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Studies in Surface Science and Catalysis 57 SPECTROSCOPIC CHARACTERIZATION OF HETEROGENEOUS CATALYSTS PART B: CHEMISORPTION OF PROBE MOLECULES
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis Advisory Editors: 6.Delmon and J.T. Yates VOl. 57
SPECTROSCOPIC CHARACTERIZATION OF HETEROGENEOUS CATALYSTS PART B: CHEMISORPTION OF PROBE MOLECULES
Editor
J.L.G. FIERRO Instituto de Catblisis y Petroleoquimica, Consejo Superior de lnvesrigaciones Cienti'ficas, Serrano 1 19, 28006 Madrid, Spain
ELSEVlER
Amsterdam - Oxford - New York - Tokyo
1990
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada:
ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655, Avenue of the Americas New York, NY 10010, U S A .
L i b r a r y of Congress C a t a l o g i n g - i n - P u b l i c a t i o n
Data
Spectroscopic c h a r a c t e r i z a t i o n o f heterogeneous c a t a l y s t s 1 e d i t o r . J.L.G. F i e r r o . 571 p. c 6 . -- ( S t u d i e s i n s u r f a c e s c i e n c e a n d c a t a l y s i s I n c l u d e s b i b l i o g r a p h i c a l r e f e r e n c e s and i n d e x . C o n t e n t s p t . A . M e t h o d s o f s u r f a c e a n a l y s i s -- p t . B . C h e n i s o r p t i o n o f probe molecules. ISBN 0-444-88243-X ( V . 2 ) ISBN 0-444-88242-1 ( v . 11. 1. C a t a l y s t s . 2 . Spectrum a n a l y s i s . I . F i e r r o . J. L . G . . 194811. S e r i e s . OD505.S688 1990 541,3'95--dc20 90-3560 CIP
.
--
.
ISBN 0-444-88243-X (Part B) ISBN 0-444-88812-8 (Parts A and B) 0 Elsevier Science Publishers B.V.. 1990 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 & EngineeringDivision, P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Special regulationsfor readers in the USA -This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Although all advertising material is expected to conform to ethical (medical) standards, inclusion in 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. Printed in The Netherlands
V Contents
Preface
.............................................................
CHAPTEK 1 (J.L.G.
Fierro)
.................................... .................................................
CHEMISOKPTION OF PROBE MOLECULES
1.1. I n t r o d u c t i o n
............................. .......................................... ...............................
1.2. Aspects of c h e m i s o r p t i o n systems 1.2.1. Scorekeeping 1.2.2. S e l e c t i v e cheriiisorption 1.2.3. B a s i c imolecules 1.2.4. E l e c t r o n c o n f i g u r a t i o n o f t h e probe 1.3. Q u a n t i t a t i v e d e t e r m i n a t i o n 1.3.1. Cheniisorption 1.3.1.1. V o l u m e t r i c methods 1.3.1.2. G r a v i m e t r i c methods
....................................... ...................
................................... ......................................... ...........................
..........................
....................... ............................
1.3.1.3. Continuous f l o w method 1.3.1.4. P u l s e f l o w method
............................... ............................. X-ray 1 i n e broadening (XLBA) ......
1.3.2. Physicochemical methods 1.3.2.1. X-ray t e c h n i q u e s
1.3.2.1.1. 1.3.2.1.2. Small a n g l e X-ray s c a t t e r i n g (SAXS) 1.3.2.2. E l e c t r o p h o r e t i c m i g r a t i o n t e c h n i q u e 1.3.2.3. XPS peak i n t e n s i t y measurements 1.3.2.4. E l e c t r o n microscopy 1.3.3. M i s c e l aneous methods
1.4.
xi11
..........
.............. .......................... .................................. I d e n t i f i c a t i o n o f s u r f a c e species ............................ 1.4.1. I n f r a r e d spectroscopy ................................. 1.4.2. N u c l e a r magnetic resonance (NMR) ...................... 1.4.3. E l e c t r o n s p i n resonance (ESR) ......................... 1.4.4. Temperature programmed d e s o r p t i o n (TDP)
...............
............................. ...................................... ..................................... .................................... .......................................
1.5. A p p l i c a t i o n t o c a t a l y t i c systems 1.3.1. Supported m e t a l s 1.5.1.1. P l a t i n u m 1.5.1.2. P a l l a d i u m 1.5.1.3. N i c k e l 1.5.1.4. Copper
.......................................
81 B1 83 83 85
87 88
811 811 812 814 817 818 820 820 820 821 822 823 824
825 825 825 827
830 834 836 836 836 B38 838
840
VI
....................................... ..........................................
1.5.1.5. S i l v e r 1.5.2. M e t a l o x i d e s 1.5.2.1. N i c k e l o x i d e 1.5.2.2. Chromia 1.5.2.3. Copper o x i d e s 1.5.2.4. I r o n o x i d e s 1.5.2.5. Molybdena .................................... 1.5.2.6. C o b a l t o x i d e s
................................. ......................................
................................ ..................................
................................ ................................................... ..........................................................
1.6. C o n c l u s i o n References
CHAPTER 2 (J.L.G.
Fierro)
...............................................
867
................................................. ........................................ ................................................
867 869 869
.........................................
869
INFRARED SPECTROSCOPY
2.1. I n t r o d u c t i o n 2 . 2 . I n f r a r e d spectroscopy 2 . 2 . 1 . Theory 2.2.1.1. C l a s s i c a l c a l c u l a t i o n o f v i b r a t i o n a l f r e q u e n cies
2.2.1.2.
Quantization o f the i n t e r a c t i o n o f r a d i a t i o n w i t h matter
..................................
2.2.1.3. A n h a r m o n i c i t y o f m o l e c u l a r v i b r a t i o n ......... 2.2.1.4. I n t e n s i t y o f t h e a b s o r p t i o n bands ............ 2.2.1.5. R o t a t i o n a l bands
2.3.
............................. 2.2.2. U u a n t i t a t i v e aspects .................................. 2.2.2.1. B e e r ' s l a w ............................... 2.2.2.2. A d s o r p t i o n s t u d i e s ........................... Experimental t e c h n i q u e s ...................................... 2.3.1. Transmission .......................................... 2.3.2. Emission .............................................. 2.3.3. R e f l e c t i o n methods .................................... 2.3.4.
842 842 842 848 849 850 853 859 860 860
872 873 876 877 879 879 879 B81 881 883 884
P h o t o a c o u s t i c (PAS) and photothermal d e f l e c t i o n (PDS)
.......................................... Spectrophotometers .................................... spectroscopy
2.3.5. 2.4. General aspects o f c h e m i s o r p t i o n o f CO and NO ................ 2.5. T r a n s m i s s i o n - a b s o r p t i o n IR spectroscopy ...................... 2.5.1. T r a n s i e n t k i n e t i c s t u d i e s .............................
....................................... ............................ ...............................
2.5.2. M e t a l carbonyl s 2.5.2.1. Chromium c a r b o n y l 2.5.2.2. I r o n c a r b o n y l s
B86 887 B88 889 889 892 892 894
VII
.............................. .......................... .................................................
2.5.2.3. C o b a l t carbonyl 2.5.2.4. Molybdenum c a r b o n y l
2.6. Metal o x i d e s 2.6.1. Vanadium o x i d e ........................................ 2.6.2. Chromium o x i d e ........................................ 2.6.3. I r o n o x i d e ............................................ 2.6.4. C o b a l t o x i d e 2.6.5. N i c k e l o x i d e 2.6.6. Molybdenum o x i d e ...................................... 2.6.7. Copper o x i d e .......................................... 2.7. R e f l e c t i o n - a b s o r p t i o n spectroscopy ........................... 2.7.1. E x t e r n a l r e f l e c t i o n spectroscopy 2.7.2. I n t e r n a l r e f l e c t i o n spectroscopy 2.8. P h o t o a c o u s t i c spectroscopy (PAS) 2.9. P h o t o d e f l e c t i o n beam spectroscopy (PDBS) .....................
.......................................... .......................................... ...................... ...................... .............................
................................................... ..........................................................
2.lO.Concl u s i o n References
896 898 8100 8100 8101 8106 8112 8117 8121 8127 B129 8129 B132 8133 8135 8137 8138
CHAPTER 3 (A.M. Bar6) ELECTRON VIBRATIONAL SPECTROSCOPY
...................................
3.1. I n t r o d u c t i o n ................................................. 3.2. The e x c i t a t i o n o f v i b r a t i o n s by t h e e l e c t r o n probe 3.2.1. The t h e o r y o f d i p o l e s c a t t e r i n g ....................... 3.2.2. Impact s c a t t e r i n g 3.2.3. Resonant s c a t t e r i n g 3.3. EELS v i b r a t i o n a l s p e c t r a o f adsorbed molecules 3.3.1. Atomic a d s o r p t i o n 3.3.1.1. Hydrogen adatoms 3.3.1.2. Oxygen 3.3.2. Carbon monoxide 3.3.3. Water 3.3.4. Unsaturated hydrocarbons .............................. 3.3.5. I d e n t i f i c a t i o n o f new s p e c i e s ......................... 3.3.5.1. The thermal p r o c e s s i n g o f e t h y l e n e on P t ( l l 1 ) 3.3.6. Study o f t h e r e a c t i v e c o a d s o r p t i o n o f D2 and CO on a stepped P t ( 111) s u r f a c e ............................... 3.3.7. Supported metal c a t a l y s t 3.4. Summary and o u t l o o k ..........................................
...........
..................................... ................................... ............... ..................................... ............................. ....................................... ....................................... .................................................
..............................
References
..........................................................
B145 8145 8147 8150 8156 B157 8160 8163 8165 8174 8180 8184 8186 8189 B190 8192 8194 8195 B197
VIII CHAPTER 4 ( J
. Fraissard)
NMR OF ADSORBED MOLECULES USED AS PROBES FOR SURFACE INVESTIGATION
.................................................
4.1.
Introduction
4.2.
NMR i n t e r a c t i o n s
Dipolar nuclear i n t e r a c t i o n
4.2.2. 4.2.3.
Chemical s h i f t E f f e c t o f unpaired e l e c t r o n s
4.3.
4.4.
8201 8201
.............................................
4.2.1.
4.2.4.
..
8202
...........................
........................................ .......................... Quadrupole i n t e r a c t i o n s ............................... J-Coupling ( I n d i r e c t n u c l e a r - n u c l e a r i n t e r a c t i o n s .....
8202 8205 8207 8210
4.2.5. NMR techniques f o r t h e study o f adsorbed molecules
8214
4.3.1.
Experimental c o n d i t i o n s and d i f f i c u l t i e s
8216
4.3.2.
Measurement o f resonance s h i f t s
4.3.3. 4.3.4.
Broadening and magnetic s h i e l d i n g a n i s o t r o p y Exchange e f f e c t s
........... .............. ....................... ..........
...................................... 4.3.4.1. F i r s t method ................................. 4.3.4.2. Second method ................................ NMR study of s o l i d s w i t h chemisorbed molecules ............... 4.4.1. Diamagnetic systems . Study o f t h e a c i d i t y o f c a t a l y s t s : f a s t exchange ......................................... 4.4.1.1. Background ................................... 4.4.1.2. 'H NMR chemical s h i f t o f NHdY z e o l i t e ........ 4.4.1.3.
Acid-base r e a c t i o n s a t a s o l i d s u r f a c e
.
8216 8218 8220 8220 8222 8223 8224 8224 8224 8226
B r h s t e d a c i d s t r e n g t h ; chemical s h i f t and c o n c e n t r a t i o n o f OH groups
4.4.2.
protons
4.5.
4.7.
............................................... .........................................
Paramagnetic systems
4.5.1. 4.5.2. 4.6.
...................
.........
A d s o r p t i o n o f o l e f i n s on paramagnetic c e n t e r s Decomposition o f f o r m i c a c i d on electron-donor c e n t e r s
............................................. 4.6.1. 'H NMR study o f hydrogen chemisorbed on p l a t i n u m . A p p l i c a t i o n t o t h e d i s p e r s i o n ......................... NMR o f physisorbed molecules used as probes .................. Supported metals
4.7.1. 4.7.2.
8226
High r e s o l u t i o n s o l i d s t a t e NMR o f n u c l e i o t h e r than
Chemical s h i f t o f xenon adsorbed i n a pure z e o l i t e Influence o f structure
....
4.7.2.2.
................................ Na o r H - f a u j a s i t e ............................ I n f l u e n c e o f t h e s t r u c t u r e ...................
4.7.2.3.
R e l a t i o n s h i p between t h e chemical s h i f t 6,
4.7.2.1.
and t h e v o i d space
...........................
8230 8233 8233 8234 8239 8239 8243 8243 8244 8244 8245 8245
IX
4.7.3.
4.7.4.
References
4.7.2.4. CXNal-AY
C r y s t a l l i n i t y and p o r e b l o c k i n g zeolites Influence o f cations
4.7.3.1.
Diamagnetic c a t i o n s
4.7.3.2.
Paramagnetic c a t i o n s
.
..............
............... ..........................
.........................
8247 8249 8249 8252
Chemical s h i f t o f xenon adsorbed on m e t a l - l o a d e d zeo-
................................................. Chemisorption o f hydrogen .................... 4.7.4.2. Chemisorption o f o t h e r gases G ............... 4.7.4.3. Successive c h e m i s o r p t i o n o f s e v e r a l gases .... .......................................................... 1i t e s
8253
4.7.4.1.
8256 8258 8260 8261
.
.
CHAPTER 5 (M Che and E G i a m e l l o )
..................................... .................................................
ELECTRON PARAMAGNETIC RESONANCE 5.1.
Introduction
5.2. The EPR t e c h n i q u e
............................................
......... .........
5.2.1.
The e l e c t r o n paramagnetic resonance p r i n c i p l e
5.2.2.
The b a s i c i n s t r u m e n t a t i o n o f EPR spectroscopy
5.2.3.
The b a s i c i n t e r a c t i o n s o f t h e u n p a i r e d e l e c t r o n w i t h
....... .................................
5.2.5.
8265 8266 8266 8269
i t s environment and t h e f e a t u r e s o f EPR s p e c t r a
8272
5.2.3.1.
The g t e n s o r
8272
5.2.3.2.
The e l e c t r o n s p i n - n u c l e a r s p i n i n t e r a c t i o n
5.2.3.3.
Superhyperfine s t r u c t u r e
5.2.3.4.
The case o f S > 1/2 ( f i n e s t r u c t u r e )
(hyperfine interaction)
5.2.4.
8265
...................... .....................
.........
8275 8278 8279
5.2.4.2.
.............. I s o t r o p y o f g ................................ A x i a l symmetry o f g ..........................
8280
5.2.4.3.
Orthorhombic symmetry o f g
8281
The f e a t u r e s o f model powder EPR s p e c t r a
8279
5.2.4.1.
8280
...................
The r e a l EPR powder spectrum: a p r a g m a t i c approach t o
............................................ Mu1 t i f r e q u e n c y approach ...................... I s o t o p i c l a b e l l i n g ........................... T h i r d d e r i v a t i v e s p e c t r a ..................... S p e c t r a s i m u l a t i o n ...........................
resolution
8285
5.2.5.1.
8285
5.2.5.2. 5.2.5.3. 5.2.5.4.
8285 8289 8291
5.3. C h a r a c t e r i z a t i o n o f c a t a l y t i c s u r f a c e s by means o f probe molecules and EPR 5.3.1.
............................................
8292
D e f i n i t i o n o f a probe m o l e c u l e f r o m t h e s t a n d p o i n t o f EPR
................................................... C l a s s i f i c a t i o n o f t h e probe m o l e c u l e s ........
5.3.1.1.
8292 8292
X
......... ................................
5.3.1.2. L o c a t i o n o f t h e paramagnetic c e n t e r s 5.3.2. S u r f a c e c r y s t a l f i e l d s 5.3.2.1. The s u p e r o x i d e 0; r a d i c a l i o n as a s u r f a c e c r y s t a l f i e l d probe
..........................
........... ....................... .......
5.3.2.2. NO as a s u r f a c e c r y s t a l f i e l d probe 5.3.3. Redox p r o p e r t i e s o f t h e s u r f a c e 5.3.3.1. O x i d i z i n g p r o p e r t i e s o f o x i d e s u r f a c e s 5.3.3.2. Reducing p r o p e r t i e s o f o x i d e s u r f a c e s 5.3.4. A c t i v e s i t e s i d e n t i f i c a t i o n v i a p o i s o n i n g 5.3.5. S u r f a c e groups morphology
........ ............. ............................. s p e c i e s ..........................
5.3.6. M o b i l i t y o f adsorbed 5.3.6.1. G a s - s o l i d systems ............................ 5.3.6.2. L i q u i d - s o l i d systems 5.3.7. C o o r d i n a t i o n c h e m i s t r y o f s u r f a c e t r a n s i t i o n m e t a l i o n s 5.3.7.1. S u r f a c e c o o r d i n a t i o n c h e m i s t r y o f t r a n s i t i o n
.........................
8293 8294 8297 8303 8304 8306 8307 8310 8311 8314 8314 8315 8315
m e t a l i o n s homogeneously d i s p e r s e d i n t o a s o l i d framework
5.3.7.2.
..............................
8315 8322 8328 8329
Coordination chemistry o f extraframework ions
.................................................. ..........................................................
5.4. Conclusions References
.
CHAPTER 6 (P M a l e t )
.......................................... .................................................
8333
THERMAL DESORPTION METHODS
................................. ...............................
8333 8335 8335 8338 8341
......................................
8341
6.1. I n t r o d u c t i o n 6.2. Experimental systems 6.2.1. Flow and vacuum systems 6.2.2. Experimental p i t f a l 1s 6.3. K i n e t i c a n a l y s i s o f TPD curves 6.3.1. Q u a l i t a t i v e a n a l y s i s : number and r e l a t i v e s t a b i l i t y o f
......................................... ...............................
a d s o r p t i o n forms
6.3.2. Q u a n t i t a t i v e a n a l y s i s : k i n e t i c parameters o f t h e desorpt i o n process
6.3.2.1. 6.3.2.2. 6.3.2.3.
..........................................
One parameter a n a l y s i s o f a s i n g l e TPD c u r v e One parameter a n a l y s i s o f s e v e r a l TPD curves
. .
B341 8348 8354
Whole l i n e - s h a p e a n a l y s i s o f a s i n g l e TPD curve
........................................
6.3.2.4. Whole l i n e - s h a p e a n a l y s i s o f s e v e r a l TPD curves 6.3.2.5. E f f e c t o f sample r e a d s o r p t i o n on t h e shape o f TPD curves
...................................
8355 8362 B364
XI 6.4. S u rf ac e h e t e r o g e n e i t y 6.4.1.
........................................
8367
Temperature and coverage dependence o f t h e pre-exponen-
........................................... .......... ............................................ ......................................................
t i a l factor
8367
6.4.2. Coverage dependence o f t h e d e s o r p t i o n energy 6.5. D i f f u s i o n c o n t r o l
8368 8376
6.6. Summary
8377
References
..........................................................
8380
This Page Intentionally Left Blank
XI11
PREFACE
To gain insight into catalytic processes on an almost atomic scale is one of the major objectives of researchers in heterogeneous catalysis. This can be achieved through spectroscopic analysis of the interactions of probe molecules with catalyst sites. Chemisorption studies have largely concentrated on well-defined surfaces, viz. single crystals, because of their inherent simplicity. Many progress reports and a large body of publications in specialized Surface Science Journals have provided convincing descriptions of the processes involved on model systems. However, when one turns to catalysts of practical interest, many difficulties arise in the re1 iable characterization of heterogeneous catalysts as compared to single crystals catalysts, chemisorption usually takes place on an energetically heterogeneous. In heterogeneous surface, which leads to overlap of several phenomena well understood only in the case of single crystals. This may explain the lack of systematic discussion of chemisorption on poorly defined surfaces, such as practical catalysts. It has been our endeavour to offer the reader in each chapter an overview of the physical foundations, basic concepts and capabilities of the relevant techniques applied in the field of measurements, in addition to examples of proven and potentially important applications. Part B of this work, in two volumes, is meant to fill this gap. Chapter 1 surveys the methods employed in the quantification o f the extent of chemisorption of molecules on metals and metal oxides. Special emphasis is placed on the methods which best describe the dispersion of the active components in practical catalysts. Chapter 2 provides a comprehensive appraisal of the infrared ( I R ) technique. IR has proved to be the most widely applied vibrational spectroscopy technique in both qua1 itative and quantitative determinations of molecular species and atoms in catalyst surfaces. Chapter 3 deals with Electron Vibrational Spectroscopy (EELS). Although EELS has been mainly applied to well-defined surfaces, refinements accomplished in recent years with regard to resolution and sensitivity have made it as a powerful technique in the study of model systems. Subsequently, chapters 4 and 5 provide a basic understanding of certain aspects of Magnetic Resonances (NMR and ESR) and describe its recent applications to the characterization of heterogeneous catalysts. Chapter 6 deals with desorption methods and contains a complete mathematical description of desorption curves. We acknowledge with gratitude the contributions made by the different authors, the technical staffs of the Institutes supporting this work and the
XIV
c o l l a b o r a t i o n o f t h e publishers, a l l of whose cumulative e f f o r t s have made t h i s book possible. Last but not l e a s t , our thanks a r e due t o Mrs R. Pomares whose i n d e f a t i g a b l e s e c r e t a r i a l assistance has been invaluable. J.L.G.
Fierro
Chapter 1
CHEMISORPTION OF PROBE MOLECULES
J.L.G. FIERRO I n s t i t u t o de C a t L l i s i s y P e t r o l e o q u i m i c a , C.S.1 .C., (Spain)
Serrano, 119, 28006 Madrid,
1.1 INTRODUCTION
F o r c a t a l y s t technology, t h e most s e n s i t i v e probe o f c a t a l y s t s w i l l c o n t i n u e t o be t h e r a t e and s e l e c t i v i t y o f a chemical r e a c t i o n . However, t h e s e macros c o p i c o b s e r v a t i o n s , adequate f o r d e t e r m i n i n g how good a c a t a l y s t i s , r e q u i r e supplementary m i c r o s c o p i c i n f o r m a t i o n t o remove a m b i g u i t y i n t h e d e d u c t i o n o f a c a t a l y t i c mechanism. T h i s i n f o r m a t i o n , almost down t o t h e a t o m i c l e v e l , concerni n g t h e s t r u c t u r e and r e a c t i v i t y o f t h e i n t e r m e d i a t e s , t h e n a t u r e o f a d s o r p t i o n s i t e s (and sometimes t h e a c t i v e s i t e s ) and t h e i r number, i s t h e main o b j e c t i v e o f t h e s c i e n c e o f c a t a l y s i s . The most p r o m i s i n g approach t o t h i s problem i s t h e use o f s u i t a b l e molecules f o r t h e q u a n t i t a t i v e t i t r a t i o n o f s i t e d e n s i t y and qua1 i t a t i v e c h a r a c t e r i z a t i o n o f t h e i r n a t u r e by means o f s u r f a c e s p e c t r o s c o p i c techniques o f t h e chemisorbed molecules (1-4). T h i s framework o f a c t i o n f o r a w o r k i n g c a t a l y s t i s s c h e m a t i c a l l y r e p r e s e n t e d i n F i g . 1. W h i l e w i t h o r d i n a r y supported metal c a t a l y s t s t h e e s t i m a t i o n of t h e s u p p o r t s u r f a c e uncovered by t h e a c t i v e i n g r e d i e n t s may be o b t a i n e d b y t h e d i f f e r e n c e between t h e t o t a l s u r f a c e area as measured by t h e BET method and t h e metal s u r f a c e a r e a measured by c h e m i s o r p t i o n methods, s i t u a t i o n s e x i s t where a d i r e c t e s t i t n a t e o f t h e s u r f a c e area o f a nonmetalliccomponent i s d e s i r a b l e . T h i s i s t h e case f o r supported m e t a l o x i d e c a t a l y s t s , which o f t e n c o n t a i n s e v e r a l d i s t i n c t phases. The number o f cases where c h e m i s o r p t i o n measurements have been success f u l l y a p p l i e d t o t h i s end has i n c r e a s e d i n r e c e n t y e a r s , e s p e c i a l l y i n t h e f i e l d o f h y d r o t r e a t m e n t o f p e t r o l e u m f e e d s t o c k s where molybdena- ( o r t u n g s t a - ) c o n t a i n i n g c a t a l y s t s , promoted w i t h m i n o r amounts o f c o b a l t o r n i c k e l , a r e t h e a c t i v e c a t a l y s t s f o r such r e a c t i o n s . The f a s t development a t t a i n e d i n t h e 1960s on m e t a l l i c c a t a l y s t s r e l i e d m a i n l y on thorough knowledge o f c h e m i s o r p t i o n t e c h n i q u e s as w e l l as t h e p a r a l l e l development of surface t e c h n i q u e . Moreover, t h e e l e c t r o n i c i n t e r p r e t a t i o n o f simple r e a c t i o n s upon metal s u r f a c e s , a l t h o u g h n o t always s a t i s f a c t o r y , was succ e s s f u l , f o r i n s t a n c e , i n e x p l a i n i n g t h e r e l a t i o n s h i p between t h e e l e c t r o n i c s t r u c t u r e of t h e c a t a l y s t and i t s a c t i v i t y found by Beeck, Schwab, and E l e y ( 5 , 6). When one t u r n s t o r e a c t i o n s upon oxides, t h e s i t u a t i o n i s l e s s c l e a r , and
a l t h o u g h some u s e f u l g e n e r a l i z a t i o n s may be drawn r e g a r d i n g o x i d e t y p e and
Probe Adsorbed layer Catalyst surface
\
'Reactants
F i g . 1.l.Schematic r e p r e s e n t a t i o n o f t h e c a t a l y s t i n t e r f a c e o f a working c a t a l y s t .
. reactivity, experime:,Lal
d e t a i l e d c o r r e l a t i o n s a r e r a t h e r d i f f i c u l t . A p a r t from t h e d i f f i c u l t y i n d e f i n i n g and reproducing a c l e a n surface oxide, t h i s
i s inainly due t o : (1) t h e f o r m a t i o n o f s u r f a c e defects, i . e . , CUS s i t e s , 0d u r i n g a c t i v a t i o n and pretreatments; ( 2 ) t h e d i f f i c u l t y i n
vacancies, etc.,
f i n d i n g a s e l e c t i v e probe t o q u a n t i f y s i t e d e n s i t y (Mnt and 0'-)
on t h e surface;
and ( 3 ) t h e r e l a t i v e ease w i t h which l a t t i c e oxygen l y i n g on t h e s u r f a c e o f t h e c a t a l y s t can t a k e p a r t i n c a t a l y t i c r e a c t i o n s i n v o l v i n g oxygen-containing gases. The b e s t known a p p l i c a t i o n o f s e l e c t i v e cheniisorption f o r t h e s u r f a c e area measurement o f a n o n m e t a l l i c component i s t h e use o f C02 chemisorption f o r t h e K20 s u r f a c e area i n s t a b i l i z e d iron-promoted amnonia s y n t h e s i s c a t a l y s t s ( 7 ) . I n these c a t a l y s t s t h e o x i d e component contained up t o about 10 w t % A1203 as a s t a b i l i z e r and up t o 1.6 w t % K20 as a promoter. A d s o r p t i o n was c a r r i e d o u t a t 195 K up t o a C02 pressure of about 60 kN/m2; t h e e q u i l i b r i u m uptake under these c o n d i t i o n s i n c l u d e s both chemisorbed and p h y s i c a l l y adsorbed gas, and t h e former was evaluated as t h a t f r a c t i o n which c o u l d n o t be desorbed by pumping a t 273 K (see F i g . 1.4b).However,
i n t h e l i g h t of r e c e n t s t u d i e s (8, 9 ) , t h e o r i g i n a l
B3 assumption t h a t C02 was chemisorbed o n l y upon t h e K20 s u r f a c e i s open t o d i s c u s s i o n because C02 i s known t o be r a p i d l y and s t r o n g l y chemisorbed on c l e a n i r o n a t 195 K. Even a l l o w i n g f o r t h e f a c t t h a t a t monolayer coverage o f C02 on i r o n t h e c h e m i s o r p t i o n s t o i c h i o m e t r y was about 10, i t i s c l e a r t h a t some o f t h e C02 a d s o r p t i o n p r e v i o u s l y a t t r i b u t e d t o K20 must have occured upon t h e i r o n . I t may be p o s s i b l e , however, t o m i n i m i z e C02 c h e m i s o r p t i o n on i r o n by O2 p r e a d s o r p t i o n . Another example o f s e l e c t i v e s u r f a c e area measurements i n two-component o x i d e systems i s p r o v i d e d by V o l t z and W e l l e r ( 1 0 ) and by MacIver and T o b i n (11) who used t h e amount o f oxygen t o e s t i m a t e t h e s p e c i f i c s u r f a c e a r e a o f chromia i n supported c a t a l y s t s . I n t h e 1970s t h i s method was renewed and a p p l i e d t o s e v e r a l two-component o x i d e systems, e s p e c i a l l y t o molybdena-containing c a t a l y s t s . A t t h e same time, much work was done by t h e r e s e a r c h group a t t h e F o r d Motor Co. (U.S.A.)
upon c h e m i s o r p t i o n o f LO2, C O Y and NO probes on t h e f i r s t
t r a n s i t i o n row m e t a l o x i d e c a t a l y s t s w i t h i n t h e framework o f NO + CO removal from t h e exhaust gases o f c a r engines. The c h e m i s o r p t i o n o f such molecules as p y r i d i n e , ammonia, hydrogen s u l f i d e , and boron t r i f l u o r i d e i s s p e c i f i c f o r c e r t a i n t y p e s o f adsorbents, t h i s beh a v i o u r being g e n e r a l l y r e l a t e d t o t h e o c c u r r e n c e o f s p e c i f i c surface s i t e s ( a c i d i c o r b a s i c ) which a r e p r e s e n t i n a s u r f a c e c o n c e n t r a t i o n t h a t i s n o t known. Data o f t h i s k i n d a r e u s e f u l f o r t h e i d e n t i f i c a t i o n and e s t i m a t i o n
of
s p e c i f i c t y p e s o f a d s o r p t i o n s i t e s . D e t a i l s o f t h i s s u b j e c t can be found i n a r e c e n t r e v i e w by Jacobs ( 1 2 ) . I n t h i s r e v i e w we p r e s e n t methods based on s e l e c t i v e c h e m i s o r p t i o n o f probe molecules w i t h which t o c h a r a c t e r i z e t h e s p e c i f i c s u r f a c e area o f m e t a l s and metal o x i d e s i n b o t h supported and unsupported c a t a l y s t s . I n o r d e r t o understand t h e n a t u r e o f d i f f e r e n t s i t e s i n v o l v e d i n t h e c h e m i s o r p t i o n o f a probe molecule, a few physicochemical t e c h n i q u e s ( I R , ESR and NMR) a r e considered i n S e c t i o n I V . F i n a l l y , t h e a p p l i c a t i o n o f t h e most s u i t a b l e probes t o measure surface areas o f t h e a c t i v e components i n some c a t a l y t i c systems a r e reviewed i n S e c t i o n s V and V I . 1.2 ASPECTS OF CHEMISOKPTION SYSTEMS 1.2.1 Scorekeeping Three q u a n t i t i e s of g r e a t e s t i n t e r e s t i n t h e u n d e r s t a n d i n g of t h e chemisorpt i o r i process s h o u l d be mentioned: i ) t h e c h e m i s o r p t i o n energy AE; i i ) t h e induced d i p o l e moement 1-1; and i i i ) t h e change i n d e n s i t y o f s t a t e s (DOS) f l p ( ~ ) on a d s o r p t i o n . F o r a g i v e n m o l e c u l e and s o l i d s u r f a c e AE may be d e f i n e d as t h e d i f f e r e n c e between t h e energy o f t h e s e p a r a t e molecule and s o l i d s u r f a c e and t h e energy o f t h e system a f t e r a d s o r p t i o n . A t T = 0, AE r e p r e s e n t s t h e energy r e q u i r e d t o iiiove
re-
t h e molecule from t h e s u r f a c e . Chemisorption, as opposed a t a p u r e l y van
d e r Waals i n t e r a c t i o n ( p h y s i c a l a d s o r p t i o n ) , i m p l i e s l a r g e r AE. A s i m p l e
B4
c r i t e r i o n f o r c h e n i i s o r p t i o n m i g h t be A E > 1 eV ( 1 3 ) . F o r comparative purpose a v a l u e o f AE o f about 0.4 eV has been f o u n d f o r t h e a d s o r p t i o n o f Xe on W ( 1 1 1 ) ( 1 4 ) , t h i s b e i n g t h e case o f v e r y weak i n t e r a c t i o n pressumably a case o f p h y s i c a l a d s o r p t i o n . The above e s t i m a t i o n is u s e f u l f o r monoatomics such as Xe, K r and n o n - d i s s o c i a t i v e m o l e c u l e s as CO and
NO on some s u r f a c e s . However, d i -
atomics such as H 2 , O2 and N2 f r e q u e n t l y d i s s o c i a t e on c h e m i s o r p t i o n ; t h e n t h e r e l a t i o n s h i p between 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 energy AE,
and t h e a t o m i c
c h e m i s o r p t i o n energy AE i s g i v e n by,
where D i s t h e d i s s o c i a t i o n energy o f t h e d i a t o m i c m o l e c u l e . A p r a c t i c a l r u l e can be e a s i l y d e r i v e d f r o m Eq. 1.Since t h e adsorbed s p e c i e s l o s e t r a n s l a t i o n a l and r o t a t i o n a l degrees o f freedom, t h e r e i s c o n s i d e r e d t o be a decrease i n t h e e n t r o p y o f c h e m i s o r p t i o n . Thus, i f AE< 0/2, 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 o f d i atomics is thermodynamically u n f a v o u r a b l e . F o r i n s t a n c e , f o r H2, D = 4.746 eV, so t h e m e t a l s a d s o r b i n d H2 may be assumed t o be t h o s e w i t h AE>2.37 eV, which i n c l u d e a l l t h e t r a n s i t i o n m e t a l s , Ca, Ba and Ge ( 1 5 ) . D i a t o m i c s CO and N 2 show a somewhat s i m i l a r c h e m i s o r p t i o n p a t t e r n t o H 2 . The c h e m i s o r p t i o n bond u s u a l l y i n v o l v e s a c e r t a i n amount o f charge t r a n s f e r t o o r f r o m t h e s u r f a c e ( m e t a l ) . The magnitude and t h e s i g n o f t h i s t r a n s f e r can be determined f r o m a knowledge o f t h e d i p o l e moment IJ o f t h e adatom. P
is
r e l a t e d t o t h e change i n t h e m e t a l work f u n c t i o n
A0 by t h e f o l l o w i n g e q u a t i o n ,
where n i s 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 adatoms.
A0
and n can be measured
e x p e r i m e n t a l l y b y u s i n g t h e r e t a r d i n g f i e l d method i n c o n j u n c t i o n w i t h Low Energy E l e c t r o n D i f f r a c t i o n (LEED) measurements ( 1 6 ) . As can be expected f o r a l k a l i a d s o r p t i o n on t r a n s i t i o n m e t a l s , t h e r e i s a maximum decrease o f s u b s t r a t e work f u n c t i o n o f up t o 3.5 eV, i n d i c a t i n g a r a t h e r l a r g e charge t r a n s f e r f r o m t h e adsorbate. Hydrogen and oxygen a d s o r p t i o n , on t h e o t h e r hand, t e n d s t o i n c r e a s e t h e m e t a l work f u n c t i o n by o n l y up t o 0.5 eV ( 1 7 , 1 8 ) , i n d i c a t i n g t h a t t h e r e i s a small e l e c t r o n t r a n s f e r f r o m t h e m e t a l t o t h e probe. The change i n DOS AP(E),
i n t r o d u c e d by t h e a d s o r p t i o n process, p l a y s an
i m p o r t a n t r o l e i n t h e u n d e r s t a n d i n g o f t h a t process. T h i s q u a n t i t y i s d i r e c t l y r e l a t e d t o t h e U l t r a v i o l e t Photoemission S p e c t r a (UPS). One i s tempted t o e s t a b l i s h a c l o s e correspondence between t h e s t r u c t u r e as r e v e a l e d by UPS below t h e b o t t o m o f t h e metal v a l e n c e band and l o c a l i z e d s t a t e s induced by chemisorpt i o n . T h i s s t r u c t u r e has been a t t r i b u t e d t o t h e presence o f a bonding s t a t e , w h i c h suggests a p i c t u r e i n w h i c h t h e adatom bonds s t r o n g l y t o a l i m i t e d number
B5 o f s u b s t r a t e atoms, i . e . ,
H a d s o r p t i o n on N i (19). I n o t h e r cases, s t r u c t u r e i n
t h e UP spectrum seems c l e a r l y a s s o c i a t e d with a t o m i c o r m o l e c u l a r o r b i t a l s l o c a l i z e d on t h e probe. T h i s i s t h e case o f Hg adsorbed on N i whose UP spectrum shows a d o u b l e t a s s o c i a t e d w i t h t h e photoemission o f t h e Hg5d l e v e l ( 2 0 ) . 1.2.2 S e l e c t i v e Chemisorption
While t h e n o n - s e l e c t i v e p h y s i c a l a d s o r p t i o n o f gases a t l o w temperatures, m o s t l y a t 77 K, was used t o measure t h e t o t a l 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 , Emmett and Brunauer ( 7 ) developed t h e s p e c i f i c c h e m i s o r p t i o n as a p o w e r f u l t o o l f o r t h e measurement o f t h e a c t i v e area o f d i f f e r e n t s p e c i e s exposed on t h e s u r f a c e . The c l a s s i c a l work o f Emmett and Brunauer
on t h e measurement o f t h e areas
o f K , alumina, and Fe i n a d o u b l y promoted K-A1203-Fe ammonia s y n t h e s i s c a t a l y s t , was subsequently a p p l i e d t o o t h e r c a t a l y t i c systems. The advent of r e f o r m i n g i n 1950s, u s i n g Pt/A1203 c a t a l y s t s , needed r a p i d l y t o know how w e l l t h e p r e v i o u s
m e t a l was d i s p e r s e d on t h e c a r r i e r . So, a knowledge o f t h e P t d i s p e r s i o n became v e r y v a l u a b l e n o t o n l y f o r comparison o f a c t i v i t i e s o f d i f f e r e n t r e f o r m i n g c a t a l y s t s b u t a l s o f o r p e r i o d i c t e s t s on t h e performance o f t h e c a t a l y s t s i n i n d u s t r i a l r e f o r m i n g u n i t s . T h i s i m p e r a t i v e was f u r t h e r s t r e n g t h e n e d when supp o r t e d P t c a t a l y s t s r a p i d l y found o t h e r i m p o r t a n t a p p l i c a t i o n s i n p e t r o c h e m i c a l processes, e.g.,
benzene hydrogenation, p a r a f i n and x y l e n e i s o m e r i z a t i o n , and
dehydrogenation o f n - p a r a f i n s . Hydrogen chemisorb r a p i d l y a t room temperature on t h e exposed P t atoms and i t r e a d i l y forms a monolayer. T h e r e f o r e , t h e s p e c i f i c P t a r e a can be e a s i l y
d e r i v e d from t h e e x t e n t o f H2 c h e m i s o r p t i o n p r o v i d e d t h a t t h e f o l l o w i n g c o n d i t i o n s a r e e s s e n t i a l l y f u l f i l l e d : i)a d e f i n i t e H / P t s t o i c h i o m e t r y i s assumed; i i j t h e p h y s i c a l a d s o r p t i o n o f hydrogen on b o t h metal and c a r r i e r s u r f a c e s i s n e g l i g i b l e ; and i i i ) o t h e r overimposed phenomena, e.g.,
bulk diffusion, hydride
f o r m a t i o n , H 2 - s p i l l o v e r , 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 (SMSI e f f e c t ) ,
are
excluded. The advantages and disadvantages o f c h e m i s o r p t i o n o f probe m o l e c u l e s f o r measurement o f m e t a l (and metal o x i d e ) areas a r e summarized i n T a b l e l . 1 . W i t h o u t doubt, t h e most i m p o r t a n t l i m i t a t i o n o f t h e c h e m i s o r p t i o n t e c h n i q u e i s t h a t most o f t h e probe-molecules a r e n o t so s p e c i f i c f o r a g i v e n atom o r i o n . T h e r e f o r e , c h e m i s o r p t i o n becames i n a p p l i c a b l e t o c a t a l y s t s c o n t a i n i n g two o r more a c t i v e ingredients w i t h s i m i l a r chemisorption properties. For c a t a l y s t s containing o n l y one a c t i v e
component, c h e m i s o r p t i o n i s t h e most p r e c i s e and cheapest
a l t e r n a t i v e f o r measurement o f metal (and m e t a l o x i d e ) area. The metal o r metal o x i d e / s u l p h i d e a r e a i n supported c a t a l y s t s i s an e x t r e m e l y u s e f u l parameter t o compare c a t a l y t i c a c t i v i t i e s o f s e r i e s o f c a t a l y s t s c o n t a i n i n g t h e same i n g r e d i e n t b u t w i t h v a r y i n g number o f exposed atoms i n t h e s u r f a c e . I t i s t h e r e f o r e i n f e r r e d t h a t t h e a c t i v i t y / c e n t e r r a t i o , which r e f l e c t s how good a c a t a l y s t
B6 TABLE 1.1. S p e c i f i c chemisorption f o r measurement o f metal and metal o x i d e areas Probe Advantages Disadvantages Phase T (K) . R e l a t i v e l y simple .Sensitive t o impurities .Physical a d s o r p t i o n .Formation o f h y d r i d e s negl ig 1 b l e .Misleading r e s u l t s due . P r a c t i c a l l y no adt o the strongly held s o r p t i o n on c a r r i e r hydrogen a f t e r r e d u c t i o n
HZ
co
No d i s s o l u t i o n
. D i f f e r e n t chemi sorbed species .Physical a d s o r p t i o n a t low temperature .Danger o f carbonyl formation
Stronger a d s o r p t i o n
NO
P r a c t i c a l l y no ads o r p t i o n on c a r r i e r
O2
Clean a d s o r p t i o n
N2°
Pt Ni
273 77, 195
Pd , P t
273 77, 195
N i ,Fe,Co
;I,
Ni co
Fe3+ 77, 195
.Danger o f o x i d a t i v e . D i f f i c u l t t o manipulate . D i f f e r e n t chemisorbed species .Physical a d s o r p t i o n a t low temperature
Ni,Fe,Co
.Physical a d s o r p t i o n a t l o w temperature .Bulk o x i d a t i o n o f metal o r reduced oxides a t temperature above ambient .Sensitive t o impurities
Pt,Ag N i Cr,Fi+ Mod+, W
77, 195
Cu.Ag
195, 273
2t
77, 195 3+
Ni2+,Fe co
,
No s u l p h i d e f o r m a t i o n .Physical a d s o r p t i o n Complex a d s o r p t i o n mechanism
HZO
i s , must be t h e r e a l q u a n t i t a t i v e term t o be used i n k i n e t i c s t u d i e s ( t u r n o v e r frequency). T i t r a t i o n i s another method used f o r measurement o f metal d i s p e r s i o n . The method, developed by Benson and Boudart (21) f o r supported P t c a t a l y s t s , i s based on t h e chemisorption o f H2 and
O2 on t h e exposed P t atoms as w e l l as on
t h e chemical r e a c t i o n o f H2 w i t h preadsorbed O2 and conversely t h e r e a c t i o n o f O2 w i t h preadsorbed H2. The s t o i c h i o m e t r y o f these processes a r e as f o l l o w s , Pt +
tH2
-
H (H-chemisorption)
(1.3a)
0 (0-chemisorption)
(1.3b)
+
Pt
-+
Pt
2PtH t 3/2 O2
+
2 P t O + H20 ( 0 - t i t r a t i o n )
(1.3~)
Pt-0 t 3/2 H2
+
P t H + H20 ( H - t i t r a t i o n )
(1.3d)
P t t to2
When P t i s supported on alumina, t h e water formed i n 0- and H - t i t r a t i o n s i s adsorbed on t h e c a r r i e r and does n o t i n t e r f e r e t h e t i t r a t i o n i t s e l f . N o t i c e t h a t , according t h e s t o i c h i o m e t r y numbers, t i t r a t i o n s p r o v i d e a v e r y simple procedure t o v e r i f y t h e goodness o f t h e d i r e c t chemisorption t e s t s .
1.2.3.
B a s i c Molecules
A l t h o u g h i t i s n o t t h e aim o f t h i s review, we w i l l b r i e f l y c o n s i d e r how t h e a c i d i c f u n c t i o n o f t h e o x i d e s can be d e t e r m i n e d by means o f t h e most w i d e l y used t e c h n i q u e : i n f r a r e d spectroscopy o f chemisorbed b a s i c probe molecules. The s u b j e c t has been c o n s i d e r e d i n d e t a i l i n s e v e r a l monographs (22-24), and r e f e r e n c e s t o o r i g i n a l papers can be found t h e r e . P y r i d i n e and s u b s t i t u t e d TABLE 1.2. V i b r a t i o n a l modes o f p y r i d i n e ( P y ) chemisorbed on B r i j n s t e d and Lewis ( L ) a c i d s i tesa PyHt
Vibration
Mode
v-CC(N)
A1
1655’
v-CC( N)
%
1627’
v-CC(N)
B1
1550m
v-CC(N)
A1
1490”
Py: L
1695” 1575m 1455-1442’ 1490’
avs = v e r y s t r o n g , s = s t r o n g and m = medium i n t e n s i t y (2,6-dimethyl)pyridine
a r e t h e f a v o r e d molecules t o probe, s e p a r a t e l y , t h e A1203, s i n c e B r i j n s t e d ( I ) and
B r i j n s t e d and Lewis a c i d i t y o f a n oxide, e.g.,
Lewis (11) h e l d p y r i d i n e and substituted-(2,6-dimethyl) p y r i d i n e can e a s i l y be d i s t i n g u i s h e d by t h e i r i n f r a r e d s p e c t r a . The wavenumbers o f d i f f e r e n t v i b r a t i o n a l modes used t o d i s t i n g u i s h between ( I ) and (11) s t r u c t u r e s f o r t h e alumina s u b s t r a t e a r e l i s t e d i n Table 1.2. A1:N
CH3
(o>
CH3
The d e t e r m i n a t i o n of B r o n s t e d and Lewis s i t e s r e q u i r e s t h e a p p l i c a t i o n o f q u a n t i t a t i v e I R spectroscopy. F o r t h i s purpose, s e l f - s u p p o r t i n g wafers o f t h e powdered m a t e r i a l can be pressed, d u s t y mounted i n s p e c i a l c e l l s which a l l o w degassing and thermal t r e a t m e n t s , and c o n t a c t e d w i t h p y r i d i n e vapour. If t h e spectrum i s scanned i n t h e absorbance mode, a b a s e l i n e can be drawn t a n g e n t t o t h e s p e c t r a l areas o f l o w a b s o r p t i o n . By a p p l i c a t i o n o f t h e i n t e g r a t e d form of B e e r ’ s law, t h e Lewis/Brijnsted r a t i o can be c a l c u l a t e d .
B8 1.2.4 E l e c t r o n c o n f i g u r a t i o n o f t h e probe The p a r t i c u l a r s p e c i f i c i t y o f chemisorption o f a probe molecule, namely, 02, CO, and NO, toward t r a n s i t i o n i o n s may l i e i n t h e f a c t t h a t i n t r u e cheniisorp-
t i o n an e l e c t r o n t r a n s f e r process i s i n v o l v e d . Hence, i t seems r e l e v a n t t o consider t h e e l e c t r o n i c c o n f i g u r a t i o n o f t h e probes. For s i m p l i c i t y we c o n s i d e r o n l y t h e most u s e f u l homonuclear (02) and h e t e r o n u c l e a r (CO and NO) d i a t o m i c probes. The treatment o f t h e homonuclear O2 molecule by LACOMO t h e o r y g i v e s an 2 2 2 4 e l e c t r o n c o n f i g u r a t i o n t h a t can be represented by (al) (ol*) (a,) (IT) IT*)^ o r 2 4 ( I J ~ ) ~ ( O , * ) (IT) (o,)'(IT*)~. I n both cases t h e bond o r d e r i s p r e d i c t e d t o be 2, and two unpaired e l e c t r o n s a r e expected. I t i s a l s o i n t e r e s t i n g t o n o t e t h a t t h e a d d i t i o n o f e l e c t r o n s t o O2 causes t h e bond l e n g t h t o i n c r e a s e (do
2
= 0.121 nm;
doi = 0.126 nm; d02- = 0.149 nm). These c a l c u l a t i o n s a r e t h e r e f o r e i n e x c e l l e n t 2 agreement w i t h t h e MO e l e c t r o n c o n f i g u r a t i o n , s i n c e t h e o r b i t a l t o which e l e c t r o n s a r e added i s an a n t i b o n d i n g one. Hence, c o n t r a r y t o t h e usual s i t u a t i o n , t h e a d d i t i o n o f e l e c t r o n s weakens t h e bond ( t h e bond o r d e r s i n 0;
and 02; i o n s
a r e 1.5 and 1.0, r e s p e c t i v e l y ) . T h i s treatment i s extended t o t h e h e t e r o n u c l e a r CO and NO d i a t o m i c molecules, which a r e n o t fundamentally d i f f e r e n t from t h e homonuclear diatomics, except t h a t t h e MO's a r e n o t symmetric r e l a t i v e t o a p l a n e p e r p e n d i c u l a r t o , and bisecting, the internuclear axis. The heteronuclear molecule CO may be regarded as a p e r t u r b e d N2 ( i s o e l e c t r o n i c ) molecule, C and 0, d i f f e r i n g i n atomic number by o n l y two, have atomic o r b i t a l s which a r e q u i t e s i m i l a r ; t h e f o r m a t i o n o f MO's w i l l t h e r e f o r e be almost t h e same as f o r a homonuclear d i a t o m i c ( 2 5 ) , a l t h o u g h t h e energies o f both s e t s o f atomic o r b i t a l s w i l l n o t match e x a c t l y . I n f a c t , t h e oxygen o r b i t a l s w i l l be somewhat more s t a b l e , so t h a t they w i l l c o n t r i b u t e more t o t h e bonding MO's than t h e carbon o r b i t a l s , whereas t h e C o r b i t a l s w i l l c o n t r i b u t e more t o t h e a n t i bonding MO's. Thus, although t h e 10 e l e c t r o n s a r e comprised o f s i x from t h e 0 and f o u r from t h e C, t h e low p o l a r i t y (0.1 Debye) o f t h e molecule i s e x p l a i n e d on t h e b a s i s t h a t e i g h t o f them a r e i n bonding o r b i t a l s where t h e y a r e h e l d c l o s e r t o 0 than t o C, thus t e n d i n g t o n e u t r a l i z e t h e g r e a t e r n u c l e a r charge a t t h e oxygen core. As f o r N 2 , a bond o r d e r o f 3 i s p r e d i c t e d and i s i n agreement w i t h t h e h i g h bond energy (1.07 x
lo3
kJ/mol).
The e l e c t r o n c o n f i g u r a t i o n of NO m i g h t be d e r i v e d by e i t h e r removing one e l e c t r o n from O2 o r adding one t o N2. Using MO t h e o r y , t h e e l e c t r o n i c s t r u c t u r e 2 2 1 o f NO i n t h e ground s t a t e i s u s u a l l y w r i t t e n (26) as (a1) (a1*) ( O , : * ( ~ ) ~ ( I T * ) , t h e l a s t e l e c t r o n e n t e r i n g i n t o an a n t i b o n d i n g o r b i t a l and accounting f o r t h e paramagnetism. Furthermore, t h i s t h e o r y p r e d i c t s t h e bond o r d e r t o be 2.5. and t h a t i t should n o t be t o o d i f f i c u l t t o remove one e l e c t r o n t o form t h e NO' i o n , w i t h a s h o r t e r , s t r o n g e r bond than NO i t s e l f . I n f a c t , these t h r e e p r e d i c t i o n s
B9 a r e c o r r e c t : t h e bond l e n g t h o f NO (0.114 nm) l i e s between t h a t o f a d o u b l e (0.118 nm) and a t r i p l e bond (0.106 nm); t h e i o n i z a t i o n p o t e n t i a l o f 9.25 eV i s a p p r e c i a b l y l o w e r t h a n f o r s i m i l a r molecules (N2, 02, and CO have 15.6, 12.1, and 14.0 eV, r e s p e c t i v e l y ) ; and t h e s t r e t c h i n g frequency o f t h e NO' n i t r o s y l s a l t s (2150-2400 cm-')
ion i n
i s h i g h e r t h a n o f NO i t s e l f (1888 c m - l ) . The NO'
i o n i s a l s o i s o e l e c t r o n i c w i t h N2, C O Y and CN-, which can be c o n s i d e r e d by MO t h e o r y as h a v i n g an e l e c t r o n i c s t r u c t u r e c o n t a i n i n g no a n t i b o n d i n g
IT
electrons
and a bond o r d e r o f 3. Thus, i t i s n o t s u r p r i s i n g t h a t a wide range o f NO complexes analogous t o c a r b o n y l s i s known. I n t h e s e complexes t h e accepted bonding p i c t u r e i s t h a t of d o n a t i o n o f e l e c t r o n d e n s i t y t o t h e metal and backd o n a t i o n by t h e d e l e c t r o n s ( i n t h e f i r s t t r a n s i t i o n - r o w m e t a l s ) i n t o t h e
IT*
o r b i t a l o f NO (23, 1 7 ) . The modes o f NO bonding on metal i o n s depend on parameters such as t h e i r e l e c t r o n i c s t r u c t u r e and t h e i r c o o r d i n a t i o n . D u r i n g NO a d s o r p t i o n on s e v e r a l n o n e q u i v a l e n t i o n s , l i n e a r monomers, dimers, d i n i t r o s y l s , and p o l y n i t r o s y l species were found i n l i t e r a t u r e . Such s t r u c t u r e s have been m a i n l y s t u d i e d by t r a n s m i s s i o n i n f r a r e d spectroscopy. F o r i n s t a n c e , NO has been r e p o r t e d t o form a m i x t u r e o f mononers and dimers on Cr0203/Si02 (28), w h i l e more complex s t r u c t u r e s were found on Cr203/A1203 c a t a l y s t s ( 2 9 ) . Furthermore, t h e c a t a l y s t s may undergo r e a c t i o n s even a t room temperature. One o f t h e most r e l e v a n t examples has been r e p o r t e d b y Davydov and B e l l (30) who found t h a t t h e i n i t i a l a d s o r p t i o n o f NO on reduced Ru/Si02 c a t a l y s t occurs d i s s o c i a t i v e l y and p a r t i a l l y o x i d i z e s
t h e Ru s u r f a c e . F u r t h e r , NO a d s o r p t i o n was observed t o t a k e p l a c e p r e d o m i n a n t l y on such o x i d i z e d species. The above statement i n d i c a t e s t h a t d u r i n g t i t r a t i o n o f t h e surface i o n i c s i t e s i n an o x i d i c c a t a l y s t , t h e e l e c t r o n i c c o n f i g u r a t i o n o f t h e probe m o l e c u l e as w e l l as t h a t o f t h e s u r f a c e c e n t e r t o g e t h e r w i t h i t s c o o r d i n a t i o n a r e t h e most i m p o r t a n t parameters which c o n t r o l s e l e c t i v i t y o f t h e probe f o r a g i v e n surface s i t e . The most d i r e c t l i g h t on t h e e l e c t r o n i c s t r u c t u r e o f chemisorbed CO i s g i v e n by c o n s i d e r i n g t h e
by u l t r a v i o l e t p h o t o e l e c t r o n s p e c t r a (UPS). We b e g i n
c h e m i s o r p t i o n o f CO on N i , because t h i s system has been w i d e l y s t u d i e d (31-35). The UP s p e c t r a f o r CO adsorbed on t h r e e faces o f N i a r e i l l u s t r a t e d i n F i g . 1.2a. N o t i c e t h a t on t h e N i (111) and N i (100) faces t h r e e peaks below t h e N i 5p band a r e r e s o l v e d . Three peaks i n t h e UP spectrum o f t h e gas-phase CO a r e a l s o found and t h e s e peaks a r e as f a r as p o s s i b l e l i n e d up w i t h t h o s e o f t h e adsorbed CO by d i s p l a c i n g t h e gas phase peaks upwards by about 4 eV. The assignment o f t h e gas phase peaks t o t h e CO m o l e c u l a r o r b i t a l s 40, ln, and 50 a r e a l s o marked i n F i g . 1.2a.The MOs b e i n g sketched i n F i g . 1.2.b.
F o r CO chemisorbed on a N i f i l m
t h e dependence on t h e photon frequency up t o 105 eV o f t h e peak a t 11 eV vs. t h a t o f t h e 6-7 eV d o u b l e t shows t h e 11 eV peak t o a r i s e f r o m t h e 40
orbital.
B10
I
(3
1n
40
n
su
I I
1
-15 I
I
-10
I
I
-s
I
I
E-EF(~V)O
t -
40
-
50
I
Nickel surface
F i g . l . 2 . a ) U l t r a v i o l e t p h o t o e l e c t r o n s p e c t r a f r o m CO chemisorbed on t h r e e d i f f e r e n t N i faces a t hv = 40.8 eV. (Readapted f r o m r e f . ( 3 1 ) ) . b ) M o l e c u l a r o r b i t a l s o f CO molecule. c ) Sketch o f energy l e v e l s f o r chemisorbed CO on n i c k e l surfaces. The dependence o f t h e i n t e n s i t y o f t h e peak a t 11 eV on t h e o u t g o i n g e l e c t r o n i n c i d e n c e has been measured f o r t h e Co/Ni (100) system and compared with t h e o r e t i c a l c a l c u l a t i o n s s u g g e s t i n g t h a t t h e 40
peak i s a s s o c i a t e d w i t h a CO
normal t o t h e s u r f a c e and w i t h 0 outwards ( 3 4 ) . W i l l i a m s e t a l . ( 3 5 ) have measured t h e i n t e n s i t y o f a l l t h e peaks f o r CO on N i (100) as a f u n c t i o n o f t h e a n g l e o f i n c i d e n c e o f t h e o u t g o i n g e l e c t r o n . The c o n t r a s t between t h e s i m i l a r
B11 behaviour o f t h e two most s t r o n g l y bound peaks and t h a t o f t h e weakly bound peak suggests t h e assignment o f t h e peaks t o be 4u, 50, and l a as a f u n c t i o n o f decreasing b i n d i n g energy. The UP s p e c t r a o f CO on o t h e r n o n - d i s s o c i a t i v e c a t a l y s t s , such as Pd (111) (33, 3 6 ) , Pd (110) ( 3 2 ) , Ru (100) ( 3 7 ) and I r (100) (38) and even o f t h e c a r b o n y l s Rh6(C0)16 ( 3 2 ) and Ir4(C0)12 ( 3 3 ) resemble t h a t f o r CO on N i t o a c e r t a i n e x t e n t , a l t h o u g h n o t a l l a u t h o r s r e s o l v e t h e 50-111 s p l i t t i n g . The work o f Fuggle e t a1.(37) on t h e system CO/Ru (100) i s i m p o r t a n t i n h a v i n g e s t a b l i s h e d t h e assignment o f t h e most s t r o n g l y UPS peak by an a n g l e - r e s o l v e d technique.
A s i m p l i f i e d p i c t u r e o f t h e bonding o f CO t o t h e s u r f a c e i s i n an a t o p p o s i t i o n . I t i s assumed t h a t t h e main CO-metal i n t e r a c t i o n i s v i a t h e 5a l o n e p a i r o r b i t a l , d o u b l y occupied i n CO gas. I t i s f i r s t necessary t o promote an e l e c t r o n from t h e 50 o r b i t a l t o t h e empty 2n* o r b i t a l , a s t e p r e q u i r i n g
the
r a t h e r l a r g e energy o f about 6 eV. T h i s would be analogous t o t h e d i s s o c i a t i o n o f t h e H2 molecule i n i t s 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 . The s i n g l y o c c u p i e d 5u o r b i t a l i s now analogous t o an H 1s o r b i t a l , and may i n t e r a c t w i t h metal v a l e n c e s t a t e s t o form a bonding combination, which w i l l be doubly occupied. T h i s energy diagram i s sketched i n F i g . 1 . 2 ~ . There i s assumed t o be a r a t h e r l a r g e upward r e l a x a t i o n s h i f t o f a l l t h e CO o r b i t a l on approach o f t h e CO gas m o l e c u l e t o t h e s u r f a c e . The s h i f t down i n t h e 5 a o r b i t a l on bonding t o t h e surface i s a t t r i b u t e d t o t h e f o r m a t i o n o f a bonding s t a t e . When comparing p h o t o e l e c t r o n s p e c t r a w i t h t h e o r e t i c a l i n t e r p r e t a t i o n , one must bear i n m i n d t h e p r o b a b i l i t y deduced from t h e s p e c t r a o f more t h a n one b i n d i n g s i t e under g i v e n e x p e r i m e n t a l conditions. 1.3 QUANTITATIVE DETERMINATION 1.3.1
Chemi s o r p t ion
S p e c i f i c c h e m i s o r p t i o n methods have found c o n s i d e r a b l e use i n t h e s e l e c t i v e a d s o r p t i o n o f a gas-probe m o l e c u l e o n t o t h e a c t i v e components of a s u p p o r t e d c a t a l y s t . These methods, u n l i k e those o f p h y s i c a l a d s o r p t i o n , t h e most common o f which i s t h e BET procedure (39), a r e d i s c r i m i n a t e i n t h a t , i d e a l l y , t h e adsorpt i o n occurs o n l y o n t o a p a r t i c u l a r component o f t h e c a t a l y t i c system. The probe molecule ( a d s o r b a t e ) and t h e temperature and p r e s s u r e must be c a r e f u l l y cons i d e r e d t o achieve d e s i r a b l e d i s c r i m i n a t i o n . F r e q u e n t l y , c o r r e c t i o n s f o r ads o r p t i o n o n t o t h e i n a c t i v e p o r t i o n o f t h e system, t h a t i t i s , i n a b l a n k exp e r i m e n t w i t h support, o n l y a r e r e q u i r e d . The t h e o r y o f c h e m i s o r p t i o n i s q u i t e w e l l e s t a b l i s h e d , and d e t a i l s have been p r e s e n t e d i n e x c e l l e n t monographs (4042).
B12
7
10
11
F i g . l . 3 . S t a t i c a d s o r p t i o n apparatus: 1, v o l u m e t r i c r e a c t o r ; 2, c a l i b r a t e d volume; 3, capacitance-pressure transducer; 4, r o t a r y vacuum pump; 5, o i l - d i f f u s i o n pump; 6, i o n i z a t i o n gauge; 7, P i r a n i gauge; 8. gas r e s e r v o i r s ; 9, gas entrance; 10, e l e c t r o n i c vacuum microbalance; 11, d e t a i l o f t h e g r a v i m e t r i c r e a c t o r . Open c i r c l e s a r e high-vacuum backeable v a l v e , and f u l l c i r c l e s a r e Hoke vacuum valves. The e x t e n t o f gas a d s o r p t i o n i s a b a s i c parameter r e q u i r e d i n a d s o r p t i o n s t u d i e s . E i t h e r t h e e q u i l i b r i u m amount adsorbed o r t h e r a t e o f a d s o r p t i o n (adsorbed amount vs t i m e ) i s measured as a f u n c t i o n o f temperature and time. The amount adsorbed may be c a l c u l a t e d from t h e v a r i a t i o n s o f t h e gas pressure i n a c a l i b r a t e d volume ( v o l u m e t r i c d e t e r m i n a t i o n ) o r from v a r i a t i o n s of t h e weight o f t h e c a t a l y s t sample i n a s t a t i c o r continuous- f l o w apparatus ( g r a v i m e t r i c d e t e r m i n a t i o n ) . An a d s o r p t i o n apparatus i s s t a t i c when t h e gas i s brought i n t o c o n t a c t w i t h t h e c a t a l y s t sample i n successive doses,either d i r e c t l y ( c l a s s i c a l v o l u m e t r i c method) o r through a c a p i l l a r y ( f l o w method). I n a dynamic apparatus, t h e gas flows over t h e c a t a l y s t sample f o r t h e d u r a t i o n o f t h e experiment. A l l o f these methods, t o g e t h e r with t h e p u l s e method, a r e considered i n d e t a i l i n
the following sections.
1.3.1.1. Volumetric Methods With v o l u m e t r i c methods t h e amount o f t h e gas adsorbed i s determined from t h e v a r i a t i o n o f t h e gas pressure i n a known volume. These methods have been used f o r chemisorption measurements f o r t h e l a s t 50 years, and a r e s t i l l t h e most popular and common techniques. U s u a l l y made o f g l a s s , t h e y a r e equipped w i t h a p p r o p r i a t e vacuum devices, such as d i f f u s i o n and roughing pumps, pressuremeasuring devices, vacuum l i n e , detachable c a t a l y s t r e a c t o r from which t h e c a t a l y s t can be taken, and f a c i l i t i e s f o r i n t r o d u c i n g gases (see, e.g.,
Refs. 40,
43-50). A combined v o l u m e t r i c - g r a v i m e t r i c system i s g i v e n i n F i g . 1.3. Modern
B13 equipment i n c l u d e s t h e e l i m i n a t i o n o f mercury f r o m t h e vacuum l i n e , t h e use o f g r e a s e l e s s stopcocks o r v a l v e s , t u r b o m o l e c u l a r pumps, more a c c u r a t e t e m p e r a t u r e c o n t r o l and p r e s s u r e transducers, and so on. The general p r i n c i p l e i n v o l v e s measuring t h e amount o f gas r e m a i n i n g i n t h e system a f t e r c o n t a c t w i t h t h e c a t a l y s t sample. By knowing t h e amount o f gas i n i t i a l l y present i n a c a l i b r a t e d
volume and
s u b t r a c t i n g f r o m i t t h e amount
remaining a f t e r e q u i l i b r i u m w i t h t h e c a t a l y s t sample, t h e e x t e n t o f a d s o r p t i o n can be o b t a i n e d . U s u a l l y an e l e c t r o n i c p r e s s u r e - t r a n s d u c e r i s used t o f o l l o w t h e p r e s s u r e changes caused by gas a d s o r p t i o n on t h e s o l i d s u r f a c e . Pressure changes i n t h e s t a t i c system, a t pressures n o r m a l l y b e l l o w atmospheric, would t h e n be p r o p o r t i o n a l , t h r o u g h t h e i d e a l gas laws, t o t h e a d s o r p t i o n o f a g i v e n amount o f gas. The c a l i b r a t e d volume t r a n s d u c e r and r e a c t o r volumes must be a c c u r a t e l y determined. P r i o r t o t h e a d s o r p t i o n t e s t , i t i s common p r a c t i c e t o p r e t r e a t o r c o n d i t i o n t h e c a t a l y s t s u r f a c e . F r e q u e n t l y , h i g h temperatures, about 770 K, and h i g h vacuum (-10- 6 t o r r , 1 t o r r = 133.3 N m- 2 ) a r e used. These s t r o n g c o n d i t o n s a r e n o r m a l l y r e q u i r e d because contaminant molecules, e.g.,
w a t e r , carbon d i o x i d e ,
e t c . , a r i s i n g from t h e c a t a l y s t p r e p a r a t i o n and hand1 i n g may remain s t r o n g l y h e l d by t h e s u r f a c e . A f t e r p r e t r e a t m e n t and c o o l i n g t o t h e a d s o r p t i o n temperat u r e , i t i s necessary t o determine t h e dead volume o f t h e r e a c t o r . Helium ( i n e r t gas) i s most o f t e n used f o r t h i s purpose. A f t e r e v a c u a t i o n , t h e gas probe i s dosed by means o f t h e c a l i b r a t e d volume, and t h e p r e s s u r e i s noted. I t i s t h e n expanded i n t h e r e a c t o r chamber, and t h e p r e s s u r e i s m o n i t o r e d u n t i l e q u i l i b r i u m i s reached. The p r e s s u r e o v e r t h e sample can t h e n be i n c r e a s e d w i t h new doses and readings a g a i n taken u n t i l a new e q u i l i b r i u m i s e s t a b l i s h e d . T h i s procedure i s r e p e a t e d t i l l t h e d e s i r e d p o r t i o n o f t h e i s o t h e r m i s complete. I n p r a c t i c e , t h e amount o f gas (volume i n cm3 STP o r micromoles) adsorbed a t c o n s t a n t temperature i s g i v e n as a f u n c t i o n o f t h e e q u i l i b r i u m p r e s s u r e (ads o r p t i o n i s o t h e r m ) . I f a Langmuir-type a d s o r p t i o n i s o t h e r m i s found ( F i g . 1.4a), i t i s easy t o c a l c u l a t e t h e amount o f gas r e q u i r e d t o form an u n i m o l e c u l a r l a y e r
on t h e c a t a l y s t s u r f a c e by e x t r a p o l a t i n g t h e l i n e a r p o r t i o n o f t h e i s o t h e r m t o z e r o p r e s s u r e . However, i n supported-metal o x i d e ( o r metal 1 i c ) c a t a l y s t s t h i s i d e a l i z e d i s o t h e r m i s r a r e l y obeyed; e x p e r i m e n t a l isotherms do n o t show a c l e a r l y 1 i n e a r h o r i z o n t a l p a r t . Furthermore, t h e o v e r a l l a d s o r p t i o n i s o t h e r m s r e s u l t as a c o n t r i b u t i o n o f t h e a d s o r p t i o n o f t h e probe m o l e c u l e on t h e c a r r i e r and on t h e a c t i v e phases o r promoters. I n o r d e r t o overcome t h e s e d i f f i c u l t i e s o r u n c e r t a i n t i e s , t h e most commonly p r a c t i c e d method i s t h e one i n i t i a l l y d e v i s e d by Brunauer (51), i l l u s t r a t e d i n F i g . 1.4band r e c e n t l y r e v i t a l i z e d by W e l l e r (52-55).
I t c o n s i s t s o f t h e d e t e r m i n a t i o n o f t h e o v e r a l l isotherms
( p h y s i c a l a d s o r p t i o n + c h e i i i i s o r p t i o n ) , subsequent o u t g a s s i n g t o remove t h e p h y s i c a l l y adsorbed amount, and a second i s o t h e r m ( p h y s i c a l a d s o r p t i o n ) under
B14
u Equilibrium pressure
Equilibrium pressure
F i g . 1.4. T y p i c a l measurement o f a d s o r p t i o n . ( a ) E x t r a p o l a t i o n o f t h e h o r i z o n t a l p a r t o f t h e Langmuir-type i s o t h e r m t o z e r o p r e s s u r e . ( b ) I , C h e m i s o r p t i o n + p h y s i c a l a d s o r p t i o n ; 11, p h y s i c a l a d s o r p t i o n . C h e m i s o r p t i o n i s determined b y t h e I 1 i n t h e e q u i l i b r i u m p r e s s u r e range where b o t h i s o t h e r m s a r e difference I e s s e n t i a l l y para1 l e l
-
.
t h e same e x p e r i m e n t a l c o n d i t i o n s . The d i f f e r e n c e between t h e f i r s t and t h e second i s o t h e r m g i v e s t h e e x t e n t o f i r r e v e r s i b l e a d s o r p t i o n . The main advantage o f s t a t i c v o l u m e t r i c methods i s t h e p o s s i b i l i t y o f comb i n i n g them w i t h o t h e r methods o f s t u d y i n g t h e a d s o r p t i o n . I n s t a t i c equipment, b o t h small and l a r g e amounts o f adsorbed gas and r a t e s o f a d s o r p t i o n may be r e a d i l y measured. When t h e e x t e n t o f gas a d s o r p t i o n i s measured a t l o w temperat u r e s , i t i s e x t r e m e l y d i f f i c u l t and t i m e consuming t o a t t a i n e q u i l i b r i u m u s i n g v o l u m e t r i c o r g r a v i m e t r i c methods. However, t h e p r e c i s i o n o f t h e measurement can be g r e a t l y increased, and t h e t i m e r e q u i r e d can be shortened, by measuring t h e amount adsorbed a t h i g h e r t e m p e r a t u r e and t h e n l o w e r i n g t h e t e m p e r a t u r e w h i l e keeping t h e amount adsorbed c o n s t a n t b y m o n i t o r i n g t h e gas p r e s s u r e ( d i r e c t measurement o f t h e a d s o r p t i o n i s o s t e r e s ( 5 7 ) ) . 1.3.1.2.
G r a v i m e t r i c Methods
I n t h e g r a v i m e t r i c methods t h e e x t e n t o f a d s o r p t i o n i s measured d i r e c t l y by w e i g h i n g t h e c a t a l y s t sample. The w e i g h t o f t h e adsorbed probe m o l e c u l e t o g e t h e r w i t h t h e w e i g h t o f t h e c a t a l y s t sample may be determined w i t h a m i c r o b a l a n c e i n one o f two ways: (1) by measuring t h e d e v i a t i o n o f t h e b a l a n c e from i t s z e r o p o s i t i o n and c a l c u l a t i n g t h e f o r c e r e s p o n s i b l e f o r t h i s d e v i a t i o n , t h e s e n s i t i v i t y o f t h e i i i i c r o b a l a n c e b e i n g known; and ( 2 ) by compensating f o r t h e d e v i a t i o n f r o m t h e z e r o p o s i t i o n by means o f a known f o r c e , e.g.,
a mechanical f o r c e such
as t h e s t r e t c h i n g o f a s p r i n g (58) o r a t o r s i o n a l f o r c e ( 5 9 ) , an e l e c t r i c f o r c e ( 6 0 ) . o r a magnetic f o r c e (59, 61-64). The d e v i a t i o n o f t h e m i c r o b a l a n c e from
i t s z e r o p o s i t i o n may be d e t e c t e d by d i f f e r e n t procedures which g i v e s p e c i f i e d
B15 s e n s i t i t y l e v e l s . The accuracy o f t h e g r a v i m e t r i c d e t e r m i n a t i o n of t h e e x t e n t o f a d s o r p t i o n o f a probe m o l e c u l e i s determined by t h e s e n s i t i v i t y o f t h e m i c r o balance a t a g i v e n l o a d i n g and by t h e p r e c i s i o n w i t h which i t s c h a r a c t e r i s t i c p r o p e r t y , e.g.,
l e n g t h , angle, o r compensating device, can be measured.
S i n c e t h e a d s o r p t i o n microbalances must be v e r y s e n s i t i v e , even v e r y small e x t e r n a l f o r c e s may cause i n t e r f e r e n c e s . I n o r d e r t o m i n i m i z e o s c i l l a t i o n s o f t h e microbalance when i n use, i t may be f i x e d i n t o a s u p p o r t i n g frame on a s o l i d w a l l o r i t s j a c k e t may be a t t a c h e d t o t h e s u p p o r t i n g c o n s t r u c t i o n by dampers. The o p e r a t i o n o f a d s o r p t i o n microbalances may be u n f a v o r a b l y a f f e c t e d by e l e c t r o s t a t i c charges on t h e w a l l s o f t h e p r o t e c t i o n t u b e s . These charges may be e l i m i n a t e d e i t h e r by c o v e r i n g t h e w a l l s c o m p l e t e l y w i t h a c o n d u c t i v e f i l m , e.g.,
A l , o r by p l a c i n g a n i o n i z i n g r a d i a t i o n s o u r c e i n s i d e t h e volume o f t h e m i c r o balance. I n experiments a t v a r i a b l e pressure, buoyancy changes have t o be t a k e n i n t o account, e s p e c i a l l y a t h i g h pressures ( 6 5 ) , t h e compensation b e i n g p e r formed e i t h e r by c a l c u l a t i o n (58, 63, 65) o r i n a b l a n k experiment. I n t h e l a t t e r case a nonporous m a t e r i a l , whose d e n s i t y i s almost t h e same as t h a t o f t h e c a t a l y s t sample, should be used. When a d s o r p t i o n measurements a r e performed a t temperatures c o n s i d e r a b l y d i f f e r e n t from room temperature, i n t e r f e r i n g e f f e c t s due t o gas f l o w and h e a t t r a n s f e r must be avoided. When a d s o r p t i o n i s o t h e r m s a r e measured, t h e r e a l p r e s s u r e o f t h e gas o v e r t h e sample must be known, i n a d d i t i o n t o t h e e x t e n t o f a d s o r p t i o n , and t h e r e f o r e t h e i n f l u e n c e o f thermal t r a n s p i r a t i o n has t o be c o n s i d e r e d ( 6 6 ) . The t h e o r e t i c a l a n a l y s i s o f t h e f o r c e s generated by t h i s thermal t r a n s p i r a t i o n e f f e c t on t h e arm-sample s i d e o f t h e m i c r o b a l a n c e has been c a r r i e d o u t by P o u l i s e t a l . (67, 68), who compared t h e i r e x p e r i m e n t a l d a t a w i t h ) some t h e o r e t i c a l r e s u l t s i n b o t h t h e f r e e m o l e c u l a r (Knudsen's number > > l and continuous (Knudsen's number < < l )regime l i m i t s . However, i t i s i n t h e t r a n s i t i o n regime (Knudsen's 2 l ) , where most a d s o r p t i o n experiments a r e performed, t h a t t h e most i n t e r e s t i n g e f f e c t s appear. F o r an i n e r t sample t h e mass change vs p r e s s u r e p l o t s a r e c h a r a c t e r i s t i c s volcano curves (60, 67) w i t h t h e maximum p l a c e d near 10 N rn-'(Fig.1.5).A q u a n t i t a t i v e d e s c r i p t i o n o f these volcano curves f o r t h e e n t i r e range o f Knudsen's number has been made by L o y a l k a (70, 71) on t h e b a s i s o f a complete k i n e t i c t h e o r y t r e a t m e n t u s i n g t h e BoltzmannGauss-Kassel model, and a p p l i e d ( 6 9 ) t o a few gases (He, K r , H2, O2 and C02). G r a v i m e t r i c methods have a number o f advantages f o r a d s o r p t i o n s t u d i e s . The main advantage i s t h a t t h e e x t e n t ( o r k i n e t i c s ) o f a d s o r p t i o n i s d i r e c t l y measured. Thus, e f f e c t s due t o gas a d s o r p t i o n i n o t h e r p a r t s o f t h e a p p a r a t u s do n o t i n f l u e n c e t h e r e s u l t s . Furthermore, i n g r a v i m e t r i c methods t h e dead volume o f t h e apparatus does n o t a f f e c t t h e p r e c i s i o n o f measurements as i t does i n v o l u m e t r i c methods. I t i s , t h e r e f o r e , p o s s i b l e t o r e c o r d t h e e x t e n t ( o r k i n e t i c s ) o f a d s o r p t i o n ( o r d e s o r p t i o n ) as a f u n c t i o n o f temperature, pressure, and t i m e .
B16
C 01
=-0.3 E
i 0.2
0.1
0
lo-’
100
10’
102 103 Pressure (Nm-2)
104
Fig.l.5.Volcano p l o t s o f t h e l o n g i t u d i n a l f o r c e s i n t h e Knudsen t r a n s i t i o n regime f o r d i f f e r e n t gases. (0) Hydrogen, T2 = 77 K. (A) Oxygen, T2 = 88 K. (0) Carbon d i o x i d e , T2 = 195 K. W i t h a g i v e n m i c r o b a l a n c e t h e e x t e n t o f a d s o r p t i o n - d e s o r p t i o n may be g r a v i m e t r i c a l l y determined w i t h t h e same p r e c i s i o n o v e r a w i d e range o f p r e s s u r e s , i n c l u d i n g p> 1 atm, which, t h e r e f o r e , p e r m i t s h i g h - p r e s s u r e a d s o r p t i o n s t u d i e s (65, 7 2 ) . I t i s e q u a l l y u s e f u l t o a p p l y g r a v i m e t r i c methods t o s t a t i c as w e l l as dynamic t y p e s o f apparatus. Another advantage o f t h e s e methods i s t h e r e l a t i v e l y s h o r t t i m e r e q u i r e d t o measure t h e e x t e n t o f adsorbed ( o r desorbed) gas. The main disadvantage o f t h e g r a v i m e t r i c methods i s t h e c o m p l i c a t e d d e s i g n o f t h e balances ( w i t h t h e e x c e p t i o n o f s p r i n g b a l a n c e s ) and t h e c o m p l i c a t e d m d e o f operation, p a r t i c u l a r l y working w i t h high s e n s i t i v i t y
r e q u i r e m e n t s . Another
disadvantage i s t h e s u s c e p t i b i l i t y t o e x t e r n a l i n f l u e n c e s . The g r e a t e s t d i s a d vantage o f a l l
g r a v i m e t r i c methods, however, i s t h e d i f f i c u l t y i n s i m u l -
t a n e o u s l y u s i n g o t h e r methods o f s t u d y i n g a d s o r p t i o n . The o n l y e x c e p t i o n s a r e methods u s i n g r a d i a t i o n , i o n s , o r e l e c t r o n s . I n t h i s r e s p e c t , an e x p e r i m e n t a l arrangement w i t h e x c e l l e n t p r o s p e c t s i n s u r f a c e r e s e a r c h has r e c e n t l y been r e p o r t e d by Czanderna e t a l . ( 7 3 ) , where t h e system i s capable o f combining t h e measurements o f w e i g h t change, elemental c o m p o s i t i o n by Auger spectroscopy, and r e s i d u a l gas a n a l y s i s b y mass s p e c t r o m e t r y . Several a d s o r p t i o n m i c r o b a l a n c e s and t h e i r b a s i c parameters a r e summarized i n Table1.3.It
can be seen f r o m T a b l e l 3 . t h a t s p r i n g balances do n o t u s u a l l y have
t h e same maximum s e n s i t i v i t y as beam ones, and t h a t t h e r a t i o o f t h e maximum l o a d t o t h e measuring range i s l o w e r f o r s p r i n g balances t h a n f o r beam ones. The g r e a t advantage o f beam m i c r o b a l a n c e s i s t h e i r c o n s i d e r a b l e simp1 i c i t y o f d e s i g n and o p e r a t i o n .
B17 TABLE 1.3. A d s o r p t i o n Microbalances Sensitivity
Maximum l o a d
(kg)
weighted ( k g )
Type Beam
2.5
Beam
10-l~
1 x 10-l0
Beani
2.5 x 10-l'
2
Refs.
5
1x
24, 62
6 x
63
Spring
2 x
1.5 x
28
Spring
1x
1
64
lo-'
Null zero
1x
N u l l zero
5 x 10-l'
1.3.1.3.
1
81
6 x
82
Continuous Flow Method
The f l o w c h e m i s o r p t i o n method was designed p r i m a r i l y f o r r o u t i n e measurements o f c a t a l y s t d i s p e r s i o n , e i t h e r o f m e t a l s o r o f metal o x i d e s . A l t h o u g h s i m i l a r i n p r i n c i p l e t o o t h e r dynamic a d s o r p t i o n t e c h n i q u e s (41, 74, 7 5 ) , t h e c u r r e n t approach has c e r t a i n advantages o v e r p l u g - f l o w methods. It b a s i c a l l y c o n s i s t s o f a stream o f gas composed o f ( i n e r t ) c a r r i e r gas and a probe gas which f l o w s t h r o u g h a p r e v i o u s l y evacuated c a t a l y s t - s a m p l e , u s u a l l y purged w i t h t h e c a r r i e r gas a t h i g h temperature, t h e c o n c e n t r a t i o n o f t h e probe m o l e c u l e b e i n g m o n i t o r e d f r e q u e n t l y w i t h a thermal c o n d u c t i v i t y c e l l . Experimental c o n d i t i o n s a r e a d j u s t e d so t h a t no gas adsorbs and t h e b r i d g e i s balanced. When c o n d i t i o n s a r e such t h a t gas w i l l adsorb, t h e d e t e c t i o n system responds l i n e a r l y t o changes i n t h e c o m p o s i t i o n o f t h e gas m i x t u r e . Although a b l a n k experiment i s r e q u i r e d t o e l i m i n a t e t h e e f f e c t o f dead volume, t h i s measurement i s made on t h e same sample used i n t i t r a t i o n , and t h e r e f o r e cancels o u t any a d s o r p t i o n on the support. The f l o w apparatus, shown s c h e m a t i c a l l y i n F i g . 1 . 6 . i ~ assembled f r o m a v a i l a b l e hardware used i n chromatographic s t u d i e s ( 7 6 ) . To measure t h e e x t e n t o f a d s o r p t i o n o f t h e gas probe, t h e f l o w i s s w i t c h e d f r o m t h e i n e r t c a r r i e r t o t h e gas p r o b e - i n e r t c a r r i e r m i x t u r e by means o f a four-way v a l v e . The stream c o m p o s i t i o n i s f r e q u e n t l y m o n i t o r e d w i t h a thermal c o n d u c t i v i t y c e l l . I n e r t - g a s p u r g i n g i s needed t o remove t h e weakly adsorbed f r a c t i o n o f t h e gas probe on t h e s u p p o r t . Subsequently, t h e f l o w i s a g a i n s w i t c h e d t o t h e gas p r o b e - i n e r t c a r r i e r m i x t u r e t o determine t h e a d s o r p t i o n on t h e s u p p o r t p l u s t h e dead volume. T h i s method has a l l t h e advantages o f o t h e r f l o w methods, such as e l i m i n a t i o n o f vacuum systems, mercury vapour,and o t h e r common contaminants. Another i m p o r t a n t advantage o f t h e method i s r a p i d d e t e r m i n a t i o n . However, d i f f u s i o n a l
1 i m i t a t i o n s and s l o w s i g n i f i c a n t c h e m i s o r p t i o n processes may n o t be r e a d i l y d e t e c t e d . The o v e r a l l a d s o r p t i o n o f t h e measurement may a l s o i n c l u d e some
B18
F i g . 1.6. Chromatographic apparatus: 1, chromatographic column; 2, dosage system f o r e l u t i o n chromatography; 3, needle valves f o r f l o w c o n t r o l ; 4, apparatus f o r t h e p u r i f i c a t i o n o f t h e c a r r i e r gas; 5, pressure r e d u c t i o n v a l v e ; 6, valve; 7, r e s e r v o i r f o r t h e gas under study; 8, h e a t c o n d u c t i v i t y d e t e c t o r s ; 9, f l o w meters. p a r t i c i p a t i o n o f p h y s i c a l adsorption. However, t h i s can be minimized by proper temperature and pressure adjustment. An i n t e r e s t i n g set-up and procedure, w i t h promising prospects, f o r measuring t h e a c t i v e s u r f a c e area of supported metal oxides ( o r m e t a l s ) i n t h e low temperature range where p h y s i c a l a d s o r p t i o n dominates has r e c e n t l y been g i v e n by M i l l e r and Lee ( 7 7 ) . 1.3.1.4.
Pulse Flow Method
The pulse technique d e r i v e s from t h e f l o w technique. I t i s described i n many papers, b u t E b e r l y ' s pioneer work (78, 79) i s , perhaps, a l s o t h e b e s t i n d e s c r i b i n g both i t s mathematical and experimental aspects i n c o n s i d e r a b l e d e t a i l . The usual procedure i s t o i n j e c t a p r e c i s e gas volume (probe molecule + c a r r i e r gas) o f known chemical composition i n t o t h e stream o f t h e c a r r i e r gas which f l o w s through t h e c a t a l y s t bed. I f t h e probe gas i s completely taken up by t h e c a t a l y s t surface, t h e d e t e c t i o n system, n o r m a l l y a thermal c o n d u c t i v i t y c e l l , w i l l n o t sense any change i n thermal c o n d u c t i v i t y . When t h e n e x t p u l s e f l o w s through and o n l y a p o r t i o n i s taken up, t h e d e t e c t o r responds i n proport i o n t o t h e amount adsorbed. When s a t u r a t i o n o f t h e c a t a l y s t s u r f a c e i s achieved, subsequent pulses o f gas w i l l n o t be taken up (Fig.1.7a) adsorbed w i l l be unchanged (Fig.l.7b). m i x t u r e (probe gas
and t h e o v e r a l l amount
Knowing t h e chemical composition o f t h e
+ c a r r i e r ) and t h e number o f pulses r e q u i r e d f o r s a t u r a t i o n
a l l o w s c a l c u l a t i o n o f t h e e x t e n t o f chemisorption. The major disadvantage o f t h i s technique i s t h a t t h e weakly chemisorbed p o r t i o n o f t h e probe gas i s n o t h e l d by t h e c a t a l y s t and t h e r e f o r e low uptakes a r e obtained. I n o r d e r t o a v o i d t h i s problem, a s l i g h t l y d i f f e r e n t technique can be used t o d e t e c t t h e r e v e r s i b l e a d s o r p t i o n processes. I n t h i s case a p u l s e
B19
1
1
2
3
4
5
6
7
8
Pulse number
Fig.1.7. T y p i c a l chromatogram f o r t h e c h e m i s o r p t i o n o f probe m o l e c u l e s . ( a ) E l u t i o n mode. ( b ) I n t e g r a t e d c h e m i s o r p t i o n amounts. c o n s i s t i n g of a m i x t u r e o f c a r r i e r , e.g.,
A r , and t h e probe gas i s i n j e c t e d i n t o
t h e i n e r t c a r r i e r (He) stream. The c o n d u c t i v i t y c e l l w i l l now respond t o e i t h e r
A r o r t h e probe molecule. I f o n l y one e f f l u e n t p u l s e i s recorded, i t i s e v i d e n t t h a t most o f t h e probe molecules t r a v e l e d t h r o u g h t h e column a t t h e same r a t e as
A r and, hence, no a d s o r p t i o n o c c u r r e d . On t h e o t h e r hand, i f t h e probe m o l e c u l e s a r e r e t a r d e d i n t h e i r passage by a d s o r p t i o n on t h e c a t a l y s t s u r f a c e , two peaks
w i l l r e s u l t , t h e f i r s t b e i n g t h a t o f A r and t h e second t h a t o f t h e probe. I n t h i s manner, r e v e r s i b l e a d s o r p t i o n can be observed f o r systems e x h i b i t i n g o n l y a v e r y small a d s o r p t i o n c a p a c i t y . I t has been found (79) t h a t t h e movement o f t h e maximum o f a p u l s e t h r o u g h
a packed column obeys t h e e q u a t i o n
where
L i s t h e l e n g t h o f t h e packed column, t, i s t h e r e t e n t i o n t i m e o f t h e
p u l s e maximum, v1 i s t h e l i n e a r gas v e l o c i t y ( v e l o c i t y t h a t would r e s u l t i f t h e column were c o m p l e t e l y empty), and Ka i s t h e a d s o r p t i o n e q u i l i b r i u m c o n s t a n t . Ka i s d i r e c t l y p r o p o r t i o n a l t o t h e s l o p e o f t h e a d s o r p t i o n i s o t h e r m , and i s
a t r u e c o n s t a n t o n l y f o r those systems w i t h H e n r i a n ( l i n e a r ) a d s o r p t i o n i s o therms. T h i s c o n s t a n t can be t r e a t e d as a thermodynamic e q u i l i b r i u m c o n s t a n t , and b y making t h e a p p r o p r i a t e s u b s t i t u t i o n i n Eq. (4), t h e f o l l o w i n g e x p r e s s i o n i s obtained:
B20
l o g t,
=
A
-
AH/4.576 T
where A i s a constant which depends on t h e e n t r o p y o f adsorption, t h e dimensions of t h e column, and t h e c a r r i e r gas f l o w r a t e ; and AH i s t h e a d s o r p t i o n heat. By keeping constant t h e f a c t o r s which have i n f l u e n c e on t h e parameter A, a p l o t o f t h e l o g a r i t h m o f t h e r e t e n t i o n time ( c o r r e c t e d t o STP c o n d i t i o n s ) a g a i n s t t h e r e c i p r o c a l temperature ( K ) should y i e l d a s t r a i g h t l i n e whose slope i s proport i o n a l t o t h e a d s o r p t i o n heat. T h i s method i s o f g r e a t i n t e r e s t because i t i s a very simple way o f o b t a i n i n g AH a t h i g h temperatures. The a d s o r p t i o n heats obtained i n t h i s manner a r e b e l i e v e d t o be average values o v e r t h e range o f part i a l pressures explored. Greene and Pust (80), working a t l o w temperatures, found good agreement between t h e a d s o r p t i o n heats evaluated i n t h i s way and those obtained from t h e i s o s t e r i c and c a l o r i m e t r i c methods.
1.3.2.
Physicochemical Methods
1.3.2.1.
X-Ray Techniques
There a r e two main methods which use x - r a y techniques f o r t h e e s t i m a t i o n o f average p a r t i c l e size. These are: (1) d i f f r a c t i o n l i n e broadening (KLBA), which i s based on t h e a n a l y s i s o f t h e peak shape o f one o r more d i f f r a c t i o n l i n e s o f t h e sample; and ( 2 ) small-angle s c a t t e r i n g (SAXS), which used t h e i n f o r m a t i o n obtained from t h e x-rays s c a t t e r e d by t h e inhomogeneities o f t h e samples. These techniques w i l l be o u t l i n e d b r i e f l y below.
1.3.2.1.1.
X-Ray L i n e Broadening (XLBA)
X-ray d i f f r a c t i o n l i n e s broaden when t h e c r y s t a l l i t e s i z e f a l l s below about 100 nm. This technique i s a p p l i c a b l e t o metal o x i d e ( o r m e t a l ) c r y s t a l l i t e s o f
3.5-60 nm; below 3.5 nm t h e l i n e i s v e r y broad and d i f f u s e o r i s even absent, w h i l e above about 60 nm t h e change i n l i n e s h a p e i s t o o small. There a r e f a c t o r s o t h e r t h a n p a r t i c l e s i z e which can c o n t r i b u t e t o t h e observed (experimental) l i n e w i d t h . I n f a c t , o t h e r causes o f 1 i n e broadening e x i s t , among which t h e most i m p o r t a n t i s t h e c o n t r i b u t i o n o f l a t t i c e s t r a i n s . A complete mathematical a n a l y s i s o f powder d i f f r a c t i o n data a l l o w i n g f o r t h e
separate e s t i m a t i o n o f p a r t i c l e s i z e and l a t t i c e s t r a i n s has been developed by Warren and Averbach (83). Furthermore, when two a c t i v e phases can form s o l i d s o l u t i o n s , e.g., CuNi a l l o y s on s i l i c a (84), t h e i n d i v i d u a l p a r t i c l e s w i l l have compositions which s c a t t e r around t h e average composition. L i n e broadening i s then, i n p a r t , due t o t h e d i f f e r e n c e i n l a t t i c e parameters o f t h e i n d i v i d u a l p a r t i c l e s . The s i m p l e s t approach i s t o assume t h a t t h e p a r t i c l e s i z e c o n t r i b u t e s m a i n l y t o t h e l i n e w i d t h i n excess o f t h e i n s t r u m e n t a l w i d t h . The a n a l y s i s o f t h i s s i t u a t i o n i s v e r y simple. I f we assume t h a t t h e l i n e shapes are Gaussian, then t h e squares o f t h e c o n t r i b u t i n g f a c t o r s a r e a d d i t i v e ,
B21
62 = B 2 + b 2 where B i s t h e experimental w i d t h , b i s t h e i n s t r u m e n t a l w i d t h which can be o b t a i n e d by a c a l i b r a t i o n procedure, and 6 i s t h e l i n e w i d t h due t o p a r t i c l e s i z e broadening. Having e v a l u a t e d B, t h e mean c r y s t a l l i t e d i a m e t e r dB i s g i v e n by t h e c l a s s i c a l S c h e r r e r ' s e q u a t i o n (see, e.g., dB = KX/B cos
Refs. 42, 85, 86),
(1.7)
8
where X i s t h e x - r a y wavelength, K i s S c h e r r e r ' s c o n s t a n t , and B i s t h e a n g u l a r w i d t h expressed i n r a d i a n s . The v a l u e o f K depends on how t h e peak w i d h t i s measured. F o r i n s t a n c e , i f t h e w i d t h i s measured as t h e f u l l w i d t h a t h a l f maximum (FWHM), K t a k e s v a l u e s o f 0.84-0.89,
depending on t h e assumed p a r t i c l e
shape. Values o t h e r t h a n these can be found i n t h e l i t e r a t u r e ( 8 6 ) . Since t h i s method analyzes t h e w i d t h o f x - r a y d i f f r a c t i o n l i n e s , i t i s r e s t r i c t e d t o measurements o f t h e s i z e o f c r y s t a l 1 i t e s . T h e r e f o r e , XLBA g i v e s r a p i d i n f o r m a t i o n about t h e d i s p e r s i o n degree o f a m e t a l o r metal o x i d e p r e s e n t a t t h e s u r f a c e o f t h e s u p p o r t o r even embedded i n i t . Thus, t h e method has an i m p o r t a n t p l a c e i n c a t a l y s t technology. 1.3.2.1.2.
Small -Angle X-Ray S c a t t e r i n g (SAXS)
T h i s t e c h n i q u e i s based on t h e a n a l y s i s o f t h e s c a t t e r e d r a d i a t i o n w i t h i n v e r y l o w angles ( < 4 " ) o f t h e i n c i d e n t beam. A l t h o u g h t h e t h e o r y and p r i n c i p l e s o f t h e method have been w e l l e s t a b l i s h e d ( 8 7 - 8 9 ) , i t s a p p l i c a t i o n f o r c r y s t a l -
1 i t e s i z e measurement i n supported c a t a l y s t s has been l i m i t e d . L e t I s be t h e i n t e n s i t y o f t h e s c a t t e r e d x - r a y s : t h i s i s a f u n c t i o n o f a v a r i a b l e s d e f i n e d as s = 28/A, where 28
i s t h e s c a t t e r i n g a n g l e and X i s t h e
x - r a y wavelength. I f t h e sample c o n s i s t s o f p a r t i c l e s a l l i d e n t i c a l i n s i z e , I, follows the relation I n I s = I n (NpNe)
Where N
P
-
(4r2/5)Rg 2 s 2
and Ne a r e t h e number o f p a r t i c l e s i n t h e sample and t h e number o f
i s a constant c a l l e d radius o f 9 g y r a t i o n o r G u i n i e r ' s r a d i u s and i s a measure o f t h e p a r t i c l e r a d i u s . A c c o r d i n g 2 t o Eq. ( 8 ) , R can be c a l c u l a t e d f r o m t h e s l o p e o f t h e I n I, vs s p l o t . I n 4 supported c a t a l y s t s , t h e s u p p o r t c o n t a i n s s c a t t e r i n g c e n t e r s o f more o r l e s s e l e c t r o n s p e r p a r t i c l e , r e s p e c t i v e l y , and R
t h e same s i z e as t h e c r y s t a l l i t e s under study. Since a c l e a r i n t e r f e r e n c e e x i s t s between b o t h s c a t t e r i n g systems, t h e c o n t r i b u t i o n o f t h e p o r e s must be avoided.
B22
A simple way t o overcome t h i s d i f f i c u l t y i s t o suppress t h e s c a t t e r i n g o f t h e pores by f i l l i n g them w i t h a l i q u i d , c a l l e d pore maskant, whose e l e c t r o n i c dens i t y should be almost t h e same as t h a t o f t h e support. Heinemann e t a l . (90) described t h i s method f o r Pt/y-A1203 r e f o r m i n g c a t a l y s t s , u s i n g methylene i o d i d e as t h e impregnant o f t h e alumina pores, w i t h o n l y t h e P t p a r t i c l e s a c t i n g as main s c a t t e r i n g centers. U n f o r t u n a t e l y , t h e p a r t i c l e s i z e d i s t r i b u t i o n o f c r y s t a l 1 i t e s i n supported c a t a l y s t s i s nonuniform, i.e., a p l o t o f I n I, vs s2 i s n o n l i n e a r . For t h i s case, a method o f a n a l y s i s has been suggested by Harkness e t a l . ( 9 1 ) , p r o v i d e d t h e p a r t i c l e s i z e d i s t r i b u t i o n f u n c t i o n i s a reasonable approximation t o a l o g normal f u n c t i o n . I n p r a c t i c e , some d i f f i c u l t i e s may appear i n t h e accurate e v a l u a t i o n o f R because o f d i m i n i s h e d s e n s i t i v i t y a t l a r g e s values. However, 9 f o r t h i s s i t u a t i o n t h e d i s t r i b u t i o n parameters r and u can be r e l a t e d t o t h e 9 s c a t t e r i n g parameters R and R by 9 P In;
9
=lnR
9
-
.714 I n (Rg/Rp) (1.10)
( I n o ) =~ 0.286 where
r9
and u a r e t h e geometric mean and t h e square r o o t o f t h e v a r i a n c e o f t h e
d i s t r i b u t i o n , r e s p e c t i v e l y . A more d e t a i l e d t h e o r y a p p l i c a b l e t o o t h e 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 s may be found i n Ref. (92). The m a j o r advantage o f SAXS f o r p a r t i c l e diameter, and t h e e f f e c t i v e range i s up t o about 10 nm. 1.3.2.2.
E l e c t r o p h o r e t i c M i g r a t i o n Technique
E l e c t r o p h o r e t i c m i g r a t i o n was r e c e n t l y used t o e v a l u a t e t h e apparent s u r face coverage (ASC) o f t h e support by a metal o x i d e i n supported-metal o x i d e c a t a l y s t s (93). I n one o f t h e p i o n e e r i n g works i n t h e f i e l d , Parks (94) showed t h a t t h e zero p o i n t o f charge (ZPC) i s d i r e c t l y r e l a t e d t o t h e composition of t h e samples, although t h e r e i s experimental evidence t h a t t h e ZPC
measured by
e l e c t r o p h o r e t i c m i g r a t i o n depends on t h e s u r f a c e composition. The ZPC o f samples w i t h more than one species w i t h o u t s t r u c t u r a l change i s g i v e n by (1.11)
ZPC = I(IEP)iXi
where ZPC i s t h e o v e r a l l zero p o i n t o f charge o f t h e c a t a l y s t sample, I E P i i s t h e i s o e l e c t r i c p o i n t o f species i, and Xi
i s t h e molar f r a c t i o n o f species i
a t t h e surface. I f one assumes t h a t t h e coverage i s r e l a t e d t o t h e ZPC, t h e n f o r one metal o x i d e supported c a t a l y s t Eq. (11) becomes
B23 ZPC = Xs(IEP)s
-
X0(IEP),
(1.12)
where s u b s c r i p t s s and o r e p r e s e n t t h e s u p p o r t and t h e metal oxide, r e s p e c t i v e l y . According t o E q . ( 1 2 ) , Xo i s o b t a i n e d f r o m e x p e r i m e n t a l measurements. F u r t h e r t r a n s f o r m a t i o n s o f E q . ( 1 2 ) g i v e t h e ASC d i r e c t l y ( 9 3 ) and, consequently, t h e d i s p e r s i o n degree. T h i s method p r o v i d e s a r a p i d and a c c u r a t e procedure f o r t h e d e t e r m i n a t i o n o f d i s p e r s i o n i n supported metal o x i d e c a t a l y s t s . 1.3.2.3.
XPS Peak I n t e n s i t y Measurements
X-ray p h o t o e l e c t r o n spectroscopy (XPS) i s a " s u r f a c e " s e n s i t i v e t e c h n i q u e used e x t e n s i v e l y f o r assessing d i s p e r s i o n o f m e t a l s and metal o x i d e s ( o r s u l f i d e s ) i n supported c a t a l y s t s . S i n c e t h e p h o t o e l e c t r o n s have, u n f o r t u n a t e l y , an escape depth corresponding t o s e v e r a l a t o m i c l a y e r s , t h e XPS s i g n a l probes a zone o f f i n i t e t h i c k n e s s which i s i n t h e o r d e r o f t h e s i z e o f t h e c r y s t a l l i t e s i n h i g h s u r f a c e c a t a l y s t s . Thus t h e " s u r f a c e " i s d e f i n e d h e r e as t h e p e r i p h e r y o f a macroscopic sample. A t c o n s t a n t l o a d i n g t h e number of p h o t o e l e c t r o n s I,,, escaping from t h e supported o x i d e i n c r e a s e s w i t h decreasing p a r t i c l e s i z e , whereas t h e XPS s i g n a l I, f r o m t h e s u p p o r t decreases as d i s p e r s i o n o f t h e supp o r t e d phase i n c r e a s e s . Thus, t h e $,/Is i n t e n s i t y r a t i o of two peaks a s s o c i a t e d w i t h t h e supported phase and t h e s u p p o r t r e s p e c t i v e l y , i n c r e a s e s w i t h i n c r e a s i n g d i s p e r s i o n degree. The Im/Is i n t e n s i t y r a t i o was f i r s t used o n l y i n a q u a l i t a t i v e manner (95, 96) t o f o l l o w changes i n c a t a l y s t d i s p e r s i o n as a f u n c t i o n o f p r e p a r a t i o n and c a t a l y s t p e t r e a t m e n t . However, models have r e c e n t l y been developed t h a t p r o v i d e a n a l y t i c a l e x p r e s s i o n s f o r t h e r e l a t i o n s h i p between t h e $,/Is intensity ratio and t h e d i s p e r s i o n o f t h e supported phase (97-99). These models have r e c e n t l y been reviewed by Defoss6 ( 1 0 0 ) . P h o t o e l e c t r o n l i n e s w i t h s m a l l d i f f e r e n c e s i n k i n e t i c energy may o f t e n be chosen f o r measuring I,,,and I,. I n such cases t h e i n e l a s t i c mean f r e e p a t h s o f p h o t o e l e c t r o n s (Ai)
do n o t depend on t h e e m i t t i n g atom b u t o n l y on t h e ma e r i a l
may t h e n which s c a t t e r s t h e s e e l e c t r o n s a l o n g t h e r p a t h t o t h e s u r f a c e . $,/Is be w r i t t e n as a p r o d u c t , 1.13) where Qh i s t h e i n t e n s i t y r a t i o expected f o r a n i n f i n i t e c r y s t a l ( b u l k ) , and Fm and F, depend on t h e n a t u r e o f t h e supported phase and i t s d i s p e r s i o n , and on t h e n a t u r e o f t h e s u p p o r t and i t s s u r f a c e area, r e s p e c t i v e l y . A n a l y t i c a l express i o n s have been d e r i v e d by assuming t h a t e i t h e r t h e supported phase (98, 99) o r t h e s u p p o r t (97-100) c o n s i s t s o f i d e n t i c a l p a r t i c l e s .
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U n f o r t u n a t e l y , experimental s t u d i e s aimed a t a q u a n t i t a t i v e d e s c r i p t i o n of these extreme approaches a r e s t i l l scarce. Extensive XPS s t u d i e s o f t h e d i s p e r s i o n o f t h e a c t i v e components deposited on a support c l e a r l y account f o r t h e v a r i a t i o n o f Im/Is ( o r M/S).
The o n l y systematic study by Defoss6 (97) shows
t h e i n f l u e n c e of t h e s u r f a c e area o f Mo03/A1203 c a t a l y s t s on Fs (Eq. 1 3 ) . Good agreement between experiment and t h e o r y was found. On t h e o t h e r hand, t h e as a f u n c t i o n o f t h e p a r t i c l e s i z e on t h e f a c t o r Fo r e q u i r e s dependence of Im/Is comparison o f XPS data w i t h those p r o v i d e d by another independent technique. Good agreement was o b t a i n e d by H o u a l l a e t a l . (101) u s i n g XPS and t r a n s m i s s i o n e l e c t r o n microscopy i n t h e case o f l a r g e N i O aggregates supported on s i l i c a . However, more experimental evidence i s needed i n o r d e r t o e s t a b l i s h more f i r m l y t h e accuracy o f t h e a n a l y t i c a l expressions proposed f o r Fo. An i n t e r e s t i n g q u e s t i o n i n heterogeneous c a t a l y s i s i s t o
ascertain i f the
s u r f a c e and b u l k l a y e r s have i d e n t i c a l compositions. SbSnO c a t a l y s t s , used f o r propylene o x i d a t i o n i n t o a c r o l e i n , were employed t o s t u d y i o n enrichment a t t h e surface. B o u d e v i l l e e t a l . (102) found t h a t c a l c i n a t i n g t h e SbSnO samples a t h i g h temperature r e s u l t e d i n a m i g r a t i o n o f Sb i o n s toward t h e s u r f a c e o f t h e p a r t i c l e s and a s i n t e r i n g o f these p a r t i c l e s . I t was then found t h a t t h e s e l e c t i v e c a t a l y s t was composed o f a s u r f a c e Sb e n r i c h e d s o l i d s o l u t i o n o f Sb i n t o Sn02. That such a s u r f a c e Sb e n r i c h e s t h e c a t a l y s t w i t h p a r t i c u l a r l y good s e l e c t i v i t y f o r a c r o l e i n was a l s o evidenced by XPS f o r Fe203-Sb204 c a t a l y s t s f o r Sb/Fe r a t i o s l a r g e r than 1 (103). The dynamic c h a r a c t e r o f c a t a l y s t surfaces i s a l s o evidenced by XPS. I n coal g a s i f i c a t i o n , carbon has been doped w i t h potassium carbonate. Yokoyama e t a l . (104) observed t h a t t h e XPS peaks f o r
K 1s and 0 Is decreased d r a s t i c a l l y upon
c a l c i n a t i o n a t 923 K b u t were r e s t o r e d by c o n t a c t w i t h C02 a t t h e same temp e r a t u r e . This r e v e r s i b l e behaviour c l e a r l y shows t h a t t h e dynamics of a catal y s t m a t e r i a l depends on t h e experimental c o n d i t i o n s . One must o b v i o u s l y t a k e t h i s p o i n t i n t o c o n s i d e r a t i o n i n t h e c h a r a c t e r i z a t i o n o f r e a c t i v e surfaces. A serious problem f o r a q u a n t i t a t i v e d e s c r i p t i o n o f XPS d a t a remains un-
solved: Determination o f t h e p h o t o e l e c t r o n c r o s s s e c t i o n s (am and a s ) and e l e c t r o n mean f r e e paths (Am and A s ) i s f a r from p r e c i s e . Therefore, XPS can be considered as a powerful technique f o r measuring r e l a t i v e changes i n t h e d i s p e r s i o n degree o f t h e a c t i v e i n g r e d i e n t i n a f a m i l y o f c a t a l y s t s . 1.3.2.4.
E l e c t r o n Microscopy
P r i n c i p l e s and d e t a i l s o f t r a n s m i s s i o n e l e c t r o n microscopy (TEM) as w e l l as i t s a p p l i c a t i o n t o t h e study o f supported c a t a l y s t s a r e g i v e n i n a r e c e n t review by Delannay (105). As p o i n t e d o u t by F l y n n e t a l . (106), t h e measurement o f p a r t i c l e s i z e d i s t r i b u t i o n s o f c r y s t a l l i t e s from TEM images i s based on t h e f o l l o w i n g i m p l i c i t assumptions: (1) t h e s i z e measured on t h e image i s equal t o
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t h e t r u e s i z e o f t h e p a r t i c l e ( m u l t i p l i e d by t h e m a g n i f i c a t i o n ) ; ( 2 ) a l l p a r t i c l e s have t h e same p r o b a b i l i t y o f b e i n g observed on t h e image, whatever t h e i r s i z e ; and
( 3 ) c o n t r a s t s a r i s i n g f r o m t h e s u p p o r t and f r o m t h e c r y s t a l l i t e s must
be c l e a r l y d i f f e r e n t i a t e d . P o i n t ( 3 ) i s r a r e l y obeyed f o r s u p p o r t e d metal oxides, e s p e c i a l l y when c r y s t a l l i t e s i z e i s v e r y s m a l l . Consequently, TEM p a r t i c l e s i z e d i s t r i b u t i o n s , as measured on p r o p e r l y prepared specimens, a r e i n c r e a s i n g l y s u b j e c t t o e r r o r when t h e p r o p o r t i o n o f p a r t i c l e s smaller
t h a n a l i m i t i n g s i z e i n c r e a s e s . Taking i n t o account t h i s
l i m i t a t i o n , t h e r e i s no doubt t h a t t h e y p r o v i d e d i r e c t evidence of t h e d i s p e r s i o n t o be compared w i t h c h e m i s o r p t i o n data. Moreover, t h e s i z e d i s t r i b u t i o n s o b t a i n e d from TEM images a r e l i k e l y t o be more r e a l i s t i c t h a n t h o s e found f r o m XLBA data. 1.3.3.
M i s c e l l aneous Methods
I n some c h e m i s o r p t i o n s t u d i e s , l a b e l i n g t h e probe m o l e c u l e w i t h a r a d i o a c t i v e i s o t o p e may be used t o determine t h e e x t e n t o f a d s o r p t i o n , e x p e c i a l l y on v e r y s m a l l s u r f a c e s (107-109).
I n t h i s case t h e amount o f gas adsorbed on t h e
c a t a l y s t s u r f a c e i s measured d i r e c t l y , e.g.,
by means o f a G e i g e r - M u l l e r t u b e o r
s c i n t i l l a t i o n d e t e c t o r . I n a d d i t i o n t o t h e s e measurements, t h e number o f molecules bound by a s p e c i f i c f o r c e on t h e s u r f a c e may be o b t a i n e d f r o m t h e e x t e n t o f t h e exchange between i s o t o p e s o f t h e m o l e c u l e under study. For i n s 12 tance, t h i s method can be used t o s t u d y t h e exchange o f l2C0 by 14C02 and C02 by I 4 C O 2 on t h e s u r f a c e of metal and o x i d e s covered by a bound l a y e r o f CO o r C02 (110). T h i s method i s a1 so a p o w e r f u l t e c h n i q u e t o s t u d y t h e adsorbed l a y e r i n t h e
case o f mixed probe molecules. M a g n e t i z a t i o n i s another p r o p e r t y t h a t may b e used t o d e t e r m i n e d i s p e r s i o n degree i n s u p p o r t e d m e t a l s and o x i d e s when t h e s u p p o r t e d i n g r e d i e n t s i s paramagnetic. These t y p e s o f measurements, i n s t r u m e n t a t i o n , and analyses have been v e r y w e l l d e s c r i b e d by Selwood (111). 1.4. IDENTIFICATION OF SURFACE SPECIES 1.4.1.
I n f r a r e d Spectroscopy
W i t h o u t doubt t h e most common method f o r d e t e r m i n i n g t h e v i b r a t i o n a l modes o f a chemisorbed molecule on a s o l i d s u r f a c e i s t h e d i r e c t o b s e r v a t i o n o f l i g h t a b s o r p t i o n i n t h e i n f r a r e d r e g i o n o f t h e e l e c t r o m a g n e t i c spectrum. I n f r a r e d spectroscopy o f adsorbed molecules has been used f o r many y e a r s t o i d e n t i f y t h e n a t u r e o f s u r f a c e s i t e s i n h i g h s u r f a c e area o x i d e s . Up t o 1967 t h e m a i n approach ahs been t h e use o f t h e c o n v e n t i o n a l t r a n s m i s s i o n I R t e c h n i q u e , and t h e work has been summarized i n t h r e e e x c e l l e n t monographs ( 2 2 , 23, 112). Working i n t h i s mode, frequency s h i f t s o f i n t e r n a l v i b r a t i o n s o f a s u r f a c e comp l e x between t h e probe and a s p e c i f i c s i t e o r t h e appearance o f o t h e r v i b r a t i o n s
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due t o t h e f o r m a t i o n o f a new s u r f a c e s t r u c t u r e can be observed. Such i n f o r m a t i o n p e r m i t s t h e c h a r a c t e r i z a t i o n o f t h e chemical n a t u r e o f t h o s e s u r f a c e complexes by comparing t h e i r i n f r a r e d s p e c t r a w i t h t h o s e o f known compounds.
A s e r i o u s l i m i t a t i o n i n t h e c o n v e n t i o n a l t r a n s m i s s i o n mode i s t h e i n f r a r e d 1 a b s o r p t i o n by t h e s o l i d i n t h e low-frequency range ( u s u a l l y below 1000 cm- ) , which can p r e c l u d e o b s e r v a t i o n o f i n t e r e s t i n g bands o f t h e s t r e t c h i n g mode o f t h e c h e m i s o r p t i o n bond. F o r t u n a t e l y , t h e p a s t decade has w i t n e s s e d t h e i m p a c t o f t h e e l e c t r o n i c r e v o l u t i o n on s u r f a c e spectroscopy. W i t h r e g a r d t o i n s t r u m e n t a t i o n f o r t r a n s m i s s i o n I R , t h e commercial development o f F o u r i e r t r a n s f o r m i n f r a r e d (FT-IR) spectrophotometers has l e d t o s i g n i f i c a n t advantages i n t h e d e t e r m i n a t i o n o f t h e v i b r a t i o n a l s p e c t r a o f chemisorbed p r o b e m o l e c u l e s . I t s o p t i c a l p r i n c i p l e s , comparison w i t h g r a t i n g s p e c t r o p h o t o m e t e r s , and a p p l i c a t i o n s as w e l l as d a t a a c q u i s i t i o n and s t o r a g e have been r e v i e w e d by B e l l ( 1 1 3 ) . The s e n s i t i v i t y g a i n s achieved w i t h t h e FT-IR become v a l u a b l e e i t h e r i n cases where t h e adsorbent a b s o r p t i o n i n t h e l o w - f r e q u e n c y range i s s u f f i c i e n t l y l o w o r i n t h e s t u d y o f l i q u i d - s o l i d i n t e r f a c e s ( 1 1 4 ) . A g e n e r a l o v e r v i e w of t h e I R spect r o s c o p y and i t s a p p l i c a t i o n t o many c a t a l y t i c systems i s g i v e n i n c h a p t e r 2. T h i s low-frequency r e g i o n i s e a s i l y a c c e s s i b l e t o Raman spectroscopy, s i n c e o x i d e s a r e u s u a l l y bad Raman scatterers.Consequently, t h e a p p l i c a t i o n o f Raman spectroscopy i n c o n n e c t i o n w i t h s i g n a l - a v e r a g i n g t e c h n i q u e s g i v e h i g h c a p a b i l i t y t o surface region
s t u d i e s , A c o n s i d e r a b l e body o f work has been e s s e n t i a l l y
concerned w i t h t h e p h y s i c a l , r a t h e r t h a n chemical a d s o r p t i o n o f some h i g h l y p o l a r i z a b l e probes, i.e.,
p y r i d i n e . However, o t h e r i n t e r e s t i n g a s p e c t s o f t h e
n a t u r e and r e a c t i v i t y o f s u r f a c e h y d r o x y l groups o f i n s u l a t o r o x i d e s can a l s o be s t u d i e d by Raman spectroscopy. F o r i n s t a n c e , m e t h o x y l a t i o n ( 1 1 5 ) and r e a c t i o n s w i t h hdyrogen s e q u e s t e r i n g a g e n t s (TiC14, BF3, A1(CH3)3 ( 1 1 6 ) , b e s i d e s m o d i f y i n g t h e s u r f a c e p r o p e r t i e s o f a n adsorbent o r c r e a t i n g new r e a c t i v e s i t e s on a c a t a l y s t , have a l s o been w i d e l y used as probe m o l e c u l e s t o s t u d y t h e conf i g u r a t i o n s o f s u r f a c e h y d r o x y l groups (116-118). The above s t u d i e s were f e a s i b l e because t h e s e t y p e s o f o x i d e s a r e r e l a t i v e l y poor Raman s c a t t e r e r s , which makes i t p o s s i b l e t o observe t h e Raman spectrum o f an adsrobed molecule. However, t h e l o w background s c a t t e r i n g has proven t o b e u s e f u l i n r e c e n t Raman s t u d i e s o f heterogeneous o x i d e c a t a l y s t (119-124),
mainly
molybdenum-containing c a t a l y s t s which a r e e x t e n s i v e l y used i n h y d r o t r e a t i n g processes i n c l u d i n g h y d r o d e s u l f u r i z a t i o n , h y d r o d e n i t r o g e n a t i o n , and demetalat i o n . Good q u a l i t y , complex Raman s p e c t r a have been o b t a i n e d i n t h e f r e q u e n c y range 50-1100 cm-'.
Due t o t h e d i f f i c u l t y o f a p p l y i n g t h e "group f r e q u e n c y "
concept t o molybdenum oxides, J e z i o r o w s k i and Knozinger ( 1 1 9 ) concluded t h a t Raman bands (Fig.1.8) i n t h i s r e g i o n may be a s s i g n e d as f o l l o w s : 200-250 cm-' (Mo-0-Mo d e f o r m a t i o n ) , 310-370 cm-l (Mo=O bend), 400-600 cm-'
-Mo s t r e t c h i n g ) , 700-850 cm-'
( s y m m e t r i c Mo-O-
( a n t y s y m m e t r i c Mo-0-Mo s t r e t c h i n g ) , and 900-1000
B27 I
I! I
I I
I I
I I I
I
1200
I
I
800
I
I
LOO
I
I
0
Fig.l.8. Raman spectrum o f an 8 w t % MOO /A1 03 c a t a l y s t . ( a ) A f t e r i m p r e g n a t i o n a t pH = 6. ( b ) A f t e r d r y i n g a t 393 K. i c ) g f t e r c a l c i n a t i o n a t 773 K . Redrawn from R e f . 119.
(Mo=O s t r e t c h ) . On t h i s b a s i s , s e v e r a l groups c a r r i e d o u t d e t a i l e d Raman
cm-'
s t u d i e s (120-124) w h i l e v a r y i n g d i f f e r e n t p r e p a r a t i o n parameters, namely s u p p o r t c a t a l y s t , p r e p a r a t i o n method, Mo l o a d i n g , and e f f e c t o f promoters, i n o r d e r t o understand how t h e s e v a r i a b l e s i n f l u e n c e t h e f i n a l molybdena s t r u c t u r e . Ratnan spectroscopy i s i d e a l l y s u i t e d t o " i n s i t u " s t u d i e s because i t has no i n h e r e n t 1 i m i t a t i o n on pressure, temperature, o r t h e presence o f r e a c t i o n gases d u r i n g a n a l y s i s . Changes i n t h e Raman f e a t u r e s o f c r y s t a l l i n e phases a t h i g h temperatures a r e due t o thermal broadening and a r e e l i m i n a t e d by c o o l i n g t h e sample. The Raman bands o f t h e amorphous, supported o x i d e s sharpen and s i m u l t a n e o u s l y s h i f t i n frequency a t e l e v a t e d temperatures due t o d e s o r p t i o n o f w a t e r from t h e s u r f a c e . The removal o f c o o r d i n a t e d w a t e r molecules from t h e s u p p o r t e d metal o x i d e species decreases t h e degree o f d i s o r d e r and a f f e c t s t h e symmetric
Mo=O s t r e t c h . The s h i f t i n t h e Raman band w i t h coverage o f t h e s u p p o r t appears t o be r e l a t e d t o t h e e x t e n t o f h y d r a t i o n o f t h e surface, t h u s c o n f i r m i n g t h e general c o n c l u s i o n t h a t t h e metal o x i d e s a r e p r e s e n t on t h e s u p p o r t as a h i g h l y d i s p e r s e o x i d e s p e c i e s bound t o t h e s u p p o r t s u r f a c e . 1.4.2.
Nuclear Magnetic Resonance (NMR)
The most r e l e v a n t p r o s p e c t s o f t h e v a r i o u s NMR t e c h n i q u e s as a p p l i e d t o ads o r p t i o n s t u d i e s as w e l l as t o t h e i d e n t i f i c a t i o n o f s u r f a c e s i t e s have been
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@
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I
773 KOvacuo I
Fig.l.9. ( a ) 1H-NMR s p e c t r a o f Ce02 sample outgassed a t 773 K, reduced i n h y d r o gen (133.3 kN/m2) f o r 2 h a t d i f f e r e n t temperatures, and outgassed a t 295 K. 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 on t h e i n t e g r a t e d i n t e n s i t y ( b ) and t h e second moment ( c ) o f t h e s i g n a l . reviewed by s e v e r a l a u t h o r s (125-131). H i g h r e s o l u t i o n 1H-NMR has been used t o s t u d y adsorbed species, a1 though t h e i n t r i n s i c chemical s h i f t o f adsorbed s p e c i e s i s s t r o n g l y i n f l u e n c e d by o t h e r e x t e r n a l f a c t o r s . Furthermore, t h e p r o t o n chemical s h i f t s a r e o f t h e o r d e r o f 10 ppm, so t h a t t h e r e s o l u t i o n o f n e i g h b o r i n g resonance l i n e s may be p o o r due t o l i n e b r o a d e n i n g i n t h e adsorbed s t a t e , and l i n e s may even be unobserved. These l i m i t a t i o n s a r e overcome by u s i n g p u l s e d F o u r i e r s p e c t r o m e t e r s o p e r a t i n g a t h i g h magnetic f i e l d s and h a v i n g s p e c i a l d e v i c e s f o r magic a n g l e s p i n n i n g (MAS-NMR). The NMR s p e c t r a f o r n u c l e i o f v a r i o u s elements p r e s e n t i n s o l i d c a t a l y s t s , i n c l u d i n g t h o s e w i t h small m a g n e t i c momenta and l o w n a t u r a l
B29 abundance, as w e l l as t h e h i g h r e s o l u t i o n o f t h e s e c a t a l y s t s and chemisorbed probes a r e t h e r e b y e a s i l y o b t a i n e d . I n t e r e s t i n g examples t h a t demonstrate t h e p o s s i b i 1 i t i e s f o r s t u d y i n g i n t e r m e d i a t e p r o d u c t s o f heterogeneous c a t a l y t i c r e a c t i o n s u s i n g NMR can be found i n s e v e r a l works (127, 129, 132-134). 'H-NMR
has been s u c c e s s f u l l y used t o s t u d y t h e s u r f a c e (and b u l k ) s p e c i e s
t h a t hydrogen forms on H2-reduced c e r i a samples ( 1 3 3 ) . The s p e c t r a o f t h e samp l e s reduced a t temperatures above 673 K p r e s e n t e d two components ( F i g . 1.9a), namely a narrow s i g n a l o f l o w i n t e n s i t y c e n t e r e d a t t h e resonance magnetic f i e l d and a broader component w i t h l a r g e r i n t e n s i t y s h i f t e d r e l a t i v e t o Ho. The i n crease o f b o t h i n t e g r a t e d i n t e n s i t y (Fig.1.9b) and second momentum ( F i g . 1 . 9 ~ )up t o 673 K i s due t o t h e i n c o r p o r a t i o n o f hydrogen as h y d r o x y l o r h y d r i d e s p e c i e s . The p o s s i b i l i t y o f f o r m a t i o n o f w a t e r molecules by r e d u c t i o n was excluded because t h e f r e e w a t e r molecule o r w a t e r bound t o c a t i o n s would g i v e a narrow l i n e o r two s p l i t l i n e s ( d o u b l e t ) , r e s p e c t i v e l y , which were n o t d e t e c t e d . Hydrogen d i f f u s e d i n t o Ce02 b u l k where i t a f f e c t e d t h o s e paramagnetic c e n t e r s (135) b y f o r m a t i o n o f diamagnetic h y d r i d e s p e c i e s . R e c e n t l y , Zamaraev and M a s t i k h i n (127) e l u c i d a t e d by means o f t h e NMR t e c h n i q u e t h e a c t i v a t i o n mechanism o f H2 and CO molecules on a Rh/La203 c a t a l y s t , and f u r t h e r r e a c t i v i t y o f those a c t i v a t e d molecules gave "oxygenates" and hydrocarbons. When m o l e c u l a r H2 i s chemisorbed on t h i s c a t a l y s t , t h r e e l i n e s ( F i g . 1. 10A) a r e observed: ( 1 ) a l i n e w i t h a chemical s h i f t o f about -10 t o -15 ppm from a s u r f a c e h y d r i d e species; ( 2 ) a d o u b l e t r e s u l t i n g f r o m t h e d i p o l e - d i p o l e i n t e r a c t i o n o f t h e n u c l e a r s p i n s o f two p r o t o n s i n w a t e r m o l e c u l e s formed as a r e s u l t o f s u r f a c e r e d u c t i o n o f La203;and ( 3 ) a narrow l i n e f r o m weakly adsorbed H2. T h i s spectrum i n d i c a t e s t h a t t h e a c t i v a t i o n o f t h e dihydrogen m o l e c u l e on Rh/La203 c a t a l y s t t a k e s p l a c e w i t h h y d r i d e f o r m a t i o n ; t h e subsequent s p i l l o v e r o f such s p e c i e s t o t h e s u p p o r t l e a d s t o r e d u c t i o n o f i t s s u r f a c e ( t h e w a t e r molecules formed i n t h i s process a r e s t r o n g l y h e l d by m e t a l l i c Rh). By u s i n g carbon monoxide e n r i c h e d w i t h 13C0 i s o t o p e , t h e s e a u t h o r s a1 so i d e n t i f i e d t h e s u r f a c e species formed when p u r e CO o r CO Rh/La203 c a t a l y s t . The "C-NMR
+
H2 m i x t u r e a r e chemisorbed on
s p e c t r a o f CO a t 293 and 423 K ( F i g . 1.lOB) a r e
t y p i c a l o f c a r b o n y l - t y p e species. However, f u r t h e r h e a t i n g o f t h e samples r e s u l t e d i n t h e f o r m a t i o n o f s a t u r a t e d hydrocarbons ( s e e 1 i n e s a t h i g h f i e l d ) , presumably v i a t h e r e a c t i o n s o f these carbonyl - t y p e s p e c i e s w i t h t h e h y d r o x y l groups o f t h e s u r f a c e o r w a t e r molecules which may be p r e s e n t i n t h e c a t a l y s t . The 13C-NMR s p e c t r a observed d u r i n g t h e r e a c t i o n o f CO w i t h H2 o v e r t h e same c a t a l y s t a r e g i v e n i n ( F i g . 1.lOC). As can be seen, t h e i n t e r a c t i o n between b o t h molecules f o r a CO:H2 = 1:l m i x t u r e ( t o t a l p r e s s u r e o f 50.7 kN md2) s t a r t s from c a r b o n y l - t y p e species having 13C-NMR l i n e s a t a b o u t 180 ppm. A t temperatures above 473 K, a l a r g e amount o f o x y g e n - c o n t a i n i n g compounds i s observed, seem-
B30
hydride
I
1
2 00
100
0
PPm
200
100
0
ppm
I 1
1
Fig.l.lO.(A) H-NMR spectrum o f H2 c h e m i s o r p t i o n on Rh/A1203 c a t a l y s t . ( 6 ) 13CNMR s p e c t r a o f CO chemisorbed on t h e same c a t a l y s t a t ( a ) 273 K, ( b ) 423 K, and ( c ) 473 K. ( C ) ISC-NMR s p e c t r a o f CO chemisorbed on t h e same c a t a l y s t a t ( a ) 293 K , ( b ) 353 K f o r 0.4 h, and ( c ) 503 K f o r 0.25 h, and ( d ) 513 K f o r 0.25 h. Taken f r o m Ref. 127. i n g l y c o n t a i n i n g c a r b o n y l - t y p e fragments ( l i n e s a t 180-220 ppm), f o r m a t e - t y p e fragments ( l i n e s a t 150-180 ppm), and a l k y l groups ( l i n e s a t 5-30 ppm). The f o r m a t i o n o f t h e s e compounds i s p r o b a b l y r e l a t e d t o c o n d e n s a t i o n o f t h e c a r b o n y l - c o n t a i n i n g compounds under c o n d i t i o n s o f hydrogen d e f i c i e n c y . However, upon i n c r e a s i n g t h e temperature, t h e f i n a l p r o d u c t s observed w i t h t h e NMR t e c h n i q u e were a g a i n s a t u r a t e d hydrocarbon a s t h e m a j o r compounds. 1.4.3.
E l e c t r o n S p i n Resonance (ESR)
The ESR t e c h n i q u e has been e x t e n s i v e l y used t o s t u d y paramagnetic s p e c i e s on v a r i o u s s o l i d s u r f a c e s . These s p e c i e s may b e s u p p o r t e d m e t a l ions, s u r f a c e
B31 d e f e c t s , o r chemisorbed molecules, i o n s , e t c . Each s u r f a c e e n t i t y must have one o r more u n p a i r e d e l e c t r o n s i n o r d e r t o be o b s e r v a b l e by t h i s t e c h n i q u e . I n a d d i t i o n , o t h e r f a c t o r s such as s p i n - s p i n i n t e r a c t i o n s , t h e r e l a x a t i o n t i n e , and t h e c r y s t a l f i e l d i n t e r a c t i o n havea s i g n i f i c a n t i n f l u e n c e upon t h e spectrum. The e x t e n t o f i n f o r i i l a t i o n o b t a i n a b l e f r o m ESR spectra v a r i e s f r o m t h e s i m p l e f i n d i n g t h a t an Icrknown paramagnetic species i s p r e s e n t t o a d e t a i l e d d e s c r i p t i o n o f t h e bonding o f t h e s u r face complexes. The t h e o r e t i c a l basis, e x p e r i m e n t a l c o n s i d e r a t i o n s , and a p p l i c a t i o n s o f t h i s t e c h n i q u e ta c a t a l y t i c systems have been reviewed i n d e t a i l by L u n s f o r d (136). Few simple i n o r g a n i c molecules e x i s t w i t h a n odd number o f e l e c t r o n s and a r e t h e r e f o r e paramagnetic. The NO (15 e ) m o l e c u l e i s
one example o f such a
molecule. The NO spectrum f o r t h e NO chemisorbed on ZnO and ZnS (137) has been used t o determine c r y s t a l and magnetic f i e l d i n t e r a c t i o n s a t s p e c i f i c adsorpt i o n s i t e s . From v a l u e s o f ,g,
(magnetic f i e l d p a r a l l e l t o t h e N-0 a x i s ) , t h e
l e v e l s by t h e c r y s t a l f i e l d has been determined. Y One m i g h t e x p e c t t h a t m o l e c u l a r oxygen c o u l d be r e a d i l y s t u d i e d when chemi-
s p l i t t i n g o f t h e 2pvx and 2pv
sorbed s i n c e i t i s a s t a b l e paramagnetic molecule, b u t c o n s i d e r a b l e d i f f i c u l t y a r i s e s because t h e molecule c o n t a i n s two u n p a i r e d e l e c t r o n s i n i t s ground s t a t e . This, together w i t h a strong s p i n - o r b i t coupling, r e s u l t s i n a h i g h l y anisotrop i c spectrum f o r t h e r i g i d molecule. These d i f f i c u l t i e s a r e overcome by s t u d y i n g t h e m o l e c u l e i o n s , t h e most common b e i n g t h e s u p e r o x i d e i o n (0;).
Such
a species has been s t u d i e d i n d e t a i l by many r e s e a r c h e r s , e i t h e r i n t h e c o n t e x t o f o x i d a t i o n and o x i d a t i v e dehydrogenation r e a c t i o n s o v e r o x i d e c a t a l y s t s ( 1 3 8 ) o r from t h e p o i n t o f view o f t h e ESR t e c h n i q u e (139-141). Another i n t e r e s t i n g example o f t h e f o r m a t i o n and r e a c t i v i t y o f 0;
species
on a Vycor q u a r t z supported s i l v e r c a t a l y s t f o r t h e o x i d a t i o n o f e t h y l e n e was r e p o r t e d by C l a r k s o n and C i r i l l o ( 1 4 2 ) . These a u t h o r s f o u n d t h a t t h e k i n e t i c s o f 0; f o r m a t i o n i n t h e temperature range 298-333 K y i e l d e d a v a l u e o f t h e apparent a c t i v a t i o n energy o f d e s o r p t i o n o f 61.9 kJ "01-'
f o r 0;
as analyzed
by a s i m p l e mechanism f o r t h e s u p e r o x i d e f o r m a t i o n . F o r t h i s system t h e s a t u r a t i o n u p t a k e ( 0 2 adsorbed o n l y a O;/02
+
0; a t 298 K reached a coverage o f 0.44,
b u t i t showed
r a t i o o f 0.02% a t t h a t temperature. Consequently, t h e v a s t m a j o r i t y
o f oxygen chemisorbs as m o l e c u l a r species which i s rendered nonparamagnetic by a n i s o t r o p i c s u r f a c e e l e c t r i c f i e l d s , w h i l e 0; may f o r m o n l y on t h e h i g h i n d e x planes o f t h e s u r f a c e o x i d e o r on s u r f a c e d e f e c t s where t h e r e i s c o n s i d e r a b l e h e t e r o g e n e i t y as evidenced by t h e inhomogeneously broadened ESR s i g n a l d e r i v e d form t h e 0; 0;
species.
species f r o m a f t e r O2 a d s o r p t i o n on a v a r i e t y o f a c t i v a t e d o x i d e s (and
s u l f i d e s ) . The a c t i v a t i o n can be achieved by means o f thermal t r e a t m e n t s i n vacuo, by high-energy i r r a d i a t i o n , o r by exposure t o a r e d u c i n g atmosphere. The g e n e r a l l y accepted approach i s t o adopt an i o n i c model f o r t h e 0;
i o n on t h e
s u r f a c e , which assumes e l e c t r o n t r a n s f e r from t h e s u r f a c e t o 02 m o l e c u l e t o
B32
Fig.l.ll.(a) Energy l e v e l diagram f o r 02 and t h e c o o r d i n a t e system o f t h i s i o n i n t e r a c t i n g w i t h an i o n (M") a t t h e s u r f a c e ( t h e unpaired e l e c t r o n i s i n a molecular o r b i t a l made up o f t h e two xx atomic o r b i t a l s ) . ( b ) M o l e c u l a r o r b i t a l d e s c r i p t i o n o f t h e s p i n - p a i r i n g i n t e r a c t i o n i n dioxygen b i n d i n g t o a 3d7 comp l ex. form 0; w i t h an e l e c t r o s t a t i c i n t e r a c t i o n between t h e c a t i o n s i t e and t h e supero x i d e anion. The t h e o r e t i c a l values o f t h e diagonal elements o f t h e g-tensor have been d e r i v e d by Kanzig and Cohen (143) n e g l e c t i n g second-order terms. These g-values a r e
(1.14)
where h i s t h e s p i n - o r b i t c o u p l i n g constant o f oxygen ( u s u a l l y considered as 135 cm-I), E i s t h e s e p a r a t i o n between t h e energies o f u z and 7~ x o r b i t a l s , and 9 9 TI antibonding o r b i t a l s s p l i t by t h e c r y s t a l f i e l d o f 9 t h e Mnt i o n onto which 0; i s chemisorbed. The p l o t o f energy l e v e l s and t h e
A i s t h a t between t h e two
c o o r d i n a t e a x i s system f o r an 0; i o n i n t e r a c t i n g w i t h Mnt on t h e s u r f a c e i s schematically i l l u s t r a t e d inFig.l.11 a 0;
. The
e l e c t r o s t a t i c model f o r adsorbed
i s successful f o r many o x i d e systems where l a b e l i n g w i t h 170 shows t h e oxygen
B33
n u c l e i t o be e q u i v a l e n t , c o n s i s t e n t w i t h s i d e - o n a d s o r p t i o n o f oxygen i o n s . However, t h e r e a r e s e v e r a l examples o f o x y g e n - c o n t a i n i n g complexes w i t h none q u i v a l e n t oxygen n u c l e i , i n d i c a t i n g t h a t c o v a l e n t bonding between t h e adsorpt i o n s i t e and t h e oxygen species must be c o n s i d e r e d ( 1 4 4 ) . A q u i t e d i f f e r e n t approach has t h u s been proposed (145) by c o n s i d e r i n g a s e r i e s o f 3d 7-dioxygen adducts. I n t h i s model t h e bonding i n t e r a c t i o n i n v o l v e s s p i n p a i r i n g o f a n unp a i r e d e l e c t r o n i n an a n t i b o n d i n g ( n ) o r i b i t a l o f oxygen w i t h t h e u n p a i r e d e l e c t r o n i n t h e dz2 o r b i t a l o f t h e 3d97 i o n t o f o r m a aMO c o n t a i n i n g two e l e c The second n o r b i t a l o f oxygen i s o r t h o g o n a l t o t h e dl0 and 9 c o n t a i n s t h e u n p a i r e d e l e c t r o n , The oxygen i s regarded as f o r m i n g a n o n l i n e a r
trons (Fig.l.llb).
M-0-M complex where t h e a n g l e formed by t h e t h r e e n u c l e i i s about 126'. I n a r e c e n t study, Che e t a l . (146) c o n s i d e r e d t h e h y p e r f i n e t e n s o r (Axx, Azz) v a l u e s f o r 1 7 0 - c o n t a i n i n g 0; species a t t h e s u r f a c e o f c a t a l y s t s ac*YY t i v a t e d i n d i f f e r e n t ways and concluded t h a t t h e two oxygens a r e e q u i v a l e n t . T h i s has been i n t e r p r e t e d t o mean t h a t t h e oxygen i s adsorbed i n i o n i c f o r m w i t h t h e i n t e r n u c l e a r a x i s ( z - d i r e c t i o n ) p a r a l l e l t o t h e s u r f a c e . The Ax,
v a l u e s were
found t o l i e i n a narrow range, 74-80.5 G, i n d i c a t i n g t h a t l o c a l i z a t i o n o f t h e u n p a i r e d e l e c t r o n o f oxygen i s l a r g e l y independent o f t h e c a t a l y s t s u p p o r t . Few cases o f 0;
i o n s w i t h n o n e q u i v a l e n t n u c l e i have been observed on o x i d i c
surfaces. W i t h o u t doubt, t h o s e o f molybdenum o x i d e s supported on alumina (147), s i l i c a (147, 148), and bismuth molybdates w i t h v a r y i n g Mo/Bi r a t i o s s u p p o r t e d on s i l i c a (149) a r e t h e most r e p r e s e n t a t i v e . I n such cases t h e ESR s p e c t r a a r e complex and t h e a n a l y s i s i s n o t s t r a i g h f o r w a r d . Two h y p e r f i n e s p l i t t i n g s a r e f r e q u e n t l y observed i n an a n a l y s i s o f t h e l i n e s f o r species c o n t a i n i n g o n l y one l a b e l e d oxygen, i . e . ,
(170160)-, and two e x p l a n a t i o n s f o r t h i s o b s e r v a t i o n a r e
p o s s i b l e . One i s t h a t t h e s p l i t t i n g s r e f e r t o oxygen n u c l e i i n two 0;
ions
chemisorbed on d i f f e r e n t s i t e s , t h e o t h e r i s t h a t t h e y r e f e r t o two none q u i v a l e n t 0 - n u c l e i i n t h e same 0;
species ( 1 4 7 ) . T h i s second e x p l a n a t i o n has
been confirmed by h i g h r e s o l u t i o n s p e c t r a which e x h i b i t e d h y p e r f i n e l i n e s due 17 17 - . 0 0) i o n cheinisorbed on Most/Si02 ( 6 a b o u t 4 ) c a t a l y s t s . The s i m i l a r i t y
to (
o f 0; w i t h n o n e q u i v a l e n t oxygen n u c l e i t o t h e o r g a n i c p e r o x y r a d i c a l s has l e d t o t h e s u g g e s t i o n t h a t t h e molecule should be regarded as adsorbed w i t h one 0 nucleus c l o s e r t o t h e a d s o r p t i o n s i t e . To d a t e , t h e r e i s no complete e x p l a n a t i o n f o r t h i s k i n d o f a d s o r p t i o n , a l t h o u g h t h e s u r f a c e t o p o l o g y and t h e n a t u r e o f t h e o r b i t a l s a t t h e a d s o r p t i o n s i t e a v a i l a b l e t o o v e r l a p w i t h t h e oxygen o r b i t a l s c o u l d be t h e main reason f o r such behaviour. The t h e o r y of t h e g - t e n s o r i s w e l l documented f o r many t y p e s o f r a d i c a l s , b u t i t cannot always be a p p l i e d f o r i d e n t i f i c a t i o n purposes, e s p e c i a l l y f o r p o l y c r y s t a l l i n e m a t e r i a l s . I n t h e s e cases t h e l 7 0 - l a b e l i n g t e c h n i q u e i s u s e f u l t o c h a r a c t e r i z e unambiguously v a r i o u s oxygen s p e c i e s .
B34
1.4.4.
Temperature Programmed D e s o r p t i o n (TDP)
The TDP t e c h n i q u e developed by C v e t a n o v i c and Amenomiya (150) i s a s i m p l e b u t v e r y u s e f u l procedure f o r i n v e s t i g a t i n g i n t e r a c t i o n s between a p r o b e molec u l e and a c a t a l y s t s u r f a c e , and i t has been a p p l i e d by s e v e r a l w o r k e r s t o s t u d i e s o f gas a d s o r p t i o n on m e t a l s o r m e t a l o x i d e s (151-154). W i t h o u t doubt,
O2 has been t h e most e x t e n s i v e l y used probe t o s t u d y b i n d i n g e n e r g i e s , populat i o n s , and r e a c t i v i t i e s o f m e t a l o x i d e s i n t h e c o n t e x t o f a l a r g e number o f c a t a l y t i c o l e f i n o x i d a t i o n r e a c t i o n s . F o r example, t r a n s i t i o n metal o x i d e s c a t a l y z e s e l e c t i v e o x i d a t i o n . I t i s r e a s o n a b l e t o ask i f t h e s e p a t t e r n s a r e r e l a t e d t o t h e k i n d s of oxygen s p e c i e s i n v o l v e d , e.g.,
l a t t i c e oxygen and ad-
sorbed oxygen. I t i s a l s o o f i n t e r e s t t o know whether t h e oxygen s p e c i e s e x i s t i n d i s c r e t e s t a t e s o r w i t h a broad energy d i s t r i b u t i o n . TABLE 1.4. D e s o r p t i o n o f Oxygen from V a r i o u s M e t a l Oxides Group
A
;T
Oxide
(K)
'2'5 Mo03Bi 203 wo3 B i 203-2Mo03
B
C
Cr203
723
2.13 x
Mn02
323
543
633
L203
328
623
758
Co3O4 NiO
303
438
653
308
608
698
CUO
398
663
A1203
338
Si02
373
Ti02
398
ZnO
463
593
2.45
SnO,
353
423
2.11
813
6.54 x 4.05 3.30 x
823
1.12 x
lo-*
1.42 x 10-1 2.05 2.99 x I O - ~
463
593
5.52
:These values were o b t a i n e d a t 6 = 20 K/min The o v e r a l l amount o f oxygen desorbed below 823
K, a f t e r oxygen a d s o r p t i o n .
Iwamoto e t a l . (153) i n v e s t i g a t e d t h e oxygen a d s o r p t i o n p r o p e r t i e s o f 16 o x i d e s by means o f t h e TPD t e c h n i q u e . They observed t h a t t h e oxygen a d s o r p t i o n
B35
I
I
I
A$
I
I
I
200
600
400 (kJ glatomO)
Fig.1.12. C o r r e l a t i o n o f t h e amounts o f oxygen desorbed a t 839 K w i t h t h e h e a t o f f o r m a t i o n o f o x i d e s p e r g-atom o f oxygen. Redrawn f r o m Ref. 153. phenomena were v e r y d i f f e r e n t depending on t h e metal oxides, which c o u l d be c l a s s i f i e d i n t h r e e t y p e s : (A) V205, Moo3, Bi203, W03, and Bi203'2Mo03, which e x h i b i t e d no oxygen a d s o r p t i o n o v e r t h e range 283-833
K
( s e e T a b l e l . 4 ) ; (B)Cr203,
Mn02, Fe203, Co30q, NiO, and CuO, which always g i v e r e l a t i v e amounts of oxygen d e s o r p t i o n ; and (C) Ti02, ZnO, Sn02, A1203 and Si02, f o r which e v a c u a t i o n a t h i g h temperature f o l l o w e d by oxygen a d s o r p t i o n a t r e l a t i v e l y l o w t e m p e r a t u r e i s r e q u i r e d f o r oxygen d e s o r p t i o n t o appear o v e r t h e range 283-673 K, e x c e p t f o r t h e l a s t two o x i d e s . I t i s observed t h a t Group A o x i d e s have l a y e r s t r u c t u r e s 1 9 except f o r Bi203, w h i l e Group B c o n s i s t s o f o x i d e s w i t h c a t i o n s of a d - d e l e c t r o n i c c o n f i g u r a t i o n . Among t h e oxygen s p e c i e s adsorbed on Group C o x i d e s , t h e superoxide (0;)
i o n was d i r e c t l y i d e n t i f i e d b y means o f ESR s p e c t r o s c o p y ( s e e
above) and was assigned t o s p e c i f i c d e s o r p t i o n peaks o f t h e r e s p e c t i v e TPD s p e c t r a . The amounts o f oxygen desorbed a t 833
K f o r Group B and C o x i d e s were
o n l y a few p e r c e n t o f t h e sample coverage, s u g g e s t i n g t h a t t h e a d s o r p t i o n s i t e s a r e some s o r t o f s u r f a c e d e f e c t s . There i s a f a i r l y good c o r r e l a t i o n between t h e amount o f oxygen desorbed a t 833 K and t h e h e a t o x i d e f o r m a t i o n p e r g-mol o f 0 (-AH;)(Fig.l.lZ).This
tendency
p r o b a b l y r e f l e c t s t h a t t h e l e s s s t a b l e an o x i d e i s , t h e more e a s i l y t h e s u r f a c e i s reduced t o f o r m s u r f a c e d e f e c t s f o r a d s o r p t i o n . N o t i c e a l s o t h a t Group A oxides, which a r e s e l e c t i v e o x i d a t i o n c a t a l y s t s , show no s i g n i f i c a n t oxygen d e s o r p t i o n . I n o l e f i n o x i d a t i o n o v e r Bi203.2Mo03, many studies have shown t h a t l a t t i c e oxygen i s a p r i m a r y source o f t h e 0 atoms r e q u i r e d i n t h e
B36
r e a c t i o n (155). I t i s i n f e r r e d t h a t t h e s c a r c i t y o f adsorbed oxygen p r e v e n t s these oxides from lowering t h e r e a c t i o n s e l e c t i v i t y . I n contrast, considerable amounts o f oxygen adsorbed on Group B o x i d e s m a i n l y c a t a l y z e t h e complete o x i d a t i o n o f o l e f i n s . The Group C o x i d e s a r e i n t h e i n t e r m e d i a t e s i t u a t i o n , ads o r b i n g moderate amounts o f O2 and c a t a l y z i n g b o t h t h e s e l e c t i v e and complete o l e f i n o x i d a t i o n t o r o u g h l y t h e same e x t e n t . Such comparisons l e a d t o t h e assumption t h a t t h e adsorbed oxygen i s d e e p l y connected t o complete o x i d a t i o n , w h i l e t h e l a t t i c e oxygen i s more i m p o r t a n t f o r s e l e c t i v e o x i d a t i o n . S i m i l a r t r e n d s have a l s o been r e p o r t e d by G e l b s t e i n e t a l . ( 1 5 6 ) between t h e c a t a l y t i c a c t i v i t y f o r 1-butene o x i d a t i o n and t h e a c t i v a t i o n energy o f i s o t o p i c exchange o f s u r f a c e oxygen w i t h gaseous oxygen. These c o r r e l a t i o n s seem t o i n d i c a t e t h a t t h e c a t a l y t i c a c t i v i t y f o r complete o x i d a t i o n i s d i r e c t l y r e l a t e d t o t h e amount o f adsorbed oxygen. 1.5. APPLICATION TO CATALYTIC SYSTEMS 1.5.1. 1.5.1.1.
Supported M e t a l s Platinum
Many works have been devoted t o d e t e r m i n e t h e p l a t i n u m d i s p e r s i o n i n supp o r t e d p l a t i n u m c a t a l y s t s . I t i s due t o t h e e x t e n s i v e use of P t as a c a t a l y s t f o r a l a r g e v a r i e t y of p e t r o c h e m i c a l r e f i n i n g processes. A good r e v i s i o n o f t h e methods p u b l i s h e d b e f o r e 1975 a r e p r e s e n t e d by F a r r a u t o ( 8 5 ) and Anderson ( 1 5 ) . They concluded t h a t i n g e n e r a l hydrogen c h e m i s o r p t i o n i s t h e most s u i t a b l e method, b u t t h e b e s t c o n d i t i o n s f o r a g i v e n c a t a l y s t system must be experiment a l l y determined. When t h e a d s o r p t i o n on t h e s u p p o r t i s n e g l i g i b l e o r r e a s o n a b l y sillall, t h e measurement o f t h e e x t e n t o f hydrogen c h e m i s o r p t i o n i s recommended t o be conducted a t temperatures near ambient and p r e s s u r e s no l a r g e r t h a n 0.2 kPa
. O f c r i t i c a l importance i s t h e q u a n t i t a t i v e n a t u r e o f t h e adsorbed hydrogen
species. Spenadel and B o u d a r t ( 1 5 7 ) were t h e f i r s t t o p o i n t o u t , f o l l o w e d by Anderson ( 1 5 ) t h a t hydrogen adsorbs on P t v i a a d i s s o c i a t i v e process. Each hydrogen atom adsorbs on each s u r f a c e
P t atom f o r t h e e n t i r e P t s i z e range.
Atomic d i s p e r s i o n of P t was demonstrated by H2-chemisorption, and f o r P t p a r t i c l e s between 5 and 100 nn good agreement between c h e m i s o r p t i o n d a t a and X-ray 1 i n e broadening c a l c u l a t i o n s was o b t a i n e d ( 1 5 7 ) . The above s i m p l i f i c a t i o n i s t h u s i d e a l i z e d , t h e r e a l b e h a v i o u r b e i n g more complex. As o c c u r s w i t h o t h e r Group V I I I m e t a l s , t h e s t r o n g H 2 - c h e m i s o r p t i o n i s accompanied by weak c h e m i s o r p t i o n , w h i c h i s d i f f i c u l t t o d i s t i n g u i s h . Only i n e s p e c i a l cases, t h e thermal programmed d e s o r p t i o n t e c h n i q u e i s c a p a b l e t o d i s c e r n t h e weakly chemisorbed hydrogen from t h e s t r o n g l y h e l d t y p e . The o c c u r r e n c e o f t h e hydrogen s p i l l o v e r e f f e c t i s a n o t h e r problem w h i c h masks t h e r e s u l t s o f hydrogen c h e m i s o r p t i o n . T h i s o c c u r s t o a d r a m a t i c e x t e n t on P t / c a r b o n
B37
c a t a l y s t s . For i n s t a n c e , R o b e l l e t a1
. (158)
have r e p o r t e d t h a t H 2 - c h e m i s o r p t i o n
a t 620 K y i e l d s a n e t number o f adsorbed H-atoms which exceeds t h e t o t a l number o f P t atoms p r e s e n t i n t h e sample by a f a c t o r o f 3 t o 10.
The use o f gases o t h e r t h a n H2 t o d e t e r m i n e t h e s u r f a c e a r e a o f s u p p o r t e d P t has a l s o been r e p o r t e d . Carbon monoxide was f r e q u e n t l y used f o r t h i s purpose
(159, 160). The p i o n e e r work o f Lanyon and T r a p n e l l (161) on t h e a d s o r p t i o n of CO on unsupported P t e s t a b l i s h e d a CO:H=1:1 r a t i o f o r b o t h adsorbed gases, sugg e s t i n g a l i n e a r a d s o r p t i o n f o r CO on P t . U s i n g i n f r a r e d spectroscopy, Eischens (162) demonstrated t h a t about 15% o f t h e CO m o l e c u l e s adsorbed on P t / S i 0 2 i s i n t h e b r i d g e d form, w h i l e t h e o t h e r 85% is o f a l i n e a r t y p e . These percentages a r e 0
(linear)
0
II
....:.. .. ( b r i d g e d )
$
Pt’
Pt
Pt
s t r o n g l y i n f l u e n c e d by t h e n a t u r e o f t h e c a r r i e r and t h e P t d i s p e r s i o n . I t has been shown e i t h e r on Pt/A1203 (159) o r on P t / S i 0 2 c a t a l y s t s (163) t h a t a change i n t h e mode o f CO a d s o r p t i o n o c c u r s as i n d i c a t e d by t h e CO/Pt r a t i o v a r y i n g from 1 ( l i n e a r form) f o r h i g h l y d i s p e r s e d P t , t o 0.5 ( b r i d g e d form) f o r P t c r y s t a l -
l i t e s l a r g e r t h a n ca. 5 nm. Hydrogen-oxygen t i t r a t i o n s have a l s o been used as an a1 t e r n a t i v e method f o r t h e e s t i m a t i o n o f t h e s u r f a c e a r e a o f P t ( 1 6 4 ) . From t h e s t o i c h i o m e t r y O:H=1:3,
i t f o l l o w s t h a t t h e s e n s i t i v i t y o f t h e method i s t h r e e t i m e s h i g h e r t h a n t h a t
o f t h e hydrogen c h e m i s o r p t i o n alone. I n s p i t e o f t h i s advantage, t h e hydrogenoxygen t i t r a t i o n method f o r d e t e r m i n i n g t h e P t d i s p e r s i o n i s n o t w i t h o u t c r i t i c i s m ; t h e p r i m a r y one being t h e q u e s t i o n a b l e 0:Pt r a t i o . I t was n o t e d t h a t t h i s r a t i o changes w i t h p a r t i c l e s i z e ( 1 6 5 ) , i . e . ,
a 1:l s t o i c h i o m e t r y o c c u r s
on P t p a r t i c l e s l a r g e r t h a n 2 nm, w h i l e 1:2 p r e v a i l s f o r s m a l l e r p a r t i c l e s . Furthermore, as c l a i m e d i n s e v e r a l works (50, 76, 166), t h e mechanism f o r t h e t i t r a t i o n r e a c t i o n can be d i f f e r e n t , P t - 0 + 2H2-Pt-H2
t
H20
(1.16)
s e n s i t i v i t y being t w i c e as h i g h t h a n t h a t o f H2-chemisorption. Hydrogen-oxygen t i t r a t i o n s s u f f e r f r o m d o u b t f u l 0:Pt r a t i o s and t h e r e f o r e cannot be recommended as a standard. The i n c r e a s e i n s e n s i t i v i t y may j u s t as w e l l be a t t a i n e d by u s i n g l a r g e r c a t a l y s t samples and a p p l y i n g normal hydrogen chernisorption t e s t s .
B38 1.5.1.2.
Palladium
Many s e l e c t i v e h y d r o g e n a t i o n s a r e u s u a l l y conducted on p a l l a d i u m s u p p o r t e d c a t a l y s t s . Among these, t h e benzene h y d r o g e n a t i o n t o cyclohexane, removal o f a c e t y l e n e from o l e f i n streams i n p o l y m e r i z a t i o n p l a n t s , r e d u c t i o n o f n i t r i c a c i d t o hydroxylaniine, and r e d u c t i o n o f phenol t o cyclohexanone a r e a few representative palladium catalyzed reactions. Schol t e n and van M o n t f o o r t (167) a p p l i e d CO c h e m i s o r p t i o n t o s u r f a c e a r e a measurements o f s u p p o r t e d Pd. They compared s i x i n d u s t r i a l c a t a l y s t s f r o m d i f f e r e n t m a n u f a c t u r e r s . I n cases i n which s u c c e s s i v e l y h i g h e r p r e t r e a t m e n t temperatures were used, t h e Pd s u r f a c e area c o n s i s t e n t l y decreased independent of t h e s u p p o r t area. C o n s i s t e n t l y , e l e c t r o n microscopy, X-ray d i f f r a c t i o n , and
s m a l l a n g l e s c a t t e r i n g gave v a l u e s ca. 30-40% h i g h e r f o r t h e c r y s t a l s i z e o f Pd t h a n t h e gas c h e m i s o r p t i o n method. I n s p i t e o f p o s s i b l e d i f f i c u l t i e s , such as CO/Pd r a t i o s v a r y i n g w i t h p a r t i c l e s i z e below 3 nm, p e r c e n t a g e o f l i n e a r v e r s u s b r i d g e d forms, and d i f f e r e n t exposed c r y s t a l l o g r a p h i c planes, t h e s e a u t h o r s recommended t h e CO c h e m i s o r p t i o n method. The same c o n c l u s i o n was reached by Pope e t a l . ( 1 6 8 ) s t u d y i n g t h e d i s p e r s i o n o f Pd on c h a r c o a l c a r r i e r s , and by T u r k e v i c h and K i m (169) comparing e l e c t r o n microscopy and CO c h e m i s o r p t i o n o f Pd p a r t i c l e s o f u n i f o r m s i z e . E l i m i n a t i o n o f most o f t h e disadvantages o f u s i n g CO has been s u c c e s s f u l l y achieved by Aben (170) f o r Pd/Si02 c a t a l y s t s by u s i n g H2 as an adsorbate. The e x p e r i m e n t a l c o n d i t i o n s must be e x t r e m e l y c o n t r o l l e d i n o r d e r t o a v o i d hydrogen a b s o r p t i o n i n t o t h e m e t a l , which o c c u r s t o an u n d e s i r a b l y h i g h e x t e n t . A t 343 K and H2 p r e s s u r e s o f 133 Pa, t h e a b s o r p t i o n o f hydrogen by Pd c o u l d be reduced t o 0.002 H/Pd atom. The same c o n d i t i o n s were used by S c h o l t e n and van M o n t f o o r t (167) f o r d i s p e r s i o n measurements on a s e r i e s o f Pd/A1203, Pd/Si02, and Pd b l a c k p r e p a r a t i o n s . These a u t h o r s a l s o i n t r o d u c e d a r e f i n e m e n t i n t h e c o n d i t i o n s used by Aben. Since Aben's c a t a l y s t s were m o s t l y reduced i n H2 a t 670 K, f o l l o w e d by e v a c u a t i o n f o r 16 h a t t h i s temperature, about a 3% o f t h e s u r f a c e Pd atoms remained covered by hydrogen. T h i s r e s i d u a l hydrogen can be r e a d i l y removed by evacuationat850
K, b u t o n l y a t t h e expense o f s i g n i f i c a n t m e t a l s i n t e r i n g .
A l t h o u g h t h e o c c u r r e n c e o f weakly chemisorbed hydrogen i n excess of t h e s t r o n g l y h e l d monolayer, which has been c o n v i n c i n g l y demonstrated by Lynch and F l a n i g a n (171), t h e m e t a l l i c d i s p e r s i o n o f Pd i s measured by hydrogen chemisorpt i o n , b u t c o n d i t i o n s where H2 absorbs i n t o t h e m e t a l have t o be avoided. 1.5.1.3.
Nickel
N i c k e l c a t a l y s t s a r e w e l l known as e f f e c t i v e systems i n many h y d r o g e n a t i o n , dehydrogenation, i s o m e r i z a t i o n , and h y d r o c y a n a t i o n r e a c t i o n s . The s p e c i f i c m e t a l a r e a o f n i c k e l i n s u p p o r t e d N i c a t a l y s t s i s measured most commonly by H 2 c h e m i s o r p t i o n a t temperatures near ambient and p r e s s u r e s o f up t o about 10-20
B39 kPa ( 1 5 ) . The f i r s t s t u d i e s by S c h u i t and van R e i j e n (172) o f N i / S i 0 2 c a t a l y s t s showed an approximate hydrogen monolayer a t 195 K and 13.3 kPa o f hydrogen. They a l s o r e p o r t e d CO and C2H4 c h e m i s o r p t i o n d a t a on a s e r i e s o f N i / S i 0 2 c a t a l y s t s and determined t h e o p t i m a l c o n d i t i o n s t o conduct t h e c h e m i s o r p t i o n t e s t , t a k i n g i n t o account t h e e x t e n t o f t h e p h y s i c a l a d s o r p t i o n o f t h e s e molecules and t h e und e s i r a b l e s i d e r e a c t i o n s , such as Ni(CO)4 f o r m a t i o n and d i s p r o p o r t i o n a t i o n o f C2H4. The h y d r o g e n o l y s i s o f ethane u s i n g N i c a t a l y s t s was t h e t e s t r e a c t i o n
s e l e c t e d by S i n f e l t e t a l . (173-175). These a u t h o r s found a c o r r e l a t i o n between t h e r a t e o f ethane h y d r o g e n o l y s i s and t h e N i s u r f a c e area as measured by H2 c h e m i s o r p t i o n . The N i s u r f a c e area v a r i e d as a f u n c t i o n o f t h e c a l c i n a t i o n temperature o f t h e o x i d i c p r e c u r s o r s . However, t h e i n t r i n s i c c a t a l y t i c a c t i v i t y (C2H6 molecules c o n v e r t e d p e r square meter o f N i and p e r h o u r ) was e s s e n t i a l l y c o n s t a n t , t h u s i n d i c a t i n g t h a t v a r i a t i o n s i n t h e r e a c t i o n r a t e were o n l y due t o changes i n t h e exposed N i area b u t n o t i n t h e r e a c t i o n mechanism
.
The i n f l u e n c e o f t h e c a r r i e r on N i d i s p e r s i o n and on t h e s p e c i f i c c a t a l y t i c a c t i v i t y f o r C2H6 h y d r o g e n o l y s i s was a l s o s t u d i e d (174). B o t h parameters were found t o v a r y s u b s t a n t i a l l y w i t h t h e c a r r i e r (A1203, Si02,and Si02.A1203) a l t h o u g h t h e h i s t o r y o f t h e p r e p a r a t i o n and t h e p r e t r e a t m e n t s were i d e n t i c a l . The n i c k e l d i s p e r s i o n f o l l o w e d t h e o r d e r : Ni/A1203>Ni/Si02.A1203.
Somewhat s u r -
p r i s i n g were t h e r a t e r e s u l t s , n o r m a l i z e d f o r t h e N i s u r f a c e area, i n d i c a t i n g t h a t t h e i n t r i n s i c a c t i v i t y f o r ethane h y d r o g e n o l y s i s on N i / S i 0 2 c a t a l y s t s was about t w i c e t h a t on Ni/A1203 c a t a l y s t s . T h i s p a r t i c u l a r b e h a v i o u r can be exp l a i n e d on t h e b a s i s o f a s p e c i f i c chemical o r 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 n i c k e l p a r t i c l e s and t h e alumina s u r f a c e , which i s absent i n t h e s i l i c a support. The measurement o f N i areas by hydrogen c h e m i s o r p t i o n has been s u p p o r t e d by and f r e q u e n t l y c o r r e l a t e d t o c r y s t a l s i z e measurements by p h y s i c a l t e c h n i q u e s , e.g.,
X-ray l i n e broadening, e l e c t r o n microscopy, e t c . I t i s t r u e , however, t h a t
t h e c h e m i s o r p t i o n t e c h n i q u e i s much more s e n s i t i v e t h a n t h e p h y s i c a l t e c h n i q u e s , b u t a l s o s t r o n g l y dependent on t h e temperature o f t h e c h e m i s o r p t i o n t e s t . Theref o r e t h e agreement between N i s u r f a c e areas o b t a i n e d by b o t h procedures can be f o r t u i t o u s . I n g e n e r a l , t h e isotherms o f hydrogen on N i c a t a l y s t s p r e s e n t an i m p o r t a n t and s t r o n g
c h e m i s o r p t i o n a t v e r y l o w hydrogen p r e s s u r e s , which i s f o l l o w e d by a weak c h e i i i i s o r p t i o n a t h i g h e r p r e s s u r e s . The N i d i s p e r s i o n i s b e s t determined from t h e e x t e n t o f a s t r o n g l y h e l d hydrogen monolayer (15, 1 7 6 ) . However, i t i s v e r y i m p o r t a n t t o f o c u s a t t e n t i o n on t h e f a c t t h a t t h i s does n o t i m p l y t h a t t h e weak hydrogen c h e m i s o r p t i o n i s n o t i n v o l v e d i n c a t a l y t i c r e a c t i o n s . On t h e c o n t r a r y , i t i s b e l i e v e d t h a t a f r a c t i o n o f t h e chemisorbed hydrogen i s r e s p o n s i b l e f o r t h e h y d r o g e n a t i o n c a p a b i l i t y o f t h e c a t a l y s t a t l o w temperatures, t h i s b e i n g determined by t h e q/RT r a t i o , where q i s t h e a d s o r p t i o n
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heat and T t h e r e a c t i o n temperature, b u t n o t by t h e magnitude o f q. The n o n - d i s s o c i a t i v e chemisorption o f CO a t room temperature has a1 so been used t o estimate t h e s u r f a c e area o f N i p a r t i c l e s . However, t h e chemisorption o f Ci) i s much more complicated than t h a t observed w i t h hydrogen. For instance,
Hayashi and Kawasaki (177) found f o u r d i f f e r e n t forms o f bonding f o r CO adsorbed on N i metal. Even c o n s i d e r i n g t h i s s e r i o u s l i m i t a t i o n , an assumption was made t h a t CO molecules a r e adsorbed predominantly i n t h e l i n e a r form. Another disadvantage o f u s i n g CO t o t i t r a t e exposed N i atoms i n supported c a t a l y s t s i s t h e p o s s i b i l i t y o f Ni(CO)4 formation, e s p e c i a l l y w i t h t h e s m a l l e r N i c r y s t a l l i t e s (178, 179). 1.5.1.4.
Copper
Copper c a t a l y s t s a r e e x t e n s i v e l y used i n low pressure methanol s y n t h e s i s (180, 181) and water g a s - s h i f t r e a c t i o n (182), i n t h e dehydrogenation o f a1 coho1 s t o t h e corresponding ketones ( 183) , c y c l ohexanol t o c y c l ohexanone ( 3 2 ) , and i n t h e h y d r a t i o n o f a c r y l o n i t r i l e t o acrylamide (33, 34). One o f t h e f i r s t attempts t o e s t i m a t e copper metal areas was made by S i n f e l t ' s group (187) who used hydrogen chemisorption. They p o i n t e d o u t , however, t h a t hydrogen a d s o r p t i o n on Cu i s slow a t room temperature and consequently a l s o used CO. More r e c e n t l y , P r i t c h a r d e t a l . (188) s t u d y i n g t h e H2 chemisorption on p o l y c r y s t a l l i n e copper and on v a r i o u s c r y s t a l l o g r a p h i c faces o f Cu, demonstrated t h e low M i l l e r ' s index faces t o be i n a c t i v e a t room temperat u r e , w h i l e those w i t h h i g h indexes showed an a c t i v a t e d hydrogen chemisorption. Once hydrogen i s chemisorbed, t h e hydrogen atoms may m i g r a t e o v e r t h e Cu surf a c e t o t h e low index planes. Therefore, due t o t h i s n o n - s p e c i f i t y and r e l a t i v e l y low hydrogen coverage a t ambient temperature and moderate pressure, hydrogen chemisorption i s n o t a good choice f o r measurement o f Cu area i n supported catalysts. Thurber (182) and l a t e r V a s i l e v i c h e t a l . (189) p o i n t e d o u t t h a t t h e surface area o f copper may be determined from oxygen chemisorption a t low temp e r a t u r e s . A t temperatures as low as 137 K t h e chemisorption i s n o t complicated by b u l k o x i d a t i o n nor p h y s i c a l a d s o r p t i o n . I n these s t u d i e s , an oxygen monolayer was found a t extremely low e q u i l i b r i u m pressures. A t h i g h e r pressures t h e
isothernis were v e r y f l a t , and t h e mono1 ayer cal c u l a t e d by back e x t r a p o l a t i o n t o z e r o O2 pressure. I n both cases, a chemisorption s t o i c h i o m e t r y Cu:02 = 4 : l was derived, which corresponds t o a Cu20 s u r f a c e compound, A much more w i d e l y used procedure i s t h e adsorption-decomposition o f N20 a t
temperatures near 370 K, as described by several authors (184, 186, 190, 191): (1.17)
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Fig.l.13. M i c r o g r a v i m e t r i c a d s o r p t i o n - d e c o m p o s i t i o n i s o t h e r m s (273-360 K) o f N20 on a 27.6 w t % Cu/Kieselguhr c a t a l y s t used i n t h e h y d r a t i o n o f a c r y l o n i t r i l e . S i n c e no p r e s s u r e change t a k e s p l a c e d u r i n g t h e a d s o r p t i o n - d e c o m p o s i t i o n t e s t , w e i g h t changes o r n i t r o g e n enrichment i n t h e gas phase have t o be measured. The advantage t o use N20 i n s t e a d o f O2 l i e s i n t h e l o w e r r e a c t i v i t y towards o x i d a t i o n w i t h N20 i n b o t h s u r f a c e and b u l k . T h i s f e a t u r e can be e x p l a i n e d o n t h e b a s i s o f d i f f e r e n c e s i n e l e c t r o n i c s t r u c t u r e o f b o t h molecules. The N20 i s a l i n e a r molecule s t a b i l i z e d by resonance, t h u s h a v i n g t h e l o w e s t r e a c t i v i t y , w h i l e O2 has t h e c h a r a c t e r o f a r a d i c a l , due t o t h e u n p a i r e d e l e c t r o n s and i s t h e r e f o r e more r e a c t i v e . The m i c r o g r a v i m e t r i c " a d s o r p t i o n - d e c o m p o s i t i o n " isotherms of N20 on a Cu/ k i e s e l g u h r c a t a l y s t used i n t h e h y d r a t i o n o f a c r y l o n i t r i l e a r e shown i n Fig.1.13. I t r e s u l t s from t h i s f i g u r e t h a t t h e s u r f a c e coverage i n c r e a s e s w i t h t e m p e r a t u r e
i n agreement w i t h t h e a c t i v a t i o n energy i n c r e a s i n g w i t h coverage. Back e x t r a p o l a t i o n t o z e r o p r e s s u r e o f t h e i s o t h e r m a t 360 K g i v e s t h e monolayer coverage by oxygen. I t i s v e r y i m p o r t a n t n o t t o conduct t h e N20 a d s o r p t i o n - d e c o m p o s i t i o n t e s t a t temperatures above 370 K, s i n c e a t t h a t temperature p a r a l l e l o x i d a t i o n of copper atoms of t h e s u b s u r f a c e l a y e r s o c c u r s ( 3 2 ) . I t i s a l s o i n t e r e s t i n g t o n o t e t h a t t h e c a t a l y s t s u p p o r t may e x h i b i t a
c e r t a i n a c t i v i t y w i t h r e g a r d t o t h e a d s o r p t i o n - d e c o m p o s i t i o n of N20. TO d e t e r mine t h e e x t e n t o f t h i s e f f e c t , a b l a n k experiment w i t h t h e s u p p o r t a l o n e i s recommended. I n t h i s r e s p e c t , Evans e t a l . (191) found t h a t p r o m o t i o n o f b u l k o x i d a t i o n o f Cu i n c a t a l y s t c o n t a i n i n g o x i d e s o f e s t i m a t i o n o f Cu d i s p e r s i o n .
Al, Zn and C r l e a d s t o an over-
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1.5.1.5.
Silver
S i l v e r c a t a l y s t s a r e used i n i n d u s t r y f o r t h e s e l e c t i v e o x i d a t i o n o f e t h y l e n e t o e t h y l e n e o x i d e . The f i r s t a t t e m p t s t o measure t h e s u r f a c e area o f s i l v e r were c a r r i e d o u t by Meisenheimer and W i l s o n ( 1 9 2 ) . These a u t h o r s s t u d i e d t h e oxygen a d s o r p t i o n on s i l v e r powders
i n t h e t e m p e r a t u r e range o f 473-523 K
and compared t h e e x t e n t o f a d s o r p t i o n t o t h a t o b t a i n e d a t 77 K u s i n g K r as a n a d s o r b a t e and a p p l y i n g t h e BET e q u a t i o n ; maximum oxygen coverage was 0.93 a t 473 K a t 4 Nm-'.
S i m i l a r s t u d i e s on s i l v e r f i l m s were conducted by Bagg and
Bruce ( 1 9 3 ) . They s t a t e d t h a t e q u i l i b r i u m i s d i f f i c u l t t o d e f i n e , due t h e s l o w a d s o r p t i o n processes beyond a p a r t i c u l a r s u r f a c e coverage, and found 02/Kr r a t i o s approaching 1 a t oxygen p r e s s u r e s o f a b o u t 1 Nrn-'. The above s t u d i e s on unsupported Ag powders o r f i l m s were f u r t h e r a p p l i e d t o supported s i l v e r c a t a l y s t s by Kholyavenko e t a l . ( 1 9 4 ) , who s t u d i e d Ag supp o r t e d on corundum p o r c e l a i n , and Be(OH)2-Si02 c a r r i e r s . They used and recommended O2 c h e m i s o r p t i o n a t 473 K f o r s u r f a c e a r e a d e t e r m i n a t i o n o f s i l v e r . As f o r copper, t h e a d s o r p t i o n decomposition o f N20 can a l s o be used f o r s i l v e r c a t a l y s t s . S c h o l t e n e t a l . ( 1 7 6 ) have a p p l i e d t h i s method as w e l l as O2 chemis o r p t i o n t o measure t h e s p e c i f i c Ag m e t a l s u r f a c e on unsupported and a l s o on alumina-supported Ag c a t a l y s t s . These a u t h o r s found good agreement between t h e e x t e n t o f a d s o r p t i o n and c r y s t a l s i z e measurements by X-ray l i n e broadening and t r a n s m i s s i o n e l e c t r o n microscopy. The e x t e n t o f O2 c h e m i s o r p t i o n a t 423 K was i n v a r i a b l y h i g h e r t h a n t h a t f o u n d f r o m N20 a d s o r p t i o n - d e c o m p o s i t i o n a t t h e same temperature, p r o b a b l y due t o a d s o r p t i o n o f m o l e c u l a r oxygen. Even c o n s i d e r i n g t h i s disadvantage o f O2 v i s a v i s N20, t h e oxygen a d s o r p t i o n measurements on Ag may perhaps p r o v i d e a m o r e m e a n i n g f u l measure o f t h e a c t i v e Ag s p e c i e s on t h i s t y p e o f c a t a l y s t because t h e e p o x i d a t i o n o f e t h y l e n e i s b e l i e v e d t o f o l l o w a r e a c t i o n pathway which i n v o l v e s adsorbed m o l e c u l a r oxygen s p e c i e s . 1.5.2. 1.5.2.1.
M e t a l Oxides N i c k e l Oxide
Many papers about t h e a d s o r p t i o n o f CO(C02), NO, and O2 have been p u b l i s h e d f o r supported n i c k e l o x i d e c a t a l y s t s . Perhaps CO i s one o f t h e most i n t e r e s t i n g probes f o r such a purpose; i t has n o t r e c e i v e d t o t a l acceptance because i t can become a t t a c h e d t o t h e s u r f a c e as a r n u l t i c e n t e r e d , b r i d g e d , o r l i n e a r s p e c i e s , w i t h s t o i c h i o m e t r i c COa/Msnt
v a l u e s r a n g i n g between 0.5 and 1. I n t h e case o f
i i i e t a l l i c N i t h i s s i t u a t i o n i s even more complex because o f t h e p o s s i b l e format i o n o f subcarbonyl s p e c i e s i n v e r y d i s p e r s e c a t a l y s t s (195. 196) i n which 2 ( o r even 3 ) CO m o l e c u l e s a r e bonded t o one N i atom. However, a s u i t a b l e c h o i c e o f e x p e r i m e n t a l c o n d i t i o n s can f a v o r t h e p r e f e r e n t i a l f o r m a t i o n o f one o f t h e s e species, i.e.,
a well-determined s t o i c h i o m e t r i c r a t i o .
B43
-
-
chemisorption -/-+.oxidation
I
. /
Ni3+
-.
Exposure ( L ) Fig.1.14. C o n c e n t r a t i o n o f s u r f a c e species observed d u r i n g t h e exposure o f a N i (210) s i n g l e c r y s t a l t o oxygen a t 295 K . I t i s also important t o consider t h e d e f e c t nature o f n i c k e l oxide. Although
t h e r e have been e x t e n s i v e s t u d i e s o f oxygen i n t e r a c t i o n w i t h n i c k e l s u r f a c e s by a v a r i e t y o f experimental techniques, e.g.,
work f u n c t i o n and photoemission
(197, 198), q u e s t i o n s r e l a t i n g t o t h e d e f e c t n a t u r e o f t h i n o x i d e (NiO) o v e r l a y e r s , i.e., evidence f o r Ni3+ s t a t e s , have remained unanswered. The a n a l y s i s o f t h e N i 2p peaks i n t h e p h o t o e l e c t r o n spectrum has r a r e l y p r o v i d e d any q u a n t i t a t i v e d a t a on t h e p a r t i c i p a t i o n o f v a r i a b l e o x i d a t i o n s t a t e s i n N i - 0 i n t e r a c t i o n , however, t h e r e e x i s t some examples where f e a t u r e s p r e s e n t i n c o r e l e v e l s p e c t r a a f t e r e x t e n s i v e o x i d a t i o n , u s u a l l y a t h i g h temperature, have been e x p l a i n e d as b e i n g due t o mixed o x i d a t i o n s t a t e s , e.g.,
N i 2 + and N i 3 + (198,
199). More r e c e n t l y , C a r l e y e t a l . (200) developped a method f o r p r o c e s s i n g t h e raw p h o t o e l e c t r o n s p e c t r a , which i n c l u d e s t h e comparison and s u b s t r a c t i o n of spectra t o
h i g h l i g h t t h e changes due t o oxygen i n t e r a c t i o n .
The oxygen chernisorption a t 295 K on N i ( 2 1 0 ) s u r f a c e d i d n o t p r o v i d e evidence i n t h e N i 2 p d i f f e r e n c e s p e c t r a f o r N i 2 + and Ni3+ s p e c i e s even though t h e s u r f a c e oxygen c o n c e n t r a t i o n i s about 1.6 x l O I 5 atoms
ern-'
( F i g . 1 . 1 4 ) . As
t h e oxygen c o n c e n t r a t i o n i n c r e a s e s above t h e s e values, e v i d e n c e f o r N i 2 + and N i 3 + s t a t e s emerge and t h e r e s p e c t i v e c o n c e n t r a t i o n s have been c a l c u l a t e d . These
a u t h o r s a l s o found r e l a t i v e l y h i g h c o n c e n t r a t i o n o f Ni3+ s t a t e s , t h a t a r e a t t h e
B44
surface, by comparison w i t h what m i g h t be expected f r o m b u l k d e f e c t e q u i l i b r i u m values. Q u a n t i t a t i v e a n a l y s i s o f t h e s t o i c h i o m e t r y o f t h e surface r e g i o n o f t h e b u l k o x i d e s , u s i n g 01s and NiPp s p e c t r a always i n d i c a t e an oxygen excess; t h e r a t i o u s u a l l y b e i n g a b o u t 2 : l . The c o n c e n t r a t i o n of Ni3'
s p e c i e s corresponds t o
s u b s t a n t i a l f r a c t i o n s o f t h e t o t a l n i c k e l w i t h i n t h e p h o t o e l e c t r o n sampling d e p t h ( c a . 1.6 nm). Furthermore, angular-dependent s t u d i e s suggest t h a t t h e y a r e a t t h e s u r f a c e , and have a s s o c i a t e d w i t h them 0- species, surface
Ni2'
0-
Ni3'
0-
Ni3+
02-
Ni2'
A r e d o x - t y p e s u r f a c e r e a c t i o n i s suggested t o account
f o r t h i s c o r r e l a t i o n which i s , i n t u r n , c o m p a t i b l e w i t h t h e c l a s s i c a l s t u d i e s on n i c k e l o x i d e (201, 202). D e l l and Stone ( 2 0 3 ) r e p o r t e d h e a t s o f a d s o r p t i o n o f oxygen, carbon monoxide, and carbon d i o x i d e on a t h i n N i O f i l m formed o v e r m e t a l l i c N i . They assumed t h a t t h e ( l o o ) , (110), adn (111) p l a n e s o f N i O were randomly d i s t r i b u t e d and t h a t each Ni2'
s p e c i e s adsorbed one CO o r C02 m o l e c u l e and one oxygen atom
( 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 o f oxygen). The p r o p e r t i e s o f t h e carbonate complex formed d u r i n g a d s o r p t i o n were discussed. Almost s i m u l t a n e o u s l y , T e i c h n e r and M o r r i s o n (204) s t u d i e d t h e s u r f a c e complex produced by CO and O2 on N i O p r e p a r e d by thermal decomposition o f N i h y d r o x i d e . T h e i r r e s u l t s i n d i c a t e d t h a t t h e s t o i c h i o m e t r y o f t h e s u r f a c e complex was between C02 and C03, t h e complex b e i n g s t a b l e a t ambient t e m p e r a t u r e b u t decomposing t o C02 a t about 473 K. These s t u d i e s , e i t h e r on N i O f i l m s o r powder c a t a l y s t s , were t h e f i r s t a t t e m p t s t o i d e n t i f y t h e n a t u r e and r e a c t i v i t y o f i n t e r m e d i a t e s which c o u l d be i n v o l v e d i n t h e c a t a l y t i c o x i d a t i o n o f CO t o C02, b u t t h e y a r e n o t d e t a i l e d p r o c e d u r a l methods a p p l i c a b l e f o r a q u a n t i t a t i v e d e t e r m i n a t i o n o f t h e s u r f a c e a r e a o f N i O i n s u p p o r t e d c a t a l y s t s . The f i r s t r e l i a b l e a t t e m p t t o q u a n t i f y s i t e d e n s i t y i n s u p p o r t e d c a t a l y s t s was t h a t o f Gandhi and S h e l e f (205), who t r i e d t o model t h e e q u i l i b r i u m d a t a o f a d s o r p t i o n o f b o t h CO and NO on A1203-supported N i O c a t a l y s t s . A d s o r p t i o n s t u d i e s were p e r f o r m e d a t t e m p e r a t u r e s between 273
and 373 K f o r CO, and monolayer coverage was o b t a i n e d b y e x t r a p o l a t i o n t o t h e common p o i n t o f t h e i n t e r s e c t i o n o f t h e l o g - l o g p l o t s o f a l l F r e u n d l i c h adsorpt i o n isotherms. By a p p l y i n g t h e same model f o r a n i s o t h e r m s e t f o r NO a t temper a t u r e s between 299 and 413 K , t h e y f o u n d t h a t t h e amount o f NO adsorbed a t
B45
I
I
I
10'
104
105
Pco (N m2) Fig.l.15. L i n e a r p l o t s of F r e u n d l i c h ' s e q u a t i o n f o r CO c h e m i s o r p t i o n a t d i f f e r e n t temperatures on two NiO/A1203 c a t a l y s t s . Redrawn f r o m Ref. 206. monolayer coverage was h i g h e r t h a n t h a t o f CO. The NO/CO r a t i o observed a t 0 = 1 was 1.34. The s m a l l e r amount of CO chemisorbed was accounted f o r b y assuming t h a t o n e - t h i r d of t h e adsorbed speces was i n t h e b r i d g e d form, i.e.,
one CO
molecule bonded t o two s u r f a c e N i 2 + i o n s . On t h e o t h e r hand, NO was p r i m a r i l y adsorbed i n a l i n e a r mode (N0/Ni2+ s t o i c h i o m e t r y i s l ) , and t h e r e f o r e t h e adsorbed amount of NO a t 0 = 1 a l l o w s f o r t h e c a l c u l a t i o n o f t h e d i s p e r s i o n o f nickel. Chemisorption of CO a t temperatures below 373 K has been used r e c e n t l y (206) t o e v a l u a t e t h e d i s p e r s i o n o f N i O i n a s e r i e s o f NiO/A1203 c a t a l y s t s whose N i O l o a d i n g v a r i e d between 1 and 15% N i O . From t h e e q u i l i b r i u m data, i s o s t e r i c h e a t s of a d s o r p t i o n were c a l c u l a t e d by means o f t h e Clausius-Clapeyron e q u a t i o n as a f u n c t i o n o f CO coverage. They decreased e x p o n e n t i a l l y w i t h coverage, i n d i c a t i n g s u r f a c e h e t e r o g e n e i t y . Such h e a t s o f a d s o r p t i o n were remarkably l o w e r t h a n t h e i n i t i a l d i f f e r e n t i a l h e a t o f CO on unsupported N i O a t 303 K (175 kJ/mol) found by G r a v e l l e e t a1
.
(207), and h i g h e r t h a n t h a t g i v e n by Gandhi and S h e l e f (205)
( 2 5 kJ/mol) f o r CO a d s o r p t i o n on 8.78% NiO/A1203 c a t a l y s t ( s e e above).
On t h e b a s i s o f t h e above f i n d i n g s , t h e a d s o r p t i o n d a t a a t e q u i l i b r i u m were f i t t e d t o F r e u n d l i c h ' s model (Fig.l.15).
A c o r r e c t i o n due t o CO a d s o r p t i o n on
A1203 uncovered by N i O was i n t r o d u c e d by s u b t r a c t i n g t h i s v a l u e from t h e o v e r a l l
a d s o r p t i o n . The monolayer coverage i n c r e a s e d a l m o s t l i n e a r l y w i t h N i O l o a d i n g up
B46
t o about 8% and then decreased. T h i s suggested t h a t NiO i s p r i m a r i l y spread on t h e support s u r f a c e as a h i g h l y dispersed phase, b u t a t h i g h e r N i O c o n t e n t s (above 8%) b u l k l i n e N i O c r y s t a l l i t e s could presumably form. The CO a d s o p r t i o n per u n i t area o f N i O o b t a i n e d i n Ref. 206 was l a r g e r ( b y a f a c t o r >2) than t h a t found by M a r c e l l i n i e t a1
.
(208) and G r a v e l l e e t a1
.
(207) f o r t h e system CO/unsupported N i O . T h i s d i f f e r e n c e c o u l d be due t o t h e a d s o r p t i o n o f a s i n g l e CO molecule as a b r i d g e d species i n t h e l a t t e r s t u d i e s ; however, i n t h e c a t a l y s t s o f Tablel.5,only
l i n e a r species (except f o r t h e 13.6%
Ni0/A1203 c a t a l y s t ) were detected. On t h e o t h e r hand, t h e CO a d s o r p t i o n a t monol a y e r coverage o f 5.98 mmol/g N i O on 8.78% NiO/A1203 c a t a l y s t , found by Gandhi and Shelef (205), was o n l y s l i g h t l y l a r g e r than t h a t o f 4.36 mmol CO/g N i O found on 8% NiO/A1203 c a t a l y s t (Table 1.5).This
reveals a higher dispersion o f
t h e o x i d e i n t h e former case, which i s p o s s i b l y due e i t h e r t o d i f f e r e n c e s i n t h e p r e p a r a t i o n method o f c a t a l y s t s o r t o t h e h i g h e r s p e c i f i c s u r f a c e area o f t h e a1 umi na used by those authors. TABLE 1.5 Parameters o f CO Adsorption on NiO/A1203 C a t a l y s t s Mono1ayer coverage (mnol CO/g)
O ispersion
1
0.046
38.6
Wt% NiO
(%I
Adsorption heata (kJ/mol )
4
0.171
33.2
38.9
8
0.303
31.4
47.4
13.6
0.262
15.0
-
aThe a d s o r p t i o n heat (nRT) a t a coverage o f 0.37 decreased w i t h i n c r e a s i n g temperature; t h e r e f o r e t h e Halsey and T a y l o r m o d i f i c a t i o n (212) was i n t r o d u c e d f o r i t s c a l c u l a t i o n ( n R T / ( l - r T ) ) ( f o ca a l y s i s c o n t a i n i n g 4 and 8 w t % N i O , r values were 2.14 x los3 and 2.49 x K-!, respectively). E q u i l i b r i u m a d s o r p t i o n data (Fig.l.15)
were supplemented w i t h i n f r a r e d
measurements. I R s p e c t r a f o r CO a d s o r p t i o n on t h e samples w i t h 8 and 13.6% N i O were obtained (Fig.1.16). CO a d s o r p t i o n on 8% NiO/A1203 c a t a l y s t y i e l d s bands a t 2050 and 2175 cm-',
which undergo a remarkable decrease i n i n t e n s i t y ( t h e l a t t e r
s h i f t i n g t o 2180 cm-')
a f t e r evacuation f o r 1 h. A f t e r a d s o r p t i o n o f CO on 13.6%
NiO/A1203 c a t a l y s t , two shoulders appear a t 1980 and 1930 cm" bands a t 2050 and 2180
as w e l l as two
ern-'.
A l l i n t e n s i t i e s decreased
remarkably
a f t e r evacuation
, with a s h i f t t o
h i g h e r wavenumbers. I n f r a r e d bands a t 1960-1970 and 2060 cm-l found by C o u r t o i s and Teichner (209) and a t 2050, 1985. and 1925 cm"
by Alexeyev and T e r e n i n
(210) a f t e r a d s o r p t i o n o f CO on pure N i O d i f f e r from t h a t above 2200
0-l
B47
13.6% N iO/Al 203
8 %N iO/Al, 0,
0 0
0 N
I
I
I
I
I
I
2200
2000
1900
2200
2000
190(
Wavenumber (cm-1)
2 Fig.1.16. ( a ) I n f r a r e d s p e c t r a o f CO (3.3 kN/m ) a t room t e m p e r a t u r e f o r 0.5 h. ( b ) A f t e r o u t g a s s i n g a t 0.1 N/m2 a t r om t m p e r a t u r e f o r 1 h. ( c ) Background s p e c t r a a f t e r o u t g a s s i n g a t 1.3 x 10-i N/m5 a t 773 K f o r 16 h. Redrawn f r o m Ref. 206. observed by Bore110 e t a l . (211) f o r t h e a d s o r p t i o n o f CO on supported N i O . The band a t 1980 cm-'
n-
and y-alumina-
s h o u l d correspond t o a b r i d g e d s p e c i e s
produced by a d s o r p t i o n o f CO on two n e i g h b o r i n g Ni2'
i o n s , and t h e band a t 1930
cm-l c o u l d be due t o a d i f f e r e n t b r i d g e d s p e c i e s formed by i n t e r a c t i o n o f CO w i t h N i 2 + and one 02- i o n o f t h e o x i d e l a t t i c e ( 1 9 6 ) . Another p o s s i b i l i t y would be a b r i d g e d s t r u c t u r e w i t h two N i 2 + i o n s o f a d i f f e r e n t n a t u r e . The bands a t 2050 and 2180 cm-l should be due t o a l i n e a r s p e c i e s i n which one CO m o l e c u l e i s a t t a c h e d t o one Ni2'
i o n . The l i n e a r s p e c i e s i s n o t t o t a l l y r e v e r s i b l e , as
t h e bands above 2000 cm-l do n o t disappear upon evacuation. T h i s suggests t h a t
t h i s species is more s t r o n g l y h e l d by t h e supported t h a n b y t h e unsupported o x i d e . On t h e o t h e r hand, t h e l i n e a r species y i e l d s bands o f h i g h e r i n t e n s i t y , i.e.,
t h e y a r e much more f r e q u e n t on t h e s u r f a c e t h a n b r i d g e d CO, w i t h bands o f
l o w e r i n t e n s i t y . A s w i t h supported n i c k e l ( 1 9 5 ) , l i n e a r s p e c i e s s h o u l d be more f r e q u e n t l y formed i n t h e more d i s p e r s e d NiO/A1203 c a t a l y s t s . T h i s b e i n g so, t h e p r o p o r t i o n o f b r i d g e d species on 1 and 2% NiO/A1203 c a t a l y s t s w i l l be l o w e r t h a n t h a t i n 8 and 13.6% Ni0/Al2O3 ones. Therefore, i n c a t a l y s t s w i t h l o w N i O l o a d i n g
B48 (Table 1.5),the l i n e a r species i s predominant. On t h e b a s i s o f t h e above d i s c u s s i o n , i t i s concluded t h a t s u r f a c e s i t e d e n s i t y measurements i n supported N i O c a t a l y s t s r e q u i r e n o t o n l y d e t e r m i n a t i o n
o f t h e monolayer coverage o f a g i v e n probe molecule (CO) b u t a l s o t h e p r e c i s e stoichiornetry between t h e probe and t h e N i 2 + s i t e s . For t h i s purpose, i n f r a r e d spectroscopy i s a s u i t a b l e technique t h a t enables one t o e s t a b l i s h unambiguously how two parameters ( c a t a l y s t p r e p a r a t i o n and t y p e o f probe used i n t h e cheniisorption t e s t ) i n f l u e n c e t h e s t i o c h i o m e t r y . 1.5.2.2.
Chromia
Unsupported and supported chromia c a t a l y s t s were i n v e s t i g a t e d f o r many years i n t h e c o n t e x t o f o l e f i n p o l y m e r i z a t i o n and dehydrogenation r e a c t i o n s o f hydro-
carbons. Because t h e s u r f a c e c h e m i s t r y o f chromia was v e r y promising, much o f t h e work was o f a fundamental nature. Some y e a r s ago, V o l t z and W e l l e r (213) suggested t h a t t h e c a t a l y t i c a l l y a c t i v e s i t e i n reduced chromia i s a coordinat i v e l y unsaturated (CUS) C r 3 + s i t e . Chemisorption o f oxygen gave r i s e t o t h e f o r m a t i o n o f Cr6'
i o n s which were r e s p o n s i b l e f o r a l o s s o f c a t a l y t i c a c t i v i t y .
From t h e e x t e n t of oxygen chemisorption ( o r excess), as measured by water e x t r a c t i o n and iodometric t i t r a t i o n o f Cr3'
ions, t h e y e s t a b l i s h e d t h e a b s o l u t e
o x i d a t i o n - r e d u c t i o n l e v e l f o r unsupported chromia. In a
f u r t h e r step, t h i s approach was extended t o alumina-supported chromia
(214). By means o f i o d o m e t r i c t i t r a t i o n s , t h e e x t e n t o f t h e r e v e r s i b l e adsorbed
oxygen a t 773 K was determined. From measurement o f t h e chmisorbed oxygen on a few unsupported chromia samples o f known BET s u r f a c e areas, an average v a l u e o f 2 0.163 hi o f Cr203/wnol o f chemisorbed oxygen was found. I f t h e assumption i s made t h a t t h e surfaces o f unsupported and supported chromia c a t a l y s t s behave i d e n t i c a l l y , t h e above value can be taken as a conversion f a c t o r between t h e e x t e n t o f chemisorbed oxygen on an a1 umina-supported chromia c a t a l y s t and t h e s p e c i f i c area o f t h e chromia. The o r i g i n a l ideas o f V o l t z and W e l l e r were subsequently a p p l i e d by MacIver and co-workers (11, 213) t o Cr203/A1203 c a t a l y s t s whose C r l o a d i n g ranged between 2.1 and 10.1%. They observed t h a t H2-treated c a t a l y s t s ( b o t h unsupported and supported) evacuated a t 773 K r a p i d l y cheiiiisorb oxygen and CO a t 77 K. Again, from a comparison o f unsupported and alumina-supported chromia, a c a l c u l a t i o n o f t h e apparent s u r f a c e coverage (ASC) o f t h e support can e a s i l y be made. Charcosset e t a1
.
(216) compared t h e p o l y m e r i z a t i o n r a t e s of propylene and
t h e chromia areas as determined g r a v i m e t r i c a l l y from t h e e x t e n t of i r r e v e r s i b l y adsorbed oxygen a t 195 K. By assuming a s p e c i f i c s t r u c t u r e f o r chromia and a two-dimensional l a y e r o f chromia on t h e s i l i c a - a l u m i n a support, they c a l c u l a t e d t h e c a t a l y s t s u r f a c e area. A v a l u e o f 0.223 m 2 o f Cr203 per umol of chemisorbed oxygen, s l i g h t l y h i g h e r than t h e one found by V o l t z and W e l l e r (214), was t h u s
B49
o b t a i n e d . Maxinium d i s p e r s i o n s were reached a t l o w C r l o a d i n g s , and maximum c a t a l y s t s u r f a c e area and c a t a l y t i c a c t i v i t i e s were found a t a l o a d i n g of 6.84% Cr203. The average o x i d a t i o n s t a t e o f chromium i o n s v a r i e d w i t h t h e C r l o a d i n g . Furthenilore, w i t h a c o n s t a n t C r l o a d i n g o f 3%, an i n c r e a s e i n b o t h s u r f a c e a r e a and c a t a l y t i c performance up t o an average o x i d a t i o n s t a t e o f 5.6 (where t h e d i s p e r s i o n was a l s o a maximum) was found. By u s i n g NO as t h e probe molecule, O t t o and S h e l e f ( 8 1 ) f o u n d t h a t t h e e x t e n t o f NO c h e i i i i s o r p t i o n on b o t h reduced and unreduced chromia-a1 umina c a t a l y s t s obeyed Freundl i c h ' s isotherm. As s t a t e d above f o r n i c k e l o x i d e c a t a l y s t s , e x t r a p o l a t i o n of t h e l i n e a r p o r t i o n s o f t h e l o g - l o g p l o t s t o t h e p o i n t o f common i n t e r s e c t i o n g i v e s t h e monolayer coverage. F o r t h e reduced c a t a l y s t t h i s method y i e l d e d 1.1 NO molecules p e r s u r f a c e chromium i o n . The number o f s u r f a c e chromium atoms was e s t i m a t e d by measuring t h e amount o f oxygen r e q u i r e d t o comp l e t e l y o x i d i z e a reduced c a t a l y s t . However, l o g - l o g p l t o s o f t h e o x i d i z e d samp l e s f a i l e d t o show a common i n t e r s e c t i o n p o i n t . The e x t e n t o f CO c h e m i s o r p t i o n was found t o be d i r e c t l y p r o p o r t i o n a l t o t h e r a t e of e t h y l e n e p o l y m e r i z a t i o n o f a s i l i c a - s u p p o r t e d chromium (6+) o x i d e c a t a l y s t ( 2 1 7 ) . Furthermore, i t was observed t h a t t h e r e d u c t i o n o f t h e c a t a l y s t a t 373 K i n CO y i e l d e d a b e t t e r a c t i v i t y t h a n one p r e t r e a t e d i n e t h y l e n e atmosphere, t h e l a t t e r a l s o decreasing w i t h i n c r e a s i n g CO c h e m i s o r p t i o n . When CO c h e n i i s o r p t i o n i s performed a t 323 K, t h e c a t a l y s t becomes much l e s s a c t i v e , t h u s i n d i c a t i n g t h a t CO poisons t h e p o l y m e r i z a t i o n c e n t e r s . Only a small f r a c t i o n o f t h e o v e r a l l CO a d s o r p t i o n i s on a c t i v e s i t e s , b u t t h e e x t e n t o f CO c h e m i s o r p t i o n i s p r o p o r t i o n a l t o t h e e t h y l e n e p o l y m e r i z a t i o n . A l l t h e s e f i n d i n g s were d i s c u s sed i n t h e c o n t e x t o f t h e o x i d a t i o n s t a t e o f chromium ions, which i s most i m portant for t h i s c a t a l y t i c reaction. 1.5.2.3.
Copper Oxides
Copper o x i d e c a t a l y s t s were f i r s t s t u d i e d i n t h e e a r l y 1950s by Garner e t a l . (218) t o determine t h e a c t i v e s i t e s f o r CO o x i d a t i o n , and more r e c e n t l y f o r use as automobile exhaust p u r i f i c a t i o n c a t a l y s t s (219-221), f o r d e h y d r a t i o n dehydrogenation o f a1 coho1 s (222) , and f o r methanol s y n t h e s i s (223, 224). A l t h o u g h a few o f t h e s e s t u d i e s were r e s t r i c t e d t o unsupported c a t a l y s t s , t h e y focused on supported c a t a l y s t s . London and B e l l (225) s t u d i e d t h e i n t e r a c t i o n of CO and NO probe molecules on s i l i c a - s u p p o r t e d cooper o x i d e s by means o f b o t h
i n f r a r e d spectroscopy and k i n e t i c measurements. T h e i r r e s u l t s i n d i c a t e t h a t t h e species w i t h CO, and t h e NO, N20, and 140; s p e c i e s w i t h NO, appeared a t t h e i n t e r f a c e . They a l s o r e p o r t e d t h a t CO chemisorbs m o s t l y on
adsorbed CO, C02 and C0:-
Cut s i t e s (and p o s s i b l y on Cu2'),
and t h a t c o m p e t i t i o n f o r s i t e s between b o t h
probes e x i s t s . The s i t e s i n v o l v e d i n NO c h e m i s o r p t i o n d i s s o c i a t e d NO m o l e c u l e s .
B50 F o l l o w i n g t h e same l i n e o f a n a l y s i s as f o r a l u m i n a - s u p p o r t e d chromia, Gandhi and S h e l e f (221) a t t h e F o r d M o t o r l a b o r a t o r i e s f o u n d a d s o r p t i o n o f NO a t e q u i l i b r i u m on b o t h unsupported and alumina-supported copper o x i d e s t o be q u a n t i t a t i v e l y d e s c r i b e d by F r e u n d l i c h ' s e q u a t i o n f o r a s e t o f i s o t h e r m s w i t h i n t h e temp e r a t u r e range o f 297 t o 413
K. By e x t r a p o l a t i o n o f t h e l i n e a r p o r t i o n s o f t h e
l o g - l o g p l o t s o f t h e s e i s o t h e r m s , t h e y found a common i n t e r s e c t i o n p o i n t (0 = 1 ) a t an e q u i l i b r i u i n p r e s s u r e o f NO o f 11.3 kNm-2. From t h e s e d a t a , t o g e t h e r w i t h t h e BET s u r f a c e area o f unsupported copper o x i d e , t h e same a u t h o r s concluded t h a t a 1:l s t o i c h i o m e t r y e x i s t s between t h e adsorbed NO on t h e s u r f a c e Cu i o n s , which a l l o w s c a l c u l a t i o n o f t h e s u r f a c e a r e a o f t h e a l u m i n a s u p p o r t covered by copper o x i d e . Supported copper o x i d e behaves d i f f e r e n t l y f r o m t h e o t h e r o x i d e s w i t h r e s p e c t t o NO c h e m i s o r p t i o n . O x i d i z e d c a t a l y s t s w i t h Cu2+ i o n s chemisorb NO f a s t e r t h a n cuprous ones. I t i s p o s s i b l e t h a t Cut i o n s do n o t chemisorb NO a t a l l , and t h a t NO c h e m i s o r p t i o n on a g i v e n copper i o n o c c u r s o n l y a f t e r i t s o x i d a t i o n t o t h e c u p r i c s t a t e . There i s a g r e a t resemblance t o t h i s phenomenon i n t h e c h e m i s t r y o f n i t r o s y l complexes: s o l u t i o n s o f Cu2'
i o n s g e n e r a l l y adsorb NO,
w h i l e s o l u t i o n s o f Cu+ do n o t . C o o r d i n a t i o n o f t h e N O m o l e c u l e , r e q u i r e d i n b o t h complex f o r m a t i o n and c h e m i s o r p t i o n , i n v o l v e s e l e c t r o n t r a n s f e r f r o m t h e a n t i bonding n - o r b i t a l o f t h e NO t o an u n p a i r e d d - o r b i t a l o f t h e c o o r d i n a t i n g i o n . 9 t I n Cu2+ i o n s ( d ), such an u n p a i r e d o r b i t a l i s a v a i l a b l e , b u t i n Cu i o n (dlO) i t i s n o t . T h i s assumption has r e c e n t l y been c o n f i r m e d by s t u d y i n g NO chemisorp-
t i o n on zinc-promoted HDS c a t a l y s t s ( 2 2 6 ) i n w h i c h t h e Zn2'
i o n s w i t h a d10
c o n f i g u r a t i o n do n o t show i n f r a r e d bands o f chemisorbed NO. The a d s o r p t i o n o f NO on diamagnetic adsorbents, such as ZnO, t a k e s p l a c e w i t h o u t e l e c t r o n p a i r i n g and t h e r e f o r e i s o b s e r v a b l e by ESR. Such c h e m i s o r p t i o n i n v o l v e s o n l y a m i n o r p a r t o f t h e LnO s u r f a c e ( 1 3 7 ) . On t h e o t h e r hand, on s u p p o r t e d copper o x i d e t h e p a r a l l e l i s m between t h e c h e m i s o r p t i o n r a t e and t h e r e l a t i v e a c t i v i t y f o r t h e c a t a l y t i c r e d u c t i o n o f NO by CO, observed i n 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 s , does n o t h o l d . Gandhi and S h e l e f ( 2 2 1 j observed a much s l o w e r c h e m i s o r p t i o n r a t e o f NO on s u p p o r t e d copper o x i d e t h a n on t h e o t h e r supported t r a n s i t i o n m e t a l o x i d e s , a l t h o u g h t h e f o r m e r c a t a l y s t i s among t h e more a c t i v e ones. T h i s p e c u l i a r i t y may be e x p l a i n e d on t h e b a s i s o f t h e r e l a t i v e ease w i t h which oxygen can r e l e a s e t h e s u r f a c e ( c f . Fig.1. 1 2 ) , t h u s i n c r e a s i n g t h e t u r n o v e r number. A c l u s t e r c o n t a i n i n g a few Cu atoms
i n an o x i d a t i o n s t a t e below 2+ i s p r o b a b l y t h e s t a t i o n a r y a c t i v e c e n t e r . 1.5.2.4.
I r o n Oxides
The p i o n e e r i n g work o f O t t o and S h e l e f ( 2 2 7 ) i n d i c a t e d t h a t NO i s a good probe m o l e c u l e f o r measurement o f t h e exposed Fe i o n s on t h e s u r f a c e o f aluminas u p p o r t e d c a t a l y s t s . They r e p o r t e d t h a t t h e amount o f NO m o l e c u l e s adsorbed p e r
B51
t
150%FelA1203
lo-'
ld
I
I
ld
10'
I
Pco (Nm?
Fig.l.17. F r e u n d l i c h ' s isotherms o f CO c h e m i s o r p t i o n a t d i f f e r e n t temperatures on two alumina-supported i r o n o x i d e c a t a l y s t s . Redrawn from Ref. 229. i r o n atom i n t h e s u r f a c e can be assessed by employing t h e p r e v i o u s l y observed 1 : l s t o i c h i o m e t r y i n chromia and t h e s t r u c t u r a l s i m i l a r i t y between Cr203 and
a-Fe203 ( t h e s e compounds a r e isomorphous and d i f f e r i n t h e i r l a t t i c e parameters by o n l y 0.5% ( 2 2 8 ) ) . The a p p l i c a t i o n o f t h i s c r i t e r i o n t o a 8.15% Fe/A1203 c a t a l y s t shows t h a t 66% o f t h e Fe3+ i o n s a r e exposed i n t h e s u r f a c e as opposed t o 25% i n t h e case o f t h e Cr203/A1203 c a t a l y s t prepared i n a s i m i l a r manner. By u s i n g t h e l i m i t i n g amount o f NO c h e m i s o r p t i o n as 10.4 covered by i r o n o x i d e i s about 90 m2.g-l.
mol.m-2(BET),
the surface
Again, t h i s r e s u l t , p r e d i c a t e d on t h e
assumption t h a t t h e s u r f a c e p o p u l a t i o n s on p u r e and s u p p o r t e d o x i d e s a r e equal, should serve o n l y as an approximate v a l u e . More r e c e n t l y , F i e r r o e t a l . (229) used t h e CO probe t o measure t h e d i s p e r s i o n degree o f Fe203 on s e v e r a l potassium-promoted i r o n c a t a l y s t s f o r ammonia s y n t h e s i s . I n t h i s s u t d y t h e F r e u n d l i c h i s o t h e r m was found t o a p p l y t o two c a t a l y s t s c o n t a i n i n g 1.84 and 15.0% Fe, r e s p e c t i v e l y (Fig.1.17). p o i n t o f t h e F r e u n d l i c h p l o t s (C),
The i n t e r s e c t i o n
as w e l l as t h e d i s p e r s i o n percentage a r e
summarized i n Table 1.6.From t h e a d s o r p t i o n a t monolayer coverage (C,),
and
assuming a FeSur/COad s t o i c h i o m e t r y o f 1:1, t h e s u r f a c e Fe203 and Fe203 d i s p e r s i o n ( g i v e n by t h e r a t i o between t h e exposed atoms and t h e o v e r a l l atoms i n t h e sample) were c a l c u a l t e d . From CM
v a l u e s and assuming a d e n s i t y o f m e t a l
i o n s i n t h e (001) p l a n e f o r p u r e a-Fe203 equal t o 9 . 8 ~ 1 0n ~ '~~ ,t h e a r e a covered
B52 by Fe203 per g c a t a l y s t was a l s o calculated. S i m i l a r l y , the c r y s t a l l i t e p a r t i c l e size, d, f o r each c a t a l y s t was deduced from d = ~ / P . s , where P i s t h e d e n s i t y o f metal oxide. S, t h e surface area o f Fe203 per g o f metal oxide, i s a l s o included i n Tablel.6.The high c r y s t a l s i z e obtained f o r both c a t a l y s t s , and e s p e c i a l l y f o r the c a t a l y s t containing 15% Fe. suggests t h a t the i r o n oxide i s poorly dispersed on t h e alumina surface, i.e.,
bul k l ike Fe203 c r y s t a l 1i t e s may
be present a t the i n t e r f a c e . TABLE 1.6 Dispersion, Surface Area, and Crystal Size o f Fe/A1203 Catalysts Fe content
('1 1.84 15.0
S
d (nm)
CM (mmol .g-'cat)
Dispersion
37.1
11.2
124
7.2
70.0
2.6
29
32.9
(%I
(m
I n a c a r e f u l study o f NO chemisorption on alumina-supported Fe and alkalyzed Fe Fischer-Tropsch c a t a l y s t s , King and Peri (230) observed t h a t the o x i d i z e d c a t a l y s t w i t h 10% Fe e x h i b i t e d an intense i n f r a r e d adsorption band near 1800 cm-l and assigned i t t o NO chemisorbed on Fe2+ s i t e s , w h i l e a second band near 1720 cm-l, present o n l y i n t h e reduced c a t a l y s t s , disappeared a f t e r r e o x i d a t i o n . They a l s o found a d d i t i o n a l bands i n t h e 1880-1920 cm-' r e g i o n and i n the region below 1600, which are consistent w i t h production o f e l e c t r o n - d e f i c i e n t and o x i d i z ing s i t e s , r e s p e c t i v e l y
.
I t i s r a t h e r w e l l established t h a t reduced i r o n c a t a l y s t s are unique among
coiiunon Fischer-Tropsch c a t a l y s t s i n t h a t t h e CO molecule can d i s s o c i a t e t o form a thermodynatiiically more s t a b l e surface carbide and surface oxide, even a t room temperature (231-233). This i s probably the major reason f o r the i n a b i l i t y t o observe chemisorbed CO on supported i r o n by I R spectroscopy, although t r a c e s o f oxygen have been a l s o i m p l i c a t e d (234). I n view o f t h e appearance o f bands ascribable t o FeO when NO i s chemisorbed on these c a t a l y s t s , incomplete c a t a l y s t reduction cannot e x p l a i n the l a c k o f CO bands. I n f r a r e d spectroscopy provides a p o t e n t i a l t o o l f o r monitoring t h e i n t e r a c t i o n o f i r o n s i t e s w i t h various adsorbates. Probe molecules such as CO o r NO are s e n s i t i v e i n d i c a t o r s of the nature o f t h e adsorption s i t e . Despite t h e obvious appropriateness of CO as a probe o f i r o n c a t a l y s t s , i t i s n o t a u s e f u l adsorbate w i t h reduced i r o n . With t h e exception o f the I R studies o f Blyholder and N e f f using i r o n c a t a l y s t s immersed i n o i l (235). attempts t o observe CO chemisorbed on supported i r o n have been l a r g e l y unsuccessful. T h i s may be r e l a t e d t o problems i n o b t a i n i n g clean reduced surfaces (236). Therefore, i t seems t h a t the
B53
i 0 inolecule i s a s u i t a b l e probe t o q u a n t i f y t h e i o n i c i r o n s i t e s a t t h e s u r f a c e o f supported i r o n c a t a l y s t s . However, t h e i r n a t u r e , environment, and r e l a t i v e appearance when reduced can o n l y be i n v e s t i g a t e d i n d e t a i l by means of I R spectroscopy. 1.5.2.5.
Molybdena
Molybdenum-containing c a t a l y s t s a r e e x t e n s i v e l y used i n h y d r o t r e a t i n g p r o cesses, i n c l u d i n g h y d r o d e s u l f u r i z a t i o n ( H I S ) , h y d r o d e n i t r o g e n a t i o n (HDN), and a e n i e t a l a t i o n (HDI4), d i s p r o p o r t i o n a t i o n , ammoxidation, and s e l e c t i v e o x i d a t i o n o f o l e f i n s . I n a1 1 these supported molybdena c a t a l y s t s , t h e a c t i v e component (Moo3) i s v e r y s e n s i t i v e t o b u l k r e d u c t i o n ( o r t o r e o x i d a t i o n when reduced). T h i s f a c t makes i t d i f f i c u l t t o develop a c h e m i s o r p t i o n procedure t h a t p e r m i t s d i s c r i m i n a t i o n between p u r e c h e m i s o r p t i o n and b u l k-phase r e a c t i o n . One o f t h e f i r s t a t t e m p t s t o use c h e m i s o r p t i o n o f a p r o b e m o l e c u l e f o r t h e ineasurenient o f iliolybdena d i s p e r s i o n was t h a t o f Massoth ( 2 3 7 ) . T h i s a u t h o r found t h a t 1-butene chemisorbs on a p a r t i a l l y reduced 10% Mo/A1203 c a t a l y s t i n an amount c a l c u l a t e d t o be 0.63 ( m o l a r r a t i o ) t o t h e t o t a l molybdenum p r e s e n t , which was t a k e n as evidence t h a t molybdena i s spread as a monolayer. I n a f u r t h e r s t u d y , H a l l and Massoth (238) c a l c u l a t e d t h e r e v e r s i b l e and i r r e v e r s i b l e H2 amounts on a 8% M6/A1203 c a t a l y s t prereduced t o v a r i o u s degrees. F o r a l o w
e x t e n t o f b u l k r e d u c t i o n , t h e i r r e v e r s i b l e H2 corresponded t o two H atoms p e r 0 vacancy. However, above t h i s l e v e l o f b u l k r e d u c t i o n i t i s n o t c l e a r whether t h e h i g h temperature r e t e n t i o n o f H2 i s a s u r f a c e o r a b u l k phenomenon. I n t h e l i g h t o f t h e e x t e n s i v e work c a r r i e d o u t by W e l l e r e t a l . (12, 170)
and by B r i d g e s e t a l . (11, 214) on chromia-alumina c a t a l y s t s , W e l l e r ( 5 2 ) sugg e s t e d a method, based on t h e c h e m i s o r p t i o n o f oxygen a t l o w temperature (LTOC), f o r measuring t h e s u r f a c e a r e a o f molybdena i n molybdenum-containing c a t a l y s t s . T h i s method i n v o l v e s d e t e r m i n a t i o n o f two s u c c e s s i v e O2 a d s o r p t i o n i s o t h e r m s a t 77 K ( o n reduced c a t a l y s t s ) w i t h an i n t e r m e d i a t e o u t g a s s i n g a t 195 K. The d i f f e r e n c e between t h e f i r s t and t h e second i s o t h e r m s was t a k e n as t h e amount of chemisorbed O2 ( s e e Fig.l.4b).
Dynamic oxygen c h e m i s o r p t i o n (DOC) has a l s o been
used by s e v e r a l r e s e a r c h e r s . However, t h e e x t e n t o f O2 c h e m i s o r p t i o n o b t a i n e d by t h i s l a s t t e c h n i q u e i s n o r m a l l y s m a l l e r t h a n t h a t found by v o l u m e t r i c methods, s i n c e t h e o u t g a s s i n g s t e p o f t h e c a t a l y s t s a f t e r r e d u c t i o n i s more e f f e c t i v e when u s i n g s t a t i c methods. I t i s g e n e r a l l y accepted t h a t c o o r d i n a t i v e l y u n s a t u r a t e d (CUS) Mo i o n s on
reduced and s u l f i d e d c a t a l y s t s a r e t h e a c t i v e s i t e s f o r h y d r o g e n a t i o n and hydrod e s u l f u r i z a t i o n , and t h a t oxygen, when chemisorbed a t temperatures around 195 K , does so d i s s o c i a t i v e l y (239, 240) on t h e CUS. However, on a n a l y z i n g t h e LTOC r e s u l t s on Irlo/A1203 c a t a l y s t s , i t i s c o n s i s t e n t l y observed t h a t o n l y a s m a l l f r a c t i o n o f t h e t o t a l CUS i s t i t r a t a b l e by LTOC. T h i s c o n c l u s i o n i s d e r i v e d f r o m
B54
.-c0 -06 0
f
0
z -04
-.02 3
0
6 9 12 15 Surface concentration (pmole Mo m2)
F i g 1 1 8 . Oxygen and NO i r r e v e r s i b l e chemisorption amounts on t h e reduced (a = 0.92-0.95) Mo/Al203 c a t a l y s t s a s a function of t h e Mo-loading. (0) O/Mo r a t i o . (0) NO/Mo r a t i o . For comparative purposes the O/Mo r a t i o s ( v ) from Ref. 244 a r e a l s o included.
=
t h e low O/Mo r a t i o reported by several researchers. This r a t i o i s s i g n i f i c a n t l y lower than expected because under normal conditions, i . e . , 773 K in H2, i n Mo/A1203 c a t a l y s t s about 75% of Mo6+ ions a r e converted t o Mo4+ (241, 242) generating an equivalent amount of CUS t h a t should, i n p r i n c i p l e , serve a s a potential centers f o r O2 chemisorption. The low O/Mo r a t i o t h e r e f o r e implies t h a t only a minority of the CUS a r e capable of holding chemisorbed oxygen a t low temperature. The extent of oxygen and NO chemisorptions a s a function of Mo loading f o r an unextracted (and extracted by ammonia) , reduced Mo/A1203 c a t a l y s t s s e r i e s (reduction degree almost constant: 0.92-0.95) , a s studied by Caceres e t a l . (243) a r e given in F i g . l l 8 . LTOC data given by Nag (244) on a reduced Mo/A1203 c a t a l y s t s e r i e s a t 773 K f o r 6 h a r e a l s o included f o r comparative purposes. From t h e reduction c h a r a c t e r i s t i c s i t i s c l e a r t h a t the O/Mo r a t i o i s r a t h e r low (much lower f o r Nag's d a t a ) . Furthermore, i f a l l the CUS a r e a b l e t o hold oxygen, the monotonic decrease i n O/Mo ( o r NO/Mo) r a t i o w i t h increasing Mo loading does not conform with t h e well-documented f a c t t h a t with increasing Mo loading t h e f r a c t i o n of more reducible octahedral Mo oxide, Moo, increases w i t h a concurrent increase of the number of CUS (204, 245). A monotonic increase i n t h e O/Mo r a t i o u p t o O/Mo = 1 as a function of Mo loading, r a t h e r than a
B55 decrease, should have been observed i f a l l o f t h e CUS generated by r e d u c t i o n could chemisorb oxygen. This c o n f l i c t i n g behaviour has been explained i n two d i f f e r e n t ways. On the one hand, Nag ( 2 4 4 ) considers t h a t a t very low surface coverage the m a j o r i t y , i f not a l l , o f the surface Mo remains as the t e t r a h e d r a l . MoT, and i t s low reducib i l i t y permits only a small f r a c t i o n o f the t o t a l Mo t o generate Species 1,
0
\ /" Mo
0'
0'
U 1
t h a t does chemisorb
oxygen. As coverage increases, the p r o p o r t i o n o f polymeric
Moo a l s o increases. Although Moo has a higher r e d u c i b i l i t y than MoT (241), t h e CUS t h a t are generated by reduction o f Moo probably do n o t h o l d chemisorbed
oxygen, as observed from the decreasing O/Mo r a t i o w i t h increasing Moo, i.e., increasing Mo loading. On the o t h e r hand, a q u i t e d i f f e r e n t explanation has r e c e n t l y been given by Caceres e t a1
. (283).
To compare the O2 and
NO chemisorp-
t i o n capacities of the catalysts, a l l o f them were prereduced a t a s i m i l a r degree ( a = 0.32-0.95)
4+) . From
(a = 1 means t h a t a l l Mo ions are reduced t o Mo
the i r r e v e r s i b l e O2 and NO uptake on both prereduced, Mo extracted and unextracted Mo/A1203 c a t a l y s t s (Fig.IJ8),
i t i s c l e a r t h a t i r r e v e r s i b l e NO chernisorption
on a l l c a t a l y s t s i s much lower than the corresponding
O2 uptake, a t r e n d known
t o occur when CO and O2 are used as probe molecules ( 5 5 ) . The decrease i n both O2 and NO uptake observed f o r unextracted c a t a l y s t s i n d i c a t e s t h a t Mo dispersion
c e r t a i n l y decreases when Mo loading increases on such c a t a l y s t s . I n the case of Mo-extracted c a t a l y s t s , O2 chemisorption a1 so decreases w i t h increasing Mo loading, b u t l e s s so and more g r a d u a l l y than on unextracted c a t a l y s t s , w h i l e
NO
chemisorption changes o n l y s l i g h t l y . I t i s evident that, except f o r c a t a l y s t s prnol M0.m- 2 ) a l l unextracted c a t a l y s t s present
w i t h very low Mo loading (1.3
much lower (3-4 times) O2 chemisorption c a p a c i t i e s than t h e corresponding Moextracted c a t a l y s t s . This f a c t i n d i c a t e s t h a t Mo i s poorly dispersed i n t h e high Mo-loading unextracted Mo/A1203 catalysts, since a f r a c t i o n o f the e a s i l y reduc i b l e Mo i s present on the alumina surface i n m u l t i l a y e r s o r i n Moo3 c l u s t e r s o f a few Mo atoms, as previously proposed (246). The removal o f Mo, predominantly Moo (polymeric and i n m u l t i l a y e r s ) , by ammonia leaves the remaining unextracta-
b l e Mo f r a c t i o n re1 a t i v e l y more dispersed than before e x t r a c t i o n , decreasing o n l y s l i g h t l y w i t h increasing Mo loading. This conclusion i s strengthened when the NO/Mo r a t i o s a r e also compared, Such r a t i o s f o r Mo-extracted c a t a l y s t s decrease only s l i g h t l y as Mo loading increases, suggesting t h a t such c a t a l y s t s
B56
4
m
rl
I
1900
I
rl 0
2 I
I
1800 1700 1600 Wavenumber (cm- '1
Fig.159. I n f r a r e d s p e c t r a o f adsorbed NO a t room temperature on p a r t i a l l y reduced c a t a l y s t s w i t h d i f f e r e n t Mo-loadings: ( a ) 1.04, ( b ) 3.5, ( c ) 5.1, ( d ) 9.2, and ( e ) 14.8 pmol M-m-2. present a s i m i l a r d i s p e r s i o n (based on NO a d s o r p t i o n ) and a r e v e r y c l o s e t o t h a t 2 measured f o r t h e u n e x t r a c t e d Mo/A1203 c a t a l y s t w i t h 1.3 pmol M0.mI n the l a t t e r c a t a l y s t t h e m a j o r i t y o f Mo i s probably p r e s e n t as a monolayer, s i n c e
.
b o t h O/Mo and NO/Mo r a t i o s remain v i r t u a l l y equal b e f o r e and a f t e r Mo e x t r a c t i o n . I n o t h e r words, r e l a t i v e decreases i n Mo e x t r a c t e d an a d s o r p t i o n capac i t i e s a r e s i m i l a r f o r t h e l o w e s t Mo-loading c a t a l y s t . When comparing these chemisorption data w i t h those o f t h e l i t e r a t u r e , t h e O/Wo r a t i o s found on t h e u n e x t r a c t e d Mo/A1203 c a t a l y s t s c o n t a i n i n g above 4 pmol M o . ~ - ~a r e comparable w i t h those p r e v i o u s l y r e p o r t e d on reduced and s u l f i d e d
c a t a l y s t s (213, 247). However, t h e NO/Mo r a t i o s a r e s u b s t a n t i a l l y l o w e r than t h e corresponding values c a l c u l a t e d from t h e chemisorbed NO amounts determined by e x t r a p o l a t i o n o f t h e l i n e a r p o r t i o n s o f t h e NO isotherms a t zero pressure (243). Note t h a t i f we c o n s i d e r t h e t o t a l NO uptake, t h e NO/Mo r a t i o s became l a r g e r . For instance, f o r t h e H2-reduced ( a = 0.94) Mo/A1203 c a t a l y s t s w i t h 9.1 t h e NO/Mo r a t i o o b t a i n e d by c o n s i d e r i n g t h e i r r e v e r s i b l e NO amount
pmol M.m-2,
i s 0.045, whereas by c o n s i d e r i n g t h e t o t a l NO amount i t i s almost 5 times
B57 higher. The nature and number o f exposed (CUS) Mo atoms a f t e r reduction i n hydrogen have been investigated by i n f r a r e d spectroscopy o f the NO probe (240, 243, 248251). Typical i n f r a r e d spectra o f NO chemisorbed on H2-reduced Mo/A1203 catal y s t s w i t h Mo loadings ranging between 1.04 and 14.8 Fig.l.19.
mol Mo.m'2
are given i n
A l l these spectra show two absorption bands a t 1810 and 1710 cm'l,
as-
signed t o the symmetric and antisymmetric NO fundamental s t r e t c h i n g , respect i v e l y , o f paired NO molecules h e l d e i t h e r as a dimer (248, 251) o r as a d i n i t r o s y l (252) on the surface Mo ion, probably Mo4' o r Mo3' (253). The r e l a t i v e i n t e n s i t y o f the two bands i s r e l a t e d t o the angle 0 between t h e two N-0 o s c i l 2 l a t o r s by the equation Isy/Ianti = c t g (0/2). The r e l a t i v e i n t e n s i t i e s , t h e angle 0, and the f u l l width a t h a l f maximum (FWHM) r e s u l t s f o r the reduced Mo/A1203 c a t a l y s t s s e r i e s are summarized i n Table 1.7.The angle @ decreases TABLE 1.7
I n f r a r e d Parameters f o r the Molybdenum (NO), % Molybdenum
I, ,,,/Iant e(o)
Adsorption Complexes
FWHM
1.04
0.66
102
28
3.5
0.68
101
30
5.1
0.73
99
32
9.2
0.81
96
34
14.8
0.78
97
35
a Band a t 1810 cm-'. s l i g h t l y w i t h increasing Mo loading, whereas t h e FWHM's show an o p o s i t e trend. These differences can be explained i n terms o f the reduction degree o f t h e catal y s t s . As stated above, and i n agreement w i t h several researchers (121, 254-257), Mo i s present i n both MoT and Moo environments, b u t the former species i s r e s i s t n a t toward reduction. Thus, a lower amount o f reduced Mo s i t e s i s expect e d t o be present i n the low Mo-loading Mo/A1203 c a t a l y s t s and, consequently, the extent o f NO adsorption should be lower. I n t h i s s i t u a t i o n the (NO)2 dimers o r n i t r o s y l s may i n t e r a c t w i t h o t h e r reduced i o n i c Mo neighbors, thus increasing the angle 0 between the two N-0 o s c i l l a t o r s . Furthermore, i t i s i n t e r e s t i n g t o observe i n Fig.1.19 t h a t the band a t 1710 cm-l i s considerably broader than the one a t 1810 cm-l (243, 248). This may be explained by assuming t h a t the v i b r a t i o n a l t r a n s i t i o n moment vector p a r a l l e l t o the surface w i l l detect surface inhomogeneities more e f f e c t i v e l y than the one perpendicular t o the surface. The i n t e g r a t e d i n t e n s i t i e s o f the band a t 1710 cm-l as a function o f t h e Mo loading are given i n Fig.120. These i n t e n s i t i e s increase steeply w i t h increasing
B58
.,
Fig.1.20. I n t e g r a t e d i n f r a r e d i n t e n s i t i e s (0)o f the band a t 1710 cm-I o f t h e s t r o n g l y held NO on prereduced Mo/Al203 c a t a l y s t s as a f u n c t i o n o f t h e Mo loading. readapted from r e f . 250. Mo loading up t o about 8.7 pmol Mo.m-',
and then decreased markedly. This l a s t
decrease cannot be explained i n terms o f a lower r e d u c t i o n degree of the catal y s t s , because the experimental r e s u l t s o f r e d u c t i o n show an opposite trend. Peri ( 2 5 0 ) found a s i m i l a r behaviour when using NO as the probe molecule, although the decrease occured a t lower Mo loadings, and suggested t h a t the exp l a n a t i o n may l i e i n t h e formation o f aluminum molybdate during c a l c i n a t i o n , which might occur more r e a d i l y w i t h an excess o f Mo beyond t h e monolayer and i n t e r a c t s t r o n g l y w i t h the alumina surface. I n the l i g h t o f t h e abundant l i t e r a t u r e about t h e chemisorption o f probe molecules on reduced ( o r s u l f i d e d ) alumina-supported Mo oxides, i t seems t h a t LTOC measurements may be used as a powerful t o o l t o e s t a b l i s h the r e l a t i v e d i s -
persion o f Ma i n Mo/A1203 c a t a l y s t series. C o r r e l a t i o n o f these data w i t h catal y t i c a c t i v i t y are, i n general, unsuccessful because oxygen chemisorption t i t r a tes more successfully i n a nonsensitive way w i t h reduced s i t e s than w i t h t h e act i v e s i t e s involved i n c a t a l y s i s . A d d i t i o n a l l y , I R o f t h e NO probe chemisorbed on reduced ( o r s u l f i d e d ) c a t a l y s t s can y i e l d i n f o r m a t i o n on Mo atoms exposed on the surface under f a i r l y r e a l i s t i c conditions.
B59 1.5.2.6.
C o b a l t Oxides
C o b a l t o x i d e (Co304) i s a v e r y a c t i v e c a t a l y s t f o r t h e combustion o f h y d r o carbons and CO and NH3 o x i d a t i o n , as w e l l as b e i n g t h e most used promoter i n h y d r o t r e a t i n g c a t a l y s t s . Pope e t a l . (258) measured t h e s u r f a c e a r e a o f b o t h unsupported and s i l i c a - s u p p o r t e d c o b a l t o x i d e f r o m t h e CO i s o t h e r m s a t 306 K, and found them t o be s t r o n g l y dependent on t h e h i s t o r y o f p r e p a r a t i o n and p r e t r e a t m e n t . F o r i n s t a n c e , o u t g a s s i n g i n vacuum a t 623 K p r i o r t o t h e a d s o r p t i o n o f CO was r e q u i r e d f o r c o b a l t o x i d e c a t a l y s t s prepared f r o m a n i t r a t e p r e c u r s o r ,
w h i l e o u t g a s s i n g a t 723 K r e q u i r e d f o r t h o s e p r e p a r e d f r o m o x a l a t e p r e c u r s o r . These a u t h o r s o b t a i n e d a s e t o f CO isotherms as a f u n c t i o n o f p r e t r e a t m e n t , as w e l l as o f t h e BET s u r f a c e areas, a l s o u s i n g CO as a d s o r b a t e as a f u n c t i o n o f t h e s i n t e r i n g temperatures. H e r t l (259). u s i n g i n f r a r e d spectroscopy, has i d e n t i f i e d a surface c a r b o n a t e as t h e r e a c t i v e s p e c i e s f o r CO o x i d a t i o n on Co304. T h i s c a r b o n a t e s t r u c t u r e i s formed by t h e i n t e r a c t i o n o f one adsorbed CO m o l e c u l e w i t h two a c t i v e oxygen atoms (Os). The number o f these a c t i v e oxygens can be determined by u s i n g t h e r e c t a n g u l a r p u l s e t e c h n i q u e (260) coupled w i t h t h e r e a c t i o n o f NH3, i . e . ,
2NH3 + 40,
-
N20 + 3H20
(1.20)
Niyamoto e t a l . (260) observed t h a t t h e c o n c e n t r a t i o n p r o f i l e o f N2 produced by R e a c t i o n ( 1 9 ) shows an i n i t i a l sharp N2 response f o l l o w e d by a t a i l i n g p a r t . The c o n c e n t r a t i o n a t t h e t a i l i n g p a r t i n c r e a s e s w i t h i n c r e a s i n g temperature. T h i s t a i l i n g p a r t i s due t o t h e r e o x i d a t i o n o f t h e s u r f a c e by s u b s u r f a c e oxygen atoms. The i n c r e a s e i n t h e c o n c e n t r a t i o n a t t h e t a i l i n g p a r t w i t h i n c r e a s i n g temperature means t h a t r e o x i d a t i o n o f t h e s u r f a c e t a k e s p l a c e more r e a d i l y a t h i g h e r temperature. However, because t h e number o f Os on Co304 depends o n l y on t h e s t r u c t u r e o f t h e c a t a l y s t , i t s h o u l d n o t change w i t h e x p e r i m e n t a l v a r i a b l e s , as was observed. A c c o r d i n g t o t h e r e s u l t s o f a d s o r p t i o n o f NO and CO on Co304 c a t a l y s t s , one s u r f a c e Co i o n can adsorb a p p r o x i m a t e l y one NO or CO m o l e c u l e . The a r e a occupied b y one adsorbed NO o r CO m o l e c u l e i s 0.128-0.233 nm2, which i s 2 c o n s i d e r a b l y s m a l l e r t h a n t h a t f o r a c t i v e oxygen (0.51-0.61 nm ). T h i s i n d i c a t e s t h a t 3 t o 4 s u r f a c e Co i o n s can p r o v i d e a s i t e f o r an a c t i v e oxygen. Topsfie and Topsfie (249) c a r e f u l l y s t u d i e d t h e e x t e n t o f
NO c h e m i s o r p t i o n and
t h e i n f r a r e d s p e c t r a o f NO chemisorbed on alumina-supported c o b a l t o x i d e s w i t h v a r y i n g Co l o a d i n g s . From t h e i n f r a r e d s p e c t r a , one can d i s t i n g u i s h between Co atoms
l o c a t e d a t t h e s u r f a c e o f t h e c a t a l y s t s , and changes i n t h e n a t u r e o f t h e
phases i n which t h e s e atoms a r e s i t u a t e d may a l s o be r e v e a l e d . Thus i t may be p o s s i b l e o t o b t a i n i n f o r m a t i o n r e g a r d i n g t h e r e l a t i v e importance o f d i f f e r e n t
B60
phases, namely CoA1203 and Co304, which when combined determine t h e a c t i v e species o f these c a t a l y s t s . 1.6. CONCLUSION Standard methods f o r c a t a l y t i c surface area determinations f o r m a l l y e x i s t f o r several supported-metal c a t a l y s t s . However, t h e r e i s n o t general acceptance o f such methods f o r supported-metal oxides ( o r s u l f i d e s ) , There are inherent d i f f i c u l t i e s i n s e l e c t i n g any method as a standard f o r surface area measurements, since c a t a l y s t manufacturers throughout the world prepare t h e i r m a t e r i a l s from d i f f e r e n t precursors and i n d i f f e r e n t ways. These differences can Cause marked v a r i a t i o n s i n t h e procedure r e q u i r e d t o measure accurately t h e surface area o f metal oxides. Without doubt, t h e chemisorption o f s u i t a b l e probe molecules i s t h e method of choice f o r such purposes. I t i s a l s o o f g r e a t i n t e r e s t t o combine the chemisorpt i o n measurements w i t h appropriate surface spectroscopic techniques i n order t o determine p r e c i s e l y the stoichiometry between the probe and the surface area o f the supported a c t i v e component and t h e number of s i t e s responsible f o r a given reaction. The answer t o t h i s question i s most often p o s i t i v e , b u t contrary exanples e x i s t i n the 1 i t e r a t u r e . It i s t h e r e f o r e obvious t h a t one may n o t s e l e c t a catalyst on the basis of onw "standard t e s t " . High surface area is, however, o f such basic importance t o any c a t a l y t i c process t h a t i t s should always be measured as a necessary, b u t not s u f f i c i e n t ,
c h a r a c t e r i s t i c o f the system.
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B67
Chapter 2
INFRARED SPECTROSCOPY J.L.G.
FIERRO
I n s t i t u t o de C a t i l i s i s y P e t r o l e o q u i m i c a , C.S.I.C. Spain.
Serrano 119, 28006 Madrid,
2.1 INTRODUCTION
I n f r a r e d spectroscopy i s t h e most w i d e l y used t e c h n i q u e f o r s t u d y i n g t h e gas-solid interface
. The reason
f o r t h i s c e r t a i n l y l i e s i n the f a c t t h a t i t s
e x p e r i m e n t a l requirements have been modest i n comparison w i t h many o t h e r s p e c t r o s c o p i c t e c h n i q u e s b e i n g o f t e n capable o f p r o v i d i n g more i n s i g h t i n t o t h e v i b r a t i o n a l and r o t a t i o n a l motions o f t h e atoms i n an adsorbed molecule. Even i n p h y s i c a l a d s o r p t i o n t h e r o t a t i o n a l and t r a n s l a t i o n a l m o t i o n s o f m o l e c u l e s a r e r e s t r i c t e d . As a r e s u l t , t h e r o t a t i o n a l s t r u c t u r e o f t h e a d s o r p t i o n bands disappears, m a i n t a i n i n g o n l y t h e bands o f v i b r a t i o n a l m o t i o n s . However, t h e s p e c t r a l changes become much more marked i n t h e case o f s t r o n g c h e m i s o r p t i o n and i n t h e f o r m a t i o n o f s u r f a c e compounds by i n t e r a c t i o n o f t h e m o l e c u l e s w i t h t h e s u r f a c e . I n t h i s p a r t i c u l a r case, s e v e r a l i n f r a r e d a b s o r p t i o n bands d i s a p p e a r from t h e spectrum and o t h e r new bands appear. The s t r u c t u r a l a n a l y s i s o f t h e s u r f a c e compounds formed i n such cases i s based on t h e same p r i n c i p l e s
as f o r t h e a n a l y s i s o f molecules i n t h e b u l k phase by i n f r a r e d spectroscopy. The m a j o r problem i n a l l these s t u d i e s a r i s e s f r o m t h e l i m i t a t i o n s due t o t h e s c a t t e r i n g o f i n f r a r e d r a d i a t i o n by t h e s o l i d and a l s o f r o m t h e presence o f r e g i o n s o f complete a b s o r p t i o n o f r a d i a t i o n . T h i s d i f f i c u l t y g r e a t l y h i n d e r s t h e o b s e r v a t i o n o f a b s o r p t i o n bands f o r a l l t h e p o s s i b l e v i b r a t i o n s o f t h e adsorbed molecules, t h u s making d i f f i c u l t t h e r e 1 i a b l e i n t e r p r e t a t i o n o f t h e spectral manifestations o f t h e adsorption. E a r l y r e s e a r c h o f T e r e n i n ' s group ( 1 ) showed t h a t c o n v e n t i o n a l i n f r a r e d spectrophotometers c o u l d be used t o o b t a i n t h e v i b r a t i o n a l s p e c t r a of h y d r o x y l groups on t h e s u r f a c e o f high-, o r m o d e r a t e l y h i g h - , s u r f a c e o f i s o l a t i n g oxides, i . e . ,
s i l i c a , alumina, s i l i c a - a l u m i n a , e t c . , and i n t e r a c t i o n s between
adsorbed molecules and h y d r o x y l groups. These s t u d i e s were m o s t l y conducted i n t h e near i n f r a r e d r e g i o n (4.000-10.000 cm-l), where p h o t o e l e c t r o n i c d e t e c t o r s a r e more s e n s i t i v e t h a n t h e c o n v e n t i o n a l evacuated thermocouples. I n t h e 1950s Eischens and coworkers demonstrated t h e u t i l i t y o f i n f r a r e d spectroscopy f o r s t u d y i n g small metal p a r t i c l e s d i s p e r s e d on a n o x i d e powder s u p p o r t ( 2 ) . By t h e l a t e 1960s, s u c c e s s f u l e x t e n s i o n o f i n f r a r e d spectroscopy f r o m t h e above a p p l i c a t i o n s t o many o t h e r a d s o r p t i o n and c a t a l y t i c processes i s w i d e l y
B68 confirmed i n t h e l i t e r a t u r e ( 3 , 4) and reviews (5, 6 ) . I n t h e l a s t f i f t e e n years, new a p p l i c a t i o n s and approaches have been achieved t o o b t a i n t h e i n f r a r e d spectrum. Among them, i n t e r n a l and s p e c t r a l e x t e r n a l r e f l e c t i o n , and emission spectroscopy have shown considerable promise b u t have n o t y e t found widespread a p p l i c a t i o n . The b a s i c p r i n c i p l e s o f these techniques, t h e c e l l s employed, and a l i m i t e d number o f c a t a l y t i c a p p l i c a t i o n s a r e examined i n t h i s chapter. Another novel, b u t q u i t e d i f f e r e n t , technique t h a t can be used t o o b t a i n an i n f r a r e d spectrum i s i n e l a s t i c e l e c t r o n s c a t t e r i n g spectroscopy. T h i s technique i s based on t h e s c a t t e r i n g o f low o r h i g h energy e l e c t r o n s which l o s e energy when an e l e c t r o n beam impinges on an adsorbed l a y e r . I t s a p p l i c a t i o n i s , however, r e s t r i c t e d t o very w e l l d e f i n e d systems, i . e . ,
single crystals. A
complete survey o f t h e technique and i t s a p p l i c a t i o n s t o t h e i n t e r f a c e o f model c a t a l y s t s i s g i v e n i n Chapter 3. There e x i s t o t h e r r e l a t i v e l y new a b s o r p t i o n c a l o r i m e t r i c techniques. As i s we1 1 known, i n conventional spectrophotometers t h e small f r a c t i o n o f t h e absorbed r a d i a t i o n i s d e t e c t a b l e w i t h r e s p e c t t h e l a r g e t r a n s m i t t e d beam. D i r e c t d e t e c t i o n o f o n l y t h e absorbed energy c o n s t i t u t e s p e r se a c a l o r i m e t r i c spectroscopy procedure. F o l l o w i n g t h i s procedure, good q u a l i t y spectra, comparable t o those obtained w i t h a commercial instrument, o f CO chemisorbed on n i c k e l were obtained ( 7 ) . A much h i g h e r s e n s i t i v i t y was r e c e n t l y achieved by an i n d i r e c t d e t e c t i o n method, where t h e i n f r a r e d r a d i a t i o n absorbed by t h e s o l i d i s p a r t i a l l y t r a n s f e r r e d t o t h e phase i n t h e form o f an a c o u s t i c wave, being e a s i l y d e t e c t a b l e over a microphone. Although t h e r e i s n o t a f u l l body o f 1 i t e r a t u r e on photoacoustic spectroscopy (PAS), many pub1 i c a t i o n s (8-11) have p o i n t e d o u t t h a t F o u r i e r t r a n s f o r m PAS i s capable o f producing s p e c t r a from s o l i d c a t a l y s t s o f v a r i o u s morphologies which c l o s e l y resemble conventional a b s o r p t i o n spectra. The i n h e r e n t d i f f i c u l t i e s i n v o l v e d i n t h e use of PAS were overcome by t h e advent o f photothermal d e f l e c t i o n spectroscopy ( I R - P D S ) ,
which
i s based on t h e d e f l e c t i o n o f an i n f r a r e d beam by t h e change o f t h e r e f r a c t i v e index o f a gas i n t h e c l o s e v i c i n i t y o f a h o t s o l i d s u r f a c e ( 1 2 ) . As p o i n t e d o u t by Low (13-16), t h i s technique has proved amply s u i t a b l e f o r t h e examination o f a v a r i e t y o f o r g a n i c and i n o r g a n i c s o l i d s , t h i c k s u r f a c e l a y e r s and b l a c k m a t e r i a l s such as carbon-supported metal c a t a l y s t s . I t i s n o t t h e aim o f t h i s chapter t o discuss experimental d e t a i l s o r t h e
r e s u l t s o f t h e l e g i o n o f p u b l i c a t i o n s found i n t h e l i t e r a t u r e on t h e v i b r a t i o n a l modes o f t h e adsorbed molecules i n c a t a l y s t surfaces. What we wish emphasize i s t h e p r a c t i c a l usefulness o f t r a n s m i s s i o n - a b s o r p t i o n spectroscopy v i s a v i s o t h e r more s o p h i s t i c a t e d techniques. We w i l l a l s o a t t e m p t t o h i g h l i g h t t h e advantages o f a g i v e n i n f r a r e d technique f o r p a r t i c u l a r a p p l i c a t i o n s t o t h e i n v e s t i g a t i o n o f molecule a d s o r p t i o n t o surfaces o f c a t a l y t i c i n t e r e s t .
B69 2.2
INFRARED SPECTROSCOPY
2.2.1 T 2.2.1.1
W C l a s s i c a l C a l c u l a t i o n o f V i b r a t i o n a l Frequencies
The i n t e r a c t i o n between t h e e l e c t r i c a l component o f electromagnetic r a d i a t i o n and t h e e l e c t r i c a l d i p o l a r motions w i t h i n molecules i s t h e b a s i s o f r o t a t i o n a l , v i b r a t i o n a l and e l e c t r o n i c spectroscopy. Since bands i n a l l these s p e c t r a a r e observed a t s p e c i f i c frequencies, i t f o l l o w s t h a t t h e r o t a t i o n a l , v i b r a t i o n a l and e l e c t r o n i c motions l e a d i n g t o e l e c t r i c a l d i p o l e changes must occur a t s p e c i f i c frequencies. On t h e b a s i s o f a c l a s s i c a l model i t i s o n l y p o s s i b l e t o e x p l a i n t h e e x i s t e n c e o f r o t a t i o n a l frequencies,
i f electronic
motions occur a t t h e same frequencies. By c o n t r a s t , a c l a s s i c a l model p r o v i d e s considerable i n f o r m a t i o n on t h e e x i s t e n c e o f v i b r a t i o n a l frequencies of atoms w i t h i n molecules. Hence i t i s i n f o r m a t i v e t o examine how f a r spectroscopic observations i n t h e i n f r a r e d spectrum o f d i a t o m i c molecules can be e x p l a i n e d i n terms o f a c l a s s i c a l model. For diatomic molecules i t i s observed t h a t h e t e r o p o l a r molecules (A-B) absorb i n f r a r e d r a d i a t i o n a t a p a r t i c u l a r frequency c h a r a c t e r i s t i c of t h e molecule. The homomolecular diatomics (A-A), however, do n o t absorb i n f r a r e d r a d i a t i o n . This can be understood, i f t h e d i a t o m i c molecule i s considered as two masses mA and mg j o i n e d by a bond which m a i n t a i n s atoms
A
and B a t a c e r t a i n
e q u i l i b r i u m distance. Assuming spring-1 i k e p r o p e r t i e s f o r these atoms, t h e s t i f f n e s s o f t h e bond can be c h a r a c t e r i z e d by a f o r c e constant k , which i s t h e f o r c e r e q u i r e d t o produce u n i t s e p a r a t i o n i n accordance with Hooke's law. This law s t a t e s t h a t , f o r small displacements around t h e e q u i l i b r i u m d i s t a n c e of a spring, i s an opposing r e s t o r i n g f o r c e f which i s p r o p o r t i o n a l t o t h e d i s p l a c e ment
The change i n p o t e n t i a l energy o f t h e system i s g i v e n by t h e product o f t h e instantaneous f o r c e and displacement, so t h a t i n t e g r a t i o n leads t o t h e p a r a b o l i c re1 a t i o n
For t h e diatoiiiic molecule A-B, x represents t h e displacement o f t h e atoms from t h e e q u i l i b r i u m s e p a r a t i o n re t o some value r, and i s g i v e n by r-re. I f t h e values o f k and re a r e known, t h e n t h e p o t e n t i a l v i b r a t i o n a l energy o f t h e system may be p l o t t e d , i f s u i t a b l e values f o r r a r e chosen ( F i g . 2.1, dashed curve). I t can r e a d i l y be shown by combining Hooke's law w i t h Newton's second law of motion t h a t a small displacement of one o f t h e masses r e l a t i v e t o t h e
B70
Fig. 2.1. Potential-energy f u n c t i o n f o r a r e a l diatomic molecule w i t h a dissoc i a t i o n energy and e q u i l i b r i u m bond lenght r e ( b ) . The dashed ( a ) i s t h e potent i a l energy f u n c t i o n f o r the harmonic o s c i l l a t o r t h a t approximates t h e p o t e n t i a l a t small displacements from re. The l i n e s p a r a l l e l t o the abscissa a x i s represent a1 lowed energy 1eve1 s
.
other w i l l cause the system t o v i b r a t e i n simple harmonic motion. The frequency a t which the system v i b r a t e s i s termed t h e e q u i l i b r i u m frequency ve, and i s given by
where p i s the reduced mass o f t h e system, p = mA.mB(mA+mB). I f the frequency v o f the i n c i d e n t r a d i a t i o n on t h e molecule i s tuned t o ve, i t couples w i t h t h e d i p o l e moment o f A-8 and induces t h i s molecule t o v i b r a t e . I f the diatomic
molecule i s symmetrical (A-A), no e l e c t r i c a l coupling can take place, and i t becomes apparent why molecules o f t h i s category do no adsorb i n f r a r e d r a d i a t i o n . The r e l a t i o n s h i p between frequency, f o r c e constant and reduced mass given i n Eq. (2.3) i s o f t e n used i n conjunction w i t h i s o t o p i c exchange t o assign observed frequencies i n the i n f r a r e d spectrum. One i n t e r e s t i n g example, which i l l u s t r a t e s t h i s a p p l i c a t i o n , has been reported by Kokes e t a l . ( 1 7 ) f o r the d i s s o c i a t i v e chemisorption o f hydrogen on ZnO. The experimental r a t i o s o f wavenumbers f o r ZnH and ZnD, and f o r OH and OD are compared i n Table 2.1 w i t h t h e t h e o r e t i c a l r a t i o s predicted by Eq. (2.3).
H2(D2)
+
ZnO
-
B71 HZn-OH (DZn-OD)
As a f i r s t approximation, i t i s considered t h a t t h e 0 and Zn atoms a r e a t r e s t , i n which case t h e c a l c u l a t e d r a t i o s f o r both cases would be t h e r e c i p r o c a l o f t h e square r o o t o f t h e mass r a t i o s o f t h e *D and 1H isotopes, j2/1 = 1.414. By assuming t h e s u r f a c e Ln-H (Zn-D) and 0-H (0-D) species as d i a t o m i c s and u s i n g Eq. (2.3), as was done t o c a l c u l a t e t h e (uD-pH) f values g i v e n i n Table 2.1, t h e r e i s good agreement w i t h t h e experimental r a t i o s . However, i n o r d e r t o bring the calculation i n the l i n e w i t h TABLE 2.1. Experimental and c a l c u l a t e d r a t i o s o f wavenumbers f o r ZnO
0-H(D) Zn-H(D)
VH
VD
3490 1705
2585 1225
1.35 1.39
1.37 1.40
t h e experiment u s i n g Eq. (2.3), i t would be necessary t o a s s i g n an e f f e c t i v e mass t o t h e Zn and 0 o f about one h a l f t h e i r t r u e value, because t h e Zn and 0 atoms a r e n e i t h e r a t r e s t nor a r e
they f r e e l y v i b r a t i n g , b u t v i b r a t i o n a l l y
coupled t o t h e r e s t o f t h e s o l i d thus rendering inadequate t h e simple harmonic o s c i 11a t o r we had considered. For more complex molecules than d i a t o m i c s t r u c t u r e s t h e c a l c u l a t i o n o f v i b r a t i o n a l frequencies i s n o t s t r a i g h t forward, according t o t h e approach o f hamionic o s c i l l a t o r . I n p r i n c i p l e , t h e same approach can be used, i.e.,
we
d e r i v e equations which express t h e t o t a l ( k i n e t i c and p o t e n t i a l ) energy, i n terms o f t h e displacement o f t h e atom, and determine t h e degrees o f freedom f o r v i b r a t i o n a l motion, under t h e c o n s t r a i n t t h a t t h e t o t a l energy i s constant. The number o f s o l u t i o n s ( o r v i b r a t i o n a l degrees o f freedom) w i l l be 3N-6 ( o r 3N-5 f o r a l i n e a r molecule), where
N i s t h e number o f atoms i n t h e molecule. T h i s i s
due t o t h e f a c t t h a t t h e N atoms r e q u i r e 3N Cartesian coordinates t o d e s c r i b e t h e i r motion: t h r e e o f them associated w i t h t r a n s l a t i o n a l degrees o f freedom, and an o t h e r t h r e e (two f o r a l i n e a r molecule) associated w i t h t h e r o t a t i o n a l degrees o f freedom. F i n d i n g t h e r e l a t i o n s h i p between normal v i b r a t i o n a l frequenc i e s and f o r c e constants i s a t e d i o u s work, and hence i s beyond t h e scope o f t h i s chapter. The i n t e r e s t e d reader i s r e f e r r e d , f o r complete treatment o f t h e problem, t o Nakamoto (18) and S t e e l e (19),books.
B72
I
f Excitation !
I
Fig. 2.2. Quantized energy s t a t e s f o r which t h e frequency o f emission or adsorption o f r a d i a t i o n f o r a t r a n s i t i o n i s defined by Eq. 2.5. Since the c l a s s i c a l formulation i s i n c o r r e c t i n i t s d e t a i l e d p r e d i c t i o n s o f v i b r a t i o n a l spectra and cannot be adequately a p p l i e d t o e l e c t r o n i c spectra. I n a d d i t i o n , i t cannot e x p l a i n r o t a t i o n a l spectra, explanation o f these phenomena i s sought i n terms o f t h e quantum theory. This theory demands r e c o g n i t i o n o f discontinuous, d i s c r e t e p a r t i c l e s o f matter, and a l o g i c a l extension would be t o regard energy a l s o i n terms o f t h e same a t t r i b u t e s . 2.2.1.2
Q u a n t i z a t i o n o f t h e I n t e r a c t i o n o f Radiation w i t h Matter
According t o the quantum theory, t h e energy o f a molecule i s given i n terms o f a series o f d i s c r e t e energy l e v e l s Eo, El, E2, etc. ( F i g . 2.2). Each d i s c r e t e molecule must e x i s t a t one o r t h e o t h e r o f these l e v e l s . I n an assembly o f molecules there i s a somewhat complicated s i t u a t i o n , where a l l molecules w i l l be d i s t r i b u t e d between the Eo, El,...., molecules a t l e v e l s Ei and Ef (Ei/Efj
En 1evels;the r e l a t i v e population o f being given by the Maxwell-Boltzmann
equation
N i / N f = (gi/gf).e-(Ni-Nf)/kT where gi and g . are t h e number o f permitted s t a t e s w i t h energies Ei and Ej,
J
respectively, k i s Boltzmann's constant (1.3805 x
J s-l) and T i s the
absolute temperature o f t h e system. The energy l e v e l s are f u n c t i o n s o f an
B73 integer
n (quantum number) and t h e f u n c t i o n i s r e l a t e d t o t h e p a r t i c u l a r
m o l e c u l a r process undergone by t h e molecule, e.g.,
a change i n s p i n o r i e n t a t i o n ,
r o t a t i o n a l o r v i b r a t i o n a l energy, o r t h e e l e c t r o n i c c o n f i g u r a t i o n . Each o f t hese m o l e c u l a r processes r e q u i r e s t h a t t h e quantum o f r a d i a t i o n e x a c t l y matches t h e d i f f e r e n t i a l energy El-EO,
E2-E1,
e t c . The magnitude of t h e energy quantum i s
r e l a t e d t o t h e frequency o f r a d i a t i o n by P l a n c k ' s equat ion. Hence, t h e frequency of eniission o r a b s o r p t i o n o f r a d i a t i o n f o r a t r a n s i t i o n between t h e energy s t a t e s Eo and El i s g i v e n by
v
(E1-EO)/h
=
where i s P l a n c k ' s c o n s t a n t (6.6256 x
J s ) . The concept o f q u a n t i z e d
energy l e v e l s t h u s e x p l a i n s t h e o b s e r v a t i o n t h a t t h e f r e q u e n c i e s o f t h e bands i n t h e a b s o r p t i o n s p e c t r a a r e t h e same as t h o se i n t h e emission spect ra, i n t h e absence o f o t h e r complex process, v i z . phosphorescence. 2.2.1.3
A nharmo n i c i t y o f M o l e c u l a r V i b r a t i o n
The p o t e n t i a l energy based on t h e harmonic o s c i l l a t o r model, g i v e n b y Eq. ( 2 . 2 j , i s n o t v a l i d f o r l a r g e values o f x. A t l a r g e p o s i t i v e values o f x t h e p o t e n t i a l energy i n c r e a s e s t o some t h r e s h o l d v a l u e which corresponds t o t h e d i s s o c i a t i o n o f t h e molecule w i t h no i n t e r a c t i o n between t h e atoms. A t l a r g e n e g a t i v e values o f x t h e p o t e n t i a l energy i n creases d r a m a t i c a l l y because t h e f o r c e s o f r e p u l s i o n between t h e atoms become v e r y l a r g e . These c o n s i d e r a t i o n s suggest a m o d i f i c a t i o n o f t h e c l a s s i c a l model t o t a k e account o f t h e d e v i a t i o n from t h e harmonic o s c i l l a t o r . A number o f mathematical f o r m u l a t i o n s f o r t h e p o t e n t i a l energy o f an anharmonic o s c i l l a t o r have been proposed. The s i m p l e s t one i s T a y l o r ' s power s e r i e s
ux
= -1a x2 2 0
where ao, al,
-
1 alx 3 + - 1 a 24 a2, e t c
x4 2
..., a r e
.....
(2.6)
constants.The second e x p r e s s i o n o f t h e p o t e n t i a l
energy o f a d i a t o m i c molecule i s Morse's f u n c t i o n
ux
=
where De i s t h e d i s s o c i a t i o n energy o f t h e molecule, and cx i s a c o n s t a n t c h a r a c t e r i s t i c o f t h e i n t e r n u c l e a r bond. A t h i r d e q u a t i o n f o r t h e p o t e n t i a l energy o f t h e d i a t o m i c molecule was d e r i v e d b y L i p p i n c o t t , based on quantum mechanical p r i n c i p l e s
B74 Ux = De(l-e- ax2/2r)
where
(I
has t h e same meaning as i n Morse's equation.
Equations (2.6-2.8)
may be p l o t t e d onto a graph and g i v e s i m i l a r values, i f s u i t a b l e values f o r t h e constants a r e chosen F i g . 2 . l b d i s p l a y s t h e curve f o r molecu e based on Morse's p o t e n t i a l an e x c i t e d e l e c t r o n i c s t a t e o f t h e I2 equation (Eq. 2.7). I t i s shown t h a t f o r sma 1 values o f x t h e p o t e n t i a l energy curves f o r b o t h harmonic and anaharmonic o s c i l a t o r s a r e p r a c t i c a l l y c o i n c i d e n t . The e n t i r e p o t e n t i a l energy f u n c t i o n f o r d i a omics, which have been considered above, i s based on c l a s s i c a l mechanics and assumes energy v a r i a t i o n s t o be continuous. I n terms o f t h e quantum t h e o r y , t h e v i b r a t i o n a l energy can be expressed by d i s c r e t e energy l e v e l s which a r e d e f i n e d by a v i b r a t i o n a l quantum number V. These l e v e l s , which a r e reproduced f n Fig.2.1.as
dotted l i n e s p a r a l l e l
t o t h e abscissa,represent t h e energies t o which v i b r a t i o n a l energies a r e conf ined
.
The p r e c i s e d e s c r i p t i o n o f t h e e x i s t e n c e o f d i s c r e t e energy l e v e l s i n molecular systems and o f o t h e r aspects o f t h e p r o p e r t i e s o f m a t t e r a t a submolec u l a r s c a l e has been p r o v i d e d by t h e methods o f wave mechanics. The energy o f a p a r t i c u l a r v i b r a t i o n a l l e v e l i s determined by s o l v i n g t h e SchrBdinger wave equation f o r t h e a p p r o p r i a t e system (20)
,
A p o t e n t i a l energy f u n c t i o n Ex represented i n terms o f t h e c o o r d i n a t e x may be s u b s t i t u t e d i n Eq. (2.9) and t h e equation be s o l v e d t o g i v e a s e t o f v i b r a t i o n a l wave f u n c t i o n s Yv and energy l e v e l s Ev, b o t h o f then being a f u n c t i o n o f t h e v i b r a t i o n a l quantum number V. The p o t e n t i a l energy f u n c t i o n s commonly used f o r t h i s purpose a r e based on e i t h e r t h e simple harmonic o s c i l l a t o r model o r a model which a l l o w s d e v i a t i o n s from t h e harmonic behaviour. The expressions o f t h e p o t e n t i a l energy f u n c t i o n f o r these two models, which correspond t o Eqs. (2.2) and (2.6) a r e summarized i n Table 2.2,
t o g e t h e r w i t h those f o r t h e energy
(EV) o f t h e system d e r i v e d from t h e SchrBdinger equation. The p o t e n t i a l energy f u n c t i o n o f t h e anharmonic o s c i l l a t o r has been taken f o r s i m p l i c i t y as a Tayl o r ' s power s e r i e s i n x (Eq. 2.6),
although t h i s e q u a t i o n approaches Morse's
(Eq. 2.7) f u n c t i o n o r L i p p i n c o t t ' s (Eq. 2.8) f u n c t i o n . The s o l u t i o n o f t h e Schrtidi nger equation assuming t h e harmonic o s c i l l a t o r p o t e n t i a l f u n c t i o n r e s u l t s i n allowed, e q u a l l y spaced energy l e v e l s , hve(V ti). I f t h i s o s c i l l a t o r i s i n t h e ground s t a t e , i.e., r e t a i n s fhv,
when t h e v i b r a t i o n a l quantum number V = 0, t h e system
o f energy. T h i s i s t h e z e r o p o i n t energy which i s o f c o n s i d e r a b l e
importance i n thermodynamic and k i n e t i c s t u d i e s . One i m p l i c a t i o n o f t h e zero
B75 TABLE 2.2 Frequency o f t h e I n f r a r e d Bands Harmonic v i b r a t o r Ux = f k x
Quantum energy 1eve1 s
EV = h.ve(V
Zero p o i n t energy
Eo = fhv,
Fundamental frequency
V
F i r s t overtone
v0-t 1 = 2Ve
0+1
=
Ux = f aox 2
2
P o t e n t i a l energy f u n c t i o n
Anharmonic v i b r a t o r
t
f)
'e
3 - -16 alx
EV = h v e ( V t f ) - h v e X e ( V + f ) Eo = f hve 1 hVeXe
2
-a
v o * 1 = Ve(1-2Xe) V
o+
= 2ve(l-3Xe)
k = f o r c e c o n s t a n t o f t h e bond; x = displacement f r o m t h e e q u i l i b r i u m s e p a r a t i o n o f t h e atoms; h = P l a n k ' s constant;V = v i b r a t i o n a l quantum number;v = e q u i l i b r i u m v i b r a t i o n a l frequency o f t h e molecule;xe = a n h a r m o n i c i t y c o f f s t a n t f o r the vibration. p o i n t energy i s t h a t t h e d i s s o c i a t i o n energy d i f f e r e n t , s i n c e v e and De
-
D o f t h e i s o t o p i c s p e c i e s w i l l be
f hve w i l l be d i f f e r e n t because o f t h e mass e f f e c t
on frequency ( c f . Eq. 2.3). Note a l s o t h a t t h e v a l u e s f o r t h e v i b r a t i o n a l energy l e v e l s i n t h e anharmonic v i b r a t o r d i f f e r f r o m those i n t h e harmonic v i b r a t o r approach. These d i f f e r e n c e s f r o m t h e harmonic p o t e n t i a l become i m p o r t a n t , as t h e energy o f t h e system r i s e s , and a r e observed s p e c t r o s c o p i c a l l y as a c l o s e r spacing between t h e energy l e v e l s t h a n t h e u n i f o r m l y spaced p a t t e r n , as p r e d i c t e d by hve(V t
4).
I f b o t h fundamental vo+
and f i r s t o v e r t o n e vo+
f r e q u e n c i e s a r e observed, i t i s p o s s i b l e t o e s t i m a t e t h e a n h a r m o n i c i t y c o n s t a n t (x,)
f o r the vibration. The p r o b a b i l i t y o f i n d u c i n g t r a n s i t i o n s between d i f f e r e n t energy l e v e l s can
be d e r i v e d f r o m t h e quantum mechanical approach. F o r t h e harmonic o s c i l l a t o r , t h e allowed t r a n s i t i o n s are w e l l generalized i n t h e form o f a s e l e c t i o n r u l e AV =
2
1, which means t h a t V must change by f 1 f o r t h e t r a n s i t i o n t o be a c t i v e
i n t h e i n f r a r e d . These t r a n s i t i o n s u s u a l l y t a k e p l a c e between t h e ground s t a t e (V = 0) and t h e f i r s t e x c i t e d s t a t e (V = l), s i n c e t h e number of m o l e c u l e s n o t i n t h e ground s t a t e , as p r e d i c t e d by t h e Maxwell-Boltzmann d i s t r i b u t i o n (Eq. 2 . 4 ) , i s v e r y sinall a t room temperature, i . e . , kT i s s m a l l as compared t o hv. F o r t h e anharmonic o s c i l l a t o r t h e c a l c u l a t i o n o f wave f u n c t i o n i s more complex, because h i g h e r terms i n t h e p o t e n t i a l energy e x p r e s s i o n (see T a b l e 2.2) must be t a k e n i n t o account. The e f f e c t o f t h e s e h i g h e r terms i s t o smooth t h e s e l e c t i o n r u l e so t h a t t h e t r a n s i t i o n between energy l e v e l s d i f f e r i n g by one, two o r t h r e e V u n i t s may be observed (AV = 1, -I 2, 3 ) , however t h e bands a r e always v e r y weak.
B76
2.2.1.4.
I n t e n s i t y o f t h e Absorption Bands
Besides t h e frequency o f t h e i n f r a r e d adsorption bands, t h e i r i n t e n s i t i e s are a l s o used t o characterize the mode o f the i n t e r a c t i o n o f molecules w i t h s o l i d surfaces. The dependence o f the i n t e n s i t y o f bands on t h e s t r u c t u r e o f the adsorbed molecule i s more complex than t h a t o f frequency, and t h e e s t a b l i s h ment o f such a r e l a t i o n s h i p i s o f t e n d i f f i c u l t . Due t o t h i s d i f f i c u l t y , the purpose o f t h i s chapter i s o n l y t o discuss t h e general features o f t h e dependence o f t h e i n t e n s i t y o f t h e v i b r a t i o n absorption bands on the parameters o f the molecule which may throw some l i g h t on t h e r e l a t i o n s h i p between changes i n the i n t e n s i t y o f these bands and changes i n t h e e l e c t r o n c o n f i g u r a t i o n o f molecules during the course o f adsorption. The i n t e n s i t y o f an i n f r a r e d band associated w i t h a t r a n s i t i o n from a lower t o a higher l e v e l i s p r o p o r t i o n a l t o t h e square o f t h e d e r i v a t i v e o f the d i p o l e moment w i t h respect t o t h e normal coordinate. I n t h i s c a l c u l a t i o n i t i s assumed t h a t t h e d i p o l e moment o f a molecule i s composed o f t h e d i p o l e moments o f bonds. Thus,
d i f f e r e n t charges and a c e r t a i n d i p o l e moment i s a t t r i b u t e d t o every
bond i n molecules w i t h a d d i t i v e properties. I t r e s u l t s t h a t t h e measurement o f the i n t e n s i t i e s o f the absorption bands f o r adsorbed molecules may reveal t h e change i n the d i p o l e moments o f the bonds d u r i n g adsorption, i.e.,
a direct
c h a r a c t e r i s t i c o f the change i n the e l e c t r o n i c s t r u c t u r e o f t h e molecule upon adsorption. Furthermore, according t o t h e d e f i n i t i o n o f band i n t e n s i t y , the s e n s i t i v i t y t o changes i n the e l e c t r o n i c s t r u c t u r e o f t h e molecule i s higher i n the case o f i n t e n s i t y than i n t h e case o f frequency. Together w i t h t h i s fundamental i n f o r m a t i o n about the bond we can e x t r a c t a q u a n t i t a t i v e measure o f the coverage o f adsorbed molecules. V i b r a t i o n s o f groups o f f a i r l y c h a r a c t e r i s t i c i n t e n s i t y , which are n o t involved i n s p e c i f i c i n t e r a c t i o n should n o t lead t o any dependence o f the i n t e n s i t y on the coverage. This i d e a l i z e d p i c t u r e i s almost never appropriate since both band shape and e x t i n c t i o n c o e f f i c i e n t change w i t h coverage ( 4 ) . The most r e l i a b l e approach i s t o measure independently t h e e x t e n t o f adsorption t o e s t a b l i s h a l i n e a r i t y between coverage and absorbance. Such a r e l a t i o n s h i p has been found by Eischens e t a l . (21, 22) who simultaneously measured t h e weight change of samples by microgravimetry and t h e i n f r a r e d spectrum by suspending the sample i n t o the i n f r a r e d beam i n the same sample-arm water (1630 cm-')
o f microbalance. The bending band o f
and t h e carboxylate band (1570 cm-')
r e s u l t i n g from the
decomposition o f acetylene on alumina show a l i n e a r r e l a t i o n s h i p between weight change and absorbance. I t must be remembered t h a t the c h a r a c t e r i s t i c i n t e n s i t y o f t h e i n f r a r e d bands o f adsorbed molecules i n d i c a t e a p e r t u r b a t i o n o f the corresponding fragment o f the molecule during adsorption. I n t h e i n t e r p r e t a t i o n o f spectra i t i s a l s o important t o consider t h a t both frequency and i n t e n s i t y o f a given band are functions o f d i f f e r e n t parameters. For instance, v i b r a t i o n s o f
bonds o f t h e t y p e OH, CH, e t c .
, are
assumed t o be c h a r a c t e r i s t i c w i t h r e s p e c t t o
i n t e n s i t y and frequency, w h i l e v i b r a t i o n s o f t h e symmetric bonds C=C, G X , e t c . , a r e c h a r a c t e r i s t i c w i t h r e s p e c t t o frequency, b u t n o t w i t h r e s p e c t t o i n t e n s i t y , as t h ey a r e l a r g e l y determined by t h e environment o f t h e bond. T h i s i m p o r t a n t f e a t u r e must be c o n s i d e r e d on a n a l y s i s o f t h e s p e c t r a o f adsorbed molecules, s i n c e c onc lus ions on band i n t e n s i t i e s based e x c l u s i v e l y on a n a l y s i s of frequenc ies may be erroneous. The s t udy o f t h e changes o f band i n t e n s i t i e s o f some v i b r a t i o n s of adsorbed molecules r e s u l t s o f i n t e r e s t f o r f i n d i n g t h e i r o r i e n t a t i o n w i t h r e g a r d t o t h e su rf ac e. F o r adsorbed molecules w i t h r o t a t i o n axes p a r a l l e l t o t h e surface, t h e v a r i a t i o n o f t h e band i n t e n s i t y may be r e l a t e d t o t h e r e t a r d a t i o n o f t h a t v i b r a t i o n by t h e a d s o r p t i o n f i e l d . S i n c e a p l a n a r arrangement of molecules o n t o t h e s u r f a c e is thermodynamically p r e f e r a b l e (see, e.g., r e f . ( 2 3 ) ) , t h e i n d i c i d e n c e o f t h e a d s o r p t i o n f i e l d on t h e changes o f band i n t e n s i t i e s i s a f a i r l y extended phenomenon. Laminar a l u m i n o s i l i c a t e s p r o v i d e excel l e n t examples t o s t u dy t h e o r i e n t a t i o n o f adsorbed molecules o r s u r f a c e groups. I n micas i t has been demonstrated t h a t t h e s t r u c t u r a l h y d r o x y l groups a r e d i r e c t e d t oward vacant oc t a hedra l p o s i t i o n s ( 2 4 ) . Z e o l i t e s a l s o c o n s t i t u t e good m a t e r i a l s f o r t h i s purpose. The e f f e c t o f t h e change o f band i n t e n s i t y is v e r y s t r o n g i n t h e s p e c t r a o f adsorbed molecules s u b j e c t e d t o t h e s t r o n g and non-uniform e l e c t r o s t a t i c f i e l d o f t h e c a v i t i e s o f z e o l i t e s . Fo r t h e C-C and C-H bond v i b r a t i o n s of t h e e t h y l e n e adsorbed on c a t i o n i c z e o l i t e s , t h e change i n t h e i n t e n s i t y o f t hese v i b r a t i o n s was found t o be more t h a n one o r d e r o f magnitude h i g h e r t h a t i n t h e gas s pec t ra. The s t u d i e s on t h e o r i e n t a t i o n o f adsorbed molecules m i g h t be expected t o be conducted o n l y on s i n g l e c r y s t a l s u r f a c e s , however, i t has been demonstrated t h a t a t h i n f i l m o f copper s u r f a c e s ( 2 5 ) o r d i s c r e t e p a l l a d i u m c r y s t a l l i t e s ( 2 6 ) a r e examples which s a t i s f y t h e requirements o f f a i r l y f l a t surf aces; t he i n t e r n a l o r e x t e r n a l r e f 1 e c t i o n spectroscopy b e i n g t h e usual experiment al t echnique f o r t h i s purpose (s e e s e c t i o n 2.7.). 2.2.1.5
R o t a t i o n a l Bands
The i n f r a r e d s p e c t r a o f molecules, when determined a t h i g h r e s o l u t i o n , d i s p l a y a d e t a i l e d f i n e s t r u c t u r e around t h e fundamental a b s o r p t i o n band. Such a s t r u c t u r e i s due t o simultaneous changes a t both, v i b r a t i o n a l and r o t a t i o n a l l e v e l s . The values o f these energy l e v e l s may be c a l c u l a t e d on t h e b a s i s o f t h e Born-Oppenheimer a p p r o x i m a t i o n which s t a t e s t h a t v i b r a t i o n s and r o t a t i o n s i n molecules a c t independently. The q u a n t i z e d energy l e v e l s f o r v i b r a t i o n s (T able 2 . 2 J a r e obt a ined b y s o l v i n g t h e SchrSdinger e q u a t i o n f o r t h e anharmonic
o s c i l l a t o r . The r o t a t i o n a l energy may be o b t a i n e d i n a s i m i l a r manner f o r a r o t a t o r i n which t h e e f f e c t o f c e n t r i f u g a l d i s o r t i o n i s considered. The energy
B78
Er associated w i t h the r o t a t i o n a l quantun number J i s given by the equation Er = h.B.J(J+l)
-
h.D.J2(J+1)
(2.10a)
h/8dI
B
(2. l o b )
where B i s the r o t a t i o n a l constant, h i s Planck's constant, I i s t h e moment o f i n e r t i a , and 0 i s a measure o f the d e v i a t i o n from i d e a l behaviour i n the same manner as the anharmonicity constant. Since D i s u s u a l l y 104 times smaller than B, the l a s t term i n Eq. (2.10a) can be ignored. The r o t a t i o n o f a diatomic
molecule creates a c e n t r i f u g a l f o r c e t h a t couples w i t h the v i b r a t i o n , and t h e r e s u l t a n t spectral absorption i s due t o the combined v i b r a t i o n - r o t a t i o n , whose energy (E ) i s given by the Born-Oppenheimer approximation. V,J EV ,J
= EV
+
Er = h.uo(V+f)
+
(2.11)
82I
For simultaneous t r a n s i t i o n s between two r o t a t i o n a l l e v e l s (from J ' t o J " ) and two v i b r a t i o n a l l e v e l s (from V ' t o V " ) AE = h.vo(V1-V")
+
8d I
J1(J'+l)
-
i t follows t h a t
J"(J"+1)
(2.12)
For a pure r o t a t i o n a l spectrum, t h e s e l e c t i o n r u l e i s t h a t J I - J " = f 1. For a fundamental v i b r a t i o n V ' - V " =
2
= 1, the a p p l i c a t i o n o f t h e s e l e c t i o n r u l e J ' - J "
=
1 i n Eq. (2.12) gives t h e f o l l o w i n g r e l a t i o n s h i p s f o r AE and v.
AE = h.vo
+h2 m
(2.13a
8 2I v = vo + 2 Bm
where m i s a molecular quantum number t h a t can have values ? 1,
(2.13b
?
2,
and B i s the r o t a t i o n a l constant, as already defined by Eq. (2.10b).
2
3 , etc.,
The
p r a c t i c a l i m p l i c a t i o n s o f Eq. (2.13b) are t h a t , when t h e frequency c o n d i t i o n s are f u l f i l l e d , t h e value o f uo determines t h e p o s i t i o n o f the center o f the fundamental v i b r a t i o n - r o t a t i o n band and the second term defines t h e r o t a t i o n a l f i n e s t r u c t u r e . I f m i s p o s i t i v e . l i n e s are observed i n t h e high-frequency region o f wo, b u t i f m i s negative, l i n e s are observed on t h e low-frequency s i d e o f uo. These a r e known as the R and P branches, r e s p e c t i v e l y . I n some molecules i t i s possible f o r t h e molecule t o have an angular momentum about t h e a x i s o f
the n u c l e i . I n these cases the v i b r a t i o n a l change can occur w i t h o u t r o t a t i o n , i n which case m i s equal t o zero. Very small changes i n Er g i v e r i s e t o a f i n e
B79 s t r u c t u r e contour around vo, and t h i s i s known as t h e Q branch o f t h e spectrum. The Q branch i s common i n polyatomic molecules (H20, NH3, e t c . ) ,
but f o r
d i a t o m i c molecules i t i s n o t observed, unless t h e molecule has an unpaired e l e c t r o n , e.g.,
a NO molecule.
2.2.2 q u a n t i t a t i v e Aspects 2.2.2.1
Beer's law
When l i g h t o f i n t e n s i t y I, impinges upon a l a y e r o f t h i c k n e s s 1, t h e i n t e n s i t y o f t h e emerging r a d i a t i o n i s g i v e n by t h e Lambert-Bouguer law,
I = I,.
emE*'
(2.14a)
or I n (Io/I) = ~ . 1
(2.14b)
This law provides t h e b a s i s f o r a l l q u a n t i t a i v e i n v e s t i g a t i o n s i n spec roscopy The q u a n t i t i e s E and 1 a r e t h e e x t i n c t i o n c o e f f i c i e n t and t h e sample thickness,
.
r e s p e c t i v e l y , and t h e product ~ . i1 s t h e o p t i c a l d e n s i t y . The e x t i n c t i o n c o e f f i c i e n t gives a complete d e s c r i p t i o n o f t h e s o r p t i o n c a p a c i t y o f t h e s o l i d , and, provided t h e number o f molecules i n p a t h 1 remains constant, t h e absorbance
w i l l a l s o describe t h e a b s o r p t i o n c h a r a c t e r i s t i c s o f t h e s o l i d a t any r a d i a t i o n wavelength. This i s v a l i d f o r s o l i d s and pure l i q u i d s , b u t becomes n o t a p p l i c a b l e f o r s o l u t i o n s and gases. The number o f molecules along 1 depends d i r e c t l y on t h e concentration, t h e r e f o r e t h e absorbance i s a l i n e a r f u n c t i o n o f t h e concentration. A = E.C.1
(2.15a)
o r I = I o . e -E.C.l
(2.15b)
Equations (2.15a) and (2.15b) a r e t h e w e l l known Beer's and Beer's-Lambert laws, r e s p e c t i v e l y . I f t h e c o n c e n t r a t i o n C i s expressed i n moles p e r l i t e r and t h e path l e n g t h 1 i s g i v e n i n centimeters, t h e p r o p o r t i o n a l i t y f a c t o r
E
i s called
t h e molar e x t i n c t i o n c o e f f i c i e n t and has t h e dimensions 1 i t e r / c e n t i m e t e r / m o l e . The values o f E v a r y widely. For instance, t h e bands o f t h e CO s t r e t c h i n g v i b r a t i o n s i n metal carbonyls g i v e values o f about 103 l i t e r s mole-'cm-' , w h i l e t h e C-H v i b r a t i o n bands o f s a t u r a t e d hydrocarbons a r e c o n s i d e r a b l y weaker, 1 3 l i t e r s mole- 1cm". E l y i n g i n t h e range 10 -10 2.2.2.2
Adsorption Studies
The above concept can be extended t o a d s o r p t i o n s t u d i e s . I n a d s o r p t i o n s t u d i e s i s much more u s e f u l t o express t h e p r o d u c t ~ . 1i n u n i t s of molecules p e r
B80 u n i t area. The corresponding values of E i n u n i t s o f an2 molecules-' a r e o b t a i n e d from t h e previous m o l a r values by d i v i d i n g by 6 x lo2', g i v i n g €1.10 18 cm2 molecules-' f o r carbonyls and 10-20-10-19 cm2molecules-' f o r C-H bonds, For
-
comparative purpose, values o f
E
f o r v a r i o u s adsorbate-substrate systems a r e
summarized i n Table 2.3. Using these values, i t i s p o s s i b l e t o e s t i m a t e values f o r t h e a b s o r p t i o n of a monolayer o f adsorbate. A monolayer o f CO adsorbed on a metal surface and a monolayer o f p h y s i c a l l y adsorbed ethane w i l l have s i m i l a r d e n s i t i e s of about 6 x molecules Considering t h e e x t i n c t i o n coe f f i c i e n t f o r these molecules (Table 2.3.)
we f i n d a b s o r p t i o n o f 0.28% f o r CO
and 0.012% f o r C2H6. As i t i s d e s i r a b l e t o r e c o r d good q u a l i t y s p e c t r a w i t h conventional i n f r a r e d spectrophotometers, i t i s recommended t o use h i g h s u r f a c e area samples so t h a t t h e i n f r a r e d beam may cross through several hundreds of monolayers. T h i s i n t e r e s t i n g f e a t u r e can, t h e r e f o r e , be e x p l o i t e d t o o b t a i n spectra even a t low coverages. However, t h i s procedure cannot be f o l l o w e d f o r t h e a d s o r p t i o n o f molecules on smooth s i n g l e - c r y s t a l s o r p o l y c r y s t a l l i n e s u r faces. I n t h e l a t t e r case, t h e r e c o r d i n g o f s p e c t r a i s v e r y demanding, and i t i s n o t s u r p r i s i n g t h a t most o f t h e experiments conducted t o t h i s end have
used molecules w i t h v e r y h i g h e x t i n c t i o n c o e f f i c i e n t s , e.g., CO. I t i s i m p o r t a n t t o emphasize t h a t t h e s e n s i t i v i t y o f conventional transmission i n f r a r e d spectroscopy cannot be c o n t i n u o u s l y increased by i n c r e a s i n g sample thickness. The s e n s i t i v i t y i s t h e r e s u l t o f two o p p o s i t e phenomena, v i z . ,
t h e a b s o r p t i o n and s c a t t e r i n g o f r a d i a t i o n by t h e s o l i d . W i t h
i n c r e a s i n g sample thickness t h e a b s o r p t i o n band becomes a l a r g e r and l a r g e r f r a c t i o n o f a s m a l l e r and s m a l l e r amount o f a v a i l a b l e energy o f t h e i n c i d e n t beam. TABLE 2.3. E x t i n c t i o n C o e f f i c i e n t o f Adsorbed Molecules Adsorbent
Ad so r bat e
Zn
co co
P t / S iO2
N i/ S iO2
(cm2mol ecul e - l ) 5.5 x 1 0 - l 8 2 x 4 x 10-l8
Porous g l a s s
N2 C2H2 (2950 cm-')
CoAl 204
NO
1.3
Mo/A1 203
NO
2.5
9 x
Ref.
(27) (28) (29)
(30) 10-l~
(31)
(31)
B81 2.3 EXPERIMENTAL TECHNIQUES 2.3.1 Transmi s s i on The most extended procedure t o o b t a i n t h e absorption bands o f adsorbed molecules i s the examination o f h i g h l y dispersed m a t e r i a l s by passing i n f r a r e d r a d i a t i o n across them. Such a procedure should ensure the maximum transparency w i t h many hundreds of interfaces covered by t h e adsorbed molecules under i n v e s t i g a t i o n i n t h e path o f the i n f r a r e d beam. High surface area s o l i d s , namely, s i l i c a , alumina, silica-alumina, and z e o l i t e s have wide spectral regions o f transparency w i t h o n l y 1i m i t e d absorption bands o f t h e i r skeleton, u s u a l l y A t low coverages, a s u f f i c i e n t amount o f sample w i t h moderate
below 1000 cm-'.
o r high s p e c i f i c surface must be placed i n t h e path o f the beam. This i s feasible by increasing the thickness o f the adsorbed layer, however, the s c a t t e r ing of r a d i a t i o n increases simultaneously. Therefore, i n t h e most favorable case, w i t h r e l a t i v e l y poor scatterers, preparations r e s u l t i n s a t i s f a c t o r y spectra only f o r adsorbents w i t h a s p e c i f i c surface area not l e s s than ca. 100 m'9-l.
As
the s c a t t e r i n g o f r a d i a t i o n by p a r t i c l e s whose diameter d i s small compared t o the wavelength
x
o f the r a d i a t i o n i s proportional t o d3/14, t h i n l a y e r s combine
l a r g e surface areas w i t h low s c a t t e r i n g losses. Perhaps t h e best method t o prepare samples f o r i n f r a r e d examination i s t h e pressed-disk technique. This consists i n spreading the material on a d i e and then pressing i t t o g i v e a t h i n self-supporting wafer. The main advantage o f t h i s technique i s the l a r g e surface area a v a i l a b l e w i t h i n the i n f r a r e d beam, g i v i n g l a r g e concentrations o f adsorbed molecules, even a t low coverages. For instance, f o r a t y p i c a l alumina d i s k o f 10 mg cm-2 weight w i t h a surface area o f 200 m2gm1, the t o t a l area o f a 2 d i s k o f 1 cm would then be 2 in2; i f a metal phase, w i t h a loading o f 5 w t % and an average p a r t i c l e s i z e o f 5 nm and a d e n s i t y o f 10 g c ~ n - ~i,s then i n corporated, the r e s u l t i n g metal surface area i s 103 cm2
.
One important t r o u b l e i n t h e examination o f supported c a t a l y s t s by the transmission mode i s the s c a t t e r i n g losses and the absorption losses o f radiat i o n , both depending on the nature o f the support and the p a r t i c l e s i z e o f the supported ingredient, i.e.
, metal,
metal oxide, sulphide. Although many supports
can be used w i t h a wide i n f r a r e d transmission range, some present important absorption phenomena by the oxide l a t t i c e . S i l i c a c o n s t i t u t e one example o f the l a t t e r category. S i l i c a wafers l o s e transparency progressively below 1300 cm-l, thus l i m i t i n g the observation o f v i b r a t i o n bands o f the absorbed molecules t o t h a t region. Alumina i s somewhat b e t t e r i n t h i s respect, as i t begins t o absorb r a d i a t i o n a t about 1000 cm-',
A notable exception, however, i s provided by
mangesia which r e t a i n s complete transparency over the wide i n f r a r e d spectrum, although s u r p r i s i n g l y l i t t l e use as a support has been given t o t h i s m a t e r i a l . The great profusion o f i n f r a r e d c e l l s may be due t o t h e f a c t t h a t t h e study o f every c a t a l y t i c problem presents special requirements. Many designs provide
B82
F i g . 2.3 a) Transmission i n f r a r e d c e l l f o r continuous-flow t r e a t m e n t s : l ) o u t l e t o r vacuum l i n e ; 2) V i t o n O-ring; 3) magnet; 4) g o l d chain; 5 ) thermocouple;6) sample; 7 ) furnace; 8 ) K B r windows; 9 ) gas i n l e t (From r e f . ( 3 2 ) ) . b) Highpressure i n f r a r e d c e l l : 1) CaF2 windows; 2 ) gas o u t l e t ; 3 ) gas i n l e t ; 4) Sample holder; 5) thermocouple; 6 ) O-ring; 7 ) heater; 8 ) c o o l i n g c o i l (Readapted from ref. (38)).
B83 f o r t h e sample t o be moved, e i t h e r m a g n e t i c a l l y o r by windlass arrangements, so t h a t c a t a l y s t pretreatments can be conducted w i t h o u t danger f o r t h e windows. As an example, F i g . 2.3.a shows a c e l l used i n t h e study o f NO chemisorption on i n s i t u sulphided h y d r o t r e a t i n g c a t a l y s t s (32). The sample can be t r e a t e d i n a continuous f l o w a t a g i v e n temperature, and then moved down m a g n e t i c a l l y t o t h e i n f r a r e d beam by t h e h e l p o f an o u t e r magnet capable t o be moved along a l a t e r a l branch. Extensive reviews o f t h e methods a r e c o l l e c t e d i n c l a s s i c a l works ( 2 - 6 ) . An e x c e l l e n t account o f t h e p r a c t i c a l aspects o f sample p r e p a r a t i o n and c e l l designs has been g i v e n by Parkyns (33). Various types o f i n f r a r e d c e l l s have been designed t o o b t a i n t h e s p e c t r a o f adsorbed species d u r i n g r e a c t i o n s a t h i g h temperature (34-36). Most o f them, however, have been used a t atmospheric o r reduced pressure, and less a t t e n t i o n has been p a i d t o h i g h pressure operations (36, 37). I n a d d i t i o n and i n such c o n d i t i o n s , an e f f i c i e n t f l o w o f t h e r e a c t a n t m i x t u r e i n t h e c e l l i s u s u a l l y required, i n o r d e r t o f o l l o w t h e k i n e t i c behaviour o f t h e s u r f a c e species. Tagawa and Amenomiya ( 3 8 j have r e c e n t l y developed a r e a c t i o n system w i t h an i n f r a r e d c e l l which can be used as a continuous f l o w - r e a c t o r f o r t h e c a t a l y t i c synthesis o f methanol (C0t2H2-
CH3-OH) up t o 1.8 MPa and 580 K. A schematic
diagram o f t h i s c e l l i s shown i n F i g . 2.3.b.
I t can be seen t h a t t h e pressure i n
t h e c e l l i s l i m i t e d by t h e window c r y s t a l s . For t h e case o f t h e CaF2 window
( 6 mm t h i c k , 38 mm diameter, w i t h 25 mm support diameter) t h e maximum pressure was c a l c u l a t e d as 1.8 MPa w i t h a s a f e t y f a c t o r o f 4 (39). 2.3.2.
Emission For many c a t a l y s t s working a t temperatures above 473 K t h e r e e x i s t s t h e
p o s s i b i l i t y t o r e c o r d t h e i r i n f r a r e d emission spectra. This may be advantageous i n some cases. The spectrum i s obtained by measuring t h e r a t i o o f t h e e m i t t e d r a d i a t i o n a t any wavelength t o t h a t e m i t t e d by t h e i d e a l blackbody a t t h e same wavelength a t t h e same temperature. I n theory, t h e r e l a t i v e s e n s i t i v i t y of t h e spectroscopic emission and a b s o r p t i o n modes i s g i v e n by t h e r a t i o o f t h e b l a c k body reference used f o r t h e e m i s s i v i t y measurements and t h e incandescent source used f o r t h e a b s o r p t i o n spectra. T h i s has been e x p e r i m e n t a l l y achieved f o r a blackbody e m i t t e r heated a t 460 K compared t o an incandescent nichrome w i r e source. Under these c o n d i t i o n s t h e s i g n a l - t o - n o i s e r a t i o was found t o be f i v e times g r e a t e r f o r a b s o r p t i o n around ca. 800 cm-l, and about 200 times g r e a t e r f o r absorption a t ca. 3000 cm-'
(40). Since K i r c h h o f f ' s law s t a t e s t h a t t h e
emittance o f any body i s equal t o i t s absorbance, i t f o l l o w s from t h e BeerLambert law o f absorption t h a t emission o f a t h i c k nonopaque e m i t t e r w i l l approach t h a t o f a blackbody. Thus, t h e i d e a l e m i t t e r would be a t h i n l a y e r supported on a p e r f e c t r e f l e c t o r . For X-type z e o l i t e s , i t has been shown t h a t a c o a t i n g o f 13 1-19mm-'
on a g o l d f i l a m e n t i s enough,the r o l e of t h e g o l d f i l a m e n t
B84 5 I
5
Fig. 2.4. Simpre i n f r a r e d emission c e l 1 : l ) NaCl window; 2 ) g o l d filament; 3 ) vacuum l i n e and gas-hand1 i n g gases; 4) power supply; 5) V i t o n O-ring (Readapted from r e f . (41)). being double, i.e.,
i t acts as a temperature c o n t r o l l e r and support. The simple
c e l l f o r t h i s purpose has been constructed by Dewing (41). and i s i l l u s t r a t e d i n Fig. 2.4. Emission spectra are most s u i t a b l e f o r processes t a k i n g place on surfaces a t temperatures a t which c a t a l y t i c reactions occur. Although i t i s a l s o possible t o use the transmission mode t o o b t a i n the v i b r a t i o n a l spectra o f t h e adsorbed molecules a t h i g h temperatures, these e x i s t s an i n t r i n s i c l i m i t a t i o n i n the conventional spectrophotometers, because they cannot separate the r a d i a t i o n emitted by t h e hot s o l i d (and the heated p a r t s o f t h e c e l l ) from t h e i n f r a r e d beam passing through t h e sample. This t r o u b l e has been overcome i n a few cases
(42, 43) by modulating t h e i n f r a r e d beam before t h e passage through t h e sample. As a r e s u l t , t h e unmodulated r a d i a t i o n emitted by t h e h o t sample i s n o t recorded by the detector of t h e instrument.
2.3.3. R e f l e c t i o n Methods R e f l e c t i o n methods, i n c l u d i n g specular external from metal m i r r o r s , specular i n t e r n a l and d i f f u s e external spectroscopy, can o n l y be a p p l i e d t o w e l l defined surfaces, i.e.,
s i n g l e c r y s t a l s o r f i l m s . The development o f these
methods was somewhat belated probably as a consequence o f t h e demanding vacuum conditions r e q u i r e d f o r cleaning s i n g l e c r y s t a l surfaces, t h e necessity t o introduce instrumental m o d i f i c a t i o n s i n conventional spectrophotometers, and the low s e n s i t i v i t y a t t a i n e d f o r such surfaces. The low s e n s i t i v i t y i s evidenced by the f a c t t h a t i n a s i n g l e specular r e f l e c t i o n an i n f r a r e d beam may i n t e r a c t w i t h about 1015 adsorbed molecules, as compared t o the 1018-1019 molecules i n a t y p i c a l transmission experiment. The simplest procedure t o increase s e n s i t i v i t y i s by increasing t h e number o f r e f l e c t i o n s . Up t o 35 r e f l e c t i o n s a t near normal incidence were used by Pickering and Eckstrom (44) t o observe weak bands o f CO
B85
5
4
Fig. 2.5. a ) Si ngle-pass multiple internal reflection arrangement (from Wilks Scientific Co.) b ) Double-pass multiple internal reflaction arrangement (from Harrick Scienti f i c Co.). c ) Diffuse reflection rotation ellipsoid: 1) source; 2 ) mirror; 3 j 1ens; 4 ) sample (ellipsoid focus); 5) detector (ellipsoid focus).
.
adsorbed on nickel and rhodium mirrors, b u t only four reflections a t 72" were required in the work of Francis and Ellison (45) t o record the spectra of monomolecular Blodgett films of metal stearates on s i l v e r mirrors. The principle of internal reflection spectroscopy ( I R S ) is s l i g h t l y different from specular reflection by metal mirrors. In t h i s case, the infrared radiation approaches the gas-solid interface from the solid. In the absence of
B86
absorbing molecules, t h e i n f r a r e d r a d i a t i o n w i l l be t o t a l l y r e f l e c t e d back i n t o the s o l i d when t h e incidence angle exceeds t h e c r i t i c a l angle. This f a c t makes i t very advantageous t o use m u l t i p l e r e f l e c t i o n s . Two a u x i l i a r y o p t i c a l layouts t h a t accomplish m u l t i p l e r e f l e c t i o n s i n conventional spectrophotometers are shown i n Figs. 2.5.a and 2.5.b. Wilks S c i e n t i f i c Corporation and H a r r i c k S c i e n t i f i c Corporation market o p t i c s and vacuum c e l l s using e i t h e r t h e s i n g l e pass ( F i g . 2.5.aj
o r t h e double pass geometry ( F i g . 2.5.b).
The l a t t e r o p t i c s
but i s more d i f f i c u l t t o a l i g n o p t i c a l l y . I n t e r n a l r e f l e c t i o n spectroscopy has a l s o been a p p l i e d t o the examination o f l i q u i d - s o l i d interfaces. Although I R S c e l l s made w i t h f l a t p l a t e s have been used i n the analysis of aqueous s o l u t i o n s (46) t h i s design i s n o t e f f i c i e n t w i t h respect t o energy throughput i n the c i r c u l a r beam o f t h e FTIR spectrophotometers. To increase t h i s energy throughput o f the system, Wilks (47) proposed a new i n t e r n a l r e f l e c t i o n element design employing a polished c y l i n d r i c a l rod w i t h cone-shaped ends. This c y l i n d r i c a l i n t e r n a l r e f l e c t a n c e (CIR) c e l l has been shown t o perform very w e l l f o r q u a n t i t a t i v e analyses o f aqueous s o l u t i o n s (48). D i f f u s e r e f l e c t i o n , which i s very useful f o r adsorption studies i n the v i s i b l e (49), can be extended t o the near i n f r a r e d . D i f f u s e r e f l e c t i o n i n t h e i n f r a r e d becomes d i f f i c u l t , because t h e i n f r a r e d detectors a r e s e n s i t i v e t o the thermal r a d i a t i o n o f t h e environment. This background noise increases as t h e square r o o t o f the d e t e c t o r surface, thus being minimized w i t h small area detectors. This leads t o a serious problem o f c o l l e c t i n g t h e r e f l e c t e d r a d i a t i o n , since i t i s n o t possible tofocus a p l a n e o n t o a very small plane, i.e., the sample surface must be as small as t h e detector. Kortum (50) was able t o solve t h i s problem using t h e c h a r a c t e r i s t i c s o f t h e r o t a t i o n e l l i p s o i d . As shown i n Fig. 2 . 3 . ~ t~h e sample i s placed a t one focus o f the h a l f - e l l i p s o i d which has an aluminum m i r r o r and t h e detector i s placed i n the o t h e r focus. This design has been used by Klirtum t o o b t a i n t h e d i f f u s e r e f l e c t i o n spectrum o f CO adsorbed on ZnO (31). has the advantage t h a t the i n t e r n a l r e f l e c t i o n element can be of any length
2.3.4.
Photoacoustic (PAS) and Photothermal D e f l e c t i o n (PDS) Spectroscopy Several configurations have been used f o r PAS c e l l s f o r i n f r a r e d studies o f
adsorbed molecules (9.52).
However, as G r i f f i t h s and F u l l e r ( 8 ) pointed out,
none o f these permit spectra t o be obtained under r e a c t i o n conditions. Such a c e l l should be able t o operate a t h i g h temperatures, w i t h p r o v i s i o n made f o r gases f l o w i n g through t h e c e l l t o ensure constancy o f the gas and surface comp o s i t i o n . A Hemholtz resonator c e l l c o n f i g u r a t i o n (53-55) seems t o be a s u i t a b l e arrangement t o separate the microphone from t h e sample chamber, which can then be taken t o high temperature. The c e l l can be coupled t o an a c o u s t i c a l l y i s o l a t e d gas h a n d l i g h t system t o o b t a i n the PAS spectra w i t h gas f l o w i n g through
t h e c e l l a t temperatures o f i n t e r e s t . I n t h i s case, t h e n o i s e a r i s i n g from pressure f l u c t u a t i o n s of t h e gas f l o w i n g above t h e sample must be minimized by m a i n t a i n i n g t h e f l o w i n t h e laminar regime. Very simple c o n f i g u r a t i o n s and c e l l designs have been a l s o employed t o o b t a i n photothermal d e f l e c t i o n beam (PDS) spectra. I n most cases, t h e samples were compacted t o form s e l f - s u p p o r t i n g wafers and then i n s t a l l e d on a q u a r t z b l o c k (56) which served as sample h o l d e r and t h a t assembly remained exposed i n t h e sample c e l l t o a v a r i e t y o f pretreatments. The instrument used f o r t h i s purpose i s r e l a t i v e l y simple. I t b a s i c a l l y c o n s i s t s o f an i n t e r f e r o m e t e r coupled w i t h s i u t a b l e o p t i c s optimized f o r use w i t h a "mirage" d e t e c t o r (12). The o p t i c a l system i s i n t e r f a c e d t o a computer and i t s p e r i p h e r a l s . Data a c q u i s i t i o n and handling a r e done as w i t h a conventional FT spectrophotometer. Low and M o r t e r r a (16) used a compensation system based on t h e premiss t h a t two beams t r a v e r s i n g t h e same path would be subjected t o t h e same disturbances, so t h a t these could be cancelled. The l a s e r beam i s s p l i t i n a beam s p l i t t e r , and a f t e r passing almost t h e same o p t i c a l path, each beam f a l l s on a p o s i t i o n - s e n s i n g d e t e c t o r . The photothermal e f f e c t i s induced on t h e sample by t h e modulated i n f r a r e d r a d i a t i o n coming from t h e i n t e r f e r o m e t e r and t h e photothermal l y induced beam d e f l e c t i o n i s p i c k e d up by t h e probe beam and r e s u l t s i n t h e photothermal i n t e r f e r o g r a m which i s then processed as i n a conventional FT spectrometer. 2.3.5.
Spectrophotometers The advent o f FT spectrometers i n t h e e a r l y 60's overcome, i n p a r t , t h e
serious 1 i m i t a t i o n s o f t h e conventional g r a t i n g spectrophotometers,
and have
been f r e q u e n t l y used i n transmission, specular and d i f f u s e r e f l e c t a n c e , emission, and photoacoustic and photothermal d e f l e c t i o n beam spectroscopy. A comparison o f t h e performance o f FT and d i s p e r s i v e spectrophotometers and t h e areas o f a p p l i c a t i o n i n which FT spectroscopy has proven advantageous f o r t h e study o f surface species i n heterogeneous c a t a l y s t s has been reviewed by B e l l ( 5 7 ) . More d e t a i l e d discussions concerning t h i s s u b j e c t and many examples of t h e a p p l i c a t i o n s o f FT spectroscopy t o o t h e r f i e l d s o f chemistry can be found i n s p e c i a l i z e d monographs (see, e.g.,
(55-60)).
The modern FT spectrometers have combined f a s t response i n f r a r e d d e t e c t o r s , l a s e r s t o p r o v i d e a reference i n t e r f e r o g r a m t o d i g i t i z e t h e s i g n a l i n t e r ferogram, and o n - l i n e microcomputers capable o f r a p i d l y t r a n s f o r m i n g i n t e r ferograms t o spectra. I n can be demonstrated t h a t an FT spectrometer can a c q u i r e
un'l 400 times f a s t e r a t S/N o f (4000:l)& i n t h e same t i m e
t h e spectrum from 0 t o 4000 cm-l w i t h a r e s o l u t i o n o f 1 t h e same S / N r a t i o o r w i t h an improvement i n
r e l a t i v e t o t h e d i s p e r s i v e instrument i f t h e n o i s e i s d e t e c t o r l i m i t e d . T h i s improvement i s known as F e l l g e t t ' s advantage and r e s u l t s from t h e f a c t t h a t a l l s p e c t r a l elements a r e sampled simultaneously d u r i n g t h e r e c o r d i n g o f t h e i n t e r -
ferogram r a t h e r than consecutively, as occurs i n the case o f the d i s p e r s i v e instrument. I t has, however, been observed (57) t h a t the p r a c t i c a l advantage o f the FT spectrometer c l o s e l y f o l l o w s t h e p r e d i c t e d t h e o r e t i c a l advantage, b u t i t i s a f a c t o r o f 4 smaller. The o r i g i n o f t h i s discrepancy has been analyzed by G r i f f i t h s e t a l . ( 6 1 ) who proposed t h a t i t might be ascribed t o the manner o f c a l c u l a t i n g F e l l g e t t ' s advantage. Based on t h e r e s u l t s o f Tai and H a r w i t t (62), t h i s author suggested t h a t t h e advantage should be (4000:D)' instead o f (4000:l)'. I t should a l s o be noted t h a t the inagnitude o f F e l l g e t t ' s advantage i s more diminished, i f e i t h e r a narrower wavenumber range i s selected o r a lower r e s o l u t i o n i s considered. 2.4 GENERAL ASPECTS OF CHEMISORPTION OF CO AND NO
The chemisorption o f CO and NO has been e x t e n s i v e l y used t o e l u c i d a t e t h e nature o f the adsorption and/or c a t a l y t i c s i t e s i n metals and metal oxides i n heterogeneous c a t a l y s t s (63, 64). This i s mainly due t o the l a r g e body o f t h e o r e t i c a l and experimental work done on the i n t e r a c t i o n s o f these molecules a t the surface o f many c a t a l y t i c systems. An overview o f the abundant l i t e r a t u r e concerning the use o f these molecules t o i n v e s t i g a t e the nature o f t h e adsorption s i t e s reveals t h a t three major t o p i c s , namely the symmetry o f t h e surface atoms o r ions, the n e t charge, and t h e degree o f coordination have been examined. These p o i n t s w i l l be discussed i n ensuing sections w i t h special emphasis on metal oxide systems. For a b e t t e r understanding o f the i n t e r a c t i o n mode o f the CO and NO a t the surface o f heterogeneous c a t a l y s t s , a b r i e f desc r i p t i o n o f t h e bonding o f both molecules t o t h e surface w i l l be considered first. The o r b i t a l s o f CO and NO are v e r y s i m i l a r . NO has one more e l e c t r o n than CO t h a t occupies an antibonding
n*
o r b i t a l . Both molecules form o-bonds w i t h t h e
atoms o r ions a t the c a t a l y s t surface through t h e i r lone p a i r e l e c t r o n s o f C o r N ( 5 0 - o r b i t a l ) . Moreover, the electrons from t h e surface can f l o w back t o t h e
antibonding
n*
o r b i t a l o f t h e molecule; t h i s being known as n-back-donation.
These two opposing e l e c t r o n i n t e r a c t i o n s determine t h e C-0 and N-0 bond s t r e n g t h and hence the CO and NO s t r e t c h i n g frequency, namely the o-donation ( h i g h frequency s h i f t ) and t h e I[-back-donation from the adsorption s i t e i n t o the K* antibonding o r b i t a l (low frequency s h i f t ) . N i t r i c oxide i s w e l l known as a l i g a n d i n coordination complexes o f t r a n s i t i o n metals and, as w i t h metal carbonyls, t h e accepted bonding p i c t u r e i s t h a t described above. The g r e a t e r the back-bonding t h e II* antibonding o r b i t a l , the weaker the N-0 bond and t h e lower i t s i n f r a r e d s t r e t c h i n g frequency. I n cont r a s t t o CO, however, t h e r e i s already an e l e c t r o n i n the II* o r b i t a l i n the neutral NO molecule. This e l e c t r o n can be e a s i l y t r a n s f e r r e d t o a n e u t r a l o r i o n i c s i t e , nominally producing NO',
and t h e t r a n s f e r o u t o f the antibonding I[*
B89 o r b i t a l s accordingly strengthens the N-0 bond and r a i s e s t h e i n f r a r e d s t r e t c h i n g frequency. According t o t h i s p i c t u r e , NO adsorbs more s t r o n g l y than CO on t r a n s i t i o n metals and metal oxides, and i n few instances t h e data suggest t h a t NO reacts a t comparably lower temperatures than O2 and leads t o surface o x i d a t i o n a t temperatures s l i g h t l y below room temperature, thus g i v i n g r i s e t o t h e appearance o f N2 N20 i n the gas phase. 2 3. TRANSMISSION-ABSORPTION I R SPECTROSCOPY A review o f the transmission-absorption I R spectroscopy o f adsorbed molecules on metals and metal c l u s t e r s i s beyond t h e scope o f t h i s book. There a r e e x c e l l e n t reviews concerned w i t h the a p p l i c a t i o n s o f chemisorption o f CO and NO probes t o the s t r u c t u r a l c h a r a c t e r i z a t i o n o f supported metal (1-4, 36, 65, 66) and metal c l u s t e r (67-69) c a t a l y s t s . However, what i s important i n t h i s context i s t o examine the s i g n i f i c a n c e o f intermediates formed a t c a t a l y s t surfaces during actual operation. 2.5.1.
Transient K i n e t i c Studies
As a1 ready reviewed by Tamaru (70). Kobayashi and Kobayashi (71), and Benn e t t (72), t r a n s i e n t methods are necessary t o determine k i n e t i c a l l y s i g n i f i c a n t structures and a generalized mechanism o f surface reactions. But the knowledge o f the mechanism w i t h i d e n t i f i c a t i o n o f the r e a c t i o n intermediates requires a combination o f t r a n s i e n t and spectroscopic methods. The advent i n t h e e a r l y 80s o f f a s t scanning F T I R spectrophotometers provided transmission-absorption I R spectroscopy as a very useful technique f o r t h i s purpose. To i l l u s t r a t e t h i s , the a p p l i c a t i o n o f FTIR methods t o c a t a l y t i c converters f o r automobile emission reduction operating under t r a n s i e n t conditions i s examined. The c a t a l y s t s used f o r the reduction o f NO w i t h Cooperate near t h e s t o i c h i o m e t r i c a i r - f u e l r a t i o (narrow A/F window). Therefore, a closed-loop a i r f u e l c o n t r o l system i s required which, i n turn, necessarily introduces
OS-
c i l l a t i o n s i n the feed stream. The changes i n t h e composition o f t h e feed induce r e l a x a t i o n t o a new steady s t a t e which may be monitored by transmission-absorpti o n I R spectroscopy. I t has already been observed (73) t h a t CO and NO conversions s t e a d i l y increased w i t h increasing c y c l i n g frequency. One explanation f o r t h i s l i e s i n t h e f a c t t h a t c y c l i n g decreases the extent o f formation of the r a t h e r s t a b l e surface isocyanate (-NCO) complex and a l s o lowers the average CO concentration. I t should be noted t h a t i n t h e work o f Hegedus e t
al. (73) I R was used t o
measure time-averaged surface concentrations under steady o s c i l l a t i o n o f t h e gas-phase concentration. Another a1 t e r n a t i v e has been r e c e n t l y used by Regal but0 e t a l . (74), which consists i n the fast temporal and spectral scanning w i t h I R Michel son interferometers (FTIR technique). These authors studied the con-
B90
c e n t r a t i o n and temperature-programmed CO t NO (02) r e a c t i o n over Pt/Si02 cat a l y s t s w i t h FTIR spectroscopy. Bands were observed f o r the l i n e a r surface species o f both CO and NO probes, and s h i f t e d t o lower frequencies w i t h decreasi n g coverage, from 2074 t o 2044 cm-l f o r CO and from 1774 t o 1735 cm-’
f o r NO.
I n a d d i t i o n , NO was a l s o found t o adsorb as bridged NO species, responsible f o r a band a t 1620 cm”,
b u t disappeared upon exposure t o CO o r 02. I n general , the
adsorbed NO species were seen t o be s t a b l e i n NO-N2 mixtures f o r l o n g periods o f time, b u t the CO surface species displaced t h e NO species w i t h o u t t h e formation o f COP. However, the temporary presence o f oxygen caused an a c t i v a t i o n o f t h e NO + CO reaction. The r e a c t i o n pathway was found, therefore, t o depend markedly
on the operation regime. Thus, i n t h e s i t e l i m i t e d regime, w i t h the P t p a r t i c l e s predominantly covered by CO, CO o x i d a t i o n i s most e f f e c t i v e using NO and O2 simultaneously as the oxidant, w h i l e i n the r e a c t i o n
l i m i t e d conditions o r low CO coverage o f P t , the CO + O2 i s much f a s t e r than the NO + CO r e a c t i o n . On the basis of the above f i n d i n g s , a r e a c t i o n mechanisms, i n v o l v i n g d i s s o c i a t i o n o f NO,
NO.site t s i t e
-
N.site
+
0.site
(2.16)
as the r a t e - l i m i t i n g step was proposed. The p r e r e q u i s i t e f o r the decomposition of molecularly adsorbed NO i s the presence o f an open adjacent s i t e (75). Other a l t e r n a t i v e explanations i n v o l v i n g more than one s i t e are possible. Mummey and Schmidt (76) have proposed t h a t
NO decomposition proceeds v i a a precursor
operating i n p a r a l l e l w i t h NO.sites, t h e l a t t e r being unreactive. On the other hand, Lambert and Coinrie (77) proposed t h a t adsorbed
NO decomposed from a
8-state, whereas NO adsorbed i n t h e a - s t a t e was i n a c t i v e . These r e s u l t s support the p l a u s i b i l i t y o f e i t h e r m u l t i s i t e o r dual chemisorption models. I n an attempt t o understand t h e r o l e o f t h e ad-alignment s i t e s i n the d i s s o c i a t i o n of NO, Regalbuto and Wolf (78) studied the k i n e t i c s o f the NO + CO r e a c t i o n and t h e promotional e f f e c t o f tungsten on Pt-WO,/SiO, c a t a l y s t s by FTIR i n the same i n s i t u recycle I R microreactor. They found t h a t a d d i t i o n o f W03 t o a Pt/Si02 c a t a l y s t increased a c t i v i t y toward t h e NO
+
CO r e a c t i o n . A r e -
presentative s e r i e s o f spectra i l l u s t r a t i n g t h e CO concentration programmed r e a c t i o n a t 493 K over the Pt-W03/Si02 c a t a l y s t i s shown i n F i g . 2.6.a.
and
compared t o the experiment performed over the tugnsten-free counterpart ( F i g . 2.6.b.).
I n both c a t a l y s t s , C O - P t adsorbates displaced q u i c k l y NO-Pt species and
dominated the P t surface over most o f the experiment. I n a d d i t i o n a small band a t ca. 1400 cm-’ and a shoulder a t ca. 2100 cm-’ 2.6.a.
are a l s o observed i n F i g .
Neither o f these two l a t t e r bands was found when o n l y one reactant,
e i t h e r CO o r NO, was present; being o n l y observed when NO and CO were simultaneously present.
F i g . 2.,6. I R s p e c t r a d u r i n g CO concentration-programmed r e a c t i o n (0-14% CO) i n t o 10% NO a 493 K, a t v a r i o u s time i n t e r v a l s showing t h e species present a t cat a l y s t surface. a) Pt-WO-JSi02 c a t a l y s t . A f t e r Regalbuto and Wolf (78). The k i n e t i c s d i s p l a y e d an i n h i b i t i o n behaviour w i t h i n c r e a s i n g CO conc e n t r a t i o n , b u t W03 served t o decrease CO s u r f a c e coverage d u r i n g t h e i n h i b i t e d regime. However, t h e r a t e o f NO d i s s o c i a t i o n over P t was slowest i n t h e tungstapromoted c a t a l y s t . An explanation f o r t h i s m i g h t be t h e r e d u c t i o n i n t h e adjacent s i t e s r e q u i r e d f o r NO d i s s o c i a t i o n , caused by t h e d e c o r a t i n g patches o f WOx ( x c 3). The increased a c t i v i t y i n t h e promoted c a t a l y s t i s n o t a consequence o f increased NO d i s s o c i a t i o n on P t . Since tungsta i t s e l f does n o t adsorb NO o r CO, t h e o n l y source f o r increase i n
NO
t CO r e a c t i o n r a t e s i s v i a s i t e s a t t h e
i n t e r f a c e o f t h e d e c o r a t i n g WOx patches on P t c r y s t a l l i t e s , v i z . ,
adlineation
s i t e s . This type o f s i t e s , which have been proposed t o e x p l a i n t h e increased a c t i v i t y i n SMSI c a t a l y s t s (79. 80), c o u l d be responsible f o r t h e small I R bands a t 2100 and 1400 cm-l f o r t h e CO and NO adsorbates, r e s p e c t i v e l y ( F i g . 2.6.a). F i n a l l y , t h e t u r n o v e r number f o r t h e promoted c a t a l y s t s was found t o be more than two orders o f magnitude. However, t h i s can be taken as o n l y an o r d e r o f magnitude e s t i m a t e s i n c e t h e c o n c e n t r a t i o n of a d l i n e a t i o n s i t e s i s o n l y t h e best estimate t h a t can be made w i t h t h i s complex c a t a l y s t . Therefore,
t h e loss
o f P t s i t e s by d e c o r a t i o n i s more than compensated by t h e h i g h NO d i s s o c i a t i o n a c t i v i t y o f these s i t e s .
B92 2.5.2.
Metal
Carbonyls
Metal carbonyls deposited on h i g h surface area supports can provide s u i t able model c a t a l y s t s t o study l i g a n d s u b s t i t u t i o n r e a c t i o n s catalyzed by supported metal carbonyls such as a1 kene hydrogenation (81-83) and metathesis (84). Fischer-Tropsch synthesis (85) , hydrocarbon o x i d a t i o n (86) and hydrodesul p h u r i z a t i o n (87). An approach i s t o use t h e anchored carbonyls as c a t a l y s t precursors f o r the preparation o f h i g h l y dispersed t r a n s i t i o n metals and t o use m e t a l l i c c l u s t e r s f o r heterogeneons c a t a l y s t s (88, 89). I n t h i s context, c h a r a c t e r i z a t i o n o f the supported metal carbonyl i s o f prime importance t o understand subsequent chemical process. I n f r a r e d spectroscopy, becomes extremely u s e f u l f o r t h i s purpose. A few metal carbonyls are examined i n t h e next sections. 2.5.2.1.
Chromium Carbonyl
The i n t e r a c t i o n o f Cr(C0)6 w i t h h i g h l y dehydroxylated y-A1203 has been studied by Zecchina e t a l . (90) by t h e use o f IR spectroscopy. The frequencies o f the adsorbed C r ( C O j 6 carbonyl are summarized i n Table 2.4 and compared w i t h those o f the s i m i l a r M(C0)6 (M = Mo, W) carbonyls. I n general two q u a r t e t s ( A l -
A4 and Bl-B4), whose components increase w i t h coverage were observed. A much weaker and broader component ( l a b e l e d D ) was observed i n the low frequency s i d e o f the A4 component. The A q u a r t e t was predominant during t h e i n i t i a l adsorption stages w h i l e t h e B one was observed a t the highest coverages. Upon outgassing, the two A and B quartets decrease i n i n t e n s i t y (B f a s t e r than A), and simultaneously a new q u a r t e t C and a very weak band a t ca. 1530 cm-l ( l a b e l e d E ) were a1 so observed. I t i s w e l l known t h a t CO groups i n 3d-metal carbonyls show a d i s t i n c t bas-
s i c i t y ;; the carbonyl oxygen. Since alumina possesses Lewis a c i d s i t e s , i t i s i n f e r r e d t h a t Cr(C0)6-A1203 should be o f t h e donor-acceptor type w i t h the formation o f oxygen-bonded surface compounds o f t h e type: clr(co)6 + AI+:
-
(C0)5Cr-CO--A1 3+
(2.17)
I n the gas phase Cr(C0)6 i s characterized by an I R - a c t i v e CO s t r e t c h i n g mode o f and E modes become symmetry a t ca. 2000 cm-l. When adsorbed, the o t h e r A TIU lg 9 IR-active, as i l l u s t r a t e d i n Fig. 2.7. According t o t h i s , t h e most l i k e l y structures have an C4v symmetry. The CO groups 0-bonded t o A13+ Lewis s i t e s should have s t r e t c h i n g frequencies several hundred wavenumbers below t h a t o f t h e
Alr,
gas molecule. The A4 and B4 components were assigned t o the s t r e t c h i n g v i b r a t i o n s o f the carbonyls 0-bonded t o
AlF
and r e s p e c t i v e l y . This assignment, i s consistent w i t h the data i n t h e l i t e r a t u r e , i s i n agreement w i t h t h e which stronger acceptor character o f t e t r a h e d r a l A13' s i t e s .
B93
5 pet t r u m
2200
Species
1900 1600 Wavenumber (cm-$1
Fig. 2.7. Normal v i b r a t i o n modes o f the Cr-carbonyl species i n t e r a c t i n g a t the alumina surface. The normal modes of the f i v e unperturbed CO ligands have 2A1, B2 and E symmetries, a l l of them being u s u a l l y IR-active i n complexes w i t h an X-M(C0)5 s t r u c t u r e . The frequencies o f the f o u r modes follow, i n general, the order (A1)1>B2>E&(A1)Z
w i t h the E mode showing t h e highest i n t e n s i t y and t h e (A1)2
band overlapping w i t h the main E peak. Therefore, the A1-A3 ascribed t o the (All1,
components were
B2, and E modes, respectively, o f t h e Cr(C0)5 s t r u c t u r e ,
the remaining (A1)2 mode being obscured by the strong i n t e n s i t y o f the adjacent E absorption. Another i n t e r e s t i n g p o i n t t o be considered i s the s h i f t toward
higher frequencies observed f o r t h e carbonyl groups not d i r e c t l y involved i n t h e bonding t o the alumina surface, w i t h respect t o t h a t o f f r e e carbonyls. I t i s w e l l known t h a t 0-coordination t o A13+ s i t e s causes n o t o n l y a decrease o f the d i r e c t l y perturbed C-0 group but also an increase o f the s t r e t c h i n g frequency o f the remaining carbonyls not d i r e c t l y invalved i n 0-bonding ( 3 1 ) . I t i s worth mentioning t h a t t h e l a r g e r t h e downward s h i f t o f the C-0 bonding carbonyl, the l a r g e r the upward s h i f t o f the remaining carbonyls. The formation o f the q u a r t e t C was o n l y observed upon outgassing. Subsequent CO adsorption removed these bands completely, and simultaneously the q u a r t e t A increased again. Therefore i t was concluded t h a t C1-C4
bands a r i s e
from p a r t i a l l y decarbonylated species o r i g i n a t i n g from A upon l o s s of GO 1 igands.
B94 This r e a c t i o n i s expected because 0- bonding o f metal carbonyls t o s t r o n g Lewis a c i d s i t e s i n homogeneous phase
favours CO e l i m i n a t i o n ( 9 2 ) . F i n a l l y , t h e D
peak i s associated with surface carbonyls anchored t o t h e h i g h l y a c i d i c def e c t i v e s i t e s . T h i s assignment i s supported by t h e l o w e r i n g of t h e frequency o f t h e p e r t u r b e d carbonyl, and a l s o f o r t h e v e r y small i n t e n s i t y of t h e peak, i n agreement w i t h t h e v e r y low c o n c e n t r a t i o n a t t h a t s u r f a c e s i t e . TABLE 2.4. I n f r a r e d Frequencies (cm-')
o f M(C0)6 (M = C r . Mo, W ) Carbonyls Adsorbed o n t o
A1 umi na Bands
Cr( CO)6
Mo(CO) 6
w(c0)6
A1
2142'
2145'
2140'
*2
2090'
2090'
2080'
A3
2040'
2040'
2030'
A,
1770-80'"
1760-70m
-
17 5 5 7Om ~~
B1
2130'
2132'
2130'
B2
2075'
2060'
2060Sh
B3
200OS
200OS
1995'
B4
1850-75m
1835-60m
1825-45m
D
1670'
1670'
1670'
2075"'
2080'
c2
201OSh
2010m
c3
1922'
1925m
C,
1605'
1620-00m
E
1530'
1520'
w = weak; s = strong; m = medium; vw = v e r y weak; sh = shoulder 2.5.2.2.
I r o n Carbonyls
N i t r i c o x i d e has been used f o r assessing t h e adsorbed s t a t e o f i r o n species deposited on Si02 (93, 94) and Fe2+ exchanged z e o l i t e Y (95, 96) (see s e c t i o n 2.6.3.). I n these cases, a p a i r o f i n f r a r e d bands a t ca. 1900 (weak) and 1800 -1 cm ( s t r o n g ) have been assigned t o t h e symmetric and antisymmetric s t r e t c h i n g modes, r e s p e c t i v e l y , o f a d i n i t r o s y l species bond t o t h e m e t a l . Recently, Morrow e t a l . (97) p r o v i d e evidence t h a t these bands should probably be assigned t o a t r i n i t r o s y l species. The a d s o r p t i o n o f Fe(C0)2(N0)2 complex on NaY z e o l i t e which was p r e v i o u s l y a c t i v a t e d i n vacuum a t 723 K, a f t e r exposure t o excess gaseous NO f o r l h , gave
B95
5 1920 . 1860 -1800
1740
Wavenumber (ern-')
F i g . 2.8. A ) I R s p e c t r a o f Fe(C0) ( N O ) adsorbed on z e o l i t e i n t h e presence o f excess gaseous NO; B ) I R s p e c t r a 8f t h g symmetric NO s t r e t c h i n g band o f t h e complex adsorbed on z e o l i t e ( a ) and subsequent s p e c t r a ( b - g ) showing t h e e f f e c t o f p r o g r e s s i v e l y i n c r e a s i n g exchange w i t h 15NO up t o ca. 90%. V e r t i c a l l i n e s i n d i c a t e t h e approximate p o s i t i o n o f t h e f o u r mixed i s o t o p i c t r i n i t r o s y l s p e c i e s . r i s e t o a d o u b l e t a t ca. 1914 and 1806 cm”
( F i g . 2.8.A);
no bands o f CO b e i n g
d e t e c t e d a f t e r t h i s p r e t r e a t m e n t . Morrow e t a l . ( 9 7 ) found t h a t t h e NO s p e c i e s r e s p o n s i b l e f o r t h i s d o u b l e t r a p i d l y exchanged w i t h 15N0, and used t h e I R t e c h n i q u e t o determine t h e number of NO l i g a n d s around t h e Fe atom. As a l r e a d y shown i n F i g . 2.8.B, t h e h i g h wavenumber symmetric band s p l i t i n t o f o u r components f o r p a r t i a l l y exchanged d o u b l e t s and i n t h e case o f 50% exchanged i s o t o p i c species, t h e r e l a t i v e i n t e n s i t i e s were 1:3:3:1. T h i s r e s u l t was i n t e r p r e t e d as due t o t h e f o r m a t i o n of t r i n i t r o s y l species, t h e f o u r bands b e i n g
B96
due t o t h e f o u r 14NO/15N0 combinations from a Fe(N0)3 s t r u c t u r e . These r e s u l t s imply t h a t a r e i n t e r p r e t a t i o n o f t h e e a r l i e r studies o f t h e adsorption o f NO on Fe/Si02 (93, 94) and Fe-exchanged z e o l i t e Y (95, 96) cat a l y s t s might be appropriate. F i n a l l y , i t i s emphasized t h a t t h e d e p o s i t i o n technique used by Morrow and coworkers permitted the formation o f o n l y one n i t r o s y l species, thus g i v i n g r i s e t o intense and very narrow bands, which perm i t t e d t o resolve the f o u r expected components o f a mixed i s o t o p i c t r i n i t r o s y l surface species. 2.5.2.3.
Cobalt Carbonyl
The most usual approach t o assess the nature o f m e t a l l i c c o b a l t i n reduced c a t a l y s t s i s t o use a probe molecule, such as NO (63, 64). There i s , however, another complementary approach which consists b a s i c a l l y o f the anchorage o f a cobalt complex containing NO as a l i g a n d . Although t h i s l a s t s t r a t e g y has been f r e q u e n t l y used t o deposit C O ~ ( C O onto ) ~ alumina (98, 99), o n l y few studies regarding the i n t e r a c t i o n s o f n i t r o s y l complexes w i t h alumina (100) and z e o l i t e s (97, 101, 102) have been reported. The deposition o f CO(CO)~NOgas onto alumina p r e a c t i v a t e d a t 723 K was s t u died by Roustan e t a1
. (100).
These authors found t h a t d e p o s i t i o n o f t h i s com-
plex occurs r e a d i l y a t room temperature t o y i e l d two mononitrosyl species w i t h s t r e t c h i n g frequencies a t ca. 1795 and 1700 cm-’,
r e s p e c t i v e l y . Using time-
resolved FTIR they found t h a t w i t h i n t h e f i r s t few minutes t h e adsorbed species s t a r t s t o r e a c t and y i e l d more than one type o f isocyanate, (NCO),,
as w e l l as
o t h e r n i t r o s y l species a t the alumina surface. Without evacuation o f the reactant, the isocyanate formation proceeds slowly, accompanied by t h e appearance o f a doublet a t 1880 and 1800 cm”,
r e s p e c t i v e l y , and CO desorbed t o
the gas-phase. As i s well-known, NO adsorbed on various Co-supported c a t a l y s t s , such as c o b a l t oxide (31) and Co-exchanged X and Y z e o l i t e s (103, 104) gives r i s e t o a p a i r o f intense bands a t 1816-1900 and 1780-1800 cm-’ w i t h the one a t t h e lower frequency being the strongest. I n a l l these studies both bands have been assigned t o a d i n i t r o s y l r e s u l t i n g from t h e complexation o f NO on Co
2+
ions. Following t h i s p a r a l l e l i s m , Roustan e t a l . (100) assigned the 1880 and 1800 doublet t o the d i n i t r o s y l Co(NO)F, f o r which simple removal o f the NO 1 igands would then leave Co2+ ions. The formation o f isocyanate species was found t o be g r e a t l y accelerated under vacuum conditions , however t h e predominant NO species generated a t the surface was a mononitrosyl characterized by a band a t 1830 cm-’. The l a t t e r band was e a s i l y converted t o the 1880 and 1800 cm-’ d i n i t r o s y l species i n the presence o f CO.
B97
I
I
1880
I
I
1820
I
I
I
1760 Wavenumber (cm-1)
L
17C
F i g . 2.9. I R s p e c t r a of C o ( C 0 ) NO on z e o l i t e ( a , and subsequent s p e c t r a (b-e) showing t h e e f f e c t o f i n c r e a s i j g exchange w i t h 5NO up t o about 90% (spectrum e ) . The v e r t i c a l l i n e s i n d i c a t e t h e approximate p o s i t i o n s o f t h e t h r e e peaks f o r t h e symmetric and antisymmetric s t r e t c h i n modes o f a d i n i t r o s y l species o f i s o t o p i c composition (14NO),, 1 4 N 0 1 5 N o and (TSNO),.
1
Furthermore Morrow e t a l . (97, 101) have r e p o r t e d r e s u l t s of t h e d e p o s i t i o n o f C O ( C O ) ~ N O on z e o l i t e - Y , w i t h t h e aim o f comparing t h e i n f l u e n c e of t h e supp o r t on t h e course o f t h i s r e a c t i o n . The s p e c t r a l f e a t u r e s observed f o l l o w i n g t h e a d s o r p t i o n o f C O ( C O ) ~ N O i n t h e presence o f excess gaseous NO on N a - z e o l i t e Y which had been p r e v i o u s l y a c t i v a t e d a t 723 K a r e shown i n F i g . 2.9. The spectrum recorded one hour a f t e r t h e i n i t i a l a d s o r p t i o n i n d i c a t e d t h a t CO was r e l e a s e d t o t h e gas phase. By i s o t o p i c l a b e l l i n g t h e p a i r o f bands a t 1892 and 1810 cm-'
was
shown t o be due t o a d i n i t r o s y l species. Once t h e CO species has formed on z e o l i t e Y, i t i s r a p i d l y exchanged w i t h I 5 N O . T h i s i s i l l u s t r a t e d i n F i g . 2.9. spectra b-e, which show t h a t t h e p r e d i c t e d t h r e e components f o r a d i n i t r o s y l were generated upon p r o g r e s s i v e l y i n c r e a s i n g exchange w i t h I 5 N O up t o about 90% exchange (spectrum e ) .
B98 I n alumina and z e o l i t e cases, the t o t a l i n h i b i t i o n o f t h e o x i d a t i o n r e a c t i o n by an excess o f CO i m p l i e s t h a t a t l e a s t one CO l i g a n d must be removed from the surface species i n i t i a l l y formed, whatever t h e nature of t h e support. I t f o l l o w s t h a t t h e most l i k e l y o x i d a t i o n r o u t e f o r A1203 support
i n v o l v i n g the
i n c o r p o r a t i o n o f an 0-atom i n t o the coordination sphere o f Co i s a l s o probably operative on Na Y. However, a s t r i k i n g d i f f e r e n c e i n t h e z e o l i t e case i s the absence o f isocyanate formation. Thus Roustan e t a1
. (100)
have shown t h a t t h e
intermediate formation o f a surface n i t r i d e i s the most s a t i s f a c t o r y hypothesis t o account f o r t h e formation o f (NCO),, (2.18) The n i t r i d e and isocyanate formation according t o Eq. (2.18) would be prevented, i f the supercages o f t h e z e o l i t e could o n l y accomodate one m e t a l l i c fragment
which i s u n l i k e l y . Therefore, assuming t h a t r e a c t i o n (2.18) i s n o t prevented by geometrical constraints, the most obvious d i f f e r e n c e between t h e two supports i s the presence o f A13+ s i t e s on A1203 surfaces. Here, t h e r e a c t i n g n i t r i d e i s considered t o be i n i n t e r a c t i o n w i t h surface A13+ s i t e s through the n i t r o g e n p a i r , which
( co )CO-N+-AI
could be expressed as:
--
CO( N+CO)-
-
*
~ i
Co+
+
Al--NCO
C ~ ( N C O ) + AI~+
(2.19)
An extension o f t h i s proposal would be t o p o s t u l a t e t h a t A13+ s i t e s p a r t i c i p a t i n g already i n an i n t e r a c t i o n w i t h the i n i t i a l surface NO and w i t h t h e n i t r i d e product could a l s o be involved i n the oxygen t r a n s f e r step f o r the o x i d a t i o n o f co
.
2.5.2.4.
Molybdenum Carbonyl
The c h a r a c t e r i z a t i o n o f supported molybdenum oxide has received considerable a t t e n t i o n because o f i t s relevance i n many commercial h y d r o t r e a t i n g catal y s t s , b u t complete i d e n t i f i c a t i o n o f t h e a c t i v e s i t e s has n o t i n general been possible. A t t h i s point, the r e g u l a r c r y s t a l l i n e s t r u c t u r e o f z e o l i t e s a l l o w t h e preparation and c h a r a c t e r i z a t i o n o f molybdenum c a t a l y s t s i n which t h e surface environment o f t h e a c t i v e i n g r e d i e n t i s more c l e a r l y defined than t h a t o f most conventional heterogeneous c a t a l y s t s . This i n c e n t i v e persuaded many researchers t o examine the p r o p e r t i e s o f Mo-zeolites, from the p o i n t o f view o f modifying the c a t a l y t i c p r o p e r t i e s and w i t h t h e aim o f modeling the a c t i v e s i t e s on conventional supported molybdenum oxide c a t a l y s t s . I n f r a r e d spectroscopy has been used t o study the adsorption and decomposit i o n o f Mo(CO)~i n HY and NaY z e o l i t e s (105-108). Abdo and Howe (107) found two forms o f adsorbed Mo(CO)6 i n both HY and NaY z e o l i t e s : a weakly adsorbed complex
B99 and a more s t r o n g l y h e l d species. I n HY, t h e weakly adsorbed species character i z e d by two CO s t r e t c h i n g bands a t ca. 2123 and 2003 cm-l (107). w h i l e two f u r t h e r bands a t ca. 2045 and 1905 cm'l
were explained as due t o chemisorbed
species (106, 107). I n t h e same z e o l i t e
another t h r e e d i s t i n c t subcarbonyl
species were formed r e v e r s i b l y . On h e a t i n g a t 473 K and above, d e c a r b o n y l a t i o n o f t h e complex was i r r e v e r s i b l e , O x i d a t i o n o f t h e z e r o v a l e n t Mo-species was r e markable as i n d i c a t e d by t h e l o s s o f t h e z e o l i t e hydroxyl groups and by t h e appearance o f oxomolybdenum species. These l a t t e r species a r e e a s i l y d e t e c t e d by chemisorption o f CO, as they g i v e r i s e t o a c h a r a c t e r i s t i c band a t 2170 cm-l. I t has a l s o been revealed t h a t z e o l i t e c r y s t a l l i n i t y i s s t i l l r e t a i n e d on h e a t i n g i n vacuum up t o 773 K, b u t l o s s o f s t r u c t u r e was found t o occur on h e a t i n g i n oxygen a t h i g h temperature. This l o s s o f c r y s t a l l i n i t y has r e c e n t l y been observed i n Mo-loaded HY z e o l i t e prepared by a i r - c a l c i n a t i o n o f t h e oxomolybdenum precursors (109) , although a somewhat h i g h e r c r y s t a l l i n i t y was r e t a i n e d by t h e z e o l i t e , i f c a l c i n a t i o n o f t h e oxomolybdenum impregnate was c a r r i e d o u t f o l l o w i n g a non-conventional constant r a t e decomposition procedure (110) a t a v e r y low r e s i d u a l water vapour pressure. I n both cases t h e l o s s o f z e o l i t e s t r u c t u r e seems t o be due t o a strong i n t e r a c t i o n o f t h e molybdenum species w i t h t h e o x i d e i o n s o f t h e z e o l i t e framework. I n t h e NaY z e o l i t e a s i n g l e subcarbonyl species i s formed r e v e r s i b l y upon i n i t i a l decomposition o f adsorbed Mo(CO)~. Upon a c t i v a t i o n a t 473 K o r h i g h e r temperatures, t h e subcarbonyl species i s completely destroyed. The b l a c k c o l o u r o f t h e r e s u l t i n g c a t a l y s t i s probably due t o z e r o v a l e n t Mo, which can r e a c t w i t h oxygen a t room temperature y i e l d i n g Mo5+ species (108). I f a c t i v a t i o n i s conducted a t 673 K, t h e r e s u l t i n g z e r o v a l e n t Mo species a r e r e s i s t a n t t o oxidat i o n by oxygen a t room temperature. One e x p l a n a t i o n f o r t h i s behaviour i s t h a t t h e metal Moo m i g r a t e s i n t o s i t e s n o t a c c e s i b l e t o t h e O2 molecule a t room temperature, i.e.,
t h e 8-cages.
I n recent work, Kazusaka and Howe (111) s t u d i e d t h e o x i d a t i o n of CO w i t h N20 on MO(CO)~ supported on HY z e o l i t e , and alumina and K-modified alumina. The Mo(CO)~/HY c a t a l y s t , a c t i v a t e d a t 673 K, exposed t o CO gave r i s e t o f o u r bands a t 2175, 2165, 2135 and 2115 cm-',
b u t o n l y t h e f i r s t o f these was s t a b l e under
outgassing a t room temperature. When t h e c a t a l y s t w i t h adsorbed CO, r e s p o n s i b l e f o r t h e 2175 cm-' band was exposed t o N20 a t room temperature, t h i s band was removed and new bands a t 2360 cm'l
(adsorbed C02) and 2290 and 2235 cm-I (ad-
sorbed N20) were simultaneously observed. However, when CO was added t o t h e cat a l y s t c o n t a i n i n g adsorbed N20 o n l y t h e band a t 2360 cm"
(adsorbed C02) and t h e
f o u r bands o f adsorbed CO were found. From these data, i t seems l i g i t i m a t e t o conclude t h a t t h e r e a c t i v e form o f adsorbed CO i n t h e z e o l i t e c a t a l y s t s i s r e s p o n s i b l e f o r t h e 2175 cm-' band. P e r i (112) and Millman e t a l . (113) have observed a band a t 2190 cm-l, when CO i s adsorbed on reduced Mo/Al203 c a t a l y s t s ,
BlOO which i s a t t r i b u t e d t o CO chemisorbed on Mo4+ o r Mo3+, and probably Mo2+ ions. I t i s , therefore, l i k e l y t h a t s i m i l a r Mo-species might be present i n z e o l i t e
and alumina surfaces. Exposure o f Mo(CO)~/HY c a t a l y s t containing the chemisorbed CO t o excess N20 causes complete removal o f the CO and formation o f C02 which weakly adsorbed i n the z e o l i t e .
N2°( ph)
+
"( ch) -"2(
ph)
+
(2.20)
N2
I n the case of M o ( C O ) ~ / A ~ t~hOe ~chemisorbed CO i s l e s s s t r o n g l y adsorbed than i n the Mo-zeolite, since i t can be removed a t room temperature. This r a i s e s t h e question as t o why the IR band o f chemisorbed CO i s so much weaker f o r Mo(CO)~/ A l 2 O 3 than f o r Mo(CO)~/HY c a t a l y s t s , given t h e almost three times higher a c t i -
v i t y o f the former preparation. This d i f f e r e n c e must r e f l e c t a lower i n t r i n s i c a c t i v i t y o f the s i t e s i n z e o l i t e c a t a l y s t ; i.e., a lower preexponential f a c t o r i n the r a t e constant. I t i s suggested t h a t t h e more w e l l - d e f i n e d and s t e r i c a l l y r e s t r a i n e d environment o f the low dimension o f the z e o l i t e r e s u l t s i n a more negative entropy of a c t i v a t i o n than on alumina surfaces.
2.6. METAL OXIDES 2.6.1.
Vanadium Oxide I n f r a r e d spectroscopy o f adsorbed CO has been used t o study surface V3+
s i t e s i n reduced s i l i c a - s u p p o r t e d vanadium oxide c a t a l y s t s (114). I n c o n t r a s t t o other t r i v a l e n t M3+ (M = A l , Ga, In, Fe, Sc, Y, Lanthanides) ions, which g i v e only weak CO adsorption complexes, CO i s adsorbed r a t h e r s t r o n g l y on V3+ ions. Rebenstorf e t a1
. (114)
reported I R spectra o f CO adsorbed on prereduced cata-
l y s t s w i t h CO a t 870 K f o r 1 h, and a l s o f o r 3 h. I n both cases, the spectra consisted o f two absorption bands a t ca. 2174 and 2187 cm-',
although some
changes i n l i n e shape were observed on varying the r e d u c t i o n time. The most important d i f f e r e n c e caused by increasing r e d u c t i o n time was the increase
in
the i n t e n s i t y o f the band a t 2187 cm-'. As stated i n the preceding sections, the accepted model f o r the adsorption o f CO by cations i s based
i n two d i s t i n c t i n t e r a c t i o n s : i ) donation o f elec-
trons from teh 5s o r b i t a l o f CO .to the cation, and ii)n back donation from t h e c a t i o n i n t o the antibonding n* o r b i t a l o f CO. The l a t t e r e f f e c t should o n l y be effective, o f V3+,
i f the c a t i o n has f i l e d d o r b i t a l s , e.g.,
3d metal ions. I n the case
CO adsorption i s expected t o show d i f f e r e n t c o n f i g u r a t i o n s (Table 2.5).
CO complexes o f the type 'IT" ( n = 1). "Top" ( n = l ) , and " S t a r t " ( n = 1 o r 2 )
w i l l g i v e o n l y one I R band, w h i l e the other f o u r (two "T" and two "Top") com-
plexes should d i s p l a y two I R bands. Since CO adsorption on V3+ ions shows a
BlOl TABLE 2.5. Structures and CO complexes o f Reduced Vanadium Oxide Catalysts Configuration
Structure
Complex
IITII 0
0
: o
i /
..... v - 0
"Top"
2 . .
I
v
(CO),-.-.
0
i '0
-
0 0
0
n=1-3
(CO) i '0 v - 0
i o
- 'V
/
I
0
'IS t a r t 'I
n=1-3
0' n=l,2
doublet, the " s t a r t " c o n f i g u r a t i o n o f CO molecules i s excluded, the "T" and "Top" configurations being the o n l y ones possible. I t i s l i k e l y t h a t the "Top" species i s predominant i n prereduced c a t a l y s t s a t r e l a t i v e l y shorter r e d u c t i o n times ( l h ) , w h i l e t h e "TI' c o n f i g u r a t i o n r e s u l t s most abundant a t longer reducti o n times. This assignment i s supported by t h e f a c t t h a t on c a t a l y s t r e d u c t i o n the t e t r a h e d r a l l y V5+ i o n loses t h e terminal oxygen and a "Top" c o n f i g u r a t i o n should most l i k e l y r e s u l t . The r e s u l t i n g V3+ i o n i n a "Top" c o n f i g u r a t i o n can then rearrange i t s e l f a f t e r longer reduction times t o adopt a "T" c o n f i g u r a t i o n . Further support f o r t h i s assignment i s provided by the i n t e n s i t y increase o f the band a t 2187 cm-'
i n the c a t a l y s t reduced f o r 3 h. By comparing the "Top" and
"T" complexes w i t h two CO ligands, one can expect t o f i n d a l a r g e r angle between the two CO molecules i n the case o f "T" complexes, because o n l y one oxygen l i g a n d i s r e p p e l l i n g the two CO molecules.
A l a r g e r angle between the two CO
oscillators w i l l result i n a larger intensity ratio. 2.6.2.
Chromium Oxide The i n d u s t r i a l importance o f supported chromium c a t a l y s t s f o r several
processes such as o l e f i n polymerization, aromatization o f a1 kanes, etc.,
has
stimulated numerous i n v e s t i g a t i o n s t o determine the nature of the a c t i v e s i t e s (115-118). Probe molecules such as CO and NO provide i n s i g h t i n t o the o x i d a t i o n s t a t e and a v a i l a b l e coordination s i t e s o f supported chromium ions.
B102
c
100
$ -J 80E
m c
c
A
E 60-
k-
w)-
20 -
2200
2220
2160 a40 Wavenumber (cm-1)
2180
F i g . 2.10. I n f r a r e d spectrum o f adsorbed CO a t room temperature on a reduced 0.5 w t % C r / S i O c a t a l y s t a t v a r y i n g adsorbate pressures: a ) 5.3 kNm-2; b ) 0.53 kNm-2; d ) 4.0 6rn-2; and d i f f e r e n t outgassing times; e) 0.5 min; f ) l m i n ; 9 ) 3 min. (Readapted from r e f . ( 1 1 9 ) ) . Using I R spectroscopy, Zecchina e t a l . (119) s t u d i e d t h e i n t e r a c t i o n o f CO w i t h t h e s u r f a c e o f a 0.5% Cr/Si02 c a t a l y s t and found bands a t 2181, 2186 and 2191 cm-l,
and a v e r y weak band a t 2095 cm-'
( F i g . 2.10).
The a d s o r p t i o n was
r e v e r s i b l e a t room temperature and t h e background spectrum was f u l l y recovered by prolonged pumping. Accordingly, band i n t e n s i t y was l a r g e l y dependent on CO pressure; i n p a r t i c u l a r t h e band a t ca. 2095 cm-' above 1.3 kNm-'.
was o n l y p r e s e n t a t pressures
The band t r i p l e t a t ca. 2185 cm"
was assigned t o t h e s t r e t c h -
i n g mode o f 1 : l CO-Cr l i n e a r complexes. Since no i s o l a t e d Cr-ions were detected, a schematic p i c t u r e o f these complexes i s g i v e n i n s t r u c t u r e I :
co
I Cr
co
(1)
I Cr
co
\
I Cr
(11)
co
co
co
co
\ ,...."'...., / Cr Cr
(111)
co
co
co
co
\ /. ..:::.-:: ......\ / Cr Cr
(IV)
B103
No c o u p l i n g between t h e two o s c i l l a t o r s i s expected and t h e two C r i o n s may be regarded as independent. On t h e o t h e r hand, t h e s i m p l e s t s t r u c t u r e t h a t can be expected t o form, when more than one CO i s adsorbed, i s d e p i c t e d i n s t r u c t u r e 11, b u t has t o be discarded, because i t cannot account f o r t h e important s h i f t o f ca. 1UO cm-l below t h e o r i g i n a l t r i p l e t . Thus, o t h e r s t r u c t u r e s i n v o l v i n g n o n - l i n e a r CO l i g a n d s have t o be considered, S t r u c t u r e 111, i n which a CO l i g a n d i s shared by two neighboring C r ions, can account, however, f o r CO s t r e t c h i n g frequencies as low as 2095 m-l, as t h e b r i d g i n g CO l i g a n d must possess a C atom w i t h some sp 2 h y b r i d i z a t i o n and, accordingly, a lower bond o r d e r . I n a d d i t i o n , t o account f o r t h e several bands observed i n t h e 2200-2050 cm-l s p e c t r a l region, when t h e spectra were recorded a t 195 K (119), another more complicated s t r u c t u r e ( t y p e I V ) can be a l s o envisaged. S t r u c t u r e s o f types I11 and I V may occur on t h e s u r f a c e d i f f e r i n g by C r - C r distance, valence and c o o r d i n a t i o n s t a t e o f t h e ions. The CO c o o r d i n a t i o n i s a l s o explained i n terms o f u donation and
n back
donation. n - d o n a t i o n from t h e i o n s i s n o r m a l l y small, and hence t h e h i g h frequency band i s j u s t i f i e d . I n p a r t i c u l a r , CO-Cr3' s o c i a t e d w i t h t h e band a t 2191 cm-',
complexes should be as-
s i n c e t h e s m a l l e s t back donation would
occur. Support f o r t h i s assignment comes a l s o from t h e f a c t t h a t C r 3 + p a i r s a r e more s t a b l e than t h e corresponding C r 2 + ions, as a d d i t i o n a l oxygen l i g a n d s tend t o increase t h e CFSE o f C r ions. To confirm t h i s hypothesis, t h e same authors (119) s t u d i e d t h e I R spectrum o f CO o f samples outgassed a t v e r y h i g h temperatu3+ r e s and found t h a t t h e band a t 2191 cm-l was unchanged ( C r ), w h i l e t h e o t h e r two bands a t 2181 and 2186 cm-l decrease w i t h i n c r e a s i n g thermal treatments. This l a t t e r phenomenon can be understood i n terms o f an i n c r e a s i n g r e d u c t i o n o f t h e surface, which, a t l e a s t i n p a r t , i s due t o t h e t r a n s f o r m a t i o n of C r 3 + i n t o Cr2+ ions.
Some a d d i t i o n a l i n f o r m a t i o n on t h e type o f surface s i t e s can be d e r i v e d by u s i n g NO as a l i g a n d . Zecchina's group a l s o s t u d i e d t h e a d s o r p t i o n o f NO on a reduced 0.5 Cr/Si02 c a t a l y s (120). They found t h a t a f r a c t i o n of p a r t i a l l y shielded C r i o n s s a t u r a t e s i t s c o o r d i n a t i v e sphere by a d s o r p t i o n of one NO molecule, g i v i n g r i s e t o a band a t ca. 1810 cm-'.
The s u r f a c e c o n c e n t r a t i o n o f
t h i s k i n d o f Cr i o n s was v e r y small on t h e reduced c a t a l y s t and increased by h i g h temperature treatments and by h i g h C r loadings. As thermal treatments d i d n o t a f f e c t t h e average o x i d a t i o n number of chromium, i t was concluded t h a t t h e N O - C r complex m a i n l y i n v o l v e s C r 2 + ions, which are n o t revealed by CO a d s o r p t i o n
a t room temperature. Apart from t h i s small f r a c t i o n o f Cr2+,
most C r 2 + i o n s
coordinate two NO molecules t o form v e r y s t a b l e Cr(N0)2 s u r f a c e complexes, responsible f o r t h e bands a t ca. 1865 and 1747 an-'. u n s t a b l e a t h i g h temperatures because o f t h e i r
These i o n s were found t o be
low c o o r d i n a t i o n s t a t e and t r a n s -
form i n t o t h e p a r t i a l l y shielded species mentioned above. These species a r e t h e
B104 same as those revealed w i t h CO adsorption and o r i g i n a t e two d i f f e r e n t 1:l complexes, absorbing a t 2186 and 2181 cm-l,
r e s p e c t i v e l y . Please note t h a t CO i s
able t o d i s c r i m i n a t e between two types o f exposed Cr2+ ions ( s t r u c t u r e 11). I n the case o f NO adsorption, however, t h e strong i n t e r a c t i o n o f NO cannot be brought i n t o evidence. The i n f r a r e d spectrum a l s o showed a doublet a t ca. 1880-1755 cm-'.
due t o
the in-phase and out-of-phase v i b r a t i o n s o f surface complexes i n v o l v i n g two NO molecules on Cr3'
ions ( s t r u c t u r e V ) . They were the same as those t h a t o r i g i n a t e
t h e 2191 cm-l CO band. I n agreement w i t h s t r u c t u r e 11, the adsorption o f two CO o r NO molecules on Cr3'
s i t e s requires the a v a i l a b i l y t y o f two coordinating
unsaturation s i t e s . However, t h e s t a b i l i t y o f C r ( C 0 ) F and C r ( N 0 ) P complexes i s s u b s t a n t i a l l y d i f f e r e n t . The s t r i k i n g s t a b i l i t y o f t h e two n i t r o s y l complexes l i e s i n the cooperative e f f e c t between t h e two unpaired electrons. The cooperat i o n between these two unpaired electrons i s probably a t t r i b u t a b l e t o the formation o f a molecular o r b i t a l delocalized over the whole complex, so t h a t i t s
s t r u c t u r e would be b e t t e r described by s t r u c t u r e V . Evidence of such s t r u c t u r e comes from the DRS spectra, which showed t h a t a f t e r NO adsorption, e l e c t r o n i c t r a n s i t i o n s w i t h some appreciable mixing o f metal and strong n-acceptor l i g a n d
(NO) o r b i t a l s occur. I n f r a r e d spectroscopy o f NO has a l s o been used by Pearce e t a1 study surface s i t e s i n dehydrated Cr3+-Y
and Cr3+-X
. (121)
to
z e o l i t e c a t a l y s t s . The most
s t r i k i n g f e a t u r e o f C r - Y and C r - X z e o l i t e s a f t e r NO adsorption was t h e appearance o f an intense doublet i n t h e t y p i c a l region o f N-0 s t r e t c h i n g frequencies. The spectrum o f C r - Y showed these bands a t 1900 and 1775 cm-', which were s h i f t e d by ca. 30 cm-l t o 1870 and 1745 cm-', when nitrogen-15-labeled NO was used (Table 2.6). The surface complex g i v i n g r i s e t o these bands i s a l s o i n t h i s case a geminal C r - d i n i t r o s y l species ( t y p e V )
, containing
s t r o n g l y coupled and
equivalent n i t r o s y l ligands. This assignment explains t r i p l e t s p l i t t i n g and t h e 1:2:1 i n t e n s i t y r a t i o observed, when the isotope m i x t u r e was used, since one would expect the r e s u l t a n t d i n i t r o s y l complex t o be composed of 25% 14N0, 50% 141Kl 15N0 and 25% 15N0. Additional support o f the assignment o f the i n f r a r e d spectra t o a Cr(N0);' complex was provided by the observation t h a t t h e i n t e n s i t i e s f o r the symmetric and asymmetric s t r e t c h i n g modes o f t h e complexes were s t r i c t l y proportional
,
i n d i c a t i n g t h a t both bands a r i s e from the same surface
complex. Furthermore t h e 15N isotope s h i f t i s t h e same f o r both absorption and
B105 agrees w e l l w i t h o t h e r r e p o r t e d i s o t o p e s h i f t s f o r d i n i t r o s y l complexes (122). TABLE 2.6. Assignment o f I R Absorption Frequencies (cm- 1) o f NO Adsorbed on Chromia Catalysis. Cr-SiOza
Cr-Y b
Cr( 14N0)23+
1880-1775
1990-1775
1895-1770
Cr( l 5 N O ) r
-
1870-1745
1865-1700
weak
1845-1710
-
1650-1260
Species
Cr( l 5 N O ) r
Cr-X
b
different site 14w;
1620-1235 1370 a
reduced 0.5 Cr/Si02 c a t a l y s t exposed t o NO a t room temperature. (Taken from Ref. ( 1 2 0 ) ) . b e h y d r a t e d z e o l i t e s prepared from t h e Na z e o l i t e s by aqueous i o n exchange i n s o l u t i o n s o f CrC13. (Taken from Ref. (121)). The adsorption o f NO on dehydrated C r - X z e o l i t e c a t a l y s t s a l s o showed t h e same doublet, b u t a t 1895 and 1770 cm-l, a some 5 cm-l lower wavenumber than i n C r - Y c a t a l y s t s , t h i s d i f f e r e n c e being a t t r i b u t e d t o t h e g r e a t e r n e g a t i v e charge
d e n s i t y o f t h e X-type l a t t i c e . The two bands were assigned t o a C r - d i n i t r o s y l complex. I n a d d i t i o n , two new peaks were observed a t ca. 1650 and 1260 cm-l, and these were very s t a b l e . The use o f 15N-labeled NO gave r i s e t o bands a t 1865, 1845, 1710, 1620, 1370 and 1235 cm-'. f o r Cr(NO);+
By u s i n g t h e same i s o t o p e s h i f t observed
complexes i n C r - Y z e o l i t e s t h e bands a t 1865 and 1740 cm-l were
assigned t o t h e symmetric and asymmetric s t r e t c h i n g modes o f NO i n t h e C r ( l 5 N O ) P complex. The o t h e r two bands a t 1845 and 1710 cm-l were a s c r i b e d t o d i s t i n c t d i n i t r o s y l a r i s i n g e i t h e r from mixed C r 2 + and Cr3'
c a t i o n s o r from C r
3+
i o n s a t non-equivalent c o o r d i n a t i o n s i t e s . The frequency and t h e d o u b l e t s p l i t t i n g o f the bands a t 1620 and 1236 cm-' suggested a l s o t h e presence of NO; s t r u c t u r e s , r e s u l t i n g probably from t h e d i s p r o p o r t i o n a t i o n o f NO o r t h e d i r e c t i n t e r a c t i o n w i t h l a t t i c e 0'-
i o n s . Species o f t h i s t y p e have been r e p o r t e d by
Kugler e t a l . (123) upon NO a d s o r p t i o n on chromia. The band a t 1370 cm-l appears i n t h e r e g i o n where surface n i t r a t e s (NO;)
u s u a l l y absorb (18). A f i n a l remark
concerning t h e s t a b i l i t y o f chromium i o n s i s t h a t Cr3+ i s r e s i s t a n t t o r e d u c t i o n by H2 o r CO when supported on z e o l i t e Y, then being a b l e t o form C r ( N O ) p com-
B106
1
1
2130
I
I
I
1
2170
2190 Wavenumber [cm-1)
Fig. 2.11. IR spectra of CO adsorbed by iron cations on S i l i c a gel subjected t o different pre reatments. CO adsorption on outgassed reparations (spectra a-c) containin Fe5+ cations; on a mixture of Fezt and Feg+ cations (spectra d - f ) , and on Feg+ cations (spectra g - i ) . The adsorption temperatures were: 128 K f o r a , d and g ; 203 K f o r b , e and h ; and 263 K f o r c , f and i . plexes. The situation was, however, more complex for Cr-X z e o l i t e s . The inertness toward NO adsorption in the l a t t e r case was attributed t o the migration of chromium ions into s i t e s where NO cannot penetrate, v i z . , s i t e I and possibly s i t e s I ' and 11' which are inside the sodalite cages and accesible only through a 0.22 nin diameter window. 2.6.3. Iron Oxide Infrared spectroscopy of the CO probe adsorbed on silica-supported iron species was used by Rebenstorf and Larsson (124) to investigate the type of surface s i t e s . After adsorption of CO a t temperature below 260 K, a band near 2171
B107 cm-l ( F i g . 2.11, s p e c t r a a-c) was observed. I n t h e experimental c o n d i t i o n s used, v i z . low temperature and h i g h CO pressures, t h e r a t i o o f t h e adsorbed CO was close t o two CO molecules per Fe2' c a t i o n . With t h i s r a t i o i n mind, i t i s c l e a r t h a t t h e band a t 2171 cm-'
should be assigned t o a c o n f i g u r a t i o n , such as: 0eF, -
co
I n F i g . 2.11,
i c,o
-0
;
s p e c t r a a-c, a shoulder near 2165-2169 cm-'
i s a l s o observed,
which becomes t h e o n l y band on deeply o x i d i z e d samples ( s p e c t r a d - f j . I n view o f t h e a n a l y s i s o f t h e o x i d a t i o n number o f Fe, which i s s i m i l a r t o t a h t o f nono x i d i z e d samples (2.22)
, the
same authors prepared another Fe/Si02 c a t a l y s t by
impregnation w i t h FeC13 i n o r d e r t o o b t a i n p r e p a r a t i o n s w i t h h i g h e r o x i d a t i o n numbers (2.58). The spectra ( F i g . 2.11, s p e c t r a g - i ) showed t h e expected enhancement o f t h e shoulder observed i n s p e c t r a a-c. This p a r t i c u l a r behaviour 1
was taken as i n d i c a t i v e o f t h e f a c t t h a t t h e band between 2164 and 2169 cma r i s e s from CO adsorbed on c o o r d i n a t i v e l y unsaturated Fe3' species.
To overcome t h e d i f f i c u l t i e s encountered i n t h e d e t e c t i o n o f I R band o f CO adsorbed on i r o n c a t a l y s t s a t moderate o r low pressures o f CO and temperatures near ambient, an NO probe was s e l e c t e d i n many cases f o r t h e same purpose (93, 95, 96, 125-127). U n l i k e CO, NO was found t o r e a d i l y chemisorb n o n d i s s o c i a t i v e l y on supported i r o n , g i v i n g r i s e t o a v a r i e t y o f i n t e n s e I R bands. A g r e a t v a r i e t y o f supported i r o n c a t a l y s t s were examined by i n f r a r e d spectroscopy o f adsorbed NO, t h e most re1 evant systems b e i ng discussed be1 ow.
Yuen e t a l . (93) s t u d i e d t h e a d s o r p t i o n o f
NO on s i l i c a - s u p p o r t e d i r o n
o x i d e c a t a l y s t s , c o n t a i n i n g ca. 1%Fe, reduced i n a C02/C0 gas m i x t u r e (85% mole C02) a t 653 K o r i n H2 a t temperatures i n t h e range 498-723 K . A f t e r a d s o r p t i o n o f NO, t h e I R spectrum showed bands a t 1910, 1830, 1810 and 1750 cm-',
whose
i n t e n s i t y was s t r o n g l y a f f e c t e d by t h e d i f f e r e n t c a t a l y s t pretreatments. Among these, t h e bands a t 1910, 1810 and 1750 cm-I were assigned t o NO adsorbed on Fez' c a t i o n s o f t e t r a h e d r a l coordination, w h i l e t h e band a t 1830 cm-' was ascribed t o NO adsorbed on hexahedrally coordinated Fe2+ c a t i o n s . Based a l s o on MBssbauer spectroscopy data, t h e y suggested t h a t t e t r a h e d r a l l y c o o r d i n a t e d Fez+ c a t i o n s a r e i n s t r o n g i n t e r a c t i o n w i t h t h e topmost s i l i c a l a y e r , e.g. p r e s e n t as i r o n r a f t s o r as an i r o n s i l i c a t e l a y e r . This t y p e o f Fe2+ c a t i o n s chemisorb NO and g i v e r i s e t o t h e doublet a t 1910 and 1810 cm-', species, and t o another band a t 1750 cm-',
attributed to dinitrosyl
assigned t o mononitrosyl species. On
t h e o t h e r hand, t h e o c t a h e d r a l l y coordinated Fez+ cations, which were p r e s e n t i n small p a r t i c l e s o f i r o n oxide, chemisorbed NO as mononitrosyl species (band a t 1830 cm-l). These p a r t i c l e s were i n i n t i m a t e c o n t a c t w i t h t h e s i l i c a s u r f a c e due t o t h e s t a b i l i t y a g a i n s t r e d u c t i o n t o t h e m e t a l l i c s t a t e d u r i n g H2-reduction. I n competition w i t h t h e r e d u c t i o n t o m e t a l l i c Feo, these i r o n o x i d e p a r t i c l e s were
BlO8
converted into the Fe2' r a f t s , strongly interacting w i t h the c a r r i e r , and a greater fraction of the remaining Fez' cations of octahedral coordination becomes accessible t o the NO probe, viz. increasing the dispersion. A final remark t o be made i s t h a t i n contrast t o t h i s behavior of Fez' cations in reduced Fe/Si02 preparations, NO did not adsorb t o an appreciable extent on Fe3' cations. Alumina-supported iron oxides were also studied by IR spectroscopy of adsorbed NO (94, 128). On a reduced 10% Fe/A1203 catalyst King and Peri (128) found two broad and overlapping bands a t ca. 1800 and 1720 cm-' along w i t h l e s s intense bands a t 1920, 1840 and 1225 cm-'. According t o the work of Bandow e t a1 . (124). the bands a t 1800 and 1720 cm'l were assigned t o mononitrosyl species adsorbed on Fe2' and FeO s i t e s (Table 2.7). The band a t 1920 an-'was tentatively TABLE 2.7.
Summary of Infrared Bands of NO adsorbed on Fe/A1203 Catalysts Catalysts
Petreatment
10%Fe/A1203 H 2 , 773 K , vacuum 10%Fe/A1203 02, 773 K , vacuum 5%Fe/A1203
H2, 773 K , vacuum
Major Bands( cm-l) 1800,NO/Fe2' 1720,NO/Fe0 1810,NO/Fe2' 1800,NO/Fe2+
Remarks Strongly held NO Lower frequency weak bands indicate oxidized NO Less FeO compared with 10%Fecatalyst
assigned to a NO' species formed on highly oxidizing s i t e s and t h a t a t 1225 cm-' attributed t o n i t r i t e (NO;) species. The possibility of dimeric species responsible f o r the bands a t 1840 and 1720 cm-' was not excluded. Additional information was also derived from the oxidized 10% Fe c a t a l y s t s . In t h i s case, the intensity of the band near 1800 cm-' was substantially increased upon oxidation. The band near 1720 an-', assigned t o Feo, disappeared and new bands i n the region 1880-1920 cm-', and below 1600 cm-' were observed, being consistent with the production of electron deficient and oxidizing s i t e s , respectively. Rethwisch and Dumesic (94) have also used NO t o probe the nature of Fe2+ cations supported on Si02, A1203, Ti02, MgO and ZnO. To convert essentially a l l iron ions to the Fe2' s t a t e , a l l catalysts were i n i t i a l l y pretreated f o r 4 h in a CO/C02 mixture a t 660 K and evacuated t o lo-' Nm-' f o r 1 h a t 660 K. The infrared bands found f o r adsorbed NO on supported iron oxide catalysts are summarized i n Table 2.8. I t i s evident t h a t the mononitrosyl species f a l l into two groups, one with a stretching frequency near 1815 and another a t about
ern-'
B109 TABLE 2.8. I n f r a r e d Bands (cm-l) f o r NO Adsorbed on Supported Fe Oxide Catalystsa ~
Catalyst
Mononi t r o s y l Species
1%Fe/Si02
D i n i t r o s y l Species
1815
1755
1897
1791
1824
1740
1900b
1824b
l%Fe/Al203
1805
1843
l%Fe/MgO
1800
1720
l%Fe/MgO
1813
1%Fe/Ti02
1838
1835 ~~
~
~
~~
~
a A f t e r Rethwich and Dumesic (94); bTentative assignment by t h e authors 1750 cm-'.
A t t e n t i o n must be paid t o the f a c t t h a t the band a t ca. 1750 cm-'
is
i n the same region as the n i t r o s y l species formed upon outgassing o f the d i n i t r o s y l species associated w i t h Fe2+ cations o f low coordination. This suggests 2+ t h a t t h e band a t 1750 cm-l corresponds t o mononitrosyl species adsorbed on Fe s i t e s o f lower coordination than those r e s u l t i n g i n the mononitrosyl species a t 1815 cm-'. This i s i n agreement w i t h other studies where t h e s t r e t c h i n g frequency o f NO was c o r r e l a t e d w i t h the coordination o f Fez+ cations (93, 96). Sites of lower coordination have l e s s s t e r i c hindrance than those o f higher coordination, thus allowing the formation of bent n i t r o s y l s . I n s i t e s o f higher coordination, the n i t r o s y l species would be expected t o remain more l i n e a r due t o the s t e r i c hindrance by o t h e r ligands.
The c a t a l y s t s were arranged i n the f o l l o w i n g order w i t h respect t o formation o f d i n i t r o s y l species with NO: Fe/Si02 > Fe/Ti02> Fe/A1203 = Fe/ZnO = Fe/MgO. This order can be r e l a t e d t o the coordination o f oxygen i n the c a r r i e r . I n the bulk s t r u c t u r e s o f these c a r r i e r s , each l a t t i c e oxygen i s coordinated t o two cations i n s i l i c a , three cations i n t i t a n i a , three o r f o u r i n alumina, f o u r i n z i n c oxide, and s i x i n magnesia. For a c a t i o n w i t h a given charge, t h e coordination o f t h e c a t i o n generally decreases as t h e coordination of oxygen decreases. This explains why Fe2+ cations of low coordination g i v i n g r i s e t o d i n i t r o s y l bands i n t h e I R spectra were present i n high concentration on s i l i c a i n lower concentration on t i t a n i a and absent on the other c a r r i e r s . The same NO probe was used by Segawa e t a l . (96) t o study the i n t e r a c t i o n
of t h i s molecule w i t h i r o n cations i n Y-zeolites. The i n f r a r e d spectra revealed the presence o f several kinds o f s i t e s . On reduced Fe-Y preparations, they observed bands a t 1845 on 1870 cm-' which were formed r a p i d l y and remained constant, w h i l e two o t h e r bands a t ca. 1917 and 1815 cm-I (Fig. 2.12A) grew slowly w i t h time. The i n t e n s i t y o f these l a t t e r bands was always proportional, and they
BllO
2
I
2000
1
1
1
1
1800 1600 Wavenumber lcrnq
Fig. 2.12. A) I R spectra o f NO adsorbed on Fe- z e o l i t e containing 6.3 x lo2' Fe ions g - l . a) reduced c a t l y s t w i t h o u t exposure t o NO; b) reduced c a t a l y s t contacted w i t h 0.67 kNm-$ NO f o r 1 min; c ) reduced c a t a l y s t contacted w i t h NO f o r 3 h; d) f o l l o w i n g room temperat r e outgassing o f sample c f o r 0.5 h; e) a f t e r heating sample d i n 0.67 kNm-j a t 770 K f o r 15 min; f ) o x i d i z e d c a t a l y s t f o l l o w i n g outgassing a t room temperature f o r 15 min and exposure t o 0.67 kNm-2 nds a t d i f NO. B ) Relationship between t h e i n t e n s i t i e s o f 1917 and 1815 cm-1 f e ent coverages of NO on reduced Fe-Y z e o l i t e containing 6.3 x 10" Fe ions g- *
F
were t h e r e f o r e a t t r i b u t e d t o a d i n i t r o s y l species ( c f . Fig. 2.128). A f t e r o u t gassing a t room temperature t h e d i n i t r o s y l species were transformed t o a monon i t r o s y l species (band a t 1767 cm'l).
A l l bands were removed by outgassing a t
520 K , b u t o n l y those corresponding t o d i n i t r o s y l s dissappeared on heating i n NO, the other bands i n the region 1855-1870 cm"
being maintained. This f e a t u r e
was taken as conclusive o f the f a c t t h a t the o x i d i c form of t h e c a t a l y s t chemisorbs NO, probably as mononitrosyl species.
Blll
Wavenumber (cm-c)
F i g . 2.13. I R s p e c t r a o f CO adsorbed by Co2' c a t i o n s on s i l i c a g e l : a ) 0.13 kNm-2 CO a t 293 K; b ) 10.5 k N r 2 CO a t 263 K; c ) 5.7 kNm-2 CO a t 128 K . Readapted from r e f . (124). MGssbauer spectroscopy was a l s o used (96) t o probe those s i t e s i n t h e z e o l i t e s t r u c t u r e r e s p o n s i b l e f o r t h e d i f f e r e n t n i t r o s y l complexes observed by i n f r a r e d spectroscopy o f t h e NO. The reduced Fe-Y z e o l i t e showed two Fe2+ doublets due t o i r o n c a t i o n s coordinated t o d i f f e r e n t oxygen environments i n t h e l a t t i c e . Upon exposure t o NO a t room temperature, t h e Fe2' c a t i o n s o f l o w coo r d i n a t i o n which were accessible t o NO, v i z . s i t e s I 1 and 11' formed n i t r o s y l complexes, thus corresponding t o t h e I R bands a t 1845 and 1870 cm-'.
On t h e
o t h e r hand, Fez+ c a t i o n s o f h i g h coordination. v i z . s i t e I , formed n i t r o s y l complexes by m i g r a t i n g t o s i t e s o f h i g h e r a c c e s s i b i l i t y , g i v i n g r i s e t o d i n i t r o s y l species. F i n a l l y , t h e mononitrosyl species a t 1767 cm-'
rendered a M h s b a u e r
spectrum c h a r a c t e r i s t i c o f h i g h l y coordinated i r o n , probably l o c a t e d a t s i t e s 111' i n t h e supercages. This study i s a n i c e p i e c e o f research, which i l l u s t r a t e s how t h e combination o f d i f f e r e n t techniques l e a d s t o a p r e c i s e d e s c r i p t i o n o f t h e s t r u c t u r e o f t h e a c t i v e components a t an almost atomic scale.
B112 2.6.4.
Cobalt Oxide
I n f r a r e d spectroscopic studies o f adsorbed CO and NO on supported c o b a l t oxide were conducted t o monitor the Co atoms exposed a t t h e surface. The ads o r p t i o n o f CO on a 0.3% Co/Si02 c a t a l y s t (124) a t temperatures i n t h e range 293-133 K gave bands a t 2176, 2188 and 2195 cm-l ( F i g . 2.13).
The peak a t t h e
highest wavenumber disappeared r e l a t i v e l y f a s t , w h i l e the two remaining bands stayed f o r the whole temperature range. The band p o s i t i o n was found t o be dependent on the adsorption temperature and a l s o on CO pressure. The band a t 2176 cm-' moved t o ca. 2176 cm"
a t 263 K and t h e r e a f t e r t o 2177 cm-'
(spectrum c), and t h a t a t 2188 moved t o 2189 and 2191 cm-',
a t 128 K
r e s p e c t i v e l y . I n ad
d i t i o n , the i n t e n s i t y o f these two bands was remarkably d i f f e r e n t a t low temperatures. Such a change would be expected f o r an increase i n the number o f CO ligands around the Co2+ cation. The values o f the i n t e n s i t y r a t i o o f t h e
bands f o r spectra a, b and c a r e 1.68,
1.86 and 2.62,
r e s p e c t i v e l y , lower than
t h a t expected f o r i d e a l CO-complexes w i t h one, two o r t h r e e CO ligands. Rebenst o r f and Larsson (124) explained t h i s behaviour i n terms o f a c i s - c o n f i g u r a t i o n o f the surface complexes, and assigned the bands a t 2195 cm-l t o a complex :
0
-
i/ co
co
- co
0
-
cp /co co "...
0';
containing one CO l i g a n d ( I ) , the bands a t 2176-2175 and 2188-2189 cm-l t o the A1 and B1 v i b r a r i o n s o f a cis-complex containing two CO l i g a n d s (II),and t h e two
bands a t 2177 and 2191 un'l
t o the E and A1 v i b r a t i o n s o f a cis-complex contain-
i n g t h r e e CO ligands (111). Alumina-supported cobalt oxide c a t a l y s t s have been studied by Ratnasamy and Knozinger (130) by o p t i c a l spectroscopy and adsorption o f CO a t 353 K. These authors found a very weak band a t ca. 2185 cm-'
and assigned i t as s t r e t c h i n g
v i b r a t i o n s o f CO molecules coordinated t o exposed Co2+ cations i n Co304 s t r u c t u res. The absence o f any other I R band l e d t o t h e conclusion t h a t the Co atoms associated w i t h the alumina are n o t exposed a t the surface, b u t are l o c a t e d i n the subsurface region. For the Coo-MgO c a t a l y s t , i t was found t h a t NO adsorbs much more s t r o n g l y than CO (131). Thus NO appears t o be a b e t t e r probe molecule than CO f o r studying t h e exposure o f Co2+ ions i n supported c a t a l y s t s . This i n t e r e s t i n g f e a t u r e was t h e r e a f t e r e x p l o i t e d by Topsfie's group (31) t o probe the changes occurring i n Co/A1203 and Co-promoted h y d r o t r e a t i n g c a t a l y s t s subjected t o d i f f e r e n t pretreatments. Unsupported CoAl 204 and Co304 and a1 umina-supported c o b a l t c a t a l y s t s showed two IR bands i n the region o f 1850-1900 cm-' adn 1780-1800 cm-'
(31). I n
B113 c o n t r a s t t o the findings o f Yao and Shelef (132), Topsfie and Topslde (31) found t h a t some adsorption o f NO on CoA1204 occurs (Table 2.9),
g i v i n g r i s e t o two
bands a t ca. 1875 and 1790 cm-l (Fig. 2.14, spectrum a). This adsorption was n o t very strong, since about 30% o f t h e NO could e a s i l y be removed by outgassing a t room temperature. A f t e r increasing the outgassing temperature t o 373 K , the i n t e n s i t y o f these bands decreased markedly, and complete removal occurred
on
f u r t h e r heating a t 423 K. These authors a l s o examined the I R spectra and the extent o f NO adsorption on CoA1204 calcined a t higher temperatures, and concluded t h a t t h e f r a c t i o n o f Co surface atoms adsorbing NO increases w i t h increasing pretreatment temperature. I n contrast t o CoA1204, Co304 showed a l a r g e r degreeof NO adsorption (Table 2.9), g i v i n g r i s e t o a complex I R spectrum (Fig. 2.14,
spectrum b). Spectrum b
TABLE 2.9. Adsorption o f NO on Calcined Cobalt-based Preparations (a) Cata 1ys t
(lo CoAl 204
NO Uptake
C a l c i n a t i o n Temp.
mol e ~ N O x l O - ~ ~ x c mol m - ~ NO/mol CO
-
1073
0.14
973 773
0.51
2 Co/A1203
773
-
0.49
6.5 Co/A1203
973 773
-
0.26 0.078
Co3O4 0.26 Co/A1 203
-
1.05
aReadapted from Ref. (31). consists of c o n t r i b u t i o n s around 1780, 1850 and 1865 which d i d n o t vary w i t h time o f exposure t o NO, but i t s i n t e n s i t y and shape changed g r e a t l y upon outgassing, t h i s being the o n l y sample w i t h almost the same i n t e n s i t y f o r both bands, The Co/A1203 c a t a l y s t showed two absorption bands i n t h e same frequency region as those of the reference compounds (Fig. 2.14, spectra c-e), b u t the nature of the adsorption o f NO depended s t r o n g l y on Co-loading and t h e c a l c i n a t i o n temperature. The I R spectrum o f a 0.26% Co/A1203 c a t a l y s t showed upon 140 adsorption two bands a t ca. 1885 and 1795 cm-l (Fig. 2.14,
spectrum c) which
were s h i f t e d downwards i n frequency w i t h increasing Co-loading, as seen f o r 2%Co-(spectrum d) and 6.5%Co-A1203 (spectrum e) c a t a l y s t s . From the spectra o f Fig. 2.14 i t r e s u l t s t h a t the adsorption o f NO
on t h e
0.26 and 2% Co c a t a l y s t s
has s i m i l a r i t i e s w i t h t h a t o f CoA1203 and d i f f e r s l a r g e l y from t h a t o f Co304. I n contrast, the 6.5%Co c a t a l y s t showed a NO doublet resembling t h a t o f C03O4.
B114
I
ZOO0
I
I
I
I
1800 160 Wavenumber (cm-0
Fig. 2.14. IR spectra o f NO adsorbed on d i f f e r e n t samples: a ) CoAl204; b ) Co 04; c ) 0.26% Co/Al293; d) 2.0% Co/A1203; e ) 6.5% Co/Al203. Redrawn from r e f . (317. Topstie and Topstie (31) a t t r i b u t e d t h e adsorption of NO on the low-loading Coc a t a l y s t s t o the presence of octahedral Co located a t t h e c a t a l y s t surfaces. This assignment i s c o n s i s t e n t with the findings of Ashley and Mitchell (133), who estimated t h a t i n a 0.34% Co/A1203 c a t a l y s t 66% of t h e Co atoms were octahedral. For t h e 0.26% Co c a t a l y s t , t h e value of 1.05 mole NO ( o r 0.525 mole per mole Co (Table 2.9) corresponds t o 52.5% o f t h e Co atoms adsorbing NO, a somewhat lower percentage of octahedral Co than t h a t found by Ashley and Mitchell. Even a t c a l c i n a t i o n temperatures a s high as 923 K a f r a c t i o n of ca. 132 of Co atoms was found t o be located in octahedral positions a t the s u r f a c e . The observation t h a t the IR spectrum o f NO adsorbed on the 6.5% Co c a t a l y s t resembled t h a t of t h e Co304 i s i n d i c a t i v e of t h e presence of t h i s l a t t e r species on t h e c a t a l y s t surface. I n f a c t , XRD p a t t e r n s o f t h e 6.5% Co c a t a l y s t revealed the presence o f Co304 c r y s t a l l i t e s , a s already found in o t h e r s t u d i e s (134-136). The appearance of Co304 i n Co/A1203 c a t a l y s t s with high Co loadings could be explained by t h e existence of only a limited number of s i t e s on t h e alumina f o r
B115
<1%b
P
1-2%co
-2%b
0
ooooo
'\
octahedral Co 0 tetrahedral co 0
0
0 0
F i g . 2.15. Models f o r t h e Co/A1 0 c a t a l y s t s . I n scheme 1 Co e n t e r s t h e alumina u n t i l s a t u r a t i o n i s a t t a i n e d ; e x c 6 s j Co i s i n c o r p o r a t e d i n t o Co304 phase. I n scheme 2 t h e f o r m a t i o n o f C03O4 i s accompanied by a decrease i n t h e amount o f Co a s s o c i a t e d w i t h t h e alumina. b i n d i n g Co atoms. These s i t e s a r e p r o g r e s s i v e l y f i l l e d a t i n c r e a s i n g Co c o n t e n t , and once a l l surface s i t e s have been f i l l e d , excess Co i s i n c o r p o r a t e d i n t o t h e Co304 phase. According t o t h i s model, which i s i l l u s t r a t e d by scheme 1 i n F i g . 2.15, an i n c r e a s e of t h e amount o f NO adsorbed w i t h i n c r e a s i n g Co l o a d i n g would be expected, u n t i l t h e s u r f a c e s i t e s become s a t u r a t e d . T h i s i s o p p o s i t e t o t h e t r e n d observed e i t h e r i n t h e NO uptakes ( T a b l e 2.9) o r i n t h e I R s p e c t r a ( F i g . 2.14).
I t appears t h e r e f o r e t h a t n o t o n l y Co304 i s formed i n t h e h i g h C o - l o a d i n g
c a t a l y s t s , b u t a l s o t h e number o f s u r f a c e Co atoms decrease even below t h e number i n t h e l o w - l o a d i n g c a t a l y s t s . A schematic model t o a c c o u n t f o r t h e s e f e a t u r e s i s d e p i c t e d by scheme 2 i n F i g . 2.15. The a u t h o r o f t h i s c h a p t e r has e x t e n s i v e l y used I R spectroscopy of NO adsorbed t o c h a r a c t e r i z e Co c a t i o n s , as promoters, i n molybdena-based h y d r o t r e a t i n g c a t a l y s t s , e i t h e r i n t h e o x i d i c (137-139),
reduced (139-141) and s u l p h i d e d
(142-145) s t a t e . Even i n t h e case o f complex c a t a l y t i c systems, t h e I R t e c h n i q u e i n i t s t r a n s m i s s i o n mode was f o u n d v e r y s u i t a b l e t o d i s t i n g u i s h among more t h a n
one t y p e o f promoters i n t h e c a l c i n e d form o f t h e p r e p a r a t i o n s . F o r i n s t a n c e , i n a s e r i e s o f d o u b l y promoted (Zn t h e r a t i o r = Zn(Zn
+
Co) molybdena-alumina c a t a l y s t s , i n which
+ Co) ranged between 0 and 1, t h e I R s p e c t r a o f NO adsorbed
on o x i d i c p r e p a r a t i o n s i n d i c a t e d t h a t NO adsorbs o n l y on Co2+. b u t n o t on Zn2+ c a t i o n s ( 1 3 7 ) . The IR s p e c t r a o f NO adsorbed on o x i d i c c a t a l y s t s i s shown i n F i g . 2.16A. I t i s seen t h a t t h e Z n - c o n t a i n i n g c a t a l y s t ( r = 1) does n o t show any I R band. T h i s b e h a v i o u r i s c o n s i s t e n t w i t h t h e small e x t e n t o f NO a d s o r p t i o n
found i n t h e same c a t a l y s t . I n c o n t r a s t , t h e I R s p e c t r a o f a l l c o b a l t - c o n t a i n i n g c a t a l y s t s showed a d o u h l e t a t ca. 1888 and 1800 cm-l, whose i n t e n s i t y decreased w i t h i n c r e a s i n g Co-content. The c o r r e l a t i o n diagram between absorbance o f t h e
B116
'1800
I
00
I
I
I
1500 Wavenumber lcm-ll
1900
F i g . 2.16. A) I R s p e c t r a o f NO adsorbed on doubly promoted (Zn + CO) supported molybdena c a t a l y s t s a f t e r outgassing i n h i g h vacuum a t 773 c o n t a c t i n g w i t h NO a t 297 K f o r 0.5 h. The atomic r a t i o r = Zn/(Zn + i n d i c a t e d . B ) P l o t o f t h e i n t e n s i t i e s o f 1888 and 1800 cm-1 bands a t coverages o f NO on t h e r = 0.00 ( z i n c - f r e e ) c a t a l y s t . two quoted frequencies, shown i n F i g . 2.168,
aluminaK and then Co) i s different
c l e a r l y indicates t h a t the inten-
s i t i e s o f t h e two bands a r e s t r i c t l y p r o p o r t i o n a l . T h i s f a c t s t r o n g l y suggests t h a t these two bands a r e due t o two normal modes o f t h e same s t r u c t u r e . Thus, i t i s reasonable t o e x p l a i n t h e d o u b l e t a t ca. 1888 and 1800 cm-'
i n terms of
d i n i t r o s y l s t r u c t u r e s bonded t o octahedral Co2+ c a t i o n s . The r e l a t i v e i n t e n s i t y o f t h e two bands i s r e l a t e d t o t h e angle 20 between t h e two N - 0 o s c i l l a t o r s by t h e r e l a t i o n (146, 147). (2.21) A d d i t i o n a l c a l c u l a t i o n s on t h e I R spectra, u s i n g Eq. (2.21),
indicated that the
angle 20 decreased from 112" f o r t h e c a t a l y s t r = 0 t o 105" f o r t h e c a t a l y s t
r = 0.78. T h i s change i n t h e average v a l u e o f t h e angle 28
i n d i c a t e s a decrease
o f t h e NO-Co2+ i n t e r a c t i o n , when Zn c o n t e n t increases. The p o s i t i o n o f t h e bands o f adsorbed NO i n t h e I R spectrum depends markedly on t h e e l e c t r o n d e n s i t y and t h e c o o r d i n a t i o n o f Co2+ ions. To ill u s t r a t e t h i s , Fig. 2.17 d i s p l a y s t h e I R s p e c t r a o f adsorbed NO on a c a l c i n e d
B117
0 U C
n m L
In 0
n
a
+& 7&7ik Wavenumber (cm -1)
F i g . 2.17. IR s p e c t r a o f NO adsorbed on a 1.7% Co/Al203 c a t a l y s t s u b j e c t e d t o d i f f e r e n t t reat me n t s : a ) outgassed a t 773 K; b ) H2-reduced a t 723 K f o r 1 h, f o l l o w e d by out g a s s i n g a t 773 K; c ) s u l p h i d e d i n a 10% ( v / v ) H2S:Hz m i x t u r e a t 673 K f o r 4 h, f o l l o w e d by o u t g a s s i n g a t 773 K. (spectrum a ) , reduced (spectrum b ) and s u l p h i d e d (spectrum c ) 1.7% Co/A1203 c a t a l y s t . N o t i c e t h a t t h e symmetric band a t ca. 1885 cm-l o f adsorbed NO on t h e c a l c i n e d c a t a l y s t s h i f t s downwards i n frequency by ca. 10 o r 33 cm”
i n the
reduced and s ulph i d e d c a t a l y s t r e s p e c t i v e l y . Conversely, t h i s f e a t u r e can be e x p l o i t e d f o r a s e m i q u a n t i t a t i v e d e t e r m i n a t i o n o f t h e chemical s t a t e of CO atoms i n Co/A1 203 c a t a l y s t s s u b j e c t e d t o v a r i o u s r e d u c t i o n - s u l p h i d a t i o n p r e t r e a t m e n t s , s i m i l a r t o t h a t which occurs d u r i n g a c t i v a t i o n o r r e g e n e r a t i o n o f h y d r o t r e a t i n g c a t a l y s t s (141-145). 2.6.5.
N i c k e l Oxide I n f r a r e d s t u d i e s u s i n g adsorbed CO and NO as m o l e c u l a r probes were used f o r
many y e a r s t o i n v e s t i g a t e t h e n a t u r e o f exposed n i c k e l atoms i n support ed n i c k e l o x i d e s ubjec t e d t o a v a r i e t y o f p r e t r e a t m e n t s (36, 148-154). The l i t e r a t u r e
on
t h i s t o p i c i s abundant and sometimes c o n f l i c t i v e because o f t h e numerous e f f e c t s , e.g.,
t y p e o f c a r r i e r , presence o f i m p u r i t i e s , N i - l o a d i n g and n a t u r e o f t h e
B118 precursor, which d e c i s i v e l y i n f l u e n c e t h e s u r f a c e exposure o f N i atoms a t catal y s t i n t e r f a c e s . For instance, s i l i c a regarded as an i n e r t support, can s t r o n g l y i n t e r a c t w i t h N i under c e r t a i n c o n d i t i o n s , t h i s i n t e r a c t i o n being minimized a t h i g h N i - l o a d i n g s (148). A t low N i concentrations, and on a more r e a c t i v e supp o r t , such as alumina. N i c a t i o n s become s t a b i l i z e d a t t h e s u r f a c e (159, 155, 156) through s p i n e l formation, even w i t h a b u l k c r y s t a l s t r u c t u r e . Despite t h e above d i f f i c u l t i e s ,
i t i s p o s s i b l e t o determine some general
c h a r a c t e r i s t i c s . Table 2.10 summarizes t h e wavenumber r e g i o n o f I R band appearance a f t e r exposure t o CO o f a Ni/A1203 c a t a l y s t and t h e assignment o f t h e ads o r p t i o n s i t e . I t i s g e n e r a l l y b e l i e v e d t h a t bands i n t h e r e g i o n 2030-2060 cm-' correspond t o CO adsorbed s t r o n g l y o r moderately s t r o n g l y i n a l i n e a r form, and bands i n t h e r e g i o n 1910-1970 cm-'
r e f e r t o s t r o n g l y b r i d g e d CO (154). I n ad-
d i t i o n , l i n e a r more weakly adsorbed CO appeared t o g i v e r i s e t o a band i n t h e r e g i o n 2070-2090 cm-'.
Somewhat d i f f e r e n t bands were observed by Primet e t a l .
(148) i n t h e N i / S i 0 2 c a t a l y s t s , who found two bands 2070-2080 cm-I corresponding t o i r r e v e r s i b l y h e l d l i n e a r CO on Ni2' and a t ca. 2040 cm"
c a t i o n s i n t e r a c t i n g w i t h an o x i d e phase,
assigned t o CO on unperturbed N i atoms. The bands a t ca.
2200 cm-l g e n e r a l l y show weak a d s o r p t i o n o f CO on s u r f a c e Ni2'
c a t i o n s (150,
153, 154), and those near 2140-2150 a r i s e from CO adsorbed on Ni'
species (150).
The bands i n t h e r e g i o n 2020-2065 cm-l a r i s i n g from s t r o n g l y h e l d CO a r e thought TABLE 2.10 Assignments o f bands from CO and NO on Ni/A1203 C a t a l y s t s Wavenumber range (cm-l)
Probe
Site
Reference
2180-22OOw1
co co co co co co co
Ni2'
(150,153)
Ni' Nit Ni2+
(154)
NO
Ni' Ni2+b
2 130- 2 16OW1
2080- 2120'' 2075-2O8Os1 2050-2O9Ow1 2020-2065s1 1930-1980Sb 1910-1920s1 185O-188Os1
NO
Ni' a N iO Nit
(150) (153 1 (154) (150) (153,154) (154) (154)
w l = weak l i n e a r ; s l = s t r o n l i n e a r ; sb = s t r o n g b idged; aspecies o f t h t y p e N i " associated w i t h 0'- o r S cannot be excluded; Sba ds a t ca. 1860 un-! were assigned t o moderately s t r o n g 1i n e a r NO absorbed on N i f t surrounded by s u l phide ions.
il-
t o show CO h e l d l i n e a r l y by N i " i n small c r y s t a l s . The e x a c t p o s i t i o n i n t h e spectrum a p p a r e n t l y depends on c r y s t a l f a c e exposure, coverage, s i z e and pos-
s i b l e inductive effects o f the support. A t t e n t i o n must be p a i d t o t h e f a c t t h a t t h e t h e o r y r e l a t e s CO band f r e quency t o t h e s t r e n g t h of bonding s t r o n g l y h e l d CO which y i e l d s bands i n t h e r e g i o n 2060-2090 cm-l
, thus
appearing i n c o n s i s t e n t . The e x p l a n a t i o n t o t h i s ap-
p a r e n t c o n t r a d i c t i o n i s g i v e n i n terms o f t h e d o n a t i o n o f t h e l o n e p a i r e l e c t r o n s f r o m t h e carbon atom o f CO t o a N i which t h e n back-donates e l e c t r o n s t o a n t i b o n d i n g o r b i t a l s o f t h e CO, t h u s weaking t h e C-0 bond b u t s t r e n g t h e n i n g t h e N i - C bond, e.g.,
l o w e r i n g t h e frequency o f t h e CO m o l e c u l e (2142 cm").
i s adsorbed by Ni2'
When CO
c a t i o n s , w h i c h cannot back-donate e l e c t r o n s , s u r f a c e bonding
i s weak, and t h e CO frequency i s u s u a l l y n o t lowered b u t i n c r e a s e d i n s t e a d . One reasonable e x p l a n a t i o n m i g h t be t h a t e l e c t r o n s a r e donated t o a n t i b o n d i n g o r b i t a l s o f CO, d e r i v e d n o t from t h e N i atoms, b u t from t h e a d j o i n i n g a n i o n s . P e r i (154) has c l e a r l y shown t h a t , i f s u l p h a t e was i n i t i a l l y p r e s e n t i n t h e alumina c a r r i e r , f u r t h e r H 2 - r e d u c t i o n tended t o g i v e s u r f a c e s p e c i e s a b l e t o h o l d CO weakly. The S2- i o n generated by r e d u c t i o n o f s u l p h a t e c o u l d somewhat i n c r e a s e t h e C-0 frequency by a b s t r a c t i n g e l e c t r o n s o t h e r w i s e a v a i l a b l e f o r back-donation from N i " atom. A d s o r p t i o n o f NO on Ni/A1203 shows o n l y one m a j o r band i n t h e r e g i o n 18701890 cm-l ( T a b l e 2.10). Another s h o u l d e r around 1800 cm-'
i s u s u a l l y observed,
b u t n o t a c l e a r d o u b l e t s u g g e s t i n g d i n i t r o s y l o r adsorbed NO dimer, as i s t h e case i n chromium and c o b a l t c a t i o n s . The NO may be h e l d p r e d o m i n a n t l y on N i c a t i o n s o f one t y p e , exposed i n such a way t h a t t h e y do n o t r e a d i l y adsorb more t h a n one NO l i g a n d . As shown by P e r i (154), i t r e s u l t s t h a t t h e decrease i n t h e CO band a t ca. 2190 cm-'
(Ni"
s i t e s ) , a l o n g w i t h decrease i n t h e band of NO a t
ca. 1870 cm-l as r e d u c t i o n temperature i s i n c r e a s e d , suggests t h a t t h e l a t t e r band p r o b a b l y a r i s e s f r o m NO adsorbed on t h e same t y p e o f Ni2' o c t a h e d r a l s i t e s . The f a c t t h a t t h e Ni2'
c a t i o n s , e.g.,
c e n t e r s gave much more i n t e n s e bands
w i t h NO t h a n w i t h CO i n d i c a t e s t h a t CO i s weakly adsorbed ( 6 3 ) , t h e a d s o r p t i o n s i t e s b e i n g i n c o m p l e t e l y occupied a t t h e l o w p r e s s u r e s used. The f a c t t h a t NO adsorbed on Ni2' 1890 cm-',
c a t i o n s g i v e s o n l y a m a j o r band a t 1870-
d i f f e r i n g f r o m t h e d o u b l e t a t ca. 1890 and 1805 cm-l has been ex-
p l o i t e d by Caceres e t a l . (157) t o determine t h e f r a c t i o n s o f Ni2'
and
CO2'
c a t i o n s exposed a t t h e s u r f a c e o f doubly promoted (NitCo)-molybdena alumina c a t a l y s t s . The I R s p e c t r a o f NO adsorbed on a s e r i e s o f o x i d i c c a t a l y s t s , i n which t h e o v e r a l l amount o f promoters was c o n s t a n t b u t t h e a t o m i c r a t i o r = Co/ /(Co+Ni) ranged between 1 ( o n l y Co) and 0 ( o n l y N i ) , a r e g i v e n i n F i g . 2.18a. The c a t a l y s t w i t h r = 0 gave r i s e t o a s i n g l e band a t ca. 1880 m-', w h i l e t h a t w i t h r = 1 e x h i b i t e d two bands a t 1805 and 1890 cm-'.
The o t h e r c a t a l y s t s w i t h
O < r < 1 a l s o showed two bands a t ca. 1810 and 1887-1890 cm-'.
T h i s l a t t e r band
c o n t a i n s c o n t r i b u t i o n s f r o m NO adsorbed on b o t h CoZt and Ni2'
c a t i o n s , and i t s
i n t e n s i t y was observed t o remain v i r t u a l l y c o n s t a n t , s i n c e t h e CotNi c o m p o s i t i o n
B120
-6
P
1900
8
1900 17W Wavenumber km-Y
1700
F i g . 2.18. I R spectra of NO adsorbed on doubly promoted (Ni+Co)-molybdena a l u mina c a t a l y s t s w i t h Mo and t o t a l promoter c o n t e n t c o n s t a n t b u t v a r i a b l e Co/(Co+ +Ni) atoiiiic r a t i o . a ) o x i d i c c a t a l y s t s a f t e r t r e a t m e n t under vacuum a t 773 K; b ) reduced c a t a l y s t s f o r l h a t 773 K; c ) e f f e c t of t h e atomic r a t i o on t h e i n t e g r a t e d a b s o r p t i o n of NO on Co ( 0 ) (band a t about 1805 cm-l) and on,Co+Ni ( 0 ) (band a t about 1885 cm-l). was constant. On t h e o t h e r hand, as t h e low frequency band i s e s s e n t i a l l y ass o c i a t e d o n l y w i t h Co, i t s i n t e n s i t y decreased w i t h decreasing r r a t i o s . I n add i t i o n , a weak band a t ca. 1705 cm-'
i n d i c a t e d t h e appearance o f some reduced Mo
species d u r i n g t h e outgassing s t e p (see n e x t s e c t i o n ) . The i n t e n s i t y o f t h e bands was dependent on t h e r a t i o r. As shown i n F i g . 2.18c,
t h e Co-Ni-containing
c a t a l y s t s showed h i g h e r i n t e n s i t i e s f o r t h e 1810 cm-l ( a d s o r p t i o n on Co) and 1890 cm-l ( a d s o r p t i o n on both Co and N i ) bands than expected, i f an a d d i t i v e behaviour e x i s t between t h e extreme ( r = 1 and r = 0 ) c a t a l y s t s .
B121 On hydrogen-reduced c a t a l y s t s , n o t i c e a b l e changes i n t h e I R s p e c t r a o f adsorbed NO occur. As can be seen i n F i g . 2.18b, a s h i f t o f ca. 3-5 cm-l toward l o w e r wavenumbers was observed i n t h e bands a s s o c i a t e d w i t h N i and Co, i . e . , from 1890 t o 1887 cm-l f o r r = 0 and from 1805 and 1890 t o 1802 and 1885 cm-', r e s p e c t i v e l y , f o r r = 1. Note a g a i n t h a t i n reduced p r e p a r a t i o n s a broad band i s observed, much more i n t e n s e t h a n i n t h e o x i d i c c a t a l y s t s , a t ca. 1705 cm- 1, due t o a d s o r p t i o n o f
NO i n reduced Mo s i t e s . The o r i g i n o f t h i s l a t t e r band i s
analysed i n t h e n e x t s e c t i o n . 2.6.6.
Molybdenum Oxide F o r t h e i r r e l e v a n c e i n s e v e r a l commercial h y d r o t r e a t i n g processes, molyb-
dena-alumina-based c a t a l y s t s have been i n v e s t i g a t e d i n d e t a i l i n r e c e n t y e a r s . Many s p e c t r o s c o p i c techniques have been a p p l i e d t o t h e s t u d y o f t h i s t y p e o f c a t a l y s t . Among these, i n f r a r e d spectroscopy, i n i t s c o n v e n t i o n a l t r a n s m i s s i o n mode, remains prominent ( c f . ( 6 4 ) and r e f e r e n c e s t h e r e i n ) . I n t h i s t y p e o f s t u d i e s , b o t h CO (64,158-161)and NO (23, 63, 64, 162-168)have been w i d e l y used as probe molecules. These molecules y i e l d i n f o r m a t i o n on t h e c o o r d i n a t i v e l y uns a t u r a t e d s i t e s which a r e exposed on t h e s u r f a c e o f reduced o r s u l p h i d e d cat a l y s t s . I n c o n t r a s t t o t h e m u l t i p l i c i t y o f bands observed upon CO o r NO adsorpt i o n on reduced chromia ( s e c t i o n 2.6.2.),
s p e c t r a r e p o r t e d f o r t h e s e probes
absorbed on reduced molybdena-alumina c a t a l y s t s appear t o be remarkably s i m p l e . A d s o r p t i o n o f CO a t room temperature on reduced Mo/A1203 c a t a l y s t s shows i n g e n e r a l an a b s o r p t i o n band a t ca. 2190 cm-'
( F i g . 2.19,
spectrum a) (112,
113, 158, 159). As a l r e a d y shown by Delgado e t a l . (158), b o t h t h e i n t e n s i t y and t h e shape o f t h e spectrum was s t r o n g l y dependent on CO p r e s s u r e and sample temperature. A t 80 K a second band appeared a t ca. 2160 cm-l ( F i g . 2.19,
spec-
trum b ) , which was c o m p l e t e l y removed by o u t g a s s i n g a t t h e same temperature, w h i l e t h e band a t 2188 cm-' was s l i g h t l y s h i f t e d toward h i g h e r f r e q u e n c i e s (spectrum f ) . The band a t ca. 2160 cm-l almost disappeared b y i n c r e a s i n g sample temperature up t o 125 K, w h i l e t h e i n t e n s i t y o f t h e 2190 cm-l band decreased o n l y s l i g h t l y i n t h e temperature range above 200 K. From t h e s e f i n d i n g s i t was concluded t h a t CO i s weakly h e l d a t 80 K, g i v i n g r i s e t o t h e band a t 2160 cm-'. The more s t r o n g l y adsorbed CO species was e x p l a i n e d as due t o CO c o o r d i n a t e d t o c o o r d i n a t i v e l y u n s a t u r a t e d Mo s i t e s , which s h o u l d be i n a r e l a t i v e l y h i g h p o s i t i v e o x i d a t i o n s t a t e , s i n c e t h e frequency o f t h e r e m a i n i n g band l a y above t h e gas phase frequency by ca. 45 cm-l ( 1 5 0 ) . Delgado e t a l . (158) a l s o proposed t h a t t h e o x i d a t i o n s t a t e o f Mo i n reduced c a t a l y s t s was Mo4+, a l t h o u g h f o r t h e 3+ same t y p e o f c a t a l y s t s and s i m i l a r p r e t r e a t m e n t s P e r i (112) d i d n o t e x c l u d e Mo o r ivlo2+ species. I n f r a r e d spectroscopy o f adsorbed CO a t ambient t e m p e r a t u r e was a l s o used by Zaki e t a l . (160) t o s t u d y reduced and s u l p h i d e d molybdena on s e v e r a l sup-
B122
1
1
1
1
Zoo0
2300
2300 Wavenumber k m - Y
2000
i
Fig. 2.19. Low temperature I R spectra o f CO adsorbed on a H -reduced 11%Moog /A1203 c a t a l y s t : a) 80 K; b ) 125 K; c ) 195 K; d ) 235 K; e ) $95 K a t 1.3 kNmCO pressure; f ) a f t e r evacuation a t 80 K. p o r t s , e.g.,
A1203, T i 0 2 , Ce02 and Zr02. On A1203 s t r o n g i n t e r a c t i o n w i t h t h e
support takes p l a c e and r e d u c t i o n i n H2 occurs o n l y under severe c o n d i t i o n s ( T ~ 7 7 3K ) . I n these c o n d i t i o n s , a band a t ca. 2186 cm-l was observed and assigned t o Mo4+(CO) species. The a d s o r p t i o n o f CO on t h e s u l p h i d e d c a t a l y s t s gave r i s e t o a band a t 2118 an-', which broadened upon outgassing a t room temperat u r e . Consequently i t was a t t r i b u t e d t o Mo2+ (CO) species. I t s width. however, may f a v o u r t h e presence o f Mo5'
6+ species, since, by v i r t u e o f t h e i r nature, MO
s i t e s induce m u l t i p l e a d s o r p t i o n c e n t e r s o f average o x i d a t i o n s t a t e below Mo
4+
(112). On reduced Mo/Ti02 CO a d s o r p t i o n gave r i s e t o bands a t 2186, 2172 ( s h o u l d e r ) , and two weak bands a t 2135 cm-' and 2110 cm-' which a r e assigned r e s p e c t i v e l y t o Mo3+ (CO) and
M04'
(C0l2
species. Upon s u l p h i d a t i o n , bands a t 2188, 2154 and
2135 cm-l were observed, t h e f i r s t being assigned t o Ti3+ (CO) species and t h e l a t t e r two t o p a r t i a l l y and completely s u l p h i d e d Mo-sites o f average o x i d a t i o n s t a t e between +2 and+4. Carbon monoxide a d s o r p t i o n
on reduced Mo/Ce02 gave r i s e t o
bands a t 2189 and 2120cm-'. The former a d s o r p t i o n was e x p l a i n e d by t h e f o r m a t i o n o f Mo4+ (CO ) species and t h e l a t t e r may i n v o l v e c o n t r i b u t i o n s from COheld by uncovered c e r i a and by reduced Mo i o n s i n average o x i d a t i o n s t a t e below t4. On s u l p h i d i n g , a s i n g l e band a t ca. 2110 cm-l was observed and e x p l a i n e d i n terms o f c o e s i x t i n g
2300
2200
2100
2000
Wavenu mber ( c m -'I
F i g . 2.20. I R s p e c t r a o f CO adsorbed on H2-reduced 8.9% Mo/A1203 c a t a l y s t . a ) background; b ) a f t e r c o n t a c t i n g w i t h 0.94 kNm-2 CO a t 297 K on prereduced cat a l y s t a t 783 K f o r 1 h; c ) a f t e r c o n t a c t i n g w i t h CO i n t h e same c o n d i t i o n s on prereduced c a t a l y s t a t 1093 K. c o n t r i b u t i o n s f r o m CO adsorbed on Ce02 and on Most s i t e s . F i n a l l y , t h e same a u t h o r s d i d n o t observe any s i g n i f i c a n t r e d u c t i o n i n Mo/Zr02 c a t a l y s t s , p r o b a b l y due t o a s t r o n g i n t e r a c t i o n d e v e l o p i n g i n t h e r e d u c i n g atmosphere. It i s w e l l known t h a t t h e r e d u c t i o n o f Mo6+ i o n s w i t h H2 a t temperatures
about 770 K g i v e s p r e f e r e n t i a l l y Mo4+ species, however, i f t h e r e d u c t i o n i s conducted a t temperatures above 920 K, m e t a l l i c Mo" can be o b t a i n e d . I n t h i s l a t t e r case, CO has been s u c c e s s f u l l y used t o probe m e t a l l i c Mo s i t e s (112, 168) because i t i s l e s s l i k e l y t h a n NO t o r e o x i d i z e m e t a l atoms. On a H2-reduced 8.9% Mo/A1203 c a t a l y s t a t 783 K , a d s o r p t i o n o f CO gave r i s e t o bands a t 2230 and
2185 cm-l ( F i g . 2.20,
spectrum b ) t y p i c a l o f CO h e l d on exposed A13+ s i t e s ( 1 6 9 )
and on i o n i c Mo s i t e s , p r o b a b l y Mo4+, r e s p e c t i v e l y . A f t e r r e d u c t i o n a t 1093 K t h r e e bands a t 2230, 2140 and 2045 ( v e r y s t r o n g ) cm-l were observed ( F i g . 2.20, spectrum c ) . The f i r s t two bands may be t h e same as f o r t h e c a t a l y s t reduced a t 783 K and t h e l a t t e r i s u s u a l l y assigned t o CO t y p i c a l l y h e l d i n l i n e a r c o n f i -
g u r a t i o n on f u l l y reduced s i t e s , i . e . m e t a l l i c Mo (112, 150, 168).
B124
I
I
1900
la00
1
I
1700 1600 Wavenum ber (cm -1)
F i g . 2.21. A ) I R s p e c t r a o f adsorbed NO a t room temperature on p a r t i a l l y reduced Mo/A120 cata l y s t s w i t h v a r i a b l e Mo c o n t e n t ( % ) : a ) 1.8; b) 6.1; c ) 8.9; d ) 15.9; e j 25.6 B ) Spectra o f NO on t h e p a r t i a l l y reduced Mo-ammonia e x t r a c t e d c o u n t e r p a r t s : a ' ) 1.1; b ' ) 3.5; c ' ) 5.5; d ' ) 9.5; e l ) 9.7.
#.
I n f r a r e d spectroscopic s t u d i e s o f NO adsorbed on reduced ( o r sulphided) rnolybdena-alumina c a t a l y s t s are, however, much more abundant (31, 112, 147, 157, 162-168). P e r i (1701, Kazusaka and Howe (162), as w e l l as H a l l and Millman (171) r e p o r t e d t h a t NO adsorbs on reduced Mo/A1203 c a t a l y s t s i n p a i r s , i.e.,
as d i -
n i t r o s y l o r dirneric species. The a d s o r p t i o n o f NO on reduced molybdena c a t a l y s t s was found t o be much s t r o n g e r than t h e a d s o r p t i o n o f CO (112). Two reasons can be g i v e n t o e x p l a i n t h i s behaviour. The f i r s t i s t h a t NO can form a cis-dimer, which i s s t r o n g l y chemisorbed on reduced molybdenum oxide, as r e p o r t e d by Kugler e t a1
.
(123) f o r t h e a d s o r p t i o n o f NO on chromium oxide. The second l i e s i n t h e
B125 bonding p i c t u r e t h a t c o n s i d e r s d o n a t i o n o f e l e c t r o n d e n s i t y f r o m N atom (0
t o Mo
bond) and back-bonding by t h e Mo d - e l e c t r o n s i n t o t h e a n t i b o n d i n g n * o r b i t a l
of NO. I n c o n t r a s t t o CO, NO has a l r e a d y one e l e c t r o n i n t h e a n t i b o n d i n g o r b i t a l which i s t r a n s f e r r e d t o t h e Mo atom, t h u s weakening t h e N-0 bond. The I R s p e c t r a o f NO adsorbed on H2-reduced molybdena-alumina c a t a l y s t s u s u a l l y show two bands near 1810 and 1710 cm-l ( F i g . 2.21)
( 1 6 4 ) , which have
been assigned t o symmetric and a n t i s y m m e t r i c fundamental NO s t r e t c h i n g , r e s p e c t i v e l y , o f p a i r e d NO molecules h e l d e i t h e r as a d i n i t r o s y l (162) o r as a dimer (170) on an exposed Mo i o n p r o b a b l y as Mo4+. It i s i n t e r e s t i n g t o n o t e t h a t bands a t s i m i l a r f r e q u e n c i e s were observed f o r NO adsorbed on H2-reduced Mo/ /A1203 c a t a l y s t s (31, 170, 171) and on t h e r m a l l y a c t i v a t e d M o ( C O ) ~ / A ~prepara~O~ t i o n s (162). The r e l a t i v e l y i n t e n s i t i e s o f t h e two bands g i v e d i r e c t l y t h e a n g l e 8 between t h e two NO o s c i l l a t o r s (Eq. 2.21). Yao and R o t h s c h i l d (147) s t u d i e d t h e a d s o r p t i o n of NO on H2-reduced Mo03/A1203 c a t a l y s t s o f t h e h i g h and l o w Mol o a d i n g t y p e and e v a l u a t e d t h e IR s p e c t r a i n terms of a
c i s - d i m e r (C2v)
as t h e p r i n c i p a l adsorbed species. The r e s u l t s f o r R and 8 a r e summarized i n Table 2.11.
I t i s observed t h a t 8 changes l i t t l e w i t h t h e c o n c e n t r a t i o n o f NO
TABLE 2.11 The a n g l e between two NO o s c i l l a t o r s on H2-reduced Mo/A1203 c a t a l y s t s * Mo(wt%)
R
Pretreatment
8
%trans-(N0)2
3.4
Outg.,
298 K
0.40
116
0
3.4
Outg., 513 K
0.31
120
0
3.4
Outg.,
633 K
0.30
120
0
10.1
Outg.,
513 K
0.34
119
trace
10.1
Outg., 633 K
0.22
132
59
10.1
Outg.,
0.0
180
100
*
673 K
Readapted f r o m r e f . (147).
on t h e lower Mo-loading c a t a l y s t . I n a d d i t i o n , t h e r e i s a much l o n g e r i n c r e a s e o f 8 f o r t h e dimers cheniisorbed on t h e h i g h e r Mo-loading c a t a l y s t a f t e r desorpt i o n temperatures above 523 K. Furthermore, a f t e r d e s o r p t i o n a t 673 K t h e band a t ca. 1810 cm-l (symmetric s t r e t c h i n g ) disappeared w h i l e t h e band a t ca. 1710 cm -1 ( a n t i s y m m e t r i c s t r e t c h i n g ) was s t i l l observed; t h e a n g l e 8 appeared t o approach 180". Since a C2h t r a n s - d i m e r has o n l y one i n f r a r e d a c t i v e NO s t r e t c h i n g v i b r a t i o n , i t cannot be r u l e d o u t t h a t t h e l a s t p r e t r e a t m e n t i n T a b l e 2.11 r e p r e s e n t s remaining t r a n s - d i m e r . Hence, a p o s s i b l e e x p l a n a t i o n f o r t h e l a r g e
B126 change o f 9 a t v e r y l o w NO coverages on t h e h i g h e r Mo-loading c a t a l y s t i s t h a t a small amount o f C2h trans-dimer,
more s t a b l e than t h e CZv c i s - d i m e r , i s
present on h i g h l y unsaturated s i t e s , e.g.,
a t corners o r edges.
I t i s w e l l known t h a t Mo o x i d e may e x i s t as both t e t r a h e d r a l monomeric
( M o ) ~species and as polymeric species i n octahedral c o o r d i n a t i o n (Moo). The MoT species a r e r e s i s t a n t toward r e d u c t i o n , whereas t h e Moo species a r e e a s i l y r e d u c i b l e . Therefore, i n t h e former case a l o w e r NO band i n t e n s i t y i s expected than i n t h e l a t t e r . T h i s has been assessed by Caceres e t a l . (168), who s t u d i e d
NO a d s o r p t i o n on H2-reduced u n e x t r a c t e d and ammonia-extracted ( p a r t i a l removal o f Moo species) Mo03/A1 203 c a t a l y s t s prepared by a d s o r p t i o n from t h e s o l u t i o n procedure. They found an i m p o r t a n t decrease i n t h e i n t e n s i t y o f NO bands i n t h e e x t r a t e d p r e p a r a t i o n s ( F i g . 2.21)
, which
i n t u r n , become much more d i f f i c u l t t o
reduce. Furthermore, i n both c a t a l y s t s e r i e s t h e band a t l o w e r frequencies was considerably broader than t h a t a t h i g h e r frequencies. T h i s was i n agreement w i t h many o t h e r s t u d i e s (31, 147, 168), and may be e x p l a i n e d on t h e b a s i s t h a t t h e v i b r a t i o n a l t r a n s i t i o n moment v e c t o r which i s p a r a l l e l t o t h e surface, w i l l d e t e c t t h e s u r f a c e inhomogeneities more e f f e c t i v e l y than t h e one which i s p e r p e n d i c u l a r t o t h e surface. I n t h e case o f Mo/Si02 c a t a l y s t s t h e i n t e r a c t i o n o f Moo3 w i t h t h e s u r f a c e i s much weaker than w i t h t h e alumina. Using I R o f NO adsorbed on reduced catal y s t s , P e r i (112) s t u d i e d t h e exposure o f Mo s i t e s on a Si02 support. Two bands a t ca. 1810 and 1710 cm-l were found, although a t h i g h NO pressures p h y s i c a l a d s o r p t i o n and/or o t h e r e f f e c t s gave some apparent new bands n o t observed on t h e A1203 support. However, a f t e r outgassing t h e spectrum o f t h e r e s i d u a l
NO species
appeared t o be almost i d e n t i c a l t o t h a t o f Mo/A1203 c a t a l y s t s . I n a d d i t i o n t o t h i s , t h e f a c t t h a t b o t h types o f Mo/Si02 and Mo/A1203 c a t a l y s t s a r e r e a d i l y poisoned by amounts o f s t r o n g l y h e l d
NO suggests t h a t t h e c a t a l y t i c a l l y a c t i v e
Mo s i t e s a r e s i m i l a r on both supports. As a l r e a d y shown f o r Co/A1203 c a t a l y s t s , t h e s u l p h i d a t i o n of Mo03/A1203 p r e p a r a t i o n s gave r i s e t o two bands, b u t a t lower band frequencies than i n t h e reduced p r e p a r a t i o n s (32, 163, 166, 167). F o r i n s t a n c e
L6pez Agudo e t a l . (32)
found t h e d o u b l e t a t ca. 1703 and 1792 cm-' on a sulphided 8% Mo/A1203 c a t a l y s t , which was q u i t e s i m i l a r t o t h a t r e p o r t e d by Valyon and H a l l (166) and s l i g h t l y h i g h e r than t h a t obtained by Topsbe and Topsbe (167) and by Okamoto e t a l . (163) f o r o t h e r sulphided unpromoted c a t a l y s t s . The e x p l a n a t i o n o f t h e " r e d s h i f t " o f t h e NO bands o f NO adsorbed on sulphided Mo s i t e s i s t h e same than t h a t advanced f o r Co/A1203 c a t a l y s t s . The e l e c t r o n d e n s i t y around t h e s u l p h i d e d Mo s i t e s becomes h i g h e r upon s u l p h i d a t i o n , r e s u l t i n g i n a s t r o n g e r Mo-N bond and a weaker
N-0 bond. T h i s i s most l i k e l y due t o a complete o r p a r t i a l replacement o f t h e oxygen i o n s surrounding t h e Mo atoms i n t h e c a l c i n e d s t a t e by t h e l e s s e l e c t r o n n e g a t i v e s u l p h i d e ions. A f i n a l remark t o be made i s t h a t t h i s s h i f t between t h e
B127
I R bands of NO adsorbed on reduced and on s u l p h i d e d c a t a l y s t s can be used t o
deduce whether Mo i o n s a r e reduced and/or s u l p h i d e d d u r i n g H2/H2S p r e t r e a t m e n t s as t h e H2:H2S r a t i o and temperature v a r y o v e r a wide range ( 1 6 7 ) . 2.6.7.
Copper Oxide S e l e c t i v e c h e n i i s o r p t i o n of NO and NH3 has been used by B e c k l e r and White
(172) t o d e s c r i b e t h e a c i d i c ( B r o n s t e d and L e w i s ) p r o p e r t i e s o f cooper- c o n t a i n i n g model c a t a l y s t s . The c a t a l y s t s c o n s i s t e d o f p o l y n u c l e a r m e t a l (M3+ and Cu2+) complexes o f t h e form M[(P-OH)CU(P-OCH,CH~NR~)]~( C104)3(M=A1 ,Cr,Fe) Cab-0-Sil.
s u p p o r t e d on
I n t h i s complex, t h e M3+ i o n i s f u l l y c o o r d i n a t e d w i t h an o c t a h e d r a l
symmetry and t h e s i x c u p r i c i o n s a r e connected t o t h e c e n t r a l i o n t h r o u g h s i x OH b r i d g e s . Each Cu2' e x h i b i t s a square p l a n a r symmetry w i t h f o u r l i g a n d s (OH b r i d g e , two a l k o x i d e b r i d g e s , and a -NR2 g r o u p ) . Another OH b r i d g e , l i n k i n g an a d j a c e n t Cu2+ t o t h e c e n t r a l i o n , occupies t h e J a h n - T e l l e r p o s i t i o n of t h e Cu
2+
i o n as a f i f t h l i g a n d . Thus, each Cu2' shows one CUS, and t h e d i s k - l i k e complex as an e n t i t y has t h r e e CUS p e r face, s i n c e t h e b13+ i o n i s c o o r d i n a t i v e l y s a t u r a t e d . I n t h i s c o n f i g u r a t i o n , t h e Cu2+ has been shown t o be t h e s o l e c e n t e r o f Lewis a c i d i t y , w h i l e t h e b r i d g i n g OH groups between Cu2' and
M3+ i o n s were
i d e n t i f i e d as B r o n s t e d a c i d s i t e s , t h e l a t t e r b e i n g e a s i l y probed by p r e v i o u s blockage o f Cu2+ i o n s t h r o u g h NO a d s o r p t i o n . The e x t e n t o f NO a d s o r p t i o n on t h e t h r e e (M=Al, C r , Fe) complexes showed a s t o i c h i o m e t r y o f t h r e e NO molecules p e r complex, i n d i c a t i n g t h a t a l l t h e Cu
2+
i o n s a r e a v a i l a b l e f o r NO a d s o r p t i o n , when t h e complexes a r e supported on Cab0 - S i l . B e c k l e r and White (172) found one I R band a t ca. 1890 cm-l f o r t h e A13+ and C r 3 + complexes and a t ca. 1900 cm-l f o r t h e Fe3+ complex, which compare f a i r l y w e l l t o t h e NO band observed a t 1890 cm-l on CuO/Si02 (173). The s i n g l e v i b r a t i o n a t 1890-1900 cm-l o f NO adsorbed on t h e supported complexes suggests t h a t o n l y one t y p e o f Lewis s i t e i s p r e s e n t . T h i s b e h a v i o u r c o n t r a s t s markedely w i t h t h e p a i r o f bands observed on Cr3+,
Fe2+ and Co2+ i o n s ( s e e p r e c e d i n g
s e c t i o n s ) . On t h e o t h e r hand, t h e absence o f m u l t i p l e I R a b s o r p t i o n can be t a k e n as c o n c l u s i v e o f t h e f a c t t h a t t h e c o o r d i n a t i v e l y s a t u r a t e d M3+ i o n (M=Al, C r , Fe) i s n o t a s i t e f o r NO a d s o r p t i o n , o n l y t h e CUS on Cu2+ b i n d s NO. The observed I R f r e q u e n c i e s a r e 14-24 cm-'
h i g h e r t h a n t h e v a l u e r e p o r t e d (1876 cm-')
for
gaseous NO ( 1 8 ) . I t has been proposed t h a t NO s t r e t c h i n g v i b r a t i o n s above 1850 cm-l i n d i c a t e a l i n e a r MNO o r i e n t a t i o n (174); t h u s i t i s i n f e r r e d t h a t NO i s bonded t o t h e Cu2+ s i t e i n t h e l i n e a r form. The CO probe can be a l s o used t o s t u d y t h e exposure o f Cunt
( 0 5 n 5 2) ions
i n supported copper c a t a l y s t s . T h i s probe i s t h e b e s t s u i t e d f o r c a t a l y s t s subj e c t e d t o r e d u c t i o n p r e t r e a t m e n t s , because t h e NO can be decomposed on reduced copper s i t e s ,
B128
I
2200
I
I
I
2100 2000 Wavenumber (cm-0
F i g . 2.22. I R spectra o f CO adsorbed by Cu2+ c a t i o n s on alumina; a ) 4 Nm-2 CO a t 295 K on outgassed sample a t 673 K; b ) on Hp reduced sample a t 523 K f o r 2h; c ) on p a r t i a l l y r e x i d i z e d sample b) by 1 N r 2 O2 a t 523 K; d ) on r e o x i d i z e d sample c) by 0.67 kNm-' O2 a t 523 K. CU;
+ 2 NO -CU-O,
+
N20
(2.22)
I t has been shown t h a t CO a d s o r p t i o n on outgassed CuO/A1203 c a t a l y s t s g i v e s r i s e t o a s i n g l e band a t 2128 cm-'
(175) due t o CO-Cu2+ species; t h i s carbonyl i s
very weak, s i n c e outgassing f o r 2 min removes t h e CO band completely: H2-reduct i o n a t 523 K f o l l o w e d by CO a d s o r p t i o n a t room temperature shows a g a i n a s i n g l e band which i s l e s s i n t e n s e t h a n i n t h e unreduced p r e p a r a t i o n and s h i f t e d t o 2114 cm-l ( F i g . 2.22,
spectrum b ) . Dosing oxygen t o t h e reduced c a t a l y s t r e s -
t o r e s t h e p o s i t i o n o f t h e o r i g i n a l band, however, t h e loss o f i n t e n s i t y i s n o t recovered ( F i g . 2.22,
spectrum d ) . The observed r e d - s h i f t can be taken as con-
c l u s i v e evidence t h a t reduced copper s i t e s a r e generated on H2-reduction. I n add i t i o n , t h e i n t e n s i t y decrease o f t h e CO band i n t h e r e o x i d i z e d p r e p a r a t i o n s i n d i c a t e s an increase i n CuO c r y s t a l l i t e s i z e .
B129 2.7. REFLECTION-ABSORPTION SPECTROSCOPY 2.7.1.
E x t e r n a l R e f l e c t i o n Spectroscopy
A b s o r p t i o n o f CO on s i n g l e c r y s t a l s has been s u c c e s s f u l l y s t u d i e d by 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 spectroscopy (RAIRS) methods (176-178). One o f t h e f i r s t p i e c e s o f r e s e a r c h u s i n g t h e R AI R S t e c h n i q u e was conducted by Chesters e t a l . ( 1 7 0 ) , who s t u d i e d t h e a d s o r p t i o n o f CO on a s i n g l e c r y s t a l o f Cu assembled i n an UHV c e l l and equipped f o r s u r f a c e p o t e n t i a l measurements. On Cu ( 1 1 1 ) these a u t h o r s found a s i n g l e band a t 2085 cm-',
whose i n t e n s i t y growth a t t a i n e d
up t o t h e maximum s u r f a c e p o t e n t i a l . T h i s band was, however, q u i t e d i f f e r e n t from t h e one observed w i t h copper f i l m s o r copper-supported p r e p a r a t i o n s (21002105 c m - l ) . F o r t h e h i g h e r i n d e x faces, namely Cu (IIO), Cu (211), Cu (311), and Cu (755), both i n f r a r e d frequency and s u r f a c e p o t e n t i a l d a t a were i n b e t t e r agreement w i t h t y p i c a l p o l y c r y s t a l l i n e Cu, and l e d t o t h e c o n c l u s i o n t h a t s t e p ped Cu s u r f a c e s o r h i g h index Cu p l a n e s a r e p r e d o m i n a n t l y exposed i n f i l m s and i n copper-supported c a t a l y s t s . A s l i g h t l y d i f f e r e n t p i c t u r e emerges, however, when magnesia i s used as a c a r r i e r f o r t h e Cu phase. F o r t h e Cu/MgO system, t h e CO band appears a t 2080 cm-'
i n b o t h t r a n s m i s s i o n (pressed d i s k ) and r e f l e c t i o n
( f i l m ) s p e c t r a . These r e s u l t s a r e c o n c l u s i v e w i t h r e s p e c t t o t h e i m p o r t a n t r o l e p l a y e d by t h e c a r r i e r i n a metal supported c a t a l y s t , which determines t o some e x t e n t , through t h e n u c l e a t i o n o f p a r t i c l e growth, t h e o r i e n t a t i o n o f t h e c r y s t a l faces. Time r e s o l v e d F o u r i e r t r a n s f o r m RAIRS has been r e c e n t l y used by Chenery e t a l . (178) t o r e c o r d r e f l e c t i o n - a b s o r p t i o n s p e c t r a o v e r a w i d e s p e c t r a l range o f t h e d e s o r p t i o n o f CO f r o m Cu ( 1 1 1 ) and t h e d e p r o t o n a t i o n o f methanol on o x i d i z e d Cu (111) as a f u n c t i o n o f t h e d e s o r p t i o n temperature. They observed two bands: a weak a b s o r p t i o n band a t t r i b u t e d t o b r i d g i n g CO on t h e s a t u r a t e d surface, which disappears a t 120 K, and a second l i n e a r CO s t r e t c h i n g band which becomes sharper
and i n c r e a s e s i n i n t e n s i t y t o a maximum a t 146 K; t h e l i n e a r CO absorp-
t i o n band then d e c r e a s i n g i n i n t e n s i t y and a l m o s t d i s a p p e a r i n g a t 185 K. The i n t e g r a t e d a b s o r p t i o n o f t h e two bands i s g i v e n as a f u n c t i o n o f t h e temperature i n F i g . 2.23a. By d i f f e r e n t i a t i n g t h i s curve, Chenery e t a l . (178) o b t a i n e d a " d e s o r p t i o n r a t e " curve, which i s comparable t o t h e i n f o r m a t i o n o b t a i n e d i n temperature-programmed d e s o r p t i o n experiments ( F i g . 2.23b). The d e s o r p t i o n r a t e t h u s c a l c u l a t e d i s o n l y v a l i d i n t h a t coverage r e g i o n , where absorbance v a r i e s l i n e a r l y w i t h coverage, as f o r t h e s t a g e o f l i n e a r CO coverage corresponding t o t h e peak i n t h e d e s o r p t i o n curve. From t h e a n a l y s i s of t h i s d e s o r p t i o n spectrum an a d s o r p t i o n h e a t o f 50 k J m o l - I was c a l c u l a t e d , which i n t u r n i s i n good agreement w i t h t h a t o f 61 kJ m o l - I o b t a i n e d f r o m s u r f a c e potent i a l i s o s t e r e s ( 1 7 9 ) . T h i s method o f m o n i t o r i n g CO d e s o r p t i o n has t h e advantage o f o b s e r v i n g t h e CO species and d i f f e r e n t i a t i n g i t s a d s o r p t i o n a t d i f f e r e n t
B130
Temperature (KI
F i g . 2.23. a) I n t e r a t e d absorption ( A ) o f CO bands versus temperature. Res o l u t i o n was 4 cm-?, and spectra o f one scan r a t i o e d against 1000 scan background, f o r CO desorbing from Cu (111). b) Change o f i n t e g r a t e d absorption w i t h temperature (-dA/dT) o f CO desorbing from Cu (111). Readapted from r e f . (178). sites. Another i n t e r e s t i n g example i s the decomposition o f methanol on an o x i d i z e d Cu (111) face. The R A I R S bands observed by Chenery e t a l . (178) f o r m u l t i l a y e r adsorption o f methanol on the preoxidized Cu (111) face w i t h 200 Langmuirs O2 a t room temperature are summarized i n Table 2.12, as compared t o those o f a methoxide species, and assigned by t a k i n g as a reference t h e methanol gas-phase v i b r a t i o n s . Note t h a t the g r e a t number o f bands seen i n t h e methanol spectrum i s TABLE 2.12. The assignment o f i n f r a r e d bands o f methanol and methoxy species
Assignment
Frequency (cm-l)
Mu1t i layered
Methoxide
CH30H gas (180)
CH30H i c e (RAIRS)
(RAIRS)
1030
1050
1112
1126
1036( very strong)
1470
1458
2060 2837
2833
2816 2882 29 18
2942 2940 3670
2945
B131 due t o t h e low symmetry o f t h e molecule, which c o n t r a s t w i t h t h e l e s s complex spectrum o f t h e methoxy species, as t h i s l a t t e r species adopts a CgV symnetry. The desorption o f methanol i n i t i a l l y leads t o a decrease i n i n t e n s i t y o f a l l bands begining a t ca. 135 K, u n t i l a s i g n i f i c a n t change occurs i n t h e temperat u r e range o f 150-165 K, which may be c o r r e l a t e d w i t h t h e d e p r o t o n a t i o n o f methanol ( c f . scheme) t o form t h e methoxy species. The C-0 s t r e t c h i n g band a t
(methanol )
1043 cm-'
iH3 (methoxy species) 0
y 3 OH
i s t h e s i n g l e band observed a t 150 K, and presumably corresponds t o
about one monolayer o f methanol. On i n c r e a s i n g t h e temperature a sharper band a t 1036 cm-l appears, w h i l e t h e C-0 s t r e t c h i n g band decreases i n i n t e n s i t y and simultaneously new bands i n t h e C-H s t r e t c h i n g v i b r a t i o n a r e recorded. T h i s new spectrum i s , t h e r e f o r e , associated w i t h t h e methoxy species, whose i n t e n s i t y remains unchanged up t o ca. 300 K, when t h i s species decomposes i n t o CO and H2, as p r e v i o u s l y r e p o r t e d by Madix (181). An i n t e r e s t i n g example o f s u r f a c e r e a c t i o n s has r e c e n t l y been r e p o r t e d by Hoffman and Robbins (182), who s t u d i e d whether CO d i s s o c i a t i o n d u r i n g t h e w e l l accepted methanation r e a c t i o n ( c f . 2.23a-c) i s t h e r a t e - d e t e r m i n i n g s t e p and
co + * -
CO,
Cs t 4H
CH
9
-
0, t 2H - H
-
cs
+
os
49
0 2 9
(2.23a) (2.23b) (2.23~)
whether H2 promotes t h i s r e a c t i o n . The CO s t r e t c h i n g a b s o r p t i o n band f o r t h e absorbed molecule on Ru (001) was s t u d i e d under methanation c o n d i t i o n s , w i t h pure CO (2.5 T o r r ) and H2:C0 m i x t u r e s (2.5 T o r r each). I f pure CO i s used, react i o n temperatures as h i g h as 650 K a r e r e q u i r e d t o observe a decrease i n C-0 s t r e t c h i n g i n t e n s i t y , thus i n d i c a t i n g d i s s o c i a t i o n o f CO. I n t h e presence o f t h e H2:C0 m i x t u r e , t h e r e a c t i o n r e q u i r e s a lower r e a c t i o n temperature, i . e . 500 K, and occurs o b v i o u s l y on a s h o r t e r t i m e scale. The dependence of CO coverage on t h e r e a c t i o n temperature shows t h a t CO d i s s o c i a t i o n i s a c t i v a t e d on Ru (001) w i t h an apparent a c t i v a t i o n energy o f 84 kJ mole-'.
The t u r n - o v e r frequencies
o f ~ x ~ O - ~ S f- o' r CO a t 650 K and 3 ~ 1 0 - ' s - ~ c a l c u l a t e d f o r t h e H2:C0 m i x t u r e c l e a r l y i n d i c a t e t h a t t h e presence o f H2 increases t h e d i s s o c i a t i o n r a t e o f CO by about two o r d e r s o f magnitude.
B132
The e f f e c t of H2 i n the d i s s o c i a t i o n o f CO was explained (182) as due t o the r e a c t i o n o f hydrogen w i t h the adsorbed oxygen ( r e a c t i o n 2 . 2 3 ~ ) a r i s i n g from dissociated CO t o form molecular water which i s the major r e a c t i o n product i n the methanation r e a c t i o n over Ru (001). A t h i g h temperatures both CO dissociat i o n and recombination can occur, b u t the presence o f H2 removes adsorbed oxygen (equation 2 . 2 8 ~ ) v i a water formation t o prevent recombination o f Cs and 0, (reversal o f equation 2.28a) i n favour o f CO d i s s o c i a t i o n . The decrease i n t h e i n t e n s i t y o f CO i s e s s e n t i a l l y due t o carbon formation on t h e surface, because, i n the absence o f H2, CO d i s p r o p o r t i o n a t i o n occurs, i.e.,
2 C O S e COZg
+ Cs.
This r e a c t i o n scheme has been confirmed by oxygen t i t r a t i o n o f the remaining surface carbon, which shows features i n the TPD spectrum c h a r a c t e r i s t i c o f carb i d i c carbon, which i s considered a r e a c t i v e form o f carbon i n the methanation o f CO (183). 2.7.2.
I nternal Ref 1ec t i on Spectroscopy I n t e r n a t i o n a l r e f l e c t i o n spectroscopy (IRS) i s another complement o f t h e
more usual external r e f l e c t i o n spectroscopy (RAIRS). A s depicted i n F i g . 2.5a and b, t h e i n f r a r e d r a d i a t i o n approaches the sol id-gas i n t e r f a c e through the s o l i d . I f the surface i s f r e e o f adsorbed molecules t h e i n f r a r e d r a d i a t i o n w i l l be completely r e f l e c t e d i n t o t h e s o l i d , when t h e incidence angle exceeds the c r i t i c a l angle. This f a c t makes t h e technique very d e s i r a b l e t o use w i t h mult i p l e r e f l e c t i o n s . I t r e s u l t s a l s o advantageous over o t h e r spectroscopic r e f l e c t i o n methods, since i t allows f o r a s e n s i t i v i t y increase by incrementing the number o f r e f l e c t i o n s , t y p i c a l l y i n the order o f several hundred (184). As occurs w i t h the RAIRS technique, the i n c i d e n t and r e f l e c t e d waves superimpose and form a standing e l e c t r i c f i e l d which has i t s maximum value a t the i n t e r f a c e and extends i n a l l the three spacial d i r e c t i o n s . This c h a r a c e r i s t i c can, therefore, be e x p l o i t e d t o determine the o r i e n t a t i o n o f adsorbed molecules on f l a t surfaces o r surfaces o f o r i e n t e d c r y s t a l s . Thus, IRS proves t o be adequate f o r the study o f adsorption on s i n g l e c r y s t a l oxides. The adsorption o f n-pentanol on t h r e e c r y s t a l faces o f u-A1203 was i n v e s t i g a t e d by Rice and H a l l e r (185), who found strong c o r r e l a t i o n s between the i s o s t e r i c adsorption heat and t h e coordin a t i o n unsaturation o f A1 i o n p a i r s i t e s , and a l s o between the number o f adsorbed pentanol molecules and t h e number o f A1 ions on t h e i d e a l surface. These observations were taken as evidence t h a t pentanol forms a surface pentoxide on vacant surface oxide s i t e s , each o f these being discoordinated w i t h A1 ions a t the surface. E r l e y e t a l . (186) have i d e n t i f i e d a very low CO s t r e t c h i n g band a t ca. 1520 cm-’ assigned t o steps on N i s i n g l e c r y s t a l s which probably r e s u l t from i n t e r a c t i o n w i t h t h e surface a t both ends o f the molecule, both species being precursors t o CO d i s s o c i a t i o n and accepted as an important step i n the methanation reaction.
B133 I t i s i m p o r t a n t t o n o t e t h a t t h e i n f r a r e d s t u d y o f b l a c k m a t e r i a l s , e.g. P t b l a c k , carbon, e t c . ,
p r e s e n t g r e a t d i f f i c u l t i e s . One o f t h e s e c o n c e r n i n g t h e
passage o f r a d i a t i o n through a t h i n l a y e r o f t h e s o l i d can be c i r c u m v e n t e d by u s i n g I R S techniques (187). However, I R S i s d i f f i c u l t and sometimes u n p r e d i c t a b l e , and t h e d a t a a r e r e l a t i v e l y poor, t h u s r e s t r i c t i n g i t s use. The d i f f i c u l t i e s encountered w i t h I R S measurements can be p a r t i a l l y overcome by t h e use o f i n f r a r e d p h o t o a c o u s t i c spectroscopy (PAS). 2.8.
PHOTOACOUSTIC SPECTROSCOPY (PAS) An i m p o r t a n t problem w i t h t h e i n v e s t i g a t i o n o f b l a c k c a t a l y s t s by conven-
t i o n a l t r a n s m i s s i o n I R i s t h e severe l o s s o f t r a n s m i s s i o n s i g n a l i n t e n s i t y , even f o r v e r y t h i n p r e p a r a t i o n s . The FTIR-PAS t e c h n i q u e has been s u c c e s s f u l l y used t o examine opaque supported c a t a l y s t s ( 1 8 8 ) . One i n t e r e s t i n g example o f t h e a p p l i c a t i o n o f PAS has been g i v e n by McGovern e t a l . ( 5 5 ) who s t u d i e d t h e a d s o r p t i o n o f CO on P t b l a c k and on P t / /A1203 c a t a l y s t s . F o r P t - b l a c k t h e y found a l a r g e narrow a b s o r p t i o n f e a t u r e a t 2057 cm-l and another, much l e s s i n t e n s e a t 422 cm-',
superimposed on t h e broad
b l a c k body spectrum due t o t h e gas-phase n i c k e l c a r b o n y l , N i ( C 0 ) 4 , formed f r o m t h e r e a c t i o n w i t h t h e s t a i n l e s s s t e e l o f t h e c e l l . I n no case
t h e spectrum f o r
CO adsorbed on P t - b l a c k c o u l d be recorded. T h i s r e s u l t i n d i c a t e s one o f t h e
l i m i t a t i o n s o f PAS i n t h e s t u d y o f massive c a t a l y s t s , as t h e s t r o n g a b s o r p t i o n t h r o u g h o u t t h e s p e c t r a l r e g i o n , r e s u l t e d i n s a t u r a t i o n o f t h e PAS s i g n a l gen e r a t e d by t h e s o l i d . The a d s o r p t i o n o f CO on H2-reduced alumina-supported P t c a t a l y s t s gave r i s e t o a band a t 2030 cm-',
due t o t h e C-0 s t r e t c h i n g v i b r a t i o n
o f CO adsorbed on P t atoms, and a n o t h e r band a t ca. 1590 cm-',
u s u a l l y as-
s o c i a t e d w i t h t h e f o r m a t i o n o f coke on a c i d s i t e s o f t h e A1203 c a r r i e r . Support f o r t h i s assignment a l s o stems f r o m a FTIR s p e c t r o s c o p i c s t u d y on coke f o r m a t i o n d u r i n g t h e c r a c k i n g o f 1-hexene o v e r HY z e o l i t e c a t a l y s t s , which showed a simi l a r a b s o r p t i o n band a t ca. 1585 cm-l growing d u r i n g t h e r e a c t i o n ( 1 8 9 ) . PAS s t u d i e s o f p y r i d i n e adsorbed on Mo- and Co-Mo/A1203 c a t a l y s t s were conducted by Riseman e t a l . (190) t o r e v e a l t h e presence o f B r i j n s t e d and Lewis a c i d s i t e s on c a l c i n e d and s u l p h i d e d molybdenum-based h y d r o t r e a t i n g c a t a l y s t s . The PA spectrum f o r c a l c i n e d Mo/A1203 showed a band a t 1541 cm-',
indicating that
i n c o r p o r a t i o n o f Mo6' i o n s o n t o t h e alumina s u r f a c e generates BrBnsted (BPY) a c i d i t y . Other bands a t 1451, 1616 and 1622 cm-',
a t t r i b u t a b l e t o Lewis (LPY)
a c i d i t y , were a l s o observed. The spectrum changed s u b s t a n t i a l l y , when Co was i n c o r p o r a t e d i n t h e c a l c i n e d Mo/A1203 c a t a l y s t . The LPY band a t 1622 cm-l was g r e a t l y a t t e n u a t e d w i t h r e s p e c t t o t h a t a t 1616 cm-',
which was s h i f t e d t o
lower wavenumbers( 1612 cm-l). The o r i g i n o f t h i s band was e x p l a i n e d i n terms o f p y r i d i n e c h e m i s o r p t i o n on Co2'
i o n s . The BPY a c i d i t y was a l s o a t t e n u a t e d ,
i n d i c a t i n g t h a t p a r t o f t h e Co i n c o r p o r a t e d m o d i f i e s t h e n a t u r e o f t h e Mo phase.
B134
J
300
I
1
I
I
I
1700 1500 Wavenumber (cm-1)
F i g . 2.24. P y r i d i n e adsorbed on g o e t h i t e . a ) t r a n s m i s s i o n spectrum o f p y r i d i n e adsorbed on g o e t h i t e (17 m2g-1); b ) PBD spectrum o f t h e ame sample; c ) PBD spectrum o f p y r i d i n e adsorbed on a low s u r f a c e area ( 9 m g-1) g o e t h i t e sample. The p o s i t i o n s o f t h e Lewis (LPY) coordinated p y r i d i n e a r e shown. Readapted from r e f . (56).
3
Another i n t e r e s t i n g f e a t u r e o f t h e PAS spectrum o f t h e Co-Mo/A1203 c a t a l y s t was t h e o b s e r v a t i o n o f a broad a d s o r p t i o n a t ca. 1365 cm-l, which was absent i n t h e spectrum o f t h e parent Mo/A1203 c a t a l y s t , thus i n d i c a t i n g
i t s oriqin
from
Co species associated w i t h t h e c a r r i e r . Using d i f f e r e n t pure reference compounds, Riseman e t a1
. (190)
concluded t h a t Co-aluminate l i k e phase seems t h e
most l i k e l y e x p l a n a t i o n from these r e s u l t s . The PAS spectrum o f p y r i d i n e adsorbed on s u l p h i d e d Mo/A1203 and Co-Mo/A1203 c a t a l y s t s were e s s e n t i a l l y s i m i l a r ; no absorbances due t o BPY being unamb i g u o u s l y i d e n t i f i e d . T h i s r e s u l t i s somewhat s u r p r i s i n g because t h i s t y p e of c a t a l y s t u s u a l l y y i e l d s r e a c t i o n products s i m i l a r t o those found i n t y p i c a l B r h s t e d a c i d c a t a l y s t s . I t must be s t r e s s e d t h a t t h e l a c k o f d e t e c t a b l e Bronst e d a c i d i t y on t h e sulphided p r e p a r a t i o n s r e s i d e s i n t h e p r e c o n d i t i o n i n g o f t h e samples p r i o r t o PAS examination. As a l r e a d y shown by Ramachandran and Massoth (191) t h e r e i s an enchancement i n t h e c r a c k i n g o f 1-hexene when H2S i s added t o
B135 t h e r e a c t o r feed, w h i c h can be e x p l a i n e d as due t o t h e g e n e r a t i o n o f p r o t o n s from t h e h e t e r o l y t i c d i s s o c i a t i o n o f H2S, c o n d i t i o n s f a r f r o m t h o s e used d u r i n g p r e t r e a t m e n t s f o r PAS measurements. 2 .Y. PHOTODEFLECTION BEAM SPECTROSCOPY (PDBS) The r e l a t i v e l y new technique o f i n f r a r e d photothermal d e f l e c t i o n s p e c t r o s copy (IR-PDBS) was r e c e n t l y d e s c r i b e d by Low and coworkers ( 1 9 2 ) , as w e l l as exaniples o f i t s a p p l i c a t i o n t o s t u d y t h e s u r f a c e p r o p e r t i e s o f m a t e r i a l s d i f f i c u l t o r i m p o s s i b l e t o examine by c o n v e n t i o n a l methods. The PDBS t e c h n i q u e was used by M o r t e r r a e t a l . ( 5 5 ) t o r e c o r d t h e i n f r a r e d s p e c t r a o f monolayers o f p y r i d i n e adsorbed on T i 0 2 , a-FeO(OH), S i O E and A1203 which absorb i n f r a r e d r a d i a t i o n weakly, and o f pyrene adsorbed on medium and h i g h temperature carbons, which a r e v e r y s t r o n g I R absorbers. I n g e n e r a l , t h e r e s u l t s i n d i c a t e t h a t , w i t h s o l i d s o f t h e f o r m e r group, t h e d e t e c t i o n o f adsorbed p y r i d i n e species by PDBS i s s i g n i f i c a n t l y worse t h a n by t h e c o n v e n t i o n a l t r a n s m i s s i o n - a b s o r p t i o n technique. To i l l u s t r a t e t h i s , i n F i g . 2.24 t h e I R s p e c t r a , i n t h e c o n v e n t i o n a l t r a n s m i s s i o n - a b s o r p t i o n mode, o f an a-FeO(0H) ( g o e t h i t e ) pigment sample (spectrum a ) o f r e l a t i v e l y low s u r f a c e area ( 1 7 m2g-l) b e a r i n g adsorbed p y r i d i n e , i s compared t o t h a t o f t h e same m a t e r i a l , examined under t h e same c o n d i t i o n s , w i t h t h e PDBS t e c h n i q u e (spectrum b ) . The comparison 1 shows t h a t t h e i n t e n s i t i e s o f t h e s r u f a c e p y r i d i n e modes i n t h e 1600-1400 cms p e c t r a l range w i t h r e s p e c t t o t h e i n t e n s i t i e s o f t h e p a i r o f b u l k h y d r o x y l groups o v e r t o n e and combination modes 1780 and 1650 cm-l ( 1 9 3 ) a r e g r e a t e r i n t h e t r a n s m i s s i o n - a b s o r p t i o n t h a n i n t h e PDB spectrum. W i t h t h e l a t t e r , t h e same a u t h o r s were a b l e t o d e t e c t t h e 8a-8b band o f adsorbed p y r i d i n e o n l y by u s i n g t h e spectrum o f t h e u n t r e a t e d sample as a r e f e r e n c e . Poorer s p e c t r a were r e corded w i t h a g o e t h i t e sample o f an even l o w e r s u r f a c e area ( 9 m2g-l) ( F i g . 2.24,
spectrum c j , f o r which t h e s t r o n g e s t p y r i d i n e bands was i n t h e o r d e r o f
noise, so t h a t t h e PDB spectrum proved t o be u s e l e s s . M o r t e r r a e t a1
. (56)
have a l s o s t u d i e d t h e a d s o r p t i o n o f pyrene on s e v e r a l
carbons. They used t h i s probe owing t o t h e l a c k o f ' p y r i d i n e a d s o r p t i o n e i t h e r on medium (CF18 and P500) and s t r o n g (CF18 da and P700) carbon absorbers. CF18 was a carbon prepared by c h a r r i n g a p h e n o l i c r e s i n a t ca. 773 K. A f t e r vacuum h e a t i n g a t ca. 1273 K i t was termed CF18 da. P500 was a c h a r o b t a i n e d by h e a t i n g p u r e c e l l u l o s e a t ca. 773 K i n vacuum; P700 s i m i l a r l y prepared a t 973 K. The two medium carbon absorbers a r e i n t e r m e d i a t e temperature carbons and s t i 11 c o n t a i n numerous f u n c t i o n a l groups. These m a t e r i a l s have a narrow band gap which l e a d s t o a continuum a b s o r p t i o n o v e r t h e e n t i r e I R range. When t h e s e carbons a r e f u r t h e r heated a t h i g h e r temperatures, t h e number o f f u n c t i o n a l groups decreases, t h e H/C r a t i o d e c l i n e s r a p i d l y , and t h e continuum a b s o r p t i o n i n c r e a s e s . Whatever t h e carbon type, t h e y absorb so s t r o n g l y t h e I R r a d i a t i o n t h a t even i n t h e
B136
I
'000
I
1
1500
3000
I
1
1000
1
,
SO(
2000 1000 Wavenumber (cm-11
F i g . 2.25. Pyrene adsorbed on carbons. a) s o l i d pyrene; b ) pyrene adsorbed on carbon CF18; c ) pyrene adsorbed on P500; d ) pyrene adsorbed on P700; e ) pyrene adsorbed on carbon CF18da. I n each case t h e spectrum was compensated u s i n g t h e r e s p e c t i v e s t a n d a r d carbon. Readapted f r o m r e f . ( 5 6 ) . t h i n n e s t l a y e r s t h e y cannot be s t u d i e d by t h e c o n v e n t i o n a l t r a n s m i s s i o n - a b s o r p t i o n mode. F i g u r e 2.25 shows t h e PDB s p e c t r a o f pyrene adsorbed on t h e f o u r carbon samples, u s i n g t h e s p e c t r a o f each c o u n t e r p a r t p r i o r t o pyrene a d s o r p t i o n as a r e f e r e n c e . F o l l o w i n g t h i s procedure, t h e PDB s p e c t r a o f CF18, P500 and P700 samples become c l e a r . The spectrum o f t h e CF18da sample, however, was p o o r e r whatever t h e r e f e r e n c e used t o compensate t h e carbon s i g n a l . The f a c t t h a t t h i s l a t t e r carbon i s a s t r o n g a b s o r b e r p o i n t s t o t h e d i f f i c u l t i e s i n h e r e n t i n t h e n o r m a l i z a t i o n process (194, 195), w h i c h a r e encountered, where s p e c t r a o f v e r y
B137
s t r o n g I R absorbers a r e concerned. I n s h o r t , t h e s e r e s u l t s i n d i c a t e t h a t weak a b s o r p t i o n s o f an adsorbate can b e d e t e c t e d on t o p o f v e r y s t r o n g absorbers, b u t as t h e absorbers become b l a c k e r , t h e r e s u l t s become p o o r e r . I t i s a l s o apparent t h a t , a l t h o u g h some f e a t u r e s o f t h e spectrum o f adsorbed pyrene a r e c l e a r l y v i s i b l e , t h e pyrene bands i n t h e 1600-900 cm-l were n o t d e t e c t e d . The l a c k o f d e t e c t i o n o f these bands i s n o t w e l l understood,and f u r t h e r work i s needed t o s u p p l y an e x p l a n a t i o n . 2.10 CONCLUSION
The most common approach a p p l i e d t o t h e s t u d y o f v i b r a t i o n a l modes o f adsorbed molecules on c a t a l y s t s u r f a c e s i s I R spectroscopy. The message which should be d e r i v e d f r o m t h i s b r i e f r e v i e w i s t h a t among t h e v a r i o u s i n f r a r e d techniques (transiiiission-absorption, r e f l e c t i o n - a b s o r p t i o n ,
emission, PAS, PDBS),
which i n p r a c t i c e have been used t o s t u d y t h e chemical s t a t e of adsorbed molecules, t h e t r a n s m i s s i o n - a b s o r p t i o n t e c h n i q u e remains p r o m i n e n t . None o f t h e i n f r a r e d techniques a r e recommended,if t h e problem t o be i n v e s t i g a t e d can be answered by u s i n g c o n v e n t i o n a l t r a n s m i s s i o n - a b s o r p t i o n spectroscopy. A1 1 o f them should be viewed as complementary. By v i r t u e o f t h e s p e c i a l advantages o f sens i t i v i t y , r e s o l u t i o n , e t c . , a g i v e n t e c h n i q u e may be a p p l i e d t o a p a r t i c u l a r combination o f adsorbate and c a t a l y s t . From a r e v i e w o f t h e most r e c e n t p u b l i c a t i o n s , i t i s e v i d e n t t h a t c a r e f u l a n a l y s i s o f t h e I R s p e c t r a o f adsorbed s i m p l e molecules, such as CO and NO, p r o v i d e r e l e v a n t i n f o r m a t i o n on t h e s u r f a c e o f t r a n s i t i o n metal o x i d e ( o r c a r b o n y l ) c a t a l y s t s . The f o r m a l o x i d a t i o n s t a t e , t h e degree o f c o o r d i n a t i o n unsatur a t i o n and t h e c o o r d i n a t i o n symmetry can be r e v e a l e d . I n any case, i t must be ensured t h a t surface s i t e s a r e n o t o x i d i z e d b y t h e adsorbate, e s p e c i a l l y by NO, d u r i n g t h e t e s t o f c h e m i s o r p t i o n . A t t e n t i o n must be p a i d t o t h e f a c t t h a t , w h i l e t h e q u a l i t a t i v e i n f o r m a t i o n r e s u l t s with confidence, q u a n t i t a t i v e i n f o r m a t i o n i s r a r e l y achieved. Therefore, s u r f a c e s i t e d e n s i t y d e r i v e d f r o m absorbance d a t a must be used w i t h c a u t i o n . The s t r o n g dependence o f t h e s u r f a c e p r o p e r t i e s on c a t a l y s t petreatments i s a n o t h e r i m p o r t a n t f a c t o r t o be considered. The d e n s i t y o f s u r f a c e s i t e s , t h e c o n f i g u r a t i o n o f t h e adsorbed molecules and t h e CUS s i t e s can be a f f e c t e d . Hence, p r o p e r comparison o f e x p e r i m e n t a l r e s u l t s among r e s e a r c h e r s r i g u r o u s l y r e q u i r e s t h e same c a t a l y s t p r e t r e a t m e n t . F i n a l l y , t h e p o t e n t i a l o f I R i s p r o b a b l y c o n f i n e d t o t h e i n s i t u combinat i o n o f p r a c t i c a l c a t a l y s t s a c t u a l l y o p e r a t i n g under t r a n s i e n t c o n d i t i o n s and f a s t reponse FTIR spectrometers, which can r e v e a l m e c h a n i s t i c d e t a i l s w i t h i n a very short timescale.
B138 GLOSSARY OF ACRONYMS AND ABREVIATIONS AES
Auger e l e c t r o n spectroscopy
EELS
E l e c t r o n energy loss spectroscopy
ER
External r e f 1e c t i o n
ES
Emission spectroscopy
FWHM
F u l l w i d t h a t h a l f maximum
RAIRS
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 spectroscopy
LEED
Low energy e l e c t r o n d i f f r a c t i o n
EXAFS
Extended X-ray a b s o r p t i o n f i n e s t r u c t u r e
PAS
Photoacustic spectroscopy
PDBS
Photodefl e c t i o n beam spectroscopy
SIRS
Surface i n f r a r e d spectroscopy
SVS
Surface v i b r a t i o n spectroscopy
UHV
U1 t r a h i g h vacuum
UPS
U1t r a v i o l e t p h o t o e l e c t r o n spectroscopy
XPS
X-ray p h o t o e l e c t r o n spectroscopy
CIR
C y l i n d r i c a l i n t e r n a l r e f 1 ectance
SFSE
C r y s t a l f i e l d s t a b i l i z a t i o n energy
DRS
D i f f u s e r e f l e c t a n c e spectroscopy
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7 8 9 10 11 12 13 14 15 16 17
18 19
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.
B139 20 21 22 23 24 25 26 27 28 29 30 31 32
34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
50 51 52 53 54 55
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B145 Chapter 3
ELECTRON VIBRATIONAL SPECTROSCOPY A.i.1.
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Departamento de F i s i c a de l a M a t e r i a Condensada C-111, U n i v e r s i d a d Aut6noma de Madrid, 26049 idadrid ( S p a i n ) . 3.1. INTRODUCTION
The i n t e r a c t i o n o f e l e c t r o n s w i t h m a t t e r g i v e s a v e r y v a l u a b l e i n f o r m a t i o n about i t s s t r u c t u r e , b o t h atomic, e l e c t r o n i c and v i b r a t i o n a l . S i n c e t h e d i s covery o f t h e wave n a t u r e o f e l e c t r o n s by Davisson and Germer i n 1925 ( l ) , e l e c t r o n d i f f r a c t i o n has been e x t e n s i v e l y used t o c h a r a c t e r i z e t h e a t o m i c s t r u c t u r e o f s u r f a c e s . Low Energy E l e c t r o n D i f f r a c t i o n (LEED) c o n t i n u e s t o be a t t h e s e t i m e s t h e most p r a c t i c a l way t o o b t a i n t h a t i n f o r m a t i o n . The surface s e n s i t i v i t y i s produced by t h e s t r o n g i n t e r a c t i o n o f t h e low energy e l e c t r o n s w i t h t h e c r y s t a l l a t t i c e ( 2 ) . T h i s f a c t i s a l s o r e s p o n s i b l e f o r t h e most s e r i o u s drawback o f t h e technique, because m u l t i p l e s c a t t e r i n g methods a r e needed i n o r d e r t o i n t e r p r e t c o r r e c t l y t h e e x p e r i m e n t a l d a t a . An a d d i t i o n a l problem a r i s e s f r o m t h e small v a l u e o f t h e coherence l e n g t h o f e l e c t r o n s f o r d i f f r a c t i o n ( w t h e d i a m e t e r o f t h e i n c i d e n t LEED beam (
100 A ) ,
~ mm) 1 and t h e i r r e g u l a r n a t u r e o f most
s u r f a c e s t r u c t u r e s a t t h i s s c a l e . As a consequence o f a l l t h e s e s e v e r a l f a c t o r s , comparison o f LEED d a t a w i t h models, has been o f l i m i t e d success. I n s p i t e o f t h a t , LEED has been l a r g e l y used t o o b t a i n t h e p e r i o d i c l a t t i c e o f s u r f a c e s t r u c t u r e s and i n some cases i n f o r m a t i o n on a d s o r p t i o n s i t e s , bonding
l e n g t h s , o r i e n t a t i o n o f t h e adsorbate w i t h r e s p e c t t o t h e s u r f a c e can be obt a i n e d . The l a b e l - l i n g o f s u r f a c e s t r u c t u r e s from LEED d a t a i s g e n e r a l l y used ( 3 ) and r e p r e s e n t s a mandatory s t a r t i n g p o i n t f o r any s u r f a c e s t u d y . T h i s c h a p t e r d e a l s w i t h t h e use o f E l e c t r o n Energy Loss Spectroscopy (ELLS) t o o b t a i n t h e v i b r a t i o n a l s t a t e o f species chemisorbed on m e t a l s . H i s t o r i c a l l y , s c a t t e r i n g o f l o w energy e l e c t r o n s by v i b r a t i o n s o f adsorbates on a s u r f a c e was f i r s t demonstrated by P r o p s t and P i p e r i n 1967. The main problem a t t h a t t i m e was t h e l a c k of r e s o l u t i o n o f e l e c t r o n spectrometers l i m i t e d a t 50 meV. Improvement on r e s o l u t i o n was i n c r e a s i n g i n t h e 1970s where an i m p o r t a n t e f f o r t was pursued by H. Ibach ( 4 ) . Nowadays t h e t e c h n i q u e has reached i t s n a t u r e s t a t e being spread o v e r t h e w o r l d and h a v i n g a c u r r e n t r e s o l u t i o n below t h e 5 meV range. T h i s t o g e t h e r w i t h a v e r y h i g h s e n s i t i v i t y has p l a c e d EELS as a v e r y i m p o r t a n t t o o l i n S u r f a c e Science. Much can be l e a r n e d on c h e m i s o r p t i o n of m o l e c u l a r o r a t o m i c s p e c i e s b y t h e knowledge o f t h e v i b r a t i o n a l frequencies of t h e s e e n t i t i e s . The i n f o r m a t i o n i s
B146 n o t l i m i t e d t o t h e adsorbates b u t i n c l u d e s t h e metal s u r f a c e as w e l l . We w i l l t r e a t e x c l u s i v e l y adsorbates, however. The s t u d y o f t h e v i b r a t i o n a l motion o f molecules g i v e s a very u s e f u l i n f o r m a t i o n about i t s s t r u c t u r e . I n molecular chemistry, t h i s i s c u r r e n t l y o b t a i n e d by I n f r a r e d and Raman Spectroscopy o f molecules which i s o f g r e a t h e l p t o r e s o l v e t h e i r s t r u c t u r e . The i n f o r m a t i o n about t h e v i b r a t i o n o f a molecule i s o f several types. For example, i f we have a d i a t o m i c molecule, t h e v i b r a t i o n a l movement can be t r e a t e d as i f we had a s i n g l e mass o f value t h e reduced mass 11 o f t h e molecule s u b j e c t t o a r e s t o r a t i o n f o r c e which can be t r e a t e d as a r e s t o r a t i o n s p r i n g o f constant k . T h i s harmonic o s c i l l a t o r has a frequency R = Jk/p, and from t h e measurement and supposed t h a t t h e n a t u r e o f t h e molecule i s known, i n f o r m a t i o n i s ob-
of
t a i n e d about k. The chemical bond between t h e atoms o f t h e molecule i s o f t e n equated t o t h e s p r i n g c o n s t a n t o f t h e harmonic o s c i l l a t o r . T h i s i s n o t e x a c t l y t r u e because i n f a c t t h e f o r c e constant measures t h e c u r v a t u r e of t h e p o t e n t i a l w e l l , r a t h e r than t h e depth. I f we have a polyatomic molecule, a d d i t i o n a l v i b r a t i o n s occur and t h e s i t u a t i o n i s more complicated. Any complicated m o t i o n can be r e s o l v e d i n t o t h e s o - c a l l e d normal modes o f v i b r a t i o n . I n t h e case o f gas-phase molecules, t h i s has been s t u d i e d and a v e r y complete l i t e r a t u r e e x i s t s . From t h e v i b r a t i o n a l spectrum we g e t t h e number o f normal modes and t h e i r frequency. T h i s i s due t o t h e f a c t t h a t i f a molecule has symmetry, t h e normal v i b r a t i o n s have c e r t a i n symmetry p r o p e r t i e s and as a consequence considerable s i m p l i c a t i o n s i n t h e d e t e r m i n a t i o n o f t h e normal v i b r a t i o n s i s brought about. I f we consider now molecules adsorbed on a surface, i m p o r t a n t p r o p e r t i e s
have t o be added a t t h e general view described above. The more i m p o r t a n t p o i n t i s t h e change o f t h e molecular s t r u c t u r e due t o t h e bonding t o t h e surface, which i n many cases leads even t o i t s d i s s o c i a t i o n . T h i s i s i n f a c t what we want t o know. There a r e a l s o o t h e r s u b t l e changes such as those r e l a t e d w i t h t h e l o w e r i n g o f t h e symmetry imposed by t h e surface.. The o t h e r i m p o r t a n t p o i n t connected w i t h v i b r a t i o n a l spectroscopy r e f e r s t o t h e a c t i v i t y o f these normal modes. In I n f r a r e d and Raman v i b r a t i o n a l spectra, any motion which i s connected w i t h a change i n d i p o l e moment leads t o t h e emiss i o n o r a d s o r p t i o n o f r a d i a t i o n . During t h e v i b r a t i o n a l motion t h e charge d i s t r i b u t i o n undergoes a p e r i o d i c change and t h e r e f o r e i n general t h e d i p o l e moment o s c i l l a t e s . So normal modes o f v i b r a t i o n l e a d i n g t o a change o f d i p o l e moment a r e c a l l e d i n f r a r e d a c t i v e because t h e frequencies i n v o l v e d appear i n t h e i n f r a r e d r e g i o n o f t h e l i g h t spectrum. The e x c i t a t i o n iiiechanism i n t h e case o f t h e e l e c t r o n probe desserves a careful examination. H i s t o r i c a l l y , d a t a on v i b r a t i o n a l modes came from CO adsorbed on d i f f e r e n t m e t a l l i c s u b s t r a t e s , I n t h i s case and due t o t h e s t r o n g v i b r a t i n g d i p o l e o f t h e CO molecule, i t was assumed t h a t t h e e l e c t r o n i n t e r a c t s
B147 w i t h t h e v i b r a t i o n v i a t h e induced d i p o l e inasmuch as I R l i g h t e x c i t a t i o n does. L a t e r on, i t was seen t h a t f o r o t h e r systems l i k e hydrogen adsorbates, new exc i t a t i o n mechanisms s h o u l d be invoked. The e x i s t e n c e o f a d d i t i o n a l e x c i t a t i o n mechanisms c e r t a i n l y complicates t h e s i t u a t i o n , b u t , i n s p i t e o f t h a t complicat i o n , a more complete c h a r a c t e r i z a t i o n o f t h e system i s o b t a i n e d . Thus, t h e s t u d y o f v i b r a t i o n a l e x c i t a t i o n mechanisms by t h e e l e c t r o n p r o b e has t a k e n an i n c r e a s i n g importance i n EELS. T h a t i s t h e reason why we t r e a t e x t e n s i v e l y t h i s p o i n t i n S e c t i o n 3.2. S e c t i o n 3.3 i s e n t i r e l y devoted t o t h e a n a l y s i s o f t h e v i b r a t i o n a l spectrum o f adsorbates on m e t a l l i c s u r f a c e s . We f i r s t d i s c u s s t h e importance o f symmetry groups i n o r d e r t o o b t a i n t h e maximum i n f o r m a t i o n f r o m EELS d a t a . We s t r e s s t h e f a c t t h a t a s u r f a c e produces s p e c i f i c changes on symmetry o p e r a t i o n s , which a r e o f c r u c i a l importance. A f t e r t h i s i n t r o d u c t i o n , we t r e a t t h e s i m p l e s t case o f atomic a d s o r p t i o n , by making s p e c i a l emphasis on hydrogen a d s o r p t i o n . Both t h e s i m p l e n e a r e s t neighbour s p r i n g model, and t h e more e l a b o r a t e d t o t a l - e n e r g y c a l c u l a t i o n s a r e d e s c r i b e d , so t h a t t h e reader has a good f e e l i n g o f t h e p r e s e n t s t a t e o f t h e a r t . We a l s o t r e a t q u i t e e x t e n s i v e l y oxygen a d s o r p t i o n b o t h a t o m i c and m o l e c u l a r . Carbon monoxide i s t h e most p o p u l a r adsorbate by EELS experiment a l i s t s . The enormous amount o f r e s u l t s cannot c e r t a i n l y be covered by t h e p r e s e n t r e v i e w . Thus, a f t e r an h i s t o r i c a l i n t r o d u c t i o n we d e s c r i b e t h e case of CO on P t ( l l 1 ) as an i l l u s t r a t i v e example. E x c e p t i o n s t o t h i s g e n e r a l v i e w a r e , however, t r e a t e d . I n p a r t i c u l a r , we d e s c r i b e t h e l o w CO s t r e t c h i n g frequency systems e i t h e r due t o t h e t r a n s i t i o n metal element o r t o t h e presence o f a l k a l i ads o r p t i o n . We go on w i t h p o l y a t o m i c adsorbates, i n p a r t i c u l a r w a t e r and u n s a t u r a t e d hydrocarbons. These systems show c l e a r l y t h e i n t e r e s t o f t h e EELS probe i n s u r f a c e c h e m i s t r y . We choose e t h y l e n e as a p a r t i c u l a r case which a l s o p r o v i d e s an example o f s u r f a c e r e a c t i o n which has been e x t e n s i v e l y s t u d i e d i n t h e p a s t . T h i s s e c t i o n , r a t h e r t o r e p o r t t h e enormous amount o f d a t a e x i s t i n g i n t h e l i t e r a t u r e , i n t e n d s t o g i v e a f u l l and comprehensive d e s c r i p t i o n about t h e way t h e r e l e v a n t i n f o r m a t i o n i s e x t r a c t e d f r o m EELS d a t a . The examples a r e s e l e c t e d e i t h e r by i t s i l l u s t r a t i v e c h a r a c t e r , by i t s n o v e l t y o r because o f i t s i m p o r t a n ce i n s u r f a c e Chemistry. 3.2.
THE EXCITATION OF VIBRATIONS BY THE ELECTRON PROBE
I n t h i s s e c t i o n we w i l l t r e a t t h e o r e t i c a l l y t h e way e l e c t r o n s e x c i t e s u r f a c e v i b r a t i o n s . T h i s i n f o r m a t i o n i s v e r y i m p o r t a n t i n o r d e r t o a s s i g n and i n t e r p r e t c o r r e c t l y t h e v i b r a t i o n a l spectrum. S c h e m a t i c a l l y , we can c o n s i d e r t h r e e l i m i t i n g cases t o d e s c r i b e v i b r a t i o n a l e x c i t a t i o n by e l e c t r o n s : ( i ) D i p o l e s c a t t e r i n g , ( i i ) Impact s c a t t e r i n g , ( i i i ) Resonant s c a t t e r i n g . D i p o l e s c a t t e r i n g i s due t o t h e e l e c t r o n i n t e r a c t i o n w i t h t h e t i m e - v a r y i n g e l e c t r i c f i e l d a s s o c i a t e d w i t h t h e a d s o r b a t e v i b r a t i o n . The
B148 more i m p o r t a n t p h y s i c a l p r o p e r t y o f d i p o l e s c a t t e r i n g i s i t s l o n g range charact e r . T h i s i s due t o t h e f a c t t h a t t h e s c a t t e r i n g a n g l e i s s m a l l , and as a consequence, t h e i n e l a s t i c c r o s s s e c t i o n f o r d i p o l e s c a t t e r i n g i s s t r o n g l y peaked a l o n g t h e f o r w a r d d i r e c t i o n . I n an EELS experiment, due t o t h e r e f l e c t i o n b y t h e s u r f a c e , f o r w a r d s c a t t e r i n g produces an i n e l a s t i c i n t e n s i t y s t r o n g l y c o n c e n t r a t e d around t h e s p e c u l a r d i r e c t i o n . I n t h e impact s c a t t e r i n g regime t h e e l e c t r o n e x c i t e s v i b r a t i o n s v i a t h e s h o r t range a t o m i c p o t e n t i a l . Thus, t h e dependence o f t h e i n e l a s t i c c r o s s sect i o n on t h e s c a t t e r i n g a n g l e i s c o m p l e t e l y d i f f e r e n t . We e x p e c t n o n - n e g l i g i b l e o u t o f s p e c u l a r i n t e n s i t y due t o d i f f u s e s c a t t e r i n g . Resonant s c a t t e r i n g
ik
t h e more common process i n e l e c t r o n - m o l e c u l e s c a t -
t e r i n g i n t h e gas phase where t h e v i b r a t i o n a l e x c i t a t i o n i s enhanced v i a t h e f o r m a t i o n o f r e s o n a n t n e g a t i v e i o n s t a t e s due t o t h e e l e c t r o n c a p t u r e i n t h e l o w e s t - l y i n g u n o c c u p i e d o r b i t a l o f t h e molecule. I t i s q u i t e e v i d e n t t h a t r e s o n a n t s c a t t e r i n g wi.11 g i v e i n e l a s t i c i n t e n s i t y o u t o f s p e c u l a r t o o . I n a d d i t i o n t o t h a t , t h e energy dependence o f t h e i n e l a s t i c c r o s s s e c t i o n i s q u i t e charact e r i s t i c . Thus, i t i s i m p o r t a n t t o measure t h e v i b r a t i o n a l spectrum as a f u n c t i o n o f t h e p r i m a r y energy. T h i s w i l l denote t h e k i n d o f e x c i t a t i o n mechanism, b u t more i m p o r t a n t , i t can g i v e i n s i g h t on t h e s t r u c t u r e o f t h e m o l e c u l e . The key p o i n t about d i p o l e s c a t t e r i n g is t h e narrow a n g u l a r d i s t r i b u t i o n o f i n e l a s t i c i n t e n s i t i e s . T h i s has been d i s c u s s e d by I b a c h ( 5 ) u s i n g t h e f o l l o w i n g argument. The p r i m a r y e l e c t r o n sees t h e d i p o l e p o t e n t i a l s e t up by t h e v i b r a t i o n a t a c o n s i d e r a b l e d i s t a n c e f r o m t h e s u r f a c e . T h i s comes o u t by expanding t h e d i p o l e p o t e n t i a l i n two-dimensional waves. A c c o r d i n g t o L a p l a c e e q u a t i o n , t h e c o n t r i b u t i o n f r o m a s c a t t e r i n g wave v e c t o r p a r a l l e l t o t h e s u r f a c e o f v a l u e (6Q)II
extends above t h e s u r f a c e a d i s t a n c e z = ( 6 Q l l ) - l
. Thus,
contributions
from l o n g wavelenqths e x t e n d q u i t e f a r i n t o t h e vacuum. F o r example, a t y p i c a l z value, c a l c u l a t e d f o r t h e d i p o l e a s s o c i a t e d t o t h e C-0 s t r e t c h i n g mode, e x c i t e d by e l e c t r o n s o f energy 6 eV a t an a n g l e o f i n c i d e n c e o f 70'.
i s z = 39
8.
Now imagine an e l e c t r o n approaching t h e s u r f a c e w i t h p r i m a r y energy E and wavevector Q ( F i g . 3 . l a ) . The i n t e r a c t i o n w i t h t h e d i p o l e p o t e n t a1 w i l l s c a t t e r t h e e l e c t r o n i n t o a s t a t e o f energy E l = E
-
. Now,
- f1s2 and
wavevector Q
the condition f o r excitation o f the vibration where 6 t i s t h e i n t e r a c t i o n t i m e . 6 t can be c a l c u l a t e d as = Q'(1
Qll
s i n c e (6Q
il)-'
.
Thus, S Q l i = s (6t)ii0 % 1
i s t h e r e g i o n where t h e d i p o l e p o t e n t i a l extends up. vo i s t h e
v e l o c i t y o f the electron.
As we w i l l see t h e argument l e a d s t o s m a l l s c a t t e r i n g a n g l e s . I n t h a t case we can w r i t e ( s e e F i g u r e ( 3 . l b ) .
B149
where v i s t h e s c a t t e r i n g angle. Therefore
and t h e c o n d i t i o n ( 6 t ) f l o a 1 t r a n s f o r m s i n t o
F i g . 3 . l . ( a ) S c a t t e r i n g geometry o f t h e p r i m a r y e l e c t r o n by t h e surface v i b r a t i o n . A p r i m a r y e l e c t r o n c h a r a c t e r i z e d by t h e i n c i d e n t wave v e c t o r Q i s s c a t t e r e d i n e l a s t i c a l l y i n t o wave v e c t o r Q ' . The a n g l e o f i n c i d e n c e i s 8 . 9 and Q ' a r e decomposed i n t o p a r a l l e l and p e r p e n d i c u l a r t o t h e s u r f a c e components, Q = 911 t k , Q ' = Q ' I I t k ' . ( b ) S c a t t e r i n g a n g l e u f o r a p a r t i c u l a r s i t u a t i o n showing t h a t 6 Q 11 m Qw
.
I n f a c t , a more d e t a i l e d t h e o r y would g i v e u = (tWo)/2E. T h i s produces a c o n d i t i o n f o r u , v a 1
in most o f t h e u s u a l e x p e r i m e n t a l
c o n d i t i o n s . F o r example, i f we t a k e a t y p i c a l v a l u e f o r t h e C-0 s t r e t c h i n g f r e quency (Tic2 = 0.26 eV) and a p r i m a r y energy o f 5 eV, vs = 0 . 2 6 / ( 2 x 5) = 1.5".
B150 A f t e r t h i s q u a l i t a t i v e e x p l a n a t i o n , l e t us go now t o a more q u a n t i t a t i v e d e s c r i p t i o n o f t h e d i f f e r e n t e x c i t a t i o n mechanisms. We w i l l make s p e c i a l emphas i s on t h e t h e o r y o f d i p o l e s c a t t e r i n g , and t h e consequences about t h e energy and a n g u l a r dependence o f i n e l a s t i c i n t e n s i t i e s . 3.2.1.
The t h e o r y of d i p o l e s c a t t e r i n g The i n t e r a c t i o n o f e l e c t r o n s w i t h a m o l e c u l e adsorbed on a m e t a l s u r f a c e
has been e x t e n s i v e l y t r e a t e d . We a r e m o s t l y i n t e r e s t e d on t h e a n g u l a r dependence o f such i n t e r a c t i o n . T h i s problem has been i n i t i a l l y worked o u t b y Sokcevic e t a l . ( 6 ) and we w i l l f o l l o w h i s way o f r e a s o n i n g . The system i s d e s c r i b e d by means o f a h a m i l t o n i a n H = K t U t H v i b +
V
which c o n t a i n s t h e e l e c t r o n k i n e t i c energy K , t h e p o t e n t i a l o f t h e m e t a l l i c s u r f a c e U, t h e m o l e c u l a r v i b r a t i o n a l modes Hvib
and t h e i n e l a s t i c t e r m V which
a r i s e s f r o m t h e e l e c t r o n c o u p l i n g t o t h e d i p o l a r f i e l d induced by t h e v i b r a t i o n . The geometry of t h e experiment i s d e s c r i b e d i n F i g . 3 . l a .
Q and Q ' a r e t h e
symbols f o r t h e i n c i d e n t and s c a t t e r e d e l e c t r o n wave v e c t o r s . The p o t e n t i a l U o f t h e m e t a l l i c s u r f a c e produces a s p e c u l a r l y r e f l e c t e d wave c h a r a c t e r i z e d by an a m p l i t u d e f a c t o r R and a phase s h i f t 6. As i t i s normal i n problems c o n t a i n i n g a s u r f a c e , t h e wave v e c t o r s a r e d i v i d e d i n t o p a r a l l e l and normal t o t h e s u r f a c e components. T h e r e f o r e we w r i t e
The m o l e c u l a r v i b r a t i o n i s approximated b y a p o i n t d i p o l e l o c a t e d a t a d i s t a n c e zo above t h e s u r f a c e . The i n t e r a c t i o n between t h e i n c i d e n t e l e c t r o n and t h e v i b r a t i n g m o l e c u l e i s d e s c r i b e d by t h e l o n g range d i p o l e p o t e n t i a l
The f i r s t t e r m comes f r o m t h e permanent d i p o l e o f t h e m o l e c u l e and c o n t r i butes o n l y t o t h e e l a s t i c s c a t t e r i n g . The second t e r m i s due t o t h e o s c i l l a t i n g d i p o l e i n t h e d i r e c t i o n ko, where s i s t h e v i b r a t i o n
normal c o o r d i n a t e
i s t h e p o s i t i o n v e c t o r o f t h e e l e c t r o n w i t h r e s p e c t t o t h e d i p o l e . We r e s t r i c t
o u r s e l v e s t o t h e s i n g l e harmonic v i b r a t i o n o f f r e q u e n c y no. M i s t h e reduced mass o f t h e v i b r a t i o n .
B151
I n o r d e r t o t r e a t t h e problem we c o n s i d e r two t r a n s i t i o n m a t r i x elements: The v i b r a t i o n a l one corresonds t o t h e t r a n s i t i o n between t h e ground s t a t e 10> + and t h e s i n g l y e x c i t e d s t a t e 112 = a lo>. I t s v a l u e i s
The m a t r i x element f o r t h e s c a t t e r i n g o f t h e e l e c t r o n i s g i v e n b y
With t h e s e a p p r o x i m a t i o n s t h e d i f f e r e n t i a l i n e l a s t i c s c a t t e r i n g c r o s s s e c t i o n f o r a single loss i s
where S i s t h e area p e r adosrbed molecule, 8 i s t h e i n c i d e n t angle, Scos
e is
t h e e f f e c t i v e area p e r adsorbed molecule, E ' = E-ROO, edu/ds i s c a l l ed t h e dynamic e f f e c t i v e charge which i s t h e most i n t e r e s t i n g p h y s i c a q u a n t i t y e n t e r i n g the i n e l a s t i c cross section. I f we use r e f l e c t e d p l a n e waves, t h e s c a t t e r i n g a m p l i t u d e i n c l u d e s f o u r
c o n t r i b u t i o n s which a r e s c h e m a t i c a l l y r e p r e s e n t e d i n F i g . 3.2.
(bl
R
F i g . 3.2. The f o u r c o n t r i b u t i o n s t o t h e s c a t t e r i n g a m p l i t u d e . R and R ' a r e t h e r e f l e c t i v i t y c o e f f i c i e n t s o f t h e s u r f a c e f o r e l e c t r o n s b e f o r e and a f t e r t h e scattering
.
O n l y events b and c a r e connected with a small s c a t t e r i n g angle. a ) i s t h e o n l y process which would o p e r a t e f o r a m o l e c u l e i n t h e gas phase.
B152 Due t o t h e extension o f t h e d i p o l e p o t e n t i a l above t h e c r y s t a l governed by t h e term exp(-KZ), where K = k
-
k ' , t h e s c a t t e r i n g amplitudes a r e a f f e c t e d by
t h e f o l l o w i n g terms M(a)
--- [QII +
i(k+k')l-l
M(b)
--- [QII t
i(k-k')I-'
#(c)
--- [QII -
i(k-k')I-'
M(d)
--- [Q11
-
i(ktk')]-l
Using these d i f f e r e n t c o n t r i b u t i o n s we can now work o u t t h e m a t r i x t r a n s i t i o n elements. This can be done more e a s i l y assuming t h a t t h e r e f l e c t i v i t y and phase s h i f t s do n o t change a f t e r s c a t t e r i n g by t h e d i p o l e : R = R ' and 6 = 6'. I n a d d i t i o n we consider t h e h i g h r e f l e c t i v i t y l i m i t R = 1. Then t h e terms a t d g i v e (zifo)
=
f / j (2,)
*
Q/[K2 + ( k + k ' ) 2 ] = Q/i[K2
[Ke-KZO
*
+ (k+k')2]
-
[-Ke-KzO
2 ( k + k ' ) sen ( k + k ' ) z o ]
+
2K Cos ( k + k ' ) z 0 ]
and t h e terms b + c
*
f i ( z o ) = Q/[K2 + ( k - k ' ) 2 ]
fll
(zo) = Q/i[K2 + ( k ' - k ) 2 1
[Ke-KZo
*
-
2 ( k ' - k ) sen ( k ' - k ) z o ]
[-Ke-KZo t 2Kcos ( k - k ' ) z o l
Now, when t h e molecule i s adsorbed on t h e metal surface, t h e conduction e l e c t r o n s i n t h e metal w i l l screen t h e e l e c t r i c f i e l d of t h e v i b r a t i n g d i p o l e . normal o r p a r a l -
- 0
0
0
+;;
:+; ..,
0 ..,....
I-.
:-. .., . . I
-
I01
I bl
F i g . 3.3. A d i p o l e placed near t h e s u r f a c e produces a r e d i s t r i b u t i o n o f t h e charge which i s e q u i v a l e n t t o p l a c e an image d i p o l e . The f i g u r e shows how t h e image d i p o l e produces a d o u b l i n g o f t h e d i p o l a r moment when t h e d i p o l e i s normal t o t h e surface (panel a ) , whereas when t h e d i p o l e i s p a r a l l e l , t h e d i p o l e moment cancels o u t .
B153 Thus, i f we t a k e i n t o account t h e image d i p o l e s , t h e c r o s s s e c t i o n can be o b t a i n e d t h r o u g h t h e expressions
T h i s g i v e s i n t h e l i m i t zo+ 0, i . e . f o r zO
and z0< I k
k'1-I
f l ( t o t a 1 ) = 2f1(z0)
fll
(total) = 0 T h i s corresponds t o t h e metal s e l e c t i o n r u l e which s t a t e s t h a t o n l y molecu-
l a r v i b r a t i o n s p r o d u c i n g a v a r i a t i o n o f t h e d i p o l e moment p e r p e n d i c u l a r t o t h e s u r f a c e w i l l be e x c i t e d by t h e i n c i d e n t beam. T h i s r u l e h o l d s f o r I R r a d i a t i o n too; even b e t t e r because zo<
A. I n t h e case o f e l e c t r o n e s we can c a l c u l a t e t h e
e f f e c t o f t h e image d i p o l e by working o u t t h e e x p r e s s i o n s f i and
fll
.
These r e s u l t s a r e t h e f o l l o w i n g . F o r terms a and d,
and f o r terms b and c
Both f
and
fII
a r e small i n (1) due t o t h e f a c t t h a t t h e denominators K 2 t
( k t k ' ) 2 a r e l a r g e . I n (1) and ( 2 ) t h e f a c t o r exp (-Kzo)-exp(Kzo) s t r o n g decrease o f f / I ,
produces a
We can g i v e some numbers o f a p a r t i c u l a r case: E = 6 eV, ei = Or = 70°, hao = 68 meV z 0 = 0.7 f i 2 = 5.2 x
2
f i = 1.1
f o r processes a
x 105 f o r processes b
t
it
d
t
c
Thus we should c o n s i d e r o n l y processes b t c.
NOW, f o r processes b 2.39,
t
c, t h e a m p l i t u d e f o r t h e p a r a l l e l component i s f
f o u r o r d e r s o f magnitude l o w e r t h a n t h e p e r p e n d i c u l a r component.
11
2=
B154 The expressions g i v e t h e angular dependence too. We w i l l c a l c u l a t e t h a t f o r processes b and c. L e t us assume
E
= 6 eV, 0.,
= 70,
ei
= 65.3,
hR = 68 meV, zo = 0.7
ii
f i 2 = 76.3
t h r e e o r d e r s o f magnitude down by 5" o u t o f specular fll
= 5.03 x
lo-*
i t decreases too, although t h e dependence i s much lower.
These examples demonstrate unambiguously and q u a n t i t a t i v e l y t h e two f o l lowing statements: ( i ) Only v i b r a t i o n s producing a v a r i a t i o n o f d i p o l e moment p e r p e n d i c u l a r t o t h e s u r f a c e a r e e x c i t e d by t h e d i p o l e s c a t t e r i n g ( i i ) Going o f f - s p e c u l a r t h e i n t e n s i t y o f t h e v i b r a t i o n s e x c i t e d by t h e d i p o l e mechanism decreases very s h a r p l y . These two r u l e s a l l o w t o know about t h e EELS d i p o l e act i v i t y o f l o s s peaks. The t h e o r y o f d i p o l e s c a t t e r i n g can be a l s o used t o c a l c u l a t e t h e i n e l a s t i c i n t e n s i t y i n t h e specular c o n d i t i o n s , and subsequently c o r r e l a t e i t w i t h t h e experimental measurements. The t h e o r e t i c a l expression has been g i v e n by several authors. I w i l l be u s i n g t h e expressions o b t a i n e d by Sokcevic e t a l . ( 6 ) , da/dR = 1/(R2Scos 0 )
*
J(E'/E)
*
(m/M)
*
e*'/(Elino)
*
f i2
(3.3)
2 where f i a r e t h e s c a t t e r i n g amplitudes described b e f o r e . The o n l y non-negl i g i 2 2 b l e value o f f i i s f o r processes b ) and c ) . f i c o n t a i n s a l s o t h e amplitudes
due t o t h e image
dipole.
The more i m p o r t a n t q u a n t i t y t o be e x t r a c t e d from t h e use of t h i s expression i s e* = e(dv)/ds which represents t h e dynamic charge a c t i n g i n t h e v i b r a t i o n process. For t h e s t r e t c h i n g mode o f CO adsorbed on P t ( l l l ) , we o b t a i n e d a v a l u e o f e* = 0.54. I n t h e case o f CO adsorbed on Ni(100), Andersson e t a1 ( 7 ) o b t a i n
.
e* = 0.61. These values a r e not s u b s t a n t i a l l y d i f f e r e n t from t h a t corresponding t o f r e e CO which i s e* 0.64. which a l l o w s t o e l i m i n a t e R from Experimentally, we measure Iloss/Ielastic i s t h e number of s c a t t e r e d e l e c t h e t h e o r e t i c a l expressions. The measured Iloss t r o n s i n t e g r a t e d over t h e s o l i d angle subtended by t h e entrance s l i t of t h e analyzer. Expression ( 3 ) g i v e s a l s o t h e energy dependence o f t h e i n e l a s t i c specular i n t e n s i t y . Such dependence as p r e d i c t e d by t h e t h e o r y i s q u i t e weak. T y p i c a l experimental curves obtained by t h i s a u t h o r a r e presented i n F i g . 4. The data can
B155 be qua1 i t a t i v e l y understood by examining t h e e x p r e s s i o n d 2o/dRldE'. Two terms a r e r e s p o n s i b l e f o r t h e energy dependence. The t e r m 1/E, and t h e s c a t t e r i n g am2 p l i t u d e f a c t o r Ifi dn. The l a s t t e r m depends on hQ/E, becoming c o n s t a n t a t s m a l l . T h i s i s t h e reason why t h e c u r v e corresponding t o t h e C-0 s t r e t c h i n g mode shows a maximum a t E = 6 eV. I n t h e case o f t h e CO f r u s t r a t e d t r a n s l a t i o n a l mode (CO-metal s t r e t c h i n g ) , t h e maximum would appear a t an energy s m a l l e r t h a n t h a t r e p o r t e d on t h e curve. Thus, t h e d a t a i n t h i s case show o n l y a monotonous decrease. A s i m i l a r c u r v e has been presented by Anderson and Davenport ( 7 ) f o r CO adsorbed on N i .
o
2100crn-' band
LBOcm-!band
INELASTIC I N T E N S I T Y I I X P ~ R I M f N l A L I 0 INELASTIC I N T E N S I T Y UALCULATEOI
1
2
I
4
I
6
I
8
1
I
1
0
2
1
4
1
6
1
8
1
1
0
PRIMARY ENERGY kVI
Fig.3.4. I n e l a s t i c i n t e n s i t i e s c o r r e s p o n d i n g t o t h e -0 and C-metal s t r e t c h i n g bands of CO adsorbed on P t ( l l 1 ) a t 2100 and 480 cm- as a f u n c t i o n of p r i m a r y energy,from ( 8 ) . The d a t a a r e r e f e r r e d t o t h e e l a s t i c i n t e n s i t i e s .
f
A d e t a i l e d s t u d y o f t h e o u t o f s p e c u l a r i n e l a s t i c i n t e n s i t i e s has been p e r -
formed by t h i s a u t h o r on t h e system C O / P t ( l l l ) f o r b o t h C-0 and metal-C0 s t r e t c h i n g modes. The measured d a t a have been p l o t t e d and compared w i t h t h e express i o n s g i v e n above by t h e d i p o l e t h e o r y ( F i g . 5 ) . An i m p o r t a n t c o n c l u s i o n o f t h i s s t u d y i s t h a t t h e e x p e r i m e n t a l o f f - s p e c u l a r i n t e n s i t i e s a r e much l a r g e r t h a n those p r e d i c t e d by t h e d i p o l e t h e o r y . T h i s i s n o t s u r p r i s i n g , as d i p o l e s c a t t e r i n g i s an adequate d e s c r i p t i o n o n l y i n t h e l i m i t o f s m a l l s c a t t e r i n g a n g l e s . On t h e o t h e r hand, t h i s g i v e s an i d e a o f how i m p o r t a n t t h e o t h e r mechanisms a r e , s i n c e CO i s p r o b a b l y t h e most i n t e n s e d i p o l e s c a t t e r e r due t o i t s l a r g e dynamic e f f e c t i v e charge. The e x p e r i m e n t a l d a t a show t h a t t h e d i p o l e i n t e n s i t y i s about a f a c t o r 100 l a r g e r t h a n t h e n o n - d i p o l e one. As t h e d i p o l e i n t e n s i t y depends on e*2, and t h e v a l u e o f e* i s u O . 5 f o r CO, t h i s means t h a t when t h e dynamic charge i s a f a c t o r 10 s m a l l e r , i . e . 0.05, b o t h d i p o l e and n o n - d i p o l e mechanisms produce simil a r inelastic intensities,
B156
3 . 2 . 2 . Impact s c a t t e r i n q F o r l a r g e s c a t t e r i n g a n g l e s we a r e i n t h e i m p a c t s c a t t e r i n g regime and a t h e o r y has been developed by Tong e t a l . ( 9 ) . H i s t o r i c a l l y , t h e f i r s t experiment a l evidence about t h e importance o f l a r g e a n g l e s c a t t e r i n g e v e n t s was p r o v i d e d by t h e work o f Ho e t a1 . ( 1 0 ) who s t u d i e d t h e v i b r a t i o n a l modes o f H adsorbed on W ( l o o ) , and measured t h e i n t e n s i t i e s o f t h e v a r i o u s modes as a f u n c t i o n o f the angle o u t o f the specular d i r e c t i o n . A s i m i l a r experiment was performed by t h i s a u t h o r now a b o u t t h e v i b r a t i o n a l
modes o f H adsorbed on P t ( l l 1 ) . F o r t h e geometry o f t h i s e x p e r i m e n t we can deduce t h a t t h e e l e c t r o n i n t e r a c t s a t a d i s t a n c e zo = 4
8
f r o m t h e surface,
which i s an o r d e r o f magnitude l o w e r t h a n t h e v a l u e i n s p e c u l a r c o n d i t i o n s .
Fig.3.5. Angular p r o f i l e s o f t h e e l a s t i c and i n e l a s t i c i n t e n s i t i e s c o r r e s p o n d i n g t o t h e C-0 s t r e t c h i n g band a t 2100 cm-1, f r o m ( 8 ) . E p r i m a r y energy, E ' = E-hR, C( c u t o f f a n g l e o f t h e e l a s t i c beam. The d a t a show t h a t t h e measured o f f specul a r i n e l a s t i c i n t e n s i t i e s a r e almost a f a c t o r 10 h i g h e r t h a n t h o s e c a l c u l a t e d by u s i n g t h e d i p o l e t h e o r y .
The t h e o r y o f impact s c a t t e r i n g by Tong i s r e l a t e d w i t h t h e t h e o r y o f e l a s t i c s c a t t e r i n g o r LEED t h e o r y e x c e p t t h a t we have t o l e a v e t h e s u r f a c e atoms t h e freedom t o v i b r a t e . I f Ri(o) a r e t h e e q u i l i b r i u m p o s i t i o n s o f atoms, t h e v i b r a t i o n i s r e p r e s e n t e d by new p o s i t i o n s Ri = Ri(o)
+ ui
Then, t h e i n e l a s t i c c o n t r i b u t i o n f r o m v i b r a t i o n s e n t e r s t h r o u g h t h e d e r i vatives
B157
where f i s t h e s c a t t e r i n g a m p l i t u d e which depends p a r a m e t r i c a l l y on t h e atom positions. I n o r d e r t o c a l c u l a t e t h a t , Tong e t a l . use a p h y s i c a l p i c t u r e analogous t o t h a t employed f o r t h e o r e t i c a l a n a l y s i s o f l o w energy e l e c t r o n d i f f r a c t i o n and a n g u l a r r e s o l v e d photoemission. They use a p o t e n t i a l f o r t h e s u b s t r a t e and t h e adsorbate l a y e r o f t h e " m u f f i n t i n " t y p e . A s i m p l e e s t i m a t e o f t h e r e d u c t i o n o f t h e s p e c u l a r i n t e n s i t y by t h e adsorb a t e v i b r a t i o n i s g i v e n by t h e Debye-Waller f a c t o r
I b a c h ( 1 1 ) has e s t i m a t e d f r o m t h i s argument, t h e o r d e r o f magnitude expect e d f o r i n e l a s t i c s c a t t e r i n g f r o m t h i s approach. The d i f f e r e n t i a l c r o s s s e c t i o n is ~ / 4 ncx 1/4
do/dR
*
Q 2/ 2
* h/(2MR)
where h/2MR i s t h e z e r o p o i n t a m p l i t u d e u2 T h i s g i v e s a v a l u e da/dQ =
.
cm2Sr-l E w i t h E g i v e n i n eV.
F o r CO we measure a v a l u e of 4 ~ 1 0 - cm2Sr-' l~ a t E = 2 eV, which i s two o r d e r s o f magnitude b i g g e r . I n f a c t t h i s number i s even l a r g e r , ifwe t a k e i t from t h e e l a s t i c i n t e n s i t y which i s t h e a p p r o p r i a t e q u a n t i t y , t o be compared 2 -1 a c c o r d i n g t o ( 1 1 ) . For CO a t 9 eV i t g i v e s 2 5 . 4 ~ 1 0 - l cm ~ Sr
.
I n a d d i t i o n t o t h a t , t h e energy dependence f o r E o < 10 eV i s n o t d e s c r i b e d a p p r o p r i a t e l y . F o r impact s c a t t e r i n g , we e x p e c t a monotonous i n c r e a s e o f t h e d i f f e r e n t i a l cross s e c t i o n . T h i s i s n o t t h e case e x p e r i m e n t a l l y , s i n c e we g e t a monotonous decrease. Thus, we have t o conclude t h a t an i n e l a s t i c d i f f r a c t i o n t h e o r y does n o t d e s c r i b e a p p r o p r i a t e l y t h e a c t u a l s i t u a t i o n f o r l o w p r i m a r y energies 3.2.3.
.
Resonant s c a t t e r i n g The t h i r d p o s s i b i l i t y i s r e s o n a n t s c a t t e r i n g , v i a t h e f o r m a t i o n of a tem-
p o r a r i l y n e g a t i v e i o n . T h i s process r e a d i l y o c c u r s i n e l e c t r o n s c a t t e r i n g o f gas molecules. Such phenomena has been e x t e n s i v e l y reviewed by G . J .
Schultz (12),
and we r e f e r t o t h a t a r t i c l e f o r a more i n v o l v e d c o n s i d e r a t i o n . Data r e a d i l y e x i s t f o r the simplest molecules. The q u e s t i o n a r i s e s about t h e p r o b a b i l i t y f o r such process when t h e molecul e i s adsorbed on a s u r f a c e , a l t h o u g h some arguments can be advanced. An argument would i n d i c a t e t h a t t h e l a r g e a v a i l a b i l i t y o f s t a t e s p r o v i d e d by t h e metal s u r f a c e would t e n d t o decrease s t r o n g l y t h e n e u t r a l i z a t i o n 1 i f e t i m e , t h e r e f o r e d e s t r o y i n g t h e resonance. Another argument goes i n t h e o p p o s i t e d i r e c t i o n and
B158 t e l l s us t h a t by t a k i n g CO as example, a d s o r p 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 molecular s t r u c t u r e , and t h e corresponding process o f t h e gas phase should occur. There i s s t i l l another p o s s i b i l i t y . I t i s known from LEED s t u d i e s , and i t has been p r e d i c t e d t h e o r e t i c a l l y ,
t h a t e l e c t r o n s may be trapped on t h e surface,
i n a s t a t e o f t h e image p o t e n t i a l w e l l i n t h e d i r e c t i o n p e r p e n d i c u l a r t o t h e surface. These s t a t e s have a l s o r e l a t i v e l y l o n g l i f e t i m e s as can be decuded from t h e sharp e l e c t r o n energy w i d t h . The most i n t e r e s t i n g experiment t o know about resonant s c a t t e r i n g i s t h e measurement o f t h e energy dependence o f t h e i n e l a s t i c in t e n s i t y
. Anderson and
Davenport i n 1978 ( 7 ) r e p o r t e d t h e l o s s s p e c t r a o f CO on Ni(100) and OH on N i O ( l l 1 ) as a f u n c t i o n o f p r i m a r y e l e c t r o n energy. As we have a l r e a d y mentioned t h e d i p o l e i n t e r a c t i o n describes c o r r e c t l y t h e data f o r t h e CO molecule, b u t f a i l s f o r t h e OH group. I n a d d i t i o n , t h e overtone i n t e n s i t y i s much l a r g e r than t h e d i p o l e estimate. The authors suggest t h a t a n e g a t i v e i o n l o n g - l i v e d resonance c o u l d e x p l a i n t h e data. Data on CO were r e p o r t e d by t h i s author i n t h e case o f chemisorption on P t ( l l 1 ) ( 8 ) . The energy dependence was s t u d i e d by p l o t t i n g t h e l o s s i n t e n s i t y of t h e s t r e t c h i n g C-0 band (normalized t o a constant p r i m a r y i n t e n s i t y v a l u e ) , taken a t an o f f - s p e c u l a r angle v a l u e of 24" ( F i g . 6 ) .
wl
18
-
16
-
14 -
t
9, = 70'
be, =24O 21M)cm-'bond
1 '
',
\
wl
0
There i s a maximum a t 2 eV
2
L
6
PRIHARY ENERGY
8
10
lev1
F i g 3 . 6 . Absolute loss i n t e n s i t y o f t h e 2100 cm-l band corresponding t o t h e e x c i t a t i o n o f t h e C-0 s t r e t c h i n g mode o f CO adsorbed on P t ( l l l ) , from (8). The d a t a a r e measured o f f specular by 24". as a f u n c t i o n o f p r i m a r y energy. The i n t e n s i t i e s a r e normalized w i t h r e s p e c t t o t h e p r i m a r y beam c u r r e n t .
B159
and a monotonous decrease a t h i g h e r e n e r g i e s . The d i f f e r e n t i a l c r o s s s e c t i o n was measured t o be 0.04 A'Sr-l.
The f r e e - g a s v a l u e i s 4 A 2 S r - l ( 1 3 ) and t h e
c a l c u l a t e d v a l u e f o r an u p r i g h t o r i e n t e d CO m o l e c u l e i s 1 A 2 S r - l . The w i d t h o f t h e energy-dependent peak can be e s t i m a t e d t o be 9 eV ( 1 1 ) . T h i s would c o r r e s pond t o a l i f e t i m e o f 5 ~ 1 0 - sec. l ~ The f l i g h t t i m e o f a 2 e V - e l e c t r o n t h r o u g h a
co
m o l e c u l e i s 1 . 2 x 1 0 - l ~ sec. Demuth e t a l . ( 1 3 ) found evidence f o r r e s o n a n t s c a t t e r i n g i n t h e a d s o r p t i o n
o f N2, CO, 02, H2 on p o l y c r y s t a l l i n e CO a t 20 K. A l l t h e v i b r a t i o n a l d a t a show evidence o f resonant s c a t t e r i n g . The resonance energy i s reduced f r o m t h a t observed i n t h e gas phase, which g i v e s a d d i t i o n a l s u p p o r t t o t h e f o r m a t i o n o f a negative i o n state. An i n t r i n s i c adsorbate case i s hydrogen a d s o r p t i o n . Due t o t h e l o w e f f e c t i v e d i p o l e o f hydrogen (ek = 0.018 i n t h e case o f P t ) , impact s c a t t e r i n g becomes v e r y i m p o r t a n t . The energy dependence o f t h e e l a s t i c and i n e l a s t i c intens i t y has been measured by t h i s a u t h o r f o r o f f s p e c u l a r c o n d i t i o n s (Fig.3.71.The t h r e e curves show a s u b s t a n t i a l enhancement o f t h e i n t e n s i t y i n t h e 6-8eV range. The q u a l i t a t i v e t r e n d i s t h e same f o r t h e t h r e e curves a l t h o u g h small d i f f e r e n -
PlI1111t 60 L H, T.90K q - 8 , .IL*
1000
100 CS'
10
BEAM ENERGY IcVI
F i g . 3.7. E l a s t i c and i n e l a s t i c i n t e n s i t i e s (bands A and B) versus energy t a k e n 14" o u t o f specular, from ( 1 4 ) . A b s o l u t e l o s s i n t e n s i t i e s a r e p l o t t e d . Note, however, t h e l o g a r i t h m i c s c a l e i n t h e e l a s t i c p l o t . Bands A and B correspond t o t h e symmetric and asymmetric s t r e t c h i n g v i b r a t i o n s o f hydrogen adsorbed on t h e t h r e e f o l d s i t e o f P t ( l l 1 ) (see s e c t i o n 3 . 3 ) . ces can be d i s t i n g u i s h e d . We a l s o measured t h e s p e c u l a r r e f l e c t e d i n t e n s i t y as a f u n c t i o n o f p r i m a r y energy f o r c l e a n and hydrogen-covered s u r f a c e . There i s an i n t e r e s t i n g c o r r e l a t i o n which i s w o r t h t o n o t i c e . The maxima o f t h e i n e l a s t i c bands o c c u r a t e n e r g i e s where t h e r e f l e c t i v i t y is minimum ( F i g . 3 . 8 ) . These
B160
PHllll t 60L H1
PRIMARY BEAM ENERGY
lev1
F i g . 3.8. I n t e n s i t i e s o f t h e specular beam versus p r i m a r y energy I o o ( V ) f o r clean and hydrogen covered P h ( l l 1 ) surfaces from ( 1 4 ) . A, A, correspond t o two d i f f e r e n t experimental runs. The e l a s t i c i n t e n s i t y f o r t h e covered s u r f a c e shows two minima which correspond v e r y c l o s e l y w i t h t h e maxima o f t h e o u t o f specular i n t e n s i t i e s .
minima can be r e l a t e d w i t h t h e c o n d i t i o n o f emergency o f a new d i f f r a c t e d beam. I n t h e case o f H / P t ( l l l ) t h i s beam c o u l d be t h e ( 0 , l ) . I n o r d e r t o know t h e cond i t i o n f o r emergency o f t h a t beam, t h e plane o f i n c i d e n c e w i t h r e s p e c t t o t h e s u r f a c e l a t t i c e i s needed. We have estimated t h e t h r e s h o l d c o n d i t i o n f o r t h e emergency o f t h e ( 0 , l )
beam as a f u n c t i o n o f t h e azimuthal angle formed by t h e
s u r f a c e beam energy i s comprised between 6.8-9.4
eV, which i n c l u d e s t h e range
where t h e i n e l a s t i c cross s e c t i o n i s maximum. Erskine e t a l . (15) have a l s o measured t h e energy dependence of t h e i n e l a s t i c losses o f t h e
B1
H phase on W(OO1) and c o r r e l a t e d w i t h t h a t of t h e e l a s t i c
i n t e n s i t y . They f i n d t h a t t h e maximum on t h e i n e l a s t i c l o s s corresponds t o a minimum i n t h e e l a s t i c peak. T h i s minimum corresponds t o t h e k i n e t i c a l condit i o n f o r t h e (0,-1) LEED beam t o emerge from t h e s u r f a c e . The i n c r e a s e i n impact s c a t t e r i n g cross s e c t i o n can be associated w i t h d i f f r a c t e d e l e c t r o n s being temp o r a r i l y trapped i n s u r f a c e resonance s t a t e s (16). A s i m i l a r d e s c r i p t i o n has been g i v e n f o r atom s c a t t e r i n g ( 1 7 ) . 3.3. EELS VIBRATIONAL SPECTRA OF ADSORBED MOLECULES When a molecule i s adsorbed on a s u r f a c e i m p o r t a n t changes occur which i n fluence t h e v i b r a t i o n a l motion o f i t s atoms. The f i r s t change i s r e l a t e d w i t h t h e chemisorption o f t h e molecule, i . e . which t h e bonding t o t h e metal s u r f a c e . This can l e a d q u i t e o f t e n t o t h e d i s s o c i a t i o n o f t h e molecule. T h i s of course produces, i n t h e case o f d i a t o m i c molecules, t h e non-observation of t h e frequency o f t h e s t r e t c h i n g mode o f v i b r a t i o n . By t h e way, t h i s i s p r o b a b l y t h e most
B161 unambiguous d e m o n s t r a t i o n o f a t o m i c a d s o r p t i o n o f a molecule. Ift h e m o l e c u l e i s n o t d i s s o c i a t e d , we expect a l s o s i g n i f i c a n t changes due t o t h e bonding b r o u g h t about by t h e i n t e r a c t i o n o f t h e m o l e c u l e w i t h t h e metal s u r f a c e . Those w i l l change t h e frequency v a l u e o f some s p e c i f i c modes. I n t h e case of a d i a t o m i c molecule, t h e s t r e t c h i n g f r e q u e n c y w i l l change. T h i s g i v e s a v e r y v a l u a b l e i n f o r m a t i o n about t h e c h e m i s o r p t i o n process o f t h e molecule. Another more s u b t l e change concerns t h e number o f v i b r a t i o n a l modes assoc i a t e d w i t h an adsorbate species and t h e number o f those which can be observed by EELS. As p o i n t e d o u t i n t h e i n t r o d u c t i o n , t h i s i s connected w i t h t h e symmetry o f t h e adsorbate. Now, a d s o r p t i o n on a s u r f a c e produces an i m p o r t a n t l o w e r i n g o f symmetry due t o t h e f a c t t h a t t h e s u r f a c e does n o t a l l o w some symmetry elements t o occur. F o r example, an i n v e r s i o n c e n t e r i s n o t a l l o w e d . A n o t h e r consequence comes from t h e f a c t t h a t t h e symmetry elements have t o be c o n s i s t e n t w i t h t h e t r a n s l a t i o n a l symmetry c h a r a c t e r i s t i c o f a p e r i o d i c l a t t i c e . These two f a c t o r s reduce c o n s i d e r a b l y t h e number o f symmetry p o i n t groups o p e r a t i n g i n t h e case of an adsorbate bonded t o a metal s u r f a c e . These a r e C
P
w i t h p = 2 , 3, 4, 6 and
Cnv w i t h n = 1, 2, 3, 4, 6 ( n = 1 i s n o r m a l l y l a b e l l e d
P o i n t groups C
Cs).
a r e c h a r a c t e r i z e d by a s i n g l e symmetry a x i s o f o r d e r p
P which o f course has t o be p e r p e n d i c u l a r t o t h e s u r f a c e Cnv has an a x i s of symm e t r y o f o r d e r n ( p e r p e n d i c u l a r t o t h e s u r f a c e ) , and n symmetry planes around t h i s a x i s o f symmetry. Some examples o f t h e p o i n t groups showing t h e d i f f e r e n t elements o f symmetry a r e g i v e n i n F i g . 3.9. The s u r f a c e a1 so i n t r o d u c e s changes c o n c e r n i n g t h e number o f v i b r a t i o n a l modes which can occur. I f we have a f r e e m o l e c u l e o f N atoms, t h e number o f degrees o f freedom i s 3N. Three o f them correspond t o t h e t r a n s l a t i o n a l movement o f t h e whole molecule, another s e t o f t h r e e ( t w o i n t h e case of a l i n e a r molecul e ) correspond t o t h e r o t a t i o n a l movement o f t h e whole molecule, and t h e r e s t 3N-6 (3N-5) a r e v i b r a t i o n a l movements. T h i s means t h a t i n t h e gas phase we have a t o t a l o f 3N-6 (3N-5) normal modes o f v i b r a t i o n . Now when t h e m o l e c u l e i s adsorbed, i t s t r a n s l a t i o n a l and r o t a t i o n a l modes a r e impeded by t h e bonding t o t h e s u r f a c e , so t h e y t r a n s f o r m a c t u a l l y i n t o v i b r a t i o n a l modes. Thus, t h e t o t a l number o f v i b r a t i o n a l modes f o r a m o l e c u l e o f
N
atoms adsorbed on a s u r f a c e i s
3N. These d i f f e r e n t modes c a r r y a l s o a d i f f e r e n t i n f o r m a t i o n . The m o l e c u l a r v i b r a t i o n s g i v e i n f o r m a t i o n on t h e changes produced i n t h e m o l e c u l e by t h e chemis o r p t i o n process. The 6 ( 5 ) r e m a i n i n g modes i n v o l v e t h e movement o f t h e m e t a l s u r f a c e atoms ang g i v e i n f o r m a t i o n about t h e s t r e n g t h o f t h e a d s o r p t i o n process. They a r e d e s c r i b e d by some a u t h o r s as f r u s t r a t e d t r a n s l a t i o n s and r o t a t i o n s . S t r i c t l y speaking, through t h e i n t e r a c t i o n between m o l e c u l a r and s u b s t r a t e modes, m o l e c u l a r modes can have some m e t a l l i c c h a r a c t e r , and s u b s t r a t e modes can have some adsorbate c h a r a c t e r .
B162
C
C
F i g . 3.9. Examples of adsorbate-surface systems showing some o f t h e p o i n t groups which may occur. a) Atomic a d s o r p t i o n on t h e h o l l o w s i t e o f an hexagonal l a t t i c e i s C3., I f t h e a d s o r p t i o n i s on t o p t h e p o i n t group i s C6". N o t i c e t h a t due t o t h e s t a c k i n g o f t h e (111) planes, C6v o n l y occurs i f we consider e x c l u s i v e l y t h e s u r f a c e plane. C3 occurs when t h e adsorbate i s n o t symmetric w i t h r e s p e c t t o t h e symmetry elements of t h e surface. b ) P o i n t groups C4,, C z V and C z a s s o c i a t e d t o a square l a t t i c e . c ) P o i n t groups CzV and C, produced by a d s o r p t i o n on a r e c t a n gular l a t t i c e . The symmetry p o i n t group f o r a g i v e n adsorbate i s f i x e d by t h e symmetry o f t h e adsorbate and t h e symmetry o f t h e a d s o r p t i o n s i t e . With t h a t i n mind we can work o u t t h e poss b l e v i b r a t i o n a l modes and then a n a l i z e which of them w i l l be EELS a c t i v e .
I n order t o
e c i d e about t h e l a s t question, i t i s necessary t o know what
mechanism o f v i b r a t i o n a l e x c i t a t i o n i s a c t u a l l y o p e r a t i n g . I n t h e case o f d i p o l e e x c i t a t i o n t h e s i t u a t i o n i s simple. I n t h e gas phase, t h e i n f r a r e d a c t i v e modes are those which have a symmetry species c o i n c i d e n t a t l e a s t w i t h one o f t h e components o f t h e d i p o l e moment (18). I n t h e case o f adsorbed molecules, t h i s r u l e i s transformed i n t h e sense t h a t t h e EELS d i p o l e a c t i v e modes a r e those which have a symmetry species i n coincidence w i t h t h e normal component of t h e d i p o l e moment, i . e . w i t h a t r a n s l a t i o n a l movement p e r p e n d i c u l a r t o t h e surface. For t h e d i f f e r e n t symmetry p o i n t groups o p e r a t i n g i n t h e case o f an adsorbate bonded t o a surface, t h e species a r e A 1 f o r , ,C,
A ' f o r C,
and A f o r Cp ( 5 ) . As these a r e
t o t a l l y symnietric modes, t h i s leads t o t h e i m p o r t a n t r u l e t h a t o n l y t h e t o t a l l y
B163
symmetric v i b r a t i o n a l modes a r e EELS d i p o l e a c t i v e . From these c o n s i d e r a t i o n s , i t i s q u i t e c l e a r t h a t e x a m i n a t i o n o f t h e v i b r a t i o n a l spectrum can h e l p enormously t o t h e d e t e r m i n a t i o n o f t h e a d s o r p t i o n s i t e i f t h e d i p o l e s e l e c t i o n r u l e o p e r a t e s . I n o r d e r t o know a b o u t t h a t , and as we
p o i n t e d o u t i n S e c t i o n 3.2, an a n a l y s i s o f t h e e x c i t a t i o n mechanisms i s a l m o s t unavoidable. T h i s can be o b t a i n e d by p e r f o r m i n g a n g u l a r dependent measurements which i n c l u d e o f f s p e c u l a r c o n d i t i o n s . More d i f f i c u l t b u t a l s o v e r y u s e f u l i s t o determine t h e energy dependence o f t h e i n e l a s t i c i n t e n s i t i e s of t h e d i f f e r e n t modes. W i t h t h i s i n mind, we can now work o u t some s p e c i f i c examples o f adsorpt i o n . The s i m p l e s t case i s a t o m i c a d s o r p t i o n . 3.3.1.
Atomic A d s o r p t i o n T h i s i s an i m p o r t a n t case because many molecules w i l l d i s s o c i a t e a f t e r i n -
t e r a c t i o n w i t h t h e s u r f a c e l e a v i n g adsorbate atoms as a f i n a l p r o d u c t . The most i m p o r t a n t case i s p r o b a b l y hydrogen, because o f i t s i m p l i c a t i o n s i n many i n t e r e s t i n g r e a c t i o n s . I n a d d i t i o n hydrogen is d i f f i c u l t t o d e t e c t by t h e o t h e r more conimon s u r f a c e techniques 1 ike LEED, Auger E l e c t r o n Spectroscopy, e t c . When an atom i s adsorbed on a metal s u r f a c e i t can undergo t h r e e normal modes o f v i b r a t i o n which correspond t o t h e f r u s t r a t e d t r a n s l a t i o n a l modes o f t h e atom by t h e bonding t o t h e s u r f a c e . I t i s q u i t e e v i d e n t t h a t t h e symmetry p o i n t group i s imposed u n i q u e l y by t h e a d s o r p t i o n s i t e . A c e n t r a l f i r s t n e a r e s t neighbour f o r c e model has been e x t e n s i v e l y used t o c a l c u l a t e t h e frequency values o f t h e t h r e e v i b r a t i o n a l modes. The c a l c u l a t i o n can be done a n a l y t i c a l l y f o r t h e movement p e r p e n d i c u l a r t o t h e s u r f a c e and f o r t h o s e corresponding t o t h e movement p a r a l l e l t o t h e s u r f a c e . An example of such c a l c u l a t i o n i s g i v e n below and corresponds t o a d s o r p t i o n t o t h r e e s u b s t r a t e atoms w i t h a p o i n t symmetry group
The geometry i s d e s c r i b e d i n F i g . 3.10.
F i g . 3.10. Adsorbed atom t h a t s i t s on a t h r e e f o l d s i t e . a is t h e bond a n g l e . The f r e q i i e n c y o f t h e p a r a l l e l movement o f t h e adsorbate atom i s c a l c u l a t e d by moving t h e adsorbate a d i s t a n c e 6. T h i s produces a f o r c e due t o t h e bonding t o atoms 1, 2 and 3. The frequency i s t h e f o r c e a l o n g 6 a c t i n g on t h e a d s o r b a t e p e r u n i t displacement and u n i t mass.
B164 The i d e a i s t o c a l c u l a t e t h e frequency as t h e f o r c e p e r u n i t o f displacement and u n i t o f mass which i s a c t i n g on t h e adsorbate atom. The f o r c e c o n s t a n t o f t h e adsorbate-substrate bond i s K. For t h e movement p a r a l l e l t o t h e s u r f a c e we assume a displacement v e c t o r o f value ( 6 , 0, 0 ) . The d i s t a n c e t o atom 1 increases by a f a c t o r (J3/2) sen a i f we n e g l e c t second o r d e r terms i n 6. a i s t h e angle between bond d i r e c t i o n and
t h e s u r f a c e normal. Thus. t h e f o r c e i n t h e bond d i r e c t i o n p e r u n i t displacement is K(J3/2)
sen
ci
The p r o j e c t i o n o f t h i s f o r c e p e r u n i t displacement along t h e d i r e c t i o n o f t h e displacement i s K(3/4) sen2 a The same argument can be used i n o r d e r t o c a l c u l a t e t h e f o r c e p r o j e c t i o n associated t o atom 2. The o b t a i n e d v a l u e i s t h e same as f o r atom 1. The bonding between t h e adsorbate and atom 3 does n o t c o n t r i b u t e t o t h e f o r c e because i t i s second o r d e r i n 6. Thus t h e frequency o f t h e p a r a l l e l movement i s (J3/2)*(K/Ma)
sen a
where Ma i s t h e mass o f t h e adsorbed atom. We assume t h a t s u b s t r a t e atoms have i n f i n i t e mass. We can a l s o c a l c u l a t e t h e frequency o f t h e movement along t h e o t h e r p a r a l l e l d i r e c t i o n . The frequency i s t h e same. T h i s shows t h a t t h e p a r a l l e l mode i s degenerate. S i m i l a r l y , we can a l s o c a l c u l a t e t h e frequency o f t h e p e r p e n d i c u l a r mode. The same idea can a l s o be a p p l i e d t o t h e o t h e r a d s o r p t i o n s i t e s . The f r e quencies o f t h e adatom s t r e t c h i n g modes f o r t h e d i f f e r e n t a d s o r p t i o n s i t e symm e t r i e s a r e g i v e n i n Table 3.1. The values obtained by t h i s way assume a s u b s t r a t e o f i n f i n i t e mass. I f t h e mass o f t h e s u b s t r a t e i s taken as f i n i t e , small c o r r e c t i o n s have t o be a p p l i e d ( 5 ) . These c o r r e c t i o n s a r e more i m p o r t a n t when t h e adatom-substrate frequency
f a l l s c l o s e t o t h e phonon band o f t h e metal. Black (19) has worked o u t t h a t s i t u a t i o n , i n such a way t h a t frequency s h i f t s i n t r o d u c e d i n t o t h e adatom normal modes by t h e f i n i t e s u b s t r a t e mass, can be d i r e c t l y determined.
B165 Table 3.1. C h a r a c t e r i s t i c s o f t h e v i b r a t i o n o f s i n g l e - a t o m adsorbates, as a f u n c t i o n of t h e a d s o p r t i o n s i t e . We r e f e r t h e r e a d e r t o a book on group t h e o r y f o r a d e s c r i p t i o n o f t h e s y m e t r y t y p e s o f v i b r a t i o n a l eigenmodes ( s e e f o r examp l e Herzberg ( 1 8 ) ) . The frequency values based on a s p r i n g model a r e g i v e n . K i s t h e f o r c e c o n s t a n t p e r adsorbate-metal atom bond; Ma i s t h e mass o f t h e adsorb a t e and a i s t h e bond angle. Site from
S!!mmetrv t v p e and p o l a r i z a t i o n
Denomination
S p r i n g model Frequency values
Symme t r ic s t r e t c h ing Deformation
Q2
KIMa
TOP
Al(1) E ( Ill
2-fol d Bridge
A 1 (1) B1( Ill B2( II)
Symmetric s t r e t c h i n g Asymmetric s t r e t c h i n g Deformation
Q2 = (2K/Ma)cos 2ci fl2 = (2K/Ma)sen 2ci
3-fol d Bridge
A 1 (1) E( Ill
Symmetric s t r e t c h i n g Asymmet r i c s t r e t c h ing
Q2 = (3K/M )COS 'a R2 = (3K/2fla)sen
4-fol d Bridge
A (1)
Symmetri c s t r e t c h ing Asymmetric s t r e t c h i n g
R2 = ( ~ K / M ~ ) c o 2,s Q2 = (2K/Ma)sen 2a
h 11)
=
More r e c e n t l y , frozen-phonon t o t a l - e n e r g y l o c a l - d e n s i t y c a l c u l a t i o n s have been used t o o b t a i n t h e e q u i l i b r i u m d i s t a n c e b y t h e adatom t o t h e s u b s t r a t e and t h e f r e q u e n c i e s of symmetric and asymmetric s t r e t c h i n g modes o f hydrogen adsorbed on s e v e r a l s u b s t r a t e s ( 2 0 ) . As we w i l l show i n t h e d a t a s e r i o u s d i s c r e p a n c i e s appear when comparing these c a l c u l a t i o n s w i t h those based on t h e s i m p l e s p r i n g model. T h i s would i n d i c a t e t h a t t h e s i m p l e f i r s t n e a r e s t neighbour harmon i c p o t e n t i a l model does n o t d e s c r i b e a p p r o p r i a t e l y hydrogen a d s o r p t i o n . T a l k i n g s h a r p l y , t h e d i s c r e p a n c y l e a d up t o r e v e r s e t h e frequency v a l u e o f symmetric and a s y m e t r i c s t r e t c h i n g modes. The s p r i n g model g i v e s a t o o l o w v a l u e o f t h e symm e t r i c and a t o o h i g h v a l u e o f t h e asymmetric. According t o F and H, t h i s o c c u r s because t h e p a i r w i s e s p r i n g model n e g l e c t s t h e i n t e r a c t i o n between t h e adsorbed H and t h e d e l o c a l i z e d e l e c t r o n s a t a metal s u r f a c e (21).
I n t h i s a r t i c l e we w i l l g i v e d a t a based on t h e s p r i n g model t o g e t h e r w i t h d a t a based on t o t a l - e n e r g y c a l c u l a t i o n s . As a r e s u l t o f t h e d i s c r e p a n c i e s o f b o t h approaches, c a u t i o n has t o be p l a c e d r e g a r d i n g t h e c o n c l u s i o n s reached by t h e use o f t h e s p r i n g model, p a r t i c u l a r l y f o r hydrogen. 3.3.1.1.
Hydrogen adatoms
Due t o i t s s i m p l i c i t y t h e a d s o r p t i o n o f hydrogen r e p r e s e n t s a case s t u d y t o understand t h e p r o p e r t i e s a s s o c i a t e d w i t h t h e c h e m i s o r p t i o n process. I t a l s o r e p r e s e n t s a n e c e s s i t y i n o r d e r t o know about more complex processes as hydrogen a c t s as a r e a c t a n t i n many cases. I t i s n o t a s i m p l e system, however, due t o t h e small i n e l a s t i n g s c a t t e r i n g c r o s s s e c t i o n o f i t s v i b r a t i n g d i p o l e . We s t a r t w i t h t h e H / P t ( l l l ) system, as we p u b l i s h e d t h e f i r s t r e s u l t s on it. The v i b r a t i o n spectrum o f hydrogen adsorbed on P t ( l l 1 ) i s p r e s e n t e d i n F i g . 1 3.11. Two broad bands l a b e l l e d A and B a r e observed a t f r e q u e n c i e s 550 cm-
B166
I
ENERGY LOSS km-'I F i g . 3.11 V i b r a t i o n spectra o f hydrogen adsorbed on P t ( l l 1 ) a t low temperature ( 9 0 K) f o r an exposure o f 60 L (1L = 10-6 t o r r - s e c ) , as a f u n c t i o n o f t h e o u t o f specular angle 8 r - B i , from (14). Two broad bands appear a t 550 and 1230 cm-1. N o t i c e t h a t t h e band a t 1230 cm-1 i s h a r d l y v i s i b l e i n - s p e c u l a r c o n d i t i o n s .
(band A) and 1230
(band B). The s p e c t r a a r e presented as a f u n c t i o n o f t h e
o u t o f specular angle as band B i s h a r d l y v i s i b l e when t h e spectrum i s taken i n specular conditons. The angular dependence o f i n e l a s t i c i n t e n s i t i e s i s r e p o r t e d i n F i g . 3.12.
I t shows t h a t t h e i n t e n s i t y o f band A increases s u b s t a n t i a l l y i n
specular whereas t h e i n t e n s i t y o f band B i s almost independent of t h e s c a t t e r i n g angle. As we have mentioned i n S e c t i o n 3.2.
t h e energy dependence i s a l s o impor-
t a n t . T h i s f a c t t o g e t h e r w i t h t h e angular dependence a l l o w s us t o conclude t h a t t h e r e i s an i m p o r t a n t c o n t r i b u t i o n which i s n o t due t o d i p o l e s c a t t e r i n g . By l o o k i n g a t t h e angular dependence o f bands A and B, however, i t i s p o s s i b l e t o determine t h a t t h e r e i s a s i g n i f i c a n t d i p o l e c o n t r i b u t i o n t o t h e e x c i t a t i o n o f band A. This a l l o w s us t o conclude t h a t band A i s an EELS d i p o l e a c t i v e mode and consequently corresponds t o a t o t a l l y symmetric v i b r a t i o n a l mode. T h i s i s t h e
A1 mode which i n v o l v e s a displacement o f t h e hydrogen atom i n t h e d i r e c t i o n p e r p e n d i c u l a r t o t h e surface. We a l s o conclude t h a t band B i s due t o a movement p a r a l l e l t o t h e surface,
i.e.
a non-dipole a c t i v e mode.
As explained i n Ref. (14) we i n t e r p r e t e d t h e d a t a by making use o f t h e s p r i n g model. The expected modes o f atomic a d s o r p t i o n on t h e h i g h e s t symmetry p o s i t i o n s e x i s t i n g on t h e (111) face have been described i n Table 3.1. Adsorpt i o n on t o p is r u l e d o u t because t h e bending mode has a frequency l o w e r t h a n t h e
B167
F i g . 3. 12. Angular p r o f i l e o f e l a s t i c and i n e l a s t i c i n t e n s i t i e s t a k e n a t 5 eV, from ( 1 4 ) . The experimental a n g u l a r p r o f i l e f o r t h e C-0 s t r e t c h i n g band ( b r o k e n l i n e ) is shown f o r comparison. s t r e t c h i n g mode. Thus as band A i s f r o m i t s d i p o l e a c t i v i t y a s t r e t c h i n g mode, band B cannot be a bending mode. A d s o r p t i o n on b r i d g e i s e x c l u d e d f r o m t h e numb e r o f observed f r e q u e n c i e s . An a d d i t i o n a l argument i s t h a t f o r a b r i d g e s i t e t h e low frequency o f band A i s d i f f i c u l t t o r e c o n c i l e w i t h t y p i c a l P t - H bond l e n g t h s which comes about by a p p l y i n g t h e e x p r e s s i o n s o f T a b l e 3.1. Thus, t h e h o l l o w o r t h r e e f o l d s i t e i s t h a t which p r o v i d e s t h e b e s t agreement with t h e experiment. Within t h i s i n t e r p r e t a t i o n , band A i s assigned t o t h e H-Pt symmetric s t r e t c h i n g , and band B t o t h e H - P t asymmetric s t r e t c h i n g . We can o b t a i n , on t h e b a s i s o f t h e f o r c e c o n s t a n t model, a d d i t i o n a l i n f o r m a t i o n by u s i n g now t h e frequency v a l u e s o f t h e s t r e t c h i n g modes:
1230 = J 3 / 2 (JKIM) sen a which g i v e s i n m e d i a t e l y , ci = 72.5", From
~ 1 ; we
o b t a i n t h e complete geometry o f t h e a d s o r p t i o n , i n p a r t i c u l a r a
P t - H bond l e n g t h o f 1.68"
applying the expression K =
R2 ( 2 n c ) 2 M .
JK/M = 1053 cm-'.
4.
We a l s o o b t a i n t h e v a l u e o f t h e f o r c e c o n s t a n t by
B168 If Q i s g i v e n i n cm-',
c = 3x1O1O un-',
and M i s g i v e n i n g, K i s o b t a i n e d
i n dynxcm-l. The force constant o f t h e H-Pt bond i s 66 Nm- 1 , The H - P t bond l e n g t h compares v e r y w e l l w i t h t h a t o b t a i n e d from t h e coval e n t r a d i u s o f hydrogen (22). By t a k i n g a hydrogen r a d i u s v a l u e o f 0.30 a P t - H bond l e n g t h o f 1.69
k:
k:
we g e t
almost equal t o t h e v a l u e deduced by u s i n g t h e ex-
perimental frequencies. T h i s i n t e r p r e t a t i o n i s a t v a r i a n c e w i t h t h a t o b t a i n e d from t o t a l -energy c a l c u l a t i o n s . The c a l c u l a t i o n s were performed by Feibelman and Hamann ( F and H ) (21). They f i n d t h a t t h e bonding s i t e i s t h e 3 - f o l d f c c , t h a t i s . t h e s i t e w i t h o u t a second-layer P t d i r e c t l y below t h e H. They a l s o f i n d an e q u i l i b r i u m H - P t l a y e r s e p a r a t i o n o f 0.95
A,
e f f e c t i v e H-radius o f 0.48
corresponding t o a H - P t bond l e n g t h o f 1.86
fi
and an
much l a r g e r than t h a t o b t a i n e d from t h e s p r i n g
model. They c a l c u l a t e a value o f t h e symmetric s t r e t c h i n g mode o f 1340 cm-' a v a l u e o f t h e asymmetric s t r e t c h i n g o f 920 cm-'.
and
While t h e 1430 cm-' value i s
reasonably c l o s e t o o u r experimental 1230 cm-l band, t h e o t h e r number i s d e f i n i t e l y d i f f e r e n t from o u r experimental data. They a l s o c a l c u l a t e t h e dynamic d i p o l e moment change brought about by t h e two modes. They o b t a i n e* = 0.054 f o r t h e 1430 and e* = 0 f o r t h e 920 cm-l mode. The r e s u l t s o f these c a l c u l a t i o n s do n o t f i t w i t h o u r experimental data, F and H mention i n h i s paper some p o s s i b l e sources o f e r r o r i n o u r measurement.
They q u e s t i o n t h e assignment o f t h e 550 cm-l mode due t o t h e p r o x i m i t y o f t h i s band w i t h t h a t corresponding t o t h e CO-metal s t r e t c h i n g . We r e p o r t i n our experiment, a CO contamination below 0.01 monolayer. T h i s contamination i s due t o t h e l o n g t i m e needed t o perform t h e experiment and i s due t o t r a c e s o f CO i n t h e r e s i d u a l gas pressure. The behaviour o f t h e 550 cm-'
band i n t h e angular and
energy dependence i s s u f f i c i e n t evidence t h a t t h i s band has n o t h i n g t o do w i t h a CO mode. I t c o u l d be argued t h a t t h e 550 cm-'
band, although r e l a t e d t o a
hydrogen mode, appears i n connection w i t h t h e m o d i f i c a t i o n o f t h e s u r f a c e by CO t r a c e s . However, i n o u r experiments o f H and CO coadsorption t o be r e p o r t e d l a t e r on, we do n o t observe a mutual i n f l u e n c e o f H and CO bands a t t h i s low temperature,
F and H mention another problem o f our experiment. The exposure t o hydrogen was done a t 90 K which according t o LEED data, does n o t g i v e an ordered p a t t e r n . They suggest t h a t i n these c o n d i t i o n s , o t h e r a d s o r p t i o n s i t e s c o u l d be occupied. There i s s t i l l another p o i n t o f disagreement between o u r experiment and F and H theory, which we would l i k e t o p o i n t o u t . I t concerns t h e v a l u e o f t h e dynamic d i p o l e moment. We measure e* = 0.02 f o r band
A and e*
= 0
f o r band B . F
and H c a l c u l a t e e* = 0.05 f o r band B, i n disagreement w i t h t h e experiment. New r e s u l t s using i n f r a r e d r e f l e c t i o n - a b s o r p t i o n on H/Pt( 111) a r e a b l e t o f i n d a s i n g l e v i b r a t i o n a l band a t 1245 cm-' d i p o l e moment o f 0.02 (23).
i n t h e range 800-4000 cm-',
with a dynamic
B169 The second system t h a t we r e p o r t i s H on W(OOl), where a l a r g e amount o f d a t a e x i s t . T h i s was i n f a c t t h e f i r s t EELS s t u d y i n v o l v i n g hydrogen ( 2 4 ) . A more complete s t u d y was done l a t e r by Ho e t a l . (10). I n F i g . 3.13, we reproduce t h e more i m p o r t a n t d a t a . The spectrum t a k e n i n t h e s p e c u l a r d i r e c t i o n f o r a s a t u r a t i o n coverage o f hydrogen, i s c h a r a c t e r i z e d by a s i n g l e loss peak a t 130 1 meV (1 meV = 8.068 cm- ) . T h i s l o s s i s i n c o i n c i d e n c e w i t h t h a t r e p o r t e d b y F r o i t z h e i m e t a1.(24). The i n t e r e s t i n g p o i n t i s t h a t t h e spectrum t a k e n o u t o f s p e c u l a r ( F i g . 3.13b) shows new l o s s peaks which a r e e x c i t e d b y a n o n - d i p o l e mechanism. As t h e number of observed bands i s t h r e e , a b r i d g e s i t e i s immediat e l y deduced. A l s o as t h e 130 meV l o s s peak i s t h e o n l y d i p o l e a c t i v e mode, t h i s band i s assigned t o a displacement o f t h e hydrogen adatom normal t o t h e s u r f a c e ,
i.e. t o t h e symmetric s t r e t c h i n g mode. The r e s t i s s t r a i g h t f o r w a r d as e x p l a i n e d i n the figure.
F i g . 3.13. Normalized e l e c t r o n e n e r g y - l o s s s p e c t r a f o r s a t u r a t i o n coverage (bl phase) o f H-chemisorbed on W(100) f o r 0 i = 23' i n c i d e n t a n g l e and an impact energy Eo = 9.65 eV: a ) s p e c u l a r beam d i r e c t i o n ; b ) +17' o f f t h e s p e c u l a r d i r e c t i o n towards t h e surface, f r o m ( 1 0 ) . The e l a s t i c beam c o u n t r a t e I ( i n k i l o h e r t z ) and t h e energy r e s o l u t i o n a r e i n d i c a t e d i n t h e f i g u r e . The fundamental v i b r a t i o n a l modes [ i n s e r t F i g . 3.13bl correspond t o b r i d g e s i t e C2v-symmetry bonding. The i n c i d e n t beam i s a l o n g t h e [loo] c r y s t a l d i r e c t i o n .
B170
T h i s spectrum corresponds t o t h e rage of hydrogen. The
a,
o1
phase o b t a i n e d f o r a s a t u r a t i o n cove-
phase corresponding t o a l o w e r coverage o f hydrogen
has a l s o been s t u d i e d ( 2 5 ) . I n t h i s case t h e s u r f a c e s t r u c t u r e i s ~ ( 2 x 2 )and t h e losses a r e now a t h R 1 = 155 meV, hR2 = 60 meV and hR, = 125 meV. I t i s i n t e r e s t i n g t o n o t i c e t h a t t h e bands 1 and 3 a r e interchanged i n energy, when going from low t o s a t u r a t i o n coverage o f hydrogen. The spectrum can a l s o be e a s i l y i n t e r p r e t e d by t h e e x i s t e n c e o f a non-dipole e x c i t a t i o n mechanism. The adsorpt i o n s i t e i s a l s o bridge, and t h e d i f f e r e n c e i n frequency comes from t h e reconst r u c t i o n o f t h e s u r f a c e r e s p o n s i b l e o f t h e ~ ( 2 x 2 )s t r u c t u r e ( F i g . 3.14).
From
t h e assignment o f t h e modes, i t i s p o s s i b l e t o deduce t h e a d s o r p t i o n geometry, and i n p a r t i c u l a r t h e H-W bonding l e n g t h , and t h e corresponding f o r c e constant. The r e s u l t o f these c a l c u l a t i o n s i s r e p o r t e d i n Table 3.2 t o g e t h e r w i t h t h e value obtained f o r P t ( 111).
F i g . 3.14. Surface r e c o n s t r u t e d - l a y e r model from ( 2 5 ) f o r W( 100) c[2x2]H showing ( a ) t h e b r i d g e - s i t e p o s i t i o n s o f t h e chemisorbed H atoms ( s m a l l c i r c l e s ) between two W atoms b e f o r e (dashed l a r g e c i r c 1 e s ) a n d a f t e r ( f u l l - l i n e l a r g e c i r c l e s ) r e c o n s t r u c t i o n ; t h e c[2x2]H u n i t mesh(dashed-1 i n e square) and t h e d i r e c t i o n o f t h e i r W-atomic s h i f t s (arrows) a r e i n d i c a t e d ; ( b ) t h e change i n t h e W-H-W bond angle 2a, which produces a decrease i n l a t t i c e spacing from a = 3.16 8 t o a ' = 2.74
8.
.
New d a t a on t h i s system have been r e c e n t l y r e p o r t e d by E r s k i n e e t a1 (15). The measurement o f t h e v i b r a t i o n s p e c t r a i s done by c o n t r o l l i n g c a r e f u l l y t h e
B171 s c a t t e r i n g geometry w i t h r e s p e c t t o t h e two-dimensional B r i l l o u i n zone o f t h e s u r f a c e . I n t h e case o f t h e
phase on W(100) a new f e a t u r e i s observed a t
118 meV. T h i s new loss does n o t behave as a d i p o l e l o s s , and i t shows a s t r o n g energy dependence. I t i s e x p l a i n e d by t h e a u t h o r s as t h e o p t i c mode o f t h e 8 1 - ~ o v e r l a y e r . T h i s mode c o n s i s t s o f coupled p e r p e n d i c u l a r v i b r a t i o n s o f a d j a c e n t hydrogen atoms o s c i l l a t i n g 180" o u t o f phase. As t h e y a r e two atoms p e r u n i t c e l l o f t h e B l phase, s i x v i b r a t i o n a l modes o f hydrogen a r e p o s s i b l e . Due t o t h e l a t e r a l c o u p l i n g i n v o l v e d on t h i s movement, d i s p e r s i o n o f t h i s mode r e s u l t s and i t has been e x p e r i m e n t a l l y determined. The d a t a g i v e i n s i g h t i n t o t h e H-H i n t e r -
actions. I t is i n t e r e s t i n g t o compare these d a t a w i t h t o t a l - e n e r g y c a l c u l a t i o n s .
Whereas reasonable agreement e x i s t s c o n c e r n i n g t h e bending and t h e symmetric and asymmetric s t r e t c h i n g modes, t h e c a l c u l a t i o n s g i v e a v a l u e o f 150 meV f o r t h e o p t i c mode ( 2 6 ) , much h i g h e r t h a n t h e 118 meV band assigned t o be o p t i c by Erskine e t a l . (15). V i b r a t i o n a l d a t a have a l s o been r e p o r t e d f o r H chemisorbed on W(110) and W(111), ( 2 7 ) . The o b s e r v a t i o n o f a l o s s peak a t t h e same energy of 160 meV would perhaps i n d i c a t e t h a t hydrogen occupies t h e same b r i d g e s i t e i r r e s p e c t i v e l y o f t h e d i f f e r e n t s u r f a c e geometry o f each face. The d a t a are, however, n o t as complete as those r e p o r t e d f o r t h e (100) face. The system H/Mo(100) has s t r o n g s i m i l a r i t i e s w i t h t h e W(100) case and i t has been s t u d i e d by Zaera e t a l . ( 2 8 ) . A t h i g h H coverages, t h e surface i s n o t r e c o n s t r u c t e d and two bands a r e observed a t 1030 and 1125 cm-l. The a u t h o r s propose a b r i d g e s i t e on t h e u n r e c o n s t r u c t e d s u r f a c e as i n t h e H/W(100) case. A t l o w hydrogen coverage, where t h e s u r f a c e i s r e c o n s t r u c t e d , t h e two bands
appear i n t h e 1220 t o 1260 cm-l range, and a r e i n t e r p r e t e d as due t o t h e occup a t i o n o f a m o d i f i e d b r i d g e s i t e . T h i s i n t e r p r e t a t i o n i s s u p p o r t e d by t h e s i m i l a r i t y o f t h i s system w i t h t h e H/W(100) case. We have a l s o measured t h e H / F e ( l l O ) system, The i n s p e c u l a r v i b r a t i o n a l spectrum t a k e n a t 300 K shows a s i n g l e loss peak a t 1060 cm-'. another l o s s a t 880 cm-'
Off
specular,
i s v i s i b l e ( F i g . 3.15). A n a l y s i s o f t h e d i f f e r e n t ad-
s o r p t i o n s i t e s l e a d us t o conclude t h a t t h e s h o r t b r i d g e s i t e i s t h e most adequate choice. The 1060 and 880 cm-'
bands a r e assigned t o t h e symmetric and
asymmetric modes on t h e b a s i s o f nearest-neighbour f o r c e model. We a r e aware, however, t h a t LEED c a l c u l a t i o n s f a v o u r t h e q u a s i t r i a n g u l a r s i t e ( 3 0 ) . I f we now t a k e i n t o account t h e t r e n d g i v e n by t o t a l - e n e r g y c a l c u l a t i o n s , t h e 1060 and 880 cn1-l bands c o u l d i n f a c t be i n t e r p r e t e d as due t o t h e symmetric and asymm e t r i c s t r e t c h i n g modes o f hydrogen occupying t h e q u a s i t r i a n g u l a r s i t e . Calcul a t i o n s on t h i s system would t h e r e f o r e be v e r y h e l p f u l t o d e f i n i t e l y d e c i d e between these two p o s s i b i 1 i t i e s ,
B172 ~
='KO
12.10:
'[ 0
Fe(110)+5L H2 T= 300 K !?60
m
EM]
ENERGY
1503
LOSS lcrn-'l
F i g . 3.15. V i b r a t i o n a l spectrum o f hydrogen chemisorbed on F e ( l l 0 ) a t room temp e r a t u r e , from ( 2 9 ) : ( a ) i n s p e c u l a r c o n d i t i o n s , i . e . a n g l e o f i n c i d e n c e O i equal t o a n g l e of r e f l e c t i o n Or; ( b ) O f f - s p e c u l a r ei = Or 9".
-
Another i m p o r t a n t case where a q u i t e numerous s t u d y b y EELS has been p e r formed i s n i c k e l . We have been measuring on t h e N i ( l l 0 ) face. T h i s i s n o t a s i m p l e case, because s e v e r a l s u r f a c e s t r u c t u r e s e x i s t , as have been r e p o r t e d i n t h e l i t e r a t u r e ( 3 1 ) . F o r t h e purpose o f t h i s s t u d y , i t i s c o n v e n i e n t t o m e n t i o n t h e 2x1 s t r u c t u r e , which i s completed a t a coverage o f one monolayer o f h y d r o gen, and t h e r e c o n s t r u c t e d 1x2 s t r u c t u r e , which grows c o n t i n u o u s l y a t t h e expense o f t h e 2x1 u n t i l s a t u r a t i o n i s reached a t 1.5 monolayer. The 2x1 s t r u c t u r e i s i n t e r p r e t e d as due t o hydrogen atoms f o r m i n g a z i g - z a g c o n f i g u r a t i o n on t h e n i c k e l rows a l o n g t h e (110) d i r e c t i o n ( 3 2 ) . V i b r a t i o n a l d a t a c o r r e s p o n d i n g t o hydrogen a d s o r p t i o n on N i ( 1 1 0 ) a r e r e p r o duced i n F i g . 3.16. A t l o w exposures two bands a t 650 and 1060 cm-l appear, which t r a n s f o r m a t h i g h exposure, i n t o two bands s i t u a t e d a t 940 and 610 cm-'. Both l o s s peaks have a n o n - n e g l i g i b l e d i p o l e c o n t r i b u t i o n , and t h e q u e s t i o n a r i s e s about i t s assignment. B e f o r e a n t i c i p a t i n g any i n t e r p r e t a t i o n , i t i s w o r t h t o m e n t i o n h e r e t h e new s t u d y performed by I b a c h ' s group on t h a t system ( 3 4 ) . Using a new s p e c t r o m e t e r which has much b e t t e r c h a r a c t e r i s t i c s and r e c o r d i n g v i b r a t i o n a l d a t a a t w e l l - d e f i n e d s c a t t e r i n g g e o m e t r i e s , a v e r y complete s e t o f d a t a has been r e p o r t e d . The main c o n c l u s i o n o f t h a t s t u d y t e l l s us t h a t t h e 2 x 1 s t r u c t u r e i s due t o a s i n g l e a d s o r p t i o n p o s i t i o n , which i n accordance w i t h o t h e r a u t h o r s , i s a m o d i f i e d t h r e e f o l d s i t e p r o v i d e d by t h e l o c a l (111) t r i a n g l e s e x i s t i n g i n t h e
(110) f a c e . The symmetry s i t e i s C,, which p r o v i d e s w i t h t h r e e non-degenerate v i b r a t i o n s , two o f them h a v i n g d i p o l e c h a r a c t e r , t h e t h i r d o n l y observed o u t o f 1 s p e c u l a r . T h i s l a s t band, w h i c h we d i d n o t observe, i s s i t u a t e d a t 870-910 cm- . An i m p o r t a n t p o i n t on t h e c o n t r o v e r s y about t h e p o l a r i s a t i o n o f t h e modes i s r e s o l v e d i n t h i s s t u d y . A c c o r d i n g t o ( 3 4 ) , t h e h i g h f r e q u e n c y mode a t 1060 cm-l i r p r e d o m i n a n t l y p e r p e n d i c u l a r t o t h e s u r f a c e , whereas t h e low f r e q u e n c y mode
B173
ENERGY LOSS (crn-11
F i g . 3.16. V i b r a t i o n a l s p e c t r a corresponding t o two d i f f e r e n t doses o f H on N i (110) a t 90 K, f r o m ( 3 3 ) . The exposure a t 0.6L i s t h a t r e q u i r e d t o form $he 2x1 s t r u c t u r e . A t 1.5L t h e s t r u c t u r e formed i s t h e 1x2. a t 650 cm-'
i s p a r a l l e l . T h i s i s i n agreement w i t h t h e t r e n d suggested by t o t a l -
energy c a l c u l a t i o n s . Also, t h e h i g h frequency ( p e r p e n d i c u l a r ) mode has an e f f e c t i v e v i b r a t i n g d i p o l e s m a l l e r t h a n t h e l o w f r e q u e n c y ( p a r a l l e l mode). T h i s i s t h e same behaviour observed on t h e H / P t ( l l l ) system. The 1x2 r e c o n s t r u c t e d s u r f a c e can a l s o be i n t e r p r e t e d . Our d a t a g i v e bands a t 610 and 940 an-',
a l t h o u g h a band a t l o w e r frequency appears, t h a t we c o u l d
n o t r e s o l v e . I b a c h ' s group f i n d s t h i s band a t 450 cm-'.
The d a t a a r e i n t e r p r e t e d
( 3 4 ) by suggesting t h e o c c u p a t i o n o f two d i f f e r e n t pseudo t h r e e f o l d s i t e s , t h e second one p r o v i d e d by t h e s u r f a c e r e c o n s t r u c t i o n . Data f o r t h e two o t h e r n i c k e l faces a l s o e x i s t . On t h e (100) face, hydrogen e x h i b i t s a v i b r a t i o n a l mode a t 74 meV ( 3 5 ) , i n t e r p r e t e d as
H occupying t h e f o u r -
f o l d s i t e . On t h e ( 1 1 1 ) face, H i s bonded t o a t h r e e f o l d s i t e . TWO i n e l a s t i c l o s s e s a r e observed a t 89 and 139 meV assigned r e s p e c t i v e l y t o t h e symmetric and asymmetric s t r e t c h i n g modes ( 3 6 ) . A d s o r p t i o n o f hydrogen on Ru(0001) has been r e p o r t e d by Barteau e t a l . (37). Previous s t u d i e s suggest t h a t two d i s t i n c t b i n d i n g s t a t e s a r e occupied. The EELS d a t a show, however, two p r i n c i p a l l o s s f e a t u r e s a t 105 and 138 meV which have t h e same i n t e n s i t y r e l a t i v e t o each o t h e r f o r a l l hydrogen coverages. Neverthel e s s , a t l o w exposures, t h e l o s s peaks appear a t 88 and 138 meV, w i t h t h e l o w energy peak s h i f t e d . The H-induced l o s s e s have a c o n s i d e r a b l e i n t e n s i t y f o r ang l e s away from t h e s p e c u l a r d i r e c t i o n . The a u t h o r s i n t e r p r e t t h e spectrum as c h a r a c t e r i s t i c o f hydrogen adsorbed on t h e t h r e e f o l d h o l l o w s i t e . The two bands
B174 are assigned t o the symnetric (low energy) and asymmetric ( h i g h energy) s t r e t ching modes. They a l s o suggest t h a t t h e change i n energy o f the low frequency band i s due t o the difference between the two threefold s i t e s provided by the Ru( 0001 ) surface. Table 3.2. Data of hydrogen adsorbed on various surfaces. n corresponds t o the symmetric s t r e t c h i n g and R2 t o the asymmetric s t r e t c h i n g v i b r a t i o n a l mode. The dynamic d i p o l e e* i s given i n u n i t s o f e. F. C. stands f o r t h e s p r i n g model, whereas T.E.stands f o r total-energy c a l c u l a t i o n s .
K
rH
Nm-'
8
1 ' cm-l
'2 cm-l
Site
P t ( 111) Pt(ll1)
550 1340
1230 920
3-fold 3-fold
66
W(100) c( 2x2) W ( 100) 1x1 LJ( 100) Fe(ll0) Ru(0001)
1250
1010
2-fold
77
1050
1290
2-fold
82
0.6
0.053
F.C.
1140 1060 1110
1360 880 850
2-fold 2-fold 3-fold
57
0.54 0.7 0.45
0.032 0.02
T.E. F.C. T.E.
Surface
0.3 0.48
e* 0.02 0.05
Model F.C. T.E.
F.C.
A recent experiment by Conrad e t a l , (38) confirmed the experimental data b u t reversed t h e assignment on the basis o f an a n a l y s i s o f combination and overtone bands. Total-energy c a l c u l a t i o n s confirm t h i s reversal f o l l o w i n g the same t r e n d as obtained on t h e H / P t ( l l l ) system. The H radius i s found t o be o f 0.45
A. A summary of data concerning hydrogen adsorption i s given i n Table 3.2. The values o f hydrogen
radius and f o r c e constant obtained by using the spring model
and the values obtained from total-energy c a l c u l a t i o n s are reported. 3.3.1.2.
Oxygen
Oxygen i s an important element t o study because o f i t s c r u c i a l a c t i v i t y i n phenomena l i k e oxidation, corrosion and metal-oxide interphases. As i n the case o f hydrogen, i t dissociates upon adsorption on many surfaces. Thus, i t i s another example o f atomic adsorption. One o f t h e most controversial systems i s oxygen adsorption on n i c k e l . On Ni(100), oxygen forms two d i f f e r e n t surface s t r u c t u r e s : ~ ( 2 x 2 )a t low ( 0 = 0.25) and 4 2 x 2 ) a t high ( 0 = 0.5) coverage. The losses corresponding t o t h e oxygennickel s t r e t c h i n g v i b r a t i o n s appear a t 435 and 330 cm"
(Fig. 3.17) and are i n -
t e r p r e t e d as due t o oxygen adsorbed on the f o u r f o l d hollow s i t e (39). I n addit i o n t o these losses i n v o l v i n g oxygen, s t r u c t u r e appears a t t h e low energy s i d e . These new losses can be i n t e r p r e t e d as due t o the e x c i t a t i o n of surface phonons o f the m e t a l l i c substrate (40). These new peaks may cause a complication o f t h e
B175
ENERGY LOSS
F i g . 3.17. EELS spectrum of Ni(100) covered w i t h oxygen a f t e r d i f f e r e n t exposur e s a t 150 K , from ( 4 0 ) . The sample was warmed up i n o r d e r t o develop a sharp LEED p a t t e r n . v i b r a t i o n a l spectrum, b u t t h e a d d i t i o n a l i n f o r m a t i o n a l l o w s t o b e t t e r assignment and i n t e r p r e t a t i o n t h e adatom v i b r a t i o n a l modes. The i n t e r e s t i n g p o i n t about O/Ni(100) i s t h e l o w e r i n g i n frequency f r o m
435 t o 330 cm-' accompanying t h e t r a n s f o r m a t i o n f r o m ~ ( 2 x 2 )t o ~ ( 2 x 2 ) . T h i s o b s e r v a t i o n has been t h e s u b j e c t o f s t u d y and many arguments have been g i v e n i n o r d e r t o e x p l a i n i t . Upton and Goddard (41) have suggested t h a t two d i s t i n c t ads o r p t i o n s t a t e s c h a r a c t e r i z e d by two d i f f e r e n t e q u i l i b r i u m d i s t a n c e s e x i s t . SEXAFS d a t a a r e , however, i n c o n t r a d i c t i o n w i t h t h i s a n a l y s i s ( 4 2 ) . A l l a n and
LBpez (43) remark t h a t i n t h e ~ ( 2 x 2 )s t r u c t u r e , a n i c k e l s u r f a c e atom has two oxygen neighbors, so we may e x p e c t a s t r o n g i n d i r e c t 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 adatoms v i a t h e s u b s t r a t e . D i r e c t adatoni i n t e r a c t i o n has a l s o been
i nvo ked
.
Demuth e t a l . ( 4 4 ) made an e x t e n s i v e LEED a n a l y s i s and concluded t h a t t h e oxygen does n o t s i t on t h e f o u r f o l d s i t e , b u t r a t h e r i s d i s p l a c e d f r o m i t a l o n g t h e (100) d i r e c t i o n by 0.3
i.
Rahman e t a l . (45) have made a v e r y c a r e f u l a n a l y s i s o f t h e f u l l s e t o f v i b r a t i o n a l d a t a on t h e ~ ( 2 x 2 )and ~ ( 2 x 2 )s t r u c t u r e s , i n c l u d i n g t h e phonon l o s ses. T h e i r c o n c l u s i o n i s t h a t on b o t h cases oxygen s i t s a t t h e same d i s t a n c e o f
B176 0.9
w above t h e surface.
A n a l y s i s o f t h e EELS data by Rahman e t a l . (45), sug-
g e s t t h a t t h e change i n frequency can be f i t t e d by assuming a r e d u c t i o n of t h e
0-Ni f o r c e constant by a f a c t o r 0.55. The p a r a l l e l movement which produces a band a t 450 cm-l on t h e ~ ( 2 x 2 )s t r u c t u r e , can be a l s o f i t t e d by u s i n g t h e same experimental parameters. The f o r c e c o n s t a n t i s 90 Nm-'. The system O / N i ( l l l ) has been s t u d i e d by Ibach and Bruchmann ( 4 6 ) . They observe a l o s s a t 580 cm-l t o g e t h e r w i t h l o s s e s due t o t h e e x c i t a t i o n of s u r f a c e phonons. The l o s s a t 580 cm-'
i s assigned t o t h e symmetric s t r e t c h i n g mode o f
oxygen adsorbed on a 3 - f o l d s i t e . The author o f t h i s chapter has s t u d i e d t h e system O / N i ( l l O ) ( 4 7 ) . T h i s case
i s complicated by t h e e x i s t e n c e o f a (2x1) s u r f a c e r e c o n s t r u c t i o n induced by oxygen. The v i b r a t i o n a l spectrum i s complex a t low oxygen coverage b u t a t t h e coverage where t h e (2x1) s t r u c t u r e i s sharp, a s i n g l e l o s s a t 420 cm-l occurs. We i n t e r p r e t e d t h i s l o s s as due t o t h e symmetric s t r e t c h i n g mode of oxygen adsorbed on a l o n g b r i d g e s i t e . This a u t h o r has r e c e n t l y r e p o r t e d r e a l space images o f t h e 2x1 s t r u c t u r e o f oxygen chemisorbed on N i ( l l 0 ) (48) by t h e Scann i n g Tunneling Microscope, a technique which i s a b l e t o g i v e three-dimentional images o f s u r f a c e s t r u c t u r e s a t o m i c a l l y r e s o l v e d ( 4 9 ) . A n a l y s i s of t h e data on N i has been made by u s i n g a n e a r e s t neighbour f o r c e constant model, t o g e t h e r w i t h t h e assumption which l i e s t h e f o r c e constant f o r a Z - m u l t i p l e bond w i t h a s i n g l e one ( o n t o p ) by t h e expression
1 = 670 Nm- we o b t a i n a good v a l u e f o r t h e frequency o f t h e top o t h e r a d s o r p t i o n s i t e s . We o b t a i n f 3 - f o l d = 227 Nm" by u s i n g t h e Ql value and By t a k i n g f
an oxygen r a d i u s of 0.7 I ( . For t h e l o n g b r i d g e s i t e , we o b t a i n (47) fbridge = 335 Nm-',
i n coincidence w i t h ftop/2.
We a l r e a d y mentioned t h e c a l c u l a t i o n s performed on t h e O/Ni( 100) system. L e t us j u s t t e l l t h a t f o r t h e ~ ( 2 x 2 )t h e f o r c e c o n s t a n t o b t a i n e d i s 164 Nm-' very close t o f / 4 = 167 1qm-l. A p p l i c a t i o n o f a n e a r e s t neighbor f o r c e constop 0 t a n t model g i v e s 0.86 A f o r t h e d i s t a n c e o f t h e oxygen atom above t h e surface. On Cu(OOl), oxygen a l s o occupies t h e f o u r f o l d h o l l o w s i t e (50). The s u r f a c e s t r u c t u r e s a r e a ~ ( 2 x 2 )and a c(J2x242)R45. The oxygen l o s s e s appear a t 290 cm-' corresponding t o t h e p e r p e n d i c u l a r motion and a t 700 cm"
f o r the parallel
motion. The p a r a l l e l mode l i e s a t a c o n s i d e r a b l e h i g h e r frequency t h a n t h e c o r responding v a l u e f o r t h e Ni(001) case. T h i s experimental o b s e r v a t i o n i s i n d i c a t i v e o f a r e c o n s t r u c t i o n o f t h e s u r f a c e induced by oxygen. The authors suggest indeed t h a t t h e (J2x2J2)R45 s t r u c t u r e i s t h e o n l y w e l l - o r d e r e d o v e r l a y e r a t room temperature, which can be d e r i v e d from a ~ ( 2 x 2 )s t r u c t u r e by a d i s p l a c i v e t r a n sition.
ENERGY LOSS (cm-’)
F i g . 3.18. S p e c t r a o f a s a t u r a t e d oxygen l a y e r on P t ( l l 1 ) a t 90K and 6 Langmuir ( 5 1 ) . A t l o w temperature, oxygen i s i n a m o l e c u l a r s t a t e w i t h a l o w 0-0 s t r e t c h i n g frequency a t 875 cm-1. The f e a t u r e a t 700 cm-1 i s a s s i g n e d t o t h e 0-0 s t r e t c h i n g mode o f oxygen adsorbed i n a d i f f e r e n t s i t e . The spectrum shows a l s o s e v e r a l m u l t i p l e l o s s e s and o v e r t o n e s ( 5 1 ) . A f t e r h e a t i n g t o 300 K, oxygen p a r t i a l l y desorbs, p a r t i a l l y d i s s o c i a t e 2 . The loss a t 480 cm-1 i s due t o t h e 0 - P t s t r e t c h i n g mode. The loss a t 800 cm- i s assigned t o an o v e r t o n e o f t h e p a r a l l e l a n t i s y m m e t r i c movement o f t h e oxygen atom. Oxygen can a l s o be adsorbed i n a m o l e c u l a r species. T h i s was f i r s t r e p o r t e d on P t ( l l l ) , ( F i g . 3.18). The v i b r a t i o n a l spectrum i s i n t e r p r e t e d as O2 adsorbed m o l e c u l a r l y i n a p e r o x o - l i k e species w i t h 875 cm-I b e i n g t h e 0-0 s t r e t c h i n g f r e quency and 380 cm-l, t h e symmetric 0 - P t s t r e t c h i n g v i b r a t i o n . The frequency a t 700 cm-l i s i n t e r p r e t e d as due t o a d i f f e r e n t c o n f i g u r a t i o n o f m o l e c u l a r oxygen. Another i n t e r e s t i n g o b s e r v a t i o n ( 5 1 ) concerns t h e v i b r a t i o n a l spectrum a f t e r d i s s o c i a t i n g t h e oxygen by thermal a c t i v a t i o n . The spectrum shows a m a j o r peak s i t u a t e d a t 480 cm-I which i s i n t e r p r e t e d as oxygen s i t t i n g on a t h r e e f o l d h o l l o w s i t e . B u t i n a d d i t i o n l o s s e s a t h i g h e r energy can be i n t e r p r e t e d as due t o t h e e x c i t a t i o n of o v e r t o n e s . An i n t e r e s t i n g p r o p e r t y o f o v e r t o n e s i s t h a t even i f t h e fundamental i s n o n - d i p o l e a c t i v e , t h e f i r s t o v e r t o n e i s d i p o l e . T h i s i s used t o e x p l a i n t h e band a t 800 cm-l as an o v e r t o n e o f t h e p a r a l l e l a n t i s y m m e t r i c movement of t h e oxygen atom. Due t o t h e s t r o n g c h a r a c t e r o f t h e 0 - P t bond, i t i s reasonable t o assume t h a t t h e f r e q u e n c y o f t h e o v e r t o n e i s j u s t two times t h a t of t h e fundamental. Therefore, by t h i s way, one o b t a i n s f o r O-adsorbed on P t ( l l l ) , t h e frequency o f t h e two modes:
B178 Ql = 480
un-l
Now, w i t h t h e two values, i t i s p o s s i b l e t o a p p l y t h e s p r i n g model. We g e t a bond angle of 49.7",
and an oxygen r a d i u s o f 0.71
A.
Notice the s i m i l a r i t y
with t h e v a l u e obtained f o r oxygen on n i c k e l . The f o r c e c o n s t a n t i s f 3 - f o l d = = 174 Nm-',
s m a l l e r than i n t h e case o f n i c k e l .
We a l s o measured t h e v i b r a t i o n a l spectrum o f oxygen chemisorbed on a stepped P t surface. I n F i g . 3.19,
t h e s p e c t r a corresponding t o oxygen adsorbed on
P t ( l l 1 ) and stepped P t ( l l 1 ) can be compared. They correspond t o an oxygen exposure o f 10 L a t 90 K. The i n t e r e s t i n g o b s e r v a t i o n i s t h e d i f f e r e n t i n t e n s i t y o f t h e 0-Pt bands a t 380 and 480 cm",
which correspond t o t h e s t r e t c h i n g v i b r a -
t i o n s f o r t h e molecular and t h e atomic species. We see t h a t t h e 480 cm-'
band
has a h i g h e r i n t e n s i t y i n t h e stepped than i n t h e f l a t case, i n d i c a t i n g t h a t oxygen p a r t l y d i s s o c i a t e s on t h e stepped s u r f a c e even a t t h i s low temperature. This i s i n agreement w i t h t h e i d e a t h a t steps p r o v i d e more a c t i v e s i t e s f o r i n t e r a c t i o n o f t h e molecule w i t h t h e surface. I n f a c t a small band a t 480 cm-'
is
observed a l s o on t h e f l a t surface, which probably corresponds t o 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 on d e f e c t s e x i s t i n g on t h a t sample.
ENERGY LOSS lcm-'l
F i g . 3.19. V i b r a t i o n a l s p e c t r a o f O2 chemisorbed on a f l a t and a stepped P t ( l l 1 ) surfaces, from ( 5 2 ) . The h i g h frequency l o s s e s a t 680 and 870 cm-1, a r e due t o molecular oxygen. The band a t 475 cm-1 i s due t o oxygen a t o m i c a l l y adsorbed on t h e surface. The r e l a t i v e i n t e n s i t y o f t h e 475 cm-1 band, which i s h i g h e r on t h e stepped surface, shows t h e h i g h e r a c t i v i t y o f steps t o d i s s o c i a t e oxygen.
B179 A d s o r p t i o n o f oxygen on Pd a l s o r e s u l t s on two l o s s e s w h i c h a r e c h a r a c t e r i s t i c o f m o l e c u l a r oxygen. T h i s system has been s t u d i e d by I m b i h l and Demuth
(58) on t h e P d ( l l 1 ) face. A d s o r p t i o n i s made a t 30 K and two modes a t 850 and 1035 cm-l a r e assigned t o peroxo and superperoxo species. I n c r e a s i n g t h e coverage, a new l o s s i n t e r p r e t e d as due t o p h y s i s o r b e d oxygen i s observed a t 1585 cm-l. The p h y s i s o r b e d oxygen i s absorbed on t o p o f t h e chemisorbed l a y e r b u t n o t on t h e bare metal s u b s t r a t e . D i s s o c i a t i o n of oxygen produces a new l o s s a t 485 cm-'
due t o t h e adatom
s t r e t c h i n g frequency. N o t i c e t h e s i m i l a r i t y o f t h e v a l u e w i t h t h a t r e p o r t e d f o r oxygen adsorbed on P t ( l l 1 ) . T h i s i n d i c a t e s t h a t t h e same 3 - f o l d s i t e i s occupied i n Pd. On C r ( l l O ) , oxygen i s adsorbed s i m u l t a n e o u s l y on a d i s s o c i a t i v e and on a mol e c u l a r s t a t e ( 5 4 ) . The d i s s o c i a t i v e s t a t e i s c h a r a c t e r i z e d by a s i n g l e l o s s a t 605 cm-'
i n t e r p r e t e d as due t o oxygen atoms occupying t h e t w o f o l d symmetric s u r -
f a c e h o l l o w s i t e . T h i s loss i s observed when t h e exposure i s made a t 300 K, and t h e adsorbate forms a ~ ( 4 x 2 )s t r u c t u r e . A t 120 K a m o l e c u l a r s t a t e c h a r a c t e r i s t i c o f super-oxo O2 i s populated. T h i s s t a t e produces a band a t 1020 an-'.
By
u s i n g EELS d a t a t o g e t h e r w i t h E l e c t r o n S t i m u l a t e d D e s o r p t i o n I o n Angular D i s t r i b u t i o n (ESDIAD), a model i s invoked i n which an O2 m o l e c u l e i s bonded t o chromium v i a o n l y one end o f t h e m o l e c u l e t o f o r m a n o n - l i n e a r arrangement. The v i b r a t i o n a l e x c i t a t i o n s o f oxygen adsorbed on F e ( l l 0 ) have been r e p o r t e d by E r l e y and I b a c h ( 5 5 ) . As f o r many oxygen-metal systems, t h e d a t a a r e analysed i n terms of t h r e e d i f f e r e n t processes, i.e. c h e m i s o r p t i o n , s u r f a c e o r p r e c u r s o r o x i d e and b u l k o x i d e . The frequency v a l u e s o f t h e adatom-metal s t r e t c h i n g v i b r a t i o n s a r e o b t a i n e d and a f o r c e c o n s t a n t model i s used i n o r d e r t o a s s i g n d e f i n i t e a d s o r p t i o n s i t e s f o r oxygen. The a n a l y s i s i s pursued by u s i n g t h e frequency o f a f r e e FeO molecule. T h i s i s 880 cm-I and i f we t a k e t h a t v a l u e as t h e f o r c e c o n s t a n t f o r a l i n e a r bond we can deduce t h e bond l e n g t h f r o m t h e experimental adatom-metal s t r e t c h i n g f r e q u e n c i e s . The i n t e r e s t i n g new p o i n t about t h a t system o c c u r s when t h e exposure
of
oxygen i s made a t a sample temperature o f 780 K. I n c o n t r a s t w i t h t h e d a t a observed f o r an exposure o f oxygen a t room temperature, a s i n g l e l o s s around 910 cm-l i s observed. T h i s h i g h f r e q u e n c y l o s s i s a t t r i b u t e d t o t h e f o r r a t i o n o f an i r o n o x i d e . High frequency v a l u e s o f oxygen have been observed on o t h e r m e t a l s , and a r e a l s o r e l a t e d w i t h t h e a n n e a l i n g o f t h e s u r f a c e ( 5 6 ) . These a u t h o r s ass o c i a t e t h a t species t o s u b s u r f a c e oxygen. HREELS has a l s o been a p p l i e d t o c o n t r o v e r s i a l O/metal systems. A s i g n i f i c a n t case i s t h e i n t e r a c t i o n o f oxygen w i t h A l . A r e c e n t s t u d y b y A s t a l d i e t a1 .
( 5 7 ) shows t h a t oxygen chemisorbs on A l ( 1 1 1 ) i n oxygen d o u b l e - l a y e r i s l a n d s cont a i n i n g oxygen atoms above and below t h e f i r s t aluminium s u r f a c e l a y e r . T h i s i s deduced from t h e o b s e r v a t i o n o f two oxygen-metal v i b r a t i o n a l modes due t o oxygen
B180 above (65 meV) and below the surface (70-80 meV). A l a t e r s t a t e o f oxygen described as o x i d i c species produces a l o s s a t 105 meV. The development o f t h i s
loss gives a very good i n d i c a t i o n o f the r a t e o f o x i d a t i o n of t h a t surface. I t i s worth t o mention t h a t the EELS v i b r a t i o n spectrum i s a very s e n s i t i v e probe t o detect oxide c l u s t e r s , because they produce strong v i b r a t i o n a l modes. 3.3.2.
Carbon Monoxide CO i s probably t h e most studied molecule i n v i b r a t i o n a l spectroscopy o f ad-
sorbates. I t c o n s t i t u t e s a t y p i c a l example o f diatomic molecule. I t has a very high d i p o l e moment which furnhishes intense v i b r a t i o n a l 1osses.It plays a major r o l e i n important c a t a l y t i c reactions, l i k e the Fischer-Tropsch syntesis. Hist o r i c a l l y , v i b r a t i o n a l C-0 s t r e t c h i n g bands were f i r s t measured by I n f r a r e d Spectroscopy (58) i n i t s conventional I R transmission mode. The samples used f o r t h i s purpose u s u a l l y c o n s i s t o f f i n e l y d i v i d e d metal p a r t i c l e s , deposited on inorganic oxides o f high surface area. The experimental range o f C-0 observed s t r e t c h i n g frequencies was i n t e r preted by Eischens and h i s colleagues (58) i n the f o l l o w i n g way: f o r CO adsorpt i o n bands between 2000 and 2100 cm-l, the assigment was t o l i n e a r CO, e i t h e r i n t h e configurations 1A and 16 (Fig. 3.20);
i t was also assumed t h a t the region
between 1800 and 1950 cm-l could be assigned t o bridged species, i n the configur a t i o n 2. These assignments were based on experimental data on metal carbonyl compounds.
I
C
l
C
l
C
Metal Fig. 3.20. Different c o n f i g u r a t i o n s o f the CO-metal bonding proposed by Eischens and Blyholder, from (59). Blyholder (60) questioned the necessity t o assign the lower frequency t o bridged species and suggested t h a t t h e whole range o f CO bands could be i n t e r preted i n terms o f l i n e a r species. I n f a c t , Eischen's view seems t o agree b e t t e r w i t h t h e experimental evidence. Thus, the range o f CO s t r e t c h i n g frequencies i s c u r r e n t l y c o r r e l a t e d w i t h the number o f metal atoms which a r e a c t u a l l y bonded t o the CO molecule. Nguyen and Sheppard ( 5 9 ) suggest a range o f frequencies as f o l 1ows:
2130-2000 cm-l f o r terminal CO 2000-1880 cm-l f o r t w o f o l d b r i d g e d CO 1880-1800 cm-l f o r t h r e e f o l d coordinated CO 1800 cm-l f o r f o u r f o l d coordinated CO The f i r s t EELS data were r e p o r t e d by Ibach ( 6 1 ) on CO chemisorbed on P t (111). The important breakthraugh o f EELS data comes from t h e o b t e n t i o n o f t h e metal-C0 s t r e t c h i n g mode. I R transmission was l i m i t e d t o t h e frequency range above 1300 cm-l due t o absorption o f l i g h t frequencies below t h a t value. F o r terminal CO t h i s appears a t 480 cm-' whereas t h e C-0 s t r e t c h i n g band i s a t 2100 cm-'.
For b r i d g e CO, metal-C0 s t r e t c h i n g occurs a t 380 cm-I and C-0 a t 1850
cm-l ( F i g . 3.21). T h i s a l l o w s t o unambiguously a s s i g n t h e l o w e r C-0 s t r e t c h i n g value t o multiply-bonded CO. A d d i t i o n a l c o n s i d e r a t i o n s l i k e t h e a n a l y s i s o f surface s t r u c t u r e s , assigns t h e mu1 t i p l e bonded CO t o b r i d g e d between two metal atoms.
v, L
Pt (111) +1 L co
T:90 K
2120
F i g . 3.21. T y p i c a l v i b r a t i o n a l spectrum o f CO chemisorbed a t s a t u r a t i o n on t h e P t ( l l 1 ) surface. The h i g h energy losses a t 1875 and 2120 cm-1 correspond t o C-0 s t r e t c h i n g modes of CO adsorbed r e s p e c t i v e l y on a b r i d g e and on a l i n e a r c o n f i g u r a t i o n . The low energy losses a t 390 and 480 cm-1 a r e due t o CO-metal s t r e t ching modes o f CO on b r i d g e and l i n e a r a d s o r p t i o n . T h i s experiment was used t o c o r r e l a t e t h e C-0 frequency v a l u e w i t h s p e c i f i c adsorption s i t e s , t h e lower t h e frequency corresponding t o t h e h i g h e s t bonding c o o r d i n a t i o n and t h e h i g h e s t frequency t o t e r m i n a l CO. The l o w e r i n g o f t h e f r e quency w i t h respect t o f r e e CO i s explained as a r e s u l t o f backdonation by t h e metal e l e c t r o n s t o t h e 2n*
antibonding s t a t e o f CO. T h i s e f f e c t t o g e t h e r w i t h
e l e c t r o n t r a n s f e r from t h e h i g h e s t occupied CO o r b i t a l (50) t o t h e metal i s nor-
B182
m a l l y taken t o describe the chemisorption o f CO on t r a n s i t i o n metal surfaces. Since t h e p u b l i c a t i o n o f t h i s f i r s t study o f CO adsorption on P t ( l l l ) , many EELS r e s u l t s have been reported. By changing the geometry o f the surface and the
element i n the p e r i o d i c t a b l e an enormous amount o f data e x i s t . We do n o t i n t e n d t o review them. The idea explained above about CO adsorption i s confirmed, b u t there are important exceptions which i t i s worth t o t e l l about. A f i r s t p o i n t i s r e l a t e d w i t h t h e observation o f low CO s t r e t c h i n g frequen-
cies w e l l below the range reported by Nguyen and Sheppard. This puts i n question the conventional p i c t u r e o f CO binding t o t r a n s i t i o n - m e t a l surfaces, whereby i t adsorbs i n an end-on o r i e n t a t i o n w i t h the C end towards the surface. I n cont r a s t w i t h t h a t i n t e r p r e t a t i o n , frequencies i n the range 1150-1330 cm-l, observed on C r ( l l 0 ) (62), have been associated w i t h a lying-down CO (al-CO) r a t i o n (Fig. 3.22). ESDIAD data (63) suggest t h a t al-CO
configu-
i s bonded w i t h the C-0
a x i s nearly p a r a l l e l t o the C r ( l l 0 ) surface. A s i m i l a r s i t u a t i o n occurs i n Fe (100) (64).
I
0
I
1000
2000
ENERGY LOSS
3000 (C177-l)
Fig. 3.22. V i b r a t i o n a l spectrum o f CO chemisorbed on C r ( l l 0 ) a t 120 K as a funct i o n o f gas exposure, from ( 6 2 ) . The spectrum i n panel b) shows low-energy bands a t 1150 and 1330 cm-1 which a r e a t t r i b u t e d t o the alC0 species chemisorbed w i t h the molecular a x i s o r i e n t e d p a r a l l e l t o the surface, Higher exposures reveal the c1 CO and a CO species which have normal C-0 s t r e t c h i n g frequencies. They are i n t z r p r e t e d 2 s bridge and 1 i n e a r c o n f i g u r a t i o n . A t higher coverage two o t h e r CO states (a2-CO and a3-CO) w i t h s t r e t c h i n g
frequencies i n the 1865-2070 cm'l range become populated on C r ( 110) and Fe( 100). They have been assigned t o t h e t r a d i t i o n a l end-on bonded CO on the b r i d g i n g and
B183
atop
s i t e s . T h i s has been e x p l a i n e d on t h e b a s i s o f a r e d u c t i o n o f t h e l a t e r a l
r e p u l s i v e i n t e r a c t i o n s w i t h t h e a l r e a d y p r e s e n t CO i n t h e p a r a l l e l o r i e n t a t i o n . T h e o r e t i c a l l y ( 6 5 ) , t h e l o w - l y i n g CO m o l e c u l e has been examined and a c a l c u l a t i o n o f o r b i t a l s e x i s t . B r i e f l y speaking 50 and 40 d o n a t i o n s t o t h e m e t a l and metal backdonation t o CO n* o r b i t a l s a r e enhanced.
These i n t e r a c t i o n s c o n t r i -
bure t o t h e s t r e n g t h o f c h e m i s o r p t i o n and t o l o w e r i n g o f CO d i s s o c i a t i o n . The changes i n CO c h e m i s o r p t i o n a r e dependent on t h e s i t u a t i o n of t h e element i n t h e t r a n s i t i o n s e r i e s . Broden e t a l . (66) observed t h a t t h e energy b a r r i e r s f o r d i s s o c i a t i o n o f d i a t o m i c molecules, such as CO, decrease g o i n g t o t h e l e f t i n t h e p e r i o d i c t a b l e , and proposed t h e e x i s t e n c e o f b o r d e r l i n e e l e ments, i n t h e t r a n s i t i o n s e r i e s , on t h e l e f t o f which a d s o r p t i o n i s d i s s o c i a t i v e , and on t h e r i g h t o f which i t i s m o l e c u l a r . From o b s e r v a t i o n t h a t CO adsorbs m o l e c u l a r l y on t h e (110) s u r f a c e s o f Fe and W and d i s s o c i a t i v e l y on t h e (100) s u r f a c e s a t room temperature, t h e y proposed t h a t Fe and W mark t h e b o r d e r l i n e region. O t h e r e f f e c t s can a f f e c t t h e frequency v a l u e o f t h e C-0 s t r e t c h i n g v i b r a t i o n . An i m p o r t a n t case i s t h e change produced by t h e presence of a l k a l y adsorpt i o n . A l k a l i m e t a l s a r e v e r y e l e c t r o p o s i t i v e elements. I n i t i a l i n t e r e s t o f ads o r p t i o n o f a l k a l i m e t a l s i s t o f i n d h i g h e l e c t r o n e m i s s i o n cathodes t h r o u g h t h e l o w e r i n g o f t h e work f u n c t i o n . O t h e r t e c h n o l o g i c a l i n t e r e s t i n heterogeneous cat a l y s i s and e l e c t r o c h e m i s t r y i s r e l a t e d w i t h t h e s t r o n g i n t e r a c t i o n o f t h e adsorbed a l k a l i s p e c i e s w i t h molecules. A t y p i c a l example r e p o r t e d by S o m o r j a i ' s group ( 6 7 ) i s t h e i n f l u e n c e o f potassium on t h e a d s o r p t i o n o f CO on P t ( l l 1 ) . The s t r e t c h i n g v i b r a t i o n a l f r e q u e n c y o f CO decreases f o r b o t h l i n e a r and b r i d g e d , when t h e potassium coverage
eK i n c r e a s e s and/or t h e CO coverage de-
creases. The process i s s i t e s e l e c t i v e as t h e p o p u l a t i o n o f l i n e a r CO decreases i n f a v o r o f b r i d g e CO. A l o w frequency of 1400 cm'l
i s f o u n d f o r OK = 0 . 3 . T h i s
l o w frequency v a l u e i n d i c a t e s t h a t t h e CO bond o r d e r i s 1.5 i n c o n t r a s t w i t h t h a t o f 2.4 f o r f r e e CO. The l o w e r i n g o f t h e C-0 s t r e t c h i n g f r e q u e n c y i s e x p l a i n e d by an i n c r e a s e d 2rr* backdonation f r o m d - e l e c t r o n s . T h i s b a c k d o n a t i o n i s m e d i a t e d by K which would t r a n s f e r e l e c t r o n s t o p l a t i n u m and subsequently t h e s e would produce an i n c r e a s e d backdonation. UPS d a t a s u p p o r t t h i s i n t e r p r e t a t i o n as t h e experiment a l work f u n c t i o n changes a c c o r d i n g l y . The s i t e s e l e c t i v i t y i n f a v o u r o f b r i d ged CO can a l s o be i n t e r p r e t e d i n t h e sense t h a t t h e b r i d g e s i t e p e r m i t s a b e t t e r o v e r l a p between d - o r b i t a l s o f P t and t h e symmetry o f t h e 2n* CO o r b i t a l . Data f o r CO on R h ( l l 1 ) i n t h e presence o f a small amount of K by t h e same a u t h o r s (68), d i f f e r from t h e p r e v i o u s d a t a on P t ( l l 1 ) . The l o w e s t CO s t r e t c h i n g frequency appears a t 1500 cm-l, w h i l e a t h i g h e r CO coverages, h i g h e r f r e q u e n c i e s a r e observed ( F i g . 2 . 2 3 ) . A t h i g h CO coverages, t h e l o s s e s a t 1790 and 2030 cm-' a r e s i m i l a r t o t h o s e observed on u n p e r t u r b e d R h ( l l 1 ) s i t e s . From t h e d a t a one
R184
EllERGY LOSS Icm 'I
F i g . 3 . 2 3 . V i b r a t i o n a l s p e c t r a o f CO on K - c o v e r e d R h ( l l 1 ) a t 300 K, B K = 0.1, frorii ( 6 8 ) . The l o w CO s t r e t c h i n g f r e q u e n c y a t 1500 cm-1 i s f o r t h e l o w e s t CO exp o s u r e . I n c: r e a s i n g t h e e x p o s u r e , i n c r e a s e s t h e CO s t r e t c h i n g f r e q u e n c y u n t i l i t reaches Val ues c l o s e t o c l e a n R h ( l l 1 ) . can e x t r a c t t h e f o l l o w i n g f a c t s : ( i ) two k i n d s o f CO a d s o r p t i o n s i t e s e x i s t : K proitioted s i t e s and n e a r l y u n d f f e c t e d o r " c l e a n " s u r f a c e s i t e s . A d s o r p t i o n e n e r g y i n t h e K - a f f e c t e d s i t e s i s h i g h e r t h a n i n t h e c l e a n s u r f a c e s i t e s . ( i i ) K-CO i n t e r a c t i o n i s s h o r t - r a n g e d . New [measurements f o r CO o n t h e P t ( l l l ) / K s u r f a c e show a b e h a v i o u r s i m i l a r t o t h e Rh case. A new o b s e r v a t i o n i s t h a t s e v e r a l K-promoted CO s p e c i e s a r e d e t e c t e d t h r o u g h t h e change i n t h e CO s t r e t c h i n g f r e q u e n c y . The
a u t h o r s s u g g e s t t h a t t h e e l e c t r o n i c e f f e c t o n CO b o n d i n g m e d i a t e d b y K i s s h a r e d by s e v e r a l CO m o l e c u l e s i n t h e c l o s e v i c i n i t y o f t h e p o t a s s i u m atom. Thus, t h e s t r e n g t h o f i n t e r a c t i o n p e r CO m o l e c u l e w i l l v a r y w i t h t h e CO/,K r a t i o ( 6 9 ) . 3.3.3.
La* 'This i s a l s o a n i n t e r e s t i n g m o l e c u l e , p a r t i c u l a r l y because o f i t s i m p o r t a n -
ce i n e l e c t r o c h e m i s t r y . The f i r s t EELS s t u d y was r e p o r t e d b y I b a c h and L e h w a l d
( 7 0 j . They s t u d i e d t h e a d s o r p t i o n o f w a t e r on P t ( 1 0 0 ) . I w i l l d e s c r i b e h e r e my r e s u l t s on w a t e r a d s o r p t i o n o n F e ( l l O ) , w h i c h has s i m i l a r i t i e s f r o m t h e p r e v i o u s systeiii, b u t a l s o i n t e r e s t i n g d i f f e r e n c e s .
A v i b r a t i o n a l spectruni o f t h e i n t e r a c t i o n
o f water w i t h F e ( l l 0 ) i s presen-
t e d i n F i g . 3 . 2 4 . A t l o w e x p o s u r e s , b e l o w 0.2 L, two s h a r p 0-H s t r e t c h i n g bands appear a t 3620 and 3300 cm-'.
The i m p o r t a n t p o i n t i s t h e absence o f t h e s c i s s o r -
t y p e b e n d i n g tmode, w h i c h s h o u l d a p p e a r a t 1600 c m - l . T h i s i s t a k e n a s an e v i d e n -
B185
F i g . 3.24. V i b r a t i o n a l spectrum o f w a t e r chemisorbed on F e ( l l 0 ) a t 130 K, from ( 7 1 ) . A t l o w exposures (0.2 L ) w a t e r d i s s o c i a t e s i n t o OH groups c h a r a c t e r i z e d by sharp OH s t r e t c h i n g bands. H i g h e r exposures r e s u l t on m o l e c u l a r a d s o r p t i o n char a c t e r i z e d by t h e s c i s s o r i n g mode a t 1630 m-1. Hydrogen bonding i s deduced f r o m t h e frequency v a l u e o f t h e OH s t r e t c h i n g bands and i t s broadening, and t h e obs e r v a t i o n o f t h e l i b r a t i o n a l mode a t 700 cm-1. ce t h a t a t l o w exposure w a t e r d i s s o c i a t e s l e a d i n g
OH h y d r o x y l groups on t h e s u r -
face. The o b s e r v a t i o n o f two OH s t r e t c h i n g bands i s i n t e r p r e t e d as due t o t h e o c c u p a t i o n o f two d i f f e r e n t a d s o r p t i o n s i t e s . The 1190 an-' band i s i n t e r p r e t e d as corresponding t o t h e 0-H d e f o r m a t i o n mode. From t h e f a c t t h a t o n l y one mode i s observed which i s EELS a c t i v e i t i s deduced t h a t one o f t h e h y d r o x y l groups i s adsorbed u p r i g h t whereas t h e o t h e r has i t s a x i s i n c l i n e d w i t h r e s p e c t t o t h e surface. A t h i g h exposure ( > 0.2 L ) , we observe t h e band a t 1630 m - l c h a r a c t e r i s t i c o f m o l e c u l a r water. The 0-H s t r e t c h i n g r e g i o n i s a l s o d i f f e r e n t and two bands a r e observed. The one a t 3380 cm-l i s c l o s e t o t h e frequency i n i c e . The o t h e r band appears a t l o w e r frequency o f 3070 cm-', bonding t o t h e m e t a l . The 690 cm-'
which c o u l d be due t o hydrogen
band i s due t o t h e e x c i t a t i o n o f t h e l i b r a -
t i o n a l mode. The broad c h a r a c t e r o f t h e l i b r a t i o n a l and s t r e t c h i n g modes i s char a c t e r i s t i c o f hydrogen bonding o f t h e w a t e r m o l e c u l e . The o b s e r v a t i o n o f hydrogen bonding i n d i c a t e s t h a t w a t e r aggregates f o r m a t t h i s coverage. A n a l y s i s of t h e a n g u l a r dependence o f t h e i n e l a s t i c i n t e n s i t i e s can be used t o deduce t h e o r i e n t a t i o n o f t h e w a t e r m o l e c u l e w i t h r e s p e c t t o t h e surface. The e x c i t a t i o n o f t h e l i b r a t i o n a l mode by t h e d i p o l e mechanism, i n d i c a t e s a p o i n t group symmetry l o w e r t h a n CZv, which i s i n f a c t imposed by h y d r o gen bonding.
B186 On clean Pt(100) and P t ( l l l ) , water i s adsorbed molecularly. However, i n the presence o f adsorbed atomic oxygen, water dissociates above 150 K t o form adsorbed hydroxyl species ( 7 2 ) . I t i s worth n o t i c i n g t h a t t h e d e f i n i t e evidence f o r OH, can o n l y be obtained from EELS.
As reported i n the preceeding paragraph,
t h i s i s deduced from several pieces o f evidence: the absence o f the s c i s s o r i n g mode, t h e frequency value o f t h e 0-H s t r e t c h i n g band and i t s sharpness, the observation o f an 0-H deformation band. 3.3.4.
Unsaturated Hydrocarbons V i b r a t i o n a l data o f chemisorbed hydrocarbons were f i r s t obtained f o r ace-
t y l e n e and ethylene bonded t o P t ( l l 1 ) . We w i l l concentrate our a t t e n t i o n on C2He From the number o f atoms, we deduce t h a t t h i s molecule has 12 normal modes o f v i b r a t i o n . These are reproduced i n F i g . 3.25 taken
from (18). I n a d d i t i o n , we
have t h e modes associated w i t h the bonding o f the molecule t o the metal, which are n o t very important i n t h i s discussion. CZv i s t h e highest symmetry group compatible w i t h t h e symnetry o f both molecule and surface. From i n s p e c t i o n of Fig. 3.25 we see t h a t o n l y f o u r modes are EELS-dipole a c t i v e .
Fig. 3.25. Normal v i b r a t i o n s o f an X2Y4 molecule o f p o i n t group vh (18). I t i s assumed t h a t t h e mass of X i s l a r g e r than t h a t o f Y as i n C2H4 o r C2D4. The modes are described as: symmetric s t r e t c h i n g ( n l ) , asymmetric s t r e t c h i n g ($25, Rg, f i l l ) ! C-C s t r e t c h i n g ($221, CH2 s c i s s o r i n g ($23, $ 2 1 ~ )CH2 ~ wagging ($27, Q8),0CH2 rocking (a6, nl0).
B187 A n a l y s i s of t h e v i b r a t i o n a l d a t a o f chemisorbed hydrocarbons p r o v i d e s i n t e r e s t i n g i n f o r m a t i o n about t h e p r o p e r t i e s which c h a r a c t e r i z e t h i s process. T h i s
is due t o t h e e x i s t e n c e o f t h e c h a r a c t e r i s t i c f r e q u e n c i e s a s s o c i a t e d t o s p e c i f i c groups o f t h e molecule. As i t i s known f r o m t h e s t u d y o f t h e s e molecules i n t h e gas phase, t h e y a r e b e t t e r d e s c r i b e d by independent groups, l i k e f o r example t h e CHn groups ( 7 3 ) . An i m p o r t a n t c h a r a c t e r i s t i c i s t h e f r e q u e n c y of C-C and C-H
s t r e t c h i n g modes. T h i s g i v e s i n s i g h t on t h e bonding mechanism o f t h e m o l e c u l e t o t h e s u r f a c e . Two d i f f e r e n t bonding mechanisms a r e c o n s i d e r e d f o r t h e chemisorpt i o n o f hydrocarbons. A r - d o n o r bond w i t h t h e m e t a l s u r f a c e atoms, and a o-bond which i n v o l v e s r e h y b r i d i z a t i o n o f t h e m o l e c u l a r o r b i t a l s . Going t o t h e s p e c i f i c case o f e h t y l e n e a d s o r p t i o n on P t ( l l l ) , i t s v i b r a t i o n a l spectrum and t h e assignment t o s p e c i f i c modes o f t h e m o l e c u l e a r e r e p o r t e d i n F i g . 3.26,
a c c o r d i n g t o ( 7 4 ) . We f i r s t f i x o u r a t t e n t i o n t o C-C and C-H
s t r e t c h i n g frequency values. From t h e d a t a we deduce t h a t t h e m o l e c u l e i s rehyb r i d i z e d t o sp3 upon c h e m i s o r p t i o n , d i T bonding w i t h t h e m e t a l . T h i s i s
-
b e t t e r seen by l o o k i n g t o Table 3.3, which compares t h e d a t a o f chemisorbed e t h y l e n e w i t h o t h e r r e s p r e s e n t a t i v e species.
x
Pt (lllI+2 L C2HL T=90K Ep=3eV
1000-
0
1000
2000 ENERGY LOSS Icrn-'1
3000
F i g . 3.26. V i b r a t i o n a l spectrum o f e t h y l e n e chemisorbed a t 90 K on P t ( l l 1 ) . The assignment o f bands t o s p e c i f i c v i b r a t i o n a l modes i s shown ( 7 4 ) . Another i m p o r t a n t i n f o r m a t i o n concerns t h e geometry o f t h e a d s o r p t i o n . We see f r o m F i g . 3.26.
t h a t t h e number o f m o l e c u l a r modes which a r e e x c i t e d b y t h e
e l e c t r o n probe i s s i x . I t i s i m p o r t a n t t o a n a l y z e t h i s r e s u l t , i n t h e l i g h t o f t h e e x c i t a t i o n mechanisms o f v i b r a t i o n s . T h i s p o i n t has been l a r g e l y i n v e s t i g a t e d by t h i s a u t h o r i n ( 7 4 ) . The measurement o f t h e a n g u l a r dependence o f t h e
B188 i n e l a s t i c losses shows t h a t o n l y the C - P t s t r e t c h i n g mode, corresponding t o the f r u s t r a t e d t r a n s l a t i o n o f the whole molecule i n t h e d i r e c t i o n normal t o the surface, i s a c t i v a t e d mainly by t h e d i p o l e mechanism. This i s deduced from a higher in-specular i n t e n s i t y o f t h i s mode. A c a r e f u l examination o f the i n t e n s i t i e s o f the other modes shows t h a t a l s o the CH2 wagging and t h e CH2 rocking modes have a small d i p o l e c o n t r i b u t i o n . The e x c i t a t i o n o f the CH2 rocking mode by the d i pole mechanism i n d i c a t e s a lowering o f the symnetry t o Cs w i t h a plane o f symmetry perpendicular t o the C-C a x i s . Another i n t e r e s t i n g observation i s t h a t the CH2 rocking mode i s n o t e x c i t e d a t low coverage, which i n d i c a t e s t h a t i n t h i s
case t h e p o i n t group symmetry i s C2,,.
Thus, bonding o f i n d i v i d u a l molecules on
P t ( l l 1 ) i s high symmetry, b u t by i n t e r a c t i o n w i t h t h e r e s t o f the adsorbed l a y e r , i t changes t o lower symmetry. I n the case o f N i ( l l l ) , a n a l y s i s o f the angular dependence o f i n e l a s t i c i n t e n s i t i e s allows t o deduce t h a t C2H4 i s adsorbed on C2 o r C1 group, even a t low coverage (74). This i s i n c o n t r a s t w i t h the behaviour reported f o r P t ( l l 1 ) . I t i s worth n o t i c i n g t h a t ethylene behaves very d i f f e r e n t l y by thermal dehydrogenation on both surfaces as we w i l l r e p o r t l a t e r on. Table 3.3. Assignment o f frequencies o f chemisorbed ethylene on P t ( l l l ) , N i ( 111) and A g ( l l 0 ) . For the purpose o f comparison, we a l s o i n c l u d e the frequency values o f f r e e C2H4 and Zeise's salt[K[(C2H4)PtC13]H20]. The Z e i s e ' s s a l t i s taken as an example o f strong metal -ethylene i n t e r a c t i o n , which forces the C-C s t r e t c h i n g frequency t o go down t o 1240 cm-1.
Mode CH2 s t r e t c h CH2 scissor C-C s t r e t c h CH2 wag CH2 t w i s t CHz rocking C-metal
C2H4 P t ( 111) cm-
C2H4 Ni(ll1)
2960 1440 1220 1010 780 650 450
2970 1430 1200 1100 880 740 450,600
75
C2H4 Ag(ll0) cm-l(76) 3020 1323 1525 970
C2H4 free cm-l
Zei se' s salt 77
3019 1342 1623 950 825 1000
----
V i b r a t i o n a l spectra o f chemisorbed ethylene have been measured on o t h e r surfaces 1i k e Rh( l l l ) , Ag( 110). etc. The frequencies o f t h e C2H4/Ag( 110) system, incorporated i n Table 3.3.,
show t h a t
ethylene i s T-bonded on the s i l v e r sur-
face. This i s deduced from t h e s l i g h t s h i f t o f the C-C and C-H s t r e t c h i n g f r e quencies. Acetylene has a l s o been t h e o b j e c t o f several studies by EELS. The f i r s t study was performed on P t ( l l 1 ) by Ibach, Hopster and Sexton (78). For the highe s t symmetry s i t u a t i o n , i . e . C2v, the number o f t o t a l l y symmetric modes o f adsorbed acetylene i s three, which correspond t o C-H bending (770 cm-
1 1, C-C
B189 s t r e t c h i n g (1440 cm-')
and C-H s t r e t c h i n g (3015 cm- 1) where t h e values c o r r e s -
pond t o P t ( l l 1 ) . On N i ( l l 1 ) t h e s h i f t o f frequencies t o lower values i s even 2 l a r g e r . I n both cases, t h e s h i f t i s due t o t h e r e h y b r i d i s a t i o n t o sp brought about by t h e bonding t o t h e surface. 3.3.5.
I d e n t i f i c a t i o n o f New Species One o f t h e most spectacular i n f o r m a t i o n coming from t h e a n a l y s i s o f v i b r a -
t i o n a l spectra, i s t h e observation o f new species formed due t o t h e i n t e r a c t i o n o f gas molecules w i t h a m e t a l l i c surface. T h i s s u b j e c t i s a l s o c l o s e l y r e l a t e d w i t h r e a l c a t a l y s t s and i s one o f t h e c h a l l e n i n g problems o f s u r f a c e science l a b o r a t o r i e s . We w i l l consider t h r e e examples which were a t t h e s t a r t i n g p o i n t o f t h i s subject.
I n F i g . 3.27 we show a s e r i e s o f v i b r a t i o n a l s p e c t r a o c c u r r i n g by thermal processing o f a methanol l a y e r chemisorbed on N i ( l l l ) , as r e p o r t e d by Ibach ( 5 ) . A t low temperature, t h e molecule i s chemisorbed a l t o u g h b i g changes occur as can be seen from t h e s h i f t and broadening o f t h e 0-H s t r e t c h i n g band (3681 cm-
1
i n t h e gas phase, 3328 cm-l i n t h e l i q u i d phase and 3215 un-l i n t h e chemisorbed phase). The s i m i l a r i t y o f t h e chemisorbed and l i q u i d phase frequency values o f t h e 0-H s t r e t c h i n g band t o g e t h e r w i t h i t s broadening i n d i c a t e s t h a t a hydrogen bond e i t h e r with t h e s u b s t r a t e o r between t h e molecules i s formed upon chemisorption. Thermal processing up t o 180 K produces t h e disappearance o f t h e 0-H s t r e t ching band i n t e r p r e t e d by a dehydrogenation process l e a d i n g t o a methoxy species (CH30). The most prominent peak a t 1035 cm-'
corresponds t o t h e C-0 s t r e t c h i n g
mode of both CH30 and CH30H. The frequencies of t h e CH3 group appear a t 2920 and 2825 cm-l f o r t h e C-H s t r e t c h i n g and a t 1440 cm-'
f o r t h e CH3 deformation mode.
F u r t h e r processing up t o 300 K produces complete dehydrogenation and f o r m a t i o n o f CO c h a r a c t e r i z e d by t h e band a t 1830 cm-l due t o t h e e x c i t a t i o n o f t h e C-0 s t r e t c h i n g mode. Lehwald and Ibach have r e p o r t e d t h a t dehydrogenation o f methanol occurs even a t 150 K on a stepped N i surface, which again shows t h e h i g h e r a c t i v i t y of defects t o i n t e r a c t w i t h a molecule. The second example t h a t we present here corresponds t o t h e decomposition o acetylene chemisorbed on N i ( 111) by thermal processing. The experiment i s reproduced i n F i g . 3.28 from data o f Oemuth and Ibach ( 7 9 ) . The new species develops by warming a t 450 K. The v i b r a t i o n a l spectrum shown i n panel c ) presents two c h a r a c t e r i s t i c losses a t 2980 and 790 cm-l which a r e assigned t o t h e C-H s t r e t ching and bending modes o f an i s o l a t e d CH species adsorbed on t h e surface. The CH bending v i b r a t i o n o f 790 cm-'
i s characteristic o f a state o f hybridization
somewhere between sp and sp2. As i n t h e case o f methanol dehydrogenation, a stepped n i c k e l s u r f a c e i n t e r a c t s d i f f e r e n t l y w i t h t h e molecule. I n t h i s case
B190
ENERGY LOSS
&-'I
Fig. 3.27. Thermal decomposition o f methanol ( 5 ) . ( a ) V i b r a t i o n a l spectrum corresponding t o a 3 Langmuir exposure o f methanol on N i ( l l 1 ) . The peak a t 3215 cm-1 i s due t o the e x c i t a t i o n o f the 0-H s t r e t c h i n g mode. The formation o f a methoxy species (CH 0) i s deduced from the disappearance o f t h i s peak upon anneal i n g (panel b). f u r t h e r heating (panel c ) produces t h e incomplete conversion o f methoxy t o chemisorbed CO (band a t 1820 cm-1). acetylene dehydrogenation occurs already a t 150 K, and the new species formed by t h i s process i s an i s o l a t e d C=C group (75). 3.3.5.1.
The Thermal Processing o f Ethylene on P t ( l l 1 )
This represents one o f t h e most c o n t r o v e r s i a l systems, which has been t h e o b j e c t o f m u l t i p l e studies and i n t e r p r e t a t i o n s . As d i r e c t l y involved, I w i l l t r e a t t h i s case r a t h e r extensively. Several d i f f e r e n t species have been postulated f o r the phase observed by room temperature adsorption o f ethylene on P t ( 111). The same species i s a l s o formed by r e a c t i o n o f acetylene and hydrogen on the same surface. These species are C2H2 (acetylene) (80) ,CH-CH3
C-CH3 ( e t h y l i d i n e ) ( 8 3 ) . The o l d e s t proposal was sharp (2x2) LEED p a t t e r n f o r posure a t 300 K . I t was shown e i t h e r acetylene o r ethylene
( e t h y l idene) (81). CH-CH2 ( v i n y l group) (82) and
based on the i d e n t i c a l i n t e n s i t y p r o f i l e s o f t h e t h e species formed from acetylene o r ethylene exl a t e r t h a t the formation o f i d e n t i c a l species by was due t o u n i n t e n t i o n a l hydrogenation of acetylene
B191
xfOOO
I\
3L C2H2.T-300K
~
I8L C&,
~
T-300K
0
18L CpDz, 1-300K
DO0 ZOO0 ENERGY LOSS Ion-')
3000
F i g . 3.28. Thermal processing of acetylene (79) ( a ) and ( b ) . v i b r a t i o n a l spectrum corresponding t o i n c r e a s i n g exposures o f C2H2 on N i ( l l 1 ) . I n panel ( c ) , t h e change i n t h e spectrum shows t h a t acetylene has been transformed on a new e n t i t y upon h e a t i n g up t o 450 K. The losses a t 790 and 2980 cm-1 a r e assigned t o t h e CH bending and s t r e t c h i n g modes o f a C-H group bonded t o t h e surface. (81), probably by preadsorbed o r coadsorbed hydrogen from t h e r e s i d u a l gas pressure. The v i b r a t i o n a l spectrum was i n t e r p r e t e d by Ibach as CH-CH3 (81). An a r gument against t h i s i n t e r p r e t a t i o n was p u t forward by Demuth (841, who found t h a t r o u g h l y one q u a r t e r o f t h e t o t a l amount f o hydrogen o f t h e 300 K phase desorbs w i t h a d e s o r p t i o n maximum a t 360
K w i t h o u t s i g n i f i c a n t changes
i n the
photoemission spectrum o f t h e hydrocarbon phase. Then, based on photoemission and v i b r a t i o n a l data, he proposed a v i n y l species t o account f o r 1/4 dehydrogenation,
I n t h i s r a t h e r confusing s i t u a t i o n I was doing new measurements i n t h e hope of f i n d i n g a d d i t i o n a l arguments f a v o r i n g a d e f i n i t e species. The most i n t e r e s t i n g r e s u l t i s reproduced i n F i g . 3.29 ( 8 5 ) . We could d e f i n i t e l y c o r r e l a t e t h e frequency value o f t h e CH s t r e t c h i n g mode w i t h t h e i n t e n s i t y o f t h e 1360 cm-'
loss. When t h e i n t e n s i t y of t h e 1360 cm-l band i s large, t h e CH s t r e t c h i n g band i s centered a t 2880 cm-'.
This a l l o w s t o conclude t h a t t h e 1360 and t h e 2880
cm-I modes correspond t o t h e deformation and s t r e t c h i n g v i b r a t i o n s of t h e same CHn group. The frequency values o f t h e modes a r e o n l y compatible w i t h CH3.
B192
x2LOO
PI I l l l I . 2 L O I c
x 2000
I
l l t l
0
I
1000
I
I
2000 ENERGY LOSS Icrn"I
3000
Fig. 3..29. V i b r a t i o n a l spectrum o f t h e room temperature phase o f ethylene chemisorbed on P t ( l l 1 ) taken in-specular. The more i n t e r e s t i n g p o i n t i s t h e observ a t i o n o f the bands a t 1360 and 2880 cm-l which are a f i n g e r p r i n t o f a CH3 group. The CH s t r e t c h i n g region shows a s i n g l e band. The second p o i n t i s the non-observation o f the 3025-3105 cm-' band on t h e room temperature phase. This band has been taken as a proof f o r the existence o f a CH group. But the i r r e p r o d u c i b i l i t y o f t h i s band would i n d i c a t e t h a t i t i s n o t r e a l l y representative o f t h e species which produces the we1 1-ordered LEED pattern. Experiments by f u r t h e r processing t h e surface a t higher temperatures i n d i c a t e t h a t t h i s band i s due t o dehydrogenation o f the surface species. The t h i r d p o i n t i s r e l a t e d w i t h t h e measurement o f the o f f - s p e c u l a r spectrum ( F i g . 3.30).
together w i t h a very c a r e f u l and complete study by the group
o f Sheppard (86) on t h e organometall i c compound CH2CCo3(C0)9. The important p o i n t i s t h a t t h e o f f specular data which g i v e new bands can be c o r r e l a t e d w i t h v i b r a t i o n a l modes measured by Sheppard on t h a t compound. The assignment o f modes i s presented i n Table 3.4, assuming a C.CH3
species. There i s s t i l l a problem
r e l a t e d w i t h t h e 910 and 780 cm-I bands which do not f i n d an easy i n t e r p r e t a t i o n . I n s p i t e o f t h a t the degree o f agreement w i t h e t h y l i d i n e C-CH3 i s s u f f i c i e n t l y l a r g e t o s t r o n g l y support t h a t t h i s i s i n f a c t the surface species present. P r i v a t e discussions w i t h Sheppard and Ibach suggest t h a t the 900 and 780 cm-' bands could be r e l a t e d w i t h adsorbed hydrogen close t o hydrocarbon species, which could be d i f f e r e n t from adsorbed hydrogen on a bare P t ( l l 1 ) surface. 3.3.6.
Study o f the Reactive Coadsorption o f D2 and CO on a Stepped P t ( l l 1 ) Surface
This experiment was made by sequencial adsorption o f D2 and CO a t 90 K on the surface. The exposures were 10 L o f D2 and 0.5 L o f CO. The v i b r a t i o n a l
B193
1360 I
I 2500
1130 I
-
PI 11111.1.5 1 &HA 7.300 K EpzBcV
v)
c
z 3 K >
0
1000 200 ENERGY LOSS I cm.'p
3000
F i g . 3.30. V i b r a t i o n a l spectrum o f t h e room t e m p e r a t u r e phase t a k e n o u t o f t h e s p e c u l a r d i r e c t i o n . The o f f - s p e c u l a r spectrum makes a p p a r e n t new modes which a r e hidden i n t h e i n - s p e c u l a r spectrum. spectrum corresponding t o t h i s s i t u a t i o n i s shown i n panel b ) o f F i g . 3.31 ( 8 7 ) . The l o s s e s a r e t y p i c a l o f modes o f CO and D adsorbed on t h i s surface. On t h e l o w frequency s i d e o f t h e C-0
s t r e t c h i n g band, t h e r e i s a d d i t i o n a l s t r u c t u r e , which
has been i n t e r p r e t e d t o CO adsorbed on d e f e c t s p r o v i d e d by t h e stepped s u r f a c e (88). A thermal p r o c e s s i n g o f t h i s s u r f a c e up t o 300 K produces a change i n t h e v i b r a t i o n a l spectrum (panel b ) . The more i m p o r t a n t p o i n t i s t h e o b s e r v a t i o n o f a band a t 2225 cm-l which can o n l y be i n t e r p r e t e d as a C-D s t r e t c h i n g mode. Table 3.4. Assignment o f v i b r a t i o n s o f t h e s u r f a c e species formed by room temper a t u r e c h e m i s o r p t i o n o f e t h y l e n e on P t ( l l 1 ) . The s i m i l a r i t y o f f r e q u e n c i e s o f e t h y l i d i n e t r i c o b a l t nanocarbonyl with t h o s e o f t h e s u r f a c e s p e c i e s s t r o n g l y s u p p o r t a C-CH3 group as t h e more reasonable i n t e r p r e t a t i o n . CH3CCo3(C0)9
Assignment CH3 s t r e t c h i n g asymmetric CH3 s t r e t c h i n g symmetric CH3 d e f o r m a t i o n asymmetric CH3 d e f o r m a t i o n symmetric C-C s t r e t c h i n g CH3 r o c k i n g ?
Surface species
cm-'
cm-
2930
2940
2888
2895
1420
1420
1350
1360
1163
1130 1010 910,780
1004 ?
B194
F i g . 3.31. t o a dose (b) After biguously
D2 + CO r e a c t i v e c o a d s o r p t i o n . ( a ) v i b r a t i o n a l spectrum corresponding o f 10 K o f D2 + 0.5 L o f CO on t h e stepped P t [6(111)x(111)] surface. a f l a s h a t 330 K. The o b s e r v a t i o n o f a CD s t r e t c h i n g band shows unamt h a t t h e molecules have r e a c t e d t o form a CDx species.
T h i s i s a c l e a r i n d i c a t i o n t h a t a s y n t h e s i s process between t h e two molecules has taken place. I t i s n o t c l e a r , however, what i s t h e species r e s p o n s i b l e f o r t h a t band, s i n c e t h e small amount o f CDx present, does n o t a l l o w t o show any o t h e r mode o f t h e new s u r f a c e species. From t h e frequency value o f t h e CD s t r e t c h i n g mode, t h e two more probable species a r e =CD2, o r -CD w i t h sp 3 h y b r i d i z a t i o n . The r e a c t i o n o n l y occurs when t h e h e a t i n g o f t h e sample i s done by e l e c t r o n bombardment, which i n d i c a t e s t h a t t h e process i s a c t i v a t e d by i n t e r a c t i o n w i t h t h e e l e c t r o n s . This example shows again t h e i n t e r e s t o f EELS t o s t u d y s u r f a c e chemical r e a c t i o n s . 3.3.7.
5 q p o r t e d Metal C a t a l y s t The a p p l i c a t i o n o f s u r f a c e techniques t o r e a l c a t a l y s t s i s one o f t h e cha-
l l e n g i n g o b j e c t i v e s o f Surface Science. Several attempts w i t h a d i f f e r e n t degree o f p r o x i m i t y t o t h a t o b j e c t i v e have been r e p o r t e d . The p o s s i b i l i t i e s o f EELS a r e associated w i t h t h e o b s e r v a t i o n o f non-dipole e x c i t a t i o n . T h i s i s so, because we do n o t expect h i g h e l a s t i c i n t e n s i t i e s on non-single c r y s t a l s . An i n t e r e s t i n g example has been r e c e n t l y r e p o r t e d by Venus e t a l . (89). Previous work on supported Rh p a r t i c l e s was performed by Dubois e t a l . (90). They use an A1 p o l y c r y s t a l l i n e f o i l as a s u b s t r a t e , which i s cleaned i n UHV by cycles o f A r s p u t t e r i n g and annealing. The A1 f o i l i s o x i d i z e d i n vacuum w i t h an estimated o x i d e depth o f 6
8.
M e t a l l i c p a r t i c l e s o f P t a r e d e p o s i t e d on t h e
s u b s t r a t e by vacuum e v a p o r a t i o n and c h a r a c t e r i z e d by Transmission E l e c t r o n M i croscopy. I s l a n d s o f P t o f diameter 1.2 nm a r e found. EELS data a r e reproduced i n F i g . 3.32. The i n t e n s i t y o f t h e e l a s t i c peak decreases when P t i s deposited on t h e s u b s t r a t e , b u t t h e i n e l a s t i c l o s s e s due t o s u b s t r a t e modes decrease much more s t r o n g l y . A d s o r p t i o n o f CO produces a s i n g l e
B195
Fig. 3.32 EELS spectra corresponding t o an A1203 substrate which has a deposit i o n o f 6.6 P t atoms nm-2 and was exposed t o 50 L o f CO (89). The CO s t r e t c h i n g region shows a s i n g l e band a t 2080 cm-1 c h a r a c t e r i s t i c o f l i n e a r bonding. The increase o f i n e l a s t i c i n t e n s i t y f o r frequencies below 2000 cm-1 is due t o t h e substrate. Primary energy was 3.9 eV. band a t 2080 cm-',
c h a r a c t e r i s t i c o f terminal CO bonding. No bands due n e i t h e r
t o the C - P t s t r e t c h i n g mode, nor t o the b r i d g i n g bond are observed. The d i p o l e mechanism i s responsible f o r the e x c i t a t i o n o f the substrate modes, b u t t h e i m pact s c a t t e r i n g i s the main mechanism f o r the v i b r a t i o n s o f CO adsorbed on the metal p a r t i c l e s . 3.4.
SUMMARY AND OUTLOOK
I n t h i s chapter, I have t r i e d t o g i v e an idea o f the EELS technique. I have p a r t i c u l a r l y emphasized
the importance o f t h e e x c i t a t i o n mechanism, which even i f i t can complicate the i n t e r p r e t a t i o n , i t gives a d d i t i o n a l information r e s u l t i n g on a b e t t e r knowledge o f the system. I have i l l u s t r a t e d the use o f EELS by g i v i n g reference t o work w i t h which I am c l o s e l y f a m i l i a r . My i n t e n t i o n was t o show how powerful i s the technique f o r surface chemistry. Due t o the l i m i t e d space a v a i l a b l e t h e r e are other important issues which I d i d n o t t r e a t b u t which
I would l i k e t o mention s h o r t l y . ( i ) Study o f metal surface phonons. This is a very important c o n t r i b u t i o n from EELS, made possible by the increased r e s o l u t i o n o f e l e c t r o n spectrometers. Even if the i n t e r p r e t a t i o n o f v i b r a t i o n a l data i s r e l a t i v e l y easy, simple cases such hydrogen chemisorption are s t i l l n o t f u l l y understood. The information on
B196 the substrate surface modes should h e l p enormously t o resolve t h i s problem. ( i i ) The use o f t h e o p t i c a l probe. Reflexion-Absorption I n f r a r e d Spectroscopy (RAIRS), has a l s o been used t o study the v i b r a t i o n a l spectrum o f s i n g l e c r y s t a l surfaces. The major shortcomings o f RAIRS are i t s l i m i t e d energy range and i t s low s e n s i t i v i t y . On t h e other hand, EELS has t h e problems o f low resol u t i o n ( > 2 meV) and t h e requirement o f a vacuum ambient. The use o f a commerc i a l l y a v a i l a b l e F o u r i e r transform (FT), allows t o reach s e n s i t i v i t i e s compar a b l e t o those obtained w i t h EELS. This has been very n i c e l y demonstrated by E r l e y (91).
Fig. 3 . 3 3 shows the v i b r a t i o n a l spectra o f a monolayer o f TCNE
(tetracyanoethylene) adsorbed on Cu( 111). The data by FT-RAIRS are obtained and compared w i t h those obtained by EELS i n the same UHV experimental chamber. These data show t h a t t h e use o f the o p t i c a l probe w i t h such high s e n s i t i v i t y can be very important t o o b t a i n r e s u l t s on high pressure conditions, needed f o r r e a l c a t a l y t i c studies.
F i g . 3 . 3 3 . ( a ) EELS spectrum and ( b ) RAIRS spectrum o f TCNE monolayer coverage ( c ) R A I R S spectrum of TCNE a t 2 monolayers (91).
( iii) Dynamic experiments. Recent experiments have demonstrated t h e poss i b i l i t y t o o b t a i n time-resolved e l e c t r o n energy l o s s spectra (92). This allows t o enter i n the f i e l d of surface k i n e t i c s and dynamic p r o p e r t i e s of t h e adsorbate-substrate system. The processes o f adsorption, d i s s o c i a t i o n , r e a c t i o n and desorption o f molecules from t h e surface and t h e r o l e o f precursors can be observed. A l l these developments together with the more established methods, g i v e t o
B197 EELS a v e r y open f u t u r e t o he1.p s o l v i n g t h e many c h a l l e n g i n g problems o f s u r f a c e p h y s i c s and c h e m i s t r y . I t s c o n t r i b u t i o n c o n t i n u e s t o be e s s e n t i a l f o r a b e t t e r u n d e r s t a n d i n g o f s u r f a c e processes.
3.5 ACKNOWLEDGEMENTS The a u t h o r has b e n e f i t e d o f a c o l l a b o r a t i v e s t a y i n t h e I n s t i t u t f u r Grenzflachenforschung und Vacuumphysi k (IGV) d e r Kernforschungsanlage, Jiil i c h (FRG), thanks t o a f e l l o w s h i p by t h e Alexander von Humboldt (AvH) Foundation. I t i s a p l e a s u r e t o acknowledge t h e f r i e n d l y atmosphere o f t h e I G V , t h e a d v i c e
and t h e many h e l p f u l d i s c u s s i o n s with H. Ibach, S. Lehwald, W. E r l e y and H.D. Bruchman. P a r t of t h e work r e p o r t e d h e r e was performed w i t h an EELS system donated b y t h e AvH Foundation. 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 29 30 31
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.
B198 32 W. Reimer, V. Penka, M. Skottke, R.J. Behm, G. E r t l and W. M o r i t z , Surf. Sci 186 (19871 45. 33 L. O i l 6 and A.M: Bard, Surf. Sci., 137 (1984) 607. 34 B. Voigtlander, S. Lehwald and H. Ibach, (1988), t o be published. 35 S. Andersson, Chem. Phys. L e t t . , 55 (1978) 185. 36 W. Ho. N.J. DiNardo and E.W. Plumner, J . Vac. Sci. Technol. 17 (1983) 134. 37 M.A. Barteu, J.Q. Broughton and 0. Menzel, Surf. Sci. 133 (1983) 443. 38 H. Conrad, R . Scala, W. Stenzel and R. Unwin, J . Chem. Phys., 8 1 (1984) 6371. 39 S. Andersson, S o l i d State Commun.. 20 (1976) 229. 40 S. Lehwald and H. Ibach, i n R. Caudano, J.M. G i l l e s and A.A. Lucas, "Vibrat i o n s a t Surfaces" Plenum Press, New York, 1982. 41 T.H. Upton and W.A. Goddard 111, Phys. Rev. L e t t . , 46 (1981) 1635. 42 J . StBhr, R. Jaeger and T. Kendelewitz, Phys. Rev. L e t t . , 49 (1982) 142. 43 G. A l l a n and J. L6pez, i n R. Caudano, J.M. G i l l e s and A.A. Lucas. "Vibrat i o n s a t Surfaces" Plenum Press, New York, 1982. 44 J.E. Demuth, N.J. DiNardo and G.S. C a r g i l l , Phys. Rev. L e t t . , 50 (1983) 1373. 45 T.S. Rahman, D.L. M i l l s , J.E. Black, J.M. S z e f t e l , S. Lehwald and H. Ibach, Phys. Rev. 8, 30 (1984) 589. 46 H. Ibach and 0. Bruchman, Phys. Rev. L e t t . , 44 (1980) 36. 47 A.M. Bar6 and L. 0116, Surf. Sci., 126 (1983) 170. 48 A.M. Bar6. G. Binnig, H. Rohrer, Ch. Gerber, E. S t o l l , A. B a r a t o f f and F. Salvan, Phys. Rev. L e t t . , 52 (1984) 1304. 49 G. Binnig and H. Rohrer, I B M J . Res. Dev., 30 (1986) 355. 50 M. Wuttig, R. Franchi and H. Ibach, J . E l e c t r o n Spect. Rel. Phenom., 44 (1987) 317. 51 H. Steininger, S. Lehwald and H. Ibach, Surf. Sci., 123 (1982) 1. 52 A.M. Bar6 and H. Ibach, J . Chem. Phys., 7 1 (1979) 4812. 53 R. Imbihl and J.E. Demuth, Surf. Sci., 173 (1986) 395. 54 N.D. Shinn and T.E. Madey, Surf, Sci., 176 (1986) 635. 55 W. E r l e y and H. Ibach. S o l i d State Commun., 37 (1981) 937. 56 A.G. Baca, L.E. Klebanoff, M.A. Schulz, E. Papanazzo and D.A. S h i r l e y , Surf. Sci., 171 (1986) 225. 57 C. A s t a l d i , P. Geng and K. Jacobi, J . E l e c t r o n Spect. Rel. Phenom., 44 (1987) 175. 58 R.P. Eischens, W.A. P l i s k i n and S.A. Francis, J . Chem. Phys., 22 (1954) 194. 59 T.T. Nguyen and N. Sheppard, i n R.E. Hester and R.H.J. Clark, Eds., "Advances i n I n f r a r e d and Raman Spectroscopy", Vol 5, Heyden, London, 1978. 60 G. Blyholder, J. Phys. Chem., 68 (1964) 2772. 6 1 H. Froitzheim, H. Hopster, H. Ibach and S. Lehwald, Appl. Phys., 13 (1977) 147. 62 N.D. Shinn and T.E. Madey, Phys. Rev. Lett., 53 (1984) 2481. 63 N.D. Shinn and T.E. Madey, J. Chem. Phys., 83 (1985) 5928. 64 C. Benndorf, B. Kriiger and F. Thieme, Surf. Sci., 163 (1985) L675. 65 S.P. Mehandru and A.B. Anderson, Surf. Sci., 201 (1988) 345. 66 G. Broden, G. Gafner and H. Bonzel, Appl. Phys., 13 (1977) 331. 67 J.E. Crowell, E.L. Garfunkel and G.A. Somorjai, Surf. Sci., 121 (1982) 303. 68 J.E. Crowell and G.A. Somorjai, Appl. Surf. Sci., 19 (1984) 73. 69 H. Bonzel , Surf. Sci Reports, 8 (1988) 43. 70 H. Ibach and S. Lehwald, Surf. Sci 91 (1980) 187. 7 1 A.M. Bar6 and W.Erley, J . Vac. Sci. Technol., 20 (1982) 580. 72 G.B. Fischer and B.A. Sexton, Phys. Rev. L e t t . , 44 (1980) 683. 73 P.C. Cross and J.H. Van Vleck, J. Chem. Phys., 1 (1933) 350. 74 A.M. Bar6, S. Lehwald and H. Ibach, i n R. Caudano, J.M. G i l l e s and A.A. Lucas, V i b r a t i o n s a t Surfaces", Plenum Press, New York, 1982. 75 S. Lehwald and H. Ibach, Surf. Sci , 89 (1979) 425. 76 C. Backx, C.P.M. de Groot and P. Biloen, i n Proceedings o f the 4 t h I n t e r n a t i o n a l Conference on S o l i d Surfaces and t h e 3 r d European Conference on Surface Science, Cannes. 1980, p. 248. 77 E. Maslowski, J r . " V i b r a t i o n a l Spectra o f Organometall i c Compounds", John S i l l e y & Sons, New York, 1977, 240.
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B201 Chapter 4
NHR OF ADSORBED MOLECULES USED AS PROBES FOR SURFACE INVESTIGACION
J. Fraissard L a b o r a t o i r e de Chimie des Surfaces, 4 Place Jussieu, 75252 P a r i s Cedex 05 (France) 4.1
INTRODUCTION Porous s o l i d s such as s i l i c a , t i t a n i a , alumina, z e o l i t e s p l a y an i m p o r t a n t
r o l e i n many processes i n t h e chemical i n d u s t r y where t h e y a r e used m a i n l y as c a t a l y s t s o r as adsorbents. The s t a t e o f adsorbed molecules has been t h e s u b j e c t o f numerous NWR i n v e s t i g a t i o n s , m a i n l y f o r two reasons: i)I t i s c l e a r t h a t t h e d e t e r m i n a t i o n of r e a c t i o n niechanisms i n heterogeneous c a t a l y s i s r e q u i r e s knowledge about t h e s t a t e o f t h e chemisorbed complex; i i ) B u t chemisorbed o r p h y s i sorbed molecules can a l s o be used t o i n v e s t i g a t e t h e chemical and p h y s i c a l properties o f sol i d s surfaces. There a r e e x c e l l e n t books on NMR (1-3) and r e v i e w a r t i c l e s on NMR s t u d i e s o f a d s o r p t i o n ( 5 . 1 5 ) . For a deeper u n d e r s t a n d i n g t h e s e p u b l i c a t i o n s s h o u l d be c o n s u l t e d . T h i s s e c t i o n g i v e s o n l y a s h o r t r e v i e w o f t h e p h y s i c a l background o f t h e main t y p e s of n u c l e a r magnetic i n t e r a c t i o n s a f f e c t i n g t h e NMR s p e c t r a o f s o l i d s and adsorbed molecules, and o f t h e v a r i o u s e x p e r i m e n t a l t e c h n i q u e s used i n this field. NMR i s w e l l e s t a b l i s h e d among chemists and has proven t o be e x t r e m e l y
u s e f u l i n t h e i n v e s t i g a t i o n o f l i q u i d s . However s p e c t r a cannot n o r m a l l y b e measured i n s o l i d s o r chemisorbed molecules i n t h e same way as t h e y a r e r o u t i n e l y o b t a i n e d i n s o l u t i o n . The reason f o r t h i s i s t h e e x i s t e n c e o f n e t a n i s o t r o p i c i n t e r a c t i o n s which i n t h e l i q u i d s t a t e a r e averaged by t h e r a p i d thermal m o t i o n o f molecules. The h i g h - r e s o l u t i o n NMR s p e c t r a o f m o l e c u l e s i n t h e l i q u i d s t a t e c o n t a i n a w e a l t h o f i n f o m a t i o n . The p r e c i s e v a l u e of t h e r a d i o frequency absorbed by t h e v a r i o u s n u c l e i depends s u b t l y on t h e i r chemical environments g i v i n g r i s e t o "Chemically s h i f t e d " s i g n a l s which a r e g
0.1 Hz
wide. The parameters d e r i v e d f r o m such s p e c t r a ( p o s i t i o n s , w i d t h s , i n t e n s i t i e s and m u l t i p l i c i t i e s of 1 ines, r e l a x a t i o n mechanisms and r a t e s ) p r o v i d e u n i q u e i n f o r m a t i o n about t h e s t r u c t u r e , c o n f o r m a t i o n and m o l e c u l a r motion. When t h e m o t i o n o f t h e species i s c o n s i d e r a b l y r e s t r i c t e d , even more so when i t i s suppressed, c o n v e n t i o n a l NbIR, i n s t e a d o f sharp s p e c t r a l l i n e s , y i e l d s a b r o a d l i n e up t o 100 kHz wide which conceals i n f o r m a t i o n s o f i n t e r e s t t o t h e chemist. T h i s s i t u a t i o n i s f u r t h e r c o m p l i c a t e d i n s u r f a c e s t u d i e s by t h e reduced number o f n u c l e a r s p i n s and t h e chemical inhomogneity o f t h e adsorbed s t a t e s due t o t h e heterogeneity o f t h e a c t i v e s i t e s .
B202
NMR s t u d i e s on s o l i d s have mushroomed these l a s t t e n y e a r s s i n c e t h e
development of instruments and s o p h i s t i c a t e d techniques has made i t p o s s i b l e t o e l i m i n a t e , o r a t l e a s t , t o s u b s t a n c i a l l y reduce t h e l i n e widths, and consequently, t o o b t a i n h i g h r e s o l u t i o n s p e c t r a on r i g i d systems (16-24). We a r e now going t o r e c a p i t u l a t e t h e v a r i o u s n u c l e a r magnetic i n t e r a c t i o n s and t h e means o f suppressing those which a r e a nuisance. 4.2
NMR INTERACTIONS
I n general t h e n u c l e a r s p i n Hamiltonian i s represented by t h e sum (4.1)
..
HZ accounts f o r t h e Zeeman i n t e r a c t i o n o f t h e n u c l e a r magnetic moment w i t h t h e a p p l i e d f i e l d Bo. HRF r e s u l t s from t h e i n t e r a c t i o n between t h e n u c l e a r s p i n and *
r
t h e time-dependent radiofrequency f i e l d
ill.
These two terms a r e under t h e expe-
r i m e n t a l i s t ' s c o n t r o l , w h i l e t h e f i v e others, now b r i e f l y t r e a t e d , depend on t h e
.
nucleus i n q u e s t i o n and i t s environment. These f i v e i n t e r a c t i o n s Hi can be represented by t h e equation:
.
+ = + Hi = A. T. B
(4.2)
-+ + A and B a r e v e c t o r s such as t h e magnetic f i e l d
2,.
t h e n u c l e a r spins o r e l e c t r o n
spins. To t a k e i n t o account t h e i r amplitude and o r i e n t a t i o n , t h e t r i - d i m e n s i o n a l
.
n a t u r e o f t h e i r mutual i n t e r a c t i o n i s d e s c r i b e d by a 3x3 m a t r i x o r t e n s o r . Whatever t h e n a t u r e o f t h i s i n t e r a c t i o n Hi,
t h i s t e n s o r can be d i a g o n a l i z e d by
chosing an a p p r o p r i a t e c o o r d i n a t e system and i s t h e r e f o r e d e f i n e d by t h r e e p r i n c i p a l element ul,
uZ2 and u33.
Because o f r a p i d (on t h e NMR time-scale) o r n o r m a l l y i s o t r o p i c movement o f t h e molecules, f o r l i q u i d s a mean v a l u e equal t o (ull
+
oZ2 + u33)/3 i s detected.
Sometimes ( i n t h e case o f d i p o l a r and quadrupolar i n t e r a c t i o n s ) t h i s mean v a l u e i s zero. The spectra o f s o l i d s , on t h e o t h e r hand, a r e more complex, because they i n c l u d e an o r i e n t a t i o n f a c t o r , b u t f o r t h e same reason t h e y c o n t a i n more tinformation than those o f 1 i q u i d s . 4.2.1
D i p o l a r Nuclear I n t e r a c t i o n I t a r i s e s from t h e d i r e c t d i p o l e - d i p o l e i n t e r a c t i o n s between n u c l e i . The
c l a s s i c a l i n t e r a c t i o n energy E between two magnetic moments pi and P . which a r e J separated by a d i s t a n c e r j j i s :
B203
F o r a s o l i d c o n t a i n i n g a s i m p l e t y p e o f s p i n I o f magnetogyric r a t i o y 4 , t h i s i n t e r a c t i o n niay be w r i t t e n f o r two i s o l a t e d s p i n s Ii and I
j
(4.4) where r . . i s t h e i n t e r n u c l e a r d i s t a n c e and D t h e d i p o l a r c o u p l i n g t e n s o r . I f t h e
-+
1J
i n t e r n u c l e a r v e c t o r i j makes an a n g l e 8 . . w i t h t h e magnetic f i e l d Bo, t h e d i p o l e
vD can be w r i t t e n
i n t e r a c t i o n frequency
2
9 =
,3
WD
Y?fi
4
(1-3 cos 2
1
1J
ei .) (4.5)
3 r ij
I n t h e same way, t h e i n t e r a c t i o n between two u n l i k e s p i n s I and S i s
~
YiYSh
A
H D = Hi s =
3
2
+
=
+
I . D. S
js and t h e d i p o l e i n t e r a c t i o n frequency vD
2mJ” =
WD
=
71
Yi
Ys n
(1-3 cos 2 8 . .) 1J
(4.7)
3 r is
These equations r e v e a l t h a t t h e d i p o l a r c o u p l i n g a r e : i ) independent o f t h e magn e t i c f i e l d s t r e n g t h 6,;
i i ) p r o p o r t i o n a l t o r-3and a r e t h e r e f o r e s e n s i t i v e t o
s l i g h t changes i n i n t e r n u c l e a r d i s t a n c e s . A 10%change i n a bond l e n g t h w i l l cause a 33% change i n t h i s i n t e r a c t i o n ; i i i ) s t r o n g e s t a t 8 = 0 and v a n i s h f o r e = 54O.44’ ( c o r r e s p o n d i n g t o 1-3 cos 2 8 = 0) known as t h e magic a n g l e . I t can be shown t h a t f o r r a p i d i s o t r o p i c m o t i o n t h e mean v a l u e < cos2 8.. > = 1/3 and 1J
t h e d i p o l a r c o u p l i n g vanishes. T h i s i s why d i p o l a r i n t e r a c t i o n s a r e e l i m i n a t e d from t h e NMR s p e c t r a o f l i q u i d s . Consider, f o r example, a p o p u l a t i o n o f s p i n s 1/2 w i t h p a i r w i d e d i p o l a r i n t e r a c t i o n i n a c r y s t a l , and assume t h a t these p a i r s , i j , a r e v e r y d i s t a n t f r o m each o t h e r ( i s o l a t e d two-spin system) and a l l p a r a l l e l . I n s t e a d o f a s i n g l e l i n e
B204
Fig. 4.1 a) Resonance+at the Larmor frequency. b) S p l i t t i n g i n a cr- i t a l . 8 i s f i x e d w i t h respect t o B VD i s given by equation 4.4. c ) Powder p a t t e r n due t o nuclear d i p o l a r i n t e r a c ? i o n w i t h a s p i n -1/2 I nucleus. Dashed l i n e s show t h e c o n t r i b u t i o n s from the MI = 1/2 and MI = -1/2 s t a t e s o f I . Au i s the frequency s h i f t due t o t h e d i p o l a r i n t e r a c t i o n . 04 i s t h e value o f e f o r the mI = 1/2 case. C rresponding numbers i n parentheses show values f o r mI = -1/2. D = = 3/2 yqh
.
a t the Larmor frequency defined by uL = YEo
(Bo f i x e d ) ( F i g . 4.1.a) one would
d e t e c t two components separated from t h e above t h e o r e t i c a l p o s i t i o n by 3/4 Y$ M;&)- (named the Pake doublet) ( F i g . 4.1.b). The s i g n corresponds toriJthe o r i e n t a t i o n s of t h e spins r e l a t i v e t o 30.
B205
As a c a t a l y s t i s always a powder, t h e sample c o n t a i n s m i c r o c r y s t a l l i t e s o r i e n t e d w i t h r e s p e c t t o t h e f i e l d a t a l l p o s s i b l e angles w i t h equal probabil i t i e s . Then t h e NMR spectrum c o n s i s t s o f a band o f l i n e s which can be used t o i n f e r t h e i n t e r n u c l e a r distance. The d i p o l a r p a t t e r n represented F i g . 4.l.c,
which has some broadening
i n c l u d e d i n o r d e r t o simulate t h e i n f l u e n c e o f a l l o t h e r neighbours, shows a 2 w i d t h o f roughly 3Yifi. For Ca SO4 2H20 o r CC13-C-COOH which a r e examples o f
.
homonuclear two-spin systems, t h i s w i d t h corresponds t o about 40 and 20 kHz, r e s p e c t i v e l y . For a d i r e c t l y bonded 1 3 C - l H p a i r , t h e l i n e w i d t h o f t h e 13C NMR component o f a s o l i d i s about 30 kHz. Hence t h e d i p o l a r i n t e r a c t i o n c o n s t i t u t e s a dominating line-broadening, unless a s u i t a b l e technique i s used t o e l i m i n a t e it.
4.2.2
Chemical S h i f t T h i s i n t e r a c t i o n i s due t o t h e s h i e l d i n g e f f e c t on t h e n u c l e u s o f t h e f i e l d s
produced by t h e surrounding e l e c t r o n s . I t i s described by t h e Hamiltonian *=+ . HCS = Y.h 1.o.R
(4.8)
1
where
i s a second rank tensor known as t h e chemical s h i e l d i n g tensor. T h i s
i n t e r a c t i o n i s l i n e a r w i t h the applied f i e l d .
can be described b y t h r e e
p r i n c i p a l elements ( u l l , u22, a33) and t h e t h r e e angles which d e f i n e t h e o r i e n t a t i o n o f t h e p r i n c i p a l a x i s . From these elements we may d e f i n e t h e i s o t r o p i c chemical s h i f t : 1
uIso = 3
(91
+
O22
+
(4.9)
O33)
The s h i e l d i n g a n i s o t r o p y
6 = u~~ - oIs0 = 2/3 ~ 3 -3 1/3 (all
+ aZ2)
(4.10)
and t h e s h i e l d i n g asymmetry f a c t o r
ri'
9 2 O33
- all - aIso
The t h e o r e t i c a l chemical s h i f t a n i s o t r o p y i s i l l u s t r a t e d i n F i g . 4.2.
(4.11)
In a
s i n g l e c r y s t a l , i n a f i x e d o r i e n t a t i o n w i t h r e s p e c t t o t h e magnetic f i e l d , a s i n g l e sharp l i n e w i l l be observed f o r each m a g n e t i c a l l y unique o r i e n t a t i o n o f a p a r t i c u l a r nucleus t o t h e f i e l d d i r e c t i o n . The p o s i t i o n o f these l i n e s w i l l
B206
I
‘GO Chemical shift
Fig. 4.2 a ) Single crystal of fixed orientation. b ) and c ) rheoreti cal chemical s h i f t spectrum of a randomly oriented powder of spins subje t t o b) an asymmetric s h i f t anisotropy, c) an axially symmetric s h i f t anisotropy.-d) Chemical s h i f t isotropically averaged through rapid thermal motion.
change with the orientation of the crystal. For a powder sample a signal w i l l result from each random c r y s t a l l i t e orientation and a broad l i n e will r e s u l t , the shape of which will depend on the principal element of the shielding tensor. As i s shown in Fig. 4.3, i t i s possible t o obtain u l l , uZ2 and 033 experimentally for spin 1/2 nuclei directly from the NMR spectrum o f the s t a t i c sample, provided dipolar interactions a r e small (uI1 # uZ2 # u ~ ~ ) .
B207
F i a . 4.3 Powder D a t t e r n r e o r e s e n t i n o t h e 13C chemical s h i f t a n i s o t r o o v o f H LK-O . u2 and u3 a r e ' resonance' f r e q u e n c i e s c o r r e s p o n d i n g t o uzz v a l u e s o r o l l , 022 and a3 V j i s t h e resonance frequency c o r r e s p o n d i n g t o u which r e s u l t s from jy t h e jt2 o r i e n t a t i o n o f a c r y s t a l .
V19
.
When two o f t h e elements a r e i d e n t i c a l , t h e s h i e l d i n g p a t t e r n i s " a x i a l l y symmetric". The s h i e l d i n g element o f t h e a x i s i s d e s c r i b e d as uII and t h e two o t h e r s as a., I n t h a t case n= 0. F i n a l l y , f o r a s p h e r i c a l l y symmetric t e n s o r , t h e a n i s o t r o p y 5 and asymmetry rl a r e z e r o . 4.2.3
E f f e c t o f Unpaired E l e c t r o n s We have j u s t seen t h a t an NMR spectrum can be i n f l u e n c e d by t h e m a g n e t i c
c o u p l i n g o f e l e c t r o n s t o t h e nucleus. I n d i a m a g n e t i c m a t e r i a l s t h i s c o u p l i n g a r i s e s f r o m magnetic f i e l d s generated by t h e m o t i o n o f e l e c t r i c charges, r e s u l t i n g i n chemical s h i f t s . B u t i n paramagnetic m a t e r i a l s t h e r e a r e a l s o f i e l d s which o r i g i n a t e f r o m t h e magnetic moment a s s o c i a t e d w i t h e l e c t r o n s p i n . These moments a r e much l a r g e r t h a n t h o s e a s s o c i a t e d w i t h n u c l e a r s p i n . Thus t h e presence o f u n p a i r e d e l e c t r o n s can have p r o f o u n d e f f e c t s on t h e appearance o f an NMR spectrum. T h i s i n t e r a c t i o n was f i r s t i n t r o d u c e d by Fermi t o account f o r
h y p e r f i n e s t r u c t u r e i n a t o m i c s p e c t r a . The c o r r e s p o n d i n g " h y p e r f i n e Hamil t o n i a n " can be w r i t t e n i n general as
.
+ = + HHF = I. T. S
(4.12)
where T i s a second-rank t e n s o r . T h i s t e n s o r may be decomposed i n t o t h e sum o f two terms + + - + = + . HHF = a. I . S t I . T i . S +
(4.13)
+
a. I . S i s a t e r m r e p r e s e n t i n g t h e i s o t r o p i c c o n t a c t i n t e r a c t i o n between t h e + =
-+
nucleus and u n p a i r e d e l e c t r o n s h a v i n g 2 c h a r a c t e r . I . T i . S i s a t e r m represen-
B208
t i n g t h e a n i s o t r o p i c d i p o l a r coup1 i n g between the nucleus and unpaired e l e c t r o n s i n non-spherically symmetric o r b i t a l s . The i s o t r o p i c hyperfine i n t e r a c t i o n has i t s o r i g i n i n t h e f i n i t e e l e c t r o n density a t the nucleus, which i s c h a r a c t e r i s t i c o f s-type o r b i t a l s . Fermi has shown t h a t f o r systems w i t h one e l e c t r o n the i s o t r o p i c i n t e r a c t i o n energy i s given approximately by
(4.14) 2 where [+ ( o ) ] i s the square amplitude o f the e l e c t r o n i c wavefunction a t t h e nucleus. Since t h i s i n t e r a c t i o n requires a f i n i t . e e l e c t r o n d e n s i t y a t the nucleus, i t i s a l s o known as a spin-contact term (as opposed t o a s h i e l d i n g effect). Non-spherically symmetric o r b i t a l s (e.g. p,d,f..
.) having unpaired s p i n
also can provide hyperfine s p l i t t i n g . I n t h i s case, t h e coupling between e l e c t r o n and nuclear s p i n i s d i p o l a r i n character. I t s c l a s s i c a l expression i s e x a c t l y analogous t o equation (4.3)
where
f
i s the radius vector from
ce t o GN.
The quantum mechanical correspondance i s obtained w i t h
(4.15)
(4.16)
where 8 i s the angle between t h e a x i s o f the dipoles and the l i n e j o i n i n g them. KNIGHT S h i f t I t i s the doininant e f f e c t i n the NMR spectra o f metal n u c l e i and molecules
adsorbed on the metal p a r t i c l e s . I t i s caused by the Fermi contact i n t e r a c t i o n o f conduction e l e c t r o n s w i t h n u c l e i (25, 26). I t i s t h e same type as the previous i s o t r o p i c hyperfine i n t e r a c t i o n . The i n t e r a c t i o n o f a g i v e n nuclear s p i n I
B209 w i t h t h e e l e c t r o n s i s obtained by summing t h e mean values o f t h e h y p e r f i n e i n t e r a c t i o n s o f t h e nuclear s p i n w i t h a l l t h e conduction e l e c t r o n s . I t r e s u l t s i n a s c a l a r h y p e r f i n e i n t e r a c t i o n s constant
(4.17)
where <[$ (0)12 >F denotes t h e average d e n s i t y o f t h e conduction e l e c t r o n wavef u n c t i o n s w i t h t h e Fermi energy EF. Due t o t h e r a p i d motion o f t h e conduction e l e c t r o n s o n l y t h e average value o f a l l p o s s i b l e h y p e r f i n e s p l i t t i n g s i s observed, l e a d i n g t o t h e s h i f t
(4.18)
x i s t h e P a u l i s u s c e p t i b i l i t y o f t h e conduction e l e c t r o n s . T h i s chemical P s h i f t K was observed f o r t h e f i r s t t i m e by K n i g h t (1949) i n copper,whence t h e
where
name "Knight s h i f t " (25). T h i s s h i f t has some general c h a r a c t e r i s t i c s : i)As a paramagnetic s h i f t i t normally causes a p o s i t i v e s h i f t . ( i . e . i t decreases t h e resonant f i e l d o r increases t h e resonant frequency). However, due t o p o l a r i z a t i o n e f f e c t s , c o r r e l a t i o n , etc... a l s o negative s h i f t can appear; i i ) I t i s t y p i c a l l y on t h e o r d e r o f 0.1 t o 1.0% o f t h e Larmor frequency. I t s a b s o l u t e v a l u e increases w i t h t h e atomic number o f t h e resonant s p i n and w i t h t h e s t r e n g t h o f t h e e x t e r n a l f i e l d ; i i i ) Since t h e paramagnetic s u s c e p t i b i l i t y 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 t h e temperature, whereas t h e Paul i s u s c e p t i b i l i t y i s temperature independent, complet e l y d i f f e r e n t temperature v a r i a t i o n s a r e expected f o r t h e paramagnetic species ( f o r example r a d i c a l s ) and t h e K n i g h t S h i f t ; i v ) This l a s t one can be a l s o ani s o t r o p i c due t o a n i s o t r o p i c h y p e r f i n e i n t e r a c t i o n s , w i t h t h e same o r i e n t a t i o n dependence as was discussed i n t h e case o f chemical s h i f t s . K o r r i nga Re1a t i o n The Knight s h i f t and 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 T1 of t h e n u c l e i s t u d i e d obey K o r r i n g a ' s equation:
(4.19)
The f a c t o r S i s equal t o 1 i n t h e approximation t h a t t h e e l e c t r o n s a r e i n dependant
i n a t h r e e dimensional system. I f e l e c t r o n c o r r e l a t i o n s a r e taken
i n t o account and/or i n low-dimensional system (as example a surface) S can
d e v i a t e d r a s t i c a l l y from t h e v a l u e 1. 4.2.4
Quadrupole I n t e r a c t i o n s N u c l e i w i t h s p i n I>1/2 possess an e l e c t r i c quadrupole moment eQ which i s a
measure o f t h e departure o f t h e n u c l e a r charge d i s t r i b u t i o n f o r s p h e r i c a l symmetry. The n u c l e a r e l e c t r i c quadrupole i n t e r a c t i o n i s t h e e l e c t r o s t a t i c i n t e r a c t i o n between t h i s charge d i s t r i b u t i o n i n t h e nucleus and t h e e l e c t r i c f i e l d g r a d i e n t o f t h e environment o f t h i s nucleus, As i n t h e case o f t h e d i p o l a r i n t e r a c t i o n s and t h e chemical s h i f t , t h e iiamil t o n i a n f o r t h e quadrupole i n t e r a c t i o n may be represented by t h e e q u a t i o n + = +
~
H = I.Q. I
(4.20)
Q
where
i s a second-rank t e n s o r
(4.21)
I n t h e p r i n c i p a l a x i s frame t h e e l e c t r i c f i e l d g r a d i e n t tensor nuclear s i t e i s d e f i n e d by t h r e e p r i n c i p a l elements , , ,V i n the order
lVxxl
<
lVyyl
<
lVzzl.
Vyy and Vzz
a t the arranged
I n t h i s case
(4.22)
where t h e f i e l d g r a d i e n t q and t h e asymmetry parameter ri a r e g i v e n by eq =
ri=
vzz vyy
(4.23)
-
vxx
(4.24)
vZZ
For t h e usual h i g h - f i e l d case, i n which t h e magnitude o f t h e n u c l e a r Zeeman i n t e r a c t i o n , HZ, i s much-large than t h a t o f t h e n u c l e a r e l e c t r i c quadrupole i n t e r a c t i o n HQ, p e r t u r b a t i o n t h e o r y y i e l d s expressions f o r t h e quadrupole c o r r e c t i o n s t o t h e t o t a l energy o f t h e n u c l e a r s p i n . F o r s i m p l i c i t y we assume t h a t t h e e l e c t r i c f i e l d g r a d i e n t i s a x i a l l y symm e t r i c ( i . e q = 0) and we denote
B211
uQ =
efqe, I
21
(1
uL =
- a)
= quadrupolar coupling constant
(4.25)
= Larmor frequency
(4.26)
a = I ( I t l )
(4.27)
The d i f f e r e n t energy l e v e l s can be w r i t t e n i n t h e form:
(4.28)
where t h e s u p e r s c r i p t s denote t h e o r d e r o f t h e c o r r e c t i o n
) :E
=
-
nuL m
(4.29)
I n these e q u a t i o n s mfi i s t h e component of I s p i n a n g u l a r momentum a l o n g t n e ieernan a x i s Z, and 8 i s t h e a n g l e between t h i s l a t t e r and t h e a x i s Z o f t h e q u a d r u p o l a r p r i n c i p a l a x i s system. Because o f t h e c o r r e c t i o n Ekl) e t EA2) t o t h e energy l e v e l s , i n s t e a d o f a s i n g l e resonance frequency
(4.32)
i n t h e absence of q u a d r u p o l a r i n t e r a c t i o n , t h e r e a r e now s e v e r a l resonance f r e q u e n c i e s which can be w r i t t e n :
(4.33)
The f i r s t o r d e r frequency s h i f t i s
B212
-1
0
+l
single crystal spectra
spin 3
7
Fig. 4.4. Diagram of energy l e v e l s , t r a n s i t i o n s (double headed arrows) and s i t i o n frequencies a t t h r e e l e v e l s f perturbation theory f o r t h e Zeeman, E and quadrupole e f f e c t s ( E A l ) and & ) ) . V L i s t h e Larmor frequency in t h e a ce of a quadrupole i n t e r a c t i o n . a = spin 1; b = spin 312. VQ
= 213
&
B213 (l) m
= Em-1
n
E(l) ITI
1 v (m-1)(3 cos 2
= -
2
Q
e
-1)
(4.34)
I t vanishes f o r m = 1/2. Consequently, t h e c e n t r a l t r a n s i t i o n f o r non-
i n t e g e r s p i n n u c l e i i s n o t a f f e c t e d t o f i r s t o r d e r by quadrupole i n t e r a c t i o n s . On t h e c o n t r a r y t h e t r a n s i t i o n s -1 - 0 always s h i f t e d .
and 0 -1
f o r integer spin nuclei are
The second-order c o r r e c t i o n
(4.35)
i s o b t a i n e d from eq. (4.31).
For example, f o r t h e c e n t r a l l i n e
-
1/2
++
+ 1/2
(4.36)
T h i s term 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 v
L and t h e r e f o r e t o Bo. Whence t h e
i n t e r e s t o f u s i n g h i g h magnetic f i e l d s f o r which t h e chemical s h i f t s a r e h i g h e s t and t h e quadrupole e f f e c t weakest. I t s h o u l d be n o t e d t h a t : i ) t h e f a c t o r which depends on 8 does n o t have t h e
same form f o r vil)
and
"A2);
and i i ) Em i s an uneven f u n c t i o n of
in.
The second
o r d e r frequency s h i f t s
(4.37)
a r e t h e r e f o r e equal and t h e i r d i f f e r e n c e
cancels o u t . Consequently, t h e f i r s t o r d e r d i s t a n c e between t h e s a t e l l i t e s l i n e s (m-l)-(m) and (-mj-m(m-1)
A" = v ( m - t j ( 3 cosL
Q
e
-1)
(4.39)
expresses t h i s d i s t a n c e c o r r e c t l y t o t h e second o r d e r . F i g . 4.4 r e p r e s e n t s t h e e f f e c t t h a t t h e quadrupole i n t e r a c t i o n has o n t h e energy l e v e l s and t r a n s i t i o n s ( m =
* 1) i n t h e h i g h f i e l d case f o r
I = 1 (e.g.
B214
These two cases a r e r e p r e s e n t a t i v e o f t h e s i t u a t i o n i n which I i s
23iia o r "B).
an even o r an odd m u l t i p l e o f 1/2, r e s p e c t i v e l y . L e t us c o n s i d e r t h e s p e c i a l case o f a x i a l symmetry ( q = 0) w i t h f i r s t - o r d e r c o r r e c t i o n t o t h e energy as r e p r e s e n t e d i n eq. (4.30). t i o n s f o r t h e two
t r a n s i t i o n s m: -1-0
For
I
= 1 t h e two equa-
and m: kl a r e s i m i l a r t o t h o s e f o r t h e
n u c l e a r d i p o l e i n t e r a c t i o n s ( E q . 4.5 and 4 . i ) .
I
ua =
+4
UL
v
Q
( 3 cos
2
e
-1)
(4.40)
These two e q u a t i o n s l e a d t o t h e q u a d r u p o l a r powder p a t t e r n ( F i g . 4.4a) analogous t o F i g . 4.1 f o r d i p o l a r powder p a t t e r n . The s p l i t t i n g between t h e two maxima i s u / 2
and t h e t o t a l p o w d e r - p a t t e r n w i d t h i s v
Q
F o r t h e case I = 3/2, t h e r e a r e t h r e e t r a n s i t i o n s 1 -32 - 2 ,
in :
1 1 m : - ? H 7
and
1
m:2+-.;i
Q'
3
I n t h e same c o n d i t i o n s (rl = 0, f i r s t - o r d e r c o r r e c t i o n s ) t h e t r a n s i t i o n f r e q u e n cies are
V
-9c.-2
V
-1,r 2
V
-
( 3 cos 2
e
-1)
=o
3 2
(4.42)
2
--- - 1 2
=
2
-
( 3 cos 2
e
-1)
F i g . 4.4b shows t h e c o r r e s p o n d i n g powder p a t t e r n . 4.2.5 &Coupling ( I n d i r e c t N u c l e a r - N u c l e a r I n t e r a c t i o n s ) The n u c l e a r s p i n s may be coupled, each t o t h e o t h e r s , by way o f s i m u l t a taneous c o u p l i n g o f t h e e l e c t r o n s t o b o t h n u c l e i . The H a m i l t o n i a n of t h i s s p i n s p i n c o u p l i n g i n t e r a c t i o n between a p a i r o f s p i n s I and S may be w r i t t e n *=+
HJ =
1.J.S
(4.43)
B215
1,
0 A l
, -10
,
I
kHz
10
0
F ' g . 4.5. A s u i n m u l t i p l e t r e s o l v e d i n t h e s o l i d s t a t e by magic a n g l e s p i n n i n g . 1 4 NClR s p e c t r a o f p o l y c r y s t a l l i n e KAsF ( a ) S t a t i c specimen; ( b ) specimen i n n i n g 7 e t 5.5 kHz d i s p l a y i n g a q u a r t e f s t r u c t u r e due t o J-coup1 i n g between ?F! and A s n u c l e i .
.
-
where J i s a second-rank t e n s o r . These i n t e r a c t i o n s produce t h e well-known m u l t i p l e t s o f l i n e s i n NMR spect r a o f l i q u i d s . For i n s t a n c e , t h e p r o t o n s i g n a l o f t h e methylene group i n e t h a n o l i s s p l i t i n t o a q u a r t e t by t h e p r o t o n s i n t h e n e i g h b o u r i n g m e t h y l group.
A good example o f a s p i n m u l t i p l e t r e v e a l e d i n t h e s o l i d a f t e r t h e removal o f t h e o t h e r i n t e r a c t i o n s ( d i p o l a r and chemical s h i f t a n i s o t r o p y u s i n g t h e 19F spectrum o f c r y s t a l l i n e KAs F6 ( F i g . 4.5).
MAS-NMR) i s
The q u a r t e t i n spectrum ( b )
i s due t o J - c o u p l i n g between 19F ( s p i n 1/2) and 75As ( s p i n 3/2) ( 2 7 ) . The Jc o u p l i n g i s f i e l d independent and i s u s u a l l y s m a l l e r t h a n t h e o t h e r i n t e r a c t i o n s under c o n s i d e r a t i o n . T y p i c a l v a l u e s range up s e v e r a l kHz f o r common n u c l e i . B r i e f l y , i n a p a r t i c u l a r s o l i d - s t a t e system, one o r two of t h e terms w i l l u s u a l l y dominate t h e H a m i l t o n i a n and hence d e t e r m i n e t h e c h a r a c t e r i s t i c s o f t h e spectrum. T y p i c a l v a l u e s a r e p r e s e n t e d i n T a b l e 1. TABLE 4.1.
Space, s p i n and f i e l d dependencies o f n u c l e a r m a g n e t i c i n t e r a c t i o n i n s o l i d s ( 2 8 ) Interaction
Field dependence
Honionuclear d i p o l a r ,
f .i o , ~ ~
H e t e r o n u c l e a r d i p o l a r HD,IS Chemical s h i f t
iCs
none none linear
S o l i d s magnitude I s o t r o p i c average vDaI16
40 kHz
vDiIS. < 20 kHz Ao
0
Space dependence
(1-3 C O S ~e ) 3 2 1 aiso p i n e +T(3cos2a..
,.
~
-l)Caij Quadrupole, I1
H^? I'
f i r s t order second o r d e r
none
vA< 100 MHz
none
vJ,II<
1 kHz
f i r s t row element H e t e r o n u c le a r s c a l a r coupling, HJ, IS
none
vJ,Is<
1 kHz
f ir s t row e l ement
cos
xL
J 2 (1-3 cos e)
0
o
BO' Homonucl ea_r s c a l a r coupling, H j , I 1
e)
( 1 - 3 cos'
0
(i-cos20)(9cos2e-i)
Joo
(1-3cos 2 0 )
B216
4.3
NMR TECHNIQUES FOR THE STUDY
OF ADSORBED MOLECULES
Porous s o l i d s such as s i l i c a , alumina and z e o l i t e s p l a y an i m p o r t a n t r o l e i n many i n d u s t r i a l processes i n t h e chemical i n d u s t r y where they a r e used m a i n l y as c a t a l y s t s o r as adsorbents. The s t a t e o f adsorbed molecules, which i s i n several respects i n t e r m e d i a t e between t h e l i q u i d and s o l i d s t a t e s , has been t h e s u b j e c t o f numerous i n v e s t i g a t i o n s . Most o f them make use o f c l a s s i c a l thermodynamic methods such as measurement o f a d s o r p t i o n isotherms and heats o f adsorpt i o n o r o f we1 1-establ ished spectroscopic techniques, p a r t i c u l a r l y i n f r a r e d spectroscopy. One o f t h e f i r s t h i g h r e s o l u t i o n NMR s t u d i e s c a r r i e d o u t around 1961 ( 2 9 ) on molecules adsorbed on diamagnetic s o l i d surfaces i n d i c a t e d t h a t p r o t o n resonance frequencies d i f f e r from f r e e molecule values. T h i s s h i f t ,
due t o a d i s -
turbance o f t h e e l e c t r o n d i s t r i b u t i o n , v a r i e s w i t h t h e n a t u r e o f t h e f u n c t i o n a l group, s p e c i a l l y when t h e r e i s a p r e f e r r e d o r i e n t a t i o n o f t h e chemisorbed inolecule w i t h r e s p e c t t o t h e surface. However, because o f v a r i o u s d i f f i c u l t i e s analysed below, NMR was l i t t l e used f o r a l o n g time. But t h e r e s o l u t i o n o f these d i f f i c u l t i e s has l e d t o a considerable i n c r e a s e i n t h e number of papers on t h e study o f s u r f a c e phenomena by HR-NMR. T h i s has a l r e a d y l e d t o several reviews (8-10, 12, 29-33). 4.3.1
Experimental c o n d i t i o n s and d i f f i c u l t i e s S e n s i t i v i t y . Since t h e appearance o f F o u r i e r t r a n s f o r m spectrometers t h e
same spectrum can be accumulated a v e r y l a r g e number o f times. The r e s u l t i n g s e n s i t i v i t y increase makes i t p o s s i b l e t o d e t e c t many d i f f e r e n t n u c l e i i n homogeneous media. Despite t h i s , i n t h e case o f adsorbed phases, NMR s i g n a l s can be 2 detected o n l y when t h e adsorbent have h i g h s p e c i f i c area ( > 10 m .g-’). Moreover, o n l y n u c l e i w i t h s p i n o f 112 a r e e a s i l y s u i t a b l e . I n Table 4.2 are l i s t e d t h e c h a r a c t e r i s t i c s of some n u c l e i which a r e p a r t i c u l a r l y i n t e r e s t i n g f o r t h e study o f adsorption. The minimum number Nmin o f n u c l e i necessary f o r producing a s i g n a l - t o noise r a t i o o f 10 a f t e r z repeated measurements (accumulations) i s g i v e n by (34)
(4.44)
where t h e symbols have t h e f o l l o w i n g meaning: 1, i s t h e r e l a t i v e NMR i n t e n s i t y (Table 4.2).
VIOo
i s t h e resonance frequency i n MHz i f Bo = 2.34869 Tesla; T i s
t h e sample temperature i n K; T1 i s t h e l o n g i t u d i n a l r e l a x a t i o n t i m e i n s; 6w
is
t h e observed h a l f - w i d t h o f t h e whole r e c e i v e r system i n Hz and V c i s t h e volume o f t h e r e c e i v e r c o i l i n cm3. As an example l e t us consider measurements o f 13C NMR s i g n a l s f o r adsorbed molecules having n a t u r a l abundance o f 13C n u c l e i (1.1%).
B217
TABLE 4 . 2 . NMR c h a r a c t e r i s t i c s of some n u c l e i w i t h s p i n 1 / 2 o f t e n used f o r t h e s t u d y o f
adsorbed phases.
Resonance frequency (MHz) f o r B = 2.348 69 T e s l a R e l a t i v e i n t e n s i t y o f t h e NMR signal f o r a given value of B N a t u r g l abundance o f t h e i s o t o p e
(%I
100
1 99.985
Product o f r e l a t i v e i n t e n s i t y and n a t u r a l abundance ) . , I ( I n t e r v a l o f chemical s h i f t ( i s o t o p i c values)(ppm) T y p i c a l NMR l i n e w i d t h s o f adsorbed molecules (Hz) a t about room temperature
Taking vloo
= 25.1
MHz, T
= 300 K,
25,14
10 ,13
27,66
1.59~10-~
1.04~10-~
2.12~10-~
1.108
0.365
0,264
100
1.8~10-~
3.8~10-~ 5.60~10-~
10
300
500
1000
500
100
100
10
V = 1 cm3, and 1, = 1 . 8 ~ 1 0 - (~T a b l e 4.2) i t
follows:
Nmi n
(4.45)
(13C) = 8x10
T y p i c a l values f o r t h e remaining parameters a r e T = 0.1 s, 6 0 = 100 s - l 4 (” 20 Hz), Av = 500 Hz and z = 10 accumulations, l e a d i n g t o
N , ( l 3 C ) = 5.6 x 1020 min
w i t h o u t Overhauser e f f e c t enhancement. Overhauser e f f e c t ( N O E ) . The s i g n a l enhancement due t o m a g n e t i z a t i o n t r a n s f e r f r o m t h e c o u p l i n g p r o t o n s ( i n d e x S ) t o t h e n u c l e i i n v e s t i g a t e d (C, N . . . index I ) i s g e n e r a l l y l e s s t h a n t h e optimum f a c t o r
(4.46)
w i t h y S and YI TIIS
t h e gyroinagnetic r a t i o , TI
t h e l o n g i t u d i n a l r e l a x a t i o n t i m e and
t h e c r o s s - r e l a x a t i o n t i m e due t o t h e d i p o l e - d i p o l e c o u p l i n g between I and
S s p i n s . T h i s r e d u c t i o n i s due t o t h e r e s t r i c t e d m o l e c u l a r m o b i l i t y (34) and t h e
predominance o f o t h e r r e l a x a t i o n mechanisms.
B218 4.3.2
Measureiient o f resonance s h i f t s There a r e several methods f o r measuring NMR chemical s h i f t s ( 3 5 ) . However,
f o r a v a r i e t y o f reasons (wide l i n e s , c o m p e t i t i v e adsorption, e t c ...), o n l y t h e method o f s u b s t i t u t i o n can be used f o r adsorbed phase s t u d i e s : The r e f e r e n c e and t h e sample (adsorbate-adsorbent system) a r e p u t i n t o i d e n t i c a l g l a s s tubes and a r e s t u d i e d successively. I t i s necessary t h e r e f o r e t o c o r r e c t t h e observed resonance s h i f t ,
fjObs,
o f t h e sample i n o r d e r t o o b t a i n t h e r e a l ( c o r r e c t e d )
resonance s h i f t ,
fjreal
(36, 37). Indeed, i f Bo i s t h e magnetic f i e l d a p p l i e d t o
a s o l i d o f volume s u s c e p t i b i l i t y
xv, t h e r e s u l t i n g f i e l d can be w r i t t e n as (35,
36, 37)
B
= B
0
t
bl t b2 -+ b3 + b4
(4.47)
b4 corresponds t o t h e i n f l u e n c e o f t h e e l e c t r o n i c environment surrounding each nucleus. The f i r s t c o r r e c t i o n , bl = (4n/3)XvBO, i s t h e f i e l d due t o t h e s p h e r i cal Lorentz surface (38). The f o l l o w i n g term b2 = -clxvBo i s t h e demagnetizing f i e l d . The parameter
c1
depends on t h e sample shape and on t h e Bo d i r e c t i o n , and
i s 4 ~ / 3f o r a sphere, 2n f o r a c y l i n d e r ( p r o v i d e d t h e h e i g h t i s a t l e a s t f i v e times t h e diameter), t h e a x i s o f which i s p e r p e n d i c u l a r t o Bo, o r z e r o f o r t h e same c y l i n d e r p a r a l l e l t o Bo. The c o r r e c t i o n b3 i s t h e f i e l d which depends on t h e d i s t r i b u t i o n of magnetic d i p o l e s i n s i d e t h e L o r e n t z sphere. I t i s a f u n c t i o n of
xV: b3
= kXVBo. T h i s f i e l d i s zero f o r an i s o t r o p i c o r c u b i c d i s t r i b u t i o n .
But i n c e r t a i n cases i t can be l a r g e r than e i t h e r bl o r b2 ( 4 1 ) . Using t h e same geometry f o r t h e r e f e r e n c e and t h e sample, t h e c o r r e c t i o n i s
(4.48)
The chemical s h i f t o f t h e adsorbed molecules i s g e n e r a l l y determined r e l a t i v e t o t h a t o f t h e i s o l a t e d molecules and t h e r e f o r e t h e b e s t r e f e r e n c e sample i s t h e corresponding gas a t v e r y low pressure ( 3 7 ) . I n t h i s case xv(ref)% 0. F u r t h e r , according t o Wiedeman's r u l e t h e t o t a l volume s u s c e p t i b i l i t y x
~
can be w r i t t e n as t h e weighted average
XV(samp1e) = xV(sp)
' Fa CXV(admolecule) - xV(sp)l
where Fa i s t h e volume f r a c t i o n o f t h e adsorbed molecules and
(4.49)
xv ( s p ) t h e b u l k
s u s c e p t i b i l i t y o f t h e s o l i d powder. F i n a l l y , i f t h e gas i s o n l y p h y s i c a l l y adsorbed, t h e r e i s no v a r i a t i o n o f -t 0 f o r v e r y small f r a c t i o n s Fa 0. Then t h e e x t r a p o l a t i o n o f b49 and 'real -f
(
~
~
B219
F i g . 4.6. The l i n e a r r e l a t i o n between e x p e r i m e n t a l chemical s h i f t and t h e conc e n t r a t i o n ( s c a l e 1 ) o r t h e p r e s s u r e ( s c a l e 2) o f adsorbate. AObs
v a l u e s t o z e r o coverage y i e l d s t h e s u s c e p t i b i l i t y c o r r e c t i o n f o r t h e s o l i d
powder: 4a
&obs =
(7-
xV(sp)
(4.50)
T h i s method f i r s t proposed by F r a i s s a r d and co-workers (36, 37) l e a d s t o good agreement w i t h d i r e c t measurements o f volume magnetic s u s c e p t i b i l i t i e s on t h e b a s i s of an a n a l y s i s o f p r o t o n resonance s h i f t s . F o r example, F i g . 4.6 shows t h a t t h e observed chemical s h i f t s o f CH4,TMS and C6H12 observed on v a r i o u s o x i d e s a r e l i n e a r f u n c t i o n s o f t h e amounts o f adsorbed gas. B u t 6obs f o r Fa = 0 i s o f course independent o f t h e n a t u r e o f t h e gas p h y s i c a l l y adsorbed. The usual c o r r e c t i o n s f o r 1H-NMR w i t h d i a m a g n e t i c sol i d s v a r y a p p r o x i m a t e l y between 0.1 and 2 ppm. The magnetic s u s c e p t i b i l i t y c o r r e c t i o n s must be made 1 p r e f e r a b l y by H-NMR. With h e a v i e r atoms t h e van d e r Waals i n t e r a c t i o n s may n o t be n e g l i g i b l e ( 4 2 ) . F o r example, w i t h 13C t h e above method i s v a l i d f o r s u f f i c i e n t l y l a r g e tiiolecules such as n-butane. cyclohexane o r TMS. F o r s m a l l molecul e s such as methane t h e cheiiiical s h i f t due t o van d e r Waals i n t e r a c t i o n s i s g r e a t e r t h a n t h a t due t o t h e volume s u s c e p t i b i l i t y .
B220
F o l l o w i n g on from what we have j u s t s a i d about t h e e f f e c t o f van der Waals i n t e r a c t i o n s , t h e c h o i c e o f t h e r e f e r e n c e f o r measuring t h e chemical s h i f t due t o adsorption i s d i r e c t l y r e l a t e d t o t h e d i f f e r e n c e between t h e resonance frequencies of t h e gaseous s t a t e and t h e l i q u i d s t a t e . While t h i s i s n e g l i g i b l e f o r protons ( < 0 , 5 ppni) i t can be several ppm f o r 13C and I 5 N , p a r t i c u l a r l y f o r t h e n u c l e i a t t h e o u t e r edge o f t h e molecules. I n r e f e r e n c e (43) t h e r e a r e several examples o f chemical s h i f t s r e f e r e n c e d t o t h e l i q u i d and gaseous s t a t e s . I t i s convenient t o use t h e gaseous s t a t e f o r s t r u c t u r e a t l o w coverages. I n t h i s case t h e van d e r Waals i n t e r a c t i o n s between molecules a r e g e n e r a l l y n e g l i -
g i b l e . On t h e o t h e r hand, a t h i g h coverage c o r r e c t i o n must be made f o r t h e cont r i b u t i o n o f molecule-molecule i n t e r a c t i o n s by using t h e l i q u i d s t a t e as t h e reference. 4.3.3
Broadening and magnetic s h i e l d i n g a n i s o t r o p y Molecules p h y s i c a l l y adsorbed a r e g e n e r a l l y v e r y m o b i l e a t t h e s u r f a c e o f
s o l i d s ( i f t h e temperature i s n o t t o o low). The s p e c t r a l components a r e then bell-shaped and f a i r l y narrow. Consequently, as i n t h e case
01
l i q u i d s , the iso-
t r o p i c value of t h e s h i e l d i n g t e n s o r can be measured. Conversely when t h e temp e r a t u r e i s n o t v e r y high, t h e m o b i l i t y o f s t r o n g chemisorbed molecules i s low. The c h a r a c t e r i s t i c s p e c t r a a r e then g e n e r a l l y v e r y broad which makes i t imposs i b l e t o measure t h e chemical s h i f t s . To determine them i t i s necessary t o narrow t h e s p e c t r a l components. Two methods can be used. i ) r a p i d exchange between chemisorbed and physisorbed molecules; and i i ) MAS-NMR.
I n a d d i t i o n t o t h e l i n e broadening, chemical s h i f t a n i s o t r o p y can be measured f o r molecules which a r e n o t f r e e t o move: adsorbed molecules a t l o w temperatures (44, 45) o r l o c a t e d i n small pores (46, 47) etc..
. In this
case
a l s o t h e above techniques can be used t o reduce t h e l i n e - w i d t h and t o measure t h e i s o t r o p i c chemical s h i f t ( 4 7 ) . 4.3.4
Exchange E f f e c t s The new kI4R l i n e - n a r r o w i n g techniques have n o t r e s o l v e d a l l t h e problems
o f reducing l i n e - w i d t h . For example, w i t h MAS-NMR a l i n e cannot be narrowed unless t h e sample r o t a t i o n speed i s a t l e a s t equal t o t h e l i n e - w i d t h ( i n Hz) i n t h e absence o f r o t a t i o n . T h i s technique t h e r e f o r e w i l l n o t work when t h e w i d t h of
the,
components c h a r a c t e r i s t i c o f t h e chemisorbed complexes i s g r e a t e r
than t h e maximum sample r o t a t i o n speed (about 5000 Hz w i t h sealed tubes). T h i s i s , f o r example, t h e case o f molecules chemisorbed on paramagnetic s i t e s , which i s important i n c a t a l y s i s . T h e o r e t i c a l l y , t h e spectrum o f an adsorbate-adsorbent system must have as many components as t h e r e a r e r e g i o n s where t h e molecules can be found: chemis o r p t i o n s i t e s o f d i f f e r e n t f o r c e and/or type, p h y s i c a l adsorption, i n t e r c r y s -
B221
F i g . 4.7. H i g h - r e s o l u t i o n NMR s p e c t r a o f e t h a n o l a t v a r i o u s degrees of coverage: ( a ) p u r e l i q u i d , ( b ) 0 = 20, ( c ) 0 = 10, and ( d ) 8 = 3. t a l l i t e space, e t c
... However,
t h i s i s o n l y t r u e i f t h e residence time o f t h e
molecules a t each s i t e i s s u f f i c i e n t l y l o n g ( o n t h e NMR t i m e s c a l e ) . G e n e r a l l y , ( e x c e p t a t v e r y l o w temperature) t h e s e r e s i d e n c e t i m e s a r e s h o r t ; consequently, because o f exchange t h e spectrum c o n s i s t s o f o n l y few f a i r l y narrow components due t o t h e coalescence o f t h e above, and whose chemical s h i f t i s a l i n e a r combin a t i o n o f t h e cheniical s h i f t s a s s o c i a t e d w i t h t h e v a r i o u s s t a t e s and w e i g h t e d by t h e c o n c e n t r a t i o n o f these l a s t ones. The most i m p o r t a n t problem i s t h e n t o deduce, f r o m t h e dependence o f t h e s e s h i f t s on t h e number o f adsorbed molecules, t h e s h i f t s c h a r a c t e r i s t i c o f t h e chemisorbed molecules. G e n e r a l l y , t h e s e v a l u e s a r e n o t deduced d i r e c t l y f r o m experiment, s i n c e t h e e x p e r i m e n t a l s h i f t s e x t r a p o l a t e d t o z e r o coverage a r e n o t n e c e s s a r i l y i d e n t i c a l t o t h o s e of t h e chemisorbed coinpl exes. A s an example F i g . 4.7 shows t h e v a r i a t i o n o f t h e NMR spectrum o f e t h a n o l
adsorbed on alumina w i t h t h e s u r f a c e coverage. The p r e f e r e n t i a l broadening o f t h e OH component when t h i s l a t t e r decreases i n d i c a t e s t h a t t h e CH3-CH2-OH
mole-
c u l e s a r e a t t a c h e d t o t h e s u r f a c e by t h i s group. Even a t l o w 0 t h e CH component remains r e l a t i v e l y narrow and d e t e c t a b l e because o f t h e CH3 r o t a t i o n . Long b e f o r e t h e appearance o f t h e MAS-NMR on commercial spectrometer, t h e exchange t e c h n i q u e was proposed f o r a d s o r p t i o n on d i a m a g n e t i c s o l i d s by Bonardet
B222
e t a l . (26, 27), developed f o r paramagnetic s o l i d s b y Borovkov e t a l . (28) and E nriq uez e t a1.(29),
t h e n expressed i n an a n ot her mathematical form by Michel
e t a l . (30, 31). L e t us c o n s i d e r o n l y one s o r t o f c h e m i s o r p t i o n s i t e , A , ( t o t a l number NA) and t h e e q u i l i b r i u m P t A = C
(4. 51)
where P r e p r e s e n t s t h e p h y s i s o r b e d m o l e c u l e s (number N ) , and C t h e chemisorbed P complexes (number NC). The e q u i l i b r i u m c o n s t a n t i s g i v e n by
(4. 52)
K =
N i s t h e t o t a l number o f adsorbed m o l e c u l e s (N=N +N ) , and NA-NC. i s t h e number P
o f unoccupied s i t e s .
C
I n t h e case of f a s t exchange ( g e n e r a l l y N >NC), t h e experiment al chemical s h i f t 6 o f t h e s i n g l e l i n e observed i s g i v e n by
6 = -NC N
-
N
6 t C
N
NC (4.53)
&P
a r e t h e chemical s h i f t s o f chemisorbed and physisorbed molecuP 1es, r e s p e c t i v e l y .
where 6c and
4.3.4.1
6
F i r s t method
G e nera lly , f o r l i g h t n u c l e i , t h e chemical s h i f t o f t h e physisorbed phase, tip, i s n e g l i g i b l e . 6 %Lreference
P
gas
'L
0. E quat ion
(4.53) becomes:
6 = -NC 6 N C
(4.54)
When t h e chemical and p h y s i c a l a d s o r p t i o n s a r e s u f f i c i e n t l y d i f f e r e n t , t h e v a r i a t i o n 6 = f ( l / N ) o f e q u a t i o n (4.54) shows a l i n e a r p a r t as soon as NC i s co ns t a nt , t h e r e f o r e when t h e number o f chemisorbed molecules i s maximal and equal t o t h e t o t a l number o f c h e m i s o r p t i o n s i t e s (NC)MAX. From t h e s l o p e o f t h e s t r a i g h t l i n e , we can deduce t h e v a l u e o f 6c i f (NC),,,AX
can be determined by o t h e r means (28, 29).
B223
4.3.4.2
Second inethod
L e t eq. (4.54):
(4.55) b
When N
bmo
+
0,
n
+
n
mo
=
m0 -
6c
(4.56)
i s t h e rliaximuiii s h i f t v a l u e o b t a i n e d f r o m a s i m p l e e x t r a p o l a t i o n o f 6 f o r
N + O
Combining eq. ( 4 . 5 3 ) and ( 4 . 5 2 ) :
(4.57)
nmo
=
iT-q KNA
(4.58)
When K i s e l i m i n a t e d from eq. (4.57) and ( 4 . 5 8 ) , we o b t a i n t h e e q u a t i o n
(4.59)
(4.60)
(4.61)
The q u a n t i t i e s X and Y a r e g i v e n by t h e experiment. From t h e p l o t o f Y versus X t h e s h i f t d C and t h e number o f c h e n i i s o r p t i o n s i t e s NA can be determined
(32, 53). We g i v e i n S e c t i o n 4.4 some s i m p l e examples o f t h e use o f t h e r a p i d exchange techniques. Note: Gedeon
(541 have shown t h a t one can i n t h e same way d e t e r m i n e t h e
p o p u l a t i o n weighted chemical s h i f t s f o r molecules p h y s i c a l l y adsorbed on two f a i r l y different s i t e s , given a j u d i c i o u s choice o f references.
B224
4.4. NMR STUDY OF SOLIDS WITH CHEMISORBED MOLECULES 4.4.1
Diamagnetic Systems. Study o f t h e a c i d i t y o f c a t a l y s t s : F a s t exchange.
4.4.1.1
Background
Chemists working on s o l i d s have f o r many years sought a f t e r a s c a l e o f Bronsted a c i d i t y comparable t o t h a t o f pKA f o r compounds i n aqueous s o l u t i o n . Independently o f the p o t e n t i a l o f NMR, which we s h a l l come back t o , several methods have been used t o measure t h e Bronsted a c i d i t y o f s o l i d s . Those i n v o l v i n g t i t r a t i o n a r e t h e most obvious, b u t they have however c e r t a i n disadvantages: t h e i n a c c e s s i b i l i t y of some s i t e s t o t h e reagents, m o d i f i c a t i o n s o f t h e s u r f a c e p r o p e r t i e s of t h e s o l i d s , e t c . .
. Temperature
programmed d e s o r p t i o n i s s e n s i t i v e
n o t o n l y t o a c i d i c b u t a l s o t o non a c i d i c s i t e s . I R spectroscopy i s n o t q u a n t i t a t i v e ; moreover, t h e v i b r a t i o n a l frequencies o f OH groups depend markedly on t h e i r environment. NMR spectroscopy, on t h e o t h e r hand, i s a powerful t o o l f o r
investigating
Bronsted a c i d i t y and t h e n a t u r e o f t h e s u r f a c e c o n s t i t u t i v e water o f s o l i d s ( 5 5 -
5 7 ) . One o f t h e g r e a t e s t advantages o f NMR i s t h a t i t i s q u a n t i t a t i v e s i n c e t h e areas under t h e a b s o r p t i o n curves a r e p r o p o r t i o n a l t o t h e number o f resonant n u c l e i . The d e t e c t i o n t h r e s h o l d i s about lo1'
hydrogen n u c l e i i n a magnetic
f i e l d o f 8.5 Tesla; consequently, i f t h e OH groups a r e d i s t r i b u t e d over t h e surface o f t h e sample o f t o t a l area S i n t h e c o i l , t h e d e t e c t i o n
threshold i s
IO'~/S. O f t h e v a r i o u s p o s s i b l e techniques, p r o t o n chemical s h i f t d e t e r m i n a t i o n
would appear t o be a d i r e c t means o f d e t e r m i n i n g t h e a c i d i t y of protons, as i t i s r e l a t e d t o t h e e l e c t r o n i c d e n s i t y o f t h e H atoms. The chemical s h i f t s being
a second rank tensor, i t s a n i s o t r o p i c components as w e l l as t h e mean i s o t r o p i c one (61so) may be determined (see s e c t i o n 4.2).
The a n i s o t r o p i c 6 v a l u e i s
obtained by n i u l t i p u l s e techniqeus which cancel o u t d i p o l a r c o u p l i n g , and t h e i s o t r o p i c value by a magic angle s p i n n i n g (MAS) experiment. I s o t r o p i c p r o t o n s h i f t s i n s o l i d s can be measured d i r e c t l y , f a s t e r and more a c c u r a t e l y , by experiments w i t h simultaneous use o f mu1t i pul se techniques and MAS (combined r o t a t i o n and mu1 t i p u l s e spectroscopy, CRAMPS) (55-61).
Eckman ( 6 2 ) has proposed
another method i n which t h e r e s i d u a l protons o f perdeuterated m a t e r i a l s s p i n n i n g
1 a t t h e magic angle a r e observed. The d i l u t i o n o f H removes 'H-lH
dipolar
c o u p l i n g and t h e MAS technique e l i m i n a t e s t h e r e s i d u a l e f f e c t s of inhomogeneous 2 1 2H-2H and H- H couplings.
B225
P (Torr)
F i g . 4.8. Chemical s h i f t v a r i a t i o n o f NH4Y
+
NH3 versus c o n c e n t r a t i o n .
B e f o r e these h i g h r e s o l u t i o n methods f o r s o l i d s were a v a i l a b l e , F r a i s s a r d and h i s group (48, 49, 63) had proposed a means of d e t e r m i n i n g d I s 0 .
The i d e a
was t o i i i o b i l i z e t h e OH group p r o t o n s so as t o g i v e a sharp NMR s i g n a l whereas p r e v i o u s l y when t h e y a r e s t a t i c t h e y g i v e a b r o a d s i g n a l . T h i s i s t h e method of l i n e narrowing by r a p i d exchange which we s h a l l now d e s c r i b e b r i e f l y b e f o r e g i v i n g socne examples w i t h MAS-NMR. However, t h i s method needs t h e know1edge o f t h e v o l umi c magnetic s u s c e p t i b i l i t y o f t h e s o l i d which can a l s o be measured by NMR ( 6 4 ) ( S e c t i o n 4.3).
-
B226
4.4.1.2
1H NMR chemical s h i f t o f NH;
The l i n e w i d t h of 'H due t o NH;
Y Zeolite
i s 32 kHz a t
77 K ( r i g i d l a t t i c e ) and 8 kHz a t room temperature. Bonardet e t a l . (48, 49) have shown t h a t a f t e r ammonia a d s o r p t i o n t h e r e i s o n l y one s i g n a l and t h a t i t s p o s i t i o n and w i d t h depend on t h e r e l a t i v e c o n c e n t r a t i o n s o f NH: and NH3 species ( F i g . 4.8). This s i g n a l i s c h a r a c t e r i s t i c o f t h e exchange: N~H:
t
N.H + 5 3 *
N ~ +H N.H+ ~ 3 4
(4.62)
E x t r a p o l a t e d t o zero c o n c e n t r a t i o n o f adsorbed ammonia, t h e r e a l chemical s h i f t
+-
o f t h e ammonium z e o l i t e p r o t o n i s 7.0 0.1 ppm r e l a t i v e t o NH3 gas. I n s o l u t i o n + 15 + 6H (NH4) = 6.9 ppm and bSlN ( NH4) - 43.5 ppm. T h i s shows: f i r s t l y , t h a t t h e e f f e c t o f chemical exchange on t h e s h i f t 6 i s i n d e p e n d e n t o f t h e s i g n a l w i d t h s ( b e f o r e exchange) and on t h e i r overlap; secondly, t h a t t h e ammonium i o n i s ass o c i a t e d w i t h t h e same chemical s h i f t whether i t i s i n s o l u t i o n o r i n z e o l i t e cages. 4.4.1.3
Acid-Base r e a c t i o n s a t a s o l i d surface. Bronsted a c i d s t r e n g t h ; chemical s h i f t and c o n c e n t r a t i o n o f OH groups
The Brijnsted a c i d s t r e n g t h depends on t h e p o l a r i z a t i o n o f t h e OH bond and t h e r e f o r e on t h e e l e c t r o n i c environment o f H which i s determined by i t s screening constant. T h i s i s why Bonardet e t a l . (49) suggested t h e use o f t h e OH c h e m i c a l - s h i f t as a measure o f t h e Brlinsted a c i d i t y . T h i s seems t o be p a r t i c u l a r l y i n t e r e s t i n g f o r a c i d c a t a l y s t s f o r which i t i s impossible t o d e f i n e c h a r a c t e r i s t i c a c i d i t y values a k i n t o pK values f o r homogeneous media. A t t h e time t h e r e was no technique o t h e r t h a n r a p i d exchange. L e t us consider a s u r f a c e S c o n s i s t i n g o f a c e r t a i n number of a c i d i c groups, denoted S-OH. Because o f t h e s t r o n g d i p o l a r i n t e r a c t i o n o f spins, t h e NMR l i n e s o f these OH groups a r e v e r y wide ( o f t h e o r d e r o f one gauss). On t h i s s u r f a c e l e t us adsorb a molecule AH (base) c o n t a i n i n g a t l e a s t one o t h e r nucleus A which can be detected by NWR, e.g.
I 5 N o r I7O. AH can capture a s u r f a c e p r o t o n by
e q u i l ibriurii:
s
-OH
t
AH
z s-0-
t
AH;
(4.63)
I f r a p i d exchange occurs between t h e s u r f a c e p r o t o n S-OH and those of t h e adsor-
bed riiolecule AH, t h e a c i d p r o t o n must a f f e c t t h e chemical s h i f t o f t h e adsorbed 1 phase. The H spectruin should c o n t a i n o n l y one l i n e a t 6obs due t o t h e coalescence of t h e l i n e s a t
and 60H. Thus
(4.64)
B227
For t h e same reason (4.65)
where Pi and P i a r e t h e concentrations o f H and A n u c l e i i n t h e group i. Knowing 6 t H and 6 i H $ ,
t h e r e l a t i v e concentrations, P i H and PiH$, t h e d i s s o c i a t i o n coef-
f i c i e n t o f S 0-H i n t h e presence o f AH can be c a l c u l a t e d from eq. (4.65). By eq. (4.64) i t i s then p o s s i b l e t o c a l c u l a t e
H
which could n o t be measured d i r e c t l y .
Note: I n f a c t t h e study i s more complex than i s i n d i c a t e d by t h e above a n a l y s i s which assunies t h a t a l l the AH molecules a r e adsorbed on t h e SO-H. A t h i g h coverage, physical a d s o r p t i o n i s important and g i v e s r i s e t o many e q u i l i b r i a such as : SOH
..... AH
2 so-
(AH-H)+ + AH' S OH
.... AH
t AH;
(4.66)
2 (AH-H') + AH
+ AH'
S OH
(4.67)
...... AH'
+ AH
(4.68)
Furthermore, several types o f s i t e f o r chemisorbing AH can c o e x i s t on t h e surface ( f o r example BrBnsted and Lewis a c i d s i t e s ) . The problem then becomes above a l l a chemical one since, amongst t h i s more o r l e s s complex s e t of equil i b r i a , t h e c h a r a c t e r i s t i c s o f r e a c t i o n s (4.63) alone have t o be determined. Using t h e a d s o r p t i o n of AH = I5NH3 Bonardet e t a l . have found (48, 49)
S i l i c a gel H
..................
dOH ppm (reference: gas TMS)
2
...............
4
-
5
Gay (65) proposed a method based on t h e same p r i n c i p l e s t o determine 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 t h e a c i d i c OH groups o f a s o l i d . This a u t h o r s t u d i e d t h e I3C cheiiiical s h i f t o f p y r i d i n e adsorbed on a s i l i c a gel ( c o n t a i n i n g a monolayer o f S i - O H groups) i n t h e presence o f an i n c r e a s i n g amount o f HC1. The r e s u l t s show a
l i n e a r dependence o f t h e s h i f t of each 13C i n t h e adsorbed phase on t h e HCllpyr i d i n e r a t i o up t o a value o f u n i t y ( F i g . 4.9). Above t h i s value t h e s h i f t no longer varies.This t e h a v i o u r expresses t h e v a r i a t i o n o f t h e r e l a t i v e concentrat i o n s o f p y r i d i n e niolecules and p y r i d i n i u m i o n s formed i n r e a c t i o n py t HC1
9+ b
py H t C1
(4.69)
B228
-1 5
I
0.2
I
x
x
0
0
A
A
I
I
0.6 1.0 HCllPyri dine
1.4
F i g . 4.9. 13C chemical s h i f t s w i t h r e s p e c t t o l i q u i d p y r i d i n e ( c o r r e c t e d ) as a f u n c t i o n o f H C l / p y r i d i n e r a t i o . ( X ) C-2; ( 0 ) C-3; ( A ) C-4. P o s i t i v e s h i f t s a r e t o higher f i e l d . according t o t h e equation (4.70) where, M = molecule; I = i o n ; k = C2, C3 o r C4; adsorbed molecules ( p y + pyH').
P
I
= n /n,
1
n = t o t a l number o f
U n f o r t u n a t e l y , o n l y t h e resonance l i n e s o f C2 and C3 a r e i n g e n e r a l w e l l separated i n t h e 13C spectrum. T h i s i s why Gay proposed t h e e q u a t i o n s : (4.71) The use of such an e x p e r i m e n t a l q u a n t i t y i s more c o n v e n i e n t because t h e c o n s t a n t c o n t r i b u t i o n s t o t h e resonance f r e q u e n c i e s , such as b u l k s u s c e p t i b i l i t y o r i n t e r m o l e c u l a r i n t e r a c t i o n s a r e e l i m i n a t e d . T h i s method f o r d e t e r m i n i n g t h e c o n c e n t r a t i o n of a c i d i c OH groups i s e a s i l y a p p l i c a b l e i n t h e case o f v e r y strong a c i d i t y , i . e . when r e a c t i o n 4.69.a i s complete. I f t h i s i s n o t t h e case, t h e d i s s o c i a t i o n c o e f f i c i e n t o f t h e OH groups i n t h e presence of p y r i d i n e must b e t a k e n i n t o account.This c o e f f i c i e n t can be determined by s t u d y i n g a n o t h e r n u c l e a r s p i n such as 15N.
B229
measuring temperature
ref.
[gH6 I
C6H6 I
200 Hz
I
ref.
I
U
I
I
4
pyridine molecules
pyridinium ions
C N‘’
id F i g . 4.10. C NMR s p e c t r a o f p y r i d i n e molecules and p y r i d i n i u m i o n s i n t h e l i q u i d and i n t h e adsorbed s t a t e . P y r i d i n e m o l e c u l e s : ( a ) p u r e l i q u i d ; ( b ) adsorbed i n I i a Y 2 . ~ . t h r e e molecules p e r supercage, 16k scans; ( c ) adsorbed i n Nay47 ( U s - E x ) , t h r e e molecules p e r supercage, 4k scans. P y r i d i n i u m i o n s : ( a ) i n HzSO4 ( m o l a r r a t i o 1:3); ( b ) produced by c o - a d s o r p t i o n o f HC1 t o p y r i d i n e molec u l e s i n NaY2.6 (np :nHC1w3:6j, 16k scans; ( c ) produced i n Nay47 (Us-Ex) by coa d s o r p t i o n o f HC1 d t h p y r i d i n e molecules ( n p y : n H c 1 ~ 3 : 6 ) , 16k scans.
B230
4.4.2 H i g h - r e s o l u t i o n s o l i d s t a t e NMR o f n u c l e i o t h e r than p r o t o n s
i t i s p o s s i b l e t o i n v e s t i g a t e t h e a c i d i t y o f a c a t a l y s t by NAS-NMK o f
n u c l e i o f an adsorbed phase (13C, 15N, " S i , 13C and - - "N "C
31P....)
NMR
spectra o f p y r i d i n e molecules and p y r i d i n i u m i o n s as compared i n t h e
pure l i q u i d and i n t h e adsorbed s t a t e show i n t e r e s t i n g d i f f e r e n c e s ( F i g . 4.10). Dawson e t a l . (66. 67) c h a r a c t e r i z e d s u r f a c e a c i d s i t e s on y-alumina by 13C CP/MAS experiments o f adsorbed amines, The ambient temperature CP/MAS p y r i d i ne
spectrum a t 0.5% BET monolayer s u r f a c e coverage proposed by Dawson i s completely r e s o l v e d w i t h separate resonances f o r each o f t h e t h r e e types o f carbon atoms present whose p o s i t i o n s c o i n c i d e w i t h t h e values f o r l i q u i d p y r i d i n e ( 6 6 ) . A broadening o f t h e l i n e s i s due t o t h e n e i g h b o r i n g 14N. The i n t e n s i t i e s o f t h e l i n e s a r e n o t i n t h e a:p:y carbon r a t i o . This must a r i s e from a d i f f e r e n c e i n t h e e f f i c i e n c y of t h e CP dynamics f o r each carbon. The authors conclude t o motions i n c l u d i n g a precession o r wagging motion o f t h e C2 a x i s of t h e p y r i d i n e molecule. The authors completed t h e i r r e s u l t s w i t h v a r i a b l e - t e m p e r a t u r e s t u d i e s . independently o f t h e above mentionned C2 a x i s motion, a r e s t r i c t e d o v e r a l l motion and perhaps a p r e f e r e n t i a l r o t a t i o n about t h e C2 a x i s occur. The same authors show t h a t when n-butylamine i s adsorbed on y-alumina,
(66) t h e spectrum
contains two s i g n a l s f o r t h e a and f o r B carbon atoms r e s p e c t i v e l y ( F i g . 4.11). So, a t l e a s t two types o f c h e m i c a l l y d i f f e r e n t butylamine species a r e present on t h e surface, a t t h e c l a s s i c Lewis and Bronsted s i t e s . The n i t r o g e n of t h e amine i s f i r m l y anchored t o t h e surface.
I
60
1
I, I 40
,
I,
I
I
I
20
F i g . 4.11. Carbon-13 CP-MAS s p e c t r a o f n-butylamine adsorbed t o t h e s u r f a c e o f y-alumina. The v e r t i c a l bars i n d i c a t e carbon chemical s h i f t s f o r l i q u i d - p h a s e n-butylamine.
B231 Ripmeester ( 6 8 ) s t a t e d t h a t 15N NMR seems t o s u i t b e t t e r t h e s u r f a c e s i t e s t o i d e n t i f y t h a n t h e 13C. Maciel e t a l . ( 6 9 ) e x p l a i n e d t h a t t h e success o f "NNMR
in
d i s t i n g u i s h i n g d i s c r e t e s u r f a c e s p e c i e s is p r o b a b l y due t o t h e g r e a t e r chemical s h i f t range of '%and t h e more d i r e c t i n f l u e n c e o f b i n d i n g on n i t r o g e n s h i f t s as compared t o t h e s h i f t s of t h e more remote carbons ( T a b l e 4.3). As mentioned by Haw e t a l . (70)
15
.
I\iIWR has r e c e n t l y emerged as a p r o i n i s s i n g t e c h n i q u e i n surface studies.
When p y r i d i n e - 1 5 N i s adsorbed on Y-alumina and m o r d e n i t e (68) p y r i d i n i u m i o n s a r e n o t formed on Y-alumina i n t h e absence o f m o i s t u r e whereas t h e y a r e a s s o c i a t e d w i t h B r o n s t e d a c i d s i t e s on an a c i d leached Na m o r d e n i t e .
.
blaciel e t a1 ( 6 9 ) used 13C and 15N CP/MAS experiments t o s t u d y t h e s t r u c t u r e and dynamics o f 30%15N-enriched p y r i d i n e adsorbed on s i l i c a - a l u m i n a . F i r s t , t h e y measured t h e d i f f e r e n c e s o f chemical s h i f t betweena and B carbons and betweena and
Y ones. A t low-coverage (%0.2) n e a r l y a l l o f t h e p y r i d i n e i s i n a l o w - m o b i l i t y environment: as t h e p y r i d i n e s u r f a c e coverage decreases, Lewis acid-base complexes become i n c r e a s i n g l y i m p o r t a n t i n t h e d e s c r i p t i o n o f p y r i d i n e a d s o r p t i o n . A t 0.65 nionolayer coverage, t h e m o t i o n o f t h e adsorbed p y r i d i n e must have an a n i s o t r o p i c component, a necessary c o n d i t i o n f o r c r o s s p o l a r i s a t i o n of s p i n s i n h i g h l y m o b i l e environments. The a u t h o r s found t h a t t h e i r 13C r e s u l t s a r e c o n s i s t e n t
w i t h i n f r a r e d data, which i n d i c a t e t h a t t h e p r i m a r y a c i d i c s i t e s on s i l i c a - a l u inina a r e Lewis-type centered on t r i g o n a l aluminium atoms and t h a t t h e a p p a r e n t Bronsted a c i u i t y r e s u l t s f r o m t h e i n t e r a c t i o n between a m o l e c u l e adsorbed on a Lewis s i t e and a s u r f a c e h y d r o x y l a t t a c h e d t o an a d j a c e n t S i atom. The a u t h o r s have a l s o compared t h e chemical s h i f t s o f 15N-spectra w i t h t h o s e f o r model systems o f p y r i d i n e i n i n t e r a c t i o n w i t h e i t h e r H20, CH30H, A l ( b l ~ ! ) ~of p y r i d i n i u m i o n . For 0.27 coverage t h e chemical s h i f t i s i n t e r m e d i a t e between t h o s e f o r n e a t p y r i d i n e and p y r i d i n e complexed by t h e Lewis a c i d A1(We)3. F o r 0.82 coverage hydrogen bonding is t h e dominant i n t e r a c t i o n between p y r i d i n e and s i l i c a - a l u m i n a ; b o t h r e s u l t s a r e i n agreement w i t h t h e c o n c l u s i o n s drawn f r o m 13C s p e c t r a r e s u l t s . I f t h e s u r f a c e has been p r e t r e a t e d w i t h HC1 t h e spectrum r e v e a l s two d i s c r e t e n i t r o g e n s p e c i e s t h a t i n t e r c h a n g e s l o w l y ( i f a t a l l ) on t h e NMR t i m e s c a l e . The chemical s h i f t o f t h e s t r o n g e s t s i g n a l , is t h e average o f n e a t p y r i d i n e and p r o t o n a t e d p y r i d i n e , which suggests r a p i d exchange between t h e two s p e c i e s w i t h each p y r i d i n e p r o t o n a t e d a p p r o x i m a t e l y h a l f t h e t i m e . The chemical s h i f t o f t h e l o w - i n t e n s i t y s i g n a l i s i n d i c a t i v e o f a f u l l y p r o t o n a t e d p y r i d i n e s p e c i e s , poss i b l y a non-exchanging Bronsted s u r f a c e complex. I n t h e same way, t h e 15N NMR o f 15N l a b e l e d ammonia molecules adsorbed i n Y z e o l i t e s has been s t u d i e d by M i c h e l e t a l . ( 7 1 ) and t h a t o f p y r i d i n e adsorbed i n d e c a t i o n i z e d Y z e o l i t e by Freude e t a l . ( 7 2 ) . These a u t h o r s showed t h a t , i n t h e case o f s h a l l o w bed a c t i v a t e d z e o l i t e s , t h e adsorbed m o l e c u l e s ( 3 p e r c a v i t y ) a r e f u l l y p r o t o n a t e d a t room temperature. F o r deep bed a c t i v a t i o n , t h e amount o f p y r i d i n i u m is l o w e r a l t h o u g h t h e number o f a c i d p r o t o n s is q u i t e h i g h .
B232 31P NMR 31P MAS NMR has a l s o been a p p l i e d t o phosphine molecules adsorbed on cata-
l y s t s . Lunsford e t a l . (73, 74) used t r i m e t h y l p h o s p h i n e as a probe f o r HY z e o l i t e s . They focused on t h e t r a n s f o r m a t i o n s i n a c i d s i t e s t h a t occur i n a HY z e o l i t e upon c a l c i n a t i o n a t successively h i g h e r temperatures ( 7 4 ) . The emphasis has been on t h e types o f a c i d s i t e s t h a t a r e formed r a t h e r than on t h e c o n c e n t r a t i o n o f these s i t e s . The study b e n e f i t s from t h e e x c e l l e n t s e n s i t i v i t y and t h e l a r g e range o f 31P chemical s h i f t s . For a sample c a l c i n e d a t 673
K, t h e spectrum i s
dominated by a resonance which i s assigned t o [(CH3)3 P - H I t complexes t h a t a r i s e from chemisorption a t Bronsted a c i d s i t e s and, depending on t h e d e s o r p t i o n cond i t i o n , physisorbed (CH3)3 P i s detected. C l e a r evidence f o r a phosphonium i o n i s found from w e l l - r e s o l v e d Jp-H coupling. A t l e a s t two [(CH3)3 P - H I
+
species
e x i s t : an immobilized complex and one w i t h a h i g h degree o f m o t i o n on t h e NMR time scale. For samples c a l c i n e d a t 773 K and higher, a d d i t i o n a l resonances a r i s e from chernisorption on Lewis s i t e s and from t h e presence o f A1203 c l u s t e r s
i n t h e z e o l i t e . A 773 K sample a l s o e x h i b i t s a resonance a t t r i b u t e d t o a s i t e where triniethylphosphine i s c o o r d i n a t e d both t o a Lewis and a Bronsted a c i d s i t e . B a l t u s i s e t a l . ( 7 5 ) used t h r e e t r i a l k y l p h o s p h i n e s t o s t u d y a c i d i c s i t e s on amorphous s i l i c a - a l u m i n a .
Phosphines bound t o Brgnsted and Lewis s i t e s have
been d i s t i n g u i s e d by chemical s h i f t a n a l o g i e s w i t h model systems. Only a c i d i c s i t e s t h a t a r e s t r i c t l y on t h e s u r f a c e and t h u s c a t a l y t i c a l l y a c c e s s i b l e a r e detected. V a r i a t i o n o f t h e s u r f a c e phosphine c o n c e n t r a t i o n y i e l d s a
titration
o f a c i d i c s i t e s . I t i s p o s s i b l e t o assay t h e a b s o l u t e numbers o f s u r f a c e BrBns-
t e d s i t e s d i r e c t l y though d i f f e r e n t phosphines count d i f f e r e n t numbers o f Bronsted s i t e s , demonstrating s p e c i f i c i t i e s . I t has n o t been p o s s i b l e t o quant i f y t h e number of Lewis s i t e s on s i l i c a - a l u m i n a because o f small d i f f e r e n c e s i n
chemical s h i f t s and b i n d i n g constants o f Lewis-complexed and physisorbed phosp h i nes. 29Si-NMR n-
M a c i e l ' s group has proposed "Si-NMR
a n a l y s i s o f s i l y l a t e d species u s i n g
t h e s i l y l a t e d p a r t as a probe. For instance, L i n t o n e t a l . ( 7 6 ) s t u d i e d t h e s u r f a c e r e a c t i v i t y o f hydroxyl groups on s i l i c a w i t h t r i m e t h y l c h l o r o s i l a n e u s i n g 29Si CP/MAS NMR and o t h e r techniques. The r e a c t i o n i s (CH3)3SiC1 ---t
+ -OH
+
-0Si (CH3)3 + HC1. The authors i n d i c a t e d t h a t s o l i d s t a t e NblR i s p a r t i c u l a r l y
s u i t e d t o t h e examination o f d i f f e r e n c e s i n t h e r e a c t i v i t y o f geminal and s i n g l e s i l a n o l s . Geminal hydroxyl groups a r e found t o be much more r e a c t i v e . A t coverages approaching s t e r i c l i m i t a t i o n s , n e a r l y 100% o f t h e geminal s i t e s r e a c t as opposed t o o n l y 20% of t h e s i n g l e s i t e s . The r e a c t i v e subset o f s i n g l e s i l a n o l s appears t o i n c l u d e hydrogen-bonded ( v i c i n a l ) groups. Many o t h e r r e s u l t s o b t a i n e d by t h e ''Si-llMR
technique can be found i n references (76-82).
B233
40 c
E n
-a. ‘Q
30-
20 -
10 -
F i g . 4.12. Dependence o f s h i f t o f NMR l i n e o f e t h y l e n e adsorbed on a e r o s i l cont a i n i n g Ni2+ i o n s i n t o h i g h f i e l d on r e c i p r o c a l a d s o r p t i o n . I n conclusion, we have shown t h e p o t e n t i a l o f NMR i n t h e s t u d y o f t h e a c i d i c p r o p e r t i e s o f c a t a l y s t surfaces, whether one observes t h e resonance o f t h e p r o t o n o r o f o t h e r n u c l e i b e l o n g i n g t o chemisorbed molecules a c t i n g as probes. NMR can be used i n a q u a n t i t a t i v e f a s h i o n w i t h o u t a m b i g u i t i e s and can a l s o y i e l d i n f o r m a t i o n on t h e dynamics o f adsorbed molecules. 4.5
PARAMAGNETIC SYSTEMS The r a p i d exchange t e c h n i q u e i s p a r t i c u l a r l y i m p o r t a n t f o r t h e NMR s t u d y
o f paramagnetic systems. I n heterogeneous c a t a l y s i s t h e s p e c t r a l 1i n e s c o r r e s ponding t o inolecules chemisorbed on paramagnetic s i t e s a r e t o o broad f o r more inodern methods such as MAS-NMR t o be used. 4.5.1
A d s o r p t i o n o f o l e f i n e s on paramagnetic c e n t e r s Kazansky and coworkers (83) have a p p l i e d t h e p r e v i o u s method t o t h e s t u d y
o f t h e a d s o r p t i o n o f v a r i o u s molecules such as o l e f i n e s , c y c l o a l k a n e s , benzene, etc
... on
paramagnetic c e n t e r s supported on a e r o s i l . The p h y s i c a l model i s v e r y
c l o s e t o t h a t which i s used i n t h e c h e m i s t r y o f complex compounds i n s o l u t i o n :
B234
on e n t r y i n t o t h e c o o r d i n a t i o n sphere o f a paramagnetic ion, a s p i n d e n s i t y can a r i s e i n t h e n u c l e i of t h e adsorbed molecules as a r e s u l t o f c o n t a c t i n t e r a c t i o n which leads t o a s h i f t i n t h e s p e c t r a l components. D i p o l e - d i p o l e i n t e r a c t i o n may be another source of paramagnetic s h i f t s . For example, t h e NMR spectrum ( a t 223 K) o f e t h y l e n e on Ni2'
( t r i g o n a l l y coordinated) supported on s i l i c a c o n s i s t s
o f one l i n e s h i f t e d u p f i e l d w i t h r e s p e c t t o t h e spectrum o f t h e same molecules condensed i n pores of t h e adsorbent. The s u r f a c e complexes formed can be c l a s s i f i e d as " s t r o n g " as i s i n d i c a t e d by t h e l i n e a r dependence o f t h e s h i f t on t h e r e c i p r o c a l of adsorption. The n e g a t i v e s p i n d e n s i t y d e t e c t e d on t h e p r o t o n i s e a s i l y explained by s p i n p o l a r i z a t i o n i n t h e C-H u bond, induced by a p o s i t i v e s p i n d e n s i t y t r a n s f e r r e d i n t o t h e n - o r b i t a l o f t h e molecule by formation o f a complex w i t h t h e metal i o n . T h i s example i s a simple case which does n o t o f f e r many a l t e r n a t i v e s f o r i n t e r p r e t a t i o n
( F i g . 4.12). However i n more complicated cases, i n o r d e r t o deduce t h e mechanisms o f
ligand-metal bond formation, one has t o be a b l e t o i n t e r p r e t c o r r e c t l y t h e mechanism o f s p i n c o r r e l a t i o n , which causes t h e measured s p i n d e n s i t i e s . Such an i n t e r p r e t a t i o n very often r e q u i r e s : i)measurement o f s p i n d e n s i t i e s o f a l l t h e n u c l e i , and i i ) c a l c u l a t i o n o f t h e MOOS o f t h e complexes formed, t h e most s o p h i s t i c a t e d quantum mechanical methods being used t o take account o f t h e effect o f electron correlation. 4.5.2
Decomposition of formic a c i d on electron-donor c e n t e r s The f o l l o w i n g simple example shows why NMR i s i n t e r e s t i n g i n t h e study o f
heterogeneous c a t a l y s i s . I t concerns t h e decomposition o f f o r m i c a c i d on t i t a nium d i o x i d e . ( T h i s decomposition i s a t e s t r e a c t i o n w i d e l y used i n t h e s t u d y o f dehydrogenation o r dehydration r e a c t i o n s ) . I t i s known t h a t , by vacuum treatment a t d i f f e r e n t temperatures 8, e l e c -
tron-donor c e n t e r s a r e released o r c r e a t e d a t t h e T i 0 2 - s u r f a c e (84, 85). For example, above 523 K, oxygen vacancies a r e c r e a t e d and a t t h e same t i m e Ti3' i o n s which can be assayed by t h e now-classical method o f adsorbing an e l e c t r o n acceptor such as TCNE, and d e t e c t i n g t h e TCNE- s i g n a l by ESR (86, 87). The decomposition o f f o r m i c a c i d i s almost e x c l u s i v e l y a d e h y d r a t i o n and t h e corresponding r a t e constant i s observed t o v a r y w i t h 8 i n e x a c t l y t h e same way as t h e number of donor centers: t h e l a t t e r can t h e r e f o r e be assumed t o p l a y
an important r o l e i n t h e c a t a l y s i s r e a c t i o n . I t can be shown moreover t h a t HCOOH and Ti3'
i n t e r a c t s i n c e t h e Ti3+ ESR s i g n a l is s h i f t e d when t h e a c i d i s
adsorbed (87). I t i s n o t p o s s i b l e however t o d e t e c t any h y p e r f i n e c o u p l i n g s i n c e t h e n a t u r a l 13C and 170 c o n c e n t r a t i o n s a r e r e a l l y t o o s m a l l . I n f a c t t h e f i n a l r e s u l t s show t h a t even w i t h a s u f f i c i e n t l y h i g h c o n c e n t r a t i o n of these n u c l e a r s p i n s t h e components due t o e l e c t r o n nucleus c o u p l i n g c o u l d n o t be resolved, t h e Ti3+ ESR s i g n a l of t h e Ti3+--HCOOH complex being t o o broad.
B235
-E.
a.
~
e = 673 K
1
cg
- 1.5 - 1.0 - 0.5
0.013
0.018
n-1 x
1
Fig. 4.13. 6( H ) against reciprocal adsorption. Sample temperatures: 1: 300 K ; 2: 320 K ; 3: 340 K; 4: 358 K.
e=
c
E n
-
mK
H13C00H
(0
172.:
1754
177 I
F i g . 4.14.
6(
I
I
3.0
3.5
13 C ) against reciprocal adsorption.
I
4.0 n-lx 102
B236
’HCOOH
e = mK
1HCOOH e = 673 K
cg
-100
1
(3401 I,
2.95 F i g . 4.15.
C a l c u l a t e d 6h,
(3201
(300) (K1
315
3.25 K
I 1
I,
-’
o f cheniisorbed molecules vs. T-’.
A p p l i c a t i o n s of NMR has confirmed t h e ESR r e s u l t s b u t has made i t p o s s i b l e t o d e f i n e t h e form of t h e cheniisorbed complex b e t t e r ( 8 7 ) . We summarize here t h e 1 r e s u l t s obtained by H and I3C-NMR o f t h e CH group o f f o r m i c a c i d adsorbed on amorphous T i 0 2 t r e a t e d under vacuum a t 673 K. Experiments were c a r r i e d o u t a t
B237 h i g h s u r f a c e coverage. The number 3.101*< n = [HCOOH]ads/m
2
For comparison t h e number
(E) = (Ti
3+ ) = 1 0 . / 5
<11.10
1o f
adsorbed molecules i s
18
E o f electron-donor centers i s
. 1 0 l 6 spin/m 2
The e x i s t e n c e o f a unique s i g n a l f o r b o t h 1H and 13C shows t h a t c h e m i c a l l y and 1 p h y s i c a l l y adsorbed molecules exchange r a p i d l y . The observed s h i f t s 6( H) and 13 6 ( C) ( r e l a t i v e t o t h e gas phase) a r e l i n e a r f u n c t i o n s o f (1/n)T<363K ( F i g s . 4.13 and 4.14) and o f ( l / T ) n ( F i g . 4.15). These v a r i a t i o n s show t h a t t h e chemisorbed molecules i n t e r a c t s t r o n g l y w i t h paramagnetic c e n t e r s . SO, a c c o r d i n g t o Eq. 4.54 S
(4.72)
6 = - 6 n cn
S = number o f c h e m i s o r p t i o n s i t e s p e r m2, and 6ch = chemical s h i f t o f a n u c l e u s
o f t h e cheinisorbed molecule. Assuming t h a t t h e o n l y paramagnetic e l e c t r o n c e n t e r s a r e t h e e l e c t r o n - d o n o r cent e r s assayed by ESR (E=S) i t i s p o s s i b l e t o c a l c u l a t e 6ch: 1
6ch ( H) =
-
75 ppni ( u p f i e l d )
6ch (13C) = 3934 ppin ( d o w n f i e l d )
6ch ( H ) and 6ch (
(4.73)
13 C) a r e a l s o l i n e a r f o n c t i o n s o f ( l / T ) n
The a u t h o r s have checked t h a t t h e s e chemical s h i f t s a r e due s i m p l y t o Fermi-cont a c t i n t e r a c t i o n . Whence t h e h y p e r f i n e coup1 i n g c o n s t a n t s a r e : a = 2.85 MHz o r 1.019 gauss (4.74) a = -37.30 MHz o r -13.33 gauss Assume f i r s t l y t h a t t h e HCOOH molecules remain p l a n a r a f t e r a d s o r p t i o n . The above r e s u l t s can o n l y be e x p l a i n e d as f o l l o w s . Formation o f a ( T i -HCOOH) n-complex
t r a n s f e r s p o s i t i v e e l e c t r o n s p i n d e n s i t y P, d i r e c t l y i n t o t h e r*-MOs
o f t h e molecule. By p o l a r i z a t i o n o f t h e s p i n s a l o n g t h e o(C-H) bond, t h i s
B238
~~
~
-
~
F i g . 4.16. S p i n p o l a r i z a t i o n i n t h e (C-H) bond induced by t h e t r a n s f e r r e d s p i n d e n s i t y i n t h e K 0 2 H v*-MO. C d e n s i t y p,,
near t h e carbon, induces a s p i n d e n s i t y which i s p o s i t i v e on I 3 C and 1 H ( F i g . 4.16). From t h e v a l u e o f t h e c o u p l i n g c o n s t a n t
consequently n e g a t i v e on
a i t i s p o s s i b l e t o c a l c u l a t e p i i n t h e n*-MO a t t h e carbon atom by u s i n g
McConnell and Chesnut's r e l a t i o n s h i p (88). a (gauss) = -25 P,"
(4.75)
Whence p;
=
4 x 10-2
(4.76)
From t h i s r e s u l t and t h e quantum mechanical study o f (HCO0H)- one deduces t h a t an e l e c t r o n i c charge d e n s i t y o f about 0.1 TI*
- MO
e is
t r a n s f e r r e d from T i 3 + i n t o t h e
o f HCOOH.
However, when t h e carbon atom i s sp h y b r i d i z e d ( p l a n a r c o n f i g u r a t i o n ) and t h e p o s i t i v e and n e g a t i v e s p i n d e n s i t i e s on C and H, r e s p e c t i v e l y , a r e due u n i q u e l y t o s p i n p o l a r i z a t i o n along t h e a(C-H) bond induced by t h e e l e c t r o n d e n s i t y t h e absolute values o f t h e c o u p l i n g constants l a H l and /a13 I a r e of t h e C same o r d e r o f magnitude (89, 90). The f a c t t h a t la13CI i s much g r e a t e r than l a H \ proves t h a t a p a r t o f t h e e l e c t r o n s p i n d e n s i t y on t h e IT*-MO
i s d i r e c t l y t r a n s f e r r e d i n t o t h e 2s(C) o r b i -
t a l , and t h e r e f o r e , t h a t t h e adsorbed species (HC0OH)'-
i s no l o n g e r planar. The
d e v i a t i o n from p l a n a r i t y , a, can be c a l c u l a t e d from t h e f o l l o w i n g e q u a t i o n ( 9 1 ) . ac(a)(gauss) = [ac ( o j t 1190
. 2 t a n 2 @I P:
(4.77)
where a c ( a ) i s t h e t r u e c o u p l i n g constant, ac(o) i s t h e c o u p l i n g constant corresponding t o t h e t h e o r e t i c a l p l a n a r s t r u c t u r e , and a i s t h e angle between t h e bonds and t h e plane normal t o t h e C3 symmetry a x i s , t r e a t i n g (HC0OH)'CH3 group.
as a
B239
F i g . 4.17.
HC02H-Ti3+ chemisorbed complex.
The v a l u e of a c ( o ) f o r t h e p l a n a r s t r u c t u r e can be c a l c u l a t e d f r o m K a r p l u s and F r a e n k e l ' s e q u a t i o n ( 9 1 ) and t h e 0-n s p i n p o l a r i z a t i o n c o n s t a n t s ( 8 9 ) : a c ( o ) = 0.57 G and consequently CY
= 6.5"
These c a l c u l a t i o n s a r e o n l y approximate. However t h e y g i v e w i t h s u f f i c i e n t relative to p r e c i s i o n t h e o r d e r o f magnitude o f t h e d i s t o r s i o n o f (HCOOH) E t h e t h e o r e t i c a l p l a n e . F i n a l l y , by s t u d y i n g t h e 1H-WMR s i g n a l o f C-H and OH, i t has been p o s s i b l e t o compare t h e d i s t a n c e s ,
r, between
T i 3 + and each o f t h e H
atonis, and t o d e t a i l t h e f o r m o f t h e chemisorbed complex ( F i g . 4.17). 4.6. SUPPORTED METALS
4.6.1
'H NMR s t u d y o f hydrogen chemisorbed on p l a t i n u m . A p p l i c a t i o n t o t h e
dispersion. The p h y s i c a l and e l e c t r o n i c p r o p e r t i e s o f metal p a r t i c l e s , i n p a r t i c u l a r t h e i r magnetism, must depend on t h e i r s i z e , a t l e a s t when t h e y a r e s u f f i c i e n t l y s m a l l . F o r t h i s reason i t c o u l d be i n t e r e s t i n g t o use NMR t o s t u d y t h e s e p r o perties. I n 1973, K n i g h t ( 9 2 ) p r e d i c t e d t h a t t h e n u c l e a r s p i n s o f even and odd p a r t i c l e s would have d i f f e r e n t chemical s h i f t s . The NMR s p e c t r a o f s m a l l copper p a r t i c l e s show a l i n e which i s l e s s s h i f t e d and b r o a d e r t h a n t h a t o f t h e m e t a l , and which c o u l d be t h e envelope o f t h e two l i n e s p r e d i c t e d by K n i g h t , ( 9 3 ) . The form o f t h e NMK spectrum o f metal n u c l e i s h o u l d t h e r e f o r e depend on p a r t i c l e s i z e . Moreover, a t v e r y low temperature i t s h o u l d be p o s s i b l e t o observe two l i n e s c h a r a c t e r i s t i c of even o r uneven p a r t i c l e s . As i t has been shown by S1 i c h t e r e t a1
. (94,
95) NMR s t u d y of t h e supported
metal can indeed g i v e d i r e c t i n f o r m a t i o n a b o u t t h e d i s p e r s i o n . B u t i t i n v o l v e s
B240
considerable t h e o r e t i c a l and t e c h n i c a l problems. I n a d d i t i o n , i t cannot be gener a l i z e d t o a l l metals used i n c a t a l y s i s . For t h i s reason, F r a i s s a r d e t a l . (49, 96) had t h e i d e a o f studying metal surfaces by
NMR b u t by u s i n g a gas, such as
hydrogen, which c o u l d be chemisorbed by a l l m e t a l s as t h e r e f o r e used as a u n i versal probe. T h i s l a t t e r has a l s o t h e advantage o f being v e r y s e n s i t i v e t o
NMR
d e t e c t i o n and one of t h e gases t h e most used i n c a t a l y s i s . The i n t e r e s t o f these s t u d i e s has s i n c e been confirmed by o t h e r authors b o t h w i t h p l a t i n u m and o t h e r metals (97, 98). A f t e r desorption a t
t o r r a t a temperature T, u s u a l l y 673 K and p r i o r t o any hydrogen adsorption, t h e 1H-NMR s i g n a l c o n s i s t s o f a s i n g l e s y m e t r i c a l l i n e whose chemical s h i f t i s c l o s e t o zero, more o r l e s s i n t e n s e depending on t h e n a t u r e o f t h e s o l i d . This s i g n a l has t h e advantage o f being a b l e t o serve as an i n t e r n a l r e f e r e n c e f o r t h e chemical s h i f t s . We have a l r e a d y p o i n t e d o u t t h e e f f e c t o f t h e magnetic s u s c e p t i b i l i t y , e s p e c i a l l y i n t h e case o f protons ( P a r t 4.1). But t h i s s i g n a l a l s o has t h e disadvantage o f washing a p a r t o f t h e adsorbate spectrum. T h i s problem must be overcome: i ) e i t h e r by s u b s t r a c t i o n o f t h e OH spectrum recorded b e f o r e adsorption;
b u t t h i s s o l u t i o n i s n o t always
r e l i a b l e as t h e OH r e l a x a t i o n changes w i t h t h e chemisorption on t h e metal, i i ) o r by p r i o r complete OH-OD exchange w i t h D20 and D2. I n t h i s case t h e f i r s t H atoms t o be cheinisorbed exchange w i t h t h e OD n e a r e s t t h e p a r t i c l e s ; b u t i n a l l cases t h e s i g n a l o f these OH groups i s much narrower and s m a l l e r t h a n t h e i n i t i a l signal. A f t e r a d s o r p t i o n o f a small amount o f
H2, t h e spectrum shows a l i n e
a
s h i f t e d u p f i e l d (6a n e g a t i v e ) due t o t h e resonance o f t h e hydrogen chemisorbed on t h e platinum. I t s i n t e n s i t y 1, increases w i t h t h e amount of H2 chemisorbed b u t i t s s h i f t remains about t h e same up t o a coverage, OH,
-
o f t h e o r d e r o f 0.45
0.60 (depending of t h e i s o t h e r m used f o r t h e d e t e r m i n a t i o n of t h e monolayer)
i f t h e sample i s monodispersed ( t h a t i s , i f t h e p a r t i c l e
s i z e i s homogeneous)
(segment OP F i g . 4.18). 6,
depends on OH b u t f o r a g i v e n value o f t h i s l a t t e r i t a l s o depends on
the p a r t i c l e
diameter,
( F i g . 4.18). The i n c r e a s e o f (6,) F i g . 4.19 f o r coverage OH = 0.5 and 1.0.
w i t h i i s displayedin
The p r o t o n s h i f t i s g e n e r a l l y small (0-10 ppm) and p o s i t i v e . The n e g a t i v e and o f t e n very l a r g e s h i f t , 6,
can o n l y be o f t h e K n i g h t s h i f t . The P a u l i para-
magnetism o f P t i s m a i n l y due t o unpaired s p i n s i n t h e bonding o f each H atom l o c a l i z e s a metal
4
band. Thus c o v a l e n t
e l e c t r o n and l e a d t o a decrease o f
t h e n e t s p i n d e n s i t y . T h i s concept i s confirmed by UPS s t u d i e s which i n d i c a t e t h a t substrate decrease o f (6,)
4
e l e c t r o n s a r e i n v o l v e d i n t h e s u r f a c e bond ( 9 9 ) . The l i n e a r w i t h i n c r e a s i n g OH ( > 0.5) was q u i t e easy t o e x p l a i n . The
K n i g h t s h i f t i s p r o p o r t i o n a l t o t h e paramagnetic s u s c e p t i b i l i t y of P t which, i n t u r n , i s p r o p o r t i o n a l t o t h e o v e r a l l s p i n d e n s i t y ; t h i s must decrease l i n e a r l y
B241
F i g . 4.18, H-Knight s h i f t versus coverage. x, Pt/AlzO3 ( d = 1.5 nm); 0 , Pt/A1203 ( d = 2.5 mi); , Pt/A1203 ( d = 7 nm), d a t a t a k e n f r o m r e f . 96; * P t / S i O ( d = = 1 nm), from r e f . 97; A , P t ( d = 12 n m ) ; ~ , P t ( d = 40 nm), from r e f . 96.
B242
F i g . 4.19.
H-Knigth s h i f t v e r s u s p a r t i c l e s i z e D.
a:
eH
= 0.5
b: OH = 1.0
when OH increases, as i s shown e x p e r i m e n t a l l y (segment PQ). Furthermore, t h e r e l a t i v e v a r i a t i o n
when
eH goes f r o m 0 t o 1
appears t o decrease w i t h i n c r e a s i n g p a r t i c l e s i z e . T h i s seems t o i n d i c a t e t h a t t h e s p i n d e n s i t y change i s r e s t r i c t e d t o t h e zone o f P t - H bonds (one o r two atom l a y e r s ) . Consequently,
must be c l o s e t o z e r o a t OH = 1, f o r t h e mono- and
d i a t o m i c l a y e r s . I t i s observed t o be t h e case o f t h e most h i g h l y d i s p e r s e d lg5Pt-NMR (94, 9 5 ) .
samples. S l i c h t e r e t a l . have c o n f i r m e d t h e s e p o i n t s by
I t was somewhat s u r p r i s i n g t o f i n d a p l a t e a u OP f o r O <
e
< 0.5.
I n the
case o f P t - z e o l i t e samples with v e r y s m a l l m e t a l p a r t i c l e s , ly9Xe-NMR ( P a r t 4.7) showed t h a t c h e m i s o r p t i o n o f a v e r y s m a l l amount o f H2 a t 298 K t r a c e s o u t two zones i n t h e z e o l i t e c r y s t a l l i t e s ; t h e c l o s e r t o t h e s u r f a c e c o n t a i n s o n l y p a r t i c l e s w i t h 2H; i t i n c r e a s e s w i t h
eH a t t h e expense o f t h e c e n t r a l p a r t which
c o n t a i n s o n l y b a r e p a r t i c l e s (100, 101). The OP h o r i z o n t a l corresponds t h e r e f o r e t o a c o n s t a n t coverage equal t o 2H o f a number o f p a r t i c l e s i n c r e a s i n g w i t h OH. A t p o i n t P a l l t h e s e v e r y s m a l l p a r t i c l e s b e a r 2H. The r e s u l t i s s i m i l a r f o r c o n v e n t i o n a l s u p p o r t s . The chemisorbed hydrogen a r r i v e s by t h e t o p of t h e NMR tube: C h e m i s o r p t i o n o f a s m a l l amount o f H2 a t 298 K c r e a t e s two zones: t h e f i r s t b, towards t h e t o p o f t h e tube, c o n t a i n s o n l y p a r t i c l e s w i t h a coverage c l o s e t o 0.5. The second, towards t h e b o t t o n , cont a i n s o n l y b a r e p a r t i c l e s . The f i r s t extends w i t h OH a t t h e expense o f t h e second a l o n g t h e l e n g t h o f t h e segment OP. I t i s t h e r e f o r e l o g i c a l t h a t da does
B243 n o t v a r y i f t h e sample i s monodispersed. I f t h e sample has two w e l l - d i f f e r e n c i a t e d p a r t i c l e d i s t r i b u t i o n , two ‘H-NMR
s i g n a l s a r e d e t e c t e d upon t h e H2 a d s o r p t i o n , each depending on OH as above. F i n a l l y , t h i s h o r i z o n t a l i s d i f f i c u l t t o d e t e c t i n t h e f o l l o w i n g cases: wide p a r t i c l e s i z e d i s t r i b u t i o n o r sample monodispersed b u t w i t h l a r g e p a r t i c l e s . 4.7
NMR OF PHYSISORBED MOLECULES USED AS PROBES
The c e n t r a l i d e a o f t h i s g e n e r a l r e s e a r c h was t o f i n d a molecule, non-react i v e , p a r t i c u l a r l y s e n s i t i v e t o i t s environment, t o c o l l i s i o n s w i t h o t h e r chem i c a l species and t o t h e n a t u r e o f a d s o r p t i o n s i t e s , which c o u l d be used a s a probe f o r d e t e r m i n i n g i n a new way s o l i d p r o p e r t i e s which a r e d i f f i c u l t t o det e c t b y c l a s s i c a l physicochemical techniques. I n a d d i t i o n t h i s probe s h o u l d be d e t e c t a b l e by IlMR s i n c e t h i s t e c h n i q u e i s p a r t i c u l a r l y s u i t a b l e f o r i n v e s t i g a t i n g e l e c t r o n p e r t u r b a t i o n s i n r a p i d l y moving molecules. Xenon i s an i d e a l probe because i t i s an i n e r t gas, monoatomic, w i t h a l a r g e s p h e r i c a l e l e c t r o n cloud. Any d i s t o r t i o n o f t h e e l e c t r o n c l o u d i s t r a n s m i t t e d d i r e c t l y t o t h e Xe nucleus and g r e a t l y a f f e c t s t h e NMR chemical s h i f t . From t h e NMR p o i n t of view, t h e 12’Xe
i s o t o p e t o be s t u d i e d has a s p i n of one-
h a l f . I t s n a t u r a l abundance i n xenon i s 26% and i t s s e n s i t i v i t y of d e t e c t i o n r e l a t i v e t o protons i s We r e p o r t h e r e t h e main r e s u l t s r e l a t i v e t o z e o l i t e s ( 1 0 1 ) . B u t r e c e n t s t u d i e s prove t h a t i t can be a p p l i e d t o any s o l i d (102). 4.7.1
Chemical s h i f t o f Xenon adsorbed i n a p u r e z e o l i t e
As i n t h e gas phase (1031, we have shown t h a t t h e chemical s h i f t of xenon s i t u a t e d i n any g i v e n system i s always t h e sum o f terms c o r r e s p o n d i n g t o t h e d i f f e r e n t p e r t u r b a t i o n s t o which i t i s s u b j e c t e d 4, 5 ) . Consequently t h e chemical s h i f t 6 o f xenon adsorbed i n a p u r e z e o l t e i s : 6 = 6 + 6 p C + 6 + 6 0 M Xe
(4.78)
6o i s t h e r e f e r e n c e , g e n e r a l l y t h e chemical s h i f t o f gaseous xenon e x t r a p o l a t e d
t o z e r o pressure, 6 c i s due t o t h e e l e c t r i c f i e l d c r e a t e d by t h e c a t i o n s . The l a s t t e r m corresponds t o t h e i n c r e a s e i n s h i f t caused by Xe-Xe c o l l i s i o n s ; i t i s p r o p o r t i o n a l t o t h e l o c a l d e n s i t y o f xenon adsorbed i n t h e c a v i t i e s and ( o r ) channels. 6s, due t o Xe-wall c o l l i s i o n s , depends on t h e cage and channel s t r u c t u r e . 6M depends on t h e magnetic p r o p e r t i e s o f t h e s o l i d , f o r example on t h e paramagnetism o f c a t i o n s . The xenon probe i s u s u a l l y adsorbed a t 298 K. A t t h i s t e m p e r a t u r e t h e NMR a b s o r p t i o n o f 12’Xe
i s g e n e r a l l y observed. A l l resonance s i g n a l s of I2’Xe
bed on z e o l i t e s a r e s h i f t e d t o h i g h e r frequency r e l a t i v e t o t h e reference.
adsor-
B244
'OT
Ji. XeatOmslg
-
lo2' '
F i g . 4.20. Dependence o f t h e chemical s h i f t 6 on t h e number o f xenon atoms adsorbed p e r grain o f z e o l i t e : 0 Nay; 0 Na-ZK4; A K-L; o Na-Q; A z%-5; V ZSM-11. F i g . 4.21. Dependence o f t h e chemical s h i f t 6 on t h e number o f xenon atoms adsorbed p e r gram o f z e o l i t e : A m o r d e n i t e ; x Rho; 0 K - F e r r i e r i t e ; 8 Rho a t 718K.
4.7.2
Influence o f structure
4.7.2.1
Na o r H - f a u j a s i t e
L e t us c o n s i d e r f i r s t t h e case o f N a - f a u j a s i t e denoted Nay,
where x r e -
p r e s e n t s t h e S i d A l r a t i o ( 1 . 2 8 ( x < 54.2). When t h e r e i s no s u b s c r i p t t h e sample i s c o n v e n t i o n a l Y . The s i g n a l s h i f t i n c r e a s e s l i n e a r l y w i t h t h e adsorbed xenon c o n c e n t r a t i o n [Xe] b u t i s p r a c t i c a l l y independent o f t h e v a l u e o f x, t h e r e f o r e o f t h e number o f Na
t
c a t i o n s (104) ( F i g . 4 . 2 0 ) . T h i s r e s u l t p r o v e s t h a t i n t h e Y
supercages t h e time-average e l e c t r i c f i e l d due t o t h e s e c a t i o n s i s n e g l i g i b l e a t 298 K ;
SO
6,
Y
0.
The r e s u l t s r e l a t i v e t o HY a r e s i m i l a r t o t h e p r e v i o u s ones and p r o v e t h a t t h e mean e l e c t r i c f i e l d i s a l s o n e g l i g i b l e a t 298 K i n t h e supercages o f t h i s z e o l i t e . A t v e r y l o w [Xe] t h e m o t i o n o f each atom i s d i s t u r b e d o n l y by cage w a l l s . Consequently t h e chemical s h i f t 6,(58
f
2 ppm) o b t a i n e d by e x t r a p o l a t i o n
o f t h e l i n e 6 = f [ X e j t o [Xe] = 0 can be c o n s i d e r e d as c h a r a c t e r i s t i c o f t h e z e o l i t e w i t h r e s p e c t t o xenon a d s o r p t i o n . The i n c r e a s e o f 6 w i t h [Xe] r e s u l t s f r o m mutual i n t e r a c t i o n s between Xe atoms.
B245 4.7.2.2
I n f l u e n c e of t h e s t r u c t u r e
The v a r i a t i o n o f 6 a g a i n s t xenon c o n c e n t r a t i o n p e r gram i s c h a r a c t e r i s t i c o f t h e z e o l i t e s t r u c t u r e ( F i g . 4.21) (105, 106). And t h i s t e c h n i q u e i s p a r t i c u l a r l y s e n s i t i v e since i t i s able t o d i s t i n g u i s h z e o l i t e s with small s t r u c t u r a l d i f f e r e n c e s such as LSivl-5 and ZSM-11 o r e r i o n i t e and o f f r e t i t e . F i g u r e 4.29 shows t h a t : i ) t h e s l o p e o f t h e 6 =f[Xe] c u r v e depends on t h e v o i d volume o f t h e pores; t h i s i s expected s i n c e , f o r a g i v e n number o f Xe atoms p e r gram, t h e l o c a l d e n s i t y and, thus, t h e Xe-Xe i n t e r a c t i o n s depend on t h i s volume; i i ) t h e chemical s h i f t tis a t z e r o c o n c e n t r a t i o n i s r e l a t e d t o t h e s t r u c t u r e : t h e s m a l l e r t h e channels and t h e c a v i t i e s , o r t h e more r e s t r i c t e d t h e d i f f u s i o n , t h e g r e a t e r becomes 6,. NMR i s a l s o a b l e t o l o c a t e v a r i o u s zones o f Xe a d s o r p t i o n i n a s i n g l e samp l e . F o r example t h e r e a r e two s i g n a l s f o r xenon adsorbed i n t h e c a v i t i e s and t h e prisms of K h o - z e o l i t e ( 1 0 7 ) , i n t h e two d i f f e r e n t channels of F e r r i e r i t e ( F i g . 4.21) ( l o g ) , o r i n t h e channel and s i d e - p o c k e t s o f m o r d e n i t e (106, 108). 4.7.2.3
R e l a t i o n s h i p between t h e chemical s h i f t 6, and t h e v o i d space
Table 4.3 g i v e s t h e v a l u e o f 6, f o r v a r i o u s z e o l i t e s and t h e c h a r a c t e r i s t i c s o f t h e i r v o i d spaces. I t i s c l e a r t h a t 6, depends on t h e f o r m and t h e dimensions o f t h e v o i d space i n which i t i s adsorbed. B u t t h e s e r e s u l t s a r e p u r e l y q u a l i t a t i v e . I n o r d e r t o o b t a i n by t h i s t e c h n i q u e more p r e c i s e d a t a on t h e v o i d space o f a z e o l i t e o f unknown s t r u c t u r e and on t h e dimensions o f s t r u c t u r a l d e f e c t s , F r a i s s a r d e t a l . (110, 111) have been a b l e t o c a l c u l a t e t h e means f r e e path, 1 o f xenon
adsorbed i n some model z e o l i t e s and t o d e t e r m i n e t h e
dependence o f 6, on 1 ( F i g . 4.22).
2.054 & s = 243 2.054 t 1
(4.79)
T h i s e q u a t i o n has been determined on c o n s i d e r i n g t h a t t h e v a l u e g s measured a t 299 K i s t h e average v a l u e o f t h e s h i f t o f xenon i n r a p i d exchange between a p o s i t i o n on t h e surface o f area A (adsorbate d e f i n e d by
and a p o s i t i o n i n
t h e volume V of t h e c a v i t y o r channel ( d e f i n e d by 6 v ) .
(4.80)
dV i s n o t c o n s t a n t and depends on t h e magnitude o f V. I t i s equal t o
at
t h e moment t h e xenon leaves t h e s u r f a c e and can f a l l t o z e r o i f t h e volume i s g r e a t enough f o r t h e p e r t u r b a t i o n s o f i t s e l e c t r o n i c environment t o be c a n c e l l e d out.
B246
Fig. 4.22.Variation o f the chemical 6 (ppm) against t h e mean f r e e path V a r i a t i o n o f “1 x 10-4(ppm-l) against 1 (8) TABLE 4.3. Chemical s h i f t 6, o f 12’Xe space
Zeol it e
l(1);b .
adsorbedon z e o l i t e s a n d c h a r a c t e r i s t i c s o f the v o i d
6, PPm
C h a r a c t e r i s t i c s o f the v o i d space
A
w i t h f o u r prisms openings a t logo,
60
Sphere, diameter 13 12-rings: 8 A .
A, ZK4
87
Sphere, diameter 11.4 1; s i x 8 - r i n g openings 4 depends on t h e cation.
L
90
Unidirnensional barrel-shaped channels: 1 2 - r i n g openings o f 7.1 8, maximum diameter 9 8.
s2
73
Unidimensional channels, r e g u l a r c y l i n d e r s 12-ring diameter 7.4 A.
ZSM-5
113
Trid’mensional i n t e r onnecting channels 10-rings 5.1 x 5.5 and 5.4 x 5.6
ZSN- 10
110
Tridimensional interconnecting channels 10-rings 5 . 1 x 5.5 A.
Mordenite9Z
C 115 SB = 250
Faujasite, Y
FerrieriteyF
Rho
B
= 110
C
165
C = P =
114 230
8
-
5
8
8.
Unidimensional channel 12-ring 6.7 x 7.0 8 - r i n g : 2.9 x 5.7 8.
8.
8-pseudo-spherical c a v i t y o f 7 diameter w t t h two 8r i n g openings. Two dimens‘onal interconnecting channels C-10 r i n g 4.3 x 5.5 Tridimensional interconnecting channels sphere 10 8. Prisms: 8 - r i n g 3.9 x 5.1 A h e i g h t 3.2 A.
8.
B247
F i g . 4.23. Spectrum of xenon adsorbed on t h e NaY and Ca A m i x t u r e ; xenon p r e s sure: 400 t o r r 4.7.2.4
C r y s t a l l i n i t y and p o r e b l o c k i n g
By t h e 129Xe-NHR of adsorbed xenon i t i s a l s o p o s s i b l e t o determine t h e z e o l i t e c r y s t a l l i n i t y a t s h o r t range. I t s h o u l d i n p a r t i c u l a r l e a d t o a s o l u t i o n of t h e f o l l o w i n g a m b i g u i t y : i s t h e amorphous p a r t d e t e c t e d b y x - r a y s r e a l l y due t o t h e d i s o r d e r e d s t a t e o f a f r a c t i o n o f t h e s o l i d o r t o c r y s t a l l i t e s which a r e t o o small t o g i v e r i s e t o an x - r a y d i f f r a c t i o n spectrum? F o r example, l e t us go back t o t h e s t u d y o f z e o l i t e Nay. We saw ( F i g . 4.20) t h a t whatever [Xe] t h e r e e x i s t s a s i n g l e - v a l u e d r e l a t i o n s h i p between 6 and t h e average number o f Xe atoms p e r supercage. Furthermore, a t each v a l u e o f 6 t h e s i g n a l s t r e n g t h i s p r o p o r t i o n a l n o t o n l y t o t h e q u a n t i t y adsorbed p e r cage b u t a l s o , o f course, t o t h e number o f cages. Consequently, i f t h e sample c o n s i s t s o f a m i x t u r e o f v a r i o u s s o l i d s t h e spectrum must i n c l u d e as many components as t h e r e a r e d i f f e r e n t s t r u c t u r e s , t h e i r i n t e n s i t i e s b e i n g a d i r e c t measure o f t h e c o m p o s i t i o n o f t h e sample. F o r example, F i g . 4.23 r e p r e s e n t s t h e spectrum o f xenon
adsorbed on a m i x t u r e o f 58% NaY + 42% CaA w/w (pXe = 400 t o r r ) . By com-
p a r i n g t h e i n t e n s i t y o f t h e NaY s i g n a l w i t h t h a t o f a p e r f e c t l y c r y s t a l l i z e d Nayx used as a r e f e r e n c e 1, one f i n d s t h a t t h e c o m p o s i t i o n of t h e m i x t u r e i s t h a t g i v e n above t o w i t h i n f 1%(112). Rencent r e s u l t s (110, 111) have shown t h a t t h e mean f r e e p a t h of xenon i n z e o l i t e s i s v e r y s m a l l . The s h o r t - r a n g e environment o f t h e s e atoms can t h e r e f o r e be s t u d i e d . F i g u r e 4.24 shows s p e c t r a o f xenon adsorbed under t h e same c o n d i t i o n s i n v e r y we1 1 c r y s t a l 1 i z e d , s l i g h t l y d e f e c t i v e o r c o m p l e t e l y d i s o r g a n i z e d Y samples. ( 112). I n p r a c t i c e t h e problem i s n o t always so s i m p l e . I t o f t e n happens t h a t t h e s p e c t r a o f two z e o l i t e s NaY and HY d i s p l a y s l i g h t l y d i f f e r e n t 6 s v a l u e s f o r many
B248
ppm
150
100
50
0
F i g . 4.24. A: Spectrum o f xenon adsorbed r e f e r e n c e Nay; B:HY w i t h some defects; C : completely d i s o r g a n i z e d HY reasons: t h e s i z e o f t h e c r y s t a l l i t e s , t h e presence o f A1 c a t i o n s i n t h e supercages a f t e r p a r t i a l dealumination, e t c . The s i m p l e s t s o l u t i o n t o t h i s problem c o n s i s t s then i n p l o t t i n g t h e l i n e a r v a r i a t i o n o f t h e i n t e n s i t y I of t h e s i g n a l a g a i n s t 6, ( F i g . 4.25).
By comparison w i t h a standard substance t h e number o f
well-formed supercages can be determined from t h e slope (112). N a t u r a l l y , t h i s t y p e o f s t u d y can be c a r r i e d o u t , more o r l e s s e a s i l y , whatever t h e s t r u c t u r e o f t h e z e o l i t e . This technique can a l s o be used f o r s t u d y i n g t h e o b s t r u c t i o n o f channels and c a v i t i e s (101, 106). The pore volumes
o f v a r i o u s samples can be compared by u s i n g t h e a d s o r p t i o n isotherms. But i t
F i g . 4.25. Signal i n t e n s i t y a g a i n s t t h e chemical s h i f t 6. X : r e f e r e n c e Nay; NaY t CaA; 0 : dealuminated H Y .
:
B249
Iu
0
Xe atoms/g
F i g . 4.26. Dependence o f t h e chemical s h i f t on t h e number o f xenon atoms adsorbed on z e o l i t e s : f u l l l i n e SAPO; channels: 0 v o i d , p a r t i a l l y blocked; d o t t e d l i n e R; channels: A void, 7 p a r t i a l l y blocked. appeared t o us t h a t t h e xenon t e c h n i q u e i s more p r e c i s e f o r s t u d y i n g t h e blockage o f c e r t a i n pores. F o r example, F i g . 4.26 d i s p l a y s t h e v a r i a t i o n o f 6 w i t h [Xe] o f two samples, A and 8, which a c c o r d i n g t o X-rays have i d e n t i c a l u n i dimensional channel s t r u c t u r e s . I t i s t h e r e f o r e e v i d e n t t h a t t h e v a l u e s o f 6 s and
1 should be t h e same. B u t t h e g r e a t e r s l o p e o f c u r v e B i n d i c a t e s t h a t , f o r
t h e same c o n c e n t r a t i o n o f xenon p e r gram o f s o l i d , t h e e f f e c t of Xe-Xe c o l l i s i o n s i s g r e a t e r t h a n i n A and, t h e r e f o r e , t h a t t h e l o c a l d e n s i t y i s h i g h e r . T h i s r e s u l t corresponds t o p a r t i a l o b s t r u c t i o n o f t h e channels, p r o b a b l y due t o incomplete e l i m i n a t i o n o f t h e t e m p l a t e s . The same problem c o u l d concern any o t h e r s o l i d , such as R z e o l i t e ( F i g . 4 . 2 7 ) ( 1 0 7 ) . 4.7.3
_CxNdl-x
4.7.3.1
Y Zeolites. Influence o f Cations
Diamagnetic Cations
L e t us assume now t h a t t h e r e a r e i n t h e supercages o f a Y z e o l i t e C2' t
magnetic c a t i o n s which i n t e r a c t w i t h xenon much more s t r o n g l y t h a n Na cage w a l l s . I n t h i s case, and p a r t i c u l a r l y a t l o w [Xe],
dia-
on t h e
each Xe atom w i l l have a
r e l a t i v e l y l o n g r e s i d e n c e t i m e on these C2+ c e n t e r s . The c o r r e s p o n d i n g chemical s h i f t 6 w i l l be much g r e a t e r t h a n i n t h e case o f p u r e Nay. When [Xe] i n c r e a s e s , 6 must decrease i f t h e r e i s exchange o f these atoms adsorbed on C2' w i t h t h o s e adsorbed on t h e o t h e r s i t e s (Na', w a l l s ) , and t h e n i n c r e a s e w i t h t h e number of Xe-Xe c o l l i s i o n s (113, 114). T h i s i s e x a c t l y what i s found a t 298 K f o r MgXY and
B250
O
1020
xe atoms/a
1021
Fig. 4.27. Dependence o f the chemical s h i f t on t h e number o f xenon atoms adsorbed per gram o f samples: v Ca54Y; 1 Ca71Y; A CaT9Y; A Ca85Y; 0 Mg47Y; @ Mgb2Y; 0 %71Y t
CaXY z e o l t i e s , where X denotes the degree o f exchange w i t h Na
.
When C2+ cations are i n t h e s o d a l i t e cage o r i n the hexagonal prisms (1 < 55%) 6 i s a l i n e a r functions o f [Xe], i d e n t i c a l w i t h t h a t f o r Nay, whatever the extent o f dehydration o f the sample. CJhen some C2+ cations a r e s i t u a t e d i n t h e
supercages (A > 55%), one observes f o r CY (dehydrated under vacuum above 773 K) v a r i a t i o n s i n 6, compared t o Nay, which are greatest when
A i s high, e s p e c i a l l y
a t low [Xe] ( F i g . 4.28) and which correspond t o the e l e c t r i c f i e l d e f f e c t i n the supercages ( 6
# 0 ) . More p r e c i s e l y t h e d i f f e r e n c e 6,
-
where 0, i s p r o p o r t i o n a l t o the square
i s the experimental value o f 6 f o r [Xel
= 6s,c
%,c o f the e l e c t r i c f i e l d a t the Xe n u c l e i o f Xe atoms adsorbed on C2+ cations (114). Some values o f 6 and tiCaare l i s t e d i n Table 4.4. I t i s observed t h a t f o r 1% the same c a t i o n these values a r e independent o f A ( l e s s than 53%). This r e s u l t confirms t h a t t h e r e i s a very strong e l e c t r i c f i e l d gradient and shows t h a t o n l y the c a t i o n i n contact w i t h t h e xenon atom has a r e a l e f f e c t on t h i s l a t t e r . TABLE 4.4.
Chemical s h i f t of xenon adsorbed on cations Mg2+ o r Ca2t %
71 69 85
'Mq
ppm
122 f 10 130 15
6Ca PPm
72 70
.r
10 10
B251
and ACa must be p r o p o r t i o n a l t o t h e square o f t h e e l e c t r i c f i e l d a t t h e 6M9 n u c l e i o f Xe atoms adsoi-bed on Mg2' and Ca2', i . e . a t a d i s t a n c e , 1,o f 2.86 and 3.20 A, r e s p e c t i v e l y , from t h e c e n t e r o f t h e s e c a t i o n s :
(4.81)
I n t h e absence of any r e l a t i o n s h i p o f t h e f o r m 6 = f ( E ) we a r e o n l y a b l e , f o r t h e moment, t o compare R w i t h t h e c a l c u l a t e d v a l u e s o f E. The e f f e c t s o f t h e d e h y d r a t i o n and r e d e h y d r a t i o n o f CXNal-AY t h e i n f l u e n c e of C2'
z e o l i t e s on
c a t i o n s can a l s o be s t u d i e d by t h i s t e c h n i q u e . F o r example,
F i g . 4.28 shows t h a t f o r each xenon c o n c e n t r a t i o n t h e chemical s h i f t s
4 and
the
s i g n a l w i d t h AH i n c r e a s e w i t h t h e e x t e n t o f d e h y d r a t i o n o f t h e s o l i d . A t l o w d e h y d r a t i o n temperatures 0
( 6 473K) t h e Mg2' c a t i o n s a r e a s s o c i a t e d w i t h w a t e r molecules which h i n d e r t h e Mg2'-Xe c o n t a c t . When ea 573-623 K t h e remain-
i n g w a t e r molecules a r e d i s s o c i a t e d because o f t h e e l e c t r i c f i e l d (115) w i t h f o r m a t i o n o f [Hg(OH)] Mg2'
increases w i t h
.At
i
h i g h e r d e h y d r a t i o n temperatures t h e number o f bare
0 , as i s shown by t h e changes i n 6 and
g r e a t e s t f o r 0 I 773 K, i . e . when d e h y d r a t i o n i s complete.
AH. T h e i r v a l u e s a r e
B252
E f f e c t on t h e xenon spectrum o f r e h y d r a t i n g t h e Mg Y sample (P = ( b ) ,&on i n t h e supercages c o n t a i n i n g hydrated Mg2+.
F i g . 4.29.
= 200 t o r r ) : ( a ) xenon i n t h e supercages c o n t a i n i n g o n l y bare71Mg2+;
The above r e s u l t s a r e confirmed by t h e changes i n t h e s p e c t r a when t h e sample i s rehydrated; l i n e
a t 92 ppm ( F i g . 4.29) due t o xenon i n t h e supercages c o n t a i n i n g o n l y bare Mg2+ c a t i o n s , decreases i n f a v o u r o f l i n e b a t 72 ppm as water i s adsorbed. 4.7.3.2
Paramagnetic c a t i o n s
The problem i s n a t u r a l l y more d i f f i c u l t i n t h e case o f paramagnetic c a t i o n s e s p e c i a l l y when t h e e x t e n t o f exchange i s so h i g h t h a t t h e magnetic term AM i n equation 4.78 becomes large, as has been shown by Gedeon e t a l . (116). However, Scharf e t a l . (117) have succeeded i n using t h i s technique t o f o l l o w t h e reduct i o n and t h e r e o x i d a t i o n o f Ni-NaY z e o l i t e corresponding t o 15% Na exchange and desorbed under vacuum a t 623 K. F i g u r e 4.30 shows how t h e spectrum depends on t h e r e d u c t i o n temperature. I n t h e sample reduced a t t h e l o w e r temperatures ( ~ 3 7 3K) two types of environment f o r t h e xenon atoms a r e evident, one corresponding t o xenon i n t h e nickel-exchanged m a t e r i a l , t h e o t h e r t o xenon i n c o n t a c t w i t h NaY o r HY. Reduction a t h i g h e r temperatures produces an u p f i e l d s h i f t o f t h e f i r s t resonance, i n d i c a t i n g t h a t t h i s environment becomes more l i k e t h e environment o f xenon i n HY z e o l i t e . For t h e h i g h e s t temperature, 643 K, o n l y t h e l i n e corresponding t o xenon i n HY i s detected. These r e s u l t s prove t h a t n i c k e l i o n s a r e removed from t h e supercages upon r e d u c t i o n . I n t h e same way these authors have shown t h a t t h e r e o x i d a t i o n i n 600 t o r r o f oxygen a t v a r i o u s temper a t u r e does n o t r e v e r s e t h e process o f r e d u c t i o n .
B253
No Reduction
I5O
loo
505PP:
F i g . 4.30. '*'Xe NMR s p e c t r a o f xenon adsorbed i n NiNaY z e o l i t e s reduced a t v a r i o u s tetiiperatures f o r 3 hours i n 600 t o r r o f hydrogen. From t h e above r e s u l t s one may conclude t h a t t h e "'Xe-NMR
s t u d y o f adsor-
bed xenon i s p a r t i c u l a r l y i n t e r e s t i n g f o r d e t e r m i n i n g c e r t a i n p r o p e r t i e s o f z e o l i t e s such as t h e dimensions o f c a v i t i e s and channels, t h e s h o r t - d i s t a n c e c r y s t a l l i n i t y , t h e n a t u r e o f s t r u c t u r e d e f e c t s and, f i n a l l y t h e e l e c t r i c i n fluence o f cations.
4.7.4
Chemical s h i f t o f Xenon adsorbed on m e t a l - l o a d e d z e o l i t e s I n what f o l l o w s we s h a l l reason on t h e b a s i s o f f a u j a s i t e s t r u c t u r e z e o l i -
t e s , b u t t h e r e s u t l s o b t a i n e d a r e e a s i l y g e n e r a l i z e d t o o t h e r t y p e s of z e o l i t e s and porous s o l i d s . L e t us assume t h a t d i s t r i b u t e d i n v a r i o u s NaY z e o l i t e supercages t h e r e a r e s o l i d p a r t i c l e s w i t h c h e m i c a l l y d i f f e r e n t surfaces Si. i n d e x i denotes a p a r t i c l e t y p e o f p o p u l a t i o n ni,
The
amongst t h e p t y p e s which
e x i s t ( 0 < i < p ) . Corresponding t o each of t h e s e p o p u l a t i o n s t h e r e i s a s h i f t di c h a r a c t e r i s t i c o f t h e Xe-Si f o r t h e chemical n a t u r e o f Si
c o l l i s i o n s , b e i n g t h e p r o d u c t o f two terms:
) ( x
term f o r t h e frequency o f Xe-Si c o l l i s i o n s
The spectrum depends on t h e l i f e t i m e of Xe on each a d s o r p t i o n s i t e . i ) I f t h e l i f e t i m e o f xenon on each s u r f a c e Si i s l o n g (from an NMR p o i n t of v i e w ) t h e spectrum o f t h e adsorbed xenon s h o u l d t h e o r e t i c a l l y c o n t a i n as many components 6i as t h e r e a r e t a r g e t t y p e s , t h e i n t e n s i t y o f each b e i n g p r o p o r t i o n a l t o
B254
F i g . 4.31. Model of d i s t r i b u t i o n o f Si and S . p a r t i c l e s grouped according t o J t h e i r n a t u r e i n volumes V1 and V j t h e number ni of t a r g e t s o f t h e same type; t h e t o t a l b e i n g p t l s i n c e t h e comdue t o xenon s t r i k i n g t h e supercage surfaces and t h e o t h e r
ponent ( s h i f t
Xe must a l s o be taken i n t o accout. T h i s case c o u l d be o b t a i n e d by r e c o r d i n g t h e spectrum a t s u f f i c i e n t l y low temperature. But then t h e components a r e v e r y broad and almost undetectable by c l a s s i c a l NMR methods. i i ) I n t h e o p p o s i t e case where t h e xenon has a v e r y s h o r t l i f e t i m e a t each a d s o r p t i o n s i t e and can, moreover, d i f f u s e r a p i d l y across
several supercages contained i n t h e same c r y s t a l 1 i t e o f
the z e o l i t e a l l t h e above s i g n a l s coalesce. The spectrum c o n s i s t s of o n l y one component whose p o s i t i o n depends on t h e values o f 6i, b a b i l i t y ai o f Xe-Si
each weighted by t h e pro-
collison: (4.82)
I n t h i s case, i t i s of course v e r y d i f f i c u l t t o g e t much i n f o r m a t i o n about t h i s system from t h i s s i n g l e l i n e . F o r t u n a t e l y , t h e spectrum depends a l s o on t h e d i s t r i b u t i o n o f t h e d i f f e r e n t p a r t i c l e s i, j... i n s i d e z e o l i t e c r y s t a l l i t e s . Among t h e d i f f e r e n t models considered (118, 119) we should note t h e f o l l o w i n g case which i s fundamental i n t h e s t u d y o f chemisorption on metal-zeol i t e s . (Fig. 4.31). L e t us suppose t h a t i n each z e o l i t e c r y s t a l l i t e t h e p a r t i c l e s i. j... a r e grouped according t o t h e i r n a t u r e i n volume Vi,
V..
J
Unless t h e c o n c e n t r a t i o n o f
one o f t h e species i s r e a l l y v e r y small, t h e q u a n t i t y o f xenon exchanging b e t ween two adjacent zones Vi,
V . i s n e g l i g i b l e compared t o t h e q u a n t i t i e s o f xenon
J
i n each zone. E v e r y t h i n g i s t h e n as though t h e r e were several independent samples. The NMR spectrum must t h e r e f o r e c o n t a i n p components corresponding t o each t y p e o f t a r g e t , w i t h s h i f t 6i 6. = 1
Ai
A . 6. i
1
t
p. 6 1
NaY
such t h a t :
(4.83)
and pi a r e t h e probab l i t i e s o f xenon c o l l i s i o n w i t h S i and Nay, r e s p e c t i v e l y .
B255
a
I n g e n e r a l , such a s i t u a t i o n can a r i s e whenever t h e r e a r e i n t h e z e o l i t e c r y s t a l l i t e s zones which a r e c l e a r l y d i f f e r e n t i a t e d e i t h e r b y t h e n a t u r e o f t h e p a r t i c l e s o r by t h e d i s t r i b u t i o n o f t h e p a r t i c l e s which can, i n t h i s case, be of t h e same t y p e . L e t us c o n s i d e r now a c a t a l y s t Elx-BG-NaY c o n t a i n i n g _?. m e t a l p a r t i c 1 e s . g - ' o f sample (each c o n t a i n i n g on average 5 atoms o f nietal M) w i t h a t o t a l amount 6 o f adsorbed gas G. There i s an atom G ( o r m o l e c u l e ) d i s t r i b u t i o n o v e r t h e t o t a l i t y o f t h e i i i e t a l , t h e number
1o f
G atoms ( o r molecules) p e r p a r t i c l e b e i n g
p o s s i b l y d i f f e r e n t f r o m one p a r t i c l e t o a n o t h e r . Now, t h e chemical n a t u r e o f a metal p a r t i c l e changes w i t h t h e number For each number
Io f
G atoms ( o r m o l e c u l e s ) p e r p a r t i c l e .
t h e r e i s t h e r e f o r e a c o r r e s p o n d i n g term tii = tiMxtiG
characte-
r i s t i c o f t h e c o l l i s i o n between Xe and t h e p a r t i c l e s MxtiG. A s we have seen above, t h e f o r m o f t h e spectrum o f xenon adsorbed on such
sample w i l l depend on t h e Xe-(MxtiG) i n t e r a c t i o n , on t h e nutiibers i, j... o f atoins o r molecules o f G chemisorbed on t h e v a r i o u s p a r t i c l e s , and on t h e d i s t r i b u t i o n o f MxtiG, IVlxtjG
... p a r t i c l e s w i t h i n
a z e o l i t e c r y s t a l l i t e . We g i v e
below some examples o f c h e m i s o r p t i o n on P t - Y samples.
B256
F i g . 4.33. S p e c t r a O f xenon adsorbed on P t - H-NaY a t d i f f e r e n t nos misorbed hydrogen molecules O < nH < n 2 p 4.7.4.1
n o f cheH2
Chemisorption o f hydrogen
The spectrum of xenon adsorbed a t 299 K on a Ptx-NaY sample c o n s i s t s o f a s i n g l e l i n e (denoted 3 ) whose chemical s h i f t 6,
i s always much g r e a t e r t h a n
whatever t h e xenon p r e s s u r e ( 1 2 0 ) ( F i g . 4.32 and 4.33). t o t h e coalescence of t h e l i n e . o f h i g h chemical s h i f t fipt,
T h i s s i g n a l i s due due t o xenon adsor-
bed on t h e p l a t i n u m p a r t i c l e s , and t h a t o f s h i f t dNay c o r r e s p o n d i n g t o Xe atoms c o l l i d i n g w i t h t h e w a l l s o f t h e supercages D r w i t h o t h e r Xe atoms. F o r t h e sake
g i s c h a r a c t e r i s t i c o f Xe-Pt c o l l i s i o n s , A f t e r c h e m i s o r p t i o n o f a v e r y s m a l l amount o f hydrogen ( t h e number, nH , o f
o f s i m p l i c i t y we s h a l l say t h a t l i n e
H2 molecules b e i n g much s m a l l e r t h a n t h e number o f m e t a l p a r t i c l e s d e t e r m i n i d by e l e c t r o n microscope, nE,,), i s oetwee;i t h a t of 6,
and
a second s i g n a l ( d e n o t e d
b)
i s detected; i t s s h i f t Ab
and i t corresponds t o Xe atoms adsorbed on p a r -
t i c l e s which have chemisorbed hydrogen. The e x i s t e n c e o f t h e s e two s i g n a l s p r o v e t h a t t h e d i s t r i b u t i o n o f H atoms on t h e p a r t i c l e s d i v i d e s t h e z e o l i t e c r y s t a l l i t e i n t o two zones.At ambient t e m p e r a t u r e hydrogen i s chemisorbed by t h e f i r s t p a r t i c l e s encountered when i t p e n e t r a t e s t h e z e o l i t e c r y s t a l 1 i t e s , t h u s d e f i n i n g t d o zones: a c e n t r a l one c o r r e s p o n d i n g t o b a r e p a r t i c l e s and a p e r i p h e r a l one c o n t a i n i n g p a r t i c l e s which have chemisorbed hydrogen ( F i g . 4.33). Furthermore, when nH i n c r e a s e s w h i l e r e m a i n i n g v e r y s m a l l , l i c e ses a t t h e expense o f
a,
b
increa-
g u t w i t h o u t any change i n t h e i r Ni-iR c n a r a c t e r i s t i c s
B257
F i g . 4.34. Spectrum of xenon adsorbed on P t x - H-IiaY a f t e r h e a t t r e a t m e n t a t ( l ) , 299; ( 2 ) , 363; ( 3 ) , 393; ( 4 ) , 448 K. (such as chemical s h i f t and l i n e w i d t h ) , showing t h a t each p a r t i c l e
b
bears t h e
same amount of hydrogen. We have demonstrated (119, 123) t h a t t h e amount chemisorbed i s 2H atonis p e r p a r t i c l e . L i n e 2 d i s a p p e a r s when
b
i s maximal, which
occurs when t h e t o t a l number o f cheinisorbed H2 molecules, n
i s equal t o t h e H2’ r e a l number of p a r t i c l e s , n , which one can d e t e r m i n e e x a c t l y f r o m nH One P 2 determined by e l e c t r o n microscopy, f i n d s t h a t n i s always much g r e a t e r t h a n nEEM
.
P
c o n f i r m i n g t h u s t h a t , except v e r y r a r e l y , t h i s l a t t e r t e c h n i q u e i s u n a b l e t o a s i g n a l c appears P c o r r e s p o n d i n g t o p a r t i c l e s which have
d e t e c t v e r y small p a r t i c l e s . When nH becomes g r e a t e r t h a n n ( 6 a / 6 b ) , g e n e r a l l y p o o r l y r e s o l v e d f?om b, more t h a n 2H on t h e i r s u r f a c e ( F i g . 4.32).
T h i s t e c h n i q u e can a l s o be used t o f o l l o w t h e d i f f u s i o n o f hydrogen chemisorbed on p l a t i n u m towards t h e i n s i d e o f t h e z e o l i t e c r y s t a l l i t e ( 1 1 9 ) . Consider, 19 f o r example, a P t 7 . 6 - H-NaY sample c o n t a i n i n g n = 6.0 0.2 x 10 molecu1es.g-l P ( F i g . 4.34). The s u b s c r i p t 7.4 denotes t h e average number o f P t atoms p e r metal p a r t i c l e . The r a t i o o f t h e i n t e n s i t i e s 1, and Ib o f lines
5 and
i s naturally
t h a t of t h e number of P t 7 . 4 and P t 7 . 4 t 2H p a r t i c l e s . A f t e r h e a t i n g t h i s sample
B258
F i g . 4.35. Cheliiical s h i f t o f xenon s i g n a l s 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 hydrogen cheniisorbed a t Various temperatures T, ( K ) : ( A ) , 299; ( x ) , 343; (o), 383; (o), 448. i n a s e a l e d t u b e t o i n c r e a s i n g t e m p e r a t u r e one observes a p a r a l l e l d i m i n u t i o n o f t h e l i n e g and i n c r e a s e i n t h e s t r e n g t h and t h e s h i f t o f l i n e zone
b
b,
which show t h a t
i n c r e a s e s a t t h e expense o f zone 4 as a r e s u l t o f hydrogen d i f f u s i o n and
t h a t i t c o n s i s t s now o f b a r e p a r t i c l e s and p a r t i c l e s w i t h 2H. The e x a c t composit i o n can be determined f r o m t h e p o s i t i o n o f l i n e
b.
The s i n g l e l i n e o b t a i n e d
a f t e r h e a t i n g a t 448 K i s t h e b a r y c e n t e r o f t h e i n i t i a l l i n e s a t 299 K; i t s pos i t i o n i s e x a c t l y d e f i n e d by t h e number o f p a r t i c l e s e i t h e r b a r e o r w i t h 2H and d i s t r i b u t e d randomly i n t h e z e o l i t e c r y s t a l l i t e s . One can a l s o f o l l o w t h e v a r i a t o n o f t h e xenon spectrum w i t h t h e chemisorpt i o n t e m p e r a t u r e Tc>,299 K. I t i s seen a g a i n t h a t t h e two s i g n a l s
.
a
and
b
co-
e x i s t f o r T c < 448 K and t h a t A b depends on Tc and nH B u t whatever Tc one 2 always f i n d s t h a t t h e s l o p e o f 6b a g a i n s t nH changes a b r u p t l y f o r nH2 = np 2 (119) ( F i g . 4.35). B o u d a r t e t a l . have c o n f i r m e d t h e s e r e s u l t s b u t t h e i r i n t e r p r e t a t i o n i n t h e c a l c u l a t i o n o f t h e number of m e t a l l i c p a r t i c l e s i s s l i g h t l y d i f f e r e n t from t h e p r e v i o u s one (121, 122). 4.7.4.2
Cheiiiisorption o f o t h e r gases G
The d i s t r i b u t i o n o f t h e f i r s t m o l e c u l e s o f t h e gas G chemisorbed on t h e p l a t i n u m p a r t i c l e s depends above a l l on t h e n a t u r e o f t h i s gas. F o r example, a t 299 K, oxygen behaves v e r y s i m i l a r l y t o hydrogen: one O2 m o l e c u l e on t h e f i r s t p a r t i c l e s encountered. B u t t h e s e same p a r t i c l e s i m m e d i a t e l y chemisorb much more oxygen i f c h e m i s o r p t i o n t a k e s p l a c e a t 443 K ( 1 1 9 ) . On t h e o t h e r hand, a t 299 K carbon monoxide h a l f - s a t u r a t e s t h e f i r s t P t p a r t i c l e s encountered ( a p p a r e n t s t o i c h i o m e t r y 1C0/2Pt (123) , t h e n d i f f u s e s towards t h e i n t e r i o r o f t h e z e o l i t e c r y s t a l l i t e s when t h e t e m p e r a t u r e i s r a i s e d .
B259
F i g . 4.36. (1) Spectrum o f t h e s o l i d b e f o r e any c h e m i s o r p t i o n - l i n e a i s due e s s e n t i a l l y t o b a r e p l a t i n u m p a r t i c l e s ; ( 2 ) a f t e r c h e m i s o r p t i o n G f oxygen; no. molecules, - 0.85 npt; l i n e s a and b correspond t o b a r e p a r t i c l e s and t o = particles w i t 6 0 molecules, r e s p e c t i v e l y ; ( 3 ) a f t e r c h e m i s o r p t i o n of 2npt niolecules, on?y l i n e b can be detected; ( 4 ) carbon monoxide i s t h e n i t t r o duced (r~,-~ = 6 n p t ) and t h e CO formed e l i m i n a t e d (b = 3.6 n p t ) . The spectrum t h e n c o n s i s t s o f l i n e b f o r p i r t i c l e s w i t h 2 0 2 ( t h e r z f o r e n o t a f f e c t e d by t h e CO) and l i n e C corresponding t o p a r t i c l e s with 1 CO; ( 5 ) a f t e r a d s o r p t i o n of a l a r g e amount o f CO and e l i m i n a t i o n of C02, t h e spectrum c o n t a i n s o n l y l i n e d, corresponding t o a l l t h e p a r t i c l e s b e i n g s a t u r a t e d w i t h CO; ( 6 ) one can come back t o t h e s t a t e o f spectrum ( 4 ) by r e a d s o r p t i o n o f oxygen ( n o = 0.83 npt) and e l i m i n a t i o n o f CO ; ( 7 ) a f t e r s a t u r a t i o n w i t h 0 2 and e l i m i n a t i o i of Co2, spectrum ( 7 ) i d e n t i c a y w i t h spectrum ( 3 ) i s o b t a i n e d ; (8) a f t e r c h e m i s o r p t i o n of H2 (nH = 1.25 n p t ) and e l i m i n a t i o n of w a t e r molecules, t h e sample c o n t a i n s b a r e p a r ? i c l e s ( l i n e a ) and p a r t i c l e s w i t h 2 O2 ( l i n e b ) ; ( 9 ) a f t e r c h e m i s o r p t i o n of about t w i c e as much hydrogen as t h e r e remains oxygen and e l i m i n a t i o n of t h e w a t e r formed, one o b t a i n s p r a c t i c a l l y t h e i n i t i a l spectrum 1 ( t h e i n t e n s i t y of t h e r e s i d u a l l i n e b i s l o w ) ; (10) f i n a l l y , t h e i n t r o d u c t i o n o f a f u r t h e r s m a l l amount o f H2 and e l i m i n a t i o n of t h e r e s i d u a l w a t e r l e a v e s two u n r e s o l v e d l i n e s . One, a, corresponds t o a zone o f t h e c r y s t a l l i t e s ( o r o f t h e sample a t l e a s t ) c o n t a i n i n g o n l y b a r e p a r t i c l e s ; t h e o t h e r , e, t o a zone c o n t a i n i n g a m i x t u r e Of b a r e p a r t i c l e s and p a r t i c l e s w i t h 2H.
2
Thus by means o f t h i s t e c h n i q u e i t i s p o s s i b l e i n a l l cases t o d e t e r m i n e q u a n t i t a t i v e l y t h e d i s t r i b u t i o n of gases chemisorbed on metal p a r t i c l e s and t h e
B260
d i s t r i b u t i o n w i t h i n t h e Y c r y s t a l l i t e s o f P t p a r t i c l e s d i s t i n g u i s h e d by t h e amount chemisorbed. There i s a f u r t h e r p o i n t o f i n t e r e s t concerning t h e chemisorption o f CO a t 299 K (118). Under t h e experimental c o n d i t i o n s employed t h e xenon technique can o n l y d e t e c t changes occuring i n s i d e t h e supercages. I t i s t h e r e f o r e i n s e n s i t i v e t o t h e chemisorption o f CO on t h e P t p a r t i c l e s l o c a t e d on t h e e x t e r n a l s u r f a c e o f t h e Y c r y s t a l l i t e s . I f v e r y t h i n l a y e r s o f t h e s o l i d a r e used t h e xenon w i l l o n l y d e t e c t t h e f i r s t CO molecules chemisorbed on t h e i n t e r n a l metal p a r t i c l e s when t h e o u t e r p a r t i c l e s a r e s a t u r a t e d . Whence t h e p o s s i b i l i t y of d e t e r m i n i n g t h e number o f P t atoms l o c a t e d on these l a t t e r .
4.7.4.3.
Successive chemisorption o f several gases
T h i s technique can a l s o be used t o determine t h e d i s t r i b u t i o n of several gases chemisorbed on zeol i t e - s u p p o r t e d metal p a r t i c l e s (118, 119).
As an example l e t us consider a Pt6-NaY sample, where t h e npt p l a t i n u m part i c l e s c o n t a i n 6 atoms on average. T h i s sample i s heated under vacuum a t 573 K and k e p t a t t h i s temperature d u r i n g t h e v a r i o u s successive chemical a d s o r p t i o n s o f r e a c t i v e gases. However, a f t e r each a d s o r p t i o n t h e sample i s brought back t o 299 K, a t which temperature t h e xenon probe i s p h y s i c a l l y adsorbed (PXe = 500 t o r r ) and detected by NMR. F i g u r e 4.36 d i s p l a y s t h e s p e c t r a corresponding t o t h e d i f f e r e n t chemisorptions, as e x p l a i n e d i n t h e c a p t i o n s . On t h e b a s i s of t h e above f i n d i n g s one may conclude t h a t when supported m e t a l l i c p a r t i c l e s a r e v e r y small and i n v i s i b l e t o e l e c t r o n microscopy t h e 129Xe-NivlR enables us t o count them and t h e r e f o r e t o c a l c u l a t e t h e average number o f atoms they c o n t a i n . I n a d d i t i o n , xenon a d s o r p t i o n makes i t p o s s i b l e t o d e t e r mine q u a n t i t a t i v e l y t h e d i s t r i b u t i o n o f a phase chemisorbed on metal1 i c p a r t i c l e s : number o f bare p a r t i c l e s , s a t u r a t e d p a r t i c l e s , homogeneous d i s t r i b u t i o n , CO-adsorption, e t c . APPENDIX: SYMBOL NOi.IENCLATURE
a
Bo D
-. ~
ge
9,N H
= Scalar h y p e r f i n e i n t e r a c t i o n c o n s t a n t = External s t a t i c magnetic f i e l d = Second
-
rank d i p o l a r c o u p l i n g t e n s o r
= electronic g factor = nuclear g f a c t o r = Hamil t o n i e n
h
I
= n u c l e a r s p i n angular momentum ( i n u n i t fi = -)
5
= spin
K
= Knigh s h i f t
Q
=
2ll
~~
-
s p i n ( o r J) c o u p l i n g t e n s o r
Quadrupole tiioment
B261 = Quadrupole i n t e r a c t i o n t e n s o r -
f -
= second
-
rank tensor
T'
= pseudo
-
contact coupling tensor
T1
=
spin
-
l a t t i c e r e l a x a t i o n time; l o n g i t u d i n a l r e l a x a t i o n t i m e
T2 -
= spin
-
s p i n r e l a x a t i o n time; t r a n s v e r s e r e l a x a t i o n t i m e
V
= e l e c t r i c f i e l d gradient tensor
Be RN
=
ye yN
= e l e c t r o n magnetogyric r a t i o (expressed i n radian.sec-'.gauss-l) II = n u c l e a r magnetogyric r a t i o ( II II
6
= chemical s h i f t
eh Bohr magneton = - w h e r e m i s t h e e l e c t r o n mass 2mc eh = Nuclear magneton = 2 ~ where c M i s t h e p r o t o n mass
1
s h i e l d i n g a n i s o t r o p y parameter
6
=
q
=
e
= a n g l e between t h e s t a t i c f i e l d Bo ( l a b o r a t o r y a x i s system) and t h e ( d i p o
"
11
chemical s h i f t s h i e l d i n g asymmetry parameter l a r , chemical s h i f t o r q u a d r u p o l a r ) p r i n c i p a l a x i s system
e,A,
= magic a n g l e
uL = Larmor frequency vQ = quadrupolar c o u p l i n g c o n s t a n t
-
-
u
=
u.
= i s o t r o p i c chemical s h i f t
+1 so
second
r a n k chemical s h i f t t e n s o r
pN = n u c l e a r magnetic moment = gNBNt
+
pe
4.8
=
yNfi
-+
I
e l e c t r o n i c magnetic moment = -geoe3 = y e t
+
s
REFERENCES
A. Abragam, "The P r i n c i p l e s o f N u c l e a r i.lagnetism", London, O x f o r d U n i v e r s i t y Press, 1961. 2. A. C a r r i n g t o n and A.D. Mc Lachlan, " I n t r o d u c t i o n t o Magnetic Resonance", Harper and Row, New York, 1967. 3. C.P. S l i c h t e r , " P r i n c i p l e s o f Magnetic Resonance", B e r l i n Heidelberg New York, Springer-Verlag, 1978. 4. C.A. F i f e , " S o l i d S t a t e NMR f o r Chemists", Guelph, 1984. D.E. O ' R e i l l y , Adv. Catal., 12 (1960) 31. 5. 6. H. W i n k l e r , B u l l . Ampere, 1 0 t h Year, S p e c i a l E d i t i o n , (1961) p. 219. 7. K.J. Packer, Progr. Nucl. Magn. Reson. Spectrosc., 3 (1967) 87. 8. ( a ) H.A. Resing, Advan. Mol. R e l a x a t i o n Processes, 1 (1968) 109; ( b ) H.A. Resins, Advan. Mol. R e l a x a t i o n Processes, 3 (1972) 199. 9. E.G. oerouane, J. F r a i s s a r d , J . J . F r i p i a t and W.E;E. Stone, C a t a l . Rev., S c i Eng., 7 (1972) 121. 10. ( a ) H. P f e i f e r , NMR: B a s i c P r i n c i p l e s and Progress, V o l . 7, S p r i n g e r , NewYork, 1972, p. 53; ( b ) H . P f e i f e r , Phys. Rept., 26C (1976) 293. 11. J.H. Lunsford, CRC C r i t i c a l Rev. S o l i d S t a t e Sci., 6 (1976) 337. 12. J.J. F r i p i a t , J. Physique C o l l o q . C4, 38 (1977) 44. 13. W.N. Delgass, G.L. H a l l e r , R. Kellerman and J.H. L u n s f o r d , SpeCtrOSCOpy i n Heterogeneous C a t a l y s i s , Academic Press, New York. 1979, Chap. 7 14. J . Tabony, P r o g r . N u c l . Magn. Reson. Spectrosc., 14 (1980) 1. 15. T.M. Duncan and C. Dybowski, Surf. S c i . Rep. 1,N"4, H o r t h - H o l l a n d P u b l i s h i n g Co., November 1981.
1.
-
.
-
B262
16.
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Chapter 5
ELECTRON PARAMAGNETIC RESONANCE
M. Che’ and E. Giamello2 1. Laboratoire de Rkactivitk de Surface et Structure, UA 1106, CNRS. UniversitC P. et M. Curie, 4, Place Jussieu, 75252 Paris, France 2. Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universith di Torino, Via P. Giuria 9, Torino, Italy
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5.1 INTRODUCTION
Electron paramagnetic resonance (EPR) has been applied to surface chemistry and catalysis for about thirty years to study a variety of paramagnetic species (i.e. with one or several unpaired electrons) including typically: - adsorbed atoms, molecules and/or ions which are in some cases intermediates of catalytic reactions, - intrinsic surface defects or defects formed by irradiation, grinding etc., - transition metal ions supported on an oxide surface or included in the solid, - spin labels interacting with a surface. Several review articles (1-7)have been devoted to the applications of EPR to surface problems since the early review by O’Reilly in 1960 (1). Particularly important to the surface chemist, the high sensitivity of the technique permits the study of low concentrations of active sites. The fact that diamagnetic species are not observed is both a limitation and an advantage of the technique, since, although a limited type of species can be observed, many highly reactive paramagnetic intermediates can be studied without any spectroscopic interference. From EPR spectra, previously unknown oxidation states of metal ions have been detected (81, while in other studies intermediates suspected from previous work have been directly observed and identified (9, 10). In the first part of the present review, the basic principles of EPR are given with particular emphasis on the interpretation of the spectra of polycrystalline materials, which are those most usually encountered in surface chemistry and catalysis work. In the second part, the characterisation of catalytic surfaces by adsorption of probe molecules will be considered without, however, attempting an exhaustive literature review of the field. Instead, attention will be devoted to the approaches used for studying important properties such as surface crystal field, surface redox properties, active site identification, surface groups morphology, mobility of adsorbed species and coordination of surface metal ions.
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5.2 THE EPR TECHNIQUE
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56.1 The electron paramagneticresonanceprinciple
A free electron has a spin angular momentum (or simply spin) S which, in a given direction, can only assume two values. The direction usually specified is the z direction, so that the z-component of the spin Sz can have exclusively the two values MS = Y2 and MS= -1/2 in units .li=h/2n . An electron carries a magnetic moment ps which is colinear and antiparallel to the spin itself and given by the expression:
where ge is the free electron g value (g,= 2.0023) and pg is the Bohr magneton pg = eh/4rrmc with e and m being the electron charge and mass respectively and c the light velocity. The value of the Bohr magneton is p~ = 9.27 10-21 erg gauss-1. The interaction energy of the electron magnetic moment with an external applied magnetic field is classically given by :
where B is the magnetic flux density, i.e., the effect of a magnetic field strength in the matter. B is measured in Tesla (T) or i n Gauss (G,1T = 104 GI. In quantum mechanics, the p vector is replaced by the corresponding operator leading t o the following Hamiltonian, i.e., the energy operator:
Assuming B lies in the z direction, the interaction energy corresponds to :
which is the simplest example of a spin-Hamiltonian. The energies corresponding to the two allowed orientations of the spin are therefore:
These two energy levels are often referred to as Zeeman levels. The lower energy level corresponds to Ms=-U2, the situation where B and p are parallel. The latter two are antiparallel when Ms=+Y2 which corresponds t o the upper energy level. The energy difference between these two levels is:
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At thermal equilibrium, under the influence of the external applied magnetic field, the spin population is split between the two levels according to the MaxwellBoltzmann law: L5.71
nlIn2 = e - A E k T
where k is the Boltzmann's constant, T the absolute temperature and n l and n2 are the spin populations characterised by the Ms values of +1/2 and -112 respectively. At 77 K, in a field of about 3000 gauss, n l and n2 differ by less than 0.005
.
Fig. 5.1-The Zeeman energy levels of a free electron in an external applied magnetic field. The transition between the two Zeeman levels can be induced by irradiating the paramagnetic system with a suitable electromagnetic radiation providing its frequency v fulfills the resonance condition:
From eqn. 181, it is easily deduced that the frequency required for the transition to occur is about 2.8 MHz per gauss of applied field. This means that, for magnetic fields usually employed in the laboratory, the radiation required belongs to the microwave region. The energetic scheme of the Zeeman levels and of the corresponding transition is reported in Fig. 5.1 as well as the absorption line and its first derivative. The energy absorption necessary to promote electrons from the lower to the upper energy level represents the resonance signal. By this absorption process, the populations of the two energy levels n l and n2 tend to equalize. The odd electrons
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from the upper level give up the hv quantum to return to the lower level and satisfy the equilibrium Maxwell-Boltzmann law. This energy may be dissipated within the lattice as phonons, i.e., vibrational, rotational and translational energy. The mechanism by which this dissipation occurs is known a s the "spin-lattice'' relaxation. It is characterized by an exponential decay of energy as a function of time. The exponential time constant is denoted T1e or spin-lattice relaxation time. The initial equilibrium may also be reached by a different process. There could be an energy exchange between the spins without transfer of energy to the lattice. This phenomenon, known as the "spin-spin relaxation", is characterised by a time constant T2e called the spin-spin relaxation time. When both spin-spin and spinlattice relaxations contribute to the EPR line, the resonance line width can be written as :
In general, TI, > T2e and the line width depends mainly on spin-spin interactions: T2e increases on decreasing the spin concentration, i.e., the spin-spin distances in the system. On the other hand, when TI, becomes very short, below roughly 10-7 sec, its effect on the lifetime of a species in a given energy level makes a n important contribution to the linewidth. In some cases, the EPR 1'ines are broadened beyond detection. T1e is inversely proportional to the absolute temperature ( T1e = T-n) with n depending on the precise relaxation mechanism. In such a case, cooling down the sample increases T1e and usually leads t o detectable EPR lines. Thus, quite often, EPR experiments are performed a t the boiling point of liquid nitrogen (77 K)or liquid helium (4.2 K). On the other hand, if the spin-lattice relaxation time T1e is too long, electrons do not have time to return to the initial state. The populations of the two levels ( n l and n2) tend therefore to equalize and the intensity of the signal decreases, being no longer proportional to the number of spins present in the sample. This effect, known as "saturation", can be avoided by exposing the sample to low microwave power. The typical shape of EPR lines is Gaussian or Lorentzian. The analytical expressions of the two functions are:
[5.10]
Fig. 5.2 gives the typical features of the two types of lines, in terms of normalized absorption (2a) and first derivative curves (2b).
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ai
b)
Fig. 5.2 - Lorentzian and Gaussian line shapes. a) absorption, b) first derivative.
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5.2.2 The basic instrumentationof EF'R specbrascopy
A modern EPR spectrometer is designed to measure with high sensitivity the microwave absorption in a sample as a function of the external applied magnetic field : the actual EPR experiment consists of scanning the magnetic field a t constant microwave frequency until the resonance condition [8]is fulfilled. Then a significant amount of energy is absorbed by the sample. MlcrowavQ SourCQ
Circulator
bktector Amplit ior
Isolator AttQnUatOr Osci I Ioscope
'=-+
k-i L
Cavity
Modulation
Fig. 5.3 - Schematic view of an EPR spectrometer.
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The opposite approach, i.e., scanning the microwave frequency a t constant magnetic field is not practical because the common microwave sources emit in a limited range of frequencies. The basic components of the spectrometer (Fig. 5.3)are : a) a microwave source (usually a klystron but recently gun diodes have been introduced) supplying an electromagnetic radiation at constant frequency whose power is controlled by a n attenuator between the klystron and the sample. The most employed frequencies are those corresponding to X and Q bands. The experimental features of these and other available bands are reported in Table 5.1. b) a microwave guide system to propagate the microwave radiation from the source to the resonant cavity. c) a cavity, made from a highly conductive metal and having reflecting walls to accumulate power on the sample by multiple reflexions of the microwave radiation. The internal dimensions of the cavity are similar to the wavelength of the microwaves. A t resonance, standing waves (modes) of various configurations are formed.
TABLE 5.1. Typical conditions for EPR experiments at various frequencies. Band
S X K
Q
v/GHz 3 9.5 25 35
Resonant Field T 0.11 0.35 0.89 1.25
h / m 9.3 2.9 1.1 0.8
d) a powerful electromagnet capable of providing a homogeneous field, within the range 0 - 2 Tesla approximately, which is controlled by a field probe. The range and rate of scanning are adjusted to provide the most suitable conditions for observation of the microwave absorption. el a detector diode to measure the energy absorbed by the sample a t resonance. f) a convenient amplifier system, a recorder and a n oscilloscope. Superimposed on the main magnetic field is a n oscillating field obtained by applying a n alternating current (typically 100 kHz) to a set of coils in the cavity walls. This modulating field converts the resonance to a n alternating signal which can be separated from random noise using a phase sensitive detection system. It is this method of detection that gives a very high sensitivity and makes the output appear in the form of a first derivative curve of absorption as a function of magnetic
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field (Fig. 5.4). The experimentalist selects the amplitude of the modulating field which should be large enough to obtain a good signal to noise ratio but small enough to obtain a good first derivative curve, i.e., to prevent the distortions which occur if the amplitude value becomes similar to the resonance linewidth. The samples employed in surface chemistry and catalysis studies are usually polycrystalline solids. Solid samples are usually placed in quartz tubes which may be connected to gas handling or vacuum lines so as to activate samples or adsorb the desired reagent a t the desired temperature and pressure. The sample tubes are then placed in the cavity at the centre of the electromagnet gap. The EPR tubes (5mm.in diameter for the most common X band measurements) are filled with the investigated powder, up to an average height of 10 - 20 mm. Spectra are often obtained a t 77 K using a special dewar fitting into the cavity. The temperature of the sample can, however, be easily altered within the range 90 to 500 K using a flow of cooled dry nitrogen: this is particularly useful when structural evolution of the paramagnetic species with temperature has to be followed.
t
.-b LA
Fig. 5.4 - Modulation of the magnetic field and corresponding trend of the crystal detector current. The digital aquisition of the spectra, done by means of on-line computers, is now becoming very common: this allows to improve the signal to noise ratio both by spectra accumulation and spectral curve smoothing. Furthermore, mathematical treatments of the data are also possible for spin counting based on the double
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integration of the signal and to obtain second and third derivatives of the spectra which are very useful to detect overlapping signals.
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5.2.3 The basic interactionsof the unpaired alectFonwith its environment and the featuresofEPR spectra
5.2.3.1 - The g $,ensor
When the unpaired electron belongs to a "real"chemica1 system, the g value is, in general, different from ge, i.e., when the orbital angular momentum is different from zero. In this case, the spin is no longer exactly quantized along the direction of the external magnetic field and, as shown below, the g value cannot be expressed by a scalar quantity but becomes a tensor. The angular momentum L is associated with a magnetic momentum given by
PL = - PBL
[5.111
Let us consider a system with a doublet (S = 1/21 non degenerate electronic ground state. For the sake of simplicity, the system contains exclusively nuclei with zero nuclear magnetic moment (pn = 0): the effect of p n + 0 nuclei, which contribute to the spin-Hamiltonian and complicate the spectrum, will be described in the following section. For such a system, the interaction with the external magnetic field can be expressed, in terms of a perturbation of the general Hamiltonian, by the following three terms:
The first and the second term correspond respectively to the electron Zeeman and orbital Zeeman energies. The third one represents the energy of the spin-orbit coupling and h is the spin-orbit coupling constant which mixes the ground state wave function with the excited states. Through the effect of the spin orbit coupling, the electron can acquire some orbital angular momentum. Standard h. values for various atoms have been obtained from atomic spectra. The extent of the interaction between L and S mainly depends on the nature of the system considered: in fact the orbital angular momentum of the unpaired electron can be aligned along a specific molecular axis of the paramagnetic species by interaction with the molecular electric field. In many instances, this interaction is stronger than that between the magnetic field and the orbital angular momentum. Depending on the strength of the molecular electric field, two limiting cases can be distinguished:
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a) strong fields: L must align itself along the field so that only S can orient itself with respect to the external magnetic field and contribute to the paramagnetism. In this case it results g=ge. A number of organic radicals experience this situation. Yet, g values close to 2 may be also observed, whenever the odd electron is in a molecular orbital delocalized over a large molecule, approaching the situation of a n almost free electron. b) weak fields: L is no longer under the constraint of this weak field and the spin-orbit coupling L + S can take place giving a resultant total angular momentum : [5.13]
J=L+S
associated with the magnetic moment L5.141
M=-JZJCLB J gJ is called the Land6 g factor and is given by:
a=
J(J+1) + S(S+1)- L(L+I) 2J(J+1)
+
[5.15]
This situation occurs in the case of rare earth elements. When intermediate fields are present in the paramagnetic centre, L is only partially blocked by the molecular field (transition metal ions, inorganic radicals) and the vectorial coupling model no longer holds. The system must be treated in terms of the perturbation Hamiltonian in [121 which can be synthetically written as:
H = ~BgS
[5.161
which is the new spin-Hamiltonian analogous t o that reported in [3]. The ge (scalar) value in [3] is now replaced by g , a second rank tensor (or a symmetric 3x3 matrix) representing the anisotropy of the interaction between the unpaired electron and the external magnetic field and outlining also the fact that the orbital contribution to the electronic magnetic moment may be different along different molecular axes. In other words, the magnetic moment of the odd electron in a real paramagnetic system is not exactly antiparallel to the spin and its magnitude is not that of a free electron but depends on the orientation of the system in the applied magnetic field. This concept can be summarized by the relationship
B214
which is the analogous of [l].
X
Y
Fig. 5.5 - Orientation of the magnetic field Bo in the crystallographic frame x,y,z. X,Y,Z are the laboratory axes. 8 and @ are the characteristic angles defining the orientation of Bo. The g tensor may be depicted as an ellipsoid whose characteristic (principal) values (gxx , gyu , g,,) depend upon the orientation of the symmetry axes of the paramagnetic entity with respect to the applied magnetic field (Fig. 5). The most general consequence of the anisotropy of g, from an experimental point of view, is therefore that the resonance field of a paramagnetic species, for a given frequency, depends on the orientation of the paramagnetic centre in the field itself. The g value for a given orientation depends on 8 and 0 values (Fig. 5.5) according to the following relation: 2
g2 = ( gxxcos2$ sin2 e + g,
2
2
sin24 cos2e+ g,,cos2e
)
[5.18]
Accordingly, the Zeeman resonance will occur at field values given by:
B,,, =
hv 2 ( %tX cos2@sin2 8 + gyysin%$ cos% + g,,cos% 2
2
)-I/*
15.191
PB
In the most general case, the resonance observed for a paramagnetic centre in a single crystal is obtained at distinct field values Bx,By,or B, when the magnetic field is parallel to the x, y or z crystal axis respectively: the g values corresponding
B275
to these three orientations (gxx,gyy, gzz ) are the principal (diagonal) elements of the g tensor. Absolute determinations of the g values may, in principle, be carried out by independent and simultaneous measurements of B and v using a gaussmeter and a frequency meter respectively, following the equation: hv g=PBB
L5.201
In practice, the g value is often determined by comparing the field values a t resonance for the sample investigated and that of a reference sample. As one can write:
provided v be invariant through the whole experiment, g is given by: [5.22] The usual reference samples are diphenyl-picryl-hydrazyl (DPPH, g = 2.0036), Varian Pitch (g = 2.0029) and Cr3+ in MgO matrix (g = 1.9797). These reference samples give rise to narrow lines necessary for accurate determinations. The reference sample could be placed in one of the two compartments of a dual cavity or stuck to the investigated sample quartz tube in the case of a single cavity. 5.2.3.2 - The electron sDin - n&r
.e -. min
interact ion)
Several nuclei possess spin and corresponding magnetic moments. The nuclear spin quantum number ( I ) of a given nucleus can assume integral or halfintegral values in the range 0 - 6. The magnetic moment pn associated to a nucleus is collinear with the spin vector I according to the relation:
similar to [l]. gn is the nuclear g factor and Pn the nuclear magneton which is smaller than the Bohr magneton by a factor 1838, i.e., the ratio of the proton to electron mass. When the paramagnetic centre contains one o r more nuclei with non zero nuclear spin (I#O), the interaction between the unpaired electron and the nucleus with I#O gives origin to further splitting of the Zeeman energies and, consequently,
B276
to new transitions responsible of the so-called hyperfine structure of the EPR spectrum. Two types of electron spin-nuclear spin interactions must be considered, of isotropic and anisotropic nature respectively. The former one is a quantum interaction related to the finite probability of the unpaired electron t o be at the nucleus and is termed the F e d contact interaction. The corresponding constant, called the hyperfine isotropic coupling constant ajeo,is given by:
where gn and Pn are the nuclear analogues of g, and PB respectively and IY(0) 12 is the square of the absolute value of the wavefunction of the unpaired electron evaluated at the nucleus. -+
1/2
= - 1/2
3CY W
z
hE- h%
W
‘ \
MI=- 112
Fig. 5.6 - Energy scheme of the levels produced by the interaction of an unpaired electron with a I = 1/2nucleus,
The isotropic interaction concerns exclusively a-type orbitals or orbitals with partial s character (like hybrid orbitals constructed from s-type orbitals) because these orbitals only have finite probability density at the nucleus. The spherical symmetry of s-orbitals accounts for the isotropic nature of the contact interaction. A typical isotropic hyperfine interaction is the one observed for the hydrogen atom. The electron spin is interacting with the proton (I = 1/21 spin. This latter may assume two possible orientations with MI = f 1/2. Thus the nuclear magnetic moment further splits each Zeeman level into two sub-levels. The EPR selection rule (AMs = 2 1/2 , AM1 = 0) allows only two transitions and therefore the EPR spectrum of the hydrogen atom is composed of two resonance lines separated by approximately 508 Gauss as shown in Fig. 5.6, where, in the lower part, a schematic view of the resulting spectrum is also reported. More generally, in the case of n equivalent nuclei (i.e., equally interacting with the unpaired electron) having spin I, the EPR spectrum consists of 2nI + 1 lines which form the hyperfine structure. Conversely, the knowledge of the number and separation of a hyperfine structure leads to the number and the nature of the interacting nuclei. The spacing between two consecutive lines is called the hyperfine constant and equals the coupling constant ajso to a first approximation. More complicated spectra are observed when two, o r more, sets of inequivalent nuclei are present in the paramagnetic species, as well as in the case of the presence of nuclei with I > l/2. Anisotropic electron-nuclear (hyperfine) couplings are due to the dipolar interaction between the nuclear and the electron magnetic moments when the unpaired electron is in non spherically symmetric orbitals (p, d, f orbitals). The coupling arises from the classic dipolar interaction between magnetic moments whose energy is given by: [5.251
where r is the vector relating the electron and nuclear moments and r is the distance between the two spins. The quantum mechanics analogue of 1251 is obtained by replacing ps and pn by their expressions given in [ll and [231: [5.26]
Eq. [26] must be averaged over the entire probability of the spin distribution. Ha* is averaged out to zero when the electron cloud is spherical (s orbital) and comes to a finite value in the case of axially symmetric orbitals (p orbitals, for instance). Also in the case of very rapid tumbling of the paramagnetic species (as it occurs in a low
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viscosity solution) the anisotropic term of the hyperfine interaction is averaged to zero and the isotropic term is the only one observed. In general, both isotropic and anisotropic hyperfine couplings occur when one or more nuclei with I f: 0 are present in the system. The whole interaction is therefore dependent on orientation and must be expressed by a tensor. The effective spin-Hamiltonian for a S = 1/2 system containingj nuclei with I z 0 thus becomes:
where A is the second rank hyperfine tensor. The third term represents the nuclear Zeeman interaction which is included for the sake of completeness, because i t influences the energy of the spin levels but, due to EPR selection rules, does not influence the energy of the transitions. The A tensor may be split into an isotropic and an anisotropic part a s follows:
[5.28]
with aiso = (Al+A2+A3) 1 3 . In a number of cases, the second term matrix of 1281 is a traceless tensor (Ti+T2+T3= 0) and has the form ( -T, -T, +2T). The anisotropic part of the A tensor corresponds to the dipolar interaction as expressed by the hamiltonian in [241. The s and p characters of the orbital hosting the unpaired electron are given by the following relations:
[5.291
where A0 and Bo are theoretical hyperfine coupling constants assuming pure s and p orbitals for the element under consideration. m 5.2.3.3 - BuDerhwerfine s The superhyperfine and the hyperfine interactions have common physical grounds as both are related to the coupling between the unpaired electron and a nucleus with I # 0. The term superhyperfine is usually employed when this nucleus does not belong to the species containing the unpaired electron. The most relevant cases are encountered in coordination chemistry (unpaired electron of a metal ion, interacting with a I z 0 nucleus of the ligand) and in surface chemistry
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(unpaired electron of an adsorbed species interacting with a nucleus at the surface or vice versa). An example of the latter case will be given in Section 5.3.2. A spinHamiltonian similar to that reported in [27]and containing the hyperfine term can be used to describe the cases of superhyperfine interaction.
5.2.3.4- The case of S > 112 (fine stucture) If two or more unpaired electrons are present in the system, so that the total spin is larger than l/2,a new term must be added to the spin-Hamiltonian in [27] to account for the interaction between unpaired electrons:
%=SDS
l5.301
which may be written:
[5.31] D is a dipolar traceless tensor. D and E are the zero-field splitting terms indicating the energy splitting of the spin states occurring in absence of magnetic field. The spin coupling is direct in the case of organic molecules in triplet state and occurs through the orbital angular momentum in the case of transition metal ions. In the latter case, the D and E terms depend on the symmetry of the crystal field acting on the ions. It is therefore evident that the EPR of such ions, when located onto a surface, is very difficult to study because the inhomogeneities and strong distortions of the crystal field give rise to very broad signals of difficult interpretation. For these reasons, the investigations of surface transition metal ions mainly concern cases with spin S=U2 (Cu2+,Mo5+, V4+,Ni+, etc...1.
-
62.4 The features of model powder EPR spectra
The samples usually investigated by EPR in surface chemistry and catalysis are polycrystalline materials, composed of numerous small crystallites randomly oriented in space. The resultant powder EPR spectrum is the envelope of spectra corresponding to all possible orientations of the paramagnetic species with respect to the magnetic field: provided the resolution is adequate, the magnitude of the g and A tensor components can be extracted from powder spectra whereas no information can be obtained on the orientation of the tensor principal axes. The profile of a powder spectrum is determined by several parameters among which the symmetry of the g tensor, the actual values of its components, the line shape
B280
and the line width of the resonance. As far a s the symmetry of the g tensor is considered, three cases are possible which are now discussed. 5.2.4.1 - IsotroDv Qfg. In this case, the g tensor is characterised by g, = gyy = g,, = giso and a single symmetrical line is observed. This simple case is not very often encountered in powders excepted for some solid state defects and transition metal ions in highly symmetric environment. In several inorganic radicals, the unpaired electron is in oriented atomic or molecular orbitals; additionally, when the symmetry of the g tensor is directly influenced by that of the crystal field (for example in the case of ions of the first transition series), the occurrence of structural deformations or vibrational distortions may reduce the symmetry of the crystal field giving rise to anisotropic g tensors. Nevertheless, isotropic g tensors have been observed in the case of solid state defects and transition metal ions in highly symmetric environment. An apparent isotropy may be observed in low-viscosity solutions where, whatever the symmetry of the g tensor, a radical is subjected to very rapid tumbling and reorientation. The observed g value becomes the average of the three main components:
15.321 Mobility effects may involve rotation about a particular axis. In this case, the two components perpendicular to the rotation axis are averaged: for rotation , say, about the z axis, two values are observed, i.e., g,, = gll and gavl= (gxX+gyy)/2. 5.2.4.2 - Axial svmmetrv ofe. Paramagnetic species isolated in single crystals exhibit resonances a t typical magnetic fields depending on their orientation and given by the equation [191. In the particular case of axial symmetry of the system, if z is the principal symmetry axis of the species and 8 the angle between z and the magnetic field, the x and y directions are equivalent and the angle I$ becomes thus meaningless. Relation [19] reduces to:
where 811 = g,, and g l = gyy = g, are the g values measured when the axis of the paramagnetic species is respectively parallel and perpendicular t o the applied magnetic field. The powder spectrum is the envelope of the individual lines
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corresponding to all possible orientations in the whole range of 8. Assuming that the microcrystals are randomly distributed, simple considerations show, however, that the absorption intensity, which is proportional to the number of microcrystals a t resonance for a given 8 value, is maximum when 8 = d 2 (BJ)and minimum for 8 = 0 (BII): this allows the extraction of the gll and g l values which correspond to the turning points of the spectrum (6).
Fig. 5.7 - Powder EPR spectra of a paramagnetic centre in axial symmetry. a) absorption , b) first derivative. Fig. 5.7 gives a schematic representation of the absorption curve and of its first derivative, for a polycrystalline sample containing a paramagnetic centre in axial symmetry: the solid lines have been calculated assuming a zero width for each individual line, whereas the dotted ones correspond to a finite individual linewidth. The actual width of the lines strongly influence the spectral profile and the resolution. The latter becomes very poor for increasing line widths. 5.2.4.3 - Orthorhombic svmmetrv of g Three distinct principal components a r e expected in this case. For polycrystalline samples, the absorption curve and its first derivative exhibit three singular points corresponding to gl, g2, g3 (Fig. 5.8). For powder spectra, the assignment of gl, g2 and g3 to the components g,,, gyu and g,, related to the molecular or crystal axes of the paramagnetic centre is not straightforward and must be based on theoretical grounds or deduced from measurements of the same paramagnetic species in single crystals.
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The situation becomes much more complicated if we now consider the effect of a hyperfine splitting due to the presence of neighbours with I z 0. The typical equation giving the angular dependence of the hyperfine term is very complex and its discussion is beyond the scope of the present paper.
B
Fig. 5.8 - Powder EPR spectra of a paramagnetic centre in orthorhombic symmetry. Symbols as in Fig. 5.7. Fortunately, in several cases a first interpretation of the experimental spectra can be achieved using the simple analysis described for the g tensor and neglecting second order effects. The A tensor has in fact the same type of angular dependence as the g tensor and provided the principal axes are the same, as in most cases, then each of the three possible lines of the g tensor (gl, g2, g3) will be split into a number of lines depending on the nuclear spin (21+1 components) with the spacing corresponding t o the appropriate component of the A tensor ( Al, A2, A31 : gl is split by A1, g2 by A2 and g3 by &. The number of distinct cases deriving from the different possibilities of combining the anisotropies of both g and A is very high and cannot be easily summarized. Some particular cases are thus given in the next two figures which have been obtained calculating the spectral profile for simple examples. The spectra in Fig. 6.9 as well as those in Fig. 5.10 have been obtained by means of the simulation program SIM14A (11). In Fig. 5.9, the effect of the nuclear spin on a spectrum totally anisotropic both in g and A tensors is reported. The first spectrum G O , no hyperfine structure) exhibits well separated gl, g2 and g3 features. The g features are split in the successive spectra when the hyperfine interaction is considered and values for I (from 112 to 3/2) and A l , A2 and & (the latter is
B283
constant in the series) are introduced in the computation. For the sake of simplicity, the abundance for the I& nucleus has been considered to be 100%.
A =A =O 1 2
A3 I
I
I
Fig. 5.9 - Effect of the nuclear spin on the calculated EPR spectra of an orthorhombic species. I varies from zero to 312. When I assumes an integer value the features of the original 1=0 spectrum are maintained at the centre of the spectrum whereas when I is an half integer the hyperfine lines dispose symmetrically about the centre. The complexity of the spectra and the number of lines increase with increasing I values.
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I = 112
rnult = 2
I = 312
Fig. 5.10 - Calculated EPR spectra for species with a n orthorhombic g tensor, I=l and various sets of hyperfine constants. In Fig. 5.10, the effect of different sets of A values on the shape of calculated spectra having an orthorhombic g tensor, with gl> g2 > g3 and I = 1, is reported to illustrate the dramatic changes of the spectral profile occurring with the variations of the hyperfine splittings. It must be noticed that in some instance the hyperfine lines related to different elements of the A tensor lie very close one to the other causing possible ambiguities in the assignment.
B285
-
5.25 The real EPR powder spectrum: a pragmatic approach to resolution Even in the case of model spectra such as those shown in Figs. 5.9 and 5.10,the assignment based on the simple analysis of the line separations can be somewhat difficult. Moreover, the experimental spectra are usually complicated by various effects such as: i) the presence of species having different (or slightly different) parameters; ii) the presence of various nuclei with different nuclear spins; iii) the broadening of the lines due to dipolar spin-spin interactions or, in some cases, to motional phenomena causing loss of resolution; iv) second order effects and/or nuclear quadrupolar effects (particularly occurring in the case of transition metal ions) influencing the regular spacing of the hyperfine lines. As far as point i) is concerned, it has to be recalled that it is rather unlikely that different species behave in the same way when the microwave power or the temperature are varied. This difference in EelzaPicr is very often used to separate and identify the various species in a composite spectrum. For all the above mentioned reasons, the evaluation of the spin-Hamiltonian parameters in the case of powder spectra is often performed adopting additional techniques that may help in decision.
5.2.5.1- Multifreauencv a mroach. The most relevant case is that of the coupling between X-band and Q-band (Table 5.1). The principle of the method is based on the fact that the separation AB between the lines due to the g tensor anisotropy varies linearly with the microwave frequency according to the equation: l5.341
easily derived from 183. This does not apply for the hyperfine separation that, arising from the interaction between the nuclear spin and the unpaired electron, is unaffected by microwave frequency changes a s shown by equations [24]and [26] The distinction between g and A features is usually easy since AB will change by a factor of about four on going from X (9.5 GHz) to Q band (35 GHz), i.e., the ratio of the frequencies, while the A tensor features will not be affected (Fig. 5.11).
5.2.5.2 - M D i c labellin? Two different cases are considered in which the isotopic labelling technique is employed for different goals. In the first one, atoms with nuclear spin 1=0 are substituted in the paramagnetic species by other atoms having k 0 : in this case,
B286
obviously, the scope of the isotopic labelling technique is not that of helping in the assignment of the spin-Hamiltonian parameters of the starting signal, as more complicated spectra are obtained. The goal instead is to obtain an hyperfine structure and gain information about the orbital hosting the unpaired electron.
Fig. 5.11 - Experimental (solid line) and simulated (dashed line) EPR spectra of [Co(CH3CN)6]2+ in the X band (a) and Q band (b).The isotropic line at the centre of the parallel structure is due to another species (ref. 12). It can be mentioned, for instance, the case of oxygen radical species produced with mixtures enriched with the 170 isotope (1=5/2) instead of the natural oxygen mainly containing the 160 one (I=O) (13).The structure obtained by the labelling
B287
technique is much more complicated but the information about the paramagnetic species much more exhaustive..
Fig. 5.12 - EPR spectra of the CNO32- radical on MgO obtained with four different isotopic mixtures : top a) 12CO/14NO ,b) 12CO/15NO bottom a) 13C0/14NO, b)13C0/15NO, (ref. 14).
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In the second case, the isotopic labelling is aimed at elucidating the structure of complex signals by comparing spectra of species having different isotopic composition. An example of this technique is reported in Fig. 5.12 whith spectra of a surface radical containing nitrogen, carbon and oxygen atoms. The radical is formed by coadsorption of carbon monoxide and nitric oxide onto MgO and exhibits a very complex spectrum due to the anisotropy of both g and A tensors (14).
-+
1 s t derivative
qDpba 25G
1/ 2
I
- 1/2
Fig. 5.13 First-, second- and third-derivative spectra of DPPH (top of the Figure); Comparison between (a) third-derivative and (b) first-derivative spectra of 13CO adsorbed onto V4+ ions at the surface of V2OdSiO2 (ref.16).
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By employing the four possible mixtures of 14NO (14N. I=l), 15NO (15N, 1=1/2), 12CO (12C, I=O) and 13CO (13C, 1=1/2), the four spectra in Fig. 5.12 have been obtained. It becomes therefore possible to differentiate the features due to the g tensor from those due to the A tensor. Additionally, it has been learned that the radical species contains one carbon and one nitrogen atom thus allowing the assignment of the spectra to the CNO32- radical anion (14,151. 5.2.5.3 - Third derivative soectr a In the case of overlapping signals, a useful approach for spectra resolution consists in recording, beside the conventional first-derivative spectrum, the thirdderivative one. Fig 5.13 (a) gives the different shapes of the first-, second-, and thirdderivative EPR spectra of a solid sample of DPPH (diphenyl pycrylhydrazyl, usually employed as reference standard in EPR).
Fig. 5.14 - Experimental and computer simulated EPR spectra of cupric ions in a CuO/ZnO catalyst. The third derivative has the same shape as the first derivative apart from the two outermost features, making the interpretation very simple. The major
B290
advantage of the method, however, is that it allows the enhancement of resolution of the first derivative spectrum. This is documented in Fig.5.13 (b) where the first- and third- derivative spectra of 13CO adsorbed onto V4+ ions at the surface of silica supported V2O5 are compared (16): in the third-derivative spectrum the superhyperfine lines, due to the interaction of the unpaired electron on V4+ with the 13CO nucleus , are clearly distinguished whereas in the first derivative spectrum they are unresolved. The acquisition of a third-derivative spectrum can be done both by suitable electronic devices, supplying further modulations to the magnetic field or by computer elaboration of digitalized experimental spectra.
Fig. 5.15 - Decomposition of the simulated spectrum in Fig.5.14 into its components. All components are reported with a normalized height: the relative populations of the three species are indicated.
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5.2.5.4- QDectra simulatioq The set of spin-Hamiltonian parameters obtained from an experimental EPR spectrum may be confirmed, to avoid ambiguity, by computer simulation of the spectrum on the basis of a given set of data including g and A values, lineshape and linewidth. The best spin-Hamiltonian parameters are those obtained when the fit between experimental and simulated spectra is found to be satisfactory. A n example of simulation of a EPR powder spectrum is given in Fig. 5.14. It concerns the case of Cu2+ ions present in a CuO/ZnO polycrystalline catalyst with 3% copper loading (17). Cu2+ ions possess a d9 electron configuration (S=1/2) and are usually found in distorted octahedral coordinations. In a pure octahedral environment, the EPR spectrum of Cu2+ would not be observable a t room temperature due to the degeneracy of the eg orbitals: however either tetragonal deformations (leading, in the extreme case, to the square-planar configuration) or strong coupling with lattice vibrations (Jahn-Teller theorem) usually remove the degeneracy and lower the energy of the ground state to lead to EPR spectra, with axial g tensor, observable even a t room temperature. The axial powder spectrum is, however, complicated by the nuclear hyperfine interaction with 63 Cu and 65 Cu nuclei (both with I=3/2), whose total abundance is about 100%.The parallel and perpendicular features are split into four lines by the hyperfine interaction with separation All and A1 respectively, with All > A l . The perpendicular hyperfine structure of Cu2+ powder spectra is often unresolved. Typical model spectra for Cu2+ are reported in Fig. 5.15 (second and third spectrum) which, however, are less complex than the experimental spectrum in Fig. 5.14. The fit between the two spectra in Fig. 5.14 has been achieved considering the simultaneous presence in the solid of three distinct paramagnetic species. Two are isolated Cu2+ ions exhibiting an axial spectrum with g l >gll with two sets of hyperfine lines due to the I = 312 copper nuclei. The third species is constituted by "clustered" copper ions in magnetic interaction giving rise t o a broad and structureless signal. The simulation, therefore, leads to a set of rather accurate spin-Hamiltonian parameters (17) for each species contributing t o the whole spectrum and, furthermore, supplies additional information on their populations. This is shown in Fig. 5.15 where the spectrum of Fig. 5.14 is deconvoluted into its components with the corresponding relative abundances. The broad signal due to interacting cupric ions contributes to about 40%of the total detected spins.
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-
5.3 CHARACTEFUSATIONOF CATALYTIC SURFACES BY MEANS OF PROBE
MOLECULESANDEPR
-
5.3.1 Definition of a probe molecule &om the standpoint of EPR
As mentioned in the introduction, the EPR technique has been successfully employed to investigate various aspects of surface chemistry and catalysis, and allowed to obtain valuable information on the nature and properties of the bulk and surface centres of the catalyst and on intermediates formed during the catalytic processes. Nevertheless, a non negligible fraction of the applications of the technique to surface problems requires the adsorption of probe molecules. In the field of EPR spectroscopy,one can define a probe molecule as a molecule whose properties in the adsorbed state can be monitored by EPR and bring useful information about the surface. The properties monitored by EPR concern either the probe molecule itself or the surface site and the changes of its properties upon adsorption. As a consequence, the probe molecules employed in EPR are not necessarily paramagnetic, the only condition required being the paramagnetism of the species resulting from the interaction of the probe with the solid surface. 5.3.1.1 - Classification of the Drobe molecula
A first rough classification of the different probes can be done on the basis of two criteria: their magnetic properties and the nature of their interaction with the solid system. The first criterion leads to two types of probe molecules: a) the molecules which are paramagnetic (or become paramagnetic upon adsorption). Among this type of molecules it is worth mentioning the paramagnetic probes of the surface crystal field such as NO which is adsorbed as such onto Lewis acid sites and 0 2 which, upon transfer of an electron from the surface, is adsorbed in the form of the paramagnetic 02-sensitive to the surface electrostatic field (section 5.3.2). Paramagnetic probes such as VCl4 o r MoCl5 retain their paramagnetism during reaction with surface hydroxyl groups allowing to investigate the distribution of the surface groups themselves (section 5.3.5). Another class of paramagnetic probes is that of the so called "spin-probes", usually nitroxides but in some cases transition metal ions, that are employed to investigate the molecular motion in specific locations near the surface (pores, cavities) (section 5.3.5). Finally, some probe molecules can be diamagnetic. Because of the peculiar values of their ionisation potential o r electron affinity, these probes can easily function as one-electron donor or one-electron acceptor respectively, thus becoming paramagnetic. The intensity of their EPR signal is a measure of the one electron redox properties of the catalytic surface (section 5.3.3).
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b) the molecules which are diamagnetic and remain diamagnetic upon adsorption. Such probes allow to investigate the properties of a paramagnetic adsorption site. The best example is the study of the coordination sphere of surface transition metal ions using the method of coordination vacancy filling by probe molecules such as CO, H20, CH30H, NH3 and other N-containing bases. The EPR parameters investigated by means of the diamagnetic probes are mainly the g tensor variations upon coordination and the superhyperfine structures for ligands containing nuclei with I # 0. This approach has allowed, in a variety of cases, the identification of the nature and the precise counting of the ligands coordinated to a surface paramagnetic centre under various atmospheres (section 5.3.7). The second criterion to classify the probe molecules is based on the type and strength of their interaction with the surface. One can start with the weak interactions undergone, say, by a nitroxide spin probe whose motion is influenced, for instance in the pores of a system, by the ions a t the solid surface. Another example is the physisorption of a paramagnetic gas (usually oxygen) onto a solid containing paramagnetic centres, in order t o ascertain whether the latter are located a t the surface or in the bulk (section 5.3.1). The stronger "chemical" interactions can range from weak Lewis interactions as in the case of NO adsorbed onto metal ions such as Al3+ or Ti4+ to more complex and stronger metal ligand interactions, involving surface transition metal ions and ligand molecules such as CO, NO and H2O. Finally, there are examples of chemical reactions involving electron transfer between the solid and the adsorbed probe (oxygen, redox probes) or the anchoring of the probe molecule to the surface as in the case of the reaction between paramagnetic transition metal halides and surface hydroxyl groups with water elimination. 5.3.1.2 - Location of the Daramamet'IC centra When the applications of EPR to surface studies are considered, it must be kept in mind that this technique concerns the whole sample and that one of the major questions is to distinguish a surface from a bulk species. The best criterion is to observe the effect of the adsorbed gases. If the signal of the species is destroyed or changes while previously stable in vacuo a t the same temperature, then it is likely that the signal corresponds to a species within a few angstroms from the surface. If no reaction occurs, the signal may be broadened by addition of a paramagnetic gas. A dipolar o r exchange broadening by 02 is commonly used because of its reversibility and pressure dependence. It arises from the interaction of the two unpaired electrons of the oxygen molecule with that of the surface species. Some other criteria to distinguish a surface from a bulk centre are also given in the following table.
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TABLE 5.2. Experimental approach to distinguish between bulk and surface centres. EXPERIMENT Adsorption of a reactive gas
Surface centre Fast reaction
Bulk centre No or very slow
reaction Adsorption of a paramagnetic gas
Signal broadens
No effect
reversibly Variation of the specific surface (S.S.) area
Signal intensity proportional
No effect
to the S.S. area Different from those g and A values
of the compondiig bulk centre Lower than that
g and A symmetry
of the corresponding
bulk centre
-
5.33 surface crystal fields The crystal field created by ions on a solid surface can be measured, in some cases, by means of a suitable paramagnetic adsorbed species mainly on the basis of the values of the g tensor elements. Assuming the spin-Hamiltonian defined in [251 as a perturbation of the total Hamiltonian, the perturbation theory allows to derive the following relation: [5.351
where Li is the angular orbital momentum operator in the i direction (i = x, y, z), Yo and Yn are the space wave functions for the ground and excited states respectively and Eo and En the corresponding energies. Equation [351 leads to expressions for the g tensor diagonal elements which take the general form: [5.36]
where AE is the energy difference between two orbital states and k is a constant; ge and h have been defined in section 5.2.1.On the basis of the g tensor components, however, the paramagnetic species fall into two categories: those which are crystal field insensitive and those which are crystal field sensitive. In the first class of
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radicals, the ground state is non degenerate and the energy levels are well separated and insensitive to the influence of local crystal fields.
Fig. 5.16 - The Walsh diagram for A B 2 molecules with 19 and 21 electrons (ref. 18). In that case, the g tensor is a fingerprint of the radical. Several radicals, mainly of the AB2 type, belong to this class. Most AB2 radicals in fact are nonlinear because of the stabilization of the high energy nu orbitals occurring when the molecule is bent (18);this in turns accounts for well separated energy levels (Fig. 5.16). Table 5.3 reports the g tensor values for some 17 and 19 electron radicals of the AB2 type in different environments: the nature of the host matrix only slightly influences the g values. In the second class radicals, the ground state is usually degenerate so that the surface crystal field can remove the degeneracy, splitting the two levels proportionally to the intensity of the crystal field itself. The g values are therefore drastically modified (eq. [35] and [36] ). Nitric oxide NO and the superoxide radical ion 02-are the most representative cases of this class of radicals. In view of equation [35] the following observations may be made: a) the shift of the g tensor components (Agii = gji - gel depends upon the orientation of the paramagnetic species axes with respect to the magnetic field. Indeed, the Li components have different abilities to mix ground and excited states and, therefore, generate distinct Agij shifts whose magnitude essentially depends on the ratio of the spin-orbit coupling constant h to the excitation energy (En- Eo). b) the sign of En - Eo changes depending on whether the excited state Yn results from the promotion of the odd electron from a filled to a half-filled orbital (in
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this case (En - Eo)< 0 and Ag > 0 as in the case of 0 2 - ) or from a half-filled to a n empty orbital ((En - Eo)> 0 and Ag < 0 as in the case of the NO molecule). The schematic energy level diagram for 0 2 - and NO, illustrating the energy splittings induced by the surface crystal field, is shown in Fig. 5.17. When the magnetic field is parallel to the internuclear axis (taken as the z axis), the excited states are those corresponding to the electron jump from the 2plr,* and the 2pn,* orbitals for both NO and 0 2 - . However, while for 0 2 - a positive g shift is expected, the opposite is predicted for NO. These expectations are in agreement with the observed g values for both radicals adsorbed on MgO (Table 5.4).
TABLE 5.3. g tensors for some bulk and surface inorganic radicals. number of electrons
radical and matrix
gl
17
C02- on MgO C02- in CaC03 SO2- on MgO SO2-in K2S2O5 0 3 -on MgO 0 3 - in KC103 NO$ on MgO N022-in KCl
2.0029 2.0032 2.0097 2.0103 2.0172 2.0174 2.0068 2.0099
19 19 19
NO
s2
I
2.0017 2.0016 2.0052 2.0028 2.0055 2.0018 2.0100 2.0025 2.0068
Oi
Fig. 5.17 - The molecular orbital energy diagram for the NO molecule and the superoxide ion in the surface crystal field (A is the energy splitting)
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TABLE 5.4. g values for NO and 0 2 - radicals adsorbed onto MgO Radical NO 02-
gl = g,, 1.89 2.077
g2 = g, 1.996 2.007
g3 =gxx Reference 25 1.996 n 2.001
. ion . as a s u r f a c e l d 5.3.2.1- The suDeroxide 02-deal
Drobe
Among adsorbed gases, oxygen has been one of the most extensively studied since it leads easily to the superoxide ion 02-whose energy levels depend on the surface crystal field (10).The EPR signals have only been observed in the case of 0 2 - adsorbed on non paramagnetic ions since otherwise there will be a strong interaction between the unpaired electrons leading to line broadening. The usually accepted approach is to adopt an ionic model for the superoxide ion on the surface. In this model, an electron is transferred from the surface to the oxygen to form 0 2 - and there is an electrostatic interaction between the cation at the adsorption site and the superoxide ion (Fig. 5.18 ). The process can be written as follows:
02-usually exibits three g values with gz,>gyy>gxx.The g components derived by Kanzig and Cohen for a superoxide ion trapped in an anionic vacancy of a rocksalt cubic lattice (281,neglecting second order terms and assuming h < A << E , are as follows: gxx = ge gyy = ge + 23JE gn = ge + 2UA
r5.371
The energy splittings A and E have been defined in Fig. 5.17. A typical spectrum of 0 2 - adsorbed on a surface is reported in Fig. 5.19: the adsorption sites are Ti4+ ions anchored to a glass support (29).It is clear that the magnitude of the g,, component, which depends on the energy splitting A, can be used to determine the nature of the adsorption site since it gives a measure of the cationic charge. A, in fact, is the smallest energy splitting present in [37]and gives rise for g,, to the largest deviation from ge. The calculated spectra for various 0 2 ' species characterised by different g,, values are reported in Fig. 5.20.
B298
Y
I
Fig. 5.18 - Model for 02-adsorption on a cationic surface site: z is the internuclear axis, the unpaired electron resides in the antibonding molecular orbital built from the two shaded atomic orbitals (x axis) (ref. 10).
Fig. 5.19 - EPR spectrum of 0; on Ti4+ anchored on porous Vycor glass. The g components are evidenced in the figure (ref. 29).
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The figure simulates the spectral profile for 02-adsorbed on cations with different charges which thus produce different electric fields. A plot of the g,, value as a function of the cationic charge at the adsorption site, for a series of cases collected in the literature (10) shows a fairly good correlation (Fig. 5.21).
Fig. 5.20 - Calculated spectra of four superoxide species adsorbed on different cations. The cationic charge increases with increasing g,,, The linewidth and the g, and gyyvalues have been kept constant for all the spectra. The above correlation must, however, be used with care as, in some cases, a wide range of g,, values are observed when only one oxidation state of the cation is present, for example on Ti02 and MgO (30,311 : in these systems, the various g,, values represent adsorption sites where the effective crystal field is changed either as a result of different crystal planes or of different local cation coordination. In particular, evidence is now available (32)that, in the case of MgO, the multiplicity of the g,, lines (up to four) arises from the presence of various Mg2+ sites having only two possible first coordination spheres (with coordination number 3 and 4 respectively) but different successive coordination spheres. In Fig. 5.22 (b) a computer simulation of the EPR spectrum of 02-on MgO is reported: the different
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peaks in the g,, region are labelled (A-D) and the corresponding g values indicated in the figure. Fig. 5.22 (a) reports the schematic view of the MgO surface with the four adsorption sites (A-D) corresponding to the g,, components. The differences of the coordination sphere of the various sites are sufficient to markedly change the crystal field as shown by the large variations of the Madelung constant of these ions also reported in Fig. 5.22 (a). A linear correlation is observed between the g,, shift from the free electron value (Ag,, = g,, - gel and the calculated Madelung constants at the adsorption sites (32). The g tensor can give only limited information on the nature of the oxygen species. However, further details can be obtained from the hyperfine tensor.
217. 216. 215214213212211210
209208207206 ~
205 -
0
0
@ I 8
206 203 202. 2.01
.
zooo
1
2
3
4
8
8
5
6
Fig. 5.21 - The variation of g,, of 02-as a function of the charge of the adsorption cationic site (ref. 10). The use of oxygen enriched in 170 ( 1 4 2 , natural abundance 0.04%) has been particularly valuable in their characterisation (10,131. The method was first used by Tench and Holroyd (33) to unambiguously identify the superoxide ion adsorbed on MgO. From the number of hyperfine lines, it was concluded that the radical has indeed two equivalent oxygen nuclei and is adsorbed with its internuclear axis parallel to the surface. From the values of the hyperfine constants ( A,,=77 G, Ayy=O G , Az,=15 G ) , it was calculated that the spin density, equal on both nuclei,
€3301
amounts to more than 0.9 showing that the electron is almost entirely delocalized on the radical and little on the surface.
I.07IU
0 .A 8738 I . 5669
AJ
t.l.079q
D
I.085Y
D 1.5912
Fig. 5.22 - (a) Schematic view of the MgO surface with the adsorption sites and the corresponding Madelung constants. (b) Computer simulation of the EPR spectrum of 02-on MgO (ref. 32). Alternatively, there are also cases where the 170 hyperfine tensor indicates that the two oxygen nuclei do not bear the same spin density as in the case of 0 2 on Mo6+ a t the MoOdSiOz surface (Fig. 5.23). There is indication that the source of the non equivalency of the two oxygen nuclei is indeed due to steric hindrance a t the surface caused by the presence of surface molybdenyl ions with a short M=O bond (34). There are now many cases where the non equivalency of the oxygen has been observed (10,351. Finally, one can obtain information on the site where the species is adsorbed by means of the superhyperfine tensor (i.e., the tensor related to the electron-nucleus interaction when the electron and the interacting nucleus belong to different parts of the chemical system, for instance metal-ligand o r surface-adsorbed molecule). Again in the case of molybdenum, using 95Mo (I=5/2) enriched Mo/SiO:! catalysts , it has been possible t o show that 02-ions were adsorbed a t Mo6+ sites, thus confirming the conclusion drawn from the analysis of , the g values (Fig. 5.24).
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Fig. 5.23 - The EPR spectrum of superoxide ions on MoO$SiOz showing the hyperfine interaction with two inequivalent nuclei (ref. 10).
91
I I
Ill I I
91
I I Ill I I
Fig. 5.24 - The superhyperhe interaction between 02-and Mo6+ as shown by the spectrum of superohde on 95Mo/SiO2 (ref. 36).
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The most favourable cases for the production of the superhyperfine structure correspond to nuclei with I # 0 in sufficiently high natural abundance. Among these Al (I=5/2, loo%),V (1=7/2,loo%), Co (I=7/2, 100%)and Cu (I=3/2, 100%) have been the most studied. 5.3.2.2 - JYO as a surface crvstal field Nitric oxide has been also employed, though to a lesser extent than 0 2 , to study the influence of surface crystal fields. The molecule is easily coordinated t o various cationic surface centres via the nitrogen end. As discussed above, the expressions for the g tensor principal values are similar to those of 0 2 - but with the opposite sign for Ag. A typical example is the spectrum of NO adsorbed on Ti02 (37) as reported in Fig. 5.25. q , =2.003
SOG
A z 0.3 8 0
Fig. 5.25 - The EPR spectrum of NO adsorbed on Ti02 (ref. 37). The spectrum of adsorbed NO has been studied for several systems such as MgO (38), ZnO (39), ZnS (39) and a variety of exchanged natural and synthetic zeolites (40). As in the case of the adsorbed superoxide 02-ion, the splittings A of the 2pxX*and 2pn,* levels by the surface crystal field have been determined from the values of g,, (z is the internuclear axis). Table 5.5 reports some selected values of A for adsorbed NO. Table 5.5 shows that the A values are not ordered according to the oxidation state of the cation. It must be kept in mind, however, that the electric field exerted by a cationic centre depends on the charge of the cation itself but also on its ionic radius and coordination number. Similar apparent inconsistencies have recently been found for a series of samples whose surface
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crystal field were monitored studying the adsorption of CO by microcalorimetry and IR spectroscopy (43). TABLE 5.5. Selected A splitting values for NO adsorbed onto different solids. System NaY zeolite MgO ZnO Ti02
NeV 0.21 0.30 0.33 0.39 0.50 ZnS HYzeol.(A13+ 0.60 0.60 SnO2 A1203
~~
Leference 42 30 39 37 39 42 37
0.75
4
In conclusion, both the superoxide 0 2 - radical ion and the NO radical molecule can be used as probes of surface crystal field. The former is employed when electron transfer from the surface to the dioxygen molecule easily occurs, due to the presence of reducing centres such as surface defects or transition metal ions in suitable oxidation states. The latter is a convenient probe of the surface crystal field from Lewis acid centres which can not lead t o electron transfer. The chemical bond between NO and surface positive ions is, however, more complex than a simple polarization of the adsorbed molecule as it certainly involves the unpaired electron. A good example is provided by the case of NO adsorbed on cuprous ions (Cu+, 3dlo4so) in Y zeolites. A hyperfine structure due to the copper nucleus (I = 3/21 is observed (44,451 indicating a remarkable degree of delocalization of the unpaired electron on the adsorption centre. The nitric oxide, moreover, also reacts with various surface transition metal ions to produce the nitrosyl (NO+) complex whose properties will be discussed in section 5.3.7.
-
53.3 Redox properties of the 8urfBce Metal oxides are often involved i n catalytic systems, either as such or as supports. In many cases, their surface possesses redox sites whose nature, number and strength are often required to be known in order to understand their catalytic behaviour. This has been made possible using charge transfer reactions involving solid-liquid or solid-gas systems. Charge transfer complexes a re generally produced between the solid surface (S) and electron acceptor (A) or donor
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(D)organic molecules usually dissolved in benzene.The reactions can be written as: S + A --f S + + A S + D + S- + D + The ability of the surface to form radical ions depends on the ionization potential or on the electron affinity (IP and EA respectively) of the organic molecule. The threshold of IP or EA a t which the charge transfer starts t o be observed characterizes the strength of the sites whose number is given by the intensity of the paramagnetic signal of A- or D+. In this way the distribution of sites as a function of their strength can be determined. Typical molecules with low IP are perylene, anthracene, naphtalene (a-bases) while high EA compounds which are commonly used are tetracyanoethylene, dinitrobenzene, trinitrobenzene etc.
Fig. 5.26 - EPR spectra of the perylene cation radical generated: (a) in H2SO4 , (b) on 13% A1203 silica-alumina (ref. 46). Electron transfer studies have been conducted on a wide range of acid or basic catalysts. Lunsford (3) and Flockhart (46) have reviewed the subject while Loktev and Slinkin (47) published a survey in the particular case of zeolites. The
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mechanism of formation of A- and D+ is still not very clear but the implications in catalysis are important: the correlation between redox sites and catalytic activities has been the subject of active research.
5.3.3.1 - Qzi&&y DroDerties of oxide s u r b Low IP aromatic molecules such as anthracene and perylene, when adsorbed on certain silica alumina catalysts form paramagnetic species which are generally attributed to the appropriate cation radical with splitting constants similar to those found in solution (Fig. 5.26) (47). From the observed linewidth, it can be concluded that the cation radical is relatively free to move on the surface. The nature of the surface oxidizing site is not clear and various, even contrasting, hypotheses, have been put forward also on the mechanism (48-51). Similar studies have been performed in the case of zeolites. In freshly prepared NHdY, for instance, the NH4+ ions are associated with the AlO4tetrahedra (52,531. As the zeolite is heated, NH4+ ions decompose evolving NH3 and releasing protons which subsequently form acidic hydroxyl groups by interaction with oxide ions. Further heat treatments result in water elimination, as evidenced by the drastic weight loss, by condensation of a proton with a neighbouring hydroxyl group. When removed, the hydroxyl group leaves a lattice defect o r tricoordinated aluminium ion.
.
I
SI
+ H20
The close correspondence (Fig. 5.27) between the weight loss conferring to the Y zeolite its Lewis acid character and the occurrence of the maximum of the electron acceptor ability, as measured from the concentration of the positive radicals created upon contact with the perylene molecule, confirmed both the existence and the nature of the electron acceptor sites created by the dehydroxylation of a NH4Y zeolite. It appears that half of the (A1043 units are converted from a four into a three-coordinated state. The three-coordinated A1 ions present in the defect structure are supposed to have a high electron affinity. Dollish and Hall (54) have suggested that both oxygen and tricoordinated aluminium are responsible for the generation of radical cations by decationated zeolites.
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The oxidizing activity of zeolites has also been investigated in the presence of multivalent cations. Ben Taarit et a1 (56) have shown that Ce3+ ions exchanged in Y zeolites were unable of ionizing adsorbed anthracene while upon oxidation t o Ce4+ ions the radicals were easily formed.
4
- 2.10’8
I-
I
2
2‘
- 1.1018 200
300
400
500
Fig. 5.27 - Correlation diagram between weight loss and amount of positive perylene radicals generated on NH4Y zeolites treated a t various temperatures (ref. 55). In a study of the interaction between antracene and a copper exchanged Y zeolite (571, the role of the transition metal ion introduced into the zeolite framework as electron trap was clearly evidenced. When anthracene was adsorbed into Cu2+Y zeolite, a signal appeared at g = 2.002 and increased steadily at the expenses of the Cu2+ signal, which eventually disappeared (57). These results also indicate that all the Cu2+ ions are accessible to the anthracene molecule. 5.3.3.2 - Beducinp Dr-ies of oxlde s u r f a w Anion radicals can be formed when hydrocarbons with large electron affinity (>lev) such as tetracyanoethylene (TCNE) and various nitrocompounds are adsorbed on an oxide surface. Radical anions have been observed in particular on partially dehydrated aluminas, silica-aluminas, titanium oxide and magnesium oxide. Naccache and coworkers (58,591 and Pink and coworkers (60) have reported the spectra of TCNE and of various nitroaromatic compounds adsorbed on the alumina surface. The adsorption of TCNE at room temperature on activated alumina produced a reddish colour and gave a 9-line hyperfine spectrum (Fig. 5.28) due to the interaction of the unpaired electron with four equivalent nitrogen
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nuclei. This spectrum can be attributed to the anion radical of TCNE. Trinitrobenzene (TNB) gave a three line spectrum attributed to the TNB- anion radical (Fig. 5.29).The EPR spectrum of the nitro-compound can be analyzed in terms of an anion with restricted rotational freedom in which the unpaired electron has strong anisotropic interaction with one nitrogen atom (I=l).
.f
-
10 Gauss H
Fig. 5.28- EPR spectrum of the TCNE- radical anion adsorbed on alumina (ref. 46). The degree of rotational freedom is reduced as the number of nitrogroups in the molecule is increased (61)as shown for the TNB- in Fig. 5.29.The source of the electrons is still not completely clear: some hypotheses on the role of low coordination 02-and O H were however put forward (61,62).
Fig. 5.29 - EPR spectrum of a TNB- radical adsorbed on activated titanium dioxide (ref. 61)
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Nitrobenzene (NB) and related compounds were also used to study the electron donor properties of ionic oxides such a s MgO. Che et al., for instance, used nitrocompounds and TCNE to study the electron donor properties of MgO as a function of activation temperature and found two maxima in the concentration of surface negative radicals at about 220 and 7OOOC (61). The first was attributed to OH- groups acting as donors and the second to low coordination 0 2 - ions. Cordischi et al. (63a) reported similar results but attributed the electron donor centre a t low temperature of activation to 02---OH- pairs with the oxide ion in low coordination . These ideas have been taken h r t h e r by Coluccia et al. (63b) who found a much higher concentration of NB- radicals on MgO smoke that had been etched by water vapour than on normal smoke. The latter solid is built by very regular cubic particles and these results could therefore be interpreted as a confirmation of the role of low coordination 0 2 - ions, which are more abundant after etching the solid, in the surface redox process. It has to be noticed, however, that none of the work described above on the donor properties of oxide ions reported the EPR signal from the hole centre 0’formed when 0 2 - gives up an electron. This aspect has been discussed recently and it appears that the 0- ion further reacts with electron acceptor molecules to release oxygen (64). The electron donor properties of several oxides (CaO, MgO, ZnO, Al2O3, and SiOz/A1203) have been studied using the formation of NB- a s a probe (65). After activation a t high temperature, the electron donor power of the oxide surfaces decreases from calcium oxide to silica-alumina in the order reported above, i.e., in the same way as the Lewis basic strength of the surface. An interesting comparison between the electron donor properties of pure MgO and of Coo-MgO solid solutions, i.e., a system with Co2+ ions homogeneously dispersed in the ionic MgO matrix, was performed using the formation of various anion radicals on both surfaces (66). In particular the formation of the perylene anion radical was observed exclusively rqt the COO-MgOsurface. The donor power of the latter solid has a non monotonous dependence on the COO concentration and exhibits a maximum for a value of about 3% (66). The following sequence of donor strength was proposed for the COO-MgOsystem: Co2+ > 02-~cus)-OH-> 02-ccus),where CUS stands for coordinatively unsaturated ions. As mentioned above the mechanism of electron transfer from basic oxides to various molecules is not completely understood. It has recently been shown that some oxide 0 2 - ions a t the surface of MgO behave as strong Brcensted bases and are able to capture a proton from even weakly acidic molecules (67). The anions formed are unstable and are capable of further complex reactions (31,681. The above mentioned results indicate, as already stated (91,that the mechanism of reaction of basic oxides with high EA molecules have probably to be further investigated.
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The formation of both radical cations and radical anions has also been observed on the same silica or silica-alumina surface (60). In this case, a marked enhancement of the radical anion spectrum for trinitrobenzene is observed, on the one hand, when perylene is adsorbed onto the alumina surface. The radical perylene cation signal, on the other hand, is reinforced by adsorption of trinitrobenzene.
-
5.3.4 Active sites identification via poisoning
The high sensitivity of the EPR technique has been often very useful in the identification of the catalytic active sites when these represent a very minor fraction of the surface atoms or ions. The isomerisation of 1-butene to 2-butene (cis and trans) catalysed by yalumina has been studied using NO as a probe molecule to detect the exposed Al3+ ions at the surface of the catalyst (69).
Fig. 5.30 - Experimental (a) and computer simulated (b) EPR spectra of NO adsorbed on yAl2O3 (ref. 69). The EPR spectrum of NO adsorbed onto activated y & o 3 reported by Lunsford (69) is similar to those described earlier in section 5.3.2, except for the presence of a resolved superhyperfine structure due to the interaction of the NO unpaired electron with an Al nucleus (I=5/2).The spectrum is reported in Fig 5.30. The effect of H2S on the catalytic activity and on the intensity of the EPR signal of adsorbed NO was followed as a function of increasing H2S doses. The results shown in Fig. 5.31 indicate that the intensity of the EPR signal of adsorbed NO and
B311
the catalytic activity both decrease with increasing H2S doses. This similarity indicates that the exposed Al3+ ions capable of coordinating the NO molecule are indeed the active sites for the catalytic isomerisation reaction and that H2S is an effective poison of both the adsorption of NO and the catalytic reaction.
0.80.7 0.6 -
sb 0.55
0.L -
0.30.2 0.1 -
0
i
2 3 L 5 6 Hydrogen sulfide/lO”molec crn-2
Fig. 5.31 - Variation of the relative rate constant for 1-butene isomerisation (open symbols) and of the relative NO spin concentration (solid symbols) as a function of H2S dosage on y-Al2O3 (ref. 7,69).
-
5.55 Surface groups morphology
A typical example is the investigation of the distribution of surface hydroxyls on fumed silica and alumina employing vanadium tetrachloride a s a paramagnetic probe (70). VC4 reacts at 363K with surface OH groups with HCl elimination. From the amount of HC1 evolved, the stoichiometry of its reaction with surface hydroxyl groups is determined. From the parameters and width of the EPR spectra of V4+ ions, it is possible to determine further the steric disposition of these groups. Owing to a short relaxation time, VC4 leads to EPR spectra which are too broad to be detected between 300 and 77K. However, ligand substitution within the VCl4 molecule removes the tetrahedral symmetry and orbital degeneracy and the EPR spectrum of vanadium, in this modified environment, can be readily observed even at room temperature.
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Upon reaction of rehydrated Cabosil (fumed silica) with small amounts of VCl4 (between 5 and 7% coverage), an EPR spectrum was obtained a t both 298 and 78K (F'ig. 5.32). The spectral parameters were gl=2.0250,gli =1.9699, Al=79.3 G and A I I = 189.9 G (70). This spectrum is attributed to those vanadium ions which are attached to the surface through reaction with two "vicinal" hydroxyl groups with evolution of two HCl molecules.
Fig. 5.32 - The EPR spectrum of VC14 attached to Cabosil (ref. 70). At a coverage of lo%,different spectra were obtained due to the overlap of EPR signals of doubly attached and singly attached vanadium. The latter species gives rise to a broad signal at room temperature due to the rotation about the Si-0-V bond. The use of VC4 as a paramagnetic probe has allowed to determine the type and distribution of different hydroxyl groups. Three types have been identified: geminal pairs, with two hydroxyls on the same silicon atom, vicinal pairs, with two hydroxyls separated by a distance of about 0.32 nm and thus ideal for reaction with the two C1 atoms of VC4 and finally single or isolated hydroxyls separated by at least 0.40 nm from other hydroxyl groups (70) A similar approach was followed for a different purpose, the preparation of well dispersed Mo/SiOz catalysts (active in methanol oxidation) by grafting the paramagnetic MoCl5 to a silica surface. The grafting process and the successive steps of catalyst preparation were followed by EPR (71). When silica is heated in presence of MoCl5 at 47313, a reaction between MoCl5 and silanol groups occurs and a n EPR spectrum is readily observed (Fig. 5.33). The EPR parameters (gl=1.952 and g, =1.968; A, =37G and A, ,= 70G) and the presence of hyperfine lines due to 95M0 and 97Mo indicate that it arises from pentavalent molybdenum. The EPR spectrum has axial symmetry whereas that of MoCl5 in cyclohexane is isotropic at 77K. The change of symmetry shows that the molybdenum
,
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coordination sphere is affected on grafting and confirms the formation of bonds with silica. The EPR parameters as well as the UV-Visible spectra observed for the grafted sample have been compared with those of different pure compounds such as MoCl5, [MoOC14]- and [MoOC15]2- and found to be very similar to those of the [MoOC14]- ion (71), suggesting the following grafting reaction scheme: MoCl5+ SiOH
+ SiOMoCl4 + HCl
Fig. 5.33 - EPR spectra recorded a t 77K aRer reaction of MoC15 with silica. a>first derivative b) third derivative (ref. 71).
When, after grafting, the sample is directly evacuated a t increasing temperature without any contact with air or water, dramatic changes are observed in the EPTE ~ y c t r x tnese ~ : are related to changes of the coordination sphere of molybdenum and are +scribed in section 5.3.7.
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EPR spectroscopy has proved to be a very powerful tool in the study of molecular motions in various conditions (5). Information on the motional behaviour of paramagnetic probes present or incorporated in a system is obtained from the line shape of their EPR spectrum. The nature of the motion and the related correlation times can be studied, however, only by means of highly sophisticated theoretical approaches developed in recent years (72). The usual approach is to compare the experimental spectrum with theoretical lineshapes calculated for differential motional models and correlation times. Several dynamic studies by EPR concerning molecules or ions adsorbed on solid surfaces are reported in the literature but a n exhaustive description of this field is beyond the scope of the present review. We will limit ourselves to examples of particular relevance to surface chemistry: paramagnetic species created upon adsorption at a gas-solid interface and spin probes introduced in a system to investigate the mobility at a liquid-solid interface 5.3.6.1 One of the early studies of the mobility of adsorbed species in the slow motion regime (correlation time between 10-9 and 10-'seconds) was performed by Pietrzak and Wood (73) on the mobility of NO2 and C102 adsorbed in zeolites. The lineshape of the EPR spectrum of both paramagnetic molecules changes with increasing temperature from an anisotropic powder-like form a t low temperature to an almost isotropic liquid-like form a t room temperature. The mobility of the superoxide 02-radical adsorbed on surfaces has been studied by several authors. An exhaustive study has been reported, for instance, by Shiotani et al. (74) on the motion of 02-adsorbed on Ti4+ ions supported on porous Vycor glass in the temperature range 4-400 K. Of the several types of 02-, a species noted as 0 2 ' (111) (gzz = 2.071, gyy = 2.0092, gxx = 2.0025) was characterised by highly anisotropic motion. While g, and g,, varied with increasing temperature accompanied by drastic lineshape changes, gyy was found to remain constant. This observation indicates that the molecular motion of the radical ion can be described by rotation about the y axis (perpendicular to the surface and to the internuclear axis, Fig. 5.18). The EPR line shapes were simulated for different possible models and it was found that a weak jump rotational diffusion gave the best fit with the observed spectra below 57.4 K, whereas several models could fit the data above this temperature. The rotational correlation time was found to range from 10-5sec below 14.5 K to 10-9 sec a t 263 K and the activation energy for rotation was found to be 0.5 KcaYmol above 100 K. Similar studies have been performed with tungsten cations dispersed on silica (75). With 170 labelled oxygen, it has also been observed a decrease of the hyperfine constant value corresponding t o a
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motional averaging of g,, and gyy due to the rotation about the y axis. The data concerning the mobility of oxygen species have been discussed earlier (10). 5.3.6.2 - bauid-solid s v s m A very interesting approach to investigate the dynamic properties of liquids adsorbed onto solid supports is to study the EPR spectra of suitable paramagnetic probes introduced in the liquid phase. The probes can be transition metal ions in aqueous solution (76-78) or nitroxide radicals in various kinds of solutions (79-81). The systems investigated are usually porous solids such as silica gel, alumina, zeolites and clay minerals. Different motional properties have been observed for the liquid alone or in the adsorbed state. The informations are extracted from EPR spectra measured at various temperatures, concentrations and ionic charge of the ionic probes. A more detailed analysis of the EPR approach to motional studies including that of the saturation transfer technique, STEPR is available in ref. 5.
-
5.3.7 Coordinationchemistry of surface transitionmetal ions
A molecular approach of catalysis requires a deep understanding of the coordination processes occurring a t the catalyst surface. The EPR technique has been employed, as well as a number of other techniques, to identify various types of coordination chemistry which occur on catalytic systems involving transition metal ions and oxide matrices (82). EPR has proved to be a powerful tool to investigate the surface coordination chemistry by means of paramagnetic and non paramagnetic molecules used to fill up coordination vacancies.
. .
5.3.7.1 - Surface coordination che&trv of-rt dimersed into a solid framework
..
homoveneouslv
Typical examples of these systems are the solid solutions of transition metal oxides (NiO, COO)in MgO which have been widely investigated in recent years (83) because they form solutions over the entire molar range and the dispersion of the transition metal ions can be easily controlled and modulated. Under suitable conditions, microcrystalline, high surface area samples can be prepared and have been employed as model compounds in adsorption and catalysis studies (83). The Coo-MgO system has been found recently t o function as an heterogeneous oxygen carrier (84). The Co2+ ions a t the surface of this system, whose capability to form superoxides upon contact with oxygen was already known (85). are able to coordinate reversibly the oxygen molecule according to the process c02++02
2
c03+02-
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Fig. 5.34 - EPR spectra recorded at 77 K of oxygen adducts at the surface of CoOMgO solid solutions. a) Species I and I1 stable at low temperature. b) Species I11 and lV stable at higher temperatures (ref. 84).
which is a simplified scheme of the more complex process. The cobalt-oxygen adduct given above is paramagnetic and exhibits complex EPR spectra (vide infra): on the basis of its spin-Hamiltonian parameters and of studies with 170 enriched oxygen, it has been shown that the oxygen moiety does not lie flat
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on the surface as in most cases of superoxide on metal oxides (Fig. 5.18) but is in a "bent end-on" position at the cobalt site (86) similar to that of the oxygen adducts of cobalt complexes in solution. Although an important electron transfer from cobalt to oxygen has been evidenced in the case of the adducts at the surface of COO-MgO (601, a complex and weak covalent bonding occurs between cobalt and oxygen instead of the purely ionic interaction described in Fig. 5.18. In the case of homogeneous oxygen carriers, which can be both of natural and synthetic origin, the reaction of oxygen addition can be represented by the following structure model:
B where B is an axial Lewis base, coordinated to the cobalt ion. By adsorption of oxygen at 77K onto dilute solid solutions with isolated pentacoordinated Co2+ surface ions in C4" symmetry (C05~2+),the spectrum of Fig. 5.34(a) has been obtained. The large number of hyperfine lines indicates that cobalt (59C0, I=7/2, 100% naturally abundant) is involved in the paramagnetic species. The spectrum is the superimposition of two distinct signals labelled I and 11. Both signals exhibit hyperfine structure overlapping in the high field region of the spectrum. They are, by contrast, better resolved in the low field part with distinct g and A values. The notations I and I1 are assigned to superoxide adducts adsorbed at slightly different angles onto Co2+ surface ions (58):
2-
o=o
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The spectrum in Fig. 5.34 (a) corresponds to the initial stage of the oxygen interaction with the COO-MgOsystem since the two species, stable at 77K,undergo different evolutions upon evacuation and increase of temperature to 120-150K. This treatement leads to the spectrum of Fig. 5.34(b) recorded at 77K and composed of two superimposed signals, I11 and N.The first one belongs to a new species with Co hypefine structure and orthorhombic g and A tensors (species 111) and has been assigned to a superoxide adduct, similar to species I and 11, but further stabilized by interaction with a neighbouring Mg2+ cation (Fig 5.35). The species J Y is a superoxide 02-ion adsorbed onto the MgO matrix. Fig. 5.35 represents a COO-MgOsurface with C05~2+ions merging at the (100) face: they are surrounded by five 02-ions of the MgO lattice. Co2+ ions can be found also at edges and corners of the crystals where they are four and three coordinated respectively. However, the Coo-MgO solid solutions are mainly in the form of microcrystals of cubic shape with the (100) faces prevalently exposed (87). Hence about 90% of the exposed cobalt ions are in the same C4" symmetry as that of the synthetic oxygen carrier shown in model [381.The role of the axial base B is indeed played by a lattice basic 02-ion. 0 c03.
0 Oxygen 0
Mg2'or Co2'
0 02-
Fig. 5.35 - The COO-MgOsolid solution surface showing penta-(face), tetra-(edge) and tri-(corner) coordinated cobalt ions. The oxygen adducts represented in the figure are of type I11 (see text). The adducts represented in Fig. 5.35 are of type I11 and further stabilized, with respect to I and 11, by electrostatic interaction with the ionic MgO matrix. The oxygen molecule therefore acts as a selective probe for a fraction of the pentacoordinated cobalt ions present at the surface. The remarkable feature of the
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COO-MgOsolid-solution is that it is the first example of an oxide system capable of reversibly binding oxygen according to the same model 1381 used for homogeneous oxygen carriers (88,89): this analogy is further discussed in (90). A second example concerns the coordination of nitric oxide onto the surface of NiO-MgO and COO-MgOsolid solutions. When NO is adsorbed onto NiO-MgO (911, two EPR signals, A and B, are readily formed (Fig. 5.36 ). The spectral parameters of signal B coincide with those of the signal observed after NO adsorption on MgO (25) and can be assigned to NO# radicals formed on low coordination Mg2+ 0 2 - ion pairs in nickel-free regions of the surface. Signal A is one order of magnitude more intense than B and is due to Ni+ ions in axial symmetry. The g values are respectively g11=2.174 and gl=2.131. The intensity of signal A diminishes with decreasing NO pressure and becomes very weak after evacuation: readmission of NO restores the signal. u
9, 'gY
Fig. 5.36 - EPR spectra of the signals of Ni+ (A) and N022- (B) obtained upon adsorption of NO onto NiO-MgO (ref. 91).
As Ni+ ions are formed from Ni2+ ions upon NO adsorption, the coordination process may be described as: Ni$+
+ NO
2 NisC+NO+
with ligand to metal electron transfer which is a common feature of chemical bonding of many inorganic nitrosyls. Also for the Ni+-NO+ surface complexes, the majority of metal ions involved in the reversible coordination of nitric oxide are the
B320
5-coordinated ones (Nise2+)at the (100) crystal face: the nitrosyl structure is thus tetragonal.
0
$"
0-2 The molecular orbital scheme in Fig. 5.38 better describes the chemical bonding of the surface complex and accounts for its paramagnetic behaviour. Adsorption of NO onto COO-MgOat room temperature gives rise immediately to the same signal as the one labelled B in Fig. 5.36. A second signal grows with time (Fig. 5.37) and after a few hours dominates the spectrum (91).
Fig. 5.37 - EPR spectrum obtained upon adsorption of NO onto COO-MgOsolid solutions after a few hours. The latter signal is, however, about one order of magnitude less intense than the type A obtained for NiO-MgO and shows a hyperfine structure due to the 59Co nucleus. The parameters are respectively gx= 2.11, gy= 2.06, g,= 2.00 and Ax= 38.5 G, Ay= 37 G , &= 93 G . Evacuation a t room temperature causes the disappearence of the spectrum. On the basis of the molecular orbital scheme of Fig. 5.38, the cobalt nitrosyl complex a t the (100) face, which has also been observed by infrared spectroscopy (911,is predicted to be diamagnetic. The EPR spectrum in Fig. 5.37 is
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instead due to nitrosyls formed on four-coordinated Co2+ ions that are formally reduced to the Co(0) zerovalent state upon NO coordination. I
3E
I
-\
0-
Fig. 5.38 - Molecular orbital diagrams of the metal nitrosyls formed a t the (100) cubic face of NiO-MgO and COO-MgO solid solutions upon adsorption of NO a t room temperature. The nickel complex is paramagnetic while the cobalt one is diamagnetic (ref. 91). Two possibilities exist to describe the formation of the paramagnetic cobalt complex:
Co&2+
+
2NO
2
[Co(O)(N0+)2]
In the former reaction, the matrix assists the reduction of cobalt to Co+ which is then coordinated by NO to form the zerovalent cobalt complex, while in the second case a dinitrosyl is directly formed upon NO coordination. EPR does not discriminate between the two hypotheses. The data presented above allow to give the main features of the coordination chemistry of surface framework ions:
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a) a t low concentrations, isolated transition metal ions can be found a t the surface of the host matrix. b) the structure of the transition metal ion environment is imposed by the matrix oxide which, as shown in the examples above, functions as a macroligand which is sterically demanding: for ions located a t the cubic faces, the matrix oxide acts as a pentadentate ligand whereas it becomes tetra and tridentate €or ions a t edges and corners respectively. c) the reactivity of the transition metal ion depends on its position, and thus on its coordination, at the surface of the matrix. d) no mobility is observed in the coordination sphere of the metal ion because of the structurally demanding nature of the matrix oxide ligands, a t least under the Tamman temperature, i.e., the temperature at which ionic mobility appears (92,93).
5.3.7.2
- Coordination chemistrv of extraframework io m
This type of coordination chemistry concerns isolated transition metal ions in extraframework positions, i.e., ions which can be anchored onto the surface of an oxide support by ion exchange, grafting or any other suitable method leading essentially to isolated ions. It is reasonable t o anticipate that, when transition metal ions change from framework to extraframework positions, the support oxide changes from a sterically demanding to a sterically non demanding ligand. This expectation has been tested using a method based on the pressure dependent chemisorption of various molecules on paramagnetic ions (94-96). The important characteristic of the transition metal ions in extraframework positinnr is their ability to change their coordination reversibly a s the ligand pressure is varied. An illustration of such a behaviour is given by silica supported nickel catalysts prepared by ion exchange: after a slight thermal reduction by hydrogen a t 170°C, paramagnetic Ni+ ions (d9) are formed and detected by EPR (95). On varying the pressure of 12CO from 1 to 600 torr, a series of spectra can be reversibly recorded at 77K (Fig. 39). The analogous of spectrum (d) in Fig. 5.39, but obtained with 13CO is reported in Fig. 5.40: the superhyperfine structure due to the 13C nucleus (I = 1/21 is clearly evidenced. The spin-Hamiltonian parameters of the spectra obtained under various 12CO or 13CO pressures are collected in Table 5.6 together with the different structures assigned to the Ni+ carbonyl complexes. The structure of these complexes have been derived from a self consistent analysis o f: i) the g tensor components which give the type of symmetry (axial or orthorhombic) of the environment of the paramagnetic Ni+ ion, ii) the relative order of the g tensor components which leads to the probable ground state of the Ni+ ion,
B323
iii) the superhyperfine structure from which the type of CO ligands (equivalent or inequivalent) and their number can be obtained. These informations allow to derive a precise picture of the coordination sphere of the transition metal ion.
Fig. 5.39 - EPR spectra at 77K of Ni+ carbonyl complexes formed on silica under various pressures of 12CO. (a) 10 torr CO and evacuation a t 340K , (b) 10 torr,(c) 100 torr (d) 400 tom (ref. 94). Other examples of studies of the coordination sphere of transition metal ions include silica supported vanadium and molybdenum ions. In the former system, several probe molecules have been adsorbed on reduced V2O5 supported on silica (16). The spectrum resolution indicates a n orthorhombic symmetry for the V4+ ions after CO adsorption (gl G g2= 1.985, g3=1.931, Aln A2 = 71.4 G , A3=191.4 G).The use of W O leads the authors to conclude that the V4+ surface ions can coordinate two CO molecules from the gas phase to form, together with four lattice ions, a distorted octahedron with one pronounced vanadyl character bond.
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A,(lCO)= 51 5 G A,
I
( 3C0)s 2 5G
I
A
A,(lC0)=55G
A,/ (3CO) 2 5 G r
13co
Fig. 5.40 - EPR spectrum at 77K of the Ni+carbonylcomplex obtained upon adsorption of W O (400torr equilibrium pressure) on reduced Ni/SiOz catalyst (ref. 94). In the case of silica supported molybdenum ions (961, H20 and CO have been used as probe molecules to study the coordination of molybdenum ions grafted on silica, and prepared according t o the method illustrated in section 5.3.5 (71).The thermal reduction of grafted Mo/SiOa samples induces the formation of three Mo5+ species in hexa-, penta-, and tetra-coordination (respectively M o d + , MosC5+and Mo&J+). They all possess a molybdenyl bond. During adsorption of water, the Mo5+ EPR spectra, recorded at 77K,undergo a stepwise transformation: in the first step, for low water pressure (1 torr), the M0gc5+ signal disappears, whereas that of Mosc5+ increases, then in the second step, for higher water pressure (18 torr), the Mosc5+ signal disappears whereas that of M o ~ increases ~ ~ + (Fig. 5.41).The double integration of the spectra, before and after adsorption, shows that the number of M o ~ +ions remains constant within experimental error, indicating that the signals transform ones into the others and that water only acts as a ligand.
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TABLE 5.6. Spin-hamiltonian parameters for different Ni+ carbonyl complexes observed a t the surface of Ni/SiOz. 0 2 - are the surface oxygen ions acting as ligands and ax and eq stand for axial and equatorial respectively.
pcdtor <1
1-40
ComDlex
g.
g1=2.392 g2=2.350 g3=2.020 gl=2.191 g2=2.086 g3=2.066 g1=2.200 g2=2:162
unresolved A1=30.0 A2=32.5 &=32.5 A1 unres. A2 unres &=37 (1CO,)
g3=2.005
gl=2.130 400-600
gl I=2.009
The whole scheme of water coordination onto surface Mo5+ ions is summarized in the following Table (reproduced from ref. 67). MO'+ species
hydrated species
after reduction
notation
\do/o 0
g,
g,
M o :
1.925
1.750
MoZ
1.944
1.892
0
II
0
0
IP
0-Moo-0
o/ll 0
h0
I/"
0-MO-0
bll
0
nzo
IP
O-MO-OH,
o'rl
0
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I
I
Fig. 5.41 - EPR spectra recorded at 77K of a reduced grafted Mo/SiOs catalyst : (a) after reduction, (b) after adsorption of about 1torr water, (c) after adsorption of about 18 torr water (ref. 96). The mechanism of water adsorption occurs thus in two steps: i) admission of a first water molecule in the coordination sphere of M0dc5+.The molecule is located in the equatorial plane since the EPR signal of the hydrated ion is similar to that of the non hydrated M05~5+which possesses a square pyramidal structure. ii) admission of a water molecule in the axial position of both the non hydrated M05~5+ and the monohydrated Mod$+. The previous scheme can be understood considering, as done in ref. 34, that the g values of a molybdenyl species in symmetry depends on the spin orbit coupling of the ligands surrounding the central atom. As the water ligand is connected to the Mo5+ ion by the oxygen atom, the spin orbit coupling constant is the same as that of 02-. Hence the M o ~ species, ~ ~ + hydrated by two molecules of water, is expected to give a signal similar to that of M06~5+.
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A second example involves the coordination of CO to supported molybdenum ions. After adsorption of CO a t room temperature and low pressure (2 torr) on reduced grafted Mo/SiOa catalyst, the Modes+ signal disappears while a single line signal appears as proved by the third derivative recording. It is similar to the signal of Mosc5+,but with g l = 1.960 and is more intense than that of M06~5+.As the pressure increases, it becomes apparent that this line is composed of the Mo5c5+ signal and of a line at g=1.965. At 100 torr, the latter becomes predominant (Fig. 5.42). Quantitative measurements performed before and after CO adsorption indicate that the spin concentration remains constant within experimental error showing that the signals transform ones into the others and that CO only acts as a ligand a t room temperature. The adsorption of CO is reversible at room temperature and the shape of the spectrum is restored by desorption of CO. The model for CO adsorption is analogous to that given for water. We summarize now the characteristics of the coordination chemistry of ions in extraframework positions on the basis of the preceeding examples: a) i t is possible, using suitable preparation methods, to obtain isolated transition metal ions on the surface of a support. b) The structure of the metal environment, which vanes depending on the experimental conditions, is imposed by the matrix immobile ligands andor by the mobile ligands coming from the gas phase. As a consequence the oxide surface macroligand can change from poly to monodentate. The surface is thus, in this case, a versatile ligand.
1,965-
t
SOG
npr~ I
@
& 7969-
@
Fig. 5.42 - EPR spectra of Mo5+ recorded at 77K under 200 Tom CO pressure (ref. 96)
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An important consequence of this versatility is that the surface complexes tend
to become more similar to their solution analogues when the ligand gas phase pressure is increased. c) Because of their accessibility, transition metal ions are able to participate in catalytic reactions. d) Apart from the change of symmetry of the transition metal ion, depending on the ligand gas phase pressure, there is no mobility ( for instance i n terms of translation or rotation) for the transition metal ion under the experimental conditions described above.
-
6.4 CONCLUSIONS
In the present review, we have given the basic principles of the EPR technique relevant to polycrystalline materials and the various ways to extract the magnetic parameters from powder EPR spectra, particularly in the case of composite spectra, i.e., corresponding to the presence of several paramagnetic species. The use of several microwave frequencies, third derivative presentation, isotopic labelling and spectra simulation have been found very helpful. From selected examples from the literature, we have then reviewed typical applications of EPR to the study of surfaces by means of probe molecules, which have been defined from the standpoint of EPR. The most significant progress has been made in the determination of the surface crystal field, the redox properties of surfaces, the identification of catalytically active sites, the morphology of surface groups, the mobility of adsorbed species and finally the coordination chemistry of surface metal ions. It is likely that EPR will continue to bring important informations not only on surfaces but also on adsorbed species and catalytic processes, at the molecular level. It has, however, to be kept in mind that, beside the probe-molecule approach to the study of surfaces, other kinds of EPR investigations concerning the surface in itself and even the bulk of the solid are of capital importance for the understanding of surface chemistry and catalysis phenomena. A final remark concerns the technological evolution of the EPR spectroscopy. We have limited our survey to the classic technique (continuous wave EPR or CWEPR) but more sophisticated approaches are becoming available due to the rapid evolution of both microwave and microelectronic technology. It is the case, for instance, of the EPR spectroscopies i n the time domain (essentially the ESEM, Electron Spin Echo Modulation technique) that up to now has found limited diffusion due to the absence of commercial equipments. Nowadays, however, ESEM spectrometers are becoming available on the market and it is easy to predict a rapid increase of their applications.
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The fundamental advantage of ESEM is the possibility of identifying the properties of a single line in a n unresolved, inhomogeneously broadened signal (97). In the field of surface coordination chemistry, in particular, the ESEM technique can provide, as shown by some recent articles (98-1011, informations about the number of ligands around a paramagnetic metal ion as well as about the bond lengths. The description of surface complexes is rather accurate even employing the classic CW-EPR technique and will probably undergo further development.
ACKNOWLEDGEMENTS. E. Giamello gratefully acknowledges a NATO collaborative research grant (No. RG 86/0556) and a n invited professorship a t the Universitb P. et M. Curie, Paris, during the summer of 1988. The authors wish to thank P. Gihr and F. Delvalle for technical assistance.
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Chapter 6
THERMAL DESORPTION METHODS
P. MALET Instituto de Ciencia de Materiales y Departamento de Quimica Inorganica, C.S.1.C.-Universidad de Sevilla. P.O.Box 1115, 41071 Seville (Spain)
6.1 INTRODUCTION
In a general way a thermal desorption experiment can be A gas is adsorbed on a surface at a described as follows. given temperature (tipically room temperature). After the fraction reversibly adsorbed is flushed out, the sample is heated by increasing the temperature (T) with time (t) usually following a linear programme (heating rate, P=dT/dt). Since the temperature is increased with time in a programmed way the technique is referred to as Temperature Programmed Desorption (TPD). During the heating the temperature and the evolution of adsorbed species to the gas phase are continuously monitored. The TPD spectrum is given by the concentration of the desorbed gas vs. temperature, and is determined by the changes in the desorption rate with the temperature. The rate of desorption of a gas from a surface, (-dN/dt), can be expressed as:
where N is the number of molecules adsorbed on the surface, Ed the activation energy for desorption, A the Arrhenius pre-exponential factor and fd(N) a function describing the dependence of the desorption rate on N. From Eq. 6.1, as the temperature increases the desorption rate will increase, initially at an exponential rate following the exp (-Ed/(RT)) term. When desorption progress , the surface starts to become depleted of adsorbed gas, N decreases and fd(N) decreases. The result during temperature programming is shown in Fig. 6.1, desorption rate increases, goes through a
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maximum and drops back to zero when the surface is depleted of adsorbed gas. Since concentration in the gas phase is determined by the desorption rate, the lineshape of the TPD peak and the position of its maximum are related to Eq. 6.1 and contains information on the kinetics of the desorption process. On the other hand, when a gas can be adsorbed on the surface on several adsorption forms, each of them is characterized by particular values of Ed and A , thus giving a peak in the TPD spectrum. Therefore, the first information available after a TPD experiment is the number of adsorption forms and their relative stability. If the dependence between (-dN/dt) and T can be obtained from the experimental curve, also information about the kinetics of the desorption process from each particular adsorption form is contained in the TPD profile. This chapter is centered in the possibilities of obtaining such information from experimental TPD curves.
*0°
T/K
Fig. 6.1. Plot of the rate of desorption from a surface versus the temperature in a TPD experiment (linear programme of temperature: T = T .
1
+
Pt’
B335 6.2.
EXPERIMENTAL SYSTEMS 6.2.1. Flow and vacuum systems Two types of experimental set-ups, flow and vacuum systems, are used in TPD measurements. Vacuum techniques were first applied to metallic systems in classical flash desorption (FD) experiments (1,2), where a metallic filament or ribbon is resistively heated at high heating rates ( > 10 K/s) in a high vacuum chamber that is continuosly pumped. Desorption processes are followed by pressure changes in the desorption chamber, using a total pressure gauge and/or a mass spectrometer very close to the sample, that allows to determine the composition of the evolved gas. The application of these vacuum techniques to non-metallic systems, that usually are bad conductors of heat, requires the use of slow heating rates ( < 5 0 K/min) and a careful design of the system ( 3 ) in order to avoid heat-transfer problems that are specially severe in vacuum conditions. These vacuum systems are schematized in Fig. 6.2. In any case, in a vacuum system continuosly pumped during the TPD run, the material balance in the system leads to the equation: -(dN/dt) = kv (dp/dt + SP/V)
(6.2)
where 5 is the pumping speed of the vacuum system, 1 its volume and a constant (k = l/(RT); R = gas constant; T = gas phase
TEMPERATURE
I
I
I
1
REACT I ON
CHAMBER
FURNACE
Fig. 6.2. Scheme of a system for TPD in vacuum (MS: mass spectro-
meter 1
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temperature). This expression, allows to obtain the rate of desorption, (-dN/dt) from the changes in the pressure and simplifies in two limiting cases: a) when the pumping speed is much higher than the gas evolution rate from the solid, the following equation results:
In such directly b) material
a case the response of the pressure the desorption rate from the surface. If the pumping speed is very low (s+O) balance simplifies to:
gauge
,
gives
then
the
being the pressure curve a direct measurement of the number of molecules adsorbed on the surface. Although these limiting situations simplify the treatment of experimental data, and they were sistematically used in the earliest works of TPD in vacuum, they have some disadvantages. In the case (a) the high pumping speed reduces too much the total pressure in the system, so its sensitivity is considerably lowered. On the other hand, lower pumping speeds lead to higher pressures in the desorption chamber, and readsorption processes which obscure the interpretation of the TPD profiles obtained may occur. These problems can be avoided by using an intermediate 5 value and evaluating Eq. 6.2 to get the reaction rate and surface population at each temperature, provided 5 and 1 are previously known. A data adquisition system interfaced with a microcomputer will simplify the necessary calculations. Alternatively to those vacuum systems, temperature programmed desorption experiments are carried out in flow systems similar to that described by Amenomiya and Cvetanovic (4). These systems (Fig. 6 . 3 ) , whose experimental design has been recently reviewed by Falconer and Schwartz (51, use an inert carrier flow as gas vector (Helium, Nitrogen or Argon) in which adsorbed species are desorbed from the polycrystalline sample during its heating (usually slow, < 5 0 K/min, in order to improve heat transfer in the sample). Desorbed species are
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transported to the detectors by the carrier gas, where its concentration is continuosly measured. The mass balance for a single species between the surface and the gas phase, leads to the expression:
assuming an experimental system where there is no accumulation of the desorbed phase in the bed, and diffusional processes are not rate limiting. The average desorption or reaction rate through the bed, -(dN/dt), is proportional to the concentration in the gas phase in such experimental conditions, and the response of the detector gives a direct measurement of the rate of the surface process. TPD flow techniques use catharometric detectors (5-91, although in some cases flame ionization detectors have been With the used when studying organic adsorbed species (10). most usual catharometric detectors, the carrier gas (Nitrogen, Helium or Argon) is chosen according to the thermal conductivity of the substance to be detected (111, being Helium the best carrier for most of the processes studied, and Argon the selected one to follow hydrogen desorption. Nitrogen as carrier gas gives poorer results (worse sensibility and base line stability). As shown in Fig. 6 . 3 a vacuum system to pretreat the sample and a classical volumetric system to measure adsorbed amounts previously to the TPD run, complete
PR0G RAMME R
I
DETECTOR
REACTION CHAMBER
FURNACE
Fig. 6.3. Scheme of a system for TPD under carrier gas flow. (MS: mass spectrometer; GC: gas chromatograph)
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the basic arrangement of such flow TPD systems. Alternatively, some authors ( 6 , l O ) pretreat the sample under gas flow, and adsorb the species by adding a fixed concentration of them in the carrier flow during a measured time. After the adsorption, reversible species can be desorbed by flowing a pure carrier stream until the base line has been recovered. More sophisticated systems add to this basic arrangement a gas cromatograph ( 1 0 , 1 2 ) or a mass spectrometer ( 1 3 ) to analyze the composition of the evolved gas, the latter requiring an additional setup (pumping, leak valve, etc) in order to obtain a suitable pressure at the inlet of the mass spectrometer. An analog/digital data adquisition system interfaced with a microcomputer can be used to obtain on-line data processing facilities. Experimental pitfalls Any attempt to analyze the TPD-peaks implies that the recorded signal is faithful to desorption processes from the surface. That is, the application of the material balance given by Eqs. 6 . 2 or 6 . 5 should allow to obtain the desorption rate from the surface, and its dependence with temperature and surface coverage, that should not be significantly distorted by the experimental conditions employed. Distortions in the curves could be produced when the adsorbate has left the sample by adsorption/desorption phenomena in the walls of the experimental system. In flow systems, diffusion processes of the adsorbate in the carrier could also distort the TPD profile. Moreover, the time-lag between the desorption and the detection by the catharometer leads to errors in the temperatures determined for each value of the desorption rate. Such phenomena should be carefully tested to determine the range of experimental conditions where they are not distorting in a significant way the desorption curve. In a flow apparatus, it could be done by injecting nearly instantaneous pulses in the carrier stream at different flow rates, and recording'the response of the system. The time between injection and detection gives the time-lag of the system. The width of the recorded pulse is determined by diffusion processes in the carrier and adsorption-desorption on the walls of the system. Flow conditions where such phenomena are negligible should be selected, i.e. peak widths should be 6.2.2.
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independent of the gas injected, so that adsorption/desorption processes on the walls of the system can be neglected, and the response of the system should be nearly instantaneous, so that in such conditions time-lags and diffusion effects could also be neglected. In vacuum systems the pumping speed should be determined for each desorbed species at several pressures, in order to be sure that it is constant during the whole desorption process ( 3 ) . On the other hand, the equations of material balance yield a mean value of the desorption rate throughout the sample for each measured temperature value. Mass transport effects inside the bed of a porous catalyst could lead to concentration gradients in the sample, and/or pressure built-ups inside the cell or the catalyst that may distort the TPD profile. The problem has been analyzed by Gorte et al. (14,151 who propose which dimensionless groups of catalyst parameters (Table 6.1 allow to determine ''a priori" the limit values that may be used in order to obtain a significant TPD-curve. These authors (14,15) conclude that if the experimental design is not carefully selected, and maintained within these limi s, TPD profiles will be strongly distorted, and only qualitative features could be obtained. The effect in the shape of TPD-peaks produced by the diffusion of the adsorbent in the carrier gas through the way from the sample cell to the detectors has also been examined (16,171. This effect is not important with a reasonable design TABLE 6.1 Recommended limits for experimental parameters in a TPD experhnt (refs. 14, 15) Parameter
Comments
Limits
B F(Tf-Ti)
Pressure built-up in the cell
(0.01
Pressure built-up inside the catalyst
D(Tf-Ti)
Concentration gradients in < O . l >20 the sample V = bed volume; P= heating rate: Tf= final temperature: Ti= initial temperature: 1= bed lenght; E p = particle porosity; D= effective diffusivity; F= carrier flow: & B = bed porosity.
E A2 DV
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of the experimental system and causes a slight broadening of TPD-peaks. An important source of error for non-metallic catalysts could be the non-homogeneity of the temperature across the sample bed, due to the low thermal conductivity of the sample. The effect of the temperature distribution has been examined (16) by assuming that the sample is a thin disk heated by an outer cylindrical furnace, so that the heat transfer is always radial. Let us suppose a radial distribution of temperature (18) where the temperature (Tr) is uniform throughout differential elements between r and r+dr:
a being the diffusivity of the sample (a = k/(ec); k=thermal conductivity, ?=density, c=calorific capacity). The distortion that this effect produces in TPD peaks has been determined by simulation for different experimental conditions. As shown in when the ratio 2a/r2 decreases, peaks are broadened Fig. 6 . 4 , and shifted to lower temperatures. It can be concluded (16) that the value of this parameter should be maintained higher than 0.1 in order to obtain non-distorted TPD-profiles.
r
Temperature Distribution
500
T/K
600
Fig. 6.4. Effect of temperature unhomogeneity across the sample bed
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6.3 KINETIC ANALYSIS OF TPD CURVES 6.3.1 Qualitative analysis: number and relative stability of adsorption forms In a qualitative way, TPD diagrams can be used to obtain information about the number of adsorption forms on the surface and their relative stability. This analysis assigns one adsorption form to each desorption peak observed in the TPD diagram, and it is generally assumed that the higher the temperature the peak is observed, the higher the stability of the species giving rise to such a peak. It is also usually accepted that a better resolution between overlapped peaks is achieved by using lower heating rates. Although most of the papers concerning with TPD use this sort of analysis, such oversimplified interpretation must be handled with care. As stated by Ozawa (19) for derivatographic thermogravimetric curves (DTG) two independent reaction processes (in TPD desorption of two adsorbed species) can give overlapping curves which resolve with decreasing heating rates, as it is usually assumed, but it is also possible to achieve a better resolution by using a higher heating rate, depending on the relative values of kinetic parameters for both desorption processes. Fig. 6.5 shows the changes in resolution for two overlapping TPD curves when the heating rate is changed, and how in one (Fig. 6.5A1, case resolution can be improved by decreasing improves resolution in the another case while an increase in (Fig. 6.5B).
(P)
p
P
6.3.2. Quantitative analysis: kinetic parameters of the desorption process The quantitative analysis of TPD-peaks allows the calculation of Arrhenius kinetic parameters for the desorption or reaction processes. The practical interest of the knowledge of such parameters has enhanced the efforts in the development of kinetic analysis methods of TPD-peaks, although line-shape analysis methods are not extensively used. For desorption from homogeneous surfaces Ed is a constant, without dependence of the surface coverage, while in the case of heterogeneous surfaces Ed(@) is a function of surface coverage. In some cases also the changes of A associated with the changes of T (20) or 6l (21) have been considered. In this
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section only homogeneous surfaces with constant Ed and A values throughout the whole desorption process have been considered. Kinetic analysis methods of TPD curves usually express the general relationship given by rate of desorption by the Eq. 6.1, that can be expressed in terms of surface coverage, 8=N/No, where No is the number of molecules adsorbed when the surface is fully covered:
For desorption processes from single crystals in high vacuum conditions, first (fd(@)=@) and second order (fd(e)=e2) desorption kinetics have been considered. In TPD from porous catalysts readsorption of the desorbed species is possible and the net desorption rate from the surface is given by:
I
'
\
I
Fig. 6.5. Effect of the heating rate ( 8 ) in the resolution of overlapped TPD peaks. A ) state 1: E=20 kcal/mol, A= 107s-1; 2 : E=35 kcal/mol, A=lO13s-l. B) state 1: E=15 kcal/mol, A=7x10 s-l; s t a t e 2 : E=30 kcal/mol, A3lO13s-1.
stfie
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In a flow system, taking into account the mass balance given by Eq. 6.5: C
=
No/F.(-d8/dt)
(6.9)
and by substitution in Eq. 6.8 the following equation can be obtained: (6.10)
where F' = F/ (Noka). In order to simplify Eq. 6.10 two extreme situations have been considered in the literature. In the no-readsorption limit F>>Noka, readsorption can be neglected, and Eq. 6.10 simplifies to:
In this case only the desorption kinetics determines the TPD profile. Like in desorption from single crystals, first and second order desorption processes are usually considered in the literature ( 4 ) . Thereafter, these no-readsorption kinetics will be referred to as 1W (first order, fd(8)=8) and 2W (second order, fd(e)=e21 . In the second extreme situation usually considered ( 4 ) (free-readsorption limit), the condition F<
where (kd/ka) is Kd, the equilibrium constant for desorption. In this case Kd = exp(AS/R) .exp(-AH/ (RT)), where AH is the heat of desorption and AS the change of entropy in the desorption process, and the TPD profile is determined by the displacement of the adsorption/desorption equilibrium to the gas phase as the temperature is increased. Eqs. 6.11 and 6.12 could be expressed by the general relationship:
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taking into account that in the no-readsorption limit E, A and f(8) correspond to kinetic parameters in the Arrhenius equation for the desorption process, while in the free readsorption limit E corresponds to the desorption enthalpy of the process, A stands for exp ( h S / R ) (F/No), and f (8)=fd ( 0 ) /fa (8) First order desorption processes with first order equilibrium (1-8) ) and second order readsorption (hereafter 1R1, f (@)=€I/ desorption processes with second order equilibrium readsorption (2R2, f ( Q ) = Q 2 / (1-8)’) have been considered in the literature (4,201. f(Q) expressions for different desorption kinetics are collected in Table 6.2. Analysis methods for TPD curves from such homogeneous surfaces consider also the integral form of Eq. 6.13. If the temperature of the sample is increased at a constant rate dT/dt (linear temperature programme) , Eq. 6.13 can be integrated as:
.
.
p=
(6.14) where g(8) is a function depending on the desorption and I is the integral of the Arrhenius equation:
kinetics
(6.15) g(Q) expressions for first and second order desorption kinetics from homogeneous surfaces without readsorption and with free-readsorption are also collected in Table 6.2. Several methods have been proposed in the literature to evaluate the integral of the Arrhenius equation when the value of E is independent of the surface coverage (Table 6.3). The confidence limits of these methods when applied to the kinetic analysis of thermogravimetric curves have been analyzed by Criado and Ortega ( 2 2 , 2 3 1 , concluding that the expressions developped by Senum and Yang (24) are the most exact to simulate theoretical curves, although in most cases, and for kinetic analysis purposes the approximations proposed by Coats and Redfern (251, Doyle (26) and Gyulac and Greenhow ( 2 7 ) can be used for determining the activation energy in spite of
TABLE
6.2
Algebraic expressions of f(e), f'(@), and g ( 0 ) for different desorption kinetics Kinetics First order without readsorption
1w
First order with free readsorption
1R1
Second order without readsorption Second order with free readsorption
f(e)
Symbol
e
(e)
g(e) -In ( e/ei
1
e
2w
2R2
fi
e2 e2/
f(0) dependence of the adsorption-desorption rate on 8
f'(9) first derivative of f ( e ) with respect to 8
28
- ei
- ln(8/ei)
TABLE 6.3 Approximate equations of the Integral of the Arrhenius equation ( I ) ~~
Method
Ref
Approximation t o I
Coats-Redfern
-
Doyle
In I = In (E/R
Gyulai-Greenhow
In I = 8.15
Senum-Yang
I = T.exp (-E/(RT))
-
5.34
-
0.916 In E
1.05 E/(RT)
-
0.620 E
0.9583
(26 (1o3/~)
(E/(RT))2+ lO(E/(RT))
+
18
(E/(RT))3+ 12(E/(RT) )2+ 36(E/(RT))+24
(27
(24
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giving a poor accuracy to simulate the theoretical profiles. Therefore, the different approximations for I collected in Table 6.3 could be used in developping the different kinetic analysis methods for TPD curves. At this point, it is worth noting that following the Senum and Yang's method (3rd degree rational approximation, ref. 24), g ( 8 ) can be expressed as:
being x = E/ (RT) and h(x) h(x)
=
x(x2+10x+18)/ (x3+12x2+36x+24)
(6.17)
h(x) has been plotted vs. x in Fig. 6.6 showing that for values of x = E/(RT))15, h(x) is nearly constant and differs from unity less than 10%. Therefore g ( 8 ) could be taken in a first approximation as: g(e
=
(6.18)
ART~/(EP)exp (-E/(RT))
for E / (RT)215 the ki etic Another basic equation usu lly employed i analysis of TPD curves is the expression for the maximum rate of desorption in the TPD profile. At the maximum (d2e/dt2)
0
10
20
30
50
40 X
=
E/(RT)
Fig. 6 . 6 . Plot of the h(x) term (Eq. 6.17) vs. x= E/(RT) showing that h(x) is nearly c o n s t a n t for E/(RT)a 15.
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equals zero, and the following relationship between the temperature of the maximum and the kinetic parameters of desorption is obtained: (6.19)
where f'(6,) is the value of the first derivative of the f(6) function vs. 8 at the maximum of the TPD peak (see Table 6.2). Table 6.4 summarizes the basic equations employed in the kinetic analysis of TPD curves from homogeneous surfaces. Table 6.4 Basic equations in kinetic analysis of TPD curves from homogeneous surfaces Differential equation (6.13)
-(d€I/dt) = A.exp(-E/(RT)).f(€I)
Maximum condition (6.19)
One parameter analysis of a single TPD curve As stated above, several methods have been proposed in the literature for the kinetic analysis of single TPD curves. Some of them (Table 6.5) attempt to simplify this analysis by making use of some characteristic parameters at well defined points of the curve. At the maximum of the desorption rate, 8, and T, are related through Eq. 6.19, and both parameters can be easily obtained from the TPD profile (see Fig. 6 . 7 ) . On the basis of this relationship, the method initially proposed by Redhead (28) is still used to determine the preexponential factor from Tm when the activation energy has already been determined. In a recent paper, Tronconi and Lietti (29) propose the application of Eq. 6.19 to TPD peaks normalized with respect to the height ((2,) and temperature (Tm) of the maximum (normalized height, Cn= C/Cm; normalized temperature, Tn= T/Tm). By 6.3.2.1
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TABLE 6.5 One parameter kinetic analysis of a single TPD curve Parameter
Procedure
Information A for a known E
Temperature at A.exp(-E/(RT ) ) = the maximum 2 m PE/(RTmf' (em)) f '
Coverage at the maximum
Shape Index
Width at half height
400
(em).em
E/(RT,)=
E for a known kinetics f(em).
sR
Compare ( 0 / @ . ) with c alcu 1atedmva iues (Fig. 6.8) Determine S, and S1 (see Fig. 6.9) Compare with calculated values (Fig. 6.10) Compare with calculated values
500
T/K
E and kinetics
E and kinetics
E for a known kinetics
600
Fig. 6.7. Characteristic parameters of a TPD curve at its maximum rate of desorption: T =temperature: 0 =coverage; C =desorption rate m m m
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combining E q s . fulfilled:
expression that reduces to
6.13
and 6.19,
the following condition must
for n order desorption without
be
readsorption
and for m-order adsorption-desorption processes:
Taking into account the definitions of C,
ern / ( (-dQ/dT),Tm)
=
/:
CndTn = SR
and T, : (6.23)
where SR is the area of the portion of the normalized spectrum lying in the right-hand side of the peak maximum. considering E q . 6.23, E q s . 6 . 2 1 and 6.22 simplify to:
TPD By
and :
The
assumption of the kinetics allows, through E q s . 6.24 and 6.25 the determination of E/RT, One of the main limitations of the above method is that desorption kinetics must be assumed in order to obtain E/(RTm). The relation between coverage at peak maximum, initial coverage, and kinetics of the desorption process had already been studied by Rasser ( 2 0 ) in an attempt of determining both, desorption kinetics and activation energy, from this parameter. By combination of Eqs. 6.14 and 6.19 the following relationship that applies at peak maximum is obtained:
.
(6.26)
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where 1, is the integral of the Arrhenius equation between T = 0 and T = Tm. From Eq. 6.26 it can be concluded that only the kinetics and E/RTm determine the coverage at peak maximum, since I/T only depends on E/RT (see Table 6.3). Values of em vs. E/(RTm) for different desorption kinetics and Bi = 1 have been plotted in Fig. 6.8A showing that, although some ambiguities still remain, in some cases it is possible to estimate the desorption kinetics and E from the coverage at peak maximum. It is worth noting that for desorption processes without readsorption the ratio Bm/Bi is independent of the initial surface coverage (Fig. 6.8B) , while for free readsorption kinetics Qm/Qi changes for different Qi values, so that the analysis of this dependence in experimental TPD curves recorded at different initial coverages could help in the selection of the right desorption kinetics and E. As an alternative method, the peak width at half height (W112) can be used to analyze a single desorption curve, following the relations bewteen E/(RT,) and W1I2 fitted by Chan et a1 (30) for first and second order desorption processes without readsorption, valid in the range 10gEg50 kcal/mol:
B)
A) 0.60
0.40
0.30 0
40
80
(WRT,)..
0
0.50
1.00
0,
Fig. 6.8. Ratios between coverage at peak maximum and initial cove rage for several kinetics of desorption (ref. 20). A ) Dependence on E/(RT,) at B i = 1; B ) Dependence on B i at E/(RT,) = 20.
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E/(RTm) = 1
+
(1+5.832Ti /W:/2 2
)1/2 2
E/(RTm) = 2 (-1+(1+3.117 Tm /W1/2 )ll2)
1st order
(6.27)
2nd order
(6.28)
or by comparing the experimental width at half height with the graphs reported by Rasser (20) for readsorption processes at several values of the initial coverage of the surface for 15(E/ (RTm)g40. A simple method has been also proposed by Konvalinka and Scholten (31) for the kinetic analysis from a single TPD-curve, by applying to these curves the method originally developed by Kissinger (32) to determine the reaction order of thermal decompositions of solids, from the shape-index, S, of the curve, defined as the ratio between the slopes of the tangents to the TPD curve at the inflection points (see Fig. 6.9), and related to the desorption order by the relationship: n = 1.26
S1/2
(6.29)
assuming a ratio of 1.08 between the temperatures at the inflection points. However this method has been criticised by Criado et al. (33,341 as the free readsorption assumed by Konvalinka et al. has no counterpart in the irreversible decomposition of the solid, where the shape index method does apply
-
method has been assessed by Criado et al. (34) in connection with its use in TPD kinetic analysis, and results indicate that the shape index of a TPD peak, when a linear temperature programme is used, always depends on E/(RT) and on the ratio between the inflection points temperatures (T2/T1), while when readsorption of the desorbed species takes place, dependence on the initial coverage, Oil is also observed. Fig. 6.10 collects S1 values ( S = S1(T2/T1) 4, for different desorption kinetics vs. the value of E/(RT). It is possible to conclude that the analysis of the shape index of single TPD peaks might represent an easy and quick procedure for determining kinetics of desorption, although a detailed study may be required to examine the influence of the initial coverage on the shape index to definetively decide if the reaction takes place with or without readsorption of desorbed species. "hc
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S
=
A/B
Fig. 6.9. Shape Index ( S ) and S1 parameter of a TPD curve. ( T and T2: temperatures at the inhlection points)
600
Fig. 6.10. Calculated S1 values for several desorption kinetics at two values of the initial coverage of the surface (6J.). (ref. 34) 1
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One parameter analysis of several TPD curves The analysis method based on the change in the position of the maximum with the heating rate, initially proposed by Redhead (28) and then developed by Lord and Kittelberger ( 3 5 ) for flash desorption experiments, was applied by Amenomiya and Cvetanovic ( 4 ) in TPD experiments as well. The method is based on the use of Eq. 6.19 in logarithmic form: 6.3.2.2
2 In Tm
-
In j3 = E/(RTm)
+
In Z.A
(6.30)
where 2 is a constant that depends on the desorption kinetics. This method only requires several TPD profiles at different l/Tm should heating rates, and a plot of [2 In Tm - In p] vs. yield a straight line with a slope -E/R. So, the activation energy of the process is obtained, without assuming any desorption kinetics, although the method does not give any information about this desorption kinetics or the value of the Arrhenius preexponential factor. This method of analysis is simple, and suitable for TPD profiles with multiple peaks that overlap in some extent, and it has been applied extensively in the determination of activation energies from TPD experiments. Nevertheless, it has been criticised in the literature ( 3 6 , 3 7 1 because of the limited dependence of Tm with f3, although Yakerson et al. ( 3 6 ) have concluded that this dependence can be improved by using heating rates in the range 2-20 K/min. Morec*~~L', Hucul et al. (37) have pointed out a lack of accuracy in the determination of Tm as the main failures source of the method, so that this authors claim that a careful design of the experimental system avoiding long time-lags between the desorption cell and the detector, and using fast response thermocouples embedded in the catalyst bed could avoid such meaningless use of this analysis method. The use of the lower range of heating rates ( 2 - 2 0 K/min) and well designed systems leads to E and A values which coincide within reasonable ranges with those determined by another techniques (flow reactors, or direct determination of the surface concentrations by quantitative i.r. measurements) as shown in Table 6.6.
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TABLE 6.6
Kinetic parameters for the oxidation of propylene and the dehydra t . i o n of alcohols as determined by TPD and isothermal measurements Adsorbed species propylene ethanol 2-propanol t-butanol
TPD
Isothermal A( s-l) 1.0x10; 1.0XlO6 4.8x104 2.0xlO
E(kcal/mol) 10
18.9 22.3 13.6
E (kcal/mol)
12 18.4 22.2 12.9
A(s
-1
)
ref.
1 . 2 ~ 1 0 4 3 37a,b
1 . 4 ~ 1 0 ~7 4 . 7 ~ 1 0 ~7 9.8~10 7
6.3.2.3 Whole line-shape analysis of a single TPD curve Analysis of the curve shape, prodigally used in flash desorption, has been scarcely applied in TPD, and these line-shape analyses are usually suitable only for well resolved peaks. Line-shape analysis methods try to obtain kinetic information from the dependence between desorption rate, surface coverage and temperature, since a single TPD profile contains infinite data of desorption rate vs. temperature. Provided that the initial surface coverage is known, the area under the curve between any temperature and the end of the desorption process is related to the surface coverage at this temperature. Table 6.7 summarizes line-shape analysis methods proposed in the literature that employ a single TPD profile, together with the information that could be obtained by applying them. A general characteristic of these methods is that they are only strictly suitable for desorption processes from homogeneous surfaces, being E and A constant throughout the whole desorption process. Since rate of desorption, temperature and coverage are related through Eqs. 6.13, the following relationship should apply: In [(-de/dt) /f (el]
=
In A
-
(El ( R T ) )
(6.31)
Therefore, a plot of ln[(-de/dt)/f(Q)] vs. 1/T should yield a straight line with a slope -E/R and an intercept InA, when the adequate f(e) function is used. The method (hereafter differential method) has been applied (38) to distinguish between first and second order desorption processes, by using
B356 TABLE
6.7
Line-shape kinetic analysis of a single TPD curve Method
Procedure
Information
Differential
Linearize ln((-d8/dt)/f(e)] vs. 1/T with trial f(B)
E,A for each kinetics f(8)
Integral
Linearize In g(8) - 21nT vs. 1/T with trial g(0)
E,A for each kinetics g ( B )
Normalized curves
Plot (d@/dt)/(de/dt) vs. T/Tm. Compare with cgmputer simulated curves for each kinetics and E/(RTm) (Fig.6.11)
E and kinetics
Reduced Rates
Plot (T/ToI2. (de/dt)/(de/dt) vs. B/B0. Compare with the0-O retical curves for each kinetics (Fig.6.12)
Kinetics
Freemann and Carrol's method
Plot Aln(-de/dt)/ 41n0 A(~/T)/P Lne
apparent desorption order (kinetics), E
~
-
vs.
TABLE 6.8 Application of the differential method: Test-Curve: first order desorption without readsorption (E = 24 kcal/mol, A = 108 s-',
p
= 2 0 K/min)
Trial f(e)
E(Kcal/mol)
e
24.0
2s
e2
36.0
e2/ ( 1-81
r L
1w
2R2
A 6 5 l.Oxl0
8
1 .oooo
13 2.7~10
0.9787
45.2
16 2.3~10
0.9983
78.2
1.5~10
30
0.9996
B357
trial functions f(e) = e and f(e) = Q2. The trial function giving a straight line describes the kinetics of desorption, and the parameters E and A are obtained from the slope and intercept of the straight line. Unfortunately, when free readsorption kinetics are considered, the method is ambiguous, and several f(e) functions lead to straight lines when the differential plot is done. As an example, Table 6.8 includes the values obtained for E, A and the linear correlation coefficient when a computer simulated first-order TPD-profile is analyzed by using Eq. 6.31 and f(e) functions corresponding to first and second order desorption with and without freely occurring readsorption. Data in Table 6.8 show that free readsorption kinetics (1R1, 2R2) yield straight lines as well as the "right" first order kinetics without readsorption, and the method itself is unable to distinguish the true set of kinetic parameters. Similar ambiguities result when the integral Eq. 6.18 is 2 In T] vs. employed, assuming h(x)-l. The plot of [ln g(e) 1/T also yields straight lines for g(0) functions corresponding to different desorption kinetics, and an unambiguous determination of E, A and g(8) is not possible. Another methods use combinations of the integral and differential equations. Amenomiya et al. ( 4 ) already used a line-shape analysis method to determine both the kinetics of the desorption process and the Arrhenius kinetic parameters. They compared an experimental TPD-curve, previously normalized for Tm and the height of the peak at its maximum, with master curves calculated for first order desorption kinetics from homogeneous surfaces, and for several E/ (RT,) ratios. Konvalinka et al. (39) extended this method to second order desorption processes with free readsorption, as according to these authors, only free-readsorption experimental conditions should be expected in flow temperature programmed desorption experiments. General expressions for such normalized TPD-curves from homogeneous surfaces have been given (40) in a more recent paper, and their solutions for first and second order kinetics are plotted in Fig. 6.11. An alternative method has been proposed by Criado (41) to analyze the shape of TPD curves, in order to overcome some difficulties existing in the master curves developed by Amenomiya et al. ( 4 1 , too close each other in some cases, so
-
B358 11.0
~
c
11.0
1 N
~
I
0.8
0.9
1.0
1.1
1.2
lN
TN
1.0
.o
0.8
0
1.2
1.4 l N
0.6
0.8
1.2
1.0
1.4
l N
Fig. 6.11. Normalized peak-shapes for different desorption k i n e t i c s ( C n = C/C,; Tn= T/T,). (refs. 4, 39, 40)
B359
that the experimental curve can be mistakenly fitted to two or more master curves, thus leading to some doubtfulness in the kinetics of the desorption process and subsequently in the calculated Arrhenius parameters. From Eqs. 6 . 1 3 and 6 . 1 6 : -g(0)
= l / f (0)
. (d0/dt).RT2/ (PE),h(x)
(6.32)
expression that for a particular value of 0 = e 0 : 2
-g(0,)
=
l / f ( e o ) . (d@/dt)o.RTo / (PE) h(xo)
where To and (d0/dtIo are the temperature and the desorption when 8 = B 0 . From Eqs. 6 . 3 2 and 6 . 3 3 a rate" can be defined as:
(6.33)
rate of I' reduced
(6.34)
Since for values of x = E/ (RT))15, h (x) is nearly constant, when this condition is fulfilled h(x) /h(xo) can be taken as unity, and the reduced rate would be only dependent on the desorption kinetics and the B0 value. Therefore, a plot of Vr vs. 0/0, should provide a "master curve" characteristic of each desorption kinetics, and independent of the E, A and f3 values for the desorption process. According to this method, the experimental TPD profiles are reduced by plotting the 'reduced rate' (Vr = (T/,To) 2 where the (de/dt)/ (dQ/dt)o) vs the relative coverage 0/0,, reference values (0, and (d0/dt),) are arbitrarily chosen for B0 = 0.5. In doing so, the shapes of the resulting RR-curves, Fig. 6 . 1 2 , only depend on the kinetics of the process, thus allowing to discriminate the actual desorption kinetics when the initial coverage is Bi = 1. Once the kinetic law has been established, both the activation energy and the preexponential factor of Arrhenius can be determined by the differential method, from the plot of [In{ (-de/dt)/f(e)] vs. 1/T. As an example the analysis of a computer simulated TPD curve is included in Fig. 6 . 1 2 . The application to TPD curves of the Freeman and Carroll's ( 4 2 , 4 3 1 for determining method, used in the literature
RR CURVES ( d e f i n e s k i n e t i c s )
1.0 0.8
0.6 0.4 0.2
I Y
u
-5
E
rl
-7
Fig. G.12. Analysis of a TPD curve b y a combination of t h e R H and differential methods. A plot of VR vs. 8/8, defines t h e desorption kinetics ( 1 W in t h e e x a m p l e ) . Once t h e kjnetics is known, f ( 8 ) 1 s selected ( f ( B ) = 8 in t h e example). A plot of ln{(-de/dt)/f(e)l vs. 1/T yields t h e value of E .
B361
simultaneously both the activation energy and the "order" of solid state reactions fulfilling a "n order" kinetic law, has been recently proposed ( 4 4 ) . The main advantage of this method is that the experimental data of the TPD profile are transformed into a straight line with a slope (-E/R), where E is the activation energy of the process, and an intercept 2 characteristic of the reaction kinetics. Under no-readsorption experimental conditions, and for a n-order desorption kinetics Eq. 6 . 1 3 can be written in the form: (-dQ/dt) = A exp (-E/RT) 8"
(6.35)
By differentiating the logarithmic form of Eq. 6 . 3 5 respect to lne, we obtain:
with
(6.36)
Therefore the plots of the left-hand side of Eq. 6 . 3 6 vs. d(l/T)/dln€I should yield a straight line with an slope (-E/R) and an intercept equal to the desorption order 2. It has been shown ( 4 4 ) that under readsorption conditions Eq. 6 . 3 6 is also fitted for a pseudo desorption order 2. Therefore, the plot of [Aln (-de/dt)/ Ah@] values, as calculated from a TPD curve, vs. [A(l/T)/ A In@] leads to a straight line whose intercept is characteristic of the kinetic model fitted by the desorption process, and the slope yields an apparent activation energy El. Once the kinetic model has been established from the value of p, the actual E value can be calculated from the apparent one through the relationship E'/E. The values of the apparent order (n) and the ratios E'/E are summarized in Table 6 . 9 . TABLE 6 . 9
Analysis of TPD curves by the Freeman and Carroll's method (ref.44) Apparent order of desorption and ratios between the apparent (El) and true (E) desorwtion eneraies. ~~
kinetics 1w 1R1 2w 2R2
apparent order 1.000 0.738
E'/E
2.000
1.000 0.480 1.000
1.128
0.306
B362
6.3.2.4 Whole line-shape analysis of several TPD curves The method (2) applies eqn 6.13 to several TPD curves obtained at different heating rates or different initial coverages of the surface. The application to TPD curves from homogeneous surfaces allows to determine both, the desorption kinetics and E, while for desorption processes from heterogeneous surfaces the function E(B) can be determined. For n-order desorption processes without readsorption f(B) = 8" and Eq. 6.13 can be written in logarithmic form: In(-dB/dt) = 1nA
-
E/ (RT)
+
nine
(6.37)
Following this relationship, for the TPD curves recorded under different experimental conditions the plots of the values of ln(-dQ/dt) vs. 1nB obtained at constant temperatures (desorption isotherms), will yield straight lines with a constant slope 2 (desorption order). Changes in the desorption order determined at different temperatures indicate either a significant readsorption over the sample or a dependence of E and/or A on the coverage. When the desorption order is constant, the at each temperature. intercepts stand for [lnA - E/(RT)] Therefore, a plot of the intercept vs. 1/T should yield a straight line that allows to determine the values of E and A. Alternatively, the plots of ln(-d@/dt) vs. 1/T at constant coverages (desorption isochores) will yield straight lines with a constant slope (-E/R) for homogeneous surfaces. An example of this type of analysis for TPD curves from homogeneous surfaces is shown in Fig.6.13. In some cases (45,46) these methods have been also applied to desorption processes with freely occurring readsorption by using the relation:
A plot of In(-de/dt) vs. ln[Q/(l-B)] at several constant temperatures (desorption isotherms) will yield straight lines with a constant slope m. The value of E can be obtained as in the no-readsorption case.
1
&&?& I
e
!\ C W
c
d
\
CD
in
e
d-
r i g . 6 . 1 3 . K i n e t i c a n a l y s i s o f d fani Sy of TPD c u r v e s a t s e v e r a l i n i t i a i coveriges . Desorpt i 011.-)rd"I : n ) i s d t - t p r c i i n e d a t s e v e r a l t c x p e r a t u r - e s ( i s o t h e r : i a l p l o t s ) . I f n i s c o n s t a i - , t cvci tide w h o l e !-enpcr.3ture r a n g e , a c t i v n t - i o n e n e l - g y c a n be o b t a i n e d f r o m t h e p l o t of t h e j . n t e r c e p t vs. 1/T.
B364
6.3.2.5 Effect of sample readsorption on the shape of TPD-curves As stated above, two limiting cases are generally considered when analysing TPD-curves, i.e. the noreadsorption limit and the adsorption-desorption equilibrium limit (free readsorption). When analyzing TPD curves from single crystals in high vacuum conditions, it is usually assumed that readsorption processes are negligible (2). On the contrary, several authors claim (14.47) that readsorption is always important in desorption from porous catalysts, either under vacuum or flow conditions, and thereafter the free readsorption approximation should always be considered. In such sense Herz et a1 (47) studied adsorption effects during TPD of CO from Platinum over porous supports, and they considered desorption into a vacuum and into a Helium carrier gas by assuming that the gas phase in the desorption cell is well mixed. Their results indicate that adsorption effects during CO TPD from Pt dispersed over porous supports are always important, leading to experimental conditions near to the adsorption-desorption equilibrium. At low €low-rates (desorption in a carrier gas) the accumulation of gaseous CO in the sample controls the results, while at high flow rates (desorption in a vacuum) diffusional limitations due to The accumulation of CO within the sample become controlling. results show that it is only possible to determine the heat of adsorption of CO over such a system by using TPD techniques. Gorte (14) obtain similar conclusions in a more general study of this phenomenon. However, the real case should lie between these two ones, with partial readsorption from the desorbed gas phase, without reaching the equilibrium conditions assumed in free readsorption. The effect of this partial readsorption on the shape of the curve and the kinetic and/or thermodynamic parameters of the adsorption/desorption process that can be calculated from such an analysis has been studied in vacuum experimental conditions by several authors ( 4 8 - 5 0 ) .
The effect of these intermediate situations in flow systems is similar and could be studied by solving Eq. 6.10. Assuming an un-activated readsorption (Ea = 0 ) , ka can be taken as a constant; in such a case:
B365 (6.39)
with
being fa and fd the 0 functions for the desorption and readsorption processes, respectively. Equation 6.39 has been solved for first order adsorption-desorption processes (E = 24 kcal/mol, A/P = 108s- 1 I ei=l, 1 0 0 0 ~ F ' > 0 . 0 0 1 ) . TPD profiles thus obtained have been plotted in Fig. 6.14Al where it can be seen that for F'2100 the curve remains unchanged and coincides with the no-readsorption limit, while f o r F'd100 it broadens and shifts to higher temperatures. In order to determine the influence of partial readsorption in the results of the kinetic analysis of TPD curves, the desorption kinetics for curves in Fig. 6.14A have been calculated using the reduced rate (RR) method. Results are shown in Fig. 6.14B and it can be concluded that for F'30.2 a first order desorption without readsorption (1W kinetics) is
Fig. 6.14. Effect of the progressive extent of readsorption in the s h a p e of TPD curves. A ) Computer simula ed curves for different valucs of I" (E = 24 kcal/rnol, A@ = ~O'S-'): B ) Kinetic analysis by the RX method: 1W kinetics is determined for F ' 2 0.2: 1R1 kinetics is determined for F ' , <
-.? ., ... .l'I?
fmpd,
while
f o r F ' , < 0 . 0 1 a first order desorption
with
first
fcrcler readsorption kinetics 1R1 would be determined, these zes:,lt-; being qeneral- whirh?ver the kinetic narameters ?i-i..--iously assumed for t h e desorption process. Once the kinetics of the process is kncvq by the Kii-method, the kinetic parameters can be determined by the differential method. As shown in Table 6.10 for F ' > , 1 0 1W) and r"d0.01 (1R1), E values coincide with the actual Ones within 2 % , while for O.l(F',
- 21nTm
= In
RAf'(Bm)/E
For first order readsorption:
-
(6.40)
E/(RT,)
desorption
with
partial,
un-activated
So, if readsorption is ignored, the actual value of E is calculated, while A depends on f'(Qm) and, subsequently, on F'. Comparison of the kinetic parameters obtained by both the variable-P and the differential methods show a lack of aqreement if partial readsorption takes place, and this fact c c u l d be used to check the existence of such phenomenon at some qi1:en experimental conditions.
kinetics 1w 1R1
10-1 -
24.0
25.0
14.4 28.6
1 20.1
-
10 23.5
-
100 24.0
-
B367
SURFACE HETEROGENEITY The main simplification introduced in the methods of analysis previously described is the homogeneity of adsorption sites. This assumption suppose that Arrhenius parameters are constant at all the coverages of the surface. Nevertheless, it has been shown that actual surfaces are heterogeneous, in some cases even when adsorbing on single crystal planes, and specially when using microcrystalline adsorbents. This heterogeneity can be due to an intrinsic change in the nature of the adsorption sites, or to lateral interactions between the molecules of the adsorbate, and leads to changes in the values of E and/or A with the coverage and, therefore, during TPD experiments. Two problems arise when considering the possibility of heterogeneity of the surface. The first question is whether the methods of kinetic analysis for homogeneous surfaces described in section 6 . 3 . 2 are able to distinguish the existence of such heterogeneity, since the fit of a TPD curve from an heterogeneous surface to the equations corresponding to desorption from an homogeneous surface could yield unreliable kinetics parameters. On the other hand, the TPD profile contains information about the dependence between the kinetic parameters and the coverage of the surface, and some methods of analysis have been proposed in the literature in order to obtain this information. 6.4.
6.4.1. Temperature and coverage dependence of the pre-exponential factor Several authors (20,51) consider the temperature dependence of the preexponential factor, A, according to the expression: A=Ao Tb
(6.42)
where b is any real exponent that may range between 0 and 2.5 (52,531 depending on the mobility of the adsorbed molecule on conclude that in the the surface. These authors ( 2 0 , 5 1 ) non-readsorption limit the changes of the width and shape of the TPD profiles with this effect are practically negligible, and therefore it would not be possible to obtain information about this dependence from the experimental TPD curves.
B368
However, of
it can be f o r g o t t e n w i t h o u t a f f e c t i n g t h e
the
kinetic
Rasser
desorption
p a r a m e t e r s o b t a i n e d from
also
(20)
analyzed
processes
concludes
with
the effect freely
the
in
analysis
for
readsorption
and
f o r t h e h i g h e r v a l u e s o f b (b = 2 ) t h e
that
(51).
profiles
TPD
occuring
reliability
kinetic
a n a l y s i s y i e l d s t h e v a l u e of E w i t h a s i g n i f i c a n t e r r o r 0 1 0 % ) Besides recently
lo3
or
more
surface. this it
this
dependence
reported
between
A and T ,
it
when
measured
at
different
been
factor
coverages
Although some a t t e m p t s have been made o f
dependence
has
( 2 1 ) t h a t A o f t e n d e c r e a s e s by a
of
from t h e a n a l y s i s o f t h e TPD p r o f i l e s
distinguish
between a c o v e r a g e dependence o f
f a c t o r and a c o v e r a g e dependence o f t h e
exponential
the
determining (2,54)
h a s been r e p o r t e d (51,541 t h a t TPD measurements a l o n e
hardly
of
,
could
the
pre-
activation
energy f o r d e s o r p t i o n . Coverage dependence o f t h e d e s o r p t i o n e n e r g y
6.4.2.
s t u d i e s o f TPD p r o f i l e s c o r r e s p o n d i n g t o s u r f a c e s w i t h
The variable
v a l u e s of E a r e u s u a l l y b a s e d on t h e
phenomenological
equation: ( - d @ / d t ) = A ( @ ) . e x p ( - E ( @ ) / ( R T ) )f. ( @ )
(6.431
t r y t o o b t a i n t h e dependence between E and 0 .
and
Usually, t w o
extreme
models a r e c o n s i d e r e d , t h e p a t c h e s model, w i t h
patches
of
continuous the
different, model,
surface
coverage.
to
important
that
get
constant
adsorption
several and
the
suppose a c o n t i n u o u s change of
E
vs.
Within
it
is
maxima
in
t h e former
energy, situation,
t h e b e s t r e s o l u t i o n between
the
order
t o a n a l y z e them a s s i n g l e p e a k s .
in
f3
c o u l d h e l p t o improve t h e r e s o l u t i o n
is
n o t p o s s i b l e t o a c h i e v e a good r e s o l u t i o n , b e s t - f i t programs
should
be
parameters
used
to
for
the
obtain
In t h i s sense,
desorption
individual
changes
(see 6 . 3 . 1 ) .
kinetics
TPD-peaks,
a r i s e when u s i n g t h i s s o r t o f
If
and
kinetic
although analysis
ambiguities
may
When
i s a c o n t i n u o u s change i n t h e e n e r g y o f t h e
there an
sites,
assumed, any King
infinite and
number
of
E
vs. 0
t r i a l f u n c t i o n f o r t h i s dependence. (2)
different
functions
t h e most g e n e r a l a n a l y s i s methods do n o t
some (55). active
could
be
suppose
The method p r o p o s e d by
uses a family of desorption t r a c e s i n i t i a l coverages.
it
corresponding
to
The dependence between (3 and T i s
B369
obtained by integrating the experimental curves (Fig. 6.15). Now desorption isochores (pairs of values (-d@/dt), T at a given 0 ) are obtained. Following Eq. 6.43 the plot of In(-dQ/dt) vs. 1/T should yield a straight line whose slope gives (-E(Q)/R) at the value of the coverage considered. Repeating this plot for different values of 0 the function E(Q),Q can be constructed. The intercepts of the plot stand For n order desorption kinetics (e.9. for [In A ( @ ) + In f ( Q ) ] non-readsorption 1W or 2W kinetics) and assuming that A is independent of the coverage the plot of the intercept vs. h e should yield a straight line with an slope g . If this plot is non-linear A is changing with coverage, and the analysis is not unambiguous (2,51). This method of analysis requires a high experimental resolution and it has been employed in the analysis of desorption processes from single-crystal planes (56). The procedure is illustrated in Fig. 6.15. The necessity of self-consistency of the performed analysis (i.e. the set of kinetic parameters determined by the analysis of the family of experimental TPD curves must be able to generate the line-shape of each particular curve) has been stressed by Soler and Garcia (51). In a few cases, the dependence between E and e has been determined in real catalysts by using Eq. 6.43. Lee and Schwartz (45) studied the interaction of hydrogen with Ni/Si02. They determined the adsorption kinetics by pulse chemisorption, concluding that the activation energy for adsorption is nearly zero, and the adsorption process is second order (fa(@)=(1-@)2). Once the adsorption kinetics had been determined, a set of TPD curves f o r several initial coverages was analyzed concluding that the desorption process of hydrogen from the surface is also second order, the activation energy for desorption decreases from 89 to 55 kJ/mole as the coverage of the surface decreases, and in the experimental conditions employed the free-readsorption limit should be considered. In a similar way, Zowtiak and Bartholomew (46) studied the interaction of hydrogen with cobalt catalysts, concluding that the adsorption process of hydrogen on Co is activated, the heat of adsorption changes with surface coverage, and the free-readsorption limit should also be considered in their experimental conditions.
.
B370
A)
\
TPD CURVES AT SEVERAL
-
I N I T I A L C O V F .3..A.G - -F -S
T/K
I
ARRHENIUS PLOTS
I T2T3T4
T
1/T
Fig. 6 . 1 5 . Kinetic analysis of a family of TPD curves (A) allowing to determine the dependence between activation energy and coverage of the surface: The integral ( 0 , T ) curves ( B ) are obtained from the experimental TPD profiles at several initial coverages; the integral curves allow to obtain the temperature ( T i ) for a given coverage of the surface (8,); now pairs of values (T,-de/dt) at this constant coverage are obtained from the experimental TPD curves. An Arrhenius plot ( C ) yields the activation energy at 8 j and the procedure is repeated for several 8. values. 3
B371 Some
methods
described caFes and
of
more
analysis
restrictive
than
above have been proposed i n t h e l i t e r a t u r e .
a p x r t i..cii.
3.r
those
In these
iin?c:t;on d e s c r i b i n g t h e dependence betinreen E
is p r e v i o u s l y assumed. I n t h i s s e n s e Tokoro e t a 1 non-l.!-near variations of a c t i v a t i o n energy
@
simu1:te
(57)
with
c o v e r a g e assuming t h e r e l a t i o n : PJ
1
E ( G ~=
o(
J
j =1
Where
i and
experimental constant
A
is
order
!i.-.e~j
(6.44)
M a r e i n t a g e r s , and
o(
adjustable parameters.
The
TPD c u r v e i s a n a l y z e d by u s i n g E q . 6.43 f o r and a n-order d e s o r p t i o n k i n e t i c s . The d e s o r p t i o n
supposed, s e v e r a l N v a l u e s (2,(N(13)
the
c o r r e s p o n d i n g A and
the
experimental
curve
are
tested,
and
parameters a r e obtained t o b e s t - f i t j The N by t h e l e a s t s q u a r e s method.
o(
t h a t y i e l d s t o a best f i t of t h e e x p e r i m e n t a l c u r v e ,
its
set
of p a r a m e t e r s , and t h e E ( 0 ) f u n c t i o n o b t a i n e d from Eq.
6.44
are
assumed
value
avoid
t o represent the desorption process.
ambiguities,
analysS.c
-f
heating
zates,
Less
propose
the
to
simultaneous
at
different from
A?.
qeneral the
authors
desorption curves obtained
and a p p l y t h e method t o Oxygen d e s o r p t i o n
ZnO and i w t , r l l i r between
the
several
In order
i s t h e assumption o f a
actjvation
linear
m e r g y of 3 e s n r p t i o n and
relationship the
surface
c o v e r a q s y i v e n by t h e e x p r e s s i o n :
T;ilis
-1
..
hydroq?n influence
. : f
L r f ~ : : ? i3?:-:1->g-:rei"_.i'
-:
3
o f t e n o b s e r v e d i n TPD
?.?.sorbed. on metal s u p p o r t e d o x i d e s u r f a c e s
(58).
of
The
of t h i s t y p e o f f u n c t i o n E ( Q ) i n t h e shape o f t h e TPD
w a s c o n s i d e r e d by Cvetanovic and Amenomiya (59). The c a l c u l a t e d c u r v e s a r e s p r e a d o v e r a wide t e m p e r a t u r e r a n g e and have n o t w e l l d e f i n e d maxima ( F i g . 6.16) , t h e r e f o r e f o r marked dependences between E and Q (changes i n E - 3 0 % o v e r t h e whole coveraqa range) t h e y a r e d i s c i n y u i s n a b l e by v i s u a l i n s p e c t i o n of WD-mrve5 corresponding to an homogeneous s u r f ace. M o r e o w r , Fia. 6 . 1 6 a l s o i n c l u d e s a n example o f l i n e - s h a p e k i n e t i c a n a l y s i s o f t h i s t y p e of c u r v e s assuming s u r f a c e hpmn-..-.nD: -.. ( 2 3 v , o t - X . \ A see s e c t i o n 6 . 3 . 2 1 , and it c a n be . .> .. - , . ?.-. . . z e o f h e t e r o g e n e i t y (E profil.-o
2
>:..
changes
o a
A)
M =
ti-=
2.5 10
1,
0,
0. 700
0.4
0.8
1.2
1.6
T/K Fig. 6.16. TPD curves from heterogeneous surfaces following a linear dependence between the activation energy of desorption and the coverage ( E = O + M ( 1 - e ) ) : A) Simulated profiles for E o = 30 kcal/mol, A/p = 10E2K-1 , 8i= 1 and several degrees of heterogeneity ( d ) ; B ) Kinetic analysis by the Reduced Rates method showing that these curves are easily distinguished from those corresponding to desorption from homogeneous surfaces (lW, 2W).
independent of the initial coverage (ei) of the surface (30,541. The dependence between E and Bi as determined from a family of TPD curves obtained at different initial coverages is employed to discriminate the existence of a linear dependence between desorption energy and surface coverage. Therefore, and although first and second order desorption kinetics are considered, the actual values of the kinetic parameters cannot be obtained in an unambiguous way from such sort of analysis. Yakerson et al. (36) have studied another types of distributions of surface sites, including an exponential one that can be represented by the relationship: E
= EO
-
Sine
(6.46)
An easy method to analyze TPD-curves corresponding to this logarithmic relationship between E and 8 has been reported (61). In this case, for first order desorption kinetics without readsorption the desorption rate (Eq. 6.43) can be expressed as : (-d0/dt) = A exp(-Eo/ (RT)) O(l+S/(RT))
(6.47)
The analysis uses the reduced rate and differential methods (see section 6.3.2) having regard to the fact that such TPD curves (Fig.6.17) are similar to those corresponding to desorption processes from homogeneous surfaces following a n-order kinetics of the form: (-dQ/dt) = A.exp(-EO/(RT)). 8"
(6.48)
where f! is larger than one, since the TPD curve spreads over a narrow temperature range and S/(RT) is nearly constant. In fact, the line-shape kinetic analysis of TPD-curves in Fig. 6.17A using the reduced rate method (Fig.6.17 B) shows a very good fitting of the data to 'desorption pseudo-orders' higher than one, being n-l+S/(RP) (P is the average temperature of the TPD-curve). This allows the evaluation of Arrhenius parameters (Eoand A) by the differential method applying the equation: In
[ (-d8/dt)/Qn] =
InA
-
Eo/(RT)
(6.49)
B374
"I?
1.0
0,6
0.2
600
400
800
T/K
F i g . 6 . 1 7 . TPD c u r v e s from h e t e r o g e n e o u s s u r f a c e s f o l l . m * i n r ; c l o g a r i t h m i c dependence aetwceii t-he a c t i v a t i c n energ!? cf .I, S C J - . : . and t h e c o v e r a g e ( E = EO- q l n e l : A ) S i m u l a t e d p T - o f i 1 0 , : TL - . 6 - 3 -
k c a l / m o l , A/P = 1 O l 2 K - l , 6;.= 1 and s e v e r a l d e g r e e s of : : e t c , r c q - n s l a . , i d ) ; B ) K i n e t i c a n a l y s i s b y t n e Reduced H a t e s rnethoci, s t l c ~ ~ ~ it.h: nq t h e s e TPD p r o f i l e s f i t t L dc?soxi:t..cn order:; ( r ' r hig?if>T: +PA;? w e . ( l w , n = l ; 2 W , n = 2 ; 3W, n = 3 ; 5 W , ri=S)
taking
for
previously
n
the
value
calculated.
corresponding Although
curves the
pseudo-order
could
illustrated
in
Fig.
of
obtain+
be
s e v e r a l i n i t i a l c o v e r a g e s i s recommendea
at
self-consistency
way
$ and E"
the
=1, t h e u s e of a tan,iIr
a s i n g l e TPD p r o f i l e f o r Bi
from
to
the
analysis.
to
6. 1 8 , and E ( Q i ) v a l u e s O b t a d E d
from s i m u l a t e d TPD cur'ves w i t h d i r l e r e n t i n i t A d l r a t h e r w e l l w i t h t h o s e deduced
when
v a l u e s o f Eo and
using
the
This
B i = 1.
method
water
to
(61)
c
tliis
LA
coverages
from
4 dedoced from t h e
h a s been a p p l i e d
ensure
n~i.t~io.3-
T'he
(Qisl) c o i n c i d e
I'?-
_ &
Eq.
6.Ct
CUJ?
d t
experinencal
TPD-curves
of
desorption
e n e r g i e s f o r t h e s u r f a c e between 15-L0 k ~ 1 , ' m o l an-
32-14
kcal/mol,
and ammonia from A 1 2 b 3
respectively,
(611,
i n the surface
lea2oS
coverage
tc
range
0.01<9<1.00.
Finally, TPD
curves
numerical of
the
it s h o u l d be n o t i c e d t h a t i n some c a s e s from h e t e r o g e n e o u s s u r f a c e s haJe bee;,
methods which +.ry t o di scriiiTina intrinsic
he
aeneiLl
-0
h*+* r-
'
:b2-64)
b;/
ial; z r ?
-'
-.
,
- -* .
1.
-a
A)
0.50
G.25
0.75
I..!.' 0
400
c)
600
T/K
*O0
n)
REDUCED RATES METHOD .. ...
DIFFERENTIAL METHOD _ _ _ _ _ _ _ ~ _ _ . _
determines E ( B . I
I
"r 1.0-
0.6-
0.2
4
I
1
I
0.4
0.8
1.2
I
I
I
I
.-._
1.6
e/oc Fig. 6.18. Kinetic analysis of simulated TPD curves (A) from it! heterogeneous surface following ?I 1 ogarithmic dependence bet./::.: ... E and B ( E = EO- QlnB; in this e x m p l e E o = 30 kcal/mol, 3 = 5 ; . The analysis is applied separated> to each TPD curve and gi-ccs two results: the 'reduced rates' method ( C ) determines the A C T - . ; tion pseudo-order E; the differential method with f ( 0 ) = en, (P! determines the activation energy at the initial coverage E ( B , ) . The analysis of curves at sever&-. € 1 ensures self-consistency ~ . c i i the same is determined at each B i , 2nd the values of E ( B j ! ,'I in agreement with the function obtained from Eo (0i=l) and (3 : nR'?). I n the example from ( C ) n = 3 , therefore 9 = 5 since i'-- >u31..; from ( D ) the values of E(Bi) plcl:ei. as points in ( B ) are oi
B376
lateral adsorbate-adsorbate interactions. In this sense, Sales and Zgrablich ( 6 4 ) suppose two dimensional square lattices and simulate desorption by a Monte-Carlo method, concluding that desorption spectra can be strongly affected by energy heterogeneity, site structure and adsorbate-adsorbate interactions. Following these authors, attractive adsorbateadsorbate interactions can mask heterogeneity by mixing-up different peaks, while repulsive interactions can split desorption peaks, due to the formation of intermediate ordered phases. The method has been applied to TPD of CO fron MgO ( 6 4 ) and the distribution of the adsorption sites is obtained by fitting the experimental curves by a trial and error procedure. DIFFUSION CONTROL In the study of high surface porous samples by TPD techniques, two phenomena related to the own nature of the samples should be considered. First at all, the high surface area of the samples could favour readsorption of the desorbed species. This phenomenon has been considered previously. On the other hand, the existence of micro or mesopores can lead to desorption conditions controlled by the diffusion of desorhed species from the pores to the outer surface of the samples. In order to study the influence of such phenomena in TPD profiles, several models have been proposed in the literature. Cvetanovic and Amenomiya ( 4 ) used a simple model assuming Knudsen diffusion control in a thin oxide layer with cylindrical pores opened by both sides. These authors conclurltd that TPD curves arising from this diffusion control are not distinguishable of TPD curves corresponding to f i r s t order desorption without readsorption from an homogeneous surf,ice. Alternatively, Chan and Anderson ( 6 5 ) describe? another model assuming activated diffusion control of the r.r,sorption process. These authors supposed mono and tridimensional 'n a 1D diffusion conditions (1D and 3D, respectively) 6.5
.
process it is possible to obtain the equation: (-dO/dt) = (Do/a2).( n 2 / 4 ) .exp(-E/ (RT) . 0 That is a pseudo-first order desorption rate, being activation energy of the diffusion, Do the diffusivit: the thickness of the catalyst particles.
(6.50)
the and
E
€3377
On the other hand, assuming a 3D process and cubic crystallites, two different solutions are available for the rate expression. In the beginning of the desorption 1 ) 8 ) 0 . 1 7 ) :
equation that gives TPD profiles that cannot be distinguished in practise (66) from those corresponding to a 1R1 desorption process, and is valid for almost all the coverage range. At the end of the process another solution is valid: (-de/dt)
= (3/4fY2).
(Do/a2).exp(-E/ (RT)).e
(6.52)
this expression is also a pseudo-first order equation and only is useful for small coverages (8<0.17) Chan and Anderson (65) apply these equations to TPD profiles of N 2 , Ar and C02 encapsulated in a NaA zeolite, taking into account the particle size distribution, and obtain Do values similar to those reported in the literature. Recently, Fraenkel (67) has studied activated diffusion from uniform spheres (radius ro ) in order to apply the obtained equations to TPD peaks of encapsulated species in zeolites. Under these assumptions, this author obtains the following relationship between Tm and p: 21nTm
-
1 . 9 3 = E/(RT,)
+ In (Ero/TT2RDo)
(6.53)
analogous to the corresponding first-order TPD equation. In order to distinguish between desorption and diffusion, Fraenkel proposes a few supplementary experiments under constant temperat1,re to establish the actual kinetics of the process, since TPP peaks for such a model of 3D diffusion and first order TPD peaks are not distinguishable. 6. 6.
S[TMMARY
TPD experiments are often interpreted only in a qualitative way, by looking at the number of peaks present in the diac,ram and assigning each of them to a different adsorbed species. However, the TPD profile obtained in adequate experim ,;I conditions can give a thorough information about the ki ICS of th2 desorption process, or the type and
4istlribiition of the adsorbed sites. The methods of analysis ?rrFc?erl in the literature have heen reviewed. It should be ~:,.ni:r':-J.that some of these methods tr.1 to simplify the .!!. .. . -., by using selected parameters of a minimum number of .I,,.:
-
:rr;
r:.?aks. These attempts are valuable, but keeping in mind wnen the analysis is oversimplified the ambiguities cannot 1 ; ~ .x;.oided. It is highly recommended to test the x:.f--mnsistency of the analysis by checking that the set of ?.i.rametr?rs obtained is able to generate the curves obtained un?.er different experimental conditions or coverages of the .:crfoce. It is also worth noting here that the extension to d p v m c i o n processes of kinetic analysis methods initially --,-y)r 4 for other temperature programmed techniques (e.g. =rm.?ravimmetry)has shown to be useful. t:i:t
?,>,.
L L
<.''r,C?bL,SDGEMENT rp., author wishes to thank Profs. G. Munuera, J.M. Criado - - -a :', Sives-Arnau for their fruitful help during the ?r,;?r.lzation and redaction of the manuscript at different stqea. Stimulating encouragement of Drs. A. Mufioz-Paez and P. Andren is also gratefully acknowledged.
I
'I
-7
we-exponential factor f;.r desorption (no-readsorption limit) or exp(AS/R). ( F / N c \ (free readsorption limit) thickness of the catalyst ?articles (m) exponent that defines th? dependence between A and the temperature (A=AOT~) gas phase concentration ( 3ol/m3) gas phase concentration at the maximum of the TPD curve (m01/m3) concentration normalized with respect to peak maximum (Cn=C/Cm) diffusivity (m2/s) Ed (non-readsorption ' L i - l i t ) , A H (free-readsorption limit) (J/mol) E at full coverage of the surface (e=1) (J/mol) activation energy for descrption (J/mol) volumetric flow rate of t"2 carrier (m3/s) F/(Noka), dimensionless L'arameter, defines the extent of readsorption
B379
function describing the dependence of the desorption rate on N function describing the dependence of the adsorption-desorption rate on 0 (Table 6.2) first derivative of f(8) with respect to 8 (Table 6.2) function describing the dependence of the desorption rate on 0 function describing the dependence of the adsorption rate on 8 integral function depending on the adsorption-desorption kinetics (Table 6.2) series to evaluate the integral of the Arrhenius equation (I), (Eqs. 6.16, 6.17) integral of the Arrhenius equation I= exp(-E/RT)dT equilibrium constant for desorption
i
desorption rate constant (s-') adsorption rate constant (m3/mol/s ) lenght of the catalytic bed (m) adsorption-desorption order molecules adsorbed on the surface (mol) molecules adsorbed on the surface at the beginning of the TPD experiment (mol) (-dN/dt) desorption rate (mol/s) (-d0/dt) desorption rate (s-l) n desorption order pressure (atm) P R gas constant (J/K/mol) or (atm.l/K/mol) area on the right hand side of the peak maximum for a SR TPD spectrum normalized with respect to peak maximum s pumping speed of the experimental vacuum system (mol/s) S shape index of the TPD peak S1
T Ti Tnl Tn Tl'T2 t V
S.(Tl/T*) absolute temperature ( K ) temperature at the beginning of the TPD experiment ( K ) temperature at the maximum of the TPD profile (K) temperature normalized with respect to peak maximum (Tn=T/Tm) temperatures at the inflection points of the peak (K) time ( s ) volume of the catalytic bed (m3)
B380
'.
reduced rate, (T/To) (dQ/dt)/(d0/dtIo, where To (dQ/dtIo are taken at 8 = 0 . 5 . volume of the experimental vacuum system (m3) width at half height of the TPD peak (K)
r' V
w1/2
and
E/ (RT) degree of heterogeneity (linear heterogeneity) (J/mol) heating rate(K/s) P surface coverage (Q=N/No) e surface coverage at the beginning of the TPD experiment Qi surface coverage at peak maximum em heat of desorption (J/mol) AH AS change of entropy in the desorption process (J/K/mol) $ degree of heterogeneity (logarithmic heterogeneity) (J/mo11 Symbols for desorption kinetics 1w first order desorption without readsorption 2w second order desorption without readsorption 1R1 first order desorption with first order equilibriucl readsorption 2R2 second order desorption with second order equilibriun readsorption X
d
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B383 SUBJECT lNDEX Ac;d-base
r e a c t i o n s , B227
Background f l u o r e s c e n c e , A163
A d i a b a t i c t r a n s i t i o n s , A119
Barium r u t h e n a t e , A363, A365
A d s o r p t i o n , A196
B e e r ' s law, 679
-
amnionia, A196
-
mobile, 8314
Blackbody, 683
-
p y r i d i n e , A196, BZ27-8229
Bohr magneton, 8275
Bismuth molybdate, A363, A364
A e r o s i l , A169
Born-Oppenheimer a p p r o x i m a t i o n , 678
A l c o h o l s d e h y d r a t i o n , B355
B r o n s t e d a c i d s i t e s , A161, 67, 8133, 6226
A1 umi na
-
a d s o r p t i o n o f CO and NO, A192-A193
-
forms, A185
-
a d s o r p t i o n on m e t a l s 8181-8183 a n t i bondi ng o r b i t a l s , A324
h y d r o x y l groups, A186, A196
-
d i s s o c i a t i o n , A323, A324
s u r f a c e c o o r d i n a t i o n , A186, A196
-
e l e c t r o n c o n f i g u r a t i o n , 68, 688, 889
Alumina supported c a t a l y s t s
-
gold, A337-A339
i n f r a r e d bands, 890-692, 8180
Carbon-supported c a t a l y s t s
i r o n - i r i d i u m a l l o y s , A358, A359
A1 1O
-
Carbon monoxide
dehydroxyl a t i o n , A190
,
~ S A265-A274
cobal t - i r i d i u m , A266-A268
-
CoMoS phase, A325, A329, A367-A370 i r o n , A329, A330
Carbonyls, 892
copper-ruthenium,
A265, A266
-
iridium-platinum,
A268-A270
-
molybdenum, 898-8100
-
i r o n , 694-896
copper-osmium, A265
platinum-rhenium, A273, A274 platinum-ruthenium, A271, A272
chromium, 692-694 c o b a l t , 896-698
C h a r a c t e r i s t i c isochromat spectroscopy,
p l a t i n u m - t i n , A273, A274
A49
Anatase, A198
Chemical isomer s h i f t , A307-A309, A312
Angular moment, 8272
Chemical s h i f t , A24, A108, A125, A215, 6205
Anharnionic o s c i 11a t o r , 873
Chemisorption
Appearance p o t e n t i a l spectroscopy, A9, A49 A r r h e n i u s equation, 8344 Atomic a b s o r p t i o n spectroscopy, A6 Auger e l e c t r o n spectroscopy, A9, A51
-
d e p t h p r o f i l e s , A114-A116
-
i n t e n s i t i e s , A105-A107
-
nomenclature, A97
l i n e shapes, A108, A109
-
g r a v i m e t r i c , 613, 814
-
pulse, 618-620
bases, 67 d i s s o c i a t i v e , A171 energy, 83 f l o w , 617, 618 i n f r a r e d spectroscopy, 846, 847, 852
-
selective, 85
p r i n c i p a l e n e r g i e s , A104 p r i n c i p l e , A9, A51, A90, A91
-
v o l u m e t r i c , 812, 813
y i e l d s , A103
Automobile emissions r e d u c t i o n , 889
t i t r a t i o n , 86
Chromia c a t a l y s t s , 848, 849, 8101
-
i n f r a r e d spectroscopy, 8102-8106
B384
-
CO and NO a d s o r p t i o n , 8102-8106
C o b a l t o x i d e c a t a l y s t s , 859
-
-
scanning, A9
i n f r a r e d spectroscopy, 8114, 8115
-
t r a n s m i s s i o n , A9, 824, 825
oxygen t i t r a t i o n , 859
C o o r d i n a t i v e l y u n s a t u r a t e d s i t e s , A161, A190, A331, 854 Copper c a t a l y s t s , 640
E l e c t r o n mean f r e e path, A85, A86, 823
-
u n i v e r s a l curves, A14, A15, A88, A89
E l e c t r o n probe m i c r o a n a l y s i s , A9, A49 E l e c t r o n s p e c t r o s c o p i e s , A85, A161 E l e c t r o p h o r e t i c m i g r a t i o n , 822
oxygen c h e m i s o r p t i o n , 840, 841
E l e c t r o n s p i n resonance, A6, 8265
-
c r y s t a l f i e l d s , 8294
i n f r a r e d spectroscopy, 8127, 8128
-
d e r i v a t i v e s p e c t r a , 8289
NO a d s o r p t i o n , 8127, 8128
-
energy l e v e l s , 8266
-
molybdenum o x i d e s , A30-A33
-
vanadium o x i d e s , A30
Core l e v e l s
-
E l e c t r o n induced n e u t r a l s , A52
hydrogen c h e i n i s o r p t i o n , 840
Copper o x i d e c a t a l y s t s , 849, 850, 8127
-
E l e c t r o n microscopy
c h e m i s o r p t i o n , 853, 8112-8117
C o o r d i n a t i o n number, A259
-
E l e c t r o n d i f f r a c t i o n , A10, A50, A94
l i n e w i d t h and l i f e t i m e , A130
- e q u a t i o n s , A131 Debye temperature, A330
i n s t r u m e n t a t i o n , B269 l i n e - s h a p e , B268
z e o l i t e s , A29
Debye-Waller f a c t o r , A44, A231
E l e c t r o n s p i n r e l a x a t i o n time, A309
D e e x c i t a t i o n processes, A91
E l e c t r o n s p i n echo m o d u l a t i o n , B328
Dehydroxyl a t i o n , A170
E l e c t r o n s t i m u l a t e d d e s o r p t i o n , A10, A53
D e n s i t y o f s t a t e s , A10, A46, A83, 84
E l e c t r o s t a t i c analyzers
Depth a n a l y s i s , A12
-
c y l i n d r i c a l m i r r o r , A94, A95
D e s o r p t i o n energy, 8370-8378
-
s p h e r i c a l , A95
Deuterium exchange, A193
E l e c t r o r t a t i c r e t a r d i n g f i e l d , A94
D i f f u s e r e f l e c t a n c e spectroscopy, A7
EXAFS, A7, A44, A226
D i p o l a r i n t e r a c t i o n s , 8151, 8277
-.a n a l y s i s
D i p o l e moment, B4
-
c r y s t a l s i z e and shape, A249
Doppler v e l o c i t y , A204, A207
-
d e t e c t o r s , A237
-
f o r m u l a t i o n , A231
E l e c t r o m a g n e t i c spectrum, A5 E l e c t r o m a g n e t i c waves, A38
o f s p e c t r a , A238
F o u r i e r t r a n s f o r m s , A239 p a r t i c l e s t r u c t u r e , A252
E l e c t r i c f i e l d g r a d i e n t , A309
E x c i t e d s t a t e s , A302
E l e c t r o n energy l o s s spectroscopy, A9,
E x t i n c t i o n c o e f f i c i e n t , 879, 880
A50, 8145
-
amplitude, 8153
F e l l g e t t ' s advantage, 887
h i g h r e s o l u t i o n , 8179
Fermi l e v e l , A83, A121
i n e l a s t i c i n t e n s i t y , 8154, 8155, 8159
F i scher-Tropsch s y n t h e s i s , A327
o f f - s p e c u l a r i n t e n s i t y , 8157 s e l e c t i o n r u l e s , 8153
- v i b r a t i o n a l modes, B161, 8165
-
s u p p o r t e d Fe-Ru a l l o y s , A345 s e l e c t i v i t i e s , A 3 5 , A346
F l a s h f i l a m e n t , A69
B385
F o u r i e r t r a n s f o r m techniques, A163
-
F r e u n d l i c h isotherms, 845, 851
-
F l i c k e r e f f e c t , A12 Formic a c i d
-
a d s o r p t i o n on Ti02, 8234 decomposition, 8234
F u n c t i o n a l groups, A162
Gamma rays, A45, A300
-
-
-
d e t e c t o r s , A304
Germanes, A178 Growth modes
-
-
AllO
random d e p o s i t i o n , A l l O
G u i n i e r r a d i u s , 821 H a m i l t o n i a n , B150, 8272
-
p h o t o a c o u s t i c , 88, 8133 p h o t o t h e m a l , 886, 887 relfection-absorption,
A35, A36, B84-
886, 8129-8132 r o t a t i o n a l bands, 877 s e l e c t i o n r u l e s , 875, 878 t r a n s m i s s i o n , A35, A162, A164, A177,
-
v i b r a t i o n a l f r e q u e n c i e s , 869, 875
i n s t r u m e n t a l s e n s i t i v i t y , A12
i s l a n d i n g , A113 i s l a n d i n g on monolayer, A l l l , A112 layer-by-layer,
f a r i n f r a r e d , A222
826, 881, 889
Gold c a t a l y s t s , A337
-
c e l l s , 881-887 DRIFTS, A164, A181
e l e c t r o n s p i n , 8272, B278
- h y p e r f ine, 8207
I n v e r s e photoemission, A84 I o n spectroscopies
-
c h a r a c t e r i s t i c s , A10
-
comparison, A64
-
n e u t r a l i z a t i o n , A10 s c a t t e r i n g techniques, A l l , A59
I n t e r a t o m i c d i s t a n c e s , A231,'A254, Iron catalysts
-
b i m e t a l 1i c , A339-A344
-
c a r b i d e s , A318, A323-A325, A356
Harmonic o s c i l l a t o r , 871, 8146
-
CO and NO a d s o r p t i o n , 8106-8111
Hook's law, B69
-
o x i d e s , A321, A324-A327, 850-852
-
n u c l e a r s p i n , 8202
Hydrocarbon a d s o r p t i o n , B186
-
i n f r a r e d spectroscopy, 8106-8111
f r e q u e n c i e s assignment, B188
-
reduced, A325
v i b r a t i o n a l s p e c t r a , B186
-
superparamagnetic, A316
H y d r o d e s u l p h u r i z a t i o n c a t a l y s t s 8367-8369
Isomer s h i f t , A45
Hydrogen a d s o r p t i o n , 8165, 8169-B174
I s o t o p i c l a b e l l i n g , B25, 8230-B232
-
a n g u l a r dependence, 8166
- v i b r a t i o n a l s p e c t r a , 8167
J - c o u p l i n g , 8214
Hydrogen bonded, A168, A181 Hydrogen s e q u e s t e r i n g agents, A176
K-edge a b s o r p t i o n , A228, A267
Hydroxyl groups, A161, A190
K i r c h h o f f ' s law, B83
-
number o f , A194
K n i g h t s h i f t , 8208
-
p a i r e d , A177
Knudsen regime, 815
H y p e r f i n e i n t e r a c t i o n s , A306, A309, A310
K o r r i n g a r e l a t i o n , 8209
I n f r a r e d spectroscopy, 867
I.ambert-Bouguer 1aw, 879
-
band i n t e n s i t i e s , 876
Larmor frequency, 8204
A264
B386
L a t t i c e v i b r a t i o n s , A162, A310 Laser m i c r o p r o b e mass a n a l y s i s , A8 ?EED, 84
Lewis a c i d i t y , 87, 8133
-
ESR, 8327
-
s t r u c t u r e , 855
i n f r a r e d o f NO, 856
Morse f u n c t i o n , 873
L i p p i n c o t t e q u a t i o n , 873, 874 L o w e n s t e i n ' s r u l e , A225 Magic a n g l e s p i n n i n g , A161
-
c r o s s p o l a r i z a t i o n , A174
-
v a r i o u s n u c l e i , A225-A227, 8230
M a g n e t i t e , A318
-
Mossbauer s p e c t r a , A318-A320
Mag n e t iza t ion, 825 Magnesium oxide, A202
-
Neutral s c a t t e r i n g
-
atom beams,
-
m o l e c u l a r beams,
-
h y d r o g e n o l y s i s a c t i v i t y , 839
All
All, A56 - neutrons, All, A55 N i c k e l c a t a l y s t s , B38, 8322-8325 - CO a d s o r p t i o n , 840, B322 - hydrogen a d s o r p t i o n , B39 N i c k e l o x i d e , 842
in f r a r e d , A203-A206
-
c h e m i s o r p t i o n , B42-846, 8117-8121
h y d r o x y l groups, A203
-
d e f f e c t s , B43
supported c a t a l y s t s , A337
-
i n f r a r e d spectroscopy, 8117-8121
s u r f a c e model, 8300
N i t r i c oxide d i n i t r o s y l s , A334, A337, 857, 8321
M e t a l - s u p p o r t i n t e r a c t i o n , A256, A347
-
Microbalances, 816, B17
-
s u r f a c e probe, 8303
Maxwell-Bol tzmann equation, 872 M e t a l - m e t a l i n t e r a c t i o n , A347 Metal -semiconductor i n t e r a c t i o n , A331
Microscopy
-
f i e l d emission, A66 f i e l d i o n i z a t i o n , A66
Mossbauer spectroscopy, A299
-
b a c k s c a t t e r i n g techniques, A305
e l e c t r o n c o n f i g u r a t i o n , B8, 888, 889 i n f r a r e d bands, 856-858, B90 mononi t r o s y l s, A334
N u c l e a r m a g n e t i c resonance, B201, 827-B30
-
broadening, 8220 chemical s h i f t , B205, 8218 energy l e v e l s , 8212 exchange e f f e c t s , B220
- c e l l s , A312
-
h i g h r e s o l u t i o n , 8230-8232
-
c o n v e r s i o n e l e c t r o n s , A305, A306
-
i n t e r a c t i o n s , 8202, B215
magnetic s p l i t t i n g , A306, A309
-
MAS t e c h n i q u e , B220
n u c l e i , A301, A312
-
s e n s i t i v i t y , B216
s e l e c t i o n r u l e , A309
-
t r a n s i t i o n s , 8212
spectrometer, A304
N u c l e a r processes, A306
- t r a n s m i s s i o n , A302 Flol e c u l a r o r b i t a l s , 88
-
CO gas, A142
A144 Molybdena c a t a l y s t s , A362, 853, 8121 - c h e m i s o r p t i o n , B53, 854, 8121-8127 - d e a c t i v a t i o n , A370 CO on inetals,
O p t i c a l t r a n s p a r e n c y , A162 Oxide s u r f a c e s , A161 Overhauser e f f e c t , 8217 Oxygen
-
c o o r d i n a t i v e l y u n s a t u r a t e d , A189, A190 e l e c t r o n c o n f i g r r a t i o n , 88
B387
Oxygen a d s o r p t i o n , 8174
-
- m o l e c u l a r form, -
Quadrupole i n t e r a c t i o n , 8210
metals, 8174-8179
Quadrupole s p l i t t i n g , A45, 1\306,A308
8177-8179
Q u a r t z , A183
s t a t e s , 8175 s u r f a c e r e c o n s t r u c t i o n , 6176
Raman spectroscopy, A6, A39, A162, A171,
A172 P a l l a d i u m c a t a l y s t s , 838
-
hydrogen chernisorption, 638
CO c h e m i s o r p t i o n , 838
Paramagnetic s i t e s , 8233 P a r t i c l e s i z e d i s t r i b u t i o n , A43, 822
-
f l u o r e s c e n c e , A172
-
R a y l e i g h t s c a t t e r i n g , A187 scanning technique, A163 s c a t t e r s , A162 s u r f a c e enhanced, A40-A42
P a u l i n g ' s bond r u l e , A334
Reduced mass, 870
Phonons, A176
R e a c t i v e c o a d s o r p t i o n , 8192, 8193
Phosphines, 8232 P h o t o a c o u s t i c spectroscopy,
Relaxation
A7, A37, A38,
A164, A180
-
s p i n - l a t t i c e , 8268 spin-spin,
8268
Photodesorption, A8
R e s o l v i n g power, A94
Photoemission, A121
Resonance c o u n t e r , A306
-
a n g l e - r e s o l v e d , A149-Al51
Resonant a b s o r p t i o n , A305, A303
-
i n t e r f a c e s t a t e s , A146
gesonant f i e l d , 8270
-
i o n i z a t i o n , A141
Ru-Fe b i m e t a l 1 ics , A339
s o l i d s , A145
Ruthenates, A366
Phototherrnal spectroscopy, A7, A37, A38,
Rutherford backscattering,
A164, 8135 Plasrnons, A87
R u t i l e , A200
P l a t i n u m c a t a l y s t s , 836
-
CO chemisorption, B36, B37
-
ageing, A201
-
h y d r o x y l groups, A200
All, A61-A63
i n f r a r e d bands, A200
hydrogen c h e m i s o r p t i o n , 837 t i t r a t i o n , 637
Scattering d i p o l e , 8147-8150
Potassium doping, A317, A319
-
P o t e n t i a l energy f u n c t i o n , 870
-
resonant, 6147, 8148, 8157, 8158
Poisoning, 6310 Poisson law, A16
P r e - e x p o n e n t i a l f a c t o r , B369
amplitude, 8151 impact, 8147, 8148, 8156
S c h e r r e r equation, A42
-
coverage dependence, 6370
Schrodinger e q u a t i o n , 874
-
temperature dependence, 8369
S i g n a l - t o - n o i s e r a t i o , A306, 8216
Probe molecules, 61, 8292
S i l a n e s , A182-Al93
P r o p s t diagram, A3
S i l a n o l groups, A167-A170, A176, A181
Pyridine adsorption
S i l i c a , A166
B388
-
-
a e r o s i l s , A167 BrGnsted a c i d i t y , A167
-
i n f r a r e d bands, A209 TPD, A209
g e l s , A167, A172
Time o f f l i g h t , A67
hydrophobic, A170
T i t a n i u m c a t a l y s t s , A260-A262, A348, A3E
h y d r o x y l groups, A166
Titanium dioxide
S I M S , A l l , A63, A65
-
S p e c t r o s c o p i c techniques, A2
-
i n f r a r e d s p e c t r a , A97
S p i n e l s , A185
-
l o w e r o x i d a t i o n s t a t e s , A97, 8234
s i l o x a n e b r i d g e s , A166, A171
S i l v e r c a t a l y s t s , 842
a d s o r p t i o n , A199, 8235-8239 c o o r d i n a t i o n s i t e s , A96, A97 c r y s t a l l i n e forms, A96
S t a t i s t i c a l noise, A15
TPD
S t r e t c h i n g modes, A15
-
computer-simulated curves, 8359, B36C
Superoxide r a d i c a l , 8297
-
d i f f u s i o n c o n t r o l , 8378, 8379
Supported m e t a l s , A239-A253
-
e x p e r i m e n t a l systems, 8335-B338
Supported oxides, A279-A289 Supported complexes, A288, A289 Surface
-
c o o r d i n a t i o n , 8315 d e f e c t s , A80, A81
k i n e t i c parameters, 8341 l i n e - s h a p e a n a l y s i s , B357 master curves, 8357 p r o f i l e f a c t o r s , 8338 shape index, B352
-
e l e c t r o n i c s t r u c t u r e , A82
T r a n s i e n t s t u d i e s , 889
phonons, B195
-
T r i d y m i t e , A183
r e c o n s t r u c t i o n , A80, 8170, 8171
T r i m e t h y l g a l l i u m , A77, A79, A80, A94
-
s t a t e s , A84
T u n n e l i n g spectroscopy, A68
s t r u c t u r e , A80, A81
Symmetry, 8279
-
f u n c t i o n s , A68
-
i n f u s i o n s p e c t r a , A68
a x i a l , 8280 i s o t r o p i c , 8280
U l t r a v i o l e t p h o t o e l e c t r o n spectroscopy,
orthorhornbic, 8281-8284
84
p o i n t groups, 8161
S y n c h r o t r o n r a d i a t i o n , A210, A221, A222, A234 Tensor
-
-
d i p o l a r , 8279 g, 8272, 8274
Vacuum m i c r o b a l a n c e , A167 Vanadium o x i d e c a t a l y s t s , E l 0 0
-
CO a d s o r p t i o n , 8100, 8101
ESR, 8311, 8312 i n f r a r e d spectroscopy v i b r a t i o n a l s p e c t r o s c o p i e s , A161
second-rank, 8207-8210
T e t r a c y a h o e t h y l e n e , 8307-8309
Wavelength, A162
Thermal t r a n s p i r a t i o n , 815 Thermal v i b r a t i o n , A308
X-ray p h o t o e l e c t r o n spectroscopy, A182
Thorium o x i d e - h y d r o x y l groups, A209; A210
Zeeman energy l e v e l s , 8266, 8267
B389
Zeolites
-
c a t a l y s t s , A211, A212
-
NMR chemical s h i f t , 6226, B253
-
Xenon a d s o r p t i o n , 6244-8253
- i n f r a r e d and Ranian o f , A211
-
metal exchanged, A277, A278
Zero p o i n t charge, 822 Zeta p o t e n t i a l , 822 Z i n c s u l p h i d e , A165
Zinc oxide
-
i n f r a r e d bands, A211 hydrogen a d s o r p t i o n , A211 h y d r o x y l groups, A210
Z i r c o n i u m o x i d e , A207
-
CO a d s o r p t i o n , A208 i n f r a r e d bands, A207, A208 h y d r i d e , A208 h y d r o x y l groups, A207
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B391
STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisoty Editors: 8.Delmon, Universite Catholique de Louvain, Louvain-la-Neuve,Belgium J.T. Ides, University of Pittsburgh, Pittsburgh, PA, U.S.A.
Volume 1 Preparation of Catalysts I. Scientific Bases for the Preparation of Heterogeneous Cataiysts. Proceedingsof the First International Symposium, Brussels, October 1417.1975 edited by B. 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, wjth 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 6 . 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 Socibtb de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Volume 5 Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9- 1 1, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Volume 6 Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Froment Volume 7 New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Volume 8 Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsov and V.A. Zakharov Volume 9 Physics of Solid Surfaces. Proceedings of a Symposium, Bechyiie, September 29October 3, 1980 edited by M. UzniEka 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. Rouquerol and K.S.W. Sing Volume 1 1 Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an InternationalSymposium, Ecully (Lyon), September 14-1 6,1982 edited by B. 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. JirG and G. Schulz-Ekloff Volume 13 Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Volume 14 Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz
B392 Volume 15 Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Volume 16 Preparation of Catalysts Ill. 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 International Symposium, LyonVilleurbanne, September 12-1 6, 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. Kazansky 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 Yu.Sh. 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, Portoroi-Portorose, September 3-8, 1984 edited by B. DrZaj, S. HoEevar and S. Pejovnik Volume 25 Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Volume 26 Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, 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. Cervenq Volume 28 New Developments in Zeolite Science and Technology. Proceedings of the 7th InternationalZeolite 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 30 Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-1 1, 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 B. 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 3 4 Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1 , 1987 edited by B. Delmon and G.F. Froment
B393 Volume 35 Keynotes in Energy-Related Catalysis edited by S. Kaliaguine Volume 36 Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, 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 40 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 41 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. Perot 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 Requirementsand 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 InternationalSymposium, Wurzburg, September 4-8, 1988 edited by H.G. Karge and J. Weitkamp Volume 47 Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Volume 48 Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-1 6, 1988 edited by C. Morterra, A. Zecchina and G. Costa Volume 49 Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and 6 edited by P.A. Jacobs and R.A. van Santen Volume 50 Hydrotreating Catalysts. Preparation, Characterizationand Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony Volume 5 1 New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Volume 52 Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowski and P.J. Barrie Volume 53 Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara
B394 Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura Volume 55 New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, September 18-22, 1989 edited by G.Centi and F. Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25, 1989 edited bv T. Keii and K. Soaa Volume 57A SpectroscopicAnalysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 578 SpectroscopicAnalysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro
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