EXAFS STUDY ON SiO2 - SUPPORTED Rh-Fe AND Pd-Fe BIMETALLIC CATALYSTS

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EXAFS STUDY ON SiO2 - SUPPORTED Rh-Fe AND Pd-Fe BIMETALLIC CATALYSTS

H. Kuroda, T. Yokoyama, N. Kosugi, M. Ichikawa, T. Fukushima

To cite this version:

H. Kuroda, T. Yokoyama, N. Kosugi, M. Ichikawa, T. Fukushima. EXAFS STUDY ON SiO2 - SUPPORTED Rh-Fe AND Pd-Fe BIMETALLIC CATALYSTS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-301-C8-304. �10.1051/jphyscol:1986858�. �jpa-00226180�

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JOURNAL D E PHYSIQUE

C o l l o q u e C 8 , s u p p l 6 m e n t a u n o 1 2 , Tome 47, d6cembre 1 9 8 6

EXAFS STUDY ON S i 0 , - SUPPORTED R h - F e AND P d - F e BIMETALLIC CATALYSTS

H. KURODA, T. YOKOYAMA, N. KOSUGI, M. ICHIKAWA* and T . FUKUSHIMA' *

Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo 113, Japan

he Research Institute for Catalysis, Hokkaido University, Sapporo 060, Japan

.

Department of Chemical Process Engineering, Faculty of Engineering, Yokohama, National University, Tokiwadai, Xodogaya-ku, Yokohama 156, Japan

Rh K-edge and F e K-edge EXAFS w e r e measured o n t h e Rh-Fe b i m e t a l l i c c a t a l y s t s u p p o r t e d o n s i l i c a , by u s e o f s y n c h r o t r o n r a d i a t i o n . F e a t o m s i n t h e Rh-Fe/Si02 c a t a l y s t a r e c o n c l u d e d t o b e i n t h e m e a t l - s u p p o r t i n t e r f a c e f o r m i n g Rh-Fe- O(support) bondings, and anchor Rh p a r t i c l e s o n t o t h e s u p p o r t s u r f a c e . On t h e o t h e r hand, Fe a t o m s i n t h e Pd-Fe/SiOZ c a t a l y s t a r e u n i f o r m l y d i s t r i b u t e d i n t h e m a t a l p a r t i c l e f o r m i n g a Pd-Fe a l l o y .

INTRODUCTION

The Rh-Fe b i m e t a l l i c c a t a l y s t s u p p o r t e d o n s i l i c a c a t a l y z e s t h e CO+H2 c o n v e r s i o n r e a c t i o n t o g i v e a m i x t u r e o f MeOH a n d EtOH w i t h a h i g h e f f i c i e n c y , w h e r e a s R h / S i 0 2 c a t a l y s t g i v e s a m i x t u r e o f AcH a n d AcOH a n d F e / S i 0 2 g i v e s a m i x t u r e o f h y d r o c a b o n s [ l - 3 ] . T h e Pd-Fe b i m a t a l l i c c a t a l y s t c a t a l y z e s t h e CO+H2 c o n v e r s i o n r e a c t i o n t o g i v e MeOH, b u t t h i s a c t i v i t y d i s a p p e a r s a n d g i v e h y d r o c a r b o n s a s F e / P d r a t i o e x c e e d s 0.3. I t w a s c o n f i r m e d f r o m M o s s b a u e r s p e c t r o s c o p y [ 4 ] , t h a t F e a t o m s i n t h e Rh-Fe c a t a l y s t a r e m o s t l y i n t h e s t a t e of Fe3+ e v e n a f t e r t h e r e d u c t i o n w i t h H2, w h i l e t h e y a r e i n t h e s t a t e o f FeO i n t h e r e d u c e d s t a t e o f t h e Pd-Fe c a t a l y s t . I n o r d e r t o u n d e r s t a n d t h e s e d i f f e r e n c e s b e t w e e n Rh-Fe/Si02 a n d Pd-Fe/Si02 s y s t e m s , we h a v e c a r r i e d o u t EXAFS s t u d i e s o n t h e s e two c a t a l y s t s y s t e m s .

EXPWIMENTAL

Both Rh-Fe/Si02 and Pd-Fe/Si02 c a t a l y s t s were p r e p a r e d by t h e i m p r e g n a t i o n of a m e t h a n o l s o l u t i o n c o n t a i n i n g t h e c h l o r i d e s o f t h e c o n s t i t u e n t m e t a l s i n t o a s i l i c a g e l ( A e r o s i l 300). The impregnated s i l i c a g e l was d r i e d and p r e s s e d i n t o a d i s k ( 20 mm i n d i a m e t e r w i t h a w e i g h t of a b o u t 7 0 mg), a n d t h e s e c a t a l y s t d i s k e w e r e s e t i n t o a P y r e x , i n s i t u EXAFS c e l l w i t h K a p t o n window, t o p e r f o r m a r e d u c t i o n by h e a t i n g t h e s a m p l e t o 400 OC i n t h e f l o w of H2 [5]. The w e i g h t % of Rh

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986858

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C8-302 JOURNAL DE PHYSIQUE

( o r Pd) i n t h e p r e p a r e d c a t a l y s t s was a b o u t 4 W i n a l l t h e s a m p l e s s t u d i e d h e r e . EXAFS measurements were c a r r i e d o u t by u s e of t h e EXAFS s p e c t r o m e t e r [ 6 ] a t BL 10-B i n Photon F a c t o r y of t h e N a t i o n a l Laboratory f o r High-Energy P h y s i c s (PF-KEK).

man AND DISCUSSION

F i g u r e l ( a ) s h o w s t h e F o u r i e r t r a n s f o r m o f t h e k3 w e i g h t e d Rh K-edge EXAFS o s c i l l a t i o n w h i c h w a s m e a s u r e d o n t h e Rh-Fe (1:0.3)/Si02 c a t a l y s t . T h i s F o u r i e r t r a n s f o r m e x h i b i t s o n l y o n e p r o m i n e n t p e a k t h a t is a t t r i b u t a b l e t o Rh-Rh. The c u r v e - f i t t i n g a n a l y s i s of t h e above-mentioned peak gave t h e Rh-Rh d i s t a n c e of 2.66 A w i t h t h e c o o r d i n a t i o n number o f 11 o r more, w h i c h a r e a l m o s t t h e s a m e t o t h e v a l u e s f o r t h e bulk Rh metal. The F o u r i e r t r a n s f o r m of t h e Fe K-edge EXAFS d a t a of t h e s a m e c a t a l y s t i s shown i n F i g . l ( b ) . The s t r o n g peak a t a b o u t 2.5 A i n t h i s F o u r i e r t r a n s f o r m i s a t t r i b u t a b l e t o Fe-Rh. The s m a l l e r peak a r o u n d 2 A i s a n a r t i f a c t a r i s i n g from t h e s p l i t t i n g of t h e Fe-Rh peak because of t h e n o n - l i n e a r i t y o f t h e u s e d p h a s e - s h i f t f u n c t i o n . From t h e c u r v e - f i t t i n g a n a l y s i s , t h e Fe-Rh d i s t a n c e w a s d e t e r m i n e d t o b e 2.63 A a n d t h e c o o r d i n a t i o n number t o be 4.7. The peak a t a b o u t 1.5 A i s d u e t o Fe-0, t h e i n t e r a t o m i c d i s t a n c e and t h e c o o r d i n a t i o n number w e r e c o n c l u d e d t o b e 2 A and 1.5, r e s p e c t i v e l y . Note t h a t no p e a k a t t r i b u t a b l e t o Fe-Fe w a s found i n t h i s F o u r i e r transform.

We a l s o i n v e s t i g a t e d t h e c h a n g e o f t h e Rh K-edge a n d Fe K-edge EXAFS o n exposing t h e c a t a l y s t t o a i r a t room t e m p e r a t u r e and on performing a s u c c e s s i v e H2 r e d u c t i o n a t t h e s a m e t e m p e r a t u r e . The r e s u l t s o f t h e c u r v e - f i t t i n g a n a l y s e s o f t h e s e EXAFS d a t a a r e l i s t e d i n T a b l e 1, t o g e t h e r w i t h t h e r e s u l t s f o r t h e c a t a l y s t s o f h i g h e r Fe/Rh r a t i o . The r e s u l t s shown i n t h i s T a b l e , i n d i c a t e t h a t , when t h e c a t a l y s t i s e x p o s e d t o a i r , t h e o x i d a t i o n t a k e s p l a c e b o t h on F e a n d Rh a t o m s , r e s u l t i n g i n t h e c l e a v a g e of Fe-Rh bonds, b u t t h e c a t a l y s t e a s i l y goes back t o t h e

Fig.1 F o u r i e r t r a n s f o r m of k3x(k) f o r t h e Rh K-edge EXAFS (a) and t h a t f o r t h e Fe K-edge EXAFS (b) of t h e Rh-Fe(l:0.3)/Si02 c a t a l y s t i n t h e ruduced state.

36 12

RhFe/Si02 (Fe/Rh=0,3)

- 6

I

*k

DISTANCE / DISTANCE / W

r

I

RhFe/Si02 (Fe/Rh=0.3) Fe K-edge Rh K-edge

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Table 1 Results o f Rh K-edge and Fe K-edge EXAFS d a t a f o r RhFe/Si02 c a t a l y s t s

No. Fe/Rh treatment Fe3+,Fe* Fe-Oo Fe-Rb Rh-Oo Rh-Rb Rh-Fe

N R(A) N R(A) N R(A) N R(A) N ' ~ ( i ) 1 0.3 H z r e d n . a t 6 7 3 K 0.91 1.5 2.05 4.7 2.63 - - 11.4 2.66 - -

2 0.3 1 + i n a i r 0.99 5.2 1.94 - - 3.3 2.01 4.9 2.66 - -

3 0.3 2 + H 2 r e d n . a t 3 0 0 K - 0 . 9 1.1 1.99 2.0 2.61 - - 10.7 2.66 - -

4 0.5 Hz redn. a t 673K 0.9 1.99 4.0 2.62 - - ^{9.5 2.66} - -

5 1.0 H 2 r e d n . a t 6 7 3 K 0.76 1.1 2.02 3.4 2.61 - - ^{6.3 2.66} ^{2.1 2.60}

6 1.0 5 ⁺i n a i r 4.0 1.90 - - 3.2 2.02 4.5 2.66 - -

7 1.0 6 + H 2 r e d n . a t 3 0 0 K 1.7 2.03 3.1 2.60 - - 8.7 2.66 2.2 2.62

* from Mossbauer data

o r i g i n a l s t a t e when it i s t r e a t e d w i t h H2 a t room temperature. A s i m i l a r b e h a v i o r is observed a l s o f o r t h e c a t a l y s t s o f h i g h e r Fe/Rh r a t i o .

According t o t h e o b s e r v a t i o n of e l e c t r o n micrographs, t h e a v e r a g e d i a m e t e r o f m e t a l p a r t i c l e s s u p p o r t e d o n s i l i c a i s a b o u t 2 5 A i n a l l t h e c a t a l y s t s s t u d i e d here. When t h i s f a c t i s combined w i t h t h e r e s u l t s of t h e a n a l y s e s of EXAFS d a t a , we c a n d e r i v e t h e f o l l o w i n g s t r u c t u r a l model f o r t h e Rh-Fe b i m e t a l l i c c a t a l y s t . I n t h e c a s e o f t h e c a t a l y s t w i t h Fe/Rh r a t i o o f 0.3, t h e b u l k o f e a c h m e t a l p a r t i c l e i s composed o f Rh a t o m s , w h e r e a s F e a t o m s w h i c h a r e m o s t l y i n t h e s t a t e o f Fe3', a r e l o c a t e d i n t h e i n t e r f a c e between Rh p a r t i c l e and t h e s u p p o r t , f o r m i n g c h e m i c a l bonds w i t h t h e oxygen a t o m s o f t h e S i 0 2 s u r f a c e , and anchor Rh m e t a l p a r t i c l e s o n t o t h e s u p p o r t . M o s t o f F e a t o m s a r e i n t h e m e t a l - s u p p o r t i n t e r f a c e a l s o i n t h e c a t a l y s t s o f h i g h e r Fe/Rh r a t i o , b u t a p a r t o f F e a t o m s a r e l i k e l y t o b e d i s t r i b u t e d on t h e s u r f a c e of Rh p a r t i c l e .

The d i s t r i b u t i o n o f F e a t o m s i n t h e Pd-Fe b i m e t a l l i c c a t a l y s t i s m a r k e d l y d i f f e r e n t f r o m t h a t i n t h e Rh-Fe b i m e t a l l i c c a t a l y s t . T h e r e s u l t s o f t h e Curve- f i t t i n g a n a l y s i s of t h e EXAFS d a t a o f t h i s c a t a l y s t are l i s t e d i n T a b l e 2.

Table 2 Results o f Pd K-edge and Fe K-edge EXAFS data f o r PdFe/Si02

NO. Fe/Pd treatment Fe-Fe Fe-Pd Pd-Fe Pd-Pd

N ~ ( i ) N ~ ( i ) N ~ ( i ) ^N ~ ( i )

1 0.3 H2 redn. a t 673K - - 9.9 2.64 3.5 2.62 7.9 2.76

2 0.3 1 + i n a i r - - ^6.6 2.65 ; 3. 2.64 8.8 2.73

3 0.45 H 2 r e d n . a t 6 7 3 K 0.9 2.46 9.0 2.62 4.6 2.63 7.7 2.76

reference: Fe-Fe (Fe metal ) 2.482 Pd-Pd (Pd m e t a l ) 2.751 A

For FePd3 Fe-Pd 2.715 (N=12), Pd-Fe 2.715 i (N=4), Pd-Pd 2.715 i (N=8)

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JOURNAL DE PHYSIQUE

11 17

0

DISTANCE / A

0 2 3 % 5 6 O L - r

DISTANCE / A

( a ) (b)

Fig.2 F o u r i e r t r a n s f o r m of Fe K-edge EXAFS of Pd-Fe(1 :0.3)/Si02 c a t a l y s t i n t h e reduced s t a t e ( a ) and a f t e r t h e exposure t o a i r ( b )

The Pd-Pd and Pd-Fe c o o r d i n a t i o n n u m b e r s i n t h e r e d u c e d s t a t e o f t h e Fe(l:0.3)/Si02 c a t a l y s t were 7.9 and 3.5, r e s p e c t i v e l y , and t h e Fe-Pd c o o r d i n a t i o n number was 9.9. No i n d i c a t i o n f o r t h e presence of Fe-Fe bonding was o b t a i n a b l e from t h e a n a l y s i s of t h e Fe K-edge EXAFS. The r e s u l t s mentioned above means t h a t Pd and Fe a r e forming a l m o s t uniform a l l o y , most probably w i t h t h e composition of Pd3Fe.

T h i s i s i n marked c o n t r a s t t o t h e c a s e o f t h e Rh-Fe b i m e t a l l i c c a t a l y s t . No c o n t r i b u t i o n of Fe-O(support) b o n d i n g was f o u n d i n t h i s c a t a l y s t . Fe-0 b o n d i n g d o e s a p p e a r when t h i s c a t a l y s t i s expos'ed t o a i r , a s c a n b e s e e n i n t h e F o u r i e r t r a n s f o r m of F e K-edge EXAFS shown i n Fig.P(b), w h e r e t h e peak a t a b o u t 1.3 A i s t h e one due t o Fe-0. The Pd-Fe c o o r d i n a t i o n number derived from Pd K-edge EXAFS a s w e l l a s t h e Fe-Pd c o o r d i n a t i o n number f r o m F e K-edge EXAFS d e c r e a s e d a f t e r t i s t r a t m e n t . The above-mentioned r e s u l t s seem t o i n d i c a t e t h a t some f r a c t i o n of F e atoms comes o u t from t h e bulk t o t h e s u r f a c e of a l l o y p a r t i c l e t o form Fe-0 bonding.

I n t h e Pd-Fe(1 :0.45)/Si02 c a t a l y s t , t h e Pd-Pd and Pd-Fe c o o r d i n a t i o n numbers d e r i v e d from t h e Pd K-edge EXAFS were a l m o s t t h e same a s t h o s e of t h e c a t a l y s t w i t h Fe/Pd r a t i o o f 0.3, b u t t h e F e K-edge EXAFS g a v e t h e Fe-Fe c o o r d i n a t i o n number of a b o u t 1. T h i s f a c t s u g g e s t s t h a t t h e c o m p o s i t i o n of t h e a l l o y r e g i o n i n t h i s c a t a l y s t i s n e a r l y t h e same a s i n t h e Pd-Fe(l:0.3)/Si02 c a t a l y s t , and t h e e x c e s s Fe atom a r e forming Fe c l u s t e r s ( o r i s l a n d s ) by themselves.

REFERENCES

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189(1984)

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1222 (1 986)

6 ) H.Oyanagi, T.Matsushita, M.Ito, H.Kuroda, KEK REPORT 83/30 (1984)