Studies in Surface Science and Catalysis 144 CHARACTERIZATION OF POROUS SOLIDS VI
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Studies
in S u r f a c e
Science
and Catalysis
Advisory Editors: B. Delmon and J.T. Yates Vol. 144
CHARACTERIZATION OF POROUS SOLIDS VI Proceedings of the 6th International Symposium on the Characterization of Porous Solids (COPS-VI), Alicante, Spain, May 8-11, 2002
Edited by F. R o d r i g u e z - R e i n o s o
1, B. M c E n a n e y
2, J. R o u q u e r o l
3 a n d K. U n g e r
University of Alicante Alicante, Spain 2 University of Bath, Bath, UK -~CNRS Marseille, France Johannes Guttenberg-Universitat Mainz, Germany
0
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Contents
Preface Committees Financial support
XV XVII XVII
Scanning probe microscopies for the characterization of porous solids: strengths and limitations J.I. Paredes, A. Martinez-Alonso, J.M.D. Tasc6n Role of gas adsorption in nanopore characterization K. Kaneko, T. Ohba, Y. Hattori, M. Sunaga, H. Tanaka, H. Kanoh
11
Reconstruction method for the characterization of porous carbons J. Pikunic, C. Clinard, N. Cohaut, K.E. Gubbins, J.-M. Guet, R.J.-M. Pellenq, I. Rannou, J.-N. Rouzaud
19
A new method for microporosity detection based on the use of the corrugated pore structure model (CPSM) C.E. Salmas, V.N. Stathopoulos, A.K. Ladavos, P-J. Pomonis, G.P. Androutsopoulos
27
Physisorption in nanopores of various sizes and shapes: a Grand Canonical Monte Carlo simulation study B. Coasne, A. Grosman, C. Ortega, R.J.M. Pellenq
35
Induced porosity in cross-linked polymer networks: Mean field theory and simulations S. Srebnik
43
Microbean small angle X-ray scattering (ttSAXS): a novel technique for the characterization of activated carbon fibers D. Lozano-Castell6, E. Raymundo-Pifiero, D. Cazorla-Amor6s, A. Linares-Solano, M. Miiller, C. Riekel
51
"Real time" determination of porosity development in carbons: a combined SAXS/TGA approach J.M. Calo, P.J. Hall, S. Houtmann, D. Lozano-Castell6, R.E. Winans, S. Seifert
59
SANS investigations of adsorption mechanisms in model porous silicas S. Kallus, A. Hahn, J.D.F. Ramsay
67
Preparation and surface characterization of novel ceria-copper and ceria-manganese mixed oxides M. Christophidou, C.R. Theocharis
75
About the exclusive mesoporous character of MCM-41 A. Berenguer-Murcia, J. Garcia-Martinez, D. Cazorla-Amor6s, A. Martinez-Alonso, J.M.D. Tasc6n, A. Linares-Solano
83
Incorporation of appropriate contact characterization by mercury porosimetry J.C. Groen, L.A.A. Peffer, J. P~rez-Ramirez
angles
in
textural
A two-stage Horvath-Kawazoe adsorption distribution analysis R.J. Dombrowski, C.M. Lastoskie
model for pore size
91
99
Textural and chemical characterization of NaX zeolite exchanged with Zn(ll) ions J. Silvestre-Albero, A. Sepfilveda-Escribano, F. Rodriguez-Reinoso
107
Evaluation and comparison of the pore structure and related properties of particulate and monolithic silicas for liquid phase separation processes K.K. Unger, B. Bilingmaier, C. du Fresne von Hohenesche, D. Lubda
115
Percolation phenomena in micropores: influence on single and multicomponent adsorption equilibria S. Ismadji, S.K. Bhatia
123
Characterization of the surface chemistry of activated carbon by molecular simulation of water adsorption M. Jorge, N.A. Seaton
131
Comparison of porous carbons developed via templating approaches P.M. Barata-Rodrigues, T.J. Mays, N.A. Seaton, G.D. Moggridge
139
Simulation of adsorption in 3-D reconstructed mesoporous materials by a simulated annealing algorithm M.E. Kainourgiakis, E.S. Kikkinides, G.Ch. Charalambopoulou, A.K. Stubos
147
Understanding adsorption hysteresis in porous glasses and other mesoporous materials H.-J. Woo, L. Sarkisov, P.A. Monson
155
vii Structural studies of mesoporous alumina membranes by small angle X-ray scattering J.C. Dore, D. Grandjean, R.E. Benfield, M. Kroll, D. Le Bolloc'h
163
Assessing microporosity by immersion microcalorimetry into liquid nitrogen or liquid argon J. Rouquerol, P. Llewellyn, R. Navarrete, F. Rouquerol, R. Denoyel
171
The lower closure point of the adsorption hysteresis loop of fluids in mesoporous silica materials A. Schreiber, S. Reinhardt, G.H. Findenegg
177
New metodologies in mercury porosimetry S.P. Rigby
185
Theoretical calculation of high micropore volumes on activated carbons J. Alcafiiz-Monge
193
Assessment of ultramicroporosity on carbon molecular sieves by water adsorption J. Alcafiiz-Monge, D. Lozano-Castell6
201
Active surface area of carbon materials determined by different methods A. Arenillas, F. Rubiera, J.B. Parra, J.J. Pis
209
Heterogeneity of sewage sludge derived materials as a factor governing their performance as adsorbents of acidic gases A. Bagreev, S. Bashkova, B. Reznik, V. Zibat, T.J. Bandosz
217
Evaluation of microporous structure of carbon molecular sieves using the pycnometric method M. Balys, B. Buczek, E. Vogt
225
Characterization of porous carbonaceous sorbents using adsorption data in wide temperature and pressure ranges G. De Weireld, M. Fr6re
231
Ammonia accessibility to the porosity of several activated carbons measured by flow adsorption microcalorimetry M. Domingo-Garcia, A.J. Groszek, F.J. L6pez-Garz6n, M. P6rez-Mendoza
239
An IGC and TA study of acetaldehyde adsorption on activated carbons Y. E1-Sayed, T.J. Bandosz
247
viii A novel approach for characterizing carbon catalysts by TAP experiments V. Fierro, M.T. Izquierdo, Y. Schuurman, B. Rubio, C. Mirodatos
255
Preparation of activated carbons with controlled pore size M.M.A. Freitas, J.L. Figueiredo
261
Characterization of porous solids using low pressure VOC adsorption data J.Nokerman, S. Dutour, M. Fr~re, S. Limborg-Noetinger, S. Jullian
267
Energetics and mechanism of physical sorption by carbonaceous solids: Evaluation of surface area and porosity factors E.L. Fuller, Jr.
275
Phenanthrene adsorption on a carbonaceous material: moisture and CO2 influence A.M. Mastral, T. Garcia, R. Murillo, M.S. Call6n, J.M. L6pez, M.V. Navarro
283
Water adsorption on micro and mesoporous silicas J. Alcafiiz-Monge, D. Lozano-Castell6
291
The effect of surface functionalization of mesoporous silicas with propylimidazol on porosity, pore connectivity and tortuosity G.S. An~atas, C.E. Salmas, G.P. Androutsopoulos, P.J. Pomonis
299
Preparation of porous silica by acid activation of metakaolins C. Belver, M.A. Bafiares, M.A. Vicente
307
The effect of particle shape on the filtration rate and shear strength of quartz and dolomite mineral filter cakes B. Benli G6ntil, O. Ozcan
315
Characterisation of silica low-density xerogels in presence of additives by image analysis and nitrogen adsorption-desorption S. Blacher, C. Alia, C. Gommes, P. Lodewyckx, R. Pirard, J.-P. Pirard
323
Texture characterization of ultramacroporous materials using nondestructive methods S. Blather, V. Maquet, A. L~onard, G. Chapelle, M. Crine, R. J~rome, J.-P. Pirard Formation of hierarchically ordered silicas prepared by spray drying of nanosized spheres C. du Fresne von Hohenesche, V. Stathopoulos, K.K. Unger, A. Lind, M. Lind6n
331
339
Ultrathin porous glass membranes with controlled texture properties D. Enke, F. Friedel, F. Janowski, T. Hahn, W. Gille, R. Mtiller, H. Kaden
347
Reconstruction of mesoporous silica glass Gelsil| 50 N. Eschricht, E. Hoinkis, F. M/adler, P. Schubert-Bischoff
355
Pore structural characteristics of mesostructured materials prepared under different conditions A.E. Candeias, M.M.L. Ribeiro-Carrott, P.J.M. Carrott, K. Schumacher, M. Grtin, K.K. Unger
363
A grand canonical Monte Carlo simulation study of water adsorption in a Vycor-like disordered mesoporous material at 300K J. Puibasset, R.J.-M. Pellenq
371
A review of the application of the BET equation to experimental data: the C parameter F. Salvador, C. Sfinchez-Jim~nez, M.J. Sfinchez-Montero, A. Salvador
379
Thermogravimetric and sorption measurement techniques/instruments J.U. Keller, E. Robens, C. du Fresne von Hohenesche
387
Influence of method of washcoat preparation on hydrothermal stability of alumina support M. Kulazynski, J. Trawczynski, J. Walendziewski
395
Limestone influence on PAH emissions from coal AFBC. Catalytic or/and adsorption effect? A.M. Mastral, T. Garcia, M.S. Call6n, J.M. L6pez, R. Murillo, M.V. Navarro
403
Freezing point elevation in nanospace detected directly by atomic force microscopy M. Miyahara, M. Sakamoto, H. Kanda, K. Higashitani
411
Is it possible to obtain a coherent image of the texture of a porous material? F. Noville, C. Gommes, C. Doneux, A. Brasseur, R. Pirard, J.-P. Pirard
419
Preparation and characterisation of Cr-Co spinels for methane oxidation in presence of sulphur compounds J.R. Paredes, S. Ord6fiez, F.V. Diez, H. Sastre
427
Characterising the porous structure of Egyptian mortars using thermoporometry, mercury intrusion porometry and gas adsorption manometry J. Raga'f, T. Poyet, I. Beurroies, F. Rouquerol, P. Llewellyn
435
Monitoring fast pressure changes in gas transport and sorption analysis G. Reichenauer, H.-J. Fella, J. Fricke
443
Effect of coke deposition on catalyst texture during catalytic cracking reaction J.P. Reymond, C. Delattre, M. Forissier
451
Precision of porosity measurements on cementitious mortars K. Rtibner, Th. Fritz, F. Jacobs
459
Freezing in mesopores: aniline in silica glasses and MCM-41 M. Sliwinska-Bartkowiak, G. Dudziak, R. Radhakrishnan, K.E. Gubbins
467
Transport characteristics of porous solids derived from chromatographic measurements O. Solcovfi, P. Schneider
475
Effects of porous solid structures on the electrical behaviour: prediction key of transport properties in sedimentary reservoir rock A. Cerepi, R. Burlot, S. Galaup, J-P. Barde, C. Loisy, L. Humbert
483
Pore and surface characteristics of porous melamine- and phenolicformaldehyde polymers by sorption and XPS measurements A. Derylo-Marczewska, J. Goworek
491
Porous structure of multifunctional mineral-carbon and zeolite-carbon sorbents E.B. Drag, M. Kulazynski, J. Kaczmarczyk
499
Waste air cleaning using activated carbon fibre cloths regenerable by direct electric heating E.Schippert, C. M6hner, B. Pannwitt, H. Chmiel
507
Adsorption of inflammatory cytokines and endotoxin by mesoporous polymers and activated carbons M.C. Murphy, S. Patel, G.J. Phillips, J.G. Davies, A.W. Lloyd, V.M. Gun'ko, S.V. Mikhalovsky
515
Separation of adsorption isotherms of N2 in internal and interstitial nanopores of single-walled carbon nanohorn. A comparative study with experiment and simulation T. Ohba, H. Kanoh, K. Murata, M. Yudasaka, S. Iijima, K. Kaneko
521
Visualizing the porous structure of different carbon materials: a scanning tunneling microscopy study J.I. Paredes, A. Mart/nez-Alonso, J.M.D. Tasc6n
529
Textural characterisation of activated carbons obtained from poly(ethylene terephthalate) by carbon dioxide activation J.B. Parra, C.O. Ania, A. Arenillas, J.J. Pis
537
A Monte Carlo study on the structure of carbon dioxide adsorbed in
microporous carbons Th.A. Steriotis, G.K. Papadopoulos, A.K. Stubos, N. Kanellopoulos
545
Preloading of GAC by natural organic matter: effect of surface
chemistry on TCE uptake J.E. Kilduff, R. Srivastava, T. Karanfil
553
Structural studies of saccharose- and anthracene-based carbons by high-energy X-ray scattering A. Szczygielska, A. Burian, S. Duber, J.C. Dore, V. Honkimaki
561
The dynamic adsorption behaviour of volatile organic compounds on activated carbon honeycomb monoliths M. Yates, J. Blanco, M.A. Martin-Luengo
569
Structural properties of Cu-MCM-41 and Cu-AI-MCM-41 (Si/AI=30) catalysts G. Ferraris, G. Moretti, G. Fierro, M. Lo Jacono
577
Comparative study of the textural properties of alumina-pillared saponites synthesised from the intercalation with various aluminium oligomers L.M. Gandfa, M.A. Vicente, A. Gil
585
Chord length distribution and pore size distribution of porous VYCOR glass W. Gille, D. Enke, F. Janowski
593
A structural study of dehydration/rehydration of tobermorite, a model
cement compound A. Gmira, R.J.-M. Pellenq, I. Rannou, L. Duclaux, C. Clinard, T. Cacciaguerra, N. Lequeux, H. Van Damme
601
Support mesoporosity: a tool for better control of catalytic behavior of cobalt supported Fischer Tropsch catalysts A. Griboval-Constant, A.Y. Khodakov, R. Bechara, V.L. Zholobenko
609
Textural characterization of montmoriHonite pillared with aluminum/lanthanum polyoxycations J.A. Morante, C. Pesquera, C. Blanco, F. Gonzdlez
617
xii Influence of pH in mesoporous silica aluminas (MSA) synthesis C. Rizzo, A. Carati, C. Barabino, C. Perego, G. Bellussi
625
The adsorption of l-hexene and 3,3-dimethyl-l-butene on Ru-MCM 41 U. Singh, R.T. Williams, I.D. Salter
633
XPS studies of MCM 41 postmodified by a Schiff base copper complex U. Singh, R.T. Williams, I.D. Salter, K.R. Hallam
639
Influence of synthesis conditions on surface heterogeneity of M41 type materials studied with lattice Monte Carlo F.R. Siperstein, K.E. Gubbins
647
Porosity of chemically modified silica gels by nitrogen adsorption, positron annihilation and small angle X-ray scattering J. Goworek, A. Bor6wka, S. Pikus, J. Wawryszczuk
655
Preparation and characterization of porous sorbents for hot gas desulfurization F. ToroAs-Alonso, I. Orenes-Femfindez, J.M. Palacios-Latasa
663
Synthesis and characterization of MWW zeolites L.M. Vtjurina, S.S. Khvoshchev
671
Structural analysis of oxyanion-cation complexes anchored by organic group in mesoporous silicas H. Yoshitake, T. Yokoi, T. Tatsumi
677
Adsorption equilibrium of polar/non-polar mixtures on MCM-41: experiments and Monte Carlo simulation J.-H. Yun, Y. He, M. Otero, T. Dtiren, N.A. Seaton
685
Nondestructive characterization of porous ceramics by X-ray refraction K.-W. Harbich, F.E. Fensch-Kleemann, B. R6hl-Kuhn, P. Klobes
693
lon-exchange properties of a novel layered titanium(IV) phosphate O.A. Khainakova, A. Espina, C. Trobajo, S.A. Khainakov, J.R. Garcia, A.I. Bortun
701
The anomalous sorptive behaviour of ZSM-5 and Silicalite-l: observation of low pressure hysteresis in nitrogen adsorption G. Kyriakou, C.R. Theocharis
709
Characterization of the textural properties of chemically dealuminated Y zeolites R. L6pez-Fonseca, B. de Rivas, J.I. Guti~rrez-Ortiz, J.R. Gonz~ilez-Velasco
717
xiii Complementarity of microcalorimetry, manometry and gravimetry in the study of gas adsorption by microporous solids up to 50 bar S. Motet, T. Poyet, D. Bigot, B. Polster, S. Crispel, J. Rouquerol, P. Llewellyn
723
The applicability of the Dubinin-Radushkevich equation to the very low pressure region of isotherms of various microporous solids P. Lodewyckx, L. Verhoeven
731
Controlled porosity and surface area titania gels as novel photocatalytic washcoats M. Yates, E. Garcia
737
The effect of geometry and axial orientation of spheroidal particles on the adsorption rate in a granular porous medium F.A. Coutelieris
745
The effect of Peclet on the Sherwood number in high porosity granular media F.A. Coutelieris, M.E. Kainourgiakis, A.K. Stubos
753
The application of J~intti's method for the fast calculation of equilibrium in case of multilayer adsorption J.A. Poulis, C.H. Massen, E. Robens, G. Reichenauer
761
Characterization of controlled pore glasses by small angle X-ray scattering and evaluation of the scattering data by the indirect Fourier transformation method A. Christoforides, N. Kanellopoulos, A. Mitropoulos, K.L. Stefanopoulos, K. Tarchanides
769
Author Index
775
Other volumes in the series
781
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XV
PREFACE
The Sixth International Symposium on the Characterization of Porous Solids (COPS-VI) was held at Alicante, Spain, on May 8-11, 2002 with about 220 participants from 29 countries. There were three Keynote lectures, 29 lectures and 154 poster presentations, with several sessions devoted to general discussion. The intensive three-day programme provided a stimulating forum for the exchange of information on novel research findings, concepts, techniques and materials. This volume contains 99 of the papers which were selected for publication. Taken together, the papers collected in this volume represent an up-to-date and authoritative account of recent developments around the world in the major methods used to characterize porous solids. Most of the leading international specialists in the subject are contributors to the book. I hope that the book will prove to be a useful work of reference for anyone interested in characterizing porous solids. Also included in the bookare papers giving accounts of new developments in the synthesis of porous solids, such as MCM-41 mesoporous materials, pillared clays and porous carbons. As usual at this series of meetings, papers on pore structure determination using gas adsorption feature strongly, together with papers on small angle scattering methods, mercury porosimetry, microcalorimetry, scanning probe microscopies, and image analysis. Papers reporting recent developments in molecular simulations of adsorption of gases and vapours in pores are also included. In addition to theoretical papers there are a number of papers dealing with practical applications of porous solids such as activated carbons, mineral, cements and polymers. I would like to express my special thanks to the members of the Scientific Committee (B. McEnaney, J. Rouquerol, K.K. Unger, and the Honorary member K.S.W. Sing) and to the Local Committee (M. Molina-Sabio, A. Septllveda-Escribano, F.J. Narciso-Romero) for their efforts in composing a scientific programme of great quality and in running a smooth and interesting symposium. The generous financial support of Bancaja, University of Alicante, Ministerio de Ciencia y Tecnologia, and OCIT-Generalitat Valenciana is gratefully acknowledged. It has been decided that COPS-VI will be held at Aix at Provence, France in 2005. Francisco Rodriguez-Reinoso Chairman of the Scientific Committee
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xvii Scientific Committee
F. Rodriguez-Reinoso, Universidad de Alicante, Spain (Chairman) B. McEnaney, University of Bath, Bath, United Kingdom J. Rouquerol, CTM du CNRS, Marseille, France K.K. Unger, Johannes Gutenberg-Universit~it, Mainz, Germany
Local Committee
F. Rodriguez-Reinoso, Universidad de Alicante, Spain M. Molina-Sabio, Universidad de Alicante, Spain A. Sept~lveda-Escribano, Universidad de Alicante, Spain F.J. Narciso-Romero, Universidad de Alicante, Spain
Financial Support The organizers gratefully acknowledge the financial support of the following sponsors: Fundaci6n Bancaja-Universidad de Alicante Ministerio de Ciencia y Tecnologia OCIT. Generalitat Valenciana
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Scanning probe microscopies for the characterization of porous solids: strengths and limitations J.I. Paredes, A. Martinez-Alonso, J.M.D. Tasc6n Instituto Nacional del Carb6n, CSIC, Apartado 73, 33080 Oviedo, Spain
Due to their ultrahigh resolution capabilities, scanning probe microscopies (SPM), and particularly scanning tunneling and atomic force microscopy (STM/AFM) are, a priori, ideally suited techniques for the direct visualization of porosity in solids of interest. In the work presented here, the application of STM/AFM for the mentioned purpose is reviewed and discussed. In general terms, the main factors which limit the ability of SPM to resolve small pores are the tip size (i.e., a blunt tip vs. a sharp one) and the degree of sample corrugation (rough vs. smooth surfaces). The examples presented are divided into three main groups: carbon materials (e.g., activated carbon fibres), inorganic materials (such as silica, alumina or zeolites) and organic/biological materials (polymeric membranes, dentin, nacre). 1. INTRODUCTION The term scanning probe microscopy (SPM) comprises a whole class of techniques that measure surface properties of materials on an extremely local scale. Although at the very beginning only topography could be examined, further advances extended the measurement capabilities to other surface properties, such as friction, adhesion, electric and magnetic forces or specific interactions between molecules. The scanning tunneling microscope (STM) was the first instrument of this family to be conceived and put into practice, some 20 years ago [ 1], for the study of conducting surfaces in vacuum. As a logical sequel to this invention, a few years later the atomic force microscope (AFM) was developed [2], expanding the realm of SPM application to nonconducting materials. Since then, the number of SPM-based techniques has been growing at a rapid and steady pace and currently amounts to a few tens of variants [3,4]. Nevertheless, the bulk of SPM work performed to date has been (and still is) carried out using either STM or AFM in any of its operation modes. The unique resolution capabilities of SPM are based on the combined use of two elements: an ultrasharp tip, which probes the sample surface and enables their mutual interaction (and therefore the measured property) to be highly localized, and a piezoceramic scanner, which controls the tip-sample relative position in the three spatial directions with subangstrom precision. This approach has allowed the imaging of surfaces with atomic resolution in STM/AFM, as many examples have demonstrated [5]. To attain such resolution level, the sample under study is mainly required to be atomically fiat and highly crystalline.
As a result of the development of probe microscopy, many areas of research have experienced a remarkable progress and the variety of materials that have been visualized and characterized by STM, AFM and/or related techniques is correspondingly broad. These include metals [6], semiconductors [7] and superconductors [8], layered inorganic materials [9] and self-assembled monolayers [10] or polymers [11] and macromolecules (including biomacromolecules) [ 12,13]. In this context, the SPM techniques (and especially STM and AFM) appear, a priori, ideally suited for the direct visualization of the porous structure of materials at scales which are not so readily accessible by other means (e.g., scanning and transmission electron microscopies). However, the performance of such a task is confronted with two major limitations. The first one arises from the fact that detection with SPM is exclusively restricted to the outermost surface of the sample. Accordingly, this implies that only the most external porosity of the material can be probed, whereas no information on the bulk (inner) porosity, which might not be identical to the former, is revealed. The second drawback is related to the finite dimensions of the probing tip, which limits the size of the voids (pores) physically accessible (and thus detectable) by the tip on the sample surface. Obviously, pores significantly smaller than the tip diameter will pass unnoticed to the instrument when the surface is scanned. As a specific example, the tips normally employed in AFM are not sharp enough to provide access to the whole mesopore range (between 2 and 50 nm). In spite of the mentioned disadvantages, useful information has been obtained from SPM imaging of a number of porous materials. To illustrate such point, the present review examines the research that has addressed the visualization of the porous structure of solids by SPM. A wide variety of materials is covered, such as porous silicon, activated carbon materials, aluminas, synthetic membranes or biological materials. 2. OVERVIEW OF THE SCANNING PROBE MICROSCOPY TECHNIQUE All SPMs are based on the same principle of operation: a sharp tip and the sample surface of interest are brought to a close proximity (several A or nm), or to mechanical contact. As a result, a physical interaction is established between tip and sample, which, in turn, gives rise to a measurable signal. The signal usually has a sharp dependence on the tip-sample distance, so it can be employed to accurately control such separation and track the surface topography of the sample under study. This is accomplished by a feedback system, which, as the tip scans the sample, adjusts the tip-sample distance in order to keep the measured signal constant. The relative motion between tip and sample in the three spatial directions is realized by a piezoceramic scanner, whose position can be controlled with subangstrom precision, enabling atomic-scale imaging. The main differences between the different SPM techniques lie in the type of interaction that is used to control the tip-sample distance. Although the SPM offspring are remarkably numerous [3,4], here we will focus only on STM and AFM, as they remain the most widely used and the best suited for high resolution imaging of surface structures.
2.1. Scanning tunneling microscopy The STM was the first scanning probe microscope to be developed [ 1]. Although the original aim was to image the surface topography of metals on the atomic scale and in
ultrahigh vacuum, it was later applied for the study of other (conducting) materials [5] and in additional environments (air and liquids). In STM, a bias voltage is applied between the sample and a metallic tip. On approaching both electrodes to just a few A, a tunneling current is established. This current decreases exponentially with tip-sample separation and is used as the signal to be controlled (i.e., to be kept constant) by the feedback system. Provided that the surface to be studied is flat enough, STM can render images with atomic resolution [5,14]. However, in general such images contain both topographic and electronic information and their interpretation is frequently a difficult task. By contrast, on large distance scales topographic effects usually dominate over electronic ones and the features observed in the images are more straightforward to elucidate [15]. 2.2. Atomic force microscopy The AFM was developed to overcome one of the main limitations of the STM, namely, the necessity of electrically conducting samples [2], and is based on the interaction forces (short- or long-ranged, attractive or repulsive) that exist universally between atoms and molecules. To measure the tip-sample interaction force, the former is attached to one end of a flexible lever, referred to as cantilever [5]. The forces exerted on the tip (~10 "9 N [16]) induce a measurable deflection of the cantilever, which is, essentially, the signal to be controlled by the feedback system. Basically, an atomic force microscope can be operated in three different modes: contact, intermittent contact (also referred to as tapping) and noncontact mode [5]. In contact mode, the tip is brought into physical contact with the sample and the repulsive force that originates thereof is used to track the sample topography. In the intermittent contact mode, the cantilever is vibrated at its resonance frequency and the tip allowed to intermittently contact the sample surface. Such interaction reduces the cantilever oscillation amplitude (compared to the noninteracting situation), which is then used as feedback signal [ 17]. In the noncontact mode, the instrument is operated in the net attractive force regime and the frequency shift arising from the interaction is employed as feedback [ 18]. 2.3. Resolution in STM and AFM Provided that the sample to be imaged is reasonably rigid, resolution in STM/AFM at the nano- and micrometer scale is fundamentally limited by the shape of the probing tip [19,20]. Thus, a certain degree of distortion will always be present on the images [21,22], so that prominent features on the surface will appear wider than they really are (the blunter the tip, the wider the apparent size). By contrast, depressions in the topography, such as pores, will appear narrower (again, the blunter the tip, the narrower the pore will appear) or will not be detected at all. Such effect has important implications for imaging the porous structure of surfaces by these techniques. Considering the tip radii of curvature of commercial AFM
cantilevers (a few tens of nm), the micropore and small mesopore (< 10 nm) range will not be accessible by AFM, unless the surface is molecularly fiat and highly ordered. This is the most serious constraint for the imaging of pores on corrugated and nonconducting samples, where only AFM can be employed. In the case of STM, it is possible to prepare probes with very sharp protrusions or minitips [23], one of which (the closest to the surface) will act as the tunneling tip, enabling the detection of smaller pores (even micropores).
3. APPLICATION OF SCANNING PROBE CHARACTERIZATION OF POROSITY IN SOLIDS
MICROSCOPY
TO
THE
3.1. Carbon materials
Shortly after its invention, the STM was employed for the study of highly-ordered carbons, particularly highly oriented pyrolytic graphite (HOPG) [24]. Due to the chemical inertness and atomic flatness of this material, atomic resolution images could be easily obtained in air, which made HOPG one of the most thoroughly investigated materials by STM. Such intensive research on graphite provided a solid background of knowledge which enabled subsequently the STM study of other (less ordered) carbon materials, such as carbon fibers [25]. In this context, the porous structure of carbonaceous materials has been studied by STM (and, to a much lesser extent, by AFM) since the early 90"s, when the first reports on the topic began to emerge [26,27]. Since porous carbons possess a highly disordered structure, their surface tends to be very rough on the atomic scale and, insome cases, also on the nanometer scale. As a result, the resolution attained for such systems will be in general lower than that achieved on atomically fiat, highly crystalline surfaces (e.g., HOPG) [15], which, in turn, may complicate the visualization of very small pores (i.e., micropores). For example, on studying by STM active carbons prepared from poly(ethylene terephthalate) and cellulose, B6ta et al. [28] failed to attain a resolution comparable to the size of the regions governing the microporous behavior of the specimens. Therefore, their micropore structure could not be visualized. Such negative result was attributed by the authors to the amorphous and very rough surface structure of these samples. A similar example can be found when trying to image the ultramicroporous structure (pore size < 0.7 nm) of nonactivated carbon materials prepared by pyrolysis of a polymeric precursor [ 15]. In this case, it was concluded that such tiny features cannot be probed by the STM tip with sufficient accuracy so as to completely rule out that they are artifactual. Thus, both examples underline the necessity of probing considerably flat surfaces if micropores are to be detected with STM. By choosing relatively fiat areas, Paredes et al. were able to image the mesoporous and microporous structure of activated carbon fibers (AFCs) prepared from Kevlar pulp [ 15]. A highly spongy mesoporous texture (pore sizes between 4 and 16 nm) was noticed on specific areas of the sample. The presence of such type of porosity was also deduced from the observation of hysteresis in the N2 adsorption (77 K) isotherms of the sample and was attributed as developing from low crystallinity regions of the polymer precursor. Concerning smaller pores, a large number of slit-shaped or elongated micropores (-1 nm wide) forming irregular and interconnected networks were observed, as shown in Fig. 1, consistent with the data obtained from the N2 adsorption measurements. The porosity of ACFs prepared from phenolic precursors was also visualized with STM by Economy et al. [29] and Daley et al. [30]. Again, elongated micropores (-1-2 nm wide) were detected, but in this case not only on the surface but also at cross-sections of the fibers, i.e, the inner porosity was also probed. Further, the whole range of mesopore sizes was ascertained in the different samples studied. A quantitative evaluation of the shape and size of pores in activated carbon spheres prepared from phenolic resin was attempted by Vignal et al. [31]. To this end, the authors proposed a numerical method based on contour maps from the STM images. Although
reasonable agreement was found between the STM results and the adsorption capacity (N2 and CHC13) of the samples, some inconsistencies arose and were ascribed to the chemical nature of the pore walls, which cannot be determined by STM. We will mention now two very recently published examples of STM on activated carbons. In one of them, Shi and Shiu [32] studied glassy carbon electrodes before and after electrochemical activation. They reported the development of pores following activation and a change in the electrochemical behavior of the sample, which was related to these structural changes. In the second example, Pfeifer et al. [33] examined a series of activated carbons which displayed an extended fractal network of channels. As expected from such structure, only sparse entrances (-1.3 nm wide) were observed by STM on the surface of the samples. Finally, we note that, although most of the SPM work on active carbons has been performed with STM, there exist a few examples in which AFM was used as well [34,35]. However, in this case the relatively large radius of curvature of the tips employed compromised the resolution and only macropores/large mesopores could be detected (Fig. 2).
Figure 1. STM image of the microporous structure of activated carbon fiber prepared from Kevlar pulp. Reprinted with permission from ref. 15. Copyright 2001 American Chemical Society.
Figure 2. AFM image of activated carbon fiber prepared from rayon. Macropores and large mesopores can be observed. Reprinted with permission from ref. 35. Copyright 2000 Elsevier Science.
3.2. Inorganic materials Due to their high electrical conductivity, metals constitute the most typical class of materials that can be studied by STM. In the context of porosity, silicon has been, by far, the most frequently studied metal using STM. Parkhutik et al. [36] made a rather pioneering application of STM to study the effect of silicon electrochemical anodization regime on the resulting porosity. Closely packed cylindrical mesopores were shown to form at low current densities, whereas branched, fibrous-like mesopores were obtained at high current densities. A simulation model was used to justify the formation of these two different types of pores. Enzel et al. [37] applied AFM to nanoporous semiconductors based on tin (IV) sulfide and tin (IV) selenide. The surface structure of these materials displayed regular pattems of micropores, in agreement with expectations from the crystallographic projections of their bulk structures. AFM studies of silica have dealt with, e.g., amorphous silica gels [38] or
monolithic mesoporous silica [39]. As an illustration, Fig. 3 shows AFM images of monolithic silica where mesopores ca. 4.5 nm in diameter can be easily identified, the pore size being in good agreement with data deduced from N2 adsorption. Xu et al. [40] have very recently prepared alumina with U-shaped pores of a uniform diameter (40-50 nm) whose bottoms were opened in a controlled way by Ar§ milling. Fig. 4 shows an AFM image of the resulting pores after short-term ion milling, which produced pores 8-14 nm wide. By contrast, long-term ion milling led to a hole diameter equal to nanopore diameter, indicating that the U-shaped bottom cap was etched away completely.
Figure 3. Three-dimensional AFM image of the surface of monolithic mesoporous silica. Taken from ref. 39 - Reproduced by permission of The Royal Society of Chemistry.
Figure 4. AFM image of Ar+-milled (9 min) hexagonally ordered nanoporous alumina. Reprinted with permission from ref. 40. Copyright 2002 American Chemical Society.
Many applications of AFM to pillared clays or zeolites have not specifically addressed the porosity characteristics, but rather the occurrence of adsorbed surface A1 species in, e.g., pillared montmorillonite [41 ], or the crystal growth processes, adsorption on porous surfaces and the surface structure of natural zeolites [42]. Sugiyama et al. [43] succeeded to reveal the ordered pore structure of the (001) surface of mordenite after removal of impurities that clogged the pores. The authors indicated that resolution in AFM imaging of zeolites is significantly affected by the magnitude of the periodical corrugation on the crystal surface, so that if the surface contains deep pores only the pore structure, but not the atomic structure, can be resolved. AFM has also been applied to characterizing the porosity of various oxide-based materials such as vanadia-silica glasses, where AFM provided an explanation for the increased microporosity when vanadium was incorporated to the xerogel [44]: while pure silica exhibited a mesopore structure consisting of voids between pillowlike structures, filling of voids between these globular structures took place when vanadium was present, leading to the creation of micropores. Inorganic membranes have also been studied. Thus, AFM has been used to probe the surface morphology and pore structure of micro- and ultrafiltration membranes, both in contact and noncontact mode, the latter being very suitable for soft and delicate materials. One of the first reports concerned alumina microfiltration membranes (Anapore) [45] and the authors performed statistical analysis to obtain the pore size distribution from the AFM
images. Alumina microfiltration membranes have been studied by Bailey et al. [46] and Jones et al. [47]. Differences in pore morphology were observed depending on the precursor material used to prepare the membrane. Oyama et al. [48] have inspected glass membranes employing AFM (Fig. 5), as well as silica membranes produced by chemical vapor deposition of a silica precursor onto the former substrate. Morphological differences between both samples were observed, which helped to explain their different permeability properties. Also, nanoporous silica membranes prepared by sol-gel methods have been characterized by Fujii et al. [49]. Void nanospaces between particles in such membranes, consistent with nitrogen adsorption measurements, were evidenced in their AFM images. Finally, macroporous silicate films fabricated by the sol-gel method have very recently been examined by Kramov et al. using AFM [50]. Uniform pores of several tens of nm were visualized.
3.3. Organic and biological materials Much work on AFM of polymer membranes has been carried out by Bowen et al. [45,51,52]. An example is provided in Fig. 6. Membranes'made of regenerated cellulose, polysulfone or poly(co-(acrylonitrile-vinyl chloride)), among others, were investigated by these authors. It was concluded that pore dimensions could be best determined when the membrane roughness was low, as tip-sample convolution effects were reduced. Polyethersulfone ultrafiltration membranes were studied by Ochoa et al. [53]. Again in this case, the resolution of the pore structure was deeply conditioned by the sample surface roughness. Stamatialis et al. [54,55] applied AFM to the characterization of the active layer of a series of cellulose ester membranes. The permeation performance of such membranes was improved when their surfaces displayed lower roughness as measured by AFM. Other applications of AFM to polymeric membranes addressed changes in morphology following plasma etching to improve their hydrophilicity [56] or the quantification of interactions between the membranes and other materials of interest (e.g., cellulose) [57]. A new class of artificial membranes, based on lipid bilayers supported on porous alumina, were recently studied by Hennesthal and Steinem using AFM [58]. In this case, tipsample convolution effects affecting the detection of pores (see Section 2.3) were clearly demonstrated by the authors on comparing the average pore size of the alumina support as obtained by SEM (60 nm) and by AFM (50 nm). STM has been successfully applied to image nanopores produced in a self-assembled monolayer (SAM) of alkanethiol deposited on a silver electrode [59]. Pore diameters of 5-7 nm and submicrometric pore depth (corresponding to the tickness of the SAM) could be measured, confirming the good resolution of STM. Marshall et al. [60] studied the effect of treatment with NaC10 (a typical deproteinizing agent used in endodontic treatment) on dentin, and evidenced by contact mode AFM a certain development of channels and canaliculi (i.e., macropores) following removal of collagen fibrils when pre-etching with citric acid had been performed. Jandt [61 ] has recently reviewed in depth the applications of AFM to biomaterials surfaces and interfaces; interestingly, pore creation by nanoindentation, using AFM tips made from diamond, has become a useful tool in characterizing the mechanical properties of tooth tissues [61,62]. Due probably to the characteristics of the materials themselves, most results shown for this type
of materials were obtained at low resolution; the main advantage of AFM in this case would seem to be relevance to living conditions and lack of sample damage thanks to the simplicity of preparation for microscopic examination. This also seems to apply in a study of abalone nacre growth, where AFM coupled with scanning ion conductance microscopy showed the existence of pores, 5-50 nm in diameter in the interlamellar organic sheets, coming in support of a new model of nacre growth by mineral bridge formation through these sheets [63].
Figure 5. AFM image of Vycor glass membrane showing pore mouths about 4 nm wide. Reprinted with permission from ref. 48. Copyright 2001 Kluwer Academic Publishers.
Figure 6. Three-dimensional AFM image of a Desal GN polymeric membrane. Reprinted with permission from ref. 52. Copyright 2000 John Wiley & Sons.
4. CONCLUSIONS This brief review has shown the possibilities of the scanning probe microscopy techniques, particularly scanning tunneling and atomic force microscopy (STM/AFM), for the characterization of porous materials and the work that has been undertaken to date on the topic. Regarding the resolution of pore structures, and due to the higher resolution capabilities of STM compared to those of AFM, conducting materials (the only class of samples that STM can be applied to) have benefited from these techniques to a far greater extent than insulating materials. Particularly, carbon materials (e.g., activated carbon fibers) have been studied down to the micropore level using STM, thus providing direct visual information about their porosity at scales not readily accessible by means of other techniques. By contrast, in the case of AFM and, therefore, of nonconducting materials, the visualization of pore structures has been much less successful (restricted to the macropore and large mesopore range). The origin of this limitation can mainly be traced to the relatively large curvature radius of the tips available in AFM. It is believed that improvements in this respect (for example, by using the recently developed carbon nanotube AFM tips) will alleviate the problem to some extent. ACKNOWLEDGEMENT The authors are grateful to DGICYT and CICYT for financial support (projects PB98-0492 and 1FD 1997-1915, respectively).
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Role of Gas Adsorption in Nanopore Characterization K . K a n e k o * ' t, T. O h b a t ,y. Hattori*, M. S u n a g a t ,H. T a n a k a * , and H. K a n o h , *) Department of Chemistry, Faculty of Science, *) Center for Frontier Electronics and Photonics, Chiba University, 1-33 Yayoi, Inage, Chiba 263--8522, Japan Characterization of nanopores must be carried out with integrated information from different levels and angles. It is shown that gas adsorption can provide a key information in the integrated characterization of nanopores. In particular, further understanding of pore filling is necessary; an evidence on quantum effect in pore filling and a new method of adsorption isotherm from P/Po = 10 -9 are introduced with the relevance to the nanopore characterization. 1.
INTRODUCTION
Recently a variety of new nanoporous materials have been developed, stimulating fundamental sciences and technologies. These nanoporous materials have an intensive feature that their nanoporosity can be controlled. Templating method using the molecular assemblies of surfactant molecules provides a variety of geometrical structures of pores, as shown in MCM-41 series.Ill The organic--inorganic hybrid nanoporous solids have a great possibility in the structure designing. [2,3] Also assembly of single wall carbon nanotube (SWNT)s which are composed of a single graphene sheet have gathered a great attraction from H2 storage and electronic devices; several modification forms have been prepared. [4,5] Hence, a better characterization of nanoporous solids has been requested more in wide fields such as pharmacy, organic chemistry, and medicine in addition to adsorption, catalysis, and separation sciences. Although the routine analytical method of porosity such as BET analysis is established for mesoporous and macroporous solids, nanoporosity evaluation method is not necessarily established. As the term of nanoporosity is not recommended by IUPAC, we need to define the nanoporosity here. The classical capillary condensation theory using the Kelvin relation has a catastrophe for the pores whose pore width w is less than about 4 nm in the N2 adsorption isotherm at 77 K; N2 adsorption isotherms even on cylinderical mesopores of w < about 4 nm and open both ends at 77 K disappear, which cannot be described by the classical theory. [6,7] Hence, it is quite convenient to use the nanopores in this articles for the pores whose pore width is less than about 5 nm. Generally speaking new nanoporous solids have well-defined pore structures.
However, a better structure designing requests a finer characterization of nanopores. We need to know structural features of nanopores as accurate as possible in order to develop the best nanostructured materials for the specific function. Nevertheless, nanopores are hidden in the bulk of solids. Consequently, established surface science tools cannot be directly applied to the nanopore characterization, leading to necessity of an inherent characterization method for nanopores on the basis of gas adsorption. This paper summarizes main characterization methods, which can be applied to nanopore systems, and essential roles of gas adsorption will be described. 2.
R O L E OF GAS ADSORPTION
Basically gas adsorption provides average information on the nanoporosity on the basis of thermodynamics. The typical nanoporosity description can be done by pore volume, surface area, and nanopore width distribution. If we use a specific molecule other than N2, the nanoporosity inherent to the probe molecule can be obtained. This is very important even if the crystallographic data can be applied to derive the nanoporosity. The X-ray crystallographic data cannot provide the effective nanoporosity for specific molecules or ions. This is because imperfect pore wall structures such as roughness in the sub--nanometer scale cannot be neglected in the nano-range performance of nanoporous solids. Then the effective nanoporosity evaluation should be recommended for newly developed nanoporous solids. The temperature dependence of the adsorption isotherm gives the isosteric heat of adsorption, which is a scale of the interaction. The hydrophobic or hydrophilic nature of the nanopore can be quantitatively evaluated by gas adsorption using water and organic vapor. If we use the supercritical gases for nanopore characterization, subnanometer structures (ultramicropores: w < 0.7 nm) can be elucidated. However, the difference in the molecular size of probes is not necessarily the pore width difference. At the same time, supercritical gas adsorption itself is not sufficiently understood. Better understanding of the supercritical gas adsorption is desired for application to nanopore characterization. Murata and Kaneko proposed the determination method of the absolute adsorption isotherm in addition to the surface excess adsorption isotherm; their analysis provides the information on the interaction energy and adsorbed state transformation, giving further understanding of supercritical gas adsorption. [8,9] Adsorption using different probe molecules is available for determination of the pore entrance structure. N2 molecules adsorbed near the pore entrance at 77 K often block further adsorption, indicating the presence of ultramicropores and pore-neck structures. The preadsorption technique is also effective for elucidation of the pore entrance structure. Fractal analysis using adsorption is helpful to understand the fine structure of nanopore walls. [10,11] However, the probe molecules must be carefully chosen and the monolayer capacity must be evaluated precisely, because the BET monolayer should not be used. Thus gas adsorption provides essentially important information on the nanopores from the sub--nanometer level, which is often inherent to a specific molecule. Also the density measurement is necessary for the exact porosity evaluation. The accurate particle and true densities give close porosity. Even He molecules are adsorbed in small nanopores at room temperature and thereby the He replacement method is not fit for determination of the particle density. Authors developed the high pressure He buoyancy method until 10
MPa for determining the particle density.[12] The accurate particle density is indispensable to evaluate the high pressure adsorption amount for supercritical gas, in particular, in hydrogen storage problem. Thus, the accurate determination of density is still not easy. 3.
I N T E G R A T E D C H A R A C T E R I Z A T I O N OF N A N O P O R E S
We need to use electromagnetic waves which can penetrate solids to elucidate the nanopores. At the same time, characterization of nanoporous solids must be examined from different angles and levels using optimum methods. The necessary levels are electronic, atomic, and morphological structures.
3.1 Electronic Level Characterization of Nanoporous Systems The electronic structures of porous solids have been examined by X-ray photoelectron spectroscopy (XPS). However, the penetration depth of electrons is 1 nm at best and XPS cannot examine electronic structures of inner pore-walls. XPS has been often used for the determination of surface chemical structures such as surface functional groups in activated carbon. Ar etching leads to the depth profile of electronic structures. This depth profile is often effective to evidence the presence of nanoporosity. Most of porous solids are diamagnetic and their bulk is transparent from the magnetic field. Activated carbon of relatively high purity is diamagnetic at room temperature and shows paramagnetism below 20 K. Then the magnetic susceptibility measurement over the wide temperature from 2 K to an ambient temperature is effective to elucidate spin structural changes of activated carbon. [13] If we use the magnetic probe such as an oxygen molecule which shows paramagnetism, nanopore structures can be estimated. [14] This is because the magnetism of oxygen confined in nanopores is sensitive to the nanopore structure. The electronic conductivity measurement can provide the information on the electronic structure of nanoporous solids. The electronic carriers transport through highly conductive paths in nanoporous solids and thereby the electronic conductivity measurement gives information on the intergranular contact. 3.2 Atomic-Level Characterization of Nanoporous Systems X-ray and neutron are quite useful and thereby their diffraction and scattering techniques have been used to analyze the pore structures from the atomic level. The diffraction techniques can determine the regular structures of long--range order. Regular mesoporous silica is a good example that the diffraction is effective for determination of the nanopore structures regardless of noncrystalline pore-wall structures. The radial distribution function analysis of the diffraction patterns can provide important structure information even for less-crystalline solids. The radial distribution function of activated carbon depends on the precursor and activation conditions. At the same time, in-situ diffraction measurements are very useful to determine the molecular state of adsorbate molecules in nanopores. Iiyama et al showed that water adsorbed in micropores of activated carbon fiber (ACF) at 303 K has a highly ordered structure and the structure does not vary even at 150 K. [15] Ohkubo et al showed that alcohol molecules form an oriented structure along the pore--walls of ACF at 303 K. [16] Thus, the diffraction technique is useful even for less-crystalline solids as well as well-crystalline solids. If
14 neutron facility is available, the comparative application of X-ray and neutron is quite powerful. The small angle scattering of X-ray 2.5 (SAXS) or neutrons can provide inhomogeneous structures of 0.3 nm to 100 nm.[17] In case of nanoporous solids, pores 2 @ are highly populated and thereby the Guinier analysis should not be applied routinely. The Debye analysis is more useful to determine both of the correlation length which is associated with the pore or pore-wall unit ~3 structures and the surface area of all interfaces in solids. The difference of the surface areas O. from SAXS and N2 adsorption at 77 K leads to the surface area of inaccessible pores. If the 0 I I , I 0 0.2 0.4 0.6 0.8 1 SAXS, N2 adsorption, particle density, and Fra ctiona l Filling true density are examined, the average size Figure 1. The zero angle scattering and surface area of inaccessible pores can be intensity ratio vs. fractional filling for derived. [18] In situ SAXS is also useful to water adsorption on pitch-based activated understand the adsorbed state of molecules in carbon fiber (average pore width = 12 nm) nanopores. Iiyama et al measured the water at different temperatures. Solid and adsorption and SAXS at the same time using broken lines denote adsorption and ACE They compared with the scattering desorption branches, respectively. O :303 profiles on the courses of adsorption and desorption at the same adsorption amount. K, /k: 313 K, and D: 323 K. [19] The scattering profiles were analyzed by the following Ornstein--Zernike (OZ) plot [20], which is composed of the scattering intensity I(s) at the scattering parameter s (s = 2~sine/)~), the scattering intensity at the zero angle I(0), and the OZ correlation length ,~. Here 2.0 and )~ are the scattering angle and X-ray wave length. I(0) is directly associated with the density fluctuation of the system, which provides the distribution state of molecules adsorbed in nanopores. The relationship between I(0) and the fractional filling on the courses of adsorption and desorption indicates that molecular cluster on adsorption is larger than that on desorption. Ohba et al measured the I(0) and fractional filling at 303 K, 313K, and 323 K, as shown in Fig. 1. The I(0) vs. fractional filling on the course of adsorption at 313 K is close to that at 303 K, while the relation on the course of desorption at 313 K resembles to that at 323 K. The relation on adsorption at 323 K is similar to that on desorption at 303 K. These results suggest that the molecular cluster sizes on adsorption and desorption are different from each other and the stability of the large clusters changes above 313 K on adsorption, giving rise to smaller clusters. Thus, diffraction and scattering techniques give the complementary structural information from atomic and molecular levels. Another new technique from the atomic structure is X-ray absorption spectroscopy (XAFS) which needs high quality X--ray from synchrotron. XAFS can focus a specific atom to give the local structure of the coordination number and the coordination distance. XAFS consists of XANES and EXAFS. XAFS has been actively applied to characterize the surfaces of catalysts. However, there are not so many studies on application of XAFS
to characterization of nanopores. Recently nanoporous Ni metal was characterized with XAFS. [22] Nanoporous Ni metal was produced by heating Ni(OH)2 in the texture of polyvinylalcohol (PVA) in air at different temperatures. Figure 2 shows Ni K-edge EXAFS spectra of Ni(OH)z-PVA heated at 673 273 K ~] K to 923 K. The EXAFS feature varies with the heating temperature. The products heated at 773 K and 673K j ' 923 K were close to Ni metal. The Ni-Ni distance of product at 923 K is 0.250 nm which agrees with that of metallic Ni (0.249 nm). Although the basic structure of this sample cannot be determined by X--ray diffraction, EXAFS analysis is effective for this 82'50' 83'00 83'5o 84'00 ' 8& Photon energy / eV sample. XPS of this sample coincides with that of Ni foil. N2 adsorption isotherm has a clear hysteresis Figure 2. Ni K-edge EXAFS giving the average pore width of 4rim and surface area spectra of PVA-Ni(OH)2 o f 120 m2/g. heated at high temperatures. NMR should be powerful to characterize nanopore structures. Fraissard et al proposed 129Xe NMR technique. [23] Although the chemical shift of 129Xe is associated with the pore width, its application to unknown porous system is not established yet. '
i
,
i
,
i
,
i
3.3 Morphological-Level Characterization of Nanoporous Systems There are two kinds of pores according to the origin. Pores come from intraparticle and interparticle structures. In case of nanoparticles, the interparticle pores play an essential role. The predominant nanopores in the above nanoporous Ni stem from the intergranular gaps. The morphological characterization using microscopes with different ranges of resolution is especially important in interparticle pore systems. Basically statistical analyses of images measured by the microscope give an average information on the nanoporosity and thereby the comparison with information from gas adsorption provides quantitatively the contribution by the intraparticle porosity. High resolution transmission electron microscopy (HR-TEM) has contributed to determine the subnano-structures of nanoporous materials. If nanoporous solids have a periodical structure, HR-TEM is more efficient for determination of three dimensional pore structures than X-ray diffraction. [24] The effectiveness of HR-TEM has been shown in studies on SWNT. Iijima et al showed the presence of nano-scale windows on the pore-wall of the single graphene sheet of single wall carbon nanohorn (SWNH) particle and the essential role of nano-scale windows in gas adsorption properties of SWNH is evidenced. [25,26] Another powerful technique is scanning probe microscopy (SPM) such as STM and AFM. This subject was well reviewed in the keynote lecture given by Tasc6n et al. [27]
3.4 Inter-Level Characterization of Nanoporous Systems The above information from electronic, atomic, and morphological levels must be integrated together with gas adsorption data to understand structures and functions of nanoporous systems. We must stress the importance of calorimetry and molecular
16
simulation. Calorimetry provides the interaction energy, being indispensable to understand gas adsorption in nanopores. Recent progresses in heat of immersion are quite powerful to analyze nanopore structures. [28,29] The fundamental information from calorimetry is necessary for molecular simulation and DFT studies which accelerate to elucidate adsorption mechanism of gas in nanopores. The detailed paper on these techniques was given by Neimark in COPS IV as the Keynote lecture.J30] Molecular simulation is available for development of the optimum analysis of gas adsorption data. Setoyama et al showed the effectiveness and limitation of as-plot for slit-shaped nanopores with GCMC simulation. [31] Recently Ohba and Kaneko pointed the necessity of inherent as--analysis for cylindrical nanopores such as SWNT with GCMC simulation. [32] The reverse Monte Carlo method is now. giving an insight to nanopore structures using the information from different levels together with gas adsorption data. [33] 4
New Directions in Nanopore Characterization with Gas Adsorption
It is stressed that gas adsorption should play a key role in nanopore characterization. However, understanding of gas adsorption mechanism and gas adsorption technique are not enough to elucidate nanopore structures exactly. The main mechanism of gas adsorption in nanopores is pore filling. The basic feature of pore filling for vapor can be understood using molecular simulation and DFT except for water adsorption, as shown above. However, an efficient gas adsorption technique for subnano--range pores must be developed. Kaneko et al attempted to apply He adsorption at 4.2 K for evaluation of ultramicropores in activated carbon. [34] Although He adsorption at 4.2 K is efficient for detection of presence of ultramicropores, quantitative evaluation of ultramicroporosity is still difficult; a kind of quantum effect is speculated. Johnson et al pointed that quantum effect is predominant in hydrogen adsorption in SWNT from their theoretical studies. [35] We need a small probe molecule for ultramicropore characterization. At the same time, contribution by quantum effect must be understood in order to establish nanopore characterization using the small probe molecule. 80 < Tanaka et al measured Ne adsorption on well-crystalline A1PO4-5 at 27 K to 33 K near -~ boiling temperature of Ne and calculated the ~ DFT isotherm, suggesting the presence of Z /,, quantum effect.[36] Thus, even Ne molecules 40 @ show quantum effect. Small probe molecules o~ such as He and H2 should show a pronounced quantum effect. Recently, N2 adsorption isotherm of 0 ' nanoporous solids is measured at 77 K over the -8 -6 -4 -2 0 wide relative pressure range of 10-6 to 1. Such (LogP/Po) a wide pressure range adsorption isotherm leads Figure 3. N2 Adsorption isotherms of to high resolution (xs-plot which provides carbon black at 77 K with SWPA (C)) accurate surface area and pore volume even for and WPA (A) methods
17 nanopores after subtraction of the effect of enhanced adsorption by nanopores. Nevertheless, we have a question whether the low relative pressure range until 10 -6 is enough to evaluate nanoporosity. Sunaga et al constructed a new gravimetric adsorption apparatus covering the wider relative pressure range from 10-9 to 1. This new method is named super-wide pressure range adsorption (SWPA) technique. [37] The background pressure is less than 10-Tpa. The N2 adsorption isotherms of nonporous carbon black samples were measured with the SWPA and WPA methods. Both of WPA and SWPA isotherms are representatives of type ]I of IUPAC classification in the ordinary expression of the relative pressure. The isotherm feature is indicative of nonporosity. However, the SWPA isotherm shifts upward. The BET plot of the WPA isotherms gives a complete straight line, which gives the surface area of 56 m2g-1, being close to that evaluated by TEM observation. Accordingly, carbon black sample may be presumed to be a nonporous solid from the conventional WPA analysis. Why does the SWPA isotherm shift upward ? The detailed information is shown in Figure 3, in which the abscissa is expressed by the logarithm of the relative pressure. The SWPA isotherm has a clear step at P/P0 = 2>( 10 -8 , accompanying with a wide plateau region until P/P0=10 "4. This low pressure uptake must be caused by the presence of open ultramicroporosity which cannot be evaluated by the WPA method. The pore width is estimated to be 0.58 nm with the aid of GCMC simulation. If we subtract the additional adsorption detected by the SWPA method, the resultant isotherm coincides with the observed isotherm by the WPA method. Consequently, even the WPA method using N2 cannot evaluate precisely the ultramicropores due to a predominant entrance blocking effect. Active studies on nanoporous materials request further development of fine characterization with gas adsorption. New methods should be challenged more.
Acknowledgement These works were funded by a Grant-in-Aid for Scientific Research on Priority Areas (No.288) and General B from the Japanese Government.
REFRENCES M. Kruk, M. Jaroniec, and A. Sayari, J. Phys. Chem. B, 101 (1997) 583. M. Kondo, T. Okubo, A.Asami, S. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka, and K. Seki, Angew. Chem. Int. Ed., 38 (1999) 140. 3. D. Li and K. Kaneko, J. Phys. Chem. B, 104 (2000) 8940. 4. S. Iijima, Nature, 354 (1991) 56. 5. K. Tanaka, T. Yamabe, K. Fukui Eds., The Science and Technology of Carbon Nanobubes, Elsevier, Amsterdam (1999). 6. P.I. Ravikovitch, S. C. 6 Domhnaill, A. V. Neimark, E Schfith, and K. K. Unger, Langmuir, 11 (1995)4765. 7. A.V. Neimark, P. I. Ravikovitch, and A. Vishnyakov, Phys. Rev. E, 62 (2000) R1493. 8. K. Murata, M. EI-Merraoui, and K. Kaneko, J. Chem. Phys., 114 (2001) 4196. 9. K. Murata and K. Kaneko, J. Phys. Chem. B, 105 (2001) 8498. 10. P. Pfeifer and D. Avnir, J. Chem. Phys., 79 (1983) 3558. 11. M. Sato, T. Sukegawa, T. Suzuki, K. Kaneko, J. Phys. Chem., 101 (1997) 1845. 1. 2.
18 12. K. Murata, K. Kaneko, E Kokai, K. Takahashi, M. Yudasaka, and S. Iijima, Chem. Phys. Lett., 331 (2000) 14. 13. K. Kaneko, C. Ishii, H. Kanoh, Y. Hanzawa, N. Setoyama, and T. Suzuki, Adv. Colloid Interface Sci., 76 / 77 (1998) 295. 14. A. Tohdoh and K. Kaneko, Chem. Phys. Lett., 340 (2001) 33. 15. T. Iiyama, K. Nishikawa, T. Otowa, and K. Kaneko, J. Phys. Chem., 99 (1995) 10075. 16. T. Ohkubo, T. Iiyama, K. Nishikawa, T. Suzuki, and K. Kaneko, J. Phys. Chem. B, 103 (1999) 1859. 17. M. C. Fairbanks, A. N. North, and R. J. Newpore eds., Newtron and X-ray Scattering: Complementary and Techniques, Institute of Physics, Bristol (1990). 18. M. Ruike, T. Kasu, N. Setoyama, T. Suzuki, and K. Kaneko, J. Phys. Chem., 98 (1994) 9594. 19. T. Iiyama, K. Nishikawa, T. Suzuki, and K. Kaneko, Chem. Phys. Lett., 274 (1997) 152. 20. L. S. Ornstein and F. Zernike, Proc. Sect. Sci. K. Med. Akad. Wet., 17 (1914) 793. 21. T. Ohba, T. Iiyama, H. Kanoh, and K. Kaneko, J. Phys. Chem., to be submitted. 22. Y. Hattori, H. Kanoh, and K. Kaneko, Adv. Mater., in press. 23. M. A. Springuel-Huet, J. L. Bonardet, and J. Fraissard, Appl. Magn. Reson., 8 (1995) 427. 24. S. H. Joo, S. J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, and R. Ryoo, Nature, 412 (2001) 169. 25. S. Bandow, M. Takizawa, K. Hirahara, M. Yudasaka, and S. Iijima, Chem. Phys. Lett., 337 (2001) 48. 26. K. Murata, K. Kaneko, W. A. Steele, E Kokai, K. Takahashi, D. Kasuya, K. Hirahara, M. Yudasaka, and S. Iijima, J. Phys. Chem., 105 (2001) 10210. 27. J. I. Paredes, A. Martfnez--Alonso, and J. M. D. Tasc6n, Abstract of 6 th International Symposium on the Characterization of Porous Solids, Alicante, Spain (2002) pp. 9. 28. A. V. Neimark, P. I. Ravikovitch, A. Vishnyakov, Abstract of 6 th International Symposium on the Characterization of Porous Solids, Alicante, Spain (2002) pp. 8. 29. F. R.ouquerol, J. Rouquerol and K. Sing, Adsorption by Powders and Porous Solids, Academic Press (1999) pp. 227. 30. E Rodriguez-Reinoso and M. Molina-Sabio, Adv. Colloid Interface Sci., 76 / 77 (1998) 271. 31. N. Setoyama, T. Suzuki, and K. Kaneko, Carbon, 36, (1998) 1459. 32. T. Ohba and K. Kaneko, J. Phys. Chem., in press. 33. E R. Siperstein and K. E. Gubbins, Fundamentals of Adsorption 7, K. Kaneko, H. Kanoh, and Y. Hanzawa eds., International Adsorption Soc., Chiba, (2002) pp. 434. 34. N. Setoyama, K. Kaneko, and E Rodribuez--Reinoso, J. Phys. Chem., 100 (1996) 10331. 35. S. R. Challa, D. S. Sholl, and J. K. Johnson, J. Chem. Phys., 116 (2002) 814 36. H. Tanaka, M. E1-Merraoui, T. Kodaira, and K. Kaneko, Chem. Phys. Lett., 351 (2002) 417. 37. M. Sunaga, T. Ohba, T. Suzuki, H. Kanoh, and K. Kaneko, J. Phys. Chem., to be submitted.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Reconstruction
M e t h o d for the C h a r a c t e r i z a t i o n
19
of Porous Carbons
J. Pikunic a, C. Clinard b, N. Cohaut b, K.E. Gubbins a, J.-M. Guet b, R.J.-M. Pellenq b, I. Rannou b and J.-N. Rouzaud b a Department of Chemical Engineering, North Carolina State University, 113 Riddick Labs, Raleigh, NC 27695-7905, USA
b Centre de Recherche sur la Mati6re Divis6e (UMR 131 CNRS), 1B rue de la Ferollerie, 45071 Orl6ans C6dex 02, France We implement a modified version of the reconstruction method developed in a previous work to model two porous carbons produced by the pyrolysis of saccharose and subsequent heat treatment at two different temperatures. We use the Monte Carlo g(r) method to obtain the pair correlation functions of the two materials. We then use the resulting pair correlation functions as target functions in our reconstruction method. Our models present structural features that are missing in the slit-pore model. Structural analyses of our resulting configurations are useful to characterize the materials that we model.
1. INTRODUCTION In most simulation studies of fluids confined in porous carbons, the pore volume is modeled as the space between two parallel infinite graphite walls forming slit-shaped pores of a specified width. After performing molecular simulations at several different pore widths, any physical property is calculated by averaging that property over the pore sizes. If the experimental adsorption isotherm is available, it is possible to obtain the pore size distribution. This approach relies on an important assumption: the heterogeneity of the carbon can be described with a distribution function that depends only on the pore size. In other words, the material is described as a collection of graphite-like slit pores that are independent and unconnected. However, in most real carbons of industrial interest, the pore walls are nongraphitic and the pores are highly connected. These sources of heterogeneity must be included in the model to accurately predict the equilibrium properties and especially the dynamics of fluids confined in porous carbons. More realistic models have been proposed in the last few years and they have been recently reviewed [1 ]. Most of these models are based on very simple and qualitative reconstruction of experimental structure data. Our aim is to develop a method to generate atomic configurations of carbon atoms that quantitatively match the structural properties of real porous carbons. Thomson and Gubbins [2] applied Reverse Monte Carlo (RMC) to develop a model composed of rigid and perfect graphene segments for an activated carbon. The RMC model of Thomson and Gubbins is reasonable for many graphitisable carbons. However, the use of graphene segments as the basic structural units fails to allow for the formation of defects (i.e., rings of 5 or 7 carbon atoms) that are important sources of heterogeneity of many porous carbons [1,3]. In a more recent work [3], we presented an atomistic approach, also based in
20 RMC, that allows for the formation of non-aromatic rings which are responsible for the curvature and cross-linking in carbon plates. We combined our atomistic approach with Simulated Annealing (SA) to develop a reconstruction method [4] that gives better fits to the experimental data than the original RMC technique. In this work, we explore the use of the pair correlation function as the target function in our reconstruction method [4]. This speeds up the simulations, allowing us to construct models in much larger simulation boxes. We build models for two saccharose-based carbons treated at different temperatures. We compare the exact pore size distributions and perform Grand Canonical Monte Carlo (GCMC) simulations of nitrogen at 77 K in the resulting models. 2. M E T H O D S 2.1.
Reconstruction Method
We use a reconstruction method based on Reverse Monte Carlo (RMC) [5] and Simulated Annealing (SA) [6] developed in our previous work [4]. The goal is to produce a three dimensional array of carbon atoms that is consistent with a set of experimental data. The method consists of changing the atomic positions of some initial atomic configuration through a stochastic procedure. Changes in the atomic configuration, or moves, are accepted or rejected based on the agreement between some simulated structural property and the corresponding target structural property, which is usually obtained experimentally. Throughout the simulation, the differences between the simulated and the target functions are minimized. In this work, we use the pair correlation function, g(r), as the target function. The quantity to be minimized is: neap
X 2 =Z[gs,m(r,)-gexr(r~)]
2
(1)
l=l
where n~xp is the number of experimental points, g~m(r~) is the simulated g(r) and g~p(r~) is the experimental g(r) evaluated at ri. The answer to whether or not a configuration obtained by RMC is unique is implicit in the uniqueness theorem of statistical mechanics [7]. For systems in which the potential is pairwise additive, we can show that for a given pair correlation function there is one and only one pairwise potential, and this potential determines all the higher-order correlation functions [8,9]. However, the forces acting on carbon atoms in porous carbons do not correspond to pairwise potentials. There are angular contributions to the interaction potential that are, by definition, many-body interactions. When three-body forces are present, we can show that for a given set of a pair correlation function and a three-body correlation function, all the higherorder correlation functions are determined. Therefore, assuming that only two- and three-body forces are important in disordered porous carbons, we use the pair correlation function along with a set of simple expressions to describe the three-body correlation to completely specify the structure of the system [4]. We assume that carbon atoms in our model h a v e sp 2 hybridization. Each carbon atom is bonded to one, two or three other carbon atoms. We further assume that the angular contribution to the potential is proportional to the sum over all bond angles of the squared difference between the cosine of the actual bond angle and the cosine of the equilibrium bond angle (120 degrees). In our previous work, we used a harmonic approximation that is valid only for small angles. The expression used in this work is a better model of angular contributions for all bond angles. The fraction of carbon atoms with three neighbors may be estimated from the experimental composition (hydrogen:carbon or H/C and oxygen:carbon or
21 O/C ratios). The goal of our reconstruction method is thus to minimize three quantities; the usual 22, given by Eq. (1), along with: ~ 2 = ~ - , c o s ( ~ ) - cos l=l
d2
E/
N3
]2
and
(2)
(3)
where ~ are the different bond angles in radians, no is the total number of bond angles, N3 is the number of C atoms with three neighbors and N is the total number of C atoms. As in our previous work [4], we minimize a linear combination of the cost functions, 2 2, and oa using Simulated Annealing (SA) [6]. The procedure is as follows. We fix the parameters T r z and T J Tz (see Eq.(4)) and place a number of carbon atoms at random positions in a simulation box. The density of the system is equal to the density of the real material at the length scale of the simulation box (excluding macropores), obtained from Hg porosimetry results. The simulation starts at a high temperature Tz. The functions 2A, ~ and oa are calculated. An atom is then randomly selected and displaced to a new position and the functions 2 2, ~ and oa are calculated for the new configuration. The move is accepted with a probability:
Pace = min 1, exp -
2"n~w- 2"otJ) +
1
(2
2
1
(4)
The system runs at this temperature for a number of random moves. This is called an annealing step. At the end of each annealing step, the temperature Tz is multiplied by a constant between zero and one and the simulation proceeds to the next annealing step at this new temperature. The initial temperature is given a high enough value so that nearly all the moves are accepted. The simulation is completed when no significant changes in the three functions to be minimized are observed.
2.2. Obtaining the pair correlation function from diffraction experiments In the Reverse Monte Carlo (RMC) method [5], the pair correlation function or the structure factor is calculated after each random move (Ssim(q) or gsim(r)) and compared to the respective target function obtained from experimental diffraction data (Sexp(q) or gexp(r)). It is possible to calculate Ssim(q) with full periodicity from the atomic positions. This method is best in principle [10], but the computational cost is much greater than for any of the other available methods. It is also possible to obtain Ssim(q) by first calculating gsim(F) from the atomic positions and then Fourier transform this function and calculate Ssim(q). The disadvantage of this approach is that there is an additional computational cost associated with the Fourier transform of gsim(F) after each move. An alternative approach is to u s e gexp(r) as the target function in the RMC simulations. The analysis of the experimental diffraction data to obtain Sexp(q) involves a sequence of corrections that are generally well understood. However, in order to obtain gexp(r), it is necessary to Fourier transform Sexp(q). This operation is particularly vulnerable to the limitations of the experimental data [10-12]. For example, Sew(q) is obtained up to a maximum value of q, at which there may still be oscillations. Since the Fourier transform involves an integral from q equals zero to infinity, this limitation yields to truncation errors
22 that are reflected in the resulting gexp(r). Therefore, direct Fourier transform of Sexp(q) to obtain gexp(r) is not desirable. A way round this problem is the so-called Mote Carlo g(r) (MCGR) method [ 11,12]. MCGR is an inverse method, rather than a direct Fourier transform. The idea is to randomly modify a numerical g(r) until its Fourier transform is consistent with S~xp(q). The procedure is analogous to a one-dimensional Reverse Monte Carlo [10]. Since the numerical g(r) can be generated for arbitrarily large r-values (limited only by the experimental q resolution), truncation errors are avoided in the Fourier transform. The resulting g(r) may be used a s gexp(r) in the RMC procedure (instead of S~w(q)), significantly reducing the computational cost and thus allowing the study of larger systems. The MCGR method has been successfully applied in other RMC studies [ 13]. 3. RESULTS We prepared two carbons by pyrolysis of saccharose followed by heat treatment at two different temperatures: 400~ (CS400) and 1000~ (CS1000). We performed X-ray diffraction and SAXS on each of these porous materials and obtained the structure factors, S(q), following the procedure described by Franklin [14]. The resulting S(q)'s are shown in Figure 1 (bold line). We performed Hg porosimetry to obtain the density of both carbons, accounting for the volume occupied by C atoms, closed pores and smaller open pores. We also measured the H/C and the O/C ratios by combustion experiments. The results are summarized in Table 1. a b
21
I 0
V 1
2
3
4
5
q O/A)
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
q O/A)
Figure 1. Structure factors of saccharose-based carbons obtained from diffraction experiments (bold line) and MCGR fit (thin line), a) CS400, b) CS 1000 We applied MCGR to extract the pair correlation functions from the experimental structure factors. The resulting pair correlation functions and the corresponding structure factors are shown in Figure 2 (solid line) and Figure 1 (thin line), respectively. The MCGR structure factors are in very good agreement with the experimental data. The deviations can be attributed to statistical errors in the structure factor data and in other experimental measurements that directly determine the coordination constraint used in the MCGR procedure, e.g. density, H/C and O/C ratios. The peaks are in the same positions as the experimental peaks and the deviations are not systematic. The peaks in the pair correlation functions (Figure 2) are in similar positions to those observed in the pair correlation function of graphite. The peaks of the pair correlation function of CS 1000 are more pronounced than those for CS400, indicating that higher pyrolysis temperatures produce more ordered structures. We observed that when the structure factor data is included up to only 7.5 ~-1 in the MCGR procedure, important structural features are lost in the pair correlation function.
23 Table 1. Experimental densities and compositions H/C 0.53 0.15
CS400 CS 1000
O/C 0.123 0.041
Hg p (g/ml) 1.275 1.584
We used the pair correlation functions obtained by the MCGR method shown in Figure 2 (solid line) as the target function in our reconstruction method (explained in section 2.1). The simulation boxes are 50 A long and the density of C atoms was determined using the experimental results summarized in Table 1. Each annealing step consisted of 32 MC moves / atom, for a total of 4800 MC moves / atom. We fixed the temperature of an annealing step as 90% of the temperature of the previous annealing step. We changed the maximum atomic displacement to set the acceptance ratio to 40%. The pair correlation functions and snapshots of the resulting structures are shown in Figure 2 (dashed line) and Figure 3, respectively. a b
A
4
2
0
2
4
6
8 r (A,)
10
12
14
0
2
4
6
8 r
10
12
14
(&)
Figure 2. Pair correlation functions of saccharose-based carbons obtained with MCGR (solid line) and reconstruction method (dashed line), a) CS400, b) CS 1000 The pair correlation functions of the resulting models are in excellent agreement with the target functions. The fact that the agreement is better for CS400 seems to indicate that the minimization method is more effective for more disordered materials. The region of configurational space in which the global minimum is encountered narrows down as the material becomes more ordered. Therefore, the use of a more sophisticated minimization technique, such as simulated tempering, should improve the results. A comparison between Figure 3a and b (look at the left side for clarity) reveals how the size of the graphene segments increase with the pyrolysis temperature, as expected. Most C atoms in CS400 seem to be part of chains that link the small graphene segments. On the other hand, most C atoms in CS 1000 are placed in defective graphene segments. To further characterize these models, we calculated the pore size distributions of the resulting structural models following the procedure described in ref. [ 15]. We define the pore size in terms of the reentrant surface in the volume accessible to nitrogen. In other words, it is equivalent to the size probed by a nitrogen molecule. We define the pore size distribution in the following way: p(H)dH is the fraction of pore volume corresponding to pore sizes in the range H to H + dH. The resulting pores size distributions are shown in Figure 4. As expected, the pore size distribution becomes narrower with pyrolysis temperature. In fact, there is a mode in the distribution for CS400 at larger pore sizes that is not observed in the distribution for CS 1000. Also, the density of smaller pores is larger for CS 1000.
24
Figure 3. Snapshot slabs of the models resulting from the reconstruction method. The rods represent C-C bonds, a) CS400, b) CS 1000. We performed Grand Canonical Monte Carlo (GCMC) simulations of nitrogen at 77 K in the resulting structural models at different nitrogen chemical potentials to obtain the nitrogen adsorption isotherms at 77 K. The acceptance criteria and other details of the Monte Carlo simulations in this ensemble are given in [16,17]. We modeled each nitrogen molecule and each carbon atom as a Lennard-Jones sphere, with the same interaction parameters used in a previous work [2]. The oxygen and hydrogen atoms were not included. We related the chemical potential with the bulk pressure using the ideal gas equation. The excess amount adsorbed is presented as a function of the relative bulk pressure for both carbons in Figure 5. Snapshots of the simulations at a relative pressure of 1 are shown in Figure 6. The resulting isotherms are type I, characteristic of microporous solids. The adsorption capacity of CS400 is higher than that of CS1000, due to the higher porosity of CS400. The filling process starts at much lower (approximately 2 orders of magnitude) relative pressures in CS 1000 than in CS400. This is because CS 1000 has a higher fraction of smaller pores (see Figure 4), which produces regions in the pore volume with higher attractive potential energy. 0.8 0.7-
:~.
0.6-
!
6 4..
5-
0.5-
s
0.40.3-
2" 0.2 1-
0.1" 0 0
,
,
,
,
,
,
!
2
3
4
5
6
H
(A)
0 I 7
8
9
10
1.0E-10
. 1.0E-08
.
.
.
1.0E-06
1.0E-04
1.0E-02
1.0E+00
P/Po
Figure 4. Pore size distribution of the volume Figure 5. Nitrogen adsorption isotherms at 77 K accessible to nitrogen in the resulting models of obtained from GCMC simulations in the resulting CS400 (solid line) and CS 1000 (dashed line), models of CS400 (squares) and CS 1000 (circles). Although many nitrogen molecules are adsorbed in pores that have slit shape (see Figure 6), most of them are in pores with shapes that significantly differ from a slit geometry. It is interesting to note that a significant fraction of the pores are quasi-one-dimensional, and the
25 nitrogen molecules form chain-like structures in these pores, similar to those formed by adsorbate molecules in carbon nanotubes. a
Figure 6. Snapshots of GCMC simulations of nitrogen at 77 K in the resulting models at bulk pressure of P/Po = 1. The rods and the spheres represent C-C bonds and nitrogen molecules, respectively, a) CS400, b) CS 1000 4. CONCLUSIONS We have used a modified version of the reconstruction method developed in a previous work [4] to model two porous carbons produced by the pyrolysis of saccharose and subsequent heat treatment at two different temperatures. We used the Monte Carlo g(r) method to obtain the pair correlation functions of the two materials. We used the resulting pair correlation functions as target functions in our reconstruction procedure. Our models evidence that the size of the defective graphene segments increases with the pyrolysis temperature. We calculated the geometric pores size distribution of the resulting models. The carbon heated at higher temperatures has a narrower pore size distribution and smaller pores. We also performed GCMC simulations to obtain the simulated nitrogen adsorption isotherms at 77 K. The filling process in the carbon heated at a higher temperature starts at relative bulk pressures that are about 2 orders of magnitude lower than those in the carbon heated at the lower temperature. A significant fraction of the pore volume has shapes that significantly differ from a slit geometry. Some of the pores in both models provide quasi-one-dimensional confinement environments. 5. ACKNOLEDGEMENTS We thank the Department of Energy for support of this research under grant no. DE-FG02-98ER14847. Supercomputer time was provided under a NSF/NRAC grant (no. MCA 93 SO11). We also thank Henry Bock for helpful discussions. REFERENCES
[1]
T.J. Bandosz, M.J. Biggs, K.E. Gubbins, Y. Hattori, T. Iiyama, K. Kaneko, J. Pikunic and K.T. Thomson, in: L.R. Radovic (Ed.), Chemistry and Physics of Carbon, Marcel Dekker, New York, 2002 (in press).
26
[2] [3] [41
[5] [6] [7] [s] [9] [10] [11] [12] [13] [ 14] [ 15] [16] [17]
K.T. Thomson, K.E. Gubbins, Langmuir 16 (2000) 5761. J. Pikunic, R. J.-M. Pellenq, K.T. Thomson, J.-N. Rouzaud, P. Levitz, K.E. Gubbins, Studies in Surface Science and Catalysis 132 (2001) 647. J. Pikunic, R.J.-M. Pellenq, K.E. Gubbins, in: K. Kaneko, H. Kanoh, Y. Hanzawa (Eds.), Proceedings of Fundamentals of Adsorption 7, IK International, Chiba, 2002. p. 377. R.L. McGreevy, L. Pusztai, Mol. Simul. 1 (1988) 359. S. Kirkpatrick, C.D. Gelatt, Jr., M.P. Vecchi, Science 220 (1983) 671. R. Evans, Mol. Simul. 4 (1990) 409. R.L. Henderson, Phys. Lett. 49A (1974) 197. C.G. Gray and K.E. Gubbins, Theory of Molecular Fluids, Clarendon Press, Oxford, 1984, p. 178. R.L. McGreevy, in: C.R.A. Catlow, ed. Computer Modeling in Inorganic Crystallography. Academic Press, San Diego, 1997. p 151. A.K. Soper, in: M. Davidovic, A.K. Soper, eds. Springer Proceedings in Physics, vol. 40. Springer-Verlag Berlin, Heidelberg, 1989. p 189. A.K. Soper, Institute of Physics Conference Series 107 (1990) 57. R.L. McGreevy, Materials Science Forum 166-169 (1994) 45. R.E. Franklin, Acta Cryst. 3 (1950), 107. L.D. Gelb, K.E. Gubbins, Langmuir 15 (1998) 305. M.P. Allen, D.J. Tildesley, Computer Simulation of Liquids, Clarendon Press, Oxford, 1987. D. Frenkel, B. Smit, Understanding Molecular Simulation: From Algorithms to Applications, second edition, Academic Press, San Diego, 2002.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
27
A New Method for Microporosity Detection Based on the Use of the Corrugated Pore Structure Model (CPSM). C.E. Salmas a, V.N. Stathopoulos b A.K. Ladavos b, P.J. Pomonis b and G.P. Androutsopoulos a
a Department of Chemical Engineering, National Technical University of Athens, Athens 15 780, Greece. b Department of Chemistry, University ofloannina, Ioannina 45 100, Greece. This paper deals with the application of the CPSM and a,. methods to determine microporosities of silica gels and microporous clays pillared with All.xFexOy oxidic species. A reasonably good agreement between the two methods was ascertained. However, the CPSM method evaluates slightly higher relative microporosities in the case of pillared clays. By raising the Fe content a decrease of the relative microporosity of pillared clays was observed. The CPSM method is most appropriate for the tracing of supermicropores i.e. pore sizes Dp=l-2 nm, as it is based on the assumption that an adsorbed gas monolayer exists. The CPSM model through the simulation of gas sorption hysteresis, enables a unified analysis of porous materials exhibiting mixed meso-micropore structures, involving evaluation of mesomicro pore size distribution, determination of pore surface area in good agreement with the corresponding values from a Langmuir equivalent model, estimation of tortuosity factors rCPSM and detection of relative microporosity in satisfactory agreement with relevant data obtained by the as method. 1.
INTRODUCTION
The characterization of porous materials exhibiting a composite pore structure encompassing micro-meso-and perhaps macro-pore sizes, is of particular significance for the development of separation and reaction processes. Among the characterization methods for materials exhibiting ultramicropore structures, Dp2, Zv=5 proves to be more efficient than both Zv=2 and Zv=8. In addition, mixing interactions (ZF_
_m n, 0.4
0.2
0
-16
J
-12
i
~
l
-8
.4
0
F
4 Beam position (rim)
~
i
8
12
16
Figure 4.- Relative Porod Invariant values (calculated relative to the maximum Porod Invariant value for each sample) corresponding to the measurement carried out across the fiber diameter for the samples CFC50 and CFS48. In this figure, it can be observed that, for both samples, the maximum scattering corresponds to the measurement carried out in the external zone of the fibers, indicating a higher concentration of pores in the outer zone of the fiber. In addition, it can be observed that the PI profiles, as a function of the position of the fibers, are different for CO2 and steam activated materials. In the case of steam, the scattering from all the internal zones is very similar and much lower than from the external part of the fiber. On the other hand, in the case of CO2, the porosity is more developed in the inner regions of the fibers compared to the steam activated carbon fibers. These results constitute a direct proof of the different behavior of CO2 and steam as activating agents, and show the suitability of this novel experiments for characterizing different regions of activated carbon fibers.
3. 2. 2. Effect of the fiber diameter. Another example to show the usefulness of this technique is to study the effect of the fiber diameter in the porosity development. In doing that, two activated carbon fibers prepared
57
from different precursors and using the same activating agent have been selected: sample CFC50 and sample CFBC63. In both samples the scans were divided in 10 steps. However, due to the difference in fiber diameter, the widths of the scans were different: 40 ~tm in the case of the thinnest sample (CFC50) and 80 lam in the case of the sample (CFBC63). Similarly to the previous section, the relative Porod Invariant corresponding to each measurement for the two carbon fibers has been calculated and plotted versus beam position in Figure 5.
1
l
- ~ - CFCSO
0.8
-A- FCBC63
~.. 0.6
m
~ 0.4
0.2 & 0
-28
I
I
I
I
i
1
I
I
I
i
i
i
I
-24
-20
-16
-12
-8
-4
0
4
8
12
16
20
24
~8
Beam position (rLm)
Figure 5.- Relative Porod Invariant values (calculated relative to the maximum Porod Invariant value for each sample) corresponding to the measurement across the fiber diameter. In this figure it can be observed that the porosity development across the fiber diameter is different depending on the diameter of the carbon fiber used as precursor. In the case of the thick fiber (CFBC63), the CO2 molecules do not reach the internal zone of the fiber, while in the case of the thinnest activated carbon fiber (CFC50), the porosity is much more developed in the center compared to the thick fiber. These results indicate that the starting carbon fiber does not contain transport pores (i.e. macro and mesopores) that permit an easy diffusion and a homogeneous distribution of CO: molecules. Thus, for a similar burn-off degree, the higher the diameter of the fiber, the activation process focuses mainly in the extemal zones, where most of the porosity will be located. 4. CONCLUSIONS The examples presented in this work illustrate the suitability of IaSAXS technique to characterize activated carbon fibers. It has been shown the isotropy features in activated carbon fibers prepared from different precursors and using different activating agents. In addition, this technique is able to obtain scattering measurements across the fiber diameter, which has allows us to obtain maps of pores distribution. The present results show that
58 interesting research can be carried out for porous materials science by using ~SAXS technique. Acknowledgements
The authors thank ESRF (Experiment number ME-93) for the facilities and financial support. Also we would like to thank MCYT (Project MAT-2000-0621) for financial support. 5. REFERENCES
[1] Alcafiiz-Monge J, Cazorla-Amor6s D, Linares-Solano A, Yoshida S, Oya A. Carbon 1994; 32(7): 1277-1283. [2] Alcaftiz-Monge J, Cazorla-Amoros D, Linares-Solano A. Carbon 1997; 35(10-11):16651668. [3] Lozano-Castell6 D, Cazorla-Amor6s D, Linares-Solano A, Hall, PJ, Fernandez JJ. In Unger KK, et al. Editors. Studies in Surface Science and Catalysis Vol. 128, Characterisation of Porous Solids V, The Netherlands: Elsevier Science, 2000. p.523532. [4] Cazorla-Amor6s D, Salinas-Martinez de Lecea C, Alcaftiz-Monge J, Gardner M, North A, Dore J. Carbon 1998; 36(4), 309-312. [5] Riekel, C. Rep.Prog.Phys. 2000, 63,233. [6] MOller M, Czihak C, Vogl,G, Fratzl P, Schober H, Riekel C. Macromolecules 1998; 31: 3953-3957. [7] MOller M, Burghammer M, Riekel C. ESRFNewsletter 1999; 33: 12-13. [8] MOller M, Riekel C, Vuong R, Chanzy H. Polymer 2000; 41:2627-2632. [9] MOller M, Czihak C, Burghammer M, Riekel C. Journal of Applied Crystallography 2000; 33: 817-819. [10]Paris O, Loidl D, Peterlik H, Mtiller M, Lichtenegger H, Fratzl P. Journal of Applied Crystallography 2000: 33: 695-699. [ll]Paris, O.; Loidl, D.; Mtiller, M.; Lichtenegger, H.; Peterlik, H. Journal of Applied Crystallography 2001; 34: 473-479. [12]Guinier A, Fournet G. Small Angle Scattering of X-Ray, John Wiley&Sons Inc., New York, 1995. [13] Ridgen JS, Dore JC, North AN. In Rouquerol J. et al. Editors. Studies in Surface Science and Catalysis Vol. 87, Characterisation of Porous Solids III, The Netherlands: Elsevier Science, 1994. p.263-271.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
59
"Real time" d e t e r m i n a t i o n of porosity d e v e l o p m e n t in carbons: a c o m b i n e d SAXS/TGA approach J.M. Caloa, p.j. Hall b, S. Houtmannb, D. Lozano Castello c, R.E. Winans(l, and S. Seifert d aDivision of Engineering, Brown University, Providence, RI, USA bDepartment of Chemical Engineering, Strathclyde University, Glasgow, Scotland, UK ~Dpto. de Quimica Inorgfinica, Universidad de Alicante, Alicante, Spain dChemistry Division, Argonne National Laboratory, Argonne, 1L, [JSA Time-resolved X-ray scattering data are presented for three different carbons that were activated in sit~t in the beamline at the Advanced Photon Source at the Argonne National Laboratory in Argonne, IL, USA. The resultant data exhibit varied behavior, depending on the carbon type. The data suggest a dynamic porosity development mechanism. A net population balance model of pores within a particular size range may explain these observations. 1. INTRODUCTION Small angle scattering (SAS) techniques offer a number of advantages for the investigation of the nature and behavior of porous materials. In particular, with respect to carbons, the nonintrusive nature of SAS means that, in principle, characterization can be performed on carbons in situ during activation processes, allowing real time resolution of porosity development. These types of studies have not been practical nor possible heretofore, primarily due to the prohibitively long counting times required to span the requisite wave vector (q) range over tile entire burn-off process. However, the availability of new instruments, such as the small angle X-ray scattering (SAXS) facility at the Advanced Photon Source (APS) at the Argonne National Laboratory (ANI,), has now made these types of experiments possible. Some preliminary data from these efforts have been reported previously [1]. Here we propose to present results incorporating a thermogravimetric analyzer in the beamline to directly correlate burn-off with small angle X-ray scattering (SAXS) data. Results on the behavior of some different carbons during activation in oxygen are presented, as well as the effect of a catalytic agent, calcium.
60 2. E X P E R I M E N T A L
The SAXS instrument was constructed at ANL and used on the Basic Energy Sciences Synchrotron Radiation Center (BESSRC-CAT) undulator beamline ID-12 at the APS. Monochromatic X-rays (8.98 keV here) were scattered offthe samples and collected on a 15 cm x 15 cm Mosaic CCD array (,1538 x 1538 pixels). This system is capable of a resolution of ,-~0.5 s. The scattered intensities were corrected for absorption, empty cell scattering, and instrument background. A Cahn TG-121 therlnogravimetric apparatus (TGA) was adapted for use directly in the beamline. Two holes, approximately 2 cm in diameter, located 180 ~ apart, were drilled through the walls of the TGA furnace to accommodate the X-ray beam. Two corresponding holes were also made in the quartz hangdown tube of the TGA to admit the X-ray beam. Char samples, on the order of tens of nag, contained in aluminum foil packets with multiple perforations (to allow for adequate transport of oxygen through the sample), were suspended from the microbalance on a platinum wire. The experimental arrangement and procedures were gradually improved with increasing experience with the system. Since the TGA system was operated in an "open" mode, the reaction gas (oxygen) was used to purge the quartz hangdown tube to maintain uniform cornposition and temperature. With the flow rate set too high, the sample tended to oscillate on the platinum wire. This resulted in some observable oscillation in scattering intensity at high time resolutions. This problem was solved by lowering the oxygen flow rate to the point where sample oscillation vanished. Also, the location of the sample thermocouple, necessitated by the experimental arrangement, resulted in a systematic offset in the temperature measurements. This problem was rectified via calibrations of the sample temperature in a laboratory TGA. 10 g samples of saran and microgranular cellulose, obtained fi'om the Aldrich Chenlical Co., were heated in a tube furnace at 10Ir to 900~ with a l h heat soak time, under flowing nitrogen. Calcium-loaded cellulose was prepared by mixing with calcium acetate solution, followed by drying. The resultant calcium content was approximately 3% tbllowing carbonization. 3. R E S U L T S AND DISCUSSION 3.1. Saran (.'hat"
The TGA data presented in Figure 1 were obtained under nonisothermal conditions in an atmosphere of oxygen. As shown, the sample was first rapidly heated to 200~ held at that temperature for 2 lnin, and then heated to 560~ at 6K/rain, followed by an isothermal period at the final temperature. The bulk of the sample burnt out at about 52 rain, prior to attaining the final temperature. From this point on, the mass signal appears to be due to slow burnout of residual, presumably more unreactive, material. SAXS curves corresponding to selected points during the activation history are presented in Figure 2. In the absence of other effects, scattering at high wave vector (q) values
61 is primarily due to small microporosity, while that at low values is due to meso- and larger porosity [2]. The plateau at high q is indicative of a significant population of microporosity. At low-burn-offs there is a notable decrease in the mesoporosity, accompanied by microporosity development at high q. Therealter, scattering in the microporosity region falls off markedly as the carbon in the micropores is burned off. Somewhat similar results were obtained for saran char under near-isothermal conditions in oxygen. The sample temperature was increased at 60K/rain to 435~ and then held there. The sample mass actually began to decrease prior to attaining isothermal conditions. This illustrates the difficulty of resolving the porosity development history under isothermal conditions where the burn-off behavior typically varies widely with the nature and preparmion history of the sample. Nonisothermal temperature programs appear to be more efficient in this regard. The corresponding scattering data are presented in Figure 3. The vm'iation in scattering intensity with activation appears to be somewhat complex. It is important to note that only net porosity differences are observed. Thus, the predominant behavior in the low burn-off regime (0-7.5 rain) appears to be net microporosity development, while scattering from the larger pores remains relatively more constant. However, with continuation of the burn-off history in Figure 3, the process becomes inverted such that the predominant behavior appears to become net consumption of mesoporosily, while the net microporosity remains relatively constant.
Figure 1. Saran char sample mass history during nonisothermal activation in oxygen. "l'he symbols correspond to the scattering curves in Figure 2.
Figure 2. SAXS curves lbr saran char during nonisothermal activation in oxygen to 565~ to the indicated mass losses corresponding to the symbols in Figure 1.
62 The apparent behavior of porosity with activation obtained from raw scattering data is somewhat distorted by the variation of the solid volume fraction. Os. in the following manner. The total scattering intensity is given by:
I(q) = l),,2 V ~(I- ~,,) ./'o'~y[sin(qr)/'qr] 4nv':dr
(])
where I(q) is the scattering intensity at a value of the scattering wave vector, q = 4a sin(0)/Z, 0 is the scattering halt-angle, )~ is the neutron wavelength, "/the correlation function, by is the contrast thctor between solid and voids, and V is the scattering volulne. The mean square of the fluctuations of the scattering length density is bv:~s(l- qb~),which varies with ~s, and thus acts as a scaling factor for the scattering intensity. One way to account for this effect is to correct the scattering intensities with the Porod invariant (PI) [3]: (2) Since all tile parameters on the righthand side of this expression should be constant with activation, except for ~s. the value of tile Porod invariant should follow the behavior of ~s(1- ~0. Thus, division of the scattering intensities by this factor can be used to correct for the variation of qbs. It is emphasized, however, that this is only an approximation for a number of reasons, including the thct that when I(q) varies with q to an exponent less than --2 (as at high q for the current data), the integral diverges [3], and accurate values of the Porod invariant cannot be obtained.
Figure 3. SAXS curves for saran char activated at 435~ in oxygen to the indicated mass losses.
Figure 4. SAXS curves tbr saran char from Figure 3), divided by the corresponding Porod invariants.
63
Figure 5. "Corrected" scattering data from Figure 4 tbr saran char activated at 435~ in oxygen to the indicated mass losses, amplifying the behavior of the smaller pores. The Porod invariants calculated from the scattering curves presented in Figure 3 were found to increase initially up to about 10.29 nag mass loss, and then decrease monotonically thereafter. The scattering curves in Figure 3, corrected by the Porod invariant, are presented in Figure 4. 'I'he principal effect of the correction is to draw the curves much closer together, especially for the smallest pores (i.e., at high q). As shown in Figure 4, the curves all seem to intersect in the vicinity of q ~ 0.045~x~-~. The population ol" the larger micro- and mesopores at lower q decrease monotonically with activation up to about 29.59 mg mass loss, and then increase with activation beyond this point. As shown in Figure 5, the population of the smaller micropores (q > 0.045 A -1) develops slowly with activation until about 10.29 mg mass loss. It then increases more rapidly to 32.47 mg mass loss, and decreases thereafter. It is noted that the decrease in the smallest pores coincides with the point at which tile population of the larger pores begins to increase. 'l'his behavior is consistent with pore wall collapse among the smallest pores. These data suggest a dynamic porosity development mechanism beginning with depletion of the larger micro- and mesoporosity and the development of tile smaller microporosity. At higher burn-offs the process changes to one of net loss of the smallest microporosity while the net population of the larger micro- and mesopores increases. Of course, as indicated previously, only net porosity differences are observed. Thus, tbr example. when tile porosity in one particular size range is relatively constant, this could be due either to no appreciable reaction occurring in those pores at that particular stage in the activation, or that the rates of consumption and development are in relative balance over this burn-off range. Since it would be difficult to reconcile the former mechanism with the nature of the activation process, it appears that the net population balance of pores in a particular size range is responsible for these observations.
64
Figure 6. SAXS curves for cellulose char activated at 425~ in oxygen to the indicated mass losses.
Figure 7. SAXS curves for cellulose char activated at 425~ in oxygen (q/. Figure 6) to the indicated mass losses, divided by the corresponding Porod invariants.
3.2. Cellulose C h a r Scattering data for a cellulose char sample activated isothermally at 425~ are presented in Figure 6. "l'hese data show that this carbon, like the saran char, initially exhibits a significant amount of microporosity, as indicated by the plateau centered at about q = 0.1A l. The Porod invariants lbr these data initially increase and then decrease with activation, also like the saran char. The scattering data, corrected by the PI values, are presented in Figure 7, which shows somewhat different behavior than for the saran char. As shown in Figure 7, initially the population of the smallest micropores centered c a . 0.1A -~ increases much more markedly than for the saran char. This is accompanied by relatively little development of the larger porosity up to 9.42 mg mass loss, following a significant initial loss at low q values, l towever, continued net microporosity development with activation then slows significantly beyond 13.00 mg mass loss, and finally begins to decrease al 14.05 mg mass loss, while simultaneously the population of the larger pores begins to increase at an accelerating rate. Once again, .just as for the saran char, this change ill behavior coincides with a decreasing population of the smallest pores, consistent with pore wall collapse among the smallest pores.
65
Figure 8. SAXS curves for Ca-loaded cellulose char during isothermal activation at 340~ in oxygen as a function of time. l'he total burn-off was 73%.
Figure 9. SAXS curves tbr Ca-loaded cellulose char during isothermal activation at 340~ in oxygen (c/. Figure 8), divided by the corresponding Porod invariants.
Differences in the behavior of the microporosity in the saran and cellulose chars may be related to :'closed porosity." It is well known that "glassy" carbons can exhibit a considerable amount of microporosity that is initially "'blocked" from access to the activating agent by more reactive, less ordered carbon. Early activation of these types of carbons primarily involves preferential burn-off of this carbon, exposing the intrinsic underlying microporous structure. For example, using contrast-matching small angle neutron scattering (SANS), no new microporosity development was observed upon activation of a char produced from a highly crosslinked phenol-formaldehyde resin up to 21% burn-off [21. The only mechanism observed was unblocking of initially "closed" microporosity. Unfortunately, SAXS is not amenable to contrast-matching as is SANS. Consequently, this approach could not be used in the current experiments. I lowever, a similar mechanism may be at least partially responsible for the relatively slower and less marked evolution of microporosity with activation in the saran char than in the cellulose char. 3.3. Calcium-Loaded Cellulose Char Mass loss data tbr the Ca-loaded char are unavailable since these data were obtained prior to the TGA being incorporated into the experimental system. A 1 mm diameter quartz capillary was used as the sample holder. The scattering data for this sample are presented in Figure 8. The Porod invariants ['or the data in Figure 8 increase monotonically with activation. No maximum was observed as for tile saran char and unpromoted cellulose char samples. The
66 scattering data corrected for the PI values are presented in Figure 9. As shown, the behavior of the Ca-loaded cellulose char differs significantly from that of the unpromoted cellulose (of Figure 7). Initially. the scattering intensity remains relatively constant and actually decreases slightly with activation over the entire q range. This behavior is similar to that observed fbr phenolic resin char at low q at low burn-offs, that was attributed primarily to preferential gasification of disordered carbon clusters ot" characteristically large sizes [2]. This is followed by a pattern of rapidly increasing scattering intensity at lower q values (q < 0.045 A-~) than for the unpromoted cellulose char, which continues to progressively shift to lower q with activation. As shown, there appears to be little or no small microporosity development (q > 0.045 A ~) over the entire activation history. The preferential development of mesoporosity rather than microporosity in the Ca-loaded char may be due to the catalytic effect of calcium, which may not have been well dispersed in the smallest pores. 4. CONCLUSIONS The SAXS/TGA approach has been demonstrated to be a usethl technique tbr timeresolution of porosity development in carbons during activation processes. Qualitative interpretation of the data obtained thus )hr suggests that a population balance approach focusing on the rates of production and consumption of pores as a fiJnction of size may be a fi'uittul approach to the development of quantitative models of activation processes. These then could become uselhl tools tbr the optimization of pore size distributions for particular applications by providing descriptions and predictions of how various activating agents and time-temperature histories affect resultant pore size distributions. ACKNOWLEDGEMENTS
The authors acknowledge EPSRC support under grant number GR/N03006. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract No. W-31-109-ENG-38. REFERENCES
[1] [21 [3]
J.M. Calo, P.J. Hall, S.D. Brown, R.E. Winans, and S. Seifert, Proc. Eurocarbon 2000, Berlin, Germany, 113-114 (2()00). M.M. Antxustegi, P.J. Hall, P.J., and J.M. Calo, J. Coll. Intf. Sci., 202 (1998), 490-498. J.S. Higgins and H. C. Benoit, Polymers and Neutron Scattering, Oxlbrd University Press, 1994.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
67
SANS Investigations of Adsorption Mechanisms in Model Porous Silicas S.Kallus a*, A.Hahn b and .J.D.F.Ramsaya a
Institut Europ6en des Membranes, CNRS, 1919 Route de Mende, Montpellier 34293, France
h Institute for Inorg. Chem., Johannes Gutenberg University, Mainz, Germany present address: Grace Davison, 67545 Worms, Germany A recent method for the characterisation of the structure o'f porous materials involves the measurement of small angle neutron scattering (SANS) performed in situ during isothermal gas adsorption. In this technique the coherent scattering length of the condensed gas or vapour is selected to match that of the porous solid. This method is illustrated by investigations on several mesoporous silica gels, having a highly ordered structure (MCM-41 and MCM-48), and pore sizes in the range from 1.9 to 6.4 nm. From measurements, made in situ with contrast matched benzene, adsorbed at 31 OK, and cyclohexane at 298K, it has been possible to differentiate this ordered mesoporosity from that due to the larger pores which arise from the micron-sized grain texture in these materials. The role of these two different types of porosity on kinetics of adsorption of hydrocarbon gases in mesoporous silica have also been investigated by the contrast matching technique. Results for n-hexane and cyclohexane show that the latter is adsorbed more slowly in MCM41 silica (pore diameter 2.3 nm) possibly because of hindered diffusion, due to the larger kinetic diameter of the molecule. Further kinetic investigations with hexane isomer mixtures, having different scattering lengths, have also provided an insight into the rate of exchange and partitioning of gas mixtures in MCM-41 silica. 1. INTRODUCTION An important and recent development of the technique of small angle neutron scattering (SANS) in the characterisation of porous solids has been the application of the contrast variation technique (1-7). More detailed insight into surface and porous properties, are obtained with this method by performing SANS measurements on samples in which the pores are filled or partially filled with an adsorbed vapour. In general, the scattering length density, Pb, of the condensed liquid adsorbate, which occupies the pore space, is selected to be the same as that of the solid matrix. Under such conditions "contrast matching" occurs. Such contrast matching is possible with SANS for a wide range of adsorbates (eg. water, hydrocarbons) because Pb can be varied readily by an isotopic substitution of H for D in the molecule (see Table 1). Here we illustrate two applications of the contrast matching technique applied to model mesoporous silicas (MCM-41 and MCM-48), using different adsorbates (benzene, n-hexane, cyclohexane). In the first, the larger inter-granular pores of the silica is highlighted and differentiated from the ordered cylindrical pores within the grains. These measurements have been made under
68 equilibrium isothermal conditions, at different pressures. In the second, kinetic measurements of n-hexane and cyclohexane in a smaller pore diameter MCM-41 silica are illustrated (under isothermal and isobaric conditions). Table 1
Molecular scattering lengths 5',bcohfor different liquid sorbates and corresponding scattering length densities, Pb, for mass densities, ~5, compared with amorphous silica. Dmol*, corresponds to kinetic diameter of molecule in vapour phase. Material Ebcoh/10-12cm ~5/ g cm-3 Pb/ 101~cm-2 Drool* (293K) 0.88 1.75 1.18 5.9 benzene -h6 0.95 5.44 7.99 -d6 -0.50 0.78 -0.28 6.0 cyclohexane -hi2 12.00 0.89 6.70 -d12 -hi4 - 1.25 0.66 -0.57 4.3 n-hexane -d14 13.33 0.77 6.18 water -h2 -0.17 1.00 -0.56 4.3 -d2 1.91 1.11 6.36 nitrogen (77K) 1.88 0.81 3.27 3.6 silica 1.58 2.20 3.47 _
Such kinetic investigations are feasible at high flux sources, such as ILL, Grenoble, where measurements having good statistics can be obtained in time frames of minutes. From these measurements differences in the rate of uptake are evident. It is furthermore possible to distinguish two distinct stages in the adsorption process 9A rapid initial adsorption is ascribed to uptake in the inter-granular texture; subsequent adsorption then occurs within the ordered mesoporous intra-granular structure.
2. MATERIALS AND EXPERIMENTAL PROCEDURES
The mesoporous silica powders were kindly provided by Dr Ulla Junges. The MCM-41 sample (S1M), containing the largest mesopores, was prepared as previously described by Stucky and coworkers (8). A commercial copolymer (Pluronic-123) was used in the synthesis mixture during the hydrolysis of tetra ethyl orthosilicate (TEOS). High resolution field emission SEM (Hitachi $4200) showed the powder consisted of small small rhombohedral grains having a uniform shape and size (width-~ 0.25 ~tm and length 0.8 to 1.2 ~tm). Within the grains, the aligned cylindrical mesopores (diameter 6 nm), oriented parallel to the facetted surfaces of the grains, had a periodicity of-~ 12 nm. The other MCM-41 and MCM-48 samples were prepared using cationic n-alkyl-trimethyl ammonium surfactant templates (ClaTAB and CI6TAB), and contained pores of smaller diameter in the range 1.9 to 3 nm (see Table 2). Adsorption isotherms isotherms of nitrogen (77K) and benzene (310K) were measured as previously (9). For the S1 sample, the isotherms were of type IV, showing hysteresis loops, corresponding to capillary condensation in the larger mesopores. All the other samples (S2M, S3M and S4M) contained smaller mesopores, and the nitrogen adsorption/desorption isotherms were reversible and sigmoidal in shape. Surface and porous properties of different silica samples (Table 2) were derived from nitrogen adsorption data.
69 SANS measurements were made at HMI, Berlin, LLB, Saclay and ILL, Grenoble, using the V4, PAXE and D22 spectrometers respectively. In situ measurements were made on outgassed samples after exposure to different sorbates using two different procedures. In the first, the sample, at a temperature of 31 OK, was exposed to controlled relative vapour pressures, p/p0, which allowed the hysteresis loop to be scanned, as previously described (10). In the second, controlled doses of liquid sorbate were injected with a calibrated Hamilton micro-syringe through a silicone rubber septum into rectangular quartz (Hellma) cells (pathlength 2mm), containing precisely weighed amounts of the outgassed samples at 298K. Prior to dosing the samples were outgassed in-situ in the quartz cells at 398K for approximately 8 hours in a vacuum oven (- 104 torr) and closed-off with the septum cap before exposure to ambient atmosphere. By using the same mass and packing density in the cells for a given series of samples it was thus possible to make quantitative comparisons of SANS data as previously demonstrated (1). Table 2 Properties of Mesoporous Silica Samples. Sample S 1M MCM-41 Structure 6.4 Pore Size / dp* nm Pore Volume / 1.01 cm 3 g-1
S2M MCM-41 2.3
S3M MCM-41 3.0
S4M MCM-48 1.9
0.63
0.46
0.38
610
630
0.14 (0.25) 45
0.19 (0.36) 33
830 920 Specific Surface Area / m 2 0.065 Diffraction Peak, 0.17 Qmax/ A "I (0.13) (0.33) d-spacing / A 96 36 * Derived from N2 adsorption isotherms at 77K.
g-1
In the experiments described here the volume of liquid adsorbate which was injected was sufficient to saturate the intra-granular mesopores. This volume was calculated from a knowledge of the sample mass in the cell and the mesopore volume, derived from the nitrogen adsorption isotherms. (eg for the kinetic measurements with the S2M silica, this mass was 42mg for each of the two samples and the injected volume was 40 gl.) This volume was injected via the hypodermic needle, to the base of the cell which was below the zone of the incident neutron beam. The transmissions for the different samples was in a range of 0.7 to 0.9, being typical for such porous silicas in these standard quartz neutron scattering cells.
3. RESULTS AND DISCUSSION
The SANS curve for the evacuated S 1M sample shown in figure 1 is typical of that observed for the other ordered mesoporous silicas. This features Porod scattering ( viz. I(Q) oc Q-4 ) in the low Q range (where Q = 4nsin0/~,) and small angle diffraction features at higher Q, corresponding to the cylindrical pores ordered in a hexagonal array. Thus the peaks at 6.5 x 10 "2 and ~ 1.3 x 10"l A l can be ascribed to the (100) and (200) reflections associated with a hexagonal pore structure (space group p6mm) as described previously by Stucky et al (6) from XRD analysis. Here the (100) peak corresponds to a d-spacing of-~ 96 A "~ indicating a large
70 unit-cell parameter (a0---111 A). The positions of the first maximum and corresponding dspacings for the other silica samples are given in Table 2. We will consider the SANS in these two different Q regions separately. POROD SCATTERING REGION
[ 1000
DIFFRACTION REGION
100 I(Q) 10 1
o.i
O v
xd-"-
0.01 . . . . . . . . . . . . . . . . . . . 0.01
r
t..~ 0.1
0
0.0
015
1.0
t9
Q I A "1
Figure 1. SANS of evacuated mesoporous silica, S1M (dp - 6nm). SANS contains two components: (a) at low Q - Porod Scattering; (b) at high Q - Diffraction.
Figure 2. Dependence of I(Q) for sample S1M in the Porod Scattering region (at Q = 10-2A-l) on fractional volume filling of mesopores, O, with matched benzene.
3.1. Porod scattering The Porod scattering has been ascribed tentatively to the larger-scale granular texture of the samples (9). Further evidence for this comes from the evolution of the SANS during progressive isothermal adsorption (310K) of contrast matched benzene. Under these conditions the Porod scattering remains, but increases progressively as the relative pressure, p/P0, is increased. Such a feature corresponds with the progressive filling of the mesopores in the grains. We can consider this to occur as follows: Thus for Porod scattering from a two phase system we have (11, 12): I(Q) ~ S/V. Q-4 [pl.p212
(1)
Where Pl and 192are respectively the scattering length densities of the two phases and S/V is the surface to volume ratio of the system. In this case S/V corresponds to the surface to volume ratio of the MCM silica grains, which will remain unchanged during the progressive filling of the mesopores. Furthermore pl will correspond to the "effective" scattering density of the grains and P2 the large inter-granular pores ( p2 will be zero as these pores are unfilled with sorbate at this stage ). pl will be determined by that of the solid phase (Psilica) and the intra-granular pore space (pps) respectively, and will be given by Pl = PsilicaCsilica + Psorbatet~meso .l~
(2)
where (~silica and d~meso a r e the respective volume fractions of the solid and mesopores in the silica grains and O is the fractional filling of the mesopores with liquid sorbate at a particular p/p0. For the case where Psiliea = Psorbate , ( v i z . contrast matching) we then have for a fixed value of Q (in the Porod region) I(Q) oc [~silica+~mesoO] 2
(3)
71 This relationship is represented in figure 2, which shows the relative intensity of scattering, I(Q)rel compared with the evacuated sample ( as measured at Q = 10.2 A "1) for three successive pressures (p/p0 of 0.24, 0.54 and 0.80 respectively). The linearity is satisfactory over a range during which the scattered intensity increases by a factor of 6.3.
3.2. Low-angle diffraction Changes in the low-angle diffraction region during progressive uptake of sorbate were examined in detail with three different samples: S1M (benzene); S3M (cyclohexane); S4M (cyclohexane), having different MCM structures and pore sizes (cf. Table 2). For these three samples, the main diffraction peaks were observed at 6.5 x 10"2, 1.4 x 10"1 and 1.9 x 10"l A 1 respectively, and changes in the peak intensity were noted during the process of mesopore filling with matched benzene and cyclohexane respectively. Changes in the relative intensity compared to that of the evacuated sample, I(Q)rel, showed the same trend when determined as function of increasing volume fraction fillings of the mesopores, t9.: Firstly there was a significant, although modest, increase in intensity on initial adsorption which reached a plateau when the mesopore filling was in the region of 50%. This was followed after approximately 70% filling by a marked reduction in intensity which effectively approached zero at mesopore saturation. Other SANS measurements were made on a series of identical S3M samples to investigate the changes in the diffraction region resulting from a variation in the contrast of the cyclohexane filling the mesopores (viz for 19 = 1). These, measurements made with different volume fraction mixtures of C6D12 and C6nl2, are illustrated in figure 3. (The same sorbate injections were made in all cases, so as to saturate the mesopore volume). The intensity reaches zero at a composition of 50% C6D12, corresponding to effectively the contrast matching condition. The scattering length density of this composition is 3.21 x 10 l~ cm "2 and is close, although slightly less, to that calculated for amorphous silica (Table 2). Such a small negative difference could arise from a slightly lower silica density but might also occur if the cyclohexane molecule was inaccessible to the total porous structure (eg. micropores). Such differences have been noted previously with microporous oxides (1) although for mesoporous solids good agreement has generally been reported (1,3). More quantitative comparison is complicated by changes in the scattering in the Porod region (see discussion 3.1 above) which occur with the highly deuterated compositions. These changes can be ascribed to the differences in the effective contrast of the grains of MCM-41 when filled with different cyclohexane compositions. 4
3
(a)
(b)
(c)
(d)
(e)
(f)
~
.
.
(g)
~ 0.0
0,1
0.2 Q I A "~
0.3
0.4
0.1
0.2
0.3
Q / A "~
.
.
.
. QIA
O. "~
-
9 QIA
"~
,4
.
.
.
Q I A "1
.
.
.
. Q/A
. "~
Q I A "~
Figure 3. Contrast variation for S3M mesoporous silica samples. The mesopores in the different samples are filled with cyclohexane having the following isotopic compositions, % C6D12: (a) 0%; (b) 25%; (c) 45%; (d) 50%; (e) 60%; (f) 75%; (g) 100%.
72
3.2.1. Kinetics of adsorption Another application of the contrast variation technique concerns the kinetics of adsorption of different adsorbate molecules in ordered mesoporous silica. In particular we compare the uptake of n-hexane and cyclohexane in the silica S2M. S2M has the same MCM-41 structure as S1M and S3M (cf. Table 2) but a much smaller pore diameter (2.3 nm). The kinetic evolution of the SANS in the high Q diffraction region of S2M is shown in figure 4 after exposure to the saturated vapour pressure of matching cyclohexane (50% C6DI2) [sample A]. (Here a measured injection was again made so as to saturate the mesopore volume). Firstly we note that the first diffraction peak and the weak secondary maximum are now observed at much higher Q (0.17 and 0.31 A "l respectively) than for the S1M silica (figure 1). This indicates a correspondingly smaller unit-cell parameter (a0 - 42 A), which is in accord with the smaller pore diameter of the hexagonal structure. On exposure to matching cyclohexane the intensity of the diffraction peak is progressively reduced (figure 4). There is an apparent induction period of-~ 9 minutes and then a rapid reduction, which is virtually complete after 25 minutes. Also of note, particularly at higher Q, is the increase in the flat background. This is due to the incoherent scattering of the adsorbed cyclohexane arising from the protons in the molecule. [N.B. These have an extremely large incoherent scattering cross-section (C~incoh(H) 80 barns) compared with other nuclei (t~incohfor SiO2 is zero).] This incoherent background reaches a limiting value somewhat before the elimination of the diffraction feature. This can be ascribed to a process in which the cyclohexane is firstly adsorbed in the inter-granular space before subsequently diffusing into the ordered mesoporous structure- a process which depresses the diffraction feature.
Figure 4. Kinetic evolution of SANS at high Figure 5. Kinetic changes in the intensity of Q of Silica S2M after exposure to matched the diffraction peak of silica S2M samples after exposure to cyclohexane and n-hexane. cyclohexane (50% C6D12) at 289 K. (a.l) Sample A (evacuated) exposed to 50% D cyclohexane; (b.l) Sample B (evacuated) exposed to 50% D n-hexane; (a.2) Sample A (after (a. 1)) exposed to 100% D n-hexane; (b.2) Sample B (after(b. 1) exposed to 100% D cyclohexane. A similar SANS experiment was also made with n-hexane (50% C6D14) [sample B]. Here the rate of adsorption, as determined from the suppression of the diffraction peak, was markedly faster. This is illustrated in figure 5, by the time dependence of changes in intensity of the diffraction peaks for both samples A and B. Thus for sample B there was no apparent
73 induction period, and the uptake appears to have reached completion between 7 and 11 minutes. (N.B. For sample B a constant residual diffraction feature remains after 11 minutes because total contrast matching was not achieved at the 50% D isotopic composition with nhexane as compared to cyclohexane. This is due to a small, but significant, difference in the scattering length densities of the two alkanes: 3.21 and 2.81 x 10 ~~ cm "2 for 50% D cyclohexane and n-hexane respectively; cf. Table 2.) The smaller kinetic diameter of n-hexane (4.3 A "l compared to 6.0 A "l for cyclohexane) may partly account for the faster diffusion into the cylindrical mesopores of the MCM-41 structure, although other effects due to differences in the confinement and specific adsorption of the hexane isomers may play a role. 3.3.2. Kinetics of counter-diffusion and displacement Further SANS measurements were also conducted on these two SM2 samples, loaded with 50% D cyclohexane and 50% D n-hexane ( Samples A and B ). These measurements were designed so as to compare the rates of displacement of the two isomers when further exposed to 100% D n-hexane and 100% D cyclohexane respectively. By comparing the kinetic changes in the diffraction peaks, after exposure of equivalent volumes of the two different isomers, insight into the counter-diffusion and mixing processes was obtained. It can be shown that for the condition of complete mixing (viz. equivolumes of (a) 50% D cyclohexane + 100% D nhexane [Sample A] and (b) 50% D n-hexane + 100% D cyclohexane [Sample B]) the scattering length density in both cases will be almost identical, viz. 4.70 and 4.75 x 101~ cm 2 respectively. This would imply that the intensity of the diffraction peaks in the two experiments would be the same, if there was complete mixing, and the partitioning of the two isomers was equivalent in the mesopore space of the two identical MCM samples. The results of this further experiment are also illustrated in figure 5. For Sample A there is only a slow and small increase in the diffraction intensity on exposure to 100% D n-hexane. This implies that there is little displacement of the cyclohexane adsorbed in the mesopores by n-hexane in the vapour phase. In contrast, when Sample B is exposed to 100% D cyclohexane, there is initialy a rapid reduction in intensity to zero, followed by a steady increase, which then exceeds that of Sample A. This implies that the nhexane in the mesopores exchanges more rapidly with the cyclohexane in the vapour phase. The initial reduction in intensity is caused by the increase in the scattering length density of the sorbed phase as the 100% D cyclohexane is exchanged. At the beginning, the scattering density of the sorbed n-hexane is not completely matched, being less than that of the silica matrix. However after approximately 20 minutes of exposure, the matching condition is reached and thereafter the intensity begins to increase. These conclusions can only be tentative with the limited data here. However the potential scope for future studies of the kinetics of adsorption and partitioning of mixed sorbates is evident. This is of particular interest with microporous sorbents, such as carbons, especially where molecular size effects may be predominant. Such an aspect is indeed particularly relevant in gas separation processes which may involve membranes and pressure swing adsorption.
4. REFERENCES 1. 2. 3. 4.
J.D.Ramsay and G.Wing, J.Coll. Int. Sci., 141 (1991) 475 E.Hoinkis and A.J.Allen, J. Coll. Int. Sci., 145 (1991) 540 J.C.Li, D.K.Ross, M.J.Benham, J.Appl. Cryst., 24 (1991) 794 D.W.Hua, J.V.D.Souza, P.W.Schmidt, D.Smith, Stud. Surf. Sci. Catalysis, 87 (1994) 255
74 5.
J.D.F.Ramsay and E.Hoinkis, in , Royal Society of Chemistry, London, 1997, p.33 6. J.D.F.Ramsay and E.Hoinkis, Physica B 248 (1998) 322 7. See eg. J.D.F.Ramsay in "Handbook of Porous Solids", F.Schtith, K.S.W.Sing and J.Weitkamp (eds.), Wiley-VCH, pp. 135-181 (in press) 8. D.Zhao,. J.Feng, Q.Huo, N.Melosh, G.H.Fredrikson, B.F.Chemelka, G.D.Stucky, Science, 279 1998) 548 9. J.D.F.Ramsay, S.Kallus, E.Hoinkis, Studies in Surface Science and Catalysis, 128, K.K.Unger et al. (eds.), 2000, p.439 10. E.Hoinkis, Langmuir, 12 (1996) 4299 11. G.Porod, Kolloidn. Zh., 124 12. A.Guinier and A.Fournet, "Small Angle Scattering of X-rays", Wiley, New York, 1955
5. A C K N O W L E D G E M E N T S We are indebted to Dr. U. Junges for kindly providing the mesoporous silica samples, to Mr. G. Nabias for the SEM measurements and Drs. Th. Steriotis and E.S. Kikkinides for benzene isotherm data. Technical support at the neutron scattering facilities at ILL, Grenoble, LLB, Saclay and BENSC, Berlin, together with the help and co operation of Drs. P. Timmins, L. Auvray and E. Hoinkis are gratefully acknowledged.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
75
Preparation And Surface Characterisation Of Novel Ceria-Copper And Ceria-Manganese Mixed Oxides Mafia Chdstophidou, and Chaffs R. Theochafis*, Porous Solids Group, Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus The paper presents surface texture and catalysis results for a series of copper and manganese-containing ceria samples. These samples, especially the 20% manganese containing one, had significantly higher surface area than the pristine ceria. The NO denox performance of the 20% copper containing sample indicated high selectivity towards nitrogen formation. 1. INTRODUCTION There has been much recent interest in the preparation of inorganic solids with controlled porosity in the microporous and mesoporous range, or with other desirable surface properties. The interest has somewhat shifted in the recent few years away from microporous towards mesoporous materials not least because of the increased interest in the use of porous materials in air or water pollution control. Ceria (CeO2), in particular, has attracted much interest because of its use as an additive in the socalled triodic automobile exhaust catalyst [1,2], but also because of its use as a catalyst in its own right, or as a catalyst support. Ceria acts as an excellent oxygen store [3-5] in the catalyst, which is thus rendered a very effective catalyst for combustion [6]. Moreover, addition of ceria to the automotive exhaust catalysts minimises the thermally induced sintering of the alumina support and stabilises the noble metal dispersion [7]. Ceria also enhances nitric oxide dissociation when added to various supported metal catalysts [8], which is another important function of the automotive exhaust catalyst. Recent investigations by Harrison et al have shown that ceria doped with certain lanthanides and promoted with copper and chromium have catalytic activities comparable to that of the noble metal catalysts [9]. In addition to the reactions described above which relate to the internal combustion engine emissions questions, the catalysed low temperature oxidative coupling of methane, the water gas shift reaction and many other catalytic reactions are also promoted by ceria [10-12]. A study of alkali and alkaline earth metal doped ceria
76 catalysts has shown that barium or calcium doped ceria were the most active catalyst for the oxidative coupling of CH4 [13]. Zhang and Baerns explained the observed dependence of C2 selectivity on the Ca content in terms of oxygen-ion conductivity [13]. Because of the promoting effects of ceria in many catalytic reactions, the preparation of high surface area and thermally stable ceria phases as well as the study of the parameters which control the structural, textural and redox properties of the material are of particular interest [14-15].
Ceria or metal supported on ceria have
been found to catalyse such diverse reactions as methanol oxidation to CO and HE [ 16], CO oxidation to CO2 [ 17], and reduction of NO and N20 to N2 [ 18]. In addition, several reports have appeared on the catalytic activity of mixed metal-cerium oxides.
At the University of Cyprus there is a long-standing interest in the synthesis and study of porous ceria [ 19-21 ]. Work so far has concentrated studying ways of enhancing and stabilising higher porosity, as well as studying the chemistry of the surface. Two strategies have been used, the insertion of controlled amounts of dopants on the one hand, and the use of organic matrices on the other. The mesoporosity of these solids is not intra-crystalline in the sense that the porosity of zeolites is, but depends upon aggregation of primary particles, and as such is susceptible to synthesis conditions such as pH and concentration of the cerium and other cations precursors used. In the present paper we present the results of an investigation into the use of copper and manganese ions as dopants. Copper was used because of reports in the literature about the catalytic potency of copper bearing ceria [22,23], and manganese because of it possesses a number of stable oxidation states. 2. EXPERIMENTAL
The chemical reagents used for the preparation of stock solutions were reagent grade and were used without further purification. The chemicals were in the form of the nitrate salts and were obtained commercially from Aldrich or Fluka. Pure ceria samples were prepared by precipitation of ceria from aqueous solutions containing 0.01M Ce 4+ by adding 1 M NH3 solution. The precipitate was dried at 523K. The surface acid sites concentration was measured using Hammett titrations, with phenolphthalein as indicator, and a 0.1M NaOH solution as base. Mixed oxide samples were prepared by the co-precipitation from aquatic solutions of the cations ([Ce(IV)] = 0.01 M) by adding equal volume of 1 M NH3 solution under
77 controlled conditions. The surface properties were investigated by nitrogen adsorption isotherm analysis and FTIR spectroscopy. The nitrogen adsorption isotherms were measured at 77 K using an ASAP 2000 analyser (Micromeritics) after outgassing the samples under vacuum (0.15Pa) at 423K. FTIR spectroscopy was carried out with a Shimadzu spectrophotometer (FTIR-8501) using both KBr and the DRIFTS method. The total pore volume referred to below, corresponds to the pore volume measured from the nitrogen isotherm, at p/p~
Thermogravimetric recordings were
obtained using a Shimadzu TGA apparatus in flowing air. The catalytic efficacy of ceria samples was measured vis ~ vis to oxidation of NO to N2. A continuous flow method was used, and measurements were carried out at various catalyst temperatures. The gaseous mixture used was 0.25% NO, 1% H2, 5% 02 and the rest helium. Mass flow controllers were used to control concentrations. The effluent gas synthesis was determined by quadrupole mass spectrometry. A Baker mass spectrometer was employed. 3. RESULTS Table 1 shows the adsorption isotherm data and acidity measurements for the samples under examination. Figures 1 and 2 show the nitrogen adsorption isotherms for the copper-containing and manganese-containing samples respectively. TABLE 1 Adsorption and Surface Acidity Measurements Pore Surface Acid Real Cu Content by Volume Sites Concn ICP (%) (cm3g"1) (mol/100g)
Sample
SBET (m2g-1)
CeO2 2% Cu 8% Cu 20% Cu 2% Mn 20% Mn
128 88 63 54 113 232
0.08 0.13 0.12 0.11 0.19 0.31
1.73 1.604 1.662 3.845 3.974
0 0.34 1.34 2.97 0 0
Figure 3 contains a graph showing the variation in N2 selectivity during catalysis runs for a variety of CuxCel_xO4 solids, and Figure 4 the corresponding percent conversions, as a function of temperature. Figures 5 and 6 contain the FTIR spectra for CuxCel_xO4and MnxCel.xO4 solids respectively.
78 4. Discussion
Examination of Figure 1 reveals that the FTIR spectrum of ceria and mixed metal oxide-ceria solids has three main features: first a manifold of broad bands centred 2%C u, B%C u & 2 0%C u
['
.
(,,)(' ' [ )2o.(c,,8" 2.,,c. . oc,,)
'
(c) I.-
(b) (a) ,
4000
3100
,
.
,
2 200
I
llcm
3'o0
'
' 400
Figure 1 FTIR spectrum of CeO2 containing (a) 2%, (b) 8% and (c) 20% Cu 2+ Around 3100 cm-1 second a band at 1630 crnl which is attributable to H-O-H bending mode and is indicative of the presence of molecular water in the sample, and thirdly a sharp peak at 1383 cm-1 which has been attributed to Ce-OH stretching and superimposed to higher and lower wavenumbers by a broad band, attributable to
2%Mn(d1) & 20%Mn(cl)
~ I-.-
~
(b)
f \ (a)
4000
I
I 31 O0
I
I 2200
I
I 1300
llcm Figure 2 FTIR spectrum of CeO2 containing 2%, and 20% Mn 2§
I 400
79 Ce-O-Ce modes which are characteristic of the fluorite structure of ceria [20]. The FTIR spectrum of pure CeO2 is characterised by a broad band at 1383 cm-1 which is narrower than that found in mixed oxide spectra, as that shown in Figure 1. This suggests that inclusion of the hetero-atoms in the ceria structure leads to a distortion of the lattice. No extra bands were observed in the FTIR spectra on inclusion of the hetero-atoms. Comparison of X-ray dit~actogrammes for pure and doped ceria, has indicated that there is a single phase, with cell parameters which are very similar to those of the pure phase. This apparent lack of distortion can be explained by the fact that the copper ion content found by ICP (see Table 1) was significantly lower than that in the mother liquor, and reached a maximum of only 2.97%. The significant differences between the spectra in Figure 1 were in the OH stretching frequencies: in the spectrum of pure ceria, only one band is observed [20] at 3400 cm-1, whereas the mixed copper-cerium oxide samples had three distinct peaks, at 3020, 3130 and 3400 cm-1. The fact that the intensity of the H-O-H bend band at 1640 cm-1 was very similar in the mixed oxide and pure ceria spectra, indicated that the differences between the pure and mixed oxides in the OH stretching bands, was due to differences in surface OH bands, attributable to Cu-OH groups, and may indicate that the heteroatoms are present as a dispersed microphase within the pores of the ceria phase.
In similar fashion, the FTIR spectra for Mn 2+ containing samples in Figure 2, showed that the samples showed two OH stretching bands at 3200 and 3400 cm-~, the latter band being attributable to OH stretch in molecular water. The differences between the spectra for CeO2, CuxCel.xO2 and MnxCel.,,O2 suggests that the extra bands are due to OH stretch of groups which are linked to the hetero-atom, suggesting the presence of the dispersed microphase. The differences in the surface acid sites concentration between the two mixed oxides (Table 1) confirms the presence of these surface groups. It is noteworthy that cation concentration had no effect on the acid site concentration, corroborating the probability o f a microphase. Figures 3 and 4 s u ~ i s e
the catalysis experiments results for the mixed copper-
cerium oxide and pristine ceria samples for the NO denox reaction. It can be seen from Figure 3 that the selectivity towards N2 was very high, and for one of the samples higher than for pristine ceria. However, as it can be seen from Figure 4 the
80 conversion rate achieved for these samples was not high, and significantly lower than those achieved by pristine ceria. lee r o
,m,
m
=em
4-W
u o m o (/)
Z
4=
.=..
60
o c
~J 2o
31
o
eM
(.I
ze=
.e
=o=
.o
Temperature
=o
do
see
I=
o Z
(~
e
'~ee
25e
lIQ
Temperature
350
415
4SO
see
(OC)
Figure 3 Figure 4 Nitrogen selectivity in NO denox NO conversion for CuxCel_xO2samples reaction for CuxCe].xO2 samples (In both diagrams: O: 2% Cu, A 8%, square: 15% and inverted triangle, 20%)
100
, , = -=----
80
E6o
Vads-2%Cu Vdes-2%Cu Vads-8%Cu Vdes-8%Cu Vads-20%Cu Vdes-20%Cu
|
J
............
rt
e~
E
i
I
..........................
'i
o40
>o20
0
0 p/po0 dg
Figure 5 Nitrogen adsorption isotherms for CuxCel.xO2samples Figure 5 contains the nitrogen adsorption isotherms for the 20% Cu containing sample, whilst the surface properties of all samples are summarised in Table 1. The isotherms are of Type IV with hysteresis loops of the H4 type. There are no significant differences in the shape of the isotherm or the hysteresis loop, but as observed for other mixed oxides, there was a decrease in the SBET values for the
81
mixed oxide compared with the pure oxide, whilst there is a significant decrease in
SBET values with increasing copper ion inclusion. On the other hand, there is an increase in apparent pore volume for the mixed oxides compared with the pristine oxide. Significantly, there is no change with increasing hetero-atom concentration.
250
Vads-2%Mn Vdes-2%Mn Vads-20%Mn Vdes-20%Mn
200
! i t !
!
i
i
] l
>o
50
o
Figure 6 Nitrogen adsorption isotherms for
Mno.20Ceo.8oO2
samples
Figure 6 shows the nitrogen adsorption isotherm for the 20% manganese containing samples. It is noteworthy that these mixed oxides have a much higher apparent pore volume than the pristine sample. The solid with a 2% manganese content has a BET surface area comparable with the pristine sample, but the sample with 20% manganese has a significantly higher BET surface area (Table 1). These observations combined with those from the FTIR spectra suggest that the presence of the heteroatoms leads to a distortion of the lattice. The significantly higher value of surface acid sites concentration for the MnxCel.xO2 samples compared to the CuxCel.xO2 ones may be partly due to the higher apparent surface area for the former compare to the latter, but presumably is also linked to the differences in polarisation of the Mn-OH and CuOH groups, as well as the effect of the presence of the hetero-atoms to the properties of the Ce-OH groups. The significant increase in apparent surface area achieved for ceria with the controlled incorporation of hetero-atoms, as well as the interesting
82 catalytic activity exhibited by these solids, augurs well for the development of novel ceria-based solids with interesting properties. Acknowledgement We thank Dr I. Pashalidis and Ms G. Kyriakou for useful discussions, and the University of Cyprus and the European Union for financial assistance.
References 1. G.J.K. Acres, in "Perspectives in Catalysis", (Eds J.M. Thomas and K.I.Zamaraev), Blackwell Scientific Publications, London, (1992) 2. J.G. Nunan, H.J. Robota, M.J.Cohn and S.A. Bradley, J. Catal., 133 (1992) 309 3. D. Terribile, A. Trovarelli, C. de Leitenburg and G. Dolcetti, Chem. Mater., 9 (1997) 2676 4. T. Bunluesin, R.J. Gorte and G.W. Graham, Applied. Catal. B-Environmental, 15 (1998) 107 5. V.P. Zhdanov and B. Kasemo, Applied Surface Science, 135 (1998) 297 6. Se H. Oh and C.C. Eickel, J. Catal., 115 (1988) 543 7. Thi X.T. Sayle, S.C. Parker and C.Richard A. Catlow, J. Phys. Chem., 98 (1994) 13625 8. A. Takami, T. Takemoto, H. Iwakuni, K. Yamada, M. Shigetsu, and K. Komatsu, Catal. Today, 35 (1997) 75 9. P.G. Harrison, W. Azelee, A.T. Mubarak, C. Bailey, W. Daniell, Studies in Surface Science and Catalysis, 116, 495, (1998) 10. W. Liu, C. Wadia, and M. Flytzani-Stephanopoulos, Catal. Today, 28 (1996) 391 11. P.K. Rao, K.S.R. Rao, S.K. Masthan, K.V. Narayana, T. Rajiah, and V.V. Rao, Applied. Catal. A-General, 163 (1997) 123 12. A. Naydenov, B. Stoyanova, and D. Mehandjiev, Journal of Molecular Catalysis A-Chemical, 98 (1995) 9 13. Z.-L. Zhang and M. Baems, J. Catal., 135 (1992) 317 14. J.Z. Shyu, W.H.Weber and H.S.Gandhi, J. Phys. Chem., 92 (1988) 4964 15. D. Terribile, A. Trovarelli and G. Dolcetti, J of Catalysis., 178 (1998) 299 16. W.J. Shen and Y. Matsumura, Phys Chem Chem Phys 2,1519, (2000) 17. S.H. Overbury, D.R. Mullins, and D.R. Huntley, J Catal 186, 296, (1999) 18. E.S. Putna, R.J. Gone, J.M. Vohs, J Catal 178, 598, (1998) 19. I. Pashalidis and C.R. Theocharis, Second European East West Workshop on Chemistry and Energy, Sintra, Portugal, March 1995 20. I. Pashalidis, and C. R. Theocharis, in (Eds K.K. Unger, G. Kreysa and J.P. Baselt), "Studies in Surface Science and Catalysis", Elsevier Science Publishers, 128, 643- 652, (2000) 21. G. Kyriakou, I. Paschalidis and C.R. Theocharis, Fundamentals of Adsorption, 7, In the Press 22. A.Martinez-Arias, M. Femandez-Garcia, J. Sofia, J Catal 182, 367, (1999) 23. J. Soria, J.C. Conesa, A. Martinezarias, Solid State Ionics 63-5, 755, (1993)
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
83
A b o u t the exclusive m e s o p o r o u s c h a r a c t e r of M C M - 4 1 A. Berenguer-Murcia 0), j. Garcia-Martinez (1), D. Cazorla-Amor6s Aionso -3
~9 ~
~
~
--..,,,,
CO
2
~4
,
800
1000
I
I
I
)2(kJim~ ~ ~ .
>-3
600
c02
E J - 4
-5 -6
Figure 5. Characteristic curves for N2 and CO2 on PS
-7
Figure 6. Characteristic curves for N2 and CO2 on MCM-41.
If we draw the characteristic curve for N2 at 77 K and CO2 at 273 K (at subatmospheric and high pressures), both adsorptives fit the same curve for both materials for (A/13)2 values lower than 150 KJ/mol, clearly indicating that both adsorptives follow the same adsorption mechanism. If the N2 adsorption data obtained at low relative pressures (10 -6 to 10"2) are plotted it is clear that the resulting characteristic curve falls below the one corresponding to CO2. This observed diIfusional problems of N2 molecules to enter a part of the porosity of MCM-41 at 77 K must be due to the presence of narrow micropores (150~) and cement-like materials (_ -10 J In this contribution we -20 n," have evaluated the -30 ' ' importance of incorporating 110 120 170 130 140 150 160 the measured contact angle 0 / degrees (by advancing and static angle methods) in the textural Fig. 1. Relative deviation in the calculated characterization of porous porediameter dporeas a function of the contact angle materials by MIP. 0 (reference value 140~ i
,
J
I t
i
,
i
,
,
2. E X P E R I M E N T A L 2.1. M a t e r i a l s Different porous materials, including oxides (a-AleO3, SiOe, TiOe), zeolites (NaZSM-5, H-ZSM-5, H-beta, Na-Y, and H-USY), carbon (activated carbon, Norit, and mesoporous carbon (Novacarb from MAST Carbon Ltd.)), cement-like materials (enci and tras, with low and high lime content, respectively), and catalysts (1 wt.% Pt/A1203, 5 wt.% Pt/A1203, and 16 wt.% Ni/A1203) were used, as well as home-made samples (A1203, MCM-41, and SBA-15).
93
2.2. C h a r a c t e r i z a t i o n
2.2.1. Contact angle measurement The contact angle of mercury on the various materials was m e a s u r e d using two different methods. Prior to the pellet preparation, the samples were milled to obtain a particle size < 10 pm. 9 Advancing angle method, using a Quantachrome contact anglometer (Fig. 2a) [9]. Approximately 30-60 mg of powder was brought into the sample holder and pelletized (pressure applied is 12 MPa) around a pin having a well-defined diameter. In this way a hole of known diameter (0.813 mm) is obtained in the pellet. Subsequently, the pressure difference needed to force a mercury droplet (50 pl) through the hole is measured. From this result the advancing contact angle is calculated by application of eq. (1), since the pressure, pore diameter and surface tension are known and the remaining variable cos 0is obtained. 9 Static method, using a Krfiss G-1 optical microscope equipped with a goniometer (Fig. 2b) [10]. Approximately 100 mg of the powder is compressed into a pellet at a pressure of 1000 MPa. Subsequently a mercury droplet of 26 ~1 is placed on the pellet and a goniometer in the ocular of the microscope is then used to visually determine the specific contact angle (so-called static contact angle).
Fig. 2. Schematic representation of the (a) advancing angle and (b) static angle method for determination of the contact angle 0.
2.2.2. Mercury intrusion porosimetry (MIP) MIP experiments were performed on CE I n s t r u m e n t s PASCAL 140 and 440 porosimeters, which operate in the pressure range of vacuum to 400 kPa and 100 kPa to 400 MPa, respectively. Prior to the intrusion experiments the samples were degassed in vacuum at 625 K for 16 h. The PSD was determined from the W a s h b u r n equation, taking the surface tension of mercury being 480 N.m -1. The contact angle was experimentally determined as described above.
94
2.2.3. N2 physisorption N2 a d s o r p t i o n m e a s u r e m e n t s a t 77 K w e r e p e r f o r m e d on a Q u a n t a c h r o m e A u t o s o r b - 6 B gas a d s o r p t i o n a n a l y z e r . P r i o r to t h e a d s o r p t i o n m e a s u r e m e n t s t h e s a m p l e s w e r e d e g a s s e d in v a c u u m a t 625 K for 16 h. A full ad- a n d d e s o r p t i o n i s o t h e r m w a s m e a s u r e d in t h e r e l a t i v e p r e s s u r e (p.p0 -1) r a n g e of 0.1-1. T h e P S D w a s d e t e r m i n e d from t h e B r o e k h o f f - d e B o e r (BdB) m o d e l a p p l i e d to t h e d e s o r p t i o n b r a n c h of t h e i s o t h e r m [3].
3. R E S U L T S AND D I S C U S S I O N 3.1. D e t e r m i n a t i o n of the c o n t a c t angle T h e c o n t a c t a n g l e m e a s u r e m e n t s on t h e v a r i o u s m a t e r i a l s a r e p r e s e n t e d in T a b l e 1. T h e r e s u l t s of t h e a d v a n c i n g a n g l e m e t h o d a r e v e r y s i m i l a r to t h o s e determined with the static angle measurement.
Table 1. Average contact angles of the samples investigated, as m e a s u r e d with the advancing angle and static angle method. ........................................
Method ~
Advancing angle 0 +SD 1
Materia 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Static angle 0 +SD
a-A120~
degre e 138 + 0.2
. . . . degree 137 + 0.3
A120~
141 + 0.2
141 + 0.7
Amorphous Si02
141 + 0.2
140 + 0.9
n.d. 2
140 + 0.3
TiO2 1 wt.% Pt/A1203
142 + 0.4
143 + 0.8
5 wt.% Pt/A1203
136 + 0.4
138 + 0.3
16 wt.% Ni/A1203
139 + 0.2
139 + 1.3
H-ZSM-5, Si/Al=15
138 + 0.8
n.d.
Na-ZSM-5, Si/AI=40
140 + 0.7
140 + 0.7
H-beta, Si/Al=12.5
128 + 0.4
n.d.
H-beta, Si/Al=150
127 + 0.2
n.d.
Na-Y, Si/AI=2.5
132 + 0.2
n.d.
H-USY, Si/Al=16
138 + 0.5
n.d.
C e m e n t enci (low lime content)
128 + 0.9
129 + 0.7
C e m e n t tras (high lime content)
129 + 0.4
n.d.
Novacarb mesoporous carbon
150 + 0.4
151 + 0.7
Activated carbon
155 + 0.4
155 + 0.6
Norit
163 + 1.1
n.d.
1 Values for 0 and S~ ~(stan~ measurements. 2 n.d." not determined.
on three r e p e t i t i v e
95 The p r i m a r y particle size of the powdered samples is to a certain extent of minor importance on the measured contact angle. The high pressure applied (101000 MPa) upon pelletizing causes the original particle size and shape to be distorted in such a way t h a t a very smooth surface and a well-defined hole (advancing angle method) is obtained. This has been supported by Scanning Electron Microscopy m e a s u r e m e n t s of the powders and pellets (see e.g. Fig. 3 for a Ni/A1203 catalyst).
Fig. 3. SEM micrographs of a 16 wt.% Ni/A1203 (a) powder and (b) pellet. The contact angle for most metal oxides, including A1203 and SiO2, as well as for supported catalysts is close to the presumed 8= 140 ~ In zeolites contact angles range from 127 ~ to 140 ~, and assuming 140 ~ may lead to erroneous interpretation if meso- or macroporosity determinations of these materials are required [11]. The zeolite framework type seems to be the most i m p o r t a n t variable influencing the interaction between mercury and the solid material. The most striking results are found for cement-like materials and carbonaceous materials, which show a relatively low (< 130 ~ and high (> 150 ~ contact angle, respectively. From this perspective, a more detailed study on a well-defined mesoporous carbon (MAST Novacarb) and metal oxide (A1203) is performed to verify the influence of the measured contact angle on the calculated pore size and finally the correlation between the MIP and N2 physisorption techniques.
3.2. Mercury intrusion porosimetry The intrusion curves on both A1203 and Novacarb show a well-defined step as a result of mercury filling the pores in a narrow pressure range (Figs. 4a and 4b). The inflection point in the intrusion curve is found around 200 MPa for A1203 and 100 MPa for the mesoporous carbon, indicating a smaller average pore size is found in A1203 t h a n in the carbon. The intrusion at pressures < 1 MPa in Fig. 4b is due to filling of interparticle voids between the carbon particles and results in their redispersion. Both materials exhibit an extrusion curve, parallel to the
95 0.5
0.45
(
(b)
0.4 "T 03
,~.030.30
,4 0.3
E 0
,-0.2 - 0.15 0.1
1
10
100
0.00 0.01
1000
p / MPa
0.1
1 10 p / MPa
100
1000
F i g . 4. M e r c u r y i n t r u s i o n a n d e x t r u s i o n curve on (a) A120:~ a n d (b) N o v a c a r b . O p e n s y m b o l s : i n t r u s i o n curve, solid symbols: e x t r u s i o n curve.
intrusion curve and a final volume decrease close to the total i n t r u d e d volume, indicating t h a t most of the mercury in the porous structure is released upon pressure decrease. Re-intrusion experiments on the same samples give similar results, suggesting no significant modification of the porous structure upon pressurizing and depressurizing.
3.3. N2 physisorption N2 adsorption m e a s u r e m e n t s at 77 K on both the A1203 and Novacarb samples show a type IV isotherm with a type A hysteresis loop, indicative of the presence of mesopores with cylindrical pore geometry (Figs. 5a and 5b) [1]. The almost vertical capillary condensation step indicates a relatively narrow distribution of mesopores. Application of the Broekhoff-de Boer (BdB) model leads to a n a r r o w PSD centered around 8 and 15 nm for A1203 and carbon, respectively. 450
350
J
(a)
300
a..
n
I-- 250 0o
300
V
03 200
03
I
`4 E
co
E ~ 150 -8
0
.8 150
~'~ lOO 50 0
0.0
0.2
0.4
0.6
p.po -1/-
0.8
1.0
0
0.0
0.2
0.4
0.6
p.po -1 I-
Fig. 5. N2 physisorption isotherm at 77 K on (a) A1203 and (b) Novacarb. Open symbols: adsorption isotherm, solid symbols: desorption isotherm.
0.8
1.0
97
3.4. Comparison of t e c h n i q u e s and evaluation Fig. 6a shows the good correlation between the PSD derived from MIP and N2 adsorption on A1203. No incorporation of the m e a s u r e d contact angle has to be implemented, since the contact angle of this sample is very close to the oftenp r e s u m e d 140 ~ However, the carbon sample (Fig. 6b) initially shows a discrepancy between the PSDs of both techniques, where MIP results predict a ca. 15% smaller pore size t h a n the N2 adsorption results. Incorporation of the appropriate contact angle for this sample (8= 150 ~ shifts the PSD towards the N2 adsorption results and both PSDs show an excellent agreement.
(b)
(a)
r
0
5
10 dpore /
15 nm
20
5
10
.
15
dpore /
.
.
.
.
20
_-,
25
nm
Fig. 6. Comparison of the pore size distribution on (a) A1203 and (b) Novacarb: (A) MIP, with 8 = 140 ~ (o) MIP, with 8 = 150 ~ (m) N2 physisorption, using BdB model. For cement-like m a t e r i a l s an overestimation of the pore size is expected, since the contact angle of _< 130 ~ is much lower t h a n the s t a n d a r d value of 140 ~ However, the availability of cement-based materials with an ordered pore structure in the appropriate pore size range is limited and as a consequence these results cannot be discussed here. The presence of pore network effects and interconnectivity can of course strongly influence the reliability of the data obtained via both techniques. Therefore development of new models describing these effects combined with the m e a s u r e m e n t of the contact angle will strongly contribute to assess the PSD from MIP m e a s u r e m e n t s . Additionally one should realize t h a t pore geometry is an i m p o r t a n t a s s u m p t i o n in the underlying models used in both techniques. A pore geometry different from the p r e s u m e d cylindrical shape (in W a s h b u r n equation, eq. (1)) would result in severe deviations in the calculated pore diameter. From this point of view, model materials like MCM-41, and SBA-15, showing a hexagonal a r r a n g e m e n t of almost cylindrical pores, should be interesting to evaluate MIP results. However, upon pressurizing the porous structure collapse, as can be derived from a relatively flattened intrusion curve and a relatively broad PSD (not shown here).
98 The extrusion curve for these materials hardly shows any release of mercury and reintrusion experiments exhibit a negligible intrusion, indicating the original hexagonal porous structure is damaged and as a consequence these materials are inappropriate as model materials in MIP.
4. C O N C L U S I O N S Incorporation of the measured contact angle in mercury intrusion porosimetry data is essential for an accurate determination of the pore size distribution. Both the advancing and static angle methods are suitable to carry out this measurement, leading to very similar results. For most oxidic materials and supported oxides, the contact angle is -140 ~ and incorporation of the actual contact angle is less critical in the pore size determination. However, important deviations are observed in carbon and cement-like materials, with contact angles of__ 150 ~ and __ 130 ~ respectively. This has been shown by comparison of the pore size distribution obtained from mercury porosimetry and N2 adsorption measurements.
Acknowledgements The authors t h a n k MAST Carbon Ltd. for providing the Novacarb mesoporous carbon. V. Butselaar and R. Pavan are gratefully acknowledged for performing the SEM and contact angle measurements, respectively. S. Brouwer is t h a n k e d for fruitful discussions.
REFERENCES
[1]
[2] [3]
[4] [5] [6] [7] is] [9]
S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, 2 nd ed., Academic Press, London, 1982. J. van Brakel, S. Mod:~ and M. Svatfi, Powder Technol., 29 (1981) 1. J.C. Groen, M.C. Doorn and L.A.A. Peffer, in D.D. Do (Ed.), Adsorption Science and Technology, World Scientific, Singapore, 2000, p. 229. M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 13 (1997) 6267. P.I. Ravikovitch, G.L. Haller and A.V. Neimark, Adv. Colloid Interface Sci., 76-77 (1998) 203. K.L. Murray, N.A. Seaton and M.A. Day, Langmuir 15 (1999) 8155. S.P. Rigby, J. Colloid Interface. Sci., 224 (2000) 382. A.W. N e u m a n n and R.J. Good, Surface and Colloid Science, Vol. 11, Plenum Press, New York, 1979, chapter 2. S. Lowell and J.E. Shields, Powder Surface Area and Porosity, 3 ~d ed., C h a p m a n and Hall, London, 1991, p. 225.
[10] http://www.erma.co.jp/ENG/pageO2e/O204e/O204e.htm. [11] J.C. Groen, J. P~rez-Ramirez and L.A.A. Peffer, Chem. Lett., 2002, 94.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
99
A Two-Stage Horvath-Kawazoe Adsorption Model for Pore Size Distribution Analysis Robert J. Dombrowski and Christian M. Lastoskie* Department of Chemical Engineering Michigan State University East Lansing, MI 48824-1226 USA The Horvath-Kawazoe (HK) method is capable of generating model isotherms more efficiently than either molecular simulation (MS) or density functional theory (DFT) to characterize the pore size distribution (PSD) of microporous solids. A two-stage HK method is introduced that accounts for monolayer adsorption in mesopores prior to capillary condensation. PSD analysis results from the original and two-stage HK models are evaluated. 1. INTRODUCTION Porosity and pore structure are properties that control diffusive transport, selective reaction, and sorption-based separations of gases in adsorbents and catalysts [1,2]. Sorption porosimetry may be used to characterize the porosity of both mesoporous and microporous solids. The pore size distribution F(H) is obtained from the experimental isotherm/-(P) F(P) = .~ F(P, H)F(H)dH
(1)
using an assumed thermodynamic pore filling model F(P,H) that relates the specific excess adsorption at bulk pressure P in pores of physical pore width H. The PSD obtained depends on the choice of the thermodynamic model used to solve equation (1). Pore filling models based upon the Kelvin equation are not generally applicable for adsorbents with micropores or small mesopores, where continuum thermodynamic models do not accurately represent the properties of the confined fluid [1,3,4]. For example, the Barrett-Joyner-Hallenda (BJH) method becomes inaccurate for pores smaller than 20 nm, while the Derjaguin-Broekhoff-de Boer (DBdB) method yields significant errors for pores smaller than 7 nm [5]. The HK method yields an analytic pore filling correlation calculated semi-empirially from an estimated mean free energy change of adsorption that accounts for adsorbateadsorbent interactions. In the original HK approach [6], the pores of the adsorbent are assumed to be either completely filled or completely empty, depending on whether a pore is above or below its respective filling pressure. The HK model also assumes that filling occurs in a single step and that no pore wall wetting or compression of the condensed phase occurs as the bulk pressure increases. Model isotherms constructed per the original HK method do not exhibit wetting or film growth, features observed in DFT- and MS-derived isotherms. In spite of such limitations, it has been shown that the HK method reproduces the DFT pore filling pressure correlation with surprising accuracy when the HK and DFT models are compared using the same gas-solid potential and the same dispersion parameter values [7]. Various modifications of the original HK method have been proposed. These involve corrections to either the shape of the pore [7,8,9] or the method of calculating the mean *Author to whom correspondence should be addressed. Current address: Department of Civil & Environmental Engineering, Universityof Michigan, Ann Arbor, M148109-2125 USA. Email:
[email protected].
100 adsorption free energy change [3,6,10]. Other analytic adsorption models have been suggested that derive a relationship between the pore filling pressure and the Gibbs energy of adsorption [11,12]. As with the variations of the HK method, however, these adsorption models do not account for the multiple filling events that occur in mesopores. To improve the realism and the range of applicability of the HK method, it would be useful to modify the original HK model so that the significant effect of pore wall wetting is accounted for in some manner. A new adsorption model based upon this concept, referred to as the two-stage HK pore filling model, is presented in this paper. The two-stage HK model is described in Section 2.2, and a comparison of the PSD predictions of this model with results obtained for the predecessor "single-stage" HK method are presented in Section 3. Original DFT calculations are reported in Section 3 for comparison with the two-stage HK model results. 2. DESCRIPTION OF TWO-STAGE HORVATH-KAWAZOE METHOD 2.1. Review of the Original HK Model
The premise of the HK method is that the pressure at which pore filling occurs may be obtained from the mean free energy change of the adsorbate molecule as it is transferred from the bulk gas phase to the adsorbed phase, through the equation kTIn(PHK leo)= ~,K (H) (2) where r H~H) is the mean heat of adsorption, k is the Boltzmann constant, T is the temperature, PHX is the condensation pressure, and P0 is the bulk saturation pressure. In the original HK method, the mean free energy change for adsorption in a slit pore is calculated as
e(H,z)dz -
~HK (H) =
"~'~f
(H,z)dz =
~'sf
(3)
f-o'sf dz
H - 2O'sf
where z is the spatial coordinate across the width of a pore in which the surface layer nuclei of the walls are located at z=0 and z=H; and ~sf is the arithmetic mean of the adsorbent and adsorbate diameters. The gas-solid potential ~ selected for the original HK model is a 10-4 potential for an adsorbate molecule between two infinite parallel planes of adsorbent atoms
~(H,z)=INsA~+NfAf/II(-~)I~ 2d 4
(-~L)4 -
( d~ )1~ / d~ + H-z - H-z
)41
(4)
In equation (4), N is the number density of atoms per unit surface area; A is the dispersion constant; the subscripts s and f refer to the adsorbent and adsorbate, respectively; and do 0.85~f is the z-coordinate at which the 10-4 potential for a single planar surface passes through its zero-point value. The 10-4 potential is obtained by integration of the LennardJones 12-6 potential over an infinite planar surface. The dispersion constants AsS and Aff represent the adsorbate-adsorbent and adsorbate-adsorbate interactions, respectively; these coefficients are calculated from the Kirkwood-Muller equations in the original HK paper [6]. Combining equations (2-4) yields an equation that relates filling pressure to pore width:
In(PHKI ~[ - f ~ I N {Hs A ~ ( +i N . ) f| A y ] I I ( d ~ 1 7 6 ~, Po J = L k Td3 __ - 2 o'~., , JL-9 ~ ~, j - -3 k crss )
do
/ 9+31/H--o'sf do 131
(5)
H - osr
In the original HK model, the density of the condensed phase in a pore is set equal to the liquid adsorbate density ,ol if the bulk gas pressure is greater than the filling pressure given by
101
equation (2). Otherwise, the pore is assumed to be empty. The model isotherms/-(P,H) generated in the original HK method can thus be represented as Heaviside step functions /-(P,H) = pt O[P- P/-/r(H)] (6) where O[x] - 0 for x < 0, and | - 1 for x > 0. Gas uptake at bulk pressure P in the experimental isotherm can be uniquely assigned to condensation in pores of width H as given by equation (5). The HK pore filling correlation of equation (2) can therefore be used to expediently determine the PSD of an adsorbent, by substituting equation (6) into equation (1). Several modifications of the original HK method have been suggested. These involve the use of different potential parameters [7] or alternative methods of calculating the mean heat of adsorption [3,7] so as to bring the HK pore filling correlation into closer agreement with MS and DFT results. In the original HK method, the adsorbate-adsorbate interaction energy is calculated in a physically inconsistent manner. However, for nitrogen adsorption on a planar carbon surface, adsorbate-adsorbate interactions contribute modestly (approximately 7%) to the total adsorption energy. It is therefore reasonable to assign the free energy change on adsorption to that arising solely from the adsorbate-adsorbent interaction potential. Improved HK pore filling correlations have been obtained using this approach [7,10]. 2.2. Description of the Two-Stage HK Pore Filling Model One of the principal shortcomings of the original HK method is that it is unable to portray the full sequence of filling events that occur in mesopores. The original HK method does not generate Type IV isotherms for mesoporous adsorbents, in which pore wall wetting precedes capillary condensation. A two-stage HK method is now introduced that incorporates two steps in the model isotherm, rather than just a single step as in the original method. The first step corresponds to formation of a monolayer, and the second step represents capillary condensation. In this new model, the basic methodology of the original HK model is retained, with the addition of new features that describe wetting of a monolayer. The twostage HK model is generalized so that it applies for any adsorbate/adsorbent pair at any subcritical temperature. The adsorbate-adsorbate and adsorbate-adsorbent interactions are modeled using Lennard-Jones potentials with respective well-depth parameters eft andesf. The 2 - - - I ~ ----
k~ c
.R Q. L.
0 "0
.
------......~..~;..:
~
.......
2
3.64 (81) 3.81 (80) 3.61 (79) 3.66 (78) 3.77 (79) sorption and
-- PK111 (19 pm)
.......... !--?- [-?; ~.T~...... T-7--!--i-? ! !;i .......... ?-!-~-? !-!Si . . . . . . . .
.
vp (total) /mi g-I (porosity / %)
~.-L"
.
.
.
.
.
. n
E
1
' I1""
._L%..'=
'
'
'
'
' ' " I
'
10
'
'
'
' | " I
IO0
'
'
'
'
'
' " I
'
1000
"~-'
L ' ' " I
10000
'
'
'
'
' ' "
10O000
mean pore diameter Pa/nm
Figure 2- Mercury intrusion measurements of monolithic silicas PK111 to PK118. Characterisation methods based on gas sorption have the drawback of using small tracer molecules like nitrogen or argon to evaluate the surface and pore structural parameters of porous materials. The solutes to be resolved and purified in liquid phase separation processes are much larger in size and volume and might interact with a smaller portion of the BET surface area. The same considerations can be put to the pore size distribution. Thus, we prefer to assess the mesopore size distribution from size exclusion chromatographic measurements and using inverse size exclusion chromatography (ISEC) to assess the distribution curves. Such curves are shown for the monolithic silicas in Fig. 3. The average pore diameter at the maximum of the five curves is approximately between 10 to 12 nm and is slightly shiiting to larger values with increasing macropore diameter.
120
1,5 L
:
:
:
',:
:::
;
:
', ', ' , . ' , i l l
:
:
;
:',
',:',
(1)
E :3 O
~> r O
....... ,~,,,,.,,-,
1,0
. a
:3 .Q L_
.__.
--
p.117(4.6~)
.
i : :~:i:
:,
"o
0~ 0,5
N
,
,
,
,
,
. m
(1) o ca.
i i l ii:~-a;
i
,
,
i
J ?~l???
,
,
i lilill "~!
, i~,.:,|
:
i ? K ilil
, , ,
,
,
:
: ::::':
! ) :?!:"
~;I~,,,:: i::ii
, , ,
,
L_
,
,
,
,
!
0,0
.
(]1
;
: : ~ . .
.
.
:
.
.
.
1
.
.
.
.
.
.
.
.
10
'
:
i
; 1111
......
leo
transverse pore diameter R / nm
Figure 3" Pore size distributions o f monolithic silicas PK111 to PK118 according to I S E C data. 3.1.3
Scanning electron microscopy (SEM)
121
Figure 4: SEM micro graphs of the silica monoliths PK111 (a), PK114 (b), PK115 (c), PK117 (d), PK118 (e) at a constant magnification level of 5,000.
SEM provides a direct image of the macropore size and allows one to estimate the skeleton size of the silicas. It is clearly seen from the SEM magnifications of Fig. 4 a - 4e that with increasing macropore diameter the skeleton size also increases, i.e. the number of macropores per unit of pore volume decreases. 3.2
Chromatographic properties
3.2.1
Hydrodynamic characteristics
It is interesting to compare the flow characteristics of both types of silica columns with respect to the column pressure drop and column permeability. The column pressure drop / flow rate curves of both types indicate a much lower slope of the monolithic columns (not shown) than the particulate columns. In other words, the column pressure drop of the monolithic columns corresponds to that of a column packed with 15 ~tm silica particles [6]. The column permeability values are listed in Table 3. Silica
P a r t i c u l a t e silica
type
Gi-G
G2-G
G3-G
/ 10Kr -14 m z
5.1
4.2
4.7
.
.
.
.
M o n o l i t h i c silica
G4-G G5-G P K l i l 3.0
3.2
2.1
PKll4 PKll5 PKll7 PKll8 2.6
2.6
2.7
2.9
.
Table 3: Column permeability KF of particulate and monolithic silica columns. The KF values for the particulate silica columns indicate a decrease of the permeability with increasing average mesopore diameter, increasing total porosity and increasing pore connectivity at constant average particle diameter of 10 ~tm. The monolithic column show a slight increase of the permeability with increasing macropore diameter at constant total porosity which is to be expected. 3.2.2
Column performance characteristics
The plate height-linear velocity dependencies are depicted in Fig. 5 and Fig. 6. At constant particle diameter the plate height of the particulate silica columns drops with increasing particle porosity and increasing average mesopore diameter and increasing pore connectivity. There is a marked difference between silicas G1-G and G2-G and the group of G3-G, G4-G and G5-G. The latter show similar behaviour: the theoretical plate height corresponds to approximately one particle diameter and only slightly enhances at higher linear velocities.
122
In contrast, the monolithic columns together exhibit a more common feature. The scatter in plate height is less pronounced. However, a clear trend can be recognised: the columns with the average macropore diameter of 1.9, 2.9, 3.6 and 4.6 pm have a similar performance as compared to the column with 6.0 lam macropore diameter which is worse. In general, the slope of the H vs. u curves is twice as higher than those of the columns with silicas G3-G, G4G and G5-G, respectively. ~0-
[
.......... ~ ......... Ga-~3 .
[ 60
I
i
.
.
.
G4-G
GSG
.
[.
...........
/ ...-"
.....
o-
...., "'"
-'-
...........
~_~
PK118 PKl17 PKl15 PKl14 PK111
. /
...,..
,
-
"-
'
411 ...,'""" 9
. ...........
....
............
=:: = ::; =-.= = - : - - = -
~__-.::-.~'~~ 0 0
1
2
3
linear flow velocity u~ m m s
H vs. Figure 5: particulate materials.
I\
.............. ,,"" .......
u
plot
o 0-
4
5
; ....... 0
"1
of
..... .---..--.--"
1
2
3
4
5
l i n e a r f l o w v e l o c i t y u / n T n s -~
the
Figure 6: materials.
H vs. u plot of the monolithic
4. CONCLUSIONS The pore connectivity nr of two types of silica (highly porous beads, monolithic silicas) was calculated according to the pore network model proposed by Meyers and Liapis. Nr was proportional to the particle porosity in the case of highly porous beads. The differences in the pore connectivity for both types of silica were reflected in the mass transfer kinetics in liquid phase separation processes by measuring the theoretical plate height-linear velocity dependencies. In a future study, monolithic silicas possessing different macro- as well as mesopores will be investigated and compared with the presented results. 5. ACKNOWLEDGEMENT The authors thank Prof. A.I. Liapis and B. Grimes for supplying the software for calculating the pore connectivity parameter and for helpful discussions in the interpretation of the results. We thank the Stiflung Innovation, Rheinland-Pfalz for financial support. 6. LITERATURE K. Nakanishi, Journal of Porous Materials, 1997, 4, 67-112. K. Nakanishi, H. Minakuchi, N. Soga and N. Tanaka, Journal of Sol-Gel Science and Technology, 1997, 8, 547-552. K.K. Unger, J. Schick-Kalb, and K.F. Krebs, J. Chromatogr., 1973, 83, 5-9. J.J. Meyers, A.I. Liapis, Journal of Chromatography A, 1998, 827, 197-213. J.J. Meyers, A.I. Liapis, Journal of Chromatography A, 1999, 852, 3-23. F.C. Leinweber, D. Lubda, K. Cabrera and U. Tallarek, Anal. Chem., 2002, 74, 24702477.
Studies in Surface Science and Catalysis 144 F. Rodriquez-Reinoso, B. McEnaney, J. Rouquerol and K.K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved
123
Percolation Phenomena in Micropore Adsorption: Influence on Single and Multicomponent Adsorption Equilibria S. Ismadji and S.K. Bhatia Department of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia A novel and simple method for determination of micropore network connectivity of activated carbon using liquid phase adsorption is presented in this paper. The method is applied to three different commercial carbons with eight different liquid phase adsorptives as probes. The effect of the pore network connectivity on the prediction of multicomponent adsorption equilibria was also studied. For this purpose, the Ideal Adsorbed Solution Theory (lAST) was used in conjuction with the modified DR single component isotherm. The results of comparison with experimental data show that incorporation of the connectivity, and consideration of percolation processes associated with the different molecular sizes of the adsorptives in the mixture, can improve the performance of the lAST in predicting multicomponent adsorption equilibria.
I. Introduction Activated carbons are materials having complex porous structures with associated energetic as well as chemical inhomogeneities. Their structural heterogeneity is a result of the existence of micropores, mesopores, and macropores of different sizes and shapes. Characterization of the pore structure of the activated carbons is crucially important to adsorption and separation processes. The latter is generally made in the terms of pore size distribution. Quantitation of the pore size distribution is often performed by employing the generalized adsorption isotherm (GAI) [1] in conjunction with a local isotherm model. Another important characteristic of the structure of porous materials, which governs their reaction and transport properties is the connectivity of the pore network. The most common method of quantifying this is in the terms of a coordination number Z, defined as the number of pores intersecting at a junction or node. The pore network connectivity is usually determined by gas sorption analysis [2-4] or mercury intrusion [5] based on percolation theory. Recently, Ismadji and Bhatia [6] have successfully employed the liquid phase adsorption isotherms to determine the pore network connectivity and the pore size distribution of three commercial activated carbons. In our recent study [7], the pore network connectivity of three commercial activated carbons was characterized using liquid phase adsorption isotherms of eight different compounds. In that study we used ester molecules with complex structure, as probe molecules. The structural heterogeneity of activated carbon is a result of the existence of micropores, mesopores, and macropores of different sizes and shapes, randomly connected in a pore network. In a pore network, some of the pores, that are large enough to accommodate the probe molecules, may be accessible only through smaller pores that only permit the passage of molecules having sufficiently small size. Therefore only a part of the available pores is
124
actually accessible to a given probe molecule. The fraction of actually accessible pores is a function of the network topology, pore network connectivity, and the fraction of available pores. In the adsorption process, especially involving large and complex probe molecules, the pore network connectivity is very important, and governs the transport and reaction properties of the pores. In this paper, we present the influence of incorporation of the pore network connectivity on the prediction of binary liquid phase adsorption of large molecules using the Ideal Adsorbed Solution Theory (lAST).
2. Theory For randomly connected pore networks, some of the supercritical pores (those larger than the molecular size) are connected only through subcritical pores (those smaller than the molecular size) and are therefore not accessible. In a given network, in which not all the pores can accommodate the probe molecules, the number fraction of the pores that are available is
~[f (H) dH (1)
~f (-g ) ag where f(H) is the normalized pore volume distribution function, H is the pore width, and q~ is the fraction of available pores. A slit pore model is assumed, as is now well established for carbons. In randomly connected pore networks the fraction of the pores that are actually accessible to the probe molecule ~ a , is less than the number fraction of the pores that are available. Based on the simple expression obtained by Zhang and Seaton [8] upon fitting simulation results for a cubic lattice (Z=6), subsequently generalized by Lopez-Ramon et al. [3], the actual accessible pore fraction for an arbitrary coordination number Z can be written as follows: 9 '~ = O, 9 < (1.494 / Z)
(2)
q~a [1.314(~Z - 1.494) TM + 3.153(q~Z - 1 . 4 9 4 ) - 3.48(q~Z- 1.494)2 + 1.433(q~Z- 1.494)3]/Z,
(1.494 / Z) < 9 < (2.7 / Z) ~a = ~ , ~ > (2.7 / Z)
(3) (4)
The coordination number Z can be obtained using nonlinear least-square fitting by comparing the theoretical value of V~,cc (i) a
V,,~c= ~
oo
V,o, ~f (H)dH
(5)
de
with actual experimental accessible pore volume, Vfxp, obtained from the DubininRadushkevich characteristic plots. In eq. (5) Vtotis the total specific pore volume.
125 The differences in critical molecular sizes of the different components involved in the multicomponent adsorption of large molecules give additional complexities in the problem of the pore network accessibility. Here we will briefly describe the influence of percolation phenomena and accessibility on binary adsorption. Further details of this are presented elsewhere [9,10]. Here, we consider the binary adsorption of two components, component 1 has a critical molecular size, d~l, and component 2, dc2. The critical molecular size of component 1 is smaller than component 2. Figure 1 depicts the adsorption behaviour of the binary mixture in the pores.
Figure 1. Schematic diagram of binary adsorption process in micropore network. We envisage that there are four different zones of pore space available to the mixture components in activated carbon. 9 Zone I In the pore size range between dc~ and dc2 (dcl 20 A) and therefore clearly visible with the resolution of the microscopes used (Fig. 22). The carbons produced are expected to have pores with walls of similar dimensions to the pore diameters of the template. This worm-like carbon structure is also easily distinguished by the TEM technique applied. All these materials are however quite unstable to the electron beam.
145 PXRD plots (Fig. 24) show that most of the produced carbons in this routte are molecular replicas of the templates AHS 1 and AHS3, confirming TEM results. This is seen by the retention in the upper plot of the template diffraction peak at 2 ~ 2 0. It is however difficult to resolve as low as this angle in a wide-angle powder x-ray diffractometer such as the one used. Therefore, if the peaks are not of sufficient intensity, they can show up as ,~o~-~ shoulders of the characteristic high background [ ~ 2.1. . . . . . . . . AHSIFC73DN 800~ ~42.50A Tomplated&Non~AHS3FC73DF scatter for porous carbons below 3 ~ 20, as seen t'"'" . . . . . . . . X " Tcmplatcd Carbons . . . . . AHS3FC73P74DF 600~. .... _..... k .... --FC73DF in Fig. 24 (top) for the propylene treated carbon ":t,..t.'~"'-.- . . . . . . . . . . . AHS3FC73P74DF. In addition to these findings, 40O .......~.,~ the carbon demineralised with NaOH shows a ~0o~. ......~,__~_. . -... ... .. . .. . .. . .. . .. . . . . . . . . .,, ~--.................. " - ~ Z S " : : : - - 7 : ? " .............. L~ broad peak at 25 ~ more intense than the ~ ~ ? . . . . . ".--~ other templated carbons. This peak has previously been related to a stacking structure of carbon layers and ascribed to deposits of the ~,sc~ _ _ ~c~,~ precursor on the external surface of the template particles [4]. ~ The carbons in Fig. 24 are also compared with SNU2, which is a carbon produced by a Korean research group [6] using a similar 5 template, but a different carbon precursor H f ..... ... '.~A "--~ "\ MMS AHSl (phenol formaldehyde) intercalated in the gas 60o I \ Templates - AHS3 phase. The comparison between these templated carbons indicates that the choice of precursor ~ 3 ~~ does not significantly alter the replication in this 3~ 3.98 A route. Fig. 25 shows the adsorption isotherms for the AHS3 route. The template is mesoporous Diffraeti~a angle 20 (o) with 714 m 2 g-1 surface area and 0.93 cm 3 g-i Fig. 24 PXRD: Templating routes via AHS 1 and AHS3 (included pattern for the Korean carbon SNU2, [6]) pore volume. Templated carbons AHS3FC73DF and AHS3FC73P74N93DF were very similar, with 1467 m 2 g-1 surface area and 1.3 c m 3 g-I pore volume, twice that of the SNU2's. Both DFT and BJH models were used for PSDs (Figs. 26-30). For AHS3, the hybrid DFT model yielded a narrow PSD centred at 45 A, and the BJH analysis gave a single peak at 34 A. The templated carbon PSDs are expected to reflect the dimension of the pore walls of the template rather than the template pore diameters. AHS materials have been reported to have pore walls 10 A thick [2,7]. However, Hybrid DFT PSDs show pores of 20-30 A for the AHS3 templated carbons, indicating that the pore walls of our template might be thicker than the reported estimate or that, more likely, the carbon shrunk during carbonisation. In this route, the addition of propylene was found not to make much difference to the porosity of the resulting carbon. 22
146
4. C O N C L U S I O N S Three different routes of templated carbons were successfully prepared, and a range of different directing structures was used as inorganic matrices, allowing the analysis of template effects on the synthesised carbons. A clear replication of the template morphology is seen in scanning electron micrographs in all the synthesis routes. Nitrogen adsorption, powder x-ray diffraction and transmission electron microscopy permitted evaluation of how far the replication extended to the molecular level. The clay route produced microporous carbons, with high surface areas (>1000 m 2 g-l) but no long-range order. It was found that the demineralisation step in the synthesis procedure produced part of the porosity in its own, and that the layered structure of the carbon-mineral composite collapsed on heating. The zeolite-based route resulted in microporous carbons, also with high surface areas (ca. 1000 m 2 gl) which were doubled by additional propylene CVD treatment. CVD treatment resulted in a more precise structure replication of the starting template, which is thought to be due to more complete filling of the mineral pores during impregnation and subsequent carbonisation. Mesoporous molecular sieve-templated carbons have larger pores than the previous two routes, but equally high surface areas, and were found to retain the repeating distance characteristic of the template. This is probably due to the larger pores of the template resulting in a more robust carbon framework which better withstands the chemical treatment necessary for demineralisation. The synthesis routes studied provide the tools to produce carbons with different nanoscale properties. This is potentially useful for various adsorption applications given the carbons' high surface areas and pore size distributions which vary according to the chosen template. ACKNOWLEDGMENTS P.M. Barata-Rodrigues would like to thank the Portuguese Foundation of Science and Technology (PRAXIS XXI/BD/18002/98). Acknowledgmentsare due to the EPSRC and MoD for sponsoring this research project and to all those who contributed to this work, in particular T.Bustnes, T. Kyotani, J. Lee, R. Ryoo and J. Olivier. REFERENCES [1] Reichle, W.T., Solid State Ionics, 22 (1986) 135. [2] Tanev, P.T. and Pinnavaia, T.J., Science, 267 (1995) 865. [3] Bandosz,T., Putyera, K., Jagiello, J., Schwarz, J.A., Applied Clay Science, 10 (1995) 177. [4] Kyotani,T., Ma, Z. and Tomita. A., Chemical Communications,23 (2000) 2365. [5] Ryoo, R., Joo, S.H., Jun, S., Journal of Physical Chemistry, 103 (1999) 7743. [6] Lee, J., Yoon, S., Oh, S.M., Shin, C.-H. and Hyeon, T., Advanced Materials, 12 (2000) 359. [7] Zhang, W., Pauly, T.R. and Pinnavaia, T.J., Chemistry of Materials, 9 (1997) 249 I. [8] Drezdon, M. A., 'Pillared hydrotalcites', US Patent 4,774,212 (1988). [9] Brunauer, S., Emmett, P.J. and Teller, E., Journal of the American Chemical Society, 60 (1938) 309. [ 10] Dubinin, M.M. and Ashtakov, B.A., Advanced Chemical Series, 102 (1971) 69. [ 11] Barrett, E.P., Joyner, L.S., Halenda, P.P., Journal of the American Chemical Society, 73 (1951) 373. [ 12] Olivier, J. P. and Conklin, W. B., The DFT Plus Models Library (1996) Appendix A. [13] Tarazona, P., Phys. Rev. A, 31 (1985) 2672; Tarazona, P., Phys. Rev. A, 32 (1985) 3148; [ 14] Jaroniec, M., Kruk, M., Olivier, J.P. and Koch, S., Proceedings of COPS-V, Heidelberg, Germany (1999). [15] Olivier, J.P., Carbon, 36 (1998) 1469.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
147
Simulation of Adsorption in 3-D Reconstructed M e s o p o r o u s Materials by a Simulated Annealing Algorithm M.E.Kainourgiakis a, E.S. Kikkinides b, G. Ch. Charalambopoulou a and A.K. Stubos a a
National Center for Scientific Research "Demokritos", 15310 Ag. Paraskevi Attikis, Greece
b Centre for Research and Technology Hellas, Chemical Process Engineering Research Institute, 6th km. Charilaou- Thermi Road, 57001 Thermi, Thessaloniki, Greece Aim of the present work is the determination of the spatial distribution of a condensable adsorbate in reconstructed porous domains of Vycor membranes. The digital representation of the dry porous structure is achieved by a stochastic method. The produced binary domains have the same statistical content as the actual materials. The distribution of the condensable adsorbate in the reconstructed porous structure, for a given degree of saturation, is found assuming that in equilibrium the total interfacial free energy reaches a global minimum value. The configuration leading to the minimisation of this objective function is determined through a simulated annealing method. In a further step, the effective diffusivity of an inert gas such as He, in the reconstructed Vycor porous glass, is determined by a combination of the mean square displacement method with a standard random walk process. The gas relative permeability, PR, is then calculated by normalising the computed effective diffusivity at each degree of saturation by that computed for the dry material. The comparison of the simulation results with experimental relative permeability data exhibits good agreement, supporting the validity of the suggested adsorption simulation process.
1. Introduction The spatial distribution of an adsorbate in mesoporous domains is of primary importance for a wide range of applications such as separations, catalytic reactions etc. The study, in particular, of the flow of a non-adsorbable gas through such a porous structure partially blocked by a condensed phase, can provide a useful tool for either its structural characterization [1 ], or the evaluation of its performance in various industrial processes [2,3]. What is sought after in such an effort, is to determine the effect of pore blocking on the overall transport process by combining dynamic (permeability) with static measurements (adsorption). This can be achieved by introducing a stationary condensed phase of an adsorbed vapour into the porous material and subsequently measuring the permeability of a second non-adsorbable gas, which does not condense in the pores, at least under the conditions of the experiment. The above type of experiment is known as gas relative permeability [4], and since it is usually performed at low pressures, intermolecular collisions are rare and the transport process is determined primarily from the collisions between the molecules and the solid walls. This mechanism is within the Knudsen regime and thus permeability reduces to Knudsen diffusivity.
148 Eventhough there is a well-established theoretical description of both adsorption and transport mechanisms in mesoporous media [5], the adequate representation of the three-phase system (vapour- condensed adsorbate - solid matrix) occurring during adsorption and transport processes, is extremely difficult due to the complexity of the porous structure. Although considerable progress has been made in the efficient representation of the internal structure of porous media using pore networks [6], there still exists a strong need for a direct quantitative description of the complex microstructure. The exponential growth of computational resources has recently led to advanced methods for the binary representation of biphasic media. These can be divided in two major categories: (a) the statistical reconstruction methods [7,8,9,10,11], where the binary array respects a number of statistical properties of the actual biphasic medium, and (b) the process based methods [12,13,14], where the computational procedure tries to account the physical processes underlying the formation of the medium. The binary domains resulting from either approach can be used for the simulation of either dynamic processes, such as diffusion [15,16] and flow [8] or equilibrium properties such as sorption [17,18]. In previous studies [15,19], we adopted the stochastic reconstruction technique proposed by Crossley et al. [ 11 ] to generate 3D images of Vycor glass, a model mesoporous material, and showed that this particular method not only reproduces the actual porous structure accurately, but also constitutes a kind of process-based approach [19]. In the present work, the same methodology is employed for the digital representation of dry Vycor geometry and an attempt is made to determine the concentration profile of a condensable adsorbate in the reconstructed porous domains. The distribution of the condensable adsorbate in the reconstructed porous structure, for a given degree of saturation Vs (fraction of pore space occupied by the adsorbate), is defined assuming that in equilibrium the total interfacial free energy is minimal. A simulated annealing method is employed for the optimisation procedure. In a further step, the effective diffusivity of an inert gas such as He, in the reconstructed Vycor porous glass, is determined by a combination of the mean square displacement method with a standard random walk process. The gas relative permeability, PR, is calculated by dividing the computed effective diffusivity at each degree of saturation by that computed for the dry material. The simulation results are compared with experimental relative permeability curves. 2. Representation of porous structure - Dry Vycor geometry
The spatial distribution of matter in a porous medium can be typically represented by the phase function Z(x), defined as follows: if x belongs to the pore space (1) otherwise where x is the position vector from an arbitrary origin. Due to the disordered nature of porous media, Z(x) can be considered as a stochastic process, characterized by its statistical properties. The porosity, e, and the auto-correlation function Rz(u) can be defined by the statistical averages [8,20]:
149
Rz (u):
(z<x
2b)
2
Note that indicates spatial average. For an isotropic medium, Rz(u) becomes onedimensional as it is only a function of u=]u[. Ideally, a representative reconstruction of a porous medium in three dimensions should have the same correlation properties as those measured on a single two-dimensional section, expressed properly by the various moments of the phase function. In the present work the first-two moments (porosity and auto-correlation function) are considered. In general, information on the porosity is obtained through sorption experiments, while information on the correlation function is obtained either directly from 2D images of sections of the material (assuming isotropy in the third dimension) or indirectly from the scattering spectrum measured by Small Angle Scattering techniques. The basic idea in most reconstruction techniques is to convolute a purely random process to a correlated one through the use of an appropriate kernel. The resulting correlated process is transformed into a binary phase function by thresholding it according to the porosity of the medium. The final 2D or 3D binary medium possesses the same porosity and correlation function with the actual porous medium. In the present work, the reconstruction method proposed by Crossley et al. [11 ] has been selected, since it is relatively simple, while it gives an excellent fit of the whole SANS spectrum and thus the correlation function of the material. According to this method, the reconstructed Vycor image is generated by starting with an initial three-dimensional random from a uniform probability distribution in [0,1]. The initial random number array image becomes correlated through a convolution with a Laplacian-Gaussian Kernel, which introduces a correlation length 09:
Io(x,y,z)
K(x,y,z;co)
+oo+oo+~
I(x,y,z): ~ ~ ~Km(x-x/,y- y/,z-z/)'lo(x/,Y',z')dx'dy'dz' where Km is given by:
(3a)
Km(x,y,z)=(_6+4.(x2+
(3b)
-oo..-ot>-oo
y2+
zZ)/wZ).exp(_(x:+ y2+ zZ)/w2)
I(x,y,z)
Finally the correlated array is binarised by thresholding with the porosity of the material. The resulting three-dimensional array represents a binary medium with properties similar to those of actual Vycor porous glass. This is demonstrated in Figure 1, where the auto-correlation function of the reconstructed domain is compared with the corresponding curve for the real dry-state material, derived from SAXS experiments and TEM images as described in earlier works [15,21,22]. A 2-D image of the reconstructed dry Vycor at porosity e=0.28 is presented in Figure 2a.
3. Determination of the spatial distribution of adsorbate in reconstructed Vycor Consider a sorption experiment, where a mesoporous solid, denoted hereafter as S, is progressively loaded by a condensable gas or vapour. Initially, a layer of adsorbate L is building up on the walls of the pores. When condensation occurs, all the pores with radii smaller than a critical value, given by the Kelvin equation, are progressively blocked, and the adsorbate is in equilibrium with its vapour, V. The distribution of the condensed phase in a reconstructed Vycor structure for a given degree of pore filling (saturation), is determined by
150 1.0 0.8
TEM Image [10]
0.6
Stochastic reconstruction
~-~ O.4
0.2 0.0 -0.2
100 6-'--- ~
200
ai.
u(~
300
-
~
I
I
400
Fig. 1. Autocorrelation function of actual and reconstructed dry Vycor assuming that in equilibrium the total interfacial free energy, Gs, associated with the multiphase system, is minimal [23]. Gs is defined as: Gs
=ZA, Yi
(4)
i
where Ai is the interfacial area i and ~i is the interfacial free energy of area i. In the present case, the total interfacial energy has contributions from three different interfaces: (a) condensed phase L - solid S, (b) vapour V - solid S, and (c) condensed phase L - vapour V. The corresponding ~i components always obtain positive values and for complete wetting of the solid surface (contact angle equal to 0 ~ cancel according to Young-Dupr6 equation: YsL = Ysv - 7"Lv
(5)
The actual distribution of phase L is represented by the arrangement of the fluid pixels that minimizes the total interfacial energy. This Gs minimum is determined through a simulated annealing algorithm (SA). This method was introduced by Kirkpatrick et al. [24], who identified the analogy between the annealing process in solids, the behaviour of systems with many degrees of freedom in thermal equilibrium at a finite temperature and the optimisation problem of finding the global minimum of a multi-parameter objective function. SA is based on the Metropolis algorithm at every step of which the configuration of the system under study, changes randomly causing a variation of the corresponding objective function by AE. If at a certain step AE0 the new configuration is accepted with a probability given by:
P(AE)= exp(- ~-~T 1
(6)
where k8 is Boltzmann constant and T is the temperature (or an arbitrary analogue of it, used only to symbolically represent the degree of randomness in the spatial distribution of the system phases). This step prevents the system being trapped in a local lowest-energy state. After a sufficient number of iterations, the system approaches the equilibrium state where the free energy reaches its minimum value. Subsequently the procedure is repeated for several T values lower than the initial one (using every time as initial configuration the one found as
151 equilibrium state for the previous T value). The process ends when despite the change in T, the number of accepted changes in different configurations becomes lower than a prespecified value. In our case the objective function to be minimized, is the total interfacial energy, Gs. In order to find the equilibrium distribution of liquid and vapour for a certain Vs, the required number
Fig. 2.2D sections of the corresponding 3D reconstructed images: (a) dry, (b) wet (Vs=0.5) of liquid elements, L, are first randomly selected and thus an initial state of the system is defined corresponding to a reference total energy value, Go. Progressively, the positions of one site in the pore space occupied by L phase and one V element are swapped, preserving the specified saturation. Let us consider the n-th trial of this process. If AG=(Gtrial- Gn_l) Gn-l, the new configuration is accepted with a probability given by exp(-AG/Gref), where Greyis a control parameter analogous to temperature T, having the same units as the objective function Gs. The calculations are repeated until no change in Gn is observed. A 2D section of the reconstructed 3D images for saturation equal to 0.3 is presented in Figure 2b.
4. Simulation of Knudsen Diffusion in Reconstructed Vycor The orientationally averaged effective diffusivity of He in the reconstructed Vycor structure (dry or wet), is calculated from the mean-square displacement , of a statistically sufficient number of identical particles injected in the void space of the medium, according to: D = l i m (42) t---~ 6t
(7)
where t is the travel time of the particles. The displacement is monitored throughout the distance, s, traveled by the particles assuming that they move at a constant speed equal to the mean thermal speed, "- ( 8 R T [ ~ M ) 1/2 [15,25,26]. The travel time has to be large enough to ensure that the molecules feel the effect of all the structural details of the porous medium. In this sense, the material can be considered as macroscopically homogeneous in terms of its structural and diffusion characteristics. At first, a random position in the pore space is defined as the initial position of the molecule to travel within the porous medium. Subsequently, direction angles are randomly assigned to the molecule, which starts its random walk moving from voxel to voxel along this direction. At each step it is checked whether the molecule hits a solid wall, and if this happens it undergoes a diffuse reflection according to the cosine law.
152 At all times a test is made to determine whether the molecule reaches the boundaries of the 3D medium. Periodic boundary conditions are employed in this case and two sets of coordinates have been used: local coordinates of the molecule inside the medium and the global ones used for the computations of the total displacements. Using the exact solution for an infinitely long tube of diameter le, in the Knudsen regime, D, -- ~3 le and substituting t=S/, Equation (7) obtains the dimensionless form:
D I D k = lim
(42/le2)
(8)
2ST(e
In addition, the incorporation of porosity in diffuSivity yields effective diffusivity D e# = D" c , also known as permeability, P, for the case of inert gases. Computer simulations have been performed in 3-D images of recons~ucted dry Vycor porous glass, with pixel size/e=30/~ and sample size of 100xl00xl00. A total number of 3000 test molecules was used in the majority of the simulations, while the total number of time steps was greater than 105. The effective Knudsen diffusivity, for the case of a Vycor domain with porosity e=0.28, has been found equal to 0.073xDk [15]. Since the value of Dk for He at 298 K, is 0.0126 cm2/s, it follows that the computed Knudsen diffusivity of He in dry Vycor porous glass is ~9x 10"4 cm2/s, in excellent agreement with experimental results [27,28]. The same computational procedure was repeated for the case of binary domains representing the wet Vycor structure for different degrees of saturation, Vs varying from 0.1 to 0.48. In order to evaluate the effect of the adsorbate nature on the adsorbate distribution and therefore on the transport properties of the system, three sets of interfacial energies 7i, were examined. For each of the three systems gas relative permeability, PR, was determined by normalising the effective diffusivity computed for a certain Vs value by the one for the dry material. The simulation results are compared against experimental data obtained from the measurement of He permeability in Vycor preadsorbed with CH2Br/[28], in Figure 3. A good agreement between simulation and experiment is observed, demonstrating that the suggested approach can reproduce the concentration profile of a condensable adsorbate in reconstructed Vycor domains. Most importantly, this distribution does not seem to be affected by the actual values of Yi. Eventhough ~i components are significantly varied, always preserving the complete 1.0~ []
0.8
0
0.6
Experiment [28] 1,s~22, 1,LV=72,1,9_--350 7s-v=lO0, YLV=30,7SL=70 YS~, YLV=1, ~'S=1 L
ct'~: 0.4-
0.2 .0
o.o
'
~w
ola
'
o14
i
o16
o18
|
1'.o
Fig. 3. Comparison between simulated and experimental gas relative permeability of He on Vycor porous glass partially filled with a sorbed phase
153 wetting condition (7sv>YsD, the computed relative permeability values fall upon a single curve. One can therefore claim that the spatial distribution of an adsorbate in a mesoporous medium depends mainly on the morphology of the porous structure and not on the absolute values of the set of the interfacial energies, given that proper physical conditions are satisfied. 5. Conclusions
In the present work the spatial distribution of a condensable adsorbate in reconstructed porous domains of Vycor membranes is determined. A stochastic method is employed to generate digital pictures of the dry mesoporous structure. The distribution of a condensable adsorbate in the reconstructed domains, for a given degree of saturation, is found assuming that in equilibrium the total interfacial free energy reaches a global minimum value. The adsorbate concentration profile leading to the minimisation of this objective function is identified through a simulated annealing algorithm. In addition, the effective diffusivity of He in the reconstructed Vycor, is determined by a combination of the mean square displacement method with a standard random walk process. The gas relative permeability, PR, is then calculated by normalising the computed effective diffusivity at each degree of saturation by that computed for the dry material. The comparison of the simulation results with experimental relative permeability curves exhibits good agreement, proving that this methodology can reproduce the concentration profile of a condensable adsorbate in reconstructed Vycor domains. In addition, this distribution is not affected by the nature of the solid-adsorbate interaction. References
[1] A.K. Stubos, Th. A. Steriotis, A.Ch. Mitropoulos, G.E. Romanos and N.K. Kanellopoulos, "Inorganic membranes: pore structure characterisation. In: Physical Adsorption Experiments, Theory and Applications, J. Fraisard (eds.), Kluwer Academic Publishers, The Netherlands, 1997 [2] R.J.R. Uhlhorn, K. Keizer and A.J. Burggraaf, J. Membrane Sci., 66, 259 (1992) [3] D.P. Sperry, J.L. Falconer and R.D. Noble, J. Membrane Sci., 60, 185 (1991) [4] R. Ash, R.M. Barrer and C.G. Pope, Proc. Roy. Soc., A271, 1 (1963) [5] S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd Ed., Academic Press, London, 1982 [6] D. Nicholson and J.H. Petropoulos, J. Chem. Soc. Farad. Trans. I., 80, 1069 (1984) [7] J. A. Quiblier, J. Colloid Interface Sci., 98, 84 (1986) [8] P.M. Adler, 'Porous Media: Geometry and Transports', Butterworth, London, 1992 [9] C.L.Y. Yeong and S. Torquato, Phys. Rev. E, 57, 495 (1998) [10] P. Levitz, V. Pasquier and I. Cousin, in Characterization of Porous Solids IV, edited by B. Mc Enaney, T. J. Mays, J. Rouq6rol, F. Rodriguez-Reinoso, K. S. W. Sing and K. K. Unger (Bath, UK, 1996), pp. 135-140 [11] P.A. Crossley, L.M. Schwartz and J.R. Banavar, Appl. Phys. Lett., 59, 3553 (1991) [12] S.L. Bryant, C.A. Cade and D.W. Mellor, AAPG Bulletin, 77, 1338 (1993) [13] S. Bakke and P.E. t~ren, SPE Journal, 2, 136 (1997) [14] P.E. Oren, S. Bakke, and O.J. Arntzen SPE Journal, 3, 324 (1998) [15] M.E. Kainourgiakis, E.S. Kikkinides, A.K. Stubos, N.K. Kanellopoulos, J. Chem. Phys., 111, 2735 (1999) [16] M.E. Kainourgiakis, E.S. Kikkinides, Th.A. Steriotis, A.K. Stubos, K.P. Tzevelekos, N.K. Kanellopoulos, J. Colloid Interface Sci., 231, 158 (2000)
154 [17] L. D. Gelb and K. E. Gubbins, Langmuir, 14, 2097 (1998) [18] H.J. Woo, L. Sarkisov and P.A. Monson, Langmuir, 17, 7472 (2001) [19] M.E. Kainourgiakis, E.S. Kikkinides and A.K. Stubos, "Diffusion and flow in porous domains constructed using process-based and stochastic techniques", J. Porous Mat., in press [20] J.G. Berryman, J. Appl. Phys., 57, 2374 (1985) [21] P. Levitz, G. Ehret, S.K. Sinha and J.M. Drake, J. Chem. Phys., 95, 6151 (1991) [22] P. Levitz and D. Tchoubar, J. Phys., 1 2, 771 (1992) [23] R. Knight, A. Chapman and M. Knoll, J. Appl. Phys., 68, 994 (1990) [24] S. Kirkpatrick, C.D. Gelatt, M.P. Vecchi, Science, 200, 671 (1983) [25] M.M. Tomadakis and S.V. Sotirchos, AIChE J., 39, 397 (1993) [26] V.N. Burganos, J. Chem. Phys., 109, 6772 (1998) [27] C.N. Satterfield and T.K. Sherwood, The Role of Diffusion in Catalysis, AddisonWesley, Reading, 1963 [28] P.K. Makri, G.E. Romanos, Th.A. Steriotis, N.K. Kanellopoulos, A.Ch. Mitropoulos, J. Colloid Interface Sci., 206, 605 (1998)
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Understanding Adsorption Hysteresis and Other Mesoporous Materials
155
in Porous
Glasses
H.-J. Woo, L. Sarkisov, and P. A. Monson Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, USA The origin of adsorption hysteresis in porous glasses is investigated using a mean field theory and Monte Carlo simulations, utilizing a coarse-grained lattice model. Results in the hysteresis region closely agree with the behavior seen in the experiments of xenon adsorption in porous glass by Everett and co-workers (Everett, 1982). Hysteresis and scanning behavior occurs due to the presence of a very large number of metastable states. Adsorption hysteresis is dynamic in origin, but the long time dynamics is so slow that on accessible time scales, the system appears equilibrated. 1. I N T R O D U C T I O N Structural and energetic heterogeneity in various types of porous materials often profoundly influence fluid behavior such as adsorption properties. Among other developments, recent progresses in molecular modeling research are beginning to have impact on our view on the microstructure characterization. Use of molecular models (Gelb and Gubbins, 1998; Page and Monson, 1996; Sarkisov and Monson, 2000) as well as more coarse-grained lattice models (Kierlik et al, 2001; Woo et al, 2001) allows us to examine many of the phenomenologies observed in experiments within the statistical mechanical perspective. In the current paper, we discuss some of the new approaches and results that have been developed and obtained recently within the context of such molecular modeling research, and in particular with the mean field and Monte Carlo studies of a lattice model. The next section describes the Gaussian random field method (Woo et al, 2001), which provides a computationally efficient route to generate realistic representations of the disordered mesoporous glasses. Application of the mean field theory, and Monte Carlo simulations are described in Secs. 3 and 4, respectively.
156 2. M E S O P O R O U S
GLASSES
Vycor or controlled pore glasses are synthesized by quenching a binary mixture of silica and boron oxide to low temperatures, which induces spinodal decomposition and a glass transition, resulting in the disordered network of pores interconnected throughout the space (Levitz et al, 1991; Gelb et al, 1999). The Gaussian random field method considers a stochastic random field in the threedimensional space, which is constrained to have a number of statistical properties corresponding to the physical material. For Vycor glasses, the porosity and the structure factor measured in Figure 1" Two-dimensional crossthe small-angle neutron scattering exper- section of a Vycor glass realization geniments serve as the minimal set of con- erated by the Gaussian random field straints (Woo et al, 2001). The three- method. The cubic grid size is 10/~. dimensional space is discretized into a cubic grid. Random fields are generated on the grid points in the Fourier space with correlations designed to match the experimental structure factor of the Vycor glass, and inverse Fourier-transformed into the real space via fast Fourier transform (Press et al, 1992). A level-cut of the resulting field (Cahn, 1965) yields a realization of the solid density distribution in the real space. In contrast to the approach of simulating the physical synthesis of the material, the computational cost involved in the Gaussian random field method is minimal. Statistically independent realizations with the desired porosity and spatial correlations can be repeatedly generated simply by changing the random number seed of the Gaussian random field. 3. M E A N F I E L D T H E O R Y A lattice model Hamiltonian that has been successfully used to model fluids in mesoporous glasses reads H - - J ~_, n i t i n j t j - # ~ niti - y J ~ 8 nm (pore condensation pressures p/po > 0.7), for which the Kruk prescription overestimates the true pore size, a corrected value of D was derived from an in-situ SANS nitrogen adsorption study. Details of the synthesis and characterisation of the materials are given elsewhere. 8'9 Some properties of the materials are summarized in Table 1. A disordered mesoporous silica material (sample C1) was obtained by the route outlined for SBA-15, when the Pluronic P123 used as the template was replaced with an equal-weight mixture of Pluronic and trimethylbenzene. SEM micrographs indicate that this material constitutes a system of spherical pores of wide size distribution, connected and accessible by small mesopores and/or micropores only. The nitrogen adsorption isotherm indicates a wide pore size distribution and a H2 type hysteresis loop. Some properties of this material are included in Table 1. The experimental fluids, trifluoromethane (CHF3) and hexafluoroethane (C2F6) were obtained from Linde (purity 99.95%) and were used as received. 2.2. Adsorption measurements Adsorption measurements were made by a gravimetric method using two ultra-microbalances (Sartorius S3D), suitable for measurements in the low-pressure region up to 1 bar (isotherms up to 188 K for CHF3), and the region of elevated pressures (isotherms up to the critical temperature), respectively. Due to the symmetrical two-pan construction of these balances, buoyancy effects of the sample can be compensated by using combinations of taring materials. In this way it is possible to determine the mass of the fluid in the pore space in a direct way. Details of the experimental setup and procedure are given in ref. 9.
179
Table 1 Properties of materials used in this study: Ordered mesoporous silicas MCM-41 (M1 - M3) and SBA-15 (S1 - $3), and a cellular silica material (C1).
p/Po p/po 'Sampie ao nm Ads. Des. M1 4.04 0.280 0.280 M2 4.63 0.354 0.354 M3 4.92 0.418 0.411 S1 9.33 0.66 0.59 $2 11.1 0.75 0.68 $3 12.3 0.81 0.74 C1 0.88 0.49
D
Vp
nm
cm 3 g-1 m 2~1
3.4 3.8 4.2 7.2 8.9 9.5 ~18
0.79 0.90 0.99 0.87 1.06 1.42 0.57
as
1060 1040 870 760 750 910 540
4.0
9 ~t ,~ 1 ~'E
0.o
~'~
0.0
,,,K / r ~ - - - t I"K ~ ~ ~ -
0.2
0.4
1~/~233
0.6 P/P0
9
l
0.8
9 -
K
?
1.0
~.,]
/ ~ 4
I''K
~'oE 1.S
0.0
0.0
2G3 K
0.2
0.4
0.6 PIP0
0.8
1.0
Fig. 1. Sorption isotherms o f CHF3 in the SBA-15 sample $2 (Fig. 1A, lett) and in the cellular material C1 (Fig. 1B, right) for 8 different temperatures in the range 168 K to 293 K, as indicated in the graphs. The adsorbed amount is expressed by the mean density p of the pore fluid. For clarity the isotherms are displaced along the ordinate by increments of 0.3 gcm "3 (Fig. 1A) or by 0.2 gcm -3 (Fig. 1B). The critical temperature of CHF3 in the bulk fluid state, To, is 299.1 K.
180 3. RESULTS 3.1. Sorption hysteresis in MCM-41 and SBA-15 Sorption isotherms of C H F 3 and C2F6 in the MCM-41 and SBA-15 samples listed in Table 1 were recorded over the entire pressure range (0 < p/p0 10 l/h). The flow rate bypasses the TG-DSC and is released to the hood until the end of the outgassing procedure. Once the outgassing is completed, we switch from the pure helium flow to the VOC-He flow in the TG-DSC. The total flow rate passing through the saturators is split in two parts: the first one (2 l/h) goes to the TG-DSC; the second part (> 8 l/h) goes to the FID gas analyzer to perform the concentration measurements. During the adsorption step, the sample is then theoretically submitted to an instantaneous concentration step. In fact, a finite time is required to reach the equilibrium concentration. The mass uptake (TG signal) and the heat flow (HF signal) versus time as well a s Ts, Ptot and y are recorded during the entire adsorption experiment. The adsorption equilibrium is reached when the TG signal is constant (TGeq). The difference between TGeq and TG,,s gives the adsorbed mass at equilibrium (meq).
271 3.
T R E A T M E N T OF T H E R E S U L T S
3.1 Determination of the V O C partial pressure in the V O C - H e flow
The VOC partial pressure (Pvoc) in the helium flow passing through the calorimeter is given by: Pvoc = Y eatm
(1)
In this equation" - Patmis the atmospheric pressure (TG-DSC operating pressure), - y is the VOC molar ratio in the gas flow. y and thus Pvoc depend on the VOC temperature in the saturators (T~). y may be determined by two differem methods : A. By direct measurement from the gas analyser
The VOC molar ratio is recorded as a function of time with a frequency of one value per 10s. B. By use o f a vapor pressure law
The VOC molar ratio is given by: y=
P,,,,-voc(Ts)
(2)
Ptot
in which" Ptot is the total pressure at the outlet of the saturators, - Psat-vocis the VOC vapour pressure at temperature Ts. This equation is based on the assumption of a helium flow saturated by the COV. P, at-VOC are calculated from a correlation based on the DIPPR database . The validity of this correlation in the low pressure domain has been checked by comparison with experimental data from Mokbel et al [ 10]. Discrepancies between the two series are lower than 2%. -
3.2 Treatment of the HF signal
A. Thermodynamical meaning o f the H F signal
The HF signal is given by" HF = -fads "qst
in which" - f~dsis the adsorption flow rate, qst is the isosteric heat of adsorption. -
(3)
272
The integration of the HF signal gives the heat released during adsorption for the corresponding adsorbed quantity on the basis of the isosteric heat. Eq. 3 shows that this heat corresponds to a global variation of enthalpy (AH). B. Calculation o f the experimental and &osteric heats o f adsorption
The experimental heats presented in the "results and comments" part of this paper are calculated by dividing AH (result of the integration of the HF signal) by the outgassed sample mass (mos) "
AH Qoxp = ~
(4)
m os
Those experimental heats are pseudo integral heats of adsorption. They are indeed related, as shown by Eq. 3, to the isosteric heat and not to the differential heat of adsorption. Accuracy on Qexp is 5%. The isosteric heat is calculated from the derivation of Qexp versus meq (adsorbed mass at equilibrium) 9
0Qo~
(5)
q st = m os " C~m e q
Qexp(meq) is obtained by performing several adsorption experiments for different VOC concentrations. The correct calculation of the isosteric heat requires the derivation of Qexp(meq). Very otten, Ameq between two experiments. This procedure provides the average isosteric heat of adsorption q~t~. q~t~ is equal to q~t if the mass uptake between two adsorption experiments remains low. This assumption is difficult to check for systems exhibiting type I isotherms with a high initial slope. That is the reason why we present the calorimetric results on the basis of Qexp as a function of meq. qst is calculated by dividing Qexp by the mass uptake
3.3 T r e a t m e n t o f the T G signal
The difference between the TGms and TGe signals gives the outgassed sample mass mos. Such a procedure does not take into account the buoyancy effect on the volume of the sample. The real sample m a s s (mosr) Can be calculated from the measured sample mass by: mos = mosr - p'Vaa ~
(6)
in whichV,,dsis the volume of the sample, - p is the gas density. -
The pVads term is the buoyancy effect on the volume of the adsorbent. It can be neglected as it leads to an error on the sample mass of about 0.04%.
273
The difference between TGeqand TGms gives the adsorbed mass at equilibrium (meq).The buoyancy effect on the adsorbed phase is neglected. The coverage ratio is calculated by: | =
m eq
(7)
mo,M
in which: M is the molar mass of the adsorbate. -
4.
RESULTS AND COMMENTS We studied the NaY/toluene system at 150~
and for high partial pressure of toluene The adsorption isotherm and the experimental heats of adsorption are shown on Fig. 2 and 3. Such data correspond to the end of the Henry area and to the saturation plateau of the isotherm. The average isosteric heat calculated for a coverage ratio of 2.081 mmol/g is 97.8 kJ/mol. In this area it is possible to compare our values to existing data. The discrepancies appear to be less than 5% on the adsorption isotherm. We provide a first result in the Henry area (Pcov=l 1Pa) for which it is not possible to achieve a comparison with other data. Going down to the Henry area leads to very long experimental times due to the low VOC concentration in the He flow. Such long experimental times are not a problem for the adsorbed mass measurements but could lead to important errors (up to 10%) on the determination of the heat of adsorption due to the long integration time.
(Pcov>30 Pa) corresponding to saturation temperatures (Ts) higher than -40~
As a conclusion, we provide a new experimental procedure which allows to reach the Henry area for VOC/adsorbent systems. It provides both the adsorption isotherms and the heat of adsorption. Comparison studies with existing data at high temperature should be performed prior to the data acquisition at lower temperatures.
~._m.2"5
f
i2 m 90
......,i~
.......
3E............... T
& ....................
&
1
u 0.5
0
0.2
0.4
0.6
0.8
1 1.2 P lkPa
Fig. 2 "NaY/Toluene 9Isotherm/150~
1.4
1.6
1.8
2
2.2
274
300 250 200
8-
d
~5o
100 50
0.5
1
1.5
Coverage ratio I m m o l . g "1
2
2.5
Fig.3 9NaY/Toluene 9Experimental Heats REFERENCES
D.M. Ruthven, Principles of adsorption and adsorption processes, John Wiley, New York (1984) [2] J.F.M. Denayer, G.V. Baron, Adsorption 3 (1997) 251-265 [3] J.R. HuRon, D.M. Ruthven, R.P. Danner, Microporous Materials 5 (1995) 39-52 [4] D.M. Ruthven, B.K. Kaul, Ind. Eng. Chem. Res. 35 (1996) 2060-2064 [51 S. Palmas, A.M. Polcaro, R. Carta, G. Tola, J. Chem. Eng. Data 36 (1991) 1-4 [6] J.C. Moise, J.P. Bellat, A. M6thivier, Microporous and Mesoporous Materials 43 (2001) 91-1001 [7] Z. Chen, J. Lu, X. Liu, T. Ding, Chinese J. of Chem. Eng. 8(4) (2000) 283-286 [8] D.M. Ruthven, I.H. Doetsch, AIChE journal 22(5) (1976) 882-885 [9] J.-H. Yun, D.K. Choi, S.-H. Kim, AIChE journal 44(6) (1998) 1344-1350 [10] I. Mokbel, E. Rauzy, J.P. Meille, J. Jose, Fluid Phase Equilibria 147 (1998) 271-284
[1]
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
275
Energetics and Mechanism of Physical Sorption by Carbonaceous Solids" Evaluation of surface area and porosity factors. E. Loren Fuller, Jr. Lorela Enterprises, PO Box 355 Stanton NE 68779-0355, USA The physical adsorption of inert gases (nitrogen, argon, etc.) is the most informative method for evaluation of surface area and porosity of finely divided materials. Small (in the molecular size range) micropores interact very strongly -- sorbing at very low pressures (higher sorption potential). Nitrogen adsorption was analyzed in terms of the AutoShielding Potential (ASP) theory for a series of nonmicroporous carbonaceous chars, microporous (activated) chars and nonmicroporous deactivated chars.
i. Introduction Current industrial and environmental processes are increasing the need for storage, separation, purification, sorbents and techniques for characterizing sorbent systems. Physical sorption techniques I traditionally involve the measurement of the amount of sorbate associated with a sorbent at a fixed (isotherm) temperature with volumetric or gravimetric techiques. 2 There are few, if any, techniques that can probe in great detail into and around composites, molecular sieves, agglomerates, etc. Granted the resulting isotherm, expressing sorption, F, as a function of pressure, P:
r=r(#)
(1)
is inherently complex and requires statistical and thermodynamic interpretation with respect to the relevant properties of interest (surface area, pore volume, pore sizes, sorp~ion energy, etc.) 3 ' This work examines physical sorption (physisorption) as the result of attraction of gaseous species into the force field (sorption potential) that exists at the heterogenic boundary at the interface between solid and fluid (vacuum) phases. Modeling of the physisorption processes involves the considerations: m Low energy, less than that noted for chemical bond formation (chemisorption) and more than that for condensation of the liquid. The sorbed entity is statistically associated with the substrate with the loss of only a few degrees of freedom. In the limit, there is only one degree of translational motion lost in the physisorption process. 9 Loose association with the surface with free exchange with the gas phase. Even for crystalline materials, the periodic potential fields cannot be very great or else the
276
9
9
9
9
9
9
process of binding would be in the classical realm of chemisorption (and undoubtedly irreversible at the temperature of the isotherm experiment). There is no association of sorbed entities with any given site. This precludes analyses of the Langmuir and the related BET a theory for physisorption isotherm analyses. Each sorbed molecule is free to move across the surface unhindered with completely elastic collisions and/or escape to the headspace. There is no way to specifically identify any molecule except as a statistical approximation as to its location. Rapid equilibration limited to the transport to the surface. The only rate effects to be noted are enthalpic equilibration (exothermic heat transfer) and/or diffusional over very short distances. The exception would be for small micropores if they exist in the inner reaches of a bulk form, such as monolithic zeolites. Liquid like properties with the concept of density meaningless in the classic sense of a three dimensional definition. The density of the sorbed species can have only statistical meaning and achieves physical meaning as the liquid film state is formed. Energetic trends are continuous in, and through, the monolayer and multilayer formation stages -- ever decreasing as the surface concentration increases. This in contrast to the assumptions inherent in the BET "multilayer' theory 4 where all layers beyond the first are assumed to be formed with energy equal to that of liquid condensation. In reality this condition occurs only at the equilibrium vapor pressure of the sorbate liquid at the temperature of the isothermal experiment. Wide pressure range for data acquisition to allow more complete description of the system of interest. It all to easy to overlook important trends and construct an incomplete or inaccurate paradigm. One should avoid limiting the analyses to restricted data regimes such as the traditional "BET region" of the sorption isotherm. Maximum number of data points should be acquired to further elucidate trends or show the absence of thermodynamic and/or kinetic trends. Modem automated computer driven equipment has permitted the efficient use of operator time and equipment. 5 Low pressure data should be included to reveal the limiting pressure below which there is no physisorption. In the limit, the first physisorbed molecules must have a finite vapor pressure below which there is no physisorption per se, There is no perfect vacuum limit for physisorption isotherms!
Inherent in all of these concepts is that a monolayer (or any other multilayer) does not exist as a discrete entity, but is only described (exists) in a statistical sense. This is not to detract from the importance and utility of such a parameter as a means of describing a substrate in terms of surface area, porosity, etc. The AutoShielding Potential (ASP) theory 6 has shown an exponential relationship for the Polyani (Verb. Dtsch. Phys. Ges. (1914) 16 1012) sorption potential -- defined as the reversible work required to achieve the equilibrium pressure, P, in a closed system. This is conveniently expressed for the single vapor component with respect to the liquid sorbate reference pressure, P(0), at a given temperature, T, E = -RTIn[P/P(0)]
(2)
277 with respect to the monolayer/multilayer sorption. In essence this is the amount of reversible work required to be done on a system to achieve the given state with respect to the liquid phase at the temperature of the isotherm. This is a minimum amount of work that must be done on the system in industrial processing. The ASP theory casts the adsorption isotherm into a rectilinear coordinate format with respect to the initial sorption potential, E*: E = [E*]exp(-0)
(3)
where 0 = fractional monolayer coverage = F/F(m)
(4)
with F expressed in the desired sorption units [cc(STP), mg/g, etc.] relative to the monolayer capacity. The first order rectilinear form r = r ( m ) {In(E*/RT)- In(E/RT)}
(5)
This can be represented in rectilinear form: F = b(0) - b(1) In(E/RT)
(5a)
a simple mathematical relationship with only two adjustable parameters that accurately defines physical sorption isotherms for numerous organic7 and inorganic8 substrates with only two (2) adjustable parameters over a wide pressure range. The relationships of Equations 5 and 2 are unquestionably valid for unlimited surface coverage on ideal external open (flat, planar, accessible) surfaces ranging from nil at E* to infinity at E=0. All of the inherent assumptions (tabulated above) are equally valid as models for physical adsorption in internal constricted regions. These are classically denoted as ultramicropores ( 1.0 > 1000 (loss of structure) 5 2-4.5 > 1.0 > 1000 MCM-48 (loss of structure) 5-6 2-10 0.2-1.0 300-1000 MCMoidal 5-6 (structure remains) 8-9 50-1000 0.2-4.0 1-50 Xerogel 8-9 Table 1" Comparison of chemical and pore structural data of highly ordered and amorphous silica adsorbents, aOH: silanol group concentration; rehydroxylation: treatment with hydrochloric acid under reflux for 12 h. MCM-41
2-4
As previously published [9], the MCMoidal material is classified as bridging the gap between highly ordered (MCM-41, MCM-48) and totally amorphous (xerogel) silica adsorbents in respect to their crystallinity, pore structure and behaviour towards water.
343
In other words, the loss in long range order and specific surface area is accompanied with an increase in stability towards water or water vapour as shown in Tab. 1. This advantage can be exploited by applying a hydrothermal treatment in order to enlarge the pore size. The hydrothermal treatment is conducted under reduced basic conditions as the mother solution is diluted with water. However, a pH adjustment aider dilution to a value of 10 did not show any changes in the resulting pore size enlargement. The concentration of water was found out to have a significant effect on the hydrothermal process (Tab. 2).
Ratio R a, (BET) vp (G) pa (BJI~ / ml g-1 / nm mother solution / water / m z g-1 1 no treatment 908.6 0.64 2.7 2 1.2 413.5 0.69 5.1 3 1.7 367.3 1.04 9.1 Table 2 Effect of the ratio between volume of mother solution to volume of added water prior to hydrothermal treatment on the resulting pore structural parameters. Material
.
.
.
.
.
.
.
When no water was added (R-value of oo) and hydrothermal conditions were applied, the material totally lost its pore structure. With increasing amount of water (at constant total volume) and decreasing ratio R, we could create graded porosity starting from a parent silica (material 1 in Tab. 2). The pore size distributions of these MCMoidal beads are displayed in Fig. 4.
9v
-E ~,
A
8
il
ii ,' I!
i
II il,, I i
II | I I |
Iliil, l
.;
t
t
.I
1
i
i
as synthesised, 2.7 nm --. - -- hydrothermally treated, 5.1 nm -- -- hydrothermally treated, 9.1 nm
t
i
i
i'l"
lO
i
i
I
i
1
i
i
lOO
I
average pore diameter Pd (BJH) / nm
Figure 4: Pore size distributions ofhydrothermally treated samples in comparison to the untreated parent silica. In order to understand the mechanism of the pore size enlargement by hydrothermal treatment, transmission electron microscopy (TEM) was applied. Both treated materials in Fig. 5 consist of primary particles of the same dimension. The difference in contrast indicate that the pore size enlargement is accompanied by partial dissolution and rearrangement of those spheres.
344
Figure 6: Comparison of bimodal macro- and mesopore arrangements of agglomerates in dependence of the synthesis temperature of primary MCMoidal particles. The spray drying process is a common technique for particle size enlargement [10] and was applied to create agglomerates in the micron size range l~om suspensions of nano-particles with different dimensions. The synthesis of the primary particles was carried out like mentioned above with temperature control between 15 and 50 ~
345 The resulting suspensions were spray dried directly and the products were characterised by SEM and nitrogen sorption at 77.4 K. The bimodal pore size distributions of the agglomerates are shown in Fig. 6. The syntheses lead to primary pore diameters of approximately 2.2 nm regardless from the temperature applied. A slightly broader primary pore size distribution is observed at high reaction temperatures indicating a modified behaviour of the n-alkylamine. The secondary pore size distribution depends on the primary particle size which decreases with increasing synthesis temperature and the inter-particle voids create the second pore system. Thus, the packing of the spheres in the agglomerates is a crucial parameter. It was found out for spherical particles, however, that the average interstitial void diameter corresponds to about 40 % of the particle diameter. The mean particle diameters were calculated according to this correlation and show good consistency with the observed values obtained by SEM pictures (Tab. 3). synthesis temperature
secondary pore mean particle size mean particle size diameter (SEM) (2.s xt,,O T/~ pa2 (BJH) / nm dpso / nm dpso / nm 15 200.1 500 400- 750 25 166.4 416 300-500 30 103.3 258 200-450 35 43.2 108 -100 40 26.1 65 -* 45 27.9 70 -* 50 26.0 65 -* Table 3" Comparison of primary particle sizes in agglomerates. *: too small to measure.
Figure 7: SEM pictures of primary particles (left) and resulting agglomerates (fight). The bimodal agglomerates possess spherical morphology with sizes between 4 and 20 gm depending on the parameters of the spray drier. Agglomeration of smaller primary particles results in smoother agglomerate surface and better morphology.
346 4.
SUMMARY AND OUTLOOK
Bimodal spherical agglomerates where prepared using a two step procedure. The porosity, shape and size of the primary particles were adjusted using a templated St0ber synthesis in combination with temperature control and hydrothermal alter-treatment of the products. In a second step, the primary particles were subjected to agglomeration and the secondary pore size and morphology of the materials was tailored by the applied primary particles and the spray drying parameters. Thus, the hierarchy exists on two levels. On the first level spherical particles of 10 nm build up the primary pores and on the second level the packing of primary particles build up the secondary pores in the agglomerates. The possibility of tuning both pore systems independently makes these hierarchical materials valuable for designed applications. The secondary pore system enables a fast access (high mass transfer kinetics) to the primary pore system which are responsible for the obtained high specific surface areas (high loadability). 5. L I T E R A T U R E
10
K. Nakanishi, Journal of Porous Materials, 1997, 4, 67-112. K. Nakanishi, H. Minakuchi, N. Soga and N. Tanaka, Journal of Sol-Gel Science and Technology, 1997, 8, 547-552. K. Cabrera, G. Wieland, D. Lubda, K. Nakanishi, N. Soga, H. Minakuchi and K.K. Unger, Trends in Analytical Chemistry, 1998, 17, 50-53. W. Strber, A. Fink, E. Bohn, J. of Colloid and Interface Sci., 1968, 26, 62-69. Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing, C.J. Brinker, G.W. Scherer, Academic Press, London, 1990. C.G. Tan, B.D. Bowen, N. Epstein, J. Colloid Interface Sci., 1987, 118, 290. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.TW. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc., 1992, 114, 10834. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature, 1992, 359, 710. D. Kumar, K. Schumacher, C. du Fresne von Hohenesche, M. Gr'tin and K.K. Unger, Colloid and Surfaces, A: Physiochem. and Eng. Asp., 2001, 187, 109. H.R. Wuthrich, Ullmann "sEncyclopedia oflndustrial Chemistry, Volume B2, 5 th Edition, John Wiley & Sons, Chapter 7, 1988.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
U l t r a t h i n p o r o u s glass m e m b r a n e s
347
with controlled texture properties
D. Enke a, F. Friedel a, F. Janowski a, T. Hahn a, W. Gille b, R. Miiller c, H. Kaden c Institute of Technical Chemistry and Macromolecular Chemistry, University of Halle, SchloBberg 2, D-06108 Halle/Saale, Germany b Department of Physics, SAS-Laboratory, University of Halle, Hoher Weg 8, D-06120 Halle/Saale, Germany c Kurt-Schwabe-Institute of Measuring and Sensor Technique Meinsberg, Fabrikstrasse 69, D-04720 Ziegra-Knobelsdorf, Germany a
Porous glass membranes were prepared in shape of ultrathin plates from a sodium borosilicate initial glass by phase-separation and combined acid and alkaline leaching. The thickness of the membranes varies between 100 and 300 gm. The textural characteristics of these materials were determined using nitrogen physisorption, mercury intrusion, scanning electron microscopy and small angle X-ray scattering. Gas permeation measurements were used to investigate the transport parameters of the porous glass membranes. Furthermore, the mechanical stability and the optical transparency (by UV-VIS spectroscopy) were characterized. The pore structure of the membranes can be tailored in the range between 1 and 120 nm. 1.
INTRODUCTION
In the last years increasing research activities in the fields of membrane science [1, 2], chemical sensors [3], confined matter [4] and micro-reaction engineering [5] have evoked a new interest on porous glass membranes. Furthermore, such membranes are ideal model systems for the investigation of transport processes in porous structures. This broad spectrum of applications demands variable texture properties. Porous glass membranes are currently available only on the basis of Porous VYCOR Glass (PVG) in shape of tubes or discs. That means, the pore size is fixed between 4 and 10 nm [6]. However, pore sizes of 1 nm or lower are required for an adequate selectivity in gas separations. That's why several procedures were developed to reduce the pore size in the VYCOR substrate down to the molecular size level (below 1 nm). Levy et al. [1] modified porous VYCOR tubes using low pressure chemical vapor deposition of SiO2. Beltsios et al. [2] modified the surface of VYCOR membranes by a commercial silicone which is subsequently subjected to oxygen plasma and converted to SiO2. In another approach, these authors used the multilayer Langmuir-Blodgett (LB) deposition of fatty acid salts, sesquioxane polymers and pitch products, followed by a plasma treatment to prepare gasseparating asymmetric membranes on the basis of PVG [5]. Dong and Wong [7] prepared composites on the basis of a microporous MFI-type zeolite on the surface of a flat mesoporous glass membrane by substrate self-transformation. On the other hand, pore sizes above the range covered by PVG are demanded for a good performance of a porous glass membrane in optical sensor devices (fast sensor response) and for the systematical investigation of confinement effects. The various applications require the determination of the texture and transport characteristics of such porous glass membranes. This can be done by combination of equilibrium
348 and dynamic methods, as nitrogen adsorption, mercury porosimetry, molecular probes and permeability measurements, with small angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) [8]. This contribution describes the preparation of mechanically stable flat porous glass membranes with variable texture properties on the basis of phase-separated sodium borosilicate initial glasses and their characterization with a series of techniques. 2.
EXPERIMENTAL
Two sodium borosilicate initial glasses were used in this study. The initial glass (1) in shape of 1 0 - 15 mm thick plates was prepared from a glass melt of the composition 70 wt.-% SIO2, 23 wt.-% B203 and 7 wt.-% Na20 by roller quenching, followed by an optical fine cooling at 300~ for 1 h and a subsequent cooling in air. The initial glass (2) was prepared by moulding the same glass melt in a thicker plate in air, followed by an optical fine cooling. The latter is characterized by a more progressive phase-separation. The initial glasses were cut with a diamond circular saw (SAW 15, Logitech) in smaller glass blocks (i.e. 25x25xl 0 mm), which were phase-separated in the temperature range between 530 and 720~ The initial glass (1) was used up to 610~ The initial glass (2) was employed above this temperature. In the next step, the phase-separated glass blocks were cut in ultrathin flat plates (i.e. 25x25x0.1 mm) using an annular precision (Annular 55, Logitech) and a diamond band saw (SAW 15, Logitech). After that, the ultrathin plates were leached in special baskets with 1 N hydrochloric acid at 90~ for 1 h. The colloidal silica deposits remaining in the pore system of the glass membranes after acid leaching of initial glass plates phase-separated above 550~ were removed by treatment with 0.5 N sodium hydroxide solution at room temperature for lh. Nitrogen sorption measurements were performed by use of a Sorptomatic 1900 Turbo apparatus by Carlo Erba Instruments. All samples were degassed at 393 K before measurement for at least 24 hours at 10-5 mbar. The mercury porosimetry measurements were carried out on a Porosimeter 2000 apparatus by Carlo Erba Instruments. A contact angle of 141.3 ~ for Hg was used. The samples were degassed at 393 K before measurement for 24 h. SEM of the porous glass membranes was carried out on a Phillips ESEM XL 30 FEG microscope. SAXS measurements were performed using nickel-filtered CuK~ radiation. The membranes were studied directly. About 120 points of the smeared relative scattering intensities in the region of the scattering vector h, 0.05 < h < 3 per nm, were recorded point by point using a Kratky camera. After background subtraction about 2500 impulses from the scattering of the sample were obtained at the position hmax= 3 per nm. The experimental intensities have a dynamic of 5 decimal powers after the application of the special collimation correction procedure [9]. The SAXS curves measured were analyzed applying the theory of chord length distribution (cld) [ 10, 11 ]. The ultrathin porous glass membranes were examined for the permeability of air at 25~ The measurements were performed as follows: The membrane was sticked on a brass plate containing a bore with 2 and 5 mm diameter, respectively. After that, the plate was fixed with a special dome on a turbo molecular pump (TMU 261, Pfeiffer). Now, the dome was evacuated. The gas flow was determined by measuring the pressure below the membrane. Finally, the integral permeability was estimated using the pumping speed, the measured pressures and the membrane thickness. The UV-VIS spectroscopic studies on the ultrathin porous glass membranes were performed in the wavelength range 2 0 0 < ~ , < 8 0 0 n m using a 8452 Diode Array Spectrophotometer by Hewlett Packard. A standard cuvette possessing a special membrane
349
holder was used. The self absorption of the cuvette was estimated prior to each measurement and deducted from the spectrum of the investigated membrane. The mechanical stability of the membranes was characterized using the material specific fragility which was standardized on the membrane thiclmess. The measurements were performed with an universal material testing machine Zwicki 1120 by Zwick. The membranes were placed on a mounting plate possessing a bore with 4.4 mm in diameter and breaked through by a punch with 3.2 mm in diameter. The force required was recorded in combination with the distance covered by a 200 N force detector. The test velocity of the punch was 0.01 mm/min. The measurement was finished after a force drop of 85 %. 3.
RESULTS AND DISCUSSION
The texture properties of the ultrathin porous glass membranes prepared in our laboratory were initially characterized by the equilibrium based methods nitrogen gas adsorption and mercury porosimetry. The nitrogen sorption isotherms of two membranes are shown in Fig. 1. The fully reversible isotherm of the membrane in Fig. 1 (A) can be classified as a type I isotherm according to the IUPAC nomenclature which is characteristic for microporous materials. The membrane in Fig. 1 (B) shows a typical type IV isotherm shape with hysteresis of type H1 (IUPAC classification). This indicates the presence of fairly uniform mesopores. The texture characteristics of selected porous glass membranes are summarized in Tab. 1. The variable texture demanded the application of various characterization techniques and methods of evaluation. It is obvious that the porosity of the membranes can be varied by the conditions of heat treatment of the initial glass blocks. The time and temperature of the heat treatment for phaseseparation act in combination with the leaching conditions as structure directing parameters in the preparation process of porous glasses [6]. The great flexibility of the practicable pore sizes allows a tailoring of the membrane properties to many different applications. However, there are no general relationsships between the specific surface areas and pore volumes and the average pore diameters of the membranes. This may originate from the peculiarities of the characterization techniques, residues of colloidal silica in the pore system of the meso- or macroporous glass membranes or from differences in the microstructure of the initial glasses. Systematic investigations are in progress. The mercury porosimetry intrusion plot of a mesoporous glass membrane is given in Fig. 2. That curve and the isotherm shapes in Fig. 1 are indicators for a relatively narrow pore size distribution of the ultrathin porous glass membranes. 200 180160140~'g 120"~ 10080-
60i
B
t
300] . B ~ ii . . L ~ ' L T J ' l l ~
~---
-
~
200
l.l"
,? ,
40202
0
0,0
t-t d
0;2
0;4
0',6
P/P0
0;8
1,0
O/
0,0
T
.
.
0,2
.
0]4
.
0]6
P/P0
Fig. 1 Nitrogen sorption isotherms at 77 K of two ultrathin porous glass membranes (A microporous, B mesoporous)
.n"
0',8
1,0
350 450 ,---, 400 e~
& o o
350 300
250 200 150 100 50 0
.....
O,1
9..-:7"-
1
~~
. . 7 o -
o~
~~
10 1 O0 pressure [bar]
~"7--7-. .~] .... , 1000
Fig. 2 Mercury porosimetry intrusion curve of a mesoporous glass membrane Table 1 Textural properties of some ultrathin porous glass membranes prepared from a sodium borosilicate initial glass by varying of the conditions of heat treatment Temperature Time Spec. surface area Spec. pore volume Avg. pore dia[~ [h] [m2/g] [cm3/g] meter [nm] _ _ 4121, 2 0.1461 540 24 2861'2 0.1681 570 24 1811'3 0.7191 111'4 570 24 1795 0.5645 125 610 24 1955 0.7515 185 630 24 665 0.4705 475 680 24 525 0.4825 725 700 24 365 0.4465 965 720 24 415 0.4635 1125 1N2 sorption at 77 K, 2DUBININ-RADUSHKEVICH, 3BET, 4BARRETT-JOYNER-HALENDA, desorption branch, 5Mercuryporosimetry
Fig. 3 Scanning electron microscope images of a mesoporous (A) and a macroporous glass membrane (B)
351
0,14 0,12 lO
_
0,10
-
0,08 -
'|
-
,~, "z3
0,06
=
0,04
-
'~1.................... "- ....
0,02 0,00
".
I 1
'
I 2
'
d/nm
I 3
'
I 4
' 5
Fig. 4 Chord-length distribution 7"(d), 0.5 n m < d < 5 nm, inset: the corresponding SAXS curve obtained for the microporous glass membrane Scanning electron micrographs given in Fig. 3 clearly show a homogeneous texture of the corresponding meso- or macroporous glass membranes. Fig. 4 shows the chord-length distribution 7"(d), obtained from a SAXS experiment, of the microporous glass membrane, see Fig. 1 (A). 7"(d) allows, in combination with the results of other characterization techniques (nitrogen sorption, mercury porosimetry, scanning electron microscopy), a detailed characterization of the microstructure of this sample. Here, the lower resolution limit of the SAXS experiment, dmin = 0.5 n m , leads to difficulties in the assignment of the peaks and shoulders obtained. However, it is possible to explain all peaks using a linear simulation model, recently published by Gille [12]. Furthermore, the porosity of the membrane determined by nitrogen sorption at 77 K can be inserted as a check. In Fig. 4 the main maximum of the chord-length is observed below d = 1 nm. This maximum can be interpreted in agreement with the results of the nitrogen sorption experiment. It reflects the mean chord-length of the micropores. The resolution limit of the SAXS experiment slightly influences this peak position. It is shifted to larger chord-lengths. Additionally, a shoulder is found at about 1.5 nm. Based on the simulation model, Fig. 5, the peak and the shoulder can be explained in terms of = 0.5 nm (mean chord-length of a pore) and <m> 1.55 nm (mean chord-length of the wall). This is supported by the porosity c of the investigated membrane. On one hand, the porosity is given by c = /(+<m>)= 24 % [11 ]. On the other hand, c follows from the network density of the membrane, 9 = 2.2 g/cm 3, and the specific pore volume, Vp= 0.146 cm3/g, determined by nitrogen sorption. Therefore c = 1 / ( I + I / ( 9 " V P ) ) = 24%. Based on the curvature behaviour of the SAS correlation function 7(0, the distribution laws of random distances between the interfaces were analyzed [12]. This is examplified for a sequence of 4 pores and 3 walls in Fig. 5. Furthermore, the appearance of all minima and maxima in the chord-length distribution, see Fig. 4, at d = 2.1 nm, 2.5 nm, 2.9 nm .... is traced back to the mean chord lengths and <m>. The permeabilities of air at 25~ were determined to get first informations about the transport properties. This was done by comparison of the values of the individual ultrathin porous glass membranes. The results obtained (Tab. 2) were used to check the quality (homo-
352
Fig. 5 Linear simulation model geneity, absence of cracks or defects, reproducibility) and to characterize the correlations between the texture and transport properties. All membranes are completely porous. The reproducibility of the permeabilities of membranes of the same pore size indicates the absence of cracks, defects or pinholes. This is confirmed by scanning electron microscope images (Fig. 3). The transport characteristics of the ultrathin porous glass membranes are strongly related to their textural characteristics. The permeabilities of the investigated membranes vary in dependence on the average pore diameter in a range between 3.10 -5 and 3" 10-2 cm2/s. The low air permeability of the membranes prepared by leaching of the roller-quenched initial glass of 3.5"10 -5 cmZ/s, is remarkable. These membranes possess in combination with their microporous texture (Fig. 1 and 4) potential for molecular sieve applications. However, only a "rough" estimation of the transport properties of the ultrathin porous glass membranes was performed in this study. Currently, the permeabilities of various gases, the relations of the textural and structural characteristics to the transport properties, the flow regime and molecular sieve effects are investigated. Table 2 Permeability of air for various membranes Temperature of Time of phaseAvg. pore phase-separation separation [h] diameter [~ [nm] 540 570 610 630
24 24 24 24
< 11) 1.42) 123) 183) 473)
Permeability [cm3(SyP).cm .minl .cm2. atm -1]
Permeability at 25~ [cm2. s -1]
1.95.10-3 9.6" 103 0.37 1.56 1.72
3.5.104 1.7" 10-4 6.6.10 .3 0.028 0.031
~SAXS, 2Nz sorption 2"Vp/Os,3Mercuryporosimetry The optical transparency of the porous glass membranes is interesting for sensor applications. It depends on the structural features of the membranes [ 13]. The UV-VIS spectra of micro- and mesoporous glass membranes are compared in Fig. 6. The membranes with an
353
.
200
.
.
.
300
.
.
.
400
.
.
500
18nm,
600
.
700
800
Z[nm] Fig. 6 UV-VIS spectra of porous glass membranes with different pore sizes average pore diameter of < 1, 12 and 18 nm show a high transmittance within in spectral range )~ = 200 - 800 nm. In contrast to that, the membrane with an average pore size of 47 nm is characterized by a fourfold higher extinction at k = 300 nm. This is also manifested in the strong opalescence of these membranes. Regarding the optical properties, the membranes with < 1, 12 and 18 nm average pore size are better suited for optical sensor applications. The fast response is another important parameter characterizing the quality of an optical chemosensor. Here, membranes with larger pore sizes are to be preferred. This means, these contrary parameters must be optimized for a given sensor application. The great variability of the membrane textures represents an important advantage compared to other porous materials used for the immobilization of chromophors. The characterization of the mechanical stability of the ultrathin porous glass membranes is an essential precondition to the application in micro-reaction engineering or for gas separations in the high pressure range. The material specific fragility was used in this study. The force and the distance covered by a punch were measured up to the fracture of the membrane. This is shown in Fig. 7 for porous glass membranes with < 1, 12, 18 and 47 nm pore size. In all cases, the increase of the force is linear up to the fracture, followed by a vertical decrease. This is characteristic for brittle materials. However, there are important differences between the investigated membranes. The fracture of the microporous membrane occurs at a force which is approximately twice as high as for the various mesoporous membranes. Consequently, the microporous membrane possesses the highest mechanical stability. The transformation of the network SiO2 to colloidal SiO2 and its alkaline removal in the preparation process leads to a decrease of the wall thickness of the resulting mesoporous membranes. This results in a decrease of the mechanical stability. The fine three-dimensional network of the microporous membrane reduces formation and propagation of fractures. The longer distance of the punch and the reduced specific fragility are indicators for a slightly higher "flexibility" of the membrane with an average pore size of 47 nm. 4. C O N C L U S I O N S Ultrathin porous glass membranes with variable texture properties were prepared from a SiO2-rich sodium borosilicate initial glass by careful fine tuning of the conditions of heat treatment for phase-separation. Pore sizes between < 1 and 120 nm can be realized. The membranes are characterized by a narrow pore size distribution. The transport, optical and mechanical properties vary with the pore size. The tailorable texture and transport characteris-
354
3,5 3,0 i
! 2,5
/
/
0). Table 1. Summary of freezing point elevation Approx. number of 6T[K] Distance [nm] molecular layers Cyclohexane 1.0 2.0 OMCTS
7.0 3.5
1.5
4.6
5
2.8 4.0
2.8 1.7
3 2
14 7
417 10
'
I
'
I
'
I
~
I
/,' //
E
/
//
/
/
/
/
/
/
/
/
/
/
//
0
/
/ /// 0
/ // / //
/
/
/
/
/
/
//
L.)
//
0
,
I
J
I
,
I
t
I
,
10 Observed pore width Wobs [nm] Fig.8 Comparison in terms of the width of nanospace for cyclohexane. The "excess" potential A~b is defined as the difference between the pore potential ~ore and that for a fictitious pore whose wall was made up of the liquid's solid state ~iquid. The underlying assumption includes: The perturbation in the structure of the solid phase is negligible in the slit pore; thus the temperature dependence of the chemical potential of solids in slit pore can be expressed with the entropy of the bulk solid Ss. Further details on the model are available in Ref. 3. For a strongly attractive wall A~b is negative, thus resulting in elevated freezing point. For larger pores [A~ becomes small and the freezing point approaches to the bulk value. The model was tested quantitatively with the simulation results, and showed good agreement. The potential function that is suitable to express carbon/graphite surfaces would be the so-called 10-4-3 potential [11]. If we look at, however, the potential energy at a distance in the range of a few nanometers, it can be approximated by the so-called 9-3 potential (qfl-3(z)=(2/3)rcpsCo3[(2/15)(cr/z)9-(cr/z)3]), and only the attractive part will be prevailing. Thus a single wall contributes to the A~b by the solid's ~b9-3 minus the liquid's one. Taking the potential energy at the center of the pore of width w as the simplest representative value, the A~b in question can be expressed as follows. A~b "-
w
2[Op9or3e J.
-- Oliquid( z )] =
0
W3
6
6
; where C = -~Jr(p,e,SCrsS - pse•Cr•)
(2)
With the above treatment, all that has to be determined for estimation by the model is now the parameter C. The definition of the parameter naturally reminds us of its tight relevance with the Frenkel theory for adsorption on a planar surface. Utilizing the theory, a method to determine the above parameter from a standard adsorption isotherm was developed by the authors based on the molecular simulation as an ideal experimental system [12], and successfully applied to a real experimental system [13]. Here to be needed for the determination is an adsorption isotherm of cyclohexane and OMCTS on a nonporous carbon or graphite. We found the data for cyclohexane [14], but not for OMCTS: The comparison can only be made here for the former liquid, whose parameter was determined as C=2.3x10 -22 [Jnm3]. Equations (1) and (2), together with the parameters, connect the freezing point elevation and the width of the nanospace. For a given elevation the comparison was made in terms of the width as shown in Fig. 8. Though the number of data points is quite limited, the predicted widths are roughly in good accord with the observed results. Note that the "parameter" needed for the prediction is NOT a fitting parameter but is the one determined from adsorption data, independently, based on its physical meaning. Considering the above, the agreement would be thought as a satisfactory one to demonstrate the validity of the model.
418 An examination of the results of OMCTS shall be made in the near future to confirm the reliability. 4. CONCLUSION Following the earlier molecular simulation work by the authors, in which freezing point elevation in nanospace had been predicted, an experimental trial for finding the elevation phenomena in the simplest geometry, a slit, was conducted. The employed technique for this purpose was colloidal-probe Atomic Force Microscopy. A carbon microparticle with high degree of carbonization was attached to the top of the cantilever tip, forming the colloidal probe, and its interaction force with cleaved graphite was measured within a liquid cell filled with organic liquid, controlled at a desired temperature above the bulk freezing point of the liquid. The two surfaces will form a slit-shaped nanospace because the radius of the particle is far larger than the separation distance concerned here. The results demonstrated that cyclohexane in the nanoscale slit space between the carbonaceous solids freezes when the distance comes down to 4 nm, even at 8.4~ which is above the bulk freezing point of 6.4~ Measurements with octamethylcyclotetrasiloxane, bulk freezing point being 18~ also detected the freezing behavior, e.g., at the distance of 1.7 nm at 22~ Further, the degree of elevation was in good accord with the prediction by our model based on the attractive potential energy of the wall. Though the extent of the elevation itself might look rather small, we believe that the finding of the definite existence of the elevation, and its accord with the prediction of our model would be of much importance in the research field of the phase behavior in nanopores. REFERENCES
[ 1] Over more than half a century many studies were reported: e.g., W.A. Patric and W.A. Kemper, J. Chem. Phys., 42 (1938) 369; J.A. Duffy, N.J. Wilkinson, H.M. Fretwell, M.A. Alam and R. Evans, J. Phys. Cond. Matter, 7 (1995) L713. [2] J. Klein and E. Kumacheva, Science, 269 (1995) 816. [3] M. Miyahara and K.E. Gubbins, J. Chem. Phys., 106 (1997) 2865. [4] H. Kanda, M. Miyahara and K. Higashitani, Langmuir, 16 (2000) 8529. [5] M. Miyahara, H. Kanda, M. Shibao and K. Higashitani, J. Chem. Phys, 112 (2000) 9909. [6] H. Dominguez, M. P. Allen, and R. Evans, Mol. Phys., 96 (1999) 209. [7] R. Radhakrishnan and K. E. Gubbins, Mol. Phys., 96 (1999) 1249. [8] K. Kaneko, A. Watanabe, T. Iiyama, R. Radhakrishnan and K.E. Gubbins, J. Phys. Chem. B, 103 (1999) 7061. [9] A. Watanabe and K. Kaneko, Chem. Phys. Lett., 305 (1999) 71. [ 10] M. Sliwinska-Bartkowiak, R. Radhakrishnan and K.E. Gubbins, Molecular Simulation, 27 (2001) 323. [ 11 ] W. Steele, The interaction of gasses with Solid Surfaces, Pergamon, Oxford (1974). [ 12] M. Miyahara, T. Yoshioka, J. Nakamura and M. Okazaki, J. Chem. Eng. Jpn., 33 (2000) 103. [13] H. Kanda, M. Miyahara, Y. Yoshioka and M. Okazaki, Langmuir, 16 (2000) 6622. [ 14] A.L. Myers, C. Minka and D.Y. Ou, AIChE J., 28 (1982) 1.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
419
Is it possible to obtain a coherent image o f the texture o f a porous material? Francis Noville a, C6dric Gommes a, Catherine Doneux b, Alain Brasseur a, Ren6 Pirard a, Jean-Paul Pirard a a Universit6 de Li6ge, Laboratoire de G6nie chimique, Institut de Chimie, B6a, B-4000 Li6ge b Solvay Research & Technology, Analytical Technologies Department, rue de Ransbeek, 310, B- 1120 Brussels
This study consists in verifying the coherence of a few commonly used analysis methods of nitrogen adsorption-desorption isotherms. These methods were tested on model samples obtained by mechanically mixing two micro- and mesoporous solids respectively with known mass proportions. Although the individual analysis methods may lead to discrepancies in the interpretation of the isotherms, their systematic comparison allows drawing a coherent picture of the porous texture. 1. INTRODUCTION The analysis of nitrogen adsorption-desorption isotherms is one of the most commonly used methods to assess the texture of porous materials. It has given rise to numerous theoretical studies and many mathematical models have been developed to analyze the results. These models establish a relationship between pressure and pore size on the basis of the real physicochemical adsorption mechanisms. However, the user is often bewildered by the diversity of the models, the disparity of basic hypothesis, the difficulty checking them and the apparent incoherence of the results. The aim of this work is to test and to compare the performances of various nitrogen adsorption-desorption isotherms analysis methods. These models were applied to model samples obtained by mechanically mixing two micro- and mesoporous solids respectively in perfectly known proportions. The relevant morphological characteristics of the porous texture of the mixtures, such as the specific surface and volume, are physically additive. A criterion that allows determining the reliability of the analysis methods tested is thus to check the linearity of the relation between a given parameter and the weight percentage of the pure solids.
420
2. E X P E R I M E N T A L Two materials have been used: a mesoporous alumina (Pural SCC30 - Condea) and a microporous amorphous Si-Ti co-gel. Five mixtures were prepared by mixing these materials in various mass proportions. The nomenclature of samples is given as X(y), y is the weight percentage of microporous material. The nitrogen adsorption-desorption isotherms were determined at nitrogen boiling temperature by the classical volumetric method with a CE Instruments SORPTOMATIC 1990 series of THERMOQUEST. The apparatus is equipped with an additional 10 torr pressure gauge and a turbomolecular pump. Nitrogen of high purity (99.999%) was used. The samples were first outgased at 10.4 Pa during 16 h, then heated from 25 to 200~ at a rate of 1~ and kept at 200~ during 12 h. The weight loss of each sample during this treatment was determined and removed from the sample weight.
Table 1" Samples' textural properties. Sample
CBET
SBET m2/g
St m2/g
Sext
SBdB SBJH
m2/g
m2/g
m2/g
m2/g
SDH
X(0)
98
173
169
(a)
157
282
280
X(t 3)
166
228
233
194
126
229
228
X(29)
205
317
335
203
104
191
190
X(50)
264
408
442
152
74
135
134
X(76)
393
545
536
92
35
64
64
X(85)
333
603
582
92
28
44
43
X(100)
380
698
691
_(a)
_(.)
_(a)
_(a)
Sample
VBaB
VBjH
VDH
•m
cm3/g
cm3/g
cm3/g
cm3/g
VD
VHK
VB
cm3/g
cm3/g
cm3/g
Vp cm3/g
X(0)
0.527
0.747
0 744
0.060
_(a)
_(a)
_(a)
0.563
X(13)
0.435
0.607
0 605
0.078
0.082
0.107
0.015
0.507
X(29)
0.366
0.487
0486
0.110
0.116
0.145
0.048
0.487
X(50)
0.256
0.326
0 324
0.141
0.160
0.181
.0114
0.452
X(76)
0.129
0.164
0 164
0.189
0.218
0.237
0.186
0.374
X(85)
0.099
0.139
0 139 (a)
0.208
0.234
0.259
0.283
0.354
0.241
0.252
0.307
0.347
0.327
X(100) _(a) (a) not applicable
_(a)
421 400
@
Adsorbed Volume (cm31g)
400
Adsorbed Volume (cm 3/0 )
4,
~,~
-
,~,"
t
=
~3
>=
0.2 0.
~ 0.2
1
lO
10o
Pore Diameter / nm
Figure 8 9Comparison of the pore size distributions obtained with sample mN2.
lOOO
0, 10
100
Pore Diameter / nm
1000
Figure 9 9Comparison of the pore size distributions obtained with sample mVT4.
The case of mN2 is once again interesting. It would seem that an overlap is obtained for the smaller pore size whereas adsorption manometry is not able to probe the larger pores. However, the smaller pores are at the limit at which the BJH method is valid. That is to say that from the adsorption data alone, one would discard the peak obtained at around 4 nm as an artefact. From the mercury intrusion data though, a peak is observed which would seem to confirm the presence of a meaningful porosity. Finally, the case of mVT4 again shows a good coincidence between the pore size distributions obtained between the three methods.
441 4. CONCLUSIONS Each of the three characterisation techniques explored in the present study can be criticised in terms of experimental procedure or mathematical interpretation of the raw data. Nevertheless, in the present study, a remarkable agreement is obtained between the results obtained with the three techniques. It is interesting to note that this is the case with such large pore samples of heterogeneous pore size distribution. These results are in good agreement with comparison studies carried out on other more ordered pore systems [6]. We are now left with our initial set of queries, in terms of different 6poques, different regions of manufacturing and different ageing conditions. We hope to address these questions in some following work to the present research. REFERENCES 1. A. Lucas, J. R. Harris, "Ancient Egyptian Materials and Industries", Edward Arnold Ltd., 1962. 2. M.L. Brun, A. Lallemand, J.-F. Quinson and E. Eyraud, Thermochimica Acta 21 (1977) 59. 3. W.A. Hammond, J. R. Withrow, Ind. Eng. Chem., 25 (1933) 1112. 4. E. Badens, P. Llewellyn, J. M. Fulconis, C. Jourdan, S. Veesler, R. Boistelle, F. Rouquerol, J. Sol. State Chem., 139 (1998) 37. 5. J. Raga'f, K. S. W. Sing, M. Yates, in "Characterization of Porous Solids II" (F. Rodriguez-Reinoso, J. Rouquerol, K. Unger and K. S. W. Sing Eds.), Elsevier, Amsterdam, 1991, p.693. 6. Non published results presented at SILICA 2000 and in the present conference by I. Beurroies and R. Denoyel.
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
443
M o n i t o r i n g fast pressure changes in gas transport and sorption analysis G. Reichenauer a'b, H.- J. Fella a and J. Fricke a aphysikalisches Institut, Universit~it Wtirzburg, Am Hubland, 97074 Wtirzburg, Germany bcurrent affiliation : Bavarian Center for Applied Energy Research, Am Hubland, 97074 Wtirzburg, Germany,
[email protected] Due to the limited response time of suitable sensors fast sorption or gas transport processes on a time scale below a second are hard to monitor. To significantly improve the resolution in time an interferometric pressure sensor can be applied. The central part of the interferometric pressure sensor presented is a Michelson-interferometer; this set-up is sensitive to changes in gas pressure as the index of refraction, and thus the optical path length for a laser beam within the interferometer, is a function of the gas density. We have built a prototype of such a pressure sensor with a resolution in differential pressure of about 0.3 %. The resolution in time achieved was about 50 ms, limited by features of the manifold rather than the interferometric device itself. 1. I N T R O D U C T I O N Gas pressures in vacuum applications are usually either recorded via membrane transducers, systems that monitor the gas density via partial ionisation of the gas or sensors that make use of the fact that the thermal conductivity or diffusivity of a gas is pressure dependent. The first type of transducer is sensitive to the total gas pressure while the other methods yield gas dependent signals. In terms of application properties such as the response time of the sensor, the sensitivity and the pressure range that the sensor covers are important technical specifications. The response of a membrane pressure sensor to a step-like pressure change is essentially an exponential function characterized by a relaxation time r ; for a MKS transducer, type Baratron 220 [1], z- was determined to be 0.227 s (see Fig. 1), the actual pressure and the value as recorded by the transducer therefore do not match within the error bars given for the sensor until more than a second passed. Gas pressures can also be monitored via the analysis of the refractive index of the gas under investigation. This non-invasive method is a fast, gas sensitive technique that provides a high resolution, both, in time and pressure, combined with the option of its operation in vacuum, at ambient pressure or above. 2. T H E O R E T I C A L B A C K G R O U N D High resolution gas pressure measurements based on the evaluation of the refractive index can be performed with a Michelson-interferometer: The interferometer compares the phases of two beams that result from splitting the light of a coherent source. Using a beam splitter the two coherent beams travel different paths before
444 105 ,'c-
104
s loa t._
(n 102
Q" 101 100
_ _1
o
o'.5 . . . . .
....
1.5
2
t(s) Fig. 1. Response of membrane pressure sensor (Baratron 220, MKS, full pressure range : 1000 mbar, resolution : 0.15 % of full range) after a step-like increase in gas pressure at t=0. The full line represents a fit to the experimental curve (o) with an exponential relaxation function. The dashed lines indicate the error bars at equilibrium pressure. they are reflected into the same direction and finally interfere. As shown in Fig. 2 the interference of the beam that is crossing Lreo/2 twice and the reference beam in the interferometer yields an interference pattern consisting of concentrical rings. The intensity I at a fixed point (e.g. the point where an optical diode is positioned) of the pattern as a function of a change in gas pressure Ap is given by
E /
1~ 1 + cos I ( p ) = --~
-2~a~e~
+
r
.
(1)
Io is the intensity of the incident beam, ALopt is the change in the optical path length of the beam through the gas under investigating, /],Laser is the wavelength of the incident beam and ~o is a phase shift that can be adjusted e.g. through the position of mirror #2 in Fig.2. ALop t is related to the geometric path Lceo via
(2)
ALopt (Ap) = LGeoAn(Ap) ,
where An is the change in refractive index upon a change in gas pressure Ap. For ideal gases the refractive index for a given wavelength is related to the pressure via 1
p
(n - 1) oc -- = ~ , V v.R.T
therefore
An = Ap. TR (n R _ 1). T PR
(3)
Here 1/V is the density of the atoms or molecules in the gas, v is the number of atoms/molecules per mol, R is the gas constant and T the temperature, nR denotes the reference refractive index at temperature TR and gas pressure pR. For pressures and temperatures for which the ideal gas equation represents no longer a good approximation, Eq.
445
Fig. 2. Schematic representation of the Michelson-interferometer for gas pressure monitoring attached to a manifold (e.g. a volumetric sorption unit). (3) has to be modified by including higher terms of the virial representation. With Eqs. (2) and (3) relation (1) transforms into
'~
I(Ap) = -~- 1 + cos
.......... /~Laser " T ' p
il
+ q9 R
(4)
.
Examples of the gas specific refractive indexes nR are given for four different gases in Table 1. Since besides the polarizability of the molecules the gas density is the second factor affecting the index of refraction, the value for He is about an order of magnitude lower than for the other gases listed. 3. E X P E R I M E N T A L SET-UP
3.1. Design We have built a prototype of an interferometric pressure transducer that was designed to be attached to a non-commercial room temperature sorption instrument [3] equipped with a differential membrane sensor (MKS, Baratron 220CD, full range 10 mbar, pressure resolution: 0,15 % of full range value). An additional absolute pressure sensor (MKS, Baratron 220 CA) integrated into the set-up provides the average pressure in the manifold and Table 1 Relative refractive indizes for different gases at pR = 1013 mbar, TR= 273 K, Gas 103 (nR- 1)
He 0.036
N2 0.297
Ar
C02
0.281
0.451
2= 589 nm [2]
446 therewith the reference value for the differential transducers. According to Eq. (4) the highest resolution in pressure can be achieved in the linear part of the cosine (see Fig. 3). Restricting ourselves to + re/6 around the center of the linear range, the full range in differential pressure mpmax is given by the condition rc
2"--"-
2~r . L G e o 9Apmax 9T R (n R 2Laser " T . p R
6
with
C=
2~. TR(n R - 1)
1)
~
1
::~ Apmax = - - ~
(5)
3 Laeo " C
.
(6)
/]'Laser " T " P l~
To make sure that upon the pressure change to be recorded the intensity of the interference pattern on the diode stays within the linear range, the interference pattern can be shifted making use of the arbitrary phase in Eq. (4). In practice the phase shift is provided by changing the position of mirror #2 (Fig. 2) via a piezoelectric transducer. The only free parameter in Eq. (5) is LGeo , the geometrical length of the path that the light travels through the cell containing the gas under investigation. To cover a range in pressure change of about 1 mbar for gases like Ar and N2 (Table 1), Laeo has to be about 0.5 m. On the other hand, the related volume should be as compact as possible to minimize the dead space introduced by the sensor. Therefore the long distance needed was provided by multiple reflections of the beam off the reflectively coated walls of the volume connected to the manifold. That way the additional dead space, incl. the connecting pipes, was only 10cm 3, corresponding to an increase in volume of 10% of the sample cell. To suppress effects of changes in the environmental pressure on the optical paths that the two beams travel separately before they interfere they are fed through evacuated tubings. Changes in the interference pattern can also be induced by fluctuations in temperature resulting in variations of the laser wavelength, the geometrical distances (e.g. between mirrors) or a change of the temperature of the confined gas. The interferometric device is
Fig. 3. Intensity at the diode according to Eq. (4) vs. pressure change at room temperature for nitrogen and LGeo= 0,5 m.
447 therefore shielded from these effects by a thermostated chamber that also contains the sorption unit. A combination of active and passive elements were used to set the temperature for a given experiment to a value between 5 ~ to 40~ on one hand and to effectively damp all external thermal fluctuations on the other hand. With the design of the interferometric sensor we are restricting ourselves to a small part of one period in the interference pattern for the sake of a high resolution in pressure; at the same time we want to achieve a high resolution in time. Fast vibrations of the device resulting in fast and small, but possibly crucial changes of distances within the interferometer therefore have to be avoided. Building a set-up rigid is possible, but not an option for a prototype were the position of the mirrors have to be adjustable on site. The instrument was therefore placed onto a table resting on air cushions (by Spindler & Hoyer GmbH, Germany) that efficiently damp vibrations above 2 Hz. 3.2. Performance of the interferometic pressure sensor
The interferometric set-up was first calibrated at constant pressure by comparing the signal to the reading of the membrane differential pressure sensor. Applying Eq. (4) a geometrical distance LGeo = (0.575 + 0.003) m (Fig. 2) has been determined for N2, Ar and CO2. The value derived for He is with (0.715 + 0.003) m significantly off. Gas impurities in the order of 1 % or less would be sufficient to explain this effect, since the relative refractive index of He is at least an order of magnitude smaller than for most other gases and vapors. An analysis of the gas will clarify the situation in the near future. Analysis of the stability of the signal at constant pressure versus time reveals that the relative error in pressure for analysis periods t < 2 s is dominated by the inaccuracy of the calibration parameter LGeo (caused by thermal drifts during the long calibration runs) rather than vibrational contributions or thermal effects upon analysis itself. Fig. 4 shows the comparison of the signals detected via the Baratron differential transducer and the
Fig. 4. (a) Pressure change as a function of time as detected after a step-like change in gas pressure of about 5 mbar via the interferometric (o) and the membrane (x) pressure sensor (average gas pressure : 500 mbar, N2 ). (b) Interferometric versus membrane transducer (data taken from Fig. 4 (a)). The gray shaded regime (t > 1.5 s) indicates the range where the two signals coincide. The dashed lines are guides to the eye.
448 interferometric sensor aider a step like change in pressure of about 5 mbar. The pronounced relaxation in pressure (Fig. 4 (a)) is caused by the expansion of flexible connections in the manifold (to extract equilibrium data for sorption analysis this effect is taken into account via a volume calibration routine). Fig. 4 shows that the agreement between the two datasets is excellent for t > 1.5s while for t < 1.5s the response of the membrane sensor shows the expected damping behavior. The resolution in time of the interferometric device is demonstrated in Fig. 5 that shows the response of the optical sensor to a pressure step. The pressure step was induced by opening a dosing valve for 50 ms. This period is reflected in the interferometric signal by a sharp change with vibrations superimposed that are induced by the opening and closing of the valve, respectively. Note that the signal in Fig. 5 presents the raw data, i.e. the voltage at the diode. 4. A P P L I C A T I O N OF THE I N T E R F E R O M E T R I C P R E S S U R E S E N S O R
Every sorption set-up also allows for the extraction of information about sorption dynamics and gas transport provided that the resolution in time of the sensor applied is adjusted to the process under investigation. In an earlier publication we studied the transport of gases into the mesopores of monolithic materials by analyzing the pressure relaxation after the dosing of a defined amount of gas onto the sample [3]. An improvement of the resolution in time allows to investigate also small monoliths or samples with larger pores. Fig. 6 demonstrates the advantage of a fast pressure monitoring for a diffusion experiment on a silica aerogel with a porosity of 96 %, a mesopore size of about 60 nm and a volume of 2x3x4 cm 3. The fit of the analytical solution for the related diffusion problem [3] to the experimental data taken with the interferometric transducer yields a diffusion coefficient for nitrogen of (19.5 +1.5). 10-6 m2/s. |
1.5
i
|
i
i
i
w
~
!
,
,
,
I
!
m
i
I
,
~
~
i
1.0
0.5
6/V
-0.5
-1.0
I
0
,
,
,
!
,
0.1
0.2
0.3
t/s Fig. 5. Change of voltage measured at the diode as a function of time after changing the gas pressure by opening a dosing valve for about 50 ms. The oscillations within the first 30 ms and between about 60 and 120 ms are due to vibrations induced by opening and shutting the valve.
449
7.5 7"6LI "u"
~-........... -r~---,--~, . . . .
, ....
i
7.4
E 7.a
7.2I
0
/ membrane transducer 1
2
3
i 4
5
t(s) Fig. 6. Dynamic expansion experiment : Relaxation of the gas pressure, after a step like change of the pressure in the sample cell, due to molecular diffusion of the gas into the open porous monolithic material. 5. SUMMARY AND CONCLUSIONS A prototype of an interferometric pressure transducer has been built and tested with a room temperature sorption unit. The sensor proved to provide a resolution in time clearly below 50 ms; in practice the resolution is limited by features of the manifold (e.g. characteristic times of valves, gas transport through tubing thermal equilibration) to which the senor is connected rather than its intrinsic properties. The resolution in pressure of currently 0.01 mbar for He (at full range 6 mbar) and 0.002 mbar for Ar, N2 and CO2 (at a full range of 0.3 to 0.7 mbar) can be further increased by - improving the stability of the laser : highly stabilized laser diodes (AX/2) < 0.0003 are state of the art (e.g. by LASER2000) - increasing the geometrical distance Lgeo that the light is traveling through the gas to be analysed; special optical set-ups, e.g. with spherical mirrors, are available that provide up to several hundreds of meters compared to 0.5 m used in our prototype. The pressure range covered can drastically be extended by using a diode array instead of a single diode, so that the restriction to only part of the full cosine can be avoided. Alternatives to the presented interferometric configuration are Mach-Zehnder-type interferometers that are also available as integrated optical devices; the performance of such optical sensors will be tested for the above application in the near future.
REFERENCES
[ 1] MKS Instruments Inc., Andover, MA, USA. [2] CRC Handbook of Chemistry and Physics, CRC Press, 63th Edition. [3] C. Stumpf,K. von G~issler, G. Reichenauer, J. Fricke, Journal of Non-Cryst. Solids 145 (1992) 180.
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
451
Effect of coke deposition on catalyst texture during catalytic cracking reaction. J.P. Reymond, C. Delattre and M. Forissier. LGPC ; ESCPE ; 43 Bd 11 Novembre, 69616 Villeurbanne cedex, France The active phase of industrial catalytic cracking catalysts is a Y-type zeolite. Deactivation occurs during reaction by deposition of large aromatic molecules (coke). Changes in porous texture of an industrial catalyst have been investigated for different coke quantities, using physisorption isotherms of argon (77K and 87K) and nitrogen (77K). Apparent microporous texture of the catalyst and its change with increasing coke content depend on both the adsorbate type and the sorption temperature.
1. Introduction 9 Fluidized catalytic cracking (FCC) is still the major refining operation to convert heavy petroleum compounds to lighter products The industrial catalyst, shaped as small spheres (70 pm), circulates in the process trough a fluidized bed reactor. It is composed of an active phase, Y-zeolite, embedded in a matrix [1]. Two ways of catalyst deactivation occur during the cracking reaction. First, the deposition of polyaromatic carbonaceous compounds (coke) produced during by-side reactions leads to a fast but reversible deactivation [2,3]. Burning coke can regenerate the initial activity. Then, an irreversible deactivation occurs due to the partial destruction of the zeolite cristallinity, induced by the drastic hydrothermal conditions existing during the regeneration, and to the harmful presence of metallic ions in the feed (Ni, V...). Study of the irreversible deactivation is beyond the scope of this paper. Coke can induce texture modifications, such as active surface covering, pore diameter reduction or pore closing. Typically, the analysis [4,5] of solid texture consists in measuring gas physisorption isotherms. Texture representative parameters (specific surface area, volume, diameter and diameter distribution of each pore c l a s s e s - micro, meso, macropores) can be extracted using models (B.E.T., B.J.H., t-plot, Horvath-Kawasoe, Dubinin .... [4]). This paper aims at locating coke inside the porous catalyst particles, using the texture modifications induced by coke deposition on an industrial catalyst. Isotherms using Nz and Ar are used in order to extract texture data for samples containing an increasing quantity of coke. Due to the presence of Y type zeolite, a large fraction of FCC catalyst volume is mainly microporous. Literature studies [6] emphasize that Nz is not suitable to determine microporous parameters characteristic and instead strongly recommend using Ar at its boiling temperature. However, N2 is a more commonly used and less expensive adsorbat than Ar. As this paper is devoted to studying texture evolution and not measuring exact surface area and porous volume, does argon have to be used ? In order to answer to this question, different adsorbates have been used at different temperatures and the results obtained from these experiments are compared.
2. Experimental Adsorption isotherms were recorded with a Micromeritics ASAP 2010 instrument. Experimental porous data (surface area, volume, pore diameter distribution) have been
452 obtained with the models and the related parameters provided by Micromeritics. N2 was used at 77.35 K (adsorption and desorption branches), while Ar was used at 77.35 K (adsorption branch only, up to P/P0 = 0.7) and 87.29 K (adsorption and desorption branches). Prior to any measurement, each sample was outgased under vacuum (~0.1Pa) during 24 hours, with a heating time of 2 hours at 623 K. Coke deposition was performed via cracking reactions of a real feedstock (gas-oil) operated in a fixed bed reactor which allows a wide range of experimental conditions [7] : catalyst mass from 0.5 to 10 g ; reaction temperature from 723 K to 873 K ; pressure from 1 to 4 bar ; injected feed mass between 0.4 and 4 g ; feed injection time from 10 to 300 s. This reactor induces a coke formation very similar in quantity and nature to that observed on industrial plant catalysts [7]. Coke combustion was performed at 1773 K under oxygen flow in a Leco CR12 carbon analyzer. The global carbon content was extracted from the total volume of carbon dioxide produced during combustion
3. Materials Two different catalyst samples were obtained from TotalFinaElf : a fresh carbon-free catalyst, (reference catalyst) and an equilibrium catalyst, which was working in an industrial FCC unit. So, the equilibrium catalyst is a mixture of regenerated catalyst particles, plus some spent catalyst particles and steamed fresh catalyst particles (the fresh catalyst is introduced in the regenerator of the industrial FCC unit). It contains a residual carbon rate : 0.276 wt%. Catalyst samples with various carbon contents (from 1.7 wt% to 4.2 wt%) were also prepared with the MAT reactor. Moreover, two reference microporous solids have been also tested : NaY and USY zeolites (from Zeocat).
4. Results and discussion 4.1 Physisorption isotherms The shape of adsorption-desorption isotherms gives the first pieces of information on a porous solid texture. Existence of an inflexion point in the low-pressure region of the high-resolution isotherm is a sign of the microporous character of a solid [6]. NaY zeolite appears microporous (type I isotherm) regardless of probe molecule and temperature. Similarly, USY zeolite is mainly microporous,but a small hysteresis loop at high P/P0 values reveals the presence of some mesopores created during dealumination of Y zeolite [8,9]. Table 1 gives the values of SBET (specific surface area), Smicro and Vmicro (surface area and volume of micropores, calculated from the t-plot model), VT, the total pore volume is measured at P/P0 = 0.97 with N2-77K and Ar-87K, and at P/P0 = 0.7 with Ar-77K (corresponding values are in italic) as well as the ratio Smi~ro/SazT and VmiCroNT ,for the two zeolites and the fresh catalyst with all sorption conditions. Table 1 : total and micropore areas and volumes of NaY and USY zeolites. Adsorbate SBET Smicro Smicro/SBE T VT Vmicro mZ/g m2/g cm3/g cm3/g Vmicro/Vtotal Temperature 644 580 0.901 0,27074 0,32578 0,831 Nz-77K 0,894 541 491 0.908 0,25060 0,2802 Ar-77K NaY 450 371 0.824 0,18813 0,26329 0,715 Ar-87K 0,577 564 430 0.762 0,23353 0,40443 Nz-77K 0,634 449 321 0.715 0,16178 0,25523 Ar-77K USY 0,416 520 336 0.670 0,16756 0,40262 Ar-87K 0,654 222 176 0,8 0,14960 0,09788 Na-77K 0,549 215 163 0,76 0 , 1 4 9 6 0 0,0822 Ar-77K Fresh catalyst 0,569 213 169 0,8 0,1412 0,08034 Ar-87K
453
Both the values in this table and the position of the inflexion point on high-resolution isotherms strongly depend of probe molecule and adsorption temperature. Values calculated from N2 isotherm are greater than values calculated from Ar isotherms. On one hand, the differences could be attributed to interactions between Nz molecule and the adsorbent (strong electrostatic fields exist in zeolites), leading to a preferential orientation of the N2 molecule, more perpendicular to adsorbent surface than expected, leading to higher calculated surface areas [10]. On another hand, the cross-section of Nz molecule (0.3 nm) is smaller than that of Ar atom (0.38 nm). Nz could penetrate in pores remaining non accessible to Ar, leading to larger adsorbed volumes for N2 and consequently higher surface areas. Due to the presence of the binder, the surface areas (SBET, Smicro) of the fresh catalyst are smaller than that of zeolites. It is possible to provide some estimate of the amount of zeolite in the catalyst, assuming that : the fresh catalyst contains USY zeolite, micropores are exclusively present in the zeolite (Szeol = Smicro and SBEX= Szeol + Sbinder), binder and zeolite have same density. From Ar isotherms the fresh catalyst would contains --50 wt% zeolite and --40 wt% from Nz isotherms., From similar calculations, binder surface area is near 90 mZ/g. Figure 1 shows the isotherms obtained for the equilibrium catalyst : adsorption-desorption branches and the hysteresis loops are similar for all isotherms (Ar and Nz ). These isotherms are close to type I (microporous solid) or type II (adsorbed volume increases at high P/P0.) 100
30
80
25
"~
60
E o ">"
40
20
Ar-87K Ar-77K
~ N2-77K
20
10
_
5 0
0.2
0.4
0.6
0.8
P/Po
Figure 1 9Ar (87K and 77K) and N2 (77K) isotherms of equilibrium catalyst.
1
0 1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
P/Po
Figure 2 9low pressure region of high resolution isotherms of equilibrium catalyst.
High-resolution isotherms of the equilibrium catalyst are represented in Figure 2. The observed sigmoid curves demonstrate the microporous character of the catalyst, but the relative pressure corresponding to the inflexion point depends on both the adsorbate and the adsorption temperature (Ar at 77 K or 87 K). Thus, mesopores and micropores are qualitatively evidenced from each physisorption isotherm but quantitative differences exist. The presence of hysteresis loop (H3 type, corresponding to slit pores) reveals the presence of mesopores. Typically, in a FCC catalyst, micropores are attributed to the zeolite fraction while mesopores are mainly related to the amorphous matrix. Upper part of Table 2 gives specific surface area (SBE~), surface areas of micropores (Smicro) and mesopores (Smeso), total pore volume (VT) and micropore volume (Vmicro) of fresh, equilibrium, 2.17 %C and 4.2 %C catalyst samples. Lower part of table 2 gives the same characteristics of equilibrium, calcined equilibrium, calcined coked 2.17 %C catalysts (calcination is operated at 873 K, during 24 hours, in air flow) as determined from N2-77 K or Ar-87 K isotherms. All values depend on the probe molecule and sorption temperature, which confirm the observations deduced from the isotherm shape. Comparison of fresh and equilibrium catalysts (lines 3 and 4) shows that irreversible deactivation strongly changes the porous texture of the catalyst, mainly the microporous
454 fraction (see Smicro and Vmicro values). Coke deposition induces a decrease of all measured surfaces and volumes. Calcination of coked samples leads to a reproducible state (lines 7 and 8), demonstrating the reversible effect of carbon deposition on the catalyst texture. Table 2 9texture characteristics as determined from N2-77 K and Ar-87 K isotherms. Nitrogen 77.35 K Catalyst Fresh Equilibrium (0.276% C) Coked cata. (2.17 % C) Coked cata (4.2 % C)
m 2 / g cm3/g cm3/g mZ/g 46 0.1496 0.0978 212.5
Smicro mZ/g 169
Smeso Vtotal Vmicro mZ/g cma/g cm3/g 43.4 0.14120 0.0803
2 5 . 2 0.1093 0.0350
78.2
48.5
29.7
0.0970 0.0241
57
20.6
0.1011 0.0264
64.5
37.5
26.9
0.0959 0.0186
62.7
41.3
21.4
0.0849 0.0191
54.8
30.5
24.3
0.0712 0.0151
89.6
64.4
2 5 . 2 0.1093 0.0350
78.2
48.5
29.7
0.0970 0.0241
94.2
68
24.3
0.1116 0.0369
88.3
65.2
23.1
0.1353 0.0298
94.4
67.3
27
0.1115 0.0365
90
59.7
3 0 . 4 0.1134 0.0296
Smicro
mZ/g 221
mZ/g 176
89.6
64.4
77
Equilibrium (0.276% C) Calcined eq. cata (o % c) Calcined coked cata (0 % C)
Argon 87.29 K SBET
SBET
Smeso
Vtotal
Vmicro
Figure 3 presents the variations of specific surface area (SBET) and micropore area (Smicro) as a function of carbon content for all the tested catalysts. Similarly, figure 4 presents the corresponding variations of total pore volume (VT) and of micropore volume (Vmicro). As evidenced by curves on figures 4 and 5, changes in texture characteristics are quasilinearly related to increasing carbon content. It also appears that calculated surface areas strongly depend on the physisorption conditions, while measured adsorbate volumes are less affected. This observation is in favor of erroneous assumptions on nitrogen molecule area for the calculations of specific surface area [10]. But, as mentioned above, nitrogen molecule could penetrate in smaller pores, increasing measured micropore volume and surface areas. 100 9N2,77K 9A r, 87K o A r, 77K
o~
~
80
O ..........
E
v
60
VT
0.08
E
m O >
(D O t~
't: :3 co
0.12
-9
O n
40
20 0
|
|
|
|
1
2
3
4
Carbon content (%)
Figure 3" SBET and Smicro of coked catalysts.
0.04
9N2 77K OAr 77K l i a r 87K
Vmicro ,~--__ . . . .
1
2
3
4
Carbon content (%)
Figure 4" Micropore and total pore volume (liquid) of coked catalysts.
On one hand, extrapolation of each line to 0% carbon content gives values very close to that of calcined catalysts. On the other hand, extrapolation to a null micropore or BET surface area gives an estimation of the maximal carbon content of the catalyst. From Ar-87 K experiments
455 Smicro and SBZTdrop to zero for a carbon content of 1 1 % of the catalyst weight. In the case of Nz experiments, these areas become null for respectively 19 % and 27 %. Once again, it has to be emphasized that the description of changes in catalyst texture due to coke deposition strongly depends on physisorption conditions. 4.2 M i c r o p o r e c h a r a c t e r i z a t i o n f r o m H o r v a t h - K a w a z o e plots : The zeolite fraction, the active phase of FCC catalysts, looses its activity through deactivation by coke deposition. Change in the zeolite texture or the microporous texture will give some insight on the deactivation phenomena. Changes in micropore diameter and volume have been studied from the curves calculated from the Horvath-Kawazoe model (denoted H-K) applied to slit pore geometry (hysteresis loop type H3). This quantitative model, developed for adsorption on carbons [11], is only a qualitative measure of pore diameter for others solids but it can be used to compare changes in micropore diameter for a given solid. Pore size distributions (PSD) of NaY and USY zeolites are presented on figure 5 and 6 respectively. 8 E ,= O
9 E
3 0.45
_ . _ N2,77K + Ar,77K ~Ar,87K
6
0.65
4
o
E
O
>
N2, 77K = Ar,77K :. At, 87K
~" 2.5 ,= 2
~ 0.68 ~1
IT
1.5
O >
2
O
......
9
|
|
,
0.5 0.6 0.7 micropore diameter (nm)
0.4
,.
0.4
0.3
-
0.5
=
'
0.6
t'
i
0.7
0.8
0.3 0.25 02
E 0.2 _=
E 0.15 _=
O >
O >
~
0
,-, 0.1 o E
|
-
Figure 6" Pore size distributions (H-K) of U S Y zeolite.
E =
'~
"--
-
0.4
micropore diameter (nm)
Figure 5" pore size distributions (H-K) of NaY zeolite.
E ~
0
0.8
t
0
J
)
~ ,
0.4
J
A
r ,
,
0.6 0.8 1.0 micropore diameter (nm)
Figure 7 9Cumulative pore volume plots for NaY zeolite
0.1
0
__,_At, 771< , 87K
.~_o 0.05 , 1.2
E
0
'
04
I
06
018
....i:o
,12
mieropore diameter (nm) Figure 8" cumulative pore volume plots (H-K) for USY zeolite.
Experiments using Ar at 77K and 87 K lead to similar PSD and maxima of pore mean diameter, but resuts obtained with N2 are very different, with smaller mean pore diameters (particularly for NaY) and a bimodal PSD for USY zeolite centered on 0.46 nm and 0.58 nm. Aperture of Y zeolite micropore is 0.7-0.8 nm (cristallographic data [1]) and diameter of internal cage is --1.2 nm. N2 leads to underestimate Y zeolite micropore diameter, while Ar
456
leads to a correct value.Bimodal PSD encountered with N2 for USY zeolite has been previously described attributed to the dealumination process [9]. H-K cumulative pore volume plots as a function of pore diameter for NaY and USY zeolites are presented on Figure 7 and 8 respectively : micropore volumes calculated from nitrogen isotherm are overestimated compared to values calculated from argon isotherms. Figures 9 to 14 present the PSD and cumulative pore volume plots (H-K) relative to several coked samples (from 1.76 to 4.2 wt%C) and the equilibrium catalyst as calculated with nitrogen (fig. 9 and 10), argon at 77 K (fig. 11 and 12) and argon at 87 K (fig. 13 and 14). As shown by figure 9 (Nz-77K), the PSD for both the equilibrium catalyst and the coked samples have two micropore populations, centered on 0.59 nm (DI(N2)) and 0.45 nm (Dz(N2)). With increasing coke content, the mean diameter of micropores corresponding to DI(N2) is not affected but the number of pores tends to decrease. The mean diameter of pores corresponding to Dz(Nz) seems to increase (from 0.45 nm to 0.51 nm) as carbon content increases.Curves of figure 10 show that increasing of carbon content leads to a decreasing of the sample micropore volume. Their relative position to each other suggest that the volume of larger micropores (diameter > 0.58 nm) is more affected than the volume of smaller ones (curves are merged for low micropore diameters).
I N , 77K J
0.4 E
r
i
(0.51)
0.3
X
IN , 77K ~
0.04
D. (0,59)
A
_
~tE 0.03 o v
E 0.2
D2
\
...... Eq. Cata
O >
E := o 0.02 ,- 0.01 ._o E
0
E X~
0.5 OJ6 017 micropore diameter (nm)
0.4
Figure 9 9pore size distribution (H-K method ) of coked catalysts with N2 at 77K.
E 0.5 d~ t-
_.,,_2.8%0 --0--3.5% C - x - 4.2 %C ..... Eq. Cata
E 0.4 .9.o E -, 0.3
O
i
~
/,~
/I 1
"5 2 .9 E
~ 2 (0 50) D
0.4
i
1.2
IAr, 77K I
0.03
0.02
i
I
|
,
0.5
0.6
0.7
0.8
micropore diameter (nm)
Figure 11 9pore size distribution (H-K method ) of coked catalysts with Ar at 77K.
"
/.~, /~9
0
i
1.0
a) E
0.2
0
i
0.8
Figure 10" cumulative pore volume plot (H-K method) of coked catalysts with N2 at 77K.
2 0.1
!
0.6
0.04
i /1/ !1/
O >
_ x _ 4.2 % C _7 Eq. Cata
micropore diameter (nm)
......... 1 Ar, 7rK i I D, (0.67)
0.6 ,.-.,
._ E
0.4
018
; 3.s%c
~'~ ,~x~~, J ;5
0
: 1.8 %C __ 2.6 %C
l#
O
o. 0.1
O
~'#,,'t,--
O
0
0.01
,~x
_~:_ 4.2 %C ..... Eq. Cata
XX"
0 0.4
06
1.8 % C 218 %C
08
micropore diameter (nm)
Figure 12" cumulative pore volume plot (H-K method) of coked catalysts with Ar at 77K.
Argon adsorption experiments at 77K (figure 11) shows that PSD for the equilibrium catalyst is monomodal, with a mean pore diameter centered on 0.67 n m (DI(Ar)). This value is higher
457 than that reported for nitrogen (0.59 nm). PSD for coked samples is slightly bimodal with appearance of a population of smaller micropores centered on --0.5 nm (DZ(Ar)). As the carbon content increases, both mean diameters (DI(Ar) and DZ(Ar)) remain constant and the micropore volume decreases (fig. 12) with a larger drop for pores larger than 0.67 nm compare to smaller ones. Same observations can be made from the results of argon adsorption experiments at 87 K (fig. 13 and 14), with for DI(Ar) = 0.67 nm and D2(Ar) = 0.47 nm. The calculated values of D1 (fig. 9, 11 and 13) match the corresponding values of micropore diameter for the USY zeolite (fig. 6). Ar sorption results (77 and 87 K) show that the micropores of diameter D2 do not exist neither on the U SY zeolite nor on the equilibrium catalyst but appear only for coked samples. In this case the presence of these small pores would be a consequence of the coke coating which would decrease the diameter of larger pores. On the contrary, nitrogen sorption indicates their presence on both the zeolite (fig. 6) and the equilibrium catalyst. So, existence of these small pores would be an intrinsic property of the solids, not linked to the coke coating. Whether the adsorption experiment is run with nitrogen or argon, micropore population centered on Dz and its changes are different. A general trend in the data shows that when carbon content increases the mean diameter of micropores remains constant, while the volume of micropores decreases. --,--E ~E
0.3
I Ar, 87KI
_._
' 1.8 %c
~2.8 %C . - o - - 3.5 %C - x - 4.2 %C ....... Eq. C a t a
0.2
E
I
IAr, 87K I
;~D1 (0,67) /1/
.-.
/ I~ / li //\ / ~l ~
0,03
E ._. o ~
0,02
o o ..~-
0,01
~l,~.~s,........
>
~t. o
F=
~' x-x/" xXJ /
E
0
0.4
, 0.5
016
, 0.7
_1i
,~~
I
'
018
micropore diameter (nm)
Figure 13 : pore size distribution (H-K method ) of coked catalysts with Ar-87K.
0,00
,
0,4
~. _x_ __
J~, ,~J',t
,
1,76% C 2,83% C 3,5%C 4,2% C Eq. C a t a .
0,6 0,8 10 micr0p0re diameter (nm)
|
1
Figure 14 : cumulative pore volume plot (H-K method) of coked catalysts with Ar-87K.
The modification of pore size distribution and the decrease of pore volume could be explained by two simultaneous phenomena described previously [12] : Coke deposition on the walls of larger micropores (diameter > D1), appearing predominant at the beginning of cracking reaction, e.g. for the lowest coke content. As a result, smaller micropores centered on Dz are created (argon experiments). - Coke deposition progressively closing the entrance of micropores centered on D], in such a way that D1, representing the diameter of open pores, remains constant. The decrease of pore volume is then induced by the diminution of the number of pores accessible to adsorbate. As carbon content increases, the volume of large micropores decreases because coke coating of largest pores can occur, leading to the formation of the 0.51 nm pores appearing on figure 10. Nitrogen molecule, due to its lower cross-section, could be able to penetrate in these small pores, detecting changes of pore diameter while argon cannot do it. Thus, nitrogen could give a better description of changes in the population of the small micropores than argon do [13]. However, it has been seen that N2 underestimate the zeolite micropore diameter (fig. 6 and 7) and overestimate micropore surface area and volume. Only qualitative changes in micropore characteristics can be detected when Nz is used as the probe molecule. -
458
5. Conclusions The cracking of complex hydrocarbon feedstocks in presence of an industrial catalyst leads to the deposition of carbonaceous compounds (coke) on the catalyst, inducing a loss in activity of catalytic sites. As a general observation, increase of coke content leads to a decrease in micropore area, BET area, micropore and mesopore volumes, and the micropore diameter distribution. The mean diameter of zeolitic micropores (0.67 nm) is slightly modified. Mesoporosity of the solids studied in this work is satisfactorily described from Nz or Ar (87 K) isotherms. Description of the microporosity and its changes is more complicated. We have studied the micropore texture modifications due to the coke deposition using the HorvathKawazoe model (slit pore geometry). From a qualitative point of view, N2 and Ar isotherms describe similarly a great part of changes observed during catalyst coking. However, all micropore surface and volume values depend strongly on the adsorbate nature. N2 physisorption leads to overestimate micropore surface and volume and underestimate micropore diameters. These overestimations can be explained by the differences of molecular cross-section between argon and nitrogen , or by interactions between Nz and the solid, inducing a more perpendicular orientation of the Nz molecule towards the surface. Due to a smaller cross-section N2 molecule could penetrate in smaller pores than Ar, leading to a better description of the micropore populations. But H-K diameters of the zeolitic micropores calculated with N2 as probe molecule are very different of the real diameters of Y zeolite micropores. As H-K diameters calculated with Ar are very similar to real zeolitic diameters, and as Ar-adsorbent interactions are very weak, Ar (at 87 K) must be preferred to Nz in order to evaluate micropore texture. Understanding of coke deposition process can be improved from Ar experiments (77 K and 87 K) : at low carbon content coke deposition occurs predominantly on micropore walls leading to the formation of smaller micropores. Then, coke progressively plugs the micropore entrances in such a way that the number of pore decreases while the diameter of open micropores remains constant.
Acknowledgements: The authors want to thank TotalFinaElf for providing financial support.
References: [1] P.B. Venuto and E.T. Habib Jr, Catal. Rev. Sc. Eng., 40 (1975) 1. [2] E.E. Wolf and A. Alfani, Catal. Rev. Sc. Eng., 24 (1982) 329. [3] T.C. Ho, Ind. Eng. Chem. Res., 31 (1991) 2281. [4] S.J. Gregg, K.S.W. Sing in "Adsorption, Surface Area and Porosity" (Ac. Press, London, 1982). [5] A.J. Lecloux in "Catalysis, Science and Technology" (J.R. Anderson, M. Boudart eds, 1981), 171. [6] S. Storck, H. Bretinguer and W.F. Maier, Applied Catal. A: General 174 (1998) 137. [7] C. Delattre, M. Forissier, I. Pitault, D. Schweich, J.R. Bernard, Chem. Eng. Sci. 56 (2001) 1337. [8] G de la Puente and U.A. Sedran, Microporous Materials 12 (1997) 251. [9] J. Lynch, F. Raatz and C. Delalande, in "Characterization of Porous Solids" (K.K. Unger and al. Eds, Elsevier, Amsterdam, 1988) 547. [10] P.L. Llewellyn, C; Sauerland, C. Martin, Y. Grillet, J.P. Coulomb, F. Rouquerol, J. Rouquerol, in "Characterization of Porous Solids IV" (B. Mc Enaney, and al. Eds., R.S.C, Cambridge, 1997) 111. [11] G. Horvath and K. Kawazoe, J. Chem. Eng. Japan, 16 (1983), 470. [12] M. Guisnet and P. Magnoux, Applied Catal. 54 (1989), 1-27. [13] S.W. Weber, W.C. Conner, in "Characterization of Porous Solids II" (F. Rodriguez-Reinoso and al., Elsevier, Amsterdam, 1991) 31.
Studies in Surface Science and Catalysis 144 F. Rodriquez-Reinoso, B. MeEnaney, J. Rouquerol and K.K. Unger (Editors) ' 9 2002 Elsevier Smence B.V. All nghts reserved
459
Precision of porosity measurements on cementitious mortars K. Rtibner a , Th. Fritz a and F. Jacobs b aFederal Institute for Materials Research and Testing (BAM), D-1200 Berlin, Germany bTechnical research and consulting on cement and concrete (TFB), Lindenstrage 10, CH-5103 Wildegg, Switzerland
1. INTRODUCTION Mercury porosimetry has been a well-established method to study the porosity and the pore size distribution of cement pastes, mortars and concretes for many years [1-6]. It provides information about a wide range of pores that are intimately associated with engineering properties of building materials such as strength, permeability and durability. However, the measurements are strongly influenced by test parameters, like sampling, preparation techniques and drying methods, because of the heterogeneity of the cementitious material and the delicate colloidal structure of their cement paste matrix. A correct analysis requires a knowledge of the effect of test parameters and the accuracy of the measuring method. A key issue is whether differences in the results of measurements on various samples are caused by the difference between the samples or by the inaccuracy of the measuring method. The heterogeneity of the material, the discrepancy between test conditions and/or the dissimilarity between laboratories produce further problems. To find an answer, the uncertainty of a measuring method has to be determined by repeat measurements on identical test items. The use of a uniform test material that is produced artificially under controlled conditions is necessary to investigate the precision. Furthermore, the test material has to represent its material class with reference to the precision of the method. The precision of the measuring method mercury porosimetry on the material class cementitious mortar was investigated by an interlaboratory study, which was organized by BAM and TFB from 1997 to 1999 [6]. The interlaboratory tests were based on ISO 5725 [7]. Six different samples of a Portland cement mortar were analysed. The repeat and the reproduce standard deviations of the numerical characteristics total pore volume at 200 MPa, median pore radius and bulk density were determined. Furthermore, the complete pressure/volume curve became the focus of analysis by calculating laboratory average curves and total average curves with uncertainty intervals. The results reported here are discussed with regard to the influence of drying technique and sample geometry.
460 2. E X P E R I M E N T A L
2.1 Laboratories 30 institutes and companies from Germany, Switzerland, Austria, Italy and Slovakia took part in the interlaboratory test, involving 32 testing laboratories that used 13 measuring instruments. 2.2 Samples The test material was a cementitious mortar made of a Portland cement CEM 142.5 and sand with maximum grain size of 4 mm. The blend ratio was 1 : 3 cement to sand by mass. The water/cement ratio was 0.44. Five 140 x 140 x 560 mortar prisms were produced. After demolding they were stored under water for 90 days. The mortar is characterized by a compressive strength of 61.9 N mm 2, a bending strength of 8.83 N mm -2 and a bulk density of 2.37 g cm -3 (determined according to DIN-EN 196-1 [8] at 28 days). Six different samples were prepared from the 90 day old mortar. Two groups of samples were produced: ~ 1 0 mm x 25 mm cylinders by drilling and 3-8 mm granules by crushing and sieving. Each sample group was pre-treated by three methods: 105 ~ drying, vacuum drying at 5 kPa, 20 ~ and wet storing until drying by the participating laboratories. The characteristics of the samples are given in Table 1. Table 1. Samples of the interlaboratory test Sample B1 G1 B2 G2 B3 G3
Bulk density
Density
(g cm-3)
(gcm -3)
2.202 similarto B 1 2.330 similarto B2 2.352 similar to B3
2.539 2.554 2.421 2.541 ---
Material cylinders, 105 ~ drying granules, 105 ~ drying cylinders, vacuum drying granules, vacuum drying cylinders, wet material granules, wet material
Cement content Residual moisture content (M.-%) (M.-%) 20 18 20 18 20 18
0.89 1.10 6.00 3.60 ---
2.3 Test design Each testing laboratory performed four measurements of each sample B 1, G1, B2, G2, B3 and G3. The mercury intrusion measurements were carried out according to DIN 66133 [9]. Additionally, the bulk density was determined pycnometrically by mercury immediately after the low-pressure measurement. The experiments were performed according to routine procedures of the participating laboratories and instructions of the test coordinator. 2.4 Statistical evaluation The data delivered by the laboratories were sample mass, bulk density and pressure/volume values (p/V curve) within minimal range from 0.01 to 200 MPa. The evaluation was performed in the following steps: - critical examination of the data in order to identify anomalies and irregularities;
461 -
determination of the total pore volume at p = 200 MPa by extra- or interpolation of the pressure/volume data for every single measurement; - calculation of pore size distribution (pore radius/volume curve) according to DIN 66133 [9] (8 = 140 ~ cr = 0,48 N m -1) and the median pore radius (as the radius, at which the pore volume reaches 50 % of the total volume); discarding of invalid values due to erroneous computation or obviously irregular application of the test method; - statistical evaluation according to ISO 5725 [7] for the numerical characteristics total pore volume, median pore radius and bulk density; statistical evaluation of the pressure/volume curves. According to ISO 5725 [7] the total mean X, the repeat standard deviation Sr, the between laboratory standard deviation So and the reproduce standard deviation SR were determined. The relation is given by -
-
The repeat standard deviation describes the scattering of the measuring results under repeat conditions (same laboratory, same equipment, same staff). Whereas, the between laboratory standard deviation expresses the differences between the laboratories. The reproduce standard deviation contains the two above mentioned scatter components. It is the deviation under reproduce conditions (different laboratories, different equipment, different staff). To get a unique repeat standard deviation it must be assumed that it does not vary (significantly) with the laboratory. For this reason the standard recommends a statistical outlier test (Cochran test) for the individual standard deviations of the laboratories. Furthermore, the individual laboratory means are a subject to an outlier test (Grubbs test). ISO 5725 [7] relies on a statistical analysis of variance model with two variance components "laboratory" and "repetition". Hence, a homogeneous material is assumed. Only under this homogeneity condition the calculated precision values are true method characteristics. For a heterogeneous material, like the tested cementitious mortar, the precision values are contaminated by the variance component of the material. Therefore, the precision values represent both material and method characteristics. To analyse the complete pressure/volume curve simple statistical methods were applied: - The p/V curves of every single measurement are linear interpolated and then evaluated on a fixed lattice of points at the logarithmic pressure scale. - For each laboratory and for each sample B1, G1, B2, G2, B3, G3 the average pressure/volume curve is calculated. Then the total average curve (non-weighted) of these laboratory averages and their standard deviation (uncertainty interval) are calculated for each sample. -
3. RESULTS AND DISCUSSION The results of the statistical analyses according to ISO 5725 [7] are presented in Tables 2, 3 and 4. Fig. 1, 2 and 3 show the total average curves with uncertainty interval of some particular samples. Full results are reported and discussed in [6].
462 Table 2. Results of statistical evaluation for the total pore volume
Sample
Number Number oflaboratories i
of values
Mean
Repeat standard deviation
X
Sr
mm 3 g-1 mm 3 g-1
Between laboratory standard deviation
Reproduce standard deviation
Sb
%
mm 3 g-I
SR %
mm 3 g-i
%
B1
24
102
59.34
4.131
6.96
1.659
2.80
4.451
7.50
B2
26
108
35.19
4.025
11.44
7.324
20.82
8.357
23.75
B3
27
111
53.74
3.909
7.28
4.205
7.83
5.741
10.68
G1
22
92
48.14
2.622
5.45
3.035
6.31
4.011
8.33
G2
25
101
37.79
2.468
6.53
3.735
9.88
4.477
11.85
G3
26
106
45.10
2.771
6.15
5.044
11.18
5.755
12.76
Table 3. Results of statistical evaluation for the median pore radius Mean Number
Sample
oflaboratories i
B1
Number of values
Repeat standard deviation
X
Between laboratory standard deviation
Sr
Reproduce standard deviation
Sb
SR
nm
nm
%
nm
%
nm
%
41.8
3.13
7.5
1.46
3.5
3.45
8.3
25
104
B2
24
100
17.1
2.56
15.0
2.21
12.9
3.38
19.8
B3
26
107
31.2
1.82
5.9
8.37
26.8
8.57
27.5
G1
22
92
37.3
1.60
4.3
2.30
6.2
2.80
7.5
G2
24
98
24.7
1.43
5.8
3.10
12.6
3.41
13.8
G3
22
89
31.2
0.80
2.6
6.70
21.5
6.74
21.6
Table 4. Results of statistical evaluation for the bulk density Mean Number
Sample
of laboratories i
Number of values
Repeat standard deviation
X
Between laboratory standard deviation
Sr
Reproduce standard deviation
Sb
SR
g c m -3
g c m -3
%
g c m "3
%
g c m "3
%
1.31 1.10
0.014 0.016
0.62 0.70
0.032 0.030
1.45 1.31
B1 B2
24 23
103 99
2.19 2.32
0.029 0.026
B3
24
103
2.20
0.027
1.21
0.011
0.49
0.029
1.31
G1
24
103
2.27
0.020
0.88
0.024
1.08
0.031
1.39
G2
24
101
2.31
0.019
0.84
0.038
1.64
0.042
1.84
G3
26
108
2.26
0.025
1.10
0.028
1.25
0.038
1.67
The mercury intrusion measurements were performed with a good precision within each individual laboratory (repeat standard deviation) as well as in different laboratories (reproduce standard deviation) for all mortar samples B1, G1, B2, G2, B3, G3. The reproduce standard deviations are 1.5 to 2 times higher than the repeat standard deviations. In particular cases,
463
like the total pore volume of B 1 e.g., they have almost the same value due to a marginal between laboratory standard deviation. This factor differs a little from findings for other measuring methods [ 10, 11 ]. Obviously, the participating laboratories are able to apply the measuring method correctly. But the precision values are affected by the inhomogeneity of the test material. It is remarkable that the pressure/volume curves of all samples have similar shapes regardless of sample preparation methods. They can be called a fingerprint of the porous material tested here, i.e. of the cementitious mortar with this composition. The techniques of sample preparation and sample drying influence the pore size distribution, the total pore volume, the median and the bulk density as well as the uncertainty of the determined characteristics.
3.1 Sample drying The preparation methods of 105 ~ drying and vacuum pre-treatment yield samples with different residual moisture contents (see Table 1). The pressure/volume curves of 'dry' (B1; G1) and 'wet' (B2; G2) samples are compared in Fig. 1 and 2. Compared with B2; G2 the 'dry' samples B1; G1 show p/V curves with a greater slope. Their pore size distribution is shifted to smaller pressures (i.e. greater pore radii). They have the highest values for the total pore volume and the median pore radius. First of all, the residual water in the pores of B2; G2 is responsible for these effects. The adsorbed water reduces the pore size and their whole volume as well. Additionally, irreversible changes of the microstructure of B1; G1 due to the heat treatment, such as the breakdown of pore walls, the collapse of small pores or the degradation of hydrate phases, have to be associated with the findings [5, 6]. The influence of the contact angle should also be considered, as it is not constant as assumed in the calculations [ 1, 12]. The repeat and the reproduce standard deviations of the total pore volume and the median pore radius are only slightly affected by different moisture contents of the samples (Tab100 000 . . . . .
70
B1
r Inm] 1 000
10 000 i. . . . . . . .
i . . . . . . . .
100
I. . . . . . . .
10
I. . . . . . . .
.....
1
i . . . . . . . .
I-"
P = 200 MPa
B2
4O: V [m~/gl 3O
10 0
0.001
.
.
.
0.01
.
.
.
.
.
0.1
.
~ ,~
1 p [aPa]
.
.
.
.
.
.
.
.
10
.
.
.
.
.
.
.
.
.
.
.
100
.
.
.
.
1 000
Fig. 1. Total average curves with uncertainty interval of cylindrical samples dried by various methods (B 1 - 105 ~ drying, B2 - vacuum pre-treatment)
464 100 000 ':"~
"
9
"
i . . . . . .
70
G1 .....
60
G2 ~
10 000 '
"
9
'1""
. . . . . .
rInml 1 000 I . . . . . . . .
100 I .....
10 " .
.
.
.
1
.i . . . . . .
"
P = 200 MPa
i "4
I
i
50
o..-
l
40 V [mrn3/g]
30 20 10 (]
-
0.001
__
. . . . . .
0.01
"0.1
1 p [MPa]
10
100
1 000
Fig 2. Total average curves with uncertainty interval of granular samples dried by various methods (G1 - 105 ~ drying, G2 - vacuum pre-treatment) les 2 and 3). B2 is an exception with a very high reproduce standard deviation of 8 mm 3 g-1. Obviously, the high moisture content of 6 % and the relatively large cylindrical bodies together cause specific measuring problems. So the testing laboratories reported difficulties with outgassing of B2. The results of B3 and G3 dried by the laboratories by a method of their own choice happen to fall in between that of the other two samples. They were averaged from samples, which were dried by 13 different procedures. As expected, the reproduce standard deviations are relatively high. But it is remarkable that they lie below the value of the 'wet' sample B2. Every laboratory treated B3 and G3 by their own method very well so that proper dried samples with low moisture contents were produced. Thus measuring samples of almost similar state could be measured relatively precisely.
3.2 Sample geometry The preparation techniques drilling and crushing yield samples with different geometry. Fig. 3 shows the p/V curves of the cylindrical sample B1 and the granular sample G1. The volume increase of the B1 cylinders is very small up to a pressure of about 1 MPa (corresponding pore radius 735 nm). Then, at this threshold pressure, the pore volume goes steadily up and at 10 MPa (74 nm) the slope becomes steeper. In contrast, the pore volume of the G1 granules increases steadily direct from the beginning of the measurement. A sharp bend at 10 MPa leads to the second part of the p/V curve. The increase of the pore volume slows down. And in the end the granules have a smaller total pore volume than the cylinders. These findings can be explained by the different geometrical forms of the sample bodies themselves as well as by differences of the drilling and crushing techniques. Thus, the granules consist of fine-grained particles with a rough surface, which can simulate additional large pores and interparticle spaces. Pore walls, blockades and narrow pore entrances can be broken due to the crushing. This leads to a greater amount of large pores. Possibly, they lie outside of
465 rInml I00 000 .....
70
!. . . . . . . .
B1
.....
G1
m
I0 000 i........
1 000 i.......
I00 '.'
10
'l ........
i ........
p = 200
1 t-.
MPa
50
40
V [mm3/g] 30
20
10
0
0.001
0.01
0.1
1 p [MPa]
10
100
.... i'b00
Fig. 3. Total average curves with uncertainty interval of cylindrical sample B 1 prepared by drilling and granular sample G1 yielded by crushing and sieving (105 ~ drying) the measuring range too. Additionaly, the granular material is richer in unporous aggregates than the cylinders (see section 2.2). However, the cylindrical samples can contain pore entrances that are blocked or partially reduced by fine abrasion particles of the drilling. The large cylindrical bodies can act as a barrier against the intrusion of mercury at low pressures. The same, but poorer effects are found for the samples B3 and G3. The 'wet' samples B2 and G2 do not show significant differences because the influences of sample geometry are interfered by other irregularities of the measurement. The repeat standard deviations of total pore volume show the tendency to be better (lower) for granules than for cylinders (Table 2). Therefore the inhomogeneity of the material supplies an explanation. So a measuring sample of B-group consists of single cylinders, i.e. a defined part of the sample volume. But a measuring sample of the granules G is produced by dividing of whole sample volume, which partly equalize the material inhomogeneities. The reproduce standard deviations of all samples, except B2 (see section 3.1), have almost the same value independently of sample geometry. The median pore radius does not show clear correlations to sample geometry (Table 3). The repeat standard deviations of the granules are a little smaller than that for the cylinders. 4. CONCLUSIONS The precision of the mercury porosimetry method on the material class cementitious mortar was investigated by an interlaboratory study of six different samples. The results are summarized in the following points. 9 Standard deviations and uncertainty intervals indicate a good precision of mercury intrusion measurements on cementitious mortars. The reproduce standard deviations are 1.5 to 2 times higher than the repeat standard deviations because the precision values are contaminated by the variance component of the heterogeneous material.
466 9 All results are significantly influenced by the techniques of preparation (sample geometry) and drying (moisture content). 9 The mercury intrusion measurements show the smallest deviations for samples with low residual moisture content (< 1%), granules and measuring samples with masses above 5 g. 9 For comparisons between several laboratories it is recommended to use samples of same geometry, i.e. same preparation technique, that are well dried or pre-treated by the same method. 9 The precision values are both method and material characteristics. They are valid for the tested material class, i.e. a cementitious mortar with this composition. 9 Based upon the determined precision values, laboratories can estimate the uncertainty of their single measurements on mortars of a comparable composition. According to [ 13] the uncertainty of a result S can be calculated from 1 2 S 2 -- S 2 q- ~ S r
(2)
m
if a mean X of k single measurements is determined. Sb is the between laboratory standard deviation and Sr is the repeat standard deviation of the measuring method. REFERENCES
[1] D.N. Winslow and S. Diamond, J. of Materials, JMLSA, 5 (1970) 564 [2] K. Hinrichsmeyer, S. Abdul-Maula, U. Diederichs and F.S. Rostfisy, Quecksilberporosimetrie- Ringversuche an erh~irtetem Zementstein, DFG-Bericht des Instituts for Baustoffe, Massivbau und Brandschutz der Technischen Universit~it Braunschweig, 1988 [3] L. Zhang and F.P. Glasser, Adv. in Cement Research 12 (2000) 79 [4] S. Diamond, Cem. Con. Res. 30 (2000) 1517 [5] C. Gallr, Cem. Con. Res. 31 (2001) 1467 [6] K. Rfibner, Th. Fritz and F. Jacobs, Ringversuch zur Quecksilberporosimetrie an Zementm6rtel, Forschungsbericht 250 der Bundesanstalt f'tir Materialforschung und -prtifung (BAM) Berlin, Wirtschaftsverlag NW, Verlag f'fir neue Wissenschaft GmbH, Bremerhaven, 2001 [7] ISO 5725, Accuracy (trueness and precision) of measurement methods and results, December 1994 [8] DIN EN 196-1, Prtifverfahren f'tir Zement- Teil 1: Bestimmung der Festigkeit, Mai 1995 [9] DIN 66133, Bestimmung der Porenvolumenverteilung und der spezifischen Oberfl~iche von Feststoffen durch Quecksilberintrusion, Juni 1993 [10] B. R6hl-Kuhn, K. Meyer, P. Klobes and T. Fritz, Fresenius J Anal Chem 360 (1998) 393 [ 11 ] W. Unger, Th. Gross, O. B6se, Th. Fritz and U. Gelius, Nachrichten aus der Chemie 48 (2000) 1108 [12] J. Adolphs and M.J. Setzer, GDCh-Monographie Bauchemie yon der Forschung bis zur Praxis, Bd. 11, Gesellschaft Deutscher Chemiker, Frankfurt, 1998, pp. 178-180 [ 13] ISO/IEC Guide 43-1, Proficiency testing by interlaboratory comparisons, part 1, 1999
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
467
Freezing in Mesopores" Aniline in Silica Glasses and M C M - 4 1 M.Sliwinska-Bartkowiak ~, G. Dudziak a, R.Radhakrishnan b, and K.E.Gubbins e a Institute of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland bMassachusetts Institute of Technology, Chemical Engineering Dept., Cambridge, MA 02139-4307 c North Carolina State University, 113 Riddick Labs, Raleigh, NC 27695, USA
1. ABSTRACT We report a study of the freezing of aniline in silica porous materials, using dielectric relaxation spectroscopy and light transmission measurements. The porous materials include controlled pore glasses with pore sizes H in the range 7.5 to 50 nm, Vycor glass (H=4.1 nm) and MCM-41 (H=2.8 nm). The freezing temperature is lowered due to the confinement, and in the larger pores crystallization occurs. In the MCM-41 material no crystallization is observed; instead a glassy phase is formed at low temperatures. In Vycor the experiments indicate a mixture of microscopic domains of crystal and glass at low temperature. The results are consistent with recent molecular simulation results. 2. INTRODUCTION Both experimental and molecular simulation studies of freezing and melting in porous materials suffer from several difficulties [1]. Difficulties in the experiments include the lack of well-characterized porous materials, difficulty in identifying the nature of the adsorbed phases, and long-lived metastable states. In the simulations, large systems and finite size scaling analysis may be needed to feel confidence in the results, and models of the porous materials may be over-simplified. The somewhat complementary nature of the difficulties in experiment and simulation make it profitable to use both approaches in a combined study. We have therefore adopted such a strategy in our recent work in this area [2-7]. For simple adsorbates composed of spherical molecules in pores of simple geometry, it is possible to map out freezing phase diagrams based on the simulation studies [3,6]. A corresponding states analysis [3] of the partition function for such a system shows that the freezing temperature in the pore, Tfpore, relative to the value for the bulk material, Tfbulk, is a function of three variables: H/try, a, and af~/trff, where H is pore width, tr is the molecular diameter, subscripts f and w refer to fluid and wall, respectively,
468 and a is a dimensionless ratio of the fluid-wall to the fluid-fluid attractive interaction when the molecular centers are at the minimum of the potential well. In practice it is found that the freezing and melting behavior is insensitive to the diameter ratio, trf,c"trff, for small molecules except when the pore width approaches the molecular diameter (molecular sieving regime). The qualitative behavior (whether the freezing temperature goes up or down, types of new phases occurring, etc.) is found to be controlled to a large extent by a, while H/aft determines the magnitude of shills in transition temperatures. The global freezing behavior predicted in the simulations is in qualitative agreement with the experimental measurements that have been reported [6]. In this paper we report new measurements of freezing and melting behavior for aniline in controlled pore silica glasses (CPG), Vycor and MCM-41 having a range of pore widths from 2.8 to 50 nm. The results are discussed in terms of the global freezing behavior predicted in molecular simulations. 3. METHODS
Freezing of a dipolar liquid is accompanied by a rapid decrease in its electric permittivity [8-10]. Following solidification, dipole rotation ceases and the electric permittivity is almost equal to n2, where n is refractive index, as it arises from deformation polarisation only. Investigation of the dynamics of a confined liquid is possible from the frequency dependences of dielectric properties, which allows both the determination of the phase transition temperature of the adsorbed substance and characteristic relaxation frequencies related to molecular motion in particular phases. The temperature of the phase transition to the solid state has also been determined recently by the light transmission method, recording changes in intensity of the light beam passing through the medium studied. At the melting/freezing transition, the number of absorption centers (nuclei of the new phase) changes discontinuously, leading to a sudden change in intensity of the light beam. 3.1. Dielectric relaxation spectroscopy The complex electric permittivity, • = ~:' + i~:", where K' = C/C 0 is the real, and K" - tan(6) / •'is the complex part of the permittivity, was measured in the frequency interval 300 Hz - 1 MHz at different temperatures by a Solartron 1200 impedance gain analyser, using a parallel plate capacitor made of stainless steel. From the capacitance, C, and the tangent loss, tan(6), the values of K' and ~" were calculated [2]. The temperature was controlled within 0.1K using a platinum resistor Pt(100) as a sensor and a K30 Modinegen external cryostat coupled with a N-180 ultra-cryostat. The aniline sample was twice distilled under reduced pressure and dried over A1203. The conductivity of purified aniline was on the order of 10.9 ~1m1. The porous silica samples used were the commercially available Controlled Pore Glass (CPG) from CPG Inc., with a pore size distribution of about 5% around the mean pore diameter. Different CPG samples having average pore diameters ranging from 50 nm to 7.5 nm were used. We also studied confinement effects in Vycor glass from Coming Inc., having a mean pore size of 4,1 nm, and a silica - based MCM-41 material with mean pore diameter of 2.8 nm. The pore samples were heated to about 600 K, and kept under vacuum (-~10.3 Tr) for 6 days prior to the introduction of the fluid. The MCM-41 samples
469
were synthesized at A. Mickiewicz University, and were characterized using x-ray diffraction and nitrogen adsorption measurements [11]. The characterization results for MCM-41 showed that these crystalline materials consisted of uniform pores in a hexagonal arrangement with a narrow pore size distribution (dispersion less then 5%). For an isolated dipole rotating under an oscillating field in a viscous medium, the Debye dispersion relation is derived in terms of classical mechanics, .
tr =tr
,
+
K ' s + K'~
, (1) 1 + (i6o r) where co is the frequency of the applied potential, and x is the orientational relaxation time of a dipolar molecule. The subscript "s" refers to static permittivity (low frequency limit when the dipoles have enough time to be in phase with the applied field). The subscript oo refers to the high frequency limit, and is a measure of the induced component of the permittivity. The dielectric relaxation time was calculated by fitting the dispersion spectrum of the complex permittivity near resonance to the Debye model of orientational relaxation of the induced component of the permittivity.
3.2. Light transmission method The light source was a He-Ne laser of 6 mW power, whose light beam was split into two beams by a set of prisms. One beam was collected directly by a photodiode, while the other was passed through the measuring cell and the focusing lens and was directed to the second photodiode. The signals from both photodiodes were analysed by the differential amplifier. The measuring cell was thermostatted by a cryostat N-180, and temperature was measured by a copper-constantan thermocouple, with an accuracy of _+ 0.1 K. The relative change in the light intensity after passing through the sample studied is proportional to the voltage change at the photodiodes and characterises the absorption of a given medium. A strong increase in the light absorption is observed in the vicinity of the phase transition as a consequence of the appearance of many nuclei of the new phase, which act as absorption centres. The systems studied are transparent in the liquid state, and the change in light absorption at the solid - liquid phase transition is easily seen. 4. EXPERIMENTAL RESULTS The dielectric relaxation method was applied to study the process of freezing and melting for samples of confined liquid aniline in CPG, having mean pore sizes of H = 50, 25, and 7.5 nm, and in Vycor with H = 4.1 nm. The sample was introduced between the capacitor plates as a suspension of CPG or Vycor in pure aniline. Therefore, the capacitance measurement yields an effective relative permittivity of the suspension of porous silica glass in pure aniline. Results of the measurements of C for aniline in these silica glasses as a function of T at a frequency of 0.6 MHz are shown in Fig.1 (a-d). There is a sharp increase in C at T=267 K corresponding to the melting point of pure aniline, due to the contribution to the orientational polarisation in the liquid state from the permanent dipoles [2,7]. The second sharp increase at 262K (Fig. l a), 260K (Fig. l b), 246K (Fig.lc) and about 227K (Fig.1 d ) is attributed to melting in the pores. The signal corresponding to melting becomes increasingly rounded as the pore size becomes smaller (Fig.ld), suggesting a heterogeneous melting process in the pores.
470
Similar results (not shown) were obtained from the light transmission measurements, which were carried out for aniline in CPG, Vycor and MCM-41 as a function of temperature. Melting temperatures were indicated by sharp increases in the photodiode voltage, which is proportional to the intensity of the light beam passing through the sample. Melting temperatures were in good agreement with those determined from dielectric relaxation spectroscopy, except for Vycor where a somewhat lower melting temperature of 229.8 K was observed. For MCM-41 a melting transition was observed at 223.1 K. '
|
'
Aniline
i
-
!
,
|
'
+ C P G 50 nm.
!
' i
,/*',-k.~
400
r
,
240
9 -
!
:
!d
,
i d
.
.
.
.~,...~.
i i: '
--...,....~,= |
;ZOO.
1; ill,
300,
.
+ CPG 25nm,
Aniline
220.
~.
180-
(,3
160.
I40 200,
120. f,~
193
233
....
ti
!
300
|
"
"
Xl~]
"
'
!
i '
+ CPG 7.5 nm . . . . .
Aniline
!~
273
2Y3
r~l '
!
"
!
Aniline
,
+ Vycor
i
"i
'1
9
|
9
i .....
,
4,1 ~Im.
i
"
..,,..m
,._,..,_~ : -..=.-
,1
73 rO
m.=- . - r 72
.... ....
,
t"/
,.--,~-'~"
.
,
,
t
:~
,
i.~
i~
|
.
,k . . . .
r~
r~s . . . .
z~'
'
"
i~ ~-'
Fig. 1 (a-d) Capacitance C vs. temperature T for Aniline in CPG and Vycor for different pore widths at frequency c0=0.6 MHz. 50
II
.
.
.
.
I
Melting
,
,
!
of Aniline
,
!
in C P G
,
20
10 9
0
o.oo
,
/
o o5
,'
i
o~o
,
i
0,,5
,
0,20
H'l[nm
Light transmission
,~ D i e l e c t r i c M e t h o d 9 i . i ,
i
0,25
"11
0,30
Method
!
o.~
,
o 40
'
471
Fig. 2. Shift in the melting temperature ATm as a function of 1/H for Aniline in CPG, Vycor and MCM-41. The straight line is a fit to the data for H values of 25 nm and above, and is consistent with the Gibbs-Thomson equation. The size dependence of the melting temperature of the confined aniline is shown in Figure 2. The linear relationship between the shift in the pore melting temperature and the inverse pore diameter for larger pore widths is consistent with the Gibbs-Thomson equation [1], but strong departures from this linear behaviour are observed for smaller widths, particularly below pore widths of about 10 nm. Such departures arise from the assumptions made in the derivation of the Gibbs-Thomson equation, and have been observed in other systems [e.g. 1,2,4]. The dielectric and optical measurements are in generally good agreement. Aniline in C P G 7.5nm | Bulk ~d
!
f
!
'
I
'
Maxwell - Wagner
Pore phase r
om ~ ~ - ir-II~-. I II
\,t;" "'m..
"" v
o) c3
-3
.
m
"-n i
m
"A m
-4
Contact layer phase - _
-
2)3
'
2~
'
3)3
'
373
'
423
T [K] Fig. 3. z vs. T for aniline in CPG with average pore size of 7,5 nm. The spectrum of the complex permittivity (~:' and •" vs ~o ) was fit to the dispersion relation, Eq.(1), to determine the dielectric relaxation time z. The frequency range in this study encompasses the resonant frequencies corresponding to the dielectric relaxation in solid and glass phases. According to Eq.(1), K' shows a point of inflection and 1267K (Maxwell-Wagner effect), 230 ~l/KT
(3)
in
~l/D~c
Here D~c is the molecular diffusion coefficient of the pair C-T and K T - (2/3)~/(8RgT/~MT) is the Knudsen constant for the tracer T, Rg is the gas constant, T temperature, and MT the tracer molecular weight. ~ and ~ are parameters characterizing the porous medium (transport parameters). stands for the integral mean radius of pores through which the
cmB/g
9
I
o=84nm ,=73nml
c=302nm ] !~2nm]
2 7nm
/J o v
oo t/
-o
j ~
--'
o
'
r
1
.
.
.
.
2
KT/DmTc 10-5 (cm 4)
Fig. 5. Diffusion times for G4 pellets from parameter matching in co-ordinate: tdifKT versus KT/DmTC
0.0
}L
II
c=330nm d=670 nm ~,-
I)/I l1 I
,
I
nm nm T ~233~ , ,
50 nm (Fig. 1, a and b) is important with respect to adsorption of such biopolymer molecules as endotoxins or inflammatory cytokines of peptide/protein nature. 3.2. Carboxens From the analysis of the standard nitrogen adsorption isotherms (Fig. 1), and structural parameters (Table 1) related to the total porosity (Vp, SBEr) and microporosity (VDs, SDs), it follows that Carboxen 1010 is almost a purely microporous carbon, with VDs close to Vp, and SDs close to SsEr. Carboxen 1003 is a bi-porous carbon with a marked contribution of mesopores, as there is a substantial difference between VDs and Vp, as well as between SDs and SBeT (Table 1). Textural features of these carbons are also presented in Fig. 2, showing the pore size distributions. For Carboxen 1010 high intensity off(x) (Fig. 2c) is observed only at x < 1 nm. Significantly lower intensity is seen at x > 1 nm both for narrow and broad mesopores, as this adsorbent is almost purely microporous. The PSD for Carboxen 1003 (Fig. 2c) is akin to that of MN200 (Fig. 2a) with a marked contribution of mesopores at x > 10 nm that can be of importance for adsorption of larger biopolymer molecules. Although MN200 has parameters of pore structure superior to MN500 (Table 1), the latter is characterised by relatively larger contribution of mesopores (Fig. 2). 3.3. Adsorption of TNF-(z and LPS Results of adsorption experiments are presented in Table 2. In solution, TNF-cz, a polypeptide with MW 17 kD, exists as a trimer with MW 51 kD. The monomeric units are kept together due to the hydrophobic interaction. From the experimental data it is clear that large mesopores play a relatively minor role in adsorption of TNF-cz, as both MN resins adsorb it rather efficiently. Isoelectric point of TNFcz is similar to that of serum albumin (pI between 4.8 and 5.6) [11]. It has been shown that TNF(z can be removed from biological media using non-polar cross-linked polystyrene adsorbents of the Amberlite | type (XAD-2, XAD-4 and XAD-6) [12]. This observation confirms that hydrophobic interactions play an important role in the adsorption of TNFcz at neutral pH. Our data support this conclusion. a
0.050.04.
j~ II
MN200 1--0-- DFT
I
0.03-
2
CONTIN
~" 0"010t
0.02
llll
1 - - 0 - - DFT
c 0.04.
~
--O--Carboxen 1003 Carboxen 1010
2 0.02"
0.005]
0.01.
0.00
0.2
1
10
Pore Half-width
(nm)
100
0.000;
0.2
.
.
.
.
1
.
.
. .. . .. . .. ... . . 10
Pore Half-width
.
.
.
(nm)
100
0.00~ 0.2
.....................
1
10 (nm)
60
Pore Half-width
Figure 2. Pore size distribution in (a) MN200, (b) MN500, and (c) Carboxen 1003 and Carboxen 1010, calculated using a model of slit-like pores and the regularisation procedure CONTIN (a, b, c); and Micromeritics DFT (a, b).
519 Table 2 Removal of TNF-tx and LPS from solution, % of initial concentration Adsorbent TNF-ot* LPS** LPS*** MN200 82 negligible negligible MN500 96 35 81 Carboxen 1003 not measured 89 not measured Carboxen 1010 not measured 18 not measured * From a solution with initial concentration of 500pg mL l after 360 minutes, at 25~ Similar results were obtained at other initial concentrations. ** After 360 minutes, at 25~ *** After 24 hours, at 25~ Molecular mass of LPS varies, and its subunits of 10-20 kD normally aggregate into complexes with a molecular mass between 100 kD and 1,000 kD [ 13, 14]. Both hydrophobic interactions and electrostatic forces are responsible for the aggregation, which is also promoted by bivalent cations. In case of LPS, MN200 demonstrates much lower adsorption capacity than MN500. Additionally, MN500 has a very different surface chemistry (strong acidic groups) from MN200 (nonionic neutral surface), which perhaps is responsible for such a striking difference in their adsorption affinity towards LPS. Despite significant efforts put into finding an efficient adsorbent for LPS removal from biological media, factors affecting its adsorption are yet not well understood and the data published so far, are controversial. It is generally accepted that both hydrophobic and electrostatic interactions contribute to the retention of LPS by surfaces [15]. There is no agreement, however, about their relative contribution to LPS adsorption. The isoelectric point of endotoxins is between 1.3 and 2.0; therefore, endotoxins are negatively charged at neutral pH [15, 16]. For this reason, anion-exchangers and affinity adsorbents with basic ligands, such as poly-L-lysine, polyethylenimine, histidine or polymyxine B have been used to purify protein solutions and other biological media from endotoxins. On the other hand, it is well known that LPS is strongly adsorbed by hydrophobic polystyrene and bound to serum albumin, the latter also being negatively charged at pH above 5.0, thus indicating an important role of non-polar interactions [ 17]. At this stage it is difficult to explain the role of the surface chemistry in adsorption of such a very complex molecule as LPS. Strong acidic groups on the surface of MN500 under the experimental conditions (neutral pH, serum albumin and Tyrode buffer containing Ca 2+) most likely exist in (-SO3)2 Ca 2+ form. It has been shown that metal ions, particularly divalent cations, strongly interact with the LPS molecule via phosphate and carboxylic groups of its core region [ 18]. Hence, LPS may be attached to the surface via Ca 2+ bridge. The real picture may be even more complicated taking into account possible adsorption of serum albumin and other proteins that could bind LPS in the adsorbed state. Presence of mesopores in Carboxen 1003 (Fig. 2c) correlates with its higher adsorption capacity towards LPS in comparison with purely microporous Carboxen 1010. Although this study raises more questions than provides answers, the results are very encouraging from the practical point of view, as they suggest that manipulating pore structure and surface chemistry of adsorbents it would be possible to design adsorbents with high capacity towards removal of LPS and inflammatory cytokines - a task, which other methods failed to fulfil.
520 ACKNOWLEDGMENTS This work was supported by the Wellcome Trust grant 051017 (M.C.M) and EPSRC grant GR/R05154. V.M.G. thanks the Royal Society for financial support of his visit to the University of Brighton, UK. Authors thank Dr. W.R. Betz (Supelco, Inc.) and Dr. J.A. Dale (Purolite Int., Ltd.) for providing samples of carbons and resins, respectively. REFERENCES
[ 1] S.V. Mikhalovsky, Microparticles for Hemoperfusion and Extracorporeal Therapy, in: Microspheres, Microcapsules & Liposomes, R. Arshady (ed.), Vol. 2, Cirrhus, London, (1999) 133-169. [2] R.L. Patterson and N.R. Webster, J. R. Coll. Surg. Edin., 45 (2000) 178. [3] L.C. Green, D.A. Wagner, J. Glogowski, P.L. Skipper, J.S. Wishnok and S.R. Tannenbaum, Anal. Biochem., 126 (1982) 131. [4] M.M. Bradford, Anal. Biochem., 72 (1976) 248. [5] A.W. Adamson and A.P. Gast, Physical Chemistry of Surface, 6th ed., Wiley, New York, 1997. [6] S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, 2 nd ed., Academic Press, London, 1982. [7] M.M. Dubinin and F. Stoeckli, J. Colloid Interf. Sci., 75 (1980) 34. [8] C. Nguyen and D.D. Do, Langmuir, 16 (2000) 7218. [9] V.M. Gun'ko and D.D. Do, Colloids Surf. A, 193 (2001) 71. [10] S.W. Provencher, Comp. Phys. Comm., 27 (1982) 213; 229. [ 11 ] G.S. Rees, C.K. Gee, H.L. Ward, C. Ball, G.M. Tarrant, S. Poole and A.F. Bristow, Eur. Cytokine Netw., 10 (1999) 383. [12] M. Nagaki, R.D. Hughes, J. Lau and R. Williams, Int. J. Artif. Organs, 14 (1991) 43. [ 13] L.P. Li and R.G. Luo, Separ. Sci. Technol., 34 (1999) 1729. [ 14] G.S. Worthen, N. Avdi, S. Vukajlovich and P.S. Tobias, J. Clin. Invest., 90 (1992) 2526. [ 15] F.B. Anspach, J. Biochem. Bioph. Meth., 49 (2001) 665. [ 16] Y. Shi, H. Hogen-Esch, F.E. Regnier and S.L. Hem, Vaccine, 19 (2001) 1747. [ 17] H. Yokota, H. Kiyonaga, H. Kaniwa, T. Shibanuma, J. Pharm. Biomed. Anal., 25 (2001) 1001. [ 18] R.D. Lins and T.P. Straatsma, Biophys. J., 81 (2001) 1037.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
521
S e p a r a t i o n of a d s o r p t i o n i s o t h e r m s of N2 in i n t e r n a l i n t e r s t i t i a l n a n o p o r e s of s i n g l e - w a l l e d c a r b o n n a n o h o r n c o m p a r a t i v e s t u d y with e x p e r i m e n t and s i m u l a t i o n
and - A
T. Ohbaa, H. Kanoha, K. Muratab, M. Yudasakab, S. Iijimab, and K. Kaneko a aGraduate School of Natural Science and Technology, Chiba University 1-33 Yayoi, Inage, Chiba 263-8522, Japan bjapan Science and Technology Corporation, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan
N2 adsorption isotherms in internal and ineterstitial nanopores of single wall carbon
nanohorn
(SWNH)
were calculated
compared with experimental one.
by GCMC
simulation
and is
Fitting of the GCMC-simulated isotherm
to experimental one in internal nanopores gave the average pore width w - 2.9 nm.
The N2 adsorption isotherm in the interstitial nanopores of the bundled
SWNH particles well coincided with the observed one.
I. Introduction Single wall carbon nanohorn (SWNH) known as a new nanocarbon material is expected as a hopeful applicant for methane and hydrogen storage. [1-3] A single particle of SWNH has a horn shaped structure and the tube part is similar to single wall carbon nanotube (SWNT). "Dahlia-flower" mesopores.
like
aggregate,
giving
SWNH particles form a
external
micropores
and
internal
Thus the SWNH assembly has both of micropores and mesopores.
Then the term of nanopores covering micropores and small mesopores (pore width < - 5
nm) is used in this article.
SWNH is highly pure, because the
SWNH is prepared by CO2 laser ablation of graphite without metal catalysts. Therefore, SWNH has an advantage for a better comparison with simulation study. [4-6]
Authors divided experimentally adsorption isotherms of N2 in
internal pores and external pores of SWNHs. [7]
Elucidation of adsorption in
522 internal and external nanopores
should be helpful to design an o p t i m u m
adsorbent for methane and hydrogen. In this study, N2 adsorption in the internal pore of single SWNH particle and on external
pores
of bundled
SWNH
canonical Monte Carlo (GCMC)
particles
is simulated
with
grand
method and the simulated isotherms are
compared with the experimental results.
2. Simulation and Experiment The preceding high resolution TEM examination suggests the shape o f a single SWNH particle. [1 ]
One end of a single SWNH particle has a corn like
structure whose tip has the average angle of 7t / 9 rad from the TEM image and closed pore structure in Fig. assemblies
must be studied
properties.
In
this
1.
The average pore structures of SWNH
in order to understand
GCMC
simulation,
a
their
single
gas adsorption
SWNH
particle
is
a p p r o x i m a t e d by the smoothed wall structure consisting of the corn and tube parts. The intermolecular interaction between N2 molecules was a p p r o x i m a t e d by one center Lennard-Jones (L J) potential function in eq (1).
(1)
Here, ooff and off are the potential well depth between N2 molecules and the effective diameter,
respectively.
The
used
LJ
p a r a m e t e r s for an N2 molecule are e f f / kB = 104.2
K
and
off
=
0.3632
nm.
The
m o l e c u l e - p o r e interaction was a p p r o x i m a t e d by the smoothed graphitic wall function for the
structure-less
tube.
[4,5]
Even the
m o l e c u l e - p o r e interaction for the corn part was a p p r o x i m a t e d by the spinning fishing rod model. [6]
Here, the 6cc / kB = 30.14 K
and o'c~ = 0.3416 nm were used for a carbon
Fig. 1 TEM image of SWNH (a) and the Model (b).
523
atom.
The
cross
parameters
L o r e n t z - B e r t h e l o t rule.
N2 and
of
carbon
were
given
by
the
The r a n d o m m o v e m e n t , creation, and r e m o v e m e n t o f
a molecule
give
a new
configuration
whose
total
potential
calculated.
As the c o n f i g u r a t i o n is a c c e p t e d in the c o n d i t i o n o f M e t r o p o l i s ' s
s a m p l i n g s c h e m e , the s y s t e m r e a c h e s an e q u i l i b r i u m state.
energy
was
The r e l a t i o n s h i p
of the tube d i a m e t e r D at the c a r b o n atom p o s i t i o n and the e f f e c t i v e pore w i d t h w w h i c h can be d e t e r m i n e d by the gas a d s o r p t i o n , is d e s c r i b e d by the f o l l o w i n g equation. w = D-0.3
/nm
(2)
In this study, s i m u l a t e d a d s o r p t i o n i s o t h e r m s o f N2 in the internal n a n o p o r e s were c a l c u l a t e d over the r e l a t i v e p r e s s u r e range o f 10 .6 to 1 for different tube d i a m e t e r s f r o m D = 2.0 nm to 3.6 nm by every 0.1 nm.
The a d s o r p t i o n
i s o t h e r m was c a l c u l a t e d on the e x t e r n a l n a n o p o r e s o f b u n d l e d S W N H p a r t i c l e s , whose D is 3.2 nm.
Here 0.4 nm was a d o p t e d as the i n t e r p a r t i c l e spacing for
the oriented SWNH assembly using the XRD data. [8] The S W N H used in this study is p r e p a r e d by CO2 laser a b l a t i o n o f graphite under Ar a t m o s p h e r e
at 101 kPa.
A s - g r o w n S W N H s were only used for
a d s o r p t i o n in the e x t e r n a l n a n o p o r e s .
The a s - g r o w n S W N H s were p a r t i a l l y
oxidized at 693 K; o x i d i z e d S W N H have n a n o - s c a l e w i n d o w s on their walls and t h e r e b y a d s o r p t i o n occurs in the e x t e r n a l and internal pores.
The w e i g h t
d e c r e a s e o f as g r o w n S W N H on the o x i d a t i o n t r e a t m e n t is less than 3 % and thereby
it
does
not
affect
the
s e p a r a t i o n o f a d s o r p t i o n in the internal and
external
pores.
adsorption
The
was
N2
measured
v o l u m e t r i c a l l y after the p r e h e a t i n g at 423
K and
1 mPa.
in
obtained
f r o m the s u b t r a c t i o n of the
adsorption
the
The a d s o r p t i o n
isotherm
internal
isotherm
pores
of
was
500 _
~.7~ ~7'
~4oo
-
~'~
-
E
300-l.35 nm. Following experimental CO2 permeability (through a microporous AC membrane) results [3], exhibiting a maximum at 308 K and a mean pressure of around 35 bar (P/Pc=0.474), which is absent at 333 K (see Fig. 4), Papadopoulos has recently presented in [42] a possible explanation of the CO2 behavior based on GCMC studies. The simulation results predicted an orientational ordering transition of the Coo axis of supercritical CO2, inside individual graphite micropores, only at 308 K and not at 333K. By studying the diffusion equation in terms of the Darken factor over a real porous medium, he concluded that this transition has a significant influence on the adsorbate permeability. On the other hand very recently, neutron diffraction experiments were performed [ 10], in conjunction with in-situ adsorption of CO2 at 308 K on the same carbon membrane. Based on these experiments the intermolecular part of the total radial distribution function, G(r), for adsorbed CO2 can be calculated (Fig. 5).
550
201 :~'~, ,.--,
/
4O
,,
, PIPc=0.813 ~
~
P/Pc=0.474
0
30
g
20
1.2
X
% ,-
0.8 10
o
0
I
I
I
0.2
0.4
0.6
'
P/Pc
Fig. 4. C O 2 permeancethroughan AC membrane versus P/Pc [3]
I
0.8
0.4
-------
0.3
0.5
0.7
0.9
r (nm)
Fig. 5. Radial distribution function of adsorbed CO2 molecules in microporous AC [ 10]
The G(r) functions exhibit a first neighbor peak (0.4 nm), followed by a damped oscillation and a second neighbor peak (0.8 nm). At the low-pressure state (10 bar) and especially at the high-pressure one (60 bar) the main peak reveals the presence of two structures centered at 0.37 and 0.415 nm respectively. The first is attributed to the C-O and OO interacting distances between orientationally correlated molecules whereas the latter arises mainly from the C-C distance between orientationally uncorrelated molecules. Such effects extend to the second neighbor molecules, since a split of the second neighbor peak appears in both curves (10 and 60 bar). In the case of the medium pressure (35 bar) no splitting is observed, while a shoulder appears at 0.5 nm, implying a different molecular arrangement. Considering the above experimental evidence of orientational transitions of adsorbed CO2 molecules, the detailed structure of the layer near the wall is of special interest. As implied by the density distribution and the pertinent 0 values of our simulations in H-l.15 nm pores (Fig. 2), this layer consists of two distinct sub-layers, one closer to the wall (sub-layer a) with molecules lying almost parallel to it and a second (sub-layer b) with molecules oriented at an average 45 ~ with respect to the pore wall. Similar sub-layers have also been observed in the case of ethane adsorption by Vishnyakov et al. [31 ]. The presence of these two entities has been attributed to the quadrupole-quadrupole interactions, which contribute essentially to the stability of such a herringbone like configurations [8]. This is further supported by GCMC simulations, since the absence of the coulombic term in Eq. (1) leads to the degeneration of the sub-layers into one uniform layer [36]. In Fig. 6 we present detailed plots of CO2 singlet density distributions in 1.15 nm pores at 308 and 333 K, "focused" on the layer close to the pore wall. The two maxima located at approximately 0.18 and 0.24 nm from the pore center indicate the position of CO2 molecular centers that constitute the two sub-layers. By tracing the density distribution changes with pressure at 308 K (Fig. 6a) it is observed that when the pressure increases the molecular arrangement changes in a way that sub-layer b is favored, since the fraction of molecules that lie flat on the pore wall is decreasing while the fraction of molecules that are oriented in a 45 ~ angle with the pore wall, is increasing. These simulation results imply that in the pressure range under consideration an orientational transition takes place, since molecules are re-oriented by literally turning away from the wall in order to achieve the most energetically "convenient" configuration.
551
40
o'2x j~ ~ ','6 t
1.15 nm, 308 K . . . . . P/Pc=0.542
40 ] i
35 1/) _.e
30
1.15 nm, 333 K
~-*-- P/Pc=O.542 I'-"-- P/Pc=0.474 35 PIPc=0.407
~
~ 3o
,::_~,-...~
25
25
20
j
0.
0.15
0.2 Z (rim)
(a)
0.25
0.3
20
0.1
0.15
0.2
0.25
0.3
Z (nm)
(b)
Fig. 6. Details of the CO2 density distribution within a 1.15 nm slit carbon pore at (a) 308 and (b) 333K, revealing an ordering transition at 308 K.
On the contrary, this not the case at 333 K, where the pressure increase leads to a proportional increase in the density of both sub-layers, implying that more and more molecules are simply added over the already adsorbed ones without any significant configurational changes. We may also note that this orientational transition was only observed for the 1.15 nm pore, at 308 K within the 30-40 bar pressure range, since the results for other pore sizes and pressure ranges follow the general trend shown in Fig. 6b (i.e. proportional increase of density all over the pore with increasing pressure). These results indicate the possible nature of the orientational transition proposed for the explanation of the observed permeability and neutron diffraction experimental data. More work is underway to confirm the present findings. In such a case, GCMC simulations will once more prove their potential as a tool to follow closely and probe adsorption phenomena in the molecular level. 5.
CONCLUSIONS
A series of experimental results imply that an orientational transition of CO2 molecules adsorbed in graphitic micropores takes place at the supercritical temperature of 308 K (but not at 333K) and at an equilibrium pressure of approximately 35 bar. In this work we employed the GCMC method to reveal structural details of adsorbed CO2 molecular configurations within microporous carbons. After a detailed examination of the local density profiles and angular distributions of molecular axes at different pressures within pores of different size at 308 and 333K, we observed an orientational transition of molecules confined in 1.15 nm slit graphitic pores at 308 K and at c.a. 35 bar. Specifically in this pore size two distinct sublayers are forming one being closer to the wall with molecules lying almost parallel to it and a second one with molecules oriented at a 45 ~ angle with the pore wall. At 308 K as the pressure increases from 30 to 35 and finally to 40 bar, the molecules in order to account for the increased total density, tend to re-orient themselves by turning their molecular axes away from the pore wall in a way that the second sub-layer is enriched in the expense of the first. This transition is absent at different pore sizes and outside the specific pressure range or even within the same pore and pressure range but at higher temperature (333 K). Our results could serve as a plausible explanation of the orientational transition implied by experimental data.
552
REFERENCES
[1] K-H. Lee and S-T. Hwang, J. Colloid Interface Sci., 110 (1986) 544. [2] R.J.R. Uhlhom, K. Keizer and A.J. Burggraaf, J. Membr. Sci., 66 (1992) 259. [3l F.K. Katsaros., Th. A. Steriotis, A. K. Stubos, A. Ch. Mitropoulos, N. K. Kanellopoulos, and S. Tennison, Microporous Mater., 8 (1997) 171 R. Evans, U.M.B. Marconi and P. Tarazona, J. Chem. Soc. Faraday Trans., 82 (1986) 1763. Bitsanis, S.A. Somers, H.T. Davis and M. Tirrell, J. Chem. Phys., 93 (1990) 3427. D. Nicholson, and R. Cracknell, Langmuir, 12 (1996) 4050. A. Akgerman in "Supercritical fluids: Extraction and Pollution Prevention", ACSSS No 670, M. A. Abraham and A. K. Sunol (eds.), American Chemical Society, Washington, 1997, pp. 208-231. [8] P. Cipriani, M. Nardone, F.P. Ricci and M.A. Ricci, Mol. Phys., 99 (2001) 301. [9] T. Morita, K. Nishikawa, M. Takematsu, H. Iida and S. Furutaka, J.Phys. Chem. B, 101 (1997) 7158. [lO] Th.A. Steriotis, K.L. Stefanopoulos, A.Ch. Mitropoulos, N.K. Kanellopoulos, A. Hoser and M. Hofmann, Appl. Phys. A, (2002) in press [11] Z. Tan and K.E. Gubbins, J. Phys. Chem., 96 (1992) 845. [12] E. Kierlik and M.L. Rosinberg, Phys. Rev. A, 44, (1991) 5025. [13] C. Lastoskie, K.E. Gubbins and N. Quirke, J. Phys. Chem., 97 (1993) 4786. [14] V. Neimark and P. I. Ravikovitch, Microporous Mesoporous Mater., 44-45 (2001) 697. [15] P.N. Aukett, N. Quirke, S. Riddiford and S.R. Tennison, Carbon, 30 (1992) 913. [16] J.P. Olivier, W.B. Conklin and M. Szombathely, in "Characterization of Porous Solids (COPS-lID, Proceedings of the IUPAC Symposium", F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing, K.K. Unger (eds.), Elsevier, Amsterdam, 1994, p.81. [17] A.V. Neimark, P.I. Ravikovitch, M. Grun, F. Schuth and K.K. Unger, Langmuir 13 (1995) 4765. [18] K.A. Sosin and D.F. Quinn, J. Porous Mater., 1 (1995) 111. [19] S. Scaife, P. Kluson and N. Quirke, J. Phys. Chem. B, 104 (2000) 313. [2o1 P I Ravikovitch, A. Vishnyakov and A V Neimark, Phys. Rev. E, 64 (2001) 11602. [211 G.M. Davies and N.A. Seaton, Carbon, 36 (1998) 1473. [22] G. M. Davies and N. A. Seaton, Langmuir, 15 (1999) 6263. [23] G.M. Davies, N.A. Seaton and V.S. Vassiliadis, Langrnuir, 15 (1999) 8235. [24] P.I. Ravikovitch, A. Vishnyakov, R. Russo and A.V. Neimark, Langmuir, 16 (2000) 2311. [25] Vishnyakov, P.I. Ravikovitch, and A.V. Neimark, Langmuir, 15 (1999) 8736. [26] R.F. Cracknell, D. Nicholson and N. Quirke, Mol. Phys., 80 (1993) 885. [27] R.F. Cracknell, D. Nicholson and N. Quirke, Mol. Sim., 13 (1994) 161. [28] R.F. Cracknell and D. Nicholson, J. Chem. Soc., Faraday Trans., 90 (1994) 1487. [29] M.B. Sweatman and N. Quirke, J. Phys. Chem. B, 105 (2001) 1403. [301 D. Keffer, H.T. Davis and A.V. McCormick, Adsorption, 2 (1996) 9. [31] Vishnyakov, M.E. Piotrovskaya and N.E. Brodskaya, Langmuir, 12 (1996) 3643. [32] S. Samios, A.K. Stubos, N.K. Kanellopoulos, R.F. Cracknell, G.K. Papadopoulos and D. Nicholson, Langmuir, 13 (1997) 2795. [331 V.I. Gusev, J.A. O'Brien and N.A. Seaton, Langmuir, 13 (1997) 2815. [34] M.V. Lopez-Ramon, J. Jagiello, T.J. Bandosz and N.A. Seaton, Langmuir, 13 (1997). 4435. [351 S. Samios, G.K. Papadopoulos, Th.A. Steriotis and A.K. Stubos, Mol. Sim., 27 (2001) 441. [36] S. Samios, A. Stubos, G.K. Papadopoulos, N.K. Kanellopoulos and F. Rigas, J. Colloid Interface Sci., 224 (2000) 272. [37] C.S. Murthy, S.F. O'Shea, and I.R. McDonald, Mol. Phys., 50 (1983) 531. [38] W.A. Steele, The Interaction of Gases with Solid Surfaces, Pergamon, Oxford, 1974. [39] D. Nicholson, J. Chem. Soc., Faraday Trans., 90 (1994) 181. [40] K. Kaneko, R.F. Cracknell, and D. Nicholson, Langmuir, 10 (1994) 4606. [41] M. Allen, and D.J. Tildesley, Computer Simulation of Liquids, Clarendon, Oxford, 1987. [42] G. K.Papadopoulos, J. Chem. Phys., 114 (2001) 8139.
[4] [5] [6] [7]
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
553
Preloading of GAC by natural organic matter: effect of surface chemistry on TCE uptake James E. Kilduff ~, Reena Srivastava a, and Tanju Karanfil b aRensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, USA bClemson University, 342 Computer Court, Anderson, SC 29625, USA Trichloroethylene (TCE) adsorption by activated carbon previously loaded ("preloaded") with natural organic matter (NOM) is reduced significantly in comparison to asreceived carbon. Granular activated carbon (GAC) surface chemistry was modified as a means to preferentially reduce the uptake of natural organic matter foulants and thus increase the uptake of TCE. Heat treatment (up to 1000 ~ under N2) and oxidation (conc. NO3) were successful in changing the surface chemistry while not extensively changing the pore structure of two GAC samples. The uptake of organic foulants decreased significantly with increasing surface acidity (as measured by NaOH neutralization). While the subsequent effects of preloading on TCE uptake were mitigated, TCE uptake by hydrophilic carbons was not satisfactory. In contrast, heat treatment of acid-activated wood-based carbons mitigated the effects of preloading while maintaining high TCE uptake. The observed effects could not be explained by a reduction in NOM uptake, suggesting a shift in the sorption mechanism, possibly resulting from the mesoporous structure of these carbons. 1. INTRODUCTION Granular activated carbon (GAC) is widely used for removing synthetic organic contaminants from potable water supplies, contaminated groundwaters, and industrial wastewaters. All natural waters and many industrial wastewaters contain natural macromolecular dissolved organic matter (NOM) that can compete with TCE and other target compounds, and thus significantly reduce the efficiency of activated carbon adsorbers [1-8]. As shown in Fig. 1 (left panel), preloading occurs when the mass transfer zone of NOM moves through a fixed bed at a faster rate than the target compound, and loads the carbon ahead of it. As shown in Fig.1 (middle panel), such preloading by NOM can result in direct competition for adsorption sites, pore blockage or both [9-11 ]. This results in a reduction in equilibrium uptake of TCE (Fig. 1, right panel). In addition, a reduction in the rate of synthetic organic compound adsorption is often observed, which may include an increased diffusion resistance at the particle surface [12], an intraparticle diffusion resistance [6,8], or both. Several factors could potentially influence the impact of preloading on adsorber performance. One such factor is the composition of the background NOM; for example, in many systems the effect of preloaded organic matter on SOC uptake is related to the amount of the smaller size fraction adsorbed [13,14]. Characteristics of the adsorbent, especially the pore size distribution [15] and surface functional group composition, are also important. Surface acidity has been implicated as a factor controlling the uptake of organic compounds [ 16-19]. Surface acidity may reduce the catalytic activity of the surface, increase
554 selectivity for water, modify the charge distribution on graphitic basal planes, change the adsorbate configuration, or produce some combination of these effects. It has been postulated that increasing the number of polar oxygen molecules within the carbon matrix or oxygencontaining surface functional groups increases the polarity of carbon surfaces and their selectivity for water. Previous work in our laboratories has shown that increasing carbon surface acidity and negative surface charge caused a significant reduction in the uptake of natural organic foulants [20]. Given that preloading effects are related to the loading of foulant [14], and that the loading of foulant can be controlled by modification of carbon surface chemistry, we hypothesized that such surface chemistry modification may be a viable route to producing carbons that resist fouling by natural organic matter. The objectives of this work were to (i) investigate carbon surface chemistry modification as a means to preferentially reduce the uptake of natural organic matter foulants and thus increase the uptake of TCE by GAC preloaded with these foulants; (ii) investigate the effects of surface chemistry and pore structure using a series of carbons prepared from different starting materials, and having a wide range of surface acidity; and, (iii) evaluate the effects of natural organic matter foulant physicochemical properties (e.g. molecular weight, acidity, hydrophobicity) by using several different natural and model humic substances (humic and fulvic acids) and natural waters.
2. EXPERIMENTAL METHODS All carbons used in this research were pretreated to remove fines by sonication (Bransonic 220, Branson Cleaning Equipment) for 30 seconds followed by a rinse with Type I reagent-grade water (Millipore Corp). Coal-based carbons were then extracted with 2-N HC1 in a soxhlet extractor for 42 hours to remove ash components and alkaline impurities. After extraction, the sample was boiled for four hours, then exhaustively rinsed until the pH of the suspension was in the range 5 to 6. All carbon samples were dried at 100 ~ under vacuum, and stored in a vacuum desiccator. The surfaces of a thermally-activated coal-based carbon (Calgon F400) and an acidactivated wood based carbon (Westvaco WVB) were modified using liquid-phase oxidation (HNO3) and heat treatment in an inert atmosphere (N2). Adsorbents were heat treated in a tube furnace (Model LSTF341C, Lindberg). An oxygen-free atmosphere was maintained by flowing purified nitrogen gas at a rate of 30 ml/min with a back pressure of 4 to 5 psi.
Figure 1.
Preloading in fixed bed adsorbers (left); preloading mechanisms (middle); effects of preloading on trichloroethylene uptake by as-received WVB GAC (right).
555 Adsorbents were de-gassed at 100~ for 24 hours prior to heat treatment. Target temperatures were attained at a ramping rate of 10 ~ per minute and maintained for a "soak" time of 24 hours, after which the furnace was allowed to cool to ambient conditions. Heat treated samples are coded as either HT1000 (1000 ~ or HT650 (650 ~ Activated carbons were oxidized in 70% HNO3 using a solution-to-solid ratio of about 17:1 w/w. Solutions were kept stirred for the duration of the contact, and temperature was maintained constant within 3~ After oxidation, the carbon was rinsed exhaustively until the pH of the suspension was in the range 5 to 6. Oxidized samples are coded by the symbol "OX" followed by the duration of the treatment in hours and the temperature in degrees Celsius (e.g., OX 2/50). Surface area and pore size distribution were measured with a Gas Adsorption Analyzer (Quantachrome Autosorb 1). Aliquots of carbon weighing approximately 50 mg were transferred to a glass sample cell and outgassed for 48 hours at 120 ~ The weight of the sample cell containing GAC did not change after outgassing, confirming negligible loss of material during this step. Nitrogen isotherms were measured at 77 K, and surface area was determined by fitting the Brunauer, Emmett and Teller (BET) adsorption isotherm equation to at least five low-pressure isotherm points (P/Po < 0.2) using a non-linear regression algorithm. Precision of replicate samples averaged 5%. Pore size distributions were calculated using the Barret, Halenda and Joyner method. Carbon surfaces were wetted either by boiling in water for 30 min followed by centrifugation to remove interstitial water, or by equilibrating washed samples with water in a thermostatted bath at 25 ~ for 48 hours, followed by centrifugation. The two techniques gave similar water uptake values, and both were reproducible within 2%. Water uptake values measured using the second technique are reported in Table 3. The acid/base characteristics of adsorbents were characterized using the Boehm technique [21 ] and are reported in Table 3. A 0.4-gram aliquot of carbon was equilibrated for at least 48 hours with 20 g of a solution 0.05N in HC1, NaOH, Na2CO3 or NaHCO3. After equilibration, the solution was filtered and titrated with either standard base (NaOH) or acid (HC1); endpoints were determined from the first derivative of the titration curve. NaOH titrant was standardized against 0.05-N potassium hydrogen phthalate. All solutions were prepared using reagent-grade chemicals, in CO2-free water. Reproducibility of the technique was within a few percent. It is often assumed that NaHCO3 uptake corresponds to strong carboxyl acidity, while NazCO3 further reacts with weak carboxyl and f-lactonic functionality, and NaOH further reacts with phenolic acidity [22]. Model NOM included polymaleic acid (PMA), a fulvic acid surrogate; natural humic (LaHA) and fulvic acids (LaFA) extracted from Laurentian soil; and Aldrich humic acid (AHA) purified to remove ash components. Physicochemical characteristics of these compounds have been reported in one of our previous publications [23]. Surface water samples were collected from the Edisto River, Charleston, S.C; the Intercoastal Waterway, Myrtle Beach, S.C.; and the Tomhannock Reservoir, Troy, NY. Physicochemical characteristics of these compounds have been reported previously [20]. Preloading was done in 250-ml well-mixed batch reactors (amber bottles sealed with teflon septa). First, 10 to 15 mg of adsorbent was contacted with an organic matter solution (6 to 20 mg/L as organic carbon) for 14 to 45 days. After preloading, reactors were sampled with glass syringes, and the solution was filtered through a 0.45-micron polysulfone filter. TCE isotherms were also measured in 250-ml well-mixed batch reactors (amber bottles sealed with teflon septa). Aliquots of TCE stock solution (4,000 mg/L in methanol) were added to reactors containing 10 to 15 mg of either as-received or surface treated carbons to yield initial TCE concentrations ranging from 0.030 to 8 mg/L. The solution phase consisted
556 of either buffer solution or NOM solution. TCE was equilibrated for 21 days, a time that was sufficient to reach an operational equilibrium. Hexane extracts of TCE were analyzed using gas chromatography. All isotherms were conducted in the presence of a 0.01 M phosphate buffer, at pH 7 and room temperature of 21 + 3 ~ 3. R E S U L T S A N D D I S C U S S I O N Examination of the data shown in Table 1 reveals that surface treatments were successful in changing the surface chemistry while not extensively changing the pore structure. For the coal-based carbon (i.e., F400), heat treatment and surface oxidation did not change the total surface area or the pore volume significantly. As-received coal-based carbons were more microporous than the acid-activated wood-based carbons, and a maximum shift of 14% in the surface area distribution from micropores to mesopores was observed with increasing extent of surface treatment. For the wood-based carbon, the initial heat treatment decreased the total surface area and pore volume by up to 35%. However, subsequent chemical oxidation had little impact on the pore structure, and a shift of only 4 to 7% in the surface area distribution from mesopores to micropores was observed. In contrast to these minor changes in pore structure, a wide range of surface acidity and functional group density was created on both types of carbon surfaces. In general, heat treatment decreased the surface acidity selectively (depending on the temperature) whereas oxidation in HNO3 solution increased the acidity, presumably by creating new carboxylic, lactonic and phenolic type functionality, as reported previously by others [21]. As expected, the acid-activated woodbased carbons had a more acidic character as exemplified by a negative surface charge (pHpzc values ranging from 3.45 to 5.35), whereas coal-based carbons had basic surfaces (pHpzc values ranging about 9.0-9.5). As a result, heat treatment to remove oxygen functionality had a large effect on the wood based carbons, decreasing their initially high surface acidity, but had little effect on the as-received coal-based carbons, which had low initial surface acidity. On the other hand, oxidation of the coal-based carbons had a dramatic effect on their surface chemistry, making them more hydrophilic and negatively charged. Table 1 Carbon characterization Physical Analysis Carbon Type
Surface Treatment
Surface Area, m2/g
Water Uptake gig
Chemical Analysis HCI Uptake l~eq/m2
NaOH Uptake ~eq/m2
Na2CO3 Uptake peq/m 2
NaHCO3 Uptake peq/m2 0.00
F400
AW
1044
0.63
0.34
0.14
0.00
F400
AW HT1000
1000
0.64
0.37
0.00
0.00
0.00
F400
OX 2/70
1041
0.67
0.22
0.94
0.40
0.22
F400
OX 9/70
1031
0.68
0.11
1.71
0.96
0.61
F400
OX 2/70 HT650 1029
0.67
0.29
0.26
0.00
0.00
F400
OX 9/70 HT650 1066
0.72
0.24
0.55
0.22
0.05
WVB
AW
1745
1.55
0.18
0.57
0.24
0.12
WVB
AW, HT1000
1169
1.13
0.23
0.38
0.08
0.00
WVB
OX 2/50
1272
1.14
0.17
0.76
0.40
0.19
WVB
OX 2/70
1295
1.13
0.13
1.14
0.62
0.35
557 The ability to control NOM adsorption through modification of surface chemistry was demonstrated previously [20]; the uptake of several model humic substances and natural organic matter isolated from surface waters decreased significantly with increasing surface acidity (as measured by NaOH neutralization). The uptake was partially restored by subsequent heat treatment of the oxidized surfaces (i.e. OX 9/70 HT650). For the wood-based carbons, the impact of surface treatment on adsorption of organic matter was surprisingly small or absent. Overall, the reactivity of carbon surfaces to DOM uptake depended on the raw material type, activation conditions and surface treatment. Previous work in our laboratories has shown that surface acidity also plays an important role in the uptake of TCE [19]. Because TCE is adsorbed by a hydrophobic adsorption mechanism, and because increasing surface acidity reduces the hydrophobicity of the surface, it was found that in general, increasing surface acidity reduced uptake. Examples of such behavior are shown in Fig. 2. For the F400, having low oxygen content initially, uptake decreased significantly upon exposure to nitric acid, which increased acidity. For the WVB carbon, having high acidity initially, solute TCE uptake increased significantly after heat treatment to reduce surface acidity and polarity. Heat treatment temperature had a significant effect on uptake; however, the effects of heat treatment were the same for reaction times ranging from 2 to 16 hours (data not shown). It was our goal to optimize the effects of surface treatment- to decrease the uptake of organic matter, thereby minimizing the effects of preloading, but without making the surface so hydrophilic as to reduce the uptake of TCE to an unacceptable degree. In assessing the success of various surface treatments, two comparisons are of interest. These are depicted schematically in Figure 3. For a given surface treatment, a comparison of preloaded and non-preloaded carbons gives the effect of preloading. This is shown as comparison 1 in Figure 3. The effect of preloading can then be assessed for different surface treatments, and the role of surface treatment in mitigating the impact of preloading can be determined. However, only those treatments for which uptake by preloaded carbon is higher than preloaded as-received carbon are of economic interest. Therefore, a comparison of preloaded as-received carbon with preloaded surface-treated carbon gives the overall effectiveness and economic viability of surface treatment for preloading control. This is shown as comparison 2 in Figure 3. 100
100 WVB
F400 it II
d m O I---
1
o"
O o 9
o-
~"
~2"
el 10-
9 qtgl, i
."
~
. . . . . . . .
,
10
. . . . . . . .
9 9149
,
100
. . . . . . . .
t
1000
TCE Concentration, gg/L Figure 2.
1
9As-recieved 9HT1000 9OX 2/70 9OX 9/70 o OX 9/70 HT650
A 0.1
Ii
.
.
.
.
.
9As-recieved 0 400 ~ A 600 ~ o 800 ~ [] 1000 ~
9 9
I~ .
.
.
.
10000
.
.
.
.
0
.
.
1
.
i
10
. . . . . . . .
f
100
. . . . . . . .
i
1000
. . . . . . . .
10000
TCE Concentration,gg/L
The effect of carbon surface treatment on the uptake of TCE by GAC. Plate A: the effects of surface oxidation and subsequent heat treatment on uptake by F400 carbon. Plate B: the effects of heat treatment temperature (6 h reaction time) on uptake by WVB carbon.
558 I O0
Surfacetreated, preloaded --7 ~--- Surfacetreated
~
..~.... ~ ".............. " " " "i"
\
. . : : .......... ~ s
~
1
""I
1
I
...o,.
.....,*"
_~........ " I - / - - - A s - r e c i e v e d
..........
0.1
~..
10
As-received, preloaded
100
1000
10000
TCE Concentration, lag/L
Figure 3. Assessing effects of preloading and surface treatment.
In this figure, the surface treatment mitigates the impact of preloading, and the uptake after preloading is higher than the uptake by preloaded as-received carbon. Therefore, in this example, surface treatment would be of economic interest. The impacts of surface acidity on the effects of preloading were found to depend on the surface treatment, carbon type (wood or coal-based) and the organic matter source (i.e. aquatic, soil, or synthetic) in a complicated way. In several cases, particularly for the F400 carbon as shown in Figure 4, the effects of preloading are mitigated with increasing surface acidity, a trend consistent with the effects of surface acidity on humic loading. Not all experiments showed such trends however. Indeed, some data (not shown) suggests that specific interactions may occur between preloaded humic materials and surface functional groups, especially those remaining after heat treatment of oxidized surfaces at 650 ~ When specific interactions occur, the high affinity between the carbon surface and organic matter can cause large reductions in uptake resulting from preloading. In general, the effects of preloading were smallest for the most hydrophilic carbons, i.e., the as-received WVB carbon and the most oxidized F400 carbon (OX 9/70). However, even though the effects of preloading were low (or in some cases actually increased the uptake of TCE), the uptake was also low. 100
PMA
o,J
E
LU tO I-
9
9~
t:~ =k 10
9
dv .i.,., e-s
100
9
*o
.o.O~
9
E
o
LFA
9~
10
9 o
9
1
9
o
Ip
9
Io 9
As-received
o As-received, preloaded
9 OX 9/70 o o x 9/70, preloaded
A 0.1
9
Q.
10
100
1000
TCE Concentration,l~g/L
Figure 4.
10000
LU O I--
9
9 0
o o 9
9
o
1
goo
o
o
9 9
%
~ m Q
9
R 0.1 10
9As-received A s - r e c e i v e d , p r e l u d e d 9OX 9/70 o OX 9/70, p r e l o a d e d 100
1000
10000
TCEConcentration,gg/L
The effect of carbon surface treatment on the uptake of TCE by preloaded GAC. Plate A: carbon preloaded by poly(maleic acid); Plate B: carbon preloaded by Laurentian fulvic acid.
559 Therefore, even though the effects of preloading were mitigated, the use of highly oxidized carbons to control preloading effects is not economically attractive. The wood-based carbons exhibited different trends than the coal-based carbons in one important way. Figure 5 presents data for TCE uptake by heat-treated WVB carbon. A comparison of preloaded and non-preloaded carbons (comparison 1 in Figure 3) suggests that the impact of preloading on TCE uptake by heat treated carbon is less than the impact on uptake by as-received carbon- i.e., the percentage reduction in single solute uptake by heat treated carbon as a result of preloading is smaller than the reduction observed for as-received carbon. More importantly, a comparison of preloaded, as-received carbon with preloaded, surface-treated carbon (comparison 2 in Figure 3) reveals that uptake by preloaded, heat treated carbon (open symbols) is significantly higher than both that of single solute uptake by as-received carbon (black circles) and preloaded, as-received carbon (filled diamonds). The observed effects cannot be explained by a reduction in the uptake of NOM (data not shown). Consistent with previous work [20], heat treatment of wood based carbons does not significantly affect the uptake of natural organic matter (or other organic macromolecules). The data shown in Figure 5 confirm the efficacy of heat-treated WVB carbon for mitigating preloading effects of both natural organic matter from a surface water, and several different model organic macromolecules. Therefore, the results are valid for organic macromolecules having a wide range of physicochemical properties including molecular weight, acidity, and polarity [20]. A comparison of the data in Fig. 2 (Plate A, filled circles) and Fig. 5 (Plate B, open symbols) reveals that the performance of the heat-treated wood-based carbon, even under some preloading conditions, is similar to single solute TCE uptake by coal-based activated carbons in the absence of preloading [9]. The observed effect may result from some combination of optimum surface acidity, optimal type of surface functional group, and/or pore structure effects. The WVB carbon has a mesoporous pore structure, which has been observed to minimize the impacts of preloading in preliminary comparative experiments designed to isolate this effect (data not shown). Future work will employ carbon surface characterization techniques that will allow identification of functional groups and more accurate correlation with surface reactivity. 100
100
Heat Treated W V B TMK Preloaded
10
r
o~
E
E ~
Heat Treated WVB Model Compound Preloaded
Q~
o
o~
o
o
o e 9
q'
Q. U.l
1
....
9Preloaded, 4 wks o Heat treated, single sol Preloaded, 2 wks DOPreloaded, 4 wks Preloaded 6 wks
I--
A 0.1 0.1
1
10
TCE
Figure 5
100
1000
Concentration,gg/L
10000
9 g 0.1 1
9 ~ 9
9 t 9
9As-received single solute 9LFA preloaded O Heat treated - single solute LFA preloaded [] AHA preloaded PMA preloaded
lO lOO lOOO TCE Concentration,l~g/L
10000
A comparison of TCE uptake by as-received (filled symbols) and heat treated carbons (open symbols), in single solute (circles) and preloaded systems; carbon preloaded with natural organic matter (Plate A) or model compounds (Plate B)
560 ACKNOWLEDGEMENTS
James Kilduff and Reena Srivastava gratefully acknowledge partial financial support for this research from the Eastman Kodak Company. Support from the U.S. EPA (R 82815701-0, Kilduff, Karanfil), the U.S. NSF (BES 0093600, Karanfil; BES 9871241, Kilduff) and the Eastman Kodak Co. (Kilduff) is also gratefully acknowledged. This research has not been subject to funding agency peer or policy review, it does not necessarily reflect their views, and no official endorsement should be inferred. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
M. Pirbazari and W.J. Weber, Jr., J. Environ. Eng., 110 (1984) 656. E.H. Smith, S. Tseng, and W.J. Weber, Jr., Environ. Prog., 6 (1987) 18. T.F. Speth and R.J. Miltner, J. AWWA, 81 (1989) 141. R.S. Summers, B. Haist, J. Koehler, J. Ritz, G. Zimmer, and H. Sontheimer, J. AWWA, 81 (1989) 66. T.F. Speth, J. Environ. Eng., 117 (1991) 66. M.C. Carter and W.J. Weber, Jr., Environ. Sci. Technol., 28 (1994) 614. E. Orlandini, T.G. Gebereselassie, J.C. Kruithof, and J.C. Schippers, Wat. Sci. Technol., 35 (1996) 295. D.R.U. Knappe, V.L. Snoeyink, P. Roche, M. Jose-Prados, and M-M. Bourbigot, J. AWWA, 91 (1999) 97. M.C. Carter, W.J. Weber, Jr., and K.P. Olmstead, J. AWWA, 84 (1992) 81-91. J.E. Kilduff, T. Karanfil, and W.J. Weber, Jr., J. Colloid Interface Sci., 205 (1998a) 271. J.E. Kilduff, T. Karanfil, and W.J. Weber, Jr., J. Colloid Interface Sci., 205 (1998b) 280. K. Wilmanski and A.N. Van Breeman, Wat. Res., 24 (1990) 773. G. Newcombe, M. Drikas, and R. Hayes, Wat. Res., 31 (1997) 1065. J.E. Kilduff, T. Karanfil, and W.J. Weber, Jr., J AW WA, 90 (May 1998a) 76. C. Pelekani and V.L. Snoeyink, Wat. Res., 33 (1999) 1209. V.L. Snoeyink and W.J. Weber, Jr., Environ. Sci. Technol., 1 (1967) 228. L.R. Radovic, I.F. Silva, J.I. Ume, J.A. Men6ndez, C.A. Leon y Leon, and A.W. Scaroni, Carbon, 35 (1997) 1339. P. Pendleton, S.H. Wong, R. Shumann, G. Levay, R. Denoyel, and J. Rouquerol, Carbon, 35 (1997) 1141. T. Karanfil and J.E. Kilduff, Envir. Sci. Tech., 33 (1999) 3217. T. Karanfil, J.E. Kilduff, M. Kitis, and A. Wigton, Environ. Sci. Technol., 33 (1999) 3225. T.J. Bandosz, J. Jagiello, and J.A. Schwarz, Anal. Chem., 64 (1992) 891. J.J. Noh, and J.A. Schwarz, Carbon, 28 (1990) 675. T. Karanfil, J.E. Kilduff, M.A. Schlautman, and W.J. Weber, Jr., Environ. Sci. Technol., 30(1996)2187.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
561
Structural studies of saccharose- and anthracene-based carbons by high energy X-ray scattering. A.Szczygielska a, A.Burian a, S.Duber b, J.C.Dore c and V.Honkimaki d. aInstitute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland bFaculty of Earth Sciences, Interdepartmental Laboratory of Structural Researche, University of Silesia, Bedzinska 60, 41-2000 Sosnowiec, Poland CSchool of Physical Sciences, University of Kent, Canterbury CT2 7NR, UK dEuropean Synchrotron Radiation Facility, BP 220, 38043 Grenoble cedex 9, France A series of porous carbon materials, produced by pyrolysis of saccharose and anthracene and heat-treated at 1000~ 1900~ and 2300~ have been studied by wideangle X-ray scattering. The X-ray data were collected at European Synchrotron Radiation Facility (ESRF) in Grenoble on the ID 15 beam line (high-energy X-ray diffraction) using the wavelength 3~=0,1067A (E=I 16,2 keV). The data were recorded in the scattering-vector range from 0.5 to 24A-1 which allowed to conversion to real space via the Fourier transform yielding the radial distribution function of a good quality. Analysis of the experimental radial distribution function shows that carbons produced from anthracene transforms into graphite at 1900~ and this process is almost complete at 2300~ The saccharose-based carbons remain disordered even at high temperature. 1. I N T R O D U C T I O N Although pyrolytic carbons have been produced for about fifty years and studied by Xray, electron and neutron diffraction, which are the most direct probes of their structure, interpretation of experimental data and explanationof different behaviour of these materials upon annealing is rather complicated, due to interplay of order and disorder in interatomic correlations. It has been known that some carbon materials are more susceptible to graphitization than others. The first detailed study of graphitizing and non-graphitizing carbons was performed by Rosalind Franklin [1 ]. The graphitizing carbons are soft and less porous and can be easily transformed into graphite under thermal treatment. The latter are much harder, have a complicated porous structure and cannot, or can only with difficulty, be transformed into graphite, even at temperatures of 3000~ or above. The internal surface of graphitizing carbons can be enhanced by mild oxidation with carbon dioxide or steam, which makes them useful for a wide range of commercial applications. The traditional view, about the structure of such carbonaceous materials, is that their microstructure consists of twisted networks of carbon layers cross-linked by bridging groups, but nature of such cross-links is not explained precisely [2]. Earlier suggestions that sp3 bonding may be present in this kind of carbons are now questioned [3]. The discoveries of the
562 fullerenes [4] and the carbon nanotubes [5] has given a new perspective on sp2-bonded carbons structures. They have prompted several authors to develop models for nongraphitizing carbons containing non-six-membered rings (e.g. pentagons or heptagons) besides the graphitic six-membered rings [6-8]. Generally, the structure of non-crystalline forms of carbon can be described in terms of disordered graphite-like layers with the very weak interlayer correlations or random stacking [1,9]. The purpose of the present work is to trace changes in the structure of two disordered carbons obtained from anthracene (graphitizing) and saccharose (non-graphitizing) depending on temperature of preparation and annealing processes. The effect of heat treatment at temperatures in the range of 1000-2300~ on these carbons, is studied by detailed analysis of the reduced radial distribution function (RRDF) computed from the X-ray scattering data_ The model based on the limited in size graphite-like structure with very weak interlayer correlations was taken to simulate of the RRDF's. 2. E X P E R I M E N T A L
TECHNIQUES
The carbons studied in the present work were prepared by the carbonization of anthracene (Cl4H10) and saccharose (C12H22Oll) in flowing argon. The sample preparation has been described in detail in our previous paper [10]. The high-energy X-ray scattering experiment were performed on the ID15A beam-line at the European Synchrotron Radiation Facility (ESRF, Grenoble). The photon energy E=ll6.2keV 0~=0.1067 A) was used for the measuremems. The powdered samples were placed in capillary tubes of 3mm diameter and mounted on the diffractometer axis. The intensities were recorded using a Ge solid-state detector. The measured intensities cover a Krange up to 24 A -1. 3. T H E O R E T I C A L
OUTLINE
Investigation of the structure of disordered systems can be performed directly by diffraction methods. The reduced radial distribution function d(r) is related to the structure factor S(K) as follows:
d(r)
.
4rCpo[g(r . . ).
1]
2 ~ -SoK[S(K)7c
where po is the number density and
(1)
1]W(K) sin(Kr)dK
W(K) is the window function:
W(K)-sin( xK )/( ~ ) 1(max
(2)
/~max
and g(r) is called the pair correlation function and can be written as: g(r) - 1+ ~
1
eK
~~ K [ S ( K ) -
2rc2rPo ,
llW(K) sin(Kr)dK.
The structure factor is defined as S(K)-I(K)/fl, where intensity andfis the atomic scattering factor.
I(K) is
(3) the corrected and normalised
563
S(K)
I(K) -
-[
1 sin(Kr~ ) -N,:,,:, ( K r ~ )
]
e
cr~
k.;
-
+1
(4)
2
The structure factor S(K) is independent on the scattering power of individual atoms and depends only on the structure of the investigated sample. The experimental RRDF provides information about the probability of finding an atom in a spherical shell at a distance r from an arbitrary atom. Successive peaks correspond to nearest-, second- and next-neighbour atomic distribution. Assuming three-dimensional Gaussian distribution of the interatomic distances with standard deviation a .The d(r) function can be finally expressed as follows [11,12]:
4zcr2g(r)P~ =
r~,%
1
N, e x p
; Jo~-~cr, r,
2cr7
-exp
2cr2,
-
-
-
+
where
P ( r ) = --1 x.~ !rv (IC) c o s ( K r ) d K
(6)
The sum in Eq. (5) was taken over all coordination spheres. A model of the atomic distributions may be constructed, based on Eq. (5), in terms of a series of peaks convoluted with the peak-shape function P(r) to include the effects of data termination at Km~. In order to compensate for the edge effects in a finite-sized model the correction term e(r), which corresponds to probability of finding an atom at a distance r from another atom lying inside the volume of the considered model, is introduced as multiplier of the RRDF computed for the infinite model. According to [ 13], for the model in the form of a rectangular prism with the edged a, b and c, e(r) is given by:
c(r)
=
1
-
lit3
abe
4-x
3re
!- ~
(7)
2
for r 0.01
I00
1000
Pore Diameter, A
Figure 1. Pore size distribution curves of: a) S 1, b) $2, c) $3, d) $4. Hydrocarbon selectivities were calculated on carbon basis. The chain growth probabilities c~ were calculated from the slope of the curve ln(Sa/n), where n is the carbon number and S,, the selectivity to a corresponding Ca hydrocarbon. 3. RESULTS AND DISCUSSION 3.1. Catalyst structure The B.E.T. surface area, total pore volume and average pore size of the mesoporous silicas are reported in Table 1. The pore size distributions (Figure 1) are much narrower in periodic mesoporous than in the commercial silicas. After impregnation with cobalt, specific area and total pore volumes slightly decrease. The shape of the pore size distribution curves remains however unchanged after Co impregnation, even if the average pore size diameters were a little smaller. XRD, XPS and XANES techniques show [14, 15] that Co304 is the only phase present in the oxidized catalysts. In the catalysts with low cobalt loadings (5wt. %), the sizes of Co304 crystallites have been found to increase with increasing silica pore sizes. The extent of cobalt reduction also increases as a function of silica pore sizes [15]. The effect of cobalt content on the structure of cobalt species was studied using cobalt catalysts supported by $2 and $4 silicas. The characterization results are presented in Table 2. Specific area and total pore volumes decrease with increasing Co content in the catalysts. The specific area remains however, relatively high (> 90 mZ/g) even at higher cobalt loadings. Table 2 also shows that the average pore size diameter of mesoporous silicas is not affected by varying cobalt content in the catalysts. The XRD patterns (not
Table 1 Adsorption properties Silica SBET (m2/g) S1 742 $2 887 $3 311 $4 213
of lnesoporous silicas TPV Pore diameter (cm3/g)
(A)
0.59 1.91 1.38 0.84
--20 91 280 330
612 Table 2 Characterization of Co catalysts Co Co Cobalt SBET TPV catalyst content surface (mZ/g) (cm3/g) (%) density, Nco/(100 ~5CoS2 1()COS2 2()COS2 30COS2 50COS2 5COS4 10COS4 20COS4 30COS4 50COS4
5.39 10 19 27.3 31.9 4.75 10.5 17.9 26 43.4
0.82 1.61 3.93 6.88 13.07 2.32 5.52 12.01 19.79 48.13
674 634 493 405 249 206 194 152 134 92
1.11 1.03 0.83 0.63 0.38 0.76 0.75 0.63 0.52 0.35
Pore diameter (/~)
Co304 crystallite diameter from XRD
Extent of reduction measured by TGA (%)
(A) 75 75 75 75 75 -230 --330 -330 -330 -330
121 112 106 125 137 230 231 315 286 250
94.9
96.3 94.4
100.0
represented here) are shnilar for all catalysts. The size of cobalt oxide particles estimated from the width of the XRD patterns does not increase with increasing concentration of cobalt atoms on the support surface. It appears that Co304 crystallite diameter (Table 2) depends only on catalyst pore sizes. These results are consistent with the suggestion [15] that the most of the cobalt particles are located inside the pores of mesoporous silicas. The extent of reduction was close to 100% in both $2 and $4 supported catalysts with the cobalt content varying from 5 to 50 wt %. High extent of reduction of $2 and $4 cobalt supported catalysts is likely to be attributed to the presence of relative large cobalt particles (100-2()0 A). Earlier reports showed that large cobalt oxide particles supported by silica could be easily reduced to metal phases [ 16].
3.2. Catalytic behavior 3.2.1. FT synthesis at atmospheric pressure The FT catalytic results obtained at atmospheric pressure after 24 hours on-stream are presented ha Table 3 and in Figure 2. Water and hydrocarbons have been observed as reaction products. Carbon dioxide has not been detected at the reaction conditions. FT reaction rate decreases slowly with the time of stream. At atmospheric pressure the steady state conditions have been usually attained after 7 h of the reaction. Figure 2 shows that catalytic behavior of Co catalyst supported by silicas is strongly affected by catalyst porous structure. Indeed, with the same cobalt content, the reaction rate increases 1()-20 times as the average pore diameter of the support increases from 20 to 330 A~. The lowest reaction rate has been observed with the narrow pore MCM-41 type support (S 1, pore diameter _--_20 A). One of the reasons for the lower catalytic activity of 5CoS 1 catalysts seems to be related to the lower reducibility of smaller supported cobalt particles [14]. Possible reoxidation of small cobalt particles by water at FI' reaction conditions as was suggested earlier by Iglesia et al. [17] could be another reason for the lower concentration of active sites in 5COS1 catalyst. C5+ selectivity observed with Co/SiO2 catalysts is between 50 and 70% (Figure 2), the chain growth probability varies from 0.7 to
613 80
~
I ,cos. ~-------...~...._~..
9
Cs+
5CoSi3 o s--.
40 > 0
0
Or)
o q)
.~ CH4
-4-O
0.1 1oo
200
300
400
P o r e d i a m e t e r , .&.
Figure 2. Effect of support pore size on FT reaction rate and hydrocarbon selectivities. 0.8. Note that slightly lower C5+ selectivity and chain growth probability c~ are observed (Figure 2) with commercial silicas ($3, $4). Table 3 shows that for the catalysts prepared from periodic mesoporous silica ($2) FT reaction rate normalized by the number of cobalt atoms increases with increasing Co content. Higher catalytic activity at high cobalt loadings could be attributed to higher metal dispersion and higher concentrations of active sites in Co catalysts supported by periodic silicas than in those supported by commercial silicas (Table 2, 3). It appears that porous structure of periodic mesoporous silicas prevents small cobalt particles from sintering. Thus, high cobalt dispersion can be maintained at higher cobalt surface density (>10 NcJ(100 A2). On the contrary, we observe a decrease in FT reaction rate at higher cobalt content tbr the catalysts prepared from commercial silica support ($4) with wide pore size distribution (Figure 3). No significant changes however are observed for all catalysts in CH4 and C5+ selectivity. The chain growth probability remains almost the same at different Co loadmgs (Table 3). Table 3 Catalytic results (H2/CO=2, P=l bar, T=463 K, 24 h on-stream, conversion
0
0 0
0.2
0.4
0.6 P/Po
0.8
1
1E-05 0.0001 0.001
0.01
0.1
1
P/Po
Figure 1. N2 adsorption isotherms at 77K of the A1-Wy sample after thermal treatments, between 400 and 700~ Relative pressure axis" left, linear and right, logarithmic.
Figures 1 and 2 show the nitrogen isotherms at 77 K for A1-Wy and A1La-Wy-50 respectively, after successive thermal treatments at temperatures between 400 and 700~ with a linear relative pressure axis (left) and with a logarithmic relative pressure axis (right). The adsorption isotherms are type I in the Brunauer, Deming, Deming and Teller (BDDT) classification [12] at low relative pressures, which indicate the presence of micropores, generated in the pillaring process to intercalate inorganic polyoxycations between the clay layers. All samples show a hysteresis loop type H4 in the IUPAC classification [ 18]. The most important difference between the pillared samples' isotherms is shown in the adsorption branch at low relative pressures. In the A1La-Wy-50 sample (Figure 2), two levels of nitrogen adsorption are observed, a new increase in the volume of adsorbed nitrogen taking place at approximately p/p0 =0.1. This new step in the level of nitrogen adsorption is seen more clearly when the isotherms of nitrogen adsorption are represented in semilogarithmic scale, right of figure 2. The presence of these two different steps at the low relative pressure of nitrogen adsorption could be due to the different sizes of polyoxycations inserted between the clay sheets. Moreover, this is reflected in the greater basal spacing, thus generating a greater
620
140
140
120
120-
~100
100-
Or) ETJ
0
-
0
-
0
-
--.- ~La-Wy-50 ~ - AILa-Wy-50-500 --~ AILa-Wy-50-6(X) --,- ~La-Wy-50-700
"t3
-a60 t,_
0
"o
m40
E
V
20
0
>
0
i
0
i
i
i
0.2
0.4
0.6
i
1
i
i
0.8 P/Po 1 0.00001 0.0001 0.001
i
0.01
i
I
0.1 P/Po 1
Figure 2. N2 adsorption isotherms at 77 K of the A1La-Wy-50 sample after thermal treatments, between 400 and 700~ Relative pressure axis: left, linear and right, logarithmic. space interlayer and, consequently, pores of larger diameter. This adsorption took place in micropores at the limit between microporous and mesoporous size. However, in the A1-Wy sample we observed that there is a single level of adsorption at low pressures, in figure 1 (right) the step at approximately p/p0 =0.1 does not appear, which indicates a unique micropore size. Figure 2 shows that the step at p/p0 =0.1, present in the samples pillared with A1/La, remained after treatments up to 700~ Although as temperature is increased, a decrease in intensity of the step is observed with inferior nitrogen adsorptions when the samples are treated at higher temperatures. This relationship between the level of nitrogen adsorption and the increase in thermal treatment, is also noted in the A1-Wy sample (figure 1). The adsorption branches of the isotherms (figures 1 and 2), at high relative pressures P/P~ remain parallel to each other after the consecutive thermal treatments, which indicates that the mesoporosity of the materials is not modified, affecting the decrease in the adsorption of nitrogen at low relative pressure values and therefore in the microporous zone. In addition, the adsorption branches of the Al-pillared material (A1-Wy), are seen to display lower values which decrease more quickly with the thermal treatment. Table 1 shows the values for specific surface area, SBET and volume of N2 adsorbed at P/P~ Vad, of the pillared samples and the raw material. All the pillared samples show a more developed porosity than the raw material (montmorillonite). The specific surface area and pore volume increase in all the pillared samples, but the increase is greater in the A1/Lapillared samples than in the Al-pillared sample. The increase in specific surface area was consistent with the expansion of the structure observed by XRD. The A1-Wy sample had a SBETof 195 m2/g while the A1La-Wy-100 had a value of 349 m2/g.
621 Table 1 Structural and textural parameters of the samples. Samples
d (/~)
A1La-Wy-25 A1La-Wy-50 A1La-Wy-75 A1La-Wy- 100 A1-Wy Wy
SBET(m2/g)
19.9 21.2 23 24 18.8 13.9
Vad (cm3/g)
299 321 317 349 195 33
Vmp (cm3/g)
0.181 0.190 0.192 0.210 0.133 0.042
0.109 0.115 0.120 0.131 0.061 0
Figure 3 shows the t-plot for two of the pillared samples. From this plot, the micropore volume, Vmp, was calculated (Table 1). The micropore volume is seen to increase in the pillared samples, and again the increase is greater in the A1/La-pillared samples. This increase in micropore volume, which is greater than that in specific surface area shows a two-fold increase in the A1La-Wy-100 sample over the A1-Wy sample (0.131 and 0.061 cm3/g respectively). The former sample presents a second step in the adsorbed volume at greater thickness, t, than in the A1-Wy sample, which presents only one step. This second step is related to relative pressure of adsorption around p/p0 = 0.1, which corresponds to the abovementioned increase in the adsorption isotherm.
80
120
Al-Wy ---4-- Al-Wy-500
70
100
a. 60 1-O9 o)
80
50
60
o 40 uf -o 30
o 20
--e-- AILa-Wy-50 --e-- AILa-Wy-50-500
20
10
--~,-.- AILa-Wy-50-600 --.-I- AILa-Wy-50-700
0
0
5
10
Thickness-Harkins-Jura (A)
15
--~'
0
i
5
10
15
Thickness-Harkins&Jura (A)
Figure 3. t-plot of pillared samples: A1-Wy and A1La-Wy-50 after thermal treatments between 400 and 700~
Table 2 shows the values of the specific surface area, SBETof the samples after thermal treatments. The A1/La pillared samples can be seen to show much higher percentages of conservation of the surface area than the A1-Wy sample. Thus the A1La-Wy-100 sample
622 Table 2. Thermal evolution of the specific surface area (mVg) of the pillared samples. Sample A1-Wy A1La-Wy-25 A1La-Wy-50 A1La-Wy-75 A1La-Wy-100
400 (~ 195 299 321 317 349
500 (~ 185 248 230 194 296
600 (~ 82 142 228 153 237
700 (~ 41 110 138 103 181
modifies its specific surface area of 349 m2/g at 400~ maintaining 181 m:/g at 700~ whereas sample A1-Wy, in the same range of temperatures, decreases from 195 m2/g to 4 lm2/g, indicating a total collapse of the pillars, with microporosity falling sharply to values similar to those of the initial sample. This can be attributed to the presence of lanthanum in the pillars which gives an increase in the basal spacing of these samples and seems to display greater thermal stability than in the sample that only incorporated A1 in this process.
Table 3. Thermal evolution of the micropore volume Sample A1-Wy A1La-Wy-25 A1La-Wy-50 A1La-Wy-75 A1La-Wy-100
400 (~ 0.061 0.109 0.115 0.120 0.131
500 (~ 0.056 0.088 0.080 0.071 0.109
(cm3/g)of the pillared samples. 600 (~ 0.024 0.050 0.079 0.050 0.080
700 (~ 0.005 0.034 0.046 0.031 0.057
Table 3 presents the micropore volume of the samples, calculated by the t-method, after thermal treatment at 400, 500, 600 and 700~ A high micropore volume is generated with the pillared process in all the pillared samples, because the raw material has no microporosity. Again, this increase is greater in the A1/La- pillared samples than in the A1-Wy sample. Table 3 shows that the volume of micropores remains high in the A1/La-pillared samples after the successive thermal treatments. On the other hand, the thermal stability of the micropore volume developed after the pillaring process is lower in the A1-Wy sample, being reduced practically to zero after thermal treatment at 700~ whereas in the A1/La-pillared samples it is 0.031-0.057 cm3/g at 700~ In order to study the evolution of the microporosity generated in these materials with thermal treatments, the micropore size distribution and cumulative pore volume of the pillared samples have been analyzed. The DFT method has been applied accordingly assuming the slit-like pores model, taking into account the laminar structure of the samples. Figures 4 and 5 show the micropore distribution (fight) and cumulative pore volume (left) for the A1-Wy and A1La-Wy-50 samples after thermal treatments. The A1/La-pillared sample presented micropores of two sizes whereas, the A1-Wy sample presented micropores of only one size, which corresponds to the smallest diameter pore of the A1/La-Wy-50 sample. Moreover, these figures also reveal that up to 700~ the cumulative pore volume in the A1La-Wy-50 sample for the smaller diameter type of pore is not greatly altered, whereas in the A1-Wy sample the decrease is easily observable in this type of pores.
623 0.04
0.14 "0
--~l-VVy
ro
"~ 0.0a5
--~/~i-Wy-500 --*-~WtO:t) /~t-Wy-700
0.12 0.1
0.03 0
> 0.025
E
t,...
_q 0.08
g_ 0.02
O
> e 0.06
o.o15
r
O
a. 0.04
0.01
t,,..
> . - -
0.005
~ 0.02
=_
E O
0 1(10
10
10
100
Fbre ~dth ( ~ o r m )
F ~ Wdth ( , m : ~ m s )
Figure 4. Distribution (right) and cumulative pore volume (left) by the DFT method of the A1-Wy sample after thermal treatments between 400 and 700~
0.0:35 0.14
--~ ~La-W~~" --~ta-V~ --~La-V~
O}
"b 0.12 tO
9
0.1
~>
0.08
~O 13.
0.06
E
0.03 0.025
v
O
> 0.02
--.-/~iLa-V~
0
o_ 0.015 t,-
E
e> 0.04 ~
0.01
L
oe- 0.005
-5 0.02 E
O
AA AAAA--~"
0 1
10 PoreV~dth( ~ )
100
1
10
Pore Wdth ( ~ )
Figure 5. Distribution (right) and cumulative pore volume (left) by the DFT method of the pillared sample A1La-Wy-50 after thermal treatments between 400 and 700~
100
624 4. CONCLUSIONS The montmorillonite pillared with aluminum and lanthanum has inorganic polyoxycations incorporated between the clay sheets, which modifies the textural characteristics of the raw material. An increase is observed both in the specific surface area and also in the porous structure, particularly the micropore volume, with generation of pores at the limit between microporosity and mesoporosity. The A1/La-pillared samples have larger pores than those generated in the A1-Wy material, whose pores clearly belong to the micropore region. In addition to the fact that the textural parameters show higher values, they are thermally more stable, maintaining high values of specific surface area and micropore volume up to 700~ This greater thermal stability in the A1/La-pillared samples, which show two types of pores, could be due to the fact that the larger sized pores prevent the collapse of the smaller-sized pores, or it may be that these types of pores generate smaller-sized pores. ACKNOWLEDGMENT: Our acknowledgment to CICYT for financial support of this work under Project MAT 99/1093-CO2-O2. REFERENCES
[1] F. Figueras, Catal. Rev.- Sci. Eng. 30 (1988) 457. [2] J. T. Kloprogge, J. Porous Mater. 5 (1998) 5. [3] A. Gil, L.M. Gandia, M.A. Vicente, Catal. Rev.- Sci. Eng. 42 (2000) 145. [4] D. Tichit, F. Fajula, F. Figueras, B. Ducouraut, G. Mascherpa, D. Gueguen, J. Bousquet, Clays Clay Miner. 36 (1988) 369. [5] M.A. Martin-Luengo, H. Martins-Carvalho, J. Ladriere, P. Grange, Clay Miner. 24 (1989) 495. [6] F. Figueras, A. Mastrod-Bashi, G. Fetter, A. Therier, J.V. Zanchettr, J. Catal. 34 (1986) 658. [7] B.M. Choudary, V.I.K. Valli, J. Chem. Soc., Chem. Commun. (1990) 1115. [8] X. Tang, W. Q. Shu, Y.F. Shen, S.L. Suib, Chem. Mater. 7 (1995) 102. [9] M.J. Hernando, C. Pesquera, C. Blanco, I. Benito, F. Gonzfilez, Chem. Mater. 8 (1995) 76. [10] J. Sterte, Clays Clay Miner. 39 (1991) 167. [11] J.R. McCauley, U.S. Patent No. 4,818,737 (1988). [12] S.J.Gregg, K.S.W. Sing, Adsorption Surface Area and Porosity, Academic Press, London, 1982. [ 13] W. D. Harkins, G.J. Jura, J. Chem. Phys. 11 (1943) 431. [ 14] J. P. Olivier, J. Porous Mater. 2 (1995) 9. [ 15] S. M. Bradley, R.A. Kydd, J. Yamdagni, J. Chem. Soc., Dalton Trans. 7 (1990) 102. [ 16] C. Pesquera, F. Gonz~lez, I. Benito, S. Mendioroz, J.A. Pajares, Appl. Catal. 8 (1991) 587. [17] F. Gonz~lez, C. Pesquera, C. Blanco, I. Benito, S. Mendioroz, Inorg. Chem. 31 (1992) 727. [18] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
625
Influence of pH in mesoporous silica aluminas (MSA) synthesis C. Rizzo*, A. Carati, C. Barabino, C. Perego and G. Bellussi EniTecnologie, Via Maritano 26, 1-20097 San Donato Milanese, Italy Fax 0039-02-520-56364:
[email protected] The effect of the pH ranging between 11.9-14.0 in the sol-gel synthesis of silicaaluminas MSA and ERS-8 has been studied. A progressive variation from white opalescent to transparent gels was observed. After drying and calcination, the gel textural properties were compared by N2 adsorption/desorption. The pH increase gives rise to evolution from irreversible Type IV to reversible Type I isotherms and a progressive decrease of medium pore size is observed.
1. INTRODUCTION The overall performance of a catalyst is known to depend not only on the inherent catalytic activity of the active phase but also on the textural properties of the solid. The ability to control the specific surface area and the pore size distribution during the synthesis of amorphous silica-aluminas has been described for both surfactant micelle templated syntheses (M41-S (1), FSM-16 (2), HMS (3), SBA (4), MSU (5), KIT-1 (6)) and cluster templated sol-gel syntheses (MSA (7), ERS-8 (8)). All these families of silica-aluminas are characterized by high surface area (larger than 500 m2/g) and narrow pore size distribution in the micro and/or meso region. The synthesis route affects not only the formation mechanism and the mean pore size but also the long-range order of pore system, that can be related to the presence of reflection at low angles in the X-ray diffraction pattern (XRD) from powders. For each family of silica-aluminas several synthesis parameters can be identified and applied to control the textural properties of final products. For example the role of the type and the amount of gelling agent (8, 9), the solvent role (10), the silica/alumina molar ratio (9) have been discussed for MSA and ERS-8 formation. Another important parameter that has to be deeply considered is the pH. It is well known the role of pH on silica chemistry: it affects dissolution and polymerization rate, gel or precipitate formation and the textural properties of the final silica (11). Also for surfactant micelle or cluster templated syntheses of silica-aluminas, the effect of pH on porosity remains relevant and it is strongly influenced by the kind of material and by the synthesis route selected for its preparation. For example, in order to synthesize a MCM-41-type material at room temperature a low pH (about 8.5) is essential, while a micropore material is obtained at pH=ll (12). By contrast when the MCM-41 synthesis is performed by hydrothermal treatment, the pH
626 adjustments at value about 11 permit to increase the structural order and the textural uniformity (13). Besides, in syntheses of MCM-41 by the system cetyltrimethylammoniumtetraethylortosilicate, the pH affects the particle size and the wall thickness (14). SBA syntheses performed at different pH give rise to only poorly-ordered mesoporous solids out of the pH range 10-12, while the best ordered samples are obtained at pH between 11 and 12 (15). In this work different pH values ranging from 11.9 and 14.0 has been studied in the solgel synthesis of MSA and ERS-8. The pH values have been modified using different amount of tetrapropyl ammonium hydroxide (TPAOH). In the synthesis of MSA and ERS-8, TPAOH plays several roles: it is the catalyst in the sol-gel reactions via alkoxides (e.g. hydrolysis, alcoholysis and polymerization/ depolymerization) and the gelling agent too. Besides, it can act as a templating agent, organizing SiO2 and A102"units in polymeric structure and addressing the pore formation through the TPA+ clusters. In particular in this work the contribution of TPAOH to the alkalinity of the reagent mixture during the preparation of silica-aluminas is discussed. Several molar ratios TPAOH/SiO2 (0.050, 0.075, 0.100, 0.125, 0.150, 0.175 and 0.200) have been considered and their effect on porosity and surface area has been examined.
2. EXPERIMENTAL Samples were prepared via sol-gel in alkali-free medium using Si(OC2H5)4 (Dynasil-A, Nobel), Al(iso-OC3H7)3 (Fluka), tetrapropylammonium hydroxide (TPAOH, Sachem), and ethanol (Fluka). All syntheses were performed at the same molar ratio: SIO2/A1203=300, H20/SIO2=8, CzHsOH]SiO2=8. The TPAOH/SiO2 molar ratio was changed between 0.05 and 2 and consequently the pH between 11.9-14. A typical synthesis preparation is following described. AI(i-OC3HT)3 was dissolved at 60 ~ in TPAOH (12 wt % in aqueous solution). The solution was cooled to room temperature, then Si(OC2H5)4 in CzHsOH was added. As the monophasic clear sol was obtained, the pH value was measured with a Ross Sure-Flow Combination, Orion model 8172, after pH meter calibration with two buffers (pH 7 and
lO). Then the sol was transformed in a homogeneous gel (from opalescent white to transparent) without separation of phases. After 15 hour aging at room temperature, the gels were dried at 100 ~ and calcined 8 hour in air at 550 ~ All the syntheses performed are reported in Table 1. The textural properties of all calcined samples were determined by nitrogen isotherms at liquid N2 temperature, using a Micromeritics ASAP 2010 apparatus (static volumetric technique). Before determination of adsorption-desorption isotherms the samples (-~ 0.2 g) were outgassed for 16 h at 350 ~ under vacuum. The specific surface area (SBET) was evaluated by full 3-parameters BET equation and by 2-parameters linear BET plot in the range p/pO 0.01-0.2. The total pore volume (VT) was evaluated by Gurvitsch rule. BJH method was applied on the desorption isotherm branch only for mesoporous materials, in order to evaluate the mesopore width. Mean pore size (dDFT) was calculated using DFT method (Micromeritics' DFT Plus | software) for all materials with the cylindrical pores in oxide surface model.
627
Table 1 Synthesized materials and their textural properties. SAMPLE pH Gel Molar ratio Isotherm SBET(2p) SBET(3p) TPAOH/SiO2 Type (m2/g) (m2/g) ET- 01 11.9 W 0.050 IV 790 1076 ET- 02 12.4 W 0.075 IV 770 823 ET - 03 12.6 O 0.100 IV + (I) 690 800 ET- 04 12.8 O 0.125 IV + (I) 710 852 ET- 05 13.0 T 0.150 I + (IV) 710 1046 ET- 06 13.6 T 0.175 I 690 1041 ET- 07 14.0 T 0.200 I 740 1108 Synthesis molar ratio: SIO2/A1203=300, H20/SIO2=8, CEHsOH/SiO2=8 W = white opalescent; T = transparent; 0 = opalescent
.
.
dajn (.A) 55 37 36 34 .
.
dDFT VT (A) 51.6 36.3 28.8 22.2 15.6 11.6 13.8 .
.
(ml/~) 0.901 0.684 0.505 0.442 0.374 0.310 0.353 .
3. RESULTS The gelation time is related to the TPAOH/SiO2 molar ratio: at low values a fast gelation is observed and opalescent white gels are obtained, by contrast a slow gelation is observed for the samples synthesized at high values and transparent gels are obtained. Intermediate behaviors are observed between the conditions above described. In Tab. 1 the textural properties of the calcined samples are reported. In Fig. 1 and Fig. 2 isotherms and cumulative pore volume for all samples are shown. The samples derived from white gels show an irreversible Type IV isotherms characteristic of MSA-type materials, while, those derived from transparent gels are characterized by a reversible Type I isotherms typical of microporous ERS-8-type materials. Irreversible Type I + IV isotherms are observed for the samples with intermediate values of molar ratios. Particularly as the TPAOH/SiO2 molar ratio increases a progressive shift of the hysteresis loop toward low relative pressure, till the total disappearance, is observed (Fig.l). All materials are characterized by a very high specific surface area. As general trend the 2-parameters BET specific surface area results underestimated with respect to that evaluated by the full 3-parameters BET. With the exception of the sample ET-01, the observed differences increase with the contribution of micropores. For mesoporous (Type IV isotherms) or essentially mesoporous (Type IV +(I) isotherms) samples, the pore diameter has been evaluated with both BJH and DFT methods obtaining a consistent values. The DFT method gives rise to lower values with respect to BJH, the difference observed for each sample gives a qualitative information about the contribute of micropore in the sample and is in agreement with the isotherm above described. Only DFT method was applied for samples with a large contribution of micropores. Indeed, DFT, based on a molecular statistical approach, is applied over the all range of the relative pressure.
628
TPAOH/SiO2 = 0.050
T P A O H / S i O / = 0.075
S TPAOH/SiO2 = 0.100
m
O
TPAOH/SiO2 = 0.125
TPAOH/SiO2 = 0.150
r~ TPAOH/SiO2 = 0.175
TPAOH/SiO2 = 0.200
r
0
0,25
~5
~75
Relative Pressure (p/pO)
Fig. 1" N2 adsorption/desorption isotherms.
629
T P A O H / S i O 2 = 0.050
T P A O H / S i O 2 = 0.075
E E
f
m
j
o
T P A O H / S i O 2 = 0.1 O0 .... ,,,,,,,., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I,i
T P A O H / S i O 2 = 0.125
o
m
E j
"-
1
_.,=~.
T P A O H / S i O 2 = 0.175
I
I
10
100
Pore Width
"
I
1000
10000
(Angstroms)
Fig. 2" DFT cumulative pore volume
The mesopore diameter (ET-01 - ET-04), evaluated with BJH method, increases when the molar ratio TPAOH/SiO2 decreases. The sample prepared at TPAOH/SiO2 =0.050 shows the largest mesopore diameter.
630 In the range of conditions studied (ET-01 - ET-07), the mean pore size, evaluated by DFT, decreases when TPAOH content increases. Accordingly both the increase of micropore volume and the shift of hysteresis loop are observed. The total pore volume decreases as the amount of TPAOH and the pH increase.
4. DISCUSSION The variation of TPAOH amount during the synthesis of silica-aluminas has two macroscopic effects: the change of the pH and of the relative amount of TPAOH to silicoaluminate oligomers. The textural properties of the final silica-aluminas may result from the different sequence of hydrolysis and condensation reactions (and the reverse reactions: esterification and alcoholic or hydrolytic depolymerization) of the Si and A1 alkoxides. Indeed an increase in TPAOH content and therefore of the pH corresponds to an increase of O H availability that can favor the hydrolysis and depolymerization reactions, giving rise to different gelation rate and to different network formation. As well-known drift fast hydrolysis and slow condensation favor formation of linear oligomers; on the other hand, slow hydrolysis and fast condensation result in larger, bulkier and more ramified oligomers (16). Also the appearance of gels is in agreement with the type of network produced: the presence of highly cross-linked gel networks yields white opalescent gels (17, 18), as those obtained at pH < 12.5 by a fast gelation. Linear chain of polysilicates yields transparent gels (17, 18), as those obtained at pH > 13 by a slow gelation. The kind of network affects the textural properties of final products: highly branched network favor the formation of open structures (i.e. mesoporous materials), while weakly branched networks tend to collapse forming more densely packed structures (i.e. microporous materials). Besides, according to the mechanism of formation of MSA-type and ERS-8-type silicaaluminas (Fig. 3) proposed in (8), the relative amount of TPAOH with respect to silicoaluminates is very important. Indeed mesoporous MSA is obtained when the silicoaluminate oligomers are enough to completely surround the TPA + solvated clusters. By contrast microporous ERS-8 is obtained when the silico-aluminate oligomers grow as sheets because they can not surround the solvated clusters. According with this mechanism ERS-8 is obtained for molar ratio TPAOH/SiO2 > 0.150. At high TPAOH content both the high availability of O H and the formation mechanism do not allow formation of large, ramified silico-aluminate oligomers favoring the growth in sheets, i.e. ERS-8 type silica-aluminas. By contrast at low TPAOH content the lower availability of OH- favors large and ramified oligomers, surrounding the TPA + solvated clusters. The dimension of TPA + clusters seems independent on the TPAOH amount, indeed the pore diameter decreases when the amount of TPAOH increases.
631
Fig. 3: Formation mechanism of ERS-8 and MSA as proposed in (8).
5. CONCLUSIONS In the sol-gel synthesis of silica-aluminas the TPAOH amount affects the textural properties of the final material modifying the pH and the relative amount of TPAOH with respect to silico-aluminate oligomers. At low TPAOH content (pH = 11.9 - 12.4) a slow gelation with white opalescent gel formation is observed. This behavior is typical of highly branched networks that after drying and calcination give rise to mesoporous materials. At high TPAOH content (pH = 13.6-14.0) a fast gelation with transparent gel formation is observed. This behavior is typical of linear networks that after drying and calcination give rise to microporous materials.
References
1. J. S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonoswicz, C.T. Kresge, K.D. Schmitt, C.TW. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 2. S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc., Chem. Commun. (1993) 680. 3. A. Tuel, S. Gontier; Chem. Mater., 8 (1996) 114. 4. Q. Huo, D.I. Margolese, G.D. Stucky; Chem. Mater. 8 (1996) 1147. 5. S.A. Bagshaw, E. Prouzet, T.J. Pinnavaia; Science 269 (1995) 1242. 6. R. Ryoo, J.M. Kim, C.H. Shin, J.Y. Lee; Studies in Surface Science and Catalysis, 105 (1997) 45. 7. G. Bellussi, C. Perego, A. Carati, S. Peratello, E. Previde Massara, G. Perego; Studies in Surface Science and Catalysis, 84 (1994) 85. 8. G. Perego, R. Millini, C. Perego, A. Carati, G. Pazzuconi, G. Bellussi; Studies in Surface Science and Catalysis, 105 (1997) 205 9. C. Rizzo, A. Carati, C. Barabino, C. Perego, G. Bellussi, Studies in Surface Science and Catalysis, 140 (2001) 401.
632 10. A. Carati, C. Rizzo, M. Tagliabue, C. Perego, Studies in Surface Science and Catalysis, 130 B (2000) 1085. 11. R.K. Iler, The chemistry of silica, John Wiley & Son Eds., (1979), chapter 3. 12. A.C. Voegtlin, A. Matijasic. J. Patarin, C. Sauerland, Y. Grillet, L. Huve, Microporous Materials 10 (1997) 137. 13. R. Ryoo, J.M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711. 14. F.Di Renzo, F. Testa, J.D. Chen, H. Cambon, A. Galaeneau, D. Plee, F. Fajula, Microporous and Mesoporous Materials, 28 (1999) 437. 15. H.M.A. Hunter, P.A. Wright, Microporous and Mesoporous Materials, 43 (2001) 361. 16. L.L. Hench, J.K. West, Chem. Rev., 90 (1990) 33. 17. S.D. Jones, T.N. Pritchard, D.F. Lander, Microporous Materials, 3 (1995) 419. 18. B. Handy, K.L. Walther, A.Wokaun, A.Baiker, Studies in Surface Science and Catalysis, 63 (1991) 239.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
633
The adsorption of 1-hexene and 3,3-dimethyl-l-butene on R u - M C M 41 Udayshanker Singh a, Ruth T. Williams a, lan D. Salter b a Department of Chemistry, The Open University, UK. b School of Chemistry, University of Exeter, UK.
The effect of heat-treatment under reduced pressure (> 10-5 torr) at 623 K on a sample of MCM 41 prepared using a ruthenium surfactant as a template (Ru-MCM 41) has been studied by sorption of nitrogen, 1-hexene, and 3,3-dimethyl-1-butene. Nitrogen sorption (77 K) shows a slight decrease in the BET surface area (330 to 282 m2gl ) and pore volume (0.19 to 0.16 cm3gl ) as a result of the heat-treatment. However, adsorption of 1-hexene at 303 K shows a doubling of the monolayer uptake of 1-hexene on heat-treatment, which corresponds to an increase in both (1-hexene derived) BET surface area (144 to 295 meg-1) and pore volume (0.12 to 0.16 cm3g-~). For the non heat-treated sample, less than-~50 % of the available surface was accessible to 1-hexene, suggesting the 1-hexene is sterically hindered compared with nitrogen. However heat-treatment of this sample resulted in improved accessibility of the surface and the pore system to the 1-hexene molecules. This increase in 1-hexene uptake without a corresponding change in the nitrogen BET surface area may explain the observed increase in catalytic activity for 1-hexene hydrogenation following heat-treatment of the RuMCM 41. The adsorption at 303 K of 3,3-dimethyl-l-butene, which is more bulky isomer of 1-hexene, yielded a (3,3-dimethyl-l-butene derived) BET surface area of 161 m2g-1 on the heat-treated sample, i.e. about half that of the 1-hexene derived value (295 m2g-1). The pore volume determined from this sorptive was also lower (0.13 cm3g-~) than that for 1-hexene (0.16 cm3g-~) on the heat-treated sample. This suggests that adsorption of 3,3-dimethyl-1butene, which approximates to a spherical shape, is more sterically hindered compared with that of the long chain 1-hexene.
1 INTRODUCTION The mesoporous solids developed by Mobil group 1 in 1991 were found to be catalytically inactive and have attracted a considerable interest from researchers throughout the world to introduce catalytically active sites within these materials. For example, doping of aluminium into the silica2 generates Bronsted acid sites and the resulting materials can be used as solid acids in acid-catalysed reactions. It is also possible to deposit metal particles within the pores and to use these materials as redox catalysts in many chemical reactions. 3 Another avenue for catalytic functionalisation is to tether metal complexes within pores in order to prepare heterogeneous catalysts. 4 It has been observed in the process of functionalisation that MCM materials can lose mesoporosity, surface area and pore volume as shown by nitrogen
634 sorption5. Most of the functionalised materials are produced using a two-step procedure, which involves preparing the mesoporous solids, and then introducing catalytic sites. However Bruce et al. 6 developed a one-step method for production of a mesoporous solid containing deposited metal particles. This method uses a metal (ruthenium) surfactant, exploiting the dual functionality of the ruthenium complex as both structure directing and catalytic site generation agent in the synthesis of Ru-MCM 41. The catalytic properties of RuMCM 41 for the hydrogenation of 1-hexene to n-hexane were found to improve upon heattreatment. 6 Therefore, we wished to investigate the effect of heat-treatment on the surface characteristics ofRu-MCM 41. In this paper, we report the adsorption of 1-hexene at 303 K by Ru-MCM 41 before and after heat-treatment at 623 K. We also report the adsorption at 303 K of 3,3-dimethyl-1butene, which is an isomer of 1-hexene, after heat-treatment ofRu-MCM 41. 2 EXPERIMENTAL
The sample of Ru-MCM 41 was supplied by Bruce and co-workers. 6 Powder x-ray diffraction data were obtained with a Philips PW1050/25 diffractometer operating in Bragg-Brentano geometry with CrKa radiation, (~ = 2.29 A). Data were collected in the 20range 1~ 20 ~ with a step size of 0.05 ~ and dwell time of 6 s per point. Nitrogen sorption at 77 K was performed using an automated Micromeritics Tristar 3000 Surface Area Analyser. The sample was outgassed by purging using the Micromeritics Flowprep 060 at 423 K under nitrogen flow for 6 h prior to analysis. Adsorption studies of 1-hexene and 3,3-dimethyl-l-butene at (303 K) were performed using a McBain-Bakr gravimetric balance: The samples were outgassed at 423 K under reduced pressure (> 10-5torr) for 1.5 h to remove any physisorbed vapours prior to adsorption. The heat-treatment of the sample was performed at 623 K for 16 h under reduced pressure (> 10.5 torr). Isotherms are presented as plots of amount adsorbed (cm3 g-l) against relative pressure, p/pO. The sorptives were exposed to three '~eeze-pump-thaw' cycles to remove dissolved gases before sorption studies. For each data point the sample was exposed to the sorptive and allowed to reach equilibrium, this being defined as the point at which no further decrease in pressure and no further increase in mass of the sample were observed. Sorptive vapour pressure readings were recorded using a pressure transducer (Digitron Instruments, Model P400). 3 RESULTS AND DISCUSSION Powder x-ray diffraction data of Ru-MCM 41 show a dl00 peak at 3.7 ~ on 20 scale, which confirms the hexagonal phase is formed. The powder x-ray diffraction analysis performed on the sample after heat-treatment continued that the material retains the hexagonal symmetry of the mesophase. Nitrogen, 1-hexene and 3,3-dimethyl-1-butene sorption isotherms for both heat-treated and non-treated samples are shown, e.g., Figs. l, 2 and 3, respectively. For nitrogen sorption both the samples display Type IV isotherms, indicating that these materials are mesoporous. The isotherm for the non-treated sample displays a lack of hysterisis on desorption, which is characteristic of nitrogen sorption on MCM 41 materials. 7 The hysterisis observed in the nitrogen sorption isotherm for heat-treated sample suggests that heating of the sample (623 K
635
0.2 ~-
~ "~
o9 O o e
f
0.15 [ e ~ ~ n
~ ' 0 O oO AA A
,
g~
9
9 A"
c9
dj
dDe~e:e~~
JA A AA AA AA ~A ~
'
o
o 9 o 9 o~
AA~6~,A~/,
A
A
A
0.1
A
adsorption (heat-treated) adsorption (non-treated)
o 0.05
00
,
!
=
I
0.2
i
,
i
i
0.4
,
,
f
0.16
i
,
relative pressure, p/pO
0.I8
L
i
~
Fig. 1 Sorption isotherms of N2 at 77 K on Ru-MCM 41, where filled symbols denote desorption.
0.2 ~
'
I
A A
~ o.~5i
A
A
A
A
0 A
o
0
O
0
A
0
0
0
0
0.1
~
F 0.05
Q A
adsorption on non-treated sample adsorption on heat-treated sample !
0
0
o12
,
i
,
,
,
I
,
0.4 0.6 relaffve pressure, p / p o
,
,0.18,
,
,
i
1
Fig. 2 Adsorption isotherm of 1-hexene at 303 K, before and after heat-treatment of Ru-MCM 41.
636
~ F 9
>o,5 I 0
0.1
i
9
0
(>
0
A
A
0
A
O
A
0
0
0
0
2
E
0
1-hexene adsorption on non-treated sample
3, 3-di-methyl- 1-butene adsorption on
0.05
heat-treated sample
o~ . . . . .
!
o. 0.2
,
1-hexene adsorption on heat-treated sample
i
i
I
,
J
~
I
,
,
0.4 0.6 relative pressure, p / p o
1_
t
9
,
0.8
Fig. 3 Comparison of the adsorption of 1-hexene and 3,3-dimethyl-1-butene at 303 K on Ru-MCM 41.
under the reduced pressure of> 105 torr) has brought about a change in the nature of the surface characteristics. The sorption data are summarised in Table 1. Table 1 Sorption data for nitrogen, 1-hexene and 3,3 dimethyl 1-butene on Ru-MCM 41. Pore volume / cm 3 g-l
Molecular
BET derived surface
area of the
area / m 2 g-1
sorptive /
Non-
Heat-
A2
treated
treated*
nitrogen
16.2
330
282
0.19
0.16
1-hexene
38.2 ~
144
295
0.12
0.16
3,3-dimethyl-
39.0 ~
Sorptives
161
Non-treated
Heattreated*
0.13
1- butene * Sample heated at 623 K under vacuum (> 10-5 torr) for 16 h. t Molecular area (a,,) calculated from liquid density (pL) using the equation am -- ( M / p z Z ) 2Is • 1016, where M is the molar mass (g mol 1) and L is the Avagadro number.
637 A slight decrease in the BET surface area (330 to 282 mZg-l) and pore volume (0.19 to 0.16 cm3g1) as a result of the heat-treatment was observed for nitrogen sorption.
1-Hexene adsorption carried out on the non-treated and the heat-treated sample yielded a Type I isotherm in each case (Fig. 2). A BET specific surface area of 144 m/g l for non-treated sample was obtained, which was found to be almost doubled (295 m2gl) for the heat-treated sample. The heat-treated material showed a high initial uptake of 1-hexene which levels off around a relative pressure of p/pO = 0.25. The final plateau of the 1-hexene isotherms gives a Gurvitsch pore volume of 0.12 cm3g-1 and 0.16 cm3g"l for non-treated and heat-treated samples, respectively. Heat-treatment was shown to result in improved accessibility of surface and pore system to the 1-hexene molecules. This observed increase in 1-hexene uptake without a corresponding change in the nitrogen BET surface area (Table 1) may partly explain the observed increase in catalytic activity for hydrogenation of 1-hexene following heat-treatment of the Ru-MCM 41. 6 The large size of the 1-hexene molecule with respect to the nitrogen molecule may explain the lower uptake of the 1-hexene (Table 1) compared with nitrogen for the sample prior to heat-treatment. The adsorption of 3,3-dimethyl-l-butene at 303 K on the heat-treated sample is shown in Fig. 3. The 3,3-dimethyl-l-butene adsorption yielded a Type I isotherm. The BET surface area value of 161 m2g-1 obtained, from 3,3-dimethyl-l-butene is lower than those derived from nitrogen and 1-hexene (Table 1). The pore volume determined from this sorptive was also lower (0.13 cm3g1) than that for 1-hexene on the heat-treated sample. This suggests that adsorption of 3,3-dimethyl-l-butene, which approximates to a spherical shape, is sterically hindered 8, whereas in 1-hexene, which is a straight chain molecule, is less sterically hindered to interact with the surface. 4 CONCLUSIONS Heat-treatment (under reduced pressure of > 10-5 torr, at 623 K) of Ru-MCM 41 has been shown to alter the surface characteristics of the material with respect to sorption of nitrogen and hexene isomers, although the change was more marked with 1-hexene. Nitrogen sorption yields a Type IV isotherm, before and after heat-treatment, with hysterisis observed in the isotherm of heat-treated sample. A slight decrease in the BET surface area (330 to 282 m2g-l) and pore volume (0.19 to 0.16 cm3g-~) as a result of the heat-treatment was observed for nitrogen sorption. However, adsorption of 1-hexene yields a Type I isotherm and a doubling of the monolayer uptake of 1-hexene on heat-treatment, which corresponds to an increase in both BET surface area (144 to 295 mZg-l) and pore volume (0.12 to 0.16 mZg-l). For this sample, the heat-treatment resulted in improved accessibility of the 1-hexene molecules to the surface and the pores. This increase in the 1-hexene uptake without a corresponding change in nitrogen BET surface area partly explains the observed increase in catalytic activity for 1hexene hydrogenation following heat-treatment of Ru-MCM 41 sample. The adsorption of 1hexene isomer, 3,3-dimethyl-l-butene (303 K), after heat-treatment showed a Type I isotherm and yielded a BET surface area of 161 m2g-l, i.e. about half of the 1-hexene (295 m2gl) derived value. The derived pore volume (0.13 cm3g-1) was also lower that that of 1-hexene (0.16 m2g1) on heat-treated sample. The lower uptake of 'spherical' 3,3-dimethyl- 1-butene, compared with the 'straight chain' 1-hexene suggests that the former isomer is more sterically hindered on the adsorption sites of Ru-MCM 41 sample.
638 5 ACKNOWLEDGEMENTS
We thank M. Danks and Prof. D. W. Bruce (University of Exeter) for the mesoporous ruthenium sample. U. Singh thanks the Open University for financial support, and University of Exeter, School of Chemistry for the use of its facilities. REFERENCES
1. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J.C. Vartuli and J.S. Beck, Nature, 1992, 359, 710. 2. B. Lindlar, A. Kogelbauer and R. Prins, Microporous and Mesoporous Materials, 2000, 38, 167. 3. W. S. Ahn, D. H. Lee, T. J. Kim, J. H. Kim, G. Seo, R. Ryoo, Applied catalysis A: General, 1999, 181, 39. 4. R. J. Clarke and I. J. Shannon, J. Chem. Sot., Chem. Commun., 2001, 1936. 5. C. M. Bambrough, R. C. T. Slade and R. T. Williams, J. Mater. Chem., 1998, 8, 569. 6. H. B. Jervis, M. E. Raimondi, R. Raja, T. Maschmeyer, J.M. Seddon and D. W. Bruce, J. Chem. Sot., Chem. Commun., 1999, 2031. 7. P. J. Branton, P.G. Hall, and K. S. W. Sing, Adsorption, 1994, 1, 77. 8. C. M. Bambrough, R. C. T. Slade and R. T. Williams, Phys. Chem. Chem. Phys., 2000, 2, 3499.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
639
XPS studies of M C M 41 postmodified by a Schiff base copper complex Udayshanker Singh a, Ruth T. Williams l, Ian D. Salterb, Keith R.Hallam r Geoffrey C. AHenC a Department of Chemistry, The Open University, UK. bSchool of Chemistry, University of Exeter, UK. r Interface Analysis Centre, University of Bristol, UK.
Preliminary studies of the distribution of the copper complex in MCM 41 postmodified by a Schiff base (salen) copper complex have been carried out using x-ray photoelectron spectroscopy (XPS) and atomic absorption spectroscopy (AAS). AAS showed 1.3 at% (atomic %) (0.65 mmolg~) of copper loading, which is in close agreement with 1.5 at% obtained from XPS analysis. Nitrogen sorption (77 K) shows a change from a Type IV to a Type I isotherm on postmodification, indicating a change in porosity of the material from mesoporous to microporous. This change in porosity corresponds to a reduction in surface surface area (from 813 m2g-l to 332 m2g-l) and pore volume (from 0.39 cm3g-l to 0.02 cm3gl). The XPS argon ion etching results suggest that about a third of the copper complex is going inside the pores, thereby narrowing some pores and blocking others. The remaining copper complex goes on to the external surface of the material. This hypothesis is consistent with the observed changes detected by nitrogen sorption studies on postmodification of MCM 41.
1 INTRODUCTION The synthesis of mesoporous materials with metal-organofunctional groups has attracted a great deal of interest from many researchers due to their potential applicability as catalysts. MCM 41 materials, 1 comprising hexagonally-packed arrays of one-dimensional cylindrical pores, can provide large surface areas for functionalisation with metals and/or organic species, either by postmodification or direct synthesis. 2' 3 Copper-containing molecular sieve materials are very important catalysts in many liquid-phase oxidation reactions. 4' 5 The analysis of metal content is usually obtained using atomic absorption spectroscopy (AAS) 6, but this provides no information on the distribution of the metal within the material. In this paper, we report on the characterisation of a siliceous MCM 41 material postmodified with a Schiff base copper complex7' 8 by x-ray photoelectron spectroscopy (XPS), AAS and other standard techniques. Quantitative estimations of the copper concentrations and chemical states and its distribution within the material have been made using XPS. The effect of modification by the Schiff base copper complex on the surface characteristics of the MCM 41 was investigated by nitrogen sorption at 77 K.
640 2 EXPERIMENTAL Synthesis Preparation of the parent MCM 41 sample The parent siliceous MCM 41 material was synthesised by hydrolysis of tetraethyl orthosilicate (TEOS) under mild alkali conditions, based on a procedure described by Schmidt and co-workers. 9 Postmodification of the parent MCM 41 with a Schiff base copper complex The chemical postmodification of the parent MCM 41 material was carried out in three steps, as shown in scheme 1. (Et O)3--Si'CH 2-CH 2-CH 2-NH 2 4- HO
-
~H
I ~ reflux in MeOH for3 h
(EtO)3---Si'CH 2"CH 2-OH 2-N
-reflux in toluene
for 6h l----O
C
M ----0\
41
/
1
2
C2H50..
t-
Cu %
\
i - ~ O -Si 43 H2"C H2"C H2-N ---~)/
3
MCM 41
C2H,O
/
~ .J
# jOj c QO C2H50," ~ -~-"~, C2HsO~~'Si - C H2"C H2-C H2"~ ___...~ I.I C 2H 5 o -
Scheme 1 Postmodification ofMCM 41 by a Schiffbase copper complex. In step 1, the Schiff base ligand was prepared by reacting aminopropyl triethoxysilane with salicylaldehyde. 6 Then, in step 2, the Schiff base copper complex was prepared by refluxing for 3 h the Schiffbase ligand (0.75 g, 2.30 mmol) with copper acetate (0.23 g, 1.26 mmol) in an approximately 2:1 molar ratio, using toluene (35 cm3) as the solvent. Finally, in step 3, template-free MCM 41 (1.00 g ) (preheated at 100 ~ for 2 h) was added and the refluxing continued for a further 16 h. The resulting yellowish green solid was filtered and washed, first with ethanol (2 x 25 cm 3) and then with acetone (10 cm3), and then dried, initially under reduced pressure and finally in an oven at 50 ~ for 12 h to give the final product (1.35 g). Characterisation The postmodified MCM material synthesised was characterised by XRPD (powder x-ray diffraction), nitrogen sorption, AAS, CHN micro analysis, XPS, and IR. 9, l0 Powder x-ray diffraction data were obtained with a Philips PW1050/25 diffractometer operating in Bragg-Brentano geometry with CrKt~ radiation, 0~ = 2.29 A). Data were collected in the 20 range 1~ l0 o with a step size of 0.05 ~ and dwell time of 5 s per point. Nitrogen sorption at 77 K was performed using an automated Micromeritics Tristar 3000 Surface Area Analyser. The samples were degassed using the Micromeritics Flowprep 060 under nitrogen flow prior to analysis. The parent material was degassed at 150 ~ for 6 hrs and the postmodified material was degassed at 100 ~ for 6 hrs (the parent MCM 41 samples, which were degassed in temp range 1 0 0 - 200 ~ did not show any significant differences in the sorption results). The lower temperature was used to avoid any possible decomposition of the postmodified material.
641 The quantitative estimation of copper in the postmodified material was carried out by AAS following complete dissolution using an HF/HNO3 mixture. II The amount of copper was determined by a calibration curve method using a copper lamp of wavelength 324.8 nm and air/acetylene fuel (ratio of 3" 1). X-ray photoelectron spectroscopy is a surface analytical (1 - 5 nm depth) technique. For the analyses presented here, it should be remembered that on the scale of the XPS analysis area (4 mm x 3 mm), the individual sample grains would be randomly oriented. The analyses will be averages of measurements from all possible directions of the grains, including those presenting the outer faces of the hexagonal structure to the analyser and others which are end-on, thus contributing information on elements and species within the open ends of the pore structure. Samples were mounted on to a stainless steel holder using double-sided adhesive carbon tabs. Wide-scan survey spectra were acquired to identify the elements present, followed by higher energy resolution scans for the elements identified. The shallow take-off angle analysis was achieved by increasing the angle between the sample and the analyser input to enhance the surface selectivity of the XPS analysis. The argon ion etching for a total of 270 s (at a rate of-~l nm min-I, equivalent to - 4.5 nm depth) was performed followed by scanning the spectra to determine the distribution of complex within the pores. The adventitious hydrocarbon peak, expected at 284.8 eV binding energy 12, was used to correct for sample charging. Quantification and peak-fitting of the data were performed using linear backgrounds beneath the peaks of interest and relative sensitivity factors. 3 RESULTS AND DISCUSSION The hexagonal structures of the parent MCM 41 and the postmodified material were confirmed by XRPD. Both materials showed sharp and intense dloo peaks at lower 20 angles and smaller ones at higher 20 angles. The IR band at 1624 cm-1 for the imine group confirms the immobilisation of the Schiffbase copper complex on the MCM 41 material. 6 Nitrogen sorption isotherms at 77 K for the siliceous MCM 41 and the postmodified material are shown in Fig. 1. The parent MCM 41 material showed a Type IV isotherm, typical of mesoporous material as reported in previous studies. 9' 13 The step-wise rise of the adsorbed amount of nitrogen, caused by capillary condensation of nitrogen in the mesopores, was clearly observed for the parent MCM 41 material in the range 0.3 < p/pO < 0.4. The postmodified material, in contrast, showed a Type I isotherm with significant decreases in pore volume and surface area, as summarised in Table 1. These changes in pore parameter were expected because of the replacement of surface silanol groups by the bulky Schiff base copper complex. 4, 5, 7, ~3 The change in the isotherm from Type IV for the parent MCM 41 to Type I for the postmodified sample suggests that the copper complex is filling and/or blocking the pores, resulting in the decrease in surlhce area and negligible pore volume for the postmodified material. 14 On postmodification the pore width (DBm) was found to decrease from 2.4 nm to 1.6 nm. Assuming a cylindrical pore shape the predicted decrease in pore volume would be expected to correspond to a decrease in pore volume from 0.39 cm3g-1 to 0.17 cm3g-~ on postmodification. However, the measured pore volume for postmodified sample is only 0.02 cm3g-~, supporting the hypothesis that considerable pore-blocking has occurred upon postmodification. The postmodified material analysed by AAS shows 0.65 mmol g-1 of copper loading. The analysis of the filtrate from the preparation of the postmodified MCM 41 material and the analysis of the washings showed a combined total of 0.09 mmol of copper. This suggests
642 ,-..300 ~ ~ I'-"
'
'
'
~
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I
ct)
' ~
"
'
'
9 ~n~::~ ~
~
o
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~
'
'
'
o o o ~ o o o
~
'
'
'-]
co
"7
"" 250 i ~ - / ~ 1 7 6 1 7 6 ~
r
Parent MCM41
9
200
>0, chemically inert support) in the sulfurization step. The melting temperatures of 1000~ or higher, are required because zinc ferrite phase is prepared about 950~ One representative material from each of the aforementioned categories has been selected for the preparation of every supported sorbent: Commercial sepiolite (Tolsa, S.A., Spain), USY zeolite (Zeolyst Inc., U.K.), and ZrO2, m.p. 2500~
665 (Merck, A.R.). On the other hand, a wide range of possible binders has been selected in order to research the action on mechanical strength of zinc titanite: Silica gel, AI(PO4), bentonite, kaolin, borax, and Inorganic Oxides (Na~CO3 and NaBO2). 2.2. Preparation of sorbents. No supported sorbents were prepared by mixing the pure oxides and the selected binders (up to 10 or 20% depending on sample) with water, extrusion with a syringe, and cut to the adequate size (2x3 mm). Then they were dried at 110~ 2 hours and calcined 6 hours at 750, 950 and 1100~ Graphite was used as a way to increase porosity. Supported sorbents were prepared by two methods, in order to modify the impregnation profile of the active species. In every case, when preparing the pelleted supports, water is mixed with the powdered sorbent, resulting in a wet paste, that is extruded with a syringe and dried for 10 min at 120~ Then it is cut into 2x3 mm size extrudates. ZrO2 and zeolite required the addition of 20% of bentonite as a binder. Dried extrudates were calcined according to the preparation method selected: Method 1: Classical incipient wetness method [19] of the calcined support with stoichiometric amount of ZnO and Fe203 (1:1) ratio at 10% w/w of the support. Method 2: Graphite co-impregnation, by mixing all powders together (including graphite) and then calcinating. Thus, the pore volume of the support before the impregnation step is maximum. In brief, in the first method, at the expense of reducing the pore volume available to the ferl'ite, all supports should have the maximum resistance due to its previous calcination at 950~ In the second one, the support keeps a higher pore volume. Additional porosity is attained by using graphite, at the risk of having worse contacts between Zn and the Fe oxides. A smamamy of the prepared materials is shown in Table 1. Table 1. Summary of the ZF(1:1) prepared supported sorbents Non impregnated Support
S o r b e n t Composition/ calcination conditions
Ferrite (as oxides) % added
SEP-5%G
SEP-M1
Sepiolite, 5% graphite, 950~
10
USY-20%B
USY-M1
80%USY, 950~
ZR-20%B-5%G
ZR-M1
80% ZrO2, 20% bentonite 5% graphite, 950~ Sepiolite, 5% graphite, 1000~
SEP-M2
20%
Bentonite, 10
USY-M2 80% USY, 20% bentonite 5% graphite, 1000~ 80% ZrO2, 20% bentonite ZR-M2 5% graphite, 1000~
Method
Incipient Wetness of support
10 10 20 l0
Graphite Co-lmpreg
2.3. Techniques.
The mechanical strength of extrudates was measured with an standardized apparatus according to ASTM D-4179 and the crush strength was obtained as the average of 20 measurements. A scanning electron microscope (SEM) ISI DS-130 coupled to a Kevex Si/Li detector and a Sun SparcStation 5 for energy dispersive X-ray (EDX) analysis was used to
666 study both the chemical composition and the distribution of the elements in the sorbents. Xray diffraction (XRD) patterns were recorded by a Seifert 3000 diffi'actometer using Nifiltered CuKtx radiation in order to characterize the structure of powder samples. The stability of supports was obtained in a thermoanalyzer TGA-50H, TA Instrtmaents, with a 25~ 1500~ temperature range, a 0-99~ heating ramp, and a 20 mg top weight. The surface area was calculated by means of the simple impregnation method, and the pore volume by titration with water. The performance of sorbents in sulfidation tests was verified in a quartz upflow fixed-bed reactor with mass flow controllers and downstream traps for collecting vapours and elemental sulphur. Analysis of the outlet and inlet gases from reactor was carried out by gas chromatography (GC) using a thermal conductivity detector (TCD) and a flame photometric detector (FPD) with a sensitivity higher than 10 ppmv for sulphur compounds.
3. Results and Discussion
First of all, the mechanical strength of selected binders was compared to the one obtained for pure zinc titanite. Considering that 20% binder was used, the obtained results for classical binders, as bentonite, were very low (0.9-1.6 kg/mm) compared to the found in the literature (2-4%). This conclusion focussed our research to the inorganic oxides having high melting point (Na2CO3, NaBO2, kaolin, cryolite), and some three-dimensional gel structure oxides (AIPO4 x H20, silica gel, borax), whose surface area is expected to be very high. Table 2 shows that pure zinc titanite presents a mechanical strength of 3.1 Kg/nma, whereas by adding 20% NaBO2 it attains 10.6 Kg/mm. The addition of 5% graphite allows reaching only 6 Kg/mm. A progressive decay of mechanical strength is produced by addition of graphite to zinc titanite. This decay is abrupter in the initial stages of the porogenic reaction (low graphite percentages) because macropores break the structure. Afterwards, increasing amounts of graphite affect scarcely to strength, because CO2 can be easily removed across the macropores network. Table 2. Effect of binders on mechanical strength of zinc titanite, ZT (0.8:1) MATERIAL CALCINATION % w/w BINDER TEMPERATURE, ~ Pure zinc titanite 950 0 Zinc titanite, 5% graphite 950 0 Bentonite 1100 5-20 AIPO4, Kaolin, Cryolite Silica 1100 20 Gel Borax pentahydrate 1100 20 Sodium Carbonate 1100 20 NaBO2 950 20 NaBO2 (5% graphite) 1100 20 NaBO2 (10% graphite) 1100 20 NaBO2 (20% graphite) 1100 20
STRENGTH, Kg/mm 3.1 2.2 0.9-1.6 O
> 0
E
E
40
20
60 - o - c u 2" ]
4O
---0- C~"
20
~o-----____~
, 0
.
.
.
.
0
i
2000
,
,
,
,
I
4000
i
,
.
,
,
,
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|
i
.
i
,
6000
V:m Ratio, mug
1000
2000
V'rn Ratio, mug
Fig. 7. Ion-exchange behaviour of 1: a) Pb 2+, Cu 2+, Cd 2+ and Hg 2+ removal from the groundwater simulant, and b) Cu 2§ and Cd 2+ ion removal from the Ringer solution. It was observed that 1 is able to remove almost all the lead from 3000 volumes and copper ions from 1000 volumes of groundwater simulant, while its efficiency for Cd 2+ and Hg 2+ ions removal is much lower (200-500 volumes). The presence of organic compounds in the Ringer solution affects somewhat the affinity of the ammonium-titanium phosphate for divalent metals. Nevertheless, it removes the Cu 2+ ion efficiently from 1000 volumes of this biological liquor and the Cd 2+ ion from 200 volumes. All together, these results indicate that the novel ammonium-titanium phosphate, despite its relatively low exchange capacity, exhibits a high affinity for some toxic heavy elements, which makes it promising for some specific separations.
Acknowledgement. This work was funded by the Ministerio de Ciencia y Tecnologia (Spain), Research Project No. MAT2000-1654.
REFERENCES [1]
[21 [31 [4] [5] [6] [7]
A. Clearfield (ed.), Inorganic Ion Exchange Materials, CRC Press, Boca Raton, FL, 1982; G. Alberti, in Recent Developments in Ion Exchange, P.A. Williams and M.J. Hudson (Eds.), Elsevier, London, 1987; R.G. Anthony, C.V. Philip and R.G. Dosch, Waste Management, 13 (1993) 503; A. Clearfield, Progress Inorg. Chem. 47 (1998) 373. S. Cheng and A. Clearfield, J. Catal., 94 (1985) 455. J-M. Winand, M. Rulmont and P. Tarte, J. Solid State Chem., 93 (1991) 341. G.D. Stucky, M.L.F. Phillips and T.E. Gier, Chem. Mater., 1 (1989) 489; M.E. Hagerman and K.R. Poeppelmeir, Chem. Mater., 7 (1995) 602. R. Roy, E.R. Vance and J. Alamo, Mater. Res. Bull., 17 (1982) 585. J. Alamo and R. Roy, J. Am. Ceram. Soc., 63 (1984) 78; R. Roy, D.K. Agrawal, J. Alamo, and R.A. Roy, Mater. Res. Bull., 19 (1984) 471. A.N. Christensen, E.K. Andersen, I.G.K. Andersen, G. Alberti, M. Nielsen and M.S. Lehmann, Acta Chem. Scand., 44 (1990) 865; M.A. Salvad6, S. Garcia-Granda and J. Rodriguez, Mater. Sci. Forum, 166-169 (1994) 619; S. Bruque, A.A.G. Aranda, E.R.
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Losilla, P. Olivera-Pastor and P. Maireles-Torres, Inorg. Chem., 34 (1995) 893; M.A. Salvad6, P. Pertierra, S. Gareia-Granda, J.R. Garcia, J. Rodriguez and M.T. Fem6.ndez-Diaz, Aeta Crystallogr. B 52 (1996) 896. A. Men6ndez, M. Bfireena, E. Jaimez, J.R. Gareia and J. Rodriguez, Chem. Mater., 5 (1993) 1078; A. Espina, E. Jaimez, S.A. Khainakov, C. Trobajo, J.R. Garcia and J. Rodriguez, Chem. Mater., 10 (1998) 2490; A. Espina, J.R. Garcia, J.M. Guil, E. Jaimez, J.B. Parra and J. Rodriguez, J. Phys. Chem. B, 102 (1998) 1713. K. Byrappa and M. Yoshimura, Handbook of Hydrothermal Technology, Noyes, New Jersey, 2001. I.W.C.E. Arends, R.A. Sheldon, M. Wallau and U. Schuchardt, Angew. Chem., Int. Ed. Engl., 36 (1997) 1145; M. Hartmann and L. Kevan, Chem. Rev., 99 (1999) 635. D.M. Chapman and A.L. Roe, Zeolites, 10 (1990) 730; M.W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha and S. Lidin, Nature, 367 (1994) 347; W.T.A. Harrison, T.E. Gier and G.D. Stucky, Zeolites, 15 (1995) 408; G. Sankar, R.G. Bell, J.M. Thomas, M.W. Anderson, P.A. Wright and J. Rocha, J. Phys. Chem., 100 (1996) 449. T.K. Das, A.J. Chandwadkar and S. Sivasanker, J. Mol. Catal. A, 107 (1996) 199; C.L. Bianchi and V. Ragaini, J. Catal, 168 (1997) 70; T.K. Das, A.J. Chandwadkar and S. Sivasanker, Catal. Lett., 44 (1997) 113. Y.J. Li and M.S. Whittingham, Solid State Ionics, 63-65 (1993) 391; A.I. Bortun, L.N. Bortun, A. Clearfield, M.A. Villa-Garcia, J.R. Garcia and J. Rodriguez, J. Mater. Res., 11 (1996) 2490; A.I. Bortun, S.A. Khainakov, L.N. Bortun, D.M. Poojary, A. Clearfield, J. Rodriguez and J.R. Garcia, Chem. Mater., 9 (1997) 1805; A.M.K. Andersen and P. Norby, Inorg. Chem., 37 (1998) 4313; C. Serre and G. Ferey, J. Mater. Chem., 9 (1999) 579; S. Ekambaram, C. Serre, G. Ferey and S.C. Sevov, Chem. Mater., 12 (2000) 444; T. Loiseau, C. Mellot-Draznieks, C. Sassoye, S. Girard, N. Guillou, C. Huguenard, F. Taulelle and G. Ferey, J. Am. Chem. Soc., 123 (2001) 9642. M.A. Salvad6, P. Pertierra, S. Garcia-Granda, A. Espina, C. Trobajo, J.R. Garcia, Inorg. Chem., 38 (1999) 5944; C. Trobajo, A. Espina, E. Jaimez, S.A. Khainakov, J.R. Garcia, J. Chem. Soc., Dalton Trans. (2000) 787. S. Frankel and S. Reitman (Eds.), Clinical Laboratory Methods and Diagnosis, Vol. 1, Mosby, Saint Louis, 1963. S. Garcia-Granda, M.A. Salvad6, P. Pertierra, J.R. Garcia, A.I. Bortun, A. Clearfield, Mater. Sci. Forum, 378-381 (2001) 665. V.C. Farmer (Ed.), The Infrared Spectra of Minerals, Mineralogical Society, London, 1974. Powder Diffraction Files, International Center for Diffraction Data, Swarthmore, PA, 1996, no. 39-207 and 38-1468. K.S.W. Sing, D.H. Everett, R.A.W. Havl, L. Moscov, R.A. Pierotti, J. Rouquerol and T. Simieniewska, Pure Appl. Chem. 57 (1985) 603.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
709
The Anomalous Sorptive Behaviour Of Zsm-5 And Silicalite-I: Observation Of Low-Pressure Hysteresis In Nitrogen Adsorption Georgia Kyriakou, and Charis R. Theocharis*, Porous Solids Group, Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus A first stage of a systematic study of the nature of the low-pressure hysteresis loop observed in the nitrogen adsorption isotherms of some MFI zeolites is presemed. It was shown that the pressure at which occurrence of this hysteresis loop takes place was linked to the presence of defect sites. The role of the exchangeable cation was also investigated, and it was suggested that in addition to the numbers of defect sites present, the strength of adsorbate-adsorbem interaction, and probably pore shape was also involved.
1. INTRODUCTION Zeolites have been of considerable practical and academic interest in the past several years because of their diverse applications as catalysts, ion-exchangers, sorbents, and latterly because of their environmental applications. It has also been proposed, that zeolites may be used as model sorbent materials that could be used for calibration and comparison purposes. A EU-funded project proposed the use of zeolites of the socalled MFI structural type, such as ZSM-5 and its silica analogue Silicalite I, for this purpose. The reason for this lay in the fact that synthetic routes existed that could consistently produce well-characterised samples with crystallite size in the region of 0.1 to 0.2mm [1]. Such samples would enable the measurement of adsorption isotherms that would not exhibit secondary capillary adsorption characteristics, or effects of activated adsorption. However, Carrott and Sing [2], Unger et al as well as Rouquerol et al observed that the nitrogen adsorption isotherms of such Silicalite or H-ZSM samples, exhibited low-pressure hysteresis loops at a p/pO value of 0.15 for the former, and close to 0.10 for the latter [1, 3, 4]. Clearly such a hysteresis loop could not be attributed to capillary condensation. Other ZSM-5 samples did not exhibit such behaviour towards nitrogen adsorption but did so towards other gases [5]. Silicalite I exhibited similar behaviour with adsorbates other than nitrogen. A theoretical simulation of the adsorption of argon in Silicalite failed to reproduce the above-mentioned step in the isotherm, but has nevertheless indicated that adsorption
710 in the micropores would proceed in at least two steps, the first one being localised adsorption on the strongest sites, and the second corresponding to molecules clustering around other adsorbed molecules [6,7]. The appearance of low-pressure hysteresis was attributed to a solid-liquid transition or a phase transition of the adsorbed phase in the porous system of the MFI lattice, and neutron diffraction data appear to corroborate this. Some work with ZSM-5 samples has indicated a correlation between the pressure at which the low-pressure hysteresis loop appeared as well as its height with the presence of aluminium [4]. Sing has discussed in detail the occurrence of low-pressure in hysteresis in silicalite in several articles [8,9]. A discussion of low pressure hysteresis was presented in [ 10]. However, no systematic work has been carried out on the nature of this unusual behaviour, especially of the factors influencing its position or shape. Furthermore, all data available thus far, were on large crystalline samples, while no data are available on commercially available ones. This paper presents work, as part of an ongoing investigation in this laboratory [11,12] on commercially available ZSM-5 samples, which attempts to clarify the nature of this anomalous sorptive behaviour. It presents a first attempt to study of the effect of changing exchangeable cations on the lowpressure sorptive behaviour of these solids. 2. EXPERIMENTAL
Nitrogen adsorption isotherms were carried out using a Micromeretics ASAP 2000 automated apparatus at 77K, from a starting pressure of 0.13Pa. Prior to adsorption isotherm determination, samples were outgassed at 0.13Pa overnight 573K for all samples, except for NH4§ ZSM-5 which was outgassed at 283K, a temperature at which the ammonium ion does not decompose. ZSM-5 and Silicalite samples used in this work were donated by Degussa and were free of binder. We chose to use ZSM-5 samples with Si:Al=120. Some data were also obtained on samples with Si:Al=80. The pristine form of ZSM-5 had Na + as counterions. Other counter-ions, specifically NH+4, Ca2+, Ce 4+ and Cu2+ were introduced by repeated overnight suspensions of the samples in appropriate solutions at room temperature. Thermogravimetric analysis was carded out in the region 300-1073K in flowing air using a Shimadzu TGA-50 apparatus. FTIR spectra were obtained using an 8500 Shimadzu Spectrometer, and DRIFTS spectra using a Spectratech attachment.
711
3. RESULTS Table 1 shows the nitrogen adsorption results for the zeolite samples with Si:AI =120 under investigation, whereas Figures 1 and 2, show the corresponding curves for samples Ca 2+ ZSM-5 and NH4+ ZSM-5.
Thermogravimetry was used for sample
NH4+ ZSM-5, to determine the optimum outgassing temperature for this sample. Figures 3-5 contain the DRIFTS spectra for the ZSM-5 samples with Si:AI=120 under investigation. The corresponding FTIR spectra have also been measured, but not included. SBET was estimated from the linear form of the well-known BET equation, and the pore volume from the total amount of nitrogen sorbed at p/p~
converted
to liquid.
Table 1
Sample
SBET (mZ/g)
Pore Volume (cm3/g)
Silicalite I
340
0.16
Low Pressure Hysteresis Yes
C a 2+ ZSM-5
327
0.15
NO
Cu 2~-ZSM-5
308
0.16
No
Na + ZSM-5
405
0.20
No
-NH4+ ZSM-5
327
0.17
Yes
C e 4+ ZSM-5
381
0.19
No
Outgassing temperatures: 573K, except for NH4+ ZSM-5 which was outgassed at 383K 4. DISCUSSION
A comparison of the differences in BET surface areas for the ZSM-5 samples with Si:Al=120 and different counter-ions in Table 1 above, cannot be completely rationalized in terms of the ionic radii, since NH4+ ZSM-5 with an ionic radius of 143 pm has a similar SBEX value with Ca 2§ ZSM-5 for which the ionic radius is 97 pm. However, ignoring Na §
C e 4+
ZSM-5 has the lowest ionic radius for the counter-ion
(92 pm) and the highest BET surface area value. Ions Na +, Ca 2§ and Cu 2§ have similar ionic radii but differing BET area values. Examination of the DRIFTS spectra in Figures 3-5 shows significant differences between the various samples in the region
712
around 3500 cmq where the OH stretching frequencies occur. The rest of the spectra were, as expected, almost identical. The differences between the OH stretching bands indicates that both the nature and concentration of structural OH groups or those associated with the exchangeable cations is different from sample to sample. Comparison of the intensity of the band at 1640 crn~, which is the H-O-H bend and thus characteristic of molecular water, in the various samples, suggests that the water content is the same for all of them, thus any differences in the OH stretching bands cannot be attributed to water.
Figure 1 shows the nitrogen adsorption isotherm for Ca2+ ZSM-5 with Si:AI=120, a typical Type I isotherm in the low-pressure range, but with a type H4 hysteresis loop at higher p/pO range. This is a typical isotherm for the rest of the Si:AI=120 samples, except for sample NH4+ ZSM-5 for which the nitrogen adsorption isotherm is shown
Ca 1 1 0
L
'
'
'
i
'
2+ '
ZSM-5 120 "
i
'
'
'
i
'
"
'
i
'
'
"
......... !
100 95 g0 85 80 0
0.2
0.4
0
0.6
0.8
1
pip Figure 1 Nitrogen Adsorption isotherm at 77K for Ca 2+ ZSM-5 outgassed at 573K in Figure 2. This sample by contrast shows a low pressure hysteresis loop centered at p/p~
This is the only sample examined which exhibited such a hysteresis loop.
The shape of the low-pressure hysteresis loop in Figure 2 is different to that reported by previous workers, or that found by this group for Silicalite I. In those cases, the loop was found to be squarer, indicating that the transition occurring within the sorbed species in the micropores occurred over a smaller pressure range [8]. A loop shape similar to that shown in Figure 2 was previously observed for samples with higher
713 aluminium content. In the present work, samples with higher aluminium content (Si:Al=80) did not exhibit a low-pressure hysteresis loop for any exchangeable cation.
NH + ZSM 5-120 4
110 105 100 95 90 85 80 75 0
0.2
0.4
p/p
0 0.6
0.8
1
Figure 2 Nitrogen Adsorption isotherm at 77K for NH4+ ZSM-5 outgassed at 383K The main systematic difference between the samples used in the present work and those used previously, was crystallite size. The samples used in the previous work consisted of large single crystals, whereas the present study was carried out using polycrystalline samples. The main difference between the nitrogen adsorption isotherms for the two sets of samples was in the pressure range in which the hysteresis loop occurred: for the single crystalline samples, this was at p/p~
and moved at
lower values with increasing aluminium content, whereas for the polycrystalline material, the hysteresis loop occurred at p/p~
and appeared to be unaffected by
aluminium content. These results, in view of the observations made from the DRIFTS spectra suggest that the position of the low-pressure hysteresis loop is influenced by the presence of defects in the crystals, which are present in greater numbers in a polycrystalline sample than a single crystal. These defects represent areas of different structure than the rest, and represent the so-called areas of strong adsorption. These are also usually associated with OH groups. Theoretical work by Nicholson et al [7] has suggested that adsorption in a tubular channel system such as that of an MFI zeolite proceeds at two steps. The first of these steps corresponds to localised adsorption at preferential high-energy sites, and the second, clustering around other
714
adsorbed molecules. Clearly, defect sites present preferred sites of adsorption, so since in the samples used here such sites were present in greater concentrations, a low-pressure hysteresis loop would be more prevalent than hitherto. In addition, the greater number of strong adsorption sites, would also have the effect of requiring a higher concentration of adsorbed molecules for clustering away from the walls. Such clustering would also occur more slowly in the presence of a higher concentration of defect sites, this making the clustering of the adsorbed molecules to occur a slower rate.
Figures 3 to 5 show the DRIFTS spectra for ZSM-5 with Si:AI=120 with different
30 25 C a Z S M 5 120 20 o~ 15
10
2950
2100 1/cm
1250
400
Figure 3 DRIFTS spectrum for Sample Ca2+-ZSM-5 counterions. The structural OH groups, ie those not associated with molecular water, are associated with Broensted surface acid sites (Si---O(H)---A1 groups), as well as defects. Given that no appreciable difference in water content has been observed for the samples discussed here, the differences in the OH groups present, are due, presumably, to the counter-ions occupying different positions in the channel system, associating themselves with different OH sites on the surface. This would alter the strength of the adsorbate-adsorbent interaction, but could also alter the shape of the
715
Cu ZSM 5 120
I-.
i
I
i
2950
3800
I
i
I
2100
,
,
I
1250
llcm
400
Figure 4 DRIFTS spectrum for Sample Cu2+-ZSM-5
30
NH4 Z S M 5 120
25,
I--
10
,,
3800
i
I
i
2950
I
2100 l/cm
f
I
1250
i
I
400
Figure 5 DRIFTS Spectrum ofNH4 + ZSM-5 Pores, thus explaining why for some counter-ions a low-pressure hysteresis loop is observed, and not for others. This change of shape may have the effect of disrupting the reorganisation of the adsorbed layer.
716 A question that does remain is why should this unusual hysteresis loop occur in certain MFI zeolites, and not in other zeolitic materials, or other microporous systems. This suggests that it cannot be a case of pore diameter, or the strength of adsorbateadsorbent interaction alone, but has to be a combination of these factors. It is apparent, that the effect of the Si:A1 ratio on the presence of the hysteresis loop is because of the influence this has on the strength of the interaction. A systematic study of microporous materials should continue with the aim of locating other systems that exhibit a similar behaviour.
Acknowledgements We are grateful to Degussa for donating the Zeolite samples, and to Maria Christophidou and Maria Tingiridou for experimental assistance. The financial support of the University of Cyprus is appreciated. References 1. U. Muller, H. Reichert, E. Robens, K.K. Unger, Y. Grillet, F. Rouquerol, J. Rouquerol, D. Pan and A. Mersmann, Fresenius Z. Anal. Chem., 333,433 (1989) 2. P.J.M. Carrott and K.S.W. Sing, Chem. & Ind., 786, (1986) 3. H. Reichert, U. Muller, K.K. Unger, Y. Grillet, F. Rouquerol, J. Rouquerol, J.P. Coulomb, Studies in Surface Science and Catalysis, (Eds F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger), Elsevier, Amsterdam, 62, 535, (1991) 4. U. Muller and K.K. Unger, Studies in Surface Science and Catalysis, (Eds K.K. Unger, J, Rouquerol, K.S.W. Sing and H. Kral), Elsevier, Amsterdam, 39, 101, (1988) 5. P.L. Lllewellyn, PhD Thesis, Brunel University, (1992) 6. R J-M Pellenq and D. Nicholson, Studies in Surface Science and Catalysis, (Eds J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger), Elsevier, Amsterdam, 87, 21, (1994) 7. D. Nicholson, R.W. Adams, R.F. Cracknel and G.K. Papadopoulos, Characterisation of Porous Solids IV, (Eds B. McEnaney, T.J. Mays, J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger), Royal Society of Chemistry, London, 57, (1997) 8. K.S.W. Sing, Colloids and Surfaces, 38, 113, (1989) 9. K.S.W. Sing, 3rd Fundamentals of Adsorption (Ed. A.B. Mersmann and S.E. Scholl), Engineering Foundation, New York, p78, (1991) 10. F. Rouquerol, J. Rouquerol and K.S.W. Sing, in "Adsorption by Powders and Porous Solids", Academic Press, New York, pp 389-366 (1999) 11. M.-E. Elettheriou, PhD Thesis, University of Cyprus (1995) 12. M.-E. Elettheriou and C.R. Theocharis, Characterisation of Porous Solids IV, (Eds B. McEnaney, T.J. Mays, J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger), Royal Society of Chemistry, London, 475, (1997)
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
717
Characterisation of the textural properties of chemically dealuminated Y zeolites R. L6pez-Fonseca, B. de Rivas, J.I. Guti6rrez-Ortiz, and J.R. Gonzfilez-Velasco* Departamento de Ingenieria Quimica, Facultad de Ciencias, Universidad del Pals Vasco/EHU, P.O. Box 644, E-48080 Bilbao, Spain. Phone: +34-94-6012681; Fax: +34-94-4648500; E-mail address: iqp~ovei~l~.ehu.es
The main objective of this work is to characterise the textural properties of a series of Y zeolites dealuminated by ammonium hexafluorosilicate treatment. It was observed that the fluorosilicate treatment produced a highly crystalline product with a contracted unit cell. Both textural and XRD analysis confirmed the samples to be at least 95% crystalline for dealumination degrees 50%) the isomorphous substitution was no longer ideal resulting in the formation of a secondary pore system. ACKNOWLEDGEMENTS
The authors wish to thank Universidad del Pals Vasco/EHU (9/UPV 0069.31013517/2001) and Ministerio de Ciencia y Tecnologia (PPQ2001-0543) for the financial support. REFERENCES .
2. 3. 4. .
6. .
8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
J. Scherzer, ACS Symp. Ser., 248 (1984) 157. B.Sulikowski, Heterog. Chem. Rev., 3 (1996) 203. D.W. Breck, G.W. Skeels, US Patent No. 4 503 023 (1985). G.W. Skeels, D.W. Breck, in: D. Olson, A. Bisio, (Ed.), Proceedings of the 6th International Zeolite Conference, Butterworths, Guilford, 1984, p. 87. Y. He, C. Li, E. Min, Stud. Surf. Sci. Catal., 49 (1989) 189. H. Fichtner-Schmittler, U. Lohse, G. Engelhardt, V. Patzelova, Cryst. Res. Technol., 19 (1984) K1. J.H. de Boer, B.G. Linsen, TH.J. Osinga, J. Catal., 4 (1965) 643. Q.L. Wang, G. Giannetto, M. Guisnet, Zeolites, 10 (1990) 301. M. Neuber, V. Dondur, H.G. Karge, L. Pacheco, S. Ernst, J. Weitkamp, Stud. Sure Sci. Catal., 37 (1987) 461. A.P. Matharau, L.F. Gladden, S.W. Can, Stud. Surf. Sci. Catal., 94 (1995) 147. A. Corma, V. Forn6s, F. Rey, Appl. Catal., 59 (1990) 267. G. Garral6n, V. Forn6s, A. Corma, Zeolites, 8 (1988) 268. A.V. Abramova, E.V. Slivinskii, E.A. Skryleva, Kinet. Katal., 39 (1998) 411. J.M. Cruz, A. Corma, V. Forn6s, Appl. Catal., 50 (1989) 287. P.V. Shertukde, W.K. Hall, J-M. Dereppe, G. Marcelin, J. Catal., 139 (1993) 468. A. Gola, B. Rebouis, E. Milazzo, J. Lynch, E. Benazzi, S. Lacombe, L. Delevoye, C. Fernandez, Microporous Mesoporous Mater., 40 (2000) 73. G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Catal. Today, 41 (1998) 207. J. Lynch, F. Raatz, Ch. Delalande, Stud. Surf. Sci. Catal., 39 (1988) 547. A. Gervasini, Appl. Catal. A, 180 (1999) 71. M.J. Remy, G. Poncelet, J. Phys. Chem., 99 (1995) 773. H. Ajot, J.F. Joly, J. Lynch. F. Raatz, P. Caullet, Stud. Surf. Sci. Catal., 62 (1991) 583. D. Goyvaerts, J.A. Martens, P.J. Grobet, P.A. Jacobs, Stud. Sure Sci. Catal., 101 (1996) 731. J. Lynch, F. Raatz, P. Dufresne, Zeolites, 7 (1987) 333.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
723
Complementarity of microcalorimetry, manometry and gravimetry in the study of gas adsorption by microporous solids up to 50 bar St~phanie Moret a, Thomas Poyet a, David Bigot b, Bernd Polster b, Simon Crispel b, Jean Rouquerol a and Philip Llewellyn a MADIREL, CNRS/Universit6 de Provence, 26 rue du 141 +meRIA, 13331 Marseille cedex 3, France
a
b Air Liquide, Claude-Delorme Research Centre, 1 chemin de la Porte des Loges, Les Logesen-Josas, France This work compares and contrasts the experimental results using three different experimental techniques : manometry, gravimetry and microcalorimetry. The system studies was the adsorption of argon, nitrogen and carbon dioxide on a NaLSX zeolite between 20 and 60~ and up to 50 bars. 1. INTRODUCTION The adsorption of gases at room temperature and up to pressures in the region of 50 bars is of interest from several points of view. From an industrial standpoint, both gas separation (by pressure-swing adsorption (PSA)) and gas storage operate in this pressure range [1 ]. From a fundamental point of view however, the nature of the adsorbate-adsorbent interactions close to and within supercritical regions merit detailed study [2]. Now, these measurements are somewhat difficult to carry out, not only because of the question of the pressure but also because of the problems of accuracy, due to the corrections and errors - due to the void volume (in adsorption manometry) [3-5] or to the buoyancy (in adsorption gravimetry) [6]. For this reason, it may be found advisable to cross-check the experimental results by making use of a variety of different techniques. The aim of this paper is therefore to check the consistency of three basic techniques (adsorption manometry, adsorption gravimetry and adsorption microcalorimetry) in the 0-50 bar pressure range, using a standard NaX zeolite adsorbent and selecting three adsorbable gases, namely: - Argon, with nearly perfect behaviour and known to usually give rise to non-specific interactions during adsorption - Nitrogen, again with nearly perfect behaviour, but now giving rise to specific adsorption interactions, due to its permanent quadrupole moment - Carbon dioxide, which gives rise to more important specific interactions due to the larger quadrupole. Furthermore carbon dioxide does not behave as a perfect gas and needs important corrections to be taken into account After presenting the experimental details and the results, the latter will be compared to each other and we shall then comment on the merits and conditions of reliability of each of these approaches.
724
2. E X P E R I M E N T A L
2.1. Adsorptives and Adsorbents The zeolite sample used in the present study is a sodium containing faujasite NaX of Si/A1 ratio = 1.3. This sample was supplied by CECA S.A. (ref. G5CO2M). The sample was in bead form of diameter 1.6-2.5 mm. The adsorptives were obtained from Air Liquide (quality Alphagaz), of purity greater than 99.99%)
2.2. Adsorption manometry The commercial equipment used for this purpose makes use of a Mensor pressure gauge covering the 0 - 70 bar range, with a sensitivity of 0.01% of the full scale and it can accommodate 4 samples at the same time. The isotherms were carried out at 20, 40 and 60~ and up to a final pressure of 50 bars. A sample mass of around 3 g was used for each experiment. The sample was outgassed in the adsorption cell on a separate apparatus attached to a turbomolecular pump. The sample was heated up to a final temperature of 400~ in 10 hours with several intermediate plateau's. The sample was kept at this final outgassing temperature for 6 hours, with a final pressure of around 10-5 mbar. For the determination of each isotherm, the sample cell was kept to within 0.1~ of the experimental temperature by use of a liquid thermostat. Dead space correction was carried out using a helium calibration. A difference in the measured dead space of up to 0.4 % was observed over the experimental pressure range. When varying the temperature within the 20 60~ range however, this variation in the measured dead space was far less. The gases were introduced using a point by point procedure. The a d s o r b a t e - adsorbent equilibrium for each point was considered attained for a pressure change of less than 8 mbar over 5 minutes. Under such conditions a typical isotherm with 20 points was obtained in around 24 hours.
2.3. Adsorption microealorimetry The adsorption up to 50 bars was carried out by means of a Tian-Calvet type isothermal microcalorimeter built in the former CNRS Centre for Thermodynamics and Microcalorimetry. For these experiments, around 2 g of sample was used which were outgassed by Controlled Rate Thermal Analysis (CRTA) [7]. The experiments were carried out at 30~ (303 K). Approximately 6 hours is required after introduction of the sample cell into the thermopile for the system to be within 1/100 th of a degree Celsius. At this point the baseline recording is taken for 20 minutes. After this thermal equilibrium was attained, a point by point adsorptive dosing procedure was used. Equilibrium was considered attained when the thermal flow measured on adsorption by the calorimeter returned to the base line. For each point the thermal flow and the equilibrium pressure (by means of a 0-70 bar MKS pressure transducer providing a sensitivity of 0.5% of the measured value) were recorded. The area under the peak in the thermal flow, Qme,,s, is measured to determine the pseudo-differential heat of adsorption, Aads/l , v i a :
Aads]l .__ G e a s -- Glank o" nmeas
(1)
725 where
Qblank is the heat output measured from a blank experiment. This value takes into
account gas compression of the non-adsorbed phase, dose.
n:eas is the
amount adsorbed during the
2.4. Adsorption gravimetry and density measurements The equipment used was built in-house around a high-pressure magnetic suspension balance marketed by Rubotherm, Germany. A similar balance has been used to measure high temperature isotherms [8]. Our current set-up has added features such as CRTA control unit for the in-situ outgassing of the sample and an adsorption manometry system. The coupling of manometry and gravimetry can be used for the study of coadsorption [9]. It is equipped with three Mensor pressure gauges (0-10, 0-50 and 0-70 bar, respectively), with a sensitivity of 0.01% of the full scale. The balance, with 10 ~g sensitivity, is able to alternately weigh either the sample proper (sample mass up to 10 g) or a cylindrical titanium sinker of known volume (ca. 4 cm 3) whose apparent weight provides a direct assessment to the buoyancy effect, i.e. to the density of the gas phase. This is valuable information each time when the gas does not behave ideally, such as carbon dioxide at room temperature, in order to make an adequate buoyancy correction. The isothermal conditions are provided by a water jacket fed by a thermostat controlled within 10~ K. Prior to any new series of adsorption isotherms and also after each adsorption isotherm of carbon dioxide, the sample (ca. 5 g) was in-situ outgassed by CRTA up to 450~ The adsorptive was then automatically introduced in successive pressure steps. The "height" of the steps was selected so as to provide ca 20 points on each adsorption isotherm up to 50 bars. Equilibrium was considered to be satisfactory when the sample weight varied of less than 80 ~g in 5 min. At each equilibrium point both the sample weight and the sinker weight were recorded. The total duration of an adsorption experiment proper (without taking into account the outgassing) was ca. 15 hours. 3. RESULTS AND DISCUSSION
We report in Figures 1 and 2, with the same presentation and scale, the experimental results obtained by adsorption manometry and gravimetry, for the three gases and the three temperatures. Incidentally, this is a good example where the formerly used representation in "adsorbed volume" or "adsorbed mass" is clearly less convenient than the more universal representation in "adsorbed amount" (or still more precisely in the present case, in "surface excess amount"). The shapes of the adsorption isotherms, with very different curvatures from one gas to the other, are a clear indication of the increased gas/solid interaction as one passes from argon to nitrogen and then to carbon dioxide, for which the isotherms are practically of type I.
726
Nitrogen /%
-O
E E
/%
2
/%
60~ "T 03 --~ 0
/%
/%
20oc
9
9
9 Anr,.,uo
9 9
"uv%, ~"
2
E E ~ 1.5
/% 15
9
2.5
/%
/%
/%
3
400C
/%
/%
A
/%
Nitrogen
/% 20~
/%
/%
/%
/% /% /%
O 9 t~
o5
1"o
o
2"0
~o
Pressure / bar
;o
~o
go
A /%
21
/%
/%
1.8
/%
/% /%
/%
/%
09,
/%/%
/%
06,
A A A AAA
A
/%
15,
E
/%
/%
/%
/%20~
/% /%
/%
/%
60~
/%
o
9
200C 40~
21 '7
600C
1.8,
O E
1.5. 1.2
~
&
~
A
A
~o
dioxide
A
&
~
0
60
A
;0
0
~
2"0
40~
A
"60~
55
Carbon
9
9 9
Ooee
sl'~
5,
;0
Pressure / bar
7
20~ A
A
,,,
,
10 ~o ;0 Pressure / bar
Carbon
9
9
9
s'o
6"0
dioxide
9 9
4"0
9
9 9
9
20~
9 9
9
9 9
9
9
9
9
~0o~ 9
~O~
4'5 ' ~ O ~ O ~
A
4.
~o
o3p
7
4.5 /
~o
0.9
1"o
~
;o
2.41
I~. D
/%
/%
/%
o, ff
_3? 4.~ ~
m O E 35,
F:
~o
Pressure I bar
271
40oC
/%
03, / ~ # A
03.
2'0
Argon
24
,-
1"o
o
Argon
3 2.7
0 E
r
0
0
E 3.5~p
3,
E
3,
%;25.
2,
2
15~
1.5
1
1
05
0.5
0. 0
;
;
;
6
;5
;8
Pressure / bar
2"1 2'4
Fig 1" A d s o r p t i o n i s o t h e r m s on zeolite determined by manometry
2'~
~;o
NaLSX
0
;
;
;
;2
1"5
;8
Pressure / bar
Fig 2" A d s o r p t i o n isotherms zeolite determined by gravimetry
2"1
2"4
on
2"7
~0
NaLSX
727 Nitrogen
30"C o
,-
.=$e
o
,i,. 9 '~
9+ _ ~ 9
20
40
15
30 -~
E
E o.
J=
1.0
0.5
0.0
"'"
0
5
10
15
20
Pore Diameter (nm) Fig. 3 Pore size distributions for SG 5.7 heat-treated at 200~ (A), 300~ ( , ) , 400~ (11) and 500~ (-*).
743 The average pore size for sample SG 5.7 a in Table 2 was determined by the MP method. The average pore diameters for the heat-treated samples were taken from the maxima in the PSD curves drawn using the BJH method from the desorption branches of the corresponding isotherms presented in Fig. 3. It may be noted that the most significant change in pore diameter was caused by heat-treatment at 500~ This shift along with the severe reduction in the SBETto only c. 50 mZg1 was mainly due to the transformation into rutile. Since for photocatalytic reactions titania should be in its anatase form it is important to note that with heat-treatment up to 300~ the sample was 90% anatase. With higher temperatures both the loss in SBET and the conversion to rutile would reduce the photocatalytic activity of the material. However, it has been shown that for high photocatalytic mineralisation of VOCs the reaction temperature should be maintained below c. 150~ [9]. Thus, with this material heat-treatment at between 200~176 would be sufficient to form the anatase washcoat and maintain a high SBET. The choice of the correct heat-treatment temperature depends not only on forming a high surface area anatase support but also on the width of the pores produced since these may control limitations due to diffusion of the gases to be treated into the active layer. Thus, although in principal a higher surface area would be expected to lead to a more active catalyst, diffusion limitations of the gases to be treated onto the active surface, especially for the microporous materials produced on drying at 60~ could be a concern. 4. CONCLUSIONS These results demonstrate that by using a sol-gel technology high surface area titanias can be produced. Furthermore, by careful control over the pH of formation the pore width of the microporous gel may be controlled. With heat-treatment it was demonstrated that the microporous gel was transformed into a mesoporous material. If the heat-treatment temperature did not exceed 300~ the titania was 90% anatase. At higher temperatures the transformation into the less active rutile phase began to take place. As the heat-treatment temperature was increased there was a steady reduction of the SBET, with a corresponding widening of the mesopores, due to sintering of the material. Thus, by careful selection of the final heat-treatment temperature the pore size distribution and SBETcould be controlled. Since after any wash-coating step it is necessary to calcine the composite material in order to fix the thin active layer to the surface this sol-gel preparation technique could be readily adapted to produce photocatalytic coatings with a tailor made porosity. REFERENCES .
2. 3. 4. .
6. 7.
European Community Official Directive, 94/63, No. L 365/24 (1994). T.T. Ibusuki and K. Takeuchi, Atmos. Environ. 20 (1986) 1711. J. Peral and D.F. Ollis, J. Catal. 136 (1992) 554. J. Blanco, P. Avila, A. Bahamonde, E. Alvarez, B. S~mchez and M. Romero, Catal. Today 29 (1996) 437. L.A. Dibble and G.B. Raupp, Environ. Sci. Technol. 26 (1992) 492. L.A. Dibble and G.B. Raupp, Catal. Lett. 4 (1990) 345. M.R. Hoffmann, S.T. Martin, W. Choi and D.W. Bahnemann, Chem. Rev. 95 (1995) 69.
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17. 18. 19. 20. 21. 22.
X. Fu, W.A. Zeltner and M.A. Anderson, in Semiconductor nanoclusters, P.V. Kamat, D. Meisel (Eds.), Elsevier Science, New York, 11 (1996) 445. P. Avila, A. Bahamonde, J. Blanco, B. Sfinchez, A.I. Cardona and M. Romero, Applied Catalysis B: Environmental 17 (1998) 75. R.J. Candal, W.A. Zeltner and M.A. Anderson, J. Adv. Oxid. Technol, 3, 3 (1998) 270. J. Ragai, K.S.W. Sing and R. Mikhail, J. Chem. Tech. Bioltechnol., 30 (1980) 1. J. Ragai and K.S.W. Sing, J. Chem. Tech. Bioltechnol., 32 (1982) 988. J. Ragai and K.S.W. Sing, J. Colloid and Interface Sci. 101, 2 (1984) 369. Q. Xu, and M.A. Anderson, J. Mater Res. 6 (1991) 1073. S. Brunauer, P.H. Emmett and E. Teller, J. Amer. Chem. Soc., 60 (1938) 309. J. Rouquerol, D Avnir, C.W. Fairbridge, D.H. Everett, J.H. Haynes, N. Pericone, J.D.F. Ramsay, K.S.W. Sing and K.K. Unger, Pure and Appl. Chem., 66, 8 (1994) 173. M.M. Dubinin, E.D. Zaverina and D.P. Timofeeva, Zhur. Fiz. Khim., 23 (1949) 1129. B.C. Lippens and J.H. de Boer, J. Cat., 4 (1965) 319. B.C. Lippens, B.G. Linsen and J.H. de Boer, J. Cat., 3 (1964) 32.9. R.S. Mikhail, S. Brunauer and E.E. Bodor, J. Colloid Interface Sci., 26 (1968) 45. E.P. Barrett, L.G. Joyner and P.P. Halenda, J. Amer. Chem. Soc., 73 (1951) 373. W.D. Harkins and G. Jura, J. Amer. Chem. Sot., 66 (1944) 1362.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
745
The effect of geometry and axial orientation of spheroidal particles on the adsorption rate in a granular porous medium
F. A. Coutelieris
National Center for Scientific Research 'Demokritos', 15310 Aghia Paraskevi Attikis, Greece
The mass transport problem from a Newtonian fluid to a swarm of prolate and/or oblate spheroidal adsorbers under creeping flow conditions is considered here. The spheroidal-incell model is used for the analytical description of the flow field within the swarm. The convective diffusion equation along with the appropriate boundary conditions for the description of the adsorption upon the solid surface is solved analytically for high Peclet numbers and numerically in the low Peclet regime. In both cases, analytical expressions for the adsorption rate are obtained. It is found that the oblate geometry offers significant advantage for capturing the diluted mass compared with the prolate one even in strongly convective environments. It is also shown that the assumption of instantaneous adsorption overestimates significantly the adsorption efficiency.
1. INTRODUCTION Modeling mass transport through swarms of particles has attracted significant interest mainly in relation to fluid flow and the associated physicochemical processes. Most of the proposed models derive analytical solutions for the mass transport problem under creeping flow conditions by assuming spherical or cylindrical shape of the particles [1]. Happel and Kuwabara have presented models that solve analytically the creeping flow problem for spherical geometry [2,3]. Both these models are based on the representation of the overall solid mass of the swarm by just one spherical particle, which is embedded in a spherical or cylindrical liquid envelope keeping the porosity equivalent to that of the swarm. However, in almost all practical applications, the particles are of spheroidal shape instead of spherical [4]. An analytical model for the representation of the flowfield within the swarm of spheroidal particles has recently been proposed for both Happel- and Kuwabara-type boundary conditions [5,6]. This analytical solution has already been applied in the study of mass transport processes within swarms of spheroidal particles for both high and low Peclet values [7,8] in a way quite analogous to previous investigations concerning particles of spherical shape [9-11 ].
746 ~0=0
z
~
q=q~
I " O=n Fig. 1. Prolate and oblate "spheroid-in-cell" models The weak point of all these approaches is the postulation of instantaneous adsorption occurring on the liquid-solid interface. This approximation, based on the assumption of the very thin diffusion layer, which is valid only for high Peclet values, produced analytical expressions for the concentration profile in that regime, while, for low Pe a numerical treatment is necessary. Unfortunately, instantaneous adsorption is rather rare corresponding to a very limited range of applications. A more realistic approach is adopted here based on an adsorption - heterogeneous reaction - desorption mechanism, which describes the adsorption of the diluted mass upon the solid surface with high accuracy [12-15]. More precisely, it can be supposed that the component A, which is diluted in the bulk phase, is initially adsorbed by the solid surface where a heterogeneous reaction takes place and its products, which are considered inactive and of very low concentration, are again desorbed in the bulk phase. The adsorption is assumed to occur due to vacant sites that are normally distributed over the solid surface while the whole process can be described by an overall rate according to basic thermodynamics analysis [ 16].
2. THEORY
Consider a solid spheroid having long semi-axis a3 and semi-focal distance a = ~f~ - 1, which is surrounded by another confocal spheroidal liquid envelope, whose thickness is adjusted so that the porosity of the granular medium is equal to that of the model. The spheroidal-in-cell model, predicts the stream function, ~, for creeping flow conditions in the prolate coordinates system (r/,0) as follows [5]: a [A2G2(coshrl)+ A3 [ 5G4(cosh r/~ ) Gl(cOsh~7)+ G4(coshrl)] ~(rl,O)= --~ GI (cosh r/p )
+A4H2(coshrl)] G2(cosO)
(1)
where D, A2, A3 and A4 are r/- and 0-dependent coefficients defined in Dassios et al. [5] and G,,(x) and H,,(x) are the Gegenbauer polynomials of the first and second kind, respectively, of degree -1/2 and of order n. The governing equation for the steady state mass transport in the fluid phase within the porous medium can be written in prolate spheroidal coordinates and in dimensionless form as: OCA 9CA Pe~ "920A U q ~ + U O ~ -90 a4sinh 2 r/+sin2 0 ( - ~ 2 +c~ av
9CA
02CA + - ~ +c~
OCA 90 )
(2)
747 The above equation can be integrated with the following boundary conditions: Ca
(r/=r/~,0)= 1,
(~CA]
0 _Pe>70) and relatively low porosities (e>0.6) and an analytical approach based on the sphere-in-cell model for high porosities (e>0.8) and high Peclet values
(Pe>100). The overall Sherwood number is calculated in all cases and
comparisons between analytical and numerical results are performed.
2. M O D E L I N G APPROACH Consider a Newtonian incompressible fluid containing a component A in high dilution (
