ENVIRONMENT OF PLATINUM ATOMS IN A H2 PtCl6/Al2O3 CATALYST : INFLUENCE OF METAL LOADING AND CHLORINE CONTENT

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Submitted on 1 Jan 1986

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ENVIRONMENT OF PLATINUM ATOMS IN A H2 PtCl6/Al2O3 CATALYST : INFLUENCE OF METAL

LOADING AND CHLORINE CONTENT

J. Berdala, E. Freund, J. Lynch

To cite this version:

J. Berdala, E. Freund, J. Lynch. ENVIRONMENT OF PLATINUM ATOMS IN A H2 PtCl6/Al2O3 CATALYST : INFLUENCE OF METAL LOADING AND CHLORINE CONTENT. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-269-C8-272. �10.1051/jphyscol:1986850�. �jpa-00226172�

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Colloque C 8 , supplbment au n o 12, Tome 47, dgcembre 1986

ENVIRONMENT OF PLATINUM ATOMS IN A H, PtCl,/A1,0, CATALYST : INFLUENCE OF METAL LOADING AND CHLORINE CONTENT

J. BERDALA, E . FREUND and J . P . LYNCH

Institut F r a n ~ a i s du Petrole, 4 avenue de Bois Preau, BP 311, F-92506 Rueil-Malmaison Cedex, France

Resume: L'etude en EXAFS de l a preparation de catalyseurs 1 d i f f e r e n t e s teneurs en m6tal e t en chlore a permis d'observer en d e t a i l l e s interactions entre l e p l a t i n e e t l e support. L'influence du chlore e s t primordiale pour obtenir des dispersions @levees. La teneur en metal i n f l u e essentiellement il 1 'etape de reduction. Un complexe mononucli'aire PtCl ( 0 ) form6 par sechage sous a i r se transforme en P t O chlore lors de41a2 8t?ff&$tion. AprPs reduction on observe des p a r t i c u l e s k t a l l i q u e s bidimensionnel l e s 8 f a i b l e s teneurs en p l a t i n e , alors qu'aux f o r t e s teneurs l e s particules sont tridinensionnelles.

Abstract:EXAFS studies of the preparation of c a t a l y s t s with d i f f e r e n t metal and chlorine contents have allowed detai 1 ed observation of pl atinum-support interactions. The influence of chlorine on the obtention of high dispersions i s primordial. Influence of metal loading i s seen e s s e n t i a l l y a t the reduced stage. Mononuclear PtCl ( 0 ) complexes formed by drying i n a i r are transformed by calcination4

ti

INTRODUCTION

In industrial supported metal c a t a l y s t s , there i s a general trend towards very high dispersions. The dispersion i s determined e s s e n t i a l l y by the metal loading and the r o l e of chlorine ( 1 ) . Classical characterization techniques (XPS, X ray d i f f r a c t i o n , electron microscopy.. .) are however limited t o loadings of several percent or are not element s p e c i f i c ( 2 ) . EXAFS enables us t o follow t h e evolution of the platinum environment (3,4) as a function of these two parameters a t various stages of preparation a t loadings typical of industrial c a t a l y s t s t o achieve a b e t t e r understanding of the preparation process and t o determine whether high metal loading models are representative of real systems.

We have studied the main preparation steps f o r Pt/Al 0 c a t a l y s t s prepared by competi t i ve co-impregnation of hexachloropl a t i n i c &I % t ~ 1 1 and hydrochloric acids. Using d i f f e r e n t metal loadings (from 0.3 %

6

wefght) and chlorine contents ( 1 . l l % t o 2.20 % ) allows a study of the importance of support interactions. Experimental and data reduction techniques are detailed in these proceedings ( 6 ) and the r e s u l t s are suzzarized i n the t a b l e . Phases and backscatteri ng amp1 i tudes were derived f rom P t metal, H2PtC16 and pto references ( ~ i g . 1 ) .

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986850

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DISCUSSION .

a l The d r i e d stage

For a t y p i c a l i n d u s t r i a l p r e p a r a t i o n (0.6 % P t , 2.2 % C1) d r i e d a t llO°C, s t r o n g i n t e r a c t i o n s o f t h e complex w i t h s u p p o r t oxygens a r e seen l e a d i n g t o f o r m a t i o n o f i s o l a t e d mono n u c l e a r groups c l o s e t o PtC14(0 ) ( F i g . 2 ) . T h i s shows g r e a t e r m o d i f i c a t i o n o f t h e complex t h a n sugfe??&fO@

t h e work o f Lagarde and a l . ( 3 ) b u t i t i s i n agreement w i t h r e s u l t s o f o t h e r methods ( 5 ) ( 6 ) . F o r t h e s e l o w m e t a l l o a d i n g s , metal l o a d i n g has o n l y a small i n f l u e n c e on t h e p l a t i n u m environment : t h e r e i s a t r e n d towards more oxygen and, more s i g n i f i c a n t l y , l e s s c h l o r i n e s u r r o u n d i n g t h e p l a t i n u m atom. Although t h e p r e c i s i o n o f t h i s t y p e o f r e s u l t i s o f t h e o r d e r o f t h e observed e f f e c t , t h e i d e a o f a range of f i x a t i o n s i t e s b e i n g p r o g r e s s i v e l y f i l l e d cannot be excluded. On t h e o t h e r hand, c h l o r i n e p l a y s an i m p o r t a n t r o l e w i t h oxygen c o n t e n t o f t h e p l a t i n u m c o o r d i n a t i o n s h e l l i n c r e a s i n g as t h e i n i t i a l c h l o r i n e decreases.

b ) - The - c a l c i n a t i o n s t a g e

i;u6'ing c a l c i n a ~ l o n i n d r y a ; r , c h l o r i n e i s p r o g r e s s i v e l y r e p l a c e d by oxygen. I t r e s u l t s a complex bounded t o t h e alumina s u p p o r t c l o s e t o P t w i t h 5,5 oxygen and 1,6 C h l o r i n e as f i r s t neighbours ( F i g . 3 ) . No s i g n i f i c a n t q u a n t i t a t i v e dependance on m e t a l l o a d i n g i s seen b u t an o b s e r v a t i o n o f t h e F o u r i e r t r a n s f o r m s i n d i c a t e s t h a t t h e d i s t a n c e may be more w e l l d e f i n e d a t 0.3

%. I t i s i n d e e d p o s s i b l e t h a t h i g h e r l o a d i n g s i n t r o d u c e a heterogeneous d i s t r i b u t i o n a t t h i s stage. Once again, t h e c h l o r i n e has an i m p o r t a n t r o l e h e r e i n p r e v e n t i n g p a r t i c l e s i n t e r i n g i n a i r . Even a t l o w c h l o r i n e c o n t e n t , s i n t e r i n g does n o t occur a t h i g h t e m p e r a t u r e s i n a i r showing t h e s t r o n g i n f l u e n c e o f t h e c h l o r i n e on t h e e l e c t r o n i c s t a t e o f t h e p l a t i n u m .

c ) R e d u c t i o n

Complete r e d u c t i o n under hydrogen o f samples c a l c i n e d a t 530°C l e a d s t o f o r m a t i o n o f small m e t a l p a r t i c l e s ( 4 t o 6 first n e i g h b o u r s ) , two remarks a r e immediate ( f i g . 4 ) :

- oxygen s u p p o r t bonds a r e v i s i b l e a t l o w l o a d i n g s and absent a t 1.5 % l o a d i n g ;

- d i s p e r s i o n decreases s l i g h t l y w i t h i n c r e a s i n g m e t a l c o n t e n t b u t n o t enough t o e x p l a i n t h e disappearance o f Pt-Osupport bonds on a s i m p l e s p h e r i c a l p a r t i c l e on a f l a t o x i d e s u p p o r t model.

Two hypotheses may be p u t f o r w a r d a t t h i s stage. E i t h e r t h e morphology o f t h e p a r t i c l e s i s m o d i f i e d ( t h e v i s i b i l i t y o f s u p p o r t bonds f o r l o w l o a d i n g suggests a f l a t t e r shape whereas f o r h i g h l o a d i n g s t h e r e s u l t s suggest t h r e e dimensional c r y s t a l l i t e s ) o r t h e n a t u r e o f t h e f i x a t i o n s i t e changes (edge and k i n k s i t e s would g i v e h i g h e r P t - 0 c o o r d i n a t i o n b u t n a y be p r e s e n t i n l i m i t e d numbers and s a t u r a t e d by t h e f i r s t p a r t i c l e s t o be formed).

A n a l y s i s o f h i g h e r c o o r d i n a t i o n s h e l l s i s necessary t o c l a r i f y t h i s p o i n t . A t h i g h l o a d i n g s an i n c r e a s e d h e t e r o g e n e i t y o f P t - 0 d i s t a n c e s may l e a d t o a smearing o u t o f oxygen bond l e n g t h s p r e v e n t i n g t h e i r d e t e c t i o n . I t i s t o be n o t e d however t h a t whenever P t - 0 bonds a r e d e t e c t e d i n t h i s work t h e d i s t a n c e i s w e l l d e f i n e d a t 2.02 + 0.04 A. We a g a i n observe a s t a b i l i s i n g r o l e o f c h l o r i n e ( 5 ) , t h e d i s p e r s i ' d n d e c r e a s i n g w i t h l o w e r i n g o f c h l o r i n e c o n t e n t .

CONCLUSIONS

A t t h e d r i e d s t a g e t h e precursor i n t e r a c t s s t r o n g l y with t h e support t o produce mononuclear, i s o l a t e d PtC14(0a)su s p e c i e s . After c a l c i n a t i o n one observes i s o l a t e d complexes c l o e !gar$ chlorided Pt02 s t r u c t u r e . Mobility of t h e s e s p e c i e s under hydrogen g i v e s r i s e t o t h e c r e a t i o n of p a r t i c l e s during reduction.

An influence of metal loading i n t h e d r i e d and c a l c i n e d s t a t e s may be seen but, a t l e a s t up t o 1.5 % P t , remains within e r r o r s . On reduction, however, t h e d e t e c t i o n of support oxygen bonds f o r low loadings and a s l i g h t l y higher d i s p e r s i o n suggest t h a t two dimensional p a r t i c l e s may here be more abundant, o r t h a t d i f f e r e n t f i x a t i o n s i t e s a r e a v a i l a b l e on t h e alumina. Very high loading c a t a l y s t s ( s e v e r a l p e r c e n t ) nay not be r e p r e s e n t a t i v e of t h e i n d u s t r i a l product, a t l e a s t a t t h e d r i e d and reduced s t a g e s .

Chlorine content has a l a r g e influence a t a l l preparation s t e p s , preventing a l a r g e incursion of oxygen i n t o t h e pl a t i n u n coordination she? 1 a t t h e d r i e d s t a g e and serving t o s t a b i l i z e o r r e d i s p e r s e t h e metal during c a l c i n a t i o n and reduction

( 1 ) J . P . B O U R N O N V I L L E . T h e s i s I F P , 1 9 7 9 ( 2 ) J . B E R D A L A , T h e s i s I F P , 1 9 8 6

( 3 ) P . LAGARDE a n d a l .

J . C a t a i . 8 4 , 3 3 3 - 3 4 3 ( 1 9 8 3 )

( 4 ) S y n c h r o t r o n r a d i a t i o n r e s e a r c h p . 4 0 1 . E d i t e d b y H e r m a n W i n i c h a n d S . O o n i d c h ( P l e n u m P u b l i s h i n g C o r p o r a t i o n , 1 9 8 0 )

( 5 ) A . A . CASTRO a n d a l .

S c i e n t i f i c b a s e s f o r t h e p r e p a r a t i o n o f h e t e r o g e n e o u s c a t a l y s t s . T h i r d i n t e r n a t i o n a l s y m p o s i u m , s e p t . 6 - 9 , 1 9 8 2 , B E L G I U M , p . A 3 . 1

( 6 ) J . F . L E P A G E , C a t a l y s e d e c o n t a c t , T e c h n i p , P a r i s , 1 9 7 8 , p . 5 9 8

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