Studies in Surface Science and Catalysis 48
STRUCTURE AND REACTIVITY OF SURFACES
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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
This Page Intentionally Left Blank
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.
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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-
<
u
I
0 0
10
20 AMOUNT
30 LO 50 ADSORBED / p mol m-2
F i g . 4. d a t e r vapour a d s o r p t i o n on CaO and CaO-MnO s o l i d s o l u t i o n s CM I, CM 25 and CM 50 a t 303 K. Heat o f a d s o r p t i o n as a f u n c t i o n o f uptake. The p l o t s do n o t e x t e n d t o l i m i t i n g uptake.
t h a t oxygen i s indeed a s e l e c t i v e probe f o r manganese i o n s i n t h e s u r f a c e o f CaO-MnO, and t h i s i s c o n f i r m e d b y t h e enhanced h e a t o f a d s o r p t i o n on t h e s o l i d solutions.
The h i g h v a l u e o f t h e h e a t o f a d s o r p t i o n , which i s 150 k J mol-'
even on CaO, suggests t h a t t h e a d s o r p t i o n i s m a i n l y d i s s o c i a t i v e .
This i s
s u p p o r t e d b y t h e o b s e r v a t i o n t h a t t h e doses a r e f o r t h e most p a r t s l o w l y adsorbed and t h a t t h e oxygen, once adsorbed, cannot be desorbed a t 303 K. Assuming t h a t t h e c a t i o n s a r e t h e a c t i v e c e n t r e s f o r d i s s o c i a t i v e a d s o r p t i o n o f oxygen, an oxygen molecule w i l l d e l i v e r i t s atoms e i t h e r t o two Ca2+ i o n s , o r t o two Mn2+ i o n s , o r t o one Ca2+ and one Mn'',
t h e p r o p o r t i o n s depending on
the concentrations o f t h e respective cations i n t h e surface.
de may t h i n k o f
these as ' p a i r s ' , a l t h o u g h t h i s does n o t i m p l y t h a t t h e c a t i o n s r e c e i v i n g oxygen atoms from a g i v e n oxygen molecule are n e c e s s a r i l y n e a r e s t neighbours. L e t us now c o n s i d e r t h i s s i t u a t i o n i n r e l a t i o n t o CM 50, CM 25 and CM 1. The a d s o r p t i o n o f oxygen on CM 50 proceeds beyond 8 = e x a l t e d h e a t o v e r t h e whole range ( F i g . 2 ) . c a t i o n s must be almost e x c l u s i v e l y Mn2+.
1 and t h e r e i s an
Ue conclude t h a t t h e s u r f a c e
This i s s t r o n g l y supported by t h e
w a t e r vapour a d s o r p t i o n behaviour ( F i g . 4) where t h e r e i s v i r t u a l l y t o t a l
38
suppression of t h e r e a c t i v i t y c h a r a c t e r i s t i c o f c a l c i u m o x i d e .
#e t h e r e f o r e
r e g a r d t h e h e a t p l a t e a u a t 325 k J m o l - I e x h i b i t e d b y CM 50 as c h a r a c t e r i s i n g oxygen a d s o r p t i o n on Mn-Mn p a i r s i n t h e sense d e s c r i b e d above.
CM 25, w h i l s t
g e n e r a l l y s i m i l a r t o CM 50 i n i t s r e a c t i v i t y towards oxygen ( F i g . 2), i s markedly d i f f e r e n t i n r e a c t i v i t y towards w a t e r vapour ( F i g . 4 ) .
The l a t t e r
p a r t i c u l a r l y r e f l e c t s Ca2+02- c h a r a c t e r , and we a c c o r d i n g l y i n f e r t h a t t h e surface o f CM 25 must c o n t a i n some c a l c i u m i o n s , a l b e i t p r e s e n t w i t h a h i g h c o n c e n t r a t i o n o f manganese i o n s on account o f t h e h i g h oxygen coverage achievable.
CM 1 i s a d i l u t e s o l i d s o l u t i o n , and as such can be expected t o
p r e s e n t p r e d o m i n a n t l y Ca-Ca p a i r s and Ca-Mn p a i r s . p l a t e a u a t 150 k J m o l - '
Ca-Ca p a i r s a f f o r d a h e a t
( F i g . 1 f o r CaO), so we r e g a r d t h e observed p l a t e a u a t
190 k J m o l - l f o r CM 1 ( F i g . 3) as c h a r a c t e r i s t i c o f Ca-Mn p a i r s w i t h t h e end e f f e c t s beyond 0.6
umol
m-2 as due t o Ca-Ca p a i r s .
The i m p l i c a t i o n o f t h i s i s
t h a t CM 1 has a marked s u r f a c e e n r i c h m e n t i n manganese. ide may develop t h i s l i n e o f argument t o e s t i m a t e t h e p r o p o r t i o n o f Ca2'
In t h e s u r f a c e o f CM 25. 300 k 2 m o l - 1 ( F i g . 2 ) .
ions
#e n o t e t h a t t h e oxygen h e a t p l a t e a u f o r CM 25 i s a t C o n s i d e r i n g f i r s t Ca-Mn p a i r s and Mn-Mn p a i r s , and
t a k i n g t h e c h a r a c t e r i s t i c oxygen h e a t s f o r t h e s e as 190 and 325 k J mo1-l r e s p e c t i v e l y , t h e h e a t p l a t e a u o f 300 k J mo1-l i m p l i e s t h a t 20% o f t h e s e s u r f a c e p a i r s a r e Ca-Mn, s i n c e (0.2 x 190) + (0.8 x 325) t h e s u r f a c e c a t i o n s i n CM 25 a r e c a l c i u m .
=
300.
Thus a t l e a s t 10% o f
C o n s i d e r i n g now Ca-Ca p a i r s , we
r e g a r d them as r e s p o n s i b l e f o r t h e 'end a d s o r p t i o n ' observed on CM 25 (which, i t s h o u l d be noted, i s absent on CM 50).
The r e l a t i v e l y l a r g e e x t e n t o f t h i s
' e n d a d s o r p t i o n ' i m p l i e s t h a t c a l c i u m i o n s a r e n o t randomly d i s t r i b u t e d , b u t t h a t t h e r e i s a tendency f o r t h e s u r f a c e manganese t o be c l u s t e r e d .
This helps
t o understand t h e m a r k e d l y i n c r e a s e d r e a c t i v i t y o f CM 25 towards w a t e r vapour as compared w i t h CM 50 ( F i g . 4 ) .
I r r e s p e c t i v e o f t h i s d e t a i l , however, t h e
c o n c l u s i o n t h a t CM 25 has i t s s u r f a c e e n r i c h e d i n Mn i s i n e s c a p a b l e .
The
enrichment f a c t o r i s a p p r o x i m a t e l y t h r e e f o l d . The magnitude o f t h e ' e n d a d s o r p t i o n ' on CM 25, i . e . t h a t r e g i o n o f observa b l e uptake beyond t h e h e a t p l a t e a u ( F i g . 2 ) , i s g r e a t e r t h a n t h e 0.4 pmol K 2 o f oxygen a d s o r p t i o n observed on CaO ( F i g . 1). A p o s s i b l e e x p l a n a t i o n
o f t h i s i s t h a t Mn2+ i o n s i n &-surface c h e m i s o r p t i o n o f oxygen as O;(ads)
l a y e r s can p r o v i d e e l e c t r o n s f o r
o r 0 - ( a d s ) on Ca2'
ions i n t h e surface.
R e t u r n i n g t o CM 1, i t i s i n t e r e s t i n g t o n o t e t h a t t h e 'end a d s o r p t i o n ' i n t h a t case (0.3 pmol mm2, F i g . 3) i s
t h a n t h e a d s o r p t i o n on CaO.
T h i s may
r e f l e c t a tendency f o r t h e Mn2+ i o n s i n t h e s u r f a c e o f CM 1 t o occupy p r e f e r e n t i a l l y t h e t y p e o f a c t i v e s i t e needed f o r t h e l i m i t e d (5% coverage) oxygen a d s o r p t i o n on CaO.
This suggestion receives support from t h e observation
t h a t t h e h e a t f o r t h e 'end a d s o r p t i o n ' on CM 1 i s much lower t h a n t h e
39
150 k J m o l - I found f o r CaO.
The oxygen on t h e Ca-Ca p a i r s i n CM 1 i s h e l d on
l e s s a c t i v e s i t e s t h a n on CaO.
These s i t e s p o s s i b l y come i n t o p l a y on CM 1
because o f t h e a v a i l a b i l i t y o f e l e c t r o n s donated b y sub-surface manganese. REFERENCES
1 2 3 4
d . Low, S o l i d S t a t e Phys., Suppl. 2, Academic Press, New 'fork, 1960. F.S. Stone and J.C. Vickerman, Trans. Faraday SOC., 67 (1971) 316-328. A. Cimino, Chim. I n d . ( M i l a n ) , 56 (1974) 27-38. J.C. Vickerman, Spec. Per. Rep. C a t a l y s i s , V o l . 2, p . 107-144, Chemical S o c i e t y , London, 1978. 5 F.S. Stone, i n J.L. P o r t e f a i x and F . F i g u e r a s ( E d i t o r s ) , Chemical and P h y s i c a l Aspects o f C a t a l y t i c O x i d a t i o n , E d i t i o n s C.N.R.S., P a r i s , 1980, pp. 457-490. 6 F.S. Stone and J.C. Vickerman, Proc. Roy. S O C . London, A, 354 (1977) 331-347. 7 A. Cimino, B.A. De A n g e l i s , G. M i n e l l i , T. P e r s i n i and P. Scarpino, J . S o l i d S t a t e Chem., 33 (1980) 403-411. 8 C . O t e r o Arean and F.S. Stone, J.C.S. Faraday I , 75 (1979) 2285-2294. 9 P . H . Emmett and S. Brunauer, J. Am. Chem. SOC., 59 (1937) 310-315. 10 B. F u b i n i and F.S. Stone, J.C.S. Faraday I , 79 (1983) 1215-1227. 11 6. F u b i n i , V . B o l i s , M. B a i l e s and F.S. Stone, Proc. 1 1 t h I n t . 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 , NJ, U.S.A., June 20-24, 1988, S o l i d S t a t e Ionics, i n press.
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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
A FIRST APPROACH TO GASEOUS AMINOACID-OXIDE INTERACTION Pietro Baraldi and Gianfranco Fabbri Dipartimento di Chimica, Universith di Modena Via G.Campi,l83 41 100 MODENA (ITALY) ABSTRACT Infrared spectra recorded after interaction of gaseous aminoacids with activated silica-alumina show that different surface species are formed. The structures of the species change as a function of the position of the amino-group relative to carboxyl group. Both ends of the molecules considered are involved in the interaction. The formation of different species may be explained by the relative stability of the rings formed by aminoacids with the Lewis acid sites. INTRODUCTION Aminoacids are the constituents of peptides and proteins and therefore their physical and chemical properties can be related to the ability of living matter to interact with inorganic compounds and originate particular pathologies such as asbestosis and silicosis (1 ). In spite of this, few works have appeared in the literature on the the interaction of aminoacids with oxides (2-11). An analysis of these papers shows that they mostly deal with experimental situations where strong interferences by other compounds are present. In natural biological situations, aminoacids and parent compounds such as peptides and proteins are in contact with water which therefore influences the interaction with oxides and aminoacids. Such competition can mask or confuse the overall results. The problem of eliminating the interaction with water can be partially overcome by dissolving the aminoacids in solvents of lower polarity (2,6),but unfortunately here the solubility is limited. A decade ago Groenewegen and Sachtler (7,8) simplified the approach by exploiting a different property of arninoacids: almost all of them sublimate at a rather low temperature and, as ascertained by them and by previous researchers (3), the sublimation, when carried out at low pressure, takes place without decomposition. This enables the
41
42
gaseous aminoacids to be brought into contact with activated oxides at a given temperature. The adoption of this experimental approach is an oversimplification biologically speaking, but it should supply interesting basic information regarding the nature, intensity and structure of the interactions involved. In this paper we report the results obtained in a simplified experimental situation of the kind discussed above, namely obtained by eliminating water from the oxide surface as far as possible. We plan to undertake the study of the possible interactions between aminoacids and oxide surfaces covered with increasing amounts of water and to compare the overall results in subsequent research. In this paper, the three simple aminoacids glycine, a alanine and p alanine are considered. Silica-alumina was chosen as adsorbent since, because of its Lewis acidity (12-15) under high dehydration conditions it might interact in different ways with aminoacids, which are bifunctional compounds (16). EXPERIMENTAL The aminoacids employed (glycine, a alanine and p alanine) were reagent grade Merck products. Silica alumina was a Houdry-Huels S90 industrial product with a 12.5 % alumina , pore volume 0.77 mIA, mean pore size 70 A and a surface area of about 430 m2/g. For the recording of the i.r. spectra the silica alumina was slightly pressed in a dye to form a thin pellet. This was fixed in a ring which was placed inside a cell specially designed for the sublimation process, as shown in Fig.1. The central part of the cell is made of quartz with an external heating attachment. The pellet was activated by heating it to 700°C under a vacuum of about 10-4 Pa for at least 30 min and then the temperature was lowered to some degrees above the aminoacid sublimation temperature. A little cap containing about 0.5 g of aminoacid was lowered from the top to the heated zone near the activated pellet and allowed to slowly sublimate for increasing periods. The aminoacid container was then raised to the top, the sample was cooled to ambient temperature and the spectra were recorded by means of a FTlR spectrometer (Perkin Elmer 1730) in the range 4000400 cm-1. The general transmittance was low and only the range from 2000 to 1200 cm-1 was practically accessible with a good signal-tonoise ratio. The spectra of pure aminoacids in the solid state, of some of their metal salts and of aminoacid hydrochlorides were also recorded for the purpose of comparison with the data obtained. The aminoacid salts and hydrochlorides were prepared according to the method
43
“4 Fig.1. A section of the cell employed. A: KBr windows,B: quartz tube, C: silica-alumina disc, D: heating coil, E: thermocouple, F:aminoacid container, G vacuum attachment.
reported in the literature (17), the former from a suspension of metal carbonate and aminoacid heated to 60°C and then filtered and dried at llO°C, and the latter from an aqueous solution of the aminoacid titrated with HCI 0.1 N to neutralization and then slowly evaporated until the salt precipitated..
RESULTS AND DISCUSSION The frequencies of the bands recorded on the sample obtained from glycine, a alanine and p alanine sublimated and adsorbed on silica alumina are summarized in Tab.1 while in Tab.11 the frequencies of the bands observed in the spectra of pure aminoacids, their hydrochlorides and salts in the region 2000-1200 cm-1 are reported together with the assignments known from the literature (9,17,18,24). Some of the spectra recorded are shown in Fig.2. A comparison of the data of Tab.11 with the spectra recorded by us suggests the assignments reported in Tab.1.
TABLE I Wavenumbers (cm-1) and assignments of the bands observed in the spectra of glycine (a), a alanine (b) and p alanine (c) adsorbed on activated silica-alumina. (a) 1680 s,b 1620 s,b 1598 m,sh
1528 1473 1441 1427 1392 1329
m w m s m,sh w
(b) 1688m 1642 rn 1616 s 1511w 1473 rn 1450 w 1381 w 1273 w
(c)
attribution V G O
1645 s 1612 1575 sh 1496 m 1482 m 1384 w 1324 w
6 NH v coov coo-
6 NH3+ 6 NH3+ 6 CH v cooV CoowCH
The recorded spectra are decidedly different from those of the pure crystalline aminoacids and from those of their salts and hydrochlorides. Moreover, it can be observed that the adsorption on silica-alumina leads to fairly complex situations where more than one adsorption form of aminoacids can be supposed. Let us examine in detail the three cases considered.
Fig.2. i.r. spectra (1800-1200 cm-1) of glycine (a), a alanine (b) and p alanine adsorbed on activated silica-alumina.
(b)
(a) 1609 1591 1528 1507 1443 1411 1390
s S s sh rn s w
1333 s 1314 w
1623 1593 1530 1519 1454 1410 1385
(c) w s w w rn rn w
1356 s 1308 s
1633 1572 1508 476 448 414 403 390 334 294
s s s w w s
(d) 1751 s 1595 s 1490 s 1442 w 1422 s
(el 1723 s 1620 rn 1499 rn 460 w 408 rn
(f)
(g 1
1610 sh 1592s,b
1619 s,b
1331 m 1319 w
321 rn 299 s
GNH3+/NH2
v coo 6NH3+ I t "
1440 s 1412 s
460 w 415 s
1349 m 1305 rn
384 s 323 s 264 s
s s m rn
attribution
vcco
vCH vC00I
I,
11
"
wCH2 tCH2
TABLE I1 Wavenurnbers (crn-l) and assignments of the bands observed in the spectra of glycine (a), a-alanine (b), p-alanine (c), glycine hydrochloride (d), p-alanine hydrochloride (e), glycinate (f) and P-alaninate (9).
1) G l y c i n e The spectrum of glycine adsorbed on activated silica-alumina shows a close similarity to the crystalline aminoacid rather than to hydrochloride. Bands attributable to deformations of NH3+ groups at about 1620 and 1528 and 1473 cm-1 are at a wavenumber slightly higher than those of the crystalline aminoacid. The bands referrable to CO vibrations are found at 1690, 1600 and 1417 cm-1, the first being attributable to C=O vibrations and the other two to symmetric and antisymmetric vibrations of the COO- groups. If the relative intensities of the bands of the -NH2 and COO- groups vibrations are compared, it can be seen that the latter are more intense than the former. This could indicate that both ends of the molecule are differently perturbed by the si Iica-al u mi na surface. The presence of both COO- and C=O bands supports the hypothesis that two different surface structures are formed, one with the aminoacid in ionic form and the other with neutral carboxylic form. The former could be bound by means of Hydrogen bonding to the surface, the second could be bound by esteric bonding. These data support Hipps and Masur 's hypothesis (10) that glycine adsorbed on alumina presents two adsorption forms of this kind. A band of weak intensity is present at 1443 cm-1. Since it is attributed to the unperturbed CH deformations, it is concluded that these groups do not interact with the surface. 2) a alanine exhibits: i) a large, intense band near 1700 cm-1 assigned to esteric bonds; ii) bands at 1616 and 1381 cm-1 associated with COO- group vibrations and iii) three bands due to NH deformations, as does glycine. The behaviour of a alanine is expected to be analogous to that of glycine, since it exhibits a similar steric configuration and the same relative position of the carboxyl and aminogroup. However, when the intensities of the bands are compared, a difference can be seen which suggests that the proportions of the two hypothetical forms are different in the two cases.. 3) For fl alanine a narrow but multiple band is observed between 1650 and 1600 cm-1; the bands due to the carboxyl group are present with particularly low intensity by comparison with those of NH deformations. The band due to the CeO bond is not observed. It may be supposed that in this case the ionic form of the aminoacid prevails over the esteric form,and that the interaction of the aminoacid with the silica-alumina surface takes place preferentially through the aminogroup.
47
Such a difference in the interaction between an aminoacid and a solid can be understood if it is borne in mind that the simpler aminoacids in the solid phase are zwitterions, but in the gaseous phase are neutral (16) with two groups showing high propensity for strong interaction with Lewis acid centres. This can lead to the formation of rings . Owing to the different distance between the carboxyl and aminogroup, different rings will be formed as a consequence of the interaction with a surface site. The number of atoms in a ring will be the determining factor. As is known from the relative stability of the complexes formed by aminoacids with metals (25,26), five member rings are more stable than six (or more ) member rings. The rings formed by the CO group on the one hand and the coordinated Nitrogen on the other hand allow different levels of stability to be achieved for the various aminoacids. In the cas of glycine and a alanine it is thought that a pentaatomic rin is favoured, as reported in the following scheme:
f,
,H
where the M centre has Lewis acid character
CONCLUSIONS From the spectra of some aminoacids adsorbed on activated silica alumina it has been found at least two surface structures can be hypothezized on the basis of the i.r. spectra, one with ionic character and the other with esteric character. For the three aminoacids considered, the proportions of the two forms is different. In fact, whereas a aminoacids tend to form a five member ring with a surface Lewis site, possessing a relatively higher stability, p aminoacids react in a different way forming a surface species characterized by free carboxyl groups.. The present data are the results of an initial study. We are planning further research into the subject, varying the experimental conditions, the kind of aminoacid employed, and also using other probe molecules, such as the esters of aminoacids, which are capable of interacting by one end. REFERENCES 1 V.Bolis, B.Fubini, G.Venturello, J.Thermal Anal. 30 (1985) 1283. 2 D.J.OConnor and F.Bryant, Nature 170 (1952).84
48
3 H.Seifert, Zeit. fuer Elektrochem. 70 (1966).409 4 R.J.Colton,J.S.Murday, J.R.Wyatt and J.J.deCorpo, Surf.Sci., 84 (1971) 235. 5 A.Hatta, Y.Moriye and W.Suetaka, Bull.Chem.Soc.Japan 48 (1975) 3441. 6 P.R.Kavasmanek and W.A.Bonner, J.Arner.Chern.Soc. 99 (1977) 44. 7 J.A.Groenewegen and W.M.H.Sachtler, J.Catal. 27 (1972) 369. 8 J.A.Groenewegen and W.M.H.Sachtler,ibid. 33 (1973) 176. 9 K.W.Hipps and U.Mazur, Inorg.Chem. 20 (1981) 1391. 10 S.D.Williams and K.W.Hipps, J.Catal. 78 (1982) 96. 11 J.S.Suh and M.Moskovits, J.Arner.Chem.Soc. 108 (1986) 471 1. 12 M.R.Basila and T.R.Kantner, J.Phys.Chem. 71 (1967) 467. 13 K.H.Bourne, FRCannings and R.C.Pitkethly, ibid. 74 (1970) 2197. 14 J.B.Peri, J.Catal. 41 (1976) 227. 15 S.M. Riseman,F.E.Massoth,G.M.Dhar and E.M.Eyring, J.Phys.Chern. 86 (1982) 1760. 16 G.S.Denisov, I.G.Rumynskaya and V.M.Shraiber, Zh.Prikl.Spektr., 31 (1979) 275. 17 Grnelins Handbuch der Anorg.Chem.,8 Auf., Band 12, Verlag ,Berlin.l958. 18 R.K.Khanna, M.Horak and E.R.Lippincott, Spectrochirn.Acta 22 (1966) 1759. 19 R.S.Krishnan and R.S.Katiyar, Bull.Chem.Soc.Japan 42 (1969) 2098. 20 C.H.Wang and R.D.Storm, J.Chem.Phys. 55 (1971) 3291. 21 J.F.Pearson and M.A.Slifkin, Spectrochim.Acta 28 (1972) 2403. 22 Y.Grenie and C.Garrigou-Lagrange, J.Mol.Spectrosc. 41 (1972) 240. 23 R.F.Adarnowicz and M.L.Sage, Spectrochim.Acta 30 (1974) 1007. 24 C.Garrigou-Lagrange, Canad.J.Chern. 56 (1978) 663. 25 H.lrving,R.J.P.Williams,D.J.Ferrett, A.E.Williams, J.Chern.Soc. (1954) 3494. 26 E.Bottari, Ann.Chim. (Rome) 60 (1976) 193.
C. Morterra, A. Zecchina and G. Costa (Editors 1, Structure and Reactivity of Surfaces 01989 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
49
ADSORPTION AND DISSOCIATION OF CARBON MONOXIDE ON Co(1120)
U. B a r d i and G. Rovida D i p a r t i m e n t o d i Chimica, U n i v e r s i t d d i F i r e n z e , V i a G. Capponi 9, 50121 F i r e n z e ( I t a l y )
ABSTRACT The a d s o r p t i o n and d i s s o c i a t i o n o f carbon monoxide on t h e (1170) f a c e o f a c o b a l t s i n g l e c r y s t a l were s t u d i e d i n t h e range o f temperatures f r o m 300 t o 470 -5 t o r r . U l t r a v i o l e t photoelectron K a t pressures equal t o o r lower t h a n 10 spectroscopy, Auger e l e c t r o n spectroscopy and t h e r m a l d e s o r p t i o n spectroscopy were used t o c h a r a c t e r i z e t h e adsorbed species. The k i n e t i c s o f t h e d i s s o c i a t i o n r e a c t i o n were measured m a i n l y on t h e b a s i s o f t h e v a r i a t i o n o f t h e work f u n c t i o n . As found i n a p r e v i o u s work on p o l y c r y s t a l l i n e m a t e r i a l , CO was found t o d i s s o c i a t e t o carbon and oxygen atoms. The a c t i v a t i o n energy f o r i t s d i s s o c i a t i o n was found t o be 23k3 k c a l / m o l e . The C o ( l l ? O ) s u r f a c e appears t o be more r e a c t i v e t h a n o t h e r low i n d e x c o b a l t f a c e s , b u t i t i s l e s s r e a c t i v e t h a n t h e p o l y c r y s t a l l i n e m a t e r i a l . Our r e s u l t s c o n f i r m t h a t t h e CO d i s s o c i a t i o n r e a c t i o n i s s e n s i t i v e t o t h e s t r u c t u r e o f t h e surface.
INTRODUCTION I n t h e l a s t years, a number o f d e t a i l e d s t u d i e s on t h e a d s o r p t i o n o f CO on c o b a l t surfaces have been p u b l i s h e d ( r e f s . 1-81.The i n t e r e s t i n t h e p r o p e r t i e s o f c o b a l t i n t h i s r e s p e c t i s r e l a t e d t o i s c a t a l y t i c p r o p e r t i e s i n t h e CO h y d r o g e n a t i o n t o hydrocarbons, f o r which
t was found t o be t h e most a c t i v e
metal ( r e f . 9 ) . C o b a l t i s expected t o have i n t e r m e d i a t e p r o p e r t i e s between t h o s e o f Fe, which i s known t o d i s s o c i a t e CO a t room t e m p e r a t u r e ( r e f s 10-11) and t h o s e o f N i , where t h e d i s s o c i a t i o n of CO i s known t o occur above 500 K ( r e f . 1 2 ) . However, t h e mechanism o f t h e d i s s o c i a t i o n o f CO on c o b a l t has n o t been s t u d i e d i n depth by means of s p e c i f i c s u r f a c e t e c h n i q u e s . Up t o now, t h e -5 s t u d i e s on s i n g l e c r y s t a l s u r f a c e s have shown t h a t a t pressures below 10 torrCO i s m o l e c u l a r l y adsorbed on t h e (0001) s u r f a c e ( r e f s . 1,2) and on t h e (1010) s u r f a c e ( r e f . 3 ) a t temperatures up t o 450 K . Some d i s s o c i a t i o n has been r e p o r t e d f o r t h e (1072) f a c e ( r e f . 4) as w e l l as f o r p o l y c r y s t a i l i n e m d t e r i a l ( r e f s . 1,131, b u t no d a t a on t h e k i n e t i c s were r e p o r t e d . D i s s o c i a t i o n o f CO on
50
t h e (1170) face has been r e p o r t e d ( r e f . 5 ) w i t h an estimated a c t i v a t i o n energy o f about 20 kcal/mole. Recently, Nakamura e t a l . ( r e f . 14) r e p o r t e d t h e dissociation o f
CO t o c a r b i d i c carbon and atomic oxygen on p o l y c r y s t a l l i n e
c o b a l t . These r e s u l t s are i n q u a l i t a t i v e agreement w i t h our previous r e s u l t s ( r e f . 6 ) , however, a l s o i n t h i s case, d e t a i l e d k i n e t i c d a t a f o r t h e d i s s o c i a t i o n r e a c t i o n were n o t r e p o r t e d . The above d i s c u s s i o n 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 CO on c o b a l t may be s e n s i t i v e t o t h e surface s t r u c t u r e . The present work was t h e r e f o r e undertaken w i t h t h e purpose o f v e r i f y i n g t h i s i n d i c a t i o n and i n general i n order t o o b t a i n q u a n t i t a t i v e d a t a on t h e d i s s o c i a t i o n r e a c t i o n on a c o b a l t s i n g l e c r y s t a l f a c e o f known s t r u c t u r e . The (1170) f a c e was chosen, since t h e r e s u l t s r e p o r t e d i n r e f . 5 c l e a r l y i n d i c a t e d CO d i s s o c i a t i o n on t h i s surface. This f a c e i s a l s o i n t e r e s t i n g because o f i t s unique s t r u c t u r e , c o n s i s t i n g o f troughs formed o f zig-zag chains o f metal atoms. The s t r u c t u r e expected from t r u n c a t i o n o f t h e b u l k s t r u c t u r e , has been confirmed by a LEED analysis ( r e f . 1 5 ) . We chose t o i n v e s t i g a t e t h e k i n e t i c s o f CO d i s s o c ; a t i o n m a i n l y by m o n i t o r i n g t h e v a r i a t i o n s o f t h e work f u n c t i o n . This technique has t h e advantage over o t h e r surface techniques o f being f a s t and
;ion
d e s t r u c t i v e , as described i n
d e t a i l i n r e f . 6. Due t o t h e use o f t h i s technique, our study was l i m i t e d t o -5 CO pressures below 10 torr.
EXPERIMENTAL A s discussed i n d e t a i l i n r e f . 6, t h e measurements were performed i n a
standard s t a i n l e s s s t e e l vacuum system capable o f base pressure i n t h e 10
-1 0
t o r r range. The system was equipped w i t h a t h r e e g r i d o p t i c s f o r LEED and AES. For t h e l a t t e r technique, we used a g l a n c i n g e l e c t r o n gun w i t h a primary e l e c t r o n energy of 2.5 keV. V a r i a t i o n s o f t h e work f u n c t i o n c o u l d be measured w i t h i n 0.01 eV by means o f t h e beam stop method. We found t h a t t h e low energy e l e c t r o n beam ( l e s s than 1 eV) used f o r these measurements causes no decomposition o f t h e adsorbed CO. Thermal d e s o r p t i o n s p e c t r a were obtained by means o f a quadrupole mass spectrometer. UPS spectra were obtained f o r t h e same sample i n a separate system equipped w i t ; , a s i n g l e pass CMA and a He I discharge lamp. The sample was a c o b a l t s i n g l e c r y s t a l c u t and p o l i s h e d along t h e (1170) plane w i t h i n 1". The c l e a n i n g procedure a f t e r i n t r o d u c t i o n i n t h e vacuum chamber c o n s i s t e d i n several c y c l e s of argon i o n bombardment f o l l o w e d by annealing a t
51
temperatures l o w e r t h a n t h e h c p - f c c t r a n s i t i o n p o i n t (around 700 K ) . A f t e r such a t r e a t m e n t , t h e sample showed t h e expected LEE0 p a t t e r n ( r e f . 1 5 ) . No oxygen c o u l d be d e t e c t e d on t h e s u r f a c e and t h e amount o f r e s i d u a l carbon was f o u n d by AES n o t t o exceed 5% o f a monolayer. The nominal p u r i t y o f CO was 99.995.
In
p a r t i c u l a r , we checked t h a t t h e oxygen c o n t e n t i n t h e gas i n t r o d u c e d i n o u r apparatus was lower t h a n 10 ppm ( a p r a c t i c a l l i m i t o f o u r mass s p e c t r o m e t e r ) .
RESULTS AND DISCUSSION A d s o r p t i o n o f oxygen and carbon I n a p r e l i m i n a r y i n v e s t i g a t i o n , t h e s e p a r a t e k i n e t i c s o f a d s o r p t i o n o f oxygen and carbon on t h e metal s u r f a c e were s t u d i e d i n o r d e r t o a s c e r t a i n : i )t h e d i r e c t p r o p o r t i o n a l i t y between t h e v a r i a t i o n o f t h e work f u n c t i o n and t h e s u r f a c e coverage, as determined by AES; i i ) t h e maximum amount o f t h e two atomic species chemisorbed a t s a t u r a t i o n ; iii t h e s u r f a c e s t r u c t u r e s observed by LEED, w i t h t h e aim t o o b t a i n a c a l i b r a t i o n o f t h e s u r f a c e coverages. The e x p e r i m e n t a l r e s u l t s o f t h i s i n v e s t i g a t i o n a r e i n p a r t r e p o r t e d elsewhere ( r e f s . 16-18). It was f o u n d t h a t oxygen i s chemisorbed a t room temperature w i t h o u t a f f e c t i n g t h e 1x1 LEE0 p a t t e r n o f t h e c l e a n s u r f a c e . AES shows t h a t t h e maximum amount o f chemisorbed oxygen ( b e f o r e t h e o x i d e begins t o f o r m ) i s about t h e same as measured f o r t h e p o l y c r i s t a l l i n e f o i l , c o r r e s p o n d i n g t o a r a t i o between t h e oxygen peak and t h e 650 eV c o b a l t peak o f about 0.6. A ( 3 x 1 ) LEED s u p e r s t r u c t u r e i s observed a f t e r h e a t i n g t h e s u r f a c e s a t u r a t e d w i t h chemisorbed oxygen a t 470 K. T h i s p a t t e r n can be t e n t a t i v e l y i n t e r p r e t e d as due t o a r e o r d e r i n g o f t h e oxygen atoms adsorbed i n h o l l o w s i t e s o f t h e z i g - z a g t r o u g h s and t h e f o r m a t i o n o f a n t i p h a s e domains. The model proposed i n r e f . 16 2 corresponds t o a coverage o f 7 . 1 ~ 1 0at/cm ~~ i n agreement w i t h t h e AES, measurements. D u r i n g t h e c h e m i s o r p t i o n process, t h e w . f .
increases l i n e a r l y w i t h
t h e oxygen AES s i g n a l , t h e n i t decreases when t h e o x i d e begins t o form. The f o r m a t i o n o f t h e o x i d e can be r e t a r d e d i f t h e sample t e m p e r a t u r e i s k e p t a t 470 K. A t t h i s t e m p e r a t u r e t h e ( 3 x 1 ) s t r u c t u r e i s formed d i r e c t l y . The maximum
increase o f t h e w.f.
t h a t can be a t t r i b u t e d t o t h e s a t u r a t i o n o f t h e
c h e m i s o r p t i o n process i s a p p r o x i m a t e l y 0.40 eV. T h i s v a l u e i s s i g n i f i c a n t l y l o w e r t h a n t h a t observed f o r t h e p o l y c r y s t a l l i n e m a t e r i a l (0.50 eV), and can be t e n t a t i v e l y a t t r i b u t e d t o t h e a d s o r p t i o n o f oxygen i n deeper s i t e s on t h e (1120) face, i n agreement w i t h t h e model d e s c r i b e d b e f o r e .
The adsorption o f carbon atoms was obtained by decompositi.on o f ethylene a t 470
I(.
t h i s temperature carbon i s adsorbed as a surface carbide (ref.18)
A:
forming a well ordered (2x51 LEE0 superstructure. For t h i s structure, a tentative model, proposed on the basis o f the structure o f compact planes o f ~ bulk cobalt carbide, would correspond t o a coverage o f 6 . 8 ~ 0 ' carbon n
atoms/cm
L
. The
maximum amount o f C adsorbed corresponds t o a C/Co(650) AES r a t i o
o f 1.4 and t o a w . f .
increase of about 0.35 eV, both values being very close t o
those observed on p o l y c r y s t a l l i n e cobalt. Also f o r C the w.f.
varies l i n e a r l y
with the coverage. Our r e s u l t s concerning C chemisorption are i n q u a l i t a t i v e agreement w i t h those of Nakamura e t a l . ( r e f . 141, where carbidic carbon i s reported t o form i n the temperature range examined i n t h e present work, while graphite i s formed a t higher temperatures.
Adsorption and decomposition o f carbon monoxide The w.f.
variations accompanying CO adsorption a t room temperature are i n
agreement with the r e s u l t s reported by Papp (ref. 5). On heating t h e adsorbed CO, a desorption maximum i s observed a t about 420 K w i t h the mass spectrometer.
I f the heating i s stopped immediately a f t e r t h i s desorption peak, the i n i t i a l
w.f.
o f the clean surface i s not restored and some residual C and 0 can be
detected by AES on the surface. The O/C atomic r a t i o , as determined w i t h AES, appears t o be roughly unity. I n agreement w i t h Papp, UPS spectra showed t h a t CO adsorbed a t room temperature i s i n molecular form, while, a f t e r heating, the detection i n UPS o f a broad maximum around 4-5 eV indicates the presence o f C and 0 atoms separately bonded t o t h e metal. These r e s u l t s i n d i c a t e t h a t some decomposition has occurred during the desorption process. On heating above 550 K, a l l oxygen and almost a l l carbon are r m v e d as CO, as detected w i t h the mass
spectrometer. A desorption maximnun around 600 K i s always observed by r a p i d l y heating the sample when 0 and C atoms are present on t h e surface. The amount o f residual C varies depending on t h e thermal history, t h e surface preparation, etc., but does n o t exceed 10%o f t h e saturation amount. I t s presence can be a t t r i b u t e d e i t h e r t o a p a r t i a l dissolution of oxygen atoms i n t h e bulk during the heating, or t o d i f f u s i o n o f some C from t h e bulk t o t h e surface. The l a t t e r process i s i n f a c t hard t o eliminate completely also f o r t h e "clean" surface. I n r e f . 14 the dissociation of CO t o C+O on p o l y c r y s t a l l i n e cobalt i s reported t o leave an excess o f C. I n t h a t study, i t was determined by XPS t h a t on heating
53
t h e adsorbed CO t h e r e i s a s i g n i f i c a n t decrease o f t h e oxygen coverage, t h a t can be a t t r i b u t e d t o oxygen p e n e t r a t i o n i n t o t h e b u l k . Evidence f o r CO d i s s o c i a t i o n can be f o u n d by exposing t h e sample a t temperatures as low as 350 K. Extended exposures t o CO a t h i g h p r e s s u r e s above t h a t t e m p e r a t u r e always l e f t t h e s u r f a c e covered by b o t h oxygen and carbon. The amount o f C and 0 reaches a maximum v a l u e showing t h a t t h e s u r f a c e becomes u n r e a c t i v e f o r f u r t h e r CO d i s s o c i a t i o n . A f t e r t h e s a t u r a t i o n of t h e d i s s o c i a t i o n process, each atomic s p e c i e s approaches about h a l f o f t h e coverage reached when adsorbed alone. A t t h e h i g h e s t temperatures s t u d i e d , a s l i g h t p r e v a l e n c e of carbon i s observed (5-10%), s i m i l a r l y t o what observed a f t e r t h e r m a l d e s o r p t i o n o f CO. A f t e r a d s o r p t i o n o f CO a t room temperature, no e x t r a beams were observed i n t h e LEED p a t t e r n . Only a f t e r h e a t i n g t h e sample above 370 K i n t h e presence o f CO we observed a p o o r l y o r d e r e d ( 3 x 1 ) s t r u c t u r e s i m i l a r t o t h a t observed w i t h oxygen alone, w h i l e AES i n d i c a t e s t h e presence o f b o t h C and 0 on t h e s u r f a c e . Papp ( r e f . 5 ) found no CO o r d e r i n g a t 100 K, b u t r e p o r t e d t h e f o r m a t i o n o f a ( 3 x 1 ) s u p e r s t r u c t u r e above 300 K, t h a t was a t t r i b u t e d t o t h e o r d e r i n g o f t h e adsorbed CO molecules. I f t h e s t r u c t u r e observed i n o u r case i s t h e same, o u r r e s u l t s seem t o i n d i c a t e t h a t t h e ( 3 x 1 ) s t r u c t u r e observed by Papp may be a t t r i b u t e d t o d i s s o c i a t i o n o f adsorbed CO under t h e e l e c t r o n beam. The k i n e t i c s o f t h e CO d i s s o c i a t i o n were m o n i t o r e d by means o f t h e w . f . v a r i a t i o n ( f i g . 1 and 21, s i n c e t h e e l e c t r o n beam used f o r AES was f o u n d t o cause a r a p i d d i s s o c i a t i o n of adsorbed CO and s u r f a c e accumulation o f carbon.
0.4 A@ (eVI
0.2
20
40
60
TIME (rnin.)
F i g . 1 and 2. A d s o r p t i o n k i n e t i c s o f CO on t h e c o b a l t (1120) f a c e m o n i t o r e d by t h e v a r i a t i o n o f t h e work f u n c t i o n .
54 As r e p o r t e d i n t h e previous study on p o l y c r y s t a l l i n e c o b a l t ( r e f . 61, t h e d i ssoci a t i on a t a given temperature and pressure was monitored by measuring t h e w.f.
a f t e r successive exposures t o CO, removing gaseous CO d u r i n g t h e w . f .
measurements. I n t h e temperature range o f 370-470
K, each t i m e t h e CO i s removed
from t h e chamber, t h e undissociated molecules a r e desorbed, thus t h e w . f . v a r i a t i o n i s a t t r i b u t a b l e o n l y t o C and 0 atoms, since t h e l a t t e r recombine o n l y a t h i g h e r temperatures. The main d i f f e r e n c e s between t h e r e s u l t s o f f i g . 1 and 2 and t h e r e s u l t s obtained on p o l y c r y s t a l l i n e c o b a l t are: i ) t h e r e a c t i o n r a t e s a t t h e same pressure and temperature are lower f o r t h e s i n g l e c r y s t a l , thus i n d i c a t i n g a lower a c t i v i t y f o r CO d i s s o c i a t i o n i n comparison t o t h e p o l y c r y s t a l l i n e surface; i i ) t h e maximum increase o f t h e w . f
.,
corresponding t o
corresponding t o t h e s a t u r a t i o n o f t h e surface w i t h C and 0 atoms, i s about 0.38 eV, as compared t o 0.45 f o r t h e p o l y c r y s t a l l i n e surface. Assuming t h a t t h e w.f.
v a r i a t i o n s caused by t h e two atomic species are a d d i t i v e , t h i s lower value
i s i n good agreement w i t h t h e lower w.f.
increase found f o r 0 chemisorption on
t h e (1170) face. I n order t o o b t a i n k i n e t i c parameters, t h e experimental curves o f f i g s , 1 and
2 were analyzed by means o f a k i n e t i c equation o f t h e same t y p e as t h a t used i n t h e case of p o l y c r y s t a l l i n e m a t e r i a l . The equation was d e r i v e d on t h e basis o f t h e f o l l o w i n g assumptions (see r e f . 6 f o r a d i s c u s s i o n o f t h e i r v a l i d i t y ) : i ) the w . f .
v a r i a t i o n i s p r o p o r t i o n a l t o t h e sum o f t h e f r a c t i o n a l coverages o f C
and 0 atoms, which are assumed t o be equal; i i ) t h e r a t e o f t h e d i s s o c i a t i o n r e a c t i o n i s p r o p o r t i o n a l t o t h e s u r f a c e c o n c e n t r a t i o n o f adsorbed CO molecules; i i i ) t h e adsorption e q u i l i b r i u m o f CO i s e s t a b l i s h e d much f a s t e r than i t s d i s s o c i a t i o n ; i v ) CO molecules can be adsorbed o n l y on t h e s i t e s n o t occupied by C and 0 atoms, b u t are h i g h l y mobile on t h e surface i n t h e temperature range o f
i n t e r e s t ; v ) CO molecules can d i s s o c i a t e o n l y i f they f i n d two s u i t a b l e s i t e s not occupied by C o r 0 atoms. I n order t o d e s c r i b e t h e a d s o r p t i o n e q u i l i b r i u m o f CO, we used t h e d a t a of Papp ( r e f . 8 ) . I n s t e a d o f a Langmuir isotherm, we found
t h a t h i s r e s u l t s can be b e t t e r described w i t h a Temkin i s o t h e r m
I n p = - I n b + qoaWRT + I n ( W ( 1 - 8 ) )
(1)
where 9 i s t h e f r a c t i o n a l coverage o f CO; b=bo exp(qo/RT) and qo and a are defined by t h e assumed l i n e a r v a r i a t i o n o f t h e
heat o f adsortpion, t h a t i s
55
q = q o ( l - a 9 ) . The v a l u e s o f t h e parameters d e r i v e d f r o m Papp's d a t a a r e : qo=35 -13 W h i l e qo i s t h e same as f o u n d f o r t h e k c a l / m o l ; a.0.2; bo=ZxlO
.
p o l y c r y s t a l l i n e m a t e r i a l , a and bo a r e d i f f e r e n t ; i n p a r t i c u l a r , a i s much s m a l l e r f o r t h e s i n g l e c r y s t a l (1120) surface,-as expected f o r a more homogeneous s u r f a c e . The e q u a t i o n d e s c r i b i n g t h e d i s s o c i a t i o n k i n e t i c s i s
dO'/dt = K f(p,T)
(l-O')n
(2)
where 8' i s t h e f r a c t i o n o f s u r f a c e s i t e s occupied by b o t h C and 0 atoms; f ( p , T ) i s t h e v a l u e o f 0 d e r i v e d from eq.1 and i s a c o n s t a n t f o r each experiment: i t r e p r e s e n t s t h e i n i t i a l coverage of CO molecules i n e q u i l i b r i u m w i t h t h e gas, t h e a c t u a l coverage b e i n g f ( p , T )
( 1 4 ' ) . The exponent n was approximated t o 3 i n t h e
p r e v i o u s study, b u t f o r t h e s i n g l e c r y s t a l f a c e we f o u n d t h a t a v a l u e o f 2 a l l o w s a f i t t i n g o f t h e e x p e r i m e n t a l c u r v e s f o r a l a r g e r coverage range. T h i s v a l u e i m p l i e s t h a t t h e p r o b a b i l i t y f o r an adsorbed CO molecule t o f i n d a s i t e
\
\
10-5
A A
4
o
Torr Tor r Torr
2 Y
c
0
-2 I
2
I
2.5 I/TXIO~(K-')
F i g . 3. Rate c o n s t a n t f o r CO d i s s o c i a x i o n on t h e Co(1120) f a c e ( f u l l symbols, broken l i n e ) and on p o l y c r i s t a l l i n e c o b a l t ( r e f . 1 ) (open symbols, f u l l l i n e ) .
56
s u i t a b l e f o r d i s s o c i a t i o n i s ( i - 9 ' ) . K can be c o n s i d e r e d t o be a measure o f t h e r a t e c o n s t a n t o f t h e d i s s o c i a t i o n process and t o v a r y w i t h T as K=Koexp(-Ea/RT), where Ea i s t h e a c t i v a t i o n energy f o r t h e d i s s o c i a t i o n process. The v a l u e s o f K derived from our experimental curves are reported i n t h e Arrhenius p l o t o f F i g . 3. From t h e s e values, t h e a c t i v a t i o n energy can be c a l c u l a t e d as Ea=23*3 k c a l / m o l . T h i s r e s u l t i s t o be compared t o Ea=17kZ f o u n d f o r p o l y c r y s t a l l i n e c o b a l t . Thus, o u r r e s u l t s show t h a t t h e ( 1 1 7 0 ) f a c e i s more a c t i v e f o r CO d i s s o c i a t i o n t h a n t h e (0001) o r t h e ( l O T 0 ) f a c e s o f c o b a l t , w h i l e i t i s l e s s a c t i v e t h a n t h e s u r f a c e o f p o l y c r y s t a l l i n e m a t e r i a l . The v a l u e o f t h e a c t i v a t i o n energy f o r t h e CO d i s s o c i a t i o n on t h e ( 1 1 2 0 ) f a c e ( 2 3 K c a l / m o l e ) can be compared t o t h e h e a t o f a d s o r p t i o n f o r t h e same s u r f a c e ( 3 4 k c a l / m o l ) ( r e f . 51, t h e (0001) ( 3 1 K c a l / n i o l e ) ( r e f . 2) and t h e ( l O i 0 ) ( 3 4 k c a l / m o l e ) ( r e f . 3 ) . These v a l u e s i n d i c a t e t h a t f o r a CO m o l e c u l e adsorbed on c o b a l t , t h e d i s s o c i a t i o n and d e s o r p t i o n paths may be c o m p e t i t i v e . Thus, r e l a t i v e l y s m a l l v a r i a t i o n s o f t h e two e n e r g i e s may e x p l a i n t h e observed s t r u c t u r e s e n s i t i v i t y f o r CO d i s s o c i a t i o n
on t h i s m e t a l . The h i g h e r a c t i v i t y o f f o r CO d i s s o c i a t i o n o f t h e ( 1 1 7 0 ) f a c e i n comparison t o t h e o t h e r low i n d e x f a c e s can be a t t r i b u t e d e i t h e r t o i t s p a r t i c u l a r s t r u c t u r e , o r t o t h e f a c t t h a t t h e degree o f o r d e r i n g a t t a i n e d f o r t h i s f a c e i s lower t h a n f o r t h e o t h e r f a c e s , due t o t h e r a t h e r l o w l i m i t f o r t h e a n n e a l i n g t e m p e r a t u r e o f c o b a l t . I n t h e l a t t e r case, t h e h i g h e r r e a c t i v i t y should be r e l a t e d t o t h e presence of s u r f a c e d e f e c t s , such as steps, k i n k s , e t c . , which c o u l d a l s o e x p l a i n t h e h i g h e r r e a c t i v i t y o f t h e p o l y c r y s t a l l i n e s u r f a c e . On t h e o t h e r hand, t h e a n a l y s i s o f t h e d i s s o c i a t i o n k i n e t i c s i n d i c a t e s a h i g h e r degree o f u n i f o r m i t y o f t h e a c t i v e s i t e s on t h e ( 1 1 7 0 ) sample i n comparison w i t h t h e p o l y c r y s t a l l i n e f o i l . A l s o t h e good q u a l i t y o f t h e LEED p a t t e r n observed on o u r s i n g l e c r y s t a l sample i n d i c a t e s t h a t t h e s u r f a c e i s m o s t l y formed o f w e l l o r d e r e d domains. Thus, i t would be d i f f i c u l t t o e x p l a i n t h e k i n e t i c s observed on t h e (1120) f a c e w i t h t h e assumption t h a t CO d i s s o c i a t e s o n l y on a minor f r a c t i o n o f s u r f a c e c o n s i s t i n g o f s t e p s o r o t h e r s u r f a c e d e f e c t s . However, i n t h e absence o f e x p e r i m e n t a l e v i d e n c e about t h e a d s o r p t i o n geometry o f CO on t h e d i f f e r e n t c o b a l t faces,
it i s not possible t o explain with
c e r t a i n t y t h e h i g h e r r e a c t i v i t y o f t h e (1120) f a c e on t h e b a s i s o f i t s p a r t i c u l a r s t r u c t u r e . I n conclusion, our r e s u l t s c o n f i r m t h a t t h e d i s s o c i a t i o n r e a c t i o n o f CO on c o b a l t i s r e m a r k a b l y s t r u c t u r e s e n s i t i v e and show t h a t t h e (1170) f a c e i s s i g n i f i c a n t l y more r e a c t i v e .than o t h e r l o w i n d e x c o b a l t f a c e s .
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
M.E. Bridge, C.M. Comrie and R.M. Lambert, Surface Sci. 67 (1977) 393. H. Papp, Surface Sci. 129 (1983) 205. H. Papp, Ber. Bunsenges. Phys. Chem. 86 (1982) 555. K.A. P r i o r , K. Schwaha and R.M. Lambert, Surface Sci 67 (1978) 393. H. Papp Surface Sci 149 (1985) 460. U.Bardi, P. T i s c i o n e and G. Rovida, Appl. Surface S c i . 27 (1986) 299. M.E. Bridge and R.M. Lambert, Surface Sci. 82 (1979) 413. H.Freund and G. Hohlneicher, Ber. Bunsenges. Phys. Chem. 83 (1979) 100. M. A. Vannice, J . Catal. 50 (1977) 228. T.W. Haas, J.T. Grant and G. Dooley, J . Vac. Sci. Technol. 7 (1970) 43. K.Y. Yu, W.E. Spicer, I . Landau, P. P i a n e t t a and S.F. L i n , Surface S c i . 5 7 (1979) 157. R. Rosei, F. Ciccacci, R. Memeo, C. M a r i a n i , L.S. Caputi and L. Papagno, J . Catal. 83 (1983) 19. R.B. Moyes and M.W. Roberts, J . Catal. 49 (1977) 216. J . Nakamura, I . Toyoshima and K . Tanaka, Surface Sci. 201 (1988) 185. M. Welz, W. M o r i t z and D. Wolf, Surface Sci. 125 (1983) 473. U.Bardi, M.Mauro, M.Maglietta and G.Rovida, Proceedingsof t h e 9 t h National Congress o f t h e I t a l i a n Vacuum Society, F i r e n z e 1986. A . A t r e i , U.Bardi, G.Rovida, M.Torrini, E.Zanazzi and M.Maglietta, Surface Sci. 189 (1987) 459. A.Atrei, U.Bardi, G.Rovida, M . T o r r i n i , E.Zanazzi and M.Maglietta, J . Vac. Sci. Technol. A5 (1987) 1006.
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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
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LCW PRESSURE O X I D A T I O N OF THE P t 3 T i ALLOY S T U D I E D BY ELECTRON SPECTROSCOPY
2
U. B A R D I l , P . N . ROSS and G. R O V I D A
1
Dipartimento d i Chirnica, Universita d i Firenze, 50121 Firenze, ( I t a l y ) Materials and Molecular Research D'ivision, Lawrence Berkeley Laboratory, Berkeley Ca 94720, (USA)
ABSTRACT
-8 The o x i d a t i o n o f t h e P t T i a l l o y i n a range o f oxygen pressures from 1x10 3 t o l x l f 4 t o r r was studied by XPS and AES. A f t e r oxidation, we observed a t l e a s t two d i s t i n c t oxidized t i t a n i u m states, evidenced by d i f f e r e n t binding energies o f the T i 2p XPS peaks and by a d i f f e r e n t lineshape o f the T i LMN Auger t r a n s i t i o n s .
The comparison o f t h e r e l a t i v e i n t e n s i t i e s o f t h e 0 1s and
T i 2p peaks t o those o f a T i 0
2
reference sample permitted t o determine t h e O / T i
atomic r a t i o i n t h e oxidized P t T i surface. The oxide which forms upon 3 o x i d a t i o n a t pressures lower than approximately l ~ l O -t o~r r has a composition close t o T i O .
The product o f o x i d a t i o n a t higher pressure i s T i 0
2'
INTRODUCTION
The i n t e r a c t i o n o f oxygen w i t h t h e surface o f a binary a;loy
a t high
temperature u s u a l l y leads t o t h e segregation on t h e surface o f a l a y e r o f oxide o f the most r e a c t i v e component ( r e f s . 1-6).
I n t h e i n i t i a l s+ages o f t h i s
process, t h e s t r u c t u r e and composition o f t h e oxide are important f a c t o r s i n determining t h e chemisorptive and c a t a l y t i c p r o p e r t i e s o f t h e surface.
I n the
case o f P t T i , t h e properties o f a p a r t i a l l y oxidized surface are o f d i r e c t
3
i n t e r e s t i n reason o f t h e c a t a l y t i c properties o f t h i s a l l o y i n o x i d i z i n g environment ( r e f . 7 ) .
Moreover, such a surface may have s t r u c t u r e and
properties s i m i l a r t o those o f t h e platinum/titanium d i o x i d e system, where t h e Strong Metal t o Support I n t e r a c t i o n (SMSI) e f f e c t was observed f o r t h e f i r s t time ( r e f . 8 ) . I n order t o understand t h e properties o f t h e oxidized P t T i surface, i t i s
3
60
obviously important t o determine t h e stoichiometry o f t h e t i t a n i u m oxide f i l m . I n r e f . 1, t h e o x i d e f o r m i n g i n t h e i n i t i a l stages o f o x i d a t i o n was i d e n t i f i e d as " T i O " on t h e b a s i s o f LEED and AES d a t a .
However, XPS can be used t o v e r i f y
t h e v a l i d i t y o f t h i s i n t e r p r e t a t i o n , a l s o p e r m i t t i n g a d i r e c t comparison w i t h t h e r e s u l t s f o r t h e o x i d a t i o n o f t h i n T i f i l m s on p u r e p l a t i n u m ( r e f s . 9,101. Although some XPS s p e c t r a f o r t h e o x i d i z e d P t T i ( l l 1 ) s u r f a c e have been r e p o r t e d 3 ( r e f . 11 1, so f a r no q u a n t i t a t i v e XPS s t u d y has been performed. We have t h e r e f o r e examined by XPS t h e o x i d a t i o n o f P t T i s u r f a c e s o f d i f f e r e n t
3
crystallographic orientation.
T h i s s t u d y showed t h a t t h e c o m p o s i t i o n o f t h e
o x i d e f i l m i s independent o f t h e s u r f a c e o r i e n t a t i o n , i n agreement w i t h t h e r e s u l t s of t h e LEED/AES s t u d i e s o f t h e same s u r f a c e s ( r e f s . 1 - 2 ) .
Therefore, i n
t h e p r e s e n t work we w i l l show o n l y r e s u l t s r e l a t i v e t o one o r i e n t a t i o n , t h e (5101, where t h e most d e t a i l e d s t u d y was performed.
EXPERIMENTAL P t T i s i n g l e c r y s t a l samples were prepared as d e s c r i b e d i n r e f . 1 .
3 w i t h a s u r f a c e c u t and p o l i s h e d a l o n g t h e ( 5 1 0 ) p l a n e was mounted on a m a n i p u l a t o r which p e r m i t t e d h e a t i n g up t o a p p r o x i m a t e l y 1200 K.
A sample
Temperatures
The s t u d y -10 torr was performed i n a vacuum system c a p a b l e o f base p r e s s u r e i n t h e l o w 10
were measured by a tnermocouple spotwelded on t h e back o f t h e sample.
range, equipped w i t h a c o n v e n t i o n a l Mg X-ray source and a s p h e r i c a l e l e c t r o n analyser w i t h a multichannel d e t e c t o r .
XPS s p e c t r a were r e c o r d e d a t a
t r a n s m i s s i o n energy of 44 eV and a t an e l e c t r o n c o l l e c t i o n a n g l e c l o s e t o t h e s u r f a c e normal.
The same a n a l y s e r was used t o examine a p u r e T i 0 ( 1 1 0 ) ( r u t i l e )
2
P t T i samples 3 o r i e n t e d a l o n g t h e ( 1 1 1 ) and ( 1 0 0 ) planes were examined i n a d i f f e r e n t vacuum s i n g l e c r y s t a l sample and a t i t a n i u m p o l y c r y s t a l l i n e f o i l .
system equipped w i t h t h e same t y p e o f XPS a n a l y z e r . The a l l o y s u r f a c e s were c l e a n e d by a t r e a t m e n t c o n s i s t i n g i n c y c l e s o f i o n O x i d a t i o n was performed -4 i n t r o d u c i n g oxygen d i r e c t l y i n t h e a n a l y s i s chamber up t o about 1x10 torr
bombardment and a n n e a l i n g as d e s c r i b e d i n r e f . 1 .
pressure.
The oxygen was pumped o u t b e f o r e r e c o r d i n g XPS s p e c t r a .
The
T i 0 ( 1 1 0 ) sample was c l e a n e d by a procedure c o n s i s t i n g o f Ar'ion bombardment and 2 a r w e a l i n g a t 800 K . The i o n bombardment cauked a d e p l e t i o n i n oxygen o f t h e s u r f a c e , however, from t h e e x a m i n a t i o n o f t h e T i 2p r e g i o n , we f o u n d t h a t a n n e a l i n g i n vacuum c o u l d r e s t o r e a s t o i c h i o m e t r i c s u r f a c e ( r e f s . 12-13).
61
RESULTS AND DISCUSSION The f o r m a t i o n o f t i t a n i u m o x i d e on t h e P t T i s u r f a c e a f t e r exposure t o oxygen
3
was d e t e c t a b l e i n XPS f r o m t h e growth o f t h e 0 1s peak and t h e i n c r e a s e o f t h e i n t e n s i t y o f a l l t i t a n i u m peaks.
Oxidation l e d also t o a reduction i n t h e
i n t e n s i t y o f t h e p l a t i n u m XPS peaks, i n d i c a t i n g t h e f o r m a t i o n o f a f i l m o f The e v o l u t i o n o f t h e T i 2p e m i s s i o n r e g i o n
t i t a n i u m oxide onto t h e surface.
f o r p r o g r e s s i v e o x i d a t i o n o f t h e s u r f a c e i s shown i n f i g . 1
I
I
I
I
I
I
I
I
I
1
F i g . 1. E v o l u t i o n o f t h e XPS T i 2p spectrum f o r progressively higher exposures t o oxygen. For each spectrum we report the conditions o f exposure t o oxygen and the. t o t a l T i 2p t o P t 4p s i g n a l area r a t i o (R). a ) no exposure. [=0.36. b) 1073 K, 5x10- T o r r f o r 30 sec. R=C58. c ) 973 K, 7x10 torr f o r 30 sec. R=l 0. d ) 973 K, l ~ l O t- o~r r and c o o l i n g a t t h e same p r e s s u r e . R=0.65.
461
451
BINDING ENERGY (eV)
XPS s p e c t r a s i m i l a r t o t h o s e o f f i g . 1 were observed a f t e r o x i d a t i o n o f t h c
P t T i ( l l l ) , ( 1 0 0 ) and p o l y c r y s t a l l i n e s u r f a c e s i n s i m i l a r c o n d i t i o n s o f 3 temperature and p r e s s u r e . A v i s u a l e x a m i n a t i o n o f t h e s e s p e c t r a shows t h a t
62
o x i d a t i o n produces a t l e a s t two d i s t i n c t t i t a n i u m o x i d e s t a t e s , c h a r a c t e r i z e d by. d i f f e r e n t b i n d i n g e n e r g i e s o f t n e T i 2 p peaks ( t a b l e 1 ) .
After oxidation a t
temperatures h i g h e r t h a n a p p r o x i m a t e l y 900 K and oxygen p r e s s u r e s l o w e r t h a n ca. 1 ~ 1 0 t- o~r r , a f t e r c o o l i n g t h e sample i n vacuum, t h e dominant f e a t u r e o f t h e T i 2p spectrum was t h e d o u b l e t o f t h e l o w e r b i n d i n g energy s t a t e .
A treatment i n
oxygen a t h i g h e r p r e s s u r e i n t h e same r a n g e o f t e m p e r a t u r e s l e d t o t h e growth o f t h e peaks o f t h e h i g h e r b i n d i n g energy s t a t e .
These peaks became t h e dominant
f e a t u r e o f t h e t i t a n i u m 2p spectrum o n l y i f t h e sample was a l l o w e d t o c o o l down -4 torr. i n t o e presence o f oxygen a t about 1x10
I t appears r e a s o n a b l e t o a s s i g n t h e s e two e l e c t r o n i c s t a t e s t o two d i s t i n c t t i t a n i u m o x i d e phases.
C a r l e y e t a l . ( r e f . 1 1 ) have shown t h a t d i s c r e t e phases
showing d i f f e r e n t b i n d i n g e n e r g i e s o f t h e T i 2p peaks f o r m on o x i d a t i o n of I n p r e v i o u s works ( r e f s . 1 - 2 ) we snowed by LEED t h a t t h e
metallic T i .
o x i d a t i o n o f t h e P t T i s u r f a c e l e a d s t o t h e f o r m a t i o n o f d i s t i n c t phases o f 3 w e l l d e f i n e d c r y s t a l l o g r a p h i c parameters. A l t h o u g h i t was n o t p o s s i b l e t o p e r f o r m i n s i t u LEED o b s e r v a t i o n s i n t h e XPS a n a l y s e r used f o r t h e p r e s e n t s t u d y , an i n d i r e c t c o r r e l a t i o n o f t h e LEED and XPS r e s u l t s can be performed by a comparison o f t h e AES r e s u l t s o b t a i n e d i n r e f . 1 and i n t h e p r e s e n t work.
The
shape o f t h e t i t a n i u m LMN Auger t r a n s i t i o n s has been shown t o be s e n s i t i v e t o t h e c o m p o s i t i o n o f b u l k t i t a n i u m o x i d e s ( r e f . 1 4 ) and i t was f o u n d i n r e f . 1 t h a t t h e f o r m a t i o n o f a m u l t i l a y e r o f o x i d e on t h e P t T i s u r f a c e causes t h e 3 appearance i n t h e spectrum o f a peak a t ca. 398 eV, w h i c h i s n o t d e t e c t a b l e i n t h e c l e a n P t T i s u r f a c e n o r f o r a s i n g l e atomic l a y e r o f o x i d e .
3
I n f i g . 2 we show T i LMN s p e c t r a r e l a t i v e t o d i f f e r e n t degrees o f o x i d a t i o n o f t h e surface.
We found t h a t t h e peak a t 398 eV was n o t d e t e c t a b l e i n t h e absence
of t h e h i g h e r b i n d i n g energy T i o x i d e s t a t e .
T h i s comparison p e r m i t s t h e r e f o r e
t o a s s o c i a t e t h e low b i n d i n g energy 2p s t a t e t o t h e phases o f t h i c k n e s s o f t h e o r d e r o f a s i n g l e atomic l a y e r which g i v e r i s e t o t h e LEED p a t t e r n s r e p o r t e d i n r e f s . 1 and 2 .
On t h e b a s i s of t h e same c o n s i d e r a t i o n s ,
the higher binding
energy phase can be a s s o c i a t e d t o t h e f o r m a t i o n of a m u l t i l a y e r .
This
i n t e r p r e t a t i o n i s consistent w i t h t h e observation t h a t t h e t r a n s i t i o n from t h e 1oi.i L i r i d i n g energy phase t o t h e h i g h b i n d i n g one was accompanied by a s i g n i f i c a n t
decrease of t h e i i . P t s i g n a l r a t i o ( s e e d a t a r e l a t i v e t o f i g 11, which may be i r i t e r p r e t e d a5 due t o t h e c o l l a p s e of t h e f l a t o x i d e l a y e r t o f o r m c r y s t a l l i t e s
o c c i i p y l n g a s m a l l e r s u r f a c e area. In t h e f o l l o w i n g , we w i l l show f r o m a
63
q u a n t i t a t i v e analysis o f the XPS r e s u l t s t h a t t h e lower binding energy oxide has a composition close t o T i O , whereas t h e higher binding energy one i s T i 0
2'
F i g . 2. Titanium LMN spectrum a f t e r oxidation o f the P t T i surface. Upper curve: Surqace covered w i t h n e a r l y pure " T i O " Lower curve: Surface covered w i t h nearly pure T i 0 2'
>
b
W
5
s
?5
a B P
"__iz TiO,
,."
,,,,,,,
Spectral f i t t i n g As pointed out by Carley e t a l . ( r e f . 11) t h e f i t t i n g o f the XPS spectrum o f
a mixture o f t i t a n i u m oxides i s a complex procedure which i s c r i t i c a l l y dependent on t h e c o r r e c t choice of t h e i n i t i a l parameters. I n t h e present study, one faces the a d d i t i o n a l d i f f i c u l t y t h a t , because o f t h e Pt-Ti i n t e r a c t i o n , i t may n o t be possible t o r e l y on t h e parameters obtained from reference bulk phases f o r the f i t t i n g .
I t has been shown t h a t the presence o f
P t - T i bonds i n P t 3 T i causes a l a r g e s h i f t i n t h e binding energy o f t h e T i 2p
peaks ( r e f s 11,151.
I t has also been c a l c u l a t e d ( r e f . 16) t h a t i n a t e r n a r y
P t / T i / O compound, where t i t a n i u m i s bonded t o both oxygen and platinum, t h e r e i s
a substantial charge t r a n s f e r from T i t o P t .
I n f a c t , as shown i n t a b l e 1, t h e
peak o f the low b i n d i n g energy oxide does not 312 correspond t o any o f t h e known bulk t i t a n i u m oxides. Therefore, i t appears binding energy o f t h e T i 2p
d i f f i c u l t t o determine w i t h c e r t a i n t y by curve f i t t i n g i f , i n a d d i t i o n t o t h e two oxidized states which can be visually.observed, f u r t h e r states o f lower
64
i n t e n s i t y a r e p r e s e n t o r i f t h e peaks a t t r i b u t e d t o a s i n g l e e l e c t r o n i c s t a t e a r e a c t u a l l y t h e r e s u l t o f t h e s u p e r p o s i t i o n o f two o r more s t a t e s a t c l o s e However, t h e p o s s i b l e presence o f one o r more m i n o r
binding energies.
components can a f f e c t o n l y m a r g i n a l l y t h e d e t e r m i n a t i o n o f t h e c o m p o s i t i o n of Moreover, t h e 2p b i n d i n g e n e r g i e s o f b o t h t i t a n i u m s t a t e s were
t h e main s t a t e s .
found t o v a r y v e r y l i t t l e as a f u n c t i o n o f d i f f e r e n t r e l a t i v e i n t e n s i t i e s i n t h e spectrum, a f a c t which appears t o r u l e o u t t h a t e i t h e r one i s t h e r e s u l t of t.he sum o f two s u b s t a t e s o f comparable i n t e n s i t y a t d i f f e r e n t b i n d i n g e n e r g i e s . We chose t h e r e f o r e t o f i t t h e e x p e r i m e n t a l d a t a as t h e sum o f t h r e e s t a t e s : two f o r o x i d i z e d T i and one f o r m e t a l l i c ( a l l o y e d ) T i .
The t r e a t m e n t o f t h e
d a t a c o n s i s t e d i n i ) smoothing by t h e f o u r i e r t r a n s f o r m method, i i )background s u b t r a c t i o n by t h e S h i r l e y method ( r e f . 171, i i i ) f i t t i n g o f t h e e x p e r i m e n t a l d a t a as t h e sum o f c u r v e s which were assumed t o be an a d j u s t a b l e m i x t u r e of l o r e n t z i a n and gaussian components.
The f i t t i n g procedure was based on t h e
d e t e r m i n a t i o n o f t h e b i n d i n g e n e r g i e s , l i n e w i d t h s and g a u s s i a n / l o r e n t z i a n r a t i o s f o r s p e c t r a where one o f t h e s t a t e s was l a r g e l y t h e m a j o r component.
The same
parameters were t h e n k e p t f i x e d i n t h e f i t t i n g o f more complex s p e c t r a , a l l o w i n g o n l y t h e r e l a t i v e i n t e n s i t i e s o f t h e components as v a r i a b l e parameters.
In this
way i t was p o s s i b l e t o o b t a i n a s a t i s f a c t o r y f i t f o r a l a r g e number o f d i f f e r e n t
-.
11
2p s p e c t r a .
Confidence i n t h e r e l i a b i l i t y o f t h i s p r o c e d u r e was d e r i v e d f r o m
t h e f a c t t h a t t h e r e s u l t s o f t h e f i t t i n g were f o u n d t o be independent o f t h e c h o i c e o f t h e i n i t i a l i n t e n s i t i e s and because t h e f i t t i n g l e d t o t h e expected
2:1 area r a t i o o f t h e t w o components o f each s t a t e .
We e s t i m a t e as b e t t e r t h a n
5% t h e e r r o r i n t h e area measurements, m a i n l y due t o n o i s e i n t h e e x p e r i m e n t a l d a t a and u n c e r t a i n t y i n t h e procedure o f background s u b t r a c t i o n .
Composition of t h e o x i d e s From t h e e x a m i n a t i o n o f t h e T i 0
2
r e f e r e n c e sample, t h e r a t i o o f t h e 01s t o
T i 2 p atomic s e n s i t i v i t y f a c t o r s o f o u r a n a l y z e r was f o u n d t o be .48 +/- 0.04. Since t h e b i n d i n g e n e r g i e s o f t h e T i 2 p (Ca. 450 eV) and 01s (Ca. 530 eV) s t a t e s a r e n o t v e r y d i f f e r e n t , t h e e l e c t r o n mean f r e e p a t h s may be c o n s i d e r e d t o be a p p r o x i m a t e l y t h e same.
T h e r e f o r e , u s i n g t h i s r e l a t i v e s e n s i t i v i t y f a c t o r , one
can d i r e c t l y measure t h e average O / T i atomic r a t i o o f t h e t h i n o x i d e l a y e r s formed on t h e P t T i s u r f a c e .
3
The s t o i c h i o m e t r y o f a s i n g l e o x i d e phase can be
determined i f a p u r e l a y e r o f such a phase can be prepared.
I n p r a c t i c e i t was
65
F i g . 3. Examples o f curve f i t t i n g o f t h e t i t a n i u m 2p spectrum. Top c u r v e : T i 0 =8Y 2 " T i 0" =82%, met a1 11 c Ti=lO%. Lower c u r v e : T i 0 =85%, 2 " T i O"= 15%.
P'
0
I
I
I
460
450
BINDING ENERGY (eV)
n o t p o s s i b l e t o p r e p a r e c o m p l e t e l y "pure" l a y e r s i n o u r e x p e r i m e n t a l c o n d i t i o n s . However, s p e c t r a where a s i n g l e component c o n t r i b u t e d f o r more t h a n 80% t o t h e o v e r a l l T i 2p spectrum were r e l a t i v e l y easy t o o b t a i n .
I n f i g 3, we show t h e
f i t t i n g o f a spectrum where 85% o f t h e T i 2p s i g n a l was f o u n d t o be due t o t h e h i g h b i n d i n g energy phase. found t o be 1.9.
The O/Ti average atomic r a t i o f o r t h i s l a y e r was
We c a l c u l a t e d t h a t t h e O/Ti r a t i o o f t h e major component c o u l d
o n l y v a r y between 2.0 and 2.2 as a f u n c t i o n o f t h e assumed O/Ti r a t i o o f t h e m i n o r i t y component ( f r o m 0.5 t o 1.5).
Considering t h e uncertainty i n t h e
measurement, i t appears t h a t t h e c o m p o s i t i o n o f t h i s o x i d e i s T i 0 2 . T h i s r e s u l t
66
was c o n f i r m e d by t h e f i t t i n g o f o t h e r s p e c t r a where t h e h i g h b i n d i n g energy s t a t e was t h e main component.
F o r t h i s phase, t h e b i n d i n g energy o f t h e T i 2p
peaks and t h e AES l i n e s h a p e correspond w e l l t o t h e l i t e r a t u r e d a t a f o r Ti02 ( t a b l e 1 ) . I n t h i s case, t h e r e f o r e , t h e P t - T i i n t e r a c t i o n does n o t appear t o a f f e c t t h e e l e c t r o n i c s t r u c t u r e o f T i , w h i c h i s i n agreement w i t h t h e h y p o t h e s i s t h a t t h i s o x i d e forms a m u l t i l a y e r on t h e s u r f a c e . I n r e f . 1 ,
no LEED p a t t e r n
attributable t o Ti0
was observed ( a l t h o u g h t h i c k l a y e r s o f T i 0 c o u l d be 2 2 observed by X-ray d i f f r a c t i o n ) . T h i s r e s u l t may be c o n s i d e r e d as an i n d i c a t i o n
grows i n t h e f o r m o f m u l t i l a y e r 2 c r y s t a l l i t e s , t o o s m a l l and/or d i s o r d e r e d t o produce a d e t e c t a b l e d i f f r a c t i o n t h a t i n our experimental c o n d i t i o n s T i 0
p a t t e r n i n LEED. I n f i g 3 we show t h e T i 2p spectrum f o r a s u r f a c e where 82% o f t h e t i t a n i u m was found t o be i n t h e f o r m o f t h e l o w b i n d i n g energy phase. atomic r a t i o f o r t h i s s u r f a c e was f o u n d t o b e 0.9.
The average O / T i
A l s o i n t h i s case, t h e
c a l c u l a t e d c o m p o s i t i o n o f t h e m a j o r component v a r i e s v e r y l i t t l e as a f u n c t i o n o f d i f f e r e n t assumptions about t h e c o m p o s i t i o n o f t h e o t h e r o x i d e . The c a l c u l a t e d O / T i atomic r a t i o o f t h e l o w b i n d i n g energy phase remains equal t o
0.9 i f t h e c o n t r i b u t i o n s o f t h e o t h e r two components a r e t a k e n i n t o account and t h e c o m p o s i t i o n o f t h e o t h e r o x i d e i s t a k e n as “ T i 0 2 ” .
A s shown i n t a b l e 1, t h e
b i n d i n g energy of t h e T i 2p that o f bulk TiO.
peak o f t h i s o x i d e does n o t c o r r e s p o n d w e l l t o 3/2 I t appears l i k e l y t h a t t h i s i s due t o t h e e f f e c t o f t h e P t - T i
bond, a f a c t which i s i n agreement w i t h r e f . 1, where i t was f o u n d t h a t t h i s phase forms a t h i n f l a t l a y e r on t h e s u b s t r a t e s u r f a c e . I n c o n d i t i o n s where b o t h o x i d e phases e x i s t i n comparable q u a n t i t i e s on t h e
surface, one can v e r i f y by a c a l c u l a t i o n i f t h e measured average O / T i atomic r a t i o i s c o n s i s t e n t w i t h t h e assumption t h a t t h e two phases a r e T i 0 and T i 0
2’
Obviously, t h e d e t e r m i n a t i o n of t h e r e l a t i v e abundance o f t h e two phases by c u r v e f i t t i n g w i l l n o t be a c c u r a t e i f one of t h e t w o phases i s m a i n l y l o c a l i z e d above t h e o t h e r .
This factor,
however, does n o t a f f e c t t h e c a l c u l a t i o n s o f t h e
c o m p o s i t i o n since, as mentioned b e f o r e , i t may be assumed t h a t t h e 01s and T i 2 p e l e c t r o n s a r e a t t e n u a t e d i n t h e same measure as a f u n c t i o n o f t h e p a t h i n t h e solid.
Over a number of measurements f o r v a r i a b l e s u r f a c e c o m p o s i t i o n s , we
found t h a t i n o r d e r t o f i t t h e e x p e r i m e n t a l ,data i t was necessary t o assume t h a t some v a r i a t i o n i n t h e s t o i c h i o m e t r y of one o r b o t h t h e o x i d e phases o c c u r s as a f u n c t i o n of t h e c o n d i t i o n s of p r e p a r a t i o n .
If we assume t h a t t h e c o m p o s i t i o n o f
67 TABLE 1 BINDING ENERGY (eV) OF THE T i 2P3/2 TRANSITION PHASE OBSERVED LITERATURE T i (metal)
454.0
453.8
(13)
Pt3Ti
455.5
455.6
(11)
"TiO"/Pt3Ti
456.3
456.9
(11)
"TiO"/Pt
456.2
(9)
Ti0
455.3 454.8
(12) (13)
T i 203
457.5
(12)
459.4
(11)
________________________________________------------
Ti02/Pt3Ti
458.8
458.9 (9) 457.9-458.6 (70)
Ti02/Pt
Bulk Ti02
459.0
459.2 459.0
(18) (12)
t h e h i g h b i n d i n g energy s t a t e i s always e x a c t l y Ti02, t h e n t h e O / T i r a t i o o f t h e low b i n d i n g s t a t e was found t o v a r y between 0.6 and 1.0.
T h i s v a r i a t i o n appears
t o be s i g n i f i c a n t l y l a r g e r t h a n t h e e x p e r i m e n t a l u n c e r t a i n t y . I n g e n e r a l , t h e above r e s u l t c o n f i r m s t h e i n t e r p r e t a t i o n o f t h e LEED and AES d a t a o f r e f . 1, where t h e t h i n l a y e r of t i t a n i u m o x i d e on P t T i was i d e n t i f i e d
3
as d e f e c t i v e T i O . It appears a l s o t h a t t h i s o x i d e has t h e same c o m p o s i t i o n as t h a t o f o x i d i z e d T i f i l m s on P t ( r e f . 9).
I n some s t u d i e s ( r e f s . 9-11],
the
lower b i n d i n g energy T i 2p peaks were a t t r i b u t e d t o t h e "t3" f o r m a l charge, a l t h o u g h t h e c o m p o s i t i o n was f o u n d t o be " T i O " i n r e f . 9.
I n f a c t , as shown i n
t a b l e 1, t h e b i n d i n g energy o f 2p peaks o f t h e low b i n d i n g energy s t a t e i s i n t e r m e d i a t e between t h a t o f T i 0 and o f T i 0 so t h a t i t i s n o t p o s s i b l e t o 2 3 a s s i g n a f o r m a l o x i d a t i o n number t o t h e t i t a n i u m i o n s .
ACKNOWLEDGEMENT T h i s work was supported i n p a r t by t h e C o n s i g l i o Nazionale d e l l e R i c e r c h e ( I t a l y ) and by t h e Department o f Energy (USA).
68
REFERENCES
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 l! 18
U. Bardi and P.N. Ross, J. Vac. S c i . Technol., 2 (19843 1476. U. Bardi, A. Santucci, P.N. Ross and G. Rovida, Surface Sci., i n press. U.Bardi, B.C. Beard and P.N. Ross, J. Vac. S c i . Technol., A 6 (1988) 665. S.E. Greco, J.P. Roux and J.M. B l a k e l y , Surface Sci., 120 11982) 203. C.R. Brundle, E. Silverman and R.J. Madix, J. Vac. Sci. Technol., 16 (1979) 474. M. Ahmad and J. M. B l a k e l y , Surface and I n t e r f a c e Analysis, 10 (1987) 92. P.N. Ross, E P R I r e p o r t EM-1553, 1980. S.J. Tausier, S . C . Fung and R.L. Garten, J. An. Chem. SOC. 100 (1978) 170. C.M. G r e e n l i e f , J.M. White, C.S. KO and R.J Gorte J. Phys. Chem. 89 (1985) 5025. D.J. Dwyer, S.D. Cameron and J. Gland, Surface Sci., 159 (1985) 430. J . Paul, S.D. Cameron, D.J. Dwyer and F.M. Hoffmann, Surface Sci., 177 (1986) 121. A.F. Carley, P.R. Chalker, J.C. R i v i e r e and M.W. Roberts, J. Chem. SOC., Faraday t r a n s . , 83 (1987) 351. Handbook o f X-Ray P h o t o e l e c t r o n Spectroscopy. Phys c a l E l e c t r o n i c s d i v i s i o n , Perkin-Elmer c o r p o r a t i o n , Eden P r a i r i e , Minnesota 979. G.D. Davis, M. Natan and K . A . Anderson, Appl. S u r f Sci., 15 (1983) 321. G . N . Derry and P.N. Ross, S o l i d S t a t e Commun., 51 1984) 151. J.A. Horsley, J. Am. Chem. SOC., 101 (1979)2870. D.A. S h i r l e y , Phys. Rev. B., 5 (1972) 4709. M. Scrocco, Chem. Phys. L e t t . , 61 (1979) 453.
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
69
INTERACTION OF ATOMIC HYDROGEN WITH THE (111) AND (100) SURFACES OF DIAMOND-LIKE CRYSTALS
Vincenzo BARONE Dipartimento di Chimica, Universita di Napoli via Mezzocannone 4, 1-80134 Napoli, Italy. Giancarlo Abbate Dipartimento di Fisica, Universita di Napoli Pad.20 Mostra d'oltremare, 1-80125 Napoli, Italy. Nino RUSSO and Marirosa TOSCANO Dipartimento di Chimica, Universita della Calabria 1-87030 Arcavacata di Rende (Cosenza), Italy.
ABSTRACT The interaction of atomic hydrogen with model clusters designed to simulate (111) surfaces of diamond-like crystals has been investigated using the MNDO method. The results confirm the local nature of chemisorption and justify the use of small model clusters saturated by hydrogen atoms. The most stable adduct is always obtained by chemisorption at the on-top position of the (100) surface. Chemisorption at the same position on the (111) surface is slightly less exothermic. Silicon and germanium have very similar characteristics, whereas chemisorption on diamond is more covalent and exothermic. (100) and
INTRODUCTION Hydrogen adsorption on diamond-like crystals has been studied in great detail by experimental and theoretical methods in view of the technological importance o f this process in several fields, such as, for instance, photovoltaic applications 11-31. However, a comparative theoretical study o f all the substrates (C,Si,Ge) and of the most significant low-index surfaces ( (100) and (111) ) has not yet been performed. The purpose of the present work is to give a contribution in this direction with special emphasis on geometric and energetic parameters in the low-coverage 1 imit. The sol id-state approach is certainly more adequate for the computation of spectroscopic properties (e.g. work functions, optical transitions, etc.) or for the study of complete overlayer chemisorption, but it is not so well suited for the study of situations (e.9. irregular surfaces, defects, or sub-monolayer chemisorption) where the periodicity along the surface i s broken. Furthermore in the case of covalent crystals geometric and thermodynamic properties are generally very local in character, so that in these situations the
70
cluster-model approach remains competitive provided that: (i) the selected quantum-mechanical procedure is re1 iable enough to provide at least physically significant general trends and (ii) the model clusters used to simulate the crystal are large enough to minimize unwanted boundary effects. Concerning the first point, it has been recently shown that the MNDO (Modified Neglect of Differential Overlap) semiempirical method [4,5] gives a poor description o f the band structure of the silicon crystal [6], too short equilibrium bond lengths, and exaggerated charge-transfers [7-91. The same probably applies to germanium, in view of the similar incongruences in the parametrization [lo]. In spite of these deficiencies, several studies have shown that useful information can be obtained by this method concerning the sites and energetics of chemisorption [11-13]. This is probably due to the local nature of the phenomenon, which avoids the come into play of the "solid-state shortcomings" of MNDO. On these grounds a systematic MNDO study can be still useful by proper check o f some key results. Small models of the (111) surfaces are thus studied at the same time by ab-initio and MNDO methods, and the results of the MNDO method for larger systems and/or other surfaces are then critically examined with reference to their performances and limits evidenced in the test computations. Once the computation technique has been chosen, a model must be sought which is sufficiently representative of the infinite solid, and its performances must be critically examined. If a finite cluster of atoms of the solid is used, boundary conditions have to be imposed artificially to simulate the effect of the remaining solid. In covalent crystals these boundary conditions take the form of geometrical constraints imposed by the periodicity of the crystal, and of artificial saturation of the bonds which would link the cluster with the rest o f the solid. Care must be taken, however, that the boundary conditions do not disturb the relevant properties. As a general procedure, embedding hydrogen atoms are used to form a bulk environment for the clusters employed and to effectively allow for sp3 hybridization [3,14]. This procedure is surely appropriate in the case of diamond due to the similar electronegativity of C and H and o f the comparable strength of CC and CH bonds. It is more questionable in the case of silicon and germanium, where appropriate pseudo-hydrogen atoms with a lower electronegativity could be profitably used [15]. In the present study we also report some preliminary results obtained by a general implementation of this procedure in the MNDO method.
METHODS
MNDO computations were performed by the AMPAC package using standard parameters [4,5,10], whereas ab-initio computations were performed by the PSHONDO package using the PP-ZZ* basis set and the pseudopotentials described in refs.16 and 1 7 .
71
The clusters used t o simulate the (111) and (100) surfaces have been often reported in previous studies (e.g. figs.1 and 2 of ref.6) and are not shown again here. Because of previous results, only on-top chemisorption has been considered for the (111) surface, whereas bridge and open sites have also been considered for the (100) surface. The chemisorption energy (CE) is defined, as usual, as the difference between the total energy of the adsorbate-cluster supermolecule and the total energies of the isolated partners. Only low-spin wave-functions (i.e. singlet states for an even number of electrons and doublet states for an odd number of electrons) were considered as possible ground states. Monovalent pseudoatoms (labelled Y) are special hydrogen atoms whose single atomic ( s ) orbital has parameters roughly corresponding to those of an sp3 hybrid of the appropriate substrate atom (denoted by X). In particular (with the notation of ref.4)
Zs(Y) =
0.25Zs(X) + O.75Zp(X)
The a parameter was finally determined to obtain XY distances equal to XX ones. Although further slight adjustments would have in principle been necessary to improve the results, the very good results obtained for XY4 and XaYs compounds convinced us to not modify the parameters obtained by the above procedure. RESULTS AND DISCUSSION The MNDO results
for chemisorption of hydrogen atoms on (111) and (100) surfaces of diamond-like crystals (see Table) show an increase of equilibrium bond lengths with the atomic number of the substrate and a decrease in the same order of the binding energies. The most stable adduct is obtained by chemisorption at the on-top position on the (100) surface, but chemisorption at the same site on (111) surfaces is only slightly less exotermic. Chemisorption on high-coordination sites i s much less favorable, and, in fact, open sites correspond to energy minima only in particular cases and also in those circumstances chemisorption becomes an endothermic process. Hydrogen is quite peculiar in this respect because all other atoms and molecules prefer higher coordination sites (especially the bridge site on the (100) surface) [6]. These trends are furthermore essentially insensitive to the dimensions of the clusters used to mimic the substrate. The similar chemical properties of Si and Ge are well evidenced by the
72
comparable chemisorption energies obtained for both surfaces of the two substrates by all the methods and clusters employed. The characteristics of carbon are, on the other hand, somehow different due to the more covalent character of the CH bond and to its consequent greater strength. As far as charge transfer and chemisorption energy are concerned, the results provided by ab-initio pseudopotential computations are very similar to those obtained at the MNDO level, but the equilibrium X-H bond lengths are slightly underestimated in the latter case. Furthermore both theoretical methods provide chemisorption energies on (111) surfaces of diamond and silicon in remarkable agreement with the experimental values of 4.2 eV [18] and 3.2 eV [19], respectively. TABLE Computed MNDO parameters for hydrogen chemisorption on (111) and (100) surfaces of diamond-like crystals. R, and R (both in A ) indicate respectively the X-H bond length and the distance between the adatom and the plane defined by X atoms of the first layer. CE (eV) indicates the chemisorption energy and q(H) (in a.u.) indicates the net charge of the adatom. Both hydrogen (H) and pseudoatom (Y) terminators have been used to terminate some of the clusters. The values in parentheses have been obtained by PP-2Z* computations (see text). (111) Surfaces
site top top open top top top open top top top open
R, 1.137 (1.094) 1.114
R
CE
q(H)
1.137 (1.094) 1.114
4.15 (4.16) 4.20
0.02 0.08 0.02
1.380 (1.484) 1.386 1.387
1.380 (1.484) 1.386 1.387
1.502 (1.556) 1.508 1.502 2.529
1.502 (1.556) 1.508 1.502 1.030
no minimum 3.28 (3.23) 3.35 3.21
0.03 (-0-11) -0.05 0.03
no minimum 2.70 (2.94) 2.89 2.72 -0.89
-0.11 (-0.09) -0.19 -0.11 -0.43
CE 5.24 3.16 -0.55 3.43 2.28
q(H)
(100) Surfaces
site top bridge open top bridge open top bridge open
R.
R
1.073 1.401 1.962 1.361 1.934
1.073 0.614 0.823 1.361 0.235
1.487 2.018
1.487 0.264
0.08 0.09 0.24 -0.23 -0.36
no minimum 3.44 2.51
no minimum
-0.26 -0.44
73
The net charges of the substrate atoms not directly involved in the chemisorptive bond and o f the embedding atoms are sometimes significant, but they are always essentially the same before and after hydrogen chemisorption. The essential o f adatom-substrate charge transfer is thus related to the atoms directly bonded to the adsorbate and this explains why the "molecule-based" parametrization of MNDO is sufficient in the study o f low-coverage chemisorption. The low computational cost of MNDO has allowed the study of quite large clusters. We find that only negligible changes occur going from X4H9 to XloHls models (i.e. including from 2 to 4 layers of substrate atoms), thus confirming the local nature of the chemisorptive bond. REFERENCES
[l] K.Fujiwara, Phys.Rev. E l 2036 (1982). [2] L .Papagno X.Y. Shen,J.Anderson ,G.Schirripa Spagnolo, G. J. Lapeyre,Phys .Rev. B34,7188 11986) and refs.cited therein [3] V.Barone,F.Lel j,N.Russo,M.Toscano,F.I1 las,J .Rubio, (1986) and refs. cited therein.
Phys .Rev.
E, 7203
[4] M.J.S.Dewar,W.Thiel, J.Am.Chem.Soc. 99,4899,4907 (1977).
.
.
.
[ 51 M. J S . Dewar E F. Healy ,J. J. P.Stewart,J E. Fr iedheim, G. L .Grady, Organometal-
lics, 5,375 (1986).
[6] P.Deik, L.C.Snyder, Phys.Rev. 836,9619 (1987). [ 71 W, S . Verwoerd, J .Cornput.Chem. 3,445 (1982).
[8] P. Deik ,L .C. Snyder ,R.K. Singh,J. W .Corbett, Phys .Rev. 836,9612 (1987).
[9] P.Deik, L.C.Snyder, 1nt.J.Quantum Chem. ,Quantum Chem.Symp. 21,547 (1987). [ 101 M. J. S . Dewar ,G.L .Grady,E . F .Healy, Organometallics, 6,186 (1987).
[ll] N.RUSSO, M.Toscano, V.Barone, F.Lel j, Surf.Sci. 180, 599 (1987). [12] V.Barone, Surface Sci. 189/190, 106 (1987). [ 131 N.Russo ,M.Toscano,P.Amodeo,V.Barone, Sol id State Commun. 65,945 (1988).
[14] A.C.Kenton, M.W.Ribarsky, Phys.Rev. 823, 2897 (1981).
[15] I.Laszlo,Int.J.Quantum Chem. G I supp1.2,105 (1977); 2 , 8 1 3 (1982). [16] V.Barone, F.Lelj, N.RUSSO, M.Toscano, F.Illas, J.Rubio, Surf.Sci. (1985).
162, 230
[ 171 V. Barone ,N .Russo,M.Toscano,F. I 1 las,J .Rubio, Sol id State Commun. , in press.
[18] H.Froitzheim,H. Ibach,S.Lehwald, Phys.Lett. A55,247 (1975). [19] M.Seel ,P.S.Bagus, Phys.Rev. g , 5 4 6 4 (1981).
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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
75
A TRANSMISSION FTIR STUDY OF THE ADSORPTION OF CARBON MONOXIDE AND CYCLOHEXANE ON EVAPORATED SILVER FILMS
J.E. BATEMAN and M.A. CHESTERS School o f Chemical Sciences, U n i v e r s i t y o f East Anglia, NORWICH, NR4 7TJ, U.K.
ABSTRACT
on t h i n ( ~ 5 0 ~s i)l v e r f i l m s has been The adsorption o f CO and Ce6H,, studied using Transmission F o u r i e r Transform I n f r a r e d Spectroscopy. Recent r e s u l t s have shown t h a t f o r f i l m s deposited a t TD < 150 K ' a c t i v e ' s i t e s are a v a i l a b l e f o r CO chemisorption whereas a t higher deposition temperatures CO w i l l not chemisorb. I n the present study CO adsorption on a Ag f i l m (TD = 130 K) leads t o two absorption bands i n the i n f r a r e d : an i n i t i a l broad band a t 2087 cm-1 followed by a sharper band a t 2137 cm-1 which grows i n w i t h no s h i f t i n frequency w i t h increasing pressure, A c o r r e l a t i o n i s made between t h i s r e s u l t and t h e r e s u l t s o f previous studies on m a t r i x - i s o l a t e d Ag c l u s t e r s . The I R spectra o f C H, adsorbed on a Ag f i l m deposited a t TD = 130 K presented here show t h a t t h e C H, i s chemisorbed w i t h a 'softened' CH s t r e t c h i n g v i b r a t i o n i n d i c a t i n g hyjrogen-bonding w i t h the metal. However, on a f i l m annealed t o room temperature only physisorbed C,H12 i s observed.
INTRODUCTION Evaporated t h i n metal studies
of
molecules
transmittance/high temperature
f i l m s have proved useful i n transmission i n f r a r e d adsorbed
on
metal
surfaces,
surface area substrate ( r e f .
(T~C150 K )
1).
providing
high
are known t o be rough on an atomic scale due t o
adatoms, k i n k and step s i t e s and porous on a nanometre scale ( r e f . 2). Tunnelling Microscopy
a
Films deposited a t low
(STMI studies
have f u r t h e r
shown t h a t
Scanning
low TD f i l m s
annealed t o room temperature r e t a i n some o f t h e i r porous s t r u c t u r e ( r e f . 3). This dependence o f surface topography on temperature has been the subject o f much i n v e s t i g a t i o n i n r e l a t i o n t o surface enhanced Raman s c a t t e r i n g (SERS) (ref. 4).
SERS a c t i v e s i t e s e x i s t on f i l m s deposited a t low temperature b u t
are removed on annealing a t room temperature ( r e f . 5 ) .
I t i s now generally
accepted t h a t sJrface roughness i s a necessary p r e r e q u i s i t e f o r SERS b u t the scale
3f
roughness i s s t i l l under debate.
Low TD f i l m s o f s i l v e r have been
shovrn t o support C9 chemisorption whereas f i l m s deposited a t room temperature o r postannealed f i l m s do not
(ref.
6).
Cyclohexane shows dramatic e f f e c t s
in
v i b r a t i o n a l spectra Nhen chemisorbed so might equally prove t o be a useful probe o f t h e a c t i v e s i t e s on low TD f i l m s ( r e f . 7 ) .
76
We r e p o r t here transmission i n f r a r e d spectra o f both CO and cyclohexane on t h i n s i l v e r f i l m s deposited a t l o w temperature and a f t e r annealing. EXPERIMENTAL The apparatus used was s i m i l a r t o t h a t f i r s t used by Bradshaw and P r i t c h a r d (ref. 8).
A glass transmission c e l l was evacuated and a f t e r baking a t IlOOC, a
background pressure o f t y p i c a l l y 2 x
torr was achieved by an o i l d i f f u s i o n
pumping system. S i l v e r (99.99%, Goodfellow Metals Ltd.) tungsten f i l a m e n t on to a l i q u i d nitrogen-cooled decrease
i n i n f r a r e d transmission
was evaporated from a
K B r p l a t e a t 130 K u n t i l a
o f approximately
30% had occurred due t o
The bulk t h i c k n e s s o f t h e r e s u l t i n g f i l m was
absorption by t h e metal f i l m .
estimated a t -508, ( r e f . 8). Temperature measurements were made by l o c a t i n g a chromel-aluminel plate.
thermocouple i n a small h o l e d r i l l e d i n t o the face o f t h e K B r
Pressure measurement was made u s i n g a Spectramass 100 quadrupole mass
spectrometer,
which a l s o served as a r e s i d u a l gas analyser.
The c e l l was
s i t u a t e d i n the sample w e l l o f a Mattson Cygnus 25 FT-IR spectrometer which was f l u s h e d w i t h d r y a i r t o remove atmospheric water vapour and CO,. taken using an
Spectra were
IR Associates InSb d e t e c t o r over the range 4000-1800 cm-l w i t h
1000 scans co-added a t a r e s o l u t i o n o f 4 cm-'.
Background spectra o f t h e c l e a n
f i l m were obtained a g a i n s t which sample spectra were r a t i o e d . RESULTS ( i) Carbon Monoxide A d s o r p t i o n
No absorption bands r e s u l t e d from b r i e f exposures t o CO b u t when a dynamic pressure o f 1 x lo-* t o r r was e s t a b l i s h e d a broad band ( h a l f w i d t h 4 0 cm-l) appeared a t 2087 c n r l (Fig.
1).
I n c r e a s i n g t h e pressure l e d t o a f u r t h e r
sharper band appearing a t 2137 cm-1 w i t h a shoulder o f 2125 cm-l.
By 1 x
t o r r the band a t 2087 cm-I had s a t u r a t e d a t 0.15% T b u t t h e 2137 cm-1 band continued to grow w i t h i n c r e a s i n g pressure and no s h i f t i n frequency u n t i l i t s i n t e n s i t y had reached 0.45% a t a pressure o f 1 x a t which spectra were recorded.
t o r r , the h i g h e s t pressure
A f t e r pumping t h e c e l l t o 2 x
torr,
a
f u r t h e r spectrum was taken which showed no bands due t o CO i n d i c a t i n g t h a t CO a d s o r p t i o n i s t o t a l l y r e v e r s i b l e a t t h i s temperature.
The f i l m was l e f t t o
anneal t o room temperature o v e r n i g h t and t h e experiment repeated.
A drop i n the
t r a n s m i t t a n c e o f the f i l m from 70% t o 65% T i n d i c a t e d t h e c o a g u l a t i o n o f s i l v e r aggregates r e s u l t i n g i n a f i l m o f o n l y s l i g h t l y decreased surface area.
No
i n f r a r e d bands due t o carbon monoxide were detected on t h e annealed f i l m a t 130
K f o r co pressures up t o 1 x 1 0 - ~t o r r . (i i) Cyclohexane A d s o r p t i o n
On exposing the f i l m deposited a t 130 K t o a dynamic pressure o f 1 x
(ii)
0.05% I T (iii)
WAVENUMBER/CM
Fig. 1 CO/Ag (i) (ii) (iii)
Film 0 130 K; TD 0 130 K Clean Film 1 x 10-8 torr 1 x 10-7 torr
-’
(ivl 1 x (v)
torr 1 x 10-5 torr
3 (ii)
3200
2900
2(
WAVENUMBER/CM -1
Fig. 2 CyclohexanelAg Film Q 130 K (i ) 5 x torr (ii) 1 x 10-7 torr (iii)5 x 1 0 - ~torr
3200
2900
2 00
WAVENUMBER/CM
Fig. 3
Multilayered Cyclohexane/Ag Film Q 130 K 5 x
torr.
79
t o r r o f cyclohexane two bands were seen: 2903 cm-'
w i t h an 'enhanced'
a h i g h l y asymnetric band centred a t
transmission feature to high frequency (shoulders
appear on both sides o f t h i s band a t 2920 cm-l and 2891 cm-l) along w i t h another Increasing the dynamic pressure to 5
more symnetric band a t 2848 cm-1 (Fig. 2). x
torr
l e d t o an increase i n i n t e n s i t y o f both these bands and the 50 cm-l) a t 2778 cm-l. A weak feature ( <
appearance o f a broad band (;1/2
0.01%) a t 2660 cm-1 was also d i s c e r n i b l e above the noise.
A t a pressure o f 5 x
t o r r the 2920 cm-1 band w i t h a shoulder a t 2901 cm-1 dominated the spectrum together w i t h the 2848 cme2 band.
The broad band a t 2778 cm-l saturated i n
i n t e n s i t y a t =a048 (Fig. 3). The f i l m was l e f t to anneal a t room temperature overnight and a subsequent spectrum confirmed t h a t the cyclohexane had completely desorbed. The f i l m was again cooled t o 130 K and exposed t o cyclohexane (Fig. 4).
At a
pressure o f 2 x t o r r two bands appeared a t 2926 cm-1 and a t 2843 cm-1 the higher frequency band having a shoulder a t 2901 cm-'. The i n t e n s i t y o f these bands increased l i n e a r l y w i t h pressure accompanied by the appearance o f a weak feature a t 2660 cm-l.
No evidence f o r a broad band was seen.
(iii)
3200
2900
2600
WAVENUMBER/CM -1
F i g . 4 Cyclohexane/Ag f i l m 0 130 K; TD B 130 K RT annealed (i)5 x torr; ( i f ) 2 x
torr
80
DISCUSSION Carbon Monoxide Adsorption The present experiment shows that CO adsorption is reversible w i t h i n the stated pressure-temperature regime and only occurs a t certain active s i t e s On annealing the film to room temperature and present in low TO films. recooling no chemisorption i s seen. T h i s r e s u l t i s i n agreement w i t h that of Dumas et a1 ( r e f . 6 ) where only physisorbed CO i s observed on Ag films annealed above a threshold temperature of 150 K whereas on low TD films ( 4 K ) CO chemisorption occurs. The number of active chemisorption s i t e s was estimated a t 4%of the surface based on the saturation intensity of the CO band. As saturation was not observed i n the present work the surface coverage cannot be estimated w i t h accuracy. However, the intensity of the bands obtained is consistent w i t h a coverage of l e s s t h a n a monolayer when compared t o the intensity of CO bands on copper films ( r e f . 9) b u t too intense t o be due to only 1%of the surface. The fact that no s h i f t i n frequency with exposure was seen i s evidence for a small number of widely spaced s i t e s being available for chemisorption so that coverage-dependent dynamic coupling a n d s t a t i c chemical e f f e c t s are not observed. In comparing the results of previous studies of CO adsorption on Ag films and clusters ( r e f . 9 - 111, a general trend i s seen i n which the CO frequency decreases w i t h decreasing TD. This can be explained by an increase in backdonation from metal o r b i t a l s into the Zn* antibonding orbital of the CO molecule as the size of the Ag clusters decreases. For small Ag clusters a greater densfty of s t a t e s near the Fermi level leads to a greater charge transfer into the CO antibonding orbital resulting i n a weaker CO bond ( r e f . 2,6).
T h i n metallic films produced by evaporation do not contain particles
of uniform size ( r e f . 13). The broad band a t 2087 cm-1 may be explained as due t o smaller Ag c l u s t e r s . The lower frequency in the infrared, large band width and the f a c t t h a t these s i t e s are populated f i r s t indicate a greater reactivity towards CO t h a n i s shown by the sharper, higher frequency band, as would be expected with smaller diameter particles. Cycl ohexane Adsorption The two bands a t 2903 cm-l a n d 2848 cm-’ on the low temperature film can be readily assigned to the asymmetric and symnetric CH, stretching vibrations respectively. In comparison with frequencies i n the polycrystalline sol id of 2930 cm-1 and 2860 cm-1 respectively ( r e f . 141, these modes are shown t o be mildly perturbed on adsorption. The shoulder a t 2920 cm-’ i s probably due t o cyclohexane physisorbed on p a r t s of the films which do n o t support
81
chemisorption. The broad band a t 2778 cm-1 i s assigned t o a CH s t r e t c h i n g v i b r a t i o n broadened and s h i f t e d t o low frequency due t o an e l e c t r o n i c i n t e r a c t i o n between c e r t a i n hydrogen atoms i n the molecule and t h e metal
More s p e c i f i c a l l y , t h i s i s described as a two-wqy charge t r a n s f e r from bonding u CH o r b i t a l s t o o r b i t a l s i n the metal and from f i l l e d metal o r b i t a l s i n t o the antibonding 8 CH o r b i t a l s w i t h the l a t t e r process dominating ( r e f . surface.
15). The degree o f softening observed i s d i r e c t l y r e l a t e d t o the r e a c t i v i t y o f the surface and has been shown t o f o l l o w the series
Ru > P t
> Pd > N i > Cu
( r e f . 16)
This r e s u l t f o r cyclohexane on s i l v e r i s comparable i n frequency t o s o f t modes observed on copper surfaces c f .
2775 cm-l on C u ( l l 1 ) and 2790 cm-l on
Cu(100) (ref. 16). However, the bands on copper are much wider w i t h band width of =160 cm-' i n d i c a t i n g a stronger i n t e r a c t i o n on copper. Cyclohexane i s t h e r e f o r e seen t o be chemisorbed b u t bound less s t r o n g l y t o s i l v e r than t o copper ( i n accordance w i t h the series proposed above). On increasing the exposure, bands grow i n i n t e n s i t y which are assigned t o physi sorbed c y c l ohexane superimposed on the spectrum o f the chemi sorbed species.
For the room temperature annealed f i l m only physisorbed cyclohexane i s
observed a t a l l exposures and no evidence f o r a s o f t mode i s seen. The adsorption behaviour o f cyclohexane on s i l v e r f i l m s i s t h e r e f o r e s i m i l a r t o t h a t o f CO i n t h a t chemisorption,
as evidenced by
the
'softened'
CH
s t r e t c h i n g mode, only occurs on low temperature f i l m s where a c t i v e s i t e s are present and n o t on RT annealed films.
This r e s u l t runs contrary t o the argument
o f Hoffmann and Upton ( r e f . 171 t h a t chemisorption and mode s o f t e n i n g are n o t c l o s e l y r e l a t e d as the s o f t mode i s only observed when cyclohexane i s chemi sorbed. A SERS study o f cyclohexane on s i l v e r f i l m s has shown t h a t by f a r the most intense band i n the spectrum occurs a t 2789 softened CH stretch.
cm-' ( r e f . 7) which we assign t o the
A d i r e c t c o r r e l a t i o n can therefore be made between the
a c t i v e chemisorption s i t e s which lead t o CH mode softening and SERS a c t i v e sites. Active Sites The STM studies o f Gimzewiski e t a1 ( r e f . 3) show t h a t roughness due t o pores o r c a v i t i e s i s n o t f u l l y removed on annealing t o room temperature. This suggests t h a t the a c t i v e s i t e s t h a t support CO and cyclohexane chemisorption are associated w i t h sub-nanometre scale structure. A low concentration o f atomic scale s i t e s i s i n d i c a t e d by photoemission studies ( r e f .
18) where =12% o f surface s i t e s on low TD s i l v e r f i l m s show
e x t r a s t r u c t u r e assigned t o l o c a l i s e d d-electron s t a t e s o f surface defects.
Our
82
r e s u l t s suggest t h a t i t i s these e x t r a m e t a l l i c s t a t e s which are i n v o l v e d i n both the chemisorption o f CO and o f cyclohexane on the low TD f i l m s .
Discrete
m e t a l l i c s t a t e s a t s i t e s o f atomic scale roughness are a l s o b e l i e v e d t o be involved i n metal-adsorbate charge t r a n s f e r t r a n s i t i o n s observed i n EELS ( r e f . 19).
Such
transitions
adsorption ( r e f . 20).
have been used t o
e x p l a i n CH mode s o f t e n i n g on
I t i s i n t e r e s t i n g t o note t h a t SERS o f the cyclohexane/Ag
f i l m system ( r e f . 7) shows the s o f t mode t o be the most enhanced feature i n the spectrum.
This
result
therefore
provides
evidence
for
a
charge
transfer
mechanism f o r SERS. The asymmetric
lineshape
of
the
i n f r a r e d absorption bands
observed
is
c o m n l y encountered i n transmission i n f r a r e d studies i n v o l v i n g t h i n metal f i l m s ( r e f . 8.9).
No c o r r e l a t i o n can be made between band asymmetry and the presence
of s i t e s of atomic scale roughness.
The anomalous peak shape must r a t h e r be
associated w i t h the bulk p a r t i c u l a t e nature o f the f i l m s . CONCLUSIONS The adsorption o f CO and cyclohexane a t 130 K on f i l m s deposited a t low temperature
and
room temperature
annealed
films
has
been
studied
using
transmission i n f r a r e d spectroscopy. Carbon monoxide i s seen t o chemisorb only on low TD f i l m s . Cyclohexane a l s o chemisorbs on low Tg f i l m s b u t a d d i t i o n a l l y physisorbs on room temperature annealed f i l m s recooled t o 130 K.
C o r r e l a t i o n o f the observations w i t h the
r e s u l t s o f SERS i n v e s t i g a t i o n s ( r e f .
7) leads t o the conclusion t h a t the s i t e s
responsible f o r SERS a c t i v i t y are those necessary f o r chemisorption o f carbon monoxide and cyclohexane. ACKNOWLEDGEMENT One o f us (JEB) i s g r a t e f u l to t h e B r i t i s h Vacuum Council f o r a t r a v e l grant. REFERENCES
1.
J. Heidberg and H. Weiss i n "Thin Metal Films and Gas Chemisorption", ed.
2.
P. Wissmann, (Elsevier, Amsterdam, 1987). A. Otto, i n L i g h t S c a t t e r i n g i n Solids, Vol. 4, Eds. M. Cardona and G.
3. 4. 5. 6.
7. 8. 9.
Guntherolt, (Springer, B e r l i n 1984). J.K. Gimzewiski, A. Humbert, J.G. Bednorz and B. Reihl, Phys. Rev. L e t t . , 55 (1985) 951. R.K. Chang, T.E. Furtak, Surface Enhanced Raman Scattering, Plenum Press, 1982. I . Pockrand "Surface Enhanced Raman V i b r a t i o n a l Studies on Solid/Gas I n t e r f a c e s " ( Springer, B e r l i n 1983). P. Dumas, R.G. Tobin and P.L. Richards, Surf. Sci. 171 (1986) 555. I . Mrozek and A. O t t o i n V i b r a t i o n s a t Surfaces 1987, Jour. Elec. Spec. Rel. Phenom. 45 (1987) 143. A.M. Bradshaw and J. P r i t c h a r d , Surf. Sci. 17 (1969) 372. A.M. Bradshaw and J. P r i t c h a r d , Proc. Roy. SOC. Lond., A316 (1970) 169.
83
10. H. Abe, K. Manzel, W. Schulte, M. Moskovits, D.P.
D i l e l l a , J. Chem. Phys.
74, 793.
11. D.P. D i l e l l a , A. Gohin, R.H. Lipsom, P. McBreen, M. Moskovits, J . Chem. Phys. 73 (1980) 4282. 12. F. Frank, W. Schulze, B. Tesche and F.W. Froben, Chem. Phys. L e t t . 103 (1984) 336. 13. R.S. Sennett and G.D. Scott, Jour. Opt. SOC. Amer. 40 (1950) 203.R.J. 14. Obremski, C.W. Brown, E.R. L i p p i n c o t t , J. Chem. Phys. 49 (1968) 185. 15. J.Y. S a i l l a r d and R. Hoffmann, J.Am.Chem.Soc. 106 (1984) 2006. 16. R. Raval, Ph.D. thesis, U n i v e r s i t y o f East Anglia 1986 and refs. therein. 17. F.M. Hoffmann and T.H. Upton, J. Phys. Chem. 88 (1984) 6209. 18. E.E. Koch, J. Barth, J-H Fock, A. Goldmann and A. Otto, S o l i d State Corn. 42 (1982) 897. 19. D. Schmeisser, J.E. Demuth and Ph.Avouris, Chem. Phys. L e t t . 87 (1982) 324. 20. B.N.J. Persson and R. Ryberg, Phys. Rev. L e t t . 48 (1982) 549.
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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
HEAT-CAPACITYANOMALIES OF TWODIMENSIONAL SYSTEMS STUDIED BY THE MUISTAN-HOCK MODEL
F. BATTAGLIA,l T. F. GEORGE2 and Y. S. KIM2
lstituto di Chimica, Universith della Basilicata, Potenza 85100 (Italy) 2Departments of Physics 8 Astronomy and Chemistry, State University of New York at Buffalo, Buffalo, NY 14260 (USA.)
PBSRACT After deriving an analytic closed form of the heat-capacity signatures for the 2 X N quasi-monodimensionalMcQuistan-Hock model of a lattice gas, we apply it to various adsorbed systems for which the lateral interaction varies from a few meV to about 300 meV. It is shown that whenever the adsorption system can be described by a two-dimensional gas on which the substrate effects are less important than the adatom-adatom interactions, the computed temperature at which the heat-capacity signatures display their maximum are in very good agreement with the experimental measurements. INTRODUCTION The recent discovery that submonolayer films of adsorbates on some substrates have two-dimensional phases has opened a new field: the study of two-dimensional matter, its phases, and the transition between them. A common way to realize two-dimensional systems is to physisorb gaseous particles (atoms or simple molecules) onto solid surfaces (substrates) that might condense as a homogeneous film with two-dimensional properties. Experimentally, the essential task in a thermodynamic investigation of a two-dimensional system is the construction of a complete group of adsorption isotherms. In practice, for an accurate determination of the entropy, calorimetric experiments to determine the specific heat signatures are performed. Such experiments offer the possibility of studying low-dimensional order-disorder phenomena such as structural and melting phase transictions, whose order, for two-dimensional systems has not been yet definetly determined. In the present paper we use the McQuistan-Hock(MQH) model (ref. 1) to compute the heat-capacity signatures for several two-dimensional systems for which the substrate effects are less important than the adatom-adatom interactions. The MQH model provides an exact solution for the distributionfunction of q indistinguishable particles on a 2 X N lattice, where N is a positive integer. One might wonder about the justificationof even considering a 2 X N model given that there are excellent numerical techniques for finding, with
well-controlled accuracy, the solution to the full two-dimensional problem. The justification
85
86
resides in the very fact that in going from the one dimensional model to the 2 X N one, there is an abrupt significant improvement in the prediction of the temperatures at which the heat-capacitysignatures display their maximum. This implies that moving continously from the 2 X N to the N X N model should not improve the results significantly, and that most of the phyisics of the transition depends on the quasi-monodimensionalityof the model. Furthemore, the model has the advantage of being exactly solvable and containing very few parameters,
,
namely V, , Voo and V,,
that are the interaction strengths, respectively, among two
nearest-neighbor adparticles, two vacant sites and between an adparticle and its own site.
THEORY The theory of the 2 X N lattice and its application to the practical computation of the heat capacity signatures have been extensively described elsewhere (refs. 1-3). In this section we give only the relevant results. The isothermal-isobaric partitionfunction for a system of q indistinguishable particleson a 2 X N lattice is given by
where
p
is the inverse temperature, p the spreading pressure, q = exp(4gp),
t = exp(-pVo), and
In Eq. (2), A", q, noo, n,
,I,
obtained by making use of a 15-termrecursion relation, is the
number of unique ways q indistinguishable particles can be arranged on a rectangular 2 X lattice to form n,,
occupied nearest-neighbor pairs and no, vacant nearest-neighbor pairs.
After a partial fraction expansion, the isothermal-isobaric partition function can then be re-written as
( O < q < 2N)
where the zi s are the solutions of the equation
1 + D,
N
z t 0 2 z 2 + 03z3 = 0 ,
and
x(z) =
-
c,+c,z+czz2 D,
+ 2D,
z + 3D,z2
The definition of the coefficients appearing in eqs. (4) and (5) can be found in refs. 2 and 3 and shall not be reported here again. The heat capacity is therefore given by
where the prime denotes the derivative with respect to
p.
The value T, = l/pc of the
temperature at which the heat capacity displays its maximum, is given by the solution of the algebraic equation 2 A(A')'+ 3 p M ' A " - 2p( A')
- pAzA "-2A'A"
=
0
RESULTSAND DISCUSSION In this paper we are interestedat the simulation of the critical temperatures as determined by the heat-capacity signatures of some realistic two-dimensional systems. Although there is no
, reliable estimate for ,V
, we can safely set it to zero because its effect is expected to be small
in comparison to the other interaction parameters; moreover, although the partitionfunction and the Gibbs free energy do depend on V, , the heat capacity, in the thermodynamic limit, does not: the role of the substrate is analogous to that of a third body in the description of the interaction between two particles embedded in a three-dimensional medium, and the only task accomplished by the substrate is to force the adsorbed particlesonto a plane. The all-important parameter here is V,
, i.e., the interaction strength between two adsorbed particles, whose
value is, of course, different than the free-space value. We have chosen a wide variety of systems and have been limited only by the availability of accurate estimates of the interaction strengths and accurate experimental heat-capacity signatures to which compare our calculations. In particular we have considered: neon, argon, kripton. xenon, molecular oxygen and methane on graphite, and copper, silver, gold, niche1 and palladium on tungsten. To compare with data available from literature, calculations have been performed at the coverages of 50% for the systems on graphite and 30%for the systems on palladium, and results are reported in Table 1.
TABLE 1 Computed (T,)
and experimental
(TExdcritical temperatures of Ne, Ar,
Kr, Xe, 0, and CH4
on graphite (coverage 50%) and Cu, Ag, Au, Ni and Pd on W[110] (coverage 30%). The nearest-neighbor interactionstrengths are also reported. All data are in Kelvin.
-Vila
TExpb
TI2
Ne
35
16
15
Ar
120
50
51
Kr
170
85
71
)ce
236
118
102
@2
54
CH4
177
25 75
23 75
2850 2360 2910 3430 2900
170 980 130 400 1170
120 930 140 350 1140
CLI ps Au Ni
w
a Taken from refs. 4 and 5 for rare gases and methane, from refs. 6 and 7 for oxygen and from
ref. 8 for metals. Taken from ref. 4 for Ne, Xe and methane, from ref. 9 for Ar, from refs. 10-12 for Kr, from ref. 13 for 0, , from ref. 8 for Cu, Ag, Ni, Pd, and from ref. 14 for Au.
In the MQH model the dependence of T, on V,,
is essentially linear with a slope that depends
smoothly on the coverage (in particular, in the range of coverages between 20% and To%, the
slope varies between 0.37 and 0.44). As it can be seen from Table 1, with the exception of Kr and Xe, the agreement between the predictionsfrom the MQH model and the experimental values, is very good, at least as far as the systems on graphite are concerned. In order to understand why the model gives such excellent predictions for Ne, Ar, 0 2 and CH4 but not for Kr and Xe on graphite, we must first notice that, being a lattice-gas model, it cannot properly describe adsorbates in a commensurate phase, in which the adsorbate-substrate interactions dominate. In an incommensurate phase, on the contrary, the interactens among the adparticles dominate on the substrate influence, a condition well described by the MQH model.* A careful analysis of the phase diagrams for the systems we have been considering, shall therefore explain some of the features of our results.
Adsorbed systems whose two-dimensionalspace symmetry results from the two-dimensional space symmetry of the substrate surface by adding or subtracting symmetry elements, are named commensurate, while those adsorbed structures whose symmetry is not related to that of the substrate are named incomrnensurale.
89
In the temperature range 1-20 K, neon submonolayers on graphite melt at 13.5 K (ref. 15), therefore the anomaly at 16 K cannot involve any solid commensurate phase (the barely stable (47 X d7)R19" structure exists only at temperatures below 4 K (ref. 16)). Argon was found
incommensurate at all conditions studied (refs. 17-18) and also 0,does not show any evidence of a commensurate phase (ref. 19). The phase diagram of two-dimensional methane adsorbed on graphite shows a temperature-trigged commensurate-incommmensuratetransition at 47 K and at coverages below 0.8 monolayers (refs. 20-21), and therefore the anomaly at 75 K, reported in Table 1, occurs in a region where the adsorbate-adsorbate interactions play a major role. Kripton on graphite prefers a commensurate structure (ref. 22) and the MQH model would not be suitable to it (a commensurate-incommensuratetransition in the range 70-130 K does occur, but only at coverages greater than unity (ref. 23)). Xenon is expected to be highly mobile because, when the (1 111 plane is isolatedfrom the bulk, its lattice constant, that in the bulk is already larger than the lattice constant of the 43 structure (which is, for graphite, equal to 4.26 A), becomes even larger, and the incommensuratephase is generally more stable; however, xenon has already shown, with its unusually high value of the ratio Tc(2D)/ Tc(3D), a peculiar behavior that has suggested that some special mechanism of localization causes the formation of a structure which is in registry with the solid substrate (ref. 11). In Table 1 we have also reportedresults from calculations for systems in a range of much stronger lateral interaction energies (200-300 mew. As we can see, the results are, in absolute value, not as satisfactory as for Ne. Ar, 0,and CH, on graphite; nevertheless, the relative error is sometimes very small (less than 1% for Au on tungsten) and, moreover, the accuracy with which TExp is measured in these systems, is not as good as in the systems adsorbed on graphite. It is important to stress the fact that we cannot ascribe the discrepancies to a bad estimate of the adatom-adatom interaction strengths that, for the systems on tungsten has not indeed been determined as accurately as for the systems on graphite. On the contrary, we expect that V,
,, as given in Table
less satisfactory result for T.,
1, is an upper bound to the real value, thereby giving a
Again, the reason for these discrepancies has to be found in the
structuralpropertiesof these metal layers on tungsten surfaces, being the lateral periodicity of the adsorbates very close, if not identical, to that of the substrate (ref. 24), whose effects on the Ts,'
are not taken into account by the MQH model.
The interestingaspect of the MQH model is that it does not allow for any first-order transition. Yet, when applied to "realistic" systems, it predicts maxima, in the heatcapacity signatures, at temperatures often very close to the experimental ones. A simple model that forbids a first-ordertransitionbut displays maxima in the heatcapacity curve is the one-dimensional
99
Ising lattice with nearest-neighbor interactions; unfortunately, this model predicts those maxima at temperature values well below the experimental data. In conclusion, the MQH model is very appealing for describing heat capacity anomalies of adsorbate-substratesystems, provided (i) no s o i i commensurate phase is involved in the region around T,
(ii) the pairwise interaction V,, among nearest-neighbor adparticles is the
parameter that plays the major role in the transition process, with only lower-order effects due to the substrate, and (iii) the transition is not of first order. These conditions are not very restrictive and, moreover, they are not independent because very often the first one implies the second, and in many instances also the third one, since several transitions from or into commensurate phases have been predictedto be of first order (ref. 25).
1 2 3 4
5 6 7 8 9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
R. 8. McQuistan and J. L. Hock, J. Math. Phys., 25 (1984) 261. F. Battaglia. Y. S. Kim and T.F. George, J. Phys. Chem., 91 (1985) 414. Y. S. Kim, F. Battaglia and T.F. George, J. Chem. Phys., 88 (1988) 7066. J. R. Klein and M. W. Cole, Faraday Discuss. Chem. Soc., 80 (1985) 71. S. Rauber, J. R. Klein and M. W. Cole, Phys. Rev. B 27 (1983) 1314. R. D. Etters, Ru-Pin Pan and V. Chandrasekharan, Phys. Rev. Lett., 45 (1980) 645. C. A. English and J. A. Venables, Proc. R. Soc. London A, 340 (1974) 57. J. Kolaczkiewicz and E. Bauer, Surf. Sci., 151 (1985) 333. T. T. Chung. Surf. Sci., 87 (1979) 348. A. Thomy and X. Duval, Carbon, 13 (1974) 242. Y. Larher, J. Chem. Soc. Faraday Trans. I 7 0 (1974) 320. Y Larher and B. Gilquin, Phys. Rev. A 20 (1979) 1599. R. Maw, Phys. Rep. 125 (1985) 1. J. Kolaczkiewicz and E. Bauer, Phys. Rev. Lett., 53 (1984) 485. G. B. Huff and J. G. Dash, J. Low Temp. Phys., 24 (1976) 155. H. Wecherl, C. Tiby and H. J. Lauter, Physica B, 108 (1981) 785. H. Taub, K. Cameiro, J. K. Kjems, L. Passal and J. P. McTague, Phys. Rev. B 16 (1977) 4551. H. Taub, L. Passal, J. K. Kjems, K. Carneiro, J. P. McTaQueand J. G. Dash, Phys. Rev. Lett., 34 (1975) 654. M. Nielsen and J. P. McTague, Phys. Rev. B, 19 (1979) 3096. A. Maw and E. F. Wassermann, Surf. Sci., 117 (1982) 267. R. Man, Z. Phys. B,46 (1982) 237. C. Tessier and Y. Larher, in S. K. Sinha (Editor), Ordering in Two Dimensions, North-Holland, Amsterdam, 1980, p. 163. A. Thomy, X. Duval and J. Regnier, Surf. Sd. Rep., 1 (1981) 1. E.Schlenk and E. Bauer, Surf. Sd., 93 (1980) 9. E. Domany, M. Sdii and J. S. Walker, Phys. Rev. Lett., 38 (1977) 1148.
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in T h e Netherlands
91
SURFACE CHEMISTRY OF CARBONACEOUS SPECIES
ALEXIS T. BELL Center for Advanced Materials, Lawrence Berkeley Laboratory, and Department of Chemical Engineering, University of California, Berkeley, CA 94720, U.S.A.
ABSTRACT The interactions of hydrocarbons and carbon monoxide with metal surfaces leads to the deposition of various carbonaceous species. The structure and composition of these species can be ascertained using modem spectroscopic techniques. Insights into the thermochemistry of forming carbonceous species on metals can be obtained through an application of the bond-orderconservatiodh4orse-potentialapproach. INTRODUCI'ION Carbonaceous species present on the surface of metal catalysts can act either as intermediates in the processing of carbon-containing feeds or as precursors to the deposition of coke and graphite. These latter forms of carbon can cause catalyst deactivation by blockage of metal sites, encapsulation of metal crystallites, and occlusion of the pores in the catalyst support. Because of this dual role, it is desirable to know how feed and catalyst composition, as well as temperature, influence the chemistry of carbonaceous species. The current knowledge of coke formation and the surface chemistry of carbonaceous species have been reviewed in references (refs. 1-8). This paper will summarize what is known about the structure and thermochemistryof carbon and partially hydrogenated carbon species present on the surfaceof metals.
THE CHEMSTRY OF CARBON AND COKE DEPOSITION Figure 1 illustrates the principal pathways for forming carbon-containing fragments starting from carbn monoxide and hydrocarbons. Dissociation of carbon monoxide produces elemental carbon, Ca(a). This species can polymerize to form amorphous or graphitic carbons, Cp(a) and C,(a), respectively, dissolve into the bulk of the metal or form a metal carbide, Cy(s). The dissolved carbon can reprecipitate and form whiskerlike, or vermicular, carbon, Cv. The decomposition of hydrocarbons is more complex than that of carbon monoxide and leads to a variety of partially hydrogenated species, CHx and CnHx,as well as &(a). The hydrogencontaining fragments can loose hydrogen to produce G f a ) , polymerize to produce coke or add hydrogen to produce gaseous hydrocarbons. It should be noted that coke can also be formed during the hydrogenation of CO, since hydrogen addition to C,(a) forms the necessary precursors.
92
C in N i (Carbon in solid soln.)
E-
Cr(s)
C,(a)-
iH(a)
C,(Vermicular
carbon)
(Metal carbide)
Cg(s) - C c ( s )
4H(a)
-
(Amorphous and graphitic carbons)
CH4(g)
H2(a)--H2(g)
+14 - x)H(a)
CH4(g)
8' Cc + H2(g)
Condensed high mol. wt. HC(a) --c Ca, C (Coke)
+ ... + Y
C H 2
(Car b o d
C,Hz
Fig. 1 Formation, gasification, and transformation of coke and carbons on metal surfaces from carbon monoxide and hydrocarbons (a, g, s refer to adsorbed, gaseous, and solid states, repectively).
93
The reactivity of carbonaceous species with respect to hydrogen decreases progressing from &(a) to graphite. An idea of the reactivity of the different forms of carbon can be obtained from temperature-programmedsurface reaction (TPSR)experiments. McCarty and Wise (ref. 9) have used this technique to charaterize the carbonaceous species produced on NiAh03 by adsorption of ethylene. The population of TPSR states they observed at different ethylene exposure temperaturesis given in Table 1. Table 1
-
Population of TPSR states for carbon deposited on an alumina-supported nickel catalyst following ethyleneexposure C2H4 exposure
State
a'
a Y
P 6' 6
a Peak
Identification Chemisorbed carbon Chemisorbed carbon Nickel carbide b-Carbon film Filamentous carbon (TEM,SEM) Encapsulating carbon (TEM)
CH4/H2 TPSR TP(K)%
573
773
873
x
x
x
X
480 +/- 25
X
X
X
X
550b 660 +I- 30b 875 +/- 20
X
410 +/- 15
960 +I-15
X
1073 1273
X
x
x X
x
x
temperature (Tp)for heating rate = 0.9 KJs. The TPSR product is CH4, the reactant is H2 increasing coverage; Tp for low coverage limit.
b Increases witrh
CARBIDIC AND GRAPHITIC CARBON The two modifications of carbon devoid of hydrogen that have been identified on metal surfaces are carbidic and graphitic carbon. Carbidic carbon consists of isolated carbon atoms, whereas graphitic carbon consists of one or more sheets of carbon in which the carbon atoms exhibit sp2 coordination. Studies of carbon adsorption on a Ni(100) surface using LEED have been reported by several authors (refs. 10-13). A (2 x 2) surface structure is observed, independent of whether the surface carbon is obtained by CO disproportination, C2H4 decomposition, or segregation from the bulk. Initial attempts to interpret the structure using dynamical LEED theory (ref. 12) were not successful, but did show that the substratelayer expansion was an important ingredient. Onuferko et al. (ref. 13) fmt realized the existence of certain characteristicmissing spots in the LEED pattern attributable to glide symmetry lines in the surface structure. Through detailed comparisons of L E D intensity data with dynamical calculations they were able to show that the top layer nickel atoms are displaced 0.35 +/- 0.05 A parallel to the surface, 0.20 +/- 0.04 A outward from the surface. and that the carbon atoms are in fourfold hollows at a spacing of 0.1 +/- 0.1 A from the
94
Fig. 2 Distortion of the top layer of nickel atoms (full large circles) producing p4g symmetry. Dashed circles represent the undistorted top nickel layer atom. The small circles represent carbon atoms in 4-fold hollows. (From ref. 13).
(b)
& 100 200 300 400
Single Crystal Graphite
100 200 300 400
(d)
Nickel Carbide
-200
250 300 350 Energy ( e v )
200 250 300 350 Energy (eV)
Fig. 3 Comparison of AES carbon Signals on Ni(100) crystal with those from single-crystal graphite and nickel carbide: (a) following IoOOs of heating at 700 K in 24 torr C O (b) single-crytal graphite; (c) following lo00 s heating at 600 K in 24 torr CO; (d) nickel carbide. (From ref. 15).
95
surface. An illustration of the distortion in the top layer of nickel atoms and the location of the carbon atoms is given in Fig. 2. Confiation of the surface structure of carbon an a Ni(100) surface determined by Onuferko et al. (ref. 13) has been obtained more recently by Riedler and Wilsch (ref. 14) using He diffraction. Auger electron spectroscopy (AES) provides an effective means for differentiatingbetween carbidic and graphitic carbon. Goodman et al. (ref. 15) and Bonzel and Krebs (ref. 16) have demonstrated that the AES signatures for the two modifications of carbon differ substantially. Figure 3 compares the AES carbon signals on a Ni(100) surface with those from single-crystal graphite and nickel carbide. Carbon deposition on the nickel surface was achieved by CO disproportionation. It is evident from Fig. 3 that CO disproportionation at 600 K produces a C AES signature comparable to that of nickel carbide, whereas disproportionation at 700 K produces an AES signature similar to that of graphite. Goodman et al. (ref. 15) have demonstrated further that AES can be used to differentiate between carbidic and graphitic carbon deposited during CO hydrogenation, and to quantify the amounts of carbon deposited. Similar applicationsof AES have been made by Bonzel and Krebs (ref. 16) in their studies of CO hydrogenation over an Fe(100) surface. Caracciolo and Schmidt (refs. 17,18) have used AES to study the effects of crystallographic variations on carbon deposition on nickel upon exposure to CO, a mixture of CO and H2,and CzH4. As can be seen in Fig. 4, exposure to pure CO at 600 K for 10 min produces up to one-half monolayer of carbon on the (100) and (1 11) planes and approximately a monolayer on the (1 10) and high index planes. Auger spectra show that the carbon is carbidic on the (100) and (111) planes and a mixture of carbidic and graphitic on all other planes. Figure 5 shows that the carbon produced from a CO/Hz mixture has a very different variation with surface crystallographic orientation. The' carbon coverage is significantly lower along the [ 1111-[1lo] zone, although the (100) and (111) planes within 200 of (100) showed only slightly less carbon than in the case of pure CO. The carbon deposited from C& is quite different from that from CO disproportionation,both in absolute quantity of carbon and variation with orientation. In ethylene maximum carbon build up occurs on (100) and (1 11) while minima occur from CO (see Fig.6). With ethylene decomposition, an epitaxial carbon overlayer is formed on these planes with its own crystal structure. When the accumulationsof carbon on the metal surface become very large, the carbon can be observed directly by transmission electron microscopy (TEM). An example of the structure of vermicular carbon produced by the decomposition of acetylene on the surface of nickel particles is shown in Fig. 7 (ref. 5). A nickel particle is evident at the head of each filament. The surface of the particle is largely encased in the growing filament. High-resolution micrographs reveal the presence of a hollow coaxial channel within the filament and a graphitic outer sheath. When the graphitic sheath completely encapsulates the the nickel particle, growth of the filament ceases, since hydrocarbon adsorption onto the particle surface is precluded.
96
iiool
i ns
(3311
ciior
OR I ENTAT ION
Fig. 4 Carbon coverage versus orientation after exposure to pure CO at 600K and 0.6 ton for (a) > 10 min and (b) 3 min. The inset shows a sketch of the curved Ni crystal with orientations and dimensions indicated. (Fromref. 17).
0.6
-
0.5
-
0.4
-
0.3
0.2
-
0.1
-
Fig. 5 Carbon coverage versus orientation after exposure at 600 K to (a) 0.6 torr for 10 min, and (b) 2 tom 1/3 m ixm ofCO/H2 for 1Omin. Surface in (b) afterreaction with 2 torrH2 fop 10 min is shown as curve (c). Two sets of data under similar conditions are shown for curve (b). (From ref. 17).
97
ORIENTATION
Fig. 6 Comparison of carbon versus orientation for deposition from CzH4 and CO. (From ref. 17).
Fig. 7 TEM micrographof vermicular carbon Ni. @om ref. 5).
YY
CxHy SPECIES The interaction of unsaturated hydrocarbon molecules with a variety of low Miller-index
single-crystal metal surfaces has been investigated by means of vibrational and NMR spectroscopies. These studies provide information about the initial bonding and ordering of the adsorbate and its subsequent rearrangement and/or decomposition upon thermal many of the observed species can lead to the formation of coke or graphitic carbon, the results of these investigations are relevant to the present discussion. Steininger an co-workers (ref. 19) have used electron energy loss spectroscopy (EELS) to chwdcterize the the surface reactions of C2H4 with Pt( 11 1) surface. An EELS spectrum for C2H4 adsorbed at 92 K is illustrated in Fig. 8. Comparison of the peak positions with those observed in infrared spectra of compounds with well-defined structures leads to the conclusion that C2H4 is essentially di-a-bonded. This phase of ethylene is stable to about 220 K. Above this temperature, the spectrum changes progressively from that shown in Fig. 8 to that shown in Fig. 9. The new spectrum is attributable to ethylidyne on the basis of infrared data for organometallic complexes. The proposed assignment has been confirmed by LEED intensity analysis carried out by Kesmodel et al. (ref. 20).
In addition to Pt( 1 1 l), di-o-adsorbed C2H4 has also been observed at low temperatures on Pt(100), Rh(lll), Pd(100), Ru(001), Ni(llf), Ni(llO), Fe(I11), and Fe(ll0) (refs. 19,21-27). EELS observations suggest two distinct pathways for this species: (a) On Pt(l1 l), Pt(100), Rh( 11I), and Pd(ll1) C2H4 converts at about 300 K to an ethylidyne species which subsequently decomposes at higher temperatures (refs. 19,28-34); (b) On Ni( 11l), Ni(1 lo), and Fe(l10) CiH4 dehydrogenates to form acetylene, which subsequently decomposes (refs. 2 1,23,24). Both decomposition processes occur in parallel on Ru(001). For each of the metal surfaces cited, EELS has been used to identify the species formed upon decomposition of the adsorbed ethylene. Acetylene adsorption has been studied with EELS on a wide variety of metal surfaces (refs. 22,24,33,35-44). Below 200 K acetylene is x/di-a-adsorbed and the carbon-carbon bond is reduced to approximately 1.5. The rehybridizationof the carbon atoms also affects the bonding of the hydrogen atoms, and in adsorbed acetylene, the hydrogen atoms are located cis- and trans- to the carbon-carbon bond. The mechanism by which molecularly adsorbed acetylene decomposes is a function of the metal composition. Direct cleavage of the carbon-carbon bond to form CH groups has been proposed to occur on Ni( 111) (ref. 24) and Fe( 110) (ref. 40). On Pt( 1 1If, formation of a CCH2 species via hydrogen transfer has been proposed as the initial step in acetylene decomposition (ref. 39). while on Pd( 100), hydrogen abstraction results in the formation of a CCH species (ref. 39). A more complex behavior is observed for Pd(ll1) and
Ru(001) surfaces. Here, molecularly adsorbed acetylene decomposes to form both CCH and CCH3 species (refs. 37,41). These species then decomposeto carbon via CH intermediates. Ultraviolet photoelectron spectroscopy (UPS) and metastable noble gas de-excitation spectmscopy (MDS) can provide additional information regarding the interactionsof unsaturated
99
Energy Loss (cm-'1
Fig. 8 In- and off-specular EELS spectrum (0, = 0; - 0,) of a saturation coverage of ethylene. (From ref. 19).
c ._ "7
*
-
0
1000
2000 3000 Energy Loss (cm-l)
Fig.9 In- and off-specular EELS spectrum (8,= 0i - 0,) after annealing a layer of ethylene, or after adsorption at room temperature. (From ref. 19).
100
hydrocarbons with metal surfaces. For example, Sesselman et al. (ref. 45) have shown that at 140 K and small exposures, acetylene adsorbs on Pd( 111) to produce x/di-o-bonded species. Higher exposures at 140 K lead to the formation of benzene. However, if the molecularly adsorbed acetylene is annealed to 300K, it hansforms irreversibly to a species believed to be CCH3. Similar observations have also been reported by Tysoe et al. (ref. 46). Recently, Wang and co-workers (refs. 47-49) have applied 13c Nh4R spectroscopy to characterizethe structure of acetylene and ethylene adsorbed at room temperatureon Pt/Alz03. By probing the 13c-13C, 13C-1H, and 1H-IH dipolar interactions among the nuclei in the adsorbed molecule, it was established that 77 +/- 7 % of the adsorbed acetylene is present as CCH2 and 23 +/- 7 96 as HCCH. Using a similar approach, is was established that all of the adsorbed ethylene is present as CCH3. When the adsorption temperature was raised to 690 K, C-C bond scission occurred and the predominant species observed were isolated carbon atoms. The carbon atoms were found to be very mobile on the Pt crystallite surfaces, and the activation energy for translational motion was estimated as 7 +/-1 kcal/mol. 13C N M R spectroscopy has also been used to characterizethe carbonaceous species present on the catalyst during CO hydrogenation. Duncan et al. (refs. 50-52) have found that the surface of Ru/Si& and bulk Ru catalysts are covered primarily by a mixture of carbidic carbon and alkyl chains. A small amount of graphitic carbon can also be observed after the other forms have been reduced by hydrogenation. Figure 10 shows a spectrum taken following exposure of Ru/Si& to WO/Hz for 5 min, and subsequent displacement of adsorbed 13CO by exposure of the catalyst to 12CO/Hz for 30 s. The broad peak at -400 ppm is attributed to carbidic carbon, whereas the narrow peak at -1 5 ppm is assigned to carbon in alkyl chains. Magic-angle spinning of the sample allows the latter feature to be resolved into five separate lines each of which can be attributed to a specific type of CHz or CH3 group as shown in Fig. 11 and Table 2. Isotopic tracer studies have confirmed that the carbidic carbon is a primary intermediate in the synthesis of CH4 and higher molecular weight hydrocarbons, whereas the other carbonaceous species are believed to contribute to catalyst deactivation. Table 2 13C N M R spectral assignments
Peaka (a) (b) (C)
(4 (el a
Center of massb
Relative
Halfwidth
Area
(Hz)
35.1+/-0.8 28.3+/-0.2 21.1 +/-0.1 18.4+/-0.1 11.7+/-0.1
0.28+/-0.06 0.43+/-0.03 om+/-0.01 O.OS+/-0.01 0.14+/-0.01
See Fig. 11 b In ppm relative to TMS.
300 90 60 65 75
Assignment Ru-C*H~-CH~Ru-CHZ-(C*HZ)-CHZ-CH~ Ru-CHZ-(CHZ)-C*H~-CH~ Ru-CH~-C*HZ-CH~ (?) Ru-(CHZ)-C*H~
101
CONVERTED; H2
Fig. 10 13c N M R spectra of a Ru/Si@ catalyst quenched after 5 min of CO hydrogenation at 463 K (Pm= 360 m,P co = 200 ton): (a) after exchange of adsorbed 13CO with 12CO and quenching the reaction; (b) after aging the freshly deposited carbonaceous species in He at 463 K for 120 S. (From ref. 51).
60
50
40
30
20
10
0
Frequency, in ppm, Relotive to TMS
Fig. 11 13c MAS-NMR spectrum of a ru/SiOz catalyst quenched after 5 min of CO hydrogenation. Dashed lines indicate a least square fit of a superposition of five Lorentzian peaks. The characteristicsof the peaks and their assignment are given in Table 2. (From ref. 52).
102
THERMOCHEMISTRY OF CARBONACEOUS SPECIES The thermochemishy of the interactions of CHx (x = 1-4)and C2Hx (x = 1-6) species with transition metal surfaces has recently been examined by Shustorovich and Bell (refs. 53,54) using the bond-order-conservationMorse-potential (BOC-MP) approach. In particular they have calculated the heat of adsorption for CHxand Cfl, species and the effects of metal composition on the activation energy for C-H and C-C bond cleavage. The BOC-MP approach has also been used to establish the influence of metal composition on the energy profiles for ethane, ethylene, and acetylene decomposition. Table 3 lists the calculated heat of adsorption for CHxspecies and the activation energies for the reversible hydrogenation/dehydrogenation of these species. The heat of adsorption for methane is predicted to be 9-10 kcal/mol for Ni(l1 I), Pd(ll1). and Pt( 11l), which is in reasonably good agreement with the experimentally observed value of 6 kcal/mol for polycrystalline Rh (ref. 55). There is also good agreement between the predicted and observed activation energy for the dissociative adsorption of methane on Ni(ll1). The BOC-MP method predicts an activation energy of 22 kcal/mol, whereas recent molecular beam studies (ref. 56) suggest that the activation energy is > 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.
108
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.
109
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.
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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
-
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).
-
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
\<
x
--<
1) has now been s t u d i e d as a f u n c t i o n of the frequency
and a t various temperatures. The c a t a l y t i c p r o o e r t i e s o f these compounds are h i g h l y m o d i f i e d by o x i d a t i o n . For t h i s reason i t seemed i n t e r e s t i n g t o study this effect i n detail. The experimental data have been i n t e r p r e t e d by means o f a hopping conduction model, using the n o t i o n o f l o c a l i z e d s t a t e s ( r e f s . 5 - 8 ) . The frequency dependent c o n d u c t i v i t y uac, can be w r i t t e n i n the f o l l o w i n g form ( r e f s . 5-9) :
where Udc i s t h e c o n d u c t i v i t y i n d i r e c t c u r r e n t ; ~ ' ( w ) the r e a l p a r t o f t h e p o l a r i z a t i o n c o n d u c t i v i t y i s r e l a t e d t o the frequency by expression ( 2 ) :
= A ( T ) ~ The experimental values have been t r e a t e d by u s i n g a numerical method r e c e n t l y developed ( r e f . l o ) , which d i f f e r s s i g n i f i c a n t l y from the a n a l y t i c a l methods ( r e f s . 5 - 9 ) . It appears t h a t f o r the amorphous molybdenum sulphides and i t s o x i d i z e d compounds , t h e conduction phenomenon can be explained by t h e r m a l l y activated
hopping model.
112
EXPERIMENTAL Preparation of MoS2
.+
The preparation of amorphous molybdenum sulphides r e s u l t s from the thermal decomposition of ammonium thiomolybdate (NH4)2 KoS4 under i n e r t atmosphere ( r e f s . 11-14). The l a t t e r has been prepared from pure commercial products by the usual methods ( r e f s . 15,16). A thermal treatment a t 195°C f o r 48 hours results in a finely powdered compound with a stoichiometry close t o MoS3. The treatment a t higher temperatures leads to a different stoichiometry, MoS2 + X . The quality of the ammonium thiomolybdate i s essential f o r the preparation of highly pure sulphides, f r e e from any oxide impurities. Due t o the hydrophilic character of the oxides ( r e f . 17), a d i f f e r e n t behaviour of the sulphides, with respect t o their conduction properties, can be induced. The elemental analysis f o r each compound indicates the t o t a l absence of oxygen. In table 1 the treatment temperatures f o r different compounds are l i s t e d . TABLE 1 Stoichiometry of the sulphides as a function of the treatment temperatures.
TT( " C )
195
235
285
315
350
440 -
Compound
MoS3
MoS2 .74
MoS2 .64
MoS2 .60
MoS2.25
"OS2
Preparation of an oxidized sample The oxidized sulphide i s formed a t 120°C in an oxygen flow f o r about 4 hours, s t a r t i n g from MoS3. The elemental analysis ( 0 :12.9, S : 35.8, Mo : 51.3 ( w g h t % 1 ) indicates the existence o f a non-stoichiometric compound. Conductivity measurements Measurements have been performed on powdered samples compressed a t about 6 k b a r s . The measuring method and assembling have been widely described ( r e f s . 2 , 3 ) . The explored frequency domain varies from 5 Hz to 13 MHz in the temperature range from 100 K t o 400 K . The reproducibility i s limited t o an e r r o r of about 3.5 X . T a k i n g into account the compacity of the material ( 0 . 9 in our case), allows t o obtain the values o f the real conductivity by correcting measured values ( r e f . 3 ) . EXPERIMENTAL RESULTS Sulphides MoS2 +
113
I n f i g . 1, the e v o l u t i o n o f the ac c o n d u c t i v i t y as a f u n c t i o n o f t h e ~ ~c ~l a.r i t y o n l y a few frequency i s reported f o r MoS2-25 and E ~ I o S ~For
representative graphs are shown. The dc
c o n d u c t i v i t y o f these compounds
appears t o be r e l a t i v e l y high i n comparison t o o t h e r types o f chalcogenides ( r e f . 5). -2
MoS2.14
-
255 K
E-3
Y
a*
142 K
*a*
*** :*
aaaaa.*aaa
a* a*
b" 0, 0-4
a a
d
1 0 9 Kaaa**
o -4
a**
a".aaaa.a.*
I . . . . . k
2,25
:
a a*
aaaa.a.aaaaaaa.a..**
U
3
MoS 292 K
7
E
[
5
6
1
-L 3
l o g f (Hzl
4
log f
5
6
7
(Hz)
Fig. 1. Evolution o f the r e a l p a r t o f the c o n d u c t i v i t y as a f u n c t i o n o f the frequency f o r MoS2.25 and MoS2 ~ 4 .
Fig. 2 shows f o r the s i x studied samples, the temperature dependence o f the dc conductivity
.
a
'0.
e-
Q a
*a
40
'
60
80
40
60
T-' 1 O4 ( K') Fig. 2. Temperature dependence o f the dc c o n d u c t i v i t y f o r FloS2
+
x.
80
114
It c l e a r l y shows up t h a t the high temperature behaviour o f Udc follows the
experimental law : BE r)
u = o0 exp(-
(3)
Such high values o f adc make i t impossible t o i n t e r p r e t the p o l a r i z a t i o n conductivity, which i s not observed w i t h a s u f f i c i e n t precision i n the explored frequency range. A t intermediate and low ?mpratures. where the p o l a r i z a t i o n conductivity i s more important. an Arrhenius type law i s no longer v a l i d . This has already been observed i n previous works ( r e f . 18). Nevertheless the s u f f i c i e n t l y high values o f the p o l a r i z a t i o n conductivity allow t o i n t e r p r e t the observed phenomenon. Oxidized sulphide Fig. 3 represents the evolution o f the frequency dependent conductivity, f o r the oxidized compound. The composition o f which has already been indicated (see EXPERIMENTAL).
-1
Y
Y
b"
b"
OI 4 0 d
-5
3
L
5
log f (Hz)
6
7
3
4
log f
5
6
7
(Hz)
Fig. 3. Evolution o f the ac conductivity f o r the oxidized compound.
It becomes c l e a r t h a t the dc conductivity values are i n f e r i o r compared t o those o f the pure compounds. This may be a t t r i b u t e d t o the low conductivity o f the pure molybdenum oxides. The temperature dependence o f the dc conductivity o f the oxidized sulphide i s i l l u s t r a t e d i n Fig. 4. I n t h i s case. the values o f the p o l a r i z a t i o n conductivity remain s u f f i c i e n t l y hiah and can therefore be i n t e r preted even a t elevated temperatures.
115
X Y X
x
*
x
X X
30
40
50
X
60
T - ’ I o ~ ( K’) Fig. 4. Temperature dependence of the dc c o n d u c t i v i t y f o r o x i d i z e d sulphide. DATA ANALYSIS
Using eqn. 1 we determined the values o f 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 u ’ ( u ) f o r a l l samples. I n these t h e o r e t i c a l models the fundamental hypothesis
consists o f assuming t h a t the t r a n s p o r t o f the c u r r e n t i s due t o charge c a r r i e r hops between l o c a l i z e d s i t e s . We o n l y consider the case o f monoelectronic hops ( r e f s . 2 , 4, 10, 19, 20). The expression o f the r e a l p a r t o f the c o n d u c t i v i t y i s therefore ( r e f s . 21-23) :
J
RO
N i s the number o f s i t e s per u n i t volume ; a(R) =
e2R2 12k~ is
the p o l a r i z a -
b i l i t y ; p(R) i s the p r o b a b i l i t y of f i n d i n g two s i t e s a t a distance R between R - - dR and R t dR ; T i s t h e r e l a x a t i o n time associated w i t h the hops. 2 The r e l a t i o n between T and R depends on the k i n d o f mechanism whereby the charge c a r r i e r s hop over the p o t e n t i a l b a r r i e r . The r e l a t i o n allows the numerical c a l c u l a t i o n o f i n t e g r a l 4 ( r e f . l o ) , t a k i n g
T
as a v a r i a b l e .
The c o r r e l a t e d B a r r i e r Hopping (CBH) model developed by E l l i o t t ( r e f . 23) i s based on three hypotheses :
-
the e l e c t r o n i c hops being thermally activated, the r e l a x a t i o n time
r e l a t e d t o the hopping mechanism takes t h e form ( r e f . 24) ’: T
= T~ exp (-
W
RT
)
(5)
116
where
T~
i s the inverse o f the v i b r a t i o n a l frequency o f the p a r t i c l e traoped
i n i t s site.
-
i n the case o f monoelectronic hops, the h e i g h t o f the p o t e n t i a l b a r r i e r
k i s r e l a t e d t o the distance R by eqn. 6 ( r e f s . 8,25)
khere
$,
i s the energy r e q u i r e d t o move an e l e c t r o n from i t s s i t e t o i n f i n i t y
( r e f . 2 3 ) , and
-
:
i s the s t a t i c d i e l e c t r i c constant o f the s o l i d .
the p r o b a b i l i t y
p(R), f o r an i s o t r o p i c medium (amorphous compounds) i s :
p(R) dR = 4nR2dR
(7)
These hypotheses l e a d t o the expression :
This k i n d o f data treatment r e s u l t s i n f i n d i n g u ' ( w ) i n the form .I(.)
= A(T)w'(~),
:
where the expression o f parameter s i s ( r e f s . 5-8) :
6kT
s = l -
WW + kT I n w
(9) lo
Fig. 5 (few representative curves) shows t h a t t h e numerical c a l c u l a t i o n o f eqn. 8 r e f l e c t s a s a t i s f y i n g i n t e r p r e t a t i o n o f the experimental data obtained on FbS2 +X. Therefore, the c h a r a c t e r i s t i c parameters N and GIw have been determined and represented g r a p h i c a l l y (Fig. 6 ) . I t may be n o t i c e d t h a t N and WM decrease w i t h d i m i n i s h i n g values o f x. This
has t o be c o r r e l a t e d t o the progressive microscopic o r g a n i z a t i o n from MoS3 (amorphous) t o MoS2 ( p a r t i a l l y organized). The temperature dependent paraneter s has been determined f o r a l l samples. T ~1 -sas a f u n c t i o n o f T f i t s c o r r e c t l y eqn. 9 proposed
The v a r i a t i o n s of
by Long ( r e f . 8). Two r e p r e s e n t a t i v e examples, E . I o S ~ . and ~~ respectively,are reported i n F i g . 7. I t i s i n t e r e s t i n g t o p o i n t o u t t h a t , i n a l l cases, the g r a p h i c a l l y e x t r a p o l a t e d values o f b$, c a l c u l a t e d from eqn. 8 .
are i d e n t i c a l t o those
117
-2
m 0
-4
1
-
Calculated curve
xxxx
Experimental curve
J .
.
d
1
2
3
4
5
6
1
log f (Hz) Fig. 5. Comparison o f the experimental and theoretical results.
-a .,-
- - -*
I
I
l
a
-
I
i
1 I
i
>
r
I I f
I I
I #
I
_-
/
0
4.
1I 0
X
0
L X
1
Fig. 6. Variation of the number of s i t e s N(a) and of the potential energy WM(b) as a function of x ( M O S ~ +compound). ~ For the oxidized compound, the solution of eqn. 8 i s again satisfying ; the corresponding graphs for some significant temperatures are given in Fig. 8. For this compound the values o f WM and N are rather different according t o the explored temperature range (see TABLE 2 ) .
118
2 000
7 50
-
-
v)
-
Y
Y
I
-
In
I
' = t
I
+-
1750
500 100
T
lK1
200
Fig. 7 . Representation of T/l-s law for I I o S ~ and . ~ ~M O S ~ . , ~ .
T (K1
150
as a function of T, according t o Long's
-2
-Calculated xxxx
- Calculated -2
xxrX
250
curve
ExDerimental curve
curve
Experimental curve
I
3
4
5
log f ( H z )
6
1
I
3
4
5
6
1
log f (Hz)
Fig. 8 . Representation of the experimental d a t a compared t o the theoretical values of log uac as a function of the frequency, f o r the oxidized compound.
I t appears that the variation of 1 - s as a function of the temperature i s perfectly 1 inear and r e f l e c t s correctly,, in b o t h temperature ranges, the evolution of the parameter s , given by eqn. 9 ; t h i s i s i l l u s t r a t e d in Fig. 9 .
119
TABLE 2
N and WM i n two temperature domains ( o x i d i z e d compound).
Values o f parameters
150
N (sites/cm3)
-
250 K
300
-
400 K
6.55 10’’
5.01
42 50
2 000
1 7 50
4000 150
T
(K)
250
30 0
T (Kl
400
F i g . 9. V e r i f i c a t i o n o f L o n g ’ s l a w (eqn. 9) f o r t h e o x i d i z e d sample. The e x t r a p o l a t i o n o f t h e graphs t o T = OK c o n f i r m s t h e c a l c u l a t e d values o f WM ; W,4 = 1,3 eV i n t h e l o w temperature range and Wr, = 2,7 eV i n t h e h i g h temperature range.
DISCUSSION AND CONCLUSION T h i s s t u d y c l e a r l y p o i n t s o u t t h a t t h e c o n d u c t i o n b e h a v i o u r i n amorphous molybdenum s u l p h i d e s and one o f i t s o x i d i z e d compounds i s an e x c e l l e n t example f o r t h e a p p l i c a t i o n o f t h e CBH model. F o r t h e amorphous MoS2
+
compounds i t shows up t h a t t h e parameters \IM
and N a r e d i f f e r e n t , depending on t h e values f o r x. While 1 WM and
wM
N are constant
: Wr+, = 1.3 eV ; N
= 1.2 eV ; N = 3.7 x
i
>
x
>
6.3 x 1 0 l 8 ~ m - ~F o. r x = 0.6
cmm3, and f i n a l l y , f o r x = 0.25 and x =O
0.64,
,
120
r e s p e c t i v e l y , LJM = 0.7 eV and N = 0.6 and 0.5 x 10l8
N i s t h e r e f o r e somewhat
The number o f s i t e s
l a r g e r f o r h i g h l y s u l p h u r i z e d compounds ; i n c o n t r a r y ,
N diminishes s i g n i f i c a n t l y f o r compounds w i t h a composition c l o s e t o MoS2. S i m i l a r y , i t becomes c l e a r t h a t t h e WF, values f o r compounds r i c h i n sulphur (amorphous) a r e obviously h i g h e r than those f o r compounds o f a composition close t o floS2. I n f a c t , i n t h e l a t t e r , t h e s u l p h u r chains ( r e f s . 26-28) disappear d u r i n g the thermal treatment l e a d i n g t o a beginning o f o r g a n i z a t i o n . This induces d i m i n i s h i n g values o f WM. The nature o f the conduction s i t e s c o u l d be t h e Mo(V) ( r e f . 2 ) which seems t o be confirmed by ESR s t u d i e s ( r e f s . 1, 2 9 ) . For a given heat treatment and up t o a c e r t a i n composition
(XN
0.6) t h e
environment o f t h e Mo(V) i s o n l y s l i g h t l y m o d i f i e d and t h e r e f o r e WF, and N a r e r a t h e r constant.
0 , 3 t h e I,lo(V) environment i s s t r o n g l y m o d i f i e d by t h e disappearing sulphur chains i n d u c i n g a l o w e r i n g o f till and N. We r e c a l l t h a t f o r h i g h For x
<
treatment temperatures (
N
900-1000°C) t h e o x i d a t i o n s t a t e o f t h e metal i s
IV.
The existence o f two d i s t i n c t energies f o r t h e o x i d i z e d compound c o u l d be due t o a m o d i f i c a t i o n o f t h e Mo(V) environment by oxygen forming F1003+ e n t i t i e s coexisting with
Nos3+ species ( r e f . 2, 28). A systematic study o f t h e oxydation
e f f e c t on amorphous molybdenum sulphides should g i v e f u r t h e r i n f o r m a t i o n s on t h e nature these
s u l p h u r i z e d and o x i d i z e d species.
REFERENCES
1
2 3
4 5
6 7
a 9 10
11 12 13 14 16
16 1;
iE 19 -. ? '
P. Belougne and J.V. Zanchetta, Rev. Chim. YinGr, 16 (1979) 565. P. Belougne, 3.C. K i u n t i n i and G . Y . Zanciietia, P h i l . Nag, B 53 (1986) 233. B. Deroide, These d ' E t a t , M o n t p e l l i e r , 1986. B. Deroide, P. Belougne, J.C. G i u n t i n i and J.V. Zanchetta, J. o f Non-Cryst. S o l i d s , 85 (1986) 79. S.R. E l l i o t , Adv. Phys, 31 (1987) 132. N.F. t-tott and E.A. Davis, E l e c t r o n i c Processes i n N o n - c r y s t a l l i n e m a t e r i a l s , Clarendon Press Oxford, 1979. S.R. E l l i o t t , Physics o f Amorphous l l a t e r i a l s , Lonomann, London, 1984. A.R. Long, Adv. Phys. 31 (1982) 553. S. Golin, Phys. Rev. 132 (1953) 173. J.C. G i u n t i n i , B. Deroide, P . Belougne and J.V. Zanchetta, S o l i d S t a t e Communications 6 2 (1987) 739. 2.C. Wildewanck and F. J e l l i n e k , Z . Anorg. A l l g . Chem. 323 (1964) 309. ;. l l e r i n g and F.. L i e v a l d i , C . R . Acad. S c i . , 213 (1911) 793. B i l t z - K o c h e r . Z . h o r g . P,llo. Chem. 172 (1941) 148. T . P . Prasad, E . Diemann and f i . b l u l l e r , J . I n o r g . Nucl. Chem. 35 (1973) 1895. E . C o r l e i s s . L i e b . Ann. 232 (1886) 245. , - . L . Bernard and G . T r i d o t , B u l l . SOC. Chim. 5 (1961) 810. E . Y . a a l l o u and S . Ross, J . Phys. Chem. 57 (1954) 653. 5 . F . El-llahalak~yand B.L. Evans, Phys. S t a t . S o l . , B 79 (1977) 713. L.C. G i u n t i n i and J.V. Zanchetta, J . o f Non-Cryst S o l i d s , 34 (1979) $19. .-.L. G i u n t i n i . 3 . J u l l i e n and J.Y. Zanchetta, Carbon, 23 (1935) 25. 1
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-
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21 22 23 24 25 26 27 28
M. P o l l a k , P h i l . Flag., 23 (1971) 519. M. P o l l a k , P h i l . Mag., B 42 (1980) 781. S.R. E l l i o t t , P h i l . Mag., 36 (1977) 129. II. P o l l a k and G.E. Pike, Phys. Rev. L e t t . , 28 (1972) 1449. G.E. Pike, Phys. Rev., B 6 (1972) 1572. L. Busetto, A . Vaccari and G. M a r t i n i , J. Phys. Chem., 85 (1981) 1927. G.C. Stevens and T. Edmonds, J . Catal,, 37 (1975) 544. Y. Bensimon, P. Belougne, J.C. G i u n t i n i and J.V. Zanchetta, J . Phys. Chem., 88 (1984) 2754.
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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
123
METAL-SUPPORT I N T E R A C T I O N PHENOMENA I N SOME H I G H METAL L O A D I N G LANTHANA SUPPORTED RHODIUM CATALYSTS S . BERNAL, F.J.
BOTANA, R .
G A R C I A and J.M.
RODRIGUEZ-IZQUIERDO
Departamento de Q u h i c a I n o r g k n i c a . F a c u l t a d de C i e n c i a s . Univ e r s i d a d de CQdiz. Apartado 40. P u e r t o R e a l . 11510 C Q d i z . S p a i n ABSTRACT
T h i s work r e p o r t s on t h e c h a r a c t e r i z a t i o n o f two Rh/La203 c a t a l y s t s w i t h 1076 m e t a l l o a d i n g , r e d u c e d a t 623 K and 7 2 3 K , q e s p e c t i v e l y . The c h e m i s o r p t i o n s t u d i e s showed t h a t H/Rh r a t i o s were much l a r g e r t h a n t h o s e d e t e r m i n e d for CO/Rh. T h i s h a s been i n t e r p r e t e d as due t o t h e o c c u r r e n c e o f hydrogen s p i l l o v e r . The s t u d y o f t h e c a t a l y s t s by XRD and HRTEM r e v e a l e d t h a t no rhodium c o n t a i n i n g phases o t h e r than t h e metal a r e p r e s e n t i n t h e reduced p h a s e s . The XRD and HRTEM r e s u l t s a l s o showed t h a t t h e m e t a l p a r t i c l e s a r e w e l l d i s p e r s e d w i t h amean s i z e n o t l a r g e r t h a n 5 nm. These r e s u l t s , which s u g g e s t t h a t t h e m e t a l p a r t i c l e s a r e b u r i e d i n t o t h e s u p p o r t , a r e d i s c u s s e d i n r e l a t i o n t o t h e mechanisms i n v o l v e d i n t h e p r o c e s s o f decoration/encapsulation o f t h e rhodium crystallites. INTRODUCTION
S i n c e t h e p i o n e e r i n g works by I c h i k a w a (1) and B e l l ( 2 ) , l a n t h a n a s u p p o r t e d m e t a l s c a n be z o n s i d e r e d r a t h e r u n c o n v e n t i o n a l c a t a l y s t s . A s f u r t h e r s t u d i e s were r e p o r t e d , i t became more and more o b v i o u s t h e complex n a t u r e o f t h e s e p h a s e s , and t h e r e f o r e , t h e d i f f i c u l t i e s t o a r r i v e at a simple i n t e r p r e t a t i o n of t h e i r c h e m i s o r p t i v e and c a t a l y t i c p r o p e r t i e s . A s s e v e r a l a u t h o r s (3,4)
have s u g g e s t e d , t h e h i g h r e a c t i v i t y
a g a i n s t H20 o f t h e r a r e e a r t h s e s q u i o x i d e s , s p e c i a l l y l a n t h a n a , c a n p l a y a n i m p o r t a n t r o l e i n d e t e r m i n i n g t h e b e h a v i o u r as s u p p o r t o f t h e s e o x i d e s . L a t e r o n , we have shown ( 5 - 9 ) t h a t n o t o n l y t h e h y d r a t i o n o f l a n t h a n a b u t a l s o i t s c a r b o n a t i o n s h o u l d be c o n s i d e r e d t o u n d e r s t a n d i n d e p t h t h e c h e m i s t r y as s u p p o r t o f t h i s o x i d e . P r i m a r i l y b a s e d on c h e m i s o r p t i o n , I R s p e c t r o s c o p y and XPS d a t a B e l l e t a1 ( 1 0 , l l ) i n a r e c e n t s e r i e s o f p a p e r s d e a l i n g w i t h l a n t h a n a s u p p o r t e d p a l l a d i u m c a t a l y s t s have p r o p o s e d t h e o c c u r r e n c e of d e c o r a t i o n o f t h e m e t a l c r y s t a l l i t e s by t h e s u p p o r t . I n r e f s . (10,11) i t i s suggested t h a t p a r t i a l l y reduced l a n t h a n a m o i e t i e s ,
Lao,,
a r e involved i n t h e d e c o r a t i o n of palladium. This p r o p o s a l ,
124
kith close resemblances to the mechanism suggested in ref.(l2) to interpret the classic SMSI effect, however, contrasts with the low reducibility of lanthana (13). In some other recent papers (14,15), the singularities observed in the behaviour of lanthana supported (promoted) metal phases hava Seen related to processes, other than the reduction of the support, occurred during the preparation of the catalyst. This work reports on the characterization by conventional chemisorption, TPD, XRD and HRTEM techniques of two lanthana supported rhodium catalysts with a high metal loading, 10 %. In agreement with several earlier studies dealing with La203 supported (promoted) metal catalysts (11,14,16,17), the phases investigated here show an anomalous chemisorptive behaviour, which suggests the occurrence of decoration/encapsulation phenomena. The results of our study are used to discuss the nature of the mechanisr..responsible for the occurrence of the phenomena mentioned above. :IE THODS
Catalysts The La203 sample used here was a 99.9% pure oxide from Ventron. The rhodium salt was Rh(N03)3.xH20, 3676 Rh, also from Ventron. The catalysts were prepared following the incipient wetness impregnation technique. To achieve the desired metal loading, 1076, several wetting-drying (in air, 373 K ) cycles were necessary. The reduction of the precursor/support system was carried out in flowing H2 at two different temperatures, 623 K and 723 K. Details of the preparation procedure, as well as information about the nature of the support phases resulting from both the impregnation and reduction treatments are reported elsewhere (6-8). Characterization Studies The volumetric adsorption studies of H2 and CO were performed in a conventional high vacuum system, at room temperature. The equilibrium time was considered to be 30 minutes. The temperature progrF.hmed desorption (TPD) studies were carried out in a flow of A r of 0.4 cm3.s-1 using a catharometer as detector. The X-ray diffraction studies were recorded on a Siemens instrument, model D500 Radiation CuK and a nickel filter were used. The high resolution transmission electron microscopy (HRTEM) images were obtained on a Jeol 200 CX instrument, belonging to National Facilities at the Arizona State University in Tempe (USA).
125
RESULTS AND DISCUSSION E a r l i e r s t u d i e s d e a l i n g w i t h t h e impregnation and r e d u c t i o n process e s l e a d i n g t o t h e c a t a l y s t s i n v e s t i g a t e d h e r e have been r e p o r t e d i n r e f s . ( 6 , 7 ) . T h i s p r e v i o u s work h a s shown t h a t t h e impregnation s t e p induces a f u r t h e r c a r b o n a t i o n on t h e s u p p o r t w i t h formation of a c r y s t a l l i n e a n c y l i t e - l i k e s t u d y by T G , TPR-MS,
hydroxycarbonate.
Likewise,
the
I R s p e c t r o s c o p y and XRD of t h e r e d u c t i o n proc-
e s s ( 6 , 7 , 9 ) h a s allowed u s t o e s t a b l i s h t h e r e d u c t i o n c o n d i t i o n s used i n t h i s work. According t o r e f s . ( 6 , 7 ) , r e d u c t i o n t e m p e r a t u r e s
as h i g h as 723 K are n e c e s s a r y t o p r e p a r e t r u e La203 s u p p o r t e d rhodium c a t a l y s t s . I f t h e r e d u c t i o n temperature i s lower, t h e supp o r t a c t u a l l y c o n s i s t s of a p a r t i a l l y h y d r a t e d and carbonated phase t h e n a t u r e of which would depend v e r y much on t h e s e l e c t e d reduct i o n c o n d i t i o n s . We have a l s o shown ( 8 ) t h a t t h e impregnation t r e a t m e n t , as w e l l as t h e m a n i p u l a t i o n s t o which t h e p r e c u r s o r / s u p p o r t . s y s t e m a r e submitted b e f o r e i t s r e d u c t i o n can a l s o p l a y an i m p o r t a n t r o l e i n determining t h e a c t u a l c o n s t i t u t i o n of t h e supp o r t i n t h e f i n a l c a t a l y s t . I n t h e s p e c i f i c c a s e of t h e samples reduced a t 623 K i n v e s t i g a t e d h e r e , i t h a s been found t h a t t h e supp o r t a c t u a l l y c o n s i s t s of a mixture of dioxymonocarbonate and oxyhydroxide phases ( 6 ) . Table 1 summarizes t h e c h a r a c t e r i z a t i o n r e s u l t s o b t a i n e d from t h e H 2 and C O chemisorption s t u d i e s . W e have i n v e s t i g a t e d t h e ads o r p t i o n on b o t h , t h e c a t a l y s t s and t h e s u p p o r t only. I n t h i s l a t t e r c a s e , l a n t h a n a w a s p r e v i o u s l y reduced as i n t h e p r e p a r a t i o n of t h e c a t a l y s t s . N o s i g n i f i c a n t a d s o r p t i o n of e i t h e r H 2 o r CO could be observed on t h e s u p p o r t . I t would be n o t e d , however, t h a t t h e p r e s e n c e o f t h e d i s p e r s e d metal has a s t r o n g i n f l u e n c e on t h e thermal e v o l u t i o n , i n flow of H 2 , of t h e s u p p o r t , and, t h e r e f o r e , that t h e n a t u r e of t h e s u p p o r t i s n o t t h e same depending on t h e metal i s p r e s e n t o r n o t ( 6 , 7 ) . For comparative purposes, Table 1 a l s o i n c l u d e s data o b t a i n e d by u s f o r Rh/La203 c a t a l y s t s w i t h 1% metal l o a d i n g . I n accordance w i t h Table 1, t h e H / R h r a t i o s a r e always l a r g e r than those estimated f o r C O / R h , the l a t t e r being negligeable i n the c a s e of t h e h i g h l o a d i n g samples. Q u i t e similar chemisorptive propp e r t i e s have been r e p o r t e d e a r l i e r f o r a number o f l a n t h a n a supp o r t e d (promoted) metal c a t a l y s t s ( i l ,16,171. I n r e f s . ( 1 6 , 1 8 ) t h e hydrogen chemisorption r e s u l t s a r e i n t e r p r e t ed assuming the occurence o f s p i l l over. F i g u r e 1 shows r e p r e s e n t -
126
TABLE 1 A p p a r e n t H/Rh a n d CO/Rh r a t i o s as d e t e r m i n e d f r o m t h e c o r r e s p o n d i n g v o l u m e t r i c a d s o r p t i o n i s o t h e r m s a t 298K
REDN.
CATALYST
(K)
TEMP.
H/Rh
CO/Rh
0.05
Rh( 10%)/La203 ( )
623
0.55
Rh(l0%)/La2O3
723
0.35
0.05
R h ( 1%) /La203 ( * )
623
0.80
0.30
Rh(l%)/La~Og
723
0.60
0.10
(*)
Mot p r o p e r l y l a n t h a n u m o x i d e . of b r e v i t y .
373
Fig.
,
573
,
T e r m used f o r sake
?p ,
973
,
1\73 K
1.- TPD-Hz d i a g r a m s c o r r e s p o n d i n g t o : a ) R h ( l O % ) / L a 2 0 3 r e d u c e d a t 623 K, t h e n e v a c u a t e d i n f l o w i n g He, f o r 1 h , a t 623 K , and f i n a l l y t r e a t e d i n f l o w i n g H2, f o r l h , a t 2 9 8 K b ) Sample t r e a t e d l i k e e x c e p t i t w a s n o t i n cont a c t w i t h H2 a t room t e m p e r a t u r e .
a,
127
a t i v e TPD-H2 diagrams o f o u r c a t a l y s t s . These diagrams c l e a r l y d e m o n s t r a t e t h e e x i s t e n c e o f s e v e r a l h i g h t e m p e r a t u r e hydrogen f o r m s , presumably c o r r e s p o n d i n g t o a d s o r p t i o n on t h e s u p p o r t . A l s o w o r t h n o t i n g , t h e comparison o f t h e two s p e c t r a d e p i c t e d i n F i g . 1 s t r o n g l y s u g g e s t s t h e o c c u r r e n c e i n our case o f s p i l l o v e r . T h i s p r o p o s a l r e c e i v e s f u r t h e r s u p p o r t from t h e c h e m i s o r p t i o n s t u d i e s , i n a c c o r d a n c e w i t h which, superimposed t o a r e l a t i v e l y f a s t p r o c e s s much s l o w e r H 2 u p t a k e does o c c u r . F i g u r e 2 r e p o r t s on t h e XRD s t u d y o f t h e reduced c a t a l y s t s . I n s p i t e o f t h e h i g h metal l o a d i n g ,
lo%,
o f t h e s e c a t a l y s t s , t h e y do
n o t show any d i f f r a c t i o n l i n e t h a t c a n be a s s i g n e d n e i t h e r t o t h e m e t a l l i c rhodium n o r t o any o t h e r rhodium c o n t a i n i n g phase. I t i s n e c e s s a r y t o c a l c i n e t h e c a t a l y s t r e d u c e d a t 723 K a t much h i g h e r t e m p e r a t u r e s , a b o u t 1 1 7 3 K , t o d e t e c t by XRD t h e p r e s e n c e o f t h e metal, Fig. 22. The c h a r a c t e r i z a t i o n o f o u r c a t a l y s t s h a s a l s o i n c l u d e d t h e i r i n v e s t i g a t i o n by HRTEM, F i g u r e 3. A s some e a r l i e r p a p e r s ( 1 6 ) , t h e s t u d y by TEM o f r a r e e a r t h o x i d e s u p p o r t e d metal c a t a l y s t s p r e s e n t s n o t a b l e d i f f i c u l t i e s . I n s p i t e o f t h i s , t h e images we c o u l d o b t a i n have a l l o w e d u s t o g e t some i n t e r e s t i n g p i e c e s o f i n f o r m a t i o n . The
metal c r y s t a l l i t e s o b s e r v e d i n o u r c a t a l y s t s were n o t l a r g e r t h a n 5 nm. F u r t h e r m o r e , upon i n c r e a s i n g t h e r e d u c t i o n t e m p e r a t u r e from
623 K t o 723 K , no s i g n i f i c a n t s i n t e r i n g o f t h e m e t a l p a r t i c l e s c a n be n o t e d . I n c o n t r a s t t o t h a t r e p o r t e d f o r Rh/Ce02 c a t a l y s t s ( 1 9 ) , i n t h e p r e s e n t c a s e , t h e m e t a l p a r t i c l e s show v e r y o f t e n
t w i n n i n g , which as i s d i s c u s s e d i n r e f . ( 2 0 ) c a n be c o n s i d e r e d a mechanism of s t a b i l i z a t i o n o f s m a l l m e t a l p a r t i c l e s . The HRTEM images o f t h e c a t a l y s t s d i d n o t b r i n g any s u p p o r t t o t h e e x i s t e n c e of p h a s e s c o n t a i n i n g o x i d i z e d forms o f Rh. T h i s o b s e r v a t i o n , which
i s i n agreement w i t h t h e XRD r e s u l t s r e p o r t e d i n F i g . 2 , i s a l s o c o n s i s t e n t w i t h some p r e l i m i n a r XPS d a t a of o u r c a t a l y s t s , i n acc o r d a n c e w i t h which f o r r e d u c t i o n t e m p e r a t u r e s h i g h e r t h a n 573 K , t h e s i g n a l f o r Rh(32) i s found a t a b i n d i n g e n e r g y o f 306.9 eV, w i t h no i n d i c a t i o n s of t h e e x i s t e n c e o f f e a t u r e s a t h i g h e r v a l u e s .
The r e s u l t s o b t a i n e d from o u r s t u d y by b o t h X R D and HRTEM p o i n t a l l a t t h e e x i s t e n c e i n o u r c a t a l y s t s of h i g h l y d i s p e r s e d m e t a l l i c
rhodium. T h i s s t r o n g l y c o n t r a s t s w i t h t h e c h e m i s o r p t i o n d a t a i n c l u d e d i n Table 1 , i n d i c a t i n g t h e l i k e l y o c c u r r e n c e of d e c o r a t i o n / e n c a p s u l a t i o n phenomena on t h e m e t a l p a r t i c l e s . T h i s h y p o t h e s i s , a l r e a d y advanced i n t h e l i t e r a t u r e (lo), r e c e i v e s s u b s t a n t i a l sup-
128
A
Fig. 2.- XRD study of the catalysts investigated here: a) Rh(lO%)/La203 reduced at 623 K. b ) Rh(10%)/La203 reduced at 723 K. c ) Catalyst 2 further calcined in flow of He at 1173 K.
129
Fig. 3.- HRTEM image corresponding t o a c a t a l y s t s Rh(lO%)/ LazOg, reduced a t 723 K. S t r u c t u r a l d a t a f o r t h e l a t t i c e f r i n g e s observed i n t h e twinned Rh c r y s t a l l i t e a r e i n d i c a t e d on t h e p i c t u r e .
130
from o u r s t u d y by HRTEM. The q u e s t i o n t h a t can now
be posed i s
about t h e mechanism(s) r e p o n s i b l e f o r t h e decoration/encapsulation of t h e rhodium c r y s t a l l i t e s i n l a n t h a n a c o n t a i n i n g c a t a l y s t s . I n an i n t e r p r e t a t i o n r e s e m b l i n g t h a t proposed i n r e f . ( l 2 ) t o e x p l a i n t h e s o - c a l l e d SMSI e f f e c t , B e l l e t a1 ( 1 0 ) have s u g g e s t ed t h a t t h e anomalous c h e m i s o r p t i v e b e h a v i o u r of a s e r i e s of Pd/ /La203 c a t a l y s t s i s due t o t h e d e c o r a t i o n o f t h e m e t a l p a r t i c l e s by L a o x p a t c h e s . T h i s would imply some s o r t of r e d u c t i o n of La203, an o x i d e h a r d l y r e d u c i b l e , s p e c i a l l y , u n d e r t h e r e l a t i v e l y m i l d conditions used i n ( 1 0 ) . Vannice e t a1 ( 3 , 1 4 ) have a l s o r e p o r t e d q u i t e u n c o n v e n t i o n a l r e s u l t s f o r H2 and CO c h e m i s o r p t i o n o v e r a s e r i e s o f r a r e e a r t h o x i d e s u p p o r t e d p a l l a d i u m c a t a l y s t s . According t o t h e s e a u t h o r s , t h e o r i g i n of t h e s i n g u l a r i t i e s o b s e r v e d might w e l l be r e l a t e d t o t h e s t r o n g c h e m i c a l and s t r u c t u r a l r e a r r a n g e m e n t s undergone by t h e s u p p o r t s d u r i n g t h e r e d u c t i o n t r e a t m e n t . The d e h y d r a t i o n o f t h e s u p p o r t s , a c t u a l l y c o n s t i t u t e d by h e a v i l y h y d r a t e d o x i d e s , w a s cons i d e r e d t o be r e p o n s i b l e f o r s u c h r e a r r a n g e m e n t s . A s we have d i s c u s s e d i n r e f . ( 5 ) , however, t h e a l t e r a t i o n s s u f f e r e d by t h e s u p p o r t t h r o u g h o u t t h e r e d u c t i o n t r e a t m e n t a r e due n o t o n l y t o i t s dehyd r a t i o n b u t a l s o t o t h e r e d u c t i o n o f t h e c a r b o n a t e d p h a s e on i t . According t o r e f . ( 6 ) , i n t h e p a r t i c u l a r c a s e o f t h e h i g h m e t a l l o a d i n g l a n t h a n a s u p p o r t e d rhodium c a t a l y s t s i n v e s t i g a t e d h e r e , t h e c a r b o n a t i o n i s v e r y i n t e n s e , s o t h a t i t s h o u l d be e x p e c t e d t o p l a y
a s i g n i f i c a n t r o l e i n t h e e v o l u t i o n undergone by t h e s u p p o r t d u r i n g the reduction process. A t h i r d mechanism proposed i n t h e l i t e r a t u r e t o e x p l a i n t h e oc-
c u r r e n c e of d e c o r a t i o n of t h e m e t a l p a r t i c l e s by t h e s u p p o r t would imply t h e r e d i s s o l u t i o n o f t h e s u p p o r t f o l l o w e d by i t s c o d e p o s i t i o n with t h e metal
( 2 1 ) . T h i s p r o c e s s , w h i c h would t a k e p l a c e d u r i n g t h e
impregnation s t e p , i s discussed i n r e f . ( 2 1 ) f o r vanadia supported rhodium c a t a l y s t s . A s far as l a n t h a n a c o n t a i n i n g c a t a l y s t s i s c o n c e r n e d , i t h a s
been shown i n a v e r y r e c e n t p a p e r ( 1 5 ) t h a t t h e i m p r e g n a t i o n s t e p i s c r i t i c a l t o u n d e r s t a n d t h e promoting e f f e c t t o o x y g e n a t e s o b s e r v
ed i n t h e C O + H 2 r e a c t i o n o v e r a s e r i e s of Rh/La203/Si02 c a t a l y s t s .
I n e f f e c t , as i s r e p o r t e d i n r e f . ( l 5 ) , t h e i m p r e g n a t i o n of t h e Rh precursor s a l t a l s o i m p l i e s r e d i s s o l u t i o n of t h e promoter, pred e p o s i t e d o n t o t h e s i l i c a i n t h e form o f La2O3. To summarize, upon r e v i e w i n g t h e c u r r e n t l i t e r a t u r e , t h e r e have
131
been found proposals where some of the three mechanisms commented on above is considered to be responsible for the singularities observed in the behaviour of lanthana supported (promoted) metal catalysts. In our opinion, however, the decoration/encapsulation of the metal particles by lanthana is more likely to be associated to the chemical and structural evolution undergone by the support throughout both the impregnation and reduction steps of the preparation process. In this latter case, the reactions of dehydration and decarbonation of the support rather than its reduction should be considered the main responsible for the decoration of the metal particles. From our point of view, the relative contribution of the impregnation and reduction steps to the formation of the lanthana patches on the metal, and eventually the complete encapsulation of it, can be variable, depending on the specific preparation procedure. In this sense, it would be recalled the considerable influence of the nature of the rhodium precursor on the intensity o f the decoration phenomena (21). It would be noted,however, that such an influence has not been checked in the case lanthana containing catalysts. The results of the chemisorption study included in Table 1 lend some support to our proposal in the sense that both the impregnation and reduction processes can contribute to a variable extent to the decoration of the the rhodium particles. In effect, for the high metal loading catalysts, the inhibition of the CO chemisorption is practically complete even for that reduced at 623 K, i.e. when the decomposition o f the starting support phase to oxide is at an intermediate stage. In contrast with this, for Rh(l%)/La20, catalyst reduced at 723 K there is a relative inhibition of the CO adsorption capability with respect to that of the sample reduced at 623 K. Taking into account that both the number of impregnation cycles and the concentration of the impregnating solution were much larger in the case of the 10% Rh catalysts, it can be suggested that for the high loading catalysts the impregnation step would represent the major contribution to the hidding of the metal crystallites. On the contrary, in the case of the 196 rhodium loading catalysts, the rearrangements undergone by the support upon increasing the reduction temperature from 623 K to 723 K seem to have a significant influence on the CO/Rh ratios.
132
ACKNOWLEDGEMENTS
T h i s work h a s r e c e i v e d f i n a n c i a l s u p p o r t from t h e Comisibn I n t e r m i n i s t e r i a l de C i e n c i a y T e c n o l o g i a (CICYT). *eject PB87-0961. REFERENCES 1 M . I c h i k a w a , B u l l . Chem. SOC. J a p a n , 51 (1978) 2273-77 2 Y . A . Ryndin, R . F . H i c k s , A . T . B e l l and Y . I . Yermakov, J . C a t a l . , 70 (1981) 287-97 3 M . D . M i t c h e l l a n d M.A. V a n n i c e , I n d . Eng. Chem. F u n d . , 23 (1984) 88-96. 4 S.S. Chan and A . T . B e l l , J . C a t a l . , 89 (1984) 433-441 5 S . B e r n a l , F . J . B o t a n a , R . Garcia, F. Rarnirez and J.M. R o d r i g u e z I z q u i e r d o , Appl. C a t a l . , 21 (1986) 379-382 6 S. B e r n a l , F . J . B o t a n a , R . Garcia, F. Ramirez a n d J.M. R o d r i g u e z I z q u i e r d o , Appl. C a t a l . , 31 (1987) 267-273. Rodriguez 7 5 . B e r n a l , F . J . B o t a n a , R . G a r c i a , F . Rarnirez and J . M . I z q u i e r d o , J . Chem. SOC. F a r a d a y T r a n s . I , 83 (1987) 2279-2287. 8 S . B e r n a l , F . J . B o t a n a , R . G a r c i a , F. Ramirez a n d J . M . R o d r i g u e z I z q u i e r d o , J . Mater. S c i . , 22 (1987) 3793-3800. 9 S . B e r n a l , E . B l a n c o , F . J . B o t a n a , R . G a r c i a , F. Ramirez a n d J . M. R o d r i g u e z - I z q u i e r d o , Mater. Chem. P h y s . 17 (1987) 433-443. 10 T.H. F l e i s h , R.F. H i c k s and A . T . B e l l , J . C a t a l . , 87 (1984) 398-413. 11 R . F . H i c k s , Q . J . Yen and A . T . B e l l , J . C a t a l . , 89 (1984) 498-510 12 J . S a n t o s , J . P h i l l i p s and J . A . Dumesic, J . C a t a l . , 8 1 (1983) 147-167. 13 D . K . S m i t h , J . Chem. Ed., 63 (1986) 228-231. 14 C . S u d h a k a r and M.A. V a n n i c e , Appl. C a t a l . , 14 (1985) 47 15 R. K i e f f e r , A . Kiennemann, M . R o d r i g u e z , S . B e r n a l and J . M . Rod r i g u e z - I z q u i e r d o , Appl. C a t a l . , 42 (1988) 77-89. 16 R.P. Underwood and A . T . B e l l , Appl. C a t a l . , 34 (1987) 289-310. 17 R.P. Underwood and A.T. B e l l , J . C a t a l . , 109 (1988) 61-75 18 N . K . Pande and A . T . B e l l , J . C a t a l . , 98 (1986) 7-16 19 S. B e r n a l , F . J . B o t a n a , R . Garcia, 2 . Kang, M.L. Lbpez, M. Pan, F. Ramirez and J.M. R o d r i g u e z - I z q u i e r d o , Catal. Today, 2 (1988) 653-662. 2 0 M . P a n , J.M. Cowley and R . G a r c i a , Micron Microscop. A c t a , 18 (1987) 236-241 21 G.v.d. Lee and V . P o n e c , C a t a l . Rev. S c i . E n g . , 29 (1987) 183-218.
C. Morterra, A. Zecchina and G.Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Pri.ntedin The Netherlands
SPECTROSCOPIC SILICALITE
CHARACTERIZATION
M.R. Boccuti’, Petrini’
K.M.
Raol,
A.
OF
SILICALITE
Zecchina’
and G.
AND
133
TITANIUM-
Leofanti’,
‘Dipartimento di Chimica Inorganica, Chimica Fisica e Materiali, Universita di Torino, Via P. Giuria 7, (Italy).
G.
dei
‘Montedipe, Bollate (Milano), (Italy). ABSTRACT Surface characterization of silicalite and titanium-silicalite is made by combined use of IR and W-Vis reflectance spectroscopy. The nature of the IR modes and of the electronic transitions associated with framework titanium is discussed in detail. The perturbation caused by the adsorption of small polar molecules (H20, NH3, CH30H) on both the IR and W-Vis spectra is discussed in terms of the formation of six-coordinated complexes by ligand insertion in the Ti(1V) coordination sphere. All the Ti(1V) is accessible to the interaction with the molecules. INTRODUCTION Titanium-silicalite (TS) is a zeolite of the pentasil family, where the titanium is thought to be incorporated instead of Si at the tetrahedral sites of the framework (1, 2 , 3 , 4). In the recent years, the TS has enabled remarkable progress to be made in oxidation reactions with H202 (4-6) and ammoxidations of ketones and aldehydes to oximes (7). The reactions proceed with high selectivity: however the role of titanium (either in internal or in external position) and of the porous nature of the material in promoting the high performances of this catalytic material is still not fully elucidated. In particular the characterization by means of physical methods of the electronic and coordinative state of titanium centres before and after interaction with simple molecules is still missing. In this contribution, an IR, UV-Vis and thermogravimetric investigation of TS surface properties is reported and the results are compared with the parallel ones obtained on the isostructural silicalite ( S ) and on amorphous silica.
134
EXPERIMENTAL Ti-silicalite (TS) and silicalite (S) have been synthesized in the Montedipe laboratories following the methods described in (8) and (9) respectively. The titanium content, is = 1% as determined by elemental analysis. The IR spectra have been carried out on a Brucker FTIR 113V, by using a specially designed silica cell (optical path : 0 . 5 cm) permanently attached to a vacuum system (ultimate vacuum :10-5 torr), allowing in situ outgassing procedures and gas dosing. The sample was in the form of thin pellets. The UV-Vis reflectance spectra have been performed with a Perkin Elmer Lambda 15-DRS. Thermogravimetric analysis has been performed with a DTG apparatus (Metler TA 3000).
RESULTS
In Fig.1 the IR spectra in the 4000-1500 cm-I range of the TS outgassed in vacuo at increasing temperatures are illustrated. In the inset the thermogravimetric curve obtained for the same temperature range is also reported.
Fig.1. IR spectra of virgin TS (1) and outgassed at 323, 6 2 3 and 773 rc 1 2 - 4 ) .
135
Outgassing at the beam temperature causes the reduction of a strong and broad, structureless, absorption in the 3700-2500 cm-' range and of a medium intensity peak at Y = 1650 cm-l. This spectral modification corresponds to the low temperature thermogravimetric loss shown in the inset and associated with the desorption of weakly adsorbed water ( = 10 molecule per unit cell). This step is totally reversible: reexposure to H20 vapour restores the original situation. Outgassing at progressively higher temperatures causes a progressive disappearance of the absorption in the 3750-2500 cm-I and a gradual thermogravimetric loss as a conseguence of the progressive dehydroxilation of the solid. In particular in spectra 2-4 a distinct peak is observed at 3510 cm-I (with a shoulder at = 3450 cm-l) which does not find correspondence in the titanium free sample (silicalite: vide infra). At the highest dehydration stages only a sharp peak at 3740 cm-I is dominating (isolated silanols). The same peak is observed on pure silicalite: vide infra and ref. (10).
-
b & crn -1
Fig. 2. IR spectra of virgin S (1) and outgassed at 323, 673, 1073K.
136
In Fig. 2, the IR and thermogravimetric results obtained in a parallel experiment on S are illustrated. By comparison with the behawiour of TS, the following main differences are observed: i) the weight loss due to desorption of adsorbed water is smaller than on TS (corresponding to = 7 molecules of H20 and = 1.0 OH groups per unit cell); ii) the overall intensity of the OH stretching bands (3750-2500 an-') 1s smaller than on TS (in agreement with 1); iii) the peak at 3510-3450 cm-I observed on TS at intermediate dehydration temperatures is absent on S; iv) the scattering profile of S and TS are very different. A detailed comparison of TS and S spectra in the 1000-400 cm-I range, is not possible on the basis of the spectra of self supported pellets. This comparison can be made if thin films of S, TS and amorphous silica are used (Fig. 3).
SlLlCALlTE
T i - SlLlCALlTE
r
AMORPHOUS SILICA
%Of
~
C r n -1
3. IR sp tr (Aerosil:380 )'-g'm
Fig.
cm-'
(1200-4-00 cm-l) of S, TS and amorphous silica outgassed at T = 323K.
The spectra of the same samples after outgassing at progressively higher temperatures are not reported for sake of brevity and because they do not change significantly with dehydration (the only observed effect is the narrowing of the 960 cm-l peak). In the context of this paper we shall focus our attention only
137
on the major difference which is represented by the 9 6 0 cm-l peak observed only on TS (a shoulder at = 1000 cm-I is also present). Beside the absence of the structure sensitive peaks (1240 and 5 5 0 cm-') (14), the amorphous silica shows a clearly = 9 8 5 - 9 9 0 cm-l. This peak has been assigned visible shoulder at (11) to the Si-OH stretching mode of surface hydroxylated tetrahedra 03SiOH (12). A similar broad shoulder is also present on S.
a)
I 10
K)
30
cm
' 10.)
40
A
50
Fig. 4 A . Reflectance spectra (Kubelka-Munk function wawenumber) of virgin S (a) and TS (b) Fig. 4B. Effect of water dosing on TS (4 torr and saturated vapour) The presence of a 0.013 molar fraction of Ti not only influences the IR spectrum but also the W - V I S one. This is shown in Fig.4A where the diffuse reflectance spectra of S (a) and TS (b) are compared. The strong transition at 48.000 cm-l observed only on TS is undoubtedly associated with an electronic transition iocalized on titanium containing structures. In order to ascertain if the Ti-sites are active in adsorption, the effect of dosing H20, NH3 and CH30H on the IR modes of Ti-sites is represented in Fig. 5.
138
CH30H
i
t
m
,
’
EOO
_-
10
Fig. 5. Effect of H 0 (0.5 Kpa), NH (26 KPa) and CH30H (13 KPa) torr) on the 960 cm-’ peak. Open circ?Les indicate gas phase bands. The broken line is the spectrum without the gas phase contribution. The following can be seen: a) adsorption of H20 shifts upwards the 960 cm-l peak to = 9 7 5 cm-l; (some intensity decrement and broadening being also observed); b ) in presence of NH3 ( 2 6 Kpa) the peak is shifted to 9 7 5 - 8 0 cm-I with pronounced broadening and weakening; c) in presence of CH30H (13 Kpa) the peak is shifted to = 980 cm-’. These modifications are reversible: desorption in vacuo at the beam temperature restores the initial situation. The adsorption of H20, NH3 and CH30H not only modifies the IR spectrum but also the W - V i s (diffuse reflectance) one. In this paper only the effect of water dosing will be documented (Fig. 4B). It can be immedately seen that the original peak at 4 8 . 0 0 0 cm-I is progressively eroded with formation of a lower frequency absorption. Under pore filling conditions the new broad peak is observed at 42.000 cm-l. Also in this case, the effect is totally reversible upon outgassing at the beam temperature.
139
DISCUSSION 1) IR modes associated with hydroxyl groups and molecular water. The virgin TS and S samples exposed to the normal atmosphere, contain molecular water as demonstrated by the peak at 1650 cm-l (H20 bending modes) and by the thermogravimetric results (10 and 7 molecules of weakly adsorbed H20 per unit cell respectively, i.e. much less than the maximum value found in presence of the saturated vapours: 15 molec. per unit cell) (11). Outgassing at T 5 373K eliminates this form of water presumably hydrogen bonded to surface silanols and titanols. For outgassing temperatures T > 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-
-
.
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).
-
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-
151
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
153
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.
154
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-
-
-
155
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).
ACKNOWLEDGEMENTS Work done in the author’s laboratory has been generously supported by Deutsche Forschungsgemeinschaft and by the Fonds der Chemischen Industrie. REFERENCES J.C. Angus, P. Koidl and S. Domitz, in J. Mort and F. Jansen (Editors), Plasma Deposited Thin Films, CRC Press, Boca Raton, FL, 1986, pp. 89-127; A.L. Robinson, Science (1986) 1074-1076. W. Ruland, Acta Cryst. 18 (1965) 992-996. P.A. Marsh, A. Voet, T.J. Mullens and L.D. Price, Rubber Chem. Technol. (1970), 470-481. L.L.Ban, in Surface and Defect Properties of Solids, Vol. 1, M.W. Roberts and J.M. Thomas (Editors), The Chemical Society, London, 1972, p. 54-94. R. Sappok and H.P. Boehm, Carbon 6 (1968) 283-295. P.S. de Carti and J.G. Jamieson, Science laa (1961) 1821; P.S. de Cadi (Allied Chemical Corp.). US-Pat. 3,238,019 (1966). T. Noda, Y. Sumiyoshi and N. Ito, carbon 6 (1968) 813-816; N. Yoshima, M. Koyama, H.Yoshida, Sh. Matsuo and H.Nagashima (Toshiba Ceramics Co. Ltd.), Ext. Abstr., Internat. Symp. Carbon, Toyohashi, Japan, 1982, pp. 523-526. A. Thorny and A.X. Duval, J. Chim. Phys. 66 (1969) 1966-1973; C. Bockel and A. Thomy, Carbon 19 (1981) 142; R.J. Birgeneau and P.M. Horn, Science 232 (1986) 329-336. A.W. Moore, in P.L. Walker, Jr. and P.A. Thrower (Editors), Chemistry and Physics of Carbon, Vol. 17, M. Dekker, New York, 1981, pp. 233-286.
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10 G.M. Jenkins and K. Kawamura: Polymeric Carbons, Cambridge Univ. Press, Cambridge, 1976; F. Rousseaux and D. Tchoubar, Carbon (1977) 55-61,63-68. 11 J.L.G. Da Silvaand D.J. Johnson, J. Mater. Sci. (1984) 3201-3210; D.J. Johnson, in P.A. Thrower (Editor), Chemistry and Physics of Carbon, Vol. 20, Marcel Dekker, New York, 1987, pp. 1-58; A. Oberlin, in A.R. Bunsell (Editor), Composite Materials Sciences, Vol. 2, Elsevier, Amsterdam, 1988, pp. 150-210. (1974) 1933 -1939; A. 12 T. Koyama, M. Endo and Y. Hishiyama, Jap. J. Appl. Phys. Oberlin, M. Endo and T. Koyama, J. Cryst. Growth (1976) 335-349. 13 G.R. Hennig, Proc. 5th Bienn. Conf. on Carbon, Vol. 1, Pergamon Press, Oxford, 1962, pp. 143-146; M. Barber, E.L. Evans and J.M. Thomas, Chern. Phys. Lett. (1973) 423-425. 14 R. SchlUgl and H.P. Boehm, Carbon 21 (1983) 345-358. 15 S. Mrozowski, Carbon 19 (1981) 365-373; L.S. Singer an I.C. Lewis, Appl. Spectroscopy (1982) 52-57. 16 H.P. Boehm, Adv. Catal. 16 (1966) 179-274. 17 B.R. Pun, in P.L. Walker, Jr. (Editor), Chemistry and Physics of Carbon, Vol. 6, M. Dekker, New York, 1970, pp. 191-282. (1963) 2344-2346. 18 G.G Fedorov, Yu.A. ZariEyants and V.F. Kiselev, Zh. Fiz. Khim. 19 R. Sappok and H.P.Boehm, Carbon6 (1968) 573-583. 20 R.M. Bauer, J. Chem. SOC.(London) 1936.1256-1261; J.P. Redmond and P.L. (1960) 1093-1099; R.H. Savage and C. Brown, J. Walker, Jr., J. Phys. Chem. (1948) 2362-2366. Am. Chem. Soc. (1977) 75-86. 21 P. Cadman, J.D. Scott and J.M. Thomas, Carbon 22 P. Cadman, J.D. Scott and J.M. Thomas, J. Chem. SOC.,Chem. Comrn. 654-655. 23 O.V. Nikitina, V.F. Kiselev, N.N. Lejnev, Carbon8 (1970) 402-404. 24 H. Tobias and A. Soffer, Carbon (1985) 281-289. 25 B.R. Pun and K.C. Sehgal, Indian J. Chem. 4 (1966) 206-208; 3 (1967) 379-380. 26 E. Scharrer and H.P.Boehm, in preparation. 27 R. Schl6gl and H.P. Boehm, Synth. Met. (1988) 407-413. 28 J.P. Wibaut and G. La Bastide, Rec. Trav. Chim. Pays-Bas (1924) 731. 29 H.P. Boehm, B. Tereczki and K. Schanz, in. J. Rouquerol and K.S.W. Sing (Editors), Adsorption at the Gas-Solid and Liquid-SolidInterface, Elsevier, Amsterdam, 1982, pp. 395-401. 30 J. Mrha, Coll. Czech. Chem. Comm. (1966) 715-733; 31 R.Kurth, B. Tereczki and H.P. Boehm, Ext. Abstr., 15th Bienn. Conf. on Carbon, Philadelpha, PA, 1981, p. 244-245. (1984) 106132 H.P. Boehm, G. Mair, Th. StCihr, A.R. de Rindn and B. Tereczki, Fuel 1063. 33 S. Mazur, T. Matusinovic and K. Camann, J. Am. Chem. SOC.99 (1977) 3888-3890. 34 N. Oyama, A.P. Brown and F.C. Anson, J. Electroanat. Chem. (1978) 435-441. (1988) 50735 B. Marchon, J. Carrazza, H.Heinemann and G.A. Somorjai, Carbon 514. 36 G.R. Hennig, in P.L. Walker, Jr. (Editor), Chemistry and Physics of Carbon, Vol. 2,M. Dekker, New York, 1966, pp. 1-49. 37 J.M. Thomas, in P.L. Walker, Jr. (Editor), Chemistry and Physics of Carbon, Vol. 1, M. Dekker, New York, 1965, pp- 121-202. 38 R.T. Yang, in P.A. Thrower (Editor), Chemistry and Physics of Carbon, Vol. 19, M. Dekker, New York, 1983, p. 163. 39 R.T. Yang and C. Wong, J. Catal. 82 (1983) 245-252. 40 C. Wong, R.T. Yang and B.L. Halpern, J. Chem. Phys. L8 (1983) 3325-3328.
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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.
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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
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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-
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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 (<125 k J m o l - l ) .
TPR measurements have been
c a r r i e d o u t w i t h t h e s e c a t a l y s t s , b u t t h e e x t r a €I2 u p t a k e s t o which f u l l o r p a r t i a l r e d u c t i o n of t h e promoter might g i v e r i s e a r e s m a l l :
peaks a d d i t i o n a l
t o those f o r t h e Ru p r e c u r s o r are however s e e n i n t h e c a t a l y s t s c o n t a i n i n g C r
and Mn. Space does n o t p e r m i t a f u l l p r e s e n t a t i o n of t h e p r o d u c t d i s t r i b u t i o n s o b s e r v e d , and i n d e e d t h e r e s u l t s a r e s t i l l b e i n g a n a l y s e d :
c e r t a i n trends a r e
however c l e a r , and b e c a u s e of t h e i r importance i n u n d e r s t a n d i n g a d d i t i v e e f f e c t s , they d e s e r v e mention.
( i ) The v a l u e of F' i s t h e same (0.22 20.01) f o r e a c h
type o f p r e t r e a t m e n t w i t h t h e unpromoted Ru/Si02 and w i t h c a t a l y s t s c o n t a i n i n g Ca, Sc o r T i :
t h i s v a l u e i s a l s o shown by Mn-Ru/Si02 a f t e r t h e HTR's.
In these
c a s e s t h e v a r i a t i o n s i n rate ( F i g . 2 ) and i n A r r h e n i u s p a r a m e t e r s ( F i g . 3) do n o t a f f e c t t h e v a l u e of F ' .
The s i t u a t i o n w i t h t h e c a t a l y s t s c o n t a i n i n g V and
C r i s more complex, and d i s c u s s i o n on t h e s e i s d e f e r r e d .
( i i ) The v a l u e of t h e
' is always g r e a t e r a f t e r LTR than a f t e r e i t h e r HTR; f o r most 3 c a t a l y s t s ( e x c e p t i n g t h o s e c o n t a i n i n g V and Cr) i t s v a l u e s a f t e r HTRl and HTR2
parameter T
a r e similar. Although i t i s t o o e a r l y t o o f f e r a d e t a i l e d e x p l a n a t i o n o f t h e s e r e s u l t s , c e r t a i n b e h a v i o u r a l p a t t e r n s a r e c l e a r l y e v i d e n t , and t h e s e w i l l c e r t a i n l y assist interpretation.
S k e l e t a l i s o m e r i s a t i o n of n-alkanes o v e r Ru c a t a l y s t s S k e l e t a l i s o m e r i s a t i o n o f n-alkanes
t o i s o - a l k a n e s i s well-known t o o c c u r
o v e r P t c a t a l y s t s , b u t i t i s n o t u s u a l l y o b s e r v e d w i t h Ru c a t a l y s t s , whose e x c e p t i o n a l a c t i v i t y f o r h y d r o g e n o l y s i s i s well-documented.
Hydrogenolysis
seems t o r e q u i r e a l a r g e r a c t i v e c e n t r e than does i s o m e r i s a t i o n , and it t h e r e f o r e seemed p o s s i b l e t h a t t h e i n t r o d u c t i o n of a d d i t i v e s s e r v i n g t o d e c r e a s e t h e a v e r a g e number of a d j a c e n t Ru atoms might d i m i n i s h h y d r o g e n o l y s i s a c t i v i t y , and t o p e r m i t s k e l e t a l i s o m e r i s a t i o n t o p r o c e e d on t h e r e m a i n i n g smaller a c t i v e
centres.
We have a l r e a d y p u b l i s h e d ( r e f . 7 ) p r e l i m i n a r y r e s u l t s which showed
t h a t Ru/Ti02 c a t a l y s t s e s p e c i a l l y a f t e r HTR gave f a i r s e l e c t i v i t i e s t o i s o m e r i s e d p r o d u c t s (SI), and t h a t t h e s e w e r e enhanced by s u l p h i d a t i o n .
Since
175
TABLE 3 Rates o f h y d r o g e n o l y s i s ( r ) and of i s o m e r i s a t i o n ( r ) of nyhexane o v e r Ru/TiO
I
H
2
a t 160 o r 36OoC
Catalyst
H/Ru
HTRl LTR HTR2
0.27 0.84 0.24
HTRl LTR HTR2
0.26 0.95 0.19
172
HTRl LTR HTR2
0.15 0.62 0.11
22 550 13
0.1% Ru/Ti02 Impr.
0.1% Ru/Ti02 I E
0.5% Ru/Ti02 Impr.
R a t e s i n mmol g-' h-'; Ru
360
r 160 H
Pretreatment
rH
-
3.9 269
-.
60
-
35
-
-
160 I
3.7
-
-
360 I
6.1 15
sI
-
-
-
49
-
64
51
-
3.5 5 3.2
57
-.-
0.61 0.05 0.45 0.65 0.23 0.94 0.14 0.01 0.19
SI = r I / ( r H + r I )
were g r e a t e r f o r 0.5% Ru l o a d i n g t h a n f o r 5X, we have now e x t e n d e d 1 o u r s t u d i e s t o embrace c a t a l y s t s h a v i n g o n l y 0.1% Ru.
v a l u e s of S
Table 3 shows t h e r e s u l t s o b t a i n e d f o r t h e r e a c t i o n of n-C6H14 on t h r e e Ru/Ti02 (Degussa P-25)
catalysts:
0.1 and 0.5% Ru/TiO
i m p r e g n a t i n g t h e s u p p o r t w i t h a s o l u t i o n o r RuCl
3'
made by ion-exchanging t h e s u p p o r t w i t h [Ru(NH3)d
were made by 2 2H20, and 0.1% Ru/TiO
'+.
was 2 The f o l l o w i n g a r e t h e
s a l i e n t f e a t u r e s of t h e r e s u l t s .
( i ) R a t e s o f h y d r o g e n o l y s i s a r e always much
l a r g e r a f t e r LTR t h a n a f t e r HTR.
( i i ) I s o m e r i s a t i o n s e l e c t i v i t i e s a r e always
l e a s t a f t e r LTR, and g r e a t e s t f o r t h e I E c a t a l y s t a f t e r any p r e t r e a t m e n t . Values of H/Ru measured v o l u m e t r i c a l l y a t room t e m p e r a t u r e a f t e r p r e t r e a t m e n t s d u p l i c a t i n g t h o s e used b e f o r e t h e c a t a l y t i c r u n s a r e a l s o shown i n Table 3. d i f f e r e n c e s i n SI between t h e two 0.1% Ru/TiO
The
c a t a l y s t s cannot be a t t r i b u t e d t o
2 d i s p e r s i o n e f f e c t s , b u t a r e more p r o b a b l y a s s o c i a t e d w i t h r e s i d u a l i m p u r i t i e s ,
s u c h as C1- remaining from t h e p r e p a r a t i o n u s i n g RuC13.
The lower H/Ru v a l u e s
found w i t h t h e 0.5% Ru/Ti02 c a t a l y s t a r e a s s o c i a t e d w i t h h i g h e r h y d r o g e n o l y s i s
r a t e s and lower i s o m e r i s a t i o n r a t e s . DISCUSSION The m o t i v a t i o n u n d e r l y i n g t h e work d e s c r i b e d i n t h i s p a p e r i s t o seek t o encompass t h e e f f e c t s of a l l o y i n g , b i m e t a l l i c c l u s t e r f o r m a t i o n , s e l e c t i v e p o i s o n s , a d d i t i v e s and s u p p o r t e f f e c t s ( i n c l u d i n g t h o s e i n d u c i n g SMSI), C d e p o s i t i o n and p a r t i c l e s i z e e f f e c t s w i t h i n a s i n g l e model based on ensemble s i z e and e l e c t r o n i c environment of t h e a c t i v e c e n t r e .
T h i s concept i s now
i l l u s t r a t e d by r e f e r e n c e t o ( i ) t h e Pt-Re/A1203 system, ( i i ) Ru/Ti02 and Ti02Ru/Si02 c a t a l y s t s , ( i i i ) o t h e r t r a n s i t i o n m e t a l o x i d e a d d i t i v e s , and
176
( i v ) n-hexane i s o m e r i s a t i o n on low-loading Ru/Ti02 c a t a l y s t s . The r e s u l t s i n T a b l e 3 c o n f i r m t h a t 0.1% Ru/TiO e f f e c t i v e f o r n-C6H14 i s o m e r i s a t i o n :
c a t a l y s t s a f t e r HTR are 2 t h e h i g h e r d i s p e r s i o n and c l e a n m e t a l
s u r f a c e produced by o x i d a t i o n and LTR g i v e mainly h y d r o g e n o l y s i s .
Ru atoms a r e u n a f f e c t e d by T i 0
However t h e
s u g g e s t t h a t a f a i r p r o p o r t i o n of t h e s u r f a c e
H/Ru r a t i o s a f t e r HTR (0.19-0.27)
s p e c i e s , which however r e d u c e t h e a v e r a g e Ru
ensemble s i z e t o t h e p o i n t where i s o m e r i s a t i o n becomes p o s s i b l e .
The lower H/Ru
r a t i o s found a f t e r HTR w i t h 0.5X Ru/Ti02 are a s s o c i a t e d w i t h lower r a t h e r than h i g h e r v a l u e s o f SI.
T h i s i s p r o b a b l y due t o a combination of l a r g e r p a r t i c l e
s i z e and a d e c r e a s e d e l e c t r o n i c m o d i f i c a t i o n by t h e reduced s u p p o r t . Table 2 p r o v i d e s e v i d e n c e t h a t T i 4 a d d e d t o Ru/Si02 (Ti:Ru = 5 . 4 ) can produce a f t e r HTR e f f e c t s analogous t o SMSI b o t h on r a t e s and on p r o d u c t d i s t r i b u t i o n s . Values of S 2 above one and o f t h e s p l i t t i n g f a c t o r F' g r e a t e r than 0 . 3 a r e a l s o i n d i c a t i v e of r e a c t i o n a t s i t e s c o n s t r a i n e d by a d j a c e n t l y adsorbed s p e c i e s such
.
as T i 0
TOF v a l u e s a r e however s t i l l t h o s e t y p i c a l of Ru/Si02, s u g g e s t i n g T i 0
d e c o r a t i o n does n o t i n f l u e n c e s i t e r e a c t i v i t y i n t h e way t h a t o c c u r s when T i 0 2 i s t h e s u p p o r t . O x i d a t i o n and LTR r e s t o r e s t h e normal p r o p e r t i e s o f Ru/SiO
2;
presumably T i 0
becomes r e - o x i d i s e d and m i g r a t e s back t o t h e s u p p o r t .
A second
HTR c a u s e s i t t o r e t u r n . i n low c o n c e n t r a t i o n (Ti:Ru = 0 . 2 5 ) h a s l i t t l e 2 e f f e c t on c a t a l y t i c b e h a v i o u r ( F i g . 2 ) , p e r h a p s b e c a u s e i t i s mainly a s s o c i a t e d Ti0
2
when added t o Ru/SiO
with the Si02.
O t h e r o x i d e s (Cr, Mn, e s p e c i a l l y V) however have much g r e a t e r
e f f e c t s , a l t h o u g h o x i d e s of t h e e a r l y t r a n s i t i o n e l e m e n t s have l i t t l e e f f e c t ( F i g . 2).
We suppose t h a t much depends upon t h e e x t e n t of i n t e r a c t i o n o f t h e
o x i d e s w i t h Ru02 a f t e r c a l c i n a t i o n ;
i t a p p e a r s t h a t mixed p h a s e s may b e formed
between RuO, and t h e o x i d e s of V , C r and Mn.
There i s much d e t a i l e d c h e m i s t r y
L
t o be u n r a v e l l e d b e f o r e a c l e a r p i c t u r e emerges. F i n a l l y , o u r r e s u l t s on A1 0 - s u p p o r t e d P t , R e and P t - R e
2 3
g i v e c l e a r p r o o f of
t h e f o r m a t i o n of b i m e t a l l i c c l u s t e r s when b o t h e l e m e n t s are p r e s e n t .
These
c l u s t e r s have s i t e s f o r h y d r o g e n o l y s i s a t which v a l u e s o f F a r e h i g h e r than f o r P t , and s o i n some s e n s e they are ' c o n s t r a i n e d ' a s a r e Ru s i t e s i n contaminated
Ru c a t a l y s t s .
We b e l i e v e , f o l l o w i n g i d e a s i n t h e l i t e r a t u r e ( r e f . 5 ) , t h a t
a l k a n e s chemisorb on a c t i v e s i t e s comprising b o t h P t and Re atoms. Pt/SiO
2
However
c a t a l y s t s do n o t show t h e same r e a c t i o n c h a r a c t e r i s t i c s as Pt/A1203;
indeed t h e y behave r a t h e r more l i k e Pt-Re/A1203.
C l e a r l y t h e r e i s much more t o
be l e a r n t about small metal p a r t i c l e s and c l u s t e r s .
177 ACKNOWLEDGEMENTS F i n a n c i a l s u p p o r t f o r R.R.R. Research Council, and f o r R.Y. acknowledged.
and M.R.G.
from t h e Science and Engineering
from t h e Government of Malaysia, i s g r a t e f u l l y
Both t h e SERC and t h e EEC (under t h e i r S t i m u l a t i o n Action
Programme) provided funds f o r a p p a r a t u s .
REFERENCES
1 G.C. Bond, R.R. Rajaram and R. Burch, i n M . J . P h i l l i p s and M. Ternan ( E d i t o r s ) , Proc. 9 t h I n t . Congr. Catal., Calgary, Canada, J u l y 1 9 8 8 , Chem. I n s t . Canada, 1 9 8 8 , v o l . 3 , pp. 1130-1137. 2 Z. Hu, H. Nakamura, K. Kunimori, H. Asano and T. Uchijima, J. C a t a l . , 1 1 2 ( 1 9 8 8 ) 478-488, 3 4
Tang Sheng, Xiong Guoxing and Wang Hongli, J. C a t a l . , 111 ( 1 9 8 8 ) 136-145. J . C . Kempling and R.B. Anderson, Ind. Eng. Chem. Proc. Des. Dev., 11 ( 1 9 7 2 )
5
S.M. Augustine and W.M.H. S a c h t l e r , J. C a t a l . , 1 0 6 ( 1 9 8 7 ) 417-427. G.C. Bond and Xu Yide, J. Chem. SOC. Farad. Trans. I , 80 ( 1 9 8 4 ) 969-980. R. Burch, G.C. Bond and R.R. Rajaram, J. Chem. SOC. Farad. Trans. I , 82
146-151. 6 7
( 1 9 8 6 ) 1985-1998.
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C. Morterra,A.Zecchinaand G. Costa (Editors),Structure and Reactiuity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A
Study of Supported Pt Catalysts by X-Ray Absorption Spectroscopy
M.J.P. Botman, A.J. den Hartog and V. Ponec Gorlaeus Laboratories, Leiden University P.O. box 9502, 2300 RA Leiden (The Netherlands)
ABSTRACT A series of well reduced Pt catalysts with a known amount of extractable ('ionic') Pt has been studied by an analysis of the X-ray absorption near the edge (white line). Two methods were applied to evaluate the white line data. One of these methods reveals some correlation between the intensity of the edge absorption and the content of 'ionic' Pt as determined by chemical extraction, but probably the particle size effects complicate this correlation.
INTRODUCTION
It has been known for some time that catalysts of the same macroscopic composition can differ in their catalytic properties when different preparation methods are used or when the conditions during the preparation are different. It is of great theoretical and practical importance to determine the relation between the different synthesis routes and the catalytic behaviour of the catalysts. Earlier studies have shown that the state of the metal component in supported metal catalysts can influence the selectivity patterns (1-4). For example, in the case of Pt/A1203 it was shown that the ionic Pt plays a role in the dehydrocyclization of hexane ( 4 ) . In the case of Pd catalysts on various supports a much larger effect was found in the synthesis gas chemistry (5). In that case, a linear correlation was found between the activity and selectivity to methanol and the amount of ionic Pd as determined by a chemical ion extraction method. An indirect effect of the existence of the ionic form of the metal component is that the ions can influence the dispersion of
179
the catalyst, acting as an anchor and stabilizing the smallest particles ( 6 ) . It is desirable to compare the results of the above mentioned extraction techniques with those obtained by another, physical method. In the literature a relation has been found between the intensity of the white line in the X-ray absorption spectrum (the absorption at the X-ray absorption edge) and the presence of the ionic form of a metal like Pt in the catalysts (7-11). This can be understood by considering that in ionic Pt the occupation by the electrons of the 5d shell is less than that of metallic Pt. This results in a higher probability of transitions of core level electrons to this state and thus in a larger absorption. In the literature two methods of analysis of the absorption spectrum are described: one by Gallezot and coworkers (method I) (10) and another by Mansour et al. (method 11) (8). These two methods are compared in this study and both methods are compared with the results of the chemical analysis of the amount of ionic Pt in the catalysts. EXPERIMENTAL
Illbe X-ray absorption experiment The X-ray absorption spectra have been measured at the Synchrotron Radiation Source (SRS) at Daresbury (U.K.), EXAFS station 9.2. The ring energy was ca. 2 GeV, ring current 150-250 mA. Both the LII and LIrI edges were monitored since the 'method 11' uses both the edges for the data evaluation. The time required to measure one spectrum was about 3 hours. The samples were reduced in situ (for 3 hours at 725 K)in an absorption cell developed at the Technical University of Eindhoven, the Netherlands. The cell allows experiments to be performed either with the gas present or in vacuum. Details have already been published (12). During the experiment the cell was kept at 90 K in H2. The sample was in the form of a self-supporting wafer. The amount of catalyst used was calculated using the absorption coefficients of the various components (typically about 150 mg is used). ation and characterization Most catalysts studied contained 1 %wt. Pt on 7 A1203 (CK300, Ketjen Amsterdam); only the catalyst denoted 'F'
181
contained 5 Bwt Pt/A1203 and EUROPT I (catalyst H) is a catalyst with 6.3 % wt supported on SiO2 (13-15). Except for the EUROPT I, all catalysts were prepared by impregnation of y A1203 with various precursors; the coding of the catalysts is as follows: Catalyst A was made by stirring H2Pt(OH)6 in H20 at 8OoC for several days during which adsorption on the A1203 took place. Hereafter H20 was evaporated and the catalyst dried. Catalysts B and C were made from precursors obtained by dissolution of H2Pt(OH)6 in HNO3. Catalyst D was made by dissolving H2PtC16'6H20 in water. E and F were prepared by dissolving the same precursor in HC1 and catalyst G was prepared from a precursor formed by dissolving Pt black in aqua regia. Catalyst B was dried in air at 475 K, all other catalysts at 385 K. Catalysts 0 , C, D and E were calcined at 725 K, catalyst G at 625 K. Catalyst E was reduced in flowing H2 containing water vapour, in order to produce large Pt particles. These variations in the preparation were applied in an attempt to prepare catalysts that differ in particle size and/or Pt ion content. The mean particle size was determined by the High Resolution Electron Microscope (HREM), Jeol 200 (Antwerp, Belgium). This information has been checked by CO adsorption under standard condition and using the EUROPT I as a standard. Adsorption of CO was determined from the flow of gasses at 273 K, by using a Quantasorb apparatus (Quantachrome). The chemical extraction method of Ptn' determination has been described elsewhere (1-3). Extraction was performed under exclusion of air by oxygen free liquid, ethylenediamine containing 4 % acetic acid, at 383 K, during 15 hours (or for a longer time). Pt was determined in the extracts by Atomic Absorption Spectroscopy (AAS). eval&ation of t& white line The raw data were first subjected to the same procedures as in the EXAFS determination: background correction is performed and the position of the inflection point in the absorption edge spectra is fixed. In method I only the single LIII edge data are used. With this method the primary spectrum is normalized by dividing all points of it by the value of the absorption at the foot of the white line. (That is, at the deepest point of the first valley followthe edge absorption). Finally the area under the
182
absorption curve is determined for the part of the curve exceeding unity. This is demonstrated in figure 1.
I -10.
0.
:c.
-
60.
40.
ElPVl
FIGURE 1 The normalized absorption spectra at the L I ~ Iedge of Pt-foil (dotted line), catalyst C (solid line) and the difference spectrum of both lines (the lowest curve) as functions of the energy (ev). In method 11, the absorption of both the L I and ~ LIII edge is determined. The authors ( 8 ) derived an equation, which can be used when the number of d-holes in a reference compound is known:
where hT is the total number of d-holes per atom Pt for the sample and the reference respectively. The values Ai are obtained by calculating the area under the absorption curve (abs. coefficient versus energy) at edge i up to a certain standard energy. 6 Ai is the difference in this area for the sample As and the reference compound A, (Pt foil); 6 Ai = Ais Air. The denominator divided by hT (ref) indicates the area under the curves per one unoccupied d-electron state (per one hole). The reference used here is Pt foil, for which the number of unoccupied d-states is taken as 0.3/atom. Other details of the procedure as well as the derivation of the formula are given in the original paper by Mansour et al. (8). However there are some problems to be mentioned. The first problem concerns the vertical normalization, the second the most appropriate region
183
of energies for determining the Ai-areas under the absorption curves. The procedure applied in this paper starts with normalization of absorption curves obtained with the sample to that of the reference. This has not been done exactly as in the original reference, but in a slightly different way. First the LII and LIII EXAFS were fitted as well as possible to each other and to the reference in the range between 30 and 100 eV. This range is dominated by the Pt structure and since in all cases Ptcatalysts are compared with Pt foil, this seems to be an appropriate method. Hereafter, edge alignment was performed to fit the absorption curves as in ref 0 . The LII edge is used here as a reference since the intensity of the white line of this edge is hardly influenced by the occupation of electrons of the 5d level and is thus insensitive to changes in the amount of ions in the samples tested. Therefore, the inflection point does not shift to lower energies. The problem of the proper energy range over which the area under the absorption curve is determined is more complicated. Fortunately, the consequences of the arbitrariness in this choice are not very pronounced. Theoretically the influence of the white line, i.e. the broadening of the primary absorption line, reaches up to 30 eV. However, in this region the broadening starts to overlap with the effects of the coordination spheres, i.e. with the effects of the structure of the surroundings (described usually as 'scattering'). The authors in ref. 0 suggest explicitly the use of the region between -10 and +13 eV round the inflection point, but in one type of determination they used the spectrum from -10 to +40 ev. We tested the results obtained by integration up to 30 eV and found that the absorption amplitude of the Pt reference spectrum above 13 eV were such that the contribution of the white line to the absorption in Pt catalyst samples appears to be negative (as compared with the reference). The same problem of apparently 'negative area' also occurs below the 0 eV limit. When one integrates from 0 up to 13 eV such non physical behaviour is excluded. This confirms the suitability of the restricted 0 to 13 eV energy region for the determination of Ai. This choice is probably the best that can be done at the moment but it is not without problems either. 'Negative areas' can still occur in the region just above the
184
-
inflection point and it is not clear how to improve (or whether to accept) this situation.
RBSOLTS
Figure 2 shows the LIII-edge absorption spectra of the reference material (Pt foil) and of two catalysts ( A and D).
Abs
I
FIGURE 2 The LIII X-ray absorption edge spectra of a: the Pt foil, b: catalyst A and c: catalyst D
a.u.
The most serious problem encountered with this method is immediately noticeable. Whereas the Pt foil and catalyst A show a structure with two minima above the absorption edge, catalyst D shows only one minimum which doesn't coincide in energy with any minimum of the other two samples. Apparently, there is no criterion available to decide whether one can identify the minimum found with catalyst D with the first or the second minimum and -perhaps - in the second case integrate only up to the inflection point of the descending side of the edge absorption peak. However, the area above the value of unity (see 'experimental' for details) is quite different with both alternatives and so also is the conclusion concerning the number of unoccupied d-states. Inspection of table I leads to the conclusion that the disappearance of the first minimum (or the shift) probably occurs when the metal particles are very small (catalyst D) or when a high percentage of Pt is present in its ionic (extractable) scate. This is useful information which
185
is difficult to quantify, but which might have value as a fingerprinting feature. The problem just mentioned throws some doubt on the applicability of this method to samples that do show a two minimum structure in the discussed energy range. The value of absorption at the minimum is probably not a good point to normalize the spectra and no point is clearly better €or this purpose. This conclusion has also been reached by other authors, on different grounds (16). TABLE 1 Determination by method I for selected catalysts.
Pt bulk A B C
D G
1St minimum E (eV) Ao(a.u.)
2nd minimum E (eV) Ao(a.u.)
# nm
D* %
%
10.69 12.83 12.97
21.39 21.52 21.67 20.67 21.22 21.61
-
28 51 54 112 115
0 0.5 7.2 9 2
1.58 1.53 1.83
not detectable
___ __-
I I
---
I I
---
-
2.18 2.63 2.56 3.0 2.51 2.28
2.6 2.2 0.8 1.3
I+
4
*
: H/Pt or CO/Pt in % I+ : Pt"+/Pttot. : A is area under the line, above the value of unity # : Pt particle diameter (EM)
This method has also some inherent problems. First, the information required (the variations in the number of d-holes caused by changes in preparation of the samples) is obtained by comparing the area under the absorption curves, as determined by integration in a chosen energy range. However it is not a priori clear how to choose this range and thus various ranges are used in the literature. To examine the effect of the energy range over which the absorption curves are integrated, the values 6 = 6 A3 + 1.11 6 A2 were calculated for the -10 to + 4 0 eV range and for the -10 to 1 3 eV range (the value b is proportional to the changes in the occupation of the d-states). The maximum observed change in 6 was about 2 0 % .This might seem to be large on first glance but the uncertainty is still acceptable (it is often the trend one wants to see not the absolute values). Much more dangerous
is the error one can make in the alignment of the various absorption curves. An error in the alignment of the curves of the reference and a catalyst sample can cause variations in 6 as high as 100 %.Although with this method the method of alignment of the curves can be criticized there is no better procedure available at the moment. Results obtained with equation 1 are presented in table 2 and figure 3 . Integration has been performed using the energy range 0 to 13 eV. Table 2 also presents the data on the content of extractable, presumably ionic, Pt. We shall turn to this comparison in the discussion but the reader should be aware of an important difference between the two methods, X-ray absorption and chemical extraction. With the X-ray absorption, the di- and tetravalent Pt contributes to the white line absorption enhancement, wherever the species might be in the sample. This is probably also true €or the Ptl+, although this is less sure. The chemical extraction method on the other hand determines all the ionic states of Pt (including Ptl+) but only in the uppermost layers of the catalysts. Ionic Pt burried deeply in the support or hidden under metallic particles is probably not reached by the extraction means used in this study ( 1 - 3 ) . It is difficult to judge but it is probably a good guess to expect that the white line method will show either the same or a higher ionicity than the chemical extraction.
WL- area (a.u.1
Po X H
0.35- iTFq
-
x
*
5
XG
-
/
/
B+ ' A
White line absorption area relative to Pt foil as determined by method I1 (Mansour c.s.). x = (a) in table 2 + = (b) in table 2
The results of the experiments shown in table 2 and figure 3 suggest that the correlation between the two methods is rather poor, although for some catalysts a correlation does seem to be present. In general the differences between various catalysts are less pronounced when looking by method I1 than by the chemical extraction method. Then, there is catalyst D, which has a large white line area, larger than is to be expected according to the amount of ionic Pt, even when the limitations of the techniques used are taken into account. The method of preparation of this catalyst does not really justify speculations on Pt ions being hidden or buried in the support. Rather, one would tend to relate the intense white line in this case to the particularly small Pt particles present in this sample. TABLE 2
Determination of the white line area by method I 1 catal.
5da
5db
%Pt2+ c
I(%)d
#(nm)e
0.343 0.352 0.387 0.381 0.347 0.341 0.340 0.359
0.336 0.357 0.387 0.375 0.333 0.336 0.333 0.351
2.5 3.1 5.1 4.8 2.8 2.4 2.4 3.5
0.5 7.2 9
1.2-8.0 2.6 2.2 0.85 1.3 2.5 1.3 1.9
2 4 0.3 4 0.4
a- number of unfilled d-states per Pt atom. Integration from 0 to 13 eV b- as a but integrated over -10 to 13 eV c- under the assumption that all oxidized Pt is Pt2', integrated from 0 to 13 eV d- Ptn+!Pttot determined by chemical method e- particle size determined by E.M. DISCUSSION
The first point to be discussed is the information one can expect to gain by an X-ray absorption study. Obviously, the normalization of spectra is still an open problem and the question is whether the method can work at all in the range of Pt ion concentration which can be expected to exist in the Pt catalysts (0-10%). At this stage the answer should probably be negative.
Irrespective of the quantitative aspects of the white line analysis it is interesting to consider the shape of the absorption curves. Apparently, catalysts with small particles show a 'one minimum' structure instead of a 'two minimum' structure. Application, without an additional analysis, of the procedure I leads to a conclusion that small particles behave as if the d-states of the Pt were much less occupied then in the bulk material. However the basis of this conclusion is questionable. The process of primary photon absorption and its probability can be changed by other aspects of the electronic structure than the occupation of d-states alone. It is indeed known that the occupation changes when going from a massive metal (d9-7s0.3) to free atoms of Pt (d9s1), but it is difficult to predict with available knowledge, what the occupation of the d-states would be of particles in between the above mentioned limits, e.g. in the 1-3nm size region, Consequently there is no firn point which would help to identify the contribution of the various possible effects. The situation is actually even worse than just described. One has to realize that the wiggles following the edge absorption are related to the structure round the absorbing atom. If the structure is disordered, the maxima and minima are less pronounced (like with X-ray line broadening) and a curve like that of catalyst D results (figure 2), where we cannot, with any certainty identify the energy region which would be comparable with that over which one integrates in the case of Pt foil. When comparing the two methods, it is clear that method I is difficult to use whereas the potential information on ionic Pt being overshadowed by other effects. However, method 11 seems to give more information although in this case again, additional effects such as particle size differences may play a role too. In various catalysts with a comparable particle size, the correlation with the chemical analysis of ionic Pt probably does exists. In general, the white line results probably reflect two seperate changes in the catalysts: variations in the metal particle size and variations in the Ptn+/Pttot ratio. A study in literature (16,17) on bulk platinous compounds (Pt', Pt2+ and P t 4 + ) has shown that measurement of the white line absorption is a reliable indication of the changes in the Pt charge. However, monitoring of the white line is not a suitable method to establish quantitatively and next to each other, changes in
189
low amounts of ionic Pt and changes due to the particle size effects when these coexist. ACKNOWLEDGEMENT This research has been made possible by generous financial support provided by S.O.N. ( Stichting Scheikundig Onderzoek, Nederland) and the help extended to them by the laboratory for Inorganic Chemistry at the Technical University of Eindhoven. REFERENCES 1 2 3
4
M.J.P. Botman, L.Q. She, J.Y. Zhang, W.L. Driesen and V. Ponec, J . Catal. 1 0 3 ( 1 9 8 7 ) 2 8 0 T.L.M. Maesen, M.J.P. Botman, T.M. Slaghek L.Q. She, J.Y. Zhang and V. Ponec, Appl. Catal. 2 5 ( 1 9 8 6 ) 3 5 M.J.P. Botman, L.Q. She, J.Y. Zhang, T.L.M. Maesen, T.h. Slaghek and V. Ponec, in ’Strong Metal-Support Interactions‘, eds. R.T.K. Baker, S.J. Tauster and J.A. Dumesic, Am. Chem. SOC. Symp. Ser. 2 9 8 ( 1 9 8 6 ) p. 1 1 0 M.J.P. Botman and V. Ponec, Adv. in Chem. Series, Symp. Series Am. Chem. SOC., Div. Petr. Chem., New Orleans, 32 ( 1 9 8 7 ) 7 2 2
J.M. Driessen, E.K. Poels, J.P. Hinderman and V. Ponec, J.Cata1. 8 2 ( 1 9 8 3 ) , 2 6 T. Huizinga, PhD thesis, Technical University Eindhoven, the Netherlands, 1 9 8 3 Y.I. Yermakov in ‘Studies in Surface Science and Catalysis vol. 7a‘, eds. T. Seiyama and K. Tanabe, Elsevier Scientific Publishing Cie., Amsterdam, 1 9 8 1 p. 5 7 L.L. Murrel and R.L. Garten, Appl.Surf.Science 1 9 ( 1 9 8 4 ) 2 1 8 E.A. Stern, Phys. Rev. B 8 ( 1 9 7 4 ) 3 0 2 7 A.N. Mansour, J.W. Cook jr. and D.E. Sayers, J. Phys. Chem.
5
6
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88 (1984) 2330
F.W. Lytle, J. Catal. 4 3 ( 1 9 7 6 ) 3 7 6 P.Gallezot, R. Weber, R.A. Dalla Beta and M. Boudart, Z. Naturforsch. 3 4 a ( 1 9 7 9 ) 4 0 11 A.N. Mansour, J.W. Cook jr., D.E. Sayers, R.J. Emrich and J.R. Katzer, J. Catal. 8 9 ( 1 9 8 4 ) 4 6 2 B. Moraweck, P. Bondol, D. Goupil, P. Fouillaux and A.J. Reaonprez, J. de Phys. Colloq. C8, Suppl. 1 2 ,
9 10
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D.C. Koningsberger and J.W. Cook in ‘EXAFS and Near Edge Structures’, eds. A. Bianconi, L. Incoccia and S . Stopchich, Springer Verlag, Berlin (W), 1 9 8 3 p. 4 1 2 G.C. Bond and P.B. Wells, Appl. Catal. 18 ( 1 9 8 5 ) 2 2 5 J.W. Geus and P.B. Wells, Appl. Catal. 1 8 ( 1 9 8 5 ) 2 3 1 P.B. Wells, Appl. Catal. 1 8 ( 1 9 8 5 ) 2 5 9 F.W. Lytle, R.B. Greegor, E.C. Marques, D.R. Sandstrom, G.H. Via and J.H. Sinfelt, J. Catal. 9 5 ( 1 9 8 5 ) 5 4 6 F.W. Lytle, R.B. Greegor, E.C. Marques, V.A. Biebesheim, D.R. Sandstrom, D.R. Horsley, G.H. Via and J.H. Sinfelt in ’New Surface Science in Catalysis’, ACS series 2 8 8 ( 1 9 8 5 ) p. 2 8 0 .
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C. Morterra, A. Zecchina and G . Costa (Editors), Structure and Reactivity of Surfaces
191
01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A SIMS AND AES STUDY OF NICKEL DEPOSITION ON T i 0 2 (100).
INFLUENCE
OF THE
STOICHIOMETRY OF THE SUPPORT
S. BOURGEOIS, 0. DIAKITE, F. JOMARD,
F a c u l t 4 des Sciences Mirande, des S o l i d e s , B.P.
M. PERDEREAU and R. POIRAULT.
L a b o r a t o i r e de Recherche s u r l a R 4 a c t i v i t 4
138, 21004 D i j o n cedex (France).
ABSTRACT N i c k e l d e p o s i t i o n on a c l e a n T i 0 2 (100) s u r f a c e was s t u d i e d m a i n l y b y Auger E l e c t r o n Spectroscopy (AES) and secondary i o n mass s p e c t r o m e t r y (SIMS). D i f f e r e n t k i n d s o f s u p p o r t s e x h i b i t i n g a more o r l e s s h i g h d e v i a t i o n f r o m s t o i c h i o m e t r y were used. F o r a q u a s i - s t o i c h i o m e t r i c s u p p o r t a l a y e r - b y - l a y e r growth o f 2 o r 3 n i c k e l l a y e r s i s o b t a i n e d f o l l o w e d b y a n u c l e a t i o n stage. F o r a n o n - s t o i c h i o m e t r i c s u p p o r t n i c k e l i s found t o d i f f u s e i n t o t h e b u l k T i 0 2 even a t room temperature. The i n f l u e n c e o f temperature on t h i s d i f f u s i o n was a l s o s t u d i e d . INTRODUCTION M e t a l - o x i d e i n t e r f a c e s a r e o f fundamental importance i n a l o t o f phenomena such as f o r example
p h o t o c h e m i s t r y o r heterogeneous c a t a l y s i s .
The o x i d e
s u p p o r t seems t o p l a y a p r o m i n e n t p a r t i n p a r t i c u l a r when t h e o x i d e i n v o l v e d i n t h e system i s a r e d u c i b l e o x i d e
: numerous s t u d i e s have been c a r r i e d
o u t on t h e s t r o n g - m e t a l - s u p p o r t - i n t e r a c t i o n concerned w i t h t h e s p e c i a l
phenomena ( r e f .
1-2) which i s
p r o p e r t i e s o f t r a n s i t i o n m e t a l s when t h e y a r e
supported on r e d u c i b l e o x i d e s such as Ti02. Model systems have been s t u d i e d i n p a r t i c u l a r N i / T i 0 2 ( r e f . 3-4).
The s t o i c h i o m e t r y o f T i 0 2 i s v e r y i m p o r t a n t
f o r t h e r e a c t i v i t y o f t h i s o x i d e ( r e f . 5). The purpose o f t h i s work was t o use a model system i n o r d e r t o s t u d y t h e metal growth on t h e o x i d e s u p p o r t and t h e i n f l u e n c e o f t h e s t o i c h i o m e t r y of
the
s u p p o r t on t h e s t r u c t u r e o f t h e d e p o s i t : w e l l - c o n t r o l l e d
amounts were d e p o s i t e d on T i 0 2 possible.
Depositions
were
(100) s u r f a c e s ,
performed
on
nickel
characterized
as w e l l
as
quasi-stoichiometric
samples
on
one hand and on samples e x h i b i t i n g h i g h e r d e v i a t i o n f r o m s t o i c h i o m e t r y on t h e o t h e r hand. The f i r s t stages i n t h e growth were i n v e s t i g a t e d u s i n g m a i n l y Auger E l e c t r o n Spectroscopy (AES) and Secondary I o n Mass Spectrometry (SIMS).
192
EXPERIMENTAL Methods Two e x p e r i m e n t a l ( r e f . 6)
s e t up were used f o r t h i s s t u d y
: i n t h e f i r s t one
n i c k e l d e p o s i t i o n was performed and t h e d e p o s i t was c h a r a c t e r i z e d
b y AES and LEED.
I n t h e second one S I M S e x p e r i m e n t s were c a r r i e d o u t on
the deposit. N i c k e l d e p o s i t i o n was done v i a a r e s i s t i v e l y heated n i c k e l w i r e (Johnson Matthey 99.9997
% pure,
0.25
mm i n d i a m e t e r ) .
The q u a n t i f i c a t i o n
o f the
n i c k e l f l u x was performed : p r e l i m i n a r y c a l i b r a t i o n s were done b y chemical analysis o f the deposit using atomic absorption. Typical
deposited
nickel
quantities
varied
corresponds t o 3 t o 7 e q u i v a l e n t monolayers.
from
1.5
t o 4.10-79.
This
T y p i c a l d e p o s i t i o n r a t e s were
between 0.05 and 0.2 e q u i v a l e n t monolayers p e r m i n u t e . Auger e x p e r i m e n t s were performed w i t h a V a r i a n r e t a r d i n g g r i d a n a l y s e r . Auger i n t e n s i t i e s a t 62 eV (NiL2,3VV),
382eV (TiL2,3M2,3M2,3)
and a t 510eV
( O K L L ) were f o l l o w e d d u r i n g t h e e x p e r i m e n t s . Auger i n t e n s i t i e s o f contaminants were a l s o
recorded.
S I M S e x p e r i m e n t s were c a r r i e d o u t w i t h a R i b e r Q156
apparatus i n a n o t h e r e x p e r i m e n t a l s e t - u p ( r e f . 7 ) . P r e p a r a t i o n and c h a r a c t e r i z a t i o n o f t h e s u p p o r t s The specimen used was a T i 0 2 s i n g l e c r y s t a l w i t h r u t i l e s t r u c t u r e s u p p l i e d by
ti.
Djevahridjan
determined
(Switzerland).
Ti02
slices with
by Laue back r e f l e c t i o n t e c h n i q u e were
u s i n g a diamond embedded s t r i n g saw.
(100)
o r i e n t a t i o n as
c u t from t h i s c r y s t a l
The s l i c e s were m e c h a n i c a l l y p o l i s h e d
u s i n g v a r i o u s grades o f a l u m i n a powders f r o m 3 p m t o D . 3 ~ m i n d i a m e t e r . Mechanical
p o l i s h i n g was f o l l o w e d b y chemical
e t c h i n g i n NaOH 5N heated
a t 340K f o r 1 hour. Samples were t h e n annealed a t 1300K f o r 24 h o u r s i n an open f u r n a c e under an atmospheric p r e s s u r e o f oxygen.
These a n n e a l i n g t r e a t m e n t s were performed
on one hand t o remove t h e damaged zone c r e a t e d b y t h e p o l i s h i n g and on t h e o t h e r hand t o s e t t h e s t o i c h i o m e t r y o f t h e sample. D i f f e r e n t k i n d s o f s u p p o r t s were t h e n used : i ) some s u p p o r t s were
used j u s t a f t e r t h e a n n e a l i n g t r e a t m e n t
previously
d e s c r i b e d : t h e s t o i c h i o m e t r y o f t h e s e samples was t h a t o f T i 0 2 and t h e i r c o l o r was y e l l o w . i i ) some o f t h e s e samples were heated under vacuum (lDm5Pa) a t lOOOK u s i n g an i n f r a r e d lamp (Osram-150W)
f o r 30 min. A f t e r t h i s t r e a t m e n t t h e samples
exhibited a blue color characteristic vacancies).
Their
conductivities
were
o f T i 0 2 w i t h p o i n t d e f e c t s (oxygen about
lo5
higher
than
that
of
193
the yellow deposing t o
samples.
Both k i n d s o f samples were s u b m i t t e d i n s i t u b e f o r e
repetitive
cycles
o f Ar-ions
s p u t t e r i n g (600eV,
2.5 p A.cm-2)
and h e a t i n g a t 800K f o r 10 min.
It s h o u l d be n o t i c e d t h a t t h e c l e a n i n g procedure m o d i f i e d t h e s u r f a c e s t o i c h i o m e t r y o f t h e sample : t h e severe i o n bombardments induced v a r i a t i o n o f t h e s t o i c h i o m e t r y b y p r e f e r e n t i a l s p u t t e r i n g o f oxygen. The s u r f a c e s t o i c h i o m e t r y was checked b y t h e O510/Ti382 AES r a t i o . F o r t h e y e l l o w samples these r a t i o s v a r i e d f r o m 1.5 t o 1 depending upon t h e d e t a i l s o f t h e c l e a n i n g procedure. a (1x3) pattern.
The c o r r e s p o n d i n g LEED diagram was
These AES r a t i o s and t h i s LEED p a t t e r n a r e c h a r a c t e r i s t i c
o f a T i 0 2 s u r f a c e w h i c h does n o t e x h i b i t a h i g h d e v i a t i o n f r o m s t o i c h i o m e t r y ( r e f . 5,8).
These samples w i l l be r e f e r r e d t o as q u a s i - s t o i c h i o m e t r i c samples.
F o r t h e b l u e samples t h e AES r a t i o s v a r i e d f r o m 0.7 t o 0.5. a
(1x5)
LEED p a t t e r n .
s t o i c h i o m e t r y . They
This
i s consistent
with a
They e x h i b i t e d
higher d e v i a t i o n from
w i l l be r e f e r r e d t o as n o n - s t o i c h i o m e t r i c samples.
RESULTS AND DISCUSSION The same procedure was used b o t h f o r t h e q u a s i - s t o i c h i o m e t r i c and f o r t h e n o n - s t o i c h i o m e t r i c
support
one : w e l l determined n i c k e l q u a n t i t i e s were
evaporated and t h e AES peak h e i g h t s o f b o t h adsorbate ( N i ) and s u b s t r a t e ( T i , 0) were f o l l o w e d versus d e p o s i t i o n t i m e i n o r d e r t o determine t h e n i c k e l g r o w t h mechanism ( r e f . 9). Several experiments were performed w i t h d i f f e r e n t t o t a l d e p o s i t i o n t i m e s which corresponded t o 3 t o 7 e q u i v a l e n t monolayers.
SIMS experiments were c a r r i e d o u t t o c o n f i r m t h e AES r e s u l t s c o n c e r n i n g t h e morphology o f t h e d e p o s i t : t h e n i c k e l d e p o s i t was s p u t t e r e d and t h e SIMS i n t e n s i t i e s N i '
f o r t h e d e p o s i t and Ti'
v e r s u s t h e s p u t t e r i n g time.
f o r t h e s u b s t r a t e were f o l l o w e d
F o r t h e s t u d y o f t h e s e t h i n d e p o s i t s SIMS was
used i n a m i d d l e mode ( p r i m a r y i o n d e n s i t y Jp = 1 0 l 2 ions.cm-*.s-l,
primary
i o n energy = 1 keV) t h a t i s t o say w i t h t y p i c a l s p u t t e r i n g r a t e o f about
0.2
1 min-1.
The r e s u l t s were model i z e d u s i n g t h e s e q u e n t i a1 1a y e r s p u t t e r i n g (SLS) model (ref.
10-11) : t h e SLS model can be used t o m o d e l i z e t h e s i g n a l s t h a t should
be o b t a i n e d f o r t h e s u b s t r a t e and f o r t h e d e p o s i t i n t h e v a r i o u s cases o f c r y s t a l growth ( r e f .
12).
the experimental r e s u l t s .
The s i m u l a t e d i n t e n s i t i e s were t h e n compared w i t h
Quasi s t o i chi ometri c support The Auger i n t e n s i t i e s o f nickel, titanium and oxygen were recorded versus deposition
breaks a t regular t i m e i n t e r v a l s
These curves e x h i b i t
time.
characteristic o f a layer-by-layer
growth mechanism ( r e f .
12).
A f t e r the
completion of 2 o r 3 monolayers (depending on the t o t a l deposition time) the experimental curves e x h i b i t deviations from the layer-by-layer theoretical curves
: the experimental
v a r i a t i o n i s slower than the theoretical
one.
This can be explained by the formation o f nickel islands. The c o e f f i c i e n t s o f attenuation of
0) through a monolayer
the substrate Auger peaks (Ti,
o f adsorbate ( N i ) derived from these experimental
curves are i n correct
agreement w i t h the values o f the i n e l a s t i c mean f r e e path obtained by other authors (ref. 13).
SIMS experiments were performed on these deposits.
One example o f the
SIMS i n t e n s i t i e s variations as a function o f the sputtering time i s shown i n f i g u r e 1 f o r a deposit o f about 7 equivalent monolayers.
20
The r e s u l t s
Ni
0
100
200
t (min.)
Fig. 1. Normalized SIMS i n t e n s i t i e s as a function o f sputtering time f o r a N i deposit on quasi-stoichiometric Ti02 (100).
concerning
Tit
i n t e n s i t y c@n be well
f i t t e d by the SLS model w i t h a
deposit composed o f 3 H i monolayers w i t h islands on then. I n the case o f the curve presented here, the islands would occupy 40 % o f the surface and would be 13 monolayers high b u t t h i s depends on the deposition time. This r e s u l t i s i n correct agreement w i t h the evaporated n i c k e l amount which
i s 7 equivalent monolayers i n t h i s experiment.
195
Moreover N i t i n t e n s i t y cannot be f i t t e d b y t h e same model : t h e h y p o t h e s i s t h a t t h e i o n y i e l d i s c o n s t a n t d u r i n g t h e s p u t t e r i n g cannot be v a l i d f o r t h i s k i n d o f d e p o s i t . The chemical environment o f n i c k e l changes a l o t between t h e n i c k e l i n t h e i s l a n d s surrounded o n l y b y o t h e r n i c k e l and i n t h e l a y e r s where oxygen o f t h e s u b s t r a t e g r e a t l y i n f l u e n c e s t h e n i c k e l i o n y i e l d .
Non-stoichiometric support AES r e s u l t s For the non-stoichiometric
support w i t h consecutive n i c k e l
depositions
up t o a b o u t 1 e q u i v a l e n t monolayer t h a t i s t o say w i t h t h e same procedure as
for
the quasi-stoichiometric
observed. nickel
support,
no n i c k e l
AES
signal
could
be
Another e x p e r i m e n t a l procedure has t h e r e f o r e be chosen : l a r g e r ( f r o m 2 t o 4 e q u i v a l e n t monolayers) were d e p o s i t e d a t
quantities
once. The n i c k e l AES peak h e i g h t was t h e n r e c o r d e d and i t s v a r i a t i o n f o l l o w e d versus t i m e . T i t a n i u m i n t e n s i t y was a l s o recorded. T y p i c a l r e s u l t s a r e shown in
2
figure
- r m Z I
-
0
for
-
I X i
,zg I
20
samples
.
I
40
.
with
different
L Ti(380eV) I
.
60
I Z I r I
, b
80
deviations
t (min)
0
10 20
I
from
I
I
io
40
stoichiometry
Ti(380eV)
50
’
t (rnin)
F i g . 2. Auger s i g n a l s ( a r b i t r a r y u n i t s ) as a f u n c t i o n o f t i m e a t room temperat u r e f o r d i f f e r e n t deviations from stoichiometry.
characterized
by
different
O/Ti
ratios.
An i m p o r t a n t decrease o f t h e n i c k e l s i g n a l i s observed on b o t h samples f o l l o w e d by a s t a b i l i z a t i o n a f t e r about 30 t o 40 min. The T i Auger i n t e n s i t y remains c o n s t a n t d u r i n g t h e experiment. signal
T h i s decrease o f t h e Auger n i c k e l
can h a r d l y be e x p l a i n e d b y d e s o r p t i o n o f n i c k e l because a l l
work i s performed a t room temperature.
this
196
One p o s s i b l e e x p l a n a t i o n i s t h e d i f f u s i o n o f n i c k e l i n t o t h e b u l k TiO2. Owing t o t h e s h o r t mean f r e e p a t h o f Auger e l e c t r o n s i n s o l i d s , n i c k e l atoms d i f f u s i n g i n t o t h e b u l k T i 0 2 b y more t h a n a b o u t 3 monolayers w i l l n o t be detected. I t s h o u l d be n o t i c e d t h a t t h e decrease o f t h e n i c k e l AES i n t e n s i t y i s
a f u n c t i o n o f t h e s t o i c h i o m e t r y o f t h e sample. F o r t h e sample w i t h O/Ti = 0.7 t h e decrease o f t h e N i s i g n a l i s about 30 % whereas i t i s about 40 % f o r t h e l e s s s t o i c h i o m e t r i c sample w i t h O/Ti from s t o i c h i o m e t r y ,
= 0.5
: the higher the deviation
t h e h i g h e r t h e decrease o f t h e n i c k e l
t o say t h e d i f f u s i o n o f n i c k e l
i n t o t h e b u l k Ti02.
signal t h a t i s
The n i c k e l d i f f u s i o n
processus can be p r o b a b l y d i r e c t l y r e l a t e d t o t h e oxygen vacancies i n t h e s u b - s u r f a c e zone o f t h e s u p p o r t . I n order t o confirm the d i f f u s i o n o f nickel i n t o Ti02 the influence o f temperature
has been s t u d i e d
: after stabilization
the nickel
of
signal
a t room t e m p e r a t u r e t h e sample was heated up t o 700K. F i g u r e 3 shows t h e
100
50
0
I
Ni (60eV)
Ti(380eV)
! . , , , * , . , .P ,
200
300
400
500
600
T(K)
700
F i g . 3. Auger s i g n a l s ( a r b i t r a r y u n i t s ) as a f u n c t i o n o f temperature. v a r i a t i o n s o f d i f f e r e n t Auger peak h e i g h t s as a f u n c t i o n o f temperature, a f t e r s t a b i l i z a t i o n o f t h e s i g n a l s a t each temperature. c o n s t a n t up t o 550K and t h e n i n c r e a s e s s l i g h t l y . temperature.
Ti
signal
remains
N i s i g n a l decreases v e r s u s
T h i s decrease can be e x p l a i n e d e i t h e r by a s i n t e r i n g o f n i c k e l
o r by d i s s o l u t i o n o f n i c k e l i n t o Ti02.
This l a s t hypothesis i s consistent
w i t h t h e f a c t t h a t t h e T i i n t e n s i t y i s c o n s t a n t up t o 550K.
197
SIMS r e s u l t s SIMS e x p e r i m e n t s were t h e n performed on t h e n i c k e l / n o n s t o i c h i o m e t r i c s u p p o r t systems a f t e r s t a b i l i z a t i o n a t room temperature. T y p i c a l v a r i a t i o n s o f Ti'
SIMS i n t e n s i t i e s as a f u n c t i o n o f t h e
and N i f
s p u t t e r i n g t i m e a r e shown i n f i g u r e 4 i n t h e case o f a d e p o s i t o f a b o u t
4 e q u i v a l e n t monolayers. using the
R e s u l t s o f s i m u l a t i o n s o f Ti'
Ti
,,i 20
n
"0
and N i t
intensities
SLS model a r e a l s o shown i n f i g u r e 4. The f i t drawn here f o r Ti'
"R 20
4
l-
20
'I:
Ni
0
. -0 40 60 80 t (min.) Experimental
4z
NI
m 20
1
40
60 8'0 t (min.) Theoretical
F i g . 4. a. n o r m a l i z e d SIMS i n t e n s i t i e s as a f u n c t i o n o f s p u t t e r i n g t i m e f o r a N i d e p o s i t on n o n - s t o i c h i o m e t r i c T i 0 2 (100). b. S i m u l a t i o n o f SIMS i n t e n s i t i e s u s i n g t h e SLS model. T : Average l i f e t i m e o f one N i monolayer.
corresponds t o t h e s p u t t e r i n g o f one monolayer having a d e n s i t y o f Z . l O I 5 at.cN2
and a s p u t t e r i n g y i e l d o f 2.
T h i s v a l u e i s i n c o r r e c t agreement
w i t h t h e s p u t t e r i n g y i e l d s g e n e r a l l y o b t a i n e d f o r Ni'
s p u t t e r e d b y Ar'
ions
w i t h a p r i m a r y energy o f 1 keV ( r e f . 14). The e x p e r i m e n t a l N i t
i n t e n s i t y can be c o n s i d e r e d as t h e sum o f t h e N i t
f i t c o r r e s p o n d i n g t o t h e s p u t t e r i n g o f one monolayer and o f t h e c o n t r i b u t i o n o f a n o t h e r N i + s i g n a l . T h i s s i g n a l i s maximum a f t e r about 15 min o f s p u t t e r i n g and t h e n decreases f o r h i g h e r s p u t t e r i n g t i m e s .
The v a l u e o f t h i s maximum
i s c o n s i s t e n t w i t h t h e t i m e needed t o s p u t t e r t h e f i r s t n i c k e l monolayer. T h i s Nit s i g n a l corresponds t o t h e c o n t r i b u t i o n o f n i c k e l which has d i f f u s e d
i n t o bulk TiOp. The these
influence o f
heating treatments
non-stoichiometric
samples
:
has a l s o been s t u d i e d b y
samples
were
heated
up
to
SIMS on 550K and
198
sputtering of
experiments
T i + and N i +
were
performed
after
cooling.
Typical
evolutions
i n t e n s i t i e s are shown i n f i g u r e 5 as a f u n c t i o n o f the
Fig. 5. Normalized SIMS i n t e n s i t i e s as a f u n c t i o n o f s p u t t e r i n g time a f t e r heat ing treatment a t 550K.
s p u t t e r i n g time.
The N i t
an
i n t e n s i t y e x h i b i t s a behaviour which i s s i m i l a r
t o the s i g n a l o f t h e d i f f u s e d n i c k e l seen i n f i g . 4 : a f t e r h e a t i n g a t t h i s temperature a l l t h e n i c k e l has d i f f u s e d i n t o t h e b u l k Ti02.
CONCLUSION The s t o i c h i o m e t r y o f the support p l a y s a prominent p a r t i n t h e growth mechanism which can occur when n i c k e l i s deposited on Ti02 (100) surfaces. This i s i n agreement w i t h t h e r e s u l t s showing the importance o f the d e f e c t s and of the s t o i c h i o m e t r y o f t h e support on t h e n a t u r e o f t h e i n t e r f a c e i n a Pd/Al203 system ( r e f . systems ( r e
. 16-17).
13) and on the k i n d o f growth i n P t o r Fe/A1203
I n the case o f a q u a s i - s t o i c h i o m e t r i c support t h e growth o f n i c k e l i s a layer-by- ayer one up t o about 3 monolayers. Then i s l a n d s o f n i c k e l grow.
For a n o n - s t o i c h i o m e t r i c support the mechanism i s d i f f e r e n t : no i s l a n d growth i s
found and d i f f u s i o n
of
nickel
i n t o T i 0 2 occurs even a t room
temperature. The amount o f n i c k e l which d i f f u s e s i n t o t h e b u l k Ti02 i s c l o s e l y r e l a t e d t o the amount o f oxygen vacancies present i n t h e support. This
study
shows
t h a t morphological
informations
can be obtained
in
199
the case o f a system deposit-support even w i t h large-area techniques such as AES o r SIMS.
Moreover SIMS used i n a "middle mode" (midway between s t a t i c and dynamic SIMS) can provide fundamental information on the f i r s t 20
1
of
the
system.
This p r o p e r t y o f SIMS associated w i t h i t s a b i l i t y , when used i n a s t a t i c mode,
t o provide information:
very useful
on s u p e r f i c i a l
molecular species should be
i n the study o f the morphology and o f the r e a c t i v i t y o f metal-
oxide systems.
REFERENCES
1
S.J.
Tauster,
S.C.
Fung and R.J.
Garten, J . Amer. Chem. SOC.,
100 (1978)
170. 2 3 4 5
Sadeghi and V.E. Henrich, J. Catal., 109 (1988) 1. Kao, S.C. Tsai and Y.W. Chung, J. Catal., 73 (1982) 136. S. Takatani and Y.W. Chung, Appl. o f Surface Sci., 19 (1984) 341. S. Bourgeois, C. Gimenez and M. Perdereau, Mater. Sci. Monogr., 288 (1985) H.R. C.C.
931.
S. Bourgeois, R. P o i r a u l t , J.M. Plociennik and R. Randriamanivo, J. Microsc. Spectrosc. Electron., 13 (1988) 89. 7 S. Bourgeois, M. Perdereau, J. Microsc. Spectrosc. Electron., 6 (1981)
6
335. Y.W. Chung, W.J. Lo and G.A. Somorjai, Surface Sci., 64 (1977) 588. For example G.E. Rhead, J. Vac. Sci. Technol., 13 (1976) 603. A. Benninghoven, Z. Phys., 230 (1970) 403. S. Hofmann, Appl. Phys., 9 (1976) 59; S. Bourgeois, 0. D i a k i t e , F. Jomard, M. Perdereau and R. P o i r a u l t , Submitted f o r p u b l i c a t i o n i n Surface Sci. 13 0. Brugniau, S.D. Parker and G.E. Rhead, Thin S o l i d Films, 121 (1984)
8 9 10 11 12
247. 14 15 16 17
M.P. Seah, Thin S o l i d Films, 81 (1981) 279. E. G i l l e t , C. Legressus and M. G i l l e t , J. Chim. Phys., 84 (1987) 167. G.I. Altman and R.J. Gorte, J. Catal., 110 (1988) 191. H. Poppa, C.A. Papageorgopoulos and E. 8auer, Ultramicroscopy, 20 (1986)
145.
The variations o f AES nickel and titanium i n t e n s i t i e s versus time a f t e r nickel deposition on non stoichiometric support ( t h a t i s t o say decrease o f N i i n t e n s i t y w i t h no increase o f T i ) are well explained by a d i f f u s i o n
o f nickel i n t o bulk Ti02. This i s also consistent w i t h sputtering
SIMS r e s u l t s where the titanium
and the nickel i n t e n s i t i e s are well accounted f o r by the sputtering o f one monolayer o f n i c k e l plus the sputtering o f nickel which has d i f f u s e d i n t o bulk Ti02 : i n p a r t i c u l a r there i s no i n i t i a l decrease o f the titanium i n t e n s i t y ( t h i s would be the case i f TiOz,, surface).
SIMS
species crept over the nickel
C. Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactivity ofSurfuces 01989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
201
STRUCTURE AND BONDING OF ADSORBATES: INVESTIGATIONS WITH SYNCHROTRON RADIATION
A. M. BRADSHAW Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, Germany ABSTRACT Using x-ray absorption, photoemission and photoelectron diffraction i t is possible to obtain important information on the structure and bonding of molecules adsorbed on single crystal metal surfaces. These spectroscopic techniques become particularly powerful when used in conjunction with synchrotron radiation. INTRODUCTION Fundamental studies in surface chemistry are necessarily concentrated in three main areas. Firstly, the elucidation of the structural details of the adsorbed layer plays a n important role, in particular, the determination of bonding site and molecular orientation. Related problems such as adlayer growth, order-disorder transitions and adsorbate-induced reconstructions also receive considerable attention. Secondly, methods are available to probe the electronic energy levels of the adsorbatekubstrate system in an attempt to obtain a better understanding of the chemisorption bond. Concomitant advances in theory have enabled molecular orbital energy diagrams in the so-called cluster approximation and, in the case of ordered overlayers, surface band structures to be calculated. Thirdly, the investigation of the dynamics of adsorption and desorption processes has recently become a highly topical area. The aim here is to elucidate the energy exchange processes occurring between particle and surface and, in the long term, a t deriving empirically the corresponding potential energy surface. The investigatory techniques used in all these studies rely heavily on spectroscopies and scattering experiments involving electrons, photons, ions or atoms. In this article we concentrate specifically on the use of photons in the vacuum UV and soft x-ray regions (XUV) for solving problems in two of these areas, namely, in studies of the structure and bonding of molecules and molecular fragments on metal surfaces. The experiments that will be described - x-ray absorption, photoemission and photoelectron diffraction are based on the phenomena ofphotoabsorption and photoionisation. X-ray absorption spectroscopy (ref. 1) and energy-scanned photoelectron diffraction (ref, 2) have recently become important structural tools in adsorption studies because, unlike low energy electron diffraction (LEED), they do not rely on the presence of long-range order in the adlayer. Since both techniques require a
202
continuously tunable source of soft x-rays, usually in the region 200 - 1000 eV photon energy, their use is dependent on the availabability of synchrotron radiation a s well as on suitable "grazing incidence" grating monochromators. (This spectral range covers the IS levels of C, N and 0, which tend to feature in most adsorbate studies.) The measurement of the polarisation dependence of the x-ray absorption spectrum of a n adsorbed species near a n absorption edge (NEXAFS = near edge x-ray absorption fine structure) is a particularly straightforward technique when applied correctly. The most direct technique for probing surface electronic structure is undoubtedly photoemission, or photoelectron spectroscopy (ref. 3). In a reasonable approximation, referred to as Koopmans' theorem, the measured ionisation potentials can be equated with the ground state orbital energies calculated in the IIartree-Fock approximation. When the molecule is only weakly adsorbed a comparison with the photoelectron spectrum of the corresponding gas phase species normally suffices to assign t h e various adsorbate-induced features. For this purpose, laboratory line sources (HeI, etc.) a r e sometimes adequate. Where a strong perturbation of the energy levels of the molecule takes place, assignment can only be carried out using selection rules preferably in conjunction with polarised light. This is particularly true of molecular fragments for which no comparison with gas phase species is possible. For the application of photoemission selection rules prior knowledge of the orientation of the molecule is necessary. Even when the molecule or molecular fragment possesses no symmetry (meaning t h a t the selection rules cannot be applied), a mere change in photon energy often helps in distinguishing, or even assigning, adsorbate-induced features. In the x-ray absorption and photoelectron emission experiments described in this article, i t is always the excited photoelectron, a n Auger electron or secondary electrons which are detected. On account of the low inelastic mean free path for electrons in solids, only the first few atomic layers are generally sampled and the techniques are very surface-sensitive. (This may not be the case in x-ray absorption spectroscopy where the use of total electron yield detection can lead to high bulk sensitivity.) The inelastic mean free path is strongly energy-dependent (ref. 4)so t h a t in photoemission the surface sensitivity can in fact be maximised by choosing a photon energy which produces a photoelectron kinetic energy corresponding to the shortest mean free path (between 50 and 100 eV for most solids - see ref. 4). In the following section the specific properties of synchrotron radiation are described. I then go on to discuss the three experiments in more detail, giving a n example of the application of each to a particular adsorbate system. Lastly, their combined application is demonstrated in a study of the surface formate species which is formed as a n intermediate in the catalytic decomposition of formic acid. SYNCHROTRON RADIATION The generation, properties and uses of synchrotron radiation have been reviewed in many places. The reader is therefore referred only to a very recent book by
203
Margaritondo (ref. 5)which serves as a useful introduction to the subject. Synchrotron radiation (SR) is produced when relativistic electrons (or positrons) are centripetally accelerated by the bending magnets of an accelerator or storage ring. It is emitted tangentially to the electron orbit as shown in Fig. 1. In recent years a new generation of storage rings has been built whose sole function is to serve as synchrotron radiation sources. Probably the most striking property of SR is the quasi-continuous spectrum extending from the infrared into the x-ray region. The characteristic spectral distribution curve for synchrotron radiation is shown in Fig. 2. An SR source is characterised by its critical wavelength, A,, which corresponds approximately to the position of the maximum of this curve. A, is inversely proportional to the third power of the electron energy and directly proportional to the radius of curvature of the orbit. Thus, a t the German XUV source BESSY in Berlin A, is 20 A, whereas for the DORISII storage ring (E = 3.7 GeV) in Hamburg (HASYLAB) Ac = 1.4 A. Experiments with hard x-rays are thus also possible on a high energy ring such a s DORIS. Synchrotron radiation is also characterised by its small source size and strongly directional emission. Both properties lead to a source of high spectral brilliance, a quantity which is defined as the number of photons per second in unit bandwidth, per unit solid angle and per unit area of source. Other important properties are the high degree of linear polarisation (exactly 100 % in the plane of the storage ring), the time
1
0.1
I
I
1.0
10
I
100 hl'h,
I
1000
I
10000
Fig. 1. (left) Schematic representations of the spatial distribution of electromagnetic radiation emitted by centripetally accelerated charged particles. Above: low energies; below: relativistic energies (for electrons > 40 MeV). Fig. 2. (right) The characteristic spectral distribution curve for synchrotron radiation.
204
structure (useful for certain time-resolved experiments) and the fact t h a t its properties are entirely calculable via the Schwinger equations (see ref. 5). For the purposes of the surface chemical investigations described in this article photon energy tunability and polarisation are the main assets of synchrotron radiation. SR is, however, a source of "white" light and must be used in conjunction with a suitable monochromator. The so-called normal incidence region extends up to about 40 eV photon energy (- 300 A); a s the name suggests, reflections a t the optical components (gratings and mirrors) of the monochromator take place a t normal, or near to normal, incidence. At higher photon energies the reflectivity at normal incidence falls drastically and i t is necessary t h a t reflections from the optical components take place a t angles near to grazing. As a result, grating monochromators for the range 40 1000 eV are difficult to construct and the aberrations produced by the optical components are high. (Crystal monochromators can generally only be used above 800 eV.) These and other problems result in a resolution ( M A ) much lower than t h a t in the normal incidence region. Nonetheless, the advent of new designs of monochromator has recently given considerable impetus to the use of synchrotron radiation in this spectral region. Before concluding this brief account i t should be mentioned t h a t undulators, produced by causing the electron beam to oscillate transversally in a periodic magnet structure inserted into the straight section of a storage ring, can further increase the intensity of synchrotron radiation. These devices produce pseudo-monochromatic r a diation due to constructive interference between the light emitted at successive oscil lations. The intensity can be as high as a factor 10 and the brilliance a factor 103 greater than that of the radiation emitted from the bending magnets on the same storage ring. Already storage rings are being planned or constructed where the main sources of radiation are provided by undulators. XUV facilities of this type - the ALS in Berkeley, Sincrotrone Trieste and BESSY II - will provide considerable impetus in surface science. X-RAY ABSORFTION SPECTROSCOPY Below the photoionisation threshold a core electron in a free molecule can be excited into empty anti-bonding molecular orbitals (m.o.'s) as well as into Rydberg states. These transitions are observable as sharp features directly below the corresponding absorption edge (carbon K, oxygen K etc.) Above the photoionisation threshold further transitions into higher-lying anti-bonding virtual m.o.'s will occur, their broad spectral features being superimposed on the background continuum absorption. This i s shown schematically for a diatomic molecule in Fig. 3. All these features associated with the existence of the molecular potentia.1 constitute t h e near edge x-ray absorption fine structure (NEXAFS) or x-ray absorption near edge structure (XANES). At higher energies above the edge further very weak structure may be visible due to a scattering phenomenon. This is the extended x-ray absorption fine
t electron yield (-absorption)
..
.. .- *.. .-.we
hv
Fig. 3. Left: schematic representation of the transitions observed in the x-ray absorption spectrum of a diatomic molecule. Right: typical near edge spectrum (actually for CO on a Ni(100) surface). structure (EXAFS) resulting from interference between the emitted photoelectron wave and waves backscattered from other atoms in the molecule. (The excitation into anti-bonding m.0. above the threshold may also be viewed as an EXAFS-type scattering resonance. Hence the term "shape" resonance: the energy and width of the features depend on the shape of the molecular potential.) When the molecule is placed on the surface, several factors come into play. Firstly, the experiment has to be performed quite differently. Since the substrate is usually a compact solid surface, a measurement of the absorption spectrum in transmission is no longer feasible. As shown schematically in Fig. 4a, the yield of Auger electrons, or the high energy portion of the secondary electrons (partial electron yield) are measured, both signals being proportional to the number of excited core electrons in the immediate surface region. Upon adsorption the near edge resonances may be shifted and their degeneracy lifted or they may even disappear as a result of the formation of the bond to the substrate. New resonances (so-called substrate resonances) may be observed. Since the chemisorption process invariably results in a molecule with a single, fixed orientation and since the excitations are subject to dipole selection rules, the transitions in NEXAFS are polarised. Thus, in the case of the diatomic of Fig. 3 the transition 1s + u is polarised along the molecular axis, i.e. its intensity in the absorption spectrum is given [E.M,]2 = cosea, where a is the angle between the E vector and the molecby .I ular axis and M, in a Cartesian component of the dipole matrix element. Similarly, the [E.MXy]2= transition 1s + 'II is polarised perpendicular to the internuclear axis: I, sin2a. These equations form the basis for the determination of molecular orientation, although care must be taken in their application to polyatomic molecules (ref. 7). As an example, Fig. 5 shows N 1s near edge x-ray absorption spectra of the HCN molecule adsorbed on Pd(ll1) for two orientations of the E vector relative to the surface normal (ref. 8). It is immediately clear that the 'II and u resonances behave very
-
-
a) X - r a y absorptioP
T
core level
threshold
bl Photoemission
,L E L
Eco
Fig. 4. Schematic representation of the. three. experiments described in this article: (a) x-ray photoabsorption (NEXAFSISEXAFS), (b) photoelectron spectroscopy (photoemission)and (c) photoelectron diffraction.
cI Photoelectron diffraction
similarly a s the angle BE is changed. In fact, a plot of the uln intensity ratio as a function of 0~ (Fig. 5, bottom) shows that it remains essentially constant. Assuming random azimuthal orientation and a lowering of the C-N bond order to two (as confirmed by vibrational spectroscopy - see ref. 9). this result is only possible if the C-N axis lies parallel to the surface (6 = 90"). The calculated intensity ratios for other angles of inclination 6 = 85,70" and 45" are also shown. (This analysis assumes t h a t the hydrogen atom does not lower the effective symmetry.) Although the result shown in Fig. 5 looks quite definitive, closer examination shows that the accuracy of such determinations cannot be much better than about 10". If the experiment is performed for CO on the same surface, the u and H resonances have exactly the opposite polarisation dependences and show that the molecular axis is perpendicular to the surface. Returning to the general case, we note that surface atoms are likely to be relatively strong backscatterers and that the extended fine structure will be dominated by scattering from the substrate. These relatively weak modulations at higher energy are referred to as the surface EXAFS, or SEXAFS. An analysis of the SEXAFS can give
207
further structural information, in particular, on the adsorption site. The technique has often been used in recent years and reviewed extensively by several authors. The reader is referred to ref. 1. PHOTOEMISSION In the photoelectron spectroscopy experiment shown schematically in Fig. 4b, the electrons emitted as a result of their excitation into states above the photoionisation threshold are analysed according to their kinetic energy. The energy balance is given simply by hv = EB E K Cp ,where EBis the ionisation (or bonding) energy of a bound level, EKthe kinetic energy of the photoemitted electron and the work function. The technique is often referred to as XPS (x-ray photoelectron spectroscopy) or ESCA (electron spectroscopy for chemical analysis), the latter being the original acronym proposed by K. Siegbahn. If the photon energy is low (hw < 50 eV), such that essentially only valence electrons are excited, then we normally refer to UPS (ultraviolet photoelectron spectroscopy) or simply photoemission. At these low photon energies the photoionisation cross-section for valence (or outer shell) electrons is also highest. For a molecule on a surface the primary excitation is a dipole transition from a molecular orbital, modified by the interaction with the substrate, into a continuum level (Fig. 6). The same photoabsorption selection rules thus apply as in NEXAFS. Whereas in NEXAFS, however, the final state is a bound or continuum level associated
+
+
+
G
......... . . . e E = 90' . . . . . . .................... ........
... ......._.
.
..... . . ............ ..................... .. ..
20°
..
400
390 1
1
1
410 1
1
E'
420 1
1
1
Photon energy ( eV 1 Fig. 5. Left: X-ray absorption spectra of HCN adsorbed on Pd(ll1) a t the N 1s edge. Right: Intensity ratio of the u and 'II resonances as a function of the angle between the E vector and the surface normal, BE. After ref. 8.
208
nw
Fig. 6. The photoemission process in the single particle picture for a n adsorbate level on a metal surface.
metal
adsorbate
Energy balance. E~
0
electron energy distributian
-nw -.
E,,#"- a~
with the molecule itself and thus of (potentially) known symmetry, the final state continuum wavefunction in photoemission must be specifically selected in a n angleresolved experiment in order to define its symmetry. When the molecular orientation is also known, the symmetry of the initial state can then be determined. How this procedure can be used in assignment will become clear in the following example. Fig. 7 shows photoemission data from the adsorption systems Cu{ lOO}-CO and Cu{1OO}-COiK (ref. 10). It is known from NEXAFS studies (ref. 11) t h a t the C - 0 axis is perpendicular to the surface in both the pure CO layer and the coadsorption system. For most experimental geometries - meaning E vector orientation and photoelectron emission direction - a spectrum such as that of Fig. 7a is obtained. The two main features at 8.7 eV and 11.5 eV are due to the 4 0 orbital and to the overlapping 5 a a n d 171 orbitals, respectively. The shoulder a t 13 eV is a satellite indicating that charge transfer screening plays a role in this system (see ref. 12). The adsorbate-induced features are generally referred to using the same nomenclature a s the orbital in the free molecule but it should be remembered that they will be altered by the interaction with the substrate. The two overlapping features have been assigned by choosing a particular geometry for which emission from levels of o symmetry i s forbidden (Fig. 7b). This is often referred to as the "forbidden geometry": the E vector is parallel to the surface and orthogonal to the emission plane for a n adsorbate with a symmetry axis. Taken together, the two spectra show that the I n level occurs at a binding energy of 8.4 eV relative to EI: and the 50 level at about 8.7 eV. In the corresponding gas phase spectrum the (vertical) ionisation energy of 5 0 is 3 eV lower than that (if 1 TI. 'I'he increase in binding energy of the 50 level is largely due to the formation of the chemi
-
209
sorption bond: the 50 orbital is located mainly a t the C end of the molecule and interacts most strongly with the metal states. In the co-adsorption system Cu{lOO}-CO/K a strong interaction between the species occurs a t high K:CO concentration ratios. This is characterised by a considerably lowered C-0 stretching frequency (ref. 13) and is probably indicative of a shortrange effect rather than a substrate-mediated, long range interaction. The forbidden geometry experiment for this case is shown in Fig. 7c . Immediately apparent is the splitting of the I n level due to the lower symmetry of the CO/K adsorption complex. Either a direct bonding interaction takes place with one or more K atoms or, alternatively, an electrostatic effect is responsible (the K atoms may be partially ionised). Notice, however, that the 40 feature is also absent (or very nearly absent) which indicates that the forbidden geometry experiment still works, i.e. as far as the 40 orbital is concerned, the rotational axis of the CO molecule still remains an effective symmetry element of the system. Two further remarks should be made before concluding this section. The first concerns Koopmans' theorem which turns out in some cases not to be such a trivial approximation. In most photoemission studies to date i t has simply been assumed that the relative ionisation energies can be compared directly with calculation, i.e. that relaxation indeed plays a role but the shift is the same for all orbitals. When intramolecular relaxation and image charge screening are dominant this may be a reasonable approximation. In the presence of charge transfer screening (the "unfilled
CU I1001 - CO/K L;
Fig. 7. Photoelectron spectra from the adsor tion systems Cu{lOO}-CO and Cu(OO}-CO/K. a) CO in the ( d 2 x d 2 ) R 45" structure; b) as (a) but "forbidden" geometry; c) CO/K coadsorption a t high WCO concentration ratio, also in the forbidden geometry. After ref. 10.
8.600
\a=o: € 1 kll I
1s
I
I
I
10 5 EF Energy below EF (eV)
210
orbital” mechanism) this may not be the case. The second point concerns dispersion. Due to lateral interactions in ordered overlayers, overlap of orbitals on adjacent molecules leads to band formation. Energy dispersion E,(kll) is then observable in the experimental spectrum: as the polar emission angle is varied, and thus the component of photoelectron momentum parallel to the surface, kll, the binding energy E, of a feature will change. This effect can be very pronounced in strongly-bound atomic overlayers with bandwidths of 2 eV or more, but is also observed for molecular adsorbates. I t was first discovered in CO overlayers some years ago (ref. 14). PHOTOELECTRON DIFFRACTION In photoemission there is a n interesting experimental variation known a s constant initial state (CIS) spectroscopy, in which the photon energy and the analysed kinetic energy are scanned simultaneously. For the excitation out of a given occupied level, a range of final states is then sampled. This approach is particularly useful in band structure studies of solids when the momentum, or wavevector, of the final state is also known, i.e. when the emission direction is also defined. In such a n experiment on a free molecule a differential partial cross-section is measured. If the photoelectron current is angle-integrated, meaning that all the photoemitted electrons are collected, then the photoabsorption spectrum, or partial photoionisation cross-section, would be obtained. Putting the molecule (or a n atom) on a surface and taking a CIS spectrum of a core level introduces, in the same way a s in SEXAFS, scattering from the substrate (Fig. 412). The resulting interferences, which depend on the adsorption site and orientation modulate the differential cross-section; the phenomenon is normally referred to a s photoelectron diffraction (PED). I t has recently been shown that the modulations in intensity are of the order of 50 90of the background signal in experiments on the C and 0 1s levels (ref. 15). (Photoelectron diffraction effects are also observed when the photoelectron emission angle is varied a t fixed photon energy (ref. 16). I t is t h u s sensible to distinguish between energy-scanned and angle-scanned PED.) By performing model calculations for a given structure and emission direction and comparing with experiment, information on adsorption site and orientation is obtained. The data are collected by measuring a short (- 30 eV) photoelectron spectrum around the appropriate core level peak a t a particular emission angle for a series of photon energies at typically 1 - 3 eV intervals. The peak areas are integrated and normalised to the background on the high kinetic energy side to give a plot of intensity against photoelectron energy. One of the main problems is that substantial changes in the background (inelastic) photoemission signal may occur when the photon energy passes through an absorption edge of the substrate or when Auger electron features and photoemission peaks (generated by second-order light from the monochromator) are superimposed. One way of recognising these features is to record the core level peaks in the form of a group plot as shown in Fig. 8 (ref. 17). Here, the 0 1s level from the methoxy species on Cu{lOO}has been investigated in normal emission. The feature
211
-
in the background a t 270 eV is due to carbon K W Auger electrons. We note in these raw data the strong intensity modulations referred to above which are due to backscattering from surface Cu atoms. The desired structural information is obtained by calculating such photoelectron intensitylenergy curves for various geometries and then comparing theory with experiment. There is still some controversy about the level of sophistication required in these calculations, in particular as to whether a full multiple scattering treatment is necessary. How does the technique of photoelectron diffraction relate to SEXAFS? By varying the photon energy in PED the scattered electron intensity is distributed between the various final state manifolds corresponding to different emission directions. If i t were possible to angle-integrate over the whole photoelectron current, then we would recover the SEXAFS signal. Again, we note the difference between a partial crosssection and a differential partial cross-section. In practical terms, however, i t means that in PED the interference between the directly emitted and scattered waves occurs at the detector and is determined by the path length differences. These in turn depend on the direction and separation of emitter and neighbouring scatterers. PED is thus particularly sensitive to the adsorption site of the emitter. In SEXAFS this real space information is only obtainable via the polarisation dependence of the modulation amplitude. Note too that PED modulations are typically a factor ten larger than those in SEXAFS.
0
1
I
100
200
I
300
I
400
Photoelectron kinetic energy (eV) Fig. 8. Raw 0 1s photoelectron diffraction data a t normal emission from the system Cu(100)-methoxy. The monochromator has been ramped in steps of 3 eV. Note the feature in the background a t 270 eV due to the carbon K W emission. After ref. 17.
-
212
Before concluding this section i t should perhaps be noted t h a t SEXAFS h a s so far been used more widely in surface structural studies than photoelectron diffraction although only with mixed success in the soft x-ray region. On the other hand, t h e potential of PED has not yet been fully realised; the PED analysis for the surface formate species described below illustrates this point. THE SURFACE FORMATE SPECIES ON C u ( l l 0 ) The decomposition of formic acid on metal and oxide surfaces i s a model heterogeneous reaction. Many studies have shown that i t proceeds via a well-defined reaction intermediate - the surface formate species - which can be isolated and investigated with spectroscopic techniques. Adsorption of formic acid at low temperatures gives rise to a molecular species on the surface which deprotonates on warming. On a Cut1 10) surface thisoccurs at 270 K, giving the formate species, which in turn decomposes a t 450 K with evolution of H2 and CO (Ref. 18).Various studies, in particular with vibrational spectroscopy, have indicated that the two C - 0 bonds a r e equivalent and that the symmetry is CB,(ref. 19). In a n x-ray absorption study Puschmann et al. (ref. 20) have recently shown t h a t the molecular plane of the surface formate species on Cu{llO} is aligned along the < 110> azimuth (the direction of the close-packed rows) and oriented perpendicular to the surface. Fig. 9 shows NEXAFS spectra at the oxygen edge with the E vector aligned in ( a ) the < 110> azimuth and (b) the < 100> azimuth. The n-type resonance corresponds to excitation into the 2b2 orbital and is expected to be polarised perpendicular to the molecular plane. There are actually two o-type resonances, corresponding to 7al and 5bl; a t the C edge they can actually be resolved (ref. 21). I n the azimuth (Fig. 9b) we note that the intensity of the A resonance varies drastically a s the angle between the E vector and the surface normal is changed. In <110> this effect is hardly observed, indicating that the molecular plane i s perpendicular to the surface and aligned in the < 110> azimuth, i.e. parallel to the close packed rows. (The measurable intensity in the TI resonance when the E vector is oriented in the <110> azimuth is due to incomplete polarisation of the incident radiation.) This qualitative conclusion is supported by comparing the BE = 90" spectra in Figs. 9a and 9b (full lines). Here we observe a drastic change i n the d o intensity ratio as the E vector (parallel to the surface) is moved around from the < 110 > 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 <110> azimuth and in (b) the <110> 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 <110> 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 <110> 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 <110> 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.
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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.
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
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<. Nakamoto. Infrared and Raman spectra of inorganic and coordination compounds, Wil Chichester (1986)pp. 324-329. 4. Au er Element %ray, Technical Report, V.G. Scientific Ltd., East Grinstead, 19%3 5. F. Forstmann, W. Berndt, P. Buttner, Phys. Rev. Letts. 30 1973) 17. 6. R.G. Jones, S. Ainsworth, M.D. Crapper, C. Somerton, D. . Woodruff, Surface Sci. 179 (1987) 425. 7. (a) A.D. Mitchell, A.E. SomerfieM, L.C. Cross (Editors), Tables of Interatomic Distances, The Chemical Socie ,London 1965). G. GArton, D.E.Henn, H.M. owell, L. .Venanzi, JChem.Soc. (1963) 3625. J. Mizuno, J.Phys.Sco.3 n. 16 (1961) 1574. B. Morosin, Acta C sta ,23 (1967) 630. A.D. Mighell, C.W. Xeimann, A. Santoro, Acta Crystal. 825 (1969 595. 8. .C. Shouten, E.Te Brake, 0.LJ. Gijteman, G.A. Bootsma, SurfaceLci. 74
!i
"M
b
P
V
6
ItF17111 1
9. b.k-hewns in Vibrational Spectroscopy of Adsorbates, Ed. R.F. Willis, p.7, Springer Verlag, 1980. 10. H. lbach and D. L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, 1982, . 121. 11. M.A.Chesters and N. Sheppard in & t r o s c o p y of Surfaces, Eds. R.J.H. Clark and R.E. Hester, Wiley 1988, p. 377.
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
Calcination-Dependenceof Platinum Cluster Formation in NaY Zeolite: A Xenon-129 NMR Study
B.F. Chmelka, L.C. de MenorVal*, R. Csencsits, R. Ryoo**, S.B. Liu***, C.J. Radke, E.E. Petersen, and A. Pines Department of Chemistry and Chemical Engineering, University of California-Berkeleyand Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 U S A
*Present address: Laboratoire de Chimie Organique Physique et Cinetique Chimique Appliqubes, UA-418, CNRS-ENSCM, 8 rue Ecole Normale, 34075 Montpellier Cedex France. **Present address: Department of Chemistry, Korea Institute of Technology, 400 Kusong-dong, Chung-gu, Taejon-shi, Chung Chong nam-do, Korea. ***Present address: Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan, R.O.C.
ABSTRACT Xenon-129 NMR spectroscopy of adsorbed xenon is used to monitor the location of metal clusters and cluster precursors as a function of calcination conditions for NaY zeolite-supportedplatinum catalysts. Our results indicate that for the 673 K reduction conditions imposed, the formation of highly dispersed platinum clusters within the Y-zeolite matrix is best achieved by employing a calcination temperature close to 673 K. incomplete decomposition of the ion-exchanged Pt(NH3),2+ complex during calcination at 473 K results in migration of nearly all platinum to the exterior surface of the zeolite crystallite during reduction. Calcination temperatures significantly above 673 K induce decomposition of the shielded precursor species and subsequent migration of the metal into the sodalite cavities. A substantial amount of the platinum confined within the sodalite cavities migrates back into the supercages during reduction at 673 K. The 129XeNMR data are corroborated by hydrogen chemisorption and transmission electron microscopy experiments.
269
270
INTRODUCTION Preparatory treatments are critically important to the performance of zeolite-supported metal catalysts.' To achieve reproducibly high metal dispersion, the conditions of calcination and reduction must be carefully ~ o n t r o l l e d , ~though -~ the detailed chemical transformations undergone by the metal guests during these processes have been difficult to e ~ t a b l i s h . ~ - ~ Recent studies have demonstrated the usefulness of 129Xe NMR as a sensitive probe of the intracrystalline environment of molecular sieves.8-20 Xenon's large spherically symmetric, polarizable electron cloud is extremely sensitive to its local environment as manifested by a chemical shift range of over 5000 ppm. As a result of xenon's chemical inertness, 129Xeprobe molecules undergo only physisorptive interactions within the interior pore spaces of the zeolite crystallites. Rapid diffusion through the zeolite's interior cavities at room temperature motionally averages the 129Xeresonance signal, producing a single chemically-shifted peak. Correlating 29Xe chemical shift data with catalyst preparatory treatments provides a convenient diagnostic probe of the clustering process. EXPERIMENT Fifteen weight-percent (dry basis) Pt-NaY samples were prepared by introducing the platinum tetraammine cation Pt(NH3),2+ into the zeolite lattice via the ion-exchangeprocedure of Gallezot et al.3 Pt(NH3)42+-NaY catalyst samples were calcined in purified flowing oxygen to temperatures ranging from 473 K to 873 K in a manner described in detail elsewhere.20 Xe-129 spectra of adsorbed xenon were obtained on a Bruker AM-400 spectrometer operating at 110.7 MHz. Typically, 2000 to 20000 signal acquisitions were accumulated for each spectrum with a recycle delay of 0.2 s between 90° pulses. Chemical shift measurementsare precise to within 0.5 ppm and are expressed relative to xenon gas at very low pressure.21 Following the different calcination treatments and 129XeNMR experiments, all samples were reduced in purified flowing hydrogen for 4 h at 673 K. After evacuation and cooling to ambient temperature, 129XeNMR spectra were obtained on the various reduced Pt-NaY samples as outlined above. Selected bright field transmission electron microscopy experiments (TEM) were performed in a JEOL 200CX microscope with a top entry holder and a high resolution pole piece at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory. Powdered specimens were suspended in 200 proof ethanol and sonicated for several minutes to break up large aggregates of the micron-sizedzeolite crystallites. A drop of the suspension was placed on a holey carbon film supported on a copper mesh grid, which was subsequently dried in air prior to TEM analysis. The microscope was operated at its maximum accelerating voltage, 200 KeV, to minimize electron beaminduced damage of the z e ~ i i t e . * ~ ~ ~ ~
271
RESULTS AND DISCUSSION Calcination Correlation of 129Xechemical shift measurementswith adsorption isotherm data for Pt-NaY samples calcined at different temperatures provides insight on sample differences arising from unique catalyst preparation histories. In particular, the changes observed in the 129Xechemical shift data mirror changes in the average environment experienced by xenon atoms adsorbed within the supercages as a result of chemical changes undergone by the metal species during calcination. For a metal loading of 15 wt% (dry basis), stoichiometric requirementsdictate that an average of two metal complex cations are present in each supercage within the uncalcined sample. Under the very mild calcination conditions of 473 K, the data in Figure 1a indicate that the zeolite matrix dehydrates without significant decomposition of the metaltetraammine complex. The relatively large chemical shift observed for 129Xe adsorbed in this sample is a result of the interaction of xenon molecules with the divalent Pt(NH3)42+complex which must be shared between two monovalent framework anion sites. Fraissard et al. have reported large chemical shifts for 129Xeadsorbed in Y-zeolites exchanged with a large fraction of divalent alkaline earth cations.12 Indeed, the chemical shift behavior we observe for xenon adsorbed in the dehydrated Pt(NH3)42+-NaYsystem is consistent with previously reported data for Ba2+-NaY.12 The barium cation and the tetraammine platinum complex are both large divalent species possessing
200
150
E, a 100
Figure 1. Variation in 129Xechemical shift with the concentration of adsorbed xenon (T=295 K) in 15 wt% Pt(NH&Z+-NaY zeolite samples calcined to different temperatures: (0)473 K, @) 573 K,(A) 673 K, (+) 773 K , (A) 873 K, @) NaY dehydrated at 873 K.
272
similar charge densities for which similar perturbation of the xenon probe molecule is anticipated. Under more severe calcination treatments at 573 and 673 K (Fig. 1b, lc), the metal complex decomposes to yield a shielded metal intermediate located in the zeolite supercages. In Figures 1b and l c , the transition from the linear chemical shift behavior observed for xenon adsorbed in the dehydrated Pt(NH&2+-NaY sample to the curved behavior for the sample calcined at 673 K reflects the decomposition of the tetraammine complex. The temperatureprogrammed desorption experiments of Exner et and Reagan et al.25 confirm that decomposition of the tetraammine complex occurs over the 573-673 K temperature interval. As the tetraammine complex decomposes at these higher calcination temperatures, perturbation of the r29Xe resonance diminishes (Fig. 1b, lc). This is opposite to the 129Xechemical shift behavior expected if the calcination products were Pt2+or autoreduced platinum metal. A higher charge density or the existence of conduction electrons acts to perturb the local xenon field to a significantly greater extent, thereby producing a much larger 129Xechemical shift.16 Large 129Xechemical shifts also occur in the presence of metal clusters covered with chemisorbed oxygen because xenon atoms are ineffectively shielded from interaction with the metallic core.17 Our 129Xechemical shift data indicate the extent to which the decomposition of the metal-tetraamminecomplex has progressed and reveal the shielded nature of the metal calcination product. Separate Raman spectroscopy and temperature-programmed-reductionexperiments suggest that the metal intermediate species is Pt0.2s Calcination of the catalyst samples at 773 K and 873 K produces additional change in the zeolite-supported metal species beyond the point where decomposition of the tetraammine complex is complete (Fig. I d , le). Between 673 K and 773 K, little qualitative difference is observed in the shape of the 129Xechemical shift data as a function of the adsorbed xenon concentration (Fig. lc, Id). However, the magnitude of the chemical shift registered at a given xenon uptake is significantly less for the sample treated at the higher temperature (773 K). This indicates that some fraction of the metal species is inaccessibleto the molecular xenon probe following calcination at 773 K and implies partial platinum depopulation of supercage sites. At calcination temperatures below 773 K, our work and that of others3$’ indicate that nearly all of the platinum is located in the supercages and is, therefore, accessible to the molecular xenon probe. Penetration of the metal species into sodalite cavities during calcination at elevated temperatures is confirmed by combined application of TEM and 129XeNMR techniques. Following calcination at 773 K and 873 K,the TEM micrographs shown in Figures 2 and 3 reveal the absence of large metal oxide clusters on the exterior of the zeolite crystallites, providing evidence that most Pt is intracrystallineand in aggregates less than the 3 nm bright field resolution limit. 129XeNMR is an effective probe of only the supercage environment, since a xenon atom’s 0.40 nm atomic diameter prevents it from entering the sodalite cages through the small 0.25 nm apertures of the six-membered oxygen rings. The short-range nature of the metal perturbation
273
Figure 2. Transmission electron micrograph of a Pt-NaY crystallite (15 wt% Pt) calcined to 773 K. A few clusters (of PO) are visible near the 3 nm bright field resolution limit. The grainy appearance, seen also in Figures 3, 5, and 6 , is due primarily to scattering of electrons through the heavily loaded supported-metal catalyst and does not indicate the presence of very small aggregates.
Figure 3. Transmission electron micrograph of a Pt-NaY crystallite (15 wt% Pt) calcined to 873 K.
274
on the local xenon magnetic field results in all metal species in the sodalite cages being invisible to the xenon probe. Removal of a fraction of metal from the supercage cavities results in the 129Xeprobe molecule interacting with the shielded metal calcination product still remaining in the supercages, but with lower probability of such an interaction occurring due to the diminished population of the accessible metal species. As a result, the downfield shift of the 129Xeresonance at a given adsorbed xenon concentration is smaller in the sample calcined to 773 K (Fig. Id), while the functional behavior is qualitatively unchanged from that observed for the sample calcined to 673 K. At 873 K, essentially complete penetration of the metal species into the sodalite cavities results in the adsorption of 129Xeprobe atoms into a supercage environment nearly identical to that of the nonmetal-loaded NaY support. Thus, as seen in Figures l e and I f , 129XeNMR chemical shift data for the Pt-NaY sample calcined at 873 K nearly superimpose with those for the nascent NaY support material dehydrated at 873 K.
Reduction After reduction of all the samples at 673 K, the fraction of surface platinum metal present within the supercages, shown in Table 1, has a large dependence on the temperature of the earlier calcination. The amount of surface platinum present in the reduced samples was determined from the amount of hydrogen gas irreversibly chemisorbed at room temperature. Since the 0.29 nm kinetic diameter of the hydrogen molecule exceeds the 0.25 nm opening of the Y-zeolite sodalite cage, the fraction of surface platinum reported in Table 1 reflects only that accessible (to both hydrogen and xenon) in the supercages at room temperature. From Figure 4, we find that the downfield shifts in the 129Xechemical shift data correlate well with the amount of surface platinum metal accessible to the xenon probe molecules within the zeolite supercages. Large downfield shifts in the 129Xeresonance are observed when the quantity of dispersed platinum in the supercages is high. The maximum
TABLE 1 Calcination-Dependenceof Surface Pt in Pt-NaY Samples
Sample
a b C
d e
Calcination (K)
473 573 673 773 a73
Reduction (K)
Fraction of surface Pt in supercagesa
673 673 673 673 673
aDetermined from irreversible hydrogen chemisorption at 293 K.
0.06 0.61
0.74 0.42 0.15
215
accessible surface Pt measured in the sample first calcined to 673 K agrees well with the maximum chemical shift perturbation of the 129Xeresonance plotted in Figure 4c. de Menorval et al. have previously documented large downfield shifts of the 129Xeresonance signal in the presence of bare platinum cIusters.l6 Conversely, the 129Xechemical shifts shown in Figures 4a, 4b, 4d, and 4e are substantially smaller for a given xenon uptake due to diminished amounts of exposed platinum metal. A distinguishing feature between aggregation of Pt into large clusters versus Pt penetration into sodalite cavities is the vastly different sizes characteristic of the metal species in the two cases. TEM is capable of delineating between these two possible distributions and can, thus, determine the location of the inaccessible metal. For the Pt-NaY sample calcined to 473 K and reduced at 673 K, the TEM micrograph shown in Figure 5 reveals very large platinum clusters (ca. 40 nm) on the exterior of the micron-sized zeolite crystallites. Accordingly, xenon adsorbed in predominantly empty NaY cavities in the crystallites' interior accounts for the low 129Xechemical shift values observed in Figure 4a. For the Pt-NaY samples calcined to 573 K and 673 K, more complete decomposition of the Pt(NH&2+ complex has occurred, providing a greater amount of the preferred platinum oxide intermediate necessary for producing high metal dispersion within the supercage cavities upon reduction. This is manifested by both higher fractions of exposed Pt (Table 1) and the larger 129Xechemical shift values shown in Figures 4b and 4c. For the Pt-NaY samples calcined to 773 K and 873 K prior to the 673 K reduction, the lower fractions of accessible Pt and smaller 129Xechemical shift values
700
600 500
400
300 200 100 . - . -
0
5
I
4 I. L . L -
10
(Mol Xe/g-catalyst)
15
xlO4
Figure 4. Variation in 129Xe chemical shift with the concentration of adsorbed xenon (T-295 K) in reduced 15 wt% Pt-NaY zeolite samples previously calcined to different temperatures: (0)473 K, (0)573 K, (A)673 K, (+) 773 K, (A) 873 K, @) NaY dehydrated at 873 K.
276
Figure 5. Transmission electron micrograph of a reduced Pt-NaY zeolite crystallite (15 wt% Pt) previously calcined to 473 K.
Figure 6. Transmission electron micrograph of a reduced R-NaY zeolite crystallite (15 wt?%Pt) previously calcined to 873 K.
211
observed in Figures 4d and 46 are due mainly to penetration of the metal species into the sodalite cages during the calcination treatment. Temperatureprogrammed-reductiondata indicate that a substantial fraction of the platinum species in the sodalite cavities is reduced at 673 K in hydrogen.2s During reduction, some of the metal migrates back into supercage environments, where it is accessible to both hydrogen and xenon. The subsequent small fraction of surface Pt (Table le) and low 129Xechemical shift values (Figure 48) suggest agglomeration of the mobile metal. Aggregation of platinum in the supercages under these circumstances is further supported by the TEM micrograph shown in Figure 6 which reveals numerous reduced metal clusters larger than the ca.3 nm resolution limit of the bright field imaging technique. CONCLUSIONS Our results indicate that for the 673 K reduction conditions imposed, the formation of highly dispersed platinum clusters within the Y-zeolite matrix is best achieved by emy!oying a calcination temperature close to 673 K. Below this temperature, incornp!ete decomposition of the Pt(NH,),2+ complex during calcination results in migration of platinum to the exterior surface of the zeolite crystallite during reduction. Calcination at temperatures significantly above 673 K induces decomposition of the shielded precursor species (most likely platinum oxide) and migration of the metal into the sodalite cavites. During reduction at 673 K,a substantial amount of the platinum confined within the sodalite cavities migrates back into the supercages where it forms Iow-surface-area aggregates. The sensitivity of physisorbedxenon to the influence of metal species makes 129XeNMR spectroscopy an important diagnostic probe of the metal clustering process in zeolites. Hydrogen chemisorption and transmission electron microscopy provide complementary informationwhich correlates well with the 129Xe NMR results. Acknowledgment. This work was supported in part by the Director, Office of Energy Research, Office of Basic Energy Sciences, Material Sciences Division of the U.S.Department of Energy under Contract DE-AC03-76SF00098, by the National Science Foundation under Grant NSF 83-14564, and by a grant from the Shell Foundation.
REFERENCES 1 Jacobs, P.A., Stud. Surf. Sci. Catal., 12, (1982), 71. . . 2 Dalla-Betta, R.A. and Boudart, M., Proc. Int. Cona. on-C I Julv. 1972, North American Pub. Co., v.2, (1973), 1329. 3 Gallezot, P., Alarcon, A., Dalmon, J.A., Renouprez, A.J., and Imelik, B., ,.J 39, (1975), 334. 4 Ribeiro, F.R. and Marcilly, C., Geteroa. Katl., 5, (19831, 383. 5 Braun, G., Fetting, F., Haelsig, C.P., and Gallei, E.,pcta Phvs. Chem., (1978),93.
Felthouse, T.R. and Murphy, J.A., J. C a t 98, (1986), 41 1. Tzou, M.S., Jiang, H.J., and Sachtler, W.M.H., React. Kinet. CataI. Lett,, 35(1-2), (1987), 207. 8 Ito, T. and Fraissard, J., Chem. Phvs,, 76(11), (1982), 5225. 9 Ito, T. de Menorval, L.C., Guerrier, E., and Fraissard, J.P., Chern. Phvs. Lett., 111, (1984), 271. 10 Riprneester, J.A., J. Maan. Res., 56, (1984), 247. 11 Johnson, D.W. and Griffiths, L., 7eolites, 7, (1987), 484. 12 Ito, T. and Fraissard, J., J, Chem. SOC..Fa,-r 83, (1987), 451. I3 Scharpf, E.W., Crecely, R.W., Gates, B.C., and Dybowski, C., J. Phvs. Chern., 90, (1986), 9. 14 Bansal, N. and Dybowski, C., J. Phvs. Chem,, 92, (1988), 2333. 15 Davis, M.E., Saldarriaga, C., Montes, C., and Hanson, B.E., J. Phvs. Chern., 92, (1988), 2557. 16 de Menorval, L.C., Ito, T., and Fraissard, J., L Chem. SOC..Faradav. Trans. 1,78, (1982), 403. 1' Fraissard, J., Ito, T., and de Menorval, L.C., & Q C . 8th Inter. Cona. C W Berlin. Julv 2-6. 1984, v.3, Verlag-Chernie, (1984), 25. Shoemaker, R. and Apple, T., J. Phvs. Chem., 91, (1987), 4024. '9 Sarnant, M.G., de Menorval, L.C., Dalla-Betta, R.A., and Boudart, M., L Phvs. Chern., 92, (1988), 3937. 20 Chrnelka, B.F., Ryoo, R., Liu, S.B., de Menorval, L.C., Radke, C.J., Petersen, E.E., and Pines, A., J. Arner. Chern. SOC., 110, (1988), 4465. 21 Jarneson, A.K., Jarneson, C.J., and Gutowsky, H.S., J. Chem. Phvg, 53(6), (1970), 2310. 22 Csencsits, R. and Gronsky, R., Ultramk, 23, (1987), 421. 23 Bursill, L.A., Lodge, E.A., and Thomas, J.M., Nature,286, (1981), 111. 24 Exner,D., Jaeger, N., Moller, K., and Schulz-Ekloff,G., J. Chern. SOC. Faradav. Trans. 1, 78, (1982), 3537. 25 Reagan, W..J., Chester, A.W., and Kerr, G.T., , .J 69, (1981), 89. 26 Chrnelka, B.F., Ph.D. thesis, University of California-Berkeley, manuscript in preparation. 6 7
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces
279
0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
XF'S AND ESR STUDY OF NICKEL VALENCY STATES IN NiO-MgO HIGH SURFACE AREA SOLID
SOLUTIONS
A. Ciminol, D. Gazzoli',
V. Indovina',
M. Inversi',
G. Morettil and M. Occhiuzzi1
'Centro CNR "SACSO", Dip. Chimica, Universita "La Sapienza", Roma (Italy) 'Dip.
Chimica, Universita di Sassari, Sassari (Italy)
ABSTRACT High surface area NiO-MgO solid solutions of various nickel content ( 1 to 5 atomic percent ) were studied as prepared and after exposure to nitrogen dioxide by X-Ray Photoelectron Spectrosco y (XPS) and Electron Spin Resonance (ESR). The formation and the stability of Nis+ species were investigated. The results obtained by the two techniques on the same specimens were compared, and the formation of Ni3' in the outer layers was deduced. A quantitative assessment is also given. INTRODUCTION Oxide solid solutions containing transition metal ions (t&)
have received a
great deal of attention as model catalysts because of their potential features in elucidating the role of structural and electronic parameters. One
of these
features was the stabilization of valency states when the tmi is incorporated in an oxide such MgO. The role of specific valency states and of their electronic configuration could thus be established (refs. 1-3). The study of NiO-MgO solid solutions provided a demonstration of the role o f a localizedmodel on the one hand, and of the possible intervention of Ni3+ on the other. The positive identification of Ni
3+ .
in the solid solution was accomplished by Electron Spin
Resonance (ESR), but the principle of controlled valency, through lithium oxide addition, had to be utilized for low surface area (LSA) materials to overcome the problem of heterovalent ions incorporation in a matrix, MgO, not prone to accomodate point defects (ref. 4 ) . The potentiality of solid solutions is amplified with high surface area(HSA) solids. Here, the special properties of surfaces can be of help in stabilizing heterovalent ion incorporation, unusual symmetries and less common valency states. In fact, upon exposure of HSA NiO-MgO solid solutions to NO2, the formation of surface Ni
3+ . i o n s could be proved by ESR (refs. 5 , 6 ) .
The X-Ray Photoelectron Spectroscopy (XPS) is a very useful tool for
the
study o f oxidation states, although some care must be taken in the interpreta-
280
tion of the spectra. Charging effects, inhomogeneity of the surface and shifts due to the contemporary presence of more thanonechemical species can render the assignment of peaks ambiguously. This was the case of Ni3+ in NiO, whose peak is practically coincident with that of Ni(I1)
hydroxide. Therefore, the presence
of a peak, or shoulder, at about 1.5 to 1.8 eV higher binding energy (BE) inthe X i 2p peak of NiO was assigned to Ni3+ by most authors, but could also be attrib
uted
KO
surface hydroxi-species (refs. 7,8). Quite recently, Roberts and co-
workers have made a thorough study of nickel oxides and assignment of the fea3+ . ture 1.5 eV higher BE to Ni is established beyond any doubt (ref. 9). Turning now to NiO-MgO solid solutions, it was of interest to identify
the
Ni3+species by XPS. In variance with ESR, all Ni3+ ions within the probingdepth of the outcoming electrons can be seen by X P S . The weaker intensity of the Ni2p feature in solid solutions if compared to NiO is compensated by the absence of the high BE feature in the former materials before the interaction with an oxidizing gas. Hence the formation of the specific feature of Ni3+ is more easily brought out. EXPERIMENTAL
Mate ria1s NiO-MgO samples were prepared by impregnation of magnesium hydroxide with
a
solution of nickel nitrate. HSA solid solutions were obtained by controlled decomposition under vacuum, as reported elsewhere (ref. 5). The samples are designated as MN followed by a figure giving the nominal Ni content expressed as
N i atoms/100 Mg atoms. The actual Xi content (molar fraction, x) and the BET 2 -1 surface area values (S, m g ) are collected in Tables 1 and 2 respectively. A specimen of the solid solution(ca.0.3
g) was placed in a silica bulb
equipped with two side tubes, for the ESR and X P S measurements. After the activation procedure (evacuation at 1173 K, 5 h) the specimen was exposed to NO
2 '
at 298 K for 10 min., and then evacuated at 443 K 0.5 h or at 773 K for 1 h (ref. 5 ) . ESR measurements The ESR spectra were recorded at 77 K at X-band frequencies on a Varian E-9 spectrometer. The absolute number of spins was determined from integrated spectra using Varian "strong pitch" as a standard. U S measurements
The X P S spectra were obtained with a Leybold Heraeus LHS 10 Spectrometer, operating in FAT mode, and connected t o a
Hp
2113 B computer, using A1
(1486.6 eV) radiation (12 KV, 20 mA). The spectrometer is equipped with a spe-
281
cia1 chamber which allows the introduction of a previously treated powder contained in a sealed tube without exposure to air. The analysis chamber was evacuated to better than
Torr.
The spectra were recorded in a sequential manner: Ni 2p, 0 I s , N I s , C I s , Mg 2s, Mg 2p. The Mg 2p line at 49.4 eV was used as reference. The dataanalysis procedure involved: smoothing, background subtraction by a non linear " S type" integral background profile, spectra subtraction, curve
fitting using a mixed Gaussian-Lorentzian function by a least square method (ref. 10).The last two operations will be illustrated below. RESULTS AND DISCUSSION
X-Ray Photoelectron Spectroscopy
(i) Data analysis and identification of Nickel ion species. After activation 0.1 eV all MN samples show one main peak in the Ni 2p(3/2) region at 855.7 (FWHM
=
3.5 eV), with a satellite at 6.4 eV higher BE (FWHM = 4.8 eV) (Fig. la).
This was obtained from the application of curve fitting with one single 2p(3/2) peak and the corresponding satellite with free BE and FWHM values for both fea2+ tures. The main peak can be assigned to Ni ions. The BE value is higher than 3+ that found in NiO, and there is no component ascribable to Ni (ref. 9 ) . The 0 I s region shows a main peak at 530.0 5 0 . 2 eV (FWHM
= 2.6
eV) due to lattice
oxygen, with a component at about 2 . 3 eV higher BE. This last component is more likely to arise from surface ions than from adsorbed species (including surface hydroxyls) in view of the drastic pretreatments in vacuum (Fig. la). The quantitative analysis of surface composition, to be reported elsewhere (ref. ll), showed no appreciable variation of surface
bulk composition for
these HSA specimens, as already found for LSA materials (ref. 12). Samples exposed to NO2 showed a shift to higher BE for the Ni 2p feature, and a larger FWHM (Fig. lb).
Evacuation at 773 K in situ restored the situation
observed for untreated samples. (Fig. lc). These observations strongly pointed to the presence of an additional peak at higher BE, concomitantly present with that of Ni
2+
. To have direct evidence for it, after smoothing and background
removal, a spectra subtraction was made. The spectrum of the untreated sample was subtracted from that of the NO
2
treated one, after correction for the ener-
gy shift to have the Mg 2p peaks coincident and scaling the Ni 2p intensities. For all the samples investigated the subtraction led to the appearance o f a weak but significant
feature with a maximum at about 1 to 1.7 eV higher
BE
and a FWHM comparable to that of the main peak. An example for MN5 is illustrated in Fig. 2. This result is a good evidence that NO2 treated specimens show a composite spectrum having a higher BE component (including corresponding 2+ feature.
satellite) ascribable to Ni3+ in addition to the Ni
a
I
MN 1 b
1.
...............
1
C
1
-
.~
I
I I
.....
1
I
I
I
I
MN2 a
MN 3
I
--
. ._
by
....
C-
- _-..-I_ . . ..
.
..
a
~.
.
I
I
MN 5 b C
I
1
I I
I
I
I
I
I
I
I
1
I
I
I
I
I
I
I
I
I
I
855
057
859
861
863
529
I
I
53
533 0 E/eV
..
Ni 2p(3/2) with corresponding satellite and 0 1s BE values for MN samples: a) after activation; b) after exposure to NO2 at 273 K and evacuation at 443 K (0.5 h); c) after subsequent evacuation at 773 K (1 h). Fig. 1
For a quantitative evaluation, a curve fitting procedure with two peaks and the corresponding satellites was made, and the influence of the data treatment on the results was tested. In all the procedures both nickel components were considered with the corresponding satellite. The Ni2+ parameters were fixed (BE,
F W , satellite/main peak intensity ratio) using the values found for the untreated samples, whereas for the Ni3+ component different routines were adopted. In routine 5 the Ni3+---- Ni2+ separation, A , was taken 1.5 eV, in accordance with ref. 9 and free FfsIM, while the main peak-satellite separation was equal to that of Ni2+; in routine b the FWHM and A values were left to the best fit. The results obtained by applying routine 5 gave Ni3+ FWHM values cmparable to that of Ni2+ (2.6
- 3.3
eV). The influence of A on the intensity
ratio I( Ni3+ )/I( Ni2+ ) = 13/12 was also checked, and was found to give modest variations, except for MN3, were a 50% variation was observed. Routine b either did not affect much the 13/12 ratio, or introduced abnormal and untenable FWHM or A values. It was apparent therefore that the decrease of the error obtained in the higher freedom allowed in the parameters choice was at the expense of any logical expectation on the spectroscopic values known from earlier work. Pig. 3 shows the results obtained by applying the routine 8 to the MN5 sample. The 0 1s region was modified on exposing the samples to NO2. A shoulder on
283
a
b C
I
871
867
863
859
855
851
B E/eV Fig. 2 . Ni 2 p ( 3 / 2 ) region for MN5 samples: a) after activation; b) after NO2 exposure; c) after subtraction of spectrum a from spectrum b. the higher BE side (532.7 eV) became apparent (Fig. lb).
The contemporary de-
tection of the N Is peak ( 4 0 7 . 8 eV) indicates the presence of adsorbed nitrates left after the reaction with NOg. Both features disappeared after evacuation at 773 K (1 h).
872
868
864
860
856
852
B E/eV Fig. 3. Illustration of curve fitting routine 5 for the MN5 sample after NO2 interaction.
2a4
-
(ii) Wntitative evaluation of Ni3+ concentration. The particulate solid is assumed to consist of cubic particles of dimension D 6/Sp , where S is the specific surface area, D the density. The D values found range from 5 to 7 nm. The excess charge introduced by Ni3+ ions is assumed to be present either in the outermost layer, or in the last two layers. It is to be recalled that even in pure NiO the excess charge tends to be located at the exterior of the crystallites, a tendency which will be even more stringent in a matrix like MgO, and which ls supported by the absence of the ESR signal due to bulk Ni3+ions. We can therefore distinguish two regions in each cubic particle, an external
-
region a, of thickness t 0.2 nm (one layer) or 0.4 ~llp(two layers), which ac2+ comodates Ni3+ and Ni ions, and an internal core 6 o f dimension D-2t, which
-
only accomodates Ni2+ ions. The concentrations of Ni3+, Ni2+ in a and B are designated as n,(a), n2(a), n2(6) (remember n3(6) 0). Furthermore, since no appreciable variation of the surface bulk composition was found. n3( a + n,(a)
* n2(6) (ref. 11).
The intensity due to Ni3+ and Ni2+ species, I3 and L2 can now be calculated 3+
(ref. 13). We assume the same Inelastic Mean Free Path X for both species Ni and Ni2+. Since we are measuring intensity ratios, the absoluteovalue is im-
material. The parameter X , however, enters because t e r m such as exp(-t/X) and exp(-D/A) appear. Following Seah and Deach (ref. 14) A is calculated as 1.6 nm. If deduced from the "universal" )i Energy curve, a value of about 1.3 nm would have been expected, introducing small variations only. For the sake of simplicity, the cube is assumed t o have one face parallel to the specimen surface, with the main emission direction at an angle 0 such that sin 6 ~ 1 .Different orientations will enhance the signal from the outer layers, because they correspond to more grazing angles. Hence, the Ni3+ concentration calculated below is likely to be an upper limit. because a given measured intensity can be accounted for by lower concentrations but more grazing angles in the average, With reference to the model one can calculate the contribution t o the inten2
sity arising from 4 zones: 1. an external thin slab (area D , thickness t) made of a; 2. four "walls", also with a composition, cross section Dt, depth D-t Q D ; 3. a core of composition6, attenuated through zone 1, of depth D - 2 t ~ D, and area D2-4Dt; 4. the "bottom" of the cube, composition a, area D2-4Dt, thickness t. The contribution of each zone to I3 and I2 is then computed, obtaining the following expressions ( F being a constant):
I3 = Pn,(a)s I2 6
-
F[n,(a)s
+
I[1-exp(t/X)
n2(6)p]
1+
(4t/D)exp(-t/X)
+ [ 1-(4t/D) ] exp(-D/A) 1
285
p = I [ 1-(4t/D)]exp(-t/i)l
It is of interest to note that the contribution from zone 1 and 2 are roughly equivalent, while that of zone 4 is much smaller. The measured quantity by X P S is the intensity ratio R = 13/12. By rearrangements one obtains: n3(a) = [R/(I+R)] [(s+p)/s]n2(B) The Ni3+ concentration per unit area is given by n ( a ) t , where from the "appar3 ent Ni3+ surface fraction", 03(XPS) can be calculated as (Ni3+ ions per unit area)/(total Ni ions per unit area). The results of the calculations are presented in Table 1 for the different specimens. The two values given for MN2 refer to distinct preparations, while those for MN3 refer to separate NOq treatments of the same starting material. Attention is drawn to the last column, reporting the 03(XPS) values. They are found in some cases above unity, which would imply that also the second layer must be involved, the outermost population of Ni atoms notbeing sufficient even if completely transformed into Ni3+
to
account f o r the Ni3+signal intensity. It
is however significant that they are never higher than 2, which is the upper limit corresponding to complete transformation into Ni3+ of all Ni ions of the first and second layer considered in the model. TABLE 1 XPS evaluation of Ni3+ concentrations. ~~
Sample
§
n*
R/(l+R)
MN1
0.010
0.5
0.37
MN2
0.020
1.1
0.23
MN2
0.020
1.1
0.23
MN3
0.033
1.8
0.18
MN3
0.033
1.8
0.17
MN5
0.043
2.3
0.40
n,(a)*" 7.8 4.4 9.3 5.4 8.8 5.1 11.6 6.8 10.7 6.3 36.4 20.8
n3(a)t
+O
1.6 1.8 1.9 2.2 1.8 2.1 2.3 2.7 2.1 2.5 7.3 8.3
*x = Ni molar fraction. -26 m-3 *(Ni atoms) 10 -17 m-2 +(Ni3+ ions) 10 'The two values given for each sample correspond t=0.2 and t=0.4 nm.
03(XPS) 1.4 1.6 0.8 0.9 0.8 0.9
0.6 0.7 0.6 0.7 1.5 1.7
Electron Spin Resonance The ESR results follow those reported elsewhere for MNO.1 and MN1 (ref. 5), where details of assignment are given. It is recalled that HSA MN specimens 2+ show an isotropic signal (g 2.21) due to Ni in octahedral sites. The line width of HSA specimens is very large (AH = 1500 g) because of the departure PP from exact cubic symmetry. Upon adsorption of NO2 two nickel paramagnetic species were observed, having g,, = 2.02, gl= 2.36 and g,,- 2.11, gL= 2.25 respec-
-
tively. These species were respectively assigned to a coordinatively unsaturat3+ L), ed Ni3+, (Nis 1, and to a distorted octahedral surface complex, ( N i ? where L can be an adsorbed species or a surface anion.
-
The specimens MN1 to MN5 here investigated show both species, and the intensities are reported in Table 2, where the total number of spins, corresponding to X i 3 + in any surface configuration, is also given as n3(exp). The n (exp) val3 ue does not vary much with total Ni concentration, implying a decrease of the fraction of Ni3+ formed as the total nickel content increases, as shown by the n3(exp)/Nx ratio, where N is the total number of sites per unit area, x is the nickel molar fraction, and hence Nx is the number of nickel ions per unit area. TABLE 2 ESR evaluation of ~ Sample
"0.15 MN1 MN2 MN3 MN5
i concentrations. ~ +
s(m2g-l)
Nir
Nis--L 3+ *
n3(exp)*
253 275 307 312 261
0.7 3.6 3.1 3.5 3.3
0.8 3.5 3.0 3.2 2.9
1.5 7.0 6.1 6.7 6.2
n3(exp) /Nx
1.4 0.6 0.3 0.2 0.1
B3(ESR)
1.5 1 .o 0.7 0.9 1 .o
§from ref, 5. -16 m-2 *spin 10
Surface concentration of Ni3+: a comparison between XPS and ESR The decrease of Ni3+ molar fraction with increasing Ni content measured by ESR can be due at least in part to the magnetic interactions, since ESR detects "isolated" Ni3+ ions only. A Ni2+ can be considered "isolated" if the exchange 2+ is smaller than 0.3 cm" at a distance r interactions with nearby Xi of about 0.8 nm (ref. 15). No similar study exists for Ni3+-- Ni3+or Ni3+--Ni2+ interactions, and a precise r value for the present case cannot be given. Turning now to statistical aspects, the number of isolated Ni3+ ions detectable by ESR, n (ESR), can be calculated imposing the condition that no para3
287
magnetic ion should be found within a distance r from a given Ni
3+
located at
the surface. If b is the number of cationic sites in the hemisphere of radius r, n3(ESR)
=
Nx(l-x)b,
where N is the total number of sites per unit area, x is the
molar fraction of Ni, ( l - ~ )gives ~ the probability that no paramagnetic ion
is
found within the distance r (ref. 16). We can now proceed to a comparison of the results of the two techniques. On the one hand we observe that X P S suggests a fairly constant value for the surface Ni3+ molar fraction relative to nickel, and about equal to the surface fraction (Table 1, last column). On the other hand we note that
nickel gives Ni MNO.l,
3+
ESR
surface content equal to the surface nickel fraction only for
the
where the distance between two paramagnetic ions can be expected to be
fairly large. Putting the two observations together, we can hypothesize that the apparent decrease of Ni3+ molar fraction detected by ESR is entirely due to the decrease of detectability, because of magnetic interactions. In this hypothesis, the decrease of Ni3+ molar fraction should parallel the decrease of "isolated" Ni3+ ions. A b value is then chosen to fit the observed n (ESR) va3
lue with the total number of nickel ions present on the surface, thereby giving weight to the suggestion of XPS that all of them are oxidized to Ni3+. The best fit is obtained with b=48, where an r value of 0.75 nm is derived. This value well agrees with the value quoted above as well as with values (0.7 to 1.0) obtained for other
3+
(Fe
, Mn2+ , Cr3+)
in various oxides (ref. 17). Adoption
of b=48 brings the observed ESR signal in line with the hypothesis that the Ni3+ surface concentration relative to the sites occupied by nickel is
about
one (Table 2, last column). CONCLUDING REMARKS The formation of Ni3+ in NiO-MgO solid solutions exposed to NOZ, already demonstrated by ESR, is also made evident by XPS. Both techniques point to a location of Ni3+ in the outermost layers of the particles of the high surface area material used. A quantitative comparison of the concentration is possible provided that limitations of detection by ESR to magnetically non interacting ions is taken into account. The results encourage further investigation of oxide solid solutions, and a more quantitative assessment of the effect of gas interactions leading to unusual valency states of transitions metal ions. REFERENCES
1 A . Cimino, Chim. Ind. (Milan), 56 (1974) 27-38. 2 J.C. Vickerman, in Catalysis (Specialist Period. Rep.), Vo1.2, The Chem. SOC. SOC., London, 1978, pp. 107-144. 3 F.S. Stone, in J.L. Portefaix and F. Figueras (Editors), Chemical and Physical Aspects of Catalytic Oxidation, CNRS, Paris, 1980, pp. 457-490.
288 4 5 6 7 8
9 10 11
12 13
14 15 16 17
A. Cimino, D. C o r d i s c h i , G. Guarino and A. M i c h e l i , Trans. Faraday SOC., 67 (1971) 1776-1 786. V. Indovina, D. C o r d i s c h i , S. F e b b r a r o and M. O c c h i u z z i , J . Chem. SOC. Faraday T r a n s . 1 , 81 (1985) 37-48. D. C o r d i s c h i and V. I n d o v i n a , i n M. Che and G.C. Bond ( E d i t o r s ) , A d s o r p t i o n and C a t a l y s i s on Oxide S u r f a c e s , E l s e v i e r , Amsterdam, 1985, pp. 209-223. K. Wandelt, S u r f . S c i . Rep., 2 (1982) 1-121. S . J . Cochram and F.P. L a r k i n s , Aust. J . Chem., 38 (1985) 1293-1300. A.F. C a r l e y , P.R. C h a l k e r , M.W. R o b e r t s , P r o c . Roy. SOC. London ser.A, 399 (1985) 167-179. W.F. Maddams, Appl. S p e c t . , 34 (1980) 245-267. In preparation. A . Cimino, B.A. D e A n g e l i s , G. M i n e l l i , T . P e r s i n i and P. S c a r p i n o , J . S o l i d S t a t e Chem., 33 (1980) 403-411. C.S. Fadley, i n C.R. Brundle and A.D. Baker ( E d i t o r s ) , E l e c t r o n Spectroscopy: Theory, Techniques and A p p l i c a t i o n s , Vol. 2 , Academic P r e s s , London, 1978, pp. 2-156. M.P. Seah and W.A. Dench, S u r f . I n t e r f a s e A n a l . , 1 (1979) 2-11. R. Guermeur, J . J o f f r i n , A. L e v e l u t and J . Penne, S o l i d S t a t e Comm., 5 (1967) 563-568. R.E. B e h r i n g e r , J . Chem. P h y s . , 29 (1958) 537-539. D. C o r d i s c h i , M. Lo Jacono, G. M i n e l l i and P. P o r t a , J. C a t a l . , 102 (1986)
1-9.
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
SURFACE CHARACTERIZATION OF IONIC MICROCRYSTALLINE OPTICAL SPECTROSCOPIES
289
SYSTEMS BY
COLUCCIA Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali dell'Universith di Torino - Via P. Giuria, 7 - 10125 Torino (Italy) S.
ABSTRACT Optical spectroscopies, such as U.V.-Vis. reflectance, photoluminescence, infrared transmission and reflectance techniques, are shown to provide relevant information both on the structure of the surface sites of highly dispersed systems and on the chemical nature and structure of the adsorbed species. Examples are described relative to ionic solids which are generally adopted as models for the simplicity of their structure (alkali halides and alkaline earth oxides). U.V.-Vis. reflectance and photoluminescence spectra show the presence of surface point defects (F and V centres) on microcrystalline solids and also evidence the surface heterogeneit.y associated with families of intrinsic surface sites which are ions differing for the coordination. The use of CO as probe molecule in infrared studies is illustrated in selected cases to show how a simple diatomic molecule is able to monitor the nature of the adsorbing sites and the various types of interactions which may operate in an adsorbed layers. INTRODUCTION Solids in the form of powders attract increasing attention both for the interest in fundamental studies of materials science and for their technological uses (catalysts, pigments, etc.) and risks (environment). Amorphous and crystalline systems may be obtained in highly dispersed forms and their overall chemical and physical properties are largely determined by the presence of a specific surface area which often reaches values of several hundreds square metres per gram of substance. The average sizes of the particles in such powders may easily go down to 10-50 A. Surface characterization of dispersed systems may be pursued by several physical and chemical approaches according to the nature of the material under investigation and to the specific information which is required. However, optical spectroscopies have certainly been the most widely used and have provided an immense mass of detailed information on the nature and the structure of surface
290
sites and of the species adsorbed on such sites. There are several reasons for such popularity: 1) the relatively high sensitivity, which allows also surface sites and adsorbed species present in very low concentration to be analyzed; 2 ) the applicability to almost any kind of materials and, thanks to the development of interpherometry, also to the most opaque ones; 3 ) the relatively iow cost of commercial instruments and cells (at least for routine work); 4) the facile preparation of samples, which are generally in the form of pellets and, in some cases, even of loose powder. In this context, infrared and U.V.-Vis. absorption, emission and reflectance spectroscopies may be referred to as "classical", to stress the fact that they have been used for many decades in the studies of dispersed materials. New powerful spectroscopic techniques are continuously developed for surface analysis of bulk materials (refs. 1-3) but most of them are intrinsically unsuitable for powdered samples. The exciting progress in single crystals studies, however, gives news hints in interpreting the results on powders. Indeed, electron microscopy throws a bridge between the "ideal" systems (single crystals) and the largely "imperfect" materials (powders). Modern high resolution microscopes reveal the surface geometric structure of the smallest particles and allow the comparison with the surface features of the extended planes of bulk materials as first indicated by Sheppard et al. (ref. 4 ) . Few selected examples of surface characterization by optical spectroscopic studies will be described, referring to ionic microcrystalline powders which, due to the semplicity of their crystal structure, are generally assumed as model systems (refs. 5-6). TECHNIQUES U.V.-Vis. spectroscopies proved able to give detailed information on the nature and the structure of surface sites particularly in the case of ionic microcrystalline systems. These techniques provide data on electronic transitions localized at the outer layers of the particles. The related bands may be readily recognized because they are affected by the adsorption of gases and vapours whereas the bands associated with bulk phenomena are not. Excellent books describe the reflectance spectroscopy (ref. 7) and the luminescence spectroscopy (refs. 8-10) and a very brief account may be found in ref. 5.
291
Reflectance spectroscopy Unlike the case of homogeneous samples, transmission spectfa in the u.V.-Vis. range cannot be obtained with dispersed systems because scattering of photons is the dominant phenomenon. The scattered light may, however, be collected by proper instrumental attachment (the integrating sphere) and analysed. Diffuse reflectance values (R,) are determined in the relevant spectral range and related to the absorption (K) and scattering ( S ) properties of the powder by the Kubelka-Munk function (ref. 7). Reflectance spectroscopy was first devoted to study of transition metal ions in/on various matrixes and supports as shown in the original works by A . Cimino (ref. 11) and K. Klier (refs. 1213). Subsequentely, it was shown that intrinsic surface states of pure oxides (refs. 14-15) and adsorbed species (ref. 16) may be evidenced by this technique. Photoluminescence spectroscopy Following excitation by U.V.-Vis. light of appropriate wavelength, luminescence may be observed and emission and excitation spectra are obtained. Emission spectra are directly related to the emitting sites and are obtained by exciting the sample with light of fixed wavelength (generally in correspondence of an excitation peak) and analizing the emitted light. Excitation spectra give information on the absorbing sites and are obtained by measuring the intensity of the light emitted at a given wavelength (generally a maximum in the emission spectrum) as a function of the wavelength of the exciting light. Absorption sites and emitting sites may be either the same or different centres. In the studies of dispersed systems, as for reflectance spectroscopy, photoluminescence was originally used to reveal point defects and transition metal ions at the surface of powders (refs. 5 , 1 3 ) . Infrared spectroscopy Photon scattering depends on the frequency of the incident radiation and is much lower in the IR range ( V < 4 0 0 0 cm-l) than in the U.V.-Vis.-NIR range (4000 cm-lc W < 5 0 . 0 0 0 cm-l). Consequently, standard transmission instruments could be used without modification to analyse a great number of dispersed systems starting from the classical studies concerning hydroxyls on the surface of oxides (refs. 17-18) and CO adsorbed on metals supported on silica (refs. 19-20). Application of infrared spectroscopy has widen out
292
considerably following the introduction of Fourier transform spectrometers and diffuse reflectance attachments (DRIFT) so that the most opaque and/or highly scattering samples can now be dealt with. Though able to give direct information on the structure of the adsorbent evidencing vibrational surface states (ref. 2 1 ) , infrared spectroscopy showed to be unrivalled for the characterization of adsorbates and the study of interactions in the adsorbed layers. Vibrational spectra of adsorbed species allow their structures to be determined and give indirect, and nevertheless extremely valuable, information on nature and structure of the sites exposed on the surface of finely divided solids. OPTICAL TRANSITIONS AT THE SURFACE OF IONIC MICROCRYSTALLINE SYSTEMS Reflectance and photoluminescence spectroscopies have shown that quite often electronic transitions localized at the surface of ionic dispersed solids are observed in the near U.V.-visible range (refs. 5 , 2 2 ) . Such surface states have been related essentially to two different types of sites: a) point defects associated with cation and anion vacancies and b) cations and anions located on the geometric features typical of crystalline surfaces (microplanes, edges, corners, kinks) and characterized by coordination lower than the coordination of their companions in the bulk of the particle. There are materials for which the assignment to either a) or b) has been firmly established but in some cases the assignment is not straightforward. Results obtained with alkali halides and alkaline earth oxides will be reviewed in the following. These solids are generally regarded as model systems for the simplicity of their crystalline structure and the high ionicity. Point defects on alkali halides Following the original work by Smart (ref. 2 3 1 , dispersed alkaline halides were studied recently by U.V.-Vis. spectroscopy (ref. 2 4 1 . Microcrystalline films of these materials, easily obtained by vapour condensation the supports cooled at 77K, are deeply coloured due to the presence of high concentration of defects. Reflectance spectra allow a detailed analysis of the colour centres as shown in Fig. 1 for KC1. Numerous bands are observed which monitor electron rich (F) and electron deficient (V,) centres associated
293
cm" 15ooo I
@
30
I
I
3
F I
Y
LL
20-
g=2.069
1
10-
9.1.9995
Fn
r H
----* / -
I
nm700
I
I
I
I
600
500
400
300
Fig. 1. Point defects in microcrystalline KC1. Sect. A: U.V.-Vis. diffuse reflectance spectrum of a KC1 film deposited at 77K; F, Fn and V indicate the nature of' the defects related to the various bands. Sect. B: ESR spectrum of the same KC1 sample. Both spectra were taken at 77K (based on data in ref. 24).
with vacant cation and anion lattice sites. Exceedingly high concentrations of such Schottky defect are frozen into the particles and on their surface during vapour condensation at 77K. Aggregated F centres (Fn) are also evidenced by the reflectance spectra. The assignments to the various defects are quoted by each band in Fig. 1A. The ESR spectrum (Fig. 1B) confirms the nature of the paramagnetic defects. The reach literature data on bulk single crystals support the above model (ref. 25). A large proportion of such colour centres are located on the surface, as show by the fact that adsorption of CO at 77K strongly reduces the intensity of the reflectance bands and of the ESR signal (ref. 24). Infrared spectra (refs. 24,261 show that a great number of different surface species are formed by this interaction. Most of them are similar to those observed on other ionic solid, that are a) anionic polymeric CO structures and b) CO molecules a-bonded to the acidic cation intrinsic sites ( K ' ) exposed
in low coordination on the cube faces of the particles (ref. 26). However two bands at 2240 and 1250 cm" are typical of CO adsorbed on coloured KC1 films and further confirm the nature of the defects. In fact, the 2240 cm-' band is due to CO+ species formed by neutralization of the positive holes associated with V centres and the 1250 cm" band is due to CO- species produced by reaction of CO molecules with trapped electrons (F centres) (refs. 24,261. Intrinsic surface sites on ionic systems It has been mentioned above that, together with point defects, cations and anions are also exposed on the surface of alkali-halides films, and that an indirect evidence of their presence is given by infrared spectra of adsorbed CO showing CO--K+ adducts. Such sites might be referred to as "intrinsic" or "normal" sites because they are the sites which are expected to show up simply as consequence of the truncation of a crystal to generate a crystal plane. No direct evidence of such sites may be found in the U.V.Vis. spectra shown in Fig. 1. Should any spectral feature of those sites be present in that range, it would likely be overshadowed by the extremely intense manifestation of the colour centres. Another difficulty in studying alkali halides films is their instability in highly dispersed forms. Indeed, when a microcrystalline alkali halide sample obtained by vapour condensation at 77K is allowed to warm up to 273K, the specific surface area decreases dramatically as sintering occurs, accompanied by the disappearance of the bands shown in Fig. 1 (refs. 24,261. Other powdered ionic materials are much more stable even when treated at high temperature. Most oxides may be easily prepared in very finely divided forms. Among these, alkaline earth oxides have the same advantages of alkali halides (simple cubic structures, particles are relatively well shaped cubelets, highly ionic character, etc.) and, when prepared by thermal decomposition of parent compounds, provide microcrystalline samples with specific surface areas in the 10-200 m2g-l range. They may be safely heated in vacuo up to 1173K with no relevant change in surface area. Sintering accurs by heating at high temperature in the presence of gases or vapours. Starting from pioneering work by Nelson and Hale (ref. 14), the optical spectra of dispersed alkaline earth oxides were studied in many laboratories both by reflectance and photoluminescence techniques (refs. 14-15,27-45). The results of such studies may be
295
summarized as follows: a) the microcrystalline samples which only underwent thermal treatments were white; the related reflectance spectra showed a few bands in the U.V. range at energy lower than the energy required for bulk excitation of the same samples in large single crystal forms; no significant ESR signals were observed (refs. 14-15,27-28); b) when the microcrystalline samples were ?-irradiated deep colours developed in the originally white samples; intense bands appeared in the visible range of reflectance spectra accompanied by new intense ESR signals; c) the same spectroscopic features described in b) for the irradiated samples, developed when doping an originally white MgO powder by heating in Mg vapours at 853K (ref. 46). The conclusion was drawn that colours centres, of the same types as those described in the previous section for alkali halides, can be produced in alkaline earth oxides by 7-irradiation and by doping but are absent in the samples which are simply thermally treated at high temperature (ref. 14). The U.V. bands typical of the reflectance spectra of undoped and unirradiated oxides were assigned to surface states associated with cation-anion couples exposed in low coordination on the surface of the microcrystals (refs. 14-16). In the case of MgO, this proposal was confirmed by comparing the results obtained with microcrystalline samples showing different morphologies (refs. 32,47-50). Photoluminescence spectra of alkaline earth oxides Systematic work on the MgO, CaO, SrO and BaO series convincingly proved that the various U.V. bands in the reflectance spectra of the unirradiated samples are associated with sites differing for the number of next-neighbours and that the lower the coordination the lower the energy which is required to excite the sites (refs. 14-16,27-28,31-32). The Madelung constant decreases together with the coordination of the ions and the excitation energy decreases in the same order (refs. 27-28, 51-54). was also shown that alkaline earth oxides powders are photoluminescent. The emission may originate either from species adsorbed on the surface (refs. 30,45,55-56) or from sites exposed on the surface after all contaminants, namely hydroxyls, have been eliminated by outgassing at high temperature (refs. 14-15,29,31). The study of the latter phenomenon is specifically useful for
296
surface characterization and will be the only one to be dealt with here. The information obtained by the reflectance spectra may be compared with the information given by photoluminescence spectroscopy, namely by the excitation spectra. It was mentioned above that an excitation spectrum monitors the intensity of the emission as a function of the wavelenght of the light and is therefore related to the sites which absorb the radiation. Excitation spectra of MgO, CaO, SrO and BaO were studied in detail, also considering the results obtained with samples of different origins, particles sizes, etc. (refs. 3 2 - 3 3 ) . It was found that in all cases a) the number of the bands observed in the excitation spectra was the same as the number of bands in the reflectance spectra and b) the bands were much in the same positions in both spectra. This may be taken as evidence of the fact that the sites which are excited in the first act of the photoluminescent phenomenon (excitation spectrum) are the same which are revealed by the reflectance spectrum, viz. ions exposed in low coordination on the surface of microcrystals. The analysis of the emission spectra of alkaline earth oxides is not completely convincing so far, and more work is needed on this topic. It has been sustained that the emitting sites are more likely to be of the same nature of the absorbing sites (refs. 5 , 3 1 1 than point defects (refs. 3 7 , 4 0 1 , so that excitation and emission might be described schematically as follows:
V'being lower than V. The emitting cation-anion couple, however, has not to be necessarily the same couple which was originally excited by light absorption. Mobility of surface excitons is not surprising (refs. 8 9, 31) and was also investigated for MgO on the basis of reflectance data and theoretical work (ref. 2 8 ) . Comparison of the emission spectra obtained at different temperatures with the same SrO sample pretreated at 1173K further supported the idea that energy transfer may occur on the surface (refs. 3 3 , 3 9 1 .
297
VIBRATIONS AT THE SURFACE OF MICROCRISTALLINE MATERIALS The infrared spectrum of a dispersed material consists of two superimposed spectra: one is the spectrum of the solid itself (support) and the other is the spectrum of chemical species which are present as "contaminants" on the surface of the powder (adsorbates). The "puretqspectrum of the support may be obtained by outgassing the pelleted powder at a temperature high enough to remove all (generally most) adsorbed species. Such spectrum is the background to be subtracted from the spectra obtained after test molecules have been allowed onto the surface; this operation gives the "pure" spectrum of the adsorbed species. It is often observed, however, that the adsorption of gases and vapours not only produces extra bands due to the newly formed surface species, but also causes changes in the background spectrum. Such changes generally consist in the erosion of bands which are present in spectrum of the clean microcrystalline powder and not in the spectrum of the same material in single crystal form. Vibrational surface states It has been shown above that U.V.-Vis. spectroscopies give direct informations on the surface structure, thanks to the fact that electronic transition localized on the surface may be easily distinguished from the optical transition associated with the bulk of the particles. Also the infrared spectra of dispersed systems are expected to be different from those of single crystals due to (ref. 5 7 ) : a) size and shape effects (first-order effects); b) variation in the force constants of the bonds present on the clean surface (second-order effects). As for the latter effects, bond relaxation occurring at the surface is expected to cause softening of the vibrations (ref. 5 7 ) , though changes in the chemical bonds and local geometries may well produce stiffening effects (refs. 5 8 - 5 9 ) . Following the original work by Morrow and Cody (ref. 6 0 1 , surface modes on silica were studied a l s o by Boccuzzi et al. (ref. 61). Two absorptions at 1 0 2 0 - 9 7 5 and 8 1 0 - 8 6 0 cm-I in the spectra of dehydroxylated SiOz were assigned to coupled vibrations localized on surface Si-0-Si bridges. Other bands at 940, 9 0 8 and 8 8 8 cm-l were assigned to extrimely strained siloxane bridges on the surface (refs. 6 0 - 6 1 ) and it was suggested that uncoupled Si-0 oscillators containing non/or weakly bridging oxygen atoms might
298
be involved in these vibrations. Similar studies were carried out on other partially covalent or highly ionic materials such as ZnO (ref. 6 2 1 , Al2O3 (refs. 21,63641, LiF (ref. 64). Relaxation effects on the infrared spectra of microcrystalline MgO were also investigated by IR spectroscopy both by varying the average particle size and by chemisorption of pyridine (ref. 65). Adsorption affects the surface vibrational modes only, leaving the bulk ones unaffected (refs. 21’61-65). The results on MgO were compared with theoretical studies of the relaxation effect on the density of phonon states in MgO microcrystals of different sizes (ref. 6 6 ) . Vibrational surface states have recently been evidenced on polycrystalline a-Cr203 (refs. 67-68) at 7 2 0 - 6 9 5 and 480-410 cm-l. Erosion of these absorption is observed when CO is allowed onto the sample, as adsorptions on coordinatively unsaturated Cr3+ ions causes surface relaxation with subsequent outward movement of the positive i o n s . Modifications of the spectrum are also observed in the multiphonon region at 882 and 810 cm-’ upon contact with CO. These effects are suggested to be associated with lattice expansion and decrease of anharmonicity caused by adsorption on ionic solids [ref. 68). Vibrational spectra of adsorbates The primary interest in the IR spectra of adsorbed species is the information on their chemical nature and structure. The production of data on this topic is massive and rapid and the results are frequently summarized in books and reviews (refs. 6,69-71). Adsorbed species are also powerful tests for surface structure. In general, the same molecule produces several surface species, so evidencing the presence of various types of exposed sites. The differences in the spectrum of a given adsorbed molecule with respect to the spectrum of the free molecule suggest nature and structure of the surface site which stabilize that species. Moreover, the spectra of a specific adsorbed species at various coverages show differences which are related to adsorbate-adsorbent and adsorbate-adsorbate interactions. Only some results on these latter subjects, strictly related to surface structure studies, will be shortly rewieved here. Carbon monoxide is known to be a powerful probe molecule because its stretching frequency is extremely sensitive to any varia-
299
tion in the structure of the adsorbing site, electronic properties of the adsorbent and structure of the adsorbed layer (ref. 6). The latter effects are normally very large on metals (refs. 72-73) and on transition metal oxides (refs. 67-68,74). It appears interesting to show that similar, though much weaker, effects have been detected on highly ionic insulator systems (refs. 26,64,7576).
For such purpose the C-0 stretching bands are the most informative and, consequently, only the 2250-2000 cm-l region is reported in Fig. 2, which shows spectra of CO adsorbed at 77K on KC1 films in high (Sect. A ) and low (Sect. B) surface area forms (ref. 26).
1.
2149.3
'2cO
10.1
A
2149.3
B
2103.3
2124
2166, 2240
4 2250
2i 05/
,u \1
y 2200
2150
cm
I
-'
\
20' 210
2160
2140
2120
2lOO
em-'
Fig. 2. Surface heterogeneity and adsorbate-adsorbate interaction on the surfa of KC1 microcrystalline film. Sect. A: Infrared adsorbed at 77K (p=40 Torr) a igh surface spectrum of "CO area KC1 film. Sect. B: Infrared spectra of psCO-1'3C0 mixtures adsorbed at 77K (p=40 Torr) on a sintered low surface area K C 1 film (based on data in ref. 26).
The information given by the spectra in Fig. 2 may be summarized as follows. (i) Surface heterogeneity. It has been shown above that the K C 1 film as collected at 77K contains very small cubic microcrystals showing a relatively large population of sites on edge and corner positions (ref. 2 4 ) and also some colour centres of the F and V
300
type (Fig. l i . The IR spectrum of CO adsorbed on this sample (Fig. 2 A ) consists of a peak at 2149.3 cm-' and a few much weaker components on b o t h sides of the maximum. The main absorption (2149.3 ema1) is assigned to CO molecules o-bonded to 5-coordinated K+ cations exposed on {OOlI microfaces and the shoulders at higher frequencies are assigned to CO molecule similarly bonded to 4coordinated cations on edge (2166 cm" 1 and 3-coordinated cations on corner (2185 cm") positions. These assignments agree with theoretical results (refs. 77-78) showing that the lower the coordination of the adsorbing sites the higher is the interaction energy and, consequently, the stretching frequency of the polarized a-bonded CO molecules. Among the shoulders at low frequency, the 2124 ' n a one is assigned to a CO molecule adjacent to a step and, for this reason, in a position to interact both via the carbon atom and via the oxygen atom with two cations. As shown for homogeneous carbonyls (ref. 79) the interaction through the oxygen lone pair moves the CO stretching at frequencies lower than that of the gas. All these co structures are illustrated in the following scheme. 2166
1
2149.3
Schemati rapresentation of CO molecules adsorbed on variou sites exposed in 1-&w coordination on the surface of cubic microcrystals in KC1 films. The figures by each species show the related CO stretching frequencies (cm'l 1
.
The component at 2135-2140 cm" monitors the presence of physisorbed CO and the weak 2101 cm" component is due to 13C0 present in natural abundance in the OC' product. As already mentioned in a previous section, the 2240 cm" band is assigned to CO+ species formed on electron-deficient sites (V centres). When the KC1 film originally obtained at 77K is sintered at room temperature, the colour centres are totally quenched, the
301
surface area drops and the proportion of sites on edge and corner positions decreases (refs. 24-26). Consequently, the spectrum of CO adsorbed on the sintered sample is not only much weaker, but also simpler because the IR components due to CO adsorbed on 3and 4-coordinated sites vanish. Moreover, upon sintering the halfwidth of the main component at 2149.3 cm-' decreases from 15 cm-' to 8 and this reflects the higher regularity of the planes exposed on larger microcrystals. (ii) Dynamic and static adsorbate-adsorbate interactions. It was shown that the maximum of the band of CO adsorbed on KC1 films shifted to higher frequencies as the coverage decreased. This behaviour indicates the existence of adsorbate-adsorbate interactions which may be associated with dynamic and static effects. When dealing with insulators, image effects (dynamic) are absent and when the adsorption enthalpies a.re low (<30 KJmol-l) also the "through-solid" vibrational coupling is negligible. In such cases Khe entire dynamic effect is associated with the direct "throughspace" dipole-dipole coupling (refs. 67-68,74-76,80),which can be evaluated comparing the spectra obtained with various 12CO-13C0 isotopic mixtures (dilution methodl as suggested by Crossley and King (ref. 81) and Hollins and Pritchard (ref. 82). Fig. 2B illustrates the results of this method in the case of sintered KC1. The overall chemical effects being equal in the two cases (same 8 ) , the dynamic effect (2.3 cm-l) is obtained from the difference in frequency between the l2C0 peaks in 8 8 ~ 1 2and 8:92 mixtures. In fact in the latter case the few l2C0 oscillators, dispersed in a majority of I3CO molecules can be considered as dynamically isolated. The difference in the stretching frequencies is large enough to avoid any dynamic coupling of l2CO and 13C0 oscillators. Finally, the total static shift ( 6 cm-l) is given by the difference between the l2C0 peak frequency measured at maximum coverage in the 8:92 mixture (where the l2C0 oscillators are dinamically decoupled but not statically isolated) and the l2C0 frequency measured at very low coverage in the same 8:92 mixture (where the l2C0 oscillators are both dynamically and statically isolated) (refs. 26, 75). Though it is difficult to separate the individual contributions of "chemical" and "solvent1' static effects (ref. 83), the "solventfteffect is believed to play a minor role (refs. 75-761 .
302
0
I
I
I I I OP 0
I I
I I
9 I
I
I I 0
I I
I
i’
Fig. 3. Intensity as a function of frequency for infrared bands of adsorbed CO. 0 from ref. 84; A from ref. 8 5 ; I) from ref. 75; !TiOZ) from ref. 8 6 ; 0 (a-Cr 03) from ref. 6 8 ; A (a-Fe203) from ref. 90; m CO gas from ref. $4, a lower value has been recently determined (refs. 86-89).
Correlation between intensities and frequencies in adsorbed CO bands In the case of dispersed systems, more attention has been generally devoted to the frequencies of the bands of adsorbed CO than to the intensities. As a full structural information requires both parameters to be known, the above quoted studies appear particularly useful because they add more information about intensities. In fact, the study of the dynamic effects allows the dynamic polarizability (a,,)to be calculated, via the modified Hammaker equation (refs. 7 2 , 7 5 ) , and the dynamic polarizability is related to the extinction coefficient. Seanor and Amberg were the first to analyze the relation and to propose a plot of extinction coefficients of various CO adsorbed species as a function of the stretching frequencies (ref. 8 4 ) .
303
Successive studies (ref. 85) and the most recent results on halides and oxides contribute in filling the gaps present in the original curve, as show in Fig. 3. Broadly speaking, the overall trend described by Seanor and Amberg (Fig. 3, dashed line) is respected (ref. 841, though the data relative to CO adsorbed on Ti02 (ref. 86) suggest a milder increase of the extinction coefficient at the highest frequencies (Y>2180 cm-l). This is the range where CO a-bonded to cations of weak polarizing power absorb. The steep rise of the curve at low frequencies ( U CvCOgas) , corresponding to CO bonded via U-K orbitals to cations with suitable d orbitals, is confirmed. Only the value of the dynamic polarizability of CO on a-Cr203 appears to be much higher than expected (ref. 68). This behaviour is likely due to the contemporary presence of a) a significant partecipation of d-K* orbitals to the Me-C bond and b) a strongly polarizing power of the small and highly charged Cr3+ cation (ref. 68). It has to be mentioned that in the course of the studies on adsorption of CO on Ti02 (refs. 86-88) and Zr02 (ref. 89) the value for the extinction coefficient of the stretching vibration of Cogas was determined to be lower than previously reported (ref. 84). The vibrational polarizability (a,) of CO adsorbed on a-Fe203 has been rec'ently obtained by the limit dilution method and it has been found that CO is bonded to Fe3+ cations by a very weak uinteraction with no significant rc backdonation.
Aknowledgments. Financial support from Progetti di Interesse Nazionale MPI is gratefully aknowledged. The author thanks prof. C. Morterra and A. Zecchina for fruitful discussions and dott. L. Marchese for helping with the manuscript.
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66 T.S. Chen, F.W. de Wette, L. Kleinmann and D.G. Dempey, Phys. Rev. B, 17 (1978) 844. 67 I>. Scarano and A. Zecchina, Spectrochim. Acta, 43A (1987) 1441. 68 D. Scarano, A. Zecchina and A. Reller, Surface Sci., 198 (1988) 11. 69 L.H. Little, Infrared Spectra of Adsorbed Species, Academic Press, New York, 1966. 70 A. Zecchina, E. Garrone and E. Guglielminotti, in Specialist Periodical Reports, Catalysis, Royal Society of Chemistry, London, 1983, vol. 6, p. 90. 71 A. Zecchina, S. Coluccia and C. Morterra, Appl. Spectr. Rev., 21 (1985) 259 and references therein. 72 B.M. Persson and R. Ryberg, Phys. Rev. B, 24 (1981) 6954. 73 G.D. Mahan and A.A. Lucas, J. Chem. Phys., 68 (1978) 1344. 74 E. Escalona Platero, S. Coluccia and A. Zecchina, Surface Sci., 171 (1986) 465. 75 E. Escalona Platero, D. Scarano, G. Spoto and A. Zecchina, Faraday Discuss. Chem. SOC., 80 (1985) 183. 76 A. Zecchina, D. Scarano and E. Garrone, Surface Sci., 160 (1985) 492. 77 E.A. Colborn and W.C. Mackrodt, Surface Sci., 143 (1984) 391. 78 R.F. Willis, A.A. Lucas and G.D. Mahan, in D.A. King and D.P. Woodruff (Editors), The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Elsevier, Amsterdam, 1983, vol. 2, p. 59. 79 C.P. Horwitz and D.F. Shriver, Adv. Organomet. Chem., 23 (1984) 219. 80 E. Escalona Platero, S. Coluccia and A. Zecchina, Langmuir, 1 (1985) 407. 81 A. Crossley and D.A. King, Surface Sci., 68 (1977) 528; 95 (1980) 131. 82 P. Hollins and J. Pritchard, Surface Sci., 89 (1979) 486. 83 G.B.Griffin and J.T. Yates Jr., J. Chem. Phys., 77 (1982) 3744. 84 D.A. Seanor and C.H. Amberg, J. Chem. Phys., 42 (1965) 2967 and refences therein. 85 L.A. Denisenko, A.A. Tsyganenko and V.N. Filimonov, React. Kinet. Catal. Lett., 25 11984) 23. 86 C. Morterra, E. Garrone, V. Bolis and B. Fubini, Spectrochim. Acta, 43A (1987) 1577. 87 V. Bolis, B. Fubini, E. Garrone and C. Morterra, J. Chem. SOC. Faraday Trans. 1, in press. 88 E. Garrone, V. Bolis, B. Fubini and C. Morterra, Langmuir, submitted 89 C. Morterra, R. Aschieri, V. Bolis and E. Borello, in C. Morterra, A. Zecchina and 0 . Costa (Editors), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989, in press. 90 A. Zecchina, D. Scarano and A. Reller, J. Chem. SOC. Faraday Trans. 1, 84 (1988) 2327.
.
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C.Morterra,A. Zecchina and G . Costa (Editors),Structure and Reactivity ofsurfuces
01989Elsevier Science PublishersB.V.,Amsterdam -Printed in The Netherlands
307
PHOTOREACTIVITY OF IRON-DOPED TITANIUM DIOXIDE POWDERS FOR DINITROGEN REDUCTION TO AMMONIA 1 1 2 2 J. C. CONESA , J. SORIA , V. AUGUGLIARO and L. PALMISANO 1
Instituto de Catalisis y Petroleoquimica, CSIC, Serrano 119, 28006 Madrid (Spain) 2 Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita di Palermo, Viale delle Scienze, 90128 Palermo (Italy)
ABSTRACT The production of NH3 from N2 and H20 on Fe/Ti02 has been studied in a continuous photoreactor under W radiation. An ESR study of the catalysts shows that Fe3+ ions are better electron traps than Ti4+ ions. This trapping can be irreversible if the corresponding holes originate stable species adsorbed on the catalyst surface. The charge separation, thus favoured by the Fe3+ ions, helps the N2 adsorption by its reaction with surface species activated by holes. The best activity is found when no excess Fe is segregated at the surface; overall turnover for N2 reduction as high as 6 e-/Fe atom can be reached before catalyst deactivation.
INTRODUCTION The conversion of solar energy into chemical energy by means of heterogeneous photocatalytic systems based on semiconductor particles has attracted a great deal of attention. Although the major part of the investigation in this area aims at decomposing water into hydrogen and oxygen, the reduction of dinitrogen to ammonia with concomitant oxidation of water is of comparable practical significance. For N photoreduction different types of semiconducting 2 oxides, either pure, mixed, or variously doped, have been used in gas-solid and liquid-solid systems (refs. 1-2). Among these various semiconducting oxides, cation-doped titanium dioxide has been proved to be effective for N photore2 duction (refs. 3 - 6 ) . The present paper reports the results of a study devoted to investigating the influence of the iron content on the formation of ammonia over a series of Fe-doped Ti0 specimens irradiated by near-W light. The powders, prepared by 2
308
the coprecipitation and wet impregnation methods, were characterized by XRD, and ESR. On the basis of all the information obtained a likely picture of
SEM,
the process is proposed.
EXPERIMENTAL METHODS The reactivity experiments were performed in a continuous fixed bed
reactor
consisting of a Pyrex glass tube (i.d.tO.4 cm) filled with 1 g of powder and irradiated by a 160 W OSRAM Hg lamp. The reactor was thermostated in a water bath between 295 and 353 K and fed with a flow of dinitrogen saturated with water at 295 K.
NH in the exit gas was trapped in aqueous HC1 and analyzed
3 colorimetrically. All runs lasted three hours but for one whose time was of 28 h.
At
the end of the runs the reactor with the powder was warmed at 573 K for
several hours under N
in order to recover and determine the ammonia adsorbed 2 onto the powder. A few experiments of NH desorption were performed with He in3 stead of N but no difference was observed. 2 The Fe-doped Ti0 powders, containing nominal Fe concentrations of 0.2 and 2 2 cation X , were prepared by the coprecipitation and wet impregnation methods; they are named (1M)x and (CP)x,
with x equal to Pe concentration. A pure Ti0 2
powder, TD(hp),
was also prepared with the same precipitation technique but
without adding iron. Details on the powders preparation can be found elsewhere (ref. 6 ) . Before the reactivity runs, the powders were reconditioned in air at K
823
90
for few hours, then crushed and sieved and those particles in the 37-
rum size range were used. BET surface areas were determined from nitrogen isotherms at 7 7 K
using a
Micromeritics Flowsorb apparatus; the values obtained for (CP)O.2, (CP12, 2 (IM)0.2, and (IM)2 were 44.4, 73.1, 68.7, and 90.3 m /g respectively. X-ray diffraction patterns were obtained at room temperature with a Philips diffractometer using Ni-filtered CuKa radiation. SEM observations were carried out using a Philips 505 microscope equipped with a K E W [ analytical apparatus. Xband ESR spectra were recorded at 77 K with a Bruker spectrometer (mod. Er 200D
SRE);
the samples were placed in silica cells with greaseless stopcocks al-
309
lowing thermal treatments under vacuum or gases when required.
EXPERIMENTAL RESULTS The X-ray results showed that the (CP) powders consist practically of pure anatase while the (IM) ones are anatase with small amounts of rutile. The specimens with 2 i! of iron showed also a disordered phase identified as pseudobrookite (Fe Ti0 ). 2 5 From the SPI results a very large distribution of shape and dimensions of the particles was observed. Although pure TD(hp) did not show macropores (with diameter >SO nm), analyses of (CP)
the Pe-doped specimens did present them.
The surface micro-
specimens showed a homogeneous distribution of Fe content
within particles, with values close to the nominal ones.
On
the contrary the
(IM) specimens showed a very inhomogeneous distribution of iron not only between particles but also within one particle. The values of Fe content measured at various points of the particle surface resulted always higher than the nominal one.
No NH
production was detected when specimens of TD(hp),
Fe 0 and (CP)2 2 3 were used for reactivity tests. The reactivity results of all the other speci3
mens at the used reactor temperatures are reported in Fig. 1 as total amount of
NH
produced during the 3-h runs. In this diagram the amount of ammonia J
released to the gas phase may be distinguished from that adsorbed onto the powder surface. From these results it may be noted that the activity of
(IM)
powders increases by increasing the temperature while for (CP)O. 2 specimen the activity is independent of the reactor temperature. For the long time run performed with (CP)0.2 at 353 K,
a net decline, until almost disappearance, of
reactivity with time was observed; the total ammonia produced in this run was of 823 pg. Heating in air at 823 K (but not under N
flow) restored the activi-
2
ty of the sample to its initial level.
The 7 7 K ESR spectra of the four samples, outgassed at 295 K, are shown in Fig. 2. For sample (CP10.2 the spectrum (Fig. 2,a) can be described as a super3+
position of several Fe
signals. The peaks at g-8.18,
5.64, 3.43,
and 2.6
3 10
..:I. m f
z
+DPPH
100
0
295
303
313
T
333
353
K
Fig.1- Total ammonia produced by a) (IM10.2; b) (CP)O.2; c f (IM)2 The shaded area indicates the amount of adsorbed ammonia.
Fig.2- ESR spectra of the FeTio, samples outgassed at 298 K. a ) (CP10.2; b) (CP)2; C ) (IM10.2; d) (IM)2.
ti
c----l
1000 G
IOOOG
4 l d ‘y 1.966
d
xi
A
Fig.3- Effect of the outgassing temperature on the ESR spectra. ( A ) (CP10.2; (B) (IM10.2. a) 298 K; b) 473 K; C) 573 K; d ) 673 K.
311
3+
(signal A ) have been assigned to substitutional Fe
ions in the rutile lattice
(ref. 7 ) . This is confirmed by preliminary measurements of the influence of the 3t
calcination temperature on the Fe
signals, which show these peaks, with
sharper lineshape, for samples calcined at 1073 K in which most of the Ti0 has 2 been transformed to the rutile phase. Therefore, although XRD only shows anatase peaks, there must exist small rutile nuclei that can retain iron ions in 3t
their lattice. Signal B at g-4.26 has been assigned to Fe
ions in rhombic
symmetry (ref. 8). It is observed with smaller intensity for (CP) samples which have been calcined at higher temperatures, indicating that these centers are mainly located in the anatase phase. The peaks with g values closer to 2 can be tentatively interpreted as a superposition of an axial signal C with g,=2.004 and unresolved g,@
e
centered in 8'2,
probably due to Fe
in octahedral symmetry. The intensity of
these peaks decreases after calcination at high temperature, supporting their 3+
assignment to Fe
ions in anatase (ref. 9).
The effect of a higher iron
content in the (CP) samples (Fig. 2,b) consists mostly in an increase in intensity and linewidth of the lines C and D, which coalesce in a single peak. Sample (1M)O.Z (Fig. 2,c),
shows essentially the same signals as sample (CP)0.2
but here all of them appear broader and, with the exception of
signal B, show a lower amplitude. The larger spectrometer gain used here allows the observation of a broad underlying component, centered around g=2, which is 3t
probably due to aggregated Fe
ions with mutual magnetic coupling. Also
the (IM) catalysts, a higher amount of iron (sample (IM)2),
for
produces mostly an
increase in lines C-D, while the other lines appear with lower amplitude and intermediate linewidth (Fig. 2 ,d). 3t
In order to determine the redox stability of the Fe
ions, the spectra of
these samples were recorded after different thermal treatments under vacuum.
The effect of these treatments is relatively similar for the four samples; Fig. 3 presents the results obtained for samples (CP10.2 and (IMI0.2. cases a progressive increase in the outgassing temperature, T V
,
In both ultimately
< :
leads to a decrease in the intensity
I
1000 G
of the Fe signals. It is to be noted
that, for T =573 K (a temperature at V
I
which this treatment produces in the 3t
Fe-free TD(hp) sample surface Ti ions associated to oxygen vacancies), only a narrow signal at g-g , assie gned to electrons trapped in oxygen vacancies, is found (Fig. 3B.c). 673 K,
I
At
the spectra are mainly formed
by an axial signal with g,=1.966
and
g,,=1.946 due to the mentioned surface
\
1
I
3t
ions (ref. 10);
Ti
this signal
appears with higher intensity for the
--
(CP)0.2 sample. The
-
x1
possible effect of the pho-
toreaction process on the state of
Fig.4- ESR spectra of (CP)0.2 sample W-irradiated under N2+H20 flow for: a) 0 h; b) 2 h; c) 14 h and after subsequent heating in air at 823 K for d) 1.5 h and e) 20 h.
the catalyst was checked by examining a (CP)O.Z sample placed in a special ESR cell which allows irradiation under controlled gas flow. Figure 4 presents the spectra obtained after W illumination under flow of wet nitrogen at 313 K during different time intervals and after heating treatment in air at 823 K. It can be seen that, while an irradiation period of 2 hours produces no substan3+ tial change in the spectrum, a significant loss of Fe signals 3t does occurr for more prolonged irradiation. No such decrease in Fe signal was observed if the irradiation was carried out under wet, N -free, helium flow. 3+ 2 The initial intensity of the Fe signal is recovered after a prolonged treatment under air at 823 K (Fig. 4.e);
.
no such recovery occurred by heating under
The ESR spectrum of a (CP10.2 sample which had been irradiated in 2 the photoreactor until almost complete deactivation showed also a strong air at
flowing N
decrease in the iron signals.
313 DISCUSSION The structural characterization of the catalysts shows that most of the Ti0 n
L
in them exists in the anatase phase. The solubility of iron in anatase is ca. 1 X (ref. 9),
so
that below this limit a homogeneous distribution of iron
in the Ti0 is possible. This is confirmed by SEM for the (CPlO.2 sample; for L
(IM)0.2, a concentration gradient is found, so that here some enrichment of the external surface in iron could exist. Such an enrichment is indeed present in the 2 4 samples; if the quoted solubility limit is considered, an average 2 surface concentration of (at least) 1.0 and 0.8 excess Fe ions/nm is expected in (CP)2 and (IM)2 samples, respectively. XRD shows that a substantial fraction of this is actually forming pseudobrookite microcrystals. This difference in the distribution of Fe is reflected also in the ESR spectra. The more concentrated samples do not show signals which can be ascribed to the Fe Ti0 phase. The spectrum of sample (CP)2 differs from that 2 5 of (CP)O.2 mainly in the expected higher signal intensity and in the larger 3t
linewidth of the lines due to isolated Fe
ions; this broadening is almost
certainly due to the dipolar magnetic interactions induced by the higher con3+ centration (probably close to the solubility limit) of Fe inside the Ti0z The fact that, for sample (IMl0.2, the signals found are broader and smaller
.
than for (CP)0.2 indicates that, due to the different preparation method, in 3t
this (IM) sample Fe
ions have penetrated into Ti0 to a much lower level, so 2 that the corresponding signals are smaller and the proximity of most of such ions to the surface is reflected in distortions of their coordination environment and broadening of the peaks. The broad underlying signal detected here 3+
(thanks to the higher gain) can correspond to the agglomerates of excess Fe ions remaining at the surface. In the case of sample (IM)2,
the presence of
larger amounts of iron ions may allow their penetration in the Ti0 grains to a 2 larger extent; the sharpness of the signal detected here suggests that the saturation level has not been reached everywhere in the Ti0 , allowing a smal2 ler dipolar line broadening in part of the dissolved Fe ions. All these observations are in agreement with the concentration gradients found in (IM) sam-
314
ples
.
The activity tests show that, besides Ti0 and N , H 0 vapour (not H I ) , Fe 2 2 2 2 An excess of Fe ions and W light must all be present in order to obtain NH 3
.
is detrimental, even if the Ti0 surface is not fully covered by
iron com-
2
pounds; in fact, sample (CP10.2, where no surface enrichment in Fe is likely to exist, gives the highest activity in most tests. The favourable effect of iron must arise therefore from the Fe ions inside the grains of Ti0
, and the
L
smaller activity of the other samples indicates that surface iron acts as a trap (or recombiner) of the photogenerated charge carriers and/or blocks specifically some particular surface sites which represent only a fraction of the atoms at the Ti0 surface. L
Since only non-surface Fe is active, its role must consist in influencing the photogeneration and/or the evolution of electrons and holes in the Ti0 L
grains. As previously suggested (ref. 9).
this can be explained assuming that
3+
Fe
ions trap the electrons, allowing the holes to reach the surface and give
rise to highly reactive OH radicals which originate the chemical reactions at the surface
-
hv
e j+
Fe
+
+
e
A
+ h
-
2+
Fe
(2)
OH surf surf 3+ This electron-accepting role f o r the Fe ions is confirmed by
h t OH-
(3) the ESR
results given here. First, we find that an outgassing of the catalysts at T(573 3+ 3+ K does not produce Ti as in Fe-free Ti0 but instead the Fe signals de2
3+
crease, and only after most of the Fe has disappeared (upon outgassing at 3+ T>693 K ) can Ti ions be found. This shows clearly that the excess electrons 3+
are trapped in bulk Fe
2+
ions (to give Fe
undetectable at 77 K ) preferential-
ly to being located in the Ti d-orbitals (which form the conduction band).
Furthermore, the spectra of (CP)O.2 sample after prolonged irradiation under reaction conditions in the special ESR flow cell do show the disappearance of 3+
the FP
signals; this reveals a redox process which must correspond to a 3+
reduction of Fe
, since it can be reversed by treating at 823 K under air but
315
.
not under N The fact that prolonged irradiation of this sample in the pho2 toreactor leads to its deactivation and also causes a nearly complete sup3t pression of the ESR signals due to bulk Fe ions, strongly suggests that this redox state of iron is essential for photoactivity (by acting in the separation of charge carriers through reaction 2) and that the deactivation is caused by 2t its reduction to Fe
.
It remains to be clarified whether the role of iron is catalytic or not.
In
the case of (CP)0.2 sample, the total quantity of NH produced before complete 3 deactivation corresponds to ca. 2 molecules per Fe atom in the sample. Since 3 electrons must be transferred per NH , a turnover number of 6 electrons/Fe is 3 found, suggesting that a catalytic role is possible. Of course this rules out a possible (but unlikely) mechanism in which Fe would act as a reductant in the 4t generation of NH , by being itself oxidized stoichiometrically to Fe : 3t 3 4t 6Fe + N t 6H 0 6Fe t 2NH t 60H-. (4) 2 2 4t 3 Such an oxidation to Fe would be also in disagreement with the ESR results, as said above. The only species able to act as a reductant of N
is 2 therefore water (or OH-) which would be oxidized to peroxide species as is the
case in the photogeneration of H on M/TiO systems in absence of sacrificial 2 2 reagents (refs. 11-12). Describing this peroxide species as 0 H- (although 2 alternatives with a larger or a smaller number of protons can be also formulated) the overall reaction would be: N t 30H- + 3H 0 2NH t 30 H(5) 2 2 3 2 if the role of Fe were exclusively catalytic. Otherwise, the mentioned turnover 3t stoichiometry would imply that the reduction of one Fe ion is accompanied by the activation and reduction of one N molecule, giving an overall stoichiomeL
try such as: 3t 2t 2Fe + 2N t 90H- + 5H 0 -4NH t 70 H- t 2Fe 2 2 2+ 3 2 In the first case, the Fe intermediate ions formed in reaction 2 would
.
transfer electrons to the activated nitrogen species at some step in the reaction mechanism, reverting reaction 2; in the second one, reaction 2 in the overall mechanism would be irreversible, and the initial [N2-- OH] complex
316
would react further with OH- and H 0 to give NH and peroxide, without the need 2 3 of the generation of another electron-hole pair. The high stability of Ti-bound peroxides at the surface of Ti0 would provide then the driving force for the 2 completion of reaction 6 . 3t In both cases, the role of bulk Fe ions would be to help the separation of electrons and holes by trapping the fornter and allowing the thus generated surface
OH to have an increased lifetime, making possible for these highly
reactive radicals to give at the surface some type of activated
[N - - OH] 2
complex:
+ OH _.lt [N - - OH] (7) 2 2 whose further reactions would lead to the breaking of the N-N bond and the N
.
-- OH in2 teraction is supported by the experimental observation that, in the absence of
generation of NH
The hypothesis of the existence of a specific N
3
N
long-time irradiation of the (GP)0.2 sample does not lead to a decrease in 3+ 2 the Fe ESR signal suggesting that, when the only surface radical species
formed is OH, the recombination reaction: 2+ 3+ Fe + OH -Fe + OH-
.
is, although slower than the e- t OH reaction in Ti0 alone, fast enough to 2 . prevent the net generation of H 0 by combination of two OH radicals, which 2 2 would lead to the overall photoreaction: hv 2t 3t 2Fe t 20H2Fe + H 0 (9) 2 2 Finally a comment can be given to the effect of excess Fe at the sample
-
.
surface. While Fe Ti0 and Fe 0 have no activity (ref. 51, other Fe species at 2 5 2 3 the surface act as poisons; their specific role cannot be ascertained yet with certainty. It can however be observed that some change in the mechanism
is
induced by them, since in the samples where they seem to exist, (IM) samples, the total activity
increases with reaction temperature, while for sample
(CP10.2 the activity is nearly constant between 295 and 353 K.
Possibly, some
of the intermediate species in the reaction becomes stabilized by the Fe ions at the surface, and its further reaction requires some activation energy, which
317
would be negligible in the absence of such surface iron (sample (CP)0.2). Further work to clarify this matter, as well as the details of the deactivation process, is in progress.
ACKNOWLEDGEMENT One of the Authors (V. A.) wishes to thank the "Consiglio Nazionale delle Ricerche" (Roma, Italy) €or the award of a NATO Senior Fellowship to enable part of this work to be carried out at the "Instituto de Catalisis y Petroleoquimica" (CSIC, Madrid)
I
REFERENCES 1 M. Schiavello and A. Sclafani, in M. Schiavello (Editor), PhotoElectrochemistry, Photocatalysis and Photoreactors. Fundamentals and Developments, D. Reidel Publishing Co., Dordrecht, 1985, pp. 503-519. 2 V. Augugliaro and L. Palmisano, in M. Schiavello (Editor), Photocatalysis and Environment. Trends and Applications, Kluwer Academic Publishers, 1988, pp. 425-444. 3 N. Schrauzer and T.D. Guth, J. Am. Chem. SOC., 99 (1977) 7189-7193. 4 V. Augugliaro, A. Lauricella, L. Rizzuti, M. Schiavello and A. Sclafani, Int. J. Hydrogen Energy, 7 (1982) 845-850. 5 V. Augugliaro, F. D'Alba, L. Rizzuti, M. Schiavello and A. Sclafani, Int. J. Hydrogen Energy, 7 (1982) 851-855. 6 V. Augugliaro, L. Palmisano, M. Schiavello, A. Sclafani, J.C. Conesa and J. Soria, Proc. XI Iberoamerican Symp. on Catalysis, Guanajuato, Mexico, June 12-17, 1988, pp. 217-224. 7 N.G. Maksimov, 1.L. Mikhailova and V.F. Anufrienko, Kinet. Catal., 13 (1973) 1162-1167. 8 J.S. Thorp and H.S. Eggleston, J. Mater. Sci. Lett., 4 (1985) 1140-1142. 9 D. Cordischi, N. Burriesci, F. D'Alba, M. Petrera, G. Polizzotti and M. Schiavello, J. Solid State Chem., 56 (1985) 182-190. 10 J.C. Conesa, P. Malet, G. Munuera, J. Sanz and J. Soria, J. Phys. Chem., 88 (1984) 2986-2992. 11 D. Duonghong and M. Gratzel, J. Chem. SOC. Chem. Corn., (1984) 1597-1598. 12 G. Munuera, J. Soria, J.C. Conesa, J. S a m , A.R. Gonzilez-Elipe,A. Navio, E.J. Lopez-Molina, A. Muiioz, A. Fernandez and J.P. Espinos, Stud. Surf. Sci. Catalysis, 19 (1984) 335-346.
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C.Morterra,A. Zecchina and G. Costa (Editors),Structure and Reactivity of Surfaces
319
0 1989Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
ANGLE-RESOLVED UPS STUDIES OF CN ON Cu(ll0) AND Cu(100) M. CONNOLLY, T. McCABE, D.R. LLOYD and E.TAYLOR Chemistry Department, Trinity College, Dublin 2, Ireland. ABSTRACT Cyanogen, (CN)2, and HCN both react on the Cu(ll0) surface to give very similar photoemission spectra. HCN does not react with Cu(100). but reacts with an oxygen pre-covered surface to give spectra which are very similar to those from the reaction product of C2N2 with Cu(100). The spectra on the two surfaces have substantial intensity differences, particularly at normal emission, but can all be assigned to CN. The spectra from the Cu(llO)/CN system are interpreted as indicating that the CN axis is approximately parallel to the surface and to the I l l 0 1 direction. On the Cu(100) surface and probably also on an oxygen pre-covered Cu(ll0) surface the CN axis appears to lie at an angle to the surface. INTRODUCTION Carbon monoxide is probably the best-studied adsorbate on metal surfaces. A wide variety of techniques has been applied to this system, and it is gener-
ally agreed that in the vast majority of cases CO is aligned perpendicular to the surface with the C atom 'down' (ref. 1). a geometry analogous to that observed in metal carbonyls (ref. 2 ) . Cyanide ion is isoelectronic with CO, and coordination complexes of CN-, coordinated through C, are very well known (ref. 3 ) , so the study of CN on surfaces is of considerable interest. CN is known to be present on some surfaces following adsorption of cyanogen, but until recently few studies (ref. 4 ) have addressed the question of the geometry of the adsorption; it has sometimes been assumed to be perpendiccular to the surface (ref. 5,6) probably by analogy with the coordination compounds. However, unlike CO, CN is frequently coordinated at both ends (ref. 3 ) . Recent work on palladium surfaces by HREELS, NFXAFS and ARUPS (ref. 7-10), and HREELS on Cu(ll1) (ref. 11,12) provides strong evidence in support of a parallel orientation of CN, though in the presence of co-adsorbed 0, CN on Cu(ll1) appears to tilt (ref. 11).
We present here some data which suggest
a parallel-bonded CN on Cu(ll0) but an inclined one on Cu(100). EXPERIMENTAL Angle-resolved He1 ultraviolet photoelectron spectra (ARUPS) were obtained on an instrument based on the Vacuum Science Workshop analyser HA45, mounted on
320
a single circle goniometer. The light source is a Vacuum Generators WL-100, and the vacuum system also incorporates argon ion cleaning, an electron gun for Auger spectroscopy, and a mass spectrometer. Copper crystals cut to expose (100) and (110) faces were mounted together on a heater plate attached to a rotatable liquid nitrogen cooled probe, and were cleaned by cycles of argon bombardment and annealing to 40OoC. Initial checks on surface contamination were made using Auger spectroscopy, but UPS was found to be much more sensitive to the presence of impurities, and cleaning was carried out until the spectra of the crystals were reproducible. This was checked using the ratio of the d band height to the scattered tail maximum, and the position of the zero cutoff (i.e. the work function).
Since the analyser has
e
lens input it discriminates
against low energy electrons, and the scattered tail is suppressed at kinetic energies less than 5 eV.
Cleanliness checks were carried out at normal exit
with the probe biased to -9 V. Data were accumulated on a home-built system based on the Apple IIt microcomputer; spectra are presented as connected lines joining the 250 channels of data. It is difficult to compare intensities of bands over a wide angular range since small misalignments, and ill-defined geometric effects of intersection of the analyser acceptance cone and the illuminated spot, tend to reduce intensities at high angles. In an attempt to minimize these effects we have normalised our angular sets of spectra to a constant integrated count over the total spectrum. This procedure essentially uses the scattered tail intensity for normalising. Samples of HCN and C2N2 were fractionated on a vacuum line and the mass spectra were checked frequently for impurities:
no significant ones were
found. Oxygen exposures were carried out using a procedure which is reported to give the p(2xl)O
pattern on Cu(llO),
i.e. 20L 02 at 2OoC, followed by heating to
130OC. (ref. 13).
The coverage on Cu(100) under these conditions is likely to
be rather less than
4,
of
1,
since approx 1000 L 02 are required to reach a coverage
but 1L O2 is sufficient to produce the surface
-
0 vibration with about 5%
of the intensity of the ordered ( 4 2 x 242) R4S00 structure. (ref. 14). RESULTS AND DISCUSSION At about 130K. multilayers of HCN are readily formed on the Cu surfaces, and for C2N2 there is substantial molecular adsorption, probably in excess of a monolayer. Room temperature adsorption of both species occurs readily on but HCN does not affect the UPS spectrum from Cu(100) unless the surface is pre-dosed with oxygen. Figure 1 shows the spectra from Cu(ll0) with
Cu(llO),
a thin multilayer of CzN2, from room temperature saturation exposures of C2N2 and HCN, and of HCN following oxygen exposure. The upper part of Figure 1 shows
321
spectra from Cu(100) with a saturation exposure of C2N2, and with saturation exposure of HCN following oxygen exposure. cu
Bands within 5 eV of EF are due to
.
Figure 1. He I UPS spectra for adsorption on Cu(ll0)
(A - D) and on Cu(100) (E, F ) . A : 3L C2N2 at 130K, B : 2L C2N2 at 300 K, C : 8L HCN at 300K, D : Cu(ll0) (2 x 110 exposed to 20L HCN at 300K.
E : 2L C2N2 at 200K. F : Cu(100) pre-exposed to 02 and then 20L HCN at 45OC.
It is clear that C2N2 and HCN react with the Cu(ll0) surface to give very similar products whose spectra are dominated by a band at about -6 eV. C2N2 gives approximately twice as much product as HCN. Similar spectra have been obtained for adsorption of C2N2 on Pt(100) (ref. 151, Pd(ll1) (ref. 71, Pd(lOO)(ref.
161, Ru(OO1) (ref. 17), Pd(ll0) (ref. lo), and in an earlier study of adsorption on Cu(ll0) (ref. 5 ) . In all these studies the -6 eV band has been assigned to CN, and we agree with this assignment. The spectra from Cu(100) exposed to C2N2 and to HCN/O are similar to each other, but distinctly different from the single band spectrum which has been assigned to CN. Although there is a -6 eV band, this is much broader than the band in the spectrum from CN on Cu(llO), and there is also a fairly strong component at -9 eV. The spectrum from HCN/O on Cu(ll0) appears to be intermediate between these two spectrum types. The possibility of reaction between 0 and CN on the surface to form NCO (ref. 18, 19) has been considered. We have obtained spectra from HNCO adsorbed on Cu(lOO)/O and Cu(llO)/O, systems expected to generate NCO on the surface. These spectra are very different from those in Figure 1, and we conclude that NCO is not a reaction product under our conditions : similar conclusions have been reached in HREELS work on Cu(ll1)
322
(ref. 12). A l l the spectra shown in Fig. 1 are taken at normal emission, and there are
clear differences between the two surfaces. However examination of angular variations of the different types of spectra suggests that all of them are due t o a common species.
Figure 2 shows spectra from the saturation Cu(llO)/HCN
-
system as a function of the exit angle from the normal, 0, in the azimuths which project on to the [I101 and [OOl] directions of the Cu(ll0) surface. The direction Ill01 is parallel to the close-packed lines of atoms which form ridges and channels on the surface.
The comparison of the azimuths is discussed below;
for the moment we note that as 0 is increased there are substantial increases in
high angles in [ l l O l ,
intensity at lower, more negative, energies.
A component at -9.1 eV is clear at
and careful examination of the spectra, and reasonable
assumptions about the form of the scattered background, suggest
that this
component is present even at 0 = O o , though with low intensity.
In the [OOl]
azimuth the increase in intensity occurs closer to the - 6 eV band; a component at - 7 . 4 eV has maximum intensity at about 0 = 20'.
Appreciable shifts in the
-
position of the -6 eV peak are visible, at least in the [1101 azimuth; variations are smaller in [OOl].
.,.
Figure 2 . Angle-resolved He1 UPS spectra on Cu(ll0) surface exposed to 20L HCN a t room temperature. Photon incidence angle a 6 0 0 , electron exit angles 0 as shcwn. Left-hand p a n e l : azimuth intersecting [ 1 1 0 ] , right-hand panel : intersecting [OOl].
323
The angular variation of the spectra from HCN adsorbed on the Cu(lOO)/O surface is shown in Figure 3 . As for the spectra of Figure 2 (Cu(llO)), the intensity of a lower lying component at -9.1 eV increases with increasing 0 , and there is a shift of the -6 eV band, intermediate between those in the two azimuths on Cu(ll0).
There is clearly substantial intensity in the region of - 7
eV, though a separate component is not resolved
so
clearly as on the (110)
surface; we suggest that there may be a small shift to higher energy of the -7.4 eV band observed for Cu(ll0) adsorption. In summary, although the two types of spectra appear very different in Figure 1, this is essentially an intensity effect; bands which are only clearly evident at high 0 for Cu(ll0) are present The spectra from Cu(llO)(2xl)O exposed to HCN are at all angles for Cu(100). shown as a function of 0 in Figure 4 : these show some of the features of both
sets.
Figure 3. He1 spectra from a Cu(100) crystal pre-exposed to 0 exposed to 2OL HCN at 45 C.and then Photon incidence angle 60°, electron exit angle 0 as shown.
2
Figure 4 . He1 spectra from a Cu(ll0) crystal pre-exposed to 02 and then exposed to 20L HCN at 100°C. Photon incidence angle 60°, electron exit angles as shown, all in the azimuth intersecting [110].
324
The most plausible interpretation of these data is that all the spectra are due to CN on the surfaces, but that there are orientational differences. The clear difference between the two azimuths on Cu(ll0)
is a strong indication that
the CN group can not be perpendicular to the surface; however a parallel or
-
nearly parallel orientation, in which the CN axes are ordered with respect to the channels along the Ill01 direction, provides a ready interpretation of an azimuthal difference. The variation in position of the - 6 eV band with 13 suggests the presence of two components. Although some of this effect might be due to dispersion, there is also a shift with the photon incidence angle a. The maximum shift observed is from -5.7 eV to - 6 . 2 eV, so the constituent bands are probably separated by slightly more than this. Thus for the (110) surface there are four components of the CN spectrum, at approximately - 5 . 7 eV, - 6 . 2 eV, - 7 . 4 eV, -9.1 eV. Detailed assignments are most easily derived by a comparison of the spectrum on the (110) surface with that of 'free' CN.
Spectra from solid NaCN
show three high-lying valence bands which have been assigned as 5 0 (-10.8 eV), 11 (-12.1 eV) and 4a (-14.1 eV); (ref. 2 0 ) . A parallel or inclined geometry will destroy the
B degeneracy, and the a' and a" components are likely to have different angular variations. By analogy with CO (ref. 11, the 50 and 4a
derived levels are expected to have emission which is concentrated along the CN axis,
so
a parallel orientation provides a partial explanation of the apparent
simplicity of the normal emission spectrum from CN on Cu(ll0). spread of the spectrum is similar to that of NaCN,
The overall
it is reasonable to assign the lowest band at -9.1 eV, most evident at high 8 , to 40. The band is only
-
so
-
observed unambiguously in the Ill01 azimuth, so it is very likely that the CN groups are oriented along [llO], parallel to the ridges. The band at -7.4 eV shows clear azimuthal differences. It is generally stronger in the [OOl] azimuth, and at 0 = ZOO it is resolved as a separate peak, "
while in [110] it is only apparent at high 8. Major azimuthal differences are expected for the %a" emission since this orbital has a node plane perpendicular to the surface. Electric vector components which lie in this plane will generate final state components which maintain the node plane,
so away from the normal much less intensity is expected in the azimuth which includes this plane. However, some intensity is always possible within the plane because the light is
unpolarised. The strong azimuthal asymmetry at -7.4 eV both assigns the
-
corresponding orbital as na" and defines the orientation of the CN axis as being along 11101, since m a x i m intensity occurs in the azimuth perpendicular to this direction. This is in agreement with the conclusions for 4a. The components of the -6 eV band are therefore na' and Sa, separated by a The band intensity increases with a, confirming that at least one
small gap.
325
component is totally symmetric. We believe that the highest lying component at -5.7 eV is probably associated with 50. Although in the ,C symmetry of CNthis orbital is totally symmetric, it is derived from 3uu of a homonuclear diatomic, and
so
the wave function is likely to have a substantial component
which changes sign across a plane perpendicular to the CN axis. The band is thus expected to have a rough inverse relationship with na" which has a true plane of symmetry parallel to this axis. We interpret the much smaller shift with a and 0 f o r the -6 eV band in the [OOlI azimuth as meaning that 50 is too weak to affect the position of the band greatly. In the [liO] azimuth, where na' is weak, 5a is stronger and influences the overall band position; the decrease in energy (more negative) as a increases is due to the greater sensitivity of na' to the perpendicular vector component, and the increase in energy with 0 up to about = 4 5 O is due to an increasing intensity from 50. We believe that all the data for CN on the (110) surface are consistent with the CN groups lying parallel to the surface and parallel to [liO], though our data do not exclude a small inclination to the surface plane. Similar conclusions have been reached recently from ARUPS studies of adsorption of CN on Pd(ll0) (ref. lo), and it seems likely that a strong -6 eV band with little intensity at other energies is the characteristic normal emission spectrum of parallel-bonded CN.
The normal emission spectrum spectrum from CN on Cu(100)
is noticeably different, and the most obvious interpretation of the substantial intensity for the -9.1 eV band is that the CN groups are tilted with respect to the surface, since on (110) we only observe this band towards the CN axis.
The
low energy side of the -6 eV band is much broader than for the Cu(ll0) system, and as indicated above we believe this to be due to the third component. We have attempted to estimate a position for this component by subtracting the Cu(ll0) normal emission ([liOl azimuth) from the Cu(100) band. This procedure gives a reasonably symmetric component at -7.3 eV, in good agreement with the position derived from high 0 measurements on the (110) surface, though in the latter case the value is a direct measurement and may be biased to higher energy by the more intense -6 eV band. The angle of tilt is difficult to assess but is unlikely to,exceed 45O since the general pattern of the spectrum is
so
similar
to that on the (llJ) surface. The n splitting may be slightly smaller on the (100) surface, as expected for a tilt, but otherwise the two sets of spectra are essentially identical. The spectra from Cu(ll0) which has been pre-dosed with oxygen before adsorption of HCN (Fig. 4) are interesting since they are intermediate between the two sets discussed above. HREELS measurements on 0-dosed Cu(ll1) surfaces have been interpreted as indicating that the presence of 0 causes CN to tilt (ref. 11. 12).
It appears from Figure 4 that a similar phenomenon may be
326
occurring on Cu(ll0). Cu(100).
However, any tilt is probably substantially less than on
The -9.1 eV band, assigned to 40, is only clearly evident at high 0
values, and is never as intense as in the normal emission for Cu(lOO),
and the
shift of the -6 eV band with 0 is similar to that for CN on clean Cu(ll0).
The
principal effect of pre-adsorbing 0 is to increase the intensity of the low so that the band maximum shifts about 0.3 eV away from EF. An alternative interpretation, which is difficult to exclude, is that the surface 0
energy side
modifies the ma" orbital. In summary, we believe that we have clear evidence for CN lying a
approximately parallel to the Cu(ll0) surface along the [110] direction. On Cu(100) the CN is inclined away from the surface, and this may also be true for Cu(ll0) when 0 is present on the surface. We thank the National Board for Science and Technology, Ireland, (now EOLAS) for support of this work. REFERENCES 1. See, for example, E.W. Pluomter and W. Eberhardt, Adv. Chem. Phys., 49(1982)
533. 2 . F.A. Cotton and G. Wilkinson, "Advanced lnorganic Chemistry", 4th edition, Wiley, New York, 1980, ch. 25. p.1049.
3. N.N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon, Oxford, 1984, p.339. 4. Earlier literature is sunmrarised in ref. 7. 5. D.A. Outka, S.W. Jorgensen, C.M. Friend and R.J. Madix, J . Mol. Catal., 21(1983) 375. 6. J.A. Rodriguez and C.T. Campbell, Surf. Sci., 185(1987) 299. 7. J. Somers, M.E. Kordesch, Th. Lindner, H. Conrad, A.M. Bradshaw and G.P. Williams, Surf. Sci., 198(1988) 400. 8 . M.E. Kordesch, Th. Lindner, J. Somers, W. Stenzel, H. Conrad, A.M. Bradshaw and G.P. Williams, Spectrochim. Acta, 43A(1987) 1561. 9. M.E. Kordesch, W. Stenzel and H. Conrad, Surf. Sci., 186(1987) 601. 10. M.G. Ramsey, G. Rosina, F.P. Netzer. H.B. Saalfeld and D.R. Lloyd, unpublished : a preliminary report was presented at the Deutsche Physikalische Gesellschaft meeting, Karlsruhe 1988. 11. M.E. Kordesch, W. Stenzel, H. Conrad and M. Weaver, J. Am, Chem. SOC., 109(1987) 1878. 1 2 . M.E. Kordesch, W. Feng, W. Stenzel, M. Weaver and H. Conrad, J. Electron Spec., 44(1987) 149. 13. K.A. DiDio, D.M. Zehner and E.W. Plummer, J. Vac. Sci. Tech. A2(1984) 852. 14. a ) M. Wuttig, R. Franchy and H. Ibach, J. Electron Spec., 44(1987) 317. b ) M.H. Mohamed and L.L. Kesmodel. Surf.Sci., 185(1987) L467. 15. H. Conrad, J. Kiippers, F. Nitschke and F.P. Netzer, J. Catal. 52(1978) 186. 16. K. Besenthal, G. Chiarello, M.E. Kordesch and H. Conrad, Surf. Sci., 178(1986) 667. 17. N . J . Gudde and R.M. Lambert, Surf. Sci., 124(1983) 372. 18. F. Solymosi and J . Kiss, Surf. Sci., 108(1981) 368. 19. F. Solymosi and A. Berko, Surf. Sci., 122(1982) 2326. 20. H. Palm, B . Marquardt, H.-J. Freund, R. Engelhardt, K. Seki, U. Karlsson, E.E. Koch and W. von Niessen, Chem. Phys. 92(1985) 457.
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reuctiuity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
327
SURFACE REACTIONS OF HYDROCARBONS AS A PROBE FOR THE CHARACTERIZATION OF SUPPORTED RUTHENIUM BIMETALLIC CATALYSTS.
B.COQ, A.BITTAR and F.FtGUERAS L a b o r a t o i r e de Chimie Organique Physique e t C i n e t i q u e Chimique Appliquee, URA.00922 du C.N.R.S.,
E.N.S.C.M.,
8,
Rue E c o l e Normale,
34075 M o n t p e l l i e r
Cedex, France.
ABSTRACT Hydrogenolysi s o f n-hexane (nH) and 2,2,3,3-tetramethylbutane (2233TeMB) a r e s t u d i e d on b i m e t a l l i c RuGe,Sn,Pb,Sb/Al 0 catalysts. Changing t h e p a r t i c l e s i z e on Ru/A1 O3 shows t h a t s e l e c t i v i g i i k c l o s e l y depend on c o o r d i n a t i o n o f t h e s u r f a & atoms. Deep h y d r o g e n o l y s i s p r e v a i l s on l a r g e Ru p a r t i c l e s and s i m p l e s p l i t t i n g o f C - C bonds on s m a l l p a r t i c l e s f o r b o t h r e a c t a n t s . A t h i g h d i s p e r s i o n ( p a r t i c l e s i z e : 1.1 nm) o f Ru/A1 0 a small addit i o n o f Sn, Pb o r Sb enhances deep h y d r o g e n o l y s i s and sup$r&ses demethylat i o n o f 2233TeMB. T h i s can be r a t i o n a l i z e d assuming t h a t Sn,Sb and Pb occupy s i t e s o f l o w c o o r d i n a t i o n ( c o r n e r s and edges) i n t h e b i m e t a l l i c aggregates and Ru stands p r e f e r e n t i a l l y on p l a n e s keeping a h i g h c o o r d i n a t i o n . RuGe exh i b i t s an o p p o s i t e b e h a v i o u r .
INTRODUCTION
The e x t e n t t o which t h e p r o p e r t i e s o f metal c a t a l y s t s can be m o d i f i e d by a l l o y i n g i s a s u b j e c t of
i n t e r e s t f o r b o t h s c i e n t i f i c and t e c h n o l o g i c a l
reasons. I n c l u s i o n of i n a c t i v e metal atoms i n t h e s u r f a c e o f an a c t i v e m e t a l o f t e n l o w e r s c a t a l y t i c a c t i v i t y f o r r e a c t i o n s i n v o l v i n g t h e r u p t u r e o f C-C bonds. Even r e l a t i v e l y small amounts of t h e i n a c t i v e atoms may show a l a r g e e f f e c t i f t h e y a r e p r e f e r e n t i a l l y segregated t o t h e s u r f a c e .
Thus i n b i -
m e t a l l i c systems, t h e component w i t h t h e l o w e r h e a t o f s u b l i m a t i o n o f t e n accumulates a t t h e surface ( r e f . 1 ) . S i t e s o f d i f f e r e n t t o p o l o g i e s e x i s t a t t h e s u r f a c e o f s m a l l m e t a l l i c p a r t i c l e s , such as corners, edges, and f l a t planes, which s h o u l d p r e s e n t d i f f e r e n t c a t a l y t i c p r o p e r t i e s . Thus,
the distribution
o f t h e two components o n t o t h e s u r f a c e i s o f g r e a t importance b u t remains s t i l l obscure. T h e o r e t i c a l c a l c u l a t i o n s on b i m e t a l l i c c l u s t e r s p r e d i c t t h a t t h e d i s t r i b u t i o n of t h e two m e t a l s between t h e s e d i f f e r e n t s i t e s
should not
328
be random ( r e f . 2 ) . However s p e c t r o s c o p i c t e c h n i q u e s ,
l i k e EXAFS, which p e r -
m i t t o e s t i m a t e t h e e x t e n t o f s u r f a c e e n r i c h m e n t do n o t p r o v i d e c l e a r evidence o f t o p o l o g i c a l s e g r e g a t i o n a t t h e s u r f a c e up t o now. C a t a l y t i c r e a c t i o n s c o u l d c o n s t i t u t e an a p p r o p r i a t e s o l u t i o n f o r t h i s problem.
Thus a l a r g e body o f works show t h a t s e l e c t i v i t y i n a l k a n e hyd-
r o g e n o l y s i s depends on t h e s i z e o f t h e m e t a l l i c p a r t i c l e s . O t h e r w i s e it exi s t s a s t r a i g h t f o r w a r d c o r r e l a t i o n between p a r t i c l e s i z e and mean c o o r d i n a t i o n number o f s u r f a c e atoms. H i t h e r t o s e l e c t i v i t y can be c o r r e l a t e d t o t h e c o o r d i n a t i o n o f t h e s e s u r f a c e atoms. S e v e r a l s t u d i e s , on Rh c a t a l y s t s f o r i n stance,
s u b s t a n t i a t e d t h i s p r o p o s a l ( r e f s . 3,4).
Thus a l k a n e h y d r o g e n o l y s i s
was used r e c e n t l y as a p r o b e f o r t h e s t u d y o f RhCu/Si02 c a t a l y s t s ( r e f . 5).
I t appeared o f i n t e r e s t t o a p p l y t h i s t o o l f o r s t u d y i n g Ru based c a t a l y s t a l l o y e d w i t h Ge,Sn,Pb
and Sb as an i n a c t i v e
second component.
Ruthenium
catalysts e x i b i t usually high hydrogenolytic a c t i v i t y w i t h a large production o f C1 and C 2 fragments ( r e f s . 6 - 1 0 ) .
The a d d i t i o n o f a l e s s a c t i v e second
metal (Cu,Au) decreases b o t h a c t i v i t y and deep h y d r o g e n o l y s i s ( r e f s . 11-14) ; a random d i l u t i o n o f t h e a c t i v e s u r f a c e was proposed t o e x p l a i n t h i s beha v i o u r . The changes i n c a t a l y t i c p r o p e r t i e s o f s u p p o r t e d Ru p a r t i c l e s when a l l o y e d w i t h Ge,Sn,Pb
o r Sb s h o u l d g i v e i n f o r m a t i o n s on t h e s u r f a c e p r o p e r -
t i e s o f t h e s e b i m e t a l l i c p a r t i c l e s . T h i s was t h e aim of t h i s work.
EXPERIMENTAL
A l l t h e c h e m i c a l s and r e a c t a n t s were of a h i g h p u r i t y g r a d e . The s u p p o r t used f o r t h e c a t a l y s t s wasx-alumina (200 m2 / g ) . the c a r r i e r
Ruthenium was d e p o s i t e d on
by l i g a n d exchange w i t h ruthenium-acetylacetonate
in
benzene
s o l u t i o n . The r e s u l t i n g m a t e r i a l s were d r i e d o v e r n i g h t under vacuum a t room temperature. The samples were reduced under f l o w i n g hydrogen a t 673 K.
In
o r d e r t o o b t a i n c a t a l y s t s w i t h l a r g e Ru p a r t i c l e s some samples were s u b j e c t e d to
a
sequence
of
reduction-calcination-reduction.
The
final
reduction
temperature was 673 K . The f i n a l amount o f Ru ranged f r o m 0.3 w t % t o 0.9 w t % .
A r u t h e n i u m b l a c k i n t h e f o r m o f powder was t e s t e d as a sample o f v e r y l o w metallic dispersion. The b i m e t a l l i c c a t a l y s t s were p r e p a r e d by u s i n g t h e C o n t r o l l e d S u r f a c e R e a c t i o n method ( r e f s . 15,161.
A reduced Ru/A1203 c a t a l y s t was c o n t a c t e d w i t h
Sn(C4Hg)4, Pb(C4H9)4, Ge(C4H9I4, o r Sb(C4Hg)3 temperature. reduced
at
i n n-heptane s o l u t i o n a t room
The r e s u l t i n g m a t e r i a l s were d r i e d under 673
K
under
o r g a n o m e t a l l i c s o f Sn,Pb,Ge
flowing
hydrogen.
The
vacuum and f u r t h e r
interaction
between
the
and Sb w i t h Ru i s h i g h l y s e l e c t i v e and no d e t e c t -
a b l e amount o f t h e second m e t a l can be f o u n d on t h e s u p p o r t when p u r e a l u m i n a
329
i s used ( r e f s . 15,171.
The main advantages o f t h i s method are : f i r s t t h e ob-
t e n t i o n o f t r u e supported b i m e t a l l i c aggregates w i t h i n t i m a t e contact between t h e two components, second t h e m o d i f i c a t i o n , under m i l d conditions,
of a
parent monometallic c a t a l y s t by changing e i t h e r t h e amount o r t h e nature o f the second metal. The chemisorption
of
hydrogen
was
volumetric apparatus a t 298 K i n t h e 0
carried
-
out
in
a
conventional
20 kPa pressure range. We used t h e
method o f t h e double isotherm ( r e f . 18) i n order t o estimate t h e m e t a l l i c dispersion. The s i z e o f t h e m e t a l l i c p a r t i c l e s was checked by Transmission Electron
Microscopy
(TEM).
The
preparation
and
characterization
of
monometallic and b i m e t a l l i c ruthenium c a t a l y s t s w i l l be d e t a i l e d i n a f o r t h coming paper ( r e f . 19). The
hydroconversions
of
n-hexane
(nH)
and
2,2,3,3-tetramethylbutane
(2233TeMB) were c a r r i e d out a t atmospheric pressure i n a microflow reactor i n l i n e w i t h a gas chromatograph equipped w i t h an apolar c a p i l l a r y column. The r e a c t i o n temperature ranged from 433 K t o 513 K and t h e molar r a t i o s o f the r e a c t i o n mixtures were : H2/nH = 14, H2/2233TeMB = 44.
I n order t o avoid
e i t h e r d i f f u s i o n a l l i m i t a t i o n s , o r secondary reactions, t h e reactant conversion was u s a l l y l e s s than 1 %. RESULTS AND DISCUSSIONS The conversion o f alkanes around 460 K on Ru c a t a l y s t s gives e s s e n t i a l l y hydrogenolysis ; t h e d i f f e r e n t possible pathways f o r nH and 2233TeMB conver-
--
sions are t h e f o l l o w i n g s : nH
t
H2
C1 t nC5, C2 t nC4, 2C3’ and deep hydrogenolysis
2233TeMB t H2
C1 t 223TrMB, 2iC4,
and deep hydrogenolysis ;
2Z3TrMB symbolize~2,2,3-trimethylbutane. The a c t i v i t y i s expressed as t h e number o f reactant molecules converted per surface Ru atom and per hour (Turnover Frequency, TOF). The changes o f s e l e c t i v i t y were characterized by two
If:=
ECi/
E (i/n)Ci
parameters : t h e fragmentation f a c t o r
defined by Paal and Tetenyi ( r e f . 201, and a S e l e c t i v i t y
F i n g e r p r i n t (SF). The f i r s t
parameter evidences t h e extent o f deep hyd-
rogenolysis and varies from 2 (simple rogenolysis t o methane) ;
n
hydrogenolysis) t o
(total
hyd-
i s t h e number o f carbon atoms i n t h e reactant.
The second parameter characterizes r e a c t i v i t i e s o f t h e d i f f e r e n t C-C bonds. Thus we defined f o r nH, SFH = ( C2 t nC4)/nC5,
and f o r 2233TeMB,
SFT =
iC4/223TrMB. C1, C2... 223TrMB represent t h e concentrations o f these r e a c t i o n products i n t h e e f f l u e n t s .
330
TABLE 1 Catalytic
properties
dispersion
iH/Ru),
of
Ru/A1203
catalysts
i n t h e hydrogenolysis
of
as
a
function
n-hexane
at
of
metallic
K, and o f
448
2,2,3,3-tetramethylbutane a t 473 K H/Ru
1
0.01
0.07
0.35
0.65
0.75
TOFa
350
400
60
50
42
10
3
4.46
3.77
2.96
2.30
2.24
2.10
2.5
2.85
3.05
3.9
Ea
31.5
24.8
29.4
27.8
26.8
31.3
TOFa
140
22
2.5
1.5
1.2
0.1
3
4.0
3.88
2.51
2.17
2.25
20
4.54
4.16
2.0 1.92
36.5
41.2
36.7
42
nH
SFH
~~
2233TeMB
SFT
0100)
Eab a : i n h-’
0100)
34
34
; b : i n kcal.mo1-’
The d a t a f o r t h e chemisorption o f hydrogen and t h e c a t a l y t i c p r o p e r t i e s o f supported Ru c a t a l y s t s a r e sumnarized i n Tables 1-3. data f o r t h e monometallic Ru c a t a l y s t s .
Table 1 g i v e s t h e
A c l e a r e f f e c t of
t h e Ru p a r t i c l e
s i z e i s observed on b o t h a c t i v i t y and s e l e c t i v i t y . The decrease of a c t i v i t y i s moderate (
>
f o r nH conversion (
< 30)
b u t l a r g e f o r 2233TeMB hydrogenolysis
1000). Large Ru p a r t i c l e s favour deep hydrogenolysis of alkanesto methane
( J > 4).
This behaviour s u b s i s t s a t conversion as low as 0.1-0.3
t h a t no r e a d s o r p t i o n ,
and new fragmentations,
of
% indicating
products occurs.
t y p i c a l o f m u l t i p l e f i s s i o n i n one s o j o u r n a t t h e s u r f a c e
(ref.
This i s 7).
By
decreasing t h e p a r t i c l e size, deep hydrogenolysis tends t o be suppressed and simple cleavage o f C - C bonds occurs
( J E2 ) .
>
w i t h t h e s m a l l e s t Ru p a r t i c l e s (H/Ru
0.651,
Moreover, concerning t h e samples we can observe changes i n t h e
r e a c t i v i t i e s o f t h e d i f f e r e n t C-C bonds. Thus t h e i n c r e a s e o f SFH shows t h a t i n t e r n a l f i s s i o n s (C3 and nC4) i n c r e a s e a t t h e c o s t o f t e r m i n a l f i s s i o n (nC5) f o r nH hydrogenolysis. C e n t r a l cleavage o f 2233TeMB t o i C 4 i s p r e v a i l i n g on medium s i z e p a r t i c l e s (H/Ru
,< 0.651,
b u t demethylation
l e a d i n g t o C1
223TrMB becomes n o t i c e a b l e on Ru aggregates s m a l l e r than 1 nm (H/Ru
>/
and 1).
These r e s u l t s are i n l i n e w i t h those p r e v i o u s l y r e p o r t e d f o r t h e hydrogenolys i s o f alkanes ( r e f s . 6,7,9). rogenolysis i s
explained
The s t r o n g tendancy o f Ru t o g i v e deep hyd-
usually
by
the
facility
of
t h i s metal t o form
331
TABLE 2 C a t a l y t i c p r o p e r t i e s o f RuSn/A1203 c a t a l y s t s as a f u n c t i o n o f t i n content,
in
t h e hydrogenolysis o f n-hexane a t 448 K, and o f 2,2,3,3-tetramethylbutane a t 473 K . (Sn/Ru 1 at H/Ru TOF~
nH
2233TeMB
0
0.26
0.63
0.84
0.75
0.39
0.31
0.27
42
21
0.45
0.1
3
2.24
3.13
2.49
2.43
SFH
3.05
2.8
4.1
3.9
Eab
26.8
26.3
28.8
26.9
TOF~
1.2
0.9
0.04
0.01
Y
2.25
2.81
2.59
2.05
SFT
4.16
13.4
3.16
1.19
Eab
36.7
38.3
36.6
23.8
a : i n h - l ; b : i n kcal.mo1-’
multiply-bound species (metallo-carbenes, metallo-carbynes)
( r e f s . 7,211,
the
formation o f such species being favoured on dense planes ( r e f . 22). Therefore these p a t t e r n s o f a c t i v i t y and s e l e c t i v i t y a r e c h a r a c t e r i s t i c o f t h e s i z e o f Ru p a r t i c l e s , and then o f t h e c o o r d i n a t i o n o f t h e s u r f a c e a t -
oms. F l a t planes, i . e . h i g h c o o r d i n a t i o n o f surface atoms ( l a r g e p a r t i c l e s ) ,
will
p r i v i l e g e deep
hydrogenolysis,
terminal
fission
cleavage o f 2233TeMB. A t variance s u r f a c e s i t e s of and corners (small p a r t i c l e s ) ,
will
of
nH and c e n t r a l
low c o o r d i n a t i o n ,
f a v o u r simple hydrogenolysis,
edges central
f i s s i o n o f nH and demethylation o f 2233TeMB. Table
2
presents
the catalytic
properties
of
RuSn/A1203
bimetallic
c a t a l y s t s . The p a r e n t m a t e r i a l was a monometallic Ru/A1203 sample w i t h H/Ru = 0.75
(mean p a r t i c l e s i z e ‘v 1.1
nm).
Three d i f f e r e n t
amounts o f Sn were
deposited on t h e Ru aggregates o f t h i s Ru/A1203. For t h e smallest Sn a d d i t i o n (Sn/Ru = 0.261,
t h e c a t a l y t i c p r o p e r t i e s s h i f t towards those o f l a r g e r Ru
p a r t i c l e s (mean s i z e s around 3-5 nm): an increase o f deep hydrogenolysis, a s h i f t o f c e n t r a l t o t e r m i n a l cleavage o f C-C bonds i n nH, and a decrease o f demethylation o f
2233TeMB.
TEM examinations
show no growth o f
metallic
p a r t i c l e s upon Sn a d d i t i o n (mean s i z e between 1.0 and 1.2 nm). A l l these f a c t s r e f l e c t an i n c r e a s e o f t h e mean c o o r d i n a t i o n number o f s u r f a c e Ru atoms
332
TABLE 3 C a t a l y t i c p r o p e r t i e s o f RuMe'/A1203 b i m e t a l l i c c a t a l y s t s as a f u n c t i o n of t h e nature o f t h e second metal Me', i n hydrogenolysis o f n-hexane a t 448 K,
and
o f 2,2,3,3-tetramethylbutane a t 473 K . Me'
Pb
(Me /Ru 1at H/Ru
TOF~ nH
Ge a
0
0.33
0.13
0.18
0.65
0.28
0.38
0.41
50
22.7
50
5.2
3
2.30
2.40
2.42
2.20
SFH
2.85
2.74
2.60
4.3
E ac
27.8
28
31.2
1.5
1
2.5
0.02
3
2.17
2.40
2.31
2.02
SFT
4.54
8.85
14.1
E ac
41.2
37.8
TOF~
2233TeMB
Sb
0.61 37.3
a : Ge was deposited on t h e same p a r e n t Ru/A1203 c a t a l y s t as t h a t used f o r
:-'
RuSn/Al 0
c a t a l y s t s , w i t h a m e t a l l i c d i s p e r s i o n o f H/Ru
b : in
; c : i n kcal.mo1-I
=
0.75 ( t a b l e 2 ) ;
i n RuSn b i m e t a l l i c aggregates. The c a t a l y t i c p r o p e r t i e s o f a l l o y s are o f t e n w e l l e x p l a i n e d i n t h e framework o f t h e ensemble t h e o r y ( r e f s . 22-24). Based
on these concepts one can expect a sharp decrease of a c t i v i t y and a s h i f t o f s e l e c t i v i t y p a t t e r n towards those o f small p a r t i c l e s when adding a l e s s act i v e second metal t o a more a c t i v e one.
This behaviour comes f r o m random
d i l u t i o n o f t h e a c t i v e surface by t h e second component,
and NiCu c o n s t i t u t e s
t h e archetypal system f o r t h i s ( r e f . 23). The d a t a o b t a i n e d w i t h RuSn/A120g (H/Ru = 0.39) cannot e n t e r i n t h i s scheme, s i n c e t h e decrease o f a c t i v i t y i s
low and t h e s e l e c t i v i t y p a t t e r n s move towards those o f l a r g e p a r t i c l e s . These behaviours can be r a t i o n a l i z e d assuming t h a t Sn i s n o t d i s t r i b u t e d randomly a t t h e s u r f a c e . Sn tends t o occupy s i t e s o f
low c o o r d i n a t i o n
(edges and
corners) l e a v i n g l a r g e i s l a n d s o f Ru atoms on t h e f l a t planes. Thus t h e mean c o o r d i n a t i o n o f s u r f a c e Ru atoms i n c r e a s e s and
the
p a r t i c l e s i s reproduced f o r a1 kane hydrogenolysis.
Larger Sn c o n t e n t s (Sn/Ru
o f Ru i s l a n d s ,
and i s o l a t e t h e Ru atoms,
= 0.65 and 0.84) decrease t h e s i z e
behaviour
of
larger
333
t h e n r e p r o d u c i n g t h e c a t a l y t i c b e h a v i o u r o f v e r y small Ru p a r t i c l e s . T h i s t o p o l o g i c a l s e g r e g a t i o n o f Sn on t h e s u r f a c e l a y e r o f RuSn aggregates i s c o n s i s t e n t w i t h t h e t h e o r e t i c a l p r e d i c t i o n s o f B u r t o n e t a l . ( r e f . 2 ) . T h e i r mathematical t r e a t m e n t shows t h a t , f o r t h e same reasons which produce an enrichment o f t h e s u r f a c e i n one component,
an enrichment o f t h e
same component t o s i t e s o f l o w e r c o o r d i n a t i o n s h o u l d occur t o o .
I n the
p r e s e n t case, t h e d r i v i n g f o r c e f o r t h e s e g r e g a t i o n o f t i n i s t h e l o w e r h e a t o f s u b l i m a t i o n o f t h i s metal w i t h r e s p e c t t o t h a t o f Ru. From hydrogen c h e m i s o r p t i o n one atom o f t i n b l o c k s 1.5 atom o f r u t h e n i u m a t l o w t i n c o n t e n t . T h i s i n h i b i t i n g e f f e c t o f t i n decreases a t h i g h e r Sn/Ru r a t i o s s i n c e o n l y 0.3 Ru atom p e r Sn atom a r e b l o c k e d f o r hydrogen chemisorpt i o n when Sn/Ru = 0.84.
One can p o s t u l a t e t h a t ,
whereas 2D i s l a n d s o f t i n
develop a t l o w c o n t e n t , t i n e i t h e r forms 3D patches o r d i f f u s e s i n t o t h e b u l k a t high content. For s i m i l a r Me'/Ru atomic r a t i o s (Me' = Ge,Sn,Pb,Sb) m e t a l l i c RuGe,Sn,Pb,Sb
t h e b e h a v i o u r of b i -
aggregates s u p p o r t e d on alumina i s r a t h e r d i f f e r e n t
(Tables 2 , 3 ) . Sn,Pb and Sb reproduce t h e same b e h a v i o u r s : s m a l l amounts o f t h e second m e t a l s h i f t t h e p r o p e r t i e s towards t h o s e o f l a r g e p a r t i c l e s . The e f f e c t appears t h e l a r g e s t f o r t i n and t h e l o w e s t f o r lead. A t v a r i a n c e t h e a d d i t i o n o f Ge moves t h e p r o p e r t i e s i n t h e o p p o s i t e d i r e c t i o n : t h e p r o p e r t i e s o f RuGe/A1203
l o o k l i k e t h o s e o f v e r y s m a l l Ru aggregates : sharp
decrease o f b o t h a c t i v i t y and deep h y d r o g e n o l y s i s , c e n t r a l cleavage o f nH and d e m e t h y l a t i o n o f 2233TeMB. T h i s p a r t i c u l a r b e h a v i o u r o f RuGe aggregates can be r a t i o n a l i z e d assuming a random d i s t r i b u t i o n o f Ge on t h e surface,
and
maybe a p r e f e r e n t i a l l o c a l i z a t i o n o f t h i s component on f l a t p l a n e s .
CONCLUSIONS
The main c o n c l u s i o n concerns t h e p o t e n t i a l i t y o f c a t a l y t i c r e a c t i o n s as a probe f o r t h e d e t e r m i n a t i o n of metallic clusters.
surface p r o p e r t i e s o f h i g h l y dispersed b i -
They a1 lowed t o evidence f o r RuGe,Sn,Pb,Sb/Alp03
b i-
metallic catalysts that :
1) a t o p o l o g i c a l s e g r e g a t i o n o f Sn,Pb and Sb t a k e s p l a c e a t t h e s u r f a c e : t h e m o d i f i e r occupies s i t e s of l o w c o o r d i n a t i o n , i . e . c o r n e r s and edges, and pushes Ru o n t o t h e dense planes. 2 ) Ge seems more randomly d i s t r i b u t e d a t t h e s u r f a c e , and maybe l o c a l i z e d p r e f e r e n t i a l l y on f l a t planes.
334
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335
THEORETICAL CALCULATIONS ON THE TRANSITION METAL SURFACES
AT
STABILITY OF
CARBIDE LAYERS
G.R. DARLING1, R.W. JOYNER2 and J.B. PENDRY1 'The Blackett Laboratory, Imperial College, London SW7 2BZ 2Leverhulme Centre for Innovative Catalysis, Department of IPI Chemistry, University of Liverpool, P.O. Box 147, Liverpool L69 3BX
ABSTRACT Local density of states (LDOS) calculations have been performed on bulk and surface carbides of iron, nickel and copper. Foccussing o n the splitting of the carbon p-orbitals and the degree of filling of the anti-bonding p-orbital, we conclude that the stability of the bulk carbides decreases from iron to nickel to copper, in agreement with experimentally known fact. This trend is also observed in the surface carbides, and it is concluded that a carbide layer is unstable on copper surfaces, with respect to graphite. The implications of these results for the mechanism and selectivity in Fischer-Tropsch synthesis on these metals is discussed. Using ideas from Effective Medium Theory, we have also examined the behaviour of various carbon layers on a Ni(ll1) surface, with a view to examining the mechanism for carbide to graphite transformation. Some preliminary results are presented here.
INTRODUCTION The chemical nature of carbon on transition metal surfaces is of considerable industrial importance and scientific interest. Two main forms are known, carbidic carbon, which is highly coordinated to metal atoms but not to any others, and graphitic carbon, comprising fragments of single graphitic layers. Goodman et a1 have shown that carbidic carbon is a precursor to the catalytic formation of methane from CO and H,, on Ni(100)
336
surfaces, whereas graphitic carbon inhibits this reaction (ref. 1). In this paper we examine the stability of bonding between carbidic carbon layers and several metal surfaces, relating the trends observed to reactivity patterns in the Fischer-Tropsch hydrocarbon synthesis. The initial product of CO dissociation on both nickel (ref. I ) and cobalt, (ref. 2 ) is carbidic carbon. Above a given coverage, however, and at suitably high temperatures, it has been shown that the observed carbon species becomes graphitic rather than carbidic (ref. 1). We also present some preliminary results suggesting reasons why such a transformation takes place. CALCULATIONS FOR CARBIDES Methods We have chosen to examine the density of states as a guide to the strength of the carbon-metal bond in both bulk and surface carbides. For the bulk carbides, we have used a self-consistent layer-KKR scheme. This is particularly suitable as we have used the Ni3C structure, which is most easily represented as a superlattice (ref. 31, for all the bulk carbides. Although this is not the correct structure for Fe3C (ref. 4 1 , the local environment of the C atom is very similar. The scheme is essentially the same as that used for LEED calculations (ref. 5); scattering is divided into intralayer events, described by spherical waves, and interlayer events, described by plane waves. This results in computational efficiency as the times for matrix inversion scale only as the cube of the number of unique atoms per layer multiplied by the number of unique layers, which is usually much less than the number of unique atoms per bulk unit cell. The method gives reliable results for the bulk densities of states and band structures of metals (ref. 6), and we have also found that it gave densities of states for TiC in agreement with other calculations (ref. 7 ) . For the surface carbides, we used published structures for the Fe(100) and Ni(100) structures (ref. 8 ) . The density of states has been calculated with a non self-consistent cluster program, which has been shown before to be capable of giving reliable qualitative results for these systems (ref. 9). The phase shifts have been calculated with the MUFPOT program, using selfconsistent bulk potentials for the substrate (ref. lo), and atomic
337
wavefunctions for C in a metallic environment. The density of states in the cluster representation can be written as -Ia(r,E) = Im{G (r,r,E)I, (1) + where G is the multiple scattering Green function. This can be decomposed into different angular momentum components on the central atom as (ref. 11) + aLLl(r,E)= Im { GLLl(r,r,E)I . (2) It is important to note that although both our calculation methods employ muffin-tin potentials, covalent effects can still be described and contribute to the result, because only the potential, not the charge density, is spherically symmetrized. Results To probe the local density of states which will be most indicative of the extent and nature of metal-carbon bonding, we choose states with p-symmetry with respect to the carbon atom, located at the point where the carbon and metal muffin tins touch. Fig. 1 shows the results of these calculations for the bulk carbides (a-c) and the surface carbides (d-f). Each curve has two peaks, corresponding to a bonding and an antibonding peak, produced by the hybridisation of the metal d-states with the carbon p-states. To determine the relative bonding strengths two quantities must be considered, the separation of the two peaks, displayed in table 1, and the occupation of the anti-bonding peak. TABLE 1 Separation of peaks in the LDOS with p symmetry on the carbon atom in bulk and surface carbides. Peak Separation Hartrees eV
Carbide Bulk:
Fe3C Ni3C cu3c
Surface:Fe(lOO)c(2~2)C Ni(lOO)p(2x2)C cu(1oo)p(2x2)c
0.39 0.31 0.24
10.6 8.4 6.5
0.28 0.21 0.12
7.7 5.8 3.2
For the bulk carbides, there is a clear decrease in peak separation from iron to nickel to copper. Also, while the
338
0.1 1
0.17
f
U
i
UP,
OPP
I
O.08. B 0.1
0.48 0. B Energy (Hartrees)
i'
b
O.08 5
0-12
i
0.35 0.65 Energy (Hartrees)
e I
UPP
Y L 0.3
:
0.08
i
Energy (Hortrees)
c
0.08
f
1
I
I
Fig. 1
I
:I
UPP
*PP
O.9
C
O-oS.0
0.3 Energy (Hartrees)
O.P
I
0.3 0.6 Energy (Hartrees)
O.9
0.3 Energy (Hartrees)
C
LDOS with p-symmetry on C atom in: a) Fe3C, b) Ni3C, cu3c { a and c in same structure as b 1 , d ) Fe(lOO)c(2~2)C, Ni ( 100 p(2x2)C, f ) Cu(lOO)p(2x2)C. The Fermi energy is shown by a dashed line.
339
anti-bonding peak is completely unoccupied in Fe3C and Ni3C, it is about 30% full for Cu3C. Taken together, these suggest that iron carbide is more stable than nickel carbide which is more stable than copper carbide. This has been known experimentally for a long time; Fe3C is marginally stable, Ni3C is metastable and copper carbide has never been observed. We can extend this analysis to the case of surface carbon. Similar trends are observed, namely the strength of the carbonmetal bonding decreases from iron to nickel to copper. In an earlier study (ref. 121, Feibelman showed that there is a similar decrease in metal-carbon interaction on moving from ruthenium to rhodium. Since ruthenium carbide is highly unstable, rhodium carbide probably does not exist, and mono-carbon species must be very weakly bound to the surface. This is probably also true for palladium. Catalytic Siqnificance There is general agreement that the dissociation of carbon monoxide is a necessary early stage in Fischer-Tropsch reactions on iron, cobalt and nickel surfaces (ref. 13). These metals form the basis of the most commonly used catalysts, although in the case of nickel, the product is mostly methane. It has also been noted that CO dissociates on those metals which form stable bulk carbides (ref. 14). More specifically, carbidic carbon has never been demonstrated to exist on copper, rhodium or palladium surfaces. In those studies where carbon has been deposited from hydrocarbon decomposition (ref. 15) we believe that it is more * likely to exist in either a graphitic or a partially hydrogenated form. We therefore feel confident in maintaining that the carbidic precursor to methanation does not exist on these non Fischer-Tropsch metals. Considering, in addition, their very low propensity for dissociating CO, we can conclude that methanol is a much more likely product than methane over catalysts made from these metals This is of course observed experimentally, eg. the ratio of methane to methanol formed on industrial copper catalysts is (ref. 16). However, this does not exclude the formation of hydrocarbons on surfaces where surface carbide is unstable. Although the selectivity to methanol is always high, up to 10% of the product obtained over palladium catalysts can be hydrocarbons (ref. 17).
340
Our results show that carbidic carbon becomes unstable on moving to the right across the transition series, by contrast, the results of surface science experiments show that oxygenated species such as formyl or hydroxycarbenes become more stable. It would seem more probable that a mechanism such as that of Emmett et a1 (ref. 18), involving the elimination of water between two hydroxycarbene species, is responsible for hydrocarbon formation over copper and palladium. We therefore envisage two distinct mechanisms for hydrocarbon formation on transition metal surfaces. On conventional Fischer-Tropsch catalysts, fracture of the carbon-oxygen bond occurs at an early stage, well before the start of chain growth. On other metals, such as palladium, platinum or copper, the carbon-oxygen bond is not broken until during the chain growth process. We note that Bell and Shustorovich have recently made similar proposals based on the latter's bond-order conservation theory, (ref. 19). FORMATION OF GRAPHITE The formation of graphite from or on top of carbide layers has been studied experimentally on at least two nickel surfaces, (100) (ref. 1) and (110) (ref. 20). For the (100) surface, Goodman has concluded that the graphite layer is only formed when the carbide coverage has reached a value of 0.5, corresponding to the formation of the well known structure with p4g symmetry (ref. 2 1 ) . Recently, Tanaka et a1 have confirmed this result and further, have reported that the graphite layer grows on top of the carbide layer (ref. 2 2 ) . This has also been shown to be a possible structure for carbon on a Pt(ll1) surface (ref. 2 3 1 , however no results have so far been presented regarding the growth of graphite on a Ni(ll1) surface. We have nonetheless decided to consider this surface because of the obvious geometric simplicity; filling all available sites with carbon atoms results in a graphite layer with a lattice constant almost exactly equal to that in bulk graphite. Method Most attempts to determine atomic geometries at surfaces have relied upon the density functional theory. A variant of this, the Effective Medium theory of Norskov (ref. 2 4 1 , has the simple
341
interpretation that, to first order, an adatom occupies a site where the density reaches some optimal value, characteristic of that adatom. For transition metal surfaces, it is necessary to include in the energy sum extra terms which take account of purely covalent contributions, which cannot be well represented in the local density approximation. Jacobsen and Norskov ( J&N ) found that for C on Ni (ref. 251, this results in the introduction of an arbitrary parameter, which has to be determined by a semiempirical method. We have attempted to circumvent this problem by considering only the density on the surface as a guide to the preffered geometry. Our approach is to integrate the LDOS in the proposed vacant carbon site, obtained with the multiple-scattering cluster method, over the conduction band. For the optimum density we use the value calculated in a vacant C site in Ni3C. We have determined this to be 0.014-0.015 /(a.u.) 3 , which compares well for the equilibrium density achieved in a homogeneous freeelectron gas (ref. 25).
Fia. 2. Electron densitv in a 3-fold hollow site on a clean Niilll) surface versus distance from the surface. The dotted line indicates the optimum density and the distance at which this occurs.
342
Resu 1ts Using the optimum density as a probe, we have determined the carbon to surface distance for a variety of geometries. Included in these calculations is a surface barrier of two layers of muffin-tins with a repulsive potential equal to the sum of the Fermi energy and the work-function of nickel. Fig.2 shows the density profile in a 3-fold hollow site on a clean surface, with the optimum density occurring at 1.4 a.u.. This corresponds to a C-Ni bond length of 3 a.u., compared to J&N’s value of 3.35-3.45 a.u.. For a graphite layer, J&N suggested that the density could be found by superimposing the nickel and carbon densities, and that one would find it rising off the surface in an effort to reduce the electron density to the optimum value. Fig. 3 shows the results of our calculations for this system.
’
1.25.
3 1.24
I
0
1.23
E 1.22
g1.21 C
% 1.20
i 1.19
c 0
2 1.18 0 1.17
Fig. 3. Electron density in a vacant carbon site in a graphite layer on a Ni(ll1) surface versus separation of layer and surface planes. It should be noted that for large C-surface distances, there is a consistent underestimate of the density in the vacant C site. This results from the poor representation of the potential between the graphite and the surface. In effect, we have integrated to a position below the true Fermi energy in the layer. A very approximate determination of the Fermi energy of graphite from a comparison of a on a C atom with band structure calculations PP
343
(ref. 2 6 ) suggests that one should use an effective Fermi energy about 0.08 Hartrees above that of nickel Using this, the density for a free graphite layer is about 0.013/(au)3 The clear trend in fig. 3 is one of increasing density with increasing surface-layer distance. This is reinforced by our observations above. The graphite layer is obviously having to rise off the surface in an effort to increase the electron density to the optimum value. We can explain this by consideration of the phase-shifts. A carbon atom will behave attractively to electrons with energy below the p-resonance, but repulsively above. As the resonance occurs around 0 . 2 2 Hartrees, a large number of the electrons from the nickel d-bands will be repelled. When the graphite layer is forced closer to the surface, the nickel ( with a d-resonance at higher energy than the carbon p) attracts pelectrons from the carbon atoms to covalent bonds, while its own d-electrons are repelled by the layer. Repulsion wins over C-Ni bonding and the net result is a decrease in the electron density available to each carbon atom. Therefore, we can say that due to complex hybridisation effects, the simple approach of adding the individual atom densities will not result in an accurate picture in terms of electron densities, and, in some cases, may lead to trends completely opposite to the correct ones. On a Ni(100) surface, a graphite layer as considered above, will only grow after a carbon coverage of 0.5 has been achieved (refs. 1 , 2 2 1 . However on the (111) surface we have found that the density in an empty site when the C coverage is 0.5 is almost indistinguishable from that of the clean surface. We have therefore started our investigation of graphite growth by considering a 1x1 coverage of carbon on the surface. Fig. 4 shows the density at the C position versus the C position for this coverage. Also included are the results of the same calculation for a clean surface with only one carbon atom in a nearestneighbour site to the test site. We can see that due to the repulsion of d-electrons, the 1x1 layer would be forced into the surface to find the optimal density. This may not be physically possible, however, because of repulsion between the Ni and C core electrons (ref. 2 5 ) . For the single nearest-neighbour case, the same situation holds. However, for C-surface distances 21.6 a.u., which is a reasonable C-surface
.
.
344
distance, the density is greater for the one case
nearest-neighbour
'21 . 5 $1.4
5 " 1.3
-b 1.2 C
I
g 1.1
-8 1.0 $0.9 0
-
0.8
Fig. 4 . Electron density in vacant C site - (a) in a 1x1 layer of I , (b) in a nearest neighbour site to one other carbon ( - at the position of the other C atom(sI on a carbon (------I Ni(ll1) surface versus the distance from the C to the surface plane. This may suggest a mechanism for the conversion from carbide into graphite. A C atom in a 1x1 coverage site could achieve a density closer to optimal by transfering to a nearest- neighbour site, thus initiating the growth of a graphite layer. Thus, we can say that due to the repulsion of the Ni d-electrons by C atoms in next nearest-neighbour sites, the electron density is less than optimal. In an effort to achieve a position of optimum electron density, the atoms move to adjacent sites. Eventually, fragments of graphite are formed, which rise off the surface to minimise the effects of electron repulsion. Since the density is greater when there is only one nearestneighbour than when there is none, it would appear that a carbide layer is unstable on a Ni(ll1) surface, as the energy can always be reduced, when the coverage is of the order of 1, by the atoms moving into neighbouring sites. This is different from the situation on a Ni(100) surface, where experimental studies have shown that the graphite is formed above a carbide layer (ref. 2 2 ) . However, Tanaka et a1 have also indicated that carbide layers are indeed unstable with respect to graphite on a Ni(ll1) surface.
346
REFERENCES 1
2 3
4 5 6 7 8
9
10 11 12 13 14 15 16 17 18
19 20 21 22 23 24 25 26
D.W. Goodman, R.D. Kelley, T.E. Madey and J.T. Yates, J. Catal., 63 (1980) 226.; D.W. Goodman, R.D. Kelley, T.E. Madey and J.M. White, J. Catal., 6 4 (1980) 479. J. Nakamura, I. Toyoshima and K. Tanaka, Surface Sci., 201 (1988) 185. S. Nagakura, J. Phys. SOC. Jap., 12 (1957) 482. R.G. Wyckoff, "Crystal Structure", Interscience, New York, 2nd. Ed., 1963. J.B. Pendry, "Low Energy Electron Diffraction", Academic Press, London, 1974. S. Crampin and D.D. Vvedensky, private communication. Results for Tic were compared with densities of states obtained by A. Neckel, P. Rastl, R. Eibler, P. Weinberger and K. Schwarz, J. Phys. C, 9 (1976) 579. J.M. MacLaren, J.B. Pendry, P.J. ROW, D.K. Saldin, G.A. Somorjai, M.A. Van Hove, D.D. Vvedensky, "Surface Crystallography Information Service", D. Reidel, Dordrecht, 1987. J.M. MacLaren, J.B. Pendry, R.W. Joyner, P. Meehan, Surface Sci., 178 (1986) 856; J.M. MacLaren, J.B. Pendry, D.D. Vvedensky, R.W. Joyner, Surface Science, 162 (1985) 322; J.M. MacLaren, J.B. Pendry, R.W. Joyner, Surface Sci., 165 (1986) L80. V.L. Moruzzi, J.F. Janak, A.R. Williams, "Calculated Electronic Properties of Metals", Pergamon, Oxford, 1978. J.M. MacLaren, J.B. Pendry, R.W. Joyner, Surface Sci., 178 (1986) 856. P.J. Feibelman, Phys Rev B, 26 (1982) 5347. R.W. Joyner, Vacuum, 38 (1988) 309. R.W. Joyner in "Catalysis" Vol. 5 ed. G.C. Bond and G. Webb, (Royal Society of Chemistry, London, 1982). T.S. Marinova and P.K. Stefanov, Surface Sci., 191 (1987) 66; D.A. Outka, C.M. Friend, S . Jorgensen and R.J. Madix, J. Amer. Chem. SOC., 105 (1983) 3468. G. Haddeland, SRI Process Economics Program Vol. 43b, "Methanol", Stanford Ca., 1982. M.L. Poutsma, L.F. Elek, P.A. Ibarbia, A.P. Risch and J.A. Rabo, J. Cat., 52 (1978) 157. J.T. Kummer, T.W. de Wit and P.H. Emmett, J. Am. Chem. SOC., 70 (1948) 3632; W.K. Hall, R.J. Kokes and P.H. Emmett, J. Am. Chem. SOC., 79 (1957) 2983; 2989. A. T. Bell, Proc. 9th Intern. Congr. Catal., to be published; E. Shustorovitch, Acc. Chem. Res., 21 (1988) 183. R. Sau and J.B. Hudson, J. Vac. Sci. Tech., 16 (1979) 1554. J.H. Onuferko, D.P. Woodruff and B.W. Holland Surface Sci., 87 (1979) 357; see also (ref. 10) p. 125. K. Tanaka, T. Kawamura, T. Yamada, J. Nakamura, private communication of results presented at ECOSS 10. H. Zi-pu, D.F. Ogletree, M.A. Van Hove and G.A. Somorjai, Surface Sci., 180 (1987) 433. J.K. Norskov and N.D. Lang, Phys. Rev. B, 21 (1980) 2131; J.K. Norskov Phys. Rev. B, 26 (1982) 2875. K.W. Jacobsen and J.K. Norskov Surf. Sci., 166 (1986) 539. C.F. McConville, D.P. Woodruff, S.D. Kevan, Surface Sci., 171 (1986) L447.
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Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V.,Amsterdam Printed in The Netherlands
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347
CRITICAL METAL CONCENTRATION EFFECT ON AVERAGE SIZE SELECTED (Pd-Ag) CLUSTERS IN THE SELECTIVE HYDROGENATION OF 1.3-BUTADIENE
V. de GOUVEIA, B. BELLAMY, A. MASSON Laboratoire de Physico-chimiedes Surfaces associk au C.N.R.S, U.A. 425, Ecole Nationale Superieure de Chimie de Paris 11 rue Pierre et Marie Curie, Universitk P. et Marie Curie 75005 PARIS Cedex 05 - FRANCE and M. CHE Laboratoire de Rkactivitk de Surface et Structure associk au C.N.R.S., U.A. 1106, Universitk P. et Marie Curie 75252 PARIS Cedex 05 - FRANCE
ABSTRACT The size and the concentration of palladium-silver aggregates are varied with atomic beam technique and by the successive evaporations method using a SiO, amorphous substrate. In this way it is possibleto define an averagesize and compositionfor which we observe an importantimprovement in the olefin selectivity . INTRODUCTION The most remarkable aspect of bimetallic catalysts concerns the selectivity for a given reaction. One finds, generally, that the bimetallic systems have selectivity and activity which differ markedly fromthose observed for the pure metals. In the case of alloys constituted by an active and an inactive metal for a given reaction, two different effects can be considered (1,2) : i)a geometric or ensemble size effect. Indeed, it is possible by alloying to change the number of identical neighbouringatoms. This effect is expected to lead to an important fall of activity when the reaction requires a site with a given number of active atoms. However, the selectivity for a given reaction can be increased. ii) an electronic effect or 4igand effectn. One assumes that the bond strength between the metallic atom and the adsorbed speciesdepends strongly on the neighbours chemical nature. It is assumed that theelectronicpropertiesofan atomor foran ensembleof active atomscan bechanged by the neighbour
348
atoms if the latter have different electronegativities.These electronic variations can induce changes in the chemisorption energies and therefore modify the activity and selectivity. The bimetallic system Pd-Ag has been the subject of different studies, with the aim to evidence the above effects (3-5). The atomic beam technique and the use of successiveevaporations, are well suited for the synthesis of such catalytic systems. In this way, the alloy formation is made safer in that the aggregatesofthe first metal deposited form the nucleation germsof the second metal (6).This approach that requires ultra-vacuum conditions, is suitable for the use of in-situ surface analysis methods (A.E.S.,U.P.S.,)tocharacterize the bimetallic aggregates.This workisacontinuationof earlier studies on the size- induced electronic effect (intrinsic size effect) (7,8).In the present paper we have investigated the influence of silver on the palladium activity and selectivity in 1,3 butadiene hydrogenation (extrinsic size effect) EXPERIMENTAL The experimental apparatus used for the synthesis and the characterization of the samples has been described previously (6-8).The amorphous silica support has been obtained by thermal oxidation of a silicon monocrystal. The metallic phase (Pd-Ag) was prepared by the atomic beam method under ultra high vacuum conditions ( 109T0rr),by successive evaporationsof palladium and silver, using two peculiar Knudsen cells (9).After synthesis, the samples are analyzed and characterizedby U.P.S. and A.E.S.. Finally, the samples are transferred under ultra high vacuum in the recirculation chemical reactor where the catalytictest is performed. The reaction conditionsthat we have used are as follows: reaction temperature 343 K, a hydrogen 1 1,3 butadiene ratio of 75 (300 Tom hydrogen and 4.7 TOK butadiene).The totalpressureof73OTorr isobtainedwith argon.The analysisofthereactionproducts as a function of time is followed by gas phase chromatography. RESULTS AND DISCUSSION Metallic clusters stud.led bv transrm'ssion electron microscouy By T.E.M., we have performed comparative studies between the samples with the palladium clusters and the bimetallic aggregates (Pd - Ag) deposited in the two cases onto the same type of amorphous silica substrate.The first observation concernsthe numberof nuclei (N,)at saturation.Table 1gives the experimentalvalues for the different samples used in this TEM study. The number of nuclei at saturation is found to be similarfor sampleswith the bimetallicclustersor with palladiumonly.This fact confirms the idea (10) that the silver atoms impinging the surface do not play a great part in the formation of the new nuclei but in the growing of the palladium nuclei already present. Another observation concerns the average size of the bimetallic clusters which are greater than the palladium aggregates for similar coverages. This discrepancy for the size cannot be asssociated solely with the supplementaryamount of silver because a sample having the same total coverage (5.6 lOI4 atoms.cm*) has a lower average size. We can explain this discrepancy of size by a silver-induced spreading of the Pd-Ag particles, due to changes in surface energy and in the interface adsorbate-substrate energy .In other words, the addition of silver increases the wetting of the Pd-Ag particles in comparison with the pure palladium clusters (6) on the silica substrate.
349
TABLE 1 The valuesof the number N, of nuclei at saturation and the particle sizefor different W-Ag coverages
sample
Pd at.mr2)
Ag (1013 at.cm-,)
cm3
3.8 4.0 5.6 3.8 4.0 3.9 4.0
0 0 0 8.0 8.0 8.0 2.0
3 2.8 3 2.7 2.5 2.4 3.2
(1014
Pd/SiO, Pd/SiO, Pd/SiO, Pd-Ag/SiO, Pd-Ag/SiO, Pd-Ag/SiO, Pd-Ag/SiO,
Nuclei (101~
size
A
16 18 21 27 25 30 21
Electronic Dropen'ies Auger electron spectroscopy The study of a particular transition like the C-VV transition represents an adapted probe to monitor the formation of the bimetallic Pd-Ag clusters. We have used the Auger M45VV transi-
I
305
315
1
325
335 Ec,eV
Figure 1: Palladium M,, VV Auger Transition for a) pure palladium, b) Pd o,82- Ag
o,,8
system
tion of palladium. In figure 1 are showed the different palladium M-VV Auger spectra with a) 1.1 10 '' atoms (Pd).cm-*and b) the same palladium amount and in addition 2.4 10 l4 atoms (Ag).cm-'. It can be seen that the spectra exhibit the smoothed appearance. From a qualitative point of view, a slight variation of the shape is observed between spectra a) and b). Upon depositing silver onto the substrate, the Auger signal intensity given by the M-VV peak integral decreases. This decrease has been assigned to an attenuation effect of the palladium M-VV transition by silver atoms. Ultra-violet Dhotoelectron S D ~ C ~ O S C O D V . A different manner to follow the formation of the bimetallic alloy by the successiveevaporations method consists in studying the valence band of the corresponding metals. Thus in figure 2 are represented the valence band spectra of a) 3.8 10 I satoms.cm-2silver deposited onto silica, b) of the system Ag-Pd/SiO, with 3.8 10 Is atoms (Ag).cm-*+ 3.8 10 l4 atoms (Pd).cm-2and c) 3.8 10 l4 atoms. cm~20fpalladiumontosilica.We haveestabished adecreaseofthevalence band intensity of spectrum b) in comparison to a). We observe also that a same coverage produces a narrowing of the palladium
.
Pd/Si02
.."".., f
..: .'.
:i
a
'
I I J
I
I
I
I
I
'..\
Ag/Si02j
, I
I
/ I Y
I
!
I
t
Figure 2 :Valence band spectra for a) Ag 3.8 lOI5 atoms.cm-z b) Ag 3.8 10l5atoms.cm.2 + Pd 3.8 IOl4 atoms.cm-zc) Pd 3.8 lOI4 atoms.cm'2. The contribution of the silica substrate has been substracted
351
valence band and a diminution of its intensity when it is condensedonto silver clusters. These effects are clearly seen if we compare spectra c) (PdSiO,) and b) (Ag-PdSiO,). We have made the same observationsfor the Pd-Ag/SiO, system i.e., with palladiumdeposited before,with the same coverage as previously but in this case the substraction of the silica valence band becomes more complicated. These effects can be attributed on one hand to the formation of smaller aggregates when a metal is evaporated onto a second (dilution effect), and on the other, to an attenuation effect induced by the second evaporated metal ,but also by the first metal indicating the formation of the alloy. Reactivity Activity In order tocorrelate the possibleintrinsiceffects induce by the addition of ccAp in the bimetallic Pd-Ag aggregates to their chemical properties (activity and selectivity), we have chosen as catalytic test the 1,3 butadiene hydrogenation. The main features of the AglSiO,, Pd/SiO, and Pd-Ag/SiO, samplesused in this study are summarizedin table 2. All samples have a palladiumcoverage of about 4 10 l4 atoms.cm-z(column b). In this series of samples, we have evaporateddifferentamountsof silver TABLE 2
Ag /SiO, PdSiO, Pd-AglSiO, Pd-AglSiO, Pd-AglSiO, Pd-AglSiO, Pd-Ag/SiO,
0 3.8 4 3.8 3.9 4 4.2
100
0
0 2.3 8 8 8 12
100 95 83 83 83 78
100 0 5 17 17 17 22
0 7.2 21.5 100.8 116.4 107.5 10.1
-
16 18 27 30 25 20
(column c) giving a palladium concentration in the range from 0% to 100% (column d).In this approach we cannot determine the palladium surface concentration for the bimetallic clusters. Even by chemisorption, the latter cannot be measured owing to the small quantity of metal involved. For this reason, we express the activity by the ccpseudo turnover (V/Nt)*(columnf) obtained by the ratio of the experimentalreaction rate by the absolute values of the palladiumcoverages obtained independently by R.B.S (column b). From the results of column (f) we establish that the activity of the
352
W-Ag samples is greater than that for pure palladium Thus the reaction rate is multiplied by a factor of 15forsampleshavingasilverconcentration of 17%.Thesilverpmrnotereffectoccursinthedomain 0% to around 20% Ag. It must be noted that the maximum of activity corresponds to particles with sizes larger than 20 A for which we have observed a wetting effect induced by silver. This wemng effect can provoke the creation of a new phase with specific catalytic properties. It has been shown earlier (11) that the use of an electrondonor like piperidine on the system Pd- butyne produces a promoter effect but also changes the selectivity. The increase of activity should be linked to an electronic effect at the active site level leading to a decrease of the alkyne adsorption strength and therefore to an increase of activity. We suggest that the association of silver with palladium can induce, (at the active site level ) modificationsof the electronic density leading to, like for piperidme (1 I), a decrease in the diolefin adsorption strength. This effect often called uligand effect, has been evidenced by Hendrickx and Ponec ( 5 ) for the supported Pd-Ag catalyst. This increase agrees with the scheme of figure 5 where it can be Seen that an increase of activity correspondsto a decrease in the diolefin adsorption strength. Selectivity A careful analysis of the results presented in figures 3 and 4 leads to establish that in the case
of palladium aggregates, the successive hydrogenations of 1- butene and 2- butene continues (fig 3) whilein thecaseof Pd-Ag aggregatesthesereactionsarecompletelystopped(fig4).Thisfactconfirms
>
A 2-trans-Butene o 1-Butene A Butane
75 0 Butadiene
J
A 2-trans-Butene o 1- Butene A Butane 2-cis- Butene
tl -0
Figure 3 : Evolution of reactant and products during 1,3 butadiene hydrogenation on the Pd/ SiO, system
25
50
75 t,I
Figure 4 : Evolution of reactant and products during 1,3 butadiene hydrogenation on the PdAg / SiO, system
353
the improvement of selectivity by the additive effect. In contrast to the increase observed for 1,3 butadiene hydrogenation,here the increase of the electronic density at the active site level produces an opposite effect in the case of olefins. This behaviour is expected on the basis of the scheme given in figure5.
a olefins
00 Y
K , adsorption coefficien, Figure 5 : Volcano curve of the reactivities of various unsaturated hydrocarbons. (reproduced with permission From (12))
CONCLUSIONS The use of two evaporationcells has allowed by the successive evaporations method to prepare bimetallic Pd-Ag/ SiO, catalysts. The characterizationof such systemsby surface analysis techniques (U.P.S., A.E.S.) has been important to conclude on the alloy state. Theuseofthe 1,3 butadiene hydrogenationastestreactionhasled toevidence the promotereffect of silver in the bimetallic Pd-Ag aggregates.This effect which is important for a critical set of size and concentration, appears to be an important improvement for the olefin selectivity.
REFERENCES: 1234567-
V. Ponec and V.M.H. Sachtler, J. Cata1.,24 (1972) 250 Y. Soma-Noto and V.M.H. Sachtler, J. Cata1.,34 (1974) 162 Y.Soma-Noto and V.M.H Sachtler, J. Cata1.,32 (1974) 315 K. Christmann and G.Ertl, Surf. Sci., 33 (1972) 254 H.A.C.M. Hendrikx and V. Ponec, Surf. Sci., 192 (1987) 234 V. De Gouveia, Thesis Paris VI (1988) V. De Gouveia, B. Bellamy, Y. Hadj Romdhane, A. Masson and M.Che , ISSPIC 4, Aix en Provence, July 1988, in press 8- Y. Hadj Romdhane, B. Bellamy, V. De Gouveia , A. Masson and M. Che , Applied Surf. Sci., 31 (1988) 383 9- B. Bellamy and G. Colomer, J. Vac. Sci. Technol., A2 (4) (1984) 1604 10- E.Gillet, Thesis Marseille (1969) 11- S. Vasudevan, Thesis E.N.S.P.M., Paris (1982) 12- J.P. Boitiaux, J. Cosins and E. Robert, Applied Catal., 32 (1987) 145
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C.Morterra,A. Zecchinaand G. Costa (Editors),Structure and Reactivity of Surfaces 0 1989Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands
355
COMPARISON OF Cu-SUPPORT INTERACTION IN MODEL AND REAL CATALYSTS
V. DI CASTRO, C. FURLANI and G. POLZONETTI Department of Chemistry, University "La Sapienza", P.le A. Moro 5, 00185 Rome, [ Italy1 ABSTRACT We have compared the copper-support interaction in Cu/A1 0 Cu/Cr 0 and 2 3' 2 3 Cu/MnO model systems prepared under ultra-high-vacuum conditions, with the surface structure of the corresponding catalysts. UPS and Auger spectroscopy have been used to study the copper growth in the model systems, XPS has been used to analyze the real catalysts. A good correlation has been found between model and real systems for the copper-support interface, in spite of the different preparation procedure. INTRODUCTION It is well known the influence of metal-support interaction on the catalytic activity of many supported systems (refs. 1-3). The interface between dispersed metal and metal oxide substrate is often studied in model systems prepared in situ by evaporation of the metal overlayer on an oxidised metal substrate (refs. 4-6). This method has the advantage to avoid appreciable levels of contaminati-
on, the possibility to control metal film deposition from submonolayer to rnultilayers, the opportunity to study the interface behaviour as a function of thickness, temperature and controlled chemical modifications. With the aim of a better understanding of copper interaction with the support we have undertaken an Auger and photoemission study of some model systems (namely Cu/A1203, Cu/MnO, Cu/Cr 0 ) prepared by progressive evaporation of the metal 2 3 on the carrier (refs. 7-11). In this paper we will compare the copper interaction with the support observed in the model systems with the structure of the corresponding real catalysts prepared by reduction of supported CuO to metallic copper in H
2'
Our end is to evaluate in which measure copper-support interface obtained by Cu evaporation in model systems can be considered rappresentative of the surface structure of the corresponding actual catalyst.
356 EXPERIMENTAL The model systems were prepared in situ by evaporation of pure copper from a supporting tungsten filament onto an oxide substrate, obtained by exposing to 0 a 500 8, tick metal film evaporated on a stainless steel base. The compositi2 on and cleanliness of the sample was tested by UPS and Auger spectroscopy.
The actual catalysts were prepared by adsorbing on the support the tetramino complex of Cu (11) according to the procedure described by Koritala for CuG/Al 0
2 3
(ref. 12). The catalysts were calcinated in air for 5 h at 350 OC.
The reductive pretreatment was performed by heating the sample at 110 OC for 15 min under vacuum, then in H 1 atm at 270 O C for 30 min. 2 The UPS measurements were taken using the vacuum ultraviolet beam line equipped with a Jobin-Yvon monochromator at the synchrotron radiation facility "PULS" in Frascati (Italy). A l l photoemission measurements are angle integrated and
were taken with a double-pass cylindrical mirror analyser (CMA). Auger measurements were performed in the above descripted equipment using the same CMA with a coaxial electron gun. XPS determination were recorded using a VG ESCA3 spectrometer employing an
Ai K
source (h
=
1486.6 eV). The catalyst samples were dusted as thin films
onto a gold probe. Cls (b.e. 285.0 eV) from pump-oil contamination was used as an internal standard for the correction of charging effects. The intestigated samples proved to be sufficiently stable to X-ray exposure under the experimenta?. conditions. The quantitative composition was determined by considering a solid solution and using the cross-sections calculated by Scofield (ref. 13). RESULTS AND DISCUSSION The adsorption of evaporated copper on the oxidised support has been studied by Auger and photoemission spectroscopies because these two techniques give complementary informations on the structure of the metal-support interface. The plot of the Auger signal intensity as a function of adsorbate coverage allows to distinguish between different growth modes (layer-by-layer. by clustering etc.) (refs. 14-16). On the other hand, the position and shape of the Cu 3d band in the UV-photoemission spectra are indicative of the degree of interaction between metal and substrate more than Auger and XPS peaks. In the analysis of the real catalysts the UPS spectra are covered by the signal due to surface contami-
357
nation intrinsic to the preparation procedure, so that the XPS core peaks become more useful1 to study the copper-support interaction. We will review the results obtained for the Cu/A1 0 Cu/MnO, Cu/Cr 0 sys2 3' 2 3 tems and compare them with the information on the catalyst surface structure
obtained by XPS measurements. Cu/Al 0 -2-3As copper is evaporated on an oxidised aluminium film, a Cu 3d band appears and grows up in the valence photoemission spectra in a position close to that of bulk metal (ref. 7). This behaviour indicates a weak interaction between the evaporated metal and the substrate at room temperature. Plot of Cu LMM Auger intensity as a function of copper coverage (Fig. la) shows only one break for a coverage of 2.0
A; after that a continuous linear in-
crease of the Cu intensity takes place in all the analysed range of coverage. This behaviour indicates a better dispersion of copper at low coverages followed by formation and growth of copper clusters. A comparison of the experimental Auger intensity ratio with the calculated ones for both layer and cluster formation, previously discussed (ref. 8 ) has shown that the experimental values are sistematically lower than the values expected for a layer-by-layer growth. Thus, probably the evaporated copper forms small dispersed clusters also in the first 2 I( of coverage, corresponding to the first linear section of Fig. la, which
afterwards increase in size. The weak interaction with the support and the attitude to form cluster observed in the Cu/A1 0 model system can be correlated with the behaviour observed 2 3 during the reduction of the corresponding impregnated catalyst. The unreduced catalyst is composed of a CuO phase supported on the A1 0 substrate without 23 strong interaction between each other as shown from the XPS measurements which give a Cu 2p peak, typical of the original CuO species in terms of both the 3/2 binding energy value and of the.intense satellite structure (ref. 17). The Cu/A1 atomic ratio measured at the surface by XPS (Tab. 1) is 3 times higher than the bulk value, due to a surface segregation of the CuO phase. During the reductive pretreatment a drastic decrease in the measured atomic ratio (see Table) is observed together with the complete reduction of Cu (11) to Cu (0). The decrease of the intensity of the Cu 2p peak could be due or to diffusion of copper into the substrate of to clustering of copper. In the analised sample the
358 formation of cluster is strongly suggested from the comparison with the sample heated in vacuum were Cu (11) is reduced to Cu (I) and just a minor decrease of the atomic ratio is observed. Moreover XRD measurements made on a similar system showed lines features characteristic o f Cu(0) crystallites (ref. 18). Thus, in the reduced catalyst is observed a weak interaction between Cu (0)and A 1 0 2 3 and formation of metallic copper clusters such as in the model system.
Cu/Cr 0
-2-3-
Cu was progressively evaporated on a Cr 0 substrate prepared by exposure to 2 3
0 of a tick layer of evaporated chromium. The plot of the intensity of Cu LMM 2 and Cr LMM Auger transition versus coverage (Fig. lb) is characterised by linear
section with twobreaks at 3.3
d
and 9.6 8, of Cu coverage, this trend indicates
a layer-by-layer growth mode, in which a change of slope occurs every time a copper layer is completed. In the photoemission spectra the progressive growth of a Cu 3d peak is observed. This peak has a binding energy close to that of bulk copper in all the analysed range of coverage with only a maximum shift of 0.3 eV towards higher b.e. for the coverage of 3
A . These results are indicati-
ve of the tendency of Cu to disperse on the Cr 0 surface and grow layer-by-la7 3 yer, without intermixing or strong interaction with the support, as already discussed in more details (ref. 11). The corresponding real catalyst is prepared by impregnation of C r 0 powderby Cu (111, calcination in air at 350 OC and re2 3
duction in H . The unreduced catalyst is composed of CuO supported on Cr 0 2 2 3 with weak interaction between each other. In fact, the XPS spectra show
Cu 2p
(b.e. 934.3) and Cr 2p
(b.e. 577.0) peaks characteristic for the 3/2 pure oxides and no signals due to mixed compounds. The Cu/Cr atomic ratio mea3/2
sured by XPS (Table l) are 5 times higher than the expected ones on the basis of the bulk composition in agreement with the surface enrichment in CuO. During the reduction in H2, a decrease of the Cu/Cr atomic ratio is observed, but it is substantially minor than the one observed in Cu/A1 0
2 3'
This dif-
ference in copper agglomeration between the two catalysts is in agreement with xhe different behaviour of copper growth in the two model systems: clustering layering for copper on Cr 0 On the other hand, 2 3' we have to deal with the partial clustering of copper during the reduction that
for copper evaporated on A 1 0
2 3'
is in contrast with the model behaviour, it can be due to various factors, such as the different surface structure o f the support, the higher loading of copper,
359
TABLE 1 Atomic ratio between Cu and oxidised metal in the support
untreated
27OoC/UHV/30min
H2/27OoC/30 min
Cu/A1203
0.27
0.21
0.08
Cu/Cr203
0.61
0.50
0.36
cu/Mno
0.59
0.15
0.15
a
b
a
8
S
IS
S
1s
F i g . 1: Auger intensity plot of a) Cu/Al 0 b) Cu/CrpOg and c) Cu/MnO 2 3’
or the higher temperature used in the preparation. Cu/MnO Copper evaporated on MnO interacts strongly with the substrate. The photoemission spectra show two Cu 3d signals (ref. 91, one shifted by 1.4 eVtowar.ds higher b.e. due to copper intermixed with the MnO substrate, a second one in a position close to the bulk value which has been attributed to an upper copper layer. The Auger intensity plot (Fig. lc), shows a layer-by-layer growth, but the amount of Cu needed to reach the first break is
3 times the
thickness of a pure copper monolayer. As already discussed in more details (ref. lo), these results suggest that one part of the evaporated copper diffuse into the MnO substrate, while another part forms a copper layer on top of the intermixed region. A similar copper-support interaction is observed in the actual catalyst.
The Cu/MnO catalyst is prepared by reduction of CuO/Mn02 precursor. During the reduction in H the MnO support is reduced to MnO as shown from the XRD lines, 2 2 but the surface analysis by XPS shows a structure similar to that of Mn 0 3 4 (ref. 19). In the mean time copper remains partially unreduced as Cu (I) (ref. 191, when exposed to H after heating in vacuum, while in the same conditions 2 CuO supported either on A1 0 and on Cr 0 is completely reduced to Cu (0). 2 3 2 3 These results, together with the decrease of the atomic ratio during the hea-
ting in vacuum comparable to the one observed in H (Table l), suggest that one 2 part of copper migrates in the support. On the other hand, the absence of signals due to copper crystallites in the XRD spectra indicates that Cu (0)present on the catalyst surface is well dispersed. Thus, the structure of the catalyst surface is consistent with the picture obtained in the model system: a copper layer on top of an intermixed region. CONCLUSIONS The copper-oxide interaction observed in the Cu/A1 0 Cu/Cr203 and Cu/MnO 2 3' model systems prepared by evaporation of copper on supports prepared in situ has been compared with the surface structure of the corresponding catalysts prepared by impregnation with Cu (11) and reduction in H
2'
In spite of the diffe-
rent support structure and preparation procedure, a good correlation has been found between the structure of copper-support interface in models and real ca-
361 talysts.
I n Cu/A1 0 t h e s u p p o r t e d copper i n t e r a c t s weakly w i t h t h e s u p p o r t and forms 2 3 b i g c l u s t e r s , w h i l e a b e t t e r d i s p e r s i o n o f t h e m e t a l is observed i n Cu/Cr 0 2 3' Copper adsorbed on MnO, i n t e r a c t s s t r o n g l y and d i f f u s e i n t h e s u p p o r t forming a mixed l a y e r between t h e MnO s u b s t r a t e and t h e pure copper a d l a y e r . The good agreement found for t h e copper behaviour i n t h e a n a l y s e d model and
r e a l s y s t e m s , seems t o s u p p o r t t h e v a l i d i t y of u s i n g model systems t o s t u d y t h e c a t a l y s t s t r u c t u r e and r e a c t i v i t y mechanism, a l t o u g h some c a r e i s needed t o extend these r e s u l t s t o other
systems.
ACKNOWLDGEMENTS The a u t h o r s are g r a t e f u l t o p r o f . M. Rossi and p r o f . G . Gargano f o r t h e prep a r a t i o n o f c a t a l y s t s . We wish t o thank t h e F r a s c a t i P e r f e t t i , D r . C . Quaresima, M r . M. Capozi)
PULS
Labs team ( p r o f . p.
for h e l p f u l a s s i s t a n c e .
REFERENCES
1 A.W. S l e i g h t , S c i e n c e , 208 (1980) 895-900. 2 G . C . Bond, i n B. I m e l i k e t a l . ( E d s . ) , Metal-Support and Metal-Additive E f f e c t s i n C a t a l y s i s , E l s e v i e r , Amsterdam, 1982 pp. 1-10. 3 M. Boudart, J. Mol. Cat., Rev. Issue (1986) 29-41. 4 H.R. Sadeghi and V.E. Henrich, Appl. S u r f . S c i . , 19 (1984) 330-340. 5 S.D. Bischke, D.W. Goodman and J . L . F a l c o n e r , S u r f . S c i . 150 (1985) 351-357. 6 S. Takani and Y.W. Chung, Appl. S u r f . S c i . , 19 (1984) 341-347. 7 V . D i C a s t r o , G. P o l z o n e t t i and R. Zanoni, Surf. S c i . 162 (1985) 348-353. 8 V. D i C a s t r o and G. P o l z o n e t t i , S u r f . S c i . , 189/190 (1987) 1085-1090. 9 V. D i C a s t r o and G. P o l z o n e t t i , Chem. Phys. L e t t . , 139 (1987) 215-218. 1 0 V . D i Castro and G. P o l z o n e t t i , Appl. S u r f . S c i . 29 (1987) 242-248. 11 V. D i C a s t r o , C. F u r l a n i , G. P o l z o n e t t i and C . Cozza, 3. E l e c t r . S p e c t r . and Rel. Phen. i n p r e s s . 12 S. Koritala, J. Am. O i l Chem. SOC., 49 (1972) 83-88. 13 H. S c o f i e l d , J. Electr. S p e c t r . and Rel. Phen. 8 (1976) 129-137. 14 J. Benard "Adsorption on Metal S u r f a c e s " , Vol. 13 o f s t u d i e s in S u r f a c e S c i e n c e s and C a t a l y s i s ( E l s e v i e r , Amsterdam, 1983). 15 B.G. Baker and C . Klauber, Appl. Surf. S c i . , 13 (1982) 190-197. 1 6 E . Bauer, Appl. S u r f . S c i . , 11/12 (1982) 479-494. 17 V. D i C a s t r o , C. F u r l a n i , C. F r a g a l e , M. Gargano, M. Rossi and N . Ravasio, Gazz. Chim. I t a l . 11.7 (1987), 43-49. 18 B.R. S t r o h m e i e r , D.E. Leyden, R.S. F i e l d and D.M. H e r c u l e s , J . Catal. 94 (1985) 514-530. 19 V. D i C a s t r o , C . F u r l a n i , M . Gargano and M. R o s s i , Appl. S u r f . S c i , 28 (1987) 270-278.
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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
363
STUDIES OF SKELETAL REARRANGEMENTS OF LABELED HEXANES ON PLATINUM-RUTHENIUM CATALYSTS. CORRELATION BETWEEN THE PRODUCT
DISTRIBUTIONS AND THE STRUCTURE OF THE CATALYSTS G I V E N BY E.X.A.F.S.
G . D I A Z l , P.ESTEBAN2, L . G U C Z 1 3 , F . GARIN*4r P . BERNHARDTI, J . - L . I I n s t i t u t o de F i s i c a
U.N.A.M.
SCHMITT4 and G . MAIRE4.
- Apdo
P o s t a l 20-364 Mexico 01000
2 U n i v e r s i d a d Autonoma d e B a r c e l o n a , F a c u l t a t de C i e n c i a s , Unidad d e Quimica I n o r g a n i c a D e l l a t e r r a - Espana. 3 1 n s t i t u t e o f I s o t o p e s of t h e Hungarian Academy of S c i e n c e s . 1525 Budapest, P . O . Box 7 7 , Hungary. 4 L a b o r a t o i r e d e C a t a l y s e e t Chimie d e s S u r f a c e s U . A . 423 du CNRS, U n i v e r s i t e L o u i s P a s t e u r , 4 r u e B l a i s e P a s c a l 67070 S t r a s b o u r g C6dex - F r a n c e .
*
t o whom a l l c o r r e s p o n d e n c e s h o u l d b e a d d r e s s e d
ABSTRACT R e a c t i o n s of two 13C-labeled m e t h y l p e n t a n e s , namely, 2- [2-I3C] m e t h y l p e n t a n e a n d 2- [4-13C] m e t h y l p e n t a n e w e r e p e r f o r m e d a t 22OoC on t w o Pt-Ru b i m e t a l l i c c a t a l y s t s . Under t h e s e e x p e r i m e n t a l cond i t i o n s p l a t i n u m and r u t h e n i u m a r e c a t a l y t i c a l l y a c t i v e . The r e s u l t s h a v e shown t h e f o l l o w i n g p o i n t s : i ) t h e p e r c e n t a g e o f c y c l i c mechanism o b s e r v e d a t 22U°C, i n t h e i s o m e r i z a t i o n o f 2m e t h y l p e n t a n e t o 3-methylpentane i s v e r y h i g h ( 6 5 f 2)%, ii) A h i g h s e l e c t i v i t y i n t h e c r a c k e d p r o d u c t s i s f o u n d where b r a n c h e d h y d r o c a r b o n s a r e f a v o r e d , iii) a p a r a l l e l i s m c a n be e s t a b l i s h e d between t h e l a r g e amount o f s e l e c t i v e l y c r a c k e d p r o d u c t s and t h e s e l e c t i v e c y c l i c mechanism. 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 b e i n t e r p r e t e d by t h e p r e s e n c e o f s i t e s formed o f P t and Ru where a c o o p e r a t i v e e f f e c t e x i s t s between P t and Ru which i s due t o metalm e t a l i n t e r a c t i o n s and n o t t o a merely g e o m e t r i c e f f e c t . These s i t e s were named " a c t i v a t e d Pt-Ru c l u s t e r s " [ 6 ]
.
INTRODUCTION
P l a t i n u m and ruthenium have r e c e i v e d c o n s i d e r a b l e a t t e n t i o n among t h e Group V I I I metals b e c a u s e t h e y a r e v e r y a c t i v e ,
f o r Rut
and v e r y s e l e c t i v e f o r P t . Taking o n l y t h e more r e c e n t p u b l i c a t i o n s a b o u t t h e s e m e t a l s w e may mentioned t h a t C o n g r e s s on C a t a l y s i s h e l d a t C a l g a r y , ruthenium c a t a l y s t s
i n t h e gth I n t e r n a t i o n a l
two p a p e r s w e r e d e v o t e d t o
[ l ] and t o s u p p o r t e d Pt-Ru b i m e t a l l i c c l u s t e r s
[ 2 ] . The f i r s t one d e a l s w i t h t h e manner o f p r e t r e a t i n g Ru supp o r t e d on T i O 2 , ratio,
and
on
SiO2 o r A1203 which h a s major e f f e c t s on t h e H/Ru the
rate,
turnover
frequency
(TOF)
and product
364
distribution for n-butane hydrogenolysis [ll.The second paper focused on the effect of pretreatment on the surface composition and structure of supported bimetallic Pt-Ru/SiOz catalysts I21 These two studies are very interesting and give a lot of informations about modification of the metal particle size , the particle morphology and the influence of contaminants, especially . Cl'ions Isomerization and hydrogenolysis of hexanes on a Pt (96 at % ) Ru (4 at %)/A1203 (31 and CO hydrogenation on a series of Pt-Ru/Al203 catalysts [4]have been studied.In this paper we pay more attention to the catalytic properties of two alumina supported Pt-Ru catalysts previously characterized by EXAFS [SI and by TEM [61. The total metal loading is maintained to 10 w t % which might decrease the influence of the support on the catalytic activity.The atomic percentages of Pt in the two samples are 44% and 68%. The aim of this paper is to correlate the physical data to the reactions of labeled hexanes and to propose a mechanism which could probably occur on the active centers already proposed [S].
.
EXPERIMENTAL Woelm alumina is used as support. Its characteristics are already mentioned [7].The sources of metals are hydrogen hexachloroplat inate (IV) (Caplain-Saint Andre) and Ruthenium I11 chloride (Johnson-Matthey).
-
The impregnating solutions contained the two metals in a ratio appropriate t o the desired Pt-Ru "cluster" composition. Support is brought into contact with a large excess of solution following the Ipatieff impregnation method [8]. Then the solution is stirred and heated progressively up to boiling point and slowly evaporated. The material is dried overnight at llO°C and is reduced. The reduction procedure is as follows : (i) Purged in oxygen-free nitrogen (2S°C, half an hour) (ii) Reduced in hydrogen (4OO0C, 24 hours) (iii) Cooled down in oxygen-free nitrogen. The catalysts are then stored in small bottles firmly closed. Two "bimetallic" catalysts of 10% (W/W) metal loading were prepared with 67% and 4 4 % atomic percentages of platinum respectively which corresponds in weight to 8% Pt and 6% Pt respectively.
365
tic expa) The 13C-labeled hydrocarbons (2-rnethylpentane-2-l3C and 2methylpentane-4-13C) are prepared by synthetic methods already described [9,10]. The isotopic purity is around 90%. b) The isomerization reactions are carried out in an allglass, grease-free flow system already described [lo]. The catalyst bed (10 to 200mg) is isothermal and isobaric. A small amount of labeled hydrocarbon ( 5 mg) is used for each run and injected into the reactor at a constant pressure (6 and 2 Torr for 2methylpentane and methylcyclopentane respectively). The pressurevs-time curve, recorded by means of a katharometer inserted in the flow line, closely approximated a square-wave pulse. Samples for G.L.C. analysis are removed directly from the gas phase by a gas syringe as the pulse passed a rubber septum located close to the reactor. c) The chromatographic analysis and the location of the carbon-13 by mass spectrometrical analysis are already described [9, 101 *
vst characterlzatlon Transmission electron microscopy,electron microdiffraction analysis and X-Ray absorption spectroscopy were performed on these catalysts and were already mentioned [ 5 , 6 ] .The main results are as follows : -When Pt and Ru are together, the amount of small particles, lower than 2 nm is increased compared to the pure metals. On the other hand the mean metallic particle sizes are 2.7 nm and 2.3 nm for the catalysts with 68 at.% of Pt and 44 at.% of Pt respectively; for pure platinum ds nm= 6.7 and for pure ruthenium ds nm= 3. -Platinum and ruthenium form alloys in the range of concentrations used in this study. The EXAFS measurements point to the interpenetration of each metal in monometallic phases of the other leading to Pt rich and Ru rich bimetallic phases. -The electron microdiffraction analysis confirms the presence of an al1oy.A constant alloy composition of Pt 44 at.% Ru 56 at.% is found for the small particles. c r e s w The Pt-Ru catalysts composed of two metals are completely different in reactivity. Ruthenium catalysts are active for carbon-
TABLE 1 2-Methylpentane and methylcyclopentane reactions on 10Wt% Pt-Ru/A1203 catalysts at 220°C
*r
=
selectivity ratio
Upper part of the table: Product distribution (in moles) from the isomerization of 2-Methylpentane Lower part of the table: Product distribution (in moles) from methylcyclopentane hydrogenolysis
(493K)
367 carbon
bonds
rupture
already
14OoC
at
h y d r o g e n o l y s e d o n l y a t 22OoC on p l a t i n u m . on p l a t i n u m
C-C
while
bonds
are
S k e l e t a l rearrangements
occur even a t h i g h e r t e m p e r a t u r e s (260-3OO0C). s t u d i e d a t a temperature
Hence Pt-Ru c a t a l y s t s were a t which b o t h P t
(and Ru a r e a c t i v e .
(220OC)
Preliminary study a t
low
t e m p e r a t u r e (14OOC) where o n l y ruthenium is a c t i v e h a s been a l r e a d y performed [6]
1cvcloDentane (MCP) r e a c t i a I n t a b l e 1 (lower p a r t ) ,
t h e molar d i s t r i b u t i o n s o b t a i n e d f o r
MCP h y d r o g e n o l y s i s a r e p r e s e n t e d .
22OoC t h a n i n t h e range 1 4 O o C
-
The
cfj
s e l e c t i v i t y i s lower a t
16OoC [ 6 1 b u t w e s t i l l have about
2 0 % of C g p r o d u c t s desorbed from t h e " b i m e t a l l i c
c a t a l y s t s " which
i s n o t t h e c a s e on p u r e Ru c a t a l y s t (S = 0 . 5 ) .Under t h e s e cond i t i o n s t h e c r a c k i n g p r o c e s s e s a r e d i f f e r e n t . On Ru c a t a l y s t , f o r m a t i o n of t h e 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 can be observed .On Pt-Ru
c a t a l y s t s t h e simple carbon-carbon bond r u p t u r e s l e a d i n g t o
t h e i n t e r n a l f i s s i o n r e a c t i o n s becomes i m p o r t a n t . W e always have a predominance of i s o p e n t a n e over n-pentane.The
selectivity smaller
ratio,r
values
at
=
understood because under active,
have
3-methylpentane/n-hexane(r=3MP/nH),
22OoC t h a n
at
14OoC
these conditions
(ii) hydrocracking r e a c t i o n s
[6].
It
can
be
easily
( i ) b o t h P t and Ru a r e
a r e more
important,
(iii)
r e p e t i t i v e p r o c e s s e s occur and ( i v ) t h e s t a t i s t i c a l carbon-carbon bond r u p t u r e of t h e r i n g i s a more a c t i v a t e d r e a c t i o n t h a n t h e sel e c t i v e carbon-carbon
r u p t u r e excluding bonds where t e r t i a r y carbon
atoms a r e p r e s e n t [ I l l . The s e l e c t i v i t y r a t i o , r , temperature;
on
the
other
hand
the
ratio
i s lower a t h i g h e r 2-methylpentane/3-
methylpentane (2MP/3MP) i s not a f f e c t e d . entane
r e a c t i o n s a r e summarized i n Table
1 upper
part. W i t h t h i s a c y c l i c molecule s e v e r a l n o t i c e a b l e p o i n t s
underlined
and
compared
to
those
obtained
for
the
can be methyl-
cyclopentane r e a c t i o n s . *
,
a ) Activity
r
=
The t o t a l a c t i v i t y determined u s i n g t h e formula (l-=)F/W Ln 1/1-= gives t h e reaction r a t e .
hydrocarbon f l o w
, a
F
is
the
i s t h e t o t a l c o n v e r t i o n and 0 i s t h e weight
of t h e c a t a l y s t . I n Table 2 we have r e p o r t e d t h e r e a c t i o n r a t e s i n
368
pl ( m i n . q ) - l f o r t h e c a t a l y t i c e x p e r i m e n t s p e r f o r m e d a t 22OoC.
TABLE 2
I
A c t i v i t i e s ( b l .min .-'g-l) w i t h MCP I w i t h 2MP
Catalysts Pt
-
RU 1
0
44 68 Wt%lO
-
W t % 10
-
56 32 0
-
11.5 54 30 0.35
135 171 103 0.02
The f o l l o w i n g o b s e r v a t i o n s c a n be g i v e n :
-
S i m i l a r a c t i v i t i e s a r e o b t a i n e d by
Bond
et a l .
[l] on p u r e
alumina s u p p o r t e d ruthenium c a t a l y s t s for n-butane h y d r o g e n o l y s i s . - On p u r e p l a t i n u m
t h e c a t a l y t i c a c t i v i t i e s are v e r y l o w a t t h i s
temperature.
-
On p u r e p l a t i n u m , m e t h y l c y c l o p e n t a n e reacts a b o u t 1 5 times f a s t e r
t h a n 2-methylpentane.
-
reacts 12 t i m e s
On p u r e r u t h e n i u m 2 - m e t h y l p e n t a n e
f a s t e r than
methylcyclopentane.
-
Methylcyclopentane
times,respectively
-
On Pt-Ru
and
2-methylpentane
react
30
and
6000
f a s t e r on p u r e Ru t h a n on p u r e P t .
c a t a l y s t s t h e a c t i v i t i e s a r e h i g h e r t h a n on t h e p u r e
metals m a i n l y i n t h e case of m e t h y l c y c l o p e n t a n e . The a c y c l i c molec u l e reacts o n l y 3 t i m e s faster t h a n t h e c y c l i c m o l e c u l e . *
.
b)Selectivity Coming b a c k t o a c y c l i c molecule
T a b l e 1, t h e
c(j
(upper part-Table
s e l e c t i v i t y i s v e r y s m a l l . The
1) g i v e s a r o u n d 1 0 t i m e s l e s s
isomers t h a n t h e c y c l i c molecule (lower part-Table
1). N o e x t e n s i v e
c r a c k i n g o c c u r s e x c e p t s on p u r e r u t h e n i u m . The r a t i o i s o p e n t a n e / n pentane
(iP/nP)
is favored.
It
is always v e r y h i g h and the d e e t h y l a t i o n r e a c t i o n c l e a r l y shows t h a t t e r t i a r y c a r b o n atoms a r e n o t
touched.
In
Table
3
the
distributions
of
the
isotopic
varieties
o b t a i n e d f r o m t h e i s o m e r i z a t i o n of 2 - m e t h y l p e n t a 1 1 e s - 2 C ~ ~ a n d 4CI3
a r e p r e s e n t e d . T h e s e d i s t r i b u t i o n s a r e s i m i l a r on t h e t w o Pt-Ru c a t a l y s t s a n d do n o t p o i n t o u t a n y p r e s e n c e of p r o d u c t s a b n o r m a l y
labeled. W e c a n o b t a i n n e g a t i v e v a l u e s d u e t o t h e e r r o r s i n t h e
369
TABLE 3 D i s t r i b u t i o n of t h e i s o t o p i c v a r i e t i e s :
2-methylpent ane-2
2 - m e t hy l p e n t a n e -4 C1>
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<) was carried out with the Gate's method (ref. 8 ) . The oxidised samples [ O s ( C O ) , ( O M ) z J u ( x = 2, 3 ; M = Si', A1 ) were obtained from H O s s ( C O ) I O(OM) by heating in vacuo (10-3 Pa) at 473 K for 7 2 h. The TPDE experiments were performed on 30-100 mg samples in a g l a s s reactor under a total flow ( oxygen/argon 5/95 by volume) of 10 cm3 min-I at the heating rate of 2 K min-l from 293 to 673 K. The analysis of the gas stream was reported elsewhere (ref.9). The FTIR and XPS measurements were carried as previously described (raf.7). All reported 0 s 4f7 2 b.e. values for the supported species are referred to the Si 2p ionization peak (103.5 eV) and to the A 1 2p ionization peak (74.1 eV),respectively. The data are obtained with an uncertainty within t 0.2 eV. RESULTS AND DISCUSSION Surface reactivity of 0 5 3 ( C O ) I Z on silica under oxyqen atmosphere On the basis of FTIR, XPS and TPDE studies recently (ref.10) we proposed the formation of the surface species [ O s ( C O ) , (OSir)2ll
( x = 2,3) by controlled thermal degradation of
This
H O s 3 ( C O ) I O( O S i < ) :
process corresponds to a multiple oxidative addition to
the
3-77
bonds by the surface hydroxyl groups. The oxidised nature of the surfaces species was supported mainly by XPS results (ref.10) as the observed value of the 0 s 4f7/2 b.e. (53.0 eV) lies in the range typical for pure and silica supported Os(I1) carbonyl compounds (Table). In addition, it is known that a similar process occurs by treatment of 0 5 3 (CO)IZ with trifluoroacetic acid to afford [ O s (CO)3 (CF3COO)z1 2 (ref.ll ) . .Is1 0s-0s
TABLE Osmium 4f7!2 binding energies carbonyl compounds. SAMPLE
for
pure
and
0 s 4f7 1 2 b.e. (eV)
supported
Osmium
Ref.
50.7 51.2 51.0
12 13 this work
51.6 51.2 51.0
12 13 this work
52.3 52.3 52.1
10 13 this work
53.4 53.1 53.1 53.4 53.0 53.2
12 13 12 13 13 this work
The oxidised nature of these surface specie could be demonstrated by st.udying the reactivity of Os3 (C0)12 on silica under oxygen atmosphere. The physisorbed Os3 (C0)lz is stable under oxygen at 298 K , as confirmed by its infrared spectrum (Fig. 1-a). By heating at 423 K a pure three-band spectrum (2124, 2043 and 1970 cm-l), characteristic for the proposed [Os (CO)x(0SiC)zIn surface species is obtained (Fig.1-c). Usually this spectrum was obtained by thermal treatment in vacuo at 423 - 473 K either of physi.sorbed 0 5 3 (C0)lz or chemisorbed HOs3 (C0)10(OSiG) (ref.2-a). In addition, the spectrum of Fig. 1-b ,obtained after a prolonged treatment of physisorbed Os3 (C0)iz in 0 2 at 373 K indicates the presence of
378
2300
2160
2000
1850
1700
r i r i m i m h r r Itwll
Fig. 1. Infrared spectra of : a) Os3 ( C O ) I Z physisorbed on SiO2. b ) After 7 days in 0 2 at 373 K. c) After further 2 hours in 0 2 at 423 K.
a mixture of both the oxidised and the grafted cluster surface species. These observations confirm that also in an oxygen atmosphere the first surface reaction is the oxidative addition of a SiOH group to an 0 s - 0 s metal bond to give HOs3 ( C O ) I O(OSi<) , followed by oxidation and degradation of the cluster framework. The TPDE experiments are in agreement with such a suggestion. In fig. 2 is reported the TPDE of silica supported 0 5 3 ( C O ) ~ Z carried out under oxygen/argon atmosphere. The evolution of carbon monoxide occurs as a very pronounced peak at 419 K with a shoulder at 433 K. In the TPDE of the H O s p ( C 0 ) i o (OSi<) , performed under the same conditions, the CO evolution presents a rather symmetric peak centered at 433 K. The absence of the peak at 419 K is the main difference from the TPDE of fig.2. These observations parallel the
379
Fig. 2. TPDE in oxygen/argon mixture of SiOz .
Os3
(C0)iz physisorbed
on
above discussed FTIR data (fig.1). The peak at 419 K could be ascribed to the initial formation of H O s 3 (C0)io (OSiC), which immediately reacts with oxygen to give oxidised surface species,as evidentiated by the shoulder at 433 K. The COZ evolution (fig.2) starts already at 413 K with a maximum centered at 450 K, followed by a second relevant peak at ca 573 K with a simultaneous evolution of CO. The formation of COZ is mainly due to a catalytic oxidation of CO and is strictly related to the formation of oxidised osmium surface species. The TPDE of [Os(CO), (0SiC)zlu under oxygen/argon atmosphere shows a complete decarbonylation. The total evolution of CO and COz corresponds to a CO/Os ratio of 2.7. This experimental value strongly supports the stoichiometry proposed for the oxidised species, according to the equilibrium:
When
HOs3 ( C O ) I O(OSi')
is themally treated at 373 K in the
XPS
preparation chamber under oxygen atmosphere the initial b.e. value of 52.3 eV increases up to 53.0 eV. ?he same value was obtained for the proposed oxidised species [ O s ( C O ) , (OSiOi 1" formed by simple thermal degradation (Tabie). These observations give a further support to the oxidised nature of these species as Os(I1) surface organometallic carbonyls. Surface
reactivity
of
0% ( C O ) 1
2
on
alumina
under
oxygen
aL~!o~Ehfye_ The surface reactivity of Os3 (C0)iz on alu.r\ina has been reported to be quite similar to that observed on silica (ref.2). The TPDE curves of physisorbed OSJ ( C O ) 1 2 arid of H O s j ( C O ) I o ( O A l ' ) carried out under oxygenlargon atmosphere , are reported in fig. 3-a and in fig. 3-b,respectively . A simple comparison o f the CO evolutions points out the presence of the peak at 403 K only for the physisorbed cluster (fig.3-a). The difference between the total integrated CO evolutions in the two TPDE curves gives a value of 2.0 mol of CO evolved per mol of cluster, a s expected from the initial surface reaction:
The peak at 403 K can thus be attributed to the formation of the grafted cluster H O s 3 (C0)lo (OAl;), as in the case of silica. The peak at about 413 K, present in both spectra, is related to the formation of oxidised surface species such as [Os ( C O ) X (0Al:)z 1.. We have previously reported the gas evolution of physisorbed 0 5 3 (C0)lz during thermal treatment under argon. Between 373 K and 523 K about 3 mol of H2 per mol of cluster are evolved (ref. 2-a). This gas evolution is consistent with the mass balance corresponding to the equation : n HOss (C0)lo (OAl<) + 5n HOA1;
-
3 [ o s ( C O ) , (OAl;)zln
+ y CO
+ 3n HZ
The TPDE of the oxidised surface species carried out under oxygen/argon atmosphere only shows the evolution of COZ as a single broac; peak. The absence of u ( C 0 ) absorptions in the infrared spectrum of the sample recovered after the run confirms that a complete decarbonylation occurred. The total evolution of
381
A
TERPERATURE ( k o l u l n )
Fig. 3. TPDE in oxygedargon mixture of : a ) on A l p 0 3 : b) H O s s (C0)io ( O A l : ) .
053
(C0)iz physisorbed
382 COZ
corresponds to a
CO/Os
molar ratio of 2.6. These data parallel
the EXAFS measurements, in which the average 0s-CO coordination number for the osmium ions was found to be 2.8 (ref.3). XPS measurements on oxidised [ Os (CO), ( O A l O 2 by
In
species, prepared
thermal degradation, exhibit for the 0 s 4f7
2 b.e. a value of As this value lies in the range 53.1 - 53.4 eV observed for the pure mononuclear and dinuclear Os(I1) carbonyl complexes (Table),we could tentatively assign the 2+ oxidation state to the
53.2 eV.
osmium
of
the
tri
and
dicarbonyl
surface
species
.
When
physisorbed 0s3 (C0)lz was treated in oxygen at 373 K for 1 hour in
51.0 eV increased up to 52.4 eV. In agreement with the TPDE data this corresponds to the intermediate formation of the grafted HOs3 (C0)lo ( O A l ’ ) surface species. Further treatment in 0 2 for 5 hours at 373 K raised the 0 s 4f7 2 b.e. up to a value of 53.2 eV. The final infrared spectrum of this latter sample showed in the .(CO) region three absorptions at wavenumbers very similar to those presented by the [ O S ( C O ) Y ( O A l < ) z J n surface species prepared by thernial treatment (ref. 2 ) . These data confirm that the same oxidised surface species are obtained thsrmally and more easily in the presence of oxygen and that, they are surface organometallic Os(I1) carbonyls. the
XPS
chamber,
Reactivity of It
the initial 0 s 4f7 z b.e.
[Os(CO),
value
(OM)z]n (x = 2,3; M = Sic, A l . : )
is known that osmium is able to form strong 0 s - 0
stable than 0 s - 0 s and 0 s - C O bonds (ref.14). The
well
at
species bonds,
more
established
formation of Os(I1) surface species, incapsulated into the surface
of inorganic oxides suggests clearly that a very strong osmium/support interaction occurs. This interaction enhances also I
the
strenght
of
the residual 0s-CO bonds
,
since
a
complete
decarbonylation of the oxidised supported species could not achieved by treatment in vacuo below 573 K for silica and up
be to
K for alumina . In addition,in the TPDE experiments carried on Os(I1) surface species under oxygen/argon atmosphere, the formation of O s O q , was observed only in the case of silica and at 523 R (ref.15). The difference in stability arising from the nature of the support can be also inferred from the very different conditions required for the formation of metallic particles. For silica a treatment in vacuo above 523 K is sufficient, whereas for alumina a prolonged thermal reduction in hydrogen at 67.3 K is necessary (ref.5 ) . The higher stability of the alumina system 710
out
383
could be attributed to a lower covalent character of the A1-0 bond than the Si-0 bond: this produces an increase in the strenght of the 0s-0 S ~ p P O r t bond and, consequently, a higher n-backbonding in the 0 s (11)-CO bond. This is confirmed by the red shift of 5-10 cm-1 observed in the infrared spectra of the alumina system (ref.2). A cooperative effect may be also played by simple Lewis acidbase interactions between a coordinatively unsaturated A13+cus site and the basic carbonylic oxygen: Os(II)-CO -+ A13+eus. It is known that the surface Lewis acid sites A13+cus are formed by dehydration of alumina (ref.16) . Studying the mobility of metal atoms on different supports, recently we observed that the oxidised silica surface species [ O s (COIx(0SiC)zI n can regenerate stepwise either the chemisorbed grafted cluster H O s s (C0)io (OSi<) or the physisorbed oS3 (CO)12 : both are obtained by prolonged thermal treatments in CO at 523 K (ref.10). Such reconstruction of the original Os3 framework would support the existence of Os(I1) surface species arranged as three atom ensembles, which retain a topology reminiscent of the original cluster skeleton even after the break-up of the 0s-0s bonds. Direct evidence of such surface topology of the osmium atoms has been obtained by transmission electron microscopy on alumina surface species(ref.17). However, the [Os(CO), ( o A l < ) ~ ]species ~ do not undergo the reconstruction of the 0 5 3 framework under CO even at higher temperatures. This observation is again in agreement with the strong osmium alumina interactions, which may consistently lower the surface mobility and the possibility of reduction of the supported Os(I1) atoms. CONCLUSIONS The spectroscopic and thermoanalytical investigations, carried
out in the presence of oxygen , definitely confirm the formation of oxidised Os(1I) surface species during the preparation of osmium catalysts by thermal treatment in vacuo of physisorbed 0 5 3 (C0)iz. The reactivity and surface mobility of these latter species markedly depend on the nature of the support. The observed higher stability of the alumina osmium system is a clear example of the so called " strong metal support interaction ".
384
REFERENCES R. Psaro and R. Ugo, in B.C. Gates, L. Guczi and H. Knoezinger (Eds), Metal Cluster in Catalysis, Elsevier, Amsterdam, 1986, pp. 451-462 and references therein. 2 a) R. Psaro, R. Ugo, G.M. Zanderighi, B. Besson, A.K. Smith and J.M. Basset, J. Organomet. Chem., 213 (1981) 215-247; b) H. Knoezinger and Y. Zhao, J. Catal., 71 (1981) 337-347. 3 F.B.M. Duivenvoorden, D.C. Koningsberger, Y.S. Uh. and B.C. Gates, J. Am. Chem. S O C . , 108 (1986) 6254-6262. 4 G. Collier, D.J. Hunt, D.S. Jackson, R.B. Moyes, I.A. Pickering, P.B. Wells, A.F. Simpson and R. Whyman, J. Catal., 80 (1983) 154-171. 5 J.M. Basset, B. Besson, A . Choplin, F. Hugues, M. Leconte, D. Rojas, A.K. Smith, A. Theolier, Y. Chauvin, D. Commereuc, R. Psaro, R. Ugo and G. M. Zanderighi, in M. Giongo and M. Graziani (Eds), Fundamental Research in Homogeneous Catalysis, Vol. 4 , Plenum Press, New York and London, 1984, pp. 19-53. 6 B.F.G. Johnson and J. Lewis, Inorganic Synthesis, 13 (1972) 92-95. 7 C. Dossi, R. Psaro, R. Zanoni, F.S. Stone, Spectrochim. Acta (A), 43 (1987) 1507-i510. a M. Deeba and B.C. Gates, J. Catal., 71 (1981) 303-307. 9 C. Dossi and A . Fusi, Anal. Chim. Acta, in the press. 10 C. Dossi, A. Fusi, E. Grilli, R. Psaro, R. Ugo and R. Zanoni, Catal. Today, 2 (1988) 585-594. 11 A.J. Deeming, personal communication. 12 R. Zanoni, V. Carinci, Raja H. Abu-Samn, R. Psaro and C. Dossi, J. Mol. Struct., 131 (1985) 363-369. 13 R. Zanoni and R. Psaro, Spectrochim. Acta (A), 43 (1987) 1497-1501. 14 E.L. Muetterties, T.N. Rhodin, E. Band, C.F. Brucker and W . R . Pretzer, Chem. Rev., 79 (1979) 119-124. 15 The volatile 0SO.i was detected by passing the effluent gas strean? into an aqueous solution of KOH. 16 J.B. Peri, J. Phys. Chem., 69 (1965) 211. 17 J. Schwank, L.F.Allard , M. Deeba and B.C. Gates. J. Cata1.,84 (1993) 24-37.
1
C.Morterra,A. Zecchina and G.Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science PubfishersB.V.,Amsterdam
385
-Printed in The Netherlands
AB I N I T I O S T U D Y OF T H E P E R I O D I C CARBON M O N O X I D E ADSORPTION ON T H E BASAL P L A N E OF ALPHA-ALUMINA
R. D o v e s i , C. R o e t t i , M. Causd and C. P i s a n i D i p a r t i m e n t o di Chimica Inorganica, Chimica P i s i c a e C h i m i c a def Materiali dell'Unfversftd df T o r i n o Laboratorio di Chimica Teorica Via G i u r i a 5 - 10125 Torino ABSTRACT T h e Hartree-Pock ab-initio LCAO program CRYSTAL is used to study t h e periodic CO adsorption o n t h e basal plane of a - a l u m i n a , w i t h i n t h e f r a m e w o r k of a thin-film m o d e l . Equilibrium g e o m e t r y , adsorption energy, and electronic s t r u c t u r e a r e discussed. T h e relatively w e a k interaction (6.2 and 3 . 8 kcal/mole for b o n d i n g t h r o u g h t h e c a r b o n or o x y g e n a t o m , r e s p e c t i v e l y ) corresponds t o polarization o f o e l e c t r o n s of t h e adsorbed molecule in t h e field of t h e surface cation. INTRODUCTION C a r b o n monoxide is a widely characterizing a l u m i n a surfaces. The
present
work
is
a
used
theoretical
"probe"
rolecule
investigation
of
for the
" r e g u l a r " ( p e r i o d i c ) adsorption o f C O o n t h e basal (0001) p l a n e o f a-alumina (corundum). IR s t u d i e s [ l ] suggest that C O m o l e c u l e s are adsorbed o n t h i s s u r f a c e vertically atop of A9 atoms. B y c o r r e l a t i n g t h e frequency s h i f t o f adsorbed CO t o t h e strength of t h e bond t o t h e s u r f a c e [ 2 ] , t h e adsorption heat is estimated t o be about 4 kcal/mole.
T H E CRYSTAL PROGRAM. Computations
are
performed
using
the
program
CRYSTAL,
implemented in t h i s laboratory [3,4]. CRYSTAL is a n ab-initio all-electron Hartree-Pock LCAO program for t h e treatment o f s y s t e m periodic in three (CRYSTALS), (SLABS), o n e ( P O L Y M E R S ) and z e r o ( M O L E C U L E S ) dimensions.
two
386
"LCAO" in the present case means that each CRYSTALLINE ORBITAL (the equivalent of the molecular orbital) is a linear combination of Bloch functions defined in terms of local functions ( o r "ATOMIC ORBITALS". AOs). The A O s are in turn combinations of Gaussian type functions (GTP) whose
exponents and
coefficients are
defined by
input; s . sp. p and d shells of GTF can be used. The program can handle any space symmetry ( 2 3 0 space groups, 80
two-sided
plane
groups.
line
99
groups).
Point
symmetries
compatible with translation symmetry are provided for molecules. The program computes: total and kinetic energies, atomic and shell
charges,
atomic
and
shell
multipoles,
band
structures,
densities of states, charge denslty maps. CRYSTAL can handle systems with up t o 32 atoms, 65 shells and 165 AOs per unit cell.
THE SLAB MODEL A "thin film" (or slab) model is adopted to study the corundum surface and simulated
chemisorption thereon: the semi-infinite crystal is
CO
by
a
small number of
"repeat units"
parallel
to
the
exposed face. The
validity
convergence
of
of the
the
slab
surface
model
can
properties
be
judged
when
from
the
increasing
the
thickness of the s l a b , and from the ability to reproduce the bulk electronic distribution at the center of the slab. Bulk corundum [ 5 ] , its basal plane [ 6 ] . and its ( 1 O T O ) surface [7] have been treated previously using the CRYSTAL program.
These studies have shown that for the basal plane of corundum, 54
slabs are thick enough for a correct description of the surface
[the number following S is the number o f
"repeat units" parallel
to slab]. However, even the S2 slab accurate surface
description formation
of
energy
(see figure 1 ) provides
surface and
properties;
the
vertical
for
are 0.00363, 0.00342.
unit
-0.44.
area
and
respectively.
-0.39,
-0.45
A
for
rather
example
relaxation
protruding aluminum atom
a
the
of
the
0.00342 a.u.
per
S2,S3
and
S4,
387
COMPUTATIONAL FEATURES. A minimal STO-3G basis set [ 8 ] was adopted for a31 substrate atoms, while a 3-21G extended set [ 9 ] was used for the atonls 111' carbon monoxide, exactly as in previous studies o f C O a d s o r p t i o l r
on magnesium oxide [ 1 0 , 3 1 ] . "Good" truncation conditions o f infinite Coulomb and exchange sums were adopted [ T I . total
energy per
corresponding to a numerical error in t h e !
cell of
the
order
of
0.0005 a.u.
(about 0.3
Kcal/mole) [ ? I . Due to the cost of computations (see Table 1 ) . an 5 2 slab was used to simulate the adsorbing crystal. Symmetric chemisorption of C O on both sides of the slab was considered. C O was allowed to adsorb vertically atop of surface A 9 atoms, either through its carbon o r its oxygen end.
For the corundum system was used
[6],
slab, the
optimized
but inward relaxation
geometry
of
( A z ~ p )of
the
bare
surface A 9
atoms was reoptinized. Both the distance zx from the surface and the interatomic distance d of the CO molecule were optimized ( s e e figure 1).
Figure 1. Geometry of the "baaal-plane" (0001) face, with indication o f the unit c e l l , and labelling o f the atoms. Left: top view; the dot-dash segment is the trace of the vertical section of the density mapa of figures 2 and 3. C O molecules are atop of surface A Q atom8. Right: side view, with schematic indication o f adsorbed molecules.
Table 1. Cost of corundum computations. NC and NX are the number of Coulomb and exchange two-electrons integrals. respectively. t, is the C P U time required for execution of PART I (integral evaluation), and refers to a N A S / 9 1 6 0 scalar computer; t, is the computation time for PART I 1 (SCF stage). The bare slab and the bulk data refer to "fair" computational conditions [7]; in particular, the number of k points used for reciprocal space sampling was 3 in those cases. 8 in the present calculations performed wjth " g o o d " [ 7 ] computational conditions. 1
I
I I i
I 1 I
I
I
1
bare
slabs I
s2
54
I
CO-covered
I
I S2 slab
t
6 66
I
I
INC/106 INX/106
I
13
I
8
I
6
132
It, ( s e e ) It, ( s e c )
I I
I
I
293 179
1
6 102
I I
I
I I
51 44
1027 452
1 1 I
103
I I
I I
66
I I
43 28
I
525
12
I I
898
I
I I
I
I
I 90
I
I
I
i
I
77 52
I
I bulk
I
I
(point group order 1 IA O s /ce 11 I
I
I I I
RESULTS
Table
2
gives some results concerning
equilibrium
geometry,
adsorption heats and Mulliken populations. few cornrents can be made upon these data, with reference to
A
the previous studies of
CO
adsorption on the (100) [ l o ] and (110)
[ l l ] face of magnesium oxide:
a)
In
contrast
with
attack were almost equally end
of
the
molecule
systems,
CO-MgO
is
where
probable, bonding here
clearly
both
directions
through
favored.
The
of
the carbon fol owlng
comments are referred essentially to this case.
b) The adsorption energy is somewhat
larger than that est mated
from frequency shifts, b u t the disagreement is well in the range of experimental uncertainties.
389
Table 2. Energy and Mulliken population data for the corundum slab, -and C O adsorption thereon. AZAQ is the change in the spacing z between the outermost layer and the nearest one at the slab interior with respect to z value in the bulk. corresponding to minimum surface energy. A9 and 0 atoms with increasing bulklike character across the slab are labelled I , X I , etc. Distances are in A , energies in kcal/mole, charges, dipoles ( z component) and quadrupoles ( z z - 0.5[xz + yz] component) in a.u.
.
r
1
I
I
J n o n - i n t e r a c t i n g l i n t e r a c t i n g systems 1
I
I Snergy and geometry data
I I
0
I I I
C
0
Atomic quadrupoles C 0
c-0 ~~
1
5.570 8.442 11.878 11.882 8.742
5.523 8.448 11.912 11.881 8.745
-0.810 -0.044
0.019
I I I I I
0.531 0.034
-0.048
I
-0.060
I I
I 5.575 8.425 11.933 11.878 8.730 -0.7'76 0.000
I I I
0.574 -0.029
I
0.874
I 1
Bond populations
-0.38 2.23 1.127 3.8
- _ _ - _ II 1.129 I ----I -0.39
I
Atomic dipoles
-0.38 2.41 1.125 6.2
I I I
I I I I
C
AQ---OC
I 1
I
Gross atomic charges
AQ---CO
I
AZAQ (surf.AQ inw. relax.)) zx (AQ-molecule distance)l d ( C - 0 distance) I Adsorption energy I Yulliken populations
I
systems
I
A9'-X
I I I I
I I I I I I
I I I
c)
The
present
value
of
adsorption
energy
0.812
0.573
0.852
0.938
-0.006
0.480
is
similar
0.484
to
that
obtained on the relatively "open" MgO (110) face ( 6 . 6 kcal/mole), and higher than on the (100) face (4.1 and 4.5 kcal/mole in 1:2, and 1:4 CO to surface Mg coverage ratio. respectively). Note that the distance between adsorbed C O molecules is here relatively
390
large (4.55 A), and intermediate between the distances corresponding to the two coverages mentioned above. The repulsive interaction energy between CO molecules at
such distance can be
estimated to be less than 0 . 5 kcal/Role [ l o ] . The weakness of the interaction is reflected in the fact that
d)
neither AZAQ
nor the intramolecular d distance
are
changed
very
much upon adsorption. e ) The only important effect of adsorption on Mulfiken populations
i s the increase of the C - 0 bond
population
(note o n the contrary
the decrease for the bond-through-oxygen case). This strengthening of the C-0 bond is consistent with the small decrease of d. and is due to depopulation of antibonding molecular
orbitals. The bond
population between
the
surface
aluminum
AQI
and
atom
above
is
slightly negative. These features were qualitatively present also in the case of adsorption on MgO.
f ) The increase in bond energy corresponds to a stiffening of the bond and to a consequent increase of the vibrational frequency.
1CO
However. the frequency
increase estimated
theoretically
change in the curvature of the E versus d while
the experimental value
is about 1 %
from
curve, is only The
[l].
the
0.3%.
disagreement
,could be due to the poverty of the basis set here adopted for the Pubstrate. g ) The similarity of the present results with those for CO on M g O ,
suggests that the adsorption mechanism is the same in both cases. The
electrostatic
interaction
with
the
exposed
surface
cation
polarizes the CO charge distribution: in particular, antibonding a electrons are shifted towards the lone--pair region.
The total and difference charge density maps of figures 2 and 3
confirm these indications. The increase in electron density in
the
lone-pair region
of
the
carbon
atom
feature, together with depopulation o f
is
the
most
evident
s-electrons of the oxygen
atom in the molecule. Closer inspection shows that the presence of the adsorbed molecule causes rearrangement of the electron distribution around the surface A 9 ion. and consequently on nearby oxygen
ions
of
the
substrate.
Again,
all
these
features
art?
completely analogous to those evidentiated for the CO-MgO syst.t?ms, in spite of the fact that the latter crystal is much more than a-alumina.
ioiijc
391
Figure 2 . Total (left) and difference (right) charge density naps for
Figure 3. Total (left) and difference (right) charge density maps for the CO covered (0001) face, in the vertical rectangle whose trace appears in figure 1 . The difference is taken with reference to the bare face density for the substrate, plus the monolayer density for the C O molecule.
392
E (a. u. 1
P i g u r e 4. T o t a l d e n s i t i e s of s t a t e s ( D O S ) f o r t h e i s o l a t e d m o l e c u l e , for t h e b a r e 5 2 s y s t e m . a n d f o r t h e CO-S2 s y s t e m .
Densities
of
states
(see
figure
4)
provide
CO
additional
information about t h e electronic structure o f the systems.
No
appreciable
chemical
interaction
is
seen
to
take
place,
s i n c e t h e r e i s n o s h a r i n g of e l e c t r o n s f r o m t h e m o l e c u l e and from the
substrate
in
the
same
crystalline
orbitals.
The
important
e l e c t r o s t a t i c i n t e r a c t i o n b e t w e e n t h e t w o s y s t e m s is a p p a r e n t f r o m the fact
that
the highest
occupied
level
(essentially associated
w i t h l o n e - p a i r o e l e c t r o n s o n c a r b o n ) is l o w e r e d w i t h r e s p e c t
to
t h e f r e e m o l e c u l e by a s m u c h a s . 7 eV. T h i s s t a b i l i z i n g e f f e c t is larger t h a n o n
(001)MgO
[Q]
in
epite
of t h e f a c t
that
the
net
c h a r g e of s u r f a c e A Q i o n s is a b o u t o n e h a l f o f t h a t o f M g i o n s , probably because cations are here protruding from the surface.
393
corcinsiois. The present study shows that ab-iaitio calculatlanr oI' periodic adsorption are now feasible even for relatively comy.lox structures such as CO on corundun, using the thin filn rodel. This study will continue with consideration of CO adsorption on other low index faces. such as the prisnatlc (1OTO) face which has already been characterized [ 7 ] , and where interesting sites far adsorption have been identified. An inportant point to be carefully explored is the influence of basis set effects: the use of a rather poor basis set Is probably the nain linitation of the present conputations, nuch aore so than adoption of the HP approxination, truncation of infinite series. and use of a very thin slab.
Financial support by the Winistero della Pubblica Istruzione and by the Consoreio per i l Sisteaa Infornativo (CSI) Pienonte aregratefully acknowledged.
REFERENCES 1. C. Morterra, 0 . Qhiotti. E. Oarrone, and F. Boccuzzl. J. Chea. 800. Farad. Trans. I 72 (1976) 2722-2734: A. Zecchina, E. Ercalona Platero, and C. Otero Aredn, J. Catal. 107 (1987) 244-247. 2. B.A. Paukshtis, R.I. Soltanov, and E.N. Yurchenko. React. Klnet. Catal. Lett. 16 (1981) 93-96. 3. C. Pisani, R. Dovesi, and C. Roetti, Hartrer-Fock Ab Initio
4. 5. 6.
7. 8.
Treatment of Crystalline 8olids, Lecture Rote8 In Chealrtry Series, Vol. 48, Springer Verlag. Berlin, 1988. R. Dovesi, C. Pfsani, C. Roetti, 1. Causd. and V.R. Saunders, subnltted to QCPE W. Causd, R. Dovesi, C. Roetti, H. Kotonin, and V.R. Saunders, Chen. Phys. Lett. 140 (1987) 120-123. C. Pisani, 1. Causd, R. Doveai, and C. Roetti, Progress in Surf. Sci. 25 (1987) 119-197. C. Roetti, 1. Causd, R. Dovesi, and C. Pisani, presented at the 10th ECOSS Conferenoe, Bologna, 1988. W.J. Hehre, R. Ditchfield. R.F. Stewart, and J.A. Pople, J. Chen. Phyr. 61 (1969) 2657-2664: ibidem, 62 (1970)
2769-2773. 9. J.S. Blnkley, J.A. Pople, and W.J. Hehre. J. h e r . Chen. SOC. 102 (1980) 939-946. 10. R. Dovesi, R. Orlando. F. Ricca, and C. Roetti, Sari. Sci. 186 (1987) 267-278. 11. 1. Causd, E. Kotonin, C. Pisanl. and C. Roetti. J. Phys. C 20 (1987) 4991-4997.
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C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
395
ADSORBATE-ADSORBATE INTERACTION IN MIXED CO-NO OVERLAYERS ON NiO(100): AN IR INVESTIGATION
by E. ESCALONA PLATERO’, E. GARRONE2, G. SPOTO’ and A. ZECCHINA’ Departamento de Quimica Inorganica, Facultad de Ciencias, Universidad de Cadiz, Apartado 40, Puerto Real, 11510 Cadiz, Spain Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, via P. Giuria 7, 10125 Turin Italy
ABSTRACT Adsorption of CO or NO on the (100) faces of NiO microcrystals occurs on the square lattice of Ni++ ions and yields an array of parallel oscillators perpendicular to the surface. A different adsorption energy is involved: NO adsorbs reversibly at r.t., and irreversibly at 77 K; CO adsorbs readily only at 77 K. In both cases, strong repulsion among nearest neighbours causes at 8 = 1/2 a ~ ( 2 x 2 )structure to be formed. Dynamic coupling takes place among CO or NO oscillators causing a blue-shift of the stretching frequency: inductive interactions among adsorbed species brings about instead a red-shift. For 8 > 1/2 adsorption on nearest neighbour sites occurs by the tilting of surrounding admolecules. When CO is adsorbed at 77 K on presorbed NO, dynamic coupling between the two kinds of molecules does not occur, and the inductive interaction of CO on NO and vice versa may be studied. It results: i) the presence of CO adparticles in the close neighborhood of a single NO molecule induces a discrete red-shift of the NO frequency, accompanied by a continuous one: two broad bands are identified, tentatively associated with a (4x4) and a (2x2) structures of the CO overlayer; ii) the integrated intensity of the NO bands remains appreciably constant, notwithstanding the substantial frequency shifts: CO undergoes continuous static shifts of lesser extent, because it is a much worse n acceptor than NO; iii) at 8 > l/2, as in the case of CO and NO alone, adsorption occurs by tilting of surrounding molecules.
INTRODUCTION Nickel oxide can be prepared as stoichiometrically and morphologically defined powders, whose specific area may be varied by sintering from = 150 to = 1 m2 g’l (ref. 1). Along the sintering process, the shape of microcrystals approaches that of ideal cubelets, and the reactivity of the (100) faces alone may be eventually studied. Adsorption of CO or NO alone at the cube faces of NiO have been
396
studied in our group by means of IR measurements on highly sintered samples (refs. 2-31, For NO adsorption, calorimetric data are also available (refs. 3-41. Rather similar features have been found for the two molecules: i) adsorption occurs at cationic Ni++ sites which form a square array; ii) repulsion among admolecules always occurs: in the NO case this is documented by the decrease in the heat of adsorption with coverage (ref. 3 ) . Repulsion is particularly strong among NN (nearest neighbour) sites, so that no NN pair is formed up to 8 = 1/2, where the overlayer has a ~ ( 2 x 2 ) structure (Figure la). In this coverage range the admolecules are perpendicular to the surface; no displacement, either vertical or iateral, occurs along coverage on such ionic solid, in contrast with the case of metals: the stretching mode of the AB molecule is now a collective motion of the set of adsorbed molecules; iii) at 9 > 1/2 adsorption on NN takes place with difficulty by the AB:CO.NO
a)
b)
Ffqyre 1 Structure of the adsorbed phase on the square array of Ni cations: a) 6 = 1/2, ~ ( 2 x 2 )structure; bl 8 > 1/2, tilting of surronding species.
tilting of nearby molecules (Figure lb) as documented by the appearance in the IR spectrum of a couple of weak bands beside the AB stretching mode; iv) in the range 0 < 8 < 1/2 the stretching mode of either CO or NO is seen to shift somewhat with coverage: CO undergoes a -16 cm" shift in the range 0 d 8 d 1/2, and NO a t6 cm'l one (Figure 2). Indeed the shift observed comes from the balance of two different phenomena of much larger extent, affecting the CO or NO stretch in opposite ways, namely the dynamical and static (or
397
chemical) shift. The former is due to the fact that the oscillating dipole of one molecule forces the surrounding ones to oscillate: the dynamical coupling given rise leads to a positive shift. The latter is due, inter alia, to inductive effects transmitted by the solid, which cause generally negative shifts. The two effects are readily discriminated by the use of isotopic mixtures (refs. 2 - 3 ) . Figure 2 illustrates the behaviour of the dynamic and static shifts with coverage for both CO and NO on NiO (100). The similarity between the two cases is evident.
II I
CO/ NiO (77K)
DYNAMIC SHIFT
NO/
NiO ( R T )
/
TOTAL SHIFT
-10
-20
-.
STATIC S H I F T ?
-30
-3
--
(-.Xcrn-'l
I
Figure 2 Dynamic and static shifts as a function of coverage for CO (section a) and NO (section b)
Two noteworthy features of the data in Figure 2 are: i) beside the sign, the behaviour of static and dynamic shifts with coverage is much the same: this probably means that the inductive effect caused by adsorption propagates over the surface in a fashion not too different from the dipole/dipole interaction; ii) the
398
concavity of the plots is indicative of strong repulsions among admolecules. Whereas a convex plot is expected for non interacting particles, in the case of strong repulsions, where the admolecules are always at the largest possible distance, the dynamic shift is expected to behave as f33/2 (ref. 5 1. The most relevant difference between the CO and the NO case lies in the adsorption energy: NO adsorbs reversibly at room ref. temperature (the heat of adsorption is around 19 kcal mol-', 3 ) and irreversibly at 77 K; CO does not adsorb readily at room temperature, but does at 77 K . In the present work, the coadsorption of CO and NO is studied, obviously at 77 K. No dynamic coupling is expected between CO and NO oscillators, due to the difference in stretching frequency: thus, the opportunity is offered to study the chemical effect alone. EXPERIMENTAL
Details of the sample preparation and preteatment are given in ref. 1. Measurements have been run on a Perkin-Elmer PE 580 B spectrometer equipped with a data station. Gases were high purity. The IR cell for measurements at 77 K is that described in previous work (refs. 2-3). RESULTS AND DISCUSSION
Measurements have been carried out as follows. After thermal treatment, the samples have been contacted with a variable amount of NO at room temperature, then brought at 77 K . At such a temperature, NO is irreversibly held at the surface, and reversible adsorption/desorption of CO can be carried out. Figure 3 reports one of these experiments. Curve a is due to NO alone, and a single narrow peak is observed at 1799 cm-', i.e. , at a frequency very close to that of the NO "singleton". The corresponding coverage is 10%. This means that, on the average, all NO molecules are isolated, and no interaction whatsoever (either static or dynamic) is yet taking place. This situation corresponds to the top scheme in Figure 4. The other curves refer to increasing coverages of CO. It is seen that for vanishing CO coverages (curve b), the CO stretching frequency is at 2140 cm-l: in the absence of NO, the "singleton" CO stretching frequency on NiO is at 2152 cm-I !ref. 3). Upon increase of the equilibrium
399
pressure of CO up to 4 ~ 1 0 -torr ~ (1 torr = 133 Nmm2, curves b to g ) , the main CO peak is observed to shift slighlty down to 2133.5
cm-I in a continous way, as does CO when adsorbed alone on the same sample. At frequencies above 2150 cm-l a weak unresolved absorption is observed. Because such absorption is present at very low CO pressure although in small amounts, one infers that the involved CO species must be comparatively strongly held on minor structural features of the sample: indeed, unsintered high area samples show bands in this position with much higher intensity, and thoroughly annealed samples do not show them at all. In curves g-i, a component at 2080 cm-l is seen (arrow), which monitors the formation of tilted CO species (ref. 3). Also, a heavy contribution from the rotational contour of gaseous CO is seen.
0.1 ( 0 . 0 . )
I 2300
I
I
I
I 2100
I
I
1
I 1900
I
I
I
I
I 1700
I
1
cm-’
Figure 3 . IR spectra of the interaction of CO with presorbed NO on NiO at 77 K. (Absorbance vs wavelength)
Simultaneously, dramatic changes take place in the NO stretching
400
region, which roughly occur in three steps as three different NO peaks are formed. In the first step (curves a-d) the NO peak ( A ) is shifted downward in a continous way from 1799 to = 1784 cm-l. The shift is accompanied by a decrease in intensity at the band maximum and by the increase of the halfwidth from 10 to = 24 cm-l, the integrated area of the absorption remaining approximately constant. In the second step (curves e-g), the A peak decreases while a second one (B) develops at 1752 - 1742 cm'l first as a shoulder then reaching its maximum intensity in curve g . In the same spectrum, a third peak ( C ) begins to appear. Finally, in the third step (curves g-ill peak C becomes prominent at = 1715 cm-l: the related halfwidth is only = 12 cm-l. On the whole, the spectral changes between 1800 and 1700 cm-I can be interpreted as due to the superposition of a continous shift and a discrete effect. The interpretation is as follows. NiO has a great ability to transmit electronic effects of inductive type. This is the cause of the substantial static shift of CO or NO already when adsorbed alone: repulsive interactions among NO molecules have been documented by microcalorimetric measurements (refs. 3-4) and by the convexity of the plots in Figure 2. CO and NO have a quite similar bonding scheme, in that both are u donors and TI acceptors, their LUMO being in both cases antibonding. Along adsorption, charge is donated to the metal centre, which is in part backdonated to the same molecule, and in part dissipated on surrounding cationic sites with an effect that fades away over some lattice spacings. Repulsive interaction between neighbouring sites of any order is expected, the larger the extent of which the nearer the two sites. Consequently, sites are expected to fill so that the CO molecules are at the maximum possible distance. Consider the Scheme in Figure 4 . Peak A is due to isolated NO species not or little interacting, as shown in the upper part of the Figure. The adsorption on sites farer than next nearest neighbours (NNN) causes a continous shift together with a broadening. Supposing for the sake of simplicity the overlayer to be ordered, a (4x4) structure may be formed (Figure 4). Peak B arises when adsorption on NNN sites takes place, the maximum intensity occurring at a coverage where a (2x2) structure is
401
formed (Figure 4 ) . Adsorption on NN sites takes place by the tilting of surrounding admolecules (bottom scheme in Figure 4 ) :
3 L
>
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
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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.
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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
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.
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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
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
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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13. H.P.Boehm, H-Knozinger, "Nature and Estimation of Functional Groups on Solid Surfaces", in Catalysis - Science and Technology, edJ.R.Anderson, M.Boudart, SDrlnger Verlag 1983, vol.4, p.39. 14. A.A.Davidov, IR Spectroscopy in Surface Chemlstry of Oxides (in russian), Izd.Nauka, Novoslblrsk 1984, p. 138. 15. A.A.Davidov, V.G.Mlkhal tchenko, V.D.Sokolovskii, G.K.Boreskov, J.Catal. 55 '( 1978) 299. 16. T.Kondo, SSaito, K.Tarnaru, J.Am.Chem.Soc. 96 (1974) 6857. 17. C.R.Adams, Proc.3rd Intern.Congress Catalysis, Amsterdam 1964, North Holland Publ.Co., Amsterdam 1965, p.240. 18. W.M.H.Sachtler, NH.de Boer, ibid.p.252. 19. R.J.Kokes, in "Catalysis Progress in Research", Plenum Press, London 1973, p.73. 20. J.Haber, MSochacka, B.Grzybowska, A.Golebiowskl, J.Molecular Catal. 1 ( 1975) 35. 21. J.Haber, M.Wltko, Acc.Chem.Res. 14(1981) I . 22. R.K.Grasselli, J D.Burrlngton, Adv.Catal. 3 0 (19811 133. 23. J.Goodenough, Progr.Solid State Chem. 5 (1971 1 145. 24. C.A.Cat low, In "Nonstoichiometrlc Oxides", ed.O.T.Scerensen, Academlc Press 198 I , p. 25. LKihlborg, Ark.Kem. 2 I f 1963) 357. 26. J.Ziblkowski, JCatal. &(I9831 263; &(I9831 3 1 l ; u ( I 9 8 3 ) 3 1 7 ; m ( 1986) 45. 27. E.Broclawik, J.Haber, L.Unger, J.Phys.Chem.Solids 42 ( 198 1 ) 203. 28. A.B.Anderson, Y.Klm, D.W.Ewlng, R.K.Grasselli, M.Tenhover, Surface Scl. 134 1983) 237. 29. F.Ohuchi, LLFirment, U.Chowdhry, A.Ferretti, J.Vac.Scl.Technol.A2 ( 1984) 1022. 30. R.P.Groff, J.Catal. 86 (1984) 215. 3 1. E.M.McCarron I1I, A W.Sleight, Polyhedron 5 ( 1986) 129. 32. J.M.Tatibouet, J.E.Germaln, J.Cata1. 7 2 ( 198 1 ) 365. 33. J.C.Volta, W.Dequesnes, B.Moraweck, J.M.Tatibouet, Proc.7th Intern. Congress Catalysls, Tokyo 1980, Kodansha-Elsevler 1981, P. 1398. 34. K.BrUckman, R.Grabowski, J.Haber, A.Mazurklew lcz, JSloczynski, T.Wiltowski, J.Catal. 104(1987) 71. 35. K-Bruckman, J.Haber, T.Wiltowskl, J.Cata1. 106 (1987) 188. 36. JCVolta, Pl.Forissier, F.Theobald, T.P.Pham, Disc.Faraday SOC.72 ( I981) 275. 37, J,M.Tatibuet, J.E.Germain, J.C.Volta, J.Catal. 82 (1983) 240. 38. J.C.Volta, J.M.Tatibouet, C.Phichitkul, J.E.Germaln, Proc. 8th tntern. Congress Catalysis, Berlln 1984, Verlag Chemie&Dechema 198 I , vo1.4, p.451. 39. A.Guerrero-Ruiz, J Massardier, D.Duprez, M.Abon, J.C.Volta, Proc.9th Intern. Congress Catalysts, Calgary 1988, ed.M.J.Phlllips, M.Ternan, The Chemical Institute of Canada, Ottawa 1988, p.1601. 40. J.Haber, E.Serw icka, Polyhedron 5 ( 1986) 107.
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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 <5%, and c o n f i r m e d t h e absence o f any o t h e r C - c o n t a i n i n g product, i t would a l s o i n d i c a t e t h a t t h e adsorbate i s p r o b a b l y n o t an
474
I
‘L3-1w-
O.!
5
I 10
15
20
Time(h)
F i g . 2. C w d s o r p t i o n curves for CO2:H2 = 1:2 a t various t e q s .
2200
2000
1800 CI -1
F i g . 3. DRIFT spectra of Ru/RuO,/TiOZ : a. background ( i n 8 0 t o r r C 0 2 ) ;h. i n HZ(%Otorr) a t 100 C; in C0z:Hz = l : l O ( l b a r , s t a t i c ) a f t e r 3h a t 50°C. important intermediate i n t h e methanatiun process, a t l e a s t below 100°C, The amount o f coadsorbed H2 exceeded t h e i n d i v i d u a l uptake by a i n d i c a t i n g e i t h e r t h e formation o f a s u b s t a n t i a l l y f a c t o r o f -3-4, hydrogenated surface complex o r H2 s p i l l o v e r . Clear evidence f o r t h e Ia t t e r was obtained by r e p e a t i n g t h e coadsorpt i on experiment w i t h excess H2,ie 10H2:1C02, From 25-100 C, t h e C02 was completely adsorbed and p a r t i a l l y converted t o CH4 I l O - S O X I over 16-18h. In no case was a f i x e d adsorption s t o i c h i o m e t r y observed. Indeed, on sustained exposure a t 50°C f o r 6Sh, t h i s value exceeded 1 O : l (H2:C02) c o n f i r m i n g t h a t most o f t h e Hp was not associated w i t h adsorbed C02 i n some form o f metaladsorbed surface complex. FI check was made o f c a t a l y s t d e a c t i v a t i o n i n nethanation, using 0.159 sample i n H2:COz = 1 2 : l ( P t o t = 2SOtorr 1 a t 4SoC, The i n i t i a l r a t e (-4% conversion h-1) f e l I gradual l y t o zero over 15h I-12% conversion], However, on evacuation a t 12OoC f o r 30min, t h e o r i g i n a l act i v i t y was regenerated; behav iour cons i s t ent w i t h t h a t r e p o r t e d earlier Iref.5).
3 , Hodel c a t a l y s t : I n v e s t i g a t i o n s bu DRIFT, As expected,
exposure t o C02 alone
produced no s p e c t r a l features
o f i n t e r e s t i n t h e important i n f r a r e d r e g i o n around 2000 cm-l(see F i g , 3 ~ ) . I n t h e presence o f H2 (fig.3h1, horueuer, a sharp band appeared a t 1950 cm-1 superimposed on a broad absorption extending from 20001800 cm-1. As many t r a n s i t i o n metal-hydrogen v i b r a t ions fa1 I i n t h i s range, e i t h e r as supported o r unsupported metals Iref.GJ1, o r as complex hydrides ( r e f . 101, these features are a t t r i b u t e d t o s t r e t c h i n g v i b r a t i o n s o f H atoms chemisorbed on surface Ru metal atoms. I n view
475
o f t h e Ru p a r t i c l e - s i z e d i s t r i b u t i o n and t h e invariance o f s p e c t r a l shape with coverage(see r e f . I l ) , t h e broad band i s t e n t a t i v e l y associated w i t h surface heterogeneity r a t h e r than adsorbate-adsorbate i n t e r a c t ions The hydrogen i c o r ig i n o f b o t h features was conf i rmed by t h e i r absence upon exposure t o 02, producing instead t h e corresponding
.
Ru-0 s t r e t c h i n g envelope i n t h e range 1200-1300 cm-1. When a sample c o n t a i n i n g pre-adsorbed H was exposed t o C02 a t 7OoC, no change was seen i n t h e Had band envelope, suggesting t h a t no surface complex was formed on t h e metal. However, subsequent contact w i t h a m i x t u r e o f C02:H2(1: 10, lbar; s t a t i c ) a t 5OoC produced bands a t 2020
1995 cm-1 already a f t e r Smin. These bands grew s t e a d i l y over and several hours ( d u r i n g which weak bands o f gas-phase CH4 a l s o appeared a t 3020 and 1305 cm-I), t o produce spectrum 3 ~ ,which s t y p i c a l o f CO adsorbed on Ru r e t a i ( s e e e g . r e f . 8 ) . This observation i s c o n s i s t e n t w i t h an e a r l i e r study o f C02 methanation over Ru/R1203 - y Solyutosi el. ( r e f . 12) , though i n t h a t case t h e COad bands were r a t h e r weak and o n l y appeared above 100°C, D i r e c t chemisorption o f CO ( 3 0 t o r r ) a t 3OoC on t h e model c a t a l y s t gave r i s e t o a band envelope o f s i m i l a r form, showing two bands a t 2030 and 1960 cm-1. The lack o f exact correspondence i n band p o s i t i o n s was not unexpected as these are known t o be s e n ~ i t i u et o i n t e r a1 i a t h e Ru p a r t i c l e size, CO coverage, and t h e absence o r presence o f H2 (refs.8,12-13), The r e a c t i u i t y o f t h e adsorbed species was t e s t e d by purging away t h e r e s i d u a l C02/H2 mixture and heat ing i n s t a t i c Flr. The 2020 cm-1 ulas se Iect i ue Iy eroded , beg i nn i ng around 75OC , and bands o f gas-phase C02 appeared (F i g . 4 ~ )suggest i ve
I
2&
2300
2000
o f the
Boudouard r e a c t i on. Th i s process
1700 Ctv 1
F i g . 4. R e a c t i v i t y of adsorbedo species(ex.C02/H2 = 10) I: on Ru/RuOx/Ti02: g. s t a t i c Ar/750/lh; p. A r / l 2 0 / l h ; E.H2/120'/1h; 4. A r p ~ r g e / 0 2 / 2 5 ~ C / 5 n i n .
476
uras e v i d e n t l y c o r p l e t e by 120° C(Fig.lh), thus c o r r e i a t ing u i t h t h e cond i t ions f o r c a f a Iyst r e a c t i u a t i o n as seen above. The r e s i duo I band a t 1995 cr-1 mas s t a b l e i n Ar up t o t h e highest t e r p e r a t u r e explored, i e 200°C. Cooling t o 12OoC and exposure t o H2 a l s o l e f t t h i s band unaffected (Fig.Ss), d i d not produce detectable amounts o f C H t , but created a ner band a t 2015 cm-1 The o r i g i n o f b o t h bands mas f i n a l i y confirmed t o be hydrogenic by r e a c t i o n u i t h 02, as they mere r a p i d l y I
removed a t 25'C urith t h e concomitant g r o v t h o f t h e fami I l o r deforrnat i o n band o f adsorbed molecular u a t e r a t 1630cm-1. As t h e bands due t o AuO-H h a w already been I d e n t i f i e d , i t must be concluded t h a t t h e s t r o n g bands a t 2015 and 1995cr-1 represent t h e (presumably) large amounts o f s p i i t o u e r H already i n d i c a t e d f r o r t h e volumetric s t u d i e s , The p o s i t i o n o f these bands shows t h a t t h e H i s associated m i t h surface metal cations, i n t h e form o f hydrides, r a t h e r than as hydroxyi species. i n d i r e c t spectroscopic c o n f i r m a t i o n o f H2 s p i l l o u e r , apparently a s s i s t e d by t h e presence o f C02, was obtained by exposing t h e f r e s h c a t a l y s t t o C02:02 ( 1:1,80torr) a t 75OC ouernight. Clear euidence mas span f o r H/D exchange, and by i r p i i c a t i o n a l s o H2 s p i i i o u e r , bands c h a r a c t e r i s t i c o f i s o l a t e d surface appearing a t 2680 and around 2520 e s t a b l i s h e d t h a t OH groups on oxide exchange u i t h 02 i n t h e absence o f an
00 and H(D)-bonded OD groups cm-1 respect i u e l y . I t is urel i surfaces do not r e a d i l y undergo act i u a t ing trans1 t i o n metal (see
eg. r e f . l t ) , DISCUSSION The r e s u l t s f r o r e l e c t r o n r i c r o s c o p y i n d i c a t e t h a t optimum a c t i v i t y i n t h e methanat i o n o f COP i s favoured by c a t a l y s t s I n r h i c h t h e Ru i s s t a b i l i z e d i n h i g h l -dispersed f o r r , i d e a l l y w i t h p a r t i c l e diameters i n t h e range 10-20 , i n t h e "model" case o f Degussa P25 Ti02, t h i s
Y
c o n d i t i o n occurs r a t h e r s e i e c t l u e l y i n o n l y -15-201 o f t h e p a r t i c l e s , roughly corresponding t o the proport i o n o f r u t i l e (-20%) k n o w t o be present i n t h e predorlnant l y (801) anatase phase o f P25. Furthermore, Ru02 c r y s t a l l i z e s i n t h e u n d i s t o r t e d tetragonal f o r r o f r u t i i e Ti02 and u i t h a s i r i l o r l a t t i c e parameter ( r e f . l 5 ) , thereby e x p l a i n i n g t h e h i g h s o l u b i l i t y I i i i t o f Rut+ (-5 ion%) i n t h e l a t t e r ( r e f . 1 6 ) . Thus, c h e r i c a l i n t u i t i o n suggests t h a t t h e h i g h - t e r p . ( 37OOC) c a l c i n a t i o n I n a i r r a y s t a b i l i z e t h e Ru02 precursor, p o s s i b l y u i a I t s p a r t i a l d i s s o l u t i o n i n t o t h e support. This r o u i d h e l p t o e x p l a i n t h e lncoaplete reduction, which i n t u r n r i g h t prouide a r a t i o n a l e f o r t h e good s i n t e r l n g r e s i s t a n c e o f t h e Ru r e t a i . P a r t i a l l y - r e d u c e d s o l i d s o l u t i o n s c o n t a i n i n g t r a n s i t i o n metal oxides appear t o confer s t a b i i l t y t o t h e metal I I c f r a c t ion e x t r a c t e d i n t h e process (see eg, refa.17-18).
However, t h e data i n Table 1 i s i n s u f f l c i e n t t o address the question o f whether r u t l l e i s superior, under otherwise equivalent c o n d i t i o n s , t o anatase Ti02 as support. I n any event , t h e r e a l i t y i s c l e a r l y more complex, as i l l u s t r a t e d by t h e wide-ranging character and performance o f t h e two c a t a l y s t s based on r u t i l e . The physico-chemical processes o c c u h i n g i n t h e i n i t i a l Ru02 d e p o s i t i o n step are e v i d e n t l y a l s o important f a c t o r s i n t h e preparation, i n which t h e t e x t u r e and surfacechemical p r o p e r t i e s o f t h e Ti02, eg. s t a t e o f hydroxylation, e f f e c t i v e charge e t c . , may be expected t o p l a y a major r o l e . Table 1 a l s o shows t h a t t h e r e i s a support e f f e c t due t o TiO2, Both anatase and a1umina"C" c a r r y Ru p a r t i c l e s o f simi l a r dimensions but the former i s moderately-act i v e i n t h e dark r e a c t ion whereas t h e l a t t e r i s v i r t u a l l y i n a c t i v e . Rs t h e temperature o f p r e p a r a t i o n tends t o disfauour the p o s s i b i l i t y o f a s t r o n g metal-support i n t e r a c t i o n (ref.231, t h e b e n e f i c i a l e f f e c t o f Ti02 may be r e l a t e d r a t h e r t o t h e s t a b i l i z a t i o n o f ( a t least some f r a c t i o n o f ) Ru i n p o s i t i v e o x i d a t i o n s t a t e ( s ) . The e f f e c t o f reduct i o n degree on c a t a l y t i c a c t i v i t y obviously needs thorough invest i g a t ion but i s beyond t h e scope o f t h e present r e p o r t . The p i c t u r e t h a t emerges from t h e chemisorption c h a r a c t e r i z a t i o n o f t h e model c a t a l y s t and i n s i t u I R s t u d i e s i s t h a t t h e coadsorption synergy between C02 and H2 observed here, and p r e v i o u s l y by o t h e r s (refs,6,12), involves t h e intermediate conversion o f C02 t o CO, which i n t u r n f a c i l i t a t e s H2 s p i l l o v e r , The simplest conceivable r o u t e f o r t h e former process i s
t h e reverse water-gas s h i f t r e a c t i o n :
as was a l s o concluded by Solymosi e t a l . ( r e f . l 2 ) . The e q u i l i b r i u m l i e s we1 I t o t h e l e f t a t ambient temperature ( A H r s + l O k c a I ,mol-1; ref.191, so t h i s proposal seems a t f i r s t s i g h t t o be r u l e d o u t , However, t h e process o c c u r r i n g on the Ru/RuOx/Ti02 c a t a l y s t I s not well-represented by eqn ,2, Whereas t h e react ants are r a t her weak Iy-assoc i a t ed w i t h t h e cat a Iy s t , t h e CO product eu i dent I y becomes s t r o n g Iy-chem i sorbed on Ruo and t h e H20 i s a l s o l i k e l y t o be s t r o n g l y bound as,eg. OH-, on t h e Ti02 support, This would favour t h e prevent t h e i r back-reaction, No C02 ( o r adsorbed CO) i n complex Rnother a t t r a c t i v e f e a t u r e o f
s t a b i l i z a t i o n o f b o t h products and evidence has been found f o r adsorbed form(s1 with H2. eqn,2 i s t h a t t h e c r e a t i o n o f t h e H20,
OH', ( f o r which t h e r e i s s l i g h t evidence from IR ), provides a ready explanation f o r t h e preponderance o f H2 s p i l l o v e r d u r i n g coadsorption. This process i s known t o be markedly-accelerated when H20 o r OH- i s present on t h e Ti02 surface ( r e f . 2 0 1 . o r more-likely
478
I t now seems establ ished t h a t the accumulation o f an overlayer o f CO on t h e Ru metal surface Is responsible f o r c a t a l y s t deactiuation, a t 0 the least below 100 C . Regeneration by h e a t i n g t o 12OoC proceeds d i s p r o p o r t i o n a t i o n o f adsorbed CO and recovery o f , presumably, a s i g n i f i c a n t number o f metal surface s i t e s by C02 desorption.The development o f t h e Boudouard r e a c t i o n a t such low temperature i s b o t h unusual and encouraging, p r o v i d i n g a s t r o n g i n c e n t i v e t o assess8 t h e performance o f t h e c a t a l y s t i n CO hydrogenation, Elegant s t u d i e s o f t h e l a t t e r r e a c t i o n ouer Ru/SiO2 haue been r e p o r t e d by Tamaru gt a l
.
(ref.211, i n which t h e importance o f CO d i s p r o p o r t i o n a t i o n as t h e i n i t i a t i n g step i s c l e a r l y demonstrated. Furthermore, a rore-recent study o f s i n g l e - c r y s t a l Ru by t h e same group presents good evidence t h a t t h i s process i s s t r u c t u r e - s e n s i t i u e , o c c u r r i n g more-readily on stepped faces o f t h e metal ( r e f , 2 2 1 , The f a t e o f t h e deposited carbon, formed i m p l i c i t l y i n t h e Boudouard r e a c t i o n o v e r t h e model c a t a l y s t , i s not c l e a r a t t h i s stage although t h e recovery o f f u l l a c t i v i t y suggests t h a t i t may be hydrogenated o f f as CHq. Indeed, t h e question o f whether t h e p r i n c i p a l r o u t e i n t h e methanation o f CO2 a t low temperature passes uia CO (and by i m p l i c a t i o n adsorbed carbon 1 i s o f fundamental i n t e r e s t . The most recent work i n t h i s laboratory tends t o argue against a CO intermediate, a t least f o r t h e mechanism under i l l u m i n a t i o n . A CO/H2 m i x t u r e a t 50°C y i e l d e d o n l y CO2 I n t h e e a r l y stages, CH4 appearing o n l y l a t e r w i t h t h e concomitant disappearance o f t h e C02.
ACKNOULEOGtlENTS The authors would l i k e t o thank E.sz.Kouats and J , K i w i fur valuable discussions. Two o f us ( K.R.T. and fl,G.1 would l i k e t o thank t h e Gas Research I n s t i t u t e , Chicago, I I I . , U.S.R.(sub-contract with t h e Solar Energy Research In s t it ut e , Go Iden , Co Iorado1 for f i nanc i a I support, REFERENCES
1 2
3 4
5 6
N .Ha Imann,i n "Energy Resources Through Phot ocher i s t r y and Cat a Iys i s" ed. N,Graetzel, Academic Press, New Vork, 1983, Ch.15, p507. G.Chlnchen, P, J.Oenny, O,G.Parker, N.S.Spencer and D.A.Uhan, Rppl. (19871, 333. Catal., F , S o l y r o s l , R.Erdohely1 and T,Bansagi, J . C a t a l . , 68, (19811, 371, G,D.Ueatherbee and C.H.Bartholoner, J , C a t a l . , 81, (19841, 352. K.R.Tharpi, J . K i w i and tl.Graetzel, Nature, U, (19871, 506. K.R.Tharpi , J,Kiwi and f'l.Graetzel , Proc,9th Int ,Congr,Catal , , U o l . 2 , The Chemical I n a t l t u t e o f Canada, Ottawa, 1988, u.837,
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P . R . G r i f f i t h s and J.A,de Haseth, i n " F o u r i e r Transform I n f r a r e d Spectrometry", Wiley, New Vork, 1986, Ch,l7, p.536. M.A.Uannice,in Catalysis,Science and Technology, Uo1.3, J.R.Anderson and fl. Boudart , eds, , Spr i nger-Uer Iag, Ber I i n, 1982, Ch ,3, p ,139, Z,Knor, i n Catalysis,Science and Technology, Uol.3, J.R.Rnderson and M.Boudart, eds., Springer-uerlag, B e r l i n , 1982, Ch.5, p.231. K,Nakamoto, i n " I n f r a r e d Spectra o f Inorganic and Coordination Compounds", Wiley-lnterscience, 2nd,ed,, 1970, p , 2 0 2 f f , J , G . H i g h f i e l d , H.uan den Bergh, K,R,Thampi and fl,Graetzcl, Langmuir, submitted f o r pub1 i c a t ion, F,Solymosi, R.Erdohelyi and n.Kocsis, J.Cher.Soc.Faraday Trans. I, 77, (19811, 1003. E.Gugl i e l r i n o t t i , G . S p o t o and A.Zecchina, Surface Sci .j.&(lg851,202. G.C.Bond, i n " S p i l l o u e r o f Rdsorbed Species", Studies i n Surface Science and Catalysis, Uo1.17, E l s e u i e r , Amsterdam, 1983, p.1. U.H.Baur and R,R,Khan, Acta C r y s t . , B 21, (19711, 2133, (19811, 89. f l . U a l i g i and D.Gazzoli, Z.Phys.Cher. Neue Folgc tl,Boudart, A,Delbouille, J,R,Dumesic, S,Hammouma and H.Topsoe, (19751, 486, J.Catal,, J , G , H i g h f i e l d , R.Bossi and F.S.Stone, i n " Preparation o f C a t a l y s t s 1 1 ", E l s e u i e r , Amsterdam, 1983, p.181. (1980), 275. D,S.Newsore, Catai .Reu.-Sci .Eng., J,-n,Herrmann and P.Pichat, i n " S p i l l o v e r o f Adsorbed Species ' I , Studies i n Surface Science and Catalysis,Uol.l7,Elseuier,Amsterdam, 1983, p.77. V.Kobori, H.Vamasaki, S,Naito, T.Onishi and K.Tamaru, J.Chem.Soc. Faraday Trans. I , (19821, 1473, E.Shincho,C.Egawa,S.Naito and K.Tamaru, Sur ace Sci . , ~ , ( 1 9 8 5 1 , 1 5 3 , See eg "net a I -Support and net a I -Rdd i t i ue E f f e c t s i n C a t a l y s i s " , Studies i n Surface Science and Catalysis, E seuier, Amsterdam, 1982.
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481
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactiuity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ON THE ROLE OF SUPPORTS AND PROMOTORS I N C O t H 2
REACTIONS ON RHODIUM BASED
CATALYSTS
J.P. HINDERMANN, A. KIENNEMANN, S. TAZKRITT L a b o r a t o i r e de Chimie Organique Appliquge, U.A. CNRS 469, EHICS, 1, r u e B l a i s e Pascal, 67008 STRASBOURG CEDEX, France
ABSTRACT The i n f l u e n c e o f d i f f e r e n t a d d i t i v e s on carbon monoxide h y d r o g e n a t i o n has been s t u d i e d . Two d i f f e r e n t e f f e c t s c o u l d be observed depending on t h e n a t u r e o f t h e a d d i t i v e and i t s c o n c e n t r a t i o n . F o r t i t a n i a , z i r c o n i a and h a f n i a t h e o v e r a l l a c t i v i t y i n c r e a s e s a t l o w l o a d i n g s ; a t h i g h e r loadings, o n l y t h e enhanced C oxygenates a c t i v i t y remains. T.P.D. measurements a f t e r CO a d s o r p t i o n ghows t h a t t h e C O d i s s o c i a t i o n o c c u r s a t a lower temperature. These e f f e c t s were a t t r i b u t e d t o t h e p a r t i a l coverage of t h e metal s u r f a c e by t h e a d d i t i v e and t h e f o r m a t i o n o f a C and 0 c o o r d i n a t e d species.
INTRODUCTION I t has been c l e a r l y demonstrated t h a t t h e a c t i v i t y and s e l e c t i v i t y o f
rhodium based c a t a l y s t s can d r a s t i c a l l y change f r o m methane t o C 2 c o n t a i n i n g oxygenates
or
to
methanol
(refs.
1-3).
These
changes
in
activity
and
s e l e c t i v i t y have been a t t r i b u t e d t o s e v e r a l i n f l u e n c e s . The c o m p o s i t i o n o f t h e Rh p r e c u r s o r may p l a y an i m p o r t a n t r o l e ( r e f s . 1,2,4-8).
One o f t h e p o s s i b l e cause o f t h e d i f f e r e n c e i n b e h a v i o u r o f t h e
v a r i o u s p r e c u r s o r s may be t h e presence o f r e s i d u a l c o u n t e r i o n s w h i c h can affect
surface
intermediate
activity.
Thus,
Worley
e t al.
(ref.
7)
have
observed d i f f e r e n c e s i n t h e i n f r a r e d s p e c t r a d u r i n g carbon monoxide a d s o r p t i o n . They proposed t h a t t h e c o u n t e r - i o n was r e s p o n s i b l e f o r t h e s e e f f e c t s . G i l h o o l e y e t a l . ( r e f . 6 ) suggest t h a t t h e c o u n t e r - i o n s do n o t a f f e c t a l l t h e p r o d u c t s i n t h e same way,
t h a t t h e i r e f f e c t s may be s i t e s p e c i f i c and concluded t h a t
changing t h e m e t a l p r e c u r s o r c o u l d a f f e c t t h e p r o d u c t d i s t r i b u t i o n s .
However,
K i p e t a l . ( r e f . 9 ) have shown t h a t t h e d i f f e r e n c e s between c a t a l y s t s p r e p a r e d f r o m RhC13 and Rh(NO3I3 r e s p e c t i v e l y a r e n o t t h e o n l y r e s u l t o f r e t a i n e d C1 o r n i t r a t e . They r a t h e r a t t r i b u t e d t h e d i f f e r e n c e i n a c t i v i t y t o d i f f e r e n c e s i n morphology. The s i z e o f t h e d i s p e r s e d Rh p a r t i c l e s have a l s o been i n v o k e d ( r e f s . 10-12). However, van den Berg e t a l .
( r e f . 13) a f t e r a n a l y z i n g a l a r g e number
o f micrographs c l a i m t h a t t h e i r Rh/Si02 and Rh-Mn0-Mo03/Si02
c a t a l y s t s do n o t
d i f f e r s i g n i f i c a n t l y i n e i t h e r t h e i r p a r t i c l e s i z e d i s t r i b u t i o n nor t h e i r
402
r e s i s t a n c e t o s i n t e r i n g a l t h o u g h t h e y have s i g n i f i c a n t l y d i f f e r e n t a c t i v i t i e s and s e l e c t i v i t i e s . K i p e t a l . ( r e f . 9 ) have a l s o shown t h a t t h e a c t i v i t y c o u l d n o t o n l y be r e l a t e d t o t h e p a r t i c l e s i z e . The s u p p o r t c o m p o s i t i o n p l a y s undoubtedly a c t i v i t y and s e l e c t i v i t y ( r e f s .
1-3,6,13-20).
a
prevailing
role
i n the
I t has a l s o been demonstrated
t h a t some e l e c t r o p o s i t i v e i o n s such as Mn, T i , V,
Z r , Fe, Mo, Ce, La, T i , Hf,
promote t h e p r o d u c t i o n o f h i g h e r oxygenates i n a CO+H2 r e a c t i o n c a t a l y z e d on Rh ( r e f s . 11, 21-42). not well
As w i l l be d e s c r i b e d l a t e r , t h e mechanism o f p r o m o t i o n i s
understood.
We t h e r e f o r e s t u d i e d t h e a c t i v i t y and s e l e c t i v i t y of
v a r i o u s c a t a l y s t s . Thermoprogrammed d e s o r p t i o n was a p p l i e d t o d e t e r m i n e t h e i n f l u e n c e o f t h e promoter on t h e CO a d s o r p t i o n . EXPER IMENTAL
C a t a l y s t s p r e p a r a t i o n ( r e f . 66) : t h e c a t a l y s t s were prepared b y a c o n v e n t i o n a l i n c i p i e n t wetness method.
RhC13 and promoter n i t r a t e ( o r o x y c h l o r i d e ) were
added s i m u l t a n e o u s l y t o t h e c a t a l y s t s . R e a c t i v i t y measurements as i n r e f . ( 6 6 ) .
.
CO c h e m i s o r p t i o n : i t was performed b y a p u l s e method i n an He f l o w on 0.5 g.
o f catalyst. Thermoprogrammed d e s o r p t i o n CO was adsorbed a t room t e m p e r a t u r e under 1 atm. o f CO. The c a t a l y s t was
t h a n f l u s h e d by an He f l o w u n t i l no more CO was observed i n t h e e x i t gas. The desorption
was
performed under
an h e l i u m f l o w
(2 l/h.
g.
cat.)
with
a
t e m p e r a t u r e i n c r e a s e o f 6K/mi n. The apparatus used was d e s c r i b e d elsewhere ( r e f . 4 3 ) . The gases f l o w i n g f r o m t h e apparatus were analyzed b y gas chromatography (T.C.O.
and F.I.D.)
to
determine t h e i r composition. RESULTS AN0 DISCUSSION The r e s u l t s o b t a i n e d on v a r i o u s c a t a l y s t s have been summarized i n Tables 1 and 2. It c l e a r l y appears t h a t t h e s e l e c t i v i t y and a c t i v i t y changes cannot be a t t r i b u t e d t o d i f f e r e n c e s i n p a r t i c l e s i z e . P a r t i c l e s i z e has been d e t e r m i n e d assuming a CO/Rh r a t i o o f 1. F o r some c a t a l y s t s w i t h h i g h promoter l o a d i n g s , t h e p a r t i c l e s i z e d i m i n i s h e s a p p a r e n t l y s l i g h t l y b u t F.T.-I.R. shows t h a t
some twin adsorbed carbon monoxides
are
then
spectroscopy present
(refs.
36,44,45).
The CO/Rh r a t i o a t s a t u r a t i o n i s t h u s h i g h e r t h a n one f o r t h e s e
catalysts.
The appearance o f t w i n adsorbed CO i s t h e consequence o f t h e
v i c i n i t y o f t h e promoter and t h e m e t a l p a r t i c l e . The d i s r u p t u r e o f t h e m e t a l m e t a l bond i n s m a l l Rh p a r t i c l e s on some s u p p o r t s i s now w e l l known ( r e f s . 46-50) and i t seems t h a t i t i s n o t o n l y encountered on Rh o n l y b u t i s a l s o
TABLE 1 : P a r t i c l e s i z e , peak maxima i n T.P.O. of C O and a c t i v i t y i n CO+H r e a c t i o n a f o r 5%Rh-x%M/Si02. o f catalyst. (M = Z r , H f , Th, Y ) . A c t i v i t y i s measured as %CO converted p e r a : T=473K, atmospheric pressure, H2/CO r a t i o = 2 , gas f l o w : 4 l / h g . c a t .
6
% M
Mean particle size
T. Max. CO O K
25A
367K 448K
603K
0.25 27 25 0.5 1% 21 3% 20 5% 24 5%Rh-x%Hf/S '02
388K 373K 378K 378K 381K
0.25 24 0.5 24 21 1 .o 3.0 23 18 5.0 5%Rh-x%Th/Si02
0
T. Max.
Initial activity
A c t i v i t y a f t e r 24H.
C20xy
C20H
H.C.
Total a c t i v i t y
3.13
0.62
0.12
2.51
2.31
0.62
0.13
5/8K 513K 568K 563K 563K
7.6/ 6.46 4.17 4.51 5.15
1.92 2.09 1.53 2.40 3.36
0.66 1.33 1.3 2.21 3.19
5.75 4.37 3.24 2.11 2.39
3.41 5.01 2.27 2.99 3.19
1.55 2.18 1.04 1.62 1.93
0.16 1.39 0.79 1.51 1.79
3lOK 376K 3/0K 398K 383K
578K 515K 569K 563K 563K
2.89 5.92 5.7/ 3.14 5.53
0.84 2.90 1.53 1.41 3.19
0.31 2.79 1.10 1.33 3.02
2.05 3.0 4.2 1.73 2.34
2.07 3./1 2.09 2.08 4.27
0.96 2.13 1.03 1.11 2.75
0.41 1.99 0.77 1.03 2.51
373K 378K 8K 3f7K 3/8K
563K 566K K 568K 563K
4.51 5.59
1.57 3.26 . 8 2.87 4.78
0.45 3.00 .OO 2.38 4.53
2.94 2.33 .68 2.1/ 2.13
1.83 2.14 .83 0.93 2.53
0.54 1.88 . 2 0.58 2.34
Tot. a c t . 2.46 .. 1.68 1.61
MeOH 0.14 .. . . 0.15 0.44
Total a c t i v i t y
C20xy
C20H
S%Rh-x%Lr/S
'O2
0.25 0.5
23 22 .o 8 3.0 24 5.0 18.5 S%Rh/x%Y/SiO,
0.25 1 .o 3.0
~
5.04 6.91
C2 a c t i v i t y
-. . - 0.89 0.93
C20H
HCC
.7. .--
0 . 1 1.
0.80 0.69
0.64 0.24
3.59 3.45 2.04 3.52
e
m e
TABLE 2 : P a r t i c l e s i z e , peak maxima i n T.P.O. o f C O and a c t i v i t y i n C O t H 2 r e a c t i o n a f o r 5%Rh-x%M/Si02. ( M = La, P r , Sm, Ce). A c t i v i t y i s measured as %CO c o n v e r t e d p e r g o f c a t a l y s t . a : T=473K, a t m o s p h e r i c p r e s s u r e , H2/C0 r a t i o = 2 , gas f l o w : 4 l / h g. c a t . % M
0
Mean particle size
T. Max. CO O K
25A
367K 448K
T. Max.
Initial activity
c02 CH4
Total a c t i v i t y
603K
A c t i v i t y a f t e r 24H.
C20xy
C20H
H.C.
Total a c t i v i t y
3.13
0.62
0.12
2.51
2.37
0.62
0.13
1.35 1.42 1.68 1.27 1.28
0.49 0.77 0.98 0.97 1.01
0.27 0.62 0.83 0.88 0.78
0.87 0.65 0.85 0.30 0.27
0.88 0.8 0.85 0.73 0.82
0.39 0.39 0.37 0.55 0.62
0.21 0.24 0.30 0.50 0.53
2.16 1.59 1. 9 l 2.02 2.48
1.02 0.85 1.11 1.39 1.92
0.56 0.60 0.94 1.27 1.67
1.14 0.74 0.8 0.61 0.56
1.37 0.94 0.99 1.16 1.22
0.74 0.54 0.57 0.81 0.93
0.31 0.29 0.39 0.67 0.8
2.61 2.22 2.11 1.56 1.57
0.93 1.16 1.35 1.06 1.04
0.59 0.97 1.21 0.95 0.91
1.74 1 .06 0.90 0.61 0.53
0.81 0.80 0.55 0.56 0.60
0.42 0.6 0.44 0.47 0.54
1.39 1.68 3.34
0.67 0.91 2.37
0.48 0.60 1.96
0.65 0.77 0.97
C20xy
C20H
5%Rh-x%La/Si02 0.25 0.75 1% 3% 5% 5%Rh-x%Pr/Si02 0.25
25 23 20 1 .o 3.0 18 5.0 18 S%Rh-x%Sm/SiO2
0.5
0.25 0.5 1 .o 3.0 5.0 5%Rh-x%Ce/SiO,
393K 397K 393K 383K 41 8K
583K 583K 576K 573K 571K
L
0.5 2.0 3.0
1.92 1.46 0.86 0.83 0.91
485
observed on Ru ( r e f . 51 1. I The presence o f t w i n s p e c i e s i s a t t r i b u t e d t o t h e presence o f Rh s p e c i e s 0% and a r e c e n t t h e o r e t i c a l s t u d y showed t h a t
‘\Rhl/
i s a stable
s p e c i e s ( r e f . 5 2 ) . However, Solymosi e t a l . ( r e f . 49) observed t h a t a t h i g h e r temperature, s i n t e r i n g o c c u r s a g a i n and t h e s e s p e c i e s d i s a p p e a r . Activity
measurements
have
shown
(Tables
1
and
2)
that
different
b e h a v i o u r s can be observed. W i t h z i r c o n i a , h a f n i a and t h o r i a , t h e o v e r a l l a c t i v i t y passes t h r o u g h a maximum a t 0.25 t o 0.5% promoter w i t h a simultaneous i n c r e a s e i n hydrocarbon and C 2 a l c o h o l a c t i v i t y . W i t h h i g h e r promoter l o a d i n g , t h e hydrocarbon a c t i v i t y drops almost t o i t s i n i t i a l v a l u e b u t t h e C2 a l c o h o l a c t i v i t y remains c o n s t a n t and t h a n i n c r e a s e s a g a i n a t 5% a d d i t i v e s . When y t t r i a i s used as a promoter, t h e a c t i v i t y i s o r i e n t e d more towards methanol s y n t h e s i s . F o r samarium, praseodymium and c e r i u m o x i d e , t h e r e i s f i r s t a sharp d r o p i n hydrocarbon a c t i v i t y and t h a n an i n c r e a s e i n C2 oxygenates. The decrease i n hydrocarbon s e l e c t i v i t y i s accompanied i n t h e I . R .
s p e c t r a by a decrease i n t h e
band which was a t t r i b u t e d t o t h e a d s o r p t i o n on b r i d g e d s i t e s ( r e f . 3 6 ) . T h i s i s i n accordance w i t h a s e l e c t i v e a d s o r p t i o n o f t h e promoter on t h e s i t e o f m u l t i p l e bonding o f CO as proposed by S a c h t l e r and I c h i k a w a ( r e f . 5 3 ) . T h i s s e l e c t i v e p o i s o n i n g o f t h e s i t e s r e s p o n s i b l e f o r t h e CO d i s s o c i a t i o n r e s u l t s i n a decrease i n t h e hydrocarbon s e l e c t i v i t y . A t t h e same t i m e , new s i t e s f o r CO d i s s o c i a t i o n appear a t t h e boundary o f m e t a l and s u p p o r t as w i l l be seen i n t h e r e s u l t s o f thermoprogrammed d e s o r p t i o n . Examples o f thermoprogrammed d e s o r p t i o n s p e c t r a a r e r e p r e s e n t e d on F i g . 1 f o r Rh/Si02 and Rh-Th/Si02 c a t a l y s t s r e s p e c t i v e l y . On Rh/Si02, CO d e s o r p t i o n i s observed w i t h two maxima a t 373K and 448K. T h i s d e s o r p t i o n corresponds t o t h e 1 i n e a r l y bonded CO. The maximum of t h e carbon monoxide d e s o r p t i o n i s somewhat s h i f t e d when compared w i t h t h e r e s u l t o f Underwood and B e l l ( r e f . 541,
b u t these authors
p r e p a r e d t h e i r c a t a l y s t s w i t h Rh(NO3I3-2H20 whereas RhC13-2H20 was used i n o u r case. Our a d s o r p t i o n was made a t 130°C and t h e c a t a l y s t was c o o l e d t o room t e m p e r a t u r e under a CO atmosphere.
Jackson ( r e f .
55) has shown t h a t a f t e r
r e p e a t e d CO c h e m i s o r p t i o n , a d e s o r p t i o n peak f o r CO i s observed a t 373K f o r a Rh/Si02 ( e x RhC13). A s h i f t f o r t h i s bond ( t o a h i g h e r t e m p e r a t u r e ) i s observed f o r Na m o d i f i e d Rh/Si02. A t 603K, a simultaneous d e s o r p t i o n o f C02 and CH4 o c c u r s and can p r o b a b l y be a t t r i b u t e d t o t h e d i s s o c i a t i o n o f i r r e v e r s i b l y adsorbed CO. When t h e promoters a r e added, t h e d e s o r p t i o n o f l i n e a r l y adsorbed CO is s h i f t e d t o somewhat h i g h e r t e m p e r a t u r e ( f r o m 373K t o 388-39310.
The d e s o r p t i o n
486
I
A
/
I
F i g . 1 : Thermoprogrammed d e s o r p t i o n a f t e r CO a d s o r p t i o n A : 5% Rh/SiO B : 5% Rh-0.25 Th/Si02 C : 5% Rh-1% ?h/Si02 D : 5% 5Rh-5% Th/Si02 a) T o t a l response o f t h e T.C.D. b ) CO c ) C02 d ) CH4
407
peak becomes broader and asymmetric, showing t h u s probably a s u r f a c e h e t e r o g e n e i t y which can, i n our opinion, t e n t a t i v e l y be a t t r i b u t e d t o a p a r t i a l coverage o f t h e s u r f a c e by t h e promoter. The peak a t t r i b u t e d t o CO d i s s o c i a t i o n i s s h i f t e d t o a lower value from 603 t o 503-563K and increases w i t h t h e amount o f promoter. T h i s peak i s accompanied i n t h e I.R.
s p e c t r a by t h e appearance o f
a band a t 1725cm-1 i n t h e case o f c e r i a which was a t t r i b u t e d t o a C and 0 c o o r d i n a t i o n o f t h e carbon monoxide t o t h e surface. Several authors have observed r e c e n t l y such bands i n t h e same area and a s c r i b e d them t o a simultaneous c o o r d i n a t i o n (Table 3 ) .
TABLE 3 : I n f r a r e d s p e c t r a o f C and 0 coordinated carbon monoxide. On rhodium c a t a l y s t s Rh-La/Si02 Rh-Mn/Si02 Rh-Zr/Si02 Rh-V/SiO Rh-Th/Sia2 Rh-K/Si02 Rh/La O3 Rh/Zrt2 Rh/Ce02
: : : : : : : : :
1725cm:; ( r e f . 56) 1 5 3 0 ~ m - (~r e f s . 22,!i1,57,58) ; 1520~rn-~ ( r e f . 65) 1 8 3 0 ~ m - and ~ 1670cm ( r e f . 65) 1760cm w i t h a shoulder a t 1650cm:; ( r e f . 53) broad kvnd between 1300 and 1750cm ( r e f . 59) 1 7 6 0 ~ m - (~r e f . 61) 1 6 8 5 ~ m - (~r e f . 62) 1 6 2 0 ~ m - (~r e f . 63) 1690cm ( r e f . 44) On o t h e r metals
Pd-K/Si02 Ru/A1203
: 1 7 4 0 ~ m - (~r e f . 60) : broad band between 1500 and 1 7 0 0 ~ m - (~r e f . 64)
C and 0 coordinated species a r e evidenced by I . R .
spectroscopy and t h e
simultaneous d e s o r p t i o n o f C02 and CH4 a t lower temperature i s observed on a l l these c a t a l y s t s . The d i f f e r e n c e i n a c t i v i t y o f these c a t a l y s t s a t low promoter loadings i s not absolutely clear. Various p o s s i b i l i t i e s can be invoked. The r e s u l t s i n T.P.D.
show t h a t t h e
s h i f t i n t h e C02 and CH4 d e s o r p t i o n i s n o t as l a r g e w i t h P r than w i t h Z r , Hf o r Th. T h i s can perhaps be due t o t h e f a c t t h a t t h e 0-support bond i s n o t as s t r o n g w i t h Pr as w i t h Z r , H f and Th. The d i s s o c i a t i o n o f CO by t h e promoter on t h e new s i t e would t h e r e f o r e be slower.
However,
our r e s u l t w i t h Lanthana
d i f f e r s from t h a t one obtained by Underwood and B e l l ( r e f . 54) who observed an increase o f a c t i v i t y a t low La loadings. The p r e p a r a t i o n o f our c a t a l y s t i s somewhat d i f f e r e n t , s i n c e Underwood and B e l l added La a f t e r r e d u c t i o n o f t h e catalyst.
The decorat-ion
o f t h e surface would t h e r e f o r e be e a s i e r and t h e
r e d u c t i o n o f La203 t o Laox favoured.
On z i r c o n i a , activity
thoria
desappears
at
and h a f n i a c a t a l y s t s , higher
the
l o a d i n g s whereas
additional
the
alcohol
hydrocarbon producibility
remains c o n s t a n t o r even i n c r e a s e s . T h i s i s i n r e l a t i o n w i t h a c l e a r decrease of
l i n e a r CO d e s o r p t i o n
desorption
i n t h e T.P.D.
peaks i n c r e a s e .
s p e c t r a whereas
T h i s seems t o
indicate that
t h e C02 most
and
CH4
part of
the
m e t a l l i c s u r f a c e i s covered by t h e promoter and t h e h y d r o g e n a t i o n a b i l i t y of t h e c a t a l y s t would d i m i n i s h .
Therefore,
t h e h y d r o g e n a t i o n o f s u r f a c e carbon
which i s v e r y demanding i n hydrogen would become slower and t h e f o r m a t i o n o f C 2 oxygenates would be favoured. T h i s i s i n agreement w i t h t h e s h a r p d i m i n u t i o n o f methane s e l e c t i v i t y on t h e s e c a t a l y s t s . I n conclusion,
C and 0 c o o r d i n a t i o n appears on t h e promoter c o n t a i n i n g
c a t a l y s t s . The appearance o f t h i s s p e c i e s i s l i n k e d t o an enhancement o f C O d i s s o c i a t i o n p r o b a b i l i t y , b u t a t t h e same t i m e ,
t o an i n c r e a s e i n C 2 oxyge-
nates. The f o r m a t i o n o f gem-dicarbonyl s p e c i e s can a l s o be r e l a t e d t o t h i s C and 0 c o o r d i n a t i o n s i n c e an e l e c t r o n t r a n s f e r between Rh and t h e s u p p o r t can be mediated t h r o u g h such a s p e c i e s .
I n our opinion,
e l e c t r o n i c f a c t o r s must be
d i s c a r d e d . B r e a u l t ( r e f . 44) has shown t h a t t h e a d s o r p t i o n band o f l i n e a r CO i s n o t s h i f t e d on Rh-Ce/Si02 recent
comment o f
c a t a l y s t s . T h i s statement i s i n agreement w i t h a
Angewaare e t
al.
(refs.
60,611.
They w r o t e
:
"It i s
n o t e w o r t h y m e n t i o n i n g t h a t t h e presence o f a K-promoter on t h e Pd s u r f a c e s does n o t i n f l u e n c e t h e s i n g l e t o n f r e q u e n c y o f t h e s i n g l e c o o r d i n a t e d CO o n l y i n a r a t h e r l i m i t e d way ( a t most about 10cm-'1. into
account
before s t a r t i n g t o
It i s important t o take t h i s value
speculate
on
the
ligand effects
of
CO
a d s o r p t i o n and/or p r o m o t i o n by potassium compounds".
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6. 7. 8. 9. 10. 11.
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-
D r . H. Miessner Akademie der Wissenschaften der DDR
Z e n t r a l i n s t i t u t f u r physikalische Chemie, BERLIN You explained
the
promoting e f f e c t
of
all
additives
by
the
formation o f C and 0 bonded CO groups w i t h t h e promotor atoms as bonding s i t e f o r oxygen. How do you e x p l a i n then t h e r a t h e r d i f f e r e n t a c t i o n o f t h e promotors used on t h e CO hydrogenation ?
49 1
Response o f D r . J.P. Hindermann : The i n f r a - r e d s p e c t r a o b t a i n e d i n o u r l a b o r a t o r y as w e l l
as by
o t h e r a u t h o r s r e f e r e n c e d i n t h i s paper show t h a t t h e C and 0 bonded s p e c i e s i s p r e s e n t on most o f t h e promoted c a t a l y s t s . C02 and CH4 desorb b o t h a t a l o w e r t e m p e r a t u r e i n t h e TPD experiments and t h i s i s i n agreement w i t h an enhanced CO d i s s o c i a t i o n p r o b a b i l i t y . The a n a l y s i s o f t h e produced hydrocarbons p o i n t s t o a reduced
hydrogenation
ability
of
the
catalysts
with
low
hydrocarbon
p r o d u c t i v i t y s i n c e t h e d r o p i n hydrocarbon f o r m a t i o n happens a t t h e expense of methane.
More,
an i n c r e a s e d C O i n s e r t i o n r a t e i s observed
upon C
and 0
c o o r d i n a t i o n ( 1 ) . Thus t h e d i f f e r e n c e s i n s e l e c t i v i t y and a c t i v i t y may be a t t r i b u t e d t o changes i n t h e k i n e t i c s o f h y d r o g e n a t i o n o f CHx e n t i t i e s and o f CO i n s e r t i o n .
References 1 ) A. Kiennemann, J.P. Hindermann "Keynotes i n Energy r e l a t e d c a t a l y s i s " S. K a l i a g u i n e Ed. Stud. C a t a l . 35, (19881, 181
G.C.
Surf.
Sci.
Bond
Brunel U n i v e r s i t y Department o f Chemistry (UK) In
view
of
your
method
of
catalyst
preparation,
involving
c o - i m p r e g n a t i o n o f metal s a l t and p r e c u r s o r s a l t , I would have t h o u g h t i t v e r y l i k e l y t h a t t h e presence o f t h e l a t t e r m i g h t a f f e c t t h e p a r t i c l e s i z e o f t h e f o r m e r . Have y o u any evidence f o r t h i s ? One a l s o has t o remember t h a t , f o r a g i v e n w e i g h t c o n c e n t r a t i o n o f promoter, t h e e x t e n t t o w h i c h i t c o a t s t h e metal p a r t i c l e s w i l l depend on t h e i r s i z e .
Response o f D r . J.P. Hindermann : We a r e aware o f t h e p o s s i b i l i t y o f d i f f e r e n c e s i n m e t a l p a r t i c l e s i z e i n o u r p r e p a r a t i o n method.
F o r t h e moment we have o n l y measured t h e
p a r t i c l e s i z e by CO a d s o r p t i o n . No s i g n i f i c a n t d i f f e r e n c e s i n m e t a l p a r t i c l e
.
Only a s l i g h t decrease f o r h i g h s i z e measured by t h i s method were observed promoter l o a d i n g s which i s a s s o c i a t e d w i t h t h e appearance i n t h e I R s p e c t r a o f gem-dicarbonyl
s p e c i e s was o b t a i n e d . E l e c t r o n microscopy measurements w i 11 be
performed and changes i n t h e Rh l o a d i n g w i l l b e s t u d i e d i n a near f u t u r e .
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C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
493
HIGH RESOLUTION VIBRATIONAL SPECTRA OF CO ADSORBED ON CLEAN AND POTASSIUM-COVERED PLATINUM (111)
D. HOGE, M. TUSHAUS, P. GARDNER and A. M. BRADSHAW Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4 - 6, D-1000 Berlin 33, F. 8. Germany
ABSTRACT The technique of infrared reflection-absorption spectroscopy (IRAS) is characterised by a n intrinsically high spectral resolution which can be used to full advantage in adsorbate lineshape studies as well as in the investigation of lateral interactions, phase transitions, island growth processes and co-adsorption effects. We illustrate some of these applications in recent studies of the metal-carbon stretch in the system Pt{l11}CO and of the C - 0 stretch in the system Pt{ 111)-COK. INTRODUCTION Vibrational spectroscopy of adsorbates on single crystal metal surfaces using optical excitation has recently received considerable impetus as a result of instrumental developments (refs. 1, 2). The technique, usually referred to as infrared reflectionabsorption spectroscopy (IRAS), has profited particularly from the introduction of a new generation of Fourier transform spectrometers as well as from concomitant advances in detector technology. It is now feasible to monitor a wide frequency range (440 - 4000 cm-I) with a baseline containing less than 0.03 % noise at high resolution (2 cm-') in a reasonable time. Recent studies in this laboratory demonstrate, for example, the importance of high resolution in lineshape studies even for low frequency bands. The measurement of the temperature dependence of such linewidths gives important information on the damping and dephasing processes operating a t surfaces. Although the system Pt(l1l)-CO has been investigated with a large number of experimental techniques, recent IRAS studies (refs. 3, 4) at high resolution and sensitivity have revealed further important details. For instance, the complex question as to the extent and coverage dependence of the dipole-dipole coupling contribution to the C - 0 stretching frequency has been resolved (ref. 3). Further, the dephasing mode for the C - 0 stretch of on-top bonded CO has been identified as the frustrated translation with a frequency near 60 cm-' (ref. 4). In the same study, the relative occupation of on-top and bridge sites, a s determined from their vibrational intensity
494
ratio as a function of temperature, was used to derive a semi-empirical potential energy curve linking the on-top and bridge sites. The metal-carbon stretch of adsorbed CO has hitherto been the almost exclusive domain of electron energy loss spectroscopy or EELS (ref. 5). Indeed, this is true of all adsorbate-derived bands below about 800 cm-'and has been largely a detector problem: the commonly used narrow-band MCT detector only functions down to this frequency. However, the successful observation of the M-C mode by Chiang et al. in the system Ni{lOO}-CO (ref. 6) and by Tobin et al. for Pt{lll}-CO (ref. 7) with IR emission, prompted us to attempt the same experiment in IRAS using a Ge:Cu detector. We thus present here the first observation of the metal-carbon stretch for CO on Pt{lll}. The intrinsic high resolution of IRAS also permits the investigation of lateral interactions, phase transitions, island growth processes and co-adsorption effects. Taking a n example from the latter area, our recent data for CO/K on P t { l l l } show that in addition to several strongly reduced C - 0 stretching frequencies indicative of a strong, short-range interaction the on-top band is shifted by a few wavenumbers, which appears to be due to a longer-range, substrate-mediated interaction between the two species. This point is particularly important, because even among theorists (refs. 8,9) there is a disagreement as to the exact nature of the interaction. We thus extend the EELS work of Crowell et al. (ref. 10) and Pirug et al. (ref. 11) and present some preliminary high resolution vibrational data for Pt{lll}-CO/K which also appears to be the most widely investigated COlalkali metal coadsorption system. EXPERIMENTAL The main features of the apparatus are a UHV-chamber (base pressure < 10 mbar) equipped with LEED and TPD facilities and an evacuable Fourier transform infrared spectrometer. The IR light is focussed via f/5 optics and CsI windows onto the Pt{lll} sample and monitored with either a narrow-band MCT detector or a Ge:Cu detector. Substrate temperatures between 40 and 1500 K could be maintained; they were measured with a Ni-NiCr thermocouple spot-welded to the edge of the crystal. A detailed description of the apparatus is given elsewhere (ref. 2). Potassium was dosed from a carefully outgassed SAES-getter source located at a distance of 2 cm in front of the crystal during dosing. The deposition was performed a t 100 K by cycling with heating periods of 70 sec and intervals of a t least 120 sec. With an increasing number of K-exposures, a series of LEED patterns was observed i n agreement with the work of Schweizer (ref. 12) and Pirug et al. (ref. 11).From the diffraction patterns the potassium coverage was determined. A linear relationship between the number of doses and the coverage derived in this way also enabled the preparation and determination of intermediate coverages.
"'
495
RESULTS AND DISCUSSION The metal-carbon stretch The spectrum in fig. 1 shows the absorption bands from the adsorption system Pt{lll}-CO a t 8 ~ 0 = 0 . 5that are currently accessible with IRAS. The data were taken using a KBr beamsplitter and a Ge:Cu detector. As indicated in the figure, the bands are assigned to the C-0 stretch of on-top (2104 cm-I) and bridge-bonded (1855 cm-l) CO and to the Pt-C stretch of the on-top species (467 cm-I). The KBr beamsplitter was replaced with Mylar foil in an unsuccessful attempt to observe the Pt-C stretch of bridging CO (expected at 380 cm (ref. 5)). The absence of this band implies that its intensity at the centre frequency is below 0.05 % absorption. As can be seen from fig. 1, FT instrumentation allows the measurement of the Pt-C and C - 0 stretches simultaneously. The CO coverage may therefore be determined from the coveragedependent behaviour of the latter which has been established in previous work (ref. 3). Inspection of the band due to the Pt-C stretch reveals a slight asymmetry towards lower frequencies a t low coverages which seems to disappear a t 8 ~ =00.5 when the well ordered c(4 X 2) overlayer is formed. At this particular coverage the linewidth (fwhm) is 4 . 5 ( + l ) c m 1 and thus comparable to that of the internal mode. The coveragedependent shift of the centre frequency is depicted in fig. 2. Both the IRAS values and the corresponding IR emission data of Tobin et al. (ref. 7) are shown. The pronounced red shift measured with IRAS as a function of increasing coverage (starting at -476 cm-I) parallels the observed reduction in the isosteric heat of adsorption (ref. 13). We suggest
p
PtIllll -co 8, = 0.5
0 I
T=95K
~0.004
~0.0001
0 CI
&Q on-top
0 € C 410 I
500
4hO
5jO 1
900
-
bridge I
1
1300 1700 Wavenumber (cm-1)
I
2100
Fig. 1:Infrared reflection-absorption spectrum of CO on Pt{lll} between 440 and 2150 cm.'. KBr beamsplitter, Ge:Cu detector, 2 cm-' resolution: The band assignments are shown.
496
I 1
Pt (11 11
- co
L75-
-i
L70-
Fig. 2:Centre frequency of the metalcarbon stretch for on-top CO on Pt{lll} as a function of increasing coverage. ( 0 ) this : work (2cm-' resolution, 100 K),( 0 ) IR : emission data taken from R. G. Tobin and P. L. Richards (ref. 7) (2.8 cm.' resolution, 275 K).
tl
n
E
465-
L60j
-fo
IR emission T=275K
00
455
1 1
I
01
I
0:2 013 014 015 CO coverage
I
016
that the red shift is largely "chemical" in origin because e* is small (see below) and there can only be a weak contribution from dipole-dipole coupling. Although the IRAS and IR emission results agree at 8 ~ =00.5 (note the different substrate temperatures) they differ somewhat at lower coverages. From the substantially greater fwhm (about a factor of two a t the same instrumental resolution), the pronounced asymmetry of the bands and the spectra of the internal modes it is possible that adsorption a t defect sites and additional inhomogeneous effects may have played a role in the IR emission experiments. The temperature dependence of the Pt-C centre frequency and linewidth is not pronounced. Typically, slight red shifts of 3 cm at €lCo = 0.25 and of -1 cm * a t 0c;o = 0.5 were found simlar to that observed for the corresponding C-0 stretch. In the latter case the shifts are caused largely by relaxation dephasing (ref. 14) and, as mentioned above, the frustrated translation has been identified as the dephasing mode (ref. 2). We suspect that the shift of the Pt-C stretch has the same origin. The direction of shift would then be due to the change in the Pt-C stretching frequency as the dephasing mode is excited, consistent with the notion that the frequency decreases when the molecule is displaced towards a bridge (or even threefold hollow) site. No corresponding increase in linewidth could be detected: any temperature-dependent increase in line-
497
width due to phase relaxation would have to be less than 1 cm-'. Even the effect of the order-disorder transition at -260 K (ref. 4)is not observed. Apart from inhomogeneous effects, the linewidth appears to be largely dictated by electron-hole coupling: damping via multiphonon emission can be ruled out because the Pt-C stretching frequency is still more than twice that of the maximum bulk phonon frequency. We have used the relation between the integrated absorbance and the transition dipole moment to determine the effective charge, e*, from the IR data (see for example (ref. 15)).Taking into account the nominal angle of incidence and the f number in our experiment, we find values of 0.17e for the Pt-C stretch and 1.5e for the corresponding C-0 stretch. These values are in qualitative agreement with predictions of cluster calculations. Bagus et al. (ref. 16), for instance, show for a CuloCO cluster with CO in the on-top site that e* (C-0)=-1.9e but le*(Cu-CO)Ic0.05. In this connection, an important question to address is why the Pt-C stretch is approximately a factor 80 weaker than the C-0 stretch in IRAS, whereas the two bands are of comparable intensity in EELS. Assuming that excitation occurs only via the dipole mechanism and taking into account the angular acceptance of a typical electron energy analyser Ibach and Mills (ref. 17) have derived an expression for the vibrational loss intensity which contains a factor ll(AE)3,where AE =hcT is the loss energy. We thus expect the metalcarbon stretch in EELS to be "enhanced" by a factor of about 85 relative to the C-0 stretch. There is thus no discrepancy here between IRAS and EELS, however disappointing the result may be for IR spectroscopists! Chesters and Sheppard also demonstrate this effect in a recent article (ref. 18) when they compare the IRAS and EELS spectra of cyclohexane in the frequency range 1000 - 3000 cm-'. CO/K coadsorption Fig. 3a shows the effect on the C-0 stretching frequency of dosing CO to a Pt(l11) surface with a potassium coverage 0 ~ = 0 . 2at 100 K. After CO exposure of 0.04.10 mbarss a C-0 stretch frequency at 1390 cm with a shoulder to higher frequency is observed. On the clean surface the (on-top) band is observed initially a t 2090 cm (ref.3). At an exposure of 0.1.10'6 mbares this feature develops into a broad absorption which appears to contain three components at 1390, 1440 and 1510 cm.l. Whereas the first component disappears on further additional CO exposure the latter two shift to higher frequency, Simultaneously, additional bands a t 1560 cm-' and 1740 cm are seen. After an exposure of 0.6.10-6mbar-s the second lowest frequency band has also disappeared and further blue shifts have taken place. Furthermore, a small absorption band has appeared at 2056 cm-l, a frequency which is only slightly lower than on K-free surface. On saturation this feature consists of maxima at 2003 cm-' and 2060 cm-I; all bands below 1600 cm-' have completely disappeared. Similar behaviour 6 In was observed for CO adsorption at all K precoverages investigated (0.02s 6 ~ 0.33). particular, we note that the largest reduction in the stretching frequency is seen at low CO exposure. Increasing CO exposure always leads to the appearance of additional
498
Ptllll]-CO/K T-1OOK
Pt(ll1J-CO/K T=lOOK B,=O.2
b?: 0.001
-
0.2
0.15
00s 1&0
1Lbo
Id00
ld00
I
2000
I
2200
1Lbo
I
I
I
1
1600
1800
2000
2200
Wovenumber (cm-11
Fig. 3:IR spectra after CO adsorption on a Pt(ll1)surface pre-covered with potassium a t T = 100 K as a function of increasing CO exposure. (a) BK =0.2,(b) 8K = 0.06. higher frequency bands, whereas the original ones lose intensity. At a precoverage of 8K20.25, however, no absorption above 1900 cm’ is observed. Conversely, for K precoverages O K 5 0.06no bands below 1500 cm are observed. As an example of a comparatively low K pre-coverage the spectra for 0 ~ = 0 . 0 6 are shown in fig. 3b. The linewidth of the absorption bands also depends on K pre-coverage: they are clearly narrower for low K doses. Absorption features similar to those seen at higher coverage are observed; the splitting of each feature into several bands actually suggests that the broad bands a t higher K coverage also result from a superposition of sharper peaks. At the lowest CO exposure three bands a t 1536, 1568 and 1596 cm are present in fig. 3b. With increasing exposure the latter band shifts to 1605 cm a t 0.1-10 mbarss and remains constant in frequency on further CO uptake whereas the others disappear. At a CO exposure of 0.15-10 mbar-s six additional and 2083 cm The feature a t 1782cm * bands are seen at 1645,1673,1703,1782,2057
499
its shoulder indicates that it consists of two bands - grows until an exposure of 0.2.10-6 mbares is reached and then remains constant up to the saturation coverage of 1OL. CO exposure beyond 0.3.10-6 mbar-s causes an increase in intensity and a slight blue shift of the highest frequency peak which occurs only 10 cm” lower in frequency than that of on-top CO on clean Pt{lll}. At an exposure of 0.5.10-6mba~sanother band a t 1841 cm’ appears which is very similar to bridge-bonded CO on the K-free surface; this grows in intensity until the saturation exposure of 10.10-6mbar-s. The spectra a t such low K pre-coverages suggest that several different strongly perturbed CO species coexist on the surface together with CO molecules which are only weakly influenced by the presence of the alkali metal. The same general conclusion appears to hold for high K pre-coverages except that the bands due to weakly perturbed CO are shifted slightly lower in frequency and considerably broadened. To obtain more information on this effect potassium was dosed onto a surface precovered with CO and the band due to the on-top C-0 stretch monitored as a function of K exposure. Fig. 4 shows that an almost linear decrease in frequency occurs accompanied by a monotonic, clearly not linear, increase in the linewidth. Attempts were also made to investigate the metal-carbon stretch in this system. Only the M-C band of the weakly perturbed CO could in fact be observed; it was also investigated by dosing potassium to the COprecovered surface. At 0co=0.3 the band is located a t 475 cm-l and surprisingly does not change in position as the potassium coverage is increased. The corresponding C-0 stretch shifted from 2096 cm-I for the clean surface to 2081 cm for higher K coverages, a t which point the intensity of both the M-C and C-0 stretches are already reduced drastically and the M-C band is no longer measurable. The EELS data of Pirug et al.
’
Fig. 4: Peak width (fwhm) and peak position of the C-0 stretch of on-top bonded CO on Pt(ll1) as a function of potassium exposure to a Pt(ll1) surface pre-covered with CO at T = 100 K. Note that one K dose gives a coverage of approximately 0.03 monolayer on a CO-free surface. Error bars are only shown when they exceed the size of the points drawn.
i I
E
““1
u mgo a
K exposure (doses)
500
(ref. 11)show that the M-C stretch associated with the strongly perturbed species is expected near 350 cm-' i. e. outside the accessible spectral range of our instrument at present. It appears to be very strongly dependent on coverage: For a K pre-coverage of 0.09 it shifts from 350 cm' a t very low CO coverages to 430 cm" and subsequently disappears. A new band a t 310 cm-' shifting to 370 cm-' is then apparent as the CO coverage is further increased. The M-C stretch associated with the less strongly perturbed species appears in their spectra a t 520 cm The corresponding C - 0 stretch is also higher than ours a t 2100 cm-'. Comparing these results with previous EELS work, the following picture emerges. The EELS data of Crowell et al. (ref. 10) were taken with CO saturation coverage at 300 K. Apart from differences of up to 50 cm I in the peak positions, the agreement with our saturation coverage data at 100 K is reasonable bearing in mind their resolution of 85 cm I . The agreement with the more comprehensive EELS work (resolution 50 cm I ) of Pirug et al. (ref. 11)is generally good; these authors also made measurements with increasing CO coverage for different K-precovered surfaces mainly at 110 K. In addition to the details revealed by the much higher resolution of the present work, the only important discrepancy is the fact that they observe no absorption above 1900 cm €or 8 ~ > 0 . 1 1 ,whereas we observe a band a t 2050 cm' even at 8 ~ = 0 . 2 .Contrary to Crowell et al. but in agreement with Pirug et al. we interpret the strongly reduced CO stretch frequency below 1820 cm as due to a short-range interaction between K and CO. The appearance of a weakly perturbed CO band between 2050 and 2100 cm in spectra with B ~ 5 0 . 2is symptomatic of a long range interaction mediated by the substrate. Note that the presence of only 0.03 monolayers of K is sufficient to shift the on-top band by 4 cm-I to lower frequencies and to almost double the linewidth (from 6 cm I to 11 cm I - see fig. 4).A detailed explanation of the various C - 0 bands due to the strong, short-range interaction is very difficult a t present until the evaluation of existing data has been completed and further experiments have been performed. Pirug et al. observed only four bands in the region below 1820 cm and explained this number in terms of the local C O K stoichiometry. In this picture it is clear that the lowest C - 0 stretching frequency is expected a t high K and low CO relative coverages and that on further CO exposure sucessively higher frequency bands appear. Unfortunately, the observation of eight or nine bands below 1820 cm cannot be explained by this model, a t least not in its present form. The present data do, however, throw source light on a controversy concerning the adsorption system Pt(ll1)-COK. Dose et al. (ref. 19) in an inverse photoemission study distinguished between two sorts of CO. The first species (observed a t low relative CO concentration) gives rise to a n*-derived feature a t 3 eV or lower above the Fermi level, Ep. This shifted further away from Ep as the CO coverage was increased. At even higher CO coverages a feature at -4.2 eV above Ep appeared indicative of a second species similar in, CO on the K-free surface, for which the n*-derived feature is found a t 4.3 eV. This interpretation was rejected by Bonze1 et al. (ref. 20) who claimed that the
'.
'
'
501
double peak structure is due to a splitting of a n*-derived orbital due to direct bonding between CO and K. Whilst this splitting due to symmetry lowering (also a feature of the electrostatic description in ref. 19 and clearly observed in photoemission e. g. ref. 21) certainly occurs, it does not explain the two peak structure in inverse photoemission. The present data distinguish between the strongly perturbed CO and the weakly influenced CO and can be correlated with the inverse photoemission data for five different coverages between O K = O and 0.15. The original explanation of the inverse photoemission spectra that the two peak structure is caused by sequential filling of adsorption sites is substantiated. We reserve comment on the bonding model of Bonzel et al. (ref. 20)until a later paper. CONCLUSION We have demonstrated in this short account how the recently improved sensitivity of IRAS can now be used to obtain adsorbate vibrational spectra in the range 440 4000cm-'.In particular, it has been shown that low frequency vibrations - in the present case the metal-molecule stretch of adsorbed CO - can be detected. A full characterisation was, however, not possible because of the inherent low oscillator strength. We also point out that typical EELS spectra give a misleading impression of the strength of such bands. Soft vibrations are very often (but not always) of low intensity, so that a further improvement in the sensitivity of IRAS is still desirable. In the second application described in this paper - coadsorption studies of CO and K - i t is demonstrated how the superior resolution of IRAS reveals a considerable number of extra details not seen in EELS. On the other hand, the additional information is found to complicate the interpretation considerably! ACKNOWLEDGEMENTS This work has been supported by the Deutache Forschungsgemeinschaft through the Sonderforschungsbereich6-81and the Fonds der chemischen Industrie. REFERENCES
1 Y.Chabal, Surf. Sci. Re 8 (1988)211-357. 2 A. M.Bradshaw and E. khweizer, Adv. Spectroscopy, 23 (1988)413-483. 3 M.Tiishaus, E.Schweizer, P. Hollins and A. M. Bradshaw, J. Electr. Spectr. Rel. Phen., 44 (1987)305-316. 4 E. Schweizer, B. N. J. Persson, M. Tiishaus, D. Hoge and A. M. Bradshaw, to be published. 5 H. Steininger, S. Lehwald and H. Ibach, Surf. Sci., 123 (1982)264-282. 6 S. Chiang, R. G. Tobin, P. L. Richards and P. A. Thiel, Phys. Rev. Lett., 52 (1984) 648-652. 7 R. G.Tobin and P. L. Richards, Surf. Sci., 179 (1987)387-403. 8 1. K.N#rskov, S. Holloway and N. D. Lan Surf. Sci., 137 (1984)65-78. 9 P. J. Feibelman and D. R. Hamann, Surf.bci., 149 (1985)48-66. 10 J. E. Crowell, E. L. Garfunkel and G. A. Somorjai, Surf. Sci., 121 (1982)303-320. 11 G. Pirug and H. P. Bonzel, Surf. Sci., 194 (1988)159-171;Surf. Sci. 199 (1988)371390.
602
12 E. Schweizer, Dissertation, Freie Universitat Berlin (1986). 13 G. Ertl, M. Neumann and K. M. Streit, Surf. Sci., 64 (1977) 393-410. 14 B. N. J. Persson and R. Ryberg, Phys. Rev., B24 (1981) 6954-6970. 15 B. N. J. Persson, Sol. State Comm.,30 (1979) 163-166. 16 P. S. Bagus, C. J. Nelin, W. Miiller, M. R. Philpott and H. Seki, Phys. Rev. Lett., 58 (1987) 559-562. 17 H. Ibach and D. L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York (1982) p. 121. 18 M. Chesters and N. Sheppard, Adv. Spectroscop , 2 3 (1988) 377-416. 19.V. Dose, J. Rogozik, A. M. Bradshaw and K. C. Ibnce, Surface Sci., 179 (1987) 90-100. 20.H. P. Bonzel, G. Pirug and R. P. Messmer, Surface Sci., 184 (1987)L415-L420. 21.M. Surman, K. C. Prince, L. Sorba and A. M. Bradshaw, Surface Sci., in press.
C. Morterra,A. Zecchina and G.Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
503
A PARTICLE SIZE EFFECT IN THE OXYGEN DESORPTION FROM PLATINUM SUPPORTED WITHIN A FAUJASITE MATRIX
N.I. JAEGER, A.L. JOURDAN,
G.
SCKULZ-BKLOFF and A. SVENSSON
Institut fiir Angewandte und Physikalische Chemie FB 2, UNIVERSITAT BREMEN, D-2800 Bremen 33, FRG
ABSTRACT Platinum-faujasite catalysts with significantly different and at the same time narrow particle size distribution between 1-8 nm are characterized by electron microscopy, electron diffraction, Xray powder diffraction and Nz-physisorption. The thermal desorption of oxygen was studied in the temperature range 300 - 900 K. A shift to higher temperature of the desorption maxima is found with decreasing average particle size. During consecutive adsorption/desorption cycles a shift of the desorption maxima to lower temperature occurs.
INTRODUCTION The importance of platinum in heterogeneous catalytic oxidation reactions has led to the investigation of oxygen adsorption as an elementary step by many authors. While most experiments were carried out on low index platinum single crystal surfaces (1-12) or on polycrystalline material (13,141 only few results are available for supported platinum catalysts (8) in spite of their widespread and growing practical application. Chemisorption studies on highly dispersed platinum phases are therefore justified even though they are complicated by possible metal support interactions and by a lack of knowledge regarding the structure of very small metal crystals. The use of a faujasite as a support allows the preparation of distinctly different metal dispersions which are embedded within the zeolite matrix and remain fully accessible to chemisorbing molecules (15-17). Due to the inclusion of the platinum crystals within the zeolite lattice the application of most physical methods for surface analysis is rendered more difficult.
504
In the following results obtained by thermal desorption spectroscopy (TDS) will be evaluated and discussed. The method is well suited for the investigation of zeolite supported metals.
EXPERIMENTAL Sample preparation Iron free faujasites NaX (Si/Al ratio 1.2) and NaY (Si/Al ratio 2.76) were prepared by hydrothermal crystallization (18). Platinum was introduced as the tetra-ammine-ion complex from aqueous solutions of the chloride. The loaded zeolites were washed until chloride free and dried at 353 K. The autoreduction of the specimens was carried out using a temperature program (1 - 5.75 K/min) in either a streaming gas medium (argon, oxygen) or under vacuum (15,19,20). In the case of samples treated under oxygen the reduction was completed with hydrogen at 573 K for 1 h (21). The materials investigated, the reduction treatment and the surface average diameter of the platinum are listed in Table 1. The surface average diameter XS = ni di / ni di was calculated from the histograms of the platinum dispersions which in turn were evaluated from electron micrographs (Table 2). For the chemisorption experiments the catalyst was pressed (1 GPa, 30 s ) , ground up again and sieved to 0.45 - 0.71 mm grains. The crystallinity of the samples was checked by X-ray diffraction, Nz-physisorption and selected area electron diffraction (15). No global or local collapse of the zeolite lattice could be observed even in the case of metal particles growing beyond supercage dimensions. Electron microscopy and electron diffraction A suspension of the fine crystalline platinum loaded zeolite powder was dropped onto carbon support films on copper grids and allowed to dry. The micrographs and the selected area electron diffraction patterns were taken with a Philips EM 420 electron microscope at 120 k V (15). The dark field technique was used to overcome phase contrast problems connected with the identification of particles less than 2 nm in size ( 2 2 ) . Thermal desorption sp_ectr_oscopy The temperature programmed desorption (TPD) was carried out in a stainless steel HV-chamber equipped with a quadrupole mass spectrometer CQ200, Leybold-Heraeus), a turbo molecular pump
505
TABLE 1 Materials, treatment and surface average diameter & of the platinum dispersion catalyst
degree of Pt-ion exchange in atom %
Pt 12x/02
12
Ptl7Y/02
17
Pt42X/V
42
Ptl2X/V
12
Ptl7Y/Ar
17
Ptl2X/Ar
12
reduction treatment
3, in nm
heated to 6 7 3 K at 1 - 2 K/min in flowing 0 2 , held 5-8 h followed by treatment in flowing H2 at 5 7 3 K for 1 h
1,1
vacuum dehydration followed by heating at 5 , 7 5 K/min to 8 9 3 K, held 15 min
1,6
1,6
2,8
heated to 6 7 3 K at 1-2 K/min in flowing argon, held 4-5 h
(Turbovac 220, Leybold-Heraeus), an ionisation gauge and a controlled leak valve. The base pressure was about Pa. The instrument was calibrated with oxygen to convert the partial ion current into the partial pressure. The electrical furnace inside the HV-chamber could be heated up to 893 K. The heating rate was in general 5.75 K/min. The sample [5 - 15 mg1 was introduced into the furnace in the HV-chamber via a first vacuum chamber equipped with a gas inlet valve. To ensure that no loss of the sample occurred during the transfer, the weight was controlled before and after the experiment. The mass spectrometer, thermoelement and ionisation gauge were connected with a PDP11-computer (DEC). Every minute a complete mass spectrum in the range of 1 to 60 amu was recorded, so it could be checked wether further gases desorb from the sample. Besides an unavoidable water desorption from the zeolite and the normal background in this vacuum range no further substances were detected. The desorption curve was evaluated and smoothened by a cubic spline function (23). In a first temperature desorption the samples stored in air were heated up to 893 K, thereby removing the main part of the water at the same time. The vacuum reduced samples were used directly after reduction in the HV-chamber. The subsequent TPD cycles for the samples reduced in A r or 0 2 and the first TPD cycle
506
TABLE 2 Typical electron micrographs (bright field, 1:300 000) and measured particle size distributions. The bars represent integer numbers.
particle %
I
mass %
0
surface %
1: Ptl2X/02
2: PtlIY/02 3: Pt42X/V 4 : Pt12X/V 5: PtllY/Ar 6: Ptl2X/Ar
507
for the vacuum reduced samples were carried out as follows. After cooling down to room temperature the Pt-catalysts were transported into the first vacuum chamber and there exposed to dried oxygen at l o 5 Pa for 1 h. Thereafter they were transferred back into the HVchamber and kept there for 2 h until the pressure was lower than 4 ~ 1 0 -Pa~ before starting the TPD cycle. With this adsorption condition equal coverage for all samples was assured (24).
RESULTS The temperature programmed desorption spectra for oxygen obtained for the samples listed in Table 1 are plotted in Fig. 1. No oxygen desorption could be observed from a blank NaX sample. The position of the desorption maximum and the shape of the curves vary in dependence on the catalyst in the temperature range between 500 and 900 K. Residual water still adsorbed on the Ptzeolite has no effect on the shape of the desorption spectrum or on the metal particle size distribution following the desorption cycle. This could be demonstrated by adsorption experiments with oxygen saturated with water at ambient temperature in agreement with observations that molecular bound water desorbs from Pt (111) and (110) surfaces already between 100 and 200 K (25,26). Oxygen evolving from the catalytic decomposition of water can be excluded since no hydrogen could be detected. The nearly linear shift of the maximum of the oxygen desorption to lower temperatures with increasing surface average diameter of the platinum crystals is depicted in Fig. 2. The results of repeated adsorption/desorgtion cycles are shown in Fig. 3a and 3b. The desorption maxima shift to lower temperatures from the first to the second cycle. The position of the desorption peak remains constant thereafter. A comparison of the histograms of the platinum dispersion after the first adsorption/desorption cycle is presented in Table 3 for two samples. The histograms reveal a slight growth of the metal crystals (see Table 1). The desorption curves plotted in Fig. 1 for Ptl2X/02 and Pt17Y/02 were obtained for the second adsorption/desorption cycle. No oxygen desorption could be observed during the first cycle for these catalysts. All samples were characterized by X-ray diffraction, electron microscopy and a selected area electron diffraction. Fig. 4 shows
508
o PflZXlAr 121
.
m
0
0
RlZXIAr i l l
P f i n i A r III Rl2XIV
111
wnx/v
111
R L Z X I V I21
.
a RL2XIV i l l Pt12Xlo, 121
T/K
T/K
Fig. 1. Oxygen desorption from platinum single crystals with a surface average diameter ranging from 1-5 nm. The desorption peak shifts to higher temperature with decreasing particle size. rD: desorption rate in 10-6 rnol/mol Pt 1: Ptl2X/Oz 2: Ptl7Y/Oz 3: Pt42X/V 4 : Ptl2X/V 5: PtllY/Ar 6 : Ptl2X/Ar
2 73
473
T/K
673
873
Fig. 2. Plot of the surface average diameters versus the desorption peak maxima. Ptl2X/Oz and Pt42X/V show two desorption peaks. The range from the first to the second maximum is marked, respectively. (1): first, ( 2 ) : second adsorption/desorption cycle
273
67 3
413
873
T/K
Fig. 3 a . (Pt42X/V) Fig. 3b. (Ptl2X/Ar) Shift of the desorption peak for consecutive adsorption/desorption cycles. After the first desorption the peak shifts to lower temperature, then its position is nearly fixed. rD: desorption rate in mol/mol Pt an electron diffraction pattern of Ptl2X/V following the temperature programmed desorption of oxygen. The typical orientation of the platinum single crystals with respect to the
509
zeolite lattice (15) is maintained. The formation of bulk platinum oxide could not be observed in the experiments. TABLE 3 Particle size distribution and surface average diameter for 2 representative samples following 3 adsorption/desorption cycles. The bars represent integer numbers. particle %
mass %
surface %
catalyst
as
particle size distribution
Pt4 2x/v
Ptl2X/Ar
in nm
2.0
5.3
l
20
1
2
3
4
nm
5
6
A& 7
8
DISCUSSION
The main results which have to be explained are the temperature range of the desorption maxima and their shift to lower temperatures with increasing metal particle size. The temperature range of the maxima in the experimentally observed desorption spectra is indicative for the chemisorption of atomic oxygen. This could be expected for chemisorption at ambient temperature even though the exposure to oxygen was very high. No indication of bulk oxide or subsurface oxide formation could be extracted from the electron diffraction patterns. The formation of a subsurface oxide should have been detected in the diffraction
510
20
-
22 0
Fig. 4. Selected area diffraction pattern from P t l 2 X / V with Bragg reflections from the platinum particles and the zeolite matrix. The diagram shows the reflex of the [110] zones of the zeolite and the platinum single crystals. The orientation relationship is *100>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
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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
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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.
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
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)
This Page Intentionally Left Blank
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 wt%) whereas the transmission mode ( i n t e n s i t y expressed i n absorbance u n i t s ) appeared t o be more useful f o r h i g h s i l i c a conc e n t r a t i o n s (>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 (%<2 w t % ) .
1,000-
0,800.
0,600-
0,400-
0,200-
o,ooo/
0
5
10
15
20
Concentration of oxygen ( wt % )
F i g . 12. The peak h e i g h t s (absorbance u n i t s ) a t 850 ( 0 ) and 1100 cm-1 (A) f o r S i c UF 15 heated a t l l O O ° C d u r i n g 3 h were p l o t t e d a g a i n s t t h e c o n c e n t r a t i o n o f S i c i n KBr.
F i g . 13. The r a t i o o f t h e peak h e i g h t s a t 1100 and 850 cm-l plotted against the wt% o f oxygen i n S i c .
GENERAL DISCUSSIONS AND CONCLUSIONS The r e s u l t s show t h a t FTIR s p e c t r o m e t r y i s a u s e f u l means o f a n a l y z i n g t h e oxygen c o n t e n t o f f i n e grade commercial a-Sic powders. The e x t e n t o f s u r f a c e o x i d a t i o n c o u l d be e s t i m a t e d f r o m t h e peak h e i g h t s o f t h e Si-C-Si band ( 8 5 0 cm-l) and t h e S i - 0 - S i band (1100 cm-1).
The DRIFT mode ( u s i n g Kubelka-Munk
t h e o r y ) was found t o be p a r t i c u l a r l y s u i t a b l e f o r e s t i m a t i n g l o w concentrat i o n s o f s i l i c a and t h e s p e c t r a were n o t e f f e c t e d b y t h e powder p a r t i c l e s i z e (1-3 pm) s i n c e t h e y were g e n e r a l l y below t h e wavelength o f t h e a b s o r p t i o n bands
(8-9 pm t o 11-12 pm r e s p e c t i v e l y f o r Si-0-Si and S i - C - S i ) .
The t r a n s m i s s i o n
mode ( u s i n g Beer-Lambert's l a w ) was f o u n d t o be more u s e f u l f o r e s t i m a t i n g h i g h e r S i 0 2 c o n c e n t r a t i o n s and was r e l a t i v e l y i n s e n s i t i v e t o S i 0 2 concentrat i o n s below 2 wt%. A l s o DRIFT was more c o n v e n i e n t and does n o t r e q u i r e s p e c i a l
558
powder p r epa r a t ion. Using the q u a n t i t a t i v e approach developed i n t h i s p r e l i m i n a r y study we are proceeding t o i n v e s t i g a t e the o x i d a t i o n k i n e t i c s and a c t i v a t i o n energies o f S i c powders over a range o f temperatures. This w i l l be reported i n a f o r t h -
coming p u b l i c a t i o n . ACKNOWLEDGEMENT The authors would l i k e t o thank Prof. P. Stenius f o r discussions and c r i t i c a l comments on the manuscript and the Swedish Technical Board f o r Development, f o r f i n a n c i a l support. Also KemaNord, Sweden f o r LECO a n a l y s i s o f powders. REFERENCES
1 Standard Methods o f Chemical Analysis o f S i c Abrasive Grains and Abrasive Crude, American National Standard I n s t i t u t e , B74.15 (1971). 2 Methods f o r Chemical Analysis o f S i c Abrasives, Japanese I n d u s t r i a l Standard R6124 (1980). 3 K. Motzfeldt, Acta Chemica Scand. 18 (1964) 1596-1606. 4 G. Socrates i n John Wiley & Sons ( E d i t o r ) , I n f r a r e d C h a r a c t e r i s t i c Group Frequencies, (1980). 5 B.E. Deal and A.S. Grove, J. Appl. Phys. 36(12) (1965) 3770. 6 J.A. Costello and R.E. Tressler, J. Am. Ceram. SOC. 64(6), 327. . . (1981) . 7 G. Ervin, J. o f Am. Ceram. SOC., 41(9), (1958) 347-352. 8 V. L o r e n z e l l i e t al., IUPAC Conference (May 1987) Sofia, Bulgaria, 40-52. 9 A. Tsuge e t al., Applied Spectroscopy, 40(32 (1986) 310-313.
C. Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactivity of Surfaces 01989 Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands
ELECTRON MICROSCOPY
AM)
559
MICRODIFFRACTION STUDY OF THE INTERACTION
OF Pd WITH Si02
R . LAMBER', N. JAEGER' and G . SCHULZ-ELOFF'
'Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wroclaw 2 (Poland) 'Institut fur Angewandte und Physi kalische Chemie, Universitlit Bremen, Postfach 330 440, D-2800 Bremen 33 (F.R.G.) ABSTRACT
The annealing of small palladium particles on silica thin film substrates i n hydrogen above 840 K leads t o the formation of a Pd2Si phase as could be
shown by electron microscopy, electron diffraction and microdiffraction. The Pd2Si phase grows epitaxial ly with respect to the palladium crystal 1 i tes. I t was observed t h a t the lattice parameters or/and the hexagonal structure of the Pd2Si precipitates are altered or distorted. Sometimes the distortion of the Pd fcc structure was also observed. Thicker Pd2Si crystallites formed a t high;?emperatures attain a structure close t o the bulk due to the less pronounced influence of the Pd/Pd2Si interface on the growth of the intermetallic phase. In some cases the intermediate phase formed between Pd and Si02 appears to be nearly amorphous, but i t i s crystallized slowly under the electron radiation. INTRODUCTION Metal dispersion supported on a porous ceramic substrate represent an important class of materials widely applied in heterogeneous catalysis. I t was demonstrated experimentally that the nature of the support can strongly affect the catalyst (refs. 1-5). The term "strong metal-support interaction" was introduced in 1978 to describe the drastic changes in the chanisorption properties of group VIII noble metals that were observed when supported on titanium oxide (refs, 6,7). Although i t was claimed that only transition metal oxides such as Ti02 give rise to strong interactions with metals (ref. 7 ) there i s now increasing evidence for similar phenomena involving nontransition metal oxides such as the widely used silica (refs. 8-16) and alumina (refs. 17-23). In a previous study (ref. 16) i t was shown that heating o f the palladium-silica system in a hydrogen atmosphere may lead to chemical interaction between metal and support and the growth of palladium silicide a t the metal-support interf ace. The present paper gives evidence: ( i ) of the formation of a Pd2Si phase, ( i i ) of the alteration of the l a t t i c e parameters or/and the distortion of the structure of the Pd,Si precipitates and the Pd crystallites i n the early stage
560
o f the palladium s i l i c i d e growth, ( i i i ) o f an amorphous Pd-Si phase formation, and ( i v ) o f the c r y s t a l l i z a t i o n o f a w e l l ordered Pd,Si phase f o l l o w i n g the heat treatment a t 875 K. EXPERIMENTAL Thin f i l m SiO, substrates were prepared by o x i d a t i o n o f S i O f i l m s a t 900 K i n a i r f o r 30 h. The l a t t e r were obtained by sublimation i n a vacuum o f S i O onto a NaCl s i n g l e c r y s t a l . Palladium was deposited by vacuum evaporation from a r e sistance heated tungsten boat. The EM g r i d s w i t h t h e Pd-Si02 specimens were placed i n a resistance heated tantalum holder located i n t h e vacuum b e l l j a r . The tantalum holder allowed the heating o f t h e EM g r i d s w i t h t h e SiO, supports before and a f t e r Pd evaporation. Thermal treatment o f t h e Pd-Si0, system was c a r r i e d o u t i n hydrogen ( 6 x I O - , Pa) supplied a t a constant f l o w r a t e . A l l specimens were examined i n a P h i l i p s EM 420 transmission e l e c t r o n microscope operated a t 120 kV and equipped with an energy d i s p e r s i v e X-ray analyzer (EDX). With EDX no contaminants were detectable i n the i n v e s t i g a t e d specimens. RESULTS E l e c t r o n micrographs o f t h e Pd-50, specimens as obtained a f t e r t h e thermal treatment i n hydrogen a t 820 K f o r 10 h, 845 K f o r 90 h, and 875 K f o r 60 h a r e shown i n F i g . 1 , Following heat treatment a t 820 K palladium p a r t i c l e s w i t h d i a meters i n the range 2 -7 m a r e formed (Fig. la ) . Heat treatment a t 845 K r e s u l t e d i n s m e changes i n t h e p a r t i c l e morphology and t h e formation o f a consid-
F i g . 1 . E l e c t r o n micrograph o f Pd on Si02 substrates f o l l o w i n g heat treatment i n hydrogen: a ) a t 820 K, 10 h ; b) a t 845 K, 90 h ; c ) a t 875 K, 60 h
.
561
erable number o f p a r t i c l e s w i t h sizes i n t h e range o f 7 -15 nn (Fig. 1 b). Some p a r t i c l e s reveal t h e presence o f f r i n g e s w i t h spacings exceeding the Pd l a t t i c e f r i n g e spacing by far. With the instrument used t h e Pd l a t t i c e f r i n g e s could n o t be resolved. The appearance o f Moire f r i n g e s due to overlapping Pd and Pd2Si c r y s t a l phases was discussed i n a previous paper ( r e f . 16). S i m i l a r r e s u l t s were obtained f o r 875 K. The e l e c t r o n diffractograms obtained from these specimens a r e shown i n F i g , 2.
F i g . 2 . Selected area d i f f r a c t i o n patterns f o r specimens heated a ) a t 820 K, 1 0 h ; b) a t 845 K, 90 h ; c ) 875 K, 60 h
.
Samples heated a t 820 K show the presence o f d i f f r a c t i o n r i n g s c h a r a c t e r i s t i c f o r f c c c r y s t a l l i t e s i n random o r i e n t a t i o n s (Fig. 2a). A f t e r heating a t 845 K the i n t e n s i t y o f t h e f c c phase r i n g s i s attenuated and i n t h e v i c i n i t y o f the Pd (111) r i n g an a d d i t i o n a l d i f f r a c t i o n r i n g corresponding t o an i n t e r p l a n a r spacing d = 0.237 m i s v i s i b l e (Fig. 2b). Furthermore two v e r y scattered a d d i t i o n a l r i n g s a r e v i s i b l e between t h e Pd (111) Ed = 0.225 rm3 and Pd (200) [ d = 194 nnl r i n g s (Fig. 2b). The e l e c t r o n d i f f r a c t o g r a m obtained a f t e r thermal treatment a t 875 K i s shown i n F i g . 2c. Measured i n t e r p l a n a r spacings a r e presented i n Table 1. They correspond t o the Pd2Si i n t e r m e t a l l i c compound and t o pal 1a d i um For f u r t h e r c h a r a c t e r i z a t i o n o f t h e Pd-SiO, system heated a t 845 K the method o f convergent beam d i f f r a c t i o n was employed. The m i c r o d i f f r a c t i o n p a t t e r n s can be explained by t h e formation o f a Pd,Si phase growing a t t h e metal support i n t e r f a c e . I n Fig. 3 an example f o r the superposition o f the Pd and t h e Pd,Si d i f f r a c t i o n p a t t e r n s i s presented. The d e t a i l e d a n a l y s i s o f F i g . 3a as g i v e n i n Fig. 3 b r e s u l t e d i n t h e f o l l o w i n g o r i e n t a t i o n r e l a t i o n s h i p s between Pd2Si and 11 [I113 Pd and (2201) Pd,Si 11 (220) Pd. I n some cases Pd: [8 17 ZEl Pd,Si the l a t t i c e parameters o f t h e Pd,Si p r e c i p i t a t e s were s l i g h t l y a l t e r e d or/and t h e hexagoml Pd,Si and t h e f c c Pd s t r u c t u r e s were d i s t o r t e d . This behaviour i s i l l u s t r a t e d by the m i c r o d i f f r a c t i o n p a t t e r n shown i n F i g . 4a. The p a t t e r n i n d i -
.
562
TABLE 1 Measured interplanar distances d (nm) f o r specimen heated a t 875 K i n comparison w i t h interplanar distances reported f o r palladium and Pd,Si 1243. Pd,Si has a hexagonal superstructure w i t h a = 1.3055 nm and c = 2.749 nm.
pal 1adium
d [nml
(hkl)
d [nml
11 1
0.2246
200
0 .I 945
palladium s i l i c i d e
.
exp 0.270 0.237 0.230 0.226 0.218 0.212
0-196 0.188 0.179
d [113133 0.2811 0,2366 0.2290 0.2240 0.2182 0.2136 0.2082
(hkl) 400 228 201 1 1 oll;-502 408 240 3012, 243
0 .I884 0.1815
600 248
Po2
b) .
C
I
@
0'22 .
.
.
@
.
.
...*..... ......... 2736
000 2101 @ . . . @ . . . @
22 0
F i g . 3 . Microdiffraction pattern obtained from a Pd p a r t i c l e with epitaxially grown PdzSi a t the metal-support interface. a ) The orientation i s as follows: P d [ l l l l , PdZSi [8 17 25181. b) Interpretation of Fig. 3a: open c i r c l e s represent palladium r e f E c t i o E the ( 1 1 1 ) reciprocal l a t t i c e section, f u l l circlesthe (8 17 - '25 18) Pd2Si reciprocal l a t t i c e section.
-
-
cates the presence of the palladfum c r y s t a l l i t e in the 12331 orientation. Comparison of the diffractogram w i t h the (233) Pd reciprocal l a t t i c e section shown in F i g . 4b reveals some distortion of the palladium f c c structure. Even though the accuracy of the determination of the interplanar spacings from the microdiffraction patterns is n o t h i g h due to the large diameters of the spots the observed distortion c l e a r l y exceeds the e r r o r of the measurement. In F i g . 4b the super-
563
Fig. 4. Microdiffraction pattern obtained from a p a r t i c l e w i t h epitaxially grown PdzSi a t the metal-support interface. a ) The orientations a r e a s follows: Pd 12331, PdgSi [ 6 -i-ir 4 13. b ) Interpretation of Fig. 4a: large f u l l c i r c l e s the (233) Pd reciprocal l a t t i c e section, large open c i r c l e s the observed Pd reflections, small f u l l c i r c l e s - the (6 7u 4 1 ) PdzSi reciprocal l a t t i c e section, small open c i r c l e s the observed Pd$i reflections.
-
-
position of the [6 m 4 I]Pd2Si l a t t i c e section and the observed Pd,Si spots a r e sbwn. I t i s seen t h a t f o r the reason of not completely commensurable l a t -
distances '(0 2 Z)Pd and
'(1
0
'i z ) P d 2 S i
[d(l 0
7
z)Pd2Si
6*d(0
5
2)Pd
F i g . 5. Microdiffraction pattern obtained from p a r t i c l e i n an "amorphous s t a t e " . Under the electron radiation some diffraction spots appeared.
1
564
the l a t t i c e parameters of the Pd,Si precipitates a r e s l i g h t l y altered or/and the hexagonal structure i s distorted. I t i s interesting to note t h a t the microdiffraction patterns revealed in some cases the presence of two o r three diffuse diffraction rings only (F'ig. 5). Under the electron radiation the appearance of some diffraction spots was observed. This observation could indicate t h a t the reaction of Pd p a r t i c l e s w i t h the SiO, substrate can lead in some cases t o the formation of an amorphous Pd-Si phase and to the crystallization of this phase induced by the electron radiation. DISCUSSION Electron diffraction and microdiffraction analysis provides evidence f o r strong, chemical interaction between palladium and s i l i c o n dioxide. The reduct i o n of Sid, and the formation of palladium s i l i c i d e most probably occur due t o the presence of atomic hydrogen produced by the dissociative adsorption of H, on Pd . The microdiffraction pattern ( F i g . 3 ) showed a well crystallized palladium s i l i c i d e phase following the h e a t treatment a t 845K. On the other h a d the selected area diffraction pattern ( F i g . 2 ) showed only two or three strongly scattered Pd2Si diffraction rings. This inconsistency could be explained by an a l teration of the Pd,Si and Pd l a t t i c e parameters and/or by a distortion of their structures. For Pd crystals in the [I111 orientation (the (111) plane oriented parallel to the substrate) a good matching between the palladium and the palladium s i l i c i d e l a t t i c e planes can be obtained ( F i g . 3 ) . For some orientations of the Pd crystals a good matching o f the atomic rows across the interface might require a s l i g h t a l t e r a t i o n of the l a t t i c e parameters of the Pd,Si precipitates and the Pd c r y s t a l l i t e s (Figs. 4 a , b ) . The distortion of the PdZSi and Pd structures r e s u l t s i n broad diffraction rings visible i n the diffraction pattern (Fig. 2 b ) . Only a f t e r heating a t higher temperatures (875 K) when thicker Pd2Si c r y s t a l l i t e s a r e formed the influence of the Pd-Pd,Si interface on the growth of the palladium s i l i c i d e phase becomes l e s s pronounced and the Pd2Si c r y s t a l s a t t a i n a more uniform structure close to the bulk Pd,Si. This change r e s u l t s i n the sharpening of the Pd2Si diffraction rings (Fig. 2 c ) . The growth of larger palladium s i l i c i d e c r y s t a l l i t e s allowed the resolution of the Pd,Si l a t t i c e fringes (Fig. 1 c ) . Microdiffraction analysis of the s i l i c a supported palladium particles a f t e r heating a t 845 K revealed that i n some cases these particles a r e i n an "amorphous s t a t e " . I t is known t h a t i n the Pd-Si system the formation of e believe that the formation of such an amorphous phase i s possible ( r e f . 25). W an amorphous Pd-Si phase might be also a r e s u l t of reaction of the Pd particles with the SiO, substrates.
565
CONCLU S I0N Prolonged heating o f the Pd-Si0, system in a hydrogen atmosphere a t a temperature of 845 K can lead t o strong chemical interaction between the palladium particles and SiO, substrate. The r e s u l t i s a t h i n layer of palladium s i l i c i d e sandwiched between the substrate and palladium. The Pd2Si phase grows epitaxiall y w i t h respect to the palladium c r y s t a l l i t e s . I t was observed t h a t the l a t t i c e parameters of the t h i n Pd,Si precipitates or/and their hexagonal structure a r e altered or distorted. Sometimes also the distortion of the Pd fcc structure was observed. When a f t e r heating a t higher temperatures thicker Pd2Si c r y s t a l l i t e s a r e formed the influence of the Pd-Pd2Si interface o n the growth of the palladium s i l i c i d e phase becomes l e s s pronounced and most of the Pd2Si crystals a t t a i n a structure close t o the bulk Pd2Si. I n some cases the intermediate phase formed between Pd and SiO, appears t o be nearly amorphous, b u t i t i s c r y s t a l lized slowly under the influence of the electron beam.
ACKNOWLEDGEMENTS R .L. gratefully acknowledges f i nancia 1 support by Deu tscher Akademisc her Au stauschdi ens t a nd Univers i t;i t Bran en. REFERENCES
Schwab, J . Block, W. Mijller and D . Schultze, Naturwissenschaften 44 (1957) 582. G.M. Schwab, J . Block and D . Schultze, Argew. Chem., 71 (1958) 101. G.M. Schwab, Adv. Catal., 27 (1978) 1 . Z.G. Szabo and F. Solymosi, in Proc. of the 2nd Int. Congress on Catalysis, Paris 1961, p. 1627. F. Solymosi, Catal. Rev., 1 (1967) 233. S.J. Tauster, S.C. Fung and R . L . Garten, J . Amer. Chem. SOC., lOO(1978) 170. S.J. Tauster and S.C. Fung, J . Catal., 55 (1978) 29. G . R . Wilson and W . K . Hall, J . Catal., 24 (1972) 306. R.L. MOSS, D . Pope, B.J. Davis and D . H . Edwards, J . Catal. 58 (1979) 206. G.A. Martin and J.A. Dalmon, Reaction Kinet. Catal. Lett., 16 (1981) 325. G.A. Martin, R . Dutartre and J.A. Dalmon, Reaction Kinet. Catal. Lett., 16 (1981) 329. G.A. Martin and J.A. Dalmon, J . Catal., 75 (1982) 233. H . Praliaud and G.A. Martin, J . Catal. , 7 2 (1981) 394. R . Lamber, T h i n Solid Films, 128 (1985) L29. R. Lamber and W. Romanowski, J . Catal., 105 (1987) 213. R. Lamber, N. Jaeger and G. Schultz-Ekloff, J . Catal., 1987 submitted. F.M. Dautzenberg and H.B.M. Wolters, J . Catal., 51 (1978) 26. G.J. Den Otter and F.M. Dautzenberg, J . Catal., 53 (1978) 116. K . Kunimori, T. Okouchi and T. Uchijima, Chem. Lett., 1 2 (1980) 1513. K . Kunimori, T. Ikeda, M. Soma and T. Uchijima, J . Catal., 79 (1983) 185. J.E.E. B a g l i n , G.J. Clark, J.F. Ziegler and J.A. Cairns, J . Mol. Catal., 20 (1983) 299. J.A. Cairns, J.E.E. Baglin, G.J. Clark and J.F. Ziegler, J . Catal., 83 (1983) 301. T. Ren-Yuan, W. Rong-An and L. Li-Wu, Appl. Catal., 10 (1984) 163. A. Nylund, Acta Chem. Scand., 14 (1960) 2381. P . Duwez, R . H . Willens and R . C . Crewdson, J . Appl. Phys., 36 (1965) 2267.
1 G.M.
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C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands
APPLICATION OF OPTICAL METHODS (50000-400 cm-')
567
TO STUDY THE OXIDATION OF
NICKEL. THEORETICAL APPROACH AND EXPERIENCE
M. LE CALVAR' and M. LENGLET' C. F.R., IRDI/DCAEA/SEA/SCAA, Centre d'Etudes Nuclhaires, B.P. 92265 Fontenay-aux-Roses Cedex (France)
Laboratoire de Physico-chimie des Mat&iaux, 76131 Mont Saint Aignan Cedex (France)
I.N.S.A,
B.P.
06,
08,
ABSTRACT Theoretical UV-Vis-NIR d i f f u s e r e f l e c t a n c e and I R specular r e f l e c t a n c e spectra f o r t h i n f i l m s o f N i O on a n i c k e l substrate have been calculated. The experimental spectra are i n r e l a t i v e good agreement w i t h the predicted behavior. UV-Vis-NIR r e f l e c t a n c e spectra reveal the presence o f small amounts o f N i (111) i n NiO. I n the I R r e ion, N i O i s i d e n t i f i e d by the absorption bands a t 580-615 cm-l and 400 cm-' which are a t t r i b u t e d t o l o n g i t u d i n a l and transversal o p t i c a l modes respectively. Both spectroscopies provide thickness measurements
.
INTRODUCTION The idea o f using o p t i c a l methods f o r non-destructive t e s t i n g i s a t t r a c t i v e i n many ways, p a r t i c u l a r l y f o r the study o f the corrosion o f metals and alloys. They can be used without exposure o f the sample t o high vacuum ( c f . XPS,
SIMS,
...) and could be used i n s i t u .
the use o f r e f l e c t a n c e techniques.
AES,
This type o f studies r e q u i r e s
But there i s l i t t l e work i n the l i t e r a t u r e
which includes t h e t h e o r e t i c a l and experimental behavior o f the r e f l e c t a n c e o f absorbing f i l m s o f oxides on metal substrates ( r e f . 1-4). The I R r e f l e c t a n c e spectroscopy has shwon i t s a b i l i t y t o i d e n t i f y q u a l i t a t i v e l y compounds which are i n f i l m s as t h i n as few nanometers ( r e f . 5-10).
EXPERIMENTAL Polycrystal n i c k e l specimens were used. P o l i s h i n g was done i n water w i t h 1200 g r i t s i l i c o n carbide p o l i s h i n g paper. The samples were oxidized i n d r y a i r a t temperatures between 275OC and 1100°C f o r times i n the range 5 min t o 96 hr.
A Perkin-Elmer
lambda 9 model spectrophotometer,
barium s u l f a t e i n t e g r a t i n g sphere,
f i t t e d with a
was used t o o b t a i n r e f l e c t a n c e spectra
568
i n t h e UV-Vis-NIR r e g i o n 50000-4000 cm-'
(200-2500 nm). The s t a n d a r d employed
was b a r i u m s u l f a t e . R e f l e c t a n c e I R s p e c t r a were t a k e n w i t h t h e P e r k i n Elmer 1760 F o u r i e r Transform spectrophotometer, equipped w i t h two d i f f e r e n t r e f l e c t a n c e a t t a c h e ments,
i n t h e wavenumber range o f 7000-370 cm-'.
The a n g l e s o f i n c i d e n c e
and t h e r e f e r e n c e s were 16" and an aluminium m i r r o r , 80" and a g o l d m i r r o r for
t h e accessories
f r o m Perkin-Elmer
and
Spectra-Tech
respectively.
The
i n c i d e n t beam was u n p o l a r i z e d .
RESULTS The UV-Vis-NIR r e g i o n The e q u a t i o n s developed b y Kubelka and Munk which g i v e t h e most comprehens i v e t r e a t m e n t o f d i f f u s e r e f l e c t a n c e and t h e e f f e c t s o f parameters, as p a r t i c l e s i z e , by Wendlandt e t a l .
such
s p e c u l a r p a r t o f t h e r e f l e c t e d l i g h t , have been r e p o r t e d (ref.11)
and Kortiim ( r e f . 1 2 ) .
The a b s o r p t i o n c o e f f i c i e n t
o f N i O f r o m Newman e t a l . ( r e f . 1 3 ) and o p t i c a l d a t a f o r n i c k e l m e t a l f r o m Johnson e t a l . ( r e f . 1 4 ) were used i n c a l c u l a t i o n s w i t h a l i n e a r i n t e r p o l a t i o n procedure between t h e e x p e r i m e n t a l p o i n t s . F i g u r e l a shows t h e r e f l e c t a n c e s p e c t r a o f N i O i n a K B r d i s c and t h e assignment o f t h e observed bands ( r e f . 15-16).
U W
U W
c
c
m
+
m
-
L U
Y-
Y d -
W
W
U
W
W
a
cz
m
-
0
0
0 I
d
I
. . . . . . . . . . . . . . . . . . . . . . . . 300 500 700 900 1100 1300nm
Fig. 1
3
2 Fig. 2
1 eV
.
Fi 1. R e f l e c t a n c e s p e c t r a o f N i O and band assignments, ( a ) i n a KBr d i s c , bq on n i c k e l s u b s t r a t e ( n i c k e l o x i d i z e d a t llOO°C, 15 m i n ) (ground s t a t e 3A2g 'T2g(ol), 'Eg(Oa) 9 3T1g(02) 9 'T2g(Ob) 3 3T1g(03) , 'A21g(oc) 1Fia.
2.
C a l c u l a t e d r e f l e c t a n c e sDectra o f N i O on n i c k e l m e t a l f o r v a r i o u s
569
The i n t e n s i t i e s o f t h e a b s o r p t i o n bands - o f c a l c u l a t e d s p e c t r a i n c r e a s e i n t h e way i n which i t m i g h t be expected t o v a r y w i t h t h e a b s o r p t i o n c o e f f i c i e n t and i n c r e a s i n g t h i c k n e s s quartz,
(Fig.2).
For a t h i n f i l m ( 8 nm) o f N i O on
o n l y two bands a t 240 nm and 290 nm (sideband),
which c o u l d be
a t t r i b u t e d t o charge t r a n s f e r bands, a r e a b t a i n e d . The 3Azg-1Eg
transition
i s observed a t l e a s t f o r a f i l m o f N i O 25 nm t h i c k on q u a r t z . But, f o r f i l m s o f n i c k e l o x i d e on n i c k e l s u b s t r a t e , t h e s p e c u l a r p a r t o f t h e r e f l e c t e d l i g h t provides
a decrease o f a b s o r p t i o n
i n t h e 4 eV
(300 nm) r e g i o n ( r e f . 1 3 )
(Fig. l b ) . When u s i n g such a r e f l e c t a n c e technique,
interference fringes could also
a f f e c t t h e a b s o r p t i o n s p e c t r a b u t p r o v i d e t h i c k n e s s measurements. The a b s o r p t i o n spectrum o f o x i d i z e d n i c k e l t h e non s t o i c h i o m e t r i c N i O , Ni
NiOl+x
c o u l d be f l a t t e n e d due t o
w i t h x <0,01. The o p t i c a l
spectrum o f
(111) has been i n v e s t i g a t e d i n d i f f e r e n t m a t r i c e s and i s c h a r a c t e r i z e d
by i n t e n s e charge t r a n s f e r bands i n t h e range 39000-24500 cm-l and d-d t r a n s i t i o n s i n t h e range 21700-9000
cm-l
(ref.
17-21).
The non s t o i c h i o m e t r y o f
NiO i s c h a r a c t e r i z e d b y an i n c r e a s e o f t h e background a b s o r p t i o n , e s p e c i a l l y i n t h e 30000-10000
cm-l
region (ref.
22),
and a decrease o f t h e i n t e n s i t y
o f t h e d i s c r e t e d-d t r a n s i t i o n s o f N i 2 + (Fig.3).
Fig. 3
Fig. 3. R e f l e c t a n c e s p e c t r a o f n i c k e l o x i d i z e d 4 h r ( t h i c k n e s s ) a t , ( a ) 400°C ( - 120 nm), ( b ) 500°C (530 nm), ( c ) 6OOOC (1680 nm, an i n t e r f e r e n c e f r i n g e i s c l e a r l y observed a t 6000 cm-l), ( d ) 800°C (8120 nm).
570
The I R r e g i o n P r e v i o u s work by Berreman ( r e f . 2 3 )
has shown t h a t t h i n f i l m s o f c u b i c
i o n i c c r y s t a l s c o u l d have s t r o n g i n f r a r e d a b s o r p t i o n bands a t t h e f r e q u e n c i e s of
polar
longitudinal
optical
modes.
Interpretation o f
r e f l e c t i o n spectra
o f s u r f a c e f i l m s a r e based on r e f l e c t a n c e c a l c u l a t i o n s . f r o m Swallow's paper ( r e f .
3),
Equations d e r i v e d
i n c l u d i n g t h e i n t e r a c t i o n s o f a b s o r p t i o n bands
w i t h i n t e r f e r e n c e f r i n g e s f o r o b l i q u e a n g l e s o f i n c i d e n c e , a r e used.
In this
( o b l i q u e a n g l e s o f i n c i d e n c e ) , t h e r e f l e c t a n c e R depends on t h e s t a t e o f p o l a r i z a t i o n o f t h e i n c i d e n t beam, f o r n a t u r a l r a d i a t i o n :
p a r t i c u l a r case
A s i m p l e system o f c l a s s i c a l o s c i l l a t o r s i s employed t o c a l c u l a t e o p t i c a l p r o p e r t i e s o f t h e oxide :
(
E
~
=5.7
;t
o
= 11.75
; ( d T = 405 cm-l ; y = 18 cm-')
O p t i c a l d a t a f o r N i metal
f r o m Ordal e t a l .
(ref.24-25) (ref.
26) were used w i t h
a l i n e a r i n t e r p o l a t i o n procedure between e x p e r i m e n t a l p o i n t s . The observed band near 600 cm-l i s a t t r i b u t e d t o t h e l o n g i t u d i n a l o p t i c a l
(LO) o f N i O . The l o w f r e q u e n c y band near 400 cm-l i s due t o t h e t r a n s v e r s a l o p t i c a l mode (TO) o f N i O . The f i r s t band (LO) i s more i n t e n s e a t 80"
mode
t h a n a t 16" angles o f i n c i d e n c e and d i s a p p e a r s a t normal
incidence f o r a
f i l m 1 p m t h i c k (Fig.4-5).
aJ
U
c
rn
c
U
aJ
L
01
CL
LOO0 2000
Fig. 4
LOO cm-'
Fig. 5
Fig. 4.. C a l c u l a t e d r e f l e c t a n c e s p e c t r a f o r a N i O f i l m l,um T h i c k on n i c k e m e t a l a t v a r i o u s angles o f i n c i d e n c e , ( a ) 0", ( b ) 16O, ( c ) 80". Fig. 5. R e f l e c t a n c e s p e c t r a of n i c k e l o x i d i z e d 5 m i n a t 800°C ( t h i c k n e s 1.3 p n ) o b t a i n e d a t , ( a ) 16", ( b ) 80" a n g l e s o f i n c i d e n c e .
571
For t h i n n e r films,
t h i s band (LO) i s the f i r s t t o appear a t 80" incidence
and could be t h e o r e t i c a l l y observed even f o r a f i l m as t h i n as 1 nm (Fig.6). A s h i f t t o l a r g e r wavelengths,
580 t o 615 cm-',
experimentally observed f o r the LO band a t 80" thickness (Fig.7).
i s t h e o r e t i c a l l y and
incidence,
w i t h increasing
a U 0
V
b
c m c U Q
4
\c
Q
cc
U Q
c
m
t u
of
* 0 cc d
.
.
200 cm-'
1' 10
Fig. 6
o
e
cm"
Fig. 7
Fig. 6. Calculated r e f l e c t a n c e spectra of N i O f i l m s on n i c k e l metal a t 80" an l e o f incidence f o r various thickness, (a) 1 nm, (b) 10 nm, ( c ) 100 nm, (dq 500 nm. Fig. 7. S h i f t t o longer wavelengths v i t h increasing thickness. Reflectance spectra o f n i c k e l oxidized obtained a t 80" angle o f incidence (thickness) ; (a) 275"C, 96 h r ; (b) 800"C, 1 min (0,3 pm) ; ( c ) 800", 5 min (1.3 pm) ; (d) 800"C, 1 h r (3.5 pm).
512
Fig. 8
Fig. 9
Fig. 8. R e f l e c t a n c e s p e c t r a o f n i c k e l o x i d i z e d a t 800°C f o r v a r i o u s a n g l e s o f i n c i d e n c e and t h i c k n e s s e s ; ( a ) 1 h r , 80°, 3 . 5 p m ; ( b ) 4 h r , 16", 7 . 5 p m ; ( c ) 4 h r , 80°, 7 . 5 p m . Fig. 9. C a l c u l a t e d R_L (-------- 1 and R I I ( ) f o r a NiO f i l m 7.5 p m t h i c k on n i c k e l metal a t 80" a n g l e o f i n c i d e n c e .
I n t e r f e r e n c e f r i n g e s c o u l d a l s o appear i n t h e a b s o r p t i o n r e g i o n ( F i g . 8 ) . When a 80" a n g l e o f i n c i d e n c e i s used, a secondary f r i n g e system i s formed. Each component o f t h e r a d i a t i o n has i t s own system o f i n t e r f e r e n c e (RI (Fig.9.).
The band a t
f o r thick films
-
,RII)
600 cm-l c o u l d be hidden b y an i n t e r f e r e n c e minimum
(thickness
2
5 pm). A t " i n f i n i t e "
thickness the f a m i l i a r
r e s t s - t r a h l e n spectrum o f NiO i s obtained. The e f f e c t s o f s c a t t e r i n g on t h e r e f l e c t a n c e s p e c t r a a r e t o reduce t h e total
r e f l e c t a n c e and t h e i n t e n s i t y o f t h e i n t e r f e r e n c e f r i n g e s . 1 3000 cm-
i n t e r f e r e n c e f r i n g e s a r e n o t o b s e r v a b l e above
-
.
Usually,
CONCLUSION p r e s e n t work on o x i d i z e d n i c k e l s u r f a c e s has shown t h a t t h e UV-Vis-NIR d i f f u s e r e f l e c t a n c e spectroscopy and t h e FTIR s p e c u l a r spectroscopy can p r o v i d e c o n s i d e r a b l e i n f o r m a t i o n about t h i n f i l m s
(thickness-composition).
The most
i m p o r t a n t problem o f d i s t i n g u i s h i n g p h y s i c a l and chemical e f f e c t s i n s p e c t r a i s c l e a r l y demonstrated and solved.
513
The r e s u l t s o b t a i n e d i n t h e I R r e g i o n f o r t h e s i m p l e c u b i c d i a t o m i c c r y s t a l o f N i O can be g e n e r a l l y a p p l i e d t o more c o m p l i c a t e d c r y s t a l s Cu20,
Cr203
( r e f . 27). Both s p e c t r o s c o p i e s have been used t o f o l l o w t h e growth process o f o x i d e l a y e r s i n t h e i n i t i a l o x i d a t i o n o f 80 Ni-20 C r a l l o y ( r e f . 28).
REFERENCES 1 2 3 4 5 6 7 8 9 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
S. T h i b a u l t Mat. Chem., 1 (1976) 71-86. G.C. A l l e n , P.J.F. H a r r i s and G.A. Swallow, S u r f . Tech., 6 (1977) 111129. G.A. Swallow and G.C. A l l e n , Oxid. Met., 17 (1982) 141-156. G.C. A l l e n and G.A. Swallow, Oxid. Met., 17 (1982) 157-176. S.A. F r a n c i s and A.H. E l l i s o n , J. Opt. SOC. Am., 49 (1959) 131-138. R.G. G r e e n l e r , J. Chem. Phys., 44 (1966) 310-314. G.W. P o l i n g , J. Electrochem. SOC., 116 (1969) 958-963. R.G. Greenler, J. Chem. Phys., 50 (1969) 1963-1968. F.P. Mertens, S u r f a c e Sci., 7 1 (1978) 161-173. R. J a s i n s k i and A. Lob, J. Electrochem. SOC., 135 (1988) 551-556. W.W. Wendlant and H.G. Hecht, R e f l e c t a n c e Spectroscopy, I n t e r s c i e n c e , New-York, 1966. G. Kortum, R e f l e c t a n c e Spectroscopy, S p r i n g e r Verlag, B e r l i n , 1969. R. Newman and R.M. Chrenko, Phys. Rev., 114 (1959) 1507-1513. P.B. Johnson and R.W. C h r i s t y , Phys. Rev., B9 (1974) 5056-5070. D.Reignen, Ber. Bunsenges. Physik. Chem., 69 (1965) 82-87. C.K. JBrgensen, M. L e n g l e t and J. A r s h e , Chem. Phys. L e t t e r s , 136 (1987) 47 5- 477. D.S. Mc Lure, J. Chem. Phys., 36 (1962) 2757-2779. A.E. Cherkashin, F.I. V i l e s o v , SOV. Phys. S o l i d S t a t e , 11 (1969) 1068- 1072. H.H. T i p p i n s , Phys. Rev., B1 (1970) 126-135. M. Lo Jacono, A. S g a m e l l o t t i and A. Cimino, Z. Physik. Chem. Neue Folge, 70 (1970) 179-194. M. Le C a l v a r and M. L e n g l e t , Personal communication. E. Escalona P l a t e r o , G. Spoto, S. C o l u c c i a and A. Zecchina, Langmuir, 83 (1987) 291-297. D.W. Berreman, Phys. Rev., 130 (1963) 2193-2198. P.J. G i e l i s e , J.N. P l e n d l , L.C. Mansur, R. M a r s h a l l , S.S. M i t r a , R. Mykolajewycz and A. Smakula, J. Appl. Phys., 36 (1965) 2446-2450. J.L. Rendon, J.E. I g l e s i a s and C.J. Serna, O p t i c a Pura Y Aplicada, 14 (1981) 117-122. M.A. Ordal, L.L. Long, R.J. B e l l , S.E. B e l l , R.R. B e l l , R.W. Alexander and C.A. Ward, Appl. Opt., 22 (1983) 1099-1119. J.M. Machefert, M. Le C a l v a r and M. L e n g l e t i n p r e p a r a t i o n . M. Le C a l v a r and M. L e n g l e t t o be p u b l i s h e d i n A. Zecchina, G. Costa and C. M o r t e r r a (Eds. ), S t r u c t u r e and R e a c t i v i t y of Surfaces, T r i e s t e 13-16 September 1988.
This Page Intentionally Left Blank
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactiuity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands
515
UV-Vis-NIR AND FTIR REFLECTANCE STUDIES OF THE I N I T I A L STAGE OF OXIDATION OF 8 0 Ni-20 C r ALLOY.
M. LE CALVAR' and M. LENGLET'
'C.F.R., IRDI/DCAEA/SEA/SCAA, C e n t r e d ' E t u d e s NuclCaires, B.P. 06, 92265 Fontenay-aux-Roses Cedex (France) ' L a b o r a t o i r e de Physico-chimie des m a t d r i a u x , I.N. S.A, 76131 Mont S a i n t Aignan Cedex (France)
B.P.
08,
ABSTRACT UV-Vis-NIR and FTIR r e f l e c t a n c e s t u d i e s o f t h e i n i t i a l o x i d a t i o n process o f a 80 Ni-20 C r a l l o y i n oxygen o r a i r a t v a r i o u s temperatures a r e d e s c r i b e d . NiCr204 and Cr2O3 a r e i d e n t i f i e d i n b o t h r e g i o n s . I R r e f l e c t a n c e s p e c t r a o f N i C r 04 i n t h i n f i l m s ( - 1 0 0 nni) i s c h a r a c t e r i z e d by a main band a t 690700 cm-1 which i s a t t r i b u t e d t o a l o n g i t u d i n a l o p t i c a l mode. N i O i s never formed i n l a r g e amounts. O p t i c a l techniques can be u s e f u l t o understand t h e mechanisms and k i n e t i c s o f o x i d a t i o n processes.
INTRODUCTION This
paper
by o p t i c a l
investigates
the
possibility
methods t h e n a t u r e o f o x i d e
of
films
identifying qualitatively formed d u r i n g t h e
initial
o x i d a t i o n process o f a 80 N i - 2 0 C r a l l o y . The o x i d i z e d l a y e r s can be s t u d i e d d i r e c t l y on t h e m e t a l s u b s t r a t e u s i n g r e f l e c t a n c e techniques. The main p r o d u c t s o f t h e o x i d a t i o n o f N i - C r a l l o y s a r e Cr203, N i C r z O q and N i O ( r e f . 1-3). N i c k e l o x i d e s N i D and NiCrp04 a r e produced on t h e 80 N i - 2 0 C r a l l o y b e f o r e a Cr203 p r o t e c t i v e l a y e r i s formed ( r e f . 4-8). A d e t a i l e d i n v e s t i g a t i o n o f c h a r a c t e r i s t i c spectra from oxidized n i c k e l
s u r f a c e s has been d e s c r i b e d e a r l i e r ( r e f . 9 ) . can be i d e n t i f i e d on n i c k e l
T h i s s t u d y has shown t h a t N i O
s u b s t r a t e even f o r t h i n f i l m s b y UV-Vis-NIR
r e f l e c t a n c e spectroscopy and I R r e f l e c t a n c e spectroscopy and has a l s o d e s c r i b e d t h e changes which o c c u r i n s p e c t r a w i t h i n c r e a s i n g t h i c k n e s s o f n i c k e l o x i d e films. EXPERIMENTAL UV-Vis-NIR d i f f u s e r e f l e c t a n c e s p e c t r a and I R s p e c u l a r r e f l e c t a n c e s p e c t r a were recorded u s i n g Perkin-Elmer
Lambda 9 and FTIR 1760 spectrophotometers
r e s p e c t i v e l y . The r e f l e c t a n c e attachements and r e f e r e n c e s have been d e s c r i b e d i n a p r e v i o u s paper ( r e f . 9 ) .
576
All
specimens were prepared f r o m 0.125
mm t h i c k f o i l s o f 80 Ni-20 C r
a l l o y f r o m Goodfellow M e t a l s I n c . The average g r a i n s i z e was 23pm.
The samples
12 mm x 16 mm were washed w i t h d e z i o n i z e d w a t e r and u l t r a s o n i c a l l y c l e a n e d
i n acetone.
The o x i d a t i o n t e s t s were c a r r i e d o u t i n t h e t e m p e r a t u r e range
f r o m 600°C t o 900°C f o r 1 m i n t o 18 h r i n d r y a i r o r p u r e oxygen. RESULTS The UV-Vis-NIR r e g i o n N i O shows t h e u s u a l p i c t u r e o f o c t a h e d r a l chromophores N i (11) O6 w i t h t h r e e bands (ol, 02, 0 3 ) which correspond t o t r i p l e t - t r i p l e t t r a n s i t i o n s and a few s i n g l e t t r a n s i t i o n s (oa, Ob, o c ) w h i c h a r e s p i n f o r b i d d e n ( r e f . 10-11).
The o p t i c a l spectrum o f NiCr204 i s c h a r a c t e r i s t i c o f N i (11) i n a
tetragonal s i t e (ref.
12-14). For C r ( 1 1 1 ) i n an o c t a h e d r a l environment (Cr203)
c o n s i d e r a t i o n s o f symmetry p r e d i c t t h r e e main a b s o r p t i o n bands and a number of l o w i n t e n s i t y q u a r t e t - d o u b l e t t r a n s i t i o n s i n a d d i t i o n . The two f i r s t s p i n a l l o w e d bands a r e s p l i t by t h e d e v i a t i o n f r o m r e g u l a r o c t a h e d r a l symmetry ( r e f . 15-17) (Table 1, F i g . 1 ) . TABLE 1 O p t i c a l t r a n s i t i o n energy and l i g a n d - f i e l d parameters
OPllCAL IRANSlIlON EIIERGY AN0 LIMNO-FIELO PARANElERS
.
Formula NiO
Spectral parameters (cm-ll
Observed band energy Icn'Il
Coordination 'A 29I'-
29 i O l i
Irp ( 0 , )
3 ~ 1 9( 0 2 i
'izg (obi
3 ~ 1 9( 0 ~ 1
l i I g toc)
NIZtOh
8900
14100
15400
22100
24000
26450
Inn1
1130
710
650
450
420
380
HlCr204
oq
8
890
aoo
400
750
1660
480
3
a] 1 1 - c 3 1 2
11 NIZ' Td
4500°'
8440
15230(1'
(nnl
2220
1185
651
Cr203
'A
CPO~ IMll
v
29
2
E
9
21
21
29
19
41
lg
11100
14300
16600
I9700
21100
7 30
100
603
508
460
( 1 1 Centre O F g r a v i t y O F doublet band
571
U u
m c U
u u
d
u-
0: 01 0 I
d
2400 nm
2b0 Fig. 2
Fig. 1
Fig.1. UV-Vis-NIR r e f l e c t a n c e s p e c t r a o f ( a ) NiO, ( b ) NiCr204, ( c ) Cr203 i n KBr p e l l e t s . Fig.2. UV-Vis-NIR r e f l e c t a n c e s p e c t r a o f 80 Ni-20 C r a l l o y o x i d i z e d a t 900°C i n oxygen ( t h i c k n e s s ) , ( a ) 1 min, ( b ) 5 m i n ( - 6 0 nm), ( c ) 30 m i n ( - 1 1 0 nm), ( d ) 1 h r ( - 1 6 0 nm), ( e ) 2 h r ( - 3 7 0 nm), ( f ) 18 h r ( - 0 . 9 m).
I t appears t h a t f o r v e r y t h i n f i l m s (20-40 nm) o f o x i d e s t h e UV-Vis r e g i o n
i s v e r y complex t o a n a l y s e f o r t h r e e main reasons :
-
The s p e c u l a r p a r t o f t h e
reflected
l i g h t could destroy i n f o r m a t i o n
i n t h e UV r e g i o n ( r e f . 9 ) .
-
I n t e r f e r e n c e f r i n g e s which p r o v i d e approximate measure o f o x i d e f i l m t h i c k n e s s appear a l s o i n t h i s r e g i o n ( r e f . 18-20).
-
The c o n t r i b u t i o n o f each o x i d e N i O ,
NiCr204 o r Cr203 and even C r ( V I )
( r e f . 21) i s d i f f i c u l t t o determine i n t h e 200-500 nm r e g i o n . 4 However Cr203 c o u l d be i d e n t i f i e d by t h e band a t 460 nm ( A v 4 T 29 l g1 4 f o r t h i n f i l m s and f o r t h i c k e r f i l m s b y t h e bands a t 460 nm ( A -4~1g3 29 and 603 nm (4A 4T). 29 29 There i s no c l e a r evidence o f N i O . T h i s i s due t o t h e f a c t t h a t Cr203 has
higher
absorption
( a Cr2O3- > 10aNiO)
coefficients
than
NiO
in
the
UV-Vis-NIR
region
( r e f . 22-23).
NiCr204 i s c h a r a c t e r i z e d by a s h o u l d e r a t 680 mm (maximum of i n t e n s i t y 3 3 o f t h e e l e c t r o n i c t r a n s i t i o n a T l d b T1). T h i s s h o u l d e r d i s a p p e a r s w i t h i n c r e a s i n g t i m e and temperature o f o x i d a t i o n . For v e r y t h i c k f i l m s o n l y Cr203 i s i d e n t i f i e d ( F i g . 2).
578
The I R region Cr203 is one of the main oxidation products of 80 Ni-20 Cr alloy. The behavior of the specular reflectance of Cr203 on Cr metal has been investigated (ref. 24). With a 80" angle of incidence and for thin films (-10 nm) only the band at 720 cm-l (LO) (longitudinal optical mode) is obtained. With increasing thickness the bands at 595 cm-l (LO), 412 cm-l (LO) and 610 cm 1 (TO) (transverse optical mode), 530 cm-l (TO), 445 cm-l (TO), 405 cm-l (TO) appear. This confirms the calculated values of the longitudinal optic frequencies obtained by Renneke et al. (ref. 25) and is in close agreement with experimental spectra. Calculated and experimental spectra show that the main band is obtained from 720 to 760 cm" with increasing thickness, at 80" angle of incidence. At the beginning of the oxidation process of 80 Ni-20 Cr alloy, a band at 690 cm"' (690 to 700 cm- 1 ) is usually obtained and is attributed to a longitudinal optical mode of NiCr204. The assignment of vibration modes is clearly demonstrated by using 16" and 80" angles o f incidence (Fig.3). The band at 690 cm" is more intense at 80" than at 16" angle of incidence (ref. 26). This band i s associated with a small band at 630 cm-l and with the bands at 460 and 475 cm-l. I
&
,
,
,
,
,
I
I
400 c m-'
1000
Fig. 3
I 400 cm-' Fig. 4
1000
Fig.3. Assignment of the new band at 690 cm-'. IR reflectance spectra o f 80 Ni-20 Cr alloy oxidized 2 hr at 900°C in oxygen (thickness-370 nm) at various angles o f incidence, (a) 16", (b) 80". F i .4. IR reflectance spectra o f 80 Ni-20 Cr alloy oxidized at 600°C in oxygen, (a7 5 min, (b) 30 min, (c) 2 hr 30 min, (d) 18 hr (80" angle of incidence).
579
U aJ m c
U aJ
c m
L
L
U
U
QJ
aJ d
c
Y-
aJ
aJ
a
a
.
.
.
1
,
LO 0
1000
Fig. 6
1000
Fig. 5 Fig. 5. I R r e f l e c t a n c e s p e c t r a o f 80 Ni-20 C r a l l o y o x i d i z e d i n a i r a t 75OoC, (a) 1 min, ( b ) 3 min, ( c ) 10 min, ( d ) 20 min, ( e ) 60 min (80" angle o f incidence). Fig. 6. I R r e f l e c t a n c e spectra o f 80 Ni-20 C r a l l o y o x i d i z e d a t 900°C i n oxygen, ( a ) 1 min, ( b ) 5 min, ( c ) 30 min, ( d ) 1 hr, ( e ) 2 hr, ( f ) 18 hr.
Cr203 i s always detected a t t h e beginning o f t h e o x i d a t i o n process and i s t h e major o x i d e formed a t h i g h temperatures (Fig. 4-5-6). The 590 cm"
r e g i o n c o u l d be composed o f overlapping bands o f Cr203,
N i O and even NiCr204.
However,
the obsorption
intensity i n this
region,
compared w i t h t h e a b s o r p t i o n i n t e n s i t y observed i n t h e same r e g i o n f o r t h e same t h i c k n e s s o f Cr203 on C r metal, NiO.
Nevertheless,
80 Ni-20
N i O i s n o t formed i n l a r g e amounts on t h i s p a r t i c u l a r
C r alloy.
bands a t 590 cm-'
may be a t t r i b u t e d t o t h e presence o f
With i n c r e a s i n g t i m e o f o x i d a t i o n and temperature, and of NiCr204 become weaker.
the
The o x i d e f i l m i s m a i n l y
composed of a t h i c k Cr203 l a y e r (assuming t h a t t h i s l a y e r i s pure Cr20g a t h i c k n e s s o f 0.9,m
i s c a l c u l a t e d f o r a sample o x i d i z e d a t 9OO"C,
18 h r i n
oxygen). Minor bands a t 750-850, Cr (VI)
880-900 and 970-1050 cm-l appear t o be due t o
o r "chromate l i k e " groups ( r e f .
80 Ni-20 C r samples. o x i d e layers.
27) a r e seen on a l l t h e o x i d i z e d
These species may be p r e s e n t o n l y a t t h e surface o f
580
CONCLUSION From t h i s s t u d y t h e f o l l o w i n g g r o w t h process i s proposed : i n t h e f i r s t s t a g e o f t h e o x i d a t i o n process, NiCr204 and Cr203 a r e t h e m a i n formed compounds, NiCr204 i s due t o t h e s o l i d r e a c t i o n between N i O and Cr203. I n t h e second stage,
o n l y o b t a i n e d a t h i g h t e m p e r a t u r e (900°C), a t h i c k Cr203
l a y e r i s formed. R e f l e c t a n c e s p e c t r o s c o p i e s can p r o v i d e i m p o r t a n t i n f o r m a t i o n a b o u t o x i d i z e d l a y e r s o f a l l o y s . They can be u s e f u l t o understand t h e mechanisms and k i n e t i c s o f o x i d a t i o n processes.
O p t i c a l s t u d i e s a r e non d e s t r u c t i v e and no vacuum
i s r e q u i r e d i n c o n t r a s t t o many o t h e r s u r f a c e s techniques.
REFERENCES 1
2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27
C.S. G i g g i n s and F.S. P e t t i t , Trans. Met. SOC. AIME, 245 (1969) 24952507. i b i d , 245 (1969) 2509-2514. B. Chattopadhyay and G.C. Wood, J. Electrochem. SOC., 117 (1970) 11631171. A. Takei and K. N i i , Trans. JIM, 17 (1976) 211-219. P. Moulin, F. Armanet, G. Bdranger and P. Lacombe, Mm. S c i . Rev. M t a l l u r g . , 3 (1977) 143-160. J.C. P i v i n , C. Roques-Carmes and P. Lacombe, Mem. S c i . Rev. % t a l l u r g . , 3 (1977) 161-175. Wood, D.P. W h i t t l e and B.D. Bastow, Oxid. Met., T. Hodgkiess, G.C. 12 (1978) 439-449. J.C. P i v i n , C. Roques-Carmes, P. M o u l i n , A.M. Huntz and P. Lacombe, Me'm. S c i . Rev. M t a l l u r g . , 11 (1978) 638-654. M. Le C a l v a r and M. L e n g l e t i n A. Zecchina, G. Costa and C. M o r t e r r a (Eds. ), S t r u c t u r e and R e a c t i v i t y o f Surfaces, Proceedings of t h e European Conference, T r i e s t e , 13-16 Seotember 1988. D. Reinen,-Ber. Bunsenges, Physik. Chem., 69 (1965) 82-87. C.K. Jdrgensen, M. L e n g l e t and J. Arsene, Chem. Phys. L e t t e r s , 136 (1987) 475-477. H.A. Weakliem, J. Chem. Phys., 36 (1962) 2117-2140. 19 (1984) J. Arsene, M. L e n g l e t and C.K. Jdrgensen, Mater. Res. Bul 1281-1291. M. L e n g l e t , A. d'Huysser, J. Arsgne, J.P. B o n n e l l e and C.K. Jdrgensen, J. Phys. C. S o l i d S t a t e Phys., 19 (1986) L363-L368. C.K. Jdrgensen, Adv. Chem. Phys. , 5C (1963) 33-146. D. Reinen, S t r u c t u r e and Bonding, S p r i n g e r V e r l a g Ber in, 6 (1969) 30-51. __ ___ C.K. Jdrgensen, O x i d a t i o n Numbers and O x i d a t i o n S t a t e s , S p r i n g e r Verlag, B e r l i n , Heidelberg, 1969. N.J.M. W i l k i n s , Corros. Sci., 5 (1965) 3-18. A. ROOS, M. B e r g k v i s t and C.G. Ribbing, T h i n S o l i d Films, 125 (1985) 221-227. A. Roos and C.G. Ribbing, Phys. L e t t . , A 108 (1985) 225-227. M. Le C a l v a r and M. L e n g l e t , A n a l u s i s 1988, i n press. R. Newman and R.M. Chrenko, Phys. Rev., 114 (1959) 1507-1513. 8. K a r l s s o n and C.G. Ribbing, J. Appl. Phys., 53 (1982) 6340-6346. J.M. Machefert, M. Le C a l v a r and M. L e n g l e t t o be p u b l i s h e d . D.R. Renneke and D.W. Lynch, Phys. Rev., 138 (1965) A530-A533. D.W. Berreman, Phys. Rev., 130 (1963) 2193-2198. A. Zecchina, S. Coluccia, L. C e r r u t i and E. B o r e l l o , J. Phys. Chem., 75 (1971) 2783-2790.
.,
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
581
REACTIVITY OF MIXED Zn-Cr OXIDE TOWARDS LINEAR C4 OXYGENATED MOLECULES BY THE TPSR METHOD*.
L. LIETTI, E. TRONCONI, P. FORZATTI**, and I. PASQUON Dipartimento di Chimica Industriale ed Ingegneria Chimica "G. Natta" del Politecnico, Piazza Leonard0 da Vinci 32, 1-20133 Milano (Italy)
ABSTRACT The TPSR method is applied to assess the reactivity of the surface of a mixed Zn-Cr oxide towards linear C4 aldehyde, alcohol and acid molecules. The results indicate that molecularly adsorbed aldehyde, C4 alkoxide and C4 carboxylate species are formed at the surface together with the corresponding c8 intermediates originating from the aldol-like condensation of two aldehyde molecules. The functions of the surface of the mixed ZnCr oxide involved in the desorption upon decomposition or reaction of the above intermediates have been identified, including aldol-like condensation, hydrogenation, dehydrogenation, dehydration, decarboxylation and double bond isomerization. INTRODUCTION The Temperature Programmed Surface Reaction (TPSR) technique is usually referred as a TPD experiment in which the desorbed species are not produced by simple desorption of the initially adsorbed molecules but originate from surface catalyzed decomposition of the adsorbate or reaction between co-adsorbates. If suitable operating conditions are selected, intermediate species can in principle be desorbed and detected as soon as they form. Thus, valuable information on the reactivity of the surface and the underlying chemistry can be extracted from the desorption spectra of the products released upon decomposition or surface reaction of the adsorbates. Accordingly, the TPSR method focuses attention on the chemical functions of the catalyst and on the reactivity of the catalyst surface towards specific molecules.
* This is note no. XVI in the series "Synthesis of Alcohols from $$rbon Oxides and Hydrogen". Author to whom correspondence should be addressed.
582
In this paper we show the application of the TPSR technique to assess the reactivity of a mixed Zn-Cr oxide surface towards linear C4 aldehyde, acid and alcohol molecules. The present study is also of relevance for elucidating the mechanism of the alcohol chain growth over Zn-Cr mixed oxides based catalysts in the direct synthesis of methanol and higher alcohols from CO and H2. METHODS The TPSR apparatus and the related experimental procedures have been described in details elsewhere (ref. 1). For the present study, the analysis section has been modified including a 16 loops gas. sampling valve, so that several on-line G.C. analyses of the gas exiting the reactor can be performed during a single experiment. The G.C. analyses of the products were performed using a OV1 25m long capillary column (0.32 mm I.D.) with high phase thickness (3 pm) which allows to detect both light and heavy products. Typical heating program included 5 min at 3OoC, followed by heating up to 100°C (heating rate 5OC/min) and then up to 23OOC (heating rate 9°C/min). Profiles of the single desorbed species were obtained by interpolating the experimental points. A commercial Zn-Cr-0 catalyst with Zn/Cr atomic ratio = 3/1 and BET surface area = 120 m2g-l has been used in this study. XRD analysis indicated the presence of both ZnO and ZnCr204 phases. Chromatographic helium, further purified by molecular sieves, and Fluka puriss. p.a. reagents were used during all runs. RESULTS AND DISCUSSION The overall TPSR trace and the profiles of the main species desorbed upon n-butanal adsorption at 35OC on the mixed Zn-Cr oxide surface is shown in Figs. la and lb. Table 1 gives the relative amounts of the various products, as percentage of the G.C. area of the peak to the area of the overall TPSR trace, together with the corresponding peak temperature. Most of the observed products can be explained by assuming the presence of two classes of intermediate surface species represented in Figs. 2a and 2b (ref. 1). The molecularly adsorbed species A1 is formed by interaction of n-butanal with a Lewis acid site. The alkoxide species A2 and
583
100
200
300
400
+
Temperature ( ' C ) Fig. la. TPSR traces obtained after n-butanal adsorption at 35°C. a) Overall FID-TPSR trace; b) n-butanal; c) 1-butanol; d) propylene; e) butenes. the carboxylate ions A4 can be formed either by reduction with H2 adsorbed during catalyst's pretreatment and by oxidative dehydrogenation of the molecularly adsorbed C4 aldehyde respectively, or by a Cannizzaro-type disproportionation reaction of species A1. The molecularly adsorbed species A3, on the other hand, is formed by interaction of 1-butanol, once produced from n-butanal, with a Lewis acid site.
TABLE 1 Desorption products, peak temperatures and relative amounts following n-butanal adsorption at 35°C on Zn-Cr-0. desorption products n-butanal 2-ethyl-2-hexenal 1-butanol 4-heptanone c8 dienes C7 linear olefins C5 hydrocarbons methane + ethylene propylene butenes c8 ketones others
peak temperatures ("C)
relative amount( % )
79 97 90, 200
8 27 6
100, 303 250
5
337 378 380 381 300, 400
11 2 2 19 5
-
3
2 10
584
t
L
200
100
300
Temperature
400
b
('C)
Fig. lb. TPSR traces obtained after n-butanal adsorption at 35OC. a) Overall FID-TPSR trace; b) 2-ethyl-2-hexenal; c) c8 dienes; d) 4-heptanone; e) C7 linear olefins. n-BUTANAL
BUTE NE S
1 - BUTANOL
PROPYLENE
Fig. 2a. Intermediate surface species originating adsorption of n-butanal, and products desorbed therefrom.
upon
Since species A2, A3 and A4 can also originate upon adsorption of 1-butanol and n-butyric acid, the TPSR of these reagents was performed in order to provide complementary
585
information on the nature of the surfaces intermediates and on the mechanism by which the different desorption products are formed. Tables 2 and 3 report the relative amounts and the temperature of the maximum of the desorption peaks for each product during 1-butanol (ref. 2) and n-butyric acid TPSR from the mixed Zn-Cr oxide respectively. In the following, results of n-butanal TPSR are discussed and compared with those of 1-butanol and n-butyric acid TPSR.
-MFig. 2b. Intermediate surface species originating from the aldol-like condensation of two molecules, and products desorbed therefrom. The desorption of n-butanal in the low temperature region originates from species Al, which is reversible in nature, and is characterized by a temperature of the peak maximum which closely corresponds to the evaporation temperature of n-butanal. The 1-butanol desorption peak exhibits a broad shape with two maxima at about 90 and 2OO0C, respectively. These temperatures roughly correspond to those observed during TPSR of 1-butanol (Table 2) and are associated with the desorption of physisorbed 1-butanol (species A 3 ) and with the hydrolysis of the butoxy species A2 by surface OH groups, followed by desorption, respectively. Dehydration of species A2 at higher temperatures (TW=4000C) accounts for the evolution of butenes during n-butanal and 1butanol TPSR.
586
Propylene desorption is primarily responsible for the peak at 381°C in the overall TPSR trace of n-butanal and is accompanied by the evolution of large quantities of C02. Propylene is believed to originate from a decarboxylation reaction of species Ad:
A similar mechanism has been suggested to account for the decarboxylation of carboxylic acids over oxide surfaces (ref. 3). The strict correspondence observed in the temperature of the maximum of the propylene peak during n-butyric acid TPSR (TM=38loC in both Table 1 and Table 3) confirms that propylene desorption originates from decomposition of a carboxylic intermediate. On the other hand, propylene is detected at the same temperature also during 1-butanol TPSR (Table 2). This implies that species A4 is also formed at high temperatures upon adsorption of 1-butanol, in line with previous reports on the formation of carboxylate species after alcohol adsorption on various oxide surfaces (ref. 4).
TABLE 2 Desorption products, peak temperatures and relative following 1-butanol adsorption at 35OC on Zn-Cr-0. desorption products 1-butanol C5 hydrocarbons methane + ethylene propylene butenes others
peak temperatures ("C) 85, 185 365 380 381 390
-
amounts
relative amount( % 1 53 3 3 36 3 2
Species Bl, B2 and B3, on the other hand, result from the surface aldol-like condensation of two aldehyde molecules. In addition to the molecularly adsorbed species, the corresponding carboxylate and alkoxy species are likely to be present, and are produced through oxidation and reduction reactions or Cannizzaro type disproportionation. Condensation of a surface C4 carboxylate species with a neighbouring molecularly adsorbed n-butanal molecule, leading to species B2, is also possible (ref. 3).
587
TABLE 3 Desorption products and peak tem2eratures following n-butyric acid adsorption at 35OC on Zn-Cr-0
.
desorntion prodkts n-butyric acid 4-heptanone C5 hydrocarbons methane + ethylene propylene butenes
peak tempeiatures
( oc
100, 225 333 364 378 381 398
* Analytical problems in the 24O+31O0C temperature range prevented evaluation of the relative amounts of the single desorbed species. Formation of 2-ethyl-2-hexenalI in the low temperature region, is explained by dehydration of species Bl, followed by desorption. Dehydrogenation and decarboxylation of species B2 originates 4-heptanoneI whereas dehydration and decarboxylation leads to C7 linear olefins. Accordingly, the olefins/ketone ratio should be governed by the dehydration-dehydrogenation capabilities of the oxide surface. As a matter of fact, the evolution of C7 linear olefins occurs to a lesser extent, whereas the desorption of 4-heptanone is greatly increased during TPSR of n-butanal from the K-promoted Zn-Cr oxide (ref. 5). The desorption of 4-heptanone occurs in the same temperature range during n-butyric acid TPSR, but no C7 linear olefins have been detected in this case. The decarboxylative condensation of carboxylic acid over metal oxides is a reaction of wide applicability, and may proceed according to alternative mechanisms involving either the condensation of two acid molecules to form P-keto acids and a subsequent decarboxylation reaction or a decarboxylative alkyl anion transfer leading to the intermediate species C and D shown in Fig. 3, respectively (ref. 3). Keto-en01 tautomerism and decarboxylation of species C leads to 4-heptanoneI whereas keto-enol tautomerism is involved in the formation of 4-heptanone from species D. It is worth noting that neither species C nor species D can lead to olefin formation, in line with experimental results. The slightly different temperatures of the maximum of 4-heptanone desorption during TPSR of n-butyric
588
i
-M-
C
Fig. 3 . Intermediate surface condensation of carboxylic acids.
D
species
originating
from
acid and of n-butanal (333OC vs 303OC) may be due to the different reactivity of the intermediate species (C or D in Fig. 3 vs B2 in Fig. 2b). On the other hand, desorption of 4-heptanone in the low temperature region (TM = 100°Cl during n-butanal TPSR is likely to originate from decomposition of a different intermediate species, whose formation involves aldehydic intermediate molecules, since only traces of 4-heptanone have been detected in the low temperature region of n-butyric acid TPSR. Finally, species B3 is involved in c&3 dienes and c g ketones desorption. Evolution of Cf3 dienes is associated with dehydration on both Ca and Cb carbon atoms of species B3, whereas c&3 ketones are likely to result from dehydration on the Cb carbon atom of the same species, followed by double bond migration and keto-enol tautomerism. It is worth noting that the formation of cg ketones can also be associated with an aldol-like condensation reaction of two n-butanal molecules with oxygen retention reversal (ref. 6), as already suggested (ref. 1). The above results point to the reactivity of the Zn-Cr-0 surface in performing a series of characteristic chemical reactions, which are summarized below. (a) Aldol-like condensation. The capability of the Zn-Cr-0 system to promote aldolic condensation, is worth noting. Actually, the condensation products of n-butanal, 2-ethyl-2hexenal, is observed already in the low temperature region during n-butanal TPSR. Besides, compounds with more than four carbon atoms are predominant among the desorbed species. This function
589
is also effective for carboxylic molecules, as reflected by the formation of 4-heptanone during n-butyric acid TPSR. (b) Hydrogenation. The desorption of 1-butanol and butenes from prereduced Zn-Cr-0 surface is related to the presence of a C4 alkoxy species, which can originate either by hydrogenation of the molecularly adsorbed n-butanal or by a Cannizzaro-type disproportionation of a surface dioxybutylene intermediate. (c) Hydrolysis. This function is likely to account for 1butanol desorption in correspondence of the peak with T ~ = 2 0 0 ~ C . (d) Dehydrogenation. The formation of dienes, trienes and aromatics from the corresponding olefins, as well as the production of 4-heptanone and propylene by decarboxylation, requires a dehydrogenation function. (e) Decarboxylation. This is one of the most evident functions of the Zn-Cr-0 surface: intermediate surface species A4, B2 and C undergo decarboxylation in the high temperature region. (f) Dehydration. The formation of a number of desorbed products involves a dehydration step, including butenes and c8 dienes from the corresponding alkoxy species and C7 linear olefins from species B2. (9) Double bond isomerization. This function is revealed by the presence of isomers for both hydrocarbons and oxygenated unsaturated compounds (ref. 1). Also, keto-enol tautomerism, which can be seen as a particular form of double-bond migration, is involved in the formation of 4-heptanone and c8 ketones. ACKNOWLEDGEMENTS This work was supported by Progetto Finalizzato Energetica/2 and Minister0 Pubblica Istruzione (Roma). REFERENCES 1 2 3 4 5 6
L. Lietti, D. Botta, P. Forzatti, E. Mantica, E. Tronconi and I. Pasquon, J. Catal., 111 (1988) 360-373. L. Lietti, E. Tronconi and P. Forzatti, J. Mol. Cat., 44 (1988) 201-206. M. Jayamani and C. N. Pillai, J. Catal., 87 (1984) 93-97. D. Chadwick and P. J. R. O'Malley, J. Chem. SOC., Faraday Trans. 1, 83 (1987) 2227-2241. Unpublished results from our laboratories. K. Klier, Plenary Lecture, XI Simposio Iberoamericano de Catalisis, Guanajuato, Mexico (1988).
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C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
591
"SURFACE ORGANOMETALLIC CHEMISTRY : NEWSELECTIVE BIMETALLIC RUSn/Si02 CATALYSTS PREPARED BY REACTION OF TETRA- N-BUTYL TIN WITH R u 0 2 SUPPORTED ON SILICA"
P. LOUESSARD'I), (1) : I.R.C.,
J.P.
CANDY''),
2 avenue Albert Einstein
(2) : I.F.P., B.P. 311, 92506
-
J.P.
-
BOURNONVILLE(2) and J.M.
BASSET'')
69626 VILLEURBANNE Cedex FRANCE
RUEIL MALMAISON Cedex FRANCE
ABSTRACT The reaction of tetra-n-butyl tin with superficially oxidized supported ruthenium particles occurs at moderate temperature, from 323 to 673 K, via a two steps process. Only butane and butenes a r e evolved in a total amount close to 4 C4 radicals per anchored tin atom. The Sn/Ru ratio can reach a value as high as 3 via this procedure. Electron microscopy studies indicate t h a t the size of t h e particles increases slightly as the reaction between SnBu4 and superficial R u 0 2 oes on. These articles have an homogeneous bimetallic composition as demonstrate% by STEM a n 8 no tin signal is. detected ,in ,s! nificant amount on the support. Chemisorption of both H2 and CO is strongly inhibifed by the addition of tin. Depending on t h e Sn/Ru ratio, the selectivity for the hydrogenolysis of ethylacetate can be quite different. With RulSi02, t h e reaction is not selective : CH4, C2H6, CO, C 0 2 as well as ethanol and acetic acid a r e produced. Low tin content catalysts a r e fairly selective for the formation of acetic acid ; while at higher Sn/Ru ratios, ethanol is the only detected product. A mechanism based on the concept of "site isolation" is proposed. INTRODUCTION A new generation of bimetallic catalysts, which exhibit unusual activity and selectivity, can be obtained via t h e reaction of organometallic compounds of main group elements with a group VIII metal supported on silica ( I t o 5). This method of preparation, based on a controlled surface reaction between an organotin compound and supported rhodium, palladium and nickel particles, leads to an homogeneous bimetallic phase (6).
The effect of tin addition on the activity and selectivity of silica supported rhodium catalysts was first evidenced by the selective transformation of ethylacetate into ethanol (4). The presence of tin not only suppresses the multiple hydrogenolysis of ethylacetate into light hydrocarbons (methane and ethane) but also enhances the r a t e of alcohol formation (7). Tin has apparently three effects ( 8 ) :
-
I t significantly decreases the amount of C O and H2 adsorbed. I t isolates rhodium
metal atoms from their neighbours.
- I t slightly increases the electron density of t h e supported metal.
This paper is devoted to the extension of t h e concept of surface controlled reaction
592
with a n o t h e r supported group VIII metal. Among others, supported ruthenium was chosen f o r i t s well known hydrogenating and hydrogenolyzing properties (9). EXPERIMENTAL Silica (SSSO B 1.5 f r o m SHELL) was used as t h e support m a t e r i a l . A f t e r drying a n d calcination under flowing dry a i r at 773 K during 2 hours, t h e s u r f a c e a r e a of 2 t h e support w a s found to b e 350 m /g. E l e c t r o n Microscopy (CTEM and STEM) T w o t y p e s of e x p e r i m e n t s w e r e c a r r i e d o u t using Conventionnal T ransmission E l e c t r o n Microscope (CTEM) and Scanning Transmission Electron Microscope (STEM). CTEM and STEM e x p e r i m e n t s w e r e respectively performed on a JEOL 100 C X and a Vacuum G e n e r a t o r HB 5 microscope. CTEM was used to d e t e r m i n e t h e histogram of supported ruthenium p a r t i c l e s s i z e s b e f o r e and a f t e r r e a c t i o n with t e t r a b u t y l tin in order to c a l c u l a t e t h e m e t a l l i c djspersion and t o monitor t h e p a r t i c l e g r o w t h a f t e r tin addition. STEM e x p e r i m e n t s w e r e c a r r i e d o u t on t h e bimetallic c a t a l y s t to d e t e r m i n e t h e composition of t h e p a r t i c l e s (Ru, Sn).
H
a n d C O chemisorption These e x p e r i m e n t s w e r e c a r r i e d o u t in a conventionnal s t a t i c volumetric device.
T h e equilibrium pressures w e r e measured with a Texas Instruments gauge operating in t h e pressure r a n g e O-O.IMPa. The e r r o r in t h e a c c u r a c y of t h o s e m e a s u r e m e n t s was equal t o 2 I %. Vacuum was achieved using a turbomolecular pump, which avoids c o n t a m i n a t i o n of t h e c a t a l y s t s by mercury. T h e g a s phase p r e s e n t i n t h e vessel was analyzed by a Mass S p e c t r o m e t e r Supavac f r o m VG. Pressure m e a s u r e m e n t s w e r e c a r r i e d o u t at 294 K. In t h o s e conditions, t h e e r r o r in t h e d e t e r m i n a t i o n of adsorbed g a s e s w a s less t h a n 5 %. Catalytic reaction The hydrogenation of e t h y l a c e t a t e to e t h a n o l w a s chosen as t h e test-reaction. Ethanol is selectively produced according to t h e following equation :
CH3
- C, Yo
+ 2 H2
_j
2 C2H50H
0 - C2H5
Various o t h e r products w e r e identified, particularly for t h e case of ruthenium/SiO2: CO, C02, CH4, C2H6 and CH3-CHO. The r e a c t i o n w e r e c a r r i e d o u t in a fixed bed flow differential r e a c t o r at b e t w e e n 473 and 573 K under a pressure of 4.2 MPa of hydrogen and 0.5MPaof e t h y l a c e t a t e . T h e weight hourly s p a c e velocity (WHSV) was 16 (wt/wt/h).
The conversions w e r e
less t h a n 15 % w h a t e v e r t h e operating conditions. Before t h e reactions, c a t a l y s t s
593
w e r e reduced in flowing hydrogen at 673 K. Activity w a s defined as t h e number of mole of e t h y l a c e t a t e transformed per g r a m m e of c a t a l y s t per hour (mol/g/h). Selectivity w a s as t h e molar r a t i o of a given product to t h e sum of t h e products resulting from t h e transformation of e t h y l a c e t a t e . RESULTS Preparation and characterization of t h e c a t a l y s t s
* Monometallic c a t a l y s t s (Ru/Si02 ) The R U ~ ( C O cluster )~~ was adsorbed onto t h e s u r f a c e of t h e silica support t h e n thermally decomposed in absence of w a t e r and oxygen. The remaining carbon monoxide ligands w e r e removed by heating under vacuum at 773 K (10-11). C a t a l y s t s loadings w e r e varied between 0.8 and 1.14 % weight depending on t h e sample.
- Average value : 1.80nm
3.0
4.0
1
Average value : 2.65 nm
I
-€L 1.0
:
r! 3.0 4.0
Fig. I : Histograms of particle size A : Monometallic c a t a l y s t B : Bimetallic c a t a l y s t (Sn/Ru = 2.7)
@ RuSn/SiOz (Sn/Ru = 2.7)
L 5.0
6.0
7.0
Particle size (nm)
594
P a r t i c l e s i z e distribution w e r e determined by CTEM experiments. A typical histogram i s shown in figure 1. The main d i a m e t e r for t h e d e t e c t e d particles i s 1.8 nm (particles less than 0.7 nm in size could n o t b e d e t e c t e d and w e r e t h e r e f o r e not counted in t h e mean d i a m e t e r calculation). This value corresponds to a dispersion of 0.6, assuming
a cubo-octahedral particle shape (12). These results a r e in good a g r e e m e n t with t h e chemisorption measurements (fig. Z), if t h e respective stoechiornetries for hydrogen and carbon monoxide adsorption a r e considered to be : H/RyS) = I (13 to 15) and CO/Ru($ = 1.5.
1O(
Fig. 2 : Hydrogen and Carbon monoxide adsorptions A : Monometallic c a t a l y s t
B : Bimetallic c a t a l y s t (Sn/Ru = 0.1)
* Bimetallic
catalysts
The reaction, between t h e organotin compound and t h e superficially oxidized supported monometallic precursor, was performed in a volumetric apparatus. The controlled oxidation of t h e monometallic particle led to t h e formation of RuO
2
at
t h e surface of t h e particle (16). The tetrabutyl tin was c o n t a c t e d with t h e monometallic Ru catalyst at room t e m p e r a t u r e , followed by increasing by 50 K up t o 673K. A t e a c h step, t h e n a t u r e and t h e amount of t h e reaction products w e r e determined by volumetric measurement and g a s chromatography.
595
Only b u t a n e and b u t e n e s w e r e d e t e c t e d during t h e i n c r e a s e in t e m p e r a t u r e . The t o t a l a m o u n t of evolved C
hydrocarbons, versus t h e t e m p e r a t u r e , a r e shown in figure 4 3 for t w o tin on ruthenium r a t i o s and for tin o n silica. Tetrabutyl-tin r e a c t s slightly with silica over t h e e n t i r e t e m p e r a t u r e r a n g e studied.
Butane is t h e only initially evolved product
. The
organotin compound r e a c t s with
t h e s u r f a c e silica hydroxyl groups, thus allowing t h e formation of b u t a n e and t h e anchoring of t i n in t h e f o r m a t i o n of a s u r f a c e c o m p l e x
such as - Si-0-SnBu remaining bound butyl radicals a r e thermally eliminated- at higher t e m p e r a t u r e .
X*
The
In presence of supported oxidized ruthenium particles, t h e loss of approximatively
4 C
4 hydrocarbons per f i x e d t i n a t o m was observed. A t t h e e n d of t h e r e a c t i o n t h e
final r a t i o of butane/butenes w a s found to depend on t h e tin c o n t e n t in t h e following way : --
f o r Sn/Ru = 1.4 : R u O l a 3 + 1.4 Sn ( C , ) , - ) R U O ~ . ~ ~
f o r Sn/Ru = 2.7 : R u O I a 3 t 2.7 Sn(C4)4->
+ 3.6 C4 + 2.1 C 4 + 0.7 H 2 0
RuOOe4 Sn2.7 + 6.3 C4- + 4.6 C 4 + 0.85H20
T h e variation of t h e b u t a n e l b u t e n e s r a t i o i n d i c a t e s t h e m o r e o r less got on reduction of t h e oxidized ruthenium depending o n t h e tin amount.
A f t e r t h e r e a c t i o n of t e t r a b u t y l t i n with supported oxidized ruthenium p a r t i c l e s
at 673 K, t h e m e t a l l i c p a r t i c l e s w e r e found to b e of homogeneous bimetallic composition (Ru and Sn) as d e m o n s t r a t e d by STEM experiments. T h e addition of tin w a s found
to lead to
p a r t i c l e g r o w t h (fig I). The m e a n d i a m e t e r of t h e observed p a r t i c l e s
increased f r o m 1.8 n m (monometallic R u / S i 0 2 ) t o 2.6 nm (bimetallic Ru-Sn/Si02 with Sn/Ru = 2.7). T h e chemisorption properties of t h e s e bimetallic p a r t i c l e s a r e strongly modified with r e s p e c t to t h e monometallic particles. Whereas t h e R u / S i 0 2 c a t a l y s t chemisorbs 0.6 a t o m
of hydrogen a n d 0.86 molecule of carbon monoxide per ruthenium a t o m
at room t e m p e r a t u r e , t h e b i m e t a l l i c Ru-Sn c a t a l y s t s chemisorb less t h a n 0.1 a t o m
of hydrogen and 0.2 molecule of carbon monoxide per ruthenium a t o m f o r very low added tin a m o u n t (fig. 2). C a t a l y t i c p r o p e r t i e s : hydrogenation of e t h y l a c e t a t e T h e t r a n s f o r m a t i o n of e t h y l a c e t a t e i n t o t w o molecules of e t h a n o l requires a h,ighly s e l e c t i v e r e a c t i o n which involves t h e breaking of only o n e carbon-oxygen bound. The R u / S i 0 2 c a t a l y s t is a c t i v e in t h e hydrogenation of e t h y l a c e t a t e , b u t i t s selectivity to e t h a n o l is very low, less t h a n 10 %. The main side r e a c t i o n s a r e hydrogenolysis to CH4, C2H6, CO a n d H 20, d u e to t h e very high hydrogenolyzing a c t i v i t y of ruthenium. T h e a p p a r e n t a c t i v a t i o n energy of t h e overall r e a c t i o n is 75 KJ/mole (16).
596
CdSn
CdM.cata (mol/g)
I
104
t4
Pc Fig. 3 : C4 Hydrocarbons evolvements during SnBu4 reaction with R u 0 2 supported on silica A : SnBu4/Si02
B : SnBu4/Ru-Si02 (Sn/Ru = 1.5) C : SnBu4/Ru-Si02 (Sn/Ru = 2.7)
Butane
a
aTrans-2 butene Cis-2 butene
597
25
T: 521K
C
.-0
fa!
10
> C 0
u
\
P : 4.2MPa HdEster: 9 WHSV: 16
8-
6-
42
Sn/Ru
0
1
2
Fig. 4 : Variation of the Ethylacetate conversion as a function of atomic Sn/Ru ratio : 4.2 b ; H2/EtAc = 9 ; T : 521
K
; WHSV : 16 h-l)
Two principle features of the Ru-Sn containing catalysts a r e shown in f i g u r e 4 and 5. As the atomic ratio Sn/Ru increases from 0 to I, the overall conversion of e t h y h c e t a t e decreases sharply whereas t h e selectivity t o ethanol increases significantly. Above a ratio Sn/Ru of 1, the conversion of ethylacetate increases with tin content and, simultaneously, the selectivity to ethanol reaches very high values (
> 90
%),
When the Sn/Ru ratio exceeds I , t h e apparent activation energy decreases t o SO KJ/mok (16), thus indicating a change in the way t h a t ethylacetate is activated as compared
to t h e monometallic catalyst. The influence of tin content on the conversion of ethylacetate has already been observed for t h e cases of Rh-Sn (4-7) and Ni-Sn (17) catalysts. Nevertheless, Ru-Sn catalysts exhibit a quite different behaviour. The formation of acetic acid is observed, when the Sn/Ru ratio is lower 0.8. The molar selectivity to acetic acid formation increases from I0 % (Ru/Si02) to 40 % (Ru-Sn/Si02 with SnfRu = 0.25) and then decreases t o 0 % when Sn/Ru ratio equals 0.8 (fig. 5).
As t h e tin content increases, t h e size of ruthenium atoms ensembles decreases. Consequently, the first affected reactions a r e those which require multiple bond adsorption, such as deep hydrogenolysis. When small ruthenium atoms ensembles remain, bridging bidentate carboxylate species can be formed, which lead t o t h e desorption of one mole of a c e t i c acid and ethane. These products can r e a c t again with t h e metallic
598
'WI
T = 548 K, P = 4.2 MPa, Alp/AcOEt = 9,WHSV = 16
Fig. 5 : Hydrogenation of Ethylacetate : Influence of t h e Sn/Ru r a t i o on t h e selectivity
phase to give ethanol and methane respectively. Finally, when ruthenium a t o m s a r e isolated from e a c h other at high tin content, t h e only allowed reaction is t h e hydrogen a t t a c k of t h e unsaturated carbon-oxygen bond, leading to t h e formation of a hemiacetal. This r a t h e r unstable adsorbed intermediate is quickly decomposed into ethanol and acetaldehyde, which is hydrogenated into ethanol (fig. 6 ) .
CONCLUSION
The reaction of
tetrabutyltin with superficially oxidized ruthenium particles
proceeds in a similar way than with oxidized rhodium particles : partial reduction of t h e surface metallic oxide leading to t h e formation of ruthenium-oxygen -tin bonds and t o t h e formation of butenes and butane. This method of preparation leads to
a well defined and homogeneous bimetallic phase. Nevertheless t h e mechanism of reaction between tetrabutyltin and oxidized ruthenium is not fully elucidated. Depending on t h e tin content, t h e bimetallic phase c a n selectively lead to a c e t i c acid or ethanol. This change is selectivity could b e governed by t h e size of t h e ruthenium a t o m s ensembles which is a function of t h e amount of tin added. The d e c r e a s e in hydrogen and carbon monoxide adsorption is in a g r e e m e n t with t h e concept of ruthenium a t o m s isolation by inactive tin a t o m s but cannot account for t h e overall process, such as t h e increase in t h e r a t e of ethanol formation. The influence of others factors, such as a n electronic e f f e c t due to tin, leading to an increase in t h e electron density on ruthenium
, is also likely involved.
0 It
4
II
d
hl It
rn
I
I
I 2
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ln
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- -+ 2"
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599
ACKNOWLEDGEMENTS The authors wish to thank C. CAMERON (I.F.P.) for helpful discussions. REFERENCES 1
-
Y.I. YERMAKOV and L.N. ARZAMASKOVA in L. Cerveny (Editor), C a t a l y t i c Hydrogenation, Studies in s u r f a c e s c i e n c e and Catalysis, Vol. 27, Elsevier, Amsterd a m , 1986, pp 459-497. 2 - Y.I. YERMAKOV, B.N. KUZNETSOV and V.A. ZAKHAROV in Catalysis by supported corn lexes, Studies in s u r f a c e s c i e n c e and catalysis, Vol 8, Elsevier, A m s t e r d a m , I9g3. 3 - J. MARGITFALVI, S. SZABO and F. NACY in L. Cerveny (Editor) C a t a l y t i c hydrogenation, Studies in s u r f a c e s c i e n c e and catalysis, vol 27, Elsevier, Amsterdam, 1986, pp 373-409. 4 - C. TRAVERS, J.P. BOURNONVILLE a n d C. MARTINO, P r o c of 8 t h International Congress on Catalysis, Berlin (West), West-Germany, July 2-6, 1984, Verlag c h e m i e Ed,IV, pp 891-902. 5 - US p a t e n t 4,380,673 assigned to I.F.P. US p a t e n t 4,456,775 assigned t o I.F.P. US p a t e n t 4,504,593 assigned to I.F.P. US p a t e n t 4,628,130 assigned to I.F.P. 6 - O.A. FERRETTI, L.C. BETTECA DE PAULI, J.P. CANDY, C. MABILON and J.P. BOURNONVILLE in 8. Delrnon, P. Grange, P.A. J a c o b s and G. P o n c e l e t (Editors ) P r e p a r a t i o n of c a t a l y s t s IV, Scientific bases f o r t h e preparation of Heterogeneous Catalysts, Studies in s u r f a c e s c i e n c e and catalysis, vol 31, Elsevier, Amsterdam, 1987, pp 713-724. 7 - J.P. CANDY, O.A. FERRETTI, J.P. BOURNONVILLE and C. MABILON, J. Chern. Soc, C h e m comrnun (1985) 1197-1198. 8 - A. EL MANSOUR - Thesis - Lyon (1986). 9 - J.H. SINFELT and D.J.C. YATES - J of Catal. 8, (1967)82-90. 10 - A. THEOLIER, A. CHOPLIN, L. DORNELAS and J.M. BASSET, Polyhedron VOI. 1 - No 1 (1982). I 1 - J.M. BASSET and A. CHOPLIN, J of Molecular Catalysis 21 (1983) p p 95-108. 12 - W. ROMANOWSKI, surf. sci., 18 (1969) pp 373-388 M.J. YACAMAN, surf. sci., 87 (1979)p. 263. M.J. YACAMAN, surf. ~ c i .106 (1981)p. 472. R.M. PILLIARD and J. NU'TTING, J Phil Mag, 16 (1967)p. 181. 13 -R.A. DELLA BETTA, J of Cata., 34 (1974)pp 57-60. 14 -K.C. TAYLOR, J of Catal., 38 (1975) pp 299-306. 15 -K.LU and B.J. TATARCHUK, J of Catal., 106 (1987)pp 166-175. 16 -P. LOUESSARD - Thesis in progress 17 -0.A. FERRETTI - Thesis - Paris - (1986). 18 -M. ACNELLI, Work in progress, I.R.C. (1988).
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
601
INFRARED SPECTRA OF CARBONS
x.
THE SPECTRAL PROFILE OF MEDIUM-TEMPERATURE
CHARS*
M. J. D. LOW and C. MORTERRA Chemistry Department, New York University, New York, N. Y.,
10003 (U. S. A.)
ABSTRACT I t i s f r e q u e n t l y found t h a t when various organic m a t e r i a l s are pyrolyzed a t temperatures i n excess o f about 55OoC, the i n f r a r e d spectra o f the chars assume s i m i l a r , a l m o s t i d e n t i c a l , p r o f i l e s . That would i n d i c a t e t h a t t h e c h a r s had become almost o r completely spectroscopically and chemically i n d i s t i n g u i s h a b l e and u n r e l a t a b l e t o t h e i r precursors. Some r e s u l t s o f pyrolyses o f c e l l u l o s e based and s i m i l a r o x y g e n - c o n t a i n i n g p r e c u r s o r s , as w e l l as p r e c u r s o r s n o t c o n t a i n i n g oxygen, a r e considered. The s p e c t r a l p r o f i l e s , and t h e s p e c t r a l changes preceeding and leading t o them, are described and discussed. INTRODUCTION I f one compares sequences o f i n f r a r e d ( I R ) s p e c t r a r e c o r d e d a t v a r i o u s
s t a g e s o f p y r o l y s i s o f a w i d e v a r i e t y o f o r g a n i c m a t e r i a l s [l-111, one f i n d s t h a t the data appear t o f a l l i n t o several groups. I n one o f these, the spectra o f the p y r o l y t a t e s (and sometimes even the spectrum o f the s t a r t i n g m a t e r i a l ) show one o r more v i b r a t i o n a l s p e c t r a l f e a t u r e s w h i c h p e r s i s t t h r o u g h o u t t h e p y r o l y s i s range u n t i l the highest temperatures (or b u r n o f f ) are reached. These s p e c i a l s p e c t r a l f e a t u r e s can be v e r y i n t e n s e as i s found, f o r example, i n s p e c t r a o f c h a r r e d bones o r t e e t h ; t h e a p a t i t e bands dominate t h e s p e c t r a . A s i m i l a r s i t u a t i o n i s encountered w i t h s i l i c a - c o n t a i n i n g vegetable matter. With r i c e husks charred a t even very low temperature, f o r example, the spectra show Si-0 absorptions which are so intense as t o overwhelm and dwarf o t h e r spectral f e a t u r e s [12].
D e s p i t e t h e presence o f such i n t e n s e and c h a r a c t e r i s t i c
" i m p u r i t y " bands, i t i s p o s s i b l e t o observe c o n t i n u o u s changes o f t h e o t h e r v i b r a t f o n a l f e a t u r e s as t h e c h a r r i n g proceeds a t p r o g r e s s i v e l y h i g h e r temperatures. That, however, appears n o t t o be so w i t h a second group o f data which e x h i b i t n o t a continuous b u t an abrupt change o f the v i b r a t i o n a l features when c e r t a i n temperatures are reached. That i s i l l u s t r a t e d d r a m a t i c a l l y by I R spectra o f intumescent chars [13]. When a p e n t a e r y t h r i tol-(NH&HP04 mixture i s m o d e s t l y heated, f o r example, t h e r e i s an a b r u p t , a l m o s t " e x p l o s i v e " l o s s o f
*
: Part I X : r e f . 9
602
v i b r a t i o n a l features,
and the spectrum assumes the featureless appearance o f
t h a t o f a high-temperature char, showing o n l y the I R continuum absorption [l]. A v a r i a t i o n o f t h i s behavior i s found w i t h polyvinyledene c h l o r i d e ; t h e r e i s a r e l a t i v e l y r e g u l a r disappearance o f v i b r a t i o n a l f e a t u r e s accompanied b y t h e build-up o f the I R continuum, b u t v i b r a t i o n a l features i n d i c a t i n g t h e f o r m a t i o n o f aromatic intermediates a r e n o t observed, as w i l l be described elsewhere. The most common behaviour, however, i s found w i t h a n o t h e r g r o u p o f d a t a : what i s u s u a l l y found i s t h a t as the temperature i s p r o g r e s s i v e l y r a i s e d t h e r e are progressive changes i n the spectra i n t h a t the v i b r a t i o n a l f e a t u r e s o f the p r e c u r s o r d i s a p p e a r and a r e r e p l a c e d w i t h new ones w h i c h i n t u r n change and d e c l i n e i n i n t e n s i t y and t h e n e v e n t u a l l y d i s a p p e a r as t h e t e m p e r a t u r e r i s e s above a b o u t 700'. What r e m a i n s i s t h e c o n t i n u u m a b s o r p t i o n . The i n i t i a l spectrum o f a p y r o l y s i s sequence i s o f course, c h a r a c t e r i s t i c o f t h e precursor, as a r e the spectra obtained a t the l o w e r decomposition temperatures. Also, the r a t e s and d e t a i l s o f changes v a r y w i t h t h e n a t u r e o f t h e p r e c u r s o r . E f f e c t i v e l y , t h e I R s p e c t r a o f m a t e r i a l s heated b e l o w a b o u t 400 o r 450' a r e h i g h l y c h a r a c t e r i s t i c o f t h e p r e c u r s o r . What i s i n t e r e s t i n g and, we t h i n k , q u i t e p e r t i n e n t i s t h a t i f t h e p y r o l y s i s t e m p e r a t u r e s proceed i n t o t h e 500
600'
-
range, t h e spectra change f u r t h e r and assume a g e n e r a l l y s i m i l a r p r o f i l e .
This paper i s concerned w i t h some aspects o f t h e spectra o f chars showing such s i m i l a r i t i e s o f p r o f i l e .
METHODS The technique employed was I R Fourier transform Photothermal Beam D e f l e c t i o n spectroscopy (PBD). The PBD technique, t h e apparatus, and the vacuum c e l l s used f o r i n s i t u work have been described elsewhere [l, 2, 14, 151. The spectra were recorded a t 8 cm-'
r e s o l u t i o n w i t h scan periods varying from several minutes t o
hours. The o r d i n a t e l a b e l S/So i n d i c a t e s t h a t a spectrum
S was compensated [l]
by r a t i o i n g i t against the spectrum So o f a high-temperature carbon o r P t black r e f e r e n c e . The o r d i n a t e s a r e a r b i t r a r y . Note t h a t the photothermal responses a r e p o s i t i v e , i.e.,
I R bands p o i n t "up,"
away f r o m t h e abscissa. The number
beside each spectrum i n d i c a t e s the p y r o l y s i s temperature i n
OC.
RESULTS AND DISCUSSION Fig. 1 shows spectra o f a v a r i e t y o f chars. PWC i s a char derived from p i n e wood a t 55OoC ( t h e numbers appended t o t h e symbol i n d i c a t e t h e p y r o l y s i s temperature).
F derived from c e l l u l o s e fibers;
NOC from oxidized c e l l u l o s e ; SC
from a South Carolina coal; PO from a polycarbonate resin; S from sucrose; COC f r o m coconut s h e l l ; and PVB f r o m p o l y v i n y l bromide. S p e c t r a showing s i m i l a r p r o f i l e s a r e shown i n t h e o t h e r f i g u r e s , e.g.,
t h e t o p t w o s p e c t r a o f Fig. 2
603
w h i c h d i s p l a y s a p y r o l y s i s sequence o f a pheno 1-formaldehyde "Novolac"-type
r e s in (PFN).
E n t i r e l y s i m i l a r r e s u l t s have been o b t a i n e d w i t h peat, peach p i t s , w a l n u t s h e l l s , and t h e PWC550
l i k e [12].
Such m a t e r i a l s are,
o f course,
F530 NOC540
s i m i l a r and can be regarded as cellulose-based mixtures. Analogous r e s u l t s are t o be found i n
SC500
t h e l i t e r a t u r e , such as Zawadski's s p e c t r a o f carbon f i l m s [16], o r Rouxhet e t al.'s s p e c t r a o f sporopollenin and l i g n i t e chars [17],
o r the
e a r l i e r cellophane char spectra o f Kmetko [18]. PO580
s550
2000
There i s no s p e c i f i c t e m p e r a t u r e a t w h i c h a
COC550
s p e c t r a l p r o f i l e s u c h as t h a t o f PWC500
PVB575
(Fig. 1) o r PFN6OO (Fig. 2) i s o b t a i n e d w i t h a l l materials; the p y r o l y s i s r a t e s are a f f e c t e d by the precursor's composition.
1000 C d
The prominent
features are a d i s t i n c t band peaking near 1600
Fig. 1 Char spectra
cm-l, t h r e e pronounced, p a r t i a l l y overlapping bands i n t h e aromatic C-H deformation region, and a prominent, apparently l a r g e l y s t r u c t u r e l e s s a b s o r p t i o n e x t e n d i n g f r o m a b o u t 1500 t o
1000 cm'l.
The r a t i o o f the i n t e n s i t i e s o f the
1600 cm"
and t h e C-H d e f o r m a t i o n bands may
v a r y somewhat f r o m s p e c t r u m t o spectrum. F o r b r e v i t y , we w i l l term a spectrum which has such features,
much l i k e spectrum PFN600,
the
"standard" p r o f i l e . There are, o f course, changes i n t h e C-H s t r e t c h i n g region d u r i n g a p y r o l y s i s sequence, s u c h a s t h e e x a m p l e shown i n F i g . 3. The spectra shown are those o f pure c e l l u l o s e chars b u t can be taken as being representative o f the changes
generally
observed.
Obviously,
a l i p h a t i c CHn s t r e t c h i n g modes are replaced by a r o m a t i c ones as t h e p y r o l y s i s proceeds. The p o i n t t o be made i s t h a t when the p y r o l y s i s has p r o g r e s s e d t o t h e s t a g e when t h e "standard" K)
Fig. 2 PFN spectra
p r o f i l e i s assumed i n t h e f i n g e r p r i n t range, very few i f any o f the a l i p h a t i c CHn s t r u c t u r e s remain. Therefore,
t h e r e i s very l i t t l e i f any
c o n t r i b u t i o n o f any t y p e o f CHn mode t o t h e
604
Fig. 3
C-H Stretching region
lsbo
Spectra of Cellulose Chars
absorptions in the fingerprint range, specifically t o t h e broad absorption i n t h e 1500-1000 cm" range. A second p o i n t t o be made concerns t h e carbonyl absorptions i n the 1900-1600 cm-' range. When o x y g e n - c o n t a i n i n g p r e c u r s o r s a r e pyrol yzed, the spectra invariably show absorptions i n t h e 1900-1600 cm" range a s c r i b a b l e t o a c i d c a r b o n y l s ; t h e number o f bands and t h e i r i n t e n s i t i e s depend on the nature of the precursor. The s t r u c t u r e s causing t h e s e bands are, however, r e l a t i v e l y unstable. An example i s shown i n Fig. 4. The 400' spectrum of a c e l l u l o s e char shows a r e l a t i v e l y s t r o n g complex o f bands c e n t e r e d n e a r 1700 cm-l. A s t h e p y r o l y s i s temperature is increased, the absorptions diminish substantially. Minor absorptions of this type can a l s o be seen i n t h e s p e c t r a of Fig. 1. Such a c i d carbonylic s p e c i e s a r e thought t o be destroyed and/or p a r t i a l l y converted t o o t h e r oxygenated species absorbing i n the 1300-1100 cm" range [ Z ,
PVC
0
Fig. 4
lob0
cb 5
Fig. 5 Spectra of PVC
31. As has been shown by means of experiments involving the formation of a b s o r p t i o n s i n t h a t range by oxidizing high-temperature carbons with 0l8, however, oxygen-containing groups contribute l i t t l e t o t h e absorption [lo]. Note a l s o F i g . 5; i t shows a p y r o l y s i s s e q u e n c e o f p o l y v i n y l c h l o r i d e (PVC). The l a t t e r obviously c o n t a i n s no oxygen, y e t t h e s p e c t r a evolve i n t o t h e standand
605 p r o f i l e as t h e t e m p e r a t u r e i s increased. S i m i l a r changes a r e f o u n d w i t h t h e p y r o l y s i s o f p o l y v i n y l b r o m i d e (spectrum PVB575 o f Fig. 1) and p o l y v i n y l f l u o r i d e [19]. Consequently, t h e r e i s l i t t l e o r no c o n t r i b u t i o n t o t h e a b s o r p t i o n s shown b y t h e s t a n d a r d p r o f i l e b e l o w a b o u t 1500 cm'l b y oxygenc o n t a i n i n g species. Further, t h e 1600 cm'l
band, about which t h e r e has been so
much c o n t r o v e r s y , a l s o appears i n s p e c t r a o f n o n - o x y g e n - c o n t a i n i n g chars, although l e s s d i s t i n c t l y . What a11 t h i s appears t o mean i s t h a t , w i t h t h e e x c e p t i o n o f t h e t h r e e d i s t i n c t bands i n t h e 900-700 cm"
r e g i o n due t o a r o m a t i c C-H o u t - o f - p l a n e
bending modes, the b u l k o f the absorptions g i v i n g r i s e t o the standard p r o f i l e are b r o u g h t a b o u t b y s u r f a c e f u n c t i o n a l groups o f t h e t y p e n o r m a l l y associated w i t h carbon surfaces. Rather, i t would seem t h a t the m a j o r i t y o f the absorptions are caused o n l y by t h e s k e l e t a l modes o f t h e chars. A complete explanation o f the e f f e c t s i n terms o f d i s t i n c t band assignments i s n o t a t hand a t present. So f a r , o u r band r e s o l v i n g c a l c u l a t i o n s i n v o l v i n g the 1600-1000 cm-l region o f t h e p r o f i l e have n o t been f r u i t f u l because these r e q u i r e a p r e c i s e knowledge o f t h e number o f bands; t h a t i s n o t a v a i l a b l e . S i m i l a r l y , o u r s t a t i s t i c a l r e s o l u t i o n enhancement c a l c u l a t i o n s have n o t been s a t i s f a c t o r y because these r e q u i r e a p r e c i s e d e l i n e a t i o n o f band shapes; t h a t i s a l s o n o t a v a i l a b l e a t present. Work on t h a t t o p i c i s proceeding. However, i t seems feasible t o suggest a t e n t a t i v e o r i n t e r i m mechanism, based on several generalities.
One o f t h e s e i n v o l v e s changes o b s e r v a b l e i n t h e s p e c t r a a t
temperatures below t h a t a t which the standard p r o f i l e i s obtained. A p y r o l y s i s sequence o f a p o l y c a r b o n a t e p l a s t i c i s shown i n Fig. 6. Note t h a t t h e r e i s a pronounced " d i p " i n t h e 48OoC s p e c t r u m (suggested b y t h e a r r o w ) r o u g h l y i n t h e 1500-1200 cm" range. A t a s l i g h t l y h i g h e r p y r o l y s i s temperature of 51OoC the d i p decreases and e v e n t u a l l y disappears. The changes suggest t h a t a b r o a d i s h a b s o r p t i o n had "grown
in"
under
the
existing
a b s o r p t i o n s . A s i m i l a r change i s shown
by t h e
PFN
spectra o f Fig. 2 and t o some
e x t e n t b y t h e PVC s p e c t r a o f Fig. 5. Spectra o f c e l lulose-based precursors 650
600
580 51 0 2000
500
om-'
a l s o show t h i s i n c r e a s e i n a b s o r p t i o n , b u t t o a s m a l l e r e x t e n t . An example i s shown i n Fig. 4. The region marked w i t h the double l i n e s , roughly centered near 1350 cm",
shows a d i p which, i n spectra
recorded a f t e r p y r o l y s i s a t h i g h e r t e m p e r a t u r e s , f i l l s i n , a g a i n much as
Fig. 6
Polycarbonate Spectra
i f a another absorption had appeared.
606 The a d d i t i o n a l a b s o r p t i o n appears r e l a t i v e l y suddenly, i.e., o v e r a s h o r t p y r o l y s i s t e m p e r a t u r e range, and a l s o becomes r e l a t i v e l y pronounced i n i n t e n s i t y . It thus seems u n l i k e l y t h a t some f u n c t i o n a l group o r small moleculel i k e moiety responsible f o r the absorption grows suddenly. A t e n t a t i v e and n o t unreasonable explanation o f the absorption centered near 1350 cm-l i s t h a t i t has t h e same o r i g i n as a b s o r p t i o n s observed n e a r 1355 o r 1360 cm" i n Raman and I R spectra o f comminuted g r a p h i t e and hydrogenated amorphous carbon f i l m s [21-261. W i t h g r a p h i t e , t h e i n t e n s i t y o f t h e 1355 cm'l band i s i n v e r s e l y p r o p o r t i o n a l t o c r y s t a l l i t e s i z e and i s thought t o be caused by a breakdown of t h e k s e l e c t i o n r u l e s ; due t o t h e f i n i t e c r y s t a l s i z e , an Alg mode o f t h e l a t t i c e becomes active. The same band i s observed w i t h amorphous carbon f i l m s ; t h e l a t t e r c o n t a i n sp2 s i t e s c l u s t e r e d i n g r a p h i t i c c l u s t e r s c o n s i s t i n g o f f o u r o r more fused s i x f o l d rings, these c l u s t e r s being interconnected b y sp3 carbon. Parenthetically, resonant Raman studies o f such f i l m s show t h a t t h e E2g mode n o m i n a l l y n e a r 1600 cm" and t h e 1355 cm" band s h i f t and change i n r e l a t i v e i n t e n s i t y as f u n c t i o n o f Raman e x c i t a t i o n frequency, resonance a r i s i n g when the e x c i t a t i o n w a v e l e n g t h approaches t h e a p p r o p r i a t e p i - p i * t r a n s i t i o n [23, 241; t h e r e a r e pi-bonded c l u s t e r s o f d i f f e r e n t s i z e s . Carbon f i l m phenomena were r e c e n t l y reviewed [25, 261. The e f f e c t s o f such a band a r e e a s i l y shown by a d d i n g an a b s o r p t i o n t o a spectrum, as i n POLYCARBONATE the crude example shown i n Fig. 7. I f a Gaussian Q c e n t e r e d a t 1350 cm'l i s added t o t h e 48OoC spectrum, t h e d i p i n t h e l a t t e r i s removed and t h e r e s u l t i n g s y n t h e t i c , hypothetical spectrum 48O+Q becomes more l i k e t h e standard p r o f i l e . A second g e n e r a l i t y i n v o l v e s " t h e 1600 cm" band." I t i s i n v a r i a b l y f o u n d t h a t when a p r e c u r s o r i s charred, a f a i r l y i n t e n s e band centered a t o r near 1600 cm'l i s formed a t q u i t e
c 1-
1350
1170
Fig. 7 Polycarbonate Spectra
low temperatures and p e r s i s t s u n t i l q u i t e high temperatures a r e reached. The band's appearance seems t o coincide w i th t h a t o f t h e C-H d e f o r m a t i o n bands. The 1600 c m - l band decreases i n i n t e n s i t y , a l o n g w i t h t h e decrease o f t h e C-H deformation bands, a t t h e highest temperatures. As shown d i s t i n c t l y by t h e sequence o f Fig. 5 (and a l s o by spectra o f PVBr and PVF t o be described elsewhere), the 1600 cm'l band i s a l s o formed w i t h precursors which do n o t contain oxygen. Also, the 1600 cm" band does n o t show an i s o t o p i c s h i f t when formed i n t h e presence o f 0l8, which can lead t o the conclusion t h a t t h e band i s n o t formed b y an o x y g e n - c o n t a i n i n g species. These o b s e r v a t i o n s p a r a l l e l those made almost two decades ago by F r i e d e l and Retcofsky 1271. They were concerned w i t h t h e formation o f coal and attempted t o generate c o a l - l i k e
607 substances v i a low-temperature pyrolyses, i n emulation o f geologic processes. I R t r a n s m i s s i o n s p e c t r o s c o p y was used t o c h a r a c t e r i z e t h e i r products. They showed, i n p a r t i c u l a r , i n one s e r i e s o f elegant experiments, t h a t w i t h a h i g h l y conjugated system such as dibenzoylmethane there were no 018-induced i s o t o p i c s h i f t s , so t h a t d a t a c o n c e r n i n g t h e 1600 cm'l band o b t a i n e d w i t h 018-tagged chars were inconclusive, a r e s u l t which we and others had neglected. F r i e d e l and R e t c o f s k y f a v o r e d a s s i g n i n g t h e 1600 cm'l band t o a c h e l a t e d carbonyl structure, b u t d i d n o t r u l e o u t some c o n t r i b u t i o n t o i t from aromatic structures. We t h i n k t h a t a more p e r t i n e n t explanation, e s p e c i a l l y i n view o f the f a c t t h a t an 1600 cm'l which
band i s formed w i t h chars derived from precursors
do n o t c o n t a i n oxygen,is t h a t t h e 1600 cm"
band i s due t o an a r o m a t i c
C-C stretch. A t very e a r l y stages o f pyrolysis, when i n d i v i d u a l aromatic r i n g s o r s m a l l c l u s t e r s analogous t o s m a l l p o l y n u c l e a r a r o m a t i c hydrocarbon (PAH) molecules have formed (e.g.,
stages t y p i f i e d by t h e 250 and 3OO0C PVC spectra
o f Fig. 5; note t h a t the aromatic C-H deformation bands have already appeared, i n d i c a t i n g t h e f o r m a t i o n o f r i n g s ) , t h e 1600 cm"
band would be t h e "normal"
a r o m a t i c C-C s t r e t c h mode a r i s i n g f r o m s m a l l a r o m a t i c s t r u c t u r e s . A t l a t e stages o f pyrolysis,
when aromatization has proceeded t o such an e x t e n t t h a t
g r a p h i t e - l i k e fragments have formed, t h a t
C-C mode, which i s Raman a c t i v e b u t
I R - i n a c t i v e i n a g r a p h i t i c n e t w o r k , i s g i v e n I R a c t i v i t y by t h e presence o f o x i d i c and/or hydrogen peripheral species. E s s e n t i a l l y , the s t r e t c h i n g mode o f some peripheral C-C bonds i s imparted I R a c t i v i t y and r e t a i n s i t as l o n g as the presence o f a s u f f i c i e n t amount o f C-H groups o r oxygen on the external r i n g s o f the growing polyaromatic network guarantees some asymmetry t o those bonds, i.e.,
as l o n g as s u f f i c i e n t " m o l e c u l a r " n a t u r e i s i m p a r t e d t o t h e p e r i p h e r a l
rings. Thus, the 1600 cm'l
band should be viewed as the r e s u l t a n t o f "quadrant"
s t r e t c h i n g modes o f external r i n g s r a t h e r than as a p l a i n C-C s t r e t c h i n g mode. Also, the broad and m u t u a l l y overlapped bands peaking around 1400 and 1180 cm" such as those found w i t h intermediate temperature PVC chars (Fig. 5) must a l s o be a s c r i b e d t o t h e r e s i d u a l a c t i v i t y o f some v i b r a t i o n a l modes o f o r i n t h e peripheral r i n g s o f the growing polyaromatic network. The I R - a c t i v i t y would be made possible by the " i n d i v i d u a l " (or molecular) nature given t o the p e r i p h e r a l r i n g s b y t h e s t i l l abundant CH groups. I n t h i s r e s p e c t , t h e band e n v e l o p e s h o u l d be t h o u g h t o f as b e i n g t h e r e s u l t a n t o f t h e
around 1400 cm'l
% e m i c i r c l e " r i n g s t r e t c h modes, and the envelope a t 1200 cm" o f m a i n l y (but n o t purely) in-plane C-H deformation modes. What i s suggested, then,
as the r e s u l t a n t
i s t h a t there are t h r e e stages i n the p y r o l y s i s o f
m a t e r i a l s o f t h e t y p e mentioned. Stage I, w h i c h i n v o l v e s t h e m o s t complex chemical and spectroscopic changes, involves the d e s t r u c t i o n o f the precursor's "skeleton" along w i t h the e l i m i n a t i o n o f some f u n c t i o n a l groups. As shown by the
IR spectra, even a t q u i t e low temperatures some aromatic r i n g s and small
608 c l u s t e r s o f r i n g s r e s e m b l i n g s m a l l PAH s t r u c t u r e s a r e formed. These changes occur progressively as the temperature i s raised. A t t h i s stage t h e spectrum o f the char i s the summation o f t h e normal v i b r a t i o n a l modes o f t h e a l i p h a t i c and n e w l y - f o r m e d a r o m a t i c p o r t i o n s . Then, when t h e t e m p e r a t u r e r i s e s i n t o t h e v i c i n i t y o f 500°,
Stage I 1 i s reached and the small PAH-like c l u s t e r s condense
t o form c l u s t e r s o f intermediate s i z e which are l a r g e enough so t h a t t h e 1350 c m - l a b s o r p t i o n becomes a c t i v e and a l s o t h e I R a c t i v i t y o f t h e r i n g modes i s i n d u c e d by s u b s t i t u e n t s a t t h e p e r i p h e r a l r i n g s . A t t h i s s t a g e t h e c h a r spectrum i s t h e summation o f t h e normal v i b r a t i o n a l modes p l u s t h e induced ones. Remnants o f t h e p r e c u r s o r s t i l l p r e s e n t i n t h e m i x t u r e may make i t possible t o r e l a t e the char spectrum t o the precursor. Further heating i n t o the 550-650°C region (these temperature regions are approximate and depend on the n a t u r e o f t h e p r e c u r s o r ) l e a d s t o Stage 111: r e s i d u a l p r e c u r s o r remnants a r e removed o r incorporated i n t o the growing polyaromatic clusters. There are now no I R - d e t e c t a b l e f u n c t i o n a l groups, w i t h t h e e x c e p t i o n o f t h e C-H groups o f p e r i p h e r a l r i n g s , and t h e v i b r a t i o n a l spectrum w h i c h i s observed i s t h e summation o f t h e modes o f the aromatic C-H bands and the r i n g modes induced by them and by peripheral oxygen. As pointed o u t above, there i s no c o n t r i b u t i o n t o t h e s t a n d a r d p r o f i l e by a l i p h a t i c CHn modes and v e r y l i t t l e i f any by t h e modes o f s u r f a c e s p e c i e s o f t h e t y p e u s u a l l y a s s o c i a t e d w i t h carbons. Essentially,
there are none. The spectrum can now no longer be r e l a t e d t o the
p r e c u r s o r . F u r t h e r h e a t i n g causes t h e removal o f t h e s u b s t i t u e n t s on t h e peripheral r i n g s so t h a t the induced I R a c t i v i t y diminishes u n t i l , eventually, above about 700-750°C,
no v i b r a t i o n a l components can be observed. Chars which
have reached Stage I11 o r have passed beyond i t are chemically active. I f there a r e no c o n v e n t i o n a l f u n c t i o n a l groups, t h e s u r f a c e a c t i v i t y would seem t o reside i n the now "empty" exposed peripheral rings. Some support f o r t h i s general mechanism may come from near-IR measurements, which should provide i n f o r m a t i o n about the s h i f t o f the absorption edge o f the c h a r s f r o m t h e v i s i b l e i n t o t h e n e a r - I R and t h e I R ranges. Also, i t seems possible t h a t the changes o f the C-H deformation bands can provide i n f o r m a t i o n a b o u t t h e g r o w t h and changes o f t h e c l u s t e r s a t v a r i o u s s t a g e s o f p y r o l y s i s . Note t h e r a t h e r l a r g e and o b v i o u s changes i n t h e C-H d e f o r m a t i o n bands i n t e n s i t i e s which can be observed as the pyrolyses proceed (Fig. 2 and Fig. 5). I t s h o u l d be p o s s i b l e t o r e l a t e t h e number o f bands and t h e i r i n t e n s i t i e s t o
t h e i r spacial d i s t r i b u t i o n on the p e r i p h e r a l r i n g s . U n f o r t u n a t e l y , t h e 20001650 cm" range o f the "summation bands," which might provide such information, y i e l d s l i t t l e ; t h e summation bands a r e u s u a l l y v e r y weak and, i n t h e case o f many char spectra, n o t d i s t i n c t l y observable. Also, there appears t o be l i t t l e i n f o r m a t i o n about the e x t i n c t i o n c o e f f i c i e n t s o f t h e C-H deformation bands o f PAH species.
Measurements made w i t h t h e consensed s m a l l PAHs coronene,
609
methylcoronene, and c i r c o b i p h e l y l ( t h e s e c o n t a i n t w o a d j a c e n t p e r i p h e r a l hydrogens) and a l s o d i c o r o n e n e and ovalene ( t h e s e c o n t a i n s i n g l e as w e l l as dual peripheral hydrogens) suggest t h a t the e x t i n c t i o n c o e f f i c i e n t o f the band o f t h e hydrogen s i n g l e t may be f o u r o r f i v e t i m e s as g r e a t as t h a t o f t h e p a i r e d hydrogen [28]. Consequently,
a l t h o u g h t h e changes o f t h e c h a r C-H
d e f o r m a t i o n bands can be e a s i l y observed, q u a n t i t a t i o n i s m i s s i n g . Work on such t o p i c s , i n c l u d i n g f a r - I R measurements, i s needed.
ACKNOWLEDGEMENT Support by NSF grant No. CHE-8516257 and NATO g r a n t 268/83 i s acknowledged. REFERENCES
1 2 3 4 5 6 7 8 9 10 11 12 13 14
M.J.D. Low and C. M o r t e r r a , Carbon 21 (1983) 275. C. M o r t e r r a and M.J.D. Low, Carbon 21 (1983) 283. C. M o r t e r r a , M.J.D. Low, and A.G. Severdia, Carbon 22 (1984) 5. C. M o r t e r r a and M.J.D. Low, Carbon 23 (1985) 301. M.J.D. Low and C. M o r t e r r a , Carbon 23 (1985) 311. C. M o r t e r r a and M.J.D. Low, Carbon 23 (1985) 335. C. M o r t e r r a and M.J.D. Low, Carbon 23 (1985) 525. C. Morterra and M.J.D. Low, Langmuir 1 (1985) 320. C. Morterra, M.L. O'Shea and M.J.D. Low, Mat. Chem. Phys. 20 (1988) 123. C. M o r t e r r a and M.J.D. Low, Mater. L e t t . 2 (1984) 289. C. M o r t e r r a and M.J.D. Low, Mat. Chem. Phys. 12 (1985) 207. N. Wang and M.J.D. Low, i n preparation. P.G. Varlashkin and M.J.D. Low, Appl. Spectrosc. 40 (1986) 393. M.J.D. Low, C. M o r t e r r a , A.G. S e v e r d i a and M. L a c r o i x , Appl. S u r f . Sci. 13
(1982) 429. 15 M.J.D. Low and C. M o r t e r r a , Adsorp. Sci. Technol. 2 (1985) 131. 16 J. Zawadzki, Carbon 25 1987) 431. 17 P.G. Rouxhet, M. V i l l e y and A. Oberlin, Geochim. Cosmochim. Acta 43 (1979) 1705. 18 E.A. Kmetko, Phys. Rev. 82 (1951) 456. 19 M.S. O'Shea, M.J.D. Low and C. Morterra, i n preparation. 20 B.R. Frieden, J. Opt. SOC. Am. 73 (1983) 927. 21 F. T u i n s t r a and J.L. Koenig, J. Chem. Phys 53 (1970) 1126. 22 R.J. Nemanich and S.A. Solin, Phys. Rev. B. 20 (1979) 392. 23 M. Yoshikawa, 6. K a t a g i r i , H. I s h i d a and A. I s h i t a n i , Appl. Phys. L e t t . 52 (198) 1639. 24 M. Ramsteiner and J. Wagner, Appl. Phys. L e t t . 51 (1987) 1355. 25 J. Robertson and E.P. O'Reilly, Phys. Rev. B 35 (1987) 35. 26 J. Robertson, Adv. Phys. 35 (1986) 317. 27 R.A. F r i e d e l and H.L. Retcofsky, i n Spectrometry o f Fuels, R.A. Friedel, ed., Plenum Press, New York, 1970, p. 46 ff. 28 A. Leger and L. d'Hendecourt, i n P o l y c y c l i c Aromatic Hydrocarbons and A s t r o p h y s i c s , A. Leger, L. d'Hendecourt and N. Boccara, eds., R e i d e l Pub. Co., Boston, 1987, p. 223.
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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
611
PREPARATION OF HYDROTREATING CATALYSTS BY KNEADING. CHARACTERIZATION OF THE OXIDIC PHASE BY LASER RAMAN, X-RAY PHOTOELECTRON AND W-VISIBLE SPECTROSCOPIES
F. LUCK* and F. VIE2
RHN-PCULl3E RZHEKXE.5, Aubervilliers Research C e n t e r , 14
rue des Ganlinoux,
93308 AVBERVILLIERS PR4E.E)
ABSTRACT ESCA, FQman and W-VIS spectroscopies have been used t o characterize the structure of NiC-Moo3/y-Al2O3 catalysts prepared by pore volume hpregnation of an alumina carrier o r by kneading and extrusion of a paste containing nickel and molybdenum s a l t s mixed w i t h boehmite. The photoelectronic responses of nickel and molykdenum as a function of the molar ratios N i / A l or Mo/Al are similar for both families of catalysts, a f t e r correction of the difference in specific surface areas. Raman spectra of the various.samples present parallel evolutions : a supported moncineric molybdate species is detected for the lowest loadings, while an identical dispersion of the surface plymolybdate phase i s evidenced up t o a beyond which c r y s t a l l i t e s of W 3 are limit loading of ca. 4.5 atoms of Mo detected. W-VIS diffuse reflectance spectroscopy and alkaline hydrolysis data indicate that the low hydrodesulfurization activity of kneaded catalysts can be ascribed t o an enhanced location of N i 2 + ions i n a spinel surface layer. A correlation between the relative amount of tetrahedrally coordinated N i 2 + and HDS a c t i v i t y has been found. INTRODUCTION
Colmrercial hydrotreating catalysts consist of Co or N i promoted Mo o r W supported on alumina carriers with various shapes. These catalysts are prepared by
several methods. The two most c o m n are : 1) The incipient wetness technique, which consists i n impregnation of the alu-
mina carrier in one ore two steps w i t h solutions of nickel (or cobalt) and molybdenum s a l t s , followed by drying and calcination. 2 ) The co-mulling technique, in which a paste containing precursors of active
oxides intimately mixed with boehmite (AlCCIH) by kneading i s extruded, dried and calcined (1). Numerous studies have shown that the catalytic activity depends on the structure and the dispersion of the oxidic phase, which is the precursor of the active sulfide species ( 2 ) . Although it is known that catalysts prepared by in-
612
cipient wetness a r e generally more active than the ones made by kneading, the reasons of t h i s difference has not received much attention. An attempt has therefore been made in the present study t o elucidate the structural differences between Ni-W-Al203 catalysts obtained by *regnation or kneading, by mans of X-ray photoelectron, l a s e r Raman and W-Visible diffuse reflectance spectroscopies, and alkaline extraction. Besides these characterizations, c a t a l y t i c activities were evaluated using hydrcdesulfurization of dibenzothiophene a s model reaction. Based on these experimental results, the correlations between preparat i o n technique, surface composition and c a t a l y t i c a c t i v i t y w i l l be discussed. E x P m A L
Catalyst preparation. Two series of supported Ni-frb catalysts were prepared, i n which the atomic proportion C! = N i / N i t Mo was Set equal t o 0 . 2 9 . The first one was obtained by kneading boehmite with n i t r i c acid (Prolabo), amnonium heptarnolybdate ( C l h x Molybdenum Co.) and nickel n i t r a t e (La Pharmcie Centrale) solutions in a 2-type kneader. The paste was extruded through a 1 . 2 m die, and the extrudates were dried for 1 2 h a t 12OoC and calcined f o r 2 h a t S0O0C. The amounts of mlybdenum and nickel were incrementally increased t o achieve loadings between 3.5 % Moo3 - 0.75 % N i O and 21.0 % Moo3 - 4.5 % N i O in the calcined catalysts. The second series of catalysts was prepared by impregnation of a Procatalyse GFSC y-alumina carrier, in form of 1 . 2 mn diameter extrudates, w i t h a BET surface area of 193 m2/g and a t o t a l pore volume of 0 . 6 2 an3/g. Impregnation solutions were prepared by dissolving arnnonium heptamlybdate i n water then adding H202 (Prolabo) up t o a I% : H202 molar r a t i o of 2, t o obtain a yellow peroxomlybdate solution and adding t o t h i s the required amount of nickel n i t r a t e solution. After drying for 12 h a t 15OoC, the sanples were calcined for 4 h a t 50OoC. As f o r the first series of catalyst, t h e concentrations of molybdenum and nickel in the impregnation solutions were sequentially increased t o achieve the same loadings of active phase. Compositions, surface areas and distribution prof i l e s of metals f o r both series are s m r i z e d in Table 1. Raman measurements. The Raman spectra were recorded on a Ramanor HG 2
spebrmter. The exciting l i n e was the 514.5 nm emission l i n e from a Spectra Physics Ar+ l a s e r with a typical power a t the sample location on the order of 35 mW. The air-calcined,
powdered catalysts were pressed i n 13 m-diameter wafers
attached t o a sample rotator and scanned under ambient conditions. The spectral slidwidth was 4 an-l, and the scan r a t e was 0.33 an-l/sec.
ESCA. A Vacuum Generators ESCA MK I1 spectrometer with computer f a c i l i t y for data processing such a s curve &convolution, peak area integration and curve f i t t i n g was used t o collect data on the oxide catalysts. Cis binding energy
613 284.6 eV (adventitious carbon) was taken a s the reference f o r obtaining other B.E'S.
DRS. The reflectance spectra were recorded by a Beckman 5230 spectrophotometer equipped with a diffuse reflectance accessory with an integrating sphere, using as reference y-Al2O3 GFSC. The measurements were carried out i n the wavelength range 850 - 190 nm. AZkaline extraction. Calcined catalysts and bulk reference compounds NiO, NjAl.204, NiMoO4, M 3 and Al2(Moo4)3 were extracted with d i l u t e m o n i a , according t o t h e procedure of G i l et a l . (3) . Solubilized WI and N i were analyzed by atomic absorption. Catalyst activity. The catalysts were evaluated for t h e i r hydrcdesulfurizat i o n (HOS) a d i v i t i e s of dibenzothiophene (DBT) i n a fixed-bed, upflow automated
bench scale p i l o t unit. The following set of conditions was applied t o a l l experimental runs : presulfiding : a t 4OO0C i n s i t u i n a stream of 10% vol. H2S 90% H2, temperature = 29OoC, hydrogen pressure = 4.5 MPa, liquid hourly space
velocity = 2 hv1, diphenylmethane
synthetic feed : 15 w t . % DBT
(Fluka,
purum)
.
Liquid product
(EGA Chemie) dissolved i n samples were automatically
collected and analyzed by gas chromatography. TABLE 1
Catalyst characterization Catalyst
Canposition (weight 9 ) W 3
co-mulled A 1 A2 A 3 A 4
A 5 A 6
Surface area
NiO
btal distribution profiles (micropanalysis) cow. 0-100 pouter shell/mre M3
3.5 7.0
0.1 1.5
242
10.5 14.0
2.2
260
11.5
21.0
3.75 4.5
204
3.3
0.7
182
1.03
7.0 10.0 14.0
1.5
111
1.01 0.94 1.31
Ni
km3geneous
3.C
nacroscopic distrhtion
Co-kpqnated B 1 B 2
B3 B 4 B5 B 6
2.2
11.5
3.0 3.1
19.6
4.2
142
1.69 1.91
1.08 0.94
0.80 1.10 0.88 0.95
RESULTS AM) DISCUSSION
Raman spectroscopy. Raman spectra of co-mulled (series A) and co-irrpregnated (series B) catalysts are shown i n Fig. 1, and the assignment of the main bands
614
is reported i n Table 2. As the spectra were taken several weeks a f t e r preparation and activation, similar effects of hydration on the position of Mo bands are expected for both series, as discussed by Stencel e t a l . (4) and Payen et a l . (5). Series A and B present c m n s p e d r a l features. For Mo loadings up t o 4.5 Mo a t m m-* (catalysts A 1-A 6 and B 1-B 4), the position of the most intense band, attributed t o the syrnnetric Mo = 0 stretching mode of a hydrated polymolybdate surface species, varies between 920 and 960 afl. The position of
this band is c l e a r l y related t o the surface density of M (61, as shown i n Fig. 2, and could indicate structural change of the surface polymolyWate from a monolayer t o a bi- o r multilayer structure ( 7 ) . In addition t o the previous band, a broad shoulder centered a t 850 cm-1 is present in a l l spectra. This band has been attributed t o the moncsneric supported molybdate. Several modes can contribute t o t h i s band, the most significant being the stretching frequencies i n Mo-0-Al
and Mo-O-Mo
groups. The plots of the Raman
intensity r a t i o [ I (850) / I(950) ] versus bb surface d e n s i t y in Fig. 2 show different behavior for type A and B catalysts. For co-hpregnated catalysts the rat i o decreases i n a continuous m e r , indicating a progressive shift with increasing Mo loadings between monomeric and polymeric molykdate species. In the region 0.6 < Mo atom < 3.6, the slope of the representative l i n e of comulled catalysts A 1-A 5 i s very different from the previous one, thus suggesting that the kneading technique leads t o a rather stable r a t i o between molybdate species. It is w e l l known that pH regulates equilibrium between molybdate anions. As a result of n i t r i c acid peptization, Mo s a l t s are contacted with boehmite during kneading i n more acidic conditions than during irnpreSnation of y -Al2O3. This interpretation is supported by the presence of a weak shoulder near 975 a w l i n the spectrum of catalyst A 1 t o A 5, tentatively assigned t o an oct w l y b d a t e species, formed by acidification of heptamolybdate ( 8 ) Bulk W 3 causes the appearance of sharp peaks a t 670, 820 and 995 cm-l i n the
spectra of catalysts B 5 and B 6. The I%
loadings of these catalysts, 4 . 8 and
5.6 PB atom nm-2, are i n excess of that needed for t h e so-called " mnolayer coverage ", attained for a Mo surface density of ca. 4.5 a t ( 9 ) . Another
band is observed a t 965 cm-l i n the spectrum of catalyst B 6, and is attributed t o a-NiM~04. Concentration of bulk Alz(Mo04)3 must be extremely low, since no characteristic l i n e above 1000 cm-l could be detected in any of the type A or B catalysts. It appears t h u s that catalysts A and most catalysts B show a highly dispersed molybdenum phase, resulting from chemical interaction between molybdate anions and surface s i t e s of the carrier ( 1 0 ) . Separate experiments ascertained the %l fixation capacity for boehmite, which is ca. 10% higher compared t o y-AlzO3 (11). Wreover, the adsorption of peroxomolylxiate occurs by the sane ion exchange mchanisrn described for the heptmlybdate solution ( 8 ) . One can then admit that fixation of Mo on y-Al2O3 by pore volume *regnation and on
615
1400 1200 1000
800
A Fig. 1.
600
1000 800
WAVENUMBER ( c m
600
400
200
-'
)
spectra of NiO-Mo03/~-Al203 catalysts prepared by : a ) kneading, b) co-impregnation
2
Y
B
E
960
-
950
-
940
-
930
-
920
-
.
v)
g 0 2
a m
8
e1o' 0
.
'
1
2
3
4
.
'0.3 5
Mo Atom I nm
Fig.2. Plot of the J&I stretching band position and the intensity r a t i o 1(850)/ I(950), as functions of I%I surface density. Catalysts (A) : A , ; (B): A ,
.
616 boehmite during kneading a r e governed by similar mchanisms, not withstanding variations i n pH and concentration conditions which may provide an explanation f o r the small differences between MI species i n the two types of catalysts.
TABLE 2 Assiqnents of Rarran bands of ommlyWenun species in Nio-Mo03/y-Al203 catalysts Phse
identification
Position of characteristic bands (an-1)
m42- * W7%4 6W 8 9 6 4 - (?) Mog N W 4
850 920 to 960 975 820 995 965
Presence i n catalysts : A
(kneaded)
B (iqxegnated)
Yes
Yes
Yes
Yes
Yes M)
no for M/m2 > 4.5
( M O / d < 4.5) M)
for Wlm2 = 5.6
(Mo/nm2< 4.5)
XpS
analysis
kblybdenm. The intensity r a t i o s ( I M ~ / I ~ ) wass a function of the Mo load-
ings of series A and B catalysts are plotted i n Fig. 3 a . The number of supported Mo atoms per surface area unit is calculated by assuming a complete NJ location on the carrier. Linear regression analysis of the data shows t h a t the (I~o/Ix)~ps r a t i o s are not significantly different f o r the two types of catal y s t s , taking i n account the accuracy of XPS measurements on powders. The r a t i o increase linearly up t o 4 . 5 I% atoms run-2, l i m i t of the monolayer loading. Ratios presented by catalysts B5 and B6 (containing f o r loadings 4.8 and 5 . 6 atoms run-* bulk Moo3 a s evidenced by Rarnan spectroscopy) do not deviate from the line, i n apparent contradiction with the r e s u l t s of Kasztelan e t a 1 . ( 9 ) . Several m o d e l s have been proposed t o correlate XPS intensity r a t i o s with dispersion of
the supported active phase (12-15). A t loadings higher than 4 . 5 Mo atoms free Moo3 c r y s t a l l i t e s are growing on alumina, which can be p a r t i a l l y shielded. The apparent continuous increase is thus purely fortuitous, resulting from a simultaneous decrease of I m and I=. The presence of free Moo3 may cause variation of the partition of the deposited phase, affecting quantitative analysis of WS intensity m e a s u r m t s . Care was i n fact taken during activation of the impregnated catalysts t o minimize segregation of free Moo3 t o the external surface of the extrudates, a s shown by microprobe analysis results reported i n Table 1.
,+
617
(b)
2.0
0.0
0.5
1.0
At. MolnmP
1.5
2.0
2.5
3.0
At. Ni Inm
Fig.3. Plot of XPS ratios as a function of metal surface density, for kneaded ( 0 ) and co-impregnated ( ) catalysts. (a) MO/M and (b)Ni/Al. N i c k e l . Figure 3 b gives the variations of the N i photoelectronic response (I~i/I=))(ps as a function of the surface density of N i for catalysts A and B.
The escape depth for N i 2 p electrons is estimated t o be of about 1 m. The N i / A l intensity r a t i o should therefore reflect the concentration of nickel-containing
species at the surface of the catalysts. Both series give straight lines up t o for B 6 catalyst. Intensity ratios displayed by catalysts A prepared by kneading are ca. 12% lower than the ratios of corresponding catalysts B. In NiO-M003/A1203 catalysts the nickel species is generally distributed among a (sub)surface NiA12O4 layer, in which N i 2 + ions occupy both tetrahedral and octahedral sites, and "Ni-Mo" and p o s s i b l y a-NiEloO4 phases, N i ions occupying octahedral sites in t h e l a t t e r phases (3,16,17). Differences in ( I ~ i / I x ) ~raps N i concentration of 1.85 atom runp2 for A 6 and 2.35 atoms
t i o s can therefore be explained by differences either i n penetration of N i 2 + i n the underlying l a t t i c e or i n distribution of N i 2 + between Ni-Mo phase and (sub)surface NiAl2O4. The thickness of t h e NiAl2O4 layer estimated from i t s vol-
un-e does not exceed ca. 0 . 5 nm, so indeed a l l N i i s present near t h e surface. Moreover, results published for the NiO/A.l203 system (18) show that N i diffusion f r o m a NiAl204-like form s t a r t s a t 650°C. Since catalysts A and B were calcined a t 5OO0C for respectively 2 and 4 h, bulk N i diffusion is unlikely t o be important in such conditions, and would anyhow lead t o a lower N i concentration i n the topmost layer and a lower photoelectronic response, for catalyst B rather than for catalyst A ( 1 9 ) . Hence, the decrease i n the N i / A l r a t i o from impregnated catalysts B t o kneaded catalysts A is likely due t o a preferential location of N i 2 + ions in a sub-surface layer i n the l a t t e r .
Catalytic activity. Table 3 presents the hydrcdesulfurization activity of nickel-molyWenum/aldna catalysts obtained by several variations of kneading and impregnation techniques. Conversion data f i t t e d satisfactorily a first-order r a t e expression, and catalytic activities were expressed as f i r s t order volume
618 r a t e constants a t 29OoC, further converted t o reaction r a t e s per gram of metal oxide f o r precise comparisons between the catalysts. The major reaction product was biphenyl, with a minor amount of cyclohexylbenzene, in accordance with previous work ( 2 0 ) . The following main trends have been deduced from the analysis of these results: a)Introduction of Mo by kneading, followed by calcination and N i imprqnation, leads t o the most active catalyst, for bb loadings up t o the monolayer coverage
.
(< 4.5 atoms nn-2) introduction of both N i and Mo b) For a given Mo loading (3.5 Mo atoms oxides by kneading is detrimental t o the c a t a l y t i c activity, compared t o prepa-
ration by impregnation. The catalyst obtained by introduction of N i in t h e kneader p r i o r t o Mo introduction (NiE.lo (k) C) is less active than t h e catalyst prepared by t h e reverse sequence of introduction.
c) Inqsregnation of catalyst N&(k)C with 3%N i O causes a sharp increase in the catalytic activity, which becomes comparable with the a c t i v i t y of catalysts prepared by N i impregnation on a supported Mo oxidic phase.
TABLE 3 Catalytic activity of N i m / y
Catalyst andrrode of preparation
- a203 catalysts in dibenzothio@em? h*cxksulfurization
Ccnposition (weight % ) M 3 NiO
A t t-W/rm?
Activity constant
h s / g
:
NiO
KIDS
t-W (i) C N i (i) C
14.0
3.0
3.7
1.20
0.52 2.22
Mi (i) C
17.5
3.75
3.5
1.35
0.51 2.16
MY(k)CNi (i) C
17.5
3.10
3.5
1.50
0.59 3.33
t-W (k)CNi (i) C
24.0
5.15
4.8
1.52
0.36 1.67
mi (k)C
17.5
3.75
3.5
0.76
0.34 1.58
N&o(k)C
17.5
3.75
3.5
0.58
0.24 1.11
N&o(k)CNi(i)C
17.0
6.65
3.5
1.24
0.50 1.27
N i (i)C
-
3.75
-
0.06
-
0.15
______________-_-_-__l__________l_______--------------------------
(i): inpregnation, (k) : kneading, C : calcination.
Catalytic a c t i v i t y of hydrotreating catalysts is greatly dependent on the distribution of Ni(Co) ions between different species i n the oxidic precursor s t a t e . The precursor of t h e active sulfided "Ni-Mo-S" phase can be described a s a supported mixed Ni-m oxidic phase, for instance a N i isapolymolybdate (9), while nickel interacting with alumina i n a surface spinel structure is t o t a l l y inactive in HDS catalysis.
619
During c a t a l y s t preparation nickel ions can interact with the carrier (boehmite and y-Al2O3) by adsorption and surface hydrolysis, i f the local pH is great e r than 5.8 (21) : Al
-
A
thin
Al
Nix2+
OH 0
+
Ni(H20)62+
+ - Ni(H20)5
<=>
Al - 0
<=>
Al - 0
layer of a Feitknecht My3+ ( OH-)2x+3y-z ( NO3-),
temperatures
- Ni(H20)5' + H+ + - N i O H ( H 2 0 ) 4 + H+
H20
phase is formed, w i t h general formula nH20, and its decomposition a t moderate
.
(330-7OO0C) y i e l d s amorphous nickel aluminate ( 2 2 ) . According t o
the above r e s u l t s , preparation by kneading increases the proportion of N i located within the alumina lattice, with respect t o preparation by successive or coWregnation. Changes i n the distribution of N i between surface and subsurface sites may be attributable i n part t o structural differences between y-alumina and boehmite, the l a t t e r presenting a higher surface area, hence m r e adsorption
sites than the former. Moreover, the boehmite phase transformation t o y-alumina occurring between 350 and 45OoC, my enhance N i location i n the numerous tetrahedral vacancies of the spinel structure of y-alumina. On the other hand, exchange of N i 2 + ions with protons of t h e polymolybdate
surface phase has been proposed for t h e fonration of N i isopolymolybdate ( 9 ) . It has also been suggested that WI attached preferentially i n tetrahedral cmrdination on alumina, prevents N i from entering tetrahedral s i t e s ( 1 6 ) . Therefore, presence of oxomolybdenum species on t h e carrier inhibits the interaction of N i 2 + with carrier s i t e s .
DRS and alkaline extraction results and correlation with HDS activity. The reflectance spectra of Ni-MdAl203 catalysts exhibit the characteristic bands of the 0 -> t % ~charge transfer transitions in tetrahedral and octahedral m6+
species. In a l l spectra, a very strong broad band is observed in the W region, centered a t 300 nm. This band is typical of polymerized oxomolywenum species, and its t a i l i n g towards high wavenmkrs masks several d-d transitions bands attributed t o Ni2+ ions in octahedral environment. The reflectance spectra of type A and B catalysts in the region 500-850 nm are c-red
with spectra of catalysts prepared with variations of the basic
procedure i n Fig. 4 . Bands a t 550 (S O), 590 (S 1) and 630 nm (S 2) can be assigned t o N i 2 + tetrahedral ions in the structure of alumina (transitions 3T1g(F)
--> 3Tlg(F) and 3T1g(F) --> 3A2g), while band a t 710 (S3) and 785 nm (S4) are attributed t o octahedral N i 2 + ions (3A2g --> 3T1g(F) and 3A29(F) --> 3T2g). This spectral region was deconvoluted by assuming overlapping Gaussian curves centered a t the five above mentioned positions S 0 t o S 4, and adjusting values of the spedroscopic parameters (Kubelka-Mmk function and half-width). Table 4 sumnarizes the relative intensities of characteristic octahedral ( N i [ O ] ) and
620
tetrahedral ( N i [ T ] ) N i bands, i n relation with the mode of preparation of the catalysts. Catalysts with N i introduced on carriers already containing calcined oxomolykdenum species ( MI (i) CNi (i) C and MI (k)C N i (i) C ) show similar spectra, bands due t o o c t a h d a l N i being more intense than N i [TI bands. The intensity of N i [ O ]
and N i [TI bands displayed by the co-bpregnated catalyst are roughly equivalent. In contrast, N i [ T ] bands are stronger than N i [ O ] bands for the catalyst obtained by kneading. The spectrum of t h i s catalyst is i n fact very similar t o the spectrum of a 3.75% NiO/Al203 catalyst obtained by impregnation, with considerable absorption i n the 590-630 nm region, due t o the presence of N i [ T l interacting with the carrier. TABLE 4 Influence of the preparation factor on the intensities of Ni DRS absoqtion bands and the alkaline extraction of Mo and N i
Catalyst and prepration technique
Relative intensities of N i bands 590
710
630
785
beta1 oxide extraction (%) NiO Moo3
(4
*
i-k
/
/
i-4-t
++
19
71
* + + + +
+
9
82
* +
-
2.5
65
/
/
Mo (k)CNi (i) C
-
Mo (itC N i (i) C
-
mi (i)C MoNi (k)C N i (i) C
+ +
*
+ *
-
(k) : kneading, (i) : irrpregnation, C : calcination. Catalysts : 14% Moo3-3% NiO/Y-AlzO3, N i ( i ) C : 3,15 % NiO/y-A1203,
Extraction treatment data ( T a b l e 4 ) indicate t h a t a larger amount of Mo is extracted from the co-impregnated catalyst compared t o the double impregnated catalyst. This result is i n l i n e with t h e observations of G i l et a l . (7) , and with the proposed redispersing effect of the N i hpregnation on the Moo3 and polymolybdate phases (23). Alkaline hydrolysis removes these phases, but d a s not attack bound tetrahedral monomer. Differences i n the structure of these catalysts are especially reflected i n N i extraction data. In agreement with the l i t e r a t u r e (3, 24), N i extraction
varies i n t h e order : double bpregnation > co-impreqnation > kneading. Extract i o n treatmznt of Nit-kOq, NiAl2O4 and N i O gave respectively 80, 5 and 0.3% for N i removal. These results, along with DRS data on the semi-quantitative N i phase
composition, confirm that kneaded and (co)inpregnated catalysts notably differ
621
Mo(i)eNi(i)&
Sl 5 2
s4
Mo(k)@Ni(i)@
MoNi(i)@
MoNi(k) $
Ni(i)e I
500
I
I
I
I
600
700
800
900
WAVELENGTH (nm)
so s 1
s3 s 4
Fig.4. DRS spectra of NiO-IW+/y-A1203 and NiO/y-A1203 catalysts, w i t h deconvolution of the 500-850 nm region. with respect t o the relative distribution of N i between "Ni-Mo" NiAL204 (insoluble) phases.
(soluble) and
The absorption coefficient for N i [TI is greater than for N i [O] , so no quantit a t i v e determination of the different N i species could be obtained. However, an empirical correlation of HDS activity w i t h light absorbance of nickel-molybdenum/alumina catalysts has been published i n the patent l i t e r a t u r e (25), based on the difference of absorbance a t 625 nm ( N i [ T ] ) and 500 nm ( N i [ O l ) , the catalytic a c t i v i t y increasing a s t h i s difference gets l e s s . In the present study, the mst intense N i [ T ] and N i [ O ] absorption bands, centered a t 630 (S 2) and 710 (S 3)
nm, were chosen t o investigate a correlation with DBT hydrcdesulfurization act i v i t y of catalysts obtained by standard iqregnation and kneading, and variations of these techniques.
622
'
0.0
0
10
20
30
S2 (630nm)I S3 (710 nm)
Fig.5. HDS a c t i v i t y versus intensity r a t i o I630 (Ni[TI)/I710 (Ni[O]) , for NiDimpregnation, 0 kneading ;
W3/y-A1203 catalysts with N i introduced by : A NiO/y-A1203 (inpregnated)
.
The data in Fig. 5 show a reasonable correlation between specific HDS activi t y and the integrated intensity r a t i o S 2 / S 3. As expected, results display some scatter, due t o variations i n preparation techniques of comulled catalysts
(nature of N i s a l t , dispersion of Mo phase, e t c . ) . Such a correlation is nevertheless of interest for preliminary screening of hydrotreating catalysts, espec i a l l y i f Rmm measurements are used t o help t o ascertain separately the dispersion of the Mo phase. CoNCLUSIGiiS
The present investigation of NiC-W3/y-A1203 catalysts prepared by kneading and inpegnation techniques c l a r i f i e s the differences i n t h e structure and properties existing between them. Useful corroborative evidence has cone from Ra-
m, XPS and catalytic tests results t o show that t h e oxomolybdenum phase originating from boehmite kneading with m n i u m heptamolybdate solution is equivalent t o the phase obtained by pore f i l l i n g of y-alumina with the sane solution,
in terms of structure and dispersion. High dispersion r e s u l t s in both cases from mlyfxiate fixation on the carrier by anionic exchange.
N i c k e l cations interact w i t h alumina and t h e oxomlybdenum phase by ion-ex-
change. W-VIS DRS and alkaline extraction show clearly that, compared t o impregnation techniques, kneading increases N i interaction with the carrier, to form a surface spinel structure a t the expense of a mixed N i - W active phase, and leads t o less active catalysts.
623 S-
The authors g r a t e f u l l y acknowledge the support from Procatalyse, P a r i s L a %fense. W e thank J. Machard for the Raman r e s u l t s and M.A. P e r r i n for t h e ESCA
wasurements
.
REFERENCES 1
W. Ripperger and W. S a m , Proceedings o f t h e C l h x Second I n t e r n a t i o n a l
2
Conference on the Chemistry and Uses of Molybdenum (P.C.H. M i t c h e l l and A. Seaman, eds), C l i m a x kblykdenum Company, Ann Arbor, Michigan, 1982, p.175. F.E. Massoth and G. MuraliDhar, Proceedings of the C l h Fourth International Conference on the Chemistry and Uses of kblybdenum (H.F. Barry and P.C.H. Mitchell, eds.), C l h x MolywenUm Company, Ann Arbor, Michigan, 1982,
p.343. 3
F.G. G i l l S. Mendorioz and A. Lopez-Agudo, B u l l . Soc. C h h . Belg.,
90 (1981)
1331. S t e n c e l , L.E. Makovsky, J . R . Diehl and T.A. Sarkus, J. C a t a l . , 95 (1985) 414. 5 E. Payen, S. Kasztelan, J. Grimblot and J . P . Bonnelle, J. Raman Spectrosc., 17 (1986) 233. 6 E. Payen, J. Grimblot and S. Kasztelan, J. Phys. Chem., 91 ( 1987) 6642. 7 F.J. G i l - Llambias, M. Escudey, A. Lopez-Agudo and J.L.G. F i e r r o , J. C a t a l . , 90 (1984) 323. 8 G.A. Tsigidinos, H.Y. Chen and B . J . Streusand, Ind. Eng. Chem. Prod. R e s . Dev., 20 ( 1981) 619. 9 S. Kasztelan, J. Grimblot and J.P. Bonnelle, J. Phys. Chem., 91 (1987) 1503. 10 J . A . R . Van Veen and P.A.J.M. Hendriks, Polyhedron, 5 (1986) 75.
4
J.M.
11 F. Luck, unpublished r e s u l t s . 12 P . J . Angevine, J . L . V a r t u l i and W.N. Delgass, Proc. VIth I n t . Congr. C a t . (G.C. Bond, P. Wells, and F.C. T q k i n s , eds.), The Chemical Society, London, 1976, V o l . 2, p. 611. 13 C . Defosse, P. Canesson, P.G. Rouxhet and B. D e h n , J. C a t a l . , 51 ( 1978)
269. 14 S.C. Fung, J. C a t a l . , 58 (1979) 454. 15 F.P.J.M. Kerkhof and J . A . Moulijn, J. Phys. Chem., 83 (1979) 1612. 16 H. Knozinger, H. Jeziorowski, and E. Taglauer, Proc. VIIth I n t . Congr. Cat., 1980, p. 604. 17 P. Dufresne, E. Payen, J. Grimblot and J . P . Bonnelle, J. Phys. Chem., 85 (1981) 2344. 18 B. Scheffer, J.J. Heijeinga and J . A . Moulijn, J . Phys. Chem., 91 (1987) 4752. 19 B. Canosa-Rodrigo, H. Jeziorowski, H. Knozinger, X.Zh. Wang and E . Taglauer, B u l l . Soc. C h h . Belg., 90 (1981) 1339. 20 M. Houalla, N.K., Nag, A.V. Sapre, D.H. Brcderick and B.C. Gates, AIChE J . , 24 (1978) 1015. 21 P.K. D e Bokx, W.B.A. Wassenberg and J . W . Geus, J. C a t a l . , 104 (1987) 86. 22 K. Rikhter, S.V. Ketchik, L.G. S h n o v a and M.S. Borisova, Khet. C a t a l . (English trans.), 16 (1975) 1121. 23 H. Jeziorowski and H. Knozinger, Appl. S u r f . S c i . , 5 (1980) 335. 24 D.A. Agievskii, V . I . Kvashonkin, L . I . Pavlova, G.D. Chukin, V . I . Mikhailov, S.A. Surin, M.V. Landau and B.K. Nefedov, Kinet. C a t a l . (English trans.), 27 (1986) 160. 25 M.F.L. Johnson, US P a t . 3 900 267, 1975
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C. Morterra,A. Zecchina and G . Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam Printed in The Netherlands
-
625
C H A R A C T E R I Z A T I O N OF THE GROWTH OF AN O X I D A T I O N PRECURSOR AT LOW
TEMPERATURE ON OFHC COPPER
r
J - M . MACHE ERT1, M. LENGLET', A .D ' H U Y S S ER 1
D . BLAVETTE',
A. MENAND* a n d
L a b o r a t o i r e d e p h y s i c o c h i m i e d e s m a t t r i a u x , I N S A , BP 0 8 , 7 6 1 3 1 M o n t Saint A i g n a n C e d e x , F r a n c e
2 L a b o r a t o i r e 'de m i c r o s c o p i e i o n i q u e , UA CNRS 808, F a c u l t t d e s S c i e n c e s , BP 6 7 , 7 6 1 3 0 M o n t Saint A i g n a n C e d e x , F r a n c e 3 L a b o r a t o i r e d e c a t a l y s e h o m o g b n e et h t t t r o g b n e , UA 4 0 2 , betimerit C 3 , U n i v e r s i t t d e L i l l e I , 5 9 6 5 5 V i l l e n e u v e d'Ascq C e d e x , F r a n c e
ABSTRACT The o x i d a t i o n process of copper begins w i t h the g r o w t h of a precursor C u T O having the s a m e crystallographic s t r u c t u r e as C u z O but g i v i n g d i f f e r e n t XPS a n d UV s p e c t r a . T h e v o l t a m m o g r a m o f t h i s oxide i s d i f f e r e n t f r o m that o f b u l k C u q O a n d i t s o x y g e n c o n c e n t r a t i o n p r o f i l e e x h i b i t s a d i f f u s e z o n e 100 A d e e p . F o r longer o x i d a t i o n t i m e t h e g r o w t h o f n o n s t o i c h i o m e t r i c c o p p e r I o x i d e is s h o w n by characteristic absorption bands on i t s UV-Vis-NIR spectrum. T h e v o l t a m m o g r a m of this oxide is then q u i t e the s a m e as that of Cu20. INTRODUCTION Many
i n v e s t i g a t o r s h a v e studied the r e a c t i o n of c o p p e r w i t h
o x y g e n at l o w t e m p e r a t u r e ( r e f . 1 - 2 1 , but o n l y a f e w s t u d i e s h a v e been m a d e at a t m o s p h e r i c pressure in spite of the great i m p o r t a n c e of the p r e s e n c e of oxide
layers on copper surfaces during
m a n u f a c t u r i n g p r o c e s s e s (ref.3). C o n s e q u e n t l y in a f i r s t p a r t t h i s p a p e r a i m s at t h e c h a r a c t e r i z a t i o n o f t h e o x i d e f o r m e d at t h e beginning of the o x i d a t i o n p r o c e s s , and so several m e t h o d s h a v e b e e n u s e d to s h o w b o t h t h e k i n d o f c h e m i c a l b o n d s a n d t h e o x y g e n c o n c e n t r a t i o n p r o f i l e in s u c h a n o x i d e . I n a s e c o n d p a r t t h e l a t e r s t e p o f t h e o x i d a t i o n p r o c e s s is s t u d i e d w i t h a n e m p h a s i s o n the non s t o i c h i o m e t r y o f c o p p e r I o x i d e a n d i t s e f f e c t o n U V - V i s - N I R d i f f u s e reflectance spectra.
626
EXPERIMENTAL.
OFHC c o p p e r s h e e t s
(0,5
mm t h i c k , m o r e t h a n 9 9 , 9 5 % Cu) w e r e
etched in nitric acid and immediately rinsed in absolute ethanol. Then,
samples were
placed
into a hot
furnace at
the suitable
t e m p e r a t u r e . At t h e e n d o f the o x i d a t i o n t i m e , s a m p l e s w e r e r a p i d l y cooled
in air. T h e U V - V i s - N I R diffuse reflectance spectra w e r e
recorded on a P e r k i n E l m e r L a m b d a 9 s p e c t r o m e t e r f i t t e d w i t h a n i n t e g r a t i n g s p h e r e , a n d t h e XPS s p e c t r a w e r e m e a s u r e d on a L e y b o l d
LHS 10 s p e c t r o m e t e r . T h e e l e c t r o c h e m i c a l c e l l w a s f i t t e d w i t h a p l a t i n i u m c o u n t e r e l e c t r o d e a n d s a t u r a t e d c a l o m e d e l e c t r o d e (SCE) a s r e f e r e n c e . B e f o r e e a c h r u n the e l e c t r o l y t e w a s r i g o r o u s l y p u r g e d with nitrogen.
In t h e c a s e o f a t o m p r o b e m e a s u r e m e n t s t h e s a m p l e i s a t i p o f OFHC c o p p e r e t c h e d , o u t g a s s e d a n d f i e l d e v a p o r a t e d i n t h e a t o m p r o b e . T h e n t h e t i p is o x i d i z e d in a hot f u r n a c e (out o f t h e p r o b e ) temperature t o the a t o m probe.
and t r a n s f e r r e d at the suitabl:
A f t e r p u m p i n g to U.H.V., p o s i t i v e h o l d i n g a n d p u l s i n g v o l t a g e s w e r e applied o n
the
tip t o
produce
a
satisfactory
rate of
field
e v a p o r a t i o n . T h e e v a p o r a t e d i o n s w e r e a n a l y s e d in a t i m e o f f l i g h t m a s s s p e c t r o m e t e r.
DISCUSSION C h a r a c t e r i z a t i o n o f the p r e c u r s o r
At t h e b e g i n n i n g o f t h e o x i d a t i o n p r o c e s s t h e s a m e U V - V i s - N I R diffuse reflectance spectra are observed whatever
the temperature
o f t h e o x i d a t i o n i s . T h e s e s p e c t r a a r e c l o s e to t h a t o f m e t a l l i c c o p p e r e x c e p t f o r a w e a k a b s o r p t i o n b a n d in the 3 6 0
-
380 nm r a n g e
and for the s h i f t of the m a x i m u m o f the a b s o r p t i o n of c o p p e r f r o m 290
t o 300 n m ( s e e
spectrum of
f i g . 1 a).
As
it
can be
s e e n ( t a b l e 1) t h e
this o x i d e ( c a l l e d p r e c u r s o r ) is o b s e r v e d f o r o x i d a t i o n
t i m e s s h o r t e r t h a n 2 h a t 3 7 3 K a n d s h o r t e r t h a n 10 m i n u t e s a t 423 K
621
Table 1 E x p e r i m e n t a l results
T Ih4E
W - V i s - N I R spectra position o f peaks (nm)
potential o f electroreduction (Vvs SCE)
Phase
o x i d a t i o n temuerature : 373 K 2 min-1
h
300;360*
2 h-8
h
320;400
-0.65;-0.75
260;320;480;500 260;320;400;530
-0.6 ; - 0 . 8 -0.5 ; - 0 . 7 5
24 h 72 h
o x i d a t i o n temperature ~~
**
.+ **
*
min
cu20 cu20
K
: 423
~~
2 min-10 15 m i n 2 h
-0.58
*
260;300;355 260;320;420 260;320;440
cu 20
-0.55;-0.6 -0.6 ;-0.8 -0.6 ;-0.78
cu20
o x i d a t i o n temperature : 523 K 2 min 3 min
265;320;385 260;400 250;325;420;500 250;325;440;480 250;325;420;490
5 min 10 m i n 20 m i n
weak
**
-0.52 -0.5 ;-0.55;-0.67;-0.84 -0.5 ;-0.67;-0.73;-0.85 -0.46;-0.64;-0.79 -0.47;-0.84
maximum
Table 2 results of XPS m e a s u r e m e n t s
I
(min)
I
1;
Temperature
Eb Cu(2p3/2)
Ec C u L M 4 5M4.5
(K)
(ev)
iev,
423 423 523 523
933 * 3 933.4 933.3 934.1
916 ; 918.3 916 916.6 916.9
628
a n d o n l y o n t h e s a m p l e o x i d i z e d 2 m i n u t e s at 5 2 3 K. S o i t a p p e a r s that
the precursor
oxidation process
is a l w a y s
present
at
the beginning
of
the
for t i m e s d e c r e a s i n g rapidly as the t e m p e r a t u r e
o f the heat t r e a t m e n t increases. T h e s t r u c t u r e o f this o x i d e c a n be determinated
thanks
sufficiently
thick
thick).
It
has
crystallographic
angle X ray diffraction w h e n a
to small layer
been
of
shown
precursor that
the
is
formed
precursor
s t r u c t u r e a s C u 2 0 (ref.
(about has
3) and
this
100
the
A
same
result
is
c o n f i r m e d ( t a b l e 1) t h a n k s t o t h e d i f f r a c t o g r a m o f a c o p p e r s h e e t o x i d i z e d 1 0 m i n u t e s a t 4 2 3 K. T h e u n i t - c e l l v a l u e o f t h e p r e c u r s o r seems
to be
similar
to
that
of C u 2 0
in
spite
of
the
lack
of
precision of the m e a s u r e m e n t s . T h e e l e c t r o r e d u c t i o n c u r v e s o f the p r e c u r s o r e x h i b i t a n i n t e n s i t y p e a k n e a r - 0 . 5 V v s SCE ( f i g . 2 a). T h i s r e d u c t i o n p o t e n t i a l is v e r y h i g h i n c o m p a r i s o n w i t h t h a t o f the
reduction
o f Cu'
in C u 2 0 (-0.85V vs
S C E i n b o r a x 0.1M).
A c c o r d i n g to D e u t s c h e r a n d W o o d s (ref. 4) t h i s o v e r p o t e n t i a l
is d u e
t o a c o p p e r I o x i d e w h i c h i s l e s s s t a b l e t h a n C u 2 0 . T h i s r e s u l t is d e t a i l e d t h a n k s t o XPS s p e c t r a r e c o r d e d on c o p p e r s h e e t s o x i d i z e d during
5 minutes
and
Cu(2p3
2)
peak
a
at
c h a r a c e r i s t i c o f Cu'
10 minutes binding
at
energy
423 K
of
(fig.
E b
=
3 b,c).
933.3
eV
The is
( r e f e r e n c e : C (1s 1 / 2 ) l e v e l at a b i n d i n g
e n e r g y of 2 8 5 e V ) a n d t h e A u g e r C u (L3M4.5M4.5) p e a k s p l i t in t w o c o m p o n e n t s o n e a t Ec = 9 1 6 . 0 e V ( c h a r a c t e r i s t i c
o f Cu')
and
the
o t h e r at Ec = 9 1 8 . 2 e V ( c h a r a c t e r i s t i c o f C u " ) l e a d s u s t o d r a w t w o hypotheses.
-
The studied samples are recovered w i t h a copper I layer thinner than 15
-
A.
T h e c o m p o u n d p r e s e n t at t h e s u r f a c e m a y c o n t a i n i n t e r s t i t i a l C u "
scattered in a c r y s t a l l i n e p h a s e w i t h t h e s a m e s t r u c t u r e a s Cu20. The oxygen concentration profile has been recorded thanks to an
629 1.
b 24 h
a 30 min
a L
E
f
u
f
3bO
1
I
500
700
3d0
560
7d0
(mu)
1 *! :
72 h
a
3b0
5d0
7b0 (mu)
F i g . I. W-Vis-NIR diffuse reflectance spectra of OFHC copper sheets oxidized at 373 K f o r the times shown.
630
a 30 m i n
b 24 h
c 72 h
I
12.5 p A
-'0.3
-0.8
fig. 2 . p o t e n t i a l s c a n s at 0 . 5 m V s - ' in N a 2 B 4 0 7 f o r t h e r e d u c t i o n o f o x i d e s p r o d u c e d at 373 K ( a , b , c) f o r t h e t i m e s s h o w n .
a t o m p r o b e ( r e f . 5 ) o n a t i p o x i d i z e d at 423 K f o r 10 minutes. T h e first r e s u l t o f t h i s e x p e r i m e n t i s t h a t t h e o x y g e n c o n c e n t r a t i o n decreases c o n t i n u o u s l y f r o m the s u r f a c e t o a d e p t h o f m o r e than o n e h u n d r e d o f a n g s t r o m s in t h e m e t a l ( f i g . 3 d).
T h i s leads
to
rule
out t h e f i r s t h y p o t h e s i s d r a w n a f t e r the XPS m e a s u r e m e n t s b e c a u s e t h e o x i d e layer is m u c h thicker than the t h i c k n e s s s t u d i e d by XPS. So i t s e e m s t h a t
the precursor
c o p p e r a p p e a r s a s Cu' also shows that
i s a n o x i d e (or a m i x t u r e ) w h e r e
a n d a s Cu". T h e o x y g e n c o n c e n t r a t i o n p r o f i l e
the precursor
is n o t a m i x t u r e of c l u s t e r s o f
copper
I
o x i d e in a c o p p e r m a t r i x ( o r
copper
I
o x i d e m a t r i x ) b e c a u s e o f t h e m o n o t o n o u s a s p e c t of t h e
clusters of copper
in a
o x y g e n c o n c e n t r a t i o n a l o n g t h e interface. N o w w e c a n c o n c l u d e that this p r e c u r s o r
c o n t a i n s i n t e r s t i t i a l C u e s c a t t e r e d in a Cu'
p h a s e , a n d t h a t t h e Cu"/Cu'
oxide
ratio should vary continuously along
the o x i d e layer in o r d e r to p r e s e r v e c h a r g e n e u t r a l i t y . G r o w t h of non stoichiometric copper I oxide For
longer o x i d a t i o n t i m e s at d i f f e r e n t t e m p e r a t u r e s l o w e r than
523 K a f t e r t h e g r o w t h o f
the p r e c u r s o r
the U V - V i s - N I R diffuse
r e f l e c t a n c e s p e c t r a e x h i b i t s i n t e n s e a b s o r p t i o n p e a k s in t h e 500
-
631 560 n m range ( 2 . 4 8
noted
that
the
-
2 . 2 1 eV) ( s e e fig. 1 c a n d t a b l e 1).
of
energies
the photons
absorbed
It c a n b e in
these
t r a n s i t i o n s a r e l o w e r than the e n e r g i e s o f the transitions recorded on h i g h p u r i t y C u 2 0 ( r e f . 6 ) . T h e s e p e a k s a r e a t t r i b u a b l e to the n o n s t o i c h i o m e t r y o f the copper I o x i d e (ref. 7). T h e v o l t a m m o g r a m s o f these s a m p l e s exhibit a m a j o r peak in the - 0 . 7 6
-
-0.85
V vs SCE
range (in borax 0.1 M) a t t r i b u t a b l e to the r e d u c t i o n r e a c t i o n : Cu'
+
e-+Cu
( i n a n o x i d e l i k e C u 2 0 ) . A m i n o r p e a k n e a r - 0 . 5 V v s SCE
is s t i l l p r e s e n t i t is a t t r i b u t a b l e to a n o x i d e o f a s i m i l a r n a u r e to t h e p r e c u r s o r , a s i t c a n b e s e e n in r e f . 4 . T h i s o x i d e e x s t s under
the
copper
oxide
I
layer
or
between
patches
s t o i c h i o m e t r i c c o p p e r I o x i d e . T h e XPS m e a s u r e m e n t s
of non
show small
amount o f CuO ( t a b l e 2 ) g r o w i n g o n t h e c o p p e r I o x i d e l a y e r ( n o p h o t o r e d u c t i o n o n CuO h a s b e e n o b s e r v e d on t h e s t u d i e d samples). The
growth of
is
CuO
confirmed
on
the U V - V i s - N I R diffuse
r e f l e c t a n c e s p e c t r a t h a n k s to t h e i n c r e a s e o f a b s o r p t i o n s t a r t i n g f r o m 8 5 0 n m ( r e f . 8 ) . A m o r e p r e c i s e s t u d y will a l l o w to s h o w w h e t h e r C u O i s g r o w i n g in l a y e r or i n p a t c h e s on t h e c o p p e r I o x i d e layer. All t h e s e m e a s u r e m e n t s l e a d u s to c o n c l u d e t h a t f o r l o n g e r o x i d a t i o n t i m e s t h a n in t h e f i r s t p a r t , t h e r e i s g r o w t h o f n o n stoichiometric copper
I
oxide.
U V - V i s - N I R diffuse
reflectance
spectroscopy s e e m s to be the m o s t a p p r o p r i a t e m e t h o d to s h o w the n o n s t o i c h i o m e t r y of the s u r f a c e c o m p o u n d s . CONCLUSION
The investigation of
the initial s t a g e o f the o x i d a t i o n of
c o p p e r b y s e v e r a l m e t h o d s a l l o w s u s to s h o w that t h e o x i d a t i o n process b e g i n s w i t h t h e g r o w t h o f
a precursor h a v i n g the s a m e
c r y s t a l l o g r a p h i c structure as C u 2 0 . The p r e c u r s o r i s found to be a d i f f u s e interface the w i d t h o f which w a s approximately 1 5 0 and
copper 0 could
2. T h e
explain
the
intertwinned m i x t u r e o f copper I relative ease
to
reduce this
632
300
700 (nm)
a
B
u
0
d : copper and o x y g e n c o n c e n t r a t i o n profiles (the c o m p l e x ions CuOyH,+ ( y = 0.5, 1, 2 a n d x = 1 to 3) a r e n o t shown). precursor c o m p a r e d w i t h
the r e d u c t i o n of C u 2 0
that
is a m o r e
r e c o n s t r u c t e d o x i d e a n d that is c o n s e q u e n t l y m o r e d i f f i c u l t
to
reduce.
REFERENCES 1 H . W i e d e r , A.W. C z a n d e r n a , J . P h y s . C h e m . , 6 6 (1962) 816. 2 T.N. R h o d i n , J.A.C.S., 7 2 (1950) 5102. 3 J.M.M a c h e f e r t , A. D ' H u y s s e r , M . L e n g l e t , J . L o p i t a u x ,
D . D e l a h a y e , M a t . R e s . B u l l . (in press). 4 R.L. D e u t s c h e r , R. W o o d s , J . o f A p p l i e d E l e c t r o c h e m i s t r y , 16 (1986) 413. 5 D . B l a v e t t e , A . M e n a n d , A n n . C h i m . F r . , 1 1 (1986) 321. 6 S. B r a h m s , J . P . D a h l , S. N i k i t i n e , J. P h y s . Fr., 2 8 ( 1 9 6 7 ) C3-32. 7 J.M. M a c h e f e r t , R. F o n t a i n e , M. L e n g l e t , P. L e t e r r i b l e , J.M.Welter, M. L e C a l v a r , S u r f . I n t e r f a c e A n a l . , 1 2 ( 1 9 8 8 ) 4 3 6 . 8 F . S . S t o n e , B u l l . S O C . C h i m . F r . , (1965) 819.
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactiuity of Surfaces 0 1989 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
633
XPS AN0 SIMS INVESTIGATION ON THE CATALYST-SUPPORT CHEMICAL INTERACTION FOR
RuOX-TEFLON SYSTEMS
C.MALITESTA1 P. G. ZAMBON I N
, G.MOREA 1 , L.SABBATINI1,
N.TANGAR12,
V.TORTORELLA2 and
1
'Laboratorio d i Chimica A n a l i t i c a , Dipartimento d i Chimica, U n i v e r s i t a ' d e g l i Studi, Via Amendola 173, 70126 B a r i ( I t a l y ) 'Dipartimento Famco-Chimico, U n i v e r s i t a ' d e g l i Studi 70126 B a r i ( I t a l y )
, Via
Amendola 173,
ABSTRACT A systematic X-Ray Photoelectron Spectroscopy i n v e s t i g a t i o n was performed on s e r i e s o f RuOx/teflon samples loaded w i t h d i f f e r e n t amounts o f c a t a l y s t . The f i n d i n g s confirmed the existence o f a strong chemical i n t e r a c t i o n between r u thenium oxide c a t a l y s t and t e f l o n support, which was hypothesised i n t h e course of a previous work. The in-depth d i s t r i b u t i o n o f the supported c a t a l y s t was a l so i n v e s t i g a t e d by XPS analysis a t v a r i a b l e t a k e - o f f angle coupled w i t h s t a t i c and dynamic SIMS measurements. A p r e f e r e n t i a l l o c a t i o n o f ruthenium i n t h e r e gion j u s t underneath the t e f l o n surface was c l e a r l y evidentiated.
INTRODUCTION I n a recent paper ( r e f . 1) the basic p r o p e r t i e s o f a very a c t i v e supported c a t a l y s t , based on hydrated ruthenium d i o x i d e on t e f l o n (PTFE) substrate (RuOxPTFE) were i n v e s t i g a t e d mainly by XPS technique. The presence o f both ruthenium species chemisorbed on t h e substrate by means o f hydrogen bonds and ruthenium species bonded t o t e f l o n t o form "ruthenate 1ike compounds" was hypothesised. A model implying the formation o f intermediate Ru(VII1) compounds "anchored" t o the substrate was proposed t o explain t h e c a t a l y t i c behaviour o f t h e RuOx-PTFE system. Further work was announced concerning both a deeper i n v e s t i g a t i o n on t h e nat u r e o f t h e catalyst-support i n t e r a c t i o n and a study o f t h e in-depth d i s t r i b u t i o n o f ruthenium c a t a l y s t i n s i d e the PTFE support. For t h i s aim RuOx/teflon samples c o n t a i n i n g d i f f e r e n t amounts o f c a t a l y s t per surface u n i t were used. The f i n d i n g s o f t h e i n v e s t i g a t i o n , performed by coupling XPS (even a t v a r i -
634 a b l e t a k e - o f f a n g l e ) w i t h s t a t i c and dynamic SIMS
techniques a r e r e p o r t e d i n
t h e p r e s e n t paper. The r e s u l t s a r e d i s c u s s e d i n t e r m o f a s t r o n g chemical i n t e r a c t i o n between s u b s t r a t e and c a t a l y s t and o f a p r e f e r e n t i a l l o c a t i o n o f t h e c a t a l y s t " a t " t h e s u r f a c e o f t h e PTFE s u p p o r t r a t h e r t h a n "on" i t s s u r f a c e .
EXPERIMENTAL Reagents RuO .xH 0 commercial powder f r o m F l u k a was used w i t h o u t o t h e r p u r i f i c a t i o n . 2 2 A l l o t h e r r e a g e n t s were Merck p r o d u c t s , RP grade.
RuOx-PTFE sample p r e p a r a t i o n C a t a l y s t was s u p p o r t e d on t e f l o n as f o l l o w s . 50 m l CHC13 and 50 m l NaI04 10% aqueous s o l u t i o n were added i n t o a r e a c t i o n vessel t o g e t h e r w i t h 20mg fhr02.xH20 powder and a f o i l o f t e f l o n (1.0cmx1.0cmx0.4cm).
A f t e r s t i r r i n g f o r about a n
h o u r , t h e r u t h e n i u m d i o x i d e disappeared and t h e o r g a n i c phase became y e l l o w . Then a s o l u t i o n of an o r g a n i c s u b s t r a t e ( i . e l g N - b e n z y l - p i p e r i d i n e i n 100 m l CHC13) was added dropwise and t h e s o l u t i o n was mantained under s t i r r i n g f o r 24 hours. D u r i n g t h i s t i m e t h e t e f l o n f o i l a c q u i r e d a b l a c k c o l o r a t i o n . T h i s t r e a t ment w i l l b e r e f e r r e d as a " c y c l e " i n t h e t e x t . So, a s e r i e s o f samples of supp o r t e d c a t a l y s t have been prepared by exposing t h e t e f l o n f o i l s t o one or more r e a c t i o n c y c l e s as t h e one d e s c r i b e d . The i n t e n s i t y o f t h e b l a c k c o l o r a t i o n i n creased on i n c r e a s i n g t h e number o f c y c l e s and, so, t h e c a t a l y s t l o a d i n g .
Instrumental techniques (i)
XPS a n a l y s i s . X-Ray P h o t o e l e c t r o n Spectroscopy a n a l y s i s was c a r r i e d o u t
b y a L e y b o l d LHS 10 spectrometer, equipped w i t h a twin-anode source (Mg Ka1253.6 eV, A1 Ka 1486.6 eV) o p e r a t e d a t a b o u t 280 W. CRR ( C o s t a n t R e t a r d R a t i o ) mode was used f o r widescan s p e c t r a w h i l e CAE ( C o n s t a n t A n a l y s e r Energy) mode, w i t h a pass energy Eo= 50eV, was used f o r d e t a i l e d s p e c t r a . A t y p i c a l p r e s s u r e o f 5x10-'
mbar (loombar-1Pa) was mantained i n t h e a n a l y s i s chamber. The b i n d i n g
energy (B.E.) ce
s c a l e was c a l i b r a t e d b y u s i n g b o t h Cu 2p3/2 and Au 4 f 7 / 2 r e f e r e n -
- eV). The Cls e l e c t r o n peak (B.E.=285.1 lines(~B.E.=848+0.3
eV) due t o r e s i -
dual hydrocarbon contaminant l a y e r was t a k e n as a n i n t e r n a l r e f e r e n c e . The s p e c t r o m e t e r was equipped w i t h a n a d d i t i o n a l sample-rod p e r m i t t i n g v a r i -
635 a b l e take - o f f angl e a n a l y s i s . The s p e c t r a were c o l l e c t e d b y an Apple I 1 microcomputer i n t e r f a c e d t o t h e spectrometer. Data a n a l y s i s was c a r r i e d o u t i n two steps by two s o f t w a r e packages. The f i r s t one was a home-made program ( r e f . 2) running on Apple I 1 microcomputer. The second one was a n o n - l i n e a r l e a s t squares f i t t i n g program ( r e f . 3 ) running on a Compaq Deskpro 386 microcomputer. Conversion o f Apple-DOS t e x t f i l e s t o MS-DOS f i l e s was made b y MatchPoint-PC ( M i c r o s o l u t i o n s , Oekalb,IL 60115). ( i i ) SIMS a n a l y s i s . Secondary I o n Mass Spectrometry measurements were performed u s i n g a m o d i f i e d VG SIMSLAB instrument equipped w i t h Gat, A r t and A r " (atoms) p r i m a r y sources. Secondary i o n s were separated according t o mass u s i n g a VG MM 12-1200 quadrupole mass spectrometer w i t h a working mass range o f 0-800 a.m.u..
Both p o s i t i v e and n e g a t i v e i o n s t a t i c S I M S spectra were recorded w i t h
a l l data a c q u i s i t i o n and m a n i p u l a t i o n performed u s i n g a PDP-11 microcomputer w i t h VG designed software. I n t h e experiments described throughout t h e paper secondary i o n s were gener a t e d u s i n g a microfocused l i q u i d g a l l i u m primary i o n source. The beam c o n d i t i ons were: 2 . a ) f o r s t a t i c SIMS: i)sampled area = 1.5 mm ; ii)Gat energy = 10 KeV; -2 i i i ) c u r r e n t p e r u n i t area = 4 nAxcm -3 2 b ) f o r ( g e n t l e ) dynamic SIMS: i ) sampled area = 5 . 6 ~ 1 0 nun ; i i ) Gat ener2 -2 gy = 10 KeV; iii)c u r r e n t p e r u n i t area = 7x10 nAxcm
.
.
RESULTS AND DISCUSSION XPS analyses o f PTFE samples which were submitted t o d i f f e r e n t numbers o f r e -
a c t i o n c y c l e s (see experimental s e c t i o n ) were performed. D e t a i l spectra r e l e v a n t t o Ru3dtCls, Ru3p, Ols, F l s , F2s, 13d and N l s l e v e l s were recorded. T y p i c a l s p e c t r a i n t h e Ru3dtCls r e g i o n a r e shown i n F i g . 1 as a f u n c t i o n o f t h e catal y s t l o a d i n g . Despite t h e nearness o f t h e C I S and t h e Ru3d s i g n a l s a p r e v i o u s i n v e s t i g a t i o n ( r e f . 4 ) showed t h a t a r e l i a b l e f i t t i n g o f these spectra can be done.
An example i s shown i n F i g . 2. D i v i d i n g peak areas by r e l a t i v e s e n s i t i -
v i t y f a c t o r s ( r e f s . 5,6)
t h e atomic percent of a l l t h e species present on t h e
ana7ysed sample can be c a l c u l a t e d . Results obtained on a s e r i e s o f samples w i t h growing c a t a l y s t coverage and l a b e l l e d w i t h number i n d i c a t i n g t h e r e a c t i o n cyc l e s a r e r e p o r t e d i n Table I.
636
29tl
2%
750.
ZEZ
ZEK
BINDING ENERGY/EV
Fig. 1. RuOx supported on PTFE. XPS spectra r e l a t i v e t o Ru3d+CIs region. The number near each spectrum r e f e r s t o t h e number o f c a t a l y s t - l o a d i n g cycles t o which the sample was submitted. Two ruthenium 3d doublets were recognised (A and
B i n Fig. 21, whose i n t e n s i -
t y grew on increasing the degree of coverage, w h i l e the Cls s i g n a l o f a t y p i c a l
-CF2- group (peak C) decreased w i t h t h e coverage. A signal (peak E) a t h i g h e r B.E.
respect t o -CF
I
2
-
became more and more evident on increasing the amount o f
295.
2-
289
t8h
283
BINDING ENERGYIEV
Fig. 2. RuOx supported on PTFE. XPS spectrum of t h e Ru3d+Cls region. Sample 6 i n Fig. 1.
637 TABLE I
Atomic percent o f Ru and C species c a l c u l a t e d f o r samples i n Fig. 1. Surface species"
Sampl e O
1
A
6
0.6 0.6 1.4 1.7 2.4 4.0
0.3 0.3 0.6 0.8 1.0 2.1
C
84.1 82.3 71.5 70.9 63.8 48.8
E
D
F
G
1.4 1.4 3.0 3.3 4.5 8.4
6.2 6.4 4.3 5.3 6.4 4.2
I I
2.8 1.1 3.4 3.7 4.1 5.3
3.2 2.4 5.1 3.9
H
4.6 7.3 12.6 11.9 12.7 23.4
"Each sample i s l a b e l l e d according t o the number o f coverage cycles (see t e x t ) OOSurface species a r e l a b e l l e d as shown i n Fig. 2. supported c a t a l y s t . Peak D has been a t t r i b u t e d t o -CF2-CH2- groups and was a l s o present on the v i r g i n t e f l o n . Peak F was present on some samples b u t absent on others. Since i t s presence was always coupled t o a s i g n a l i n the Nls region, i t was a t t r i b u t e d t o i m p u r i t i e s o f N-benzyl-piperidine,
t h e organic substrate u t i -
l i s e d t o r e p r e c i p i t a t e Ru04. Peaks G and H can be ascribed t o adsorbed C02 and hydrocarbon contaminant respectively. Ru3p region was recorded t o o b t a i n information about the t o t a l amount (peak area) o f ruthenium species on the supported c a t a l y s t . Values c o n s i s t e n t w i t h those r e l e v a n t t o Ru3d doublets were found. Since the broad s t r u c t u r e o f Ru3p l e v e l a curve f i t t i n g o f the r e l e v a n t spectrum was considered inconclusive. The most s t r i k i n g r e s u l t obtained by the described XPS experiments i s t h a t peak E appears d e f i n i t i v e l y r e l a t e d t o the presence of ruthenium on PTFE. I n a previous work ( r e f . 1) i t was excluded the p o s s i b i l i t y t h a t peak E could be due e i t h e r t o t e f l o n degradation r e l a t e d t o X-ray beam damage o r t o t h e RuOx-teflon i n t e r a c t i o n f o l l o w i n g X-ray i r r a d i a t i o n . The f a c t t h a t the i n t e n s i t y o f peak E becomes h i g h e r the higher the number o f r e a c t i o n cycles (see Fig. I),i.e.
the
amount o f loaded c a t a l y s t , s t r o n g l y supports the hypothesis p r e v i o u s l y ( r e f . 1) made t h a t h i g h l y hydrated ruthenium species a r e present on t e f l o n i n p a r t as chemisorbed compounds and i n p a r t as a surface bonded "ruthenate" species ,suchas
638
h
297.
294.
2P1.
- BINDING
288.
EXERGY/EV
285.
-
Fig. 3. RuOx supported on PTFE. XPS spectra o f the Cls+Ru3d region recordedwith two d i f f e r e n t e x c i t a t i o n sources. This could j u s t i f y t h e p o s i t i v e chemical s h i f t o f about 2eV o f Cls respect t o
- species binding energy a s w e l l as t h e chemical s h i f t (Z0.8eV) o f ruthenium 2 doublets A and B (Ru(1V) and Ru(V1) species, r e s p e c t i v e l y ) respect t o unsupported
-CF
hydrated ruthenium dioxide. Observation was made during t h i s work t h a t , even when the amount o f c a t a l y s t supported on t e f l o n was such t h a t the sample had a heavy black c o l o r a t i o n , app r e c i a b l e XPS s i g n a l s could be detected from t h e substrate (i.e.
PTFE). Therefo-
r e some attempt was made t o i n v e s t i g a t e the in-depth d i s t r i b u t i o n o f the catalyst. Fig. 3 shows two XPS spectra recorded i n the Cls+Ru3d region o f a h e a v i l y l o aded PTFE based c a t a l y s t by using both Mg Ka and A1 Ka e x c i t a t i o n l i n e s . A l a r ger Ru/CF2 i n t e n s i t y r a t i o f o r t h e spectrum recorded w i t h A1 source was c l e a r l y seen, suggesting an enrichfient o f Ru i n t h e region j u s t underneath the surface ( s i n c e the A1 Ka sampling depth i s l a r g e r than t h a t f o r Mg Ka).
A s i m i l a r t r e n d was obtained by performing XPS measurements a t v a r i a b l e takeo f f angles ( e ) . On decreasing e ( t h e angle between t h e surface and the escaping electrons) from 90" t o 30" a decrease i n Ru/CF2 i n t e n s i t y could be recorded. The absence o f ruthenium species i n the " t r u e " top surface of the RUOx-PTFE systems was confirmed by SIMS experiments. The mass spectra o f both p o s i t i v e and negative ions recorded under s t a t i c SIMS conditions i n the 0-300 a.m.u.
interval
639
* C 3 0
U
20
40
60
00 100 Cycles
120
140
160
F i g . 4. PTFE supported c a t a l y s t . Ga+ source, 10 keV, 0.6 nA. Depth p r o f i l e . d i s p l a y e d t h e presence o f expected PTFE peaks , w h i l e t h e r e was no evidence o f R u - c o n t a i n i n g s i g n a l s . Then, a sample was d e p t h - p r o f i l e d u s i n g SIMS. F i g . 4 shows t h e change o f Ru+ and CF' i n t e n s i t i e s w i t h t h e d e p t h : t h e e x i s t e n c e o f a l a y e r o f Ru+ j u s t underneath t h e sample s u r f a c e would seem t o be c l e a r . From t h e
P o s i t i v e SIMS Target B l a s 63.5 V S t e p size 2 scans a t 1013 cnannels a t 101.9 ms ~ l e rchannel
-
0.296
amu
400
350
300
u1
250
Y
C 3
200
0
U
150
100
50
0
80
90
100
110 A t O m l C Mass
120 units
130
140
150
F i g . 5. RuOx supported on PTFE. P o s i t i v e i o n s p e c t r a . Recorded a f t e r bombardment w i t h Ga+ i o n s o f 10 KeV o f energy, f o r 5 min. Source : Gat, 10 KeV.
640
c a l c u l a t i o n on the s p u t t e r i n g r a t e (see Appendix) r e s u l t e d t h a t the maximum Ru' 0
concentration was reached a t a depth of about 66 A from the surface. Both v a r i a b l e t a k e - o f f angle XPS and SIMS measurements suggested t h a t gaseous RuO could migrate i n t o t e f l o n pores and t h e r e r e a c t w i t h i t and/or repre4 c i p i t a t e as RuOx a f t e r o x i d a t i o n o f the organic substrate. Moreover the p o s i t i v e mass spectra o f the c r a t e r base region a t about 60
A
under the surface showed a strong Ru' group o f c l u s t e r s (Fig. 5). The most i n t e r e s t i n g region seemed t o be the one between 108 D' and 124 D'.
The presence of,
a t l e a s t , two groups of peaks w i t h the t y p i c a l Ru-isotopes d i s t r i b u t i o n could be r e a d i l y recognised. The f i r s t one (M=108-116 0')
maximising a t 114 D' corre-
sponds t o RuC' c l u s t e r s , w h i l e the second one (M.115-123 corresponds t o RuF'.
D') maximising a t 121D'
This r e s u l t appears q u i t e i n t e r e s t i n g since i t could con-
f i r m t h e XPS f i n d i n g s r e l e v a n t t o a strong chemical i n t e r a c t i o n between RuOx and
the t e f l o n support; however, because o f the experimental conditions used i n the SIMS a n a l y s i s (dynamic conditions), i t has t o be taken only as i n d i c a t i v e . It
i s , i n f a c t , d i f f i c u l t t o extimate how much these c l u s t e r s are r e p r e s e n t a t i v e o f the r e a l molecular s t r u c t u r e , since damage o r mixing effects, induced by the Ga' bombardment, cannot be rejected. F u r t h e r work i s i n progress i n t h i s d i r e c t i o n .
ACKNOWLEDGEMENTS SIMS measurements were performed a t the Surface Analysis Service o f UMIST i n
Manchester (U.K.).
We wish t o thank Dr.Alan Paul f o r h i s s k i l l e d cooperation i n
S I M S experiments and f r u i t f u l discussion.
Work c a r r i e d o u t w i t h the f i n a n c i a l support o f "Consiglio Nazionale d e l l e R i cerche" (C.N.R.)
and "Minister0 d e l l a Pubblica Istruzione".
APPENDIX C a l c u l a t i o n o f the thickness o f the l a y e r removed during a SIMS depth p r o f i l e . The Ga'current used i s 0.6 nA. 1 nA i s equivalent t o 6 . 2 ~ 1 09 ions.sec-', so 9 the number o f ions f o r seconds, h i t t i n g the sample, i s 0 . 6 ~ 6 . 2 ~ 1 0 ions.sec-l. I f i t i s assumed a s p u t t e r i n g y i e l d o f 5, then
NUMBER OF ATOMS REMOVED PER SECOND = 5 ~ 0 . 6 ~ 6 . 2 ~ 1 0 ~ 4 2 The bombardment was performed on an area o f 9x10 pm and an e l e c t r o n i c gat-
641 i n g was applied, reducing t o a 1/16 the t a r g e t t e d area, TARGETTED AREA The normal "density" o f atoms i s I O l 5
2 9 ~ 1 O ~ x 1 0 - ~ x 1 / cm 16 CIII-~.
NUMBER OF ATOMS I N THE TARGETTED AREA = 9 ~ 1 0 ~ ~ 1 O - ~ x 1 / 1 6 ~ 1 0 ~ ~ NUMBER OF LAYERS NUMBER OF ATOMS NUMBER OF ATOMS IN = o,33, ,aprs.sec [ISEMOVED PER SECOND] =[REMOVED PER SECONd/[THE TARGETTED AREA]
-1
0
Ifone atom l a y e r i s assumed t o be approximately 5 A, i t f o l l o w s t h a t THICKNESS OF REMOVED LAYER PER SECOND = 1.65 i.sec-'
I n t h e case shown i n Fig. 4 every c y c l e l a s t s 2 seconds ( 1 second f o r each dep t h p r o f i l e d ion).' The maximum o f Ru' i n t e n s i t y i s reached a t about 20 cycles, t h a t i s a f t e r a s p u t t e r i n g o f 40 seconds. The corrispondent depth i s MAXIMUM Ru'
INTENSITY DEPTH = 1.65
sec-'x40sec = 60
REFERENCES G. Morea, L. Sabbatini, P.G. Zambonin, N. Tangari and N. T o r t o r e l l a , submitted t o publication. E. Desimoni and C. Malitesta, Computer Enhanced Spectroscopy, 3 (1986) 107110. R.O. Ansell, T. Dickinson, A.F. Povey, P.M.A. Sherwood, J. Electroanal. Chem. 98 (1989) 79. C. M a l i t e s t a , G. Morea, L. Sabbatini and P.G.Zambonin, Ann. Chim. (Rome), 78 (1988), i n press. " P r a t i c a l Surface Analysis", D. Briggs and M.P.Seah ( E d i t o r s ) , John Wiley & Sons (1983), Appendix 5. L. Sabbatini, C. M a l i t e s t a , E. Desimoni and P.G. Zambonin; Ann. Chim. (Rome), 74 (1984) 341.
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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
643
PHOTOASSISTED MECHANISMS IN HETEROGENEOUS CATALYSIS: THE ROLE OF SURFACE OH IN THE DECOMPOSITION OF ETHANOIC ACID ON MAGNESIUM OXIDE
.
.
L. MARCHESE~, s CCLUCCIA~, E BORELLO~, L .PALMISANO~, A. SCLAFANI2 and M. SCHIAVELLO' *Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei P. Giuria -7, 10125, Materiali dell'Universita di Torino, via Torino (Italy) 2Dipartimento di Ingegneria Chimica dei Processi e dei Materiali dell'Universita di Palermo, viale delle Scienze, 90128, Palermo (Italy) ABSTRACT The photodecomposition of adsorbed ethanoic acid was studied on a number of pure and mixed insulator and semiconductor systems having a wide range of acid-base features. MgO appeared to be much more active than any other oxide when oxygen was not present in the reactant mixture. On the basis of infrared, ST.V.-Vis. reflectance and photoluminescence data it is suggested that in this case the surface hydroxy-groups which are known to absorb at wavelengths longer than 300 nm are involved in the photodecomposition process. INTRODUCTION Interest in photochemistry has been qreatly stimulated in recent years by growing attention to potential applications in photocatalysis and photoelectrochemistry (ref. 1). Photocatalytic reactions typically occur on semiconductors and the pertinent liter_ature shows that the overall mechanism may be described as a "redox mechanism", the most significant step being the generation of electron-hole pairs. A test reaction which has been studied on semiconductors both in aqueous regime and in gas-solid regime is the photodecomposition of ethanoic acid (refs. 2-31. The reaction products are essentially carbon dioxide, methane and ethane. Photocatalytic reactions may occur also on insulators (ref. 4 ) , where mechanisms based on electron-hole pairs cannot be invoked in the energy range normally involved in photocatalysis. In the case of ethanoic acid photodecomposition, the activities of a large number of pure and mixed insulator and semiconductor
644
oxides have been compared showing that the activity depends much more on the acid-base characteristics of the solid than on its electrical properties, particularly when oxygen is absent in the reactant mixture. A photolytic mechanism was proposed to be operative on insulator oxides. The interaction with the surface induces modification in the optical spectrum of the adsorbed species as compared with the free ethanoic acid molecules and renders them suitable for the photolytic process (refs. 5-91, Though the above mentioned mechanisms were able to interpret the overall experimental data related to photodecomposition of ethanoic acid on oxides, there was no satisfactory explanation of the fact that the activity of MgO appeared to be much higher as compared with all other oxidic systems. The experimental conditions and the available spectroscopic information (infrared, U . V . Vis. reflectance and photoluminescence data) on hydroxylated MgO support the idea that also surface hydroxyls are photoactive species. EXPERIMENTAL Materials Most of the catalysts referred to in this report were commercially available oxides. Some of them were home prepared. All the mixed oxides were home prepared. Details on sources and preparation procedures are in refs. 5-9. Magnesium oxide for spectroscopic studies was prepared by thermal decomposition of the parent hydroxide and finally outgassed at 1073 K following the procedure fully described in ref. 10. This sample has the same catalytic behaviour as MgO from other sources but higher purity and more favourable spectroscopic properties. Reactor The reactivity tests were run in a fixed bed flat reactor. He or air flow was bubbled in a bottle containing ethanoic acid and fed to the reactor which was irradiated by a 1000 W Hg-Xe lamp. The reactor outlet was analyzed by a gaschromatograph (Varian). Details are given in ref. 5. The apparatus was kept at 3 2 3 K. Spectroscopic measurements Self supporting pellets of the catalysts were introduced in a IR cell which allowed all treatments to be carried out in situ.
645
80 -
60-
Fig. 1. Infrared spectra of ethanoic acid adsorbed on MgO. a) MgO preoutgassed at 1073 K; b)-e) after the admission of successive small doses of acid vapour (residual pressure lower than 1 Pa); f)-h) in the presence of 0.26, 0.52 and 1.1 kPa acid vapour respectively; i) (dashed) after outgassing at 300 K for 30 minutes. The spectra were obtained with a Perkin Elmer 580 B spectrometer. Because of the high intensity of the bands observed at full coverage, the I.R. spectra are better illustrated in a transmission scale than in an absorbance one. Reflectance spectra were obtained with a Varian spectrometer. The samples were contained in a quartz cell. Photoluminescence results referred to in this report were obtained as described in ref. 11-12.
646
RESULTS Catalytic tests Detailed data relative to a large number of pure and mixed oxides are listed in refs. 5-9. It is only necessary to recall here that basic oxides are more active than acidic systems and that, in the absence of oxygen, MgO is the most active of all. In the presence of oxygen the activity of the insulators does not change whereas that of semiconductors increases significantly. Infrared spectra Fig. 1 shows the spectra obtained following the adsorption of increasing amounts of acetic acid on a MgO pellet completely dehydroxylated by preoutgassing at 1073 K. No OH stretching bands are observed in the background in the 3000-3800 cm-I range (curve a). When small doses of acid are allowed onto the sample (curves b-e), the first adsorbed species produce: a) two complex absorptions in the 1400-1600 cm-I range, b) absorptions in the C-H stretching region (2800-3000 cm-l), c) absorptions in the 0-H stretching region (3400-3800 cm-l) As the coverage increases (curves f-h), the intensities of the already existing bands increase and new components appear on both sides of the main absorptions centered at 1580 and 1430 cm-l. Spectrum h, obtained in the presence of 8 torr (1.1 kPa) of ethanoic acid, simulates the condition during the catalytic runs. When the sample is outgassed at room temperature (curve i), the bands which appeared in the low frequency region (1800-1200 cm-l) in the last steps of adsorption fade away and two extremely intense absorptions are left at ~ 1 6 0 0 and ~ 1 4 5 0 crn'l. Intense and broad bands are also left at high frequency in the OH stretching and CH stretching regions.
.
U.V.-Vis. spectra The U.V.-Vis. reflectance spectra of the MgO sample in different conditions are summarized in Fig. 2. The relevant spectral features of MgO in microcrystalline form were fully discussed in a series of papers (refs. 10-14) and it is only necessary to notice here that, under given circumstances, such spectra show reflectance values larger than 100% in the high energy range. This peculiar spectral feature is actually not genuine and is due to the luminesceEt behaviour of the samples (refs. 10-14). Depending on its origin, such luminescence in some cases is quenched by oxygen,
647
l
B
ib
;"'\
I
I
I
I
L
60 nm
I
00
C
1
I
D
! !C'
I
I
I
400 nm 6c
0
Fig. 2 . U.V.-Vis. Reflectance Spectra of a MgO sample in various conditions. Section A: spectrum in vacuo of the fully hydroxylated sample (the admission of oxygen does not modify the spectrum). Section B: b) spectrum in vacuo of the completely dehydroxylated sample; b') the same as b) but in the presence of oxygen. Section C: c) the same as b'); c') spectrum of the sample after contact with ethanoic acid (the admission of oxygen or excess acid vapour does not modify the spectrum). whereas in other cases is not. Curve a in Sect. A is the spectrum of a fully hydroxylated sample; the presence of luminescence is monitored by the abnormally high reflectance values at k < 300 nrn. Spectrum a is taken in vacuo, but admission of O2 onto the sample does not modify it significantly so showing that luminescence in this case is not quenched. Section B refers to the sample fully dehydroxylated at 1073 K. Curve b is the spectrum of the sample in vacuo and shows luminescence. Curve b'is the spectrum in the presence of oxygen which in the present case quenches the luminescence; this can be assumed as the true spectrum of dispersed MgO in the absence of OH groups and any other contaminant. Quenching is reversible by pumping off oxygen at 300 K.
648
In section C the true spectrum of MgO is reported for comparison (curve c). After removing oxygen, ethanoic acid vapour was admitted on the clean MgO and finally the excess acid pumped off at 300 K. A strong effect due to luminescence is produced in the short wavelengths region. In this case neither the admission of oxygen nor the presence of acetic acid vapours are able to quench the luminescence. The bands at k>400 nm are present in the baseline of the instrument. Excitation and emission spectra of both hydroxylated (refs. 11,15) and dehydroxylated (refs. 16-18) MgO samples have been repeatedly described and will not be reported here. It has only to be recalled that the excitation band at lowest energy was found at 270 nm for dehydroxylated MgO (refs. 16-18) and at longer wavelengths (-350 nm) for hydroxylated MgO (refs. 11-15). DISCUSSION Surface sites It has been proposed that the sites active for adsorption on the surface of microcrystalline MgO pretreated at high temperature are cations and anions exposed in low coordination (MgLC2+,OLc2-) on faces, edges and corners of the cubic particles (refs. 10, 1214, 16-18). These sites are associated with bands (see Fig. 2B,b') in the reflectance spectrum at lower energy than those of single crystals (refs. 10, 13-14). Once excited, such sites decay by radiative pathways and this is the origin of the luminescence disturbing the reflectance spectrum (Fig. 2B,b). The excitation peaks coincide with the various components in the reflectance spectrum and the emission band has a maximum at -390 nm (refs. 11, 16). This luminescence is reversibly quenched by oxygen. The related infrared spectrum of this sample (Fig. 1,a) does not show any absorption due to surface species. When hydroxyls are present on the surface, bands due to OH stretching modes appear in the infrared spectrum in the 3400-3800 cm-l range (refs. 19-20). A l s o the luminescence spectra of a largely hydroxylated sample differ from those of a clean sample because both the excitation bands (-350 rm) and the emission bands (-450 nm) are at lower energy (refs. 11,171. Besides, the luminescence associated with OH groups is not totally quenchable. This means that the actual reflectance spectrum of an extensively hydroxylated sample cannot be really appreciated in the high frequency region (Fig. 2A).
649
Surface species On the basis of the above model, the spectra related to the adsorption of ethanoic acid on fully dehydroxylated MgO may be readily interpreted (for details of assignment see refs. 5-9). Acetate species appear first at low coverage (Fig. 1, b-e) monitored by the couple of absorptions in the 1400-1600 cm‘l range. These bands appear and grow in intensity together with broad and intense bands at 3400-3800 cm” due to hydroxyls produced by the reaction:
It has to be stressed here that, as the adsorption of ethanoic acid proceeds, the surface becomes more and more hydroxylated and the newly formed OH groups behave as further adsorptive sites (ref. 6). Consequently, the spectrum obtained by adsorption of ethanoic acid on a MgO surface which was originally free from hydroxyls (Fig. 1,h) is identical to that obtained by adsorbing the acid on a completely hydroxylated surface (see Fig. 3,i in ref. 9). The acetate species may be either bidentate or monodentate (the limit forms are shown in the schemes), the latter preserving the carbonyl structure which is lost in the former because of the equivalence of the two C-0 bonds:
+ CH3 I
cI = o 0 I - MgLC
I
The infrared spectra suggest that on MgO the bidentate species prevail. In fact the two bands at 1580 and 1450 cm-l are more compatible with that structure than with the monodentate (ref. 21) because a carbonyl group is expected to absorb at frequencies higher than 1600 cm-l. In fig. 1 such species may contribute to the intensity in the high frequency side of the 1580 cm-I band. P.: the highest coverages, undissociated acid molecules are weakly adsorbed mostly on OH groups. These species are monitored by carbonylic bands at frequencies higher than 1700 crn’l.
650
Mechanisms The activity of semiconductors in photocatalysis is associated with the creation of electron-hole pairs when light is absorbed. This step is the precursor in a "redox mechanism involving adsorbed species as reactants. It may be noticed that the photoactive part of the catalytic system is, in this case, the catalyst itself. When the catalysts are insulator oxides the above mechanism is not likely to contribute in the energy range adopted in this work and a different one has been proposed, based on the fact that adsorption of the reactants on the surface may induce modification of their optical spectrum moving the absorptions to longer or shorter wavelength (22). In this mechanism the "photoactive" species are the reactants. The photodecomposition of ethanoic acid on white insulator oxides which do not have intrinsic absorptions at low energy was interpreted in terms of such "photolytic" mechanism (refs. 5 - 9 ) The photoactivation is related to absorption of light by the carbony1 group of adsorbed acid species. Surface species showing this chromophore group are the monodentate acetates which are particularly abundant on acidic systems such as silica. Their role in the catalytic process is described in ref. 5. Weakly adsorbed undissociated acid molecules may undergo similar dissociation. Carbonylic species are also present on basic oxides and, consequently, the photolytic mechanism may well be operative also on these systems. However, it should be considered that basic oxides are more active and that bidentate acetates are preferentially formed on their surfaces. This does not imply necessarily that bidentates participate to the photodecomposition, but the possibility that they play a role cannot be excluded. The fact that MgO is the most active among the basic oxides may be significant and the extensive spectroscopic and structural information available on this system prompt us to examine further mechanisms. A suggestion comes from the observation that the MgO surface is rich in hydroxyls during the reaction and that the acetates are in intimate connection with the OH groups as the two species are formed on adjacent MgLC2+ and OLc2- ions by the adsorption of a ethanoic acid molecule. Furthermore, it is well docu1>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.
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
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 <10 A (ref. 11).
for
such
loadings where
DISCUSSION
The dominant absorption at 2081 cm'l in the spectra of CO adsorbed on reduced Pt/A1203, the sharp peak at 2147 cm-' and the dependence of the spectrum on Pt loading will be discussed separately for the sake of simplicity.
659
CO adsorbed on Pto sites
The assignment of the most intense absorption in Fig. l,A and B at 2081 cm-' to CO linearly adsorbed on metal Pt particles is easy as it is well documented both for single cristals (refs. 2-5) and for supported systems (ref. 1) that monocarbonyl species on Pto are associated with bands in that position. The interpretation of coverage dependence of the spectrum of adsorbed CO is less straightforward because of: a) the complexity of the absorption at 2081 cm-l suggesting the presence of families of different sites; b) some apparently contrasting evidence coming from the adsorption (Fig. 1,A) and desorption (Fig. 1,B) runs. The main result of the adsorption experiment (Fig. 1,A) was that two bands of comparable intensity were produced by the adsorption of the very first doses of CO, whose positions did not change significantly by the admission of further doses. Even at the highest coverage only very limited frequency shifts are observed. This behaviour should be compared with the data relative to a stepped face of a single crystal which may be adopted as a suitable model for the surface of real microcrystals in dispersed catalysts. Hayden et al. (ref. 5) showed that adsorption of CO on a stepped (111) face produces at the lowest coverages only one band whose intensity increases up to a certain stage of coverage ( 8 ~ 3 0 % Omax) and then decreases leaving gradually place to a new well separated band at higher frequency. At maximum coverage the latter is the only peak present. The band observed in the first stages of CO adsorption shifted, at increasing coverages, from 2065 to 2078 cm-' and the final band shifted from 2086 to ?097 cm-l with an overall shift of 32 cm-' from zero to maximum coverage. The first band has been attributed to CO adsorbed on step sites and the second one to CO on extended terraces. As the coverage increases the effects of vibrational coupling in one dimensional domains typical of step CO molecules, are gradually substituted by the effects of coupling in two dimensional domains when extended planes start be filled up (refs. 5,161. No such behaviour can be recognized in the bands of Fig. 1 , A . In particular the limited shift observed from curve a to curve g suggest that coverage effects are not really evidenced at the various steps of the adsorption experiment. These data can be rationalized admitting that the two bands at 2076 and 2065 cm-l are associated with two different microplanes
660
exposed on the Pt particles and that, because of the slow diffusion of CO, the first doses of gas fully cover the Pt particles exposed on the most external layers of the pellet with further doses progressively covering metal particles in more internal layers (refs. 12,13,17). The consequence is that at each stage of adsorption the infrared spectra monitor CO molecules on microfaces at the highest coverage. Successive doses increase the number of fully CO covered Pt particles. A true equilibrium stage at full coverage is only attained after CO as been allowed to cover all exposed metal atoms at any level in the pellet. Equilibrium states at decreasing coverage can be observed in the course of desorption experiments where outgassing at high temperature ensure on homogeneous elimination of CO molecules from all metal particles. It is relevant noticing that, as long as two bands are recognizable (Fig. lB,b-e), a low frequency shift is observed for both of them as expected for the diminishing effect of coupling on each crystal plane when the coverage decreases (ref. 18). At low coverages, the spectrum is expected to become more complex because the bands of CO on step sites should appear so that new bands should be associated with each family of microplanes. Corner sites might add further complexity and the consequence is that all such components produce the broad absorption observed in the last stages of desorption (Fig. IB, f-g). The two faces which, on the basis of the above discussion, seem to be evidenced by CO adsorption, may only tentatively be thought as the (100) and (111) planes present on cubo-octahedron particle (ref. 8). The same planes would be present on cube particles with the corners cut by (111) planes (ref. 8). A low frequency shift at low CO coverage was also observed by Primet (refs. 17,191 and the singleton frequency was determined to be 2052 cm'l. The bands in the low frequency region (1950-1800 cm'l) are due to the stretching mode of CO adsorbed in bridged position onto metal platinum sites (refs. 1,4). As observed also in the case of single crystals (ref. 41, such structures appear at the highest coverage (Fig. 1A) and are easily desorbed at mild temperature (Fig. 1B). The presence of at least two components indicate the existence of families of slightly different two-fold bridged carbonyls, though three-fold bonded structures cannot be excluded.
661
co
on Ptn+ sites The weak bands at 2152 and 2133 cm" (Fig. 1A,g and Fig. 1B,a) and the related band at 2147 cm" (Fig. 1B,b) are in a range where CO molecules adsorbed on oxidized platinum sites absorb (ref. 20). This assignment is supported by spectrum a in Fig. 2, indicating that CO adsorbed on a oxidized sample only shows bands in that reqion and a peak at 2147 cm-' is dominant. Two aspects must be considered: a) the presence of Ptn' sites on reduced samples and b) the relation of the 2147 cm" with the parents 2152 and 2133 cm-I components. Oxidized sites on reduced metal catalyst may originate from a number of processes. A mechanism may be related to dissociation of CO on the most active sites, such as corners and/or other structural defects, as observed on stepped faces of platinum crystals !ref. 211. Another possibility is a disruption of the crystals by CO and an oxidation of the metal by some of the hydroxy groups present on the support, as proposed in the case of dispersed rhodium catalyst (ref. 221. Finally, the possibility has to be considered that oxidized sites act as nucleation agent of the metal particles produced upon the reduction, so that at the end of the treatment such sites would be masked (ref. 23). However an effect of contact with carbon monoxide might be the disgregation of some metal particles and in this case the oxidized sites would be exposed and able to adsorb CO. Disruption by CO contact of metal particles has been often observed (refs. 22,241. Though Huizinga and Prins found by EPR that Pt' ions are present at the Pt-support interface on reduced Pt/A1203 and Pt/Ti02 samples (ref. 25) more information is needed to know which of the above mechanisms contribute on Pt/A1203, and none can be excluded on the basis of the present data. It may be noticed that all of them are expected to be activated, in agreement with the observation that the intensities of the 2133, 2152 and 2147 cm'l bands increase with time of contact with CO. As for the structure of the carbonyls on Ptn+ sites, it is relevant noticing that the bands at 2133 and 2152 cm'l disappear when the excess CO is outgassed and generate a single band at 2147 an". This behaviour suggests that dicarbonyls are formed on Ptn+ sites in excess of CO, associated with two stretching modes (sym. and asym.) and that CO is easily desorbed at 300K leaving a monocarbonyl species characterized by a single stretching mode at 2147 cm-'. Such a transformation has already been observed for the
662
Ni/Si02 system (ref. 26). Dicarbonyl structures on oxidized Rh sites are reported by Solymosi et al. (ref. 27) also on the basis of previous works (refs. 22,281. Nothing can be said at the moment on the actual value of the charge on the oxidized samples. However, the absence, upon outgassing, of any frequency shift of the 2147 cm'l band and its sharpness indicate that these sites are in specific position (limited heterogeneity) and isolated (absence of collective effects). Effect of Pt loading The most significant effect of increasing Pt loading is on the intensity of the bands of CO adsorbed on the support. Such bands 3+ sites (refs. 14,151. The only monitor the most uncoordinated Alcus decrease of their intensity with increasing Pt loading indicates that these sites are selectively masked by the increased amount of metal, suggesting that interaction exists between metal Pt parti3+ ions. cles and the most electron deficient AlCUs CONCLUSIONS 1) Two crystal planes are exposed predominantly on Pt metal particles on A1203. 2) Oxidized platinum sites are observed on reduced Pt/A1203 after long time contact with CO. Dicarbonyls (absorbing at 2152 and 2133 cm-l) and monocarbonyls (absorbing at 2147 cm-l) are formed on PP+. 31 Carbonyls absorbing at 2198, 2179, 2168, 2147, 2125, 2092 and 2075 cm-l are formed on oxidized Pt/A1203 catalyst. 4) Metal-support interaction is observed between Pt particles and 3+ cations. the most electron deficient AlcUs
Financial support by the Commission des Communautes Europeennes and by Italian Consiglio Nazionale delle Ricerche is aknowledged. The authors are grateful to Prof. G.C. Bond and to Prof. G.F. Froment for discussions.
663
REFERENCES 1 N. Sheppard and T.T. Nguyen, in R.J.H Clark and R.E. Hester (Editors), Advances in Infrared and Raman Spectroscopies, vol. 5, Heyden & Son Ld, 1978, pp. 67-149. 2 A. Crossley and D.A. King, Surface Sci., 95 (1980) 131-155. 3 M.D. Baker and M.A. Chesters, in R. Caudano, J.H. Gilles and A.A. Lucas (Editors), Proc. 2nd Int. Conference of Vibration at Surface, Namur, Belgium, September 10-12, 1980, Plenum Press, New York, 1982, pp. 289-299. 4 B.E.Hayden and A.M. Bradshaw, Surface Sci., 125 (1983) 787-802. 5 B.E. Hayden, K. Kretzschmar, A.M. Bradshaw and R.G. Greenler, Surface Sci., 149 (1985) 394-406. 6 R.G. Greenler, K.D. Burch, K. Kretzschmar, R. Klauser, A.M. Bradshaw and B.E. Hayden, Surface Sci., 152/153 (1985) 338-345. 7 R.T.K. Baker, E.B. Prestridge and R.L. Garten, J. Catal., 56 (1979) 390-406. 8 T. Wang, C. Lee and L.D. Schmidt, Surface Sci., 163 (1985) 181197, 9 P.J.F. Harris, Surface Sci., 185 (1987). 10 M. Pan, J.M. Cowley and I.Y. Chan, J. Appl. Cryst., 20 (1987) 300-305. 11 L. Bonneviot, D. Trong On and M. Che, Private communication. 12 E. Guglielminotti, G. Spoto and A. Zecchina, Surface Sci., 161 (1985) 202-220. 13 J. Sarkany and M. Bartek, J. Cat., 92 (1985) 388-397). 14 G. Della Gatta, B. Fubini, G. Ghiotti and C. Morterra, J. Catal., 43 (1976) 90-98. 15 A. Zecchina, E. Escalona Plater0 and C. Otero Arekn, J. Catal. 107 (1987) 244-247. 16 F.M. Leisbe, R.S. Sorbello and R.G. Greenler, Surface Sci., 179 (1987) 101-118. 17 M. Primet, J. Catal., 88 (1984) 273-282. i8 R.M. Hammaker, S.A. Francis and R.P. EiSchens, Spectrochim. Acta, 21 (1965) 1295-1309. 19 M, Primet, L.C. de Menorval, J. Fraissard and T. Ito, J. Chem. SOC., Faraday Trans. 1, 81 (1985) 2867-2874. 20 Yu. A. Lokhov and A.A. Davydov, Kinetika Kataliz, 21 (1980) 1523-1529. 21 B. Lang, R.W. Joyner and G.A. Somorjai, Surface Sci., 30 (1972) 454-474* 22 H.F.J. Van't Blik, J.B.A.D. Van Zon, T.Huizinga, J.C. Vis, D.C. Koningsberger and R. Prins, J. Physic. Chem., 87 (1983) 22642267. 23 Yu.1. Yermakov, in T. Seiyama and K. Tanabe (Editors), Proc. 7th Int. Congress on Catalysis, Tokyo, Japan, 30 June-4 July, 1980, Elsevier, Amsterdam, 1981, pp. 57-77. 24 C. Louis, L. Marchese, S. Coluccia and A. Zecchina, J. Chem. SOC. Faraday Trans. 1, in press. 25 T. Huizinga and R. Prins, J. Phys. Chem., 87 (1983) 173-176. 26 M. Kermarec, D. Delafosse and M. Che, J. Chem. SOC., Chem. Commun., 1983 411-413. 27 F. Solymosi and M. P&sztor, J. Phys. Chem. 89 (1985) 4789-4793. 28 J.T. Yates Jr., T.M. Duncan and R.W. Vaughan, J. Chem. Phys.,71 (1979) 3908-3915.
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C. Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlande
-
665
FREE AND SUPPORTED METAL CLUSTERS: STRUCTURESAND REACTIVITY
A. MASSON Laboratoire de Physico-Chimie des Surfaces, assmi6 au CNRS Ecole Nationale Sup6rieurede Chimie de Paris 11, rue Pierre et Marie Curie 75005 PARIS - FRANCE ABSTRACT Atomic beam techniquesisshown to be aversatile technique to follow the growingof condensedmatter atom by atom. Mass and size selectedclustersarethenobtainedin freeorsupportedclusters.Crystalline, electronic structure (insulator- metal transition), initial an final state effect are discussed around electron spectroscopies.Finally size effect in chemisorption and reactivity is illustrated on transition metals clusters. Metal clusters appear as a realm of condensed matter in between organometallic and surface for modelling catalysis studies. INTRODUCTION
Clusters, aggregates, or very small particles are defined as an ensemble of atoms, link in between isolated atom and the classical condensed matter of macroscopic properties. Some maintain that it is a anew state of the matter, but there is any doubt that it is a new realm of the condensed matter. Even though, theoretical problems associated with these small finite objects have been pointed out by KUBO (1) twenty years ago, it is only during the early eighties, with technological progresses in synthesis and characterization that the experimental approach has taken such an knprtane(2) (3)(4). Industrialmotivations,as semiconductorresearchandcatalysisarealsoinvolved in that burst of activity.
666 1 - SYNTHESIS
In all the cases which are of interest here, the clusters are obtained by phase transition phenomena in the gas phase, that describe the nucleation processes of the condensed matter from a supersaturated atomic vapor. The initial thermodynamicaldescription of the physical mechanism implies the concept
of cccritical nucleus, of GIBBS (5). Generaly in free beam experiments an adiabatic expansion of a supersaturedvapor is the startingpoint for obtaining a distributionof nuclei in homogeneousphase (6). As for supported beam experiments a substrate is introduced to collect the impinging atoms and the phenomenum is described by the heterogeneousnucleation formalism (7).Both of the two approaches induce a size distribution of clusters (@.Because all experiments are normally performed out of equilibrium condition, a kinetic formalism has been developped (9) for the nucleation and growth processes and LEWIS (10) has shown that the two approaches are equivalent. In the latter case only atomic parameters are necessary.
2 - SIZE AND MASS DISTRIBUTIONS OF CLUSTERS
The alldifficulty of theexperimental study of agregates,rest in the possibilitytoobtainan homogeneous distribution of sizes and of a such quantity that its become possible to undertake modem chemical and physical analysis. 2.-1 Condensed atomic beam
On a substrate a good standard is a coverage of condensed matter mound 1015atoms per cm2 (the equivalent of a monolayer in surface science study). Atomic vapour pressure in the evaporation cell of 10.’ tor, and flux of 1OI2atoms cm-*sec-I,in ultra high vacuum conditions, are now well obtained for all elements even the refractory ones by the techniques of electron bombardement (11). Rate of impingementof atoms onto the surface, and time ts of evaporation fixed, it is possible as it is shown on fig. 1to obtain a number ns of nuclei at saturation.All the things being equal for a time Dts, the impingement of atoms is then alone sufficient for their growth.
667
2t
=1 n -0
I
I
50
100
I
I
150
t,s Fig 1 : Number density of particles as a function of exposure time.(lO) and SCHMEISSER (12) has given the variation of the mean diameter of the panicle as the function
of time as.
dZ = a R exp (( E, - Ed)/ 2 kT) t
(1)
with T = substrate temperature "K R = flux of impinging atom Atom cm-*sec-'. Ea and Eb: adsorption and activation diffusion energy of adatom. A selection of size is then obtained as is shown on fig.2.
Fig 2 :Diameter d of particle as a function of exposure time.
668
Electron microscopy is the best choice to follow the nucleation ad growth of particles bigger than 10A in diameter.
TOspeak the thruth, the description of the distribution is rather more complicated, and one as to look to a more sophisticatedcutting up of the landing space of atoms by way of the VORONOY tesselation
(13) of the surface in order to render an account of a non DIRAC distribution of sizes.
2.-3. Free atomic beam In the case of the free beam, after the process of nucleation in the homogeneousphase the size selection
is a quite different matter. After the adiabatic expansion of the atomic vapour which leads to the aggregates, these latter are photoionized by laser beam, that allows after the event to make use of the time of flight spectrometry of the ionized particles, they are mass selected (14). During the last few years, laser and sputtering sources (15 ) have been developped in order to obtain all metals in good quantity, when until1 now, thermal sourcesonly allowed the experimental approach of alcaline or 2s metals. A mass selected spectrum of aluminum clusters ion can be seen of fig.3 (16).
120 130 140 150 160 170 180 190 200 mass, amu
Fig 3 : Spectrum of mass selected aluminium clusters after (32).
669
3.CRYSTALLINE STRUCTURE AND RELAXATION. The crystalline structuresof metal aggregatesare well known for particles bigger than lOA. This status is due to electron diffraction and high resolution electron microscopy. Since many years the Japanese school has given an authoritative work, until the up to date results of TAKAYANAGI.. (17) who has been able to follow the dynamics of transformation between icosaedrical particles into compact structure ones. Recently the EXAFS and XANES techniques are employed, and generally the radial distribution funtions have revealed contraction of the crystalline net. Exception for Pd particle described by DE CRESCENZI (18) et a1 with dilatation.Tacking account of the temperature and size one can obtain much more difficulty concerningcrystallographicconclusions (38).As for the very small clusters, with a very few number atoms the first results of SATTLER(19) and JARROLD (20), with the scanning tunneling microscope, have shown that it is possible to obtain atomic resolution for gold and silver aggregates scattered on graphite substrate. In the case of free cluster, the temperature of the objects are generally not known, and it is actually difficult to assume that the size selected clusters have all the same crystalline structure, due to the very tiny difference in energy for different structures (21).
4 - ELECTRONIC STRUCTURE
In the case of metal clusters, the electronic structure is the major problem to understain, and the size effect on this structure is in enfancy. Quantum size effect, the spacing of energetic levels, and the the (magic) stability with electronic parity (even or odd) of the clusters have been discused in details (22) and are not the subject of our purpose. On the contrary, the problem of insulator to metallic transition has received recently corroborations by electron spectrometries approaches and it is about we intend todiscuss.For free clusters argumentsfor an answer existsfor alcaline metals (1s) due to measurements
of appearing potental by photoionisation compared to calculated ionisation potential by the jellium model (23). The conclusion is that in alcaline metals the delocalisation of electron is present even for tetramers. For 2s metal like mercury a recent experiment of BRECHIGNAC et al(24) with synchroton radiation has shown the limit of Van der Waals behaviour to metallic behaviouraround Hg clusters of eighty atoms. For the case of transition d metals it is until1 now the domain of supported aggregates (exception 36-37)and even if some differences arise around the average size of particle, definitive answers have been obtained by photoelectron and Auger spectroscopies.
670
4.- 1. Valence band The obvious size effect and the oldest repported is the narrowing or broadening of valence band width
by decreasing or increasing of the size of the particle as it shown on fig.4. The tigh binding approximation(25) is appropriated to describe the variation of the FWHM in function of 4 Z :Zbehg the average coordination number as
w=dz
(2)
Fig 4 : U P S spectra of Nickel supported on Si02 substrate as a function of size.
671
Table: 1 a4.13 5
b0.81 27
c=1.62 54
e=8.21 270
d=3.24 108
f=16.2 540
g=32.4 1080
h=% i=97.5 18M 3240
Experimentaldatas:Palladium coverage(atom/cm2x1015.) :Number of atoms per particle.
55 10 7.9 18
147 15 9 13
309 20 9.7 10
Mode1:C:atoms per partic1e.D:diameter:E:average coordination number:FFWHM(against the bulk) On table 1 one can compare for Pd clusters measurements obtain by photoelectron spectroscopy fig4 and calculation according to equation (2). A new is emerge recently, concerning initial state effect by taking into account explanation with correlation electron effects. This can be done by looking in Auger spectroscopy to core, valence, valencetransition.MASSONet A1(26) havediscussedindetail thecaseofNicke1and Palladium which are interesting because of opposite behaviour concerning the d holes distribution in going to atom to bulk. Such a behaviour is reported on fig.5 and compared with calculation of TREGLIA et A1 (27). 0 FWHM eV
100
200
300
.
400
t,s
I
\
\ metal I
da
I
do
d"
Fig 5: Variation of FWHM for nickel clusters as a function of size. Variation of the size is then an elegant manner to change the d electronic density. Finally it is the place to recall very recent experiment of size selected, gold clusters soft landed on graphite and summitted to spectroscopyanalysis DICENZO (28) claimed that around 150 Atoms in gold particle the transition to metallic behaviour is obtained.
672
Such kinds of experimentson selectedclustersopen the door now for comparisonof free and supported clusters and by the way to be able toanswer quantitativelyon initial, final andchemicaleffects (electron transfer or something related to strong metal support interaction : SMSI).
5 - REACTIVITY
As a matter of fact, one motivation for doing chemistry on small particles is heterogeneouscatalysis. Following BOUDART (29) who has introduced the size effect in activity and selectivity.It is obvious at this point of view that one has to make a boundary between big and small clusters. Let us assume that the boundary could be particle countaining around loo0 atoms. For particle bigger than that it is
sure,that the major problem there is very close to surface science studies, surface effects, with defects as edge and comers atoms, a geometrical approach is the most important. As for the small, and very small (less than 100atoms) the experimentalapproach is just starting,and the answer is certainly plural with electronic effect and associated geometry. For the moment, the easy things to clear concerne the minimum number of atoms necessary to induce chemisorption and reactivity, and some extrema in reactivity. Starting with free clusters experiments, the chemisorption approach is dominated by American chemists of ARGONNE, EXXON and BELL, Laboratories. Some results concerning adsorption of hydrogene and deuterium on iron and Aluminium clusters are reported on fig. 6 and fig. 7. The most fascinating things concern the maximum of selectivity for 5 02
1041
I
4 L
I , On 5
.
20 30 40 50 60 70
Fig 6.7.Clusters size dependence of the rate constants for a reaction with F a n d D2 for Al. and Fe.
(after (3 1) and (30)
673
particular particle 10 atoms of iron and hexamer of aluminium. KALDOR et al have tried to make a correlation between ionisation potential and maximum of reactivity (31 ). As for supported clusters experiments CO-chemisorptiop on Palladium has been claimed by PopPA(32) as size sensitive. In our laboratory we have reported size effect in hydrogenation of ethylene on Pt and Pd ( 33). Then hydrogenolysis of n-butane on nickel has been recently exposed ( 34 1.
Fig 8: Variation of the pseudo T 0 N as a Ag/ Pd concentration and total number of atoms per particle. Finally selective hydrogenation ofbutadiene on bimetallic Pd.Ag particles is shown on fig.8.
On the Y axis the pseudo turn over is reported, on X axis the Pd.Ag concentration and on Z axis the total number of atoms per particles. One can see that a maximum in activity is obtained for particle arround 160 atoms with a concentration of 80% Pd. The selectivity has also been studied and is reported elsewhere ( 35). Electronic effets are assumed to explain this behaviour.
674
CONCLUSION Since now the begining of the eighties. The study of naked clusters has taken a tremendous activity due essentially to the technical developpement of new evaporation sources. It is obvious now that electronic structure, and crystallographic ones could be easier studied by synchrotron radiation experiments and that the chemistry is in between organometallic clusters and surface science approaches. REFERENCES 1 - R. KUBO, J. Phys. Soc.Jap. 17 (1962) 975. 2 - ISSPIC @yon) J. de Physique 11-38,juillet (1977) 3 - ISSPIC II(Lausanne) Surf. Science 106 (1981) 4 - ISSPIC III(Ber1in) Surf. Science 156 (1985) 5 - J.W. GIBBS, Scientific papers Dover publications (1961)
6 - O.F. HAGUENA <<Molecularbeams and low density gas dynamics>>, P.P. WEGENER edit. MARCEL DEKKER A.Y. (1974) and evaporation. Nucleation and growth kinetics,. 7 - J.P. HIRTH and G. POUND, <>,ed. Traver and Putlitz Springer Verag. Heidelberg 1986 22 - J.A.A.J. PERENBOOM, P. WYDER and F. MEIER, Physic. Reports 78 n'2 Nov. 1981 27 - C. BRECHIGNAC and P. CAHUZAC, Zeit fur Phys. D. Atoms, molecules and clusters, ~01.3. n 23 (1986) 24 - C. BRECHIGNAC, M. BROYER, P. CAHUZAC, P. LABASTIE,J.P. WOLFF and L. WOSTE, Phys. Rev. Letter 1988
675
25 - F. CYROT-LACMANN, M.B. GORD0NandM.C. DESJONQUIERES, Surf.Sci. 80( 1979) 159 26 - B. BELLAMY A. MASSON, G.TREGLIA, M.C. DESJONQUIERES and D. SPANJAARD,In the press, J. Phys. 1988
c.
27 - G. TREGLIA, F. DUCASTELLE and D. SPANJAARD, J. Phys. 43 (1982) 341 28 - S.DI CENZO, Personal communication, to be published 29 - M. BOUDART, Ad. Catalysis 20 (1969) 153 30 - S. RILEY ,RICHTMEIER.J.chem.phys.82.( 198q.3659. 3 1 - A. KALDOR D.M. COX D.J. TREVOR and M.R. ZAKIN, Zeit furphys. D. Atoms, molecules and clusters, vo1.3.n"23 (1983) 32 - H. POPPA, LADAS and M. BOUDART, Surf. Sci. 102 (1981) 151 33 - A. MASSON, B. BELLAMY, Y. HADJ ROMDHANE, M. CHE, H. ROULET mdG. DUFOUR,
Surf. Scr. 739 (1986) 479
34 - Y. HADJ ROMDHANE, B. BELLAMY, V. DE GOUVEIA, A. MASSON and M. CHE, Appl.
Surf. Sci. 31 (1988) 383
35- V. DE GOUVEIA,B. BELLAMY, A. MASSON, M. CHE in c&ructure andreactiviteof surface,, Trieste, sept.1988 36 - R.E. SMALLEY Proced in ccPh sics and Chemistry of small clusters>>,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.
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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
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.
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
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 <1 OO> 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 ,<, , , , , , , , , , , , , , , , , 10 20 30 40 50 60 70 80 90 POLAR ANGLE e (DEG.)
Fig. 1. Polar scans of the C 1s / 0 1s intensity ratio for the a3 state of CO on Fe(OO1). Experimental data are shown for the [loo] and (1 1OJ azimuths. Intensity ratios have been normalized to unity at the maximum in the [loo] scan. Theoretical results are shown, not to scale, for an Isolated CO molecule and for small clusters such as illustrated in Fig. 2, with varying distances z above the surface.
725
of the photoelectron emission direction due to the finite spectrometer acceptance angle was also considered. RESULTS AND DlSCUSSfON Fig. 1 shows our polar scan XPD data for both [loo] and [110] azimuths. C 1s intensities have been normalized by corresponding 0 1s intensities in order to eliminate purely instrumental intensity variations as a function of the polar emission angle 8. Whereas the data for the [110] azimuth is rather structureless, a pronounced, broad peak is observed in the [loo] azimuth, centered at 3592' above the surface. Considering the well-known dominance of forward scattering in medium-energy photoelectron diffraction, one can identify this feature as due to intramolecular forward scattering along the CO bond axis, which appears to be tilted by 55"*2O away from the surface normal into the [loo] azimuth. A bonding geometry such as illustrated in Fig. 2 is therefore proposed. Remaining structural parameters to be determined are the vertical position z of the C atom above the @ topmost Fe layer, the exact lateral "001 0 C position of the C atom within the fourfold hollow, which had been llol iii Fe indentified in a recent highresolution electron energy loss spectroscopy (HREELS) study as the most likely bonding site [51, and Fig. 2. Suggested bonding geometry of CO(a3) on Fe(001). the degree of frustrated rotational motion of the molecule.
0
Using the cluster of Fig. 2, with a CO bond distance of 1.22 A [4, 61, we carried out SSC calculations in order to discriminate between various choices of parameters. Due to the fourfold degeneracy of the geometry, four domains corresponding to a tilt into [loo], [OlO], [-loo] and [O-101 azimuths were considered. For simplicity, the C atom was assumed to be centered in the fourfold hollow, and the dependence of the XPD on lateral shifts was not studied. The polar scan calculations in Fig. 1 have not been normalized by corresponding 0 Is intensity scans, as these were found to be rather flat and structureless. All the SSC theory curves, i.e. for an isolated CO mo tecule and for CO/Fe[OOl] clusters with varying vertical distances z of the C atom from the surface Fe layer, are in general
726
agreement with the experimental data. For the [loo] azimuth, the main peak at 8-35" is well reproduced with variations only in the fine structure, and the second feature around 8-75' is found to be more or less independent of the presence of Fe scatterers. For the [110] azimuth both SSC curves are rather flat, reflecting the lack of forward scattering directions, with better agreement in fine structure for z=0.3A. Azimuthal scan XPD has often I'D proved to be more sensitive to 0.6 minor variations i n cluster 0.4 geometries [8,12]. In Fig. 3 we show a C 1s azimuthal scan at a 0.2 max polar angle of 3 5 O , thus including the CO bond direction at @=Oo a n d E 9 0 " . A strong forward scattering t z signal is again observed, with a 3 m smaller feature appearing at (p=4S0, d a which has not been seen in the v polar scan along the corresponding CI) azimuth. The relative intensity I'of z w Ithis feature above minimum z intensity (see Fig. 3) now offers the prospect of discriminating o between different vertical distances Z, as I'/I (I = Imax-Imin) AZIMUTHAL ANGLE @ (DEG.) shows oscillatory behaviour as a Fig. 3. Azimuthal scan data at a polar angle of function of z (see inset in Fig. 3). 8=35O. Theoretical results from SSC calculations The t)xperimentally observed ratio at different vertical C - Fe distances z are also of 0.35k0.05 is consistent only shown. The dependence of the theoretical patterns on z is illustrated in the inset. I/lmax denotes the with z around 0.1, 0.3, 0.6 and overall anisotropy in the experimental scan. 0.7A. More azimuthal scans at different polar angles should be measured and similar dependencies should be exploited to unambiguously determine z. Whereas the diffraction patterns are quite well reproduced with our SSC calculations, the absolute intensity anisotropies defined as ( I m a x Imin)/Imax are highly overestimated. We obtain typically 35-40% in the azimuthal scans at 8=35", as opposed to an experimental value of only 16%. This problem has been encountered in other XPD studies as well [8], and as a possible reason can be mentioned the presence of roughly 20% CO in other
*
it v)
T-
727
than the CO(a3) state, as deduced from C 1s core level analysis. Frustrated molecular motion, and rotations in particular, are also expected to reduce overall anisotropies. This effect is illustrated in Fig. 4 for a polar scan along the [loo] azimuth and an azimuthal scan at 8-35', both with a vertical C - Fe distance of Z=0.6A. The scales of azimuthal and polar angles have been aligned to allow an overall comparison of the main features in both directions. Curves for root-mean-square displacement angles of O", 5" and 1 0 " are shown. Corresponding anisotropies are 35%, 29% and 16%. At e R M s = l O o the agreement to the experimental curves (Figs 1, 3) is suffering, but a B R M S of 5" i n combination with some C present in other species could well account for the observed anisotropy of 16%.
AZIMUTHAL ANGLE @ (DEG.) Fig. 4. Illustration of the effects of frustrated rotational motion of the molecule on theoretical azimuthal scan curves. Note that polar and azimuthal angle scales have been aligned.
coNcLusloNs Applying an SSC analysis to our XPD data from CO(a3)IFe (001) we have been able to determine the structure ot this adsorbate state very accurately. The CO molecule is found to be tilted 55Of2O away from the surface normal into <1OO> azimuths, indicating a considerable contribution of CO K * orbitals to the surface bonding. This tilt angle is at the outside limit of the NEXAFS determination of 45"flO" [6],and the quality of the CO(a3) NEXAFS data was indeed not very good. ESDIAD cannot provide any precise information on tilt angles greater than about 40" away from the surface normal due to extreme ion deflection due to image forces. The vertical distance of the C atom to the topmost Fe layer is either 0.65f0.1A or in the 0.1-0.3A range. Frustrated rotational motion of the molecule about the bonded C atom with a 8RMS of 5" appears to account best for the observed overall anisotropy in our azimuthal scan data, preserving
728
at the same time agreement with the experimental XPD patterns. This value for BRMS is much smaller than the values found for CO/Ni(110) in a similar study [13J, which are in the range of 10°-200 at 300K. This low BRMS may indicate the restraints of the molecule due to a participation of the 0 atom in the surface bonding.
ACKNOWLEDGEMENTS We acknowledge the support of the National Science Foundation under Grant CHE83-20200 and the Office of Naval Research under Contract No. N00014-87-K-0512. We are also very grateful to J. L. Gland, D. J. Dwyer, and S. Cameron for helpful discussions and for providing us with the Fe sample
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
D. W. Moon, D. J. Dwyer and S. L. Bernasek, Surf. Sci., 163 (1985) 215. J.-P. Lu, M. R. Albert and S. L. Bernasek, Surf. Sci., 199 (1988) L406. C. Benndorf, B. Kruger and F. Thieme, Surf. Sci., 163 (1985) L675. S . P. Mehandru and A. B. Anderson, Surf. Sci., 201 (1988) 345. D. W. Moon, S. L. Bernasek, D. J. Dwyer and J. L. Gland, J. Am. Chem. SOC., 107 (1985) 4363. D. W. Moon, S. Cameron, F. Zaera, W. Eberhardt, R. Carr, S. L. Bernasek, J. L. Gland and D. J. Dwyer, Surf. Sci., 180 (1987) L123. R. S. Saiki, G. S. Herman, M. Yarnada, J. Osterwalder and C. S. Fadley, submitted for publication. C. S. Fadley, Prog. in Surf. Sci., 16 (1984) 275. J. J. Rehr, J. Mustre de Leon, C. R. Natoli and C. S. Fadley, J. Phys. (Paris) Colloque, 47 (1986) C8-213. M. Sagurton, E. L. Bullock and C. S. Fadley, Surf. Sci., 182 (1987) 287. P. J. Orders, S. Kono, C. S. Fadley, R. Trehan and J. T. Lloyd, Surf. Sci., 119 (1982) 371. R. Saiki, A. Kaduwela, J. Osterwalder, M. Sagurton, C. S. Fadley and C. R. Brundle, J. Vac. Sci. Techn., A5 (1987) 932. D. A. Wesner, F. P. Coenen and H. P. Bonzel, Surf. Sci., 199 (1988) L419.
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reaetiuity ofSurfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
729
A COMPETITIVE REACTION WHICH IS ABLE TO MAKE FINE DISTINCTIONS BETWEEN REACTING SURFACES
I. PALINKd, F. NOTHEISZ and M. 8ARTdK Oepartment of Organic Chemistry, Jdzsef Attila University, 06m t8r 8, H-6720 Szeged (Hungary)
ABSTRACT The hydrogenative ring-opening of 1,l-diMe- (1) and 1,1,2,P-tetraMe-cyclopropane (2) and the cmpetitive hydrogenation of cyclopropane-olefin mixtures (1 + + 2-Me-2-butene and 2 + 2,4-diMe-Z-pentene) were studied over silica-supported Pt, Pd and Rh catalysts at 318 K and 373 K. The cyclopropanes react over a metallic surface and their reaction characteristics are influenced by the hydrogen pressure. In the competitive hydrogenation the olefin creates a hydrogenrich carbonaceous overlayer which prevents the transformations of the cyclopropanes. The olefins, however, are hydrogenated over this irreversibly bound layer. These competitive reactions reveal fine differences between reacting surfaces and provide an easy method of detecting hydrogen-rich carbonaceous overlayers. INTRODUCTION A reacting catalytic system is quite complicated. The reactants tailor the catalyst, transforming it into a moiety which contains carbonaceous residues over its surface or in the interstices of the metal lattice. The stability and reactivity of these residues are influenced by several factors, e.g. the temperature, the structure of the hydrocarbon to be transformed, the hydrogen pressure, the pretreatment conditions of the catalyst, the influencing effect of the support, etc. The first two are the crucial factors determining the hydrogen content and therefore the reactivity of these layers. The accumulation of carbon is not always detrimental t o the catalyst; a hydrogen-rich overlayer may even serve as an intermediate in the further hydrogenation of olefins or acetylenes, for example. There are compounds, however, which transform over metallic surfaces (their reactions are at least influenced by the roughness of the surface), and therefore all kinds of carbonaceous residues poison these reactions by decreasing the metallic surface available and perhaps influencing the rmaining free sites electronically. Monitoring of the reacting system from this point of view is possible, but it requires sophisiticated and also very expensive instrumentation. However, more simple kinetic methods may also give information about the properties of reacting surfaces through the reactions of suit-
730
ably chosen molecules, serving as complementary techniques to surface science methods. We describe here such a system, which indirectly proves that a reactive hydocarbon-like overlayer forms in the hydrogenation of olefins (a preliminary account has already been published (ref. 1)). This is the active phase for the hydrogenation, but it is a permanent poison for the hydrogenative ring-opening of the cyclopropane ring. EXPERIMENTAL Materials The cyclopropanes (1,l-diMe-cyclopropane (11, l,l,2,2-tetraMe-cyclopropane ( 2 ) ) were prepared and purified according to the literature (refs. 2, 3 ) . The olefins (2-Me-2-pentene and 2,4-diMe-2-pentene) were products of Aldrich Chemical Co. The olefins were gas chromatographically pure and were subjected to several freeze-evacuate-thaw cycles before reaction. Further, the olefin with higher molecular weight (2,4-diMe-2-pentene) was passed through a column filled with freshly activated basic alumina (Carnag) under an inert (N2) atmosphere before preparation of the reaction mixture. During the preparation of the cyclopropanes, the isomeric olefins may also form. In some experiments these mixtures were used. Catalysts The catalysts were prepared by using a Cab-0-Sil (in short CS) M5 (BDH) Si02 support, by a conventional impregnation method (ref. 4). The impregnating salts were H2PtC16, PdC12 and RhC13. The catalysts were subjected to relatively hightemperature reduction (773 K) in flowing hydrogen for 16 hours in order to diminish or even to eliminate their chlorine content (refs. 4-71, The catalysts were characterized through H2 (for Pt/CS and Rh/CS) and CO (Pd/CS) chemisorption in both dynamic and static systems, and through determination of particle size with a Hitachi H500H electron microscope (Table 1). TABLE 1 Characteristic data on catalysts catalyst 3.0% Pt/CS
3.0% Pd/CS 3.2% Rh/CS
dispersion/% chemisorp. TEM 11.8 15.4 27.0
11.5 16.4 24.4
particle size/nm TEM chemisorp. 8.6 6.9 3.9
8.8 6.5 4.3
Methods The reactions were performed in a closed circulation system, described else-
731 where
8).
(ref.
1.33
kPa
and
The p a r t i a l pressure o f t h e cyclopropane
or the m i x t u r e was
t h a t o f hydrogen was v a r i e d between 1.33 kPa and 66.6
kPa.
accumulation was monitored by a Carlo Erba Fractovap 2150 GC
product
The
equipped
an FID. A 1.5 m l o n g a l l - g l a s s packed column (20% bis-(2-methoxyethyl)adi-
with
pate/Chromosorb PAW) was used f o r analysis. RESULTS AND DISCUSSION The
ring-opening
transition
r e a c t i o n s o f cyclopropanes over supported and
unsupported
metals are model transformations t h a t are widely used t o e x p l o r e t h e
characteristic
f e a t u r e s o f d i f f e r e n t c a t a l y s t preparations ( r e f s . 4, 5, 9, 10).
In this
much has been l e a r n t about the laws
way,
governing
these
hydrogen atmosphere;
t h e s t e r i c a l l y less hindered C-C bond breaks with
fairly
s e l e c t i v i t y , and with e l e v a t i o n o f t h e temperature t h e p r o b a b i l i t y o f mul-
high tiple
ring-rupture
tions
have been s t u d i e d as w e l l , mostly i n order t o determine the
of
reactions
4, 5, 9-14). We know, f o r example, t h a t saturated hydrocarbons form i n a
(refs.
the
pers
r i s e s . The turnover frequency vs. hydrogen pressure
func-
molecularity
r e a c t i o n . I n s p i t e o f t h e l a r g e amount o f i n f o r m a t i o n accumulated,
concerning
pa-
the transformations o f t h e s u b s t i t u t e d cyclopropanes i n ques-
t i o n (1, 2) on P t , Pd and Rh c a t a l y s t s a r e r a r e ( r e f . 1 3 ) . The face
e a r l i e r l i t e r a t u r e claims t h a t cyclopropane i s weakly h e l d over t h e surt h e c a t a l y s t , or even r e a c t s from t h e gas phase with the s t r o n g l y
of
sorbed
hydrogen. These assumptions were used t o e x p l a i n the experimentally
tained
maximum curves ( r e f s . 15, 16) and c l o s e t o s a t u r a t i o n - t y p e curves
17)
when
More
r e a c t i o n r a t e was measured as a f u n c t i o n of
the
recently,
studied
and
however,
adob(ref.
hydrogen pressure.
t h e adsorption s t r e n g t h s o f the r e a c t a n t s
have
been
i t has been e s t a b l i s h e d t h a t cyclopropane adsorbs more s t r o n g l y
hydrogen ( r e f . 18). As t h e r a t e - l i m i t i n g step is the r e a c t i o n between
than
the
adsorbed m o i e t i e s ( r e f s . 17, 191, a n a l y s i s o f t h e shapes o f t h e f u n c t i o n s under d i s c u s s i o n may r e v e a l important mechanistic d e t a i l s . curves sharp 26.7
pass and
through maximum (Figs. l / a and l / b ) . I n most cases the maximum i s appears
a t r e l a t i v e l y low hydrogen pressure (between 13.3
kPa
and
kPa) f o r b o t h compounds, w h i l e f o r 1 over P t / C S only a s l i g h t maximum was
observed. the
,
b o t h cyclopropanes (1, 2 1 , t h e ring-opening r a t e vs. hydrogen pressure
For
The Pt/CS c a t a l y s t i s more a c t i v e i n transforming t h i s cmpound
o t h e r s and t h e turnover frequencies are appreciable, even a t h i g h
than
hydrogen
pressure.
As ture
concerns t h e r e g i o s e l e c t i v i t y of t h e ring-opening of
,
predominantly t h e rup-
t h e s t e r i c a l l y less hindered C-C bond ( d i r e c t i o n a) occurs
(Table
2)
and t h e r e i s no fragmentation whatsoever. As
mentioned
before, t h e hydrogen-dependence can u s u a l l y be described by
a
732
TOF/IO-~
TOF/10-'
TOF/lO-'
12 10 8 6
4 2
Fig. 1 The turnover frequency of ring-opening vs. H pressure functions (a) for 1 and (b) f o r 2 over Pt/CS A , Pd/CS + and Rh/CS 0 Zatalysts. TABLE 2 Ring-opening selectivities catalyst
temperature /K
3.0% Pt/CS
318
3.0%Pd/CS
313
hydrogen pressure/kPa
regioselectivity
6.7 13.3
56,1a 41.4
23.9b 24.6
39.9
18.5 10.1
19.8
6.1 13.3
3.2% Rh/CS
318
a/b
39.9 6.1
13.3
53.6
0.6 4.0
7.9 5.2
82.6
3.2 184.0
16.6 16.0
141.5 6.3
acornpound 1 : a/b=2,2-diMe-propane/2-Me-butane bcompound 2: a/b=2,2;3-triMe-butane/2,4-diMe-pentane saturation-type curve or a curve passing through a maximum (ref. 20). These functions hint at noncompetitive or competitive adsorption, respectively, or at the extent of unsaturation of the reaction complexes (associative or dissociative adsorption) when one reactant (e.g. the cyclopropane) adsorbs more strongly than the other (e.g. hydrogen). Hence, this latter case applies to our system and we may conclude that 1 and 2 react mostly through dissociatively adsorbed species. The contribution of associatively adsorbed intermediates is on-
733 l y s i g n i f i c a n t f o r 1 over Pt/CS. When
t h e s t e r i c hindrance caused by t h e s u b s t i t u e n t s i s considered, i t seems t h a t n e i t h e r o f these compounds can adsorb i n a p l a n a r mode, b u t
probable
only
i n an edgewise manner. E s p e c i a l l y f o r t h e Pd/CS c a t a l y s t , t h e p i c t u r e may be more complicated, beca-
t h i s metal i s capable o f a-13 phase transformation. I t i s w e l l known t h a t
use
t h e r e a c t i v i t i e s o f these two phases d i f f e r appreciably. A t 373 K t h i s t r a n s f o r mation have
starts
at
evidence
46.7 kPa according t o l i t e r a t u r e r e s u l t s ( r e f . 211,
but
t h a t these two phases c o e x i s t even a t a H2 pressure o f 13.3
we kPa
( r e f . 22). Even more complex behaviour i s expected i n t h e c o m p e t i t i v e hydrogenation
+ o l e f i n mixtures. For our c a t a l y s t s (5 mg was used), we
of
the
cyclopropane
the
r e a c t i o n s a t 318 K or a t 373 K (Table 3): only t h e o l e f i n was hydrogenated,
and
the
min).
ran
hydrogenation was complete by t h e time t h e f i r s t sampling occurred
There
(5
was no cyclopropane hydrogenation whatsoever, even a f t e r 100 min.
TABLE 3 Reaction c o n d i t i o n s f o r t h e competitive hydrogenation o f
2 + 2,4-diMe-2-pentene catalyst
3.0% Pt/CS fresh used fresh 3.0% Pd/CS fresh used fresh 3.2% Rh/CS fresh fresh used
The
o l e f i n content
hydrogen
temperature
/mol%
pressure/kPa
10.5 11.0
4.8
6.7 20.0 39.9
318 318 373
15.6 15.6 10.5
13.3 13.3 39.9
373 373 318
4.5 10.9 10.9
39.3 26.0 26.0
373 318 318
/K
used c a t a l y s t s were capable o f hydrogenating the o l e f i n from new o l e f i n
+ cyclopropane sampling), this
mixtures
mixtures s e v e r a l times (with t o t a l conversion before the
first
w i t h o u t any impact on t h e o t h e r component. The important feature
reaction
i s t h a t t h e o l e f i n , even a f t e r i t s canplete hydrogenation,
+ of
acts
as
a permanent poison f o r t h e transformations o f t h e cyclopropane, i r r e s p e c t i v e
of
the
hydrogen pressure. Nevertheless, a f t e r t h e hydrogenation o f t h e
olefin
in
the
mixture, a f r e s h sample o f c a t a l y s t (5 mg) w i l l hydrogenate t h e
cyclo-
734 propane. of
The
saturated hydrocarbons r e s u l t i n g from t h e previous
o l e f i n and from the ring-opening of t h e cyclopropane do
the
hydrogenation not
influence
k i n e t i c s o f t h e ring-opening r e a c t i o n . That i s , t h e r a t e o f t h e
the tive
ring-opening
+5%)
as
(within
o f the pure ( o l e f i n - f r e e ) cyclopropane was the same
t h a t o f the cyclopropane a f t e r previous hydrogenation of
the
olefin.
q u a n t i t y o f t h e c a t a l y s t (from 5 mg t o 10 rng) d i d n o t change
Ooubling t h e
the
or
E s s e n t i a l l y t h e same t h i n g happened when e i t h e r Aldrich o l e f i n s
phenomenon. those
hydrogena-
olefins
which
formed d u r i n g t h e p r e p a r a t i o n o f t h e
cyclopropanes
were
we may say t h a t the o l e f i n hydrogenation proceeds on a d i f f e r e n t
sur-
used. Thus,
face than t h a t on which the ring-opening takes place. the o r i g i n a l mechanistic c o n s i d e r a t i o n f o r the hydrogenation of
In
olefins,
H o r i u t i and Polanyi presumed t h a t t h e species wRx and xRH a r e formed on t h e surm e t a l atoms ( r e f . 2 3 ) . I n h i s e a r l y paper ( r e f . 241, Maxted mentioned
face
the
toxic
e f f e c t o f compounds c o n t a i n i n g m u l t i p l e bonds on t h e various
tions
o f more saturated m a t e r i a l s over metal c a t a l y s t s . Without c i t i n g o r i g i n a l
papers,
he
claimed
transforma-
t h a t t h i s e f f e c t was due t o a mere competition:
the
h e l d substrate prevented the adsorption o f the other. I t f o l l o w s
strongly
more s t r o n g l y adsorbed compound a c t s as an apparent poison u n t i l i t s
the
that reac-
i s over, and a f t e r t h i s the other component s t a r t s t o r e a c t . L a t e r , Hansen
tion
adsorbed
a l . p o i n t e d o u t t h a t the hydrogenation might proceed on a l a y e r o f
et
more
olefin
and
t h a t the r e a c t i o n occurs v i a a hydrogen-transfer
mechanism
(refs.
The hydrogen-rich adsorbed species serve as a hydrogen p o o l f o r t h e
25). cules
mole-
adsorbing i n t h e second l a y e r . From t h e i r r e s u l t s concerning t h e separate
and
competitive hydrogenation o f ethylene and acetylene a t 298 K over supported
Pd,
Rh and Ir c a t a l y s t s , Webb e t a l . ( r e f . 2 6 ) concluded t h a t t h e adsorption o f molecules occurred i r r e v e r s i b l y i n two separate stages : a non-linear p r i -
these mary
r e g i o n , i n which the hydrocarbon species are predominantly
adsorbed,
dissociatively
and a l i n e a r secondary r e g i o n . Hydrogenation c a t a l y s i s i s
associated
the hydrocarbon species adsorbed i n t h e secondary region. From I4CO
with
ies
i n a s t a t i c system, i t was concluded t h a t the hydrocarbon primary
was
associated
with the metal, w h i l s t the secondary r e g i o n probably
studregion
involved
the formation o f overlayers on t h e p r i m a r i l y adsorbed species.
role and nature o f carbonaceous o v e r l a y e r s and the mechanism o f
The have
coking
been s t u d i e d e x t e n s i v e l y by h o r j a i and his co-workers on s i n g l e - c r y s t a l s
( r e f s . 14, 2 7 ) . I n s e v e r a l cases, t h e i r r e s u l t s can be d i r e c t l y a p p l i e d t o catalytic
c o n d i t i o n s , because they developed the technique t o combine t h e t o o l s
s u r f ace apparatus termined
of
science with r e a l - l i f e c a t a l y s i s by u s i n g a low-pressure, high-pressure
which can be used e i t h e r i n batch or i n f l o w mode ( r e f . 28). They dethe
l i f e t i m e and the s t r u c t u r e o f these l a y e r s , and t h e
effects
of
735
temperature kinds
of
t h e i r formation and transformations. I t was shown t h a t n o t
on
carbonaceous deposits caused d e a c t i v a t i o n . Hydrogen-rich can
in
catalytic
hydrogenated an
hydrocarbon
p l a y a d e c i s i v e role, e.g. i n hydrogenation r e a c t i o n s t a k i n g
species the
all
a c t i o n , by t r a n s f e r r i n g hydrogen t o t h e second l a y e r
(ref.
part to
be
29). For instance, cyclohexene i s a c t u a l l y hydrogenated
on
a c t i v e carbonaceous overlayer l i k e t h i s , between 298 and 373 K, which i s why
t h i s r e a c t i o n i s s t r u c t u r e - i n s e n s i t i v e on P t s i n g l e - c r y s t a l s with d i f f e r e n t surface
structures
i n t h i s temperature range ( r e f . 301, i n
with
t h e r e s u l t s o f Boudart e t a l . on supported P t c a t a l y s t s ( r e f . 31). I n t h e
hydrogenation o f faces,
ethylene,
(ref.
accordance
propylene and butene on P t ( l l 1 ) and Rh(ll1)
t h e a c t i v e overlayer i s
pylidyne
complete
non-linear and i s formed by e t h y l i d y n e ,
pro-
or b u t y l i d y n e surface species, r e s p e c t i v e l y ( r e f . 32). I t was found
33)
t h a t the p r o p e r t i e s o f t h e a l k y l i d y n e l a y e r depend s t r o n g l y
temperature.
There
on
the
i s no a l k y l i d y n e l a y e r below 300 K and i t decomposes a t
400 K, so i t s importance i s r e s t r i c t e d l a r g e l y t o hydrogenation
bout
sur-
a-
reactions
which may occur i n t h i s temperature range. This l a y e r i s n o t the r e a c t i o n i n t e r n e i t h e r does i t belong t o t h e r e a c t i o n pathway; i t serves o n l y t o pro-
mediate, vide
H atoms f o r the a d d i t i o n t o the molecules adsorbed on top o f
cies.
As
these
spe-
t h e temperature increases, a more and more dehydrogenated l a y e r ,
and
f i n a l l y a g r a p h i t i z e d l a y e r forms.
I t i s worthwhile t o mention, however, t h a t the s t r u c t u r e o f adsorbed ene
over
d i f f e r e n t P t surfaces ( s i n g l e - c r y s t a l s , supported and unsupported is
P t ( l l 1 ) ( r e f . 32). I n f r a r e d spectroscopy shows a v a r i e t y o f
over
over
supported numbers
species,
and
Pt
s t i l l e x t e n s i v e l y debated. As we have seen above, i t i s e t h y l i -
catalysts) dyne and
ethyl-
species
c a t a l y s t s : 6-diadsorbed species with d i f f e r i n g C-C bond orders
o f hydrogen, the E-complexed form together with the
6'-diadsorbed
e t h y l i d y n e ( r e f . 34). S o l i d - s t a t e NMR s t u d i e s show
contradictory
r e s u l t s : e.g. Wang e t a l . ( r e f . 35) found e t h y l i d y n e , w h i l e Gay ( r e f . 36) propoimpor-
a 'i?-bonded intermediate. The determination o f the exact s t u c t u r e i s
sed
and may p r o v i d e f u r t h e r i n f o r m a t i o n about the formation and p r o p e r t i e s
tant
of
the overlayer. The h i g h t e m p e r a t u r e - s e n s i t i v i t y o f t h e hydrogen-rich carbonaceous
overlay-
ers e x p l a i n s why H a t t o r i and Burwell ( r e f . 37) c o u l d f i n d no evidence o f a hydrogen-transfer mechanism through adsorbed hydrocarbon species i n t h e hydrogenat i o n o f ethylene a t 247 K. On
t h e other hand, B u r w e l l and B u t t and Somorjai a l l agree t h a t t h e
ring-o-
pening r e a c t i o n s of cyclopropanes ( r e f . 9 ) and o f Me-cyclopentane ( r e f . 38) occur
on
bare metal atoms: the C-C bond breaks on t h e k i n k s i t e s o f
the
single
crystal. Thus,
i n t h e hydrogenation of t h e r e a c t a n t p a i r s , we may conclude t h a t
over
736
silica-supported P t , Pd and Rh catalyst
-
the
o l e f i n s are hydrogenated on an active carbonaceous overlayer a t 318 K
and a t 373 K (and very probably i n t h i s temperature range);
-
the cyclopropanes do not react on t h i s surface, but on bare metal atoms;
-
i n the competitive hydrogenation, the o l e f i n acts as a permanent poison; the hydrogen does not remove the overlayer i n the course of the reaction; the used catalyst can hydrogenate the o l e f i n from new mixtures several
times, without apparent loss of a c t i v i t y . CONCLUSIONS I t was shown t h a t competitive reactions o f appropriate molecules are capable
of
revealing important d e t a i l s about c a t a l y t i c surfaces accessible d i r e c t l y on-
l y t o surface science tools. With regard t o the d i f f i c u l t i e s o f studying hydrogen-rich carbonaceous overlayers and t h e i r importance i n c e r t a i n reactions, the above-discussed
reaction system may be useful a t l e a s t i n t h e i r detection and
i n gathering preliminary knowledge about reacting surfaces. I t i s possible make such overlayers by o l e f i n hydrogenation, and the properties o f these
to lay-
ers can be studied with these molecules as well. REFERENCES 1 I. PAlinkb, F. Notheisz and M. Bartbk, Catal. L e t t . , 1 (1988) 127-132. 2 R.W. Shortridge, R.A. Craig, K.W. Greenle, J.M. Oerfer and C.E. Eoord, J. Amer. Chem. SOC., 70 (1948) 946-949. 3 R.G. Kelso, K.W. Greenlee, J.M. Oerfer and C.E. Eoord, J. Amer. Chem. SOC., 77 (1955) 1751-1755. 4 T.A. Dorling, M.J. Eastlake and R.L. Moss, J. Catal., 14 (1969) 23-33. 5 P.H. Otero-Schipper, W.A. Wachter, J.E. Butt, R.L. Burwell, Jr. and J.B. Cohen, J. Catal., 53 (1978) 414-422. 6 S.E. Wanke and N.A. Dougharty, J. Catal., 24 (1972) 367-384; H. Miura, H. Hondou, K. Sugiyama, T. Matsuda and R.O. Gonzalez, i n M.J. P h i l i p s and M. Ternan (Editors), Proc. 9th I n t . Congr. Catal., Calgary, Canada, 1988, The Chemical I n s t i t u t e o f Canada, Ottawa, Ontario, 1988, Vol. 3, pp. 1307-1313. 7 E.J. Kip, E.G.F. Hermans and R . Prins, i n M.J. P h i l i p s and M. Ternan (Editors), Proc. 9th I n t . Congr. Catal., Calgary, Canada, 1988, The Chemical I n s t i t u t e of Canada, Ottawa, Ontario, 1988, Vol. 2, pp. 821-828. 8 M. Eartbk, F. Notheisz, A.G. Zsigmond and G.V. Smith, J. Catal., 100 (1986) 39-44. 9 P.H. Otero-Schipper, W.A. Wachter, J.E. Butt, R.L. Burwell, Jr. and J.B. Cohen, J. Catal., 50 (1977) 494-507; S.S. Wong, P.H. Otero-Schipper, W.A. Wachter, Y. Inoue, M. Kobayashi, J.E. Butt, R.L. Burwell, Jr. and J.B. Cohen, J. Catal., 64 (1980) 84-100; R. Pitchai, S.S. Wong, N. Takahashi, J. 8. Butt, R.L. Eurwell, Jr. and J.B. Cohen, J. Catal., 94 (1985) 478-490. 10 Z. Karpinski, T.-K. Chuang, H. Katsuzawa, J.B. Butt, R.L. Burwell, Jr. and J.E. Cohen, J. Catal., 99 (1986) 184-197. 11 J. Newham, Chem. Rev., 63 (1963) 123-137; J.H. S i n f e l t , O.J.C. Yates and W.F. Taylor, J. Phys. Chem., 69 (1965) 1877-1882; R.A. Oalla Betta, J.A. Cusumano and J.H. S i n f e l t , J. Catal., 19 (1970) 343-349. 12 G. Maire, G. Plouidy, J.C. Prudhome and F.G. Gault, J. Catal., 4 (1965) 556-569; J.C. Prudhomne and F.G. Gault, Bull. SOC. Chim. F r . , (1966) 832-836; T. Chevreau and F.G. Gault, J. Catal., 50 (1977) 143-155, 156-161.
737 13 J.C. Prudhomme and F.G. Gault, B u l l . SOC. Chim. Fr., (1966) 827-831; T. Chevreau and F.G. Gault, J. Catal., 50 (1977) 124-142; C. Kemball and J.C. Kempling, Proc. Roy. SOC. London Ser. A, 329 (1972) 391-403; K.E. Hayes and C.J. Bowser, i n Prep. 5 t h Canadian Symposium on Catalysis, Calgary, Canada, 1977, pp. 375-383. 14 G .A. Somorj a i , Chemistry i n Two Dimensions : Surfaces, Cornell U n i v e r s i t y Press, I t h a c a and London, 1981, pp. 419-421. 15 J. Addy and G.C. Bond, Trans. Faraday SOC., 53 (1957) 368-376. 16 G.C. Bond and J. Newham, Trans. Faraday SOC., 56 (1960) 1501-1514. 17 D.W. McKee, J. Phys. Chem., 67 (1963) 1336-1340. 18 Z. Knor, V. Ponec, Z. Herman, Z. Dolejsek and 5. Cerny, J. Catal., 2 (1963) 299-309; J.R. Anderson and N.R. Avery, J. Catal., 8 (1967) 48-63; T.S. Sri dhar and D.M. Ruthven, J. Catal., 16 (1970) 363-369; H.F. Wallace and K. E. Hayes, J. Catal., 29 (1973) 83-91; L.L. Hegedds and E.E. Petersen, J. Catal., 28 (1973) 150-156; S. Cerny, M. Smutek, F. Buzek and A. Curinova, J. C ata l ., 47 (1977) 159-165. 19 J.E. Benson and T. Kwan, J. Phys. Chem., 60 (1956) 1601-1605. 20 2 . Pagl and G. Menon, Catal. Rev.-Sci. Eng., 25 (1983) 229-324; Z. PaB1, i n 2 . P a i l and P.G. Menon ( Edit or s) , Hydrogen E f f e c t s i n Catalysis, Marcel Dekker, I n c . , New York and Basel, 1988, pp. 449-497. 21 J.E. Benson, H.S. Hwang and M. Boudart, J. Catal., 30 (1973) 146-153, and reference 15 t h e r e i n . 22 I.PAlink6, F. Notheisz and M. Bartdk, i n preparation. 23 M. Po l a n yi and J. H o r i u t i , Trans. Faraday SOC., 30 (1934) 1164. 24 E.8. Maxted, Advan. Catal. Relat. Subj., 3 (1951) 129-177. 25 R.S. Hansen, J.R. Arthur, Jr., V.J. Mineault and R.R. Rye, J. Phys. Chem., 70 (1966) 2787-2792; N.C. Gardner and R.S. Hansen, J. Phys. Chem., 74 (1970) 3298-3299. 26 S.J. Thomson and G. Webb, J. Chem. SOC., Chem. Comun., (1976) 526-527; A.S. Al-Amar and G. Webb, J. Chem. SOC., Faraday 1, 74 (1978) 195-205; A.S. Al-Amnar and G. Webb, J. Chem. SOC., Faraday 1, 74 (1978) 657-664; A.S. Al-Arrnnar and G. Webb, J. Chem. SOC., Faraday 1, 75 (1978) 1900-1911. 27 G.A. Somorjai, i n Proc. 8 t h I n t . Congr. Catal., (WestIBerlin, 1984, Verlag Chemie, Weinheim, Deerf i e l d Beach, F l o r i d a , Basel, 1984, Vol. 1, 113-150, and references 22-33. 28 D.W. Bl akel y, E. Kozak, B.A. Sexton and G.A. Somorjai, J. Vac. Sci. Technol., 13 (1976) 1901; A.L. Cabrera, N.D. Spencer, E. Kozak, P.W. Davies, and G.A. Somorjai, Rev. Sci. I n s t r u . , 53 (1982) 1888. 29 G.A. Somorjai, React. Kinet . Cat al. L e t t . , 35 (1987) 37-87. 30 S.M. Davis and G.A. Somorjai, J. Cat al. , 65 (1980) 78. 31 E. Segal, R. J. Madon and M. Boudart, J. Catal., 52 (1978) 45-49; R. J. Madon, J.P. O'Connel and M. Boudart, AIChE J., 24 (1978) 904-911. 32 C. Minot, M.A. Van Hove and G.A. Somorjai, Surf. Sci., 127 (1982) 441; R.J. Koestner, M.A. Van Hove and G.A. Somorjai, J. Phys. Chem., 87 (1983) 203-213. 33 F. Zaera and G.A. Somorjai, J . Amer. Chem. SOC., 106 (1984) 2288-2293. 34 N. Sheppard and C. de l a Cruz, React. Kinet. Catal. Lett., 35 (1987) 21-35, and references t her ein; T.P. Beebe, Jr., M.R. A l b e r t and J.T. Yates, Jr., J. Catal., 96 (1985) 1-11; Y. Soma, J. Catal., 59 (1979) 239-247. 35 P.-K. Wang, C.P. S l i c h t e r and J.H. S i n f e l t , J. Phys. Chem., 89 (1985) 3060-3066. 36 1.0. Gay, J. Catal., 108 (1987) 15-23. 37 T. H a t t o r i and R.L. Burwell, Jr., J. Phys. Chem., 83 (1979) 241-249. 38 F. Zaera, 0. Godbey and G . A . Somorjai, J. Catal., 101 (1986) 73-80; F. Zaera, A.J. Gellman and G.A. Somorjai, Acc. Chem. Res., 19 (1986) 24-31.
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C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactiuity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlanh
739
ACTIVITY AND CHARACTERIZATION OF ALKALI DOPED Ni/MgO CATALYSTS
A. PARMALIANA~, F. FRUSTERI
1
, F.
ARENA',
I s t i t u t o d i Chimica I n d u s t r i a l e
N. MONDELLO' and
F;.
GIORDANO'
U n i v e r s i t a d i Messina, 98100 Messina
* I s t i t u t o CNR-TAE, S a l i t a S . ~ u ac 39, 98126 S. Lucia, Messina
ABSTRACT Ni/Mg0 c a t a l y s t s have been p r e p a r e d and t h e i r r e a c t i v i t y i n methane steam r e f o r m i n g r e a c t i o n has been e v a l u a t e d as a f u n c t i o n o f p r e t r e a t m e n t and r e a c t i o n c o n d i t i o n s . The e f f e c t s o f a l k a l i d o p i n g ( L i and K) on t h e c a t a l y t i c f u n c t i o n a l i t y and s u r f a c e p r o p e r t i e s have been i n v e s t i g a t e d . The c a t a l y s t s were c h a r a c t e r i z e d by d i f f e r e n t t e c h n i q u e s ( H c h e m i s o r p t i o n , TPR, TEM and 2 SEM w i t h EDAX). A l k a l i s t r o n g l y a f f e c t c a t a l y t i c a c t i v i t y w i t h t h e L i b e i n g more t o x i c t h a n K. A good c o r r e l a t i o n was found between metal s u r f a c e a r e a and c a t a l y t i c a c t i v i t y o f doped c a t a l y s t s . An i n t e r p r e t a t i o n o f t h e b e h a v i o u r o f a l k a l i doped c a t a l y s t s , based on t h e g e o m e t r i c e f f e c t o f a l k a l i , i s proposed.
INTRODUCTION The Ni/MgO system i s
currently
receiving
considerable
attention
mostly
w i t h r e s p e c t t o t h e f o r m a t i o n o f NiO-MgO s o l i d s o l u t i o n s and t o t h e i r p e c u l i a r physico-chemical
and c a t a l y t i c p r o p e r t i e s
(refs.
1-41.
The p r o m o t i n g r o l e
o f MgO on t y p i c a l supported 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 hydrocarbons has been a l s o i n v e s t i g a t e d ( r e f s . oxidation,
3,5).
As d e a c t i v a t i o n due t o s i n t e r i n g ,
p o i s o n i n g and c o k i n g l i m i t s t h e l i f e t i m e of
Ni-based
catalysts,
emphasis has been p u t on t h e e f f e c t s e x e r t e d by MgO i n such processes ( r e f s .
4-5).
A c c o r d i n g t o R o s t r u p - N i e l s e n MgO as a b a s i c promoter o f N i c a t a l y s t
s t r o n g l y reduces t h e c o k i n g r a t e ( r e f .
5).
The i n f l u e n c e o f MgO on t h e N i O
r e d u c i b i l i t y , mean s i z e o f t h e N i c r y s t a l l i t e s , c a t a l y t i c a c t i v i t y and mechanic a l s t r e n g t h had been i n v e s t i g a t e d by B o r o w i e c k i ( r e f . 6 ) . I t has been c l a i m e d t h a t a l k a l i added t o t h e c a t a l y s t cause a decrease o f t h e s p e c i f i c a c t i v i t y by more t h a n one o r d e r of
magnitude w h i l s t e x e r t i n g a promoter e f f e c t i n
t h e g a s i f i c a t i o n o f t h e coke p r e c u r s o r ( r e f . 5 ) . Recent advances i n t h e m o l t e n c a r b o n a t e f u e l c e l l s (MCFC) have i n d i c a t e d t h e o p p o r t u n i t y o f i m p r o v i n g t h e e f f i c i e n c y o f t h e system b y p r o d u c i n g t h e hydrogen f u e l
d i r e c t l y "on t h e c e l l " ,
by i n t e r n a l methane steam r e f o r m i n g
740
(ref.
7). As a r e s u l t o f many years o f continous research we have proposed
and t e s t e d a Ni/MgO c a t a l y s t which takes advantage o f t h e unique p r o p e r t i e s o f t h e support ( a w e l l c r y s t a l l i z e d MgO "smoke") and o f an our own preparation procedure.
As the working c a t a l y s t must be r e s i s t a n t t o t h e a l k a l i coming
from MCFC e l e c t r o l y t e ( L i CO -K CO e u t e c t i c mixture) t h e r e f o r e one aim 2 3 2 3 ' o f our studies was t o i n v e s t i g a t e t h e surface properties and c a t a l y t i c features L i ) Ni/MgO i n t h e methane steam reforming.
o f pure and a l k a l i doped (K,
I n t h e present work we r e p o r t some p r e l i m i n a r y r e s u l t s obtained from physicalchemical analysis (BET, prepared"
TEM,
catalysts i n t h e i r
SEM,
TPR,
chemisorption etc.) o f t h e "as2 r e l a t i o n s h i p s w i t h c a l c i n a t i o n and reduction H
c o n d i t i o n as w e l l as w i t h c a t a l y t i c properties.
EXPERIMENTAL Catalyst preparation Supported
n i c k e l c a t a l y s t s were prepared by impregnation t o
incipient
wetness under continuos s t i r r i n g o f MgO "smoke" powder (UBE Ind. Ltd, Japan) according t o a procedure described i n d e t a i l s elsewhere ( r e f . 8 ) . The c a t a l y s t s were d r i e d overnight a t 393 K,
then calcined under an a i r stream f o r 16
h a t 673 K (standard c o n d i t i o n s ) . Aliquots o f t h e above c a t a l y s t s were f u r t h e r c a l c i n e d f o r 6 h a t 873, 1073 and 1273 K. The a l k a l i doped c a t a l y s t s were prepared by wet impregnation o f t h e f i n i t e o r uncalcined c a t a l y s t s w i t h isopropanol solutions o f t h e K o r L i acetate. A f t e r d r y i n g f o r 10 h a t 353 K t h e samples were calcined under an a i r stream f o r 16 h a t 673 K. The N i , K and L i contents were determined by atomic absorption.
Catalysts c h a r a c t e r i z a t i o n ( i 1 Hydrogen chemisorption and t o t a l surface area measurements. The hydrogen chemisorption was determined i n a conventional gas volumetric apparatus equipped w i t h a turbomolecular vacuum pump. A f t e r reduction, i n a H
2
stream
f o r 2 h a t 673 K and f u r t h e r f o r 1 h a t 998 K, the sample (1-2 g) was outgassed a t 998 K down t o a pressure o f
(13.3
mPa) and t h e r e a f t e r
adsorption isotherms were determined 2 a t 298 K i n t h e range o f 0-6 kPA. The monolayer coverage was estimated by
cooled t o
298 K i n f l o w i n g He.
torr
H
back e x t r a p o l a t i o n t o zero pressure o f t h e s t r a i g h t
linear portion o f the
741 isotherm.
The metal surface area (MSA),
t h e metal dispersion ( D )
and t h e
average c r y s t a l l i t e s i z e were determined according t o Mustard and Bartholomew ( r e f . 9). The t o t a l surface area o f t h e c a t a l y s t s was measured by BET method using a Sorptomatic 1800 Carlo Erba N2 adsorption apparatus. (ii)
Temperature programmed r e d u c t i o n (TPR).
performed i n a conventional
The TPR measurements were
quartz
U-tube r e a c t o r using a p u r i f i e d H -N 2 2 m i x t u r e (6.15% H2) f l o w i n g a t 54 Ncm3min-' over a 200 mg sample d r i e d i n
N2 a t 673 K f o r 2 h and then cooled t o r.t. Measurements were made from r.t. t o 1223 K and a t a heating r a t e o f 10 K min-l.
From t h e change i n H 2 concentration, .detected by a thermal c o n d u c t i v i t y detector (TCO), t h e consumpt i o n o f H was determined w i t h high r e l i a b i l i t y from t h e i n t e g r a t e d peak 2 areas. The experimental operating variables used i n TPR were chosen according t o t h e c r i t e r i a suggested by Monti and Baiker ( r e f . 10). ( i i i 1 Electron microscopy. Transmission e l e c t r o n microscopy (TEM) analysis was performed by a P h i l i p s CM 12/STEM e l e c t r o n microscope operating a t 120 KV ( r e s o l u t i o n b e t t e r than 1 nm). The samples were u l t r a s o n i c a l l y dispersed
i n ethanol and deposited on a t h i n holey-carbon coated copper g r i d . Scanning e l e c t r o n microscopy (SEM) micrographs were made by a P h i l i p s 525 e l e c t r o n microscope equipped w i t h a EOAX PV 9900 energy d i s p e r s i v e X-ray spectrometer f o r microanalysis.
Apparatus and Procedure The methane steam reforming r e a c t i o n was evaluated a t atmospheric pressure i n a continous "down-flow"
fixed-bed quartz microreactor (1= 200 mm;
4 mm). The d i l u e n t n i t r o g e n (PPL grade,
SIO product), was c a r e f u l l y purged
by passing i t a t r.t. through an O2 t r a p ( A l l t e c h product), sieve
bed and f i n a l l y over
methane (2.5
a 4A molecular
a Gas P u r i f i e r System (Supelco product). The
Messer Grieshein product,
further purification.
i.d.=
purity>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.
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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
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.
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
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) .
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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.
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
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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C. Morterra, A. Zecchina and G. Costa (Editors), Structure und Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V.. Amsterdam -Printed in The Netherlands
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
In rate =
Ea/RT + In
[ C O ] t y In
[H21
(1)
The value of apparent Ea of methanation obtained for Ru(OOO1) single crystal is i n reasonable agreement with that obtained for supported polycrystalline Ru (ref 16).
820
77 -18 VJ
3
-20
Y
c
d
-
-22 -
-24 -
-26I
1.7
I
I
I
1.9
IOOOK,j.
2.1
Fig. 2 Arrhenius plot of methane production on Ru(OOO1) surface single crystal SIMS Studies It i s t o be expected that the surface w i l l be covered by a variety of hydrocarbon residues, thus a complex SSIMS spectrum is t o be expected. A wide variety of surface species could man a range of particle clusters could contribute t o a particular simal. Clean h ( O 0 0 l ) Surface. The low resolution positive SSIMS spectrum of the surface after CO hydrogenation i s shown i n Fig 3. Fig 4 shows the high resolution mass spectrum f o r the entire mass range investlgated. In the region 10-20 D the pattern i s similar t o that produced by a similar experiment carried out by Kaminsky et al. (ref 17) with the dominant species t The peaks at 16 and 18 D may imply the presence of adsorbed being CH3 oxygen and water respectively. In the 70-85 D region the observed pattern is characteristic of aliphatic C6 hydrocarbons where the odd mass peaks 75, 77 and 79 are more intense r e l a t i v e t o the even-mass peaks. However, the r e l a t i v e l y high i n t e n s i t y of the peaks a t 71, 72, 73 and 74 D imply the presence of some oxygenated C4 species. The spectrum in the 108-120 D region appears t o be due t o a mixture of RuCHxt (where x = 0 - 3 ) , RuOt and RuOH2'. Supporting evidence for the presence of RuCHxf species come from low mass data. Signals at 12 t o 15 amu suggest the presence of CHx (x = 0 t o 3 ) species on the surface after the reaction. Peaks appearing i n the 120-130 D region can be assigned t o RuCPx species while those i n t h e 135-145 D region may be a t t r i b u t e d t o RuC3Hx species. Turning t o t h e low resolution s p e c t r a (Fig 3a), one can observe a
.
821
100 120 140 160amu Fig. 3
Low resolution SSIMS spectra of Ru(0001) surface ( a ) after interrupting the FTS reaction, and (b) a f t e r hydrogenation of the carbon pre-covered
h(O00l) surface.
(a)
I
Fig.
4 High
resolution SSIMS spectra of Ru(0001) surface (a) after interrupting the FTS reaction, and ( b ) after hydrogenation of the carbon pre-covered Ru(OOO1) surface.
progression of peaks centering a t intervals of 12-14 D after the Ru isotope cluster. This would suggest the polymerisation of CH2 species on a Ru site. Carbon pre-covered Ru(OOO1) surface. "he low resolution SINS spectrum resulting from the hydrogenation of the carbon pre-covered Ru surface i s shown i n Fig 3b. The 10-20 D region i s very s i m i l a r t o t h a t produced by CO hydrogenation on clean Ru, however, there i s no oxygen peak here. A s i n the previous experiment the presence of aliphatic C6 hydrocarbons is indicated by the distinctive pattern of peaks in the 7&85 D region (Fig 4b).
822
A maximum at 114 D Indicates the presence of RuC' species whilst the peaks + at 117, 118 and l l g D Indicate presence of RuCHx species. The peak a t 121 D may be due to RuOH', the OH arising from adsorbed water. Supporting evidence for t h i s cam from the low mass spectrum which shows a peak at 18 D. Here again we observe a progression of peaks at intervals of 12-14 D after the Ru t
isotope cluster (Fig 3b), suggesting the presence of h C H X c RuC4Hx on the surface.
, RuC2H;,
RuC3Hx',
EELS Studies
The EELS spectrum obtained a f t e r exposure of the clean Ru surface t o the reaction mixture For more than 30 mlnutes is shown i n Fig 5b. The intensity of the e l a s t i c reflected electrons along the specular direction was dramatically reduced compared to that of a clean Ru surface, obviously caused by the strong carbon contamination of the surface during reaction. I n the high frequency region t h e i n t e n s e loss a t 2944 cm-l and i t s shoulder a t 3024 cm-I are i n the range f o r CH s t r e t c h i n g v i b r a t i o n s of adsorbed hydrocarbons. The loss at 1452 cm-I could be attributed t o the scissor mode of an adsorbed methylene species. The loss at 452 cm-' which is s l i g h t l y obscured by the high background i n t h i s region and hence poorly 1 resolved would be due t o the Ru-C stretch. !he vibrational loss at 564 cm-
>
H
-
(a)
0
1800
2000
3000 Energy
L O S d
Fig. 5 EEIS spectrum of (a) the clean Ru(OOO1) surface, (b) after InterrmptITg the FTS reaction, and (c) after hydrogenation of the carbon pre-covered
Ru(OOO1) surface.
823
falls i n the region of Ru-0 stretch. The appearance of loss vibrations at 1290 an-' and 750 cm-l could be Indicative of the presence of an acetylide species (CCH) where the first loss could be due to (CC) and the second to CH bend respectively (ref 18). 'Ihe broad loss vibration centering at 840 m-l suggests that there is more than one loss contributing to it. In this region v i b r a t i o n a l modes due t o CH2 deformation are t o be expected (ref 19). However, a contribution from CH bending vibration of mthylidene species can not be ruled out (23). The Intense loss at "1830 cm-l cannot be attributed t o any CH bond modes and may indicate the presence of an oxygenated species, possibly carbonyl on the surface. The EELS spectrum obtained for carbon pre-covered surface i s shown i n Fig 5c. The spectrum i s basically similar t o t h a t obtained on the clean Ru surface after reaction in regards t o the vibrational losses due to hydrocarbon species. Losses between 6451250 cm-l are rather d i f f i c u l t to distinguish because of the high background and hence poorly resolved. ?he loss appearing at 1992 cm-l could be attributed t o CO stretch of some adsorbed CO. DISCUSSION AND CONCLUSION
Although the two sets of spectra (SIMS and EELS) for the different i n i t i a l surfaces after the reaction are basically similar, significant differences can be found. The EELS spectra differed i n one important aspect. The spectrum of the carbon pre-covered Ru surface showed no oxygenated species t o be present, whereas the spectrum d t e r hydrogenation of CO over clean Ru surface did, i n 1 the form of a loss at "1830 cmThis strikirg difference is echoed i n the
.
SIMS data.
In the spectnan of the (H2 i.RuC) surface the absence of peaks a t 16 D and 120 D is significant since they appear i n the spectrum of the (COD2 + Ru) surface and are attributed to 0 and &O respectively. The relatively high intensity of the 71, 72, 73 and 74 D peaks i n the (CO/H2 + Ru) spectrum with respect to those i n the (H2 + RuC) spectrum suggest that these are due t o an oxygenated species. I n s p e c t i o n of t h e low resolution spectrum f o r both surfaces (Fig 3) showing Ru+ cluster (96-104 D) and above, indicate the presence of growC H species attached to Wu where x = l 4. This suggests that a chain X Y growth process is occurring on the surface with the basic unit being C or CH2. Finally the following general conclusions can be drawn. 1. The f a c t that the activation energy for methanation reaction over the Ru(OOO1) surface is comparable with that of polycrystalline supported Ru seems t o suggest that the same mechanism i s i n operation over both catalysts. 2. EELS spectra provide direct evidence for the presence of p a r t i a l l y
824
hydrogenated carbon on the Ru(OOO1) surface.
CHx and C2Hx are clearly evident
consistent with the hydrogenation of surface carbon.
It i s more l i k e l y that
these species are reaction intermediates. 3. SSIMS spectra apart from confirming the findings of EELS, seems t o sug@;estthe polymerisation of CH2 species on Ru site.
Hence lending support
for the postulate t h a t during reaction a surface carbide intermediate is involved i n chain growth leading t o higher hydrocarbons thereby supporting the mechanistic scheme involving carbidic intermediates.
P&"CES 1.
2.
3.
4. 5. 6. 7. 8. 9. 10. 11. 12.
13.
P. Biloen and W:M.H. Sachtler, Adv. Catalysis 30 (1981) 165. R.C. Brady and R. Pettit, J. Am. Chem. Soc. 103 (1981) 1287. C.K. Rofer - De Poorter, Chem. Rev. 81 (1981) 447. V. Ponec, Catalysis, Rev.-Sci. n7g. 18 (1978) 151. A.T. Elell, Catal. Rev.-Sci. Eng. 23 (1981) 203. J.A. Rabo, A.P. Risch and M.L. Poutsma, J. Catal. 53 (1978) 295. M.A. Vannice, J. Catal. 37 (1975) 462. J.G. Ekerdt and A.T. Bell, J. Catal. 58 (1979) 170. V. Ponec, in: Catalysis, Vol. 5, Eds. G.C. Bond and G. Webb (Specialist Periodical Reports) (The Chemical Society, London, 1982) p.48. C.K Rofer De Poorter, Chem. Rev. 81, 447 (1981). B.A. Sexton and G.A. Sormorjai, J. Catal., 46 (1977) 167. H.J. Krebs, H.P. Bonze1 and G. Gaf'ner, Surf. Sci., 88 (1979) 269. D.W. Goodman, R.D. Kelley, T.E. Madey and J.T. Yates, Jr., J. Cam. 63
-
(1980) 226. 14. B. Sakakini, N. Dunhill, C. Harendt, B. Steeples and J.C. Vickeman, Surf. Sci. 189/190 (1987) 221. 15. B. Sakakini and J.C. Vickerman, Proc. 6th Intern. Conf. on Secondary IOn Mass Spectroscopy (SIMS V I ) , Paris, 1987, p1025. 16. S.Y. hi and J.C. V i c k e m , J. Catal. 90, 337 (1984). 17. M.P. Kanimsky, N. Winograd and G.L. Geoffroy, J. Am. Chem. Soc. 108, 1315 (1986) * 18. M.M. Hills, J.E. Pamter, C.B. Mullins and W.H. Weimberg, J. Am. Chem. Soc., 108, 3554 (1986). 19. M . A . Barteau, J.Q. Broughton and D. Menzel, Appl. S u r f . S c i , 19, 92 (1984).
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
825
CO DISSOCIATION ON Ni(100) STUDIED BY AUGER ELECTRON SPECTROSCOPY
A. SANTONI1, C. ASTALD&(*), F. DELLA VALLE2v3 and R. ROSE13~4 1ENEA, Dipartimento TIB, Divisione Scienza dei Materiali, C.R.E. Casaccia, 1-00100 Roma A.D. (Italy) 2Laboratorio Tecnologie Avanzate Superfici e Catalisi del Consorzio INFM, Area di Ricerca, Padriciano 99, 1-34012 Trieste (Italy) 3Dipartimento di Fisica, Universith di Trieste, via A. Valerio 2, 1-34127 Trieste (Italy) 4Sincrotrone Trieste, Padriciano 99, 1-34012 Trieste (Italy)
ABSTRACT Auger Electron Spectroscopy (AES) has been used to investigate the kinetics of CO dissociation on a Ni(100) single crystal surface. The coverages of carbidic carbon and oxygen on the surface have been monitored as a function of time for different sample temperatures (453 K I T I 573 K) and CO pressures (3 x 10-7 mbar I P c o < 3 x 10-6 mbar). The experimental data have been compared to the results of rate equations relative to different dissociation models. This procedure led to the identification of the probable reaction mechanism, whose most relevant step in this pressure range resulted to be the rupture of the CO bond. The activation energy of the process has been evaluated. INTRODUCTION Carbon monoxide dissociation has been recognized as the initial step of the methanation reaction on transition metal catalysts. Several of these metals (Ni, Fe, Co and R u ) can adsorb carbon monoxide dissociatively through the overall reaction
to form a surface "carbidic" carbon species which has been demonstrated to be the active intermediate in hydrocarbons formation (refs. 1-4). The problem of carbon monoxide dissociation has also started to receive attention from a theoretical point of view. Several models and calculations have appeared in the literature in the last few years (refs. 5-6). In
826
principle, once the model for the path of dissociation has been established, first principle total energy calculations can be performed, providing a very important insight on the elementary reaction steps by predicting the activation energies. In Fig. 1 we show a hypothetical two-dimensional total energy surface for the interaction between a metal surface and a CO molecule. In this euristic model the energy of the system is a function of two parameters: the distance of the CO molecule from the surface (d) and the tilt (9). In reality the total energy of the system is a function of a much larger parameter set. However, it is obvious that the knowledge of the shape of the total energy hypersurface contains much of the physics of the dissociation process.
Fig. 1. Hypothetical two-dimensional total energy surface for CO adsorption and dissociation on a metal (M) surface. The points A and B are the minima of the potential energy well for adsorption and dissociation of the CO molecule respectively. (For the definiton of the other parameters see text). By using these concepts and by modelling such hypersurfaces Tully and Knowlton (ref. 7) have realized an interesting computer animation which gives precious insight on a number of elementary surface reactions. However, it should be realized that these models represent reality only to
821
the extent that the relevant parameters of the energy hypersurface, like activation barriers for dissociation and ricombination, are modelled correctly. An advance in the understanding of important surface catalytic processes relies therefore on the unequivocal individuation of the dissociation model and on accurate measurements of the activation energy barriers. In the following we describe the results for CO dissociation on a Ni(100) surface in the low CO coverage regime, obtained by Auger electron spectroscopy.
EXPERMENTAL A Ni single crystal sample was cut to expose the (100) surface within l o , polished and mounted in a Leybold UHV system. The sample was cleaned in situ by repeated ion-etching cycles at room temperature and by oxygen exposures at 800 K. LEED observations showed sharp diffraction spots with low background which indicate a low density of steps. After the cleaning procedure, the sample was heated at a fixed temperature in the 4 5 3 K .c 573 K range and carbon monoxide was admitted into the chamber through a leak valve. The operating pressure was in the range 3 x 10-7 mbar 5 Pco I 3 x 10-6 mbar. After an exposure period the leak valve was closed and CO pumped away while the sample temperature was kept constant. The pressure in the chamber dropped rapidly in the 10-10 mbar range while all residual molecular carbon monoxide desorbed from the surface. At this point an Auger spectrum was taken to assess the carbidic carbon and oxygen coverage. By repeating this procedure for new exposure periods at the same CO pressure and sample temperature we have measured the accumulation of carbon and oxygen by Auger spectroscopy. The carbon Auger line always showed the characteristic "carbidic" lineshape (ref. 4). High energy electrons are well known to induce CO dissociation so that particular care was taken to ensure that during the Auger measurements no residual undissociated carbon monoxide was left on the surface (see discussion below).
RESULTS The kinetics of a typical run of CO dissociation on the Ni(100) surface is shown in Fig. 2.
828
.6
I
I
I
I
1
.5 h
.4 h
la
d
0
-s$
Carbon
.3
.2
0
f
o
.1
P,,
.o
0
= 9
2
* lo-' mbar 4
6
Oxygen
T = 453 K 0
10
12
Time (s) ( x ~ O + ~ )
Fig. 2. Carbon and oxygen coverages as a function of CO exposure time. The lines are intended as guide for the eye. Both carbon and oxygen coverages (ec and 0 0 ) show an approximately exponential behaviour and saturate at about 0.5 monolayer which is the saturation coverage of C and 0 on the Ni(100) surface (refs. 8-9). The oxygen saturation level is slightly lower than the carbon one because of the oxygen depletion reaction
The kinetics of CO dissociation do not change much by changing the temperature, as shown in Fig. 3 (only the kinetics of carbon accumulation is shown for clarity). On the other hand, pressure has much more influence, and the rate of CO dissociation increases markedly by increasing the operating pressure. In particular we have found that the initial carbon and oxygen accumulation rates were strictly proportional to CO pressure (ref. 10).
829
.6
.5 -
I
I
P,,
= 3
I
I
* lo-’ mbar
- A
h
-
v)
.4 h d n
T = 453 K
-
A
T = 473 K
-
T = 513 K .1
.o
-
I
Fig. 3. Carbon coverages versus CO exposure time for different temperatures and constant pressure.
DISCUSSION The results we have obtained are amenable to a rather simple interpretation. Both the findings that the initial dissociation rate is linear with pressure (and hence with the CO coverage in the temperature and pressure range of our experiment) and that both carbon and oxygen accumulate on t h e surface strongly suggest that CO dissociation is a unimolecular reaction. We are therefore led to the proposition that individual CO molecules adsorbed on the nickel surface thermally dissociate into their atomic constituents. This process is obviously in competition with thermal desorption of CO in the temperature and pressure range of our experiment. Oxygen depletion, via the reaction (2), has also to be taken into account. The rate equations which describe our observations are therefore:
830
where O V is the "coverage" of vacant sites. The rate constants have the following form: K1 = (2mnkT)-1/2 (assuming a unitary sticking coefficient), K-1 = KO-1 exp(-Edes/RT), K2 = KO2 exp(-Edis/RT), K3 = KO3 exp(-Edcpl/RT). KO-, , KO2 and KO3 are the pre-exponential factors for desorption, dissociation and oxygen depletion respectively, while Edes, Edis and Edcpl are the activation energy for the same processes. (For the definition of the energy zero see below). As far as the oxygen depletion reaction is concerned it should be stressed that our experiment cannot discriminate between the Langumir - Hinshelwood and Eley - Rideal mechanisms (since we have 8 c o P c o ) . The last term in eqn. 5 is therefore only indicative. We have set up a computer program for fitting our experimental data according to eqns. 3-5. As an example the results of the fit for the kinetic run taken at P c o = 3 x 10-6 mbar and T = 493 K are shown in Fig. 4. KO-, and Edes were obtained from the literature (refs. 11-12) so that from our model calculation we were able to obtain values for K2 and K3 which turn out to be the only adjustable parameters. 0:
7 .6
1
I
I
I
I
-
7
I
(al
-
I
I
I
t T=
= 3 * lo-' mbar 493
a0a"2
K
1 -
.o
1
(b)
6 -
1
,P
I
1
I
.
1
,P = 3 * lo4 mbar
.
?=4S3K
1 1
I
1
I
I
I
0-
I
I
I
I
I
I
Fig. 4. Carbon (a) and oxygen (b) coverage versus CO exposure time. The squares are the experimental points. The solid lines are obtained by integrating eqns. 3-5. A total of 24 kinetic runs was performed in the above mentioned temperature and pressure ranges obtaining a consistent fit in each case. Figs. 5 (a) and (b) show a logarithmic plot of K2 and K3 versus 10001T. Both
831
sets of data fit nicely an Arrhenius plot giving KO2 = 5.4 x 109 (monolayer x s)-l, Edis = 23.4 Kcal/mol, KO3 = 105 m x s/kg and Edepl = 12.4 Kcal/mol. The value we obtain for Edis is about the same of a previous similar experiment on Ni(ll0) (ref. 13). and is in excellent agreement with a number of theoretical evaluations (refs. 14-15). It is worth noting also some dissimilarities in the behaviour of the (110) and (100) Ni surfaces during the CO dissociation experiment. The most striking one is that no oxygen was ever detected on the Ni(ll0) surface (ref. 13), while on the Ni(100) surface it accumulates with about the same rate as carbon. It seems that the oxygen depletion reaction (eqn. 2) proceeds much faster on the Ni(ll0) surface in the pressure range of the experiment.
1.7
I
I
I
1
1.8
1.9
2.0
2.1
I
2.2
I
2.3
Fig. 5 . Plot of In K2 and In K3 versus 1000/T. The linear fits of the experimental points are also shown. By looking at eqn. 3 it is possible to infer that, by dosing carbon monoxide into the experimental chamber, CO coverage reaches quickly, in the pressure and temperature range of our experiment, a pseudo-regime value given by Bco -- K1Pco/K.1 (where K l P c o << K-1). confirming the assumption that CO coverage is proportional to pressure in the working conditions of our experiment. Also by putting P c o -- 10-10 mbar we see that at the lowest temperature of our experiment (T = 453 K) Bco drops to negligible values within about 3 s in the worst case (i.e. assuming an initial CO saturation coverage), confirming the assumption that during our Auger measurements no molecular CO is present on the sample surface.
832
Fig. 6 shows a schematic potential energy diagram along a "reaction coordinate" (which can be considered as a cut through the two-dimensional total energy surface of Fig. 1). According to our model CO molecules adsorb first, thermalize on the nickel surface and then move away from the bottom of the potential well (point A in Fig. 6), either to the right (desorption) or to the left (dissociation, point B). For this reason we have chosen the bottom of the well as the zero of the apparent activation energy for dissociation (instead of the zero of energy as it is customary). The same choice was taken implicitly in ref. 10. In this respect we note that our results are rn in contraddiction with those reported by Steinriick et al. (ref. 16). These authors have canied out a molecular beam study of CO on a Ni(100) surface at 500 K, and have found the dissociative sticking coefficient to be 0.02, independent of beam energy from 1.6 to 20 Kcal/mol. From this finding they concluded that the value of the apparent activation energy for the process should be not larger than zero. Indeed this result and the interpretation are not at odds with our findings because, within the usual convention for the zero of energy, the apparent activation energy we have measured turns out to be E'dis = -3 Kcal/mol.
I
B
Fig. 6. Schematic potential energy diagram along a "reaction coorainare
r.
In conclusion, we have monitored the dissociative adsorption kinetics of CO on a Ni(100) surface via Auger Electron Spectroscopy. Our measurements are consistent with a model of unimolecular dissociation of CO which leaves carbidic carbon and oxygen on the surface. The activation energy of this process (as measured from the bottom of the adsorption potential well) is
833
23 Kcal/mol, corresponding to E d j s = - 3 Kcal/mol. The activation energy for the oxygen depletion reaction has also been measured and found to be Edepl = 12.4 Kcal/mol.
A C K " T S The authors would like to thank the Area per la Ricerca Scientifica e Tecnologica of Trieste for the partial support of this work.
REFERENCES (*)
1 2 3 4
5 6 7 8 9 10 11 12 13 14 15 16
To whom all the correspondence should be addressed. D.W. Goodman, R.D. Kelley, T.E. Madey and J.M. White, J. Vac. Sci. Technol., 17 (1980) 143. D.W. Goodman, R.D. Kelley, T.E. Madey and J.M. White, J. Catal., 64 (1980) 479. H.P. Bonze1 and H.J. Krebs, Surf. Sci., 91 (1980) 499. D.W. Goodman, R.D. Kelley, T.E. Madey and J.T. Yates, Jr., J. Catal., 63 (1980) 226. D. Tomanek and K.H. Benneman, Surf. Sci., 127 (1983) L111. W. Andreoni and C.M. Varma, Phys. Rev., B23 (1981) 437. J.C. Tully and K. Knowlton, "Dynamics of a Gas Surface Interaction: a Computer Generated Movie", Bell Laboratories, Murray Hill, NJ, 1978. J.H. Onuferko, D.P. Woodruff and B.W. Holland, Surf. Sci., 87 (1979) 357. F. Labohm, C.W.R. Engelen, O.L.J. Gijzeman, J.W. Geus and G.A. Bootsma, Surf. Sci., 120 (1983) 429. A. Santoni, C. Astaldi, F. Della Valle and R. Rosei, in preparation. J.T. Yates, Jr. and D.W. Goodman, J. Chem. Phys., 73 (1980) 5371. S. Johnson and R.J. Madix, Surf. Sci., 108 (1981) 77. R. Rost;i, F. Ciccacci, R. Memeo, C. Mariani, L.S. Caputi and L. Papagno, J. Catal., 83 (1983) 19. E. Miyazaki, J. Catal., 65 (1980) 84. T.H. Upton, Lectures of the International Summer School on Surface Science and Catalysis of the Methanation Reaction, Sangineto Lido, July 1-10, 1982, unpublished. H.P. Steinruck, M.P. D'Evelyn and R.J. Madix, Surf. Sci., 172 (1986) L561.
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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
835
ON THE NATURE AND THE NUMBER OF REACTION SITES ON Pt CATALYSTS
SARKANY Institute of Isotopes of the Hungarian Academy of Sciences, H-1525 Budapest, P.O.Box 77, Hungary
A.
ABSTRACT Transformation of n-hexane has been investigated over 0.8 wt% Pt/Al203 and 15 wt% Pt/Al2O3 catalysts (metal dispersion is 66 and 6.3%, respectively) at 593 K as a function of the partial pressure of hydrogen. The surface composition of the working surface has been characterized by measuring the fraction of "free" sites, total hydrocarbon coverage, the coverage of the firmly held species. The experimental results confirm that the surface state of the catalysts is affected by the variation of the Hz/n-hexane ratio. Under the experimental conditions investigated 10-20% of the surface platinum atoms might participate in forming the reaction sites responsible for the skeletal reactions. INTRODUCTION During recent years the various forms of Pt based catalysts have been systematically investigated in transformation of alkanes. In these studies considerable attention has been directed to the understanding of the mechanism of individual skeletal reactions (fragmentation, C5-cyclization, aromatization, etc.) and to the nature of the active sites (refs. 1-3). The transformation of alkanes and the selectivity of individual products formed have been confirmed to be sensitive to dispersion and to crystallographic orientation. Thus the labelling studies have proven that the percentage of C5 route of skeletal rearrangements increases with dispersion (ref. 1). The turnover number of fragmentation seems to increase with the number of platinum atoms of low coordination (refs. 4,5). The high rate of fragmentation of butane (ref. 61, n-hexane (refs. 6-8) on high index faces has been correlated with the high concentration of steps and kinks. Surface sensitivity in aromatization of n-hexane has also been reported (ref. 7). The particle or surface sensitivity of reactions might be complicated by secondary effects (ref. 9) like for example the selfpoisoning of the surface sites, In fact the less severe carbon poisoning of low coordination sites seems to contribute to the larger specific activity of the highly dispersed samples (ref. 10).
836
The present paper is aimed at investigating the surface composition of the working surface formed in transformation of n-hexane at 593 K with Pt/A1203 catalysts. Attempts have been made at estimating the number of reaction sites able to participate in transformation of n-hexane. EXPERIMENTAL Catalytic measurements Transformation of n-hexane was investigated in an atmospheric flow system. The partial pressure of n-hexane in all the catalytic measurements was 4.92 kPa. The partial pressure of hydrogen in the nH/H2/He stream was 0.81, 4.67, 17.76, 31.78, 44.98, 76.31 and 93.5 kPa. The premixed H2/He mixture taken from gas cylinders was passed through Pd/A1203 and molecular sieve columns. The impurities from n-hexane (mostly 3-methylpentane and methylcyclopentane) were removed by preparative GLC. After purification no other hydrocarbon than n-hexane could be detected at the highest electrometer sensitivity. The rate and product selectivity data (rate of product formation divided by the rate of n-hexane consumption) were calculated in conformity with earlier papers. Catalysts The catalyst samples used in this study are enumerated in Table 1. As a support material non-microporous y-A1203 (Degussa C) was applied. In order to avoid creation of acidic sites chlorine free Pt complexes were used. The 15 wt% Pt/A1203 was prepared by wetting A1203 repeatedly with Pt(NH3) (NO2)2; 0.8 wt% Pt/A1203 were obtained by treating the support with Pt(NH3)2(N02)2 at pH 9. TABLE 1 Characterization of Pt/A1203 catalysts Catalyst 15 wt% Pt/A1203 0.8 wt% Pt/A1203 OT H2 02
-
x 1 0 - ~from ~ Pt gkat OT H2 02 29.0 27.5 26.6 16.3 22.2 17.1
Dispersion (OT) %
6.3 66
02 titration of PtS-H
hydrogen chemisorption by frontal analysis (1% H2 in Ar) oxygen chemisorption by frontal analysis (0.8% 02 in He)
The catalyst precursors were dried at 393 K and reduced in 1 stream of H2 (heating rate 1 K min , 5 hr at 673 K). The regeneration of the catalysts performed after each n-hexane test reaction
837
consisted of temperature programmed oxidation up to 623 K ( 2 0 K rnin-1 heating rate, 0.65 Kpa 0 2 ) and temperature programmed reduction in H2/He up to 593. The temperature was held there for 30 min. The catalysts were characterized after a few "break in" experiments by O 2 and H2 chemisorption and by O2 titration of chemisorbed hydrogen at room temperature. The number of surface platinum atoms, PtS, used in the calculations was inferred from the average of O 2 titrations assuming one PtS-H consumed 0.75 02. Characterization of working catalysts The surface state of working surface sites was characterized by determining the fraction of surface Pt atoms capable of participating in H2 and O 2 titration at room temperature following n-hexane chemisorption. The value of f , "free site fraction" corresponds to the fraction of sites not covered by hydrocarbons. The hydrocarbon coverage of the catalysts, C/PtS (number of carbon atoms divided by surface Pt atoms) was measured by temperature programmed oxidation following the determination of 2. Details have been presented in former papers (refs. 11,12). The Ctot/Pts ratio represents the carbon coverage measured after 2 rnin He purging of the nH/H2/He reactant from the reactor at 593 K. In repeated expreriments after 5 rnin on nH/H2He stream n-hexane was plugged and the catalyst was hydrogenated with the H2/He mixture for 5 rnin unless otherwise stated. The hydrocarbons remaining on the surface were regarded as strongly bound ones, C fh/PtS. In the present study we paid special attention to the composition of the chemisorption phase that remained after He purging of the reactor. After "freezing in'' the reaction the catalyst was hydrogenated with H2/He mixture used in the kinetic experiment and the desorbed hydrocarbons were analysed by GLC. RESULTS All the rate , product selectivity and surface composition data belong to 5 rnin on n-Hexane/H;/He stream. The rate and surface composition data are presented in Tables 2 - 3 . The selectivity distributions are shown in Figure 1. The conversion was less than 10%. The rate of n-hexane consumption is higher by a factor of 3.6 on 0.8 wt% Pt/A1203, than on 15 wt% Pt/A1203, if one compares the rate data at the maximum of the consumption rate which is at about
838
TABLE 2 Rate of consumption of n-hexane (-R) and the surface composition 15 wt% Pt/A1203 (T = 593 K, 4.9 kPa nH, m = 0.05 g, with F = 35-75 ml min-l) H2 kPa 93.5 76.3 44.8 31.8 17.4 4.7 0.8
-R/s-l Ctot/Pts 0.048 0.051 0.048 0.031 0.015 0.007
0.001
Cfh/PtS
aCfh/Pt" 1OOXftOt lOOxffh
1.05 1.12 1.36 1.83 1.91 2.22 2.78
2.21 2.28 2.23 2.23 2.35 2.45 2.88
0 -45 0.61 0.78 1.01 1.76 2.11 2.73
16.2 14.7 15.5 14.3 13.5 11.5 6.3
72.1 70.2 65.5 60.3 48.5 42.1 23.2
a
After 10 min on H2/He stream, ftot and ffh were measured in presence of tot and cfh, respectively.
TABLE 3 Rate of consumption of n-hexane (-R) and the surface composition 0.8 wt% Pt/A1203 (T = 593 K, 4.9 kPa nH, m = 0.04 g, with F = 35-75 ml min-') H2, kPa
-R/s-l
93.5 76.3 44.8 31.8 17.4 4.7 0.8
0.18 0.185 0.16 0.135 0.11 0.082 0.027
Ctot/pts 2.32 2.31 2.38 2.37 2.34 2.37 2.75
Cfh/Pts 0.93 0.92 1.04 1.22 1.43 1.88 2.36
100xftot 23.3 22.5 21.7 20.4 18.5 15.1 9.3
100xffh 81.5 80.3 76.8 69.3 63.5 50.6 37.5
76 kPa H2 at this temperature and under these experimental conditions. The activity difference between the highly and poorly dispersed sample is somewhat higher if one compares the rate data at low H2/HC ratios. This observation is in accordance with earlier hydrogen sensitivity data which indicated "less deep" volcano shaped curves with the highly dispersed catalysts (ref. 13). fh The g, Ctot/Pts and C / P t S values exhibit only small changes with dispersion. The ftot value belonging to Ctot (2 min He purging at 593 K ) is in the range of 15-21% at 44.5 kPa H2; it decreases with the decrease of the partial pressure of hydrogen. The Ctot/Pts values are less in 0.8 kPa H2 over 0.8 wt% Pt/A1203 than over 15 wt% Pt/A1203. The Ctot/Pts increases with the decrease of the partial pressure of hydrogen only to a small extent, whereas fh the coverage of the firmly hela species, c /Pts, steeply increases.
839
!I $40
aJ
d
aJ
v)
I
0
H2, kPa
0
I
HZ, kPa
1
100
Fig. 1. Product selectivity over 1 5 wt% Pt/A1203 and 0.8 wt% Pt/A1203 at 593 K ( 4 . 9 4 kPa nH).0-Sfr,~-S2Mp+3Mp,~-~McP,~-SB and4
-'alkenes
As indicated in Table 2 , the coverage of the firmly held hydrocarbons is not independent of the time of hydrogenation. For the separation of the "reactive" and "firmly held" hydrocarbons we chose 5 min hydrogenation time with the consideration of mind that the surface species remaining on the surface might not contribute to product formation measured at 5 min on nH/H2/He stream. It is clear however that the coverage of the "reactive species", Ctot /PtS-Cfh/PtS, sharply increases with the partial pressure of hydrogen. The variation of Cfh/Pts is less steep on our 0.8 wt% Pt/A1203 sample than on 15 wt% Pt/A1203. The hydrogenation-desorption results which reveal the surface composition of the reactive hydrocarbons, Ctot/pts-Cfh/pts, are collected in Tables 4 and 5. The product selectivity distributions were invoked to make estimations for the number of reaction centers responsible for the reforming of n-hexane. We performed different types of experiments recognizing the fact that the "freezing in" of the reaction by the removal of the reactant inevitably perturbs the surface composition. In experiments type A after nH/H2/He stream the reactor was purged with He for 0.5 min only (because of the low dead volume of the reactor this time was sufficient to remove the reactants) and the surface was hydrogenated. The rather sharp desorption peak was either directly introduced into GLC or
840
TABLE 4 Product composition measured by "freezing in" the reaction on 15 wt% Pt/A1203 H2, kPa
Exp. nH
Product selectivity,
%
ptsr/gcat
PtSr/PtS
_ _ _ _ _ ~
93.5
HD,Aa 45.2
8.7
93.5 76.3 76.3
HD,A HD,A HD,B HD,Bb HD,B
69.5 61.1 78.3 71.5 78.8
5.2 3.3 4.2 6.3 7.5
76.3 93.5
a2 min in He;
3.7 4.7 6.1 2.3 1.5 4.3
3.3 2.1 1.3 1.7 0.8 1.3
39.1 18.5 28.2 13.5 19.9 8.1
5.54~10 'I8 0.19 0.17 4.94~10l8 6 . 2 6 ~ 1 0 ~ ~0.21 3.49~10 l8 0.12 4.58~10'~ 0.15 3.43~10 l8 0.12
bReactor cooled to 473 K.
TABLE 5 Product composition measured by "freezing in" the reaction on 0.8 wt% Pt/A1203 H2,kPa
Exp. nH
Product selectivity, %
Ptsr/gCat
PtS'/PtS
0.33 0.25 0.23 0.20 0.25 0.19
93.5 93.5 76.3 76.3 93.5
HD,Aa HD,A HD,A HD,B HD,B
41.2 55.8 60.2 66.2 57.3
29.6 18.9 9.9 10.3 10.2
14.7 2.7 3.3 4.7 7.1
4.3 1.9 1.2 2.4 2.2
10.2 20.7 25.4 16.3 23.2
5.48~10 l8 4.19X10 l 8 3.74~10 l8 l8 3.18~10 4.04~10l8
93.5
HD.B b 56.2
12.2
3.1
3.3
15.3
3.22X10 l8
a2 min in He;
bReactor cooled to 473 K.
the gas sample was collected by a syringe. In experiments type B the catalyst in the presence of nH/H2/He was cooled back to about 523 K and the gas phase was removed by He. The desorption of the chemisorbed phase was facilitated by H2 and by the simultaneous increase of the temperature back to 593 K in a few seconds. The experiments (types A and B) do not give identical product distributions. The product selectivity without n-hexane differs also from product distribution measured at nH/H2/He stream. The percentage of n-hexane transformed to fragments is definetely less than that in the kinetic tests. The amount of fragments (
841
DISCUSSION The presented results with a group of Pt/A1203 catalysts provide clear evidence that the surface state of working Pt catalysts is remarkably affected by the partial pressure of hydrogen. The fraction of surface platinum atoms not covered by hydrocarbons, ftot, slightly increases with the increase of the partial pressure of hydrogen. In principle the surface atoms not covered by hydrocarbons are available for hydrogen chemisorption. In the interpretations of alkane transformation it is generally accepted that the reaction requires a certain number of contiguous metal atoms that can be regarded as a reaction site or centre. As a consequence of the competitive adsorption between hydrogen and n-hexane with the decrease of the partial pressure of hydrogen the probability of finding free surface atoms for n-hexane should increase. However, in the absence of sufficient surface hydrogen "deep" dissociation of C-H bonds occurs resulting in a decrease in the number of the reaction centers. This is what we see in Tables 2 and 3, one has to regard the coverage of the firmly held hydrocarbons, the value of C fh/PtS steeply increases with the decrease of partial pressure of hydrogen. The firmly held deposits residing on the surface for a longer time than requiied for a given skeletal reaction (hydrogenolysis, C5 cyclization, etc.) poison the surface Pt atoms. By way of example, at 0.81 kPa H2 the value of Ctot/Pts-Cfh/Pts is only 3.6% of the value of Ceh/Pts on 15% Pt/A1203. It follows from the above considerations and from the inspection of the surface composition data that the number of reaction sites goes through a maximum as a function of the partial pressure of hydrogen. In large excess of hydrogen the hydrogen chemisorbed directly regulates the number of reaction centres. At lower hydrogen pressures the trapped hydrocarbons seem to govern the number of the reaction sites. The maximum in the number of working sites at a certain H2/HC ratio is somewhat in accordance with the change of the n-hexane consumption rate in Tables 2 - 3 . It is not likely that the turnover number on reaction centre basis would be identical for all the skeletal reactions therefore the number of reaction centres might not strictly follow the activity curves. One might safely conclude however that the number of reaction centres is affected by the reactants and by the experimental conditions. In the interpretation of the reaction sites the difficulty arises not only from the variation of the total number of the reac-
842
tion centrums but also from the change of the distribution of the individual reaction sites. As shown in Figure 1 with the decrease of the partial pressure of hydrogen non-destructive hydrocarbon transformations come into prominence (ref. 11). The firmly held hydrocarbons formed at low H2/HC ratios sit on surface platinum atoms in a more or less random distribution inducing geometric/steric and electronic variation of the working sites (ref. 12). The delocalised metal-carbon interaction might become localised and the electronic interaction between the surface and the trapped hydrocarbons should decrease the binding energy for hydrogen and carbon on the remaining sites. Finally we consider the product distributions obtained by "freezing in" the reaction and using hydrogenation desorption. As we already emphasized the experimental procedure disturbs the surface state of the chemisorbed species. Nevertheless it seems certain that a considerable fraction of the desorbed products at 76 and 93 kPa H2 consists of n-hexane indicating that among the surface centres the chemisorption sites dominate. Accepting this fact the following reaction models might be proposed: i) Hydrogen, either by desorption into the gas phase or by surface migration, creates free platinum atoms in the vicinity of a chemisorption centre and thereby the chemisorption site is transformed into a reaction one allowing skeletal transformation of the hydrocarbon species; ii) The chemisorbed n-hexane by surface diffusion reaches a reaction site which is then suitable for a given transformation (refs. 8,141. The occurrence of multiple deuterium exchange might be regarded as evidence for the migration of the chemisorbed species. The product distributions of the hydrogenation-desorption differ from those measured in the nH/H2/He stream. Nevertheless the percentage of fragments with the highly dispersed 0.8 wt% Pt/A1203 is always higher than that with 15 wt% Pt/A1203. The higher selectivity of fragmentation on highly dispersed samples might be the consequence of better utilization of surface hydrogen preventing formation of carbonaceous materials. On the basis of n-hexane desorbed we made attempts at estimating the number of surface platinum atoms, Pt", participating in the formation of the reaction sites. In these calculations we have taken identical ensamble sizes and assumed that ffh-ftot is the surface fraction which is available for reaction and chemisorption. The fraction of platinum atoms, Ptsr/Pts, which might be responsible for the
843
transformation of n-hexane is collected in the last column of Tables 4 and 5. These preliminary data seem to indicate that the fraction of PtS forming reaction centres might be somewhat larger on 0.8 wt% Pt/A1203 than on 15 wt% Pt/A1203. CONCLUSIONS i) The surface state of the working Pt particles is significantly affected by the partial pressure of hydrogen: chemisorbed hydrogen and firmly held hydrocarbons regulate the number and the nature of the working sites. ii) The surface composition data presented seem to indicate that the number of working sites is higher on small particles than on large ones. Carbon effect plays a de inite role in the higher activity of small particles. REFERENCES 1 F.G. Gault, Adv. Catal. 30 (1981 -pp. - 1-95. 2 2. Pa61, Adv. Catal. 29 (1980) pp. 273-334. 3 S.M. Davis and G.A. Somorjai, in D.A. King and D.P. Woodriff (Editors), The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Elsevier, Amsterdam, 1982, pp. 217-364. 4 J.R. Anderson and Y. Shimoyama, in J. Hightower (Editor), Proc. Int. Congr. Catal. 5th, 1973, Vol. 1, pp. 695-704. 5 J.P. Brunelle, A. Sugier and J.F. LePago, J. Catal. 43 (1976) 273-291. 6 S.M. Davis, F. Zaera and G.A. Somorjai, J. Am. Chem. SOC. 104 (1982) 7453-7461. 7 S.M. Davis, F. Zaera and G.A. Somorjai, J. Catal. 85 (1984) 206-223. 8 A. Dauscher, F. Garin and G. Maire, J. Catal. 105 (1987) 233-244. 9 W.H. Manogue and J.R. Katzer, J. Catal. 32 (1974) 166-169. 10 P.P. Lankhorst, H.C. deJongste and V. Ponec, in B. Delmon and G.F. Froment (Editors) Catalyst Deactivation, Elsevier, Amsterdam, 1980, pp. 43-52. 11 A. Sbrkdny, Catal. Today, submitted. 12 A. Sdrkbny, J.C.S. Faraday Trans. I, 84 (1988) 2267-2277. 13 L. Guczi, K. Matusek, A. Sbrkdny and P. TBtBnyi, Bull. SOC. Chim. Belg. 88 (1979) 497-515. 14 L. Guczi, A. Frennet and V. Ponec, Acta. Chim. Acad. Sci. Hung. 112 (1983) 127-151.
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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
INFRAREO S T U O Y OF THE E F F E C T S OF PREAOSORBED ON
845
0-CONTAINING COMPOUNOS ON
CO
Pt/Cab-0-Sil
J . SARKANY a n d M . B A R T d K Department o f Organic Chemistry, Jdzsef A t t i l a U n i v e r s i t y , 06m t 8 r 8 , Szeged, H-6720, H u n g a r y ABSTRACT Transmission i n f r a r e d spectroscopy revealed t h a t t h e K v i b r a t i o n a l f r e q u e n c y o f C O p r e a d s o r b e d i n l i n e a r f o r m a!1298 o n 5 % P t / S i 0 2 [Eco (average)*O.3] d e c r e a s e d b y 39-44 cm on t h e 1 3 3 . 3 P a ) , 3 0 min] coadsorption [298 K , 10 T o r r (1 Torgl= of d i o x a c y c l o a l k a n e s , and o n l y b y 29-32 crn i n t h e case o f o x a c y c l o a l k a n e s . The a b s o r b a n c e , i n t e g r a t e d a b s o r b a n c e a n d h a l f - w i d t h of t h e CO b a n d v a r i e d d u r i n g t h e c o a d s o r p t i o n . The c h a n g e i n b a n d s h a p e was a l s o s t u d i e d o n t h e t h e r m o d e s o r p t i o n o f C O f r e e f r o m any coadsorbed substance. 8 ( l o c a l ) proved t o be o f importance i n t h e above c o a d s o r p t i o n expeggments. INTROOUCTION
The v i b r a t i o n a l f r e q u e n c y
Y(C0)
o f CO l i n e a r l y a d s o r b e d o n
may b e i n f l u e n c e d b y many e f f e c t s ( r e f s 1 - 9 ) .
w i t h t h e i s o t o p i c d i l u t i o n method ( r e f .
shifts
AV(C0)
ounds ( E D C ) ,
The r e s u l t s o b t a i n e d
4 ) i n d i c a t e t h a t t h e Elco-
i s due w h o l l y or m a i n l y t o t h e d i p o l e - d i p o l e
i n d u c e d s h i f t i n Y(C0) i n t e r a c t i o n s ( r e f s 4,
Pt
10-121,
but the explanations o f the r e d
caused by t h e c o a d s o r p t i o n o f e l e c t r o n - d o n o r
comp-
f o r example, m o s t l y d i f f e r ( r e f s 12-22).
I n a g r e e m e n t w i t h o t h e r a u t h o r s , i t was f o u n d e a r l i e r i n t h e case o f 5% P t / S i 0 2 t h a t
V ( C 0 ) depended on
Bc0,
the temperature,
t h e s u r f a c e s t r u c t u r e o f t h e P t c a t a l y s t and t h e presence o f 0 (reaction),
H (weak i n t e r a c t i o n ) ,
s u r f a c e C or NO ( d i s p l a c e m e n t )
( r e f s 23-28). F o l l o w i n g t h e s e b a s i c i n v e s t i g a t i o n s , we h a v e now s t u d i e d t h e effects
o f c o a d s o r p t i o n o f some e l e c t r o n - d o n o r
i n g sp3 0 a t o m s o n t h e
compounds c o n t a i n -
V ( C 0 ) o f p r e a d s o r b e d CO. I n c o n t r a s t w i t h
t h e numerous i n v e s t i g a t i o n s o f
v(CO>,
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
<
oxane oxane
6(1)
32 22 16
r-1-01
<
30
29
19 13
18
17
10
12
7
(cm-')
Q Q d a L J
1,4-dioxane 1,3-dioxepane 1,3-dioxane 1,3-dioxolane 6(1,4) 2 7(1,3) 2 6(1,3) < 5(1,3) 1
I
10 Torr, 30 min, 298 K Evac., 15 min, 298 K Evac., 5 min, 373 K
5(1)
'
diethyl ether
31
-&CCO) Coadsorbed compounds in the sequence of ,,(a)
-
oxolane oxolane
39 32 21
1-2-0 J 42 25 15
40 23 14
44 26
11
involve a Pt. ..O (ether) dative bond. Since the investigated m o l ecules d o not have side-chains, the steric hindrance during the c o adsorption is minimal, the conformation energies are not high, and thus they can form electron-donor bonds to Pt via one (group or/and two (group [-]I 0 atoms. The sequence of AYCCO) values obtained at the ECP, and especially during evacuation between 2 9 8 and 373 K , may be closely related to the electron-donating ability of the coadsorbed molecule, i.e. to the magnitude of the first IP and to the molecular structure. Oespite the IP values for being higher than those for the coadsorption of the former compounds resulted in much larger red shifts in )/(CO> (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
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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.
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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.
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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,
,<-.
.
._ .: . -..
. . . .
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:
.. .. . . ..> .....
.
.
.
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i
:
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.
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b--
' d
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336
342
ev
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.:
,I
. : . . ... .
'-_
j
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a,-'
'-
I
1
834
839
ev
. .-.
,L
\.
\ ,
i
.
:.
.
\-
-\ I
'i.
/,;
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x
-. *-.
I
b c
/-
'
1
:
.
,
---.
'
. ' ,<:
? d 4 . .
, ;i
r.
..
,k.
i
j
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c.::
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,; '=-..& ..
:
!
:.
:
e?x;:
; "..\ ,
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*.* .
.
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; ; + ,
..;-. .. .. ._ .. --:. . L;'
.\\ I
528.5
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533.5
ev
Figure 5 XPS spectra of Pd, La and 0 elements: a) initial sample, b) after 4 hours-H2-3000C,c) after 12 hours-H*-300°C. 1) corrected from charging effects.
869
ufects of heat treatment. Due to the great affinity of La for O2and the easy decomposition of LaPd3 (at least on the surface), 0 2 is present from the start and forms mainly La(OH)3 together with some LapO3. We found that heat treatment (300°C under hydrogen) led the catalyst to decompose progressively with the duration of the treatment. favouring the segregation of La towards the surface to form more La203, while the hydroxide decomposed. The initial surface of LaPd3 catalyst , which was already 5 1 6 times more enriched with lanthanum than expected from the stoichiometry, became more and more enriched with lanthanum. Meanwhile the amount of carbon and oxygen present on the surface did not change, which means that the increase of activity as a function of the time of pretreatment was not due to a decontamination process.
lm
Model of LaPd? Cata We can imagine the surface of initial LaPd3 with islands of LaPd3 and La(OH), as follows:
5-qT;=+y7-pd OH OH OH I
OH OH OH
- ---::+La
0 0 0 . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 .-Lap%
With heat treatment these islands of LaPd3 decompose progressively into Pd metal and lanthanum oxide. So after 1 hour of treatment these islands of LaPd3 are still present, therefore the catalytic activity is very weak and the selectivity is high. After 12 hours of treatment these islands are entirely decomposed and the surface of LaPd, became like palladium particles dispersed on lanthanum oxide. The presence of Pd particles exhibits the augmentation of activity and approach the same selectivity as palladium metal.
CONCLUSION The catalytic results obtained for LaPd3 catalyst showed that the activity inscreased and the selectivity diminished linearly with the number of runs, each run consisting of a pretreatment (H2, 1h, 300°C) followed by catalytic hydrogenation reaction. After ca. 10515 runs or 15 hours of H2.3000C pretreatment the catalyst
became stable. The selectivity in isomerization for the stabilized LaPd3 was characteristic of palladium metal. This so-treated LaPd3 catalyst has the surface like palladium
870
particles dispersed on lanthanum oxide, and the catalytic behaviour is as the classical palladium catalyst deposited on inert support. In the initial state, prior to the extensive decomposition of the intermetallic compound, an electron transfer from the rare earth to palladium may be responsible for the lower activity and higher selectivity as compared to palladium particles deposited on oxide. Such a conclusion is supported by the catalytic behaviour of other palladium-rare earths intermetallic compounds [131. REFERENCES
F. Le Normand, P. Girard, L. Hilaire, M.F. Ravet, G. Krill and G. Maire J. Catal. 89, 1 (1984) V.T. Coon. T. Takeshita. W.E. Wallace and R.S. Craia, -. J. Phvs. Chem.. 8p,17 (1976) C.A. Luengo, A.L. Cabrera, H.B. Mckay and M.B. Maple, J. Catal. 47,1 (1 977). &-l.diaue. A. Percheron-GuBaan. J.C. Achard and F. Tasset. J. Less. CommGn Metals 24, 1 (1980). H.C. Siegmann, L. Schlapbach and C.R. Brundle, Phys. Rev. Lett. 972 (1978.) L. Sc'hlapbach, A. Seiler, H.C. Siegmann, T.V. Waldkirch, P. Zucker and C.R. Brundle, Int. J. Hydrogen Energ 5, 21 (1979). 3550 (1982). 10 F.U. Hillebrecht and J.C. Fuggle, Phys. ev. B 2179 (1983). 11 F.U. Hillebrecht and J.C. Fuggle, Phys. Rev. B 12 T.H. Fleisch, R.F. Hicks and A.T. Bell, J. Catal. fl,398 (1984). 13 K.S.Sim, PhD thesis, Strasbourg (1988) .
M
.
a,
A a, a,
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
871
PHASE-TRANSITION INDUCED CHANGE OF ELECTRON-SURFACE SPIN-FLIP SCATTERING REVEALED FROM CESR ON METALLIC L i GLOBULES EMBEDDED I N n-IRRADIATED L i F
A. STESMANS Departement Natuurkunde, Katholieke U n i v e r s i t e i t Leuven, 3030 Leuven, Belgium
ABSTRACT Conduction e l e c t r o n s p i n resonance (CESR) experiments on small L i o a r t i c l e s embedded i n L i F reveal the CESR l i n e shape f a c t o r K t o change s i g n i f i c a n t l y upon c o o l i n g from 300 K t o 4 K. A t 300 K, the l i n e i s Lorentzian ( K = 3.628) w i t h a peak-to-peak l i n e w i d t h ABpp = 0.065 mT and g = 2.00229, Upon c o o l i n g a second 0.218 mT, i s admixed; t h e t o t a l Lorentzian signal o f equal g, b u t w i t h AB signal i n t e n s i t y , however, remains constan!! -The CESR width being dominated by electron-surface scattering, t h i s i s ascribed t o a change i n surface s p i n - f l i p p r o b a b i l i t y E as a r e s u l t 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 (from bcc t o hcp) occurring i n bulk L i a t 70-80 K.
-
INTRODUCTION When a conduction e l e c t r o n i n a metal s t r i k e s the metal-ambient surface, along w i t h the momentum change there i s a c e r t a i n p r o b a b i l i t y
E
f o r a spin f l i p
t o occur. This spin-surface r e l a x a t i o n has been w e l l studied by conduction elect r o n s p i n resonance (CESR) [see, e.g., r e f . 1,21 and i s now accepted a l s o t o a r i s e v i a the s p i n - o r b i t (SO) coupling [ 3 ] as i s the case f o r s c a t t e r i n g a t i m p u r i t i e s , defects and phonons-the
metal-ambient d i s c o n t i n u i t y now representing
the l a t t i c e defect. For non-magnetic metal surfaces, i.e.
, no
magnetic surface i m p u r i t i e s (second
s p i n system) present, so f a r E i s found t o be temperature (T) independent [ 11. Generally, the experimental
E
values r e f e r t o the metal-ambient (mostly a i r )
i n t e r f a c e (surface) which may d i f f e r from the i d e a l metal-vacuum i n t e r f a c e , as For L i , because o f i t s high r e a c t i v i t y has r e c e n t l y been demonstrated [ 4 ]
.
w i t h a i r a t room temperature (RT), samples p r a c t i c a l l y a r e always protected by some k i n d o f i n s u l a t i n g f i l m . Thus, L i metal from various preparation and p u r i f i c a t i o n methods has been analysed, e.g.,
evaporated t h i n f i l m s [ 51
, dispersed
p a r t i c l e s embedded i n p a r a f f i n wax prepared by the aerosol method [ 61 , L i part i c l e s formed i n L i F by neutron i r r a d i a t i o n [ 7-91 ( l a t e r r e f e r r e d t o as LiF:Li material),etc.
So far, the smallest CESR l i n e w i d t h s were reported [ l o ] f o r
f i l m s ( o f thickness d
2
0.1 nun) p r e c i p i t a t e d o u t o f liquid-ammonia s o l u t i o n s and
812
p a r t i c l e s (diameterx D = 12
2 pm) prepared by a high-T e l e c t r o l y s i s technique
As measured by X-band r e f l e c t i o n CESR, t h e former study r e p o r t s aBpp = 2 . 1 2 0.2 PT, almost T-independent i n t h e range 4.2-300 K , where aspp : 2/&T2 represents t h e peak-to-peak w i d t h o f t h e a b s o r p t i o n - d e r i v a t i v e dPpa/dB; y i s t h e g x 8.7941 x lo7 mT-ls-l where g i s t h e spectrose l e c t r o n gyromagnetic r a t i o copic s p l i t t i n g f a c t o r , T2 i s t h e transverse s p i n r e l a x a t i o n time, Ppa i s t h e absorbed microwave power and B represents t h e e x t e r n a l l y a p p l i e d magnetic i n d u c t i o n . However, t h e l a t t e r i n v e s t i g a t i o n f i n d s a T-dependent aBPp, r e p o r t i n g AsPP = 1.4 0 . 3 and 2.5 0 . 2 uT a t 77 and 300 K, r e s p e c t i v e l y . To t h i s , t h e r e l a x a t i o n TI = T2 (T1 being t h e s p i n - l a t t i c e r e l a x a t i o n t i m e ) was accepted t o h o l d f o r L i , as w e l l shown by experiments [ 7 , 9 ] . Both works conclude t h a t t h e s p i n - c u r r e n t mechanism [12] i n s t e a d o f t h e SO i n t e r a c t i o n i s t h e dominant i n t r i n s i c s p i n - l a t t i c e r e l a x a t i o n mechanism,Li so f a r being t h e o n l y metal i n which t h e SO-coupling i s small enough t o a l l o w t h e SC mechanism t o be observed. The above mentioned CESR works study r e l a t i v e l y l a r g e L i samples, i . e . , small e s t dimension t y p i c a l l y > 10 nm, o f which t h e s i a n a l i n t e n s i t y I (area under [ 111.
P
a b s o r p t i o n curve) r e f l e c t s t h e Pauli-paramagnetic s u s c e p t i b i l i t y ( X L i ) .
A
second c l a s s o f experiments, however, s t u d i e s f i n e l y dispersed [ 8,131 L i c l u s t e r s (D 6 50 mm) i n which quantum s i z e e f f e c t s (QSE) [ 141 become v i s i b l e , leading, P xLi, g s h i f t s , s t r o n g l i n e shape m o d i f i c a t i o n s
among others,to a d e v i a t i o n from
and b l o c k i n g o f t h e s p i n - r e l a x a t i o n process. The intense, narrow and s t a b l e CESR o f L i F : L i has been g e n e r a l l y used f o r spectrum c a l i b r a t i o n .
The L i p a r t i c l e s , t y p i c a l l y o f D g 0 . 2 pm, formed i n t h i s
c l e v e r way a r e w e l l sealed o f f from a i r and probably, a r e among t h e p u r e s t L i so f a r produced.
I n t h e present work i t has been observed t h a t upon c o o l i n g from
RT t o 4 . 2 K, t h e L i F : L i CESR l i n e shape
s t r o n g l y v a r i e s w h i l e , apparently,
I t w i l l be o u t l i n e d t h a t t h i s may be t r a c e d back t o a
aBpp stays unaltered. change i n E, o r i g i n a t e d by a L i M a r t e n s i t i c phase t r a n s i t i o n .
EXPERIMENTAL PROCEDURES CESR experiments were c a r r i e d o u t a t X (9.0 GHz) and K (20.9 GHz) band i n t h e range 4.2 d T d 300 K u s i n g homodyne r e f l e c t i o n spectrometers d r i v e n i n t h e a b s o r p t i o n mode.
Modulation o f
B
a t 100-150 kHz r e s u l t e d i n t h e d e t e c t i o n o f
f i r s t - d e r i v a t i v e dP /dB spectra. An i n c i d e n t microwave power P d 0.11 and 0.16 Pa P mW - corresponding t o B1 < 0,0033 and 0.0074 mT a t t h e sample s i t e o f t h e X- and K-band TEO ll
cavities, respectively
- was
a p p l i e d ; up t o these power l e v e l s no
s a t u r a t i o n e f f e c t s a r e observed (B1 i s t h e in-phase rf amplitude). High p u r i t y L i F power was i r r a d i a t e d w i t h a dose o f 5 X lo1' thermal neutrons/ 2 cm w i t h o u t c o o l i n g p r o v i s i o n s during i r r a d i a t i o n . Typical CESR L i F : L i samples
'D may b o t h r e f e r t o a sphere diameter and a mean g l o b u l e s i z e .
873
TABLE 1.
Sample
L l SGc
K-band (20.9 GHz) CESR data of LiF:Li samples. Dimensiona (m3) 118x118~56
PC
L2 SG P L3 SG L4 P
227x105~22 226x151~64
b
(%12)
N b (11119~cm-3)
10.5 7.33 81.3 65.3 12.2 3200
1.35
AB
k0.03 (!!-4T) 4.2 K 295 K 0.47 0.54
15.6 0.48
0.56 0.62 0.63 0.66 0.55 0.65+0.06
aExtremal dimensions o f t h e LiF:Li grain observed by an optical microscope bEstimated r e l a t i v e e r r o r f o r data obtained on one sample = 4 %; absolute e r r o r
=
CSG
20 %
and P r e f e r t o single-grain and pulverized, respectively
had a volume < 2 . 5 ~ 1 0 -mm ~ 3 and comprised e i t h e r one s i n g l e LiF:Li grain o r a powder of many smaller grains. Spin d e n s i t i e s NV were determined r e l a t i v e t o a ruby (Al2O3:Cr3') spin standard of i n t e n s i t y ICr which exhibits a Curie-type -1 s u s c e p t i b i l i t y xCr a T . EXPERIMENTAL RESULTS Apart from L4, the LiF:Li samples studied each consisted i n i t i a l l y of one LiF:Li grain a s checked under the microscope; i n a second experiment these were crushed t o a smaller-grained powder. Some geometrical properties together w i t h K-band CESR r e s u l t s a r e surveyed in Table 1. Over t h e whole T range covered only one, always highly symmetric l i n e was observed; no signal s p l i t t i n g was ever observed. A t RT, the l i n e shape was closely Lorentzian f o r a l l samples i n agreement with previous reports [ 7,9] ; a typical K-band signal observed a t 288 K on sample L4 ( c f . Table 1) i s shown i n Fig. l ( a ) by t h e s o l i d l i n e . The dotted l i n e in t h i s f i g u r e , which f i t s the experimental curve very well, i s a computer-calculated Lorentzian f i t t e d t o t h e experimental curve a t the peak pos i t i o n s . The linewidths of a l l samples were comparable, lying i n the range 0.056 2 0.003 t o 0.065 + 0.006 mT a t RT and almost T-independent ( c f . Table 1) while NV (300 K ) varies from 0.48 t o 15.6 x lo1' cm-3. A t various T ' s , ILi was determined r e l a t i v e t o ICr by double numerical i n t e g r a t i o n of the dP /dB t r a c e s , ua the r e s u l t s o f which are presented i n Fig. 2 as the scaled r e l a t i v e i n t e n s i t y (circles) IR = ILi/Icr x T = C ILi = x L ~ , (1) shows t h a t ILi i s T-independent a s expected for a This where C i s a constant. tJ degenerate e l e c t r o n gas, i . e . , ILi a x L i . Having a v a i l a b l e the ILi data obtained by double i n t e g r a t i o n , l i n e shape f a c t o r s K - represented i n Fig. 2 by the t r i a n g l e s - could e a s i l y be extracted using the d e f i n i t i o n 2 ILi IKY'$B PP
-
874
I
fg=2.00229
Fig. 1. Typical K-band CESR spectra ( f u l l curves) o f LiF:Li sample L4 ( c f . Table 1). The dotted curve i n (a) and the broken curve i n ( b ) are t h e o r e t i c a l Lorentzian shapes. The dotted curve i n (b) represents the sum o f two Lorentz curves o f width ABpp = 0.065 and 0.218 mT, respectively,with r e l a t i v e i n t e n s i t y r a t i o o f the broad t o the narrow l i n e R = 2.22. where 2 Y ' m represents the peak-to-peak h e i g h t o f the dP /dB spectrum. Unexpecpa t e d l y , K increases from 3.8 2 0.4 ( c f . K = 3.628 f o r a Lorentzian l i n e ) a t RT t o values as high as 8.5
0.4 f o r T
.r< 25 K.
The attendant d r a s t i c change i n
l i n e shape i s i l l u s t r a t e d i n Fig. l ( b ) showing a K-band signal observed a t 14.6 K ( s o l i d l i n e ) ; the f i t t e d Lorentzian (dashed) curve c l e a r l y accentuates the
p a r t i c u l a r change i n l i n e shape t h a t has occurred.
The low-T signal develops
much broader wings then a Lorentzian l i n e which, because o f the constant AB PP' t r a n s l a t e s i n t o a s t r o n g l y increasing K w i t h decreasing T. The L i F : L i signal has a l s o been studied a t 9.0 GHz and the spectra have been compared e x p l i c i t l y w i t h the K-band ones a t 4.22 K. I t i s found t h a t the spectra are i d e n t i c a l both as regards the overall line shapeandwidth thus demonstrating the absence o f any s i g n i f i c a n t inhomogeneous ( a - d i s t r i b u t i o n ) l i n e broadening. I n l o o k i n g f o r the explanation, many mechanisms and a r t i f a c t s p o s s i b l y o r i g i n a t i n g t h e observed CESR behaviour could be considered; however, many o f them are excluded j u s t by s t r a i g h t experimental facts. (a) The signal n o t e x h i b i t i n g any inhomogeneous l i n e broadening r u l e s o u t a t once g - d i s t r i b u t i o n influences from QSE or surface e f f e c t s [15,16].
Also, i n case o f QSE, a xLi 0: T - l beha(b) Possibly, there
v i o u r would r e s u l t [ 141 a t variance w i t h observations.
could be an i n f l u e n c e o f the electromagnetic s k i n e f f e c t [17,181,
either i n a
s i n g l e L i globule o r - i n a broader sense - screening o f the i n n e r L i p a r t i c l e s by the outer ones, a l l p a r t i c l e s being w e l l i n s u l a t e d from each other by the
L i F matrix.
Indeed, the T dependence o f the s k i n depth E = ( 2 / ~ w ) ~v i /a ~the
875
Fig. 2. T-dependence o f the scaled r e l a t i v e i n t e n s i t y ( c i r c l e s ) IR :ILi/(ICrT) and lineshape f a c t o r K as derived from K-band CESR data o f L i F : L i i s obtained by d y b l e numerical i n t e g r a t i o n o f sample L4 ( c f . Table 1). ILi dPua/dB; K data are derived from K = ILi/(AB ppY'm). c o n d u c t i v i t y u ( u = permeability; w / h = rf frequency) could a l t e r the l i n e shape. e.g.,
This , however, has been excluded by measurements on pulverized samples , the s i n g l e - g r a i n sample L1 o f mean dimension
100 pm was crushed i n t o
This has no i n f l u e n c e on the lineshape; o n l y a s l i g h t
> 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)
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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
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
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.
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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
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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 (<20eV) needed to filter and guide ions. This feature allows ion-molecule reactions at low energy, and the deposition of filtered particles in the "soft-landing mode" without the necessity of slowing down the ion beam, as will be discussed in the following chapters.
Fig. 2 Mass spectrum of negatively charged sputtered silver clusters obtained by bombarding a 15 keV, 2mA Ar+-beam on a silver target and coexpanding a Cs-beam on the sputter surface in order to enhance the formation of negative ions [81. 2. Experimental setup
The experimental scheme for performing cluster-molecule reactions is shown in figure 3. The cluster ions are produced, as explained in the previous chapter, by means of sputtering. The primary ion beam can, whenever nonconducting sputter targets are used, be re-neutralized in a resonant chargeexchange cell, in order to prevent the target to be charged up. The kinetic energy distribution of sputtered particles is typically between 0 and 10 eV. A cylindrical retarding field energy analyzer allows the selection of an increment of 2 eV. In addition, it blocks all fast primary ions and sputtered neutrals. The energy filter is directly mounted at the entrance of the quadrupole mass filter at a distance of about 4 cm from the target. We used a commercial quadrupole rod system of 9.5 mm pole diameter and a length of 20 cm (Extranuclear Model 4-162-8),which reaches under normal operating conditons a maximum mass value of 1700.
928
rror A
\Analyzer
Ion Deflector ven
Fig. 3 Triple-quadrupole arrangement for performing experiments on sizeselected metal clusters: The particles are produced by sputtering, energy-analyzed, mass-filtered and introduced into the ion drift tube. All inelastic scattering events are recorded with a subsequent quadrupole mass analyzer. The monodispersed cluster beam is directly guided from the quadrupole mass filter into a RF ion drift tube, which is 60 cm long and has an identical sectional view to the mass filter. The drift tube is operated with the same RF frequency as the mass filter at a controlled phase shift. Since no DC voltage is applied, there is no mass discrimination, and all ions, reaction products or photofragments, are kept in confinement. The bias potential of the rod system allows the confined ions to be slowed down to almost thermal velocities, which correponds to residence times of up to -10 ms. The drift tube is surrounded by a scatter chamber of 10 cm length, where reactant gas can be introduced. The complete drift tube assembly is placed inside another differentially pumped ultrahigh vacuum chamber, pumped by a 500 l/sec turbomolecular pump. The background pressure under normal operation conditions is 10-8 mbar, which is sufficient to prevent undesired background collisions. It can be operated, however, at a pressure up to 10-2 mbar, when reactant gas is introduced. At the exit of the drift tube the confined ion beam enters another quadrupole mass filter (Extranuclear Model 4-162-8), which is operated at the same frequency as the first mass filter at a controlled phase shift. It is placed inside a differentially pumped ultra-high vacuum chamber pumped by a 500 l/s turbomolecular pump. The same chamber also contains a 21-stage Cu-Be secondary electron multiplier for detecting the ions. It is mounted on an electrostatic ion deflector, allowing a laser beam to enter the triple-quadrupole system in order to interact with the confined particles.
929
Ni4 after mass selection
______
Fig. 4 (a) Mass spectrum of sputtered nickel aggregates obtained by setting the first QMS and drift tube on RF only, while the last QMS was scanned. (b) Mass spectrum of a monodispersed cluster beam obtained by setting the first QMS on the mass of Ni4+ while the last QMS was scanned 1191. Figure 4a shows the mass spectrum of sputtered Nin+ - clusters that was obtained, when the final quadrupole mass filter was scanned, while the first quadrupole mass filter was kept - like the drift tube - in an ion-confinement mode at RF only. When the first quadrupole mass filter is set - by adding a DC voltage - into a discriminative mode, only clusters of a well-defined size can enter the drift tube. So the final quadrupole mass filter detects the signal of a monodispersed cluster beam (see fig. 4b, where the first quadrupole was set to the mass of Ni4+).The introduction of a reactant gas into the ion drift tube allows one to perform and trace ion-molecule reactions with cluster ions of any desired size and energy. The geometry of the whole arrangement was layed out such that a laser beam could pass accross the tripel-quadruple arrangement, in order to trace photodynamic reactions, as will be reported in chapter HI. 3. Cluster-molecule reactions If one simultaneously introduces size-selected Nin+ - clusters and carbon monoxide into the drift tube, nickel carbonyl ions (Nin(CO)k+) are revealed by scanning the last quadrupole. An example is shown in figure 5 for a beam of Ni+4 with a kinetic energy of 2 eV and a CO pressure in the ion drift tube of 3 x 10-4 mbar. Ni4+ mother ions react with one or several C O s before leaving the interaction zone. Plotting the concentration of carbon-monoxide introduced into the reaction cell versus the product intensity of Ni4CO+, a strictly quadratic behaviour is observed, as can be seen in the log-log plot of figure 6 . This emphasizes a three-body collision process for the adhesion of one CO-ligand. Since however at such low pressures the effective cross section for three-body collisions is very small, the growth of the carbonyl complexes must proceed by sequential collisions, in which the excess of energy deposited in a sticking collision Nik+(co)n + co + Nik+(CO)n+l* (1) is released through stabilizing unreactive encounters Nik+(CO)n+l*+ co + Nik+(CO)n+l+co (2) This implies that the lifetime z of the "collision complex" is long enough to allow for the stabilization process to occur, i.e. longer than the mean time between collisions.
930
J
Fig. 5 Reactive clustering of CO-ligands to a N k + cluster by introducing carbon mbar [191. monoxide into the drift tube at a CO-presssure of 3 x
Concentration of
co
Fig. 6 Log-log plot of the Ni&O+ - intensity vs. CO-concentration exhibiting a quadratic dependence 1191.
931
The ligand distribution, shown in figure 5, exhibits an exponentially decreasing pattern. This indicates that the reaction is not yet completed. More COmolecules can be added to the metal core. The CO pressure is then gradually increased to the point at which the mass spectrum no longer changes; i.e., when saturation of clusters by carbon monoxide ligands occurs (CO-pressure: 2.4 x 10-3 mbar). This situation is depicted in figure 7.
9-
k
L
10
1
3
4
5
I
7
m
Fig. 7 Products of the reaction of carbon monoxide with Ni4+ at a CO-pressure of approximately 2.4 x 10-3 mbar. The system Nia(CO)io+ appears clearly as the saturated compound D91. The mass spectrum clearly shows that no further carbonyls are observed after Ni,&O)10+. This indicates that Nk+ compounds are saturated with 10 COligands. The strong resonance on the mass of Ni4(C0)9+ is due to the charge of the observed particles, since the missing electron diminishes the cluster capability to become saturated. Applying the simple 18-electrons rule to this situation, the fully occupied s-, p- and d-orbitals accommodate 4 x 18 = 72 electrons. The four nickel atoms provide 40 electrons, the 10 CO-ligands contribute another 20 electrons. This leaves 12 electrons to establish the metal skeleton. This represents six metal-metal bonds in the most compact structure of a tetrahedron, having four terminal and six bridging CO- ligands as shown in figure 8.
-
932
Fig. 8 Structure of the tetraedal complex Ni4(CO)io having four terminal and six bridging ligands I191. The mass spectrum in figure 9 shows a result, which was obtained, when Ni6+ was treated with carbon monoxide. One observes that up to 13 CO-ligands were attached to the unfragmentized metallic skelleton. Besides that a series of carbide fragments reaching up to Ni6C(C0)11+ and fragments of the metallic skelleton reaching up to Nig (CO)ii+ can be seen. The stoichiometry of these larger aggregates shows fairly large deviations from the 18-electrons rule. This is due to a large promotion energy between the s and p orbitals found for the free atom [lo]. Thus, the bonding capacity of a given metal will depend upon the relative energy of the p orbitals. Simple electron-counting rules exist to correlate molecular geometries of metal clusters with the total number of valence electrons. Since individual CO-ligands rapidly exchange positions over the metal core, there is no one arrangement that is energetically preferred over all others [ll, 121. Indeed, it is apparent that individual ligands interact with the cluster as a whole and not just with individual atoms. The close analogies between this situation and the bonding of the so-called electron deficient boranes, BnHm, have been recognized by Wade and Mingos 113,141. They established empirical rules [15,16] based on the same delocalized molecular orbitals. These ideas have also been extended to account for some aspects of the reactivity of organometallic clusters [17].Of particular relevance to our results is the work of Lauher [lo] who not only calculated the most favourable molecular geometry for any given transition-metal cluster but also predicted the bonding capabilities of such clusters. The original calculations were based on clusters of rhodium atoms, but the results are readily transferable to most other transition metals. Since that time, many of Lauher's predictions have been experimentally verified 1181.
933
I
I
k I
m
11
7
Fig. 9 Products of the reaction of carbon monoxide with Ni6+ at a CO pressure of 2.4 x 10 -3 mbar 1191.
n=8
n=9
n=10
n=ll
n=12
n=13
Fig. 10 Shapes of the metallic skeleton of the saturated cluster compounds Ni,(CO)m as predicted and measured according to Table 1 [191.
934
Choosing the most compact (closo) configuration the application of this rule to nickel carbonyls leads to the geometries shown in figure 10 1191. The corresponding number of ligands is given in table 1 (top). The predicted values are surprisingly close to the results, which we have observed experimentally for the saturated compounds of unfragmentized reaction products (see table 1 bottom). The observation verifies the validity of Lauher's predictions. The maximum quantities of ligands attached to fragmentized reaction products like Nin-I(CO)m+or NinC(CO)m+,on the other hand, exceed those which are possible for the closo structures. We attribute more open nido or aracno geometries. The occurance can be explained by the fact that the amount of collisions still occuring after the fragmentation event has become insuffiaent to stabilize the compound to its closest structure. The results prove that stoichiometrically reasonable informations can be obtained with size-selected metal clusters in the proposed triple-quadrupole arrangement. Cluster maximum quantity of predicted CO ltpandr maximum quantity of observed CO iipandr
Table 1
+
+
+
+
+
+
+
Ni2 Ni, N$ Ni, Ni6 Ni, Nig Ni,
7 9
9 8
lo
13 15 10 12 13 15 11
17 16
19 17
+
+
+
+
+
Ni,o Nil, Ni12 Nil,
21 18
23 19
25 20
20 20*
Predicted and observed CO bonding capabilities of Ni,+ clusters 1191.
111. PHOTODYNAMICS OF METAL CLUSTERS
The attachment of CO-ligands to a metallic core rose the question, if by irradiating the CO-streching frequency, the attached ligands would get desorbed again. This question was in particular interesting, if the stretching frequency could be monitored as a function of cluster size. For this reason we irradiated Nim(CO)n+ compounds with the light of a frequency-doubled tunable COz-laser, operating in the 5,3 micron wavelength range at an output power of 100 mWatt. No desorption process was observed. Conclusive results cannot be drawn out of this negative result, because we might have been simply off resonance, or the cross section for exciting the CO-stretching vibration is too small for providing a significant effect at the power level we operated. Rather frustration than intuition provoked us then to try the Argon ion laser lines as irradiation wavelength; and we obtained very surprising results [20]. An example for the ligand system of Nilo(CO),+ is shown in figure 11. With no laser irradiation the system accumulates CO-ligands until Nilo(CO)18+is formed. The strongest peak occurs at Nilo(C0)17+,which is due to the missing electron. When 10 mW laser power of the 488 nm-line was applied, the mass peaks of the saturated complexes began to loose intensity. The phenomenon gradually increased at 30 mW, and at 100 mW no signal appeared for Nilo(C0)17+ and Nilo(CO)18+.Further ligands were stripped at 300 mW, and at a laser power of 900 m W no more ligands were seen and even the mother-ion Nile+ exhibited an increased fragmentation tendency towards Nig+, Ni8+ and Ni7+. The result shows that very efficient photochemical reaction channels are opened. They are
-
935
activated by an electronic excitation of the metallic core since the CO-molecule is transparent in this wavelength range. This process is followed by a very rapid internal energy conversion into ligand vibrations, which successively shakes them off. The frequency behaviour of the phenomenon, when tuning across different Ar + laser lines, is shown in figure 12 : The effect increases significantly, when going to shorter wavelengths. This might indicate the onset of an absorption band, which, with the laser wavelengths available, could not be crossed. The very large lineshape, on the other hand, might indicate the time t of the internal energy conversion process to be t << 10-12 sec.
-
i
$0
I
no laser
10 rnw
I
30 mW
I
A
Fig. 11 Progressive stripping of CO-ligands from Ni4+(CO), by means of photoexatation of the metallic core (h = 488nm) [201.
L
0
//
2.4
2.6 2.8 PHOTON ENERGY (aV) (LASER POWER:30rnW)
Fig. 12 Wavelength dependence of the photodesorption efficiency : the depletion of Nilo+(C0)17is plotted vs. photon energy [20l.
936
I
I
i
I
r
I
I
L
I
0.5
I
I
1
I
I
I
I
1.5
2
w
L a s e r power
Fig. 13 Plot of the laser-induced change in the steady-state Ni4+(CO)n-~ignalas a function of laser power for various values of n 1201. In figure 13 we show the results of detailed measurements of the depletion of the Niq+(CO)n - carbonyls, obtained from the difference between two mass spectra taken with the laser ON and OFF respectively, as a function of laser power (P = 5,l x 10-4 mbar and h = 488 nm). At small power levels one observes a fast depletion of the n = 9 carbonyl and simultaneously an increase of the n = 8 and n = 7 signals of the same magnitude. This shows that the photodesorption process can be described by
The measurements indicate that the branching ratios for the reactions are about 50% for desorption of one ligand, 50% for two ligands, and insignificant for more. As the laser power is increased, the photoproducts Ni4+(C0)8 and N4+(CO)7 will absorb photons as well, and in turn, are also depleted. The sequence goes on for smaller carbonyls, which exhibit similar but smoothend behaviour. The observation of the branching ratios made above allows us to set upper and lower limits to the average binding energy En of a CO-ligand to the nickel cluster ion. Indeed the photon energy absorbed by the cluster must be larger than 2 En and lower than 3 En, thus implying 0.85 < En < 1.26 eV, which is consistent with the known values for binding a CO-molecule to a single nickel atom, 1.1 eV, or to a nickel bulk surface, 1.17 eV [21] or to a small nickel cluster 1221.
937
I
I
I
15
H
Fig. 14 Oscillograph recordings showing the transients in the Niio+(CO)io-signal at large and reduced photodesorption laser power. The CO-pressure was 9 x 10-4 mbar, and the laser wavelength h = 476nm 1201. An insight into the mechanism of the competition between collisioninduced adsorption and photodesorption can be drawn from observations of changes of intermediate size carbonyls, e.g., with the number of CO-ligands being about half the maximum value. Figure 14 shows the intensity changes of Nilo+(CO)ro formed from mass-selected Niio+ travelling through CO at 9 x 10-4 mbar, when the laser beam (h = 476 mm) is switched ON and OFF. The top oscillogram, corresponding to high laser power (PL= 3 W), shows a narrow spike at the turning ON of the laser followed by a decrease of the signal down to zero. The occurence of this spike is an additional evidence for the sequential character of the photodesorption. Fast removal of one or two CO-molecules from large carbonyls produces a sudden rise of intermediate size carbonyls, immediately followed by a quick fall-off due to the removal of CO from the just formed intermediate size carbonyls. Indeed figure 11 shows that under these conditions the carbonyls have been "cleaned" off their ligands leaving only the bare metal cluster. Accordingly, when the laser is turned OFF it takes a rather long time (- Is) to recover the coverage. This is because ten successive events are needed to form one Nilo+(CO)io. The second oscillogram (fig 14 bottom) shows the same experiment under reduced laser power (200 mW). The spike at turning ON is now followed by a tail reaching a steady-state intensity which is larger than in the absence of the laser beam. Indeed the Nilo+(CO)lo - flux is "fed" by the photodetachment of ligands from larger carbonyls and simultaneously the attachment of ligands to smaller ones. The moderate photodesorption rate allows only partial "cleaning" of the clusters. Consequently when the laser is turned off, the buildup of larger carbonyls can take place efficiently and the intensity of Nilo+(CO)lo quickly returns to its smaller static value. These time-dependent behaviors can be simulated on the basis of the kinetic modeling presented in detail in ref. [201.
938
A major handicap of the experiment was the fact, that the synthesis and photodestruction of the investigated particles occured in the same reaction chamber. Quintipel-quadrupole systems or ICR-experiments combined with ultrashort UV-light sources could certainly contribute to a further understanding of the impressive effectiveness of the observed photodynamic phenomena.
IV. DEPOSITION OF IONIZED CLUSTERS A system such as metal carbonyls is of great interest for applications in catalysis. The large electronic excitation cross section of the metallic core also raises the possibility for photon-induced chemical reactions Experiments in the gas phase are sensitivity-limited, because the number of three-body collisions in the above-mentioned experimental scheme is low. This suggests depositing the size-selected particles on a surface. A classical system where metal clusters have extraordinary catalytic properties is the photographic system. Photographic plates consist of silver halide microcrystals (sized between 0.1 to several pm) supported on a substrate. All models, which describe elementary processes between the light and the silver halide, conclude that with light exposure, formation of a latent image speck occurs on the silver halide crystal. The immersion of the plate in the development bath creates the real image by reduction of crystals containing specks to metallic silver. The process is considerably slower for unexposed crystals, containing no specks. There is a general agreement, that the latent image speck is not atomic silver, but it should be a cluster [=I.
.
Size-selected Ag;
beam
nn Counting coordinate
Fig. 15 Experimental scheme for depositing monodisperse silver clusters on a photographic plate. The ITGlayer prevents the buildup of repulsive charge.
939
We exposed, therefore, photographic plates to beams of size-selected silverclusters Agn+ (n = 1 - 9) [241. The photographic specimens consisted of binder-free AgBr-monocrystals sedimented onto a gelatin coated glass plate (see Fig. 15). The grain shape was cubic with a length of 0.8 pm. In addition, the glass plate was coated with a thin conductive layer of indium-tin oxide to prevent charging up during deposition. The radiation dose was chosen such that approximately 10 clusters were deposited on each AgBr-grain. Half of the irradiated area was covered with a conductive glass plate to compensate for a spurious light background, which could come from the primary ion source. The irradiated plates were then taken out of the vacuum and developed by using a conventional developer. 0.5
A [XI Counting coordinate
Fig. 16 Fraction of developed grains vs. counting coordinate X. The result proves, that Agn+-clusters [n 2 41 deposited on a photographic plate create latent image specks [241. The results we obtained for irradiation with different cluster sizes are shown in fig. 16: The deposited clusters cause no change on the photographic plate for cluster sizes between Ag+ and AD+. At A&+, however, over 90% of the irradiated crystals became developable within the cluster-treated area. An increased developability of about 40% is also observed at larger cluster sizes. This may be due to fragmentation phenomena during the deposition process, or due to a threshold behaviour of the process, which at this point cannot be conclusively decided 1241.
940
The experiment shows, however, that clusters of individual size do exhibit extraordinary catalytic properties, and that they can successfully be deposited on surfaces, even if some impact fragmentation occurs, while they are deposited. The outcome of the experiment encourages us to propose the following experimental scheme (see fig. 17): Metal clusters are generated by means of sputtering, energyfiltered, mass-filtered and deposited on an inert target at the lowest possible energy (EkinCZ eV). Simultaneously, a molecular beam of a catalytic reaction system is scattered on the target. Resulting catalytic reactions are observed by a mass-spectrometric analysis of the scattered particle beam. Hopefully this experiment will allow us to observe in a more general sense catalytic reactions on the "in-situ" deposited, size-selected metal clusters, in order to confirm experimentally the extraordinary catalytic properties, which are attributed to these particles.
Energy filter
Quadrupole mass filter
Fig. 17 Experimental setup proposed for studying the catalytic properties of metal clusters of a specific size.
V. ACKNOWLEDGEMENTS The above-mentioned experiments are the result of a very close and fruitful collaboration with my former graduate student Pierre Fayet, who constructed the triple-quadrupole system and performed most of the experiments. Besides that we enjoyed very stimulating collaborations with other research groups, and I like to express my gratitude to several colleagues: R. Keller provided the CORDIS design, M. McGlinchey established the organometallic understanding for the carbonyl systems; C. and Ph. Brhchignac and W.Saunders participated in the ligand-stripping experiment; E. Moisar, 8. Pischel, G. Hegenbart and F. Granzer performed together with us the photographic experiment.
941
VI.REFERENCES 1. Knight, W., Clemenger, K., de Heer, W., Saunders, W., Chou, M., Cohen, M.L.: Phys. Rev. Lett.52,2141 (1984) 2. Ekhardt, W.: Surf. Sa. 152.180 (1985) 3. Brbchignac, C., Broyer, M., Cahuzac, Ph., Delacrbtaz, G., Labastie, P., Wolf, J.P., 275 (1988) Woste, L.: Phys. Rev. Lett. 4. Rademann, K., Kaiser, B., Even, U., Hensel, F.: Phys. Rev. Lett. 2319 (1987) 5. Tomanek, D., Mukherjee, S., Bennemann, K.H.: Phys. Rev. B 665 (1983) 6. Whetten, R.L., Cox, D.M., Trevor, D.J., Kaldor, A.: Phys. Rev. Lett. 1494 (1985) 7. Keller, R.: Proc. Workshop on High-Current, High-Brightness, and High Duty-Factor Ion Injectors, San Diego (1985) 8. Fayet, P., Woste, L.: Z. Phys. D 5 177 (1986) 9. Joyes, P., Leleyter, M., J. Phys. B 6 150 (1975) 10. Lauher, J.W.: J. Am. Chem. Soc. 100,5305 (1978) 11. Cotton, F.A., Jamerson, J.D.: J. Am. Chem. SOC.9 t J , 1273 (1976) 12. Benfield, R.E., Johnson, B.F.G.: J. Chem. SOC.Dalton Trans. 1743 (1980) 13. Wade, K.: Adv. Inorg. Chem. Radiochem. 1 (1976) 14. Mingos, D.M.P.: J. Chem. Soc. Dalton Trans. 133 (1974) 15. Mingos, D.M.P.: Acc. Chem. Res. -7l 311 (1984) 16. Mingos, D.M.P.: Chem. Soc. Rev. 31 (1986) 17. McGlinchey, M.J., Mlekuz, M., Bougeard, P., Sayer, B.G., Marinetti, A., Saillard, J.Y., Jaouen, G.: Can. J. Chem. 1319 (1983) 18. Johnson, B.F.G.: In Transition Metal Clusters, ed. by B.F.G. Johnson, (WileyInterscience, New York 1980) 19. Fayet, P., McGlinchey, M.J., Woste, L.: J. Am. Chem. Soc. 109.1733 (1987) 20. Brbchignac, C., Brbchignac, Ph., Fayet, P., Saunders, W.A., Woste, L.: J. Chem. Phys.-3!8 2419 (1988) 21. Christmann, K., Schober, O., Ertl, G., J. Chem. Phys J(6 4719 (1974) 22. Cox, D.M., Reichmann, K.C., Trevor, D.J., Kaldor, A., J. Chem. Phys. SS, 111 (1988) 23. James, T. H.: The Theory of Photographic Process, 4th Edn. (MacMillan Publ. Co., New York, London, 1977) 24. Fayet, P., Granzer, F.,Hegenbart, G., Moisar, E., Pischel, B., Woste, L.: Phys. Rev. Lett. 3002 (1985)
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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
-
943
T. ZE;RLIA~, A. CARIMATI~, s. MARENGO', s. MAR TI NEW^ and L. Z A N D ~ Q I I ~ 1 Stazicme sperhtale per i W t i b i l i , V.le A. l k Giisperi, 3 20097 San Donato Milanese, MI (Italy) * ccntro CN? m i bassi stati di ossidazione, UniversitA di Milano, Via G. Venezian 21, 20133 Milano (Italy) 3 Dipartimento di Chimica Pisica ed Elettrochimica, Universita di Milruro, Via G. Venezian 21, 20133 Milano (Italy)
IuErmAm CO hydrogenation catalyst precursors prepared by adsorbing Rhr(CO)iz and on ZrOz were studied by Fourier transform infrared photoacoustic spectroscopg and by temperature-programed desorption. Rh4(CO)iz/Zr0z evolves, at temperature near ambient, to a system d d m t e d by Rhl(00) z species tbrou~&a d by reducmechanism involving 00 dissociation. The stmll Rh rrystallites f tion of the sample at 523 K are disrupted and converted to the same Rhl species when exposed to 00 at 305 K. Mo enhances 00 dissociation and favours the oxidative interaction of surface hydrowls with Rho species at about 500 K; the pramtion effect of Ho in 00 hydro(ilenetionis discvssed in connection w i t h these result.. Mo(a))6
INIRXMXIoN In the formulation of heterogeneous catalysts for alcohol synthesis frao CO and Hz,Rh is among the most valuable colaponents due to its high selectivity to oxggenated products, in particular to ethenol (ref. 1). Ihe utilieation of metal carbonyl ccmplexe in the preperation of such catalysts allow a high dispersion of the active caaponents to be obtained; in addition the atalyst precursors represent an interesting model system, whose evolution during the activation process may provide a wealth of inforroation on the physicocheau '-1 properties of the active components as well as on the elementary steps involved in the &nism of 00 conversion. Ammg the techniques utilized in the study of solid surfaces, Fourier transform infrared photoacoustic spectmscopy (W-rrypAs) is particularly suitable for the shdy of unstxhle and air sensitive materials (as laoert eetal carbonyls are) as it requires no sample preparation, besides offering the possibility of observing both &ae phese and solid phase simltaneously. We present here the results obtained in an investigation on rhcdiuebzed atalysts far mklydrog-tim. ThecatalyatFaecuraors, lmepmedby&*ing Rh4(00)12 andk(m)S On m2, WXY! Studid bY' W-IR/pAs and -tlWeocdesorptian ("Pi)), in order to evidentiate the surface &ions curring in the activation process and to obtain information on the nature of the
944
active sites in the final catalyst.
nEmm6 Prepsration of eumorkd petal carbonsls ZrOz (surface area 83 d/g) YSBS pmpx-ed by drying at 383 K and calcining at 823 R a gel of hydrated zircaniun oxide obtained by m l y s i s of ZrOCl2 with aqueous -a. Rh4(a>)l2/mZ prepared bY tingthesupportwitha pentane solution of rhodium dodecacarbonyl. The samples containing molybdenua carbonyl were prepared by dispersing m 2 in powder form in a n-heptane solution of the metal cmplexes, heating at reflux lnder inert atmosphere and drying the solid phase after remwing the solvent. All of the samples had a Rh content of lwt% and an atogic ratio Rh& betwen 1:O
and 1:2
.
are named in the text after the formula of the t of any eventual mdification occurred during adsorption on
They
9 --
the support. FT-IR photoacoustic
'Ik FT-IR/PA
spectra were recorded by
a Nicolet 6000 C spectrometer equipped
w i t h a PAR photoacaustic cell (KBr windaw) at 8 car1 resolution and mirror speed
of 0.176 cm.s-1. carbon black was used as a reference. The samples were transferred into the sample cell carrier in a glove box lnder nitrogen ataephere and then flushed w i t h He for 1 minute. To stdy chemisorbed CO on the reduced sample,CO was injected by a syringe through a septun ccmmsted to the collpling gas valves. 15 seamis after CO admission the coupling gas valves were closed anl the spectra collection started.
The initial 0 partial pressure in the gas pbase above the sample wxs about 250 tom. Tmture-
deaorption TPD s t were carried out in ~~~1111111 (10-3-10-4Pa) by heating at a rate of 10 K/min a powdered sample ( - 30 n g ) placed in a qucrrtz holder connected to a v i z o l l e d quadrupole m889 spectrolDeter (-10 Erba Instrummts). The apparatus all- up to eight m/e signals to be simultaneausly recmded end plotted as a fumtion of activatian teaperature. Desorption of H2, CX4, CO, m3M, COz w?is aDnitored by d e = 2, 15, 28, 31 and 44 respectively. The peaks wlere corrected in order to account for fragaentation 'ssion; the correction facmtterns, ianization efficiency and quadruple txanm~ tor thus &temined relative to CO allawed us to obtain curves repmenting the relative desorption rates of the aelected cherical species.
945
Chtalytic tests The catalytic activity in CO hydrogenation WIS detRlmined in a stainless steel tubular flaw reactor by -sing a a F f I z mixture over a bed of 1 g of catalyst in the pressure range of 0.1-4 MPB (ref. 2). Before the catalytic test,& supported metal carbonyls were activated by hea-I
ting in Hz flaw froa 298 to 523 K at a rate of 2 Wmin and holding the latter tem-
a
z
perature for 2 h.
a.
s
RFSULTS AND DISCUSSION
0
Rh4 (m)lZ
m i !
'Ihe PT-IR/PA spec-
@a
r
E-
q
N,
N -I 6
z
E! Lo
c
2 9
I
:
R
s
F -
0
,
2400
ZlbO
l8bO
WAVENUMBERS
of solid R h r (CO)I 2 (see Fig. 1) shows bands with maxiam at 2066 and 2040 a-ldue to CO 1-4 bonded to Rh and a band at 1870 cm-1 attributedtobridghg00 (ref. 3). A d b a n d of gaseous COz at 2349 ca-1 can also be noticed. Ziramia-mpprtd Rhr(OD)iz abws quite different IR features: in the regionof linearlybondeda) three bands are observed at 2092, 2060 and 2015 cm-1, while the band of bridging CO is puch mker, coppsred with pure Rhr(CO)iz, and shifted to 1819 ca-1. Horeover, the spectrumofthesempleplacedin the photoea coustic cell at abaut 305 K urmier He atmo1 sphere exhibited a remrkble evolution 1'500 (see Fig. 2). The band of bridging OD gradually demeased and disappeared after few
a band centered at 2349 a - 1 due to COz As a consequence, carbaxstea deposition OcCuITed on tbe s u p gradmlly grew up. port, as suggested by a h o d band at 1550-1650 ca'l. chmisorbsd on Iol/lu203 (ref. (1*1 the h i s of the assignaents reported far 3-5), a description of the caplex surface stcucture of Rh4(0)xz/Za>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
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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
