Structure and Reactivity of Surfaces: Proceedings of a European Conference, Trieste, Italy, September 13-16, 1988 (Studies in Surface Science and Catalysis)

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Studies in Surface Science and Catalysis 48

STRUCTURE AND REACTIVITY OF SURFACES

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis Advisory Editors: B. Delrnon and J.T. Yates

Vol. 48

STRUCTURE AND REACTIVITY OF SURFACES Proceedings of a European Conference, Trieste, Italy, September 13-1 6,1988 The Conference was organized by

Associazione ltaliana di Chimica Fisica Divisione di Chimica Fisica della Societh Chimica ltaliana Division de Chimie Physique de la Soci6t6 de Chimie Francaise Royal Society of Chemistry, Faraday Division Deutsche Bunsen Gesellschaft fur Physikalische Chemie

Editors

Claudio Morterra and Adriano Zecchina Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, UniversitA di Torino, via P. Giuria 7, I- 10 125 Torino, Italy

and

Giacomo Costa Dipartimento di Scienze Chimiche, Universitd di Trieste, p.le Europa 1, 1-34 127 Trieste, Italy

ELSEVIER

Amsterdam

-Oxford - New York -Tokyo

1989

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 2 11, lo00 AE Amsterdam, The Netherlands Distributors for the United States and Canada:

ELSEVIER SCIENCE PUBLISHINGCOMPANY INC. 655, Avenue of the Americas New York, NY 10010, U S A .

Structurc and reactivity of surfaces.

(Studies i n surface science and catalysis ; 48) Includes index. i . Surface chemistry--Congresses. 2. Surfaces iZiysics )--Congresses. 3. Cataysis-Congresses. I . Norterra, Claudio. 11. Zecchina, Adriano. 19361x1. Costa, Ciacomo, 1922IV. Associazia?e italiana di chimica f i s i c a . V. Series. .25506.us?; 1939 541.3'3 ?9-11$9b ISBN O-h44-;:7465-3 (U.S. )

.

ISBN 0-444-87465-8 (VOl. 48) ISBN0-444-4 1801 -6(Series) Q Elsevier Science PublishersB.V., 1989

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./ Physical Sciences 81Engineering Division, P.O. Box 330,loo0 AH Amsterdam, The Netherlands. Special regulationsfor readers in the USA -This publicationhas been registeredwith the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publicationmay be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. No responsibilityis assumed by the Publisher for any injury and/or damage to personsor property as a matter of products liability, negligence or otherwise, or from any use or operationof any methods,products, instructionsor ideas contained in the material herein. Although all advertising material is expectedto conform to ethical (medical) standards, inclusionin this publication does not constitute a guarantee or endorsement of the quality or value of such product or of the claims made of it by its manufacturer. Printedin The Netherlands

V

................................................................. ...........................................................

XI11 XIV

Pbtoluminescencer photo-irduced reactivities and catalytic p r o p r t i e s of Na- and Li-doped MgO M. Anpot Y. Y d a , T. Doir I. Matsumat S. Coluccia, A. Zecchina and M. Che

1

Foreword Acknowledgments

.................................................

The characterisation of s u p p r t d m e t a l catalysts by t h e determination of differential heats of adsorption

........................................................... 11 Model studies of the SMSI phenomenon a t the T i 0 / R u ( O O O l ) interface J.P.S. EMyal, A.J. Gellman, R.W. Judd and%.M. L a r b r t ...............19 P.N. Aukett

Oxygen reactivity as a probe for the surface composition of Cao-Mno solid solutions M. W e s t B. Fubini and F.S. Stone

................................... A f i r s t approach t o gaseous aminoacid-oxide interaction P. Baraldi and G. Fabbri .............................................. Adsorption and dissociation of carbon mnoxide on Co(1120) U. Bardi and G. Rovida ................................................ Low pressure oxidation of the P t T i alloy studied by electron spectroscom U. Bardit P.N. Ross and G. h i d a .....................................

31

41 49 59

Interaction of atomic hydrogen with t h e (111) and (100) surfaces of diamndl i k e crystals V. Baronet G. Abbatet N. Russo ard M. Toscano 69

.........................

A transmission ETIR study of the adsorption of carbon mnoxide

hexane on evaporate3 silver films J.E. Baterpan and M.A. Chesters

and cyclo-

........................................

1~

Heat-capacity anomalies of two-dimnsional systems studied by the MCgLlistanHock model F. Battagliar T.F. George and Y.S. Kim 85

(*I

................................ Surface chemistry of carbonaceous species A.T. Bell .............................................................

91

Polarization corductivity of amorphous molybdenum sulphides. Influence of the oxidation Y. B e m i m n r J.C. Giuntinit P. Belougne, B. Deroide ard J.V. Zanchetta111 (invited paprs a r e marked by an asterisk)

VI

Metal-support interaction phemmem in sane high rretal loading lanthana supported rhcdium catalysts S. B e e r F.J. BotanaI R. Garcia and J.M. Rodriguez-Izcpierdo

........123 Spectroscopic characterization of s i l i c a l i t e and t i t a n i u m s i l i c a l i t e M.R. Boocutis K.M. Raor A. Zecchinar G. Leafanti and G. P e t r b i .......133 (*I Surface properties of carbons H.P. ............................................................ 145

A comparison between the Lewis acidity of n o d lretal cations in Y-zeolites

and on ionic surfaces

..........159 Promtion and selective poisoning of supported mtal catalysts G.C. Bond, M.R. Gelsthorper R.R. Rajaram and R. Yahya .................167 A study of supported Ft catalysts by X-ray absorption spectroscopy X.J.P. Bobant A.J. den H a r t q and V. P o n s ........................... 179 V. B o l h B. Fubinir E. Garroner E. Giarnello @ C. Morterra

A SIMS and AES study of nickel deposition on Ti02 (100). Influence of t h e

stoichiorretry of the support S. Bourgeoisr D. D i d c i t e r F. Janardr M. Perdereiu ard R. Poirault

.....191

( * ) Structure and bonding of adsorbates: investigations with synchrotron radiation A.X. Bradshaw

.........................................................

201

Drifts and Raman spectroscopic study of intrazeolite mtal carbonyl chemistry C. Brbardt E. Denneulinr C. Depcker and P. Lqrard

..................223

The adsorption of iodine a t GaAs (100) surfaces v i a the deconposition of CH I : a &el study of semicorductor etching %.S. suhaenkot S.M. Francisr P.A. Gouldhg ad M.E. P&le

............229

Effect of cobalt on the surface properties of Zn-Cr and Cu-Zn-Cr methanol synthesis catalysts G. Ibsfxb M.E. Pattuellir F. Trifirb ad A. Vaccari

...................239

A study of t h e adsorption of acetylene on Cu(100) using Auger electron

spectroscopy X.A.

Chesters and D.R.

Linder

.........................................

Correlation of transmission FTIR spectra of CO adsorbed on Pt/SiO pressure and ET-RAIRS of co adsorbed on a palycrystdline f o i l X.A. Chesterst D. C o a b an3 S.F. Parker

249

a t high

in%^

.............................. The adsorption of carbon tetrachloride on Ni(ll0) X.A. Chesters and D. Lennon ...........................................

257 263

VII Calcination-dependence of platinum cluster formation i n Nay zeolite: a xenon-129 NlR study B.F. ChPlelka, L.C. de Menorvalt R. Csencsitst R. Ryoot S.B. L i u t C.J. Ra3ket E.E. Petersen and A. Pines

...............................

269

XPS and ESR study of nickel valemy states i n NieMgO high surface area solid solutions A. C-t

D. Gazzolit V. Indovinat M. Inversit G. Moretti and

M. Ckchiuzzi

.........................................................

279

(*I Surface characterization of ionic microcrystalline system by optical spectroscopies S. Colmia 289

..........................................................

Photoreactivity of iron-dopd titanium dioxide pawders for dinitrogen reduction t o m n i a J.C. Canesat J. Soriar V. Augugliaro and L. Palmisano

................307 Angle-resolved UPS studies of CN on C u ( l l 0 ) and Cu(100) H. Ccmmllyt T. McCabet D.R. Lloyd ard E. Taylor ..................... 319 Surface reactions of hydrocarbons as a probe for the characterization of support@ ruthenium binetallic catalysts B. Coqt A. Bittar ard F. Figueras

327

Theoretical calculations on the s t a b i l i t y of carbide layers a t transition metal surfaces G.R. Darlingt R.W. Joyner ard J.B. Perdry

335

Critical metal concentration effect on average size selected (Fd-Ag) clusters in the selective hydrogenation of lt3-butadiene V. D e G a m e i a r B. Bellamyt A. Masson and M. Che

347

....................................

............................

...................... Canparison of Cu-support interaction in &el and real catalysts V. Di Castrot C. Furlani and G. Polzonetti ...........................

355

Studies of skeletal rearrangertents of labeled hexanes on platinurnruthenium catalysts. Correlation between t h e p r d u c t distributions and the structure of the catalysts given by E.X.A.F.S. G. D h z t P. Estfbant L. Guczit F. G a r h P. Bernhardtr J.-L. Schmitt and G. Maire 363

.........................................................

Surface reactivity u d e r oxygen atnosghere of @s3(C0ll2 s u p p r t d on s i l i c a and alumina C. Dossit A. Fusit R. Psarot R. Ugo ard R. Zanoni

....................

Ab i n i t i o study of the p r i c d i c cartori aonoxide adsorption on the basal p l a e of alpha-alumina R. D o v e s i r C. Roettir M. C a & an3 C. Pisani

........................

375

.385

Adsorbate-adsorhte jntttact ion in n i x e d C G M ) overlayers on NiO(100) : an 1R inve:,tic;aticln E. Escalona Plateror E. Gaironer C. S p t o an3 A. Zecchina 395

............

Silanol as a rrodel for the f r e e hydroxyl of anorphous s i l i c a : non-enpirical calculations of the vibrational features of H p i O H E. Garraw and P. Ugliengo 405

............................................

An I R study of ethylene hydrogenation a t RT on a W Z n O catalyst G. Ghiotti, F. Boccuzzi and A. Chiorino

415

Effect of water in the encapsulation of the m t a l l i c fiase during S I generation in W T i O catalysts A.R. Gm?Ales-diper P. Malet, J.P. Espin6sr A. Ciballero and G. Munuera

~ 2 7

...............................

............................................................ An infrared study of CO 4 ~ 0 r b e don a W Z n O catalyst E. Qxjlielrinattiand F. Boccuzzi ..................................... (*I The concept of s t r u c t u r e s e n s i t i v i t y i n catalysis by oxides J. Baber ..............................................................

437 ~ 4 7

Catalyst characterization and in s i t u FTIR studies of carbon dioxide methanation over ruthenium supported on t i t a n i a J.G. EighfieLa, P. Ruterana, K.R. Thanpi ard M. Graetzel

..............469

Ch the role of supports ard promtors i n CO + €$ reactions on rhodium based catalysts J.P. H i n d e m e m , A. Kiennemann and S. Tazkritt

481

High resolution vibrational spectra of CO adsorbed on clean and pot assiumcove r d platinum (111) D. Hoqe, M. Tilshausr P. Gardner and A.M. B r d s h w

493

........................

.....................

A particle size effect in the oxygen desorption from platinum supported within a faujasite matrix N.I. Jaeger, A.L. Jourdan, G. Schulz-Ekloff and A. Svensson

...........503

Comparison of copper and palladium catalysts in the synthesis of n-ethanol from CO/% mixtures 3.R JaningS ard M.S. spencer

........................................ Molybdena on niobium o x i d e catalysts: preparation and characterization Y.S. Jin, A. Oxpurr A. Aurow and J.C. Vedrhe .......................

525

Change i n the reactivity of a superalloy by forming surface alloys using laser glazing technique A.S. K b a m a r W.J. Quadakkersr II. Schusterf K. Wisserbacht A. Gasser ard E.W. Kreutz

535

.......................................................

Fourier transform i n f r a r d spectroscopy of oxidized ultra-fine a-silicon carbide n. K i e l h g and R.J. Pugh...............................................549

515

1x Electron microscopy and microdiffraction study of t h e i n t e r z t i o n of W with SiO R. &mbert N. Jaeger and G. Schulz-Ekloff

.559

Application of optical methods (50000 - 400 an-’) t o study the oxidation of nickel. Theoretical approach and experience H. Le Calm and M. Lenglet

567

............................

...........................................

and FTIR reflectance studies of t h e initial stage of oxidation 20 C r alloy H. Le Calm and M. Leqlet

575

Reactivity of mixed Zrr-Cr oxide towards linear C4 oxygenated molecules by the TPSR method L. L i e t t i r E. Tronconir P. Forzatti and I. Pasquon

581

W-Vis-NIR

of 80 Ni

-

........................................... ....................

Surface organorretallic chemistry: new selective birretallic Ru-Sn/SiO catalysts prepared by reaction of tetra-n-butyl tin with Ru02 supporged on silica P. fawasard, J.P. Candyr J.P. Bournonville and J.M. B a s s e t

...........591

Infrared spectra of carbons. X. The spectral p r o f i l e of medimtemperature chars H.J.D. Law and C. Morterra

............................................

601

Preparation of hydrotreating catalysts by kneading. Characterization of the oxidic phase by laser R a m n r X-ray photoelectron and W-visible spectroscopies F. Iudr and F. Viez 611

...................................................

Characterization of the growth of an oxidation precursor a t low t-rature on WHC c o p p r J.44. Ehchefertr M. L e q l e t r D. Blavetter A. Me& and A. D’Huysser ..625 X p S an SIMS investigation on the catalyst-support chemical interaction for RuOx-teflon systans C. Mitesta, G. Morear L. S&batinir N. Targarit V. Tortorella and P.G. zarbonin 633

.........................................................

Photoassistd mechanisms in heterogenecus catalysis: t h e role of surface OH in the deconposition of ethanoic acid on magnesium oxide L. lhrcheset S. Colucciar E. Borellot L. Palmisanor A. Sclafani and M. Schiavello 643

.........................................................

The WA1 0 system: infrared studies L. M.R. m c u t i r S. Colucciar S. Lavagninor A. zecchina, L. bnneviot and M. Che (*)

............................................... Free a d supprted metal clusters: structures and reactivity A. nassCn .............................................................

653 665

The formation of well defined rhodium dicarbonyl and d i n i t r o s y l with rhodium supported on highly dealuminated zeolite Y B. fiessnerr I. Burkhardtr D. Gutschick, A. Zecchina, C. Morterra and G. spot0 677

..............................................................

X

Surface characterization of WSiO catalysts prepred by ion-exchange A. MInfu, J.T. K i s s t G. Sirdrrh and M. Bart&

.......................

685

Methanol deccmpsition on Pd/Th02: relation between activity and surface structure X. l b n t a g n e r R. Boulett E. Freud and J.C. Laralley

...................695

Adsorbateadsorbate interactions a t the surface of polycrystalline monoc l i n i c zirconia C. lbrterrar R. Aschierit V. Palis and E. Borello

703

Adsorption and dissociation of C02 on lanthanide ion promted &A1203 catalysts J.A. OaLinrmlnt I. Carrhosa and R. Alwro

713

.....................

............................

3 A structure determination of the t i l t e d a s t a t e of CO on Fe(001) by X-ray

photoelectron diffraction J. Cbterwaldert G.S. Hermanr R.S. Saikit M. Y&a

....723

and C.S. Fadley

A c-titive

reaction which is able t o make fine distinctions between reacting surfaces I. PaZink6t F. Notheisz and M. Bart&

.................................

729

Activity and characterization of a l k a l i doped Ni/MgO catalysts A. Parmlianat F. Frusterit F. Arenat N. Modello and N. Giordano

.....739

Comparison of a l k a l i pronoters on silica-supported N i and N i (111):chemical statet localization and range of the proapter effect H. Praliardr B. Tardy, J.C. Bertolini and G.A. Martin 749 (*)

Metal sugport interaction R h - h r J.H.A. M a r t a and D.C.

................. KOningsbergeK ....................... 759

Surface acidity of solid acids and superacids: a Fp-IR study of the behaviour of t i t a n i a do@ with @~~s@orict sulphuric, turgstic and rrolyWic acids G. Radst G. Busca and V. Lorenzelli 777

..................................

(*) The nature and reactivity of chemisorbed oxygen and oxide overlayers a t metal surfaces as revealed by photoelectron spzctrosclopy

..........................................................

LGW. & b e r t ~

support effects in test reactions of hexanes on WU02catalysts and on a internetallic c upt3 LG Bmtmt A. DaLISCherr L. Hilaire, W. Muller and G. Maire

787

.............799 the reactivity of diamond-like semiconductor surfaces L .............................................................. 809 Hydrogenation of carbon monoxide over an Ru(OOO1) single crystal surface B. SdrakMt B. S t q l e s t N. Durhill and J.C. V i c k e m ...............817

(x1

&SSO

XI

CO dissociation on Ni(100) studied by Auger electron spectroscopy A. santaul.. C. Astaldit F. Della V a l l e and R. Rose1

...................825 On the nature and the n&r of reaction sites on Pt catalysts A. sdrkbry ............................................................ 835 In€rared study of the effects of &containing conpourds on CO preidsorbed on Pt/Ca&o-Sil J. S&&ny and M. Bart&

.............................................. Iwsitu X-ray study of the solid-state rduction of copper catalysts P.A. S e n a m r M.S.W. Voq and K. Grant .................................

845 855

Catalytic properties and characterization of Law intermetallic corrpound K.S. S b L. Hilaire, F. Le Normand, R. Tourkde, V. Paul-Boncour and A. Percheron-Gu&gan 863

...................................................

Phase-transition irrluced change of electron-surface spin-flip scattering revealed from CESR on metallic L i globules enhdded in n-irridiatd LV

...........................................................

871

Studies of the i n i t i a l stages of the adsorption of W on an extensively oxidised Zn(0001) support A.J. Sift and J.C. V i c k e r m

881

A.

.........................................

Nitric oxide adsorption on (111) and (001) surfaces of diillrond-like crystals. A theoretical study on rodel f i n i t e clusters M. Toscwo and N. RLISSO

.............................................. Surface analysis of t h e activation of getter compounds I. V d e l and L. Schla@ach ............................................ Microstructures of wmr surfaces during electroless c o p p r deposition M. Wmner and K.G. Weil ...............................................

.893 903 911

Variation of optical emissivity during t h e f i r s t stages of the oxidation of tungsten B. Weberr G. Sum Yuenr P. Pigeat and N. Pacia 919 (*)

......................... Reactivity and photodynamics of size-selected m e t a l clusters L. W k t e ..............................................................

925

Surface chemistry of zirconia-supported rhodium carbonyl clusters T. 2erliar A. C a r h t i r S. Marergor S. Martinergo and L. Zarderighi

....943

XPS and Fl'TIR investigation of y-alumina s u p r t e d catalysts derived from and NH Re0 : a comparative study H Re (CO) 3X. &l.dosr A? and L. Guczi

...................................... Author Index ............................................................... d k

Studies i n Surface Science and Catalysis (other volumes in t h e series)

955 965

.....968


XI11

FOREWORD

Between t h e area known as S u r f a c e Science, which m a i n l y d e a l s w i t h s i n g l e c r y s t a l s u r f a c e s , and t h e v a s t area o f t h e s u r f a c e p r o p e r t i e s o f d i s p e r s e d s o l i d s , t h e knowledge o f which i s w i d e l y a p p l i e d i n t h e f i e l d s o f c a t a l y s i s and m a t e r i a l s science, t h e r e i s s t i l l a remarkably wide a l t h o u g h g r a d u a l l y decreasing gap. T h i s b o r d e r l i n e area needs t o be e x p l o r e d as fundamental physico-chemical problems a r e i n v o l v e d . The aim of t h e T r i e s t e m e e t i n g was t o b r i n g t o g e t h e r s p e c i a l i s t s of v a r i o u s o r i g i n s and backgrounds, i n o r d e r t o s t i m u l a t e t h e growth o f o u r knowledge i n t h i s area. We f a v o u r e d a f o r m a t t h a t was comprised o f t h e p r e s e n t a t i o n o f p l e n a r y l e c t u r e s , s h o r t communications, and p o s t e r c o n t r i b u t i o n s on t h e a p p l i c a t i o n s o f p h y s i c a l and t h e o r e t i c a l methods t o " p e r f e c t " and d i s p e r s e d ( m i c r o c r y s t a l 1 i n e and amorphous) m e t a l s , oxides, and mixed systems. S p e c i a l emphasis was g i v e n t o m e t a l - s u p p o r t i n t e r f a c e s . The r e s u l t s a r e p r e s e n t e d i n t h i s book, which is d e d i c a t e d w i t h g r a t i t u t e t o a l l t h e s c i e n t i s t s who p a r t i c i p a t e d i n t h e T r i e s t e meeting on S t r u c t u r e and R e a c t i v i t y o f Surfaces.

XIV

ACKNOWLEDGMENTS The

Organising

"Structure

Commirtee

of

the

and Reactivity of Surfaces"

European

Conference

gratefully

financial support from the following organisations:

B r u k e r Italiana

Cassa di Risparmio di Trieste Chimica del Friuli Consiglio Nazionale delle Ricerche (C.N.R.) Commissar iato del Govern0 Comune di Trieste I EM

Illycaff@ S.p.A. Leybold S.p.A. Lloyd Adriatic0 S.p.0. Montedipe S.p.6. Provincia di Trieste Regione Friuli Venezia Giulia S.A.S.

i l Principe della Torre e Tasso

Sincrotrone Trieste UniversitA degli Studi di Torino Universita degli Studi di Trieste

on

acknowledges

C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfuces 0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

1

PHOTOLUMINESCENCE, PHOTO-INDUCED REACTIVITIES AND CATALYTIC PROPERTIES OF Na- AND Li-DOPED MgO.

M. ANPO*, Y. YAMADA, T. DOI1, I. MATSUURA1, S. COLUCCIA2, A. ZECCHINAZ, and M. CHE3 Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591 ( Japan ) 1Department of Chemistry, Faculty of Science, Toyama University, Gofuku, Toyama, Toyama 930 ( Japan ) 2Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universith di Torino, Corso Massimo dheglio, 48,10125 Torino ( Italy ) 3Laboratoire de RBactivitB de Surface et Structure, Universit6 P. e t M. Curie, UA 1106 CNRS, 4 Place Jussieu, Tour 54,75252Paris Cedex 05 ( France )

ABSTRACT Hydrated MgO-(I) catalysts exhibit photoluminescence at 320-480nm with an absorption band at 230-300nm linked with surface OH groups. Na-doped MgO(I) show a new photoluminescence at approximately 360-450 nm, while the photoluminescence due to OH groups decreases with increasing amounts of Na. Photo-induced reaction of iso-C& on undoped MgO-(I) leads to the formation of iso-CqH10 (hydrogenation), while on Na-doped MgO-(I), CzH4 and C2H6 rather than iso-CrHlo are formed (hydrogenolysis) to a n extent which depends on the amount of Na. Microcrystalline MgO-(II) catalysts doped with small amounts of Li exhibit a high activity for the oxidative coupling of methane. A good parallel is found between the intensity of the new photoluminescence observed with Li-doped MgO-(11) catalysts and their activity for the oxidative coupling. These results suggest that the low coordination surface sites produced by Na or Li doping play a significant role in the photo-induced and catalytic reactions on Na- or Li-doped MgO catalysts. INTRODUCTION It is well established that surface ions of catalysts have quite different properties than bulk lattice ions, due to their lower coordination. Therefore, those surface ions are expected to play a significant role not only in heterogeneous catalysis but also in photocatalysis. Recently, we have shown that intensities and shapes of photoluminescence spectra of oxide catalysts could give useful information on the physicochemical properties of oxide surfaces with low coordination sites (ref. 1).

Anpo et al. (ref. 21, have found that the isomerization of 2-C& proceeds on degassed MgO catalysts under UV-irradiation at 0°C and suggested from photoluminescence studies of such catalysts that four coordinated surface ions play a significant role in the photocatalysed isomerization. Matsuura et al. (ref. 31, have found that crystalline MgO doped with only 1-5Li mol% exhibits a high activity for the oxidative coupling reaction of CH4 into C2 hydrocarbons. On these Li-doped MgO-(11) crystallites, Anpo et al. (ref. 4). have found a good parallel between the intensity of a newly observed photoluminescence at around 450 nm and their activity for oxidative coupling of CHI. They have suggested that low coordination surface sites produced by incorporation of Li into MgO play a significant role in the CHq oxidative coupling reaction. Therefore, it is of special interest to study the nature of the active surface sites and their role in catalytic and photocatalytic reactions by wing a highly sensitive and nondestructive photoluminescence technique. In the present study, we deal with the relation between the photoluminescence properties and the reactivity of Na/iMgo-(I) catalysts for photo-induced hydrogenation reaction of iso-C& and the reactivity of LVMgO-UI) catalysts for the oxidative coupling reaction of CHI.

EXPERIMENTAL Na and Li-doped MgO catalysts were prepared as follows: a specified quantity of Na2CO3 (purity > 99.9%) was added to a suspension of hydrated MgO-(I) powders (supplied from the Catalysis Society of Japan as a standard MgO-2 sample). The dried substances were calcined at 623 K for 5 hs. X-ray diffraction measurements showed no evidence of sodium compounds. A specified quantity of Li2CO3 (purity > 99.0%) was added to a suspension of microcrystalline MgO-(II) (Ube # 500, purity > 99.996; BET surface area 25-40 rnz/g). The dried substances were calcined at 1073 K for 2 hs. X-ray diffraction measurements showed that the undoped original MgO-(11) samples had the pure single crystal structure with particle size of 450-600 A. Surface area measurements indicated that doping MgO-(II) samples with Li decreased the surface area typically from 40 to 1 m2/g at 10 mol 96 Li. Photoluminescence spectra were measured at 77 and 298 K with a Shimadzu RF-501 spectrofluorophotometer equipped with colour filters. Prior to the experiments, catalysts were degassed for 2 hs at the desired temperature. W-irradiation of the catalyst in presence of iso-C& (6 Torr) was carried out a t 273 K using a high pressure mercury lamp (Toshiba SHL-1OOW) through a water filter. The catalytic reaction of methane oxidation was carried out in a fixed bed reactor a t a constant flow rate, 260 d m i n of the gas mixture of C&, 0 2 , and He under a total pressure of one atmosphere (101.325 kPa) and partial pressures of 36 @a for methane and 16.5 kPa for oxygen.

3

RESULTS AND DISCUSSION .. photoluminescence and Dhoto-induced ' reactivltv of Na/MeO-(I) The degassed MgO-(I) sample exhibits a photoluminescence spectrum a t around 320-480nm. Figure 1 shows the photoluminescence spectra (right side) of the MgO-(I) sample degassed at vatious temperatures and their corresponding excitation spectra a t 77 K. The photoluminescence spectrum and its excitation analogue for MgO-(I) are strongly dependent on the degassing temperature. It was also observed that the photoluminescence spectrum is dependent on the excitation energy, i.e., the photoluminescence recorded under excitation at 240 nm shows much higher intensity at around 360 nm, while under excitation a t 280 nm, the intensity is much higher at around 410 nm.

200

2 50

300

300

400

500

Wavelength. nrn

Fig. 1 Photoluminescence and excitation spectra a t 77 K of MgO-(I) powders degassed at various temperatures (in K) ( 1-773,2-873,3-1073,4-1273.Excitation a t 240 nm, monitored emission a t 400 nm). According to previous results (ref. 21, the photoluminescence observed a t around 410 nm is due to surface OH groups. The emission is either from the lowest quartet state of the hydroxyl radicals or from a lower-laying triplet state of the hydroxyl ions. The photoluminescence observed a t 360 nm is associated with the following charge transfer processes on MgO sites involving four coordinated OdC2- ions. hv (I@"042- ) w ( M g + - 04c-)* (1)

4

The photoluminescence spectra of MgO-(I) changes upon doping with Na. For increasing amounts of Na up to 1.0 wt%, the photoluminescence observed under excitation of 240 nm decreases in intensity with a constant h,, at 365 nm. On the other hand, as indicated in Fig. 2, the excitation spectrum monitored at 450 nm, and assigned to the photoluminescence due to surface OH groups, exhibits a drastic change upon doping MgO with Na. For increasing amounts of Na, the excitation spectrum due to surface OH groups decreases in intensity and Am, of the spectrum peak shifts to shorter wavelengths, i.e., from 275 to 250 nm. These changes suggest that the Na ions destroy the emitting surface OH groups.

Y h

h

1

12

200

300

250

Wavelength.

N

Fig. 2. Effect of the amount of doping Na upon the excitation spectra of Na/MgO(I) a t 77 K. (monitored a t 450 nm emission band, amount of Na (in wt%): 1undoped, 2-0.2, 3-0.5,4-1.0, 5-3.0). The W-irradiation of a degassed MgO-(I) sample in presence of iso-C4Hs at 273 K was found to lead to iso-CqH10. This photohydrogenation reaction proceeds only under W-irradiation (h < 300 nm)and its rate is strongly dependent on the degassing temperature of MgO. As shown in Fig. 3, the yield (rate) of photoinduced hydrogenation reaction goes through a maximum a t 873 K before decreasing with further increase of the evacuation temperature.

5

Figure 3 also shows the effect of the degassing temperature upon the relative intensity of the photoluminescence due to surface OH groups. The latter goes through a maximum at 873 K before decreasing with further increase of the evacuation temperature. These changes are in good agreement with those observed with the yield of iso-Cd31o. Figure 3 finally exhibits the relation between the yield of the photo-formed isoC4H10 and the relative number of surface OH groups as deduced from photoluminescence data. Those results suggest that the hydrogen atoms incorporated into iso-CqH10 are supplied

F

Degassing temperature, K

Fig. 3. Effect of the degassing temperature upon the yield of butane formed in the photo-induced reaction of iso-butene on MgO-(I) and upon the intensity of photoluminescence of MgO-(I) due to surface OH groups. from surface OH groups of MgO-(I) catalysts. It is expected that, in its excited state, the 0-H bond weakens to produce H2 according to the process (ref. 5 ) : OH-....-HO t) 0 -.*..-0 -t H2 (2) resulting in the hydrogenation of iso-&Hg into iso-CqHlo. Alternatively, it is possible t h a t H 2 is also produced by a reaction of the type

HH+ Mg2+ 0 2 c) Mg2+ 0 2 - + H2 (3) since the reverse reaction giving hydrides and protons is known on MgO (ref. 6). Figure 4 presents the effect of Na doping upon the photo-induced reaction of isoC4H8 on MgO-(I) samples. For increasing amounts of Na, the yield of photoformed iso-C4Hlo decreases monotoneously while those of C2H4 and C2H6 (hydrogenolysis of iso-C4Hs) increase. The preceding results indicate that the surface OH groups play a significant role not only in the photo-formation of iso-CqH10 from iso-CrH8 on hydrated MgO(I), but also in the reaction with C-C bond fission on UV-irradiated Na/MgO-(I) catalysts

0

0.2

0.4

0.6

0.8

1.0

3.0

Amounts of Ha ions. v t l

Fig. 4. Effect of the amount of Na upon the yield of the photo-induced reactions of iso-butene on Na/MgO-(I).(UV-irradiation a t 275 K, reaction time: 30 min). (&.&$X

. .

.

activitv in methane oxi dative coudinp and Dhotoluminescence

ProDertles of LIJMg0-m Figure 5 illustrates the influence of the amount of Li upon the yield of C2compounds formed, the selectivity, and the oxygen and CH4 conversions on LdMgO-(II) catalysts. The undoped MgO-(11) sample exhibits a lower activity with lower selectivity for Cz-compounds (mainly C2H4 and CzH6). With Li/MgO-

7

(11) catalysts, both the yield and selectivity for Ca-compounds markedly increase with increasing amounts of Li. An optimum activity is observed a t around 3 mol% of Li. For larger Li amounts, the decrease of the catalytic activity might be

attributed to a reduction of the surface area of the catalysts.

ra

3

100

3

>

U U 3

al m

U 4

m

21 4

R

50 C rl

m

$ V 0 N

0

*

I

8

0 0

10

20

Amount of Li ions. molX

Fig. 5. Influence of the amount of Li upon the yield of Ca-compounds and the 02selectivity i n CH4 oxidative coupling on Li/MgO-(11) catalysts ( +: conversion, Ca-conversion, -o- : Cn-yield,- -o- -: Cg-selectivity).

+.

Figure 6 gives the photoluminescence spectrum of undoped MgO-(11) sample (spectrum 1) degassed a t the same temperature as that required for the methane oxidative coupling reaction. The undoped MgO-(II) sample exhibits a characteristic photoluminescence spectrum with Amax at around 360 nm. The excitation band of the emission was observed at around 220-270 nm. As mentioned above, this photoluminescence is due to charge transfer processes (eqn. 1) associated with four coordinated surface sites. Figure 6 also shows the photoluminescence spectrum of the 3 mol% Li-doped MgO-(11) catalyst (spectrum 2) and the corresponding deconvoluted curves (2-a and 2-b). It is clear that, in addition to a characteristic emission band due to MgO-(II), the Li/MgO-(11) catalyst exhibits a new emission with Am, a t around

450 nm (dotted line; 2-b). The intensity of the new emission band was found to

depend upon the amount of Li. The excitation spectrum of this new emisson was observed a t around 260-290 nm. According to previous results (ref. 21, surface sites with a coordination number of four may be associated with this new photoluminescence, on the basis that the lower the coordination number, the lower the absorption energy (ref. 5). -

14

-

12

-

4.13

354

eV 3.10 2.76

2.48

300

350

400

CLOO

. z 10C 3

n L

8 -

0

-in

A

6-

c

z

c

4-

C

2450

Wavelength, nm

Fig. 6. Photoluminescence spectra of MgO-(11) (solid line (111, and of 3 mol% Lidoped MgO-(11) (solid line (2)) and the corresponding deconvoluted curves (dotted lines (2-a) and (2-b)). Excitation wavelength: 240 nm, recording range: 500 mV for line (1) and 200 mV for line (2)). Figure 7 shows the effect of Li upon the relative intensity of the newly observed photoluminescence. For increasing amounts of Li, the intensity of this photoluminescence goes through a maximum at around 3 mol% of Li, before decreasing with further increase of Li. As shown in Fig.7, a good parallel between the intensity of the new photoluminescence and the activity of LiMgO(11) catalysts for methane oxidative coupling is obtained. This suggests that the surface sites with a coordination number of four play a significant role in the oxidative coupling of methane. The addition of methane at 0.1 Ton onto the 3 mol% Li-doped MgO-UI) scarcely quenched photoluminescence in the temperature range of 298 - 573 K. However, it was found that the addition of methane above 723 K quenched the newly observed photoluminescence (2-b in Fig.6). but not the characteristic photoluminescence at around 360 nm (2-a i n Fig. 6). The characteristic

9

photoluminescence of the undoped MgO-(11) material was found to be quenched withCH4 only at temperatures higher than 873K. The additionof @,on the

- 60

0

2

4

6

8

LI content,

1012

14

mol %

Fig. 7. Influence of the amount of Li upon the activity and selectivity of CH4 oxidative coupling , and upon the intensity of the newly observed photoluminescence of Li/MgO-(II) at around 420-520 nm. (temperature: 965 K for catalytic reaction, 77K for photoluminescence measurements, excitation: 240 nm, selectivity: for CZ-compounds)

other hand, led to the quenching of both the characteristic emission and the new emission, even at room temperature. The results of the methane quenching experiments imply that the new low coordination surface sites are more reactive than those of the undoped MgO-(TI) catalyst. Such a high reactivity might be associated with the high reactivity of the Li/MgO-(II) catalysts for the methane oxidative coupling reaction. Lunsford et al. (ref. 71,have detected (Li+-0-) or 0- sites on Li/MgO (7 Li-mol%) by EPR and suggested that those sites play a significant role in the formation of CH3 radicals from CH4. It is not clear a t present whether the newly produced low coordination surface sites are directly associated with the existence of such sites or not. However, the high activity of MgO-(TI) involving a small amount of Li seems to be related to the formation of low coordination surface sites, because the undoped MgO-(II) also exhibits an activity which is much lower than that of Li/MgO-(11) catalysts. It should be mentioned that a further increase of Li from 15 to 30 mol% leads to another maximum in the catalytic activity. The activity of

10

such higher Li-content region might be associated with the existence of (Li+-0-) or 0-sites. CONCLUSIONS MgO-(I) exhibited photoluminescence at around 320-480 nm with its absorption band a t 230-300 nm linked with surface OH groups at the lower coordination surface sites. Na/MgO-(I) showed a new photoluminescence a t 360-450 nm which is associated with newly produced surface sites with much lower coordination, accompanied by a decrease of the photoluminescence due to the surface OH groups. Photo-induced reaction of iso-C4Hs on MgO-(I) led to the formation of iso-C4Hlo. On the other hand, on Na/MgO-(I), C2H4 and CzHs formation occurred primarily, to a n extent depending on the amount of Nadoped. MgO-(11) microcrystals doped with a small amount of Li, exhibited a high activity for the oxidative coupling reaction of CH4. A good parallel between the intensity of newly observed photoluminescence a t 400-500 nm with Li/MgO-(II) catalysts and their activity for the coupling reaction was found. This photoluminescence is associated with newly produced unsaturated surface sites with coordination number lower than four. These results suggest that the newly produced unsaturated surface sites, achieved by doping with Na or Li, play a significant role in both photo-induced and catalytic reactions which take place on Na/MgO-(I) or Li/MgO-(11)catalysts.

REFERENCES 1 M. Anpo and Y. Kubokawa, Rev. Chem. Intermed., 8 (1987) 105-124. 2 M. Anpo, Y. Yamada, Y. Kubokawa, S . Coluccia, A. Zecchina and M. Che, J. Chem. Soc., Faraday Trans. I, 84 (1988) 751-764. 3 I. Matsuura, T. Doi and Y. Utsumi, Chem. Lett., 1987, pp. 1473-1474. 4 M. Anpo, M. Sunamoto, T. Doi and I. Matsuura, Chem. Lett., 1988, pp. 701-704. 5 T. Ito, Ji-Xian Wang, Chiu-Hsun Lin and J. H. Lunsford, J. Am. Chem. Soc.,107 (1985)5062-5067. 6 M. Che and A.J. Tench, Adv. Catal. 31 (1982) 77-133. 7 S. Coluccia, F. Boccuzzi, G. Ghiotti and C. Mirra, Z. Phys. Chem. NF 121 (1980)141-146. Acknowledgments: M. Anpo thanks the Universite P. et M. Curie for a n appointment as Professeur Associe in 1988 and M.Che the JSPS for an invitation which gave him the opportunity to work on this project in Japan in 1986.

C. Morterra,A.Zecchina and G . Costa (Editors),Structure and Reactiuity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam Printed in The Netherlands

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THE CHARACTERISATION OF SUPPORTED METAL CATALYSTS BY THE DETERMINATION OF DIFFERENTIAL HEATS OF ADSORPTION P N AUKETT BP Research Centre, Sunbury-on-Thames, Middlesex, England

ABSTRACT Central to understanding the mechanism of catalysis is a knowledge of the number and strength of the catalytically active sites. Only microcalorimetry provides a direct measurement of the heat of adsorption as a function of coverage at the temperature of interest. The adsorption of carbon monoxide and hydrogen on a reference catalyst, EUROPT-1, has been studied to illustrate the application of this method. Microcalorimetry can readily distinguish between reversible and irreversible adsorption and is a particularly powerful technique for investigating the heterogeneity of the surface adsorption sites. For adsorption of carbon monoxide and hydrogen on EUROPT-1 the surface appears surprisingly homogeneous. INTRODUCTION In a heterogeneous reaction, the metal function acts as a catalyst by adsorbing reactant molecules. The electronic structure of the adsorbed species is modified by electron transfer between the adsorbent and adsorbate. Clearly, the magnitude of this interaction between adsorbent and adsorbate is an important parameter in understanding the mechanism of catalysis. Ideally, the heat of adsorption as a function of coverage is needed. This is termed the differential heat of adsorption (1). A number of methods are available, such as temperature programmed desorption and isosteric measurements. However, only microcalorimetry provides a direct measurement of the heat of adsorption at the temperature of interest (2). In particular, no assumptions about the thermodynamic equilibrium of the system need to be made and a model of the adsorption/desorption process is not required. The only disadvantage is the elapsed time for accurate microcalorimetric measurements, which is necessarily long. Ideally, when attempting t o understand the mechanism of a catalytic reaction, heats of adsorption of the actual reactants, products and intermediates should be determined. Also, i t is increasingly being recognised that synergy between adsorbed components can have an important bearing on the adsorption process and hence also on the catalytic mechanism. For instance, the adsorption sites for both carbon monoxide and hydrogen on rhodium(100) are

11

12

different in a coadsorption system compared to the single species adsorption ( 3 ) . Microcalorimetry should be a particularly sensitive technique for investigating interactions between adsorbates, since any synergy will be immediately apparent in a modified heat of adsorption. This paper describes experiments carried out on a silica supported platinum catalyst. Heats of adsorption of carbon monoxide and hydrogen have been determined as a function of coverage to illustrate the application of this technique. EXPERIMENTAL Catalyst Sample The reference catalyst EUROPT-1 was used in these experiments. This platinum on silica catalyst was manufactured by Johnson Matthey for the Council of Europe's Research Group on Catalysis. It was distributed t o participating laboratories for characterisation of its chemical composition, total surface area, the size distribution of the platinum particles, and chemisorption properties. The results of this round-robin exercise have been published in a series of papers (4-8) and this catalyst is now available as a well characterised reference material. The platinum loading is 6 . 3 wtX. Apparatus Differential heats of adsorption were determined using a glass volumetric gas dosing system interfaced to a Setaram BT2.15 microcalorimeter (Fig. 1). Using this apparatus the adsorption isotherm was determined simultaneously with the adsorption heat. The BT2.15 is a Calvet type heat-flow calorimeter

lei Flgure 1. Schematic of apparatus.

-- Measurement cell Reference cell 3 - Catalyst bed 4 - Outlet for reduction gas 5 - Gas inlet 6 - Dosing volume 7 - Sorbate reservoir Icha pumps

1 2

I

Microcalorimeter

1

13

in which there is a well defined path for heat to flow from the sample to a thermostatted block of large heat capacity. Experiments were therefore carried out under isothermal conditions at a temperature of 25C. The sample was supported on a glass sinter in the calorimeter cell. This allowed for in-situ reduction by a flow of hydrogen through the catalyst bed, followed by a static adsorption measurement. Procedure A sample of EUROPT-1 (ca. 0.4 g) was reduced by heating to 400C over two hours in a stream of hydrogen (50 ml/min). The catalyst was held at 400C for two hours to ensure complete reduction. To remove residual hydrogen, the cell was then evacuated (ca. torr) for one hour while the temperature was maintained at 400C in order to remove residual adsorbed hydrogen. Finally, the sample was cooled to 25C. The surface of the catalyst was progressively covered with adsorbate by increasing the pressure in steps. For each step, dividing the heat evolved by the number of adsorbed molecules gives the differential heat of adsorption (kJ/mol of sorbate) at that coverage. Once saturation had been reached, as shown by little heat evolution for large changes in pressure, the system was evacuated to determine the heat of desorption. This distinguishes between reversible and irreversible adsorption at 25C. RESULTS AND DISCUSSION Carbon Monoxide The adsorption isotherm is shown in Pig. 2. The higher pressure portion of the isotherm is not horizontal, indicating a small degree of reversible adsorption. An estimate of the monolayer coverage can be obtained by extrapolating the high pressure region of the isatherm back to zero pressure. This gives a value of 1.0 x lozo molecules/g. Assuming an adsorption stoichiometry of one carbon monoxide molecule to one surface platinum atom, this gives a platinum dispersion of 51%. This is slightly lower than other published results (which were in the region of 58-63%), but this could be due to the presssure range over which the isotherm has been extrapolated. Since mainly irreversible adsorption was of interest in this experiment, only adsorption up to 1-2 torr was considered as opposed to 100-200 torr in other work (8). The value of 1.0 x 1020 for the degree of irreversible adsorption is confirmed by the heat of adsorption measurements (Fig. 3 ) . The total heat

I1

O.!

0

co

8

H2

0.01 0

1

1

2

3

4

5

6

Pressure torr

Figure 2. Adsorption isotherms for EUROPT-1.

of adsorption minus the heat of desorption is 24 ( + / - 0.5) J/g.

On the basis

of last molecule in, first molecule out, this corresponds to an irreversible

adsorption of 1.0 x 1020. The differential heat of adsorption of carbon monoxide is remarkably constant over a wide range of coverage (Fig. 4 ) . The surface appears almost completely homogeneous. It is only when significant reversible adsorption takes place that the heat of adsorption falls. The plateau value of 160 kJ/mole compares well with that determined on other polycrystalline surfces (9). Adsorption of Hydrogen A s observed €or carbon monoxide, there is a significant amount of reversible adsorption apparent from the adsorption isotherm (Fig. 2 ) . Extrapolating the high pressure region back t o zero yields a value of 1.3 x 1020 molecules/g for the amount of irreversible adsorption. This is confirmed by the heat of adsorption/desorption measurements (Fig 5).

15

0.2

0.0

0.4

0.6

1.0

0.8

1.2

Number adsorbed 102'/g

Figure 3. Sorption heats for carbon monoxide on EUROPT-1.

1801

a CO

Y I

130-

T

T

T

1

I

H2

T

T Y 80 . I -

Q

0,

I

30

1

I

0.5

I

1.o

I

1.5

Number adsorbed 102'/g

Figure 4. Differential heats of adsorption on EUROPT-1

16

301

0 Adsorption

I

- -(Ads)-(Des)

I

heat

00.0

0.5

1.o

1.5

Number adsorbed 102*/g

Figure 5. Sorption heats for hydrogen on EUROPT-1. As noted by many authors, this corresponds to a dispersion of 1 3 4 % , ie more than one hydrogen atom per platinum atom if an adsorption stoichoimetry of 1:l is assumed (7). Temperature programmed desorption studies on this catalyst revealed the presence of up to four states of adsorbed hydrogen (10). Only one of these is due to hydrogen atoms adsorbed at surface platinum atoms. A l s o observed at room temperature are irreversible processes associated with the supported metal interaction and conventional spillover of hydrogen o n t o the support (7). The differential heat of adsorption of hydrogen is ca. 110 kJ/mole which compares well vith values for dissociative adsorption on platinum (9). As observed for carbon monoxide, the heat of adsorption is constant whilst irreversible adsorption i s taking place. This is in contradiction to the model proposed by Frennet and co-workers where the heat of adsorption is assumed to fall from 92 kJ/mole at 2.5 :< IC,’’’ molecules/g adsorbed, to 46 kJ/mole at 5.5 x 1019 molecules/g adsorbed (11). These values were determined from desorption isotherms at a series of temperatures. This discrepancy then probably reflects a difference in the energetics of adsorption and desorption.

17

CONCLUSIONS Heats of sorption can be used to distinguish between reversible and irreversible adsorption. For the sorbentlsorbate systems in this paper the extent of irreversible adsorption determined by microcalorimetry is the same as that determined by extrapolation of the adsorption isotherm back to zero pressure. Microcalorimetry is a particularly powerful technique for investigating the heterogeneity of the surface adsorption sites. For the adsorption of carbon monoxide and hydrogen on EUROPT-1 the surface appears surprisingly homogeneous. Heats of sorption will be particularly valuable in investigating synergy between adsorbed components. To do this it is necessary to compare the heat of adsorption predicted by the single adsorbate experiments with that determined experimentally. Chemical analysis of the gas phase is needed to determine the composition of the surface phase by difference. REFERENCES 1. P.C. Gravelle, Adv. Catalysis, 22 (1972) 191-260. 2. G. Della Gatta, Thermochimica Acta., 96 (1985) 349-363. 3. H.S. Tan, A. Morawski and W.E Jones, Surface Sci., 195 (1988) L193-L198. 4. G.C. Bond and P.B. Wells, Applied Catalysis, 18 (1985) 221-224. 5. G.C. Bond and P.B. Wells, Applied Catalysis, 18 (1985) 225-230. 6. J.W. Geus and P.B. Wells, Applied Catalysis, 18 (1985) 231-242. 7. A. Frennet and P.B. Wells, Applied Catalysis, 18 (1985) 243-257. 8. P.B. Wells, Applied Catalysis, 18 (1985) 159-272. 9. G.A. Somorjai, Catal. Rev. Sci. Eng., 19 (1979) 105-159. 10. R. Kramer and M. Andre, J. Catal., 58 (1979) 287-292. 11. A. Crucq, L. Degols, G. Lienard and A. Frennet, Acta. Chim. Acad. Sci. Hung., 111 (1982) 547-571. ACKNOWLEDGEMENT Permission of the British Petroleum Company plc to publish this paper is gratefully acknowledged.


C . Morterra, A. Zecchina and G . Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

19

MODEL STUDIES OF THE SMSI PHENOMENON AT THE TiOx/Ru(000 1) INTERFACE

JAS PAL S. BADYALI, ANDREW J. GELLMAN2, ROBERT W. JUDD3 and RICHARD M. LAMBERTl IDepartment of Chemistry, University of Cambridge, Cambridge, CB2 IEP, UK. 2Permanent address: School of Chemical Sciences, University of Illinois at UrbanaChampaign, 505 S. Mathews Avenue, Urbana, IL 61801, USA. 3British Gas plc, Research & Technology Division, London Research Station, Michael Road, London SW6 ZAD, TJK.

ABSTRACT Surface phenomena pertinent to SMSI behaviour exhibited by supported metal catalysts have been examined using a well characterised model system. LEED, AES, XPS and chemisorption of CO and H2 have been used to investigate the growth morphology, structure and chemical properties of TiOx films on Ru(OOO1) as a function of oxide loading and temperature. The properties of such films is critically dependent on the method of preparation. Blocking of metal adsorption sites by highly dispersed TiO, species is believed to occur in the case of submonolayer films deposited at room temperature. INTRODUCTION The existence of the 'Strong Metal-Support Interaction' (SMSI), between Group VIII metals and titania has been the subject of considerable interest since it was first reported that such catalysts can exhibit specific activities for CO hydrogenation which are substantially greater than those for silica- or alumina-supported metals, whilst their ability to adsorb H2 or CO is diminished (refs. 1-2). Recently, experimental evidence from EXAFS (ref. 3), Rutherford backscattering (ref. 4), TEM (ref. 5), Auger electron spectroscopy (ref. 6), HREELS (ref. 71,UPS and XPS (refs. 8,9), and SIMS (ref. 10) has suggested that support migration onto the metal particle occurs, resulting in both geometric and electronic perturbation of the active metal surface (refs. 11-12). The effects of decorating metal surfaces by TiO, moieties have been studied using metal foils and single crystals (refs. 13-15). However, the nature of the titanium species which migrates (metallic or oxidized) and the type of bonding which exists between this species and the group Vm metal remain as unresolved issues.

We report here on the modelling of such systems by investigating TiO, species adsorbed onto the basal plane of ruthenium, and the effect of these moieties on the chemisorptive behaviour of H2 and CO. The basal face of ruthenium has been chosen as the model system since ruthenium catalyst particles are reported to expose (0001) faces preferentially (ref. 16). EXPERIMENTAL PROCEDURE Measurements were made in two separate ultra high vacuum chambers of conventional design which were capable of routinely achieving base pressures of c: 2 x 10-11 Torr. The first system included a multiplexed mass spectrometer for temperature programmed desorption (TPD) measurements and a 3-grid retarding field analyser for LEED/Auger measurements. The mass spectrometer ioniser was enclosed in a shield which effectively discriminated against the scattered gas signal from the rear face of the specimen. The resulting TPD signals were therefore dominated by the line-of-sight flux from the crystal front face, and effects due to finite pumping speed were thereby also minimised. XPS measurements were made in the second chamber which was a VG ADES 400 system incorporating a twin-anode (Mg/Al) X-ray source and a rotatable hemispherical analyser. All XP spectra were referenced with respect to the Fermi level and calibrated against the Ru(3d5/2) feature at 279.9 eV (ref. 17). Gas exposures were carried out using a capillary array doser; a collimated, resistively heated evaporation source was employed for titanium dosing. The Ru(0001) specimen was prepared from a 99.99 +% pure ingot by standard methods and mounted onto a sample holder which could be cooled to 140 K and resistively heated using a programmable power supply. Cleaning was acheived (ref. 18)by heating to 1350 K in 10-7 Torr oxygen, followed by flashing to 1550 K in ultra high vacuum to remove traces of subsurface oxygen (ref. 19). Extreme care was taken to ensure that in all experiments the sample was free from dissolved titanium; this was achieved by leaching out bulk titanium using repeated cycles of heating in oxygen (1250 K/lO-7 Torr) and Ar+ etching until the Auger spectrum characteristic of clean ruthenium was obtained (ref. 20). In this connection, we have already reported in detail (ref. 20) on the assignment of certain significant features which appear in the Auger spectrum of Ru(0001). As explained in reference 20, these features may be variously assigned to Ru Auger transitions, diffraction features, or the presence of impurity titanium. The Auger and diffraction features were identified and it was shown that signals from impurity titanium were undetectable with a rigorously cleaned specimen. All gas exposures were corrected for ion gauge sensitivities (ref. 21). RESULTS AND ANALYSIS TiO,- Deposition on Ru(0001)

The method of preparation of the model surface is of great importance, and by using appropriate procedures (ref. 22) we ensured that no alloying occurred between

21

surface TiOx and the ruthenium single crystal substrate. A possible method of preparing adsorbed TiOx moieties is to deposit a certain amount of titanium and then expose it to oxygen. Fig. 1 shows the uptake of oxygen on a titanium monolayer at room temperature as monitored by the O(510 eV) AES signal. There are two regimes: the initial rapid uptake corresponds to oxygen chemisorption on the titanium film; this is followed by a slower process which involves migration of some oxygen to subsurface sites. XPS measurements show that under these conditions the titanium is not fully oxidized; the Ti(2p3/2) peak shifts from 455.2 eV to 456.6 eV on exposure of 4000L 0 2 to a IML predeposited titanium film, Fig. 2a-c, compared to the reported value of 458.8 If: 0.1 eV for bulk Ti02 (ref. 23).

8o

1

0

I

2

4

6

8

10

450 0 454 0 458 0 462 0 466.0

0, Exposure/Langmuirs

Fig. 1(left): O(510 eV) AES uptake of 02 on 1.OML Ti/Ru(0001) Fig. 2 (right): XP spectrum at room temperature of (a) clean Ru(OOO1); (b) 1.OML Ti/Ru(0001); (c) 4000L 0 2 exposure to (b); (d) 1.OML 'as-deposited TiOx/Ru(OOO1). Note that the large feature around 462 eV is mainly due to the Ru(3p312) level (with some contribution from Ti(2pl/2) in (b,c,d)).

In order to overcome limitations due to oxygen migration (ref. 24) an alternative method was developed in which titanium was deposited in a background pressure of 1x10-6 Torr of oxygen at room temperature; the oxygen doser was always turned on before each titanium dose and shut off after completion of metal evaporation. The formation of adsorbed TiOx must involve a surface reaction under these conditions (mean free path at 1x10-6 Torr is 45 m (ref. W)). At a pressure of 1x10-6 Torr the rate of adsorption is approximately one monolayer of oxygen every 1.5 seconds (ref. 261, whereas the deposition rate of titanium was approximately a monolayer every 15 minutes. Hence every titanium atom should adsorb in the vicinity of an oxygen atom and in addition should experience a sufficient flux of oxygen molecules to ensure complete oxidation, and indeed XPS measurements show this to be the case (Fig. 2(d)).

22

Growth Mode of TiO,- on Ru(0001) Fig. 3 shows the AES signals of Ru(231 eV), Ti(387 eV) and O(510 eV) versus deposition time; care was taken to minimise the effects of electron irradiation on the intensity of the O(510 eV) AES signal. Breaks are evident at regular intervals, and this behaviour is characteristic of monolayer-by-monolayer growth (Frank-van der Merwe growth (ref. 27)). The ratio of the Ti(387 eV) AES signal to the Ru(231 eV) AES signal at the first breakpoint is 0.38 0.02, a value which is significantly larger than that for titanium metal deposition on Ru(0001) (0.31 0.03) (ref. 24), because of the additional attenuation of the substrate signal by oxygen atoms. A monolayer coverage of TiO, on

*

*

Ru(OOO1) exhibits a weak (1 x 1) LEED pattern which sharpens up on annealing to 400 K, consistent with the presence of an in-registry TiO, overlayer; this point will be addressed below. ~

~

~

~

_

_

_7 160 _

Fig. 3 (left): Growth mode of TiO, on Ru(0001) as determined by AES at 295 K. Fig. 4 (right): Excited H2 cleaning of 0.5ML O/Ru(0001) as followed by the O(510 eV) AES signal. Selective Removal of Oxveen from the Bare Ruthenium Surface In the past, one of the major difficulties which arises when attempting to model the SMSI effect using this type of approach has been the procedure used to selectively remove oxygen adsorbed on the bare metal patches, without disturbing the oxygen associated with the TiO, species (refs. 14,15). In the present work this problem was overcome by using a hot cathode ion gun operating in a background pressure of 2 x 10-6 Torr of hydrogen, and in line of sight with the specimen, the latter being held at 575 K (no alloying occurs under these conditions (ref. 28)). This arrangement provides a flux of hydrogen ions (and excited molecules) (ref. 29) which are very effective in removing oxygen chemisorbed on bare ruthenium. Fig. 4 shows the effect of this procedure on Ru(OOO1) saturated with oxygen; the exponential decay is characteristic of a first order rate process, as might be expected. On repeating this cleaning procedure for a deposited full monolayer of TiO,, no detectable change was observed in the O(510 eV), Ti(387 eV) or Ru(231 eV) AES signals. It is therefore apparent that this technique can be successfully employed to remove ( 0 ) a d species selectively from Ru metal

23

without significantly affecting the composition of the TiO, film. This cleaning procedure had the additional beneficial effect of allowing selective masking of the back face of the specimen by (O)& thereby suppressing H2 and CO desorption from the back face during TPD measurements. Structure and Stoichiometrv of the TiOx - Film Under ultra high vacuum conditions, a saturation exposure of oxygen to the Ru(OOO1) surface leads to a (2 x 2) LEED pattern and an 0:Ru ratio of 0.5, (ref. 30). This information can be used in conjunction with the known properties of a range of titanium oxides to draw inferences about the structure and stoichiometry of the TiOx film on Ru(OOO1). Titanium monoxide (TiO) has the NaCl structure, and Table 1 lists the lattice parameters for a range of defect oxides which exhibit non-stoichiometry due to either anionic or cationic vacancies. TABLE 1 Lattice parameters of non-stoichiometric titanium oxides (ref. 31) Oxide

Lattice Parameter a/A 4.1850 4.1780 4.1766 4.1733 4.1689 4.1661

The number of oxygen ions per unit area in the (111)planes of these oxides is plotted in Fig. 5 as a function of the non-stoichiometric parameter (x), and Fig. 6 shows the O(510 eV) AES signal observed before and after hydrogen-cleaning the surface as a function of TiOx coverage. Each point was taken for a freshly prepared TiOx film, thus ensuring ,that no electron beam damage occurred. By using Fig. 5 and cross-calibrating the amount of oxygen in a complete monolayer of the TiOx film (using Auger data for the Ru(0001)-(2x 2) 0 structure), the stoichiometry of the TiOx overlayer is found to be consistent with the presence of a structure composed of a (111) sheet of Ti interleaved between two (111)layers of oxygen atoms. The important point to emerge from this procedure is that the structure and stoichiometry of the TiOx film lie close to that of the bulk monoxide (TiO) rather than those of TiO2. In fact, in the monolayer regime, the Ti:O stoichiometry i s Ti02 since the structure corresponds to Ru-0-Ti-0. Thus in the monolayer regime Ti atoms are expected to be in a higher oxidation state than in the bulk oxide TiO; this in turn this will cause the

24

lattice parameter ofthe monolayer filmto shrink, facilitatingregistry with the Ru(OOO1) surface mesh in agreement with the LEED observations.

-

70

6s

5:

60

\ X

55

50

0.7

o . ~0 . 0

1.0

1.1

1.2

1.3 1 . 4

Oxide S t o i c h i o m a t r y . x

0

0.2

0.4

0.8

0.8

1.0

1.2

1.d

T i O , Covmragm/Wonolaycrm

Fig. 5 (left): Number of oxygen ions per unit area in the close packed planes of the defective titanium monoxides (Tiox) as a function of stoichimetry parameter. Fig.6 (right): Variation with TiO, coverage of the a 5 1 0 eV) AES s&al before and after hydrogen cleaning the surface.

For the 'as-deposited' Ti& film the Ti(Zp3/2) binding energy was 459.2eV (Fig.2d), slightly higher than that found for bulk T e i (ref. 231, and entirely consistent with the model proposed above for the monolayer TiOx film. Thus for the monolayer sheet of "Tiof lying on top of the ruthenium substrate, the presence of Ru atoms causes the Ti atoms to carry a somewhat larger net positive charge than they do in bulk T i@ (ref. 23). It is noteworthy that in this sandwich configuration the number density of oxygen atoms in the Ru contact layer is twice that observed for the Ru(OOOl)/oxygen interface under UHV conditions (ref. 30). This is in keeping with the observation that the adsorbed oxygen/surfaceRu ratio can be doubled at elevated oxygen pressures (ref. 32). CO Chemisomtion The effect of TiOx on neighbouring Ru sites was investigated using a near saturation dose of 44L of CO at 295 K and following the subsequent "PD behaviour of the chemisorbed CO. The change in "I'D behaviour as a function of TiO, coverage is shown in Figs. 7a and 7b from which it can be seen that no new desorption features

25

appear and that the TiO, exerts a simple site-blocking effect. A slight shift of the peaks towards lower temperature is seen with increasing TiO, coverage; this need not be an electronic effect, but could be ascribed to destabilisation of CO islands due to reduction in their size. Dosing at 140 K did not lead to any new features, and use of a 1:l mixture of 13C16O and 12C180 gave no evidence for any isotopic scrambling in the desorbing gas. This simple site-blocking effect is therefore consistent with a geometric effect with no apparent electronic effect arising from the presence of the TiOx moieties.

0.7

1

,

I

I

I

i

I

350 400 450 500 550 600 650 700 TEMPERATURE/K 0

0 2

0.4

0.6

0.8

1.0

1.2

1.4

TiO, C o v e r a g e / M o n o l a y e r s

Fig. 7a (left):TPD spectra of 44L CO doses at room temperature as a function of TiOx coverage. Fig. 7b (right): CO desorption yield per surface ruthenium atom, (8) as a function of TiOx coverage. H2 -Chemisorption

Thermal desorption data following a saturation dose of H2 on clean Ru(OOO1) at 140 K are in accord with our earlier work (ref. 24). Two poorly resolved peaks are evident, characteristic of two different types of chemisorbed atomic hydrogen species. Peak shifts towards lower temperature are seen with increasing TiO, coverage in the TPD spectra following H2 adsorption at 140 K (Fig. 8a); the presence of TiO, also leads to a remarkable decrease in the amount of H2 desorption (Fig. 8b). This behaviour can

also be simply understood in terms of a geometric effect if an initial ensemble of the

Ru atoms is required for the dissociative chemisorption of H2.

Fig. 8a (left): TPD spectra following 200L H2 doses at 140 K as a function of TiO, coverage. Fig. 8b (right): Hydrogen desorption yield (0) defined as hydrogen atoms per surface ruthenium atom as a function of TiO, coverage. Hz+CO Chemisorption

Peebles et al. (ref. 33) have suggested that coadsorption of CO and H2 on Ru(OOO1) leads to the formation of segregated islands of the two adsorbates. This important point was investigated further by the following method. A mixture of 6%CO and 94%H2 was used for competitive coadsorption studies, a high fraction of H2 being employed to compensate for its lower sticking probability, thus enabling approximately equal amounts of the two gases to adsorb. Dosing of this CO/H2 mixture onto clean

Ru(0001) gave identical CO TPD behaviour (Fig. 9a), as that reported for CO/Ru(OOOl), (ref. 24). This is to be expected given that H2 desorption has already occurred before the sample reaches temperatures characteristic of CO desorption. With increasing total coverage, the H2 peaks shift to lower temperatures (Fig. 9b); which would be consistent with repulsive (CO)ad-(H)ad interaction at the edges of the surface islands. Most importantly, it can be seen that for most of the total coverage range the fractional (C0)ad and (H)ad coverages show a linear relationship (Fig. 9c); this verifies the segregated islands model (ref. 33) for CO/H2 coadsorption on clean Ru(0001) (see below). At very low exposures, there is a relatively higher (H),d coverage than observed in the linear region; some enhancement of the relative H2 sticking probability is to be expected at low coverages because unfavourable effects due to limited precursor state lifetime and ensemble requirements are minimised. The independent behaviour of CO and H2 on coadsorption can be used to probe the effect of TiO, on CO and H2 chemisorption for submonolayer coverages of TiO,.

27

- -.._ .

+.-..1 OL

.

..---.

__,

._ /

'

200

5 OL

'-.,

-i--u-.-

l

'

I

.

400

300

l

0 1L

'

500

I

200 Temperature/K

'

l

300

~

400

l

-

,

'

500

Temoerature/K

Fig. 9a (top left):CO TPD spectra for 1OL exposure of a 6%cO/94%H2gas mixture at 140 K on clean Ru(OOOl), (note that the low CO content in this mixture leads to only the high temperature CO feature, which is characteristic of CO/Ru(OOOl)). Fig. 9b (top right): H2 TPD spectra for 1OL exposure of a 6%C0/94%H2gas mixture at 140 K on clean Ru(OOO1). Fig. 9c (left): Relationship between (CO)ad and (H)ad per surface ruthenium atom in competitive adsorption.

0

0

0.1

0.2

Fractional

I

I

I

0.3

0.4

0 5

H,,

Coverage 8 ,

H2 TPD behaviour for the COIH2 mixture is similar to that observed for the H2/TiOx/Ru(0001) system, Fig. IOa-b. However, the amount of CO desorbing increases for low coverages of TiO,, Fig. 1Oc-d, this can be attributed to an associated decrease in H2 chemisorption; the eventual fall in CO uptake at high TiOx coverages is due to site blocking as found for the CO/TiOx/Ru(OOO1) system.

.

__........_...&.-..._...I

.................. -r.-___

200

~

7 - 7 T 300 400

Sernoeratb-e/u

. . . . . .

_ _ _ ........

3

..... ".-

.:.. *... ......

'ijl'

7

! OMLi

500

O : ]0 \ ,

0 2

TiO,

0 4

0 6

0 8

*

1 0

Coverage/Monolayers

Fig. 10a (left): H2 TPD spectra for 1OL exposure of a 6%C0/94%H2gas mixture at 140 K for TiOx/Ru(OOO1). Fig. lob (right): Hydrogen desorption yield defined as hydrogen atoms per surface ruthenium atom, (0) as a function of TiO, coverage

TiO, C o v e r a g e / M o n o l a y e r s

Fig. 1Oc (left): CO TPD spectra for IOL exposure of a 6%C0/94%H2 gas mixture at 140 K for TiO,/Ru(0001). Fig. 1Od (right): CO desorption yield per surface ruthenium atom (0) as a function of TiO, coverage.

29

DISCUSSION The observed layer-by-layer growth of the TiO, film on Ru(0001) is consistent with TiO, moieties ’wetting’ the metal surface in a manner which resembles the behaviour of catalyst particles in the SMSI state. It would appear that by preparing a true model of the postulated decoration effect of metal catalysts in the SMSI state (i.e. no alloying between metal and TiO, species) we have duplicated the reported SMSI behaviour regarding CO and H2 chemisorption on supported catalysts. CO chemisorption is blocked in a simple linear fashion with respect to TiO, coverage, but a more dramatic effect is observed for H2 chemisorption; this can be attributed to the fact that an ensemble of surface ruthenium atoms may be prerequisite for dissociative H2 chemisorption. CO is believed to bond upright at all coverages with the carbon end attached to individual surface ruthenium atoms so that such ensemble effects are not expected to be of significance (refs. 34-36). The linear CO:H2 chemisorption behaviour for CO/H2 mixtures on clean Ru(0001) confirms the postulated island segregation model, since the formation of a mixed layer would selectively hinder H2 chemisorption through the operation of ensemble requirements. On this basis the very pronounced non-linear suppression of H2 chemisorption by TiO, can be attributed to a very hiehlv dispersed immobile TiO, species on the Ru(0001) surface. This contrasts with the hydrogen uptake behaviour exhibited by the CO+H2/Ru(0001) system in the absence of TiO, where CO mobility and concomitant island formation have much larger bare patches of Ru available for H2 chemisorption. The high dispersion of TiO, produced in this manner on the Ru(OOO1) surface at room temperature is consistent with an initial growth mode in which preadsorbed oxygen atoms on the Ru(0001) surface act as ‘anchors’ for the incident titanium atoms. CONCLUSIONS Coadsorption of titanium and oxygen under ultra high vacuum conditions yields a TiO, species of similar structure to bulk TiO, however at the metal-netal oxide interface a complete monolayer of this structure has in effect the stoichiometry of Ti02. It is possible selectively to deposit TiO, moieties which reside on the Ru surface rather than partially diffuse into the bulk. These TiO, moieties have a similar effect on H2 and CO chemisorption as reported for Ti02 supported catalysts in the SMSI state. CO+H2/Ru(0001) chemisorption experiments indicate that at submonolayer coverages these TiO, species are highly dispersed and that simple site blocking is responsible for their effect on CO chemisorption; however the loss of ensembles of surface ruthenium atoms hinders H2 adsorption in a more severe manner. ACKNOWLEDGEMENTS JPSB acknowledges financial support by the SERC and BP Research Company plc under CASE Studentship No. CB020. We are grateful to Johnson Matthey Ltd for a loan of precious metals.

30

REFERENCES M.A. Vannice, C.C. Twu and S.H. Moon, J. Cat., 79 (1983) 70-80. 1 2 G.C. Bond and R. Burch, Specialist Periodical Reports, Royal Society of Chemistry, Catalysis, 6 (1982)27-60. S. S&ellson, M. McMillan and G.L. Haller, J. Phys. Chem., 90 (1986) 1733-1736. 3 J.A. Cairns, J.E.E. Baglin, G.J. Clark and J.F. Ziegler, J. Cat., 83 (1983)301-314. 4 5 B.R. Powell and S.E. Wittington, J. Cat., 81 (1983) 382-393. D.N. Belton, Y.-M. Sun and J.M. White, J. Phys. Chem., 88 (1984) 5172-5176. 6 S. Takatani and Y.W. Chung, J. Cat., 90 (1984) 75-83. 7 H.R. Sadeghi and V.E. Henrich, J. Cat., 87 (1984) 279-282. 8 T. Huizinga, H.F.J. van't Blik, J.C. Vis and R. Prins, Surface Sci., 135 (1983) 5809 596. 10 D.N. Belton, Y.-M. Sun and J.M. White, J. Am. Chem. Soc., 106 (1984) 3059-3060. 11 D.E. Resasco and G.L. Haller, J. Cat., 82 (1983) 279-288. 12 J. Santos, J. Phillips and J.A. Dumesic, J. Cat., 81 (1983) 147-167. 13 Y.W. Chung, G. Xiong and C.C. Kao, J. Cat., 85 (1984) 237-243. 14 K.A. Demmin, C.S. KO and R.J. Gorte, J. Phys. Chem., 89 (1985) 1151-1154. 15 M.E. Levin, M. Salmeron, A.T. Bell and G.A. Somorjai, J. Chem. Zoc., Faraday Trans. I, 83 (1987) 2061-2069. 16 E. Guglielminotti, G. Spoto and A. Zecchina, Surface Sci., 161 (1985) 202-220. 17 J.C. Fuggle and N. Martensson, J. Electron Spectrosc., 21 (1980) 275-281. 18 G.E. Thomas and W.H. Weinberg, J. Chem. Phys., 70 (1979) 1437-1439. 19 G. Praline, B.E. Koel, H.-I. Lee and J.M. White, Appl. Surface Sci., 5 (1980) 296-312, 20 J.P.S. Badyal, A.J. Gellman and R.M. Lambert, Surface Sci., 188 (1987) 557-562. 21. L. Holland, W. Steckelmacher and J. Yarwood, Vacuum Manual p.52 (E. and F.N. Spon Ltd., 1974). 22 J.P.S. Badyal, A.J. Gellman, R.W. Judd and R.M. Lambert, Cat. Letts., 1 (1988) 41-50.

23

24 25 26 27 28 29 30 31 32 33

31 35 36

A.F. Carley, P.R. Chalker, J.C. Riviere and M.W. Roberts, J. Chem. Soc., Faraday Trans. I, 83 (1987)351-370. J.P.S. Badyal, A.J. Gellman and R.M. Lambert, J. Cat., 111 (1988) 383-396. W.J. Moore, Physical Chemistry, 5th Edn., (1981) p.150, (Longman). G. Ertl and J. Kuppers, Low Energy Electrons and Surface Chemistry, 2nd Edn., (1985)p.3, (VCH). F.C. Frank and J.H. van der Merwe, Prof. Roy. Soc. (London) A198 (1949) 205-225; A200 (1950) 125-134. J.P.S. Badyal, A.J. Gellman and R.M. Lambert, In Preparation. S.C. Brown, Basic Data of Plasma Physics (MIT), (1966). S.L. Parrott, G. Praline, B.E. Koel, J.M. White and T.N. Taylor, J. Chem. Phys., 71 (1979) 3352-3354. J.D.H. Donnay, G. Donnay, E.G. Cox, 0. Kennard and M.V. King, Crystal Data Determinative Tables, 2nd Edition, (1963), American Crystallographic Society. C.H.F. Peden and D.W. Goodman, J. Phys. Chem., 90 (1986) 1360-1365. D.E. Peebles, J.A. Schreifels and J.M. White, Surface Sci. 116 (1982) 117-134. P. Hofmann, J. Gossler, A. Zartner, M. Glanz and D. Menzel, Surface Sci., 161 (1985)303-320. W. Riedl and D. Menzel Surface Sci., 163 (1985) 39-50, J. Rogzik and V. Dose, Surface Sci., 176 (1986) L847-L851.

C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

31

OXYGEN REACTIVITY AS A PROBE FOR THE SURFACE COMPOSITION OF CaO-MnO SOLID SOLUTIONS

M. BAILES,'

'School

B . FUBINI'

and F.S. STONE'

o f Chemistry, U n i v e r s i t y o f Bath, Bath BA2 7AY, UK

Z D i p a r t i m e n t o d i Chimica I n o r g a n i c a , Chimica F i s i c a e Chimica d e i M a t e r i a l i , U n i v e r s i t ' a d i T o r i n o , V i a P . G i u r i a 9, 10125 T o r i n o , I t a l y

ABSTRACT Oxygen a d s o r p t i o n has been used as a probe f o r manganese i o n s i n t h e s u r f a c e of CaO-MnO. By e x p l o i t i n g a d s o r p t i o n m i c r o c a l o r i m e t r y , t h e e n e r g e t i c s o f t h e oxygen uptake have been determined f o r t h e p u r e s o l v e n t m a t r i x CaO and f o r s o l i d s o l u t i o n s o f nominal b u l k s o l u t e c o n c e n t r a t i o n s o f 1 mol%, 25 mol% and 50 mol% Mn, d e s i g n a t e d CM 1, CM 25 and CM 50. Oxygen a d s o r p t i o n on Mn2' i o n s i s c h a r a c t e r i s e d b y a much h i g h e r h e a t t h a n on CaO. I n t h i s way i t i s shown t h a t t h e uptakes on CM 1, CM 25 and CM 50 a r e much i n excess o f those expected on t h e b a s i s o f t h e nominal ( b u l k ) c o n c e n t r a t i o n s o f Mn. The c o n c l u s i o n i s t h a t t h e s o l i d s o l u t i o n s e x h i b i t s u b s t a n t i a l s u r f a c e enrichment i n manganese, and t h i s a l s o a f f e c t s t h e i r r e a c t i v i t y towards w a t e r vapour. INTRODUCTION S o l i d s o l u t i o n s o f t r a n s i t i o n metal i o n s i n diamagnetic and i n s u l a t i n g o x i d e m a t r i c e s have been s t u d i e d f o r many y e a r s as model systems f o r i n v e s t i g a t i o n s o f o p t i c a l and magnetic p r o p e r t i e s (1,Z). c a t a l y s t s (3-5).

They have a l s o been used as model

ilhere surface b e h a v i o u r i s concerned, as i n heterogeneous

c a t a l y s i s , i t i s necessary t o address t h e q u e s t i o n o f how c l o s e l y t h e s u r f a c e c o m p o s i t i o n matches t h e b u l k composition.

I n p r i n c i p l e t h i s problem can be

s o l v e d b y p h y s i c a l methods such as Auger o r X-ray p h o t o e l e c t r o n spectroscopy and chemical methods such as s e l e c t i v e c h e m i s o r p t i o n o r d i g e s t i o n i n r e a c t i v e l i q u i d s , b u t t h e a c t u a l p r a c t i c e and i n t e r p r e t a t i o n a r e o f t e n d i f f i c u l t . C e r t a i n o x i d e s o l i d s o l u t i o n s a r e w e l l behaved i n t h e sense t h a t t h e s u r f a c e c o m p o s i t i o n c l o s e l y f o l l o w s t h e b u l k composition;

examples a r e a-Cr2O3-Al2O3

( 6 ) and NiO-MgO ( 7 ) , b o t h of which have been s t u d i e d b y X-ray p h o t o e l e c t r o n spectroscopy. The g r e a t e r t h e d i s c r e p a n c y between c a t i o n s i z e s , however, t h e greater the l i k e l i h o o d t h a t t h e surface coordination w i l l a f f o r d a s u f f i c i e n t l y d i f f e r e n t s t a b i l i z a t i o n f o r the s o l u t e i o n t h a t i t s concentration i n t h e surface

w i l l be d i f f e r e n t f r o m i t s b u l k o r o v e r a l l c o n c e n t r a t i o n .

As a g e n e r a l r u l e , o x i d e s o l i d s o l u t i o n s i n quasi-close-packed s i m p l e o x i d e m a t r i c e s , e.g.

MgO and a-A1203, f o r m across t h e whole c o m p o s i t i o n range p r o v i d e d

the solute and solvent cations have the same charge and do not d i f f e r i n radius by more than about 15%. An unusual example, however, i s CaO-MnO, where the Mn2+ ion i s 20% smaller than the calcium i o n Cr(Mn2+) = 80 pm;

r(Ca2+) = 100 pml,

yet a monophasic s o l i d s o l u t i o n CaXMn-O ,l forms over the f u l l range Ocxcl (8). This s o l i d solution i s therefore a p a r t i c u l a r l y i n t e r e s t i n g one i n which t o examine surface composition. I t s high r e a c t i v i t y towards water vapour and C02, however, means t h a t i t must be prepared and studied i n s i t u . Selective chemisorption was chosen as the preferred method o f study, since by combining measurements o f uptake w i t h determinations o f the d i f f e r e n t i a l heat o f adsorption, i t was thought t h a t information might be provided on the d i s t r i bution of the solute on the surface as w e l l as i t s concentration.

The use o f

selective chemisorption t o probe the surface composition o f s o l i d s i s o f long standing. For example, Emnett and Brunauer (9) used C02 adsorption t o monitor the concentration o f potassium on the surface o f promoted i r o n catalysts over 50 years ago. However, we are not aware o f any major studies where adsorption calorimetry has been employed. An added merit o f calorimetry i s t h a t the discrimination between adsorption on the solute and adsorption on the solvent can be energetically quantified. Oxygen as a probe i s our p r i n c i p a l concern i n the work presented here, oxygen having strong a f f i n i t y f o r the solute manganese ions. also investigated H20 adsorption on CaO-MnO.

However, we have

Yater vapour i s a complementary

probe in view o f i t s high r e a c t i v i t y f o r the solvent ion p a i r s (Ca2+02-). EXPERIMENTAL

Materials CaO was prepared by thermal decomposition o f Specpure CaC03 i n vacuo a t 1073

K. The r e s u l t i n g oxide was then exposed t o water vapour a t 295 K t o form

Ca(OH)2, followed by decomposition a t 1073 K i n vacuo t o form m i c r o c r y s t a l l i n e CaO o f surface area (BET, N2, 77

K)

32.4 2 g - l .

Three CaO-MnO s o l i d solutions o f nominal concentrations 50% Mn, 25% Mn and 1%Mn (designated CM 50, CM 25 and CM 1, respectively) were prepared from the * corresponding carbonate s o l i d solutions by decomposition i n vacuo a t 925 K. X-ray powder d i f f r a c t i o n and magnetic measurements confirmed t h a t s o l i d s o l u t i o n formation occurred under these conditions. The procedure i s described i n d e t a i l elsewhere (10). The surface areas (BET, Np, 77 K) o f the oxide s o l i d solutions were 11.4 m2gm1 (CM 50). 21.1 d g - 1 (CM 25) and 46.5 m2g-l (CM 1). Oxygen was Grade X from B.O.C. Ltd. Yater was doubly d i s t i l l e d and outgassed by repeated freeze-pimp-thaw cycles. *The carbonates had been prepared by p r e c i p i t a t i o n from aqueous mixtures o f the n i t r a t e s (10).

33

Method Measurements o f oxygen uptake and h e a t e v o l u t i o n were made a t 303 K u s i n g a Calvet microcalorimeter.

The a l l - g l a s s t w i n c a l o r i m e t e r c e l l s , one o f which

c o n t a i n e d t h e o x i d e sample, were l i n k e d t o a g l a s s dosing system i n c o r p o r a t i n g a McLeod gauge and mercury manometer f o r p r e s s u r e measurement.

Successive doses

o f oxygen were a d m i t t e d u n t i l t h e uptake c o u l d no l o n g e r be measured ( t y p i c a l l y when p r e s s u r e reached 2 - 4 k P a ) .

The a d s o r p t i o n o f w a t e r vapour was s t u d i e d

similarly. The o x i d e sample was prepared i n s i t u i n t h e c e l l ( b y h e a t i n g t h e a p p r o p r i a t e carbonate i n vacuo a t 925 K ) ,

i s o l a t e d and then i n s t a l l e d i n t h e c a l o r i m e t e r .

I n t h e case o f some uptakes which, a f t e r an i n i t i a l r a p i d stage, became v e r y slow, t h e h e a t measurement was d i s c o n t i n u e d a f t e r 2 hours, b u t t h e uptake measurement was always c o n t i n u e d u n t i l t h e n e x t dose was admitted. RESULTS A d s o r p t i o n o f Oxygen R e s u l t s f o r t h e a d s o r p t i o n o f oxygen a t 3OoC (303 K ) on CaO w i l l be p r e s e n t e d f i r s t i n o r d e r t o i l l u s t r a t e t h e b a s e l i n e o f background r e a c t i v i t y from the solvent m a t r i x .

The r e s u l t s f o r t h e i n d i v i d u a l s o l i d s o l u t i o n s w i l l

t h e n be d e s c r i b e d w i t h p a r t i c u l a r r e f e r e n c e t o t h e e x t e n t o f t h e oxygen uptake and t h e magnitude and v a r i a t i o n o f t h e h e a t o f a d s o r p t i o n .

The uptakes can be

expressed as coverages i f some a r b i t r a r y d e f i n i t i o n o f s i t e d e n s i t y and a d s o r p t i o n mode i s assumed.

F o r t h i s purpose we s h a l l assume t h a t t h e s u r f a c e

o f t h e o x i d e i s made up o f { O O l ) p l a n e s (cube f a c e s ) o f r o c k - s a l t s t r u c t u r e o f a,

= 480 pm, and t h a t f u l l coverage ( 0 = 1) corresponds t o one oxygen atom

adsorbed p e r s u r f a c e c a t i o n (Ca2+ o r Mn2+). ( a ) CaO.

The a d s o r p t i o n o f oxygen on p u r e CaO was l i m i t e d t o o n l y a s m a l l

fraction o f sites. (150 k J m o l - l ) . $0.4 y o 1 m-2.

The h e a t o f a d s o r p t i o n ( F i g . 1) was n e v e r t h e l e s s h i g h

I t was independent o f uptake u n t i l t h e l i m i t was approached a t

Three doses a r e i l l u s t r a t e d ;

94% o f dose 1 was adsorbed, 50%

o f dose 2 and f o r dose 3, which l e d t o a p r e s s u r e i n c r e a s e f r o m 7.5 t o 44 Pa, o n l y 2% was adsorbed. measurable uptake.

I n c r e a s i n g t h e p r e s s u r e t o 2000 Pa l e d t o no f u r t h e r

The a d s o r p t i o n on CaO was slow.

The dashed ranges i n t h e

h e a t p l o t r e f e r t o uptake r e g i s t e r e d i n t h e p e r i o d o f 2-16 h a f t e r admission o f t h e dose b u t where h e a t measurement was n o t made:

t h e h e a t v a l u e determined f o r

t h e uptake d u r i n g t h e f i r s t 2 h has been e x t r a p o l a t e d .

A l l uptake was

i r r e v e r s i b l e , i n l i n e w i t h t h e h i g h m o l a r h e a t observed. the chemisorption i s d i s s o c i a t i v e .

T h i s suggests t h a t

Assuming t h e oxygen atoms l o c a t e on Ca2+

i o n s (see above) t h e uptake l i m i t o f 0 . 4 pmol K 2 i s e q u i v a l e n t t o 8 = 0.054.

I f a d s o r p t i o n i s assumed t o occur as atoms b o t h on c a t i o n s

and on

anions ( w i t h

34

e

COVERAGE

0

0.05

E . I -

----I

5= o1

,

0

, 0.4

0.2 AMOUNT ADSORBED

/urn01 m-2

F i g . 1. Oxygen a d s o r p t i o n on CaO a t 303 K . and 3 t o p o i n t o f l i m i t i n g uptake.

t h e l a t t e r becoming 0;value.

Heat o f a d s o r p t i o n o f doses 1, 2

as i n Ca02) t h e coverage would o b v i o u s l y be h a l f t h i s

I n e i t h e r case t h e c l e a r c o n c l u s i o n i s t h a t t h e p u r e m a t r i x t a k e s up

o n l y a v e r y s m a l l amount o f oxygen. ( b ) Ca0~50Mn0~500(CM 5 0 ) . The c a l o r i m e t r i c d a t a a r e shown i n F i g . 2. There are d r a m a t i c d i f f e r e n c e s f r o m t h e r e s u l t s f o r CaO. The h e a t o f a d s o r p t i o n f o r t h e i n i t i a l doses was c l o s e t o 400 k J mol-’

and t h i s was f o l l o w e d b y a p l a t e a u

a t about 325 k J mol-’ which extended t o h i g h coverage.

The h e a t decreased f o r

t h e f i n a l doses b u t was s t i l l much h i g h e r t h a n t h a t observed f o r CaO.

A l l doses

a d m i t t e d were c o m p l e t e l y adsorbed e x c e p t f o r t h e l a s t one shown i n F i g . 2. uptake c o u l d be measured f o r subsequent doses. v e r y a b r u p t l y a t 8.5 pmol m-’. ( c ) Ca0.75Mn0.250

No

The a d s o r p t i o n t h e r e f o r e ceased

A l l t h e a d s o r p t i o n was i r r e v e r s i b l e .

(CM 2 5 ) . R e s u l t s f o r CM 25 a r e shown i n F i g . 2, where t h e y

may be compared d i r e c t l y w i t h t h o s e f o r CM 50.

There i s a broad h e a t p l a t e a u

a l s o i n t h i s case, and a marked g e n e r a l s i m i l a r i t y w i t h t h e r e s u l t s f o r CM 50. F o r a l l t h e doses w i t h h e a t s g r e a t e r t h a n 200 k J m o l - l t h e whole o f t h e oxygen a d m i t t e d was adsorbed.

The l i m i t o f uptake, however, was l e s s a b r u p t t h a n w i t h

CM 50, and f o r t h e l a s t t h r e e doses shown w i t h CM 25 t h e p r o p o r t i o n s adsorbed were 26%, 6% and 1%,r e s p e c t i v e l y . ( d ) Ca0.99Mn0.010 (CM 1). The h e a t o f a d s o r p t i o n p l o t f o r CM 1 i s shown i n F i g . 3. There i s a p l a t e a u a t a p p r o x i m a t e l y 190 k J m o l - l , a much l o w e r v a l u e t h a n on CM 50 and CM 25, b u t g r e a t e r t h a n on CaO.

A new f e a t u r e i s t h e more

35

I

c

I

I

-

400 -

1 d

E -l

5

300

-jj%Il

-

CM25

L-

+ L

Z

2

:2 0 0 I-

0 v)

0 Q

tf

CM25

I

100-

z

W

I

0

I

I

I

COVERAGE

0

e

0.05

0.10

I

I

7

2

20c

E

7

Y

\

-z

0

h

g

100

v)

0 Q L

0 I-

Q W

I

0 0

I

I

I

I

0.2

0.4

0.6

0.8

AMOUNT ADSORBED

/ p m o i m-2

F i g . 3. Oxygen a d s o r p t i o n on CaO-MnO s o l i d s o l u t i o n CM 1 a t 303 K . a d s o r p t i o n t o l i m i t i n g uptake.

Heat o f

38

gradual approach t o the l i m i t o f adsorption.

The proportion o f oxygen g i v i n g a

beat w e l l below the heat o f the plateau i s much greater than on CM 50 o r CM 25 (Fig. 2).

The oxygen was again completely adsorbed f o r a l l doses on the

plateau, and the percentage adsorbed then decreased rapidly.

The uptake was

barely measurable f o r t h e l a s t two doses, so the molar heat values (whose accuracy depends g r e a t l y on the precision o f measuring the uptake) are only approximate i n these cases. Comparing CM 1 w i t h CaO (Fig. l), the e f f e c t o f 1%Mn i s very s t r i k i n g .

As

already noted, the heat a t the plateau i s s i g n i f i c a n t l y higher, but most noteworthy i s the g r e a t l y increased extent o f adsorption. An enhancement o f almost 0.5 pm1 m-2 over t h a t found with CaO i s indicated. This additional adsorption i s f a r more than can be accounted f o r by s p e c i f i c adsorption on an e x t r a 1% o f cations, Adsorption o f iiater Yapour A calorimetric study o f the adsorption o f water vapour on CaO, CaO-MnO and other oxides has been reported elsewhere (11). However, i n regard t o CaO and CaO-MnO, some points merit r e c a l l i n the context o f the present work w i t h oxygen. Fig. 4 sumnarises c a l o r i m e t r i c data f o r CaO, CM 1, CM 25 and CM 50 i n the form o f p l o t s o f heat o f adsorption o f H20 versus uptake. Several points should be noted. The adsorption o f H20 on well-outgassed CaO i s r a p i d and extensive, compatible with hydration i n depth t o form Ca(OH)2. This i s a consequence o f the high i o n i c i t y o f CaO, r e f l e c t e d also i n the high heat o f reaction (140-150 k J per mol H20). H20 adsorption i s therefore diagnostic f o r the solvent surface o f CaO-MnO s o l i d solutions, the t r a n s i t i o n metal oxide ( i n t h i s case MnO) being much less ionic. Fig. 4 shows t h a t as we progress from CaO t o CM 25 and CM 50 the heat o f adsorption o f H20 decreases dramatically. Indeed, f o r CM 50 the surface behaves almost l i k e t h a t o f a hydrophobic solid, e.g. r u t i l e or Aerosil Si02, with a heat o f adsorption belou the heat o f liquefaction ( 4 4 k J mol-1) and no reaction.

A s i g n i f i c a n t feature o f the H20 probe i s t h a t a

marked difference shows up between the behaviour o f CM 25 and CM 50.

H20 diagnoses CM 25 as more s i m i l a r t o CaO than CM 50, whereas oxygen adsorption, which focusses on manganese. discriminates less between these two s o l i d solations jFig. 2 ; .

DISCUSSI or; :he small c x t e n t o f t h e oxygen adsorption on CaO (Fig. 1) when seen i n comparison w i t h the s i g n i f i c a n t l y larger extent observed on CM 1 (Fig. 3) and the very much greater extents found w i t h CM 25 and CM 50 (Fig. 2) i l l u s t r a t e s

37

150

1

c

I 4

0

E

+ -I ----I

7 Y \

100

CMl

I

z

0 c

- -I

0

a

&!

CM25

1

50

LL 0 I-

18 kcal/mol. The remaining figure listed in Table 4 cannot be compared with experiment because the experimental data are not available. It is interesting to note, though, that the heat of adsorption for CH predicted by the BOC-MP approach is indistinguishable from that obtained by the ub inirio SCF-CI many-electron imbedding theory (ref. 57). As may Seen in Table 3, the heat of adsorption of CHx species decreases monotonically as the value of x increases. The activation energy for the hydrogenation of CHxfollows a similar trend. Table 3 Heat of adsorption of CH,and activation energies for CH hydrogenation and dehydrogenation

CH,

C CH CH2

CTb

cH4

QcHx(kcaVmo1) Ni

Pd

Pt

171

160 82

150 73 36 21 9

90 44 26 lo

40 24 lo

Reac.

&*r (kcalhol) Ni

C + H = CHx C H + H = CH2 CH2+H=CH3 CH3+H = C H 4

_-----

63 33 12 6

Pd

Pt

59 31 11 5

57 27 9 4

AE*T(kcaVmo1) Ni Pd Pt 0 26 41 22

0 29 43 24

0 31 43 27

The predicted heat of adsorption and the energetically preferred coordination for C2Hx species are listed in Table 4. For symmetric species HxC-CHx(x = 1-3), where H atoms prevent

the end-on (111) coordination, only the side-on (q2) geometry was considered, since this mode of

103

coordination is well documented for acetylene and ethylene on many fcc metals (ref. 58). The BOC-MP method predicts that the mode of coordination for HxCC species depends on the stoichiometry of the species and the composition of the metal. Table 4 shows that on the late transition metals, both CHC and CHzC will prefer q2 coordination. For CHC, the preferred coordination predicted by the BOC-MP method is well corroborated by EELS spectra obtained for Pd(ll1) (ref. 37), Ir(ll1) (ref. 59), and Ru(001) (refs. 41, 60) surfaces. For C H K , 112 coordination is established from EELS data for Ru(001) p(2x2)O (ref. 61) and Pt(ll1) (ref. 39), and from 13c NMR data for supported Pt particles (refs. 52-54). The only point of comparison between theory and experiment for the heats of adsorption listed in Table 4 is for ethylene. The BOC-MP method predicts values of 12, 15, and 18 kcal/mol for Pt, Ni, and F e w , respectively. These values are in excellent agreement with experimentally measured values for Pt, Pd, Ru, and Ni (refs. 27,30,62,63), which lie in the narrow range of 11-13 kcal/mol. Table 4 The coordination and heat of adsorption of Cflx species C2HX

H3C-CH3 H3C-CH2 H3C-CH HzC=CHz H3C-C H2C=CH HzC=C HeCH HC=C G C

COOrd.

Fe/w 6 35 62 20 94 41 67 25 65 70

Qcmx (kcal/ml) Ni

Pt

5 27 46 15

5 21 37 12 56 25 58 14 39 43

70

31 61 18 49 53

Figures 12-14 illustrate the energetically preferred pathways for the decomposition of ethane, ethylene, and acetylene. The activation energy (in kcal/mol) for each elementary reaction is indicated above the reaction arrow. It should be noted that activation energies for reactions involving the formation or consumption of CH2CH have been omitted because the accuracy of the estimates for these processes are considerably lower than those for all other processes. The BOC-MP calculations predict that adsorbed ethylene should not undergo direct decomposition to CH2 groups because the activation energy for this step is very high (98-118 kcaymol). Instead, Figs. 12-14 suggest that ethylene should either isomerize to CH3CI-I or loose hydrogen to form CHKH. CH3CH groups once formed can participate in three reactions -

P i (Ill)

ethane

ethylene

acetylene

Fig. 12 Reaction network illustrating the pathways between C2Hx and CHXspecies on Pt( 11 1). For the sake of clarity, the addition or loss of hydrogen atoms is not indicated. Activation energies are given in kcal/mol. (From ref. 54).

NI ( 1 1 1 )

ethane

ethylene

+C

acetylene

Fig. 13 Reaction network illustrating the pathways between C f l , and CHxspecies on Ni(ll1). For the sake of clarity,the additionor loss of hydrogen atoms is not indicated Activation energies are given in kcaVmoL (From ref. 54).

105

ethane

ethylene

ocetylene

Fig. 14 Reaction network illustrating the pathways between C ax and CHXspecies on Fe/W(llO). For the sake of clarity, the addition or loss of hydrogen atoms is not indicated. Activation energies are given in kcal/mol. (From ref. 54).

106

isomerization back to CHzCHz, dehydrogenation to form CH3CH, and dehydrogenation to form CH&N. Figures 12-14 show that an alternative path to CH 3C is via the isotnerization of CH zCH. In competition with this step is the loss of hydrogen from CHzCH to form CH2C or CHCH. Because reliable estimates of the activation energies cannot be obtained for processes involving the formation or consumption of CHzCH, it is not possible to say whether the dehydrogenation of ethylene to CH3C proceeds preferentially via CH3CH or CHzCH. The BOC-MP calculations also provide some insights into the pathways by which C-C bond cleavage occurs. On Pt, CH3C decomposition to CH3 and C has the lowest activation energy (17 kcal/mol) of any process involving C-C bond cleavage. The other process that might contribute to the dissociation of C-C bonds is the decomposition of CHC to CH and C, for which the activation energy is 31 kcal/mol. For Ni, the activation energies for CH3C and CHC decomposition decrease to 9 and 14 kcal/mol, respectively, and the activation energy for CHzC decomposition now decreases to 36 kcal/mol, versus 57 kcal/mol for Pt. The activation energies for CH3C, CHzC, and CHC decomposition on F e w are even lower than for Ni, dropping to 2,9, and 0 kcal/mol, respectively. The decomposition of CH3CH also becomes possible, since the activation energy for this process is only 27 kcal/mol for Fern, and is, in fact, lower than for the dehydrogenation of CHEH to CHKH. For acetylene decomposition, the BOC-MP calculations indicate that isomerization to form CH2C is significantly more favorable energetically the dehydrogenation to form CHC. On Pt and Ni, the isomerization of CH2C back to CHCH has a substantially lower activation barrier than those for the dehydrogenation to form CHC plus H. Only for F e w does the activation for C-C bond cleavage become smaller than that for isomerization back to acetylene. The conclusions regarding the relative stability of various C2Hx species drawn from BOCMP calculations are in good qualitative agreement with experimental observation. For example, CH3C has been readily observed during the decomposition of acetylene on close-packed surfaces (refs. 58,64) and metal particles (refs. 48,65) of Pt, Pd, Rh, and Ru, but not on Ni or more active metals. Figures 12-14 suggest that the reason for the stability of CH3C on the noble metals is the relatively high activation energy for CH3C decompositionon these metals. The value of AE*f for the process C H G CH3,s + Cs, is 17 kcal/mol for Pt( 11 1). 9 kcal/mol for Ni( 11I), and 2 kcaYmol for F N ( I 10). It should be noted that the calculated values of AE'f are comparable to the experimental (TPSSIMS) values of 17 kcal/mol for Pt( 111) (ref. 66) and 12 kcal/mol for Ru(001) (ref. 67), which is intermediate in activity between Pt( 111) and Ni( 1 1 1). Moreover, the low activation energies for C-C bond cleavage on F e w explain why CHx species, rather than CH3C, are observed on Fe surfaces (refs. 22,40). Consistency between theory and experiment can also be found in the case of acetylene decomposition on Pt. In agreement with the information presented in Fig. 12, EELS data for Pt(l11) (ref. 39) and 13c NMR spectra taken with supported Pt particles (refs. 52-54) indicate that CHzC is the first stable intermediate formed from chemisorbed acetylene, and that C-C bond scissioning occurs mainly in CHC rather than CHzC (refs. 52-54). Similar findings have recently been rewrted for RhU 111 (ref. 241. Bv contrast. onlv CHI fragments are observed on various Fe

107

surfaces (refs. 22,4Q)due to the relatively low thermal stability of CH2C on these surfaces. Also in agreement with the BOC-MP calculations that on Ni surfaces (ref. 23) acetylene decomposes rapidly to CHC and CHxspecies. Finally, the BOC-MP calculations predict that the C-C bond dissociation barrier for CH3C is smaller than that for CH-C (e.g., by 5 kcdmol for Ni( 1 1 1)). Consistent with this, it is observed that when both CH3C and CHC are formed on Ru(001), CH3C decomposition begins at lower temperatures than CHC (refs. 39,41). CONCLUSIONS Modern spectroscopictechniques provide considerable informationregarding the structure and composition of carbonaceous species deposited on the surface of metal catalysts. This information contributes to the development of a detailed understanding of how hydrocarbons and carbon monoxide form carbon-containing species and the pathways via which these species react to form refractory carbon deposists. The BOC-MP method provides a reasonable frame work for describing the thermochesitryof CHx and CzHx species and for assessing energetically preferred reaction pathways. The agreement of the predictions made by this method with experimental data is encouraging, and suggests that the BOC-h4P approach can be used to explain trends observed with changes in catalyst and reactant composition. ACKNOWLEDGMENT This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, United States Department of Energy, under contract DE-AC03-76SF00098. REFERENCES 1. L. J. E. Hofer, in P. H. Emmett (Editor), Catalysis, Vo1.4, Reinhold, New York, 1956. 2. H. B. Palmer and C. F. Cullis, in P. L. Walker (Editor), Chemistry and Physics of Carbon, Vol. 1, Marcel Dekker, New York, 1965. 3. J. Rostrop-Nielsen and D.L. Trimm, J. Catal., 48 (1977) 155. 4. D. L. Trimm, Catal. Rev. -Sci. Eng., 16 (1977) 155. 5. R. T. K. Baker and P. S. Harris, in P. L. Walker and P. A. Thrower (Editors), Chemistry and Physics of Carbon, Vol.14, Marcel Dekker, New York, 1978. 6. C. H. Bartholemew, Catal. Rev. -Sci. Eng., 24 (1982) 67. 7. J. Oudar and H. Wise (Editors), Deactivation and Poisoning of Catalysts, Marcel Dekker, New York, 1985. 8. A. T. Bell, in E. E. Petersen and A. T. Bell (Editors), Catalyst Deactivation, Marcel Dekker, New York, 1987. 9. J. G. McCarthy and H. Wise, J. Catal., 57 (1979) 406. 10. J. E. Demuth and T. N. Rhodin, Surf. Sci., 45 (1974) 249. 11. L. C. Isett and J. M. Blakeley, Surf. Sci., 47 (1975) 645. 12. M. A. Van Hove and S. Y. Tong,Surf. Sci., 52 (1975) 673. 13. J. H. Onuferko, D. P. Woodruff, and B. W. Holland, Surf. Sci., 87 (1979) 357. 14. K. H. Riedler and H. Wilsch. Surf. Sci., 131 (1983) 245. 15. D. W. Goddman, R. D.Kelley, T. E. Madey, and J. T. Yates, Jr., J. Catal., 63 (1980) 226. 16. H. P. Bonze1 and H. J. Krebs, Surf. Sci., 91 (1980) 499. 17. R. Caracciolo and L. D.Schmidt, Appl. Surf. Sci., 25 (1986) 95. 18. R. Caracciolo and L. D.Schmidt, J. Vacuum. Sci. Technol., A2 (1984) 995. 19. H. Steininger, H. Ibach, and S. Lehwald, Surf. Sci., 117 (1982) 685. 20. L. L. Kesmodel, L. H. Dubois, and G. A. Somojai, J. Phys. Chem., 70 (1979) 2180. 21. W. Erley, A. M. Baro, P. McVreen, and H. Ibach., Surf. Sci., 120 (1982) 273.

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22. U. Seip, M. C. Tsai. J. Kupper, and G. Ertl., Surf. Sci., 147 (1984)65. 23. 1. A. Smscio, S.R. Bare,and W. Ho, Surf. Sci., 148 (1984)499. 24. S. Lehwald and H. Ibach. Surf. Sci., 89 (1979)425. 25. R. G. Carr, T. K. Sham, and W. E. Eberhardt, Chem. Phys. Lett. 113 (1955). 26. J. E. Demuth, Surf. Sci., 84 (1979)315. 27. M.M. Hills, J. E. Parmenter. C. B. Mullins, and W. H. Weinberg, J. Am. Chem. Soc., 108 (1986)3554. 28. 1. A. Gates and L. L. Kesmodel, Surf. Sci.. 120 (1982)L461. 29. J. A. Gates and L. L. Kesmodel, 124 (1983)68. 30. W. T. Tysoe, G.L.Nyberg, and R. M. Lambert,J. Phys. Chem., 88 (1984)1960. 31. I. Ratajczykawa, and I. Szymerka, Chem. Phys. Lett., 96 (1983)243. 32. L. L. Kesmodel and J. A. Gates, Surf. Sci.. 111 (1981)L747. 33. L. H.Dubois, D. G. Castner, and G. A. Somorjai, J. Phys. Chem., 72 (1980)5234. 34. H.lbach, Proceedings of the International Conference on Vibrations in Adsorbed Layers, Julich, West Germany, 1978. 35. B. J. Bandy, M. A. Chester, M E. Pemble. G. S.McDougall, and N. Sheppard, Surf. Sci., 139 (1984)87. 36. J. A. Gates and L. L. Kesmodel, J. Phys. Chem., 76 (1982)4281. 37. L.L. Kesmodel, G.D. Wadill, and J. A Gates, Surf. Sci., 138 (1984)464. 38. L. L.Kesmodel, J. Phys. Chem., 79 (1983)4646. 39. H.Ibach and S. Lehwald, J. Vacuum Sci. Technol., 15 (1978)407. 40. W. Erley. A. M. B m . and H. Ibach. Surf.Sci. 120 (1982)273. 41. J. E. Parmenter, M. M. Hills, and W. H. Weinberg, J. Am. Chem. Soc., 108 (1986)3563. 42. C.Bockx, R.F. Willis. B. Feurbacher, and B. Fitton, Surf. Sci., 68 (1977)516. 43. C.Bockx, B.Feurbacher, B. Fitton, and R. F. Willis, Surf. Sci.. 63 (1977)193. 44. J. C.Hamilton, N. Swanson. B. J. Waclawski, and R. J. Cellotta, J. Chem. Phys. 74 (1981).4156. 45. W. Sesselmann, G. Enl, and J. Kuppers, Surf. Sci.. 130 (1983)245. 46. W. T.Tysoe, G.L. Nyberg, and R. M. Lambert, Surf. Sci., 135 (1983)128. 47. P.-K.Wang, C. P. Slichter, and J. H. Sinfelt, Phys. Rev. Lett., 53 (1984)82. 48. P.-K. Wang, C.P. Slichter, and J. H. Sinfelt, J. Phys. Chem., 89 (1985)3606. 49. P.-K. Wang, J. P. Ansermet, C. P. Slichter, and J. H. Sinfelt, Phys. Rev. Lett., 55 (1985) 2731. 50. T. M. Duncan, P. Winslow, and A. T. Bell. Chem. Phys. Lett., 102 (1983)163. 51. T. M. Duncan, P. Winsolw, and A. T. Bell, J. Catal., 93 (1985)1. 52. T. M. Duncan, P. Winslow, and A. T. Bell. J. Catal., 95 (1985)305. 53. E. Shusmvich and A. T. Bell, J. Catal., 113 (1988)341. 54. E.Shustmvich and A. T. Bell, Surf. Sci.. in press. 55. S.G. Brass and G. Erlich, Surf. Sci.. 187 (1987)21. 56. S. T. Ceyer, J. D. Beckerle, M.B. Lee. S. L. Tang, Q. Y.Yang, and M. A. Himes, J. Vacuum Sci. Technol.. A5 (1987)501. 57. J. L.Whitten, private communication. See also: Abstract of Papers, 194th ACS Meeting, New Orleans, LA, August 30-September 4,1987;Phys. 188. 58. H.Ohtani, M. Van Hove, and G. A. S m r j a i , Prog. Suf. Sci.. 23 (1986)155. 59. T. S. Marinova and K. L. Kostov, Surf. Sci., 181 (1987)573. 60. P. Jacob,A. Cassuto, and D. Menzel, Surf. Sci.. 187 (1987)407. 61. M. M.Hills, J. E. Parmenter, and W. H. Weinberg, J. Am. Chem. Soc.,109 (1987)597. 62. M.Salmeron and G. A. Somorjai, J. Phys. Chem., 86 (1982)341. 63. R. A. ZutV and J. B. Hudson Surf. Sci., 66 (1977)405. 64. E. M.Stuve and R.J. Madix. J. Phys. Chem..89 (1985)105. 65. T. B. Beebe, Jr. and J. T. Yams, Jr., J. Phys. Chem.,91 (1987)254. 66. K. M. Ogle, J. R. Creighton, S.Achter, and J. M.White, Surf. Sci., 169 (1986)246. 67. C. M. Greenlief, P. L.Radloff, X.-L. Zhou, J. M. White, Surf. Sci., 191 (1987)93.

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Dr. o f

H. Miessner (Zentralinstitut f u r physikalische Chemie,

&ad.

Sci., Berlin, D D R ) asked:

You breifly mentioned in your introduction the IR-spectroscopy. What kind of structural information can we obtain from the conventional IR spectroscopy regarding the nature of carbonaceous species on the surface? Answer Infrared spectroscopy provides relatively little information about the structure of carbon deposits on surfaces because of the limited range of frequencies that can be observed as a consequence of the strong absorbance of the catalyst support. Thus, for example, while it is possible to see C - H stretches, it is not possible to make reliable observations of bending and deformation modes in hydrocarbon-type species. By contrast, Raman spectroscopy does allow observation of polycondensed aromatics. The C - C vibrations in such species are different for external and internal C - C bonds. By measuring the ratio of the two bands, it is possible to estimate the average size of the islands of condensed aromatic rings.

Prof. G. C . Bond (Bi-unel University. Uxbridge, U)o

asked:

Is it your view, or is it the consensus of the literature, that formation of amorphous or graphitic carbon occurs primarily or exclusively through monocarbon species; or does polymerization of the reactant hydrocarbons, followed by dehydrogenation,play a role? Answer NMR evidence obtained in our laboratory has shown that thermal aging of carbidic carbon can produce a product the NMR signature of which is identical to that of graphite. On the other hand, there is also evidence that on metal surfaces acetylene can form benzene and butadiene can form coke. These observations suggest that coke precursors can form by polymerization of hydrocarbon reactants. The subsequent dehydrogenation of such species provides a very plausible path for the formation of coke and graphite.


C. Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactiuity ojSurjuces 0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

POLARIZATION CONDUCTIVITY OF APIORPHOUS ROLYBDENUN SULPHIDES

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111

INFLUENCE

OF THE 5XIDATION

Y.

BENSIMON, J.C. G I U N T I N I , P . BELOUGNE, 6. OEROIDE and J.V. ZANCHETTA ( U n i v e r s i t e des Sciences e t Techniques du Lanyedoc .. L a b o r a t o i r e de Chimie Physisue (UA 407 CNRS) - Place E. B a t a i l l o n 34060 FIONTPELLIER Cedex FRANCE).

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ABSTRACT The study o f the complex c o n d u c t i v i t y o f molybdenum sulphides (MoS2 + x ) i s developed. The experimental r e s u l t s show t h a t the e l e c t r o n i c t r a n s p o r t p r o p e r t i e s a r e c o r r e c t l y i n t e r p r e t e d by means o f t h e Correlated B a r r i e r Hopping (CBH) model. For a l l compounds the law U ' ( U J ) = A(T)wS i s v e r i f i e d . The p o t e n t i a l energy WM and the number o f s i t e s responsible f o r the conduction d i f f e r as a f u n c t i o n o f the composition ( 0 , 3 < x 4 0,6) The conduction s i t e s are the molybdenum V. The p o l a r i z a t i o n c o n d u c t i v i t y o f the o x i d i z e d sulphides i s somewhat l a r g e r ; the CBH model i s s t i l l t a l u a b l e . INTRODUCTION

Studies of amorphous molybdenum and tungsten s u l phides have r e c e n t l y been published ( r e f s . 1-4). C e r t a i n c o n t r a d i c t o r y

r e s u l t s l e a d t o the s u p p o s i t i o n

t h a t these compounds were o f i n s u f f i c i e n t p u r i t y ( r e f s . 1 - 3 ) . The c o n d u c t i v i t y o f a homogeneous group o f s i x amorphous molybdenum sulphides

,0

(Nos2

\
373K a further, gradual, loss of water occurs due to the process:

which involves silanols Si-OH ( S ) or both silanols and titanols (TS). Because of the different electropositive character of Si and Ti, the Si-0-Ti bridges formed upon dehydroxylation, are polarized (the Ti-0 bond being more polar than the Si-0 one). These bridges are consequently the preferential sites for polar molecoles adsorption. Comparison of the sequences of spectra of TS and S at different dehydration stages indicates that the TiOH groups absorb in the 3510-3450 cm-I range because a peak in this frequency range is clearly observed only on TS. This conclusion agrees with the known frequencies of titanols at the surface of Ti02 (12). Due to the polarization of the Si-0-Ti bridges, it is conceivable that they represent preferential sites of the H2g attack, following the reverse of eq. 1: this explains the slightly larger hydroxylation state of TS with respect to S.

2) IR modes associated with framework titanium. The major difference betwen the IR spectra of S and TS is represented by the isolated peak at 960 cm-I (present only on TS). This peak, which is considered as the IR finger-print of the TS (1, 2, 3), has been assigned to a stretching mode of a [Si041 unit bonded to a titanium ion. In order to confirm or to reject this

140

assignment, we must first answer the following question: can we exclude that the 960 cm-l peak is associated with the stretching mode of a titanyl group Ti=O (especially considering that it normally absorbs in the 900-1000 cm-' region) (13)? Our answer is positive for the following reasons: a ) the electronic transitions of the titanyl group are not present in the W - V i s reflectance spectrum of TS ( vide infra); b) the peak at 960 cm-' does not show any tendency to exchange with 1802 even at the highest temperatures (973K); c) the peak at 960 cm-l is totally insensitive to reduction in molecular H2 at 973K or to reduction in atomic hydrogen generated by a glow discharge. Having established that the titanyl groups cannot be responsible for the peak at 960 cm..', we can go back and discuss the original assignment. In order to do this, it is useful to compare the IR spectrum of SiOz (amorphous), S and TS in the 1400-400 cm-' range where the fundamental and impurity modes of the framework are expected. The predominant primary building unit (PBU) is in all cases the [Si04] tetrahedron. The tetrahedra are linked by sharing the oxygens to form disordered (amorphous silica) and ordered secondary building units (SBU) (S and TS). The zeolite framework results from the packing of the SBU units in the space. The vibrational representation of the stretching modes of isolated PBU is: rstret.= F2+A1 (F2= triply degenerate). Following several authors (19-23 and references therein) the F2 and A1 modes are at 1100-1050 cm-l and at 750-850 cm-l respectively. Another relevant mode of T2 symmetry with bending character is at 450-500 cm-'. The three main absorptions of amorphous silica at 1050-1200, =800 and =450 cm-I essentially correspond to the stretching (P2, A1) and bending modes of the isolated tetrahedron. Of course, due to the interaction with a disordered outer sphere of tetrahedra having lower symmetry, the degenerate F2 mode is broad and show a signs of splitting into ill defined components. In crystalline zeolites the situation is similar, but the surrounding structure (secondary building unit, SBU) is now ordered. We have consequently to consider the vibrational modes of the SBU some of which are considered as "structure sensitive" because they represent a finger print of the SBU. Comparison between the spectra of amorphous silica and S , clearly shows that the 1250 and 5 5 0 cm-I peaks are indeed those structure sensitive modes (14). For samples containing nydroxyl groups on the surface (amorphous

141

silica) or in the channels IS and TS) this picture is not sufficient. In fact in both cases a certain fraction of the tetrahedra is carrying an hydroxyl group and so have approximate C3v symmetry. This implies an "ab initio" splitting of the degeneracy of the F2 mode into an E (doubly degenerate) and A1 modes. For instance, it has been demonstrated that the hydroxylated unit 0 3 S i O H unit present at the surface of silicas, has a "local" mode (presumably of A1 symmetry) at =985 cm-l (11) (arrow in Fig. 3 ) . As this mode is sensitive to hydrogen-deuterium exchange, it must be essentially localized on the Si-OH bond. Of course, a similar broad band is also present on S hydroxylated samples (arrow). On TS sample it cannot be so clearly seen, because it is partially obscured by the 960 cm-I peak. However, in many cases a shoulder at 990-1000 cm-I is visible, which is suggestive of surface hydroxylation, because it tends to disappear upon dehydration at 673K. In order to assign the 960 cm-I band of TS it is worth to recall that a similar peak was found in the IR and Raman spectra of Ti02-SiO2 glasses (15-16). The usually given explanation is: the presence of substitutional Ti(IV), forming polarized TiOSi bridges with 4 adjacent [Si041 tetrahedra reduces the local "site" symmetry of tetrahedra (as does the presence of hydroxyls) from Td to C3v. New E and A1 local modes appear at 1110 r%rIC: =940 cm-l from splitting of the degenerate F2. These considerations indicate that the 960 cm'l peak is essentially a fllocal'v impurity mode of a [SiO,] structure bonded to a Ti(1V): 03SiOTi. By analogy with the assignement given for hydroxylated tetrahedra, we can hypothesize that this A1 local mode is essentially the Si-0 stretching of the polarized Si-06----Ti6+(IV) bond. On dehydrated samples there are 4 Si-06' bonds for each Ti atom (Scheme 1: structure a); on hydrated samples one or more Si-06---Ti6+(IV) bridges exposed into the channels are hydroxylated (Scheme 1: structures b and c). It is most remarkable that the Ti(OH)2 groups in structure c can be considered as an anchored form of hydrated titanyl. Of course all the Me-OH groups formed upon hydroxylation are presumably interacting by hydrogen bonding.

3 ) Electronic transitions associated with framework Ti(1V).

iiydroxylated TS is characterized by an electronic transition

142

at 48.000 cm-l. In view of the its high frequency and intensity, the 4 8 . 0 0 0 cm-I band is assigned to a transition having charge transfer (C.T.) character involving the Ti(1V) sites schematized in Scheme I(a,b,c).

Following Jfprgensen (171, the frequency of a C.T. transition is given by: Ti =30-103[ /yopt(X) - Kept (Ti)]. The )c opt(Ti) for Ti(1V) in tetrahedral coordination has been estimated to be around 1.85. The optical electronegativity of X=OH- groups is intermediate between that of F- (3.9) and C1- (3.0) and near that of H20 ( 3 . 5 ) (17): so the 3 . 4 5 figure can be safely adopted. In conclusion, hydroxylated structures should have a C.T. band at 48.000 cm-'. The agreement with the experimental results is remarkable. On the other hand, the peak at 4 8 . 0 0 0 cm-I cannot be explained in terms of a C.T. band of titanyl groups. In fact on the basis of the known spectra of titanyl compounds (13) and of the well known spectrum of the vanadyl group (la), a peak in the

143

25-35.000 cm-' range should be expected. As a matter of fact TS is not absorbing at all in this region.

4) Effect of small polar molecules adsorption on the IR and UV-Vis transition of framework Ti. This effect is shown in Fig. 4B and 5. For reasons of simplicity we shall start with the effect of H20 on the reflectance spectrum in the UV-Vis. A new C.T. transition at lower L appears whose intensity grows with the amount of adsorbed H20. In a situation of complete pore filling, the band (broad) is centered at ~ 4 2 . 0 0 0 cm-l, while the original one has disappeared. In excess of H20, it is likely that Ti(1V) assumes octahedral coordination by insertion of two ligands. The insertion of new ligands changes both the coordination state and the Aopt of Ti( IV) The new value of koptTi(IV) in octahedral coordination is 2.05 (17). Application of the equation gives the characteristic frequency of the new octahedral structure. The calculated value (42.000 cm-') completely agrees with the experimental one. Smaller amounts of H20 give intermediate situations (i.e. the original tetrahedral band is not totally eroded and the new transition is not fully developed and has intermediate frequency). An important observation deriving from Fig.6, is that a high fraction of titanium sites are perturbed by H29: this means that, as far as the small H20 molecule is concerned, all the Ti sites are accessible. In the parallel IR experiment (Fig. 5 1 1 only situations far from pore filling have been investigated in detail (because only in this case the broad bands of the liquidlike water do not partially obscure the IR manifestation of Ti sites). However even in presence of . 5 KPa only of H20 (or D20) the 960 cm-l band is distinctly perturbed and shifted (with broadening) to higher frequency. Essentially the same results are obtained with NH3 and CH30H. The process is entirely reversed by outgassing at the beam temperature, so confirming that the adsorption energy is small (as expected for a coordinative interaction). Coordination of H20, NH3 and CH30H to Ti(IV) centres, modifies the polarity of the Si-O...Ti bridges and this is reflected in the change of the frequency of the Si06vibration. The simultaneous broadening of the band is likely associated with hydrogen bonding between the Si-06- and the

.

144

coordinated molecules. Under pore filling conditions the polarity of Si-O-Ti bridges is maximized by the local increase of the dielectric constant and this can favour further formation of hydrated Ti(OH)2 structures by hydrolysis of Si-0-Ti bridges.

The authors wish to thank Mr M. Padovan who prepared the samples of TS and S used in this work.

BIBLIOGRAPHY 1 M. Taramasso, G. Perego and B. Notari, U.S. Pat. 4. pp. 410-501. 2 G. Perego, G. Bellusi, C. Corno, M. Taramasso, F. Buonomo and E. Esposito, Proceedings VII th Int. Zeolite Conference Tokyo August 17-22 1986 Elsevier, Kodansha Pub. Tokyo 1986. 3 G. Bellusi, G. Perego, A. Esposito, C. Corno and F. Buonomo Atti IXX Cong. Naz. Chim. Inorg. e VI Cong. Naz. Catal. Cagliari Ottobre 6, 1986. 4 W. Holderich, M. Hesse and F. Naiimann, Angew. Chem., Int. Ed. Engl., 27 (1988) 226. 5 C. Neri, A. Esposito, B. Anfossi and F. Buonomo, Eur. Pat., 100 119. 6 A. Esposito, M. Taramasso, C. Neri and F. Buonomo, U.K. Pat., 102 665. 7 European Patent Application 6109400 July 9, 1986. 8 Italian Patent Application 196076 A-87 March 6, 1987. 9 E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchener and J.V. Smith, Nature, 271 (1978) 50. 10 G. Brunner, Zeolites, Rev., 7 (1987) 9. 11 F. Boccuzzi, S. Coluccia, G. Ghiotti, C. Morterra and A. Zecchina, J. Phys. Chem., 82 (1978) 1298. 12 C. Morterra, A. Chiorino, A . Zecchina and E. Fisicaro, Gazz. Chim., It., 109 (1979) 691. 13 P. Comba and A. Merbach, Inorg. Chem., 26 (1987) 1325. 14 J. C. Jausen, F.J. van der Gaag and H. van Bekkun, Zeolites, 4 (1984) 369. 15 M.F. Best and R.A. Condrate, J. Mat.Sci. Letters, 4 (1985) 994. 16 B.G. Varshal, V.N. Denisov, B.N. Mavrin, G.A. Paulova, V.B. Podobedov and K.E. Stebin, Opt. Spectrosc. (USSR) 47 (1979) 344. 17 C.K. Jqrgensen, Prog. Inorg. Chem., 12 pp. 101 S.J. Lippard ed.Intersci. Pub., John wiley N.Y. 1970. 18 C.J. Ballhausen and H.B. Gray, Inorg. Chem., 1 (1962) 111.

C. Morterra, A. Zecchina and G . Costa (Editors), Structure andReactiuity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

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SURFACE PROPERTIES OF CARBONS

H.P.BOEHM lnstitut fur Anorganische Chemie der Universitiit Munchen Meiserstrasse 1, D-8000 Munchen 2 (FedAepublic Germany)

ABSTRACT The surface properties of carbons are obviously influenced by the nature of the carbon. The structural features of several, important forms of carbons are briefly described, i.8. of diamond, natural and synthetic graphites, cokes, glass-like carbons, activated carbons, carbon blacks and carbon fibers. The chemisorption of hydrogen, halogens, sulfur and nitrogen on the carbon surface is discussed, and surface oxides are dealt with in more detail. The surface oxides may have acidic or basic properties, depending on the formation conditions. Several functional groups have been identified. The last section deals with the influence of surface complexes on the properties of carbon as a catalyst or catalyst support. STRUCTURAL PROPERTIES OF CARBONS Elemental carbon may be observed in many different forms. It is necessary to understand their structures before one discusses their surface properties. Especially with carbons derived from the graphite lattice the proportion of basal and prismatic planes in the surface can vary over a wide range. Two crystalline modifications of carbon are known, diamond and graphite, the thermodynamically stable form under normal conditions. The structures of these two modifications result from sp3 and sp2 hybridization, respectively, of the valence orbitals of the carbons. In addition, there exist many forms of carbon with more or less defect structures derived from these lattices. In recent years, diamond-like films have attracted much interest, which are produced by ion beam deposition from hydrocarboniargon mixtures. These films are not crystalline and contain an appreciable quantitiy of bound hydrogen (ref.1). They should rather be called "dense hydrocarbon polymers", therefore. However, most of the disordered carbons are derived from the graphite lattice, i.e. they consist essentially of parallel-stackedcarbon layers with the honey-comb pattern of graphite (such layers are also called "graphene" layers). These layers, usually of limited size, are stacked without threedimensional order, such structures are called turbostratic. The diff ractograms of turbostratic carbons show only two-dimensional (hk) bands with distinct tailing towards larger 2 8 values, in addition to (OOL) reflections. Practically important

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forms of such carbons are, among others, cokes, activated carbons, carbon blacks and carbon fibers. In turbostratic carbons the interlayer distance is larger than in graphite. Instead of 335.4 pm one observes by X-ray diffraction (XRD) values between 344 and ca. 360 pm. Careful analysis of the diffraction profiles as well as high-resolutiontransmission electron microscopy (HRTEM) of e.g. carbon blacks revealed, however, that there is a relatively wide distribution of interlayer spacings in a given carbon; XRD yields only an average value (ref.2-4). The dimensions of the coherently scattering domains have been determined in many cases from the broadening of the X-ray reflections. With cokes, activated carbons or carbon blacks, usually very small "crystallite sizes" in the range of La = 1.5 - 2.5 nm parallel to the layers and 4: = 1.3 - 1.6 nm in the heights of the stacks have been observed. HRTEM showed however, that the layers are much larger, but bent and creased (ref.3,4). One observes by XRD only the average dimension of the planar sections between bends in the layers. Natural and synthetic d&!xx)& come in fairly large crystals, but fine powders with surface areas of 20 m2/g and more may be obtained (ref.5). Synthetic diamond made from graphite by shock compression (ref.6) has a surface area of ca. 200 m2/g. Single crystals of natural graDhite of 1-2 mm diameter are quite rare and difficult to obtain. Single crystals have also been produced syntbetically (ref.7). The small surface area of natural graphite flakes is increased to 40-80 m2/g in exfoliated araDhite produced by flash-heating of graphite intercalationcompounds. Such exfoliated graphites with very homogeneous basal surfaces are used with great success in studies of the structure of adsorbed gas layers (ref.8). Pvrolvtic araDhite is obtained in fairly large pieces by pyrolysis of methane at low pressures and high temperatures (-. 2300 K) and subsequent heat-treatment at 3300 K or higher. In pyrolytic graphite the carbon sheets are oriented parallel to the surface of the substrate. Highly-orientedpyrolytic graphite (HOPG) with less than 1' deviation from coplanarity of the layers is produced by stress-annealingof pyrolytic graphite at temperatures well above 3300 K (ref.9). HOPG has a mosaic structure consisting of crystals of ca. 1 m diameter exhibiting a random orientation of their &axes. Its three-dimensional structure is well-developed, albeit not as perfect as that of the best natural graphites, e.g. from Madagascar. HOPG is often used in studies of the properties of the basal plane (001) of graphite. Pvrolvtic carbon is made in a similar way as pyrolytic graphite, but at lower temperatures of 1000-1600 K. It has a turbostratic structure with the carbon layers stacked approximately parallel to each other and to the surface of the substrate, e.g. quartz glass. In the "crystallites" are arranged in a dense, statistical packing. There is extended cross-linking by covalent bonds, perhaps also by means of tetrahedrally bonded carbon atoms bridging the edge atoms of the layers. A quite hard structure results. The surface area is fairly small, in the range of a few m2/g or less.

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Glass-like carbon is produced by controlled pyrolysis of certain polymers, e.g. phenolformaldehyde resin. It has a very disordered structure which makes it nearly isotropic on a scale of a few tens of nm (ref.lO). It is non-porous. Activated carbons are prepared from carbonized precursors, such as wood, coconut shells, peat, coal, etc. by partial gasification, usually with steam. In this process, a highly porous texture is produced. Depending on the intended application, the activation process is controlled in such a way that predominantly micropores with diameters below 2 nm are produced or also mesopores in the range between 2 nm and 50 nm. Pore volumes are mostly in the range of 0.3 to 0.8 ml/g. The micropores are filled at low relative pressures (p/po < 0.1) in low-temperature adsorption of nitrogen, and hence absurdly high surface areas, sometimes above 1500 m2/g, are calculated by application of the BET equation. The true surface area of the external surface, and of the macro- and mesopores is usually not higher than ca. 70-200 m2/g. Carbon blacks are produced by pyrolysis of hydrocarbons (mainly oil or natural gas), at normal pressure. In the predominant furnace process, oil is burned with a limited supply of air. The surface area of carbon blacks is usually 30 - 150 m2/g, and even higher in "activated" color blacks. The particles consist of branched chains of fused approximately spherical primary particles. The graphene sheets show a preferred orientation parallel to the surface of the spherical particles. Carbon blacks contain relatively large quantities of foreign elements, mainly hydrogen but also oxygen, sulfur and nitrogen. Often aromatic compounds are adsorbed on the surface which can be extracted with hot solvents. Graphitization at 3300 K leads to a polyhedral shape of the primary particles, the surface consists almost entirely of basal planes, especially with low-surface area thermal blacks. Commercially produced Carbon fibers are obtained by controlled pyrolysis of suitable fibrous precursors, mainly poly(acrylonitri1e) or of fibers drawn from mesophase pitches. Stretching during carbonization results in a preferred orientation of the carbon layers parallel to the fiber axis. After heat-treatment at high temperatures such fibers have a high modulus and/or high tensile strength because the system of covalent bonds of the graphene layers is quite extended in the direction of the fiber axis. In the fiber cross-section, the layers are preferentiallyoriented radially in the case of pitch-basedfibers, whereas concentric shells, at last in the surface region, are observed with PAN-based fibers (ref.11). Therefore, the surface has more basal plane character in the latter case. Because of the relatively large diameter of the fibers (in the range of several m), their surface area is comparatively small. Even after heat-treatment at 3300 K there is practically no three-dimensional order of the graphene layers. Fibers of well-developed graphitic character have been prepared by thermal decomposition of benzene (or other hydrocarbons) on tinycatalyst particles, e.g. of iron, with following graphitization treatment (ref.12).

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SURFACE CHEMISTRY The atoms in the surface of diamond or at the edges of graphene layers are chemically unsaturated. The carbon atoms have free valences which may be saturated by chemisorption of foreign elements. This state has been described as "dangling bonds". Many carbons contain hydrogen originating from their formation from organic precursors. In the case of well-crystallized graphite, the basal planes are saturated, and there is no covalent bonding possible except at defect sites. It has been shown that practically no oxygen is covalently chemisorbed on the basal planes of graphite (ref.13). An 0 1s signal was given in XPS only by the prism faces of a piece of HOPG, not by the basal surface (ref.14). However, adsorption with charge transfer to or from the graphite is possible. Such an interaction is largely reversible, whereas desorption of foreign atoms bound to the "edge atoms" at the prismatic faces leads often to loss of carbon atoms as well, e.g. as CH4 or CO and C Q . Most or all carbons in the "asdelivered"state contain chemisorbed atoms, mostly hydrogen and oxygen. The surtace species are not stable at high temperature, and fairly "clean" carbon surfaces may be obtained by heating in vacuo to temperatures of 1270 K or higher. The last remnants of chemisorbed oxygen or hydrogen are lost at 1570 K. Such carbon surfaces are quite reactive, and the presence of localized free radicals has been shown by ESR (ref.15). However, their concentration was not equivalent to the quantity of "edge" atoms. Most of the work in this field deals with surface oxides whereas not much systematic work has been done on chemisorbed hydrogen, halogens. sulfur and nitrogen. The literature up to 196WO has been extensively reviewed (ref.16,17). Surface oxides on carbon will be described in the next chapter. Hydrogen, chlorine, or oxygen may be chemisorbed by treatment with these gases at elevated temperatures; oxygen reacts already at temperatures above 230 K (ref. 18). The chemisorption of hydrogen on diamond powder has been shown by infraredspectroscopy to produce CH2 groups (ref.5). With oxygen, carbonyl groups are formed, but also some EC-OH groups (ref.19). Similarly, hydrogen is chemisorbed by graphite and microcrystalline carbons (ref.20). The reaction with fluorine is difficult to control with finely divided carbons. CF, CF2, and CF3 groups have been detected by XPS after treatment at small partial pressures of F2 or F which were obtained by microwave discharges in SF6 or C2F6 (ref.21). Similar observations were made when diamond was treated either in the same way (ref.22) or with fluorine gas at room temperature (ref.5). Chlorine, too, is chemisorbed at elevated temperatures, and a CI concentration of 1.1 . 1015 atoms per cm2 of prism face has been estimated (ref.23). Fig.1 shows the C Is and Cl2p photoelectron spectra of graphite flakes which had been treated by the producer with Cla at high temperatures for purification. It is also possible to substitute hydrogen at the edge of the carbon layers by chlorine. At a reaction temperature of 723 K,HCI is relea-

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149

296

-

286eV 206 binding energy

196 eV

Fig. 1. Detection by XPS of chemisorbed CI on graphite flakes after treatment with CI2 at elevated temperatures (- 870'C) (ref. 14).

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sed, and CI is bound (ref.l6,24). Chlorine uptakes of 7.5 mmoPg, equivalent to 20 wt.%, have been observed with some carbon blacks. The same reaction has been described with bromine (ref.l7,25). Chemisorption of chlorine has also been observed during milling of graphite powder under CCl4 (ref.26). Very likely, CC14 reacts with broken bonds produced during milling, i.e. with free radicals. Graphite is attacked by CI and other free radicals. CC14 is photolyzed by UV light to CI + CCb radicals. After irradiating graphite under CCb, the presence of chemisorbed chlorine was deduced from the CI 2p and C 1s X-ray photoelectron spectra (ref.27). Apparently, also the basal planes of graphite, are attacked by the free radicals since even large Madagascar flakes were convertedto an evil-smelling sludge after prolonged irradiation. Sulfur, too, is chemisorbed on carbons at elevated temperatures. Very large uptakes by activated carbons have been described (ref.28), but it has been shown that only a relatively small quantity of the fixed sulfur was chemisorbed, the remainder being sulfur polyrnerized in the pore system during cooling. It could not transform into soluble S8 rings of u-sulfur because of steric hindrance in the narrow pores (ref.29). Chemisorptionof nitrogen is more difficult because of the stability of the N2 molecule. However, nitrogen is taken up by carbons on treatment with ammonia at 870-1170 K (ref.30-32) or with hydrogen cyanide at 1170 K (ref.32). As in the case of chemisorbed sulfur, very little is known about the bonding of the nitrogen atoms (see below). The surface free radicals produced by thermal decomposition of surface complexes can be used for the chemisorption of olefins (ref.33). A subsequent uptake of oxygen decreased stoichiometrically. The surface could be further derivatized after chemisorption of acrylyl chloride (ref.33):

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Ammonia and amines are also directly chemisorbed on carbon surfaces etched in an argon plasma (ref.34). It has been suggested that surface lactone groups are produced in an analogous fashion by chemisorption of CO2 to atomically clean graphite (ref.35): Oh

m

o=c=o

*

c- 0

r/'/y7

SURFACE OXIDES Surface oxides are an intermediate in the oxidation of carbons. It has been shown by use of the etch-decorationtechnique and electron microscopy, that defect-free basal planes of graphite are not attacked by molecular oxygen at ca. 920 K. Gasification occurs only at the edges of the graphene sheets and at vacancy sites (ref.36-39). Depending on the oxidation conditions, zig-zag or armchair structures of the oxidized edges are formed (ref.37). However, ozone and atomic oxygen are able to remove carbon atoms from the intact basal planes at 300-350 K (ref.36,40). The chemisorption of oxygen is, as to be expected, an activated process, and it has been observed with reactive graphite wear dust that no oxygen was bound at 77 K whilst chemisorption occured at higher temperatures (ref.18). Apparently identical surface oxides decomposing in the same TPD pattern were found in the adsorption of 0 2 , C02 and H20 on atomically clean polycrystalline graphite (ref.35). The authors assumed formation of semiquinone-type surface groups, i.e. carbonyl groups. Many attempts have been undertakento elucidate the structures of surface oxides on carbon. First, it should be pointed out that the surface oxides may impart basic or acidic propertiesto the carbon surface when it is immersed in water, depending on their formation conditions. When a carbon is exposed to oxygen at room temperature (after removal of surface complexes by heating in vacuo or under inert gases to 1170-1270 K), its surface acquires basic properties in aqueous suspensions, it has a positive surface charge and anion exchange properties (ref.41). When the same carbon is allowed to chemisorb oxygen at moderately elevated temperatures, e.g. 570-670 K, its surface will exhibit acidic properties and a cation exchange capacity. Protic solvents such as water are necessary for the formation of basic surface oxides. Some oxygen is chemisorbed when carbon outgassed at high temperatures is exposed to dry 02 at room temperature. The same quantity of 0 2 again is taken up on immersion of this carbon in water or aqueous acids (ref.42). One equivalent of acid is adsor-

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bed at the same time for each two oxygen atoms chemisorbed in the two steps. Garten and WeiO had ascribed the basic properties to chromene-like structures (ref.43). However, the observation that each basic site involvedtwo oxygen atoms as well as chemical evidence (ref.44) led us to assume pyrone-type structures: OH

0

(+)

X-

Very likely the carbonyl-type oxygen is bound in the first, anhydrous chemisorption step. This would agree with the observation by Marchon et,al. (ref.35). The acidic surface oxides are made up of a larger variety of functional groups. The presence of carboxylic groups (also as carboxylic anhydrides and lactones or lactols) and phenol-type hydroxyl groups has been inferred from chemical reactions (Fig.2, ref.45-49). Electrochemical studies indicate that carbonyl groups are arranged in such a way as to produce quinone-like character. They can be reduced reversibly to hydroquinones (ref.50).

(e)

(f)

(s)

(h)

Fig. 2. Possible oxygen-containing functional groups in surface oxides. (a) free carboxyl group, (b) carboxylic anhydride, (c) lactone, (d) lactol, (e) carbonyl group, (f) quinone, (9) phenol, (h) ether-type oxygen. The presence of an arm-chair structure of the layer edge must also be considered, see (f). The formation of acidic groups by oxidation with air begins slowly at temperatures as low as 373 K with high-surface area carbons such as activated charcoals (ref.51). Acidic surface groups can also be generated at room temperature by oxidation with strongly oxidizing, liquid media, e.g. concentrated nitric acid, aqueous solutions of NaOCI, (NH4)2S208 etc. (ref.45,46), or solutions of ozone in CCb (ref.52). The quantity of acidic groups on welloxidized carbons is usually much higher than the quantity of basic sites observed with the same carbon after appropriate treatment. The available space does not allow to discuss here the chemical reactions that led to the identificationof the various groups. Carboxylic groups are sufficiently strong acids to be neutralized by NaHC03 solution, whilst lactols etc., react only with Na2C03 or stronger

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bases, quite analogous to phenolphthalein. The very weakly acidic phenols need NaOH for neutralization. The acidic groups found on well-oxidizedcarbon accounted, however, for only approximately half of the oxygen bound on the carbon surface (ref.45). It is not known how the remaining oxygen was bound, very likely as carbonyl- and ether-type oxygen. It has been frequently, but by far not always, observed that the various groups of different acidity appear in equivalent concentrations in the case of well-oxidizedcarbons (ref.45,46). This has led to a speculation that the groups occur as an ensemble which is an intermediate in the gasification of carbon. However, the ratio of the functional groups seems also to depend on the type of carbon. Spectrometric methods have been used to identify the surface groups. The results are not entirely satisfying. The strong absorption of carbons makes the application of infrared spectroscopy very difficult. Some spectra with very finely dispersed carbons confirmed the results of the chemical methods. By far the best IR spectra were obtained by thermal beam deflectionspectroscopy which is related to photoacoustic spectroscopy (ref.53). However, even with this method useful spectra were obtained only with carbons (cellulose chars) which had not been heat-treated to higher temperatures than 1150 K. The spectra showed in oxidized carbons the presence of C=O groups of cyclic carboxylic anhydrides (1760 cm"), carbon-carbon double bonds turned infrared-activeby chemisorption of oxygen (1600 crn-'), and of C - 0 single bonds giving rise to a broad band centered at 1260 cm-' (ret54,55). Sensitive tools for the detection of chemisorbed oxygen are Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Both methods have been frequently used, especially with materials of low surface area such as carbon films or fibers. Oxidation of the carbon fiber surface is essential for adhesion and good reinforcement in carbon fiber - polyester resin composite materials. The presence of chemisorbed electronegative elements gives rise to small component peaks at the high binding-energy side of the C 1s peak as can be seen in Fig.1. Peak positions shifted by 1.6 eV, 3.0 eV and 4.2 eV from the main peak are ascribed to ether and hydroxyl groups, carbonyl groups and carboxyl groups, respectively (ref.56.58). The position of the 0 1s signal is influencedby the type of bonding of the oxygen. However, the range of binding energies is relatively small, and the resolutionof the signals is limited, therefore. Most studies show two main signals near 532.8 and 531.O eV which are associated with ether and carbonyl oxygen, respectively (ref.56-58). The various functional groups differ in thermal stability. The carboxyl groups are least stable. They decompose already above ca. 570 K. giving mostly CO2. Samples oxidized with aqueous oxidants at room temperature are especially rich in carboxyl groups. Such samples lose about half of their carboxyl groups between ca. 470 and 570 K (ref.45,46,59). There have been many attempts to ascribe formation of CO and CO2 during vacuum pyrolysis to definite functional groups (ref.47). However, the results of thermodesorption from oxidized carbons depend on the type of carbon used (ref.59,60), as

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well as on the experimental conditions (ref.59). Other authors concludedthat both gases may derive from the same sites (ref.61). Usually most of the CO2 is evolved from 570 K to 870 K,whereas CO formation peaks at 870 - 970 K, and is not complete at 1170 K. At high temperatures also some hydrogen is evolved (ref.47). The surface oxides on a carbon surface provide hydrophilic adsorption sites, and water adsorption is greatly enhanced (ref.62). It has been observed with samples of oxidized diamond powder and with other substances that active hydrogen-bearing groups (i.e. -OHand -COOH) are primary adsorption sites for water molecules in a 1:1 ratio (ref.63). Well-oxidized carbons become hydrophilic, and are easily dispersed in water (ref.5,52).

INFLUENCE OF SURFACE GROUPS ON CATALYTIC PROPERTIES Activated carbons have been used since long times as catalysts, e.g. in the addition of CI2 to CO or So;!in the production of COCh or SO2C12. Activated carbons are also used as cathodes in the electrochemical reduction of oxygen in fuel cells. There are vast differences in the activities of various types of carbon in these applications. It has been found that the catalytic activity can be enhanced by treatment with ammonia at high temperatures (ref.30-32). Insteadof ammonia also hydrogen cyanide can be used (ref.32). The increase of catalytic activity has been tested by several oxidation reactions with molecular oxygen, e.g. of sulfurous acid (ref.31), or of oxalic acid, Fe2+, methanol etc. (ref.32,64).

Fig. 3. N 1s photoelectronspectra of activated carbon Anthralur, treated with NH3 at 1170 K. (a) outgassed at 423 K, (b) treated with H2 at 970 K, (c) treated with H2 at 1170 K.

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Table 1 Influence of carbon-support pretreatment on activity of palladium in hydrogenolysis of ethane at 523 K (ref. 64). (Support: Anthralur; Pd applied by incipient wetness technique, reduction at 673 K, flowtype microreactor). Pretreatment vac. - 573 K

02 - 673 K NH3-1170K

Pd loading pmollg 100 200 400 100 200 400 100 200 400

dispersion (CO adsn.) 0.12 0.09 0.08 0.09 0.09 0.08 0.1 1 0.13 0.16

% -conversion Of

C2H6 0.005 0.075 0.48 0 0 0.05

0.30 5.20 23.9

specif. rate mmol/h rn2m 0.1

0.8 3.2 0 0 0.3 5.5

38.0 74.0

Considerable quantities of nitrogen atoms are bound on the carbon surface on treatment with NH3 or HCN, i.e. more than 2 mmol/g with activated charcoals of ca. 800 m2/g surface area. All attempts to identify the chemisorbed species by chemical reactions failed, but a broad N 1s signal with two main peaks was obtained in XPS (Fig.3). The peak positions at binding energies of 400.5 and 398 eV might be indicative of the presence of arnines and pyndine-like nitrogen. However, small concentrations of other species might be hidden under the broad signal. Removal of a large part of the bound nitrogen by treatment with hydrogen at 1170 K caused a decrease of the N 1s signal, especially of the peak at 398 eV. The catalytic activity was not impaired by this treatment (ref.65). Acidic surface oxide on the carbons inhibit the catalytic activity. Activated carbons are also used as supports for metal catalysts. The catalysts are usually prepared by the liquid-impregnation method. The presence of surface oxides can be beneficial in this case, since anions, e.g. PtCls2-,are adsorbed on basic surface sites, and cations, e.g. Pt(NH3)4'+, on acidic surface sites. This can lead to a better dispersion of the reduced metal as indicated by Attwood et al. (ref.66) who used [Pt(NH&](OH)2 with oxidized carbon-fiber paper. A high concentration of chemisorbed platinum ions will lead to a high number of Pt nuclei during reduction. The dispersion of platinum deposited on graphitized carbon black increased when the support had been activated by partial combustion with air (ref.67,68). This effect has been explained by the increase in heterogeneity of the surface. However, this treatment created additional "edge" atoms which were covered by acidic surface oxides which may adsorb PtCls2- ions. To enhance the effect, the authors treated the activated carbon with concentrated nitric acid (ref.69). In the same line are results by Jung et al. (ref.70) who found a much higher dispersion of iron (after reduction) on an activatedcolor black with more than ten percent of bound oxygen than on non-

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oxidized carbons, e.g. graphitized carbon black. The iron had been applied as a nonaqueous solution of Fe(N03)s. However, the presence of surface oxides or of chemisorbed nitrogen (after treatment with NH3 at 870 - 1170 K) affects also the catalytic properties of the metals doposited on the carbon (ref.71). An example is shown in Table 1. In this case, hydrogenolysis of ethane on supported palladium has been used as a test reaction. There was also an influence of the ammonia treatment on the dispersion of the palladium on the activated-carbon support. Therefore, the catalytic activities are presented in the table per m2 of metal surface. Practically no hydrogenolysis activity was observed when the suppott had been pretreated by oxidation with 0 2 , although the acidic surface oxides formed in the pre-treatment had certainly been partially destroyed during the reduction of the metal chloride with hydrogen at 673 K. Other authors described an increased activity of carbon-supportediron in the hydrogenolysis of n-butane when the support had been treated with ammonia (ref.72). The activity of carbon catalysts in the oxidation of dilute sulfurous acid to sulfuric acid (with 0 2 at 293 K) is considerably enhanced when noble metals are deposited on the carbon surface. With identical metal loadings, the increase in activity is still higher when ammonia-treated carbons are used (ref.71).

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a

a

157

41 H.R. Kruyt and G.S. de Kadt, Kolloid-Z. 47 (1929) 44; I.M. Kolthoff, J. Am. Chem. SOC. (1932) 4473-4480. 42 M. Voll and H.P. Boehm, Carbon 6 (1970) 741-752. 43 V.A. Garten and D.E. Weiss, Rev. Pure Appl. Chem. 7 (1957) 69-122. 44 M. Voll and H.P. Boehm, Carbon 9 (1971) 481-488. 45 H.P. Boehm, E. Diehl, W. Heck and R. Sappok, Angew. Chem. 16 (1964) 742-751; Angew. Chem. Internat. Ed. Engl. a (1964) 669-677. 46 H.P. Boehm, E. Diehl and W. Heck, Proc. 2nd London Carbon and Graphite Conf. (1965); SOC.Chem. Ind., London, 1966, pp. 369-379. 47 S.S. Barton, G.L. Boulton and B.H. Harrison, Carbon 1p (1972) 395-400; S.S. Barton, D. Gillespie and B.H. Harrison, Carbon 11(1973) 649-654. 48 S.S. Barton and B.H. Harrison, Carbon 13(1975) 283-288; S.S. Barton, D.J. Gillespie, B.H. Harrison and W. Kemp, Carbon 16 (1978) 363-365. 49 J.B. Donnet, Carbon 6 (1968) 161-176; 2Q (1982) 267-282. 50 K.F. Blunton, Electrochim. A c t a B (1973) 869-875; J.P. Randin and E. Yaeger, J. Electroanal. Chem. 58 (1975) 313-322. 51 H. Oda and H.P. Boehm, in preparation. 52 J.B. Donnet and E. Papirer, Bull. SOC.Chim. France=, 1912-1915. 53 C. Morterra and M.J.D. Low, Spectrosc. Lett. X (1982) 689-697. 54 C. Morterra and M.J.D. Low, Carbon 21 (1983) 283-288. 55 C. Morterra, M.J.D. Low and A.G. Severdia, Carbon 22 (1984) 5-12. 56 D.T. Clark and H.R. Thomas, J. Polymer Sci. (Polymer Chem. Ed.) (1978) 791-820. 57 S. Evans and J.M. Thomas, Proc. Roy. SOC.London A 353 (1977) 103-120. 58 A. Proctor and P.M.A. Sherwood, Carbon 21 (1983) 53-59. 59 H.P. Boehm and G. Bewer, Proc. 4th London Internat. Carbon and Graphite Conf. (1974), SOC.Chem. Ind., London, 1976, pp. 344-359. 60 S.S. Barton, D. Gillespie and B.H. Harrison, Carbon 11(1973) 649-654. 61 L. Bonnetain, X. Duval and M. Letort, Proc. 4th Bienn. Conf. on Carbon, Pergamon Press, Oxford, 1960, p. 107-114; L. Bonnetain, J. Chim. Phys. (1961) 34-46. 62 V.R. Deitz and J.A. Rehrmann, Ext. Abstr., 18th Bienn. Conf. on Carbon, Worcester, MA (1987), pp. 100-101. 63 R. Sappok and H.P. Boehm, Z. Anorg. Allg. Chem. 365 (1969) 152-156. 64 A. Vass, Th. Stohr and H.P. Boehm, Proc. Carbont36, Internat. Carbon Conf., BadenBaden, 1986, pp. 41 1-413. 65 B. Stohr, Th. Stohr and H.P. Boehm, Ext. Abstr. CarbonW, Internat. Carbon Conf., Newcastle, 1988, in print. (1981) 287-295. 66 P.A. Attwood, B.D. McNicol and R.T. Short, J. Catal. (1976) 61-67. 67 P. Ehrburger and P.L. Walker, Jr., J. Catal. 68 A. Linares-Solano, F. Rodriguez-Reinoso, C. Salinas-Martinez de Lecea, O.P. Mahajan and P.L. Walker, Jr., Carbon 20 (1982) 177-184. 69 P. Ehrburger, O.P. Mahajan and P.L. Walker, Jr., J. Catal. g (1978) 63-70. 70 H.-J. Jung, P.L. Walker, Jr. and M.A. Vannice, J. Catal. Z (1982) 416-422. 71 Th. Stohr, Thesis, Univ. Munchen, 1987; Th. Stdhr and H.P. Boehm, in preparation. 72 A. Guerrero-Ruiz, 1. Rodriguez-Ramos, F. Rodriguez-Reinoso, C. Moreno-Castillaand Jr. D. Lbpez-GonzBlez, Carbon E (1988) 417-423.

a


C . Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces Q 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

A

159

COMPARISON BETWEEN THE LEWIS ACIDITY OF NON-d METAL CATIONS IN Y-ZEOLITES AND ON IONIC SURFACES

V. BOLIS, B. FUBINI, E. GARRONE, E. GIAMELLO and C. MORTERRA Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita' di Torino. Via P. Giuria 7, 1 0 1 2 5 Torino (Italy).

.ABSTRACT Heats of adsorption and IR spectra of CO on Na-Y, 7 2 % Ca-Y and 1 5 % Zn-Y zeolites have been measured at room temperature. The data fit a correlation between stretching frequency and molar heat of adsorption already found for non-d cations in ionic systems. For the same cation, both stretching frequency and molar heat of adsorption measured on Y-zeolite are higher than the corresponding ones on ionic surfaces, probably because of the low coordination assumed by the cations in the Y-zeolite supercage. INTRODUCTION Carbon monoxide is widely used as a probe molecule to characterize the surface Lewis acidity of solids (ref. 1). On non-d metal cations it chemisorbs weakly, giving rise to stretching modes that are shifted to frequencies higher than that of the free molecule (2143cm-'), in that a plain o-coordination occurs. We have recently studied the interaction of CO with non-d cations on ionic surfaces by coupling IR spectroscopy and adsorption microcalorimetry (refs. 2-4). Correlations were found between spectroscopic and energetic data, namely between V , the stretching frequency of adsorbed CO and E , its molar extinction coefficient, on the one hand, and AaE, the molar adsorption enthalpy, on the other (ref. 3 ) . Such correlations were not unexpected. In fact it is known that the shift of the stretching frequency depends upon the polarizing power of the cation, i.e., its acidic strength (refs. 51-61, and the molar adsorption enthalpy directly measures the acid-base interaction. In the present paper we studied with the same techniques the

160

interaction of CO with non-d metal cations dispersed in the Yzeolite framework, in order to check whether vibrational and energetic data for these systems obey the same kind of correlation. Several years ago Angel1 and Schaffer (ref. 7) and Egerton and Stone (refs. 8-10) studied the interaction of CO with metal cations exchanged Y-zeolites, and linear correlations were proposed between stretching frequency and electrostatic field by the former authors, and between stretching frequency and isosteric heats by the latter.

EXPERIMENTAL Linde Na-Y was used as received. Ca-Y and Zn-Y were prepared by standard exchange methods with aqueous solutions from Na-Y [8l.The extent of exchange was 72% for Ca-Y. and 15% for Zn-Y. All samples were pretreated in vacuo at 620 K in order to eliminate completely adsorbed molecular water. CO specpure (Matheson) was employed. Infrared spectra were taken on a vacuum-purged Bruker FS 113v spectrometer, following a strictly in situ procedure, so that the absorbance spectrum of adsorbed CO could be directly computed by ratioing the spectra at the various CO pressures against the spectrum of the bare solid. All absorbance spectra were substracted of the contribution of the CO gas-phase. The heats of adsorption of CO were measured by means of a TianCalvet microcalorimeter kept at 303 K, connected to a volumetric apparatus which enables the simultaneous determination of adsorbed amounts (ref. 11). The temperature of 303 K was chosen because it is the estimated temperature of the sample during the IR measurements. Pressures were measured by means a 0-100 torr transducer gauge (Baratron MKS): the highest pressure measured with the required accuracy is some 70 torr (1 torr = 133.3 Pa).

RESULTS AND DISCUSSION Sections a and b of Figure 1 report the infrared spectra of CO adsorbed at various equilibrium pressures on .Na-Y and Ca-Y, respectively. In Fig. la (Na-Y), a band at 2169 cm-' is observed, readily

161

ascribed to CO a-coordinated to Na' ions, together with two shoulders at 2155 and 2180 cm-l. Another band is present at 2121 cm-l, i.e., at a frequency lower than that of the gas. The relative intensity of all observed bands and shoulders is fairly constant over the whole pressure range examined.

Fig. 1. Infrared spectra of CO adsorbed on: (a) Na-Y and (b) Ca-Y in the pressure range 0.2-100 torr. Fig. lb (Ca-Y) is quite similar to Fig. la. A main band is seen at 2197 cm-l with two shoulders at 2211 and 2188 cm-l, all ascribable to CO a-coordinated to Ca2+ cations. A low frequency band is also observed at 2096 cm". At high pressures, the band at 2169 cm'l, related to non-exchanged Na+ cations, appears together with its low frequency partner (2121 cm-l). The stretching modes of CO a-coordinated to Ca2+ fall at frequencies (2197-2211 cm-l) higher than those of CO a-coordinated

162

on Na' (2159-2180 cm-l), consistently with the higher polarizing power expected of the divalent cation with respect to the monovalent one. The exact nature of the low frequency bands is quite puzzling and will be discussed in more detail elsewhere. The old suggestion by Angel1 and Schaffer (ref. 7) (who, however, did not realize that the spectral location of the low V bands strictly depends on the cation) that they are due to an interaction with the lattice, has probably to be interpreted in the sense that the CO molecule, already a-coordinated to the cation through the C end, may also interact via the 0 end either with another guest cation "a+, Ca2+) or, more likely, with some Lewis centres of the lattice. The intensity of the low frequency band is some 5% of that of the high frequency one in the case of Ca-Y, whereas the ratio is about 30% in the case of Na-Y. still, from a quantitative point of view, also in the latter case the presence of the species related to the low frequency band can be thought as negligible, as the extinction coefficient become very high when the CO stretching mode is below the gas phase value (ref. 12). Figure 2 reports the adsorption data concerning the interaction of CO with Na-Y and Ca-Y. The volumetric isotherms are shown in Fig. 2a: it can be noted that, in the Na-Y case, the isotherm is still in the Henry region, whereas for Ca-Y the isotherm is of the Langmuir-type, and still far from the monolayer. Figure 2b shows that the energy of interaction of CO with Na' is fairly constant with coverage at 2 8 kJ mol-l. As for the adsorption isotherm for Ca-Y (curve 2 in Fig.Za), it is reminded that it should be thought as due to the superposition of 72% contribution from Ca2+ and 28% from Na+ ions. The latter contribution should be thus of the order of 28% of the (small) amount adsorbed when only Na' is present, and given by curve 1 in Fig. 2a. It is thus deduced that the contribution of Na+ cations to the overall adsorption can be assumed as negligible, in good agreement with the relative intensity in the IR spectra. Consequently the partial molar heat (curve 2 in Fig. 2b) may be ascribed entirely to the interaction of CO on Ca2+ (50 kJ mol-'1. The adsorption on both Na-Y and Ca-Y appears to be close to ideality (Langmuir behaviour), as far as both the isotherm and the energetic requirements are concerned. The multiplicity of IR bands observed on Fig. 1 is not in contrast with the observed overall

163

ideality, as the various species grpw together.

2

1 0 I-

0

3 0 6 0 9 0 pltorr

Fig. 2. (a): volumetric isotherm of CO adsorbed on Na-Y (1) and on Ca-Y (2) (b): partial molar heat of adsorption of CO on Na-Y (1) and Ca-Y (2) as a function of adsorbed amount.

.

'

An opposite situation is encountered with Zn-Y. Because the divalent cation is present in low amount (15%) together with an excess of Na+ centres , adsorption occurs simultaneously and competitively on both kinds of centre. This fact accounts for the simultaneous presence, at all CO pressures, of absorption bands (Fig. 3a) corresponding to the a-coordination on the two acidic centres: at 2169 cm-' (Na+/CO) and at 2210 cm-l (Zn2+/CO). The 2169 cm-l band is accompanied by its 2121 cm-l partner, but no low frequency partner is observed for the 2210 cm-l band, the latter being probably too weak to be detected. As expected, the overall isotherm (not reported for sake of brevity) is non-Langmuirian, and the partial molar heats of adsorption (reported in Fig. 3b) are observed to decrease almost linearly. The spectral counterpart of this is shown by the IR spectra in Fig. 3a: the band due to the Zn2+/C0 a-interaction at 2210 cm-l saturates quickly with pressure, whereas the band at 2169 cm-l, due to the Na'/CO interaction, present from the very

164

beginning, keeps growing in the whole pressure range explored.

t

Fig. 3 . (a): Infrared spectra of CO adsorbed on Zn-Y in the pressure range 0.2-100 torr. (b): partial molar heat of adsorption as a function of adsorbed amount. An estimate of the energy of o-interaction of CO with Zn2+ is provided by the extrapolation of the partial molar heat curve at The higher value of the zero coverage: the result is 58 kJ mol". stretching frequency (2210 ern-') with respect to that of co adsorbed on Ca2+ (2197 cm-') indicates that the polarizing power of the zinc cation, because of its smaller ionic radius, is higher than that of the calcium. The heat of interaction is consequently larger. The stretching frequency values of CO adsorbed on Ca2+ and Zn2+ cations on Y-zeolites are in good agreement with those reported by Angel1 and Schaffer (ref. 7). The same authors considered the band at 2169 cm'l as due to non-specific interaction of CO just as the low frequency one, in that the two bands were observed on all their samples regardless of the zeolite cation composition. We have indeed shown that this band is due to o-coordination of CO

165

with Na+. The heats of adsorption of CO on Na+, Ca2+ and Zn2+ measured by us are slightly larger than the isosteric heats at low coverage reported by Egerton and Stone (ref. lo), which are 25, 45 and 52 kJ mol-l , respectively. The discrepancies are not enormous, if account is taken of the difference in the measuring procedure. Moreover, in the case of Ca2+ and Zn2+, the lower values of the isosteric heat may be partially ascribable to the interference of the adsorption on the weaker Na+ sites.

c I

€ u D \

I

0

-30

-60

Aa Go/k J mot

-90

Fig. 4. Stretching frequency vs. molar enthalpy of adsorption for Na-Y, m Ca-Y, A Zn-Y CO adsorbed on various non-d cations. (present work) ; 0 NaC1, A ZnO, 0 6 e Ti02, v A 1 2 0 3 (ref. 4 and references therein). x Stretching frequency of CO gas.

In Figure 4 the stretching frequency value and the molar enthalpy of adsorption (defined as AaF = q + RT, ref. 2 ) are reported for Na', Ca2+ and Zn2+/C0 a-complexes, and compared with the analogous data for non-d metal cations in ionic surfaces (ref. 3 ) . It appears that: i) a common correlation curve holds for both zeolitic and ionic systems. The linear correlation proposed by Egerton and Stone (ref. 10) is not in contrast with

166

this finding in that the curve in Fig. 4 extends over a larger set of different systems and also comprehends the stretching frequency of the CO molecule for vanishing molar adsorption enthalpy; ii) the polarizing power of a cationic centre depends on the nature of the matrix: Na+/CO and Zn2+/C0 are characterized by different shifts of the stretching frequency and different enthalpy of adsorption according to the location of the cation in an ionic or zeolitic matrix; iii) the strength of the interaction in both cases is larger in zeolites than on ionic systems. The latter feature, i.e., the higher polarizing power of cations in Yzeolite framework with respect to ionic systems, is probably related to the low coordinative situation assumed by chargebalancing cations in Y-zeolite supercages. ACKNOWLEDGEMENTS The authors are indebted to Mme A. Auroux (Institut de Recherches sur la Catalyse, CNRS, Villeurbanne, France) for kindly supplying Linde Na-Y. REFERENCES 1 2 3

4 5

L.H. Little, Infrared Spectra of adsorbed species, Academic Press, London and New York, 1966. E. Garrone, G. Ghiotti, E. Giamello and B. Fubini, J. Chem. SOC. Faraday Trans. I, 77 (1981) 2613-2620. C . Morterra, E. Garrone, V. Bolis and B. Fubini, Spectrochim. Acta, 43 (1987) 1577-1581. V. Bolis, B. Fubini, E. Garrone and C. Morterra, J. Chem.Soc. Faraday Trans. I, in press. R.H. Hauge, S.E. Gransdenand, J.L. Margrave, J.C.S. Dalton, (1979) 745-748.

7

R. Larsson, R. Lykvist and B. Rebenstorf, 2 . phys. Chemie, Leipzig 263 (1982) 1089-1104. C.L. Angel1 and P.C. Schaffer, J. Phys. Chem., 70 (1966) 1413-

8

T.A. Egerton and

6

1418.

F.S. Stone, J. Chem. SOC. Faraday Trans. I,

66 (1970) 2364-2377. 9

T.A. Egerton and F . S . Stone, J. Coll. and Interface Sci., 38

10

T.A. Egerton and F.S. Stone, J. Chem SOC. Faraday Trans. I, 69

11 12

B. Fubini, Thermochim. Acta, 135 (1988) 19-29. D.A. Seanor and C.H. Amberg, J. Chem. Phys. 42 (1965) 2967-

(1972) 195-204. (1973) 22-38. 2970.

C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

16i

PROMOTION AND SELECTIVE POISONING OF SUPPORTED METAL CATALYSTS

G.C.

BOND, M.R.

GELSTHORF'E, R.R.

RAJARAM and R. YAHYA

Department of Chemistry, Brunel U n i v e r s i t y , Uxbridge UB8 3PH (United Kingdom)

ABSTRACT The term 'promoter' i s g e n e r a l l y a p p l i e d t o substances added t o m e t a l l i c c a t a l y s t s t o improve t h e i r performance. There i s no d i s t i n c t i o n i n p r i n c i p l e between promoters t h a t enhance s e l e c t i v i t y o r l i f e t i m e , and s e l e c t i v e poisons t h a t e l i m i n a t e unwanted r e a c t i o n s . This concept i s i l l u s t r a t e d by r e f e r e n c e t o ( i i ) t r a n s i t i o n metal oxide a d d i t i v e s t o t h e e f f e c t of ( i ) Re on Pt/Al 0 Ru/Si02, and ( i i i ) high-tempe?a&re r e d u c t i o n of Ru/Ti02, using alkane hydrogenolysis and i s o m e r i s a t i o n a s t e s t r e a c t i o n s . The r e s u l t s a r e i n t e r p r e t e d i n terms of a decreasing s i z e of t h e a c t i v e metal ensemble, and, w i t h Pt-Re, the c r e a t i o n of new a c t i v e c e n t r e s .

INTRODUCTION

Promotion and s e l e c t i v e p o i s o n i n g A very important f e a t u r e i n t h e p r a c t i c a l a p p l i c a t i o n of m e t a l l i c c a t a l y s t s i s t h e i n c l u s i o n of one o r more components which of themselves have l i t t l e o r no c a t a l y t i c a c t i v i t y b u t which c o n t r i b u t e s i g n i f i c a n t l y t o o v e r a l l performance. The support when p r e s e n t i s c l e a r l y one such component, b u t t h i s paper i s mainly concerned w i t h o t h e r s which are c o l l e c t i v e l y d e s c r i b e d a s promoters o r sometimes a s s e l e c t i v e poisons.

Such a d d i t i o n a l components however f u l f i l a v a r i e t y of

f u n c t i o n s , and it i s t h e r e f o r e d e s i r a b l e t o t r y t o d e r i v e a more s y s t e m a t i c terminology t o d e s c r i b e t h e v a r i o u s a d d i t i v e s used and t h e r o l e s t h a t they p l a y . E a r l y work on NH3 s y n t h e s i s i d e n t i f i e d t h e need f o r a d d i t i v e s t o a c t as

--

s t r u c t u r a l and e l e c t r o n i c promoters.

I n t h e development of Fischer-Tropsch

s y n t h e s i s i t w a s a l s o found necessary t o i n c o r p o r a t e a s e l e c t i v i t y promoter t o c o n t r o l t h e type of product formed, and an a c t i v i t y maintenance promoter t o e n s u r e an adequate l i f e t i m e f o r t h e c a t a l y s t .

The c h i e f f u n c t i o n of t h e l a t t e r

was t o l i m i t carbon d e p o s i t i o n , and i t simultaneously a c t e d a s a s e l e c t i v e poison of p a r a s i t i c r e a c t i o n s .

There i s t h u s no d i s t i n c t i o n i n p r i n c i p l e

between an a d d i t i v e t h a t promotes s e l e c t i v i t y o r r e t e n t i o n ofactivity

and a

s e l e c t i v e poison t h a t minimises unwanted r e a c t i z . Substances added t o supported metal c a t a l y s t s s e r v e s i m i l a r f u n c t i o n s .

They

may be c l a s s i f i e d a s follows: ( i ) e l e c t r o n - r i c h m e t a l s forming a l l o y s o r b i m e t a l l i c c l u s t e r s w i t h t h e a c t i v e metal;

( i i ) e1ec)tron-rich non-metals and

168 t h e i r compounds;

( i i i ) i o n s of e l e c t r o p o s i t i v e e l e m e n t s ;

and ( i v ) 0x0-species

of elements i n t h e c e n t r e of t h e T r a n s i t i o n S e r i e s , o r t h e corresponding m e t a l . Table 1 p r e s e n t s some examples of t h e s e a d d i t i v e s and o f t h e e f f e c t s t o which they g i v e r i s e .

When o x i d e s , which when used as s u p p o r t s show t h e phenomenon of

Strong Metal-Support innocuous oxide (e.g. produced ( r e f s . 1 , 2 ) . f o r debate ( r e f . M/SiO

I n t e r a c t i o n (SMSI), a r e added t o a metal s u p p o r t e d on an S i 0 2 ) , e f f e c t s similar t o t h o s e caused by SMSI a r e The p h y s i c a l chemistry u n d e r l y i n g SMSI i s s t i l l a matter

3 ) , b u t t h e u s e o f , f o r example, Ti02 as an a d d i t i v e t o an

c a t a l y s t may h e l p t o d i s t i n g u i s h e l e c t r o n i c from s t e r i c f a c t o r s .

The 2 procedure of u s i n g t h e o x i d e of i n t e r e s t a s an a d d i t i v e r a t h e r than a s a s u p p o r t h a s t h e f o l l o w i n g advantages: ( i ) v a r i o u s o x i d e s may b e added t o a s t a n d a r d supported m e t a l , t h e p a r t i c l e s i z e of which may remain c o n s t a n t ; ( i i ) t h e o x i d e : metal r a t i o may be v a r i e d ;

and ( i i i ) v a r i o u s ways of assembling

t h e system can

be t r i e d ,

In t h i s p a p e r we p r e s e n t some of o u r r e c e n t work, i l l u s t r a t i n g t h e u n d e r l y i n g u n i t y between t h e concepts of promotion, s e l e c t i v e p o i s o n i n g and SMSI.

TABLE 1 Examples of s e l e c t i v i t y promoters f o r m e t a l c a t a l y s t s and of t h e r e a c t i o n s they affect Class

Category and Examples

Effects

1

Electron-rich metals, e.g. Cu, Ag, Au, Hg, Sn, Pb e t c

( a ) Improved s e l e c t i v i t y i n hydrogenation of m u l t i p l e bonds (b) Suppression of h y d r o g e n o l y s i s a c t i v i t y i n alkane transformations

2

Electronegative species, e.g. S, C1, N bases e t c

( a ) Suppression of h y d r o g e n o l y s i s i n petroleum reforming ( b ) Supuression of n o n - s e l e c t i v e o x i d a t i o n i n oxirane synthesis ( c ) Improved s e l e c t i v i t y i n hydrogenation of m u l t i p l e bonds ( L i n d l a r c a t a l y s t )

3

Oxides of e l e c t r o p o s i t i v e elements, e . g . L i , Na, K , C s , Ca, Mg, L a e t c

( a ) Suppression of carbon d e p o s i t i o n ( b ) M o d i f i c a t i o n of chain-growth in F i s c h e r Tropsch s y n t h e s i s

4

Kid-Transition S e r i e s elements and 0x0-compounds, e . g . R e , MOO VOx, T i O x , X’ Ce02

( a ) Improved y i e l d s of oxygenates i n F i s c h e r Tropsch s y n t h e s i s (b) Improved a l k e n e / a l k a n e r a t i o i n F i s c h e r Tropsch s y n t h e s i s

EXPERIMENTAL. P t - c o n t a i n i n g c a t a l y s t s w e r e AKZO p r o d u c t s (0.3% Pt/A1203, CK303; Pt/A1203, CK306;

and 0.3% Pt-O.3X Re/A1203, CK433):

1 h) and reduced (49OoC, 1 h ) immediately b e f o r e u s e .

0.6%

t h e y were c a l c i n e d (49OoC, Re/A1203

c a t a l y s t s were

prepared i n t h e L a b o r a t o i r e de R&activit&’de S u r f a c e e t S t r u c t u r e , U n i v e r s i t e

169 P. and M. C u r i e , by impregnating t h e same A1203 s u p p o r t w i t h aqueous NH Re0

4 4: A l l these

a f t e r d r y i n g , p r e c u r s o r s r e c e i v e d t h e same p r e t r e a t m e n t a s above. c a t a l y s t s were used i n an unsulphided s t a t e . To p r e p a r e t h e promoted 1%Ru/SiO

2

s e r i e s of c a t a l y s t s , a b a t c h of RuC13/Si02

(Degussa A e r o s i l ) was f i r s t prepared by impregnation and d r y i n g ( a c t u a l [Ru] 0.78 wt.X).

Aqueous s o l u t i o n s of t h e f o l l o w i n g compounds were then added t o about 0.1 w t . % o f M):

g i v e an M/Ru r a t i o of 0.25 ( i . e .

Ca(OH)2, S C ( N O , ) ~ . ~ H ~ O ,

(NH4)21:VO(C204)2! .2H20, (NH4)2Cr207 and M ~ ( O A C ) ~ T. i was a p p l i e d a s an 'PrOH They a r e d e s i g n a t e d a s MOx-Ru/Si02.

s o l u t i o n of Ti(01Pr)4.

m a t e r i a l s were c a l c i n e d (475OC, 16 h) b e f o r e r e d u c t i o n . used f o r t h e hydrogenolysis of n-C4H10 485OC, 16 h (code HTR1);

1 h (code LTR);

A f t e r being d r i e d ,

Each sample was then

o r o t h e r alkane a f t e r ( i ) r e d u c t i o n a t

( i i ) o x i d a t i o n (35OoC, 1 h) and r e d u c t i o n a t 16OoC, 0

and ( i i i ) a second r e d u c t i o n a t 485 C , 16 h (code HTRL).

c a t a l y s t s employed Degussa P-25 Ti02 a s s u p p o r t ; 2 given l a t e r .

Ru/TiO

A l l r e a c t i o n s were performed i n continuous-flow

further details are

systems o p e r a t i n g a t

atmospheric p r e s s u r e .

For alkane hydrogenolysis, sample weights were u s u a l l y -1 H2 100 ml min ; N 2 , ~ 0 . 2 gand f l o w - r a t e s a s f o l l o w s : a l k a n e , 10 ml min-'; 30 ml min-'.

To o b t a i n meaningful r e s u l t s down t o a f r a c t i o n a l conversion of

p a r t i c u l a r c a r e was needed t o c o r r e c t f o r i m p u r i t i e s i n t h e r e a c t a n t alkane.

For alkane r e a c t i o n s , temperatures were r a i s e d s t e p w i s e t o o b t a i n

activation energies.

For P t , Re and Pt-Re c a t a l y s t s , r a t e s were a l s o measured

a s t h e temperature was r e t u r n e d t o i t s i n i t i a l v a l u e ; completed t h e f i r s t

cycle which

t h i s second sequence

was then u s u a l l y r e p e a t e d once o r twice more i n

o r d e r t o g i v e information on d e a c t i v a t i o n .

RESULTS Treatment of experimental r e s u l t s Products of t h e r e a c t i o n of n-C4H10 going i s o m e r i s a t i o n , S T ; C3H8,

s1

a r e f i r s t d e f i n e d by t h e f r a c t i o n under-

the hydrogenolysis products (CH4, S1;

C2H6,

S2;

and

S ) were then renormalised, and a r e d e f i n e d by t h e e q u a t i o n 3

+ 2s* +

3s2

4.

=

R e l a t e d d e f i n i t i o n s apply t o iso-C4H10

Theoretical a n a l y s i s i s based

and C3H8.

on a comprehensive r e a c t i o n scheme, f i r s t proposed by Kempling and Anderson ( r e f . 4 ) , which may be d e p i c t e d a s follows f o r n-C4H10.

.. .. . c4ti. . . .ii; . c4*

c3* F

-*

C2*

C1

gas phase

Cl*

s u r f a c e phase

.. . . . . . * t. . . . ... . . . . ... .. *

170 Each u n i t s t e p is aasigaed a rate c o n s t a n t , k! f o r converting C.* 1

i n t o i t s gas-

phase analogue, and ki* f o r i t s cracking to s p e c i e s c o n t a i n i n g fewer C atoms. Defining Ti a s

and the s p l i t t i n g f a c t o r F as shovn i n t h e scheme, s t e a d y - s t a t e a n a l y s i s ( r e f . 4) a t low conversion g i v e s S2

(1 + F

-

S3)/T2;

S3

(1

- P)T3.

Values of the t h r e e unknowns cannot be d e r i v e d from t h e v a l u e s of Si f o r n-C4H10 since only two are independently v a r i a b l e :

however i f S2 i n t h e r e a c t i o n of

C3B8 under t h e same c o n d i t i o n s is taken t o equate t o t h e v a l u e of T2 i n t h e n-C4H10 r e a c t i o n , v a l u e s of F and of T j may be found. When S2 from C H is n o t 3 8 known, the approximation T2 = 1 is introduced and v a l u e s of P and T3 so d e r i v e d o r e d i s t i n g u i s h e d by t h e use of primes. Reactions of alkanes on Pt/Al,g,

and Pt-Re/Al,03

Re is added t o Pt/A1203 c a t s l y s t s f o r petrolem reforming t o extend i t s l i f e time by a l t e r i n g t h e form i n which C d e p o s i t i o n occurs.

The p r e c i s e way i n

which t h e Re produces t h i s e f f e c t is not c l e a r , b u t t h e two elements appear t o be i n c l o s e a s s o c i a t i o n , w i t h t h e Re being sulphided under o p e r a t i n g c o n d i t i o n s . We have examined the r e a c t i o n s of C3Hs,

n-C4H10

and iso-C4Hl0

Pt/A1203 c a t a l y s t s , and on a b i m e t a l l i c Pt-Re/Al,03, S-free state.

on c o m e r c i a 1

as noted above,all i n the

W e have a l s o examined s e v e r a l Re/A1203 c a t a l y s t s f o r these

It i s very clear t h a t fully-reduced Re/A1203 i s much more a c t i v e

reactions.

than Pt/A1203 f o r hydrogenolysis.

For example, rates of n-butane hydrogenolysis

on 1.5% Re/A1203 and on 0.3% Pt/A1203

-1

a t 267OC are r e s p e c t i v e l y about 32 and

more a c t i v e by a f a c t o r o f 30. Furthermore t h e Re in the b i m e t a l l i c system is less active than

0.2 um01.g

Cat

h-':

on unit weight of metal b a s i s , Re i s t h e

when i t is by i t s e l f .

This simple f a c t argues t h a t Re i n t h e b i m e t a l l i c system

does not d i s p l a y ensembles of s u f f i c i e n t s i z e t o c a t a l y s e hydrogenolysis i n i t s own r i g h t and t h a t Re atoms must be h i g h l y d i s p e r s e d on t h e s u r f a c e of t h e Pt particles. It h a s been suggested ( r e f . 5) t h a t t h e a c t i v e centre €or hydrogenolysis

comprises both P t and Re atoms,

W e f i n d t h a t f o r both t h e 0.3% and 0.6X Pt/AI2O3

c a t a l y s t s t h e v a l u e s of F and of T3 a t 33OoC a r e almost t h e same ( s e e Fig. 1). even t o t h e e x t e n t of the values of F ( b u t n o t T3) decreasing between t h e f i r s t and second sequences.

With t h e Pt-Re/A1203 c a t a l y s t , t h i s i n i t i a l change does

n o t occur, and t h e v a l u e o f F is h i g h e r , and of T3 l o v e r , than €or t h e P t catalysts.

This p o i n t s very d i r e c t l y t o t h e i n t e r v e n t i o n of a d i f f e r e n t type oi

171

'm

0.4

0.3

0.95

a c t i v e c e n t r e a t which t h e p r o b a b i l i t y of b r e a k i n g t h e c e n t r a l C-C bond i s some t h r e e times l a r g e r than i s t h e case with pure Pt.

Related

d i f f e r e n c e s a r e a l s o seen between t h e product d i s t r i b u -

Sequence no.

1 2 3 4 Sequence no.

t i o n s from C H and iso-C H 3 8 4 10' The v a l u e s of F and T shown i n 3 Fig. 1 f o r pure P t c a t a l y s t s a r e n o t however unique f o r t h i s element:

F i g . 1. Dependence of Kempling-Anderson parameters F and T3 ( s e e t e x t ) f o r n-C H 4 10 hvdroaenolvsis a t 33OoC on seauence number. 0 ,0.32 $ t / A 1 2 0 3 ; , 0.6% Pt/A1203; @ , Pt-Re/A1203.

-

-

A

Pt/Si02,

EUROPT-1 (6.3%

a=

1.8 nm) g i v e s

F = 0 . 4 and T3 = 0.95 under

these conditions ( r e f . 6). The r e l a t i v e chances of t h e two modes of f i s s i o n t h e r e f o r e

depend s e n s i t i v e l y on p a r t i c l e s i z e , s u p p o r t e f f e c t s e t c , and provide a d e l i c a t e means f o r probing s u r f a c e morphology. Reactions of a l k a n e s on Ru/Si02 c a t a l y s t s c o n t a i n i n g T r a n s i t i o n S e r i e s oxide additives A s n o t e d i n t h e I n t r o d u c t i o n , a convenient way t o s t u d y t h e e f f e c t s of oxides

of t h e T r a n s i t i o n S e r i e s elements on a supported m e t a l i s t o i n c o r p o r a t e them a s a t h i r d compound.

We have again used alkane hydrogenolysis a s t h e t e s t r e a c t i o n ,

n o t because of i t s i n t r i n s i c importance b u t because of i t s ready response t o morphological changes i n t h e m e t a l p a r t i c l e s :

i t i s a l s o reasonable t o e x p e c t

t h a t a c t i v i t y v a r i a t i o n s observed w i t h i t w i l l be r e f l e c t e d i n o t h e r s t r u c t u r e s e n s i t i v e r e a c t i o n s such as Fischer-Tropsch and NH

synthesis. 3 We have a l r e a d y r e p o r t e d t h e e f f e c t s of adding T i 0 2 t o RulSiO,

u s i n g an atomic Ti:Ru r a t i o of -5.6

( r e f . 1 ) . The r e s u l t s o b t a i n e d w i t h t h e

former a r e t h e more e a s i l y i n t e r p r e t e d . 0.02-0.03,

and Ru/A1203,

The H/Ru r a t i o i s lowered a f t e r HTR t o

and i s r e s t o r e d a f t e r o x i d a t i o n and LTR t o a h i g h e r v a l u e (0.35) a k i n

t o t h a t found i n t h e absence of T i 0 2 .

v a l u e s of TOF (2.0.045 s-'

Rates change s y m p a t h e t i c a l l y and so

a t 16OoC) a r e much l i k e t h o s e found w i t h o u t T i 0 2 .

There a r e however s i g n i f i c a n t changes i n product d i s t r i b u t i o n s ( s e e Table 2 ) . When a s h e r e c a t a l y s t s a r e formed by r e d u c t i o n of Ru02 o b t a i n e d by c a l c i n i n g RuC13, Ru/Si02 a f t e r HTR a f f o r d s CH4 a s t h e c h i e f p r o d u c t , and t h e t r u e val'ue of T i s s o low t h a t t h e assumption t h a t i t i s about u n i t y i s i n a p p l i c a b l e . The 2 a d d i t i o n of T i 0 however so m o d i f i e s t h e Ru p a r t i c l e s a f t e r HTR t h a t v a l u e s of 2 t h e s p l i t t i n g f a c t o r F of 0.3-0.4 a r e o b t a i n e d (assuming T2 = l ) , s i m i l a r t o

172 TABLE 2

K i n e t i c parameters f o r n-C4H10 h y d r o g e n o l y s i s on p r e c a l c i n e d Ru/Si02, Ru/Ti02 and Ti02-Ru/Si0

( 0 . 5 2 Ru)

2

Catalyst

Pretreatment

H/Ru

ra

TOF/s-la

S2

s3

F'

Ru/Si02

HTRl LTR HTR2

0,21 0.38 0.10

364 874 158

0.048 0.064 0.047

0,17 0.85 0.19

0.03 0.26 0.07

-

-

0.11

0.29

HTRl LTR

0.07 0.75

(71) 1911 (0.01)

0.029 0.071

(0.95) 0.78 1.24b

(0.22) 0.34 0.17b

(0.17) 0.12 0.41b

(0.26) 0.39 0.29b

(45) 570 (34)

0.041 0.045 0.050

1.07 0.70 1.12

0.36 0.45 0.25

0.43 0.15 0.32

0.61 0.53 0.35

Ru/Ti02

Ti02-Ru/Si02

HTR2

,lo

HTRl LTR

0.03

HTR2

0.35 0.02

-1 3 a t e s 6') i n mmol g h-l. Ru A t 260 c.

-

T3'

-

-

and TOF's, a t 16OoC.

t h a t found w i t h Ru/Ti02 a f t e r t h e second HTR, which produces t h e SMSI e f f e c t . Oxidation and LTR of RufSiO produces m e t a l p a r t i c l e s t h a t produce much less 2 methane, and t h e same t r e a t m e n t s a p p l i e d t o RufTiO and Ti07--Ru/Si0 g i v e very 2 2 s i m i l a r r e s u l t s . We t h e r e f o r e d e r i v e t h e v e r y i m p o r t a n t c o n c l u s i o n t h a t , w h i l e

-

Ti0

s p e c i e s on Ru/Si02 do n o t a f f e c t t h e TOF s i g n i f i c a n t l y , t h e y modify t h e

s i t e environment t o t h a t o f a t y p i c a l SMSI s i t u a t i o n where c e n t r a l C-C bond f i s s i o n and t h e d e s o r p t i o n of i n t e r m e d i a t e p r o d u c t s a r e more favoured.

We w i l l

return t o t h i s point later. In o r d e r t o e x p l o r e t h e g e n e r a l i t y of t h e e f f e c t s produced by T i 0 2 , we have prepared and examined a s e r i e s o f Ru/SiO 2 c a t a l y s t c o n t a i n i n g o x i d e s of t h e elements (M = Ca, Oc, T i , V , C r and Mn) h a v i n g an M:Ru r a t i o of 0.25. This v a l u e was chosen on t h e b a s i s t h a t t h e e x p e c t e d Ru d i s p e r s i o n might be W . 5 , s o t h i s l e v e l of promoter would cover about h a l f t h e a v a i l a b l e metal s u r f a c e , i f i t S m a l l e r d i s p e r s i o n s would o f c o u r s e l e a d t o h i g h e r -1 -1 coverages. The r a t e s of n-butane hydrogenolysis (mmol g h at 16OoC) a f t e r Ru HTR1, LTR and HTR2 a r e shown i n F i g . 2 , and t h e Arrhenius parameters a s a p l o t

were a l l i n t h i s p o s i t i o n .

of compensation e f f e c t i n Fig. 3.

I n most c a s e s , t h e i n c l u s i o n o f Ca, Sc and T i

i n c r e a s e s t h e a c t i v i t y o v e r t h o s e found w i t h o u t them: a f t e r LTR i s p a r t i c u l a r l y h i g h . a c t as promoters.

t h e a c t i v i t y o f Sc-RufSiO

2

These elements i n t h e i r o x i d e form t h e r e f o r e

This may seem t o b e i n c o n f l i c t w i t h what was s a i d above

concerning T i O Z , b u t t h e r e t h e Ti:Ru r a t i o was much h i g h e r t h a n h e r e . that i n l o w concentration Ti0

2

It seems

i s b e n e f i c i a l , perhaps by improving t h e Ru

d i s p e r s i o n , although we do not y e t have r e s u l t s t o b e a r t h i s o u t , b u t t h a t much l a r g e r amounts a r e n e c e s s a r y t o s i m u l a t e t h e SMSI e f f e c t .

' h e a d d i t i o n of V

d r a m a t i c a l l y lowers a c t i v i t y a f t e r each type of p r e t r e a t m e n t ( F i g . 2 ) ;

t h e same

3

fj2 0

8-1 -I 0 -1

-

Ca

Sc

Ti

V

Cr

Mn

Fig. 2. Dependence of loglO(rate/nunol gRu -1 h-1 ) for n-C4H10 hydrogenolysis on added element after various pretreatments.

5( 45

4c

a

c 35

-I

3c

25

'4 90

I

100

110

I

120

130

I

140

150

E/ kJ mol-' Fig. 3. Compensation plot of Arrhenius parameters for hydrogenolysis on 1% Ru/SiO + additives 2 n-C4Hlp (symbo s as in Fig. 2).

174 i s t r u e b u t t o l e s s e r e x t e n t s f o r C r and Mn. f u r t h e r r e s u l t s on t h e V-Ru/SiO

D i s c u s s i o n w i l l b e postponed u n t i l

system have been p r e s e n t e d .

2 Fig. 3 r e v e a l s s y s t e m a t i c e f f e c t s o f p r e t r e a t m e n t and a d d i t i v e on t h e

Arrhenius p a r a m e t e r s ,

( i ) The a c t i v a t i o n e n e r g i e s E f o r a l l c a t a l y s t s

s u b j e c t e d t o o x i d a t i o n and LTR l i e i n a narrow r a n g e (121 26 k J m o l - I ) , s u g g e s t i n g t h a t h e r e t h e a d d i t i v e s have minimal e f f e c t on t h e e n e r g e t i c s of t h e a c t i v e c e n t r e , b u t p e r h a p s more on t h e i r number.

( i i ) By c o n t r a s t , a f t e r e i t h e r

of t h e two HTR's, t h e v a l u e s of E v a r y much more ( 9 2 - 1 5 4 k J mol-I):

C a , Sc, T i

and t h e unpromoted c a t a l y s t show t h e l a r g e r v a l u e s (>130 k J mol-l) w h i l e V , C r and

g i v e t h e lower v a l u e s ( to the azimuth. A quantitative analysis of the polar angular dependence in the < 100 > azimuth indicates that the molecular plane i s indeed perpendicular to the surface, the accuracy of the determination being f 10" (ref. 20). The corresponding SEXAFS analysis of Puschmann et al. (ref. 20) gave t h e socalled aligned atop site shown as inset A in Fig. 10. The adsorption site of the formate species on both Cu {llO} and Cu{lOO} has proved, however, to be controversial (refs. 22,231. The recent photoelectron diffraction data of Woodruff et al. (ref. 24) h a s indicated t h a t the same local geometry pertains on both surfaces a n d that the aligned

-

-

213

bridge site (B) is actually occupied. The 0 1s data for both surfaces in normal emission are shown in Fig. 10. It is immediately apparent that the two curves are very similar. The modulation frequency is identical and turns out to be due to backascattering from Cu atoms almost directly "behind" the oxygen atom. The calculated curves for the locally equivalent site B (using C u - 0 distances of 1.99 A for (100) and 1.94 A for (110) are also shown in Fig. 10 and provide the best fit to the experimental data. No satisfactory fits were obtained for other sites; in particular, the aligned atop site (A) can be definitely ruled out. The calculations of Woodruff et al. (ref. 24) used curved wave double scattering with clusters of typically 500 copper atoms. Although agreement between theory and experiment is not perfect (some higher order scattering and

B

A

(110) azimuth

cu I1001 (100) azimuth

Tc

I

b) I

530

v

I

550 560 Photon energy lev) 5LO

I

I

1

I

200 300 LOO Photoelectron kinetic energy (eV1

100

Fig. 9. (left) 0 1s x-ray absorption spectra from the surface formate species on Cu(llO}. The E vector is aligned in (a) the azimuth and in (b) the azimuth. After ref. 20. Fig. 10. (right) Photoelectron diffraction data (normal emission) for the surface formate mecies on Culllo) together with the calculated curves for the aliened bridge site. The insets A and B vdesignate the aligned atop and aligned bridge suites, respectively. After ref. 24. I

214

angle averaging may have to be considered), the main features of the PED spectra are reproduced. Having established the orientation of the formate species on the Cu{llO} surface we can proceed to examine the photoemission data. Fig. 11 shows the effect of deprotonation of adsorbed formic acid which occurs on warming the surface to above 270 K. Spectrum (a)can be assigned by comparison with the photoelectron spectrum of the free molecule. The formate species also gives rise to four spectral bands and, since the number of expected orbitals is the same, i t is tempting to assume a one-to-one correspondence, allowing of course for the change in symmetry from C, to Czv. The application of selection rules proves, however, that such a n assignment is incorrect (ref. 25). Fig. 12 shows three spectra a t hv = 25 eV with the E vector parallel to the surface and aligned along the azimuth, i.e. oriented in the molecular plane of the formate species. Spectrum (b) was obtained a t normal photoelectron emission, for which the selection rules tell us that only levels belonging to bl in Czv will be observed. This immediately assigns two features in the spectrum a t 4.8 eV and 9.6 eV below EF. By moving the detector off-normal into the < 100> azimuth (spectrum (a), E kll) emisssion from a2 states should be observed as well. Whereas peak 3 remains in the same place peak 1 shifts slightly to lower binding energy indicating t h a t it also contains a level of a2 symmetry. Similarly, by moving the detector off-normal into the azimuth (spectrum (c) Ellkll a , and bl states are expected. Under these conditions peak 1 moves up in binding energy, as does peak 3. In addition, peak 4 is observed. Thus, three a1 states are also present. Peak 2 is only visible with E l k 8 for

-

a)

--

Fig. 11. ( a ) Photoelectron spectrum of adsorbed formic acid on Cu{llO} and (b) correspondin spectrum after formation of the ormate species above 270 K. h v = 25 eV. After ref. 25.

A 270K V

b)

-

HC006- (ad)

I

I

15

10

k

Energy betow EF (eV)

I

0

B

215

non-normal incidence (not shown in Fig. 12), indicating that i t belongs to b2. By performing further confirmatory experiments a t other orientations of the E vectpr, in particular when it is aligned in the azimuth, a complete assignment is possible. Peak 1contains three bands due to la2(11),4bl(o) and 6al(a) a t 4.7, 4.8 and 5.1 eV below EF, whereas peak 2 consists only of lb2(n) at 7.8 eV. Peak 3 contains 3bl((I) and %'il((I)at 9.6 and 9.7 eV; peak 4 is due to 4al((I) a t 13.0 eV. These measured ionisation energies have been compared with HF-SCF calculations for the formate ion (ref. 26) as well as with an INDO Cu(llO}-HCOO cluster calculation (ref. 27). The relative orbital energies from the latter, semi-empirical treatment are in reasonable agreement with the measured binding energies although the absolute values, as expected, are way out. The important result from this calculation is the correct assignment of the photoelectron spectrum (via Koopmans' theorem), in particular, the prediction that three levels are present in the first band and only one in the second. An analysis of the percentage formate character in the adsorbate-derived orbitals reveals that the la2, 4bl and 6al orbitals are most strongly involved in the chemisorption bond. Relative to the formate ion, surface formate has both lower u and II populations but the (I population difference is the greater. The 'TI donation occurs mainly via the la2 orbital; the strongest (I donor is the 4bl orbital. Back-donation from the metal into the anti-bonding n*(2b2) orbital is neglible because, unlike the situation in adsorbed CO, the latter is too high in energy.

I

I

HCOO/Cu1110) > w = ~ e5 ~in ITIOI azimuth

Fig. 12. Angle-resolved photoelectron spectra from the system Cu(ll0)HCOO for three different emission an les. hv = 25 eV. After re&25.

9 = 60'

-..

-\--

I

15

10

I

5 Energy below EF (ev)

I

0

[iiol

216

SOME COSCLUSIONS I have attempted in this short review article to describe some of the experiments that can be performed with synchrotron radiation in order to gain a better understanding of the structure and bonding of molecules and molecular fragments adsorbed on metal surfaces. In structural studies the usefulness of x-ray absorption and photoelectron diffraction has been demonstrated, particularly in situations where the adlayer is not ordered and conventional diffraction techniques cannot be applied. In fact, most molecular adsorbates show little sign of long-range order (CO and C&fi tend to be exceptions) and, for molecular fragments formed in simple heterogeneous reactions, i t almost never occurs. The ability to independently determine the orientation of adsorbed species opens up the possibility of applying selection rules to the assignment of the adsorbate-induced features in photoelectron spectra. Photoemission studies of adsorbed molecules in recent years have tended to be more diagnostic in nature, meaning that they are largely concerned with general characterisation rather than with bonding aspects. The combination of photoemission studies, where the emphasis is on accurate determination of ionisation energy and symmetry, with theory will hopefully change this situation in future. For this purpose, more cluster calcu lations a t higher levels of sophistication will be necessary; attempts to solve some of the very difficult problems caused by the present use of Koopmans' theorem would also be highly desirable. ACKNOWLEDGEMENTS This work has been supported financially by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 6-81 and by the Fonds der Chemischen Industrie. I also acknowledge the considerable contributions of my colleagues H. Conrad, A . L. D. Kilcoyne, M. E. Kordesch, Th. Lindner, C. F. McConville, G. Paolucci, K. C. Prince, J. Somers, L. Sorba, M. Surman, G . P. Williams and D. P. Woodruff to various parts of the work reviewed in this article. REFERENCES

1

J. Stbhr, in V. R. Vanselow and R. Howe (Editors), Chemistry and Physics of Solid Surfaces, Springer, Berlin, 1984, p. 231; J. Haase, Appl. Phys. A38 (1985) 181; J. Haase and A. M. Bradshaw, in R. J. Bachrach (Editor), Synchrotron Radiation

2

3

J. J. Barton, C. C. Bahr, S. Robey, J. G. Tobin, L. E. Klebanoff and D. A. Shirley, Phys. Rev. Lett. 51 (1983) 272; M. Sagurton, E. L. Bullock and C. S. Fadley, Phys. Rev. B 30 (1984) 7332; D. P. Woodruff, Surface Sci. 166 (1986) 377. N. V. Richardson and A. M. Bradshaw, in A. Baker and C. R. Brundle (Editors),

4

Electron Spectroscopy: Theory, Techniques and Applications, Academic Press, London, 1982, p. 154; M. Schemer and A. M. Bradshaw, in D. A. King and D. P. Woodruff (Editors), The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Elsevier, Amsterdam, 1983, Vol. 2, p. 165. M. P. Seah and W. Dench, J. Surf. Interf. Anal. l(1979) 1.

Research: Physics of Low Dimensional Systems, Plenum, New York, in press.

217

5. 6

7 8 9 10 11 12 13 14

15 16 17 18 19

G. Margaritondo, Introduction to Synchrotron Radiation, Oxford University Press, New York, Oxford, 1988. J. Stbhr and R. Jaeger, Phys. Rev. B 26 (1982) 4111. (The derivation of these expressions is somewhat laborious in this first paper on x-ray absorption spectroscopy of adsorbed molecules. See Somers et al., Surface Sci. 183 (1987) 576, for a simpler approach.) A. M. Bradshaw, J. Somers and Th. Lindner, Proc. of Solvay Conference on the Physics and Chemistry of Surfaces, Springer, Berlin, in press. M. E. Kordesch, Th. Lindner, J. Somers, W. Stenzel, H. Conrad, A. M. Bradshaw and G. P. Williams, Spectrochim. Acta 43A (1987) 1561. M. E. Kordesch, W. Stenzel and H. Conrad, Surface Sci. 205 (1988) 100. M. Surman, K. C. Prince, L. Sorba and A. M. Bradshaw, Surface Sci., in press. G. Paolucci , M. Surman, K. C. Prince, L. Sorba, A. M. Bradshaw, C. F. McConville and D. P. Woodruff, Phys. Rev. B 34 (1986) 1340. D. Heskett and E. W. Plummer, Phys. Rev. B 33 (1986) 2322. L. H. Dubois, B. R. Zagorski and H. S. Luftman, J. Chem. Phys. 87 (1987) 1367. K. Horn, A. M. Bradshaw and K. Jacobi, Surface Sci. 72 (1978) 719; K. Horn, A. M. Bradshaw, K. Hermann and I. P. Batra, Solid State Commun. 31 (1979) 257. C. F. McConville, D. P. Woodruff, K. C. Prince, G. Paolucci, V. Chab, M. Surman and A. M. Bradshaw, Surface Sci. 166 (1986) 221. E. G. C. S. Fadley, Phys. ScriptaT 17 (1987) 39. Th. Lindner, J. Somers, A. M. Bradshaw, A. L. D. Kilcoyne and D. P. Woodruff, Surface Sci. 203 (1988) 333. D. H. S. Ying and R. Madix, J. Catalysis 6 1 (1980) 48. B. E. Hayden, K. C. Prince, D. P. Woodruff and A. M. Bradshaw, Surface Sci. 133

(1983) 589. 20 A. Puschmann, J. Haase, M. D. Crapper, C. E. Riley and D. P. Woodruff, Phys. Rev. Lett. 54 (1985) 2250. 2 1 J. Somers, A. W. Robinson, Th. Lindner and A. M. Bradshaw, to be published. 22 D. Outka, R. J. Madix and J. Stiihr, Surface Sci. 164 (1985) 235. 23 M. D. Crapper, C. E. Riley and D. P. Woodruff, Surface Sci. 184 (1987) 121. 24 D. P. Woodruff, C. F. McConville, A. L. D. Kilcoyne, Th. Lindner, J. Somers, M. Surman, G: Paolucci and A. M. Bradshaw, Surface Sci. 201 (1988) 228. 25 Th. Lindner, J. Somers, A. M. Bradshaw, G. P. Williams, Surface Sci. 185 (1987) 75; P. Hofmann and D. Menzel, Surface Sci. 191 (1987) 353. 26 S. D. Peyerimhoff, J. Chem. Phys. 47 (1967) 349. 27 J. A. Rodriguez and C. T. Campbell, Surface Sci. 183 (1987) 449.


C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

219

DRIFTS AND RAMAN SPECTROSCOPIC STUDY OF INTRAZEOLITE METAL CARBONYL CHEMISTRY

C. BREMARD*, E. DENNEULIN, C. DEPECKER and P. LEGRAND Laboratoire d e Spectrochimie Infrarouge et R a m a n du CNRS, bbt. C5, UniversitC d e s Sciences et Techniques d e Lille Flandres Artois, 59655 Villeneuve d'Ascq Cedex (France)

ABSTRACT Adsorption and t h e r m a l decomposition of Mo(C016, Mn2(CO)IG and C o (CO) in t h e cage s y s t e m of dehydrated z e o l i t e Na55Y have been studied with situa DRIFTS and R a m a n spectroscopies. Mo(C016 and Mn (CO)lG a r e adsorbed as i n t a c t molecules, w h e r e a s C o (CO& is absorbed as C O ~ ( C ? O ) ~and ~ cobflt subcarbonyls. The local s y m m e t r y of t h e absorbed species a r e assigned to Na OC- contacts. Thermal t r e a t m e n t of t h e m e t a l carbonyl/zeolite a d d u c t s lead to loss of C O and s o m e t i m e s to reversible formation of subcarbonyls which a r e anchored to z e o l i t e framework. The final products of t h e t h e r m a l t r e a t m e n t a r e small naked metallic c l u s t e r s and probably larger c r y s t a l l i t e s sticking at t h e o u t e r s u r f a c e of t h e z e o l i t e crystals.

...

INTRODUCTION The c h e m i s t r y of m e t a l carbonyls within t h e cavities of z e o l i t e has been developed to stabilize homogeneous c a t a l y s t s again aggregation or bimolecular deactivation (ref.1). F u r t h e r m o r e t h e z e o l i t e c a g e s y s t e m may favor unique selectivity of a c a t a l y t i c r e a c t i o n due to changed complex geometries, transition s t a t e modifications and/or diffusional selectivity for t h e s u b s t r a t e molecules. The large cages and pores

of faujasite-type z e o l i t e Y p e r m i t to a c c o m o d a t e l a r g e molecules such as carbonyl besides t h e Na55Y z e o l i t e metals: Fe(CO)5,Mo(CO)6,Co2(C0)8,Fe2(CO)9,Mn2(CO~1G d o e s n o t c o n t a i n oxidizing s i t e s a s HY zeolite. A variety of techniques a r e available for t h e study of intrazeolite chemistry : t h e s e include MAS-NMR, EXAFS, small a n g l e X-ray

s c a t t e r i n g radial distribution (ref.2,3).

However t h e most frequently

used technique is t h e i n f r a r e d transmission spectroscopy using t h e self supporting w a f e r technique (ref.2-7).

DRIFTS and R a m a n spectroscopies using powder samples

a r e c o m p l e m e n t a r y vibrationnal spectroscopic techniques, which a r e powerful tools to d e t e c t C O and metal-metal bonds respectively. The present r e p o r t indicates

t h a t it is possible to obtain p e r t i n e n t bonding information by DRIFTS and Raman spectroscopies concerning t h e chemistry of m e t a l carbonyls entrapped in zeolites.

METHODS The sodium form of faujasite-type zeolite w i t h unit cell composition Na55(A102)55 (Si02)I 37.xH20 was purchased from Strern Chemicals (Linde LLY52).

Mo(CO)~,

Co (CO) Mn (CO)Io were stored under dry argon in the dark. 2 8" 2 .All diffuse reflectance infrared-Fourier Transform spectra, were recorded on a Bruker IFS I t 3V instrument equipped with diffuse reflectance accessory. An heatable-evacuable cell i s used for the "in situ" DRIFTS experiments, in connexion with vacuum or gas lines. Prior to introduced the zeolite into the DRIFTS chamber, the sieve lot i s washed with 0.2 N NaOH then with twice distilled water and we first removed much of the adsorbed water by overnight in a vacuum oven at 385 K. The cell i s connected to the vacuum and gas lines in the sample compartment 2 of the spectrometer. The cell i s pumped down to a pressure of 10 Pa and heated stepwise up to 600 K. After 6 h at 600 K the sample was cooled to room temperature and argon was drawn through the dry NaSjY zeolite. A DRIFT reference spectrum 1 of neat zeolite was run subsequently in the mid infrared region [5000-1400] cm-

.

The carbonyl compound

Mo(CO)~,C O ~ ( C O or ) ~ Mn2(CO)I0

was then admitted into

the cell by sublimation from a side arm of the gas lines under flowing CO or Ar.

3

After each period the sample was degassed for 20 mn at a pressure of 10 Pa and argon was drawn again through the loaded zeolite, so the system is allowed to reach equilibrium and the base lines of the DRIFT spectra remain reproducible

. The DRIFT

spectra shown in this paper were corrected to the Kubelka-Munk function. The DRIFT spectra of the absorbates were measured by substraction of the neat zeolite spectrum. The Raman spectra were recorded on multichannel spectrometer DlLOR model

OM.4RS 89 at room temperature and at 220 K. Raman scattering was excited using argon laser lines with low power at sample (less than 10 mu') 457.9,

488.0,

514.5

nm. The illumination time or integration time used was 1 s. The spectra were averaged over at least 50 accumulations of spectra. The sealed tube and sample spinning techniques were used for all the Raman experiments. For the Raman experiments an heatable-evacuable glass cell was used in connection with vacuum or gas lines for zeolite dehydration and subsequent vapor phase loading with metal carbonyls. After loading, the powder i s transferred under argon in cylindrical tube and sealed off. The metal content of the loaded zeolites is controlled by elementai analysis. RESULTS The DRIFT spectrum of neat dehydrated Na55Y zeolite powder i s well suitable

221

between 5000 and 1500 cm-I to observe t h e s p e c t r a of entrapped m e t a l carbonyl in t h e v ( C 0 ) region. The R a m a n s p e c t r u m of t h e dehydrated Na55Y z e o l i t e is weak and allows potential observations of mid and low frequency modes of adsorbed species. Adsorption of Mo(CO), o n t o Nag&'

"

2.5

a

7.0

6.0 2.0

Ln

-z

5.0

t-

1.5.

...

3

\

.o

Y

z 7

E

1.0.

.

0.5

.

CK Y

3.0

2.0

m

7 Y

I

.o

0.0

0.0

-0.5

-1

.o

WRVENUMBERS CM-1

Fig. 1. DRIFT s p e c t r a in t h e V(C0) region of Mo(CO)~entrapped in Na55Y z e o l i t e (a) 0.04 Mo(C016 per supercage

. (b) 1.9 Mo(CO)~per supercage

Initial exposure t o Mo(CO)~vapor produces DRIFT s p e c t r a which exhibits s e v e r a l bands in t h e V(C0) region w h e r e a s t h e isolated Mo(CO)~exhibit one single band. All t h e bands i n c r e a s e simultaneously with t h e Mo(CO)~ adsorption c o v e r a g e and

t h e r e l a t i v e intensities do n o t vary significantly with carbonyl c o v e r a g e and f r o m o n e e x p e r i m e n t to another o n e before s a t u r a t i o n of t h e bands which o c c u r s for t h e Na55Y[Mo(C0)6]0.81 composition, which corresponds to Ca 0.1 Mo(C0)6 per supercage. Below 0.1 molecule per s u p e r c a g e a t l e a s t six infrared bands a n d shoulders c a n be observed on t h e DRIFT spectra. The s t r u c t u r e of t h e DRIFT s p e c t r a leads us to think t h a t Mo(C0l6 molecules a r e trapped at d e f i n i t e adsorption s i t e of t h e supercage, besides t h e mobility of Mo(CO)~inside t h e c a g e is less t h a n t h e t i m e s c a l e of t h e vibrational spectroscopy and t h e low local s y m m e t r y is typical of t h e environment of t h e e n t r a p p e d Mo(CO)~.Two types of adsorption s i t e s a r e evidenced a t high c o v e r a g e by t w o induced bands arising f r o m t h e infrared forbidden

VI

mode

of t h e f r e e molecule. A t high coverage, informations concerning t h e modes arising

222

f r o m v 3 and

v6 modes c a n be deduced f r o m t h e o v e r t o n e s a n d binary combination

modes w i t h t h e v 1 mode by :

where ( vi

+

v1

) is t h e observed wavenumber of t h e combination mode and

k11

is t h e a n h a r m o n i c i t y c o e f f i c i e n t which is a s s u m e d to be analogous t o t h o s e reported

for t h e isolated molecule (ref.8)

.

The number of fundamental u (CO) infrared modes deduced f r o m t h e observed okertones and binary combination modes ( v I + v ) a r e in good a g r e e m e n t with 1 those exhibited on t h e R a m a n s p e c t r a which w e r e recorded on t h e s a m e samples. Indeed, significant wel) resolved R a m a n s p e c t r a a r e only obtained for s a m p l e s with high coverage, typically 1.9 Mo(CO)~ per supercage, b e c a u s e of t h e poor R a m a n cross section of t h e d C 0 ) modes.

The t w o bands observed by R a m a n s c a t t e r i n g and DRIFTS (2122 and 2116 c m - 1)

v region (2051, 2045, I 2033, 2017 c m - ) a r e in a g r e e m e n t with t w o adsorption sites per s u p e r c a g e at high

in t h e v I region a n d t h e four observed R a m a n bands in t h e

coverage

.

All t h e R a m a n a c t i v e fundamental modes of M o ( C O ) ~e n t r a p p e d in Naj5Y z e o l i t e

c a n be observed and particularly in t h e low frequency region (Fig.2a). I t should be noted t h a t t h e low frequency modes u (MoC), 6 (MoCO) and

6 (CMoC) of t h e

entrapped Mo(C0) a r e very c l o s e t o t h o s e of t h e isolated molecule (ref.8). Indeed, 6 n o d e t e c t a b i e splitting of t h e d e g e n e r a t e d modes is observed upon adsorption and t h e wavenumber s h i f t s a r e found to be weak, Fig.2a.

The R a m a n spectroscopy is

particularly i n t e r e s t i n g for p l y m e t a l l i c c o m p l e x e s with m e t a l - m e t a l bonds, such a s Mn2(CO)I0, Fig.2b. Indeed t h e

v(meta1-metal) modes possess l a r g e R a m a n cross s e c t i o n s and give

p e r t i n e n t information of t h e g e o m e t r y of t h e c l u s t e r inside t h e z e o l i t e framework.

v (MnC) and

For hln2(CO)10, t h e

v (Mn-Mn) wavenumbers a n d r e l a t i v e intensities

a r e analogous for t h e f r e e molecule, 408 a n d 160 c m - l a n d t h e e n t r a p p e d molecule 402 and 152 c m - I respectively; w h e r e a s t h e wavenumbers a n d t h e r e l a t i v e intensities of t h e infrared and R a m a n bands of t h e u ( C O ) modes a r e d r a m a t i c a l l y modified

upon adsorption. An e x t e n s i v e study concerning t h e DRIFT a n d R a m a n spectroscopies of

t h e adsorption of

O S ~ ( C O o) n~t o~ Na

Mn2(CO)I0,

Re2(CO)10,

Fe2(COIq,

Fe3(C0Il2,

RU~(CO)~~,

Y z e o l i t e via sublimation will be t h e s u b j e c t of a n o t h e r publica-

55 tion. However t h e adsorption of Co (CO) g e n e r a t e s a set of i n t r a z e o l i t e r e a c t i o n s 2 8

223

which a r e detailed in the following paragraph.

WflVENUMBEAS CM-1

Fig. 2. Low frequency Raman spectra 1.9 molecule per supercage

per supercage

.

Adsorption of Co,(CO),

. a)

M O ( C O ) ~entrapped in Na55Y zeolite

. b) Mn2(CO)I0 entrapped in Na5jY

zeolite 0.95 molecule

onto N a 5 s 0

Sublimation of C O ~ ( C O )onto ~ Na55Y

powder samples was conducted under

slow flowing CO at room temperature t o prevent decomposition of C O ~ ( C O )in~ the gas phase, Fig.3 a

.

The DRIFT spectra of the sorbed phase were recorded under CO atmosphere a t low pressure, Fig.3.b

. The

DRIFT spectra of the sorbed phase exhibits a set

of bands which a r e not analogous whith that of C O ~ ( C O )in~ gas phase, Fig.3.a,

as well as in solution. However, the DRIFT spectra of t h e sorbed phase appear as a mixture of both the set of bands of C O ~ ( C O and ) ~ ~Co(CO);, in the region

.

V

(CO)

4

0 . PS

;-.

:

I.0 I .8

;; N

I

I

.(I

I

..t

I

.z

i

.o

in I-

5

0.2

W

U z

am :II: Lc

0.15

0.e

0.1

.

!! i

0.05

0.r 0.2

0.0

0.0

WAVENUMBERS CM-1

.

(a) Infrared spectrum of C O ~ ( C O in ) ~ gas phase (b) DRIFT spectrum of Fig.3 NaSSY zeolite a f t e r loading by C O ~ ( C O )0.04 ~ , Co per supercage Two independent reaction pathways appear to function for C O ~ ( C O )as~ it is adsorbed onto faujasites (Ref.5,6) 2

2 co2(co)8

2

co4(co)12 + 4

3 Co2 (C0l8 + 12 Oz

+

co

c+ 2 4 Co(CO< + 2 C d 0 ~ ) +~8 CO

(2)

(3)

The symbol Oz is used to refer to framework oxygen of t h e zeolite. All subcarbo~ ~ quickly oxidized by 02.This nyl species into the zeolite, except C O ~ ( C O )are feature permits t h e preparation of samples in which t h e only cobalt carbonyl species ~ . figure 4a exhibits t h e DRIFT remaining in zeolite framework is C O ~ ( C O ) ~The spectrum when CO is exhaustively removed and t h e sample exposed to small doses of 02. The individual doses provided insufficient oxygen to oxidize all the cobalt carbonyl species in t h e sample, t h e bands at 1977, 1950, 1927 and 1900 cm-I disappear and only features due to C O ~ ( C O remain )~~ at t h e end of t h e entire sequence; CO and C02 liberated upon oxidation of Co(CO$ are removed by vacuum evacuation. C O ~ ( C Oadsorbed )~~ in NaS5Y is more evidenced by low frequency Raman spectroscopy. Indeed t h e intense Raman bands observed at 250 and 185 cm-',

for the

225

C O ~ ( C O isolated )~~ molecule (Ref.9) a r e not signicantly perturbed upon adsorption ) ~ ~ t h e c a g e s y s t e m of and allow to c h a r a c t e r i z e t h e f o r m a t i o n of C O ~ ( C O within zeolite

. It

should b e noted t h a t t h e Raman s p e c t r u m of t h e isolated molecule

C O ~ ( C O exhibits )~ i n t e n s e bands a t 230, 185 and 160 c m - l (Ref.9)

.

Four bands and shoulders a r e evidenced on t h e DRIFT s p e c t r u m of entrapped C O ~ ( C O in ) ~ ~t h e terminal v ( C 0 ) region (Fig.4a) and o n e band and one shoulder c a n be observed in t h e v (CO) bridging region. The decomposition of t h e s p e c t r u m in gaussian and lorentzian band profiles with t h e four bands hypothesis points o u t t w o narrow bands at 2123, 2081 c m - I and t w o broad bands at 2054, 2008 crn-l.

These t w o l a t t e r bands contain probally t w o unresolved modes respectively. Consequently i t is probable t h a t six modes a r e a p p a r e n t in t h e v (CO) t e r m i n a l region

( 3 A , + 3 E) and t w o modes in t h e V ( C 0 ) bridging region ( A l

t

E), a n d t h e e l e c t r i c

field within t h e supercage of Na55Y d o e s n o t break down t h e Cjv molecular symmetry. On t h e o t h e r band t h e intravity e l e c t r i c field reduces t h e local s y m m e t r y of Co(COr4 f r o m Td t o lower symmetry. Indeed four bands a r e observed on t h e DRIFT s p e c t r u m shown of Figure 4b

. 0.6

Lo 0 . 8

I-

z

0.7

0.5

3 z

0.6

A

Y W

5*L

0.5

0.Y

0.3

G.+

0.3

0.2

0.2 0.1 0.1

0.0

0.0

22 0

El50

2100

2050

2000

1s

1950

1900 UPVENUMEERS 0 - 1

URVENU!?BERS CH-1

Fig.4

-

DRIFT s p e c t r a in t h e V (CO) region of (a) C O ~ ( C O entrapped )~~ in Na

z e o l i t e (b) C O ( C O ) entrapped ~ in Na55Y z e o l i t e

.

Thermal decomposition of Mo(CO)," e n t r a p p e d in Na5,Y , zeolite

.

55y

226

Thermal decomposition of adsorbed metal carbonyls as precursors for dispersed naked metallic clusters and a limited number of attemps to anchor subcarbonyls on oxide surfaces have been reported previously (Ref.2) In the present work the thermolysis is carried out under vacuum or argon by "in situ" DRIFTS and Raman spectroscopy at low and high coverage. A t 373 K the complete replacement of the set of v(C0) bands of adsorbed Mo(C016 with 0.04 molecule per supercage with a new set of band is achieved. The thermogravimetric measurements performed previoulsy (ref.7) on t h e Mo(C0)6/NaY system a r e in accordance with the following stoichiometry

.

Mo(CO)~ Z Mo(COl4 (Oz)2 + 2 CO

Mon + 4 n CO

n Mo(CO)~

s.0

\ .O

9.0

1.0

I .o

0.0

WRVENUMBEAS Cfl-1

Fig3

. DRIFT spectra. (a) during the course of the thermolysis of . (b) a f t e r spectra decomposition a t 373 K .

Mo(CO)~entrapped

in Na55Y zeolite

The low frequency Raman spectrum indicates no intense v(Mo-Mo) band for the intermediate species, nevertheless one intense band is observed at 917 cm- 1 and is indicative of the interaction of the subcarbonyl species with t h e framework oxygen Oz of the zeolite

.

227

Heating above 400 K causes c o m p l e t e and irreversible decarbonylation producing zerovalent molybdenum

cluster within t h e z e o l i t e s t r u c t u r e and probably larger

c r y s t a l l i t e s sticking at t h e o u t e r s u r f a c e of t h e z e o l i t e crystals. Unfortunately no m e t a l l i c c l u s t e r s Mon (n= 2,3

...)

have been d e t e c t e d by R a m a n spectroscopy yet.

The molybdenum c l u s t e r s a r e n o t disintegrated by C O adsorption. DISCUSSION DRIFTS allows t o observe well resolved infrared s p e c t r a in t h e

v ( C 0 ) region

of carbonyl m e t a l s adsorbed o n t o z e o l i t e at very low coverage. On t h e o t h e r hand t h e R a m a n spectroscopy provides vibrational informations over a l a r g e wavenumber range for carbonyl m e t a l s entrapped in zeolite. DRIFTS and Raman spectroscopy p e r m i t to use powder s a m p l e s under "in situ" conditions. The sampling makes e a s i e r t h e t r a n s f e r from t h e gas phase i n t o t h e solid t h a n t h e self w a f e r technique used in transmission infrared spectroscopy. The mobility of t h e molecule inside t h e z e o l i t e framework depends particularly o n t h e r e l a t i v e s i z e of t h e pores and t h e molecule. With large molecules such a s M o ( C O ) ~ , Mn2(CO)I0 o r C O ~ ( C O )t h~e ~ mobility in Na55Y z e o l i t e is reduced to adsorption s i t e s c l o s e to Na'.

Ordinarily t h e rotational d e g r e e s of freedom of t h e

g u e s t molecules a r e "frozen out" and t h e s p e c t r a consist of t h e pure vibrational transitions of t h e isolated molecule. The c h e m i c a l (see experimental section) and spectroscopic c h a r a c t e r i z a t i o n of t h e sorbed phases d e m o n s t r a t e t h a t Mo(C0)

and 6 Mn2(CO)I0 a r e adsorbed as i n t a c t molecules w h e r e a s C O ~ ( C O i)s~adsorbed as Co (CO) 4

12

and subcarbonyl. The DRIFTS and R a m a n s p e c t r a of t h e bulk solids a r e not observed and t h e vibrational s p e c t r a exhibit well resolved bands of t h e isolated molecule in t h e z e o l i t e matrix. Previous a t t e m p t s to d e t e r m i n e t h e precise location of t h e m e t a l a t o m s of M o ( C O ) ~ in HY by X-Ray diffraction method have failed (Ref.3). However t h e well resolved R a m a n and DRIFT s p e c t r a a r e n o t in good a g r e e m e n t with fast reorientational motion or fast s i t e to s i t e jump in t h e t i m e s c a l e of t h e vibrational spectroscopy. Hence Mo(C0) i s probably trapped a t adsorption s i t e s a n d t h e d C 0 ) region is particu6 larly sensitive to t h e local s y m m e t r y of t h e available s i t e of t h e host l a t t i c e and t h e o r i e n t a t i o n of t h e g u e s t molecule in their site. The e x p e c t e d Cs or C 1 local s y m m e t r y of Mo(C0l6 entrapped in Na55Y is in good a g r e e m e n t with t h e s i x v (CO) band observed on t h e DRIFT s p e c t r a at low resolution. Besides at high resolution t h e v I mode splits i n t o t w o bands (2122 and 2116 c m - 1) which is t h e most c l e a r evidence for t w o adsorption s i t e occupancy by Mo(CO)~. The wavenumber and t h e intensity of t h e V(C0) bands depend on t h e intensity and t h e orientation of t h e

228 eiectric field at the adsorption site. The electric field within the supercage induces the inactive infrared modes of the free Mo(CO)~and provide a probe of the electric field intensity at the adsorption site. The dimer C O ~ ( C O i)s~ small enough to penetrate the channels of the Na55Y zeolite. The resulting aggregate Co4(CO),,

can be accomodated by the supercage

(1.2 nm) but is prevented by i t s size from passing the openings of the supercage at adsorption site with high symmetry near the center (0.8 nm). C O ~ ( C Oi )s ~located ~

of the supercage and the intracavity electric field does not generate observable splitting of the E modes

. For all the carbonyl metals under study M O ( C O ) ~ , . M ~ ~ ( C O ) ~ ~

and C O ~ ( C O ) the ~ ~ , electric field at the adsorption site does not significantly modify the low frequency modes which are relevant of the free molecular structures. The Raman scattering gives pertinent information of the geometry of the polymetallic complexes within the zeolite framework, through the

(M-M) modes, M=Mn,

Co, which possess large Raman cross sections. The "in situ" DRIFTS and Raman spectroscopy indicate that during the thermolysis of Mo(CO)~adsorbed onto NaS5Y, transient subcarbonyls are generated and anchored to zeolite framework. No experimental evidence i s found for aggregation before exhaustive decarbonylation. REFERENCES M.C. Connaway, B.E. Hanson, Inorg. Chem., 25 (1986) 1445 T. k i n , S.J.

Mc Lain, D.R. Corbin, R.D.

Farlee, K. Moller, G.D.

Stucky,

D. Sayers, 3. Am. Chem. Soc., 110 (1988) 1801 P. Gallezot, G. Goudurier, &I. Primet, 8. Imelik, in Molecular Sieves I1 ACS

G. Woolery,

J.R. Katzer (Editor) (1977) 144 T. Bein, P.A. Jacobs, J. Chem. Soc., Faraday Trans., 79 (1983) 1919 R. Alves, D. Rallivet-Tkatchenko, G. Coudurier, N. Duc Chau, M. Santra, Bull.

Soc. Chim. France. 3 (1985) 386 R.L. Schneider, R.F. Howe. K.L. Watters, Inorg. Chem.. 23 (1984) 4593

Y. You-Sing, R.F. Howe, J. Chem. Soc., Faraday Trans., 1.52 (1956) 2887 L.H. Jones, K.S. Xlc Dowell. 31. Goldblatt, Inorg. Chem., 8 (1969) 2349 S. Onaka, D.F. Shriver, Inorg. Chem.. 1 5 (1976) 915

C. Morterra,A. Zecchina and G . Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands

229

THE ADSORPTION OF IODINE AT GaAs (100) SURFACES VIA THE DECOMPOSITION OF CH212 : A MODEL STUDY OF SEMICONDUCTOR ETCHING

D.S. BUHAENKO, S.M. FRANCIS, P.A. GOULDING AND M.E. PEMBLE Department of Chemistry, University of Manchester Institute of Science and Technology, P.O.Box 88, Sackville St., Manchester, M60 1QD.

ABSTRACT It is demonstrated that CIE;!12 adsorbs at 300 K on an As-rich GaAs (100) surface. Low energy AES data indicate the complete depletion of surface As following adsorption suggestive of the formation of volatile As species via direct reaction followed by desorption in vacuo. Residual surface carbon and iodine are believed to be bound to Ga sites and are removed completely by annealing to 870 K, revealing an As-rich surface once again. The relevance of these findings to the development of novel photoassisted etching processes for GaAs (100) is discussed. INTRODUCTION Laser/photo-assisted etching techniques for use with 111-V materials are attractive since they are potentially capable of

high spacial

selectivity, low crystallographic anisotropy and high chemical selectivity while introducing minimal damage and being cost-effective (ref. 1).

For

GaAs in particular, Osgood and co-workers have demonstrated laser/photo-

assisted etching via the photolytic production of radicals by excimer laser irradiation of species such as HBr (refs. 2 and 31, CF3Br, CH3Br, CF31 and CH3C1 (refs. 4-6) and have suggested that both halogen and

carbon-

containing radicals may react with surface Ga and As atoms (ref. 4). For these systems, the operation of a photochemical rather than a purely thermal etching mechanism has been confirmed via the

use

of illumination

both perpendicular and parallel to the substrate surface. However, in the case of the CH3Br and CF3Br etchants, the etch features observed under perpendicular illumination were much more localised than those observed under parallel illumination (ref. 4 ) . Thus despite the fact that gas-phase photolysis was assumed to be the dominant mechanism for radical production in both illumination geometries, surface effects undoubtedly influence the etching characteristics.

230

There are two possible photo-induced surface contributions to the dry etching of GaAs (100) surfaces which have been noted previously (ref. 1). These are (i) laser-assisted desorption of etch products via local heating, and !ii)

the generation of photocarriers within the substrate effectively

weakening the Ga-As bonds. We suggest here that a third possibility a l s o exists - that of adsorption of the etchant gas prior to photolysis. This process would also give rise to enhanced etch rates under perpendicular illumination via the photolysis of a concentrated adlayer of

etchant

molecules with a further possible contribution arising from an increase in photolysis cross-section via

a

perturbation of the electronic structure of

the etchant molecule upon adsorption. If adsorption does occur prior to photolysis, the spacial resolution of the etch features will be no longer inherently limited by

the gas-phase diffusion length of

the radical

species, plus the reaction would be highly chemically specific. We have an interest in the development of alternative photo-assisted etching processes for GaAs (100) and as such are attempting to employ species which (a) may be photolysed using soft uv or visible radiation, and (b) have appreciable vapour pressures at 300 K. CI$$

was selected as a

potential etchant as it satisfies the vapour pressure requirement and photolysis of this molecule at 310 nm results in

?

production at high

quantum efficiency (ref. 7 ) . We demonstrate here that in support of the pre-photolysis adsorption mechanism presented above, CH212 adsorbs on an As-rich GaAs (100) surface at 300 K in the absence of any photolytic activation and apparently removes surface As via a direct reaction (although we cannot yet eliminate the possibility of electron-beam induced processes), while surface Ga removal appears to require considerable thermal activation.

EXPERIMENTAL The UHV systedatmospheric pressure reactor employed in this work has been described previously (refs. 8 and 9). Briefly, facilities for Auger electron spectroscopy, LEED and thermal desorption are available on the while an main chamber (base pressure < 1 x 10-"mbar) 'catalysis' cell based on a design by Judd et al. (ref.

isolatable permits

la)

treatment of the sample up to pressures of 1000 mbar while maintaining UHV in the main chamber. The operation of the cell is depicted schematically in figure 1.

231

t hcrmocouplr

I

Figure 1. Schematic of the isolatable reactor employed in the routine cleaning of GaAs (100) surfaces. Samples were n-type GaAs (100) having a surface area of approximately 1 Si-doped to 1 x 1017cm-3 (MCP Electronics). Methylene iodide (CH212) was supplied as 99% pure (Aldrich) and was vacuum distilled over copper turnings to remove residual iodine prior to use. All Auger spectra were recorded using a primary beam energy of 3 KeV, 1-5 uA cm-2 primary current density, in derivative mode. SURFACE PREPARATION OF CLEAN, AS-RICH GaAs (100) SURFACES Clean GaAs (100) surfaces exhibit several reconstructions that vary with Ga and As stoichiometry (ref. 11). We have developed a technique for the production of a (1x1) surface that is As-rich which we employ in model studies of GaAs growth and etching (refs. 9 and 12). (b) and (c) depict Auger electron spectra from a GaAs Figures 2(a), (100) surface following wet chemical etching and heating in UHV to 820 K. These spectra show clearly the presence of C and 0 contamination. Figure

232

2(d)

the low energy As ( 3 4 eV) and Ga ( 5 5 eV) Auger peaks which

depicts

are employed in the determination of surface stoichiometry. The use of these peaks is more reliable than the use of the higher energy peaks shown in figure 2 ( c ) since the escape depth of the low energy electrons is only 5 - 7 Angstroms.(ref. 13).

(a)

Ga

(b)

As

0

-maz

\a , v

Dl 1000

1

r

1

1300

460

560

Electron Energy/ e V

Electron Energy/ eV

c;

tr

0) v

220

I

r

320

25

Electron Energy/ eV

1 75

Elezlron Energy/ ev

Figure 2. AES spectra from a GaAs (100) surface after wet chemical etching, mounting in UHV and heating to 700 K. Figures

3(a)-(d)

depict

corresponding Auger

spectra

recorded

after

placing the sample i n the isolatable reactor and heating to 570 K in 0.5 mbar H2 for some fifteen minutes. From figure 3 it may be seen that the hydrogen treatment results in a 'clean' surface by Auger standards. This cleaning procedure has the advantage over conventional thermal processing of GaAs in that As

is not lost via evaporation. However, figure 3(d)

indicates that the resulting surface is somewhat Ga-rich. In order to generate an As-rich surface the sample was then heated in 5 x lo5 mbar AsHj/He 1:9 for some ten minutes. The low energy Auger spectrum from such a surface is shown in figure 3(e)

and is clearly dominated by the As 34 ev

feature indicative of high surface As levels. GaAs (100) surfaces prepared in this manner exhibit (1x1) LEED patterns which we believe are due to adsorbed hydrogen which arises from the heterogeneous decomposition of ASH,

233

Ga

As

1000

1300

r

I

460

560

Electron Energy/ eV

Electron Energy/ eV

I

100

25

Electron Energy/ eV (e)

I

I

I

320

220

Electron Energy/eV Ga

I

I

25

Electron Energy/ eV

Figure 3(a)-(d)

AES spectra from GaAs (100) following treatment in the

isolatable reactor to 570 K under 0.5 mbar

%

for 15 minutes. Figure 3(e)

AES spectra obtained following further treatment in 5 x 10-5mbar As3 /He

1:9 at 770 K for 10 minutes. on GaAs (100) (ref. 12) although there are reports in the literature of a (1x1) As-stabilized surface generated at high As coverages (ref. 14). A complete account of this cleaning procedure will appear elsewhere (ref.

15).

234

RESULTS AND DISCUSSION Figures 4(a)-(d) show the Auger spectra obtained following exposure of a surface prepared using the techniques described above to 1000 L CH212 at 300 K. It is clear from these spectra that both I (520 eV MNN) and C have been deposited on the surface, whle the low energy As peak intensity has fallen dramatically. Figures 5(a)-(c)

show the result of thermal processing of

this surface up to 770 K, while figures S(d)-(f) spectra obtained

after

annealing

to

870

K.

are the corresponding These

spectra clearly

demonstrate that above 770 K surface carbon and iodine are removed, surface Ga coverage is slightly depleted and surface As coverage increases to yield an As-rich surface again.

Ga

AS I

a

z

-m.. h

a M

-z z

I

I

1000

1300

iJ.-""u'

C

Y

W

Y

r J

P-

,-

rt II) v

1

560

Electron Energy/ eV

rt

G

r 460

220

320

25

75

Electron Energy/ e V

Figure 4 . AES spectra from GaAs (100) following exposure to 1000 L CH21Z at 300 K .

235

(b)

(C)

I

C

25

E

1

I

I h

v

75

220

r

I

320 460

Electron Energy/ eV

AY

(f)

QJ

I

Y

Y

/ -

c

3 Prt

I

I

rn

Y

25

15 220

I

1

320 460

560

Electron Energy/ eV

AES spectra from GaAs (100) following exposure to 1000 L CH212 at 300 K and annealing to 770 K. Figures 5(d)-(f), corresponding spectra following annealing to 870 K.

Figures 5(a)-(c).

The disappearance of the low energy As peak following exposure to

CH212 suggests that direct reaction to yield

a

volatile As species such as

As13 occurs. This species could then sublime in vacuo. Similar behaviour has been noted in studies of the oxidation of GaAs (100) surfaces in which the formation of As406 is believed to occur followed by direct sublimation of this oxide (ref. 13). It is also possible that decomposition of CH212 is induced by the electron beam. However, at present, the reproducibility of the Auger data suggest that a direct reaction is the most likely pathway in operation. Adsorption of CH212 may occur via the coordination of surface As lone pairs to the electron-deficient carbon atom in CK$,

in a manner

similar to the Lewis acid-base complex formation proposed in order to account for the adsorption of triethyl gallium on such a surface (ref. 12). Residual levels of C and I following exposure are not high, yet these

236

signals must

represent

species

which

are

strongly

adsorbed.

Indeed

annealing in excess of 770 K is needed before these signals disappear. We tentatively suggest that fragments of the type CH21 remain on the sample surface following loss of I and subsequent removal of surface A s .

Thus

these fragments are most likely bound to exposed Ga atoms revealed upon removal of the top layer of As atoms. This suggestion is supported in part by the observation that surface Ga levels decrease slightly following annealing above 770 K coincident with the disapearance of

the C and I

signals. Activated removal of Ga atoms in this way would expose underlying As

atoms whilst

diffusion of As

atoms to the surface would

also be

facilitated, accounting for the resulting As-rich surface, figure 5(d). Assuming that these

(OK

similar) processes occur, it is possible to

speculate on the design of a surface-specific etching process for GaAs (loo),

i.e. the results presented here suggest that selected area etching

of G a A s (100) surfaces could be achieved using CH212 together with a non-

specific wavelength, focussed light source which would be employed to thermally desorb Ga species, revealing surface As atoms which would be subject to direct chemical attack and removal possibly as As$. work

involving

the

use

of

visible

lasers

and

thermal

Further

desorption

measurements is underway in order to confirm or deny this proposal.

SUMMARY We have demonstrated that CH212 adsorbs on As-rich GaAs (100) at 300 K in the absence of photo-activation. This finding may provide a route to the design of a surface-specific laser-assisted etching process for GaAs (100) surfaces. ACKNOWLEDGEMENTS The authors gratefully

acknowledge

the

support

of

the

SERC

for

equipment grants and studentships for DSB and PAG. additional support for

DSB from GEC Hirst Research (CASE Studentship) and support from the MOD for a research assistantship (SMP). REFERENCES

1

W.M.

Holber

and

R.M.

Osgood

Jr.,

Solid

State Technology,

(1987) 139-143. 2

P.D. Brewer, D. McClure and R.M. Osgood Jr., Appl. Phys. Lett., 47 (1985) 310-312.

3

P.D.

Brewer, D. McClure and R.M. Osgood Jr., Appl. Phys. Lett.,

49 (1986) 803-805.

April

237

4

P.D. Brewer, S . Halle and R.M. Osgood Jr., Appl. Phys. Lett., 45

5

P.D. Brewer, S . Halle and R.M. Osgood Jr., Mat. Res. SOC. Symp.

(1984) 475-477.

Proc., vol. 29 (1984) pp. 179-184. 6

D.J.

Ehrlich, R.M. Osgood Jr. and T.F. Deutsch, Appl. Phys. Lett.,

36 (1980) 698-700. 7

J.B. Koffend and S.R. Leone, Chem. Phys. Lett., 8 1 (1981) 136-141.

8

M.E. Pemble, Chemtronics, 2 (1987) 13-15.

9

D.S.

Buhaenko, S.M.

Francis, P.A.

Goulding and M.E.

Pemble, J.

Vac. Sci. Technol. B., December 1988, accepted for publication. 1 0 R.W. Judd, H . J .

Allen, P. Hollins and J. Pritchard, Spectrochimica

Acta, 43A (1987) 1607. 11 P. Drathen, W. Ranke and K . Jacobi, Surface Science, 77 (1973)

L162-166. 12 M.E.

Pemble, D.S.

Buhaenko, S.M.

Francis and

P.A.

Goulding, to

appear in Proc. NATO Workshop on ‘Reactions of Organometallics with Surfaces’, June 1988. 13 W. Ranke and K. Jacobi, Surface Science, 47 (1975) 525-542.

14 J. Massies, P. Devoldere, P. Etienne and N.T. Linh, in Proc. 7th

Int. Cong. on Solid Surfaces, (Vienna 1987) p.639. 15 D.S.

Buhaenko, S.M. Francis, P.A. Goulding and M.E. Pemble, to be

published.

This Page Intentionally Left Blank This Page Intentionally Left Blank

C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

EFFECT OF COBAZT ON THE SURFACE PROPERTIES OF Zn-Cr SYNTHESIS CATALYSTS

AM)

239

Cu-Zn-Cr METHANOL

2

G. BUSCA'. M.E. PATIzTELL12, F. TRIFIROt2and A. VACCARI Istituto di Chimica, Facolts di Ingegneria, Universita, 16129 Genova (Italy).

P.le

Kennedy,

Dipartimento di Chimica Industriale e dei Materiali, Universita, V.le Risorgknto 4, 40136 Bologna (Italy).

ABSTRACT The catalytic activity in the synthesis of methanol of cobalt-free and 2 atom % cobalt-containing Zn-Cr and Cu-Zn-Cr catalysts was investigated. The adsorption of hydrogen and CO was studied by FT-IR spectroscopy. It is suggested that small m u n t s of cobalt inhibit the catalytic activity of ZnCr catalysts through an inhibition of their ability to activate hydrogen dissociatively. In the case of copper-containing catalysts, cobalt ions do not mdify apparently the properties of the surface copper species. Also in this case the synthesis of methanol is inhibited by cobalt, probably again through the inhibition of the hydrogen dissociation on the spinel phase, that would represent the first step in hydrogen activation. The results are interpreted as evidences of the role of the oxide phase even in coppercontaining catalysts.

INTRODUCTION The &rn lowpressure and low-temperature methanol synthesis catalysts are constituted by copper-containing mixed oxides [l], that are reduced by activation procedures with the appearance of a copper metal phase together with the oxide phases. These mixed oxide catalysts, usually produced by coprecipitation methods, have been found to be considerably more active than the individual ccenponents [1-3]. However, different hypotheses are reported in the literature on the role of the different phases present in the working conditions in the catalytic system. According to some authors, mtallic copper is the active species while the oxide phases only constitute a support for it [ 4 ] . Klier [3] has reported that Cul+ species stabilized in the ZnO lattice are catalitically active in the methanol synthesis, and a mechanism involving Cul+ sites on ZnO has been proposed by Edwards and Schrader [5] and by Olive and Olive [ 2 ] . Finally, for some authors [6,7] the oxide phases mdify the copper reactivity and/or are involved in the reagent adsorption. Aim of this work was to give a contribution to the understanding of the

240

role of the different phases in the copper-containing catalysts, by investigating the surface properties (catalytic activity and adsorption of H2 and CO) of CuZnCr and ZnCr ethanol catalysts, and the effect of small amwnts of cobalt on them. The catalyst ZnCr was studied as an example of the oxide matrix that is also active in methanol synthesis although at higher tenpzrature and pressure [la,81. EXPERIMENTAL The catalysts used in the present work (Table I) were prepared by coprecipitation starting fran a solution of the nitrates of the elements, dried at 363 K and calcined at 623 K for 24 h [8,91. Table I. Characterization of the catalysts under study. Notation

Atan. ratio %

ZnCr COZnCr CuZnCr CoCuZnCr

50 :50 2:48:50 13:63:24 2:13:61:24

Surface area (m'/g) (a) (b) 88 64 81 96

119 145 96 89

XRD data

(a)

(b)

amorphous amorphous

nss nss

HY HY

llss

> zno nss > ZnO

(a) precursor dried at 363 K; (b) catalyst calcined at 623 K; HY = hydrotalcite-type phase; nss = non-stoichianetric spinel-type phase.

The catalytic activity in methanol synthesis was investigated in all ' h cases using a H2/CO/O02 65:32:3 v/v gas mixture, with GHSV of 16,000 . The tests were performed for copper containing catalysts at 1.2 MPa and for copper-free catalysts at 6.0 MPa, in the tenperature ranges 500-580 K and 550-620 K, respectively. The catalysts were previously reduced in the reactor by hydrogen diluted in nitrogen, with the hydrogen content and tenperature being progressively increased [7,10]. The FT-IR spectra were recorded using Nicolet MX1 and 5ZDX spectraneters, with conventional gas-manipulation lines and IR cells. In all cases, self-supporting pressed disks of the pure catalyst powders were Pa) at 673 K for 1 h, treating in 100 kPa H2 pretreated by evacuation at 673 K for 1 h, and again evacuation at 673 K for 1 h. m?lsuLTs

a) Catalytic activity. The productivities of methanol obtained in the conditions of highpressure methanol synthesis on ZnCr and CoZnCr catalysts are ccnpared in Fig. l,A. The addition of cobalt considerably decreases (about 50 % ) the productivity of ethanol without any change in selectivity (about 100 %, dry basis). It is relevant that, even in the presence of cobalt, no traces of methane and other hydrocarbons are detected. The deviation fran linearity of

241

A

80

40

0 490

530 570 T e m p e r a t u r e CK)

Fig. 1. Methanol productivity as a function of the reaction temperature on A: ZnCr ( m ) and CoZnCr ( 0 ) catalysts (P= 6.0 MPa) and B: CuZnCr ( W ) and MuZnCr ( 0 ) catalysts (P'1.2 ma). the trend of the methanol productivity on the cobalt-free catalyst at high temperature arises from thennodynamic limitation of conversion (la). Cobalt has an even stronger effect on the activity of the CuZnCr catalyst in the low-temperature conditions for the synthesis of methanol. Near 550 K the productivity of the CoCuZnCr catalyst is near one tenth that of the cobalt-free catalyst (Fig. 1,B). Even in both these cases the selectivity approaches 100 %, without any detection of hydrocarbons. The data reported in Table I1 show that cobalt has a detrimental effect on the surface area of Etallic copper. This effect is dramatic in the reaction conditions, where the copper surface has almost disappeared on the cobalt-containing catalyst, while on the cobalt-free catalyst it only decreases in accordance with its partial oxidation [7,11]. The decrease in copper surface area is in both cases reversible upon further reduction in diluted hydrogen. Therefore, no sintering phencmna can be invoked in either case: the disapperance of copper surface on the CoCuZnCr catalyst in reaction conditions must be attributed to its coverage by adsorbed species. Table 11. Copper surface area (m2/g Cu) measured by N20 titration. Sample Before reaction After reaction

242

b ) ET-IR study of t h e a d s o r p t i o n

of hydrogen.

The spectra of t h e ZnCr c a t a l y s t a f t e r a c t i v a t i o n a n d a f t e r f u r t h e r c o n t a c t w i t h hydrogen and deuterium a t r . t . are r e p o r t e d i n Fig. 2. Hydrogen adsorption produces a s l i g h t l y s p l i t band at 1815,1788 cm-', while deuterium adsorption produces a parallel s l i g h t l y s p l i t band a t 1315,1295 an-'.

The

r a t i o between t h e f r e q u e n c i e s o f t h e s e t w o c o u p l e s o f b a n d s is 1 . 3 8 , accordmy t o t h e i r a s s i g n m n t t o l i n e a r z i n c hydride and d e u t e r i d e species, r e s p e c t i v e l y [ 1 2 ] . The formation of t h e l i n e a r hydride species is r e l a t e d t o

the presence of t h e n o n - s t o i c h i m t r i c s p i n e l phase:

t h e y are i n f a c t not

observed on t h e s t o i c h i m e t r i c ZnCr204 s u r f a c e i n t h e same c o n d i t i o n s [12]. Zinc hydrides have been observed i n similar c o n d i t i o n s also on ZnO [ 1 3 ] b u t

are

c h a r a c t e r i z e d by

l m r Zn-H f r e q u e n c i e s on

ZnO (1710 cm-')

t h a n on

t h e Zn-Cr c a t a l y s t .

Fig. 2. FT-IR spectra of t h e ZnCr c a t a l y s t after a c t i v a t i o n ( a ) and after c o n t a c t with hydrogen (b) and deuterium (c) a t r.t. (40.0 W a ) . When t h e same experiments

were carried o u t on t h e CoZnCr c a t a l y s t , no

bands a s s o c i a t e d w i t h adsorbed hydrogen c o u l d be seen. Only small m o u n t s of

water were detected. Another s t r i k i n g d i f f e r e n c e between ZnCr and CoZnCr c a t a l y s t s frcm t h e spectroscopic p o i n t of view is the absence on t h e latter of f r e e s u r f a c e hydroxy g r o u p s , e v e n a f t e r water a d s o r p t i o n . The VOH

spectral r e g i o n s of t h e two c a t a l y s t s are carpared i n Fig. 3. The behaviour of t h e CoZnCr compound ( n o n - s t o i c h i m e t r i c s p i n e l ) is analogous t o t h a t of t h e s t o i c h i m e t r i c ZnCr204 c q u n d . Also t h i s last c a t a l y s t seems t o be i n a c t i v e both i n w a t e r and i n hydrogen d i s s o c i a t i v e a d s o r p t i o n [12,14]

well as being poorly a c t i v e i n t h e s y n t h e s i s of mthanol

[la,8].

as

243

I(

u C m Y

Y 4

E

m C m

L Y

4000

3600

3200

4000

3600

wavenumbers

3200

cm -4.

Fig. 3 . FT-IR spectra of activated ZnCr (a)2and activated CoZnCr ( b ) and of CoZnCr a f t e r contact with water 0.5 10 Pa ( c ) . Another effect of cobalt on the ZnCr catalyst is the displacement towards higher frequencies of t h e bands due t o semiconductivity phenomena [ 1 4 ] showing that it interferes with the electronic s t a t e of the catalyst.

I n the case of the coprecipitated CuZnCr catalyst, no new features are observed

a f t e r hydrogen adsorption a t r.t.. Similarly also coprecipitated

Cu/ZnO linear hydrides are undetectable by adsorption of hydrogen a t r.t. [6,al while on pure ZnO they are w e l l evident i n the same conditions [13]. However, i f the CuZnCr catalyst heated i n hydrogen amsphere, starting

near 420-450 K two spectroscopic features graw (Fig. 4 , a ) : 1) a very broad, -1 almost continuous absorption, centered near 2000 cm , i n s e n s i t i v e t o deuterium exchange, similar t o t h a t observed also on Cu/ZnO catalysts and

u

0

C

a

.*:. .

P

c 0 0

n a

\

3800

3000

2200

600

1400

wavenuabars

cm-i.

Fig. 4. FT-IR absorptions arising from hydrcqen adsoption on the CuZnCr (a) and CuCoZnCr (b) catalysts a f t e r contact with 1 10 Pa of H2 a t 473 K. The sharp negative bands near 1000 and 900 cm-’ correspond absorptions that disappear i n these conditions.

t o skeletal

244

assigned to electronic phenonrena [6a]; 2 ) a broad although "localized" absorption centered near 900 an-', probably containing sane mltiplicity, &served only when hydrogen is adsorbed, unlike deuterium. According to the isotopic effect, the latter band is assigned to a vibration of an hydrogencontaining species. Because of the absence of intense OH and OD stretching bands after hydrogen and deuterium adsorption respectively, this band cannot be due to a MDH deformation mode. Conseqwntly, this strong band is assigned to the stretching vibration of a multiply bridged metal hydride species. Since this band is absent on ZnCr catalysts, its relation w i t h copper is very likely. Sarre of us have proposed [15] an assigrmnt to triply bridged hydride species according to the structure of the only e l l characterized copper hydride catpound [(PR3)CuHJ6 [16]. Traces of these absorptions are also observed if contact at r.t. is carried out for several hours. A very similar band is also observed on the CoCuZnCr catalyst (Fig. 4,b), although its formation scents to be slightly m r e difficult. In this case the features of the broad absorption band due to electronic phenomena is different, shawing two maxima near 3200 and 2000 an-' with a mininun near 2450 m-l. Again it is clear that cobalt modifies the electronic state of the system.

2250

21so

2250

2150

wrvenuamrs

2050 CR-L

Fig. 5. FT-IR spectra of ZnCr (a) and CoZnCr (b) catalysts with adsorbed at 170 K.

cx)

c) ET-IR study of the adsorption of carbon monoxide.

spectra of carbon moxide adsorbed at roan and lcwer tenperatures on ZnCr and CoZnCr catalysts are shown in Fig. 5. The addition of cobalt adds new centers for CO adsorption, producing carbonyl species characterized by lcwer K O frequencies (2170 and 2145 an-') with respect to those of CO interacting with Cr and Zn centers (2205 and 2192 an-' [14,171). The new The

245

c e n t e r s are v e r y l i k e l y c o n s t i t u t e d by u r e d u c e d cobalt c e n t e r s , m o s t probably Co2+ and/or Co3+ [18I . These data agree w i t h t h e tendency of c o b a l t

t o form s p i n e l s w i t h both o x i d a t i o n states (such as Co304,

CoCr204 and

Co2Cr04) and mixed chromite s p i n e l s w i t h z i n c [19]. The spectra of CO adsorbed on reduced CuZnCr and CoCuZnCr a t room or

lower temperatures appear v i r t u a l l y i d e n t i c a l ( F i g . 6 ) . The carbonyl species i r r e v e r s i b l y adsorbed a t 170 K

show two main absorptions both m u l t i p l e .

The higher frequency one is c o n s t i t u t e d by a s h a r p band a t 2192 an-' w i t h a shoulder near 2205 an-',

almost i d e n t i c a l to t h e band c h a r a c t e r i s t i c of

s u r f a c e carbonyl on t h e z i n c c h r d t e c a t a l y s t s . T h i s r e s u l t shows t h a t z i n c chromite is exposed i n p a r t on t h e s u r f a c e of the CuZnCr c a t a l y s t . The lower frequency a b s o r p t i o n is c o n s t i t u t e d by a couple of bands a t 2110 and 2130 cm-1 ( t h e latter one due t o a species s l i g h t l y mre s t r o n g l y bonded being

more r e s i s t a n t t o e v a c u a t i o n a t r e d u c e d t e m p e r a t u r e ) w i t h an h i g h e r -1 These bands are due t o copper species, frequency shoulder near 2155 cm

.

according t o their frequencies and t h e i r absence on t h e copper-free samples. Adsorption a t r.t. causes t h e formation of o n l y a s i n g l e band a t 2125 cm-', t h a t may be assigned t o CO species on Cul+ c e n t e r s ,

stable a t r.t. [20].

The band a t 1-r

t h a t are g e n e r a l l y

frequency (2110 an-'),

observed only

a t 170 K is due t o less s t a b l e carbonyl species and is assigned t o CO on copper metal p a r t i c l e s [6a,21]. These assignments are also supported by t h e comparison w i t h the carbonyl species observed on a Zn:Cr 75:25 c a t a l y s t t h a t

B

2250

2150

I 2250

.

2150

wavenunbars

2050

catwL

Fig. 6. ET-IR spectra of CuZnCr ( A ) and CuCoZnCr ( B ) with adsorbed CO: a ) a t 170 K under evacuation; b ) a t r.t. under evacuation; c ) on t h e h y d r q e n covered s u r f a c e . d ) a 75 % Zn 25 % C r c a t a l y s t impregnated w i t h 2 % copper w i t h adsorbed CO a t 170 K.

246 has been inpregnated by 2 a t a n % of copper, where the f o m t i o n of extended

capper netal particles is unlikely. In t h i s case, i n f a c t , together with the bands due t o CO on the Zn and Cr surface cations, the band near 2130 an-’, stable at r.t., is well evident with only a very weak shoulder near 2110 an-’ (Fig. 6,A,d). The shoulder near 2150 an-’ on both CuZnCr and CoCuZnCr is assigned to co on b2+ centers 1201. The almost perfect coincidence of the bands observed on CuZnCr and on CtEoZnCr, particularly those assigned to copper centers, suggests t h a t cobalt does not interfere with t h e state of copper. I t see.ms also relevant t h a t in our experimental conditions CO on mtallic cobalt is undetected. ALSO the spectra of CO adsorbed on CuZnCr and CoCuZnCr where H2 has been preadsorbed (so showing the bands discussed above) are very similar (see Fig. 6,c). These spectra shaw a strong band at 2105 an-’ and a weaker band near 2145 an-’. The low frequency feature is assigned t o CO adsorbed on capper particles, similar t o those described by Ghiotti and Boccuzzi [6a] f o r Cu/ZnO c a t a l y s t s while t h e higher frequency f e a t u r e (2145 an-’) is l i k e l y due t o CO on Cu2+ c e n t e r s [20], almost unperturbed by hydrogen adsorption ( t h i s band is &served as a shoulder near 2150 an-’ a l s o on activated samples). The difference between the spectra recorded in the presence of adsorbed h y d r q n with respect to those recorded on the reduced samples confirms t h a t p a r t of copper c e n t e r s are involved i n hydrogen adsorption. The data may be interpreted suggesting t h a t the copper centers we have identified as Cu”, responsible f o r the carbonyls characterized by the relatively stable band near 2130 can-’ (no more &served on the hydridecovered s u r f a c e ) are blocked by hydride species, while copper metal particles are still f r e e (bands of adsorbed a3 a t 2110-2105 an-’). The m d i f i c a t i o n of the electron density i n the system, arising fran adsorbed hydrogen, might be responsible f o r the increase of the s t a b i l i t y of CD adsorbed on copper particles ( i n f a c t the band near 2105 an-’ is well evident at r.t. only on the hydride-mvered surface). w a i n , it seems t h a t cobalt does not m d i f y qualitatively the state of copper, neither unreduced nor reduced, a l s o in the presence of adsorbed hydrogen.

DISCUSSION The catalytic experiments we have reported evidence that w i n g both ZnCr and CuZnCr catalysts with a small ammt of cobalt Ckeply modifies the properties of the systems with a strong decrease of the catalytic a c t i v i t y without substantial change i n selectivity. This effect, observed also on Cu-Zn-A1 c a t a l y s t s [221 can be a t t r i b u t e d n e i t h e r t o t h e presence of metallic cobalt (typically active. i n hydrccarbon synthesis and undetcted on

247

our c a t a l y s t s ) nor to the surface coverage by other cobalt species. Also t h e s m a l l reduction of copper surface area by cobalt doping on t h e activated c a t a l y s t s cannot j u s t i f y t h e inhibition of the catalytic a c t i v i t y observed on copper containing systems. However, t h e drop of the surface area of copper a f t e r reaction c l e a r l y indicates t h a t cobalt favours the deactivation of copper by adsorbed species. Although t h e conditions of the I R e x p ? r k n t s are very f a r f r a n those of methanol s y n t h e s i s , t h e y g i v e some i n d i c a t i o n s t h a t may e x p l a i n t h e d i f f e r e n t behaviours of cobalt-free

and cobalt-doped catalysts.

Our data

suggest that even small amounts of cobalt m d i f y the electronic state of the ZnCr c a t a l y s t . This c a t a l y s t , whose electronic properties were investigated several years ago by Garcia de l a Banda [23],

is a typical semiconducting

oxide system whose behaviour is strongly dependent on i t s canposition and t h e g a s phase i n which it i s immersed. So, t h e p e r t u r b a t i o n of i t s properties even by small amounts of cobalt is not surprising f r a n this pit of view.

The main r e s u l t of this perturbation is the inhibition of the

dissociative adsorption of hydrogen, t h a t explains the drop i n the catalytic a c t i v i t y i n methanol synthesis. The doping with cobalt of t h e copper-containing c a t a l y s t CuZnCr seems t o leave unaltered the nature of the copper surface sites, as deduced by our I R data. CO adsorption s h m that on t h e surface of t h e low-temperature CuZnCr

c a t a l y s t and of the doped CuCoZnCr c a t a l y s t the spinel phase is exposed and may have a role i n c a t a l y s i s . According t o the observation that the coppercontaining c a t a l y s t s active i n methanol synthesis a h s t invariably contain an oxide phase able t o dissociate hydrogen heterolytically ( 2 4 ) , such as ZnO,

Tho2, Cr203 o r Zn-Cr oxides, as w e l l as t o the hm inhability of copper metal t o dissociate hydrogen, we may suppose t h a t i n our case the spinel phase is needed to dissociate hydrogen as a previous s t e p f o r t h e formation of copper hydrides. The inhibition of the spinel phase by cobalt

Zr02,

may then j u s t i f y also t h e inhibition of t h e copper-containing catalyst. The lack of active hydrogen species may also be responsible f o r the formation i n reaction conditions of

strongly adsorbed carbn-containing

species t h a t

cover copper particles.

As a conclusion, our data suggest t h a t a role is played by the oxide matrix even i n copper-containing methanol synthesis c a t a l y s t s , i n p a r t i c u l a r i n the hydrogen-activation

step,

although further research is needed t o

confirm t h i s hypothesis.

-

ACKNOWLEDGEMENTS F i n a n c i a l s u p p o r t s from t h e P r o g e t t o F i n a l i z z a t o h e r g e t i c e CNR-ENEA, and the Minister0 Pubblica Istruzione ( S t r u t t u r a e Reattivita d e l l e Superfici) are gratefully achowledged.

REFERENCES

(a) G. Natta, in P.H. -ttt (Editor), Catalysis, Reinhold, New York, , Rev. *i. Eng., 22 (1980) i953, voi. 111, p. 349. (b) H.H. ~ ~ n gCatal. 235. G. Henrici-Olive' and S. Olive', Catalyzed Hydrogenation of Carbon Wnoxide, Springer-Verlag, New York, 1984. K. Klier, in D.D. Eley, H. Pines and P.B. Weisz (Editors), Advances in Catalysis, Academic Press, New York, Vol 31, 1982, p. 243. (a) K. ShimnUra, K. Ogawa, M. &a and Y. Kotera, J. Catal., 52 (1978) 191. (b) S.P.S. Andrew, Plenary Lecture (Paper 121, Post Congress Symposium, 7th International Congress on Catalysis, Osaka, Japan, July 1980. (c) G.C. Chinchen, P.J. Denny, D.G. Parker, G.D. Short, M.S. Spencer, K.C. Waugh and D.A. Whan, Prepr. Am. Chem. Soc., Div. Fuel m.,29 (1984) 178. J.F. Edwards and G.L. Schrader, J. Catal., 94 (1985) 175. (a) G. Ghiotti and F. BOCCuzzi, Catal. Rev. Sci. Eng., 29 (1987) 151. (b) S. Gusi, F. Trifiro' and A. Vaccari, Reactivity of Solids, 2 (1986) 59. (c) B. Rasrmssen, P.E. Hojlund Nielsen, J. Villadsen and J.B. Hansen, in B. Delmm, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 785. S. Gusi, F. Trifiro', A. Vaccari and G. Del Piero, J. Catal., 94 (1985) 120. M. Di Conca, A. Riva, F. Trifir;, A. Vaccari, G. Del Piero, V. Fattore and F. Pincolini, Proc. VIIIth Int. Congr. Catalysis, Dechema, Frankfurt am Main, 1984,_Vol. 2, p. 173. S. Gusi, F. Pizzoli, F. Trifiro, A. Vaccari and G. Del Piero, in B. D e h , P. Grange, P.A. Jacobs and G. Ponelet (Editors), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 753. 10 P. Courty, D. Durand, E. F'reund and A. Sugier, J. Mol. Catal., 17 (1982) 241. 11 G.C. Chinchn, M.S. Spencer, K.C. Waugh and D.A. Whan, J. Chem. Soc., Faraday Trans. 1, 83 (1987) 2193. 12 G. Busca and A. Vaccari, J. Catal. 108 (1987) 491. 13 (a) R.P. Eischens, W.A. Pliskin and M.J.D. Lcw, J. Catal. 1 (1962) 180. (b) F. Boccuzzi, E. Borello, A. Zecchina, A. eoSsi and M. Camia, J. Catal., 51 (1978) 150. 14 M. Bertoldi, B. Fubini, E. Giamello, G. Busca, F. Trifir6 and A. Vaccari, J. Chem. Soc. Faraday Trans. I, 84 (1988) 1405. 15 G. Busca and A. Vaccari, J. Chem. Soc. Che3n. Carm., in press. 16 T.H. Lenunen, K. Folting, J.C. Huffman and K.G. Caulton, J. Amer. Chem. Soc., 107 (1985) 7774. 17 E. Giamello, B. Fubini, M. Bertoldi, G. Busca and A. Vaccari, J. Chem. Soc. Faraday Trans. I, in press. 18 N. Sheppard and T.T. Nguyen, in RJ.H. Clark and R.E. Hester (Editors), Advances in Infrared and Raman Spectroscopy, Heyden, London, 1978, Vol. 5, p. 67. 19 G. Fornasari, S. Gusi, F. Trifir; and A. Vaccari, Ind. Eng. Chem. Res. 26 (1987) 1500. 20 G. Busca, J. Mol. Catal. 43 (1987) 225. 21 J. Pritchard, T. Catterick and R.K. Gupta, Surf. Sci. 53 (1975) 1. 22 (a) F.N. Lin and F. Pennella, in R.G. Herman (Editor), Catalytic Conversion of Synthesis Gas and Alcohols to Chemicals, Plenum Press, New York, 1984, p. 53. (b) D.J. Elliott, J. Cata1.t 111 (1988) 445 23 (a) J.F. Garcia de la Banda, J. Catal. 1 (1962) 136.(b) J.F. Garcia de la Banda and J. Hernaez Marin, Anales Real Soc. Espan. Fis. Q u h . 52B (1957) 499 and 54B (1958) 115. 24 G. Wrobel, L. Jalawiecki, J.P. Bonnelle, F. Bali and A. Battahar, New J. Chem.11 (1987) 715.

C. Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactiuity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

"9

A STUDY OF THE ADSORPTION OF ACETYLENE ON Cu(100) USING AUGER ELECTRON SPECTROSCOPY

M.A. CHESTERS and D.R. LINDER School o f Chemical Sciences, U n i v e r s i t y o f E a s t A n g l i a , NORWICH, NR4 7TJ, U.K.

ABSTRACT The thermal e v o l u t i o n o f a c e t y l e n e on t h e Cu(100) s u r f a c e has been s t u d i e d u s i n g h i g h r e s o l u t i o n Auger e l e c t r o n spectroscopy. The C lsVV Auger spectrum o f a c e t y l e n e adsorbed on Cu(100) a t 120 K was recorded t o produce a c h a r a c t e r i s t i c Auger l i n e s h a p e o r ' f i n g e r p r i n t ' spectrum. A t 170 K, a new l i n e s h a p e was observed i n d i c a t i n g a change i n t h e adsorbed species, t h e new f o r m b e i n g s t a b l e u n t i l 275 K. The s u r f a c e species desorbed f u l l y between 300 and 350 K t o l e a v e a c l e a n s u r f a c e . Recent r e s u l t s ( r e f . 1 ) on C u ( l l 0 ) have i n d i c a t e d t h a t an a c e t y l e n e c y c l o t r i m e r i z a t i o n r e a c t i o n occurs a t 325 K r e s u l t i n g i n t h e f o r m a t i o n o f benzene. Hence t h e carbon Auger spectrum o f benzene has a l s o been recorded f o r comparison with t h e a c e t y l e n e s p e c t r a a t e l e v a t e d temperatures. E l e c t r o n beam induced damage o f t h e adsorbed a c e t y l e n e molecule a t 120 K was shown t o o c c u r a f t e r an e l e c t r o n dose o f 120 C and so a l l s p e c t r a were recorded u s i n g a dose o f n o t more t h a n 20 C r2.

INTRODUCTION The a d s o r p t i o n o f a c e t y l e n e on copper has been s t u d i e d by s e v e r a l w o r k e r s o v e r t h e l a s t few y e a r s , products o f

i t s thermal

Marinova e t a1 ( r e f . 21,

but there i s s t i l l decomposition.

a l a c k o f agreement o v e r t h e

I n t h e most r e c e n t i n v e s t i g a t i o n ,

on t h e b a s i s o f HREELS d a t a on Cu(lOO), conclude t h a t

t h e adsorbed a c e t y l e n e molecule i s c o n s i d e r a b l y d i s t o r t e d compared t o t h a t i n t h e gas phase by l e n g t h e n i n g o f t h e C-C bond and bending o f t h e hydrogen atoms away f r o m t h e metal s u r f a c e plane, On h e a t i n g ,

r e s u l t i n g i n a h y b r i d i s a t i o n c l o s e t o sp3.

simultaneous d e s o r p t i o n and decomposition o f t h e molecule i n t h e

range 300 K t o 375 K r e s u l t i n t h e f o r m a t i o n o f new hydrocarbon species, most prominent o f which i s CCH.

T h i s i s r e p o r t e d t o be s t a b l e u n t i l 550 K,

when f u r t h e r decomposition to CH fragments and carbon occurs. same surface, as a n-bonded

Demuth ( r e f . 31,

the

U s i n g UPS on t h e

however, s t a t e s t h a t a t 80 K, a c e t y l e n e adsorbs

species which i s n o t s t r o n g l y r e h y b r i d i s e d and t h a t a t h i g h e r

temperatures t h e a c e t y l e n e desorbs r e v e r s i b l y . p o l y c r y s t a l l i n e copper surface,

Yu e t a1 ( r e f .

A p p l y i n g t h e same t e c h n i q u e t o a

4) r e p o r t t h a t t h e chemisorbed

a c e t y l e n e i s s t a b l e on t h e s u r f a c e up t o room temperature b u t desorbs a t about 375

K, a f t e r which t h e copper s u b s t r a t e appears t o remain contaminated w i t h

carbon.

250

I n a study on C u ( l l 0 ) using TDS, XPS and UPS, Outka e t a1 ( r e f . 5) suggest t h a t acetylene i s n o n - d i s s o c i a t i v e l y adsorbed w i t h desorption peaks a t 280, 340 and 375 K.

According t o t h e i r model, the acetylene molecule i s d i s t o r t e d such

t h a t the hydrogen atoms are bent away from the metal surface, y i e l d i n g a surface species t h a t r e a d i l y forms ethylene by a surface r e a c t i o n w i t h a maximum r a t e a t approximately 340

K.

A t temperatures above 420 K,

acetylene desorption was

complete l e a v i n g r e s i d u a l atomic carbon on t h e surface. conclusions, surface

however,

process

on

Avery dismisses these

and r e p o r t s t h a t t r i m e r i z a t i o n to benzene i s t h e main the

has

erroneously a t t r i b u t e d mass spectrometric fragments o f benzene t o ethylene.

On

o f TDS and EELS,

(ref.

11,

Outka

t h e evidence

110 plane,

suggesting

Avery concludes

c o n c u r r e n t l y i n the r e g i o n 200 t o 400 K:

that

t h a t three

r e a c t i o n s occur

desorption o f acetylene, t r i m e r i z a t i o n

t o benzene and formation of hydrocarbon species.

D e s t r u c t i v e dehydrogenation o f

the hydrocarbon residues occurs betwen 600 and 900 K to leave atomic carbon on t h e surface.

F u r t h e r work by Avery ( r e f .

6 ) on t h e Cu(100) surface shows

benzene formation w i t h thermal desorption peaks a t 180 and 340 K .

Reversible

molecular desorption o f acetylene i s r e p o r t e d to occur a t 360 K. Auger lineshape analysis, although complex, has been c a r r i e d out s u c c e s s f u l l y on several molecules, i n c l u d i n g acetylene and ethylene ( r e f . 7). adsorbed on metal surfaces,

however,

For molecules

the spectrum i s f u r t h e r complicated by

f a c t o r s such as the screening e f f e c t o f t h e metal conduction e l e c t r o n s and as a r e s u l t , Auger spectra o f adsorbed molecules have mainly been assigned as more general " f i n g e r p r i n t s " o f adsorbed species.

A l a r g e data-base o f such spectra

has subsequently accumulated over the l a s t few years ( r e f . 8

-

18) and i n t h i s

study we have used t h i s l a r g e l y q u a l i t a t i v e approach t o i n v e s t i g a t e the thermal e v o l u t i o n o f acetylene on the Cu(100) surface. EXPERIMEMTAL The spectra were recorded w i t h a hemispherical analyser (VG ESCA 3) which was c a l i b r a t e d t o give the Cu L, W t r a n s i t i o n a t 919.0 eV i n the N(E) mode.

d, energy

emission was e x c i t e d by e l e c t r o n impact using a beam o f 0.12 defocused to i r r a d i a t e a 3.5 mn diameter spot on the sample.

3.0 keV, A l l spectra were

recorded i n d i f f e r e n t i a l mode using a spectrometer s l i t w i d t h o f 4 mn, energy o f 100 V,

Auger

pass

a peak-to-peak modulation voltage o f 2V and r e s o l u t i o n o f 2eV.

Re1 a t i v e exposures o f hydrocarbon gases were c a l c u l a t e d from uncorrected i o n gauge readings taken j u s t above t h e d i f f u s i o n pump.

P u r i t y o f the gases was

checked u s i n g a quadrupolc mass spectrometer. The Cu(100) s i n g l e c r y s t a l was cleaned by argon i o n bombardment and annealed at

700

K.

thermocouple.

The

crystal

temperature was

I n the e l e v a t e d temperature

measured

experiments,

by

a

chrome1 -alumel

a f t e r adsorption

of

251

acetylene a t 120 K,

the c r y s t a l was heated a t the required temperature f o r 15

minutes, followed by recooling t o 120 K,

a t which temperature a l l spectra were

recorded. E l e c t r o n Beam Damage When e x c i t i n g Auger spectra by e l e c t r o n impact,

care has t o be taken t o

ensure t h a t s i g n i f i c a n t damage i s not induced i n the sample during the t i m e taken t o record the spectrum.

Canning e t a1 ( r e f .

10) reported t h a t f o r

acetylene adsorbed on Cu(ll1) an e l e c t r o n dose o f 250 C m-2 can impinge on the c r y s t a l before there i s any detectable beam damage.

I n t h i s study, however, we

as shown i n Fig. 1. which shows how f i n d t h i s f i g u r e t o be as l a w as 100 C r2, the C lsVV Auger spectrum o f acetylene adsorbed on Cu(100) gradually changes w i t h increasing e l e c t r o n dose, eventually t a k i n g a form previously assigned t o g r a p h i t i c carbon ( r e f . 19).

I n a l l subsequent experiments, beam damage e f f e c t s

were minimised by recording spectra using a dose o f not more than 20 C m-*. RESULTS AND DISCUSSION The sequence o f carbon KVV Auger spectra i n Figure 2 shows the e f f e c t o f heating acetylene adsorbed on Cu(100) a t 120 K t o a given temperature f o r ten The clean surface

minutes and recording the spectrum a f t e r recooling t o 120 K. spectrum a t 120 K

includes

a small

d i f f r a c t i o n o f secondary electrons.

oscillation

a t about

275 eV due t o

A t 120 K acetylene adsorbs molecularly t o

produce a f i n g e r p r i n t spectrum w i t h s i x main features a t 260, 266, 270, 274, 279 and 283 eV.

On heating t o 170 K, immediate changes are apparent, the feature a t

279 eV being enhanced a t the expense o f those a t 266, 270 and 283 eV, w i t h the peak a t 274 eV remaining the most intense.

T h i s peak shape then p e r s i s t s u n t i l

the surface species desorbs between 300 and 350 K t o leave a clean surface. These changes are sumnarised i n Table 1, along w i t h descriptions o f other re1evant spectra. Obviously some m o d i f i c a t i o n t o the acetylene molecule has occurred as low as

170 K, although Marinova e t a1 ( r e f . 1) do not r e p o r t any decomposition below 300 K, a t which temperature they describe the formation o f CCH species, which are s t a b l e u n t i l 550 K. We see no evidence f o r t h i s , however, as a l l adsorbed species desorb before 350 K l e a v i n g no residue. Outka e t a1 ( r e f . 4) suggest the formation o f ethylene from acetylene a t elevated temperatures on Cu(llO),

so t o i n v e s t i g a t e t h i s p o s s i b i l i t y on the (100) surface we have also recorded the C KVV spectrum o f ethylene under the same experimental conditions a t 120 K (Fig. 3 ( i ) ) . There i s l i t t l e s i m i l a r i t y

between t h i s spectrum and the one o f acetylene a t 250 K,

however,

and i f we

r e f e r t o Table 1, we can confirm t h a t the f i n e s t r u c t u r e o f the two spectra show few coincident features.

4

120K Electron dose/Cm-2

20 120K

120

~~

4vy

170K 230

200 K

250 K

350K

~

240

260

,v, 280

I

300

KE/eV

F i g . 1 Carbon Auger spectra showing the e f f e c t o f beam damage on a monolayer of acetylene adsorbed a t 120 K on Cu(100).

,

240

260

I

,

280

,

,

300

KE/eV

F i g . 2 Carbon Auger spectra showing the e f f e c t of h e a t i n g a monolayer of acetylene adsorbed a t Cu(100) a t 120 K to i n c r e a s i n g temperatures.

253

TABLE 1 Adsorbate

C2" 2

CZH2

'bH6

'bH6

C2H4

'bHb

Temperature/K 120

250

170

200

120

300

Substrate

Peak Energy/eV

Cu(100)

260 266 270 274 279 283

Cu(100)

253 260 266 270 274 279 283

cu 100 1

248 269 273 277

cu 100 1

259 264 272 277 280

Cu(1001

263 271 280 284

Pt(ll1)

262 275 280

( r e f . 18)

Intensity

m m m

s W

s s m S

s s

s W

m s

s W

m W

s s

s 5

s s W

m s W

More l i k e l y i s Avery's suggestion from evidence on C u ( l l 0 ) t h a t t r i m e r i z a t i o n t o benzene i s the main surface process a t increased temperatures ( r e f . 5). In fact,

if we consider Avery's TDS r e s u l t s f o r acetylene adsorbed on Cu(100)

( r e f . 61, we f i n d good agreement w i t h the Auger spectra presented here, i n t h a t Avery r e p o r t s t h a t benzene formation occurs as low as 180 K and t h a t a l l surface species desorb by 360 K t o leave a clean surface.

I n order t o i n v e s t i g a t e t h i s

f u r t h e r , we adsorbed 3 L o f benzene on t o the clean copper surface a t 120 K. Benzene m u l t i l a y e r s are expected t o form a t t h i s l a w temperature and so the c r y s t a l was heated a t 170 K f o r ten minutes, followed by r e c o o l i n g t o 120 K t o record the spectrum shown i n F i g u r e 3 ( i i ) .

The high signal i n t e n s i t y and low

energy of the main features, however, suggest t h a t m u l t i l a y e r s are s t i l l present

254

and t h i s was confirmed by f l a s h i n g t h e c r y s t a l monolayer spectrum shown i n F i g u r e 3 ( i i i ) .

t o 200

K, which y i e l d e d t h e

The main f e a t u r e s i n t h i s benzene

f i n g e r p r i n t occur a t 272 and 277 eV, with minor peaks a t 259,

J

.

1

.

I

.

I

t

.

l

-

L

240 250 260 270 280 290 300

240 250 260 270 280 290 300

KEIeV

KEieV

(iii) J

.

(iv) I

.

I

.

L

.

I

L . .

1

.

I

.

>

L

200

230 240 250 260 270 280 290 300

KEIeV

KEhV

Fig. 3

264 and 280 eV.

Carbon Auger spectra o f on Cu(100) a t 120 K ( i ) C,H, (iii) C,H, on Cu(100) a t 200 K

( i i ) C6H6

( i v ) C,H6

300

on Cu(lO0) a t 170 on Pt(ll1) a t 300

K K ( r e f . 18)

255

Again, there i s l i t t l e s i m i l a r i t y between t h i s and the spectrum o f the acetylene thermal r e a c t i o n product.

Further i n v e s t i g a t i o n showed t h a t the molecularly

adsorbed benzene desorbed completely from the surface below 300 K, whereas t h e new species formed as a r e s u l t o f heating acetylene s t i l l showed 5 0 b o f i t s o r i g i n a l i n t e n s i t y a t t h i s temperature. p o s i t i v e evidence o f t r i m e r i z a t i o n

Hence, t h i s r e s u l t appears t o o f f e r no

o f acetylene t o benzene on Cu1100)

at

increased temperatures. By comparison w i t h work c a r r i e d out by Netzer e t a1 ( r e f .

181, however, we

noted t h a t the main features o f the C KVV Auger spectrum o f benzene adsorbed on P t ( l l 1 ) (Fig. 3 ( i v ) ) coincide w i t h the two main peaks i n t h a t o f the new species formed on Cu(lOO), as shown i n Table 1. EELS r e s u l t s o f Lehwald e t a1 ( r e f . 20) on P t ( l l 1 ) suggest a l a r g e degree o f p e r t u r b a t i o n o f the benzene molecule on adsorption, whereas EELS spectra from adsorption o f benzene on the (ill), (110) and (100) faces o f copper ( r e f . 21) i n d i c a t e much weaker p e r t u r b a t i o n o f the benzene adsorbate, as the v i b r a t i o n frequencies d i f f e r l i t t l e from the gas phase values. i t may be t h a t when benzene i s molecularly adsorbed on Cu(lOO),

Hence,

molecule i s weakly x-bonded w i t h only minor changes t o i t s geometry.

the

However,

i n the process o f i t s formation from the heating o f the acetylene i t i s able t o overcome the

activation

barrier

necessary

to

take up

the

more perturbed

c o n f i g u r a t i o n normally found on Group VIII metals, f o r which spectroscopic and LEED data show t h a t the C-C bond length becomes s u b s t a n t i a l l y closer t o t h a t o f a s i n g l e bond, w i t h several r e p o r t s suggesting regular bond length a l t e r n a t i o n around the r i n g ( r e f . 22). An a l t e r n a t i v e p o s s i b i l i t y i s t h a t the species t h a t we have formed on the Cu(100)

surface i s an intermediate i n the benzene formation reaction,

the

existence o f such a species on P d ( l l 1 ) having been studied by Patterson ( r e f .

23).

I n h i s study,

i s o t o p i c a l l y l a b e l l e d acetylene and a molecule which

behaved as an intermediate i n the r e a c t i o n were employed i n multi-mass thermal desorption t o i n d i c a t e t h a t the mechanism o f the benzene formation involves a CH, ,

intermediate.

CONCLUSION High r e s o l u t i o n Auger e l e c t r o n spectroscopy has been employed t o record the C KVV spectra o f acetylene on Cu(100) a t various temperatures between 120 and 350 K , along w i t h f i n g e r p r i n t spectra o f ethylene and benzene.

Acetylene adsorbs molecularly a t 120 K, but on heating t o 170 K a new species possibly strongly perturbed benzene, i s formed, which desorbs between 300 and 350 K t o leave a clean surface.

256

ACKNOWLEDGEMENT We are g r a t e f u l t o SERC f o r an equipment g r a n t and a research s t u d e n t s h i p (DRL)

.

REFERENCES 1. N. Avery, J. Amer. Chem. SOC. 107 (1985) 6711. 2. T.S. Marinova, P.V. Stefanov, Surface Sci. 191 (1987) 66. 3. J.E. Demuth, I B M J. Res. Develop. 22 (1978) 265. 4. K.Y. Yu, W.E. Spicer, I. Lindau, P. Pianetta, S.F. L i n , Surface Sci. 47

(1976) 157. 5.

D.A.

Outka, C.M.

Friend, S. Jorgensen, R. Madix. J. Amer. Chem. SOC.

105 (1983) 3468. 6.

18.

N. Avery, Personal Communication. C. Liegener, Chem. Phys. 92 (1985) 97. M.D. Baker, N.D.S. Canning, M.A. Chesters, Surface S c i . 111 (1981) 452. H.P. Bonzel, H.J. Krebs, Surface Sci. 91 (1980) 499. N.D.S. Canning, M.D. Baker, M.A. Chesters, Surface Sci. 111 (1981) 441. N.D.S. Canning, M.A. Chesters, J. Mol. Struc. 79 (1982) 191. M.A.Chesters, B.J. Hopkins, R . I . Winton, Surface Sci. 59 (1976) 46. R. Ducros, G. Piquard, B. Weber, A. Cassuto, Surface Sci. 54 (1976) 513. S.D. F o u l i a s , K.J. Rawlings, B.J. Hopkins, Surface Sci. 133 (1983) 377. M.P. Hooker, J.T. Grant, Surface S c i . 55 (1976) 741. M.P. Hooker, J.T. Grant, Surface Sci. 62 (1977) 21. P.V. Kamath, K . Prabhakaran, C.N.P. Rao, Ind. J. Phys. 60B (1986) 84. F.P. Netzer, J.A.D. Matthew, J. E l e c t r o n Spectrosc. & Related Phenom. 16

19. 20. 21. 22. 23.

N.D.S. Canning, Ph.D. t h e s i s . S. Lehwald, H. Ibach, J.E. Demuth, Surface Sci. 78 (1978), 577. N. Sheppard, Ann. Rev. Phys. Chem. 39 (1988) 589. D.F. Ogletree, M.A. Van Hove, G.A. Somorjai, Surface Sci. 183 (1987) 1. C.H. Patterson, Ph.D. t h e s i s .

7. 8. 9. 10.

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

(1979) 359.

C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfuces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

251

CORRELATION OF TRANSMISSION FTIR SPECTRA OF CO AOSORBED ON Pt/SiO, AT HIGH PRESSURE AND FT-RAIRS OF CO ADSORBED ON A POLYCRYSTALLINE FOIL I N UHV

M.A. CHESTERS and D. COOMBS School o f Chemical Sciences, U n i v e r s i t y o f East Anglia, NORWICH, NR4 7TJ, U.K.

S.F. PARKER Spectroscopy Branch, B r i t i s h Petroleum Research Centre, Chertsey Road, SUNBURY ON THAMES, Middlesex, TW16 7LN, U.K.

ABSTRACT A c e l l designed f o r t h e study o f c a t a l y t i c systems a t h i g h temperature and pressure has been used t o study the adsorption o f CO on Pt(6%)/SiO In a d d i t i o n t o t h e bands due t o l i n e a r and b r i d g i n g CO a weak band assigned2 t o t h e combination ( v ~ + vpt-co) i s a l s o observed. This band i s n o t seen on a r e c r y s t a l l i s e d poi1 i n r e f l e c t i o n - a b s o r p t i o n i n f r a r e d experiments. We discuss t h i s r e s u l t i n the l i g h t o f r e l a t e d spectra on a P t ( l l 1 ) s i n g l e c r y s t a l recorded by o t h e r workers using t h e EELS technique.

.

INTRODUCTION A p a r t i c u l a r advantage o f i n f r a r e d spectroscopy i s t h a t since i t s use i s n o t

restricted

to

ultra

high

vacuum c o n d i t i o n s

m o n i t o r i n g surface r e a c t i o n s under c a t a l y t i c

i t allows

conditions.

the

possibility

of

I n t h i s respect,

i n f r a r e d v i b r a t i o n a l spectroscopy has been a powerful and widely used t o o l f o r t h e study o f heterogeneous c a t a l y s i s ( r e f . 1). region o f t h e spectrum,

There i s however an important

the 300-600 cm-1 range,

t h a t i s generally neglected.

T h i s region i s important because i t includes the metal-ligand bending fundamentals.

These provide d i r e c t

s t r e t c h i n g and

i n f o r m a t i o n on the

nature and

s t r e n g t h o f the adsorbate-metal i n t e r a c t i o n . Transmission i n f r a r e d experiments using pressed discs o f oxide supported metals u s u a l l y give r i s e t o e s s e n t i a l l y zero transmission i n t h i s region.

In

t h e case o f carbon monoxide adsorbed on platimum the vpt-CO mode has been observed on P t ( l l 1 ) s i n g l e c r y s t a l s ( r e f . 2-4) and i n o t h e r i n f r a r e d experiments (ref.

5-7).

detection o f

The mode has a l s o been observed i n d i r e c t l y on Pt/TiO, a combination mode o f the

vc-o

and vpt-CO fundamentals

by the (ref.

8). We r e p o r t here the

results of

our transmission i n f r a r e d studies

of

CO

adsorbed on a P t ( 6 % ) / S i 0 2 c a t a l y s t a t h i g h pressures and r e f l e c t i o n - a b s o r p t i o n

258

i n f r a r e d spectroscopy vacuum.

(RAIRS) on a r e c r y s t a l l i s e d P t f o i l

under u l t r a h i g h

Our experiments were s e t up t o examine c o r r e l a t i o n s between working

c a t a l y s t c o n d i t i o n s and t h e r a t h e r i d e a l i z e d c o n d i t i o n s o f a s i n g l e c r y s t a l plane and l o w gas pressures. While the main C-0 s t r e t c h i n g fundamental i s seen i n each case (as would be expected)

a weak

band

attributable

to

the

vc-o +

Vpt-CO

combination

is

detected i n t h e spectrum o f the c a t a l y s t b u t n o t on t h e r e c r y s t a l l i z e d f o i l . This r e s u l t i s discussed i n the l i g h t o f r e l a t e d spectra on a P t ( l l 1 ) s i n g l e c r y s t a l recorded by t h e EELS technique ( r e f . 9). EXPERIMENTAL

High pressure transmission i n f r a r e d spectroscopy Transmission i n f r a r e d spectra were recorded from a 26.8 mg pressed d i s c o f P t ( 6 % ) / S i 0 2 (EUROPT-1).

The sample was mounted i n a commercial h i g h pressure The sample c o u l d be heated t o 600 K a t up t o 30 bar

i n f r a r e d c e l l (Accuspec). pressure.

The c e l l was m o d i f i e d i n two respects. The o r i g i n a l s t a i n l e s s s t e e l

sample h o l d e r was replaced by a two p i e c e holder c o n s i s t i n g o f a brass body and copper t i p . The i n t e r n a l c a r t r i d g e heater was mounted as c l o s e t o the sample as possible.

These m o d i f i c a t i o n s g r e a t l y reduced t h e thermal g r a d i e n t between the

heater and sample.

The i n t e r n a l path l e n g t h o f 42 mn was reduced t o 20 inn by

the use o f stepped windows. spectrometer.

The

spectra

Spectra were recorded w i t h a N i c o l e t MX1 FTIR presented

are

ratioed

against

a

clean

surface

background a t the same temperature u s i n g 400 scans a t a s p e c t r a l r e s o l u t i o n o f 4 cm- 1. F o u r i e r Transform R e f l e c t i o n Absorption I n f r a r e d Spectroscopy FT-RAIRS experiments were c a r r i e d o u t u s i n g a small p o r t a b l e uhv system and a glass

cell

i n c o r p o r a t i n g K B r windows

fixed with

a

silicone

varnish.

A

p o l y c r y s t a l l i n e p l a t i n u m f o i l was cleaned i n uhv by c y c l e s o f h e a t i n g i n 1 x t o r r o f 0,

f o l l o w e d by h e a t i n g t o 1500 K t o remove the oxide.

procedure r e s u l t s i n r e c r y s t a l l i s a t i o n o f the p o l y c r y s t a l l i n e f o i l predominantly (111) f a c e t s ( r e f .

10).

i n t o the sample w e l l o f the spectrometer.

The glass c e l l was i n s e r t e d d i r e c t l y The spectrometer was a commercial

F o u r i e r t r a n s f o r m i n s t r u m e n t (Mattson S i r i u s 100). is,

This

t o expose

a f t e r passing through the i n t e r f e r o m e t e r ,

L i g h t from a Globar Source

focused onto the sample a t a

grazing angle o f incidence ( 8 P + 3 0 ) . The r e f l e c t e d beam i s then focused onto

d

l i q u i d n i t r o g e n cooled InSb d e t e c t o r through a p o l a r i s e r t o remove the unwanted s - p o l a r i s e d component. co-added.

1500 scans a t a s p e c t r a l r e s o l u t i o n o f 4 cm-l were

259

RESULTS M D DISCUSSION The spectrum o f the Pt(6%)/Si02 c a t a l y s t i n the CO s t r e t c h i n g region i s shown i n Figure 1.

Apart from bands due t o gas phase CO a t 2120 and 2174 cm-1, there

are three bands apparent a t 2064,

1810 and 1625 c m l .

These are assigned t o

terminal COY b r i d g i n g CO and the ~ O - H mode o f a small amount o f adsorbed water t h a t was introduced from the gas handling system.

2250

2050

1850

1650

2600

WAVE NUMBER

2520

2440

WAVENUMBER

Fig. 1 I n f r a r e d spectrum o f Pt(6%)/ S i O q i n the C-0 s t r e t c h i n g region a f t e r reduction and absorption o f CO.

Fig. 2 Spectrum o f Pt(6%)/Si02 i n t h e combination band region a t room temperature.

I n a d d i t i o n t o the C-0 s t r e t c h i n g mode, a band a t 2485 cm-1 i s a l s o seen, Figure 2. the

linear

This i s assigned t o the combination band (vc-0 and vpt-CO) o f species

and allows

frequency as 432 cm-'.

the

determination

of

the

(linear)

vpt-CO

The frequency i s somewhat higher than seen f o r Pt/TiO,

( r e f . 8) b u t i n good agreement w i t h d i r e c t measurements ( r e f . 2,

5-7).

The

behaviour of the terminal CO band i s shown more c l e a r l y a f t e r removal o f the gas phase COY Figure 3. The behaviour o f the band a t 2485 cm-l, Figure 4, on thermal desorption o f CO p a r a l l e l s t h a t o f the l i n e a r band supporting i t s assignment t o the combinat ion band

.

The spectrum o f CO adsorbed t o s a t u r a t i o n on the P t f o i l a t 85 K i s shown i n Figure 5. The p o s i t i o n o f the terminal band i s a t somewhat higher frequency and does n o t e x h i b i t the asymnetry seen i n the case o f the supported c a t a l y s t . T h i s undoubtedly r e f l e c t s the more homogeneous nature o f the r e c r y s t a l l i s e d P t foil.

I t was n o t possible t o detect the bridged CO band as t h i s occurs close t o

w

Y

C 5A

0.25A

m

a

b

t

I

1

2200

1800

2000

2600

1600

WAVENUMBER

F i g . 3.

2520 2440 WAVENUMBER

F i g . 4 Spectrum o f P t ( 6 % ) / S i 0 2 i n t h e ( ~ c - 0+ Vpt-CO) r e g i o n d u r i n g desorption o f CO ( a ) 328 K, ( b ) 353 K, ( c ) 383 K, ( d ) 479 K.

Spectrum o f P t ( 6 % ) / S i 0 2 i n the ~ c - 0r e g i o n d u r i n g desorption o f CO a t ( a ) 328 K, ( b ) 353 K, ( c ) 383 K, ( d l 479 K.

o r below the c u t o f f o f the InSb d e t e c t o r used. The major d i f f e r e n c e between the two systems was t h a t i n s p i t e o f c a r e f u l and repeated study, we were unable to d e t e c t the combination band on the P t f o i l . Consideration o f Figures 3 and 4 shows t h a t the h a l f w i d t h s o f the t e r m i n a l and combination bands are s i m i l a r (-60 cm-l) b u t t h a t the amplitudes vary by almost a f a c t o r of 200.

Under optimum c o n d i t i o n s t h e maximum amplitude o f t h e l i n e a r

CO band on the f o i l was 2%.

Assuming t h a t the r e l a t i v e i n t e n s i t i e s are s i m i l a r

on the f o i l and the supported c a t a l y s t t h i s would p r e d i c t a maximum i n t e n s i t y o f the combination band o f 0.01% i n the R A I R spectrum the best noise l e v e l obtained was 0.004%, peak t o peak, thus f a i l u r e t o observe t h e band i s s u r p r i s i n g . EELS s t u d i e s o f CO adsorbed on P t ( l l 1 ) a t 92

a peak assigned t o e i t h e r a combination or a m u l t i p l e l o s s vpt-CO + 2550 cm-'. fundamental,

The almost

relative intensity an o r d e r

of

In

K, Lehwald e t a1 ( r e f . 9) observed

of

magnitude

this

band was

larger

than

-3%

the

of

Vc-o the

at

Vco

0.5% r e l a t i v e

i n t e n s i t y of the combination band observed i n the i.r. spectrum o f CO on the Pt/SiO,

sample.

A band o f s i m i l a r r e l a t i v e i n t e n s i t y a t 2550 cm-'

would be

observed i n the R A I R S experiment i f the o r i g i n i s a combination e x c i t a t i o n and would t h e r e f o r e have a peak amplitude AR/R limit.

- 0.06%,

w e l l above

our d e t e c t i o n

T h i s discrepancy between the EELS and RAIRS r e s u l t s may be e x p l a i n e d i f

t h e mechanism o f e x c i t a t i o n o f t h e EELS peak a t 2550 cm-1 i s by n u l t i p l e loss

261

I

I

2900

2800

2700

r

2140

2600 2500

2060

1980

1

1900

WAVEN U M BE R

FT-RAIR spectrum o f CO adsorbed to saturation on the P t f o i l a t 85 K. (a) combination band region (b) C-0 s t r e t c h i n g region

Fig. 5.

i.e.

sequential e x c i t a t i o n o f the two fundamentals.

Such a mechanism would n o t

be operative i n the i n f r a r e d absorption experiment.

Unfortunately t h i s means

t h a t the EELS experiment can provide us w i t h no information on the r e l a t i v e i n t e n s i t y o f the combination loss. CONCLUSIONS We r e p o r t a weak band i n the i.r. spectrum o f CO adsorbed on a Pt/SiO, catalyst

which

carbon-oxygen

is

assigned

stretches.

to

a

combination

of

the

metal-carbon

and

We have f a i l e d t o detect the equivalent absorption

band i n the spectrum o f CO adsorbed on a P t f o i l measured by RAIRS. This f a i l u r e may be explained i f the combination band were somewhat broader than t h a t o f the

vco

fundamental b u t a recent R A I R spectrum f o r CO on P t ( l 1 1 ) shows the

low frequency band to be as narrow as t h a t o f the C-0 s t r e t c h ( r e f . 4 ) .

An

a l t e r n a t i v e explanation i s t h a t anharmonic coupling o f the two fundamentals i s weaker f o r CO adsorbed on the s i n g l e c r y s t a l plane compared t o CO adsorbed on small c r y s t a l l i t e s .

An apparently s i m i l a r band a t 2550 cm-I reported i n the EEL

spectrum o f CO on P t ( l l 1 ) with a considerable r e l a t i v e i n t e n s i t y ( r e f . 9) i s t h e r e f o r e shown t o a r i s e from sequential e x c i t a t i o n o f the fundamentals r a t h e r than e x c i t a t i o n o f a combination. ACKNOWLEDGEMENTS An equipment grant from the SERC and a SERC CASE studentship (DC) B r i t i s h Petroleum are g r a t e f u l l y acknowledged. given by the B r i t i s h Petroleum Company PLC.

with

Permission t o p u b l i s h has been

262

REFERENCES

1. 3 . Mink, Mikrochim. Acta, 98 (1988) 123. 2. H. Froitzheim, H. Hopper, H. Ibach and S. Lehwald, Appl. Phys. 13 (1979) 147. 3. R. G. Tobin and P.L. Richards, Surf. Sci. 179 (1987) 387. 4. D. Hogen, M. Tflshaus, E. Schweizer and A.M. Bradshaw, Chem. Phys. L e t t . i n press.

5. R.P. Eischens and W.A. P l i s k i n , Advan. Catal. 10 (1958) 1. 6. C.W. Garland, R.C. Lord and P.F. Troiano, J. Phys. Chem. 69 (1965) 1188. 7. M. Primet, P. F o u i l l o u x and B. I m e l i k , J. C a t a l . 61 (1980) 553. 8. J.C. Robbins and E. Marucchi-Soos, J. Phys. Chem. 91 (1987) 2026. 9. S. Lehwald, H. Ibach and H. S t e i n i n g e r , S u r f . Sci. 117, 1982, 342. 10. R.A. Shigeischi, D.A. King, Surf. Sci. 58, (19761, 379.


263

THE ADSORPTION OF CARBON TETRACHLORIDE ON Ni(l10)

M.A. Chesters and D. Lennon School of Chemical Sciences, University of East Anglia, NORWICH NR4 7TJ, England.

ABSTRACT The adsorption ot carbon tetrachloride on Ni(l10) at 300 K was investigated using vibrational electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES). Adsorption was found to be dissociative with the chlorine atoms probably adsorbed in five coordinate sites in the 'channels' of the (1 10) surface. Heating the adsorbed layer above700 K resulted in dissolution of carbon but the chloride layer remained intact until above 1150 K, when it desorbed, probably as NiCl (ref. 1). ?he intensity of the nickel-chlorine stretching loss peak in the EELS spectrum was analysed to provide an estimate of the dynamic dipole moment, 'y, associated with this mode. INTRODUCTION Adsorbed layers of chlorine on metal single crystals have been little studied with vibrational spectroscopic techniques, despite the interest in this surface species in its role as both a promoter and a poison of catalytic processes. This particular study was prompted by an interest in the reaction between carbon tetrachloride and nickel oxide the investigation of which is the subject of another publication. Here we describe the dissociative adsorption of carbon tetrachloride on the clean Ni(ll0) surface and report the vibrational spectrum of adsorbed chlorine atoms. The results are correlated with those of Erley (ref. 1) on the same surface system studied by thermal desorption, work function change, AES and LEED measurements. EXPERIMENTAL The EEL spectra were recorded using a Leybold Heraeus ELS 22 spectrometer mounted in a stainless steel vacuum chamber equipped with a 3 grid LEED optics which was used to monitor surface cleanness with Auger electron spectroscopy. The Ni(ll0) crystal was cleaned by repeated samples of Ar+ bombardment and annealing at 1000 K. The crystal temperature was measured by a chromel-alumel thermocouple. The minimum background pressure was 6 x lo-" mbar and gas purity was monitored with a quadrupole mass spectrometer. The CCI, (99+%, Aldrich Chemical Company) was purified by several cycles of freeze-pump-thawing followed

264

331 x 1000

~

50 L

’I 2009

339

468

1872

Clean

Fig. 1 Electron energy loss spectra as a function of CCI, exposure (Langrnuirs)

265

by vacuum distillation before being introduced to the diffusion pumped gas manifold. Relative exposures were calculated from uncorrected ion gauge readings. The spectrometer was operated at 7-9 meV resolution (56-65 cm-') at 5.0 eV beam energy giving an elastic count rate from a clean crystal of -1 x lo5 ds. The electron beam was reflected at 60"from the surface normal. RESULTS AND DISCUSSION The EELS spectrum of the clean Ni(l10) surface is shown in Fig. la. The band at 200 cm-' has been assigned to a nickel phonon (ref. 2) while the remaining bands in the spectrum may be attributed to a small surface coverage of carbon monoxide contamination. The two peaks at near 1900 and 2000 cm-' are assigned to the carbon-oxygen stretching modes of bridged and linear species respectively and the peak at 460 cm-' is assigned to the metal-carbon stretch of the linear species, that of the bridged species is not observed. Exposure to carbon tetrachloride at 300 K resulted in the series of spectra shown in Fig. 1 b-f. Adsorption is accompanied by the growth of a single band at -340 cm" which remains at constant loss energy (+I5cm-l). A low energy shoulder appears at exposures above 20L. Molecular carbon tetrachloride, if physically adsorbed would give a loss peak near 780 cm-l corresponding to the i.r. active T, stretching mode while species of the form CCI, would be expected to produce C-CI stretching modes with energies in a similar range. The observed loss peak is in a position consistent with assignment to a nickel-chlorine stretching mode and is close to that for a

I

100

500

700

KE/eV

Fig. 2. Auger spectrum of Ni(ll0) + 50 Langmuirs CCI,.

900

266

terminal nickel chlorine stretch (ref. 3). We therefore conclude that adsorption is completely dissociative with the resulting adsorbed carbon atoms providing no discernable features in the electron energy loss spectrum. The Auger spectrum resulting from saturation of the clean nickel surface with carbon tetrachloride is shown in Fig. 2. The much greater intensity of the chlorine LMM peak results from the high electron impact ionization cross section of the chlorine L shell. After taking account of the relative sensitivity factors for carbon and chlorine (ref. 4) the peak intensity ratio in the spectrum is consistent with a carbon to chlorine ratio of 1:4. In general bridging M-CI stretching frequencies are lower than terminal M-CI stretching frequencies. The metal chlorine stretch observed is indicative of terminal CI (ref. 3) but structural information for other adsorbed halogens (refs 5,6) indicates that chlorine is likely to adsorb in a high coordination site. The Ni-CI bond length measured for a range of nickel complexes is in the range 2.3 - 2.5A (ref. 7) which is similar to the nickel metallic diameter (2.48A). Therefore the chlorine is able to sit in the rectangular hollow of the (110) surface and will have a stretching frequency arising from coordination to four Ni atoms in the surface plane and one atom in the second layer. Bonding to the fifth nickel atom directly beneath the chlorine atom is likely to 'stiffen' the Ni-CI stretch. A LEED study of chlorine adsorption on Ni(ll0) (ref. 1) showed the initial formation of a (72x2) overlayer at 8 = 0.5. Higher exposure resulted in a p(10 x 1) pattern for which the surface coverage was estimated as 8 = 0.7. The high coverage structure was believed to result from compression of the rows of chlorine atoms, originally adsorbed in equivalent sites along the 'troughs' in the (110) surface. It was not possible to identify the original adsorption sites but we suggest they are 5-fold coordinate positions described above. It is significant that we detect a low frequency shoulder appearing on the Ni-CI stretching band at exposures above 20 L (Fig. l e and 11). A plot of the normalised intensity of the Ni-CI stretching loss peak versus exposure (Fig. 3) shows that this shoulder appears at a coverage of -8 = 0.5, assuming Erley's value of saturation coverage of 8 = 0.7. This analysis assumes that the intensity of the EELS band is proportional to coverage and that the presence of carbon atoms in our experiment does not affect the chlorine adlayer; both assumptions are justified below. We therefore associate the appearance of this shoulder in the EELS spectrum with the displacement of CI atoms along the (110) direction into less favourable bonding sites. On increasing the substrate temperature in stages and re-cooling to 300 K for spectroscopic measurements we found no change in the Ni-CI stretching vibration for temperatures up to 1000 K. Auger analysis showed that the carbon signal was removed on heating to 700 K which is consistent with dissolution of carbon into the nickel crystal (ref. 8). The loss of carbon has no discernable effect on the EELS spectrum and so the carbon and chlorine atoms appear to adsorb quite

267

independently. Heating to 1000 K did result in a reduction in intensity of both the EELS and AES peaks due to chlorine and the adlayer was completely removed on heating to 1150 K. This is in agreement with the results of Erley (ref. 1).

I

1

5

10

15 20 25 30 35 40 CC14 EXPOSURE (LANGMUIRS)

45

50

Fig. 3 Plot of normalised intensity of 340 cm-' EELS peak versus CCI, exposure. Off specular measurements of EELS spectra showed the nickel-chlorine stretch to be excited by dipole scattering. The relatively large loss peak in the spectrum of the chlorine saturated surface superficially suggest that a large dynamic dipole is associated with this mode. However the insensitivity of the measured loss energy to increases in chlorine coverage up to a close-packed layer indicates that there is little vibrational coupling, for instance due to dipole-dipole interactions. This apparent anomaly is explained by a calculation of the dynamic dipole moment associated with the Ni-CI stretch from the EELS relative intensity. Using the equation given by Newn's (ref. 9), the dynamic dipole moment, y, was calculated from the EELS relative intensity and found to be 0.1D which is less than half the typical value found for the C-0 stretch of adsorbed carbon monoxide. Since the intensity of the EELS bands and the extent of dipole-dipole coupling scale as f it is not surprising that the nickel-chlorine stretching frequency shows no strong coverage dependence. The high intensity of the nickel-chlorine stretching loss peak arises from an instrumental effect in EELS which accentuate the peaks at low loss energy. This point has been discussed by lbach and Mills (ref. 10) and Chesters and Sheppard (ref. 11).

CONCLUSIONS Carbon tetrachloride has been shown to adsorb dissociatively on Ni(ll0) at 300 K. The nickel-chlorinestretching frequency of the adsorbed chlorine is 340 cm". We suggest that the chlorine adsorbs in a five-foldcoordinate site where it is bonded to four nickel atoms in the first layer and one nickel atom directly underneath in the second layer. We are not able to locate the adsorbed carbon atoms but our results indicate little interaction between the carbon and chlorine atoms. The relatively small dynamic dipole moment associated with the adsorbed chlorine indicates that the nickel-chlorine bond has little ionic character, which is typical of halogens chemisorbed on transition metal surfaces. The changes in surface structure at high coverage reported by others (ref.l) do not have a very marked effect on the EELS spectrum. It would therefore probably be of interest to repeat these experiments with a higher resolution vibrational spectroscopic technique such as reflection-absorptioninfrared spectroscopy. ACKNOWLEDGEMENTS We are grateful to the Science and Engineering fleseach Council for equipment grants and for a reseach studenship (D. Lennon). REFERENCES 1. W. Erley. Surface Sci. 114 1982) 47. 2. J.A. Stroscio, M. Person, .R. Bare, W. Ho, Phys. Rev. Letts. 54 (1985) 1428. 3. (a) J.R. Ferraro, Low freque vibrations of inorganic and coordinated compounds. Plenum, New Yo ,(1971) pp. 169-177. (b) I

1 Pt-Ru 68-32

Pt-Ru 44-56

99.6% 1.8%

T

CM=62.9

-3%, 98.1% BSA=4.9

I

-

0.1%

%CM=66.2

I I

0 91.8%

‘%BSA=8.2

I

68-32

-

0.5%

97%

44-56

I

5.5%

I I

96.4%

---------+---------1.4% 2.3% I 8.2% I 9.3% 90.4%

1

88.4%

7.3%

I

-2.9%

63.5%

I

66.1%

I Z=29.1 %CM=2Z

I I

~~~~~~

370

i s o t o p i c abundances d e t e r m i n a t i o n o f t h e p u r e labeled i s o m e r s ; t h e values

obtained

being

very

close

together

[12].

About

the

i s o r n e r i z a t i o n mechanisms w e c a n see t h a t t h e r e l a t i v e c o n t r i b u t i o n s of t h e c y c l i c mechanisms a r e : - F o r t h e r e a c t i o n 2MP-2-C13 3 3MP-3-C13

to 6 2 . 9 % and 6 6 . 2 % f o r t w o Pt-Ru - F o r t h e r e a c t i o n 2MP-4-CI3 7 3 . 6 % f o r Pt-Ru

t h e percentages a r e equal

catalysts.

t h e p e r c e n t a g e s are e q u a l t o 58.2% and

68-32 a n d 44-56 r e s p e c t i v e l y .

From t h e s e r e s u l t s w e c a n d e t e r m i n e t h e t o t a l r e a c t i o n b a l a n c e which i s a t 220°C

:

-

F o r Pt-Ru

68-32

: 2 . 6 % CM - 1 . 4 % B S - 96% c r a c k i n g

F o r Pt-Ru

44-56

: 2 . 5 % CM - 1%BS

-

96.5% c r a c k i n g

a n d on p u r e p l a t i n u m w e c a n d e t e r m i n e t h e r e a c t i o n b a l a n c e u s i n g

o n l y t h e amount o f m e t h y l c y c l o p e n t a n e f o r m e d a n d t h e s e l e c t i v i t y r a t i o r ( T a b l e 3 ) . W e o b t a i n : 1 8 % CM

-

56% BS

-

26% c r a c k i n g .

DISCUSSION T h e s e r e s u l t s l e a d t o f o c u s on t h r e e p o i n t s :

(i) I n t h e i s o r n e r i z a t i o n o f

2-methylpentane

t o 3-methylpentane,

t h e p e r c e n t a g e of c y c l i c mechanism o b t a i n e d a t 22OoC i s v e r y h i g h : (65+2) % ( i i ) The c r a c k i n g r e a c t i o n s o c c u r i n g on t h e s e Pt-Ru

v e r y i m p o r t a n t l a r o u n d 958, b u t

c a t a l y s t s are

e x t e n s i v e c r a c k i n g r e a c t i o n s do not p r o d u c t s cracked i s f o u n d where

occur. A high s e l e c t i v i t y i n t h e branched hydrocarbons a r e favored.

( i i i ) A p a r a l l e l i s m c o u l d be e s t a b l i s h e d b e t w e e n t h e l a r g e amount

o f p r o d u c t s s e l e c t i v e l y c r a c k e d a n d t h e s e l e c t i v e c y c l i c mechanism f o r which t h e s e l e c t i v i t y r a t i o , r = 3MP/n-H, i s e q u a l t o 3 . 5 4 0 . 3 . On t h e o t h e r h a n d EXAFS m e a s u r e m e n t s h a v e shown t h a t t h e s e catalysts

are w e l l

reduced

at

4OO0C

and t h e y

d i f f e r e n t k i n d s of m e t a l l i c p a r t i c l e s

are present

: f.c.c.

in

four

Pt particles,

Ru

p a r t i c l e s a n d P t r i c h o r Ru r i c h b i m e t a l l i c o r a l l o y a g r e g a t e s i n agreement w i t h T . P . R .

a n d h y d r o g e n T.P.D. s t u d i e s .

A l l t h e s e c a t a l y t i c r e s u l t s c a n be i n t e r p r e t e d b y t h e p r e s e n c e

of

s i t e s w h i c h a r e v e r y a c t i v e f o r t h e c y c l i c mechanism a n d h a v e

t h e " s e l e c t i v e c h a r a c t e r " o f t h e ruthenium f o r t h e carbon-carbon bond

rupture

and rearrangement

leading t o t h e

selective cyclic

mechanism. W e s u g g e s t t h a t t h e s e s i t e s a r e created by b o t h P t and Ru atoms. C o n c e r n i n g t h e h i g h c o n t r i b u t i o n of t h e c y c l i c mechanism i t c a n n o t be e x p l a i n e d b y a s i m p l e d i l u t i o n of t h e p l a t i n u m by t h e

371

ruthenium

.

On p u r e p l a t i n u m c a t a l y s t , a t low t e m p e r a t u r e (220OC)

t h e c y c l i c mechanism i s n o t e x p e c t e d which i s o p e r a t i v e

only a t

26O0CIand i t i s f a v o r e d o n l y on small m e t a l l i c p l a t i n u m p a r t i c l e s . I t i s a l r e a d y known t h a t two t y p e s o f c y c l i c mechanisms e x i s t on

p l a t i n u m c a t a l y s t s [ l o ] : o n e where o n l y s e c o n d a r y c a r b o n a t o m s p a r t i c i p a t e t o t h e r e a c t i o n which i s t h e s e l e c t i v e c y c l i c mechanism a n d one where s e c o n d a r y and t e r t i a r y c a r b o n atoms a r e i n v o l v e d which i s t h e non s e l e c t i v e c y c l i c mechanism t h e l a t t e r one b e i n g f a v o r e d on s m a l l p a r t i c l e s . F o r t h e r e a c t i o n 2MP+3MPlat 22OoC, t h e r e l a t i v e c o n t r i b u t i o n of t h e s e l e c t i v e c y c l i c mechanism i s , on P t Ru c a t a l y s t s ,

e q u a l t o 6 5 % . To o b t a i n t h e same v a l u e on p l a t i n u m

c a t a l y s t w e have t o work a t 260°C on a c a t a l y s t which h a s 9 7 % of m e t a l l i c p a r t i c l e s l o w e r t h a n 2nm [ 1 0 , 1 3 ] , b u t on t h i s p l a t i n u m c a t a l y s t t h e c y c l i c mechanism i s non s e 1 e c t i v e ; r = 3MP/nH = 0 . 4 f o r t h e methylcyclopentane h y d r o g e n o l y s i s . On t h e o t h e r hand, on these Pt-Ru c a t a l y s t s t h e r e a r e a b o u t 63+9% o f t h e p a r t i c l e s s m a l l e r t h a n 2 nm which i s l e s s t h a n on p u r e p l a t i n u m and t h e r e a c t i o n t e m p e r a t u r e i s 22OoC i n s t e a d of 254OC on Pt

c a t a l y s t s . I n o r d e r t o e x p l a i n t h i s u n e x p e c t e d b e h a v i o u r under

t h e s e c o n d i t i o n s w e have t o invoke t h e p r e s e n c e of " a c t i v a t e d Pt-Ru

c l u s t e r s " which are v e r y a c t i v e f o r t h e c y c l i c mechanism t o e x p l a i n the

high

value

for

this

selective

c o o p e r a t i v e e f f e c t between P t i n t e r a c t i o n s and n o t t o a merely s t r o n g metal-metal i n t e r a c t i o n w e i s o b s e r v e d f o r t h e r e a c t i o n of mechanism tertiary

involves and

a

the

carbon-carbon

secondary

cyclic

mechanism.

This

a n d Ru i s due t o m e t a l - m e t a l geometric effect. To confirm t h i s may n o t i c e t h a t t h e same tendency c h a i n l e n g t h e n i n g : 2MPjnH. T h i s

carbon

bond

atom.

We

rupture obtained

between on

a

Pt-Ru

c a t a l y s t s i n t h e r a n g e of 58 t o 7 4 % f o r t h e r e l a t i v e c o n t r i b u t i o n o f t h i s mechanism a t 22OoCl i n s t e a d o f 1 0 0 % e x p e c t e d f o r a p l a t i n u m c a t a l y s t working a t 254OC and where 6 3 % o f c y c l i c mechanism i s found p r e v i o u s l y f o r t h e 2MP+3MP r e a c t i o n [ l o ] . The v a r i o u s r e a c t i o n s can be involving, f o r secondary (CII)

r e s u l t s o b t a i n e d f o r i s o m e r i z a t i o n and c r a c k i n g r a t i o n a l i z e d by t h e f a c t t h a t t h e r e a c t i o n pathway t h e C-C bond r u p t u r e , o n l y p r i m a r y ( C I ) and c a r b o n atoms i s f a v o r e d compared t o t h e pathway

where t e r t i a r y ( C I I I )

carbon atoms a r e i n v o l v e d d u r i n g t h e carbon-

c a r b o n bond s c i s s i o n o r r e a r r a n g e m e n t .

This p a r t i t i o n

i n t o two

pathways c o r r e s p o n d s t o t h e f a c t t h a t i n t h e former case t h e carbon atoms have a t l e a s t two hydrogen atoms b u t i n t h e l a t t e r c a s e o n l y one hydrogen atom i s p r e s e n t . T h i s d i s c r e p a n c y may l e a d t o t h e

372

formation of d i f f e r e n t p r e c u r s o r s p e c i e s on t h e s u r f a c e a s carbene s p e c i e s or 0-alkyl s p e c i e s r e s p e c t i v e l y . For P t - R u systems, carbene s p e c i e s could be r e s p o n s i b l e f o r t h i s s e l e c t i v i t y . These s p e c i e s leading t o a-adsorbed olefin+x-adsorbed alkynes and f i n a l l y , a f t e r t h e s p l i t t i n g of t h e alkyne, t o t h e carbyne s p e c i e s [141; a s i m i l a r mechanism was proposed by Clauss a t a l . [15] f o r t h e study of alkyne s c i s s i o n on a t r i m e t a l l i c framework.

i s not t h e o n l y one t o p r e s e n t t h i s high s e l e c t i v i t y . I n our l a b o r a t o r y we a l s o found t h a t Pt-Co [14,16] P t The system P t - R u

Mo [ 1 7 ] and P t - I r

[18] have t h e same behavior; t h e s e systems do not

carbon bonds where a t e r t i a r y carbon atom i s present.We can

touch

propose t h e following r e a c t i o n scheme :

I

/,-[

Y

H

H is located between the first and the second layer of Pt-Ru atoms.

P t -R ‘

u ] -H

/

From t h e r e s u l t s obtained by X ray absorption w e can a s s o c i a t e the

r e a c t i v e e n t i t y t o a more p r e c i s e a g r e g a t e

formed from t h e

i n t e r p e n e t r a t i o n of each metal i n t h e monometallic phases of other

leading

to

respectively P t

rich

and

Ru

rich

the

bimetallic

phases; a f t e r a treatment a t 4OO0C only t h e l a t t e r phase e x i s t s o n the catalyst

[S].

373

CONCLUSION All these catalytic results can be interpreted by the presence of sites formed of Pt and Ru where a cooperative effect exists bet' ween Pt and Ru which is due to metal-metal interactions and not to a merely geometric effect. These sites are named "activated Pt-Ru clusters" [ 61

.

REFERENCES G.C. Bond and R.R. Rajaran, in M.J. Phillips and 1 M. Ternan (Eds), Proc. gth Int. Cong. Catalysis, 3 (1988) 1130 2 M. Asomoza, G. del Angel, R. Gomez, B. Rajai and R.D. Gonzalez, in M.J. Phillips and M. Ternan (Eds), Proc. 9th Int. Cong. Catalysis, 3 (1988) 1182 3 G.Diaz, F.Garin and G.Maire, J.Cata1. , 82 (1983) 13 4 K. Matusek, I. Bogyay, L. Guczi, G. Diaz, F. Garin G. Maire, C1 Mol. Chem., 1 (1985) 335 P. Esteban, G. Diaz, L. Guczi, F. Garin, P. Bernhardt, 5 J.L. Schmitt and G. Maire, Submitted to J. de Chimie Physique (Part I) 1988- (presented at the 3rd national congress of the "Societe FranCaise de Chimie" Nice 1988 abstr.NOlP.55) P. Esteban, G. Diaz, L. Guczi, F. Garin, P. Bernhardt, 6 J.L. Schmitt and G. Maire, Submitted to J. de Chimie Physique (Part 11) 1988- (presented at the 3rd national congress of the "SociCte FranCaise de Chimie" Nice 1988 abstr .N'lP. 5 5 ) I A. Chambellan, J.M. Dartigues, C. Corolleur and F.G. Gault, Nouv. J. Chimie, l(1976) 41 H.Pines, R.Olbe!rg, V.Ipatieff, J.Amer.Chem.S0~.,70(1948) 537 8 9 C.Corolleur, S.Corolleur and F.G.Gault, J.Cata1, 24(1972) 385 J.M.Dartigues, A . Chambellan, S .Corolleur, F .G.Gault , 10 A.Renouprez, B.Moraweck, P.Bosch-Giral and Dalmai-Imelik, Nouv.J.Chimie, 3(1979) 591 F.G.Gault, Advances in Catalysis (D.D.Eley, P.W.Selwood and 11 P.B.Weiss, Eds.), vol.30,p.l Academic Press, San Diego, 1981. A.O'Cinneide and F.G.Gault, J.Cata1 37 (1975) 311. 12 F. Garin , 0.Zahraa ,C.Crouzet,J.L.Schmitt and G.Maire, Surf. 13 Sci. 106 (1981) 466 14 S. Zyade, F. Garin and G. Maire, New. J. Chem.,ll (1987)429 A.D. Clauss, J.R. Shapley, C.N. Wilker and R. Hoffmann, 15

374

Organometallics 3 (1984) 619

16 17

18

S. Zyade, F. Garin, L. Hilaire, M.F. Ravet and G. Maire, M.F. Ravet and G. Maire, Bul. SOC. Chim. Fr., "(1985)341 C. Petit, These d'Etat UniversitC? de Strasbourg 1987 Unpublished Results.

C. Morterra,A. Zecchinaand G. Costa (Editors),Structure and Reactivity of Surfaces 0 1989Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands

SURFACE

REACTIVITY

UNDER

OXYGEN

ATMOSPHERE

375

OF

053

(C0)iz

SUPPORTED ON SILICA AND ALUMINA.

C. Dossi, A . Fusi, R. Psaro, R . Ugo and R. Zaiionil

Dipartimento di Chimica Inorganica e Metallorganica C.N.R., Via G. Venezian, 21 20133 Milano, Italy 'Dipartimento di Chimica, Universita' "La Sapienza", Moro,5 Roma, Italy

e

Centro

P.le

A.

ABSTRACT The surface reactivity of 053 (C0)lz supported on silica and alumina has been investigated under oxygen atmosphere by means of FTIR and XP spectroscopy and temperature programmed decomposition (TPDE) techniques. Experimental evidence for the formation of molecular [Os(CO)r (0M)zIn (x=2,3 ; M=SiC, Al:) surface species has been obtained. The higher stability of the alumina supported species, resulting from a strong metal-support interaction, is supported by the reactivity under 0 2 , CO and Hz. INTRODUCTION The surface organometallic chemistry originated by the interaction of 0 5 3 (CO)IZ with the surface of different metal oxides has been deeply investigated in the last few years: the first stage of chemisorption has been shown to give rise always to the organometallic species [ O m (p-H)(CO)I o (u-0-oxide)1 (ref. 1). Such surface molecular clusters decompose on heating above 423 K and the resulting surface species show in the y(C0) region three specific infrared bands (ref.2) The real nature and structure of these latter species still remains an open question a t least on silica. We first proposed the complete rupture of the original cluster cage with the formation of mononuclear oxidised Os(I1) surface species such as: [Os(CO), (O-oxide)z]n (x=2,3) (ref.2). This hypothesis has been confirmed by EXAFS measurements on the alumina samples (ref.3). However, these species have been described by other authors as larger aggregates of about 12 atoms (ref.4). Moreover, it has been suggested that on silica the oxidised osmium mononuclear species contain some hydridic hydrogens in addition to the carbonyl

376

ligands (ref.5 )

.

In order to provide a definite characterization of these surface species, the behaviour of 0% ( C O ) I 2 supported on silica alumina surfaces has been studied under oxygen atmosphere. EXPERIMENTAL The SiOi support pretreatment

was

Aerosil

-A1203

Degussa,

used

at 2 9 8 K overnight under high vacuum

B.E.T. surface area was 200 m2 The

200

and

after Pa).

a

Its

g - ' .

support was treated in

0 2

at 673

K

overnight

to

remove surface carbonates and then allowed to cool in vaciio at 298

K. Its B.E.T. surface area was 2 0 0 m2 g-I. 0 5 3 ( C O I I Z was prepared according literature's method (ref.6). The impregnation of the supports W A S performed as previously described (ref.7). All the samples had a loading of 2 % weight of osmium. The preparation of the grafted species HOSJ ( C O ) I O(OM) ( M = Si:, Al

8 8 1

em=

0.1

a,>0.4 (Tilted spaciosl

Figure 4 . Schematic description of the various stages of the CO/NO interaction.

the corresponding NO peak is peak C at = 1715 cm-', whereas the analog CO peak is observed at 2080 cm'l (arrow in Figure 3 ) . The presence of both discrete and continous inductive shifts transmitted by the solid have been reported for CO on ZnO by Tsiganenko et al. (ref. 6) and, very recently, by some of us (ref. 7). A case showing close similarity with the one under study is that of the interaction of hydrogen with CO on ZnO (ref. 81, where the hydrido band initially at 1706 cm" shifts with CO coverage continously, but discrete components are also given rise. From the inorganic literature it is well known that NO is a much better n acceptor than CO. Such a property is readily observed in the present case when both molecules are present at the surface, as NO undergoes the larger static shifts. In Figure

402

5a the static shift of NO due to CO adsorption (data from Figure 3 ) is compared with the analogous static shift due to NO adsorbed on a sample already carrying 10% NO (data from Figure 2b). For brevity, such experiments are indicated as 10% NO/excess NO. As for CO, the static shift in the NO/CO interaction has been calculated by subtracting from the experimental shift (central curve in Figure 5b) the dynamic shift of the 10% CO/excess CO experiment (upper curve in Figure 5b), to take into account of the fact that 10% of the sites are already occupied by NO molecules. Such static shift is compared in Figure 5b with the static shift in the 10% CO/excess CO experiment (data from Figure 2b), and again it is seen that NO is a better n acceptor, in that its presence decreases the extent of the static shift of CO.

r

+20l

1 DYNAMIC S H l n

CO(NO1

f

10

I + 18 c m-' I

-20-

- 30-

Figure 5. Comparison of the static shifts of NO and CO (section a and b respectively) as a function of coverage in the NOJCO interaction and the experiments 10% -/excess AB (see text)

A final observation concerns the width of peak B. Were the situation exactly that illustrated in Figure 4 at CO coverage 0.4, i.e., were a true (2x2) CO structure be formed, one would expect

403

to observe a narrow NO peak, because all NO molecules would be in the same configuration, in sharp contrast with what observed. Schemes in Figure 4 surely describe limiting situations, and the surface phase is much more disordered than the picture proposed in Figure 4. The integrated intensity of the 1800-1700 cm-I region is fairly constant with CO coverage in spite of the large shifts observed. This is a somewhat surprising result. Indeed, the vibrational polarizability of a stretching mode is usually assumed to depend markedly on the frequency: this has been widely documented in the case of CO. No straightforward interpretation of this fact is at hand. REFERENCES

1 2

3

4 5 6 7 8

E. Escalona Platero, S. Coluccia and A. Zecchina, Lang., 1 (1985) 407 E. Escalona Platero, B. Fubini and A. Zecchina, Surface Sci. , 179 (1987) 4044 E. Escalona Platero, S. Coluccia and A. Zecchina, Surface Sci., 171 (1986) 4654 E. Garrone, B. Fubini, E. Escalona Platero and A. Zecchina, to appear in Langmuir G.D. Mahan and A.A. Lucas, J. Chem. Phys., 68 (1978) 1344 A.A. Tsiganenko, L.A. Denisenko, S.M. Zverev and V.N. Filimonov, J. Catal., 94 (1985) 10 D. Scarano, G. Spoto, A. Zecchina and A. Reller, to appear in Surface Sci. F. Boccuzzi, E. Garrone, A. Zecchina, A. Bossi and M. Camia, J. Catal., 51 (1978) 160


C. Morterra, A. Zecchina and G . Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

405

SILANOL AS A MODEL FOR THE FREE HYDROXYL OF AMORPHOUS SILICA: NON-EMPIRICAL CALCULATIONS OF THE VIBRATIONAL FEATURES OF HjSiOH.

E. Garrone and P. Ugliengo Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali. University of Turin, Via P. Giuria 7, 10125 Torino ITALY. ABSTRACT Amorphous silica after thermal treatment at high temperature exhibits isolated OH groups capable of interaction with a variety of molecules. Due to the covalency of the matrix, a cluster approach to the SiOH behaviour is particularly suitable. The simplest cluster model is H3SiOH (silanol). H SiOH does not exist as such as it condenses to siloxane. The suitazility of silanol as a model compound is thus assessed in the present work by comparing calculated values of the torsional barrier, the full set of vibrations in the harmonic approximation, and the anharmonic OH stretch (including overtones) with the corresponding experimental values for silica hydroxyls. All the calculations have been carried out at ab-initio level employing basis sets larger than in the past, and including electron correlation through perturbative technique. The agreement with experiment is good already at the Hartree-Fock level, and it further improves by inclusion of the electronic correlation and anharmonicity effects. INTRODUCTION One of the most studied surface systems ever is probably the isolated hydroxyl at the surface of amorphous silica in interaction with some molecule. A review by Knozinger published in 1976 (ref. 1 ) lists 9 5 molecules, the interaction of which with the silica surface has been followed in the IR: others have been studied since, with other techniques as well. In contrast, there are comparatively few theoretical papers on the subject. KazansKy and coworkers (ref. 2 ) have studied this topic in some detail, but with semiempirical methods only. & initio computations have been carried out by Sauer and coworkers (refs. 3 - 5 ) , ChaKoumakis et al. (ref. 61, and Geerling et al. (ref. 7 ) . In all cases, a cluster approach has been adopted for the

406

surface OH, which is particularly feasible, if account is taken of the covalency of Si02. Kazansky and coworkers have used the pseudo-atom termination approach (ref. 2): all others have terminated the cluster either with H atoms or OH groups, thus considering as the model molecule H3SiOH (silanol) and H4Si04 (silicic acid) respectively (ref. 3). Silanol is evidently simpler than silicic acid, and thus preferable as a model molecule, once its suitability has been assessed. Silanol does not exist as such, as condensation to siloxane occurs (ref. 8). It is thus impossible to carry out any comparison between calculated and experimental geometries to assess the bounty of the model. The geometry of the surface OH is also not known, whereas the IR features have been studied in detail (refs. 9-11). The possibility thus arises of comparing the calculated IR features of H3SiOH with the experimental features of the silica hydroxyl, to assess the suitability of the former as a model for the latter before studying any actual interaction. This study is reported in the present paper. The quantum mechanical study of the interaction of CO with HjSiOH, mimicking the related adsorption on silica has been already reported (ref. 12): the interaction of other molecules is the subject of future work

.

COMPUTATIONAL METHOD All the calculations have been performed at ab-initio level, using GAUSSIAN82, GAMESS and CADPAC suites of programmes. Pople standard 6-31G** and Dunning double zeta (DZ) and triple zeta (TZP) basis sets have been adopted throughout. For geometry optimisation both numerical and analytical gradient of the total energy were adopted. Electronic correlation has been evaluated using perturbative Mvller-Plesset technique, truncated at the second order, called hereafter MP2. Harmonic normal-mode frequencies have been computed through analytical second differentiation of the total energy and solving the classical-mechanical equations of nuclear motion by standard methods. For any computational detail the reader is referred to reference 12. The calculations have been carried out on the NAS9160

407

mainframe of the Turin computing centre, the FPS-164 machine at Daresbury Laboratory, the Rutherford Laboratory CRAY-XMP/4 and the London computing centre CRAY-1S super-computers. CALCULATIONS AND RESULTS Geometry of silanol Full geometry optimization of the silanol keeping Cs symmetry and staggered conformation (which will be shown below to be the most stable one) has been performed adopting both Hartree-Fock SCF with 6-31G** and TZP basis set and MP2 technique with 6-31G** basis set. Figure 1 shows a representation of the optimized structures with the corresponding total energies.

Total --

Enerqy

. 36€.141 919 --66.191 730

-366.441

201

1.644 1.645 1.666

Fig. 1. Geometri.cal features of silanol as computed at three different levels of theory. HF-6-31G**, HF-TZP and MP2/6-31G** data listed from top to bottom. Bond length in A, bond angles in degrees and total energies in hartree. A thorough discussion of the basis set dependance of the geometry of silanol has been reported in a previous paper (ref. 12). Before our work, the best available calculation was that by Sauer (ref. 3), who, adopting a 6-31G* basis set (with a different

exponent of 0.39 for the orbitals on the silicon atom), obtained results very close to our 6-31G** geometry, i.e., somewhat longer Si-0, Si-H and 0-H bonds and a slightly narrower Si-0-H angle. TZP data paraliels the 6-31G** results: slight differences are only seen for the Si-0-H angle and for the 0-H bond length. The calculated geometry of the SiOH moiety in orthosilicic acid (ref. 3) shows no important differences when compared with the present results for silanol in a 6-31G** basis set. This is a remarkable result, as the orthosilicic H4SiOq molecule is certainly a better physical model of the surface silica hydroxyls than silanol. The introduction of the electron correlation through MQllerPlesset perturbative expansion results in some changes in the geometry with respect to the Hartree-Fock data; the most evident variation is the lengthening of both the Si-0 bond and the 0-H bond (about 0.02 8, for both bonds) accompanied by a narrowing of about 3" of the Si-0-H angle. This changes are small and compare well with the similar changes in geometry observed by Pople and coworkers (ref. 1 3 ) for a closely related system, methanol, when optimized at the MP2 level. The introduction of higher level of perturbation (MP3 term) is not expected to alter significantly the MP2 geometry. As discussed above, direct comparison with experimental structures cannot be made. Calculated values of the Si-0 bond fall, however, between 1.637 and 1.67 A , experimentally found respectively in cyclohexylsilanetriol (ref. 14a) and in the isolated monosilicate unit Si03(0H)3- contained in a crystal of NaCaSi03(0H) (ref. 14b). Vibrational features of silanol The full set of harmonic frequencies has been calculated at Hartree-Fock level of theory, adopting 6-31G** and DZP basis sets. The two sets of frequencies are reported in Table 1, along with the description of each normal mode. Higher levels of treatment such as TZP and MP2 were not attempted for the full set of harmonic frequencies: further on the OH stretching mode alone is studied at the MP2 level. A good agreement between the two sets of calculated frequencies is observed. Recalling that H3SiOH does not exist, the comparison with experimental data can be made, on the one hand,

409

TABLE 1. Comparison between computed harmonic vibrational frequencies of H3SiOH and experimental values: (a) data concerning the silica hydroxyl [lo]; (b) group frequencies 1151. Data in cm(-1). Symme- Description try of mode a'

OH SiH3 SiH3 SiH3 SiH3 OH SiO

stretch stretch stretch deform deform bend stretch

SiH3 rock a"

SiH3 stretch SiH3 deform SiH3 rock torsion

HF/6-31GX*

HF/DZP

Exptl.

4231 2398 2356 1107 1068 904 952

4251 2394 2350 1105 1076 905 949

736

737

3745 (a) 21602140 (b) 945920 (b) 754 (a) 980 (a) 955-835 (b) 680540 (b)

2339 1041 795 201

2329 1053 796 205

as above I' I1

with the known vibrational features of the silica hydroxyl (as far as the OH stretch, the OH bending and the Si-0 stretch are concerned), and with literature qroup frequencies as far as the Si-H modes are concerned. Note that, because of the imposed Cs symmetry, three Si-H stretching modes result, as well as three deformations. If account is taken that the rotation around the Si0 axis is almost free, as discussed below, the local symmetry of the SiH3 moiety becomes C3v, and two modes are actually foreseen, both IR active, for both stretching and deformation. HF-SCF computed frequencies are observed to be sistematically overestimated by about 15%, as it is well known to occur (ref. 13). The Si-0 stretching frequency deserves some comments. The discrepancy between computed values and that observed on silica cannot be ascribed to the fact that in this latter case the cluster is anchored to the surface and the Si atom can be given a very high apparent mass, because a still larger discrepancy would arise. The point is that Si-0 stretching modes in siloxanes have frequencies lower than in other Si-0 compounds (lower limit value in Table 1): we meet here a point of weakness of the model, which reflects the obvious fact that H-termination or OH-termination of the cluster induce a different electron density at the silicon atom.

410

Anharmonicity of the OH stretch mode Kustov et al. (ref. lla), Shen et al. (ref. llb) and Kazansky et al. (ref. llc) have measured several overtones of the OH stretching modes by means of the diffuse reflectance technique in the near IR and shown that their energy wov can be represented by the following expressions: G(v) =

we

(V

+ 1/21

-

WeXe ( V

+ 1/2)2;

wov = G(v)

-

G(0)

which are typical of a Morse oscillator. The experimental values of the three first transitions as well as the corresponding we and wexe (anharmonicity) are reported in Table 2 . An ab-initio perturbative calculation of this latter quantity has been recently carried out by Sauer and coworkers, taking into account the coupling with the bending OH mode. Through the calculation of derivatives of the potential energy higher than the second and by the use of 6-31G* basis set, anharmonicity has been calculated to be 81 cm-I (ref. 5). A simpler approach has been adopted here: the 0-H distance R has been assumed as the normal coordinate, neglecting any coupling with the OH bending mode or any other normal modes. Indeed, this is thoroughly justified by the results of Sauer and coworkers, who report for the coupling term 2 cm-I only. The R parameter has been given values up to 3 0 % larger and 1 5 % smaller than its equilibrium Re value: the potential energy calculated at each distance has been fitted using a 6th power polynomial in (R-Re). The corresponding Schrodinger equation for the nuclear motion was then solved variationally. The anharmonicity has been computed from the first two vibrational transitions wol and wO2: = wo1 + 2 WeXe WeXe = ( 2 wo1 - wo2)/2; we Three estimations were carried out, using different levels of theory for the corresponding optimum geometry of silanol: Table 2 summarizes the relevant data. Hartree-Fock calculations overestimate all frequencies, but yield a correct anharmonicity parameter. Introduction of electron correlation lowers all HF frequencies of about 7%, which come now close to the experiment. A small further decrease is observed when using the MP2 optimized geometry, which is not worth the cumbersome calculations needed. As it has been recently shown

411

(ref. 16), a further extension of the relative poor 6-31G** basis set would instead allow MP2 theory to reach an even better accuracy. TABLE 2. Fundamental woll first and second overtone wo and w we and wexe constants of the OH stretching mode o$ H SiOfi3'molecule computed at HF-SCF and MP2 levels with 6-31G** %asis set and experimental data. All data in cm(-1). Equilibrium OH distance Re in A.

wo 1 wo2 w03 WeXe we Re

HF//HF

MP 2/ /HF

MP2/ /MP2

4074 7998 11795 75 4224 .9415

3829 7500 11043 79 3987 .9594

3802 7447 10977 79 3959 .9613

Exptl (ref. 1la) 3745 7320 10730 90 f 15 3925 /

Torsional barrier to the OH free rotation The energy barrier for the rotation of the OH moiety around the Si-0 bond has been determined by studying the temperature variation of the linewidth of the IR fundamental stretching mode (ref. 9), and results to be 3.78 ? 0.3 kJ mol-l. A related quantum chemical calculation has been performed by Sauer (ref. 17) on orthosilicic acid with 6-31G* basis set and partial geometry optimization: the barrier to the rotation of a single OH group results in 3.02 kJ mol-l, estimated as an average value of the barrier deriving from the syncronous rotation of the four protons (from D2 to S4 conformation respectively). Hartree-Fock and MP2 calculations concerning H3SiOH using 631G** basis set have been carried out by us as follows. Starting from the staggered conformation of the silanol, the eclipsed conformer was reached changing w, the torsion H-Si-0-H angle from 180" to 120" by steps of 10". For each step full relaxation of the geometry was allowed at Hartree-Fock level only. MP2 calculation has then been carried out utilizing the corresponding optimized HF structure. The resulting set of energy values, reported in Table 3, is fairly well reproduced by the simple expression: Ew = V3 (1 + cos3w) / 2. V3 is the height of the torsion barrier arising from inter-bond repulsion, which cause the staggered conformation to be slightly more stable.

412

Computed energy barriers compare well with the experiment. TABLE 3. Relative energy as a function of the torsional angle H-Si-0-H and barrier height V3. Data in kJ mol(-l). HF//HF 0,.12,.46,.92 1.4,1.74,1.86 "3

1.862

MP2 / / HF

Exptl (ref. 9)

0,.17,.6,1.2 1.82,2.28,2.44 2.436

3.78

? 0.3

CONCLUSIONS Although the simplest cluster model, silanol H3SiOH reproduces in a satisfactory way nearly all the known spectroscopic features of the silica hydroxyl, namely the OH stretching mode (both fundamental and overtones, i.e., harmonic and anharmonic behaviour), the OH bending and the torsional energy barrier around the Si-0 bond. Agreement is good already at HF//HF level of treatment, if account is taken that the frequencies are always somewhat overestimated. The MP2//HF level of treatment, given only for the OH stretch yields satisfactory results, so that MP2//MP2 calculations do not seem to be worthwhile. The only datum not reproduced is the Si-0 stretch. Because H3SiOH is much simpler than the other possible cluster H4Si04, it appears as the ideal candidate for calculations mimicking the interaction with molecules.

ACNOWLEDGEMENTS Many thanks are due to Dr. V. R. Saunders for his invaluable suggestions and continuos help. We are also grateful to Dr. J. Sauer for discussions and for sending us a copy of reference 5 prior to publication and to Prof. G. Ghiotti for useful comments. Generous allowance of computer time by CSI PIEMONTE (Consorzio per il Sistema Informativo) is also acknowledged. One of us (PU) thanks the C.N.R. (Progetto Finalizzato Materiali e Dispositivi per 1'Elettronica a Stato Solido) for a grant, SERC for allowance of computer resources and Daresbury Laboratory for kind ospitality.

413

REFERENCES 1

2 3 4

5 6 7 8 9 10

11

12 13

14 15 16 17

H. Knozinger, in P. Schuster, G. Zundel and C. Sandorfy (Editors), The Hydrogen Bond. Recent Developments in Theory and Experiments, North Holland, Vol. 111, Chapter 27, pp. 1263, 1976 G.M. Zhidomirov and V.B. Kazanski, Advances in Catalysis, 34 (1986) 131. J. Sauer, J. Phys. Chem., 91 (1987) 2315. J. Sauer and R. Zahradnik, Int. J. Quantum Chem., 26 (1984) 793. H. Mix, J. Sauer, K. Schroder and A. Merkel, Coll. Czechoslov. Chem. Commun. in press. B.C. Chakoumakos and J. Gibbs, J. Phys. Chem. 90 (1986) 996. P. Geerlings, N. Tariel, A. Botrel, R. Lissillour and W.J. Mortier, J. Phys. Chem., 88 (1984) 5752. E.A.V. Ebsworth, Volatile Silicon Compounds, Pergamon, London, 1963. F.W. Lampe, in P. Ausloos (Editor), Interaction Between Ions and Molecules, Plenum, New York, 1974. P.R. Ryason and B.G. Russell, J. Phys. Chem., 79 (1975) 1276. F. Boccuzzi, S. Coluccia, G. Ghiotti, C. Morterra and A. Zecchina, J. Phys. Chem., 82 (1978) 1298. (a) L.M. Kustov, V. Yu Borovkov and V.B. Kazansky, J. Catal., 72 (1981) 149; (b) J.H. Shen and K. Klier, J. Colloid Interf. Sci., 75 (1980) 56; (c) V.B. Kazansky, A.M. Gritscov, V.M. Andreev and G.M. Zhidomirov, J. Mol. Catal., 4 (1978) 135. P. Ugliengo, V.R. Saunders and E. Garrone submitted to J. Phys. Chem. W.J. Hehre, L. Radom, P.V.R. Schleyer and J.A. Pople Ab-initio Molecular Orbital Theory., John Wiley & Sons, New York, 1986. (a) H. Ishida, J.L. Koenig and K. Gardner, J. Chem. Phys., 77 (1982) 5748; (b) B.G. Cooksley and H.F.W. Taylor, Acta Cryst. Sect. B, 30 (1974) 864. G. Socrates, Infrared Characteristic Group Frequencies, John Wiley, New York, 1980, pp. 128-130. N.C. Handy,J.F. Gaw and E.D. Simandiras, J. Chem. SOC., Faraday Trans 11, 83(9) (1987) 1577. J. Sauer, Chem. Phys. Lett., 97 (1983) 275.



415

AN IR STUDY OF ETHYLENE HYDROGENATION AT RT ON A Cu/ZnO CATALYST

G. Ghiotti, F. Boccuzzi, and A. Chiorino

Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali. Universith di Torino, Via Giuria 7, 10125 Torino, ITALY. ABSTRACT An IR spectroscopic study of ethylene hydrogenation on Cu/ZnO based catalysts showed that, under reaction conditions, Cu(0) is at the origin of the H2 activation process and that Cu(0) and ZnO are both able to activate ethylene as weakly rr-bonded species. No evidence of ethylene n-bonded to Cu(1) ions occurred. INTRODUCTION Cu/ZnO based catalysts are active and selective in the hydrogenation reactions, the methanol synthesis being one of the most studied and discussed (refs. 1,2), while alkene hydrogenation has received lesser attention (refs. 3 - 5 ) . Vedage and Klier (ref. 3 ) proposed that surface Cu(1) in solution on ZnO (as in methanol synthesis) are the active phase for olefin activation. De Rossi et al. (ref. 4 ) have correlated the catalytic activity for propene hydrogenation on Cu/AlzO3 and Cu/ZnO/A1203 with the Cu(0) surface area, they also found a beneficial influence of the ZnO on the activity and proposed that propene is activated by Cu(0) , while H2 is activated by the oxide phase. In our laboratory we recently investigated the state of copper in cu/zno catalysts at different reduction stages, mainly by IR, Vis and W studies of CO and O2 adsorption (refs. 6-7). We also investigated the adsorption at room temperature (RT) of pure H2 and ethylene on reduced samples (refs. 8 - 9 ) . The study of the reaction is thus the logical consequence of the previous investigations. The final purpose of this work is to contribute to the knowledge of the Cu and ZnO role in the activation of H2 (or D2) and ethylene. EXPERIMENTAL Oxidized samples with different copper content were obtained by decomposition o f basic carbonates CuxZnl-x(C03)2(0H)6, prepared as in ref. 10, with x ranging from 0.03 to 0.1. All data reported i n

41fi

ttis paper are relative to x=0.05. The same results were obtained for all other copper concentrations. The decomposition was followed by treatment with pure oxygen up to 588 K (activation) "in situ", the IR cell allowing thermal treatment in a controlled atmosphere or in vacuo. The reduction was performed with mixture of H 2 in N2 arid with pure H2 (red H samples) up to 493 K as illustrated in ref. 6 . When D2 was used to reduce the catalyst (red D samples) the samples were completely deuteroxylated. After reduction or oxidation the IR cell was evacuated] the pellet was cooled to RT and the IR spectrum of the clean sample was taken (background spectrum). Then the different gases were admitted ( H 2 , D2] ethylene, or H2 (D2)/ethylene mixtures) and the IR spectra were recorded at different contact times. The H 2 ( D2 ) /ethylene gas m.ixtures were subjected to gas-mass analysis immmediately after contact with the catalyst. The IR spectra were recorded with a 580B Perkin-Elmer spectrophotometer, equipped with a 3600 Data Station. The spectrum of gaseous ethylene was always subtracted. RESULTS AND DISCUSSION Pnysical and chemical properties of oxidized and reduced catalysts. The activation produced CuO and ZnO particles of 6-8 nm and 1520 nm respectively (11). As for the reduced samples, transmission electron micrographs (TEM) showed that ZnO particles are hexagonal prisms with a mean particle size of 15-20 nm, while copper particles were neither detectable by TEM nor by XRD analysis. Therefore the metal dispersion has been obtained by O2 chemisorption at 77 K. A copper surface area of 180-120 m 2/ g Cu has been measured and a mean particle size of =4 nm was calculated, assuming spheres, a not correct assumption,in fact we have evidence (ref. 7) of an extended surface crystallinity of the copper particles. This two-dimensional crystallinity is in good agreement with other evidence of a preferential growth of (211) copper planes cn the (lOi0) ZnO faces, as a consequence of an epitaxial Fhenomenon (refs. 1 1 6 1 1 2 ) . Two important features of our red H (red D) samples were the following: i) the presence of Cu(1) on their surface has been excluded by IR and microgravimetric measurements of CO adsorption. No cu(I)-CO surface species could be put in evidence on the basis of the CO stretching frequencies and of the evacuation resistance

417

(ref. 6). This does not imply obviously the absence of subsurfacial, interfacial or internal Cu(1) ions (ref. 2); ii) they exhibited, in vacuo, the same transparency as the oxidized samples (ref. 8 ) , unlike pure reduced ZnO that absorbs completely the IR radiation at F? 2000 cm-I (ref. 13), The transparency loss of pure reduced ZnO has been assigned to the photoionization, by the IR radiation, of V o + and V o o donor centres (monoionized and neutral oxygen vacancies) and/or to the scattering of the radiation by the free carriers produced during the reductive treatments (ref. 1 3 ) . However the contact between n-ZnO and metallic copper causes a transfer of electrons from the ZnO to the copper and a consequent depletion of the ZnO donor centres so that the oxide phase becomes again transparent (ref. 8). Adsorption of H2 - (D,). The H2 admission at RT on both oxidized and reduced samples gave very different results from those obtained in the case of pure ZnO. On pure ZnO two types of H2 chemisorption have been observed, generally known as type I (fast and reversible chemisorption) and type I1 (slow and irreversible chemisorption), both consisting in the etherolytic dissociation of the molecule on surface Zn2+02couples with production of surface hydroxyls and hydrides, whose vibration frequencies have been well established (ref. 14). On the contrary on the oxidized catalyst the H2 admission (spectra not reported in figure for sake of brevity) gave rise to type I chemisorption (bands at 1710 and 3500 cm-l) in very small amounts and only in the first few contact minutes, as transient species. The main process was the growth of the surface hydroxyls, easily interpretable as due to the slow, partial CuO reduction leading to the formation of water that rehydrsted the oxidic phases. This is the reason for the presence of only small amounts of type I species chemisorbed on ZnO and for their nature of transient species. When D2 was used an isotopic exchange with surface hydroxyls could be put in evidence. The easy reducibility of the CuO phase is not surprising, in fact when CO was used as reducing agent the process already occurred at 77 K (ref. 6). As for the reduced catalyst, the D2 contact with a red H sample (see Fig. 1) gave rise to a transparency loss in the overall spectral range examined, the effect being practically completed in 2 hrs and partially reversible at RT; in the same time an isotopic

418

exchange was acting between D2 and surface hydroxyls. The same effect was produced by H2, but for the isotopic exchange with’the hydroxyls, therefore the decrease in the overall sample transparency is not related to a vibrational-phenomenon, but to an electronic one. The transparency loss is due to very broad absorption, whose intensity increases with until1 to = l o 0 0 cm-l (Fig. 1 b) The exchange phenomenon, in the case of D2, is put in evidence by the negative bands in the OH stretching and bending regions (3600-3000 cm-I and 900-800 cm-l respectively) and by the rise of bands in the OD stretching region (2700-2200 cm-l).

.

T%

12

-

10

-

a6-

4-

I

Fig. 1. IR spectra of D2 adsorption (20 Torr) at RT on a red H catalyst. (a) Transmission spectra: curve 1, background; curve 2 , after 20 min contact; curve 3, after 2 hrs; curve 4, after evacuation at RT. ( b ) Absorption spectra: curve 1, difference ketween curves 3 and 1 of section (a); curve 2, an analogous difference when H2 was adsorbed (reported for comparison).

419

The nature of the large absorption and of the chemical processes related to it have already been discussed in a previous paper (ref. 8), and we only remember here the conclusions: the stepped planes exposed by the metal particles are able to dissociate H2 (or D2) , that spills over the ZnO surface. As it is known (ref. 15) atomic hydrogen can be adsorbed by ZnO in a protonic form releasing electrons, that populate the oxygen vacancies depleted by the contact with the metal; the transparency loss is the consequence of the photoionization of the repopulated donor centres. No bands characteristic of type I or type I1 H2-chemisorption could be detected. However the lack of these adsorbed species is not surprising: in fact, on one hand the well dispersed metal copper recovered an high portion of the support (ref. 7 ) , on the other hand the reduction process produced a lot of water that partially rehydrated the ZnO surface, and consequently the Zn2+ 02couples active in the H2 (D2) adsorption were very few. As for the presence of contributions due to Cu-H stretching modes to the broad absorption, data from literature indicate that metal-hydride stretches, arising from bridging adsorbed hydrogen, should absorb in the 1000-800 cm-I region. Unluckily this is also the region of the hydroxyl bending modes and of ZnO bulk multiphononic vibrations. Therefore, in spite of the use of the isotopic exchange, we are not yet completely sure about the presence or not of contributions different from those arising from hydroxyl bending modes. Ethylene adsorption. The ethylene adsorption on oxidized samples (Fig. 2a) showed mainly the formation of non dissociatively adsorbed species weakly bonded to the surface cations [Zn(II) and possibly Cu(II)I, reversible to the evacuation at RT (bands at 1000, 1440, 1605, 2980, 3060 cm-l) (ref. 6). Simultaneously very small amounts of oxidation products were formed, some of them irreversible to the evacuation (bands at 2870, 1550, 1340, 1050 cm-l, assignable to formate and carbonate like species), some reversible (bands at 1500, 1420, 1245 cm-l, not assigned). The ethylene adsorption on reduced samples (Fig. 2b) showed the presence of n-complexes on copper metal particles (bands at 920, 1290, 1550 cm-l) and of rr-complexes on Zn(I1) ions (bands at 1000 cm-l) in lower amounts than on the oxidized samples (ref. 9).

4 20

L

&

I

I

I

UOo

2600

2aX,

I

1700

1

I

I

1400

I

I

1100

800

Fig. 2 . IR absorption spectra of C H4 adsorbed at RT and p=20 Torr on :(a), an oxidized sample and ( $ 1 , a reduced sample. Curves 1, after 20 min contact; curve 2 , after 1 hr contact; curves 3, after evacuation at RT.

After 1 hr contact minor quantity of a-bonded species were produced (bands at 1480,1160,970 cm-l) irreversible to the evacuation at RT. No bands could be assigned to n-bonded complexes on Cu(1) ions, according with results obtained using CO as test molecule (ref. 7). In Table 1 the IR bands assignable to the adsorbed ethylene are summarized and assigned on the basis of refs. 9,16.

421

TABLE 1

Frequency assignment of ethylene adsorbed on cu/zno. frequencies (cm-1) Mode rt-complexes on: di-a-complexes on: (approximate CU(0) Zn(II),Cu(II) CU(0) description) CH2 sym stretch 1550 1605 CC stretch 1480 1160 CH2 scissor 1290 920 1000 CH2 wag 970 CH:, sym stretch 2990 CH2 scissor 1440 CH2 antisym stretch 3060 -

H2/ethylene mixture on the oxidized sample.

-

The interaction of 1:l H2/C2H4 mixtures with oxidized samples (Fig. 3 ) caused, as expected, the partial reduction of the CuO phase, leading to the formation of surface hydroxyls and of the ethylene oxidation products. A l s o in this case, type I H2 chemisorption on ZnO was present in very small amounts and as transient species, but the Zn-H and ZnO-H stretching modes were shifted to 1675 and 3520 cm-l as a consequence of ethylene

1675

15001420

Fig. 3 . IR absorption spectra of C2H4(20 Torr)/H2(20Torr) adsorbed at RT on an oxidized sample. Curve 1, after 20 min contact; curve 2 , after 1 hr; curve 3 , after evacuation at RT.

422

coadsorption as n-bonded species. However, the amount of ncomplexes, since the beginning of the contact, was lower than that formed by interaction of pure ethylene and decreased with the contact time; the main reason is the rehydration of the surface, another being the reduction of surface CuO phase to some C u ( O ) , whose formation was put in evidence by the slow increase of the band at 920 cm-l. However, no ethylene hydrogenation was performed: no bands due to ethane appeared, no ethane was detected by gas-mass analysis. Adsorption of H2E2€I4 mixtures on D2 reduced samples. - -The admission of 1:l H2/C2H4 mixtures on red D samples produced the following features worth of notes: (Fig. 4 , curve 1) no decrease in the sample transparency, very little or no exchange between H2 and deuteroxyls, very few or none n-complexes on both metallic and oxidic phases, the appearence of bands at 2980 and 1450 cm-I coincident with those obtained allowing ethane to contact the reduced catalyst, and reversible to the evacuation at RT. The gas-mass analysis, immediately after contact, revealed the presence of only C 2 H 6 . No deuterated ethane was detected, so we exclude that some surface deuteroxyls could be the active species in the ethylene hydrogenation. If the mixture admitted contained an excess of ethylene, the ethane formation was associated to the presence of n-complexes both on the metal and oxide phase ( Fig. 4 , curve 2 1 . If the mixture admitted contained an excess of H 2 (curve 3 ) the ethane formation was followed by the transparency loss and by the exchange phenomenon. Adsorption of D2/C2Hi mixtures on H2 reduced samples. When mixtures D2/C2H4 were admi-ited on red H samples (Fig. 4 , curve 4), results similar to those described in the above paragraph were observed. The only difference was the formation of deuterated ethane as showed by the presence of absorptions at ~ 2 2 0 0cm-' (CD stretching region), and at ~ 2 9 7 0 cm-' ( C H stretching region), reversible to the evacuation. The gas-mass analysis, immediately after contact, revealed the presence of C2H4D2 only. Surface hydroxyls are not the active species in the ethylene hydrogenation; this is consistent with the results described in the above paragraph.

423

Fig. 4. IR absorption spectra of C2H4/H (or D ) adsorbed at RT on reduced samples. Curve 1, C H4 (20 TorrS/H2 h0 Torr) on a red D sample; curve 2 C H4 (45 ?orr)/H2 ( 2 0 Torr) on a red D sample: curve 3 , C2H4 ( i 0 $orr)/H (30 Torr) on a red D sample: curve 4, C2H4 (20 Torr)/D2 (30 Torr? on a red H sample. The role of the metal and ZnO in the H2 and ethylene activation. Our first view about the different role of copper and of ZnO can now be suggested on the basis of the reported results. The hydrogenating catalytic activity of pure ZnO is well known (17), the researchers studying Cu/ZnO based catalyst for the hydrogenation have been therefore tempted to assign the role of hydrogen activator to the ZnO phase. However, our results have clearly shown that , under the showed conditions, ZnO surface is not able to activate H2 as type I [i.e. the active species in the olefin hydrogenation on pure ZnO (ref. 1711: H2 is atomically

424

dissociated by the metal copper, spills over the ZnO where it is adsorbed in a protonic form releasing electrons to the solid. Thus we have two species of activated hydrogen in equilibrium between them, the atomic hydrogen on copper surface and the protonic hydrogen on ZnO , that are consumed in the ethylene hydrogenation Whatever of the two species is responsible for the attack to the clefin, the metal copper is at the origin of the hydrogen activation process. A s for the role of copper and ZnO phases in the ethylene activation, our results have shown that they both activate ethylene as weakly rr-bonded species, both species being consumed during the reaction. On the contrary, no n-bonded ethylene on Cu(1) ions could be put in evidence on our reduced surface, so we exclude that Cu(1) should be responsible for the ethylene activation.

.

ACKNOWLEDGEMENTS We thank the Italian C.N.R., Progetti Finalizzati:" Chimica Fine e Secondaria", and "Materiali e Dispositivi per 1'Elettronica a Stato Solido", for financial support. REFERENCES 1 K. Klier, Advan. Catal.,31 (1982) 243. 2 G-Ghiotti and F.Boccuzzi, Catal. Rev. Sci. Eng., 29 (1987) 151. 3 G. Vedage and K. Klier, J. Catalysis, 77 (1982) 558 4 S. DeRossi, A.Cimino, M.LoJacono and G.Ferraris,in DECHEMA (Ed& tor),Proc.8th Int.Cong.Catalysis,Berlin,July 2-6,1984, Vol. IV, Verlag Chemie, Wheinheim, 1984, pp. 611-622. 5 G. Ferraris, S. DeRossi and R. Mancini,in Proc. VI Cong.Naziona le Catalisi, Cagliari, Italy, October 6-10, 1986, pp. 459-462. 6 G. Ghiotti, F.Boccuzzi and A. Chiorino, in M. Che and G.C. Bond (Editors), Studies on Surface Science and Catalysis, Vol. 21,EL sevier, Amsterdam, 1985, p. 235. 7 F.Boccuzzi, G.Ghiotti, A.Chiorino, Surface Sci.,162 (1985) 361. 8 F.Boccuzzi, G.Ghiotti, A.Chiorino, Surface Sci.,183 (1987) L285 9 G.Ghiotti, F.Boccuzzi, A.Chiorino, Surface Sci.,178 (19861 553. 10 G. Petrini, F. Montino, A. Bossi and F. Garbassi, in G. Poncelet, P. Grange and P. A. Jacobs (Editors), Preparation of Catalyst 111, Elsevier, Amsterdam, 1983, p.735. 11 F. Garbassi and G. Petrini, J. Catalysis, 90 (1984) 106. 12 G. Ghiotti, F.Boccuzzi and R.Scala, J. Catalysis, 92 (1985) 79. 1 3 F. Boccuzzi, G. Ghiotti and A. Chiorino, J. Chem. SOC. Faraday Trans.2, 77 (1983) 1779. 14 F. Boccuzzi, E. Borello, A. Zecchina, A. Bossi and M. Camia, J. Catalysis, 51 (1978) 150. 15 J. I. Gersten, I. Wagner, A. Rosenthal, Y. Goldstein, A . Many and R. E. Kirby, Phys Rev., B29 (1984) 2458. 1 5 E. M. Stuve and R. J. Madix, J. Phys. Chem., 89 (1985) 3183. 17 R. J. Kokes and A. L. Dent, Advan. Catal., 22 (1972) 1.

425

Dr. D. Reinalda Kon Shell Laboratorium Amsterdam, Holland.

The difference in transparency between pure reduced ZnO and reduced Cu/ZnO is explained by you assuming an electron transfer from reduced ZnO to the metallic Cu phase. If the reduction rate is decreased by the presence of cu (blocking of surface sites) than the results can be explained equally well. Have you considered this explanation?

Pr. G. G h i o t t i .

Prior to achieve the final interpretation, we obviously considered different explanations [see G.Ghiotti and F.Boccuzzi, Catal. Rev.Sci. Eng., 2 9 , (1987) 1511 , but not the one you suggested. In fact on the one hand the process we are discussing is not the reduction of ZnO particles precovered by copper metal particles, but the reduction of microcrystalline powders made of very well interdispersed CuO particles and particles of an oversaturated solution of Cu(I1) ions in ZnO (see ref. 10, 11 in the text) to obtain the reduced samples. On the other hand we thougth that the metal copper formed in the first steps of the reduction should increase the reduction rate of the ZnO phase, owing to the ability of that metal to dissociate H2. As we knew that the chosen conditions were such as to kill almost completely the transparency of pure ZnO (see ref. 13 in the text), we were very surprised to obtain reduced catalysts with the same IR transparency of the oxidized ones. The successive study of the H2 and of H2/02 interaction with the reduced catalysts, the reading of the physical literature about the metal-semiconductor contacts and of the chemical literature about the metal-support interaction, lead us to the present explanation. Recently, different experimental data have indicated that ZnO is more reducible in the presence of copper metal [see for example M.S. Spencer, Surface Sci., 192 (1987) 3231. A s a consequence we think that the copper formed during the reduction cannot do anything but to increase the reducibility of our ZnO phase.


C. Morterra, A. Zecchina and G . Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

427

EFFECT OF WATER I N THE ENCAPSULATION OF THE METALLIC PHASE DURING SMSI GENERATION I N P t / T i 0 2 CATALYSTS

A.R.

Gonzalez-Elipe,

P. Malet, J.P. Espinos, A. Caballero, G. Munuera

I n s t i t u t o de Ciencia de M a t e r i a l e s (CSIC-Universidad de S e v i l l a ) Dpto. Quimica I n o r g l n i c a . P.O. Box 1115. 41071 S e v i l l a . S P A I N ABSTRACT The e f f e c t i n t h e generation o f SMSI o f t h e presence o f water vapour d u r i n g t h e r e d u c t i o n w i t h hydrogen a t 773K o f a H C1 P t / T i O precursoy (5% by weight adsorEtign/TPD $nd XPS/Ar etching. While o f m e t a l ) has been s t u d i e d using H H f l o w r e d u c t i o n s under normal c o g d i t i o n s i n t h e range 473-773K produce a s ? e a d i l y increas+ng SMSI s t a t e , wet+hydrogen f P H 0 /P H = 0.036) leads t o an i r r e v e r s i b l e SMSI-like s t a t e . XPS/Ar - s p u t t e r i n g &ow tha? i n t h i s case a t o t a l encapsulation o f t h e m e t a l l i c p a r t i c l e s w i t h a ca. 3 nm T i 0 overlayer occurs. Meanwhile several a c t i v a t e d forms o f hydrogen are incorporated by s p i l l o v e r t o t h e support (and t o t h e decorating T i 0 o v e r l a y e r ) d u r i n g t h e r e d u c t i o n process and c o n t r i b u t e t o t h e suppression o f HX adsorption on t h e platinum, i n d i c a t i n g an a d d i t i o n a l " e l e c t r o n i c e f f e c t " . A m6del i s proposed which emphasizes t h e i m portance o f water vapour i n t h e c o n t r o l o f t h e thickness and r e d u c t i o n s t a t e o f the T i O x overlayer. INTRODUCTION

I n a previous s e r i e s o f works (1-3) we have examined by

1 H-NMR,

EPR, XPS

and I R spectroscopies t h e i n t e r a c t i o n o f H2 (and D2) w i t h Rh/Ti02 reduced up t o 773K showing t h e i n c o r p o r a t i o n o f d i f f e r e n t forms o f hydrogen i n t h i s system. One o f these forms, an h y d r i d e - l i k e species i n t h e T i 0 2 support was found t o be i n v o l v e d i n t h e e l e c t r o n i c i n t e r a c t i o n between t h e metal and t h e reduced support, thus h i n d e r i n g t h e adsorption o f H2 and CO a t 300K on t h e m e t a l l i c p a r t i c l e s (SMSI s t a t e ) which c o u l d be p a r t i a l l y recovered by removal o f such h y d r i d e - l i k e species by outgassing a t 773K (4,5).

I n a d d i t i o n , we have r e c e n t l y shown (6) that

t h i s t y p e o f hydrogen a l s o promotes a g r e a t i o n i c m o b i l i t y o f t h e reduced sLpport t h u s l e a d i n g t o decoration (and f i n a l l y t o encapsulation) o f t h e m e t a l l i c p a r t i c l e s w i t h a T i O x overlayer, a process t h a t i s w i d e l y accepted as r e s p o n s i b l e o f t h e SMSI fenomena. Recently t h i s "decoration model" has gained support, from HREM s t u d i e s ( 7 ) which have c l e a r l y shown t h e formation i n Pt/Ti02 of l a y e r s o f T i suboxides a few nanometer t h i c k , t o g e t h e r w i t h several types o f s u p e r s t r u c t u -

r e s due t o P t T i - a l l o y s a f t e r r e d u c t i o n a t 773-9OOK. I n s p i t e o f t h i s , i t i s d i f f i c u l t t o understand how t h e SMSI s t a t e can be almost completely reversed i n Pt/Ti02 j u s t by exposing t h e reduced sample t o oxygen a t 300K, as r e c e n t l y r e ported by Hermann e t a l (8) i n a d e t a i l e d c a l o r i m e t r i c study o f t h i s system, o r why outgassing a t 773K p a r t i a l l y r e s t o r e s t h e non-SMSI s t a t e i n our Rh/Ti02 c a t a l y s t (4,5)

i f such t h i c k TiOx o v e r l a y e r i s present. On t h e o t h e r hand, a

c h a r a c t e r i z a t i o n o f t h e f a c t o r s c o n t r i b u t i n g t o t h e formation o f such "decoration

'ayer" as well as the understanding o f the chemical (electronic) i n t e r a c t i o n between the T i O x and the m e t a l l i c p a r t i c l e s i s s t i l l lacking. I n the present work the r o l e o f hydrogen and traces o f water vapour i n the generation o f the T i O x decoration layer and i n the nature o f i t s i n t e r a c t i o n u i t h the m e t a l l i c phase are examined. EXPERIMENTAL

Three d i f f e r e n t catalysts (L, H and H(H20)) were prepared from t h e same batch o f Pt/Ti02 precursor (5% o f metal by weight), obtained by wetness impregnation o f nonporous Degussa P25 w i t h c h l o r o p l a t i n i c acid. Once d r i e d i n flowing a i r a t 3 :383K, reduction i n hydrogen flow (100 cm fmin, 3h) was c a r r i e d out according t o rhe scheme below: H2 473K, 3h H2(PH20/P = 0.0361,

HZ 773K, 3h

773K,3h

so t h a t samples H and H(H20) only d i f f e r i n the p a r t i a l pressure o f water present during t h e i r reduction a t 773K. The three samples were kept i n a i r i n a dissicator u n t i l f u r t h e r use together w i t h TiOZ and Pt/Si02 (EUROPt-1) taken as references. Structural and t e x t u r a l characterization o f the samples was made by XRD together w i t h H2 and N2 adsorptions ( a t 300 and 77K, respectively) using ca. 0.59 o f sample i n a 65 cm3 c e l l with a MKS Baratron capacitance gauge working i n the range 102-104 Pa. A f t e r a

3

f i r s t H2 isotherm up t o 7x10 Pa and outgassing a t 3M)K a second isotherm was measured so, t o t a l ( T I and r e v e r s i b l e (R) adsorptions could be evaluated by extrapolation t o zero pressure. TPD experiments were c a r r i e d out w i t h a conventional TCD-system connected t o a HP-30544 system for data s t o r i n g and processing. The c a r r i e r gas stream (Ar, 50 ml/min) was passed through ca.lg of sample, using a constant heating r a t e of 10Kfmin. XPS spectra were recorded w i t h a LeyboldHeraeus (mod. LHS-10) spectrometer w i t h Ar-etching f a c i l i t i e s i n the preparation chamber using the same procedure described elsewhere ( 6 ) . Unless otherwise stated, reduction an a l l other pretreatments were c a r r i e d out "in s i t u " during l h before each experiment (HZ isotherms, TPD o r XPS spectra). RESULTS Sample Characterization The main r e s u l t s o f the characterization of the three samples are summarized i n Table 1. While XRD does not detect l i n e s due t o the m e t a l l i c phase i n samples L and H, for sample H(H201, reduced under wet conditions, a broad P t ( l l l 1 l i n e was m d e d

429

P t s i z e (nm)

Sample

-

T i O2

Sample Pt/Ti 0 2 L a i r/300K Pt/Ti 0 2 y a i r/300K I1 I,

02/773K Pt/Ti02-H(H,0) air/300K 0,/673K

Pretreatments ( T / K ) REDUCTION OUTGASSING

(“9-l)

.

50 = 1 49 1 49 * 1 49 * 1

70 70 70 70

< ,a) < ,a) ca. 7b )

Pt/Ti02-L Pt/Ti02-H P t / T i 02-H(H20)

’BET

% Anatase

TOTAL

H/Ptx100 REVERSIBLE

T/R

473

473

49.6

19.0

2.6

773 773 473 473

473 773 473 473

1.4 8.7 19.7 43.5

1.4 5.4 8.8 17.5

1 .o 1.6 2.3 2.5

473 473

473 473

1.7 1.8

1.4 1.4

1.2 1.3

430 n o t modify t h e a d s o r p t i o n c a p a c i t y o f sample H(H20) which s t i l l shows t h e

ow

T / R values c h a r a c t e r i s t i c o f SMSI.

SMSI generation d u r i n g H, r e d u c t i o n c

F i g 1 shows c h a r a c t e r i s t i c hydrogen TPD p r o f i l e s f o r

he P t / T i 0 2 precu sor

reduced a t i n c r e a s i n g temperatures i n t h e range 473-773K. A t l e a s t two new forms o f adsorbed hydrogen (peaks a t 583 and 69310 a r e generated w h i l e t h e o r i g i n a l

broad peak due t o hydrogen adsorbed on t h e m e t a l l i c p a r t i c l e s decreases by ca.

50% a f t e r t h e r e d u c t i o n a t 773K f o r l h . F u r t h e r r e d u c t i o n a t 773K ( n o t shown i n the f i g u r e ) does n o t modify t h i s TPD p r o f i l e except f o r a p r o g r e s s i v e decrease o f t h e peak a t lower temperatures and a small decrease o f t h e new forms o f adsor-

Ded hydrogen. Data i n Fig.2 show t h a t removal o f these new forms o f hydrogen, by outgassing a t 773K, p a r t i a l l y r e s t o r e s t h e c a p a c i t y o f t h e metal t o adsorb hydrogen which, however, o n l y recovers i t s o r i g i n a l l e v e l a f t e r r e o x i d a t i o n a t 673K and r e - r e d u c t i o n a t 473K. I n a d d i t i o n , t h e d a t a i n t h i s f i g u r e suggest t h a t t h e qew forms of hydrogen should be generated d u r i n g t h e r e d u c t i o n a t h i g h temperatures followed by c o o l i n g down i n He, s i n c e t h e y do n o t appear a f t e r a d s o r p t i o n o f ;1*

a t 300K once t h e y have been removed by outgassing a t 773K. A comparison o f

these TPD experiments w i t h those given i n Fig.3 f o r H2 a d s o r p t i o n confirms a par a l l e l i s m w i t h t h e generation of t h e SMSI s t a t e . Moreover, t h i s s t a t e i s partially removed if t h e sample i s outgassed a t 773K, a treatment t h a t w i l l e l i m i n a t e t h e rlew forms o f hydrogen detected by TPD. A s i m i l a r s e t o f TPD experiments w i t h t h e sample H(H20) i n Fig.4 does n o t show t h e c h a r a c t e r i s t i c adsorption on t h e metal, i n good agreement w i t h data

Pt/Ti02-L/H 0

I

( e - OSa) r

E r L73K

f

Er 1 1 3 K

? , t

373 373

573 1 /K

773

Fig.1. TPD o f hydrogen from a P t / T i O precursor p r o g r e s s i v e l y redwd a t i n c r e a s i n g temperatures (T,) i n t h e range 473-773K. Right: d i f f e r e n c e TPD spectra.

573

300 K

773

1 /K

Fig.2. TPD o f hydrogen from a P t / T i O p precursor reduced a t 773K f o r l h : a) outgassed a t 773K before H adsorption a t 300K; b ) o u t ggssed a t 473K b e f o r e H adsorpt i o n a t 300K. Right: d i v f e r e n c e TPD spectra.

431 Pt/TiOz -L/H H, adsorption

PtITi0.I-

H (H701

Y Y

1 8

rnln

.m Y Y 0 -

ziz I

Y

3, 0

(d-a)

Emc. 473K

513

373

773

TIK 473

573 673 1 reduction (K)

773

F i g . 3 . Hydrogen a d s o r p t i o n ( H / P t ) a t 300K on P t / T i O samples r e d u ced a t t h e i n d ? c a t e d temperat u r e s f o r 3h and evacuated a t 473K ( 0 ) o r 773K (01 f o r l h .

Fig.4.

TPD o f hydrogen f r o m a P t / T i O - H ( H 0) sample p r o g r e s s i v e l y redused fbr l h a t i n c r e a s i n g t e m p e r a t u r e s (T ) i n t h e range 473-773K and Right: t h h c o o l e d down i n H d i f f e r e n c e TPD s p e c t r g . -

.

i n T a b l e 2, though i t s r e d u c t i o n up t o 723K l e a d s t o a small peak a t ca. 593K and t o a new one a t ca. 513K which appears a t a s l i g h t l y h i g h e r t e m p e r a t u r e s . A f t e r t h i s s e t o f TPD-runs o u t g a s s i n g a t 773K does n o t l e a d t o hydrogen adsorption on t h e metal a t 300K w h i l e r e o x i d a t i o n a t 673K f o l l o w e d by a s i m i l a r s e t o f TPD r u n s g i v e s t h e same p a t t e r n b u t now t h e two peaks have much s m a l l e r i n t e n s i t i e s and remains unresolved. Hydrogen i n c o r p o r a t i o n i n t o reduced T i 0 2 I n o r d e r t o see whether t h e new forms o f hydrogen generated d u r i n g t h e r e d u c t i o n o f t h e P t / T i 0 2 p r e c u r s o r a t T,573K

a r e r e l a t e d t o t h e reduced T i 0 2 s u p p o r t

a s e r i e s o f experiments were c a r r i e d o u t u s i n g t h e two r e f e r e n c e s , T i 0 2 and P t / S i 0 2 and a 3 : l mechanical m i x t u r e . W h i l e hydrogen r e d u c t i o n o f t h e T i 0 2 s u p p o r t a t t e m p e r a t u r e s i n t h e range 473-773K always gave f l a t TPD p r o f i l e s , f o r P t / S i 0 2 t h e c h a r a c t e r i s t i c TPD p r o f i l e f o r t h i s c a t a l y s t ( 1 0 ) was o b t a i n e d and remained unchanged f o r a l l t h e temperatures. However, as shown i n F i g . 5 f o r t h e P t / S i 0 2 + T i 0 2 m i x t u r e , r e d u c t i o n a t 773K f o r d i f f e r e n t p e r i o d s o f t i m e p r o g r e s s i v e l y gener a t e s i n t h i s case t h e d i f f e r e n t f o r m s o f hydrogen p r e v i o u s l y observed. So, after 4.5h,

t h e whole TPO p r o f i l e was v e r y s i m i l a r t o t h o s e r e c o r d e d f o r P t / T i 0 2 . Diffe-

r e n c e o f TPO s p e c t r a i n t h i s f i g u r e a g a i n i n d i c a t e t h a t t h e growth o f t h e new f o m o f hydrogen suppress t h e a d s o r p t i o n on t h e m e t a l l i c phase. Moreover, o u t g a s s i n g a t 773K f o l l o w e d by H2 a d s o r p t i o n a t 300K a t any stage between two c o n s e c u t i v e r u n s i n t h i s f i g u r e reproduces t h e o r i g i n a l TPD p r o f i l e , which i s s i m i l a r t o that of P t / S i 0 2 b u t w i t h o n l y ca. 45% o f i t s i n t e n s i t y , i n d i c a t i n g a g a i n a p a r t i a l r e c o v e r y o f t h e c a p a c i t y f o r H2 a d s o r p t i o n a f t e r removal o f t h e new forms o f hydrogen i n c o r p o r a t e d t o t h e reduced T i 0 2 .

432

TPD of hydrogen from a P t / SiO +Ti0 (1:3) mixture: a ) feducgd a t 773K and o u t gassed a t the same temperat u r e before H adsorption a t 300K. b - d ) reguced a t 773K during different periods of time and then cooled down a t 300K in H2.Right: difference TPD s p e c r Y

373

TIK

573

773

n m u

TiOx-layer thickness and SMSI r e v e r s i b i l i t y Fig.6 shows characteristic XPS/Ar+-sputtering profiles f o r samples H and H(H20). Reduction of the precursor e i t h e r a t 473 or 773K (samples H and L ) does not produce changes in the original intensity of t h e P t ( 4 f ) signal indicating a grea, dispersion b u t i t decreases by ca. 90% f o r sample H(H20). Ar+-sputtering produces a sharp i n i t i a l decay of the intensity in sample H , thus confirming a h i g h dispersion in t h i s case, while f o r sample H ( H 2 0 ) i t leads t o a progressive increase t o recover up t o ca. 75% of the original intensity a f t e r 15 min, suggesting an encapsulation of the metallic p a r t i c l e s ( 1 1 ) in t h i s sample. When the two samples, deeply reduced by d i f f e r e n t i a l Ar+-etching (O/Ti ca. 1.55), are rn treated in H2 ( b u t not in He) a t 773K, encapsulation again occurs as shown by a new Ar+-sputtering. This f a c t confirms our previous conclusions ( 6 ) on the role 01 hydrogen as promoter of T i O x migration.

I

I

0

2

4

I

I

6

I

0

I

~

1

1

1

2

A? sputtering (min)

1

u

'

~

,

X

m 0m 1

I

I

V

I

2

3

4

s

A? sputtering (mid

Fig.6. XPS/Ar+-sputtering profiles: a ] Pt/Ti02-H; b ) Pt/Ti02-H(H20).

433

300 K b)

473 K

cl

473 K

dl

81

77

73

Binding

Fig.7.

69

65

81

E n e r g y (eV)

77

73

Binding

69 65 Energy(eV1

E f f e c t o f t h e r e o x i d a t i o n i n t h e P t ( 4 f ) XPS-spectrum o f Pt/Ti02-H(H20) before ( l e f t ) and a f t e r A r - e t c h i n g ( r i g h t ) .

XPS spectra i n Fig.7 c o n f i r m t h a t t h e p l a t i n u m i n sample H(H20) remains i n a reduced s t a t e a f t e r storage i n a i r and i t cannot be o x i d i z e d a t 673K though i t does p a r t i a l l y even a t 300K, once t h e t h i c k T i O x overlayer have been removed by Art-etching.

This c o n t r a s t s w i t h s i m i l a r experiments e i t h e r w i t h samples L o r H,

which show a complete o x i d a t i o n o f t h e metal a t 673K before Ar+-etching. DISCUSSION

Since t h e t h r e e c a t a l y s t s s t u d i e d here show very s i m i l a r s t r u c t u r a l and t e x t u r a l c h a r a c t e r i s t i c s a comparative study seems o f i n t e r e s t t o t r y t o understand t h e o r i g i n and t h e mechanism o f generation o f t h e SMSI s t a t e i n P t / T i 0 2 . Data-for hydrogen adsorption i n Table 2 i n d i c a t e t h a t sample H shows a conventional SMSI s t a t e which i s p a r t i a l l y reversed upon outgassing a t 773K o r exposure a t 300K t o t h e a i r and, almost completely, by r e o x i d a t i o n a t 673K. However, sample H(H20) shows an i r r e v e r s i b l e SMSI-like s t a t e even f o r t h e most d r a s t i c c o n d i t i o n s norm a l l y used t o regenerate t h e non-SMSI s t a t e ( i . e .

02, 673K), i n d i c a t i n g t h a t this

sample i s i n a d i f f e r e n t s t a t e from what i s normally described i n t h e l i t e r a t u r e as SMSI. I n f a c t , XPS/Ar+-sputtering experiments show t h a t a t h i c k Ti0,-overlayer must cover t h e b i g m e t a l l i c p a r t i c l e s , h i n d e r i n g H2 adsorption, and t h a t t h i s l a y e r can n o t be removed even by r e o x i d a t i o n a t 673K.

A simple c a l c u l a t i o n using

t h e model given by Kerkhof e t a1 (12) f o r t h e decay i n XPS i n t e n s i t i e s going fran a m e t a l l i c monoatomic l a y e r t o metal supported p a r t i c l e s w i t h d i f f e r e n t sizes i n d i c a t e s t h a t f o r P t p a r t i c l e s o f ca. 7 nm, 75% o f t h e i n i t i a l i n t e n s i t y o f t h e P t ( 4 f ) peaks would be l o s s . So t h e observed decrease (ca. 90%) should be ascribed t o a f u r t h e r a t t e n u a t i o n o f t h e s i g n a l due t o t h e decoration l a y e r .

A rough e s t i -

mation o f i t s average thickness, t a k i n g t h e mean f r e e path f o r t h e P t ( 4 f ) e l e c -

434

trons through the titanium dioxide layer 2.62 nm (131, gives a value o f ca. 2.7 nm i n good agreement w i t h those recently observed by Wang e t a1 (7) using HREM on P t p a r t i c l e s o f a s i m i l a r size. On the other hand, hydrogen adsorption data i n Fig.3 f o r t h e precursor reduced a t increasing temperatures i n t h e 473-773K range c l e a r l y i n d i c a t e t h a t normal RSI state steadily increases reaching an almost complete loss o f the capac i t y f o r hydrogen adsorption a f t e r reduction a t 773K f o r ca. 3h. Meanwhile, TPD experiments i n Fig.1 f o r a progressively reduced Pt/Ti02 precursor under rather s i m i l a r conditions suggest t h a t generation o f t h i s SMSI state i s r e l a t e d t o the development o f the new activated forms of adsorbed hydrogen on t h e reduced samples. These can be i d e n t i f i e d , from our TPD data f o r Ti02, Pt/Si02 and t h e i r mechanical mixture, as hydrogen incorporated i n t o the reduced Ti02 support, and probably on the decorating TiO, moieties (see below). I n fact, from those data we can conclude t h a t both, exposed P t p a r t i c l e s and Ti02, are necessary t o gener a t e the new activated forms o f adsorbed hydrogen, which probably involves hydrogen s p i l l o v e r from the metal (14). So, the small amount o f such species f o r sample H(H20) i n Fig.4 and t h e i r loss a f t e r t h e f i r s t set o f TPD-runs confirm an almost complete encapsulation o f a l l the platinum i n t h i s sample. As we have previously found f o r Rh/Ti02 (2,3,11)

s p i l l o v e r o f hydrogen read i l y occurs i n these systems enhancing t h e reduction o f the support a t r e l a t i v e l y low temperatures as could be detected by t h e growing Ti3+ signal i n EPR. However, a t T*573K a decrease i n i t s i n t e n s i t y i s observed what has been explained assuming the formation o f diamagnetic hydride-like species ( i .e. TiH3+), though a residual almost sylnnetric Ti3+ signal, probably due t o extended oxygen vacani n t e r a c t i n g with hydrogen, s t i l l remains. Such hydride-l'lke cies ( i .e. (TiV,)?) species, probably i n t h e neighbourhood o f t h e m e t a l l i c particles, can be removed by outgassing a t Ta-573K regenerating the c h a r a c t e r i s t i c EPR signal due t o paramagnetic, low coordinated Ti3+ centers. Moreover, f o r both, Rh/Ti02 and Pt/Ti02, we have recently found (6,111 t h a t hydrogen i n t e r a c t i o n a t 773K w i t h samples deeply pre-reduced by d i f f e r e n t i a l Ar+-etching leads t o decoration/encapsulatian o f the m e t a l l i c p a r t i c l e s with migrating TiO, moieties producing overlayers 1 nm thick. These f a c t s can be now r e l a t e d t o t h e detection by TPD here o f d i f ferent activated forms o f hydrogen incorporated i n t o t h e progressively reduced Pt/Ti02 precursor. F i r s t a t a l l , i t should be noted t h a t these forms are generated i n a d i f f e r e n t way depending on t h e sample. So, while the sharp peak a t ca. 513K i s hardly observed i n t h e reduced Pt/Ti02 precursor i t i s t h e main form observed i n t h e Pt/Si02+Ti02 mixture reduced a t 773K f o r lh. On t h e contrar y the shoulder a t 693K which appears j u s t a f t e r reduction a t 573K i n the Pt/Ti02 precursor i s t h e l a t t e r t o be detected i n t h e mechanical mixture. A t e n t a t i v e explanation of these differences can be given by assuming t h a t t h e sharp peak

435

a t ca. 513K i s due t o i s o l a t e d diamagnetic T i H 3 + s p e c i e s formed under m i l d c o n d i t i o n s b y i n c o r p o r a t i o n o f hydrogen t o i s o l a t e d l o w c o o r d i n a t e d Ti3'

sites (i.e.

T i V3+). F u r t h e r r e d u c t i o n would produce extended oxygen vacancies (i.e. (TiV,)?) 0

-

e i t h e r d i r e c t l y o r by reactions o f t h e type: 2 TiH3+ + T i 0

9t H20 + ( T i V o I 3

/1/

where TiH3+ and Ti0 r e p r e s e n t h e r e f u l l y c o o r d i n a t e d T i 3 + and T i 4 + s p e c i e s r e s p e c t i v e l y . These vacancies, a l s o i n c o r p o r a t e hydrogen g e n e r a t i n g t h e broader s h o u l d e r a t 693K i n t h e TPD o f t h e P t / T i 0 2 p r e c u r s o r reduced a t 573K, t h e onset f o r t h e decrease o f t h e i n t e n s i t y o f t h e broad EPR T i 3 + s i g n a l s (3,111.

At still

h i g h e r r e d u c t i o n temperatures ( i .e. 67310 d e c o r a t i o n o f t h e p a r t i c l e s w i t h m i g r a t i n g T i 0 2 o r H T i O x m o i e t i e s s t a r t l e a d i n g t o t h e b r o a d TPD-peak a t ca. 583K until complete e n c a p s u l a t i o n o f t h e p a r t i c l e s t a k e s p l a c e . When t h i s o c c u r s f u r t h e r i n c o r p o r a t i o n o f hydrogen would decay due t o t h e l a c k o f metal exposed. So i n sample H(H20), where most o f t h e P t seems i n i t i a l l y covered by a t h i c k overlayer, o n l y a s m a l l amount o f t h e s e forms i s generated d i s a p p e a r i n g a f t e r t h e f i r s t s e t o f TPO-runs,

p r o b a b l y due t o a complete b u r i a l of t h e m e t a l , t h e new s i t u a t i o n

b e i n g i r r e v e r s i b l e i n t h e c o n d i t i o n s n o r m a l l y used t o r e s t o r e t h e non-SMSI s t a t e

(i.e. 02, 673K and r e - r e d u c t i o n a t 473K). Partial

SMSI r e v e r s i b i l i t y b y o u t g a s s i n g a t 773K has been p r e v i o u s l y reported

by us (4,5) i n Rh/Ti02 and r e l a t e d t o t h e removal o f t h e h y d r i d e - l i k e species i n c o r p o r a t e d i n t o t h e s u p p o r t and T i o x d e c o r a t i n g overlayer. From F i g . 3 w e c a n e s t i m a t e t h a t t h e percentage o f such SMSI r e v e r s i b i l i t y i s h i g h e r t h e lower t h e r e d u c t i o n t e m p e r a t u r e . So, w h i l e a t 623K i t r e p r e s e n t s ca.30% o f t h e whole SMSI a t t a i n e d , a t 773K i t o n l y accounts f o r ca.15%. T h i s can be e x p l a i n e d i f TiO,

m o i e t i e s incor-

p o r a t i n g hydrogen ( i .e. tlTiOx) i n i t i a l l y f o r m small patches d e c o r a t i n g t h e metal l i c s u r f a c e w i t h a s t r o n g " e l e c t r o n i c i n t e r a c t i o n " w i t h t h e m e t a l , which i s p a r t i a l l y l o s t when hydrogen i s removed by o u t g a s s i n g a t 773K. T h i s r e s t o r e s t h e c a p a c i t y f o r n o n - a c t i v a t e d a d s o r p t i o n o f H2 on t h e f r e e m e t a l l i c s u r f a c e though Ti0

X

m o i e t i e s s t i l l would remain on t h e s u r f a c e b l o c k i n g t h e m e t a l l i c atoms under

neath. T h i s e f f e c t suggests a l o n g range T i - H - P t i n t e r a c t i o n ( " e l e c t r o n i c effect") though changes i n w e t t i n g , and t h e r e f o r e i n t h e e x t e n d o f t h e i n t e r f a c e , upon hydrogen i n c o r p o r a t i o n can n o t be r u l e d o u t . However, t h e above r e v e r s i b i l i t y o f t h e SMSI s t a t e would disappear when t h e TiOx/HTiOx m o i e t i e s f o r m a complete monolayer on the m e t a l l i c p a r t i c l e s , so p h y s i c a l b l o c k i n g w i l l r e s u l t i n t h i s case even a f t e r outgassing t o remove the h y d r i d e l i k e s p e c i e s f r o m t h i s t h i n f i l m as we h a v e r e c e n t l y r e p o r t e d f o r Rh/Ti02 ( 1 5 ) . Once a t t a i n e d t h i s s i t u a t i o n l o n g e r r e d u c t i o n p e r i o d s and/or h i g h e r r e d u c t i o n t e m p e r a t u r e s w o u l d p r o d u c e e i t h e r a deeper r e d u c t i o n o f t h e t h i n T i O x o v e r l a y e r ( p r o b a b l y i n v o l v i n g t h e h y d r i d e l i k e species t h r o u g h r e a c t i o n s o f t h e t y p e g i v e n by eqn.1 above) and/or f u r t h e r m i g r a t i o n o f TiOx/HTiOx m o i e t i e s t o

436

to built up athicker TiOx multilayer depending on the actual reduction conditias as stated in the scheme below:

**w

eYUWLArlO**

in the former case a PtTi-alloyed film could be finally generated at the surface (growing toward the bulk), while in the second case, irreversible encapsulation of the metal will occur. The partial pressure of water, necessarily present due to its generation during reduction with hydrogen, being the keystone vhich should drive the whole process in one or another direction. ACKNOWLEDGEMENT We thank the CAICYT (project 0230/84) and the CSIC (project 552) for finanti a1 support. REFERENCES 1 J.C.Conesa, P.Malet, G.Munuera, J.Sanz, J.Soria, J.Phys.Chern., 88 (1984) 2986 2 J.Sanz, J.M.Rojo, J.Phys.Chem., 89 (1985) 4974 3 J .C .Conesa, P.Malet , A.Muiioz , G.Munuera, M.T.Sainz , J. Sanz, J. Sori a, Proc. 8th Int.Congr.Cata1. West Berlin, 1984, 5, p. 217 4

J.Sanz. J.M.Rojo, P.Malet, G.Munuera, M.T.Blasco, Chern. 89 (1985j 5427

.

J.C.Conesa, &Soria, J.Phys.

5 A.Muiioz, A.R.Gonzalez-El i pel G .Munuera, J P.Espi nos , V. Ri ves-Arnau, Spectrochi

-

mica Acta 43A (1987) 1599 6 G.Munuera, A.R.Gonzalez-Elipe, J.P.Espinos, J.C.Conesa, J.Soria, J.C.Sanz, J.Phys.Chem. 91 (1987)6625 7 L.Wang, G.W.Qiao, H.G.Ye, K.H.Kuo, Y.X.Chen, Proc. 9th Int.Congr.Cata1. Calgary, 1988, 3, P. 1253 Eds. M.J.Philli s and M.Ternan 8 J .M.Herrmann, M.Grave1 le-Rwau-Mai 1 lot, !.C .Gravel le, J. Catal .lo4 (1987) 104 9 J.R.Anderson in "Structure of Metallic Catalysts" Academic Press, London,1975 10 A.Frennet, P.B.Wells, Appl. Catal. 18 (1985) 243 1 1 A.R.Gonza1 ez-El i pel G.Munuera, J. P.Espinos, J .Soria, J. C.Conesa, J .Sam, Proc. 9th Int.Congr.Catal., Calgary, 1988, 3, 1392 Eds.M.J.Phil1ipsandM.Tetm 12 F.P.J.Kerkhof, J.A.Moujlin, J.Phys.Chem. 83 (1979) 1612 13 M.P.Seah, W.A.Dench, Surf.Interf.Ana1. 1 (1979) 2 14 J. C.Conesa, J.Sori a, J .M.Rojo, J.Sanz, G.Munuera, Z. Phys .Chem. (NF) 152 (1 987) a3 15 M.T. 81asco, J .C .Conesa, J. Sori a, A.R.Gonta1 ez-El i pel G.Munuera, J .M. Rojo , J.Sanz, J.Phys.Chem. 92 (1988) 4685

C. Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactivity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

AN

437

INFRARED STUDY OF CO ADSORBED ON A Ru/ZnO CATALYST.

E. Guglielminotti and F. Boccuzzi Dipartimento di Chimica Inorganica,Chimica Fisica e Chimica dei Materiali Universita di Torino, via P. Giuria 7 ,Torino (ITALY)

Abstract The Ru/ZnO (0.5% Ru) system, prepared by decomposition of R U ~ ( C O )on~ ~ ZnO, was studied. The IR s ectrum of CO adsorbed on H2 reduced samples gives evidence of Rug( CO) (x,n=2,3 surface com lexes. After heating at 423-523 K the CO further reduces the RuX' ( CO) complexes. The spectrum of CO adsorbed after this freatment can be interpreted in term of linear RuO-CO ( 3 = 2 0 0 0 cm) on very small cluste s and Ru-CO bridged anionic surface species ( v =1900-1820 cm-f). The formation of the last species, unusual on Ru, is favored in this case by an electron transfer from the reduced n-ZnO semiconductor to the small Ru particles.

Introduction It is well known that supported metal catalysts, prepared by impregnation and decomposition of a transition metal carbonyl in vacuo and at mild temperature conditions, are in general different in comparison to those prepared by a salt solution impregnation. In particular at low metal loading ( 5 1%) the interaction between che carbonyl and the surface ligands of the support such as the hydroxyl groups leads to the formation of metal ions which are difficult to reduce completely. Typical examples are the supported Rh catalysts obtained from Rh4 and Rh6 carbonyls showing, in CO atmosphere, EXAFS and IR evidences of species Rh1+(CO)2 (1-4). Ru samples, supported on Si02 and A1203, obtained by R U ~ ( C O ) impregnation ~~ in presence of 0 2 , show at 300-473 i( =oxidative interaction with the hydroxyl

438

groups, with

formation of

RuX+(CO),(x,n=2,3)

surface species

(11215t6).

Moreover, if reducibles oxides as Ti02 or ZnO are used as a support, an electron transfer from the support to the metal can occurs (7,8,9). As a consequence a change of the chemical properties of the surface metal atoms is expected, mainly on very small supported metal clusters where nearly all the metal atoms are in contact with the support. On these supports, in the strongest conditions of reduction, effects of coating or alloying between the metal and the reduced metal oxide support can occur (8,9). In this paper we present IR and microgravimetric data on CO adsorption on a Ru/ZnO system prepared by Ru3(C0Il2. Some preliminar CO adsorption results on the same kind of sample were reported in a previous papertlo).

Experimental A ZnO Kadox 25 powder (BET area 10 m2.ge1) was impregnated with a pentane solution of Ru3(C0Il2 and the solvent evaporated at 323 K. The sample,containing 0.5% Ru in weight was pressed in pellets and then decarbonylated in vacuo at 473 K in the IR cell. Then the samples were submitted to alternate oxidation and evacuation treatments at 623 K. Finally the samples were reduced with a 3% H2/Nz mixture and outgassed at the same temperature to f u l l y remove carbonyl groups and the products of reduction. The IR spectra are recorded at RT with a Perkin Elmer 580 B instrument interfaced with a 3600 Data Station. In all the spectra reported in the figures the background have been subtracted. Gravimetric data of CO and O2 adsorption are obtained with a Sartorius 4102 microbalance. A TEM analysis of freshly reduced samples was attempted with a Jeol 200 HREM: in spite of the high magnification ( * 380,000) it was impossible to have a clear evidence of the presence of Ru particles dispersed on ZnO. Only some surface roughness of ZnO in comparison with pure ZnO gives an indication of the possible presence of very small Ru clusters partially coated by ZnO.

439

Results and discussion At the end of the preliminar treatment described in the experimental section no residual bands , due to carbonyl species, are present in the IR spectra. Fig la, curve 1 show the IR spectrum of CO adsorbed on a reduced sample and, for comparison, the spectrum of R U ~ ( C O ) ~ ~ / Z ~ O impregnated sample few hours after the preparation (dotted line). The two spectra are quite similar: the first one shows an absorption with a maximum at 2065 cm-l,a quite strong shoulder at ~2000-1990cm-l and a very weak one at 2115 cm-l;in the second case a couple of intense peaks at 2076 and 1988 cm-l with a small shoulder at 2125 cm-l is visible. In hexane solution the carbonyl compound shows IR modes at 2061s(E'),2031m(A"),2018vw(E'),and 2012w(E') cm-l (11),quite different as number and frequency from the previously illustrated bands. This fact indicates that the carbonyl compound immediately change drastically by adsorption and

I

2ooo c m l Fig.

la00

I 2200

I

I

cm-1

I800

1 IR spectra of CO adsorption and desorption: , few hours after impregnation a) dotted line, Ru3(COl1 in air; curve 1, I f0 Torr of CO adsorbed at RT on a H2 reduced sample; curve 2,-0-, after heating 20 min in 10 Torr of CO at 423 K; b) curve after outgassing 20 min at 423 K; curve & the same at 473 K; curve --, the same at 523 K.

-

then during the treatments there are only small changes. Literature data of CO adsorbed on supported Ru obtained from RU~(CO) decomposition ~~ (1,2,12) indicate that at low Ru loading ( S 1%) the prevailing mechanism is the anchoring of the carbonyl to the support through an oxidative attack of hydroxyls already at RT in presence of oxygen; after decarbonylation in vacuo at 373473 K the formation of ( Ru(COb2 )n (n=1-3) species linked to oxygen ions was found on silica, alumina and basic supports (1,2,13). By comparison with analogous peaks found for Ru carbonyls heterogenized on different supports we can assign the bands at i065 and 1990 cm-l to an R u ~ + ( C O )complex ~ where the original Ruthenium(0) cluster is oxidized by interaction with the surface hydroxyls of the support in presence of the water and oxygen of the atmosphere. The thermal decomposition of these oxidized complexes in vacuo and in H2 do not leads to a fully reduced Ruo sample probably for the difficulty to fully eliminate the hydroxyls formed on the support during the reduction at 623 K. Heating in CO atmosphere at 423 K (Fig la, curve 2) produces a strong increase of a component at 2000 cm-l, a decrease of the 2115 cm-' band and a small increase of the bands at = 2065 cm-' and at= 1990 cm-l; a broad absorption,very weak at RT appears at 1920-1900 cm-l. At the same time a lowering in the overall transparency is observed. After CO desorption under vacuum at the same temperature (fig.lb, the 2065 cm is slightly red-shifted and strongly curve - ) reduced in intensity together with the 1990 cm-I shoulder, while the 2000 cm-l band is fixed in frequency and becomes prevailing in the spectrum. At the same temperature a quite strong absorption is present at= 1915-1900 cm-I and a weak band appears at 1820 cmAfter outgassing at 473 K (fig-lb, curve A )the 2060 cm-' band is almost depleted,the 2000 and 1915-1900 cm-' bands remain unchanged in frequency and slightly changed in intensity, whereas the 1820 cm-' further grows in intensity. Finally an outgassing of 20 min at 523 K leaves only quite weak bands at 2000,1900 and 1820 cm'l (figlb, curve- - -1. In conclusion, in the temperature range 423-523 K at least two bands are present at frequencies lower than 2000 cm-l,i.e. at 1915-1900 and at 1820 cm-l; the assignment of these bands can be made with the help of spectral data found on Ru homogeneous and

-'

441

heterogeneous complexes. In homogeneous Ru complexes bands at so low frequency were found for anionic bridged carbonyl complexes as for electrochemically n y anionic l R U ~ ( C O ) ~ ~H2Ru4(C0)12 ~-, 2reduced R ~ ~ ( C O ) ~ ~ - c a r b oor and [Cp R u ( C O ) ~ ] ~species (14). In heterogeneous complexes weak bands at 1910-1875 cm-' and at 1873 cm-l were found for CO adsorbed on R U ~ ( C O )decomposed ~~ on MgO (12) and Ti02 (6) and are assigned to bridged species; a band at 1950 cm-' was found for CO on alkali-doped Ru/A1203 (15). We assign the bands at 1900 and 1820 cm-l to CO bridged on negatively charged small Ru clusters. The formation of these species can be favoured in our samples by the high Ru dispersion due to the preparation ex-carbonyl. In particular on the basis of literature data (14) ,we can assign the 1820 cm-l band to a Ru2(C0I6- bridged structure and the bands at 1915-1900 cm-l to semibridged species: both these species ,are formed on very small clusters. The formation of surface anionic bridged species in our experimental conditions is probably a consequence of a surface reduction of the ZnO in CO atmosphere at 423K. This reduction is testified by two different experimental data: the loss in IR transparency and the weigth loss in the microgravimetric experiments (see Table I).

Table I Adsorbed Gas

co co co co co co 02

H2

* The loss in ** Assuming a

Sample treatments H2red., CO adsorbed at

RT 423K desorbed at 423K II II I1 473K 11 523K CO red, outgassed at 573K H2 red.,02 adsorbed at RT 11 11 I1 II H2 II

11

II

I1

II

II

moles ads.gas/Ru 0.49 0.33 0.26 0.05 -l.Ol*(O/RU) 0.33 0.47 ** =>2(spillover)

weigth corresponds to the desorption of 1 0 for Ru dissociative O2 chemisorption (16).

442

The IR transparency loss indicates that during this kind of treatment oxygen vacancies are produced on ZnO leaving electrons in shallow donor levels (10) that can be partially delocalized on the Ru small particles. Nevertheless bridged CO species on Ru(1010) surface have been recently observed in EELS experiments (17); therefore structural effects of stabilization o f this kind of species on more open Ru surface sites cannot be ruled out. Finally, we discuss the -2000 cm-I band. This peak,fixed in frequency and not shifting with the coverage as expected for CO adsorbed on extended Ru face (181,is assigned for its spectral position and general behaviour to CO linearly adsorbed on Ruo very small clusters ( < 10 A ) . A value of 2004 cm-l was in fact calculated for an hexagonal CO island of 10 A diameter on Ru (001) face (18). The T a b l e I1 summarizes these assignments.

Tablo ll

Assignmant

Fraquency range(cni’)

Ru~+(CO$(~,

2140 -2110

Ru2*(C0b

2075- 2060

St r uct u re

co

a Ruo-CO n R~~-CO R+-CO

- m85 - x)60 - 2000 - 1980

2055

(em,,) - mo

m

(8-0)

1915

- I900

1820

RuC(C0)

co

Ru<Eg

Linear CO on Ruo C ,Q. RuL-

.‘Ru

HCY

Ru ( - 1

Ru

c?( ”t i I tad,,CO

4585

/ Ru

ZnS’

443

The quantitative microgravimetric data of CO adsorption on reduced samples, are in quite good agreement with the spectroscopic results. In particular,the molar ratio CO/Ru =0.49 indicates a rather high Ru dispersion. This result is also confirmed by the adsorption of Oxygen (O/Ru =0.47 ) assuming a dissociative adsorption of this gas (16). These data indicate that nearly half of the Ru atoms are exposed at the surface and therefore that the Ru crystallites are quite smal1,so justifying the fact that HRTEM experiments give no evidence of Ru crystals and that the spectroscopic features are more similar to those of clusters than to those of extended metal surfaces. As for the CO desorption results at increasing temperatures, it is noticeable the loss in weight of the sample after CO desorption at 573 K, versus the weight before CO adsorption. The loss of wheight corresponds quite exactly to the desorption of 1 0 atom for 1 Ru atom. This datum can be explained by a sample reduction ,induced by CO itself, during the desorption at 473-573 K with elimination of C02. This fact indicates that the reductive power of CO is greater than that of the H2 mixtures. A similar phenomenon was observed by Spencer on the Cu/ZnO system, who showed that the reducing activity of a CO/CO2 mixture is two order of magnitude greater than the H2/H20 mixture, at 523-573 K (19). After CO desorption at 573 K the amount of readsorbed CO is reduced to ~0.33 CO/Ru, the readsorption of CO at RT gives also a spectrum (Fig 2, curve a) which is different from that reported in Fig la, curve 1: the predominant band in the spectrum is here at 2052 cm-l with weak shoulders at 2068 and 2112 cm-I at higher frequency and at 2007 and 1915 cm-I at lower frequency. The main band shows also a shift towards lower frequency during the desorption: it shifts from 2052 to 2007 cm-I ( fig.2 curve b) .This behaviour is typical of a coverage dependent shift due to dipole-dipole and chemical effects occurring on quite extended Ru surfaces (18,20) ( v= 2055 cm-l at 8=1, v= 1990 cm-I at 6=0 1 . CO readsorption at full coverage restores the initial frequency of CO ( 2050 cm-l, fig.2, curve c).

444

2052

Q4A

0.2-

t

Fig. 2

I

I

I

1

IE spectra of CO adsorption on CO reduced sample: curve a) 40 Torr of CO adsorbed at RT; curve b) after 20 min outgassing at 473 K; curve c) 40 Torr of CO readsorbed on b).

This kind of spectrum is more similar to those of other supported Ra sample. In fact, besides the residual presence of a couple of

weak bands at 2112 and 2070 cm-l, assigned to Ru3+(C0l2 ,the frequency of the prevailing band is at 2052 cm-l. Therefore the hzating in CO atmosphere at 423 K and the subsequent outgassing at 5 7 3 K leads to a Ru agglomeration process and/or to the formation of carbides, revealed also by the decreased amount of adsorbed CO on this kind of sample (CO/Ru=0.33 instead of 0.49). Besides weak bands at 2000 cm-l, 1900 and 1820 cm'l give evidence of some residual small Ru clusters. A strong absorption occurs also at 1585 cm-' together with weak bands at 1530 and 1345 cm-', the last two ones assigned to carbonate groups on ZnO. An outgassing at 473 K leaves only a peak at 2007 cm'l and weak components at 1915-1900 and 1820 cm-' (fig.2 ,zurve b). The intense band at 1585 cm-l, present also with minor intensity on H2 reduced samples, does not correlate to other bands and could be assigned to a single CO group tilted toward a

445

Zn ion at the borderline Ru-ZnO phase. A four electron donor carbonyl species, Ru/C=o+Zn2+ , in analogy with that found on a Cu/ZnO catalyst (2l),could account for the spectral and thermal behaviour of this species; bands at 1620 and 1580 cm-l,with a similar assignment were found for Zn doped Rh/Si02 system (22).

Conclusions The preparation of a Ru/ZnO system e x - R ~ ~ ( C 0gives ) ~ ~ a dispersed Ru phase strongly interacting with the support. CO adsorption evidences the presence of oxidized Ru species even after reducing treatments in H2. Thermal treatments in CO at 423-523 K show however the effective reducing ability of this gas with formation of reduced Ru particles. In these conditions unusual Ru2C0 bridged groups ( V co= 1915-1900 and 1820 cm-l) are observed. Besidesfafter CO evacuation at 523 K the full Ru reduction process is accompanied by agglomeration and formation of more extended Ru particles.

Acknowledgements The work was supported by the Italian C.N.R. 'I Progetto Finalizzato Materiali e Dispositivi per 1'Elettronica a Stato Solido". We also thank dr. A. Reller and dr. D. Scarano for the electron microscopy experiments.

References 1 2 3 4 5 6

R. Psaro and R. Ugo, in B. C. Gates, L. Guczi and H. Knozinger (Eds.), Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986, pp 427-496 and references therein. Y. Iwasawa, Adv. Cata1.,35 (1987) 187-264. K. Asakura, Y.Iwasawa and H.Kuroda, Bull. Chem. SOC. Jpn., 59 (1986) 647 J. Evans and G. S. Mc Nulty, J. C. S . Dalton Trans. (19841, 587 G. M. Zanderighi, C. Dossi, R. Ugo, R. Psaro, A . Theolier, A. Choplin, L. D'Ornelas and J. M. Basset, J. Organomet. Chem., 296 (1985) 127 and references therein. a) J. Evans and G. S. Mc Nulty, J. C. S. Dalton Trans., (1984) 1123 b) V. D. Alexiev, N. Binsted, J. Evans, G. Neville Greaves and R. J. Price , J. C. S. Chem. Corn., (1987) 395

7

8

9 10 11 12

13 14 15 16

17 18

19 20

21

22

a) J. R. Katzer, A . W. Sleight, P. Gajardo, J. B. Michel, E. C. Gleason and S. Mac Millan, Faraday Disc. Chem.Soc., 72 (1981) 121 b) H. R. Sadeghi and V. E. Henrich, J. Catal.,lO9 (1988) 1 L. Wenzhao, c. Yixuak Yu Chunying, W. Xiangzhen, H. Zupei and W. Zhaobin, Proc. 8 Intern. Congr. Catalysis, Berlin vol.V, Dechema, Frankfurt,(l984) p. 205 S. J. Tauster, S. C. Fung and R. L. Garten, J. Amer. Chem. Soc.,100 (1978) 170 E. Guglielminotti, F. Boccuzzi, G. Ghiotti and A . Chiorino, Surface Sci., 189/190 (1987) 331 G. A. Battiston, G. Bor, U. K. Dietler, S . F. A . Kettle, R. Rossetti, G. Sbrignadello and P.L. Stanghellini, Inorg. Chem.,l9 (1980) 1961 E. Guglielminottl, Langmuir, 2 (1986) 812 V. L. Kuznetsov, A. T. Bell and Y. I. Ermakov, J. Cata1.,65 (1980) 374 M. I. Bruce, Coord. Chem. Rev. ,76 (1987) 1-43 and references therein M. M. Mc Laughlin Mc Clory and R. D. Gonzalez, J. Cata1.,89 (1984) 392 K. C. Taylor, J. Cata1.,38 (1975) 299 G. Lauth, K. Christmann, T. Solomun and W. Hirshwald, EcosslO, 5-8 Sept.,1988 (Bologna), ECA 121, B33 H. Pfnur, D. Menzel, F. M. Hoffmann, A . Ortega and A . M. Bradshaw, Surface Sci., 93 (1980) 431 M. S. Spencer, Surface Sci.,192 (1987) 3 2 3 E. Guglielminotti, G. Spoto and A . Zecchina, Surface Sci.,155 (1985) 1 3 2 G. Ghiotti, F. Boccuzzi and A. Chiorino, J. C. S. Chem.Com., (1985) 1012 W. M. H. Sachtler and M. Ichikawa, J. Phys. Chem.,90 (1986) 4752


447

THE CONCEPT OF STRUCTURE-SENSITI VlTY IN CATALYSIS BY OXIDES J. HABER Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow (Poland)

ABSTRACT Crystallites of transitlon metal oxides exhibit t w o types of crystal facesthose w i t h a l l atoms chemically saturated but exposing metal cations w i t h empty d-orbitals which are LUMOs and free electron pairs of oxide ions which are HOPlOs, and those which are composed of coordinatively unsaturated cations and anions, whereat excess charges are ,accumulated. Their catalytic properties d i f f e r what results in pronounced structure-sensitivity of the oxidation reactions, the type of products depending on the habit of crystallites. Oxide surfaces are i n dynamic interactions w i t h the gas phase and changes of latter’s composition may induce changes i n composition and structure of the outermost layers of the oxide, generating new catalytic properties. This may result in induced structure-sensitivity. Properties of Moo3 and V205 crystallites are used as examoles. INTRODUCTION Very early in catalytic studies attention was drawn to the importance of surface structure o f the solid catalyst for i t s catalytic behaviour, the first examples being the adlineation theory of Schwab ( 1 ) and Balandin’s theory of multiplets (2).In the sixties a considerable volume of experimental data accumulated indicating that i n the case of metal catalysts the catalytic a c t i v i t y depends on the surface structure, which varies when different crystal planes are being exposed on changing the crystal habit, when imperfections in form of steps and kinks are generated at the surface or when the particle size i s varied in the c r i t i c a l ramge between 1 and 10 nm. In order t o rationalize these observations Boudart (3,4) introduced the concept of structure sens/tivf(j( the reactions o f hydrocarbons and hydrogen on metal catalysts being devided into t w o types: structure insensitive, whose rate depends very l i t t l e on such parameters as crystalline orientation, presence of surface defects or alloying, and structure sensitive, which are strongly affected by variation of these parameters. It is obvious that the notion of structure sensitivity must be closely related t o the conceDt of active sites based on the observation that i t i s not

448

the whole surface area of the solid, which is involved in the catalytic reaction, but only i t s very small fraction called catalytica//y active or simply cafa/r.tre surface area. As pointed out recently by Carberry ( 5 ) the issue of observed structure sensitivlty or insensitivity may rest upon the methodology whereby this catal~t~c area Is determined. The same reaction when related t o the total surface area may exhibit the apparent structure sensitivity whereas when only catalytically active sites are taken into account it may aopear to be structure insensitive. The question of the importance of the properties related t o ' t h e geometrical structure of the surface for the catalytic activity of non-metallic solids such as oxides or sulphides has been raised already some time ago. Schuit et a1.(6)'have drawn attention t o the implications of the structure of different crystal faces of bismuth molybdate for I t s catalytic activity i n oxidation of butene, and Farraher and Cossee (7) used the considerations of the geometry of different crystal faces of MoS2 to explain the activating influence of cobalt in hydrodesulphurization catalysts. However, It i s only in the last decade that direct experimental evidence has been accumulated of entirely different catalytic behaviour of various crystal faces of oxide and SUIphide catalysts, some of the results being reviewed by Germain ( 8 ) . It seems thus timely to undertake a discussion of the significance of structure sensitivity concept in relation to catalysis by oxides. ELECTROPHILIC At40 NUCLEOPHILIC OXIDAT ION

Molecular oxygen i n i t s ground state has two unpaired electrons, the ground state is thus a triplet. Because of the rule of spin conservation, reactions between this triplet oxygen and organic molecules which are i n the singlet state experience high activation energies. This symmetry barrier may be eliminated either by activating oxygen, or by activating the organic molecule t o make i t susceptible to the reaction w l t h molecular or atomic oxygen ions (Fig 1 ) When a hydrocarbon molecule i s contacted w i t h the surface of an oxide it may become adsorbed in diffferent forms depending on i t s structure and on the properties of active sites present at the surface of the oxide. These adsorption complexes, corresponding to different types of activation of the hydrocarbon molecule, may then react w i t h different forms of oxygen species in series of parallel and consecutive steps into different products. Molecular oxygen may be activated i n different ways: by excitation to the singlet state or by transfer of electrons to form molecular or atomic !On radicals. All three activated oxygen forms. neutral singlet ' ~and0the~ionic

02- and 0- species are strongly electrophilic reactants, which attack the organic moleculP i n the region of i t s highest electron density (9).Such attack results tn the formation of Deroxo- or superoxo- complexes which decompose

449

into oxygenated products. Along this reaction path epoxides are formed from olefins and acids from aldehydes, or oxyhydration of the double bond takes place resulting i n the formation of respective saturated ketones. At higher temperatures, i n the conditions prevailing in heterogeneous catalytic reaction, the peroxo- arid superoxo- complexes decompose by' s p l i t t i n g the C-C bond, saturated aldehydes and acids being formed from the fragments of the carbon skeleton, or by fission of the aromatic ring, resulting i n the appearance of anhydrides. The aldehydes are often very reactive and easily undergo t o t a l oxldation The second route of heterogeneous oxidation is the reaction w i t h l a t t i c e oxide ion 02-, A t variance w i t h a l l other oxygen species these ions have 'no oxidizing properties, but are nucleophilic reactants, which can perform a nucleophilic addition t o the hydrocarbon molecule a f t e r the l a t t e r has been activated by abstraction of hydrogen and made prone to such addition (10).ln the f i r s t step oxidative dehydrogenation of alkanes and alkenes t o dienes, or the dehydrodimerization and dehyrocyclization may take place. Or, by appropriate activation and bonding of the organic molecule a t the active centre of the catalyst, by which the charge distribution i n the molecule i s determined, the nucleophilic addition of oxygen may be directed t o the selected site in the molecule, resulting i n the formation of an aldehyde or ketone, The consecutive steps of hydfogen abstraction and oxygen addition may be then repeated to obtain more and more Oxygenated molecule, the corresponding number of electrons being transferred into the orbitals of the I

Electrophilic oxygen species

I

-c-c-c7c-

I I

I

I '8

activation

sctlvatfon of hydrocarbon

oxidation of metal t o form oxide lattice

I l l

I

-c-c-c-c-

I

-c-c--c-

I

Nucleophilic oxygen species

I 1 I

-C-C-C-C-O

F I Q 1 t-lechanisrri

I

c-

: I

qf

I 1

thQ nxldatlon O f hydrocarbons

-

1 1 I I I I

-c--C-L-C-oH

450

cations of catalyst l a t t i c e A f t e r the nucleophilic addition of l a t t i c e oxygen ion the oxygenated product is desorbed, leaving a vacancy at the catalyst surface. The vacancies are then f i l l e d w i t h oxygen from the gas phase, simultaneously oxidizing the reduced cations. Incorporation of oxygen from the gas phase into the oxide surface does not necessarlly take place at the same site, wherefrom surface oxygen has been inserted into the hydrocarbon molecule. Therefore, high value of the diffusion coefficient of oxide ions in the l a t t i c e of the catalyst i s often an important condition f o r the catalyst to show high a c t i v i t y ( I 1 ) . Reactions of catalytic oxidation may be thus devided into t w o groups ( 10,121: electrophilic oxidation, proceeding through the activation of oxygen, and nucleophilic oxidation, in which activation of the hydrocarbon molecule i s the f i r s t step, followed by consecutive steps of nucleophilic oxygen addition and hydrogen abstraction. The reaction path actually followed would depend on the type of adsorption complexes of hydrocarbon and oxygen molecules formed at the surface of the oxide catalyst. The surface of an oxide may contain various active sites, differing i n the type of interaction w i t h hydrocarbon molecules ( 1 3). I t may be populated by the OH groups behaving as Bronsted acid or basic sites. The "naked" cations exposed at the surface show Lewis acid properties and transition metal cations act simultaneously as redox sites. Exposed surface oxygen ions are Lewis basic centres, but may also behave as one electron donors. On the surface covered w i t h very weak Bronsted acid s i t e s hydrocarbon molecules become weakly adsorbed due t o hydrogen bonding, usually i n @-position, w i t h surface OH groups (Fig.2). This i s reflected in the s h i f t of the C-H stretching vibration in the ir spectrum of adsorbed molecules (14,15) When the strength of Brgnsted centres i s higher, acid-base interactions begin and finally complete transfer of protons t o the hydrocarbon molecules may take place resulting in the formation of carbonium ions. Disappearnace of the C=C vibrations i n the ir spectrum of olefins is observed and isomerization or other carbonium ions transformations proceed at appropriate temperatures. A t the surface w i t h exposed transition metal ions Ti-complexes are formed by donation of n-electrons of the hydrocarbon t o empty metal d-orbitals and back donation of electrons from the metal orbitals t o the antibonding n*-orbitals of the hydrocarbon. Depending on the extent of these t w o effects different s h i f t s of the C=C vibrations are observed in the ir-spectrum (14). Simultaneously the hydrogen atom of the CH3 group in the @-position t o the multiple bond may interact w i t h the adjacent surface oxide ion. When the basicity of the l a t t e r i s high enough, abstraction of hydrogen takes place and an ally1 group i s formed It may be attached t o the cationic centre side-on w i t h the participation of i t s rr-orbitals ( n - a l l y l ) or vertically w i t h the formation of a metal-carbon d-bond (a-allyl). Using the microwave analysis of the products of propylene-deuterium exchange Kondo e t al.( t 6) showed that on Bi203-t1003 catalyst 6-ally1 predominates at lower temperatures. On the other hand studles of Adams ( 17) using deuterated propylenes and C4-C8

451

olefins, and of Sachtler and de Boer (18) w i t h C I 4 labelled propylenes showed that the symmetric side-on allyiic species i s the intermediate i n the selective oxidation of olefins. Thus, Kondo et al. assumed that a dynamic equilibrium between 6- and n-ally1 exists at the surface and described i t as a dynamic allyl. The n-ally1 species formed by abstraction of hydrogen in the reaction which may be considered as an acid-base process becomes bonded t o the surface cation. Depending on the electronic structure of this cation different charge may accumulate on the allyl. Analysis of the ir spectra indicates that on such oxides as ZnO ( 19) or A1203 the adsorbed allyl i s negatively charged, isomerization via a carbanion mechanism may be thus expected. When no charge appears on the allyl, which than has a radical character, polymerization should be facilitated. ir spectra of allyl species adsorbed at the surface of transition metal oxides show a considerable s h i f t of the C=C stretching freauency and indicate that positively charged moiety i s formed. Quantum chemical calculations revealed (20,21) that depending on the occupancy of it; d-orbitals the oositive charge of the n-ally1 may be accumulated a t the terminal carbon atoms, rendering them prone to the nucleophilic addition of 02- ion. The n-ally1 transforms into an alcoxide species, which after the abstraction of a second hydrogen (22) desorbs as an oxygenated product. A fundamental question may be asked a t this point as t o whether a l l these different types of active sites are unifbrmly distributed at the surface of the catalyst crystallites or each given type of sites i s characteristic for a particular crystal plane. Chemical intuition permits a conjecture that in the case of transition metal oxides w i t h strongly pronounced crystallographic anisotropy different properties of active sites are related t o the differences of surface structure of various crystal planes what results in structure sensitivity of reactions on such catalysts. SURFACE STRUCTURE OF OXIDES The chemlstry of group V,VI and VII transition elements such as vanadium or molybdenum is dominated by the consequencies of the considerable extension of their d-orbitals and positions of the d-electron redox potentials relative t o the anion valence band edge (23). In the octahedral coordination of oxide ions, in which d2sp3 hybridized orbitals are used by the metal to form u' bonds, the remaining dxy, dyz and dxZ orbitals extend far enough to considerably overlap w l t h the np orbitals of oxygen. As the result n-bonds w i t h oxygen atoms are formed and the cations become displaced from the center of the octahedron towards terminal oxygen atoms. The softness of the metal-oxygen potential Is the cause of large cation displacement polarizabilities, which give r i s e t o the high relaxation energy and makes possible the phenomenon of crystallographic sheaf by strongly stabilizing the shear planes (24). Moreover, octahedral s i t e displacements

Fig 2 interaction of hydrocarbon molecules w i t h dif f e r m t active sites at the oxide surface

Fig.3. Arrangement of atoms at (0101, (001 1 crystal faces of Moo3

( 100) and

stabilize layered structures of oxides As examples Moo3 and V205 w i l l be discussed because of their extensive use as components of the industrial catalysts. Mooj has a layer structure In which Moo3 ocathedra are linked together by edges to form a double zig-zag chain; the chains are linked through corners into infinite sheets ( 2 5 ) .Each octahedron has one unshared corner, the free corners in one layer pointing down between those of neighbouring layers. The Idealized structure may be considered as fcc oxygen packing w i t h Mo in one thlrd of octahedral interstices.ln fact the octahedra are considerably distorted (Mo-0 distancies vary between 1.671 and 2.332A) and the displacements along the a axis are in opposite directions in the adjacent layers. The l e f t side of Fig.3 shows (0I O), ( 100) and (00 1 ) projections of the arrangement of octahedra, and the right side of Fig.3 illustrates the arrangement of molybdenum and oxygen atoms on the appropriate crystal planes. I t has been assumed that this arrangement i s the same as that i n the bulk. The crystal has been cut in such a manner that the stoichiometric cornposltion i s retained. The charges on surface ions have been calculated on the basis of the Bond Strength Model as developed by Zi64kowski (26). On the (010) plane a l l molybdenum and oxygen ions are coordinatively saturated; therefore t h i s plane, in the absence of defects, i s inert in chemisorption

453

processes lnvolvlng heteroli tyc bond breaklng of polar molecules Quantum chemlcal calculations (27,281 show that the HOMO orbital of Moo3 i s that of the lone electron pair of the bridging oxygen at the (010) plane Thls oxygen may thus Interact w i t h hydrocarbon molecules having an empty low lylng orbltal of the approprlate symmetry such as the antlbondlng n*-orbltal of the ally1 or the n*-orbital of acrolein and perform a nucleophilic addition A clean (100) plane contalns unsaturated !-lo6' lons w l t h one brldglrq 0'Ion mlsslng from their coordination sphere and three types of surroundlng 02- ions. three brldglng oxygens coordinatlng 3 Mo6' ions, one oxygen brldglng 2 Mo6' lons and one terminal oxygen w l t h bond order 2, and unsaturated bridglng 02- lons w i t h one Mo6' ion misslng Taking lnto account the distortlon of the octahedra the charges appearing on tlo6'cus and would amount t o '081 and -091 respectively A decrease of local charge dlfference could be achieved by dlssoclative chemisortplon of water w l t h a proton golng to oxygen and the hydroxyl group being attached to Mo6' Ion, thls reduces the charge dlfference to a much smaller value Analogous conslderatlons show that the charges on Mo6' and 02- lons exposed on the (001) plane would be 0.34 and -0.37 respectively, and on (101) plane (not represented In Flg 3) . 0 8 and -0 9 respectlvely, also these planes show the tendency to hydroxylat lon It may be thus concluded that the reactlons of hydrocarbons on Moo3

02-c.5

should exhibit a pronounced structure sensitivity, the steps involving proton transfer taking place on the crystal faces perpendicular t o the (010) basal plane, and steps In which nucleophilic additlon of oxygen Is performed - on the basal (010) plane Indeed, studles of the behaviour of oriented slngle crystals In the oxdatlon of methanol have shown ( 2 9 ) that on the (010) p l a w only physlcal adsorptlon of methanol takes place without any chemical transformatlons, whereas on surfaces perpendlcular to the (010) plane methoxy groups are formed at room temperature as the result of the Interactlon of methanol w i t h the surface OH groups As the temperature i s raised the methoxy groups begln to react formlng formaldehyde and water, which rapldly desorb leavlng the surface of Mooj reduced The presence of methoxy CH30- groups at the surface of polycrystalllne Moo3 has been conflrmed by Ir spectroscopy (30). The rate Iimltlng step Is the abstraction of hydrogen from the methyl group by a surface oxtde Ion Further instght lnto the structure of the surface lntermedlate was galned by syntheslzlng the analogue of the postulated lntermedtate and studylng Its propertles ( 3 1 ) The Moo3 2H20 was taken as the inltlal reactant and the reaction wlth formaldehyde was carried out 2M003 2H20 + 2Ctt30H --+ M O ~ O ~ ( O C+H5 ~ti20 )~ Dlmethoxlde Is a whlte powder w i t h plate-like crystal habit suggesting that i t has the layer structure The character of I3CNI-1R spectrum exhibltlng a slngle line lmplies that all carbon enviroments are eqirlvalent as in the

454

0 = OCH,

f ig.4. Proposed structure of M O ~ O ~ ( O C(Ha f~t e) r~r e f . 3 1 )

proposed structure shown in f i g 4 The s i m i l a r i t y of the ir spectra of H ~ ) ~t o methoxy groups bound t o the surface o f Moo3 and of M o ~ O ~ ( O C leads the conclusion that chemical properties of methoxy groups should be In both cases essentially similar Thus the fact that the formatlon of the dlmethoxlde requires the participation of coordinatively unsaturated molybdenum atoms conflrms the concluslon that i n the case of Moo3 methanol becomes chemisorbed and oxidized on the non-(010) crystal planes. On illumination at ambient tempprature or on heating i t decomposes into formaldehyde and methanol i n ;3 reaction similar t o that oostrilated for the intermediates at the surface of MoO3 M O ~ O ~ ( O C---H ~ )H2xMo205 ~

+

I 1 +xKH20

+

( 1 -x)CH30H

where x represents the degree of reduction of the molybdate residue and varies between 0 and 1 . Comparison w i t h the behaviour of the deuterated dimethoxlde Mo2D55(OC@3)2 showed that the degree of reduction of the residuefrom deuterated compound was smaller than from the undeuterated material Suwression of C-H bond breaking by the kinetic isotope effect confirms that the abstraction of hydrogen from the methyl group Is indeed rate determining At variance w i t h these results Tatibouet e t al. (32,331 in an early study of the behaviour of Moo3 prepared by slow condensation from an oxygen stream carrying the trioxide vapours, then broken and sieved to obtain crystallltes wit!-t different r a t l o s of the low index crystal planes, concluded on the basis of kmetic studies that oxidation of methanol takes place on both (010) and non-40 10) planes, whereas the pure acid-base reactions such as formation o f dimethylether and methylal are proceeding only on the non-(010) planes. The question of the role of different crystal planes of Moo3 i n the oxidation of hydrocarbons has been In recent yeasrs a subject of a lively discussion (34,35,36,37.38,39,40). As discussed i n 5 1 . selective oxidation of oleflns t o unsaturated aldehydes belongs t o the nucleophilic type oxidations

455

In the case of e.g. propene the process can be envisaged as proceeding in t w o steps: activation of propylene molecule by abstraction of hydrogen t o form an a l l y l and nucleophilic addition of an oxide ion t o the a l l y l t o form acroletn: CH -CH-CH3

2-

H abstraction addition Y -__----___ bCH2--CH=CH202_----_----_ CH2=CH-C=O

H abstraction By comparison of the behaviour of propene and a l l y l compounds which in the reactor decompose t o alve free allyl radicals It Is possible t o separate these t w o steps and thus to draw conclusions as t o the nature and location of acttve sites involved in the abstractibn of hydrogen and in nUCleOphlllC addition of oxygen In order t o answer these questions experlments were carried out, In which activity of Moo3 preparations of different crystal habit

i n the reaction w i t h ally1 compounds was determined (34). Results are shown in Figs, in which the yield of acrolein is plotted as the functton of the surface area of the (010) crystal faces A linear relatlonship was obtatned, the straight line passing through the ortgin of coordinate system It could be thus concluded that the nucleophilic addition of oxygen into allylic species to form acroleln took place at those parts of the Moo3 surface, where the (010) faces are exposed It is much more d i f f i c u l t t o localize the sites responsible for the f i r s t step of the reaction, i e the activation of propene, which i s very slow on pure Moo3 Analysis of the results obtained by Volta et a l ( 3 6 ) on studying the oxidation of propene on graphite supported Moo3 crystallites seem to indlcate that It may occur at the planes perpendtcular the (010) This step can be however considerably accelerated when Bi3+ ions are supported a t the surface of Moo3 crystallites Experiments on the impregnatton of Moo3 w i t h Bi3+ ions revealed ( 3 5 ) that the pronounced unisotropy of surface properties of Moo3 crystallites manifests itself also in the process of impregnation

Flg.5. The yield o f acrolein in the reaction of allyl compounds on No03 catalysts as function of the surface area of (010) face (after ref.34).

456

Namely, deposition o f Bi3' ions takes place only on the non-(010) crystal planes, whereas the (010) plane remains bare. Micrographs of t w o parts of the Moo3 platelet after supporttng multl-layer deposlt of Bt3' tons and annealtng clearly indicate that the basal (010) crystal plane of Moo3 remained practlcally unchanged, whereas a t the side ( 1001, (0011 and ( 1011 planes a new phase has grown. The presence of t h i s phase i s also vtstble a t those locations on the (010) plane, where steps and kinks exposed the planes perpendicular t o the (010) plane. Indeed, recent studies of Moo3 platelets by electron mlcrodiffractton techntque showed (41) that on the (010) plane very thin steps and kinks exist of the thtckness, amounting t o 115 or 415 of the l a t t i c e unit c e l l dimension b It may be thus concluded that the t w o steps of the transformation of propene l n t o acrolein take place a t dtfferent crystal planes of Mo03: abstraction of hydrogen by Bi3+ ions occurs a t the planes perpendicular t o

(OIO), the a l l y l i c intermediate must then mtgrate to the (010) plane, where nucleophllic addition of oxygen Is performed. Analysis of ESR spectra of Moo3 registered in the course of i t s reduction in different atmospheres enabled the localization of surface oxygens performing the nucleophllic addltton to the hydrocarbon molecules (101. As an example Fig.6 shows (40,421 the ESR spectra of Moo3 after outgassing a t the temperature of 43OoC for 5 mln (curve A) and 35 m l n (curve B). The values of the g-tensor reveal the presence of t w o dtfferent Mo5' centres: type A, formed at the early stage of the reductton and characterized by rhombically distorted square pyramid surroundtng of axlal symmetry along the drr double bonded oxygen, and type B of distorted octahedral symmetry and appeartng in more reduced samples Comparlson of these r e s u l t s w i t h the sltuatlon a t the surface of Moo3 c r y s t a l l l t e s leads to the concluston that the

Flg.6. ESR spectra of

Moo3 a f t e r outgasstng a t 43OoC f o r 5 min (curve A) and

35 mtn (curve B), and structural assignment of spectrum A and B respectively (after ref. 10).

457

only surface oxygen ion, which can be removed leaving the reduced molybdenum cation i n square pyramidal coordination w i t h double bonded oxygen in opposite apex, is the surface bridging oxygen. Similar conclusion concerning the removal of bridging oxygen atoms on reduction of bismuth molybdate was drawn from the studies of ir spectra (43). When concentration of vacancies increases on progressing reduction, crystallographic shear takes place and M05' cations assume octahedral coordination along shear planes. M05+ ions registered in the ESR measurements constitute only a fraction o f the reduced species, the m a j o r i t y being Mo4+ ions, which as non-Kramers ions are not expected t o give an ESR signal. As these ions are located i n the shear planes in edge linked octahedra, Mo-Mo bonds are formed as revealed by the XPS studies (27,441. UPS spectra indicate that these clusters of tetravalent molybdenurn ions constitute donor type energy levels situated i n the forbidden energy gap of the oxide (45) and may play the role of active sites f o r the chemisorption of oxygen. Very interesting conclusions could be drawn from a direct observation of the low temperature reduction of Moo3 in 1 MeV electron microscope (46). On exposing thin platelets t o H2/He mixture a t room temperature a strong diffraction contrast appeared on the (010) plane parallel t o the [I011 direction. This has been interpreted as resulting from stacking faults bound t o screw dislocations Loss of surface oxygen ions due t o reduction generates oxygen vacancies which accumulate at the surface. The m i s f i t strains at the interface between the reduced surface layer and the remainder of the crystal i s then discharged by the appearance of a dislocation bounding a stacking fault. The most spectacular example of the strong influence of surface structure on the direction of the reaction in oxidation of hydrocarbons on Moo3 based catalysts i s the behaviour of the t w o cuprous molybdates: Cu2Mo3OI0 and Cu6Mo4OI5 in the oxidatlon of 1-butene (47,48). Both are composed of the same chemical elements in the same valence states and d i f f e r only in the spatial arrangement of atoms. Yet they show entirety different catalytic properties: Cu2Mo3OI0 i s active in the isomerization and oxidative dehydrogenation, but no traces of oxygenated hydrocarbon molecules are present in the products, whereas Cu6Mo4OI5 mainly inserts oxygen into the organic molecule t o form crotonaldehyde. The most striking feature i s the complete absence of isomerization in the l a t t e r case. Unfortunately the lack of crystallographic data does not permit a more detailed discussion of the origins of this pronounced structure sensitivity. The surface properties of V205, the second catalytically important transltion metal oxide w i t h layer structure, w i l l now be discussed. The stereochemistry of V205 may be considered t o be either a distorted trigonal blpyramid approximated by a tetragonal pyramid (five V-0 bond lengths of

N30AXO HnlawNvA

0

3

Fig.7. Idealized structure of V205: (a) - projection nf 1010) and (b) - (100) plane drawn as tetragonal pyramids. (c) - (100) ptoJectlon drawn as idealized trigonal pyrarnI ds 1.58-2.02 A) or a distorted octahedron (the slxth V-0 bond length of 2.79 A). Flg.7a shows ribbons of double tetragonal pyramlds sharing edges and formlng sheets by sharlng corners w i t h adlacent ribbons on both sldes A l l Pyramlds in one row of the ribbon point up, and In the second row polnt down. The sheets form a three-dlmenslonal network belng stacked one over the other in such a manner that apices of pyramlds of one sheet are positioned over basal planes of pyrarnlds of the sheet beneath, thus cornpletlng the dlstorted octahedron (Fig.7b). These weak V-0 bonds gtve rlse to perfect cleavage between the sheets Fig7c is the same projection as Flg.7b only drawn as idealized trlgonal blpyrarnlds. Simllarly as in the case of Mo03 also here the different crystal planes differ In the type of chemical bonds and the degree of coordlnative unsaturatlon. The basal (010) plane (notation after ref.49) is composed of termlnal oxygen on-double bonded to vanadlum lons, t w o types of brldglng oxygen lons and vanadlum lons wlth a l l chemlcal bonds almost fully saturated, leaving a negliglble excess posltlve charge of 0.04 as estimated from 'the bond strength model (50). However, exposed vanadlum ions possess two non-bondlng d-orbitals sticking out of the surface and therefore may play the role of Lewis acid sltes, whereas the lone electron pairs of bridging oxygens may act as Lewis b a s k sltes. Dlfferent situation exists a t the (100) and (001) planes. Cleavage leaves on the (100) plane coordlnatively unsaturated vanadium ions w l t h about +0.5 excess charge and oxygen ions w i t h about -1.0 excess charge, and on the (001) plane - Vcus and Ocus w i t h charges of about + and - 0.7 respectively. These planes should thus show strong tendency t o dissoclatively adsorb

459

B

I

520 5M

.

,

.

520

,

.

521

I

528

.

,

,

S32 536

,

5rO

aL, ev

Flg.8. V2p and 01s photoelectron spectra of samples exposing (A) - (010) plane, (8) - t 100) plane; (a) - as recetved, (b) - after argon sputterlng for 2 min. and (c) for 5 mln (after ref.51). water and to develop acid-base Interactions w l t h the reacting molecules, as w e l l as t o generate electrophllic adsorbed oxygen specles actlve In total oxidation. Indeed, this is preclsely the result obtained from XPS studles (511. Flg.8 shows the V2p and 01s photoelectron spectra of the (010) plane and the (100) plane of V205 "as recelved" and after argon sputtering. The 01s spectrum obtained from the (010) plane consits of a simple regular line which does not change on sputtering Conversely, a complex llne appears In the range of Eb values correspondlng t o 01s electrons, which could be deconvoluted Into two gaussian lines: O1 correspondlng to the Eb of 01s electrons 529 6 e V characteristlc for the lattlce oxygen lons of V2O5 (521, and O i l corresponding to an Eb value of 532.2 eV, typlcal for OH groups i n transltlon metal hydroxldes and at surfaces of transltlon metal oxldes (53,541. After sputtering the intensity of the 011line decreased, whereas the intensity of the O1 llne assigned to lattice oxygen Increased. This indlc6tes that the specles responsible for the OIl line are the OH groups located at the surface of the investlqated crystals, screening the Iattlce oxygen Ions. It may be thus concluded that the (010) crystal plane Is inert and remalns bare, whereas It i s the ( 100) plane whlch adsorbs water molecules to become hydroxylated. Consequently the basal (010) and the non-basal planes w l l l show dlfferent behaviour In adsorption and catalysls, exhibiting a pronounced structure sensitivity Fig.9 shows (55) selectlvitles of the oxldatlon of o-xylene to phthallc anhydride and to products of total oxldatlon carried out on crystallltes of V205 of different hablt, changlng from platelets exposlng predomlnantly the basal (010) plane t o needles w l t h malnly (1001, (0011 and ( 1 10) planes The materlal was characterized by the morphological factor f deflned as the

460

r a t i o of intensity of ( 1 10) reflection t o the intensity of (010) reflection (56). Samples Characterized by low values of f i.e.composed of plate-like crystallites showed very high selectivity t o phthalic anhydride, whereas on crystallites of needle-like habit, characterized by higher values of I,t o t a l oxidation became the predominant reaction pathway. Anderson (57) studied adsorption of benzene (61, toluene (TI, pyridine ( P I and 3-methyl-pyridine (MP) on polycrystalline V2O5 samples a t 25OoC and 350°C and found that benzene showed practically no adsorption in comparison t o other hydrocarbons (Table). This indicates that n-electrons of the

mt

2 zir LL

0

Fig.9. Selectivities i n oxidation of

Fig.10. The amount of N2 obtained in

o-xylene on V2O5 as function of

i n reaction between NO and NH3 over

the morphological factor (after ref. 551.

V2O5 as function of morphological

factor (after ref.51).

TABLE Concentrations (prnole.cm-2) of adsorbed benzene (81,toluene (TI, pyridine (P) and 3-methyl-pyridine (MP) on V2O5 (after ref.57)

Temp. (OC)

250 350

B

0.00 0.00

T

0.17 0. I 7

P

0.28 0.03

(T+P)

0.45 0.20

MP

0.45 0.19

461

aromatic ring do not particlgate in the adsorption bond and the other aromatic molecules are adsorbed probably end-on through their functional groups. The amount of toluene adsorbed does not change on raising the temperature what indicates that it must be strongly held at the surface at variance with pyridine which is only weakly adsorbed. The sum of the amounts of toluene and pyrldlne adsorbed is equal to the amount of adsorbed 3-methyl-pyrldine what shows that toluene and pyridine are adsorbed on dlffel'ent crystal planes. It may be assumed that pyridine will be adsorbed on the highly polar non-(010) planes either on Bronsted or Lewis acid sites (for detailed discussion see ref.57). This leaves the basal (010) plane as the probable location of sites adsorbing toluene and - by analogy - also o-xylene. Results of in sltu ir studies of toluene and o-xylene adsorbed at the surface of V 9 5 (58,591 confirm the end-on configuration of these molecules. These conclusions are in line with the fact that oxidation of o-xylene is selectlve when plate-like crystallltes of V2O5 are used as catalysts, but total oxidation takes place on needle-like crystallltes. The latter, because of the high coverage of non-!010) planes wlth OH groups are also active catalysts for the reaction of NO with ammonia: V5+-OH + NH3 ----- V5'-ONHd 2 V5*-ONH4+ V5t-O-V5* + 2N0 ---- 2N2 + 3H20 + V5*-OH + v4*OV4' where 0 represents an oxygen vacancy at the surface of V2O5 lattice. This is shown In flg,lO, in which the amount of N2 formed i s plotted as the function of the morphological factor of V2O5 crystallltes. This amount increases when crystallites are more needle-like. In the absence of gas phase oxygen the reduced surface sites are rapidly regenerated t o their initial state by diffusion of oxide ions from the bulk of the crystallites, the reaction being in fact not catalytic but stoichlometrlc, whereas in the presence of the gas phase oxygen lattice becomes rapidly reoxidized: v4+av4+ + 1i202 _---- vS+-o-vS+ the catalytlc cycle belng thus completed STRUCTURE-SENSITIVITY OF CATALYTIC REACTIONS ON OXIDES A following Dlcture of structure Sensitivity on group V, V I and VII transltlon metal oxides emerges from the present discussion. Crystallites of oxides assuming layer structures exhibit two types of crystal faces: - those, whereat all constituent atoms are chemically fully saturated but have either exposed metal cations wlth empty d-orbitals which may play the role of LUPlO of the oxide, or free electron pairs of the oxide ions whlch may act as the oxide's HOMO; - those, whl'ch are composed of coordinatively unsaturated cations and anions, whereat excess charges are accumulated generatlng conslderable variations of the potent la1 along the surface.

462

The basal (010) Plane of e.g. V205 belongs t o the f i r s t type. Exposed vanadium ions i n the adjacent rows of square pyramids i n every two ribbons have empty dxz and dyz orbitals which are LUMO's and can play the role of a Lewis acid sites. Oxygen ions bridging the ribbons have free electron pairs a t the HOMO'S and because of the displacement of V5+ ions towards the orr-bonded terminal oxygens they acquire more negative charge and- are rendered more basic. When a toluene or xylene molecule approaches this surface the C-H bond of the methyl group reacts w i t h the empty d-orbitals of vanadium, the proton being shifted to the bridging oxide ion and the benzyl radical attached end-on to vanadium. The nucleophilic addition of the other brldging oxygen ion of the surface may be now performed t o the carbon atom of the -CH2- group of the adsorbed benzyl radical, resulting i n the formation of a precursor of the aldehyde, which i s then desorbed after abstraction of the second hydrogen, generating a surf ace oxygen vacancy. Quantum chemical calculations (60) indeed show that neither molecular oxygen nor atomic oxygen react w i t h the methyl group of o-xylene t o form the aldehyde but the reaction requires f i r s t abstraction of hydrogen from the methyl group and then a nucleophilic attack of an oxide ion from the direction perpendicular t o the plane of the ring. Such geometry i s realized by an o-xylene molecule adsorbed end-on at the vanadium ion through its methyl group, and attacked by the adjacent bridging oxide ion. Different interactions prevail at crystal planes of the second type. Here the OH groups of acidic character may interact w i t h the hydrocarbons t o form carbontum ions. On heating at higher temperatures dehydratlon of the surface takes place, leaving coordinatively unsaturated V5' cations and 02- anions, on which considerable excess charge i s accumulated. Such sites may induce a heterolytic bond scission i n the adsorbed reactant molecules. Simultaneously the reducing atmosphere of the hydrocarbon reaction medium causes usually some reduction of the catalyst surface so that V4' ions are generated. Such ions may function as sites activating the oxygen molecules t o their electrophilic active forms, which may initiate the electrophllic oxldation route. In the conditions o f a heterogeneous catalytic process this route may end i n total oxidation. The type of product formed as the result of an electrophilic attack of oxygen on the hydrocarbon molecule depends on structure of the oxygen adsorption complex. Namely quantum chemical calculations of e.g. the reaction of oxygen w i t h benzene revealed (61) that the type of product formed depends on the direction of approach and mutual orientation of the reacting molecules and on the mode of oxygen activation. Oxygen molecule may be bound t o the surface of an oxide i n many different forms. Che e t al (62) have distinguished 6 dlfferent forms of oxygen molecules adsorbed a t the surface of the Coo-MgO solid solution: species linked to the surface C03+ cations side-on and end-on, forming different angles t o the surface, from a vertical orientation t o a horizontal one. It i s noteworthy that at the steps and

463

a t the steps atid klriks a two-slte adsorption may take place. Also the surface OH groups may interact w i t h adsorbed molecules. Thus i t ' l s obvlous that the type of adsorbed oxygen species must strongly depend on the structure of the surface, rendering also the electrophlllc oxldatlon structure sensltlve.

Fig. I I. Dlfferent adsor bed oxygen specles detected a t the surface of Coo-MgO solid solutlon (after ref.62).

DYNAMICS OF THE OXIDE SURFACE

In the last decade a vast experimental evidence has been accumulated indicating that the surface of a solld i s not a r i g i d s t a t i c structure, on whlch various phenomena involving molecules adsorbed from the gas phase occur, but is always In rlynamic lnteraction'wlth the l a t t e r (63,64,65). Adaptabllity t o the change of external condltlons i s one of the most important properties of solid surfaces, responsible for many phenomena of great theoretical importance and practical consequencles When properties of the gas phase are altered, the gas/solid lnterface immedlately adapts itself t o the new conditions, very often by changes of the composition and structure of the outermost layer of the solld. This i n turn may result In changes of the properties of the solld surface, influencing strongly the course of processes, taking place a t this surface. in particular new sites actlve in catalytlc reactions may be generated, modifying strongly the behaviour of the solid as the catalyst. Dynamic interactions which may take place a t the surface of an oxlde are summarized in f i g 12 A metal oxide crystal in equilibrium w i t h the gas phase shows always a defect structure because certain number of metal cations and oxide anions pass into the gas phase leaving behind in the crystal l a t t i c e point defects S\JCh as vacancies and i n t e r s t l t i a l s or extended defects such as shear planes. in case of transition metal oxides this results in the change of stoichiornetry, these oxides belonging to the class of nonstoichlometric solids showing broader or narrower range of nonstoichlometry The equillbrium concentratlon of defects i.e. the nonstoichiometry 1s determined by oxygen pressure, temperature and concentration of forelgn tons. Such crystals may be consldered as solid

464

H IOu("00U

0.101

Fig.12. Interaction of the gas phase w i t h the oxide surface. solutions of defects in the crystal lattice As the appearance of a defect at the surface of the solid changes I t s surface free energy, adsorption of defects may take place at the gas/solid interface, the surface layer becoming enriched in those defects, whose presence decreases the surface tension (65) By changing the oxygen pressure the concentration of defects in the oxide may be altered, what i s followed by an appropriate change of surface enrichment When this latter attains a certain critical value, ordering of these defects in the surface layer may take place and a superstructure may appear at the surface Further changes of the surface composltion may cause the shifts of the posltlons of atoms resulting in surface reconstruction and formation of new bidlmensional phases A l l these phenomena may be strongly Influenced by the presence of adsorbates in the gas phase Adsorbed molecules interact w i t h the atoms located at the surface of the oxide, shlftlng the defect equilibria, changlng the surface composition and loosening the bonds between the lattlce constituents, what may lead t o the formatlon of an ordered adsorbed monolayer, followed by the migration of Ions and formation of new bldlmenslonal compounds (66). In appropriate conditions nucleation takes place at various surface defects and crystallites of the new bulk compound may appear. In the late sixties the Gent school of solid state physics undertook detalled studies of the surface structure of V2O5 monocrystals and of its changes on outgassing and exposition to various gases Gillls (67) showed that V2O5 monocrystals when outgassed easily give o f f oxygen transformlng a t the surface Into V6013 The structure of the latter may be derived from that of V2O5 by removlng every thlrd oxygen (010) layer He also drew attention t o the fact that In the V2O5 structure channels stretch along the (001) and (010) directions Thelr diameter of 2A is large enough to permit rapid transport of oxygen. Flermans and Vennlk (68) showed then by LEED that the v6013 is epitaxially formed on the (010) plane of V2O5 crystal and

465

Colpa'ert (69)demostrated that the (010) face of V205 1s fairly Inert and becomes active In selective oxidation of propene and butene only when surface transformation into V601 3 takes place. Dziembaj (70) measured the equlllbrlum oxygen pressure over these oxldes and concluded that V2O5 can accomodate only a very small concentration of oxygen vacancies and univariant redox equilibrium Is then established: v6013' O2 = V205, prevaillng In the course of the catalytic reactlon. Recently Andersson e t all711 in the i n sltu HREM experiments observed directly the formation of V60 13 in the course of the catalytlc oxidation of toluene, but this phase was found equally active and selectlve as the inltlal V2O5 It may be thus concluded that in the course of the reactlon the (010) plane of V2O5 becomes but because both have the same surface structure the covered by V6013, actlvity In selective oxldatlon Is not affected However, the surface layer of V6013 on the (010) plane plugs the channels in the V2O5 structure i n the

[OIO]direction so that only channels along the [I001direction remain open to transport easily oxygen to the (100) surface, where i t can be activated and perform the total oxidation of hydrocarbons molecules. In this way the (010) and (100) planes acquire different properties in respect to the supply of lattice oxygen available at the surface for the reaction w i t h hydrocarbon molecul es. T his phenomenon may be ca 11ed induced structure-sensiti~it)?~ REFERENCES

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C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactiuity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

469

CRTRLVST CHRRRCTERIZRTION RND /H .VW FTlR STUDIES OF CRRBON D I O X I D E nETHRNRTION OUER RUTHENlUfl SUPPORTED ON TITANIR. J. G

I

H IGHF I EL01 , P RUTERRNR2, K . A , THRnP I3 and n, GRRETZEL3 I

1Laborat o i re de Chi m i e Techn i que, Eco I e Po I yt echn i que FCdCra Ie Lausanne, Lausanne, CH-1015 Switzerland. 2lnst itut Interdepartementale de Hicroscopie, Ecole FbdCrale de Lausanne, Lausanne, CH-1015 Switzerland,

de

Polytechni que

3 I nst i tut de Ch I m i e Phys i que, Eco Ie Po I yt echn i que FCdCra I e de Lausanne, Lausanne, CH-1015 Switzerland,

ABSTRRCT Partially-reduced ruthenium dioxide on titania is an active and selective catalyst for methane formation from carbon dioxide , already at temperatures below 100 C. Results from catalytic screening together with characterization b y electron miCrOSCOpy indicate that the Ti02 support ( Degussa P26 in particular) plays a major role i n stabilizing metallic ruthenium i n highly-dispersed form (d 10-30 A), provided care is taken in the RuO2 deposition step. Chemisorption of H2 and CO2 has revealed a strong coadsorption synergy together with extensive h y d n p k spibvxr. Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy j.n situ has provided confirmatory evidence of the latter, and shown that CO, believed to arise from the reverse watergas shift reaction , accumulates on the metal causing catalyst facilitates the Bcdmuud deactivation. Heating above 100 -tkam lartian and the activity is regenerated. Implications for the mechanism of CO2 hydrogenation are briefly discussed.

INTRODUCTION The

hydrogenation

of carbon

dioxide I s of topical interest,

both

from the point of view o f producing useful chemical fuels while at the same time seeking a viable solution to the ecological problem o f

accumulating potentially dangerous levels of this gas in the atmosphere (ref,l). Industrial interest in its use as a chemical feedstock has received a boost by the recent finding that it nay be the primary carbon source in the synthesis of methanol from CO/CO2/H2 mixtures(ref -2) The main alternative route to hydrocarbon product is methanation the so-called Sabatier reaction: I

CD2

+

4H2

--b

CH4

+

2H20

* * *

1,

470

wh i ch

is

thermodynam i ca I Iy

downh i I I ( n G 0 2 9 8 ~= -27kca I ,mo 1 - 1 )

but

subject t o s e r i o u s k i net i c b a r r i e r s . I n p r a c t ice, i t t y p i ca I I y requ i r e s a c a t a l y s t and temperatures o f *300°C t o proceed a t a s i g n i f i c a n t r a t e (refs.3-4). Recently, however, t h e authors have r e p o r t e d t h a t t h e Ru/RuOx(xb2)/Ti02 system i s a c t i v e already below l0OoC and shows a s u b s t a n t i a l photoenhancement o f t h e r a t e upon band-gap i l l u m i n a t i o n o f t h e support ( r e f .5). I n t h i s work, we begin by presenting data from transmission e l e c t r o n microscopy i n an attempt t o r a t i o n a l i z e some o f t h e trends i n c a t a l y t i c act i v i t y from screening s t u d i e s already r e p o r t e d ( r e f ,151, These i l l u s t r a t e the value o f a ruthenium-containing a c t i v e phase, o f Ti02 (and Degussa P25 i n p a r t i c u l a r ) as support, and emphasise t h e importance o f c a r e f u l a t t e n t ion t o d e t a i I i n c a t a l y s t p r e p a r a t i o n a t t h e oxide precursor stage, From mechanistic considerations, we have i n v e s t i g a t e d the surface chemical i n t e r a c t i o n s o f CO2 and H2 on t h e model c a t a l y s t , r e s t r i c t i n g t h e study t o methanation i n t h e over t h e temperature range o f 1 SO'C To t h i s end , convent i ona I vo Iumet r i c met hods i n t e r e s t , i e 25'(gas chemisorpt ion) have been coupled e f f e c t i v e l y with j n s i t u D i f f u s e Reflectance I n f r a r e d F o u r i e r Transform (DRIFT) Spectroscopy, DRIFT permits examination o f t h e c a t a l y s t d i r e c t l y i n powder form, and under r e a c t i o n cond it Ions, a t sens it i v i t ies which may ue I I surpass those obtained by the conventional method o f I R transmission through pressedd i s c s t r e f .71.

.

EXPERltlENTRL The p r e p a r a t i o n and screening o f t h e "model c a t a l y s t " has been described i n d e t a i l elsewhere(ref.5), I n b r i e f , Ti02 ( Oegussa P25) uas suspended i n an a c i d i c , aqueous s o l u t i o n o f RuC13 and t h e m i x t u r e h y d r o l l z e d a t 7OoC by a d j u s t i n g t h e pH t o 4-4,s with KOH. i f t e r evaporat ion, t h e r e s i d u e was c a l c i n e d i n a i r i n two stages t 170 and 37SoC), Residual K C I was then removed by d i a l y s i s and t h e sample d r i e d a t l l O ° C . For opt i m u m a c t i v i t y i n a s t a t i c r e a c t o r a t l b a r ( Pc02=0.05; P~~'0.61, the ideal loading o f Ru02 was found t o be 5% w/w and t h e opt i mum temperature range f o r pre-reduct i o n ( i n f l o r i ng H2/Ar 1 : 1 ) was found t o be 20O125O0C. A f t e r t h i s treatment, XPS showed t h a t t h e Ru was not conpletely-reduced t o t h e m e t a l l i c s t a t e . The e l e c t r o n microscope was a Phi I i p s En430 ST model with I . 9 8 p o i n t r e s o l u t i o n , equipped with EOX and EELS a n a l y s i s f a c i l i t i e s . The volumetric measurements were made i n a conventional vacuum frame equipped with a Barocell pressure transducer, H i g h - p u r i t y gases H2,02,CO and CO2 were dosed from c y l i n d e r s and f u r t h e r p u r i f i e d by

471

r o u t i n e freeze-thaw procedures. I n t h e coadsorption studies, t h e uptake o f each component was monitored by s e l e c t i ue Iy f r e e z i n g out t h e C02 a t

77 K . Gas compos it i o n ana Iys i s was performed by quadrupo Ie mass spectrometry ( Ba Izers mod. QMG420 1 a f t e r appropr i a t e ca I ib r a t ion. I n f r a r e d d i f f u s e - r e f lectance spectra from 4000-800 cr-1 a t 4 cm-1 r e s o l u t i o n were obtained w i t h a Bomen DR3.002 FTlR spectrometer vacuum equipped with a H a r r i c k DRR-2CO diffuse-reflectance/HUP-DRP chamber accessory-combination. The spectrometer has been adapted t o operate with t h e o p t i c s compartment permanently under vacuum w h i l e t h e sample compartment i s purged with d r y N2, thus o b v i a t i n g t h e need f o r uacuum-t i g h t s e a l s f o r t h e associated flow I ines and e l e c t r i c a l leads t o t h e vacuum chamber. The l a t t e r has been f i t t e d t o permit sample obseruat i o n i n vacup, under c o n t r o l l e d pressure ( up t o 12Otorr), and under dynamic o r s t a t i c c o n d i t i o n s a t l b a r . Gases uere suppl i c d from h i g h - p u r i t y c y l inders and passed through moisture t r a p s ( and a deoxo u n i t i n t h e case o f Rr) before sample contact.Each spectrum s h o w here was obtained by t h e c o a d d i t i o n o f 500 scans, r e q u i r i n g - 3 min. per spectrum. Spectral n o r m a l i z a t i o n was achieved u s i n g powdered K B r i n f l o w i n g Fir a t t h e corresponding temperature, RESULTS 1 . Catalygt screen i n s- and cha r a c t e r i z a t i o n by e l e c t r o n microscQRy, Euidence has already been presented t o show t h a t t h e model c a t a l y s t p r i o r t o r e d u c t i o n c o n s i s t s o f well-dispersed Ru02 i n i n t i m a t e contact w i t h t h e Ti02(Degussa P25) s u p p o r t ( r e f . 5 ) . Closer inspection(see F i g . 1 )

F i g , l , Micrographs o f t h e model c a t a l y s t p r i o r t o reduction, showing: Ru02 depos i t s Ioca I i zed i n se Ie c t e d b 2 0 x ) T i 02 p a r t i c Ies; and a. 10-1 b 30-401 Ru02 dona i n s random I y-d i s t r ibut ed but more scarce(same sca I e).

58

I

472

o f t r a n s m i s s i o n e l e c t r o n micrographs, however, r e v e a l s t h a t t h e r e i s e f f e c t i v e l y a bimodal d i s t r i b u t i o n o f Ru02, e x i s t i n g m a i n l y as v e r y smal I d e p o s i t s ( d o l 0 - 1 5 8 ) l o c a l i z e d i n o n l y a modest f r a c t i o n (-20%) o f t h e T i 0 2 p a r t i c l e s . The remainder c o n s i s t s o f well-spaced,larger p a r t i c I es (* 3U-401) random Iy - s c a t t e r e d over t h e support On r e d u c t i o n a t 22OoC, the d i s p e r s i o n o f t h e r e s u l t i n g Ru metal c l o s e l y r e f l e c t e d t h a t o f t h e o x i d e p r e c u r s o r , Fln i n d i c a t i o n o f t h e s i n t e r i n g r e z i s t a n c e o f t h e c a t a l y s t was o b t a i n e d by a more-seuere r e d u c t i o n a t 500 C . This caused some Ru p a r t i c l e growth b u t even t h e l o c a l i z e d r e g i o n s were l i t t l e - a f f e c t e d , now showing a somewhat broader d i s t r i b u t i o n from 1030x. The c a t a l y t i c a c t i v i t y I ikewise s u f f e r e d o n l y a modest decrease ( ~ 2 5 % due ) t o the l a t t e r treatment, I n view o f t h e above behauiour, i t was suspected t h a t t h e wide range i n a c t i v i t y o f t h e R u - c o n t a i n i n g c a t a l y s t s under s t u d y might c o r r e l a t e e s s e n t i a l l y w i t h t h e i n i t i a l d i s p e r s i o n o f t h e Ru02, and thus a l s o t h e metal p a r t i c l e s i z e a f t e r r e d u c t i o n . The e f f e c t o f changing t h e support and v a r i o u s p r e p a r a t i o n a l d e t a i l s was subsequently e x p l o r e d . The r e s u l t s a r e p r e s e n t e d below (see Table 11, I

Table 1 . E f f e c t o f support and c a t a l y s t p r e p a r a t i o n on C02 methanation a c t i v i t y and c o r r e l a t i o n w i t h Ru02 ( o r Rul d i s p e r s i o n (5% w / w l o a d i n g as Ru02). Sumort

Preparation

&&(or

Ru

1

sized;

tlethanat i o n a c t i u i t y / 45OC I n i t i a l r a t e / y m o I , h - l CH4 dsrk UU i l l u m i n a t i o n zero zero 0.03 0.03 1.20 5.75 3.69

standard* Rerosi I Rlumina " C " T i 02(P25) I, I, r e dn. /H2/500

Ru; Ru; Ru; Ru;

rut i l e

0.78

n

Ru02;not v i s i b l e ( u n i f o r m by EOX) R ~ 0 2 ; 60-100

-

0.16

Y

R u O ~ ; 20-30

0,33

0,74

14

standard*

rut i le2

anatase3 P25

no 2nd c a l c n . (170 C o n l y )

P25

/pH10

60-100 20-30 10-15,30-40 10-30,40-60

" f r e e " RuO2 a s maasiue p a r t i c l e s Ru02;30 and

-

-

2,46

zero 0,93

" f r e e " Ru02

*

Preparat i o n Suppl i e d by Obtained b y Supplied by

as d e s c r i b e d i n EXPER IMENTRL s e c t i on (a 130 see r e f , 5 ) ,

T i o x i d e ( 6011129-1). heat l n g Oegussa P25 i n a i r a t 1000°C, Tioxide,

473

2,

tlodel c a t a lY&ik!2

Ehem i S O r D t i on c h a r a c t e r i z a t i o n and coadso r-p

i n t e r a c t i o n o f CQ2h2A f t e r r e d u c t i o n o f a sample (0.59

3

2 x 10-4 moles Ru02)

i n H2

( 1 0 0 t o r r ) a t 25OoC f o r l h and c o o l i n g t o 25'C, t h e t o t a l consumption o f H2 ( Q t o t ) was - 2 . 9 x 10-4 moles, assumed t o comprise b o t h t h e reduct ion

( Q r e d ) and chemisorbed

(Pad)

components,

The

latter

was

e s t i m a t e d by evacuat i o n a t 25OoC and r e a d s o r p t i o n a t 100°C, y i e l d i n g a r e p r o d u c i b l e v a l u e f o r gad = 0.42 x 10-4 moles,( o r 0.84 x 10-4 g.atom H 1, Rssuming an a d s o r p t i o n s t o i c h i o m e t r y o f lHad:lRuOS ( where RuOs r e p r e s e n t s a s u r f a c e Ru metal atom ),and t h e c r e a t i o n o f lRuO atom p e r 2H2 molecules consumed i n t h e r e d u c t i o n s t e p a c c o r d i n g t o t h e s i m p l i f i e d r e a c t i o n scheme: + 2H2 = Run + 2H20 Ru02 leads t o a minimum e s t i m a t e o f t h e mean d i s p e r s i o n o f t h e metal o f 0 = +pad / Qred = 0 , 6 7 , T h i s h i g h value, i n d i c a t i v e o f a mean p a r t i c l e i s c o n s i s t e n t with t h e TEtl euidence. The degree o f size of-202 r e d u c t i o n was e s t i m a t e d as -box, c o n f i r m i n g t h e presence o f a s i g n i f i c a n t f r a c t i o n o f Ru i n a h i g h e r o x i d a t i o n s t a t e ( o r s t a t e s 1 i n l i n e with t h e XPS d a t a o f r e f . 5 . R c o r r e s p o n d i n g s t u d y o f a b l a n k sample o f T i 0 2 ( Degussa P25 I , which had been g i v e n t h e same treatment, showed n e g l i g i b l e c h e m i s o r p t i o n o f H2. A f t e r e v a c u a t i o n a t 25OoC, c o o l i n g and exposure t o C02 (100 t o r r 1, t h e uptake a t 25OC was s u b s t a n t i a l 0.7 x 10;q moles ) b u t c o m p l e t e l y r e v e r s i b l e upon e v a c u a t i o n a t 25 C , R t 100 C, t h e uptake was l e s s ( 0 . 2 x 10-4 moles) and -80% r e v e r s i b l e on pumping a t 100°C. Howeuer, u i r t u a l l y i d e n t i c a l behaviour was observed on t h e b l a n k Ti02, d e m o n s t r a t i n g t h a t C02 a l o n e i n t e r a c t s o n l y weakly w i t h t h e support

6

and h a r d l y a t a l l w i t h t h e m e t a l . When a m i x t u r e o f CO2:Hz = 1 :2(80torr;Qtot=3x10-4

moles) was dosed

a t 100°C, t h e t o t a l ( c o a d s o r p t i o n ) uptake f a r exceeded t h e sum o f t h e i n d i v i d u a l components adsorbed s e p a r a t e l y , as shown i n F i g . 2 . Rdsorpt i o n a t p r o g r e s s i ue Iy- I oser temperatures ( a f t e r succes i ve r e g e n e r a t i o n o f t h e c a t a l y s t i n H2 a t 250°C) became s l o w e r but always reached a s i m i Ia r s a t u r a t i o n ua Iue , i e Q t o t = 2 . 2 - 2 . 6 ~ 1 0 - 4 mo Ies , i n wh i c h t h e C02 component was qu i t e reproduc ib I e a t 0.76-0.78x10-4mo Ie s , Rs t h e l a t t e r v a l u e i s c l o s e t o t h e e s t i m a t e o f RuO, from H2 chemisorption, i t suggests t h a t t h e metal s u r f a c e becomes covered with an o v e r l a y e r o f a carbon-cont a I n i ng s p e c i e s d e r i ued from t h e i n t e r a c t i o n o f C02 and H2. Howeuer, as mass s p e c t r o m e t r i c a n a l y s i s showed t h e c o n v e r s i o n t o CHq t o be t y p i c a l l y Z.o11t. //*lOO>Pl. pattern at least for the largest metal particles of about 5000 platinum atoms (7 nm) with more than 25% surface atoms (27). Surface, subsurface and bulk oxide are formed on Pt single crystal surfaces at temperatures beyond 773 K and for high exposures and their decomposition occurs at temperatures between 1000 and 1500 K (2,6,8). On polycrystalline Pt wire bulk oxide was found for very long exposure times at loo Pa oxygen pressure and 750 K, i.e. again at considerably more severe conditions in comparison with our experiments ( 2 8 ) . Molecular bound oxygen on the other hand desorbs at much lower temperatures. Adsorption of oxygen at 100 K is followed by desorption at 150 to 300 K (2,3,14). The small oxygen desorption peak observed in some cases at 320 K (Fig. 1) cannot be reproduced and is considered to be an artifact at the present time. A shift of the desorption maxima to lower temperatures due to

an increase in the oxygen coverage which has been observed by several authors (1,4,6,7,9,12) can be ruled out in our experiments. The ratio of atomic oxygen to surface platinum atoms

511

estimated from the electron micrographs and a quantitative evaluation of the oxygen desorption was found to be approximately constant in our experiments ( 2 4 ) . The positions of the desorption maxima result from desorption kinetics and are not affected by diffusion limitation. Considering the applied heating rate ( 5 . 7 5 K/min) and the average zeolite crystal size ( < 10 pm) the desorption kinetics would be effected by oxygen diffusion limitation only, if a diffusion coefficient for the 0 2 diffusion in the faujasite matrix D < 10-10 cmZ/s in the temperature range from 600 to 900 K could be expected. Such low values of the diffusion coefficient can be excluded definitely (29,30). Readsorption as a reason for shifting the desorption maxima to higher temperatures as implied by a model of Demmin and Gorte (31) can also be excluded. An increase in the number of adsorption centers, e.g. in the case of sample Pt42X/V does not lead to a shift to a higher terrperature. The shift of the desorption maxima to higher temperatures with decreasing metal particle size can therefore be interpreted as a true particle size effect on the basis of an increase of the surface free energy and hence the desorption energy in the same direction. The apparent contradiction from a comparison of the observed range of desorption temperatures with experimental results obtained for bulk platinum can be resolved by postulating an accompanying metal support interaction. The occurence of strong electrostatic fields inside the zeolite cavities is well-known (32). The negatively charged lattice oxygen could induce a polarisation in the platinum particles reducing the electron density in the surface layer. Electron acceptors like oxygen should therefore be bound less strongly. The shift of the desorption peaks to lower temperatures from the first to the second adsorption/desorption cycle can be explained by a growth of the small metal particles (Table 3 ) . Since we operate at the limit of precision for the determination of particle sizes from electron micrographs the reconstruction of the surface induced by the heating in oxygen might be an alternative explanation. Such reconstructions have been observed in the last years by several authors, varying from the reconstruction of the topmost layer at Pt(100) (9,11,12) or Pt(ll0) ( 3 3 ) to electrochemical induced displacements at the surface ( 3 4 ) . Even a change of the morphology

512

of a single crystal can be observed depending on the medium used during the heat treatment (35-37). The lack of oxygen desorption from Pt12X/0~ and Ptl7Y/Oz following the reduction and storage in air might be due to zeolite lattice fragmentations which accompany the formation of the metal phase. The fragments can plug the zeolite pores and impede the oxygen transport to the metal particles. The high temperatures up to 900 K in the TDS cycle facilitate the transport and the distribution of the fragments in the zeolite framework, thus opening the zeolite pores and establishing the accessibility of the metal crystallites. Proof for the fragmentation process based on dealurnination was obtained by Z9Si-HR-MAS-NMR spectroscopy (38). SUMMARY Zeolite supported platinum phases which exhibit the structural speciality that the metal crystallites are uniformly surrounded by the zeolite support show a particle size effect in the chemisorption of oxygen. The shift of the desorption curves to higher temperatures with decreasing metal particle size is interpreted in a first approach

with a shift of the global parameter surface free energy. The surface free energy is expected to increase with decreasing size of the metal crystals. The observed relatively weak bond strength for the oxygen atoms at the zeolite embedded platinum could be related to a special metal support interaction caused by the strong electrostatic field in the zeolite matrix.

ACKNOWLEDGEMENT

We thank Dr. A. Kleine for taking the electron micrographs and the electron diffraction patterns, Mr. H. Kompa for carrying out the Nz-physisorption measurements and Dr. D. Exner for supplying the Pt42X sarple.

513

REFERENCES G.N. Derry, P.N. Ross, Surface Sci., 140, 165 (1984) J.L. Gland, B.A. Sexton, G.B. Fisher, Surface Sci., 95, 587 (1980) 3 J.L. Gland, Surface Sci., 93, 487 (1980) 4 C.T. Campbell, G. Ertl, H. Kuipers, J. Segner, Surface Sci., 107, 220 (1981) 5 H. Niehus, G. Comsa, Surface Sci., 93, L147 (1980) 6 J.L. Gland, V.N. Korchak, Surface Sci., 75, 733 (1978) 7 K. Schwaha, E. Bechtold, Surface Sci., 65, 277 (1977) 8 P.R. Norton, R.L. Tapping, J.W. Goodale, J.Vac. Sci. Technol., 14, 446 (1977) 9 M.A. Barteau, E.I. KO, R.J. Madix, Surface Sci., 102, 99 (1981) 10 G. Kneringer, F.P. Netzer, Surface Sci., 49, 125 (1975) 11 K. Griffiths, T.E. Jackman, J.A. Davies, P.R. Norton, Surface Sci., 138, 113 (1984) 12 P.R. Norton, K. Griffiths, P.E. Bindner, Surface Sci., 138, 125 (1984) 13 J.H. Craig,Jr., Surface Sci., 110, 75 (1981) 14 P.R. Norton, Surface Sci., 47, 98 (1975) 15 A. Kleine, P.L. Ryder, N. Jaeger, G . Schulz-Ekloff, J. Chem. SOC., Faraday Trans. I, 82, 205 (1986) 16 G. Schulz-Ekloff, D. Wright, M. Grunze, Zeolites, 2, I0 (1982) 17 S. Briese-Gulban, H. Kompa, H. Schrubbers, G. Schulz-Ekloff, React. Kinet. Catal. Lett. 2 0 , 7 (1982) 18 H. Kacirek, H. Lechert, J. Phys. Chem., 79, 1589 (1975) 19 D. Exner, N. Jaeger, A. Kleine, G. Schulz-Ekloff, J. Chem. SOC., Faraday Trans. I, 1988 in press 20 P. Gallezot, in Studies in Surface Science and Catalysis, vo1.5, "Catalysis by Zeolites" (B. Imelik et al., Eds.), Elsevier, Amsterdam 1980, p. 227 21 D. Exner, N. Jaeger, G. Schulz-Ekloff, Chem. Ing. Techn., 52, 734 (1980) 22 M.J. Yacaman, Appl. Catal. 13, 1 (1984) 23 C.S. Duris, SIAM J. Numer. Anal. 14, 686 (1977) 24 N.I. Jaeger, A. Jourdan, G. Schulz-Ekloff, A. Svensson, G. Wildeboer, Chem. Express, 1, 697 (1986) 25 F.T. Wagner, T.E. Moylan, Surface Sci., 191, 121 (1987) 26 J. ~ u s y ,R. Ducros, Surface Sci., 176, 157 (1986) 27 W. Romanowski, S. Engels, Hochdisperse Metalle, Verlag Chemie, Weinheim 1982 28 R.J. Berry, Surface Sci., 76, 415 (1978) 29 D.W. Breck, in "Zeolite Molecular Sieves", Wiley, New York 1974, p. 671 30 R.M. Barrer, in "Zeolites and Clay Minerals as Sorbents and Molecular Sieves", Acadenic Press, London 1978, p. 256 31 R.A. Demmin, R.J. Gorte, J. Catal., 90; 3 3 (1984) 32 W.J. Mortier, R.A. Schoonheydt, Prog. Solid St. Chim., 16, 1 (1985) 33 S. Ladas, R. Imbihl, G. Ertl, Surface Sci., 197, 153 (1988) 34 J. Canullo, Y. Uchida, G. Lehmpfuhl, T. Twomey, D.M. Kolb, Surface Sci., 188 , 350 (1987) 35 S. Wong, M. Flytzani-Stephanopoulos, M. Chen, T.E. Hutchinson, L.D. Schmidt, J. Vac. Sci. Technol., 14, 452 (1977) 36 T. Wang, C. Lee, L.D. Schmidt, Surface Sci., 163, 181 (1985) 37 P.J.F. Harris, Surface SCi., 185, L459 (1987) 38 G. Schulz-Ekloff, N.I. Jaeger, Catalysis Today, in press 1 2


C. Morterra,A. Zecchina and G . Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands

515

COMPARISON OF COPPER MID PALIADIIRI CATALYSTS IN THE SYHTHESIS OF HETEANOL FitOM CO/H,

MIIXTURES

J.R. JENNINGS’ and M.S. SPENCERa. Research & Technology, Catalysis Research Centre, ICI Chemicals & Polymers Ltd, P 0 Box 1, BILLINGHAM, Cleveland, TS23 lLB, UK ABSTRACT

Various alkali-promoted palladium catalysts were used for methanol synthesis under typical industrial synthesis conditions. High selectivities for methanol were found with alkali-promoted Pd/SiO, and Pd/La,O, catalysts. Methane was the major by-product but minor quantities of ethane were also formed. Carbon dioxide has little effect on methanol synthesis over palladium catalysts in marked contrast to the strong promotion effect on typical copper catalysts. This difference is attributed to the differences in CO adsorption and adsorbed oxygen coverage on the two metals. Two types of mechanism for methanol synthesis in CO/Ha mixtures are proposed for both copper and palladium catalysts.

IXIXODUCIXOH

High yields of methanol from synthesis gas over palladium catalysts were first obtained by Poutsma et a1 (ref 1) and in the subsequent years much work has been done (refs 2-11) to elucidate the mechanism of the reaction and the factors influencing selectivity for methanol formation. The importance of palladium crystal morphology, promoter action, support influence and the oxidation state of palladium has been emphasised by different workers. However, little work has been reported for typical industrial conditions used for methanol synthesis, i.e. pressures of 50-100 bar and with synthesis gases containing carbon dioxide as well as carbon monoxide. Chinchen et a1 (ref 12) have shown that methanol is formed only from carbon dioxide in CO,/CO/H, mixtures using typical commercial catalysts and that support effects are minimal (refs 13,141 for such catalysts of high copper content prepared by co-precipitation. Other workers (ref 15.16) have found support effects with copper catalysts prepared by different methods. Support effects are more pronounced in methanol synthesis from CO/H, mixtures, where the carbon source must be carbon monoxide, than in methanol synthesis from 1

Now at R&T Department, ICI Chemicals & Polymers Ltd., P 0 Box No 90, Wilton, Middlesbrough, Cleveland TS6 8JE, UK.

2

Now at Department of Chemistry, University of Wales College of Cardiff, P 0 Box 7 8 , Cardiff CF1 1XL. UK.

616

CO,/H, and CO,/CO/H, mixtures (refs 15,16)

.

Catalyst preparation methods are

of major importance in influencing specific activity and perhaps the most striking examples have been the copper catalysts prepared by the controlled oxidation of various copper alloys (refs 17-20).

These catalysts have

exceptionally high activities for methanol synthesis in CO/H, mixtures. Fakley et a1 (refs 21,221 have proposed that two types of mechanism can operate in the synthesis of methanol from CO/H, mixtures over copper catalysts. In Type 1, occurring solely on the copper metal surface, adsorbed carbon monoxide is hydrogenated. With a base present, either as support or promoter, Type 2 mechanism is possible, when carbon monoxide reacts to give a formate intermediate on the basic support or promoter or at the copper metal/oxide interface. Similar mechanisms have been proposed (refs 2-11) for methanol synthesis from CO/H, mixtures over palladium catalysts. In this paper some experimental results with palladium catalysts under industrial methanol synthesis conditions are used with earlier data on copper and palladium catalysts to assess how far common mechanisms apply in methanol synthesis.

-AL MID RESULTS Catalyst Preparation Catalysts were prepared by addition of the appropriate alkali chloropalladite in aqueous solution to the support and kneading to form a smooth paste. The paste was then dried overnight at 100°C. and compressed into pellets, which were subsequently broken into approximately 100-600 micron particles prior to charging into the catalyst test unit. Details are given in Table 1. TABLE 1 : Catalysts Used Catalyst Number

Alkali

1

Li Li Li Na Li

2

3

4 5

Support

% Pd

Alumina Na Y Zeolite Lanthanum oxide Davison 57 Silica Davison 57 Silica

3.4 2.8 5.0

4.5 4.4

Catalyst Testing Catalyst particles were charged into the reactor and reduced in a flowing mixture of 5% hydrogen in nitrogen. The catalysts were then tested in sythesis gas comprising 67% H,, 20% N,, 10% CO and 3% CO, at GHSV of 5000 or

517

10000 hr-l, and at temperatures shown on Figures 1-5. The pressures were 50 or 100 bar absolute. Quantitative analysis was carried out by gas chromatography using a capillary column, and all components of each reaction

product were identified by g.c/mass spectrometry. Other than methane and methanol, the only products identified were dimethyl ether from the catalyst supported on alumina, and ethane from both the alumina- and silica- supported catalysts at 100 bar.

Variations of product formation with temperature are

shown in Figures 1-5.

Maximum selectivities for methanol are given in Table

+I

a

1.7

1

1.6 1.5 1.4 1.3 1.2

4

1

a .. 1I 0

,#'

0

-

/ 0.8

GHSV: lO,OOO/hr GHSV: 5,OOO/hr

-- - -- - -

/'

0.7 0.6

250

270

290

310

530

350

370

390

410

4

Reaction Temperature OC Fig. 1 - Variation of product formation with temperature for Pd/Na/SiO, catalyst. Catalyst 4. Pressure, 50 bar.

TABLE 2 : Maximum Selectivities for Methanol Catalyst Number

Temperature /"C

1 2 3 4 4 5

380 320 360 360 340 340

*

GHSV /hr-l

5000 5000 5000 10000 5000 5000

Calculated on C atom basis

Pressure /bar

100 100

100 50 100

100

Methanol Selectivity* /% 25 50 80 78 66 92

518

DISCUSSION Effects of Carbon Dioxide Methanol synthesis from CO/H, mixtures cannot be carried out under C0,-free conditions over palladium catalysts. except at negligibly small conversions.

All palladium catalysts give significant amounts of methane as a co-product to methanol, so water and carbon dioxide are also present in the reactor. Similarly, carbon monoxide is present in experiments with CO,/H, feeds (refs 23,241 because of the reverse water-gas shift reaction. Any discussion of the effects of carbon dioxide must therefore be concerned with the effects of different concentrations of carbon dioxide in CO/CO,/H, mixtures.

Poutsma et a1 (ref 1) found that added carbon dioxide decreased

somewhat the rate of production of methanol (in marked contrast to the large increase in rate given by carbon dioxide addition over copper catalysts (ref 2)).

Both the rates of methanol synthesis and the selectivities for methanol

are broadly comparable with those found by other workers with similar catalysts in CO/H, mixtures without added carbon dioxide (refs 2,251.

For

example, the high selectivities for methanol given by alkali-promoted palladium on silica and lanthanum oxide supports and the greater effectiveness of lithium over sodium (Table 2) are well established for nominally COX-free conditions (ref 2).

The results available so far indicate that the presence

of carbon dioxide has little effect on the reaction.

It is therefore likely

that the source of carbon for methanol synthesis over palladium is mainly (possibly solely) carbon monoxide rather than carbon dioxide.

Experiments

with 1hC tracers, as used with copper catalysts (ref 121, have not yet been done. The contrast between the reaction patterns of copper and palladium catalysts in CO,/CO/H, mixtures can be understood, at least qualitatively, in terms of differences in the adsorption of intermediates. working copper catalysts is covered with 0.1-0.4 (refs 14.22).

The metal surface of

monolayers of adsorbed oxygen

The state of the surface of a palladium catalyst is not known

experimentally but on comparison (ref 25) of relevant adsorption energies suggests that adsorbed oxygen coverages are much lower on palladium than on copper.

The adsorption energy for carbon monoxide on polycrystalline copper

is about 55 kJ mol-1 (ref 22) whereas values ranging from 90 to 230 kJ mo1-I have been reported (ref 26) for carbon monoxide adsorption on various palladium surfaces.

Thus the working palladim metal surface differs from a

copper metal surface in the same CO,/CO/H, mixture by having a higher coverage of adsorbed carbon monoxide and a much lower coverage of adsorbed oxygen.

519

1'd

2-

Methanol

1

, / M e t h a n e

Ib.etlon Temperature

Fig 2

"c

- Variation of product formation with temperature for Pd/Na/SiO, catalyst. Catalyst 4. Pressure 100 bar.

Ethane

0.1

Rewtion Temperature

'c

Fig 3 - Variation of product formation with temperature for Pd/Li/Al,O, catalyst. Catalyst 1.

The change from carbon monoxide to carbon dioxide as source of carbon for methanol synthesis occurs at very low CO, concentrations over copper catalysts (ref 12). This was interpreted in terms of the key role of adsorbed oxygen: it both decreases the surface concentration of carbon monoxide and facilitates the adsorption of carbon dioxide. From the discussion above palladium surfaces and adsorbed intermediates are probably much less affected by carbon dioxide. Erdohelyi et a1 (ref 24) concluded that CO, is converted to CO before hydrogenation t o methanol over supported palladium catalysts.

520

Fig 4

- Variation of product

formation with temperature for Pd/Li/La,O, and Pd/Li/NaY catalysts. Catalysts 2 and 3.

mdian lhbpW&mPc Fig 5 - Variation of product formation with temperature for Pd/Li/SiO, catalyst. Catalyst 5. Ethane and Dimethyl Ether By-products Ethane was detected as a byproduct with some catalysts but only in runs at 100 bar. Most other workers have reported methane as the sole byproduct in methanol synthesis but C,+ products have occasionally been observed (ref 25). In the absence of any established mechanism for methane formation it is not possible to discuss possible routes to ethane. One general point can be made. The surface coverage of various adsorbed C, species on the palladium metal surface will be higher, under otherwise identical conditions, at 100 bar than at 50 bar, so the probability of dimetisation to various C, species should increase.

521

The formation of dimethyl ether as well as ethane with the Li/Pd/Al,O, catalyst at 100 bar was probably a secondary reaction of methanol at acidic sites on the alumina support. Mechanisms of Methanol Synthesis Fakley et a1 (ref 21) have proposed that methanol synthesis from CO/H, mixtures can take place by two types of mechanism on copper catalysts. These were identified as:Type I. Carbon monoxide, adsorbed on the copper surface, is hydrogenated by the addition of hydrogen atoms while the C-0 bond remains intact. A second C-0 bond (as found in the formate ion) is neither formed nor broken. Type 11. Carbon monoxide, or a partially-hydrogenated intermediate (e.g. HCO), reacts with an oxygen atom on the catalyst surface to give an intermediate, typically a formate, which contains two C-0 bonds. Subsequent reaction leads overall to methanol and the re-formation of the surface oxygen atom. A s methanol is formed for carbon monoxide o n l y at very low concentrations of

adsorbed oxygen on the copper surface, the adsorbed oxygen necessary for the Type I1 mechanism must be on the support phase o r at the metal/support periphery. Formate and other possible di-oxygenate intermediates are anionic, so supports and promoters of a basic character tend to facilitate Type I1 mechanisms. Similar proposals have been made for methanol synthesis over palladium

catalysts. Hicks and Bell (ref 10) proposed a reaction mechanism in which adsorbed carbon monoxide is hydrogenated directly to methanol on the palladium surface, without proceeding via a formate intermediate. The differences in specific activity between e.g., Pd/SiO, and Pd/La,O, catalysts were attributed to changes in the morphology of palladium crystallites. An alternative view is that of Tamaru (ref 3 1 , who has identified formate intermediates by IR spectroscopy. He interpreted the promotion effects of alkali metal compounds in terms of increased stability of formate intermediates. Thus the division of possible mechanisms into types I and I1 provides a basis for the comparison of copper and palladium catalysts. TYPE I MECHANISMS These will be found in catalysts without basic supports or promoters, e.g, unsupported.or silica-supported metals. As methanol can be formed over unsupported Cu (ref 221, Cu/SiO, catalysts (refs 16,22), Pd black and many Pd/SiO, catalysts (refs 1,2,4-11) and Pd(ll0) crystal faces (ref 271, it is clear that Type I mechanisms work on both copper and palladium. On copper this route appears to be intrinsically slower than a Type I1 mechanism or Since adsorbed carbon monoxide synthesis from carbon dioxide (refs 15,16,22). is bonded to copper through the carbon atom, whereas methoxy, for example, is

522

bonded through the oxygen, the reversal of an intermediate must occur at some stage in the reaction. This may account for the relatively slow rate of an apparently simple reaction. However the differences in rates are much less marked with palladium catalysts, on which the same reversal must occur, so a more detailed understanding of the reaction steps is needed. TYPE I1 MECHANISMS The key feature of the Type I1 mechanism is the formation of an adsorbed di-oxygenate intermediate from carbon monoxide and its subsequent

-

hydrogenolysis to methanol, i.e. CO t O(a) (O-kO)(a) tH CH,OH + 0 (a) where the intermediate is an adsorbed formate species (this is a convenient and probable assumption but the argument below also applies to other intermediates. e.g. H,CO,(,)). Surface formates can be made by the reaction of carbon monoxide with alumina, magnesia and zirconia (refs 28.29). Hicks and Bell (ref 10) have also observed formates (but not methanol) produced on lanthanum oxide from CO/H, mixtures. Two main types of formate can be distinguished in catalysts : the formate is either adsorbed on the surface of the support oxide or base promoter alone or at the periphery between the metal surface and the base.

Jin et a1 (ref 30) have shown that carbon monoxide

adsorbed on platinum in a Pt/CeO, catalyst can react with lattice oxygen at the metal/oxide interface. The c o m o n feature of both palladium and copper catalysts which show enhanced specific activity for methanol synthesis in CO/H, mixtures is the presence of either a basic oxide support (La,O,. ZnO, CeO,. etc) or an alkali metal compound promoter. Thus surface formates are formed on those catalysts under reaction conditions and indeed this has been demonstrated both for some palladium catalysts (refs 3,lO) and copper catalysts (ref 25). The key question is therefore, does this adsorbed formate species take part in methanol synthesis? There is much evidence (refs 22,311 that formate hydrogenolysis is the rate limiting step in methanol synthesis from carbon dioxide.

Similarly the key to a fast Type 2 mechanism lies in a fast rate of hydrogenolysis. Adsorbed formate species on the surfaces of oxide supports or patches of alkali promoters need hydrogen spillover from metal crystallites to undergo hydrogenolysis. In contrast, an adsorbed formate species at the metal/base periphery ( m e of the possible structures for an alloy-derived Cu/CeO, catalyst is shown in Figure 6) does not need hydrogen spillover for hydrogenolysis. Clearly the reaction can proceed with hydrogen atoms adsorbed on the metal phase. Fakley et a1 (ref 21) have suggested that the very high synthesis activity of catalysts made by the controlled oxidation of copper/cerium and other alloys can be attributed to a Type 2 mechanism using peripheral formate. The

523

H

I

I I I I

periphery

Fig 6 - Formate intermediate at metal oxide periphery on alloy-derived Cu/CeO, catalyst very small size of copper clusters (refs 17-20) gives a large peripheral reaction zone. Calculations (ref 21) showed that peripheral reactions were unlikely to be significant in copper catalysts prepared by conventional co-precipitation or by impregnation. With copper crystallites of 5m or more in diameter the peripheral reaction zone is too small to be significant. Thus the enhanced activity of Cu/ZnO (ref 16) or Cu/ZrO, (ref 15) catalysts over Cu/SiO, catalysts must involve hydrogen spillover if a Type 2 mechanism is proceeding. A wide range of Pd crystallite sizes has been used in studies of supported palladium catalyst and Type 2 mechanisms involving peripheral adsorbed formate species seem possible, especially for the smaller palladium crystallites. Crysallite Size and Morphology Effects A constant specific activity should be found in catalysts operating by Type I mechanisms, when the fraction of active metal surface to total metal surface remains constant. Variations in metal crystallite morphology (as a result of epitaxy, etc) and size, giving different proportions of active faces, edges, etc, could lead to marked variations in specific activity. There is disagreement on the dependence of specific activity of Pd/SiO, catalysts on palladium crystallite size (refs 5,32-34). This can be accounted for in differences in palladium crystallite morphology affecting a Type I mechanism but the possibility of alkali impurities promoting a Type 2 mechanism cannot be discounted. A s Type 2 mechanisms involve a promoter or support phase as well as the.meta1 phase, no simple relation between catalyst activity and total metal surface area can in general be expected. A correlation between activity and metal crystallite periphery might be expected if peripheral formate were important, or if the rate of hydrogen spillover controlled methanol synthesis rate, but no such correlation has been reported. ACKMXLKWrn

The authors thank J Holt and R H Logan for help with the experimental work.

524

1.

2. 3. 4. 5. 6. 7.

8.

M.L. Poutsma. L.F. Elek, P.A. Ibarbia. A.P. Risch and J.A. Rabo, J. Catal., 52 (1978) 157. G.C. Chinchen. P.J. Denny, J.R. Jennings, M.S. Spencer and K.C. Waugh, Appl. Catal.. 36 (1988) 1. Y. Kikuzono, S . Sagami, S . Naito, T. Ouishi and K. Tamaru, Faraday Disc. Chem. SOC., 72 (1981) 143. F. Fajula, R.G. Anthony and J.H. Lunsford, J. Catal., 73 (1982) 237. K.P. Kelly, T. Tatsumi, T. Vematsu, D.J. Driscoll and J.H. Lunsford, J. Catal., 101 (1986) 396. H. Deligianni. R.L. Mieville and J.B. Peri. J. Catal., 95 (1985) 465. E.K. Poels, R. Koolstra. J.W. Geus and V. Ponec, in B. Imelik, C. Naccache, G. Coudurier. H. Praliaud, P. Merideau, P. Gallezot, G.A. Martin and J.C. Vedrine, (Eds.), Metal-Support and Metal-Additive Effects in Catalysis, Studies in Surface Science and Catalysis, Vol. 11, Elsevier, Amsterdam, 1982, p.233. J.M. Driessen, E.K. Poels, J.P. Hindermann and V. Ponec, J. Catal., 82 (1983) 26.

R.F. Hicks, Q.J. Yen, A.T. Bell and T.H. Fleisch, Appl. Surface Sci., 19 (1984) 315. 10. R.F. Hicks and A.T. Bell, J. Catal., 90 (1984) 205. 11. R.F. Hicks and A.T. Bell, J. Catal., 91 (1985) 104. 12. G.C. Chinchen, P.J. Denny, D.G. Parker, M.S. Spencer and D.A. Whan, Appl. Catal., 30 (1987) 333. 13. G.C. Chinchen and K.C. Waugh, J. Catal., 97 (1986) 280. 14. G.C. Chinchen. K.C. Waugh and D.A. Whan, Appl. Catal.. 25 (1986) 101. 15. B. Denise, R.P.A. Sneeden, B. Begiun and 0. Cherifi, Appl. Catal.. 30

9.

(1987) 353. 16.

R. Burch, J. Chem. SOC., Faraday Trans. I, 83 (1987) 2250; Appl. Catal.,

43 (1988) 141. 17. R.M. Nix, T. Rayment, R.M. Lambert, J.R. Jennings and G. Owen, J. Catal., 106 (1987) 216. 18. G. Gwen, C.M. Hawkes, D. Lloyd, J.R. Jennings, R.M. Lambert and R.M. Nix. Appl. Catal., 33 (1987) 405. 19. C.M. Hay, J.R. Jennings, R.M. Lambert, R.M. Nix, G. Owen and T. Rayment. Appl. Catal.. 37 (1988) 291. 20. S.J. Bryan, J.R. Jennings, S . J . Kipling, G. Gwen. R.M. Lambert and R.M. Nix, Appl. Catal., 40 (1988) 173. 21. M.E. Fakley, J.R. Jennings and M.S. Spencer, J. Catal., in press. 22. G.C. Chinchen, M.S. Spencer, K.C. Waugh and D.A. Whan, 3. Chem. SOC., Faraday Trans. I, 83 (1987) 2193. 23. E. Ramaroson. R. Kieffer and A. Kiennemann, J. Chim. Phys., 79 (1982) 759. 24. A. Erdohelyi, M. Pasztor and F. Solymosi, J. Catal., 98 (1986) 166. 25. G.C. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer and K.C. Waugh, 26.

to be published. G.A. Somorjai, Chemistry in Two Dimensions : Surfaces, Cornell Univ. Press, Ithaca, 1981, p. 320; P. Chou and M.A. Vannice, J. Catal., 104

(1987) 17. 27. P.J. Berlowitz and D.W. Goodman, J. Catal., 108 (1987) 364. 28. P.G. Gopal, R.L. Schneider and K.L. Watters, J. Catal.. 105 (1987) 366. 29. N.S. Jackson and J.G. Ekerdt, J.Catal., 101 (1986) 90. 30. T. Jin, T. Okuhara, G.J. Mains and J.M. White, J. Phys. Chem., 91 (1987) 3310. 31. L.L. Mueller and G.L. Griffin, J. Catal., 105 (1987) 353. 32. J.S. Rieck and A.T. Bell, J. Catal., 103 (1987) 46. 33. S. Ichikawa, H. Poppa and M. Boudart, ACS Symposium Series, 248 (1984) 439. 34. S. Ichikawa, H. Poppa and M. Boudart. J. Catal., 91 (1985) 1.


525

MOLYBDENA ON NIOBIUM O X I D E CATALYSTS : PREPARATION AND CHARACTERIZATION

Y.S.

JIN, A. OUQOUR, A. AUROUX

and J.C. VEDRINE

I n s t i t u t de Recherche5 s u r l a Catalyse, CNRS, conventionne i 1 ' U n i v e r s i t e Claude Bernard, LYON I, 2, Avenue A. E i n s t e i n 69626 V i l l e u r b a n n e Cedex FRANCE

ABSTRACT Niobium o x i d e has been chosen as a support f o r molybdenum o x i d e c a t a l y s t w i t h t h e aim o f s t u d y i n g t h e d i s p e r s i o n o f molybdenum o x i d e species on t h e s u r f a c e as a f u n c t i o n o f Mo l o a d i n g . Niobium oxide c a l c i n e d a t 500°C e x h i b i t s low a c i d i t y w h i l e Mo species a r e a c i d i c w i t h t h e f o r m a t i o n o f hydroxyl groups (3440 cm-1 band) o f low a c i d i t y and o f Lewis s i t e s . A few strGngacid s i t e s have been evidenced by m i c r o c a l o r i m e t r y (120-140 kJ.mo1-1 f o r the i n i t i a l heat o f NH3 adsorption). ESR technique c l e a r l y shows t h a t Mo5+ i o n s a r e n o t i n c o r p o r a t e d i n t o Nb2O5 m a t r i x i n a s o l i d s o l u t i o n form b u t a r e present p a r t i c u l a r l y a f t e r c a t a l y t i c r e a c t i o n due t o t h e r e d u c t i o n o f polymeric molybdate species w h i l e XPS technique shows t h a t Mob+ i o n s i n h i g h c o n c e n t r a t i o n a r e depo s i t e d on niobium oxide support. The c a t a l y s t t u r n s o u t t o be compose o f !lo polymeric species (UV-vis-band near 360 nm) w i t h OH groups depos t e d on Nb2O5. I t c o n s t i t u t e s a poor p a r t i a l o x i d a t i o n c a t a l y s t f o r propene 70 t o 80 % s e l e c t i v i t y i n CO2) and isopropanol dehydrogenation i n t o acetone b u t e x h i b i t s a c i d type behaviour f o r isopropanol dehydration. INTRODUCTION The even spreading o f an oxide over another oxide as a support has been l a r g e l y s t u d i e d , p a r t i c u l a r l y f o r Moo3, U03 and V205 on Si02, A12D3, TiD2, e t c ( r e f . 1) As a f u n c t i o n o f t h e coverage, d i f f e r e n t types o f species a r e deposit e d f o r i n s t a n c e monomeric Td Mo 0;-

species, polymeric Oh molybdate species

and f i n a l l y b u l k - t y p e molybdenum oxide ( r e f . 2 ) . These t h r e e main types o f spec i e s are e x h i b i t i n g d i f f e r e n t c a t a l y t i c and a c i d i c p r o p e r t i e s ( r e f s . 3-6). Niobium o x i d e has been e x t e n s i v e l y s t u d i e d as a p o t e n t i a l c a t a l y s t p a r t i c u l a r l y by K. Tanabe and coworkers ( r e f s . 7-9).

However i t was o n l y seldom used

as a support o f m e t a l l i c o r oxide c a t a l y s t ( r e f . 10). I n a wide programme

of

research i n v o l v i n g Mo as an a c t i v e c a t i o n f o r p a r t i a l o x i d a t i o n o f o l e f i n s o r f o r o x i d a t i v e dehydrogenation o f alcanes o r oxygenate compounds , one has been concerned w i t h t h e d e p o s i t i o n o f molybdenum o x i d e species on Nb oxide. C a l c i n a t i o n o f n i o b i c a c i d H3B03 a t temperature from 100 t o 500°C i s known ( r e f . 9) t o t r a n s f o r m s t r o n g a c i d i c m a t e r i a l i n t o a weakly a c i d one (Nb205) w h i l e the s u r f a c e area was shown t o decrease d r a s t i c a l l y . D i f f e r e n t ways o f d e p o s i t i n g an o x i d e on a support do e x i s t . The more w i d e l y used method c o n s i s t s i n impregnating t h e support w i t h an aqueous s o l u t i o n o f

526

ammonium heptamolybdate a t a given pH chosen as a f u n c t i o n o f t h e i s o e l e c t r i c p o i n t o f the support ( I E P S ) ( r e f . 11). The support e x h i b i t s p o s i t i v e charge a b l e t o a t t r a c t a n i o n i c species as molybdate ions f o r pH i n f e r i o r t o the I E P S (0.5 f o r Nb205). Other procedures o f preparation c o n s i s t i n depositing organometall i c complexes o f Mo o r .in g r a f t i n g a Mo compound by chemical r e a c t i o n w i t h surface a c t i v e groups as hydroxyls o r i n c a l c i n i n g a mechanical mixture o f Moo3 and t h e support a t h i g h temperature (> 500°C) t a k i n g advantage o f the r e l a t i v e l y low m e l t i n g p o i n t o f Moo3. I n the present work our i n t e r e s t has been focussed on the deposition o f molybdenum oxide on the surface o f niobium oxide, on the c h a r a c t e r i z a t i o n o f the m a t e r i a l s by several physical techniques and a t l a s t on the study o f cata1y t i c properties. EXPERIMENTAL The samples were prepared by i n c i p i e n t wetness impregnation Nb205 ( n i o b i c a c i d t r e a t e d i n a i r a t 500°C) w i t h an aqueous s o l u t i o n o f amnonium paramolybdate a t a pH i n f e r i o r t o 0.5 ( s e r i e s A) and 5-6 ( s e r i e s B) and f u r t h e r c a l c i n a t i o n a t 50OOC f o r 4 h. The IEPS p o i n t o f Nb205 i s 0.5 ( r e f . 12). The Mo content var i e d from 0 up t o 8 w t %.Nb205 as s t a r t i n g m a t e r i a l has a surface area close t o 35 m2g-1. Nb205 as a support was prepared by p r e c i p i t a t i n g the hydroxide by NH40H sol u t i o n added t o a methanolic s o l u t i o n o f NbC15 ( A l d r i c h 99 % pure). A TDA anal y s i s shows endothermical peaks a t 70 and 136OC w i t h dehydration completed a t 300°C. BET surface area was observed t o decrease from 216 t o 43 down t o 35 m2g-' when c a l c i n a t i o n temperature varies from 110, 400 and 500°C, r e s p e c t i -

vely. S i x d i f f e r e n t samples were prepared as described above w i t h chemical compos i t i o n equal t o 3, 5 and 8 w t % r e s p e c t i v e l y ( s e r i e s A and B) i.e. w i t h an atomic r a t i o Mo/Nb equal t o 0.042, 0.073 and 0.12 respectively. X ray d i f f r a c t i o n studies using a c l a s s i c a l X r a y d i f f r a c t o m e t e r and CuK, source shows t h a t , w i t h i n the technique accuracy, Moo3 c r y s t a l 1 i t e s are n o t detectable. I R spectra were recorded w i t h a Perkin Elmer 580 spectrometer w i t h e i t h e r s e l f supported wafers o r 1 %, KBr d i l u t e d discs. ESR spectra were obtained a t room and l i q u i d n i t r o g e n temperatures w i t h a Varian E 100 spectrometer, UV-vis. spectra were registered w i t h a Lambda 9 Perkin Elmer spectrometer working i n d i f f u s i o n r e f l e c t a n c e mode. XPS data were obtained w i t h a Helwett Packard HP 5950 A spectrometer a t room temperature w i t h the sample powder attached under pressure t o a s o f t indium holder. Charging e f f e c t was compensated w i t h

527

an electron flood gun. Cata1yti.c properties were studied a t low conversion level ( c few percents) f o r three reactions, namely : a isopropanol dehydration to propene b i sopropanol oxidative dehydrogenation t o acetone c propene oxidation into acrolein, carbon oxides, propanal, ethanal , . A flow microreactor in line with gas chromatography detectors was used f o r the reactions.

..

EXPERIMENTAL RESULTS and DISCUSSION IR study 1 %, KBr discs were analyzed in the vibrational mode region below 1100 cm-l. Two main peaks were detected a t 700 and 900 cm-' f o r Nb205. For the 5 w t % Mo/Nb205 additional peaks a t 870, 560 and 370 cm-l were observed while orthorhombic Moo3 i s known t o exhibit characteristical bands a t 990, 870, 590, 390 and 300 cm-1 assigned t o Mo=O, Mo-0-Mo and Mo-0-Mo stretching vibrations f o r the

I

Mo

f i r s t three bands and t o deformation vibrations f o r the l a t t e r two. Note t h a t the 990 cm-' band typical of Moo3 was n o t observed f o r any of our samples. This supports the XRD findings. The bands as observed may be assigned t o hexagonal Moon (ref. 13) o r heptamolybdate species ( r e f . 14).

3800

3000 WAVENUMBER

3400

3200

5 1 em'

Fig. 1. IR spectra in the 3200-3800 cm-' region f o r sample A2 ( s e r i e s A , 5 w t % Mo). ( a ) a f t e r outgassing a t room temperature. ( b ) a f t e r outgassing a t 100°C. (c) a f t e r contacted w i t h 1 t o r r NH3 a t 20°C and outgassing a t 20°C. (d) a f t e r contacted w i t h 100 t o r r NH3 a t 20°C and outgassing a t 100°C. ( e ) pure Nb205.

528

For the s e l f supported p e l l e t s outgassed a t increasing temperatures from 20 t o 300°C t h e r e s u l t s are presented i n f i g . 1. I n the 3200-3600 u n - l region a broad peak was detected f o r pure Nb 0 A t variance f o r Fto/Nb205 samples a 2 5:1 narrow peak appeared near 3440-3445 cm They disappeared under outgassing a t

.

temperatures higher than 250°C. As discussed below and by comparison t o o t h e r systems t h i s band may be assigned t o OH groups bonded t o Ma o r Nb cations ( r e f . 15) o r t o H20 bonded t o Mo o r Nb cations as i n aquo complexes described i n ( r e f . 16) f o r high surface area Moo3. Under rehydration o f the 250°C samples, the 3445 cm-'

band f o r Mo/Nb205 and

the broad band f o r pure Nb205 reappeared, a t l e a s t p a r t l y . This shows t h a t the 3445 cm-l band corresponds t o OH groups o f Nb205 modified by the presence o f

Mo ions.

Upon NH3 adsorption the 3445 cm-' band i n t e n s i t y decreased. By e l i m i n a t i n g excess NH3 by outgassing a t room temperature bands a t 3445, 3360, 3270 (see f i g . 1 c and d) and 1605, 1450 and 1215 cm-l were observed. A f t e r f u r t h e r outgassing a t 100°C the 3445 an-' band was completely restored w h i l e the bands a t 3360 and 3270 cm-l had t h e i r i n t e n s i t y sharply decreased and even disappeared upon outgassing above 200°C. The 3360 and 1215 cm-l bands are assigned t o s t r e t c h i n g and deformation v i b r a t i o n s o f NH3 coordinated 17). The 3270 and 1450 cm-' bands o f NH.;

t o Lewis s i t e s ( r e f .

bands correspond t o s t r e t c h i n g and deformation

One may then conclude t h a t the Mo/Nb205 samples e x h i b i t much

higher a c i d i t y than pure Nb205, even i f the a c i d i t y corresponds t o a medium strength compared t o strong a c i d m a t e r i a l s as z e o l i t e s . This a c i d i t y corresponds t o both Lewis and Br'dnsted s i t e s . UV-vis. spectroscopy data I t i s generally accepted ( r e f . 18) t h a t Mo6+ i n tetrahedral monomeric spe-

c i e s Moog- absorb near 230-270 nm and Mo6+ i n octahedral polymeric species absorb near 310-340 nm. For a l l samples a broad UV-vis. absorption band i s observed between 380 t o 200 nm w i t h a maximum near 300 nm as shown i n f i g . 2 due t o the charge t r a n s f e r absorption o f 4 do Nb(V) ions ( r e f s . 19, 20). Samples containing Mo e x h i b i t an increased absorption near 360 nm which may be assigned t o polymolybdate species. One may then conclude t h a t polymeric molybdate species are formed which i s i n agreement w i t h I R f i n d i n g s f o r framework v i b r a t i o n s .

529

TABLE 1 Chemical analysis and XPS data f o r a l l samples studied. Chemical analysis Samples

atomic

w t % Mo

MoINb

XPS atomic r a t i o s MoINb Nb/O

data

b i n d i n g energies (eV)% Mo 3d Nb 3d

A1 A2 A3

3 5 8

0.042 0.073 0.12

0.18 0.21 0.30

0.37 0.34 0.28

235.9 235.8 235.8

232.7 232.7 232.7

209.8 209.8 209.7

207.1 207.1 207.0

B1

3 5

0.042 0.073

0.18 0.22

0.34 0.38

235.8 235.8

232.6 232.7

209.8 209.8

207.1 207.1

B2

"refered t o Ols a t 530.0 found i d e n t i c a l t o CIS a t 284.5 eV.

Fig. 2. UV-vis. r e f l e c t a n c e spectra o f Mo/Nb O5 samples. (a) sample B3, ( b ) pure Nb2O5. ( c ) d i f f e r e n c e spectrum (a-bf.

XPS data For a l l Mo containing samples a well resolved Mo 3d doublet was observed w i t h binding energy values equal t o 232.7 and 235.8 eV, which correspond t o Mo6' ions (ref. 21). The presence o f a well resolved 3d doublet f o r Mo even a t low Mo loading i s a t variance w i t h Mo ions species on s i l i c a support ( r e f . 22 and references therein) and indicates t h a t well defined molybdate species are laying on the support surface. The atomic Mo/Nb r a t i o may be calculated f r o m the XPS peak area (A) r a t i o s and the following approximate relationship :

where n corresponds t o the number o f atoms, u t o the electron cross section calculated by Scofield (ref. 23) and Ek the k i n e t i c energy value f o r the corresponding peaks. The data f r o m XPS are compared with chemical analysis data i n table 1. It c l e a r l y appears t h a t Mo content a t the surface o f the c r y s t a l l i t e s i s much higher than i n the bulk by a factor o f 4 (samples A1, A2, B1, B2) o r 2 t o 2.5 (samples A3, B3). This corresponds t o molybdate species deposited on Nb205 c r y s t a l 1ites. Microcalorimetry data Adsorption o f amnonia i n successive doses has been followed by microcalorimetry a t 100°C using a Setaram Calvet type microcalorimeter. The samples have been outgassed a t 400°C p r i o r t o NH3 adsorption. The d i f f e r e n t i a l NH3 heats o f adsorption as a function o f NH3 coverage are p l o t t e d on f i g u r e 3. The i n i t i a l heat a t low coverage i s rather high (120-140 kJ.mol"), which corresponds t o strong acidity. The amount of strong s i t e s i s rather low and corresponds t o ca 0.1 umol g - l o r 3 . N 3 umol m-2 while on oxide material a crude approxim2 t i o n gives 10 umol per m Pure Nb205 support was observed t o e x h i b i t very low a c i d i t y w i t h a maximum adsorption heat i n the range 80 t o k.J.mo1-'. The number o f strong acid s i t e s increases with the Mo content, p a r t i c u l a r l y f o r the A series.

.

ESR spectroscopy data (refs. 24,25) ESR spectra typical o f M05' ions are detected with gav = 1.9168 and AHpp = 220 G. No structure was evidenced due t o a x i a l symnetry o r hyperfine coupling and the presence o f oxygen does not modify the peak i n t e n s i t y as could be expected f o r surface Mo5' ions due t o broadening beyond detection. These

531

NH, ADSORBED / cma.g-'

Fig. 3. Heatsof anunonia adsorption a t 100°C versus NH3 coverage f o r sample A and B compared t o Nb205 support (A). parameters i n d i c a t e t h a t Mo5+ ions are i n strong i n t e r a c t i o n one w i t h the o t h e r as i n reduced Moo3 ( r e f . 26) and do n o t correspond t o Mo5' i n Nb205 m a t r i x ( s o l i d s o l u t i o n ) as observed f o r T i O p o r Sn02 ( r e f s . 21, 27, 28). I f a mechan i c a l m i x t u r e o f 5 w t % Moo3 and Nb205 i s performed and calcined a t 500°C i n a i r a s i m i l a r ESR spectrum i s observed. A f t e r isopropanol c a t a l y t i c r e a c t i o n enhanced Mo5' s i g n a l i s obtained w i t h an i n t e n s i t y comparable t o the A1 o r B1 sample ones. Note t h a t i f the samples a f t e r c a t a l y t i c r e a c t i o n are oxidized i n a i r a t 320°C the l i n e i n t e n s i t y decreases and reaches the i n i t i a l values. These data i n d i c a t e t h a t Mo5' ions are n o t incorporated i n Nb205 l a t t i c e i n s u b s t i t u t i o n p o s i t i o n as could be expected by c a l c i n i n g i n a i r t o form a s o l i d s o l u t i o n . They i n d i c a t e t h a t polymolybdate species are formed presumably a t t h e surface o f Nb205 c r y s t a l 1ites , molybdate species r e l a t i v e l y e a s i l y reduced t o g i v e Mo5+ ions. No b i n a r y phases were detected ( r e f s . 29, 30). C a t a l y t i c data Propene o x i d a t i o n was performed a t 400°C i n a f l o w d i f f e r e n t i a l microreactor w i t h 100 mg c a t a l y s t . The main r e s u l t s are given i n t a b l e 2. It c l e a r l y appears t h a t Mo b r i n g s some a c t i v i t y t o Nb205 which i s p a r t i c u l a r l y i n a c t i v e . However the s e l e c t i v i t y i n a c r o l e i n o f Mo supported on Nb205 appears t o decrease w i t h Mo loading i n f a v o r o f t o t a l o x i d a t i o n and n o t i n f a v o r o f propanal as i t was observed f o r Mo/Si02 c a t a l y s t ( r e f . 22). This i s presumably due t o the increase

532

TABLE 2 Propene oxidation a t 400°C with 100 mg catalyst (C3H6 : O2 : N 2 = 100 : 100 : 3 -1 560) flow rate of 1.1 cm . s

.

Mo wt %

Conversion

Nb205

0

0.03

A1 A2 A3

3 5 8

0.23 0.31 0.45

26 21 14

69 73 81

3 5 8

0.13 0.21 0.53

26 28 14

69 63 80

lOOb l0OC

0.03 0.29

35 15

63 79

5

0.33

17

76

Samples

B1 52 B3 Moo3

Md

%

Selectivitya % acrolein CO2 9 88

aby products are acetone (1-3 %), propanal (1-2 X) and ethanal ( 2 %). blow surface area Moo3 prepared by calcining ammonium paramolybdate i n a i r . 2 -1 'high surface area (44 m .g ) Moo3 prepared by the torch method (oxidation of MoCl5 in a torch flame). dmechanical mixture of high surface area Moo3 (Mo 5 % w t ) and Nb2O5 calcined in a i r a t 500°C. in acidic character of such catalyst w i t h Mo loading. For isopropanol reaction in a i r a t l l O ° C the results are given in table 3. I t appears t h a t the a c t i v i t y i s rather high w i t h respect t o Moo3 on different supports, increases w i t h Mo loading which favors the formation of propene w i t h respect t o acetone. This r e s u l t supports the conclusion that the acidic properties (evidenced by propene formation) increase w i t h Mo loading. While Nb205 i s rather inactive f o r both reactions, propene oxidation and isopropanol transformation, Mo/Nb205 samples are very active even w i t h respect of Moo3 taking into account the low Mo loading. Moreover f o r propene oxidation c a t a l y t i c properties are closer t o those of high surface area Mooj (44 rn2g-l) than f o r low surface area Moo3 (0.5-1 m2g-1).A11 these data i n addition t o XPS data which show t h a t Nb205 c r y s t a l l i t e s are rich i n Mo a t t h e i r surface lead AS t o suggest t h a t molybdate species are well dispersed a t the surface of Ub205 crystal 1 i t e s

.

533

TABLE 3 Isopropanol transformation a t 110°C i n a i r w i t h 100 mg o f c a t a l y s t (isopropanol: 10 t o r r ) and a f l o w r a t e o f 0.3 cm3 . s -1

.

Samples

Mo wt %

conversion %

Selectivity % acetone propene

0

E

nd

nd

3

4

49

51

A2

5

10

30

70

A3

8

9

21

79

3

3

40

60

5

7 11

33

67

38

62

Nb205 A1

B1 *2 B3 MOO3

MC

8 100a lOOb

0.5 3

= 46 23

5

9.5

15

2

54 77

85

slow surface area Moo3 as i n t a b l e 2. 2 -1 bhigh surface area Moo3 (44 m .g ) as i n t a b l e 2. 'mechanical mixture o f high surface area Moog and Nb205 calcined a t 500°C i n a i r . nd non detectable. CONCLUSION The impregnation o f molybdenum oxide on 500°C calcined Nb205 leads t o a m a t e r i a l which e x h i b i t s d i f f e r e n t physicochemical and c a t a l y t i c p r o p e r t i e s by comparison w i t h m a t e r i a l s prepared by impregnation on o t h e r support as Si02, Ti02 o r A1203. A few strong a c i d s i t e s a r e created and hydroxyl groups r e l a t e d t o Mo b u t o f r e l a t i v e l y weak a c i d i t y as w e l l , These polymolybdate species, i d e n t i f i e d by the UV-vis. band near 360 nm, are located a t the surface o f Nb205 c r y s t a l l i tes as evidenced by XPS technique. They e x h i b i t r e l a t i v e l y high c a t a l y t i c a c t i v i t y w i t h u n f o r t u n a t e l y low p a r t i a l o x i d a t i o n f e a t u r e f o r propene o x i d a t i o n and r a t h e r a c i d i c type p r o p e r t i e s f o r isopropanol conversion. These polymeric moiybdate species are r a t h e r e a s i l y reduced under c a t a l y t i c r e a c t i o n conditions as evidenced by ESR o f Mo5+ ions. No important e f f e c t o f the pH o f preparation before c a l c i n a t i o n w i t h respect t o the i s o e l e c t r i c p o i n t o f the support ( I E P S ) has been observed f o r Mo ions dispersed on the surface and c a t a l y t i c properties.

534

REFERENCES 1 For instance : F.E. Massoth, Adv. i n Catal. 27 (1978) 265 and references therein. 2 S.R. Stampfl, Y. Chen, J.A. Dumesic, C. Niu and C.G. H i l l , J. Catal. 105

(1987) 445. 3 M. Che, F. Figueras, M. F o r i s s i e r , J.Mc Ateer, M. Perrin, J.L. Portefaix and H. Praliaud, Proceed V I I n t e r n . Cong. on Catal., G.C. Bond, P.B. Wells and F.C. Tompkins, Ed., The Chemical Society, London 1976 p. 261. 4 R. Thomas, J.A. Moulijn, V.H.J. de Beer and J. Medema, J. Molec. Catal.

8 (1980) 161.

5 6 7 8 9

C. Louis, J.M. Tatibouet and M. Che, Polyhedron 5 (1986) 123. Y. Barbaux, A. Elamrani and J.P. Bonnelle, Catalysis t o day 1 (1987) 147. K. Tanabe and T. Iizuka, Niobium Technical Report NbTR.08/85 pp 1-14. K. Tanabe, M a t e r i a l s Chem. and Phys. 17 (1987) 217-225. T. Iizuka, K. Ogasawara and K. Tanabe, J. Chem. SOC. Japan, 56 (1983)

2927-2931. 10 11 12 13 14 15

E.I. J.P. G.A. E.M. M.J. J.S.

81 16 P. 77 17 D.

KO, J.M. Hupp, F.H. Kogan and N.J. Wagner, J. Catal. 84 (1983) 85-94. Brunelle, Pure Appl. Chem., 50 (1976) 1211. Parks, Chem. Rev. 65 (1965) 177. McCarron 111, J. Chem. SOC., Chem. Commun. (1986) 336-338. Schwing-Weill and F. Arnaud-Neu, B u l l . SOC. Chim. F r (1970) 853-860. Chung, R. Miranda and C.O. Bennett, J. Chem. SOC. Faraday Trans, I,

(1985), 19-36. Vergnon, D. Bianchi, R. Benali Chaoui and G. Coudurier, J. Chim. Phys.

(1980) 1043-1049.

Bianchi, J.L. Bernard, M. Camelot, R. Benali-Chaoui and S.J. Teichner, B u l l . SOC. Chim. Fr. (1980) I 275-280. 18 A. Castellan, J.C.J. Bart, A. Vaghi and N. Giordano, J. Catal., 42 (1976)

162. 19 C.K. Jdrgensen, i n ''Absorption Spectra and Chemical Bonding i n Complexes" , Pergamon, Oxford 1964 p. 284. 20 M. Guenin, R. Frety, E. Garbowski and P. Vergnon, J. Mater. Science 23 (1988) 1009-1013. 21 J.C. Vedrine, H. Praliaud, P. Meriaudeau and M. Che, Surf. Sci. 80 (1979) 101-109. 22 T.C. Liu, M. F o r i s s i e r , G. Coudurier and J.C. Vedrine, Submitted t o J. Chem. SOC. , Faraday Trans , I , i n may 1988. 23 J.H. Scofield, J. Electron Spectrosc. 8 (1976) 129. 24 C. Louis and M. Che, J. Phys. Chem. 91 (1987) 2875-2883. 25 M. Che and L. Bonneviot, Z e i t . f u r Physik. Chem. Neue Folge, 152 (1987) 5, 113-119. 26 P.F. Cornaz, J.H.C. Van Hooff, F.J. P l u i j m and G.C.A. Schuit, Discuss. Faraday, 41 (1966) 290-304. 27 R.T. Kyi, Phys. Rev. 128 (1962) 151. 28 Ph. de Montgolfier, P. Meriaudeau, Y. Boudeville and M. Che, Phys. Rev. B 14 (1976) 1788-1795. 29 R.S. Roth and J.L. Waring, J. Res. Nat. Bur. Stand., Sect. A, 74 (1970) 485. 30 K. Tanabe and T. Iizuka, i n "Niobium Techn. Report, NbTR, p a r t I (1983) 1-30.

C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam Printed in The Netherlands

-

A.

535

S. Khannal, W. J. Quadakkers, H. Schuster,

Institut fur Reaktorwerkstoffe, Kernforschungsanlage, Julich, F.R.G. K. Wissenbach2,

A.

Gasser2 and E.

Kreutz3,

W.

Guest Scientist, IGCAR, Kalpakkam, India Fraunhofer Institut fur Lasertechnik, Aachen, F .R.G. Lehrstuhl fur Lasertechnik, Aachen, F.R.G.

ABSTRACT Investigations were

carried out to

study the effect of the formation of

surface alloys on the high temperature oxidation of an Laser glazing was

carried out

INCONEL 713 alloy.

on the alloy substrate after coating with

yttrium, cerium o r hafnium metal. A phase rich in aluminum and the coated metal was

seen on the

laser treated material. In the case of Y and Ce

alloying, both oxidation rate and scale adherence were heavy

spalling was detected on the alloy

improved, while

formed with Hf.

effect of laser glazing technique is expected provided a

A

beneficial

right choice of

laser glazing parameters and a good method to coat the alloying element on the substrate is chosen. INTRODUCTION Several nickel base alloys have high reactivity at They form thick oxide scales which

elevated temperatures.

often spa11 during thermal cycling.

Preventive measures have been tried by alloying them with small amounts of active elements like Y,

Ce, Hf,

La etc. The positive effect of the

addition of these elements in reducing the oxidation rate and modifying the scale adherence is extensively reported /1-3/. However, two serious problem have been observed by carrying out only

optimum

such bulk

alloying. Firstly,

quantities of these elements can be

added, as

large

quantities can cause a deleterious effect on the other properties of the parent

alloy, especially the mechanical properties of the alloy. Secondly,

a substantial quantity of these elements are wasted in bulk they are

alloying, as

required mainly to improve the surface properties of the alloy.

Therefore an alternative method

is needed which

can restrict

these

elements on the surface so that an effective use of these additives can be

made to improve the corrosion/oxidation, wear o r friction behaviour

of

the

alloy and at the same time reduce the cost of the alloy by using lesser qmounts Q V tnese ? w e m w a i a . taser g$aoring p w m i s e s tnese actvaptqges. Laser surface alloying is compositions by

of

on

in the melt

a

substrate by

surface

controlled laser

irradiation.

the nature and

of

relative

laser alloyed structure is

amounts

of the alloyed species,

their distribution and cooling rates that occure during laser The biggest

advantage of

laser

alloying is that

strong metallurgical bond

barrier and provides advantages of

excellent

between

them.

protection

This

then

the

alloy

to

processing.

it provides a strong

adhesion between laser alloying zone and substrate due to the a

elemental

pool and subsequent solidification establishes the

modified surface composition. Development influenced by

altering

adding small quantities of alloying elements to a pool of

molten metal. produced Diffusion

a process

formation of

acts as a strong surface.

Other

laser alloying are: (i)due to very small heat affected zone,

bulk characterstics of the substrate are

retained,

and

(ii)high cooling

rate during laser glazing helps in generating novel microstructure.

Surface

alloying using

laser irradiation is not very old. Drapper

given a chronological list of literature on technique from

1964 to

surface

Ni,

Ti

to

metals

Fe,

alloys such as steels and INCONEL. Yttrium and cerium have

not been tried yet. Kaufmann /5/ alloyed its

has

1982. Various metals and compounds have been used

for alloying on several different substrates ranging from pure Al,

/4/

alloying using this

composition using

Rutherford

hafnium on

Backscattering

nickel

and measured

technique. Many studies

ranging from corrosion/oxidation / 6 , 1 / , wear and friction /8,9/

have

been

carried out on laser alloyed materials. In the present work, an attempt has been made to form surface alloys using active elements like Y, Ce, o r Hf alloys formed were

first

on

an

characterised

INCONEL 713 substrate.

Surface

in terms their composition and

morphologies and then the reactivity of the resultant material

was

tested

by investigating their oxidation behaviour.

EXPERIMENTAL A schematic arrangement for the laser surface alloying is given in Fig. 1. The metal

to be alloyed was coated on the substrate using a hot dip method

/lo/, o r by ion-sputtering technique. In hot dip method, heated at metal

500 'C

to be

the

substrate is

for 5 min. and dipped into a solution of the salt of the

alloyed.

The

substrate

is then

reheated at

temperature for 5 min. after complete drying of the surface.

the

same

537 Laser melting

and

alloying was performed using a continuous C02 laser.

After many trial runs using various laser parameters,

following two

sets

of parameters were used for laser alloying:

Set (i)

Set (ii)

Laser power (watts):

750

720

Beam radius (mm) :

0.88

0.80

1

2

Processing speed of the workpiece (m/min.) : Laser

glazing. was

carried out under flowing argon atmosphere (flow rate:

20 l/min.). The sample was preheated to about 500 'C, in order to minimise the

heat

affected

zone and also cracking at the laser melted zone/matrix

interface. Laser glazing was carried out The displacement

by

overlapping of melt

the substrate material. Y, or Ce was coated from a 30 % solutions using

tracks.

from track to track was 0.3mm. INCONEL 713 was used as

hot

,

dip method described above.

LAYER

Y(N03) A

or Ce(N03)

thin coating (about

' A

Fig.1. Schematic arrangement of laser surface alloying.

1Oum) of Y or Ce was thus obtained. Hf

method to

a

was

coated using

ion-sputterring

thickness of 4um. The coating was very uniform in the case of

ion-sputterring method, while a

non-uniform coated

surface was

obtained

using hot dip method.

Oxidation tests were carried out in air at 1OOO'C for durations 300 to 500 hours. Characterisation of laser treated Scanning Electron Microscopy

surface was

carried out

using

(SEM) and Energy Dispersive X-ray analysis

(EDAX) and Electron Probe Microanalyser

(EPMA). Oxide

Scales

after

oxidation were analysed using SEM/EDAX and X-ray diffraction.

RESULTS AND DISCUSSIONS

Characterisation of Surface Alloys Fig.

2a

shows the transverse sections of the

Specimen

(a)

is without

laser treated

alloys.

(b), (c) and (d) are respectively Y, Ce

coated,

and Hf coated specimens. Fig. 2b gives the corresponding line

scans of

Y,

Ce and Hf along with that of A 1 through the laser melted zone using EPMA.

Following inferences can be drawn:

-

There iz not much heat affected zone. All

the

three elements Y, Ce o r Hf are concentrated on the surface. In

the case of Hf, a good amount is being present zone, while melt.

almost neglegible amount

This might

be

due

to

the

resulted in a more uniform coating of non-uniform coating seen on

Y

the

laser melted

of Y and Ce are present in the

fact that Hf

in

ion-sputtering method

compared to

the

relatively

and Ce coated specimens, coated using

hot dip method.

-

A1

was present more homogeneously in the melted zone compared to

that

in the alloy matrix.

The main

difference in the samples glazed using parameters in set (i) and

set (ii) wa:3: (i) the depth of laser melted zone was slightly the cracking in the

laser melted

smaller

(ii)

zone was relatively less and (iii) a

slightly higher concentration of Y and Ce was present on the surface.

Next, it was determined what changes take place in the composition and the morphology

of

the surface after laser glazing. Fig. 3 compares the surface

composition of the as-received alloy with that of

the

laser treated one.

539

Set (i)

Set(ii)

(ii) Fig.2. (i) Optical micrographs showing the transverse sections of the specimens after laser irradiation:(a) as-received; (b)Y-coated;(c)Ce-coated and (d)Hf-coated; (ii) Al,Y,Ce and Hf line scan using EPMA.

is c l e a r t h a t t h e r e is l a r g e difference i n t h e Al concentration on t h e

It

surface of t h e two alloys. The Al concentration has been almost the

laser

treated

specimen.

doubled on

is a l s o s o m small change

There

concentration of Cr and T i . Presence of a strong oxygen peak

in

i n the

the

laser

t r e a t e d a l l o y indicates t h a t A 1 is perhaps present a s an oxide. Surface morphologies

and compositions of t h e Y, Ce and Hf coated substrate

a f t e r laser treatment a r e given i n

Fig.

Following

4.

inferences

can be

drawn from these: On

1.

the

Y

coated sample, t h e r e were several phases of d i f f e r e n t shapes

which were r i c h only i n Al and flat,

plate

like

The

Y.

shapes of

these

of A 1 and Y, some having more Al and less Y and others and more Y.

were

phases

or e l l i p t i c a l and these were having variable amounts having

less

A1

There was, however, negligible amount of Y present i n t h e

matrix. Unlike i n t h e case of Y coated sample, Ce i n t h e Ce coated sample was

2.

d i s t r i b u t e d throughout

the

matrix and a

Ca

and Al r i c h phase was

d i s t r i b u t e d more uniformly on t h e surface. Like Ce, Hf was a l s o present i n t h e matrix and a Al and Hf

3.

was

rich

phase

uniformly d i s t r i b u t e d on t h e surface. Aa shown i n Fig. I ( c ) , t h e

Al, Hf rich phase was thicker along t h e path traversed by laser beam. X-ray d i f f r a c t i o n could not detect t h e presence of any 11l and Y on

the

Y

coated sample.

This

looks

localised only i n very small region. On t h e however, -103

r i c h phase

obvious a s t h e Al, Y r i c h phase is Ce

and Hf

coated specinens,

and Hf02 were detected.

Oxidation Behaviour Linear

plots

of

weight

gain vs tine f o r t h e oxidation of t h e a l l o y s both

with and without l a s e r treatment a r e given in Fig. 5 . results Y

Fig

S(a)

gives

the

on t h e samples oxidised under l a s e r treatmrnt (parameters setti)).

coated sample shows a s l i g h t hprovenunt while Ca coated sample shows

slight

increase

in

weight

gains.

behaviour. The scales show heavy

parameters set(ii)

(Fig.

Hf

coated sample Show8 a

spalling

on

this

a

very bad

sample. Under l a s e r

S ( b ) ) , a lam difference can be notiaed i n the

oxidation behaviour of t h e l a s e r t r e a t e d and non t r e a t e d

SmphS.

In both

541

Fig.3. EDAX results comparing the surface composition of the (a) as-received; and (b) laser irradiated specimens.

Fig.4. SEn micrographs ((a) and (b)) showing the surface morphologiea and EDX analysis ( c ) , showing the surface compositions, of the Y, Ce, H f coated, laser treated specimens.

1.50

(a)

I

u1

N P

N

h l Y

*&OF, I

0

50

/

1 Time 0 0 1 5 hours 0 2 0 0 2 ? 1 0 J o o

r

' I

1

/

0.00 0

I

I

I

I

Jo

100

150

200

Time

/

I

2!Ki

I

3w

hours

0

l M ) 2 W 5 IM ) 4 o o s M ) 6 W 7 W

Time

/

hours

F i g . 5 . Weight change vs time p l o t s for the oxidation i n

F i g . 6 . Weight change vs time p l o t s for the oxidation i n

a i r a t 1000° C for the specimens laser treated with laser parameters (a) s e t ( i ) ; and (b) s e t ( i i ) .

a i r a t 1000° C , showing the e f f e c t of (a) Y and Ce coating ; and (b) polishing a f t e r laser treatment.

543 Y

or Ce coated specimen there is a significant improvement in both the

scale adherence o r

in the reduction of the oxidation rate. Another

important point to note is that even the without

coated as-received laser

treated sample shows a reduction in the oxidation rate.

Since this

reduciton in the oxidation rate is lower than that of the Y or Ce coated samples, it indicates that Y and Ce coating has further helped in reducing the oxidation rate. Fig.

6(a)

compares the oxidation results on the as-received specimen and

the samples after coated with Y

and Ce using hot

dip method.

It is

apparant that Y coating slightly improves the oxidation behaviour while Ce coating slightly makes it worse. Fig. 6(b) gives the information about what happens when the laser coated surface is slightly polished to

remove the surface defects like cracks,

thin oxide formation during laser treatment. The useful efect

of

results show that the

laser treatment vanishes once the top few layers are

removed. By grinding 10 to 15 um of the total 200 um of laser melted

zone,

a good behaving Y coated sample showed heavy spalling on oxidation. Ce and Hf coated specimens also showed higher oxidation rates. The change in the surface composition and the

surface morphology of the

oxide scale formed after oxidation is shown in Figures 7 and 8 . Fig. I shows how the oxide scale changes after laser treatment on the without coated

sample.

The

alumina

and

chromia rich

layer with

concentration of NiO crystals on the as-received sample changes to which

is mainly

alumina and has a few nodules rich in

A1

a high a layer

and Cr.There is

also a significant change in the surface morphology of the oxide scale formed on the laser irradiated sample to that of the as-received sample. Surface morphologies and oxide scale compositions of the as-received sample and the laser alloyed samples after oxidation are given in Fig. 8. Y coated sample (Fig. 8b) shows a very

uniform and adherent layer. The

oxide scale on the Ce coated specimen (Fig. 8c) also appears same as that on Y coated sample. The oxide scale, however, spalled heavily on the Hf coated specimen

(Fig. Ed).

A

significant change in the composition of the

oxide scale must be due to the laser irradiation as well as due the difference in the coated metal.

X-ray

diffraction

results

on

the

oxide

scales

formed

on as-received,

as-received and laser treated, Y coated and Ce coated specimen treatment

indicated

scale

in

the

overall

composition

two

main

coated

Y

and

specimen

and

the

after

coated

as-received

and

scale

cross

oxidation. The scales formed on the laser treated specimens

are quite different from that on the as-received specimen. As X-ray

Ce

presence of Al2Y4O9 and Ce02 was detected

respectively on Y and Ce coated specimens. Fig. 9 compares the sections

the

differences were noticed. The concentration of NiCr204

spinel phase increased compared to the as-received and the laser

of

formed on as-received sample. However, a significant change in

the oxide morphology has been indicated (Fig. 7 ) . On the specimen,

laser

the formation of A1203 and a spinelphase (NiCr204). On

laser treatment, there was no change oxide

after

diffraction

indicated

in

results, there is more concentration of Ni rich oxide on

the laser alloyed specimens compared to that on the as-received one.

Fig.7. SEM micrographs comparing the surface morphology of the oxide scale formed on the (a) as-received and (b) laser treated specimens oxidised in air at 1000° C.

545

>

Ti

.Cr

Fig.8. SEM micrographs showing the scale morphology and EDX analysis showing the scale compositions for the oxide scales formed on the laser treated specimens after oxidation in air at

loooo c.

546

DISCUSSION From

the

alloying

present has

investigations,

definite

advantage

it

is

apparent

that

laser

techniques. A s very c l e a r l y shown t h a t t h e oxidation r a t e of coated

laser

treated

specimen

without

any

helping

as-received

in

the

specimen.

formation

surprising.

improvement

oxidation

in

Y

or

Ce

the The

oxidation

alumina

be

irradiation.

layer

much

faster

rate

of

due

t o the

This

might

than

the

R e l a t i v e l y poor o x i d a t i o n behaviour of t h e Hf coated

specimen is not very a l l o y e d with Hf.

of

the

c o a t i n g might

i n c r e a s e o f A 1 content on t h e s u r f a c e a f t e r l a s e r be

the

a f t e r laser i r r a d i a t i o n was lower t h a n t h e as-received o r

specimens

only l a s e r t r e a t e d specimen. S l i g h t improvement i n the

surface

over t h e bulk a l l o y i n g and o t h e r c o a t i n g

positive

/3/

Strafford behaviour effect

of

of Y

a

has

reported

almost

no

N i C r A 1 based a l l o y when

and

ce a l l o y o i n g is w e l l

r e p o r t e d /2,3/.

F i g . 9 . SEM micrographs comparing t h e c r o s s s e c t i o n of t h e s c a l e formed a f t e r o x i d a t i o n i n a i r a t 1000° : (a)as-received; (b) Y-coated;(c) Ce-coated and (d) Hf-coated

547

CONCLUSION In

conclusion,

great

it

potential

proviided

can be emphasised that the laser glazing technique has

in

forming surface alloys

suitable choice of

laser

is

also

equally

important which

capable of

present

shown

a

a

suitable coating technique

different

positive effect

improvement in chosing better laser parameters required

which

will

result

in

microstructure,

can form a uniform coating layer and is

forming coatings of

results have

novel

irradiation is made which can form ‘a

crack free laser melted zone. The choice of

also

of

thicknesses.

Although

of laser glazing, but an

and

coating technique

further improvement

in the

behaviour of the alloy and hence strongly recommend the

is

oxidation

utility

of

this

technique for surface alloying.

ACKNOWLEDGEMENTS The

authors gratefully acknowledge the help rendered by Messers F. Els, V.

Gutzeit, H.Schulze and H. Griibmeier of respectively

the

analysis using

this

institute for carrying out

SEM/EDAX, optical micrography, X-ray

diffraction and EPMA. They are also thankful to this

institute

for coating Hf

acknowledges with gratitude the

Mr.

A.

Gupta

also

from

using ion-sputterring method. A.S. Khanna Humboldt

foundation for

sponsering his

stay in Germany.

REFERENCES 1. J.G. Smeggil, Mat. Sci. and Eng., 87 (1987) 261 2.

K.N. Strafford, High Temp. Techn., 1 (1983) 307

3.

D.P.

4.

C.W. Draper, J. of Metals, 34 (1982) 24

5.

L.

Whittle

(1980) 309

Buene,

-

and

J.

Stringer, Phil.

-

65. 18.

Trans.

Royal SOC. London, 295

29.

E.N.

Kaufmann et

al.,

-

in

32.

Nuc.

and

Electron Resonance

Spectroscopy, E.N. Kaufmann (Editor), Elsevier, North 391

6. M. 423 7.

-

Pons,

-

Holland,

(1981)

96.

M.

Caillet and A. Galerie, Mater. Chem. and Phys., 16 (1987)

32.

P.G. Moore and E. McCafferty, J. Electrochem SOC.,

129

(1981)

1391

-

94.

8. P.J.E.

Monson

and W.M. Steen, Laser Treatment of Materials, DGM, B.L.

Mordike (Editor), (1986) 123

-

27.

9.

A. Gasser, K. Wissenbach and E.W. Kreutz, ibid., 351

10

M. Landkof, A.V. 344

-

57.

Levy, D.H. Boone and E.

Yaniv,

-

56.

Corrosion,

41

(1985)


C. Morterra,A. Zecchina and G.Costa (Editors), Structure and Reaetiuity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlands

-

549

FOURIER TRANSFORM INFRARED SPECTROSCOPY OF OXIDIZED ULTRA-FINE a-SILICON CARBIDE

M. KIZLING and R.J. PUGH

I n s t i t u t e f o r Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden

ABSTRACT Fourier Transform I n f r a r e d Spectroscopy was shown t o be a useful technique t o estimate the e x t e n t o f o x i d a t i o n o f commercial a - s i l i c o n carbide powders. The a n a l y t i c technique was based on the determination o f the peak i n t e n s i t i e s o f the S i - C - S i and the Si-0-Si bands. The D i f f u s e Reflectance mode ( i n t e n s i t y expressed i n Kubel ka-Munk u n i t s ) appeared t o be s a t i s f a c t o r y f o r determining low concentrations o f s i l i c a (%2 w t % ) and was found t o be r e l a t i v e i n s e n s i t i v e t o low s i l i c a concentrations. INTRODUCTION S i l i c o n carbide ( S i c ) powders are used i n the preparation o f ceramic composi t e s , heat r e s i s t o r s , c a t a l y s t substrates and t o r e i n f o r c e metals and p l a s t i c s . The f i n e powders u s u a l l y have f a i r l y h i g h surface areas and are e a s i l y oxidized i n a i r t o form a p r o t e c t i v e s i l i c a surface f i l m . To produce a good ( d e f e c t f r e e ) s i n t e r e d composite i t i s important t o c o n t r o l the oxygen content o f the powders and several a n a l y t i c a l methods have been standardized i n i n d u s t r y (1,2). These techniques u s u a l l y i n v o l v e d i r e c t combustion o f the powders.

A more d i r e c t method t o study the o x i d a t i o n o f Sic powders i s o f f e r e d by F T I R spectroscopy which o f f e r s the p o s s i b i l i t y t o increase the signal-to-noise

r a t i o t o acceptable l e v e l s a t medium r e s o l u t i o n (2-4 cm-1) f o r the study o f surface o x i d a t i o n and several studies have been r e c e n t l y reported (8,9). I n the present p r e l i m i n a r y i n v e s t i g a t i o n , we have applied FTIR t o study and q u a n t i f y the extent o f o x i d a t i o n o f a series o f commercial S i c powders using both the d i f f u s e r e f l e c t a n c e and transmission modes. Expe r imenta 1 The u l t r a f i n e a-Sic powders (Acheson type) were supplied by Lonza Ltd, Basle, i n three grades (UF 15, UF 25 and UF 45). The major i m p u r i t i e s as reported by the manufacturers were ( a ) UF 15; f r e e C 0.392, f r e e S i 0.12%, t o t a l 0 0.81%, Fe 0.04% and A1 0.05%, (b) UF 25; f r e e C 0.30%, f r e e S i 0.14%, t o t a l 0 1.7%, Fe 0.05% and A1 0.03%, ( c ) UF 45; f r e e C 0.58%, f r e e S i 0.22%, t o t a l 0 3.5%, Fe 0.05% and A1 0.03%. The t o t a l surface area (as determined by BET N2) were

550

as follows: UF 15; 13.4 d / g , UF 25; 22.7 m2/g and UF 45; 34.9 d / g . The part i c l e s were f r e e from processing and manufacturing aids. To study the e f f e c t s o f oxidation, the powders were heated i n a platinum c r u c i b l e i n a i r i n a clean convection m u f f l e a t temperatures between 250" and 1100°C f o r various periods o f time. A f t e r heating, the samples were cooled and stored i n a desiccator. Before and a f t e r heat treatment, the p a r t i c l e sizes o f the powders were examined using an o p t i c a l microscope equipped w i t h an innnersion o b j e c t i v e ( x 1000 magnification). These observations i n d i c a t e a s l i g h t change i n p a r t i c l e s i z e (from about 1 pm t o about 1-2 pm) a f t e r heat treatment. For example i n Fig. 1, the BET surface area (Np gas) versus temperature a f t e r 3 hours heat treatment f o r UF 15 powder i s shown. These r e s u l t s i n d i c a t e an i n i t i a l change i n surface area occurring a t about 600°C and r a p i d l y decreasing a t higher temperatures.

'4

I

1

Fig. 1 Surface area o f oxidized S i c UF 1 5 powder versus the temperature o f treatment i n a i r ( 3 hours).

9 - . . . . 0 200 400

600

800

1 0 0 0 1; D O

Temperature of treatment in air 'C

FTIR spectra o f the unoxidized and oxidized powders were obtained i n both the transmission and r e f l e c t i o n modes using a N i c o l e t 5DXB instrument w i t h a d i f f u s e r e f l e c t a n c e accessory (Barnes A n a l y t i c a l I n s t . ) .

For the D i f f u s e

Reflectance Analysis (DRIFT) the powders were i n i t i a l l y mixed w i t h KBr (Riedel and Haen, spectroscopic grade) a t a 10 w t % r a t i o . The mixture was then ground f o r a standardized time period (15 min) i n a p e s t l e and mortar. F i n a l l y the ground powder was t r a n s f e r r e d t o a 12 mm diameter sampling cup without compression and the surface o f the powder smoothed w i t h a spatula. An average o f 100 scans were made on each sample and the spectra r a t i o n a l i z e d against the KBr background. The FTIR r e f l e c t a n c e spectra was recorded between 2500 and 400 cm-1 wavelength a t a r e s o l u t i o n o f 4 m1. Transmission spectra were obtained by preparing pel l e t s a f t e r mixing the S i c w i t h KBr powder (0.02-0.12

w t % ) . I n the preparation o f the d i s c i t i s

551

important t o ensure t h a t the powders are homogeneously mixed and i n the present study i t was found convenient t o examine the d i s t r i b u t i o n using a microscope a f t e r g r i n d i n g before pressing the d i s c . Providing the sample i s homogeneously d i s t r i b u t e d throughout the KBr pel l e t the transmission spectra has the advantage over DRIFT i n t h a t Beer's law i s obeyed, i . e . absorption band i n t e n s i t i e s are proportional t o sample concentrations. The oxygen content o f the powders was accurately determined by Combustion Analysis using the LECO RO-116 equipment (LECO Corp., USA). This standardized method has been w e l l established i n the ceramic i n d u s t r y and involves the d i r e c t c a t a l y t i c combustion o f the powder i n a c r u c i b l e followed by f l u s h i n g the CO i n t o a I R d e t e c t o r system. RESULTS AND DISCUSSION FTIR Analysis o f H y d r o f l u o r i c Acid Washed Powders I n Fig. 2 the DRIFT spectra o f UF 15 powder (10 w t % S i c i n KBr) i s shown ( a ) a f t e r a water wash a t neutral pH, and ( b ) a f t e r a h y d r o f l u o r i c a c i d (40%) wash. Both spectra c l e a r l y show the pronounced antisymmetrical S i c s t r e t c h a t 850 cm-1, b u t the a c i d washed sample appears t o show a l e s s pronounced Si02 band a t 1100 cm-3 (4).

Fig. 2. Drift spectra o f S i c UF 15 powder, a) washed w i t h HF, b) washed w i t h water.

I

I

1700 1300 900 WAVENUMBERS (CM -1) This r e s u l t shows t h a t HF i s e f f e c t i v e i n removing the oxide f i l m f o r the p a r t i c l e s . I t i s a l s o o f note t h a t the corresponding transmittance spectra o f the two samples (Fig. 3) appears t o be l e s s s e n s i t i v e . This r e s u l t i s n o t p a r t i c u l a r l y s u r p r i s i n g since the DRIFT i s an e s s e n t i a l l y surface s e n s i t i v e technique ( i t enhances t h e r a t i o o f the signal from the surface t o the signal

552

f r o m t h e b u l k because o f t h e e f f e c t i v e m u l t i p a s s i n g o f t h e beam and r e p e a t e d r e f l e c t i o n s f r o m t h e sample).

s 1-04 I

W

0

w

4

0 2 4

z

c

it

Ef

4

w

z

-1

U

K

K

!-

H

bp

)o.o

iioo.0

1300.0

900.0

W A V E N U MB E R S (CM-1)

F i g . 3. T r a n s m i t t a n c e s p e c t r a o f S i c UF 15 powder; a ) washed w i t h HF, b ) washed w i t h water.

3.0

-1800

1000 WAVENUMBE R S

600 (CM-1)

F i g . 4. DRIFT s p e c t r a o f a ) Sic UF 15 b ) UF 25 and c ) UF 45.

A n a l y s i s o f t h e UF 25 and UF 45 powders ( w i t h o u t a c i d wash) was a l s o c a r r i e d o u t i n t h e DRIFT mode. From these r e s u l t s ( F i g . 4) i t may be suggested t h a t t h e h i g h e r s u r f a c e a r e a powders a r e more h e a v i l y o x i d i z e d , t h e SiO2 band (1100 cm-1) i n c r e a s i n g i n i n t e n s i t y i n t h e o r d e r UF 15>UF 25>UF 45. FTIR A n a l y s i s o f O x i d i z e d Powders I n F i g . 5 t h e e f f e c t o f temperature on t h e o x i d a t i o n o f t h e S i c powder i s shown. The f i g u r e shows t h e FTIR r e f l e c t a n c e s p e c t r a o f t h e S i c powder heated f o r 3 h a t f i v e d i f f e r e n t temperatures. As t h e temperature i n c r e a s e s t h e e x t e n t

of t h e o x i d a t i o n i n c r e a s e s which i s demonstrated b y t h e growth i n i n t e n s i t y o f t h e Si-0-Si band ( i n t h e 1100 cm-l r e g i o n ) . I n a d d i t i o n , i t can be seen t h a t t h e secondary S i - 0 - S i v i b r a t i o n band (700-400 cm-l) a l s o i n c r e a s e s i n intensity. I n F i g . 6 t h e r e l a t i v e i n t e n s i t y o f t h e S i - 0 - S i band a t 1100 cm-1 t o t h e sum o f t h e i n t e n s i t i e s o f t h e S i - C - S i

band a t 850 cm-1 and t h e Si-0-Si band

a t 1100 cm-1 was p l o t t e d a g a i n s t t h e t e m p e r a t u r e o f h e a t t r e a t m e n t . T h i s c u r v e

553

Relative Intensity

I 2500'

1700

0

900

WAVENUMBERS(CM-1)

250

500

750

Temperature

1000

1250

C0C)

F i g . 6. The r a t i o o f t h e DRIFT i n t e n s i t y o f t h e Si-0-Si v i b r a t i o n t o t h e S i - C - S i and S i - 0 - S i v i b r a t i o n f o r a-SiC powders o x i d i z e d i n a i r versus temperature.

F i g . 5. DRIFT s p e c t r a o f S i c UF15 o x i d i z e d i n a i r f o r 3 h a t 250-1100°C.

r e p r e s e n t s a s e m i - q u a n t i t a t i v e measure o f t h e e x t e n t o f o x i d a t i o n o f t h e powd e r s t h r o u g h o u t t h e temperature range. From t h e s e r e s u l t s i t can be seen t h a t t h e r a t e o f o x i d a t i o n appears t o i n c r e a s e w i t h t h e i n c r e a s e i n temperature.

1

I

2500

1700 900 WAVENUMBERS(CM-~ 1

F i g . 7. R e p r e s e n t a t i v e DRIFT spect r a o f S i c UF 15 powder o x i d i z e d i n a i r a t 1000°C f o r 0.5 t o 6 h.

554

I n Fig. 7 a d d i t i o n a l representative FTIR r e f l e c t a n c e spectra demonstrate the e f f e c t o f time on the o x i d a t i o n process. These spectra suggest an i n i t i a l r a p i d r a t e o f o x i d a t i o n occurring w i t h i n the f i r s t h a l f hour followed by a gradual reduction i n r a t e extending up t o about 3 hours. S i l i c o n carbide i s w e l l known t o owe i t s temperature resistance t o t o t a l oxidation, t o the build-up o f a Si02 p r o t e c t i v e f i l m which i s much more thermodynamically s t a b l e than S i c a t high temperature (5). Also many g r a v i m e t r i c studies have demonstrated a r a p i d i n i t i a l l i n e a r o x i d a t i o n step followed by a slower parabolic growth r a t e o f the 9 0 2 on the S i c core p a r t i c l e (6,7). To e x p l a i n the r e a c t i o n r a t e c o n t r o l l i n g step the d i f f u s i o n o f oxygen through the surface f i l m has been suggested (3). Q u a n t i t a t i v e Estimation o f Si07 i n S i c UF 15..by DRIFT The t h e o r e t i c a l foundations o f d i f f u s e r e f l e c t a n c e based on continuum theory was developed by Kubelka and Munk (K-M).

From the theory the r a t i o o f the I R

reflectance o f the sample t o t h a t o f the reference m a t e r i a l i s l i n e a r l y dependent on concentration p r o v i d i n g the terms describing the s c a t t e r i n g e f f e c t s remain unchanged. The observed r e s u l t includes scattering, absorption, transmission and r e f l e c t i o n from the sample. DRIFT signals are s t r o n g l y influenced by the p a r t i c l e s i z e o f the sample and d i l u e n t . For small p a r t i c l e ( p a r t i c l e sizes lower than the wavelength o f the measured absorbing band) the surface r e f l e c t i o n decreases and the spectra resembles the transmission mode. I n the present study the l i n e a r i t y of the K-M u n i t w i t h respect t o the concentration was determined a f t e r measuring the d i f f u s e reflectance spectra o f f i n e l y ground S i c UF 15 (before heat treatment) i n K B r a t concentration up t o 6 w t % . 80 1

0

Concentration of SIC in KBr (W %)

Fig. 8. C a l i b r a t i o n curve f o r S i c UF 15 (beofre heat treatment). Results from DRIFT spectra. The peak height a t 850 cm-1 was measured (Si-C-band).

2

4

6

8

10

12

concentration of Sic in KBr (wt O h )

Fig. 9. C a l i b r a t i o n curve f o r Sic UF 15 heated a t 1100°C during 3 h. Results from DRIFT spectra. The peak heights a t 850 ( 0 ) and 1100 cm-1 (A) were measured.

555

In Fig. 8 the peak i n t e n s i t i e s estimated from peak height of the Si-C-Si band (850 cm-l) expressed in K-M units was plotted against concentration. The linear plot a t low Sic concentrations appears t o confirm K-M theory, although deviation from l i n e a r i t y appear a t concentrations > 2 w t % Sic. Additional experiments were made t o check the l i n e a r i t y of the K-W unit w i t h respect t o concentration for strongly oxidized Sic powders ( a f t e r heat treatment a t 1100°C f o r 3 h ) . In this case the peak intensity of the Si-C-Si (850 cm-I), F i g . 9 A, and the Si-0-Si (1100 cm-I), Fig. 9 B y bands converted t o K-M units versus concentration both gave a linear plot extending up t o about 7 and 11.5 w t % Sic in KBr respectively. The relative standard deviation was generally about 5-10 w t % i n most cases, possibly due t o small differences i n the packing of the powder i n the cup. To estimate the extent of oxidation of the S i c powders a quantitative c a l i bration curve was constructed from the DRIFT analysis and the oxygen analysis obtained from the LECO combustion method. In Fig. 10 respectively ( a ) the peak height of the Si-0-Si band and ( b ) the r a t i o of the peak height of the Si-0-Si t o the Si-C-Si bands (expressed in K-M units) were plotted against the LECO analysis. Both plots appear t o be relatively linear passing through zero. However, an attempt t o plot the r a t i o of the peak height of Si-0-Si t o the sum of the peak height of the Si-0-Si plus Si-C-Si bands failed t o show d i r e c t proportionality.

F i g . 10. The peak height a t 1100 cm-1 (A) and the r a t i o of the peak heights a t 1100 and 850 cm-1 ( 0 ) were plotted against the w t % of oxygen content in Sic (LECO Analysis)

I

0

10

20

Concentration of oxygen In SiC(wl%)

In table 1, a comparison of the oxygen content of a s e r i e s of UF 15 powders are shown w i t h different degrees o f oxidation. Although satisfactory correlation was obtained a t low oxygen levels, deviations occurred a t h i g h oxygen contents. This was caused by the f a c t t h a t a t higher SiOp concentrations, interference

556

o f Si-0-Si and S i - C - S i

absorption bands occur and reduce the p r e c i s i o n o f the

measurement. An a l t e r n a t i v e method based on the accurate deconvolution o f the peaks and measurements o f the surface areas o f the peak instead o f the peak height could p o s s i b l y improve the method. TABLE 1 Sample S i c UF 15

Si-0-Si/Si-C-Si

, 100%

5.6 17.1 20.7 31.45 102.35

Oxygen wt% DRIFT

Oxygen wt%+O. 2% COMBUSTI O N

1.15 3.5 4.4

1.2

6.6

6.3

21.8

29.3

3.3 4.9

Q u a n t i t a t i v e Estimation o f S i O 7 i n UF 15 by Transmission The l i m i t a t i o n s o f DRIFT analysis w i t h p a r t i c u l a r respect t o the l i m i t a t i o n s t o low Si02 content powders focussed a t t e n t i o n on the transmission mode. The major advantage o f the transmission mode i s t h a t the signal t o noise r a t i o i s higher and the absorption bands appear more d i s t i n c t ( f o r example Fig. 11 shows the TM spectra o f an oxidized powder).

Fig. 11. Transmission spectra o f S i c UF 15 powder o x i d i z e d i n a i r a t 1100°C f o r 1.5 h.

2500

1700

900

WAVENUMBERS(CM-1)

For transmission FTIR, the well-known Beer-Lambert's (B-L) law i s d i r e c t l y obeyed; the i n t e n s i t y o f the absorption band a t any wavelength i s then d i r e c t l y proportionate t o the concentration o f the species.

557

An a t t e m p t t o v e r i f y B-L l a w f o r s t r o n g l y o x i d i z e d S i c powders (UF 15 h e a t t r e a t e d a t 1100°C f o r 3 h ) has been done. I n t h i s case t h e absorbance o f ( a ) t h e S i - 0 - S i and ( b ) t h e S i - C - S i

versus c o n c e n t r a t i o n were p l o t t e d . G e n e r a l l y

f a i r l y l i n e a r p l o t s c o u l d be o b t a i n e d ( F i g . 1 2 ) . A c a l i b r a t i o n p l o t was cons t r u c t e d r e l a t i n g t h e a b s o r p t i o n band i r l t e n s i t y ( o f S i - 0 - S i t o Si-C-Si) t o t h e oxygen c o n c e n t r a t i o n ( F i g . 1 3 ) . A l t h o u g h t h e c a l i b r a t i o n p l o t was found t o be l i n e a r i f was found t o be i n s e n s i t i v e t o r e l a t i v e l y l o w c o n c e n t r a t i o n o f oxygen (%300 nm mented that surface OH groups on MgO absorb light at (refs. 11,17). This means that surface OH groups may well contri-

.

651

t

L

Jmc.

L

WC.

bute to the photoactive species. The interaction with an adjacent excited hydroxyl may induce decomposition of bidentate species, which are not reported to absorb at 1>300 run, following the scheme shown above. It is suggested that carbon dioxide produced in the decomposition and adsorbed to form a carbonate-like species, is removed by an incoming acid molecule which forms an acetate and a hydroxyl and regenerate the condition for a new catalytic event on the same site. Such displacement will be tested spectroscopically in future work, though carbonate and carboxylate-like species absorb in the same region, which is already crowded of bands due to the numerous surface species formed upon CH3COOH adsorption. The mechanism indicated above for MgO specimens cannot be excluded for special semiconductor insulator mixtures (as MgO-Ti02) for which an exceedingly high photoreactivity was found for certain compositions (8). This work was supported by the Italian MPI under the scheme of Progetti di Interesse Nazionale. The authors thank Prof. A . Zecchina for discussion, Miss M.R. Boccuti for helping with reflectance spectra and one referee for helpful suggestions.

652

REFERENCES 1 2 3 4

5

6 7

8 9 10

11 12 13 14 15 16 17 18 19 20 21 22

M. Schiavello, in M. Schiavello (Editor), Photocatalysis and Environment. Trends and Applications, NATO AS1 SERIES, Kluwer Academic Publishers Dordrecht, The Netherlands 1988, p. 351. H. Yoneyama, Y. Takao, H. Tamura and A.J. Bard, J. Phys. Chem, 87 (1983) 1417. S. Sato, J. Chem. SOC., Chem. Comm., (1982) 26. M. Anpo, Y. Yamada, and Y. Kubokawa, J. Chem. SOC., Chem. Comm., (1986) 714. V. Augugliaro, L. Palmisano, M. Schiavello and A. Sclafani, J. Catal., 99 (1986) 62. A. Sclafani, L. Palmisano, M. Schiavello, V. Augugliaro, S. Coluccia and L. Marchese, New J. Chem., 12 (1988) 129. L. Palmisano, A. Sclafani, M. Schiavello, V. Augugliaro, S. Coluccia and L. Marchese. New J Chem., 12 (1988) 137. L. Palmisano, M. Schiavello, A. Sclafani, S. Coluccia and L. Marchese, New J. Chem., in press. M. Schiavello, V. Augugliaro, S . Coluccia, L. Palmisano and A. Sclafani, in M. Anpo and T. Matsuura (Editors), Photochemistry on Solid Surfaces, Elsevier, Amsterdam, in press. A. Zecchina, M. G. Lofthouse and F.S. Stone, J. Chem. SOC., Faraday 1, 71 (1975) 1476. S. Coluccia, A.M.Deane and A.J. Tench, in Proc. 6th. Int. Cong. Catalysis, London 1876, The Chemical Society (Editor), London, 1 (1977) 171. S. Coluccia, A.M. Deane and A.J. Tench, J. Chem. SOC., Faraday 1, 74 (1976) 2913. E. Garrone, A. Zecchina and F.S. Stone, Phil. Mag. B, 42 (1980) 683 and references therein. R.L. Nelson and J. W. Hale, Discuss. Faraday SOC., 52 (1971) 77. W.W. Duley, J. Chem. SOC., Faraday 1, 80 (1984) 1173. S. Coluccia, R.L. Segall and A. J. Tench, J. Chem SOC., Faraday 1, 75 (1979) 1769. W.W. Duley, Phil. Mag., 49 (1984) 159. M. Anpo and Y. Yamada, in K. Tanabe (editor), Advances in Basic Solid Materials, Elsevier, Amsterdam, in press. P.J. Anderson, R. F. Horlock and J. F. Oliver, Trans. Faraday S O C . , 61 (1965) 2754. S. Coluccia, L. Marchese, S.Lavagnino and M. Anpo, Spectrochim. Acta, 4% (1987) 1573. L.H. Little, Infrared Spectra of Adsorbed Species, Academic Press, London, (1966) pp. 74-84. P.A. Leemakers, H.T. Thomas, L.D. Weis and F.C. James, J. Am. Che. SOC., 20 (1966) 5075.


653

THE Pt/AlzO3 SYSTEM: INFRARED STUDIES

L. MARCHESE, M.R. BOCCUTI, s. COLUCCIA, s. LAVAGNINO, A. ZECCHINA~ L. BONNEVIOT and M. CHEL 'Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali dell'universita di Torino, Via P. Giuria 7, 10125 Torino (Italy)

2Laboratoire de Reactivitb de Surface et Structure UA1106 CNRS, Universite P. et M. Curie - 4 Place Jussieu '75252 Paris Cedex 05, ( France )

ABSTRACT The spectra of CO adsorbed on Pt/A1 0 samples indicate that two faces are dominant on the metal partic?es. Bands due to CO adsorbed on oxidized platinum sites are observed also on the reduced samples. Comparison of spectra relative to samples with different Pt loadings indicate metal-support interaction involving the most electron deficient aluminium sites. INTRODUCTION The Pt/A1203 system has been widely studied because of its importance as basis for reforming catalysts and many papers on surface characterization have been published. A very sensitive, and then currently used, technique for the characterization of highly dispersed metal catalysts is the infrared spectroscopy (ref. 1). In the last 10 years Infrared Reflection-Adsorption Spectroscopy (IRAS) data of CO adsorbed on flat (refs. 2-4) and stepped (ref. 5 ) single crystals have contributed to define a good model for dispersed platinum catalysts. The metal particles in the catalysts can be seen as crystallites showing (111), (110) and (100) stepped high index microfaces, corners and edges and finally surface defects at the atomic level (kink sites) characterized by different adsorption energies. An attempt to extend the results coming from single crystal investigation to supported platinum was illustrated by Greenler et al. (ref. 6). Electron microscopy (TEM and HRTEM) studies support and improve such model evidencing the presence of specific crystallographic

654

planes on the metal particles in relation with thermal pretreatments and structure of the support (metal-support interaction) (refs. 7-10). Vibrational spectra of CO adsorbed on dispersed metal catalysts give valuable information on: a) dispersion and morphology of the particles; b) oxidation state of the platinum; c) presence of coadsorbed species; d) adsorbate-adsorbate interactions; e) metalsupport interaction. The effects of a) coverage, b) time of contact and c) sample pretreatments on infrared spectra of CO adsorbed on commercial and laboratory prepared Pt/Alz03 systems are illustrated here. EXPERIMENTAL Two Pt/A1203 samples were used (a commercial (Akzo CK306) sample with 0.6 wt % Pt loading and a laboratory prepared (ref. 11) sample with a Pt loading of 2.86 wt % ) . Both catalysts were obtained by impregnation of A1203 (Akzo CK300). The standard IR cell for solids was permanently connected to a vacuum line ( ~ = 1 0 Torr) ~ and allowed thermal treatments and adsorption-desorption experiments to be carried out in situ. The spectrometer was a Bruker IFS 48 equipped with cryodetector MCT. The resolution was 4 cm-l. The spectra of CO adsorbed at 300K are shown in absorbance scale after subtraction of the "background" spectrum of the pelleted catalyst and of any contribution of CO gas. The samples were analyzed after reduction and oxidation pretreatments. The "reduced" samples were obtained by the following procedure: a) outgassing at 773K for 30 min., b) oxidation at 773K with o2 (150 Torr) for 1 hour; c) outgassing at 773K for 20 min. d) reduction with H2 (150 Torr) at 773K for 1 hour and e) final outgassing at 773K for 30 min. The "oxidized" samples were prepared by: a) outgassing at 773K for 30 min., b) oxidation at 773K with O2 (150 torr) for 30 min. and c) final outgassing at 773K for 30 min. 02, H2 and CO were U.H.P. grade (Matheson). RESULTS Reduced samples Adsorption experiments. Infrared spectra at increasing doses of CO on the reduced 0.6 wt % Pt catalyst are reported in Fig. 1, section A. The final pressure (spectrum g) was 80 Torr. The curves in Fig. l,A were obtained immediately after the admission of CO and, consequently, do not represent equilibrium conditions because

655

2081 4

T

T

P'

A @

2081

OOD.

cm-1

cm-1

Fig. 1. CO adsorbed on reduced Pt/A1203 catalyst (0.6 wt % ) . Sect. A: adsorption experimyt; 8-c) successive small doses of co, d-g 1 in the presence of 10- ,lo- , 10 and 80 Torr CO respectively. Sect. B: desorption experiment after contact with 80 Torr CO for 16 holurs; a) 80 Torr CO, b) after outgassing (20 min.) at 300K and at c) 363K, d) 423K, e) 473K, f) 573K and 9) 623K respectively. the diffusion of CO at ambient temperature onto the internal layers of the pellet is slow as compared with time required for scanning the spectra (refs. 12,131. Infrared spectra show bands in three spectral ranges: (i) 2300-2170 cm-'. The bands in this range are due to CO molecules adsorbed on the A1 03 support. The various components are related to families of Al,, sites exposed in very low coordina-

3+

tion states on the surface. The CO band at 2247 cm-I is associated with molecules adsorbed in the most acidic cationic sites and appers first at the lowest coverages (Fig. lA, a-c), followed by the bands (Fig. l A , d-g) at lower frequencies down to 2201 cm-' associated with less active sites. Assignments and structures of CO adsorbed on A1203 are fully described in refs. 14,15 and will not be discussed further.

656

(ii) 2170-1950 cm-'. After the admission of the first doses of CO distinct components, although heavily overlapped, are observed (Fig. lA,a-b) at 2075 and 2067 cm-'. The intensities increase with coverage and the two components gradually merge into a complex and broad absorption (Fig. lA, c-d), whose maximum is at 2081 cm" in curve d. The maximum does not shift at further stages of adsorption (Fig. lA, e-g), though the intensity increases continuously. The overall halfwidth of the 2081 cm-' absorption is about 37 cm-' in Fig. lA, g. Two very weak components, heavily overlapped to the principal absorption, appear at 2152 and 2133 cm-' (Fig. lA, 9). (iii) 1950-1800 cm-'. Two broad and weak bands are observed at 1900 and 1850 cm'l , whose intensities grow with coverage. Desorption experiments. The sample previously contacted with 80 torr of CO for 16 hours in order to reach the complete equilibration (Fig. lB, a), was than submitted to a desorption procedure in the 300-623K range. The IR spectra (Fig. 1st obtained in this way represent equilibrium states (refs. 12,131. Comparison of curve a in Fig. 1B with curve g in Fig. 1A shows that the main effect of prolonged contact with CO is the increase in intensity of the two bands at 2152 and 2133 cm-' and of the principal band at 2081 cm-l. No frequency shift is observed. (i) 2300-2170 cm-'. The behaviour of the CO bands related to the A1203 support in the desorption experiment parallel their behaviour in the adsorption experiment: the species formed in the 3+ sites final stages of adsorption on the relatively less acid Alcus (band at 2201 cm-l) are pressure dependent and are desorbed at 300K, those formed in the early stages of adsorption on the most active sites (2247 cm-l) are eliminated at higher temperature. (ii) 2170-1950 cm-l. Outgassing at 300K (Fig. lB, b) slightly affects the dominant complex absorption at 2081 cm" which shifts to 2075 cm'l but has a drastic effect on the weak bands a 2152 and 2133 cm'l which disappear while a new sharp peak appears at 2147 cm-l. This effect is reversible as readmission of CO at this stage immediately restores the original spectrum of Fig.lB,a and the cycle may be repeated. The 2147 cm" peak is progressively eliminated by outgassing at higher temperatures and no frequency shift is observed at the various stages. This peak was not observed at any stage of the absorption run (Fig. 1,A). Experiments, not shown for brevity, confirmed that the progressive increase of the intensity of the two bands at 2152 and 2133 cm'l with increasing time of contact with CO (80 Torr) parallel the

657

increase of the 2147 cm'l peak observed by outgassing at 300K. The 2081 cm-' absorption shows a complex evolution as the desorption temperature increases. Two overlapped components at 2078 and 2065 cm'l are distinguishable after outgassing at 363K (curve c). Their intensity decreases as the outgassing temperature increases and the maxima shift to lower wavenumbers. After desorbing at 423K the bands are at 2068 and 2057 cm-I (Fig. lB, d). The component at 2065 cm'l is depleted preferentially. In the final stages of desorption (curve elf1 only a broad and complex absorption is observable whose components and relate behaviours can hardly be recognized. After outgassing at 723K the maximum is at 2049 cm" and the overall half-width is 40 cm-'. It should be noticed that in this range shape and position of maxima in spectra of comparable intensities in Fig. 1A and 1B are very different, expecially at low coverages. (iii) 1950-1800 cm". The two absorptions sharply decrease by pumping off at 300K and disappear by outgassing at 423K. Oxidized 0.6 wt % catalyst For brevity we only illustrate the spectrum obtained after contact for 16 hours with 80 Torr of CO and successively outgassed 5 minutes at R.T. (Fig. 2, curve a). The absorption at 2075 cm-' which dominate the spectrum of CO on the reduced sample is absent in the spectrum of the oxidized sample, whose absorptions are observed at higher frequencies. The 2147 cm" band is common to the two spectra. Other absorptions in the spectrum of CO on the oxidized sample are a sharp peak at 2168 crn-l, two broad and overlapped bands at 2125 and 2092 cm'l and two very weak bands at 2198 and 2179 . 'm c Effect of Pt loading In Fig. 3 the spectra of CO adsorbed, after 16 hours contact, on reduced 0 . 6 wt % Pt (curve b) and 2.86 wt % Pt (curve a) samples are reported. The overall shapes and bands positions are similar, though significant differences are observed in the intensities: a) the integrated intensity of the dominant absorption at 2081 cm-l is 2.5 times larger in curve a than in b; b) the integrated is 4 times intensity of the low frequency bands (1950-1800 cm'll larger in a then in b; c) the intensities of the bands due to CO are much weaker on the adsorbed on the support (2300-2175 cm") high loading (curve a) than on the low loading sample (curve b).

658

b

Fig. 2. Spectra of CO adsorbed on oxidized la) and reduced (b) Pt/A1203 catalyst ( 0.6 wt B 1. The spectra were obtained after contact with 80 Torr CO for 16 hours and subsequent outgassing at 300K.

Fig. 3 . Spectra of CO (80 Torr) adsorbed on reduced Pt/A1203 samples with different loading: a) 2.86 wt % Pt, b) 0.6 wt % Pt

The dimension of the particles determined to be >,P.JENA, BRAD, S. KHANNA, NATO, AS1 SERE B, vol158,198737-W. LINEBERGER, in Proced ISSPIC IV, Aix-Marseille 1988. 38-P.M.AJAYAN and L.D.MARKS . Phys.Rev.Let. 60 (1988) 7.



677

THE FORMATION OF WELL DEFINED RHODIUM DICARBONYL AND DINITROSYL WITH RHODIUM SUPPORTED ON HIGHLY DEALUMINATED ZEOLITE Y H. MIESSNER~, A. ZECCHINA',

I. B~JRKHARDT~ , D. GTJTSCHICK~ , C. MORTERRA',

and G. SPOTO'

'Zentralinstitut fUr physikalische Chemie der Wissenschaften der DDR, Berlin 1199, G.D.R. 'Istituto di Chimica Fisica, Universita di Torino, Italy

Akademie

der

10125 Torino,

ABSTRACT Highly dealuminated and extracted ultrastable zeolite Y (US-Ex, Si:Al = 951 exhibits unique properties as support for the formation of Rh dicarbonyl and Rh dinitrosyl. The.interaction of CQ with Rh exchanged into US-Ex results in 1.r. spectra with unusually sharp (F.W.H.M. < 5 cm-') carbonyl bands at 2118 and 2053 cm-I and their corresponding 13C0 satellites. The local structure is CZV, as verified by isotopic exchange, with a bond angle of 106O between the CO groups. The interactjon of the dicarbony1 with nitric oxide yields in a fast reversible exchange of the carbonyl ligands by NO and the formation of Rh1(NO)2/US-Ex with NO stretching bands at 1855 and 1780 cm-' (F.W.H.M. < 20 cm-'). The spectra indicate the formation of well defined Rh complexes with zeolite lattice oxygens as ligands.

INTRODIJCT ION It has been known during the last two decades that zeolites as supports for metal catalysts exhibit some particular properties. The reducibility of cations exchanged into the zeolite is different to other supports and depends on the type and the Si:Al ratio of the zeolite. Sometimes non-typical oxidation states are stabilized hy the Zeolite and at certain conditions the cations occupy definite positions within the threedimensional zeolite framework. If reduced, the metal atoms aggregate to small metallic clusters in the cavities (or migrate to the outer surface to form larger metal particles. With carbon monoxide the formation of carbonyls, subearhonyls or cxrbonyl clusters of transition metals has been observed. Several authors have shown that the generation of variou~i CRrbor~ylspecies from rhodium in zeolites depends on the zeolite type and the preparation and interaction conditions. For Rh exchanged into NaY zeolite, two i. r. band doublets are obtained in the carbonyl stretching region after the interaction with CO, which were assigned to different dicarbonyl species (refs. 1-71.

618

The nature of these dicarbonyl species is still under debate. The interaction of Rh exchanged into Nay-zeolites with nitric oxide was investigated by Iizuka and Lunsford (ref. 2) in connection with the reduction of NO by CO. Different nitrosyl stretching bands were assigned to a Rh dinitrosyl complex, a Rh-NO", and to a mixed complex Rh I(CO)ZNO. Similar assignments were made by other authors for the spectra obtained with NO on Rh/A1203 (refs. 8-10). Dnlike this assignment, other authors assigned the bands obtained on Rh/A1203 to different rriononitrosyl species (refs. 11-13). In a preliminary communication we reported on the first results regarding the formation of a well defined dicarbonyl by the interaction of CO with both reduced and oxidized rhodium in highly dealuminated ultrastable zeolite Y (US-Ex) (ref. 1 4 ) . These extraordinary properties of Rh exchanged into US-Ex prompted us to study this system in detail including isotopic exchange and ligand exchange with NO. EXPERIMENTAL Rh/US-Ex ( l w t % Rh) was prepared by treatment of dealuminated and extracted ultrastable zeolite Y (Si:Al=95) (ref. 15) with an aqueous solution of [Rh(NH3)5C11(OH)2. The samples were dried at 383 K for 3 h and calcined in air at 673 K for 2 h. The structure of the zeolite was checked by i.r. spectroscopy of K B r wafers (ref. 7 ) . The dispersion of Rh, determined by hydrogen chemisorption of the reduced samples was estimated by 0.5-1.2 (H:Rh). The transmission i.r. studies were performed with conventional cells made from glass or fused silica, connected with a vacuum and gas dosing line for in situ pretreatment of the self-supporting wafers (20-30 mg/cm 2 ) . The spectra were recorded at room temperature after the interaction of the evacuated samples with ca. 1 kPa CO at temperatures up to 423 K. The exchange of CO with 13C0 o r NO was performed after previous evacuation with about 1 kPa of the desired gas at 300 K. The spectra were recorded with a FT-i.r. spectrometer IRF-180 (ZWG, Academy of Sciences, G.D.R.) and with an IFS 113 v FT-i.r. spectrometer (BRUKER) at a resolution of 2 cm-l. The spectra shown in this paper are corrected for the background and the contribution of the windows. RESULTS AND DISCUSSION Fig. 1 shows the doublet due to the dicarbonyl formed during the interaction of CO with the calcined sample at 423 K. The main bands at 2118 and 2053 ern-' due to the symmetric and antisymmetric CO stretching are unusual sharp. The band width (F.W.H.M.: full width at half maximum) was found to be smaller than 5 em-',

679

Fig. 1. Infrared spectra of Rh/US-Ex after interaction with natural CO at 423 K for 30 min and evacuation at 300 K (a) and subsequent exchange with "CO :13C0 = 10:90 at 300 K (b).

I 2100

2 000

1900

WAVENUMBER / c 6 '

whereas the F.W.H.M. of CO adsorbed on supported transition metals is usually larger than 15 cm-'. This indicates the formation of a very well defined carbonyl compound within the zeolite, which acts as a kind of matrix leading to an effective isolation of the carbonyl species. Owing to the intensity and sharpness of the bands, also %he 13C0 satellites are well resolved (Fig. l), even for 13C0 in natural abundance (1.1%). An analysis of the intensities of the satellite bands indicated a proportion of about 2% relative to the parent bands for a wide range of overall intensities as expected for a Rhl(CO)Z species (ref. 14). To confirm the assignment, we performed an isotopic exchange with a mixture of l2C0 and 13C0 (10:90). The expected corresponding bands due to 13C0 appear immediately after admission of the new isotopic mixture (Fig. 1). The position of these bands are in good agreement with those calculated using an approximate force field (ref. 16) for Rhl(CO), with Czv symmetry (Table 1). In a subsequent study we investigated the isotopic exchange not only with 13C160 but also with 12C180 and 13C180 including the observation of the corresponding combination bands (ref. 17). Due to the

680

sharpness of the car"bony1 stretching bands, it was possible to detect 17 out of the 20 possible stretching bands in the system Rh1(CO)2 with 12C160, 13C160, 12C180, and 13C180 as carbonyl ligands. These results completely confirm the proposed structure of the dicarbonyl formed by CO with Rh/US-Ex. TABLE 1

Stretching parameters of Rhl(XO)Z/OS-Ex; X=C,N

k( XO ) /Nm-' i( XO-XO 1

v /cm-l

obs. -

calc.

I ^

12c160~12c160

2118 2053

13C160/12C160

2101 2021

2 102 2022

13c160/ 13c 160

2068 2006

2071 2007

14N16011% 160

1855 1780

--

-

1757 55

1454 60

It is possible to estimate the bond angle between the two uarbonyl groups in the Rh I(CO)z species from the absorbance ratio of the bands due to symmetric and antisymmetric CO stretching (2118 and 2053 cm-', respectively): (ref. 16). This intensity ratio is shown in Fig. 2 for a wide range of overall intensities. The mean value is 0.57, corresponding to a bond angle of 106O as already stated in the preliminary communication (ref. 14).

-

Fig. 2. Absorbance ratio of the bands due to symmetric and antisymmetric stretching of CO in Rhl(CO)z and of NO in Rhl(NO)z on US-Ex.

Rh/US- EX

1

3 5 ABSORBANCE

I Aos

681

2100

2 000

1900 1800 WAVENUMBER / crr-'

Fig. 3 . Infrared spectra of Rh*(CO)Z/US-Ex (as in Fig. la) before (a) and after (b) the interaction with 1 kPa NO and subsequent,evacuation at 300 K.

Fig. 3 shows the i. r . spectrum of the dicarbonyl together with The bands the result. of the interaction with 1 kPa NO at 300 K. of the dicarbonyl disappear immediately and at the same time two n e w bands at 1865 and 1780 crn-l are obtained. This exchange is reversible: the admission of CO after evacuation completely restores the spectrum 3a. The ligand exchange can be repeated several times at 300 K without loss of overall intensity. Generally, %he spectra of adsorbed NO m supported Rh are by far not so well resolved as the corresponding CO spectra (refs. 2, 8-13). With a F . W . H . M . < 20 cm-I the bands of nitrosyl stretching on Rh/US-Ex are not as sharp as the carbony1 stretching bands, but compared with other surface nitrosyl species on supported metals, they are still unusual1 narrow. The nitrosyl stretching bands at 1855 and 1780 cm-l are similar to those obtained at 1860 and 1780 cm-' by Iizuka and Lunsford by interaction of NO with Rh/NaY (ref. 2). The complete reversible replacement of CO and NO as ligands support the original assignment (ref. 2) to a dinitrosyl species of Rh within the zeolite micropores. As an additional argument we analysod the absorbances of the bands due to symmetric (1855 cm-') and antisymmetric (1780 cm-') NO stretching in the Rh I (NO)Z speAs for Rhl(CO)Z this ratio is rather constant and cies (Fig. 2).

682

corresponds to a bond angle of about llOo between the two nitrosyl groups. The slightly larger bond angle between the nitrosyl ligands compared with the dicarbonyl may be explained by the higher charge density on the nitrosyl ligands compared with the carbonyl ligands. As in the case of dicarbonyl, an approximate force field allows the calculation of the force and interaction constants (Tab. 1). A similar calculation was performed for Rh(N0)2/A1 0 by Liang et al. (ref. 9 ) , who got a force constant of 1402 imS1 for 14N0 stretching. Compared with this value the force constant obtained f o r Rh1(NO)2/lJS-Ex (1454 Nm-') is significantly larger. This indicates a relative small back bonding in the Rh-NO bond due to a comparative low electron density on Rh' coordinated to the zeolite framework. A similar conclusion has been drawn from the high value of the CO-stretching constant in Rh I (COI2/US-Ex (ref. 17). Fig. 4 shows the results of admission of a CO + NO mixture The main bands (1:1) to the evacuated Rh I ( C O ) z sample at 300 K. of the dicarbonyl and dinitrosyl appear immediately with an absorbance ratio approximately corresponding to the contents of the gas phase. There is nu shift in the wavenumbers of both species due to a mutual influence. Fig. 5 shows the sum of absorbances

I

2100

Rh/US-Ex

2000

+ ICO +NO)

1800

1900

WAVENUMBER

I

crn-'

Fig. 4. Infrared spectra of Rh/lJS-Ex after the interaction of Rhl(CO)Z/US-Ex with CO + NO (1:l) at 300 K (a) and subsequent, evacuation (b).

683

Rh/US-Ex

+

[NO + CO 1

300 K

w

Fig. 5. NO and CO band absorbances after the interaction of Rhl(CO)2/US-Ex with different mixtures of NO and CO at 300 K.

9a 4 ' m

0

z

0

2 .

0

I

'

0 -

6 8 AS+A,S CO BAND ABSORBANCE 2

4

(As+Aas) obtained for NO and CO with the same sample of Rh/lJS-Ex after the interaction of Rh I (C0)2 with different mixtures of NO+CO at 300 K. It is evident from this figure that NO and CO occupy the same surface centres and reversibly replace each other, Beside the main bands due to the dicarbonyl and dinitrosyl three additional bands are obtained after interaction with CO+NO (Fig. 4 ) : a small band at 2160 cm-' which seems to be stable during evacuation at 300 K and two bands at 2102 and 1712 cm-', which disappear if the gas phase is removed (Fig. 4 ) . The band at 2160 cm-l could be assigned to CO adsorbed on higher oxidized forms of Rh, but in that case we would expect a desorption during evacuation (ref. 17). The fact that this band appears after several NO + CO interactions points to a reaction product (-CN or -NCO) but a final assignment needs additional studies. The assignment of the bands at 2102 and 1712 cm-' is also not straightforward. The band at 1712 cm-' may be due to a NO adsorbed in a bent form Rh-NO'-. A similar assignment has been made by other authors (refs. 10,13) for bands at 1690 and 1716 cm-', respectively. On the other hand, this weak band exists only in the presence of both CO and NO in the gas phase like the band at 2102 cm-'. So, it may be possible to assign these bands to a mixed species Rh'(C0) (NO). Other authors suggested these species to have bands at about 2100 and 1760 cm-' (refs. 10-12), but due to the overlapping with the main dicarbonyl and dinitrosyl bands this assignment is rather tentative. Summarizing, we can conclude f r o m our studies the formation of well defined dicarbonyl and dinitrosyl species on Rh/US-Ex and their reversible conversion, whereas the identification of other than these species during the interaction of CO and NO at 300 K on Rh/US-Ex needs further investigations.

684

ACKNOWLEDGEMENTS W e wish t o thank D r . U . Lohse (ZIPC, B e r l i n , G . D . R . ) for t h e p r e p a r a t i o n of t h e dealuminated z e o l i t e US-Ex, and Mrs. Haase (ZIPC) f o r t e c h n i c a l a s s i s t a n c e . H.M. is g r a t e f u l f o r an exchange programme between t h e Academy of S c i e n c e s ( G . D . R . ) and t h e C o n s i g l i o Nazionale d e l l e Ricerche ( I t a l y ) .

REFERENCES 1 M. Primet, J.C. Vedrine and C. Naccache, J . Mol. C a t a l . , 4 (1978) 411. 2 T . I i z u k a and J.H. Lunsford, J . Mol. C a t a l . , 8 (1980) 391. H . Arai and H. Tominaga, J. C a t a l . , 75 (1982) 188. 3 4 R . D . Shannon, J . C . Vedrine, C. Naccache and F. Lefebvre, J. C a t a l . , 88 (1984) 431. 3 E . J . Rode, M.E. Davis and B.E. Hansen, J. Catal., 96 (1985) 574. 6 H. Miessner, D . Gutschick, H. Ewald and H. Mueller, J. Mol. C a t a l . , 36 (1986) 359. 7 H.-E. Maneck, D. Gutschick, I . Burkhardt, B. Luecke, H. Miessner and I]. Wolf, Catalysis Today, i n press. 8 E.A. Hyde, R. Rudham and C . H . Rochester, J . Chem. S o c . , Faraday Trans. 1, 80 (1984) 531. 9 J . Liang, H.P. Wang and L.D. S p i c e r , J. Phys. Chem., 89 (1985) 6840. 10 T . M . Annmna, A . A . flaBblAOB, H.C. C a 3 O H O B a and B.B. nOnOBCKMt\, f f i ~ .K a i a n . , 28 (1987) 655. 1 1 H. Arai and H. Tominaga, J . Catal., 43 (1976) 131. 12 F. Solymosi and J . Sarkany, Appl. S u r f a c e S c i . , 3 (1979) 68. 13 R. D i c t o r , J. C a t a l . , 109 (1988) 89. 14 I. Burkhardt, D . Gutschick, U. Lohse and H . Miessner, J . Chem. S O C . , Chem. Commun., (1987) 291. 15 H . S t a c h , U. Lohse, H. Thamm and W. Schirmer, Z e o l i t e s , 6 (1986) 74. 16 P. S. Braterman, Metal Carbonyl S p e c t r a , Academic P r e s s , London, 1975. 17 H. Miessner, I . Burkhardt, D. Gutschick, A. Zecchina, C. Morterra and G: Spoto, J. Chem. SOC., Faraday Trans. 1, submitted f o r p u b l i c a t i o n .

C. Morterra, A. Zecchina and G . Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands

685

SURFACE CHARACTERIZATION OF Cu/Si02 CATALYSTS PREPARE0 BY ION-EXCHANGE

A.

MOLNAR, J.T. KISS, G. SIROKMAN and M. BARTOK

Department of Organic Chemistry, A t t i l a Jdzsef U n i v e r s i t y , 0dm t 6 r 8, Szeged, Hungary

H-6720,

ABSTRACT The a c t i v i t y o f copper-on-silica g e l c a t a l y s t s prepared by ion-exchange i n t h e double bond m i g r a t i o n of alkenes has been s t u d i e d i n poisoning experiments, and by temperature-programmed desorption and I R spectroscopy. The a c t i v e s i t e s are p r o t o n i c centres generated d u r i n g the a c t i v a t i o n of the c a t a l y s t s , together with Cu(1) i o n s formed as a r e s u l t of the incomplete r e d u c t i o n of ion-exchanged copper species, i n d u c i n g an i o n i c - t y p e i s o m e r i z a t i o n .

INTROOUCTION

A

r e c e n t paper ( r e f . 1) described d e t a i l e d s t u d i e s of t h e unique a c t i v i t y of g e l c a t a l y s t s prepared by ion-exchange i n t h e

reduced copper-on-silica

double

bond i s o m e r i z a t i o n of c e r t a i n alkenes i n an i n e r t atmosphere.

R I

CH2=C-CH2-R Further

'

-

R I

CH3-C=CH-R

'

r e s u l t s concerning the more exact n a t u r e o f t h i s i s o m e r i z i n g a c t i v i -

t y are r e p o r t e d here. EXPERIMENTAL Materials 2-Methyl-1-butene Pyridine kept

(99.5%, Fluka), methylenecyclohexane (98%, Fluka) and hep-

(Fluka, p u r i s s . , 100% pure by GC) were used w i t h o u t f u r t h e r

tane

(Fluka, p u r i s s . ) and 2,6-dimethylpyridine

purification.

(Merck, f o r synthesis)

were

on potassium hydroxide and d i s t i l l e d before use. Oxygen-free hydrogen was

prepared with a Matheson 8326 generator o p e r a t i n g with a palladium membrane. Helium cating

( 9 9 . 9 9 % ) was f u r t h e r p u r i f i e d by passage through an Oxy-Trap and an I n d i Oxy-Trap

Cab-0-Sil

M5

(Alltech).

S i l i c a g e l (Strem, l a r g e pore, 120-230

(50H) showed l e s s than ppm l e v e l of A l , Ca, Mg and Cu

mesh)

and

impurities

by atomic a b s o r t i o n and no detectable d i f f e r e n c e s were observed. Catalysts Catalysts

were

prepared by ion-exchange ( X )

(ref. 2) or p r e c i p i t a t i o n ( P I

686

(ref. 3) hy using silica gel or Cab-0-Sil supports (denoted S and CS, respectively). F o r example, Cu/S-X stands for a catalyst prepared by ion-exchange using the silica gel support. After preparation, all catalyst precursors were treated at 773 K for 3 hr in air. Before use, the catalysts were reduced in flowing hydrogen for 2 hr at the temperatures indicated in the Figures, then kept in flowing helium for 1 hr at the same temperature. Methods A pulse microreactor equipped with a Carlo Erba b d . GV chromatograph operating with a hot-wire detector was used with catalyst samples of 10 mg and with 1 pl pulses of the reactant (flow rate of hydrogen or helium carrier gases = 20 ml/min). In poisoning experiments, 5x10-* molar solutions of the bases in heptane were used. GC analyses were performed on a 10% ethyl N,N-dimethyloxamate (Supe1co)-on-Chromosorb P column (4 m) at room temperature. In the temperature-programmed desorption studies, catalyst samples (50 mg) were reduced in a hydrogen flow (20 ml/min, 673 K, 1.5 hr), then treated in helium for 0.5 hr under the same conditions. The adsorption of pyridine or 2,6-dimethylpyridine (2xrOp1 of molar solutions in heptane) was carried out at 423 K in helium carrier gas (10 ml/min), followed by degassing at this temperature for 1 hr. The temperature was then increased to 700 K (10 deg/min) under a helium flow and the desorbed molecules were monitored with a Carlo Erba thermal conductivity detector. Infrared spectroscopic studies were performed in a SPECORO 71 IR apparatus (Carl Zeiss, Jena) equipped with a static cell. Self-supporting wafers made from 24 2 0.5 mg catalyst were used in these studies. Evacuation after pretreatment was carried out to a residual pressure of Pa before the spectra were taken. Transmittance scales of the figures range approximately from 0 to 5025%. Mass spectra were recorded on a Q 300 C (Atomki, Hungary) quadrupole mass spectrometer (55 eV>. RESULTS AND DISCUSSION

The most important conclusions of our earlier studies are as follows: (i) Neither catalysts prepared by precipitation nor catalysts prepared by using Cab-0-Sil as a support exhibited isomerizing activity. (ii) Both the copper loading of the Cu/S-X catalysts and the reduction temperature had a strong effect on the activity. A higher loading and a higher temperature of activation in hydrogen led to more active catalysts with more stable activity (Fig. 1). (iii) Mainly alkenes containing the CH2=C:i, moiety, that is compounds able to form the most stable carbocation, participated in extensive double bond migration.

687

Fig. 1. Isomerization of 2-methyl1-butene as a function of copper loading (open symbols) and the reduction temperature (circles). (Pretreatment: 2 hr in hydrogen, then 1 hr in helium; reaction temperature: 423 K). 0 6.36% Cu/S-X, 573 K

A 3.45% Cu/S-X, 573 K

O 1.91% Cu/S-X,

5

10

573 K 0 1.91% Cu/S-X, 473 K 1.91% CU/S-X, 673 K

No. of pulses This latter observation is a strong indication of an acid-catalysed, ionictype isomerization. Further support of this conclusion is given by the results of poisoning experiments. Dilute solutions of pyridine (Fig. 2A) and 2,6-dimethylpyridine (Fig. 28) were injected onto the reduced 6.36% Cu/S-X catalyst before the transformation of 2-methyl-1-butene. These experimental results were used to calculate the decrease in activity relative to the activity in the unpoisoned reaction (Fig. 3 ) . It can be seen that both bases affected the activity of the catalysts, but to different extents. The marked decrease in activity brought about by pyridine and the much milder effect of 2,6-dimethylpyridine

No.of pulses

Fig. 2. Poisoning of the isomerization of 2-methyl 1 butene by pyridine ( A ) and 2,6-dimethylpyridine (8). Before reaction, a 5x10-' molar solution of the base , 1 0 ~ 1 .(6.36% Cu/S-X; prewas injected: A l p l , 0 3 f l 1 , 0 5 p 1 , A 7 . 5 ~ 1 V treatment: 573 K, 2 hr in H, then 1 hr in He; reaction temperature: 423 K . )

688

Fig. 3 . Relative decreases in activities in poisoning experiments (values calculated from the data in Fig. 2 ) . v and 0 : calculated by using the activities of the fifth pulse; and. : calculated by using the sum of the activities of the first 5 pulses. Triangles: pyridine, circles: 2,6-dimethylpyridine.

5

25

-8

50

= l o M base

can be attributed to their different abilities to interact with surface acidic sites. As the literature data indicate (refs. 4, 51, pyridine can interact with both Bronsted and Lewis sites, whereas 2,6-dimethylpyridine, being sterically hindered by the two methyl groups, can interact only with Bronsted acidic centres, despite its higher basicity. Although Knozinger (ref. 6) found that 2,6dirnethylpyridine was not as selective as expected, our results clearly indicate a significant difference in the poisoning strengths of the two bases. Temperature-programmed desorption studies also suggested such a difference. The peak at around 423 K, pointing to the presence of loosely held, physisorbed base was neglected; in contrast to the double peak of pyridine (Fig. 41, the' single desorption peak of 2,6-dimethylpyridine indicated that the latter really can interact with only one type of active site.

Fig. 4. Temperature-programmed desorption of pyridine (a1 and 2,6dimethylpyridine (b) (catalyst: 6.36% Cu/S-X).

373

173

573

673

temperature (K)

Additional studies of the catalysts were carried out by means of IR spectroscopy. Independently of the method of preparation, all the catalysts exhibited

689 only a broad absorption band, centred a t about 3650 cm-l, e i t h e r without activat i o n or a f t e r reduction i n hydrogen (for example, 6.36% Cu/S-X, f i g . 5 , a and b), whereas activation i n deuterium

curves

resulted i n more characteristic

and

more informative absorption patterns i n the range 2400-2800 cm-l (Fig. 5, c l .

I

I

,

2800 2600 2400 wavenumber (cm-l )

3800 3600 3 0 0

Fig. 5 . Absorption patterns o f the 6.36% Cu/S-X catalyst a f t e r d i f f e r e n t pretreatments a t 673 K . a: 2 hr evacuation; b: 2 hr i n hydrogen, then 2 hr evacuat i o n ; c: 2 h r i n deuterium, then 2 h r evacuation. and s i l i c a supports (Fig. 6 , a and b), and a l l

The Cab-0-Sil catalysts

the

inactive

(Cu/S-P and Cu/CS-P, f i g . 6 , e and f > except Cu/CS-X, gave a band of

weak i n t e n s i t y a t around 2730-2750 cm-l a f t e r pretreatment i n deuterium. I n cont r a s t , the -1 cm (Fig.

spectrum of Cu/CS-X consisted of a strong, sharp maximum a t

6,

broader towards

2740

while i n the spectrum o f Cu/S-X the absorption was much lower wavenumbers (Fig. 6 , d). A t low reduction temperatures,

c),

2800

2600

2LOO

2800

2600

2LOO

2800 2600

2LOO wavenumber

(ern-')

Fig. 6 . Absorption patterns of the supports and the catalysts a f t e r pretreatment i n deuterium f o r 2 hr a t 673 K 2 hr, followed by a 2 hr evacuation (the maximum of the transmittance scale represents 40% transmittance). a: Cab-0-Si1 c: 3.66% Cu/CS-X e : 6.62% Cu/S-P b: s i l i c a d: 6.36% CU/S-X f : 6.79% Cu/CS-P

690

the absorption band was even broader: the maximum at 2740 cm-l continued in a band of peaks and shoulders at lower wavenumbers (Fig. 7).

Fig. 7 . IR spectra of the 6.36% Cu/S-X catalyst after reduction in deuterium at different temperatures. a: after reduction at by evacuation (673 b: after reduction at by evacuation (473

673 K (2 hr), followed K, 2 hr) 473 K (2 hr), followed K, 2 hr)

2800 2600 2400 wavenumber (cm-') It follows from a comparison of the spectra of the supports, the precursor, the inactive catalysts and the active Cu/S-X specimens that a large number of hydroxyl (deuteroxyl) groups are generated on the active catalysts during reduction. Independent experiments with deuterium oxide showed that their formation can be attributed to the effect of H20 (020) formed as a result of the reduction of CuO. The IR spectra of silica gel and the 6.36% Cu/S-X catalyst treated with 020 at room temperature indicate the development of a broad feature similar to that resulting from low-temperature reduction (Figs 8A and 88, curves b). After further treatment in deuterium at high temperature, the spectrum of the silica gel changed to the sharp maximum characteristic of the inactive Cu/CS-X (Fig. EA, c). In contrast, the high-temperature reduction of the D20treated Cu/S-X catalyst did not result in disappearance of the broad bands at

04 aJ

V

d"

U 44

'E v)

t

e

U

I

l-----l

2800 2600 2400 2800 2600 2400 wavenumber (cm-l) Fig. 8. Effect of deuterium oxide treatment on the IR spectra of silica gel (A> and the 6.36% Cu/S-X catalyst (6). a: After a 2 hr evacuation at 673 K; b: after treatment with 0 0 (20 torr, room temperature, 0.5 hr), then evacuation (403 K, 0.5 hr); c: a k e r additional treatment in deuterium (673 K, 2 hr) and evacuation (673 K, 2 hr).

691 lower

but a spectrum s i m i l a r t o t h a t of the simple

wavenumbers,

high-tempera-

ture

reduction product was attained (Fig. 86, c; see also Fig. 7, b ) . These re-

sults

demonstrate that s i l i c a gel i s readily rehydrated on the action o f water.

I t also follows that these newly developing 00 bands must a l l correspond surface that while

to

groups. E a r l i e r observations ( r e f s . 7-10) had demonstrated

deuteroxyl

the strong, sharp maximum corresponds t o isolated, unperturbed 00 groups, the broad absorption feature r e s u l t s from mutually i n t e r a c t i n g OD groups.

I n our case, on the basis of a comparison o f the spectra of the O20-treated and reduced that

silica

gel

and Cu/S-X (Figs 8A and 88, curves c) , i t

i s conceivable

the possible interaction of the 00 groups with c e r t a i n surface copper spe-

cies may also induce the appearance o f the broad I R absorption. Our recent studies

by temperature-programmed reduction o f the Cu/S-X catalysts, i n d i c a t i n g the of a substantial amount (7-29%) of unreducible copper ( r e f . 111, allow

presence

assumption that these species i n t e r a c t i n g with s i l a n o l groups might be non-

the

reduced Cu(1) the

ions. Since no s i g n i f i c a n t amount of other metals were found

in

supports by elemental analysis other complexes can be excluded. On the oth-

er hand,

the observation that the broad absorption bands a t lower

wavenumbers

were exhibited only by the active catalysts indicates that the isomerization act i v i t y might be connected with t h i s special feature of the I R spectra. I n a search f o r further evidence t o support the above conclusions, the merization the

of methylenecyclohexane was carried out i n the I R cell. Admission of

alkene onto the Cu/S-X catalyst resulted i n substantial changes i n the

sorption

iso-

pattern.

During long exposures (20 h r ) , there was a s i g n i f i c a n t

abde-

crease i n the i n t e n s i t y of the 00 absorption region, while a new band characteristic

o f OH absorption developed i n the region around 3650 cm-l (Fig. 9 ) . These

1

3800 3600 3400

Fig. 9 . Change i n the methylenecyclohexane. by evacuation (673 K, K ; c: same as b, a f t e r

2800 2600 2400 wavenumber (cm-' )

I R spectrum of the 6.36% Cu/S-X catalyst a: After reduction i n deuterium (673 K, 2 h r ) ; b: a f t e r the admission of 2.7 kPa 20 h r a t 423 K and subsequent evacuation

on the action of 2 h r ) , followed o f alkene a t 423 (423 K , 2 hr).

692

changes might be attributed in part to a simple interaction of the 00 groups with the alkene, or to a simple 0-H exchange between the surface and the organic molecule. However, analysis of the reaction mixture recovered from the IR cell after different intervals proved that isomerization of the alkene had occurred under the conditions employed. A similar treatment of the inactive Cu/CS-X with the alkene did not result in any significant change in the IR absorption (Fig. 101, which proves that isomerization and exchange on Cu/S-X must take place at the same time.

3800 3600 3400

28a3 26002400 wavenumber (an-')

Fig. 10. Change in the IR spectrum of the 3.66% Cu/CS-X catalyst on the action of methylenecyclohexane. a: After reduction in deuterium (673 K, 2 hr), followed by evacuation (673 K, 2 hr); b: kept in 2.7 kPa of alkene at 423 K for 20 hr, followed by evacuation (423 K, 2 hr). If the above conclusion is valid, then deuterium incorporation in the isomerized alkene can be expected. When a large excess of methylenecyclohexane was reacted over deuterium-treated Cu/S-XI incorporation of one (M+1=13.1%) , two (M+2=2.2%) and even three (M+3=0.2%) deuterium atoms into 1-methylcyclohexene was observed by mass spectrometric analysis. This fact, together with the simultaneous decrease in intensity of the 00 absorption, confirms the participation of surface 00 (OH) groups in isomerization. The multiply deuterated products are formed by the redistribution of deuterium between the exchangeable positions ( l ' , 2 and 6 ) and also the silanol groups via the tertiary carbocation. The observed composition of the molecular ions is very near that calculated on the basis of the equilibrium distribution of the incorporated amount of deuterium between the exchangeable positions (M=83.3%, M+1=15.5%, M+2=1.2%, M+3=0.2%). This suggests that the isomerization is an equilibrium process. CONCLUSIONS In accordance with our previous observations (ref. 11, the present results (poisoning experiments, temperature-programed desorption) supply further evi-

693 dence

of

t h e presence o f two d i f f e r e n t types o f a c t i v e s i t e s ,

catalysing the

double bond i s o m e r i z a t i o n o f c e r t a i n alkenes i n p a r a l l e l . Recent

with

s t u d i e s by temperature-programed r e d u c t i o n ( r e f s .

spectroscopic

Cu(1)

ions

evidence ( r e f s . 14, 151, show

11-13)

t h e presence o f

together

non-reduced

on ion-exchanged copper c a t a l y s t s a f t e r r e d u c t i o n . These

ions

are

presumed t o p a r t i c i p a t e as Lewis acid-type a c t i v e s i t e s i n isomerization. M o n i t o r i n g o f the surface changes o f t h e c a t a l y s t s d u r i n g d i f f e r e n t p r e t r e a t and r e a c t i o n , by means o f I R spectroscopy, p u l s e experiments and d e u t e r i -

ments

um exchange, demonstrated t h a t h y d r o x y l groups generated i n h i g h c o n c e n t r a t i o n during

ion-exchange

and subsequent r e d u c t i o n on the surface o f the s i l i c a

gel

p a r t i c i p a t e i n i s o m e r i z a t i o n . A p o s s i b l e i n t e r a c t i o n between

the

support

also

silanol

groups and Cu(I), evidenced by t h e s p e c i a l f e a t u r e of the I R spectra o f

the which form the

a c t i v e c a t a l y s t s , r e s u l t s i n t h e formation of weak Bronsted a c i d i c are

centres

capable o f i s o m e r i z i n g alkenes with increased p r o t o n a f f i n i t y .

The

o f this i n t e r a c t i o n might be a c o o r d i n a t i o n o f unsaturated Cu(1) i o n s with oxygen atom o f OH groups i n s u i t a b l e p o s i t i o n r e s u l t i n g i n an increased po-

l a r i z a t i o n o f t h e 0-H bond. REFERENCES

1 A . Molnar, J.T. K i s s and M. Bartok, J. Mol. Catal., i n press. 2 H. Kobayashi, N. Takezawa, C. Minochi and K. Takahashi, Chem. L e t t . , (1980) 1197-1200. 3 V. Ruzicka and J. Soukup, Czechoslov. Pat. 91 868 (1958); Chem. Abstr., 54 (1960) 145069. M. Hgjek and K. Koechloefl, C o l l . Czech. Chem. Commun., 34 (1969) 2739-2752. 4 H.A. Benesi, J. Catal., 28 (1973) 176-178. 5 P.A. Jacobs and C.F Heylen, J. Catal., 34 (1974) 267-274. 6 H. Knozinger and H. S t o l z , Ber. Bunsenges. Phys. Chem., 75 (1971) 10551063. 7 C.G. Armistead, A.J. Tyler, F. H. Hambleton, S.A. M i t c h e l l and J.A. Hockey, J. Phys. Chm., 73 (1969) 3947-3953. 8 A. A Tsyganenko and V . N. Filimonov , J. Mol. S t r u c t u r e , 19 (1973) 579-589. 9 A.J. van Rosmalen and J.C. Mol. J. Phys. Chem., 82 (1978) 2748-2751; 83 (1979) 2485-2751. 10 G. G h i o t t i , E. Garrone, C. Morterra and F. Boccuzzi, J. Phys. Chem., 83 (1979) 2863-2869. i n press. 11 A. Molnar, I.PBlink6 and M. Bartok, J. Catal., 12 M. Shimokawabe, N. Takezawa and H. Kobayashi, B u l l . Chem. SOC. Jpn., 56 (1983) 1337-1340. 13 M. A. Kohler, H. E. Curry-Hyde, A. E. Hughes, 8. A. Sexton and N. W. Cant, J. Catal., 108 (1987) 323-333. 14 M. Shimokawabe, H. Kobayashi and N. Takezawa, Appl. Catal., 2 (1982) 379387. 15 M. Amara, M. Bettahar, L. Gengembre and D. O l i v i e r , Appl. Catal., 35 (1988) 153-168.


C. Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

METHANOL DECOMPOSITION ON Pd/ThOZ: ACTIVITY AND SURFACE STRUCTURE

X. MONTAGNEl , R. BOULET'

, E. FREUND'

695

RELATION BETWEEN

and J.C. LAVALLEY2

Institut Ranfais du Pktrole, B P 311, 92506 Rueil-Malmaison Cedex, France Laboratoire de Spectrochimie, UA 414,Institut des Sciences de la Matikre et du Rayonnement, Universitb de Caen, 14032 Caen Cedex, France.

ABSTRACT The evolution with time of adsorbed species and reaction products formed from methanol decomposition on Tho2 and Pd (0.1 wt %)/Tho2 has been studied by FT-IR spectroscopy in transient and stationary states (P = 1 atm ; T = 280OC). Besides the reaction products [CO, HZ and (CHs)20), two kinds of formate and methoxy species have been observed and correlated t o each other. The methoxy I and formate I species, bound to one surface Th4+ ion, are formed on the (1lO)faces. They are very reactive and selectively lead to Hz and CO. Type I1 species, bound to two Th4+ ions on the (211) faces are less reactive. However it is found that methoxy I1 species can react with physisorbed methanol to produce dimethyl ether. Such results show evidence of a link between the structure of the exposed Tho2 cristalline faces and their reactivity towards CH30H decomposition.

INTRODUCTION A great deal of works has been devoted to the study of the influence of the support in CO H2 reaction on metal supported catalysts (ref. 1,2). It has been found for instance that the performances of supported Pd catalysts are very sensitive to the metal oxide on which they are deposited, a mildly basic oxide leading to methanol formation (ref. 3). Recently, the concept of the structure sensitivity developped on metals has been extended to oxide surfaces (ref. 4). The aim of the present work is to study the effect of the surface ion arrangement on the different faces of a given oxide on their activity. Among the different metal oxides, thoria seems particularly convenient since results obtained by STEM and FT-IR spectroscopy showed that Tho2 presents a well defined morphology, the surface of the microcristdites being mainly composed of (110), (211) and (111) faces in about equal proportions (ref. 5 ) . Moreover, Tho2 is a basic oxide (ref. 0), active itself in the synthesis of methanol from CO H2 (ref. 2, 7). The IR study of the methanol synthesis from CO H2 is difficult to undertake in the conditions required for the reaction as it needs a cell which can be used under pressure (ref. 8). For this reason, we have chosen to study the reverse reaction, possible under atmospheric pressure. T h o 2 and palladium highly dispersed on T h o 2 have been used as catalysts. The reaction has been studied in situ in the IR cell at 280OC. Spectra have

+

+

+

696

been interpretated by reference to those obtained from adsorption of different compounds, presented at the beginning of this work. EXPERIMENTAL The ThOz (%Cine-Poulenc) used has a specific area of 120 m2g-l. The highly dispersed Pd catalyst (dispersion: 65%, 0.1 wt %) has been prepared using Pd (NH3):+ as a precursor, by cationic exchange in an aqueous solution at pH = 10-11. For IR experiments, catalysts were pressed (at a pressure of 2 x 10' MPa) into pellets of c_a.2 em2 surface, weighing m.25 mg. The infrared cell set on a FT-IR DIGILAB-FTS 15E spectrometer enables the observation of the spectrum of both the adsorbed species either at room temperature or at 28OoC and of the gaseous phase using the double beam. For static experiments (part A), adsorptions have been performed at room temperature by adding pulses of CO or CH30H on the catalysts activated under vacuum at 60OoC. In the case of dynamic studies (part B), catalysts have been activated by heating under a mixture of hydrogen and argon (1/10) at 2OO0C, and followed by heating at 45OoC under argon flow. After cooling down to 28OoC, the catalyst has been placed-under a flow of argon methanol (LHSV = 1). After reaching the stationary state, methanol has been removed to study the thermal stability of adsorbed species. Spectra have been recorded every 5 minutes and the double beam has been used to eliminate bands due to the gaseous phase.

+

RESULTS Characterization of species given by CO and CHBOHadsorption CO adsorption on a thoria sample activated at 7OO0C has already been studied (ref. 9). In the conditions used in the present study, bands due to formate species are quite strong (fig. 1). Two types of formate species can be distinguished according to their thermal stability:

m.

0

0

Type I, characterized by a band at 1581 cm-l, v, (COO-) mode ; Type 11, characterized by bands at 1567 cm-' (u.COO-), 1362 cm-' (v, COO-) and 1375 cm-' ( b CH).

Formate I1 are more stable than formate I species. The lower stability of the latter and the high wavenumber of its u, (COO-) mode are in favor (ref. 10) of species bound to only one Th4+ ion, while formate I1 are bound to two Th4+ ions:

697

H

H

I

I

\

\

/

I

I

/

I

I

Th4+

Th4+

Th'+ Type I1

Type of formates

U

10

2000

1200

1800

Fig. 1. Species given by CO adsorption on thoria FI, FII: formate I and formate I1 species Other bands appear at 1530, 1480, 1335, 1310, 1080, 1010 and 910 cm-I and correspond to species not yet clearly identified. In the presence of CO gas (fig. l), bands at 2170 and 2160 cm-' characterize physisorbed species (ref. 9). Similw spectra are obtained on the Pd/Th02 catalyst. Supplementary bands observed at 1480, 1421, 1300, 1007 and 860 cm-' show the formation of carbonate species (ref. 6). Such species occur from the following reaction :

2 co 4 coz

+ CPd

698 as already observed on Pd/Si02 (ref, 11). Contrary to observations on other supports (ref.

12), bands in the 2200-1800 cm-' range, corresponding to Pd, (CO) species (x = 1,2), are weak. CH30H. The study of methanol adsorption on Tho, has already been undertaken (ref. 5 ) . Two kinds of chemisorbed methoxy species have been observed. One, characterized by the v(C0) band at 1120 cm-' (species I), is monodentate

0

(110) faces

Methoxy I

I

Th The other (species 11), giving rise to the v(C0) band at 1060 cm-', is bidentate, which explains its higher thermal stability

Methoxy I1 Th

Th

Their structure depends on the local arrangement of the surfaces. A third species, characterized by a sharp band at 1050 cm-', is molecularly adsorbed and corresponds to physisorbed species. Similar results are observed on Pd/ThOz catalyst. Moreover, introduction of a large amount of methanol produces a set of weak bands at 1678, 1380, 1237 and 874 cm-'. They disappear by heating and could be assigned to palladium formate species, by analogy with results obtained on Pt/A1203 catalysts (ref. 13). Heating at 100°C leads to the formation of formate I species that do not appear on pure ThOz in similar conditions. Methanol decomposition in dynamic conditions ThOz. Reaction products are CO, Hz (in small quantities, less than 10 %), HzO and (CH3)20, the later being predominant. It is important to note that heavy products build up on the surface explaining a lack of stoechiometry. Methanol introduction leads successively to the formation of the following species: 0

0

methoxy I and I1 (the number of methoxy I1 species increases more quickly with time than that of methoxy I) formate I1

699

formate I (formate species are always few in number) physisorbed CO species finally, physisorbed methanol species when the steady conditions are reached. Increasing the reaction temperature up to 35OoC decreases the number of methoxy I1 and formate I1 species, and that of physisorbed methanol whereas the catalyst activity in (CH3)ZO increases. On the other hand, the number of methoxy I and formate I species is not affected and reversible CO species are always present. When the methanol flow is stopped, physisorbed CO and methanol, methoxy I and formate I species disappear while the number of methoxy I1 is constant. At the same time, the number of formate I1 tends to increase slightly. Such results show a strong relation between methoxy I, formate I species and CO formation, methoxy I1 and formate I1 species, methoxy 11, physisorbed methanol, and formation of (CH3)zO. Pd/ThOz. CO and Hz are mainly produced (conversion: 55 %) with only traces of

CH4, COz and (CH3)zO. The evolution of adsorbed species is shown in fig. 2a,b,c. Methanol introduction leads successively to the apparition of the following species: carbonates (band at 1527 cm-'), formate I1 (fig. 2a), formate I and methoxy 11, methoxy I (fig 2b, 2c). Bands due to methoxy I and formate I are much less intense than those due to methoxy I1 and formate I1 species. No physisorbed methanol is observed. Stopping the methanol flow (fig. 2d,e) leads to a sharp decrease of the number of formate I and methoxy I. The number of methoxy I1 tends to decrease slowly with time whereas that of formate I1 is constant. The results confirm the relation between formate I and methoxy I and between formate I1 and methoxy I1 species, previously established on ThOz. Moreover, activity in CO and H2 is correlated to type I species since their disappearance corresponds to a strong decrease of CO and Hz formation.

700

FIX

1

Flt

I

Mlt

MI

MI

I

2

b a 1500

1000

1

i

cm-1

I

1500

1

1000

Fig. 2. Evolution with time of adsorbed species formed from methanol decomposition on Pd/ThOz at 28OoC a,b,c: after respectively 5 , 15 and 25 min of reaction 1 and 6 min after stopping the methanol flow d,e: C: carbonates, MI, MII: methoxy I and methoxy I1 species.

DISCUSSION AND CONCLUSION The results obtained on both catalysts allow us to distinguish two reaction paths, one via methoxy I and formate I species, the other via the type I1 ones. Methoxy I are formed on the (110) faces; the structure of formate I species proposed above is compatible with the local arrangement of the surface ions on such a face. We therefore propose that

701

on the (110) face the following scheme occurs:

CH3

CH30H OThO

4

I

H

O

0

Th

I I

-HZ 0

+

0

I

I

I

-HZ

,/

\,

0

Th

+

-cO OThO +OThO

A similar transformation can also take place on the (211) faces. Taking into account the structures of methoxy I1 and formate I1 species, we propose:

I

0

H CH30H OThThO

4

I /\

0 Th

I

I

Th 0

-----)

0 Th

h! ?

The transformation of formate I1 species into CO and Hz seems quite low due to the thermal stability of species 11. Another reaction takes place on the (211) faces involving methoxy I1 and physisorbed methanol, and leading to dimethyl ether: CH3 0 CH3

CH3

CH3

I Th/O\

CH3 CH3

I

I

\LI/

0-H Th 4

/O\n/ Th

H

I

0-H Th/

+

/"\Th

Th

Such a mechanism has already been proposed on alumina (ref. 14). Results obtained for PdjThOz show that palladium favors the transformation methoxy I + formate I, perhaps through palladium formate species, which seem quite labile in the experimental conditions used. Moreover, on Pd/ThOz, traces of CHI and COz are formed. As already suggested they could come from the following reactions:

102

No (CH3)20 is produced because a great part of methanol is transformed on the (110) faces and cannot therefore accumulate on the other faces. In conclusion, our results relative to the CO, Hz and (CH3)ZO formation show a link between the structure of the exposed ThOz cristalline faces and their reactivity towards CH30H decomposition.

REFERENCES (1)

J.R. Katzer, A.W. Sleight, P. Gajardo, J.B. Michel, E.F. Gleason and S. Rlc Millan, Farad. Discuss. Chem. SOC.22, 121 (1981)

(2) (3) (4) (5)

E. Druet, Ph.D Thesis, ENSPM, Ed. Technip, Paris (1982) R.F. Hicks and A.T. Bell, J. Cat& 104 (1985) J.E. Germain, Stud. Surf. Sci. -Catal., 21,355 (1985) X. Montagne, J. Lynch, E. Freund, J. Lamotte and J.C. Lavalley, J. Chem. SOC.,Farad. Trans. I,

(G)

x,

83, 1417 (1987)

J. Lamotte, J.C. Lavalley, E. Druet and E. Freund, J. Chem. SOC.,Farad. Trans. I, 19,2219 (1983)

(7) (8) (9)

R. Bardet, J. Thivolle-Cazat and Y. Trambouze, C.R. Acad. Sci., 292,883 (1981) J. Saussey and J.C. Lavalley, J. Mol. Catal., in press J. Lamotte, J.C. Lavalley, V.Lorenzelliand E. Freund, J. Chem. SOC., Farad. Trans. I, (1985)

(10) (11) (12) (13) (11)

G. Busca and V. Lorenzelli, Mater. Chem., 1,89 (1982) X. Montagne, Ph.D Thesis, Paris VI University, Ed. Technip, Paris (1987) A. Palazov, G . Kadinov, Ch. Donev, D. Shopov, Surf. Sci., 188,505 (1987) E. Hayes, Canadian Journal of Spectroscopy, 106 (1982) V. Moravek and M. Kraus, J. Catal., 452 (1984)

a,

a,

u,215

C. Morterra, A. Zecchina and G . Costa (Editors),Structure and Reactivity of Surfaces

703

0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

INTERACTIONS AT THE SURFACE O F PCILYCRYSTCILLINE

ADSORBATE-CIDSORBATE

MONOCLINIC

C l a u d i o MORTERRA*,

ZIRCONIA

R e n a t o ASCHIERI,

V e r a BOLIS and Enzo

BORELLO

Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universith d i Torino, Via P. Giuria 7, 10125 TORINO, Italy

ABSTRACT The adsorption of CO at ambient temperature was studied by situ FTIR spectroscopy, to check the (strong) surface Lewis acidity of a monoclinic Z r O 2 preparation activated in the 25-800 OC thermal interval. Two types of r-coordinated CO species could b e detected, characterized by slightly different VCO frequency, adsorption heat, and concentration dependence on pressure and activation temperature. Concentrations and spectral features of both CO species turned out to b e influenced by the nature and surface concentration o f charge-withdrawing/releasing species (including CO itself), as well as by the surface concentration of polar species ( e . g . , OH groups). As the overall surface coverage of CO at ambient temperature is quite low, the medium/long-range transmission of electronic effects at the surface of a non conducting system is inferred. INTRODUCTION In

recent

years

traditionally

less

interest has grown on

some

oxidic

investigated, among which zirconia,

systems that

so

some systematic work of characterization o f these materials is now becoming available. Our main interest in t h i s field has been so far concentrated on the

vacuum

thermal

activation

of

3-02

and

on

the

relevant

mechanisms (ref. l ) , as well as on the development, at the surface of progressively dehydrated 21-02,of a Lewis acidity strong enough to

a-coordinate carbon monoxide at ambient temperature (refs.

1-

2 ) . Two families of Lewis acidic sites could b e revealed, and

the

3

and

r-elevant ad-species were termed ( C O ) H (absorbing at higher characterized

by

respectively).

Both

unsaturated

a

higher species

adsorption were

ascribed

heat) to

and

(CO)L

coordinatively

(cus) Zr4+ centres, which are thought to

belong

to

different coordinative and/or crystallographic situations, not yet identified.

704

The for

present contribution deals with the experimental

some

that

lateral effects, or

have

adsorbate-adsorbate

been observed to affect the spectral

evidence

interactions,

features

CO

of

adsorbed o n variously pre-treated Zr02. EXPERIMENTAL Materials Zirconia

was

propylate,

prepared

by the hydrolysis

of

pure

zirconium

following a procedure previously described

3). Structurally i t is over 95 % monoclinic and

(refs.

1,

microcrystalline,

up to the temperatures at which sintering processes begin ( T L 750 QC).

reported

As

dehydr-ation at

earlier

( T i 400

O C )

(ref.

the

11,

early

stages

must be produced in situ by

of

302

dehydrating

the desired temperatures a sample that was first activated

vacuo

at

T

400 OC, to get rid

1

contaminants,

and

of

then rehydrated at

the

abundant

ambient

carbonatic

temperature

with

saturated water vapour. Samples a r e herewith designated by the symbol ZRP ( t h e letter P r-eminiscent of the propylate, i.e.,

is

emplayed

o f the precursor that

for the preparation here adopted), carrying a

corresponding

to

the temperature

the

subscript sample

was

first activated in vacuo, occasionally followed by the letter

(r)

(OC)

at which

was

to remind that the sample was thoroughly rehydrated, and a numeral corresponding to the temperature

by

sample,

after

interacting

rehydration,

was

dehydrated

at

( O C )

in

followed which

vacuo

the

before

CO or else. Whenever an activated sample was with carbonates (all the surface carbonate-like

with

sur-face-loaded

species that form upon contact at ambient temperature with some

5

tori- C 0 2 arid remain adsorbed further to a 5 min. evacuation at the same

temperature),

the

numeral indicating

the

temperature

of

dehydration carries the superscript C. the symbol Z R P 4 0 0 ( r ) 3 0 0 stands f o r a 2 ' 0 2

CE.g.,

propylate that was first activated in vatu0 at 400 at

sample

ambient temperature, and then dehydrated at 300

O C

from

rehydrated

O C ,

for 2

hrs.

The

symbol ZRP400(r )3OOc represents a sample prepared a s

before,

and

then

C02

surface modified in that i t was contacted with

at

ambient temperature and evacuated at the same temperature]. BET

surface

Sorptomatlc up to 400

OC,

area

(determined with N 2 at 78 K by

a

C.

1800) w a s found to b e q u i t e constant (92 - 85

Erba 2 -1) m g

and to decline sharply at higher temperatures (e.g.,

705

-

at 800 OC i t is 38

28 m2g-l, depending o n activation time).

IR spectra All

IR

spectra

spectrometer

were

Bruker

run at resolution 4

113v,

adopting

an

in

cm

-1

on

situ

an

FTIR

configuration

allowing the best correspondence among different spectra. The (absorbance) spectra of adsorbed CO were normalized against the

of

spectrum

contribution

the

bare

solid,

band-simulated

and

were

subtracted

Whenever needed, CO

o f the g a s phase.

using a (Pascal) program in which only the

spectral components is imposed, whereas all other

of

the

spectra

were number

parameters,

including the percent o f gaussian shape o f each band, a r e

allowed

to float, up to the desired degree o f band fitting. RESULTS AND DISCUSSION

Fig.

1

the R T

shows

"optical"

overall isotherms and band-resolved spectral

of

patterns

adsorption

isotherms

(both

o n e s ) and the relevant overall

CO adsorbed on a 21-02sample

activated

at

three temperatures chosen within the temperature range 100-600 OC, i.e.,

within the range between the beginning o f the exibition o f a

Lewis acidic activity towards CO at RT and the virtually elimination

surface

of

hydroxyls,

but

before

an

complete

appreciable

beginning of sintering processes.

It

is

quite evident that, whereas

ad-species

and

CO

individual patterns

do

besides

band

individual

the

adsorption

isotherms

the rather different adsorptive behaviour o f

iilustrate

the

strong

dependence

uptakes on activation indicate

that other

of

both

overall

temperature,

important

the

the

spectral

intensities, change appreciably, both

features, to

I. Spectral features

I

sample

Vco

Avy

fC01" %

QUiSS.

Vco ICO1L

A u ~ % gauss.

of

CO o n ZrO2 at various coverages.

I

ZRP4m(r)330

ZRP~1r)M)O

2194.8

2192.2

2190.3

2168.7

2198.7

219S.O

2193.6

2192.7

12.5

12.3

13.0

13.4

12.0

12.9

13.2

13.3

98

9E

98

9E

9

98

98

2186.0

2183.3

2181.4

2179.8

Z189.7

2184.9

2182.8

21E2.0

16.0

17.3

15.9

16.8

15.2

15.0

15.3

13.0

61

67

60

57

79

73

72

70

8

9

8

each

another

(e.g., s e e Table I). Table

and

spectral

within

isotherm and o n passing from one isotherm

two

706

Fiq. 1. "Optical" adsorption isotherms (left-hand plots) and relevant spectral patterns (right-hand c u r v e s ) of CO adsorbed on: ( A ) ZRP600(r)600; ( B ) ZRP400(r)400; ( C ) ZRP+00(r)250.

707 t h e e x i s t e n c e o n 2'02

c h a n g e s are a f i r s t m o n i t o r of

These

some a d s o r b a t e - a d s o r b a t e

interactions,

that w i l l be briefly

of

dealt

with separately.

interaction

The LO-CO

A f i r s t effect w e o b s e r v e

a

gradual

bands,

in the spectral patterns of fig.

Vmax of V oY r the

downwards s h i f t w i t h c o v e r a g e of

while other

spectral

features ( l i k e

~

1 is CO

both

gaussian

p e r c e n t o f e i t h e r b a n d ) r e m a i n f a i r l y c o n s t a n t w i t h CO p r e s s u r e .

rig.

2cS

"optical"

ShOWS

coverage, and

species

that

various

isothei-ms

degr

on

total

t h e s l o p e b e i n y much t h e same for t h e t w o

fairly c o n s t a n t i n a l l

c i ~ ~ c - i s srar'ye d covered by

consequelire

t h e s h i f t depends l i n e a r l y

isotherms.

the straight

Chdnge5

with

CC CC

expected,

the

l i n e s corresponding t o

the

activation

As

temperature,

of

a c h a n y i n g CO u p t a k e a l l o w e d b y

2

4

the

as

d

dehydration

ye.

= -.

'C

fS 2200

2195

2185

2180

J

,

,

(cm-')

2

4

, (cox, 6

8

x)

12

hat

I

6

Fiq. 2. D e p e n d e n c e o f t h e s p e c t r a l p o s i t i o n o f t h e t w o CC b a n d s o n the over-a1 I i n t e g r a l d b s o r b a n c e o f ad so^ b e d CO ( t o p c u r d e s refer to ( C O j H , b o t t o m o n e 5 t o ( c 0 ) ~ s; e c t i o n A r-efers to un-sintered s a m p i e s , section B t o s i n t e r e d o n e s ) .

708 spectral shift effect is likely to be brought about by

an

adsor-bate-adsorbate interaction: as the coordination of either

CO

The species

occurs through a o-donation from the C-lone pair

orbital

cus cationic centres (and the upwards shift of the V C O band -1 respect o f the free gas molecule ( 2 1 4 3 cm ) measures the

to in

extent with

of the charge-release to the solid), the

downwards

shift

coverage we observe monitors the decreasing capacity of

inductive

the

the increasing

non-conducting solid to receive extra charge, i.e.,

co

effect produced on the (charge-releasing) adsorbing

moleLules by the concentration of (charge-releasing) CO

molecu es

already adsorbed. Fig.

2B shows that this effect

sintered the t w o different

is

a general one. In fact, on

material the linear dependence (with constant slope)

W.-O

frequencies

from

that

s t i l l observed, only the slope

is

the previous set

of

of

be "9 as d

isotherms,

consequence o f the different adsorbed amounts and, possibly, o f varied mechanism of transmission of inductive effects in a

a of

a

system

structurally modified, at least at the surface. It is nutewhorty that, if the C O / Z r 0 2 system

a quantitative pont of view (ref. 4 ) , the C O / C turn the

considered f r o m

is

inductive

effects

out to be transmitted at relatively long distances. In asymptotic

hydroxyls

maximum

(from

corresponds to close-packed

fig.

20

1:

integral 2

CO molecules per nm

1.1-1.2

oxygen

with

coveiage on a sample

lattice only represents

no

absorbance

,

fact

residual units)

that compared to of

c7-15%

the

a

cus

centres made available by dehydration. The OH-CO interaction If the only effect acting on the spectral position o f

adsorbed

CO species were that due to CO coverage, the isotherms relative to different constant

activation surface

temperatures o f

area

would

be

materials

represented,

of

(virtually)

2, b y

fig.

in

different portions of one straight line for each CO species (i.e., there would b e a constant

V C O at zero coverage

(

Y O ) for

species), whereas this is not the case. In fact there are straight

lines,

progressively shifted

upwards

with

each

several

proceedlng

surface dehydration. A

similar

effect was observed with a CO species

adsorbed

on

Ti02 (refs. 5-6), and was shown to depend primarily on the surface residual

concentration

of

undissociated

coordinated

molecules

(i.e.. it was mainly an inductive effect deriving

waterfrom

709

other

adsorbed species, much a5 that produced by C O

ZrOZ

molecular

Ti02,

water coordinates more weakly than on

in the earliest stages of dehydration ( T I

desorbs l),

itself). 150

OC)(r.ef.

that it cannot be resposible for the observed shift of

50

Rather,

the

variable

groups,

i.e.,

varying

concentration

residual concentration

of

surface

the polarity of the interface a s modified of OH dipols, is believed to

this

can

be regarded a s a special

case

of

Yo. OH

by

the

produce

the

shift. In view of the peculiar nature o f surface hydroxyl

1

On and

groups,

adsorbate-adsorbate

nterar t i o n .

Fig. the

YO

3 reports the

o f

both CO species a s a

function

intensity of the "free" surface OH groups: the data

cor-relate quite well, considering that they derive from sampler

and

dehydration

experiments, there

is

hvdi-oxyls (see ref. 1 )

a

and

that

residual

at

the

seem

from

to

different

lower- stages

contribution

of

of

H-bonded

that is difficult to account for.

F i a . 3. The frequency at

zero CO coverage ( YCz) of CO adsorbed on Zr02 treated at various temperatures , a5 a function of the integrated absorbance o f the bands due to "free" surface OH groups (different graphic symbo 1s refer to different experiments and/or samples).

The C02-CO interdctio_ll U

third type o f adsorbate-adsorbate interaction

was

observed

when CO was adsorbed at a Zr02 surface that pre-adsorbed C02. The Z r U 2 / C 0 2 system

is

a rather complex o n e , and will be

dealt

with elsewhere. I t is only anticipated here that several different carbonate-like

species

form, whose nature and

amounts

strongly

710 depend o n the temperature o f activation. The effect on u-coordination o f CO one would expect o f

surface

carbonates, a s well a s o f any other anionic surface species, is a n increase

of

inductive withdrawing

the

Y C O frequencies, a s

a

consequence

effect produced o n charge-releasing groups

by

of

charge-

ones. The spectral patterns in f i g . 4 show that

is indeed the case, but they also show, a s the relevant

the this

isotherms

Fiq. 4. "Optical" adsorption isotherms (left-hand plots) and relevant spectral patterns (right-hand curves) o f CO adsorbed on samples that pre-adsorbed C02: C C C ( 4 )Z R P 6 0 0 ( r ) 6 0 0 ; ( 8 ) Z R P 4 0 0 ( r ) 4 0 0 : ( C f Z R P 4 0 0 ( r ) 2 5 C J .

711

that the expected upwards shift o f both CO bands

do,

only effect produced by

not

is

the

pre-adsorption. I t is also observed:

C02

a severe reduction o f the overall CO adsorptivity, which

( 1 )

stronger

the

lower the activation temperature, and

species

(CO)”

affects

more than the (CO)L one. Note that,

if

inductive

effects were ttie only cnes produced by carbonate-like species,

___increase

of CO ddsurptivity should accompany the observed

an

upwards

shifts, in that mor-e extra charge should b e allowed

frequency

released to tne system in the presence

be

of

ic,

the

to

charqe-w~thdrawiny

groups. The decrease o f CO adsorptivity we observe indicates

that

least some of ttie carbonate-like species do not involve only 2cus anionic centres ( 0 , OH ) , but also some cus cationic ones of at

the same nature of those which ctiemisorb CO (and especially in the form

termed ( ( 3 0 ) ~ )A. direct interference between C 0 2

adsor-ption

and surface Lewis acidity of Z r 0 2 is thus evidenced. a

(ii)

dramatic alteration

inductive

of

the

mechanisms

through

effects transmit. This Is particular-ly evident

case o f t h e effect5 due to the C O - c o

wthlctl in

the

interaction, as shown by f i g .

5 (the counterpart of fig. 2 ) . First

of all, independently of the temperature of sample

Vt:,

treatment, there is only one meaning

that

negligible shift) most

role

played

likely

for each of the two CO species,

the residual OH population

on

in respect of the role

plays

(the upwards

by the presence of carbonates. This a

general

pre-

a

frequency

behaviour

B

one, a 5 shown by section

here

of

fig.

is 5

relative to a sintered material ZRP~Q~;,. Secondly, the dependence o f

V C O on CO coverage, sti.ll fairly

liiiear in the coverage ranges explored and still the same for

the

two CO species, is now peculiar of each activation temperature. In fact w e observe that for low activation temperatures ( T i 400

DC)

the slope of the straight lines is higher, and for high activation temperatures

(T

carbonate-free difficult

to

L

400 OC) is lower than

specimens. explain,

Also

this

on

the

corresponding still

behaviour,

is not fortuitous, as shown

by

quite fig.

58

relative to a sintered system. We

believe

spectral doped

that

responsible for

the

altered

and

behaviour of CO adsorbed at the surface of a

Zr02

preparation

must be the

particular

variable carbonate-

nature

of

the

anionic species (in this case bicarbonates and mono/bi/polydentate carbonates) prevailing at the surface in the various stages o f the dehydration

diid/ur sintering process. This aspect

is

presently

712

ZRPaT

Fig. 3. Dependence o f the overall integral pre-adsorbed CO2 (top section A r e f e r s to ones).

being

@

Z R P ~ T ~

the spectral position o f the two CO bands on absorbance o f CO adsorbed on sample5 which curves refer to (CO)H, bottom ones to (CO)L; un-sintered samples, section B to sintered

investigated in some detail, with the extension

to

anions

other- than carbonates, and will be reported elsewhere.

REFERENCES t 0. Morterra, R. nschieri and M. Volante, Mater. Chem. P h y s . , in press. 2 C. Morterra, R. Aschieri, V. Bolis, B. Fubini and M. Volante. tiazz. Chim. ltal., 118 (1988) 479-481. 3 a M . Bensitel. 0. Saur, J.C. Lavalley and G . Mabilon, Mater. Chem. P t i y s . , 17 ( 1 9 8 7 ) 249-258. b M. Bensitel, V . Moravek, J. Lamotte, 0. Saur and J.C. Lavalley, Spectrochim. Acta, 436I (1 9 8 7 ) 1487-1491. 4 C. Morterra et a l . , w o r k in progress. 5 C. Morterra, J. Chem. S O C . , Faraday Trans. I , 84 (1988) 16171637.

6

E.

Garrone, V. Bolis, B. Fubini and C. submitted.

Morterra,

Langmuir,

C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V.,Amsterdam - Printed in The Netherlands

713

ADSORPTION AND DISSOCIATION OF C02 ON LANTHANIDE I O N PROMOTED Rh/A1 203 CATALYSTS

J.A. ODRIOZOLA, I. CARRIZOSA and R. ALVERO Departamento de Quimica Inorginica, I n s t i t u t o de Ciencia de Materiales, Universidad de Sevilla-C.S.I.C., P.O.Box 874, S e v i l l a (Spain) ABSTRACT By FTIR, XPS and TDMS experiments, CO d i s s o c i a t i o n a t room temperature has been stated on Ln2Og-promoted Rh/A1203 c k a l y s t s (Ln =La,Lu). Both t h e r a r e e a r t h c a t i o n and the rhodium metal are involved i n t h e intermediate steps o f t h e process. I n t h e cooperative model proposed, t h e surface hydroxylation p l a y s t h e key r o l e on t h e generation o f strong Lewis a c i d centres a t t h e surface which are essential f o r t h e carbon dioxide d i s s o c i a t i o n . INTRODUCTION

Carbon dioxide a c t i v a t i o n on heterogeneous c a t a l y s t s i s a subject o f growing i n t e r e s t due t o i t s p o t e n t i a l use as C, source f o r t h e synthesis o f f i n e chemic a l s . Thus, t h e synthesis o f methane o r methanol from C02 i s f a r more s e l e c t i v e and proceeds w i t h a greater s p e c i f i c r e a c t i o n r a t e than when i t s counterpart, CO, i s used as s t a r t i n g material ( r e f . 1). Regarding t h e c a t a l y t i c systems, i t has been stated ( r e f . 2 ) t h a t t h e manipul a t i o n of t h e support and/or t h e promoters o f rhodium c a t a l y s t s lead t o select i v e obtention of methane o r methanol i n t h e hydrogenation of carbon oxides. Rhw dium i s then an optimum candidate t o study t h e a c t i v a t i o n o f COP e i t h e r i n w e l l characterized surfaces o r i n supported systems. On t h e contrary t o other metall i c systems, C02 d i s s o c i a t i o n on rhodium i s c o n t r o v e r s i a l , and so, whereas Somorj a i e t a1 ( r e f . 3 ) p o s t u l a t e i t s d i s s o c i a t i o n a t room temperature i n CO t o w i t h a s t i c k i n g p r o b a b i l i t y c o e f f i c i e n t o f 0.1 on R h ( l l l ) , Goodman e t a1 ( r e f . 4) found t h a t t h i s p r o b a b i l i t y i s n o t greater than l . l O - l l ,

and Weinberg ( r e f . 5)

concluded t h a t C02 d i s s o c i a t i o n should be n e g l i g i b l e a t 300K. The same controver-

sy appears when rhodium i s supported, Primet ( r e f . 6) and I i z u k a e t a1 ( r e f . 7) r e p o r t i n g C02 d i s s o c i a t i o n on Rh/A1203 a t 300K, w h i l e i n t u r n Solymosi e t a1 ( r e f . 8 ) r e j e c t t h i s p o s s i b i l i t y i n t h e absence o f H2. Concerning t h e r o l e o f r a r e earth oxides i n c a t a l y t i c systems, i t has been studied how they change t h e s e l e c t i v i t y of palladium ( r e f . 91 o r rhodium catal y s t s ( r e f . 10) due t o a promotion of t h e CO bond-breaking ( r e f . 11). I n s p i t e o f t h e a b i l i t y o f t h e lanthanide oxides t o adsorb C02 because o f t h e i r basic character ( r e f . 121, no studies concerning C02 adsorption on Ln203-promoted cat a l y s t s have been published t o our knowledge. I n t h e present paper, we study the C02 adsorption a t r o m temperature on

714

Ln203-promoted Rh/A1203 c a t a l y s t s i n o r d e r t o e l u c i d a t e i f t h e r e i s a p r o m o t i o n e f f e c t o f t h e r a r e e a r t h c a t i o n s i n t h e CO bond-breaking i n carbon d i o x i d e and t o c o r r e l a t e , i f p o s s i b l e , t h e r e s u l t s o b t a i n e d w i t h t h e w e l l known a b i l i t y o f boron-doped rhodium s u r f a c e s t o d i s s o c i a t e C02 ( r e f . 13,141. EXPERIMENTAL Rh c a t a l y s t s were prepared u s i n g Rh(N03)3.xH20

(Ventron, 37% rhodium c o n t e n t 1

as t h e p r e c u r s o r s a l t . The rhodium n i t r a t e , f r o m an aqueous s o l u t i o n , was deposi t e d o n t o t h e s u p p o r t s (A1203, La203/A1203 and LuZO3/Al2O3) by an i n c i p i e n t wetness i m p r e g n a t i o n t e c h n i q u e . I n o r d e r t o a c h i e v e t h e f i n a l rhodium l o a d i n g ( l % ) , s i x s u c c e s i v e c y c l e s o f i m p r e g n a t i o n a t 298K, and d r y i n g i n a i r a t 383K f o r 10h, were necessary. The 10% Ln203/A1203 (Ln = La,Lu)

s u p p o r t s were o b t a i n e d by impregnatingJ-Al2O3

(Degussa) t o i n c i p i e n t wetness w i t h a s o l u t i o n prepared by d i s s o l v i n g Ln203 ( S i g ma, 99.9%) i n HN03. The s o l u t i o n was evaporated t o dryness and oven d r i e d a t 373K b e f o r e c a l c i n a t i o n i n a i r a t 873K f o r 4h. A f t e r t h a t , Lnz03 species were n o t d e t e c t e d on t h e s u r f a c e o f t h e samples b y X-ray d i f f r a c t i o n , t h u s s u g g e s t i n g a h i g h d i s p e r s i o n o f t h e Ln3+ i o n s on t h e A1203 s u r f a c e . I R s p e c t r a were o b t a i n e d on a F o u r i e r Transform N i c o l e t 5DXE i n s t r u m e n t

(4600-225 cm-’) w i t h a r e s o l u t i o n o f 4 cm-’.

S e l f supported p e l l e t s p l a c e d i n a

h e a t a b l e and evacuable c e l l , w i t h no m e t a l l i c p a r t s , were used. When r e c o r d i n g t h e I R s p e c t r a , a s a t i s f a c t o r y s i g n a l - t o - n o i s e r a t i o was o b t a i n e d by coadding 100 i n t e r f e r o g r a m s . Thermal d e s o r p t i o n mass s p e c t r o m e t r y (TDMS) was performed by means o f a quadr u p o l e mass spectrometer (Hewlett-Packard 5992A). A l l t h e experiments were c a r a t a h e a t i n g r a t e o f 8 K.min-’.

r i e d o u t i n a h e l i u m f l o w (30 ml.min-’)

The X-ray p h o t o e l e c t r o n s p e c t r a were r e c o r d e d i n a Leybold-Heraeus LHS-10 spectrometer (MgK, r a d i a t i o n , l l k V , 20mA, vacuum b e t t e r t h a n 10-6Pa). B i n d i n g energy (BE) r e f e r e n c e was t a k e n a t 284.6 eV f o r t h e C ( l s ) peak o f t h e a d v e n t i t i o u s carbon p r e s e n t i n a l l t h e samples. V o l u m e t r i c measurements were performed i n a c o n v e n t i o n a l d i f f u s i o n pumped g l a s s system. A l l t h e gases employed were 99.998% p u r e (S.E.O.). RESULTS The supported rhodium samples were H2-reduced a t 625 o r 725K i n o r d e r t o s t u d y t h e i n f l u e n c e o f t h e r e d u c t i o n t e m p e r a t u r e i n t h e a b i l i t y of t h e prepared c a t a l y s t s t o d i s s o c i a t e C02. The r e d u c t i o n temperatures were chosen on t h e b a s i s o f o u r p r e v i o u s r e s u l t s on t h e h y d r a t i o n l c a r b o n a t i o n o f p u r e and a1 umina-support e d r a r e e a r t h o x i d e s ( r e f . 12,15-17).

Since La203 among t h e r a r e e a r t h o x i d e s

expv-

i s t h e one t h a t dehydrates/decarbonates a t t h e h i g h e s t temperature, the mental setup was designed keeping i n mind t h a t t h e t r a n s f o r m a t i o n La(OH)3

_j

715 T1

_i,

LaOOH

2

takes place a t T1 =625 and T2 = 725K ( r e f . 15), l e s s a t La203 t e n t i o n being p a i d t o the promoter decarbonation since i t i s thermodynamically favoured towards methane i n t h e presence o f H2 ( r e f . 18). The BET area o f t h e

2

reduced samples, 1anthanide-promoted o r non-promoted ones, was 95f5 m . g - l whatever the r e d u c t i o n temperature. The rhodium dispersion was c a l c u l a t e d by means o f hydrogen adsorption a t room temperature (RT). Formal atomic dispersion was obt a i n e d f o r t h e Ln203-promoted samples, decreasing t o ca. 60% i n t h e case of Rh/A1203. Figure 1 shows the I R spectra f o r t h e promoted and non-promoted rhodium catal y s t s a f t e r C02 adsorption and f u r t h e r outgassing a t RT. Special care was taken i n e l i m i n a t i n g the residual hydrogen species adsorbed a t the metal surface by prolongated pumping a t the reduction temperature. I n the case o f t h e non-promot e d sample, t h e d i s s o c i a t i o n o f C02 i s n o t observed whatever the reduction temperature, w h i l e i n t u r n f o r t h e LuPO3-promoted one d i s s o c i a t i o n o f C02 i s observed a t both reduction temperatures by means o f t h e presence o f CO bands. The La203-promoted sample l i e s i n between t h e two above described showing C02 dissoc i a t i o n o n l y a t t h e highest reduction temperature t r i e d . For t h e Lu203-promoted sample both l i n e a r and bridged CO species are observed, whereas o n l y l i n e a r spec i e s are present f o r the La203-promoted one.

2100. o

ieoo. o

WAVENUMBERS

2100.

o

ieoo. o

CCM-l>

F i g . 1. FT-IR absorption spectra o f CO2 adsorbed a t room temperature (60 t o r r ) on: a) Rh,Lu203/A1203 ; b ) RhYLa203/A1203 ; c ) Rh/A1203

.

The s t a b i l i z a t i o n o f carbon monoxide species a t the metal surface leaving beh i n d an oxygen atom i s confirmed by TDMS. I n f i g u r e 2 i s p l o t t e d the thermal des o r p t i o n p r o f i l e corresponding t o a.m.u.

28 (CO')

a f t e r adsorption o f a COP mono-

716 l a y e r over a prereduced a t 725K Rh,La203/A1203 sample, e v o l u t i o n o f CO a t ca. 450K being observed,

p a r a l l e l t o evolution o f

I

m/e = 28

O2 (a.m.

u. 32, n o t shown). These r e s u l t s p o i n t t o t h e d i s s o c i a t i o n o f COP i n C O + O and t h e desorption o f both species separately and l e t us p o s t u l a t e t h a t C02 d i s s o c i a t i o n I

occurs a t r h o d i um s i t e s keeping

1

325

525

t h e noble metal t h e oxygen atom.

-

I

725 T/K

Fig. 2. TOMS signal corresponding t o i o n CO a f t e r adsorption a t R.T. o f CO2 on Rh ,La203/A1203

.

Further support f o r t h i s hypothesis i s obtained from XPS measurements, f i g ure 3. A f t e r t h e reduction treatment t h e rhodium 3d5/2 peaks a t 307.0 eV i n close agreement w i t h t h e reported value f o r supported Rh(0) p a r t i c l e s ( r e f . 1 9 ) . A f t e r i n t e r a c t i o n w i t h 4 t o r r o f C02 t h e peak s l i g h t l y s h i f t s t o 307.2 eV and a shoulder a t ca. 308.5 eV i s observed. This one should be ascribed t o t h e presence o f Rh(I1 species according t o l i t e r a t u r e data ( r e f . 191. This f a c t againlends

2 m Fig. 3. XP spectra o f Rh,La O3/Al203 reduced a t 723K (lower t r a c e s ) and a f t e r i n t e r a c t i o n w i t h CO2 a t R.T. upper t r a c e s )

f

.

717

support t o C02 d i s s o c i a t i o n and the subsequent o x i d a t i o n o f t h e rhodium species. H2 and C02 coadsorption experiments have been also c a r r i e d out. C02 adsorp-

t i o n on a c a t a l y s t i n which a hydrogen monolayer has been preadsorbed leads t o C02 d i s s o c i a t i o n over a l l t h e c a t a l y s t s a t both reduction temperatures, f i g u r e 4 The CO band and 2059 cm-l i n t h e case o f t h e A1203 support corresponds t o CO spec i e s on t o p o f rhodium atoms, whereas f o r t h e La203/A1203 support i t peaks a t 2004 and 2034 cm-’ f o r t h e samples reduced a t 625 and 725K, respectively. This

frequency s h i f t w i t h respect t o the Rh/A1203 sample may suggest a d i f f e r e n t pathway f o r C02 d i s s o c i a t i o n t h a t w i l l be discussed l a t e r .

4

a 0.25 C

d 2100.0

reoo.0

WAVENUMBERS CCM- 1 > Fig. 4. FT-IR absorption spectra of CO adsorbed a t R.T. on samples w i t h a iydrogen monolayer: Rh,La 0 /A1203 reduced a t 723K ( a ) , 623K f b f * Rh/Al2O3 reduced a t 723K(c), 62iK(d).

a

b

-

b

C

2100.0

1eoo.o

WAVENUMBERS CCM-1)

F i g . 5. FT-IR absorption spectra o f a mixture C02tH2 (1:3) adsorbed a t R.T. on: a) Rh,Lu203/A1203 , b ) Rh,La203/A1203 , c ) Rh/A1203

.

F i n a l l y , considering t h e growing i n t e r e s t i n C02 methanation, experiments have been performed i n which t h e c a t a l y s t s underwent i n t e r a c t i o n a t R.T. w i t h a C02tH2 (1:3) mixture. I n t h i s s i t u a t i o n and without e l i m i n a t i n g t h e r e a c t i o n mixture, C02 d i s s o c i a t i o n was also observed i n a l l t h e cases, f i g u r e 5, i t s ext e n t depending on t h e nature o f t h e promoter, thus being more favoured f o r t h e Lu203-promoted sample than f o r t h e non-promoted one. I n t h e presence o f t h e mixture, t h e CO bands observed a t ca. 2040 cm-l may be ascribed t o hydrido-carbonyl species.

718

DISCUSSION The d i s s o c i a t i o n o f C02 on s i n g l e c r y s t a l metal s u r f a c e s has been t h e s u b j e c t o f a wide c o n t r o v e r s y ( r e f . 20). T h e o r e t i c a l s t u d i e s ( r e f .

21) and a c a u t i o u s se-

l e c t i o n o f UHV experiments l e a d t o t h e c o n c l u s i o n t h a t C02 i s d i s s o c i a t e d on Rh f o i l o r R h ( l l 1 ) s i n g l e c r y s t a l s s u r f a c e s o n l y i f H2, K o r B a r e p r e s e n t ( r e f . 5, 14,22,23). t o empty

These contaminants p r o v i d e Rh s u r f a c e s i t e s a b l e t o donate e l e c t r o n s

n* o r b i t a l s

o f t h e C02 molecule, f a v o u r i n g t h e s t a b i l i z a t i o n of a b e n t

m o l e c u l e t h a t c o u l d be s t a b i l i z e d i n a b i d e n t a t e c o o r d i n a t i o n s t a t e a t t h e metal s u r f ace. To t h e b e s t o f o u r knowledge, o n l y i n t h e cases o f alumina o r s i l i c a suppor-

t e d rhodium s t u d i e s c o n c e r n i n g COP d i s s o c i a t i o n have been c a r r i e d out; again, i n s p i t e o f t h e c o n t r o v e r s y on t h e a b i l i t y o f rhodium c r y s t a l l i t e s t o d i s s o c i a t e

C02 ( r e f . 241, no c l e a r evidences have been p u b l i s h e d on how t h e rhodium s u r f a c e i s a b l e t o do i t when supported. I n t h e p r e v i o u s s e c t i o n we p r o v i d e d e x p e r i m e n t a l evidences f o r

COP d i s s o c i a -

t i o n a t R.T. when t h e Rh/A1203 c a t a l y s t i s promoted w i t h r a r e e a r t h oxides, even i n t h e absence o f H2.

I n t h i s case, t h e C O bands generated, f i g u r e 1, may be as-

c r i b e d t o l i n e a r CO adsorbed on Rh(1) species, w h i l e i n t u r n , when H2 i s p r e s e n t t h e g e n e r a t i o n o f h y d r i d o - c a r b o n y l bands i s observed, f i g u r e s 4 and 5. T h i s r u l e s o u t t h e p o s s i b i l i t y o f a p r o m o t i n g e f f e c t on C02 d i s s o c i a t i o n coming f r o m t h e presence o f adsorbed hydrogen atoms. When t h e Rh/A1203 sample i s promoted w i t h Ln203 (Ln =La,Lu) two e f f e c t s have been p r e v i o u s l y r e p o r t e d : t h e r e d u c i b i l i t y o f t h e rhodium c r y s t a l l i t e s i s much e a s i e r and t h e rhodium d i s p e r s i o n i s t w i c e t h e o b t a i n e d i n t h e non-promoted samp l e ( r e f . 1 2 ) . The f i r s t one may be i n d i c a t i v e o f a decrease i n t h e rhodium work f u n c t i o n which may f a v o u r t h e d o n a t i o n o f e l e c t r o n s t o t h e e m p t y q * o r b i t a l s o f t h e C02 m o l e c u l e and t h e second one may g e n e r a t e h i g h l y d e f e c t i v e rhodium c r y s t a l l i t e s i n which t h e presence o f k i n k s and s t e p s a r e dominant, t h u s f a v o u r i n g t h e C02 d i s s o c i a t i o n , a c c o r d i n g t o Hendrickx e t a l ( r e f . 23). However, t h e r h o dium d i s p e r s i o n i s about t h e same f o r b o t h promoters a t a l l t h e r e d u c t i o n temper a t u r e s t r i e d , t h i s r u l i n g o u t any e x p l a n a t i o n based on t h e presence o f s u r f a c e d e f e c t s , s i n c e t h e d i s s o c i a t i o n e x t e n t i s n o t t h e same and i n one case has n o t been observed, f i g u r e 1. An e f f e c t on t h e rhodium work f u n c t i o n by t h e LnOx spec i e s t h a t may be p r e s e n t a t t h e s u r f a c e o f t h e rhodium metal cannot be r e j e c t e d . T h i s t y p e o f i n t e r a c t i o n have been p r e v i o u s l y proposed by B e l l e t a1 ( r e f . 9 ) t o e x p l a i n changes i n t h e s e l e c t i v i t y o f a Pd/La203 c a t a l y s t . I n a d d i t i o n t o t h e above d e s c r i b e d p o s s i b i l i t y , a c o o p e r a t i v e e f f e c t t h a t i m plies

COP a d s o r p t i o n on b o t h t h e rhodium metal and t h e promoter, i n a s i m i l a r

way t o t h a t proposed by S a c h t l e r and I c h i k a w a ( r e f . 2 5 ) f o r t h e a d s o r p t i o n o f CO on o x o p h i l i c - p r o m o t e d metal c a t a l y s t s , s h o u l d be considered. I n a p r e v i o u s paper

719 we r e p o r t e d t h e enhancement o f C02 a d s o r p t i o n on t h e Ln203/A1203 (Ln =La,Lu) s u p p o r t w i t h r e s p e c t t o t h e p u r e alumina, g i v i n g r i s e t o CO; and HCO; s p e c i e s ( r e f . 12,17).

T h i s enhancement s h o u l d be c o n s i d e r e d as a consequence o f t h e i n -

creased b a s i c c h a r a c t e r o f t h e support; however, a p a r a l l e l i n c r e a s e i n t h e weak a d s o r p t i o n o f l i n e a r C02 (Q3 = 2353 0 - l ) was a l s o observed. T h i s a b s o r p t i o n mode has t o be r e l a t e d t o t h e presence o f l o w c o o r d i n a t e d Ln3+ i o n s a c t i n g as Lewis a c i d s i t e s , a l s o demonstrated by XPS, t a b l e 1. Whereas t h e b i n d i n g e n e r g i e s f o r L u ( 4 d ) and L a ( 3 d ) i n Ln203 agree f a i r l y w e l l w i t h t h e l i t e r a t u r e r e p o r t e d v a l u e s ( r e f . 26,27),

t h e b i n d i n g e n e r g i e s when supported i s ca. 1 eV h i g h e r t h a n i n t h e

b u l k o x i d e s . C o n s i d e r i n g t h a t i n t h e cases o f b u l k y La203 and Lu203 i t has been i m p o s s i b l e t o reduce t h e l a n t h a n i d e i o n s even under s t r o n g A r t bombarding c o n d i t i o n s , t h e s h i f t f o r t h e s u p p o r t e d l a n t h a n i d e i o n s , t a b l e 1, has t o be a s s o c i a t ed t o a change i n t h e Madelung p o t e n t i a l around t h e r a r e e a r t h c a t i o n , s u p p o r t i n g t h e i d e a o f Ln3+ species d e f i c i e n t l y c o o r d i n a t e d w i t h an enhanced Lewis a c i d character.

TABLE 1 B i n d i n g e n e r g i e s f o r l a n t h a n i d e i o n s i n Ln203 and when supported on A1203 Sample

Ln* l e v e l

B.E.(eV)

Reference

Lu203 Lu203/A1 203

4d5/2 4d5/2

196.1

26

196.9

17

833.5

T h i s work

835.0

T h i s work

La203 La203/A1 203

3d5/2 3d~/2

Ln* = Lanthanide

The importance o f such Lewis a c i d s i t e s i s s t a t e d by s t u d y i n g t h e C02 d i s s o c i a t i o n as a f u n c t i o n o f t h e i n t e r a c t i o n t i m e . The r e s u l t s f o r t h e Rh,Lu2O3/A1p3 sample ( i n absence o f H2) a r e i n c l u d e d i n t a b l e 2, which shows how t h e CO band i n c r e a s e s a t t h e expenses o f t h e weak l i n e a r C02 band on Lewis a c i d s i t e s , t h u s p o i n t i n g t o a c o o p e r a t i v e e f f e c t between t h e i s o l a t e d r a r e e a r t h c a t i o n and t h e rhodium m e t a l . Even i n t h e presence o f H2 t h i s c o o p e r a t i v e e f f e c t s h o u l d be i n voked when Ln3+ i o n s a r e p r e s e n t . So, i n f i g u r e 6, t h e simultaneous e v o l u t i o n w i t h t i m e o f t h e C02 band on Rh,Lu203/A1203 and t h e CO band coming f r o m C02 d i s s o c i a t i o n i s shown. Taking i n t o account t h a t d u r i n g t h e experiment t h e C02 atmosphere ( ~ 2 0t o r r ) was k e p t , i t i s p o s s i b l e t o assume t h a t t h e s t r o n g Lewis a c i d s i t e s were poisoned by t h e r e a c t i o n w i t h LO2, s i n c e on t h e c o n t r a r y t h e p e l l e t

would have restored the o r i g i n a l

J3

absorbance.

TABLE 2 Evolution o f C02 t o CO a t R.T.

A(C02 1/A(CO)*

time ( m i n 1

Tred ( K,

25 240

4.7

723

30

10.5

723

60

8.9

623 623

*

on Rh,Lu203/A1203

3.6

A = absorbance u n i t s

- - - - - - - - t-----------l

t-1 2400.0

2300.0

WAVENUMBERS

2100.0

1900.0

CCM- 1 >

Fig. 6. Evolution w i t h time o f t h e FT-IR absorption spectra d a mixture C 0 2 t H 2 (1:3) a t R.T. on Rh,Lu203/A1203

.

On t h e basis o f the exposed r e s u l t s , we propose a cooperative model f o r CO, L

d i s s o c i a t i o n on Ln203-promoted Rh/A1203 c a t a l y s t s , i n which d i s s o c i a t i o n takes place through C02 species weakly bonded t o t h e Ln3' ions. Once the molecule has CI.

been taken by t h e LnJ+ ion, rhodium atoms i n t h e v i c i n i t y o f the r a r e e a r t h cat i o n may i n t e r a c t w i t h emptyq* o r b i t a l s o f a C02 molecule, leading t o a bident a t e C02 intermediate species, scheme I,t h a t once disrupted leads t o t h e format i o n o f a CO molecule adsorbed on a Rh(1) species. A t t h e same time a bridged oxygen species hidden the p o s s i b i l i t y o f f u r t h e r adsorption o f l i n e a r C02 since the a c i d i t y o f the Ln3' i o n has been diminished. Even t h i n k i n g t h a t t h e Rh-Ln3'

721

0

0

I1 c

J!j

C.

II

Rh

\

4b Scheme I

distance may be w e l l beyond t h e e q u i l i b r i u m distance i n COZY 1.16

A,

i f we f o l -

low t h e scheme proposed by Freund and Messmer ( r e f . 21) a distance up t o 3

a

could be regarded as adequate considering t h e p o t e n t i a l energy curve f o r C02 d i s 3 s o c i a t i o n i n t o CO (l1)and 0 ( P I . I n t h i s cooperative scheme, the hydration degree o f the support i s o f paramount importance, since i f the support i s f u l l y hydroxylated no Lewis a c i d cent r e s o f t h e described type are accesible. This explains why i n t h e case o f Rh/ /A1203 and i n t h e case o f Rh,La203/A1203 prereduced a t 625K and not f u l l y dehydroxylated, C02 d i s s o c i a t i o n does not take place. The same type o f cooperative e f f e c t could be appropriate t o j u s t i f y t h e observed C02 d i s s o c i a t i o n on borondoped surfaces i n t h e absence o f H2 ( r e f . 13,141,

on the basis o f the strong

Lewis a c i d character o f t h e B3+ species. ACKNOWLEDGEMENT We thank C A I C Y T ( p r o j e c t no. 1112/84) f o r f i n a n c i a l support and D r . A.R.

Gon-

zglez-Elipe f o r XPS measurements. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14

R.Bardet, M.Perrin, M.Primet and Y.Trambouze, J.Chim.Phys., 75 (1978) 1079. G.v.d.Lee and V.Ponec, Catal.Rev., 29 (1987) 183. L.H.Dubois and G.A.Somorjai, Surface Sci., 128 (1983) L231. D.W.Goodman, 0. E. Peebles and J. M.White, Surface Sci. , 140 (1984) L239. W . H.Wei nberg, Surface Sci , 128 -( 1983) L224. M.Primet, J.Chem.Soc.Faraday Trans, 1, 74 (1978) 2570. T.Iizuka and Y.Tanaka, J.Catal., 70 (19811 449. F.Solymosi, A.Erdohelyi and M.Kocsis, J.Catal,, 65 (1980) 428. 19 (19841315. R.F. Hicks, Q.J.Yen, A.T.Bel1 and T.H.Fleisch, Appl.Surface Sci A.Kiennemann, R.Breau1 t , J.P. H i ndermann and M .Laurin, J .Chem.Soc .Faraday Trans. 1, 83 (1987) 2119. Y.Takita, T.Yoko-o, N.Egashira and F.Hori, Bull.Chem.Soc.Jpn., 55 (1982) 2653 R.Alvero, A,Bernal, I.Carrizosa and J.A .Odriozola, Inorg.Chim.Acta, 140 (1987) 45. M.A.Henderson and S.D.Worley, Surface Sci., 149 (1985) L1. F.Solymosi and J.Kiss, Surface Sci., 149 (1985) 17, ~

.

.,

722

15 R.Alvero, I.Carrizosa, J.A.Odriozola, J.M.Trillo and S.Berna1, J.Less-Common Met., 94 (1983) 139. 16 R.Alvero, J.A.Odriozola, J.M.Tri110 and S.Berna1 J.Chem.Soc.Da1ton Trans., (1984)87. 17 R.Alvero,'A.Bernal , I.Carrizosa, J.A.Odriozola and J.M.Trillo, Appl .Catal ., 25 (1986)207. 18 R.Alvero, 1.Carrizosa and J.A.Odriozola, in preparation. 19 M.A.Baltanas, J.H.Onuferko, S.T.McMillan and J.R.Katzer, J.Phys. Chem., 91 (1987)3772. 20 F.Solymosi and L.Bugyi, J.Chem.Soc.Faraday Trans.1, 83 (1987) 2015. 21 H.J.Freund and R.P.Messmer, Surface Sci ., 172 (1986) 1 , 22 F. Solymosi and J.Kiss, Chem.Phys.Lett ., 110 (1984) 639. 23 H.A.C. M.Hendrickx, A.P. J. M .Jongenel i s and B. E. Ni euwenhuys, Surface Sci ,, 154 (1985) 503. 24 F.Solymosi and M,PBsztor, J.Cata1. , 104 (1987) 312. 25 W.M.H.Sachtler and M.Ichikawa, J.Phys.Chem ., 90 (1986)4752. 26 J.P.Espinhs, A.R.Gonz5lez-Elipe and J.A.Odriozola, Appl.Surface Sci ., 29 (1987) 40. 34 27 Y.Uwamino, T. Ishizuka and H.Yamatera, J. Electron Spectrosc.Relat.Phenom (1984)67.

.,

C. Morterra, A. Zecchina and G . Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

723

A STRUCTURE DETERMINATION OF THE TILTED a3 STATE OF CO ON FE(001) BY X-RAY PHOTOELECTRON DIFFRACTION

J.Osterwalderlr G.S. Hermas, R.S. Saikg: M.Yamada3 and C. S. Fadley2 1 lnstitut de Physique, Universite de Fribourg, CH- 1700 Fribourg

(Switzerland) ZChemistry Department, University of Hawaii, Honolulu, HI 96822 (USA) 30ptoelectronics Techn. Res. Corp., Tsukuba (Japan)

ABSTRACT There exists evidence for a strongly tilted a 3 state of CO on Fe(OO1), possibly representing a precursor to the dissociation of CO. We present xray photoelectron diffraction data of Al Ka excited C 1s electrons that exhibit strong forward scattering features along the C-0 bond axis and therefore can be used to determine the orientation of the molecule very precisely. A tilt of 55Of2" away from the surface normal is observed, oriented along the directions. Single scattering cluster (SSC) calculations are also presented, that yield further information on bonding parameters such as the distance of the C atom to the surface and the degree of frustrated rotational motion of the CO molecule.

INTRODUCTION The system COIFe(OO1) has attracted much interest recently as a model for studying a catalytic reaction in great detail [l-41.Three associatively bonded states, CO(al), CO(a2) and CO(ag), can be distinguished with thermal desorption spectroscopy, and one dissociatively bonded state CO(p) [l]. Among these, the most strongly bound molecular state CO(a3) appears to be a precursor state for dissociation: The C-0 stretch frequency is found to be abnormally low [5], and recent near-edge x-ray absorption fine structure (NEXAFS) [6] and electron-stimulated desorbed-ion angular distribution (ESDIAD) [3] experiments indicate that, in contrast to many other CO adsorption states, the molecular bond axis is far from being perpendicular to the surface in this state, with a tilt angle of about 45°f100 away from the surface normal. In this paper we will present x-ray photoelectron diffraction (XPD) data

724

from this system and demonstrate how the bonding geometry can be determined with considerably more accuracy (see also Ref. 7). EXPERIMENT' A Hewlett-Packard 5950A photoelectron spectrometer equiped with a monochromatized Al K a x-ray source and a two-axis sample goniometer [8] has been used for data aquisition. The Fe(OO1) surface was cleaned using standard recipes [ l ] and CO was adsorbed at ambient temperature. C 1s and 0 1s core level analysis indicated that a high fraction of CO was present in the CO(a3) state [l]. A slow desorption of CO was found to be a problem with typical scan durations of more than 24 hours at ambient temperature. The data presented in Figs. 1 and 3 are sums of up to 7 individual scans with sample cleaning and CO exposure in between. SINGLE SCATTERING THEORY Model calculations were done within the single scattering cluster (SSC) formalism [8], using proper spherical-wave final states [9]. A cluster size of one CO molecule and 5 Fe atoms forming a fourfold hollow pocket of Fe(OO1) was found to be sufficient for reasonable convergence. Correlated vibrational motion of substrate and adsorbate atoms was taken into account by a sitedependent Debye-Waller-type factor [lo]. In addition, we have studied the dependence of the XPD patterns on different degrees of frustrated rotational motion of the CO molecule about the C atom, characterized by a root-mean-square displacement angle BRMS. A large number of intensities corresponding to slightly different molecular orientations were summed with individual weighting factors according to a harmonic oscillator motion [ l l ] . Angular broadening

[110] AZIM.

v)

0.7

I ,99,9

Val%) was used without

The n i t r o g e n and t h e methane were bubbled together

i n t o a t h e r m o s t a t i c a l l y c o n t r o l l e d ( T = 2 0.01

K ) H20 s a t u r a t o r a t 357.6 K

t o g i v e a f i x e d volume r a t i o s : R= H20/CH4 o f 2.54 and R'= N /CH o f 1. The 2 4 r e a c t i o n mixtures was f e d i n t o t h e r e a c t o r containing t h e c a t a l y s t (0.02 g, 40-70 mesh) d i l u t e d w i t h same-sized carborundum ( l / l O , v o l / v o l ) t o ensure quasi-ideal

conditions f o r

mass and heat t r a n s f e r .

Analysis was performed

742

b y an " o n - l i n e "

gas-chromatograph

(ATC/f s e r i e s 410 C a r l o Erba, TCD d e t e c t o r )

equipped w i t h two s t a i n l e s s s t e e l columns ( 1 . r e s p e c t i v e l y molecular He as

2.5 mm;

4mm) c o n t a i n i n g

s i e v e 5A and Poropak Q and o p e r a t e d a t 347 K w i t h

t h e c a r r i e r gas ( 6 0 Ncrn3min-').

Before

the reaction,

was reduced i n a hydrogen f l o w o f ca, 50 Ncm3 min-' subsequently

i.d.=

the catalyst

f o r 2 h a t 673 K and

f o r 1 h a t 998 K w i t h t h e same hydrogen f l o w ,

then cooled

t o r e a c t i o n T i n a H atmosphere. The steam r e f o r m i n g o f methane was s t u d i e d 2 in t h e t e m p e r a t u r e range 873-923 K and space v e l o c i t y expressed as GHSV -1 -1 -1 (NlCH4h Nlcat= h ) ) r a n g i n g f r o m 37,500 to150,OOO h The c o n v e r s i o n of rne-

.

t h a n e (mol%) was t a k e n as a measure o f t h e c a t a l y t i c a c t i v i t y . f o r m a t i o n was observed.

No carbon

I n a s e r i e s o f experiments t h e methane c o n v e r s i o n

was k e p t below 10% t o o b t a i n r e l i a b l e d i f f e r e n t i a l k i n e t i c data.

RESULTS AND D I S C U S S I O N Cat a1y s t s c h a r a c t e r i z a t ion Hydrogen c h e m i s o r p t i o n and BET d a t a f o r t h e pure and f o r t h e a l k a l i - d o p e d Ni/MgO c a t a l y s t s a r e l i s t e d i n Table 1. The low values o f t h e metal d i s p e r s i o n (ca. 5%) f o r t h e MPF7 and MPF12 c a t a l y s t s w e l l agree w i t h t h o s e r e p o r t e d ( r e f s . 9,111

f o r the air-calcined e t c . 1. This 2 ( > l o wt%), t h e N i

h i g h l y N i - l o a d e d c a t a l y s t s on d i f f e r e n t c a r r i e r s (Si02, A1203, T i 0 o b s e r v a t i o n seems t o i n d i c a t e t h a t a t h i g h Ni d e p o s i t e d on s u p p o r t

builds

up as

content

a m u l t i l a y e r which i s i n s e n s i t i v e t o

t h e n a t u r e o f t h e support.

TABLE 1 Hydrogen c h e m i s o r p t i o n and BET d a t a f o r " p u r e " and a l k a l i doped N i w catalys%sa. __._

Catalyst

MPF7 MPF12 MPF12-A MPF12-B

Ni (WtX)

16.5 17.9 17.9 17.9

A l k a l i metal (WtX)

--1 (Li) 1 (K)

H uptake 2 (pmol g - l cat __ 56.2 77.1 6.5 43.5

(%I

M S A (mzN/giNi

3.8 5.1 0.4 2.8

25.0 34.3 2.9 19.3

D

BETsurface area (m2gcet) 29.2 28.2 28.9 28.7

a A l l samples were c a l c i n e d i n an a i r s t r e a m a c c o r d i n g t o t h e s t a n d a r d c o n d i t i o n s and t h e r e a f t e r reduced,as

indicated i n the experimental section,in

f a r 1 h a t 998 K.

flowing H

2

f a r 2 h a t 6 7 3 K and s u b s e q u e n t l y

743

The same l o a d i n g o f d i f f e r e n t a l k a l i e s ( 1 w t % ) added t o t h e c a t a l y s t i m p a i r s t h e c h e m i s o r p t i o n c a p a c i t y towards t h e H2 a l t h o u g h t o a d i f f e r e n t e x t e n t : t h e L i appears t o be t h e most t o x i c as MSA i s c u t by as much as 92% w i t h respect

t o original

c a t a l y s t a g a i n s t a 45% c u t f o r t h e K-doped c a t a l y s t .

A p l a u s i b i l e reason o f t h i s r e s u l t c o u l d be f o u n d i n t h e d i f f e r e n t m o l e c u l a r

radius

of

the

poisoning

species

and t h e r e f o r e a geometric f a c t o r

could

be i n v o k e d t o e x p l a i n t h e observed losses i n the z t i v e metal area. On the c o n t r a r y t h e BET s u r f a c e a r e a o f t h e c a t a l y s t s i s u n a f f e c t e d f r o m t h e presence o f a l k a l i . Moreover no changes i n metal d i s p e r s i o n and BET s u r f a c e area were observed on aged c a t a l y s t s (200 h t i m e on stream a t 898 K ) . The e f f e c t s o f t h e Tc and a l k a l i d o p i n g on c a t a l y s t s r e d u c i b i l i t y were assessed

through

TPR

experiments.

Results

a r e summarized i n Table 2 i n

terms o f t h e temperature o f t h e maximum peaks (T,) d e r i v e d f r o m t h e H2 consumption

and t h e N i O r e d u c i b i l i t y

( a t T o f r e d u c t i o n up t o 1223 K ) b o t h on

p u r e and doped c a t a l y s t s .

TABLE 2 Temperature programmed r e d u c t i o n o f Ni/MgO c a t a l y s t s . E f f e c t o f a l k a l i doping and c a l c i n a t i o n temperature.

Catalyst

Composition

Calcination temperature (K) and time ( h ) 17.9%Ni/MgO 673 (16) * 17.9XN i/MgO S.C.+ 873 (6) 17.9%Ni/MgO S.C.+1073 (6) 17.9%Ni/HgO S.C.+1273 (6) 17.9%Ni/l%Li/MgO S.C.+ 673 (6) 17.9%Ni/l%K/MgO S.C.+ 673 (6)

MPF12 MPFlZ-1 MPF 12-2 MPF12-3 MPF12-A MPF12-8

T,1 (K) 543 543 nil nil 549 548

T,2

T,3

(K) 643 708 nil nil 670 670

(K)

(K)

(%)

903 933 938 nil 908 888

1047 1101 1223

77.4 67.5 22.0 5.0 100.0 77.0

Tm4 NiO Reduction

nil 1123 948

*standard conditions (s.c.)

From t h e above i s e v i d e n t

that:

i)

t h e r e d u c t i o n i s c h a r a c t e r i z e d by

onset

o f f o u r d i f f e r e n t peaks which have been a t t r i b u t e d t o d i f f e r e n t s p e c i e s

(ref.

4);

ii) upon i n c r e a s e o f t h e c a l c i n a t i o n t e m p e r a t u r e ( T c ) , f r o m 673

t o 1273 K, t h e Tm o f a l l t h e s e peaks a r e s h i f t e d t o h i g h e r values; i i i ) t h e Tm, peak, the N i

ascribed t o Ni3+

(ref.

r e d u c i b i l i t y decreases

t o 5% f o r a Tc o f 1273 K.

41, d i s a p p e a r s a t Tc h i g h e r t h a n 873 K; i v ) m o n o t o n i c a l l y f r o m 77% f o r

a Tc o f 673 K

744

The TPR p r o f i l e s o f t h e samples a i r c a l c i n e d a t d i f f e r e n t Tc are shown i n Fig.

1.

TPR r e s u l t s f o r t h e pure c a t a l y s t s are i n t e r p r e t e d i n terms

o f a progressive d i f f u s i o n o f Ni2'

i o n s i n t o the MgO s t r u c t u r e ( w i t h inherent

formation o f s o l i d s o l u t i o n s ) increasing f o r an increase o f Tc,at o f t h e " f r e e " N i O (T

mz

of

i3 and Tm

(ref.

t h e expense

1. The progressive s h i f t towards higher temperatures

w i t h increase o f Tc denotes, i n accordance w i t h t h e l i t e r a t u r e ,

41, a more abundant presence o f N i

2+

ions i n thesub7sWaelayer and

w i t h i n t h e MgO l a t t i c e .

b

0

u

N

I

C

/ 573

n3

973

1173

T( K ) F i g . 1. TPR p r o f i l e s f o r c a t a l y s t MPF12 a i r - c a l c i n e d a t d i f f e r e n t temperature. a) MPF12 (673 K); b ) MPF12-1 (873 K); C ) MPF12-2 (1073 K);d)MPF12-3(127X). Fi g . 2 . TPR p r o f i l e s f o r Ni/MgO c a t a l y s t s . E f f e c t o f a l k a l i doping. a) MPF12-A (17.9%Ni/l%Li/MgO); b ) MPFlZ-B (17.9%Ni/lZK I M g O ) ; c ) MPF12 (17.%Ni/MgO).

The TPR p r o f i l e s (Fig. 2) o f L i and K doped N i c a t a l y s t s are vely

n o t d i f f e r e n t from those o f t h e undoped sample,although

qualitati-

t h e i r reducibi-

K modified (MPF12-B) shows t h e same

l i t y i s quantitatively

affected: t h e

degree o f r e d u c i b i l i t y

(77%) as t h e "pure"

catalyst, opposite t o t h e L i

doping which enhances, t i l l f u l l reduction, t h e N i r e d u c i b i l i t y . This behaviour looks s i m i l a r t o t h a t observed on t h e N i / A l 2 O 3

system

by Narayanan and Uma ( r e f . 12) who have a t t r i b u t e d i t t o a decreased a c i d i t y of L i modified system t h a t helps N i 2 + reduction. I n our case, as t h e a c i d i t y of

t h e c a t a l y t i c system Ni/MgO i s very low, any eventual chemical e f f e c t

o f a l k a l i should has been more marked i n case o f t h e K doped c a t a l y s t thus denying any chemical e f f e c t . Moreover t h e formation

of

a "true"

compound and/or

a s o l i d solution

i n c o r p o r a t i n g L i 0 i n the MgO l a t t i c e looks very u n l i k e l y , on account o f 2 the moderate Tc (673 K ) , as also proven by t h e f a c t t h a t t h e reduction's p a t t e r n o f t h e Li-doped c a t a l y s t i s n o t modified. Most l i k e l y t h e L i promotes t h e r e d u c t i o n o f N i 2 + i o n s located on t h e surface allowing a r a p i d formation o f metal n u c l e i ( N i l which,

i n turn,

enhance r e d u c i b i l i t y v i a a nucleation

mechanism as suggested f o r metal oxide reduction ( r e f . 13). Therefore t h e TPR r e s u l t s could be i n t e r p r e t e d t a k i n g i n t o account t h e prominent surface e f f e c t o f a l k a l i doping i n accordance w i t h H2 chemisorption r e s u l t s . TEM micrographs

o f the

support show a quasi-perfect

cubic

structure

o f t h e c o n s t i t u e n t p a r t i c l e s most l i k e l y due t o t h e p a r t i c u l a r preparation method.

Because o f very f i n e

dimensions,

i t s exposed geometric

area i s

equal t o t h a t determined from BET and thus i t can be c l a s s i f i e d as having an "open"

structure.

This i s confirmed by t h e very uniform d i s t r i b u t i o n

o f N i O evidenced by maping t h e sample w i t h EDX analysis. A l k a l i doping(sanp l e MPF12-B) a f f e c t s c a t a l y s t surface as shown i n Fig. 3.

Fig. 3. SEM micrographs o f MPF12-B c a t a l y s t

"as prepared"; ( a ) x 40, (b) x 325.

746

The surface appears covered by dark i s l a n d s which, f r o m EOX microanalysis, have been i d e n t i f i e d as due t o K p a r t i c l e s evenly d i s t r i b u t e d a l l over t h e surface, (Fig.

Furthermore,

examining a r e g i o n o f MPF12-B a t h i g h m a g n i f i c a t i o n

3 ( b ) ) i t i s evident t h a t even i f l o a d i n g o f t h e K i s low ( 1 w t % )

i t masks ca. h a l f o f t h e exposed surface. This observation i s w e l l i n agree-

ment w i t h chemisorption data which f o r t h e K-doped c a t a l y s t

MPF12-B i s

about t h e h a l f o f t h a t o f t h e pure N i c a t a l y s t (MPF12). I n order t o b e t t e r d e f i n e t h e n a t u r e o f surface e f f e c t e x e r t e d by t h e a l k a l i doping f u r t h e r work must be done.

I n p a r t i c u l a r i t seems necessary

t o evaluate whether a l k a l i covers p h y s i c a l l y t h e N i a c t i v e surface making i t inaccessible f o r H

chemisorption ("geometric e f f e c t " ) o r i f they modify 2 t h e N i a c t i v e surface by an "ensemble e f f e c t " , a f f e c t i n g H2 chemisorption and r e d u c i b i 1it y o f N i /MgO system.

C a t a l y t i c a c t i v i t y data ( i )Pure c a t a l y s t s .

E q u i l i b r i u m conversion on pure c a t a l y s t s

a t 898K

i s f u l l y a t t a i n e d already a t GHSV o f 37,500 h - l (Table 3 ) .

TABLE 3 E f f e c t s o f c a l c i n a t i o n temperature and GHSV on a c t i v i t y o f Ni/MgO c a t a l y s t s f o r t h e steam r e f o r m i n g o f methane a t TR= 873-923K; R= H O/CH =2.54and R0=N2/CHc1. 2 4 Catalyst

Tc

MPF7 MPF7 MPF7-1 MPF7-2 MPF7-3 MPF7 MPFl2 MPF12 MPF12 MPFlZ

(K) 673 673 873 1073 1273 673 673 673 673 673

*n.a.

GHSV

TR

CH4 conv.

(h-') 150,000 150,000 150,000 150,000 150,000 150,000 37,500 75,000 150,000 150,000

(K) 873 898 898 898 898 923 898 898 898 923

% 43.0 46.2 31 .O 8.0 n.a.* 56.4 67.8 52.5 46.1 54.5

Equilibrium CH4 conv. % 66 68 68 68 68 70 68 68 68 70

= non a c t i v e

The i n f l u e n c e o f t h e Tc and t h e o t h e r experimental v a r i a b l e s was i n v e s t i g a t e d a t h i g h e r GHSV (Table 3 ) .

Although,

as expected,

t h e conversion decreased

747

w i t h i n c r e a s i n g GHSV, h i g h c a t a l y t i c a c t i v i t i e s were observed

s t i l l at

GHSV

as h i g h as 150,000 h - l i n t h e whole t e m p e r a t u r e range i n v e s t i g a t e d (873-923K). Air-calcination

at

Tc

f r o m 673 t o

1273 K caused a p r o g r e s s i v e decrease

o f t h e c o n v e r s i o n f r o m t h e 46.2% f o r Tc o f 673 K t o z e r o f o r Tc o f 1273K. T h i s a l l o w s t o assess a s t r a i g h t c o r r e l a t i o n between a c t i v i t y and N i r e d u c i b i l i t y insofar,

as s t a t e d above,

h i g h e r Tc promote m i g r a t i o n o f N i O i n s i d e

t h e MgO

s t r u c t u r e t h u s l o w e r i n g t h e f r a c t i o n of

exposed.

As F i g .

4 shows,

r e d u c i b l e and a c t i v e N i

the catalyst exhibits high s t a b i l i t y indicating

absence o f c o k i n g and s i n t e r i n g . ( i i ) Doped c a t a l y s t s . As t o t h e e f f e c t s o f a l k a l i ( T a b l e 41, i t i s e v i d e n t t h a t L i depresses t h e c a t a l y t i c a c t i v i t y more s t r o n g l y t h a n K. T h i s observat i o n ‘ i s c o n s i s t e n t w i t h p r e v i o u s H c h e m i s o r p t i o n d a t a d e n o t i n g a geometric 2 e f f e c t o f t h e a l k a l i doping. Moreover i n t e r p r e t a t i o n w e l l complies w i t h r e s u l t s o f a s e r i e s o f experiments performed under d i f f e r e n t i a l conditions (GHSV~150,OOO h - ’ ) t h r o u g h which r e l i a b l e values o f t h e t u r n o v e r number, TON (molec M-’.S-’), CH4 s have been d e r i v e d . As F i g . 5 shows, TON i s independent f r o m t h e MSA f o r b o t h t h e doped and t h e undoped c a t a l y s t s d e n o t i n g t h a t a l k a l i render inaccessible t o

TABLE 4 C a t a l y t i c a c t i v i t y o f a l k a l i doped Ni/MgO c a t a l y s t s (Tc=673K) on nethane steam r e f o r m i n g a t T = 898 K. R

Catalyst MPFl2 MPF12-A MPFl2-A MPFl2-6 MPFl2-B MPF12-6 a hPF 12-61 a MPF12-61

A l k a l i content (wt%)

--

1(Li) 1( L i 1 1(K) 1(K) 1(K) 1 (K) 1 (K)

GHSV (h-’)

CH4 Conv.

150,000

46.1 26.8 2.7 52.3 45.1 28.2 28.0 5.0

75,000 150,000 37,500 75,000 150,000 75,000 150,000

(%I

a T h i s sample was prepared by i m p r e g n a t i o n o f t h e u n c a l c i n e d MPF12 c a t . w i t h i s o p r o p a n o l s o l u t i o n o f K a c e t a t e .

748

7 -

'u aa

3-

cn '3

= 2..

A-

Y

*-f

'0

c c

L: 0

0

'MSA Cm2fi.g"l(il

Reaction time Chi

F i g . 4. Methane conversion versus r e a c t i o n time a t T= 898 K a t d i f f e r e n t GHSV f o r MPF12 c a t a l y s t . GHSV: (0) 75,000 h - l , ( A ) 150,000 h-1. Fig. 5 . TON versus metal surface area. TR= 898 K . ( ( 0 ) MPF12.

1 MPF12-A; ( A ) MPF12-B;

H

and CH some f r a c t i o n of t h e a c t i v e s i t e s without hindering t h e r e a c t i v i 2 4 t y o f t h e adjacent sites.As t o t h e lower a c t i v i t y o f t h e MPF12-B1 sample i t r e f l e c t s a d i f f e r e n t poisoning e f f e c t depending upon t h e previous h i s t o r y

of

t h e sample.

An explanation could be found on t h e d i f f e r e n c e s i n t h e

i n t e r a c t i o n between K and N i species i n t h e uncalcined and c a l c i n e d sample. F u r t h e r studies, now in progress,

have been purported t o shed more l i g h t

on t h i s subject. REFERENCES H i g h f i e l d , A. B a s s i and F.S.

J.G.

Stone, i n G. P o n c e l e t , P. Grange and P.A.

Jacobs (Eds), Prepa-

r a t i o n o f C a t a l y s t s 111, E l s e v i e r , Amsterdam, 1983, p. 181.

A.

Zecchina, G. Spoto, S. C o l u c c i a and E. G u g l i e l m i n o t t i , J. Chem. SOC. Faraday Trans I. 80 (1984)

1891-1901. 1. B o r o u i e c k i , Appl. Catal.,

31 (1987) 207-220.

G.C.

Bond and S.P.

J.R.

R o s t r u p - N i e l s e n , Steam R e f o r m i n g C a t a l y s t s , D a n i s h T e c h n i c a l Press, Copenaghen, 1975, p.81.

Sarsan, Appl. C a t a l . 38 (1988) 365-377.

1. B o r o u i e c k i , Appl. C a t a l . 10 (1984) 273-289.

A.J.

Appleby, "Advanced F u e l C e l l Technology" EPRI J.,

A. Parmaliana,

F. F r u s t e r i .

P o l o A l t o CA-USA,

rence, Wien J u l y 20-24 1986, Perganon Press, d x f o r d , 1986, Vo1.3, D.G.

10 11 12 13

D.R.M. Y.J.

M u s t a r d and C.H.

Oecember 1984, p. 63.

P. T s i a k a r a s and N. Giordano, Proc. 6 t h World Hydrogen Energy Confe-

p. 1252.

Bartholomeu, J. C a t a l . 67 (1981) 186-206.

M o n t i and A. B a i k e r , J. C a t a l . 83 (1983) 323-335. Huang and J.A.

Schuarz, Appl. C a t a l . 30 (1987) 239-253.

S. Narayanan and K. Uma, J. Chem, Soc. Faraday Trans I, 81 (1985) 2733-2744. N.W.

H u r s t , S.J.

Gentry and C.Jones,

C a t a l . Rev. Sci. Eng. 24 (2)(1982) 233-309.

C.Morterra,A. Zecchinaand G . Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands

COMPARISON OF ALKALI PROMOTERS ON SILICA-SUPPORTED Ni and Ni (111) STATE, LOCALIZATION AND RANGE OF THE PROMOTER EFFECT

749

:

CHEMICAL

H. PRALIAUD, B. TARDY, J.C. BERTOLINI and G.A. MARTIN Institut de Recherches sur la Catalyse, C.N.R.S., 2 avenue Albert Einstein, 69626

- Villeurbanne CBdex

-

France

SUMMARY or The effects of two alkalis, Na and K either vapodeposited on Ni(lll), introduced as nitrates in the precursor of a silica-supported nickel before its reduction, have been compared. In promoted Ni/SiO the alkali & io;.iq and associated with a counteranion while it is under &e form of K (N ) on Ni(lll), with $r varying from 0.5 to 0 as the coverage increases. The alkali silicate decorates the metal surface for Ni/SiO catalysts. All things being 2 lower for K than for Na on equal, the vibration frequency of adsorbed CO is Ni(ll1) ; the reverse is observed for Ni/SiOZ. This is interpreted assuming a direct electrostatic interaction between the 0 atom of adsorbed CO and the ionic alkali in the promoted catalyst while an indirect interaction via the metal would be prevailing for the case of Ni single crystal precovered with vapodeposited alkalis. INTRODUCTION The influence of poisons or promoters on chemisorption and catalytic properties of metals can be interpreted either in terms of geometric effects (simple site blocking, facetting of small particles) or via electronic effects. Two kinds of electronic effects are generally considered : i) the indirect interaction, via the conduction electrons, where the additive changes the electronic and chemical properties of metal atoms interacting with it and the neighbouring metal atoms, inducing a more or less long range delocalized interaction probably associated with a change in the density of states at the Fermi level (1). ii) the direct interaction, due to an overlap between orbitals on promoter and adsorbate or to electrostatic interaction leading to a localized interaction ( m a x i m range 3-4 A) ( 2 ) . Recently, Uram et al. ( 3 ) clearly distinguished between short and long-range interaction for CO and K coadsorbed on Ni(ll1) with the help of reflection-absorption spectroscopy. For that case, the spectral feature attributed to the short-range electrostatic interaction was detected only at low CO coverage and was shown to be small, while that attributed to the long-range delocalized interaction was much larger and detected over the whole CO coverage.

750

Organometallic chemistry provides us with numerous examples of direct interactions : carbonyl ligands in RU~(CO)~~,bonded by their carbon atoms to the metal centers, may also interact by their oxygen to Lewis acids such as A1Br3 (4) : the electron doublet of oxygen is given to A1Br3 and the J CO frequency is shifted at 1535 cm-'. More recently (S), evidences of a direct interaction between the oxygen of CO ligands and alkali ions have also been yielded : CpM'(C0);M' in tetrahydrofuran (M' = Co, Cr, W and Mt = Li, Na, K) gives rise to J CO at ca. 1700 cm-l. The smaller the ionic radius of Mt, the lower the frequency : J co frequencies are 1717, 1743 and 1748 cm-' for Lit, + Na and.'K respectively, when M is cobalt. They are interpreted by an interaction of the type M' - CoS 0 .'M The 3 CO variations are attributed to the electrostatic field Z/r created by the alkali ion where Z is the ionic charge and r the i'onic radius. This field is larger for smaller ionic radius, giving rise to lower frequency. In contrast with cases where the direct interaction is prevailing, indirect interaction leads to the reverse situation : when going from Li to Cs, the ionization potential decreases, the electron transfer from the additive to the metal increases and the corresponding $ CO is expected to decrease. Accordingly, the comparison of the vibration frequencies of carbon monoxide adsorbed on metals promoted with different alkalis allows us to take up the question of the direct versus indirect electronic interaction of promoters. This work reports on the comparison of CO chemisorption on Ni(ll1) precovered with vapodeposited Na and K, and on Ni/Si02 catalysts promoted with Na and K compounds obtained by adding alkali nitrates to the catalyst precursor before reduction. A correct understanding of spectroscopic data needed additional morphological data (location and chemical state of additives) which are also presented in this work.

...

EXPERIMENTAL The Ni(0H),/SiO2 precursor was obtained by adding the silica support (Aerosil 200 m2g-' from Degussa) to a solution of nickel nitrate hexammine as described elsewhere ( 6 ) . After washing and drying the solid was stirred in a solution of KN03 or NaN03 then evacuated to dryness under reduced pressure at 351 K in a rotary evaporator ( 7 ) . The Ni amount was 24.8 wt Z and the K or Na/Ni atomic ratios span the 0-0.24 range. The samples were reduced in flowing hydrogen (5 1 h-') by heating at 2 K min-' from 298 to 923 K for 15 hours. Chemisorption, magnetic, and infrared experiments were run as described in (ref. 7).

751

Experiments on single crystals were performed in an UHV chamber equipped with an electron monochromator and a high resolution analyser for vibrationnal EELS (0 = 8 ' = 5 0 ° , Ep = 3 eV), a quadrupole mass-spectrometer for TDS, and a cylindrical mirror analyser electron spectrometer for Auger spectrometry. Potassium and sodium were evaporated from a commercial SAES getter source, (located at a distance of about 2 cm from the crystal) heated by passing a direct current through it. The calibration of deposited K or Na amounts was done by using the procedure previously described for the K/Pt (100) system ( 8 ) . The cleanliness of the K or Na adlayer was checked by AES and care was taken to avoid oxygen pollution. RESULTS AND DISCUSSION Chemical state of alkali compounds It has been shown elsewhere (7) that in potassium-promoted Ni/Si02 catalysts, the position of the K(2p

3 1 2 ) peak from XPS experiments remains

almost unchanged after reduction, suggesting that potassium is probably present as Kt in the reduced catalyst rather than as metallic potassium. A selective extraction method was used to obtain more informations on the chmmical state of the alkali compound. The principle of this technique is based on the fact that K 2 0 and KOH are soluble in water and ethanol while silicates are soluble only

in water. The results are summarized in Table 1.

TABLE

1

Chemical state of K-compounds in reduced potassium-promoted Ni catalysts.

catalysts

K/Ni at.ratio (%)

K-silicates % of total

K-Ni/Si02

17.9

41

K-Ni/Si02

4.7

64

K-Ni/MgO

2.4

0

K

K20 (KOH) % of total K 23 2.3

94

The results of Table 1 show that for the case of K-Ni/Si02 catalysts only 213 of potassium can be extracted by water. Further experiments are obviously

needed to determine the nature of the K-containing insoluble compound. However, a prima facie deduction can be drawn from this table : at low K contents, most

of the promoter is under the form of a silicate or a mixture of silicates (K2Si03, K2Si205, K2Si409...). For larger K contents, large amounts of K20 are also detected, together with silicates (the presence of KOH can be ruled out on the basis of

infrared spectroscopy

which does not reveal any hydroxyl

I52 vibrations).

Some selective extraction experiments were also carried out on

K-Ni/MgO (Ni, 15 wt X) to check the validity of the method. As can be seen from Table 1 the potassium balance is correct (94 X) and the quantity extracted by water equals that extracted by ethanol, as expected when silicates are not present. The combined use of XPS, AES and EELS techniques have revealed that with & varying from 0.5 to 0 when the K coverage increases from 0 to unity (one monolayer = 6.7 x 1014 K.cm-2) (9). The charge of vapodeposited sodium on Ni(100) has also been shown

potassium vapodeposited on Ni(ll1) is present as K"

to be near zero at 8 Na = 1 (10). As can be seen, the electronic and chemical state of vapodeposited alkalis is different from that of alkalis introduced as nitrates in supported catalysts : their electronic charge is smaller and they are not associated with a counteranion. The problem of the location of alkali promoters in supported catalysts was adressed in a previous work (12). Partial conclusions were obtained from the study of Ni/Si02 where metallic loading was made low to permit their study by an analytical electron microscopy (scanning transmission electron microscope,

STEM, equipped with a field emission gun giving sharp electron beams and an energy-dispersive X-ray spectrometer, EDX) : it was deduced that K is uniformly dispersed on the support and that it concentrates near Ni particles. Due to the large Ni concentration of the catalysts studied in this work, this technique cannot be used. The fraction of the nickel surface covered with the promoter, however, can be roughly estimated by comparing the total number of Ni surface atoms, deduced from a physical method such as magnetism (6.7) or electron microscopy, with the number of Ni surface atoms accessible to gas chemisorption. The adsorption of CO was preferred to that of oxygen where a deep oxidation of Ni may occur ( 7 ) ,

or to that of hydrogen where H atoms are so small that they can be adsorbed between the promoter layer and the Ni surface. The quantity of adsorbed CO first decreases steeply when the amount of promoter increases, then tends to a limit values (Figure lb).

This decrease

cannot be accounted for by the limited sintering of Ni particles due to the presence of the alkali compound. Indeed, the average diameter of Ni particles does not vary dramatically when the quantity of alkali promotor increases. In figure la are reported the variations of the surface average diameter of Ni particles (calculated from their superparamagnetic properties) with the atomic fraction Na or K/NiT (NiT, total number of Ni atoms) ; it shows that the variation does not depend upon the nature of the alkali compound added. The "toxicity" of alkali atoms toward CO chemisorption on Ni can be

easily

calculated from the initial section of the b curve. Its intercept with the

753

"CO

4c

30

(b)

20

10

Fig. 1 : Surface average diameter of Ni particles in promoted Ni/SiOZ catalysts calculated from their magnetic properties (curve a) and volume of adsorbed CO at room temperature and 5 Torr (curve b) as a function of the atomic ratio K or (circles and squares, respectively). Vco in m l NTP per g Ni. Na"itotal

754 x-axis is Alk./NiT

=

6.5

X. Taking into account the nickel dispersity

NiS/NiT = 0.3

(NiS,

Alk/NiS = 21.7

X. In other words, one Alkali atom inhibits ca

number of

surface Ni

atom)

one

can deduce

that

4 . 6 Ni atoms.

This value is too large to be accounted for by the coverage of the nickel surface by Na 0 or K20 which leads to Alk./NiSrir 2 (both oxides are 2 crystallized in the antifluorite cubic structure with a = 5 . 5 5 and 6 . 4 3 6 A, respectively).

In contrast, silicates yield such high Alk./NiS values. This

confirms that alkali compounds are probably present under the form of silicates and that they mask rapidly most of the surface of Ni particles. At Alk./NiT ratios in excess of 0.1 (Figure l ) , the volume of chemisorbed CO does not change very much with the alkali content. This can be interpreted by a selective deposition of the promoter on silica or by a formation of alkali multilayers on nickel.

In figure 2, are reported the variations of the CO coverage on Ni(ll1) (measured from TDS experiments, after 12 L exposure to CO at 300 K) versus the vapodeposited potassium coverage. As

8

increases, the CO coverage first

increases, goes through a maximum and then decreases. When the Ni(ll1) is

I , 0 0.1

1 ML K 1 6 - 7 ~1014 K / c d

.

03

c

0

Fig. 2 : Amount of adsorbed CO at 12 L exposure and at 300 K (arbitrary units) as a function of the potassium coverage on Ni(lll), after pumping off the gas phase.

755

covered with a K multilayer, no more CO is adsorbed. The maximum probably the increase of results from two opposite effects which compete : at low

eK,

the Ni-CO bond strength due to K addition is predominant and results in a

d co

increase. For larger potassium coverage the site blocking by K adatoms becomes important and

@ co

decreases. Thus vapodeposited alkalis and alkali compounds

added to catalysts achieve different behaviors. Spectroscopic data On K or Na predosed Ni(lll),

the vibration spectra of adsorbed CO exhibit

only one structure in the 1300-2000 cm-l range. This loss peak can be attributed to the excitation of the stretching vibration of adsorbed CO more or less modified by alkali adatoms. The dco frequency shifts downwards with increasing the alkali precoverage and shifts upward with increasing CO exposure for both vapodeposited K and Na (figure 3).

Only one

observed at low or high alkali coverage. Moreover the

J co absorption peak is L/ co frequency decreases

continuously when the carbon monoxide coverage decreases for a given alkali coverage. These observations strongly suggest that in these systems an indirect interaction via the metal is prevailing : the electron backbonding from metal d orbitals into the CO 2'N* orbitals can take place to a greater extent in presence of the alkali metal. The decreased work function associated with the alkali adsorption makes easier the filling of the

2T* orbitals of CO

:

either

the 2.K gas-phase level becomes closer to the Fermi level and the overlap between the 2 ' T level and the metal orbitals increases, or the electrons in the d-2% orbital become less localized on nickel, shifting more their charge density towards C and 0. Figure 3 shows also that, for given CO and alkali coverages, the

Jc0frequency

is lower for K. This observation is in good

agreement with the indirect interaction model : the frequency lowering is more important for the more electropositive alkali (K) which do transfer more easily its electron from the 4s electronic level to or via the metal. The infrared spectra of CO irreversibly adsorbed at 300 K on unpromoted and K or Na-promoted Ni/SiO2 catalysts have been reported elsewhere (7). On unpromoted Ni/Si02 two main bands are observed at (2070-2030) cm-' and 1930 cm-' which were assigned to CO bonded to one and two Ni atoms, respectively. Addition of alkali results in a progressive decrease of the intensity of these bands and in the emergence of a new low frequency band. As can be seen in figure 4 , the frequency of this band depends little on the alkali concentration and is lower for sodium, in contrast with the case of vapodeposited alkalis on Ni(ll1).

This observation is

better accounted for assuming a direct interaction between the oxygen atom of the adsorbed CO molecule and the alkali ion, which is expected to be larger for

+

the smaller ionic radius (Na ). This does not exclude the possibility of an indirect interaction, adding its effect to the direct interaction, but points to the fact that the latter is prevailing.

756

1800

1'100

1500

Fig. 3 : Variation of $ frequency with the alkali coverage on Ni(ll1). Values are given for low at% high CO exposures.

K

Fig. 4 : Vibration frequency of the low frequency band of CO adsorbed on promoted Ni/SiOZ catalysts as a function of the atomic ratio K or Na/Nitotal.

757

CONCLUSION It can be concluded that alkali-promoted catalysts and single crystals precovered with vapodeposited alkalis demonstrate a very different behavior : the chemical and electronic state of alkalis are not the same ; in the former case alkalis are ionized and associated with a counteranion (silicate), while in the latter no anion is present and the charge of the alkali is smaller than unity. Spectroscopic data of adsorbed CO indicates that a direct interaction between the oxygen atoms of adsorbed CO molecules and the alkali ions in promoted catalysts takes place ; in contrast, an indirect interaction via the metal would be prevailing for the case of the Ni single crystal precovered with vapodeposited alkalis. These results suggest also that it could be hazardous to model alkali promoting effects in catalysis by vapodepositing alkali metals on well-defined metal surfaces, if a counteranion is not associated with the alkali on the surface.

REFERENCES P.J. Feibelman and D.R. Hamann, Phys. Rev. Letters, 52 (1984) 1 ; Surf. 1 Sci., 149 (1985) 48. 2 N.D. Lang, S. Holloway and J.K. Norskov, Surf. Sci., 150 (1985) 24-38. K.J. Uram, L. Ng, M. Folman and J.T. Yates, J. Chem. Phys., 84 (1986) 3 2891. 4 J.S. Kristoff and D.F. Shriver, Inorg. Chem., 13 (1974) 499. M.Y. Darensbourg, P. Jimenez, J.R. Sackett, J.M. Hanckel and R.L. Kump, J. 5 Am. Chem. SOC., 104 (1982) 1521. G.A. Martin, N. Ceaphalan, Ph. de Montgolfier and B, Imelik, J. Chim. 6 Phys., 10 (1973) 1422. M. Primet, 3.A. Dalmon and G.A. Martin, 3. Catal., 46 (1977) 25. H. Praliaud, M. Primet and G.A. Martin, Applications Surf. Sci., 17 (1983) 7 107-123 - Bull. SOC. Chim. France, 5 (1986) 719. J.C. Bertolini, P. Delichiire and J. Massardier, Surf. Sci. Letter, 160 8 (1985) L 531. J.C. Bertolini, J.L. Duvault, Y. Jugnet, Ph. Ruiz and B. Tardy, J. Chem. 9 Phys., 88 (1988) 394. 10 J.E. Demuth, D.W. Pepsen and P.M. Marcus, J. Phys. Chem., 8 (1975) L25. S. Andersson and J.B. Pendry, Solid State Corn., 16 (1975) 563. 11 12 V. Pitchon, P. Gallezot, C. Nicot and H. Praliaud, submitted to Applied Catalysis.



759

METAL SUPPORT INTERACTION

R.

PRINS~,J.H.A.

MART ENS^

and D.C. KONINGSBERGER~

Technisch Chemisches Laboratorium, Eidgenossiche Technische Hochschule, CH-8092 Zurich Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, NL-5600 MB Eindhoven ABSTRACT A Rh K EXAFS study showed that the rhodium particles in a Rh/Ta205 catalyst were fully reduced and in the lnomall state after reduction in H2 at 523 K. The metal particles contained about 73 rhodium metal atoms and had a diameter of 17 A. After reduction at 858 K the catalyst was in the SMSI state. In addition to a contribution from rhodium nearest neighbours, two contributions from neighbouring tantalum ions could be detected. The tantalum ions were located in the reduced supporting oxide directly underneath the rhodium metal particles and in tantalum oxide covering the rhodium metal particles. Neither alloy formation, nor the formation of pillboxes or raftlike structures was observed. INTRODUCTION In catalysis as well as in the field of composite materials the interaction between a metal and a metal oxide is of prime importance for the mechanical properties. Sintering of supported metal catalysts (1) and adhesion in metal-ceramic composites (2) are therefore extensively studied. In catalysis there is the further interest in the possible influence of the metal-support interaction on the catalytic properties of the metal atoms in the metal-gas interface. It has been pointed out that the interaction between the ions in the metal oxide and the metal atoms in the interface occurs via a special van der Waals interaction of the ion-induced dipole type ( 3 ) . The interaction energy is of the r-4 type and not of the r-6 type as between neutral atoms. The magnitude of this metal-ion interaction therefore can be substantial. Still, the contributions of more distant ions and atoms in both lattices cannot be ignored and therefore lattice matching has been claimed to give stronger bonding ( 4 ) . In catalysis it has been suggested that special metal cations may contribute to the sintering resistance of metal particles.

760

Thus Yermakov suggested that low valent metal cations of Mo, W, Ge and Co have a strong interaction with group VIII noble metal atoms and that these metal cations function as 'anchoring' sites for the noble metal particles (5). Analogously, Gallezot suggested that sintering of metal particles inside zeolites was prevented by transition metal cations which were exchanged into the zeolite (6). He also claimed that, because of the interaction between the metal particle and this transition metal cation, the metal particle became electron deficient. Recent work by Sachtler C.S. has confirmed the stabilizing influence of Fe2+ and Cr3+ cations in Pt/NaY zeolite (7). Huizinga and Prins extended the model of Yermakov by proposing that also cations of the noble metal proper could act as anchoring sites (8). They reported that the presence of Pt+ ions makes the Pt particles in Pt/A1203 more stable against sintering. In agreement with this, Anderson et al. calculated that the bonding between the 0 2 - anions and an oxidized Pt surface in an A13+02-Pt+Pt interface is strong (9). A question which may be posed is, what makes the interaction between a metal particle and a transition metal ion different from that between a metal particle and a non-transition metal ion? For metal cations of the metal proper one may furthermore wonder why such metal cations do not become reduced. In the classic ion-induced dipole interaction there is no principal difference between a transition metal ion and a non transition metal ion: ~ = - q 2 ( 1 / 2 r 4 , in which g is the charge on the ion, (I is the polarizability of the metal atom and r is the ionatom distance. A lower valency of the metal cation even has an adverse effect on the interaction energy. Quantum mechanically, however, a lower valency of the transition metal cation leads to a smaller energy gap between the highest occupied orbitals (HOMO'S) of the metal ion in the support surface and the lowest unoccupied orbitals (LUMO's) of the metal atom. Together with an increased size of the lower valent transition metal orbitals, this will give a larger interaction. Not enough theoretical MO results are available, however, to be able to fully test these qualitative predictions. Indeed, MO calculations have indicated that a bonding between a small metal cluster and a transitionmetal oxide molecule can exist (10). But also the bonding between a Pt cluster and AlxOyn- clusters representing a A1203 fragment were found to be substantial (9). The interaction in the Pt/A1203 interface was calculated to be stronger with A13+ cations than

761

with 02- anions in the interface. One comes to the same conclusion on the basis of the induced-ion dipole formula E=-q2a/2r4 , which indicates that highly charged, small ions will have the largest interaction with a metal-atom. Cations like A13+, and 02- anions rather than OH' anions will therefore form relatively strong bonds. This suggests that the metal-support bonding might be increased by the creation of vacancies at the support surface and filling them with metal atoms. In that way metal cations in the support surface come in direct contact with metal atoms. Since the discovery of the phenomenon of Strong Metal-Support Interaction (SMSI) a lot of research has been devoted to its explanation (11-13). SMSI can be defined as follows. After reduction at low temperatures (below 400 ' C say) the adsorption capacities of metal particles supported on certain transition metal oxide supports is normal, whereas after a high temperature reduction (above 450 'C) the adsorption capacity for H2 and CO has diminished drastically, although the metal particle size remains unaffected. This change in adsorption capacity is reversible, after oxidation and re-reduction at low temperature the adsorption capacity is normal. This Strong Metal-Support Interaction has similarities with the metal-support interaction discussed in the foregoing. The transition metal oxides which are used as a support are all reducable at higher temperature and this led to the suggestion that there might be a special (strong) interaction between the reduced transition metal ions and the metal atoms in the metal-support interface, hence the name SUSI. In an Xa-SW-MO calculation Horsley showed that there indeed is bonding between a Pt atom and a Ti05" cluster (14). Whatever the explanation of the metal-support interaction, it is clear that it is important to know the structure of the metalsupport interface on an atomic scale. For that reason we started some years ago to use the Extended X-ray Absorption Fine Structure (EXAFS) technique in our study of the metal-support interface (3). But although the EXAFS-technique can very well measure the number and art of atoms or ions around a certain atom, it has the disadvantage of being a bulk technique. Therefore, in order to obtain meaningful results about the local structure of the metal atoms at the metal-support interface, we had t o study very well dispersed metal particles and even then had to take care in obtaining good signal-to-noise data and to

762

use an analysis programme suited for reliably separating small signals of the interface metal atom from large signals of the other metal atoms (15). In this way we succeeded in observing for the first time the metal-oxygen anion contacts at the metalsupport interface (3). Subsequent EXAFS studies on R h , Pt and Ir on A1203, Ti02 and MgO catalysts demonstrated that in all cases the metal particles had a three-dimensional rather than a twodimensional shape (16-23). A raftlike structure was observed only in one very special case, in which a Ir/A1203 catalyst was prepared from Ir4 (CO)12/A1203 (23). This catalyst proved to contain a carbon overlayer, which apparently determined the raftlike shape. Although no M-A13+ distances have been observed thus far in M/A1203 catalysts, we have been able to measure Rh-Tin+ distances in Rh/TiOZ (18, 19). With this and additional Rh-Rh and Rh-02' information it proved possible to obtain a detailed picture of the metal-support interface of the %/Ti02 catalyst in the normal, as well as in the SMSI state. In the SMSI state the metal particles were found to rest on reduced titania and a model was proposed as shown in Fig. 1. No indications for alloy formation were found (which is one of the proposed explanations for SMSI). Covering of the Rh particles by TiOx fragments (which is another explanation) could not be fully excluded, however, and it was argued that if covering was occurring, it had to be very loose with irregular Rh-TiOx contacts. In full agreement with this suggestion Logan et al. recently observed in high resolution transmission electron microscopy pictures that the whole surface of a Rh/Ti02 catalyst, the metal as well as the support, was covered with an amorphous layer of (probably reduced) Ti02 after high temperature reduction (24). A few problems remained from our EXAFS study on Rh/Ti02, however. The first was that it could be argued that our reduction treatment of the Rh/Ti02 catalyst was not severe enough and that therefore the covering was not tight and not observable by EXAFS. Secondly, although we did observe %-Tinf distances of 3.4 and 4.3 A, we did not observe the Tin+ ions at 2 A directly underneath the Rh atoms, as indicated in Fig. 1. On the other hand, the number of such Tin+ neighbours is relatively small and combined with the low backscattering amplitude ifor Ti, the contribution of the 2 A Rh-Tin+ distances might be too low for a reliable analysis. Thirdly, although we did not observe Rh-Ti

763

distances as in Rh-Ti alloys, others have claimed to have observed alloy distances in Rh/Ti02 (25) and in Ni/Nb205 and Ni/Ti02 (26).

Figure 1 Model of the metal-support interface in a Rh/Ti02 catalyst in the SMSI state

O R h @02-

.Tint

For these reasons we decided to extend our EXAFS studies of the metal-support interface of SMSI systems to the Rh/Ta205 catalyst. Ta2O5 is known to show SMSI behaviour (12) and, more importantly, Ta has a backscattering amplitude at high k-values which is 2 to 4 times larger than that of Ti (27). If Tan+ ions are present underneath Rh interface atoms in the SMSI state or, alternatively, if Rh-Ta alloy distances are present, one must expect that such distances can be detected by EXAFS. EXPERIMENTAL A high surface area Ta2O5 support was prepared by adding a solution of 20 g TaC15 in 100 ml concentrated HC1 to a mixture of 4 1 distilled water and ice, which was acidified with HC1 to pH=O. Approximately 300 ml of a NH40H solution was subsequently added dropwise to the TaC15 solution during a period of 100 min while vigorously stirring the solution. At the end of the ammonia addition the pH had increased to 6 . 0 . After stirring for another 100 min, the precipitated Ta(OH)5 was filtered off and washed several times with distilled water, thereafter it was carefully dried at 393 K for 24 h (heating rate 2 K min'l), cooled down to

164

room temperature, powdered, dried again as described above, and finally calcined for 1 hr at 873 K (heating rate 2 K min’l). The resulting 7 g Ta2O5 had a surface area of approximately 100 m2g-I A 3 wt% Rh/Ta205 catalyst was prepared using the urea method (28). Three gram of the Ta2O5 was added to 350 ml distilled water at 365 K and the stirred solution was acidified with 8 N HC1 to pH=2.5. Then 0.53 g of urea (a tenfold excess based on the amount of RhC13) was added and finally 0.23 g of RhCl3.3H20 was added. Because of the slow decomposition of urea at 365 K, the pH of the solution increased very slowly. At a pH value of approximately 4 , Rh(OH)3 started to precipitate. After 10 h the catalyst precursor was filtered off and dried as described above for the Ta2O5 support. To remove the remaining urea the sample was calcined at 923 K, pre-reduced in hydrogen at 773 K and finally oxidized at 573 K. This sample was stored for further use. Temperature programmed reduction experiments indicated that reduction was complete at 470 K when using 4 % H2 in N2. Hydrogen chemisorption measurements after reduction at 523, 773 and 873 K gave H/Rh values of 0.93, 0.14 and 0.06, respectively. The stored Rh/Ta2O5 catalyst was pressed into a thin self supporting wafer, whose thickness was such that px=2.5 at the rhodium K-edge, assuring an optimum signal-to-noise ratio in the rhodium EXAFS spectra. The wafer was mounted in an EXAFS cell which enabled in situ pretreatments in different gas atmospheres at temperatures ranging from 100 to 873 K. The sample, once mounted in the cell, was reduced in 100 % H2 at 523 K for 1 h (heating rate 5 K min’l). After cooling with liquid nitrogen to 100 K an EXAFS spectrum was recorded with the sample still under H2. Thereafter, the sample was reduced in H2 at 858 K for 15 min (heating rate 5 K min-l) , cooled down to 100 K and a second EXAFS spectrum was recorded. The EXAFS spectra of the reference compounds, rhodium foil, Rh2O3, RhCl3, Rh3Ta alloy, Ta powder and TaC15, were recorded at 100 K as well. The absorption spectra were recorded at the synchrotron radiation source (SRS) in Daresbury, U.K. The ring was operated at 2.0 GeV and with ring currents between 100 and 300 mA. RESULTS The backscattering amplitudes F(k) and the phase shift functions $(k) which are necessary for analyzing the EXAFS data have been obtained from the reference compounds Rh2O3 (for the

765 Rh-0

contributions), RhCl3 (for the Rh-C1 contributions) Ta powder (for the Ta-Ta contributions) and TaC15 (for Ta-C1 contributions). In Table 1 all the relevant information concerning the references is given. The crystallographic data were taken from (29).

TABLE 1 Crystallographic data and Fourier transform ranges for the reference compounds Compound

Edge

NNa

Rb

NC n

Rh foil *2O3 RhCl3 Ta TaC15

Rh R h Rh Ta Ta

Rh o C1 Ta C1

2.687 2.05 2.31 2.863 2.37

12 6

3 3

6

1 3

8 6

3

ourier transformation k-range r-range 2.90-25.48 2.64-22.17 3.00-20.15 2.95-16.97 2.44-15.87

1.80-2.90 0.68-2.12 0.00-2.33 2.14-3.48 0.18-1.88

a: Nearest Neighbour, b: Coordination Distance ( A ) , c: Coordination Number, d: Weighting factor in the Fourier transformation F(k) and g(k) for the Rh-Ta contributions could not be extracted from the the Rh3Ta alloy, because the Rh-Rh and Rh-Ta peaks overlapped almost completely in the Fourier transform of the EXAFS spectrum of the alloy. Since the backscattering amplitude is only a function of the scattering atom (27) we took EXAFS spectrum of tantalum powder to F T ~ - T ~ ( from ~) the LIII represent FRh-Ta(k). Because of the additivity of the contributions of the absorbing and scattering atoms, a phase shift function can be written as a linear combination of three other phase shift functions (27, 30). For Rh-Ta(k) this leads to: gRh-Ta(k) = PRh-Cl(k) + gTa-Ta(k) PTa-Cl(k). Therefore We measured the Rh K-edge EXAFS spectrum of RhCl3 and the Ta LIII EXAFS spectra of Ta powder and of TaC15. From these EXAFS functions we extracted gRh-Cl(k), gTa-Ta(k), and gTa-Cl(k), and a linear combination of these three functions yielded the desired phase shift function for the Rh-Ta contributions. Our procedure of analyzing the EXAFS spectra has already been described extensively in the literature (15, 16, 19). EXAFS spectra containing one or more shells are calculated by using the backscattering amplitudes F(k) and the phase shift functions g(k) of suitable reference compounds. By varying the coordination number N, the coordination distance R, the Debye Waller factor

-

766

and EOI the correction on the edge position, one tries to fit the calculated EXAFS spectra to the measured spectra as accurately as possible. In two previous studies ( 1 8 , 1 9 ) we described a recurrent optimization process for the separate analysis of the contributions from high-2 and low-2 scattering neighbours. Since in this study the contribution of the low-2 scatterer (oxygen) turned out to be very small and the remaining contributions originated from high-2 scatterers (rhodium and tantalum), this procedure could not be used. Moreover, the Rh-Rh and Rh-Ta contributions overlapped in the Fourier transform (cf. Figure 3f), making it impossible to use the difference file technique ( 1 5 , 1 9 ) . Therefore a single step multiple shell analysis with four parameters (N, R, Ao2 and Eo) for each shell was used. We calculated a Rh-Rh EXAFS function and two Rh-Ta EXAFS functions, added them, Fourier transformed the resulting spectrum and compared the result with the Fourier transform of the measured data. Because of the high4 character of the main contributions (Rh and Ta), the use of k3-weighted Fourier transforms was essential. Differences between the two spectra were minimized by varying the Rh-Rh and Rh-Ta parameters. The results of this analysis procedure are presented in Table 2 . Ao2,

TABLE 2 Final results from EXAFS data analysis _

Treat- NN ment

Coordination number (a)

Distance (A) (a)

(a)

R523

Rh

7.9

0.2

2.658

0.005

7.4

1

R858

Rh Ta Ta

7.9 1.6 0.8

0.2 0.5 0.2

2.650

0.005 0.3 0.1

7.0 5.4 5.4

1 2 2

R: a:

1.7 2.0

_

~

-

EO (ev) (a)

AU2 ( 1 0 - 3 A-2 )

5.5

1

7.8 -10

1 3

6

3

-

Reduction in H2 at 5 2 3 or 8 5 8 K . Estimated overall (experimental + systematic) error.

In Figure 2a, the raw EXAFS data for the sample reduced in pure Ha at 5 2 3 K during 1 h and the calculated best fitting Rh-Rh ‘EXAFS function are shown. The imaginary parts of the Fourier transforms of these EXAFS functions are shown in Figure 2 b and their magnitudes in Figure 2c. Only minor contributions are present next to the Rh-Rh contribution.

767

30

*10-2

0

-30 0 40

-5

0

5

10 oi5

k

[A-']

20

25

0

Figure 2 Rh/Ta2O5 after reduction at 523 K : (a) Raw data (solid) and calculated Rh-Rh EXAFS (dotted line). (b) Imaginary parts of the k3-weighted Fourier transforms of the raw data (solid) and calculated Rh-Rh EXAFS (dotted line). (c) Magnitudes of the k3-weighted Fourier transforms of the raw data (solid line) and calculated Rh-Rh EXAFS (dotted line). In Figure 3a the raw EXAFS spectrum and the calculated best fitting Rh-Rh EXAFS functions for the samples after a subsequent reduction at 858 K for 15 min are shown. In Figure 3b the imaginary parts of the k3-weighted Fourier transforms of the measured data and calculated Rh-Rh EXAFS function are shown. Figure 3c shows the magnitudes of these Fourier transforms. The differences in Figures 3b and 3c at the left hand side of the main Rh-Rh peak are due to neighbouring tantalum ions. We tried to fit these differences with rhodium and oxygen neighbours as well, but the fits resulted in physically irrelevant parameters: coordination distances of about 1 A for Rh-Rh and coordination numbers higher than 10 for oxygen. In addition, the resulting fit was worse than the fit with tantalum neighbours. Figures 3d, e and f show the raw data and the calculated best fitting Rh- (Rh+Ta) EXAFS functions, the imaginary parts of their k3-weighted Fourier transforms and the magnitudes of the Fourier

768

*lo+

-6

0

*lo-2

5

k

10 015

20

25

0

[A-']

5

k

10 015

20

25

[A-f]

30

0

-30 40

20

0

Figure 3 Rh/Ta205 after reduction at 858 K : (a) Raw data (solid) and calculated Rh-Rh EXAFS (dotted line). (b) Imaginary parts of the k3-eighted Fourier transforms of the raw data (solid) and calculated Rh-Rh EXAFS (dotted line). (c) Magnitudes of the k3-weighted Fourier transforms of the raw data (solid line) and calculated Rh-Rh EXAFS (dotted line). (d) Raw data (solid) and calculated Rh-(Rh+Ta) EXAFS (dotted). (e) Imaginary parts of the k3-weighted Fourier transforms of the raw data (solid) and calculated Rh-(Rh+Ta) EXAFS (dotted). (f) Magnitudes of the k3-weighted Fourier transforms of the raw data (solid), calculated Rh-Rh EXAFS function (dotted), the calculated Rh-Ta EXAFS functions (dashed) and the sum of the calculated Rh-Rh and Rh-Ta contributions (dash-dotted).

769

transforms of the raw data and of the three separate contributions. Clearly, the agreement at the left hand side of the main Rh-Rh peak in the Fourier transform is better. The Fourier transforms of the EXAFS spectra were complicated by the k-dependence of F(k) and pl(k). Therefore, the transforms were corrected for F(k) and p(k) from rhodium foil, the reference for the Rh-Rh contribution, which was the major contribution in all spectra. As a result, in the Fourier transforms the Rh-Rh contributions 'peaked' at the correct Rh-Rh distance and the imaginary parts of the Fourier transforms were more or less symmetric (15). But the peaks corresponding to other minor contributions shifted to seemingly longer or shorter distances and were asymmetric. However, since the same correction has been applied t o measured and calculated data, the calculated coordination numbers and distances represented those in the sample as accurately as possible. DISCUSSION Rh/Ta905 after Reduction at 523 K According to the TPR experiments, reduction of the sample should be complete at 523 K. A careful analysis of the EXAFS spectrum confirmed this. Figure 2 shows that the main peak is due to a Rh-Rh contribution. The other peaks in Figure 2b and c are very small; the shoulder at the right hand side of the main peak could not be fitted with a Rh-Rh, a Rh-0 or a Rh-Ta contribution. The peak around 1.2 A in Figure 2c is due to an artefact that will be discussed in the following. From Table 2 it can be concluded that on the average each rhodium atom had approximately 7.9 rhodium neighbours at 2.658 A, which is slightly shorter than the Rh-Rh bulk distance (2.687 A ) . Using the calibration procedure as described in (21), we estimated from the Rh-Rh coordination number that the particles were approximately 17 A in diameter and contained about 73 f 5 rhodium atoms. The H/Rh value should be about 0.95 according to an experimental calibration method as well as a computer model calculation (21). This agrees excellently with the measured value, H/Rh = 0.93, and demonstrates that the metal particles were in the 'normal' state. In the EXAFS spectra of fully reduced supported metal catalysts, a metal-oxygen contribution from the metal atoms in the metal-support interface having oxygen neighbours has

frequently been reported (15, 17, 19, 22). In the Fourier transform, such a contribution i s situated at the left hand side of the main Rh-Rh peak. Since the relative extent of the metal-support interface decreases with increasing particle size, the contribution from neighbouring oxygen ions decreases with increasing particle size as well (15, 17). For a 73 atom metal particle about 30 % of the rhodium atoms are situated in the metal-support interface. When these rhodium atoms each have 2 to 3 oxygen neighbours, the average Rh-02' coordination number is about 0.6 to 0.9 and, because of the low coordination number and the low backscattering amplitude of oxygen, the corresponding contribution in the Fourier transform should be smaller by more than an order of magnitude compared to the Rh-Rh contribution. Therefore, no contribution from neighbouring oxygen ions could be detected in the EXAFS spectrum of the sample reduced at 523 K. Rh/Ta905 after Reduction at 858 K After reduction at 773 K, the H / R h value determined with hydrogen chemisorption decreased to 0.14, and after reduction at 873 K to 0.06. Clearly, after reduction at 858 K the metal particles were in the SMSI state. Since the Rh-Rh coordination number remained unchanged (cf. Table 2) the basic structure of the metal particles was still the same. In a study of Pt/TiO2 it was shown that in the SMSI state the Pt particles are spread over the support and that 'pillboxes' are formed (31). If such a spread of the metal particles had also occurred in Rh/Ta205, it should have been accompanied by a significant decrease in the Rh-Rh coordination number. We did not observe such a decrease and therefore we conclude that in the case of Rh/Ta205 spreading of the rhodium particles did not occur. This is in agreement with literature data, which indicate that rhodium does not 'wet' TiOx surfaces (24), like Pt does (31). Another explanation was the formation of alloys (11). We did not observe contributions from neighbouring tantalum atoms at distances in the range of 2.7-2.8 A (in the Rh3Ta bulk alloy, the Rh-Ta distance is 2.729 A (29)). Therefore, alloy formation can be ruled out in our Rh/TalO5 catalyst. In three other EXAFS studies the observation of an alloy in the SMSI state has been claimed (25, 26, 32). However, we think that the presence of the carbon support might have greatly influenced PtTi alloy formation in the system Pt-Ti02/C (32) , while the assignment of a peak in

771

the EXAFS spectrum of Rh/Ti02 to a Rh-Ti alloy contribution (25) is highly questionable because of the applied analysis procedure (19). The claim of Ni-Nb and Ni-Ti alloy contributions in the EXAFS spectra of Ni/Nb205 and Ni/TiO2 (26), respectively, seems even more questionable because of the extremely narrow window used in the reverse Fourier transformation and because of the fact that others have found that a large fraction of the Ni ions in such systems is not reduced at all, but forms a Ni-Ti-0 compound (33). In our opinion therefore, no unequivocal EXAFS proof for the existence of an alloyed phase in the SMSI state of a M/Ti02 (or related) system has been presented yet. Table 2 shows that the Rh-Rh coordination distance decreased by 0.008 A when the reduction temperature was increased. This is only a small change for a metal particle which contracts, because it is no longer covered with adsorbed hydrogen in the SMSI state. A decrease of 0.05 A was reported for very small rhodium metal particles in Rh/Ti02 (19). The difference between the Rh/TazO5 and Rh/Ti02 catalysts must be due to the fact that the metal contraction is largest in the surface layer (34). Small metal particles will therefore show a larger decrease in the average Rh-Rh distance than bigger metal particles. In the EXAFS spectrum of the sample after reduction at high temperature, two more contributions were present which both originated from tantalum neighbours at notably short distances. In the Fourier transforms, the peaks from both Rh-Ta contributions overlapped to a large extent with the much larger Rh-Rh contribution. As a result, the uncertainty in the Rh-Ta coordination number is larger than the uncertainty in the Rh-Rh coordination number. The estimated uncertainty in the 2.0 A Rh-Ta coordination number is about 0.2 and in the distance about 0.1 A. The uncertainty in the 1.7 A Rh-Ta contribution is somewhat larger than that in the 2.0 A Rh-Ta contribution, because of an irregularity around k = 9 A-l in the EXAFS spectra of the Rh/Ta205 sample reduced at 858 K (see Figure 3a). This irregularity gave rise to a peak in the Fourier transform at r = 1.6 A. In the Fourier transforms of the sample reduced at 523 K (see Figure 2b and c), the contribution from this artefact was small, clearly separated from the Rh-Rh peak and caused only small deviations in the Fourier transform. The 1.7 A Rh-Ta contribution had a main peak at 2.3-2.4 A in the Fourier transform and two sidelobes at 1.8 and 1.4 A (see Figure 3f). The

112

main peak is shifted from the real Rh-Ta distance because the Fourier transform was corrected for Rh-Rh phase shift and backscattering amplitude. The two sidelobes interfered with the artefact at 1 . 6 A and thus, only the main peak of this contribution could be used to determine the accompanying parameters. This caused an additional uncertainty in the parametexs for the 1 . 7 A Rh-Ta contribution: about 0 . 2 in the Fh-Ta coordination number and approximately 0 . 1 A in the Rh-Ta coordination distance (additional to the uncertainty in the 2 . 0 A Rh-Ta contribution). In the Rh-Ta distances, another uncertainty has to be considered. The phase shift function for this absorber-scatterer pair has been composed from three phase shift functions. Hence, the resulting uncertainty in the final phase shift function was the sum of the uncertainties in the three individual phase shift functions. We estimated that based on this, the uncertainty in the resulting calculated Rh-Ta distances was about 0 . 1 A. In Table 2 , the final uncertainties of all parameters are summarized. Because the irregularity around k = 9 created problems, we tried to eliminate it from the spectrum by subtracting the best fitting calculated spectrum from the measured data. The difference spectrum consisted mainly of this artefact. Using a Hanning window between 8 and 9.5 A-1, we isolated the artefact from the difference file and subtracted it from the measured data. The Fourier transform of the resulting spectrum was indeed better than the spectrum of the raw data, the artificial peak around r = 1 . 6 A was almost completely removed. However, this procedure introduced new, smaller artefacts around 8 and 9.5 A-1 and it was impossible to completely remove the artefact without introducing new artefacts. Nevertheless, our conclusion is that the peak in the Fourier transform around r = 1 . 6 A is indeed induced by the artefact around k = 8-9 A‘l. Although the overall uncertainty in the Rh-Ta parameters is quite large and made a detailed interpretation not very meaningful, there is no doubt that tantalum ions are present at distances between 1.4 and 2.1 A. These are very short coordination distances and can arise only from Tan+ ions in direct contact with rhodium atoms in the metal-tantalum (sub)oxide interface. These ions may be located directly underneath the rhodium metal particles. This indicates that indeed the Ta205 support under the metal particles had been

773

reduced. However, the coordination numbers of the Rh-Ta contributions are rather high. When only the rhodium atoms in the metal-support interface have Tan+ neighbours , and the support does not expose a large amount of bare Tan+ ions, these coordination numbers cannot exceed 0.3. For a 73 atom half spherical metal particle about 30 % of the metal atoms is in the metal-support interface and we assumed that each interfacial rhodium atom could have up to one tantalum neighbour at such a short distance. Therefore we conclude that also the surface rhodium atoms, which are not in the metal-support interface, must be in direct contact with Tan+ ions and that the metal particles were substantially covered with reduced Ta2O5. The direct contact between rhodium atoms and tantalum ions after reduction at high temperature could result in a strong interaction between metal particle and support. In (18, 19) we reported Rh-Tin+ distances of 3.4 and 4.3 A. Such longer distances have not been observed for the Rh/Ta2O5 catalyst in the SMSI state. The reason is the following. Ti02 has a very simple and very regular structure. The 3.4 and 4.3 A distances are therefore well separated from other distances. For supports with a more complicated crystal structure like A1203 and Ta2O5 this is unfortunately not the case. At short distances, the Rh-Tan+ (or IU1-A13+) distances are well separated, but many Rh-Tan+ distances can occur between 3 and 5 A, each with a rather low coordination number. This makes makes it almost impossible to observe the longer Rh-Tan+ distances and explains also why Rh-A13+ distances have never been observed. CONCLUSIONS Apart from the Rh-Rh contribution, no other contributions could be detected in the EXAFS spectra of the Rh/Ta205 catalyst reduced at 523 K. From the Rh-Rh coordination number it was concluded that the particles were about 17 A in diameter and contained about 70 to 80 rhodium atoms. After reduction at 858 K the sample was in the SMSI state. The Rh-Rh coordination number remained unchanged. Thus, the basic structure of the metal particles remained intact. Any spread of the metal particles over the support like a pillbox formation could be excluded. Alloy formation had not taken place either. In the SMSI state the rhodium atoms in the metal-support interface had tantalum ions as neighbours at very short distances, ranging from 1.4 to 2.1 A.

774 This indicated that the Ta2O5 oxide directly underneath the metal particles was reduced to a lower oxide of Ta2O5. In addition, the metal particles were substantially covered with reduced Ta20.5. In the Rh/Ti02 samples we did not observe any covering, but we could not exclude partial covering either (19, 20). The fact that the rhodium metal particles in Rh/Ta205 are covered to a larger extent than the metal particles in %/Ti02 can be explained in several ways. First of all, the metal particles in Rh/Ta2O5 are much larger than the metal particles in the Rh/Ti02 samples (19, 20) and coverage has up to now only been reported in literature for larger metal particles. Another reason might be the fact that the Rh/Ta2O5 sample was reduced at much higher temperature (858 K) than the Rh/Ti02 Samples (673 and 723 K). A third reason may be found in the preparation method. The Rh/Ti02 samples were prepared by exchanging with a solution of Rh(N03)3 which had a relatively high pH. The Rh/TaaO5 sample was prepared using the urea method and therefore, the starting pH was low. Thus, during the preparation of the Rh/Ta205 sample, some Ta2O5 might have dissolved and later precipitated on top of the rhodium metal particles. This kind of coverage has already been reported for Rh/V2O3 by van der Lee et al. (35) and for Rh-V203/Si02 by Kip et al. (36). After reduction at high temperature, this Ta2O5 on top of the metal particles will become reduced and may have an intimate contact with the metal particle, giving rise to Rh-Tanf bonding. ACKNOWLEDGEMENT This study was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid form from the Netherlands Organization for the Advancement of Pure Research (ZWO). REFERENCES 1 P. Wynblatt and N.A. Gjostein, Progr. Solid State Chem., 9 (1975) 21. 2 J.T. Klomp, Proc. Brit. Ceram. SOC., (1984) 249. 3 J.B.A.D. van Zon, D.C. Koningsberger, H.F.J. van It Blik, R. Prins and D.E. Sayers, J. Chem. Phys., 80 (1984) 3914. 4 C.A.M. Mulder and J.T. Klomp, J. Phys. (Paris), 46 (1985) C4111. 5 Y.I. Yermakov, Catal. Rev.-Sci. Eng., 13 (1976) 77; Y.I. Yermakov and B.N. Kuznetsov, J. Mol. Catal., 9 (1980) 13. 6 P. Gallezot, Catal. Rev.-Sci. Eng., 20 (1979) 121. 7 M.S. TZOU, H.J. Jiang and W.M.H. Sachtler, Appl. Catal., 20 (1986) 231; ibid, 39 (1988) 255. 8 T. Huizinga and R. Prins, J. Phys. Chem., 87 (1983) 173.

715

9 A.B. Anderson, C. Ravimohan and S.P. Mehandru, Surf. Sci., 183 (1987) 438. 10 A.B. Anderson and M.K. Awad, Surf. Sci., 183 (1987) 289. 11 S.J. Tauster, S.C. Fung and R.L. Garten, J. Am. Chem. SOC., 100 (1978) 170. 12 S.J. Tauster and S.C. Fung, J. Catal., 55 (1978) 29. 13 S.J. Tauster, S.C. Fung, R.T.K. Baker and J.A. Horsley, Science, 211 (1981) 1121. 14 J.A. Horsley, J. Am. Chem. SOC., 101 (1979) 2870. 15 J.B.A.D. van Zon, D.C. Koningsberger, H.F.J. van It Blik, D.E. Sayers, J. Chem. Phys., 82 (1985) 5742. 16 H.F.J. van It Blik, J.B.A.D. van Zon, T. Huizinga, J.C. Vis,

D.C. Koninqsberqer and R. Prins, J. Am. Chem. SOC., 107 (1985)-3139; 17 D.C. Koningsberger, J.B.A.D. van Zon, H.F.J. van It Blik, G.J. 18 19 20 21 22

Visser. R. Prins, A.N. Mansour, D.E. Sayers, D.R. Short and J.R. Katzer, J . &ys. Chem., 89 (1985) 4075. D.C. Koningsberger, J.H.A. Martens, R. Prins, D.R. Short and D.E. Sayers, J. Phys. Chem., 90 (1986) 3047. J.H.A. Martens, R. Prins, H. Zandbergen and D.C. Koningsberger, J. Phys. Chem., 92 (1988) 1903. J.H.A. Martens, R. Prins and D.C. Koningsberger, J. Phys. Chem. , in press. B.J. Kip, F.B.M. Duivenvoorden, D.C. Koningsberger and R. Prins, J. Catal., 105 (1987) 26. D.C. Koningsberger and D.E. Sayers, Solid State Ionics,

16 (1985) 23. 23 F.B.M. van Zon and D.C. Koningsberger, to be published. 24 A.D. Logan, E.J. Braunschweig, A.K. Datye and D.J. Smith, Langmuir, 4 (1988) 827. 25 S. Sakellson, M. McMillan and G.L. Haller, J. Phys. Chem., 90 (1986) 1733. 26 G. Sankar, S. Vasudevan and C.N.R. Rao, J. Phys. Chem., 92 (1988) 1878. 27 B.K. Teo and P.A. Lee, J. Am. Chem. SOC., 101 (1979) 2815. 28 J.W. Geus, in G. Poncelet, P. Grange and P.A.' Jacobs (Eds.), Preparation of Catalysts 111, Elsevier, Amsterdam, 1983, p.1 29 Rh metal: Wyckhoff Crystal Structures, 1 (1963) 10; Rh2O3: Struct. Rep., 40a (1976) 301; RhC13: Struct. Rep., 29 (1972) 275; Rh3Ta: Struct. Rep., 29 (1972) 130; Ta metal: Wyckhoff Crystal Structures, 1 (1963) 16; TaC15: Struct. Rep., 22 (1968) 237. 30 J.H. Sinfelt, G.H. Via, F.W. Lytle and R.B. Greegor, J. Chem. Phys., 72 (1980) 4832. 31 R.T.K. Baker, E.B. Prestridge and R.L. Garten, J. Catal., 56 (1979) 390. 32 B.C. Beard and P.N. ROSS, J. Phys. Chem., 90 (1986) 6811. 3 3 H.C. zur Loye and A.M. Stacey, J. Am. Chem. SOC., 107 (1985) 4567. 34 A.D. King and D.P. Woodruff, Eds., The Chemical Physics of

Solid Surfaces and Heterogeneous Catalysis, Elsevier, Amsterdam, 1981. 35 G. van der Lee, B. Schuller, H. Post, T.L.F. Favre, V. Ponec, J. Catal., 98 (1986) 522. 36 B.J. Kip, P.A.T. Smeets, J. van Grondelle, and R. Prins, Appl. Catal., 33 (1987) 181.

776

D.

Dr.

Reinalda (Royal/Dutch-Shell

Laboratory,

bmsterdam,

The

electrons

may

Netherlands) asked: Reduced

T i 0 2 is a semiconductor and by hopping

move

through the bulk. Yet, the reduced Ti ions do tend to

over

the

Rh

particles. What may be the driving

force

of

creep this

phenomenon?

Answer to the questlon of Dr. D. Relnalda

The formation of reduced Ti02 on top of noble metal particles may take place through creeping of TiOx from the Ti02 surface onto the metal surface, or through reduction of Ti02 fragments which were deposited on top of the metal particles during the wet impregnation step. The latter mechanism is well established (Ponec c.s., see for instance J. Catal., 98,522, 1986), the former much less so. It is not clear what the driving force for this creep mechanism is, but it might have to do with metal bonding between the noble metal and Magnelli phase Tinoen-1 material, which is a metallic conductor. A third possibility for the formation of reduced transition metal oxide on top of the noble metal particles is to first form a mixed metal oxide through calcination, and then to reduce this mixed metal oxide. In this way, Kip et al. formed V2O3 on top of Rh when RhV04 on SO2 (formed via calcination of Rh(N03)3 and NHqV04 on Si02) 157, 1987). was reduced in flowing H2 (Appl. Catal.,

a,

C.Morterra,A.Zecchinaand G . Costa (Editors),Structure and Reactiuity of Surfaces 0 1989Elsevier SciencePublishersB.V., Amsterdam -Printed in The Netherlands

771

SURFACE ACIDITY OF SOLID ACIDS AND SUPERACIDS: A FT-IR STUDY OF THE BEHAVIOUR OF TITANIA DOPD WITH PHOSPHORIC, SULPHURIC, TUNGSTIC AND lrylLyBDIC ACIDS

G. RAMIS, G. BUSCA and V. LORENZELLI

Istituto di Chimica, Facolts di Ingegneria, Universit& P.le Kennedy, 16129 Genova (Italy).

ABSTRACT

The surface structure and the acid behaviour of titanias doped with sulphuric (TS), tungstic (TW), molybdic (TMD) and phosphoric (TP) acids has been studied using FT-IR spectroscopy. The acidity scale detected using the olefin oligmrization activity in the IR cell as the discriminant is TS >> TW > ?Mo >> TP >> Ti02 and seems to agree with catalytic activity data. It is deduced that the enhanced acidity of doped samples is of the Bronsted type, the Lewis sites of titania being only weakly perturbed. Moreover, &ping strongly poisons basic sites. Surface mono-0x0 anionic species are very evident on TS, TW and TMD, while on TP tridentate hydrqenphosphate ions seem to be predaninant. INTROD~ION Metal oxides containing anionic species such as sulphates, tungstates and phosphates, generally prepared by inpregnation techniques, have recently received an increasing interest in the field of heterogeneous catalysis, hecause of their enhanced activity in typically acid-catalyzed reactions (1-3)‘ and are considered as solid “superacids”. This effect, particularly evident when titania- and zirconia- based powders are concerned, has been either attributed to the enhancement of the surface Lewis acid strength by inductive effect of the surface oxoanions (1,2) or to the generation of very strong Bronsted acid sites ( 4 ) . Titanias containing sulphates (1,2) , phosphates (5) and molybdates (6) are reported to behave as very strongly acidic materials. Oxide catalysts supported on titania are also deeply investigated in relation to their activity in selective oxidations and selective catalytic reduction of nitrogen oxides (7,8). The present paper reports the results of an investigation of titanias doped with different anions using the FT-IR spectroscopic technique. The aim was to obtain data on the oxide-oxide interaction as well as on the acid sites of these materials and on the generation of the so-called “superacid” behaviour

.

118

Expmmm Sane data on the c a t a l y s t s used i n this work are reported i n T a b l e 1. The

doped t i t a n i a samples were prepared by hpregnation w i t h water solutions of

phosphoric and s u l p h u r i c a c i d s and of dodecatungstate

( a l l from Carlo Erba,

ammonium heptamolybdate Milano,

and

I t a l y ) , followed by

The o l e f i n gases were f r a n SIO (Milano, I t a l y ) .

calcination a t 723 K.

The I R spectra were recorded by Nicolet MX1 and 5ZDX spectrmters, using

pressed d i s k s of the pure pcwders preactivated by calcination i n air a t 723 K and evacuation f o r 2 h a t the s a w t q r a t u r e . Olefin o l i g m r i z a t i o n was

investigated a t r.t. by contact of the activated samples w i t h o l e f i n gases (200 Torr). TABLE 1 Characteristics of the materials used.

2.8

3.7

10l8

41

11.11

5.2

10l8

55

Moo3/Ti02

4.3

3.4

1ol8

53

T?!

P205/Ti02

3.1

3.9

10l8

50

SA

Si02/A1203 #

---

---

TS

S03/Ti02

!rw

m3/TiO2

?Mo

300

* two d i f f e r e n t samples, both f r m Degussa; # 87 % Si02, 13% A1203, f r a n Strem. RESULTS AND DISCUSSION

a ) Characterization of t h e surface 0x0-anionic species. In the Fig. 1 t h e spectrum of the surface sulphate species (1500-1000 c m-1 r e g i o n ) on t h e a c t i v a t e d TS sample is shown. I t is o b t a i n e d by subtracting t h e spectrum of a pure activated t i t a n i a disk from t h a t of activated TS. I t is very similar t o that reported f o r a similar sample by 18 16 O/ 0

Saur e t al. ( 4 ) . According t o these authors, t h a t p e r f o m d careful

isotopic exchange experiments, a strong band a t 1370 an-’

is due t o the

s t r e t c h i n g mode of a s i n g l e S=O double bond of a t r i d e n t a t e s u l p h a t e species. I n our case we detect this band c l e a r l y s p l i t i n t o two canponents a t 1385 and 1375 cm-’,

probably i n d i c a t i n g some h e t e r o g e n e i t y of t h i s

s p e c i e s . The bands i n t h e r e g i o n 1200-1000 c m - l

are t y p i c a l of S-0

stretchings i n ionic sulphates while the wak band a t 1325 cm-’

might be due

t o other sulphate species such as, f o r example, di-oxo species. So, although

779

r.4

1400

1200

wavenunbars

1000

cm"

-

Fig. 1 FT-IR spectra of the sulphate species on TS activated a t 723 K ( a ) and a f t e r successive adsorption of water (b and c ) .

a mno-0x0 species is the spectroscopically mst evident one, we cannot exclude the presence of other species on the activated surface. The mst evident spectroscopic features of the surface tungsten oxide

species are the strong band a t 1018 an-' and its f i r s t overtone a t 2020 cm-l (Fig. 2 ) . Accordingly, on a dried w03-Ti02 sample a sharp band a t 1011 an-' was observed by Laser Raman spectroscopy by Chan e t 61. (9). Fran the apparent sinplicity of both I R and Raman band, from the s i q l i c i t y of the I R f i r s t overtone (10) and from the almost coincidence of the I R and Raman bands we can assign this absorption t o the w..O stretching of a single mno0x0 tungstate species. This conclusion is similar t o that already obtained for ?Mo as reported elsewhere (ll),where the .oMo=o frequency of mno-0x0 Hawever, also i n these cases we are speaking about the mst evident species fran the point of view of I R spectroscopy. Weak features of other species are also observed.

mlybdenyl species is observed a t 1000 an-'.

780

2100 2000 Fig. 2

- ET-IR

OUT

1000

wavenumbers

900

C S - ~

spectrum of activated TW.

Phosphate-containing frm

1400

titania has been the object of previous studies

laboratory (12,13). The spectrum of the TP is canpared t o t h a t of

p u r e T i 0 2 i n F i g . 3. The s u r f a c e p h o s p h a t e s p e c i e s have very different spectroscopic f e a t u r e s than those shown above f o r s u r f a c e sulphate, mlybdate and tungstate species. In t h i s case, i n f a c t , the mst -1 i n t e n s e absorptions, centered near 1200 c m , c l e a r l y a r i s e s from t h e stretchings of PO bonds whose bond order is intenuediate between 1 and 2,

similar t o those typical of metal ortophosphates or hydrqenphosphates. A assigned t o P=O stretchings of surface phosphoryl species is only detected in ratioed spectra (12,131, after adsorption of water, due t o the small m u n t of these species. mak band a t 1340 a n-',

3800

3000

2200

wavrnuderr Fig. 3

- FP-IR spectra of

is00

cm

n i

activated T i 0 2 (a) and TP (b).

781 b) Characterization of t h e surface hydroxy-groups and of adsorbed E.

The spectra of TS and 'IW i n the 4CX-I region, a t d i f f e r e n t dehydration stages after adsorption of water are reported in Fig. 4,A and B. They m y be 14 and also

ccmpared with those of the pure support, discussed i n r e f .

Shawn i n Figs. 3 and 4, and of !Bb discussed i n r e f . 11. It is clear that t h e presence of surface oxoanions causes the ccmplete disappearance or the substantial decrease of the mst evident (sharp and intense) bands due t o f r e e surface OH'S of t i t a n i a , observed near 3720 and 3670 an-'.

On TS a band

i n t h e r e g i o n of f r e e s u r f a c e hydroxy groups 1s observed, although r e l a t i v e l y broad, having poorly resolved canponents near 3655, 3630 and 3610 -1 an Similarly a l s o on 'IW a weak and broad band is observed a t near 3660

.

an-'.

A similar r e s u l t w a s reported f o r !Bb (broadat 3650 an-' (11)). However, it is wrth noting t h a t i n the cases of TS, TW and 'IMO a very

broad "continuous" absorption without any defined maxinun is also detected i n t h e r e g i o n below 3600 c m - l and resists evacuation a t 670 K. T h i s absorption, evidenced by the non-coincidence of the transmittance curve w i t h t h e " s c a t t e r i n g l i n e " i d e a l l y o b t a i n e d as t h e p r o l o n g a t i o n of t r a n s m i t t a n c e l i n e i n t h e r e g i o n 4000-3700 cm-'

(see Fig. 4 , B ) .

the This

species. absorption must be assigned t o strangly H-bonded O...H...O Only i n the case of TP t h e supported 0x0-anions are responsibk for a

very intense and sharp 4 0 H band due t o f r e e POH groups a t 3665 an-' (Fig. 3,b). H m v e r , a l s o i n t h i s case t h e above cited continuous absorption belaw 3600 an-' w 25 a

u

;20 C

is observable.

--. ;i *-

*----a

I .

10

8

4

0 fY c

15

6

10

4

5

2 A

0

4000

3600

3200

2800

0 4000

3600

3200 W8V8nUDb8r8

2800 d C0

Fig. 4 - FT-IR spectra of TS (A) and 'IW (B) a f t e r adsorption of water and evacuation a t increasing temperatures up t o 723 K. Broken line: activated TiOZ. Dashed line: "scattering line".

The adsorption of water at high coverages is in all cases of the mlecular coordinative form, as mainly detected by the appearance of the scissoring mode, very intense and slightly split at 1625 and 1615 an-’. The presence of adsorbed water perturbs the bands of the surface oxoanions (Fig. 11, as already sham by Morterra et al. (15) for TS and by ourselves for ?Mo (11). In the case of TP, however, only the weak bands due to surface W phosphoryl groups appear clearly perturbed, while the most intense absorption of P-O bonds is not. c) Characterization of the surface acidity. The adsorption of bases of different strength such as n-butylamine, m n i a , pyridine, tetrahydrofuran, acetonitrile and carbon mnoxide has been used in order to characterize the surface acidity of these materials. All bases when adsorbed on such titania-based catalysts form coordinated species. However, the position of the more sensitive bands (for example the 48a band of pyridine, the .?cN doublet of acetonitrile and the 4 C O band of carbon mnoxide) does not reveal a significant change in the strength of Lewis sites. On the contrary, CO adsorption evidences that the strongest sites for CO adsorption, producing on pure titania a coordinated species characterized by .3co at 2210 cm-’ (14),are almost cqletely absent on all doped titanias. The weaker sites appear only slightly strengthened, if any, by the presence of surface 0x0-anions. Besicks chemisorbed species, the adsorption of the strongest bases nbutylamine, m n i a and pyridine causesthe formation of stable protonated species on all doped titanias, unlike the pure titania sample. This evidences that Bronsted acidity is generated by surface 0x0-anions. In previous works we have used the so-called hydrogen-bonding method (evaluation of the shift of the &H bands upon interaction with weak bases) to evaluate the acid strength of Bronsted sites. Unfortunately, in this case this method seems not useful, due to the presence of H-bonded hydroxyls and of species characterized by broad I)OH bands. To have a scale of the Bronsted acidity of our samples VK have then investigated the adsorption of the sinq?le olefins, looking at the formation of oligmric species. We have in fact previously observed that olefins oligomerize on Bronsted acidic surfaces, although this effect may also involve other sites besides acidic protons (12,131. In the present cases, being the nature of metal centers the sane on all surfaces, it seems reasonable to assign the different activity in oligmrization mainly to the difference in Bronsted acid strength. We have preliminarily observed that ethylene, propylene and n-butenes only form stable molecularly adsorbed species at r.t. on pure titania (16). Isobutene transforms in these conditions on Ti02 giving a dimric species (17).

783

The results of oligomerization of olefins on the titania samples as well as on silica-alumina for comparison, in the IR cell in the standard conditions we have chosen, agree with the tendency of the olefins to polymerize by a cationic mechanism, that increases as follaws: ethylene < propylene < isabutene (Table 11). It is worth noticing that the scale of acidity of the solids deduced by these data TS >> lw > SA > llb >> TP >> Ti02 agrees with several indicationsreported in the literature. For example, Tanabe and co-workers reported for sulphate-containing Ti02 higher

TABLE I1 Results of olefin oligomerization on titania catalysts. Catalyst Ethylene Propylene 1-Butene Isobutene Ti02 TP ?Mo

SA lw

TS

+

-

= 01 = 01

lomerization occurs; (+) = oliqomerization occurs to a small extent: ,omerizationundetected. 0

0 0 C

0

C 10 P

10 0

c

c

0 Ib

0

W

a

a

ro

10

3100

2900

2700

wavenumbers cm-d

3200

2700 1500 1200

wavenumbers cm''

Fig. 5 - FT-IR spectrum of poly- Fig. 6 - FT-IR spectra of polypropylene ethylene produced by ethylene produced by propylene adsorption on adsorption on TS (+3region). lw (a) and SA (b) (I)cH and JCH regions)

784

activities than for silica-alumina in several reactions (l), while Boehm and c m r k e r s (5)reported for the strongest acid sites the scale s0,2-/Ti02 > W43-/Ti02 >> Ti02 using the Benesi method. It nust be remarked that while on TS and Tp the oligamerization reactions are the only transformations detected, in the cases of Tw and IMO also hydration and oxidation reactions occur. The oligomerizing ability of sulphated titania was already s h m by Sheppard and co-mrkers (18) and is very rapid. "he 4Ci-I bands of polyethylene formed by adsorption of ethylene at r.t. are s h m in Fig. 5, while the spectra of polypropylene obtained on Tw and SA are compared in Fig. 6 These spectra agree with those reported in the literature for the corresponding polywrs. d) Characterization of the surface basicity. The surface basicity of our samples has been tested using carbon dioxide as a probe mlecule. The spectrum of the surface species arising from the adsorption of C02 on Tw is reported in Fig. 7 . The band at 2359 m-' is

.

coordinated on

due to the

2600

2200

idloo

wavenunbars Fig. 7

- FT-IR spectrum of CQ2

1400

cn-*

adsorbed on Tw.

Lewis centers, as deduced by the significant shift towards higher frequencies with respect to the gas-phase value. The reactive adsorption producing carbonate and bicarbonate species, due to the interaction with the basic surface sites, well evident on pure Ti02 (20) as generally on all ionic oxides (211 is completely suppressed by the supported phase. This result is exactly repeated also in the cases of TP and 'IMO, while only on TS small m u n t s of bicarbonates are formed. This would indicate that basic sites are a h s t suppressed by doping titania with a sufficient m u n t of 0x0-anions arising from adsorbed 0x0-acids.

785

coNcLus1ms The present data confirm that w i n g w i t h 0x0-anions such as sulphates, t u n g s t a t e s , molybdates and phosphates induces a s t r o n g e r s u r f a c e acid c h a r a c t e r on t i t a n i a s u r f a c e s . T h i s a g r e e s w i t h t h e enhanced c a t a l y t i c a c t i v i t y r e p o r t e d i n t h e l i t e r a t u r e f o r s i m i l a r l y doped t i t a n i a s f o r r e a c t i o n s s u c h as c y c l o p r o p a n e i s o m e r i z a t i o n , n - b u t a n e s k e l e t a l

aranatic acylations (1-3). H m v e r , the present r e s u l t s suggest a relevantly d i f f e r e n t picture of the mlecularl e v e l e f f e c t s of doping with respect t o those proposed i n the literature isamerization and FYiedel-Crafts-type

(1).

Our r e s u l t s in f a c t strongly support the idea that the enhanced

a c i d i t y of doped t i t a n i a s is of the Bronsted type and i s related t o the a c i d i t y of the adsorbed 0x0-acid (for example H2S04 > H3F04). The Lewis a c i d i t y is i n s t e a d o n l y p o o r l y enhanced by doping. Moreover, a f u r t h e r e f f e c t whose role cannot be undervalued is the poisoning of basic sites. This may j u s t i f y f o r exaPple an enhanced catalytic a c t i v i t y i n reactions t h a t are c o n s i d e r e d t o be Lewis-acid c a t a l y z e d such as a c y l a t i o n of

aranatics by acyl-halcqenides.

In f a c t these reactants generally deccnpose

on ionic oxides by hydrolysis on surface nucleophilic oxide ions (19). It is possible that only i f such basic sites are poisoned by acidic 0x0-anions a purely Lewis acid-catalyzed mechanism may be activated. Although the general chemical e f f e c t of &ping w i t h 0x0-anions seems t o be similar i n the four cases under study, s a w interesting differences are

observed i n the structure of the active sites. Only i n the case of phosphate anions i n f a c t , Bronsted sites are characterized by a very c h a r a c t e r i s t i c tungstate and mlybdate

4 O H band (3665 an-’). I n the case of sulphate,

species, instead, much weaker and broader bands are observed, suggesting that a major role may be played by protons that are involved even in the

activated sanples i n strong H-bondings, and are then responsible f o r very broad l t o H absorptions. This would agree w i t h the a c i d i t y scale that may be deduced by the a c t i v i t y of the d i f f e r e n t saonples in o l e f i n o l i g a w r i z a t i o n (supposed the differences being mainly due t o the d i f f e r e n t Bronsted acid strengths). In this respect, i n f a c t , t i t a n i a ckpd w i t h phosphoric acid a p p e a r s t o b e t h e less a c i d i c o n e , t h e o t h e r doped s a m p l e s b e i n g s i g n i f i c a n t l y mre active. The a c i d i t y of the W sites could be too small t o i n t e r a c t w i t h the very weak basic sites of the surface, unlike those of IMO,

are predaninantly involved i n H-bon&ngs. It is also relevant that IR spectroscopic data d e f i n i t e l y demxlstrate

‘Iw and TS, that

t h a t mono-oxo a n i o n i c species are p r e s e n t i n r e l e v a n t amounts on t h e surfaces of TS, TW and IMO, althocgh the presence of other species, such as

786 the di-oxo species hypothesized by Tan- and coll. (l), cannot be excluded. In the case of the phosphatized sample, instead, only a small amunt of mono-oxo species seem to be present, the IR spectrum being apparently dominated by the features of tricmrdinated hydrogenphosphate anions. The difference on the structures taken by the surface 0x0-anions might be sinply due to the different charge of them, the mno-0x0 form being preferred by the M042- ions, while the tricmrdinated protonated form being preferred by l r l ~ ~ species. ~REFERENCES

1. T. Jin, T. Yamaguchi and K. Tan-, J. Phys. Chem. 90 (1986) 4794. R.A. Rajadhyaksha and D.D. Chaudhari, Ind. Eng. Chem. Res. 26 (1987) 1743. 3. K. Arata and M. Hino, in M.J. Phillips and M. Ternan (Editors), Prcc. 9th Int. Congr. on Catalysis, The Chemical Institute of Canada, Ottawa, 1988, Vol. 4, p. 1727. 4. 0. Saw, M. Bensitel, A.B. M o m d Saad, J.C. Lavalley, C.P. Tripp and B.A. Morrow, J. Catal. 99 (1986) 104. 5. J. Cornejo, J. Steinle and H.P. Boehm, 2. Naturforsch. 33B (1978) 1311. 6. W.W. Swanson, B.J. Steusand and G.A. Tsiqdinos, in Prcc. IVth Climax Int. Conf. on Chemistry and Uses of Molybdenum, Ann Arbr,1982, p. 323. 7. J. Haber, in Prcc. VIIIth Int. Congr. Catalysis, Verlag Chemie, Berlin, 1984, Vol. I, p. 85. 8. P.J. Gellings, in G.C. Bond and G. Webb (Editors), Catalysis Vol. 7, Royal Society of Chemistry, London, 1985, p. 105. 9. S.S. Chan, I.E. Wachs, L.L. Murrell, L. Wang and W. K. Hall, J. Phys. Chem. 88 (1984) 5831. 10. G. Busca and J.C. Lavalley, Spectrcchim. Acta 42A (1986) 443. 11. G. Ramis, G. Busca and V. Lorenzelli, 2. Phys. Chem. 153 (1987) 189. 12. G. Ramis, G. Busca, V. Lorenzelli, P.F. Rossi, M. Bensitel, 0. Saur and J.C. Lavalley, in M.J. Phillips and M. Ternan (Editors), Prcc. 9th Int. Congr. on Catalysis, The Chemical Institute of Canada, Ottawa, 1988, vol. 4, p. 1874. 13. G. Busca, G. Ramis, V. Lorenzelli, P.F. Rossi, A. La Ginestra and P. Patrono, s m t t e d paper. 14. G. Busca, H. Saussey, 0. Saur, J.C. Lavalley and V. Lorenzelli, Appl. Catal. 14 (1985) 245. 15. C. Morterra, A. Chiorino, A. Zecchina and E. Fisicaro, Gazzetta Chim. Ital. 109 (1979) 691; C. Morterra, J. Chem. SOC. Faraday Trans. I, 84 (1988) 1617. 16. G. Busca, G. Ramis, V. Lorenzelli, A. Janin and J.C. Lavalley, Spectrochh. Acta 43A (1987) 489. 17. G. Busca, G. Ramis and V. Lorenzelli, J. Chem. Soc. Faraday Trans. I, in press 18. F. Al-Mashta, C.U. Davanzo and N. Sheppard, J. Chem. Soc. Chem. Carm. (1983) 1258; F. Al-Mashta, N. Sheppard and C.U. Davanzo, Mater. Chem. Phys. 13 (1985) 315. 19. V. Lorenzelli, G. Busca and N. Sheppard, J. Catal. 66 (1980) 28. 20. C. Morterra, A. Chiorino, F. Boccuzzi and E. Fisicaro, 2. Phys. Chem. 124 (1981) 211. 21. G. Busca and V. Lorenzelli, Mater. Chem 7 (1982) 89. 2.


The nature and reactivity of chemlsorbed oxygen and oxide overlayers at metal surfaces as revealed by photoelectron spectroscopy

M. W. Roberts School of Chemistry and Applied Chemistry, University of Wales College of Cardiff; CARDIFF CFI 3TB. United Kingdom

ABSTRACT X-ray photoelectron spectroscopy has shown that chemlsorbed oxygen and oxide overlayers at both nlckel and titanium surfaces exhibit both extensive nonstoichiometry and variable oxidation (redox) states. Assignment of core-level features has been posslble through studles of defective bulk oxldes and their Localized oxldation thermally induced conversion to 'more perfect' oxides. states, TI2+ and Ti3+ are also shown to form when carbon monoxlde (Ti2+) and nltric oxide (Ti2+, TI3+) are chemlsorbed at tltanlum surfaces. This is the first spectroscopic evidence for locallzed and discrete oxidatlon states in the chemisorption of these dlatomlc molecules at metal surfaces. Transient surface oxygen species O-(s) and 0-2(s) are shown to be intermediates In the formation of stable chemlsorbed oxygen 02-(a) at Mg(0001) These reactive transients play an important role in the and Zn(0001) surfaces. chemistry observed when dloxygen Is coadsorbed with ammonla and pyridine at Zn( 000 1) and Mg( 000 1) surfaces. ammonla and pyridlne acting as specific chemical probes for the short-lived oxygen surface Intermediates. INTRODUCTION

Photoelectron spectroscopy is sensitive to ail the elements other than hydrogen, and for the surface chemist It has the advantage of being sensitive to a depth of no more than three or four atomic layers.

The characteristlc feature

that stood out of the ploneerlng studles of Siegbahn was that the photoelectron binding energy was highly dependent on the charge associated with the emitting atom and from the chemists' partlcuiar interest this provlded, at least in principle, the opportunity of determlning the dlstrlbutlon of electronic charge around that atom. and therefore In the case of a metal surface atom hopefully its oxidation state.

Furthermore i f the photoelectron spectra could be analysed

so as to provide quantitative concentration data and also be able to distinguish

between different bonding states of the same atom then the surface chemist has available a unique experimental approach for studying molecular interactlons at solid surfaces. The Organisers of this Meeting invited me to discuss the Impact of photoelectron spectroscopy on the understanding of the chemlstry of oxide surfaces.

I have taken a rather broader vlew and have endeavoured to llnk the

chemistry of bulk oxide surfaces with both oxide overlayers at metal surfaces and also with chemisorbed oxygen at atomically clean metal surfaces.

This also is

787

tho way we have approached the general problem of unravelling the chemistry of 'oxygen' at metal surfaces:

it has involved combining the more traditional post-

reaction surface analysis approach with real-time spectroscopy.

or dynamic photoelectron

The latter has the advantage of being able to explore whether or

not highly reactive oxygen transient species play a role in the reactivity of dioxygen at surfaces.

We also show that spectroscopic evidence for such

species is more readily obtained by using probe molecules with specific chemical reactivities and working at very low temperatures when their surface life-times are sufficiently long for them to be chemically trapped and spectroscopically

We emphasise however that these short-llved surface oxygen species

identified.

are likely to be relevant to the catalytic chemistry of bulk oxide surfaces at very much higher temperatures and pressures.

Although much effort has and also is

being made to apply various other spectroscopic techniques (ESR.

IR and UV) to

investigate the nature and reactivity of oxygen at bulk oxide surfaces and to relate these studies to catalytic actlvlty (refs. 1-6),

very llttle effort has however

been made to investigate atomically clean metal surfaces.

Bulk Oxide3 The surface of bulk oxides have been characterised through monitoring the

O( 1s) and metal core-levels:

we have generally not found valence level spectra

(UPS) sufficiently diagnostic and particularly troublesome is the way in which

'unknown' surface contaminant species can complicate the UPS features characteristic of the true oxide surface.

UPS data can not.

conjunction with core-level spectroscopy,

be analysed with confidence to provide

an atomic picture of oxide surfaces. amenable to quantification.

unless used in

Vaience level spectra are also not easily

At Cardiff we therefore have relied more on the

analysis of X-ray induced spectra to provide such quantitative information as surface stoichiometry.

the presence of variable metal oxidation states and

different bonding configurations of oxygen, with UPS, for the reaons stated above. providing a supportive but less signiflcant role. Although nickel oxide has been an archetypal oxide in solid state defect chemistry.

with some evidence for an oxygen-excess type oxide (ref. 7 ) , direct

spectroscoplc evidence for surface defects has not been easy to obtain.

The

electronic configuration of both metal and oxygen species in the surface region was explored by XPS for a number of different preparations of nickel oxides and also nlckel oxyhydroxlde (refs. 6-91.

The chemistry usually associated with

thermally induced Interconversions was then related to simultaneous changes in both the Ni(2p) and O(ls) spectra.

By this means a data base was built up

from which it was possible to assign. with the help of charge-binding energy correlatlons. speclflc defect states to particular spectroscoplc features.

Classical

sottd-state defect chemistry has Interpreted the chemistry of nickel oxldss In

terms of a very restricted degree of bulk non-stoichiometry, the range 0 to 0. 02.

NiOl+x where x is in

What emerged from XPS (refs. 6-91 was surprising in the

context of this traditional view.

First there was clear evidence for both oxidation

states. Ni2+ and Ni3+ in the Ni(2p) spectra.

second the concentratlon of the

latter was substantlal (usually greater than 20% of the total catlons) ln the surface region, third there were two components to the O(ls1 spectra and fourth there was substantlai oxygen excess eg x > 0. 5 in the surface region.

N12+ and

Ni3+ species have characteristic Ni(2p) peaks at 854. 7 eV and 856.2 eV respectively. while OCls) peaks at 529.5 eV and 531 eV were assigned to 02and 0- species (refs. 8-10).

Heating the hlghiy defective 'black' nlckel oxide

with predominantly Ni3+ species present to llOO°C in vacuum led to the colour changing to green, the formation of Ni2+ species and also an increase in the concentration of 02- species, respectively (Fig.

both at the expense of Ni3+ and 0- species

1).

n

Fig. 1. Ni(2p) spectra for hlghly defective NiO.OH. NiO( 100).

'more perfect' NIO and

An important point that also emerged was that it was not a trivial matter to prepare a bulk nickel oxide with a surface stoichiometry close to NiO.

inevitably

790 there was present substantial oxygen excess ( x

0.2) even for a single crystal.

supposedly Ni0(100), after high temperature treatment in vacuum (ref. 8 ) .

Cbdde Ovsrlavrys at MQ&I 8urfaoga The application of photoelectron spectroscopy to the study of chemisorbed oxygen at metal surfaces was an obvious extension of our earlier photoemission and work function studies.

Of particular interest was the answer to the question

'under what circumstances did chemisorbed oxygen transform to an oxide overlayer and when did the latter take on the reactivity associated with the corresponding bulk oxide?' raw Ni(2p) spectra.

Earlier work was disappointing in the sense that the

although modified after oxygen chemisorption.

evidence for either discrete Ni2+ or N13+ species.

did not reveal

However encouraged by the

model spectra established with the bulk oxides Including NiO( 100) single crystals we recently reinvestigated (ref. 10) oxygen chemisorption at both Ni( 100) and Ni(210) surfaces taking advantage of the progress in data processing made through the use of microprocessors and software developed in this laboratory. Both Ni(100) and Ni(210) surfaces when exposed to oxygen at 295K gave evidence for both Ni2+, N13+, 02- and 0- species when the oxide layer was no more than about two monolayers thick (ref. 10).

Some significant points to

note are (a) That a spectral difference procedure showed that with Ni(100) for up to an oxygen coverage of 0 . 9 x 1015 atoms species,

1.0.

Ni2+ or Ni3+. were present.

no 'charged' nickel

Above this concentration ( 0 > 1.0)

clear evidence for both Ni2+ and N13+ was observed in the Ni(2p) spectra. (b)

With Ni(210) surfaces i t was feasible to separate out the individual

contributions of Ni2+ and Ni3+ to the spectra (Fig. 2 ) . with Ni( 100).

This was not possible

The open atom structure of Ni(210) apparently facilitated the

formation of a more defective oxide overlayer and with which we would associate more Ni3+ than Ni2+ species. Analogous studies of coadsorbed water: oxygen mixtures showed clear evidence for Ni3+ species at Ni(1OO) surfaces (ref. 11).

Evidence for localized

higher oxidation states within the chemisorbed oxygen overlayer at nickel surfaces was therefore firmly established. This concept was further explored with the titanium-oxygen system and a sequence of sub-oxide states. Ti2+ and Ti3+,

shown to develop with oxygen

exposure at low temperatures prior to the formation of Ti4+ species (ref.

12).

0

Ail three co-existed in the oxide overlayer (10A) at a poiycrystaiiine surface, with Ti2+ and Ti3+ being predominantly at the metal-oxide interface.

The defect

structure of the overlayer was established through angular dependent studies of the Ti(2p) spectral region (ref. 12).

in The nature of the bond formed when a molecule is chemisorbed at a metal

791

:oncentration (10Kcm-2I

I

I I

I I -CHEMISORPTION/

454p

OXIDATION-

3d /

I

I I

I I 0

7’

/

r

50

Fig. 2. Concentration of chemlsorbed oxygen. 02-, 0-. N12+ and N13+ calculated from O( Is) and Ni(2p) photoelectron spectra during exposure of Ni(2lO) surface to dioxygen at 295K.

surface has been one of the central questions In surface chemistry end heterogeneous catalysis.

Although work function data provlde evidence for the

sign of the surface dipole it provides little Information on the precise nature of the chemical bond, it for example being unable to dlscrlmlnate between (weak) dispersion and (strong) Ionic forces.

We report some recent XPS studies (ref.

25) of the chemisorptlon of carbon monoxide and nitric oxlde at polycrystalilne surfaces and show how Tl(2p) difference spectra provlde evidence for discrete Ti2+ states being generated In CO chemlsorptlon whereas TIP+ and Ti3+ forms with nltrlc oxlde (Flg. 3 ) .

Wlth both molecules dissociative chemlsorptton occurs

and this is established through the monltorlng of C(ls), O(1s) and N ( l s ) sDectral realons.

792

Ti(2p) DIFFERENCE SPECTRA

3' 2' I

2'

I

I

I I I

CARBON MONOXIDE

I

NITRIC OXIDE

.myii I I I I I

450

455

460

465

470

450

455

B E (QV)

460

465

470

B E (eV)

Fig. 3. Ti(2p) difference spectra (Tio contribution removed) for the chemisorption of carbon monoxide (25L and 375L) and nitric oxide (5L and 500L) at titanium surfaces at 295K.

Surface Omaen and the Chemical Reactivitv of Ads0 rbates Although oxygen is not usually regarded as a promoter in heterogeneously catalysed reactions at metal surfaces, roles have emerged.

a number of quite distinct promoter-type

At low temperatures H-bonding of such adsorbates as

water and ammonia to chemisorbed oxygen has been observed (refs. 13-14). whereas in other cases ( i e when oxygen is present at different metal surfaces) H-abstraction leading to surface hydroxylation occurs with little or no thermal activation being necessary the process being very fast at low temperatures

(150K). Activation of adsorbates is both dependent on the metal and the surface oxygen coverage;

in general as the oxygen coverage increases so does the

efficacy of the oxygen-induced chemistry decrease.

In the case of for example

nickel "perfectly" stoichiometric NiO is unreactive to water vapour whereas chemisorbed oxygen formed at a nickel surface at 80K is highly reactive to water vapour (ref. 15).

If we consider the oxygen present at a clean nickel surface

at 80K as being a transition state in the formation of an oxide overlayer. and there is indeed substantial evidence from work function,

photoemission and XPS

studies that this is indeed the case, then it raises the interesting question as to what is the precise electronic configuration of the reactive surface oxygen.

We

793 and it is the high efficiency of O-(s) in effecting H-abstraction from an otherwise unreactive ammonia molecule that is responsible for surface amide and hydroxyl formation.

With Mg(0001) surfaces a steady state model (ref. 22)

provided an estimate for

TO-(^) of about

lod8, at 295K.

The essential

component of the model is the reaction between the transient O-(s) species and a hopping weakly interacting ammonia moiecule whose surface concentration is

immeasurably small ( 8 < 1 % ) ,

in other words a modified Eley-Rideai mechanism.

We can consider the ammonia molecule as being a highly effective scavenger of

the electrophilic O-(s) species prior to the latter becoming the unreactive stable oxide overlayer 02-(a),

Clearly according to the model there is no B oiori

reason why the short-lived 0-2C.s)

transient might not be the dominant reactive

species and responsible for the chemistry observed. on the relative surface life-times of 0-

(5)

This will depend very much

and O-pfs)

which in turn will be a

function not only of the metal but also. for a given metal, its surface atomic structure. in order to explore whether or not these ideas were of more general

significance we extended our studies to other sp-metals and chose Zn(0001) surfaces where the sticking probability of dioxygen dissociation is almost a factor of

l o 3 smaller than for Mg(0001).

The surprising feature of these coadsorption

studies of ammonia and dioxygen was that the efficiency (rate) of formation of chemisorbed oxygen, 02-(a),

per molecular impact with the surface was

enhanced by a factor of more than 500 compared with dioxygen alone (ref. 23). This implied that a surface molecule-ion complex (O-2----NH3)

is Involved

whose surface-iife tlme and concentration is substantially greater than 0-21 s)

.

Now in view of the very small sticking probability of dioxygen it is not surprising that the first step in the dissociative chemisorption of dioxygen at Zn(0001) surfaces - involving accommodation and single electron capture inefficient.

-

is very

The role of the ammonia molecule is therefore seen to make this

first step more efficient through the formation of the transient ammonia

-

dioxygen complex which subsequently decomposes to generate 02-(a)

OH(a)

,

and NH2(a).

NH3tg)

f

: physical adsorption

NH~~s)

N H ~ ( s )+ 02-(s)

--b

(NH3---O-2)

(s)

: complex formation

794 can rule out fully Coordinated 02- species.

Thiei and Madey (ref. 70) have

suggested that the chemically reactive surface oxygen species mimics the electronlcally excited O(lD)

state known to be highly efficient in the gas phase

water-oxygen atom reaction and In contrast to the unreactive ground state oxygen with configuration O(3P).

What has also been established (for nickel oxides) is

that the surface becomes chemically more inert as the high binding energy

O(ls) component (-531 eV) decreases in intensity. O-(a)

This we have assigned to

ie dissociated oxygen molecules which have not become fully coordinated

lattice oxygens and therefore developed the full charge associated with 02lattice.

They are surface excess metastable oxygen adatoms.

Under certaln circumstances chemisorbed oxygen present at metal surfaces can induce chemlcal reactlvity leadlng to the chemlsorptive replacement of the oxygen. by for example an lmmide species, when exposed to ammonia (ref. 17)

.

Under the same erperlmental conditions the atomically clean copper

surface is unreactive to ammonia. H-abstractions with

. NH

The mechanism is likely to involve step-wise

radical formation.

the simultaneous desorption of water.

The latter is then chemisorbed with

It is the highly facile nature of this

-

reaction that is surprising. the process being complete and fast at 180K but sensitive to the oxygen surface coverage.

At high coverages ( 8

1.0)

chemisorptive replacement does not occur at low temperatures.

e S

u

w

The complex surface ohemlstry assoclated with the chemisorption of nitric oxide at for example C u ( l l 1 ) and C u ( l l 0 ) surfaces (refs.

18-19),

the role of

surface oxygen in the activation of adsorbates (such as water) and the chemistry of coadsorbates,

such as water and nitric oxide (ref. 2 0 ) . led to the questlon

being posed as to whether or not reactive short-lived species were responsible for the observed chemistry.

The metal-adsorbate systems were chosen carefully.

both the atomically clean metal and the stable oxide overlayer being chemically unreactive to the adsorbate under the experimental conditions used in the coadsorption studies.

In the Mg(OOOl)-ammonia-oxygen

system (refs. 21-22)

activation of the N-H bond was shown to be facile and a transient oxygen species, designated O-(s), was shown to be the reactive species.

The latter

could be formed as a result of the dissociative chemisorptlon of dloxygen. nitric oxide and nitrous oxide with NH2 and OH species being generated (Fig. 4) at very low temperatures (llOK).

The crucial factor in the observed chemistry of

these coadsorbed molecules i s the surface life tlme. the sequence (for example for dioxygen):

7.

of the O-(s) species in

795

NH3- 0, mixture (5:l)

30L, l l O K

BElev) Flg. 4. N(1s) and O(1s) spectra after ammonla-dloxygen mlxture exposed to Mg(0001) surface at 110K: In the absence of dloxygen NH3 Is physically adsorbed at both the clean and oxlde-overlayer surfaces.

More recently the specific chemlcai reactlvlty of 0-2(s) species associated wlth the dissoclatlve chemlsorption of dioxygen at Zn(0001) surfaces has been further established in coadsorptlon studles of dloxygen-pyrldlne

24).

mlxtures (ref.

Dloxygen bond cleavage was enhanced by a factor of nearly

lo3

compared

with oxygen (Fig. 5) alone Indicating the role of surface charge-transfer complexes formed through 0-2(s) Interaction with the pyrldlne n-electron system.

SUMMARY In heterogeneous catalysis the catalyst surface Is In a dynamic state both

surface composltlon and chemical reactlvlty being related to the reaction conditions (temperature. reactant composltion. etc).

surface and gaseous addltlves

Although we have a long standing Interest In oxygen chemlsorptlon at

metal surfaces detalled information on bondlng , the presence of locallzed redoxstates at the metal-oxide and oxide-gas interfaces, the presence of different chemisorbed oxygen specles. surface stolchlometry etc has only become avallable through photoelectron spectroscopy.

In another paper to be glven at this

Conference we see Clmlno and hls colleagues (ref. 26) uslng XPS to study NIOMgO solld solutlons.

But It Is when photoelectron spectroscopy Is used under

796

15

-2

%(lo cm A

1

1.0 -

0.5-

________I

1

20

- -I ) Lo

O2 exposure (L 1 Fig. 5. Dissociative chemisorption of dioxygen in the absence (---) and presence of pyridine (10: 1 mixtures) at a Zn(0001) surface at 180K. 220K and 295K; evidence for a pyridine-dioxygen surface complex.

real-time or dynamic conditions at low temperatures

(

100K-300K) that enables

individual reaction steps in a sequence of complex reactions to be identified. We believe this is one approach to unravel the mechanism of molecular reactions under real catalytic conditions (high pressures and temperatures) since at the present time experimental methods are not available for monitoring the surface under these conditions and one has to rely on post-reaction analysis when the surface may well relax to a metastable state which will have little relevance to its active state.

Three particular aspects are emphasised in this paper:

nature of oxide overlayers at nickel and titanium surfaces.

the defect

specific localized

oxidation states in chemisorption and the specific chemistry associated with coadsorbed molecules at two sp-metal surfaces,

Mg(0001) and Zn(0001).

The

chemistry observed with ammonia-dioxygen mixtures has been related to the specific reactivity of short-lived surface oxygen transients; the reactive species is O-(s) while with Zn(0001) transient,

NH3---O2-,

at Mg( 0001) surfaces

it is a molecule-ion

that determines the reaction pathway.

surface

Dioxygen bond

cleavage is cataiysed by coadsorbing oxygen with pyridine indicative of a (Py-

02-) charge-transfer

surface complex.

These studies also emphasise how an

otherwise unreactive probe molecule (ammonia or pyridine in this case) provides evidence for surface oxygen transients,

O-(s)

and 0 2 - ( s ) ,

being generated in

the dissociative chemlsorptlon of dioxygen at metal surfaces leading to the formation of a stable. and by comparison,

relatively unreactive oxide. 02-.

overlayer. We acknowledge support of this work by SERC and the Royal Society. London, for permission to reproduce flgures 1 and 2.

797 REFERENCES 1 2 3 4

5 6

7 8 9

70

11 12 73 14 15 16 17 18 19 20 21 22 23 24 25 26

A. Zechina, G. Spoto. S. Coluccia and E. Guglielminotti. J. Phys. Chem., 88 (1984) 2575. S. Coluccia, E. Garrone and E. Borrelo, J. Chem. SOC. Farad. Trans. I., 79 (1983) 607. M. Che and A. J. Tench in D. D. Eley et a1 (Editors) Adv. in Catalysis. Academic Press, 1982. 31, p77. J. Haber in J. P. Bonnelle et a1 (Editors) Surface Properties and Catalysis by Non-Metals D. Reidel Publishing Company, 1983. p l . See J. Nowotony and L. C. Dufour (Editors) Surface and Near-Surface Chemistry of Oxide Materials. Elsevier. Amsterdam, 1988. p219. E. Garrone and F.S. Stone, in Proceedings 8th Int. Cong. on Catalysis. Berlin. Dechema 1984. 3. p441. J. Deren and J. Stoch, J. Catalysis, 18 (1970). 249. M . W . Roberts and R. St.C. Smart, J. Chem. SOC. Faraday Trans. I. (1984). 80. 2957. L . M . Moroney, R. 3 . C . Smart and M. W. Roberts. J. Chem. SOC. Faraday Trans. I. (1983). 79, 1769. A. F . Carley, P. R. Chalker and M . W. Roberts, Proc. R. SOC. Lond. A399 (19851, 167; C.T. Au, A. F. Carley and M. W. Roberts, Phil. Trans. R . SOC. A318, (1986). 61. A . F . Carley, S . R . Grubb and M. W. Roberts. J. Chem. SOC. Chem. Communications (1984) , 459. A. F. Carley, P.R. Chalker, J . C . Riviere and M. W. Roberts, J. Chem. SOC. Faraday Trans. I. 83. (1987). 351. F. P. Netzer and T. E . Madey, Chem. Phys. Lett. 8 8 . (7982). 315. C . T . Au. M . W. Roberts and A.R. Zhu. Surface Sci. 115. (1982). L117. A.F. Carley, S. Rassias and M . W . Roberts. Surface Sci. 135, (1983). 35. P. A. Thiel and T. E . Madey, Surface Sci. Reports, 7. (19871, 211. C. T . Au and M. W. Roberts, Chem. Phys. Lett., 74. (19801, 472. D . W . Johnson, M . H . Matloob and M . W . Roberts, J. Chem. SOC. Chem. Commun. (1978), p40. D . W . Johnson, M . H . Matloob and M.W. Roberts. J. Chem. SOC. Faraday Trans. I . , 75, (1979), 2143. C . T. Au and M. W. Roberts, Proc. Roy. SOC. Lond. A396. (19841, 165. C.T. Au and M.W. Roberts, Nature, 319, (1986). p206. C . T . Au and M.W. Roberts, J. Chem. SOC. Faraday Trans. I.. 83. ( 1987) , 2047. A . F . Carley, M.W. Roberts and Song Yan. J. Chem. SOC. Chern. Commun. (1988), p267. A. F. Carley. M. W. Roberts and Song Yan. Catalysis Letters (1988) (accepted for publication) . A. F. Carley, J . C . Roberts and M. W. Roberts (to be published). A. Cirnino. D. Gazzoli. V. Indovina, G. Moretti and M. Occhinzzi (this Conference Proceedings) .


C. Morterra, A. Zecchina and G . Costa (Editors), Structure and Reactiuity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands

799

SUPPORT EFFECTS IN TEST REACTIONS OF HEXANES ON Pt/U02 CATALYSTS AND ON A UPt3 INTERMETALLIC COMPOUND.

M. ROMEO', A. DAUSCHERl, L. HlLAlRE1, W. MULLER2and G. MAIRE' 1 Laboratoire de Catalyse et Chimie des Surfaces, U.A 423 du CNRS, 4 Rue Blaise Pascal, F-67070 Strasbourg (France) 2 Bureau Central des Mesures Nucleaires, Joint Research Centre, Steenweg naar Retie, B-2440 Gee1 (Belgium)

ABSTRACT Skeletal rearrangement reactions of hexanes have been studied under the same experimental conditions, on various Pt supported on uranium oxide powder catalysts and on a polycrystalline UR, intermetallic compound. 1%-labeling allowed estimation of the relative contribution of both cyclic and bond-shift mechanisms in isomerization. Results show that Pt/U02 catalysts can lead to metal-support interaction, but this effect is not the same on low and high metal content Pt/U02 catalysts. This may be related to the difference of reducibility of uranium oxides as a function of platinum content. UPt3 shows a catalytic behaviour similar to high content Pt/U02 catalysts, but this is due to the fact, as ESCA results show, that it easily decomposes under hydrogen flow to form platinum particles and uranium oxides. INTRODUCTION One of the most interesting problems in heterogeneous catalysis is related to the extent of the interaction that could exist between the support and the metal particles. Proofs have been presented (ref. 1) that group Vlll noble metals supported on Ti02 exhibit Strong Metal Support Interaction (SMSI), when reduced under hydrogen at high temperature. Within these conditions the support can alter the metal phase behaviour in such a peculiar way that it cannot be attributed to simple dispersion. One of the mechanisms suggested in the seminal work of Tauster et al. (ref. 1) for SMSl phenomenon is the formation of a Pt-Ti alloy in the case of PtTTi02 catalysts. This is supported by a theoretical molecular orbital study (ref. 2). Lately, the formation of PtgJi or PtTi has again been evoked by several authors (refs. 3,4,5), while Baker et al. (ref. 6) and Sheng et al. (ref. 7) suggested the formation of a highly mobile phase, e.g. a mixed surface oxide PtnTiOx. Nevertheless, the most commonly proposed model for SMSl is the "decoration" of the metal particles by reduced TiOx support species. This explanation is supported by studies performed on model catalysts obtained by evaporation of Pt on Ti or vice versa (refs. 8,9,10). Moreover, group Vlll metals supported on V2O5 or Nb2O5 behave identically to TiOp-suppotted metals (ref. 11). It was then concluded that SMSl is exhibited on oxide supports which may be easily reduced during H2-treatment of the catalyst.

800

To acquire more informations about the relation existing between reducibility of the support and metal-support interaction we have performed additional study on U02-supported platinum catalysts. This oxide is normally found on its over stoichiometric form (U308,U02+x, etc.), but upon severe conditions it could undergo reduction to form suboxides; consequently, we could expect a Ti02-like behaviour. On the other hand, the catalytic behaviour of a UPt3 intermetallic compound has also been studied in order to find out if its formation, amidst Pt-U interface of Pt/U02 catalysts, is possible. EXPERIMENTAL Materials The synthesis of the 13C-labeledhydrocarbons used as reactants or references for mass spectrometry has been described elsewhere (ref. 12). The unlabeled hydrocarbons were FLUKA puriss grade. Before each experiment the reacting hydrocarbon was purified by gas-liquid chromatography and correct labeling ascertained by mass Spectrometry. e p aration Two catalysts 0.8 wt YO Pt/U02 and 8 wt YO Pt/U02 were prepared by impregnating U02, obtained from the JOINT RESEARCH CENTRE (Karlsruhe, GERMANY), with hexachloroplatinic acid solution. These catalysts were then calcined under air flow at 400 "C for n (n = 0 - 4) hours and reduced under hydrogen flow at 200 "C for 16 hours. The UPt3 intermetallic compound was prepared by cofusion (radiofrequency heating) of the two metals in a Hukin's crucible. Slices 1.5 mm thick were cut from the obtained sphere of material by spark erosion. This preparation was carried out by J.C. Spirlet at JOINT RESEARCH CENTRE (Karlsruhe, GERMANY). One of these slices was used for XPS and UPS experiments. For catalytic tests, another slice was ground and sieved to obtain grains of diameter lower than 100 pm. m

re . Apparatus and Drocedu Catalytic tests were carried out in a differential reactor on 0.25 g of catalyst (0.5 g for UPt3). The differential reactor and experimental procedure have been described elsewhere (ref. 13). In each run a very small amount of hydrocarbon (ca. 4 pi) was conveyed over the catalyst at constant pressure (ca. 3.5 Torr) and the obtained reaction mixture was analyzed by gas-liquid chromatography. Electron microscopy examinations of both 0.8 and 8 wt Yo Pt/U02 catalysts (calcined at 400 OC for 4 hours under air flow and reduced at 200 "C for 16 hours under hydrogen flow), were carried out at a magnification of 1.000.000 X. A total of 2000 particles from each sample were counted. The determined average size for these two catalysts is ca. 30 A.

801

BET was performed using krypton on untreated 0.8 wt % P W 0 2 and on both 8 wt

Yo PVU02 and 0.8 wt % PtlUO2 calcined under air flow at 400 "C for 4 hours. The

obtained specific area of these catalysts was 2.6, 4.5 and 5.4 m2/g1respectively. E.S.C.A. experiments were performed in a Vacuum Generators ESCAlll spectrometer. As photon source the At K, radiation (1486.6 eV) was used. Detection of electrons was made by using a high resolution hemispheric analyzer connected to a channeltron used as an electron multiplier. A cryogenic pump connected to the analysis chamber allowed us to reach an ultimate pressure in the low 10-10 Torr range. In situ oxidative and reductive treatments were performed in the preparation chamber of the ESCA apparatus. All peak positions were determined relative to the C 1s level at 284.8 eV.

RESULTS ESCA studies Pt 4f and U 4f peaks were recorded for both 8 wt Yo WU02 and 0.8 wt % WU02 catalysts. The corrected values of their binding energy are reported in Table 1 together with the measured charging effect. Before any treatment, the U 4f transition of 8 wt Yo Pt/U02 exhibits a poorly resolved satellite at about 3.7 eV towards higher binding energies and a minuscule one at 10.4 eV (see Figure la). These very small satellites, together with the binding energy of the main peaks are typical of UO3 (sat. = 3.7 eV and 10.6 eV, ref. 14).

512

712

u 4f 512

a

a

381.4

392.0

3.7 10.4

9.0

i air 4.5 h 400 "C

71.5 72.7

74.7 76.0

381.8

392.7

3.4 9.7

0.2

t ,"a00 "C

71.2

74.6

380.1

390.8

6.9

0.5

0.8% WU02

70.8

74.0 75.3

382.4

393.3

10.0

3.0

+ air 4 h 400 "C

71.7

74.6 76.5

380.6 381.6

392.4

7.4 9.7

0.1

71.0

74.1

380.4

391.2

6.2 7.8

0.5

Pt 4f

Catalyst 712 8% Pt/U02

A Sat.

charging effect

802

A'

375

eV

405

Fig. 1. XPS s ectra of U 4f peaks, 8 wt % WU02, (a) Untreated, (b) + air 400°C 4.5h, (c) + H2 200 4h.

'8

C

b a

68

78

eV

Fig. 2. XPS spectra of Pt 4f peaks, 8 wt % WU02, (a) Untreated, (b) + air 4OOOC 4.5h, (c) + H2 200 "C 4h.

803

..

..-

. '

. -

-c' 405

375

SV

Fig. 3. XPS spectra of 0 4f peaks, 0.8 wt % PVU02, (a) Untreated, (b) + air 4OOOC 4h, (c) + H2 200 "C 4h.

..

. ....5: -.. .. .. 68

6 . .

78

ev

Fig. 4. XPS spectra of Pt 4f peaks, 0.8 wt % Pt/U02, (a) Untreated, (b) + air 400°C 4h, (c) + H2 200 "C 4h.

804

This is confirmed by the high charging effect (UO3 is an insulator). Nevertheless, a large amount of chlorine is detected (CI/U = 1) on the surface and the formation of some kind of uranyl chloride cannot be discarded. The Pt 4f region is quite complex. On the raw spectra two humps at 71.3 and 72.1 eV are clearly detected. A third 4 1712 contribution is also included in the broad peak of the 4 f5/2 area (see Figure 2a). The respective ratios of these peaks cannot be determined. All we can say is that Pt is partly in its metallic state and partly in at least two chloride or oxychloride forms. After calcination (4.5 h at 400 "C, air) the Pt 4f peaks are split into two doublets (see Figure 2b). This is due to a partial oxidation of platinum atoms. Very little changes are observed on the U 4f peaks. At this stage CI/U ratio drops to 0.1, which means that CI/Pt ratio is equal to 1. After reduction (4 h at 200 "C, H2 each of U 4f transitions exhibits a strong shake up satellite at 6.9 eV (see Figure lc). This satellite is distinctive of U02 and it is known to be very sensitive to stoichiometry (ref. 15). The binding energy of U 4f7/2 and U 4f5/2 transitions are also in good agreement with the values of 380.0 eV and 390.8 eV found by Chadwik (ref. 16) and by Allen et al. (380.1 eV, refs. 14, 17) on U02. The Pt 4f lines for the same sample show that platinum is fully reduced after the above mentioned treatments. The CI/U ratio is not influenced by this treatment. The same series of analysis was performed on the 0.8 wt % Pt/U02 catalyst. The evolution of Pt 4f peaks is analogous to the one showed by 8 wt % Pt/U02, whilst U 4f spectra are quite different. Initially Pt is a mixture of platinum in its metallic state and Pt chloride or oxychloride (CI/Pt = 12). The large tail on the low energy side of U 4f peaks may indicate the presence of U3O8 (see Figure 3a). This oxide may be formally considered as a mixture of U02 and UO3, both contributions of which may be detected (ref. 18). The poor resolution of our spectrum is probably due to a mixture of oxides, with a major contribution of UO3. This is confirmed by the almost complete absence of satellites around 10 eV and the existence of charging effect. After calcination the U 41 spectrum becomes distinctive of pure U3O8, whereas Pt is a mixture of metallic platinum and platinum oxides. Reduction under hydrogen flow at 200 "C leads to metallic platinum. However, the tail on the Pt 4f peaks towards higher binding energies reveals that a small amount of platinum oxide is still present (see Figure 4c). The binding energy of U 4f peaks is quite the same as on U02. Nevertheless, the satellite is quite different from what was obtained on both pure U02 (ref. 19) and 8 wt % Pt/U02 catalyst. On the latter two compounds a huge satellite is detected, whereas on 0.8 wt % Pt/U02 this satellite is very small and two contributions could be identified at 6.2 and 7.8 eV. This spectrum is very similar to the findings of Allen et al (ref. 19) when studying UOZ+~. In a parallel study on UPt3 intermetallic compound (ref. 20) we have observed that this intermetallic compound readily decomposes, even under small hydrogen pressures (2 Torr) to form metallic platinum and uranium dioxide. This result is

805

somewhat surprising but one can understand that the great affinity of uranium for oxygen makes it easy for hydrogen to favour the segregation of oxygen towards the surface; another explanation may be the desorption of oxygen atoms from the walls of the ESCA chamber when hydrogen is introduced into the system. w v t i c resuIts Skeletal rearrangement reactions of both 2-methylpentane (2MP) and methylcyclopentane (MCP) were studied under the same experimental conditions on 0.8 and 8 wt Yo Pt/U02 catalysts and on UPt3 intermetallic compound. The results obtained on these catalysts are given in Table 2 and compared to 0.2 and 8 wt Yo Pt/A1203 catalysts (ref. 21), which are insensitive to SMSl phenomenon, plus 0.2 and 8 wt Yo PVTi02 catalysts (ref. 22), which does and does not exhibit SMSl behaviour, respectively. Comparison of only reduced 8 wt YO Pt/U02 catalyst with calcined and reduced 8 wt Yo Pt/U02 catalyst, shows that a calcination preceding the reduction increases the catalytic activity (z, 2MP and MCP). This effect is also observed on low metal content PtKi02 catalyst. However, the above mentioned treatment does not alter the catalytic activity when the same reaction is done on low metal content Pt/U02 or high metal content PVTi02 catalysts. These experimental findings indicate that PtKi02 catalysts behave contrarily to Pt/U02 catalysts, for what concerns the metal loading. When only reduced, the 8 wt % Pt/U02 catalyst is very inactive compared to both PVA1203 and WTi02 catalysts. After a calcination, the differences are markedly lower. An opposite behaviour is observed on low metal content Pt/U02 catalysts. When MCP is used as starting hydrocarbon, the 3-methylpentane/n-hexane ratio (3MP/n-H) decreases (increases) after a calcination for 8 wt % Pt/U02 (0.8 wt Yo Pt/U02) catalyst. For the other catalysts no modification of products distribution is found. For UPt3 a calcination preceding the reduction leads to similar variations of activity and selectivity as the ones observed on 8 wt Yo Pt/U02 catalysts, even though, only reduced catalysts have not an identical behaviour. The isomerization reactions of labeled hexanes, namely 2-[2-13C], 2-[4-13C] and 3-[3-13C] methylpentanes have been studied at 390 "C on various catalysts. In Table 2, the total contributions of cyclic (ZCM), bond-shift (ZBS) and hydrogenolysis (Zhydr.) mechanisms are presented. The detailed methods of determining the different contributions are explained in ref. 12. The total contribution of cyclic mechanism on both high and low metal content P W 0 2 catalysts does not show great differences from the one observed on Pt/A1203 catalysts. Moreover, calculation of ZCM for UPt3 at 390 "C, assuming an apparent activation energy for isomerization equal to 16 kcai/mole (temperature range = 350-390 "C), gives a value of ca. 80%. This shows that this intermetallic compound

806

behaves identically to PVU02 catalysts.

TABLE 2 Reactions of hexanes at 390 "C (UPt3: 350 "C).

Labeled hexanes

ZCM

ZBS Zhydr.

0 4

6.5 144.0

98.0 81.0

3.0 101.0

1.8 0.9

71

18

0 2

6.8 3.1

90.4 82.6

3.8 2.4

0.8 1.4

77

10

11

13 ~

0 4400.0

76.0

2600.0

0.8

68

4

28

0.2% PVAI20 0

620.0

86.0

680.0

0.7

78

8

14

0 4

131.0 180.0

80.0 81.0

100.0 128.0

0.9 0.8

61

23

16

0 4

4.0 213.0

82.0 62.0

3.1 354.0

0.8 0.7

46

39

15

0 4

0.2 7.3

77.0 52.0

0.2 5.3

1.0 1.5

40

17

43

r 0.2% m o ,

a- Duration of calcination preceding reduction. S-Selectivity in isomers, defined as the percentage of isomerized products upon the overall conversion

DISCUSSION

The catalytic results presented in the previous section clearly show that high metal content Pt/U02 catalyst calcined for 4 hours at 400 "C under air flow and reduced for 16 hours at 200 O C , behaves identically to 0.2 wt % PVA1203 catalyst (very low 3MP/n-H ratio and same values of total contribution of isomerisation mechanisms). The behaviour similarities (enhancement of catalytic activity when a calcination is performed before reduction) found between 0.2 wt YO PVTi02, which exhibits SMSl behaviour, and 8 wt % Pt/U02 catalysts, can be explained by taking into account the particle size of active phase. In fact, 3MPln-H ratio in MCP hydrogenolysis, which is very sensitive to particle size (ref. 23), drops from 1.8 to 0.9 when the above mentioned treatment is performed. This means that particle size of the active phase diminishes. The final value (0.9) is in agreement with both the very high total contribution of cyclic mechanism and by electron microscopy measurements, where an

807

average size of ca. 30 A has been found for platinum particles. Increasing the dispersion of metal particles also denotes an increase of the number of active sites. This provokes an obvious amelioration of catalytic activity. Another approach that could explain these effects is the deactivation of the only reduced 8 wt % WU02 catalyst caused by chlorine atoms (ref. 24). In fact, ESCA results show that the amount of chlorine is remarkably high when the catalyst is not calcined, whereas this amount is notably lower after calcination. Nevertheless, UPt3 intermetallic compound displays the same relative increase of activity after calcination as 8 wt % PUU02 catalysts, even though no chlorine is present. Thus, chlorine can hardly be considered as responsible for the variations encountered when the high metal content Pt/U02 catalyst is calcined. Another very important result is the formation of small particles (ca. 30 A, electron microscopy results) on a support the specific area of which is 5m2/g (BET measurements) even for a metal loading as high as 8 YO.This result is not unforeseen as dispersing properties of uranium oxides, for what concerns the metal phase, have been already found on other catalytic systems (eg. Pt/U02/A1203 ref. 20, (Ni+Mo)/U02/A1203ref. 25). ESCA results show that after calcination at 400 "C for 4 hours and reduction at 200 "C for 16 hours, the final state of the support for high and low metal content PUU02 catalysts is not the same. In fact, according to these results high metal content and low metal content catalysts are supported on U02 and U O Z + ~ , respectively. This could play an important role in the catalytic differences between the latter catalysts, for what concerns the effect of a calcination before the reduction. However, the inactivity of these catalysts compared to the high activity found on Pt/A1203 catalysts, even though they are reasonably well dispersed, remains undecipherable. One possible explanation is that a metal support interaction exists for both 8 and 0.8 wt % WU02 catalysts, but its nature is different from the one found on Pt/Ti02 catalysts. To find out the nature of this interaction further investigations are necessary; all we may say is that this interaction impedes the access to the atoms located on comers, kinks and edges, whereas the access to face atoms is not affected. In fact, Garin et al. (ref. 26) pointed out that activity on Pt stepped surfaces can be correlated to density of defects. The fact that the interaction with the support seems higher when metal loading is high is comparable to what was observed on Pd/CeO2 catalysts (ref. 27).

CONCLUSION High and low metal content WU02 catalysts exhibit a relatively good dispersion (mean particle size 30 A) even though the specific area of the support is very small. Nevertheless, the catalytic activity of these catalysts is very low, in spite of

808

some improvement when 8 wt % Pt/U02 is calcined before reduction. These findings are certainly related to a metal-support interaction, but this interaction is different from the one observed on PVTiO2 catalysts. Further investigations, in particular on the possible role of chlorine or of the difference of reducibility of uranium oxides as a function of Pt content, are necessary to decipher the exact nature of the metal-support interaction in this system. AKNOWLEGEMENT We are grateful to the Commission of the European Community (Scientific Dpt.) for the award of a grant to M. Romeo, P. Wehrer for BET measurements and J.L. Schmitt for electron microscopy. We are also grateful to F. Garin for his illuminating advices on the matter. REFERENCES 1 2 3

4 5 6

S.J. Tauster, S.C. Fung and R.L. Garten, J. Amer. Chem. SOC.,100 (1978) 170. J.A. Horsle , J. Amer. Chem. SOC.,101 (1979) 2870. L. Wang, 8.W. Qiao, H.Q. Ye, K.H. Kuo and Y.X. Chen, 9th Inter. Congress on Catalysis, Calgary, Canada, Vol. 3, 1253. A. Dauscher, L. Hilaire, W. Muller and G. Maire, Surface Sci., in press. B.C. Beard and P.N. Ross, J. Phys. Chem., 90 (1986) 681 1. R.T.K. Baker, K.S. Kim, A.B. Emerson and J.A. Dumesic, J. Phys. Chem., 90 (1986) 860.

7 8 9 10 11 12 13 14 15 16 17

18 19 20 21 22 23 24 25 26 27

T. Sheng, X. Guoxing and W. Hongli, J. Catal., 11 1 (1988) 136. M.E. Levin, M. Salmeron, A.T. Bell and G.A. Somorjai, J. Catal., 106 (1987) 401. D.J. D er, S.D. Cameron and J. Gland, Surface Sci., 159 (1985) 430. D.N. Bxon, Y.M. Sun and J.M. White, J. Catal., 102 (1986) 338. S.J. Tauster and S.C. Fung, J. Catal., 55 (1978) 29. C. Corolleur, S. Corolleur and F.G. Gault, J. Catal., 24 (1972) 385. a F. Garin and F.G. Gault, J. Amer. Chem. SOC.,97 (1975) 4466. b{ F. Fajula and F.G. Gault, J. Amer. Chem. SOC.,98 (1976) 7690 J.J. Pireaux, J. Riga, E. Thibaut, C. Tenret-Noel, R. Caudano and J.J. Verbist, Chem. Phys., 22 (1977) 113. G.C. Allen, P.M. Tucker and J.W. Tyler, J. Phys. Chem., 86 (1982) 224. D. Chadwik, Chem. Phys. Lett., 21 (1973) 291. G.C. Allen, J.A. Crofts, M.T. Curtis,P.M. Tucker, D. Chadwick and P.J. Hampson, J. Chem. SOC.Dalton Trans., (1974) 1296. J.Verbist, J. Riga, J.J. Pireaux and R. Caudano, J. Electr. Spectrosc., 5 (1974) 193. G.C. Allen, P.M. Tucker and J.W. Tyler, Vacuum, 32 (1 982) 481. M. Romeo, PhD Thesis , Strasbourg, (1987). J.M. Dartigues, A. Chambellan, S. Corolleur, F.G. Gault, A. Renou rez, B. Moraweck, P. Bosch-Giraland G. Dalmai- Imelik, Nouv. J. Chimie, 3 (19795)591 A. Dauscher, F. Garin, F. Luck and G. Maire, Stud. Surf. Sci. and Catal., in B. lmelik et al. (Editors), Elsevier, Amsterdam, 11 (1 982) 11 3. F.G. Gault, Adv. Catal., 30 (1981) 1. M. Asomoza, G. Del Angel, R. Gomez, B. Rejai and R.D. Gonzalez, 9th Inter. Congress on Catalysis, Calgary, Canada, Vol. 3, 11 82. G. Agostini, M.J. Ledoux, L. Hilaire and G. Maire, IV Intern. Symp. Prep. of Catalysts, Louvain, F6.1 (1986). F. Garin, S. Aeiyach, P. Legare and G. Maire, J. Catal., 77 (1982) 323. L. Hilaire, K. Kili and F. le Normand, To be published.


809

ON THE REACTIVITY OF DIAMOND-LIKE SEMICONDUCTOR SURFACES N. RUSSO D i p a r t i m c n t o d i Chimica, U n i v e r s i t l d e l l a C a l a b r i a , 1-87030 Arcavacata d i Rende, Cosenza ( I t a l y )

ABSTRACT The a d s o r p t i o n and decomposition o f m o l e c u l a r oxygen and water on t h e ( 1 0 0 ) s u r f a c e o f d i a m o n d - l i k e c r y s t a l s ( S i and Gel have been s t u d i e d by s e m i e m p i r i c a l quantum c h e m i s t r y MNOO method i n t h e framework o f a c l u s t e r approach. The r e s u l t s show t h a t b o t h 0 and H 0, on t h e two k i n d s o f c r y s t a l s u r f a c e s , adsorb 2 b o t h m o l e c u l a r l y and d i s s o c i a t i v e l y . The d i s s o c i a t i v e process i s always f a v o u r e d f r o m a thermodynamic p o i n t o f view, b u t i n v o l v e s an a c t i v a t i o n energy i n agreement w i t h t h e e x p e r i m e n t a l evidence, INTRODUCTION The r e a c t i v i t y o f w e l l d e f i n e d t r a n s i t i o n metal s u r f a c e s has been t h e subject

o f s e v e r a l i n v e s t i g a t i o n s i n t h i s l a s t decade ( r e f s . 1,Z).

S i m i l a r s t u d i e s on

semiconductor s u r f a c e s a r e l e s s common. The most s t u d i e d r e a c t i o n s i n t h i s f i e l d a r e t h e i n i t i a l stages o f h o t and wet o x i d a t i o n o f d i a m o n d - l i k e c r y s t a l s ( r e f s . 3-29). Many o f t h e s e works concern t o t h e s i l i c o n s u r f a c e s ( r e f s . 3-25), w h i l e much l e s s i n f o r m a t i o n i s a v a i l a b l e f o r carbon ( r e f . 29) and germanium ( r e f s . 26-28). The n a t u r e o f t h e s e a d s o r p t i o n stages however s t i l l i s c o n t r o v e r s i a l , e.g.

f o r b o t h oxygen and water, t h e d i s s o c i a t i v e c h e m i s o r p t i o n has

been a l t e r n a t i v e l y d i s p r o v e d and supported ( r e f s . 3 - 2 5 ) . and H 0 w i t h c l u s t e r s s i m u l a t i n g 2 2 t h e c h a r a c t e r i s t i c c h e m i s o r p t i o n s i t e s o f ( 1 0 0 ) s u r f a c e s o f s i l i c o n and germaI n t h e p r e s e n t work, t h e i n t e r a c t i o n o f 0

nium i s s t u d i e d a t t h e same l e v e l o f t h e o r y , so t h a t , comparison between t h e r e a c t i v i t y o f d i f f e r e n t s u b s t r a t e s con be made. METHOD As a c o m p u t a t i o n a l t o o l t h e MNDO methos ( r e f . 30) has been employed u s i n g AMPAC package ( r e f . 31) implemented t o r u n on VAX/780 o f t h e Computer Center o f U n i v e r s i t y o f Calabria. As commonly r e p o r t e d i n l i t t e r a t u r e ( r e f . 32) embedding hydrogen t e r m i n a t o r s have been used. The c l u s t e r s used f o r t h e s i m u l a t i o n o f t h e t h r e e h i g h symmetry chemisorption s i t e s o f (100) surface o f diamond-like c r y s t a l s are X H 9 12' 'gH14

810 f o r on-top. bridge and open s i t e s r e s p e c t i v e l y . I n a d d i t i o n , t h e and X15H16 chemisorption o f atomic oxygen on both (100) and (111) surfaces o f C, S i and Ge has been studied. For the (111) surfaces t h e c l u s t e r s employed t o reproduce t h e on-top and open s i t e s are X

H 15 and X10H13

respectively.

The experimental bulk geometries have been chosen: t h e X-H bond ler,gths have been assumed as 1.10,

1.50 and 1.54

A

f o r C, S i and Ge r e s p e c t i v e l y .

A schematic p i c t u r e of chemisorption s i t e s f o r atomic, molecular and dissoc i a t i v e adsorption i s given i n F i g . 1.

A

C

*-4

D

F i g . 1. Schematic. p i c t u r e o f s i t e s and d e f i n i t i o n o f geometrical parameters f o r ( A ) chemisorption o f atomic oxygen on the (111) and ( 6 ) (100) surfaces o f C , S i and Ge; (C) molecular and (D) d i s s o c i a t i v e O2 and H20 chemisorption on S i and Ge (100) surfaces.

811

RESULTS AN0 DISCUSSION a) Atomic oxygen chemisorption. As a f i r s t step o f t h e work, we have examined t h e i n t e r a c t i o n o f atomic oxygen w i t h both (100) and (111) surfaces o f C, S i and Ge. The r e s u l t s are c o l l e c t e d i n t a b l e 1. TABLE 1 Geometric and e l e c t r o n i c parameters f o r t h e chemisorption o f atomic oxygen on t h e (100) and (111) surfaces o f Carbon, S i l i c o n and Germanium. For c l u s t e r s , s i t e s and d e f i n i t i o n s of parameters see Fig. 1. q0 i n d i c a t e s t h e oxygen n e t charge. Cluster

Site

d/W

R/i

BE/eV

qo/a.u.

(111) ‘1 OH15-o ‘ l O H l 3-o Si10H15-0

Si10H13-0 Ge10H15-0 Gel OH13-o

On-top

1.385

1.385

3.64

-0.225

Open

2.061

0.711

0.81

-0.301

On-top

1.582

1.582

3.25

-0.278

Open

2.323

0.671

0.61

-0.289

On-top

1.716

1.716

0.09

-0.731

/

/

0.00

0.000

Open

(100) On-top

1.220

1.220

8.02

-0.272

Bridge

1.397

0.606

5.78

-0.307

Open

1.831

0.781

0.18

-0.404

On-top

1.541

1.541

4.05

-0.586

S i H -0 9 12 S i ,5H16-0

Bridge

1.936

0.254

5.31

-0.840

Open

2.861

-0.901

0.98

-0.701

GegH14-0

On-top

1.592

1.592

5.07

-0.733

GegH12-0

Bridge

2.021

0.289

4.95

-0.817

/

/

0.00

0.000

‘gH14-’ ‘gH1 2-o ‘1 5H16-o SigH14-0

Ge15H16-0

Open

As t o t h e (111) surface, t h e on-top p o s i t i o n corresponds t o t h e absolute chemisorption minimum f o r a1 1 the t h r e e substrates. The binding energies decrease going from C t o Ge. The e q u i l i b r i u m bond lengths are c o r r e c t l y reproduced accord i n g t o t h e p e r i o d i c t a b l e . The i n t e r a c t i o n on t h e open s i t e i s endothermic i n t h e case o f carbon and s i l i c o n w h i l e no minimum i s found f o r t h e germanium surface. I n the case o f (100) surfaces, the r e s u l t s show t h a t t h e chemisorption beha-

812

v i o u r i s d i f f e r e n t f o r t h e t h r e e substrates considered. I n f a c t , i n t h e case o f carbon surface, t h e absolute minimum i s found f o r t h e on-top s i t u a t i o n , whil e , f o r s i l i c o n , f o r t h e b r i d g e one. F i n a l l y f o r germanium, t h e on-top and b r i d g e c o n f i g u r a t i o n s are e n e r g e t i c a l l y e q u i v a l e n t (5.07 versus 4.95 eV respect i v e l y ) . As a general trend, a l s o i n t h i s case we found t h a t t h e BE o f t h e abs o l u t e minima decreases going f r o m C (8.02 eV) t o S i (5.31 eV) and Ge (4.95 eV). For t h e s i l i c o n substrates an accurate MC-SCF c a l c u l a t i o n ( r e f . 9 ) on m i n i mal Si6H12-0 c l u s t e r i s a v a i l a b l e and a comparison w i t h our r e s u l t s i s possible. The MNDO value o f BE on t h e same minimal c l u s t e r ( r e f s . 10,111 i s 4.85 eV, w h i l e t h e MC-SCF value i s 4.25 eV. A good agreement i s a l s o found f o r t h e equil i b r i u m distance,

charge t r a n s f e r and chemisorption energy, f o r b o t h on-top

and b r i d g e s i t e s . A comparison w i t h experimental d a t a f o r S i (100) surface shows t h a t our c a l c u l a t i o n s c o r r e c t l y p r e d i c t t h e oxygen p r e f e r r e d chemisorpt i o n s i t e as revealed by t h e EELS spectra ( r e f . 5 ) . Our r e s u l t s i n d i c a t e some d i f f e r e n c e s i n t h e p r o p e r t i e s o f t h e substrates considered. The l a r g e discrepancy o f BE between on-top and b r i d g e s i t e s , i n t h e case o f C (100) (about 2.2 eV) suggests t h a t , a t low coverage, o n l y t h e on-top s i t e s a r e occupied. Instead, i n t h e case o f Ge ( l o o ) , t h e two values o f BE d i f f e r o n l y o f 0.1 eV and t h i s means t h a t , i n t h e same c o n d i t i o n s , t h e oxygen atoms can occupy both s i t e s and can d i f f u s e more e a s i l y on t h e surface. The BE values f o r S i (100) a r e i n t e r m e d i a t e b u t nearer t o those o f germanium. As i n t h e case o f t h e (111) surface, a l s o f o r t h e (100) one, t h e h i g h e r

c o o r d i n a t i o n s i t e s ( i .e. open) are e n e r g e t i c a l y unfavoured l e a d i n g t o endothermic energies o r t o thermoneutral i t y . chemisorption. 2 Table 2 c o l l e c t s t h e most s i g n i f i c a n t energetic, e l e c t r o n i c and geometrical

b) 0

parameters f o r molecular and d i s s o c i a t i v e chemisorption o f 0 on t h e (100) sur2 f a c e o f S i and Ge. As concerns t h e molecular process, d i f f e r e n t p o s s i b l e i n t e r a c t i o n s i t e s have been considered ( f o r t h e d e f i n i t i o n and d e s c r i p t i o n see r e f . 11). I n agreement w i t h t h e experimental i n d i c a t i o n s , we found t h a t t h e most s t a b l e s i t u a t i o n corresponds, both f o r s i l i c o n and germanium, t o an oxygen atom coordinated a t a b r i d g e p o s i t i o n (see F i g . 11, whereas t h e second protend i n g oxygen towards t h e vacuum, shows a n e g l i g i b l e i n t e r a c t i o n w i t h t h e surface.

813 TABLE 2 MNOO r e s u l t s f o r molecular and d i s s o c i a t i v e 0 chemisorption on (100) s u r f a c e of

2 s i l i c o n and germanium. The t r a n s i t i o n s t a t e i s r e f e r r e d t o t h e f o l l o w i n g p r o ( w i t h b o t h oxygen atoms 'on cess: X H -0 ( w i t h 0 on b r i d g e !-X,5H16-20 bridge)!5qi:~qo2 i n d i c a L t h e net charge onthe two oxygen atoms. Cluster

Site

dli

d0-i/,

Ahf/eV

alo

q 011a.u.

q 02 1a.u.

SILICON

si

15H16-'2 Si15H16-20

Bridge

1.928

1.297

4.81

147.5

-0.451

-0.455

Bridge

1.949

I

2.25

1

-0.803

-0.803

1.920

2.340

0.30

142.4

Transition state

I

I

GERMAN1UM

Gel 5H16-'2 Ge 5H 6-20

Bridge

2.011

1.295

26.48

148.6

-0.495

-0.499

Bridge

2.021

I

24.00

I

-0.822

-0.822

2.024

2.410

30.22

144.7

Transition state

I

/

s t a t e (MNDO) = 1.135 i (exp. = 1.226 i 9 AHf ( 0 ) (MNOO) = 2.58 eV ; AHf (02) (MNDO) = 0.53 eV

do-O

in A '

The valence a n g l e a i s found t o be about 150" f o r both t h e s u b s t r a t e s . A BE o f 4.5 and 2.3 eV r e s p e c t i v e l y f o r S i and Ge i n d i c a t e s t h e presence o f s i g n i f i c a n t bonding t h a t decreases going from s i l i c o n t o germanium. The e q u i l i b r i u m 0-0 bond l e n g t h (1.29

and 1.295

f o r S i and Ge r e s p e c t i v e l y versus 1.135

f o r t h e i s o l a t e d molecule) i s a f u r t h e r i n d i c a t i o n o f a s t a b l e molecular adsorp t i o n . The d i s s o c i a t i o n o f 0 can y i e l d d i f f e r e n t f i n a l products and t o p o l o g i c a l 2 s i t u a t i o n s . The most s t a b l e d i s p o s i t i o n i s t h a t i n which t h e two oxygen atoms c o o r d i n a t e a t neighbouring b r i d g e s i t e s . The s t a b i l i t y o f t h i s arrangement w i t h r e s p e c t t o t h e corresponding molecular i n t e r a c t i o n , i s found t o be 2.56 and 2.48 eV f o r S i and Ge r e s p e c t i v e l y . Other s i g n i f i c a n t evidence on t h e r e a c t i v i t y o f diamond-like c r y s t a l surfaces can come from t h e computation o f t h e a c t i v a t i o n energies i n t h e d i s s o c i a t i v e processes and f r o m t h e l o c a l i z a t i o n o f t h e t r a n s i t i o n s t a t e s t r u c t u r e s . Our r e s u l t s show t h a t t h e a c t i v a t i o n energy f o r O2 d i s s o c i a t i o n i s o f t h e same o r d e r i n t h e two considered surfaces (3.49 and 3.74 eV r e s p e c t i v e l y f o r S i and Gel. F i n a l l y , i f we consider t h a t t h e h a l f o f b i n d i n g energy o f t h e 0 molecule i s 2

814

2.6 eV and t h a t t h e

BE found i n our c a l c u l a t i o n s are 5.31 eV ( f o r S i ) and 4.95

eV ( f o r Gel, we can hypotize t h a t t h e (100) surface o f S i and Ge are able t o promote t h e 0 d i s s o c i a t i o n . 2 c ) H20 chemisorption. Very recent HREELS ( r e f . 27)

AES and UPS (refs. 26,281 studies o f water on

Ge(100) surface support t h e d i s s o c i a t i v e nature o f t h e i n t e r a c t i o n i n t h e temperature range between 340 and 300 K w h i l e t h e molecular form i s present befor e heating o f the sample. I n t h e case of water adsorption on t h e S i (100) surface, d i s s o c i a t i o n , together w i t h molecular i n t e r a c t i o n , has been observed a t room temperature ( r e f s . 12-22). TABLE 3 MNDO r e s u l t s f o r molecular and d i s s o c i a t i v e chemisorption of water on t h e (100) surface o f s i l i c o n and germanium. The t r a n s i t i o n s t a t e i s r e f e r r e d t o t h e f o l lowing process: X H -H 0 ( w i t h H20 i n bridge)-X H -H, OH ( w i t h H on-top 15 16 2 15 16 and OH on b r i d g e ) . Cluster

Site

d/a

S i 15H16-H20

Bridge 2.035

dO-H/A

a/"

AHf/eV

dX-H/i

qo/a.u.

qH/a.u.

SILICON 0.962

S i 5H 6-H OH

Ch-top 1.960 0.945 Bridge Transition state 1.986 1.450a

127.4

4.36

/

-0.162

0.258

157.7

3.41

1.421

-0.525

-0.228

150.6

6.88

/

/

/

GERMAN1UM Ge15H16-H20 Ge15H16-H,0H

Bridge 2.243 0.959

127.0

25.66

/

-0.197

0.256

Ch-top 2.062 0.963

142.0

23.87

1.489

-0.701

-0.311

145.3

28.46

/

/

W-i dge

T r a n s i t i o n State

2.091 1.46Za

/

a) OH-H bond distance We have thus considered i n t e r e s t i n g t o study water adsorption on both S i and Ge (100) surfaces i n order t o compare t h e i n t e r a c t i o n mechanism. Our data (Table 3 ) show t h a t both kinds of adsorption are exthotermic but t h e d i s s o c i a t i o n i s more favoured, i n agreement w i t h experimental evidence ( r e f s . 12-22]. The exothermicity i s s l i g t h l y higher for germanium (about 1.6 eV) than for s i l i c o n (about 0.9 eV). The a c t i v a t i o n energies f o r the d i s s o c i a t i o n are p r a c t i c a l l y t h e same (about 2.5 and 2.8 eV f o r S i and Ge r e s p e c t i v e l y ) . These data can explain, from a thermodynamic p o i n t o f v i e w t h e d i f f e r e n t extent o f

815

water

d i s s o c i a t i o n on t h e two surfaces found experimentally(refs. 12-19,26-28).

I f we consider t h a t t h e MNDO a c t i v a t i o n energy f o r the d i s s o c i a t i o n o f f r e e

water i s about 5.0 eV, our r e s u l t s i n d i c a t e t h a t both S i and Ge (100) surfaces should a c t u a l l y promote water d i s s o c i a t i o n , even a t room temperature, i n agreement w i t h t h e experimental data ( r e f s . 12-19,26-28).

The most stable products

o f t h e d i s s o c i a t i v e chemisorption are found t o be H and OH coordinated on on-top and on bridge s i t e s r e s p e c t i v e l y both f o r S i and Ge surfaces. As i n t h e case of adsorption (see Table 21, t h e geometrical parameters o f the t r a n s i t i o n s t a t e 2 show a very s i m i l a r t r e n d e x p e c i a l l y f o r t h e OH-surface bond angles ( a 1 (150.6'

0

and 142.0" f o r S i

and Ge r e s p e c t i v e l y ) .

F i n a l l y we note t h a t , also f o r t h e molecular process, t h e behaviour on t h e two surfaces i s s i m i l a r . I n f a c t , i n both cases, t h e p r e f e r r e d chemisorption s i t e i s found t o be t h e bridge and t h e values o f n e t charges do not d i f f e r s i g n i f i c a n t l y t a k i n g i n t o account t h e d i f f e r e n t e l e c t r o n e g a t i v i t y o f S i and Ge. Furthermore, t h e a angles are q u i t e s i m i l a r and t h e e q u i l i b r i u m X-0 distances are consistent w i t h t h e p e r i o d i c properties. CONCLUSIONS On t h e basis o f t h e above r e s u l t s , i t i s concluded t h a t : i ) The S i and Ge (100) surfaces are able t o promote 0

2

and H20 d i s s o c i a t i o n ;

ii)The behaviour o f the two surfaces w i t h respect t o O2 and H20 i s b a s i c a l l y

s i m i l a r ; iii)For both adsorbates on S i and Ge (100) surfaces, both molecular and d i s s o c i a t i v e chemisorption are exothermic; i v ) I n t h e case o f molecular ads o r p t i o n t h e preferred s i t e i s always the bridge one

: v)

The OH and H disso-

c i a t i o n products l i e a t bridge and on t o p s i t e s r e s p e c t i v e l y on both substrates. REFERENCES 1 D.A. King and D.P. Woodruff, The Chemical Physics o f S o l i d Surfaces and Heterogeneous Catalysis, Vol. 1-4, Elsevier, Amsterdam, 1984. 2 R.P.H. Gasser, An I n t r o d u c t i o n t o Chemisorption and Catalysis by Metals, Clorendon, Oxford, 2nt ed., 1987. 3 C.Y. Su, P.R. Skeath, I.Lindon and W.E. Spencer, J. Vacuum Sci. Thecnol., 18 ( 1981 1 843-846. 4 T. Kunjunny and D.K. Ferry, Phys. Rev. 824 (1981) 4604-4608. 5 H. Ibach, H.D. Bruchmann and H. Wagner, Appl. Phys., A29 (1982) 113-124. 6 E.G. Keim, Surface Sci .,148 (1984) L641-L643. 7 A . Redondo, W.A. Goddard 111, C.A. Stewarts and T.C. Mc G i l l , J . Vacuum Sci. Technol., 19 (1981 1 498-503. 8 S . C i r a c i , S. E l l i a l t o g l u and S . Erkoc, Phys. Rev. 826 (1982) 5716-5720. 9 I . P . Batra, P.S. 8agus and K. Hermann, Phys. Rev. L e t t . , 52 (1984) 384-386. 10 N. Russo, M. Toscano, V. Barone and F. L e l j , Phys. L e t t . 113A (1985) 321-323. 11 V . Barone, F. L e l j , N. Russo and M. Toscano, Surface Sci., 162 (1985) 230-238.

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12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

K. Fujiwara, Surface Sci., 108 (1981) 124-128; J.Chem.Phys.,75(1981)5172-5178.' 0. Schmeisser, Surface Sci., 137 (1984) 197-206. H. Ibach, H. Wagner and H. 0. Bruchmann, S o l i d S t a t e Commun., 42 (1982) 457-460. F. S t u k i , J. anderson, G.J. Lapeyre and H.H. Farrel, Surface Sci., 143 (1984) 84-89. Y.J. Chabal, Phys. Rev., 829 (1984) 3677-3681. Y.J. Chabal and S.B. Christmann, Phys. Rev., 829 (1984 6974-6981. E.M. O e l l i g , R. Butz? H. Wagner and H. Ibach, Solid State Carnun., 51 (1984) 7-10. W. Ranke and D. Schmeisser, Surface Sci., 149 (1985) 485-493. S . C i r a c i and H. Wagner, Phys. Rev., 827 (1983) 5180-5185. N. Russo, M.Toscano, V . Barone and F. L e l j , Surface Sci., 180 (1987) 599-604. V. Barone, F. L e l j , N. Russo and M. Toscano, J. Chim. Phys., 84 (1987) 799-803. P. Morgen, W. Wurth and E . Umbach, Surface Sci., 152/153 (1985) 1086-1095. C. Hofer, P. Morgen, W. Wurth and H. Umbech, Phys. Rev. L e t t , 55 (1985) 2979-2982. L. I n c o c c i a , A. Balerna, C. Cram, C. Kunz, F. Senf and I. Storjohann, Surface Sci., 189/190 (1987) 453-458. J . Kuhr and W. Ranke, Surface Sci., 189/190 (1987) 420-425. L. Papagno, L.S. Caputi, 0. Frankel, Y. Chen and G.J. Lapeyre, Surface Sci., 189/190 (1987) 199-603; H.J. Kuhr and W. Ranke, Surface Sci., 187 (1987) 98-111. P. Bazdiag and S. Werwoerd, Surface Sci., 183 (1987) 469-483. M.J.S. Oewar and W. T h i e l , J . Am. Chem. SOC., 99 (1977) 4899-4907. J.J.P. Stewart, Quantum Chem. Exch. (QCPE), no 506 (1987). V. Barone, F. L e l j , N. Russo, M. Toscano, F. I l l a s and J. Rubio, Phys. Rev., 834 ( 1986) 7203-7208.

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817

HYDROGENATION OF CARBON MONOXIDE OVER AN Ru(OOO1) SINGLE CRYSTAL SURFACE

B. SAKAKINI, B. STEEPLES, N. " H I L L

AND J.C. VICKEFfMAN

Surface Analysis Research Centre, Department of Chemistry, University of Manchester I n s t i t u t e of Science and Technology, Manchester M60 lQD, UK

ABSTRACT Hydrogenation of carbon monoxide over Ru( 0001) single crystal has been studied. A combination of a surface analysis UHV system and an a m s p h e r i c H i g h resolution electron energy l o s s spectroscopy chamber has been used. (EEIS) and s t a t i c secondary mss spectrometry (SSIMS) were used t o monitor the state of the surface a f t e r reaction. I n EELS spectra vibrational losses attributable t o CH and C-C are clearly evident, consistent with the hydrogenation of sur9ace carbon. SSIMS results, apart from confirming EELS findings gave conclusive evidence that during reaction a surface carbide intermediate is involved i n chain growth leading to higher hydrocarbons. Experiments using a carbon pre-covered Ru(OOO1) surface i n pure hydrogen atmosphere is a l s o reported.

INTRODUCTION

Recent i n t e r e s t i n produciw fuels from coal has prompted new attention t o the surface reactions involved i n Fisher-Tropsch synthesis (nS) of hydrocarbons from hydrogen and carbon monoxide. The mechanism (or mechanisms) of t h e Fischer-Tropsch s y n t h e s i s and t h e methanation r e a c t i o n remain a controversial topic today ( r e f s 1-5). Euthenium Is an imprtant active catalyst i n Fischer-Tropsch synthesis (refs 7, 8). Some recent studies suggest t h a t synthesis of hydrocarbons over ruthenium involves the dissociation of GO followed by hydrogenation of the resulting carbon and polymerisation of various hydrocarbon intermediates. However, t h e chemical i d e n t i t y of the hydrocarbon i n t e r m e d i a t e s and t h e mechanism of chain growth is s t i l l f a r from being resolved. Absorption bands due t o C-H s t r e t c h modes indicating the presence of hydrocarbon intermediates have been observed i n infrared studies. Hydrocarbon e n t i t i e s of stoichimetry CHx and CnHx have been proposed as intermediates i n Fische-Tropsch synthesis (refs 9 , 10). According t o Biloen e t al. (ref 1) the active hydrocarbon intermediates consist of a mixture of CH, CH2 and C H ~species in addition to polymeric hydrocarbons. B e l l e t al. (refs 5, 8) reported evidence f o r the presence of CH2 species and swested that hydrocarbon chain growth occurs by

818

successive addition of such methylene intermediates. Determination of the s u r f a c e composition of t h e working c a t a l y s t by surface analytical techniques is an essential requirement f o r elucidation of the mechanism of the FTS and the methanation reaction. For t h i s purpose a combination of an u l t r a high vacuum (UHV) analysis chamber and atmospheric reaction chamber has proved t o be useful (refs 11-13). Such systems provides the possibility of conducting both high-pressure kinetic measurements and UHV surface characterisation without removing the sample from the controlled atmosphere. I n t h e present study both EELS and SSIMS were used t o i d e n t i f y t h e hydrocarbon surface species a f t e r the hydrogenatlon of carbon monoxide over an Ru(OOO1) single crystal. The complementary nature of these two powerful techniques i n moniterlng surface hydrocarbon intermediates i n ethene decomposition has been illustrated by us i n previous studies (refs 14, 15). EXPERIMENTAL

The apparatus used i n the present study Is schematically shown i n Fig 1. It consists of a stainless steel UHV chamber combined with reaction chamber which can be i s o l a t e d from t h e UHV chamber by a g a t e valve. A d e t a i l e d description of the UHV system and the cleanlng procedure of the Ru(OOO1) single crystal is published elsewhere (ref 15). After the transfer of the sample into the reaction position and closure of' the gate valve the gaseous mixture H2/C0 and A r w a s admitted t o the reaction c e l l a t a total pressure of 1 atm, and the adjusted flow rates. Cooling of the high pressure c e l l during the reaction was achieved by passing cold water through a loop surrounding the cell. This was a necessary precaution i n order to limit the heterogeneous reaction to the heated Ru(OOO1) single crystal. Reaction products were analysed with gas chromatography usflame-ionisation detection. For the separation of products a Poropak Q column has been used. Analysis of the surface after reaction was carried out by interruptlng the reaction by stopping the H2, CO stream, turning off the sample heating and leaving only Ar passing into the high pressure cell. The sample was moved t o a differentially pmped section where the pressure reached t o r r range i n few minutes, after which the gate valve w a s opened and to the sample was moved t o the UHV chamber where surface analysis was carried out. The t r a n s f e r time from t h e r e a c t o r i n t o t h e UHV a n a l y s i s p o s i t i o n was around 10 minutes. In sme experiments the h surface was covered with carbon by exposing the Ru surface a t 780 K t o ethene at 5 x loe7 torr f o r 20 seconds in the Iw chamber. The crystal w a s then transferred t o the high pressure c e l l and

819

Lwcl Optics

Gas Mixture

yi+ “);“ic

Ion Gun Copper Source . I.

m

Diff usI Pump

..- .. J

EELS Spectrometer

,

Cell

L

Pumping

.Fig. 1 Schematic diagram of the UHV apparatus used

exposed t o a H2/Ar mixture with the r a t i o 6:1, during which the crystal was maintained at 580 K f o r 20 minutes. The crystal was then transferred t o the UHV chamber for surface analysis. SSIMS spectra were obtained using a 2 keV, + 6 x lo-’’ A Ar beam. EELS spectra were recorded on specular using a 5 eV beam. RESULTS AND DISCUSSION

Kinetic Study Studies of FTS from CO and H2 on h(1000) single crystal were carried out a t a t o t a l pressure of 1 atm and C O B 2 r a t i o of 1314. The temperature range investigated w a s 493-573 K. Methane was the dominant product, small amounts of ethene and ethane were produced. The kinetic data presented were obtained under steady state reaction conditions and at low conversion (typically ,

n o t t o o much a t t e n t i o n h a s

b e e n p a i d t o t h e v a r i a t i o n i n t h e CO b a n d s h a p e ( r e f . a l l y under coadsorption c o n d i t i o n s .

2 9 ) , especi-

846 METHOO The 5 % P t / S i 0 2 , grade M-5)

p r e p a r e d by i m p r e g n a t i o n o f Cab-0-Sil

w i t h a s o l u t i o n o f H2PtC16, t h e g a s e s ( C O ,

transmission i n f r a r e d (IR) l,)-dioxane

(Hungary),

and 1,4-dioxane

The d i e t h y l e t h e r ,

were p r o d u c t s o f Reanal

oxane, oxepane and 1 , 3 - d i o x o l a n e

1,3-dioxepane

the

t e c h n i q u e and t h e p r o c e d u r e used i n t h i s

s t u d y were d e s c r i b e d e a r l i e r ( r e f s 23, 24). oxolane,

(BOH,

02, H2),

were f r o m F l u k a , and

was a p r o d u c t o f E a s t m a n O r g a n i c C h e m i c a l s .

T a b l e 1 g i v e s t h e n o t a t i o n s u s e d for t h e c o a d s o r b e d s u b s t a n c e s . A c c o r d i n g t o t h e O2 and CO c h e m i s o r p t i o n measurements, t h e p e r c e n t a g e o f e x p o s e d P t was 0 . 2 1 ( r e f s 2 3 , 2 4 ) . On a p r e t r e a t e d , r e d u c e d s e l f - s u p p o r t i n g 5 % P t / S i 0 2 s a m p l e , C O was f i r s t a d s o r b e d a t 298 K u p t o B C O ( a v e r a g e ) . v 0 . 3

( c a l c u l a t e d on

t h e b a s i s o f t h e CO absorbance r e l a t i v e t o t h a t o b t a i n e d a t s a t u r a t i o n CO c o v e r a g e ) .

CO,

CO(A)

( r e f s 23,

T h i s t y p e o f CO was e a r l i e r c a l l e d " a d s o r p t i v e " 25).

F o l l o w i n g e v a c u a t i o n f o r 5 min, a n EOC

was s t e p w i s e l y a f t e r - a d s o r b e d a t 298 K .

A f t e r t h e admission o f each

d o s e , a w a i t i n g p e r i o d o f 5 min was a l l o w e d , was t h e n r e c o r d e d . (9.29 x t o o (5,

and t h e I R s p e c t r u m

I n t h e case o f t h e l a s t dose

molecules/g P t ) ] , 15 and 30 min),

[lo Torr

t h e a d s o r p t i o n was f o l l o w e d i n t i m e

w i t h subsequent d e s o r p t i o n under c o n s t a n t

e v a c u a t i o n a t 298 K a n d t h e n a t h i g h e r t e m p e r a t u r e s . Some e x p e r i m e n t s w e r e a l s o d o n e w i t h " d e s o r p t i v e "

CO,

CO(0)

( r e f s 2 3 , 2 5 1 , when t h e p r e a d s o r b e d C O was p r o d u c e d b y p a r t i a l t h e r m a l d e s o r p t i o n a t 5 7 3 K f r o m B C O ( s a t u r a t e d ) ( a t 273 K )

to

B c O ~ O . 3( a c c o r d i n g t o t h e r e l a t i v e C O a b s o r b a n c e s ) . RESULTS a n d DISCUSSION Changes i n V C O ( A ) Some c h a r a c t e r i s t i c r e s u l t s o n t h e v a r i a t i o n i n i n T a b l e 1. I t c a n b e s e e n t h a t for C O ( A ) s o r p t i o n p r o c e d u r e (ECP) ( 2 9 8 K , high,

t h e compounds

y(C0)

( 3 9 - 4 4 cm-')

[u] clearly

10 T o r r ,

V(C0)

are given

a t t h e end o f t h e coad3 0 m i n ) , when BEDC was

caused l a r g e r r e d s h i f t s i n

than d i d t h e group

[H] ( 2 9 - 3 2

cm-').

Some b l a n k e x p e r i m e n t s w e r e a l s o c a r r i e d o u t and,

i n contrast

w i t h t h e r e s u l t s i n T a b l e 1, b u t i n a g r e e m e n t w i t h t h e f i n d i n g o f o t h e r s ( r e f . 1 9 1 , t h e r e was no o b s e r v a b l e AY(C0)

s h i f t during the

c o a d s o r p t i o n o f t h e c o r r e s p o n d i n g 0-free s a t u r a t e d h y d r o c a r b o n s . T h i s r e v e a l s t h a t t h e 0 - c o n t a i n i n g m o l e c u l e i s adsorbed on P t t h r o u g h t h e 0 atom,

or more e x a c t l y , t h e a d s o r b e d s p e c i e s s h o u l d

TABLE 1 The measured red shifts in Y(C0) during the coadsorption and the subsequent evacuation (desorption) in the case of pread orbed "adsorptive" CO, CO(A), on 5% Pt/Cab-0-Sil (sample 1) (average) N 0.3 according to the relativelCO absorbanc~~co('I)ICool without coadsorption was 2 0 6 8 to 2 0 7 0 cm- 1. I

Coadsorbed compounds in the sequence of IP(a)

0 0 6633 -

oxepane

7(1) 10 Torr, 30 min, 298 K Evac., 15 min, 298 K Evac., 5 min, 373 K

(Table 1).

[A)

[HI,

12-01

848

T h i s draws a t t e n t i o n t o t h e f a c t t h a t t h e molecules

c a n be

adsorbed on P t t h r o u g h two 0 atoms. The l o c a l C O c o n c e n t r a t i o n o n P t i s r e l a t i v e l y h i g h f o r C O ( A ) a n d l o w f o r CO(0)

( r e f s 23,

25).

As e x p e c t e d ,

i n accordance with

t h e r e s u l t s o f o t h e r r e s e a r c h w o r k e r s ( r e f s 12-14,

16-18),

depended s t r o n g l y on B C O ( l o c a l ) ; t h e red s h i f t i n

Y(C0)

h i g h e r f o r CO(0)

than f o r CO(A)

AY(C0)

was much

(Table 2).

TABLE 2 C o m p a r a t i v e c o a d s o r p t i o n e x p e r i m e n t s a t 298 K o n 5 % P t / C a b - 0 - S i l ( s a m p l e 2 ) i n t h e c a s e s o f C O ( A ) a n d CO(0)

P r e a d s o r b e d CO

stages o f coadsorption. T a b l e s 1 and 2 l e a d t o t h e f o l l o w i n g f i n d i n g s :

( a ) A t t h e end

YEO(AI

o f t h e c o a d s o r p t i o n e x p e r i m e n t s (10 T o r r , 3 0 m i n , 298 K )

was s h i f t e d f r o m c a . 2 0 6 8 - 2 0 7 0 c m - l t o l o w e r f r e q u e n c i e s ( b y 2 9 - 3 2

or 3 9 - 4 4 cm-l) t h a n t h e o r i g i n a l y[CO(O]

(H2048-2050 c m - I ) .

was s h i f t e d b y t h e c o a d s o r b e d compounds

v[CO(A)] cm-', and t o even l o w e r v a l u e s f o r

CO(0).

[E]t o

(b) 2025-2030

These f r e q u e n c i e s were

much l o w e r t h a n t h e s i n g l e t o n f r e q u e n c y o f CO on 5% P t / C a b - 0 - S i l ( 2 0 3 5 - 2 0 3 7 cm-').

T h i s l a t t e r was c o n c l u d e d f r o m t h e t h e r m a l

d e s o r p t i o n r e s u l t s i n F i g . 1 and i n r e f . 23, where t h e f r e q u e n c y (ca.

2037 c m - l )

obtained by "the d i l u t i o n l i m i t " using t h e i s o t o p i c

d i l u t i o n method f o r P t / S i O

was v e r y s i m i l a r ( r e f .

frequency d i f f e r e n c e y[CO(Ai

- vEO(01

o f c o a d s o r b e d compound

for t h e compounds ( 2 0 5 4 - 2 0 6 0 cm-'

160, for

[el,

for

22).

( c ) The

increased with t h e presence

T a b l e 21. ( d ) E s p e c i a l l y was h i g h e r b u t f o r [H] t o o , V[CO(A)I a n d 2 0 4 9 - 2 0 5 9 c m - l f o r @]) d u r i n g example,

t h e t h e r m a l d e s o r p t i o n ( e v a c u a t i o n ) p r o c e d u r e even a t 3 7 3 K t h a n the original

y[CO(O8

,

i . e . 2 0 4 8 - 2 0 5 0 cm-1.

I t s h o u l d be n o t e d t h a t

849

a t 373 K , a n d e v e n m o r e s o a t h i g h e r t e m p e r a t u r e s ,

not only the

desorption o f t h e 0-containing molecule, b u t a l s o t h e rearrangement

25).

and p a r t i a l d e s o r p t i o n o f CO t a k e p l a c e ( r e f . act i n the opposite direction, i.e.

they decrease

any c o a d s o r b e d m o l e c u l e i n a b l a n k e x p e r i m e n t , Y(C0) 23,

f o r CO(A)

a t 373 K was a b o u t - 5 c m - l ,

These p r o c e s s e s Y(C0).

Without

this red shift in

s i m i l a r l y a s i n ref.

decreased r a p i d l y with i n c r e a s i n g temperature.

a n d Y(C0)

present coadsorption experiments,

V(C0)

I n our

might decrease b y about

4 - 5 c m - l u p t o 373 K i n t h e d e s o r p t i o n p r o c e d u r e . The a b o v e r e s u l t s s t r o n g l y s u g g e s t t h a t t h e c o a d s o r p t i o n - i n d u c e d red shifts

a t h i g h BEDC a r e d u e p r i m a r i l y t o t h e i n c r e a s e d

AY(C0)

e l e c t r o n b a c k - d o n a t i o n ( t o t h e 25f not t o the d i l u t i o n effect,

i.e.

molecular o r b i t a l s o f CO),

and

n o t t o t h e decrease caused i n t h e

i n t e r a c t i o n s (ref. 4 ) b y t h e i n s e r t i o n o f t h e c o a d -

dipole-dipole

s o r b e d m o l e c u l e s between t h e CO d i p o l e s , as suggested by o t h e r s (ref.

22) i n s i m i l a r coadsorption experiments.

V a r i a t i o n s i n i n t e n s i t y and shape o f CO band

(i) Thermal d e s o r p t i o n o f C O ( A ) .

When C O p r e v i o u s l y a d s o r b e d

a t 2 9 8 K u p t o a b o u t B C O ( a v e r a g e ) H 0 . 3 was d e s o r b e d i n vacuum b y heat treatment, sorbance ACO, t h e CO band

Y(C0)

i.e.

w h i l e t h e CO a b -

t h e i n t e g r a t e d a b s o r b a n c e Bco a n d t h e h a l f - w i d t h

aVl,,

c h a n g e d a c c o r d i n g t o maximum c u r v e s ( F i g .

I n t h i s experiment, compound,

decreased continuously,

CO(A)

of

1).

was f r e e f r o m a n y c o a d s o r b e d o r g a n i c

€IEDC was 0 .

The r e s u l t s i n F i g . 1 a r e i n a g r e e m e n t w i t h t h e e a r l i e r o n e s (ref.

23):

t h e e x t i n c t i o n c o e f f i c i e n t o f CO i n c r e a s e d , w h i l e t h e

CO b a n d b r o a d e n e d a n d

V(CO> decreased. S i m i l a r l y , i n experiments

o n C O a d s o r p t i o n o n R u ( 0 0 1 ) a t 8 0 a n d 200 K P f n u r e t a l .

(ref.

31)

f o u n d t h a t B c o d i s p l a y e d a maximum c u r v e a s a f u n c t i o n o f Bc0 (maximum a t c a . B C O ~ 0 . 3 3 ) . A l t h o u g h E C O ( a v e r a g e ) ( i n i t i a l ) was c a . 0 . 3 i n o u r e x p e r i m e n t , i n t h e case o f CO(A)

t h e v a l u e o f B C O ( l o c a l ) ( i n i t i a l ) was much

l a r g e r t h a n 0.3 ( a t l e a s t i n t h e o u t e r p a r t o f t h e s e l f - s u p p o r t i n g sample)

( r e f s 12, 25).

Hence, a s a r e s u l t o f s u r f a c e m i g r a t i o n , re-

a r r a n g e m e n t , d e s o r p t i o n a n d r e a d s o r p t i o n o f CO (refs 2 3 , 2 5 1 , average CO-CO

distance increased (the dipole-dipole

the

interaction

decreased) on h e a t t r e a t m e n t and t h e magnitude o f t h e e l e c t r o n d o n a t i o n f r o m t h e P t d o r b i t a l t o t h e 2$

antibonding o r b i t a l o f

t h e CO presumably increased. I n accordance w i t h o t h e r s ( r e f .

31),

850

"VZ

Icml 30 *

273 473 Temperature IK] F i g . 1. ( L e f t ) V a r i a t i o n s i n a b s o r b a n c e A C O , i n t e g r a t e d a b s o r b a n c e Bco and h a l f - w i d t h during thermal desorption i n t h e case o f C O ( A ) on samplk/2 1 w i t h o u t c o a d s o r p t i o n . F i g . 2. ( R i g h t ) V a r i a t i o n s i n A B and o f i n f r a r e d abs o r p t i o n b a n d o f C O ( A ) on samplE"1 CoduringAv1/2interaction w i t h compound 6 ( 1 , 3 ) c o a d s o r b e d a t 2 9 8 K a n d s u b s e q u e n t e v a c u a t i o n (thermal desorption). i t was c o n s i d e r e d t h a t t h e t r a n s i t i o n a l i n c r e a s e s i n B C O a n d A C O a t t h e beginning o f t h e thermal desorption o f CO(A)

(Fig.

1)

s h o u l d be a t t r i b u t e d m a i n l y t o t h e decreased d i p o l e - d i p o l e c o u p l i n g s between t h e adsorbed CO molecules.

(ii) C o a d s o r p t i o n w i t h C O ( A ) . On t h e e x a m p l e o f 6 ( 1 , 3 ) , i t c a n b e s e e n i n F i g . 2 t h a t d u r i n g t h e c o a d s o r p t i o n a t l o w e r EEoC ( s t a g e 1) B c o i n c r e a s e d b y a b o u t 1 0 % ( c o m p a r e d t o t h e o r i g i n a l coadsorption-free

Bco , o ) ,

w h i l e t h e CO band broadened (

A),/,

in-

dVll2d u r i n g e v a c u a t i o n ( t h e r m o d e s o r p t i o n ) up t o 373 K ( s t a g e s 3 and 4 i n F i g . creased).

S i m i l a r f e a t u r e s were o b s e r v e d f o r BCO and

2). CO(A)

e x i s t s i n i s l a n d s o n t h e P t s u r f a c e (refs 2 3 ,

251,

and i n

t h e s t a g e s w i t h l o w EEoC a b o v e , t h e a d s o r b e d EOC l o c a t e d m a i n l y on t h e empty P t atoms a r o u n d t h e CO p a t c h e s , w h i c h r e s u l t e d i n asymmetric m o l e c u l a r e n v i r o n m e n t s a r o u n d t h e CO m o l e c u l e s . I n

851

Y(C0) a c c o r d a n c e w i t h t h e d - 2 g b a c k - b o n d i n g t h e o r y (ref. l), decreased. sequently,

However, t h e e f f e c t had t o d e c r e a s e w i t h d i s t a n c e ;

con-

on p r o c e e d i n g t o w a r d s t h e i n t e r i o r o f t h e CO i s l a n d s ,

t h e f r e q u e n c i e s o f t h e a d j a c e n t CO m o l e c u l e s d i f f e r e d f r o m one a n o t h e r t o a g r e a t e r e x t e n t i n t h e s t a g e s w i t h l o w e r BEoC t h a n i n t h e o t h e r c a s e s . These l o c a l v a r i a t i o n s i n d i p o l a r d e c o u p l i n g s between t h e CO molecules, C O b a n d w i t h an e n h a n c e d

Y ( C 0 ) caused l o c a l r e s u l t i n g i n a wide

Bco.

W i t h i n c r e a s i n g EEDC ( s t a g e 2 1 , t h e l o c a l C O f r e q u e n c i e s came

n e a r e r t o each o t h e r and t h e d e c o u p l i n g s decreased between t h e CO dipoles.

F o r t h i s r e a s o n t h e CO band narrowed and

while the red s h i f t i n

Y(C0)

Bco decreased,

was e n h a n c e d . A t h i g h e r EEoC ( i n

t h e s e c o n d p a r t o f s t a g e 2 i n F i g . 2 1 , some o t h e r e f f e c t s a l s o became i m p o r t a n t . e f f e c t ( r e f s 7, While

As a c o n s e q u e n c e o f t h e i n c r e a s e d s h i e l d i n g

31),

the values o f

Bco

and ACO f e l l s t i l l more.

Bco i n c r e a s e d i n s t a g e 4 ( F i g . 2 ) w i t h d e s o r b i n g E D C , i n

c o n t r a s t w i t h s t a g e 1,

V(C0)

s t i l l i n c r e a s e d and

A))ll2

decreased.

T h i s c o u l d be i n t e r p r e t e d as a consequence o f t h e s u p p o s i t i o n t h a t a c e r t a i n number o f d o n o r m o l e c u l e s w e r e a d s o r b e d i n s t a g e 2 by i n c o r p o r a t i o n between t h e CO m o l e c u l e s ,

thereby d i s r u p t i n g

t h e C O i s l a n d s o n P t i n t o s m a l l e r p a t c h e s o r / a n d t i l t i n g away t h e a x e s o f some C O m o l e c u l e s f r o m t h e o r i g i n a l ( n o r m a l ) d i r e c t i o n .

[At

t h e e n d o f s t a g e 3 , some C O m o l e c u l e s m u s t s t i l l r e m a i n o n

t h e P t because I n stage 5, eratures

V ( C 0 ) i n c r e a s e d i n s t a g e 4.1

Bco and A C O d e c r e a s e d ( F i g . 2 ) and a t h i g h e r temp-

v ( C 0 ) s h i f t e d again t o lower frequencies,

which were

a t t r i b u t e d t o t h e i n t e r - and i n t r a p a r t i c u l a r d i f f u s i o n o f CO on t h e s u r f a c e s o f P t c r y s t a l l i t e s and t o t h e t h e r m a l d e s o r p t i o n o f

CO. T h e s e phenomena w e r e d i s c u s s e d a b o v e i n s e c t i o n ( i ) a n d e a r l i e r i n r e f s 23 a n d 2 5 .

(iii) C o a d s o r p t i o n w i t h CO(0) with the findings f o r CO(A),

ning of coadsorption o f

I n contrast

i n t h e case o f CO(D),

60, BCO

( w i t h o u t a n y maximum) w h i l e

( e f f e c t o f Eco,.

byl/,

a t t h e begin-

and ACO decreased c o n s i d e r a b l y increased greatly.

These w e r e

t h e t e n d e n c i e s t h r o u g h o u t t h e e n t i r e c o a d s o r p t i o n p r o c e s s . On evacuation,

Bco a n d A C O i n c r e a s e d w h i l e

Aj/l,2

decreased, b u t the

i n i t i a l v a l u e s were n o t r e c o v e r e d even a t 423-473 K . I n t h e case o f CO(D), CO(A)

(ref. 2 5 ) ;

E C O ( l o c a l ) was much s m a l l e r t h a n f o r

consequently,

t h e e x t e n s i v e b a c k - d o n a t i o n p e r CO

m o l e c u l e i n c r e a s e d t h e v i b r a t i o n a l p o l a r i s a b i l i t y ( r e f s 7,

31):

852

A)/l,2 increased

considerably. In contrast with the experimental findings, the same effect should have increased B c o , too. The explanation was as follows. At low BEOC, asymmetric force fields developed around the CO molecules, which might incline their axes, resulting in a smaller BC0. At high ElEOC the asymmetry decreased, but this was overcompensated by the shielding effect (ref. 7): BCO further decreased and the CD band became even wider. CONCLUSIONS (1) The red shifts caused in ) / ( C O ) by the coadsorption of CO(A) with the compounds or [E] could b e attributed mainly to their electron-donating abilities, i.e. the magnitudes of the first ionization potentials, and to the molecular structures. (2) The adsorbed species of the 0-containing molecules studied involved a Pt-O(E0C) interaction (bond), i.e. the adsorption of EDC on Pt took place through 0 atom(s). ( 3 ) In accordance with the results of others, the local CO coverage considerably influenced the decrease caused in Y ( C 0 ) by the coadsorption. (4) In the case of CO(A), Bco increased at low BEDC, which might be due to the back-donation-induced local dipolar decouplings of the CO molecules. (5) The transitional increase in BCO in the thermodesorption of CO(A) [ec0 (initial)& 0.31 without any coadsorbed substance might be caused by the increase in the average distance between the CO molecules.

[El

REFERENCES 1 G. Blyholder, J. Phys. Chem., 68 (1964) 2772; 79 (1975) 756. 2 0 . Post and E. J. Baerends, Surface Sci., 109 (1981) 167. 3 0. P. Woodruff, 8. E . Hayden, K . Prince and A. M. Bradshaw, Surface Sci., 123 (1982) 397. 4 R . M. Hammaker, 5. A. Francis and R . P. Eischens, Spectrochim Acta, 2 1 (1765) 1295. 5 G. 0. Mahan and A. A. Lucas, J. Chem. Phys., 68 (1978) 1344. 6 M. Scheffler, Surface Sci., 81 (1979) 562. 7 N. J. Persson and A. Ryberg, Phys. Rev., 824 (1981) 6954. 8 M. Moskovits and J. E. Hulse, Surface Sci., 78 (1778) 397. 9 5. Efrima and H. Metiu, Surface Sci., 92 (1980) 433. 1 0 A. Crossley and D . A. King, Surface Sci., 6 8 (1977) 528. 11 F. J. C. M. Toolenaar, G. J. Van Der Poort, F . Stoop and V . Ponec, J. Chin. Phys., 78 (1981) 927.

m.

853 12 13 14

15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

M . P r i m e t , J . C a t a l . , 8 8 ( 1 9 8 4 ) 273. M. P r i m e t , J . M. B a s s e t , M. V . M a t h i e u and M. P r e t t r e , J . C a t a l . , 29 ( 1 9 7 3 ) 213. J . M. B a s s e t , G . O a l m a i - I m e l i k , M . P r i m e t and R . M u t i n , J . C a t a l . , 37 ( 1 9 7 5 ) 2 2 . A . P a l a z o v , J . C a t a l . , 30 (1973) 1 3 . A . P a l a z o v , Ch. B o n e v , G . K a d i n o v , 0 . S h o p o v , G . L i e t z a n d J . V o l t e r , J . C a t a l . , 7 1 ( 1 9 8 1 ) 1. F . J . C . M. T o o l e n a a r a n d V . P o n e c , J . C a t a l . , 83 ( 1 9 8 3 ) 2 5 1 . A . P a l a z o v , Ch. B o n e v , G . K a d i n o v , 0 . S h o p o v , J . C a t a l . , 8 3 ( 1 9 8 3 ) 253. A . P a l a z o v , A . S a r k i n y , Ch. Bonev a n d 0 . S h o p o v , A c t a Chim. Hung., 108 ( 1 9 8 1 ) 3 4 3 . Ch. B o n e v a n d A . P a l a z o v , Cornmun. Dep. Chem. B u l g . Akad. S c i . , 1 6 (1983) 243. C. M . Mate, B. E . Bent and G . A . Sornorjai, J . E l e c t r o n S p e c t r o scopy, 39 (1986) 205. F . S t o o p , F . J . C . M. T o o l e n a a r a n d V . P o n e c , J . C a t a l . , 73 (1982) 50. M . B a r t b k , J . S i r k i n y a n d A . S i t k e i , J . C a t a l . , 72 ( 1 9 8 1 ) 2 3 6 . J . S i r k a n y , M . B a r t b k and R . 0. G o n z a l e z , J . C a t a l . , 8 1 (1983) 347. J . S 6 r k i n y a n d M . B a r t b k , J . C a t a l . , 92 ( 1 9 8 5 ) 3 8 8 . J . S i r k a n y a n d M . B a r t b k , A c t a Chirn. H u n g . , 122 ( 1 9 8 6 ) 2 8 5 . J. S a r k B n y , A . S i t k e i a n d M. B a r t o k , A c t a C h i m . H u n g . , 1 2 4 ( 1 9 8 7 ) 419. J . Stirkany, M. B a r t 6 k and R . 0. Gonzalez, i n p r e p a r a t i o n . R . G. T o b i n , S u r f a c e S c i . , 183 (1987) 226. J . S i r k i n y and M. Bartcjk s u b m i t t e d f o r p u b l i c a t i o n . H. P f n i i r , 0. M e n z e l , F. M. H o f f m a n n , A . O r t e g a a n d A . M . B r a d s h a w , S u r f a c e S c i . , 93 ( 1 9 8 0 ) 431.


C. Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactivity of Surfaces 0 1989Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

IN-SITU X-RAY

P.A.

855

STUDY OF THE SOLID-STATE REDUCTION OF COPPER CATALYSTS

SERMON, M.S.W.

VONG and K. GRANT

Department of Chemistry, Brunel University, Uxbridge UB8 3PH (United Kingdom)

ABSTRACT

Isothermal reduction of 15% CuO/ZnO by hydrogen a t 427 K has been followed by i n - s i t u X-ray d i f f r a c t i o n . This r e v e a l s monoclinic CuO being replaced by zerov a l e n t Cuo. The k i n e t i c d a t a derived from such d i f f r a c t i o n r e s u l t s obey t h e Avrami r e l a t i o n s h i p suggesting t h a t t h e c a t a l y s t r e a c t i v i t y and i t s reductive transformation is nucleation-controlled. However, s u r p r i s i n g l y , l e s s than a h a l f of t h e copper phases w i t h i n such c a t a l y s t samples i s X-ray d e t e c t a b l e . With t h e p r e s e n t CuO/ZnO sample no intermediate Cu20 was detected. INTRODUCTION

Supported copper i s used t o c a t a l y s e t h e s y n t h e s i s of methanol (e.g. Cu/ZnO/A1203 o r Cu/ZnO/Cr203 ( r e f . 1 ) ) .

However, t h e r e i s g r e a t u n c e r t a i n t y and

c u r r e n t i n t e r e s t i n t h e chemical s t a t e of t h i s copper under r e a c t i o n conditions. There i s disagreement about i t s oxidation s t a t e ( r e f . 2), with some emphasising zero-valent components ( r e f . 3) and o t h e r s p o s i t i v e oxidation s t a t e s ( r e f . 4 ) . CuO/ZnO i s a model methanol s y n t h e s i s c a t a l y s t ( r e f . 5) and has been s t u d i e d here using i n - s i t u X-ray d i f f r a c t i o n i n a flowing hydrogen atmosphere a t a temperature r e l e v a n t t o l a t e r c a t a l y s i s of CO/H r e a c t i o n s . 2 EXPERIMENTAL Catalysts Unsupported 15% CuO/ZnO was prepared by c o - p r e c i p i t a t i o n of an aqueous s o l u t i o n of c u p r i c n i t r a t e (Johnson Matthey; P u r a t r o n i c ) and zinc a c e t a t e (AnalaR) with NH4HC03/(NH4)2C03;

t h e washed p r e c i p i t a t e was d r i e d (383 K, 16 h )

and then calcined ( 4 7 3 K, 16 h ) . C h a r a c t e r i s a t ion X-ray photoelectron spectroscopy (Kratos EZ300 spectrometer with i n c i d e n t r a d i a t i o n a t 1468.6 eV) w a s used. The r e d u c i b i l i t y of the 15% CuO/ZnO was determined i n a temperatureprogrammed manner from t h e rate and e x t e n t of H2 consumption (TPR). Total s u r f a c e areas were estimated by BET a n a l y s i s of N2 physical adsorption a t 78 K (Carlo Erba 1800 and 1826 instruments) a f t e r outgassing f o r 16 h.

The

856 a c t i v e Cuo s u r f a c e a r e a of t h e reduced Cu/ZnO sample was determined volumetric-

ally by N 0 decomposition ( r e f . 6 ) a t ambient temperature (with N 2 0 removal a t 2

78 K) and oxygen chemisorption a t 7 8 K and 5 . 3 kPa.

I n both c a s e s an a d s o r p t i o n s t o i c h i o m e t r y Cu:O of 2 was assumed and i t w a s judged t h a t t h e r e were 1 . 4 x 101 9 2 Cu atoms p e r m

.

An i n - s i t u X-ray

c e l l ( r e f . 7) was used t o f o l l o w t h e i s o t h e r m a l r e d u c t i o n o f

t h e 15% CuO/ZnO sample when t h i s was h e l d i n a v e r t i c a l d i f f r a c t o m e t e r ( P h i l i p s PW1710) o p e r a t i n g w i t h N i - f i l t e r e d

CuKa r a d i a t i o n (average wave-length

0.15418 nm) a t 40 kV and 30 m A with N2 o r H2 flowing a t 101 kPa.

RESULTS C h ar a cterisation -

2 -1 The t o t a l s u r f a c e a r e a of 15% CuO/ZnO was found t o be 18.6 m . g

.

The

presence of shake-up s a t e l l i t e s i n X-ray p h o t o e l e c t r o n spectroscopy suggested unreduced 15%CuO/ZnO contained d i v a l e n t copper.

Temperature-programmed

r e d u c t i o n confirmed t h i s i n t h a t t o t a l H2 consumption was e q u i v a l e n t t o t h e t o t a l r e d u c t i o n o f d i v a l e n t copper p r e s e n t .

TPR a l s o suggested t h a t 15% CuO

reduced i n 101 kPa H2 a t a maximum r a t e a t 4 7 3 K.

CuO i s known t o reduce

Fig. 1. X-ray d i f f r a c t i o n p a t t e r n s of 15% CuOIZnO o b t a i n e d during i s o t h e r m a l r e d u c t i o n i n H (101 kPa, 427 +2K) a f t e r d i f f e r e n t times. Unshaded and shaded peaks denote d o s e of C u ( l l 1 ) and CuO(ll1).

85 7

( r e f . 8) even a t 360 K a l b e i t slowly;

i n o t h e r words ZnO may have decreased t h e

r a t e of CuO reduction compared t o pure CuO, as p r e d i c t e d previously ( r e f . 9 ) . I t was t h e r e f o r e judged t h a t an i n - s i t u isothermal study of reduction of t h e

CuO/ZnO c a t a l y s t should be c a r r i e d out below 430 K and might then r e v e a l mechanisms, k i n e t i c s and intermediates i n reduction of CuO t h e r e i n i f analysed a s a f u n c t i o n of t i m e . I n - s i t u X-ray a n a l y s i s Fig. 1 shows r e s u l t s of i n - s i t u X-ray a n a l y s i s a s a function of t i m e i n flowing €I2 (101 kPa) a t 427 K C 2 K.

C l e a r l y t h e (111) peak of monoclinic CuO

(38.71' 28; d = 0.232 nm) p r e s e n t i n i t i a l l y decreases i n i n t e n s i t y with reduction

(+)

O/O

OO /

(x)

50 100

80 60

25 40

20 I

20

40

60

80 Hmin)

Fig. 2 . Percentages of CuO reduced (0) and Cu formed (H ) i n reduction of 15% CuO/ZnO under conditions i n d i c a t e d i n Fig. 1 deduced from i n t e n s i t i e s of CuO(ll1) and Cu(ll1) d i f f r a c t i o n peaks. and X denote Z of phases which were X-ray d e t e c t a b l e and Z of a l l phases.

+

858 t i m e , w h i l e simultaneously t h e (111) peak of Cu (43.24' i n c r e a s e s i n i n t e n s i t y (from a small b u t f i n i t e v a l u e ) .

28; d = 2.090 nm) Only l i m i t e d work on

i n - s i t u X-ray a n a l y s i s of t h i s r e d u c t i o n h a s been attempted p r e v i o u s l y ( r e f s . 10, 1 1 ) .

Samples were s e p a r a t e l y t r e a t e d a t 5 7 3 K i n flowing hydrogen u n t i l completely reduced;

t h e s e were mixed i n known amounts w i t h t h e unreduced c a t a l y s t s .

Using

t h e s e m i x t u r e s a s c a l i b r a n t s i t was p o s s i b l e t o c a l c u l a t e t h e p e r c e n t a g e of CuO reduced and t h e p e r c e n t a g e of Cu produced from t h e d a t a i n FiE. 1.

Such r e s u l t s

a r e shown i n Fig. 2 a s S-shaped r e d u c t i o n p r o f i l e s , which have t h e form which has been g e n e r a l l y seen f o r CuO r e d u c t i o n ( r e f . 12) and a t t r i b u t e d ( r e f . 13) t o a u t o - c a t a l y t i c reduction.

Although t h e % CuO reduced approximately e q u a l s t h e

Z Cu produced ( i . e . t h e r e i s a good mass b a l a n c e between t h e two), n e i t h e r corresponds t o t h e e n t i r e copper c o n t e n t of t h i s m a t e r i a l .

Nevertheless t h e r e

i s no d e t e c t a b l e i n t e r m e d i a t e phase between CuO and Cu (e.g. Cu20 ( r e f . 1 1 ) ) . However, t h e m a j o r i t y of t h e copper appears t o be i n a s t a t e which could be probed by X-rays h e r e . X-ray d i f f r a c t i o n l i n e broadening (XRDLB) e s t i m a t e d t h e average c r y s t a l l i t e s i z e of X-ray d e t e c t a b l e CuO and Cu phases i n 15Z CuO/ZnO t o be 89.7 nm and

116.1 nm r e s p e c t i v e l y , assuming a S c h e r r e r c o n s t a n t of 0.9. These correspond t o 2 -1 2 s u r f a c e a r e a s of 7.50 m .g Cu and 6.95 m .g Cu-l r e s p e c t i v e l y . For t h e same reduced sample N 0 decomposition and O2 chemisorption suggested Cu s u r f a c e a r e a s

$

of 9 . 1 and 8.5 m .g Cu-l; l i k e l y t h a t t h e non-X-ray

s i n c e t h e s e a r e a l l i n moderate agreement it seems d e t e c t a b l e m a t e r i a l i s amorphous and of low-order,

r a t h e r t h a n merely f i n e l y d i s p e r s e d (when it would have r a i s e d chemisorption a r e a s b u t n o t XRDLB o n e s ) .

It should however b e remembered t h a t N20 a l s o

decomposes on copper o x i d e s ( r e f . 14) and t h i s means t h a t such a d s o r p t i o n d a t a

i s n o t i t s e l f unequivocal. DISCUSSION The Avrami r e l a t i o n s h i p (exp(-kt")=(l-x))

f o r t h e k i n e t i c s of a n u c l e a t i o n -

growth t r a n s i t i o n r e l a t e s t h e volume f r a c t i o n of t h e transformed phase x t o

t i m e t.

When x was c a l c u l a t e d from t h e percentage of Cu formed i n X-ray

d i f f r a c t i o n measurements ( s e e F i g . 3 ) t h e d a t a do g e n e r a l l y obey t h e r e l a t i o n s h i p a t longer times w i t h a v a l u e of i n t e g e r n of 4.27 f o r r e d u c t i o n of t h e 15%

CuO/ZnO sample. Previous s t u d i e s ( r e f . 15) o f s i l i c a - s u p p o r t e d

P t have found t h a t a s l i t t l e a s

30% of t h e m e t a l may b e i n a form which i s X-ray d e t e c t a b l e , although t h i s i n c r e a s e s on h e a t i n g a t 373-773 K.

S i m i l a r l y , t h e p r e s e n t work s u g g e s t s t h a t

much of t h e copper i n such c a t a l y s t s i s i n a low-order o r amorphous

state.

This

i s c o n s i s t e n t w i t h o t h e r e a r l i e r work on Cu/ZnO which concluded ( r e f . 16) t h a t it c o n t a i n s both metal m i c r o c r y s t a l l i t e s

amorphous

Cu'

& the

ZnO l a t t i c e

859

2

1

0

3 In[1/(l-x)J

Fig. 3 Avrami p l o t s of d a t a i n Fig. 2 .

(although t h i s must be contrasted with o t h e r r e p o r t s ( r e f , 17) t h a t l e s s than 2 % of copper i n such c a t a l y s t s i s not zero-valent).

Nevertheless, much copper i n

such c a t a l y s t s cannot be resolved by e l e c t r o n microscopy ( r e f . 18) and p a r a c r y s t a l l i n e phases ( r e f . 19) may be r e l e v a n t .

Present i n - s i t u X-ray

analyses of c a t a l y s t reduction a r e c l e a r l y r e l e v a n t t o c a t a l y s t s i n CO/CO

2

hydrogenation t o methanol ( r e f . 3) and complement o t h e r methods of c h a r a c t e r i s a t i o n ( r e f s . 5, 20);

n e v e r t h e l e s s , they may need t o be augmented by

EXAFS s t r u c t u r a l a n a l y s i s , e s p e c i a l l y with regard t o t h e c r i t i c a l low-order

copper phases.

It i s q u i t e p o s s i b l e t h a t f u r t h e r i n - s i t u analyses of these

reductions w i l l reveal m e t a - s t a b l e intermediates ( j u s t as they appear during Cu oxidation ( r e f . 2 1 ) ) . The p r e s e n t approach i s not l i m i t e d t o reduction modes of c a t a l y s t treatment. There i s d i s c u s s i o n about the i n t e r a c t i o n of C02 with such Cu c a t a l y s t s , but i n - s i t u X-ray d i f f r a c t i o n p a t t e r n s produced when C02 a t 427 K and 101 kPa i n t e r a c t e d with pre-reduced CujZnO did n o t r e v e a l t h a t t h e f r a c t i o n of CuO ( o r

860 Cu 0) p h a s e s i s not i n c r e a s e d . T h i s is n o t s u r p r i s i n g s i n c e t h e thermodynamics 2 a r e v e r y u n f a v o u r a b l e ( r e f . 22) and b u l k o x i d a t i o n of Cu i s n o t t h e r e f o r e e x p e c t e d ( r e f . 2 3 ) , a l t h o u g h C02

9 i n c r e a s e t h e f r a c t i o n a l coverage of oxygen

on t h e copper s u r f a c e ( r e f . 2 4 ) .

CONCLUSIONS The p r e s e n t i n - s i t u XRD a n a l y s i s ( r e f . 25) i s b o t h non-invasive and informat i v e concerning t h e s o l i d c r y s t a l l i n e p h a s e s p r e s e n t i n Cu heterogeneous c a t a l y s t s under r e a c t i o n c o n d i t i o n s .

F u r t h e r work on t h e a n a l y s i s o f c a t a l y s t s

d u r i n g r e d u c t i o n and o x i d a t i o n r e a c t i o n s w i l l be r e p o r t e d i n due c o u r s e . T h i s emphasises t h e need f o r i n - s i t u methods of a n a l y s i s of c a t a l y s t s ;

in

t h i s p a r t i c u l a r c a s e f u r t h e r EXAFS-XRD work i s c e r t a i n l y r e q u i r e d t o e l u c i d a t e t h e s t a t e and chemical environment of copper i n t h e s e i m p o r t a n t i d e a l i s e d (and s u b s e q u e n t l y commercial) heterogeneous c a t a l y s t s .

I t should be i n t e r e s t i n g t o

know how t h e f r a c t i o n (and n a t u r e ) o f t h e low-order phase changed w i t h Cu l o a d i n g on ZnO and a l s o w i t h t h e a d d i t i o n of alumina o r chromia. P r e v i o u s l y ( r e f . 2 6 ) , i n - s i t u XRD has been used t o pro.be c a t a l y s t s ;

the

p r e s e n t work makes a s m a l l c o n t r i b u t i o n t o t h i s i n t r i g u i n g theme of i n - s i t u catalyst characterisation.

REFERENCES 1 J . C . Amphlett, R.F. Mann, C. McKnight and R . D . Weir, P r o c . I n t e r s o c . Energy Conf. 20 (1985) 2772, 52-1. 2 G.C. Chinchen and K.C. Waugh, J. C a t a l . , 9 7 (1986) 280-283; T.H. F l e i s c h and R.L. M i e v i l l e , J. C a t a l . , 97 (1986) 284-285; A . J . Rridgewater, M.S. Wainwright, D . J . Young and J . P . Orchard, Appl. C a t a l . , 7 (1983) 369-382; V. Ponec, M.S. S p e n c e r , F. T r i f i r o , J . J . F . S c h o l t e n , F. Solymosi, M.W. R o b e r t s , J. P r i t c h a r d , D. Chadwick, R.W. J o y n e r , R. Burch, R.A. Van Santen and D.W. Goodman, J . Chem. SOC. F a r a d . Trans. I , 83 (1987) 2244-2251. 3 G.C. Chinchen, K . C . Waugh and D . A . Whan, Appl. C a t a l . , 25 (1986) 101-107. 4 G.R. Apai, J . R . Monnier and M . J . Hanrahan, J . Chem. SOC. Chem. Comun. (1984) 212-213. 5 C.T. Campbell, K.A. Daube and J.M. White, S u r f . S c i . 182 (1986) 458. 6 G . C . Chinchen, C.M. Hay, H.D. Vandervell and K.C. Waugh, J. C a t a l . , 103 (1987) 79-83; G.E. Parris and K . K l i e r , J . Catal., 97 (1986) 374-384. 7 M.S.W. Vong, P.A. Sermon, V.A. S e l f , K. Grant and A . J . Blackburn, J. Phys., 21E (1988) 495-496. 8 M. P o s p i s i l , C o l l . Czech. Corn. 42 (1977) 3111-3117. 9 C.R. Alder Wright and A.P. L u f f , J . Chem. SOC. 3 3 (1678) 1 - 2 7 . 10 T. Takeuchi, 0. Takayasu and S . Tanada, J . C a t a l . , 54 (1978) 197-206. 11 P. P o r t a , R. Dragone, G. F i e r r o , M.L. J a c a n o and G. M o r e t t i , 1 1 t h I n t e r n . Symp. R e a c t i v i t y of S o l i d s , P r i n c e t o n , J u n e , 1988. 12 C . R . Alder Wright, A . P . Luff and E.H. Rennie, J. Chem. SOC., 35 (1879) 475-524. 1 3 R.N. Pease and H.S. T a y l o r , J. Amer. Chem. SOC., 43 (1921) 2179-2188. 1 4 R.M. Dell, F.S. Stone and P.F. T i l e y , Trans. F a r . SOC., 49 (1953) 195-201. 15 T.A. D o r l i n g and R.L. Moss, J . C a t a l . , 5 (1966) 111-115. 16 K. K l i e r , Adv. C a t a l . , 31 (1982) 242-313; S . Mehta, G.W. Simmons, K. K l i e r and R.G. Herman, J . C a t a l . , 57 (1979) 339-360.

861

1 7 T.H. F l e i s c h and R.L. Mieville, J. C a t a l . , 9 0 (1984) 165-172. 18 J.M. Dominguez, G.W. Simmons and K. K l i e r , J. Mol. C a t a l . , 20 (1983) 369-385. 19 J . Zhao, Y. L i , F. Ma, H. Kang and C. Zhu, Ranliao Huaxue Xuebao, 14 (1986) 201. 2 0 F. Boccuzzi, G. C h i o t t i and A. Chiorino, Surf. S c i . , 172 (1985) 361-367; Y . Okamoto, K. Fukino, T. Imanaka and S . T e r a n i s h i , J . Phys. Chem., 87 (1983) 3740-3754. 2 1 R. Guan, H. Hashimoto and T. Yoshida, Acta. Cryst., B40 (1984) 109-114; R. Guan, H. Hasimoto and K.H. Kuo, Acta. C r y s t . , B40 (1984) 560-566. 22 C.W. Dannatt and H . J . T . Ellingham, Disc. Far. Sco., 4 (1948) 126-139. 2 3 T . Van Herwijen and W.A. de Jong, J. C a t a l . , 63 (1980) 83-93; T. Van Herwijen, R.T. Guczalski and W.A. de Jong, J. C a t a l . , 73 (1980) 94-101. 24 G.C. Chinchen, M.S. Spencer, K.C. Waugh and D.A. Whan, J . Chem. SO C. Farad. Trans. I , 83 (1987) 2193. 25 E.A. Owen and E . S t . J. W i l l i a m s , Proc. Phys. SOC. London, 56 (1944) 52. 26 T . Rayment, R. Schlogl, J . M . Thomas and G. E r t l , Nature, 315 (1985) 311; R.M. Nix, T. Rayment, R.M. Lambert, J . R . Jennings and G. Owen, J. C a t a l . , 106 (1987) 216; E.M. Moroz, V.S. Bulusheva, V.A. Ushakov, S.V. Tsybulya and E.A. L e v i t s k i i , React. Kinet. C a t a l . L e t t . , 3 3 (1987) 185-189; P. Gallezot and B . Imelik, J. Phys. Chem., 77 (1973) 652-656; T.A. Kriger, D.V. Tarasova, L.M. Plyasova, A.V. Shkarin and S.S. Stroeva, React. Kinet. Catal. L e t t . , 34 (1987) 207-212.


C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactioity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

863

CATALYTIC PROPERTIES AND CHARACTERIZATION OF LaPd3 INTERMETALLIC COMPOUND

K.S. SIM', L. HILAIRE, F. LE NORMAND, R. TOUROUDE Laboratoire de Catalyse et Chimie des Surfaces, U.A. 423 du CNRS, 4 rue Blaise Pascal, 67070 Strasbourg (France) V. PAUL-BONCOUR,A. PERCHERON-GUEGAN

Laboratoire de Chimie Mdtallur ique des Terres Rares, U.A. 209 du CNRS, 1 Place A. Briand, 92195 Meudon (France!

ABSTRACT The catalytic behaviour of LaPd3 has been investigated in but-l-ene h drogenation and isomerization. The activity increased and the isomerization se ectivity decreased linearly as a function of the duration of the pretreatment catalyst (H2 ,300OC) to reach constant values after 15 hours.The evolution of catalytic activity and selectivit is due to the pro ressive decomposition of *Pd3 islands into La203 and Pd, whicl were initially founi on the surface of catalyst jointly to lanthanum oxide and hydroxide. That was deduced from photoelectron spectroscopy analysis.

r

INTRODUCTION Recently, rare earth and transition metal intermetallic compounds have been used as catalysts in reaction involving the activation and transfer of hydrogen such as hydrogenation of olefins [l , 21 and other catalytic reactions [3, 41. However, the nature and the structural changes of the catalysts occuring under the catalytic reaction conditions Is a permanent question. X-ray diffraction studies carried out by others workers have led to the hypothesis that metal -rare earth oxide catalysts are formed in situ [5,6]. In this paper, we report on the catalytic properties of LaPdg compound pretreated by thermal reduction. Additional studies of electron microscopy and X-ray photoelectron spectroscopy before and after the catalytic reaction or after the pretreatment have been done.

'Permanent address: Korea Institute of Energy and Resources, P.O. Box 5, Daedeok, Science Town, Daejeon, Chungnam, SOUTH KOREA.

864

EXPERIMENTAL METHODS Preparation LaPd3 was prepared by induction melting of the pure components under vacuum [7]. Its stoichiometry and homogeneity were verified by metallographic examination and microprobe analysis. The sample studied in the catalytic reaction was ground mechanically in argon atmosphere and sieved in order to produce powder with a grain size of less than 36pm. Characterization (i) X-rav Diffraction ( X u. Single plate and powder was analysed by XRD using an adapted Debye-Scherrer camera and CU-K radiation. The lattice spacings were derived from the diffraction patterns using the Nelson-Riley extrapolation fonction. (ii) Electron Microscopy. The bulk analysis of LaPd3 powder before and after catalytic use was performed by STEM (Scanning Transmission Electron Microscopy) and EDAX (Energy Dispersive Analysis by X-ray). liii) X-rav p h o t o m r o n S p e c t r o w ( ) ( P a The surface analysis of LaPd3 was performed with the pellets prepared in argon atmosphere, in the initial state and after the treatment (H2,3OO0C), in situ, within the chamber of the spectrometer.

..

Activitv and Selewitv measurements 0) Gas-Dhase hvdro' . This reaction was carried out in a flow reactor at 760 Torr total pressure over the temperature range, -37.5 to 30°C, with 8.2 Torr but1-ene partial pressure. Catalyst loading was about 40 mg. The selectivity was measured by the ratio of isomenzed to isomenzed plus hydrogenated products. * . Gas chromatographic analysis was effected using a 5m (ii) Product long x 3x10s rn i.d. column packed with 30% dimethyl sulfolane on fire brick. RESULTS and DISCUSSION -0Dert ies

..

..

. This evolution was tested as 0) Fvolution of the c a W actlvla function of the number of successive runs. In each run, LaPd3 was pretreated at 300% for 1 hour in a purified hydrogen flow at 760 Torr (50 ml/min) and the catalytic reaction was then performed. From the results presented in Fig.1 a significant linear increase in activity and a linear decrease in selectivity were observed during the first few runs. After ca.10 runs the catalytic activity and selectivity were stabilized . If we take another loading of LaPd3 and pretreat directly more than 15 hours with H2 at 300°C, we obtain the same activity and selectivity than after 15 funs.

s-

0

0 0 a 0

-r

40

20

hours

Fig. 1. Total activity (r) and isomerization selectivity (S) as a function of the duration of H2-3000Cpretreatment (hours). (ii) m e t i c studtes. * The activity and selectivity in the hydrogenation and isomeriration of but-l-ene obtained with various conditions are summarized in Table 1. From these results the apparent activation energy was approximately 7.3 KcaVmol, r = k PH20.5PB0.1 and the reaction orders lead to the equation where r is the initial rate of hydrogenation only, and PH2 and PBthe initial pressures of hydrogen and but-1-ene respectively. TABLE 1

No

Temp. ("c)

PH2 For)

PB Fan)

L-1 L-2 L-3 L-4 L-5 L-6 L-7

0 -18 -37.5 0

752

8.2 "

0 0 0

a

" 757 747 226 38

2.9 13 8.2 11

Activity Selectivity ( molekg. cata) total hydrogenation

("w

3.7 1.8 0.45 2.8 3.8 2.6 1.8

1.6 0.72 0.18 1.3 1.6 0.94 0.44

58 60 59 53 60 64 75

866

Characterization of LaPG (i) X-rav diffraction measy rements. The XRD results indicated that for the initial LaPd3 there was a good agreement with the LaPd3 compound, but after the catalytic tests other compounds such as palladium and La203 were observed with LaPd,. It is clear that there was a transformation of the intermetallic compound after pretreatment. (ii) Electron microscopv measurements. Fig.2 and 3 show some representative STEM and EDAX results for the initial catalyst and after catalytic tests. Initially STEM examination showed a rather good homogeneity in the dispersion of rare earth and transition metal. However after the catalytic tests, we observed an inhomogeneity of the sample, in good agreement with the results of XRD analysis. For example in the region b of the figure 3,there is a strong enrichment in lanthanum .

Pd

2

la

4

6

8

kev

Figure 2. STEM and EDAX analysis of initial sample, LaPds

867

Figure 3. STEM and EDAX analysis of LaPd3 sample after catalytic tests.

(iii) XPS rneasureme nts. XPS spectra of LaPd3 were taken for the initial sample (a), after treatment during 4,hours (b) and 12 hours (c). The Pd/La value of surface composition , shown in Fig.4, indicated an enrichment of lanthanum initially and the progress of this enrichment after the treatments (H2, 300°C).The spectra and binding energies for Pd 3d, La 3d 5/2 and 0 1s are presented in Fig.5 and Table 2. The 0 1s peak of the initial sample showed a main peak at 531.7eV, characteristic of OH- anion [8]. After treatments the secondary peak increases at 529.7eV which is characteristic of 0 2 - anion [9]. After the second treatment (c), all peaks were shifted towards higher binding energies, due to a charging effect, and the Pd signal was very weak and broadened (full width at half maximum 1.8 eV in sample (a) compared to 2.4eV in sample (c). In the latter case, the binding energies and the shape of Pd peak were similar to those of a 0.2% Pd/La203 sample studied by Fleisch et al [12], using the same reference energy (C 1s, 284,6eV). It is clear that after 12 hours treatment, the LaPd3 surface was identical to the surface of a Pd/La203catalyst.

868

T'ABLE 2

3d5l2

Pd 3d312

335.1 335.4 334.9

340.3 340.6 340.2

0

Compound

1s

1

LaPd3 (a! LaPd3 (by LaPd3 (C

529.7 529.4

531-7 531.9 531.0

La MI2 834.9 834.7 834.2

Ref.

838.8 838.9 838.3

10 335.2 340.4 Pd metal 8-9 835.9 839.6 532.1 La(OH)3 834.7 839.2 8-9 La203 530.1 335.75 341.05 10-11 832.9 837.0 LaPd3 12 834.3 334.9 3.2%Pd/La203 1- referred to Fermi level. 2- the binding ener ies are corrected from charging effects, referred to the same C l s ~ ~ o ~ as 0.2% P ~ / L lef.12).

Figure 4 Pd/La ratio (XPS intensities corrected from cross sections and escape depths differences) as a function of the duration of in situ H2-300"C treatment.

Pd

La

3d

.. .: ... ...

3d 5/2

Is

0

,.----

I,

, .....

.

.

.

a: ;*A

b'

i

:

,.

.

s

, 5500 p a r t i c l e s (powder).

decrease i n ILi a NS (the number o f spins) was noticed ( c f . Table 1).

( c ) De-

magnetization e f f e c t s could inhomogenize the signal since, l i k e l y , L i p a r t i c l e s o f various shapes and o r i e n t a t i o n s are present. inhomogeneous l i n e broadening excludes t h i s .

Clearly, the absence o f such

(d) F i n a l l y , the T-independence o f

NS r u l e s o u t any possible i n t e r f e r e n c e (growing w i t h decreasing T ) from eventual

spurious unresolved ESR signals. INTERPRETATION AND D I S C U S S I O N Surface r e l a x a t i o n D i r e c t measurement o f the L i p a r t i c l e s s i z e D are n o t available; however, the range i n which i s t o be s i t u a t e d may be estimated from the CESR data. The absence o f any QSE on t h e CESR signal r e s u l t s [ 13,161 i n the lower l i m i t D b 5 nm.

Using m / 2 ~E 20.95 -1 and mvF = 1 1 . 7 5 ~ 1 0 -g~ cm ~ s ,

The upper l i m i t r e s u l t s from the lineshape data.

GHz, the e l e c t r o n density nLi

= 4.7~10~'

where m and vF represent the e l e c t r o n mass and Fermi v e l o c i t y , respectively, one may d e r i v e from the ( b u l k )

0

data [ 191 t h a t f o r [20]

IAT

< 1 (T i s the o r b i t a l

c o l l i s i o n time) normal s k i n - e f f e c t (NSE) conditions [ 20,211 apply i n the region

876

T > 60 K s i n c e a t 60

K t h e r e l a t i o n &/A

-

42 h o l d s , where

x

i s the electronic

I n t h e NSE r e g i o n , t h e CESR l i n e shape t h e o r y o f Oyson as adap-

mean f r e e p a t h .

t e d by Webb [ 2 2 ] f o r s p h e r i c a l metal p a r t i c l e s a p p l i e s ; t h i s t h e o r y has been w e l l c o n f i r m e d by experiments.

I n t h e p r e s e n t s t u d y symmetric l i n e s a r e always

observed which, f r o m Webb’s t h e o r y , means t h a t D/6 Q 0.4. the practical restriction leads t o D ~ 0 . 1 2

xe

Q 0, where

xe

Combining t h i s w i t h i s t h e e f f e c t i v e mean f r e e path,

I n a d d i t i o n , i t f o l l o w s f r o m t h i s upper l i m i t t h a t

t h e NSE - o r b e t t e r s t a t e d , t h e s i z e - e f f e c t l i m i t e d r e g i o n p r e s e n t L i s t u d y o v e r t h e whole range 4.2 Q T I s has been w e l l s t a t e d [2,8] studied, i.e.,

< 300

- applies

t o the

K.

t h a t f o r the pure L i p a r t i c l e s o f s i z e presently

5 nm < D ~ 0 . 1 2,,my t h e CESR l i n e w i d t h i s dominated by t h e s u r -

face-induced w i d t h aBS which i s g e n e r a l l y g i v e n as [ 18,231 PP’ AB ~ E v F1+Bo) ( PP=L=

5

(3)

YT;

p r o v i d e d t h e c o n d i t i o n s [ 23,241

Herein, Bo i s t h e f i r s t Landau F e r m i - l i q u i d parameter, 6,

are f u l f i l l e d . (2DeT2)”*

t h e s p i n d e p t h and De = ( I t B 0 ) v F ~ / 3 t h e d i f f u s i o n c o n s t a n t .

=

Condi-

t i o n s ( 4 ) a r e c l e a r l y met a t RT - and a f o r t i o r i a t l o w e r T - asmaybe checked by i n s e r t i n g t h e k i n d o f cLi

values : A = 1 . 0 6 ~ 1 0 -cm, ~ g = 2.00229, b AB pp = c/yT2 ~ 0 . 0 5mT. I n s e r t i n g cLi

,

i . e . , < 5 ~ 1 0 -and ~ u s i n g t h e RT 7 vF = 5 . 4 8 ~ 1 0 cm/s and a b u l k l i n e w i d t h

g e n e r a l l y met [ Z ]

2

~ x I O - as ~ previously derived f o r LiF:Li

[2,9] i.e.,

and D = 0 . 1 2 urn indeed shows t h e p r e s e n t s i g n a l t o be s i z e - e f f e c t l i m i t e d , nBSpp 50 ,T, as expected. I t bears o u t t h e dominance o f s u r f a c e s c a t t e -

ring

s u g g e s t i n g t h e observed K-versus-T b e h a v i o u r l i k e l y t o be r e l a t e d t o e l e c -

tron-surface i n t e r a c t i o n too. Lineshape analysis -

I n t r y i n g t o s i m u l a t e ( d e c o n v o l u t e ) t h e low-T l i n e shape, two o b s e r v a t i o n s are considered c r u c i a l .

F i r s t , no inhomoqeneous broadening i s observed, t h a t i s ,

o n l y one g v a l u e and no powder o r g l a s s e f f e c t p r e s e n t [ 2 5 ] . v a l u e s up t o 8.5, Lorentzian a t

i.e.,

SeCOnd,K reaches

much l a r g e r t h a n t h e s i n g l e - l i n e l i m i t s e t by t h e

= 3.628 ( c f .

K

= 1.033 and 1.033 < K G 3 . 6 2 8 f o r t h e Gaussian

and V o i g t p r o f i l e s , r e s p e c t i v e l y ) . f a c t may be a sum o f v a r i o u s

-

T h i s suggests t h a t t h e low-T l i n e shape i n

l i k e l y L o r e n t z i a n - l i n e s o f equal g b u t o f un-

equal AB

and I. Indeed, t h e s i g n a l f o r T < 25 K i s c l o s e l y f i t t e d , (see t h e PP d o t t e d c u r v e i n Fig. 2 ( b ) ) by t h e sum o f 2 L o r e n t z i a n l i n e s o f equal g ( = 2.00229)

w i t h AB

= 0.065 and 0.218 mT, r e s p e c t i v e l y . T h e i r i n t e n s i t y r a t i o i s g i v e n as PP R E Ib/ I s = 2.22, where Ib and Is a r e t h e i n t e n s i t i e s o f t h e broad and narrow l i n e , r e s p e c t i v e l y . T h i s two-component s i m u l a t i o n c l e a r l y shows how a v a l u e

> 3.628 may a r i s e : the small component mainly determines AB

w h i l e the PP broader one accounts f o r the broader wings and the main p a r t o f the t o t a l i n -

K

tensity.

Thus, as T decreases, the l i n e gradually evolves from an almost one-

component Lorentzian l i n e o f AB T w h i l e the apparent

K,

= 0.065 mT t o the two-component l i n e a t low PP derived by keeping t r e a t i n g the signal as a one-compo-

nent spectrum (see ( 2 ) ) gradual l y increases. Two €-values model

A t h i r d main observation i s t h a t w h i l e

K

s t r o n g l y changes w i t h decreasing

T,

NS stays unaltered. This then means ( i f supposing t h a t the signal i s always t o t a l l y due t o L i CE's) t h a t w h i l e a l l C E ' s c o n t r i b u t e t o one small s i g n a l ( o f aBpp = 0.065 mT) a t RT, g r a d u a l l y more and more o f them "leave" t h e narrow signal w i t h decreasing T t o add t o t h e broad l i n e , s t i l l keeping however the same g.

Thus, the low-T CESR signal may e f f e c t i v e l y be described as comprising two I n l i g h t o f the

CE baths o f equal g b u t o f d i f f e r i n g s p i n r e l a x a t i o n times.

dominating surface r e l a x a t i o n t h i s suggests the existance o f two "kinds" o f p a r t i c l e s , i.e.,

two zLi values p r e v a i l ,

a surface s p i n - f l i p p r o b a b i l i t y

E~~

I E

A t RT, almost a l l p a r t i c l e s e x h i b i t

ILi and behave i d e n t i c a l l y i n every res-

= 0.065 mT and g = 2.00229 i s obPP As T increases, f o r some physical reason, grachallymore and more L i I1 I I p a r t i c l e s acquire the value E~~ = cLi = (0.218/O.065)zLi = 3.35 E ~ now ~ ,exhib i t i n g a broadened signal o f AB I0.218 mT. A t T < 25 K, the second subPP system comprises 2.22 x more spins than t h e f i r s t one. Since both s i g n a l s o r i I Although t h e E~~ p a r t i c l e s ginate from CE's i n L i , no g change i s expected.

pect; a one-component Lorentzian l i n e o f AB served.

represent l e s s o s c i l l a t o r strength they keep dominating AB

PP' I n a more continuous model, one could r e l a x somewhat the s t r i c t two-component

i n t e r p r e t a t i o n and p o i n t o u t t h a t the low-T signal may equally w e l l be f i t t e d by a sum o f more Lorentzian l i n e s o f d i f f e r e n t AB

b u t s t i l l o f equal g. I n t h i s PP , p i c t u r e , upon cooling various subsystems, a l l o f ( s l i g h t l y ) d i f f e r e n t E~~ a r i s e . Whatever i n t e r p r e The extreme l i m i t would be a continuous d i s t r i b u t i o n i n cLi. t a t i o n one prefers, however, the essence o f the physics involved remains unaltered. To account f o r the change i n cLi upon c o o l i n q one could invoke t h e d i f f e r e n c e i n thermal expansion o f the substances involved, i . e . , L i and LiF. Indeed, a v a r i a t i o n i n the L i - L i F "contact", t h a t i s , a v a r i a t i o n o f the L i surface, would c e r t a i n l y have an i n f l u e n c e on zLi.

The f a c t , however, t h a t the s i g n a l i s n e a t l y

accounted f o r by invoking on29 tm particZe subsystems i s f e l t c l o s e r t o r e a l i t y and i s considered t o bear o u t the e f f e c t s o f the M a r t e n s i t i c phase t r a n s i t i o n o c c u r r i n g i n L i a t low T. Physical o r i g i n : M a r t e n s i t i c phase t r a n s i t i o n I t i s w e l l documented t h a t a M a r t e n s i t i c phase t r a n s i t i o n i n L i sets i n a t

a temperatuETM = 70-80 K; L i transforms from the body-centered cubic (bcc) t o

878

the low-T hexagonal close-packed (hcp) structure attendant with a small volume expansion * 0.3 %. Generally, the transformation does not occur coapletely at once and stacking faults are built in [ 261. Is is estimated that * 50 and 90 % of Li has transformed into hcp at 60 and 4 K, respectively. This transition, among others, hinders good u measurements 1191. It is known also that the size of the small, pure Li particles formed in LiF depends on the particular irradiation and anneal treatment applied [8,91; X-ray analysis has shown that at RT there are at least two mean types of particles, i.e., flat platelets and more spherical ones, perhaps suggestive of two phases coexisting. The particles constrained by the surrounding LiF matrix may be under compression or tension depending on the sample's history. However, in light of the thermal expansion properties (cf. the RT linear expansion coefficient a = 3 4 ~ 1 0 -and ~ 46~10-~ for LiF and Li, respectively 127,281) the present samples will have a larger number of Li particles under tensile stress at RT because of the self-annealing which has resulted from the heating up during n-irradiation. Further, melting theories state that, generally, size reduction lowers the phase transition temperature due to the excess surface free energy. However, for a solid-solid phase transition it is not clear yet whether a depression or elevation is to be expected [29] . However, for the present case such considerations are of little importance since both compression and tension are likely; inevitably, TM will be depressed for some particles (under compression) and elevated for others. This leads us to the following interpretation. At high T (Q 300 K) most Li particles are in the bcc phase leading to (one) Lorentzian CESR signal (with I Perhaps there are some hcp particlespresent too ABpp = 0.065 mT; eLi = eLi). giving a weak background signal, which as yet is not detected. As T decreases gradually more and more particles under crystal constraint transform to hcp thus (Likely, enhancing the second-component signal (of A6 = 0.218mT, EL^ =.)::E P? on continued cooling all particles under tensile stress transform above T 'L 75 K in light of the small volume expansion. The remainder transform progressively below 70-80 K. The phase transition likely flattens out at w 25 K because of the stabilization effect which limits the atomic mobility. Then, at 25 K about (2.22/3.22)~100 = 69 % is expected to reside in the hcp phase. I to Thus, theclue of the CESR pattern is found in the change of E~~ from eLi ;:E upon transition of a Li particle from bcc to hcp. Physically, the change in E arises because of the nature o f the martensitic transition itself; upon transition - unlike a diffusional change - many atoms make a coordinated movement over a fraction o f an atomic distance which does not preserue the origina2 crystaZ shape. This severely influences the Li/LiF contact planes and modifies ELi.

CONCLUSION Spin-surface s c a t t e r i n g i s found t o be the dominant s p i n r e l a x a t i o n mechanism f o r CE's i n L i globules formed i n L i F by n - i r r a d i a t i o n [ 301. The s i g n i f i c a n t variat i o n i n t h e L i F CESR lineshape occurring upon cooling from 300 t o 4.2 K i s asc r i b e d t o a change i n s p i n - f l i p p r o b a b i l i t y f o r s c a t t e r i n g o f CE's a t t h e L i surface ( L i / L i F i n t e r f a c e ) .

I t represents the f i r s t r e p o r t o f a T-dependent E

c h a r a c t e r i z i n g a non-magnetic metal surfaLe.

The observation r e s u l t s from t h e

p a r t i c u l a r s o l i d - s o l i d s t r u c t u r e studied, namely, L i p a r t i c l e s suspended i n a r i g i d L i F m a t r i x . The change i n E~~ i s brought about by the M a r t e n s i t i c phase t r a n s i t i o n from bcc t o hcp occurring i n b u l k L i a t TM = 70-80 K.

For each L i

p a r t i c l e separately the change i n E~~ i s believed t o be a one-step process, i.e., I 1 upon t r a n s i t i o n . Because o f the t e n s i l e and compressive stresses from E : ~ t o E~~ present i n LiF:Li and the incomplete nature o f the t r a n s i t i o n i t s e l f , t h e r e i s a spread i n TM leading t o a gradual increase and decrease o f the broad and small components, respectively, o f the CESR signal upon cooling. The observation o f strong l i n e shape ( K ) v a r i a t i o n s from 300 t o 25 K has consequences f o r the a p p l i c a t i o n o f L i F : L i as a c a l i b r a t o r

6.9.)

s p i n marker).

I n t e n s i t y c a l c u l a t i o n s should be t r e a t e d w i t h care, e.q., adapting the f i e l d i n t e g r a t i o n range t o the growing importance o f the signal wings upon cooling. I f applying the I =

method, K needs t o be c a l i b r a t e d against T. PP F i n a l l y , E i g l e r and Schultz [ 41 have r e c e n t l y n e a t l y demonstrated the r e l a -

t i o n s h i p between

E

K

YomAB2

and t h e nmtw of adutoms a t a metal surface f o r the L i

gas ambient i n t e r f a c e . r e l a t i o n s h i p between

E

- inert

The present r e s u l t s bear o u t f o r t h e f i r s t time the and the sol i d - s o l i d i n t e r f a c e structure.

REFERENCES 1 J.R. Sambles and J.E. Cousins, S o l i d StateCommun. 32, 1021 (1979) 2 A. Stesmans, J. W i t t e r s and R. Sambles, S o l i d State Commun. 47, 71 (1983) and references t h e r e i n 3 R.J. E l l i o t t , Phys. Rev. 96, 266 (1954); Y. Yafet, S o l i d State Phys. 14, 1 (1963) 4 D.M. E i g l e r and S. Schultz, Phys. Rev. L e t t . 54, 1185 (1985) 5 J.H.Pifer and R. Magno, Phys. Rev. 83, 663 (1971) 6 M. Ya,Gen and V. Petinov, Zh. Eksp. Teor. F i z . , 48, 29 (1965)[Sov. Phys.-JETP 21, 19 (1965)l 7 Y.W. Kim, R. Kaplan and P.J. Bray, Phys. Rev. 117, 740 (1961); R. Kaplan and P.J. Bray, Phys. Rev. 129, 1919 (1963) 8 Ch. Taupin, J. Phys. Chem, Solids, 28, 41 (1967) 9 A.J. Watts and J.E. Cousins, Phys. Status S o l i d i , 30, 105 (1968) 10 P. Damay and M.J. Sienko, Phys. Rev. 13, 603 (1967) 11 F.G. Cherkasov, E.G. Kharakhash 'Yan, L . I . Medvedev, N . I . Novosjelov and Y . I . Talanov, Phys. L e t t . A 63, 339 (1977) 12 A.W. Overhauser, Phys. Rev. 89, 689 (1953) 13 J.P. Borel, C, Borel-Narbel and R. Monot, Helv. Phys. Acta 47, 537 (1974); S. Pasche and J.P. Borel, S o l i d State Commun. 58, 865 (1986) 14 R. Kubo, J. Phys. SOC. Jpn 17, 975 (1962); A. Kawabata, J. Phys. SOC. Jpn 24, 902 (1970)

880

15 16 17 18 19 20 21 22 23 24 25 26 27 28

J . B u t t e t , R. Car and C.W. Myles, Phys. Rev. 626, 2414 (1982)

A. C h a t e l a i n , 3-L. M i l l e t and R. Monot, J. Appl. Phys. 47, 3670 (1976) R. Hecht, Phys. Rev. 132, 966 (1963) F.J. Dyson, Phys. Rev. 98, 337 (1955) T.C. Chi, J. Phys. and Chem. Ref. Data (USA) 8, 339 (1979) K. Saermark, S o l i d S t a t e Commun. 20, 199 (1976) G.E.H. Reuter and E.H. Sondheimer, Proc. R. SOC. London A 195, 336 (1948) R.A. Webb, Phys. Rev. 158, 225 (1967) M.B. Walker, Phys. Rev. 63, 30 (1971) D.M. E i g l e r and S. S c h u l t z , S o l i d S t a t e Commun, 44, 1565 (1982) See e.g., P.C. T a y l o r and P.J. Bray, J . Magn. Res. 2, 305 (1970) J.S. Dugdale and D. Gugan, Cryogenics 2, 103 (1961) 8. Yates and C.H. Panter, Proc. Phys. SOC. 80, 373 (1962) Y.S. Touloukian, R.K. K i r l y , R.E. T a y l o r and P.D. Desai, Thermo-physical P r o p e r t i e s o f M a t t e r , Vol. 12 (Plenum, New York) p. 186 (1977) 29 J.R. Sambles, J. Phys. & Chem. S o l i d s , 46, 525 (1985) 30 A. Stesmans, J. Phys. D : Appl. Phys. 21 (1988); i n press


881

STUDIES OF THE INITIAL STAGES OF THE ADSORPTION OF Pd ON AN EXTENSIVELY OXIDISED Zn(OOO1) SUPPORT

A.J.SWIFT and J.C.VICKERMAN Department of Chemistry, UMIST, PO Box 88, Sackville Street, Manchester M60 1QD (United Kingaom)

ABSTRACT Thick oxide overlayers have been prepared from Zn(0001) surfaces by oxidation of the base crystal under high conditions of temperature and pressure. The so called ZnOX surfaces have been characterised by XPS, AFS, Static and Dynamic SIMS and by examining surface reactivity. Initial stages of the adsorption of metalic Pd at ZnOX has been monitored using these techniques. In each case the growth mode of Pd was found to be layer by layer (Franck-van der Merwe). No chemical reaction of the adsorbate with either host element or diff’usion effects of Zn or 0 have been detected. However, diffusion of Pd into and across the support is obsetved. INTRODUCTTON Research into the applications of metal oxides as catalysts offers not only a wealth of interesting surface chemistry, but also, is of tremendous economic importance to the chemistry industry. The incorporation of a second component at the surface of mtal oxide catalysts has been shown to exert a pronounced and varied effect on catalytic properties of these supports, for example in the fields of methanol synthesis (ref. 1) or s&nal generation in gas sensors (ref. 2). However, relatively little work has been performed modelling doped metal oxide systems and hence the role of support and dopant I n these cases is far from well understood. As part of a programne of work to investigate such catalysts, the ZnO/F’d system has recently been examined using a range of Instrumental techniques. The ZnO surface has been well studied in the past (refs. 3-7) as have the initial stages of Zn(0001) oxidation (ref. 8) and much is known of its electronic structure and physical properties. ‘he preparation and characterisation of the support used I n these experiments will be described elsewhere (ref. 9 ) . In short, the support is understood to consist of a heterogeneous ZnO surface, slightly rich in zinc and increasing in zinc concentration into the bulk. ’he objective of this experiment was to characterise adsorption of metallic Pd at the ‘ZnOX’ surface under well controlled conditions. A few salient points emerge fm a recent review of related studies (ref.

882

10) and these are: (I) Pd deposition by evaporation fm a hot wire is a well used and controllable process. (11) On ZnO substrates, Pd (with other mighbourlng metals; Ag, Au, N i (refs. 11-13) displays a noc-face specific growth mode. For polar and non-polar faces, layer by layer mechanisms are reported for low Pd coverages. (111) It i s hportant t o be aware of dynamic diffusion processes that can occur i n these experiments. Vertical' (Into surface) and 'lateral' (across surface) have been reported prevlously (refs. llr-16) f o r Pd a t a substrate surface. Mffusion of zinc (ref. 16) or oxygen through the metal overlayer may also be important. Diffusion of oxygen through N i overlayers on NiO(0001) has been reported previously (ref. 13). (IV) Chemical reactiqn of adsorbed Pd With either host elanent of ZnO has not been reported In any Pd/ZnO study. However, I n recent work by Huck e t 61. (ref. 1 4 ) , surface oxidation of adsorbed Pd t o PdO i s proposed following Pd deposition on Sn02(110). Lattice Sn is observed to reduce t o metallic form in this process. (This can have serious consequences upon the diffusion properties of Pd a t t h e surface, since the ionic form of Pd has a much enhanced diffusion r a t e i n semiconducting oxides compared with t h a t f o r metallic W (ref. 17)). Pd2SI has been detected a t PdSi phase boundaries when Pd is adsorbed on S i and SI/SI02. In the case of SI/SI02 thls occurs after d i f f u s i o n of Pd through the oxide i t s e l f ( r e f . 15) and i s a subsurface reaction, whereas Pd2SI is directly formed on Si(ll1) (ref. 18).

Two separate UHV systems were used i n these experiments, one for studies by Secondary Ion Mass Spectrometry (SIMS) and the other f o r analysis by electron spectroscopy.

XPS/AEs Experiments 'Ihese were performed In a modified Vcf ESCA mUII, details of which are published elsewhere ( r e f . 8). After mounting a f r e s h l y c u t and polished Zn(OOO1) crystal, the Instrument was baked (base pressure < 5 X 10-l' mbar) and the sample further cleaned using etch/anneal cycles. It was then extensively oxidised (ref. 9 ) to ZnOX by the crystal t o 573K I n mbar of oxygen (5N Messer Greshiem) for 30 mins. X ray photoelectron (XPS), X ray Induced Auger electron (XAES) and nanoamp Auger electron (nAES) spectra were recorded following each Pd deposition. For XPS analysis, a hi@ s e n s i t i v i t y (CAE = 50 eV) multiple scan of the Pd 3d region was used t o evaluate surface coverage. 0 Is, Zn XAES and Zn 2p regions were also multiply scanned at higher resolution (CAE = 20eV). I n the Auger analysis, the spectrometer was repeat scanned (CRR = 2 ) between 50 and 1050 eV (KE),

883

encompassing the Pd MNN, 0 KLL and Zn IMM transitions. This spectrum was also used to monltor surface cleanliness. SIMS Experiments These were performed in a custom built UHV system also described in hll detail elsewhere (ref. 19). After mounting, the Zn(OOO1) crystal was sMlarly cleaned by etch/anneal cycles, until mlnlmal impurity levels were detected by Static SIMS (SSIMS). A &OX surface was then generated as for the X P S / m experiments (ref. 9). SSIMS analyses were performed using Art primary ions at 3 kV and an ion current of 3-4 nAcn~-~(measured on target). !he spot size was 2-3 mn2 SSIMS spectra were recorded following each Pd deposition at high (for qualitative determination from isotopic distribution analysis) and low resolution (for quantitatlve analysis from peak area meaurements).

.

Palladim Deposition Pd (5N, Ooodfellows metals, 0.125 mn dim.) was deposited from a hot wire evaporation source (ref. 9) using a stabillsed power supply (0-10 A). Accurate sample positioning for deposition was calibrated during installation and optimdl degassing and operatlng conditions were evaluated from a series of trial experiments. ?he experimental configuration was reproduced exactly for each instmanent. RESULTS

Chemical Effects of Deposition Initial Inspection of the behaviour of the Pd source Is shown In Fig. 1, where the Pd 3d doublet and the Zn 2p3/2 peak intensities are plotted against exposure time. Although this type of plot is not the most useful f o r interpretation of growth mode characteristics, the smooth curve clearly demonstrates good reproducibility and stability I n the performance of the Pd evaporator. Fig. 2 shows the development of the Pd 3d doublet following sequential dosing of the surface. 'Ihe growth of Pd 3dgI2 and Pd 3d3/2 peaks at 334.8 eV and 340.0 eY respectively Is clear and these binding energies cmpare favourably with previous WS analyses of clean Pd surfaces (refs. 20-22). No significant change in binding energy of these peaks is observed throughout deposition. SMlarly, no chemical shlftlng is detected in the 0 Is spectra at 530.5eV, although this is not clear f r o m first inspection due to the influence of the devloplng Pd 3p3,2 intensity (see Fig. 3) upto saturation. At'this level of exposure the Zn 2p3/2 W S peak at 1022.9 eV is barely detectable, indicating that coverage at this exposure approaches the escape depth for the

884

XPS Peak Intensity vs Exposure

I

4.

Fig. 1. A plot of XPS peak area against exposure time for; + = Pd 3d Doublet, and 0 = Zn 2p 3,2 i n t e n s i t i e s

*

Zn 2p3/2 electron through Pd (" 7 1?he low resolution positive ion SSIW spectra f o r sequential Pd exposures are shown i n Fig. 4. The gradual developnent of new peaks around 100 and 165 t t mu upon Pd deposition is clear and these have been assigned t o Pd and PdZn species frm t h e i r isotope s p l i t t i n g patterns. No evidence i s seen i n the positive o r negative ion spectra f o r the formation of PdO ions. It can be seen from these spectra that the e f f e c t of Pd deposition is t o cover the ZnOX t t surface, suppressing the Zn and ZnO/OH s l g ~ 2 . st o a similar extent. The Pd' peak appears t o grow consistently with exposure, u n t i l , at saturation, it is the only significant spectral feature. In t h e negative ion spectra the ZnO/OK and Zn2/02K are suppressed as Pd is deposited. Growth Mode Evaluation ?he growth mechanism can be most readily deduced f o r t h i s system, using the XPS results, when the modelling plots of Biberian and Somorjal (ref. 24) are applied. Fig. 5 shows a plot of Pd 3d intensity against Zn 2p intensity 3/2 (based on peak area measurement using instrument software). From t h i s two breaks i n gradient can be clearly seen and the curve represents that f o r Franqk-van der Merwe (FM) growth, with each curve break corresponding t o the

* Calculated from the hanogemus overlayer model calculations of Seah and Dench (ref. 23) for an inorganic system.

885

CleanZnoX

53

535

-Binding EnergyIeV Fig. 2. Sequential XPS spectra of the Pd 3d doublet upon exposure of ZnOX t o Pd.

-

Flg. 3. Sequential XPS spectra of the 0 Is region upon exposure of ZnOX t o Pd.

formation of the first and second monolayers ( CJ1 and 9 2). me Inelastic mean free path f o r t h e Zn 2p 32, e l e c t r o n c a l c u l a t e d from t h e s e p o i n t s is comparable with t h a t predicted In the qUantitative plots of &ah and k n c h (see refs 10,241. For growth mode evaluation f m n nAEs and SSlMs data, plots of n o w i s e d intensity against exposure t h e have been used and these are presented a o r g wlth the XPS data In Figs. 6 (a)b(b). Auger Intensities could readily be deduced from peak t o Peak height measurement and consideration of the oxygen Intensity is now possible since, in the Auger spectra, the 0 KLL transition is Intense and well resolved (see Fig. 7). Changes In Intenslty, for the SSIMs spectra have been determined by triangulation area measurement.

886

DISCUSSION

It can be seen from these results that Pd overlayers have been successfully prepared on the ZnOX surf'ace by Pd evaporation. Ihe useflilness of XPS, SINS and nAES for detectchanical changes d Intensity ratio modelling plots following Pd deposition have been demonstrated. In all cases layer by layer growth (FM) has been deduced in accord with the obsemmtions of Gaebler e t al. (ref. 111, who have monitored Pd growth on both polar faces of ZnO a t similar substrate temperatures. All spectra indicate that no surface c h d c a l reaction has occured following adsorption and there is no spectral evidence for the oxidation of adsorbed Pd. mrther, it is concluded that diffusion of surface oxygen through the Pd adlayer(s) does not occur. Scheisser and Jacob1 (ref. 13)

c

exposure /sec

3

1

300

520

840 2000

3320

1 ) 1 ' 50 75 10 -mass/arrm-

F&.

4. Sequential low resolution SSIMS s p e c t r a for the deposition of Pd on ZnOX.

Growth Mode Plot (XPS,

Zn

F i g . 5. Growth mode plot for the depostion of Pd on ZnOX fm XPS data.

887

Comparison of Gnmth Mode Data 6) s=sims x= xp

species

I

ratio

0

10 -exjmure/min

-

Fig. 6(a). A comparison of growth mode from all three analyses for low coverages of Pd. have reported an increase in the 0 KLL:Zn lvQJN Auger intensity ratio for the adsorption of Ni on ZnO(OOO1) and thls has been attributed to oxygen diffuslan through the overlayer. No significant variance in the 0 KLL:Zn MNN Auger intensity ratio can be seen in these results. Similarly, in the SSIplls analysis the ZnO/OH+ : Zn' does not vary slgnlflcantly with exposure.

I

Comparison of Gronth Mode Data (b)

Fig. 6 ( b ) . A canparison of growth mode data from all three analyses over coverages (symbols as for Rtg. 6(a>.>

888

I " " ' " " 1 However, reexamination of the final surface, at maximum Pd coverage after 64hrs i n vacuum, revealed that the Pd Auger yield had fallen significantly and that the 0 and Z n signal Intensities had Increased concmnltantly and by shllar amounts. The spectra clearly Indicate less effective coverage of the ZnOX surface and this is thought t o be due t o migration of Pd Into or across the substrate surface. Determlnation of the dominant mechanism (ie; 'vertical' o r tlaterdll diff'usion) is impossible from these results. Jacobs e t al. (ref. 25) deduce that lateral Pd diffusion is the dominating mechanism i n this temperature regime f o r Pd adsorbed on ZnO(OOO1). However, i n these experiments, Art etching of this sample Indicated the presence of a significant level of Pd In the subsurface of ZnOX and it was clear , , , that vertical diffusion had occurred. meed Pd diffusion into ZnOX canpared with ZnO(OOO1) is not unexpected since the defect F&. 7. Sequential nA Auger spectra density a t ZnOX i s l i k e l y t o be for the deposition of Pd on considerably greater than ZnO(O001) ZnOX and defects such as grain boundaries are expected t o provide favourable tvertical' diffusion paths f o r the migrant. Excellent agreement i s seen i n the growth mode p l o t s f o r a l l of the techniques used (see Figs. 6 (a)&(b)). Best correlation is noted at e1 (=lML), e2 i s better defined and more evident after a shorter exposure t h e in the SSIlrIs analysis compared with the results from the electron spectroscopies. This can be a t t r i b u t e d t o t h e g r e a t e r surface s p e c i f i c i t y of the SSIMS analysis. As van Delft e t a l . (refs. 26,271 note, AES and XPS measure a

:v2

,M,MNN

.

889

weighted average of several layers near the surface. Therefore, despite "2ML

coverage of Pd a t the ZnOX surface, the effect on these plots can be modified by t h e presence of Zn (and 0) i n t e n s i t y i n t h e e l e c t r o n s p e c t r a from

underlying Zn (and 0) atans. Hence i n these plots the turning point at e2 is somewhat less w e l l resolved. Under the conditions used f o r the SSIMS experiments, it is proposed that the features observed are derived fm the first 2-3 atcanic layers. Consequently after e2s the Znt s i g n a l at 64 amu is t t much less intense and the Pd /Zn r a t i o rises sharply. Further evidence f o r enhanced surface specificity i n the SSIMS data can be learnt from a closer t inspection of the ZnPd cluster ion behaviour.

Mixed Positive ion Emission vs Exposure

00 10,O

250

50,O

F i g . 8. ZnPd' cluster ion r a t i o (nomalised t o the t o t a l Pd' intensity) plotted against exposure time.

t Znt

For a sampling depth of "2ML the statistical maximum secondary ion yield predicted for t h i s ion (relative t o the t o t a l Znt t Pdt ion Intensity) w i l l occur at el and this i s seen in F i g . 8. !lhe ZnPdt/[Znt + Pd'] ion intensity rises sharply t o a maxirman a t an exposure equivalent of "1ML ("15min). The mixed ion intensity then falls as the second layer develops. This is seen, not only as evidence of a shallower SSIMS sampling depth ("2ML), but also as further evidence f o r layer by layer growth of Pd. SUMMARY AND CONCLUSION Pd overlayers have been deposited a t the surface of an extensively oxidised Zn(0001) s u r f a c e (ZnOX), i n a well controlled and reproducible manner. For every experiment using XPS, nAES and SSm; (i)the growth mode of

890 Pd has been found to be layer by layer

(m),(11) no

chemical reaction between

adsorbate with e i t h e r host element has been detected and (111) preferential migration of Zn or 0 has not been detected. Evidence of diff'usion of Pd i n t o and across %OX

is forwarded, although the dominating mechanism has yet t o be

determined. The high s u r f a c e s p e c i f i c i t y of t h e SIMS a n a l y s i s has been demonstrated.

ACKNOWLEIXEMENl'S 'Ihe Health and Safety Executive and The Science and hgineering Research Council are acknowledged for t h e i r financial support. REFwENcEs

1

G. Natta, Catalysis, 3, P.H hmett (Editor), b i n h o l d , New York, 1955, p

2

3

f o r example see; T.A. Jones, Sensor Review, Jan 1982, pp. 14-19. A. Jones, T.A. Jones, B. Mann and J.G. Firth, Sensors and Actuators, 5 (19841, PP* 75-88. B. Bott, T.A. Jones and B. Mann, Sensors and Actuators, 5 (1984), pp. 65-73. T.A. Jones, P. Moseley and B. Tofield, Chemistry in Britain (Aug. 1987), PP. 749-754. G.Heiland, E. Mollwo and F. Stoclanann, Sol. State Physics, 8 (19591, p

4

H.E. Brown, "Zinc Oxide

5

6

7 8

9 10 11 12

13 14

15 16 17

349.

191.

(1976).

- Properties and Applications", (ILZRO), New York 'Zinc Oxide - Properties and Behawlour of the Bulk', E.

W. Hirschwald, Kaldis (Editor), Curr. Top. Mat. Sci., 7 (1981), pp. 143-482. G. Heiland and H. Luth, 'Adsorption on Oxides', The Chemical Physics of Solid Surfaces and Heterogeneous Catalysts", Vol. 3, D.A. King and D.P. Woodruff (Editors), Elsevier, Amsterdam. C.S. John, 'Catalysis by Zinc Oxide' , !he Chemical Society, Specialist Periodical Reports, 3, "Catalysis", pp. 169189. A.J. Swift, PhD. Thesis (1988), UMIST, PO Box 88, Sackville Street, Manchester M60 lQD, United Kingdom, C h s . 4 and 5, pp. 94-204 and references therein. A.J. Swift and J.C. Vickerman, i n preparation. A.J. Swift, PhD. Thesis (1988), UMIST, PO Box 88, Sackville Street, Manchester M60 lQD, United Kingdcan, Ch. 6, pp. 205-210. W. Gaebler, K. Jacobi and W. M e , Surf. Sci., 75 (1978), pp. 35-67, E.F. Wassermann and K. Polacek, Appl. Phys. Lett., 16 (1970), p 259. D. Schneisser and K. Jacobi, Surf. Sci., 88 (1979), pp. 138-152. R. h c k , P. Kohl and G. Heiland, Proc. Int. Symp. 'Trends and New Applications I n Thim Films', Strasbourg, March 1987, pp. 1-5. B. Schleich, D. S c h e i s s e r and W. Gopel, Surf. Sci. in press, (1987). A. Fasana and L. Braicovich, Surf. Sci., 120 (1982), pp. 239250. H.J. de Bruin and M. "antreeratana, J. Phys. Chem. Solids, 42 (1981), pp.

333-3340 18 S. Nishigaki, T. Komatsu, M. Arimoto and M. Suglhara, Surf. Sci., 167 (19861, PP- 27-38.

19 A. Brown, PhD. 'Ihesis (1980), UMIST, PO Box 88, Sackville Street, 0 United rUngaan, Ch. 2, pp. 47-53. Manchester ~ 6 lQD, 20 C.D. Wagner, W.M. Riggs, L.E. Wries, J.F. Moulder and G.E. Muilenberg, 'Handbook of X Ray Photoelectron Spectroscopy', Perkirr-Elmer Corporation, Physkal Electronics Division, Eden Prairie, Minnesota (1979).

891

21 L. Hilaire, P. Legare, Y. Holl and G. Marie, Sol. St. Comn., 32 (1979), p. 157. 22 P. Weightman and P.T. Andrews, J. Phys. C. Sol. St. Phys., 13, L815, L821 (1980)23 M.P. %ah and W.A. Dench, Surf. and Int. Anal., l(1) (1979), pp. 1-11. 24 J.P. Biberlan and G.A. Scmorjai, Appl. Surf. Scl., 2 (1979), pp. 352-358. 25 H. Jacobs, W. M o m , D. Kohl and G. Heiland, Surf. &I 160 ., (1985), pp. 217-234. 26 F.C.M.J.M. van Delft, A.D. van Langeveld and B.E. Nleuwenhuys, lhin Solid Films, 123 (19851, PP. 333-351. 27 F.C.M.J.M. van Delft and B.E. Nleuwenhuys, Solid State Ionlcs, 16 (1985), pp. 233-240.


C. Morterra, A. Zecchina and G . Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

893

N I T R I C O X I D E ADSORPTION ON ( 1 1 1 ) AND (001) SURFACES OF DIAMOND-LIKE CRYSTALS. A THEORETICAL STUDY ON MODEL FINITE CLUSTERS

M. TOSCANO and N. RUSSO D i p a r t i m e n t o d i Chimica, U n i v e r s i t s d e l l a C a l a b r i a , 1-87030 Arcavacata d i Rende, Cosenza ( I t a l y ) ABSTRACT We p r e s e n t h e r e a MNDO t h e o r e t i c a l s t u d y of t h e a d s o r p t i o n o f n i t r i c o x i d e on t h e (111) and (001 1 i d e a l i z e d u n r e c o n s t r u c t e d s u r f a c e s o f d i a m o n d - l i ke c r y s t a l s (C, S i , Gel. N i t r i c o x i d e i s p r e d i c t e d t o b i n d p r e f e r e n t i a l l y a t o n - t o p p o s i t i o n o f ( 1 1 1 ) s u r f a c e f o r a l l c o n s i d e r e d systems. F o r t h e carbon ( 0 0 1 ) subs t r a t e , o u r r e s u l t s show t h a t t h e m o l e c u l a r c h e m i s o r p t i o n o c c u r s a t t h e o n - t o p s i t e which i s s l i g h t l y p r e f e r r e d o v e r t h e b r i d g e one, whereas i n t h e case o f s i l i c o n and germanium (001) s u r f a c e s , t h e b r i d g e p o s i t i o n becomes t h e most s t a b l e one. D i s s o c i a t i v e c h e m i s o r p t i o n has been i n v e s t i g a t e d i n t h e case o f ( 0 0 1 ) s u r f a c e s and r e p r e s e n t s t h e b e s t process f r o m a thermodynamic p o i n t o f view. INTRODUCTION The i n t e r a c t i o n o f n i t r o g e n c o n t a i n i n g gas molecules (NO, NO2, NH3 e t c . ) w i t h b o t h (111) and (001) surfaces o f d i a m o n d - l i k e c r y s t a l s has been i n v e s t i gated, i n t h e s e l a s t y e a r s , by d i f f e r e n t e x p e r i m e n t a l t e c h n i q u e s ( r e f s . 1 - 1 0 ] . The most s t u d i e d s u b s t r a t e s a r e t h e surfaces o f s i l i c o n ( r e f s . 1-5,7,8,10)

and

germanium ( r e f s . 6,9), whereas f o r carbon no d a t a a r e r e p o r t e d i n l i t e r a t u r e . I n p a r t i c u l a r t h e a d s o r p t i o n of n i t r i c o x i d e (NO) on t h e ( 1 1 1 ) and (001) s u r f a c e s o f s i 1 i c o n has been i n v e s t i g a t e d u s i n g Auger E l e c t r o n Spectroscopy (AES) ( r e f s . 7,101 and Low Energy E l e c t r o n D i f f r a c t i o n (LEED) ( r e f . 1 0 ) . To o u r knowl e d g e o n l y two Laser Induced Fluorescence ( L I F ) s t u d i e s o f N O on germanium s u r f a c e s have been p u b l i s h e d ( r e f s . 6,9). N o t w i t h s t a n d i n g t h e s e i n v e s t i g a t i o n s some q u e s t i o n s remain u n r e s o l v e d : i1 The NO p r e f e r r e d c h e m i s o r p t i o n s i t e ; i i ) The n a t u r e o f t h e c h e m i s o r p t i o n process ( i .e. d i s s o c i a t i v e o r m o l e c u l a r ) . I n t h i s paper we r e p o r t t h e f i r s t t h e o r e t i c a l s t u d y performed b o t h f o r mol e c u l a r and d i s s o c i a t i v e c h e m i s o r p t i o n of NO on model c l u s t e r s r e p r e s e n t i n g t h e (111) and (001) s u r f a c e o f carbon, s i l i c o n and germanium. The s t u d y has been performed u s i n g t h e s e m i e m p i r i c a l MNDO ( M o d i f i e d N e g l e c t o f D i f f e r e n t i a l Overlap) method, p r e v i o u s l y t e s t e d as an e f f e c t i v e t o o l t o ob-

894

t a i n r e l i a b l e r e s u l t s f o r several chemisorption processes on diamond-like c r y s t a l surfaces ( r e f s . 11-16). I n p a r t i c u l a r , i n t h e case o f oxygen on s i l i c o n ( r e f . 111, t h e c a l c u l a t e d MNDO parameters are i n good agreement w i t h t h e abi n i t i o C I r e s u l t s ( r e f . 17) (1.920

A

f o r t h e dSi-O

e q u i l i b r i u m distance a t

both levels; 5.31 versus 4.85 eV o f binding energy a t MNDO and MC-SCF l e v e l s respectively, i n t h e absolute minimum). Also f o r the hydrogen chemisorption on s i l i c o n surface ( r e f . 15) a good agreement i s found, i n f a c t , t h e MNDO value o f binding energy i s t h e same (4.20 eV) o f t h a t derived by experimental i n d i c a t i o n ( r e f . 18) and d i f f e r s by o n l y 0.04 eV from a b - i n i t i o value ( r e f . 15). I n t h e case o f hydrogen adsorption on germanium, a comparison i s possible o n l y w i t h t h e a b - i n i t i o r e s u l t s ( r e f . 19). The binding energy and t h e bond length obtained by MNDO f o r GeH4 c l u s t e r are 2.70 eV and 1.502 i n agreement w i t h t h e a b - i n i t i o r e s u l t s (2.94 eV and 1.556

a respectively

A).

O f course, t h e semiempirical nature o f MNDO and t h e l i m i t a t i o n s o f t h e em-

ployed s t r u c t u r a l models (i.e. t h e i d e a l unreconstructed surfaces o f our c l u s t e r s ) suggest t h a t t h e r e s u l t s s h a l l be regarded w i t h some caution. On t h e other hand, since chemisorption i s a l o c a l phenomenon, i n some case t h e influence o f r e c o n s t r u c t i o n does n o t s i g n i f i c a n t l y change t h e values o f t h e chemis o r p t i o n parameters ( i . e . bond lengths and binding energies) ( r e f s . 16-17,211. COMPUTATIONAL DETAILS As commonly reported i n 1it e r a t u r e ( r e f s . 11-17

1, embedding hydrogen atoms

have been used t o terminate t h e model c l u s t e r s . The i d e a l bulk geometry o f d i a mond-like c r y s t a l s (i.e.

d

c-c

=1.542 A; dS-iSi

=2.345 A; dGe-Ge =2.450

l ; and

t e t r a h e d r a l valence angles) has been used i n a l l c a l c u l a t i o n s . As i n previous works ( r e f s . 11-15), t h e surface r e l a x a t i o n and r e c o n s t r u c t i o n have been neglected. For t h e embedding hydrogen a C-H, S i - H and Ge-H distance o f 1.10, 1.50 and 1.54

A

respectively, has been employed. A l l t h e c a l c u l a t i o n s have been per-

formed using t h e AMPAC package ( r e f . 22) i n t h e so c a l l e d h a l f - e l e c t r o n approximation (HE) a t MNDO l e v e l ( r e f . 231. For t h e ( i l l ) surfaces two c h a r a c t e r i s t i c chemisorption s i t e s o f high symmet r y have been considered, whereas t h r e e s i t e s have been selected f o r t h e (001)

s u r f aces.

I n t h e case o f d i s s o c i a t i v e process, several f i n a l products, i n d i f f e r e n t t o p o l o g i c a l d i s p o s i t i o n s , have been taken i n t o account. The model c l u s t e r s , chemisorption s i t e s and geometric parameter

definitions

895

are shown i n F i g . 1 and 2 . RESULTS AND DISCUSSION

a ) General considerations. As a f i r s t step o f t h e work, we have considered t h e p o s s i b i l i t y f o r NO mol e c u l e t o b i n d w i t h N o r 0 side t o t h e surface. I n a l l cases we have found t h a t t h e NO adsorption occurs w i t h t h e n i t r o g e n atom towards t h e surface and t h e oxygen atom towards t h e vacuum. This r e s u l t i s i n agreement w i t h a recent t h e o r e t i c a l study on t h e i n t e r a c t i o n of NO w i t h t h e basal plane o f graphite ( r e f . 24) and w i t h t h e expression proposed by Shustorovic ( r e f . 25) on t h e bas i s o f a study o f NO on metals. On t h e other hand, very recent experimental i n v e s t i g a t i o n s o f NO on Si(OO1) 2x1 surface ( r e f . 10) g i v e a f u r t h e r confirmation o f this result. b ) Molecular chemisorption.

P

on -top

open

on- top

bridge

Fig.1. Clusters and chemisorption s i t e s employed i n t h e study o f molecular NO adsorption on the (111) and (001) diamond-like surfaces.

896

I n t a b l e 1 we r e p o r t t h e r e s u l t s f o r t h e m o l e c u l a r c h e m i s o r p t i o n o f NO on t h e (111) surface o f diamond-like c r y s t a l s . An i n s p e c t i o n o f t h i s t a b l e shows t h a t f o r a l l t h e t h r e e c o n s i d e r e d subs t r a t e s , t h e m o l e c u l a r c h e m i s o r p t i o n p r e f e r e n t i a l l y o c c u r s a t t h e o n - t o p pos i t i o n s . We n o t e t h a t t h e e l e c t r o n i c and g e o m e t r i c a l parameters a r e e s s e n t i a l l y i n s e n s i t i v e g o i n g f r o m an X H

4 9

to X

H

10 15

cluster.

TABLE 1 Computed a d s o r p t i o n p r o p e r t i e s f o r t h e m o l e c u l a r c h e m i s o r p t i o n o f NO on t h e ( 1 1 1 ) s u r f a c e o f d i a m o n d - l i k e c r y s t a l s (C, S i , Gel. The model c l u s t e r s , ads o r p t i o n s i t e s and d e f i n i t i o n s o f geometric parameters a r e shown i n F i g . 1. Cluster

Site

R/i

d/i

dNV0/I

BE/eV

qN/a.u.

qo/a.u.

a/'

CARBON C4Hg-NO

On-top

1.534

1.534

1.158

1.15

0.110

-0.142

121.18

H

On-top

1.530

1.530

1.158

1.20

0.110

-0.143

121.22

Open

1.171

1.867

1.165

0.02

0.206

-0.302

179.99

C

-NO

10 15 C10H13-N0

SILICON Si4Hg-NO

On-top

1.855

1.855

1.151

1.32

-0.002

-0.097

122.64

Sil0Hl5-NO

On-top

1.855

1.855

1.151

1.33

-0.016

-0.096

123.84

1.141

2.493

1.132

-1.24

0.250

-0.061

178.34

Si10H13-N0 Open

GERMANIUM

Ge H -NO 4 9 Gel0Hl5-NO

On-top

1.988

1.988

1.151

0.62

0.053

-0.148

121.11

On-top

1.988

1.988

1.153

0.52

0.058

-0.147

122.05

1.483

2.754

1.151

-1.89

-0.055

-0.201

179.98

Ge10H13-N0 Open

The b i n d i n g energy ( B E ) f o r t h e o n - t o p a d s o r p t i o n remains a t t h e same v a l u e i n t h e case o f carbon and s i l i c o n s u r f a c e s (about 1.2 and 1.3 eV r e s p e c t i v e l y ) and decreases i n t h e case o f germanium. The i n t e r a c t i o n o f NO w i t h t h e open s i t e i s s t r o n g l y unfavoured. The c a l c u l a t e d BE a r e t h e r m o n e u t r a l ( f o r c a r b o n ) o r endothermic ( f o r s i l i c o n and germanium). Due t o t h e l a c k o f e x p e r i m e n t a l i n f o r m a t i o n around t h e p r e f e r r e d c h e m i s o r p t i o n s i t e o f NO, we can o n l y compar e t h e p r e s e n t r e s u l t s w i t h t h e o t h e r t h e o r e t i c a l d a t a performed f o r d i f f e r e n t adsorbates on d i a m o n d - l i k e c r y s t a l s . Our d a t a c o n f i r m t h e tendency, p r e v i o u s t y found i n t h e cases o f hydrogen ( r e f . 151, halogens ( r e f . 13) and oxygen (ref.111,

897 t o i n t e r a c t w i t h t h e monocoordinated s i t e s o f t h e C and S i (111) surfaces. The a n a l y s i s o f t h e geometrical parameters r e v e a l s a non l i n e a r e q u i l i b r i u m o r i e n t a t i o n o f adsorbed n i t r i c o x i d e and t h e valence angle assumes a value o f about 121" f o r a l l k i n d s of considered substrates. The e q u i l i b r i u m bond l e n g t h d increase

a-

going from C t o Ge as expected by t h e p e r i o d i c p r o p e r t i e s ( i . e .

tomic r a d i u s ) . I n t a b l e 2 our r e s u l t s f o r t h e (001) surfaces a r e c o l l e c t e d . TABLE 2 Geometric and e l e c t r o n i c parameters f o r t h e molecular adsorption o f n i t r i c o x i de on t h e (001) surface o f diamond-like c r y s t a l s (C, S i , Gel. The model c l u s t e r s , adsorption s i t e s and d e f i n i t i o n s o f geometric parameters are shown i n F i g . 1.

Cluster

Site

R/i

d/i

dN-/i

BE/eV

qN/a.u.

q /a.u. 0

a/"

CARBON

CgH14-N0

On-top

1.307

1.307

1.188

2.78

0.090

-0.176

135.05

CgH12-N0

Bridge

0.840

1.514

1.168

2.48

0.337

-0.030

179.99

C15H16-N0

Open

1.291

2.200

1.121

-1.85

0.368

-0.016

179.92

SigH14-N0

On-top

1.812

1.154

-0.50

0.014

-0.011

126.92

SigH12-N0

Bridge

1.178

2.09

-0.197

-0.130

179.99

1.132

-0.85

-0.014

-0.032

179.97

SILICON

S i ,5H16-N0 Open

0.557 1.091

1.812 1.999 2.926

GERMANUI M

Ge H -NO 9 14 GegH12-N0

On-top

1.944

1.944

1.162

-0.21

-0.371

-0.093

127.99

Bridge

0.872

2.856

1.167

1.61

-0.144

-0.092

179.97

/

/

/

0.0

0.0

0.0

/

Ge15H16-N0 Open

I t i s e v i d e n t from t h i s t a b l e t h a t , f o r t h e carbon substrate, both on-top

and b r i d g e s i t e s are minima i n t h e p o t e n t i a l energy surfaces. The on-top p o s i t i o n i s s l i g h t l y favoured over t h e b r i d g e one (2.78 versus 2.48 eV) whereas t h e f o u r - c o o r d i n a t e d s i t u a t i o n (open s i t e ) i s s t r o n g l y endothermic (-1.85 eV) and c h a r a c t e r i z e d by a very l o n g d i s t a n c e from t h e s u r f a c e (2.200 A). Given t h e small d i f f e r e n c e i n BE ( o n l y 0.3 eV) between t h e on-top and b r i d g e s i t e s , i t i s q u i t e p o s s i b l e t h a t NO adsorbes a t both these s i t e s .

898

For t h e s i l i c o n and germanium surfaces t h e bridge p o s i t i o n becomes the most s t a b l e one. For both these surfaces t h e on-top chemisorption i s unfavoured as w e l l as t h e coordination a t t h e open s i t e . The value o f BE f o r t h e bridge s i t e s decreases going from 2.48 eV, f o r t h e carbon surface, t o 2.09 eV f o r s i l i c o n and 1.61 eV f o r germanium. As i n t h e case o f the (111) surface and f o r other studied systems ( i . e .

atomic hydrogen

( r e f s . 14-15), halogens ( r e f . 26) and water ( r e f . 2 7 ) ) , the lowest value o f t h e binding energy i s found f o r germanium substrate. This can be regarded as an i n d i c a t i o n o f a smaller r e a c t i v i t y o f t h i s surface w i t h respect t o carbon and s i l i c o n ones.

As f a r as t h e geometrical parameters are concerned, we note t h a t f o r t h e ont o p s i t u a t i o n , t h e value o f valence angle i s near 120" i n a l l cases ( t h e same values are found f o r t h e (111) surfaces) w h i l e f o r t h e bridge s i t e s , t h e adsorp t i o n occurs w i t h t h e molecule appoaching perpendicularly t o t h e surfaces ( a = 180"). For both t h e studied surfaces t h e r e s u l t s o f t a b l e 1 and 2 reveal t h a t t h e NO distance does not s i g n i f i c a n t l y d i f f e r from t h a t o f t h e i s o l a t e d molecul e (1.124

a t HE-MNDO l e v e l ) . This r e s u l t i s an i n d i c a t i o n t h a t t h e possible

d i s s o c i a t i o n o f NO molecule must i n v o l v e an a c t i v a t i o n b a r r i e r . F i n a l l y our data i n d i c a t e t h a t a substrate-NO charge t r a n s f e r occurs i n t h e chemisorption processes. Notwithstanding some q u a n t i t a t i v e d i f f e r e n c e s and t a k i n g i n t o account t h e l i m i t o f the parametrization ( e x p e c i a l l y f o r germanium) and t h e embedding hydrogens, t h i s charge t r a n s f e r has an homogeneous t r e n d i n t h e substrate series and both N and 0 atoms c o n t r i b u t e t o t h i s phenomenon. c ) D i s s o c i a t i v e chemisorption. The d i s s o c i a t i v e adsorption o f NO has been considered i n t h e case o f (001) surfaces o f diamond-like c r y s t a l s . I n F i g . 2 t h e possible products and r e l a t i v e chemisorption s i t e s o f t h e d i s s o c i a t i v e process are shown.

heenergetic, elec-

t r o n i c and geometrical c h a r a c t e r i z a t i o n s f o r each d i s s o c i a t ve topology are c o l l e c t e d i n t a b l e 3. As i s evident from t h i s table, t h e d i s s o c i a t i v e fashion

s preferred over

t h e molecular one, from a thermodynamic p o i n t o f view i n a l l considered c r y s t a l s . The exothermicity decreases going from carbon t o s i l i c o n and germanium (about 4.1,

1.0 and 0.3 eV r e s p e c t i v e l y ) .

Only f o r t h e s i l i c o n (001) surface experimental information i s a v a i l a b l e and i n d i c a t e s t h a t t h e d i s s o c i a t i o n o f NO i s observed a t 550 K. The preferred che-

899

m i s o r p t i o n s i t e s f o r t h e p r o d u c t s a r e d i f f e r e n t i n t h e t h r e e c o n s i d e r e d subs t r a t e s . I n t h e case o f carbon t h e f o u r s u r f a c e d i s p o s i t i o n s o f t h e N and 0 atoms a r e a l l e x o t h e r m i c .

'I'

N

0

0

Nand 0 on-bridge

'I' i

0

0 N

Oon-top; N on-bridge

F i g . 2. Schematic drawing f o r s e v e r a l p o s s i b l e p r o d u c t s o f t h e d i s s o c i a t i v e NO a d s o r p t i o n on t h e (001 1 s u r f a c e o f d i a m o n d - l i k e c r y s t a l s . When we c o n s i d e r b o t h atoms i n t h e o n - t o p p o s i t i o n s , t h e h i g h e s t v a l u e o f e x o t h e r m i c i t y i s found (about 4.1 eV), w h i l e , when t h e c h e m i s o r p t i o n o f t h e p r o d u c t s o c c u r s a t t h e b r i d g e , t h i s v a l u e decreases a t about 1.5 eV. S i m i l a r v a l u e s a r e f o u n d f o r N and 0 atoms chemisorbed a t o n - t o p ( n i t r o g e n ) and b r i d ge (oxygen) o r o n - t o p (oxygen) and b r i d g e ( n i t r o g e n ) (1.9 and 1.5 eV r e s p e c t i v e l y ) . Going t o d i s c u s s t h e t o p o l o g i c a l d i s p o s i t i o n s o f t h e d i s s o c i a t i v e p r o d u c t s on s i l i c o n s u r f a c e , we n o t e t h a t o n l y t h e s i t u a t i o n i n which t h e N atom l i e s a t o n - t o p and oxygen on b r i d g e s i t e i s exothermic ( a b o u t 1.0 eV). F o r b o t h N and 0 a t o n - t o p p o s i t i o n we f i n d a t h e r m o n e u t r a l v a l u e , w h i l e f o r t h e o t h e r

cases a s t r o n g l y endothermicity i s found. TABLE 3 MNDO r e s u l t s f o r several p o s s i b l e products o f d i s s o c i a t i v e chemisorption o f n i t r i c o x i d e on t h e (001) diamond-like surfaces. For model c l u s t e r s , s i t e s and geometrical parameters see F i g . 2. The AHf o f C15H16, and 0 a r e 393.77,

Cluster

H

15 16’ Ge15H16y No’ 113.00 and 59.56 Kcallmol r e s p e c t i v e l y .

199.45, 650.12, -0.20,

Site

Si

dX-,/i

dX-N1/i

AHf/Kcal mo1-l

CARBON C1 5H1 6-N0

C15H16-N0 C H -N,O 15 16 C H -N,O 15 16 C1 5H1 6-N, 0 C

H -N,O 15 16

On-top

1.441

I

335.7

Bridge

1.516

/

334.5

On-top

1.270

1.218

240.5

Bridge On-top Bridge

1.382

1.479

300.7

1.271

1.388

290.2

1.383

1.220

299.2

Bridge On-top

SILICON Si15H16-N0

Bridge

1.998

/

155.4

Si15H16-N,0

On-top

1.545

1.547

231.5

Si15H16-N,0

Bridge On-top Bridge Bridge On-top

1.920

1.927

154.7

1.541

1.927

132.4

1.921

1.539

214.2

Si

H -N,O 15 16

S i 5H1 6-N,0

GERMANIUM

Gel 5H1 6-N0

Bridge

2.185

/

621.3

Ge15H16-N,0

On-top

1.628

1.606

654.9

Gel 5H1 6-N, 0

B r idge On-Top Bridge Bridge On-top

2.013

2.021

624.4

1.587

2.023

654.8

2.012

1.610

613.8

Gel 5H16-N,0 Ge

6-N, 0

F i n a l l y , i n t h e case o f germanium surface, we f i n d t h a t o n l y f o r t h e ads o r p t i o n o f N on b r i d g e and 0 on-top, t h e process i s s l i g h t l y exothermic (about 0.3 eV). Even i n t h i s case f o r t h e s i t u a t i o n w i t h both atoms i n b r i d g e

901

s i t e s , a t h e r m o n e u t r a l i t y i s f o u n d and f o r t h e r e m a i n i n g two combinations a s i g n i f i c a n t e n d o t h e r m i c i t y i s e v i d e n t ( a b o u t -1.5 eV i n b o t h c a s e s ) . From t h e s e r e s u l t s we can observe t h a t , though f o r a l l t h e t h r e e c o n s i d e r e d s u b s t r a t e s t h e d i s s o c i a t i v e i n t e r a c t i o n o f NO i s khermodinamically f a v o u r e d o v e r t h e m o l e c u l a r one, t h e d i s s o c i a t i o n mechanism and f i n a l t o p o l o g y o f t h e d i s s o c i a t i o n p r d u c t s on t h e s u r f a c e s are q u i t e d i f f e r e n t . T h i s means t h a t we can p o s t u l a t e a d i f f e r e n t r e a c t i v i t y o f t h e t h r e e s u b s t r a t e s w i t h r e s p e c t t o NO mol e c u l e . On t h e o t h e r hand, some d i f f e r e n c e s between carbon and s i l i c o n ( 0 0 1 ) s u r f a c e s have been a l s o observed i n t h e d i s s o c i a t i o n mechanism o f 0 molecules

2

( r e f s . 11,161. I n o r d e r t o b e t t e r understand t h e d i s s o c i a t i v e pathway o f NO, as w e l l as

and o t h e r adatoms, a c c u r a t e p o t e n t i a l energy surfaces, w i t h t h e l o c a l i 2 z a t i o n o f t h e t r a n s i t i o n s t a t e s , a r e necessary. Work i s i n p r o g r e s s i n t h i s for 0

respect i n our laboratory. CONCLUSIONS On t h e b a s i s o f o u r r e s u l t s t h e f o l l o w i n g c o n c l u s i o n s can be drawn: 1 ) I n t h e case o f m o l e c u l a r process t h e NO molecule chemisorbs, i n a l l c o n s i dered systems, w i t h t h e n i t r o g e n bonded t o t h e s u r f a c e . 2 ) The p r e f e r r e d c h e m i s o r p t i o n s i t e o f NO molecule i s t h e o n - t o p one ( s l i g h t l y

p r e f e r r e d over t h e b r i d g e one) i n t h e case o f carbon and t h e b r i d g e s i t e f o r s i l i c o n and germanium ( 0 0 1 ) surfaces. I n t h e case o f (111) surfaces t h e on-top p o s i t i o n i s always p r e f e r r e d . 3 ) Both m o l e c u l a r and d i s s o c i a t i v e i n t e r a c t i o n s a r e e x o t h e r m i c b u t t h e d i s s o -

c i a t i v e one i s thermodi nami c a l l y fovoured. 4 ) The behaviour o f t h e t h r e e s t u d i e d d i a m o n d - l i k e s u r f a c e s shows some analo-

g i e s (i.e.

f a d o u r t h e NO d i s s o c i a t i o n ) b u t a l s o some d i f f e r e n c e s ( i . e .

diffe-

r e n t t o p o l o g i c d i s p o s i t i o n o f NO d i s s o c i a t i o n p r o d u c t s ) . REFERENCES T. Isu and K. F u j i w a r a , S o l i d S t a t e Commun., 42 (1977) 477-480. M. N i s h i j m a and K. F u j i w a r a , S o l i d S t a t e Commun., 42 (1977) 101-104. M.D. Wiggins, R.J. B a i r d and P. Wynblatt, J . Vacuum S c i . Technol., 18 (1981) 965-969. M. N i s h i j i m , H. Kobayashi, K. Edamota and M. Onchi, S u r f a c e Sci., 137 (1984) 437-442. M. N i s h i j i m a , K. Edamota, Y. Kubota, H. Kobayashi and M. Onchi, S u r f a c e Sci., 158 (1985) 422-427. A. Modl, H. Robota, J . Segner, W. V i e l h a b e r , M.C. L i n and G. E r t l , J . Chem.

902

7 8 9 10

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Phys., 83 (1985) 4800-4806. D.R. He and F.W. Smith, Surface Sci., 154 (1985) 347-353. E.G. Keim, L. Wolterbeek and A. van S i l f h o u t , Surface Sci., 180 (1987) 565-571. F. Budde, T. Gritsch, A. Miodl, T.J. Chuang and G. E r t l , Surface Sci. 178 ( 1986) 798-805. A.G.B.M. Sasse, D.G. Lakerbeld and A. van S i l f h o u t , Surface Sci., 195 (1988) L167-L172; ibidem 199 (1988) 243-260. N. Russo, M. Toscano, V. Barone and F. L e l j , Phys. Lett., 113A (1985) 321325. G. Abbate, V. Barone, E. Iaconis, F. L e l j and N. RUSSO, Surface Sci., 152/ 153 (1985) 690-699. V. Barone, F. L e l j , N. Russo and M. Toscano, S o l i d State Comun., 59 (1986) 433-436. N.Russo , M. Toscano, V. Barone and F. L e l j , Surface Sci., 180 (1987) 599604; ibidem J. Chim. Phys., 84 (1987) 799-803. V. Barone, F. L e l j , N. Russo, M. Toscano, F. I l l a s and J. Rubio, Phys. Rev., B34 (198617203-7206. P. Badziag and W.S. Verwoerd, Surface Sci 183 (1987) 469-478. I.P. Batra, P.S. Bagus and K. Hermann, Phys. Rev. Lett., 52 (1984) 384-387. H. Froitzheim, H. Ibach and S. Lehwald, Phys. L e t t . , 55A (1975) 247-250. V. Barone, N. Russo and M. Toscano , t h i s volume. M. See1 and P.S. Bagus, Phys. Rev., 828 (1983) 2023-2031. N. Russo and M. Toscano, J. Vacuum Sci. Technol., A6 (1988) 1559-1560. J.J.P. Stewart, Quantum Chem. Prog. Exch. (QCPE) No 506. M.J.S. Dewar and W. Thiel, J. Am. Chem. SOC., 99 (19771 4899-4907). N. Russo, E. Iaconis, F. I l l a s and M. Toscano, Gazzetta, 118 (1988) 603605. E. Shustorovic, Surface Sci., 163 (1985) L730-L734. F. I l l a s , J. Rubio and J.M. Ricart, Phys. Rev., 831 (1985) 8086-8090. N. Russo and M. Toscano, t o be published.

.

C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reaetiuity of Surfaces 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SURFACE ANALYSIS OF THE ACTIVATION OF GE'ITER COMPOUNDS I. Vedel and L.Schlapbach Institut de Physique, Universite de Fribourg, CH-1700 Fribourg

ABSTRACT The surface study of the polycrystalline getter compounds Zr-V-Fe and ZrFe2 by means of X-ray photoeIectron spectroscopy has shown that Zr of a clean surface of Zr-V-Fe is oxidized to 45% after exposure to 1OL 400 C. 0 2 at 25 C while it remains metallic after the same exposure at Moreover, UHV annealing of Zr-V-Fe that has been previously exposed to air leads to the formation of a metallic surface. This observation corroborates recent results on this compound [l], a metallic surface is probably formed by the absorbtion of the oxygen. In contrast, the results on ZrFe2 indicate that the degree of oxidation of Zr increases with temperature. INTRODUCTION Getter materials have been used for many years in industrial and research laboratories to improve the vacuum in high vacuum systems or in tubes or to remove reactive gas impurities like 0 2 , N2, H 2 0 and CO from noble gases or from hydrogen. A class of materials which is most widely used in advanced applications are Zr based alloys like Zr-C, Zr-Ni, Zr-Al, and Zr-V-Fe. All of them require activation at elevated temperature to get higher speed and capacity for sorption [2]. The activation temperature depends on the compound. It is lowest for the ZrV-Fe alloy ( T ~ 4 0 0C). The activation mechanism is not well known, it depends on the type and strength of the adsorbate interaction with the metal surface. Recents results on as-received fractured-at-air surfaces of ZrNi, ZrAl and Zr-V-Fe alloys showed that the formation of a metallic surface is the principal process for activation [ 11 The oxygen diffuses from the surface into the bulk at elevated temperature. Other mechanisms such as surface segregation can be involved.

903

904

Up to now studies of the activation process were performed by comparing the results of a surface analysis before and after the activation treatment. The purpose of this work is to investigate the effect of the activation temperature on previously cleaned surfaces of Zr-V-Fe after different oxygen exposures (from 1L to 1OOOL) by means of the X-ray photoelectron spectroscopy (XPS) and to compare these results with those obtained on ZrFe2, which is known to have poor sorption properties in this range of temperature.

EXPERMENTAL The experiments were performed in a VG ESCALAB 5 photoelectron spectrometer using MgKa (1253.6 eV) radiation (Au 4f at 84.0 eV, FWHM = 1.2 eV). The base pressure was below 2.10-10 Torr. Both samples, Zr 70.5% - V 24.8% - Fe 4.7% (weight percent) and ZrFe2, were prepared by RF levitation melting and their crystal structure was verified by X-ray diffraction. The diffraction diagram of Zr-V-Fe showed the 220, 311, 222, 422, 511, and the 333 lines which correspond to Zr(Vo,89Feo,ll)2 in the cubic MgCu2 phase, a rather strong line at dhkl = 2.447 A and some finer lines which could not be indexed. The ZrFe2 diagram showed only lines of the cubic MgCu2 phase (ao= 7,082 k 0,002 A). Photoelectron spectra of Zr 3d, V 2p, Fe 2p and 0 1s core levels were recorded as a function of temperature and oxygen exposure. The surface concentration of the different elements was calculated from peak areas after subtracting a linear background. Theoretical atomic subshell photoionization cross sections [3], and a correction for different photoelectron mean free paths and analyser transmission at different photoelectron energies were used. The surfaces of the getter compounds were studied under two different conditions : 1- Zr-V-Fe and ZrFe2 fractured at air and annealed for one hour at 400 C in ultra high vacuum (UHV). 2- Zr-V-Fe and ZrFe2 with UHV-clean surfaces (diamond filed) and exposed to lL, lOL, 50L, lOOL and lOOOL of 0 2 (1L : 10-6 Torrs) at 25 C. Cleaning and oxygen exposure was then repeated at 200 C, 400 C and 550 C.

905

RESULTS 1- ZrFe2 and Zr-V-Fe exDosed to air and annealed at 400 C, The results of our core level analysis are shown in Table 1.Both ZrFe2 and Zr-V-Fe surfaces exposed to air are strongly oxidized. The positions of the V 2p and Fe 2p lines of the two compounds correspond to those of V02 and Fe2O3, respectively [4],[5]. The Z r 3d and 0 1s lines of Zr-V-Fe appear at different binding energies from those of ZrFe2. The Zr 3d line of Zr-V-Fe corresponds to that of Zr02, but for ZrFe2 it is about 0.8 eV lower, suggesting an intermediate oxide [6]. The 0 Is peak of ZrFe2 has its maximum at 530.4 eV and shows a shoulder on the high binding energy side (532.0 eV) while that of Zr-V-Fe is symmetric with a maximum at 531.2 eV. At 400 C, pronounced differences appear in the Z r 3d spectra of the two alloys : Zr in ZrFe2 is more oxidized than at room temperature and the Z r 3d positions correspond now to those of Zr02 while Zr in Zr-V-Fe appears to be metallic. This last result corroborates recent results on ZrV-Fe [I] : the annealing of this compound at 400 C leads to the formation of a metallic surface. The same trend is observed for the Fe in ZrFe2 and Zr-V-Fe and for the V in Zr-V-Fe, they all are in a metallic state after annealing. Indeed the enthalpy of formation of Fe and V oxide is lower r oxide. The shoulder in the 0 1s spectrum of ZrFe2 than that of Z (Table 1). disappears and the maximum is shifted to 531.3 eV

Sample

ZrJdS/z(eV)

Zr3dm(eV)

Vlpsn(eV)

Air exp. ZrFez

182.1

184.4

Air exp. Zr-V-Fe

185.3 185.3

515.9

Annealed ZrFe2

182.9 182.8

Annealed Zr-V-FB

179.1

181.4

513.0

Fe2p3/2(eV)

Ols(eV)

711.0

530.4

707.6

531.2 531.3

707.4

531.2

Table 1: Binding energies (-+0.2eV) of some core levels of Zr, V, Fe, 0 in Zr-V-Fe and in ZrFe2 after two different treatments : Air exposed and after annealing at 400 C .

Variations in the Zr to Fe atomic ratio in BFe2 indicate a strong segregation of Zr to the surface of ZrFe2 at 400 C : The.ratio increases from 0.7 (compound exposed to air) to 1.0 at 400 C. In contrast the Z r to V atomic ratio in Zr-V-Fe decreases from 5.7 for the air exposed sampIe

906

to 4.1 at 400 C, where the surface becomes metallic again.

2-ZrFe2 and 73-V-Fe cleaned surfaces and exDosed to 0 7 . at d ifferent JemDeratures, Surface cleaning with a diamond file results in metallic Zr, V and Fe according to the core lines although the 0 1 s and Cls signal are still detectable. The atomic ratios are the following : Zr/V = 4.3, Zr/Fe = 29 and V/Fe = 7.3 in Zr-V-Fe and Zr/Fe = 0.47 in ZrFe2. This last result corresponds to the stoichiometric bulk value within an accuracy of 20%. At 25 C and after oxygen exposure between 1OL and lOOL, all elements are oxidized. However at temperatures above 200 C, only the Zr is oxidized. The fraction of Zr oxide on 23-V-Fe is estimated to be 40% to 45% after exposures to 1OL 0 2 at 25 C. The Zr 3d line positions correspond to those in Zr02 (182.9 eV and 185.2 eV) and for both compounds, the oxide fraction rises to 70% approximately after lOOL 0 2 . For ZrFe2 the atomic ratio does not depend on the annealing temperature, however the binding energies increase with temperature (Table 2).

sample ZrFe2.25C.filed

Zr3d5/2(eV) 179.0

Zr3d3/2(eV) 181.4

182.3

Ois(eV) 531.2

ZrFe2.25C.lOOL

179.0

Zr-V-Fe.25C.filed

178.8

184.7

530.8

181.0

531.2

Zr-V-Fe.25C. IOOL ZrFe2,400C, IOOL

178.7

182.8

185.2

531.2

179.0

182.9

185.2

531.2

Zr-V-Fe,400C.1 OOL

179.4

181.6

531.4

On the other hand, the Zr in Zr-V-Fe is metallic for an exposure of

1OL of 0 2 at 400 C (Figure 1). Noticeable oxidation of Zr is observed only after exposures of more than 100 L at this temperature. For V and Fe we find the following behaviour : A 20% to 30% and a 40% to 50% fraction of V are oxidized after lOOL and lOOOL respectively and a 30% to 40% fraction of Fe after lOOL of 02 at 25 C.

907

A s in the first series of experiments, the 0 1s line of ZrFe2 shows a weak shoulder at the high binding energy side of the peak after low 0 2 exposures at 25 C. It then disappears for exposures above 1OL of 0 2 and at higher temperatures. The peak maximum is located at 530.9 eV and it reaches a maximum binding energy of 531.2 eV after 1OL of 0 2 at 400 C. The 0 1s peak of Zr-V-Fe for exposures of 0 2 up to lOOL is always symmetric up to 550C and its position is stable at 531.2 eV. After IOOOL of 0 2 at 550 C, its value rises to 531.7 eV. This increase in binding energy is probably due to high hydrogen desorption which starts at 500 C. At 25 C the Zr to Fe atomic ratio of ZrFe2 increases as a function of the 0 2 exposure and reaches the same maximum value as has been obtained for the surface preliminary exposed to air at 25 C (0.66). At equal exposures, the ratio increases also with temperature and reaches 1 at 400 C and 1OOL. Figure 2a shows that the Z r F e ratio increases rapidly up to 1OL and then saturates. This behaviour is less pronounced at 400 C. The Zr to V atomic ratio of Zr-V-Fe shows the same tendencies but less pronounced. The 0 atomic percentage as a function of the oxygen exposure is shown in figure 2b : In the case of ZrFe2, the curves plotted at 25 C, 200 C and 400 C are almost the same; the 0 atomic percentage increases strongly up to lL, then more slowly until 1OL and then saturates. At 400 C, the absolute oxygen concentration at the surface is lower than at 200 C. In the case of Zr-V-Fe, the concentration remains stable up to 1L at 25 C and 200 C, then it increases until 1OL and saturates at higher exposures. The behaviour is completely different from the precedent one at 400 C and 550 C; the curve is almost continuous all over the range of exposures. For 1OL of 02, the absolute surface oxygen concentration is lower than at 25 C and 200 C, probably indicating an absorption of 0 2 at low exposures. It remains slightly lower for lOOOL of 0 2 .

908

DISCUSSION This comparative study of the interaction of oxygen with the surface of Zr-V-Fe and ZrFe2 has shown that the surface of Zr-V-Fe remains metallic at moderate temperature (400 C) and low exposures (i IOL 02). whereas that of ZrFe2 oxidizes under this condition. However, both compounds show a strong surface segregation of Zr. The Zr to Fe atomic ratio in ZrFe2 strongly increases at low dosages (cIOL0 2 ) and low temperatures (25 C and 200 C) and then saturates at higher exposures (2 1OL 02)

BJNDJNG ENERGY ( e V )

Fieure 1: Zr 3d XP spectra for Zr-V-Fe exposed to 1L and 1OL of 0 2 at 25 C, at 200 C and 400 C.

while it continues to increase at 400 C. This behaviour can be correlated with the composition at the surface: At low temperatures the surface of oxidized ZrFe2 appears to be B mixture of intermediate Zr oxides, Zr02, and Fe2O3. At elevated temperature (400 C) and higher exposures the transformation of intermediate Zr oxides into ZrO2 is probably activated and the Z#+ cations can diffuse from the bulk to the surface according to

909

nrost models (71. Indeed the binding energy of the oxide component of Zr 3d is shifted towards higher binding energy. This phenomenon is less pronounced in the case of ZrV-Fe probably because of the reactivity of V which competes with that of Zr. Moreover the Zr-V-Fe is m u 1 tip h a s e . Two rnechnnisms have to be considered to explain the different behaviour of Zr-V0 7 Fe and ZrFe2 exposed to 10L a1 4 0 0 C, either the sticking coefficient of oxygen on the Zr-V-Fe surface is very low or oxygen diffuses rapidly o 07 1 from the surface into the I bulk in lhis range of teiiiperatures. But if we take into account the very O2 E x p o s u r e ( L ) differcnt behaviour of Zr-VFe and ZrFe,, the second mechanism seems to be much

probable, the sticking coefficient should be comparable for both compounds. Moreover the second mechanism could also

Figure 2: 2a, Zr to Fe atomic ratios of ZrFe2 as a function of 0 2 exposures at 25 C, 200 C and 400 C . 2b, 0 atomic percent of Zr-V-Fe (Left scale) and 0 atomic percent of ZrFe2 (Right scale) as a function of 0 2 exposure at 25 C, 200 C, 400 C , 550 C (Logarithmic scale).

explain the appearance of a metallic surface on Zr-V-Fe that has been previously exposed to air. The multiphase Zr-V-Fe might allow for a much faster oxygen diffusion and solution. We know also that ZrV2 bas a higher sorption capability for hydrogen than ZrFe2 [ 81. A next step of this study will be the determination of the mechanisms which occur on Zr-V-Fe and the synthesis of monophase crystalline or amorphous Zr-V-Fe samples to obtain information about effects of grain boundaries. We will also investigate the role of carbon at

910

the surfaces of those compounds.

ACKNOWLEDGEMENTS We gratefully acknowledge discussion with Dr Alex Stucheli, technical assistance by Urs Maier and financial support by the Swiss Nation Science Fundation (NFP 19) and Sulzer Brothers. F s m R F N m 1- Kkhimura, M. Matsuyama, K. Watanabe J. Vac. Sci. Technol. A5 (1987) pp. 220-225 2- C.Boffito, B.Ferrario. P. Della Porta, L. Rosai J. Vac. Sci. Tech. 18 (1981) pp. 1117-1120 3- J.J. Yeh, I. Lindau Atomic Data and Nuclear Data Tables 32 (1985) pp. 6-15 4- J. Kasperkiewicz, J.A. Kovacich, D. Lichtman J. El. Spect. and Rel. Phenomena 32 (1983) pp. 123-132 5- SSinha, S. Badrinarayanan, A.P.B. Sinha J Less Common Met. 125 (1986) pp. 85-95

6- C.O. de Gonzalcz, E.A. Garcia Surf. Sci. 193 (1988) pp. 305-320 7- P. Sen, D.D Sarma, R.C. Budkhani, K.L. Chopia, C.N.R. Rao J. Phys. F: Met. Phys. 14 (1984) pp. 565-577 8- D. Shaltiel, I. Jacob, D. Davidov J. Less Common Met. 53 (1977) pp. 117-137

C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V.. Amsterdam -Printed in The Netherlands

911

MICROSTRUCTURES OF COPPER SURFACES DURING ELECTROLESS COPPER DEPOSITION

Michael Wanner and Konrad G. Weil Institut fiir Physikalische Chemie der Technischen Hochschule Darmstadt, Petersenstrasse 20, 6100 Darmstadt (W.-Germany)

ABSTRACT In-situ measurements of the copper deposition rate and simultaneous determination of the total current enables us to determine the rates of the partial processes copper deposition and formaldehyde oxidation. By changing the bath composition at a constant potential we can show, that formaldehyde oxidation occurs with appreciable rates only at surfaces with a specific microstructure. The latter is characterized by unusual high values of the double layer capacitance, which is determined simultaneously by time resolved impedance spectroscopy. INTRODUCTION Electroless copper deposition is an important step in the fabrication of printed-circuit boards. It occurs in solutions of pH = 12 with formaldehyde as reducing agent. In earlier papers [1,2] we could show, that formaldehyde at the same time acts as a catalyst for the copper reduction. Copper ions in the plating bath are present as (Cu[EDTA] )4--complexes. (EDTA

=

ethylendiaminetetraacetate). Formaldehyde oxidation, on the other

hand,

occurs

with

appreciable

simultaneous copper deposition. When

rates

only

during

copper deposition is

suddenly switched off, either by fast solution exchange or by a potential

step

to

more

positive

potentials,

then

the

formaldehyde oxidation rate decreases to nearly zero with a time constant of = 400 s. We suggested [2] that during copper deposition a specific surface microstructure is maintained,

912

which

exhibits

a

specific

catalytic

activity

for

the

formaldehyde oxidation. In this paper we report on further experiments, by which we tried to verify the above mentioned suggestion.

EXPERIMENTAL One face of a gold coated AT

-

cut quartz crystal was used

as the working electrode in our experiments. Prior to each run, the gold surface was covered electrolessly with copper.

A

circuit, similar to that described in [3] was used to excite shear vibrations in the quartz. The eigenfrequency, which changes linearly with the amount of deposited copper, was measured with a Rhode and Schwarz frequency meter. In order to avoid

interference of

the

frequency measurement

with

the

impedance spectroscopy, the connection to the frequency meter was

realized

by

an

opto-coupling

element.

Impedance

measurements as a functip:i of time and potential could be done with the Fourier-Transform-Impedance-Spectrometer described in [4].

The electrolytic cell was constructed in a way which made

possible a change of the solution within a time of about one second.

Chemicals were of p. a. grade. Solutions were made using triply distilled water. Prior to each experiment, all the glassware was

cleaned with

hot

chromic/sulfuric acid

and

subsequently thoroughly rinsed with boiling triply distilled water.

In order to start each experiment from the same, well defined

913

initial state, all electrodes were plated with copper for abaut 500

s

with the following standard plating bath: 0.04 mol/L cupric sulfate 0.12 mol/L Ethylendiaminetetraacetate (EDTA)

0.15 mol/L sodium sulfate 0.30 mol/L sodium formate

0.01 mol/L formaldehyde

+ sodium hydroxide to adjust the pH at 12. Temperature; 25 During

all

experiments,

OC.

the

solutions

were

continually

deaerated with a constant stream of purified nitrogen.

RESULTS In

a

first experiment, we

switched

off

the

copper

reduction current by fast solution exchange, replacing the plating bath by a similar bath, which did not contain cupric ions and the equivalent amount of EDTA. The potential was kept constant at the mixed potential of the plating bath. As noted earlier [2], this exchange leads to a sudden decrease of the formaldehyde oxidation rate, which under these conditions can be measured by determination of the total current. Fig. 1, left hand side, shows the total current during such an experiment. The small negative current prior to the solution exchange indicates a small, insignificant deviation from the mixed potential. After

the exchange

(arrow in the figure), the

current becomes anodic, its initial value is equal to the negative current, equivalent to the rate of copper reduction before the exchange. Under the conditions of the described experiment,

when

the

copper

reduction

is

stopped,

the

914

formaldehyde oxidation decreases within about 400 s by a factor of two.

-

-

t / s

t/s

Fig. 1 Left hand side: Current through a copper electrode in a standard plating bath and after a solution exchange, indicated by the arrow. The second solution was free of cupric ions. Right hand side: Response of the differential double layer capacity of the copper electrode on the fa t solution exchange. Electrode area 0 . 3 3 cm1

From

the

right

hand

part

of

Fig.

1 we

see,

that

simultaneously with this decrease the differential double layer capacity decreases also. It should be noted on passing, that a very similar behavior is observed, when the copper reduction current is switched off by a sudden potential step to more positive potentials, where the cathodic partial current tends to zero [l].

In a second series of. experiments we tried to change the

rates of both partial processes independently by the addition of

additives. Again,

the

rate

of

copper

deposition was

915

monitored

with

the

quartz

micro

balance.

The

rate

of

formaldehyde oxidation could be calculated as the difference of the total current and the equivalent current of the copper reduction. We give the results for the two additives ethylenediamine and sodium cyanide. They are collected in Table 1. Table 1 Partial currents of copper reduction and of formaldehyde oxidation together with the differential double layer capacitance in the standard bath and in two baths with additives as stated i% the left column. Electrode area: 0.33 cm , Temperature 25 ' C

-

Standard bath 1

+ 0.02 mmol/L NaCN Standard bath 2

+ 1 mmol/L ethylenediamin

II I

-

+

108

150

I

+ 70

240

I

103

+

155

II I

100 475

.

450

The different values, obtained with the identical standard baths 1 and 2 indicate the poor reproducibility of quantitative values for the electroless copper deposition properties. As a consequence of this property, only relative changes can be discussed.

DISCUSSION It is evident from Fig. 1 that there seems to be a correlation between the formaldehyde oxidation rate and the differential double layer capacity of the electrode. This finding is corroborated by the results, presented in Tab. 1.

916

Here we see, that ethylenediamine, which decreases the copper deposition

rate

markedly,

has

only

little

influence

on

formaldehyde oxidation and on double layer capacity. Cyanide ions catalyze the copper deposition, their addition leads to a decrease of formaldehyde oxidation rate and to a decrease of the double layer capacity. Electrolessly appearance. When

deposited copper

copper

films

is deposited

have

a

brownish

galvanically

in

the

presence of higher concentrations of ethylenediamine, a bright metallic

surface

is

obtained.

During

electroless

copper

deposition, the differential double layer capacity always is markedly high, while it attains normal values < 100 pF/cm2 during

galvanic

deposition.

From

all

these

findings

we

postulate, that formaldehyde oxidation on copper is catalyzed bx a specifically active surface, which establishes itself during

copper

deposition.

This

surface

seems

to

be

characterized by a microstructure which is determined by an extremely high surface roughness, with amplitudes on an atomic scale. Only for a roughness on this size scale, such a rapid relaxation can be expected, as is seen for instance in Fig. 1.

ACKNOWLEDGMENTS This work was partly performed under a contract with the Electroplating

Division,

Schering

AG.

Further

financial

assistance by the Fonds der Chemischen Industrie is greatly acknowledged. The work is part of the Dissertation of Michael Wanner , to be presented to the Technische Hochschule Darmstadt (D17).

917

REFERENCES [ l ] H. Wiese and K. G. Weil, Ber. Bunsenges. Phys. Chem. 9 l , (1987)t 619 - 626. [2] H. Wiese and Konrad G. Weil, Special Symposium Series, AI, Proceedings of the Symposium on Electroless Metal Deposition, I. Ohno and M. Paunovic, Eds., The Electrochemical Society 1988 [3] M. Benje, M. Eiermann, U. Pittermann, and K. G. Weil, Ber. Bunsenges. Phys. Chem. 90, (1986), 435 - 439. [4] J. Uhlken, R. Waser, and H. Wiese, Ber. Bunsenges. Phys. Chem. 92, (1988), 730 - 736


C. Morterra,A.Zecchinaand G.Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science PublishersB.V.,Amsterdam Printed in The Netherlands

919

-

VARIATION OF OPTICAL EMISSIVITY DURING THE FIRST STAGES OF THE OXIDATION OF TUNGSTEN. B. WEBER, C. SUM W E N , P. PIGEAT and 1. PACIA 6 rue du Joli Coeur 54000 Laboratoire E.R.M.E.S. I.N.P.L.

-

-

NANCY

-

FRANCE

ABSTRACT Oxidation of tungsten is studied under oxygen pressure of 1.3 Pa and temperatures in the range of 800K to 1OOOK. During this reaction the oxygen consumption and the variation of the normal and spectral emissivity of the surface are measured in the IR wavelength range l m < A < 1Om. ~n induction period followed by parabolic growth of a homogeneous oxide film is observed. The induction period is interpreted by oxide islands growing by surface and volume diffusion from of nuclei randomly dispersed on the metal surface. These islands finally join to form a homogeneous layer which is limited in growth by bulk diffusion. The experimental values of the emissivity agree with the calculated values obtained from a model based on effective medium theories and the interferential theory of thin films. The complex refraction index of this oxide (UO, orthorhombic) fbr the temperature and spectral ranges considered is determined.

INTRODUCTION Among optical constants, the emissivity is the least intensively studied. Nevertheless this parameter is very important in many industrial and scientific fields (pyrometry, detection, medical thermography, energy transfer, microelectronic...). The dispersion of the emissivity results (ref. 1) is partly due to the fact that the sample physico-chemical characteristics are badly defined. With a suitable experimental technique (refs. 2-3) which makes possible the separation of the role of these physico-chemical parameters, the influence of the presence of a few monolayers of a coating material (ref. 4 ) or the thickness of an oxide layer has been shown precisely. In this paper, the variation of the normal and spectral emissivity of tungsten ribbons during the whole oxidation reaction (Po 1.3 Pa) is studied 2 in the wavelength range lm - low and for sample temperatures in the range 800K - 1000K. Starting of the experimental results, a kinetic model and a optical model are developped.

-

EXPERIMENTAL PROCEDURE AND RESULTS The home-built vacuum IR spectrophotometer used in our research has already been described (ref. 3 ) . It analyses and compares the radiation emitted by the two faces of a metallic sample ribbon. With shutters kept at ambiant temperature (To 300K) it is possible to blind one of the two optical pathes. For the sample temperatures studied, 800K < T < lOOOK (temperature usually encountered in industry), the energy emitted by the obturator surface can be reasonably neglected. A Littrow's mounting with a sodium chloride prism and a pyroelectric detector analyses the radiation emitted in a spectral range of 1 - 10140. The sample, the optical circuit and the detector are placed in a

-

920 vacuum chamber provided with a gas inlet. The ultimate pressure is in the l o p 5 Pa range. The tungsten ribbons used are 180 mm long, 2 mm wide and 2 5 m thick and are heated by Joule effect. Only the central surface (28mm x lmm) is observed. During the oxidation, the total hemispherical emissivity varies. The sample is kept at a constant temperature T by an electronic control of its resistance (potential leads are situated in the isothermal zone at each side of the observed surface). The influence of oxidation on the normal spectral emissivity E h n T is represented by the variation of the relative emissivity The emissivity of the sample before oxidation is B o h n T . After a systematic annealing for 3h at 2000K under lo-’ Pa and cleaning of the surface for lh at 2400K under Pa oxygen, oxidation is carried out at a temperature T under a pressure of 1 . 3 Pa of oxygen in the spectrometer chamber isolated from the vacuum pumping unit. During oxidation, are recorded : - the variation of the oxygen pressure &(t) in the spectrometer chamber (ionization gauge) ; the heating current I(t) and the potential drop U(t) in the isothermal zone of the sample ; - the signal of the detector E, at fixed A , 0 and T.

-

AeT

The number N(t) of oxygen atoms per cm2 of oxidized sample versus time is determined from the oxygen consumption in gaseous phase : N(t)

-

2 &(t)

$ V,

(1)

/Se

-

with K, 2.66 10 l 4 molecules of 02/Pa.cm3, V, the volume of the vacuum chamber, and S, the surface of oxidized ribbon. For different oxidation temperatures T are drawn N(t) versus \$; (See Fig. 1). The observation during the oxidation run at a temperature T of the emitted energy at a fixed wavelength A , is repeated for different wavelengths. For 800K < T < 1OOOK and lm < X < lopin, the relative emissivity ( & / & o ) A n T versus time has a typical behaviour, but as for the oxygen consumption these 2.3~). curves can be shifted in time (e.g. Fig. 2, T = 900K, A

-

0

1

2

3

b

Fig. 1 : Oxygen consumption versus t”‘

I

t (rnn)’/r

for different oxidation temperatures

921

Fig. 2 : Relative emissivity E / E o and oxygen consumption & versus t1I2 but vith at the same temperature T and same wavelelength A- 2.3 I~II different induction times. Kinetic model The parabolic growth of oxide deduced from consumption curves (See Fig. 1) is characteristic of growth limited by bulk diffusion (Ref. 5 ) . If D, is the bulk diffusion coefficient of oxygen atoms in the tungsten oxide, Fick's equations (Ref. 6 ) give the number of oxygen atoms trapped by the oxidation dN(t)/dt :

the oxygen concentration per unit area and S the surface of the with C actually oxidized ribbon. For each temperature, the constant U is deduced from the slope of the experimental oxygen consumption. Equation 2 involves a parabolic growth of the height of the oxide layer h(t) : h(t)

- kfi

(3)

The Arrhenius law fits correctly the evolution of D,. An activation energy of 36 Kcal/mol is obtained. This value is very close to those calculated from literature results (Refs. 7 - 8 ) . The induction period before parabolic growth observed at the beginning of the reaction, can be related to the time of formation of the reactional interface (Ref. 9 ) . It can be described by a model where oxide islands grow around a number (m) of pre-existing sites present at the metal surface. These islands increase and finally join together so that they cover the whole metal surface. At this moment the parabolic growth as described above begins (equation 3 ) . Around m initial sites randomly dispersed, circular islands grow. At time t, the surface area covered per unit area is (Ref. 10) :

where R

is the radius of

an isolated island. These

islands are fed by

922

chemisorbed - oxygen atoms diffusing over the metal surface S(t) where is S(t) 1 - S(t) and by the oxygen coming from the gas phase which diffuses through the oxide island. We showed (Ref. 11) that the rate of lateral growth dR/dt is constant :

-

-

dR/dt k Ds (5) with D, the surface diffusion coefficient of the oxygen atoms on the metal surface. The simple sum of lateral and transverse growth of islands (equations 3 and 5) does not show the experimental shift observed on the oxygen consumption curves (See Fig. 3 ) . The observed delay of oxygen consumption involves that the oxygen uptake from the gas phase which diffuses through S(t) also contributes to the lateral filling of the islands. The volume of an isolated island is schematized by a cylinder of height h(t). The increase in volume AV of these islands is deduced from equation 2 . With S(t) derived from equation 4, the height h(t) is calculated as : h(t

+ At)

+ AV - V(t) S(t + At)

In this kinetic model all parameters are known, the only product mDf (equation 4-5) is to be adjusted.

fh.rt”z

f

4

thoonticat curvr

wrvr

Fig. 3 : Island form and oxygen consumption versus time for a kinetic model with a simple sum of lateral and transverse growth. ODtical model During the initial stages of the oxidation, the substrate is covered by an inhomogeneous layer of thickness h(t) composed of oxide islands. It is assumed that the islands are growing outside the substrat (medium 2 ) . In this layer of thickness h(t) the interstitial volume is vacuum. The filling factor of this layer is Q(t), the ratio of the oxide volume (medium 3 ) to the total volume of the layer. The complex index of refraction n, n, - ik, of a homogeneous equivalent layer (medium 1) at any instant of the reaction is calculated from an effective medium theory.

-

923

Because during the oxidation complex index has to be calculated with component (oxide : n3 n3 - ik and symmetrical equation. Bruggeman's equation

-

Q(t) goes from zero to unity, the the complex indexes of each mixture vacuum n 1) with a topologically (Refs. 12-13) was used :

-

-

where g is the depolarization factor. We currently chose g 1/3 for simplification. With the physical solution nl(t) from equation 7, the classical theory of thin films is used (Ref. 14 ) to calculate the normal spectral reflectivity P ~ , B =of~ a sample covered with an equivalent thin film of thickness h(t) and index nl(t). The tungsten substrate being opaque (thickness 251411) KIRCHHOFF's second law and the energy conservation principle are used to calculate the normal spectral emissivity E h , e z 0 :

-

CONCLUSION For different temperatures T (800K < T < 1000K) it is verified that the proposed model correctly describes the evolution of the oxygen consumption and the emissivity E h , 8 = o (e.g. Fig. 4 for T 900K A 81.4~) (Ref. 11).

-

-

-1

Fig. 4 : Experimental (+++) and theoretical relative emissivity (-) versus t'" external islands (BR ext), internal islands (BR int).

924

With this model the complex index n, of refraction of orthorhombic W 0 3 , is determined for different temperatures and in the spectral range lclm < < amm, (Ref. 15). These indices were hitherto unknown. But in addition to these informations on optical parameters, this direct observation method of emissivity and its theoretical interpretation can give informations on surface physico-chemical parameters. For example, in this study, it is shown how the number of oxidation sites and informations about the shape of oxide islands are made accessible. The effective medium model is used with oxide islands growing within the substrate (model noted "BR int" in Fig. 4). By comparison (See Fig. 4) with "BR ext" curves (islands out of the substrate), it is shown that the islands are actually in an intermediate position (- 2/3 out of the substrate) which is exactly that of the density ratio (PILLING - BEDWORTH ratio equals 3.35). Emissivity measurements reveal beeing a powerful1 non destructive and in situ method of investigations of surface properties. This one is now applied to studies such as influence of strain, cracks, microrugosity and cristallographic phase modifications.

REFERENCES 1 Y.S. TOULOUKIAN and D.P. DEWITT, "Thermal Radiative Properties of Materials (Plenum, New - York, 1970). 2 P. PIGEAT, Brevet d'invention n' 80.02201. 3 P. PIGEAT, N. PACIA, B. WEBER and D. PAULMIER, Revue Phys. Appl. 20 (1985) 863. 4 P. PIGEAT, These Nancy (1986). 5 PER KOFSTAD : "High Temperature Oxidation of Metals" (John Wileys Sons, inc N.Y. London, Sidney 1966). 6 Y. ADDA and J. PHILIBERT : "La diffusion dans les solides" (PUF, Paris 1966). 7 W.W. WEBB, J.T. NORTON and C. WAGNER, J. Electrochem. SOC. 103 (1956) 107. 8 V.K. SIKKA and C.J. ROSA, Corrosion Sci. 20 (1980) 1201. 9 B. DELMON : "Introduction A la Cinbtique HbtBrogBne" (Technip, Paris, 1969). 10 M. AVRAMI, J. Chem. Phys. 7 (1939), 1103 : 8 (1940). 212 ; 9 (1941) 177. 11 P. PIGEAT, N. PACIA and B. WEBER, Appl. Surf. Sci. 27 (1986) 214. 12 D.A.G. BRUGGEMAN, Ann. Phys. (Leipzig) 24 (1935) 636. 13 C.G. GRANQUIST and 0. HUNDERI, Phys. Rev. B16 (1977) 3513. 14 O . S . HEAVENS, "Optical Properties of Thin Solid Films" (Batterworths, London 1955). 15 P. PIGEAT, D. PAULMIER, N. PACIA and B. WEBER. Thin Solid Films, 145 (1986) 185.

C. Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands

925

REACTIVITY AND PHOTODYNAMICS OF SIZE-SELECTEDMETAL CLUSTERS L. Woste *

Centre dApplications Laser Ecole Polytechnique Fdderale de Lausanne CE-Ecublens CH-1015 Lausannd ABSTRACT

Metal cluster ions were produced by means of sputtering, energy-filtered, and introduced into a tripel-quadrupole arrangement. The arrangement allowed us to perform well-controlled ion-molecule reactions with carbon monoxide. The amount of ligands of the observed carbonyl-reaction products correlates well with established electron counting rules. The ligands are extremely sensitive with respect to photodesorption. The controlled deposition of silver cluster ions on photographic plates shows that extraordinary catalytic properties may be attributed to metal clusters of a specific size. I. INTRODUCITON

The spectroscopy of metal clusters in the gase phase, using molecular beam techniques combined with mass spectrometry, has provided a series of impressive results, which combined with new computational methods contributed to the evolution of cluster science from what was originally a purely empirical field to a discipline, where quantum mechanical understanding confirms the experimental observations. The development of the shell model by Knight et al. 111 and related calculations by Ekhard 121 provided an excellent example. The same is true for the observation of the gradual transition of the behaviour of mercury clusters from that of van der Waals molecules to metallic by Brechignac et al. 131, Rademann et al. [41 and related calculations by Bennemann et al. [5).A bridge between spectroscopic properties of metal clusters and their corresponding chemical properties was established by Whetten et al. 161 who measured and correlated the ionization potentials of iron clusters and the corresponding hydrogen absorption affinities as a function of their size. The results prove that the investigation of optical properties of metal clusters and their relating reaction dynamics contribute not only to the fundamental understanding of these particles, but they also indicate a way to future applications.

-

* The exoeriments were Derformed at : Institut de physique exp6rimentale Ecole Polytechnique FMderale de Lausanne PHB-Ecublens CH-1015 Lausanne

-

926 11. CHEMICAL REACTIONS OF METAL CLUSTERS

1. Production of size-selected metal cluster ions

An impressive number of chemical reactions of metal clusters has been performed by injecting reactive gas into the expansion of a laser vaporization source [ 6 ] . The experimental arrangement leaves, however, some doubt about fragmentation phenomena, which might occur during the reaction itself, or as a result of the subsequent ionization process. For this reason, we have preferred to perform experiments on size-selected cluster ions, which - in a way - is a limitation. The approach, however, allows one clearly to distinguish parent-ion reactions from fragmentation products . High currents of cluster ions are conveniently generated in the sputtering process by bombarding a target with fast atom or ion beams. In order to produce large cluster currents, the extraction ion optics of the cluster transport line must be close to the target. This constraint places the primary ion source quite far away from the target. Thus the source must be of low divergence, but must provide a large current. This requirement is fulfilled by the extreme brightness of the cold reflex discharge ion source (CORDIS) developed by R. Keller [71. Figure 1 shows a sectional view of CORDIS. The primary electrons are generated by six resistively heated tantalum filaments suspended on a molybdenum rod near the source axis. 18 permanent magnets placed around the anode restrict the plasma electrons into the loss zones only. These zones are the magnetic field lines of the cusps across the anode wall. This configuration creates a cylindrical plasma of uniform density over a diameter of 3.5 cm.The source is operated with argon at 4 Pa of pressure as measured near the gas inlet tube. Ar -ions are extracted by the accel/decel lens system at 20 keV. The negatively biased intermediate electrode focuses secondary electrons created in the extraction zone by the ionizaton of the argon residual gas. Since the positive space charge of the Ar + ion beam is balanced by the presence of electrons, ions are strongly focused over a long distance. A beam current up to 10 mA can thus be measured on the target, even if it is positioned 50 an away from the source outlet plane. +

-

---

Fig. 1 Sectional view of the sputter ion source "CORDIS", developed by Keller [7]. Using argon, the source allows one to obtain at 20 kV extraction voltage primary ion currents on the target (diam. 8 mm) of 10 mA.

927

When combined with a quadrupole mass spectrometer, the sputtering arrangement is a powerful1 source of size-selected cluster ions, because quadrupole mass spectrometers have extremely high transmission functions under decent resolution values. A typical mass spectrum, as it was obtained for negatively charged silver clusters is shown in figure 2. As indicated by the mass peaks AgCs and AgCs2, the silver target contained some cesium. Indeed, in order to offer negative surface charges, a small amount of cesium was coexpanded from a Knudsen cell to the target, which increased the formation of negative cluster ions by more than a factor of 100. This allowed us, for example, to obtain beams of Ag3+ reaching up to 1012 particles per second [81. Another interesting feature arising from the mass spectrum in figure 2 is the pronounced odd-even intensity alternation, which is due to the fact, that preferentially particles with completed electron pairs emerge from the sputtering surface [9]. A further advantage of quadrupole mass spectrometers is the low ion energy (a can be proposed. The twin bands observed at 2092 and 2015 ca-1 can be -signed to g d nal dicarbcmyl on isolated R h r sites; these sites are initially p-eaemt together w i t h original R h r clusters in which is linearly and kidge bonded to W . This bridging 03 exhibits a miximm at quite lmer freqwmcy (1819 cm-1) w i t h respect to pure Rhr(CD)iz, PrObablJr due to an interaction w i t h the 8upport. In

946

fact a 50-an-' shift to lower wavemmber reported for Rh/SiOz modified with

was

2k-O~ (ref. 6 ) .

The surface structure initially observed is not stable in the conditions of IR measurements, as Rho sites are slowly removed while Rh*(a) z species tend to prevail on the surface. l h i s process does not involve consumption of surface hydroxyls, as the broad band at about 3550 an-l does not change in intensity. Concerning the cause of this evolution, 2400

19bo

1650 WF1VWUMBERS

2150

lrbo

an influence of the irradiation conditions cannot be exc.lur+xi;Gardella et al. h e r vf?d a promtion effect of the IR source on

Rh;;(CO) ~z/ZrOz'as a f&:tion of t h Ni/SiOz to 0 0 2 (ref. 7 ) . at 305 K: a) after 1 minute; b) am- describes the fter 1 h; c ) after 4 h; d ) afLer 22 h. of Rha(a)iz/ZrOz during thermal activation (see Fig.3). Coz is the miin pro-

duct desorbing below 500 K; only a minor fraction of C l leaves the surface without dissociating. At higher temperature a Hz peak developg in parallel with 0 3 2 , while CO desorption rate is quite lower. 'Ihe relat.ively good separation of the

mz

peaks suggests the presence on the surface of t w distinct types of CO ligands: a more resctive fraction which is c a n v e M to 0 0 2 at low temperature thpough a disproportionation

mechanimt and a second fraction reacting above 500 K w i t h residual surface

hydroxyls to pmduce C o z and Hz via a water gas shift laechaniam. 'zhe IR a& TPI) results indicate unambiguously that ported an ei&a,

when sup starts to decaapo-

Rh4(a))izr

se already at tempersture near ambient

speciesby cheraisorbed oxygen. This conclusion is in agreement with the

Rh, lCOllz

/Zr 02

1 c ?

e a

300

400

SO0

600

Temperature IKI

700

800

947

interpretation proposed by Primet and by van't Blik et al. for W A L z O s systers (ref. 4,8). The lack of Hz evolution at -rattmz < 500 K in the TPD experiments rules out the alternative mecham'sm based on oxidation of Rh via interaction with surface hgdroxyls, proposed for W A l z O s (ref. 9). When utilized for the catalytic hydrogenation of a), the sample Rh4(0)12/i%h WBS activated by heating for two hrrurs in H2 flow at 523 K. 02 chemisorptim messurements carried out after rerfuctianat 523 K showed that the dispersion of Rh was about 8oX,corresponding to a particle size of about 14 A (ref.2). The surface structure after this treatment WBS studied to obtain information on the active sites existing on the surface in the initial stage of catalyst life.

whenthereducedsamplewas exposed to 03 in the IR cell, 0 0 2 evolution was observed since the start of spectra recording (less than 1 minute after sample exposu~eto a)); the subsequent spectra shaued a decrease in the intensity of gasCO bends at 2171-2116 a n - 1 and the tmild up of a few bands in the 2100-2000 aa-1 range (see Fig.4). After 30 minutes gaseous 00 disappeared and the gem-dicarbonyl

bends at 2094 Bnd 2016 prevailed in the linear a) region. In parallel w i t h 0 0 2 evolution, no change of the hydroxyls band at 3550 cm-1 was obser-

ved, while the broad band of carbomtes at 1650 cp-l already visible before 03 injection, inin intensity. These results show that also in the reduced state W Z r O z tends to evolve,

in the of CO, to a surface structuredcmimted by isolated Rhr sites. In this case the mechanism involves, beside a)dissociation, also disruption of Rh metal crystallites; this phenawnon was reported to occur

at 298 K on Rh/AlzOa, when the size of metal crystdlites is below 20 A (ref.

1

2150

19bO

1650

7

1$00

WAVENUMBERS

Fig.4

spectra of at 305 K after reduction with Hz at 523 K: a) before introduction of a>; b) l-min. exposure to 00; C ) Slain. expos~re; d) 30min. exposure; e) 12O-min. exposure. FT-IWA

Rhr(a))iz/Zr0z

948

498) 'Ihe supported rhodim

clusterwas

Rh / Z r 02

in Hz at 523 K and expoeme to f l m i n g a> at 298 K f o r lh (see Fig.5). Belaw 450 K studied by TPD a f t e r reduction

Reduced at 523 K

c

misthe

main product leaving the surface, while at higher temperature Coz and Hz are wedanmn * t; thepeaks of

?

z 9

Q

these two s p e c i e s s u g g e s t t h a t a

large fractian of Coz is fo&

through a water gas s h i f t reaction. 300

400

SO0

600

700

0

Temperature I K I

Fig.5 TPD in VBCUIIBI of Rhr(a>)i2/i%h after reduction with H2 a t 523 K and adsorption of CO at 298 K. Heating rate 10 K/min.

.0 r "r

This reactivity pttern is quite diffe-

rent frola that of the supported carbonyl: a0 dissociation occurs a t much luwer rate at low temperature, wfiereas the s h i f t reaction is strongly proated, as Shawn by a 8 0 4 decrease in a>z and H2 peak mexiBa. Inconsideringtheseresults, take into m x a m t thet

wehave to

thissamplewas

lh a t 298 K; this treatment, according to IR data above

contacted w i t h flowing 00 for

I n 1

I I

U\

.

is able to generate on the surface a subatantid ammt of RhI(a>)z spe-

reported,

.hl

b

the 'IlJDspectnractllallyreflects the reactivity of these new species. To obtain a closer picture of the active 1 sites present on the catalyst i n the wor00 Zk i O 2150 19bO l&O l k b 0 king conditions, the sample Rh/zrOz, a f t e r WFIVENUMBERS activation with Hz at 523 K, 1188 exposed Fig.6 BT-IR/PA spectra of to a fluwing Hz/CO 3:l mixture at 523 K Rhr(CO)1z/Zroz a t 305 K after con- and atamspheric pressme; after cooling t o tacting w i t h H2/CO 3:l mixture at 523 K: a) before introduction of CO; 298 K in H2 and flushing w i t h He, it m s b) I-. ~qrowrreto CO; c) 4-mi.n. contactedwith a0 i n the photoamustic exposure; d)16-min. exposure; e) cell. As w i t h the sample reduced w i t h Hz, l2o-min. exposure. the IR spectra showed e reaarkable evolution (see Fig.6).

1/11u

cies:

a

949

In this case, however, the surface reactions occurred at a lower rate, and

with the reduced

disappeared only after 120 minutes (ampared to 30 m i n u t e s

sample). In addition, a band of bridging 0 was fonned at 1860 an-1 in the first minutes of spectra recording. After 120 minutes, the bawls of the gemdicarborqrl complex were predcmimant, but a significant amDllllt of CO linearly bonded to Rh crystallites was still visible. These results suggest that Hz/CO mixture brings to formation of larger Rh crystellites than plre Hz. A similar maggzv=gation effect of a0 bas been reported for Rh/A1203 sys(ref. 9 ) . The high frequency shift of the bridging 00 band with r e s p t to the supported carbony1 m y be attributed to a weaker interactian of CO with the support, probably due to carbon deposition which occumed during the t r e a m t with Hz/CO mixture. Catalyst fouling can also be responsible for the lower density of Rh sites on the surface, evaluated by the integrated areas under the CO bands. fRh4(a)llZ*(CO)6]/ZrO2

In a stdy on the effect of additives on the catalytic activity of Rh/ZrOz in a0 hydrogenation, -weobserved that Mo exerts a strong pnmtian effect both on

conversion and on selectivity to oxygemated groducts (ref. 2 and Table 1). With increasing amount of Mo the catalytic activity rises dramatically and the selectivity is strongly modified with prevalent fonmtion of Cz+ oxygenates for Rh/Mo = 1:1 and of tkC#I with Rh/Mo = 1:2 (see Table 2). To know mre on this prapotion effect of Mo, we studied the catalyst precursor [Rhrl(CO)iz+Mo(a?)a]/Zt0~ with the same methods utilized for Rhr(a))iz/ZrOz. ?he IR spectrzlm of the sauple containing an Rh/Mo ratio = 1 : l is relatively more ccmplex,in the linear CO region, than that of the mnanetallic system (see Fig.7). Gem-dicarbonyl, linear CO and bridging CO are yisible as in TABLE 1 CO Hydrogenation in a Tubular Flow Reactora Catalyst precursor

Rh

Mo

(wt%) (wt%)

CO convers. Product distribution

(%

of converted CO)

(2)

HC

MeOH

C2+-OXYG

co2

a Reaction conditions: T = 523 K, P = 3 MPa, GHSV =.1200 h-l, feed: H2/C0 1:l

950

TABLE 2 CO Hydrogenation in a Tubular Flow Reactora Catalyst Rh-Mo/ZrOZ

(Rh = 1 wt%)

Rh/Mo at .ratio

(%I

Product distribution ( % of converted CO)

CO conversion

HC

MeOH

CZ+-OXYG.

co2 8.44

1 :0.5

2.01

58.8

9.9

22.8

1:l

3.7

35.6

9.2

43.1

12

12.4

27.6

27.3

19.5

25.6

1 :2

a Reaction conditions: T = 503 K, P = 3 MPa, GHSV = 2400 h-l, feed HZ/CO 1:l

Rhc(aO)~z/Z#z,

but

the band centered a t 2026

c p - l ShOWS

on the law-frequency side. A contribution of CO adsorbed exclufed,

ttyo inflection points

M

IR bard at 2002 cm-';

1

Mo sites cannot be

because pure Mo(CO)s showed an however Mo(CO)6 is

l i k e l y to have been deeply a l t e r e d after adsorption on ZrQz, ard i n f a c t the sample

Mo(00)6/ZrOz rowxi 2000

showed only a w e a k b a n d a -

cm-1.

Amoreconvincingexplamtion seems to the presence of bridging CO bonded t o

be

Rh i n the +I oxydation state. This

species w a s

Rh/aZ03,

shown t o

form

on

giving a band at 1985-2025 on-'

( r e f . 5,lO).

co2 is v i s i b l e since the beginning of IR measurements, while i n t h e monometallic sample it appeared only a f t e r lh. I

I

2150 1900 1650 WAVWUMEERS

1900

The spectra recorded a f t e r various t i m e intervals show

that the band i n i t i a l l y

a t about 2026 cm-l gradually centred Fig.7 ET-IR/PA spectra of [Hhs(aO)12 shrank and after 16h the maxi.umm w a s at t M O ( C O ) S I / ~ Z (RWMO I : I ) a function of time a t 305 K: a ) a f t e r 2013 cm-1; a t that time, bridging 0 al1 min.; b) a f t e r 1 h; c) after 2 h ; most disappeared, l i n e a r 00 w a s still vid) a f t e r 3.5 h ; e ) a f t e r 16 h. and the s i b l e w i t h a band at 2062 twin band of gem-dicarbonyl prevailed i n

95 1

the range 2100-2000 an-'. 'Ihe IR spectrum of the sample RhMo/zrOz reduced in Hz at 523 K and exposed to CO shows CO2 evolution ard formation of f ? h I ( a ) z species with the same mechanism observed for R h/mz (see Fig.8).

The promotion effect of Mo on the rate of CO dissociation, already observed with the supported car-1, is confirmed for the HhMo/zrO2 reduced with Hz and exposed to CO; in fact the b a d at 667 ran-' due

to Coz bending grew at higher rate w i t h increasing anmnmt of Mo in the sample. The TPD q e c t r u m of the sample with Rh/Mo = 1:1 shows that CO is the predominant species desorbingbelow500 K, a fraction being also converted to 0 2 (see Fig.9); at higher temperature, a O z and Hz are evolved. At about 500 K the desorption rate of H2 is higher than that of 0 0 2 ; the excess of H2 with respect to the stoichiometry

[Rh4 (COl,2* MolCO161/Zr02

!

Rh/Mo I I

i

WRVENUMBERS Fig.8 F F W A spectra of LRh4(CO)lZ + k(CO)S]/zrOZ (Rh/Mo 1:1) at 305 K after reduction with Hz at 523 K: a) before introduction of CO; b) l-min. exposure to 0; c) G-min. exposure; d ) 40-min. exposure.

of the water gas shift reaction map be at-

tributed to the interaction of surface hy-

300

400

500

600

700

BOO

TernperaturrlKI

Fig.9 TpD in vacuum of' [Rh4(0)32 + Mo(CO)6]/zrOz with Rh/Mo 1:l. Heating rate 10 K/min.

-1s with metal atoms yielding Me*" and H2. This effect becomes =re evident with increasing content of Ho (see Fig.10). As the sample Mo(CO)s/ZrOz showed no H2 evolution below 500 K, v e can attribute the first Hz pesk in IPD spectra to the oxidative interaction of surface hydroxyls with Rh; the role of mlybdemunis t o p r a m t e the fonmation of oxidized Rh species. Also noticeable is the desorption of significant almlmts of CHI a& CH3a.I; the fonm-

952

tion of the latter poduct, which is observedonlywithMom>1,canbe

attributed to the peculiar functionality

[Rh4ICO1,pMo ICO161/Zr02

prclaoted by no. coNculsIoNs

I

400

500

EOO

Temperature(KI

mo

800

Zircmia-supported R h r ( 0 ) i z is a chemically unstable systep, evolving at temperature near ambient to Rhl (00) 2. FFIR/PA spectroscopy, combined with m, proved to be an efficient tool in describing the d e composition mechanism, which involves 00 dissociation and oxidation of Rh by Msorbed mygem, w i t h pacallel evolution of

Fig.10 TPD in vacuun of [ R h 4 ( 0 0 ) 1 2 mz. + M~(CO)SI/Z~OZ with RWMO 1:2. ~ e a - -1 ting rate 10 Wmin.

Rh crystallites formed by reduction at 523 K are converted to the same Rh* species when exposed to 00 at 305 K; at 523 K 00 can induce an apposite -tion effect. Mo pamotes 00 dissociation at tRaperature near ambient, and oxidative intenrction between Rh and surface 1s at about 500 K. The results of this s t d y suggest that in the working conditions both Rh crystallitas and oxidized Rh species are present on the catalyst surface. Rhx sites are likely to p h y an hportant role in the CO -emtion 'Slaand, according to soee Authors,are responsible for alcohol formation (ref. 11). The reto be related to the stabilization of Rh' markable pramtian effect of Mo species in the reaction conditians (ref. 12) which induces high selectivity to oxygenates,and to the enhancement of 00 dissociation.

-

The work tms supported by Consiglio Nazianale delle Ricerche - Progetto Fina1izzat.o mergetica. 'Ih Authors thank Mr. S. Scappatura (Staeione sperktale per i Cdmstibili) for performing TPD masmments and Prof. F. Pinm (vniversitii di Venezia) for supplping Z r o z support.

RmmtmaS 1 M. ,-I cheptech, (1982) 674-680. 2 A. CariPati, A. Girelli, S. PfarenBo, S. k r t i n e n g o , L. -righi and T. Zerlia, in M.J. Phillips and M. T e m m (Eaitors), Proc. 9th Int. Gmgr. Catal., 'lhcherical Institute of Canada, Ottam, 1988, V01.2, pp. 706713. 3 A.C. Yang and C.W. Garland, J. P h p . chea., 61 (1957) 1504-1512. 4 M. R h t , J . cha.Soc., Fareday Trans. 1,74 (1978) 2570-2580. 5 C.A. R i c e , S.D. Worley, C.W. Curtis, J.A. Guin and A.R. lbrrer, J. cher.

953 Phys.,74 (1981) 6487-6497. M. Ichikaaand T. lkkwhma * J. phys.Ck1~.,89(1985) 1564-1567. 7 J.A. (krdella, Jr., D. Jiang, W.P. Hd(enuyi a d E.H. Egring, Appl. Surf. Sci ,15 ( 1983) 36-49. 8 H.F.J. van’t Blik, J.B.A.D. van Zon, T. rrUizinga, J.C. Vis, D.C. Koningsberger and R. P r b , J. Am. (Xem. Soc., 107 (1985) 3139-3147. 9 P. Solymosi and H. Pasetcar, J. chen.,89 (1985) 4789-4793. 10 I.M. Ham&dehand P.R. Griffiths, Appl. Spectroec.,41 (1987) 682-688. 11 P.R. Watscm and G.A. Smrjai, J. cataL.,76 (1982) 282-295. 12 F. Solymosi, M. pasetor and G. Rakhely, J. Chtal.,llO (1988) 413-415. 6

.

m.

Dr

.

D.

R e i n s l d a ( R o v a l / D u t c h She1 1

IF t h r i I and5

1

I.abordtnry,

Am.;te~-dam.

asked :

c c ~ n : ~ : ~ t i o ncaa lr r i e i - l l k e s i l i c a ?

Dr.

!;.

Marenqo replied:

It has been shown ( 1 ) that when rhodium is dispersed on a moderately acidic support as ZrO the selectivity to C2-oxygenates is much higher than with Si02; in aikition, with Rh/Si02 catalysts, the promotion effect of ZrOl on CO dissociation is comparable to that of Ti02 (2). For these reasons, porous ZrO appeared to us a suitable starting material in our investigation aimedZ at the preparation of catalysts for the synthesis of higher alcohols. 1 M. Ichikawa, Chemtech, (1982) 674-680. 2 W.M.H. Sachtler and M. Ichikawa, J. Phys. Chem., 90 (1986) 4752-4750.

The


C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reuctiuity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

955

XPS AND FTIR INVESTIGATION OF y-ALUMINA SUPPORTED CATALYSTS DERIVED FROM H3Re3(C0)12 AND NH4Re04 : A COMPARATIVE STUDY

Z. ZSOLDOS, A. BECK and L. GUCZI Institute of Isotopes of the Hungarian Academy of Sciences, H-1525 Budapest, P.O.Box 77, Hungary ABSTRACT To clarify the formation of the catalytically active sites in Re-catalysts supported on alumina two different precursors. H3Re3(C0)12 and NHqRe04, were used a s Re-sources. Their comparison helped by TPD, FTIR and peak-synthesis aided XPS methods-revealed that the annealing in vacuo or under inert atmosphere leads to an interaction between H3Re3(C0)12 and alumina surface and via two peculiar carbonyl forms it is oxidized, while NHqRe04 undergoes a reduction due to a thermal decomposition process. Subsequent hydrogenation of both catalysts results in the formation of catalytically active zerovalent rhenium. The extent of this reduction is higher for the NHqReOq-derived catalyst which directly explains its higher catalytic activity in the CO + H2 reaction. INTRODUCTION Introduction of metallic clusters has proved to be one of the remarkable advancements in the field of catalytic research (ref. 1). Their application as catalyst precursors, mostly with carbonyl ligands, usually led not simply to metal catalysts of higher dispersion but the formation of intermediate states via which the catalytic reactions were promoted. A typical example of this behaviour is shown by cluster-derived ruthenium supported on alumina (ref. 2). Rhenium-containing catalysts have been extensively studied since their first important industrial application in the naphta reforming process (ref. 3 ) . Although the function of rhenium in this process is still unrevealed - alloy-formation or multifunctionality (refs. 4 - 8 ) - it is known that the main problem caused lies in the complex interaction between rhenium species and alumina. The primary goal of the present study is to compare the behaviour of rhenium on alumina surfaces using different sources, H3Re3(C0)12 and NH4Re04, under various treatments. This way helps us to understand the mechanism by which the catalytically active species are formed. XPS, FTIR and TPD methods as well as catalytic measurements were used for sample characterization.

956

EXPERIMENTAL Materials For the preparation of the cluster-containing sample, y-alumina (Degussa Alon C) was treated under Pa at 570 K for 16 h. Then, it was impregnated with the solution of n-penthane containing 1.6 wt% H3Re3(C0)12. The fresh sample was outgassed and kept under vacuum for 6 h afterwards it was stored under He atmosphere. Ammonium perrhenate sample was made by incipient wetness method from untreated .y-alumina and 1.2 wt% aqueous solution of NH4Re04 (Hicol b.v. Chemicals). The metal loadings of the samples prepared in these ways were 1.1 and 1.8 wt%, respectively, checked by XRF. Characterization methods The catalytic tests were carried out with 1:2 mixture of CO and H2 both in flow (15 cm3 min-l) and in circular systems. A Packard 427 gas chromatograph and a Varian-DuPont GC-MS, respectively, were used to analyse the products. The infrared spectra were recorded on sample wafers using a Digilab FTS-2OC interferometer equipped with a DATA GENERAL NOVA 3 computer. The TPD measurements were carried out with a TC detector at 20 -1 heating rate either in flowing He or H2. K min The XPS characterization was carried out by a KRATOS ES-300 ESCA equipment working in the FRR mode. The base pressure was about Pa. In situ UHV and atmospheric gas treatments were conducted in the sample preparation chamber (SPC) and in a small reactor closely attached to the vacuum system, respectively. The recording of the spectra was controlled by a microcomputer then the data acquisitions were performed by an IBM PC/XT using a peak-synthesis-type sottware. The binding energy (B.E.) scale was referenced to the A1 2p peak (73.7 eV in .y-A1203 (ref. 9)) as an internal standard. All spectra were subjected to an X-ray satellite and an inelastic (Sherwood-type) background subtraction. The peak-synthesis started with the B.E. values of Re 4f712 peak in the different valence states as: (0):40.6; (+l): 41.0; (+4): 42.9; (+6): 44.4; (+7): 46.5 eV (the Gaussian FWHM values were uniform for all the valence states) furthermore the coupling between 4f7,2 and 4f512 peaks was 2 . 4 eV as had been collected both from the literature (e.g. ref. 4) and from our own standard experiments.

957

RESULTS AND DISCUSSION Catalytic tests Table 1 shows the results of the catalytic measurements obtained over V-alumina-supported samples after the treatments of 1 bar He at 570 K for 30 min followed by an atmospheric hydrogenation at 6 7 0 X for flow and circular systems, respectively. The results of both types of measurements are in good correspondance and they TABLE 1

-1 CO + H2 reaction rates at 530 K in mol s -1 gcat and product selectivities at 550 K ~~

Catalyst

Reaction rate in Flow and Circular system

H3Re3(CO) 12/~-Al,03 8.0~10-~ 9.1~10-~ NH4Re04/y-A1203 20.2x10-~ 19.2Xi0-~

Selectivity 4.

%

C,L

Olefins

18.0 15.4

51.8 40.4

display a higher reaction rate for the perrhenate-derived catalyst than for the cluster-derived one. Nevertheless, even this latter value is less than that could be expected if one assumes that the total metal content is present in highly dispersed, zerovalent form (XRT, showed no crystallites on either sample) being catalytically active species in the CO + H2 reaction. The additional surface characterization experiments were conducted to help in explaining these catalytic findings. Catalyst derived from H3Re3(C0)12 In order to follow thg transformation of the rhenium-containing species over the alumina surface the effect of the heat treatments under vacuum was investigated. The FTIR results obtained for cluster-derived sample after such in situ treatments can be seen in Fig. 1. The spectra clearly show that while the isolated band at 2093 cm-l wavenumber, characteristic solely of the original cluster, gradually disappears, simultaneously a new three band spectrum (2035, 1920 and 1885 cm-l, resp.) develops with increasing temperature. This latter triplet can be assigned to a surface subcarbonyl ("tricarbonyl") species (ref. lo), however its first two bands of higher wavenumbers can be attributed to the presence of a pe(COb(OH)]4 complex ("cubene") as well. Although the exact

958

s

0

1 '0.

.-ma C

0 0

b.

.-i

WVENUHBERS. t/CH Fig. 1. FTIR spectra of cluster derived sample during in situ heat treatments in vacuo at (a) 300 K (b) 570 K (c) 740 K.

300

490

600

750

fernporeture/K Fig. 2 . Temperature Programmed Desorption (TPD) spectra of cluster-derived catalyst under (a) He (b) H2 atmosphere.

ratio of these two species cannot be determined we are inclined to accept that both carbonyls are simultaneously present on the surface of alumina. This.is also supported by the fact thpt the third and the other two bands change asymmetrically during the treatment. Nevertheless, on the basis of the FTIR experiments it can be stated that increasing the temperature the original cluster is transformed into a mixture of "tricarbonyl" and "cubene" species almost completely at 570 K and the latter starts to decompose at 740 K. TPD measurement shown in Fig. 2(a) also proves it since the smaller TPD peak at 4 2 0 K can obviously be attributed to the decomposition of the original cluster (CO li-jands are partly released) and the larger peak at 730 K is characteristic of the decomposition of the carbonyl species developed. The XPS results obtained after two subsequent heat treatments are collected in Table 2. The presented values are calculated on the basis of the original XPS spectra of the Re 4f region which were

959

TABLE 2 The results of XPS measurements carried out on cluster-derived sample after different treatments in UHV (ref. 11) Rea in the valence states of

%

Treatment (0)

-

75

5 7 0 K for 20 min 7 7 0 K for 2 0 min ~

-

(+I)

50 40

(+4)

(+6)

25 45

-

40

5 10

(+7)

-

10

~

aCRe = 100%. synthesized starting from the premises mentioned before. Re is in (+1) valence state both in the surface "tricarbonyl" and in the "cubene" complexes thus the first two columns represent almost the same results as were observed in FTIR and TPD experiments. The only difference is the lower extent of the decomposition of Re(+l) that may be due to the better vacuum conditions. What are the new pieces of information provided by XPS are the demonstration of the presence of the higher valence states of Re and making a possibility to determine their proportions. The values shown in Table 2 clearly show that, due to the interaction between H3Re3(C0Il2 and A1203, the original supported cluster already contains a considerable amount of Re in the ( + 4 ) valence state probably identical with the Re(+4) layer described by YaO and Shelef (ref. 12) formed over the alumina surface. Moreover, our XPS measurements show further oxidation of Re even to ( + 7 ) valence state. Under the UHV conditions applied this must be attributed to the effect of the support surface so we can rather imagine this Re(+7) as surface species having more Re-0-A1 bonds instead of Re207. The thermal decomposition carried out under H2 atmosphere shows no difference from the decomposition in vacuo as can be seen in the respective TPD spectra in Fig. 2(b). Fig. 3(a) presents the FTIR spectrum recorded after room temperature CO adsorption on the H3Re3(C0)12/V-A1203 sample decomposed in H2. The large single band of rnonoadsorbed CO at 2 0 4 5 cm-l unambiguously demonstrates the presence of reduced metallic Re. To clarify the role of surface carbonyls in relation to the CO + H2 reaction catalytic and CO titration experiments were carried out on the cluster-derived sample. The results are collected in Table 3 . After 1 h treatment in flowing He at 5 7 0 K, simulating the decomposition in vacuo, Re is mostly present in

960

1

c

Irl

U

z c

m K 0

m

m

c

Fig. 3 . FTIR spectra recorded after CO adsorption at 300 K on (a) H3Re3 (CO)12/~-Al203and (b) NHqReOq/v-A1203 samples treated in H2 at 670 K for 2 h. TABLE 3 Rates of CO + H2 reaction measured in flow system at 570 K and results of CO titrations obtained on H3Re3(CO)12/v-A1203 after different treatments Reaction rate He at 570 K for 1 h H, at 770 K for 1 h

2.6xlO-' 1. 3x10m7

Adsorbed CO at 300 K 0 16.2

surface carbonyl forms as was revealed before. These species show a very low catalytic activity and no metallic Re can be found by CO titration. After subsequent treatment in H2 at 770 K the rate of CO + H2 reaction significantly increases and metallic Re can be detected on the surface. These observations clearly show that the catalytic activity obtained after the treatments in He and H2 (presented in Table 1) can predominantly be attributed to the presence of zerovalent Re even in the case of the supported Re catalyst derived from H3Re3(C0)12 although surface carbonyls have some catalytic activity in CO hydrogenation.

961

Catalyst derived from NH4Rr+ The XPS spectra of the-Re 4f region recorded after different treatments of NH4Re04/V-A1203 are shown in Fig. 5(a). For the peak synthesis a further broad peak on the higher binding energy side had to be taken into account that may be assigned as an electron energy loss peak of the near oxygen 2s photoelectron line. The results are summarized in Table 4 . Similarly to the cluster-derived sample it is clearly seen that a part of the original catalyst already contains Re(+4) but further treatments in vacuo result in more and more reduced Re species in contrast with the oxidation experienced in the previous case. In order to explain this behaviour, beside the obvious difference between the support surfaces and thus in their reactivities, a special reductive decomposition process must be taken into account described by Pdlfi (ref. 13). During this reaction N2 gas evolves and Re(+4) is formed from NH4Re04 either in inert (vacuum) or in reductive (H2) atmosphere. Similar surface interaction was observed in the Fe(N0 ) + 3 3 + Pt(NH4I4 system on silica support (ref. 14). We think that this decomposition dominates the transformations on the alumina surface after heat treatments in vacuum. It must also be noted that during this reduction the valence state of Re does not decrease under the value of (+4) what is another evidence both for the controlling role of the reductive decomposition and for the high stability of the surface Re species in the valence state (+4). However, it is not the case after the decomposition in H2 as can be seen in Fig. 5(b) and in Table 4 . In the presence of H2 the heat treatmentleven after a pretreatment in UHV,at 670 K and 770 K, respectively, results in the occurrence of zerovalent Re probably due to the reduction of Re(+4) formed from the original NH4Re04 via the reductive decomposition not affected by H2. Simultaneously, the FTIR experiment carried out after similar hydrogenation also reveals the presence of metallic Re shown by the large single peak of adsorbed CO (Fig. 3(b)). It is also proved by comparing Fig. 3(a) and (b) spectra that the extent of the reduction into zerovalent Re is much higher in the case of the NH4ReO4/V-Al2O3 catalyst than for the cluster-derived material. This difference clearly resulted in the higher catalytic activity of the CO + H2 reaction for the perrhenate-derived sample. Although on the catalyst derived from H3Re3(C0)12 some subcarbonyl species may still be present during the reaction they are practically not active in it.

962

I

I

60.0

I

50.0

40.0

.

Bindinq E n e r q y / e 1 7

I

I

I 30.0

I

r

I

4G.0 Binding Energy/eV

60.0

Fig. 5. XPS spectra of Re 4f region recorded on NHqReOq/v-A1203 after different heat treatments (a) in UHV (b) under H2 atmosphere. TABLE 4 The results of XPS measurements obtained on NH4Re04/y-A1203 catalyst after different treatments ~~

%

Rea in the valence states of

Treatment

-

UHV at 6 7 0 K for 20 min UHV at 7 7 0 K for 20 min

-

1 bar H2at 7 7 0 K for 1 h UHV at 6 7 0 K 6 0 min 1 bar H7 at 6 7 0 K 40 min aIRe = 100%.

(0)

35 -

25

(+I)

-

-

(+4) 15 25 85 15 45 60

45

(+6)

30 -

(+7)

85 45

15 85 15 40

25

963

SUMMARY The FTIR and the XPS methods proved to be very useful for the comparison of the surface transformations of Re-containing species derived from H3Re3(C0Il2 and NH4Re04, respectively. Especially, the latter peak-synthesis aided technique helped to follow the different processes taking place among the rhenium species in various valence states and the alumina surfaces. During the heat treatments in vacuum or under inert atmosphere the cluster-compound and NH4Re04 pass through opposite trajectories. The former transforms into surface species containing Re in higher valence states,through two special carbonyl complexes,due to its interaction with the support surface. However, the latter compound undergoes a reductive thermal decomposition. Subsequent hydrogenation results in the occurrence of zerovalent Re being active in the catalytic reaction of CO + H2 and the extent of this reduction is different for the two materials derived from the cluster and from NH4Re04, respectively. Since more Re(0) appears in the case of the latter source the higher catalytic activity of the NH4Re04-derived catalyst can easily be understood. Moreover, it is proved that the surface carbonyls of Re formed in the first step from the original cluster on alumina are not active in CO hydrogenation and they do not even promote it as was in the case of ruthenium. We must suggest on the basis of our results that the most important rhenium species is the Re(+4) being present on the alumina surface in the case of both types of starting materials and at each stage of the experiments and its ability to the transformation into zerovalent Re is the controlling factor for catalytic CO + H2 reaction. Further aspects of this process are under investigations. ACKNOWLEDGEMENT The authors are indebted to Mrs Gy. Stefler and Mr K. Matusek for the catalytic and to Mr. I. Bogyay for TPD measurements. REFERENCES 1 B.C. Gates, L. Guczi and H. Knozinger (eds.), Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986. 2 A . Beck, S. Dobos and L. Guczi, Inorganic Chemistry, in press. 3 H.E. Klucksdahl, U.S. Patent, 3,415,737 (1968). 4 P.S. Kirlin, B.R. Strohmeier and B.C. Gates, J. Catal., 98 5

(1986) 308-316. M.J. Kelley, A.S. Fung, M.R. McDevitt, P.A. Tooley and B.C. Gates, Proc. Materials Research Society, to be published.

964

6 V.K. Shum, J.B. Butt and W.M.H. Sachtler, J. Catal., 99 (1986) 126. 7 M.S. Nacheff, L.K. Kraus, M. Ichikawa, B.M. Hoffman, J.B. Butt and W.M.H. Sachtler, J. Catal., 106 (1987) 263. 8 S.M. Augustine, M.S. Nacheff, C.M. Tsang, J.B. Butt and W.M.H. Sachtler, in J.W. Ward (Editor), Catalysis 1987, Elsevier, Amsterdam, 1988. 9 D. Briggs and M.P. Seah (Editors) Practical Surface Analysis, John Wiley and Sons, Chichester, 1983. 10 P.S. Kirlin, F . A . DeThomas, J.W. Bailey, H.S. Gold, C. Dybowski and B.C. Gates, J. Phys. Chem., 90 (1986) 4882. 11 L. Guczi, Proc. 9th Int. Congress on Catalysis, Calgary, June 1988, in press. 12 H.C. Yao and M. Shelef, J. Catal.., 4 4 (1976) 392. 13 S. Pdlfi, Ph.D. Thesis. Budapest, 1976. 14 L. Guczi.and F. Till, Material Science Monography, 10 (1982) 908.

965

AUTHOR

Abbate, G. 69 713 Alvero, R. 1 Anpo, M. Arena, F. 739 A s c h i e r i , R. 703 A s t a l d i , C. 825 307 Augugliaro, V. 11 Aukett, P.N. Auroux, A. 525 19 Badyal, J.P.S. B a i l e s , M. 31 41 B a r a l d i , P. 49, 59 Bardi , U. 69 Barone, V. Bartbk, M. 685, 729, 845 Basset, J.M. 591 Bateman, J.E. 75 B a t t a g l i a , F. 85 Beck, A. 955 B e l l , A.T. 91(x) Be1 lamy, B. 347 Belougne, P. 111 Bensimon, Y. 111 Bernal , S . 123 Bernhardt, P. 363 B e r t o l i n i , J.C. 749 B i t t a r , A. 327 B l a v e t t e , D. 625 B o c c u t i , M.R. 133, 653 Boccuzzi , F. 415, 437 Boehm, H.P. 145(x) B o l i s , V. 159, 703 Bond, G.C. 167 Bonneviot, L. 653 643, 703 B o r e l l o , E. Botana, F.J. 123 Botman, M.J.P. 179 B o u l e t , R. 695 Bourgeois, S. 191 B o u r n o n v i l l e , J.P. 591 Bradshaw, A.M. 201(x), 493 BrGmard, C. 219 Buhaenko, D.S. 229 Burkhardt, I . 677 Busca, G. 239, 777 Caballero, A. 427 Candy, J.P. 591 C a r i m a t i , A. 943 Carrizosa, I . 713 Causii, M. 385 Che, M. 1, 347, 653

INDEX Chesters, M.A. 75, 249, 257, 263 415 C h i o r i n o , A. Chmelka, B.F. 269 279 Cimino, A. Coluccia, S. 1, 289(x), 643, 653 Conesa, J.C. 307 Connolly, M. 319 Coombs, D. 257 Coq, B. 327 Csencsits, R. 269 625 D'Huysser, A. D a r l i n g , G.R. 335 Dauscher, A. 799 De Gouveia, V. 347 De MenorVal, L.C. 269 D e l l a V a l l e , F. 825 Den Hartog, A.J. 179 219 Denneulin, E. 219 Depecker, C. Deroide, B. 111 D i Castro, V . 355 D i a k i t e , D. 191 Diaz, G . 363 1 Doi, T. Dossi, C. 375 Dovesi, R. 385 817 Dunhill, W. Escalona P l a t e r o , E. E s p i n k , J.P. 427 Esteban, P. 363

395

41 Fabbri, G. 723 Fadley, C.S. Figueras, F. 327 581 F o r z a t t i , P. Francis, S.M. 229 695 Freund, E. F r u s t e r i , F. 739 F u b i n i , B. 31, 159 F u r l a n i , C. 355 375 Fusi, A. Garcia, R. 123 Gardner, P. 493 Garin, F, 363 159, 395, 405 Garrone, E. Gasser, A. 535 Gazzoli , D. 279 Gellman, A.J. 19 Gelsthorpe, M.R. 167 85 George, T.F.

966 G h i o t t i , G. 415 159 Giamello, E. Giordano, N. 739 G i u n t i n i , J.C. 111 Gonzales-Elipe, A.R. 427 Goulding, P.A. 229 Graetzel, M. 469 Grant, K. 855 Guczi, L. 363, 955 G u g l i e l m i n o t t i , E. 437 Gutschick, D. 677 447(x) Haber, J. Herman, G.S. 723 469 H i g h f i e l d , J.G. H i l a i r e , L. 799, 363 Hindermann, J.P. 481 Hoge, D. 493 Indovina, V. I n v e r s i , M.

279 279

Malet, P. 427 M a l i t e s t a , C. 633 Marchese, L. 643, 653 Marengo, S. 943 Martens, J.H.A. 759 749 M a r t i n , G.A. Martinengo, S. 943 Masson, A. 347, 665(x) Matsuura, I. 1 McCabe, T. 319 Menand, A. 625 Miessner, H. 677 685 Molnar, A . Mondello, N. 739 Montagne, X. 695 633 Morea, G. M o r e t t i , G. 279 M o r t e r r a , C. 159, 601, 677, 703 M u l l e r , W. 799 Munuera, G. 427 Notheisz,

Jaeger, N. 559 Jaeger, N. I . 503 Jennings, J.R. 515 J i n , Y.S. 525 Jomard, F. 191 Jourdan, A.L. 503 Joyner, R.W. 335 Judd, R.W. 19 Khanna, A.S. 535 Kiennemann, A. 481 K i m , Y.S. 85 Kiss, J.T. 685 K i z l i n g , 14. 549 Koningsberger, D.C. Kreutz, E.W. 535

729

279 Occhiuzzi, M. 713 Odriozola, J.A. Osterwalder, J. 723 Ouqour, A. 525

759

Lamber, R. 559 Lambert, R.M. 19 653 Lavagnino, S. L a v a l l e y , J.C. 695 Le Calvar, M. 567, 575 Le Normand, F. 863 Legrand, P. 219 Lenglet, M. 567, 575, 625 Lennon, D. 263 L e o f a n t i , G. 133 L i e t t i , L. 581 L i n d e r , D.R. 249 L i u , S.6. 269 Lloyd, D.R. 319 Lorenzelli , V. 777 Louessard, P. 591 Low, M.J.D. 601 611 Luck, F. I l a c h e f e r t , J.-M. 625 363, 799 idaire, G.

F.

Pacia, N. 919 P81ink6, I . 729 Palmisano, L. 307, 643 Parker, S.F. 257 Parmaliana, A. 739 Pasquon, I. 581 P a t t u e l l i , M.E. 239 Paul -Boncour, V . 863 Pemble, M.E. 229 335 Pendry, J.B. Percheron-Guggan, A. 863 Perdereau, M. 191 Petersen, E.E. 269 133 P e t r i n i , G. Pigeat, P. 919 Pines, A. 269 335 P i s a n i , C. P o i r a u l t , R. 191 P o l z o n e t t i , G. 355 Ponec, V. 179 P r a l i a u d , H. 749 Prins, R. 759(x) Psaro, R. 375 Pugh, R.J. 549 Quadakkers , W. J.

535

Radke, C.J. 269 Rajaram, R.R. 167 Ramis, G. 777 133 Rao, K.M. Roberts, M.W. 787(x)

967

R o d r i g u e z - I z q u i e r d o , J.M. R o e t t i , C. 385 799 Romeo, M. Rosei, R. 825 59 Ross, P.N. 49, 59 Rovida, G. RUSSO, N. 69, 809, 393 Ruterana, P. 469 Ryoo, R. 269 S a b b a t i n i , L. 633 S a i k i , R.S. 723 S a k a k i n i , 6. 817 S a n t o n i , A. 825 S a r k l n y , A. 835 845 Sarkany, J. 643 S c h i a v e l l o , M. Schlapbach, L. 903 S c h m i t t , J.-L. 363 S c h u l z - E k l o f f , G. 503, 559 Schuster, H. 535 643 S c l a f a n i , A. Sermon, P.A. 855 S i m , K.S. 863 Sirokmdn, G. 685 S o r i a , J. 307 Spencer, M.S. 515 Spoto, G. 395, 677 Steeples, 6. 817 Stesmans, A. 871 Stone, F.S. 31 Sum Yuen, G. 919 Svensson, A. 503 S w i f t , A.J. 881 633 T a n g a r i , N. Tardy, 6. 749

123

319 T a y l o r , E. T a z k r i t t , S. 41! 9 Thampi, K.R. T o r t o r e l l a , V. 633 Toscano, M. 69, 893 Touroude, R. 863 T r i f i r B , F. 239 T r o n c o n i , E. 581 Tiishaus, M. 493 Ugliengo, P. Ugo, R. 375

405

239 V a c c a r i , A. Vedel, I. 903 Vedrine, J.C. 525 Vickerman, J.C. 817, 881 Viez, F. 611 Vong, M.S.W. 855 Wanner, M. 911 Weber, 6. 919 911 Weil, h.G. 535 Wissenbach, K. Woste, L. 925(x) Yahya, R. Yamada, M. Yamada, Y.

167 723 1

Zambonin, P.G. 633 Zanchetta, J.V. 111 Z a n d e r i g h i , L. 943 Zanoni, R. 375 Zecchina, A. 1, 133, 395, 653, 677 Z e r l i a , T. 943 Zsoldos, Z. 955

968

STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universitb Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U S A .

Volume 1 Preparation of Catalysts 1. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 1417,1975 edited by 8. Delmon, P.A. Jacobs and G. Poncelet Volume 2 The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Volume 3 Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7,1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Volume 4 Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Socibt6 de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Volume 5 Catalysis by Zeolites. Proceedings of an InternationalSymposium, Ecully (Lyon), September 9-1 1,1980 edited by 8. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and

H. Praliaud Volume 6 Catalyst Deactivation. Proceedings of an InternationalSymposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Froment Volume 7 New Horizons in Catalysis. Proceedings of the 7th InternationalCongress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiy ma and K. Tanabe Volume 8 Catalysis by SupLrted Complexes by Yu.1. Yermakov, B.N. Kuznetsov and V.A. Zakharov Volume 9 Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29October 3, 1980 edited by M. LAzniEka Volume 10 Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an InternationalSymposium, Aix-en-Provence, September 2 1-23, 198 1 edited by J. Rouqueroland K.S.W. Sing Volume 11 Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14- 16, 1982 editgd by 8. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau,

P. Gallezot, G.A. Martin and J.C. Vedrine

Volume 12 Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. JirO and G. Schulz-Ekloff Volume 13 Adsorption on Metal Surfaces. An Integrated Approach edited by J. BBnard Volume 14 Vibrations at Surfaces. Proceedingsof the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz

969 Volume 15 Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Volume 16 Preparation of Catalysts 111. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Volume 17 Spillover of Adsorbed Species. Proceedings of an InternationalSymposium, LyonVilleurbanne, September 12- 16, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Volume 18 Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by P.A. Jacobs. N.I. Jaeger, P. JirQ, V.B. Karansky and G. Schulz-Ekloff Volume 19 Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Volume 20 Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Volume 2 1 Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29,1984 edited by M. Che and G.C. Bond Volume 22 Unsteady Processes in Catalytic Reactors by YuSh. Matros Volume 23 Physics of Solid Surfaces 1984 edited by J. Koukal Volume 24 Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, PortorobPortorose, September 3-8, 1984 edited by 8. Driaj. S. Hobvar and S. Pejovnik Volume 25 Catalytic Polymerization of Olefins. Proceedings of the InternationalSymposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Volume 26 Vibrations at Surfacer 1B86.Proceedings of the Fourth InternationalConference, Bowness-on-Windermere, September 15-1 9, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Volume 27 Catalytic Hydrogenation edited by L. &men9 Volume 28 New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Volume 29 Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Knozinger Volume 3 0 Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8- 11,1986 edited by A. Crucq and A. Frennet Volume 3 1 Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4,1986 edited by 8. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Volume 32 Thin Metal Films and Gas Chemisorption edited by P. Wissmann Volume 33 Synthesis of High-silica Aluminosilicate Zeolites by P.A. Jacobs and J.A. Martens Volume 34 Catalyst Deactivation 1987. Proceedings of the 4th InternationalSymposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment

970 Volume 35 Keynotes in Energy-RelatedCatalysis edited by S. Kaliaguine Volume 36 Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicalsfrom Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chaney, R.F. Howe and S. Yurchak Volume 37 Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-1 7, 1987 edited by P.J. Grobet. W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Volume 38 Catalysis 1987.Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Volume 39 Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29, 1987 edited by K.K. Unger. J. Rouquerol. K.S.W. Sing and H. Kral Volume 4 0 Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1, 1987 edited by J. Koukal Volume 4 1 Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-1 7, 1988 edited by M. Guisnet, J. Barrault. C. Bouchoule, D. Duprez, C. Montassier and G. PBrot Volume 42 Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by 2. Paal Volume 43 Catalytic Processes under Unsteady-State Conditions by Yu. Sh.Matros Volume 44 Successful Design of Catalysts. Future Requirements and Development. Proceedings of the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Volume 45 Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Volume 46 Zeolites as Catalysts. Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wurzburg, F.R.G., September 4-8,1988 edited by H.G. Karge and J. Weitkamp

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