 material is presented at the Fig. 6(c). The peak, midpointed around -87 ppm, seems to result in the contribution of two peaks, one centred on -85 ppm (Si(OAl)4) and the second on -90 ppm (-Si(OAl)3). This proposal is in good agreement with Si/Al ratio, provided by elemental analysis, which is close but not exactly equal to 1.0 (Si/Al = 1.3).

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(c) Si(Al)3

(a)

Si(Al)4

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Fig. 6. Al MAS NMR spectra of (a) the TM13 and (b) the TM13.5 materials, and (c) 29Si MAS NMR spectrum of the TM13.5 material.

Adsorbed Volume (cm³/g - STP)

The isotherm of TM13 material, exhibited in Fig. 7(a), is similar to type IV, characteristic of mesoporous compounds, with a capillary condensation step at p/p0 0.75. From these data sets, pore size distribution is calculated with maxima centred at ~ 5 nm (Fig. 7(a), inset). A high specific surface area of 315 m²/g is obtained. TM13.5 material presents a combination of type I and type II isotherms (Fig. 7(b)). This isotherm is characteristic of supermicroporous structures with a pore size inferior to 1.5 nm coupled with larger pores as is shown by the sharp increase in N2 adsorption located at high relative pressure (p/p0 ≥ 0.9). This confirms again the presence of a secondary porosity centred at ~ 30 nm for TM13.5 material (Fig. 7(b), inset), as it is observed by TEM imaging of the Fig. 5(b). Accessible surface area, provided by BET calculation, is about 80 m²/g. 450

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Intrusion-extrusion porosimetry measurements were done to characterize more correctly these macrostructures. Nevertheless, results are not well matching to the pore size distribution determined by SEM and TEM investigations. This is certainly due to the special morphology of those hollow shaped macrovoids. Indeed, since those macrovoids are not interconnected, mercury is not regularly intruded into pores, which is affecting the quality of results even if again, the appearance of the extrusion curve is well matching the intrusion curve aspect, meaning that those both materials are quite mechanically stable.

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3. Ordered mesoporous zirconosilicate High loading of zirconium into ordered mesoporous silica framework is still an amazing challenge but is difficult to achieve since the polymerisation rate of the two precursors are uneven. This is also affecting the interaction with the templating agent in the solution leading to weakly organized materials. Molecular precursor such as Zr[OSi(OsBu)3]4, prepared according to the literature [5], was converted, via the sol-gel process, into a zirconosilicate material with Si/Zr ~ 4 and SBET > 300 m²/g. Study of temperature and the pH during the process was required to obtain very homogeneous material with appropriate concordance of Si/Zr ratios and 29Si MAS NMR. Indeed, acidic solutions allow the hydrolysis of this single precursor, implying also too stable cationic complexes formation, leading to phase separated materials. A pH-adjustment step was required to favor heterocondensation reactions between these hydrolysed species. For a typical synthesis : 0.6 g of pure Zr[OSi(OsBu)3]4 was hydrolysed into a 10.0 ml acidic solution (pH = 0, HCl 37%) at 60°C. After a time period comprised between 4 to 12 hours, 10.0 ml of a NaOH solution is added to give a jelly solution with a pH of 12. After an ageing period of 5 days at 80°C, the material was filtrated, washed, dried at room temperature and finally characterized. Q2 The 29Si MAS NMR spectrum (Fig. 8) of this material is constituted of one large peak almost Q3 totally bared of Q4 silicon species attributed to pure silica phase (Si(OSi)4, -110 ppm). This observation, coupled with local and global Si/Zr ratios (Si/ZrEDX = 4.0, Si/ZrXPS = 4.93 and global Si/Zr = 3.91) can be interpreted as the synthesis of a very homogeneous -50 -60 -70 -80 -90 -100 -110 -120 -130 -140 δ Si (ppm) material, mainly constituted of Zr-O-Si linkages. 29

29

4. Conclusions

Fig. 8. Si MAS NMR spectrum of homogeneous zirconosilicate material.

Hierarchically structured macro-mesoporous aluminosilicates containing a higher level of tetrahedral aluminium and Si/Al ratios close to one were successfully synthesised via a single-source molecular alkoxide precursor (sec-OBu)2-Al-O-Si-(OEt)3, in controlled conditions (chelating agent or silica co-reactants). Same concept is currently applied to the conception of highly ordered mesoporous zirconosilicate materials via the aqueous conversion of Zr[OSi(OsBu)3]4 into a homogeneous mixed oxide. Efforts are now devoted to the improvement of the mesostructuration by the use of templating agents.

References [1] Yuan, 2003, Surfactant-assisted synthesis of unprecedented hierarchical meso-macrostructured zirconia, Chem. Commun., 1558. [2] Léonard, 2004, A novel and template-free method for the spontaneous formation of aluminosilicate macro-channels with mesoporous walls, Chem. Commun., 1674. [3] van den Brand, 2004, Interaction of Anhydride and Carboxylic Acid Compounds with Aluminum Oxide Surfaces Studied Using Infrared Reflection Absorption Spectroscopy, Langmuir, 20, 15, 6308. [4] Lippmaa, 1981, Investigation of the Structure of Zeolites by Solid-state High-Resolution 29Si NMR Spectroscopy, J. Am. Chem. Soc., 103, 17, 4992. [5] Kriesel, 2001, Block Copolymer-Assisted Synthesis of Mesoporous, Multicomponent Oxides by Nonhydrolytic, Thermolytic Decomposition of Molecular Precursors in Nonpolar Media, Chem. Mater. 13, 10, 3554.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Hierarchical porous catalyst support: shaping, mechanical strength and catalytic performances S. Ould-Chikh,a S. Pavan,c A. Fecant,a E. Trela,a C. Verdon,a A. Gallard,a N. Crozet,a J-L. Loubet,c M. Hemati,b L. Rouleau,a a

IFP-Lyon, Direction Catalyse et Séparation, BP-3, 69360 Solaize, France, e-mail: [email protected], [email protected] b Laboratoire de Génie Chimique (LGC) de Toulouse - UMR5503, BP84232, 4 allée Emile Monso, 31432 Toulouse cedex 4, France, email: [email protected] c Laboratoire de Tribologie et Dynamique des Systèmes - UMR 5513, Ecole Centrale de Lyon, 36 avenue Guy de Collongue, 69134 Ecully cedex, France, email: [email protected]

Abstract Palladium layered catalysts were reported to have the best performances for selective hydrogenations. A new core/shell bimaterial consisting of low specific surface area α alumina beads coated by a high specific surface area γ alumina layer is proposed and is suitable for a controlled thickness deposition of metallic palladium particles. A coating process was developed for a pan granulator where γ alumina dry powder was added under a boehmite sol pulverisation onto α-Al2O3 beads. The material exhibits a uniform and non defective coating (20 µm), a strong resistance against general attrition and local mechanical properties of coating and interface measured by nanoindentation in the order of magnitude of conventional γ-Al2O3 beads. Metallic nano-particles deposited by incipient wetness impregnation of palladium nitrate solution are more preferentially located into the γ alumina shell. Activity and selectivity of Pd core/shell bimaterial catalyst are hugely improved compared to traditional catalyst (Pd deposited onto αAl2O3 beads) in selective hydrogenation of styrene/isoprene model mixture. This metallic core/shell catalysts are thus promising candidates for reactions sensitive to intra-particular diffusion limitations. Keywords: core/shell bimaterial, alumina support, indentation, Pd catalyst, selective hydrogenation

1. Introduction Monoenes, used in petrochemistry, are achieved by selective hydrogenation of a mixture of mono and polyunsaturated hydrocarbons produced invariably by conversion processes such as steam cracking of fluid catalytic cracking. It is well known that many heterogeneous metal catalysts exhibit high activity in the hydrogenation of the carbon– carbon double and triple bond. However, palladium is reported to be the most active and selective metal to achieve selective hydrogenations [1,2,3]. Recent developments have shown that hydrogenation selectivity and activity are very sensitive to palladium distribution on porous supports. Best performances were reported for egg-shell type catalysts where the palladium particles were deposited in crust [4,5]. This palladium distribution makes it possible to reduce intraparticular diffusion limitations. This kind of catalyst can be achieved for example by impregnation of metallic palladium nano-particles in a colloidal suspension onto porous alumina [6].

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The present study is the achievement of a broader one which aims at proposing a support ready for a controlled thickness deposition of metallic palladium particles by an easy incipient wetness impregnation of palladium nitrate solution. These new supports are spherical bi-materials whose core is a low specific surface area α alumina and the coating is a high specific surface area γ alumina. Indeed, γ alumina hydroxyls are protonated in contact with an acidic palladium solution. This leads to a pH shift of the solution and hence triggers the precipitation of palladium hydroxide nano-particles into the catalyst support [7,8]. The latter mechanism is made useful to locate palladium into the high specific surface area coating as the latter comprises the highest number of surface hydroxyls while keeping a catalyst with a globally low specific surface area which has been shown to be advantageous for selective hydrogenation [9]. We will describe here the elaboration of such a catalyst as well as the mechanical strength of such bi-material - crucial for an industrial application - and its catalytic performances.

2. Experimental section The core-shell bimaterials were prepared by coating of γ alumina powder (filler) and boehmite sol (binder) on α alumina beads and were analyzed by textural and mechanical characterization.

2.1. Coating procedure The binder was obtained by peptisation of boehmite (Pural SB3-Sasol) with nitric acid solution containing polyvinyl alcool (Carlo Erba). A dispersion (HNO3/AlOOH = 3.35 %(w/w), PVA/(PVA+AlOOH) = 2.5%(w/w), AlOOH/(AlOOH+H2O) = 3%(w/w)) was agitated during 2 h before removing the unpeptized boehmite by a centrifugation at 3800 g during 20 min. Coating device was a laboratory pan granulator GRELBEX P30 equipped with a cylindrical conical bowl. In the first step, 100 g of α alumina macroporous beads (1.6 mm, 9 m2/g, 64 nm, 0.24 mL.g-1) denoted Spheralite 537c (Spheralite 537–Axens calcined at 1273 K and sieved) were placed into the bowl under cascade state of flow at rotary speed of 40 rpm and 30° angle. Coating thickness was chosen to be 20 µm and corresponded to the use of 8.3 mL (6.4 g) of homemade mesoporous γ alumina powder (2 µm, 223 m2/g, 8 nm, 0.35 mL.g-1), considering the α−Al2O3 beads external surface and process efficiency. Coating procedure started with wetting of Spheralite 537c surface with the binder during 28 min with a volumetric flow rate of 1 mL.min-1. When the cascade state of flow was about to vanish, beads had received on their surface a liquid film large enough to collect efficiently the filler. Then, the filler volume was continuously added during 81 min with a volumetric flow rate of 1 mL.min-1 under the binder spraying and with an applied hot air flow (inlet temperature : 343 K). After adding the precursors, the coated cores were dried in a ventilated drying oven at 303 K during three days. Dried coated materials were calcined in a muffle furnace at 873 K for 2 hours in air with a heating rate of 3 K.min-1. Boehmite is then converted by a topotactic transformation into γ−Al2O3 and loose some structural water during this stage [10].

2.2. Textural characterization Pore size distributions of materials were determined Hg-porosimetry (Autopore 4Micromeretics). Specific surface areas and mesopore size distributions were calculated from nitrogen physisorption measurements (ASAP 2420-Micromeretics) by B.E.T. and B.J.H. mathematical treatment, respectively. Direct observations of raw materials and

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dried coated materials were performed by SEM (Supra 40-Zeiss). Microstructure was analyzed by SEM (Supra 40-Zeiss) on polished section of dried coated beads embedded inside an epoxy resin.

2.3. Mechanical characterization Ultralow load indentation, also known as nanoindentation, is a widely used tool for measuring the mechanical properties of thin films and small volumes of material. The principle is to pushing in a hard material tip called the indenter into the analyzed sample and to measure the curve load-penetration. A modified commercial nanoindenter (Nano indenter XP – MTS) was used to characterize coated materials. The device allows to measure the contact stiffness with superimposing a harmonic oscillation (small amplitude of 3 nm, constant frequency of 32 Hz) to the continuous penetration of the indenter into the sample. This specificity allows one to continually measure the elastic modulus and hardness according to the penetration depth. Loubet et al. demonstrated that reduced Young modulus and hardness for a Berkovich indenter with a dynamic measurement method could be deduced from the following equations [11]: E* =

S 2

H=

π Aind

= 0,149.

(h

r

'

S + h0 )

P P = Aind 35.36 × (hr ' + h0 )2

Equation 1 Equation 2

with E* is the reduced Young modulus (GPa), S the contact stiffness (N/m), Aind the indentation area for a Berkovich indenter (Aind = 35.36(hr’+h0)2), hr’ the plastic depth under loading (m), h0 the tip indenter defect (10-9 m), P the applied load (N). Coating and core characterization were respectively done with 100 mN and 450 mN maximum loads with a 3.10-2 s-1 loading rate. Seven indentations were performed on polished section of coated beads embedded inside a bakelite resin either for core and coating. An attempt for characterizing coating adhesion was undertaken with a diamond cube corner indenter. The principle relies on initiating and propagating a crack at the core/coating interface. To give a common parameter of adhesives properties, Chicot et al. proposed an apparent interfacial tenacity KIC (Pa.m-1/2) depending on the critical point load Pc (0.450 N), the length of propagated flaw c (m), a calibration parameter χV (0.015) and the Young modulus and hardness ratio (E/H)I of the interface [12]: 1/ 2

PC ⎛E⎞ K IC = χV ⎜ ⎟ 3/ 2 H c ⎝ ⎠I

Equation 3

As indentation is performed at coating/core interface (I), hardness and elastic properties of core (S) and coating (R) are concerned during loading. The following relation was proposed to take into account the latter mechanical properties: 1/ 2

1/ 2

⎛E⎞ ⎜ ⎟ ⎝ H ⎠I

1/ 2

⎛E⎞ ⎛E⎞ ⎜ ⎟ ⎜ ⎟ H ⎠S ⎝ ⎝ H ⎠R + = 1/ 2 1/ 2 ⎛H ⎞ ⎛ HS ⎞ ⎟⎟ 1 + ⎜⎜ 1 + ⎜⎜ R ⎟⎟ ⎝ HR ⎠ ⎝ HS ⎠

Equation 4

2.4. Catalysts preparation The preparation method used an aqueous solution of palladium nitrate (9.85%w/w, Engelhard). Incipient wetness impregnation was realized in a rotating beaker. 0.22 cm3 of palladium nitrate at adequate dilution were impregnated per gram of support corresponding to the total porous volume. After drying at 393 K, the catalysts were calcined under airflow at 473 K during 2 h. The palladium loading was 0.2 g.100 cm-3.

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2.5. Metallic dispersion Metallic dispersion of each catalyst was determined by measuring amount of CO chemisorbed at the metallic particle surfaces. Solids were previously reduced at 423 K under H2 flow, and return to room temperature was effected under helium flow. After 1h at 303 K under helium, CO pulses were operated and CO consumption was measured. Pd dispersion was then calculated assuming that the ratio between chemisorbed CO and metallic atoms is one. The following equation allows us to obtain metallic dispersion (percentage of surface atoms relative to total metal atoms) from CO consumption. D=

Va × M Pd % M × Vm × χ

Equation 5

where Va is adsorbed CO volume (mL.g-1), MPd molar weight of palladium (106.4 g.mol-1), Vm molar volume (24000 cm3 at 293 K), χ stoichiometric coefficient CO/Pd = 1, %M metallic weight.

2.6. Metal distribution Castaing micro-probe analysis was performed to determine the radial profile of Pd concentration along the diameter of the alumina beads. The preparation for the measurement includes an embedding in a metacrylate resin, polishing with SiC paper and coating with carbon black. Measurements were performed with a JEOL 8100 microprobe a semi-quantitative way.

2.7. Catalytic performances Mixture of styrene-isoprene in n-heptane was engaged in selective hydrogenation in liquid phase using a laboratory-scale stainless-steel and perfectly stirred batch reactor with variation of the concentration of reactants and products over time. 2 cm3 of shaped catalyst initially reduced under hydrogen flow at 423 K during 2 h, was transferred under Ar in a glove bag into the batch reactor filled with 210 mL of n-heptane. Catalyst beads were fixed in an annular basket located around the stirrer. The catalyst was then put into contact with about 34g of reactants (50%w isoprene, 50%w styrene) at 318 K under 35 bars of H2 and at a stirring velocity of 1600 rpm. A pressure gauge before the batch reactor maintains the pressure constant inside the reactor at 35 bars. The course of the reaction was followed by the loss of H2 pressure in the pressure gauge and by gas chromatography analysis (PONA column, split injector, FID detection). Experimental conditions were previously selected in order to avoid mass transfer limitations. Activities of catalysts were based on the rate of consumption of H2 for the hydrogenation of isoprene and styrene to 2-methylbutenes and ethylbenzene respectively before the formation of saturated product 2-methylbutane – formation of ethylcyclohexane is not detected in our conditions (Equation 6). Their selectivities in total hydrogenation were calculated in percentage relative to i-pentane formation (Equation 7). A( mol. min −1 .cm 3 ) =

VH 2 × PH 2 Vc × R × T

Equation 6

where VH2 is hydrogen volume (L), PH2 loss of pressure per minute (bar.min-1), VC catalyst volume (cm3), R molar gas constant (0.0829 L.bar.mol-1.K-1) and T temperature (K). S (%) =

% w(i − C5= ) × 100 % w(i − C5= ) + % w(i − C5 )

Equation 7

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where %w(i-C5=) 2-methylbutenes concentration (%) and %w(i-C5) 2-methylbutane concentration (%).

3. Results 3.1. Microstructural and textural characterization SEM micrographs indicate that dry deposited coating is continuous with a 20 µm homogenous thickness (Figure 1-a). The coating microstructure is granular as expected considering the coating formulation (Figure 1-b). The binder is homogenously dispersed between γ-Al2O3 grains and at γ-Al2O3 grains and α-Al2O3 core interfaces. Some very thin longitudinal cracks rise scarcely through the coating. The latter cracks are triggered by boehmite gel shrinkage during drying. a)

b)

Figure 1. SEM micrographs of dried coated Spheralite 537c : a) whole coated bead, b) coating microstructure.

The N2 physisorption and Hg porosimetry measurements of the calcined coated materials highlight a macroporosity (64 nm), and a mesoporosity (8 nm) brought by the core and the shell, respectively. The total specific surface area of the coated Spheralite 537c has increased up to 19 m2.g-1.

3.2. Mechanical characterization Figures 2-a and 2-b exhibit load-displacement curves obtained for core and coating. The spread of the results are due to the nature of our materials which belongs to porous ceramics. The average depth into the material is 3.4 µm. Even if the loading applied has been decreased to 100mN for the shell, the proximity of the indent and core induces a modification of the measured coating mechanical properties (Figure 2-d). This was taken into account in reduced Young modulus calculation by selecting first points obtained during loading (Figure 2-c). All the calculated mechanical properties are grouped in Table 1. Table 1. Mechanical properties of calcined coated Spheralite 537c (E was calculated with ν = 0.3). Localization Core Coating

Hmoy(GPa)

E*moy (GPa)

Emoy(GPa)

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65±5 10.3±1,5

59±5 9.4±1,5

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Figure 2. Load–displacement curves for a) the core, b) the coating; c) reduced Young modulus given as plastic depth under loading for coating; d) optical observation of indents in the coating.

Ten core/coating interface loadings have produced only one crack initiation and propagation through the interface. This arises from the difficulty to accurately target the interface and from the limited applied strain by nanoindentation although a cube corner indenter was used. The measured crack length c is 43 µm. Using Equations 3 and 4, the apparent interface tenacity KIC was calculated to be ~0.135 MPa.m1/2. Courroyer et al. performed mechanical characterization of γ-Al2O3 beads commonly used in reforming operation [13]. Calculated mechanical properties were E=11.1±1.5 GPa, H=0.32±0.07 GPa, and KC = 0.178±0.021 MPa.m1/2. Comparing the mechanical properties of coating on Spheralite 537c with the latter values, it appears that Young modulus and hardness are in the same magnitude order. The apparent interface tenacity is also comparable to the bulk tenacity of γ-Al2O3 typical supports.

3.3. Catalytic performances Two Pd catalysts were prepared following the procedure indicated on experimental section, on γ-alumina coated Spheralite 537c (A) and on Spheralite 537c (B) as a reference. Properties of these materials are illustrated on Table 2. Both catalysts contain the same amount of palladium per volume. On Figure 3-a, solid exhibits a high palladium concentration into the 20-30 first micrometers from the external surface corresponding to the thickness of γ-Al2O3 coating. This means, as expected, that deposition of metal precursor was favored within the shell of high specific surface area γ alumina, whereas metal penetration occurred in much more amount along the whole diameter beads with catalyst on core Spheralite 537c (Figure 3-b). Besides, one can notice that catalyst A on core-shell bimaterial as support shows a higher metallic dispersion than on core Spheralite 537c. This could be explained by the preferential localization of metal for the first material in the outer shell of high specific surface area inducing stronger metal-support interactions during catalyst preparation causing then smaller particles than for catalyst B.

Hierarchical porous catalyst support

199

Table 2. Properties of Pd catalysts prepared on coated and non-coated Spheralite 537c. Catalyst A B

Carrier coated Spheralite 537c Spheralite 537c

Pd vol. content (g.100cm-3)

Pd mass content (%)

Apparent bulk density (g.cm3 )

Metallic dispersion (%)

0.20

0.16

1.29

22

0.20

0.17

1.18

9

a)

Palladium concentration (a.u.)

Palladium concentration (a.u.)

b)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

0.0 0.1 0.2

0.3 0.4 0.5 0.6

Beads diameter (mm)

0.7 0.8 0.9 1.0 1.1

1.2 1.3 1.4 1.5

Beads diameter (mm)

Figure 3. radial profile of Pd concentration along the diameter of the alumina beads for catalyts prepared on a) coated Spheralite 537c and b) Spheralite 537c.

Performances of both catalysts were evaluated in terms of activities and selectivities. A twice higher activity was found for the catalyst A on core-shell bimaterial compared to catalyst B (Figure 4-a). A benefit in total hydrogenation selectivity was also measured especially from 60% to higher isoprene conversion (Figure 4-b). The preferred localization of metal into the outer layer for catalyst A may induced a decrease in internal mass transfer limitations. For the same solid, a higher metallic dispersion could allow more active sites to catalyze hydrogenation reactions. These two phenomenon could be at the origin of the gain in activity. Nevertheless, it seems difficult to figure out their respective contributions. Concerning selectivities, it is already known that selective hydrogenation of poly-unsaturated compounds are not favored with higher metallic dispersion catalysts (with small particles) [14]. Thus, the better selectivity observed with catalyst on coated Spheralite 537c is certainly due to lower intraparticular mass transfer limitations induced by metal profile of catalyst A. a) 8 b) 100 95

Coated Spheralite 537c

6

Selectivity (%)

Activity (.10-03 mol.min-1.cm 3)

7

5 4 3 2

Spheralite 537c

90

85 80 75

1

70 0

0

Coated Spheralite 537c

Spheralite 537c

20

40

60

80

100

Isoprene conversion (%)

Figure 4. catalytic performances in terms of a) activities and b) selectivities for catalysts prepared on coated and non-coated Spheralite 537c.

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4. Conclusion A bimaterial catalyst support was obtained in a pan granulator with a composite sol-gel formulation based on γ-Al2O3 filler, boehmite binder and α-Al2O3 beads. The resultant catalyst support shows a homogenous coating with a twenty micrometer thickness. Local mechanical properties of coating and interface are in the magnitude order of conventional γ-Al2O3 beads. Deposited metallic palladium nano-particles on this bimaterial are very preferentially located into the γ-Al2O3 coating as expected. Activity and selectivity of the bi-material catalyst show a huge improvement compared to the reference catalyst using conventional carrier. This study demonstrated that bi-material catalyst are promising candidate for all industrial catalytic reactions that present intraparticular diffusion limitations as mechanical properties and catalytic performances are very satisfactory. It should be planned in the future to extend the concept to multifunctional catalysis.

References [1] J.-P. Boitiaux, J. Cosyns, M. Derrien, G. Léger, 1995, Newest hydrogenation catalysts, Hydrocarbon Processing, 64, 3, 51-59. [2] G.C. Bond, P.B. Wells, 1964, Advances in catalysis, Academic Press, 15, 91-226 [3] J.-F. Le Page et al., 1978, Catalyse de contact. Conception, préparation et mise en oeuvre des catalyseurs industriels, Technip, 63-80. [4] R. Krishna, S.T. Sie, 1994, Strategies for multiphase reactor selection, Chemical Engineering Science, 49,24, 4029-4065. [5] T.-B. Lin, T.-C. Chou, 1994, Selective hydrogenation of isoprene on eggshell and uniform palladium profile catalysts, Applied Catalysis A: General, 108, 7-19. [6] D. Heineke, E. Schwab, M. Fischer, G. Schmid, M. Baeumle, 1998, Palladium clusters and their use as catalysts, EP 0 920 912 B1. [7] T. Pagès, 1998, PhD Thesis, Université Pierre et Marie Curie, Paris VI. [8] S. Verdier, 2001, PhD Thesis, Université Pierre et Marie Curie, Paris VI. [9] S. Asplund, 1996, Coke formation and its effect on internal mass transfer and selectivity in Pd-catalysed acetylene hydrogenation, Journal of Catalysis, 158, 267-278. [10] Handbook of porous solids Vol 3, 2002, Edited by F. Schüth, K.S.W. Sing, J. Weitkamp, Wiley-VCH, Chapter 4.7.2. [11] J.L. Loubet, M. Bauer, A.Tonck, S. Bec, B. Gauthier-Manuel, 1993, Mechanical Properties and Deformation Behavior of Materials Having Ultra-Fine Microstructures. Kluwer academic publishers, 429-447. [12] D. Chicot, P. Démarécaux, J. Lesage, 1996, Apparent Interface toughness of substrate and coating couples from indentation tests, Thin Solid Films, 283, 151-157. [13] C. Couroyer, M. Ghadiri, P. Laval, N. Brunard, F. Kolenda, 2000, Methodology for investigating the mechanical strength of reforming catalyst beads, Oil & Gas Science and Technology, 55, 1, 67-85. [14] J.P. Boitiaux, J. Cosyns, S. Vasudevan, 1983, Hydrogenation of highly unsaturated hydrocarbons over highly dispersed palladium catalyst. Part I : Behaviour of small particles, Applied Catalysis, 6, 41-51.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Catalytic property of carbon-supported Pt catalysts covered with organosilica layers on dehydrogenation of organic hydride Keizo Nakagawa,*a,b,c Yusuke Tanimoto,c Tetsuya Okayama,c Ken-Ichiro Sotowa,a,b,c Shigeru Sugiyama,a,b,c Toshihiro Morigaa,c a

Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima, Minamijosanjima, Tokushima 770-8506 b Department of Geosphere Environment and Energy, Center for Frontier Research of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770-8506 c Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770-8506

Abstract Carbon-supported Pt metal nanoparticles were covered with a silica layer including phenyl or methyl groups using successive hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and phenyltriethoxysilane (PhTES) or methyltriethoxysilane (MTES), followed by reduction with H2. Highly dispersed Pt nanoparticles could be produced in silicacoated Pt catalysts using each organosilane. The results of N2 adsorption showed that micropores were formed in the silica layer by introducing functional groups into the silica network. Because the microporous structure of silica layers which wrapped Pt metal particles increased the diffusion capability of cyclohexane, the Pt catalyst covered with silica layers containing functional groups showed higher activity in the cyclohexane dehydrogenation, compared with Pt catalysts covered with a silica layer containing no functional groups. Keywords: Pt metal particles, silica layer, organosilane, dehydrogenation of cyclohexane

1. Introduction Metals or metal oxides supported on supports are often used as catalysts for various catalytic reactions because the deposition of metal species on the supports results in the improvement of catalytic activity and selectivity, and/or in the inhibition of their sintering at high temperatures due to the chemical interaction between the metal species and the supports. Supported metal catalysts covered with silica layers have been studied [1-13]. Metal nanoparticles supported on supports were covered with silica layers of a few nanometers thickness by hydrolysis of silicon alkoxides such as tetraethoxysilane (TEOS). We demonstrated that the metal particles in these silica-coated catalysts showed good resistance for sintering, even at high temperatures, because the metal nanoparticles were covered with silica layers. In consequence, these silica-coated metal catalysts allowed preferential formation of carbon nanotubes or nanofibers with uniform diameters through ethylene decomposition, while the metal catalysts without silicacoating formed carbon nanotubes or nanofibers with various diameters because the metal particles aggregated severely during ethylene decomposition [6,7,11,12]. The silica-coated metal catalysts also showed a high stability for the repeated potential cycling experiment as a Pt electrocatalyst for a proton-exchange-membrane fuel cell

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[8,13]. Thus, the coverage of metal particles with silica layers is an effective method to enhance the stability of catalysts. Meanwhile, the metal particles in these silica-coated catalysts also showed specific reactant shape selectivity because the silica layer which wrapped Pt metal particles have porous structure. As a result, the silica-coated metal catalysts showed a specific performance for competitive oxidation of mixed hydrocarbons because the porous silica structure controlled the diffusion rate of reactant molecules [3,4]. The development of silica-coated catalysts to provide an increase of pore size or addition of functionality of the silica layer covering the metal particles, leads to an increase in their application for catalytic reactions. Organically functionalized materials have interesting effects on the porosity, adsorption and diffusion of reactants, and ultimately on the control of the surface reactivity [14,15]. These materials can be synthesized by using organoalkoxysilanes as precursors for the sol-gel reactions in which organic groups are introduced within an inorganic network through the Si-C bond. Thus, this synthesis method can be applied to silica-coated metal catalysts. In the present study, carbon-supported Pt catalysts covered with organosilica layers using phenyltriethoxysilane (PhTES) or methyltriethoxysilane (MTES) as the silica source were prepared. In addition, these catalysts were applied to the dehydrogenation of cyclohexane. We would report the specific catalytic performance of Pt catalysts covered with silica layers containing functional groups in the dehydrogenation of cyclohexane, compared with Pt catalysts covered with silica layers containing no functional groups. The effect of the amount of SiO2 of carbon-supported Pt catalysts covered with organosilica layers using MTES was also investigated.

2. Experimental Carbon black (CB) (Vulcan XC-72 supplied by Cabot Co.) was used as a support for Pt particles. CB was immersed in an aqueous solution containing H2PtCl6, and aqueous NH3 was added to the solution to deposit Pt metal precursors onto the CB. After this solution was filtered, the sample was dispersed in a solution containing aqueous NH3, and the successive hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and other organosilanes, such as tetraethoxysilane (TEOS), phenyltriethoxysilane (PhTES) and methyltriethoxysilane (MTES) was performed at 333 K for 1.5 h to form the silica layer containing each functional group on the CB. The mole ratios of TEOS to Pt cations in the aqueous solution were changed in order to prepare the catalysts with different loadings of Pt. The obtained sample was dried at 333 K in air, then exposed to an atmosphere of H2 at 623 K for 3 h. Hereafter, the samples obtained are denoted as SiO2(each organosilane) /Pt/CB catalysts. For comparison, CB-supported Pt metal particles (Pt/CB) were prepared by conventional impregnation. Catalytic cyclohexane dehydrogenation was performed in a batch-wise reactor in the condition that catalytic dehydrogenation could be accomplished efficiently with carbon-supported Pt in the liquid-film state [16,17]. Cyclohexane dehydrogenation was performed under boiling and refluxing conditions by heating at 523 K and cooling at 278 K at atmospheric pressure. Cyclohexane (1.0 ml) and 0.3 g of catalyst were used in the catalytic reaction. The hydrogen that was evolved from the cyclohexane was collected in a gas buret and pursued volumetrically for 150 min. X-ray absorption spectra for the samples were measured at the Photon Factory in the Institute of Materials Structure Science for High Energy Accelerator Research Organization, Tsukuba, Japan, with a ring current of 2.5 GeV and a stored current of 250–450 mA. Pt LIII-edge EXAFS was measured at the beam line BL-7C and 9C

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equipped with Si(111) in transmission mode at room temperature (Proposal No.2006G343 and 2009G087). Analysis of EXAFS data was performed using an EXAFS analysis program, REX (Rigaku Co.). Inversely Fourier-transformed data for Fourier peaks were analyzed by a curve-fitting method, using phase-shift and amplitude functions estimated from EXAFS spectrum of Pt foil. The content of Pt, SiO2 and carbon in the CB-supported Pt metal nanoparticles covered with organosilica layers was evaluated by X-ray fluorescence spectroscopy (XRF) and elemental analysis. Transmission electron microscopy (TEM) images of the samples were recorded with a Hitachi H-800 instrument (Hitachi High-Technologies Co.). Specific surface areas were calculated from the adsorption isotherm obtained with a conventional BET nitrogen adsorption apparatus (BELSORP-18SP, Bell Japan Inc.). The exposed surface areas of the Pt metal particles in the silica-coated Pt/CB were evaluated by the CO adsorption method (BELCAT, BEL Japan Inc.) at 323 K, assuming an adsorption stoichiometry of 1:1 for CO/Pt. Before the measurement of CO adsorption, the samples were treated with hydrogen at 623 K for 30 min.

3. Results and discussion Table 1 presents the SiO2, Pt and carbon contents in Pt/CB and silica-coated Pt catalysts using different organosilanes. The Pt loading of all catalysts were about 1–2wt%. The SiO2 loading of silica-coated Pt catalysts were changed from about 30 to 55wt% by changing the organosilanes. Figure 1 shows TEM images of Pt/CB and silicacoated Pt catalysts using different organosilanes. CB and Pt metal nanoparticles were observed in all TEM images. The diameter of the Pt particles in Pt/CB ranged from 1 to 3 nm. However, some aggregated Pt metal particles with diameters of 8 to 10 nm were also observed in Pt/CB. In contrast, the diameter of the Pt particles in SiO2(TEOS)/Pt/CB, SiO2(PhTES)/Pt/CB and SiO2 (MTES)/Pt/CB ranged from 1 to 3 nm. Thus, highly dispersed Pt nanoparticles could be produced in silica-coated Pt catalysts using each organosilane.

Table 1. Contents of SiO2, Pt and C in Pt/CB and silicacoated Pt catalysts using different organosilanes. Sample Pt/CB SiO2(TEOS)/Pt/CB SiO2(PhTES)/Pt/CB SiO2(MTES)/Pt/CB

SiO2/wt%

Pt/wt%

55.1 31.4 41.1

1.3 1.8 1.9 0.8

C/wt % 98.7 43.1 66.7 54.2

(a)

(b)

(c)

(d)

Fig. 1 TEM images of (a) Pt/CB, (b) SiO2(TEOS)/ Pt/CB, (c) SiO2(PhTES)/Pt/CB, (d) SiO2(MTES)/Pt/CB.

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Absorbance / a.u.

Absorbance / a.u.

In order to confirm the (a) (b) existence of functional groups in 700 the silica layer of SiO2(PhTES)/ 740 1273 Pt/CB and SiO2(MTES)/Pt/CB, FT-IR measurement was performed. Si-C bond absorption derived from phenyl groups were observed at 700 and 740 cm-1 in SiO2(PhTES)/Pt/CB as shown in Fig. 2(a). Meanwhile, Si-C bond 850 800 750 700 650 600 550 1300 1200 1100 1000 900 Wavenumber / cm Wavenumber / cm absorption derived from methyl groups was observed at 1273 Fig. 2 FT-IR spectra of (a) SiO2(PhTES)/Pt/CB, (b) cm-1 in SiO2(MTES)/Pt/CB as SiO2(MTES)/Pt/CB. shown in Fig. 2(b). These peaks are in agreement with those from previous data [18]. In addition, ca. 20% weight loss in SiO2(PhTES)/Pt/CB and ca. 5 % weight loss in SiO2(MTES)/Pt/CB compared with the weight loss in SiO2(TEOS)/Pt/CB were observed in the results of thermogravimetric analysis (results not shown). These results strongly indicate that phenyl groups and methyl groups existed in the silica layers of SiO2(PhTES)/ Pt/CB and SiO2(MTES)/Pt/CB, respectively. Pt/CB covered with silica layers was prepared using the successive hydrolysis of APTES and organosilanes such as TEOS, PhTES and MTES in the presence of Pt/CB. In previous studies, APTES was adsorbed on CB supports through the interaction between graphene in CB and amino groups in APTES, which resulted in the coverage of Pt/CB with uniform silica layers of thickness (< 1 nm) [9,11]. The subsequent hydrolysis of organosilanes in the presence of Pt/CB covered with thin silica layers from APTES is expected to cover Pt/CB with silica layers of a few nanometers thickness. It should be noted that the Pt particles in silica20 coated Pt catalysts using each organosilane seem to be covered with a silica layer and were not 15 found on the outer surface of the silica layers, but in their bodies. These results suggest that the (a) surface of the Pt particles and 10 CB can be uniformly covered with silica layers by the succes(b) sive hydrolysis of APTES and 5 each organosilane. (c) Figure 3 shows Fourier trans(d) forms of Pt LIII-edge k3-weighted 0 EXAFS spectra (RSFs; radial 0 1 2 3 4 5 6 structural functions) for silicaR/ Å 3 coated Pt catalysts using dif- Fig. 3 Fourier transforms of Pt LIII-edge k -weighted ferent organosilanes. A strong EXAFS for (a) Pt foil, (b) SiO2(TEOS)/Pt/CB, (c) peak was observed at 2.7 Å in a SiO2(PhTES)/Pt/CB, (d) SiO2(MTES)/Pt/CB. The intensity RSF for Pt foil. This peak could of the peak for Pt foil was halved. be assigned to the presence of the neighbouring Pt atoms in Pt foil. In the RSFs for each silica-coated Pt catalyst using different organosilanes, a strong peak was observed at the same position as that for Pt |FT|

-1

-1

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3

Amount adsorbed [cm (STP)/g]

foil. In addition, the features of Table 2. Structural parameters estimated by the RSFs for silica-coated Pt cata- curve-fitting analysis for the Pt LIII-edge EXAFS lysts using different organosilanes spectra of each catalyst. were similar to each other. This Sample R/ Åa C.N.b result indicates that most Pt SiO2(TEOS)/Pt/CB 2.73 7.9 species in silica-coated Pt cataSiO2(PhTES)/Pt/CB 2.71 7.4 lysts using different organosilanes were present as Pt metal. The SiO2(MTES)/Pt/CB 2.70 7.7 intensity of the peak at 2.7 Å in aR, interatomic distance of Pt-Pt; bC.N., cothe RSFs of silica-coated Pt ordination number of Pt-Pt. catalysts using different organosilanes did not change. As for the RSFs of any metals, the peak intensity is sensitive to their crystallite size. These RSFs implied that the crystallite sizes of Pt metal in silicacoated Pt catalysts using different organosilanes were similar to each other. In order to confirm the structure of Pt species in silica-coated Pt catalysts using different organosilanes in detail, the curve-fitting analyses were performed for the Fourier transforms of Pt LIII EXAFS spectra shown in Fig. 3. The structural parameters estimated by the curve-fitting analysis for the EXAFS were listed in Table 2. The peak in the RSF was inversely Fourier-transformed in the ranged of R = 1.7–3.4 Å and the k3-weighted EXAFS spectra was fitted in the range of 4–15 Å by using amplitude function and phase sight extracted from the EXAFS spectra for Pt foil. All the EXAFS spectra for silica-coated Pt catalysts using different organosilanes could be fitted with a shell of Pt-Pt bond. The coordination number and interatomic distance of Pt-Pt bonds in silica-coated Pt catalysts using different organosilanes did not change very much. These results indicated that Pt metal particles with almost the same crystallite size could form silica-coated Pt catalysts using different organosilanes. Figure 4 shows the N2 adsorption isotherms at 77 K of silica-coated Pt catalysts using different organosilanes. An increase of the amount of N2 adsorption is observed below P/P0 = 0.1 for SiO2(PhTES)/Pt/CB and SiO2 140 SiO2(MTES)/Pt/CB (MTES)/Pt/CB as compared with SiO2(PhTES)/Pt/CB SiO2(TEOS)/ Pt/CB, suggesting 120 SiO2(TEOS)/Pt/CB the additional formation of micropores in the silica layers. 100 The specific surface area evaluated from the BET method was 80 208 m2/g for Pt/CB. On the other 60 hand, the specific surface area was 37 m2/g for SiO2(TEOS)/Pt/ 40 CB, 103 m2/g for SiO2(PhTES)/ 2 Pt/CB and 187 m /g for SiO2 20 (MTES)/Pt/CB. The specific surface areas of any silica-coated Pt 0.0 0.2 0.4 0.6 0.8 1.0 catalysts using different organRelative Pressure [-] osilanes were smaller than that of Pt/CB. The decrease of speci- Fig. 4 N2 adsorption measurements of silica-coated Pt fic surface area of silica-coated catalysts using different organosilanes. catalysts was because of the blocking of pores of CBs by a silica layer. Although it is difficult to measure the pore size or the specific surface area of only silica layer in the catalysts from N2 adsorption measurements because of the presence of the CB support,

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TOF / min

-1

Amount of hydrogen generation -2 / mmol m -Pt

these findings suggest that the increase of specific surface area was because of the formation of microporous silica layer which wrapped Pt metal particles by introducing phenyl or methyl groups into the silica network. Cyclohexane dehydrogenation was performed over Pt/CB and silica-coated Pt catalysts using different organosilanes. Figure 5 shows the time courses of amount of hydrogen generation from cyclohexane dehydrogenation with SiO2(TEOS)/Pt/CB, SiO2(PhTES)/Pt/CB and SiO2(MTES)/Pt/CB. In this batch reactor, the conversion of Pt/CB was about 80%. The amount of hydrogen generation of SiO2(TEOS)/Pt/CB was very low. In contrast, the amount of hydrogen generation of SiO2(PhTES)/Pt/CB and SiO2(MTES)/Pt/CB were significantly higher than that of SiO2(TEOS)/Pt/CB as shown in Fig. 5. It is assumed that this result was responsible for the diffusion of cyclohexane into the silica layer because Pt metal particles were similar with each other from the result of EXAFS spectra as shown in Fig. 3 and Table 2. Cyclohexane made contact with catalytically active Pt metal 25 particles on the CB after they difSiO2(MTES)/Pt/CB MTES 20 fused into the silica layer. The result SiO2(PhTES)/Pt/CB PhTES SiO2(TEOS)/Pt/CB of the low amount of hydrogen geneTEOS 15 ration for SiO2(TEOS)/Pt/CB implied that the pore size in the silica layer is 10 not large enough for the diffusion of cyclohexane in SiO2(TEOS)/Pt/CB. 5 On the other hand, higher amounts of hydrogen generation were obtained 0 for the SiO2(PhTES)/Pt/CB and SiO2 0 10 20 30 40 50 60 (MTES)/Pt/CB because diffusion time on stream / min capability of cyclohexane into the silica layer increased as a results of Fig. 5 Change of Amount of hydrogen generation formation of micropores in the silica with time on stream in the cyclohexane dehydrolayer considering from the results of genation over each catalyst. N2 adsorption. Thus, SiO2(PhTES)/ 0.3 Pt/CB and SiO2(MTES)/Pt/CB are effective catalyst for the catalytic reaction involved larger molecules 0.2 because higher activities were obtained nevertheless Pt metal particles are covered with silica layers. 0.1 We carried out the cyclohexane dehydrogenation over SiO2(MTES)/ Pt/CB with different SiO2 loadings 0.0 in order to examine the effects of 0 wt% 10.6 wt% 27.7 wt% 41.7 wt% SiO2 loading on catalytic performance. Figure 6 shows the turnover Fig. 6 Turnover frequency for the dehydrogenation frequency (TOF) estimated based on of cyclohexane over SiO2(MTES)/Pt/CB with difthe rate of hydrogen generation and ferent amount of SiO2. the number of Pt atoms at the surface of Pt metal particles which was evaluated by CO adsorption on SiO2(MTES)/Pt/CB. As shown in Fig. 6, the TOF for the cyclohexane dehydrogenation decreased with the higher SiO2 loading, i.e. with thickness of silica layers which wrapped the catalytically active Pt metal. It is likely that

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the TOF depends on the thickness of silica layers of SiO2(MTES)/Pt/CB because the diffusion rates of cyclohexane in silica layers affect the reaction rates over SiO2(MTES)/ Pt/CB. Although the activity of SiO2(MTES)/Pt/CB was not beyond that of Pt/CB, the activity of SiO2(MTES)/Pt/CB could be improved by decreasing the thickness of SiO2 layer which wrapped the catalytically active Pt metal. Thus, we developed Pt catalyst covered with microporous silica layers with higher catalytic activity for the cyclohexane dehydrogenation by introducing functional groups into silica layers.

4. Conclusion CB-supported Pt metal nanoparticles were covered with silica layers including phenyl groups or methyl groups using successive hydrolysis of APTES and PhTES or MTES. Micropores could be formed in the silica layers by introducing functional groups into the silica network. The Pt catalyst covered with silica layers containing functional groups showed higher activity in the cyclohexane dehydrogenation, compared with Pt catalysts covered with a silica layer containing no functional groups. This catalyst can be applied to various catalytic reactions involved larger molecules.

Acknowledgment This work was funded by a Grant-in-Aid for JST Research for Promoting Technological Seeds (2008) and a Grant-in-Aid for Young Scientists (B) KAKENHI 20750167 to K.N. The authors gratefully acknowledge Dr M. Tagami (Center for Technical Support, Institute of Technology and Science, The University of Tokushima) for his assistance with TEM experiments.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

M. Kishida, T. Tago, T. Hatsuta and K. Wakabayashi, Chem. Lett., 29 (2000) 1108. T. Tago, T. Hatsuta, K. Miyajima, M. Kishida, S. Tashiro and K. Wakabayashi, J. Am. Ceram. Soc., 85 (2002) 2188. S. Takenaka, K. Hori, H. Matsune, M. Kishida, Stud. Surf. Sci. Catal., 162 (2006) 585. K. Hori, H. Matsune, S. Takenaka, M. Kishida, Sci. Tech. Adv. Mater., 7 (2006) 678. S. Takenaka, H. Umebayashi, E. Tanabe, H. Matsune and M. Kishida, J. Catal. 245 (2007) 392. K. Nakagawa, S. Takenaka, S. Imagawa, H. Matsune and M. Kishida, Chem. Lett. 36 (2007) 252. S. Takenaka, Y. Orita, E. Tanabe, H. Matsune and M. Kishida, J. Phys. Chem. C 111 (2007) 7748. S. Takenaka, H. Matsumori, K. Nakagawa, H. Matsune, E. Tanabe and M. Kishida, J. Phys. Chem. C 111 (2007) 15133. S. Takenaka, T. Arike, K. Nakagawa, H. Matsune, E. Tanabe and M. Kishida, Carbon. 46 (2008) 365. S. Takenaka, T. Arike, H. Matsune, E. Tanabe and M. Kishida, J. Catal, 257 (2008) 345. S. Takenaka, T. Iguchi, E. Tanabe, H. Matsune and M. Kishida, Carbon. 47 (2009) 1251. T. Iguchi, S.Takenaka, K. Nakagawa, Y. Orita, H. Matsune and M. Kishida, Top. Catal. 52 (2009) 563. S. Takenaka, H. Matsumori, T. Arike, H. Matsune and M. Kishida, Top. Catal. 52 (2009) 731. J. Wen and G. L. Wilkes, Chem. Mater., 8 (1996) 1667 S. Tanaka, J. Kaihara, N. Nishiyama, Y. Oku, Y. Egashira and K. Ueyama, Langmuir, 20 (2004) 3780.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Molecular aspects of solid silica formation Istvan Halasz, Mukesh Agarwal, Robert E. Patterson PQ Corporation, R&D Center, 280 Cedar Grove Road, Conshohocken, PA 19428, USA

Abstract Raman spectroscopy indicates distinct differences between the molecular constitutions of amorphous silicas solidified from aqueous solutions at acidic and basic conditions which might have implications on the synthesis and properties of zeolites, mesostructured silicas and silica gels. The Qn connectivities of [SiO4] tetrahedra in the primary nanoparticles, which determine the ultimate molecular structure of gels, seem rarely to depend on the concentration, elemental composition, or molecular constitution of the dissolved alkaline silicate ingredients. Experimental and computational evidence support a surprisingly large volume of Qo silica monomers in many acid-set gels. Because of a good spectral resolution both in liquid and solid phases, robust and mobile instrumentation, low cost and ease of use, Raman spectroscopy is a preferred noninvasive analytical technique that allows one to follow in situ the complex solidification process of silicates. However, we found that FTIR spectroscopy is better suited in some cases, for example for studying the TEOS based synthesis of (Me4N)8Si8O20 x 65H2O, a starting material for the designed synthesis of double four ring (D4R) based nano structures. Keywords: silica gel, nano synthesis, zeolite, Raman, FTIR

1. Introduction Raman spectroscopy has been routinely used for identifying the Qn connectivity of [SiO4] tetrahedra in glasses and silica based minerals for decades [1, 2]. The usually well separated νs Si-O Raman bands near 850, 900, 980, 1060, and 1140 cm-1 (with about ±3% average tolerance) are considered to be characteristic of the presence and relative amount of Q0, Q1, Q2, Q3, and Q4 connected [SiO4] tetrahedra, respectively, where the superscripted numbers represent the number of Si-O-Si bonds connecting this unit with its neighboring [SiO4] building blocks [3]. Obviously, at < Q4 discontinuities must be present in the network of Si-O-Si connections and capping these “defect” points with protons or other cations is necessary to ensure charge neutrality. Since we have not seen any reason why this well established method would not work for other amorphous silica systems, its adoption for testing the structure of the sub-nano sized dissolved alkaline silicate molecules [3-8] and that of the variously prepared silica gels [9-11] seemed to be in order. Therefore, along with Raman-based ring identifications from the zeolite literature [3], we used this convenient in situ approach to exploring the formation and transformation of silica nanoparticles from dissolved alkaline silicate molecules, which is among the least understood hence least controlled steps in the synthesis of a wide variety of amorphous and crystalline silica products. While one can indeed identify marked differences between the Raman and FTIR spectra of these amorphous structures, we have found that the published peak assignments to siloxane rings and Qn connectivities scatter quite a bit which calls for their further theoretical study. Moreover, the dissociation in the aqueous environment and the presence of water were unexpectedly found to also affect the Si-O vibrations [3, 6].

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By investigating the effect of molecular structure, type of alkaline ion, alkaline/ silica ratio, concentration, and gelling method on the molecular structure of silica gels made from aqueous alkaline silicate solutions we demonstrated recently that the method of synthesis, i.e., drying or precipitating from alkaline or acidic solutions, has usually stronger impact on the Raman identified structural characteristics of products than the other parameters combined [9-11]. It has long been known that gelling can be performed both at acidic and at basic pH values and certain physical characteristics of such acid-set and base-set gels can characteristically differ from each other even when the same alkaline silicate ingredients are used [12-14]. However, the bulk molecular structures of differently made gels have virtually never been distinguished and, what is more, different molecular gel structures have rarely been assumed as a possibility. A notable exception is Iler’s more than half century old speculation about the possible linear siloxane-chain structure of acid-set gels versus a random 3D network of siloxane rings in base-set gels [13-15]. By systematically applying the electronegativity equalization principle, Livage [16] also concluded later that linear polymer chains should form from aqueous silicate solutions at acidic conditions and “branched species” at basic conditions. Recently the structure changing effect of pH was also implied for example in the success of template-free synthesis of RTH zeolite by excess NaOH [17]. In the course of our in situ vibrational spectroscopic studies we unexpectedly found that certain gelling processes, like drying or base setting of Na2SiO3 solutions [6, 9] or acid setting most alkaline silicates [10, 11], likely result in substantial Qo silica monomers in the solid. Here we focus on the clarification of this rather surprising issue also considering the above mentioned uncertainties of vibrational band assignments in the aqueous environment of silica. Further, we will show that the tetraethyl orthosilicate (TEOS) based non-aqueous synthesis of (Me4N)8Si8O20 x 65H2O, containing D4R (also named as cubic octamer or octa-silsesquioxane) siloxane entities [18-21], presumably proceeds without the explicit formation of H2SiO42- or similar hydrolyzed monomer ions from TEOS.

2. Experimental The dissolved alkaline silicate ingredients were commercial products from PQ Corporation. The solid Na2SiO3 x 9H2O and Na4SiO4 samples were purchased from Sigma and AlfaAesar, respectively. Forsterite (San Carlos, Arizona) was obtained from Ward’s Nat. Sci. Establishment, Inc., TEOS from Silbon, and TMA-OH from SAChem. For preparing base-set gels 3 M HCl solution was drop-wise added to the stirred silicate solution and the pH was measured with a Corning Scholar 425 pH meter equipped with an Accumet electrode. When the first visible gel particles appeared (usually near the electrode), the dosage of acid was stopped and we waited to see if the whole material gels within about 10 minutes or the visible particles dissolve again. In the former case the solidifying experiment was terminated and the vibrational spectrum of the fresh gel was measured. Acid-set gels were made by dosing the silicate solution into HCl solution following the same principles as for the base-set gels. Further experimental details including solution making and handling have been reported elsewhere [11]. For the synthesis of (Me4N)8Si8O20 x 65H2O we followed the Kuroda group’s method [21]. The crystal structure of product was identified by XRD. Raman spectra were obtained with 532 nm (180 mW) and 780 nm (400 mW) dispersive laser Raman spectrometers from Kaiser and Lambda Solutions, respectively. Both spectrometers are equipped with fiber optic connected sapphire sampling windows. FTIR spectra were measured with triple bounce diamond ATR on a Nicolet

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Magna 550 spectrometer. Details of instruments, techniques, and the concept of vibrational band assignments have been discussed elsewhere [3, 6, 7, 22, 23]. Model calculations were performed on the VAMP [24], DMOL3 [25, 26], and CASTEP [27] modules of the Materials Studio program package from Accelrys. Full geometry optimizations and vibrational frequency analyses were carried out in all electron approximation using in DMOL3 the BLYP [28, 29] functional in conjunction with the double-numeric-basis set with polarization functions (DNP) and the IR models were calculated from the Hessians [30]. In CASTEP the gradient-corrected (GGA) PBE [31] functional was selected for the density functional theory (DFT) computations with norm conserving and not spin polarized approach [32]. In the semi-empirical VAMP method we used the PM3 parameterization [33] from the modified neglect of diatomic differential overlap (NDDO) model to obtain the Hessians for vibrational spectrum models [30].

3. Results and discussion The Raman spectra in Fig. 1 illustrate that very different initial alkaline silicate solutions can result in quite similar Qn connectivity distributions in the acid- and baseset gels. A number of other Li, Na, and K silicates have shown similar trends although this phenomenon is not entirely universal [10, 11]. The specific pH values for gelling cannot be made uniform for each solution as “apple to apple comparison” type research logic would dictate since it has been long known that the gelling time might change

` Fig. 1 The Raman spectra of 3 M and 0.2 M aqueous solutions of commercial sodium (1/a) and potassium (1/b) silicates have substantially different molecular structures and compositions but similar Qn connectivity distributions when gelled at acidic (pH < 7) or basic (pH > 7) condition.

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Fig. 2 The mainly Q4 connected siliceous zeolites [22, 23] are poor Raman absorbers in the 700 to 1300 cm-1 νs and νas Si-O vibration range (2/a) and the 1140 cm-1 Raman band of a gel made from Kasil-1624 (K/Si ~ 0.76) [8, 10, 11] substantially decreases upon drying which makes its Q4 assignment dubious (2/b).

from 1 min to near infinite at a given pH simply by changing the concentration [12] because it also changes the dissociation, molweight, and constitution of dissolved silicate molecules [3, 7, 8]. Beside the predicted Q2 type Si-O-Si chains, all acid-set gels seem to contain mostly Q0 monomers and, also in line with Iler’s and Livage’s [13-16] conjecture, all base set gels seem to have predominantly a Q3/Q4 syloxane network. There are however some doubts about the validity of the glass-originated Qn assignments in Fig. 1. For example siliceous zeolites like those shown in Fig. 2/a do not have an intense Raman absorption near 1140 cm-1 although they are exclusively composed of Q4 type [SiO4] tetrahedra. We could not measure Raman spectra on many commercial silica gels either despite their assumed Q4 connected irregular siloxane rings in a 3D network. The cause of these phenomena are under investigation. Fig. 2/b suggests that the 1140 cm-1 band might also be water associated. There have been several experimental reasons [3-8] for assigning the ~780 cm-1 band to Q0 monomers in aqueous solutions in contrast to the corresponding ~850 cm-1 in glasses. Based upon this consideration we have also assigned the 750 cm-1 band in Figs. 2/c and 2/d to Q0. To clarify further the role of water with these Q0 associations of Raman bands, we collected Raman spectra of materials largely containing only monomer [SiO4] tetrahedra (proven by independent experimental methods) and also performed adequate molecular modeling calculations. Figure 3/a shows the Raman spectra of three sodium silicate solutions that have been identified in the literature as mostly monomer containing materials based on light diffraction, molybdate reactions, molweight measurements, and Si29 NMR data [6, 13, 34]. Most Raman bands overlap quite nicely hence the most intense 770 cm-1 band is reasonably characteristic for the monomer structure. The intensity differences of other bands might be associated with different levels of dissociation and perhaps also with some larger silicate impurity. It is not clear why the 850 cm-1 band appears only in the spectrum of Na/Si ~ 40 ratio silicate but not in the other spectra. It was recently demonstrated by various ab initio calculations [35] that for modeling realistic FTIR spectra of the dissolved monomers it is not enough to simulate the solvent with the usually applied conductor-like screening (COSMO) method [36, 37]: one has to explicitly include water molecules into the model, which significantly increases the computational power demand for modeling even small realistic silicate molecules. Figure 3/d shows that this is also valid for modeling the Raman spectra of H2SiO42- ions composing the bulk of dilute basic solutions of Na2SiO3 x 9H2O [6].

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Fig. 3 Raman spectra of independently verified monomer silicate solutions (3/a); model of Na4SiO4, the hydrolized variations of which compose the dilute basic monomer solutions (3/b); model of the H2Na2SiO4 molecule surrounded by 20 water molecules (3/c) and the computed and experimental Raman spectra of its dissociated H2SiO42- derivative likely present in dilute solutions of Na2SiO3 x 9H2O [6] (3/d).

In Fig. 4/a we compare the Raman spectra of some crystalline solids containing only monomer [SiO4] tetrahedra in their XRD verified structures [38, 39]. Their existence suggests that amorphous solids might also contain substantial amounts from [SiO4] monomers. There is a clear difference between the Q0 associated peak positions of the water-containing and water-free crystals: the latter ones tend to show these vibrations near 850 cm-1 like glasses while the 770 cm-1 vibration of the former one resembles that in spectra of dissolved silicates. Figure 4/b illustrates that the relative peak intensities (but not the positions!) in Raman spectra can substantially vary when measured with different wavelength lasers which has to be taken into consideration when one compares the experimental and model Raman spectra (the latter being more comprehensive). We carried out DFT calculations to model the Raman spectra of the water-free Forsterite and the aqueous Na2SiO3 x 9 H2O crystals. Their structures are shown in Figs. 5/a and 5/b. As Fig. 5/c illustrates, the model spectrum of Forsterite fits the experimental data extremely well. The more complex sodium silicate spectrum in Fig. 5/d is also a good fit but fails to be decisive exactly in the critical 750-850 cm-1 range. It is not clear yet if this is due to the above mentioned laser vawelength associated experimental artifact (this spectrum was measured with a 780 nm laser) or to the computational flaw that our system was not fully minimized before carrying out these initial vibrational calculations. These issues are currently being investigated involving also 29Si NMR studies on both these crystalline and the amorphous solids.

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Fig. 4 Raman spectra of XRD-verified aqueous and non-aqueous solid crystals containing only [SiO4] monomers (4/a) (see also Figs 5/a and 5/b); the relative intensity of Raman bands can change with the laser wavelength as these Fayalite spectra [38] demonstrate (4/b).

Fig. 5 DFT calculated and experimental Raman spectra of crystalline Forsterite (5/a and 5/c) and Na2SiO3 x 9 H2O; color assignments on 5/b are the same as those on Fig. 3 for the dissolved silicates (e.g., Figs 3/a and 3/d).

Since Raman spectroscopy can detect silicate structures both in solutions and in solids, it is well-suited to follow the poorly understood transformation of dissolved silicate molecules into the initial solid nanoparticles that ultimately agglomerate into the

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3D gels. We intended therefore to use this in situ technique to see how the monomer TEOS molecules convert into the D4R structures of the (Me4N)8Si8O20 x 65H2O crystals.

Fig. 6 In situ analysis of the synthesis of (Me4N)8Si8O20 x 65H2O; 6/a shows the structure of a D4R silicate building block presumed to form when the crystalline product in 6/b hydrolyzes; colors in 6/a have the same meaning as colors in Fig. 3 and colors in 6/b mean: red = oxygen, yellow = silicon, grey = carbon, blue = nitrogen; hydrogen atoms are not shown for clarity; the Raman spectra of the overall reaction mixtures at the beginning and at the end of reaction are largely identical owing to exact overlaps between the bands of reactants and product (6/c); additional bands have not appeared during reaction, suggesting organic reaction pathway instead of hydrolysis into [SiO4]4- type monomer ions; the FTIR spectra of reactants and product (marked as “solid D4R” for brevity) indicate that the development of a νas Si-O band near 1000 cm-1 can be a clear in situ indication for the progress of reaction (6/d)

Specifically our hope was to see the disappearance of the TEOS related Raman bands and the development of D4R related Q3 bands around 1050 cm-1 accompanied by new siloxane ring vibrations around 440 cm-1 [3]. As Fig. 6/c illustrates, experiment did not confirm this prediction. The weak but measurable (after magnifying electronically) TEOS band near 605 cm-1 disappeared after about 40 hours reaction time. This band appears in the pure material at 655 cm-1 but shifted in the reaction mixture. We could not see any other change in the Raman spectra in the course of the 65 hour reaction although spectra were frequently measured. This observation suggests that TEOS does not hydrolyze into reactive monomer ions or orthosilicic acid (H4SiO4) in the presence of alcohol as many researchers assume but rather reacts like an organic molecule breaking and making only one bond at a time. Raman bands in Fig. 6/c correspond to the typical bands of the pure organic ingredients (see list in Fig. 6/d) and the spectrum of solid D4R crystal totally overlaps with that of the TMA-OH. For clarity we do not show these overlapping spectra in Fig. 6/c. As 6/d indicates however, FTIR

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spectroscopy could be a good in situ method for following this reaction. It is possible that the 1100 cm-1 IR band is associated with Q3 connectivity [3] but regretfully it is not well resolved in the spectrum of the reactant/product mixture. The 1000 cm-1 band is well separated but we are not sure yet about its correct chemical bond assignment. References [1] B. O. Mysen, 1990, J. Geophys. Res., 95 (B10), 15733. [2] B. G. Parkinson, D. Holland, M. E. Smith, C. Larson, J. Doerr, M. Affatigato, S. A. Feller, A. P. Howes, C. R. Scales, 2008, J. Non-Crystalline Solids 354, 1936. [3] I. Halasz, M. Agarwal, R. Li, N. Miller, Microporous Mesoporous Materials, submitted. [4] I. Halasz, R. Li, M. Agarwal, N. Miller, 2005, 19th NAM, Philadelphia, USA, P-122. [5] I. Halasz, R. Li, M. Agarwal, N. Miller, 2007, Catal. Today, 126, 196. [6] I. Halasz, M. Agarwal, R. Li, N. Miller, 2007, Catal. Lett., 117, 34. [7] I. Halasz, R. Li, M. Agarwal, N. Miller, 2007, Stud. Surf. Sci. Cat., 170A, 800. [8] I. Halasz, M. Agarwal, R. Li, N. Miller, 2008, Stud. Surf. Sci. Cat., 174B, 787. [9] I. Halasz, R. Li, M. Agarwal, N. Miller, 2007, 20th NAM, Houston, USA, O-S2-04. [10] I. Halasz, 2008, IMMS, Namur, Belgium, P-037. [11] I. Halasz, M. Agarwal, R. Li, N. Miller, 2009, in “Characterisation of Porous Solids VIII”, ed. by S. Kaskel, P. Llewellyn, F. Rodriguez-Reinoso, N. A. Seaton, RSC Publ., pg. 416. [12] J. Vail, 1952, “Soluble Silicates”, Reinhold Publishing Co., New York. [13] R. K. Iler, 1979, “The chemistry of silica”, J. Wiley & Sons, New York. [14] R. E. Patterson, 2006, Surfactant Sci. Ser., 131, 779. [15] G. Alexander, 1967, “Silica and me”, Doubleday & Co., New York. [16] J. Livage, 1994, Stud. Surf. Sci. Catal., 85, 1. [17] T. Yokoi, M. Yoshioka, H. Imai, T. Tatsumi, 2009, Angew. Chem. Int. Ed., 48, 1. [18] Yu. I. Smolin, Yu. F. Shepelev, R. Pomes, D. Hoebbel, W. Wieker, 1979, Sov. Phys. Crystallogr., 24 (1), 19. [19] M. Wiebcke, M. Grube, H. Koller, G. Engelhardt, J. Felsche, 1993, Microp. Mater., 2, 55. [20] R. Goto, A. Shimojima, H. Kuge, K. Kuroda, 2008, Chem. Commun, 6152. [21] Y. Hagiwara, A. Shimojima, K. Kuroda, 2008, Chem. Mater., 20, 1147. [22] I. Halasz, M. Agarwal, E. Senderov, B. Marcus, W. Cormier, 2005, Stud. Surf. Sci. Catal., 158, 647 [23] I. Halasz, M. Agarwal, B. Marcus, W. Cormier, 2005, Microporous Mesoporous Materials, 84,318 [24] T. Clark, A. Alex, B. Beck, F. Burkhardt, J. Chandrasekhar, P. Gedeck, A. Horn, M. Hutter, B. Martin, G. Rauhut, W. Sauer, T. Schindler, T. Steinke, 2001, “VAMP Semi-Empirical Quantum Chemistry in Materials Studio”, Universität Erlangen. [25] B. Delley, 1990, J. Chem. Phys. 92, 508 . [26] B. Delley, 2000, J. Chem. Phys. 113, 7756. [27] S. J. Clark, M. D. Segall, C. J. Pickard, P. J. hasnip, M. J. Probert, K. Refson, M. C. Payne, 2005, Zeitschrift fur Kristallographie, 220(5-6), 567. [28] A. D. Becke, 1988, J. Chem. Phys., 88, 2547. [29] C. Lee, W. Yang, R. G. Parr, 1988, Phys. Rev. B, 37, 786. [30] E. B. Wilson, J. C. Decius, P. C. Cross, 1955, “Molecular Vibrations”, Dover, New York. [31] J. B. Perdew, K. Burke, M. Ernzerhof, 1996, Phys. Rev. Lett., 77, 3865. [32] D. Porezag, M. R. Pederson, 1996, Phys. Rev. B, 54, 7830. [33] J. J. P. Stewart, 1989, J. Comput. Chem. 10, 209 & 221. [34] G. Engelhardt, D. Michel, 1987, “High-Resolution Solid-State NMR of Silicates and Zeolites”, John Wiley & Sons, Chichester, NY, Brisbane, Toronto, Singapore [35] I. Halasz, A. Derecskei-Kovacs, 2008, Molecular Simulation, 34 (10-15), 937. [36] A. Klamt, G. Schüürmann, 1993, J. Chem. Soc., Perkin Trans. 2, 799. [37] B. Delley, 2006, Mol. Simul., 32, 117-123. [38] RRUFFTM database, http://rruff.info/. [39] F. Liebau, 1985, “Structural Chemistry of Silicates”, Springer Verl., Berlin, Heidelberg, NY. © 2009

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A novel continuous approach for the synthesis and characterization of pure and mixed metal oxide systems applied in heterogeneous catalysis Stefan Kaluza,a Martin Muhlera a

Laboratory of Industrial Chemistry, Ruhr-University Bochum 44780 Bochum, Germany

Abstract An extensive set of characterization methods is required to study the processes occurring during the evolution of the initially amorphous precursor towards the complex Cu/ZnO/Al2O3 system. A novel preparation method was therefore developed that provides the possibility of a systematic study of all components in the different stages of the precipitation of the ternary catalyst. As a result, a continuously operating synthesis route was established as an alternative to the industrially applied process. Keywords: continuous precipitation, aging, Cu/ZnO/Al2O3 catalyst, methanol synthesis

1. Introduction Pure and mixed metal oxide systems are of immense importance in heterogeneous catalysis today. For instance, zinc oxide is an interesting material for a wide range of applications due to its unique electronic and optical properties. It is a wide-gap semiconductor that is also luminescent, thus being a promising candidate for optoelectronic applications. Because of the good conductivity and high transparency in the visible region, thin films of ZnO have been investigated as transparent electrodes for solar cells. Furthermore, zinc oxide nanoparticles have been used as white pigment or as gas sensors, for example, for detection of hydrogen or nitrogen oxide gases. Active aluminas are also interesting materials for a large range of applications in the field of heterogeneous catalysis. Similar to zinc oxide, they are catalytically active or are used as catalyst support in many processes of industrial importance. In recent years, the role of methanol as a basic chemical has strongly increased, and therefore further development of the ternary Cu/ZnO/Al2O3 catalyst for methanol synthesis has become more important. It is widely accepted that ZnO acts both as an electronic and structural promoter and exhibits a major influence on the catalytic activity, while alumina mainly increases the long-term stability of the ternary catalyst system. Thus, the interest in the binary Zn/Al as well as the ternary CuO/ZnO/Al2O3 system as catalytic materials is very high. In general, precipitation is one of the most frequently applied methods in terms of large-scale catalyst preparation since precipitation and coprecipitation processes provide a good dispersion and a high homogeneity of the components in the catalyst precursor. Nevertheless, in spite of its high industrial importance, precipitation is still a complex process, which is difficult to study, even with the highly advanced analytical tools currently available. Recently, a novel continuous precipitation process using a micromixer was developed, in which the continuous precipitation was immediately quenched by a subsequent spray drying process.[1] Thus, it was possible to investigate the formation

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mechanisms occurring during the first few seconds of the precipitation, which is hardly feasible in a batch operation mode. This novel method was successfully used for detailed studies on the synthesis and aging of single oxides (CuO, ZnO, and Al2O3) as well as mixed binary oxides (CuO/ZnO, ZnO/Al2O3, and CuO/Al2O3). The results were combined to a comprehensive systematic data set of chemical and structural information, which was used to learn more about the formation processes of the ternary catalyst. Additionally, a continuous aging device was developed that allows aging of the continuously formed precipitate with an exactly defined residence time. By this means, a time-resolved investigation of the phase transformations occurring during the aging process of the ternary catalyst precursor became possible.

2. Results and discussion 2.1. Quenching of the precipitation reaction The immediate quenching of the precipitation reaction by applying a subsequent spray drying process enabled us to investigate the initial precursor stage by means of solidstate characterization techniques. Considering the continuous precipitation of aqueous zinc nitrate solution with sodium carbonate, the initially formed metastable precipitation product is sodium zinc carbonate Na2Zn3(CO3)4. The same observation is made in the case of the precipitation of copper nitrate with sodium carbonate, which leads to the formation of sodium copper carbonate Na 2Cu(CO3)2 (Fig. 1, left). By exposing those metastable phases to water during the subsequent washing step a phase transition into the thermodynamically more stable hydroxy carbonate phases, i.e., Zn5(OH)6(CO3)2 and Cu2(OH)2CO3, respectively, takes place (Fig. 1, right). In both cases, the phase transition taking place is an activated process due to mass transport phenomena, and its rate can be increased thermally, by intensive stirring, or by the use of ultra sound. washed/dried Zn-precursor Zn5(OH)6(CO3)2 Zn4(OH)6CO3

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Fig. 1 Diffraction patterns of the metastable initial precipitation products Na2Zn3(CO3)4 (left, top) and Na2Cu(CO3)2 (left, bottom), respectively, and their transformation into the corresponding thermodynamically stable hydroxy carbonate phase after exposure to water (right).

During the preparation of mixed ZnO/Al2O3 composites a highly X-ray amorphous binary Zn-Al phase is observed as the initial metastable coprecipitation product. Unfortunately, its structure could not be determined up to now as all attempts of thermal recrystallization result in the decomposition of this phase into ZnO and Zn/Alhydrotalcite. Nevertheless, the occurrence of this binary Zn/Al phase turns out to play a major role during the synthesis of the ternary Cu catalyst [2].

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2.2. The influence of the post-precipitation treatment After precipitation, the precursor obtained has to undergo several further steps to be transformed into the final oxidic material. The so called post-precipitation processes, i.e., washing, drying, and calcination, are often guided by intuition, and more attention should be paid to process control and reproducibility. However, the influence of the post-precipitation steps on the properties of the final material can be rather strong. By applying the novel quenching method developed in this work, the spray-dried precursors provide an excellent starting material to investigate the processes of posttreatment more detailed [3]. Considering the binary Zn/Al system, continuous precipitation followed by calcination, washing and freeze drying leads to the formation of Zn/Al-hydrotalcite, a material of high synthetic interest due to its layered structure (Fig. 2,left). By only slight changes of the post-precipitation sequence starting from the same precursor, the formation of the hydrotalcite is suppressed, and Al3+-incorporated ZnO with a specific surface area of up to 144 m2g-1 is obtained (Fig. 2,right) [2,4]. Zn(NO Zn(NO3)2 Al(NO3) 3 calcined, washed and freeze-dried Zn/Al-precursor ZnO Zn6Al2(OH)16CO3

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Fig. 2 Schematic illustration of the continuous coprecipitation of a binary Zn/Al precursor. Different final materials were obtained by systematic process control.

This example out of many shows the high importance of every single preparation step in terms of sophisticated process control.

2.3. Continuous aging The immediate quenching of the continuous coprecipitation reaction of the ternary precursor leads to the formation of Na2Cu(CO3)2 separated from a binary Zn/Al phase. This result shows that in the initial stage of the reaction there is no coprecipitation of all three components occurring, but a competition taking place in favor of a mixed Zn/Al phase and a single Cu phase. However, previous results revealed that the ternary precursor consisted of binary Cu-Zn phases as well as a ternary hydrotalcite-like phase, when it was prepared by the usual batch process including 2 h aging of the precipitate in its mother liquid. Thus, during the aging process exchange reactions between the formerly separated single copper phase and the binary Zn/Al phase took place and led to a precursor with a much more homogeneous metal ion distribution. The continuous consecutive precipitation method provides the unique possibility to simulate the initial situation during the coprecipitation process, that is, the formation of a single Cu phase separated from a binary Zn/Al phase. By applying a novel continuous method, the exchange reactions of these metastable phases can be investigated as a function of aging time (Fig. 3).

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The observed phase transitions proceed in an analogous manner to those described for the separated single copper and binary Zn/Al systems. Upon aging, the initially formed Na2Cu(CO3)2 was converted into malachite (Cu2(OH)2CO3). In the presence of the binary Zn/Al phase, the same transition took place and additionally, some zinc was incorporated into the structure to form zincian malachite (Cu,Zn)2(OH)2CO3 [5,6]. Furthermore, the amorphous Zn/Al phase transformed into a hydrotalcite-like structure during the aging process. The diffraction pattern of the continuously precipitated precursor that was continuously aged for 60 min is almost identical to that observed for the precursor coprecipitated and aged for 2 h in the usual batch process. Thus, it can be assumed that this precursor composition represents a thermodynamic minimum that all ternary precursors of the same molar ratios approach with increasing time, independent of the precipitation method. This is also reflected in the increasing crystallinity with increasing aging time compared to the initially X-ray amorphous precursor mixture.

3. Conclusions The continuous approach discussed in this work enabled us to study the precursor formation during the initial step of precipitation as well as the time-resolved transformations occurring during the process of aging. Moreover, better control of the process parameters allowing fast and reproducible parameter screening suggest that the continuous catalyst preparation is a promising alternative to the conventional batch process.

References 1. 2. 3. 4. 5. 6.

S. Kaluza, M.K. Schröter, R. Naumann d’Alnoncourt, T. Reinecke and M. Muhler, Adv. Funct. Mater., 18 (2008) 3670. S. Kaluza and M. Muhler, J. Mater. Chem., 19 (2009) 3914. S. Kaluza and M. Muhler, Catal. Lett., 129 (2009) 287. S. Miao, R. Naumann d’Alnoncourt, T. Reinecke, I. Kasatkin, M. Behrens, R. Schlögl and M. Muhler, Eur. J. Inorg. Chem., 7 (2009) 910. C. Baltes, S. Vukojevic and F. Schüth, J. Catal., 258 (2008) 334. M. Behrens, F. Girgsdies, A. Trunschke and R. Schlögl, Eur. J. Inorg. Chem., 10 (2009) 1347.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Innovative preparation of Au/C by replication of gold-containing mesoporous silica catalysts Fatmé Kerdi1, Valérie Caps1,2 and Alain Tuel1* 1

IRCELYON, UMR 5256 CNRS-Université de Lyon, 2 avenue A. Einstein, 69626 Villeurbanne Cedex, France. 2 KAUST Catalysis Center (KCC), 4700 King Abdullah University of Science and Technology, Thuwal 23955 – 6900, Kingdom of Saudi Arabia

Abstract A new strategy, based on the nanocasting concept, has been used to prepare gold nanoparticles (NPs) highly dispersed in meso-structured carbons. Gold is first introduced in various functionalized mesostructured silicas (MCM-48 and SBA-15) and particles are formed inside the porosity upon reduction of Au3+ cations. Silica pores are then impregnated with a carbon precursor and the composite material is heated at 900°C under vacuum. Silica is then removed by acid leaching, leading to partially encapsulated gold particles in mesoporous carbon. Carbon prevents aggregation of gold particles at high temperature, both the mean size and distribution being similar to those observed in silica. However, while Au@SiO2 exhibit significant catalytic activity in the aerobic oxidation of trans-stilbene in the liquid phase, its Au@C mesostructured replica is quite inactive. Keywords: mesostructured carbon, gold nanoparticles, catalysis, aerobic oxidation

1. Introduction Gold nanoparticles are efficient oxidation catalysts both in the gas and liquid phases. It is however essential to stabilize particles with diameters below a few nanometers [1]. This requires sophisticated chemical methods, which are usually support-specific. Another strategy consists in limiting particle aggregation via physical confinement. Gold particles synthesized within mesoporous titania-modified silicates exhibit significant activity for structure-sensitive CO oxidation [2]. However, aerobic epoxidations in the liquid phase require the use of low-polarity solvents [3], in which these conventional oxide-supported catalysts are poorly dispersed. The use of activated carbons as supports, which enhances mass-transfer, could be beneficial to the efficiency of the overall catalytic system. We have developed a new strategy, based on the nanocasting concept, to prepare gold nanoparticles highly dispersed but partly occluded in meso-structured carbons.

2. Experimental Calibrated gold nanoparticles were formed inside the porosity of mesostructured silicas of various pore size and architecture (MCM-48 and SBA-15) using two different routes. In both routes, silica was preliminarily functionalized before contacting with an aqueous HAuCl4.3H2O solution. The two routes essentially differed by the nature of the graft: Ntrimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TPTAC) in Route 1 and 3mercaptopropyltrilmethoxysilane (MPTMS) in Route 2. Gold was then introduced in functionalized mesoporous silicas and particles were formed upon reduction of Au3+

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cations with sodium citrate and/or NaBH4. The gold content was limited to 1-1.5wt.% to favor a high metal dispersion inside the pores and prevent the formation of large particles upon heating. In the case of MPTMS, gold nanoparticles were not formed at room temperature and it was necessary to heat the sample in air at 300°C. The silica pores were then impregnated with a carbon precursor (sucrose in H2SO4) following the recipe of Ryoo et al. [4] and the composite material was heated at 900°C under vacuum. Silica was then removed by HF leaching, leading to partially encapsulated gold nanoparticles in mesoporous carbon (Scheme 1). Gold particles inside silica pores

Impregnation Carbonization

Carbonimpregnated silica

HF washing SiO2 removal

Mesoporous carbon with partially encapsulated particles

Scheme 1. The various steps in the synthesis of gold-containing mesostructured carbons [adapted from ref 5].

All modified silicas and gold-containing mesoporous carbons were characterized by X-ray diffraction, N2 adsorption and TEM. Au/silicas and the corresponding Au/C replicas were tested in the aerobic oxidation of trans-stilbene in the liquid phase under conditions previously described [6].

3. Results All silicas were synthesized following literature procedures. Their structural and textural characteristics are reported in Table 1. Moreover, carbon replicas were also prepared on pristine silicas to evaluate their characteristics in the absence of gold particles. Table 1. Textural properties of the various silicas and the corresponding carbon replicas. Support/replica SBA-15 CMK-3 MCM-48 CMK-1

SBET (m2/g) 624 1218 1318 1507

Pore diameter (nm) 7.3 4 2.6 2.9

As shown in the Table, all silica and carbon supports possess a high BET surface area, in excellent agreement with data reported in the literature for similar materials. Moreover, the regularity and ordering of the porous network was evidenced by intense and well-defined reflections in the corresponding X-ray diffraction patterns (Figure 1).

Innovative preparation of Au/C catalysts by replication

0,6

1,6

2,6

3,6

4,6

0,6

1,6

223

2,6

3,6

4,6

2 theta (°)

2 theta (°)

Figure 1. XRD patterns of silicas (-) and the corresponding carbon replicas (-●-). SBA-15 (left) and MCM-48 (right).

The regularity and long-range ordering of the pore system was not affected by the different post-synthesis treatments: gold-containing silicas still show well defined XRD patterns, even after reduction of Au3+ cations. Both routes lead to well dispersed Au particles in silica (Fig. 2). However, a systematic study performed with TPTAC showed that the number of functional groups attached onto the silica surface is critical for the size and location of Au particles. At low coverage, TPTAC molecules are preferentially located on the surface of silica particles, leading to large Au crystals upon reduction. (a)

(b)

(c)

Figure 2. TEM pictures of Au/SBA-15 (0.3 wt. %) obtained with TPTAC (a) and MPTMS heated at 300°C (b) and Au/C obtained by replication of the sample prepared with MPTMS (c).

Au particles are significantly bigger with TPTAC (6.1 nm) than with MPTMS (3.8 nm). Moreover, for MPTMS-modified supports, the particle size is not strongly affected by temperature (3.8 and 4.2 nm at 300 and 560°C, respectively). As shown in Figure 3, the distribution of gold NPs sizes is retained after carbon pyrolysis and dissolution of silica. This is not the case for the catalytic properties of the materials: while Au/SBA-15 (MPTMS-300°C) is an efficient catalyst of the aerobic oxidation of trans-stilbene (Figure 4), the corresponding Au/C replica is essentially inactive. This is attributed to the lower accessibility to the gold nanoparticles partially embedded within the carbon walls. Ways to tune this nanocasting-based strategy towards active mesostructured Au/C catalysts are under investigation.

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1

2

3

4 5 6 Particle size (nm)

7

8

Figure 3. Distribution of gold particle sizes in SBA-15 (MPTMS-300°C, black) and the corresponding CMK-3 (grey).

Despite mass-transfer limitations, Au/SBA-15 (MPTMS-300°C) remains an interesting catalyst for this liquid phase reaction. Its selectivity is indeed markedly different from that displayed by gold nanoparticles supported on passivated high surface area silica [6]: deoxybenzoin (1,2-diphenyl-ethanone), not the epoxide, is the main reaction product, with a yield of 45% at full conversion (78 h).

Trans-stilbene Conv. %

100

trans-stilbene conversion (■), epoxide (●) and deoxybenzoin (∆) yields. Reaction conditions: trans-stilbene (1 mmol), methylcyclohexane (solvent, 20 mL), tert-butyl hydroperoxide (0.05 mmol / 7 μL of a 70% TBHP in water Aldrich solution), catalyst (91.7 mg / 2 μmol Au), 900 rpm, 80°C, air (atmospheric pressure).

80 60 40 20 0 0

10

20

30 40 50 Reaction time (h)

60

70

80

Figure 4. trans-stilbene conversion over 0.3%Au/SBA-15 (MPTMS-300°C)

4. Conclusion New Au@C materials have been obtained using a nanocasting method, starting form preliminary functionalized silicas. Excellent dispersions were obtained, with Au particles partially embedded in carbon walls. This route is very promising for the preparation of catalysts for the aerobic oxidation of olefins in the liquid phase.

References [1] M. Haruta, Gold Bull., 37 (2004) 27-36. [2] V. Caps, Y. Wang, J. Gajecki, B. Jouguet, F. Morfin, A. Tuel, J.-L. Rousset, , Stud. Surf. Sci. Catal. 162 (2006) 127-134. [3] P. Lignier, S. Mangematin, F. Morfin, J.-L. Rousset, V. Caps, Catal. Today, 138 (2008) 50-54. [4] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B, 103 (1999) 7743-7746. [5] S.M. Holmes, P. Foran, E.P.L. Roberts, J.M. Newton, Chem. Commun., 14 (2005) 1912-1913. [6] D. Gajan, K. Guillois, P. Delichère, J.-M. Basset, J.-P. Candy, V. Caps, C. Copéret, A. Lesage, L. Emsley, J. Am. Chem. Soc., 131 (2009) 14667-14669.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

TiO2 photocatalysts prepared by thermohydrolysis of TiCl4 in aqueous solutions A. Di Paola, M. Bellardita, L. Palmisano “Schiavello-Grillone” Photocatalysis group, Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy

Abstract Nanostructured TiO2 photocatalysts were synthesized by thermohydrolysis of TiCl4 at 100 °C in various aqueous solutions. Anatase or rutile, binary mixtures of anatase and rutile or anatase and brookite, and ternary mixtures of anatase, brookite and rutile were obtained depending on the hydrolysis solution. The most efficient catalysts consisted of ternary mixtures of the three polymorphic TiO2 phases. Keywords: photocatalysis, TiO2, anatase, brookite, rutile

1. Introduction Heterogeneous photocatalysis by semiconductor oxides is a promising method for the removal of many organic and inorganic pollutants from water and air (Fujishima et al., 1999). TiO2 is the most reliable photocatalyst because of its low cost and (photo)stability under irradiation. TiO2 exists in three main crystallographic forms: anatase (tetragonal), brookite (orthorombic) and rutile (tetragonal). All three crystalline structures consist of deformed TiO6 octahedra connected differently by corners and edges. Anatase is generally accepted to be a photocatalyst more efficient than brookite and rutile but mixtures of different TiO2 phases have often revealed photocatalytic activities superior than those of the pure phases (Di Paola et al., 2008; Di Paola et al., 2009). Sol-gel techniques are usually employed to produce TiO2 catalysts. The resulting materials are generally amorphous or not well crystallised and, consequently, they must be subjected to calcination to obtain active samples. In this work we report on the preparation of polymorphic TiO2 nanoparticles obtained by thermohydrolysis of TiCl4 in various aqueous solutions at 100° C. The preparation method is very simple and does not require the use of expensive thermal or hydrothermal treatments. The content of anatase, brookite and rutile is easily tailored by varying the composition of the hydrolysis solution. The degradation of 4-nitrophenol was chosen as model reaction to evaluate the photoactivity of the various samples.

2. Experimental 2.1. Preparation of the samples 1 ml of TiCl4 (Fluka 98%) was slowly added to different volumes of distilled water or aqueous solutions at room temperature. The solutions obtained after continuous stirring were heated in closed bottles and aged at 100 °C in an oven for 48 h. The bottles were allowed to cool and the resultant solids were recovered using a vacuum pump at 55°C.

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2.2. Characterization X-ray diffraction patterns of the powders were collected by a powder diffractometer employing the CuKα radiation and a graphite monochromator in the diffracted beam. The crystal phase composition of the catalysts was determined using a modified Rietveld method (Lutterotti et al., 1998). The specific surface areas (SSA) were obtained by nitrogen physisorption experiments performed at the liquid nitrogen temperature. The band gap values of the samples were obtained by diffuse reflectance spectra measurements: BaSO4 was the reference sample and the spectra were recorded in the range 200–600 nm. The position of the flat band potentials of anatase, brookite and rutile was determined measuring the photovoltage as a function of the suspension pH (Roy et al., 1995).

2.3. Photoreactivity experiments A Pyrex batch photoreactor of cylindrical shape containing 0.5 L of aqueous suspension was used. A 125 W medium pressure Hg lamp was immersed within the photoreactor and the photon flux emitted by the lamp was Φi = 13.5 mWcm−2. O2 was continuously bubbled for ca. 0.5 h before switching on the lamp and throughout the occurrence of the photoreactivity experiments. The amount of catalyst was 0.6 g L-1 and the initial 4-nitrophenol (NP) concentration was 20 mg L-1. The quantitative determination of 4-NP was performed by measuring its absorption at 315 nm. The photoactivity of the various samples was compared to that of commercial TiO2 Degussa P25.

3. Results and Discussion The physical properties of the solids obtained by thermohydrolysis of TiCl4 in aqueous solutions are strongly influenced by the synthetic variables. In particular, acidity, presence (and nature) of anions, and titanium concentration govern the composition and the photoreactivity of the TiO2 photocatalysts (Cheng et al., 1995; Koelsch et al., 2004). Depending on the experimental conditions, rutile or anatase, binary mixtures of anatase and rutile or anatase and brookite, or ternary mixtures of anatase, brookite and rutile, can be obtained. Table 1 shows the crystal phase composition of some selected samples prepared under different experimental conditions. Table 1. Crystal phase composition, specific surface area and initial reaction rate (r0) values of TiO2 powders prepared under different experimental conditions.

[Ti] mol L-1 concentrated HCl diluted HCl NaCl solution H2O Degussa P25 a

0.34 0.15 0.22 0.12

Anatase %

70.9 64.5 80.0

Brookite %

Rutile %

SSA m2·g-1

ro x 109 mol L-1 s-1

73.6 16.2 28.3

100.0 26.4 12.8 7.2 20.0

29 141 189 216 51

12.5 16.5 21.6 76.1 43.0 a

The runs were carried out at pH=3.3, obtained by adjustment with HCl.

Titanium(IV) cations form octahedral aquo-hydroxo complexes in an acid or neutral medium (Jolivet, 2000). As a consequence of hydrothermic treatments, the octahedra link together by olation, through dehydration reactions between aquo and

TiO2 photocatalysts prepared by thermohydrolysis of TiCl4 in aqueous solutions

227

hydroxo ligands. Rutile type nuclei are developed if the octahedra combine by sharing equatorial edges, whereas anatase or brookite type nuclei form if the monomers combine by sharing apical edges. Further growth proceeds by formation of linear chains from the rutile type nuclei or of skewed chains from the anatase or brookite type nuclei. The exact nature of the titanium(IV) octahedral complexes depends on the acidity and type of ligand in solution (Zheng et al., 2001; Pottier, 2001). In concentrated HCl solutions only rutile crystallites were developed whereas in dilute HCl solutions both rutile and brookite crystallites can be obtained contemporaneously. In NaCl solutions, ternary mixtures of anatase, brookite and rutile were formed. The composition of the powders obtained by thermohydrolysis of TiCl4 in water depended on the TiCl4/H2O ratio and binary or ternary mixtures of the three polymorphs were prevalently produced. The average particle sizes of all the phases present in the various samples were in the range 2-10 nm. The photocatalytic activity of the samples, as well their composition, depended on the hydrolysis solution. Table 1 reports the values of the initial degradation rate of 4NP, r0, determined in the presence of the most active TiO2 samples. The mixed systems revealed an enhanced photoactivity compared with that of the pure TiO2 polymorphic phases and some samples were more active than Degussa P25. The most efficient samples consisted of a ternary mixture of anatase, brookite and rutile. The high photocatalytic activity can be explained by the presence of junctions among different polymorphic TiO2 phases. Figure 1 shows the relative positions of the energy bands of anatase, brookite and rutile (Di Paola et al., 2009).

-1 - 0.37 V

- 0.45 V

- 0.45 V

V

0

1

2.98 eV

3.05 eV

3.27 eV

2

3 Rutile

Anatase

Brookite

Figure 1. Electrochemical potentials (versus NHE) of the band edges of anatase, brookite, and rutile at pH = 7. The coupling of semiconductors possessing different redox energies for their corresponding conduction and valence bands allows the vectorial displacement of holes and electrons from one semiconductor to another and reduces the recombination of the photogenerated electron/hole pairs, enhancing the efficiency of the interfacial charge transfer to adsorbed substrates (Serpone et al., 1995). The contact among the different phases is very efficient due to the small sizes of the crystallites.

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4. Conclusion A facile way to prepare active TiO2 photocatalysts has been developed. The crystal phase composition of the samples can be easily tailored by simply varying the type of aqueous solution. The most efficient samples consisted of a ternary mixture of anatase, brookite and rutile. The presence of junctions among different polymorphic TiO2 phases favours the separation of the photogenerated electron-hole pairs, enhancing the catalyst activity.

Acknowledgments The authors wish to thank MIUR (Rome) for financial support.

References H. Cheng, J. Ma, Z. Zhao, L. Qi, 1995, Hydrothermal preparation of uniform nanosize rutile and anatase particles, Chem. Mater. 7, 4, 663-671. A. Di Paola, G. Cufalo, M. Addamo, M. Bellardita, R. Campostrini, M. Ischia, R. Ceccato, L. Palmisano, 2008, Photocatalytic activity of nanocrystalline TiO2 (brookite, rutile and brookitebased) powders prepared by thermohydrolysis of TiCl4 in aqueous chloride solutions, Colloid. Surf. A: Physicochem. Eng. Aspects, 317, 1-3, 366-376. A. Di Paola, M. Bellardita, R. Ceccato, L. Palmisano, F. Parrino, 2009, Highly active photocatalytic TiO2 powders obtained by thermohydrolysis of TiCl4 in water, J. Phys. Chem. C, 113, 34, 15166-15174. A. Fujishima, K. Hashimoto, T. Watanabe, 1999, TiO2 Photocatalysis. Fundamentals and applications, Bkc Inc., Tokyo. J.-P. Jolivet, 2000, Metal oxide chemistry and synthesis: from solution to solid state, Wiley, Chichester. M. Koelsch, S. Cassaignon, J.-P. Jolivet, 2004, Synthesis of nanometric TiO2 in aqueous solution by soft chemistry: obtaining of anatase, brookite and rutile with controlled shapes, Mater. Res. Soc. Symp. Proc., 822, 79-84. L. Lutterotti, R. Ceccato, R. Dal Maschio, E. Pagani, 1998, Quantitative analysis of silicate glass in ceramic materials by the Rietveld method, Mater. Sci. Forum, 278-281, 87-92. A. Pottier, C. Chanéac, E. Tronc, L. Mazerolles, J.-P. Jolivet, 2001, Synthesis of brookite TiO2 nanoparticles by thermolysis of TiCl4 in strongly acidic aqueous media, J. Mater. Chem.11, 4, 1116-1121. A.M. Roy, G.C. De, N. Sasmal, S.S. Bhattacharyya, 1995, Determination of the flat band potential of semiconductor particles in suspension by photovoltage measurement, Int. J. Hydrogen Energy, 20,8, 627-630. N. Serpone, P. Maruthamuthu, P. Pichat, E. Pelizzetti, H. Hidaka, 1995, Exploiting the interparticle electron transfer process in the photocatalysed oxidation of phenol, 2-chlorophenol and pentachlorophenol: chemical evidence for electron and hole transfer between coupled semiconductors, J. Photochem. Photobiol. A: Chem. 85, 3, 247-255. Y. Zheng, E. Shi, Z. Chen, W. Li, X. Hu, 2001, Influence of solution concentration on the hydrothermal preparation of titania crystallites, J. Mater. Chem. 11, 5, 1547-1551.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Metal complex-assisted polymerization of thermosetting resins: a convenient one-step procedure for the preparation of heterogeneous catalysts Ulrich Arnold, Manfred Döring ITC-CPV, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

Abstract A series of molybdenum-doped materials based on thermosetting cyanate ester and epoxy resins was prepared and tested as catalysts for the epoxidation of cyclohexene, styrene, 1-octene and propylene with tert-butyl hydroperoxide as oxidant. Monomers with more than two functional groups yield highly stable catalysts that can be used in several consecutive reactions without any catalyst reconditioning step. Metal leaching strongly depends on the resin as well as the substrate. Keywords: epoxidation; cyanate ester resins; epoxy resins; molybdenum; polymersupported catalysts

1. Introduction Thermosetting resins such as epoxy or cyanate ester resins are valuable precursors for the preparation of high performance materials. Applications are manifold, e.g. in the coatings sector or the manufacture of composites for light-weight construction.(1) Recently, epoxy resins were polymerized using metal complexes as initiators. Thus, a variety of catalysts could be obtained by simple mixing of epoxy resins with small amounts of metal complexes, typically around 5%, followed by heating. The resulting metal-doped materials were shown to be useful catalysts for a variety of reactions, e.g. epoxidation, C-C coupling, hydroformylation and hydrogenation reactions.(2) In the meantime, this concept was extended to other thermosetting resins. Cyanate esters and cyanate ester/epoxy resin blends were polymerized in the presence of Mo(OEt)5 and the resulting materials were tested as catalysts for the epoxidation of alkenes.

2. Experimental 2.1. General

The cyanate ester resins PRIMASET® LECy, PRIMASET® BADCy and PRIMASET® PT30 were provided by Lonza. The epoxy resins TGAP and TGMDA were purchased from Aldrich and DGEBA (EPR 164) was obtained from Bakelite AG (now Hexion Specialty Chemicals GmbH). Structures of the resin monomers and oligomers are summarized in Table 1. The molybdenum alkoxide Mo(OEt)5 was obtained from Gelest. Anhydrous tert-butyl hydroperoxide (TBHP) in toluene was prepared by azeotropic drying of 70 wt% TBHP in water (T-HYDRO® solution from Aldrich). Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were carried out under N2 with a heating rate of 10°Cmin−1.

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U. Arnold and M. Döring Table 1. Resin monomers.

Cyanate ester resin monomers

Epoxy resin monomers DGEBA

LECY NCO

OCN

O

O

O

O

NCO

OCN

OCN

OCN

O

O

BADCY

TGAP O

N

O

OCN

n

PT30

N

O O

N

O O

TGMDA

2.2. Preparation of the catalysts Catalysts were prepared by vigorous stirring of resin/Mo(OEt)5 mixtures followed by curing in an oven. Typically, molybdenum contents around 0.8% were adjusted. In the case of resin blends 50/50 wt% mixtures of the resins were employed. The catalyst PT30-TGMDA0.75%Mo was prepared by combining a solution of Mo(OEt)5 in acetone with a 1:1 mixture of PT30 and TGMDA followed by removal of the solvent at elevated temperatures. The hardened materials were ground and particle diameters between 20 and 300 μm were adjusted. Catalysts, curing temperatures and some catalyst features are summarized in Table 2.

2.3. Epoxidation procedures Typically, a mixture of alkene (10 mmol), a 37.5 wt% solution of TBHP in toluene (14 mmol) and 500 mg of catalyst was magnetically stirred at 90 °C for 24 h. The catalyst was separated by filtration (PTFE filters; pore width: 0.45 μm) and employed in the next run without reconditioning. The filtrate was analyzed by GC and atomic spectroscopy. Epoxidation of propylene was carried out in a 80-ml steel autoclave charged with 50 mmol TBHP (34.0 wt% in toluene) and 1 g of catalyst. The solution was saturated with propylene and a pressure of 8 bar was adjusted. The reaction mixture was stirred for 24 h at 90 °C (operating pressure: ca. 20 bar). Propylene oxide yields were based on peroxide consumption determined by iodometric titration and GC analyses.

3. Results and discussion 3.1. Catalyst preparation and characterization Hardening of the liquid or paste-like resin/Mo(OEt)5 mixtures was carried out in aluminum molds by raising the temperature stepwise up to 230 °C (Table 2). DSC measurements revealed high polymerization enthalpies thus indicating high crosslinking. Accordingly, glass transition temperatures Tg and TGA data revealed increasing thermal stability along with an increase of functional groups in the resin monomers.

3.2. Catalytic performance Initially, catalysts based on LECY, BADCY, DGEBA and TGAP were tested in the epoxidation of cyclohexene (Fig. 1a). Alkene conversions and epoxide selectivities were between 94 and 100% in five consecutive reactions. However, extensive metal leaching was observed. Metal leaching could be vastly reduced by use of resins with more than 2 functional groups. In the case of PT30-TGMDA0.75%Mo the catalyst metal content after 10 reactions was still 99.91% of the metal content originally loaded on the polymer

Metal complex-assisted polymerization of thermosetting resin

231

(Fig. 1b). Using this catalyst cyclohexene conversion was around 63% (epoxide selectivity ≥ 96%) and values between 90 and 100% for cyclohexene conversion and epoxide selectivity were observed employing other catalysts containing PT30. Table 2. Curing, DSC and TGA data of molybdenum-doped resins. Curinga

Resin LECY0.84%Mo BADCY-DGEBA0.80%Mo BADCY-TGAP0.75%Mo PT30-LECY0.47%Mo PT30-TGAP0.75%Mo PT300.78%Mo PT30-TGMDA0.75%Mo

Tonsetb (°C) 80 132 120 160 115 147

A A A B A B C

DSC data Tpeakc ΔHd (°C) (Jg−1) 168 842 166 711 150 782 202 599 150, 218 449+g 190 404 Not determined

TGA data T5%f (°C) 242 286 291 311 316 343 317

Tge (°C) 140 163 209 229 n.d.h 225 n.d.

100

95

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0.75% Mo

BADCY-TGAP 0.80% Mo BADCY-DGEBA 0.84% Mo LECY

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a Method A: 80°C, 1h – 100°C, 1h – 120°C, 1h – 140°C, 1h – 160°C, 1h – 180°C, 1h – 200°C, 1h – 220°C, 1h – annealing at 230°C for 6h; Method B: 120°C, 1h – 140°C, 2h – 160°C, 1h – 180°C, 1h – 200°C, 1h – 220°C, 1h – annealing at 230°C for 6h; Method C: Solvent removal at 50°C for 12h – Method A. bOnset temperature of reaction peak. cPeak temperature. dPolymerization enthalpy. eGlass transition temperature of the resulting material. fTemperature at 5% weight loss. gThe value is higher but not determinable due to an unsteady baseline. hNot detected.

50

50 0.75% Mo

BADCY-TGAP 0.47% Mo PT30-LECY 0.75% Mo PT30-TGAP 0.78% Mo PT30 0.75% Mo PT30-TGMDA

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Fig. 1. Catalytic performance of molybdenum-doped cyanate ester/epoxy resin systems in the epoxidation of cyclohexene. Catalysts (a) without and (b) with PT30.

Catalytic performances of PT30-TGAP0.75%Mo, PT300.78%Mo and PT30-TGMDA0.75%Mo in the epoxidation of styrene and 1-octene were also investigated (Fig. 2). Epoxide selectivities were 100% but alkene conversions were moderate. Apart from significant metal leaching in initial reactions, metal leaching was very low and strongly depended not only on the polymer but also on the alkene. Promising results were obtained in the epoxidation of propylene catalyzed by PT30-TGAP0.75%Mo (Fig. 2a). Propylene oxide yields were around 76% and no byproducts were detected.

U. Arnold and M. Döring 100

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95

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20 10

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Fig. 2. Catalytic performance of (a) PT30-TGAP0.75%Mo, (b) PT300.78%Mo and PT30-TGMDA0.75%Mo in epoxidation reactions.

4. Conclusion Polymerization of thermosetting cyanate esters and epoxy resins in the presence of catalytically active metal species represents a facile, time- and cost-saving one-step procedure for the preparation of various catalysts. The resulting polymers exhibit outstanding stabilities, superior to most other catalyst systems based on organic polymers. Such a strategy offers several tunable parameters, e.g. different metal species, resins and (inorganic) additives. Hence, a large potential for optimization is available.

Acknowledgment The authors thank Lonza for supplying the cyanate esters.

References (1) W. Schönthaler, 2005, Chapter Thermosets, in: Ullmann’s Encyclopedia of Industrial Chemistry: Electronic release; John Wiley & Sons Inc.. (2) J. Artner, H. Bautz, F. Fan, W. Habicht, O. Walter, M. Döring, U. Arnold, 2008, Metaldoped epoxy resins: Easily accessible, durable and highly versatile catalysts, J. Catal., 255, 180-189.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis and study of mesoporous WO3-ZrO2-SiO2 solid acid S.V. Prudius, O.V. Melezhyk, V.V. Brei Institute for Sorption and Problems of Endoecology, National Academy of Sciences of Ukraine, Naumova str., 13, Kyiv, 03164, Ukraine

Abstract Three methods for synthesis of mesoporous WO3-ZrO2-SiO2 oxide with high surface area (350 m2/g) are proposed. According to determined acid site strength distributions, WZrSi (H0 ≥ -11.4) occupies the position between superacid WO3/ZrO2 (H0 ≥ -14.5) and middle acidic ZrO2-SiO2 (H0 ≥ -8.2) oxides. It was found that WZrSi provides high yield of polytetramethylene ether (PTME) in tetrahydrofuran oligomerization reaction and demonstrates high activity in the process of isobutane-isobutanol transformation into branched hydrocarbons C8. Keywords: solid acid, hammett acidity, tungstated zirconia, PMTE

1. Introduction Tungstated zirconia (WZr) is known as stable solid superacid which demonstrates high catalytic activity in numerous reactions with a proton transfer, especially in the hydroisomerization of n-C4 – C7 alkanes and Friedal-Crafts acylation of aromatic compounds. Relatively low specific surface area (< 70 m2/g ) is a certain disadvantage of WZr. Also, on today the synthesis of solid acids which occupy intermediate position between zeolites with their acidity function values (Н0 ≥ -8) and superacids (Н0 ≤ -12) is of interest. The data on synthesis and catalytic study of mesoporous WO3-ZrO2-SiO2 (WZrSi) oxide are presented in this communication.

2. Experimental The WZrSi samples were synthesized by three procedures. Zirconyl chloride octahydrate ZrOCl2·8H2O, ammonium metatungstate (NH4)6H2W12O40·xH2O, potassium silicate K2Si2O5 or tetraethoxysilane (TEOS), nonionic surfactant Triton CF-10 (Dow Chemical), and carbamide were used as starting reagents. At first, water sol of silicic acid was obtained through treatment of potassium silicate with H-exchange resin KU-2 (sulfonated styrene and divinylbenzene copolymer). In order to synthesize mixed ZrO2SiO2 (ZrSi) oxide, the aqueous solution of zirconium oxychloride was added to 1 l of polysilicic acid solution to give a mole ratio of Zr:Si = 1:2 [1]. Then, 0.5 wt.% of carbamide and 0.1 wt.% of Triton CF-10 were added. Resulting solution was held at reflux for 2 h while stirring to transform sol into gel. Synthesis of WZrSi samples were performed in the same way, but solution of ammonium metatungstate was added to SiZr sol at atom ratio W:Zr = 0.2:1. The obtained gel was simultaneously aged and dried at 120°C, 48 h. The xerogel was separated in two batches and then was calcined with two different procedures: 1. Xerogel was heated (2°C/min) up to 500°C and kept at this temperature for 4 h. During the treatment, volatile and combustible compounds were eliminated. The product was calcined at 700°C, 2 h and is denoted as WZrSi-d;

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2. Xerogel was washed with water to eliminate chloride ions, dried, and calcined at 700°C, 2 h. This sample is denoted as WZrSi-w. The third sample WZrSi-s (Zr:Si = 1:2) was prepared using TEOS. In a glass reactor, 0.2 mole of TEOS was dissolved in 20 ml of ethanol and 2M HCl mixture. Ammonium metatungstate was mixed with zirconium oxychloride solution and quickly added with vigorous stirring to silica sol (W:Zr = 0.2:1). The resulting solution was boiled for 1 h under stirring to transform sol into gel and then aged at 100°C for 24 h. The semitransparent gel was washed with water, dried at 120oC, and calcined at 700°C, 2 h. For comparison, the WZr-ht sample was synthesized according to hydrothermal procedure described in [2]. Total acidity of the samples was determined by the reverse titration using nbutylamine solution in cyclohexane with bromthymol blue as an indicator. The strength of acid sites was estimated by direct titration with n-butylamine using Hammett indicators (Aldrich): benzalacetophenone (pKBH+ = -5.6), antraquinone (-8.2), 4nitrotoluene (-11.35), 1-chloro-3-nitrobenzene (-13.16), 2,4-dinitrotoluene (-13.75) and 2, 4-dinitro-1-fluorobenzene (-14.52). All samples were dried at 500°C, 1h before testing. XRD patterns of samples were recorded on DRON-4-07 diffractometer (CuKα radiation). Surface areas, pore size distributions and pore volumes were measured by N2 adsorption at 77 K using Nova 2200e Surface Area and Pore Size Analyzer. Before analysis, the samples were treated at 300oC under vacuum. The catalytic activity of the WZrSi, WZr and ZrSi oxides were tested in two reactions: tetrahydrofuran (THF) oligomerization and isobutanol dehydration in the presence of isobutane for producing i-C8 hydrocarbons. The experiments were performed using flow reactors with fixed bed of catalyst.

3. Results and discussion The formation of predominantly tetragonal phase of ZrO2 in framework of all WZrSi samples calcined at 700ºC is registered in the XRD patterns. The ZrO2 crystallite size calculated from peak half-width using Sherrer equation is 4-5 nm for WZrSi-w and 9-10 nm for WZrSi-d. Consequently, burning of the volatile templates without preliminary washing leads to the formation of larger ZrO2 crystals. The parameters of pore structure and acidity of WZrSi in comparison with prepared WZr and ZrSi samples are presented in Table 1. Table 1. Pore structure parameters and acidity of prepared WZrSi, WZr and ZrSi oxides.

Sample

WZrSi-d WZrSi-w WZrSi-s WZr-ht ZrSi

Specific Pore volume (BJH), surface area (SA), cm3/g m2/g 250 0.16 270 0.17 350 0.27 175 0.35 300 0.15

Pore radius (BJH), nm 2.2 2.0 3.6 5.2 2.8

Average pore radius, nm 1.8 1.4 3.2 7.7 1.6

Total acidity, mmol/g

H0

1.09 1.10 1.50 0.64 1.31

≥ -11.35 ≥ -11.35 ≥ -11.35 ≥ -14.52 ≥ -8.2

The SA values of prepared WZrSi samples are in the range of 250-350 m2/g, which is close to SA for ZrSi sample and ca. 4-5 times higher than for WО3/ZrО2 samples obtained by usual co-precipitation technique without hydrothermal treatment [3]. It should be noted that WZrSi-d possesses larger pores than WZrSi-w prepared from

Synthesis and study of mesoporous WO3-ZrO2-SiO2 solid acid

235

washed out xerogels. Perhaps, carbamide and ammonium chloride perform the role of templates and pore-forming substances. Total acidic site concentrations determined using n-butylamine adsorption consist 1.1–1.5 mmol/g for WZrSi samples calcined at 700°C (Table 1). The acid site strength distributions for synthesized oxides are presented on Fig. 1. For WZr-ht sample a wide range of acid strength is observed, from medium (-5.6 ≥ H0 ≥ -8.2) up to superacidic H0 = -14.5 (~5%). About 60% of the sites on the surface of ZrSi sample corresponds to H0 = -5.6. It has been found that WZrSi samples change their color from white to light yellow in the presence of 4-nitrotoluene (H0 = 11.35). However, WZrSi is a stronger solid acid than ZrSi because about 80% of its acid sites are characterized with Hammett acidity function value H0 = -8.2 (Fig. 1). C , m m ol/g 0 .8 0 .7 0 .6 0 .5 0 .4 0 .3 0 .2 0 .1 0 .0

− 1 3 ,7 5 ≤ Η 0 < − 1 4 ,5 2 − 5 ,6 ≤ Η 0 < − 8 ,2 − 1 1 ,3 5 ≤ Η 0 < − 1 3 ,1 6 − 1 3 ,1 6 ≤ Η 0 < − 1 3 ,7 5 − 8 ,2 ≤ Η 0 < − 1 1 ,3 5

Fig. 1. Concentration - strength acid site distributions for ZrSi ( ), WZrSi-w (■) and WZr-ht (□) samples.

The WZrSi, WZr and ZrSi oxides demonstrate high activity in liquid-phase THF oligomerization process (Fig. 2). However, the WZrSi catalyst provides higher yield of polytetramethylene ether (PMTE) at feed rates 8 - 13 mmol THF/gcаt/h in comparison with WZr and ZrSi oxides (Fig. 2). 50

Conversion, %

45 40 35 30

1

25

3

2

20 0

2

4

6

8

10

12

14

16

18

Feed rate, mmol THF/gcat/h Fig. 2. THF conversion over WZrSi-d (1), WZr-ht (2) and ZrSi (3) at different feed rates (Vcat=1 cm3, 60°C, THF: acetic anhydride = 8 : 1 mol).

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Typically PTME olygomers with Mn = 500-900 are formed. In the transformation of isobutanol – isobutane mixtures into high-octane hydrocarbons i-C8, WZrSi-w shows lower activity then WZr-ht (Table 2). This solid acid provides the yield of liquid phase of alkanes i-C8 and olefin i-C′8 at the level of 28 mol % (on expended alcohol) in comparison with 50 % for WZr-ht. The lower yield of alkanes i-C8 (13%) is observed for WZrSi-w also. Table 2. The content and yield of hydrocarbons i-C8 obtained over different catalysts1.

Catalyst

Т, oC

Р, МPа

WZr-ht WZrSi-w ZrSi

210 200 200

1.5 0,7 0,7

1)

Feed rate, mmol (iC4H9OН)/gcat/h 4 2 2

Yield, mol%

іC8H18,%

iC8H16,%

50 28 25

21 13 15

79 87 85

Vcat = 3 cm3, i-C4H10 : i-C4H9OН = 6 : 1 mol

It should been noted that WZrSi catalyst, as well as WZr, can be repeatedly (>20 times) regenerated by calcining at 600оС, 2 h without loss of it activity.

4. Conclusions Three synthetic procedures for preparation of mesoporous WZrSi oxide with high specific area have been proposed. The method utilizing TEOS allows WZrSi samples to be prepared with SA = 350 m2/g and concentration of acidic sites up to 1.5 mmol/g. Acid site strength distributions show WZrSi is strong solid acid which occupies the position between superacid WZr and middle acidic ZrSi oxides. WZrSi can be considered as promising catalyst for the production of PTME via THF oligomerization. Also WZrSi demonstrates high activity in the conversion of isobutane-isobutanol into high-octane hydrocarbons i-C8.

References [1] A. Tarafdar, A.B. Panda, P. Pramanik, Microporous and Mesoporous Materials, 84 (2005) 223. [2] M.A. Cortes-Jacome, J.A. Toledo, C. Angeles-Chavez, M. Aguilar, J.A. Wang, J. Phys. Chem. B, 109 (2005), 22730. [3] V.V. Brei, A.V. Melezhyk, S.V. Prudius, E.I. Oranskaya, Polish J. Chem., 83 (2009) 537.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Citral hydrogenation over Pt-M/CeO2 catalysts (M= Zn, Zr) M. Aoun a, b, M. Chaterb, P. Marecot c, C. Especel c, G. Lafaye c, a

Centre de Recherche scientifique et technique en Analyses Physico-Chimiques (C.R.A.P.C), Alger RP, 16004BP 248, Algérie b Laboratoire d’Etude Physico-Chimique des Matériaux et Application à l’Environnement, Faculté de Chimie, USTHB, El Alia, BabEzzouar, Alger B.P.32, 16111, Algérie. c Université de Poitiers, Laboratoire de Catalyse en Chimie Organique, UMR 6503, 86022 Poitiers, France

Abstract Bimetallic PtZn/CeO2 and PtZr/CeO2 catalysts prepared by impregnation were tested for the selective hydrogenation of citral. Samples with 5wt% Pt and atomic ratio Zn/Pt=Zr/Pt=5 were reduced at 450°C. The monometallic Pt/CeO2 sample was also prepared and studied for comparisons. Samples were analysed by TPR, H2-chemisorption and cyclohexane dehydrogenation. Their catalytic behaviour was evaluated in the citral hydrogenation reaction after reduction treatments in flowing hydrogen at 450°C. Results obtained show that the presence of Zn clearly promotes the hydrogenation of the carbonyl bond. Large differences in reducibility between catalysts were determined from the TPR results. A modification of the catalytic properties of platinum has been achieved by modifying the Pt/CeO2 catalyst by addition of Zn and Zr. Keywords: platinum catalysts; Pt/CeO2; Zn-promoted catalyst

1. Introduction Citral (3, 7-Dimethyl-2, 6-octadienal) is an α , ß-unsaturated aldehyde and the main component of the lemongrass oil [1]. As an unsaturated aldehyde it is a very attractive model molecule for hydrogenation from both scientific and industrial points of view [2]. The hydrogenation products of citral have all important uses not only in the synthesis of flavors but also in pharmaceutical and cosmetic industries. Citronellal and citronellol are especially interesting to the perfume industry because of their highly pleasant odors. The presence of 3,7-Dimethyl octanol and 3,7-Dimethyl octanal detracts from this valuable quality [3]. The reduction of citral can lead to a variety of products, the hydrogenation of the C=O bond produce geraniol and nerol. On the other hand, the hydrogenation of the conjugated C=C bond leads to the saturated aldehyde citronellal, which through cyclisation can form isopolegols. The hydrogenation of the isolated C=C bond leads to citronellol and finally to 3,7-diméthyloctanol. The purpose of the present work was to prepare active and selective ceria supported platinum catalysts for the hydrogenation of citral in order to produce unsaturated alcohols, namely nerol and geraniol. Furthermore, catalysts have been characterized by a number of techniques in order to correlate their surface characteristics with their behaviour in the reaction.

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2. Experimental The CeO2 support (99,995%, Aldrich) was calcined in flowing air for 4 h at 500°C. Prior to use, it was ground and then sieved to retain particles with sizes between 0,1-0,25 mm. 5Wt.% Pt/CeO2 catalyst was prepared by successive impregnation from Cl6H8N2Pt (44% Pt, Aldrich) precursor salt. The catalyst was dried overnight at 120°C and calcined at 500°C for 4h. Bimetallic PtZn/CeO2 and PtZr/CeO2 catalysts with Zn/Pt and Zr/Pt atomic ratio of 5 were prepared as the monometallic catalysts using ZnCl2 (99,999%, Aldrich) and ZrCl4 (98% purity, Aldrich) salt precursors. Catalysts description: PtCe: Pt/CeO2 catalyst; PtZnCe: PtZn/CeO2 catalyst, PtZrCe: PtZr/CeO2 catalyst The amount of H2 chemisorbed on the catalysts was measured in the same condition used by F. Benseradj and coll [4]. TPR experiments were carried out on a pulse chromatograph described else-where [5]. Cyclohexane dehydrogenation reaction was carried out following a procedure described elsewhere [6]. The hydrogenation of citral was performed in the liquid phase following a procedure described elsewhere [7] under the same conditions.

3. Results and discussion 3.1. H2 chemisorption

Selected supported platinum catalysts on CeO2 were characterized by H2 chemisorption after reduction at 500°C. Table 1 reports the results obtained. It was observed that the high hydrogen uptake of the monometallic catalyst reflects high platinum dispersion (69%). However, the metal dispersion is not sensitive to the presence of Zr. But the chemisorption values decreases with Zn addition, reflecting a loss in the number of surface sites, suggesting either the development of a SMSI (strong metal-support interactions) or a state of sintering of the metal particles. This lower ability for H2 chemisorption must be attributed to an increase in the support coverage by Zn species and to a poorer dispersion. These results are similar to those found by Silvestre-Albero and coll. [8] on Pt/TiO2 catalysts and by Aoun and coll. [9] on rhodium catalysts. Table 1. Characterization of samples, Hc: amount of hydrogen chemisorptions (μ mole/gcata), DHc: metal dispersion (%).

Catalysts PtCe PtZnCe PtZrCe

Hc (μ mole/gcata) 176,79 112.8 169.2

DHc(%) 69 44 66

3.2. Temperature programmed reduction Figure 1 summarized the TPR curves of all catalysts which were submitted to the same treatment before the TPR experiments. We noticed that the addition of zinc modifies the shape of the TPR curve of platinum. The observed evolutions are in accordance with previous work [10]. In brief, for PtCe catalyst, the first peak centred at 250°C which is assigned to the surface reduction of ceria in close contact with the metal, and may also include the reduction of platinum. The TPR profile obtained with PtZnCe catalyst shows two broad hydrogen consumptions. The first centred at 248°C can be correlated to the same peak observed above on the monometallic catalyst. Then, the second hydrogen consumption, appears at 494°C can be attributed to the reduction of platinum

Citral hydrogenation over Pt-M/CeO2 catalysts (M= Zn, Zr)

239

and/or zinc species. Nevertheless, the amount of hydrogen consumption is higher than that of other catalysts. A displacement of the maximum reduction temperature towards a relatively low temperature (200°C) is noted on PtZrCe catalyst.

3.3. Catalytic activity for cyclohexane dehydrogenation The different catalysts were tested for a structure insensitive reaction [11], the cyclohexane dehydrogenation, at 270°C and at atmospheric pressure. The evolution of the specific activity is shown in Figure 2. It can be seen that PtZnCe catalyst is not active in this reaction. This result suggests an interaction between the two metals (PtZn). Regarding hydrogen chemisorption, there is a decrease in the chemisorption values with the Zn addition. It has been reported in the literature that the zinc interacts strongly with platinum [10]. Moreover, the specific activity is practically not modified by Zr addition to PtCe. This result would indicate a negligible electronic change or a low alloy formation after Zr addition to Pt. Quantité d'hydrogène consommée (u,a)

1,0

PtC PtZnCe PtZrCe

0,8

0,6

0,4

0,2

0,0 0

100

200

300

400

500

600

700

Température (°C)

Fig. 1. TPR Profiles of PtCe, PtZnCe and PtZrCe catalysts.

Fig. 2. Specific activity evolution over catalysts.

3.4. Citral hydrogenation The catalytic behaviour of the samples is compared in the liquid phase hydrogenation of citral. Under the present experimental conditions, the products observed are citronellal, citronellol, unsaturated alcohols (geraniol and nerol). Figure 3 shows the results of their catalytic activity. A rapid hydrogenation occurs during the first few minutes, then the catalysts activity decreases. The same observations are noted by Lafaye and colleagues on Rh/Al2O3 catalysts. The explanation generally proposed is a decomposition of the citral or unsaturated alcohols yielding chemisorbed CO and carbonaceous species that accumulate on the catalysts surface and block a part of the active sites [12]. However, PtZnCe was the most active, reaching a citral conversion value of 52% after 30 min of reaction. This value is higher to that found by Malathi and colleagues [13]. This result could be due to a lower number of accessible platinum active sites for citral hydrogenation. From these results it can be concluded that the Zn addition to PtCe produces an important electronic modification of the metallic phase. The selectivity of the different samples is shown in Fig. 4. The selectiviy to nerol + geraniol is clearly enhanced by the Zn addition to PtCe. The modification of the catalytic activity in citral hydrogenation, when Zn is added to Pt, could be related to: 1) enhancing the selective reduction of the carbonyl group of citral leading to a production of nerol and geraniol, 2) Zn addition would inhibit the hydrogenation of -C=C-bonds, which is reflected in a lower formation of citronellal, citronellol and the saturated alcohol (3,7dimethyloctanol), the effect of Zn was in agreement with the literature [10]. The selective reduction or hydrogenation of the carbonyl group and the inhibition of the hydrogenation of -C=C-bonds of citral would require a particular structure of the metallic surface. By assuming that ionic Zn would enhance the polarization of the oxygen atom in the carbonyl group, increasing the positive charge on the carbon atom of the carbonyl group and favoring its reaction withhydrogen atoms dissociated in the neighbouring Pt atoms. According to the results of cyclohexane dehydrogenation reaction, the specific activity values for PtZnCe catalyst is different to that of the monometallic one, which indicate a high electronic effect between Pt and Zn, meaning a

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high concentration of PtZn alloy particles. Another topic related to the structure of the metallic phase is the formation of PtZn alloys [10]. Taking into account the results shown in Table 1, it can be observed that H2 chemisorption value decrease when Zn is added. This would mean that the size of these new ensembles after the Zn addition appears to be such as to inhibit the -C=C- hydrogenation. On the other hand, the value obtained on PtZrCe catalyst is very close to that of the monometallic one, which indicates a low electronic effect between Pt and Zr, meaning a low concentration of PtZr alloy particles. 100

100

PtCe PtZnCe PtZrCe

60

40

PtCe PtZnCe PtZrCe

80

Selectivity (%)

Conversion (%)

80

60

40

20

20

0

0 0

20

40

60

80

100

120

Time, min

Fig. 3. Citral hydrogenation as function as function of time

0

20

40

60

80

100

120

Time, min

Fig. 4. unsaturated alcohols selectivity as function of time

4. Conclusion Zn addition to PtCe modifies the selectivity in the citral hydrogenation. The modification of the selectivity to nerol + geraniol by the zinc addition to Pt can be associated to an important change in the structure of the metallic structure. On the basis of the test reaction results of the metallic phase, the catalytic behaviour in citral hydrogenation, H2 chemisorption experiments, PtZnCe catalyst can be described as having large metallic particles. A high concentration of Pt alloys appears to exist on the metallic surface of the particles to give any activity in the cyclohexane dehydrogenation. However, a low electronic effect takes place between Pt and Zr, giving a low concentration of PtZr alloy particles.

References [1] I.M.J. Vilella, S.R. Miguel, C.S.M. Lecea, A. Linares-Solano and O.A. Scelza, Appl. Catal. A: Gen. 281 (2004) 247. [2] M. Arvela, P. Tiainen, L.P. Lindblad, M. Demirkan, K. Kumar, N. Sjöholm, R. Ollonqvist, T. Väyrynen, J. Salmi and D. Yu. Murzin, Appl. Catal. A: Gen. 241 (2003) 271. [3] M.A. Aramendia, V. Borau, C. Jimenez, J.M. Marinas, A. Porras and F.J. Urbano, J. Catal 172 (1997) 46. [4] F. Benseradj, F. Sadi, M. Chater, J. Soc. Alg. Chim. 12 (2002) 99. [5] D. Duprez, F. Sadi, A. Miloudi, A. Percheron-Guegan, Stud. Surf. Sci. Catal. 71 (1991) 629. [6] T. Ekou, A. Vicente, G. Lafaye, C. Especel, P. Marecot, Appl. Catal. A: Gen: 314 (2006) 73. [7] K. Kouachi, G. Lafaye, C. Especel, O. Cherifi, P. Marecot, J. Mol. Catal. A: Chem. 280 (2008) 52. [8] J. Silvester-Albero, A. Sepulveda-Escribano, F. Rodriguez-Reinoso, J.A. Anderson, J. Catal. 223 (2004) 179. [9] M. Aoun, M. Chater, C.R. Chim. 10 (2007) 644. [10] J. Silvester-Albero, A. Sepulveda-Escribano, F. Rodriguez-Reinoso, J.A. Anderson, Appl. Catal A: Gen. 304 (2006) 159. [11] D.W. Blakely, G.A. Somorjai , J. Catal. 42 (1976) 181. [12] U.K. Sing, M.A. Vannice, J. Catal. 199 (2001) 73. [13] R. Malathi, R.P.Viswanath, Appl. Catal. A: Gen. 208 (2001) 323.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Foam-supported catalysts tailored for industrial steam reforming processes Raphaël Faurea, Francesco Basileb, Irene Bersanib, Thierry Chartiera, Aude Cunic, Mathieu Cornillacc, Pascal Del Galloc, Grégory Etchegoyend, Daniel Garyc, Fabrice Rossignola, Angelo Vaccarib a

SPCTS (Sciences des Porcédés Céramiques et de Traitments de Surfaces), UMR 6638 CNRS/ENSCI/Université de Limoges, Limoges, France bDipartimento di Chimica e Chimica Industriale, University of Bologna, Bologna, Italy c Air liquide CRCD Research Centre, Jouy-en-Josas, France d CTTC, Limoges, France

Abstract Alumina foams coated with Rh/MgAl2O4 spinel active phases have been produced to be used as catalysts in steam reforming processes with improved thermal transfers and limited pressure drops. Those foam-supported catalysts are here fully characterised before and after aging in water-rich atmosphere at elevated temperatures. It is shown that they are stable at any architectural scale: macro- (foam), micro- (coating) and nano(Rh active phase) structures. Such catalysts are then very promising catalytic loads to be further implemented in industrial units instead of standard loads. Keywords: foam-supported catalysts, Rh, spinel, steam reforming

1. Introduction The Steam Reforming of Methane (SMR) process is today widely used to produce hydrogen. However SMR also conducts to high amounts of CO2 releases. Moreover, the catalytic loads conventionally used in SMR processes are causing thermal transfer limitations and pressure drops. In order to decrease the impact of SR processes on the environment, other solutions that use bio-fuels as reactants are developed today, amongst which bio-ethanol [1]. Thermal transfer limitations and pressure drops can be partly solved by the use of highly porous catalysts. Such supported catalysts are already in development in many processes such as pollution abatement processes (catalytic converters) or conventional catalytic processes.[2, 3] Our work takes into account industrial requirements and existing issues of SR processes. Active phases suitable for both ESR and SMR are prepared and coated on alumina foams. The stability of the as prepared catalysts is tested in hydrothermal atmospheres (coating adherence, thermal stability, compressive strength of the foams, corrosion resistance). The catalysts produced are finally tested in SMR and ESR reactions at lab-scale and pilot-scale, before being implemented in industrial plants.

2. Experiments 2.1. Preparation of alumina foams First described in 1963, the fabrication of ceramic foams by impregnation of polymeric sponge-like templates has been widely studied.[4-6] For the need of our study, alumina foams are prepared by impregnating polyurethane-ester type foams cut into cylinders

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with an alumina slurry containing additives such as dispersing agents (polymethacrylate ammonium, 2wt%), binders (polyvinylacetate, 4wt%). Additional organics can be used to improve the coating of PU foams (anti-foaming agents and wetting-agents). Impregnated foams are successively dried (r.t., overnight), organics are pyrolized (650°C, 1°C.min-1 heating rate) and alumina is sintered at 1600°C for 2h. The monoliths thus obtained exhibit a densification of 99%.

2.2. Preparation of active phases and coating techniques Active phases are made of Rh dispersed on a magnesium aluminate spinel support. Two preparation techniques are reported here. The first preparation method consists in coating active phases on alumina foams. Commercial spinel powder is attrition-milled to break agglomerates. Attrition-milled MgAl2O4 spinel powder is dispersed in an aqueous solution of rhodium nitrates under stirring at room temperature, for 2h. The amount of Rh is calculated to produce 20wt% Rh loaded catalysts. Water is then evaporated from the slurry and residues are calcined in air (450°C, 4h). Powders obtained are used to prepare slurries which are later used to coat alumina foams. Another preparation method, described in details in the poster communication, can be used. It consists in using sodium polystyrene sulfonate (PSS) polymer to create negative charges on spinel that favour adsorption of Rh3+ precursor.

2.3. Aging tests Active phases, catalyst supports and supported-catalysts are aged in hydrothermal atmospheres figuring the most extreme conditions for SR processes. Powders or foams are introduced in a tubular furnace and heated at 900°C in water/nitrogen (3:1 in molar ratio) mixed atmosphere for durations ranging from 12 hours to 30 days. It must be noticed that active phases and supported catalysts are reduced in 5% H2/Ar before aging.

3. Results and discussions 3.1. Alumina foams characterisation Alumina foams are produced in a variety of pore sizes and shapes. Their porosity can be typically controlled to create graded catalytic beds that enable to improve the thermal profile in the reactor [7, 8]. The stability of alumina foams is attested in water rich atmosphere. TG analysis (Fig. 1.C) realised on alumina foams revealed that no weight losses can be observed upon aging at 900°C (90% relative humidity). However low surface corrosion is observed after 30 days of aging (Fig. 1.A&B). Micrographs from the bulk of alumina foams (not shown here) revealed no microstructural changes before and after aging. In order to point out surface modifications, alumina foams are polished and aged in water rich atmosphere.

Fig. 1 Evolution of alumina foams upon aging. Microstructure of the foam before aging (A) and after 30 days aging (B). TG analysis of alumina foam in water rich atmosphere (C).

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After 5 days of aging treatment, grain boundaries appeared on SEM micrographs. Surface modifications is assumed to be due to formation of volatile aluminium hydroxides, as expected by themodynamic modelling [9]. As the corrosion phenomenon remains limited to the surface, it is not affecting the stability of the foam in the bulk. As a proof, mechanical resistance of alumina foams is kept constant before and after aging: the compressive crush strength of 10 ppi alumina foams with 85% apparent porosity has been measured at 2 MPa.

3.2. Active phases 3.2.1. Bulk and surface characterisation Bulk (XRD, TDA-TG, TPR) and surface (XPS) analyses allow determining the Rh-O species present in Rh/MgAl2O4 catalysts. At low calcination temperatures, amorphous rhodium oxides are formed. Such species are reduced at low temperature (Fig. 2.B) and cannot be seen on HT-XRD (Fig. 2.A). Rh-O species crystallise in α-Rh2O3 at 720°C. Crystallite size increases between 720°C and 950°C, as a shift toward higher reduction temperatures can be observed on Fig. 2.B. At 950°C RhMgAl solid solution starts to form, as identified by the new line appearing on HT-XRD and by a large reduction peak from 800°C on TPR. Similar phases have already been described in the literature [10].

Fig. 2 HT-XRD (A) and TPR (B) characterization of 20wt% Rh/spinel catalysts calcined at 500°C (RhS500), 600°C (RhS600), 720°C (RhS720), 880°C (RhS880) or 950°C (RhS950).

3.2.2. Rh dispersion stability against coalescence Nice Rh dispersions of nano-sized Rh particles (1 to 3 nm, 75% metal dispersion) are obtained by reduction of fresh spinel-supported catalysts exhibiting amorphous Rh-O species (i.e. low initial calcinations temperature). Active phases calcined at higher temperatures (from 720°C to 1000°C) exhibit larger Rh particles. Nano-sized Rh particles observed on fresh RhS500 catalyst coalesce upon aging at 900°C in water-rich atmosphere: metal dispersion is decreased to 19%.

Fig. 3 TPR of aged Rh/spinel catalysts (A) and HR-TEM.

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HR-TEM micrographs reveal that particles of 1 to 5 nm coexist together with larger particles of 10 to 30 nm after aging (Fig. 3.B, RhS500). For samples calcined at higher temperatures (720°C, 880°C), lower Rh dispersions are maintained (Fig. 3.B, RhS720&880). On any aged samples, a solid solution is clearly evidenced at 600°C to1000°C by TPR (Fig. 3.A). Such interfacial solid solution would anchor Rh particles and limit their coalescence upon aging.

3.3. Foam-supported catalysts Foam-supported catalysts are first produced by washcoating the active phases on alumina foams. Coating thicknesses of 1 to 10 µm are produced. The homogeneity of the coating thickness is difficult to control on foams, but it is influenced by the spinel loading the slurry. Similar coatings are obtained by impregnating diluted Rh nitrate solutions on foams that are already washcoated with PSS-adsorbed spinel. It is demonstrated in the poster communication that sulfonate groups of PSS create negative charges on the spinel surface, thus preventing from inhomogeneous Rh dispersion that often occur during impregnation of spinel-coated alumina foams.

4. Conclusions Stable architectures dedicated to industrial SR processes are being developed. Alumina foams are produced by impregnation of polymeric sponge-like templates. Foams can exhibit a variety of porosities and shapes. Rh/spinel active phases are currently used. Active phases can be prepared separately and then coated on alumina foams, or catalysts can be directly prepared by impregnation of spinel-coated alumina foams. An interesting methodology is currently developed to favour the homogeneous Rh loading while impregnating spinel-coated alumina foams. It is shown that the best Rh dispersions on fresh catalysts are obtained after calcinations at low temperature, i.e. by reduction of amorphous Rh-O species. An interfacial RhMgAl solid solution, appearing upon aging, could be responsible for Rh stabilisation. The foam-supported catalysts are stable at 900°C in SR-like water-rich environments. These supported catalysts are currently giving promising results both in ESR and in SMR.

References [1] A. Haryanto, S. Fernando, N. Murali, S. Adhikari, Energy and Fuels 19 (2005) 2098-2106. [2] R.M. Heck, S. Gulati, R.J. Farrauto, Chemical Engineering Journal 82 (2001) 149-156. [3] M.V. Twigg, J.T. Richardson, Industrial and Engineering Chemistry Research 46 (2007) 4166-4177. [4] P. Colombo, Philosophical Transactions: Mathematical, Physical and Engineering Sciences (Series A) 364 (2006) 109-124. [5] J. Luyten, I. Thijs, W. Vandermeulen, S. Mullens, B. Wallaeys, R. Mortelmans, Advances in Applied Ceramics 104 (2005) 4-8. [6] K. Schwartzwalder, A.V. Somers, United States Patent Office 3,090,094 (1963). [7] P. Del Gallo, M. Cornillac, F. Rossignol, R. Faure, T. Chartier, D. Gary, EPO EP 2 123 618 A1 (2010). [8] P. Del Gallo, D. Gary, T. Chartier, M. Cornillac, R. Faure, F. Rossignol, EPO EP 2 141 140 A1 (2010). [9] E.J. Opila, D.L. Myers, Journal of the American Ceramic Society 87 (2004) 1701-1705. [10] F. Basile, G. Fornasari, M. Gazzano, A. Kiennemann, A. Vaccari, Journal of Catalysis 217 (2003) 245-252.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis of ordered nanostructured CuO-CeO2 catalysts by hard template method Petar Djinovića, Jurka Batistaa, Janez Leveca, Albin Pintarb a

Laboratory for Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia b Laboratory for Environmental Sciences and Engineering, National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia

Abstract This study focuses on the hard template synthesis method, which was used to prepare CuO-CeO2 mixed oxide catalysts with 10, 15 and 20 mol % CuO content. KIT-6 silica template was synthesized using a TEOS/Pluronic P123 ratio of 60 and aging temperature of 100°C. After template removal by NaOH etching, CuO-CeO2 solids exhibited ordered mesoporous structure, which was identified by N2 adsorption/ desorption and XRD analyses as a cast of the KIT-6 silica mesostructure. BET surface area of tested materials was in the range of 147-166 m2/g and average CuO particle size between 1.3 and 1.9 nm. High activity and selectivity (over 99 %) of these solids was achieved during WGS reaction in the temperature range from 250 to 450°C. Keywords: hard template synthesis, CuO-CeO2 nanostructured catalysts, WGS reaction

1. Introduction CuO-CeO2 mixed oxides are very active and selective catalysts for preferential CO oxidation in excess H2 (CO PROX) [1] and water-gas shift (WGS) reaction [2]. Reducing particle size to nano scale and increasing surface area will provide a larger number of more active sites and consequently lead to higher activity in abovementioned reactions [3]. By using an inorganic template with high porosity and ordered arrays of mesopores, which is impregnated with a catalyst precursor solution and dissolved after their mineralization, catalysts with high surface area can be obtained, which cannot be usually prepared by conventional methods [4]. KIT-6 silica exhibits controllable pore size, interconnectivity of pores and high surface area, which can be tailored for practical applications by different synthetic pathways, thus making it an ideal template [5]. In this work, we report in detail on the synthesis and characterization of ordered CuO-CeO2 mesoporous mixed oxides (with 10, 15 and 20 mol % CuO content) by using a hard template method with KIT-6 silica acting as a template. The obtained CuO-CeO2 oxides were characterized by a variety of techniques, such as N2 adsorption/desorption, XRD, H2-TPR/TPD, H2-TPR/TPO/TPR, selective N2O chemisorption, and tested for performance in WGS reaction conducted in a continuous-flow fixed-bed reactor.

2. Experimental 2.1. Synthesis of KIT-6 silica template Mesoporous KIT-6 silica was prepared in aqueous solution by mixing 144 g of distilled water and 7.5 g of concentrated HCl (37 %, Merck). In this acidic solution, 4 g of Pluronic P123 (EO20PO70EO20 poly-(alkylene oxide) based triblock copolymer,

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MW=5800, Aldrich) as a structure directing agent was dissolved, while mixing on a magnetic stirrer. Afterwards, 4 g of butanol (absolute, p.a., Merck) was added under stirring and left for 1 h at 35º C. Finally, 8.6 g of TEOS (Si(OC2H5)4, 99.0 % purity, Fluka) was added and stirred for additional 24 h at the same temperature. Molar ratio of SiO2/P123 strongly influences the average pore size, pore volume and BET specific surface area of the prepared template [5]. At higher ratios lower pore volume, smaller pore size and lower BET specific surface area are usually obtained [6]. Furthermore, pore interconnectivity is reported to decrease when using SiO2/P123 ratios above 60, which could decrease the rigidity of the CuO-CeO2 mesostructure and cause its collapse during the subsequent process of template removal. During the synthesis of KIT-6 silica, we used SiO2/P123 ratio equal to 60, since it was reported as the best compromise between pore size (pore volume), silica wall thickness and ample 3D pore interconnectivity [5]. To obtain the desired cubic Ia3d phase, butanol must be added as a structure co-directing agent, together with Pluronic P123. In addition, HCl concentration has to be as low as possible. It was reported that cubic Ia3d phase is formed in the presence of butanol, when concentration of the acid catalyst (HCl) is 0.75 M or less [5]. On the other hand, higher acidity greatly accelerates the kinetics of mesophase formation, leading to smaller domains of ordered mesostructure and consequently greater deviance from the desired cubic Ia3d symmetry. The obtained gel was transferred into stainless steel autoclaves lined with teflon, and aged at 100ºC for 24 h under static conditions. This aging temperature was selected in order to obtain an average pore size of around 8 nm. This was reported as very favorable for subsequent impregnation with various metal oxide precursors [5]. The aged slurry was vacuum filtered and dried overnight at 100º C in a laboratory drier. The dried product was first mixed with 500 ml of ethanol (absolute, p.a., Riedelde Haën) and 30 ml of concentrated HCl and stirred on a magnetic stirrer at room temperature for 1 h. It was then vacuum filtered and washed with 250 ml of distilled water and 150 ml of ethanol. Finally, the product was dried at 60ºC overnight and calcined in air at 550ºC for 5 h to remove the polymer template.

2.2. Synthesis of ordered, mesoporous copper-cerium mixed oxides Three different CuO-CeO2 catalysts were synthesized with nominal 10, 15 and 20 mol % CuO content (they are referred to as CuCe10, CuCe15 and CuCe20, respectively). Appropriate amounts of Cu(NO3)2·3H2O (99.5 % purity, Merck) and Ce(NO3)3·6H2O (99 % purity, Aldrich) were dissolved in 25 ml of ethanol (absolute, p.a., Riedel-de Haën). The amounts of added metal salts were calculated to yield a total metal ion concentration of 0.7 M. Higher concentrations of metal ions are reported to induce formation of bulk metal oxide particles outside the pores of silica [7]. Into 15 ml of this solution, 1 g of KIT-6 was added and stirred at room temperature for 1 h in order to allow the solution to penetrate and fill the KIT-6 pore system completely. Afterwards, the solid was dried overnight at 60ºC. The obtained CuCe15 catalyst precursor was heated in a ceramic crucible in an oven at 400ºC for 3 h to completely decompose the nitrate species. CuCe10 and CuCe20 precursors were calcined at 550 and 450°C, respectively. The impregnation step was repeated with 10 ml of the ethanol-metal salt mixture. After overnight drying at 60ºC, CuCe15 precursor was again calcined at 400º C (CuCe10 at 550ºC and CuCe20 at 450ºC) for 3 h. Optimal calcination temperatures for catalysts with different CuO loadings, which were determined in our previous work [8], were employed in this study. KIT-6 silica template was removed from the resulting solids by leaching twice with 2 M NaOH (Merck) at 50ºC, while mixing the suspension on the magnetic stirrer.

Synthesis of ordered nanostructured CuO-CeO2 catalysts by hard template method 247 Traces of NaOH were removed by continuous washing with distilled water and centrifugation until pH value of the slurry reached 7. Finally, the mesoporous CuOCeO2 mixed oxide samples were dried overnight at 50ºC in a laboratory drier.

3. Results 3.1. Catalyst characterization CuO-CeO2 powders exhibited a well resolved crystalline FCC CeO2 nanostructure and Ia3d cubic symmetry (results not shown), as determined by both wide- and low-angle X-ray diffraction analyses using a PANalytical X`pert PRO diffractometer (Cu Kα radiation, λ=0.15406 nm). No characteristic peaks of copper-containing phases could be identified in any of examined solids, thus indicating very small size of copper entities. Table 1. BET surface area, total pore volume, average CeO2 crystallite size, average CuO particle size and partial CeO2 reduction of CuO-CeO2 catalyst samples. Sample

BET surface area,

Total pore volume, cm3/g

2

m /g

Average (111) CeO2 crystallite size,

Average CuO particle size,

Partial CeO2 reduction,

nm

nm

%

KIT-6 template

600

1.43

/

/

/

CuCe10

147

0.29

7.8

1.3

14.4

CuCe15

166

0.31

6.5

1.9

16.1

CuCe20

161

0.33

6.6

1.7

24.5

CuO phase dispersion at an extent of 28-40 % that was evaluated by selective N2O reduction on a Micromeritics AutoChem II 2920 catalyst characterization system at T=90°C, confirmed the above claim and revealed copper particle sizes, which are considerably smaller (Table 1) compared to chemically identical materials, prepared by either co-precipitation, sol-gel or other traditional methods [9]. BET surface area of synthesized CuO-CeO2 catalysts determined by N2 adsorption/desorption technique using a Micromeritics ASAP 2020 MP/C apparatus, disclosed values between 147 and 166 m2/g (Table 1). Decreasing surface area values follow the trend of increasing calcination temperatures. H2-TPR/TPD tests revealed facile reducibility of the CeO2 support, besides the complete reduction of CuO in the tested temperature range of -20 to 400°C (not shown). The extent of CeO2 reduction increased with CuO loading, indicating its positive influence on oxygen mobility and oxygen storage capacity in the CeO2 structure. Furthermore, H2-TPR/TPO/TPR cycling caused little change in pore size/volume and specific surface area of tested solids, demonstrating good thermal stability of the CeO2 skeletal structure under reductive and oxidative conditions.

3.2. Water-gas shift (WGS) reaction Nanostructured CuO-CeO2 catalysts were tested for WGS reaction performance in a stainless steel (9 mm I.D.) fixed-bed reactor with a 300 mg of catalyst loading. The gas feed composed of 30 % CO and 30 % H2O that was balanced with He and N2 (GHSV = 23600 h-1). Beside H2 and CO2, minute amounts of CH4 were detected throughout the tested temperature range, with the maximum value of 1500 ppm at 450°C. This implies that the selectivity of investigated CuO-CeO2 catalysts in excess of 99 % was measured. At the reaction temperature of 450°C, CO conversions of 62, 61 and 54 % were

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measured over CuCe10, CuCe15 and CuCe20 catalysts, respectively (see Fig. 1). This shows that the activity of nanocrystalline and ordered CuO-CeO2 solids in the stoichiometric WGS reaction is up to 100 % higher compared to chemically equivalent materials prepared by the co-precipitation method [10]. 0,90

CO conversion, /

0,75 0,60 0,45

CuCe10 CuCe15 CuCe20 Equilibrium

0,30 0,15 0,00 250

275

300

325

350

375

400

425

450

Reaction temperature, ° C

Figure 1. CO conversion as a function of reaction temperature obtained during the WGS reaction for CuCe10, CuCe15 and CuCe20 catalysts.

4. Conclusions Hard template preparation method enables the synthesis of highly porous and high surface area nanocrystalline CuO-CeO2 catalysts, which exhibit very small CuO entities (even at a CuO loading of 20 mol %) and strong interactions between both metal oxide phases that facilitate oxygen storage/mobility in the mesoporous CeO2 structure. Additionally, CuO-CeO2 powders proved to be very active and selective during WGS reaction in the middle-to-high temperature range.

References [1] G. Avgouropoulos, T. Ioannides, C. Papadopoulou, J. Batista, S. Hočevar and H.K. Matralis, Catal. Today, 75 (2002) 157. [2] Y. Li, Q. Fu and M. Flytzani-Stephanopoulos, Appl. Catal. B, 27 (2000) 179. [3] W. Shen, X. Doug, Y. Zhu, H. Chen and J. Shi, Microporous Mesoporous Mater., 85 (2005) 157. [4] S.C. Laha and R. Ryoo, Chem. Commun., (2003) 2138. [5] F. Kleitz, T.-W. Kim and R. Ryoo, Bull. Korean Chem. Soc., 26(11) (2005) 1653. [6] M. Choi, W. Heo, F. Kleitz and R. Ryoo, Chem. Commun., (2003) 1340. [7] A. Rumplecker, F. Kleitz, E.-L. Salabas and F. Schüth, Chem. Mater., 19(3) (2007) 485. [8] P. Djinović, J. Batista and A. Pintar, Appl. Catal. A, 347 (2008) 23. [9] A. Pintar, J. Batista and S. Hočevar, J. Colloid Interface Sci., 307 (2007) 145. [10] P. Djinović, J. Batista, J. Levec and A. Pintar, Appl. Catal. A, 364 (2009) 156.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Fine-tuning of Vanadium Oxide Nanotubes Jens Emmerich,a Marijn Dillen,a Christine E. A. Kirschhock,a Johan A. Martensa a

Centre for Surface Chemistry and Catalysis, KU Leuven, Department of Microbial and Molecular Systems, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium

Abstract The formation of vanadium oxide nanotubes (VOx-NTs) is sensitive to agitation during hydrothermal synthesis. The ordering of the VOx layers is lower and the interlayer distance increases. The pH range for VOx-NT synthesis could be extended to pH values below 5. At low pH, the VOx layers are better ordered and there is a trend towards shorter tubes. Below a critical pH around 4.5, other morphologies next to VOx-NTs appear. Only amine templates were found to be suitable while platelets and “star-like” VOx nanomaterials were obtained using alcohols and thiols. Keywords: vanadium oxide, nanotube, template, hydrothermal treatment

1. Introduction Vanadium oxide (VOx) is relevant to a multitude of catalytic reactions [1]. Amongst various open, metastable VOx phases containing organic species or metal cations, vanadium oxide nanotubes (VOx-NTs) and derivatives thereof can be prepared via hydrothermal synthesis. Thanks to their unique layered structure and the simultaneous presence of vanadium in various oxidation states, VOx-NTs are interesting with regard to catalysis, lithium intercalation in batteries and sensor applications [2,3,4]. Initially, carbon nanotubes (CNTs) were used as templates and coated with molten vanadium(V) oxide (V2O5) or VOx gel [5,6]. Alternatively, VOx gels or molten V2O5 were mixed with primary amines to yield VOx-NTs [7,8]. Recently, a safe and costefficient way to prepare VOx-NTs on a large scale from V2O5 and amine templates was published [9]. The possibility to roughly control the tube diameters, the number of VOx layers in the tube walls and the tube length by using different structure-directing amine agents has been shown [10]. However, for using VOx-NTs as heterogeneous catalyst or cathode material, better control of the NT formation is required. Therefore, we performed a systematic study of parameters affecting the formation of these nanomaterials. The preliminary results are summarized in this communication.

2. Experimental Vanadium(V) oxide (V2O5, Riedel-de Haën), dodecylamine (Acros), dodecanethiol (Aldrich), dodecanol (Fluka) and sulfuric acid (H2SO4, Merck KGaA) were used as purchased from the supplier without further purification. VOx-NTs were synthesized based on literature [9] and characterized using X-ray powder diffraction (XRD) and scanning electron microscopy (SEM): (a) Static vs. dynamic hydrothermal treatment (h.t.): V2O5 (15 mmole) and dodecylamine (15 mmole) were stirred for 2 days in a mixture of ethanol and water (20 mL) at 30°C. Ageing was followed by h.t. in teflon-lined autoclaves at 180°C for 7 days. One reaction was carried out in a stationary oven, a second batch was continuously stirred in an oven with a rotating axis for autoclaves. The final products were filtered, washed with ethanol/ hexane and dried in a vacuum oven (40°C, 24 h).

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(b) Synthesis of VOx-NTs at lower pH: The pH of a standard experiment containing V2O5 and dodecylamine (molar ratio 1/1, i.e. template/V = ½) was adjusted with sulfuric acid to pH values in the range from 4.3 – 4.7 by adding 1 – 3 mmole H2SO4. Ageing, static h.t. and product recovery were performed as described in (a). (c) Influence of the template on the formation of VOx nanomaterials: V2O5 (15 mmole) and template (dodecylamine, dodecanol or dodecanethiol: 15 mmole) were aged for 2 days in a mixture of ethanol and water (20 mL, 30°C). H.t., product filtration and drying followed the procedure outlined in (a).

3. Results and discussion Figure 1a provides a comparison of vanadium oxide nanotube (VOx-NT) XRD patterns upon synthesis under static and dynamic hydrothermal conditions. The interlayer distance is in good agreement with the original data in the former case while stirring results in a lower degree of VOx layer ordering and increased interlayer distances [10]. In Figure 1b, XRD patterns obtained for VOx materials prepared at pH = 4.3 – 4.7 are shown. While the tubular structure is well preserved above pH 4.5 (patterns 1 and 2), no pure VOx-NTs could be prepared at pH = 4.3 (pattern 3). (a)

(b)

Figure 1. (a) X-ray diffraction patterns of VOx-NTs hydrothermally treated at 180°C for 7 days without stirring and using an oven with a rotating axis for autoclaves to ensure a dynamic hydrothermal reaction (“stirring”). (b) XRD patterns of VOx materials upon pH adjustment with sulfuric acid (H2SO4) and hydrothermal treatment (180°C, 7 d). The numbers indicate the amount of H2SO4 used (in mmole).

3.1. Static vs. dynamic hydrothermal synthesis Niederberger et al. synthesized vanadium oxide nanotubes (VOx-NTs) without agitation [11]. In accordance with the original work, we observed that VOx-NTs have a tendency to stick together under this condition [4]. To overcome that problem, we synthesized the material under continuous stirring during the whole synthesis. The resulting XRD patterns of VOx-NTs obtained by different hydrothermal treatment (h.t.) are compared in Figure 1a. No significant differences between the materials concerning their morphology were observed with SEM. Thus, dissolution-precipitation and solidsolid transformation processes that occur during h.t. are only marginally affected [12]. No positive effect on VOx-NT dispersion could be identified. Besides, the sample prepared under dynamic conditions is of poorer quality regarding the ordering of VOx layers and the interlayer distance has shifted to higher values (d = 2.82 nm).

3.2. Synthesis of VOx-NTs at lower pH The influence of pH on the morphology of vanadium oxides (VOx) obtained from aqueous solutions is widely recognized [13]. In general, higher coordination numbers of vanadium(V) have been observed at pH values below 7 while tetrahedral coordination predominates with increasing pH. Around pH = 7, layered structures are formed in the

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presence of organic cations which stabilize the negatively charged polyanions [14]. In a previous publication, the influence of the pH on the formation of vanadium oxide nanotubes (VOx-NTs) has been investigated for high pH values achieved by ammonia addition [15]. Bent and scrolled VOx layers but no typical VOx-NTs were obtained at pH = 9 – 10. Since agitation during the hydrothermal treatment (h.t.) did not result in a better dispersion of the VOx-NTs, we decided to explore the lower pH range to examine the influence on the polymerization of VOx and subsequent formation of nanotubular structures. The corresponding X-ray diffractograms are presented in Figure1b. The pH of the original experiment is around 5.9 while we succeeded to synthesize VOx-NTs at significantly lower pH around 4.5 – 4.7. Remarkably, the VOx-NTs synthesized at pH = 4.6 are shorter (data not shown here) and their crystallinity is even improved (Fig. 1b). Lowering the pH below 4.5 results in the formation of a second morphology next to VOx-NTs. Possibly, an increasing positive charge on the VOx polymers prevents strong interaction with the template which is protonated at this pH. Even though interactions between the amine template and the VOx layers might not be simply ionic in nature, the structure-directing properties of the ammonium species can be assumed to be significantly weaker at lower pH [16].

3.3. Functional groups SEM micrographs of vanadium oxide (VOx) nanomaterials obtained using dodecanol, dodecanthiol or dodecylamine as structure-directing template are shown in Figure 2.

Figure 2. SEM micrographs of (from left to right) vanadium oxide (VOx) platelets using dodecanol and V2O5 as vanadium source, “star-like” VOx obtained from the reaction of V2O5 with dodecanethiol and vanadium oxide nanotubes (VOx-NTs) synthesized according to ref. [9].

As becomes clear from the SEM images, vanadium oxide nanotubes (VOx-NTs) are only formed with the amine-functionalised template. However, next to individual, open tubular structures, agglomerates of VOx-NTs can be observed as already indicated in the original work on VOx-NTs [4]. Similar to a previous study, “star-like” VOx was obtained when dodecanethiol was used as template [17]. However, in our case, the platelets that are grown together to form the six-fold rotationally symmetric VOx nanostructures are thicker and the whole assembly is larger (0.5 – 1.0 µm). These differences most probably are due to different experimental parameters. The experiment was performed at significantly lower pH (pH = 3 – 4) compared to the VOx-NT synthesis, but neither alcohols nor amines resulted in similar morphologies, so assumedly the functional group is responsible for star formation. When changing the surfactant to dodecanol, nicely dispersed, tiny laths are obtained. In contrast to the very uniform “star-like” material synthesized with dodecanethiol, the particle size is less homogeneous, ranging from 1 – 10 µm in length but having very uniform widths. Similar as in case of the VOx/dodecanethiol hybrid materials, the pH of the reaction mixture was significantly lower compared to the reference using dodecylamine as template. It seems that both pH and especially the functional group influence the

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morphology by favoring a distinct VOx structure [14]. This hypothesis is supported by a control experiment with VOx/dodecanol mixtures where the pH had been adjusted to around 6 with KOH but no nanotubes were obtained (data not shown here).

4. Conclusions The formation of VOx-NTs is sensitive to agitation during hydrothermal treatment and the pH of the reaction medium. The ordering of the VOx layers is lower and the interlayer distance is increased when a dynamic hydrothermal treatment is applied. By careful pH adjustment with sulfuric acid, the pH range for VOx-NT synthesis could be extended to values below 5 where the VOx layers are better ordered and a trend towards shorter tubes is observed. Below a critical pH around 4.5, other morphologies next to VOx-NTs appear. Only amines were suitable templates while platelets and “star-like” VOx nanomaterials were obtained using alcohols and thiols.

5. Acknowledgments J.E. acknowledges financial support granted by IWT-Vlaanderen. C.E.A.K. and J.A.M. acknowledge the Flemish Government for long-term structural funding (Methusalem).

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

I.E. Wachs, Y. Chen, J.-M. Jehng, L.E. Briand and T. Tanaka, Catal. Today, 78 (2003) 13 T. Chirayil, P.Y. Zavalij and M.S. Whittingham, Chem. Mater., 10 (1998) 2629 L. Krusin-Elbaum, D.M. Newns, H. Zeng, V. Derycke J.Z. Sun and R. Sandstrom, Nature, 431 (2004) 672 M.E. Spahr, P. Stoschitzki-Bitterli, R. Nesper, O. Haas and P. Novák, J. Electrochem. Soc., 146 (1999) 2780 P.M. Ajayan, O. Stephan, Ph. Redlich and C. Colliex, Nature, 375 (1995) 564 B.C. Satishkumar, A. Govindaraj, M. Nath and C.N.R. Rao, J. Mater. Res., 12 (1997) 604 G.T. Chandrappa, N. Steunou, S. Cassaignon, C. Bauvais, P.K. Biswas and J. Livage, J. Sol-Gel Sci. Technol., 26 (2003) 593 G.T. Chandrappa, N. Steunou, S. Cassaignon, C. Bauvais and J. Livage, Catal. Today, 78 (2003) 85 M. Niederberger, H.-J. Muhr, F. Krumeich, F. Bieri, D. Günther and R. Nesper, Chem. Mater., 12 (2000) 1995 F. Krumeich, H.-J. Muhr, M. Niederberger, F. Bieri, B. Schnyder and R. Nesper, J. Am. Chem. Soc., 121 (1999) 8324 M. Niederberger, Dissertation 13971, ETH Zürich A. Michailovski and G. R. Patzke, Chem. Eur. J., 12 (2006) 9122 J. Livage, Chem. Mater., 3 (1991) 578 J. Livage, Coord. Chem. Rev., 178-180 (1998) 999 K.S. Pillai, F. Krumeich, H.-J. Muhr, M. Niederberger and R. Nesper, Solid State Ionics, 141-142 (2001) 185 P. Liu, I.L. Moudrakovski, J. Liu and A. Sayari, Chem. Mater., 9 (1997) 2513 C. O’Dwyer, V. Lavayen, D. Fuenzalida, S.B. Newcomb, M.A. Santa Ana, E. Benavente, G. González and C.M. Sotomayor Torres, Phys. Stat. Sol. B, 244 (2007) 4157

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Plasma-assisted design of supported cobalt catalysts for Fischer-Tropsch synthesis Jingping Hong,a,b Wei Chu,a* Yongxiang Ying,a Petr A. Chernavskii,c Andrei Khodakovb* a

Department of Chemical Engineering, Sichuan University, Chengdu 610065, China Unité de Catalyse et de Chimie du Solide, UMR 8181 CNRS, Bât. C3, USTL-ENSCLEC Lille, Cite Scientifique, 59655 Villeneuve d’Ascq, France c Department of Chemistry, Moscow State University, 119992 Moscow, Russia b

Abstract Two CoIr-based catalysts were prepared via high frequency cold plasma jet following / or instead of thermal calcination, and studied using a wide range of characterization techniques (X-ray diffraction, X-ray absorption, temperature programmed reduction and in situ magnetic measurements). In the plasma assisted preparation process, the precursor was first treated by high frequency cold plasma jet for 10 min after drying. The catalyst prepared via a combination of the plasma treatment and calcination and which combined good cobalt reducibility and high cobalt dispersion exhibited an enhanced activity in Fisher-Tropsch synthesis. Keywords: high frequency cold plasma jet, plasma-assisted preparation, Fisher-Tropsch synthesis, cobalt

1. Introduction Fischer-Tropsch (FT) synthesis converts natural gas-, coal- and biomass-derived syngas into liquid hydrocarbon fuels which are totally free of sulfur- and nitrogen-containing compounds and have very low aromatic contents [1]. FT synthesis proceeds on cobalt metal sites, the overall number of cobalt metal sites on supported catalysts depends on both cobalt dispersion and reducibility. Decomposition of cobalt precursor is an important step in the catalyst preparation, which could significantly influence both cobalt dispersion and cobalt phase composition [2,3]. In our previous study, the effects of pretreatment with glow discharge plasma on cobalt FT catalysts were investigated. The glow discharge plasma was found to considerably enhance cobalt dispersion [3]. In this study, another type of plasma–high frequency cold plasma jet, which was previously found to have a great advantage in the preparation of Ni-based catalyst for methane reforming with CO2 [4], was employed for optimization of the iridium promoted Co/Al2O3 catalysts. Its effects on cobalt dispersion, reducibility and catalytic performance of CoIr-based FT catalysts are addressed in this paper.

2. Experiments 2.1. Catalyst preparation The 15 wt.% Co 0.3wt.% Ir/Al2O3 catalyst precursor used in this work was prepared by incipient wetness impregnation. The Al2O3 support was impregnated with an aqueous solution containing cobalt nitrate and iridium chloride, and dried at 383 K for 5h. The conventional prepared sample was obtained by thermal calcination (673 K for 6h) and

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labeled as CoIr/Al2O3-C. For plasma-assisted samples, the precursor was treated by the jet of plasma in a flow of 20 vol.% H2 and Ar. The catalyst prepared using plasma instead of thermal calcination is denominated as CoIr/Al2O3-P. CoIr/Al2O3-P+C sample corresponds to the sample first treating by plasma jet, and following by thermal calcination (673 K for 6h). Figure 1 shows the apparatus for the plasma treatment. An inside copper electrode was connected to a high voltage supply. The coaxial stainless steel cover served as the ground electrode, a mixture of 20 vol.% H2 and Ar was used as plasma-forming gas. When 20KHZ voltage was applied, the plasma gas was introduced into the catalyst bed. The plasma treatment was performed for 10 min.

Figure 1. Schema of the apparatus for atmospheric high frequency cold plasma jet.

2.2. Characterization X-ray powder diffraction patterns were recorded with a Siemens D5000 diffractometer and Cu Kα radiation. The average size of Co3O4 crystallites was determinated by the Sherrer equation using the Co3O4 diffraction peak at 2θ = 36.8o. X-ray absorption spectra at the Co K-edge were measured at DUBBLE beamline in ESRF (Grenoble, France). Characterization of calcined catalysts was performed using our X-ray absorption cell described in Ref. [5]. The Si (111) double-crystal monochromator was calibrated by setting the first inflection point of the K-edge spectrum of Co foil at 7709 eV. The temperature-programmed reduction profiles were obtained by passing 5% H2/Ar gas mixture through the catalyst while increasing the temperature at a linear rate. The amount of samples for all experiments was about 50 mg. The gas flow velocity was 30 ml/min, and the rate of temperature ramping was 3 oC /min. In situ magnetic measurements were performed using a Foner vibrating-sample magnetometer as described previously [6]. The experiments were conducted by passing pure H2 through the catalyst while increasing the temperature at a linear rate. The amount of samples for all measurements was around 20 mg. The appearance of metallic cobalt species in the samples was monitored in situ by a continuous increase in sample magnetization during the reduction.

3. Results Both CoIr/Al2O3-C and CoIr/Al2O3-P+C catalysts exhibited XRD patterns characteristic of Co3O4 spinel in addition to the patterns of γ-Al2O3, as shown in figure 2. Comparing with CoIr/Al2O3-C, the XRD peaks assigned to Co3O4 phase were broadened in plasma assisted CoIr/Al2O3-P+C catalyst. The particle size of Co3O4 crystallites based on the Sherrer equation, decreased from 14.7 nm in CoIr/Al2O3-C to 7.6 nm in CoIr/Al2O3-P+C. As for CoIr/Al2O3-P catalyst, all the XRD peaks were very broad, indicating very high cobalt dispersion. Since the plasma forming gas contained some concentration of

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hydrogen, new XRD peaks attributed to the reduced cobalt species (CoO and Co) were observed for this sample. The X-ray absorption results were in agreement with the above XRD findings. The XANES spectra of both CoIr/Al2O3-C and CoIr/Al2O3-P+C catalysts were almost identical, similar to those of the Co3O4 reference compound (Figure 3). While for CoIr/Al2O3-P catalyst, the XANES spectrum was rather different from those of the reference compounds (Co3O4, CoO and Co), which indicated the simultaneous presence of several cobalt phases. Linear combination fitting of XANES spectra showed that cobalt nitrate was completely decomposed during 10 min plasma jet treatment, 35.2 % of Co3O4, 59.1 % of CoO and 5.7 % of metallic Co were co-exist in plasma assisted CoIr/Al2O3-P sample.

CoIr/Al2O3-C CoIr/Al2O3-P CoIr/Al2O3-P+C

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CoIr/Al2O3-C

Norm. asorbance, a.u.

Intensity, a.u.

Co3O4

35

45

55

65

75

CoIr/Al2O3-P CoIr/Al2O3-P+C CoO Co

7700

7720

2 Theta, degree

Figure 2. XRD patterns of CoIr-based Catalysts.

7740 7760 Energy, eV

7780

7800

Figure 3. XANES spectra of oxidized CoIrbased catalysts.

The reducibility of CoIr-based catalysts was investigated by TPR and in situ magnetic measurements. There was only one broad reduction peak in all three catalysts (Figure 4). The TPR peak was shifted to lower temperature for plasma assisted catalysts, and the hydrogen consumption was much higher in catalysts prepared with thermal calcination (CoIr/Al2O3-C and CoIr/Al2O3-P+C).

CoIr/Al2O3-P+C CoIr/Al2O3-P

CoIr/Al2O3

Magnetization, emu/g

Intensity, a.u.

18

500

700

900

1100

Temperature, K

Figure 4.TRP profiles of CoIr-based catalysts.

CoIr/Al2O3-P+C

12

CoIr/Al2O3-P

9 6 3 0

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CoIr/Al2O3-C

15

300

400

500

600

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Temperature, K

Figure 5. In situ magnetization of CoIr-based Catalysts during the reduction in pure hydrogen.

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The effects of plasma treatment on cobalt reducibility in CoIr-based samples could be also seen from figure 5. Metallic cobalt was the only ferromagnetic phase present in the catalysts under the experimental conditions. This suggests that the magnetization in strong fields (saturation magnetization) is proportional to the concentration of the metallic cobalt phase. Plasma treatment resulted in a decrease in the temperature of appearance of the metallic cobalt phase. A much higher magnetization and thus a much higher concentration of cobalt metal phase were observed in calcined CoIr/Al2O3-C and CoIr/Al2O3-P+C catalysts. At 673 K, the saturation magnetization was 14.66 emu/g for CoIr/Al2O3-C, 13.26 emu/g for CoIr/Al2O3-P+C and only 6.11 emu/g for CoIr/Al2O3-P. This was consistent with the TPR results. The catalytic performances of the three CoIr-based catalysts were evaluated in a differential catalytic reactor under atmospheric pressure (Table 1). The catalyst assisted by plasma jet followed by thermal calcination (CoIr/Al2O3-P+C) showed a twice higher activity in FT synthesis than the conventional calcined sample (CoIr/Al2O3-C) and the catalyst (CoIr/Al2O3-P) which was prepared using plasma treatment instead of calcination. Table 1. Catalytic performance of conventional and plasma-assisted cobalt catalysts in FT synthesis. Selectivity, % CH4 C2-4-HC C5+-HC CoIr/Al2O3-C 8.52 11.68 13.30 75.02 CoIr/Al2O3-P 7.65 13.07 7.79 79.14 CoIr/Al2O3-P+C 17.50 20.13 21.95 57.92 Conditions: 0.5g catalyst, p = 1 bar, T=463 K, gas hourly space velocity (GHSV)=3000 ml/(g·h)-1, H2/CO = 2. Catalysts

CO conversion, %

4. Conclusion It was found that the catalysts assisted with plasma jet exhibited much higher cobalt dispersion than those prepared via conventional calcination. Smaller cobalt particles in plasma assisted CoIr/Al2O3-P catalyst displayed more difficult cobalt reducibility than cobalt particles in the catalysts prepared with thermal calcination. High catalytic acitivity of plasma assisted CoIr/Al2O3-P+C catalyst was attributed to the combination of relative high cobalt dispersion and good cobalt reducibility. Plasma jet seems to be a promising tool which can be used to control cobalt dispersion and improve catalytic performance of cobalt FT catalysts.

References 1 2 3 4

A.Y. Khodakov, W. Chu, P. Fongarland, 2007, Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels, Chem. Rev., 107, 1692-1744. W. Chu, P.A. Chernavskii, L. Gengembre, G.V. Pankina, P. Fongarland, A.Y. Khodakov, 2007, Cobalt species in promoted cobalt alumina-supported Fischer-Tropsch catalysts, J. Catal., 252, 215-230. W. Chu, L. Wang, P.A. Chernavskii, A.Y. Khodakov, 2008, Glow-discharge plasma-assisted design of cobalt catalysts for Fischer-Tropsch synthesis, Angew. Chem. Int. Ed., 47, 5052-5055. Liu, Y. Li, W. Chu, X. Shi, X. Dai, Y. Yin, 2008, Plasma-assisted preparation of Ni/SiO2 catalyst using atmospheric high frequency cold plasma jet, Catal. Commun., 9, 1087-1091.

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J.-S. Girardon, A. Y. Khodakov, M. Capron, S. Cristol, C. Dujardin, F. Dhainaut, S. Nikitenko, F. Meneau, W. Bras, E. Payen, 2005, A new experimental cell for in situ and operando X-ray absorption measurements in heterogeneous catalysis, J. Synchrotron Radiat., 12, 680-684. P.A. Chernavskii, A. Y. Khodakov, G. V. Pankina, J.-S. Girardon, E. Quinet., 2006, In situ characterization of the genesis of cobalt metal particles in silica-supported Fischer-Tropsch catalysts using Foner Magnetic method, Appl. Catal. A Gen, 306, 108-119.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Chemical vapor deposition of Fe(CO)4(SiCl3)2 for the synthesis of hydrogenation catalyst made of highly dispersed iron silicide particles on silica Jingchao Guan, Anqi Zhao, Xiao Chen, Mingming Zhang, Changhai Liang * State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China

Abstract New iron silicides based hydrogenation catalysts have been prepared by organometallic chemical vapor deposition of Fe(CO)4(SiCl3)2 precursor on silica support. Fe(CO)4(SiCl3)2 was synthesized from Fe3(CO)12 and SiHCl3 at 120 oC, as confirmed by FTIR, 13C and 29 Si NMR. The FeSi loadings have been varied by changing the amount of the precursor. XRD patterns only showed a diffraction peak due to silica, indicating that iron silicide particles were too small to be detected. TEM image showed that the lattice spacing of the particles was 0.2578 nm, which matched well with the lattice spacing 0.2591 nm of the FeSi (111) plane. TEM image showed that the size of iron silicide particles dispersed on the silica was about 3 nm. However, the so-prepared FeSi/SiO2 catalysts showed little catalytic activity in naphthalene hydrogenation. More studies would be needed to have a better understanding on how to improve the efficiency of these new catalysts. Keywords: Fe(CO)4(SiCl3)2; organometallic chemical vapor deposition; iron silicide; naphthalene hydrogenation

1. Introduction Transition metal silicides have unique physical and chemical properties, such as good electrical conductivity, high chemical inertness and thermal stability [1]. It has been shown that transition metal silicides can give excellent catalytic activity and stability in hydrogenation reactions, such as hydrodechlorination and hydrorefining, where they behave differently as conventional catalytic materials [2-4]. It was reported that the active phases of silicon tetrachloride hydrodechlorination catalyst were nickel silicide and copper silicide, but not metallic nickel or copper [2]. Panpranot et al. reported recently that formation of palladium silicide in Pd/SiO2 greatly improved the selectivity of phenylethylene in phenylacetylene hydrogenation [3]. Simultaneously, thermodynamic data also indicate that transition metal silicides have higher stability in the presence of hydrogen sulfide than other intermetallic compounds, such as transition metal nitrides, carbides and phosphides [5]. However, conventional preparation methods for silicides inherited from the microelectronic industry result in low surface area and poor catalytic activity. Chemical vapor deposition has been shown to be a powerful method for generating highly dispersed catalysts in a controlled and reproducible manner [6]. In this work, we report for the first time the metal organic chemical vapor deposition (MOCVD) of Fe(CO)4(SiCl3)2 as a single-source precursor to FeSi nanoparticles on silica support at atmospheric pressure in a fluidized bed reactor. The catalytic properties of the so-prepared FeSi/SiO2 catalysts were tested in the reaction of naphthalene hydrogenation.

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2. Experimental The precursor Fe(CO)4(SiCl3)2 was synthesized from Fe3(CO)12 and SiHCl3 at 120 oC in the absence of water and air following a procedure modified from literature [7,8]. Silica supported nanostructured iron silicides were prepared by a two-step chemical vapor deposition of Fe(CO)4(SiCl3)2 at atmospheric pressure in a fluidized bed reactor. In order to remove adsorbed water, silica was first calcined in air at 500 oC. The precursor was a moisture-sensitive solid and the adsorbed water would promote its hydrolysis. The precursor was sublimed at about 100 oC, carried downstream by argon and adsorbed on silica support. The adsorbed precursor was then treated at 420 oC in hydrogen at atmospheric pressure and a stable black sample was achieved. Especially, the iron silicide loadings could be controlled by changing the amount of the precursor. The samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Naphthalene hydrogenation was used as model reaction to test the hydrogenation activities of the so-prepared iron silicide catalysts. The reaction was carried out at 340 o C and 4.0 MPa in a continuous fixed-bed reactor, before which the catalysts (0.2 g, diluted with 2.0 g SiC) were activated in situ with H2 at 400 oC and 0.1 MPa for 4 h. The liquid reactant was composed of 1 mol% undecane (as internal standard for GC analysis), 5 mol% naphthalene reactant and varying amounts of decane (as solvent). The reaction product was analyzed by off-line gas chromatography.

3. Results and discussion 3.1. Characterization of the precursor Fe(CO)4(SiCl3)2

The IR spectrum of the synthesized precursor in CDCl3 solution revealed the characteristic peaks of carbonyl at 2129 (m) and 2060 (s) cm-1. The latter was so broad (from 2030 to 2090 cm-1) that it was difficult to find other peaks in this area. The results were in good agreement with the literature [7,8]. The 13C NMR spectrum of the precursor in CDCl3 solution showed two peaks at 197.30 and 199.44 ppm, which were attributed to the axial carbonyl group according to the literature [8]. The 29Si NMR spectrum of the precursor in CDCl3 solution showed two peaks at 39.6 and 44.4 ppm, which could be attributed to tetracoordination of the silicon atom in the isomers. This was different from 21.5 ppm reported by Novak et al. [8]. This may be due to the different isomers of Fe(CO)4(SiCl3)2, which needs further investigation. The crystalline product finally obtained was yellow or slightly green, which might correspond to the trans- and cis-isomer, and this resulted from isomerization that might occur above 90 oC during sublimation [7,8]. As we intended to do metal organic chemical vapor deposition, this point seemed to be of little importance due to their similar vapor pressure and their same composition. The reasons why Fe(CO)4(SiCl3)2 was chosen as the single-source precursor were as follows. Based on in situ UPS study and DFT calculation, it was reported that a 1:1 ratio of Fe and Si could be precisely delivered to the substrate surface according to a decomposition pathway of Fe(CO)4(SiCl3)2 with elimination of SiCl4 and formation of Fe=SiCl2(CO)4 which would decompose to FeSi [7,9]. For another, compared with conventional multi-source MOCVD, the single-source precursor approach allowed simpler and safer experimental setups because it avoids usage of highly hazardous liquid precursors (Fe(CO)5 typically for Fe and SiCl4 for Si) [7].

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3.2. Characterization of silica supported nanostructured iron silicides

Intensity (a.u.)

X-ray diffraction patterns of FeSi/SiO2 with 8.1, 13.9 and 18.8 wt% FeSi calculated loading only showed a broad diffraction peak due to silica at about 23.4 °, and did not exhibit any diffraction peak due to FeSi, Fe or FeCl3, indicating that iron silicide particles were too small to be detected (Figure 1). X-ray diffraction patterns of CoSi particles on silica support with similar loadings revealed the same results [10]. The FeSi/SiO2 samples were further determined by transmission electron microscopy (TEM) measurement. The high-resolution TEM image in Figure 2 showed that the lattice spacing of the particle was 0.2578 nm, which matched well with the reported value 0.2591 nm of the FeSi (111) plane. This confirmed the formation of iron silicide by chemical vapor deposition of Fe(CO)4(SiCl3)2 as the precursor. The TEM image also showed that the size of iron silicide particles dispersed on the silica was about 3 nm, which was in good agreement with the XRD results.

18.8%FeSi/SiO2 13.9%FeSi/SiO2 8.1% FeSi/SiO2 20

40

60

80

2Theta (deg.) Figure 1. XRD patterns of FeSi/SiO2 with 8.1, 13.9 and 18.8 wt% FeSi loading from MOCVD.

Figure 2. HRTEM image of FeSi/SiO2 with 18.8 wt% FeSi loading from MOCVD.

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3.3. Catalytic properties of the FeSi/SiO2 catalysts in naphthalene hydrogenation

The reaction results showed little catalytic activity in the hydrogenation of naphthalene over the FeSi/SiO2 with 13.9 wt% FeSi calculated loading. This might be attributed to SiO2 covered the FeSi particles due to FeSi oxidation in air. Meanwhile, the actual loading of FeSi/SiO2 was too low to exhibit any activity in naphthalene hydrogenation. We also tested the catalytic activity of the sample in phenylacetylene hydrogenation. No activity was detected. Further work is necessary to clarify why the FeSi/SiO2 catalysts were not active and to understand how to improve the efficiency of the new catalysts.

4. Conclusions Nanostructured FeSi particles on silica support have been synthesized by MOCVD of Fe(CO)4(SiCl3)2 as the single-source precursor at atmospheric pressure in a fluidized bed reactor. The results indicate that the size of iron silicide particles dispersed on the silica is about 3 nm. The resulting iron silicide catalysts showed little catalytic activity in naphthalene hydrogenation and phenylacetylene hydrogenation. Nevertheless, it is believed that chemical vapor deposition method is of great potential in controlled synthesis of transition metal silicides, which may be applied to some specific reactions in synthesis of fine chemicals.

Acknowledgments We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 20973029), the Program for New Century Excellent Talents in Universities of China (No. NCET-07-0133) and the Doctoral Fund of Ministry of Education of China (No. 20070141048).

References [1] A. H. Reader, A. H. V. Ommen, P. J. W. Weijs, R. A. M. Wolters, D. J. Oostra, 1993, Transition metal silicides in silicon technology, Rep. Prog. Phys., 56, 1397-1467. [2] H. Walter, G. Roewer, K. Bohmhammel, 1996, Mechanism of the silicide-catalysed hydrodehalogenation of silicon tetrachloride to trichlorsilane, J. Chem. Soc. Faraday Trans., 92, 22, 4605-4608. [3] J. Panpranot, K. Phandinthong, T. Sirikajorn, M. Arai, P. Praserthdam, 2007, Impact of palladium silicide formation on the catalytic properties of Pd/SiO2 catalysts in liquid-phase semihydrogenation of phenylacetylene, J. Mol. Catal. A, 261, 29-35. [4] W. Juszczyk, Z. Karpiński, D. Łomot, J. Pielaszek, 2003, Transformation of Pd/SiO2 into palladium silicide during reduction at 450 and 500 oC, J. Catal., 220, 299-308. [5] R. B. Levy, 1977, in Adv. Mater. Catal., Ed. J. J. Burton and R. L. Garten, New York, Academic Press, 101-127. [6] P. Serp, P. Kalck, 2002, Chemical vapor deposition methods for the controlled preparation of supported catalytic materials, Chem. Rev., 102, 9, 3085-3128. [7] A. L. Schmitt, M. J. Bierman, D. Schmeisser, F. J. Himpsel, S. Jin, 2006, Synthesis and properties of single-crystal FeSi nanowires, Nano. Lett., 6, 8, 1617-1621. [8] I. Novak, W. Huang, L. Luo, H. H. Huang, H. G. Ang, C. E. Zibill, 1997, UPS study of compounds with metal-silicon bonds: M(CO)nSiCl3 (M=Co, Mn; n=4, 5) and Fe(CO)4(SiCl3)2, Organometallics, 16, 1567-1572. [9] C. E. Zybill, W. Huang, 1999, Formation of FeSi and FeSi2 films from cis-Fe(SiCl3)2(CO)4 by MOCVD-precursor versus substrate control, Inorg. Chim. Acta, 291, 380-387. [10] C. H. Liang, A. Q. Zhao, X. F. Zhang, Z. Q. Ma, R. Prins, 2009, CoSi particles on silica support as a highly active and selective catalyst for naphthalene hydrogenation, Chem. Commun., 2047-2049.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Laser electrodispersion method for the preparation of self-assembled metal catalysts T.N. Rostovshchikovaa, S.A. Nikolaeva, E.S. Loktevaa, S.A. Gurevichb, V.M. Kozhevinb, D.A. Yavsinb, A.V. Ankudinovb a b

Lomonosov Moscow State University, Moscow, 119991, Russia Ioffe Physico-Technical Institute of RAS, St-Petersburg, 194021, Russia

Abstract Laser electrodispersion (LED) method makes possible to fabricate dense nanostructured catalysts with unique catalytic properties. In contrast to earlier laser ablation techniques, where nanoparticles were synthesized from vaporized matter, LED is based on the cascade fission of liquid metallic drops. Fabricated catalysts consist of ensembles of nanoparticles that are uniform in size and shape, amorphous and stable to coagulation. The catalytic activity of these self-assembled Pt, Ni, Pd, Au and Cu catalysts with extremely low metal content (0.4e(αR/ε0)1/2(Rd/(R+Rd))

(Eq.3)

which shows that Te should be raised to bring microdrops in the fission mode. Estimations made using (Eq.3) show that needed value of plasma temperature is about 20-30 eV. Laser power density required to heat laser torch plasma up to this value is more than 109 W/cm2. The development of the capillary instability of drops commonly involves two stages [4]. First, on exceeding the instability threshold, the drop loses its spherical shape, and the fission starts with a large number of finer (daughter) drops ejected from the prominence on the surface of the mother drop. Analysis shows that daughter drops are also unstable. Accordingly, the drop fission is a cascade process where the drop size decreases by approximately a factor of 10 at each stage of the cascade. The cascade fission stops suddenly when the daughter drops reach a nanometer size. As charged drops become smaller the electric field on their surface increases, that results in a dramatic increase in the field emission of electrons. After the size of the daughter drops decreases to several nanometers, the flow of electrons emerging from the drop surface becomes more than the electron flux coming in from the plasma. When the drops become discharged and stable, the fission terminates resulting in a tremendous number of nanometer drops with narrow size dispersion.

Fig. 1. Scheme of the nanoparticle deposition on granulated support.

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Figure 1. shows schematically the process of nanoparticle deposition on the surface of relatively large (1-3 mm) support grains. A laser pulse causes melting of the target surface and creates the laser torch plasma near the surface. Microdrops of molten metal, which escape from the target and arrive into the plasma, are charged and their fission occurs to give nanosize droplets. Divergent electric field concentrated near the substrate is applied to correct trajectories of charged nanoparticles. This opportunity is used to separate the nanoparticles from the residues of maternal microdrops. The electric field strength was chosen so as to direct nanosize particles to the substrate without disturbing the motion of larger drops. The support grains are placed on the oscillating piezoelectric plate, which is driven by AC source. This results in an intense vibration and random rotation of support grains. The nanodrops formed in the plasma fly apart at a velocity of ~104 cm/s, whereas the velocity of expansion of the plasma cloud exceeds 106 cm/s. Accordingly, at the final stage the expanding plasma moves away from the target and charged drops continue their movement to the substrate in a vacuum. They are cooled down to solidification and cover uniformly the substrate surface. The estimated cooling rate exceeds 106 K/s. That is why formed nanoparticles are amorphous. It is difficult to study directly a process of the drop fission in the laser torch plasma. The main obstacles are associated with the short duration of the fission process (less than 100 ns), the small size and high velocity of the particles. In addition, the charging and division of particles occur in high-density and hot laser torch plasma. The evidence for the process of microdrop fission was obtained in a special experiment when the substrate was mounted in a close vicinity of the target surface. As can be seen in Fig. 2., the surface of the substrate placed near the copper target is covered with submicrometer particles. Some particles have projections whose size is 10 times smaller than that of the main particles. The presence of such particles with projections can be interpreted as a consequence of rapid cooling and deposition onto the substrate of drops that are in the initial stage of fission. By now, nanostructured Cu, Ni, Pd, Pt and Au and Ni/Au catalysts on silicon (100), surface oxidized silicon SiO2/Si, γ-Al2O3 and carbon supports have been prepared by the LED technique. As one can see from the TEM image of Pd catalyst on γ-Al2O3 (Fig. 3.), the support grain is covered uniformly by small aggregates of separated particles. The relative particle size dispersion does not exceed 10%. Comparison of TEM micrographs of Cu, Ni, Pd, Pt and Au films shows that the average size of nanoparticles is only determined by the material of which the particles are composed, it is 5 nm for Cu particles and about 2-3 nm for other metals. In all cases, the electron diffraction patterns recorded directly in the TEM had the form of diffuse halos, which indicates that nanoparticles are in the amorphous state.

Fig. 2. Submicrometer Cu particles on the surface of a substrate near the target.

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The nanoparticles of all studied metals do not coagulate on coming in contact with each other. That may be a consequence of the amorphous state of the metal. This feature makes possible the fabrication of dense self-assembled catalysts. Unique catalytic properties of such catalysts are associated with the inter-grain electron tunneling in dense nanostructures [1,2]. Theoretical and experimental studies show a possibility of the formation of a significant amount of the charged particles due to charge redistribution within an ensemble of clusters on the dielectric support (SiO2/Si) or between supported clusters and conducting support (Si,C) [7]. These charged states provide unusually high catalytic activity (up to 105 mol(product)*mol(metal)-1*h-1) of self-assembled catalysts in hydrogenation as well as in chlorohydrocarbon conversions. This is several orders of magnitude higher compared to that for separated metal clusters, highly loaded metal films and usual supported catalysts (102 mol (product)*mol(metal)-1*h-1). Another important advantage of catalysts prepared by LED technique is their unusually high stability against oxidation and poisoning. For example, according to XPS data, the valence state of Pd remains unchanged during catalytic hydrodechlorination [6]. Catalysts consisting of ensembles of Pd and Ni nanoparticles deposited on sibunite and γ-Al2O3 with extremely low metal content ( 430 nm. The light absorbance at λ= 664 nm after exposure for 2 hours was measured to estimate the photocatalytic activity of the samples. UV-vis absorption spectra of the samples were recorded with JASCO V-550 in a transmission mode. XPS measurements were performed with a Shimadzu ESCA-3300 with Al Kα emission for the X-ray source, and the spectra of N 1s, O 1s and Ti 2p regions were recorded. Binding energy was calibrated using the C 1s peak at 284.6 eV. N K-edge X-ray absorption near edge structure (XANES) spectra of the samples were measured at the BL-8B1 station of UVSOR-II at the Institute for Molecular Science, Okazaki, Japan. Data were recorded at room temperature in total electron yield mode, and the X-ray energy dependence of the N Auger electron yield was monitored. Considering the escape depth of the Auger electrons, the spectra probe the sample from the surface up to a few nanometers in depth.

2. Results and discussion As shown in Fig. 2, in the UV-vis spectra for the as-implanted and 573 K annealed samples, the absorbance in the visible-light region from 420 nm to 540 nm increased with the fluence of nitrogen, suggesting the generation of visible light responsiveness.

Visible-light response TiO2 photocatalyst prepared by nitrogen

269

0.50 21

-2

Absorbance

3 x 10 / m (heat treat.) 0.40 21

3 x 10 / m 21

0.30

-2

1 x 10 / m TiO 2

-2

0.20 440

480 520 Wavelength

560

Fig. 2. (left) UV-vis spectra for as-implanted and 573 K annealed samples.

Amount of MB decomp. (nmol)

The N+ fluence dependences of the visible-light responsive photocatalytic activity are shown in Fig. 3 for the as-implanted and 573 K annealed samples. The TiO2 and the sample implanted by 1 x 1021 m-2 (TiO2-N(1)) showed almost the same MB degradation rate under visible-light irradiation. The sample implanted by 3 x 1021 m-2 (TiO2-N(3)) clearly showed the photocatalytic activity, while this sample became almost photocatalytically inactive after the heat-treatment at 573 K (TiO2-N(3)-H). Thus, the visible light responsiveness was not proportional to the photoabsorbance, which is consistent with the previous reports [2]. 0.4 0.3 0.2 0.1

as implanted after heat treat.

0.0 0.0

1.0 2.0 3.0 + 21 2 N Fluence ( x 10 / cm )

Fig. 3. (right) Amount of decomposed MB after visible light irradiation for 2 h as a function of N+ fluence.

The chemical states of the implanted nitrogen were characterized by XPS, and the N 1s core-level spectra of the as-implanted, the 573 K annealed and TiN samples are shown in Fig. 4. In the spectrum of a TiN powder, a peak around 397 eV was observed. The N 1s peaks observed for as-implanted and the annealed samples can be divided into two groups, one at 396 eV and another at 402-403 eV. We assigned the N 1s peak around 396 eV to nitrogen replacing one of the O sites of TiO2 [1] and the peak at 402403 eV to nitrogen species bound to various surface oxygen sites (N-O like species) [1]. It is interesting to note that XPS spectrum for the photocatalytically active sample (TiO2N(3)) exhibits only one peak at 396 eV while those for the photocatalytically inactive samples (TiO2-N(1)) and the annealed sample (TiO2-N(3)-H) show two peaks. Figure 5 shows N K-edge XANES spectra of the N +-implanted TiO2 and TiN samples. Common XANES features in (b) and (d) suggest that N in TiO2-N(3) is in a chemical environment similar to that in TiN. Careful observation led us to notice that two peaks around 400 eV for the catalyst sample shift to lower energy side than those for the TiN sample, which was well reproduced by the theoretical prediction using FEFF code when N occupies one of the O sites of TiO2. On the other hand, the XANES spectra of TiO2N(1) and TiO2-N(3)-H showed a distinct single peak around 401 eV. This peak could be empirically attributed to formation of the species such as N−O bonds near the surface [4], and which also indicates that the active nitrogen formed in TiO2-N(3) changed to the inactive N-O species by oxidation at 573 K. Thus, both XPS and XANES spectra of TiO2-N(3) exhibited the generation of the photocatalytically active nitrogen (N replacing the O site of TiO2), and the similar spectra were measured for the sample implanted with the higher N+ fluence of 5 x 1021 m-2 (not shown here). The present XPS and XANES measurements also showed that the inactive N-O species dominates in TiO2-N(1). However, these results are in conflict

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with our previous study, in which the photocatalytically active nitrogen was preferentially produced by the implantation with the lower nitrogen concentration.

c)

b)

d) Absorbance (arb. unit)

Intensity (arb. unit)

d)

c)

b)

a)

a) 404

400

396

Binding energy (eV)

392

Fig. 4. (left) XPS spectra of the N 1s region for (a) N+-implanted at 1 x 1021 m-2, (b) N+implanted at 3 x 1021 m-2, (c) N+-implanted at 3 x 1021 m-2 followed by heating at 573 K for 2 h, and (d) TiN.

395

400

405

410

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415

Fig. 5. (right) N K-edge XANES spectra of (a) N+-implanted at 1 x 1021 m-2, (b) N+implanted at 3 x 1021 m-2, (c) N+-implanted at 3 x 1021 m-2 followed by heating at 573 K for 2 h, and (d) TiN.

Then, we roughly calculated the concentrations near the surface of the two samples TiO2-N(1) and TiO2-N(3) from their XPS spectra of N 1s, O1s and Ti 2p regions, and found that the nitrogen concentration of the former sample is a little higher (ca. 5.3 atom%) than that of the latter sample (ca. 4.6 atom%). It is unclear why the nitrogen concentration of the latter sample was so low in spite of the high dose N+ implantation. However, effects of the sputtering process would not be ignored for the present low energy N+ implantation, since a Monte Carlo calculation by SRIM code actually indicated that two atoms of the sample could be sputtered by one 5 keV N+ ion. The nitrogen concentration of the latter sample would not increase by the long time sputtering process, leading to produce the photocatalytically active nitrogen near the sample surface.

References [1] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, 2001, Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides, Science, 293, 269-271.; R. Asahi and T. Morikawa, 2007, Nitrogen complex species and its chemical nature in TiO2 for visible-light sensitized photocatalysis, 2007, Chem. Phys. 339, 57-63. [2] H Irie, Y. Watanabe and K. Hashimoto, 2003, Nitrogen-Concentration Dependence on Photocatalytic Activity of TiO2-xNx Powders, J. Phys. Chem. B, 107, 5483-5486. [3] T. Yoshida, S. Muto and J. Wakabayashi, 2007, Depth-Resolved EELS and Chemical State Mapping of N+-Implanted TiO2 Photocatalyst, Mater. Trans., 48, 2580-2584. [4] J-H. Wang, P. K. Hopke, T. M. Hancewicz and S. L. Zhang, 2003, Application of modified alternating least squares regression to spectroscopic image analysis, Anal. Chim. Acta, 476, 93-109.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation and characterization of shape-selective ZSM-5 catalyst for para-methyl ethylbenzene production with toluene and ethylene Binzuo Liu, Zhaoxiang Yu,Yongtao Meng, Luhao Cui, Zhirong Zhu* Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, China

Abstract In order to improve the shape-selective catalysis of ZSM-5 for para-methyl ethylbenzene (p-MEB) production with alkylation of toluene and ethylene, the acidic sites on the external surface of ZSM-5 crystal are completely eliminated by chemical liquid deposition of silica (SiO2-CLD) using polyphenylmethyl-siloxane (PPMS) as a modifier. So, orthomethyl ethylbenzene and meta-methyl ethylbenzene cannot be formed as by-products neither on the external surface nor in the channel of modified ZSM-5. The acidity of modified ZSM-5 can be further decreased by adding some metallic oxides (1.0 wt% of La2O3 or MgO). This may reduce side reactions like ethylene polymerization and coking deactivation of ZSM-5 catalyst. As a result, ZSM-5 modified by SiO2-CLD and metallic oxide loading shows both high alkylation efficiency (ethylene conversion over 98%) and high selectivity in para-methyl ethylbenzene (over 96%) with a good stability over time. Keywords: ZSM-5 zeolite; chemical modification; para-methyl ethylbenzene; alkylation; shape-selective catalysis

1. Introduction Para-vinyltoluene is a kind of important feedstock for producing quality polymers, which may replace styrene monomer [1]. Para-vinyltoluene is produced by the dehydrogenation of para-methyl ethylbenzene [2]. Para-methyl ethylbenzene may be obtained by alkylation of toluene with ethylene. The main reactions involved in this alkylation process are as follows:

C 2H 5

k1

k 3-1 k 3-2

CH 2 = C H 2 +

C 2H 5

k 4-1

k2

k 4-2 C 2H 5

Figure 1. Schema of the main reactions occuring during the alkylation of toluene.

As the isomerization reaction among three kinds of methyl ethylbenzene isomers (K3, K4) is much easier than alkylation reaction of toluene with ethylene (K1, K2), shown in Figure 1, the mixture of para-methyl ethylbenzene, orth-methyl ethylbenzene

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and meta-methyl ethylbenzene was produced over ordinary acidic catalysts. So, the acidic catalyst for alkylation of toluene with ethylene should be designed with the shape-selective property, to prevent formation of ortho-methyl ethylbenzene and metamethyl ethylbenzene. Zeolite ZSM-5 was often used for the shape-selective catalysis in the synthesis of the para-dialkylbenzene. ZSM-5 modified by chemical vapor deposition of silica (SiO2CVD) might improve para-xylene selectivity in toluene disproportionation or alkylation [3]. In preparation of the commercial catalyst for the shape-selective disproportionation of toluene to para-xylene, the pre-coking method has been replaced by SiO2-CLD [4]. Compared with SiO2-CVD, SiO2-CLD may be more easily transferred to an industrialscale preparation, such as SiO2-CLD modification of ZSM-5 with polysiloxane in Mobil’s MTPX Process [5]. Moreover, modification by loading MgO or phosphate was used to prepare zeolite catalysts for alkylation of EB with ethylene or ethanol, improve para-DEB selectivity over ZSM-5 [6, 7]. Though MgO modification was not as effective as SiO2-CLD for obtaining the high catalytic activity, it can decrease the formation of non-aromatic hydrocarbon and benzene over ZSM-5, by reducing the side reaction of dealkylation. In this work, modification of SiO2-CLD combined with loading metallic oxides was developed to prepare a shape-selective ZSM-5 catalyst for para-methyl ethylbenzene production by alkylation of toluene and ethylene.

2. Experimental 2.1. Preparation of modified ZSM-5 catalyst ZSM-5 with Si/Al 150 was hydrothermally synthesized according to the reported method [8]. NH4-form ZSM-5 was mixed with solution of polyphenylmethylsiloxane (PPMS), at 20 wt % PPMS to ZSM-5. After dried at 393 K, the impregnated ZSM-5 was heated to 823 K at a rate of 3 K/min, and the SiO2-CLD modified ZSM-5 (SiO2CLD/ZSM-5) was obtained. As a reference, the SiO2-CVD modified ZSM-5 (SiO2CVD/ZSM-5) was prepared with TEOS from NH4-form ZSM-5 according to the reported procedure [3]. The above SiO2-CLD/ZSM-5 was mixed with solution of Mg(NO3)2 or La(NO3)3, at the ratio of 1.0 wt % oxide to ZSM-5. After the above impregnated sample was dried at 393 K and calcined at 823 K for 2 h, the SiO2-CLD&MgO or La2O3 modified ZSM-5 (SiO2-CLD&MgO or La2O3/ZSM-5) was obtained.

2.2. Catalytic reaction The catalytic reaction for alkylation of toluene and ethylene was respectively carried out over the parent ZSM-5, SiO2-CLD/ZSM-5, SiO2-CVD/ZSM-5, SiO2-CLD& La2O3/ZSM-5 and SiO2-CLD&MgO/ZSM-5. The reaction was conducted in the fixedbed reactor with WHSV 8 h-1 and 1.2 Mpa (the molar ratio of ethylene / toluene 3.0) at 673 K. The products were determined by on-line gas chromatography with 50 m - 0.32 mm i.d. FFAP capillary column and FID.

3. Results and discussion 3.1. Zeolite materials for catalyzing alkylation of toluene Catalytic performance of a variety of acidic zeolites, with different structural and acidic properties, are compared in the methyl ethylbenzene (MEB) synthesis by alkylation of toluene. These zeolites are the MCM-22, Beta, SAPO-11, MOR and the ZSM-5 with different Si/Al ratio. Among these zeolites, ZSM-5 and SAPO-11 fall into the category

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of medium 10-membered ring, mordenite and Beta the category of large 12-membered ring. MCM-22 has its specific structure with 10-membered ring channels and large intracrystalline cages. Besides, there is a wide range of acidity among the investigated zeolites, from MOR with high-strength acidity to mid-strength acidity SAPO-11. Table 1. Catalytic performance for toluene alkylation over different zeolites.

Mordente ZSM-5 /48 ZSM-5 /150 ZSM-5 /220 MCM-22 SAPO-11 Beta

Toluene Conversion /% 30.9 30.5 29.6 27.7 31.0 27.3 29.2

Ethylene Conversion /% 98.2 97.5 95.7 90.8 97.6 89.5 96.4

Total MEB Selectivity /% 79.3 86.6 91.0 91.5 84.2 87.3 82.1

p-MEB / MEB para-Selectivity /% 63.1 71.2 74.5 76.8 62.0 73.4 68.1

According to the results shown in Table 1, zeolite ZSM-5, with the high Si/Al ratio 150, 10-membered ring channels and mid-strength acidity, shows both high selectivity and catalytic activity for alkylation of toluene with ethylene.

3.2. Modification of SiO2-CLD with loading metallic oxides

In order to improve the shape-selective catalysis of ZSM-5, SiO2-CLD using PPMS as a modifier was adopted to eliminate the acidic sites on the external surface of ZSM-5 crystals. As a result, orth-methyl ethylbenzene and meta-methyl ethylbenzene cannot be formed as by-products on the external surface of modified ZSM-5 as their molecular sizes are more than the pore size of modified ZSM-5. Comparatively, SiO2-CLD/ZSM-5 shows higher catalytic activity as SiO2-CVD/ZSM-5 loses more acidic sites in ZSM-5 channels during modification. However, the selectivity of total methyl ethylbenzene over SiO2-CLD/ZSM-5 is affected by both disproportionation of toluene and dealkylation of methyl ethylbenzene. Two side reactions are mainly caused by the stronger acidic sites of SiO2-CLD/ZSM-5. Table 2. Catalytic performance for toluene alkylation over modified ZSM-5 *. Samples

Toluene Ethylene Total MEB p-MEB / MEB Conversion Conversion Selectivity para-Selectivity /% /% /% /% unmodified ZSM-5 29.4 91.2 95.5 74.9 SiO2-CLD / ZSM-5 27.9 93.5 91.3 90.7 26.1 89.7 91.0 87.2 SiO2-CVD/ ZSM-5 SiO2-CLD&MgO /ZSM-5 25.0 (28.3) 90.7 (98.1) 95.1 (94.2) 98.6 (97.3) 25.6 (28.8) 91.3 (98.5) 94.8 (93.9) 98.2 (96.8) SiO2-CLD&La2O3 /ZSM-5 Note: The data in the bracket were the reaction results after increasing reaction temperature 5 K.

In order to decrease ZSM-5 acidic strength, a little of metallic oxides, basic La2O3 or MgO, were loaded over SiO2-CLD/ZSM-5. This may effectively reduce the sidereactions of both disproportionation and dealkylation. Moreover, the amount of loaded La2O3 or MgO over ZSM-5 relates to catalytic activity and selectivity. The selectivity of para-methyl ethylbenzene and the selectivity of total methyl ethylbenzene increase with

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loaded La2O3 or MgO over ZSM-5, but the catalytic activity drops down. Taking both catalytic activity and selectivity into consideration, about 1.0 wt% La2O3 or MgO loading of over ZSM-5 is proper for the further modification of SiO2-CLD/ZSM-5. Table 3. Results of NH3-TPD characterization for the acidity of modified ZSM-5. Samples

unmodified ZSM-5 SiO2-CLD / ZSM-5 SiO2-CVD/ ZSM-5 SiO2-CLD&MgO /ZSM-5 SiO2-CLD&La2O3 /ZSM-5

Week acidic sites Temperature Desorbed NH3 /mmol/g /K 529 0.67 528 0.53 526 0.48 525 0.50 529 0.51

Strong acidic sites Temperature Desorbed NH3 /K /;mmol/g 778 0.31 775 0.25 773 0.20 750 0.11 754 0.12

Moreover, the further La2O3 or MgO modification reduces coking deactivation from ethylene polymerization over ZSM-5 catalyst, improving its stability. Therefore, SiO2-CLD& La2O3 or MgO/ZSM-5 may be considered as a promising catalyst for toluene alkylation to produce para-methyl ethylbenzene.

Acknowledgements This work is supported by China NSFC 20873091 and Shanghai 09JC141000.

References [1] [2] [3] [4] [5] [6] [7] [8]

C. Yu, C. Tan, J. of Supercritical Fluids, 44 (2008): 341. Z. Zhu, Q. Chen, Z. Xie, C. Li, 13th International Conference on Catalysis, Pairs, 2004: 114. B. Anand, Halgeri, Jagannath Das, Catal. Today, 73 (2002): 65. D. Rotman, Chemical Week, 30 (1995): 18. R. W. Weber, K. P. Moller, C. T. O’Conner, Micropor. Mesopor. Mater., 35 (2000) : 533. Z. Zhu, 8th International Conference on Mechanism of Catalytic Reaction, Russia, 2009: 82. N. Y. Chen, Stud. Sulf. Sci. Catal., 38 (1988): 153. Z. Zhu, Z. Xie, Q. Chen, W. Li, W. Yang, C. Li, Micropor. Mesopor. Mater., 2007(101): 169.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Microwave-assisted preparation of Mo2C/CNTs nanocomposites as an efficient support for electrocatalysts toward oxygen reduction reaction Min Pang, Ling Ding, Chuang Li, Changhai Liang* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China

Abstract Nanostructured Mo2C/CNTs has been synthesized by microwave-assisted thermolytic molecular precursor method. Pt nanoparticles were deposited on the as-prepared Mo2C/ CNTs by using the modified ethylene glycol method. The samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrocatalytic activity toward ORR was measured through a thin-film rotating disk electrode. The results showed the particles size of Mo2C and Pt ranged from 3 to 6 nm. The formation process of Mo2C followed the sequence: Mo(CO)6 → Mo → [Mo,O,C] → Mo2C → Mo3C2. The Pt-Mo2C/CNTs sample possessed higher ORR activity with a more positive onset potential in acid solution than that of Pt /CNTs under the same condition, which could be attributed to the synergistic effect among Pt, Mo2C and CNTs. Keywords: Mo2C/CNTs, microwave, thermolytic molecular precursor, oxygen reduction reaction

1. Introduction Proton exchange membrane fuel cells (PEMFCs) have recently attracted much attention from both a fundamental and an applied point of view for their future potential as clean and mobile power sources. High cost, low activity and poor durability are still major barriers to the commercialization of PEMFCs although lots of advances have been made within the past few decades. Therefore, an inexpensive, active and robust substitute for Pt-based catalyst is in urgent need. Transition metal carbides have received considerable attention for its exceptional noble metal-like activity in some reactions. It shows a good prospect in replacing or reducing the usage of the noble metal in manufacturing the electrode catalysts. However, there are difficulties in obtaining highly dispersed carbide nanoparticles according to the previous preparation methods. Microwave-assisted thermolytic molecular precursor has been proved to be a powerful method for generating highly dispersed tungsten carbide catalysts in a controlled and reproducible manner [1]. Here we report on the synthesis of evenly distributed Mo2C nanoparticles supported on carbon nanotubes by microwave-assisted thermolytic molecular precursor. The Mo2C/CNTs nanocomposite has shown the potential as an efficient electrocatalyst support for oxygen reduction reaction.

2. Experimental CNTs supported molybdenum carbides were prepared by the microwave-assisted thermolytic molecular precursor method. Briefly, CNTs and Mo(CO)6 were mixed in an agate mortar for 0.5 h. The mixture was placed in a quartz reactor and treated in 30

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mL/min argon flow for 2 h. The reactor was placed in a microwave oven operating at 2.45 GHz with a maximum power of 800 W. The duration of microwave exposure varied from 1 to 30 min in argon. Pt nanoparticles were deposited on the as-prepared Mo2C/CNTs by using the modified ethylene glycol method. The samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrocatalytic activity toward ORR was measured through a thin-film rotating disk electrode.

3. Results and discussion 3.1. Characterization of samples with different Mo loadings Figure 1 shows XRD patterns of the samples undergoing 15 min of microwave irradiation. No distinct peaks due to MoxC phase were detected for the 4.8 wt% sample, suggesting the highly dispersed MoxC on CNTs. The XRD pattern of the 13.0 wt% sample showed three typical diffraction peaks at 37.9, 39.4, and 61.5 °, which can be assigned to (002), (101), and (110) crystal face of β-Mo2C with hexagonal closedpacked structure. Further increasing the Mo loading to 16.7 wt%, the diffraction peaks became sharper, which could be ascribed to the growth of Mo2C particle. The average particle size of β-Mo2C estimated according to the Scherrer formula was 6.2 nm for 16.7 wt% samples, which was in good agreement with the value observed from TEM images (Figure 2). ♦

• Mo2C

* Mo3C2 Intensity (a.u.)

♦ CNTs

• *•

•

♦

♦

*

•

16.7 wt% 13.0 wt% 9.1 wt% 4.8 wt%

10

20

30

40

50

60

70

80

2 Theta (deg.)

Figure 1. XRD patterns of the Mo2C/CNTs samples with different Mo loadings.

Figure 2. TEM images of the Mo2C/CNTs sample with the 16.7 wt% Mo loading.

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3.2. Formation process of Mo2C/CNTs

To understand the formation process of Mo2C on CNTs under microwave irradiation, the samples with different irradiation time were checked. Figure 3 shows the XRD patterns of the samples undergoing different duration of microwave irradiation. After 1 min of microwave irradiation, two clear diffraction peaks at 37.3 and 44.6° due to MoOxCy phase appeared, while a weak peak at 19.4° due to Mo2C also emerged. This was attributed to precipitation of O inside the MoOxCy and carburization of external surface of MoOxCy particles [2, 3]. With increasing reaction time to 5 min, the diffraction peaks due to MoOxCy phase became weaker and those due to Mo2C became sharper. Meanwhile the diffraction peaks at 36.4 and 46.4° assigned to Mo3C2 phase appeared. Further increasing reaction time to 15 min, the diffraction peaks due to Mo2C became much sharper while the diffraction peaks due to Mo3C2 phase also became clear. It was also found that the inner wall of the reactor was covered by a layer of metallic Mo when microwave irradiation time was 0.5 min. This was due to the decomposition of Mo(CO)6 precursor to metallic Mo in the initial step, which is in agreement with literature [4]. With the increase of microwave irradiation time, MoOxCy phase first was formed by the reaction between metallic Mo and active carbon species from CO. The MoOxCy phase was further carburized to Mo2C with the increase of microwave irradiation time. Accordingly, the formation process of Mo2C/CNT can be postulated as follows: Mo(CO)6→Mo→[Mo,O,C]→Mo2C→Mo3C2. • Mo2C ♣ Mo3C2

Intensity (a.u.)

♥ MoOxCy ♦ CNTs • • ♣•

15min

•

5min

♣

•

♦

•

♦

•

♥• •

♥ •

1min

•♦

♥

•

0 8

0 7

0 6

0 5

0 4

0 3

0 2

0 1

2 Theta (deg.)

Figure 3. XRD patterns of the MoxC/CNTs samples undergoing different duration of microwave irradiation.

3.3. Electrocatalytic activity of Mo2C/CNTs toward ORR

Figure 4 shows the HRTEM image of Pt-Mo2C/CNTs sample. All the particles were in sphere shape and no agglomerations were detected. The particle size varied from 3 to 6 nm. The Pt particles obtained through the reduction of ethylene glycol method were more likely to be closed with or in conglutination with the Mo2C particles, which was quite different from the Pt-WCx/CNTs samples gained through the same way. Figure 5 shows the linear sweep sweeping curves of the Pt/CNTs and Pt-Mo2C/CNTs samples in the O2 saturated 0.5 M H2SO4 solution. The Pt-Mo2C/CNTs catalyst had a more positive onset potential of 85 mV compared to Pt/CNTs catalyst with the same Pt loading, which could be attributed to a synergistic effect among Pt, Mo2C and CNTs. This performance implied that Mo2C/CNTs was an efficient support for electrocatalyst and possessed a good ability in cutting down the Pt usage.

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0.2 0.1

Current/mA

0.0

Pt-Mo2C/CNTs Pt/CNTs

-0.1 -0.2 -0.3

△ V = 85 mv

-0.4 -0.5 -0.6

0.0

0.2

0.4

0.6

0.8

1.0

Potential/V vs. SCE

Figure 4. HRTEM images of Pt-Mo2C/CNTs sample.

Figure 5. Linear sweeping curves of O2 at Pt/CNTs and Pt-Mo 2 C/CNTs in solution of o 0.5M H2SO 4 with scan rate of 5mV/s at 25 C.

4. Conclusions Mo2C particles with 3-6 nm has been successfully synthesized and well distributed on CNTs by microwave-assisted thermolytic molecular precursor method. The formation process of Mo2C followed the sequence: Mo(CO)6→Mo→[Mo,O,C]→Mo2C→Mo3C2. The Pt-Mo2C/CNTs which were prepared by the modified ethylene glycol method exhibited higher ORR activity with a more positive onset potential in acid solution than that of Pt /CNTs under the same condition, which was attributed to the synergistic effect among Pt, Mo2C and CNTs.

Acknowledgments We gratefully acknowledge the financial support provided by the Scientific and Technical Foundation of Educational Committee of Liaoning Province, Foundation for Returness of Ministry of Education of China.

References [1] C.H. Liang, L. Ding, A.Q. Wang, Z.Q. Ma, J.S. Qiu, T. Zhang, 2009, Microwave-assisted preparation and hydrazine decomposition properties of nanostructured tungsten carbides on carbon nanotubes. Ind. Eng. Chem. Res., 48, 3244. [2] D. Mordenti, D. Brodzki, G. Djéga-Mariadassou, 1998, New synthesis of Mo2C 14 nm in average size supported on a high specific surface area carbon material. J. Solid State Chem., 141, 114. [3] C.H. Liang, P.L. Ying, C. Li, 2002, Nanostructured β-Mo2C prepared by carbothermal hydrogen reduction on ultrahigh surface area carbon material. Chem. Mater., 14, 3148. [4] H.Y. Chen, L. Chen, L. Lu, Q. Hong, H.C. Chua, S.B. Tang, J. Lin, 2004, Synthesis, characterization and application of nano-structured Mo2C thin films. Catal. Today, 96, 161.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Laser-induced photocatalytic inactivation of coliform bacteria from water using pd-loaded nano-WO3 A. Bagabas,a* M. Gondal,b A. Khalil,b A. Dastageer,b Z. Yamani,b M. Ashameria a

Petroleum and Petrochemicals Research Institute (PAPRI), King Abdulaziz City for Science and Technology (KACST), P. O. Box 6086, Riyadh 11442, Saudi Arabia b Laser Group, Physics Department and Center of Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, 31261 Saudi Arabia

Abstract Nano palladium-loaded on nano tungsten trioxide (n-Pd/n-WO3), with 10% wt Pd loading, was prepared by the impregnation evaporation method. The n-WO3 support was prepared by dehydration of tungstic acid (H2WO4). The n-Pd/n-WO3 was characterized by Raman spectroscopy, X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). This material was tested as a photocatalyst for inactivation and killing of coliform bacteria, by applying 355-nm pulsed UV laser radiations, generated from the third harmonic of Nd:YAG laser, to a model water sample, prepared using bacteria strains of Escherichia coli. The killing effect of n-Pd/nWO3 on coliform bacteria was characterized by means of selective culture media. The photocatalysis process did result in a very high irreversible injury (99%) under investigated conditions. This process is cost-effective because no bacteria re-growth was recorded under optimum environment conditions. The disinfection rate of water was estimated by exponential decay. The conventional titania (TiO2) semiconductor and commercially available WO3 display a lower decay rate than that for n-Pd/n-WO3. Keywords: Nano palladium-loaded on nano tungsten trioxide; n-Pd/n-WO3; E-Coliforms; water disinfiction; heterogeneous photocatalysis

1. Introduction The Word Health Organization (WHO) reports two million deaths worldwide annually due to consumption of infected water. The provision of clean water supplies is therefore a key issue for human health and environment. The increasing concern for pathogenic related water diseases has forced the world regulatory bodies to apply strong regulations on microbiological pollution of water to meet drinking water standards [1]. Due to these reasons, specific disinfection techniques must be developed for water treatment [2]. Various chemical processes based on activated carbon, coagulation and multimedia sand filtration have been applied for removing the microorganism [3]. Nevertheless, these conventional technologies only convert the contaminated substances from the treated water to another solid form, requiring further treatment and disposal. Chlorination is another cost-effective and efficient disinfectant method [4]. However, the residual chlorine in treated water is toxic. To overcome these disadvantages in current water disinfection methods, there is a need to develop efficient, cost-effective alternatives. Recently, TiO2 photocatalytic technology has been applied for water

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disinfection by using conventional UV lamps and solar radiation [5,6]. However, to enhance the efficiency of photocatalytic process, further improvements are required. In this work, n-Pd/n-WO3 was synthesized and was applied as a photocatalytic agent for the disinfection of water from coliforms by using 355-nm pulsed UV laser radiations, generated from the third harmonic of an Nd:YAG laser. The killing effect of n-Pd/n-WO3 on coliform bacteria was characterized by means of selective culture media.

2. Experimental 2.1. Catalyst preparation All the chemicals were commercially available (Sigma-Aldrich and Fluka) and were used without further purification. The n-WO3 support was prepared by dehydration of H2WO4 at 300oC for seven hours. The Pd was loaded on this support by the impregnation evaporation method using a sulfur-free benzene solution of palladium acetate in a rotary evaporator. The n-Pd/n-WO3 catalyst was obtained by reduction under H2 flow of 50 ml/min at 350o C for five hours.

2.2. Bacteria calturization and growth Escherichia coli K12 wild-type strain MG 1655 was grown overnight in nutrient broth at 37°C on a rotary shaker (160 rpm). Aliquots of the preculture were inoculated into a fresh medium and were incubated in the same conditions to an absorbance at 600 nm of 0.50±0.025. Cells were harvested by centrifugation at 4000 g for 10 min at 4°C, were washed twice with a sterile 0.9% NaCl solution at 4°C and were resuspended in the photocatalytical solution to a concentration of 2 × 107 CFU/ml. Culturable bacteria (tested bacteria with laser induced photocatalysis) were analyzed by plating on nutrient agar plates after serial dilution in 0.9% NaCl solution. Colonies were counted after 48 h incubation at 37°C.

2.3. Catalyst characterization Raman spectra was recorded on a Perkin Elmer NIR FT-Raman Spectrum GX spectrometer in the range of 4000-100 cm-1. The crystalline phase identification and crystallite size were determined by using a Bruker D8 Advance X-ray diffractometer, operated at 40 kV and 40 mA, using CuK α radiation, in the 2 theta range from 10 to 100 o. The particle size and morphology were determined by a Jeol JEM-2100F (HR) high resolution transmission electron microscope.

2.4. Photoreactor and photocatalytic inactivation experiments The Photocatalytic reactor, used in this study, has been described in detail in our earlier publications for hydrogen production, phenol degradation and other applications [7,8]. The contaminated water samples with bacteria (80 ml) were irradiated using Nd:YAG laser, at different incident laser energies, varying amounts of photocatalyst and for different times. To find the effect of the catalyst identity on the inactivation of the coliforms, micron-WO3 (μ-WO3), Pd-free n-WO3 and n-Pd/n-WO3 catalysts were used. Furthermore, to find out the effectiveness of either photocatalyst or laser radiation on removal of coliforms, photocatalysis was performed in the cases of photocatalyst without laser radiation as well as laser irradiation without a photocatalyst. During this study, the laser energy per pulse was kept at 100 mJ and the contaminated water samples were irradiated for 10 minutes. The treated water samples were collected at different time intervals to observe the removal.

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3. Results and Discussion The Raman spectra are depicted in Fig. 1 for both doped and undoped n-WO3. In Fig. 1a different Raman active modes of vibrations of WO3 are marked. The most intense one (~808 cm-1) is for O-W-O stretching mode of vibration while the one at ~713 cm-1 is for O-W-O bending mode of vibration and the one at ~260 cm-1 is for W=O band. In Fig. 1b for n-Pd/n-WO3, there is a significant change in the relative intensities of the O-W-O stretching and O-W-O bending modes of vibrations, but there is a slight decrease in the intensity for W=O band. It is due to the clinging of Pd atom on WO3 in the doped material.

10% Pd/WO3

Intensity (a.u.)

WO3

20

30

40

50

60

70

80

o

2Θ ( )

Fig. 1. Raman spectra of (a) 10% n-Pd/n-WO3 and (b) undoped n-WO3.

Fig. 2. XRD spectra of 10% n-Pd/n-WO3 (black) and undoped n-WO3 (red).

The crystalline phase identification and crystallite size of the synthesized n-WO3 of both doped and undoped were estimated from the XRD study (Fig. 2). The undoped n-WO3 (spectrum in red), obtained from the dehydration of H2WO4 at 300oC, adopts the triclinic phase. Its crystallite size was calculated from peak broadening (in nm) using the Scherrer’s equation, resulting in an estimated average crystallite size of 8 nm. However, the XRD for n-Pd/n-WO3 (spectrum in black) shows that the phase of n-WO3 changed to tetragonal after the reduction of Pd acetate to Pd metal. The estimated average crystallite size of the doped n-WO3 is 14 nm. However, no characteristic patterns observed due to Pd metal. This observation is attributed to the highly dispersed [9] Pd metal nanoparticles (3-4 nm), supported on n-WO3, as confirmed by TEM study (Fig. 3).

Palladium nanoparticles

Fig. 3. TEM micrograph of 10% n-Pd/n-WO3.

The growth and decay of the bacterial population is exponential in nature. Hence, the rate constant of the bacterial decay was calculated from the slope of the curves, resulted by plotting ln(N/N0) versus the light exposure time. N0 is the initial normalized population (4×107 CFU/ml) and N is the diminishing number of bacterial population in CFU/ml. The threshold time of bacterial decay was estimated from the length of time

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when the decay process sets in after the exposure of the first laser pulse. Bacterial decay curves were established for μ-WO3, n-WO3, n-Pd/n-WO3, n-WO3 without UV laser radiation, and for the UV laser radiation without photocatalyst. When using 300 mg of photocatalyst and applying 100 mj laser energy pulse, μ-WO3 shows a decay constant of 0.645 min-1 and a threshold time of 5 min and 58 sec. On the other hand, n-WO3 demonstrates a decay rate constant of 0.945 min-1 and a threshold time of 0 sec, implying that the decay process is almost instantaneous. Furthermore, in the case of n- WO3 , the complete killing of bacteria occurs in 12 minutes, but, in the case of μ- WO3, it occurs in nearly 24 minutes. This substantial improvement in the catalytic process of antimicrobial activity is due to particle size differences. When applying n-WO3 without UV laser irradiation, no decay at all was observed. However, when UV laser irradiation was applied without photocatalyst, a much slower decay rate of bacteria was observed. Doping n-WO3 with 10% wt palladium metal improved the bacterial decay process. This activity enhancement is due to increasing the band gap energy, calculated from absoprtion spectra, from 2.71 eV for the undoped n-WO3 to 3.5 eV for n-Pd/n-WO3, corresponding to around 355 nm in wavelength. The irradiation wavelength used in this study is 355 nm, which is quite closer to the band gap of the doped material. Therefore, we could achieve a near resonance condition that enhanced the transfer of electron from the valence band to the conduction band and, in turn, increased the photocatalytic process. In the case of n-Pd/n-WO3, the decay constant is about 1.1 min-1 and the threshold time is zero sec when using 80 mg of n-Pd/n-WO3 and applying 80 mJ laser pulse energy.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

WHO/UNICEF, Report. 2000, Global water supply and sanitation assessment report, New York and Geneva. W.A. Yanko, 1993, Analysis of ten years of virus monitoring data from Los Angeles country treatment plants, Meeting on California Wastewater Reclamation Criteria. Water Environ Res., 66, 221-226. V. Lazarova, and J.C. Bourdelot, et. al., 1998, Advances in wastewater disinfection: technical and economic evaluation and large scale operation, Proceedings of the WEFTEC Asia’ 98, Singapore, March 8–11, 1998, 2, 129 -39. H. Arai, M. Arai, and A. Sakumoto, 1986, Exhaustive degradation of humic acid in water by simultaneous application of radiation and ozone. Water Res. 22, 123-126. R. Armon, N. Laot, and N. Narkis, 1998, Photocatalytic inactivation of different bacteria and bacteriophages in drinking water at different TiO2 concentration with or without exposure to O2, J. Adv. Oxid. Technol. 3, 145-150. P. K. J. Robertson, et. al., 2005, Photocatalytic detoxification of water and air, The Handbook of Environmental Chemistry; Volume 2M/2005: Environmental Photochemistry Part II, Springer, Berlin, Heidelberg, 367-423. M. A. Gondal, et al., 2008, Selective Laser Induced Photo-Catalytic Removal of Phenol from Water Using p-Type NiO Semiconductor Catalyst, J. Hazard. Mater. 155, 83-89. M.A. Gondal, et. al., 2009, Efficient Removal of Phenol from Water Using Fe2O3 Semiconductor Catalyst Under UV Laser Irradiation, J. Environ. Sci. Health Part A, 44, 515521. Z. Ma, F. Zaera, 2006, Characterization of Heterogeneous Catalysts, Surface and Nanomolecular Catalysis, Taylor and Francis, Boca Raton, FL, USA, 1-37.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Effect of the carbon nanotube basicity in Pd/N-CNT catalysts on the synthesis of R-1-phenyl ethyl acetate Serap Sahin a, Päivi Mäki-Arvela a, Jean-Philippe Tessonnier b, Alberto Villa b, Lidong Shao b, Dang Sheng Su b, Robert Schlögl b, Tapio Salmi a, Dmitry Yu. Murzin a a

Process Chemistry Centre, Åbo Akademi University, Turku, FI-20500, Finland Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradagweg 4-6, 14195 Berlin, Germany

b

Abstract Catalytic activities of palladium catalysts supported on activated carbon and carbon nanotubes were investigated in the one-pot synthesis of R-1-phenylethyl acetate in combination with an immobilized lipase in toluene. Palladium catalysts on carbon nanotubes with nitrogen-containing surface groups were prepared by incipient wetness impregnation. The basic N-CNT support was synthesized by post-treating oxidized CNTs in gaseous NH3 at high temperature, prior to Pd addition. The basic character of the support was adjusted by controlling the temperature of the post-treatment step. The results showed that the desired product yield was enhanced over palladium catalysts with the lowest basicity. Keywords: one-pot cascades, carbon nanotubes, immobilized lipase, acetophenone

1. Introduction Cascade methodology, implying several reactions in one reactor pot, has gained interest recently due to the savings in equipment and separation costs in particular for production of fine chemicals. One of the ways to utilize cascades efficiently is to combine biological and chemical (i.e. heterogeneous and homogeneous) catalysis [1]. Synthesis of the R-1-phenylethyl acetate, which is an important building block for the production of biologically active pharmaceuticals, was studied over a heterogeneous palladium (Pd) catalysts supported on carbon nanotubes (or activated carbon) in combination with an immobilized lipase in one-pot under mild reaction conditions. Pd supported catalysts on activated carbon (AC) have been widely studied as catalysts for hydrogenation, dehydrogenation and oxidation reactions for the production of fine chemicals [2]. Activated carbons as catalyst supports present several advantages being relatively inexpensive and inert materials [3]. However, they also exhibit a major drawback as their surface properties can vary from batch to batch. Furthermore, typically Pd/AC catalysts exhibit acidic surface groups, such as carbonyl, carboxylic, phenolic hydroxyl, lactone and quinone groups [4]. It was recently reported that Brønsted acid sites enhance the hydrogenolysis of secondary alcohols, such as 1-phenylethanol, in the model reaction [5]. Carbon nanotubes (CNTs), thanks to their unique properties such as high surface area, electrical properties, high mechanical stability, and adjustable surface properties [2]. Pd has been deposited on CNTs with various methods such as incipient wetness

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impregnation [6], homogeneous deposition precipitation [6, 7], ion exchange [6, 8] and organometallic grafting [6, 9]. Typically, the dispersion of the metal can be improved by pre-treating the carbon nanotubes in order to introduce functional groups (e.g. oxygencontaining surface groups) to the surface to enhance interactions between the support and the catalyst precursor [10, 11]. In the present work, the catalytic activity of palladium catalysts either on activated carbon or carbon nanotubes was studied in the one-pot synthesis of R-1-phenylethyl acetate. The influence of the acid-base properties of the support on the catalytic activity has been investigated by treating oxidized CNTs with NH3 at different temperatures in order to introduce various amounts of basic N-containing groups on the surface [12].

2. Experimental 2.1. Catalyst synthesis

CNTs were first oxidized with concentrated nitric acid at 100oC for 2h. After washing and drying, the oxidized CNTs were further treated with gaseous ammonia at 200oC, 400oC or 600oC, respectively [12]. 2 % (w/w) Pd/N-CNT catalysts were subsequently prepared by incipient impregnation using an aqueous solution of Pd (NO3)2.2H2O. After drying at room temperature for overnight, the samples were calcined in air at 350oC for 2h and reduced in hydrogen at 400oC for 2h. Part of the N-containing basic sites was lost during the calcination and reduction. For comparison with Pd/N-CNT catalysts, 5 % (w/w) Pd/AC (Degussa) was also tested.

2.2. Catalyst characterization The catalysts were characterized by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 150oC for 3 hours prior to the specific surface area measurements calculated by using the BET equation. Hydrogen temperature programmed desorption was performed at 200oC for 120 min with a temperature ramp 5oC/min under H2 flow (Micromeritics, Autochem 2910). Palladium stoichiometry was taken as 2 [13]. The acid-base titrations were performed to characterize the surface chemistry of the N-CNT and AC supports. Typically, 100 mg of sample was dispersed in 50 mL of 10-3 M KCl solution and stirred for overnight. Prior to measurements, the mixture was degassed under Ar for at least 1h untill the pH value was constant. The titration was performed under Ar, using 10-2 M HCl solution. The initial pH (pHinitial) values of the solution were recorded. The pH of the Pd/AC was measured as in [14] for determination of the acidity.

2.3. Experimental procedure

Experiments were typically performed at 70oC in toluene (125 mL) in a glass reactor at atmospheric pressure under H2 flow (AGA 99.999, 295 mL/min). The initial reactant concentration was 0.02 mol/L. Ethyl acetate with the concentration of 0.06 mol/L was used as an acyl donor. The catalytic hydrogenation of acetophenone (Acros, 99%) was carried out over 2% (w/w) Pd/N-CNT (312.5 mg) and the formed R-1-phenylethanol was acylated in the same pot to R-1-phenylethyl acetate with an immobilized lipase (Novozym 435, lipase B from Candida antarctica) (62.5 mg). The supported Pd catalysts were pre-reduced at 200 oC prior to the experiment. The products were analysed by a gas chromatography equipped with a chiral column CP Chirasil Dex (250 μm × 0.250 μm × 25 m) and a flame ionization detector. The samples were analyzed by using the flowing temperature program 100 oC (1 min)0.30 oC/min-130 oC-15 oC/min-200 oC (10 min). The temperature of the injector and

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split ratio were 280oC and 100:1, respectively. The products were identified with GC-MS (Agilent Technologies 6890N Network GC System, 5973 Network MS Detector).

3. Results and discussion 3.1. Catalyst characterization results Acid-base titrations showed that during the calcination and reduction processes many basic sites were lost, thus leading to lower catalyst pH values for the catalysts than for the starting N-CNT supports (Table 1). This might be due to the loss of carboxylic acid groups during the catalyst preparation. The low specific area of Pd/N-CNT is due to the wall thickness of the CNTs [15]. Pd/AC exhibited the lowest pH. Table 1. Catalyst characterization results. Entry Catalyst

1 2 3 4 a, b, c

Pd/N-CNTa Pd/N-CNTb Pd/N-CNTc Pd/C

Metal dispersion (%)

Metal cluster size (nm)

30 41 51 54

3.7 2.7 2.2 2.0

BET (m2/gPd) 43 43 43 949

pHinitial 7.0 5.6 4.6

CNT treated with NH3 at 200oC, 400oC, 600oC, respectively.

3.2. Catalytic activity results Although entries 3 and 4 had similar metal particle sizes, Pd/AC, being the most active, displayed the highest turnover frequency (TOF). The relation between the TOF and the pHinitial can be seen in Table 2. The dispersions of Entries 3 and 4 were alike while the pHinitial were different, leading to a conclusion that the catalytic activity is significantly influenced by the acid-base properties of the support. The differences in activities cannot be attributed to structure sensitivity. It is well known that the maximum catalyst dispersion is favored when the carbon material is acidic [16]. The highest acetophenone conversion was obtained over 5 % (w/w) Pd/AC (Entry 4). However, the yield of R-1-phenylethyl acetate (R-PEAc) was only 13 %, since ethyl benzene (EB) was formed as a major product (Entry 4) due to the acidic support (Table 2). The maximum yield of R-1-phenylethyl acetate over 2 % (w/w) Pd/N-CNT (Entry 3) was 26 % at 75 % conversion level of acetophenone corresponding to 34 % selectivity over 312.5 mg of 2 % (w/w) Pd/N-CNT catalyst in combination with 62.5 mg of immobilized lipase (Figure 1a). Furthermore, the yield of R-1-phenylethyl acetate as well as the conversion of acetophenone increased with an increased basicity of the support material. At the same conversion level, the most selective catalyst was Pd/NCNT (Entry 1) in which the support was treated at 200oC with NH3 prior to Pd addition exhibiting the lowest acidity of the three studied CNT-catalysts.

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S. Sahin et al. Table 2. Kinetic results using Pd catalysts reduced at 200 oC for 120 min under H2 flow.

Entry Catalyst 1 2 3 4

Pd/N-CNTa Pd/N-CNTb Pd/N-CNTc Pd/C

Initial hydrogenation rate (mmol/min/ghyd.cat.)

TOF (s-1)

Conversion after 480 min (%)

Selectivity to R-PEAc (%)

0.002 0.020 0.030 0.10

0.0001 0.0008 0.0009 0.005

26 66 75 96

41d 36e 34e 23e

a, b, c

CNT treated with NH3 at 200 oC, 400 oC, 600 oC, respectively, d selectivity to R-PEAc at 26 % conversion after 480 min, e selectivity to R-PEAc at 66 % conversion. a)

b) 50

0.003 o

o

2 wt% Pd/N-CNT treated at 400 C

0.002

o

2 wt% Pd/N-CNT treated at 600 C

30

5 wt% Pd/C

20

5 wt% Pd/C

0.0025

EB (M)

Selectivity to R-1-PEAc (%)

2 wt% Pd/N-CNT treated at 200 C 40

2 wt% Pd/N-CNT o treated at 600 C

0.0015 0.001

10

0.0005 0

0 0

20

40

60

Conversion (%)

80

100

0

40000

80000

120000

160000

Time x mg Pd (min x mg Pd)

Figure 1. a) Selectivity to R-PEAc as a function of acetophenone conversion, b) EB formation as a function of time.

4. Conclusion Pd/N-CNT catalysts with different surface acid/base properties were prepared. The basicity increased with increased support treatment temperature with NH3. In one-pot synthesis of R-1-phenylethyl acetate via hydrogenation of acetophenone over Pd/NCNT and acylation on immobilized lipase higher acetophenone conversion was obtained with higher metal dispersion and smaller metal particle size. The yield of the desired product increased with the decreased basicity of the support material.

References [1] P. Mäki-Arvela, S. Sahin, N. Kumar, J.P. Mikkola, K. Eränen, T. Salmi, D.Yu. Murzin, 2009, Catal. Today, 140, 70-73. [2] T. Harada, S. Ikeda, M. Miyazaki, T. Sakata, H. Mori, M. Matsumura, 2007, J Mol. Catal. A: Chem., 268, 59-64. [3] S. Wang, G.Q. Lu,1998, Carbon, 36, 283-292. [4] C.-C. Huang, H.-S. Li, C.-H. Chen, 2008, J Hazardous Mat., 159, 523-527. [5] P. Mäki-Arvela, S. Sahin, N. Kumar, T. Heikkilä, V.-P. Lehto, T. Salmi, D.Yu. Murzin, 2008, J Mol. Catal. A: Chem., 285, 132-141. [6] P. Serp, M. Corrias, P. Kalck, 2003, Appl. Catal. A: Gen., 253, 337-358. [7] M.L. Toebes, M.K. van der Lee, L.M. Tang, M.H.H. in’t Veld, J.H. Bitter, A.J. van Dillen, K.P. de Jong, 2004, J Phys. Chem. B, 108 , 31, 11611-11619. [8] M.L. Toebes, F.F. Prinsloo, J.H. Bitter, A.J. van Dillen, K.P. de Jong, 2003, J. Catal., 214, 78-87.

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[9] T.G. Ros, D.E. Keller, A.J. van Dillen, J.W. Geus, D. C. Koningsberger, 2002, J. Catal., 211, 85-102. [10] T.W. Ebbesen, H. Hiura, M.E. Bisher, M.M.J. Treacy, J. Shreeve-Keyer, R. Haushalter, 1996, Adv. Mater. 8, 2, 155. [11] A. Jung, A. Jess, T. Schubert, W. Schutz, 2009, Appl. Catal. A: Gen. 362, 95. [12] R. Arrigo, M. Hävecker, R. Schlögl, D.S. Su, 2008, Chem.Commun., 40, 4891-4893. [13] P. Canton, G. Fagherazzi, M. Battagliarin, F. Menegazzo, F. Pinna, N. Pernicone, 2002, Langmuir, 18, 6530. [14] H. Markus, P. Mäki-Arvela, N. Kumar, N.V. Kul’kova, P. Eklund, R. Sjöholm, B. Holmbom, T. Salmi, D.Yu. Murzin, 2005, Catal. Lett. 103, 125. [15] J.P. Tessonnier, D. Rosenthal, T.W. Hansen, C. Hess,M.E. Schuster, R. Blume, F. Girgsdies, N. Pfänder, O. Timpe, D.S. Su, R. Schlögl, 2009, Carbon, 47, 1779. [16] J.M. Solar, V.H.J. de Beer, F. Derbyshire, L.R. Radovic, 1991, J. Catal., 129, 330.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Metal-carbon nanocomposite systems as stable and active catalysts for chlorobenzene transformations Ekaterina Lokteva, a Alexey Erokhin, a Stanislav Kachevsky, a Anatoly Yermakov, b Mikhail Uimin, b Aleksey Mysik, b Elena Golubina, a Konstantin Zanaveskin, с Anara Turakulova, a and Valery Lunin a a

M.V. Lomonosov Moscow State University, Moscow, Russia Institute of Metal Physics, Ural Branch of RAS, Ekaterinburg, Russia с Karpov Institute of Physical Chemistry, Moscow, Russia e-mail: [email protected] b

Abstract Nanocomposites based on Pd and Ni encapsulated (@) in carbon have been prepared by condensation of nanoparticles in the flow of gas mixture (Ar and hydrocarbons) and characterized by TEM, TGA-MS, XRD spectroscopy and BET adsorption measurements. Ni@C, NiPd@C nanocomposites consist of metal core 3-10 nm in size covered by a few carbon layers; Pd particles are 10-15 nm in size, have no carbon shell and are joined in chains. Catalytic properties were investigated in hydrodechlorination (HDC) of chlorobenzene in gas phase and 1,2,4-trichlorobenzene in liquid phase. Totally carbon covered particles of Ni and Pd-Ni demonstrate high activity and stability in gasphase hydrodechlorination of chlorobenzene at 100-350°C and in liquid phase HDC of 1,2,4-trichlorobenzene at 130oC under middle pressure. Keywords: metal-carbon nanocomposites, hydrodechlorination, catalysis

1. Introduction Chlorinated organics are among the most significant and widespread toxic matters in the environment. The most environmentally friendly and usable method for its treatment is hydrodechlorination [1]. Useful products such as hydrocarbons can be produced without dioxins formation. Stability in reaction medium is the weak point of known catalytic systems, so the design of active and stable catalytic systems based on not-noble metals is still the problem to be solved. Different ways of nanoparticles stabilization were developed last years, including carbon coverage [2], but none of such systems were tested as HDC catalysts.

2. Experimental 2.1. Preparation of nanocomposites Nanocomposites Pd@C, Ni@C, and NiPd@C were produced in Institute of Metal Physics, Ural Branch of the RAS. The piece of Pd, Ni or Ni-Pd alloy was heated by induction levitation melting inside of two oppositely directed turns of inductive coil in closed system filled with hydrocarbon containing inert gas (Ar). Evaporation of strongly overheated (~ 2000°C) liquid metal drop was performed in the flow of Ar, containing butane or methane. The metal vapors were taken away by the flow of argon into the colder part of reactor where nucleation and condensation of nanoparticles occurred. This

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process was accompanied with hydrocarbon decomposition (or pyrolysis) on the surface of hot metal nanoparticles and resulted in the formation of the layered carbon coating and capsulation of metal particles. Encapsulated nanoparticles were collected in a bag house. The average particle size (from few up to about 20 nm) depends on the metal nature and can be controlled by metal drop temperature, argon pressure and its flow rate. The thickness of the coating can be controlled by hydrocarbon content in Ar.

2.2. Characterisation X-Ray diffraction (XRD) study was performed using DRON-6 diffractometer and Cr Kα1 or Cu Kα1 radiation. Thermogravimetric analysis (TGA-MS) was carried out with STA 409PC Luxx Netzsch and STA 449PC Jupiter Netzsch apparatus in a temperature range from 273 to 1273 K with heating rate 5%min in air flow (40 ml/min). Analysis by high resolution transmission electron microscopy (HR TEM) was done using JEOL JEM-2010 with lattice resolution 0.14 nm. SBET was measured by Quantochrom instrument using N2 adsorption measurements.

2.3. Catalytic experiment

Vapor-phase catalytic transformations of chlorobenzene were performed at 50–300oC, 0.1 MPa in a quartz fix-bed flow-type reactor. Chlorobenzene was fed to the reactor in H2 flow at molar ratio H2:C6H5Cl =55:1. Reaction products were analyzed by GC (Agilent 6890N; DB-WAX column 30 m, flame ionization detector). For each analysis a gas probe was taken directly after the reactor by syringe. Each point on conversion vs time curves is the average value for 4-5 measurements at the stable work period. Liquid phase HDC was investigated at 130°C in N2+H2, at H 2 partial pressure of 1 Mpa and total pressure of 1.3 MPa in NaOH water solution.

3. Results and discussion 3.1. Catalysts characterization Hydrocarbons decomposition on the hot surface of metal particles in-situ in the condensation zone of the reactor leads to the nanocomposite particles formation. As it was demonstrated by HR TEM (Fig. 1) and XRD ([3]), metal core of nanoparticle is encapsulated in carbon shells composed of some graphene-like layers. According to XRD data no metal carbides formation was found in such-produced nanocomposites; this result is in strong contrast with literature data about preferential formation of carbon encapsulated metal carbides in arc-discharge (e.g. from Mo, and Ta [4]). TEM (Fig. 1) and XRD results demonstrate that in all prepared composites particles are enough uniform in size, crystalline in structure and weakly agglomerated so they could be easily dispersed by weak ultrasonic treatment. No alloy formation was found in PdNi@C by XRD; two separate phases formed were attributed to Ni and Pd and/or alpha-PdH. Perhaps, the driving force for phase decomposition is the growing role of surface chemical potential as the particle size is decreasing. Table 1. Properties of nanocomposites. SBET, m2/g Me, wt.%

Ni@C(pc) 68±4 85

Ni@C(dc) 168±9 72

PdNi@C (pc) 87±5 94.5 (95%Pd, 5%Ni)

C, wt.%

15

28

5.5

PdNi@C(dc) 115±7 68 (95%Pd, 5%Ni) 32

Pd@C 33±2 97 3

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Compositions of Ni, Pd and PdNi composites are presented in Table 1. Carbon content in the different nanocomposites varied from 5 to 35%wt., estimated by the DTA-TG data. Depending on flow rate and Ar pressure, hydrocarbons nature and its concentration in the gas mixture the more or less dense carbon shells can be formed on metal nanoparticles. Thus PdNi@C system was produced in Ar-butane mixtures having different composition: butane concentration in the first mixture was 4 times higher that in the second one. Carbon content in final nanocomposite produced in the first and second mixture was 30% and 5%, correspondingly; specific surface area also decreased, but the difference was not so prominent. The term “dc” means sample with fully dense carbon-covered particles, and “pc” means sample with poorly carbon covered particles. Also two different Ni@C samples were obtained in different synthesis conditions: hydrocarbon mixture was introduced in hot (dc) or cold (pc) condensation zone. Specific surface area of “dc” and “pc” samples was different (168 and 68 m2/g respectively).

Fig. 1. HR TEM for Ni@C(dc) (A) and Ni@C(pc) (B) and particle size distributions.

Thus, different types of nanocomposites can be produced by the method of levitation melting in Ar-hydrocarbon flow depending on the synthesis conditions. Surface properties of nanocomposites, including specific surface area and pore structure, are determined by the thickness and properties of surface carbon shell.

3.2. Catalytic hydrodechlorination (HDC) of chlorobenzene HDC of chlorobenzene (CB) proceeds according the following scheme (1) HDC reaction could be accompanied with benzene ring hydrogenation to cyclohexane. Figure 2 demonstrates the conversion of CB depending on temperature in the presence of nanocomposites, commercially available 5% Pd/C Fluka and pure carbon. In the presence of all composites HDC of CB proceeds at much lower temperatures than on carbon itself. The most active catalysts are Pd@C, Pd/C (Fluka) and PdNi@C; in the first and second catalysts Pd particles are available for reagents adsorption, but in the last one particles of metals are entirely carbon covered. It seems that encapsulating of metal particles by carbon doesn’t hinder catalytic reaction. The activity and specific surface of Ni@C (pc) both decrease in cyclic experiment, where temperature was first increased stepwise up to 350°C and then decreased. On the contrary, the activity and SBET of densely carbon covered Ni@C (dc) increase during the first and the second heating-cooling cycle (Table2); in the third cycle it begins slowly

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decrease but it is still higher than in the first cycle. SBET of Ni@C(dc) after 3rd cycle was the same as for fresh catalysts, for Ni@C (pc) after first cycle it was 15 m2/g. It means that the carbon encapsulation seems to protect active metal from deactivation under the influence of reaction mixture at high temperature. Table 2. Temperatures of 50% CB conversion on Ni@C at stepwise heating (↑) and cooling (↓) 1 cycle 2 cycle T50↑,°C T50↓,°C T50↑,°C T50↓,°C Ni@C(dc) 170 150 140 120 Ni@C(pc) 165 200 n.d.* n.d. *n.d.= not determined; catalyst was tested in one cycle only

3 cycle T50↑,°C 145 n.d.

T50↓,°C 165 n.d.

It is important to underline a high activity of Ni nanocomposites. In the presence of Ni@C(dc) T50 (temperature of 50% conversion) is about 180°C lower than on pure C. 100 90 80 70 60 50 40 30 20 10 0

Conversion , %

Ni@C PdNi@C T, °C

Pd/С (Fluka)

35 0

27 5

22 5

17 5

12 5

75

30

C

Fig. 2. Catalytic properties of nanocomposites in chlorobenzene reduction.

Liquid-phase HDC of 1,2,4-trichlorobenzene at increased pressure on Ni@C (dc) was also successive, conversion was about 70 g/g catalyst per hour.

4. Conclusion Totally carbon encapsulated nanocomposites of Ni and NiPd demonstrated good conversion of CB at the temperatures of almost 200°C less than pure C and very good stability in aggressive reaction medium; Ni@C composites are active in liquid-phase 1,2,4-trichlorobenzene transformations at middle pressure. Such prepared stable and active catalysts based on not-noble metal (Ni) could be promising for heavy chlorinated wastes processing or other hydrogenation reactions, in spite of high metal loading.

Acknowledgments The authors acknowledge financial support of Russian Ministry of Education and Science (02.513.11.3030 and 02.740.11.0026) and Russian Foundation of Basic Researches (07-03-01017a, 10-02-00323-а).

References [1] E. Lokteva, V. Lunin, 1996, Catalytic hydrodechlorination of organic compounds. Russ.Chem.Bull., issue 7, P. 1609 [2] P.Z Si. Z.D Zhang., D.Y Geng et al., // Carbon. 2003. V. 41. P. 247. [3] A. E. Yermakov, M. A. Uimin, E. S. Lokteva, 2009, Russ.J.Phys.Chem. A, Vol. 83, No. 7, p.1187 [4] F. Banhart, N. Grobert, M. Terrones et al., 2001, J. Modern Phys. B, V.15, issue 31, P.4037

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Development and design of Pd-containing supported catalysts for hydrodechlorination Elena V. Golubina, Ekaterina S. Lokteva, Stanislav A. Kachevsky, Anara O. Turakulova, Valery V. Lunin Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskie Gory 1, build.3, Moscow, Russia

Abstract Hydrodechlorination (HDC) is a remarkable environment friendly and cost saving alterative to the traditional methods for utilization of chlorinated pollutants. The development of new catalysts and revelation of general approaches to catalysts design are discussed in present work. In this work several directions for catalyst design were considered: (1) change of support nature to influence on a formation of Pd-containing active site; (2) modification of active site by second metal addition; (3) varying of metal deposition method and reduction agent on an active site formation. Keywords: Pd catalyst, hydrodechlorination, ultradispersed diamond

1. Introduction Hydrodechlorination (HDC) is a remarkable environment friendly and cost saving alterative to the traditional methods for utilization of chlorinated pollutants. The development of new catalysts and revelation of general approaches to catalysts design are discussed in present work. Study of Pd, the most active in HDC, can give the basement for understanding an active site nature and to establishing general approaches in hydrodechlorination catalysts preparation. It was recently found that active site is dual in nature: Pd0 is responsible for hydrogen activation and Pdδ+ is responsible for substrate adsorption [1]. Moreover Pd0/Pdδ+ ratio should be close to 1. Additionally, possibility of substrate activation on support, hydrogen spillover and presence of adsorption centers should be taken into account. So, catalyst needs to be multifunctional to conform with all listed facts. In this work several directions for catalyst design were considered: (1) change of support nature to influence on a formation of Pd-containing active site; (2) modification of active site by second metal addition; (3) varying of metal deposition method and reduction agent on an active site formation.

2. Experimental 2.1. Catalyst preparation Catalysts with 0.5; 1; 2 and 5%wt. Pd loading were prepared by impregnation or deposition-precipitation from PdCl2 solution. Ultradispersed diamond (UDD, detonation nanodiamond, 260 m2/g, fraction 0,5-0.16 mm) [2], activated carbon (1212 m2/g, fraction 0,5-0.16 mm), ZrO2, Y2O3, Ga2O2 and ZrO2-M2O3 (M – Al, Y, Ga) were used as supports. Oxide supports were prepared by co-precipitation by ammonia. In modified zirconia the content of second oxide was 1; 5 and 10%.

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2.2. Hydrodechlorination HDC reaction was performed in gas phase or under multiphase conditions. Multiphase reaction conditions were following: 50°С, substrate solution in iso-octane, Aliquat 336 (tricaprylmethylammonium chloride, Aldrich), KOH (5% aqueous solution), catalyst, hydrogen flow 5 ml/min, nonane as the internal standard. Samples were taken from organic phase during the reaction at fixed intervals and analyzed by GC. Gas phase reaction was performed in flow-type fixed bed reactor. The fresh catalyst (100 mg) was put in reactor between quartz filter paper. The reactor was heated in hydrogen flow to the reaction temperature (100 - 350°С). Then a substrate–H2 mixture was passed through the reactor (hydrogen was bubbled through substrate). Gaseous products were continuously analyzed by GC. 2.2.1. Catalyst characterization Specific surface area and pore size distribution were measured by low temperature nitrogen adsorption on Quantachrome. Phase composition was studied by X-ray diffraction analysis. XRD were performed on STOE powder difractometer (CuKα radiation); 2 theta range 20-70° (scanning 0.05°). The reduction behaviour of samples was studied by temperature programmed reduction (TPR). About 50 mg of the sample was heated (12%min) in a flow of 36 ml/min а 5% hydrogen in Ar. Changes in a hydrogen concentration was measured by thermal conductivity detector. IR spectroscopy of adsorbed CO was performed on Bruker Equinox 55/s spectrometer. SEM images were obtained on scanning electron microscope “JEOL JSM – 6390LA” (Japan) combined by EDS.

3. Results and discussion Several directions for catalyst design were considered: (1) change of support nature to influence on a formation of Pd-containing active site; (2) modification of active site by second metal addition; (3) varying of metal deposition method and reduction agent on an active site formation. The activity of catalysts was tested in gas phase hydrodechlorination (HDC) of chlorobenzene and multiphase HDC of various chlorinated aromatic compounds: chlorobenzene; 1,3,5-trichlorobenzene; 2,4,8-trichlorodibenzofuran and hexachlorobenzene. First approach to directional catalyst synthesis is based on chemical interaction of Pd with support. In a series of Pd supported on oxides catalysts on modified zirconia with second oxide content of 1 and 5% were the most active. Complete trichlorobenzene dechlorination in liquid phase was achieved within 20 min. Moreover, catalysts on modified zirconia were more stable than catalysts on individual oxides. Total converted amount of trichlorobenzene was 500 mol per 1 mol of Pd. According to TPR and IR of adsorbed CO data both Pd0 and Pdδ+ are presented on the catalysts surface. Pdδ+ most likely to be a part of compounds like Pd-Zr-O. Second way to obtain Pd0/Pdδ+ in active site is use of ultradispersed diamond (UDD) as support. UDD is one of the new carbon cluster substances that may be produced in large amounts by the detonation method. UDD possesses high specific surface area, almost 300 m2/g, with several types of carbonyl functional groups predominant on the surface a highly defective structure, super hardness and chemical stability. It was shown by TEM data, palladium particles are well distributed on the surface of UDD and their size lies in relatively narrow range [3]. This fact provides high activity of catalysts supported on UDD in comparison with activity of activated carbon

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supported catalysts with the same Pd loading in HDC of CB. The catalysts supported on AC and UDD showed a significant difference in activity (Fig.1). Pd catalysts supported on UDD were highly active in trichlorobenzene hydrodechlorination as well. Complete TCB dechlorination was achieved within about 40 min. During the same time period TCB conversion in the presence of 5%Pd/C was only 9%. Activated carbon possesses an amorphous structure containing a mixture of carbon fibers, layers and single agglomerates of different size. This leads to agglomeration of supported Pd particles that makes them larger and decrease of active surface. Pd on activated carbon surface is dispersed without any significant order with broad particle size distribution. Texture characteristics were measured for Pd/UDD and Pd/C. Catalysts on UDD have mesoporous structure with pore size 13 nm. Part of micropores is less than 1%. At the same time activated carbon has microporous structure with average pore size 2 nm. Probably, some parts of Pd particles could be blocked in micropores of activated carbon. This fact was confirmed by TPR analysis. Mesoporous structure of ultradispersed diamond provides accessibility of most part of Pd particles. Consequently, Pd/UDD is more active in hydrodechlorination.

Fig. 1. 1,3,5-trichlorobenzene conversion in multiphase HDC in the presence of Pd supported on UDD, activated carbon and commercially available Pd/C (Fluca).

Another way to improve the catalytic activity is modification by second metal. In this work non-noble metals such as Fe, Ni, Co and Cu were used. All bimetallic catalysts were more active than Pd/C. Conversion of hexachlorobenzene in the presence of Pd/C and modified by different metals Pd-containing catalyst are shown on Fig. 2. It was found that activity of Pd-Fe/C and Pd-Ni/C (Me/Pd = 1:4 and 1:1) was similar in multiphase hydrodechlorination of 1,3,5-trichlorobenzene and hexachlorobenzene. In this case, it becomes possible to replace part of Pd by second non noble metal without significant decrease of activity that will result in reduction of cost of catalyst.

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Fig. 2. Conversion of hexachlorobenzene in multiphase HDC in the presence of mono- and bimetallic catalysts supported on activated carbon.

By TPR method combined with magnetic measurements the composition of metal particle in Pd-Fe/C was investigated. It was found that both Pd-enriched PdFe alloy and Fe2O3 are presented on catalyst surface. Alloy formation changes the electronic state of Pd in bimetallic catalysts. At the same time chlorine could be eliminated from the reaction mixture due to FeCl3 formation to prevent palladium deactivation as a result of PdCl2 formation.

4. Conclusions Several approaches to form Pd0/Pdδ+ in active site are investigated. Highly uniform distribution of Pd may serve as one of the reason of high activity of catalysts supported on UDD in HDC of chlorinated derivatives of benzene. Use of modified zirconia as support leads to formation of intermetallic oxide Pd-Zr-O, which results in active and stable hydrodechlorination catalyst. Addition of second non-noble metal improves catalytic activity by formation of bimetallic alloy and at the same time decreases Pd poisoning by chlorine. This work supported by Russian Foundation of Basic Research (№07-03-01017) and Russian Ministry of science and education (state contract № 02.740.11.0026).

References 1. 2. 3.

L. Ma.Gomez-Sainero, X.L. Seoane, J.L.G. Fierro, A.Arcoya, 2002, Liquid-Phase Hydrodechlorination of CCl4 to CHCl3 on Pd/Carbon Catalysts: Nature and Role of Pd Active Species, Journal of Catalysis 209, 279–288. S.A. Kachevskii, E.V. Golubina, E.S. Lokteva, V.V. Lunin, 2007, Palladium on Ultradisperse Diamond and Activated Carbon: the Relation between Structure and Activity in Hydrodechlorination, Zhurnal Fizicheskoi Khimii, 81 (6), 998–1005. E.V. Golubina, S.A. Kachevsky, E.S. Lokteva, V.V. Lunin, P.Canton, P.Tundo, 2009, TEM and XRD investigation of Pd on ultradispersed diamond, correlation with catalytic activity, Mendeleev Commun., 19, 133–135.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Role of deposition technique and support nature on the catalytic activity of supported gold clusters: experimental and theoretical study Elena V. Golubina, Daria A. Pichugina, Alexander G. Majouga, Sultan A. Aytekenov Department of Chemistry, M.V.Lomonosov Moscow State University,Leninskie Gory 1, build.3, 119991, Moscow, Russia

Abstract The properties of gold nanoparticles depend on their size, which is, in turn, determined by the type of a support and method used for their deposition. In this work Au particles were supported on oxides (ZrO2 modified with Ga2O3, CeO2, SiO2, and Al2O3) and ultradispersed diamond by traditional deposition-precipitation from HAuCl4 by ammonia or by proprietary deposition method from gold nanoparticles suspension. Supporting gold on modified zirconia results in formation of large metal particles even at low Au loading. Ultradispersed diamond was found to be promising support for gold catalysts. Electronic state of supported gold particles was found to be strongly depended on preparation method. Deposition-precipitation leads to formation of partially charged metal particles, while deposition from gold nanoparticles suspension results in Au0. Proposed CO oxidation mechanism on gold particles in the presence of different catalysts was corroborated by DFT calculations. Keywords: supported catalyst, Au nanoparticles, CO oxidation, DFT

1. Introduction In the early stages of the study of heterogeneous catalytic processes, it was observed that the course of some reactions depends on the size of the catalytic center. Thus promising catalysts should contain supported metal nanoparticles. The opening of opportunities for creating high-performance catalysts with new properties attracts the interest of chemists for such nano-scale systems. Catalysis by nanoparticles may be in demand in many industries, such as the use of nanoparticles in three way catalysts of exhaust gases processing, and it will allow both to increase the efficiency of the catalyst and reduce its cost. The problem of stabilization of nanosized particles, protection of such particles from aggregation, is of great importance. Moreover, sinthesis of nanoparticles with desired properties is of current importance as well. Small clusters are unstable and tend to be agglomerated. The stability against aggregation can be achieved by anchoring the particles with other solid compounds. The possibility of obtaining highly dispersed stable gold particles on various supports has opened up new opportunities for use of gold in catalytic and sorption processes. The surface of the support stabilizes a cluster, affects its structure and charge, and modifies it or creates active centers, with the adsorption and catalytic properties of the cluster changed. One of the promising support for catalyst is ultradispersed diamond (UDD). The small size of diamond particles and the presence of nonequilibrium defects on their surface make it possible to achieve a high dispersity of metal particles in their deposition. It has been found previously that

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using UDD as a support enables stabilization of partially oxidized palladium particles with sizes less than 12 nm [1,2]. Thus, UDD surface should favor to stabilization of nanoparticles of other metals. In this work influence of support nature and deposition tecnique on the size and electronic state of gold nanoparticles was studied. Catalytic activity was investigated in CO oxidation. Quantum chemical calculations were performed to corroborate reaction mechanism.

2. Experiment Catalysts were prepared by deposition-precipitation (DP) and deposition from suspension (DS). Deposition-precipitation of nanosize gold particles was performed by precipitation onto a support from a HAuCl4 solution with ammonia at pH 9. Suspension of gold nanoparticles with average size 11 nm was synthesized by Turkevich method [3], starting from gold precursor (HAuCl4) and sodium citrate. Then gold particles were deposited on support. UDD, ZrO2 modified with Ga2O3, CeO2, SiO2, and Al2O3 were used as supports. Modified ZrO2 and CeO2 were prepared by precipitation of corresponding nitrates was by ammonia at pH=10. Precipitated hydroxides were dried at 95 C and calcinated. Calcination temperature was chosen for each oxide on the base of DTA-TG analysis. The catalytic activity was examined in the reaction of CO oxidation. Reaction was performed by a pulsed microcatalytic technique. Impulses of (2% CO + 1%O2) in He were passed through the catalysts. Products were analysed at the output of reactor on 1 m packed column (Porapak Q) coupled with thermal conductivity detector. Impulses time interval was 5 min, because of reaction mixture analysis. Specific surface area was measured by low temperature nitrogen adsorption. Phase composition was studied by X-ray diffraction analysis. XRD were performed on STOE powder difractometer (CuKα radiation); 2 θ range 20-70° (scanning 0.05°). The reduction behaviour of samples was studied by temperature programmed reduction (TPR). About 50 mg of the sample was heated (12%min) in a flow of 36 ml/min а 5% hydrogen in Ar. Changes in a hydrogen concentration was measured by thermal conductivity detector. IR spectroscopy of adsorbed CO was performed on Bruker Equinox 55/s spectrometer. SEM images were obtained on scanning electron microscope “JEOL JSM – 6390LA” (Japan) combined by EDS. Quantum chemical calculations were performed by methods of density functional theory with PBE functional and gold pseudopotential with relativistic corrections included.

3. Results and discussion Comparison of the X-ray diffraction patterns of Au/UDDDP and Au/(5% Ga2O3–ZrO2)DP samples shows that the last sample is three-phase. The X-ray diffraction pattern contains peaks of zirconium oxide in a tetragonal (2θ = 30.3, 34.6, and 35.3°) and cubic crystalline modifications (2θ = 30.6 and 35.2°) and a peak corresponding to gold clusters at θ = 38.2°. All the peaks related to zirconium oxide are broadened, which indicates a high dispersity of the resulting oxide. The content of gold in the sample prepared is as low as 0.5%. At such a low content of the metal, it can be assumed that no reflections associated with the gold phase must be observed in the X-ray diffraction pattern. However, a minor peak of gold is well seen in the diffraction pattern at 2θ = 38.2°, which may be due to precipitation of gold clusters in the form of coarse crystals.

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The presence of coarse gold particles on the surface of modified zirconium oxide leads to its low catalytic activity: the maximum CO conversion is only achieved at T = 450– 500°C and is as low as 6%. Such a low degree of CO conversion is in all probability due to an inhomogeneous distribution of gold clusters over the support surface. So, modified zirconia is not favourable to gold nanoparticles stabilization. Catalysts on UDD prepared by DP and DS methods were studied by SEM. The size of gold particles supported by DS is in correspondence with particle size in suspension. Since gold particles in solution stabilized by organic ligands, metal surface is blocked for substrate access. Indeed, DS catalysts were not active in CO oxidation if they were used as prepared. CO conversion was less than 5% in all studied temperature range (150 – 450°C). So these catalysts were additionally treated on air at 450°C. CO conversion in the presence of treated catalysts is shown on Fig. 1. Au/SiO2 was the most active at higher temperatures (350 – 450°C). At temperatures below 350°C CO conversion was highest in the presence of Au/UDD.

Fig. 1. CO conversion in the presence of 2% Au supported on different supports by deposition from gold suspension.

Comparison of catalytic activity of catalysts prepared by deposition precipitation and deposition from suspension are shown on Fig. 2 by the example of Au/UDD. Ultradispersed diamond was not active in CO oxidation. Interesting result was obtained for 0,05% Au/UDDDP. This catalyst is more active at 250 and 300°C, though the gold content is lowest. At temperatures 350 and 400°C CO conversion is nearly the same in the presence of catalysts prepared by DP. Catalysts prepared by DS at these temperatures have lower activity; CO conversion was only 35%. Such behavior most likely related with electronic state of supported metal. Activity of gold catalysts in CO oxidation depends on metal charge [4]. Study of prepared catalysts by IR-spectroscopy of adsorbed CO shows that DS results in deposition of Au0, and the support influence in this case is minimal. On contrary, in the case of DP method both Au0 and Au+ are presented on surface. Thus, in this case nature of UDD surface strongly influence on supported metal.

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Fig. 2. CO conversion in the presence of Au/UDD catalysts.

To make obtained experimental data more clear theoretical study was performed. Density functional theory calculations with PBE functional and gold pseudopotential with relativistic corrections included show that an isolated Au8 and Au10 clusters should be able to catalyze the CO oxidation reaction even below room temperature. The disklike geometry is chosen on the base of STM data. Two possible reaction paths are considered: O2 dissociates on clusters or adsorbed O2 reacts directly with adsorbed CO. Both reactions are found to be extremely facile on Au10 cluster, with reaction barriers equals to 65,6 kJ/mol indicating that the reactions should be possible well below room temperature. Calculated value of activation energy is close to result obtained from experimental data (65,6 kJ/mol).

4. Conclusions UDD was found to be promising support for stabilization of small Au particles. Due to the presence of large amount of functional groups and defects deposition of gold on UDD surface results in partial oxidation of Au particles. The influence of support surface nature on supported metal particles could be reduced by use of DS method, which leads to deposition of Au0. This work is supported by “Russian Leading School” program (grant НШ428.2008.3) and President RF grant for young scientists (МК-158.2010.3).

References 1. 2. 3. 4.

E.V. Golubina, S.A. Kachevsky, E.S. Lokteva, V.V. Lunin, P.Canton, P.Tundo, 2009, TEM and XRD investigation of Pd on ultradispersed diamond, correlation with catalytic activity, Mendeleev Commun., 19, 133–135. S. A. Kachevskii, E. V. Golubina, E. S. Lokteva, and V. V. Lunin, 2007, Palladium on Ultradisperse Diamond and Activated Carbon: the Relation between Structure and Activity in Hydrodechlorination, Russ. J. Phys.Chem. A, 81, 866-873. B.V. Enustun, J. Turkevich, 1963, Coagulation of Colloidal gold, J. Am. Chem. Soc., 85, 3317-3328. M.S. Chen, D.W. Goodman, 2006, Structure–activity relationships in supported Au catalysts, Catalysis Today, 111, 22–33.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Nanosized nickel ferrite catalysts for CO2 reforming of methane at low temperature: effect of preparation method and acid-base properties R. Benrabaaa, H. Boukhlouf,a E. Bordes-Richard,b R. N. Vannier,bA. Barama a a

Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, Faculté de Chimie, USTHB, BP32, El- Alia, 16111 Bab Ezzouar, Alger, Algérie b Unité de Catalyse et de Chimie du Solide, UMR CNRS 8181, Université Lille Nord de France, Cité scientifique, 59655 Villeneuve d’Ascq, France

Abstract Activity and selectivity of nanosized nickel ferrites have been studied for hydrogen and syngas production via the CO2 reforming of methane (DRM). The catalysts were prepared by two different methods: (i) co-precipitation (CP) route using nitrates salts as precursors and (ii) hydrothermal (HT) method using chlorides as starting salts. The materials were characterized by several techniques: HT-XRD, TGA-DTA, XRD, BET, LRS, TPR, SEM. Surface acid-base measurements were performed by 2-propanol decomposition (IPA) and catalysts were tested in DRM reaction. A relationship is established between the method of preparation, the solid structure, the surface acid-base properties and the catalytic activity of iron-nickel solids in DRM reaction. The surface acid-base properties seem to play an important role in DRM reaction. Keywords: co-precipitation, hydrothermal, NiFe2O4, reforming, methane

1. Introduction Spinel ferrite nanoparticles have been intensively studied in the recent years, because of their typical ferromagnetic properties, low conductivity, high electrochemical stability and catalytic behavior. These materials are widely used in large-scale applications: (i) in electric and electronic devices, (ii) in H2O, CO2 and alcohols decomposition and in CO and CH4 oxidation [1, 2]. Several routes are used for the preparation of NiFe2O4 catalysts such as co-precipitation, hydrothermal, sol gel, combustion [3-6] etc. However, the structural and textural properties of ferrite spinel are strongly influenced by the preparation methodology used in their synthesis and may influence the catalytic activity of these materials when used as catalysts. Hence, the effect of the preparation method on the surface acid–basic properties and therefore on the catalytic activity is a very interesting subject. Methane is the cheapest and most available carbon source for the petrochemical industry, and steam reforming of methane is currently used industrially to produce hydrogen and syngas. In recent years, methane dry reforming process has received significant attention since it allows the production of syngas with a lower H2/CO ratio which is suitable for further use in the production of oxygenated compounds as well as Fischer–Tropsch synthesis for production of liquid hydrocarbons [7]. Another advantage of this reaction is that it consumes two greenhouse gases. However, one of the major drawbacks in this process is the coke deposition and sintering of active species, which deactivate the catalysts. According to the literature data [8] the active phase insertion into a well-defined structure, such as perovskites, spinels etc, increases

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the stability of the catalyst. The present study deals with the preparation, structural, textural and acid-base characters of spinel nickel ferrite nanoparticles prepared via (i) co-precipitation (CP) route using nitrates salts as precursors and (ii) hydrothermal (HT) method. A relationship between these parameters is established. The main aim of the present work is to analyze the influence of structure, texture and surface acid–basic sites properties of CP and HT catalysts on the activity and selectivity in DRM reaction.

2. Experimental The catalysts were synthesized by CP and HT methods. (i) The samples (noted CP-650, CP-750 and CP-850) were prepared by CP route using Ni(NO3)2, 6H2O (2.6M) and Fe(NO3)3, 9H2O (3.4M) aqueous solutions as precursors and NaOH (5.3M) as precipitating agent (pH=10). The obtained precursor (noted CP-80) was washed, dried at 80°C for 24h and annealed at various temperatures (650, 750 and 850°C) for 4h. (ii)The sample (noted HT-140) was obtained by HT process using NiCl2 (0.05M) and FeCl3 (0.07M) as the starting aqueous solutions. The mixture solution was put into a teflon-lined stainless autoclave and NaOH (1M) was slowly added under constant agitation until the final pH=10. The autoclave was put into an oven at 140°C for 12h then cooled down to room temperature. The product was washed with distilled water and absolute ethanol and dried at 60°C for 12h. XRD, FTIR, LRS, BET, TPR, IPA and catalytic testing were recorded as reported in the previous works [9-11].

3. Results and discussion XRD patterns of the precursor and catalysts are shown in Fig. 1. It shows that the precursor is nearly amorphous with only one peak which could be ascribed to FeO(OH). In contrast, NiFe2O4 (PDF 01-071-3850) is evidenced as the major crystalline phase in all catalysts. A pure phase is obtained for HT-140, while for the all CP-samples, besides NiFe2O4, additional peaks ascribed to Fe2O3 maghemite-c (PDF 00-039-1346) are to be noticed. However no trace of NiO, which should be in excess due to the presence of Fe2O3, is observed due probably to low crystallinity. XRD at variable temperatures was also carried out. It showed NiFe2O4 starts to form at 450°C and remains stable up to 1000°C. ♦

♦ NiFe 2 O 4

• FeO(OH)

♣ Fe 2 O 3

HT-140

♦

♦

CP-850

♣

CP-750

♣

CP-650

♣♣

♦

332

♣

20

30

696

479

CP-850

CP-750

CP-650

•

CP-80

10

♦ ♦

♦

♦ ♦

Intensity (a.u.)

Intensity (a.u)

HT-140

♦

40

50

60

2Theta (°)

Fig. 1. XRD patterns of NiFe2O4 catalysts.

70

200

400

600

800

1000

1200

1400

-1

Wavenumber (cm )

Fig. 2. Raman spectra of NiFe2O4 catalysts.

According to XRD data, the incorporation of iron in the NiFe2O4 structure is not complete at 850°C for the CP catalysts. These results are in agreement with those Rashad et al. results [12] who showed that the formation of pure phase NiFe2O4, using CP method, is observed around 1200°C.

Nanosized nickel ferrite catalysts for CO2 reforming of methane

303

The LRS analysis (Fig. 2) is in agreement with XRD results. They revealed, for HT-140 and CP-850, the presence of three Raman bands at ca. 332, 479 and 696 cm-1 that have been assigned to NiFe2O4 [13]. Structural, surface and acid-base parameters are summarized in the table 1. The values of lattice constant (a) and the density (d) are in agreement with literature data [6]. The decrease of the unit cell parameter with annealing temperature is likely due to an evolution of the spinel composition which displays a solid solution domain. The surface area (SBET), for all CP samples, is very low (< 4m2/g) compared to the HT sample (37 m2/g). This difference is in agreement with the bigger size (Cs) of the CP-crystallites (20-50nm for CP-solids against 10nm for HT solid). These results are confirmed by SEM analysis which evidenced the highest size obviously for CP-850 (50 nm). Table 1. Structural, surface and acid-base parameters of CP and HT catalysts.

CP-650 CP-750 CP-850 HT-140

Structural and surface properties SBET Cs d a (m2/g) (nm) (g/cm3) (Å) 2 25 5.32 8.39 3 30 5.41 8.33 2 50 5.54 8.26 37 10 5.39 8.34

The catalyst reducibility was examined by H2-TPR. Results are depicted in Fig. 3. For all CP polyphasic samples, the TPR curves show one broad and asymmetric reduction peak between 559 and 579°C with a shoulder at higher temperatures (722-826°C). The first peak, whose position is function of the calcination temperature, is assigned to the reduction of both Ni2+ and Fe3+ species to Ni metallic and FeO respectively [14].

Acid-base properties at 250°C Con. IPA Sel. Pr Sel. Ac (%) (%) (%) 38 45 72 47

99 99 99 46

1 1 1 54

0,14 0,12

TCD response (a.u.)

Catalysts

CP-850

0,10 0,08 0,06

CP-650°C CP-650 HT-140

CP-750°C CP-750

0,04 0,02 0,00 -0,02

200

400

600

800

1000

Temperature (°C)

Fig. 3. TPR profiles of CP and HT samples.

The shoulder can be attributed to a deeper reduction leading to Fe°. For the HT monophasic solid, it can be seen two H2-consumption peaks at ca. 387 and 687°C. The peak, at lower temperature, could be due to the reduction of Ni2+ species and the one at higher temperature to Fe3+ into Fe2+ species and probably, in less extent, to the reduction of Fe2+ to the metallic oxidation state. The hydrogen consumption, during TPR, is similar for all samples (14-15 mmol.g-1). The HT-sample reduction, starting at lower temperature, is in agreement with the smallest grain size observed by SEM for this solid. The acid-base properties have been estimated using IPA decomposition in the temperatures range 200-350°C (table 1). In all case, propylene and acetone were detected. For the monophasic HT-sample, the IPA is mainly dehydrogenated into acetone and, in less extend, dehydrated to propylene. In contrast, for all polyphasic CPsamples, IPA is predominantly dehydrated into propylene (up to 99% of selectivity). The selectivity towards dehydrogenation and dehydration reactions was much affected by changing the reaction temperature and preparation method. The temperature of calcination did not influence markedly the products distribution.

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The CP and HT catalysts were tested in DRM reaction at 450-650°C. The results are gathered in the table 2. The HT-catalyst, exhibits the highest activity at low temperature (25% of CH4 conversion against 2-6% for CP-catalysts at 450°C). These results could be attributed to the better reducibility of this material and to its basic character and reasonable surface area. Table 2. Catalytic properties of CP and HT catalysts: CH4/CO2=1, Tr=450-650°C. catalysts CP-650 CP-750 CP-850 HT-140

Tr (°C) 450 550 650 450 550 650 450 550 650 450 550 650

% Con CH4 6 6 10 5 6 10 2 5 5 25 35 55

% Con CO2 8 9 9 10 10 10 10 12 12 18 17 35

% Yield H2 1 4 4 1 6 7 5 7 7 25 35 38

% Yield CO 2 5 5 2 4 8 3 7 8 18 37 41

4. Conclusions Nanosized NiFe2O4 was prepared by CP and HT methods. The HT method presented many advantages compared to the CP method. It led to the formation of a pure crystalline NiFe2O4 at 140°C, while a mixture of NiFe2O4 and Fe2O3 phases was evidenced for samples prepared by CP technique in the temperature range 650-850°C. In addition, the HT-sample presents the better SBET of 37m2/g. The IPA dehydration to propylene, which is an indication of acid character, predominate on CP samples; while, the dehydrogenation reaction to acetone, related to the contribution of redox (basic) site, is favoured on HT-sample. The pure NiFe2O4 solid exhibits interesting results in DRM reaction compared to CP-compound. The difference in catalytic behaviour of these materials could be explained in term of acid-base and redox properties.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

B. Baruwati, K. Reddy, S. Manorama, R. Singh and Om. Parkash, Appl. Phys. Lett. 85 (2004) 2833. M. M. Bucko and K. Haberko, Journal of European Ceramic Society 27 (2007) 723. S. Sreekumar and S. Sugunan, Journal of Molecular Catalysis A:Chemical 185 (2002) 259. J. Wang, Materials Science and Engineering B 127 (2006) 81. Dong-Hwang Chen and Xin-Rong He, Materials Research Bulletin, 36 (2001) 1369. S. Balaji, R. K. Selvan, L.J. Berchmans. S. Angappan, K. Subramanian and C.O. Augustin, Material sciences and Engineering B 119 (2005) 119. Ş. Özkara-Aydınoğlu, E. Özensoy and A E. Aksoylu. International Journal of Hydrogen Energy 34 (2009) 9711. T. Utaka, S.A. Al-Drees, J. Ueda, Y. Iwasa, T. Takeguchi, R. Kikuchi and K. Eguchi, Appl. Catal., A 247 (2003) 125. N. Haddad, E. Bordes-Richard, L. Hilaire and A. Barama, Catal. Today, 126 (2007) 256. H. Boukhlouf, R. Benrabaa, S. Barama and A. Barama, Mat Science Forum 609, (2009) 145. A. Djaidja, S. Libs, A. Kiennemann and A. Barama, Catal. Today 113 (2006) 194. M. M. Rashad and O. A. Fouad, Material Chemistry and Physics, 94 (2005) 365. Y. Shi, J. Ding, Z. X. Shen, W.X. Sun, L. Wang, Solid State Comm., 115(2000) 237. M. del Arco, P. Malet, R. Trujillano and V. Rives, Chem. Mater, 11 (1999) 624.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Hierarchical porous Ce-Zr materials for oxidation of diesel soot particulate Natalia V. Zaletova, Anara O. Turakulova, Valery V. Lunin Chemistry Department, M.V.Lomonosov MSU, Leninskie Gory, bld.1/3, Moscow, 119992, Russia

Abstract Hierarchical porous biomorphic catalyst Ce0.5Zr0.5O2 for oxidation of soot was prepared by calcination of sawdust, impregnated by solutions of Ce and Zr nitrates. SEM analysis revealed total reproducibility of biotemplate morphology by final oxide. Biomorphic Ce-Zr oxide has certain advantages over coprecipitated one: it possesses larger surface area, is thermally stable, has higher amount of mobile lattice oxygen and lower temperature of its release. Thanks to its filamentous-like morphology and improved redox properties biomorphic catalyst is more active in combustion of soot. Keywords: biomorphic Ce-Zr oxide, porous catalyst, soot oxidation

1. Introduction Hierarchical porous materials have attracted much interest in recent years due to their intensive use in different fields ranging from catalysis to ceramics [1,2]. In the present work an original environmentally friendly technique combining simple synthesis method with use of waste biomass was utilized for the production of Ce-Zr oxide, which can be used both as a support and as a catalyst in various processes. Thanks to its oxygen storage capacity (OSC) Ce-Zr oxide is the main component in three-way catalysts and catalytic filters for oxidation of soot emitted by diesel engines [3]. In the latter case the hierarchical porous structure of the systems is of great importance in order to capture the particulates of soot.

2. Synthesis of Ce-Zr catalysts In the present work Ce0.5Zr0.5O2 was prepared by the following method: pine sawdust (wood biomass with the approximate size of 0.63-1.25 mm) was impregnated by solution of Ce and Zr nitrates in the ratio sawdust:mixed oxide = 10:1 and the dried resultant system was calcined at 600°C during 4 hours to give, in a direct and simple manner, the material of desirable and predictable porosity, avoiding the use of additional chemicals in the process. The sample obtained with the help of sawdust is indicated as biomorphic. For comparison Ce-Zr oxide of the same composition was prepared by traditional coprecipitation method and calcined in the same conditions.

3. Physicochemical properties of the catalysts 3.1. SEM Analysis Reproducibility of initial biomaterial by biomorphic oxide is one of the advantages of such method of synthesis. This fact is confirmed by SEM analysis (fig. 1 a-d): biomorphic oxide completely repeats the structure of the cellulose component of the wood and is characterized by large pore distribution with significant contribution of

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macropores. Crystals of coprecipitated oxide are relatively smooth and vary greatly by dimensions (fig.1 e, f).

а)

b)

d)

e)

c)

f)

Fig. 1. SEM images of Ce0.5Zr0.5O2 a)-d) biomorphic; e), f) coprecipitated.

3.2. BET Analysis Compared to coprecipitated oxide biomorphic one possesses more developed surface area (72 over 54 m2/g). To examine the influence of initial texture on the thermal stability, both systems Ce0.5Zr0.5O2 were calcined at 1000°C, 2 hours. After calcination surface area of biomorphic oxide amounts to 21 m2/g whereas this value for coprecipitated oxide is 2 m2/g. As it is seen from pore size distribution diagrams (fig. 2), pore size of coprecipitated oxide doesn’t exceed 60Å (fig. 2a). Biomorphic oxide is characterized by larger pore distribution (fig. 2b), besides from SEM images (fig.1 a-d) the presence of macropores is evident. It is known [3], that systems characterized by wide pore distribution are less affected by high temperature treatment, that’s why we observe such a difference in surface area of calcined samples. Thus filamentous-like morphology of biomorphic mixed oxide leads to its high thermal stability, which is essential prerequisite for any catalytic application in order to maximize both the contact with the reactants and the durability of the catalyst. b)

2,1E-03 Pore volume, ml/g

2

54 -> 2 m /g 1,4E-03

7,0E-04

2,1E-03 2

Pore volume, ml/g

а)

72 -> 21 m /g 1,4E-03

7,0E-04

0,0E+00

0,0E+00 10

50 90 Pore diameter, А

130

10

50 90 Pore diameter, А

Fig. 2. Pore size distribution of Ce0.5Zr0.5O2 a) coprecipitated, b) biomorphic.

130

Hierarchical porous Ce-Zr materials for oxidation of diesel soot particulate

307

3.3. Redox properties

Hydrogen consumption, a.u.

Redox properties of Ce0.5Zr0.5O2 systems were examined in temperature programmed reduction and oxidation. Hydrogen was acting as a reducing agent. Mobility of lattice oxygen is estimated by the temperature of maximal speed of lattice oxygen release. For biomorphic Ce0.5Zr0.5O2 this temperature is 513°C, for coprecipitated oxide there are two peaks at 535 and 730°C (fig. 3). Lower reduction temperature in case of biomorphic oxide is connected with its highly porous structure, which can be easily reached by gas. Besides, element analysis of biomorphic oxide revealed the presence of K+, Ca2+ and Mg2+ ions (0.58, 1.21 and 0.24 wt.% respectively). These elements exist in wood as inorganic components and are incorporated in the structure of biomorphic oxide during synthesis. These low-valent cations are supposed to create additional oxygen vacancies in biomorphic system and in this way enhance oxygen mobility. TPR profiles

Oxygen storage capacity, %

513

biomorphic biomorphic

535

80

coprecipitated

60 730

40

78

20 0

200

400 600 Temperature, 0C

800

coprecipitated

31

0

Fig. 3. TPR profiles and OSC values of Ce0.5Zr0.5O2.

Another important characteristic of redox properties is OSC, which is estimated as the quantity of oxygen absorbed by reduced system. OSC is calculated as a ratio of reduced cerium amount to total amount of cerium in the systems (ω(Ce3+)/ ω(Ce3++Ce4+)). In case of biomorphic material OSC is 78%, which is 2.5 times higher than in case of coprecipitated one (fig. 3). It is know [4] that OSC of Ce-Zr systems depends on its phase composition, homogeneous Ce0.5Zr0.5O2 solid solution is believed to exhibit the lowest temperature of lattice oxygen release. XDR analysis of initial oxides doesn’t allow to determine phase composition unambiguously. However phase composition can be estimated by implication of TPR profiles. The only peak on TRP profile of biomorphic oxide allows to propose the presence of phases characterized by close composition and properties. TPR profile of coprecipitated sample is characterized by additional peak in high temperature region. This fact is an evidence of presence of phases with considerably different properties. Thus inhomogeneity in phase composition of coprecipitated Ce0.5Zr0.5O2 explains its low OSC while high homogeneity of biomorphic oxide provided by the method of synthesis causes its high OSC.

3.4. Catalytical properties of Ce0.5Zr0.5O2 in oxidation of soot 3.4.1. Catalysis on initial biomorphic and coprecipitated systems Catalytic properties of biomorphic and coprecipitated systems were examined in the reaction of soot oxidation. Soot oxidation rate greatly depends on intensity of contact between soot and catalyst. Fibrous, highly porous structure of biomorphic oxide provides its better contact with soot particulates. Without catalyst soot is oxidized at 628°C. In presence of both catalytic systems temperature of soot combustion considerably decreases: biomorphic catalyst oxidizes soot at 415°C, whereas coprecipitated – at 490°C (tight contact in mortar). Lower temperature in the former case is due to both

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exo

initial

415 460

calcined

50

150

250

350

450

Temperature, 0С

coprecipitated Ce-Zr

DSC Signal, a.u.

exo

biomorphic Ce-Zr

DSC Signal, a.u.

improved redox properties of biomorphic material and its hierarchical porosity, which favours improving contact between soot and catalyst. 3.4.2. Catalysis after high temperature pretreatment of the catalysts During exploitation of the catalyst temperature inside diesel particulate filter sometimes can reach 1000°C as a result of local overheatings, that’s why influence of high temperature pretreatment on catalytic properties of Ce0.5Zr0.5O2 systems was investigated. As indicated above porous structure of biomorphic system provides its improved resistance to high temperatures (1000°C, 2 hours). This is extremely important in catalytic oxidation of soot because soot combustion being a reaction between two solid substances greatly depends on contact area between catalyst and substrate. Figure 4 shows catalytic tests after additional calcination of Ce-Zr oxides.

550

650

490

initial

516 calcined

50

150

250

350

450

550

650

Temperature, 0С

Fig. 4. DSC curves of soot combustion in presence of initial and calcined Ce0.5Zr0.5O2.

In both cases calcination of the catalysts shifts the temperature of soot combustion to higher temperature region: 460 and 516°C for calcined biomorphic and coprecipitated catalysts correspondingly. It is worth mentioning that even after calcination, soot combustion in presence of biomorphic system occurs at lower temperature than in presence of initial coprecipitated catalyst. In present work small-sized fraction of wood were used as a template and thus the catalyst was in the form of powder. However if we use wood monolith as a matrix, ceramic blocks characterized by unidirectional hierarchical pores can be obtained [1]. Besides, as final bioceramics totally reproduces the structure of initial wood it’s possible to regulate the diameter of pores and its structure by varying biomatrix. So biomorphic method of synthesis allows to obtain catalytic filters of desired porosity, which opens large perspectives for its application in combustion of soot emitted by diesel engines and in other processes where hierarchical porosity of the system is of great importance.

4. Conclusions Hierarchical porous Ce0.5Zr0.5O2 was prepared by using biomaterial as a matrix. Besides unique textural properties it possesses thermal stability, low temperature of oxygen release and high OSC value. Thanks to all these factors it exhibits high activity in soot oxidation.

References 1. 2.

Rambo C.R., Cao J., Sieber H. Preparation and properties of highly porous, biomorphic YSZ ceramics // Materials Chem. and Phys, 2004, V. 87, Iss. 2-3, P. 345-352. Vogli E., Sieber H., Greil P. Biomorphic SiC-ceramic prepared by Si-vapor phaseinfiltration of wood // J. Eur. Ceram. Soc, 2002, V. 22, Iss. 14-15, P. 2663-2668.

Hierarchical porous Ce-Zr materials for oxidation of diesel soot particulate 3. 4.

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Di Monte R., Kaspar J. Nanostructured CeO2-ZrO2 mixed oxides // J. Mater. Chem, 2005, V. 15, P. 633-648. Liotta L.F., Macaluso A., Longo A., Pantaleo G., Martorana A., Deganello G. Effects of redox treatments on the structural composition of a ceria–zirconia oxide for application in the three-way catalysis // Applied Catalysis A: General, 2003, V. 240, Iss. 1-2, P. 295-307.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The role of organic additives in the synthesis of mesoporous aluminas and Ni/mesoporous alumina catalysts Faiza Bentaleb and Eric Marceau* Laboratoire de Réactivité de Surface, UMR 7197 CNRS, UPMC (Université Pierre et Marie Curie – Paris 6), 4 place Jussieu, 75252 Paris Cedex 05, France E-mail: [email protected]

Abstract Mesoporous aluminas can be prepared using organic additives as porogens even if these are not surfactants, as is the case for glucose. It is shown here that porosity arises from glucose entrapped in the material precipitated at the beginning of the synthesis, and subsequently caramelized. After impregnation by nickel(II) salts and calcination, more difficult-to-reduce nickel aluminate is detected on mesoporous aluminas than on commercial aluminas, because the surface area exposed to water is larger in the former case. The presence of citrates in the impregnating solution helps to lower the proportion of nickel aluminate, but may also lead to larger NiO particles. Keywords: alumina, nickel, glucose, citrate, temperature-programmed reduction

1. Introduction For almost 20 years now, syntheses of mesoporous aluminas combining a specific surface area higher than that of most commercial aluminas (> 250 m2.g-1), a high mesopore volume and a narrow pore size distribution have been intensively investigated [1]. Synthesis routes have often been patterned on the preparation of mesostructured silicas, with micelles of surfactants acting as porogens. In contrast, recent works have described the use in aqueous medium of non-surfactant, cheap organic additives such as glucose, as an easy way to obtain non-mesostructured, but thermally stable mesoporous aluminas satisfying the above-mentioned criteria [2, 3]. However, the role of the organic additive remains unclear and the stability of the alumina upon introduction of an active phase, such as nickel, can be questioned. For instance, Kim et al. have shown by temperature-programmed reduction (TPR) that on mesoporous aluminas, difficult-to reduce nickel aluminate phases are prominent compared to NiO [4]. The purpose of the present paper is to investigate how glucose acts as a porogen in the synthesis of aluminas and how organic additives can help to prevent the formation of mixed phases when Ni catalysts are prepared on these supports.

2. Experimental 2.1. Preparation and characteristics of aluminas The mesoporous alumina was prepared following ref. [2, 3]. 0.02 mole of glucose and 0.02 mol of Al(OiPr)3 were introduced into 54 mL of water. The pH was lowered to 5 by addition of HNO3, resulting in the precipitation of a solid. After 30 min under stirring and 5 h in static conditions, water was evaporated at 130°C in an oil bath. γ-Al2O3 was obtained by calcination of the solid in air at 600°C in a muffle oven.

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Ni/Al2O3 catalysts were prepared from this alumina (hereafter called “mesoporous alumina”) and from two commercial aluminas provided by the Institut Français du Pétrole and exhibiting pore size distributions centered on different pore diameters: a γAl2O3 sample (EC1285), and a η-Al2O3 sample presenting a higher specific surface area and a microporous contribution (EC08701) (Table 1). Table 1. Characteristics of the aluminas and Ni content of the catalysts measured after drying. Alumina

Specific surface area (m2.g-1) 350

Pore volume (cm3.g-1) 0.60

Mean pore diameter (nm) 5.6

Mesoporous γ-Al2O3 0.60 8.8 (contribution Commercial 200 > 15 nm) γ-Al2O3 Commercial 315 0.50 4.0 (contribution < 2 nm) η-Al2O3 * FWMH: full width at medium height of the pore size distribution

FWMH* (nm) 4.5

Ni wt% 6.5

4.0

8.0

2.0

7.2

2.2. Preparation of the Ni/Al2O3 catalysts

Three Ni/Al2O3 catalysts were prepared by incipient wetness impregnation of the aluminas described above, using 3.1 mol.L-1 aqueous solutions of nickel(II) nitrate ([Ni(H2O)6](NO3)2). The nickel contents after drying at room temperature are listed in Table 1. These nickel contents correspond to catalysts before calcination and the different values are linked to the different quantities of water retained in the supports. After calcination, the content in Ni for the three catalysts is 10 wt%. A fourth catalyst was synthesized by incipient wetness impregnation of mesoporous γ-Al2O3 using a 3.1 mol.L-1 aqueous solution of nickel(II) citrate (citrate/Ni = 1). 10 mL of this solution were prepared by heating under reflux a suspension of 2.91 g of Ni(OH)2 contacted with 6.63 g of citric acid monohydrate, till Ni(OH)2 was dissolved. The weight content in Ni of the dried catalyst is 7.2 wt%.

2.3. Characterization techniques The Ni and C contents of the dried catalysts were measured by ICP at the CNRS Vernaison Center of Chemical Analysis, and by catharometry after fast calcination at the UPMC microanalysis center, respectively. X-Ray Diffraction (XRD) analyses were carried out on a Siemens D500 diffractometer using Cu Kα radiation (1.5418 Å). Specific surface areas were determined by the BET method applied to N2 physisorption isotherms at –196°C, on samples outgassed at 250°C for 4 h prior to analysis, using an automatic Micromeritics ASAP 2010 instrument. Pore size distributions were calculated from the N2-adsorption curve (BJH method) due to ink-bottle shaped pores [3]. Calcination of the dried catalysts, purge and subsequent TPR were performed using an Autochem 2910 (Micromeritics), under air (25 cm3.min-1; heating rate 7.5°C.min-1 up to 500°C), Ar (25 cm3.min-1; down to 20°C) and 5% H2/Ar (25 cm3.min-1; heating rate 7.5°C.min-1 up to 900°C), respectively. H2 consumption was followed by catharometry.

3. Results and discussion The role of glucose in the formation of the alumina porous system was investigated by modifying the procedure of preparation described above (Table 2).

The role of organic addtives in the synthesis of mesoporous aluminas

313

Table 2. Influence of the preparation procedure on the porosity of aluminas. Procedure Centrifugation+calcination (no caramelization) Centrifugation+water +caramelization+calcination Prepared with 3.6g glucose (complete procedure) Prepared with 3.6g fructose (complete procedure)

Specific surface area (m2.g-1) 237

Mesopore vol.ume (cm3.g-1) 0.17

Mean pore ∅ (nm) 3.3

FWMH (nm) 2.2

325

0.45

4.6

5.0

350

0.55

5.6

4.5

200

0.22

5.0

4.0

The solid recovered by centrifugation after the 5h static step was identified by XRD and chemical analysis as a poorly crystallized boehmite containing 10% of the glucose introduced in solution. A direct calcination of this material at 600°C led to a γ-Al2O3 material exhibiting low specific surface area and pore volume, and small pores. In a second experiment, the centrifuged boehmite was placed back into 54 mL of water containing no glucose and the synthesis procedure was carried through to completion. During evaporation of water at 130°C, a change in color from white to yellow showed that glucose was caramelizing. Compared with the first experiment, a large increase in specific surface area, pore volume and mean pore size was observed after calcination.

675 760

(d) μ mol H / (°C.g ) 2 cat

400 593 790

(c)

645 785

(b) 586

10 790

(a)

100 200 300 400 500 600 700 800 900 Temperature (°C) Fig. 1. TPR profiles of calcined Ni catalysts supported on: (a) commercial γ-Al2O3; (b) mesoporous γ-Al2O3 (prepared from nickel nitrate); (c) mesoporous γ-Al2O3 (prepared from nickel citrate); (d) commercial η-Al2O3. H2 consumptions are expressed per g of dried catalysts.

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F. Bentaleb and E. Marceau

The formation of the alumina mesoporous system thus appears to be connected to the caramelization of glucose entrapped inside the precipitated boehmite. The difference with the material prepared following the complete procedure may be assigned to the diffusion of a fraction of glucose into water, as evidenced by a simple test with Fehling’s solution. It can be noted that the standard procedure carried out with a ketohexose, fructose, instead of glucose, led to a quasi non-porous γ-Al2O3, though the characteristics and content in sugar of the precipitated materials were similar. Ni/Al2O3 catalysts were synthesized from the mesoporous alumina prepared with glucose and from the two commercial aluminas mentioned above. The speciation of nickel was studied by TPR after calcination in air (Fig. 1). Three main reduction peaks were detected and attributed to: (i) larger NiO particles (Tred ≈ 400°C); (ii) smaller NiO particles interacting with the alumina surface (Tred = 500-700°C); (iii) a nickel aluminate phase originating from the migration of Ni2+ ions into the alumina surface layers (Tred > 750°C) [5-7]. On the mesoporous alumina (Fig. 1b) and η-Al2O3 (Fig. 1d), nickel is present as species (ii) and (iii). Compared with the commercial γ-Al2O3 (Fig. 1a), the proportion of nickel aluminate is higher and the reduction of the smaller NiO particles is shifted to higher temperatures (645°C instead of 586°C), despite a lower surface density of nickel ions. In contrast, and in line with earlier reports [8], protecting Ni2+ ions with citrate ligands in the impregnation solution helps to hinder the formation of mixed phases (Fig. 1c). The formation of nickel aluminate thus seems to be favoured by the high surface area of the support exposed to water during impregnation, with nickel ions penetrating in the hydrated surface layers of alumina. It should finally be noted that the citrate solution is viscous. Its difficult penetration into the ink-bottle pores of mesoporous Al2O3 may explain the high proportion of larger NiO particles reduced at 400°C. No XRD peaks assigned to NiO can be observed though.

4. Conclusions It is not glucose itself, but the caramelization of glucose entrapped in the boehmite precursor that creates the mesoporous system of the alumina. When mesoporous aluminas are used as supports for nickel catalysts, the high surface area exposed to water during impregnation is the cause for the formation of nickel aluminate. Protecting nickel(II) ions with citrate ligands helps to inhibit the formation of mixed phases, but the high viscosity of the solution may limit the penetration of the solution into the pores and as a result, larger particles of NiO may be formed after calcination.

References [1] C. Márquez-Alvarez, N. Žilková, J. Perez-Pariente and J. Čejka, Catal. Rev. Eng. Sci., 50 (2008) 222. [2] B. Xu, T. Xiao, Z. Yan, X. Sun, J. Slon, S.L. Gonzáles-Cortés, F. Alshahrani and M. L. H. Green, Micropor. Mesopor. Mater., 91 (2006) 293. [3] S. Handjani, J. Blanchard, E. Marceau, P. Beaunier and M. Che, Micropor. Mesopor. Mater., 116 (2008) 14. [4] Y. Kim, P. Kim, H. Kim, I. K. Song and J. Yi, J. Mol. Catal. A, 219 (2004) 87. [5] J. M. Rynkowski, T. Paryjczak and M. Lenik, Appl. Catal. A, 106 (1993) 73. [6] F. Negrier, E. Marceau, M. Che and D. de Caro, C. R. Chimie, 6 (2003) 231. [7] F. Négrier, E. Marceau, M. Che, J. M. Giraudon, L. Gengembre and A. Löfberg, J. Phys Chem. B, 109 (2005) 2836. [8] A. J. van Dillen, R.J.A.M. Terörde, D.J. Lensveld, J.W. Geus and K.P. de Jong, J. Catal., 216 (2003) 257.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Inverse replica of porous glass as catalyst support Sebastian Wohlrab,a Alexander Janz,a Marga-Martina Pohl,a Stefanie Kreft,a Dirk Enke,b Angela Koeckritz,a Andreas Martin,a Bernhard Lueckea a

Leibniz Institute for Catalysis at the University Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany b Institute for Technical Chemistry, University of Leipzig, Linnéstr. 3-4, D-04103 Leipzig, Germany Dedicated to Professor Uwe Rosenthal on the occasion of his 60th birthday

Abstract Porous glass granules with a pore size of about 150 nm and a specific surface area of 44 m2/g were used as crystallization matrix for ceria. A complete pore filling revealed porous granules of CeO2 after removal of the glass matrix. The new material consists of primary crystallites with sizes between 8 to 20 nm and shows a surface area of 48 m2/g. It possesses a structure comparable to the former pores of the glass matrix detected by SEM. This inverse replica was used as a support for gold particles which could be deposited from a colloidal solution (dAu,colloid = 1–2 nm). Its catalytic performance during the oxidation of ethylene glycol was compared to a likewise impregnated porous glass, previously used as exotemplate. Keywords: VYCOR glass, nanocasting, template, catalytic oxidation, glycolic acid

1. Introduction Crystallization inside a protective solid matrix, called exotemplating [1], is a powerful tool for generating advanced materials. In general, scaffolds are used for sterical shielding, suppressing uncontrolled sintering and further crystal growth during the synthesis. Several materials have already been proved suitable and established successfully as matrices, for instance: activated carbon [2], in situ generated carbonaceous foams [3], porous polymers [4], silica aerogels [5], mesoporous glass [6] or colloidal silica particles [7]. For these examples, removal of matrices can be achieved either by thermal treatment of carbonaceous matrices or by simple dissolution of glassy exotemplates yielding nanopowders or inverse replica of former incorporated materials. Porous glasses are usually manufactured from phase separated alkali borosilicate glasses by extraction. The resulting extraction products strongly depend on the former phase composition. Such glasses can be synthesized with a narrow pore size distribution and with pore sizes ranging from 1 to 1000 nm. Pore volumes between 0.1 and 1.1 cm3/g and inner surfaces up to 500 m2/g can be provided in such materials [8]. These porous glasses are promising candidates for the application either as catalyst supports or as exotemplates to generate inverse replicas which themselves can be used as support materials. With inherent nanoscale domain morphologies (redox activation) and pores above the mesoscale range (molecular transport) such inverse replicas promise new developments in the field of heterogeneous catalysis.

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2. Experimental As described in [8], sodium borosilicate glasses were used as starting materials for the generation of porous glass. Granules of this glass were etched with 1 N hydrochloric acid for 1 h at 90°C, washed and dried at 20°C, followed by characterization via nitrogen adsorption using the five-point BET method (Asurf = 44 m2/g) and scanning electron microscopy (SEM). The synthesis of porous CeO2 within this glass was achieved by a process including multiple impregnation and calcination steps: The porous glass granules were thrice impregnated with 1 M aqueous Ce(NO3)3*6H2O (Acros Organics) until complete wet impregnation was achieved and dried at 60 °C. Afterwards the material was calcined at 450°C under air at a heating ramp of 5°C/min. The resulting material was reimpregnated and recalcined according to the above described method for another five times. After this impregnation and calcination process the CeO2/glass-composite was etched in refluxing 1 N NaOH for 90 minutes yielding CeO2 quantitatively which was characterized by nitrogen adsorption, X-ray diffraction, energy-dispersive X-ray spectroscopy (EDX) and SEM. Colloidal gold was prepared from 45.6 mg (0.12 mmol) HAuCl4*3H2O (from metal source) in 100 ml H2O containing 62 mg poly(vinylpyrrolidone) (PVP, Mw = 58 000 g/mol). Fast addition of 55.5 mg (1.5 mmol) NaBH4 (Fluka) in 10 ml H2O at 0°C yielded a brown solution which was concentrated to 4.5 ml by evaporation at 40°C under vacuum. This solution was characterized via transmission electron microscopy (TEM). The Au particles from 1 ml of the concentrated colloidal solution were deposited onto 650 mg of the porous glass granules as well as onto 750 mg of the porous CeO2 by drying at 75°C for 20 minutes. Fixation and polymer removal was achieved by adding 40 ml of 0.5 M aqueous AlCl3*6H2O (Merck) to the dried powders. After stirring for 5 minutes the materials were centrifuged and washed thrice with deionized water. After drying at 60°C the whole deposition process was repeated once more. The materials were characterized by TEM and inductively coupled plasma optical emission spectroscopy (ICP-OES). Catalytic testing was performed in stainless steel autoclaves equipped with glass inlets at 70°C and 5 bar O2 under strirring. The reaction solution consisted of 621 mg (10 mmol) ethylene glycol (Riedel-de-Häen), 20 ml of 0.5 M aqueous NaOH and 216 mg of Au/glass or 394 mg of Au/CeO2, respectively. Filtered reaction samples were analyzed via high performance liquid chromatography (HPLC).

3. Results and discussion A porous glass with a BET-surface of 44 m2/g and an average pore size of 150 nm (Figure 1a) was applied as exotemplate for the synthesis of ceria, CeO2. Approaches which use a low loading of the precursor cerium nitrate result in a nanocrystalline powder consisting of single particles and loose aggregates of CeO2. In order to generate an inverse replica of the porous glass the pores have to be completely filled. This status can be achieved by multiple impregnation and calcination. Alkaline etching of the so prepared glass quantitatively yields a nearly pure cerium oxide with traces of Si, detected via EDX. XRD-analysis revealed the cubic structure of CeO2 by comparison with the powder diffraction file 89-8436 (STOE, WinXPow) (Figure 1b).

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Figure 1. a) Outer surface of the porous glass used as exotemplate for the CeO2 synthesis: SEM; b) X-ray diffraction pattern of the isolated CeO2 after removal of the glass matrix.

The reflexes appear broad and point to apparent nanocrystalline domains. Using the Scherrer method (STOE, WinXPow) the averaged elongation of the CeO2 lattice planes was calculated with {111} = 13 nm, {200} = 19 nm, {220} = 13 nm and {311} = 12 nm. The obtained CeO2 was analyzed by SEM (Figure 2a). It appears similar in size and shape compared to the former porous glass exotemplate. Besides, some present smaller structures can be ascribed to an incomplete inverse replication due to an insufficient connectivity between the ceria nanoparticles. Figure 2b shows the regular structure mainly present in the formed inverse replica. Figure 2c shows a minor part of loose and anomalous pore structures which can be ascribed to irregularities within the porous glass powder, and insufficient impregnation of this template.

Figure 2. Microscopic structure of the inverse replica: SEM; a) granule structure; b) regular inverse replica structure; c) irregularities within the inverse replica.

The surface of the inverse replica was examined via nitrogen adsorption using the five-point BET method indicating a specific surface of 48 m2/g. Due to the higher density of the ceria compared to the glass it can be proposed that a certain porosity is present within the pore walls of the inverse replica. The two porous materials, glass and ceria, were used as support for gold particles. Therefore, a colloidal solution of Au stabilized by PVP (Mw = 58 000 g/mol) was prepared according to [9] in a slightly modified approach. The dispersion was concentrated by evaporation yielding Au particles of 1-2 nm in size as well as elongated structures in the nano-range (Figure 3a). The gold particles were deposited by impregnation and drying onto the porous glass (Figure 3b) as well as onto its inverse ceria replica with an obvious inherent mesoporosity (Figure 3c).

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Figure 3. Au particles [I]: TEM a) as prepared; and deposited onto supports [II]: b) porous glass; c) ceria.

It is important to note that the PVP has to be removed after particle deposition by a washing procedure. Multivalent ions are known for their destabilizing properties concerning colloids. A 0.5 M aqueous AlCl3 solution could be utilized to achieve polymer dissolution with a parallel physical stabilization of the catalyst onto glass or ceria. After this procedure, the aluminum chloride can completely be removed by washing with water. The main appearance of the gold onto both supports is polyhedral with a main particle size fraction ranging from 4 to 12 nm indicating a particle growth during deposition. CeO2 appears nanocrystalline in the size range which was calculated from the Scherrer method. The molar loading of the gold onto the supports was determined via ICP-OES with 0.53% and 1.33% for glass and ceria, respectively. Gold catalyzed oxidations of alcohols and diols possess great potential towards selectivity [10]. As example, chemical kinetics was measured during the catalytic oxidation of ethylene glycol based on the O2 consumption. Deposited Au on the porous glass shows no catalytic activity while the Au loaded ceria replica shows an initial activity of 53 mmol·gAu-1·min-1 within the first 10 minutes of the reaction. In the latter case, after 60 minutes a conversion of 59% was obtained. As main product glycolic acid was produced at a selectivity of 94%. Oxalic acid and glyoxylic acid were detected as side products.

4. Conclusion With the synthesis of porous CeO2 granules, a preparative method for generating porous oxides via porous glass exotemplates is introduced. This method is basically applicable to a sum of other oxidic systems which are stable under the conditions of acidic or basic template removal. In comparison to mesoporous systems an appropriate substance transport during catalytic reactions can be expected in such porous materials. Deposition of Au particles onto porous glass as well as of its inverse ceria replica was performed in order to compare the catalytic activity of these two materials during the oxidation of ethylene glycol. It was shown that the inverse transformation of the pore structure into a redox active material is a promising route with respect to the advanced catalytic performance.

References [1] [2]

F. Schüth, 2003, Endo- and exotemplating to create high-surface-area inorganic materials, Angewandte Chemie-International Edition, 42, 31, 3604-3622. M. Schwickardi, T. Johann, W. Schmidt, F. Schüth, 2002, High-surface-area oxides obtained by an activated carbon route, Chemistry of Materials, 14, 9, 3913-3919.

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A. B. Panda, A. Tarafdar, S. Sen, A. Pathak, P. Pramanik, 2004, Preparation of nanocrystalline SrBi2Ta2O9 powders using sucrose-PVA as the polymeric matrix, Journal of Materials Science, 39, 3739-3744. [4] S. Wohlrab, M. Weiss, H. C. Du, S. Kaskel, 2006, Synthesis of MNbO3 nanoparticles (M = Li, Na, K), Chemistry of Materials, 18, 18, 4227-4230. [5] D. Carta, G. Mountjoy, G. Navarra, M. F. Casula, D. Loche, S. Marras, A. Corrias, 2007, Xray Absorption Investigation of the Formation of Cobalt Ferrite Nanoparticles in an Aerogel Silica Matrix, The Journal of Physical Chemistry C, 111, 6308-6317. [6] W. B. Yue, W. Z. Zhou, 2008, Crystalline mesoporous metal oxide, Progress in Natural Science, 18, 11, 1329-1338. [7] N. C. Strandwitz, G. D. Stucky, 2009, Hollow Microporous Cerium Oxide Spheres Templated By Colloidal Silica, Chemistry of Materials, 21, 19, 4577-4582. [8] F. Janowski, D. Enke; F. Schüth, K. S. W. Sing (Editors) , J. Weitkamp, 2002, Handbook of Porous Solids, Wiley-VCH, 3, 1432. [9] H. Tsunoyama, H. Sakurai, N. Ichikuni, Y. Negishi, T. Tsukuda, 2004, Colloidal Gold Nanoparticles as Catalyst for Carbon-Carbon Bond Formation: Application to Aerobic Homocoupling of Phenylboronic Acid in Water, Langmuir, 20, 11293-11296. [10] A. S. K. Hashmi, G. J. Hutchings, 2006, Gold Catalysis, Angewandte Chemie International Edition, 45, 7896-7936.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The use of small volume TOC analysis as complementary, indispensable tool in the evaluation of photocatalysts at lab-scale Stefan Ribbens,a Vera Meynen,a Koen Steert,b Koen Augustyns,b Pegie Coola a,

Laboratory of Adsorption and Catalysis University of Antwerpen (UA), Universiteitsplein 1, B-2610 Wilrijk, Belgium b Laboratory of Medicinal Chemistry, University of Antwerpen (UA),Universiteitsplein 1, B-2610 Wilrijk, Belgium

Abstract “Total Organic Carbon”-analysis (TOC) on micro volume (µV) liquids was applied for the first time by means of a special designed Shimadzu gas injection kit®. This way, it became possible to evaluate the efficiency of photocatalytic dye degradation in terms of CO2 conversion (photomineralization) simultaneously with classic UV-Vis (photobleaching) measurements within a small, lab-scale photocatalytic test setup. The possibility to allow multiple micro volume sampling in short time intervals during several hours without a substantial decrease in volume/catalyst ratio is of particular value in the evaluation of photocatalysts on lab-scale volumes (< 100 ml). By combining both complementary techniques (UV-Vis and µV TOC), indispensable, additional knowledge on the degradation process/mechanism and the catalyst efficiency can be obtained in a fast, inexpensive and easy way. In this study, the photocatalytic acitivity of mesoporous titania (EISA TNH4OH C450) and hydrogen trititanate nanotubes (H-TNT) towards the degradation of rhodamine 6G was investigated. It has been illustrated that the detailed TOC-plots add important, complementary information to the data obtained by UV-Vis analysis that can reveal rate limiting steps, surface adsorption effects and charge effects. This can lead to a better evaluation of the catalyst and improved insights in the various degradation mechanisms that can occur. Keywords: photocatalytic degradation, TOC, photomineralization

1. Introduction Various methods for assessing and characterizing the photoactivity of mesoporous titania based materials have been developed. Most of these techniques study the photobleaching process of dye molecules (e.g. methylene blue and rhodamine 6G) by measuring the decrease in concentration of the dye in function of time by applying UVVis analysis measured at only 1 wavelength, that of maximum absorption of the original dye. Although it is a particularly fast, non-destructive and inexpensive method, it only allows evaluating the decrease in concentration of the initial test molecule in function of time and not of possible existing intermediates. Even though, the photobleaching process can be studied in this way, it is not necessarily representative for the total degradation towards CO2. Therefore, in most cases, the UV-Vis study can only be correlated to the initial degradation steps (photobleaching) and not to the total process (photobleaching + photomineralization). Indeed, often photomineralization of organic compounds to carbon dioxide is a more slow and complex process [1]. The knowhow

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on both photobleaching and photomineralization processes is of great importance as it gives information on the global efficiency of the photocatalyst. “Total organic carbon”-analysis (TOC) allows measuring the total amount of organic carbon present in aqueous samples and is therefore a commonly used online technique in industry in order to evaluate wastewater streams. Nevertheless, the use of classic TOC-analysis in small, lab-scale photoreactors (volumes: 25-100 ml) can give rise to serious misinterpretations. Indeed, relatively large volumes of samples of at least 5 ml are needed for each sampling due to the substantial amount of external tubing towards the combustion tube, syringe, etc. that requires purging with part of the sample volume as well as the loss of sample in this dead volume. In case of correct evaluation of any photocatalytic experiment, the ratio of catalyst/dye solution should not be influenced to a great extent in order to avoid strongly altered reaction conditions and misinterpretations of the results due to progressively increasing catalysts concentration during sampling. The use of larger test setups is not opportune in the field of catalyst development and screening, studies of its degradation mechanism or other fundamental studies because this implies the need for a substantially increased amount of catalyst. Therefore, a method was developed to be able to inject microliter volumes in TOC. A Shimadzu® designed gas injection kit, in combination with a high precision syringe (Hamilton), was applied to inject a small quantity of sample directly on the combustion tube, therefore allowing the use of small volumes of a few microliters only (µl). This technique was used in combination with classical UV-Vis analysis in order to evaluate the photocatalytic activity of two different mesoporous photocatalysts.

2. Experimental section 2.1. Chemical reagents and synthesis All products were used as received without any modification or purification, unless stated otherwise. Ultrapure milli-Q water was used to prepare the 4.10-5 M rhodamine 6G solution. Trititanate nanotubes (TNT) were prepared using a template free, hydrothermal synthesis method identical to the one described by S. Ribbens et al [2]. Mesoporous titania was synthesized using the “Evaporation Induced Self-Assembly” method (EISA) followed by a post modification in ammonia to stabilize the structure. The surfactant was removed by calcination at 450°C [3].

2.2. Characterization The photocatalytic activity was tested by photodegradation of a cationic dye (rhodamine-6G) in aqueous solution. 16 mg of the catalyst was added to a solution of 50 ml 4*10-5 M rhodamine-6G and stirred for 30 minutes without UV irradiation to establish an adsorption-desorption equilibrium. The solution was then irradiated for 360 minutes with UV light (wavelength 365 nm) emitted by a 100 Watt Hg-lamp (Sylvania Par 38; 21.7 mW/cm² at 5 cm). During this illumination, samples with a volume of 5 ml were taken out of the suspension at fixed intervals (10 min) and analyzed using UV-VIS spectroscopy. After each measurement, the solution was returned to the initial solution to prevent large changes in volume/catalyst ratios. The absorbance was measured at 526 nm with water as a reference. A maximum of 0.5 ml of the sample volume of UV-VIS was used to analyze the photooxidation to CO2 by µV TOC. A minimum of three sequential injections of the obtained samples were injected by means of a high precision syringe (Hamilton 1725 gas-tight syringe, RN type 2). Standard deviation of the multiple injections in one measuring point is maximum 1%. This analysis was performed on a Shimadzu TOC-VCPH equipped with a manual injection kit.

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Conc (10^-5 M) R6G

TOC (ppm)

3,5 16 2.2.1. Results and discussion Figure 1 shows the results of the 14 3 photocatalytic test as obtained 12 by UV-Vis analysis. The 2,5 concentration of the initial 10 2 dye molecule is plotted as a 8 function of time. In Fig. 1 1,5 results of the TOC measure6 ments are shown. Here, the 1 4 total organic carbon present in the solution is plotted as a 0,5 2 function of time. In UV-Vis 0 0 analysis, it can be seen that 0 100 200 300 400 there is an immediate decrease Time (min) in concentration for both catalysts after the UV-light is Fig.1. TOC analysis : EISA TNH4OH C450 switched on. This indicates TOC analysis : H-TNT X UV-Vis analysis : EISA TNH4OH C450 that photocatalytic reactions UV-Vis analysis : H-TNT are initiated immediately at both catalysts, although the reaction rate of H-TNT is slow during the first 90 minutes. Both catalysts clearly cause (partial) degradation of the original dye molecule. If the analysis would be based solely on UV-VIS, one could conclude that both catalysts are very active, but the reaction rate of EISA TNH4OH C450 is three times higher compared to H-TNT. (kEISA:0.0073 and kH-TNT: 0.0025). However, an immediate decrease in carbon content is not apparent in TOC-analysis (Fig. 1B). The TOC amount remains constant during the first 60 minutes of UV irradiation, implying that only intermediate products are formed and no full degradation to CO2 has taken place. If the irradiation time is long enough (> 60 min), the intermediate degradation products can be photooxidized to very small molecules and further conversion to CO2 is possible. In case of EISA TNH4OH C450, an even longer irradiation time is needed for CO2 conversion and much less carbon has been removed from the solution compared to HTNT after 360 minutes (H-TNT: 74% removed carbon and EISA TNH4 OH C450: 39%). Therefore, H-TNT has clearly a better conversion rate to CO2 than EISA TNH4OH C450. This means that the lifetime of intermediates is much less in case of the H-TNT photocatalyst. This is of particular importance in photocatalytic degradation of pollutants since it will diminish the risk of creating harmful intermediates. Here, the importance of a complete evaluation of both photodegradation of the initial dye and photomineralization is clearly demonstrated. Using UV-Vis scans and LC/MS, the photocatalytic processes can be studied in more detail (not shown). UV-Vis scans of the dye solutions measured at time intervals of 20 minutes reveal that the degradation mechanism for both photocatalysts is completely different: for H-TNT a clear hypsochromic shift in absorption maximum can be observed, whereas for EISA TNH4OH C450 the absorption maximum of the dye solution just decreases in intensity in function of time. Furthermore, LC/MS results show that the N-deethylation of the dye in presence of H-TNT is more pronounced compared to the EISA TNH4OH C450. These results could imply that there is a stronger interaction between the cationic dye and the negativily charged surface of H-TNT compared to EISA TNH4OH C450: whereas strongly adsorbed molecules can be subjected to numerous photodegradation reactions, weakly adsorbed molecules can only interact with the catalyst surface for a short time and therefore the photooxidation of these molecules will be limited. This

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would explain the observations in Fig. 1A and Fig. 1B. Because of the weak interaction between the dye and EISA TNH4OH C450, dye molecules can easily adsorb and desorb on the surface of the catalyst. Therefore, a high photodegradation rate, as analysed in UV-Vis spectroscopy, can be observed for EISA TNH4OH C450. However, because of the short adsorption time of the dye/degradation products on the photocatalyst, the concentration of the initial dye and products will play an important role. Because the initial dye molecules are positively charged, they have a better interaction with the slightly negative charged surface of EISA TNH4OH C450 compared to the neutral intermediates. This implies that there is a competition between the initial dye molecules and the oxidation products. Therefore, oxidation to CO2 takes place after more than 200 minutes. Here, the competition between intial dye molecules and degradation products is seriously and degradation products can be further oxidized. If H-TNT is suspended in the dye solution, an oppisite effect can be observed. Due to the strong interaction between the dye and the photocatalyst, molecules are longer adsorped on the surface and will be further photooxidized. This will lead to a fast oxidation towards CO2, but slows down the photobleaching of the dye molecules. More work will be done to support this hypothesis.

3. Conclusion This study shows that a better evaluation of the catalyst and improved insights in the various degradation mechanisms can be obtained by performing UV-Vis and µV TOCanalysis simultaneously. It has been demonstrated that H-TNT has a slow photodegradation rate towards initial dye molecules, but oxidize the adsorbed dye molecules very efficiently into CO2. For EISA TNH4OH C450 opposite results are found. The small volumes (< 5µl) that are required in combination with the short analysis times (2-3 minutes) makes micro-volume TOC also suitable for other process that work with small analysis volumes such as high through put or pharmaceutical applications. The simple design of the manual injection kit could allow a further development towards auto sampling. In this way, fully automated analysis could become possible.

Acknowledgement V. Meynen is grateful to the FWO-Flanders for her postdoctoral research grant. This work has been done in the frame of the FWO project G.0237.09. The authors would like to thank Shimadzu for the technical support.

References 1.

2.

3.

R. Comparelli, E. Fanizza, M.L. Curri, P.D. Cozzoli, G. Mascolo, R. Passino, A. Agostiano, 2005, Photocatalytic degradation of azo dyes by organic-capped anatase TiO2 nanocrystals immobilized onto substrates, Applied Catalysis B : Environmental, Volume 55, issue 2, p. 81-91. S. Ribbens, V. Meynen, G. Van Tendeloo, X. Ke, M. Mertens, B.U.W. Maes, P. Cool, E.F Vansant, 2008, Development of photocatalytic efficient Ti-based nanotubes and nanoribbons by conventional and microwave assisted synthesis strategies, Microporous and Mesoporous Materials, 114, 1-3, p. 401-409. E. Beyers, P. Cool, E.F. Vansant, 2007, Stabilisation of mesoporous TiO2 by different bases inluencing the photocatalytic activity, Microporous and Mesoporous Materials, 99, 1-2, p. 112-117.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Enzymatic oxidation of phenols by immobilized oxidoreductases B. Tikhonov, A. Sidorov, E. Sulman, V. Matveeva Tver Technical University, A. Nikitina str., 22, Tver, 170026, Russia

Abstract 7 various cation-exchange resins on the basis of styrenedivynilbenzene were used as the carriers for immobilization of oxidoreductases (horseradish peroxidase and musroom tyrosinase). Ion exchangers were treated with sodium alginate, chitosan, glutaric dialdehyde and N-(3-dimethyl-aminopropyl)-N-ethylcarbodiimide hydrochloride. Synthesized biocatalytic systems on the basis of oxidoreductases were found to be highly active and stable in catalytic oxidation of phenols including sewage treatment and industrial waste products to harmless melanin-type polymers. Keywords: immobilization, oxidoreductases, waste water, phenol, oxidation

1. Introduction The use of oxidoreductases in the industrial catalysis has considerably increased recently . Their efficiency is proved in reactions of homogeneous oxidation of aromatic components, in particular, aniline and phenol in modeling solutions and waste waters [1,2]. Worldwide application of this method is limited to the high cost and poor stability of purified enzymes. These problems can be solved by the immobilization of enzymes from the aqueous extracts on inorganic or organic carriers with the obtaining as a result of a heterogeneous system [3,4]. One of the most prospective methods of enzymes immobilization is the covalent cross-linking of enzymes with the modified carrier which should be mechanically strong, water-insoluble, has high chemical and biological stability and low cost. Experimental selective oxidation of monosaccharides: L-sorpbose and D-glucose.

2. Experimental 2.1. Materials and methods In this work various biocatalytic systems were investigated. 7 various cation-exchange resins (Dowex 50WX, Dowex 50WX2, Lewatit CNP-105, Amberlite 200, Amberlite IR-120, Amberlite IRC-86, Ku 2-8) on the basis of styrenedivynilbenzene with SO3H or COOH active groups were used as the carriers. Ion exchangers were treated with sodium alginat, chitosan, glutaric dialdehyde and N-(3-dimethyl-aminopropyl)-Nethylcarbodiimide hydrochloride. Two methods of chitosan and activating agent deposition on ion exchange resin were studied. They are consecutive deposition and deposition of components mixture. The schemes of biocatalyst synthesis: (i) with primary support activation S + M Æ С-M;

S-M + A Æ S-M-A;

S-M-A + Е Æ S-M-A-Е;

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A + Е Æ A-Е;

S-M + A-Е Æ S-M-A-Е.

where S – support; M – modifier; A – crosslinking agent; E – enzyme. Also catalytic efficiency of peroxidase and tyrosinase from various sources were investigated. The activity of biocatalysts in reactions of phenol and catechol oxidation to melanin-type polymers was found as a change of optical density of reaction mixture at 440 nm. Besides, to determine the kinetic parameters of the catalysts the chronometric method was used [5].

3. Results and discussion Experiments showed that the most efficient carriers are Ku 2-8 and Amberlite 200, which functional groups have high reactivity. Besides, they can be applied for biocatalysis by surface characteristics. It has been revealed that the scheme (i) is optimal for crosslinking agent glutaric dialdehyde, while the scheme (ii) – for N-(3-dimethyl-aminopropyl)-N-ethylcarbodiimide hydrochloride. During the measurements it was determined that consecutive deposition of chitosan and glutaric aldehyde on cation-exchanger provides the strongest and more stable bounding of enzyme with the carrier. It was shown that glutaric dialdehyde provides better stabilization of enzyme on the carrier to compare with N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride. For the investigated biocatalysts the optimal conditions of phenols oxidation process with the achievement of high degree of conversion (more than 95%) were found: temperature - 25°C, intensity of mixing - 300 min-1, pH 6.5 and 7 - for peroxidase and tyrosinase, respectively. The optimal ratio of the biocatalyst components was determined (see Table 1). Table 1. The optimal ratio of the components of the biocatalysts. Biocatalyst

Concentration of chitosan solution, %

Concentration of m(E)/m(S), % glutaric dialdehyde solution, % S-М-А-E1 0,1 25 8 S-М-А-E2 0,2 25 10 where S – cation exchanger; M – chitosan; A – glutaric dialdehyde; E1 – peroxidase; E2 tyrosinase.

Physicochemical investigations (FTIR spectroscopy, XPS, nitrogen physisorption) of optimal biocatalytic systems were carried out. The result of nitrogen physisorption for the biocatalyst components is shown in Fig. 1. Kinetic and physicochemical investigations showed that biopolymer (chitosan) is distributed on the surface of the carrier as separate molecules or bidimentional clasters without formation of 3D structures. Such distribution promotes minimization of intradiffusive limitation during the oxidation. The scheme of the optimal biocatalyst is shown in Fig. 2.

Enzymatic oxidation of phenols by immobilized oxidoreductases

327

Figure 1. The result of nitrogen physisorption.

Figure 2. The scheme of the optimal biocatalyst formation.

The representation of the surface of a biocatalytic system is shown in Fig. 3. Kinetic parameters of synthesized biocatalysts are shown in Table 2. Table 2. Kinetic parameters of synthesized biocatalysts. Biocatalyst

Substrate

Vm, mM s-1

Native Peroxidase

Phenol Catechol Phenol Catechol

0.069 0.156 0.024

Km, M 3.791 11.439 29.79

0.023

54.23

S–C–A–E1

Native Tyrosinase Catechol 0.022 18.99 S–C–A–E2 Catechol 0.009 85.6 S – cation exchange resin Ku 2-8; C – chitosan; A – glutaric dialdehyde; E1 – horseradish peroxidise, E2 – mushroom tyrosinase

The decrease of immobilized peroxidase and tyrosinase activities are the consequence of heterogenization of enzymes and the influence of intradiffusive factors.

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Figure 3. Representation of a biocatalytic system surface (S – carrier, A – activator, E – enzyme).

However, biocatalysts are stable in more than 10 cycles, and heterogenization makes the system more technological. One more advantage of the developed biocatalytic systems is an essential depreciation of catalyst due to the obtaining of enzymes from vegetative raw material without expensive purification.

4. Conclusions Synthesized biocatalytic systems on the basis of horseradish peroxidase and mushroom tyrosinase were found to be highly active and stable in catalytic oxidations of phenols including sewage treatment and industrial waste products. The catalysts obtained can be used for sewage and industrial waste biocatalytic treatment as they allow transfering dangerous phenolic compounds to harmless melanin-type polymers.

Acknowledgements We sincerely thank Federal Education Agency of Russian Federation (contracts P 257 and P 1196) for the financial support.

References 1. 2. 3. 4. 5.

S. Ibrahim, H. I. Ali, K. E. Taylor., N. Biswas, J. K. Bewtra. Enzyme-catalyzed removal of phenol from refinery wastewater: Feasibility studies. Water Environ. Res. 73 (2001) 165172. M. A. Gilabert, L. G. Fenoll, F. Garcia-Molina, J. Tudela, F. Garcia-Canovas, J. N RodryguezLopez. Kinetic characterization of phenol and aniline derivates as substrates of peroxidase Biol. Chem., Vol. 385, (2004) 795–800. L. V. Bindhu, E.T. Abraham. Immobilization of Horseradish Peroxidase on Chitosan for Use in Nonaqueous Media. Inc. J. Appl. Polym. Sci. 88 (2003) 1456-1464. W. Tischer, F. Wedekind. Immobilized enzymes: methods and applications. Top. Curr. Chem., Vol. 200 (1999) 95–126. D. C. Goodwin, I. Yamazaki, S. D. Aust, and T. A. Grover, Determination of Rate Constants for Rapid Peroxidase Reactions, Anal. Bioch, 231, 333–338 (1995).

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

A coordinative saturated vanadium containing metal organic framework that shows a remarkable catalytic activity Karen Leus, a Ilke Muylaert, a Veronique Van Speybroeck, b Guy B. Marin, c and Pascal Van Der Voort, a a

Center for Ordered Materials, Organometallics and Catalysis (COMOC), Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), 9000 Ghent, Belgium b Center for Molecular Modeling, Ghent University, Technologiepark 903, 9052 Zwijnaarde, Belgium c Laboratory for Chemical Technology, Ghent University, Krijgslaan 281 (S5), 9000 Ghent, Belgium

Abstract A completely saturated Metal Organic Framework, MIL-47 was synthesized and tested for its catalytic performance in the oxidation of cyclohexene with tert-butyl hydroperoxide as oxidant. The catalyst was compared to several reference catalysts: namely VAPO-5, supported VOx/SiO2 and the homogeneous catalyst VO(acac)2. MIL-47 shows a remarkable catalytic activity and preserves its crystalline structure and surface area after a catalytic run. Furthermore MIL-47 exhibits a very high activity in successive runs. Keywords: metal organic frameworks, vanadium, oxidation, liquid phase

1. Introduction Metal Organic Frameworks (MOFs) are crystalline porous solids composed of a threedimensional (3D) network of metal ions held in place by multidentate organic molecules [1,2]. In recent years, MOFs have received considerable attention as potentially valuable gas storage and catalyst materials [3-7]. MOFs possess several attractive features: a high micropore volume, crystallinity and a high metal content offering potentially valuable active sites. So far, only a few catalytic applications of Metal Organic Frameworks have been reported. Some of their potential applications were outlined recently in two excellent reviews [8,9]. All these reports deal with Metal Organic Frameworks that have unsaturated sites. However, to obtain insight into the real nature of the active sites, it is of a paramount importance to study saturated Metal Organic Frameworks. Therefore, a completely saturated, vanadium containing MOF was synthesized, namely MIL-47. This MOF is a porous terephthalate built from infinite chains of V4+O6 octahedra, held together by dicarboxylate groups of the terephthalate linkers and has a three-dimensional orthorhombic structure [10]. In the present work, we have tested MIL-47 for its catalytic performance in the oxidation of cyclohexene. Amongst the various oxidation products of cyclohexene, cyclohexane epoxide is a highly reactive and selective organic intermediate which is widely used in the synthesis of enantioselective drugs, epoxy paints and rubber promoters [11]. Furthermore the catalytic activity of MIL-47 is compared to VAPO-5, VOx/SiO2 and the homogeneous catalyst VO(acac)2.

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2. Experimental section The hydrothermal synthesis of MIL-47 is based on a literature procedure [10]. A mixture of VCl3, terephthalic acid and H2O (molar ratio 1/0.25/100) is brought into a Teflon lined steel autoclave, which is heated at 473 K for 4 days. In a next step, MIL47as is brought at 573 K for 22 h and 30 min to remove the excess of terephthalic acid in the pores. VAPO-5 is synthesized as described previously: a solution of oxovanadium (IV) sulphate-hydrate and a solution of H3PO4 are mixed together. While stirring, pseudo boehmite (from Sasol) and triethylamine are added. In a further step, the gel is brought into an autoclave and placed in an oven at 443 K for 2 days. By centrifugation, the solid is recovered. Furthermore the catalyst is dried and calcinated under a O2-flow [12]. For the synthesis of VOx/SiO2, Kieselgel 60 is stirred in a NH4VO3-solution at 338 K for 2 h. Afterwards, the solid is filtered and dried during 2 h at 373 K, followed by a calcination at 823 K during 5 h. After a catalytic run, the MIL-47 is regenerated by a treatment in a tubular furnace under a N2-flow at 523K. This is necessary to remove the organic compounds in the pores.

3. Results and discussion The oxidation of cyclohexene was carried out in a three neck flask under an inert atmosphere. To a solution of cyclohexene (0.05 mol), tert-butyl hydroperoxide (0,14 mol) and 1,2,4-trichlorobenzene (0.05 mol) (used as internal standard) in chloroform (0.38 mol) 0,1 g of the catalyst was added. The reaction mixture was stirred at 50°C. All the samples were analyzed with a Trace GC Ultra (Finnigan), fitted with an capillary column (10m, 0,1 mm, 0,4 µm) and an FID detector. Blanc reactions were performed without catalyst.

Fig. 1. Conversion curve of cyclohexene for (■) unsupported VO(acac)2 , (○) MIL-47, (▲) VOx/SiO2 and (▼) VAPO-5.

In Figure 1, the conversion curve of cyclohexene is presented in comparison with the three reference catalysts. As can be seen in Figure 1, VAPO-5 is catalytic inactive for the oxidation of cyclohexene, whereas the three other catalysts: MIL-47, the supported VOx/SiO2 and the homogeneous VO(acac)2 exhibit a very high catalytic activity.

A coordinative saturated vanadium containing Metal Organic Framework

331

Intensity/ a.u

The turn over number (TON) of MIL-47 is calculated, based on the amount of cyclohexene that is converted. The TON of MIL-47 was approximately 108 after eight hours of reaction. Thermal Gravimetric Analysis experiments (TGA) were performed on MIL-47 before and after a catalytic run to quantify the amount of leached vanadium. In comparison with the supported vanadium oxide catalyst, only a small amount of vanadium is leached. The leaching was less than 20% in the first run with MIL-47, whereas the VOx/SiO2 showed a leaching of more then 40%. Furthermore, the catalyst was recovered after a first catalytic run. The X-ray diffraction patterns of MIL-47 before and after regeneration are shown in Figure 2.

b

a 5

10

15

20

25

30

35

40

45

50

2 theta

Fig. 2. XRD patterns of MIL-47 (a) before and (b) after regeneration.

MIL-47 preserves its crystalline structure after regeneration, as can be seen from Figure 2. Moreover, the nitrogen adsorption experiments of MIL-47 before and after regeneration are presented in Figure 3. Note that the MIL-47 shows no loss at all of surface area and pore volume after regeneration.

Fig. 3. Nitrogen adsorption isotherms of MIL-47 (■) before and () after regeneration.

To evaluate the regeneration capacity of this novel catalyst, MIL-47 was tested for a second catalytic run and compared to the vanadium oxide catalyst. The conversion of cyclohexene for MIL-47 and the VOX/SiO2 catalyst in the first and second run is shown in Figure 4.

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Fig. 4. Conversion of cyclohexene for MIL-47 in its (■) first run, (●) second run and VOx/SiO2 (▲) first and (▼) second run.

MIL-47 still shows a high conversion of cylohexene, whereas the supported VOx/SiO2 shows no activity at all in its second run due to leaching of the vanadium centers. This observation indicates that MIL-47 acts as a truly heterogeneous catalyst. In conclusion, the saturated Metal Organic Framework, MIL-47, is investigated for its catalytic activity for the oxidation of cyclohexene and compared to three reference catalysts. MIL-47, containing saturated vanadium centres, shows a high catalytic conversion. X-ray diffraction measurements and nitrogen adsorption experiments prove the stability of this new catalyst under oxidation reactions. Furthermore MIL-47 exhibits a very high catalytic activity in successive runs.

References 1.

M. Eddaoudi et al, 2001, Modular chemistry: Secondary building units as a basis for the design of highly porous and robust metal organic carboxylate frameworks, Acc. Chem. Res., 34, 4, 319. 2. S.L. James, 2003, Metal Organic Frameworks, Chem. Soc. Rev., 32, 276. 3. C. Janiak, 2003, Engineering coordination polymers towards applications, Dalton Trans., 14, 2781. 4. S. Kitagawa et al, 2004, Functional porous coordination polymers,Angew.Chem. Int.Ed., 43, 18, 2334. 5. Y.M.A. Yamada, et al, 2006, Novel 3D coordination palladium-network complex: a recycable catalyst for Suzuki-Miyaura reaction, Org.Lett, 8, 19, 4259. 6. S.H. Cho et al, 2006, A metal organic framework material that functions as an enantioselective catalyst for olefin epoxidation, Chem. Commun., 24, 2563. 7. B. Gomez-Lor et al, 2005, Novel 2D and 3D indium metal organic frameworks : Topology and catalytic properties, Chem. Mater.,17, 10, 2568. 8. A.U. Czaja et al, 2009, Industrial applications of metal organic frameworks, Chemical Society Reviews,38, 1284. 9. U. Mueller et al, 2006, Metal organic frameworks- prospective industrial applications, J. of Mat. Chem., 16,7,626. 10. K. Barthelet et al, 2002, A Breating Hybrid Organic-Inorganic Solid with Very Large Pores and High Magnetic Characteristics, Angew. Chem. Int. Ed, 41, 2, 281. 11. M.R. Maurya et al, 2008, Immobilisation of oxovanadium (IV), dioxomolybdenum (VI) and copper (II) complexes on polymers for the oxidation of styrene, cyclohexene and ethylbenzene, App.Cat.A-General, 351, 2, 239. 12. M. J. Haanepen et al, 1997, VAPO as catalyst for liquid phase oxidation reactions. Part1: preparation, characterisation and catalytic performance, App.Cat.A-General, 152, 183.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Influence of preparation conditions on properties of gold loaded on the supports containing group five elements Izabela Sobczak*, Justyna Florek, Katarzyna Jagodzinska, Maria Ziolek A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland

Abstract Gold catalysts based on MCM-41 modified with vanadium and niobium, SBA-3 and group V metal oxides were prepared by several methods. The properties of the materials obtained were characterised by nitrogen adsorption, XRD, TEM, UV-Vis, and test reactions (AcoAc cyclisation and methanol oxidation). The best dispersion of gold was reached when it was introduced during the synthesis of MCM-41 with the use of H2SO4 as pH adjusting agent and Nb source besides Na silicate. Acid/base properties were determined by the preparation methods and the presence of group five elements. Keywords: AuNbVMCM-41; AuSBA-3; Au/V,Nb,Ta-oxides; acidity/basicity

1. Introduction Following the breakthrough research results of Hutchings and Haruta, there has been a dramatic increase in the interest in gold catalysis [1]. It has been demonstrated that the physicochemical and catalytic properties of gold catalysts depend mainly on the type of support and the preparation method. Both parameters influence the size of Au clusters [1]. Interaction between gold and the metals localized in the support plays also an important role and can determine the catalytic activity of the catalysts. The idea of this work is to use two groups of materials, silicate or metalosilicate (Me=Nb, V) hexagonally ordered mesoporous molecular sieves and transition metal oxides (V2O5, Nb2O5, Ta2O5) as supports for gold. The first group of samples exhibits very high surface area and the presence of isolated metal species, whereas bulk metal oxides are characterized by smaller surface areas and much higher concentration of metals on the surface. The main focus of this study is the influence of the methods of support syntheses and gold loading and their effect on the physicochemical properties of materials prepared.

2. Experimental 2.1. Preparation of the catalysts SBA-3 material was synthesized following the procedure reported originally by Stucky et al. [2]. NbMCM-41 samples were synthesized by hydrothermal method [3] and modified with gold according to [4,5]. Au/MCM-41 (with gold loading of 1 wt%) and Au/SBA-3 (3wt % of Au) were prepared by incipient wetness impregnation (IMP) of the support with HAuCl4 (Johnson Matthey). The alternative, direct synthesis of AuSBA-3, AuMCM-41, AuVMCM-41 and AuVNbMCM-41 (COP- co-precipitation of all components in one pot synthesis) was performed in the same manner as conventional MCM-41 [6] and SBA-3 [2]. The only difference was the admission of HAuCl4,

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vanadium(IV) oxide sulphate hydrate and ammonium niobate(V) oxalate as the sources of Au, V or Nb, respectively and the use of HCl or H2SO4 as pH adjustment agent. Commercial oxides (V2O5,Ta2O5 –Aldrich, Nb2O5 –Alfa Aesar) were modified by goldsol method [7] with tetrakis(hydroksymethyl)phosphonium chloride(THPC) as reducing agent and HAuCl4 as a source of gold (1 wt.% of Au) and, additionally, by depositionprecipitation (DP) method with urea [7]. The prepared materials were calcined at 623 K.

2.2. Characterization The XRD patterns were obtained on a D8 Advance diffractometer (Bruker) using CuKα radiation (λ=0.154 nm). The surface area and pore volume of the samples were measured by nitrogen adsorption at 77 K, using the conventional procedure on a Micromeritics 2010 apparatus. The UV–visible spectra were recorded on a Cary 300Scan (Varian) spectrometer in the range from 800-180 nm. For transmission electron microscopy (TEM) measurements powders were deposited on a grid with a holey carbon film and transferred to JEOL 2000 electron microscope operating at 80 kV. The catalysts were tested in acetonylacetone (AcoAc) cyclisation at 623 K and methanol oxidation at 473 and 523 K as the probe reactions under conditions described in [4,5].

3. Results and discussion 3.1. AuMCM-41 and AuSBA-3 The main difference in the preparation of AuMCM-41 and AuSBA-3 in one pot synthesis is the use of an alkaline medium (pH=11) in the first one and an acidic medium (pH=1) in the second synthesis. Moreover, different sources of silicon (sodium silicate and TEOS, respectively) are applied. Results of this study have proved a significant difference in the surface properties of both materials containing metallic gold particles on the surface. Acetonylacetone (AcoAc) cyclisation allows us to evaluate acidity and basicity of the surface on the basis of the selectivity to methylcyclopentanone (MCP) and dimethyl furan (DMF) [8]. MCP/DMF1 – basic properties of the surface. Highly basic character of AuMCM-41 is demonstrated by MCP/DMF ratio equal 104, whereas it is only 0.11 for AuSBA-3 (Table 1). The acidity of AuSBA-3 is confirmed by dehydration activity (dimethyl ether formation) in methanol oxidation (Table 2). Interestingly, SBA-3 impregnated with gold (Au/SBA-3) reveals oxidative properties like AuMCM-41 demonstrated by selectivity to formaldehyde and methyl formate. It means that acidity of the surface is not generated during the synthesis of silicate SBA-3 and impregnation with chloroauric acid but it results from gelation of TEOS together with HAuCl4 in the presence of Pluronic and HCl at pH=1. Table 1. Texture properties of the catalysts and selectivity ratio in AcoAc cyclisation at 623 K. Catalyst AuSBA-3 AuMCM-41(HCl) Au/NbMCM-41(IMP) AuNbMCM-41(HCl) AuVMCM-41(HCl) AuVMCM-41(H2 SO4 ) AuVNbMCM-41(HCl) AuVNbMCM-41(H2SO4)

Average pore vol. BJH, Surface area (ads.) cm3g-1 BET, (ads.) m2g-1 996 0.45 886 0.81 900 1.00 870 0.86 813 0.80 1055 1.34 851 1.08 1042 1.05

* MCM = methylcyclopentenon; DMF = dimethylfuran

MCP/DMF* (AcoAc cyclisation) 0.11 104 0.23 9.00 22.0 0.19 16.0 0.05

Influence of the preparation conditions on the properties of gold loaded

335

Participation of gold source in the formation of acidic centers is clearly deduced from these results. Table 2. The results of methanol oxidation at 473 K. Catalyst

MeOH conv., % 25 8 38

AuSBA-3 Au/SBA-3 AuMCM-41

HCOH sel., % 56 2

HCOOCH3 sel., % 91

CH3OCH3 sel., % 99.9 -

CO2 sel., % 0.1 44 7

3.2. MeMCM-41(Me=Nb, V) materials containing gold – effect of preparation method and synthesis conditions MCM-41 material has been chosen for the further study of the interaction of gold with group V metals located in the structure of mesoporous material. Gold containing NbMCM-41 materials were prepared by two manners: impregnation (Au/NbMCM-41) and co-precipitation (AuNbMCM-41). The introduction of gold during the synthesis leads to the catalysts with more disordered structure and lower surface area and pore volume compared to the material obtained by impregnation (Table 1). The Au-metal crystallites are present on both Au-catalysts (XRD, UV-Vis – Fig. 1). However, the peaks assigned to the metallic gold in the XRD patterns (at 2Θ = 38.2° and 44.8°) are sharper for the impregnated material indicating larger Au agglomerates. The introduction of Au during the synthesis strongly enhances the dispersion of Au. AuVNbMCM-41 (HCl)

Intensity, a.u.

10

AuVNbMCM-41 (H2SO4)

0.5

AuVNbMCM-41 (HCl) AuVMCM-41 (HCl) 35

40

45

2Θ ,

50 o

55

60

0

AuVNbMCM-41 (H2SO4)

F (R)

Au/NbMCM-41 (IMP) AuVNbMCM-41 (H2SO4)

Au

AuVMCM-41 (HCl)

AuVNbMCM-41 (HCl)

Au/MCM-41 (IMP) 300 400 500 600 700 800

Wavelenght, nm

Fig. 1. XRD patterns, TEM images and UV-Vis spectra of selected MCM-41 materials.

To obtain the catalysts with high gold dispersion, AuVMCM-41 and AuVNbMCM-41 materials were prepared by COP method with the use of HCl or H2SO4 as pH adjustment agent. As shown in Table 1, the use of HCl leads to the catalysts with lower surface area and pore volume when compared to those of AuVMCM-41 (H2SO4) and AuVNbMCM-41(H2SO4) materials. Moreover, MCM-41 prepared with HCl shows disordering of hexagonal structure. Considering the size and dispersion of gold, it was found on the basis of XRD patterns (Fig. 1) that bigger Au agglomerates are formed on the surface of Au(V,Nb)MCM-41 (HCl). TEM images (Fig. 1) confirm this conclusion. The average size of Au crystallites in AuVMCM-41(HCl) and AuVNbMCM-41(HCl) was 45-50 nm. The application of H2SO4 to adjust pH during the synthesis leads to much smaller gold particles (~20 nm). Moreover, Nb species located in MCM-41 samples plays the role of a structural promoter that decreases the agglomeration of gold. TEM images do not show the smaller Au particles located inside the channels. The use of HCl or H2SO4 during the synthesis of MCM-41 materials influences also the acid-base properties of the catalyst surface studied by AcoAc transformation (Table 1). AuVMCM-41(H2SO4) and AuVNbMCM-41(H2SO4) materials exhibit acidic properties

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(MCP/DMF >1). There is no doubt that the presence of chlorine near Au species is responsible for a very high basicity of gold-MCM-41, as was indicated earlier for AuMCM-41 [9]. The introduction of V and mainly Nb into MCM-41 together with Au in one pot synthesis diminishes the basicity. The interaction between Au and group five elements in MCM-41 determines the surface properties also in methanol oxidation. Such interaction in AuVNbMCM-41 (H2SO4) results in the highest selectivity to formaldehyde because of the weaker chemisorption of HCHO, whereas bimetallic catalysts (AuVMCM-41(HCl) and AuNbMCM-41(HCl)) are the most active in CO2 formation because of their basicity [5].

3.3. Group V metals oxides - effect of preparation method

o

Au

Au/Nb2O5 (THPC)

Nb2O5 30

4. Conclusions

Au/Nb2O5 (DPU)

500

Intensity a.u.

Gold was introduced into group V metal oxides (V2O5, Nb2O5, Ta2O5) by two methods: via deposition-precipitation with urea and via gold-sol method with THPC as a reducing agent. In XRD patterns of all calcined materials the reflections characteristic of metallic gold are well visible on the catalysts prepared by DP method indicating bigger Au particles than that when gold sol method is used (Fig. 2 -example for Au/Nb2O5) as confirmed by TEM images. The average particle size in the Au/Nb2O5 prepared by DP method is about 20 nm, whereas in the sample prepared by gold-sol method it is of about 6 nm. THPC used during gold-sol method stabilizes the colloid gold solutions and that is why gold particle sizes are smaller and their dispersion is higher.

35

40

2Θ,o

45

Dispersion of gold is better when Au is introduced into Fig. 2. XRD patterns of Nb2O5 samples. ordered mesoporous material in one pot synthesis than in the case of using the impregnation method. Au crystallite size depends on the nature of acid used as pH adjusting agent in one pot synthesis (HCl promotes agglomeration of gold) and on the chemical composition of MCM-41 material (the presence of Nb decreases Au agglomeration because of strong Au-Nb interaction). Dispersion of Au on metal oxides is higher than on MeMCM-41 because of higher concentration of group V metals. Gold-sol method using for the modification of metal oxides gives rise to a higher gold dispersion than deposition-precipitation one. That is why the modification with Au using THPC as reducing agent is recommended. Acid/base properties of mesoporous gold – silica prepared in one pot strongly depends on pH of the synthesis medium. Acidity dominates for AuSBA-3, whereas basicity is characteristic of AuMCM-41. Oxidative properties of Au-MCM-41 catalysts are determined by the preparation methods and the presence of V and Nb.

Acknowledgements Polish Ministry of Science (Grant No. N N204 032536) is acknowledged for the partial financial support of this work. Acknowledge is made also to Johnson Matthey (UK) for supplying HAuCl4.

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References 1. 2. 3. 4. 5. 6. 7. 9.

G.C. Bond, C. Luis, D.T. Thompson, 2006, Catalysis by Gold, Imperial College Press Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schüth, G.D. Stucky, 1994, Chem. Mater., 6, 1176 M. Ziolek, I. Nowak, 1997, Zeolites,18, 377 I. Sobczak, A. Kusior, J. Grams, M. Ziolek, 2007, Stud. Surf. Sci. Catal., 70, 1300 I. Sobczak, N. Kieronczyk, M. Trejda, M. Ziolek, 2008, Catal. Today 139, 188 C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, 1992, Nature, 359, 710 S. Demirel-Gulen, M. Lucas, P. Claus, 2005, Catal. Today 102–103, 1668. R.M. Dessau, 1990, Zeolites, 10, 205 I. Sobczak, A. Kusior, J. Grams, M. Ziolek, 2007, J. Catal., 245, 259

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

High loaded Ni/SiO2 catalyst for producing ultra-pure inert gas Jung Wha Son,a Songhun Yoon,a Hee Geun Oh,b Dong Young Shin,b Chul Wee Leea,g a b

Green Chemistry Division, KRICT, Daejeon 305-600, S. Korea Korea Pionics Co. Anseong-city, Gyeonggi-do 456-833, S. Korea

Abstract A simple synthesis protocol to prepare highly loaded Ni/SiO2 was developed, based on co-precipitation method using 200L batch type reactor, where kind of raw materials, precipitation rate, aging condition and calcinations/reduction conditions are important factors for determining the quality of catalyst. Coprecipitation method shows better results than impregnation. Their performance of removing impurities such as CO, O2, H2O and H2 in inert gas was evaluated. The physical properties, H2-TPR and chemisorption were attempted to understand the performance of Ni/SiO2. Keywords: Ni/SiO2, coprecipitation, ultra-pure inert gas, metal dispersion, H2-TPR

1. Introduction Large quantities of non-reactive gases such as He, N2, Ar are used for pharmaceutical and electronic industries, particularly during fabrication of semiconductors [1]. These inert gases must be as pure as possible and particularly, they must be substantially free from impurities such as O2, CO, H2O, CO2, H2 etc, which reduce the quality and performance of the semiconductors. A growing number of industries are now requiring gases having impurities concentration of sub ppb level. Generally Ni/SiO2 catalysts containing Ni in amount of higher than 50wt% are used for industrial hydrogenation processes, and it was reported that incorporation of Mg, as a promoter, into Ni/SiO2 can improve its catalytic activity [2]. The objectives of present study is to find optimum conditions for preparing Mg containing Ni/SiO2 with high loading and dispersion and its application for eliminating impurities from N2 gas at room temperature. The relationship between performance of Ni/SiO2 and its physical properties such as BET, H2-TPR, EDS and chemisorptions were discussed and compared. It was found that for preparing desired Ni/SiO2, kind of raw materials, composition of constituents and post treatment conditions such as calcination/reduction procedure are important factors.

2. Experimental 2.1. Sample preparation For preparing Ni/SiO2, nickel nitrate or nickel sulfate, supports such as silica, sodium silicate and precipitating agent, urea, were employed [3]. Typical preparation procedure of sample A is as follows. The reaction was carried out with 200L batch type reactor(BE630 model, Pfaudler Co.) 6510g of Ni(NO3)2x6H2O was dissolved in 90L of distilled water. 2870g of Mg(NO3)2x6H2O was dissolved in 10L of distilled water and it was mixed with the aq. nickel nitrate solution. Then 1220g of sodium silicate and 35L of distilled water was added successively for 1.5h and the solution temperature was

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heated up to 90°C while stirring. When the solution temperature was reached at the desired temperature 5380g of urea was added slowly. The solution temperature was maintained at least 90°C while stirring for 24h, followed by aging at 90°C for 10h without stirring then the solution temperature was cooled down to room temperature and the green precipitate was filtered and washed with hot water. The sample was dried in an oven at 110°C and calcined at 500°C for 5h. The product yield is about 90%. Sample B was prepared by the impregnation method. For preparing final product, the sample was reduced with two different ways. (1) 5%H2/He was passed through the bed at flow rate of 50cc/min at 400°C with ramping of 5°C/min for 3h, (2) 5%H2/He was passed through the bed at flow rate of 50cc/min at 150°C and 400°C for 3h, respectively. At the completion of reduction, the sample was outgassed in N2 for 0.5h at 20°C above the reduction temperature and it was cooled to ambient temperature with the inert gas flowing. For comparison, at least three different samples were prepared. Compositions and constituents of sample used in this study were summarized in Table 1. Table 1. Compositions and constituents of sample attempted in this study. sample

Ni (NO3)2(g)

MgNO3(g)

Urea(g)

support(g)

H2O(g)

pH(4)

(2)

A 6510 2870 5380 1220 135 4.6/7.8 B 6550 2920 5450 1200(3) 130 4.2/7.7 C 6500(1) 2700 5500 1200(2) 130 4.7/7.8 Raw materials of (1)NiSO4x6H2O, (2)sodium silicate and (3)SiO2 (BET 300m2/g, particle size of 5μm) were used. (4)Initial pH after mixing reaction mixture at 90°C /final pH after aging.

2.2. Characterization For elemental analysis, EDS was monitored by Bruker (Model Quantax 200). The BET surface area, pore volume and pore size distribution were measured by N2 adsorption/ desorption at 77K using Micromeritics ASAP 2000. The nickel surface area of the reduced sample was calculated from the amount of H2 chemisorbed on the sample by using Micromeritics ASAP 2010. TPR was measured by Belcat-M of Bel Japan Inc.

2.3. Performance evaluation Prior to testing the performance, the sample was reduced indicated in the text by H2 and the performance of the sample was evaluated by monitoring breakthrough curve for adsorbing impurity gases in N2 gas. The concentration of impurities such as O2, CO, H2O, CO2 was measured by Micro TCD-GC(HP5280), API-MS(Hitachi) and RGA3 (Trace Analytical), respectively.

3. Results and discussion In this study, depending on raw materials used, three different kinds of Ni/SiO2 were prepared. As indicated in Table 2, Ni loading is higher than 60wt%. After calcinations, elemental analysis by EDS indicates that C 1.65, 2.20, 2.31wt%, O 23.7, 26.5, 27.2wt%, Mg 1.28, 2.10, 1.95wt%, Si 1.84, 5.85, 4.60wt%, Ni 71.4, 63.4, 60.6wt% for sample A, B and C, respectively. Only for sample C, 2.57wt% of sulfur was detected due to NiSO4. The precipitation procedure is reproducible. After many synthesis attempts, Ni(NO3)2, Mg(NO3)2, urea, NaSiO3 seems to be a desirable staring materials with relative molar ratio with 5.2moles of Ni(NO3)2, 2.6moles of Mg(NO3)2, 20 moles urea, 1.0moles NaSiO3 and 2.0moles H2O. Increasing the concentration of urea and nickel nitrate resulted in increased nickel deposition, but leading to decreased dispersion. The BET surface area of Ni/SiO2 was in the range of 120~140m2/g which is in a good agreement with the previous work [4]. Pore volume is in the range of 0.22~0.28cc/g and average

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pore size is in the range of 5.8~8.0nm. H2O content was measured in the range of A 4.2~6.1 wt% by thermal analysis. Figure 1 shows H2-TPR curve of sample A, B and C. Sample A shows a reduction peak centered at 310°C, followed by a hump at 460°C. B Hydrogen uptakes centred at 450°C and 700°C for sample B and C, respectively, are 100 200 300 400 500 600 700 800 900 observed. This indicates that main reducTemperature ( C) tion temperature of Ni2+ to Nio is clearly different each other. High temperature Fig. 1. H2-TPR of three different Ni/SiO2. peak of sample C can be assigned to the Pretreatment: 450oC for 2h with He. reduction of NiO having strong interaction with support, i.e. SiO2 cause an increase in the reduction temperature of the NiO phase. However, when sodium silicate was employed as a support, the reduction temperature goes down clearly. TPR provides that reduction proceeds in one step, uniform phase composition of nickel and influenced by the source of NiO and supports [5]. Calcination in air prior to reduction has little effect on the resulting metal dispersion. This implies that the reduction precursor, presumably nickel hydrosilcicate, is not drastically altered by the air treatment. Chemisorption data such as metallic surface area and crystallite size were summarized in Table 2. Metallic surface area is in the range of 0.31~17.7 m2/g Ni-metal after calcination. Although reduction of calcined sample at 400°C shows only slight improvement on metallic surface area and crystallite size, however, as shown in Table 2, step-wise reduction with 5%H2/He at 200°C, followed by at 400°C for 3h shows a remarkable change. With this process, metallic surface area is increased by about 65% and crystallite size is reduced by about 60%. This indicates that reduction under mild conditions seems to be effective for improving metal dispersion. The basic catalytic reaction can be suggested as follows. For removal, Ni + CO ÆNiCO, 2Ni+O2 Æ 2NiO, NiO+H2Æ Ni+H2O and for regeneration, NiCO2 + 4H2 Æ CH4 + 2H2O + Ni, NiCO + 4H2 Æ CH4 + H2O + Ni

TCD (uV)

C

o

Table 2. Chemisorption data of various sample prepared in this work. crystallite size (nm) Ni loading metallic surface area (m2/g Ni) (wt%) calcined reduced* calcined reduced* A 17.7 18.5(1); 29.5(2) 38.1 36.4(1); 23.0(2) 62.8 B 2.73 2.77(1); 4.21(2) 246 232(1); 150(2) 63.5 C 0.31 0.22(1); 0.51(2) 1257 1170(1); 895(2) 61.3 (1) *Reduction conditions: 5%H2/He was passed through the bed at flow rate of 50cc/min at 400°C for 3h, (2)5%H2/He was passed through the bed at flow rate of 50cc/min at 150°C and 400°C for 3h successively. sample

The reduction process generates almost 2 mol of water for every mole of nickel so that the presence of moisture may impede the thermodynamic equilibrium conversion of Ni. The reduction step may follow decomposition of the nickel silicate to the oxide, perhaps controlled by the same water removal. Varying the reduction condition was found to be a very sensitive and reproducible way of controlling the amount of reduced Ni and its dispersion. For screening the performance of the catalysts, model gas containing 50ppm of O2 in N2 gas was employed. Figure 2 shows the breakthrough point of sample A, B and C during adsorbing O2 impurities in N2 gas, which was determined by micro TCD-GC. Sample A shows the best performance indicating that 50ppm of O2 was reduced for 35h.

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Breakthrough point of 35h, 23h and 6h was detected for sample A, 0 .8 C C and B, respectively. Sample A shows the best performance 0 .6 B indicating that 50ppm of O2 was 0 .4 removed at least for 35h. This 0 .2 performance seems to be closely A related with metal dispersion and 0 .0 0 5 10 15 20 25 30 35 40 size. Highly dispersed shows higher T im e (h ) activity. Low activity of sample B is Fig. 2. Breakthrough curve for adsorbing O2 in N2 that NiO species is not fully reduced at RT with 50ppm of O2, WHSV=12,000h-1. to Nio and the active site is not fully Pretreatment: calcination under air at 500°C for 2h, developed under the reduction at reduction with 5%H2/Ar at 400°C for 3h. C1/C0 400°C. In order to evaluate the indicates O2 conc. ratio before and after reaction. performance of the sample in a real condition, the calcined powdered sample was mixed with stearic acid and graphite for making a cylindrical type pellet of 3mm diameter and 3mm length. As shown in Table 3, for sample A and B, impurities of CO, CO2, H2O and O2 of 700 m2/g) and uniform pores size ranging from 2 to 5.8 nm and from 5 to 30 nm, respectively [2,3]. In the recent years, such ordered mesoporous silicas have attracted worldwide attention as new supports for catalysts. A tailored porous structure is beneficial for catalytic applications because allows to control the optimum size of supported metal nanoparticles and/or metal oxides. Au and Pd nanoparticles dispersed on mesoporous silica are currently being explored for environmental catalysis, such as CO oxidation and CH4 combustion [4-7]. The textural properties of ordered mesoporous silicas, such as large specific surface area, a hexagonal array and uniform pore channels with size ≥ 5nm) meet the requirements for the achievement of the optimal size, between 2-5 nm, for Au nanoparticles [8]. However, due to the inertness of silica, a typical non-interacting support, sintering of noble metal particles during long time activity occurs [4,9]. Addition of modifiers, such as Cu, Fe, Nd, Ce, Ce1-xZrxO2 in appropriate concentration to mesoporous silicas is reported to modify the surface and acidic properties of the matrix and to stabilize against sintering the supported metal nanoparticles through interaction with the promoter [6,9-11]. On these grounds, the present work focus on the synthesis and characterization of ceria functionalized mesoporous SBA-15, as support for noble metals deposition. SBA15 with hexagonal porous structure was synthesized by using triblock co-polimer, as template and, then, impregnated with different amounts of cerium nitrate hexahydrate. Calcination at 400°C was performed to favor crystallization of the ceria fluorite structure, characterized by high mobility of lattice oxygen [12]. Characterizations of

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structural and morphological properties were performed by XRD, BET surface area and pore-size distribution measurements. Acidic and reduction properties were investigated by NH3-TPD and H2-TPR experiments.

2. Experimental Mesoporous SBA-15 was prepared starting from tetraethyl orthosilicate (TEOS, Aldrich 98%), as silica source and using a triblock poly (ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) (EO20PO70EO20, Pluronic P123, Aldrich), as template, according to published procedure [3]. In a typical preparation, 8.1 g Pluronic P123 was dissolved in 146.8 g de-ionized water and 4.4 g of conc. HCl (37%) and stirred over night at 35°C in a 250 ml one neck flask. To this solution 16 g of TEOS was quickly added and stirred for 24 h at 35°C. The milky suspension was annealed at 100°C for 24 h in closed polypropylene bottle. The solid product was filtered, washed with an HCl/water-mixture and calcined at 550°C for 5h in air. Five CeO2/SBA-15 samples, with CeO2 content equal to 5, 10, 15, 20 and 30 wt%, were prepared by successive incipient wetness-impregnations. The SBA-15 was impregnated with 0.8 ml/g of an ethanolic solution of cerium nitrate hexahydrate, whose concentration was selected to obtain 5 wt% of CeO2. For higher ceria loadings, the impregnations were repeated until the target value. After drying at room temperature, the resulting samples calcined at 400°C for 2h were labeled as CexSBA, where x refers to the ceria weight content. Physico-chemical characterizations were performed on the finished ceria-doped silicas. Surface area measurements (BET) and mesopore size distribution (BJH) were carried out by means of Sorptomatic 1900 (Carlo Erba) instrument. X-ray diffraction patterns were recorded with a D 5005 X-Ray Diffractometer (SIEMENS) using Cu Kα radiation coupled with a graphite monochromator. The crystallite sizes of ceria phase were calculated from the line broadening of the most intense reflection using the Scherrer equation [13]. The surface acidity of the ceria-doped SBA-15 samples was studied by a temperature-programmed desorption of ammonia (NH3-TPD). The measurements were performed with a Micromeritics Autochem 2910 apparatus equipped with a thermal conductivity detector (TCD) and a mass quadrupole spectrometer (Thermostar, Balzers). Prior to the ammonia sorption, the samples (∼100 mg) were outgassed in a flow of O2 (5% in He) at 500°C for 1h, then, cooled to room temperature under He and saturated in a flow of NH3 (5% in He, 30 mL/min) for 1h. Subsequently, the catalysts were purged in a He flow at 100°C for 1h until a constant baseline level was reached. The ammonia desorption was carried out with a linear heating rate (10°C/min) up to 1050°C under a flow of He (30 mL/min). Calibration of the TCD were carried out in order to evaluate the ammonia desorption peaks. Temperature programmed reductions with hydrogen (H2-TPR) were carried out with the same Micromeritics Autochem 2910 apparatus. The samples (∼ 50 mg) were pre-treated with O2 (5% in He) at 600°C for 30 min, cooled in He and then H2 (5% in Ar, 50 mL/min) was flowed from room temperature to 1050°C (heating rate 10° C/min).

3. Results and discussion In Table 1 the morphological properties of CexSBA samples are listed. Doping silica with ceria (up to 15 wt%) resulted into a gradual reduction of the specific surface area and cumulative pore volume, indicating deposition of ceria nanoclusters inside pores. In addition, no evidences of crystalline ceria features were found by XRD. By further

Mesoporous SBA-15 silica modified with cerium oxide: Effect of ceria loading

403

increasing the loading up to 30wt%, broad peaks indicative of fluorite-type cubic structure of ceria were detected. The calculated average particle size was of 5 nm. Surface area sintering and pore volume decrease were also observed, likely because of the crystallization process. Table 1. Morphological properties of CeO2-doped SBA-15 samples. Sample

BET (m2/g)

SBA-15 Ce5SBA Ce10SBA Ce15SBA Ce20SBA Ce30SBA

840 620 580 569 392 385

Pore size (BJH) (nm) 7.4 6.7 6.4 5.9 5.1 5.0

(a)

3

Volume adsorbed (cm STP/g)

(a) SBA-15 (b) Ce10SBA (c) Ce20SBA (d) Ce30SBA

(b) (c) (d)

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure, P/P0

Fig. 1. Nitrogen adsorption/desorption isotherms for CexSBA samples.

(a) SBA-15 (b) Ce10SBA (c) Ce20SBA (d) Ce30SBA

TCD Signal (a.u.)

(a)

(b) (c)

(d)

50

100

150

200

250

300

350

Total pore volume (cm3/g) 0.81 0.70 0.68 0.55 0.48 0.34

In Fig. 1 the nitrogen adsorption/ desorption isotherms of CexSBa samples are displayed. A H1 hysteresis, which is typical of cylindrical mesopores and wide bottleshaped mesopores, was observed in any case, although the hysteresis shape slightly changes by increaseing ceria content and the relative pressure where capillary condensation step occurs shifts to lower P/Po values. No evidences of micropores were found. According to XRD results, these findings suggest that, at high loading, ceria crystallizes, partially obstructing the mesopores. The surface acidity of ceria-doped silica samples was studied NH3TPD. Desorption of ammonia from SBA-15 was detected with a peak centered at 105 °C. No additional desorption occurred at higher temperature. For CexSBA samples the quantity of chemisorbed NH3 increased from 0.09 mmol/g for pure SBA-15 to 0.35 mmol/g for Ce30 SBA. The observed values are in line with literature results [10].

400

Temperature (°C)

Fig. 2. NH3 -TPD curves for CexSBa samples.

Moreover, the desorption started at slightly higher temperature, around 120°C, with a broad and asymmetric peak ranging up to 350-400°C. Therefore, it seems that ceria-

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modification of silica resulted in an increased acidity, in terms of higher concentration and strength of surface acid sites. Accordingly, it has been reported that modification of mesoporous silica with transition metals considerably increases the surface acidity by the generation of Lewis acid sites, to which ammonia molecules are bonded by donoracceptor bond [10]. TPR experiments were carried out for studying the reduction properties of ceria in CexSBA oxides. For all samples high reducibility was observed, the reduction started at around 300°C and two peaks, generally, were observed at ~ 400°C corresponding to surface reduction and at 700°C due to bulk reduction of ceria. Stoichiometrically, 1g of ceria requires 2905 µmol H2/g for a complete reduction to Ce2O3, therefore, ceria in CexSBA samples was almost completely reduced, being for instance the overall consumption of 834 µmol H2/gCe30SBA for Ce30SBA and of 415 µmol H2/gCe20SBA for Ce20SBA. The presence of grain boundaries and defects in the small ceria crystallites may account for the enhanced reducibility [14], which is a very important property for catalytic application, in particular, CO oxidation reactions [12].

4. Conclusions Ceria-modified SBA-15 oxides with increased surface acidity and high reducibility were prepared. The specific surface area was ranging between 620-385 m2/g, and well dispersed ceria crystallites (mean diameter ≤5 nm) were obtained. The so far reported results suggest that CexSBA oxides, prepared by incipient wetness-impregnation approach, are suitable supports for preparation and stabilization of noble metal nanoparticles.

References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] S.A. Bagshaw, E. Prouzet, T.J. Pinnavaia, Science 269 (1995) 1242. [3] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [4] J.-H. Liu, Y.-S. Chi, H.-P. Lin, C.-Y. Mou, B.-Z. Wan, Catal. Today 93-95 (2004) 141. [5] M. Ruszel, B. Grzybowska, M. Łaniecki, M. Wójtowski, Catal. Comm. 8 (2007)1284. [6] A. Beck, A. Horváth, Gy. Stefler, R. Katona, O. Geszti, Gy. Tolnai, L. F. Liotta, L. Guczi, Catal. Today 139 (2008) 180. [7] A.M. Venezia, R. Murania, G. Pantaleo, G. Deganello, J. Catal. 251 (2007) 94. [8] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. [9] F. Yin, S. Ji, P. Wu, F. Zhao, C. Li, J. Catal. 257 (2008) 108. [10] L.Chmielarz, P. Kuśtrowski, R. Dziembaj, P. Cool, E.F. Vansant, Appl. Catal. B 62 (2006) 369. [11] J.A. Hernandez, S. Gómez, B. Pawelec, T.A. Zepeda, Appl. Catal. B 89 (2009) 128. [12] J. Kašpar, P. Fornasiero, M. Graziani, Catal. Today, 50 (1999) 285. [13] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1954. [14] H. Zhu, Z. Qin, W. Shan, W. Shen, J. Wang, J. Catal. 225 (2004) 267.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis and characterization of catalysts obtained by trifluoromethanesulfonic acid immobilization on zirconia Marina Gorsd, Mirta Blanco, Luis Pizzio Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. J. J. Ronco” (CINDECA), Dto. de Química, Facultad de Ciencias Exactas, UNLP-CCT La Plata, CONICET, 47 N° 257, 1900 La Plata, Argentina

Abstract Mesoporous zirconia (zirconium oxide) materials containing mainly mesopores have been synthesized via sol-gel reactions from zirconium propoxide using urea as a template. The solid was dried, extracted with water to remove urea, calcined at different temperatures and impregnated with trifluoromethanesulfonic acid. The samples thus obtained were extracted with a mixture of dichloromethane and diethyl ether using a Soxhlet apparatus in order to remove the loosely adsorbed acid. The solids were characterized by FT-IR, XRD, DTA-TGA, and N2 adsorption-desorption measurements. The mean pore diameter of the support was higher than 3.7 nm, which increased with the increment of the thermal treatment temperature. At the same time, the specific surface area and the amount of triflic acid attached on the support decreased. The potentiometric titration with n-butylamine indicated that the catalysts present very strong acid sites. The catalytic activity of the prepared catalysts in the esterification of 4-hydroxybenzoic acid with propyl alcohol was evaluated. Keywords: zirconia, sol-gel, triflic acid, catalysts, esterification

1. Introduction The trifluoromethanesulfonic acid (CF3SO3H) has a highly acidic nature and excellent thermal stability; it also has good resistance to reductive and oxidative dissociation, with no generation of fluoride ions. The trifluoromethanesulfonic acid was used as an efficient homogeneous catalyst, but has environmental disadvantages because it generates high amount of wastes. An interesting alternative is the trifluoromethanesulfonic acid heterogeneization by immobilization on an adequate support. There are few works on this subject, though it can be mentioned its immobilization on titania and carbon [1, 2]. On the other hand, zirconia is an interesting material to be used as catalyst support due to its thermal stability in different atmospheres. The most common methods that can be used to obtain zirconia are the sol-gel method, the micellar technique or the mechanochemical synthesis [3]. Zirconia is frequently prepared by micellar method, while the sol-gel method from an alkoxide is less used [4]. Its acid properties can be modified by addition of cationic or anionic substances, such as sulfate or tungstate [5]. In the present work, trifluoromethanesulfonic acid was immobilized on mesoporous zirconia obtained by sol-gel method using urea as a low-cost template. The physicochemical and textural characteristics of the prepared catalysts are studied. In addition, the catalytic behavior of the synthesized catalysts in the esterification of 4-hydroxybenzoic acid with propyl alcohol was evaluated.

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2. Experimental 2.1. Support preparation Zirconium propoxide (Aldrich, 26.6 g) was mixed with absolute ethanol (Merck, 336.6 g) and stirred for 10 min to obtain a homogeneous solution under N2 at room temperature, then 0.47 cm3 of 0.28 M HCl aqueous solution was dropped slowly into the above mixture to catalyze the sol-gel reaction for 3 h. After that, an appropriate amount of urea-alcohol-water (1:5:1 weight ratio) solution was added to the hydrolyzed solution under vigorous stirring to act as template. The amount of added solution was fixed in order to obtain a template concentration of 10% by weight in the final material. The gel was kept in a beaker at room temperature till solidification. The solid was grounded into powder and extracted by distilled water for three periods of 24 h, in a system with continuous stirring to remove urea. Finally, it was calcined at 100, 200, 300, and 400°C for 24 h (ZrTX samples, where X is the calcination temperature).

2.2. Catalyst preparation Trifluoromethanesulfonic acid, CF3SO3H, (0.01 mol, Alfa Aesar, 99%) was added drop wise to a mixture of ZrTX (2 g) and toluene (20 cm3, Merck) at 90ºC under nitrogen atmosphere; then it was further refluxed for 2 h. Next, the sample was cooled, filtered, washed with acetone (Mallinckrot AR ) and dried at 100ºC for 24 h. The solids were extracted with a mixture of dichloromethane and diethyl ether (100 g of mixture per g of catalyst) for three periods of 8 h using a Soxhlet apparatus in order to remove the acid weakly attached to the support. Afterwards, they were dried again at 100ºC for 24 h. The samples were named TriZrT100, TriZrT200, TriZrT300, and TriZrT400. The amount of trifluoromethanesulfonic acid retained was determined by C and S elemental analysis with an EA1108 Elemental Analyzer (Carlo Erba Instruments).

2.3. Support and catalyst characterization The textural properties of the solids were determined from N2 adsorption-desorption isotherms at liquid-nitrogen temperature. They were obtained using Micromeritics ASAP 2020 equipment. The samples were previously degassed at 100ºC for 2h. Fourier transform infrared (FT-IR) spectra of the samples were recorded, using Bruker IFS 66 FT-IR equipment, pellets in BrK, and a measuring range of 400-1500 cm-1. X-ray diffraction (XRD) patterns of the solids were recorded with Philips PW-1732 equipment, using Cu Kα radiation, Ni filter, 30 mA and 40 kV in the high voltage source, 5-55°2θ scanning angle at a scanning rate of 1° per min. The thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis were carried out using Shimadzu DT 50 thermal analyzer. The analysis were performed under argon, with 25-50 mg sample, heating rate 10°C/min, and temperature range 25600°C. The acidity of the solids was measured by means of potentiometric titration. A known mass of solid was suspended in acetonitrile and stirred for 3 h. Later, the suspension was titrated with 0.05 N n-butylamine in acetonitrile solution at 0.05 ml/min, measuring the electrode potential variation with a digital pH meter Hanna 211. The catalytic activity of the samples in the esterification of 4-hydroxybenzoic acid with n-propanol was evaluated. It was carried out in liquid phase at reflux temperature in a 50 ml glass reactor equipped with a condenser and a magnetic stirrer. The reagent mixture was heated under stirring and then the catalyst was added. An n-propanol:4hydroxybenzoic acid:catalyst molar ratio of 10:1:0.1 was used. Samples were taken periodically and analyzed by gas chromatography using dodecane as internal standard.

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3. Results and discussion Mesoporous zirconia (zirconium oxide) materials containing mainly mesopores were obtained, with the total pore volume significantly higher than the micropore volume (Table 1). The mean pore diameter (DP) was higher than 3.7 nm, which increased with the thermal treatment temperature (Table 1). At the same time, the specific surface area (SBET) and the specific surface of micropores (SMicro) decreased. The textural properties of the catalysts were mainly the same as those of the supports. The amount of acid attached on the support (NTri) decreased with the increment of the thermal treatment temperature. This effect may be explained if the interaction is assumed to be of electrostatic type due to proton transfer to the -OH groups on the support surface. So, as a result of the support dehydroxylation during the thermal treatment, the amount of OH groups to be protonated decreases, and therefore NTri diminishes. Table 1. Support textural properties and CF3SO3H amount in the catalysts. Sample

SBET (m2/g)

SMicro (m2/g)

ZrT100 ZrT200 ZrT300 ZrT400

192 132 78 20

88 50 17 0

Total pore volume (cm3/g) 0.18 0.16 0.11 0.07

Micropore volume (cm3/g) 0.05 0.02 0.01 0

DP (nm)

Sample

3.7 4.7 5.5 14.2

TriZrT100 TriZrT200 TriZrT300 TriZrT400

NTri (mmol CF3SO3H/g) 0.91 0.53 0.41 0.10

By XRD it was observed that the samples have amorphous characteristics; there is not any line indicating the presence of crystalline phases. The DSC diagram of ZrT100 (Figure 1) showed two endothermic peaks at 69 and 171°C, attributed to the loss of physically adsorbed water, and the partial dehydroxylation of the solid respectively. The exothermic peak at 434ºC was assigned to the zirconia transformation from an amorphous to a metastable tetragonal phase. In the DSC diagram of TriZrT100 (Figure 1) TriZrT200, TriZrT300, and TriZrT400 samples, this peak appears at higher temperatures. Apparently, the trifluoromethanesulfonic acid retards the crystallization of zirconia. Additionally, another exothermic peak at 365ºC assigned to the elimination of acid was displayed, whose intensity decreases in parallel to the decrease of trifluoromethanesulfonic acid content. From TGA-DSC, it can be established that the catalysts are thermally stable up to 250ºC. The FT-IR spectrum of the TriZrT100 sample displayed bands at 1267, 1180, and 1040 cm-1, in addition to those present in the ZrT100 sample (Figure 2). The first two bands are ascribed to the S═O stretching mode of the adsorbed trifluoromethanesulfonic acid and the last one is assigned to the C-F stretching [6, 7]. These bands are also present in the spectra of TriZrT200, TriZrT300, and TriZrT400 catalysts, although their intensities are lower as a result of the lower amount of trifluoromethanesulfonic acid adsorbed on the support. The acidity measurements of the catalysts by means of potentiometric titration with n-butylamine let us to estimate the number of acid sites and their acid strength. As a criterion to interpret the obtained results, it was suggested that the initial electrode potential (Ei) indicates the maximum acid strength of the sites, and the value of meq amine/g solid where the plateau is reached indicates the total number of acid sites [8]. Nevertheless, the end point of the titration given by the inflexion point of the curve is a

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491

40

DSC (a.u.)

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20 434

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-5

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TriZr T200 TriZr T300 TriZrT400

ZrT100

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Transmittance (a.u.)

0

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300

400

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-10

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Temperature (°C)

Fig. 1. DSC of ZrT100 and TriZrT100 samples.

4000

3000

2000

1000 -1

W avenumber (cm )

Fig. 2. FT-IR spectra of ZrT100, TriZrT100, TriZrT200, TriZrT300 and TriZrT400 samples.

good measure to carry out a comparison of the acidity of different samples. On the other hand, the acid strength of these sites may be classified according to the following scale [8]: Ei > 100 mV (very strong sites), 0 < Ei < 100 mV (strong sites), -100 < Ei < 0 (weak sites) and Ei < -100 mV (very weak sites). The potentiometric titration with n-butylamine indicated that the catalysts present very strong acid sites with potential between 100 and 700 mV. Ei values were in the range 700-650 mV, nearly independent of NTri, and the number of acid sites decreased with the thermal treatment temperature in an almost linear relation with the NTri decrease. The acidity of the supports is higher than that of a zirconia obtained by micellar technique from zirconium oxychloride and ammonia [8], which present very weak sites. The catalysts were tested in the esterification of 4-hydroxybenzoic acid with propyl alcohol. It was observed that the amount of 4-hydroxybenzoic acid converted into the ester increased continuously with the reaction time. The conversion depended strongly on the CF3SO3H content, decreasing from 80 to 7% when NTri decreased from 0.91 to 0.10 mmol CF3SO3H/g. The catalytic activity of the samples, expressed as moles of ester formed at 5 h/mol CF3SO3H in the catalyst, decreased slightly in the following order TriZrT100 (0.88) > TriZrT200 (0.86) > TriZrT300 (0.76) > TriZrT400 (0.71). On the other hand, the catalysts were reused several times without appreciable loss of catalytic activity. These results show that the prepared solids would be appropriate catalysts for their use in acid reactions employing a clean technology, to replace the classical acids used both in the laboratory and the industry.

References 1. 2. 3. 4. 5. 6. 7. 8.

L. Pizzio, Mater. Lett. 60 (2006) 3931. D.O. Bennardi, G.P. Romanelli, J.C. Autino, L.R. Pizzio, Catal. Commun. 10 (2009) 576. M. Fernández-García, A. Martínez-Arias, J.C. Hanson, J.A. Rodríguez, Chem. Rev. 104 (2004) 4063. X. Qu, Y. Guo, Ch. Hu, J. Molec. Catal. A 262 (2007) 128. G. D. Yadav, J.J. Nair, Micropor. Mesopor. Mater. 33 (1999) 1. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules, Von Nostrand Reinhold, New York, 1945. L.J. Bellamy (ed.) The Infrared Spectra of Complex Molecules, Wiley, New York, 1960. L. Pizzio, P. Vázquez, C. Cáceres, M. Blanco, Catal. Lett. 77 (2001) 233.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Influence of precursor on the particle size and stability of colloidal gold nanoparticles A. Alshammari,a,b A. Köckritz ,a V.N. Kalevaru,a A. Martina a

Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29A, D-18059 Rostock, Germany b Petroleum and Petrochemicals Research Institute, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia

Abstract Particle size and stability of colloidal gold nanoparticles (AuNPs) have considerable importance in nanocatalysis due to their beneficial properties. Colloidal AuNPs of the present study were prepared in aqueous solution by one-step chemical reduction of various gold metal precursors. The nature of Au precursor showed a significant effect on the size and stability of colloidal AuNPs. Furthermore, the study describes the effect of different synthesis parameters such as temperature, pH-value, concentration of precursors and reducing agent on the size of the colloidal AuNPs. The obtained samples were characterized by means of UV-Vis spectroscopy, transmission electron microscopy, dynamic light scattering and zeta potential measurements. Keywords: gold metal precursor, colloidal AuNPs, size of AuNPs, stability, reaction parameters

1. Introduction The nano-scale level of metal nanoparticles is indeed an important parameter in the field of heterogeneous catalysis due to their unique and completely different catalytic properties compared to their bulk solids. For example, the catalytic properties of nanostructured gold catalyst are known to depend on the size of the gold particles [1]. It is known that metal reducibility, metal distribution, and particle size can be controlled by the preparation method. However, the nature of the metal precursor compound also shows significant influence on the particle size as well as stability. Particularly, to manufacture such catalysts in an efficient and reproducible way, it is important to gain the control over the parameters mentioned above. Therefore, efforts are being made by various researchers for the past few decades to find ways to stabilize and optimize the size of such nanoparticles in a desired way. A substantial number of studies have been published in recent times on the synthesis and use of supported AuNPs as catalysts for different reactions [e.g. 2]. Literature survey [e.g. 3] also reveals that the activity of catalyst depends not only on their composition but also on the kind of precursors used in the preparation method. However, investigations concerning the effect of precursor materials on the size of colloidal AuNPs are very rare in the literature. In the present study, we made attempts to prepare AuNPs using different gold precursors and ensure their affect on the size and stability. The aim is also to check the effect of temperature, concentration of reductants and pH-value of the reaction mixture on the size of AuNPs.

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2. Experimental 2.1. Samples preparation One-step chemical reduction of HAuCl4 solution was carried out using sodium thiocyanate (ST) as reducing agent in presence and absence of sodium citrate (SC) as stabilizing agent. In another set of experiments, SC was used as reductant. Three different gold precursors (Au-p) such as HAuCl4·3H2O (Au-pa, commercial), HAuCl4·3H2O (Au-pb, lab prepared) and NaAuCl4·2H2O (Au-pc, commercial) were used in the preparations. The syntheses were carried out similar to experiments described elsewhere [4]. The final samples are denoted as (a), (b) and (c). The reductant to Au+3 ratio was maintained at 4 : 1 (in case of syntheses with stabilizer, 7 mM of SC were used). The reactions were carried out at room temperature under stirring. In addition, the influence of increased reduction temperature (40-80°C), concentration of starting material and pH-value of reaction mixture on the size of AuNPs were also examined with sodium citrate (SC) as reductant.

2.2. Samples characterization ICP analyses were carried out to check the accurate concentration of the Au precursor compounds (Au-pa, Au-pb, Au-pc) in aqueous solution using an Optima 3000XL device (Perkin-Elmer). Optical properties (absorbance) of colloidal solutions were acquired with UV-Vis spectrometer (Avantes-2048). Particle size distribution and zeta potential values were obtained at room temperature from dynamic light scattering and zeta potential measurements, respectively, which were performed on a Malvern Instrument (ZS ZEN 4003). Size analysis (i.e. size, shape, morphology etc.) of colloidal AuNPs was further confirmed with HRTEM (JEM-2100F) at a voltage of 200 kV.

3. Results and discussion At first, ICP analyses showed comparable Au concentrations (0.045-0.05 mg/l). The corresponding changes in the morphology, size distribution and stability of the prepared colloidal AuNPs were characterized by different methods as described below.

3.1. Spectroscopic and microscopic investigations

Absorbance / a.u.

3.1.1. Ultraviolet-Visible spectroscopy (UV-Vis) In a first set of experiments colloidal AuNPs were obtained using ST as reductant and SC as stabilizer. UV-Vis spectroscopy is often used as quite sensitive technique for observing the formation of colloidal AuNPs 1.0 because they display an intense absorption (a) 0.8 band in the region of 500-550 nm, especially (b) (c) when the particle size exceeds ca. 5 nm [5]. 0.6 The position of those bands also depends on different factors (e.g. size, shape). 0.4 However, the colloidal samples prepared in b c the present study (Fig. 1) did not display a 0.2 such band in this range, which implies that the average particle size is ≤5 nm. Such 0.0 300 400 500 600 700 800 phenomenon can be explained as an Wavelength / nm indication of the quantum size effects (i.e. the loss of bulk Au character during the Fig. 1. UV-Vis spectra of colloidal AuNPs transition from bulky Au to small “quantum- using various gold precursors.

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sized” Au particles) [5]. Additionally, sample (b) reveals a slightly broadened spectrum compared to samples (a) and (c), which indirectly provides hints that bigger particles were formed with Au-pb precursor. 3.1.2. Transmission electron microscopy (TEM) The better evidence for the size and morphology of colloidal AuNPs can be easily obtained by TEM. Figure 2 (Au-pa, upper part) showed well-dispersed particles of colloidal AuNPs prepared using precursor Au-pa with average size of approx. 3 nm, while usage of Au-pb and Au-pc showed slightly bigger AuNPs. However, all obtained colloidal gold nanoparticles exhibited more or less spherical morphology. In addition, from HRTEM images (Fig. 2, lower part, final samples (a), (b) and (c)) one can clearly observe the crystal planes of gold. The distance between lattice plane fringes is estimated to be in the range from 0.19 to 0.25 nm, depending upon the type of crystal plane or precursors used. For instance, samples Fig. 2. Electron micrographs of colloidal AuNPs (a) and (c) showed (0.22 & 0.23 nm) and prepared using precursors Au-pa, -b & -c. (0.23 & 0.23 nm) that corresponds to Au(111) plane, while sample (b) displayed Au(200) besides Au(111). In sample (a), Au(111) is more exposed. 3.1.3. Dynamic light scattering (DLS) Supporting investigations regarding the effect of metal precursor compounds on the size and distribution of colloidal AuNPs were also accomplished by DLS. Figure 3 shows the result obtained from DLS using precursor Au-pa as a model. The average particle diameter was found to be 6.7 nm in this case. The other two gold precursors (Au-pb and Au-pc) gave somewhat bigger particles with the size of 13.9 and 10 nm, respectively. We have shown that DLS gives a bigger diameter value than those obtained by TEM. This phenomenon is due to the fact that DLS measures the hydrodynamic particle size in the suspension medium, where TEM shows the core particle size.

3.2. Stability of colloidal AuNPs In order to study the influence of precursor on the stability of colloidal AuNPs, the Au ions were reduced in absence of stabilizer. The expected particle growth was measured by their zeta potential (ζ), with the simultaneous intention to estimate their stability. The particles with zeta potential values ≥ +30 mV or ≤ -30 mV are usually Fig. 3. Size distribution of colloidal AuNPs considered stable. In the present study, prepared using precursors Au-pa. the ζ values varied in the range from 24 mV to -31 mV, depending upon the type of precursor used. Thus, the type of precursor used also affects the changes in the stability of the final colloidal AuNPs. Colloidal AuNPs obtained from precursor Au-pb displayed the lowest zeta potential (-24 mV) and

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hence less stable compared to other two precursors. Alternatively, the highest stability of AuNPs obtained from precursor Au-pc (i.e. NaAuCl4.2H2O) with a ζ value of -31.3 mV. Moreover, the stability of colloidal AuNPs can also be checked indirectly by a color change or precipitation time (τ). These results were in good agreement with spectroscopic results (zeta potential). The resulting suspension from precursor Au-pc neither lead to any change in color nor to appearance of agglomeration over a period of more than three weeks, whereas a precipitation of agglomerates using precursor compounds Au-pa and Au-pb was observed after two weeks and one week, respectively.

3.3. Influence of synthesis conditions on colloidal AuNPs Size Influence of the reaction parameters such as temperature, initial Au concentration, concentration of reducing agent and pH-value on the size, distribution and stability of colloidal AuNPs were studied in another row using SC as reducing agent instead of ST. Precursor Au-pa was selected as a model starting material. We observed that an optimum temperature of 80°C is suitable for the preparation of small AuNPs with reasonably good stability. However, the particles prepared at lower temperature ( ethanol > propanol > butanol. The results obtained show the ways for the further development of multi-component catalysts prepared on the basis of hydrotalcite-like LDHs and their application in oxidative transformations of EB, light alkanes and alcohols.

4. Conclusions The method of synthesis of multi-component hydrotalcite-like layered metal hydroxosalts (LDHs) was elaborated. The products obtained have been used as precursors of V-MoW-Nb-O catalysts for ODH of EB, ethane, propane and alcohols. Considerable increasing of EB, alkane and acohols conversion simultaneously with the selectivity to alkene, styrene and carbonyl compounds has been successively achieved using the purposeful progressive complication of the composition of the LDHs precursors. Investigations made it possible to formulate scientifically substantiated preparation and application of the V-Mo-W-Nb-O catalysts to obtain high activity and selectivity in the ODH processes.

References [1] O.N Krasnobaeva., I.P Belomestnykh., G.V Isagulyants., V.P Danilov., Rus. J. Inorg. Chem., 54 (2009) 485-499. [2] V.P Krasnobaeva, O.N., V. P Danilov, I.P Belomestnykh, G.V Isagulyants, Rus. J. Inorg. Chem. 52 (2007) 181. [3] P. Botella, J.M. Lopez Nieto and N. Solsona, Catal. Lett., 78 (2002) 383. Dear Collegues, Now I send a manuscript corrected according to the remarks of the reviewer It is in the attach file Best regards, yours I. Belomestnykh

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis of high-surface area CeO2 through silica xerogel template: influence of cerium salt precursor L.F. Liotta a, G. Di Carlob, F. Puleob, G. Marcìc and G. Deganelloa,b a

Istituto per Lo Studio dei Materiali Nanostrutturati (ISMN)-CNR via Ugo La Malfa, 153, 90146 Palermo, Italy. E-mail: [email protected] b Dipartimento di Chimica Inorganica e Analitica “Stanislao Cannizzaro”, Università di Palermo, Parco d’Orleans II, Viale delle Scienze pad. 17, 90128 Palermo, Italy. c Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università di Palermo, Parco d’Orleans II, Viale delle Scienze pad. 6, 90128 Palermo, Italy.

Abstract Ceria nanosized oxides with high surface area were synthesized by means of a templating approach, using a porous silica xerogel with surface area as high as 718 m2/g. After impregnation of the silica template with the cerium salt solution and further calcination at 600°C, the final ceria oxide was recovered by dissolving the silica framework in NaOH solution. The effect of cerium counteranion, nitrate or chloride, on the textural and reduction properties of the ceria oxide was examined. Characterizations by BET and pore size distribution, XRD, TPR and SEM/EDX techniques were performed. The silica xerogel templated approach resulted in the preparation of ceria with surface area of 205 m2/g and very small particle size (∼5 nm), when cerium chloride precursor was used. An enhanced reducibility, at temperature < 700°C, was also observed for the so obtained CeO2 sample. The results were discussed in terms of the influence of cerium choride and cerium nitrate thermal decomposition. Keywords: silica xerogel, cerium nitrate and chloride, incipient wetness impregnation, ceria nanostructure, reduction properties

1. Introduction Since the discovery of mesoporous silica (e.g. MCM-41, SBA-15) in the 1990s, highsurface metal oxides, such as titania, ceria, zirconia, alumina, have received worldwide attention for applications in catalysis, as chemical sensors and as electrodes in fuel cells. Various synthesis approaches, depending on the applications, are currently used such as sol-gel technique, deposition-precipitation, chemical vapour deposition, spray pyrolysis and micro-emulsion methods. Some of these procedures give materials with poor crystallinity and low thermal stability. Others are quite complicated and expensive. In recent years, a wide variety of porous materials have been obtained by means of template technique [1,2]. Recently, Fuertes [3] reported the use of an inexpensive porous silica xerogel as template for synthesis of various metal oxides. In particular, starting from a silica xerogel with specific surface area of 510 m2/g, prepared by sodium silicate in aqueous solution containing HCl, in appropriate molar ratio, the synthesis of ceria with specific surface area equal to 141 m2/g and a narrow pore size distribution centred at ∼4.6 nm was achieved. In the present work, we report the synthesis of nanosized ceria oxides, starting from cerium nitrate and cerium chloride, as precursors and using a high surface area silica xerogel, as template. After calcination of the template material impregnated with the

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cerium precursor, the ceria oxide was recovered by dissolving the silica matrix in NaOH solution. Characterizations by BET and pore size distribution, XRD, TPR and SEM/EDX techniques were performed.

2. Experimental The silica xerogel was synthesized according to a published procedure [3], opportunely modified, using sodium silicate solution, Na2O(SiO2)x⋅xH2O, (Sigma-Aldrich, with composition: Na2O ∼10.6% + SiO2 ∼26.5%), as silica source. In a typical preparation, an appropriate amount of sodium silicate solution was diluted (1:2) with distilled water and, then, was added under stirring to an aqueous solution of HCl. The final molar composition of the reagents was: sodium silicate/HCl/H2O = 1/6/194, accordingly to the literature [3]. The obtained transparent solution was stirred in the closed flask for 20h at room temperature. After that, the mixture, which turned from completely transparent to slightly opaque, was aged in a closed Teflon vessel at 100°C for 2 days. The obtained gel was filtered and washed several times with water. Finally, the solid was washed with acetone and diethyl ether and then dried at room temperature. Synthesis of ceria oxides was accomplished by incipient wetness impregnation steps of the so prepared silica xerogel with ethanol solution of cerium nitrate, Ce(NO3)3⋅6H2O, or cerium chloride, CeCl3⋅7H2O. In order to maximize pore filling and avoid the surface segregation of cerium salt, four successive impregnations were performed until a final loading of 40wt% of ceria. The resulting samples were dried at room temperature, calcined at 600°C for 4h and, in order to dissolve the silica matrix, treated with a 2M NaOH solution at 65°C. The final samples were labeled as CeO2-N (from cerium nitrate) and CeO2-Cl (from cerium chloride). As reference, a portion of silica xerogel was also calcined at 600°C for 4h. The resulting silica was labeled as SiO2-calc. Physico-chemical characterizations were performed during the different steps of ceria oxides preparation and on the finished samples. Surface area measurements (BET) and pore size distribution (BJH) were carried out by means of Sorptomatic 1900 (Carlo Erba) instrument. X-ray diffraction patterns were recorded with a D 5005 X-Ray Diffractometer (SIEMENS) using Cu Kα radiation coupled with a graphite monochromator. The crystallite sizes of ceria phase were calculated from the line broadening of the most intense reflection using the Scherrer equation [4]. Temperature programmed reductions (TPR) were carried out with a Micromeritics Autochem 2910 apparatus equipped with a thermal conductivity detector. The ceria samples (∼ 50 mg) were pre-treated with O2 (5% in He) at 600°C for 30 min, cooled in He and then H2 (5% in Ar, 50 ml/min) was flowed from room temperature to 1050°C (heating rate 10° C/min). Scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDX) analyses were obtained using a FEI quanta 200 ESEM microscope, operating at 20 kV on specimens after being coated with a layer of gold.

3. Results and discusion The physical properties of silica xerogel and of the prepared ceria oxides are listed in Table 1. In Fig. 1 the XRD patterns of ceria samples are shown. The starting silica xerogel exhibits a surface area as high as 718 m2/g and a narrow pore size distribution (mean pore size of 8.2 nm) with a total pore volume of 1 cm3/g. After calcining the xerogel at 600°C for 4h, the surface area and pore volume were reduced, whilst the pore size increased only slightly. Maintaining good textural properties after calcination makes silica xerogel attractive as template. Rather high surface area (130 m2/g) was obtained for CeO2-N, showing pore size and pore volume values almost comparable

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with the literature results [3]. Crystallite sizes of 6.9 nm were calculated from XRD peak broadening. When cerium chloride was used as precursor, a ceria oxide with improved physico-chemical properties was achieved, displaying surface area of 204 m2/g and very small particle size (4.7 nm). Accordingly, for CeO2-Cl broad and weak peaks were detected as compared to CeO2-N (Fig. 1). It is likely that the exothermic decomposition of the nitrate precursor during calcination, by increasing much more the local temperature, induces a faster crystallite growth in CeO2-N. Table 1. Structural properties of silica xerogel and synthesized ceria oxides. Sample

Calcination temperature (°C)

BET (m2/g)

SiO2 xerogel SiO2-calc CeO2-N CeO2-Cl

No calcination 600 600 600

718 560 130 205

(a) CeO2-N

Intensity (A.U.)

(b) CeO2-Cl

(a) (b) 20

25

30

35

40

45

50

2θ (°)

Fig.1. XRD patterns in the angular range 20-50 2θ for ceria oxides obtained through template method . (a) CeO2-N (b) CeO2-Cl

3

Volume adsorbed (cm STP/g)

(a)

(b)

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure, P/P0

Fig.2. Nitrogen adsorption/desorption isotherms for ceria oxides.

Mean pore size (BJH) (nm) 8.1 8.5 6.8 4.9

Total pore volume (cm3/g) 1.05 0.78 0.48 0.42

Crystallite size (nm) 6.9 4.7

In Fig. 2 the nitrogen sorption isothermes of the two samples, CeO2-N and CeO2-Cl are compared. The different shape of isotherms is indicative of different surface area and different pore size of structural mesopores. The isotherms obtained for CeO2-N shows a pronounced hysteresis in the range p/p0 ∼0.5-0.8, while in the case of CeO2-Cl the capillary condensation takes place at lower relative pressure, in the range p/p0∼0.4-0.6. Accordingly, CeO2-N shows bigger pore size than CeO2-Cl (Table1). From SEM and EDX investigation no significant differences were observed between the xerogels after impregnation with cerium nitrate or cerium chloride and drying at room temperature. Indeed the morphology and the percentage of Ce and Si in the two series of oxides were very similar. Interestingly some differences appeared after calcinations at 600°C, probably due to the different decomposition process of cerium counteranions. As far as final ceria samples are concerned, Fig. 3a,b shows two SEM images of CeO2-N and CeO2-Cl oxides. By the comparison of these pictures, it is evident that ceria from chloride shows a rougher surface with respect to the nitrate prepared one and the presence of smaller grains can be detected.

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Moreover, some interstices between the nanoparticles (textural pores in the size of macropores) can be detected. All these insights are in agreement with the higher surface area measured for the CeO2-Cl sample. The residual Si content, observed by EDX, was less than 10wt% in both ceria samples.

a

b

2μ

2μ

Fig. 3a,b. Comparison between SEM micrographs of two ceria samples: (a) CeO2-N, (b) CeO2-Cl. (a) CeO 2-N

H2 consumption (A.U.)

(b) CeO 2-Cl

(b)

(a) 200

300

400

500

600

700

800

900 1000

Temperature (°C)

Fig. 4. H2 -TPR profiles for ceria oxides obtained through template method.

It is know that the reduction profile of ceria depends on the surface area and crystallite size. H2-TPR measurements performed over the two ceria samples reflect different properties (Fig. 4). According to our previous results [5], the reduction profile of CeO2-N was characterized by an intense peak centered at 425°C (647 μmolH2/g) due to the surface reduction, while the broad peak at higher temperature, 700-800°C, was ascribed to the reduction of the bulk (446 μmol H2/g). Much more reducible appeared the CeO2-Cl sample showing the overall H2 consumption (1450 μmolH2/g) at temperature < 700°C, which accounts for almost 50% reduction of CeO2 to Ce2O3. The presence of grain boundaries and defects in such small ceria crystallites could explain the enhanced reducibility [6].

4. Conclusions Synthesis of mesoporous ceria oxides nanoparticles with surface area larger than those reported in literature for similar prepared materials can be successfully achieved by applying silica xerogel templating approach. The effect of cerium counteranion was significant, ceria from nitrate being more affected by the exothermic decomposition of the precursor. Enhanced bulk reducibility at temperature below 700°C was observed in nanosized ceria crystyallites prepared from chloride precursor, demonstrating the feasibility of this approach for the preparation of reducible oxides for catalytic purposes.

References [1] P.D. Yang, D.Y. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater. 11 (1999) 2813. [2] B.T. Holland, C.F. Blanford, T. Do, A. Stein, Chem. Mater. 11 (1999) 795. [3] A.B. Fuertes, J. Phys. Chem. Solids, 66 (2005) 741. [4] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1954. [5] A.M. Venezia, G. Pantaleo, A. Longo, G. Di Carlo, M.P. Casaletto, L.F. Liotta and G. Deganello, J. Phys. Chem. B 109 (2005) 2821. [6] H. Zhu, Z. Qin, W. Shan, W. Shen, J. Wang, J. Catal. 225 (2004) 267.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Iron based catalyst for hydrocarbons catalytic reforming: A metal-support interaction study to interpret reactivity data Luca Di Felicea,b Claire Coursona, Pier Ugo Foscolob and Alain Kiennemanna a

Laboratoire des Matériaux, Surface et Procédés pour la Catalyse, ECPM, UMR7515, 25 rue Becquerel, 67087, Strasbourg Cedex 2, France b Chemical Engineering Department, University of L’Aquila, 67040 Monteluco di Roio, L’Aquila, Italy

Abstract The addition of iron, a cheap and non toxic metal, to the natural minerals dolomite and related materials, CaO and MgO, has been investigated for biomass gasification applications. The Fe/CaO, Fe/MgO and Fe/dolomite systems have been prepared by impregnation following two preparation methods to generate Fe (+2) and/or Fe (+3) species, and carefully characterized. The improvement on tar conversion of CaO, MgO and dolomite by adding iron, has been investigated by using toluene as model tar compound in a microreactor rig. Keywords: iron catalyst, tar reforming, dolomite

1. Introduction The research in new materials with catalytic properties is an important stage on the development of the applicability of biomass gasification concepts, with the aim of overcoming problems of in-situ tar elimination for producing a clean gas suitable for feeding fuel cells. The aim of this work is to investigate synthesis methods, characterization and catalytic tests of new cheap and non toxic catalysts for gasification reactions, using toluene as model tar compound, in a fixed bed microreactor. Iron has been added to dolomite, CaO and MgO substrates, well known materials in biomass gasification processes [Delgado et al., 1997], in order to improve their catalytic activity.

2. Experimental 2.1. Catalyst preparation. The catalytic systems investigated in this work consists of pre-calcined natural dolomite (Ca,Mg)O, a pre-calcined natural lime (CaO) and magnesia (MgO) impregnated by 20% of iron by weight. There have been developed two preparation pathways: an oxidative one, focused on the evaluation of the Fe (+3)–substrate interaction (impregnation solvent: water; salt precursor: iron nitrate; samples named OxiCa, OxiMg and OxiDolo) and a neutral one to evaluate the Fe (+2, +2.5)–substrate interaction (impregnation solvent: ethanol; salt precursor: iron acetate; samples named NeuCa, NeuMg and Neu Dolo). For both preparation methods, the iron salt is solubilized in the impregnation solvent, then the substrate is added and stirred to obtain a suspension. The solvent is evaporated at the appropriated temperature and the solid recovered is dried (120°C, 5h) and crashed (80 97%, Reachim, Russia) solution. Ratio support/solution (g/ml) was equal to 1/1.3 (VTi, VAL); 1/1.8 (VZr); 1/6.3 (VSi) and determined by water-absorbing capacity of support. After impregnation the samples were dried at 110oC for 24 h and calcined in air flow (50 ml/min) at 400°C for 4 h. Series 2. The samples of series 1 were washed with a 10% water solution of nitric acid to remove the crystalline V2O5 phase. After washing, the samples were calcined

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once more at 400°C for 4 hours. The samples of series 1 and 2 are labeled as VMe and VMew, (Me = Si, Al, Zr, Ti), respectively. Catalysts were characterized by BET, the chemical analysis, Raman spectroscopy, XRD and XPS. Catalytic experiments were carried out in a flow-circulation setup with chromatographic analysis of reaction products. Composition of the initial mixture was as follows: 5% CH2O, 10% H2O, air balance. The reaction temperature was 120°C.

3. Results and discussion 3.1. Characterization of the catalysts The physicochemical properties of the catalysts are listed in Table 1. Table 1. Physicochemical properties of the catalysts. Sample

V2O5 (wt.%)

SBET, (m2/g)

Nsa, V/nm2

[V/Me]

Phase XRD

Series 1 Support catalyst bulkb surfacec VSi 20.0 200 129 10.3 0.17 0.03 V2O5, SiO2 VAl 21.9 250 146 9.9 0.14 0.07 V2O5,γ-Al2O3 VZr 17.3 120 96 11.9 0.34 0.92 V2O5, ZrO2d VTi 19.6 350 111 11.7 0.22 0.28 V2O5, TiO2 Series 2 SBET, (m2/g)e VSiw 0.0 150 150 0.0 0.00 0.00 SiO2 VAlw 1.7 250 233 0.5 0.01 0.01 γ-Al2O3 3.4 120 122 1.9 0.05 0.07 ZrO2d VZrw 11.0 150 140 5.2 0.11 0.14 TiO2 VTiw a surface density (Ns, Vat/nm2); bthe bulk atomic ratio by the chemical analysis; cthe surface atomic ratio determined by XPS; dmonoclinic (85%) and cubic (15%) modification of ZrO2; e SBET after washing and calcination at 400oC.

An increase in specific surface area of the catalysts after washing is related to removal of crystalline V2O5, which makes its value close to the surface area of support after washing and calcination at 400oC. One can see that in the series 1 samples the bulk atomic ratios [V/Me]v and the surface atomic ratios [V/Me]s are very different. Low values of [V/Me]s for VSi and VAl samples indicate that only a small amount of vanadia in the catalysts belongs to the surface, while the main part of vanadia is in V2O5 crystallites. The excess of [V/Me]s over [V/Me]v in VTi and VZr samples can be attributed to high dispersion of V2O5 over ZrO2 and TiO2. Values of [V/Me]s and [V/Me]v in the washed samples virtually coincide, indicating the presence of surface vanadia species and absence of the crystalline V2O5 phase. The same conclusions follow from XRD patterns of the series 1 and 2 samples. XRD patterns (Fig. 1). of the series 1 samples show reflections of the crystalline phases of support and V2O5. In the spectra of series 2 samples only the reflections of support phase are observed. Complete removal of vanadium, as shown by the chemical analysis data, occurs after washing the VSi sample. It means that all vanadia in the VSi catalyst is represented by crystalline V2O5. After removal of crystalline V2O5, the VAlw, VZrw and VTiw samples still contain strongly bound insoluble vanadia species, their amount depending on the support nature. The nature of strongly bound vanadia species was studied by Raman spectroscopy (Fig. 2). The Raman spectrum of VTi sample shows an intense broad band with a maximum at 840 cm–1, which is attributed to stretching V-O-V vibrations in polymeric VOx species, and a low-intensity band of V2O5 (995 cm–1).

Preparation, active component and catalytic properties

* - V 2O

465

5

(V = O) 99 4

(V = O) 702 V Ti V T iw

*

* *

**

*

*

*

*

1 03 0 ( V= O ) 990

VZ r

*

**

**

*

**

(V - O -V ) 8 40

VS i VS iw

*

*

( V = O)

9 35

V Zrw *

*

(V -O-V ) 75 0

V Zr

V Zrw

93 0 (V = O)

*

V Al

*

995

V Ti

V Alw V T iw 0

10

20

30

40

50

60

70

2 Θ

Fig. 1. XRD patterns of the catalysts.

7 00

80 0

90 0

R a m a n s h if t, c m

1 00 0 -1

Fig. 2. Raman spectra of the catalysts.

In the Raman spectrum of the washed sample only the bands of polymeric VOx species are observed. The presence of these species is confirmed by the data of chemical analysis giving a 5.2 V/nm2 density of VOx in VTiw samples, which is close to the monolayer surface coverage of polymerized vanadia species [1-4]. The Raman spectrum of VZr sample before washing shows low-intensity bands of polymeric (935 cm–1) and monomeric (1030 cm–1) VOx species and intense bands of crystalline V2O5 (994 cm–1). Similar bands are observed for the VAl sample (not shown). Washing of these samples results in disappearance of the bands from V2O5 and polymeric VOx. The density of VOx particles on the surface of the VZrw sample is 1.9 V/nm2 (Table 1), which is close to the monovanadate monolayer coverage [2]. Only the crystalline V2O5 bands are observed in the Raman spectra of VSi (not shown).

3. 2. Catalytic properties Table 2 compares the catalytic performance of the different supported vanadium catalysts with reference to the crystalline V2O5 and the supports without the addition of vanadium. The TiO2 and ZrO2 supports catalyze conversion of formaldehyde to methyl formate. Methyl formate and methanol are the main products of formaldehyde conversion on γ-Al2O3. SiO2 is inactive in both reactions. Crystalline V2O5 has low activity, but high selectivity (ca. 90%) in formaldehyde oxidation to formic acid. Some increase in activity (TOF to formic acid) is observed when vanadium is supported on SiO2, without significant changes of selectivity. Supported vanadium on γ-Al2O3 does not provide a notable increase in activity, which is explained by low dispersion of vanadium over the surface of support. Increasing activity of VAlw is related to the presence of monomeric VOx species in sample. Low selectivity to formic acid in the oxidation of formaldehyde on VAl sample is caused by a low coverage of the surface with vanadia species and by a large fraction of free areas of support that are active in formaldehyde conversion to methyl formate. The activity of vanadia sites for the formation of formic acid increases when vanadium is supported on ZrO2 и TiO2. The activity of the washed samples containing only the monomeric and polymeric vanadia species is considerably higher compared to that for the two-phase samples with prevailing crystalline V2O5. A higher activity of the monolayer samples in comparison with the two-phase those is caused by blocking a part

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of active VOx species by low-active crystalline V2O5 in the two-phase samples. The absence of methyl formate in the reaction products indicates a homogeneous coverage of the supports by VOx species. Table 2. Catalytic properties. Temperature – 120ºC. The composition of the reaction mixture (vol. %): 5% CH2О, 10% Н2О, air the balance. V=8.2 l/h. Catalyst

Weight, g

X,%a

Selectivity, %

rb,10-9 mol/m2·s

TOF,c 10-4s-1

FAd MFe Mf COx V2O5 10.5 6.1 91.8 6.1 0.0 2.1 6.6 0.02 VSi 2.9 6.6 87.7 10.0 0.0 2.3 0.4 0.20 TiO2 0.8 35.0 3.0 82.0 15.0 0.0 16.6 VTi 0.7 35.3 96.1 0.3 0.0 3.6 24.8 12.20 0.3 35.6 93.7 0.8 0.0 5.5 43.0 22.50 VTiw ZrO2 1.1 35.0 3.3 87.5 7.7 1.5 13.5 VZr 5.1 36.2 22.8 66.5 4.7 6.0 3.8 0.44 1.4 35.0 17.1 74.3 2.4 6.2 15.1 8.10 VZrw 4.7 35.0 0.0 63.8 31.8 4.4 2.0 Al2O3 VAl 7.5 37.8 3.9 85.1 4.3 6.7 1.8 0.04 VAlw 4.5 35.0 2.0 64.5 29.9 3.6 1.7 0.42 a X- conversion of CH2O; b r - rate of СН2О transformation; c TOF – normalizing the rate of oxidation formaldehyde to formic acid per vanadium site, s-1; d FA - formic acid; e MF - methyl formate; f M- methanol.

The following order of activity for formic acid formation as a function of the nature of support was established: TOF to formic acid (10–4 s) (CH2O conversion ≈ 35%): Series 1: VTi (12.2) > VZr (0.44) > VSi (0.2) > VAl (0.04) ≈ V2O5 (0.02); Series 2: VTiw (22.5) > VZrw (8.1) > VAlw (0.42) > V2O5 (0.02).

4. Conclusions Activity of the samples with monomeric (VAlw, VZrw) and polymeric (VTiw) VOx species exceeds the activity of the samples, which contain too the crystalline V2O5. The presence of crystalline V2O5 in samples leads to a partial blocking of the active sites and hence decreases the catalyst activity. The lower formic acid selectivity over VAl and VZr catalysts was explained by the fact that VOx species do not fully cover the surface of support, and it results in formaldehyde transformation to methyl formate.

Acknowledgement The authors acknowledge Federal Agency Science Innovation for financial support.

References 1. 2. 3. 4.

G.C. Bond, S.F. Tahir, Appl. Catal. 71 №1 (1991) 1-31. A. Khodakov, B. Olthov, A.T. Bell, I. Iglesia, J. Catal. 181 №2 (1999) 205-216. G.Ya. Popova, T.V. Andrushkevich, E.V. Semionova, Yu.A. Chesalov, L.S. Dovlitova, V.A. Rogov, V.N. Parmon, J. Mol. Catal. A: Chemical. 283 (2008) 146-152. G.Ya. Popova, T.V. Andrushkevich, I.I. Zakharov, Yu. A. Chesalov Kinet. Catal. 46 (2005) 217-226.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Investigation of different preparation methods of PtIr, PtIrSn and PtIrGe catalysts Christophe Poupin, Camille La Fontaine, Laurence Pirault-Roy Laboratoire de Catalyse en Chimie Organique, Université de Poitiers, France

Abstract Addition of Ir to a Pt/Al2O3 catalyst by two different methods, successive impregnation and organometallic grafting resulted in catalysts with different structure: in the first case separate Pt and Ir sites can be suggested, while in the second case, Ir was exclusively deposited on Pt, forming very small Ir clusters on Pt particles. Addition of Ge or Sn to the Pt-Ir grafted catalyst led to third component deposit on the surface of mixed metals particles. On the other hand, when the promoter is added to PtIr-IS, Ge seems to be located on Ir particles while Sn modified Pt particles. Keywords: platinum, iridium, germanium, tin, organometallic grafting

1. Introduction Reforming of hydrocarbons is an important catalytic process for the production of highoctane gasoline, aromatics and hydrogen from naphtha. The reactions involved in this process as hydrogenation-dehydrogenation reactions occur over the metallic sites of the catalyst while isomerization and dehydrocyclization proceed mostly on bifunctional metal–support acid sites. The metallic function is usually provided by Pt in the form of very small particles dispersed on the surface of the catalyst. Its properties can be finetuned by the addition of another element. This metal function promoter can be another noble metal, e.g. Ir, or/and another element with the desired properties (Sn, Ge). It was observed that Ge or Sn adding could favorably replace the sulfidation step of Pt-Ir catalysts needed to decrease the high hydrogenolytic activity of the catalyst [1]. Our aim was to investigate bimetallic Pt-Ir catalysts prepared by two different methods, namely conventional successive impregnation and by organometallic grafting [2, 3] as well as to study the effect of third compound as Ge or Sn added in both cases by organometallic grafting method.

2. Experimental 2.1. Catalyst preparation 2.1.1. Support Alumina from Degussa (Aluminum Oxid C; δ-alumina; surface area of 100 m2.g-1) was used in powder form made of microspheres of about 100 Å. This powder was first wetted with water to prepare a slurry. Then it was dried overnight at 393 K in an oven, ground and sieved to collect the fraction between 0.1 and 0.25 mm, calcined in dry air flow (4 h, 773 K) and finally reduced in H2 flow (4 h, 773 K). 2.1.2. Monometallic parent catalysts The monometallic catalysts were prepared by wet impregnation of the treated alumina using a platinum salt [Pt(NO2)2(NH3)2] to obtain 1 wt.-Pt % loading and [Ir(C5H7O2)3] precursor to obtain 0.25 wt.-Ir %. The slurry was shaken at room temperature for 12 h and after drying in a sandbath at 353 K, the impregnated alumina was left overnight in

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an oven at 393 K. Then, the sample was calcined in dry air (4 h, 773 K) and reduced under pure H2 flow (4 h, 773 K). 2.1.3. Modified catalyst: multi-metallic catalysts Different bimetallic catalysts were prepared by using successive impregnation or the surface organometallic chemistry method [2, 3]. For the successive impregnation, the Pt parent was impregnated with the Ir precursor [Ir(C5H7O2)3]. Then it was calcined in dry air at 673 K for 3 h and reduced in N2 flow containing 20% H2 for 3 h at 673 K. The samples were denoted as PtIr(IS). For the organometallic grafting, 6.6 g of the parent Pt sample was pre-reduced (in H2 flow at T = 673 K for 2 h, heating rate 2 K/min), cooled in H2 to room temperature and kept at room temperature for 1 h (H2 adsorption). It was first immersed in 15 ml toluene, kept at room under argon flow (1 h) then 15 ml toluene solution of [Ir(C5H7O2)3] was added and the sample was kept for 6 h at 343 K in bubbling Ar. The amount of [Ir(C5H7O2)3] dissolved in toluene corresponded to nominal coverage of 1/2 Ir monolayer, as calculated for surface Pt atoms. The sample was washed with toluene, dried in Ar flow at 393 K for 1 h and finally reduced in H2 flow (473 K; 4 h). This reduction temperature was found to be sufficient for producing catalytically active and reproducible samples [3]. The sample was denoted as PtIr(GS). The tri-metallic catalysts were prepared by Ge or Sn grafting on Pt-Ir catalysts (IS or GS one) using organometallic route described before. A solution of Ge(nC4H9)4 or Sn(nC4H9)4 in heptane was used and the catalysts made were denoted PtIrGe(IS), PtIrGe(GS), PtIrSn(IS) and PtIrSn(GS).

2.2. Characterization methods

The metallic accessibility was determined by two different techniques: hydrogen chemisorption and transmission electron microscopy (TEM). The volumetric hydrogen chemisorption was carried out on prereduced samples (473 K, p(H2)=75 kPa, 1 h) after evacuation at room temperature in an apparatus described previously [3]. TEM was performed with a Philips CM120 electron microscope operating at 120 kV with a theoretical resolution of 0.35 nm. Samples were included in a polymeric resin and cut into small sections (about 40 nm) using a diamond knife. Cuts were put onto copper TEM grids. The average particle size (average diameter) was determined on several pictures using the relation Σ nidi3/Σ nidi2. CO probe FTIR measurements were performed using a Nicolet Magna-750 spectrometer. The samples (about 20 mg) were pressed into pellets and reduced in situ in a dedicated cell at 473 K in H2 flow for 2 h, followed by outgassing and cooling to ambient temperature. CO adsorption was performed at room temperature by injecting pulses in the cell until the catalysts saturation. Then, the samples were evacuated at room temperature for 1 h. Difference spectra were obtained from the absorbances before and after adsorption of the probe molecule.

3. Results and discussion 3.1. Catalysts characterization by TEM and H2 chemisorption

Good correlations between the values obtained by these two methods were pointed out for monometallic samples (see Table 1) that ensured the hypothesis, 1Irs or 1Pts = 1Hads, used for the chemisorption. By successive impregnation preparation, Pt and Ir species are expected to be separate and the dispersion should be near 50% as it was measured. The addition of a third component, on the PtIr(IS), decreased the metallic accessibility but the size of the particles measured by TEM remains the same. So Ge or Sn is grafted on particles surface and hindered the chemisorption of H2, but the amount of the addition is to low to increase the size of the particles. The PtIr(GS) accessibility

Investigation of different preparation methods of PtIr, PtIrSn and PtIrGe catalysts 469 measured was lower than expected. Ir poisoned the Pt particles and such monoatomic Ir or organometallic species don’t allow the dissociative chemisorption of H2. The addition of a third compound, on the PtIr(GS), does not change the poor metallic accessibility and the size of the particles. Table 1. Characterization of catalysts by H2 chemisorption and TEM. Metal. acc. (H2 Chem.);

Particles size

Estimated size (nm)

TEM (nm)

Pt

50%; 1.9

2.2

0.2%Ir/Al2O3

Ir

50%; 1.9

2.1

1%Pt-0.2%Ir/Al2O3 (IS)

PtIr(IS)

40%; 2.9

3.5

PtIrGe(IS)

7%; 16.3

3.4

PtIrSn(IS)

11%; 11.4

3.5

PtIr (GS)

10%; 10.4

3.0

PtIrGe(GS)

10%; 10.4

3.1

PtIrSn(GS)

10%; 10.4

3.2

Catalysts

Code

1%Pt/Al2O3

1%Pt-0.2%Ir-0.15%Ge/Al2O3 (Pt-Ir (IS) parent) 1%Pt-0.2%Ir-0.15%Sn/Al2O3 (Pt-Ir (IS) parent) 1%Pt-0.2%Ir/ Al2O3(GS) 1%Pt-0.2%Ir-0.2%Ge/Al2O3 (Pt-Ir (GS) parent) 1%Pt-0.2%Ir-0.15%Sn/Al2O3 (Pt-Ir (GS) parent)

3.2. Catalysts characterization by CO FTIR spectroscopy

The left IR graph gathers the parents’ spectra as well as the bimetallic catalysts ones. For Ir/Al2O3, both Ir0 and Irδ+ specie exist. A major carbonyl band (2043 cm–1), is due to CO linearly adsorbed on fully reduced Ir sites, while an intense pair of bands observed at 2073 and 1976 cm–1 is assigned to gem-dicarbonyl species adsorbed on small Ir clusters or isolated Ir [4-7]. On Pt parent catalyst, the single band at 2075 cm–1 is due to linear CO species (LCO). PtIr (IS) catalyst exhibits two bands at 2075 cm–1 (LCO species on Pt) and 2066 cm–1 while on PtIr (GS) catalyst, three bands can be observed at 2075, 2063 and 2023 cm–1. The peak near 2060 cm–1 can be assigned to LCO species adsorbed on low coordinated Ir sites. The main difference between the two bimetallic samples is a band at ca. 2020 cm–1 observed only on the Pt-Ir grafted catalysts. It was recognized as LCO adsorbed on very small Ir clusters by Solymosi et al [6]. On the other hand, McVicker et al. [7] specifically assigned a peak around 2020 cm–1 to be LCO adsorbed on large Ir clusters on alumina support. As no gem-dicarbonyl species can be pointed out on the PtIr (GS) spectra, no large Ir cluster can exist. So, we assume that very small Ir clusters are present on the grafted catalyst. According to the preparation procedure, separate Pt and Ir sites can be suggested for PtIr (IS) catalysts, while for PtIr (GS) samples, Ir was exclusively deposited on Pt, forming very small Ir clusters on Pt particles. The right IR graph presents the spectra of bimetallic catalysts and an example of a sample modified by Ge. The presence of a peak at 2020 cm–1 for the PtIrGe (IS) suggests that Ge grafting occurred on Ir particles and divided the facets to create very small clusters. When tin is added, no peak appears but the Pt peak at 2075 cm–1

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drastically decreases. Addition of Ge or Sn to PtIr (GS) catalysts leads to a diminution of the band attributed to LCO species both on Ir and Pt. 2075 cm-1

2075 cm-1

2075 cm-1

2063 cm-1

0,07 0,06

Pt Ir PtIr (IS) PtIr (GS)

0,05 0,04

2023 cm-1

0,03 0,02

PtIr (GS) PtIr (IS) PtIrGe (IS)

0,070

2075 cm-1

2043

0,060 Absorbance (A.U.)

Absorbance (A.U.)

0,09 0,08

2063 cm-1

2066 cm-1

2066 cm-1

2075 cm-1

0,050 0,040

2075 cm-1

2023 cm-1

0,030

2020 cm-1

0,020

cm-1

2073 cm-1 0,010

1976 cm-1

0,01

-0,000

-0,00 2200

2000

1800

1600

wavenumber (cm-1)

2200

2000

1800

1600

wavenumber (cm-1)

Figure 1. Characterization of catalysts by FTIR CO spectroscopy.

4. Conclusion Addition of Ir to Pt catalysts by successive impregnation (IS) and organometallic grafting (GS) results in catalysts with different structure: in the case of PtIr-IS separate Pt and Ir particles on the alumina surface can be suggested while in the case of PtIr-GS, Ir is exclusively attached to Pt, as ensured by the organometallic grafting method. Addition of Ge or Sn to the Pt-Ir catalyst prepared by organometallic grafting leads to deposit on the surface of the bimetallic particles and blocks both. On the other hand, when Ge or Sn is added to PtIr-IS, Ge and Sn seem to be located on different sites: Ge mainly on Ir particles and Sn on Pt particles. Further experiments pointed out that PtIrGS catalyst showed very promising behavior in ring-opening reaction of MCP as resulting in ROPs at high selectivity even at high conversion.

References [1] P. Samoïla and al, 2007, “Influence of the pretreatment method on the properties of trimetallic Pt–Ir–Ge/Al2O3 prepared by catalytic reduction”, Appl. Cat. A : General, 332, 37. [2] A. Wootsch and al, 2006, “Characterization and catalytic study of Pt-Ge/Al2O3 catalysts prepared by organometallic grafting”, J. of Catal., 238, 67. [3] L. Pirault-Roy and al, 2003, “A new approach of selective Ge deposition for RhGe/Al2O3 catalysts: characterization and testing in 2,2,3-trimethylbutane hydrogenolysis”, Appl. Cat. A: General, 245, 15. [4] Y. M. Lopez-De Jesus and al , 2008, “Synthesis and Characterization of Dendrimer-Derived Supported Iridium Catalysts” J. Phys. Chem. C, 112, 13837-13845. [5] A.Bourane and al, 2002, “Heats of Adsorption of the Linear CO Species Adsorbed on a Ir/Al2O3 Catalyst Using in Situ FTIR Spectroscopy under Adsorption Equilibrium” J. Phys. Chem. B, 106, 2665-2671. [6] F.Solymosi and al. 1980, “CO and NO adsorption on alumina-supported iridium catalyst” J. Catal., 62, 253. [7] McVicker and al, 1980, “Chemisorption properties of iridium on alumina catalysts”, J.Catal., 65, 207.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Perovskite-type catalysts for the water-gasshift reaction Francesco Basilea, Giuseppe Brennaa, Giuseppe Fornasaria, Pascal Del Gallob, Daniel Garyb and Angelo Vaccaria a

Dipartimento di Chimica Industriale e dei Materiali, ALMA MATER STUDIORUM – Università di Bologna, Viale Risorgimento 4, 40136 Bologna (I) b Air Liquide, Centre Recherches Claude-Delorme, BP 126, 90124 Jouy-en-Josas (F)

Abstract Fe/La perovskite-type (PVK) catalysts - as such, or containing Cr, Ce or Cu - have been synthesized by the citrate method to be applied in the water-gas-shift reaction at moderate temperatures. A calcination temperature of 650°C made it possible to obtain stable PVK phases, with acceptable surface area values. TPR/TPO/TPR analyses evidenced the specific effects of the added cations or calcination parameters on the Fe3+ ions reducibility, which reflected on the catalytic behaviour of the final catalysts.

Keywords: water gas shift, perovskite, Fe, La, Ce, Cu , citrate method 1. Introduction Recent years have seen increasing interest towards the water-gas-shift-reaction (WGSR), CO + H2O → CO2 + H2, as an upgrading technique either to adjust the syngas (CO + H2) composition or to reduce the CO content and obtain a high H2 grade for application in low temperature fuel cells. Currently, the WGSR is performed in two steps: i) immediately after the steam reforming reactor, operating at about 350°C with stable Fe-based catalysts (HTS); and ii) operating at about 250°C with highly active Cubased catalysts (LTS) [1]. However, there is an increasing interest for new formulations able to operate at moderate temperatures (about 300°C, or MTS) with high activity, selectivity and stability with time-on-stream. Two paths may be explored: (i) to stabilize the LTS catalysts, thus avoiding methanation and sintering, or (ii) to improve the activity of HTS catalysts by introducing new active elements. The aim of this study was to develop new catalytic formulations which are active and selective in WGSR at moderate temperatures, and stable with time-on-stream and towards some poisons (sulphur, chlorine and silica) that may be present in the exit streams of steam reforming reactors. To obtain active catalysts and avoid interferences due to either structure dishomogeneity or phase segregation, reducible and flexible structures have to be selected; ABO3 perovskite-type (PVK) phases were therefore prepared, due to their capacity to cover broad composition ranges [2]. Furthermore, in order to improve this activity, it may be advantageous to prepare complex PVK formulations, with A and B substitution on the reference LaFeO3 PVK phase (considering the role of Fe-based HTS catalysts), by introducing Cr as a stabilizing element and Cu or Ce as activating elements [3,4].

2. Experimental PVK precursors have been synthesized by the citrate method [4,5], using metal nitrates as the starting materials, and regulating their amounts according to the nominal composition (Table 1). All reagents were obtained by Aldrich (≥ 99,0%). A citric

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acid/total metal nitrates molar ratio equal to 1.30 was used to avoid oxide or hydroxide segregation. The nitrate solution was added in drops to the citric acid solution, followed by evaporation to dryness at 90°C for 4h and gel decomposition at 180°C for 16h. Finally, the powder was calcined at different temperatures (450-950°C range) and times (2 or 12h). X-ray powder diffraction (XRPD) patterns were recorded using a Philips PW1050/81 diffractometer (Cu-Kα - Ni filtered, λ = 0.15418nm), investigating a 2θ range from 10° to 80°. Temperature-programmed reduction/oxidation/reduction analyses (TPR/TPO/TPR) were performed in a Thermo Quest CE TPDRO 1100 instrument; after a pre-heating step in He at 150°C for 15min to eliminate weaklyadsorbed species, samples were cooled and reduced (or oxidized) by heating from 60 to 650°C (10°C/min), with a final isothermal step of 1h. BET surface area was determined by a Carlo Erba Sorpty 1750 instrument, after a preliminary degassing step under vacuum at 200°C. Catalysts (30-40 mesh sizes) were charged in the reactor (INCOLOY 800HT) in the isothermal zone between two corundum layers. Before tests, catalysts were reduced for 2h, by progressively increasing both the temperature (to 300°C) and H2 content. The catalytic activity was investigated as a function of the temperature (250400°C), operating at 1.5MPa, with the steam / dry gas ratio equal to 0.55 (v/v) and a contact time (τ) of 0.50sec. Table 1. Composition of the investigated catalysts. Sample

La

Ce

Fe

Cr

Cu

Formula

LF LFC-Cu LFC-Ce

1 1 0.9

0.1

1 0.9 0.9

0.08 0.1

0.02 -

LaFeO3 LaFe0.9Cr0.08Cu0.02O3 La0.9Ce0.1Fe0.9Cr0.1O3

Figure 1. XRPD pattern of the LF sample calcined for 12h at different temperatures.

3. Results and discussion Figure 1 shows that an amorphous phase is also present after calcination up to 550°C; that phase disappears completely at 650°C without any further change in the pattern of sample calcined at 900°C. 650°C may therefore be considered a good compromise to obtain crystalline PVK phases [6], with acceptable surface area values (Table 2). All the investigated samples mainly contain mesopores (2-50nm) and the decrease in the surface area values is more evident for the LF sample, thus evidencing the thermal

473

Perovskite-type catalysts for the WGSR

stabilizing effect of Cr. Before and after the reaction, all the samples peak at the PVK phase (LaFeO3) [6], while those of Cu- or Ce-containing side phases were not detected, although their presence either as amorphous phases or as an undetectable amount cannot be excluded. The crystallite size calculated on the peak at about 32.3°, the peak corresponding to the crystallographic plane (121), indicates a high crystallinity (Table 2) in line with low surface area values. Moreover, the crystal size values do not show significant increases after reaction. Table 2. Bulk and surface data as a function of the calcination temperature. Sample

Calcinat. temp (°C)

LF-650 LF-450 LFC-Cu650 LFC-Ce650

650 450 650 650

Phase Fresh PVK PVK PVK PVK

Crystal size (nm) Spent PVK PVK PVK PVK

Fresh 42 47 54 49

Spent 52 54 56 52

SBET (m2/g) Fresh 14.0 9.0 9.0 12.0

Spent 9.0 7.0 7.0 10.0

The properties of the PVK samples were investigated by TPR/TPO/TPR tests. The first TPR profile of the LF-650 sample shows two overlapped H2 consumption peaks, with maxima at ca. 360 and 540°C (Fig. 2). Since La3+ is not reducible in these conditions, these peaks may be ascribed to the reduction of Fe3+ ions, which are present respectively on the surface or in the bulk [7]. Following oxidation, it is possible to observe an intense reduction peak at 480°C with two shoulders at a lower temperature. The shoulder at 320°C may be attributed to the reduction of Fe4+ ions formed during the oxidation step [8], whereas the second one may be attributed to the reduction of surface Fe3+ ions. The reduction of bulk Fe3+ ions is shifted at 480°C, i.e. at a lower temperature than in the first TPR run. Finally, it must be noted that a decrease in both the calcination temperature (450°C) and the time (2h) (Fig. 2) significantly improves the reducibility of the Fe3+ ions.

Figure 2. TPR profiles of LF-650 and LF-450 samples, before or after oxidation.

In the LFC-Cu650 PVK sample (Fig. 3A), the first H2 consumption peak (ranging from 200-350°C) may be attributed to the reduction of Cu2+ ions, that occurs in two steps (Cu2+ → Cu+ and Cu+ → Cu0) with comparable rates. The reduction peaks of Fe3+ ions are shifted to lower temperatures (340 and 510°C) if compared to the LaFeO3 PVK, suggesting that Cu0 may promote the reduction of Fe3+ ions. Cr3+ ions are not reduced under the applied conditions, therefore the shoulder at 620°C ca. may be attributed to the reduction of well-dispersed La2CrO6 (formed in the synthesis) to La2O3 and stable LaCrO3; the latter does not oxidize in the oxidation step, since the peak disappears in the second reduction [9]. Finally, the LFC-Ce650 PVK phase (Fig. 3B) shows a first peak

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F. Basile et al.

attributable to the reduction of Fe3+ ions present on the surface, whereas the peak at 490°C is related to the Ce4+ → Ce3+ reaction [10]. Finally, the broad peak with its maximum at 610°C may be attributed to the overlapping of the reduction of the bulk Fe3+ ions and chromates. After the oxidation step, a significant shift towards lower temperatures may be observed for all the reduction peaks.

Figure 3. TPR profiles of LFC-Cu650 (A) and LFC-Ce650 (B) samples, before or after oxidation.

The LF-650 sample demonstrates low activity at medium temperatures, reaching a CO conversion value of 30% ca. at 400°C, as was previously reported [3]. The partial substitution with Cr3+, Cu2+ or Ce4+ ions does not appear to have positive effects on catalytic performances, with CO conversion values ≤ 8%, also after a further reduction step, by raising the temperature to 500°C. The decrease in the calcination temperature (450°C) and time (2h) for the LF sample (i.e. starting from samples containing PVK and amorphous phases (LF-450)) significantly improves the sample’s reducibility, but does not seem to have any positive effect on the activity, with a CO conversion value of 15% at 400°C. Therefore, preliminarily, Fe-based PVK catalysts do not seem able to approach the interesting data previously reported for other PVK compositions [4].

4. References M.V. Twigg, 1989, The Water-gas Shift Reaction, Catalyst Handbook, 2nd ed., Wolfe, London, 283-339. [2] M.A. Peňa, J.L.G. Fierro, 2001, Chemical structure and performances of perovskite oxides, Chem. Rev. 101, 1981-2017. [3] M.J. Koponen, T. Venäläinen; M. Suvanto, K. Kallinen; T.-J.J. Kinnunen, M. Härkönen, T.A. Pakkanen, 2006, Water gas shift reaction studies on 2% Pd/AM1-xFexO3 (A= Ba, La, Pr; x= 0.4, 0.6) perovskites, Appl. Catal. A311, 79-85. [4] S.S. Maluf, E.M. Assaf, 2009, La2-xCexCu1-yZnyO4 perovskites for high temperature watergas shift reaction, J. Natur. Gas Chem. 18, 131-138. [5] Z. Liu, M.-F. Han, W.-T. Miao, 2007, Preparation and characterization of graded cathode La0.6Sr0.4Co0.2Fe0.8O3, J. Powder Sources 117, 837-841. [6] International Center for Diffraction Data, 1991, JCPDS Inorganic Files, Swarthmore, USA. [7] R. Zhang, H. Alamdari, S. Kaliaguine, 2006, Fe-based perovskites substituted by copper and palaldium for NO+CO reaction, J. Catal. 242, 241-253. [8] P. Ciambelli, S. Cimino, L. Lisi, M. Faticanti, G. Minelli, I. Petitti, P. Porta, 2001, La, Ca and Fe oxide perowskites: preparation, characterization and catalytic properties for methane combustion, Appl. Catal. B29, 239-250. [9] S. Ifrah, A. Kaddouri, P. Gelin, G. Bergeret, 2007, On the effect of La-Cr-O-phase composition on diesel soot catalytic combustion, Catal. Commun. 8, 2257-2262. [10] S. Ricote, G. Jacobs, M. Milling, Y. Ji, P.M. Patterson, B.H. Davis, 2006, Low temperature water-gas shift: Characterization and testing of binary mixed oxides of ceria and zirconia promoted with Pt, Appl. Catal. A303, 35-47. [1]

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Evaluation of different methods to prepare the Fe2O3/MoO3 catalyst used for selective oxidation of methanol to formaldehyde Karim H. Hassana,*, Philip C.H. Mitchellb a b

Department of Chemistry, College of Science, University of Diyala, Baqubq, Iraq School of Chemistry, University of Reading, Reading, RG6 6AD, UK

Abstract Different Fe2O3/MoO3 catalysts were prepared by kneading, precipitation and co-precipitation methods. Their activities and selectivities in the oxidation of methanol to formaldehyde were compared with those of a commercial catalyst. The iron(III) molybdate catalyst prepared by co-precipitation and filtration had a selectivity towards formaldehyde in methanol oxidation comparable with a commercial catalyst; maximum selectivity (82.3%) was obtained at 573 K when the conversion was 59.7%. Catalysts prepared by reacting iron(III) and molybdate by kneading or precipitation followed by evaporation, omitting a filtration stage, were less active and less selective. Keywords: Iron molybdate catalyst, Selective catalytic oxidation, Catalysts preparation

1. Introduction Formaldehyde, CH2O, is manufactured by the selective oxidation of methanol over a silver [1,2] or iron molybdate catalyst [3,,4]. Iron molybdate catalyst is a combination of the two oxides that produces the desired active and selective catalyst. Iron(III) oxide by itself is unselective producing carbon dioxide and water; molybdenum trioxide is selective but with low activity [5]. The overall reaction is CH3OH + 0.5O2 = CH2O + H2O. The oxidation reaction is exothermic (∆H=-159 kJ mol–1) and proceeds through reaction of methanol with the molybdate surface [6]. The technical catalyst composition is ca.(80% MoO3 and 20% Fe2O3), equivalent to an iron mole fraction 0.31. Iron may be partially replaced by a promoter, e.g. chromium. The active catalyst is considered to be Fe2(MoO4)3. The excess of MoO3 is said variously to be required to ensure the stability of the catalyst towards loss of MoO3, to maintain the active species and to enhance the surface area. It should be stressed that catalyst structure depends also on other parameters such as metal loading, and drying and calcination temperatures. The method of preparation appears to have a significant impact on the activity and selectivity of the catalyst. There are some studies of different preparations, for example sol-gel catalysts vs co-precipitated catalysts [7]. The aim of the present work is to evaluate different methods of preparation of the iron molybdate catalysts and test the activities and selectivities in the oxidation of methanol to formaldehyde by comparing with those of a commercial catalyst.

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2. Experimental 2.1. Catalyst preparation Ammonium heptamolybdate, (NH4)6Mo7O24.4H2O; and iron(III) nitrate nonahydrate, Fe(NO3)3 .9H2O, analytical grade (>99% purity), were used. 2.1.1. Kneading and evaporation: catalyst 1 Ammonium molybdate was added to the amount of water that is sufficient to obtain a homogeneous paste when added gradually to iron nitrate powder with continuous stirring. The paste was heated in an oven at 110oC for 2 h to evaporate water and calcined at 400–500°C in a current of air for 4 h. 2.1.2. Precipitation and concentration: catalyst 2 Solutions of ammonium heptamolybdate and iron (III) nitrate were mixed at pH of about 2. The precipitate formed was left to settle overnight at room temperature, the supernatant was decanted off and the precipitate dried and calcined as for catalyst 2. 2.1.3. Coprecipitation and filtration: catalyst 3 Solutions of ammonium heptamolybdate and iron(III) nitrate were prepared and mixed as in the preparation of catalyst 2. The precipitate was filtered off and washed several times with distilled water until the pH of the filtrate reached 7. The solid was dried and calcined as before. Pellets (or tablets) (7 mm diameter, 4 mm thick) were prepared in a tablet press at 2 atm. Using polyvinyl alcohol as binder and were calcined at 500oC .

2.2. Catalyst characterization and testing Iron and molybdenum were determined by standard atomic absorption methods. Pore volume, densities and hardness values were determined by the usual methods used in catalysts characterization. Activities and selectivities of the catalysts in the conversion of methanol to formaldehyde were determined in a continuous flow pilot plant described by Karim and Hummadi [8]. Test conditions were: reactor temperature 200 to 350°C (473–623 K); pressure, 10 atm (1013 kPa); flow rate, 15.858 cm3/s; methanol, 5.5% by volume in oxygen. Analysis of the reaction products was carried out periodically [9] after two hours of collection of the samples.

3. Results and discussion 3.1. Chemical composition and physical properties of the catalysts Catalyst 3 in its composition and physical properties is closest to the commercial catalyst. The most obvious difference between the different preparations is the excess MoO3, which is greatest for catalyst 3 (Table 1). Excess molybdenum (over the stoichiometric composition) appears to have little effect on the catalyst density. However, the two catalysts with the highest molybdenum (catalyst 3 and the commercial catalyst) have the greatest pore volumes and hardness.

Evaluation of different methods to prepare the Fe2O3/MoO3 catalyst

477

Table 1. Chemical composition and physical properties of the catalysts. Composition and Property

Catalyst Catalyst 1 Kneading and evaporation

Catalyst 2 Precipitation and concentration

Composition/wt. % Fe 17.2 15.6 Mo 50.2 51.7 Fe2O3 24.6 22.3 MoO3 75.4 77.7 2.32 4.57 MoO3 excess/wt.% Mo/Fe atomic 1.70 1.93 ratio Fe/(Fe + Mo) 0.371 0.341 mole fraction Colour Yellow green Yellow green Pellet size/cm 0.9×0.9 0.9×0.9 Pore volume/ 0.28 0.30 cm3 g-1 0.53 0.50 Solid density/g cm-3 1.05 1.05 Bulk density/g cm-3 Hardness/105 1.70 1.63 dyne a Received from the Ministry of Industry of Iraq.

Catalyst 3 Coprecipitation and filtration

Commerciala

13.8 53.4 19.7 80.2 7.12

14.0 53.0 20.0 80.0 6.52

2.25

2.20

0.308

0.312

Yellow 0.9×0.9 0.40

Yellow 0.45×0.4 0.35

0.52

0.50

1.05

1.10

2.1

2.3

3.2. Catalytic properties: activities and selectivities in the oxidation of methanol to formaldehyde Activities and selectivities are shown plotted vs temperature in Fig. 1. The behavior of our co-precipitated catalyst (catalyst 3) is similar to that of the commercial catalyst. The activities of all catalysts rise with rising temperature and converge to roughly the same conversion at 598 K. The significant distinction between the catalysts is in the selectivity which passes through a maximum at 573 K with the commercial and the co-precipitated catalysts having the highest selectivities. Our catalytic results are consistent with the literature [3,4], with activities tending to the same value independently of the iron (or molybdenum) content of the catalyst and selectivities passing through a maximum with increasing reaction temperature. We discuss now how the activity and selectivity depend on the composition of the catalyst with reference to our results and literature data [10]. Generally the effect of composition has been discussed in terms of excess of MoO3. However, since MoO3 is in excess it would seem logical to express the variation of catalyst composition in terms of iron added (or not) to molybdenum, i.e. the Fe/Mo ratio or the Fe/(Fe+Mo) mole fraction as for other two-component catalysts, for example, the cobalt-promoted molybdenum disulfide based hydrodesulphurization catalyst. Unfortunately most researchers have not studied a wide range of Fe/Mo compositions (and we are no exception). However, we can combine certain patent literature data [10] with our data and thereby examine a wider range of compositions. Activities and selectivities values (figure and data can be obtained from authors). For

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the activities we see a typical volcano curve, the activity rising to a maximum value as iron is added to MoO3 and then dropping off. This behavior demonstrates synergy between iron and molybdenum. Beyond an iron mole fraction of 0.4 the activity begins to increase as Fe2O3 takes over. The selectivity to formaldehyde is more or less constant until an iron mole fraction of ca 0.3 is reached. The selectivity then drops as Fe2O3 becomes dominant. This behavior tells us that the selective catalyst is MoO3 and iron is an activity promoter. 65

0.85 0.75

55 selectivity

conversion/%

60

50

0.65 0.55

45 commercial catalyst catalyst 1 kneading catalyst 2 precipitation catalyst 3 co-precipitation

40 35 450

500

550 T /K

600

650

0.45 0.35 450

500

550 T /K

600

650

Fig. 1. Conversion (left) and selectivity (right) in conversion of methanol to formaldehyde.

References [1]

J.L. Li, W.L. Dai, K. Dong and J.F. Domg, (2000) “A new silver–containing ceramics for catalytic oxidation of methanol to formaldehyde”, Materials Letters,44 (3-4), 233-236. [2] I.E. Wachs, (2003) “Extending surface science studies to industrial reaction conditions; mechanism and kinetics of methanol oxidation over silver surface”, Surface Science, 544, 1-4. [3] A.P.V. Soares, M.F. Portela, A. Kiennemann, L. Hilaire and J.M.M. Millet (2001) “Iron molybdate catalysts for methanol to formaldehyde oxidation; effect of Mo excess on catalystic behaviour”, Applied catalysis, 206, 221-229. [4] K. Ivanov and I. Mitov, (2000) “Selective oxidation of methanol on Fe-Mo-W catalysts”, Journal of Alloys and Compounds, 309(14) 57-60. [5] C.T. Wang and R.J. Willey, (2001) “Mechanistic aspects of methanol partial oxidation over supported iron oxide aerogel”, Journal of Catalysis, 202(2)211-219. [6] E.M. McCarron and A.W. Sleight, in P.C.H. Mitchell and A.G. Sykes (eds.), The Chemistry and Uses of Molybdenum, Proceedings of the Climax Fifth International Conference, Polyhedron Symposia-, Number 2, Pergamon Press, Oxford, 1986, p.129. [7] A.P.V. Soares, M.F. Portela, and A. Kiennemann, Third World Congress on Oxidation Catalysis, By Grasselli, R.K; Oyama, S.T; Gaffney, A.M, Published by Elsevier, (1997), 110, 807-816. [8] K.H. Hassan and K.K. Hummadi, (2004), “Production of formaldehyde by catalytic conversion of methanol”, Iraqi Journal of Chemical and Petroleum Engineering, 5, 33-39. [9] D. Monti ,A. Reller and A. Baiker ,(1985) “Methanol oxidation on K2SO4- promoted vanadium pentoxide catalysts”, Journal of Catalysis, 93, 360-367. [10] I.E. Wachs and L.E. Briand, US Patent 6037290 (2000) to Lehigh University, Bethlehem, PA, USA.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Formation of active component of MoVTeNb oxide catalyst for selective oxidation and ammoxidation of propane and ethane E.V. Ischenko, T.V. Andrushkevich, G.Ya. Popova, V.M. Bondareva, Y.A. Chesalov, T.Yu. Kardash, L.M. Plyasova, L.S. Dovlitova, A.V. Ischenko Boreskov Institute of Catalysis SB RAS, Prosp. Ak. Lavrentieva 5, Novosibirsk, Russia

Abstract The effect of slurry pH on the formation of active component of MoVTeNbO catalyst for selective (amm)oxidation of ethane and propane has been studied. pH affects the nature and composition of the crude and dry precursors as well as chemical and phase composition of the final catalyst. The most effective catalyst is prepared at рН=3.0, which is characterized by a maximum content of M1 phase. Keywords: MoVTeNb mixed oxide; M1, M2 phase; ethane; propane (amm)oxidation

1. Introduction The most effective catalysts to date in the propane and ethane (amm)oxidation are MoVTeNbO ones reported by Ushikubo et al. [1]. Their catalytic properties are determined by the presence of orthorhombic M1 and hexagonal M2 phases [1]. The goal of the present work is to study the effect of slurry pH on the phase formation of MoVTeNbO catalyst.

2. Experimental Mo1V0.3Te0.23Nb0.12On catalysts were synthesized from aqueous slurry according to the patented procedure [1]. 34.267g ammonium heptamolybdate (NH4)6Mo7O24*4H2O, 6.811g ammonium metavanadate NH4VO3 (Reachem, Russia) and 10.247g telluric acid H6TeO6 (Aldrich) were dissolved in 300 ml of water under stirring at 80°C to obtain a uniform aqueous solution (pH ~ 6). Then, upon adding a 50.7 ml niobium oxalate solution (42.7 мг/мл Nb, pH ~ 1) to MoVTe solution at 30°C, the pH drops from 6 to 3, and a bright-orange gel forms. Niobium oxalate solution (C2O42–/Nb = 3/1) was synthesized by the interaction of oxalic acid and made-up niobium hydroxide that was prepared by alkaline hydrolysis of NbCl5 (Acros Organics, 99.8%). Thereafter a lab spray-dryer (Buchi-290, Tinlet = 220°C and Toutlet = 110°C) was used for fast drying of crude precursors. The resulting powders were calcined stepwise in air flow at 320°C shortly and then in He flow at 600°C for 2 h. pH of Mo1V0.3Te0.23Nb0.12On slurry was varied from 1 to 4 by adding HNO3 or NH4OH. The samples prepared with HNO3 were calcined in He flow only. XRD, IR, Raman and atomic absorption spectroscopy, HTEM and differential dissolution (DD) [2] methods were used for characterization of the samples. The propane (amm)oxidation and oxidative dehydrogenation of ethane were carried out in a fixed-bed tubular reactor with on-line chromatographic analysis. Experiments were performed at 380-420°C with the feed consisting of 5%C3H8, 30%H2O, 65% air, 5% C3H8, 6% NH3, 89% air and 30% C2H6, 30% O2, 40% N2 (% mol.).

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3. Results and discussion 3.1. Precursor characterization

The following complexes were identified in the ternary MoVIVVTeVI solutions by NMR spectroscopy [3] at pH = 6: TeMo6O246– (the main one), Te-containing metavanadate derivatives with the average Te/V molar ratio of ca. 1 : 3, and small amounts of TeMo3V3O249–, MoO42– and telluric acid. Upon adding niobium oxalate to the MoVTe solution, the pH drops to 3 and a gel-like four-component material form. At that, MoO42– and mixed TeV and TeMoV complexes disappear, while a new TeMo3V5O275– complex forms. TeMo6O246– (HPA) remains the main complex in both the solution and gel, as evidenced by Raman spectroscopy data [4]. The solid precursor obtained upon drying is amorphous to X-rays (Fig. 1, pH=3-3.5). A FT-IR spectrum indicates the presence of an Anderson-type anion in the dried precursor [5] (Fig. 2). At pH = 4, the

pH=1

10

20

30

40

2 Theta,

o

50

60

Fig.1. Fig.1. XRD patter pattern of the pr preecurso ursor sors.

330

550

685

927 894

460

pH=3 375

pH=2

2000 1600

1000

pH=4 pH=3.5

540

pH=3.5 pH=3

Absorbance

pH=4

930 898

x

795

x

934 904

1714 1670

x

x x

1408

x - (NH4)6TeMo6O24

x

670 620

x

pH=2 pH=1

800

600 -1

400

Wavenumber,cm

Fig. 2. FT-IR FT-IR spec spectra of the pr preecurso ursors. so

(NH4)6TeMo6O24 phase is detected by XRD (Fig. 1) and FT-IR spectroscopy (Fig. 2). A decrease in the slurry pH leads to partial decomposition of HPA, which is indicated by decreasing intensity of the absorption bands assigned to heteropolyanion in the IR spectra of precursors (Fig. 2, pH= 1, 2). At that, a new peak at 2Θ = 22.2o appears in the XRD patterns of precursors synthesized at pH = 1 or 2 (Fig.1). According to DD method, more than 85% wt. of the dried precursor prepared at pH = 3 dissolves in water (DW), the composition being Mo1V0.28Te0.17Nb0.04On. With regard to XRD and FTIR data, we suppose HPA to be the main building block of DW. At pH = 4, along with DW (70%) there are also Mo1Te0.8 and a MoVNb oxide compound enriched with niobium. At pH below 3, the amount of DW decreases to 50% and formation of the MoVNb compound enriched with molybdenum is observed. Heat treatment at 320°C results in destruction of heteropolyanion and formation of nanosize particles with the structure similar to that of M1 and M2 phases (Fig. 3).

3.2. Catalysts characterization The transformation mechanism of amorphous precursors to M1 and M2 phases in 220600oC range was demonstrated previously [6]. Final phase compositions of the catalysts form upon heat treatment at 600°C. XRD patterns of the samples after high-temperature treatment are shown in Fig. 4. The content of constituent phases was calculated by Rietveld refinements of X-ray powder diffraction data (Table 1). Phase composition of the calcined catalysts is determined by the composition of solid precursors. The dependences of the amount of M1 phase and DW on the slurry pH are quite similar (Fig. 5, curve 4, 5). At that, their maximum amount is observed at pH = 3.

Formation of the active component of MoVTeNb oxide catalyst

481

Decreasing or increasing the pH value leads to a decrease in the content of M1 phase and appearance of Mo5–x(V/Nb)xO14 and TeMo5O16. Simultaneously Te content in the heat-treated catalysts varies. Note, minimal Te content is fixed at pH = 3, when maximal M1 phase content and maximal catalysts surface area are observed. o

x

o-phase M1 x-phase M2 Δ-Mo5-x(V/Nb)xO14

x

- TeMo5O16

oo

ΔΔ Δ

Δ Δ

Δ

x

Δ

o

x

x

pH = 4.0

ΔΔ

ooo Δ o oo

o

ooo

o

pH = 3.5 pH = 3.0 pH = 2.0

oo 0

Fig. 3. TEM image of the sample (pH=3) calcined at 350°C in air flow.

10

pH = 1.0 20

30

2 Theta,

o

40

50

60

Fig. 4. XRD patterns of the catalysts.

Table 1. Influence of pH slurry on chemical and phase composition of the catalysts. Phase composition Chemical composition of the catalysts M2b Mo5-x(V/Nb)xO14c TeMo5O16d M1a 1 Mo1V0.29Te0.21Nb013 40 42 18 2 Mo1V0.28Te0.22Nb013 53 48 3 Mo1V0.28Te0.12Nb011 80 15 3 2 58 30 9 3 3.5 Mo1V0.28Te0.16Nb011 4 Mo1V0.28Te0.16Nb011 24 45 17 13 a - [JCPDS 58-790], b - [JCPDS 57-1099], c - [JCPDS 58-788], d - [JCPDS 31-874] pH slurry

3.3. Catalytic performance

Content, % wt.

Conv ersion, %

The catalytic properties are presented in Fig. 5 and Table 2. Dependences of activity on pH slurry for propane oxidation (curve 2), propane ammoxidation (curve 3) and ethane oxidative dehydrogenation (curve 1) as well as content of M1 phase (curve 4) have the same shape. For all the reactions, the maximum activity is observed over the 80 80 5 samples obtained at pH = 3.0 and containing the greatest amount of orthorhombic phase 60 60 M1. Table 2 shows selectivities to main reaction products. Acrylonitrile or acrylic 40 40 acid and propylene are main products of 4 propane (amm)oxidation, selectivity to 20 20 3 sum of propylene and acrylonitrile or 2 1 0 acrylic acid is close to 80%. Changes in 0 1 2 3 4 the ratio of selectivities to propylene and pH slurry Fig. 5. Dependences of ethane (1) and acrylonitrile or acrylic acid are related propan e (2,3) conversion as well as co ntent with different conversion of propane over of M1 (4) phase and DW (5) on pH slurry. the catalysts under consideration and are

482

E.V. Ischenko et al.

caused by the consecutive formation of acrylonitrile or acrylic acid via intermediate propylene [4]. Ethylene is a sole product of selective conversion in ethane oxidative dehydrogenation. The studied catalytic system is most selective in this reaction. Table 2. Influence of pH slurry on catalytic performance. pH

1.0 2.0 3.0 3.5 4.0

S, m2/g 2.6 3.9 9.6 8.2 4.1

X a, %

S, %

Xb, %

S, %

S, %

Xc, %

S, %

S, %

C2H6 17.0 16.9 46.8 23.9 2.0

C2H4 92.9 95.1 93.9 95.0 81.7

C3H8 8.2 13.1 60.0 27.5 6.1

C3H6 51.5 31.8 2.3 23.1 49.0

C3H4O2 30.1 52.0 65.0 48.3 21.4

C3H8 18.4 32.9 68.8 42.9 12.1

C3H6 20.7 10.6 6.3 7.7 33.4

C3H3N 48.8 59.2 68.8 66.0 41.1

a

30% C2H6, 30% O2, 40% N2,emperature 400°C, contact time 2.4 s; b 5%C3H8, 30%H2O, 65% air, temperature 380°C,contact time 2.2 s; c 5% C3H8, 6% NH3, 89% air; temperature 420°C, contact time 2.4 s. X – conversion degree, S - selectivity

4. Conclusion Formation of the active component of MoVTeNb oxide catalysts includes the following stages: (i) formation of compounds with heteropolyanion (HPA) structure at the step of initial solutions mixture, (ii) drying of the wet precursor with the HPA structure being retained, (iii) decomposition of the dried precursor upon low-temperature treatment leading to amorphous product that contains nanosize particles with the structure and composition close to those of M1 and M2 phases, and (iv) crystallization of M1 and M2 phases upon calcination in helium flow at 600°C. The slurry pH affects the nature and composition of the crude and dry precursors as well as the chemical composition and M1/M2 ratio of the final catalyst. The most effective catalyst is prepared at рН=3.0 when a maximum content of M1 phase is formed.

Acknowledgements The authors are grateful to the Federal Agency for Science and Innovations for financial support.

References 1. 2. 3. 4. 5. 6.

T. Ushikubo, A. Oshima, T. Ihara, H. Amatsu, 1995, US Patent 5.380.933. V.V. Malakhov, I.G. Vasilyeva, 2008, Russ. Chem. Rev., Vol. 77, P. 350-372. R.I. Maksimovskaya, V.M. Bondareva, G.A. Aleshina, 2008, Eur. J. Inorg. Chem., No 11, P. 4906-4914. G.Ya.Popova, T.V. Andrushkevich, Yu.A. Chesalov, L.M. Plyasova, L.S Dovlitova, E.V. Ischenko, G.I. Aleshina, M.I. Khramov, 2009, Catalysis Today, Vol. 144, p. 312-317. Evans, Jr.H.T. 1968 J. Am. Chem. Soc. V. 90. No 12. P. 3275-3276. G.Ya.Popova, T.V. Andrushkevich, L.S Dovlitova, G.I. Aleshina,Yu.A. Chesalov, A.V. Ischenko, E.V. Ischenko, L.M. Plyasova, V.V. Malahov 2009, Appl. Catal. A: Gen. Vol. 353, p. 249-257.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Functionalization of carbon nanofibers coated on cordierite monoliths by oxidative treatment Sabino Armenisea,b, Marcos Nebraa, Enrique García-Bordejéa, Antonio Monzónb a

Instituto de Carboquímica, C/Miguel Luesma Castan,4, 50018, Zaragoza, Spain. Departamento de Ingeniería Química y Medio Ambiente, Universidad de Zaragoza, C/Pedro Cerbuna, 50009, Zaragoza, Spain

b

Abstract Carbon nanofiber coating on cordierite monoliths has been functionalized by oxidation treatments conventionally used for carbon materials. The functionalized CNF-monoliths have been characterized to assess the impact of the different oxidizing treatments on the surface chemistry, texture and CNF coating adhesion. Keywords: carbon nanofibers, monoliths, functionalization, TPD, adhesion

1. Introduction Structured reactors such as monoliths have several obvious advantages over the traditional reactor randomly filled with catalyst particles [1]. They have lower pressure drop, uniform flow distributions, less hot-spot formation and uniform residence times. However, these catalytic systems have low surface areas, making it necessary to incorporate some phase like alumina or silica, which increases the surface area. New types of structured catalysts and reactors, which in addition to high surface area, have large pore volume (mesoporous range) and without the presence of micropores, based on carbon nanofibers (CNFs) are being investigated as catalytic support [2,3]. To anchor and disperse active metal phase on CNFs, usually the CNF surface is previously submitted to oxidation treatments which generates surface oxygen complexes. Most of the literature focuses on the study of the functionalization of unsupported carbon nanofibers and carbon nanotubes [4] but works reporting the functionalization of CNF grown on structured reactors such as cordierite monoliths are scarce [5]. Here we have functionalized CNF-coated cordierite monoliths using two conventional oxidation treatments, viz. HNO3 and H2O2, at different conditions of concentration, temperature and durations. We have characterized the monoliths after functionalization to assess the affect of oxidation treatment on the generated oxygen complexes and on the adhesion strength of nanocarbonaceous materials to the structured support. Work is underway to study other unconventional oxidative treatments and supporting Pd catalyst for hydrogenation in liquid phase reactions.

2. Experimental CNF on cordierite monoliths were prepared as reported elsewhere [6]. In brief, first cordierite monoliths were coated with γ-alumina by dipcoating with a sol. Subsequently, Ni was dispersed by ion exchange. Finally, the monolithic catalyst was reduced in a H2 flow at 823 K and CNF growth was carried out at 873 K using C2H6 as the carbon source.

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The CNF-monoliths were functionalized with HNO3 and H2O2 at different conditions. The identification of the most representative samples is displayed in Table 1. AD, AC and HP stands for treatment with diluted 1M HNO3, concentrated HNO3 and H2O2, respectively. T means that the functionalization has been carried out at boiling temperature. The number is the duration of the treatment in hours. The CNF-monoliths after functionalization were characterized by different techniques. Nitrogen adsorption was performed on a Micromeritics ASAP 2020 at 77 K, SEM study was performed with a Hitachi s4300 field emission microscope. TPD was carried out in a quartz microreactor and the gas analyzed with a Pfeiffer mass spectrometer. The adhesion was characterized by ultrasound (frequency 40 KHz) and the metal leaching by ICP-OES. Table1. Code sample identification, textural characterization CNF/Monolith composite and total amount of CO2 and CO calculated from the TPD for different oxidation treatments. Code Sample

Oxidation Treatments

S(BET) (m2/g carbon)

V(pore) (cm3/g carbon)

Amount evolved µmol /g carbon CO2 CO 144 640

CNF(untreated) 150,3 0,30 * AC-1T HNO3 (65%) Refluxed 1h to 353 K 152,8 0,31 326 1310 AC-5T HNO3 (65%) Refluxed 1h to 353 K 246,8 0,47 884 2002 AD-1T HNO3 (1M) Refluxed 1h to 353 K 151,7 0,41 650 1810 AD-5T HNO3 (1M) Refluxed 1h to 353 K 190,5 0,45 460 1296 AD-1 HNO3 (1M) 1h to RT 148,8 0,33 214 1214 AD-5 HNO3 (1M) 1h to RT 154,8 0,37 253 1775 HP-20 H2O2 (30%) 20h to RT 161,9 0,38 232 578 * Surface area and micropore volume calculated by t-plot method (10m2/g carbon and 0,008cm3/g carbon)

3. Results 3.1. Textural properties of the functionalized CNF-monoliths

Textural properties of the samples were investigated by nitrogen adsorption and SEM. Table 1 shows the results of surface area and pore volume. Figure 1 shows SEM images of samples before (CNF-untreated) and after treatment with H2O2. Generally speaking, at the same magnification the samples after treatment show morphology more compact than before functionalization. Additionally, we can possibly see that the sample funcionalized with nitric acid presents a structure with greater porosity than the functionalized with hydrogen peroxide. The reason for that must be that the nitric acid is an oxidising agent more aggressive than hydrogen peroxide, which could eliminate any rest of ashes or non graphitic carbonaceous matter. Comparing all the treatments studied, it is apparent that as the severity of functionalization increases, the surface area increases, which is accompanied by an increase of the pore volume. The increase of the superficial area could be related to the generation of new pores or to the increase of the fibres roughness as reported by other authors [4].

Funtionalization of CNF cordierite monoliths

485

Figure 1. SEM images of carbon nanofibers. a) Untreated, b) treated with H2O2 (HP-20).

3.2. TPD characterization of surface chemistry

TPD methodology and interpretation have followed previous results reported by Figueiredo et al. [7]. Figure 2 shows TPD traces of CNF/Monolith before and after some selected funcionalization treatments. The quantification of evolved CO and CO2 is displayed in table 1. CO2 spectra (Figure 2a) can be divided in two temperature ranges. First range at low temperature (373-673 K) exhibits two peaks. Those peaks are characteristic of carboxylic acid with different thermal stability. This difference in stability may be related to the groups surrounding the carboxylic carbon. Sample functionalized with hydrogen peroxide (HP-20) shows only one peak attributed to carboxylic acids evolving at slightly higher temperatures than for the nitric acid treated. The evolution of CO2 at higher temperature (750-1000 K) is attributed to carboxylic anhydride and lactone groups. Furthermore, from figure 2a we can observe that the increase of acid strength (AC-5T vs. AD-5) leads to increased formation of carboxylic acid groups and to the emergence of more basic groups ascribed to lactones. Figure 2b shows CO evolution which occurs in the temperature range between 750 K to 1173 K. It is possible to associate the peak around 750 K to anhydrides, the shoulder at 1050 K to phenol groups and finally last peak at 1170 K to carbonyl/quinone groups. The sample functionalized with hydrogen peroxide does not exhibit peaks referable to phenol but only one peak referable to quinone/carbonyl in almost similar amounts as for nitric acid treatment.

Figure 2. TPD spectra for the untreated CNF/Monolith and sample after functionalization treatment: (a) CO2 traces; (b) CO traces.

3.3. Testing of CNF coating adherence after functionalization by ultrasonication

The impact of the different functionalization treatments on the adhesion strength of carbon nanofibers on ceramic monoliths has been investigated by means of ultrasonic treatment. Figure 3 a and b show the results of coating loss (% wt) of alumina, nickel and CNF, after 60 minutes of ultrasonic treatment. Figure 3a illustrates the impact of the

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temperature of functionalization on the adhesion strength of carbon nanofibers coating the monolith. After 60 minutes of treatment in ultrasound, approximately 30 wt% of the coating is removed in monoliths functionalized refluxed with 1M nitric acid during 1 h (AD-1T), whereas the functionalized at room temperature (AD-1) only entails a loss of 6 wt% of the coating weight. Figure 3b shows the effect of the type of oxidising agent and acid concentration on the coating adhesion. The samples treated with concentrated acid show a weight loss up to 42 wt% of the initial weight of the coating incorporated at the monolith. We can observe that after functionalization with hydrogen peroxide only 4% weight of coating is removed (figure 3b), similar to CNF without treatment. Possibly, only the outer fibers non-attached to the alumina are removed with the H2O2 treatment, whereas a deeper removal of nanofibers and support-alumina occurs when monoliths are functionalized with nitric acid. The leaching of metals from alumina coating and cordierite monoliths after the most aggressive oxidative treatment was confirmed by analysis of solution by ICP-OES. 40 35 30 25 20 15 10 5 0

50

a)

CNF (untreated) AD-1 AD-5 AD-1T AD-5T

45

(Al,Ni,CNF) Coating weight loss (%)

(Al,Ni,CNF) Coating weight loss (%)

50

10

20

30

40

Time in ultrasonic bath (min)

50

60

CNF (untreated) AD-1T AD-5T AC-1T AC-5T HP-20

40 35 30 25 20 15 10 5 0

0

b)

45

0

10

20

30

40

50

60

Time in ultrasonic bath (min)

Figure 3. Coating weight loss as a function of time of ultrasonication after different oxidation treatment. a) Effect of temperature. b) Effect of acid strength.

4. Conclusions CNF-coated cordierite monoliths have been functionalized with HNO3 and H2O2 at different conditions of concentration, time and temperature. All the tested treatments created oxygen functionalities on the CNFs as characterized by TPD. However, the treatment with concentrated HNO3 is always detrimental for the attachment of the coating. The same occurs when the treatment is carried out under reflux, irrespective of the oxidizing agent. The mechanical stability of CNF coating is preserved after the treatment with H2O2 or diluted HNO3 at room temperature.

Acknowledgement We acknowledge the European commission for financial support (contract 226347).

References [1] [2] [3] [4] [5]

A. Cybulski, J. A. Moulijn, Catal. Rev-Sci. Eng., 36(2) (1994) 179. K P. De Jong, J. W. Geus, Catal. Rev.-Sci. Eng., 42(4) (2000) 481. P. Serp, M. Corrias, P. Kalck, Applied Catal. A: General, 253 (2) (2003) 337. T.G. Ros, A.J. van Dillen, J. W. Geus, D. C. Koningsberger, Chem. Eur. J., 8 (5) (2002) 1151. S. Morales-Torres, A.F. Pérez-Cadenas, F. Kapteijn, F. Carrasco-Marín, F. J. MaldonadoHódar and J. A. Moulijn, Applied Catal. B: Environmental, 89 (3-4) (2009) 411. [6] E. Garcia-Bordeje, I. Kvande, D. Chen, M. Ronning, Advanced Materials, 18 (12) (2006) 1589. [7] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J. J. M. Órfão, Carbon 37 (9) (1999) 1379.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Synthesis of mesoporous silicas functionalized with trans (1R,2R)-diaminocyclohexane by sol-gel method F. Fakhfakh,a L. Baraket,a A. Ghorbela, J. M. Fraile,b J. A. Mayoralb a

Laboratoire de Chimie des Matériaux et Catalyse, Département de Chimie, Faculté des Sciences de Tunis, Campus universitaire, 2092 El Manar Tunis (Tunisie) b Departamento de Química Orgánica, Facultad de Ciencias. ICMA, Universidad de Zaragoza C.S.I.C. E-50009 Zaragoza (Spain)

Abstract N-[3-(Triethoxysilyl)propyl]-(-)-(1R,2R)-diaminocyclohexane was co-condensed with tetraethoxysilane (TEOS) under different synthesis conditions to obtain new functionalized hybrid silica materials. These materials were characterized by different spectroscopic methods. N2 sorption studies were used to confirm the mesoporous character of these functionalized materials. Keywords: trans-(1R,2R)-diaminocyclohexane, sol-gel, synthesis conditions

1. Introduction Recently the synthesis and the design of chiral mesoporous silica materials have attracted great interest because of their potential applications in heterogeneous asymmetric synthesis. The most important step in the preparation of these materials is the introduction of the chiral moiety in mesoporous silica. The co-condensation of two alkoxides is one of the most suitable methods, as it allows a homogeneous distribution of the organic groups on the materials. A wide variety of chiral groups has been incorporated into mesoporous silica, among them trans-(1R,2R)-diaminocyclohexane [1,2,3]. In this work, we report the synthesis of new trans-(1R,2R)-diaminocyclohexane functionalized mesoporous organosilica materials by sol-gel under different conditions.

2. Experimental 2.1. Synthesis of N-[3-(triethoxysilyl)propyl]-(-)-(1R,2R)-diaminocyclohexane (D)

D was synthesized by reacting trans-(1R,2R)-diaminocyclohexane and (3-chloropropyl) triethoxysilane under microwave heating on a CEM Discover apparatus. The reaction was achieved at 140°C for 45min. The yield was 52%. The obtained chiral molecule was characterized by FT-IR, 1H and 13C NMR spectroscopies. The resulting spectra are in accordance with those found in previous works [1,2].

2.2. Synthesis of functionalized organosilica materials The synthesis were conducted at 35°C either in water or propanol as a solvent, with HCl, CH3COOH or C2H5COOH as a catalyst. In propanol, the molar composition of the mixture was TEOS:D:propanol:acid:water = 1:0.018:6.5:1:6. A solution of 0.04 mol of TEOS, 0.018 mol of D and 20 mL of propanol was stirred for 30 min followed by the addition of the acid. Two hours later, the desired amount of H2O was added. In water, the molar composition was TEOS:D:acid:water = 1:0.018:1:27. In a typical synthesis 0.04 mol of TEOS, 0.018 mol of D, 20 mL of water and the corresponding amount of

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acid were mixed together. In both cases the gel was kept in the reaction medium for an additional period of 24 h. After that, it was aged in a Teflon-lined autoclave at 120°C for 24 h, then oven dried at 120°C for 24h.

2.3. Characterization

N2 physisorption, FT-IR, 29Si and 13C-CP-MAS-NMR spectroscopies were carried out as previously described [4]. 1H and 13C NMR spectra in solution were recorded on a Bruker AC-300 spectrometer.

3. Results and discussion 3.1. Preparation and textural characterization of silica materials containing trans-(1R,2R)-diaminocyclohexane The synthesis of hybrid organosilica containing trans-(1R,2R)-diaminocyclohexane was achieved D-S1 by sol-gel under different synthesis conditions. It was conducted either in water or in propanol as D-S2 solvent in the presence of an acid catalyst: HCl, CH3COOH or C2H5COOH. The choice D-S5 of propanol and a carboxylic acid was based on previous results dealing with the synthesis Relative pressure (P/P°) of ethylenediamine-functionalized mesoporous silica materials by the co-condensation method [4], showing that this combination favors the formation of functionalized materials with developed textural properties. The synthesis conditions D-S3 of the functionalized silica and their textural D-S4 properties are listed in Table 1. The N2 isotherms are represented in Figure 1. As can be seen, all Relative Pressure (P/P°) the trans-(1R,2R)-diaminocyclohexane functionFigure 1. N 2 isotherms of silica conalized materials exhibit a type IV isotherm sugtaining trans-(1R,2R)-diaminocyclogesting the formation of mesoporous materials. hexane. N2 isotherms of the samples D-S1, D-S2 and DS5 are similar. They show a H2 hysteresis loop type, indicative of the development of ink-bottle pores. 900

3 Adsorbed volume (cm /g)

800 700 600 500 400 300 200 100 0

0,0

0,2

0,4

0,6

0,8

0,0

0,2

0,4

0,6

0,8

1,0

1000

3

Adsorbed Volume (cm /g)

900 800 700 600 500 400 300 200 100 0

1,0

Table 1. Synthesis conditions and textural properties of the functionalized silicas. Sample

Solvent

Catalyst

SBET (m²/g)

Dmax (Å)b

D-S1

water

HCl

659

35

0

0.64

D-S2

propanol

HCl

663

35

0.05

0.45

D-S3

propanol

CH3COOH

124

n.d.a

0

0.49

252

a

D-S4 D-S5

propanol water

C2H5COOH C2H5COOH

466

Vmic Vp (cm3/g)c (cm3/g)d

n.d.

0.01

0.84

74

a

0.91

n.d.

a

n.d. :not determined. Maximum of the pore size distribution. c Microporous volume calculated by t-plot method. d Total pore volume at P/P0=0.95. b

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In the case of the samples D-S1 and D-S5 prepared in water with HCl and C2H5COOH respectively, the N2 isotherms show a large hysteresis loop which occurs under a wide range of relative pressure values. For D-S1, they are ranged between 0.37 and 0.89 and for D-S5 P/P° values varied between 0.55 and 0.94. As a consequence both solids show high total pore volume (0.64cm3/g for D-S1 and 0.91 for D-S5). In the case of the synthesis in propanol with HCl (sample D-S2), the total pore volume is reduced to 0.45cm3/g, confirmed by the narrow hysteresis loop, occurring under low relative pressure values (0.38-0.74). The pore size distribution of these solids (not shown) is narrow and monomodal with a maximum corresponding to 35Ǻ for D-S1 and D-S2 and 74Ǻ for D-S5. Table 1 reveals that the surface area for these solids is high (466 -663 m²/g). On the other hand, N2 isotherms of the samples D-S3 and D-S4 prepared in propanol, are similar. They exhibit a H1 hysteresis loop type, which occurs at high relative pressure values, indicating the formation of large and uniform cylindrical pores. The pore size distribution (not shown) reveals the formation of large mesopores which maximum could not be determined. Surface areas are lower than those of the samples prepared in water, 124 m²/g for D-S3 and 252 m²/g for D-S4. From these results, it can be seen that D-S1, D-S2 and D-S5 have similar textural properties, probably due to the similar mechanism of formation, as proposed by Brinker [5], consequence of the use of water as solvent or a strong mineral acid in alcoholic solution. Samples D-S3 and D-S4 show similar textural properties, but markedly different from D-S1, D-S2 and D-S5. The use of carboxylic acids in propanol might modify the mechanism of the sol-gel process. As reported before [4], carboxylic acids may modify the alkoxydes structure (TEOS and D), generating different types of precursors. The different hydrolysis rate of each precursor under sol-gel conditions would modify the textural properties. Jiang et al. [2] prepared the trans-(1R,2R)-diaminocyclohexane functionalized material by the co-condensation of TEOS and D under basic conditions, obtaining a porosity of 25Ǻ with a surface area of 890m²/g. Bied et al. [3] prepared the same kind of material by the co-condensation in water in presence of a primary amine. The chiral solids are generally microporous with a surface ranged between 600 and 1170m²/g. From the results of these works, it is worth to note that the porosities of the trans(1R,2R)-diaminocyclohexane functionalized materials are less developed than those prepared in this work. Thus the acidic conditions used here are favorable to obtain large porosities.

3.2. Spectroscopic characterization of the hybrid materials FT-IR spectra of hybrid organosilica D-S4 materials are collected in Figure 2. The spectra show the presence of typical D-S3 silica bands relative to the inorganic backbone. The vibration bands of D-S2 siloxane groups appear at about 464, D-S1 804 and 1080 cm–1. The bands at 960 –1 and 1650cm are respectively associated to the frequency of Si-OH bending and to the angular vibration of water mole4 00 0 35 0 0 30 0 0 2500 2000 1500 1000 500 -1 cules bonded to the framework. The W avenum bers (cm ) Figure 2. FT-IR spectra of chiral solids. large band centered at 3500 cm–1 is relative to OH stretching frequency of the silanol groups in the inorganic framework. The spectra show also vibrations at 2960 and 2850cm–1 which are assigned to C-H

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stretching of the organic moiety D. The -57 band at 1450cm–1 is the characteristic C-H vibration of cyclohexane of the -65 trans-(1R,2R)-diaminocyclohexane group. The N-H and N-H2 vibration bands overlap with ν(O-H) bands at 3500 cm–1 and 1630 cm–1. D-S1 NMR spectra of the different mateD-S4 rials are gathered in Figure 3 and 4. 29SiCP-MAS NMR spectra of the materials 0 -20 -40 -60 -80 -1 00 -1 20 -140 -16 0 -18 0 ppm D-S1 and D-S4 represented in Figure 3 29 show the presence of signals at −110, Figure 3. Si MAS NMR spectra of D-S1 and −101 and −92 ppm. These bands corres- D-S4. pond to Si(OSi)4 (Q4), (HO)Si(OSi)3 (Q3) 1, 4,7,8,9,10 and (HO)2Si(OSi)2 (Q2) silica species respectively. Moreover, the spectra show additional low-intensity signals at −65 and −57ppm. The signal at −65ppm is 2,5,6,11 3 assigned to CSi(OSi)3 (T3) species, whereas the signal at −57ppm is associated to C(OH)Si(OSi)2 (T2) species. The presence of these bands confirms that the chiral organic moiety is covalently bonded to (a) the silica framework. The 13C-CP-MAS70 60 50 40 30 20 10 0 NMR spectrum of D-S1, together with that ppm of the precursor in solution, are represented (b) in Figure 4. The spectrum of the solid shows prominent signals at 18 and 58 ppm, corresponding to Si-O-CH2-CH3 groups, not fully hydrolyzed in the sol-gel process. The signals in the range of 45-65 ppm, attributed to N-CH and N-CH2 groups, at 20-35 ppm corresponding to methylene 60 40 30 20 10 0 50 groups of diaminocyclohexane and propyl ppm backbones, and the signal at 10 ppm, 13 ascribed to CH2-Si, demonstrate the pre- Figure 4. C NMR spectra of (a) D-S1 and (b) the liquid precursor D. sence of the chiral moiety on the solid. Thus FT-IR and CP-MAS-NMR spectra confirm that the trans-(1R,2R)diaminocyclohexane moiety was successfully incorporated into the mesoporous silica framework. 5

7

6

8

9

11

10

NH

3

4

Si(OCH2CH3)3 2

1

NH2

4. Conclusion Trans-(1R,2R)-diaminocyclohexane functionalized silica materials were successfully prepared by sol-gel method through the co-condensation of TEOS and precursor D under acidic conditions. The use of propanol as a solvent and a carboxylic acid as a catalyst in the sol-gel process allows obtaining materials with large pores and moderate surface area, whereas the process in water leads to solids with high surface area and narrow pore distribution of around 35 Å. Due to their interesting textural properties,

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these mesoporous materials are promising as solid chiral ligands in enantioselective heterogeneous catalytic reactions.

References [1] A. Adima, J. J. E. Moreau, M. Wong Chi Man, Chirality 12 (2000) 411. [2] D. Jiang, Q. Yang, J. Yang, L. Zhang, G. Zhu, W. Su, C. Li, Chem. Mater. 17 (2005) 6154. [3] C. Bied, D. Gauthier, J. J. E. Moreau, M. Wong Chi Man, J. Sol-gel Sci. Technol. 20 (2001) 313. [4] F. Fakhfakh, L. Baraket, J. M. Fraile, J. A. Mayoral, A. Ghorbel, J. Sol-gel Sci. Technol. 52 (2009) 388. [5] C. J. Brinker, J. Non-Cryst. Solids 100 (1988) 31.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Physico-chemical and catalytic properties of effective nanostructured MnCeOx systems for environmental applications Francesco Arenaa,b,*, Giuseppe Trunfioa,§, Jacopo Negroa, Cettina Sajaa, Antonino Raneria, Lorenzo Spadaroa,b a

Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Università degli Studi di Messina, V.le F. Stagno D’Alcontres 31, I-98166 Messina, ITALY b Istituto CNR-ITAE “Nicola Giordano”, Salita S. Lucia 5, I-98126 Messina, ITALY § current affiliation: Université de Franche-Comté, Chrono-environnement, UMR 6249 UFC/CNRS usc INRA, Place Leclerc 25030 Besançon Cedex, FRANCE

Abstract A synthesis route based on the occurrence of redox reactions between MnVII, MnII and CeIII precursors leads to nanostructured MnCeOx systems with high surface area (120250 m2/g) and a quasi-atomic dispersion of the active phase. Larger accessibility and higher oxidation state enhance the redox activity of the surface active Mn sites resulting in an improved mobility and availability of surface oxygen that greatly promotes the CO oxidation activity of the MnCeOx system at low temperature (20-150°C). Keywords: oxide catalyst; nanostructure; dispersion; oxygen mobility; redox activity

1. Introduction The increasing levels of industrial pollution is pressing worldwide a great research concern on catalytic technologies for the abatement of noxious organic pollutants in gas-exhausts and wastewaters, mostly based on total oxidation reactions. Although uncommon targets and reaction conditions hinder the assessment of general rules for catalysts requirements yet, according to principles of oxidation catalysis an enhanced mobility and availability of oxygen at the catalyst surface constitutes the basic condition for an effective conversion of organic substrates to carbon dioxide [1]. This explains the exploitation of noble-metal catalysts [1-3], whose high cost remains the main drawback before an extensive development of environmental catalytic processes. On this account, a great deal of research concern has been focused onto MnCeOx systems, as a viable less-costly alternative to noble-metal catalysts [1,2]. However, the preparation method is crucial for tuning the catalytic behaviour of the title system, since high surface exposure and higher oxidation state of the active Mn sites are the main catalyst requirements [1-5]. Therefore, in an attempt to improve the total oxidation catalytic performance, we designed an alternative synthesis route of the MnCeOx system based on redox reactions of suitable oxide precursors [1-5]. In fact, larger surface exposure and oxide dispersion promote the reducibility and the oxidative strength in comparison to the conventional co-precipitation method [1-5]. Therefore, this work shows some fundamental results documenting that the nanosized arrangement of the oxide domains in a wide range of the Mn loading (9-33 wt%) confers a superior electron and oxygen mobility to redox MnCeOx systems, enhancing the reactivity at low temperature (20-150°C) in the CO oxidation, taken as a “model” reaction.

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2. Experimental 2.1. Catalysts

MnCeOx catalysts with different Mn-to-Ce atomic ratio (Mn/Ce) ranging between 0.33 and 2 were prepared via the “redox” route, consisting in the titration of a KMnO4 solution at ca. 60°C under stirring with an aqueous solution of Ce(NO3)3 and Mn(NO3)2 precursors at constant pH (8.0±0.2) [1-5]. The following main redox reactions: ⎧MnO4− + 3e − + 2 H 2 O → MnO2 ⇓ +4 HO − (1), ⎪ 2+ − − (2), ⎨Mn + 4 HO → MnO2 ⇓ +2e + 2 H 2 O ⎪ 3+ (3), Ce + 4 HO − → CeO2 ⇓ + e − + 2 H 2 O ⎩

account for the “sticking” of the MnOx and CeOx phases at a quasi-molecular level, due to the required contact of Mn and Ce ions for the electron-transfer prompting the precipitation of both oxide species [3,4]. A reference MnCeOx catalyst (Mn/Ce, 1.0) was obtained via the co-precipitation route of the MnCl2 and CeCl3 precursors [1-5]. All the solids were dried at 100°C and further calcined in air at 400°C (6h). The list of samples with the relative physico-chemical properties is given in Table 1. Table 1. List of the catalysts and main physico-chemical properties. Catalyst

prep. meth.

Mn/Ce

[Mn] (wt%)

SA (m2/g)

PV (cm3/g)

APD (nm)

M1C3-R M1C1-R M3C2-R M2C1-R M1C1-P

redox redox redox redox co-prec.

0.34 0.95 1.44 2.12 1.00

9.3 20.5 26.6 32.7 21.2

168 154 157 140 101

0.28 0.49 0.45 0.50 0.24

5.1 11.7 14.3 16.7 9.4

2.2. Methods

The physico-chemical characterization was carried out by BET, XRD, XPS, H2-TPR (5% H2/Ar) and CO-TPR (5% CO/He) techniques, while the CO oxidation activity was probed in the range of 100-150°C under kinetic regime by feeding a CO/O2/He reaction mixture in the molar ratio of 2/1/22, at the rate of 0.1 stp L/min on powdered catalyst samples (0.035 g), diluted with granular SiC in the weight ratio of 1/10. 3.0

M1C3-R M1C1-P M1C1-R M3C2-R M2C1-R 20

30

40 50 60 2θ (degree)

70

B)

2.5

(Mn/Ce)XPS

Intensity (a. u.)

A)

2.0 REDOX 1.5 1.0 CO-PRECIPITATION

0.5 0.0 0.0

0.5

1.0 1.5 2.0 (Mn/Ce)XRF

2.5

3.0

Figure 1. (A) XRD patterns and (B) surface chemical composition (XPS) of redox and co-precipitated MnCeOx catalysts.

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3. Results and discussion The physico-chemical characterization data in Table 1 show a strong enhancement in the texture of the redox catalysts, evidenced by surface are (SA) and pore volume (PV) values much larger than those of the co-precipitated one [1-5]. At variance of the coprecipitated system, showing the diffraction lines of cerianite and, to a lower extent of pirolusite [1,4], the XRD data (Fig. 1A) of the redox systems show analogous featureless patterns, irrespective of the composition. This indicates that the improved catalyst texture reflects the lack of any “long-range” crystalline order, as a consequence of the quasi-molecular mixing of the oxide phases, hindering the formation and growth of whatsoever crystalline domains [1,3]. This peculiar architecture also accounts for the quite regular pore size distribution [1-3] and the tinier average pore diameter (APD) of the redox systems (Table 1). Notably, further evidences on the homogeneity of the redox catalysts at microscopic level are provided by XPS data in Figure 1B. In spite of the high loading (9-33 wt%), the oxide dispersion keeps constant in the whole Mn/Ce ratio range, resulting even in a considerable surface enrichment of the active phase never observed for the co-precipitated systems [1]. This still depends on the singular characteristics of synthesis route that enables very effective reciprocal oxide dispersion [1-4] and, consequently, a strong synergism between ceria carrier and the active phase. Such an effective interaction promotes the electron transfer processes enhancing the oxygen mobility and the oxygen activation functionality at low temperature [1-3]. This is evident from TPR patterns under both H2 and CO, shown in Figure 2. The superior rate of H2 consumption at T 6.5 which results in the formation of Fe2O3 on the surface of TiO2 during calcination.

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Recently, ion implantation by using magnetron sputtering has been reported to yield doped photo catalysts of improved photo catalytic activity, higher than that of corresponding pure TiO2. It has been suggested that doped metals located in substitution positions of the lattice of TiO2 enhance the photo catalytic activity. While catalysts with heterogeneous phase composition like extra-lattice transition metal species or oxides were less active [4]. Encouraged by these findings, a novel chemical vapour deposition doping method has been developed in this work specially with Fe doping. The photo catalytic performance of prepared Fe-doped TiO2 materials has been investigated. The improvement of photo catalytic activity of Fe-doped TiO2 under visible light could be proven.

2. Experimental The actual Fe content of doped TiO2 was determined by atomic absorption flame emission spectroscopy (Shimadzu AA-6400F). X-ray diffraction patterns of materials were measured with a Shimadzu XRD-6100 analyzer with Cu Kα radiation (1=1.5417Å). The X-ray photoelectron spectroscopic (XPS) investigations were carried out with a Shimadzu ESCA-3200 spectrometer in order to analyze the surface elemental composition and valence state of elements of the photo catalysts. Diffuse reflectance UV-vis spectra of the catalysts were measured using a Shimadzu UV-2200A and a Shimadzu UV-vis spectrophotometer. The FT-IR spectra of the samples were measured using KBr pellets (BIO-RAD FTS-3000). The photo catalytic activity of the TiO2 and Fe-doped TiO2 photo catalysts were investigated using the oxidation of i-propanol under visible light irradiation. The experiments were carried out at room temperature. The reaction mixture was irradiated with a Hg lamp through a colour glass filter (L-42, Asahi Techno Glass). The reaction was carried out in a Pyrex tube containing TiO2 powder (25 mg) and an aqueous solution of 2.5 x 10–3 M i-propanol (50 ml). The solution was bubbled with oxygen at a rate of 30 ml min−1 for 30 min. The amount of acetone formed during the course of reaction was determined on a Shimadzu GC-14A gas-chromatograph equipped with a PEG-1000 column.

3. Results and discussion 3.1. Catalyst preparation A selected amount of TiCl4 was added dropwise into i-propanol under stirring. The resulting solution was introduced into distilled water under a vigorous magnetic stirring. Thereby, the solution was cooled in an ice-water bath. Then the pH value of the obtained acidic solution was adjusted to 7 by adding an NH4OH solution. Thereby a white gel was formed [2]. The resulting gel was aged at room temperature for 24 hours under stirring. The obtained white precipitate was filtered and washed repeatedly with distilled water until the removal of chloride ions was complete. Thereafter, the precipitate was re-dispersed in water by treating in an ultrasonic batch. Then, a 30% aqueous solution of H2O2 was added dropwise into this mixture under stirring. The resulting yellow transparent solution was poured into an autoclave and heated at 100oC for 20 h. After hydrothermal treatment, the precipitate was removed, washed and dried at 100oC to get the TiO2 powders. Fe doping was carried out by high temperature chemical vapour deposition (CVD) using FeCl3 as Fe source. The reactor was a quartz tube (2cm x 25 cm) in which a selected amount of FeCl3, was introduced at one side. On the opposite side, separated by quartz filter, was placed a selected amount of C. A scheme of the experimental set up

Novel method for doping of nano TiO2 photocatayst by chemical vapor deposition

499

for titania doping using high temperature chemical vapour deposition and postdeposition thermal treatment in inert gas is shown in Fig. 1.

TiO2

FeCl

Quartz

N2 flow Quartz

Quartz

Furnace

Figure 1. Scheme of TiO2 doping with Fe by CVD method.

For titania doping, FeCl3 was evaporated by heating at 350oC using N2 as carrier gas. The amount of deposited iron depends on the selected amount of used FeCl3 and titania, the temperature and the treatment time. After completing deposition, the samples were further heated at 500oC in N2 flow for 2h to remove all excess of chloride from samples. For comparison, additional Fe-doped TiO2 samples were prepared by impregnation of TiO2 with FeCl3 solution.

3.2. Characterization and photo catalytic testing The XRD patterns confirm that prepared titania and Fe-doped titania samples consist of pure anatase (Fig. 2). Other crystalline by-products have not been detected. This indicated that after TiO2 modification by doping with iron, the crystals structure of anatase still remained. After Fe-doping, no Fe2O3 crystalline phase was detected. This indicated that Fe has been well dispersed within the TiO2 matrix. 2500

2000

TiO2-Fe 1.82% 1500

TiO2-Fe 0.12%

1000

TiO2-Fe 0.12% (*) 500

TiO2

0 20

30

40

50

60

70

80

Figure 2. XRD patterns of TiO2 and Fe- doped TiO2 by CVD and impregnation method (*).

UV-vis absorption spectra showed the fundamental absorption edge of TiO2, appearing at about 385 nm. Iron doping leads to a red shift and increased absorbance in the visible light range with increasing doping content. This red shift may be attributed to a charge transfer transition between the iron d orbital and the TiO2 conduction or valence band [3]. XPS spectra (Fig. 3) of pure TiO2 and doped-TiO2 show that Ti 2p1/2 and Ti 2p3/2 peaks are located at binding energies of 464.2 and 458.5 eV, respectively, in excellent agreement with the values of Ti4+ in pure TiO2 [5].

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I n t e n s it y

3 2 1 1100 1000

900

800

700

600

500

400

Binding energy/eV

300

200

100

0

Figure 3. XPS spectra of TiO2 (1), TiO2-Fe 0.12% (2) and TiO2-Fe 1.82% (3) samples.

The FTIR lattice spectra reveal the formation of defect sites in the titania matrix after Fe doping at very low Fe content. Even these samples are photocatalytically active in the oxidation of i-propanol to acetone prior to all other catalysts. This finding tends to show that the improvement of photocatalytic activity might be related to the appearance of defect sites. After 10h of reaction, i-propanol conversion reached the value of 70%.

4. Conclusion The presented high temperature CVD procedure using metal chloride starting materials is an effective and easy-to-handle method for the preparation of improved metal doped photocatalytic materials. The role of defect sites in the catalytic performance needs further verification.

Acknowledgement This work was supported by the DAAD which is gratefully acknowledged.

References [1] A. Di Paola, G. Marci, L. Palmisano, M. Schiavello, K. Uosaki, S. Ikeda, B. Ohtani, Preparation of Polycrystalline TiO2 Photocatalysts Impregnated with Various Transition Metal Ions: Characterization and Photocatalytic Activity for the Degradation of 4-Nitrophenol. J. Phys. Chem. B 106 (2002) 637. [2] H. Kato, A. Kudo, Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 106 (2002) 5029. [3] W. Choi, A. Termin, M.R. Hoffmann, The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics. J. Phys. Chem. 98 (1994) 13669. [4] M. Anpo, M. Takeuchi The design and development of highly reactive titanium oxide photocatalysts operating under visible light irradiation. J. Catal. 216 (2003) 505. [5] E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca, M. Grätzel, Hydrogen production from water by visible light using zinc porphyrin-sensitized platinized titanium dioxide. J. Am Chem. Soc. 103 (1981) 6324.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Study on the preparation of active support and multi-porous supported catalyst Vu A. Tuan*, Bui H. Linh, Dang T. Phuong, Tran T.K. Hoa, Nguyen T. Kien, Nguyen H. Hao, Hendrik Kosslick*a, Axel Schulza Institute of Chemistry, Vietnamese Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam a

Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29a, Rostock 18059, Germany * Corresponding author. Prof. Dr. Vu Anh Tuan, Tel: 04.8361145. E-mail: [email protected]; Dr. Hendrik Kosslick, Tel. 0049381 4986384, E-mail [email protected]

Abstract The preparation of improved catalytic materials based on natural diatomite is reported. Aluminum incorporation by an atomic implantation methods yields supports with enhanced acidity. Nano-zeolite Y supported diatomite composite materials with a multimodaö distribution of interconnected micro- meso- and macro pores were obtained by in situ crystallization. Materials were characterized by XRD, TEM, NH3-TPD and textural studies using nitrogen adsorption / desorption measurements. The catalytic performance was tested in the cracking of a heavy petroleum residue. The results show, that diatomite due to its large pores is a superior support for the preparation of supported catalysts for the cracking of heavy oil fraction containing bulky molecules. Atomic implantation of aluminum yields catalysts of the same catalytic performance as loading with acidic nano-sized zeolite HY. Keywords: catalyst preparation, heterogeneous multi porous catalyst, zeolite composite, acidity, FCC

1. Introduction Due to the limited availability of fossil feedstock, the manufacture of high value fuel products from low value feedstock like heavy oil residues, oil sands or biomass has received great interest of research and manufacturing. Fluidized catalytic cracking (FCC) is one of the most important processes to produce gasoline and diesels. FCC catalysts consist of active components like zeolite Y, rare-earth modified zeolite-Y and USY or supported silica and /or aluminosilicates. However, they suffer from large crystal size (1-5µm) and narrow pore dimension (0.74 nm for zeolite Y) limiting access of active sites by large molecules such as polyaromatics. Improvement of catalytic performance and considering sustainability aspects needs the development of new generation of FCC catalysts. These catalysts should possess multi-modal pore structures of interconnected macro- meso- and micropores. They should allow easy access of heavy feedstock compartments to catalyst active sites. In this way large polyaromatics can be pre-cracked in the mesopores. Further cracking at acidic sites in the micropores converts the pre-cracked compounds into valuable fuel components and other valuable chemicals. Nano-sized crystals may improve the efficiency of catalyst by making use of the external surface [1-5]. Moreover, the catalyst supports are usually catalytically inactive or less active. The catalytic functionalization of the support with acidic

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aluminium site can additional improve the catalytic performance by synergetic effects between the active components and the matrix. In this paper, we report the recent results on the acidification of diatomite (Phu Yen-Vietnam) by atomic implantation aluminum and the preparation of nano-zeolite Y supported diatomite by in situ crystallization method.

2. Experimental Starting materials. Nano-zeolite Y was prepared by hydrothermal crystallization using the natural starting material kaolin. For this, the kaolin was calcined at 550°C. The Diatomite support (Phu Yen-Vietnam) was calcined at 500°C, treated with 1 M hydrochloric acid under reflux..

2.1. Characterization and catalytic testing of samples Samples were characterized by XRD (Siemens D5000 XRD spectrometer), FE-SEM (Field Emission Scanning Electron Microscope, Hitachi S-4800), textural properties were determined by N2 adsorption/desorption experiments) and NH3-TPD (temperatureprogrammed desorption of ammonia). Catalytic properties of samples were tested in the cracking of a Bach Hổ petroleum residue from Vietnam (370-500°C fraction) as heavy feedstock using a MAT 5000 micro activity testing system. The reaction conditions were 482°C, WVH=27, catalyst to feedstock ratio 3/1, reaction time 45 sec.

3. Results and discussion 3.1. Catalyst preparation 3.1.1. Acidification of diatomite Acidification of diatomite was carried out in a tubular reactor as presented in Fig. 1. Purified diatomite and excess of aluminum chloride, separated by aquartz filter, were placed into a tubular reactor. The reactor was heated from room temperature to the final temperature of up to 500° with a heating rate of 10K/min. The evaporated AlCl3 was passed through the diatomite with nitrogen carrier gas (N2). The flow rate was 50 mL/min. reaction time: 0.5- 2 h. Then the sample was calcined at 500°C to remove excess of chloride.

Fig. 1. Reactor set up for the acidification of diatomite by atomic implantation method.

3.1.2. Preparation of nano-zeolite Y and supported diatomite Nano-zeolite Y supported diatomite was synthesized according to a 2 steps procedure: Step 1: Preparation of nano-zeolite Y seeds and seeded diatomite. Nano-zeolite Y seeds were prepared according to the following: A solution of water glass and sodium hydroxide solution were added to meta-kaolin under vigorously stirring to form a homogeneous gel having the molar composition: 16NaOH:Al2O3:10 SiO2:720H2O. After aging at 25°C for 24 hours, the gel was transferred into an autoclave and crystallized at

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503

80°C for 20h under autogeneous pressure. The nano zeolite Y seeds were then mixed with purified diatomite assisted by ultrasonic agitation and at 100°C for 15h. Step 2: Nano-zeolite Y seeded diatomite from step 1 was introduced into same the gel that was used for the preparation of zeolite Y seeds under vigorous stirring to form a homogeneous mixture. The gel was aged one day at room temperature. The aged gel was transferred into an autoclave and crystallized at 90°C for 20 h under autogeneous pressure. The products were filtered, washed with deionised water and dried at 120°C over night. To obtain the acidic form, the products were ion exchanged 3 times with a 2 molar aqueous NH4NO3 solution. The obtained product was filtered off, dried and calcined at 500°C for 2 h.

3.2. Characterization and catalytic testing The crystallinty of as-synthesized zeolite Y nano particles and nano-sized supported HY has been checked by XRD. Zeolite synthesis using kaolin as starting material yields crystalline zeolite Y. The diatomite is x-ray amorphous. The seeded diatomite shows the XRD pattern of zeolite Y, however, the reflections are of low intensity. Based on the comparison of peak intensities with pure zeolite Y, the amount of nano-zeolite Y supported on diatomite was ca. 15%. Hence, diatomite allows a substantial loading with acidic active nano-sized zeolite Y components. This is obviously closely related to the macro-meso porous interconnected pore structure with large open void volumes. (c)

(a)

(b)

Fig. 2. FE-SEM photographs of diatomite (middle), nano-zeolite Y supported diatomite (right), and corresponding surface zoom (left).

The morphology and porosity of nano-zeolite Y supported diatomite was examined by FE-SEM (Fig. 2) The µm–sized diatomite support consists of cylindrically shaped tubes. The tubes have large-sized free internal pores. They are additionally accessible from the surface of the tubes by large mesopores (Fig. 2, middle). It was observed that nano-zeolite Y, providing micropores, was formed on the surface of diatomite. Additionally, voids of 20–50 nm size were formed. (Fig. 2, right). The nano-zeolite Y crystal size varied from 20 to 35 nm (Fig. 2, left). This result was also confirmed by N2 adsorption/ desorption measurement. In the NH3–TPD profiles of the samples, two peaks: a low temperature peak (150°C-180°C) and a high temperature pick (350°C-420°C) could be identified belonging to weak and strong acid sites were observed. For the Al-modified diatomite, a large amount of strong acid sites desorbing ammonia at 420oC (maximum) was noted. Besides strong sites, diatomite modified with nano-zeolite HY showed a large amount of weak acid sites (maximum desorption temperature of 185°C ).

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Catalytic testing reveals high cracking activity for Al-modified diatomite. In contrast, pure diatomite alone is not active. Under the same reaction conditions, acidified diatomite exhibited somewhat higher conversion and selectivity to LPG and gasoline compared to nano-zeolite Y supported diatomite (Tab. 1). This result is consistent with data obtained from NH3 –TPD that acidified diatomite has higher acidity compared to that of nano-zeolite Y supported diatomite. For both samples, the main products were gasoline, light and heavy cycle oil (LCO and HCO). Especially, very low yield of light gases was noted. Also, low yield of coke was observed. Table 1. Reaction conditions, conversion and products yields in cracking of Bach Ho residue over nano-zeolite Y supported diatomite and acidified diatomite. Catalyst Name Product Yield (wt%) H2 Coke Total C3 Total C4 LPG (C3,C4) Gasoline (25 oC ~216 oC) LCO (216oC~360 oC) HCO (360 oC)

Nano Y support. Diatomite

Acidified diatomite

1.45 3.14 0.01 0.10 0.21 16.60 20.37 57.78

2.20 3.72 0.03 0.34 0.65 18.60 20.30 53.52

4. Conclusion New catalytic materials based on Al-modifed diatomite was successfully prepared by using atomic implantation method. Acidity of this material is comparable to that of zeolite Y as evidenced by NH3-TPD measurements. This material is an active in hydrocarbon cracking. In addition, nano-zeolite Y supported diatomite was successfully synthesized by using a two step procedure: First step- Preparation of nano-zeolite Y seeds and seeded diatomite and the second step 2- Formation and deposition of nanozeolite Y seeds on diatomite by in-situ crystallization. The resulting supported material contains a multi modal pore structure. with interconnected macro-, meso- and micropores. Both the materials were active in the cracking of heavy petroleum residue, indicating its high application potential for FCC catalyst preparation.

References [1] Y. Liu, W. Zhang, T.J. Pinnavaia, Steam-stable aluminosilicate mesostructures assembled from zeolite type Y seeds, J. Am. Chem. Soc., 122 (2000) 8791. [2] Y. Liu, W Zhang, T.J. Pinnavaia, Steam-stable MSU-S aluminosilicate mesostructures assembled from zeolite ZSM-5 and zeolite Beta seeds, Angew. Chem. Int. Ed., 40 (2001) 1255. [3] K.S. Triantaflyllidis, T.J. Pinnavaia, A. Iosifidis, P.J. Pomonis. Specific surface area and I-Point evidence for microporosity in nanostructure MSU-S aluminosilicates assembled from zeolite seeds, Journal of Mater. Chem., 17 (2007) 3630. [4] Z. Jing, Hirotaka, K. Ioku, E. H. Ishida, Hydrothermal Synthesis of Mesoporous Materials from Diatomaceous Earth, J. AIChE, 53 (2007) 2114. [5] S.W. Rutherford, J.E. Coons, Water sorption in silicone foam containing diatomaceous, J. Colloid Interface Sci, 306 (2007) 228.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

The influence of preparation procedure on structural and surface properties of magnesium fluoride support and on the activity of ruthenium catalysts for selective hydrogenation of chloronitrobenzene Mariusz Pietrowski* and Maria Wojciechowska Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznań, Poland; *e-mail: [email protected]

Abstract The effect of preparation conditions on structural and surface properties of magnesium fluoride was studied in the aspect of its use as a catalyst support. Amorphous and spherical polycrystalline MgF2 supports were prepared and characterised by BET, XRD, TEM, and FTIR (pyridine adsorption) techniques. The influence of MgF2 properties on the performance of Ru/MgF2 catalysts in selective reduction of ortho- and parachloronitrobenzene to respective chloroanilines is reported as well. Keywords: MgF2, ruthenium catalyst, hydrogenation of chloronitrobenzene

1. Introduction Recently, a growing interest of researchers in fluoride supports, including magnesium fluoride, is observed [1-15]. In many laboratories research on the development of new active and selective catalysts supported on MgF2 is carried out and this resulted in the creation of interesting catalysts for such processes as hydrodesulphurization [15], hydrodechlorination [16], ammoxidation [4], reduction of nitrogen oxides [3], Knoevenagel reaction [5], oxidation of CO [3], photodegradation of acetone [6] and recently, hydrogenation of chloronitrobenzene to chloroaniline [8-9]. A great advantage of magnesium fluoride is its easy preparation and availability as well as low cost of parent materials. The simplest way of MgF2 preparation is the reaction of hydrofluoric acid with MgCO3 [14] which enables to obtain magnesium fluoride with specific surface area up to 43 m2·g-1 after thermal treatment at 670 K. Results of studies on the preparation of MgF2 of a higher surface area were recently reported by Kemnitz and coworkers [2]. Monodispersive spherically-shaped powders of magnesium fluoride of particle size in the range 0.25-0.36 μm was obtained as well [12]. No comparative study on properties of magnesium fluoride obtained by using different preparation methods was published in the literature yet. This is why we have performed a comparison of MgF2 samples prepared by four different methods developed in our laboratory. The subject of the comparison were structural (XRD) and surface (FTIR) properties. Moreover, usefulness of magnesium fluoride obtained in different preparation ways was evaluated from the point of view of its application as a support of ruthenium catalyst for selective reduction of chloronitrobenzene to chloroaniline. It is worth to add that the latter compound is an important intermediate for the manufacture of a wide range of pharmaceuticals, herbicides and dyes [17].

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2. Experimental 2.1. Procedures for the preparation of MgF2 supports and Ru/MgF2 catalysts

Polycrystalline MgF2 was obtained using four methods listed below. (i) Reaction of HF with MgCO3 (carbonate sample - C): 40% solution of hydrofluoric acid was added dropwise to suspension of magnesium carbonate in water. The resulted precipitate of MgF2 was stirred for 48 h and then dried at 390 K. (ii) Reaction of NH4F with Mg(NO3)2 - amorphous MgF2 (nitrate sample - N): 0.1M solution of NH4F was slowly added, with the use of peristaltic pump, to 0.1M solution of Mg(NO3)2 at 330 K. The precipitate was filtered off and dried at 390 K. (iii) Reaction of NH4Fwith Mg(NO3)2 - spherical MgF2 prepared with the use of microwave radiation, (nitrate spherical - Ns): Cold 0.01M solution of NH4F was poured into 0.01M solution of Mg(NO3)2 followed by immediate placing in a microwave oven and heating within 30 s to a temperature close to boiling point. The precipitate was filtered off and dried at 390 K. (iv) Reaction of HF with magnesium alkoxide (alkoxide sample - Alk): A solution of freshly prepared magnesium methoxide was added dropwise to a 40% hydrofluoric acid solution in methanol on vigorous stirring. The process of gelation was conducted for 6h at room temperature. The gel was aged at room temperature for 40 h, then dried at 350 K. After drying, all MgF2 samples were calcined in air at 670 K for 4 h. 20 cm g Ruthenium was loaded onto MgF2 by conventional impregnation using methanolic solution of RuCl3·3H2O. Ruthenium content was 1 wt%. The catalysts were C dried at 350 K and reduced at 670 K under hydrogen flow. 3 -1

Volume adsorbed (cm g )

3 -1

2.2. Activity test

Alk

N Ns 0.0

0.2

0.4

0.6

0.8

1.0

p/p0

Figure 1. Isotherms of low-temperature nitrogen adsorption on MgF2.

Hydrogenation of ortho-, and parachloronitrobenzenes (o-; p-CNB) to orthoand para-chloroanilines (o-; p-CAN) was performed in liquid-phase at 350 K for 2 h at 4 MPa of hydrogen pressure in a 200 cm3 stainless steel autoclave with a magnetic stirrer. 0.05 g of a catalyst and 50 cm3 of 0.1M CNB solution in methanol were loaded to the autoclave. The reaction products were analysed on a gas chromatograph equipped with a capillary column RESTEK MXT-5.

3. Results and discussion 3.1. The effect of preparation on physico-chemical properties of fluoride supports Magnesium fluoride was prepared using four different methods, however, thermal treatment of all samples was identical and consisted in calcination at 670 K. Despite the same calcination temperature, MgF2 samples considerably differ in their porous

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507

structure which is reflected in the shape of hysteresis loop in the isotherm of lowtemperature nitrogen adsorption, specific surface area, pore size and pore volume (Table 1; Figure 1). The greatest surface area (43 m2·g-1) was obtained in the case of carbonate sample (C), whereas the smallest one (13 m2·g-1) in the case of spherical MgF2 prepared from magnesium nitrate (Ns). Adsorption-desorption isotherms of all samples are of type IV, however, they differ in the shape of their hysteresis loops. The latter belong to type H1 for samples C, Alk and N, which indicates the presence of cylindrical pores, whereas hysteresis loop of the sample Ns combines features of hysteresis loops of types H1 and H2, encountered when narrow-necked pores are present. The greatest pore size (r = 13 nm) and pore volume (0.250 cm3·g-1) were found in magnesium fluoride prepared from magnesium alkoxide (Alk). The discussed sample is also characterized by the lowest thermal stability as concluded from considerable increase in crystallite size and drastic reduction in surface area with the rise in calcination temperature (Table 1). The highest resistance to sintering and recrystallisation at high temperatures were shown by carbonate (C) and nitrate-spherical (Ns) samples. Table 1. Textural and structural properties of magnesium fluoride samples obtained by different methods. BET surface area (m2·g-1) after calcination at different temperatures

Crystallite size*(nm) after calcination at different temperatures

570 K

670 K

770 K

570 K

670 K

770 K

0.172

85

43

25

12

22

41

13

0.250

81

32

24

10

22

116

N

6

0.066

62

22

12

14

59

139

Ns

9

0.066

26

13

5

28

46

86

Average pore radius (nm)

Total pore volume (cm3·g-1)

C

8

Alk

Sample

* Determined by Hall method on the ground of broadening of XRD reflections

Infrared spectra of adsorbed pyridine have shown the presence of weak Lewis acid centres on surfaces of all MgF2 samples studied. The amount of these centres, as estimated on the ground of absorption band intensities, is small and decreases in the order: C ≥ N > Alk > Ns. No Brøensted acid sites were detected on MgF2 surface, irrespective of its preparation way. Summing up, we can say that magnesium fluoride of the greatest surface area (after calcination at 670 K) and the highest resistance to sintering was obtained by the reaction of magnesium carbonate with hydrofluoric acid. It should be stressed that the preparation of MgF2 by this method is cheap and is characterised by a excellent reproducibility.

3.2. Activity of Ru/MgF2 catalysts for selective reduction of chloronitrobenzene 1 wt.% of ruthenium was loaded onto surfaces of MgF2 samples by impregnation with ruthenium(III) chloride. The samples were dried at 350 K and reduced in hydrogen flow at 670 K for 4h. In spite of applying identical preparation conditions of Ru/MgF2 catalysts, different dispersions of the metal were obtained (Table 2). The highest dispersion (15%) was found in the case of Alk-Ru catalyst. Selectivities were somewhat higher in the case of reduction of p-CNB (~90%) compared to o-CNB (~85%). No significant effect of dispersion on the selectivity to chloroaniline was observed. In general, the order of catalytic activity is as follows: Ns > C > Alk > N, both in the case

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of ortho- and para-chloronitrobenzene. All magnesium fluoride supported catalysts were more selective than commercial 0.5%Ru/Al2O3 Engelhard catalyst. Table 2. Liquid-phase hydrogenation of o-, and p-CNB to o-, and p-CAN on Ru/MgF2 catalysts. Catalyst

Dispersion, p-CNB % Conversion, % Selectivity,%

o-CNB Conversion, % Selectivity,%

Ns-Ru C-Ru Alk-Ru N-Ru

12.3 6.7 15.0 14.1

9.8 7.7 5.1 3.5

87.7 90.5 91.0 90.1

13.5 9.6 7.8 5.7

81.0 86.3 88.1 83.6

0.5%Ru/Al2O3 Engelhard

-

6.9

68.6

-

-

4. Conclusion Studies carried out recently proved that magnesium fluoride is a very good material for support in catalysis. Results reported in this paper have shown that by using different preparation methods it is possible to obtain MgF2 samples differing in their structural and surface properties. The most suitable preparation method seems to be the reaction between magnesium carbonate and hydrofluoric acid. Magnesium fluoride obtained by this method is characterised by higher surface area and higher thermal stability compared to samples prepared by other methods investigated. Simplicity of the preparation method and low cost of reagents used for synthesis are also of importance. Data obtained while studying the reaction of selective reduction of chloronitrobenzene to chloroaniline enable to draw the conclusion that preparation way and properties of MgF2 influence catalytic results. Differences in catalytic activity can reach as much as 300%.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

J.M. Winfield, 2009, J. Fluor. Chem., 130, 1069-1079. S. Wuttke, S.M. Coman, G. Scholz, H. Kirmse, A. Vimont, M. Daturi, S.L.M. Schroeder, E. Kemnitz, 2008, Chem. Eur. J., 14, 11488-11499. J. Haber, M. Wojciechowska, M. Zielinski, W. Przystajko, 2007, Catal. Lett., 113, 46-53. V.N. Kalevaru, B. David Raju, V. Venkat Rao, A. Martin, 2009, Appl. Catal. A, 352, 223233. R.M. Kumbhare, and M. Sridhar, 2008, Catal. Commun., 9, 403-405. F. Chen, T.H. Wu, and X.P. Zhou, 2008, Catal. Commun., 9, 1698-1703. J. Krishna Murthy, U. Gross, S. Rudiger, E. Unveren, W. Unger, E. Kemnitz, 2005, Appl. Catal. A, 282, 85-91. M. Pietrowski, M. Zieliński, M. Wojciechowska, 2009, Catal. Lett., 128, 31-35. M. Pietrowski, M. Wojciechowska, 2009, Catal. Today, 142, 211-214. M. Wojciechowska, A. Wajnert, I. Tomska-Foralewska, M. Zieliński, B. Czajka, 2009, Catal. Lett., 128, 77-82. M. Wojciechowska, W. Przystajko, M. Zielinski, 2007, Catal. Today, 119, 338-341. M. Pietrowski, M. Wojciechowska, 2007, J. Fluor. Chem., 128, 219-223. M. Wojciechowska, I. Tomska-Foralewska, W. Przystajko, M. Zielinski, 2005, Catal. Lett., 104, 121-128. M. Wojciechowska, M. Zielinski, M. Pietrowski, 2003, J. Fluor. Chem., 120, 1-11. M. Wojciechowska, M. Pietrowski, B. Czajka, 2001, Catal. Today, 65, 349-353. A. Malinowski, W. Juszczyk, J. Pielaszek, M. Bonarowska, M. Wojciechowska, Z. Karpinski, 1999, Chem. Commun., 685-686. X. Wang, M. Liang, J. Zhang, Y. Wang, 2007, Curr. Org. Chem. 11, 299-314.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Bimetallic Co-Mo-complexes with optimal localization on the support surface: A way for highly active hydrodesulfurization catalysts preparation for different petroleum distillates O.V. Klimov, A.V. Pashigreva, K.A. Leonova, G.A. Bukhtiyarova, S.V. Budukva, A.S. Noskov Boreskov Institute of Catalysis SB RAS, Pr. Lavrentieva 5, Novosibirsk,630090, Russia

Abstract The preparation method of the catalyst for the deep hydrotreatment of vacuum gas oil and gasoline is described. The method is based on vacuum impregnation of the carrier with required average pore diameter with the solution of bimetallic CoMo complexes. It was shown that the use of Co-Mo complexes, containing chelating ligands and having different molecule size, allows to obtain catalysts with the uniform distribution of the surface species, containing supported metals only in the form of Co-Mo-S phase type II that is located inside of the pores exposed to all reacting heteroatomic molecules of the feedstock. Keywords: hydrotreating catalyst, textural properties of carrier, catalyst active component localization

1. Introduction Nowadays, it is well known that the active sites of the hydrotreating reactions surface are the nanosized MoS2 particles with Co(Ni) atoms anchored at the edge plane[1]. From the other side, the textural properties of the supports that are optimal for different type of distillates are well distinguished [2]. The use of the pores with smaller size restricts the access of S- and N- containing compounds to the catalysts active sites. The use of pores with sizes larger than necessary results in a lower specific surface area and therefore in a decrease of the number of active sites per volume of catalyst. The catalysts designed for hydrotreating of the selected type of feedstock should contain the supported metals only in the form of the active sites, arranged in the pores with optimal size. The preparation of the carriers with required average pore diameter is easy-to-implement task. The catalysts containing Co and Mo only in the form of the active sites can be prepared using bimetallic complexes [3]. The selective synthesis of the active sites inside of the pores with the required size is much more difficult. The current contribution describes a hydrotreatment catalysts preparation method for different petroleum distillates – gasoline and vacuum gas oil (VGO). Since S- and N- containing compounds of these distillates characterized with a different molecule size, the nanoparticles of active phase have to be arranged in the pores with relatively small sizes for gasoline and with a large size for vacuum gas oil. The key factor in obtaining such catalysts is the size of the bimetallic complexes used for the catalysts preparation. Whereas the molecule size of the complex that will be supported determines in what pores the active sites will be arranged.

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The preparation route includes the following stages: - Synthesis of the bimetallic Co-Mo complexes with a different molecule size in aqueous solution with Co and Mo stoichiometry that is optimal for the active sites of hydrotreating catalysts; - Deposition of the bimetallic complexes onto support surface avoiding initial complexes decomposition. - The drying of the catalysts under conditions, excluding bimetallic complexes decomposition and migration of Co and Mo atoms through the support surface and subsequent sulfidation.

2. Experimental 2.1. Preparation of the complexes Two bimetallic Co-Mо complexes Co[Mo2O4(C2O4)2(H2O)2] (hereinafter CoMo2Ox) and Co2[Mo4(C6H5O7)2O11]•nH2O (hereinafter CoMo4CA) with the geometrical size (without its solvate sphere) of 4×4×7 Å and 7×10×12 Å, correspondingly, were used for hydrotreating catalysts preparation. Both complexes were synthesized by similar methods adding of Co(NO3)2•6H2O to the solution containing corresponding molybdenum containing anion, so that to obtain Mo/Co ratio in the resulting solution equaled 2. Initial (NH4)2[Mo2O4(C2O4)2(H2O)2] was obtained by dissolution of (NH4)2[Mo2O4(OH)4(H2O)2] (obtained by synthesis as in [4]) in aqueous solution of oxalic acid synthesized at stirring and heating at 50 ºC. The synthesis of (NH4)4[Mo4(C6H5O7)2O11]•nH2O is described in [5].

2.2. Preparation of the catalysts The catalysts were prepared by vacuum impregnation of the trilobe shaped carrier (Ø = 1.5 mm, l = 3-6 mm) by aqueous solutions of the complexes in accordance with the technique described in [6], followed by drying at 120ºC without calcination. Only for the determination of Co and Mo content the catalyst samples were calcined 550ºC 2 hours. To prepare catalyst for VGO hydrotreatment the alumina support developed deliberately by JSC Promyshlennye Katalizatory (Ryazan, Russia) was used. This support has specific surface area of 240 m2/g, pore volume of 0.75 cm3/g, average pore diameter of 120 Å (carrier 120). The concentration of CoMo4CA in the solution was chosen to prepare catalyst containing 10.5% Mo, 3.3% Co. The catalyst is further denoted CoMo4CA/Al2O3. To prepare gasoline hydrotreating catalyst the laboratory prepared carrier with specific surface area of 260 m2/g, pore volume of 0.50 cm3/g, average pore diameter of 70 Å (carrier 70) was used. The (CoMo2Ox) concentration in an impregnating solution corresponded to the content of 8.0% Mo, 2.4% Co in the catalyst. The catalyst is further denoted CoMo2Ox/Al2O3. The sulfidation of the catalysts was performed in H2S flow of 1000 ml/h during 2 hours at 230ºC and then 2 hours at 400ºC. The obtained samples are designated as CoMo2Ox-S/Al2O3 and CoMo4CA-S/Al2O3.

2.3. Complexes and catalysts characterization Equipment and techniques for NMR, FTIR, Raman, EXAFS and HRTEM characterization are described in [5,6]. Equipment and XPS characterization technique and the methods for textural properties study are given in [7].

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3. Results and discussion The synthesized complexes of (CoMo2Ox) and (CoMo4CA) were obtained in the solid form by ethanol precipitation and were characterized by physical-chemical methods. FTIR, Raman and EXAFS data for complexes in the solution and in the solid form are fully agree with the data of [5,6]. That allows to conclude that structure of (CoMo2Ox) and (CoMo4CA) is identical to that described in [6]. The structure of the initial complexes is not changed after deposition onto support surface, that confirmed by followed methods: - In accordance with FTIR data the coordination of Co towards Mo-containing anions via the carboxylic group of oxalic and citric ligands, correspondingly, are saved; - Raman data show that Mo environment in the catalysts and in the initial complexes is the same; - According to EXAFS data the all distances of Mo-O, Mo-Mo and Mo-Co corresponded for the initial bimetallic complexes were also revealed for the catalysts; - The NMR analysis of the excess of impregnating solution drained out the catalysts showed the absence of any molybdenum containing compounds differed from the initial complexes. The results of the sulfide catalysts characterization study are presented in Table 1. Table 1. The results of sulfide catalysts characterization. Characterizatio n method Element analysis XPS EXAFS

CoMo2Ox-S/Al2O3

CoMo4CA-S/Al2O3

S/Mo=2,05

S/Mo=2,00

Mo3d 228,7 eV Mo K-edge

Co2p 778,8 eV Co K-edge

Mo3d 228,6 eV Mo K-edge

Co2p 778,9 eV Co K-edge

Mo-S=2,40Å

Co-S=2,22Å CoMo=2,78Å

Mo-S=2,42Å Mo-Mo=2,61Å

Co-S=2,22Å Co-Mo=2,80Å

Mo-Mo=2,60Å

HRTEM Raman Adsorption/ desorbtion N2

MoS2 particles Average length =30Å Average number of layers=2,35 MoS2 361;402 cm-1 No Mo-O compounds Carrier 70 Catalyst S=235 m2/g S=217 m2/g Vpore=0,43cm3/ Vpore=0,34cm3/g g Øpore=62Å Øpore=73Å

MoS2 particles Average length=40Å Average number of layers=1,95 MoS2 363;426 cm-1 No Mo-O compounds Carrier 120 Catalyst S=240 m2/g S=190 m2/g Vpore=0,73cm3/ Vpore=0,53cm3/g g Øpore=111Å Øpore=120Å

The catalysts characterization and element analysis data allow to conclude that both catalysts contain cobalt and molybdenum only in the form of Co-Mo-S phase type II, described in [8,9]. The decrease of wide pore volume in comparison with carrier was noticed for both catalysts, while, the narrow pore volume remained about the same (Fig.1). It means that Co-Mo-S phase type II is localized in the pores with diameter of 50-100Å in CoMo2Ox-S/Al2O3, and in pores with diameter of 70-130Å in CoMo4CA-S/Al2O3.

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Figure 1. Pore size distribution in the carriers and in the catalysts.

Testing of CoMo4CA-S/Al2O3 sample in the hydrotreatment of VGO [7] showed that achieved hydrodesulfurization degree exceeds 99% and hydronitrogenation degree is more than 85%. Hydrotreatment of the straight run gasoline fraction over CoMo2Ox-S/Al2O3 catalyst allows to reach the product, which fully corresponds to the quality of reforming feed in terms of sulfur and nitrogen content at typical hydroprocessing conditions used in industry [10].

4. Conclusions Thus, the use of bimetallic Co-Mo complexes for the catalysts preparation allows to obtain high active hydrotreating catalysts, which mainly contain Co-Mo-S phase type II inside of the pores exposed to the reacting heteroatomic molecules of the feedstock. Depending on the molecules size of supported Co-Mo complex, the proposed method can be used for the preparation of hydrotreating catalysts both for light and heavy petroleum distillates.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

J.V. Lauritsen, J. Kibsgaard, G.H. Olesen et al., J. Catal. 249 (2007) 220. J. Ancheyta, M.S.Rana, E.Furimsky, Catal.Today, 109 (2005) 3. J. Mazurelle, C. Lamonier, Ch. Lancelot et al., Catal. Today, 130 (2008) 41. A.N. Startsev, O.V. Klimov, S.A. Shkuropat et al. Polyhedron 13 (1994) 505. O.V. Klimov, A.V. Pashigreva, M.A. Fedotov et al., J.Mol.Catal. A, 2010, in press. O.V. Klimov, A.V. Pashigreva, G.A. Bukhtiyarova et al., Catal. Today (2009), doi:10.1016/j.cattod.2009.07.095 A.V. Pashigreva, O.V. Klimov, G.A. Bukhtiyarova et al., Catal. Today (2009), doi:10.1016/j.cattod.2009.07.096 H. Topsoe, Applied Catalysis A: General 322 (2007) 3. S. Eijsbouts, L.C.A. Van den Oetelaar, R.R. Van Ruijenbroek, J. Catal. 229 (2005) 352. Song, Catal. Today 86 (2003) 211.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Mn, Mn-Cu and Mn-Co mixed oxides as catalysts synthesized from hydrotalcite type precursors for the total oxidation of ethanol Daniel Aguilera, Alejandro Perez, Rafael Molina, Sonia Moreno* Estado Sólido y Catálisis Ambiental, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, AK 30 No. 45-03, Bogotá, Colombia. E-Mail address: *[email protected]

Abstract In this work, nanoparticulates of compounds precursors of LDHs for preparing Mn, Mn-Cu and Mn-Co mixed oxides were successfully synthesized by the reverse microemulsion method. It was observed that the precursor obtained from the above method had similar characteristics for preparing mixed oxide catalysts used in the oxidation of ethanol. This method was compared with the conventional co-precipitation method. Keywords: mixed oxides, VOCs, microemulsion

1. Introduction The total catalytic oxidation of volatile organic compounds (VOCs) is one of the most effective and economically attractive methods which can be applied in order to limit emissions. Metal oxides are very promising for the obtaining of active catalysts in the oxidation of VOCs. The metallic mixture may generate a cooperative effect that can promote oxygen mobility, stabilization of the most active species and generate a redox cycle [1]. Mixed oxides can be obtained by controlled decomposition of LDH compounds which show large surface areas, high metal dispersion and stability against sintering [2]. Nevertheless, the LDH structure depends on a great number of parameters. Co-precipitation methodology is the most common synthesis technique to obtain LDHs, however nucleation and the kinetics growth can not be controlled easily. An alternative method for the synthesis of nanomaterials is reverse microemulsion (waterin-oil) in which an aqueous phase is dispersed into an oil phase stabilized by a surfactant film. Microemulsions can be used as nanoreactors leading to homogeneous nanomaterials with a narrow particle size and better textural properties [3]. In this work, Mn and binary Mn-Cu, Mn-Co hydrotalcite-like precursors synthesized in reverse microemulsion and the effects of preparation methods on the performance of catalysts for deep oxidation of VOCs have been studied.

2. Experimental 2.1. Preparation of catalysts 2.1.1. Co-precipitation Mn–Mg–Al, Mn–Cu–Mg–Al and Mn–Co–Mg–Al hydrotalcites were synthesized from the nitrates of Mg 2+, Al3+, Mn2+ and Cu2+ or Co2+ with M2+/M3+ = 3 and (Mn + M)/ Mg = 0,36 ratios where M corresponds to Cu or Co [4, 5]. The M/Mn = 0.5 ratio was

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selected to agree with preliminary tests. The nitrate solution was added drop wise to an aqueous solution of sodium carbonate at room temperature; the slurry was vigorously stirred while keeping the pH between 9 and 10 by the slow addition of a diluted solution of NaOH. After the complete addition of the metal nitrate solution, the suspension was stirred for 1 h, followed by ageing for 24 h without stirring. The solid was rinsed with deionized water and dried in air at 60°C for 24 h. The dried solid was ground into a fine powder (100 µm). The results also indicate that the material is quite stable, only the large particles will break at higher pressure during attrition testing. Nitrogen sorption measurements were preformed to determine the BET surface area, the average pore size, and the pore volume. Some of the materials were impregnated with 20% cobalt and 0.5% rhenium by the incipient wetness method using cobalt nitrate hexahydrate and perrhenic acid, and tested in a fixed bed reactor. The results from the N2-sorption measurements as well as the fixed bed testing of three selected materials are listed in Table 1. The data from the fixed bed testing are reported as CO reaction rate (rCO) and selectivity for C5+ hydrocarbons. These results are in agreement with previously reported data [12].

Spray drying of porous alumina support for Fischer-Tropsch catalysis Particle Size Distribution

13

10

9 8.5 8

10 9.5 9 8.5 8

7.5 7 6.5 6

Volume (%)

Volume (%)

Particle Size Distribution

12.5 12 11.5 11 10.5

9.5

5.5 5 4.5

7.5 7 6.5 6 5.5 5 4.5 4 3.5

4 3.5 3 2.5

3 2.5 2 1.5

2 1.5 1 0.5 0 0.1

687

1

10

100

600

1 0.5 0 0.1

1

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P article S ize (µm )

P article S iz e (µm )

35% Dispal, 3.0, 1. juli 2009 11:40:13 20% Dispal + 1/2% B inder, 3,0 bar, 1. juli 2009 11:47:24 10% Dispal + 5% Zn, 3,0B ar, 1. juli 2009 11:59:23

35% Dispal, 0,15, 1. juli 2009 11:38:33 20% Dispal + 1/2% B inder, 0,15bar, 1. juli 2009 11:49:55 10% Dispal + 5% Zn, 0,15B ar, 1. juli 2009 11:57:11

Figure 1. Particle size distributions for materials prepared from 1) 35% alumina (120 nm), 2) 20% alumina (120 nm) + 0.5% Al(NO3)3, and 3) 10% alumina (120 nm) + 5% Zn, all spray dried with a 1.0 mm nozzle measured at 0.15 and 3.0 bar, respectively. Table 1. Results from N2-sorption measurements and fixed bed testing for three different materials spray dried with a 1.0 mm nozzle and water as balance. The fixed bed data were measured at 210°C, 20 bars and 45% CO conversion. 35% Alumina (120 nm)

20% Alumina (120 nm) + 0.5% Al(NO3)3

10% Alumina (120 nm) + 5% Zn(NO3)2

Pore Volume (ml/g)a 2

0.50

0.55

0.53

a

138

137

128

a

15-20

15-20

15-20

0.051

0.051

0.039

81.9

82.8

82.5

BET Surface Area (m /g)

Mean Pore Diameter (nm) b

rCO (molCO/gCo*h)

C5+ Selectivity (%) a

b

b

N2-sorption data; Fixed bed data

The materials were furthermore characterized by scanning electron microscopy to study the morphology. The SEM study shows that all the various materials prepared will consist of densely packed spherical particles.

3.2. Agglomeration of non-porous alumina particles with polystyrene latex particles From scanning electron microscopy it was seen that increasing the alumina/polystyrene latex ratio a denser material is obtained, and that the particle size can be increased by increasing the total concentration of solid and by increasing the nozzle size. Varying the other instrumental parameters (pump speed, air flow, drying temperature) did not change the particle morphology or pore structure, but they were important to insure efficient drying and a higher yield. The material was fluffy, and there was no welldefined pore structure originating from the latex particles, as seen in a previous study on agglomeration of silica particles with polystyrene latex [10]. Two typical SEM images of a) a spray dried and b) spray dried and calcined material are shown in Figure 2. These materials are still to be tested in the fixed bed reactor.

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a)

b)

1 µm

1 µm

Figure 2. SEM images of agglomerates of 13 nm alumina primary particles together with 260 nm polystyrene latex particles, before (a) and after calcination (b).

4. Conclusions Porous alumina supports for Fischer-Tropsch catalysis have been prepared by spray drying of various primary particles. By agglomeration of differently sized non-porous alumina particles a porous material is obtained, where the porosity originates from voids between the primary particles. These particles are densely packed, with a specific pore size, and a spherical morphology. By agglomeration of non-porous alumina particles together with polystyrene latex particles, a porous material is obtained, where the porosity originates from voids between the alumina particles, as well as from annihilation of the latex particles. These particles have a spherical morphology, and as they are quite loosely packed, they have a broad pore size distribution. The catalytic performance correspond well to what has previously been reported in the litterature.

References [1] [2]

M. Fareed, Chem. Eng. Sci., 58 (2003) 2985. F. Iskandar, I.W. Lenggoro, T.-O. Kim, N. Nakao, M. Shimada, K. Okuyama, J. Chem. Eng. Jpn., 34 (2001) 1285. [3] F. Iskandar, I.W. Lenggoro, B. Xia, K. Okuyama, J. Nanoparticle Res., 3 (2001) 263. [4] C. du Fresne von Hochenesche, K. K. Unger, T. Eberle, J. Mol. Catal. Chem., 221 (2004) 185. [5] C. du Fresne von Hochenesche, V. Stathopoulos, K.K. Unger, A. Lind, M. Lindén, Stud. Surf. Sci. Catal., 144 (2002) 339. [6] A. Lind, C. du Fresne von Hochenesche, J.-H. Smått, M. Lindén, K.K. Unger, Microporous Mesoporous Mat., 66 (2003) 219. [7] F. Iskandar, Mikrajuddin, K. Okuyama, Nano Lett., 1 (2001) 231. [8] F. Iskandar, Mikrajuddin, K. Okuyama, Nano Lett., 2 (2002) 389. [9] F. Iskandar, Mikrajuddin, K. Okuyama, Encyclopedia of Nanoscience and Nanotechnology, 8 (2004) 259. [10] A. Lind, C. du Fresne von Hohenesche, D. Schmidt, V. Schädler, K.K. Unger, submitted. [11] Ø. Borg, S. Eri, E.A. Blekkan, S. Storsæter, H. Wigum, E. Rytter, A. Holmen, J. Catal., 248 (2007) 89. [12] Ø. Borg, S. Eri, E. Rytter, A. Holmen, Prepr.Pap.-Am. Chem. Soc., Div. Fuel Chem., 51 (2006) 699.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Ni/SiO2 fiber catalyst prepared by electrospinning technique for glycerol reforming to synthesis gas Prasert Reubroycharoen,a,b Nattida Tangkanaporn,a Chaiyan Chaiyac a

Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand, E-mail: [email protected] b Center for Petroleum, Petrochemicals and Advanced Materials, Bangkok 10330, Thailand c Division of Chemical Engineering, Faculty of Engineering, Rajamangala University of Technology Krungthep, Bangkok 10120, Thailand.

Abstract The Ni/SiO2 fiber catalysts is successfully prepared for the first time by sol-gel and electrospinning techniques and used as a reforming catalyst. Nickel acetate and tetraethyl orthosilicate are used as a source of nickel and silica at different Ni loading (5, 10, and 20%wt). The effect of spinning voltage on the morphology of the SiO2 fiber is studied. The Ni/SiO2 fiber catalyst is prepared by impregnation technique and characterized by SEM-EDS, XRD, and TPR. SEM results show that the average diameter of the SiO2 fibers ranged from 1.28 μm to 930 nm. The amount of Ni metal measured by EDS technique is close to that of Ni loading. The reaction test shows that the activity of the fiber catalyst is higher than that of a conventional Ni/SiO2 porous catalyst, and the synthesis gas with H2/CO ratio of 2, a raw material for Fischer-Tropsch synthesis, is obtained by using the fiber catalyst. Keywords: Ni/SiO2, fiber catalyst, electrospinning, glycerol, reforming

1. Introduction Ni-based catalysts have been widely used in the steam reforming of methane, ethanol, and glycerol [1-3] because of their appreciable catalytic activity, good stability, and low price. Although these are much cheaper than Ru- and Rh-based catalysts, they require high reaction temperature and an excess steam to prevent the sintering of Ni particles and the deposition of carbon on the catalyst surface [4]. Moreover, the deposited carbon on the catalyst surface leading to rapid catalyst deactivation which is contributed by the catalyst pore blockage [5]. Thus, a non-porous catalyst such as a fiber-like structure catalyst could avoid the pore blockage, reduce pressure drop, and increase the mass transfer rate at the same time due to no diffusion in the catalyst pore [6]. An effective production technique of the fiber catalysts is electrospinning technique commonly used for producing metal oxide and polymer fibers [7]. Previously, there were no works on the preparation and performance of the fiber catalyst for the syngas production from a glycerol steam reforming. In this study, the electrospinning is used to prepare Ni/SiO2 fiber catalyst which is then used as a glycerol steam reforming catalyst for syngas production. The fiber catalyst is characterized by SEM-EDS, XRD, and TPR.

2. Experimental The Ni/SiO2 fiber catalyst is prepared via sol-gel and electrospinning techniques following by impregnation method. Nickel acetate and tetraethyl orthosilicate (TEOS)

P. Reubroycharoen et al.

690

are selected as a source of nickel and silica. SiO2 fiber is prepared by sol-gel incorporated with electrospinning technique. Then, Ni is impregnated on the SiO2 fiber at different metal loading percentage.

2.1. Silica sol preparation via sol-gel process The silica sol is prepared from TEOS, ethanol, distilled water, and HCl at molar ratio of 1:2:2:0.01, respectively. Firstly, TEOS is mixed with distilled water in a beaker under vigorous stirring for 5 min. Then, HCl and ethanol are added to the solution under stirring for another 5 min. Finally, the solution is heated to 55oC and stirred at 55oC for 50 min before it is cooled down to room temperature.

2.2. Silica fiber preparation via electrospinning process The electrospinning apparatus consists of a high voltage generator with metal collector and a precision syringe pump shown in Fig. 1. The silica sol is filled into a disposable syringe equipped with 0.6 mm-diameter needle and placed on the syringe pump. The silica sol is electrospun and transformed to SiO2 fiber. The standard electrospinning condition is as followings: feeding rate of 10 μl/h, applied voltage of 15 kV, and the tipto-collector distance (TCD) of 15 cm. The electrospun fibers are collected and dried at 110oC overnight, then calcined at 500oC for 2 h.

Fig. 1. The electrospinning apparatus.

2.3. Catalyst impregnation Ni/SiO2 fiber catalyst is prepared by an impregnation of the fiber with different nickel acetate solution. The nickel acetate solution is prepared by dissolving a nickel acetate in the solution of glycerol/ethanol (volume ratio of glycerol and ethanol is 1:9). The solution of nickel acetate is impregnated onto the SiO2 fiber. The catalyst is dried at 120oC for 12 h and calcined at 500oC for 2 h.

2.4. Catalyst characterization The fiber catalyst is characterized by SEM-EDS, XRD, and TPR. TPR is performed in the Micromeritics II 2920. The TPR is performed by flowing Ar (50 mL/min) over 60 mg of catalyst. The reducing gas (10%H2 in Ar at 50 mL/min) is passed over the catalyst with the heating rate of 10oC/min until 600oC is reached. XRD (Philips model X’Pert) equipped with CuKα is used to investigate crystallite size. The crystalline average size is calculated by L = Kλ/Δ(2θ)cos θ0, where L is the crystalline size, K is a constant (K = 0.9–1.1), λ is the wavelength of X-ray (CuKα = 0.154 nm), and Δ(2θ) is the width of the peak at half height. SEM, JOEL JSM-6480LV, is used to investigate morphology of the fiber catalyst. The average diameter of the fiber is analyzed by a SemAfore program. EDS is used to determine catalyst composition.

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2.5. Catalyst performance on a glycerol steam reforming Catalyst performance on a glycerol steam reforming is carried out in a fixed-bed quartz tube reactor. The reaction conditions are Ptotal = 1 atm, PN2 = 0.7 atm, Pwater/glycerol = 0.3 atm, T = 550oC, catalyst = 0.1 g, reaction time = 6 h, water/glycerol = 9:1 (molar ratio), and feed rate of water/glycerol = 0.01 mL/min. The effluent gas from the reactor is analyzed by on-line TCD-GC equipped with Unibead-C column.

3. Results and discussion

Fig. 2. SEM images of SiO2 fibers with various applied voltage (a) 15 kV, (b) 20 kV, and (c) 25 kV at TCD of 15 cm.

The average diameters of SiO2 fiber prepared at 15 kV, 20 kV, and 25 kV are 1.28 μm, 1.27 μm, and 930 nm, respectively. It is shown in Fig. 2 that the shape and diameter of the electrospun fiber from 15 kV applied voltage are more uniform than those of 20 and 25 kV.

Fig. 3. SEM images of Ni/SiO2 fiber catalysts (15 kV, TCD: 15cm.) at Ni loading of (a) 5%wt, (b) 10%wt, and (c) 20%wt.

As shown in Fig. 3, the roughness of the catalyst surface depended upon Ni loading. When Ni loading increases, the roughness of the catalyst surface increases. The rough surface as confirmed by EDS is Ni particles depositing on the SiO2 fiber. Large Ni particles and non-uniform particle distribution are obtained when Ni loading percentage increases. The Ni loading percentage on the fiber analyzed by EDS is shown in Table 1. It is obvious that the actual %Ni depositing on the fiber is close to %Ni loading when the %Ni loading are 5%, and 10%. However, Ni deposited on the fiber catalyst is lower (only 14.42%) when the %Ni loading is 20% implying that 15% Ni loading is maximum percentage loading. The crystallite size calculate from L = Kλ/Δ(2θ) cos θo is indicated in Table 1. The Ni particle sizes range from 11.31 to 13.95 nm. The slight increase in crystallite of 20%Ni/SiO2 fiber catalyst derives from the agglomeration of Ni on the surface of fiber catalyst.

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P. Reubroycharoen et al. Table 1. Actual %Ni deposited on the fiber catalyst and crystallite size. %Ni loading

%Ni deposited on the fiber*

Crystallite size (nm)**

70

a

H2 consumption (a.u.).

%Glycerol conversion .

5 4.53 10 9.35 20 14.42 *Analyzed by EDS, **Calculated from L = Kλ/Δ(2θ) cos θo. Fiber catalyst

60 Porous catalyst

50

11.31 11.37 13.95 271 o C

b

Fiber

428 o C 341 o C 435 o C

Porous

40 2.5

3

3.5

4 4.5 T ime (h)

5

5.5

6

50

150 250

350 450 T ( o C)

550 650

Fig. 4. (a) Time on stream of glycerol steam reforming and (b) TPR profiles of 10%Ni/SiO2 fiber and 10%Ni/SiO2 porous catalysts.

Figure 4 (a) shows that the glycerol conversion of the fiber catalyst is higher than that of the porous catalyst. Moreover, the conversion of the fiber catalyst becomes stable faster than that of the porous catalyst. Syngas produced from the porous catalyst has H2:CO molar ratio of 7.5, while that produced from the fiber catalyst gives H2:CO molar ratio of 1.9. It could be conclude that the fiber catalyst is very selective for produce syngas from glycerol. In general, the activity of catalysts is related to the reduction peak in TPR profiles. In Fig. 4 (b), both catalysts exhibit two reduction peaks at low and high temperature, corresponding to NiO bulk with weak interaction with SiO2 and NiO with fairly strong interaction with SiO2. Ni/SiO2 porous catalyst shows the reduction peaks at 341oC and 435oC while Ni/SiO2 fiber catalyst exhibits the reduction peaks at 271oC and 428oC. It is clear that the first reduction peak of fiber catalyst shifts to lower temperature and the intensity of second reduction peak decreases. This indicates that the fiber catalyst can be easily reduced at lower temperature implying its higher activity compared to the porous one.

4. Conclusion Ni/SiO2 fiber catalyst is successfully produced by electrospinning technique. The fiber shows the average diameter of 1.28 μm–930 nm at TCD of 15 cm. The SEM-EDS results show that Ni is deposits on the surface of SiO2 fiber. Compared with a conventional porous catalyst, the fiber catalyst exhibits higher glycerol steam reforming activity.

Acknowledgement This work is financially supported by Energy Policy and Planning Office, Ministry of Energy, Thailand Research Fund (TRF) and Center for Petroleum Petrochemicals and Advanced Materials, Chulalongkorn University.

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References [1] M.C. Sanchez, R.M. Navarro and J.L.G Fierro, Int. J. Hydrogen Energy, 32 (2007) 1462. [2] S. Adhikari, S. Fernando and A. Haryanto, Renewable energy, 33 (2008) 1097. [3] S. Adhikari, S. Fernando, S.D. To, R. Bricka, P. Steele and A. Haryanto, Energy Fuels, 22 (2008) 1220. [4] J. N. Amor, Appl. Catal. A 176 (1999) 159. [5] B. Zhang, X. Tang, Y. Li, Y. Xu, and W. Shen, Int. J. Hydrogen Energy, 32 (2007) 2367. [6] A. K. Neyestanaki, P. M. Arvela, H. Backman, H. Karhu, T. Salmi, J. Vyrynen, and D. Yu. Murzin, Ind. Eng. Chem. Res., 42 (2003) 3230 [7] S.W. Lee, Y.U. Kim, S.S. Choi, T.Y. Park, Y.L. Joo, S.G. Lee, Mater. Lett. 61 (2007) 889.

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10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Selective preparation of β-MoO3 and silicomolybdic acid(SMA) on MCM-41 from molybdic acid precursor and their partial oxidation performances Tran M. Huong, Nguyen H.H. Phuc, Hironobu Ohkita, Takanori Mizushima, Noriyoshi Kakuta* Department of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi, Aichi-pref. 441-8580, Japan

Abstract SMA(Mo/MCM-41Imp) and β-MoO3(Mo/MCM-41 Evap) were successfully prepared on MCM-41 using the molybdic acid solution. The molybdic acid solution is the effective precursor to generate selectively either SMA or β-MoO3, and those species are anchored on MCM-41 through the formation of SMA. The Mo/MCM-41Imp catalyst showed a good catalytic performance for partial oxidation of methane to formaldehyde while the Mo/MCM-41 Evap is a promising catalyst for partial oxidation of methanol to formaldehyde. Keywords: SMA, β-MoO3, MCM-41, methane, methanol

1. Introduction Mizushima et al. successfully synthesized β-MoO3 through a simple evaporation of a molybdic acid solution which prepared by a cation exchange of an aqueous solution of Na2MoO4.2H2O [1]. The key was an addition of a small amount of nitric acid before the evaporation. A heating the dried molybdic acid at 573K in an oxygen stream led to a formation of a bright yellow powder, which was confirmed to be β-MoO3 and almost free from α-MoO3 by XRD and Raman analyses. Deltcheff et al. pointed out that the molybdic acid was an effective precursor for the synthesis of SMA [2]. This suggests that the formation of anchored SMA species can be expected by the reaction with Si-O species as a new SMA catalyst preparation method. Huong et al. reported the formation of the desired Mo species, either SMA or β-MoO3, on SiO2 and MCM-41 by use of the molybdic acid solution [3]. In this study, we investigated the behavior of both β-MoO3 and SMA species supported on MCM-41 catalysts and their catalytic activities were evaluated through partial oxidation of methane and methanol.

2. Experimental MCM-41(SA=1328m2g-1) was synthesized using a modified procedure reported by Grun et al. [4]. A molybdic acid precursor was prepared by a cation exchange of 1M Na2MoO4.2H2O solution with resin through a one meter long glass column in order to remove Na+ ion [1a]. Either SMA or β-MoO3 supported on MCM-41 catalyst was prepared according to the previous paper [3a]; two methods were employed.

*

Corresponding author, e-mail: [email protected].

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Impregnation method: MCM-41 powder (1.6g) was immersed in the molybdic acid precursor solution(120 mL). The impregnated powder was dried at 383K for 24h. Evaporating dry method: The molybdic acid precursor (120 mL) was mixed with 79mL of 1% HNO3 solution and MCM-41 powder (1.6g) in a flask, and then was submitted to the vacuum evaporation at 323K for 1h. The obtained powder was dried in desiccator for 12h followed by calcination at 573K for 1h under an oxygen stream. Mo concentrations in the prepared catalysts were 20wt%. They were named as 20%Mo/MCM-41Imp and 20%Mo/MCM-41Evap, respectively. Structural analysis was carried out by a JASCO Laser Raman spectrometer(NR-1800). Partial oxidation of methane (POM) [3b] was carried out in a continuous stainless steel fixed-bed reactor at 873K. Partial oxidation of methanol [1b] was also performed in a fixed-bed flow 10mm i.d. Pyrex reactor operating at atmospheric pressure, where air was bubbled in a methanol solution to maintain a constant flow rate of the liquid vapor.

3. Results and discussion 3.1. Characterization of 20%Mo/MCM-41 catalysts

Intensity

Figure 1 displays Raman spectra of catalysts. The spectra of reference materials are also added for α -MoO3 comparison. The 20%Mo/MCM-41Imp catalyst gives typical Raman bands which are corresponding to the bands of SMA. Four characteristic Mo/MCM-41 Evap frequency ranges, 160-290, 340-467, 504-680, and 900-1000 cm-1, are assigned to Mo-O-Mo deformation mode, Mo=O bending mode, symβ -MoO3 metric Mo-O-Mo stretching mode, and symmetric Mo=O terminal stretching mode, respectively [5]. Two bands at 1043 and 1057 cm-1, which Mo/MCM-41 Imp have been seen in the 20%Mo/MCM-41 catalysts, were attributed to background bands of Ar SMA laser. 200 400 600 800 1000 On the other hand, Raman spectrum of the Raman shift (cm-1) 20% Mo/MCM-41Evap catalyst is clearly Fig. 1 Raman spectra of Mo/MCM-41 different from that of the 20%Mo/MCM-41Imp catalysts. catalyst and the spectrum pattern is the same as that of β-MoO3 but not of α-MoO3. As the site symmetry of the molybdenum atoms of β-MoO3 is not known in detail, we could differentiate Raman bands of β-MoO3 from αMoO3 by the absence of the band at 997 cm-1(α-MoO3) and the presence of Mo-O-Mo symmetric stretch band at 777 cm-1(β-MoO3) [6].

3.2. Catalytic performances of SMA catalysts prepared from fresh SMA and from molybdic acid solution Since Mo phases of Mo/MCM-41Imp and Mo/MCM-41Evap catalysts were assigned to SMA and β-MoO3, respectively, partial oxidation of methane is an effective reaction for evaluating the activity of β-MoO3 in the presence of water vapor. This was reported previously that the β−MoO3catalyst(Mo/MCM-41Evap) showed the high selectivity to HCHO and the SMA catalyst(Mo/MCM-41Imp) showed the high conversion of methane [3]. It means that the SMA catalyst is effective for the activation of methane and the β−MoO3 catalyst is active for the selective oxidation to HCHO from activated methane. In order to elucidate in detail the activity of SMA on MCM-41, the SMA

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catalyst was prepared by a conventional impregnation method for comparison. A series of SMA/MCM-41 catalysts were prepared with Mo loading ranged from 1 to 20wt%. The performance of these catalysts in POM is exhibited in Figure 2. Mo/MCM-41Imp catalysts give higher activities in POM at all the ranges of Mo loading. However, at low loading of Mo (1, 5 wt%) the differences was not definitely observed both in conversion of methane and HCHO yield. The 10%Mo/MCM-41Imp catalyst has shown the highest catalytic activity with 4.2% HCHO yield in 11% conversion of methane. A distinct difference was found in the performance of the 20% Mo/MCM-41 Imp and the 20%SMA/MCM-41. The formation of SMA on MCM-41 is expected by the following reaction when the molybdic acid solution is used: SiO2 +12MoO3+2H2O = H4SiMo12O40

12 11

CO2

Yield %

10 The reaction of Si-O of MCMCO 9 HCHO 41 with the molybdic acid under 8 7 excess SiO2 leads to the highly 6 dispersed and anchored SMA 5 species on the MCM-41 surface, 4 3 resulting in the high activity even at 2 20% Mo loading. However, as SMA 1 0 species on the 20%SMA/MCM-41 20% 10% 5% 1% 20% 10% 5% 1% catalyst are unstable, the aggregation SMA/MCM-41 Mo/MCM-41 Imp and decomposition might occur in Fig. 2 Performances of SMA/MCM–41 and Mo/ the reaction condition, resulting in MCM–41 Imp catalysts. the sharp decrease in activity. Consequently, the catalyst preparation using the molybdic acid solution is the effective method for the stabilization of SMA species at higher SMA loadings. Raman spectra of the catalysts after POM demonstrate the reason of good performance of the Mo/MCM-41Imp catalyst. The presence of SMA species was revealed on 1, 5, 10%SMA/MCM-41 catalysts while 1, 5 and 10% Mo/MCM-41Imp catalysts showed both β-MoO3 and SMA after POM. As we reported previously that βMoO3 assists in structural transformation of SMA as well as in the production of formaldehyde, the both species thus are attributable to the higher activity of Mo/MCM41Imp catalyst. At 20% Mo loading, the low activity of SMA/MCM-41 may be due to the appearance of α-MoO3 peak with relatively high intensity. In contrast, the coexistence of SMA, α-MoO3 and β-MoO3 were detected in the spectrum of Mo/MCM41Imp catalyst after POM. This probably makes POM more active because of the predominant β-MoO3 phase.

3.3. Catalytic activity of 20%Mo/MCM-41Imp (SMA) and 20%Mo/MCM-41Evap (β-MoO3) catalysts in methanol oxidation

Methanol oxidation(MO) is an important chemical reaction that produces valuable intermediates used in chemical industry. The production of formaldehyde for the synthesis of phenolic resin from methanol is largely preferred. Compared with other oxidation reactions, MO has a wide selectivity pattern and set of reaction mechanisms [7]. The activities of β-MoO3(20%Mo/MCM-41Evap) and SMA (20%Mo/MCM41Imp) species in MO were investigated. The result is shown in Figure 3. Dimethyl ether (DME) and formaldehyde were main products. At low temperature (523K), DME was generated, and the yield reached up to about 20% of total products but this yield rapidly decreased to 6% at 573K and to 0% at 623K in both catalysts. This means that the dehydrogenation reaction of methanol was performed at low temperature regardless of Mo species. However, there were

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remarkable differences in the HCHO yield and in the conversion of methanol. The HCHO yields of both catalysts reached to maximum at the temperature of 623K, 39% and 79% for the 20%Mo/MCM-41Imp and the 20%Mo/MCM-41Evap, respectively. In counterpoint to the role of the catalysts in POM, β-MoO3 supported on MCM-41 demonstrated a high activity in the oxidation of methanol to formaldehyde. The conversion of methanol also exhibited excellent results, 90% at 623K and almost 100% at 723K, respectively. β -MoO3(20%Mo/MCM-41Evap)

SMA(20%Mo/MCM-41Imp)

100

100

80

80

HCHO

60

60 Conversion Formaldehyde 40 DME

DME

%

%

Conv ersion

40

20

20

0

0 523

573

623

673

723

523

573

623

673

723

Te mpe rature /K

Tempe rature /K

Fig. 3 Catalytic performances of Mo/MCM–41 catalysts in MO.

10

Since the HCHO yield of bare β-MoO3 was a maximum of 30% at 573K[1], the high dispersion of β-MoO3 species on the large surface area of MCM-41, where βMoO3 was stabilized through SMA formed on the interface of MCM-41[3], may α -MoO contribute in enhancement of its catalytic β -MoO activity. On the other hand, the Mo/MCM41Imp catalyst exhibited poor activity even β -MoO (20% Mo/MCM-41Evap) in comparison with fresh SMA catalyst. Figure 4 shows Raman spectra of MCM41 supported catalysts after MO at 623K. No peaks corresponding to SMA and β-MoO3 SMA(20% Mo/MCM-41Imp) were observed in the Mo/MCM-41 Imp 100 200 300 400 500 600 700 800 900 1000 1100 catalyst. This suggests that the low catalyric Raman shift (cm ) activity of Mo/MCM-41Imp is due to the Fig. 4 Raman spectra of 20%Mo/MCM–41 structural transformation of SMA to α-MoO3. catalysts after methanol oxidation at 623 K. In addition, SMA species even anchored on MCM-41 are decomposed easily at 623 K when water was absent in the feed. In contrast, β-MoO3 species act as active species for the formaldehyde formation although some of β-MoO3 was transformed to α-phase. Consequently, it is necessary to find the optimum conditions (temperature, oxygenmethanol ratio, etc.) for the stabilization of β-MoO3 species anchored on MCM-41. 3

Intensity

3

3

-1

References [1] a) T. Mizushima, K. Fukushima, T. M. Huong, H. Ohkita, N. Kakuta, Chem. Lett., 34(2005)986. b) T. Mizushima,, K. Fukushima, H. Ohkita, N. Kakuta, Appl. Catal. A, 326 (2007)106. [2] C. R. Deltcheff, M. Fournier, R. Franck and R. Thouvenot, Inorg. Chem., 22 (1983)207. [3] a) T. M. Huong, K. Fukushima, H. Ohkita, T. Mizushima, N. Kakuta, Catal. Commun., 7(2006)127. b) M. T. Huong, H. Ohikita, T. Mizushima, N. Kakuta, Appl. Catal. A, 287(2005)129. [4] M. Grun, K. K.Unger, A. Matsumoto, K. Tsutsumi, Character. of porous solid, Vol. IV, 81-89. [5] G. Mestl and T.K.K. Srinivasan, Cata. Rev.-Sci. Eng., 40 (1998)451. [6] S. Kasztelan, E. Payen, J.B. Moffat, J. Catal., 112 (1988)320. [7] J.M. Tatibouët, Appl. Catal. A: Gen., 148 (1997)213.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Functionalization of carbon xerogels for the preparation of Pd/C catalysts by grafting of Pd complex Chantal Diverchy,a Sophie Hermans,a Nathalie Job,b Jean-Paul Pirard,b Michel Devillersa a

Université catholique de Louvain, Unité de Chimie des Matériaux Inorganiques et Organiques, Place L. Pasteur 1, Louvain-la-Neuve B-1348, Belgium. b Université de Liège, Laboratoire de Génie Chimique, Sart Tilman B6a, Liège B-4000, Belgium.

Abstract A mesoporous carbon xerogel was functionalized by treatments with nitric acid in order to introduce oxygenated functions at its surface. The carbon supports were characterized by Boehm’s titrations, XPS and nitrogen adsorption, and the results were compared with those obtained with a microporous activated carbon. It was shown that carbons with high oxygen content were obtained. The oxygenated functions were then used as anchors for the grafting of a palladium complex. High Pd surface concentrations are observed by XPS but it appears that the Pd particles are not homogeneous in size and repartition. Keywords: carbon xerogel, palladium, functionalization

1. Introduction Carbon materials are used extensively as supports in heterogeneous catalysis because they present interesting properties such as inertness, cheapness… Moreover, it is possible to modify their surface chemistry by adding functional groups. Among these surface groups, oxygenated functions are by far the most often studied. It has been well documented that these modifications affect, amongst other parameters, their behaviour when used as catalyst supports [1]. However, because the spectroscopic characterization of carbon materials remains a challenge, the study of their surface chemistry is not easy and many developments are still to come. This work consists in the preparation of Pd catalysts supported on carbon xerogel. This kind of carbons was used because their pore texture and surface chemistry are well controlled compared to traditional activated carbons [3]. The aim is to take advantage of oxygenated functions introduced onto the surface of the carbon support in order to improve the interaction between the metallic precursors and the support. The precursor chosen in this study is a water-soluble coordination compound containing carboxylate ligands, which were found to exchange easily at least one ligand to allow metal grafting onto oxygenated surface functions [2].

2. Experimental 2.1. Support synthesis The carbon support used in this study is a mesoporous xerogel X25 (maximum mesopore size ca. 25 nm, SBET ~ 660 m².g-1), obtained by evaporative drying and subsequent pyrolysis of an aqueous resorcinol-formaldehyde gel prepared under well-

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defined conditions [3]. This carbon xerogel was submitted to oxidative treatments using HNO3 as follows: 4 g of crushed xerogel (granulometry: 50-100 µm) was stirred in 100 mL nitric acid 0.2 or 2.5 mol.L-1 under reflux. After 24 h, the solid was recovered by filtration and washed with water on a Soxhlet apparatus. The samples were then dried under vacuum at 50°C.

2.2. Catalyst preparation The synthesis of [Pd(OAc)2(Et2NH)2] was described elsewhere [2]. The preparation of the catalysts was carried out by mixing 0.1742 g of complex and 0.95 g of carbon in 50 mL of water for 24 h. The amount of complex introduced was calculated to obtain a theoretical loading of 5 wt.% of Pd in the final catalysts (after activation). After this treatment, the solution was filtered out and the solid sample was dried under vacuum at 50°C. It was then activated in a CARBOLITE tubular oven at 200°C under N2 flow during 4 h.

2.3. Characterization Boehm’s titration: This method was used to evaluate the amount of carboxylic acid, lactone and phenol groups present at the surface of carbon samples, by stirring 0.5 g of carbon in 50 mL of NaHCO3, Na2CO3 or NaOH [2]. The carbon was filtered out after 24 h and the filtrate was titrated with hydrochloric acid. All solutions were prepared using freshly distilled decarbonated water and maintained under nitrogen. XPS: X-ray photoelectron spectroscopy was carried out on a SSI-X-probe (SSX100/206) Fisons spectrometer. The binding energies were set up by fixing the C1s peak (C-(C,H) component) at 284.8 eV. Three photopeaks (C1s, O1s and N1s) were systematically analyzed for the carbonaceous materials, the Pd3d peak was added for the catalysts. The XPS results were decomposed with the CasaXPS software, using a sum of Gaussian/Lorentzian (85/15) after subtraction of a Shirley-type baseline. The constraints used for decomposition of the Pd3d peaks were as follows: imposing an area ratio Pd3d5/2 / Pd3d3/2 of 1.5, a difference in the binding energies (Pd3d3/2 - Pd3d5/2) of 5.26 eV and a FWHM ratio (for the Pd3d5/2 / Pd3d3/2 peaks) of 1. Nitrogen adsorption: The specific surface areas were measured on an ASAP 2000 Micromeritics instrument by nitrogen adsorption at -196°C. Before analysis, the sample (0.1 g) was outgassed during several hours at 150°C under a pressure of 500 Pa. Data were analyzed using the classical BET theory in order to calculate the specific surface area, SBET. SEM: Topographic SEM images were obtained using a FEG Digital Scanning (DMS 982 Gemini LEO) electron microscope fitted with an EDAX analyzer (Phoenix CDU LEAP). Atomic absorption (AA): Atomic absorption was carried out on a PERKINELMER 3110 spectrometer in order to determine the possible residual metal amounts in the filtrates.

3. Results and discussion The mesoporous carbon xerogel support (X25) was submitted to an oxidative treatment using HNO3 in order to increase the amount of oxygenated surface functions. Such treatments were previously studied using a microporous activated carbon (SX+) supplied by NORIT (SBET ~ 920 m².g-1) [2].

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Table 1. Characterization of the carbon xerogel supports compared to the SX+ carbon supports. Notation C0 C1 C2

Treatment Carbon used CHNO3 (mol.L-1) Xerogel SX+ Xerogel 0.2 SX+ Xerogel 2.5 SX+

O/C (XPS) 0.04 0.04 0.14 0.10 0.27 0.19

Total acidity (mmol.(100 gC)-1) 14 27 149 110 424 310

SBET (m².g-1) 624 922 576 913 478 756

It is found, as previously for the SX+ activated carbon [2], that the acidity or the oxygen content of the mesoporous sample increases with the HNO3 treatment. The rise is, however, more pronounced when using the xerogel than SX+ (Table 1). Boehm’s titrations indicate that this increase in acidity is mainly due to the formation of carboxylic acids and phenols (Figure 1). A decrease of the BET surface area is also observed after functionalization in both cases (Table 1).

Acidity (mmol/100g C)

400 350

X25

300

SX+

250 200 150 100 50 0 Carboxylic acid

Lactone

Phenol

T otal

Figure 1. Distribution of acid functions for the most functionalized carbon supports (C2).

The oxygenated functions introduced on the supports were then used as anchors for the grafting of [Pd(OAc)2(Et2NH)2]. It appears that the amount of incorporated Pd depends on the degree of oxidation of the carbon xerogel support whereas the Pd loading is maximum for the SX+ supports (Table 2). Moreover, Pd atomic % (XPS) measured before activation are higher for samples prepared on the mesoporous carbon xerogel support but are not related to the surface acidity. The higher Pd atomic % obtained on xerogels supports may be due to a stronger hydrophobicity of these samples compared to SX+ supports. As a result, the Pd complex (dissolved in water) may not easily penetrate into the pores and so might react preferentially on the external surface. Moreover, it seems that when the loading increases on xerogels, the Pd atomic % decreases. For example, the most functionalized support, which contains the highest quantity of Pd, presents the lowest Pd atomic %. This may be the result of an agglomeration of Pd, linked to the higher amount of Pd introduced combined with a loss of specific surface area. The grafted samples were then activated and characterized after activation by XPS and SEM. Results show that the surface Pd atomic % decreases after activation but remains higher on xerogels than on SX+ supports (data not shown). SEM images show that the samples obtained are inhomogeneous in terms of particles size and repartition. Moreover, big particles are observed in all cases, the smallest ones being obtained with the intermediate functionalized support (Figure 2). Various hypotheses may be put

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forward to explain these results. One explanation may be that the intermediate support contains an optimal number of oxygenated functions: the most functionalized support with a lot of thermally unstable functions may lead to agglomeration during the activation process, while for the non-modified support, the grafting process may be not optimal due to the smaller amount of functions. Finally, it can be seen from Table 2 and Figure 2 that the particle size and the atomic Pd % follow the same evolution, meaning that this trend was already present before activation. Table 2. Catalyst characterization by XPS before activation and Pd loading obtained by analysing the filtrate by atomic absorption spectrometry (AA). Treatment C0 C1 C2

Pd atomic % (XPS) Xerogel SX+ 2.62 0.73 3.21 0.78 1.46 0.78

Pd loading (AA) (wt.%) Xerogel SX+ 3.90 4.97 3.52 4.97 5.00 5.00

2 µm

(xerogel C0)

(xerogel C1)

(xerogel C2)

Figure 2. SEM images of the Pd/C catalysts obtained on xerogel carbon supports with variable degrees of functionalization.

4. Conclusion The functionalization of a carbon xerogel sample was carried out with success and led to higher oxygen content. These functionalized carbons were used as supports for the grafting of a palladium complex. It was shown that, by opposition with a commercial microporous activated carbon support (SX+), the degree of functionalization influenced the amount of introduced palladium.

Acknowledgement The authors wish to thank the PAI INANOMAT, and FNRS for financial support and the NORIT firm for supplying the SX+ carbon.

References [1] P. E. Fanning and M. A. Vannice, 1993, Carbon, 31, 721. [2] S. Hermans, C. Diverchy, O. Demoulin, V. Dubois, E.M. Gaigneaux and M. Devillers, 2006, J. Catal., 243, 239. [3] N. Job, R. Pirard, J. Marien and J.-P. Pirard, 2004, Carbon, 42, 619. [4] A. Deffernez, S. Hermans and M. Devillers, 2005, Appl. Catal. A-Gen., 282, 303.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of Pd-Bi catalysts by grafting of coordination compounds onto functionalized carbon supports Chantal Diverchy, Sophie Hermans, Michel Devillers Université catholique de Louvain, Unité de Chimie des Matériaux Inorganiques et Organiques, Place L. Pasteur 1, Louvain-la-Neuve B-1348, Belgium

Abstract HNO3 functionalized carbons are used as supports for the preparation of bimetallic PdBi/C catalysts. The aim is to take advantage of the oxygenated surface functions introduced on the carbon, which are expected to act as anchoring sites. Two neutral coordination complexes were selected to incorporate the two metals onto differently functionalized carbon supports. The grafted samples are then submitted to an activation treatment and characterized by SEM, XRD and XPS. It appears that the order of incorporation strongly influences the surface properties of the catalysts and thus modifies their activity in the oxidation of glucose. Keywords: carbon, palladium, bismuth, functionalization, grafting

1. Introduction Activated carbons are widely used as supports for heterogeneous catalysis. They are especially suited for applications carried out in the liquid phase because they are robust, cheap and easy to recover. However, as carbonaceous materials are difficult to characterize spectroscopically, the study of their surface chemistry remains a challenge. In this study, carbon-supported catalysts were synthesized using the following strategy [1]: (i) a chemical functionalization of the carbon surface with HNO3 to allow (ii) the grafting of selected metallic precursors. The present work deals with the preparation of Pd-Bi/C catalysts because of the interest of such an association in selective oxidation processes involving renewables like carbohydrates [2-4].

2. Experimental 2.1. Support functionalization The carbon used in this study is an activated carbon SX+ supplied by Norit (C0). It was submitted to oxidative treatment by stirring 6 g of carbon in 150 mL of nitric acid 0.2 (C1) or 2.5 (C2) mol.L-1 under reflux. After 24 h, the carbon was recovered by filtration and washed with water on a Soxhlet apparatus. The samples were then dried under vacuum at 50°C.

2.2. Catalysts preparation The complex [Pd(OAc)2(Et2NH)2] was synthesized as described elsewhere [1] while [Bi(dpm)3] (dpm = dipivaloylmethanate) is commercially available (Acros). The amount of complex introduced was calculated to obtained 5 wt.% in both metals in the final catalysts (after activation), i.e. 0.1742 g for [Pd(OAc)2(Et2NH)2] and 0.1818 g for [Bi(dpm)3] with 0.95 g of C for the grafting of the first metal; the amount of second metal grafted was determined on the basis of grafting models, assuming that the grafted

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parts were ‘Pd(OAc)(Et2NH)’ and ‘Bi(dpm)2’. The syntheses were realized in 50 mL of solvent. After 24 h of treatment, the samples were systematically recovered by filtration, washed with water and dried under vacuum at 50°C. Monometallic samples were first prepared under different grafting temperatures (room temperature (RT), 50°C and reflux) to determine the optimal grafting conditions. The solvents were chosen depending on the solubility of the metallic complexes (water and toluene for the Pd complex, toluene only for the Bi complex). Following this, bimetallic samples were prepared by consecutive grafting of the complexes, by grafting first the Pd complex (in water at RT) or the Bi complex (in toluene at RT). Other bimetallic samples were also obtained by grafting the Pd complex as described previously followed or preceded by deposition of bismuth oxoacetate in heptane, as described in the literature [4]. All bimetallic samples obtained were activated by thermal treatment for 8 h at 500°C under a N2 flow.

2.3. Characterization Boehm’s titration: This method was used to evaluate the amount of carboxylic acid, lactone and phenol groups present at the surface of carbon samples, by stirring 0.5 g of carbon in 50 mL of NaHCO3, Na2CO3 or NaOH. The carbon was filtered out after 24 h and the filtrate was titrated with hydrochloric acid. All solutions were prepared using freshly distilled decarbonated water and maintained under nitrogen. XPS: X-ray photoelectron spectroscopy was carried out on a SSI-X-probe (SSX100/206) Fisons spectrometer. The binding energies were set up by fixing the C1s peak (C-(C,H) component) at 284.8 eV. The XPS results were decomposed, with the CasaXPS software, using a sum of Gaussian/Lorentzian (85/15) after subtraction of a Shirley-type baseline. The constraints used for decomposition of the Pd3d peaks were as follows: imposing an area ratio Pd3d5/2 / Pd3d3/2 of 1.50, a difference in the binding energies (Pd3d3/2 - Pd3d5/2) of 5.26 eV and a FWHM ratio of 1. The constraints used for decomposition of the Bi4f peak were as follows: imposing an area ratio Bi4f7/2 / Bi4f5/2 of 1.33, a difference in the binding energies of 5.31 eV and a FWHM ratio of 1. XRD: Diffractograms were recorded on a SIEMENS D5000 diffractometer. Phases were identified with reference to the JCPDS database. Atomic absorption (AA): Atomic absorption was carried out on a PERKINELMER 3110 spectrometer. Catalytic tests: The catalysts were tested in the oxidation of glucose into gluconic acid, as described elsewhere [4]. The tests were carried out at 50°C during 4 h with 54 mg of catalyst. Catalytic results are expressed as glucose conversion.

3. Results and discussion The treatments with HNO3 caused an increase in the number of oxygenated (acid) functions on the surface of the carbon support. Figure 1 shows the quantification of the different acidic functions in the initial and the two functionalized supports, as determined by Boehm’s titration [5].

Preparation of Pd-Bi catalysts by grafting

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Figure 1. Distribution of acid functions in the non-modified and functionalized supports.

Bimetallic catalysts were synthesized using [Pd(OAc)2(Et2NH)2] combined with [Bi(dpm)3]. These complexes were chosen because they were thought to exchange easily at least one ligand for surface oxygenated functions. Table 1. Grafting of [Pd(OAc)2(Et2NH)2] in toluene and in water on non-modified and functionalized carbon supports.

Toluene

Solvent

Water

Carbon C0 C2 C0 C2 C0 C2 C0 C2

T RT RT 50°C 50°C Reflux Reflux RT RT

Pd atomic % (XPS) 0.90 0.75 0.18 0.72 3.84 1.10 0.68 0.81

Pd % on C (AA) 1.25 4.73 1.64 4.97 4.73 5.00 4.97 5.00

Monometallic samples were first prepared. By varying the grafting temperature and the solvent used, optimal grafting conditions were determined (Table 1). The monometallic samples obtained were analysed by XPS and the amount of metal grafted was determined by measuring the residual metal amounts in the filtrates by atomic absorption spectroscopy. As shown in Table 1, the results depend strongly on the solvent and the type of support. When working with toluene, with the non-functionalized support (C0), the amount of grafted Pd complex is highly temperature-dependent, and approaches the expected 5 wt. % only under reflux. The Pd grafting yield is in any case higher with the functionalized support (C2), and reaches 5 wt. % under reflux. Grafting from aqueous solution appears to be more efficient at RT even for the non-treated support. Water was thus selected for the preparation of bimetallic catalysts even if higher surface Pd atomic % could be obtained when using toluene. On the contrary, the Bi complex was quantitatively grafted at room temperature in toluene. Following these results, bimetallic samples were prepared by a consecutive grafting procedure at RT using soft conditions in water for the Pd complex and in toluene for the Bi one. Samples are noted PdBi when the Pd complex was grafted first and BiPd when the Bi complex was grafted first. Other bimetallic samples were also obtained by grafting of the Pd complex followed or preceded by deposition of bismuth oxoacetate in heptane. The obtained samples were then activated, characterized by XPS, SEM and XRD, and tested in the reaction of glucose into gluconic acid.

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Table 2. Characterization of the samples obtained by grafting of Pd and Bi (after activation). Support PdBi BiPd

C0 C1 C2 C0 C1 C2

Pd % 0.54 0.53 0.41 0.66 0.36 0.37

XPS Bi % 0.56 0.51 0.35 2.54 0.30 0.25

Pd/Bi 0.96 1.05 1.18 0.26 1.22 1.48

Activity (%/mgPd) 12.1 11.6 9.60 0.38 5.23 7.75

All the bimetallic samples prepared in this study contained 5 wt.% of Pd and 5 wt.% of Bi, while the surface Pd and Bi atomic % (XPS) decreased with the degree of functionalization when grafting both metals (Table 2). Pd/Bi XPS ratios reached a unique value of 1 for activated PdBi samples whereas this ratio increased with the degree of functionalization for activated BiPd samples. These experimental surface Pd/Bi atomic ratios appear to be systematically lower than the theoretical value of 1.96 calculated from the bulk composition, independently from the order of incorporation. This reflects the lower surface energy of Bi with respect to Pd, leading to surface enrichment in this element, as always observed [3]. Various PdxBiy intermetallics were suspected by XRD. The activity decreased with the functionalization in PdBi samples while it increased in BiPd samples (Table 2 and Table 3). These catalytic results are linked to the amount of Pd and Bi on the surface and also to their surface ratio. Table 3. Characterization of the activated samples obtained by the grafting-deposition method. Support PdBi BiPd

C0 C2 C0 C2

Pd % 0.53 0.40 0.26 0.32

XPS Bi % 0.62 0.35 0.29 0.27

Pd/Bi 0.84 1.14 0.90 1.18

Activity (%/mgPd) 10.2 5.39 7.22 10.3

4. Conclusion Bimetallic Pd-Bi/C catalysts were prepared on carbon supports bearing different amounts of oxygenated groups. The grafting of Pd and Bi complexes was carried out in different solvents and at variable temperatures, allowing the determination of optimal conditions. Bimetallic catalysts were then prepared either by grafting Pd before Bi or the opposite. Other bimetallic samples were synthesized by a mixed process (grafting for Pd and deposition for Bi). It was observed that the properties of these catalysts were influenced by the degree of functionalization of the support but also by the order of incorporation of the metals. Their activity in the oxidation of glucose seemed to be linked to the amount of surface metal but also to their surface ratio.

References [1] S. Hermans, C. Diverchy, O. Demoulin, V. Dubois, E.M. Gaigneaux and M. Devillers, 2006, J. Catal., 243, 239. [2] P. Gallezot, 2007, Catal. Today, 121, 76. [3] M. Wenkin, R. Touillaux, P. Ruiz, B. Delmon and M. Devillers, 1996, Appl. Catal. A-Gen., 148, 181. [4] M. Wenkin, P. Ruiz, B. Delmon and M. Devillers, 2002, J. Mol. Catal. A-Chem, 180, 141. [5] C. Diverchy, S. Hermans and M. Devillers, 2006, Stud. Surf. Sci. Catal., 162, 569.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Novel dicarboxylate heteroaromatic metal organic frameworks as the catalyst supports for the hydrogenation reaction Vera I. Isaeva,*a Olga P. Tkachenko,a Igor V. Mishin,a Elena V. Afonina,a Gennady I. Kapustin,a Ludmila. M. Kozlova,a Wolfgang Grünert,b and Leonid M. Kustov a

N. D. Zelinsky Institute of Organic Chemistry RAS, Leninsky pr. 47, Moscow 119991, Russia b Lehrstuhl Technische Chemie, Ruhr-University Bochum, D-44780 Bochum, Germany

Abstract The novel Zn-derived heteroaromatic metal organic frameworks (MOFs) based on 2(5)pyridinedicarboxylate and 2(5)-pyrazinedicarboxylate ligands were synthesized. In order to elucidate the framework nature effect, a reference sample of aromatic MOF-5 derived from Zn4O clusters and benzene-1,4-dicarboxylate linkers was prepared. The variation of the preparation procedure parameters in respect to the MOF texture (porosity, surface area) was accomplished. The synthesized metal organic frameworks were characterized by the combination of the physicochemical methods: XRD, volumetric N2 adsorption/desorption, DRIFT, and XAS. The catalytic activity of the Pd-containing MOFs in the liquid-phase hydrogenation of cyclohexene (20°C, PH2 1atm) was higher than that of Pd on activated carbon. Keywords: Metal-organic framework, characterization, hydrogenation

1. Introduction The synthesis of metal organic frameworks (MOFs) is a realization of an idea of constructing new materials with tunable physical and chemical properties. However, most of these synthesized systems are being explored from the point of view of its application in adsorption and separation processes with a relatively few citations of the use of MOF materials in catalytic reactions [1-3]. The situation has changed during the recent years. The research work number of the application of the metal organic framework in the catalysis field significantly arose. The phenylenecarboxylate MOFs present a most wide studied group of these materials due to its rigid three dimensional structure characterized by permanent porosity. As a rule these systems are used for the catalysis purposes as the traditional carriers like activated carbons and zeolites. The present contribution deals with the synthesis of the novel porous heteroaromatic dicarboxylate Zn-based MOFs, and its utilization as the supports for the Pd–containing catalysts for the liquid-phase hydrogenation of cyclohexene under mild conditions. Two alternative procedures were used for the synthesis: solvothermal (100°C) and direct mixing methods (20-80°C). In order to compare the MOF texture characteristics as well as the catalytic performance in the hydrogenation tests, a sample of the reference aromatic metal organic framework (MOF-5) derived from Zn4O clusters and benzene1,4-dicarboxylate linkers was prepared.

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2. Experimental 2.1. Synthesis Two alternative procedures were used for the MOF sample preparation: solvothermal and direct mixing methods (Table 1). The syntheses of the samples 2 and 3 were carried out according to the procedure developed by us for the preparation of MOFs based on 2(5)-pyridinedicarboxylic acid and 2(5)-pyrazinedicarboxylic acid respectively. Sample 1. Zn(NO3)2 6H2O (0.350 g, 1.177 mmol) and 2(5)-pyridinedicarboxylic acid (0.197 g, 1.179 mmol) were dissolved in a mixture of N,N’-dimethylformamide (DMF) (9 ml) and toluene (35 ml). The reaction mixture was homogenized (1 h) for solvothermal treatment. The solution was heated in a Teflon-line stain-less autoclave (95°C, 20 h), cooled to room temperature and filtered off in Ar flow. The resulted pale crystals were washed with DMF. The material was evacuated for 6 h (10-3 Hg, 150°C). Sample 2. Zn(NO3)2 6H2O (1.21 g, 4.07 mmol), 2(5)-pyridinedicarboxylic acid (0.33 g, 3.1 mmol), DMF (40 ml) were stirred (80°C, 1.5 ч). The resulting solid was centrifuged repeatedly, washed with DMF (3 x 20 mL) and dried under vacuum (10-3 Hg, 200°C). Sample 3. Zn(NO3)2 6H2O (2.3 g, 7.81 mmol), 2(5)-pyrazinedicarboxylic acid (0.623 g, 3.1 mmol), DMF (85 ml) were stirred (80°C, 1.5 ч). The resulting solid was centrifuged repeatedly, washed with DMF (3 x 20 mL) and dried under vacuum (10-3 Hg, 60°C). Sample 4 was synthesized according to [4]. Table 1. Preparation procedure and composition of MOFs and Pd/MOFs. Sample 1 2 3 4 2a, 3a, 4a

Sample composition Zn, 2(5)-pyridinedicarboxylate Zn, 2(5)-pyridinedicarboxylate Zn, 2(5)-pyrazinedicarboxylate Zn, 1,4-benzenedicarboxylate Pd-containing MOF samples 2, 3, 4

Preparation method solvothermal direct mixing direct mixing direct mixing incipient wetness impregnation

2.2. Catalyst preparation and catalytic performance Pd-containing MOFs were prepared by impregnation of the parent frameworks with a Pd(OAc)2 solution in dry chloroform (1% wt Pd) analogously to Pd(acac)2 deposition [1]. The solution of Pd(OAc)2 (0.032 - 0.050 g) dissolved in chloroform (0.30 ml) was slowly added to evacuated MOF samples (0.5 g) with formation a light orange paste. The solvent was evaporated under continuous stirring. The Pd(OAc)2/MOFs were dried under reduced pressure (20°C, 4 h). The samples of Pd/heteroaromatic MOFs (2a and 3a) were obtained by heating under vacuum (140°C, 4 h) and Pd/aromatic MOF-5 reference sample (4a) was obtained at 200°C for 4 h. The hydrogenation tests were carried out in absolute 1,4-dioxane (20°C, PH2 1atm, catalyst 0.05 g, cyclohexene 0.2 mL). The reaction products were analyzed by GLC equipped with a FID detector and a capillary column.

2.3. Characterization N2 adsorption data were obtained at -196°C by a volumetric method. Specific surface areas were calculated according to the BET equation. Powder XRD patterns were recorded with a DRON 3M diffractometer using Cu Kα radiation in Bragg-Brentano reflecting and Debye–Sherrer transmission geometry (λ=1.54 Å). The DRIFT spectra were recorded at room temperature with Nicolet 460 Protégé spectrometer with a diffuse reflectance attachment. Before IR study samples were evacuated at 200°C for 1 h. The CD3CN was adsorbed at 20°C and saturated vapour pressure. X-ray absorption spectra

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(Zn K edge at 9659 eV) were measured at the Hasylab X1. The spectra were recorded in the transmission mode at -190°C. The spectrum of a metal foil was registered simultaneously between the second and third ionization chambers for energy calibration. The EXAFS data analysis was performed using the software package VIPER. Reference spectra were taken using standard reference compounds: ZnO and Zn-foil. The fitting was done in the k- and r-spaces.

3. Results and discussion 3.1. Characterization All synthesized MOF samples are characterized by proper crystallinity. The synthesis procedure does not remarkably influence the textural parameters of the resulted metal organic framework. The specific surface areas for the 2(5)-pyridinedicarboxylate samples 1 and 2 are 270-300 m2/g. These values are lower, than for 1,4-benzenedirboxylate (MOF-5) sample 4 (1000 m2/g). Despite the crystallinity retention after evacuation, the 2(5)-pyrazinedicarboxylate sample 3 has no surface area. Tentatively, such difference in surface areas could be explained by lower micropore volume for the samples 1 and 2 or lake the permanent porosity for the 2(5)-pyrazinedicarboxylate framework (sample 3). Figure 1 shows the vibration bands in the region 1400 and 1700 cm-1 (a) corresponding symmetric and asymmetric vibrations of C=O bond in carboxylate ion and aromatic ring, while a few bands in the region ~ 3030 сm-1 (b) belong to valent and combination C-H vibrations of aromatic systems (sample 1 - 4, 4a). These data confirm the retention of heterocyclic or phenylenedicarboxylate bridge fragments in the MOF structures.

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Figure 1. DRIFT spectra of MOFs.

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2250

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Figure 2. DRIFT spectra of CD3CN.

The DRIFT spectra of adsorbed acetonitrile-d3 on the samples 3, 4, 4a (Figure 2) show the presence of strong Zn2+ Lewis acid sites at the MOF surface. The C≡N stretching vibrations frequency shift relative to the gas phase of acetonitrile-d3 (2253 cm-1) is 45-55 cm-1. The XANES (Figure 3a) evidence that zinc exists as Zn2+ ions in all synthesized organic frameworks. EXAFS spectra exhibit pronounced differences (Figure 3b) in the position and the intensity of the second peak. The analysis of the EXAFS oscillations shows that the nearest neighbors of the central Zn atom in all synthesized frameworks are O atoms with an average coordination number (CN) 3-4 and with Zn-O real distance ~ 1.95-2.04 Ǻ. Next neighbors in the sample 4 (MOF-5) are Zn atoms with average coordination numbers of 3 and 3.20-3.22 Ǻ real distance. The presence of Zn neighboring atoms in this sample suggests the presence in our samples of some Zn species and/or MOF frameworks interpenetrating each other like that observed in [5]. The introduction of Pd in this sample results in the increase of the amount of interweaved cells, the average Zn-Zn coordination number in 4a sample is 4.

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3.2. Hydrogenation reaction As it was mentioned above (see Experimental section) incipient wetness impregnation was used for the preparation of Pd-containing MOF samples. It should be noted that the solvent quantity needed for impregnation is lower, than in case of the reference sample 4 (MOF-5). Probably it could be connected with the different framework dimensionality (2D and 3D) and various pore openings of the synthesized heteroaromatic (sample 2, 3) and aromatic (sample 4) MOFs. Despite the differences in specific surface areas and framework nature the catalytic activities of synthesized frameworks in cyclohexene hydrogenation are similar. The specific surface area does not influence in the activity that indicates the localization of Pd mainly in outer surface of MOF microcrystals, i.e. all Pd species are accessible to cyclohexene. The leaching of Pd from the metal organic support is not detected. The XRD patterns indicate the retention of the crystallinity of Pd/MOFs systems both in the impregnation course and during the reaction. Using Pd/MOFs this reaction proceeds much faster, than on 5%Pd/C (Figure 4). Cyclohexene is hydrogenated selectively to cyclohexane over Pd/MOFs. Any traces of benzene due to disproportionation reaction into benzene and cyclohexane are found in hydrogenation course unlike the hydrogenation reaction on Pd/C. Probably above mentioned observations indicate the advantage of MOFs as the highly ordered and crystalline catalytic systems as compared to activated carbon with an irregular structure [1]. 100

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Figure 3. Zn K XANES (a) and FT EXAFS (b) of MOFs and ZnO.

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FT ( χ k 2 )

Normalized absorption

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Figure 4. Cyclohexene hydrogenation.

4. Conclusions The novel Zn-derived metal organic frameworks based on heterocyclic 2(5)pyridinedicarboxylate and 2(5)-pyrazinedicarboxylate bridging ligands were synthesized. The heteroaromatic MOFs were utilized as the supports of the Pd catalysts for the liquid-phase hydrogenation of cyclohexene. Pd/MOF systems showed the higher catalytic activity in this reaction in comparison with Pd supported on activated carbon.

References [1] Sabo M., Henschel A., Frode H., Klemm E., Kaskel S., J. Mater. Chem., 17(2007) 38273832. [2] Xamena, F. X. L. i.; Abad, A.; Corma, A.; Garcia, H J. Catal., 250 (2)(2007) 294-298. [3] Opelt S., Turk S., Dietzsch E., Sabo M., Henschel A., Kaskel S., Klemm E., Catal. Commun., 9(2008)1286-1290. [4] Isaeva V.I., Tkachenko O.P., Mishin I.V., Kostin A.A., Brueva T.R., Klementiev K.V., Kustov L.M., Topics in Chemistry and Materials Science. Advanced Micro and Mesoporous Materials, 1(2007) 155-162. [5] J. Havicovic, M. Bjorgen, U. Olsbye, P.D.C. Dietzel, S. Bordiga, C. Prestipino, C. Lamberti and K.-P. Lillerud J., Am. Chem. Soc., 129 (2007) 3612-3620.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Monitoring of the state of silver in porous oxides during catalyst preparation Elie Sayah, Dalil Brouri, Anne Davidson, Pascale Massiani Laboratoire de Réactivité de surface, UPMC-Université Pierre et Marie Curie, CNRS-UMR 7609, 4 place Jussieu casier 178, 75252 Paris cedex 05, France Tel.: +33 1 44274917, Fax: +33 1 44274917 email: [email protected]

Abstract Silver-supported porous oxides are promising heterogeneous catalysts for air treatment. Classical ways of preparation are incipient wet impregnation or ion exchange (in case of zeolites) of a support with silver nitrate. Nevertheless, this precursor is known for its photosensitivity. The latter can lead to preparations that are not well controlled yet. In this contribution, the influence of the nature of the oxide support and of the thermal activation conditions towards the state and the dispersion of silver in the catalyst is investigated. Complementary characterization techniques are used, i.e. Temperature Programmed Reduction (TPR) coupled with mass spectrometry (MS), UV-Visible spectroscopy (UV-Vis.) and Transmission Electron Microscopy (TEM). Keywords: zeolites, mesoporous oxide, silver cations, TEM microscopy, nanoparticles

1. Introduction Silver supported on porous oxides in the form of isolated species or nanoparticles have gained considerable attention in the last years. They can be potential catalysts for air treatment in industrial and automotive processes such as deNOx [1] and VOC's decomposition [2]. Actually, the use of silver, alone or combined to base metals [3], would represent a cheaper alternative to classic noble metals such as Pt and Pd used for this kind of reactions. Different types of active silver phases have been proposed, depending on the work. Small metallic silver nanoparticles have been reported for the SCR of NOx by methane [4]. Small charged clusters have been proposed for the SCR of NOx by propane [5]. Finally, dispersed silver oxide clusters have been considered in the case of the catalytic oxidation of methyl ethyl ketone [6]. In addition, it has been reported that the active phase may be a combination of the above species. For instance, the presence of both metallic silver and silver oxide species has been shown to enhance the catalytic oxidation of acrylonitryle [7]. These various examples illustrate the complexity of the silver-based catalytic systems and it points to the need for a deeper understanding of the evolution of the silver state under reaction conditions. Several studies were carried out on Ag-based catalysts supported on different porous oxides but no general consensus concerning the nature and the evolution of the silver species under thermal/gas treatment is reached yet. In this contribution, the influence of the oxide support, the silver deposition method (exchange or impregnation) and the activation conditions on the state of silver are investigated. The final size, the dispersion and the state of the nanoparticles are evaluated by transmission electron microscopy (TEM).

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2. Experimental Ag/13X(IWI) and Ag/SBA-15(IWI) catalysts with 3 wt.% Ag were prepared by incipient wet impregnation method [8] using an aqueous solution of silver nitrate in a rotary evaporator at 60°C. After impregnation, vacuum was applied at the same temperature to remove the excess water. The resulting powder was left to dry in air. A silver 3 wt.% exchanged faujasite Ag/13X(E) was prepared by exchanging the zeolite with the adequate amount of a diluted solution of silver nitrate in the dark [9,10]. All samples were calcined in air up to 500°C (5°C/min) then temperature programmed reduction (TPR) was performed under flowing 5%H2/Ar (7°C/min) and followed by both catharometry (TCD, Autochem 2910, Micromeritics) and mass spectrometry (MS, HPR20/DSMS). UV-Visible spectroscopy (UV-Vis.) was carried out on a Varian Cary 5000 spectrophotometer. For TEM experiments, performed after TPR at 700°C, samples powders weare deposited on Cu grid covered with a carbon film. Observations were made on a JEOL 2011 UHR (200kV accelerating voltage and LaB6 emission) microscope equipped with an Orius Gatan camera.

3. Results Transmission electron microscopy was carried out to compare the silver dispersion on the three different samples after TPR. Representative micrographs and their related histograms of particle sizes are shown in Figure 1. On Ag/13X(E), small particles are present with a mean diameter of 7 nm. Much bigger particles with a mean diameter of 22 nm are formed on Ag/SBA-15(IWI). In the case of the silver impregnated zeolite Ag/13X(IWI), two populations with a mean particle diameter of 7 and 24 nm are found.

(a)

(b)

(c)

(d)

2.8 Ǻ (oxide) 2.36 Ǻ (metallic) Figure 1. Representative micrographs of Ag/13X(IWI) (a), Ag/SBA-15(IWI) (b) and Ag/13X(E) (c) and corresponding histograms of particle diameters (in arbitrary unit). High Resolution Electron Microscopy (HREM) micrograph of Ag/13X(E) after calcinations; both metallic silver and silver oxide inter-reticular distances are found in the image: 2.4 Å and 2.8 Å characteristic of (111) fcc metallic silver and (111) cubic silver oxide (d).

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For a better understanding of the different particle sizes, TPR/MS and UV-Vis. spectroscopy were conducted after reduction of the samples at different temperatures. Water production, detected by mass spectrometry (m/e=17,18), enables to differentiate whether the metal reduction involves oxide or cationic species. Figure 2 shows the results of TPR followed by simultaneous TCD and MS measurements for the silver exchanged faujasite Ag/13X(E) sample that gives the best dispersion. Three temperature ranges are detected: I (80-200°C), II (260-400°C), III (423-540°C). Some authors attributed the low temperature zone (I) to the reduction of silver cations into charged clusters on Ag/MFI [4,5,12]. Other authors attributed this peak to the reduction of silver oxides on Ag/HY [6]. At higher temperatures (zone III), according to Baek et al. [6], reduction of cations into metallic silver occurs. Nevertheless, other authors attributed this high temperature peak to a reduction of silver clusters [4,5,12]. From our data, only the second temperature range involves a water production, being thus assignable to the reduction of oxide species as was already proposed [13]. The two other low and high temperature ranges (I and III) unambiguously correspond to the reduction of cationic species. Concerning Ag/SBA-15(IWI), no significant TCD signal is detected upon sample reduction. This suggests that the AgNO3 impregnation on this support followed by calcination led mainly to reduced Ag particles with low dispersion as is seen by TEM (Figure 1b). The formation of significantly smaller particles on the Ag/13X(E) sample is related to the reduction of exchanged silver cations stabilised by the zeolite support. Diffuse reflectance UV-Visible spectroscopy was carried out to further identify the transformations of the cationic species in the exchanged zeolite after reduction, at different temperatures (Figure 3). This allows a better comprehension of the silver reduction mechanism. In freshly calcined Ag/13X(E), cationic silver (band at 235nm) and silver clusters (246-280 nm) with a slight contribution of metallic silver were detected (>290 nm) [11]. The decrease of the cationic silver band contribution when the temperature increases to 700°C, and the simultaneous increase of the metallic and clusters bands, suggest a two step formation of the silver particles in the exchanged silver zeolite as follows: Ag+ ÆAgnm+ ÆAg.

Figure 2. H2 -TPR profile of Ag/13X(E) followed by mass spectrometry.

Figure 3. UV-Visible spectra of the Ag/13X(E) after TPO at 500 (a) followed by TPR at 350°C (b), 700°C (c).

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Finally, in addition to the cationic and oxide silver species seen above by combined TPR/MS and UV-Vis. data, the presence of a metallic phase is also found by TEM on Ag/13X(E) freshly calcined. Therefore, the coexistence of either oxidized or reduced Ag particles is deduced from the measurement of two distinct inter-reticular distances at 2.4 and 2.8 Å corresponding to oxide and metallic silver, respectively (Figure 1d).

4. Conclusion In this work, the state of silver, the formation and the final size and dispersion of the particles are investigated. The combination of TPR/MS and UV-Vis. spectroscopy allows a better comprehension of the transformations occurring during thermal/gas treatment In addition, TEM shows that the exchanged zeolite exhibits the smaller particles with best dispersion that can be attributed to the stabilization, and next, to the reduction of exchanged ions

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

M. Boutros et al, 2009, Appl. Catal. B, 91, 640-648. E. Cordi et al, 1997, Appl. Catal. A, 151, 179-191. N. Luo et al, 2009, Mater. Lett., 63, 154-156. C. Shi et al, 2004, , Appl Catal B, 51, 171-181. J. Shibata et al, 2004, J. Catal, 227, 367-374. S-W. Baek et al, 2004, Catal Today, 93-95, 575-581. T. Namba et al, 2008, J. Catal, 259, 250-259. Y. Li et al, 2009, Appl. Catal. B, 89, 659-664. J. R. Morton et al, 1987, Zeolites, 7, 2-4. T. Sun et al, 1994, Chem. Rev., 94, 4, 857-870. A. Satsuma et al, 2005, Catal. Surv. Asia, 9, 2, 75-85. J. Shibata et al, 2004, Appl. Catal. B: Environ, 54, 137-144. W. Gac et al, 2007, J. Mol. Catal. A: Chem, 268, 15-23.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V.

Strong electrostatic adsorption for the preparation of Pt/Co/C and Pd/Co/C bimetallic electrocatalysts L. D’Souza and J. R. Regalbuto* ([email protected]) Dept. of Chemical Engineering, University of Illinois at Chicago 810 S. Clinton, Chicago, IL 60607

Abstract The method of “strong electrostatic adsorption” (SEA) can be extended to the rational synthesis of bimetallic catalysts. In this study it is demonstrated that cationic ammine complexes of palladium or platinum selectively adsorb onto the cobalt oxide particles of a cobalt oxide/carbon surface. This done at an equilibrium pH of 11, where the carbon surface is negligibly charged and the cobalt oxide surface is deprotonoated and negatively charged. Reduction at high temperature leads to homogeneously alloyed particles while lower temperature reduction leads to core-shell morphologies with a core of cobalt. Keywords: bimetallic catalyst synthesis, electrostatic adsorption

1. Introduction The seminal paper of Brunelle (1978) outlined the rational method of catalyst synthesis whereby charged metal coordination complexes such as hexachloroplatinate ([PtCl6]-2) or platinum tetraammine ([(NH3)4Pt]+2) can be electrostatically adsorbed onto oxide surfaces which contain naturally occurring hydroxyl groups (-OH) that are either protonated and positively charged (--OH2+) or deprotonated or negatively charged (-O-), depending on the solution pH. We have employed this method to synthesize highly dispersed, highly loaded noble and base metals on silica, alumina, and carbon supports (Regalbuto 2007). Electrostatic control of metal complex adsorption might also be achieved at the nanoscale over surfaces containing two oxides for a scientific method to prepare a wide range of bimetallic catalysts and promoted catalysts. The idea is illustrated in Figure 1 in the simulation of surface potential versus pH for a surface consisting of a carbon with

Figure 1. Electrostatic adsorption preparation strategy for bimetallics.

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a PZC of 9, which supports particles of cobalt oxide, which has a PZC of about 7. At a pH of 8, the cobalt oxide phase will be deprotonated and negatively charged, while the carbon surface will be protonated and positively charged. Tetraammine cations of Pt and Pd should then adsorb selectively onto the cobalt oxide particles. Subsequent reduction in H2 can then be used to form bimetallic PtCo or PdCo particles.

2. Experimental The details of our experimental procedures for studying metal adsorption and characterizing synthesized materials are found elsewhere (D’Souza 2008). With a bimetallic system the idea is to study adsorption of a particular metal complex over the individual components (in this case, Vulcan XC72 carbon black with a PZC about 9 and 254 m2/g and cobalt (II,III) oxide, synthesized by calcining cobalt nitrate at 400°C, PZC = 7 and 60 m2/g). From these experiments the pH to maximize adsorption selectivity is determined. Next, adsorption is conducted at this optimal pH over a physical mixture of the carbon and cobalt oxide to permit facile characterization by electron microscopy. When the selectivity of adsorption has been confirmed in this way, the carbon supported Co3O4 sample is impregnated with the metal amine and the bimetallic nature of the material is characterized by single particle analysis in the electron microscope (single point and line EDXS and EELS scans) and other methods such as EXAFS and TPR.

3. Results and discussion The results of adsorbing platinum and palladium tetraammine (PTA and PdTA) over Vulcan XC72 and over Co3O4 are shown in Figure 2. Neither metal adsorbs to an appreciable extent over the carbon support. In another publication we have surmised that the chargeable groups on high PZC carbon, likely the pi bonds of aromatic rings, can only be protonated and positively charged, and not deprotonated; that is, virgin carbon blacks with high PZC can only adsorb anions at low pH, and not cations at high pH (Hao, Barnes, and Regalbuto, manuscript in preparation). On the other hand, both the PTA and the PdTA exhibit the volcano shape of uptake versus pH characteristic of strong electrostatic adsorption (Regalbuto 2007). Since the carbon does not adsorb either noble metal to an appreciable extent in the high pH range, a pH of 11 was employed for adsorption onto physical mixtures and onto the carbon supported cobalt oxide.

Figure 2. PTA and PdTA uptake versus pH on Co3O4 and Vulcan XC72 (200 ppm metal, 1000 m2/l surface loading.

Electrostatic adsorption for the preparation of Pt/Co/C and Pd/Co/C bimetallic

717

For adsorption over the physical mixtures, 200 ppm solutions of PTA and PdTA were contacted with cobalt oxide and carbon each with 1000 m2/l in solution. The dried materials were reduced at 150°C in hydrogen for one hour to form Pt particles (TPR results, not shown, reveal that this temperature is not sufficient to reduce the cobalt oxide). Representative micrographs of PTA adsorption are shown in Figure 3. The Pt particles predominate over the cobalt oxide fraction of the sample. From analysis of several dozen areas and hundreds of particles, the Pt and Pd (not shown) are about 80 percent partitioned onto the cobalt oxide phase. All particles are in the size range of 1 to 3 nm, typical of SEA preparations.

Co3O4

Vulcan XC72

Figure 3. PTA uptake at pH 11 over a physical mixture of Co3O4 and Vulcan XC72 (200 ppm metal, 1000 m2/l each surface loading).

The preparation of highly dispersed cobalt oxide on carbon is detailed in another paper (D’Souza et al., 2007). A 10 wt% Co/Vulcan XC72 sample is shown in Figure 4, before and after adsorption of PTA at a pH 11. The PtCo sample was reduced at 400°C. The mass ratio of the Pt to Co adsorbed is 1 to 1.5. The particles are somewhat larger, on the order of 2-6 nm. Single spot analysis of many dozens of particles has revealed the virtually complete contacting of platinum with cobalt. The appearance of the PdCo sample was similar, with a similar essentially complete contacting of the Pd with Co.

Co3O4/VXC

PtCo1.5/VXC

Figure 4. The 10 wt% cobalt oxide/carbon substrate, onto which was adsorbed PTA at pH 11.

Analysis of single particles was also done with line scans using either EDXS or EELS. An EDXS line scan of a series of PtCo particles reduced at 500°C (Figure 5) shows a coincidence of Pt and Co profiles. When the PdCo material was reduced at the lower temperature of 200°C, a core-shell morphology is apparent both from the image (bottom left of Figure 5) and the EELS line scan profiles of Pd and Co.

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PtCo1.5/VXC

PdCo3.5/VXC

Co Pd

Spectrum Image

0.01 µm

Figure 5. Line scans of PtCo (with EDXS) and PdCo (with EELS) particles.

4. Conclusions A rational approach for synthesizing bimetallic catalysts can be based on electrostatic adsorption by exploiting the PZC differences between the catalyst support and a supported metal oxide of one of the catalyst metals. The catalyst support is chosen with a PZC different from the metal oxide, and the second metal catalyst precursor is made to selectively adsorb onto the first metal oxide by appropriate control of solution pH. Carbon is a particularly good support for this type of synthesis as it can be oxidized and its PZC can be controlled (Hao 2006). As demonstrated in this work for PtCo and PdCo on carbon, the severity of reduction can be used to produce either alloyed particles or core shell structures.

References J.P Brunelle, 1978, Preparation of catalysts by metallic complex adsorption on mineral oxides, Pure Appl. Chem. 50, 1211. L. D'Souza, J.R. Regalbuto, J.T. Miller, 2008, Preparation of carbon supported cobalt by electrostatic adsorption of [Co(NH3)6]Cl3, J. Catal., 254, 157. X. Hao, L. Quach, J. Korah, W. A. Spieker, J. R. Regalbuto, 2004, The Control of Platinum Impregnation by PZC Alteration of Oxides and Carbon, J. Mol. Catal. A: Chem., 219, 97. J.R. Regalbuto, 2007, Strong Electrostatic Adsorption of Metals onto Catalyst Supports, in Catalyst Preparation: Science and Engineering, J.R. Regalbuto, ed., Taylor and Francis/CRC Press, 2007

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of gold catalysts supported on SiO2-TiO2 for the CO PROX reaction L. Gonzalo-Chacón,a B. Bachiller-Baeza,a A. Guerrero-Ruiz,b I. Rodríguez-Ramosa a

Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie, 2, Cantoblanco, 28049 Madrid b Dpto. Química Inorgánica y Técnica, Facultad de Ciencias, UNED, Senda del Rey, 9. 28040 Madrid

Abstract A titania-coated silica, as prepared and calcined, was used as support to prepare two Au catalysts. The catalysts were characterized by several techniques, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and tested in the CO Preferential Oxidation (CO PROX) reaction. The dispersion of Ti onto the silica was very homogeneous. On the other hand, the incorporation of Au was limited to less than 1 % and the particles size varied in the range 3-10 nm in both samples. Reaction studies were carried out on a fix bed reactor and on a Temporal Analysis of Products (TAP) reactor. The results reveal the participation of the surface -OH groups on the mechanism of the selective CO oxidation. Keywords: titania-coated silica, Au catalyst, deposition-precipitation method

1. Introduction The preferential CO oxidation in H2-rich streams is a reaction of great relevance due to its application in the purification of feeds for hydrogen fuel cells, and because of the scientific interest. This reaction is known to be very sensitive to catalytic surface structures and to the pretreatments. Au catalysts supported on metal oxides with high metal dispersions have been demonstrated as very effective in this PROX reaction [1]. However, the studies carried out over these systems have allowed us to conclude that several factors affect the performance of the catalyst, such as particle sizes, preparation method, supports, etc. Nevertheless, there is still some controversy with respect to the nature of the active site or about the mechanism of reaction. In the present communication and with the aim to obtain further information to elucidate these unresolved points a study on the role of the support in the CO PROX mechanism, particularly emphasizing in the aspects related to the preparation of Au-supported catalysts is presented. The use of a TAP (Temporal Analysis of Products) reactor is also applied to reveal elementary processes that are taking place on the surface under reaction conditions, since this reactor system allows the detection of reactants and products with a submillisecond time resolution [2].

2. Experimental The support material used is a titania-coated silica (Ti-Si). This titania-coated silica is prepared by deposition-anchoring of TiO2 from ethanolic solutions of Ti[O(CH2)3CH3]4 alkoxide over SiO2. The silica support (aerosil, SBET=482 m2·g-1) is suspended in ethanol (250 ml/g support) and the amount of alkoxide to reach a Ti/-OH ratio of 3 is added.

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The mixture is stirred under He flow and the temperature is increased to 343 K. Then, water is added in small doses at 30 minutes intervals; the water to Ti(OR)4 ratio being of 30. After that, the mixture is kept under reaction for 4 h. The sample is filtered, washed with ethanol and dried at 383 K overnight. The support prepared using this procedure is denoted as Si-Ti and is characterized by thermal analysis (TG). An aliquot of this support is calcined at 723 K for 24 h leading to Si-Ti-C support. Fourier Transform Infrared Spectroscopy (FTIR) is carried out on self-supported wafers. The spectra are recorded on a Nicolet 5 ZDX spectrophotometer equipped with an MCT (mercurycadmium-telluride) detector with a resolution of 4 cm-1. The Au catalysts are prepared by the deposition-precipitation method in basic medium using HAuCl4. The pH of the aqueous solution of HAuCl4 is adjusted between 10 and 11 with a 0.2 M NaOH aqueous solution, then the support is added and the pH is again corrected to 10-11. The mixture is stirred for 18 h and the solid is filtered, washed and dried overnight at 383 K. The prepared catalysts are denoted as Au-Si-Ti and AuSi-Ti-C for the uncalcined and calcined support respectively. The catalysts are characterized by XPS before and after reaction, and by STEM and STEM mapping after reaction. XPS spectra are obtained on a ESCAPROBE P spectrometer from OMNICROM equipped with a EA-125 hemispherical multichannel Electronics analyzer. The pressure in the analysis chamber is kept below 10-9 Pa. The excitation source is the Mg Kα line (hν=1253.6 eV, 300 W). The binding energy is referenced to the C 1s line at 284.6 eV. Samples for examination by STEM are prepared by dispersing the catalyst powder in high-purity ethanol, then allowing a drop of the suspension to evaporate on a holey carbon film supported by a copper grid. High-angle annular dark-field (HAADF) imaging is carried out on a JEOL microscope model JEM 2100F, which is also equipped with an Oxford Instruments Inca software package for X-ray energy dispersive spectroscopic (XEDS) mapping. The CO oxidation reaction is studied in a fix-bed reactor under atmospheric pressure and temperatures in the range 333-413 K. A mixture 1% CO, 1% O2, 40% H2 (He balance) is passed through the catalytic bed. Transient experiments are carried out in a TAP-2 reactor where single pulses or simultaneous pulsing of reactants are performed at 473 K after the sample is outgassed in vacuum at the same temperature for 1 h.

3. Results and discussion The comparison of the FTIR spectra of a self-supported wafer of the support, before and after in situ calcination, and of the original silica support is presented in Figure 1. The spectrum obtained for Si-Ti present a group of bands in the range 2850-2970 cm-1 which are assigned to the C-H stretching vibration of ligands anchored to the silica surface during the preparation process. These bands are absent in the spectrum after calcination confirming the removal of the hydrocarbon fragments. It is also important to notice some modifications occurring in the range 4000–3000 cm-1 related to stretching vibrations of hydroxyl groups. The sharp single peak at 3740 cm-1 arise from isolated surface hydroxyl groups (–Si–OH), and is typical for highly hydrophilic samples [3]. The increase in this signal for the Si-Ti sample can be related to changes in the proportion between H-bridging hydroxyl groups (–Si–OH. . .O–Si–), associated with the band at 3590 cm-1, and isolated silanol groups due to reaction between adsorbed titanium alkoxide molecules and surface OH, producing Ti-O-Si bonds. A comparison of curves also confirms the removal of a considerable amount of adsorbed water, showed by the broad band at 3350 cm-1, during the in situ calcination process. The XPS spectra of the fresh Au catalysts (Figure 2) show the characteristic peaks of Ti, Si and O, and of Na due to residual species coming from the preparation method.

Preparation of gold catalysts supported on SiO2-TiO2 for the CO PROX reaction

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However, the Au signal cannot be detected probably due to the is 0.21 and 0.12 for Au-Si-Ti and Au-Si-Ti-C respectively. The Ti 2p3/2 binding energy for the TiO2-grafted SiO2 (about 460 eV) is shifted to higher values compared to that for TiO2 (458.5 eV) . This fact reflects an intimate association of TiO2 and silica producing Ti–O–Si bonds where Ti4+ occupies tetrahedral coordination sites similar to Si in SiO2. The O 1s peak presents two components at 533.5 and 530.5 eV. The first is typical of SiO2 while the second component can be reasonably assigned to oxygen in Si–O–Ti bonds at the surface [3]. The presence of residual hydrocarbon fragments on the non calcined support is confirmed again after analysis of the C1s core level. Using the annular dark field image (Figure 3), which gives strong atomic number (Z) contrast, it is found that for these catalysts the metal particles range from 3 to 10 nm in diameter for both catalysts. STEM-XEDS mapping also shows that the Si-L1 and Ti-L1 signals are spatially coincident, indicating that the Ti coating is very homogeneous. However, a higher concentration of Ti is observed in some small areas which could reveal the existence of some TiO2 surface segregation (see the arrow in Ti map).

Figure 1. FTIR spectra of the support before and after calcination.

Figure 2. XP spectra of the Au catalysts.

20 nm

Au

Si

Ti

Figure 3. HAADF image showing small Au particles in the Au-Si-Ti-C sample (left) and corresponding STEM-XEDS maps of the Au-L1, Si-L1 and Ti-L1 signals.

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As far as the oxidation of CO reaction is concerned, both Au catalysts are active for the CO oxidation without any pretreatment (Figure 4). Comparing the fresh and the calcined support, it can be observed that the catalysts prepared on the calcined support give higher CO conversions at lower temperatures. Therefore, it is suggested that the OH groups are involved in the reaction mechanism affecting the catalyst performance. By pulsing CO and O2 over the catalysts on the TAP reactor, it is revealed that both molecules are adsorbed reversibly on the surface. The lower conversions obtained when CO, O2 and H2 are simultaneously pulsed over the catalysts compared with those obtained in the fix bed reactor, are due to thermal effects and heat transfer differences in both types of experiments. In spite of that, a shift to longer times is observed for the maximum of the CO2 peak signal compared to the transient responses of reactants. This shift suggests that H2O derived species, which are formed by oxidation of H2, and bicarbonate species, which are related to the –OH of the support, are involved on the reaction mechanism.

Figure 4. Results of CO oxidation in the fixed bed reactor over fresh and calcined supports.

4. Conclusions Therefore, all the characterization techniques indicated that the preparation method of the titania-coated silica support is very effective and that multilayer coatings of TiO2 are obtained on the SiO2 after calcination. More precisely, the grafting of titanium onto silica is produced by reaction between alkoxide precursor and surface OH groups. Additionally, the support post-treatment, although not influencing the particle size distribution significatively, determines the Au catalyst performance.

Acknowledgment Authors recognize financial support from MICINN (CTQ2008-06839-C03-01/PPQ and -03/PPQ).

References A Stephen, K. Hashmi, G.J. Hutchings, 2006, Gold Catalysis, Angew. Chem. Int. Ed., 45, 7896. G.S. Yablonsky, M. Olea, G.B. Marin, 2003, Temporal analysis of products: basic principles, applications, and theory, J. Catal., 216, 120. C.U.I. Odenbrand, S. L. T. Andersson, L.A.H. Andersson, J.G.M. Brandin, G. Busca, 1990, Characterization of Silica-Titania Mixed Oxides, J. Catal., 125, 541.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

A method of preparation of active TiO2-SiO2 photocatalysts for water purification M.P. Fedotovaa, G.A. Voronovaa,b, E.Yu. Emelyanovaa, O.V. Vodyankinaa a b

Tomsk State University, 36, Lenin str., 634050 Tomsk, Russia Tomsk Polytechnical University, 30, Lenin str., 634050 Tomsk, Russia

Abstract A new method of synthesis of highly active TiO2-containing systems to purify wastewater by photodecomposition of organic substances has been proposed. A number of catalysts with titania nanoparticles homogeneously distributed on the silica support surface have been prepared and investigated. It is shown that the photoactivity for both unpromoted and Au-containing samples increase not only under action of UVirradiation. The significant growth of kinetic constant of methylene blue degradation is observed under the action of visible light, compared to Degussa P 25. Keywords: photocatalysis, titania, supported catalysts, gold nanoparticles

1. Introduction This work deals with the preparation of active titania photo-catalysts. To obtain effective semiconductor photo-catalysts it is necessary to note that different interfacial electron processes with participation of e- and h+ must compete effectively with the main deactivation processes of e-–h+ recombination. Recombination of e-–h+ pair may occur in the bulk or on the surface. To form an active photo-catalyst it is necessary to distribute active TiO2 species homogeneously on the surface of support, and create conditions for prolongation of lifetime of charge carriers. This can be achieved by deposition of titania on a surface of suitable support and/or addition of promoter such as noble metal nanoparticles (mainly, Pd, Pt, Ag, Au). A new method of synthesis of highly active TiO2-containing systems to purify wastewater by photodecomposition of organic substances is developed in the present work. The main idea of the developed preparation method is the controlled hydrolysis of organic precursor of titania by surface hydroxyl group of the silica support in dehydrated organic solvent.

2. Experimental 2.1. Catalyst preparation The TiO2 supported catalysts are prepared by the hydrolysis of titanium tetraisopropoxide (TTIP, Merck) on the surface of silica support in the dehydrated toluene. SiO2 aerogel (Ssp = 100 m2/g) is used as a support; it is prepared as recommended in [1]. The required amount of the support is added into the solvent, continuously bubbled by dry gas (nitrogen or argon) flow. TTIP is partially added at constant temperature of 110 oC into reaction vessel. After addition of each portion of precursor the reaction mixture is thermostated for 30 min. Finally, the obtained samples are dried in vacuum at 25 oC for 2 h and then heated in inert gas flow at 550 oC for 5 h. Several TiO2/SiO2 supported samples containing 3–15 wt % of TiO2 are synthesized to study their physicochemical properties and catalytic characteristics.

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Gold deposition on the surface of the prepared TiO2/SiO2 catalysts is performed by deposition-presipitation technique using water solution of HAuCl4 (Acros) in the presence of urea according to [2]. 1 g of catalyst is added to 100 mL of an aqueous solution containing HAuCl4 (4.2 × 10−3 M) and urea (0.42 M). The initial pH is 2. The suspension is thermostated at 80 oC and vigorously stirred for 16 h. Urea decomposition leads to gradual rise in pH from 2 to 7. The solids are gathered by centrifugation (12,000 rpm for 10 min), washed in 100 mL of distilled water under stirring for 10 min at 50 oC, and then centrifuged. The sequence of operation is repeated several times. The solids are dried under vacuum at room temperature for 16 h and then at 150 oC for 2 h in air.

2.2. Photocatalytic tests The photocatalytic activity of the prepared systems is studied in the well known methylene blue (MB) dye decomposition reaction under stationary conditions [3]. The source of UV-radiation is a DRSh-250 mercury lamp; the source of visible-radiation is a Sylvania lamp (60 mW); the radiation is not filtered. An aqueous solution of MB and the required amount of the catalyst (5 or 8 mg) are placed into reactor (volume 50 ml) with a quartz window. The suspension is treated in ultrasonic bath for 10 min for homogenization. During irradiation, the suspension is thoroughly stirred using magnetic stirrer. Air is passed through the suspension at a constant rate for saturation with oxygen and more effective stirring. The activity of the catalysts is estimated from a decrease in the MB concentration. The MB concentration on solutions is determined spectrophotometrically using SF-256 spectrophotometer at equal time intervals (samples were preliminarily centrifuged for 5 min on an OPN-12 centrifuge with a 8000 rpm rotation rate). The photoactivity of the prepared samples (the degree of MB transformation in a certain time interval) is compared to the activity of titania Degussa P25 (45 m2 g−1, nonporous, 70 % anatase and 30 % rutile, purity > 99.5%). Table 1. Chemical composition and several features of prepared catalysts. Sample composition

Ssp, m2/g

Average pore diameter, nm

1%Au / 3%TiO2 / 96%SiO2

85,5

35,7

1%Au / 6%TiO2 / 93%SiO2

89,7

31

1%Au / 8%TiO2 / 91%SiO2

74,6

20

1%Au / 15%TiO2 / 84%SiO2

36,6

20,3

Phase composition, % mol. Au SG 225 Amorphous Au SG 225 Anatase Amorphous Au SG 225 Anatase Amorphous Au SG 225 Anatase Amorphous

0,5 99,5 0,5 4,5 95 C7 alkanes, were formed under the applied conditions. The metal to acid site ratio strongly influences the hydroconversion activity and selectivity [8]. TEM and XRD results suggested that Ni has high dispersion in each of our catalyst preparation. By doubling the Ni-content of the Ni,H-MCM-22 more than twofold increase was obtained in the rate of mono-branched C7 formation (Fig. 3A), whereas the rate of cracking decreased to a small extent only (Fig. 3B). Regarding the

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rate of the mono-branched C7 formation the activity of the Ni/H-dl-MCM-22/MCM-41 catalyst was between the activities of the Ni,H-MCM-22 catalysts (Fig. 3). An important finding of the present study is that the composite catalyst has a significantly lower cracking activity than any of the reference Ni,H-zeolite catalysts (Fig. 3B). The Ni/H+ ratio of the composite must be higher than the Ni/IEC ratio. It can be near to 0.5. Nevertheless, the low cracking activity of the composite catalyst can neither be accounted for its lower zeolite content nor for its higher Ni/H+ ratio. 60

Ni/H-MCM-22 + (Ni/H =0.50)

A

50 Ni/H-dl-MCM-22/MCM-41 (Ni/IEC=0.28)

40 30

20

B

Ni/H-MCM-22 + (Ni/H =0.25)

10 0.2

0.4 0.6 0.8 -1 1.0 Space time, gCATg C7h

1.2

1.4

Ni/H-MCM-22 + (Ni/H =0.25)

15 Ni/H-MCM-22 + (Ni/H =0.50)

10

20

0 0.0

25

Crack yield, wt%

Mono-branched isomer yield, wt%

70

5 0 0.0

Ni/H-dl-MCM-22/MCM-41 (Ni/IEC=0.28)

0.2

0.4 0.6 0.8 -1 1.0 Space time, gCATg C7h

1.2

1.4

Fig. 3. Yields of mono-branched isomers (A) and cracking products (B) as a function of space time. To get the Ni/H-catalysts the Ni,H-forms were pre-reduced in situ in the reactor in a 150 cm3 min-1 H2 flow of at 450°C for 2 hours.

The lower rate of cracking in the micro/mesoporous composite relative to the microporous zeolite can be explained by the shorter residence time of the C7+ carbenium ions and C7 products in the mesoporous particles having lower diffusion resistance. If the residence time was equalized by adjusting the space time to get similar conversions the selectivity difference of the catalysts disappeared.

4. Conclusions Hierarchical micro/mesopore structure can be obtained by forming composite from delaminated layered-structure zeolite and a mesoporous MTS material. At the same space time the bifunctional zeolite catalyst, having hierarchical micro/mesoporous structure, show lower n-C7 hydroconversion activity and higher hydroisomerization selectivity than the corresponding microporous zeolite catalyst.

Acknowledgement The authors thank for the financial support of the OTKA (No. K 68537), and of the National Office for Research and Technology (NKTH No. GVOP-3.2.1. 2004-04-0277/3.0).

References [1] M. Kollár, M.R. Mihályi, J. Valyon in I. Halasz (ed.), Silica and Silicates in Modern Catalysis, Transworld Research Network, India, Kerala, 2010, pp. 171. [2] J. Čejka, S. Mintova, Catal. Rev., 49 (2007) 457. [3] M. Kollár, R.M. Mihályi, G. Pál-Borbély, J. Valyon, Micropor. Mesopor. Mat., 99 (2007) 37. [4] A. Corma, V. Fornes, S.B. Pergher, Th.L. Maesen, J.G. Buglass, Nature, 396 (1998) 353. [5] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature, 359 (1992) 710. [6] B. Onida, L. Borello, B. Bonelli, F. Geobaldo, E. Garrone, J. Catal., 214 (2003) 191. [7] J.A. Martens, P.A. Jacobs, J. Weitkamp, Appl. Catal., 20 (1986), 283. [8] A. Lugstein, A. Jentys, H. Vinek, Appl. Catal., A: Gen., 166 (1998) 29.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation and characterization of nanocrystallines Mn-Ce-Zr mixed oxide catalysts by sol-gel method : application to the complete oxidation of n-butanol Saïd Azalim,a,b,c,d Rachid Brahmi,d Mohammed Bensitel,d Jean-Marc Giraudon,a,b,c Jean-François Lamonier a,b,c a

Univ Lille Nord de France, F-59000 Lille, France CNRS, UMR8181, France c USTL, Unité de Catalyse et de Chimie du Solide F-59652 Villeneuve d’Ascq, France d UCD, Laboratoire de Catalyse et de Corrosion des Matériaux, Faculté des Sciences, 24000 El Jadida, Morocco b

Abstract A series of Zr(0.4)Ce(0.6-x)Mn(x)O2 mixed oxides catalysts with different compositions (x = 0; 0.12; 0.24; 0.36; 0.48) were prepared by a sol–gel method. The samples calcined at 500°C were characterized by X-ray diffraction (XRD), surface specific areas (SSA) and H2-TPR measurements and tested in the butanol oxidation. Using a sol-gel method very high SSA, small crystallite sizes and high redox properties are obtained especially when manganese content increased in the Zr-Ce-Mn-O system. The butanol complete oxidation is easier with Mn content increasing. Keywords: VOC, butanol, sol-gel, Zr-Ce-Mn oxide

1. Introduction Volatile Organic Compounds (or VOCs) are major air pollutants and the treatment by catalytic oxidation is one of the most promising ways to reduce these pollutants in the atmosphere since this technique allows operating at low temperatures (200-500°C) and thus leading to NOx formation in lower quantity. Noble metal (Pt, Pd) catalysts supported on alumina or other oxides are usually employed for VOC oxidation [1]. But metal oxides catalysts are also studied as cheaper alternatives to noble metals. Among them manganese oxides are the most active catalyst in VOC oxidation [2]. The Mn-CeO catalytic system has been the subject of a number of studies [3-5] due to the unusual redox behavior of ceria and its high oxygen storage capacity (OSC) [6]. It was further showed that the MnOx-CeO2 mixed oxides had much higher catalytic activity than those of pure MnOx and CeO2 owing to the formation of the solid solution between manganese and cerium oxides [7]. Besides it is well known that formation of mixed oxides of ceria with Zr4+ enhanced oxygen storage properties of ceria and the so-formed mixed oxides exhibited good thermal stability [8]. The present work describes different Zr-Ce-Mn-O catalytic systems synthesized using a sol-gel method. The effect of Mn amount is particularly studied in order to optimize the performances of the Zr-Ce-Mn oxides for the complete oxidation of n-butanol. Butanol has been chosen as VOC model molecule, because oxygenated VOC concentration in the atmosphere is increasing.

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2. Experimental 2.1. Catalyst preparation The mixed metal oxides catalysts Zr(0.4)Ce(0.6-x)Mn(x)O2 (x = 0; 0.12; 0.24; 0.36; 0.48) were prepared using a sol–gel method. The Ce(NO3)3.6H2O, Zr(NO3)2.5H2O and Mn(NO3)2.5H2O (0.5 mol/L) nitrates were separately dissolved in ethanol and added together in order to get the different molar ratio of Zr:Ce:Mn. To the resulting solution heated at 80°C was added deionized water (5 vol. % of ethanol) under constant stirring. The resulting gel was gradually formed after few minutes and the temperature was maintained for 1h30. After that the gel was allowed to mature overnight at room temperature (RT) before to be heated at 80°C and 100°C respectively in order to remove ethanol and water excess. After grounding, the resulting powders were submitted to calcination from RT to 300°C (2 h) and from 300°C to 500°C (2 h) in flowing air.

2.2. Catalyst characterization The X-ray diffraction (XRD) patterns were collected with a D8 Advance-BRUKER diffractometer using Cu Kα radiation. The crystallite size was determined from the Scherrer equation. The lattice parameter was estimated using FullProf software. The textural properties were evaluated at -196 °C from the nitrogen isotherms (Micromeritics ASAP 2010). The samples were previously outgassed at 160°C for 4 h. The specific surface area (SSA) was calculated using the BET model. Temperature programmed reduction (H2-TPR) was investigated (Micromeretics Autochem II) by heating Zr(0.4) Ce(0.6-x)Mn(x)O2 samples (50 mg) in H2 (5 vol.%)/Ar flow (50 mL min-1) at a heating rate of 5°C min-1 from 20 to 900°C.

2.3. Catalytic tests The activity of the catalysts (200 mg) was measured in a continuous flow system on a fixed bed reactor at atmospheric pressure. The flow of the reactant gases (0.1 mL min-1 of butanol and 99.9 mL min-1 of air) was adjusted by a Calibrage PUL 010 and DGM 110 apparatus constituted of a saturator and three mass flow controllers. The reactor temperature was increased from RT to 400°C (0.5°C min-1). The outflow gases were analyzed by a VARIAN 3800 gas chromatograph.

3. Results and discussion XRD patterns of Zr(0.4)Ce(0.6-x)Mn(x)O2 samples are displayed in Fig.1. The pattern of Ce0.6Zr0.4O2 is rather similar to that of a reference cerianite (JCDS 81-0792) suggesting the incorporation of Zr ions in the cubic lattice to form an homogeneous Ce–Zr–O solid solution, in accordance with the size of the ionic radius of Zr4+ ion (0.84Å) which is smaller than that of Ce4+ (0.97Å). Indeed a decrease in the lattice parameter a from 5.4120 Å for pure CeO2 [9] to 5.315 (±0.001) Å is observed on the fresh Ce0.6Zr0.4O2 sample. When adding small amounts of manganese, the fluorine-type structure is preserved. No manganese and zirconium oxide phases were detected on Zr0.4Ce0.12Mn0.48O2 solid. The absence of such phases suggests that Mn and Zr related species may be incorporated into the CeO2 lattice forming solid solutions [4]. Again the formation of a Mn–Ce–Zr–O solid solution is in line with the lower value of the lattice constant a of Ce0.48Zr0.4Mn0.12O2 sample which is of 5.303 (±0.001) Å according to the low ionic radius of Mnn+ (Mn2+ = 0.83Å, Mn3+ = 0.64Å, and Mn4+ = 0.53Å). Only a broad asymmetric peak in the 2θ range of 25–35° is observed for the Zr0.4Ce0.36Mn0.24O2 sample. Considering the Zr0.4Ce0.24Mn0.36O2 sample the dominant peaks of the cerianite are observed but they appear significantly enlarged. Finally the most enriched Mn

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sample is totally amorphous. Hence with increasing content of manganese, amorphisation of the samples is enhanced. Similar qualitative observations have been already observed on Mn-Ce-O composites elsewhere [4,5]. These authors conclude to the occurrence of more defective fluorite like lattices having a lower degree of crystallinity and a smaller particle size as the sample is Mn enriched. Then comparing with Ce-Mn-O systems addition of zirconium herein retards the crystallization of the samples and/or allows forming some small oxide related crystallites. Adding manganese to the Zr-Ce-O system has a positive effect on the SSA which doubles from the free Mn sample to the higher Mn loaded one as well as on the average crystallite size which decreases from 4.5 nm for x= 0 to 1.2 nm for x= 0.36. Hence the effect of Mn in the presence of Zr is to retard the crystallization of the sample likely due the existence of Zr-Mn-Ce-O solid solution. The direct consequences are a substantial increase of the SSA concomitant with a crystallite size lowering with increasing the Mn content as observed in the MnCe-O system [4,5] but in a much more amplified manner in our case. Table1. Surface area, particle size, average crystallite size, and T50 of samples. average crystallite size (nm)

SSA (m2 g-1)

Zr0.4Ce0.6O2

4.5

98

960

216

Zr0.4Ce0.48Mn0.12O2

2.8

110

1700

184

Zr0.4Ce0.36Mn0.24O2

1.2

157

2420

176

Zr0.4Ce0.24Mn0.36O2

1.2

163

2880

172

Zr0.4Ce0.12Mn0.48O2

-

199

3470

162

Samples

H2 Consumption

T50* (°C)

-1

(µmol g )

* Temperature at 50% of butanol conversion

Fig. 1. XRD patterns of calcined samples

Fig. 2. H2-TPR profiles of calcined samples

° The H2-TPR profiles of the Zr(0.4)Ce(0.6-x)Mn(x)O2 samples are presented in Fig. 2. The onset of Ce0.6Zr0.4O2 reduction arises at 295°C. The reduction of oxygen from surface and bulk solid may account for the TPR profile of Ce0.6Zr0.4O2. When adding Mn the most interesting points are the following : (i) easily reducible species are formed as the onset of reduction is dramatically reduce all the more than the Mn content is increased ; (ii) the development of a broad complex envelop in the 100-500°C temperature range in line with the different Mnn+ whose shape is quasi-similar considering the three higher Mn loaded samples ; (iii) the H2 consumption linearly increases with Mn content (Table 1). Considering both Mnn+ to be reduced completely to Mn2+, Zr4+ not reduced in the temperature range studied, and Ce3+/Ce4+ molar ratio unchanged, an average oxidation number (AON) of manganese of circa 3.1 is obtained for each sample. The increase of

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the H2 uptake is only due to an increase of reducible sites in accordance with the SSA increase having a similar AON. Hence the rate increase of the different sites appears to be the same with Mn content increase in accordance with the similarities of the different H2-TPR envelops. The results of the catalytic activity in butanol oxidation are summarized in Table 1 through the T50 (temperature at 50% of butanol conversion) values : the butanol complete oxidation is easier all the more than the Mn content increases.

4. Conclusion Zr(0.4)Ce(0.6-x)Mn(x)O2 solid solutions were successfully synthesized by a sol-gel method, characterized and tested for the total oxidation of butanol. The butanol complete oxidation is easier with Mn content increasing. Here the results may be explained by the SSA and crystallite size measurements as observed previously in similar systems but for lower Mn content [10].

Acknowledgements The authors thank the European Community for financial supports through an Interreg IV France-Wallonie-Flandre project named REDUGAZ.

References [1] K. Okumura, 1998, Appl. Catal. B 15, 75-84. [2] C. Lahousse, 1998, J. Catal., 178, 214-222. [3] S. Imamura, 1996, Effect of cerium on the mobility of oxygen on manganese oxides, Appl. Catal. A: Gen., 142, 279–288. [4] G. Picasso, 2007, Preparation and characterization of Ce-Zr and Ce-Mn based oxides for n-hexane combustion: Application to catalytic membrane reactors, Chemical Engineering Journal, 126, 119–130. [5] H. Chen, 2001, Composition-activity effects of Mn-Ce-O composites on phenol catalytic wet oxidation, Appl. Catal. B: Environ., 32,195–204. [6] A. Trovarelli, 1999, The utilization of ceria in industrial catalysis, Catal. Today, 50, 353– 367. [7] A.M.T. Silva, 2004, App. Catal. B., 47, 269-279. [8] C.E. Hori, 1998, Thermal stability of oxygen storage properties in a mixed CeO2-ZrO2 system, Appl. Catal. B: Environ., 16, 105–117. [9] M. Wolcyrz, 1992, Rietveld refinement of the structure of CeOCI formed in Pd/CeO2 catalyst: Notes on the existence of a stabilized tetragonal phase of La2O3 in La-Pd-O system, J. Solid State Chem., 99, 409-413. [10] T. Rao, 2007, Oxidation of ethanol over Mn-Ce-O and Mn-Ce-Zr-O complex compounds synthesized by sol–gel method, Catal. Comm., 8, 1743–1747.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

SCR activity of conformed CuOX/ZrO2-SO4 catalysts S.B. Rasmussen,a* M. Yatesa, J. Due-Hansenb , P. Ávilaa R. Fehrmannb a

Instituto de Catálisis y Petroleoquímica (ICP), Consejo Superior de Investigaciones Científicas (CSIC), Calle Marie Curie 2, Cantoblanco, 28049 Madrid, Spain b Centre for Catalysis and Sustainable Chemistry (CSC), Department of Chemistry, Technical University of Denmark (DTU), Bygn. 207, Kemitorvet, DK-2800 Kgs. Lyngby, Denmark

Abstract CuOX/ZrO2-SO4 catalysts have been synthesised as conformed materials with the use of sepiolite as agglomerant and the performance in the NH3-SCR reaction with relation to biomass fired boiler units have been studied. The optimal Cu-loading of the catalysts is 3 wt.% CuO, both in terms of activity and selectivity. This catalyst constitutes a possible solution for NO removal in biomass-related applications, since it posses mainly Lewis acid sites, and therefore might be less subjected to deactivation by potassium containing fly ash particle produced during biomass combustion Keywords: CuOX, sepiolite, biomass, NH3-SCR, ZrO2-SO4

1. Introduction The use of biomass in fossil fuel based power plants is of increasing interest, since it is considered as a CO2 neutral fuel, having zero human impact on the carbon release to the atmosphere. Simultaneously, continuous efficient selective catalytic reduction of NO with ammonia (NH3-SCR) remains a very important condition for the implementation of biomass fuel as a sustainable alternative for energy production. However, the NH3SCR catalyst suffers from a number of deactivation phenomena when installed in boiler units based on biomass combustion, due to exposure to potassium containing fly ash[1]. The active V=O and V-OH sites on the commercially used V2O5-WO3/TiO2 based catalyst reacts with the potassium salts and form inactive alkali vanadates, which are unable to adsorb ammonia. Therefore new NH3-SCR catalysts more resistant to deactivation by potassium salts metals are needed. One of the possible ways to increase catalyst resistance to alkaline poisons is the use of supports revealing high or super-acidic properties, which would interact more strongly with potassium than vanadium species. Potassium oxide affects the Brønsted acid sites of the catalyst to a much larger extent than Lewis sites. Therefore, another possible solution for NO removal in biomass-related applications is the use of other metal oxides as active components, which posses mainly Lewis acidity [2,3]. In this context we have synthesised and studied CuOX/ZrO2-SO4 catalysts with respect to the performance of NH3-SCR in biomass fired boiler units.

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CuO

Sepiolite

ZrO2

Starch

H2O

(NH4)2SO4

IWI Extrusion Drying (150°C) Calcination (600°C) Figure 1. “One pot” synthesis route used for the synthesis of sepiolite supported CuOX-applied for the production of scalable pelletised catalysts.

2. Experimental 2.1. Catalyst preparation The zirconia source was freshly precipitated Zr(OH)4 from Mel Chemicals. The αsepiolite employed (Pansil 100) was supplied by Tolsa S.A. Ammonium sulphate and the Cu-precursor, Cu(NO3)2 were from Panreac, ( > 99%). The sulphated zirconia was prepared by dissolving (NH4)2SO4, and Cu(NO3)2 into a part of the water. Thereafter the solution was mixed with the homogenised powder blend of the oxides and pore generating agent (PGA, starch), and extra water was added in order to obtain a paste of adequate rheology. After digestion/drying in ambient atmosphere for 2 hours with occasional kneading, the paste was extruded from 20 ml plastic syringes with 2 mm orifices. The samples were then dried at 150°C for overnight. Calcination of the samples was performed at 600°C for 4 hours in air. Finally the extruded materials were chopped into 3-5mm cylindrical pellets. The synthesis procedure is outlined in Figure 1.

2.2. Catalyst characterisation The thermal gravimetric analysis of the samples were carried out on a Netzsch 409 EP Simultaneous Thermal Analysis device. The DSC curves were measured using approximately 20-30 mg of powered sample which were heated in an air flow of 75 ml·min-1 at a rate of 5 ºC·min-1 from room temperature to 1000 ºC, using α-alumina as reference. The specific surface areas, SBET, were obtained from nitrogen adsorption at −196 ◦C using a Micromeritics Tri-Star apparatus, after application of the BET equation at relative pressures in the range 0.05–0.35 p/p◦. Table 1. Textural characteristic of conformed catalysts. Sample

SBET (m2g-1)

SExtern (m2g-1)

Vmeso (cm3g-1)

Vmacro(cm3g-1)

1% CuO

110

104

0.20

0.71

2% CuO

80

68

0.19

0.68

3% CuO

79

70

0.20

0.73

5% CuO

84

77

0.19

0.74

Prior to N2 adsorption the samples were outgassed overnight at 150 ◦C to a vacuum of 1000 nm. Mechanical strength and SCR activity measurements suggested that 25% w/w sepiolite is the optimal catalyst composition. Keywords: Composite, extrusion, biomass, NH3-SCR, ZrO2-SO4, sepiolite

1. Introduction The NH3-SCR process is established as a robust and useful technique for the elimination of NOX from off-gases. Commercially used V2O5-WO3/TiO2 catalysts reduce NO selectively to N2 and H2O by the following reaction: O2 + 4NO + 4NH3 Æ 4N2 + 6H2O

(1)

The reaction involves two cycles, an acid cycle and a redox cycle [1,2]. Thus, the redox capacity of V(V) and V(IV) is of key importance, but also it is crucial to have sufficient acidic surface sites in order to chemisorb and administer NH3 for the SCR reaction [3]. To comply with the Kyoto protocol, substitution of fossil fuels with biomass constitutes a practical, economical and environmentally viable solution, as biomass (straw, wood chips, saw dust etc.) attains its carbon from air during photosynthesis. Thus, biomass can be regarded as a CO2 neutral fuel, and making attempts to increase the biomass content in energy production is of great interest. Though biomass combustion technology is relatively easy to implement in coal and oil-fired power plants, there are drawbacks. Among others, potassium fly ash particles, originating from the firing of biomass, poison the traditional SCR catalysts [4]. Thus, there is an urgent need for alternative potassium fly ash resistant catalysts. We report herein some special features of composite VOX-ZrO2-SO4/sepiolite catalysts. The catalyst exhibit increased surface acidity as well as a shielding effect of the working catalyst, induced by the sepiolite clay enhancing the durability of the SCR catalyst while working under exposure to fly ash that contains potassium.

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(NH4)2SO4

ZrO2

H2O

Drying (RT, 48 hrs) Drying (150°C, 24 hrs)

Calcination (500°C, 4 hrs) Sepiolite

VOX/ZrO2-SO4

VO2+

Calcination (500°C, 4 hrs)

(aq)

VOX/ZrO2-Sepiolite catalyst

Extrusion

Figure 1. Synthesis route employed to produce scalable pelletised catalysts.

2. Experimental 2.1. Catalyst preparation Melcat Zr(OH)4 with a particle size d50 = 15μm was used as the zirconia source and the ammonium sulphate was from Panreac ( > 99%). The sulphated zirconia was prepared by impregnating 101.6 g of the zirconia hydroxide with 8.3 g (NH4)2SO4 by the incipient wetness method. After digestion/drying in ambient conditions for 2 hours the samples were dried at 150°C for 3 hours. Calcination of the samples at 500°C for 4 hours in air was then performed by placing the material directly into a pre-heated furnace, in order to facilitate the preferred formation of metastable tetragonal phases. The catalysts were prepared by wet impregnation of VOSO4 onto the sulphated zirconia, to produce a slurry that was allowed to settle for an hour under stirring. Thereafter sepiolite, α-sepiolite Pansil 100 supplied by Tolsa S.A. and water were added in the desired amounts to obtain a homogeneous paste with an adequate viscosity for extrusion from a 20 ml syringe with a 2 mm orifice. The extrudates were allowed to dry slowly, sealed in a wet atmosphere for 48 hours then dried overnight at 150°C in air. Calcination was carried out at 500°C for 4 hours in air. Finally the extruded material was broken into 3-5mm cylindrical pellets. The synthesis procedure is outlined in Figure 1.

2.2. Catalyst characterisation The specific surface areas (SBET), were obtained from the corresponding nitrogen adsorption isotherms at −196°C using a Micromeritics Tristar apparatus, after application of the BET equation in the relative pressure range 0.05–0.35 p/p°. Prior to N2 adsorption, the samples were outgassed overnight at 150°C to a vacuum of ATNTA. This clearly shows that Pd catalysts are more active than gold catalysts. The highest activity of palladium based catalysts can be related to the easiest accessibility and reducibility of palladium particles (PdO) [1], which are probably outer the surface. However gold particles (Au) are more difficult to access and in our case some of them are inserted into the tube hollows [9]. The nature of the support plays also a key role in the reaction thus catalysts prepared on TNTP25 give higher performance than catalysts prepared on TNTA. This result can be related to the high surface area of TNTP25 (Table 1), leading to well dispersed palladium and gold particles. The trend for the rate of oxidation of VOCs over palladium catalysts is propene > MEK > toluene: the more the molecule is small the more it is easier to oxidize. Whereas with gold catalysts, the trend appears to be different: MEK > propene > toluene. The reactivity of oxygenated molecules seems to be more important with the gold catalysts and VOC oxidation is governed by the polarity of VOC molecule. Minico et al. [11] attributed this behaviour to the ability of highly dispersed gold to activate the oxygen in which oxygenated VOC was strongly adsorbed. Consequently alcohols and ketones showed higher reactivity than aromatics during reactions investigated on Au/Fe2O3 samples [11]. It can be deduced that oxidation mechanisms over palladium and gold catalysts are quite different.

4. Conclusion Propene, MEK and toluene total oxidation are studied with palladium or gold based catalysts (1.5wt%) supported on titanium oxide nanotubes (TNTA and TNTP25) with high surface areas. Palladium catalysts are more active than gold catalysts. The trend for the rate of oxidation of VOCs over palladium catalysts is propene > MEK > toluene: the more the molecule is small the more it is easier to oxidize. Whereas with gold catalysts, MEK is easier to oxidize, the reactivity of oxygenated molecules seems to be more

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important with the gold catalysts. The nature of the support plays also a key role in the reaction, thus catalysts prepared on TNTP25 give higher performance than catalysts prepared on TNTA.

90

propene conversion (%)

80

100

ATNTA

ATNTA

ATNT P25

90

Pd/TNTA

80

Pd/TNT P25 toluene conversion (%)

100

70 60 50 40

ATNT P25 Pd/TNTA Pd/TNT P25

70 60 50 40 30

30 20

20

0 100

10

A

10

150

200

250

300

350

0 100

400

150

200

250

300

350

C

400

temperature (°C)

temperature (°C)

Fig. 2. Light-off curves for propene (A), MEK (B) and toluene (C) total oxidation in air over catalysts supported on titania nanotubes.

100 90 80

ATNTA ATNT P25 Pd/TNTA

mek conversion (%)

Pd/TNT P25 70

Acknowledgements

60 50

This work was supported by IRENI and Interreg IV “Redugaz” projects and the European Community (European Regional Development Fund).

40 30 20

B

10 0 100

150

200

250

300

350

400

temperature (°C)

References [1] H.L. Tidahy, S. Siffert, J.-F. Lamonier, E.A. Zhilinskaya, A. Aboukaïs, Z.-Y. Yuan, A. Vantomme, B.-L. Su, X. Canet, G. De Weireld, M. Frère, T.B. N’Guyen, J.-M. Giraudon, G. Leclercq, Appl. Catal. A 310 (2006) 61 [2] H.L. Tidahy, M. Hosseini, S. Siffert, R. Cousin, J.-F. Lamonier, A. Aboukaïs, Catal. Today 137 (2008) 335 [3] G. Arzamendi, V.A. de la Peña O'Shea, M.C. Álvarez-Galván, J.L.G. Fierro, P.L. Arias, L.M. Gandía, J. Catal. 261 (2009) 50 [4] M. Paulis, L.M. Gandia, A. Gil, J. Sambeth, J.A. Odriozola, M. Montes, Appl. Catal. B 26 (2000) 37 [5] S. Ivanova, C. Petit, V. Pitchon, Appl. Catal. A 267 (2004) 191 [6] M. Hosseini, S. Siffert, H. L. Tidahy, R. Cousin, J.-F. Lamonier, A. Aboukaïs, A. Vantomme, B.-L. Su, Catal. Today 122 (2007) 391 [7] C. Gennequin, M. Lamallem, R. Cousin, S. Siffert, F. Aïssi, A. Aboukaïs, Catal. Today 122 (2007) 301 [8] V. Idakiev, L. Ilieva, D. Andreeva, J.L. Blin, L. Gigot, B.L. Su, Appl. Catal. A 243 (2003) 25 [9] V. Idakiev, Z.-Y Yuan, T. Tabakova, B.-L Su, Appl. Catal. A 281 (2005) 149 [10] Y. Takita, K. Inokuchi, O. Kobayashi, F. Hori, N. Yamazoe, T. Seiyama, J. Catal. 90 (1984) 232 [11] S. Minicò, S. Scirè, C. Crisafulli, R. Maggiore, S. Galvagno, Appl. Catal. B 28 (2000) 245

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Preparation of Alkali-M/ZrO2 (M = Co or Cu) for VOCs oxidation in the presence of NOx or carbonaceous particles Aissa Aissata,b, Stéphane Siffert*,a,b, Dominique Courcota,b a

Univ Lille Nord de France, F-59000 Lille, France ULCO, UCEIV, F-59140 Dunkerque, France *Corresponding author. E-mail address: [email protected]

b

Abstract Total oxidation of toluene is investigated on catalysts with alkali metals (Na or Cs) alone or with Co or Cu impregnated on ZrO2. The sample Cs/ZrO2 is more active than Na/ZrO2. In the presence of NOx or carbonaceous particles, the addition of Co to Cs/ZrO2 improves toluene oxidation, especially for the catalyst with a low Cs/Co ratio. Keywords: Toluene oxidation; NOx ; Carbonaceous particles; Alkali metals; Transition metals

1. Introduction The increasing environmental awareness in the last two decades has prompted the emergence of stricter regulations for industrial activities. Among these, the reduction of volatile organic compounds (VOCs) emissions is important because these molecules represent a serious environmental problem. Complete VOCs oxidation needs highly active catalysts at low temperatures [1]. Several industries are faced to the problem of simultaneous release of VOCs and NOx. A solution to this environmental problem could be a catalyst with high activity for NO oxidation to NO2 and high activity for NO2 reduction to N2 with hydrocarbons [2]. Zirconia has shown good catalytic activity, especially in hydrocarbons oxidation reactions owing to its ability to convey oxygen species [3]. The introduction of alkali metals in catalysts developed for VOCs oxidation in the presence of NOx is expected to provide interesting effects. In some cases, alkali metals are known to be promoters for the oxidation of carbon black (CB) as well as for NOx reduction [4]. Previous works of our laboratory [5,6] presented this effect of alkali metals in the oxidation of soot. Moreover, Co and Cu are interesting active phases for toluene oxidation [7]. The aim of this work is focused on the preparation of catalysts with alkali metal (Na or Cs) alone or impregnated with Co or Cu on ZrO2. The effect of alkali/Co ratio on the catalyst performance for toluene oxidation in presence or without NOx or CB is especially studied.

2. Experimental 2.1. Catalysts preparation ZrO2 support is prepared by a precipitation method, adding dropwise an aqueous solution of zirconyl (IV) chloride to an ammonia solution under continuous stirring. The precipitate is filtered and washed to remove remaining chloride ions [8]. This solid is dried at 115°C for 24 h and subsequently calcined under air flow (2 L h-1) at 300°C for

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4 h [5,6]. The alkali/ZrO2 systems are obtained by impregnation of aqueous alkali carbonates on a ZrO2 support (201 m2 g-1). Cs-Co/ZrO2 catalysts are prepared by co-impregnation of CoCO3 and Cs2CO3 salts onto ZrO2. CsCu/ZrO2 catalysts are prepared with the same method (using CuCO3·nCu(OH)2). After drying, the samples are calcined under air flow at 600°C for 4 h. The as-obtained solids are denoted Csx-M0.1/ZrO2, where M corresponds to the transition metal, x and 0.1 to alkali/Zr and M/Zr atomic ratio respectively.

2.2. Catalysts characterization The chemical composition of the samples is determined by ICP-MS (Varian 820 MS). BET surface areas are measured by N2 adsorption at 77 K (Thermo Electron Qsurf M1). XRD measurements are obtained by a Bruker D8 Fig. 1. XRD patterns of catalysts calcined Advance diffractometer. TG-DTA experiments at 600°C (A: ZrO2, B: Alkali0.15/ZrO2 are performed in air flow (75 mL min-1, -1 catalysts “a: Na0.15/ZrO2, b: Cs0.15/ZrO2”, 5°C min , Netzsch STA 409). C: Cs0.15-M0.1/ZrO2 catalysts “a: Cs0.15Toluene oxidation is carried out in a Cu0.1/ZrO2, b: Cs0.15-Co0.1/ZrO2”, continuous flow reactor with a fixed bed at (•) monoclinic phase, (♦) tetragonal phase, atmospheric pressure (1000 ppm toluene and -1 (□) CuO phase, (○) Co3O4 cubic phase). 10% O2 in N2, total flow rate: 100 mL min ). Before each activity test, the catalyst (100 mg) is dried in air (2 L h-1) at 500°C (1°C min-1) for 4 h [3]. Toluene conversion is checked using a Varian CP-4900 µGC. The effect of the presence of NOx (NO2 + NO) in the feed gas (1000 ppm of NO in N2) or CB (100 mg of catalyst + 6 wt.% of CB) is also studied. The products (NO, NO2, CO and CO2) are measured with Servomex Xentra 4900C analyzer. The volume hourly space velocity (VHSV) is 105 000 h-1.

3. Results 3.1. Characterization The theoretical alkali metals content in the catalysts is almost confirmed (Table 1). However, the content of transition metals and Cs are both lower than that expected in the case of coimpregnated samples. A XRD analysis of the different alkali0.15/ZrO2 and Cs0.15-M0.1/ZrO2 solids calcined at 600°C is carried out (Fig. 1). After calcination at 600°C, ZrO2 is a mixture of tetragonal

Table 1. Chemical composition, specific areas and mean crystallite size of tetragonal ZrO2 (2θ = 30.4° for the (111) reflex) of alkali0.15/ZrO2 and Csx-M0.1/ZrO2 solids after calcination at 600°C for 4 h. Catalyst ZrO2 Na0.15/ZrO2 Cs0.15/ZrO2 Cs0.15-Co0.1/ZrO2 Cs0.015-Co0.1/ZrO2 Cs0.15-Cu0.1/ZrO2

Experimenta l molar ratio 0.130 0.132 0.095 (Cs) 0.073 (Co) 0.013 (Cs) 0.089 (Co) 0.100 (Cs) 0.074 (Cu)

BET surface area (m2 g-1) 84 34 21

Particle size (nm) 8±3 23 ± 3 14 ± 3

34

18 ± 3

85

11 ± 3

17

21 ± 3

Preparation of Alkali-M/ZrO2 (M = Co or Cu) for VOCs oxidation

749

(JCPDS 50.1089) and monoclinic (JCPDS 65.1023) phases (Fig. 1.A.). When Co or Cu salt is impregnated with Cs onto the ZrO2 carrier (Fig. 1.C), the XRD patterns of the corresponding solids (Cs0.15-M0.1/ZrO2) calcined at 600°C show only the presence of the tetragonal phase revealing that Co or Cu favors the stabilization of this crystalline phase. Similar observations are obtained for Na0.15/ZrO2 (Fig. 1.B.a). In the case of Cs0.15/ZrO2 (Fig. 1.B.b), both tetragonal and monoclinic phases are detected. Nevertheless, the presence of the transition metal in Cs0.15-M0.1/ZrO2 leads to the stabilization of the tetragonal ZrO2 phase evidenced by the proportion of monoclinic phase which is extremely low after calcination at 600°C. Moreover, differences in middle height width are detected for the different solids. Mean crystallite size for the tetragonal phase (2θ = 30.4° for the (111) reflex) is estimated using the Scherrer equation (Table 1). No significant difference in mean crystallite size is obtained for ZrO2 and M0.1/ZrO2. On the contrary; it appears that tetragonal ZrO2 crystallites possess a bigger size in the presence of alkali containing solids and particularly in the presence of Na. ZrO2 possesses a high specific area value (Table 1), but the presence of alkali metal (Na or Cs) leads to a strong decrease of this area. These phenomena can be explained considering the increase of mean crystallite size of tetragonal ZrO2 in the presence of alkali. Moreover, the strong coverage of Cs species on ZrO2 surface could explain the low specific areas of Cs0.15/ZrO2 and Cs0.15-M0.1/ZrO2. TG-DTA analysis is performed on dried alkali0.15/ZrO2 and alkali0.15-M0.1/ZrO2 solids and the obtained DTA curves are displayed in Fig. 2. For ZrO2 solid, the exothermic peak detected at 428°C is attributed to the crystallisation of tetragonal ZrO2 [1]. DTA curves show the influence of alkali species on the ZrO2 crystallisation and its tetragonal-monoclinic transformation. Broad exothermic peaks are detected at 505°C and 564°C in Na0.15/ZrO2 and Cs0.15/ZrO2 respectively. Furthermore, additional shifts of this exothermic peak in Cs0.15-M0.1/ZrO2 versus alkali/ZrO2 seem to indicate that Cs and the transition metal have cumulative effects hindering the tetragonal ZrO2 crystallisation. An exothermic peak at 910°C for Na0.15/ZrO2 solid ascribed to a rapid tetragonal-monoclinic transformation [9] is detected. Recall that Na0.15/ZrO2 contains only a well-crystallized tetragonal ZrO2 after calcination at 600°C (Fig. 1.B.a). The absence of exothermic peak in Cs-containing solids could be explained by a different transformation kinetic in these solids. In the case of alkali-Co catalysts, an endothermic peak detected at 930°C corresponds to Co3O4 decomposition to CoO [10].

3.2. Catalytic test Between the catalysts promoted by alkali alone, the oxidation of toluene is only complete at 500°C for Cs0.15/ZrO2 (Table 2). Co and Cu impregnations on Cs0.15/ZrO2 lead to a powerful catalyst for Cs0.15Co0.1/ZrO2 but a less active solid for Cs0.15Cu0.1/ZrO2. However, the solid with a low amount of Cs (Cs0.015-Co0.1/ZrO2) allows to reach a T50 = 296°C instead for Cs0.15Co0.1/ZrO2.

Fig. 2 DTA curves of dried solids (a: ZrO2, b: Na0.15/ZrO2, c: Cs0.15/ZrO2, d: Cs0.15Co0.1/ZrO2, e: Cs0.15-Cu0.1/ZrO2).

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Therefore, the presence of a high amount of alkali metal (interesting for CB Toluene Toluene Toluene oxidation [5]) decreases the Catalyst + O2 + CB + O2+ NO + O2 activity of the Co3O4 phase for toluene oxidation, and in Na0.15/ZrO2 > 500 (283) the presence of CB, T50 is not Cs0.15/ZrO2 443 (283) 451 (276) 490 (308) enhanced, but Ti is slightly Cs0.15-Cu0.1/ZrO2 487 (300) enhanced. This is due to the Cs0.15-Co0.1/ZrO2 371 (260) 423 (220) 399 (250) oxidation of CB proceeding Cs0.015-Co0.1/ZrO2 296 (260) 269 (208) 279 (210) before that of toluene. For Cs0.015-Co0.1/ZrO2, T50 and Ti are both enhanced in the presence of CB. In this case, adsorption of toluene on CB could explain this result. Toluene oxidation is not enhanced by the presence of NOx in the case of catalysts with a high Cs content. This is due to the adsorption of NOx species by the catalyst with a high alkali amount, leading to the formation of nitrates [11]. A lower amount of Cs permits to both enhance NOx conversion and toluene oxidation compared to the same oxidation test performed without NOx. The oxidation of toluene is then very efficient in the presence of NOx but also in the presence of CB over Cs0.015Co0.1/ZrO2. Table 2. T50: temperature at which 50% of toluene is oxidized, (Ti): ignition temperature (°C).

4. Conclusion The addition of transition metals (Co or Cu) to Cs0.15/ZrO2 stabilizes tetragonal ZrO2 phase. The presence of CB or NOx leads to improve toluene oxidation, especially with the catalyst Cs0.015-Co0.1/ZrO2.

Ackowledgements The “Nord-Pas de Calais” Region, the “Syndicat Mixte de la Côte d’Opale” and European Union via Interreg IV “Redugaz project” are gratefully acknowledged for financial support.

References [1]

M. Labaki, S. Siffert, J.-F. Lamonier, E.A. Zhilinskaya, A. Aboukaïs, Appl. Catal. B 43 (2003) 261. [2] T. Holma, A. Palmqvist, M. Skoglundh, E. Jobson, Appl. Catal. B 48 (2004) 95. [3] J.-F. Lamonier, M. Labaki, F. Wyrwalski, S. Siffert, A. Aboukaïs, J. Anal. Appl. Pyrolysis 81 (2008) 20. [4] A. Bueno-Lopez, J. Soriano-Mora, A. Garcia-Garcia, Catal. Commun. 7 (2006) 678. [5] D. Hleis, M. Labaki, H. Laversin, D. Courcot, A. Aboukaïs, Colloids Surf. A 330 (2008) 193. [6] H. Laversin, D. Courcot, E. Zhilinskaya, R. Cousin, A. Aboukaïs, J. Catal. 241 (2006) 456. [7] F. Wyrwalski, J.-F. Lamonier, S. Siffert, A. Aboukaïs, Appl. Catal., B 70 (2007) 393. [8] J. Matta, J.-F. Lamonier, E. Abi-aad, E. Zhilinskaya, A. Aboukaïs, Phys. Chem. Chem. Phys. 1 (1999) 4975. [9] S. Liu, S. Huang, L. Guan, J. Li, N. Zhao, W. Wei, Y. Sun, Microporous Mesoporous Mater. 102 (2007) 304. [10] G. Gürdağ, I. Boz, S. Ebiller, M. Gürkaynak, React. Kinet. Catal. Lett. 83 (2004) 47. [11] I. Matsukuma, S. Kikuyama, R. Kikuchi, K. Sasaki, K. Eguchi, Appl. Catal. B 37 (2002) 107.

10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Design of appropriate surface sites for rutheniumceria catalysts supported on graphite by controlled preparation method J. Álvarez-Rodríguez, A. Maroto-Valiente* , M. Soria-Sánchez, V. Muñoz-Andres, A. Guerrero-Ruiz Departamento de Química Inorgánica y Química Técnica, Facultad de Ciencias, UNED, Pº Senda del Rey, 9, 28040 Madrid, Spain.

Abstract Ru-Ce supported catalyst properties were studied with the aim of improving catalytic performance in phenol abatement from aqueous solutions by investigating the effect of different thermal pretreatments under a flow of helium. Characterization using TGA, XRD, TEM and XPS shows that ruthenium acetylacetonate was more highly dispersed than CeO2. Ru-Ce/HSAG samples show increased Catalytic Wet Oxidation (CWO) of phenol compared to Ru/CeO2, Ru/HSAG and Ce/HSAG. The Ru-Ce sites generated during the preparation and activation of the Ru-Ce/HSAG catalyst at 473 K, result in higher activity, conversion and mineralization values in the phenol CWO. Keywords: phenol, CWO, Ce, Ru, graphite

1. Introduction Due to their unique properties, cerium oxides are currently one of the most employed components in the preparation of catalysts, i.e. as support for metallic heterogeneous catalysts, as stabilizers of dispersed components or as promoter ingredients [M. Boaro (2003), A. Trovarelli (2002)]. Its redox properties, oxygen storage capacity, high mechanical strength and ultraviolet absorption should be related to the formation of oxygen vacancies and oxygen mobility, which means the Ce(IV)-Ce(III) relation can play a critical role in catalytic reactions (fuel cells, three-way catalysts, CWO, etc). Moreover, it is well known that catalyst preparation methods are important to the final surface properties. It should therefore be interesting to explore the possibility of using different thermal activation pretreatments which could modify the Ru-Ce interactions, and then evaluate the effects of these modifications in the required processes (oxidation hydrogenation, decomposition, etc.) to expand and improve the properties of the catalyst. The Ru/CeO2 system is known for its efficient performance in CWO processes, which are employed to remove pollutants in wastewater [S. Imamura (1988), L. Oliviero (2000)]. It is expected that the catalytic performance could be improved if optimized Ru species are well dispersed on CeO2. Acting cooperatively they can act as active centers when deposited over a carbon material. Thus the purpose of this work is to study if it is possible to optimize a thermal pretreatment method and analyze the decomposition process of the metal precursor in order to improve Ru-CeO2 aggregates built on graphitic surface for phenol CWO.

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2. Experimental 2.1. Preparation Graphite (HSAG Lonza) was employed as support after pretreatment in a tubular reactor at 1173 K under helium flow. It was then activated by impregnation of a aqueous solution of Ce(NO3)3·6H2O (Fluka) and subsequently treated under nitrogen flow at 773 K for 2 h (5 wt.% of Ce content). The sample obtained was labeled as Ce/HSAG. Supported Ru catalysts were prepared by excess-impregnation solution (MeOH:H2O = 1:1) of Ru(acac)3 (Alfa-Aesar), over Ce/HSAG and CeO2 (Rhone-Poulenc), labeled as RuCe/HSAG and Ru/CeO2 respectively (2 wt.% of Ru content). All these samples were dried at 373 K and then stored in a desiccator.

2.2. Characterization Thermogravimetric Analysis (TGA) was carried out using a SDTQ600 5200 TA System. 10 mg samples were pretreated at room temperature for 30 min under helium atmosphere (flow rate = 100 mL min−1) and heated to 973 K, with a heating rate of 10 K min−1. The X-ray diffraction (XRD) patterns were recorded in an X-ray diffractometer (Seifert Model XRD 3000P), using Cu-Kα radiation and a graphite monochromator. Transmission Electron Microscopy (TEM) and X-Ray Energy Dispersive Spectroscopy (XEDS) studies were carried out in a JEOL JEM-2000 FX microscope at 200 kV. The samples were prepared by grinding and ultrasonic dispersal in an acetone solution before being placed on a copper TEM grid and the solvent evaporated. X-ray photoelectron spectra (XPS) were recorded with an Omicron spectrometer equipped with an EA-125 hemispherical electron multichannel analyzer and an unmonochromatized Mg Kα X-ray source having radiation energy of 1253.6 eV at 150 W and a pass energy of 50 eV. The spectral data was analyzed with CasaXPS software and RSF database by peak fitting after Shirley background correction.

2.3. Reaction test Oxidation reaction experiments were performed in a 300 mL stainless-steel high pressure reactor vessel (Parr Instruments Co., USA, 5521) operated under isothermal batch mode at 413 K, 2 MPa of oxygen pressure and stirred at 500 rpm to optimize the mass transfer in the liquid phase. For every run a fresh feed of aqueous phenol solution of 20 mmol L−1 and 4 g L−1 of the catalyst was introduced to the reaction vessel. The liquid phase was analyzed by HPLC on a Pursuit XRs 5 C18 150 while the gas phase was analyzed in a GC equipped with a Porapak Q packed column.

3. Results and discussion 3.1. Characterization TGA of the Ru-Ce/HSAG sample indicates that metal precursors, Ru(acac)2 and Ce(NO3)3, interact with the support and display three contributions to a total weight loss of up to 5%. There is a first weight decrease around 460 K, a second one close to 550 K and last weight loss at around 920 K. Derivate profiles (D-TGA), displayed in Figure 1, reveal, with accuracy, the three peaks positions are centered at 375 K, 553 K and 898 K. The first should be related to water desorption, close to 9% of weight lost. The main peak, at 553 K, can be assigned to ruthenium precursor decomposition and represents 78% of weight lost, and finally, the peak at 886 K, may be attributed to oxygen loss from the CeO2. In fact, when Ce/HSAG is studied, only the peak at 886 K was obtained. The shape of the main peak of Ru-Ce/HSAG seems to contain different contributions. So in order to identify them, three fresh samples were pretreated for 2 h in helium at 473 K, 503 K and 553 K, respectively. The TGs of these samples show that three

Design of appropriate surface sites for ruthenium-ceria catalysts supported on graphite 753 contributions can be distinguished in the main DTG peak of the Ru-Ce/HSAG sample; at 513 K (18 wt.% lost), at 563 K (54 wt.% lost) and at 617 K (7 wt.% lost). These three meta-species are attributed to multi-step decomposition of ruthenium precursor and/or with different surface site distribution of the adsorbed ruthenium acetylacetonate, namely over ceria or over graphite sites/planes.

%w / K

D-TGA

400

600

800

Temperature (K)

Figure 1. D-TGA of Ru-Ce/HSAG as made(black), pretreated under He flow at 473 K (red) and 503 K (blue).

Ce/HSAG and Ru-Ce/HSAG XRD patterns show a main diffraction peak at 2θ = 26° (0 0 2) due to graphitic carbon structures and CeO2 crystallites, which exhibit a cubic fluorite-type structure, displaying characteristic diffraction lines of CeO2 at 2θ = 28.5° (1 1 1), 33.2° (2 0 0) and 47.5° (2 2 0). The presence of crystalline ruthenium species, RuO2 and Ru0, is not confirmed at characteristic 2θ values. However, in all samples with ruthenium content, Ru-Ce/HSAG and Ru/CeO2, an increase of the diffraction peak at 56.3º (3 1 1) of CeO2 was observed, thus the presence of some crystalline Ru species was not discarded. Information concerning particle size and component distribution in these catalysts was obtained by Transmission Electron Microscopy (TEM). For the Ce/HSAG sample, dispersed CeO2 particles were observed and identified by XEDS analysis with heterogeneous sizes between 20-100 nm. Likewise, CeO2 particles were detected for Ru-Ce/HSAG, but no RuO2 or Ru0 crystallites were observed by TEM. This probably means that the ruthenium species are highly dispersed. Careful XEDS analysis was used to gain information about the Ru distribution, and Ru was detected whenever the sample was studied. Everywhere that CeO2 particles exist, ruthenium is also present. However, in the positions where ruthenium appears, CeO2 is not always detected. Following the Luo 2009 processing method, analysis of X-ray photoelectron spectra of samples showed the Ce 3d region was composed of 10 contributions, named U1, U0, V1 and V0 for Ce3+ species, and U3, U2, U, V3, V2 and V for Ce4+ species. The peak areas were then determined and the oxidation states were calculated using the following equations: Ce3+ = U1 + U0 + V1 + V0 Ce4+ = U3 + U2 + U + V3 + V2 + V Percentage of Ce3+ = Ce3+ / (Ce3+ + Ce4+)

Eq. (1) Eq. (2) Eq. (3)

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After solving this percentage equation, ratios found for Ce3+/Ce4+ were 36/64 for Ce/HSAG and 27/73 for Ru-Ce/HSAG. This suggests that the fraction of ruthenium anchored over ceria particles is preferred at those sites with Ce3+. Note that this fraction of ruthenia is similar to that fraction of first TGA-peak identified and could suggest a Ru-Ce3+ site.

3.2. CWO reaction test Phenol is commonly employed as a reference molecule for aqueous oxidation tests of heterogeneous catalysts in order to gain knowledge about its performance for abatement of aromatic compounds. Phenol CWO over our catalysts yields hydroquinone, benzoquinone, cathecol, acetic acid, formic acid, oxalic acid, fumaric acid, maleic acid, malonic acid and carbon dioxide, but with different distribution. After 5 h of reaction, Ce/HSAG and Ru/CeO2 samples shows conversion values approximately of 30%, but higher phenol transformation of up to 100%, is achieved with Ru-Ce/HSAG. However, depending on the pretreatment, total conversion is achieved in differing time periods. Similar conversion is observed after 180 min and 220 min for catalysts pretreated at 473 K and 503 K respectively. Mineralization (phenol transformed to CO2) tendencies were observed and it was seen that Ru/CeO2 < Ce/HSAG