ACTIVATION OF REFORMING CATALYSTS : REACTIVITY AND KINETIC STUDIES USING EXAFS

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ACTIVATION OF REFORMING CATALYSTS : REACTIVITY AND KINETIC STUDIES USING

EXAFS

H. Dexpert

To cite this version:

H. Dexpert. ACTIVATION OF REFORMING CATALYSTS : REACTIVITY AND KINETIC STUDIES USING EXAFS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-219-C8-226.

�10.1051/jphyscol:1986841�. �jpa-00226163�

ACTIVATION OF REFORMING CATALYSTS : REACTIVITY AND KINETIC STUDIES USING EXAFS

H. DEXPERT

LURE, ~ a b o r a t o i r e CNRS-CEA-MEN, Bgtiment 209D, Universite de Paris-Sud, F-91405 Orsay Cedex, France

RESUME

L'EXAFS e s t devenu un o u t i l p r i v i l Q g i Q pour l T 6 t u d e des c a t a l y s e u r s c o m e en thmoignent l e s nombreux travaux p u b l i e s s u r l e s u j e t . Les Q t u d e s c i n h t i q u e s en c o n d i t i o n s r 6 a c t i o n n e l l e s s o n t tr&s importantes pour mieux c o n n a r t r e e t mod6liser l e s m6canismes auxquels l e s transformations chimiques f o n t appel. On expose i c i l e s p o s s i b i l i t h s a c t u e l l e s dans l e c a s de c a t a l y s e u r s hhtQrog&nes 3 base de p l a t i n e s u r alumine u t i l i s e s en reformage.

ABSTRACT

EXAFS h a s become an usual t o o l i n t h e f i e l d o f c a t a l y s t s c h a r a c t e r i z a t i o n a s shown by t h e numerous published works. K i n e t i c and i n s i t u s t u d i e s a r e e s s e n t i a l t o model t h e mechanisms involved i n t h e v a r i o u s chemical t r a n s f o r m a t i o n s these m a t e r i a l s promote. I l l u s t r a t i o n s of t h e today p o s s i b i l i t i e s i n t h e c a s e of a reforming c a t a l y s t b a s i c a l l y made of platinum on alumina a r e given.

INTRODUCTION

During t h e p a s t decade, t h e u s e f u l n e s s of t h e EXAFS technique t o c h a r a c t e r i z e v a r i o u s c l a s s e s of c a t a l y s t s h a s been proved many t i m e s ( l s 2 ) . In f a c t one of t h e e a r l y papers checking t h e v a l i d i t y of t h e EXAFS formalism was published i n t h i s f i e l d ( 3 ) and s i n c e t h a t time, it h a s become one o f t h e major t o o l owing t o t h e p o s s i b i l i t y of i n s i t u s t u d i e s . Applications a r e wide open from academic t o i n d u s t r i a l m a t e r i a l s s o t h a t synchrotron r a d i a t i o n is no more t h e unique used source a s c a t a l y s t r e s e a r c h e r s a r e very concerned by t h e development of t h e f i r s t i n l a b a p p a r a t ~ s ' ~ ' ~ ) .

Most of t h e works d e a l s with t h e determination of t h e a c t i v e s p e c i e s a t t h e reduced s t e p of t h e p r e p a r a t i o n procedure. Numerous examples can be found i n t h e case of m u l t i m e t a l l i c reforming c a t a l y s t s where almost a l l t h e d i f f e r e n t combinations between two elements of t h e group V I I I have been i n v e s t i g a t e d ( 6 ) . A l l t h e s e e f f o r t s g i v e r i s e t o a g r e a t number of r e s u l t s and a rough e v a l u a t i o n of t h e i n t e r e s t can be drawn from t h e number of papers presented a t t h e s u c c e s s i v e InternaLional EXAFS Conferences

:.

The chemist u s u a l l y c o r r e l a t e s t h e k i n e t i c and r a t e o f a c a t a l y t i c r e a c t i o n t o t h e m e t a l l i c a r e a of t h e c l u s t e r involved i n t h e molecular transformation. In the case of reforming c a t a l y s t s , t h e m a t e r i a l s a r e made of low c o n c e n t r a t i o n s (one weight

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986841

JOURNAL DE PHYSIQUE

$ or l e s s ) of small aggregates (nanometer s c a l e ) supported by l i g h t oxides of high s p e c i f i c a r e a (alumina, s i l i c a , etc... of 200mZg-1 o r more). TO understand t h e i r behaviour, t h e chemist must handle many parameters, t h e determination of which implies t h e knowledge o f i ) t h e chemical composition, i i ) t h e i n t e r n a l s t r u c t u r e , i i i ) t h e e l e c t r o n i c s t a t e , i i i i ) t h e s i z e , the morphology, and f i n a l l y t h e r e l a t i o n s h i p between t h e p a r t i c l e and i t s surrounding.

Most o f t h e s e q u e s t i o n s have been tackled through many EXAFS o r XANES (edges s t u d i e s ) experiments and it is very d i f f i c u l t t o g i v e even an overview of t h e main conclusions. However, a common point is t h a t t h e experimental data a r e g e n e r a l l y c o l l e c t e d a f t e r t h e samples a r e cooled down t o room temperature, t h e measurements being done a f t e r t h e r e a c t i o n took place. Since two y e a r s we combine the f a c i l i t i e s developped a t LURE t o o b t a i n an a b s o r p t i o n s i g n a l using e i t h e r t h e s t e p by s t e p o r t h e d i s p e r s i v e mode. We have t h e r e f o r e been a b l e t o follow a t a time s c a l e of a few seconds t h e e v o l u t i o n of some heterogeneous c a t a l y s t s during t h e a c t i v a t i o n process and under choosen c a t a l y t i c r e a c t i o n s . These goals obliged u s t o c o l l e c t t h e d a t a a t high temperature under v a r i o u s g a s flowings. I f one keeps i n mind t h a t our samples have a t o t a l metal loading o f a few weight per c e n t , t h e o b t e n t i o n of s i g n i f i c a n t r e s u l t s is not obvious. The simple case of a monometallic compound, platinum on alumina, was consequently our s t a r t i n g p o i n t before we extended our i n v e s t i g a t i o n s t o some b i m e t a l l i c couples. S e v e r a l r e a c t i o n s have then been dynamically followed.

Besides t h e i n t e r e s t t o g e t t h e continuous i n s i t u v a r i a t i o n s of t h e a c t i v e element neighbourhood, we a l s o undertook t h e s e experiments t o c o r r e l a t e q u a n t i t a t i v e l y t h e successive e l e c t r o n i c s t a t e s with t h e s t r u c t u r a l e v o l u t i o n of t h e p a r t i c l e . Considerations about t h e shape and growth of t h e c l u s t e r , metal-metal d i s t a n c e s a t high temperature, e x i s t e n c e of i n t e r a c t ions with t h e c a r r i e r , changes i n t h e r e d u c t i o n r a t e with t h e chemical n a t u r e o f t h e s t a r t i n g precursor a r e discussed here. These c h a r a c t e r i z a t i o n s g i v e a more a c c u r a t e image of t h e m a t e r i a l before the

i n t r o d u c t i o n of hydrocarbons f l u x .

THE REDUCED STEP INVESTIGATED FROM DATA TAKEN AT ROOM TEMPERATURE

A s already pointed o u t , such s t u d i e s c o n s t i t u t e t h e main p a r t of t h e works done up t o now. The d a t a a r e c o l l e c t e d a f t e r the r e a c t i o n took p l a c e , when t h e system is thermodynamically s t a b i l i z e d . These post mortem examinations of t h e c a t a l y s t may l e a d t o informations on t h e shape and growth of the m e t a l l i c p a r t i c l e .

Figure l a r e p r e s e n t s r a d i a l d i s t r i b u t i o n f u n c t i o n s o f t h r e e 1.5 t o 2 weight

;b P t on gamma alumina c a t a l y s t s a f t e r reduction. They a r e compared t o t h e platinum f o i l which is used a s a r e f e r e n c e f o r backscattered amplitude and phase s h i f t (curve D ) . A l l t h e c a t a l y s t s a r e highly d i s p e r s e d , issued from t h e same p r e c u r s o r , H,PtCl,, b u t they d i f f e r by t h e treatment they had, 2h a t 450°C (sample A), 5h a t 450°C (sample B ) and 15h a t 200°C (sample C ) . I n each c a s e t h e four s h e l l s can be analysed, t h e Fourier transforms (FT) being c a l c u l a t e d over 500 eV on t h e k 3 x ( k ) s i g n a l .

From t h e comparison o f t h e f i r s t Pt-Pt p a i r s l o c a t e d along t h e (hho) d i r e c t i o n s of t h e f c c s t r u c t u r e , i t comes immediately t h a t t h e p o s i t i o n of t h e FT moduli peaks towards s h o r t e r d i s t a n c e s f o r t h e s m a l l e s t p a r t i c l e s ( c a t a l y s t s A and 0 ) . This displacement is not a r e a l c o n t r a c t i o n : a s these p a r t i c l e s correspond t o a low f i r s t neighbours number (FNN = 6 t o 7 ) , t h e s u r f a c e platinum atoms a r e numerous and coupled t o t h e oxygen of t h e c a r r i e r . Such assumption is drawn from t h e p l o t o f t h e

c a t a l y s t

Log metal r a t i o versus k 2 : a s t h e b a s k c a t t e r i n g amplitude of oxygen becomes

t h e Pt-Pt FNN d i s t a n c e is then c l o s e t o t h e bulk value (2.75 8 ) :

Aa and AE a r e r e s p e c t i v e l y t h e Debye Waller and edge energy d i f f e r e n c e s with t h e v a l u e s they have i n t h e r e f e r e n c e compounds.

When looking now a t t h e upper s h e l l s ( f i g u r e l b ) , it is c l e a r t h a t they a r e not s i m i l a r l y developped from samples A t o D. The t h i r d one i s h i g h e r than t h e f o u r t h f o r c a t a l y s t s A and B , and becomes lower when t h e p a r t i c l e s i z e i n c r e a s e s ( c a t a l y s t C and platinum f o i l D ) . I t is t h u s a t t r a c t i v e t o see i f t h e shape of t h e s e c l u s t e r s can be determined and i f a model of t h e i r growth may be proposed. The r e s u l t s g e t from t h e adjustment procedure a r e a s follow :

f o i l f o r :

Unmeasurable

A few y e a r s ago, Greegor and ~ y t l e ' ~ ) p l o t t e d t h e c o o r d i n a t i o n numbers (CN) of t h e upper s h e l l s versus t h e p a r t i c l e s i z e of s e v e r a l b i m e t a l l i c c a t a l y s t s f o r t h r e e i d e a l s t a c k i n g : a two l a y e r s d i s c (TLD), a cube (C) and a sphere (S). A comparison o f t h e r e s p e c t i v e CN we found with t h e i r c a l c u l a t e d v a l u e s l e a d s t o t h e morphology o f our c l u s t e r s . The t a b l e below is t h a t comparison f o r sample A (small p a r t i c l e s

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averaged diameter : 6 A) and sample C ( l a r g e p a r t i c l e s

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JOURNAL DE PHYSIQUE

The b e s t agreement f o r t h e small c l u s t e r s ( 6 FNN) is a t h r e e dimensional network, s p h e r i c a l o r cubic. The l a r g e ones ( 9 FNN) t a k e a more f l a t t e n e d two dimensional shape.

The f c c cuboctahedral symmetry of t h e 3D c l u s t e r can change t o a planar s t r u c t u r e when t h e FNN is very low. I n 1984 we found t h a t very highly d i s p e r s e d platinum on e t a alumina reduces t o a r a f t (100) plane with 4 neighbours in t h e f i r s t and second s h e l l , none i n t h e t h i r d and fourth('). F i n a l l y , a l l t h e a n a l y s i s we did on v a r i o u s samples lead u s t o conclude t h a t the most reasonable p i c t u r e f o r the growth of t h e p a r t i c l e s is a s followed :

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Using time resolved experiments, t h e k i n e t i c study of t h e d i r e c t reduction, s t a r t i n g from i s o l a t e d Pt cl2; anions, w i l l complete t h i s p i c t u r e .

THE REDUCED STEP INVESTIGATED FROM DATA TAKEN AT H I G H TEMPERATURE

I n s i t u s t u d i e s a r e important a s they provide e s s e n t i a l informations on the r e a l c a t a l y s t and temperature is t h e r e a parameter t o t a k e i n t o account. Even i f t h e s i g n a l becomes weaker due t o thermal v i b r a t i o n s , t h e s e s t e p by s t e p measurements a r e a l s o necessary t o c a l i b r a t e k i n e t i c experiments. A s a matter of f a c t , t h e o b s e r v a t i o n s undertaken i n d i s p e r s i v e mode a r e c o l l e c t e d through a photodiode a r r a y s o t h a t t h e energy is measured by p i x e l s i n s t e a d of eV a s i n conventional EXAFS.

For t h e platinum f o i l used a s a r e f e r e n c e compound, t h e s i g n a l is g r e a t l y decreased ( a f a c t o r 3 between room temperature and 450°C) and FT modulus of t h e P t FNN peaks a t a s h o r t e r d i s t a n c e (0.1 A ) due t o t h e change of t h e b a c k s c a t t e r i n g amplitude with temperature. In these c o n d i t i o n s , t h e f i r s t s h e l l of t h e c a t a l y s t s samples is s o l e l y a n a l y s a b l e and t h e i r FT moduli a r e a t a s h o r t e r d i s t a n c e compare t o t h e bulk metal a t t h e same temperature. The f i r s t question t o s o l v e i s whether o r not t h i s c o n t r a c t i o n is p h y s i c a l : a s t h e p a r t i c l e is small, we a l r e a d y saw a t room temperature t h a t l i g h t elements i n t h e v i c i n i t y of platinum can y i e l d t o t h e same r e s u l t .

To c l e a r t h i s problem, two kinds of r e d u c t i o n a t 450°C f o r lh30 were undertaken. The f i r s t one is d i r e c t from t h e d r i e d s t e p ( f i g u r e 2a) and t h e second is c a r r i e d out a f t e r t h e c a t a l y s t has been c a l c i n e d during two hours a t 520°C ( f i g u r e 2b).

Each f i g u r e r e p o r t s t h e FT moduli of t h e c a t a l y s t , t h e platinum f o i l (curve A ) and e i t h e r H2 ? t C16 (curve B ) or Pt02 (curve C ) . The h e i g h t o f t h e maximum o f t h e r e f e r e n c e +compounds have been a d j u s t e d t o match with t h e corresdonding f e a t u r e s of t h e c a t a l y s t s . On t h e l e f t p a r t of t h e main c o n t r i b u t i o n , t h e FT moduli o f t h e c a t a l y s t s p r e s e n t s i d e lobes which peak very c l o s e t o t h e Pt-C1 p a i r s of H2 P t C 1 i n t h e case of t h e d i r e c t reduction ( a ) and t o t h e Pt-0 bond when t h e c a t a l y s t h a s been previously 6 c a l c i n e d ( b ) . The second i n t e r e s t i n g p o i n t is i n d i c a t e d by t h e arrows which show t h a t t h e metal-metal d i s t a n c e i n t h e c l u s t e r is l e s s than f o r t h e bulk. These observations have been completed by a d i r e c t reduction a t 200°C on a l e s s d i s p e r s e d m a t e r i a l (averaged p a r t i c l e s s i z e about 15 A i n s t e a d of 6 t o 8 it). The c h l o r i n e c o n t r i b u t i o n is i n t h a t case s t r o n g l y decreased and t h e c o l l a p s e of t h e Pt-Pt bond is g r e a t l y reduced.

An a c c u r a t e a n a l y s i s of t h e s e f i r s t s h e l l s g i v e s t h e r e s u l t s l i s t e d below ^:

S i g n i f i c a n t informations a r e thus e x t r a c t e d from high temperature experiments even f o r low loaded c a t a l y s t s . Three main c o n c l u s i o n s can be drawn :

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These e f f e c t s , a s well a s t h e metal-metal d i s t a n c e s c o l l a p s e , a r e a t t e n u a t e d when t h e p a r t i c l e s i z e i n c r e a s e s . They disappear with longer r e d u c t i o n times ;

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THE REDUCED STEP FROM K I N E T I C EXPERIMENTS

The v a r i o u s s t r u c t u r a l and e l e c t r o n i c changes of a c a t a l y s t a r e u s u a l l y well described a t the time s c a l e of t h e s t e p by s t e p mode. D e t a i l s of the r e a c t i o n remain n e v e r t h e l e s s unknown a s t h e chemical process is sometimes too f a s t . In a r e c e n t work we shown the f e a s i b i l i t y of such s t u d i e s using time resolved E X A F S " ~ ' . The technique is a l r e a d y reported i n s e v e r a l papers of Dartyge and Fontaine ( s e e f o r example 1 3 ) and t h e b a s i c p r i n c i p l e s a r e b r i e f l y r e c a l l here.

The experimental s e t up a s s o c i a t e s a d i s p e r s i v e o p t i c s provided by a bent c r y s t a l with a cooled photodiode a r r a y used a s a p o s i t i o n s e n s i t i v e d e t e c t o r t o l o c a l i z e the synchrotron r a d i a t i o n . The X r a y beam is focussed onto the sample by t h e bent c r y s t a l , s o t h a t a wide energy band pass, up t o 600 eV, is r e f l e c t e d without any mechanical movement. The energy-direction c o r r e l a t i o n obtained with t h e d i f f e r e n t Bragg r e f l e c t i o n s is transformed i n an energy-position one o n t o t h e photodiode a r r a y . This s e t up allows t o c o l l e c t a whole spectrum a t once i n l e s s than 10 rns when a m e t a l l i c f o i l is probed. The low c o n c e n t r a t i o n s we consider Ln our c a t a l y s t s i n c r e a s e t h i s time t o a t y p i c a l f i v e t o f i f t e e n seconds per spectrum.

JOURNAL DE PHYSIQUE

I n view o f s t u d y i n g d i f f e r e n t oxydo-reduction o r o x y c h l o r a t i o n c y c l e s we f i r s t p a i d a t t e n t i o n t o t h e e v o l u t i o n o f t h e impregnated p r e c u r s o r under a c o n s t a n t hydrogen p r e s s u r e . We determined t h e hydrogen consumption t h r o u g h two k i n d s o f experiments. The f i r s t one d e f i n e s t h e o p t i m a l t e m p e r a t u r e where t h e r e a c t io&! b e g i n s by c o l l e c t i n g t h e s u c c e s s i v e s p e c t r a with g r e a t e n i n g t e m p e r a t u r e s . The second is t h e r e a l consumption s t u d y where t h e choosen t e m p e r a t u r e is maintained c o n s t a n t d u r i n g t h e d a t a c o l l e c t i o n (200°C f o r a pure p l a t i n u m s a m p l e ) .

The changes o f t h e FT moduli u n c o r r e c t e d from phase s h i f t s a r e shown i n f i g u r e 3a where t h e two r e f e r e n c e compounds a r e H 2 P t C16 ( c u r v e 1 ) and t h e m e t a l ( c u r v e 1 0 ) . Few r e c o r d e d s p e c t r a a r e g i v e n , t h e c o l l e c t i o n c o n d i t i o n s were 4 s p e r spectrum, 15s s e p a r a t e two s u c c e s s i v e s p e c t r a . The c a t a l y s t i s h e a t e d up t o 200°C i n a i r b e f o r e hydrogen i s i n s e r t e d . Curve 9 is t h e end o f t h e e x p e r i m e n t , t e n m i n u t e s a f t e r t h e g a s f l o w reached t h e powder w i t h i n t h e r e a c t i n g c e l l .

Some f a c t s a r e o b v i o u s : i ) t h e FT maxima moves towards l o n g e r d i s t a n c e s (0.16 A from c u r v e 1 t o 9 ) b u t t h e flowing time is no long enough t o s e e s e p a r a t e Pt-Pt bonds. However, p l o t s o f t h e FT imaginary p a r t s confirm t h a t p l a t i n u m c o n t r i b u t i o n is p r e s e n t s i n c e c u r v e 4 ; i i ) a two s p e e d s regime seems t o c o n t r o l t h e r e a c t i o n as t h e FT d e c r e a s e s a r e g r e a t e r a t t h e b e g i n n i n g t h a n a t t h e end.

F i g u r e 3b is t h e c o r r e s p o n d i n g change of t h e normalized L w h i t e l i n e a r e a v e r s u s time. A sudden d e c l i n e happens between spectrum 3 and 4 , o n l y s e p a r a t e d by 3 40 s. T h i s jump is succeeded by a slow d e c r e a s e .

Some o f t h e f i t s we d i d o v e r 500 eV a r e l i s t e d below :

Temperature Time Atmosphere Curve Nature and number o f p l a t i n u m neighbours

2OO0C 0 a i r -t H, 2 4.5 C1 + 0.9 0

2 0 0 9 ~ I m n 1 6 s H, 3 4.0 C1 + 0.2 0

200°C lmn 5 4 s H, 4 2.5 C 1 ⁺ 0.4 P t

2OO0C 4mn 45s H 2 7 1.7 C1 + 0.9 Pt

2OO0C 9mn 49s H, 9 1.3 C1 ⁺ 1.5 P t

A t e n t a t i v e model f o r t h e decomposition of t h e i s o l a t e d P t C 1 2- complexes c o u l d t h e n be :

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2- .

C14 10"s ;

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T h i s example i l l u s t r a t e s how promising a r e time r e s o l v e d experiments.

E x t e n s i o n s t o t h e s t e a d y s t a t e have been i n i t i a t e d and f a i r l y hopes t o d e s c r i b e a p p r o p r i a t e molecular t r a n s f o r m a t i o n s seem r e a s o n a b l e a t t h i s time o f o u r i n v e s t i g a t i o n s .

Made o f heavy elements, forming small domains embedded i n adequate c o n c e n t r a t i o n w i t h i n a q u a s i t r a n s p a r e n t matrix, heterogeneous c a t a l y s t s a r e t h e r e f o r e well s u i t e d f o r c r y s t a l l o g r a p h i c c h a r a c t e r i z a t i o n by X-ray a b s o r p t i o n spectroscopy.

General t r e n d s on t h e i n t e r a t o m i c d i s t a n c e s a l t e r a t i o n , s e g r e g a t i o n o r m i s c i b i l i t y between t h e d i f f e r e n t components, even i n t e r a c t i o n s with t h e c a r r i e r o r adsorbed molecules a r e now e s t a b l i s h e d from t h e EXAFS domain. The edge r e g i o n becomes t o o of a g r e a t i n t e r e s t . XANES p r o v i d e s i n f o r m a t i o n s on t h e e l e c t r o n i c s t a t e s t h e a c t i v e s p e c i e s t a k e d u r i n g t h e r e a c t i o n s o t h a t t h e number o f empty s t a t e s can be determined and l i n k e d with t h e chemical behavlour. Some c o n s t r a i n t s l i m i t t h e a p p l i c a t i o n s , a s t h e smearing of t h e s i g n a l w i t h temperature, but t h e r e is no doubt t h a t t h e f u t u r e of t h e t e c h n i q u e w i l l g i v e an i n c r e a s i n g p a r t t o i n s i t u k i n e t i c experiments done on r e a l m a t e r i a l s .

Acknowledgments a r e due t o many of my c o l l e a g u e s , s p e c i a l l y t h e LURE group working i n EXAFS. The p r e s e n t work could n o t have been e f f e c t i v e without t h e c o l l a b o r a t i o n of o t h e r teams a s t h o s e o f t h e I n s t i t u t F r a n g a i s du P e t r o l e o r t h e L a b o r a t o i r e de C a t a l y s e e t Chimie d e s S u r f a c e s t o whom I am very g r a t e f u l .

REFERENCES

BART, J.C.J. and V L A I C , G., Adv. Cat., (1985) 34

PRINS, R. and KONINGSBERGER, D.E., X-ray Absorption : P r i n c i p l e s , A p p l i c a t i o n s , Techniques of EXAFS, SEXAFS and XANES, J . Wiley, ed., (1986)

LYTLE, F.W., SAYERS, D.E. and MOORE, E.B., J . Appl. Phys. L e t t . , 24 (1974) 45 TOHJI, K., UDAGAWA, Y . , KAWASAKI, T . , and MASUDA, K., Rev. S c i . I n s t . , 54 (1983) 1482

VAN ZON, J.B.A.D., Ph. D. T h e s i s , Eindhoven U n i v e r s i t y of Technology (1984) V I A , G.H., MEITZNER, G., SINFELT, J.M., GREEGOR, R.B. and LYTLE, F.W., EXAFS and Near Edge S t r u c t u r e 111, S p r i n g e r Verlag, ed.. (1984) 176

GREEGOR, R.B., and LYTLE, F.W., J . C a t a l . , 6 3 (1980) 476 LAGARDE, P. and DEXPERT, H., Adv. Phys., 33 (1984) 567

MONTANO. P.A.. SCHULZE, W., TESCHE, B., SHENOY, G.K. and MORRISON, T.I., Phys.

Rev., 30 (1984) 672

( 1 0 ) MOBILIO, S.. B U R A T T I N I , E . , BALERNA, A., B E R N I E R I , E., P I C O Z Z I , P., REALE, A . , and SANTUCCI, S . , EXAFS and Near Edge S t r u c t u r e 111, S p r i n g e r Verlag, ed., (1984) 222

( 1 1 ) MORAWECK, B. and RENOUPREZ, A.J., S u r f . S c i . , 10 (1981) 635

(12) MAIRE, G., G A R I N , F., BERNHARDT, P., G I R A R D , P., SCHMITT, J.L., DARTYGE, E., DEXPERT H., FONTAINE, A . , JUCHA, A. and LAGARDE P., Accepted i n Appl. Cat. (1 986) (13) DARTYGE, E., DEPAUTEX, C., DUBUISSON, J.M., FONTAINE, A . , JUCHA, A..

LEBOUCHER, P . , and TOURILLON, G., Nuc. I n s t . Meth. Phys. Res., ~ 2 4 6 (1986) 452 (14) MANSOUR, A.N., COOK, J.W. and SAYERS, D.E., 3 . Phys. Chem. 88 (1984) 2330 ( 1 5 ) LYTLE, F.W., GREEGOR, R.B., MARQUES, E.C., SANDSTROM, D.R., V I A , G.H., and

SINFELT, J.H., J . Cat., 95 (1985) 546.

C8-226 JOURNAL DE PHYSIQUE

Figure 1: Particles growth as a function of reduction conditions

Figure 2:The reduced step at high temperature: from the dried sample (2a) or the calcined one (2b)

L edge intensity

3 a .u.

A

rk.

! H introduction(T=20O0C)

j 2

0 2 4

*

6 time (mn)

2 3 4 R(A)

Figure 3: Kinetic study of the reduction. Evolutions of the FT (3a) and the L3 edge (3b) as a function of time