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Pt-Re ON ALUMINA REFORMING CATALYSTS AT THE REDUCED STEP
Dominique Bazin, H. Dexpert, P. Lagarde, J. Bournonville
To cite this version:
Dominique Bazin, H. Dexpert, P. Lagarde, J. Bournonville. Pt-Re ON ALUMINA REFORMING
CATALYSTS AT THE REDUCED STEP. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-293-
C8-296. �10.1051/jphyscol:1986856�. �jpa-00226178�
Pt-Re ON ALUMINA REFORMING CATALYSTS AT THE REDUCED STEP
D. BAZIN, H. DEXPERT, P. LAGARDE and J.P. BOURNONVILLE*
LURE, Laboratoire CNRS, CEA, M E N , Batiment 2090, universite Paris-Sud, F-91405 Orsay Cedex, France
"7nstitut Franqais du Petrole, BP 311, F-92506 Rueil Malmaison Cedex , France
I N T R O D U C T I O N
T h i s p a p e r r e p o r t s a s t u d y o f t h e c a t a l y t i c s y s t e m Pt-Re on a l u m ~ n a a t t h e r e d u c e d s t e p . T h e s y s t e m h a s a l r e a d y b e e n a n a l y z e d a t t h e d r ~ e d a n d c a l c i n e d s t a g e s u s i n g t h e s t a n d a r d s t e p b y s t e p m e t h o d a s w e l l as t h e dispersive o n e ( 1 , Z ) . I n o r d e r t o u n d e r s t a n d t h e r o l e p l a y e d b y t h e s e c o n d m e t a l , w e c o m p a r e t h e s e r e s u l t s w i t h t h o s e o b t a i n e d o n t h e m o n o m e t a l l i c c a t a l y s t , P t / A 1 2 0 3 . ( 3 )
F X P E R I M E N T A L MFTHOD
Experiments h a v e b e e n d e s c r i b e d e l s e w h e r e (2) a n d w e r e m a d e a t L.U.R.E. We h a v e u s e d t h e D.C. I . s t o r a g e r l n g running e ~ t h e r a t 1.72 GeV o r 1.85 GeV w i t h a n a v e r a g e c u r r e n t o f 2ODmA. E x a f s d a t a h a v e b e e n t a k e n u s i n g e i t h e r a c h a n n e l - c u t (Si 2 2 0 o r 400) o r a d o u b l e c r y s t a l m o n o c h r o m a t o r (Si 31 1 ) w l t h t w o i o n c h a m b e r s as d e t e c t o r s at t h e LIII e d g e s o f p l a t i n u m ( 1 1 . 5 6 keV) a n d r h e n i u m (10.53 keV). A t t h e r e d u c e d s t e p , w e f3sed a f u r n a c e w i t h b o r o n n i t r i d e c e l l s a n a l o g t o t h a t d e s i g n e d b y L y t l e e t a1.(4). T h l s a p p a r a t u s a l l o w s a n i n s i t u t r e a t m e n t o f t h e c a t a l y s t . E x a f s s p e c t r a w e r e c o l l e c t e d a t r o o m t e m p e r a t u r e .
The c a t a l y s t s a r e h l g h l y d i s p e r s e d Pt-Re b i m e t a l l i c s y s t e m s deposited o n g a m m a c u b ~ c a l u m i n a e x t r u d a t e s ( B . E . T . s u r f a c e 2 4 0 m 2 / g ) . The d i f f e r e n t preparation p a r a m e t e r s a r e l i s t e d b e l o w :
p r e c u r s o r Hz PtC16, R e 0 4 NHq I m p r e g n a t i o n p r o c e d u r e s u c c e s s i v e i m p r e g n a t i o n
D r y i n g d r y i n g t i m e 2 4 h o u r s a t 130°C m e t a l l o a d i n g 1.44 % w t P t , 1 . 0 3 % w t Re
- -- - - - -
t e m p e r a t u r e 5 3 0 ° c C a l c i n a t i o n
t i m e 2 4 h o u r s
' 1 3 0 0 " c t e m p e r a t u r e T2 5000c
R e d u c t i o n t i m e 2 h o u r s
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986856
JOURNAL DE PHYSIQUE
D A T A A N A L Y S I S
Two k i n d s o f i n f o r m a t i o n w e r e e x t r a c t e d f r o m t h e data. Q u a l i t a t i v e ones a r e p r o v i d e d by t h e edge s t r u c t u r e : t h e L I I I edges show a w h i t e line, t h e i n t e n s i t y of which i s r e l a t e d t o t h e o c c u p a t i o n of t h e m e t a l 5d band. F i g u r e 1 r e p o r t s as an e x e m p l e t h e p l a t i n u m L I I I edge s t r u c t u r e s f o r p l a t i n u m oxide (Pt02), and t h e h e x a c h l o r o p l a t i n i c a c i d s o l u t i o n (H2Pt C16)
.
A s f o r t h e p u r e m e t a l , t h e r e a r e v e r y d i f f e r e n t , and a c o m p a r i s o n between t h e L I I I edge of the s a m p l e s w i t h t h o s e of r e f e r e n c e compounds g i v e s a q u a l i t a t i v e i n d i c a t i o n on t h e e l e c t r o n i c s t a t e of t h e p l a t i n u m a t o m s . The r h e n i u m edge behaves on t h e same way.Q u a n t i t a t i v e i n f o r m a t i o n s a r e g i v e n by t h e E x a f s s p e c t r a . They have been a n a l y z e d i n a conventionnal m a n n e r w i t h a V i c t o r e e n f i t f o r t h e pre-edge r e g i o n and a k 3 w e i g h t e d F o u r i e r t r a n s f o r m b e y o n d t h e edge. A f i r s t l i m i t a t i o n i n t h e a n a l y s i s of o u r r e s u l t s comes f r o m t h e v i c i n i t y of t h e L edges of p l a t i n u m and r h e n i u m : t h e Re L I I edge l i e s o n l y 400 eV above t h e Pt L I I I edge s o t h a t t h e e n e r g y domain a v a i l a b l e f o r t h e a n a l y s i s of t h e Pt E x a f s data i s l i m i t e d t o about 350 eV. Secondly, p l a t i n u m a n d r h e n i u m a r e v e r y c l o s e i n the p e r i o d i c t a b l e s o t h a t we c a n n o t s e p a r a t e t h e i r c o n t r i b u t i o n i n t h e Exafs s p e c t r a .
R E S U L T S
The p l a t i n u m edge s t r u c t u r e of t h e c a t a l y s t ( f i g . 2) e x h i b i t s a w h i t e l i n e i n t e n s i t y a l m o s t i d e n t i c a l t o t h a t of t h e p u r e m e t a l i n d i c a t i n g t h a t m o s t of t h e p l a t l n u m a t o m s a r e i n a m e t a l l i c z e r o v a l e n t s t a t e . F i g u r e 3 shows t h e a m p l i t u d e s o f t h e F o u r l e r t r a n s f o r m s o f t h e c o r r e s p o n d i n g E x a f s data, s u p e r i m p o s e d w i t h t h a t of m e t a l l i c p l a t i n u m a n a l y z e d i n t h e same conditions : t h e y d i f f e r e s s e n t i a l l y i n t h e i r o v e r a l l m a g n i t u d e . I n m o n o m e t a l l l c r e d u c e d P t / a l u m i n a , t h e a v e r a g e e n v i r o n m e n t o f one m e t a l a t o m h a s b e e n f o u n d composed o f 6 m e t a l atoms a t 2 . 7 5
A
and 0 . 7 (50.5) oxygen atoms a t 2 . 0 4A,
as i g n a t u r e o f t h e c l u s t e r t o c a r r i e r bonding ( 3 ) . I n t h l s b i m e t a l l i c Pt-Re case, t h e q u a n t i t a t i v e a n a l y s i s l e a d s t o 3 ( r e d u c t i o n t e m p e r a t u r e 300°C) and 5 (at 500°C) m e t a l atoms a r o u n d one platinum, w i t h an h a r d l y d e t e c t a b l e contribution f r o m o x y g e n atoms.
The w h i t e l l n e of t h e r h e n i u m L I I I edge i s d e f i n i t e l y n o t ( f l g . 4) c h a r a c t e r i s t i c of a m e t a l l i c state, and t h e F o u r i e r t r a n s f o r m e x h i b i t s a s h i f t t o l o w e r R v a l u e s ( f i g . 5). Q u a n t i t a t i v e l y it a p p e a r s t h a t one r h e n i u m atom i s s u r r o u n d e d at t h e same t i m e by 6 o r 8 m e t a l atoms and 2 oxygens. Rhenium s e e m s t h e r e f o r e t o be i n v o l v e d i n a n o x i d i z e d c o m p o u n d as w e l l as an i n t e r m e t a l l i c one.
DISCUSSION
H. C h a r c o s s e t e t a l . (5) h a v e s h o w n t h a t P t - R e / x - A I 2 O 3 may be s t a b i l i z e d u n d e r v a r i o u s states, depending on t h e p r o c e d u r e f o l l o w e d during t h e i r a c t i v a t i o n , t h e t w o e x t r e m e s i t u a t i o n s being homogeneous b i m e t a l l i c a g g r e g a t e s on one hand, and s e p a r a t e p a r t i c l e s o f p u r e P t and h i g h l y d i s p e r s e d p u r e Re on t h e o t h e r one. The f i r s t e x t r e m e c a s e has been
c o n t a c t a t t h e r e d u c e d s t e p .
I n o u r s y s t e m , t h e f o l l o w i n g c o n c l u s i o n s h a v e t o b e p o i n t e d o u t :
a- p l a t i n u m a t o m s a r e i n a m e t a l l i c state, w i t h a l o w c o o r d i n a t i o n n u m b e r of m e t a l ( p l a t i n u m o r r h e n i u m ) . The a m o u n t o f o x y g e n p e r m e t a l a t o m i s v e r y l o w , as c o m p a r e d w i t h t h e h i g h l y d i s p e r s e d n o m o m e t a l l i c c a s e . T h i s s u g g e s t t h a t t h e p l a t i n u m i s n o t d i r e c t l y b o u n d t o t h e o x y g e n o f t h e c a r r i e r ;
b - r h e n i u m c o o r d i n a t i o n i s h i g h e r ; t h i s m e t a l i s a t t h e s a m e t i m e s u r r o u n d e d b y m e t a l a t o m s a n d o x y g e n ones, a n d i t s c h e m i c a l s t a t e i s n o t m e t a l l i c .
We t h e r e f o r e c o n c l u d e t h a t r h e n i u m a t o m s f o r m a n o x i d i z e d s u b s t r a t e , l i n k e d t o t h e c a r r i e r t h r o u g h a n o x i d e base, a n d w i t h a m e t a l l i c c h a r a c t e r a t t h e t o p . The p l a t i n u m a t o m s a r e d i s p e r s e d as s m a l l p a t c h e s a t t h e t o p s u r f a c e o f t h e s e o x i d i z e d c l u s t e r s . A s t h e s e a t o m s a r e a n c h o r e d t o a r h e n i u m s u b s t r a t e s t r o n g l y b o u n d t o t h e c a r r i e r , w e u n d e r s t a n d t h e r e f o r e t h e h i g h e r t o u g h n e s s o f t h e b i m e t a l l i c s y s t e m t o s i n t e r i n g .
T h i s m o d e l , w h i c h i n v o l v e s a n o x i d e r h e n i u m i n t e r f a c e b e t w e e n t h e p l a t i n u m a n d t h e a l u m i n a , a p p e a r s c o h e r e n t w i t h t h e g r e a t e s t a f f i n i t y o f r h e n i u m w i t h o x y g e n t h a n t h e p l a t i n u m o n e . T h i s i s i n l i n e w i t h o t h e r e x p e r i m e n t a l a n d t h e o r e t i c a l r e s u l t s (7,8).
CONCLUSION :
M e t a l - m e t a l a n d m e t a l - c a r r i e r i n t e r a c t i o n s h a v e b e e n d e f i n e d t h r o u g h X r a y a b s o r p t i o n a n a l y s i s o f a Pt-Re&-A1203 c a t a l y s t a t t h e r e d u c e d s t e p . Edge a n d E x a f s a n a l y s i s g i v e c o h e r e n t a n d c o m p l e m e n t a r y i n f o r m a t i o n s w h i c h l e a d t o a t w o p h a s e s s y s t e m w h e r e a n o x i d i z e d r h e n i u m s u b s t r a t e s u p p o r t s s m a l l p l a t i n u m c l u s t e r s . T h i s i m m i s c i b i l i t y s e e m s t o p r e s e n t a s t r o n g e r c h a r a c t e r t h a n t h e one o b s e r v e d b y L y t l e e t - a 1 i n t h e P t - I r c a s e (9)
.
This r e s u l t i s i n l i n e w i t h t h e d i f f e r e n t b e h a v i o u r s t h e s e t w o b i m e t a l l i c c o u p l e s h a v e s h o w n i n t h e n u m e r o u s c h e m i c a l t r a n s f o r m a t i o n s t h e y p r o m o t e .REFERENCES
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3- P. LAGARDE, J. MURATA, G. VLAIC, E. FREUND, H. DEXPERT, J.P. BOURNONVILLE J. Cat. 8 9 (1983) 3 3 3
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JOURNAL DE PHYSIQUE
F i g 1 : P l a t i n u m LIII e d g e s f o r P t 0 2 a n d H2PtC16
F i g 2: P l a t i n u m e d g e s f o r t h e c a t a l y s t (
. .
. ) a n d t h e m e t a l(--I
F i g 3 : Rhenium e d g e s f o r t h e c a t a l y s t (
. .
. ) a n d t h e m e t a l(--I
F i g 3 : F o u r i e r t r a n s f o r m s a m p l i t u d e s a t t h e P l a t i n u m e d g e
0 2 4 6
R
( a )
F i g 5 : F o u r i e r t r a n s f o r m s a m p l i t u d e s a t t h e Rhenium e d g e
