Characterization and Catalytic Properties of Nickel Oxioe Supported on Rare Earth Oxides. Description of the Metal — Support Interaction

Characterization and Catalytic Properties of Nickel Oxioe Supported on Rare Earth Oxides. Description of the Metal — Support Interaction

A. Holmen et al. (Editors 1, Natural Gos Conuersion 1991 Elsevier Science Publishers B.V., Amsterdam CHARACTERIZATION AND CATALYTIC ON RARE EARTH ...

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A. Holmen et al. (Editors 1, Natural Gos Conuersion 1991 Elsevier Science Publishers B.V., Amsterdam

CHARACTERIZATION

AND

CATALYTIC

ON RARE EARTH OXIDES. 3. BARRAULT',

S. PROBST',

PROPERTIES OF

DESCRIPTION OF THE METAL A. ALOUCHE',

357

NICKEL

-

OXIDE

SUPPORTED

SUPPORT INTERACTION

A. PERCHERON-GUEGAN3,

V. PAUL-

60NCOUR3 and M. PRIMET4

'Laboratoire de C a t a l y s e e n Chimie Organique, U.R.A. C.N.R.S. DO 350, 40, Avenue du Recteur Pineau - 86022 P o i t i e r s (France) 2 U.F.S.C. - Chemistry Department, 88000 F l o r i a n o p o l i s , S a n t a C a t a r i n a , Brasil 'Laboratoire de Chimie Metallurgique d e s Terres Rares, E.R. C.N.R.S 209, 1 , Place A r i s t i d e Briand - 92195 Meudon (France) 41.R.C. C.N.R.S., 2, Avenue A. E i n s t e i n , 69626 Villeurbanne (France) SUMMARY The a c t i v i t y and t h e s e l e c t i v i t y of n i c k e l a r e g r e a t l y modified when supported on c e r i a ; an i n c r e a s e o f both the a c t i v i t y and C s e l e c t i v i t y is o b t a i n e d . The comparison of r e s u l t s of H a d s o r p t i5o n TPD, (CO, H ) r e a c t i o n , c o m p e t i t i v e benzene and toluene i y d r o g e n a t i o n and i n f r a re$ s p e c t r a of CO i r r e v e r s i b l y adsorbed l e a d t o t h e conclusion t h a t new c a t a l y t i c sites i n v o l v i n g reduced support a r e s e l e c t i v e l y formed a t t h e metal-support i n t e r f a c e . The i n c r e a s e of both t h e s u r f a c e a r e a and t h e r e d u c t i o n temperature s i g n i f i c a n t l y modify t h e e l e c t r o n t r a n s f e r between s u p p o r t and metal ( t h e n t h e nature of active s i t e s ) and t h e r e p a r t i t i o n o f t h e s e sites. +

INTRODUCTION

It i s well known t h a t t h e chemisorptive and t h e c a t a l y t i c p r o p e r t i e s o f metals a r e s i g n i f i c a n t l y modified when they a r e d e p o s i t e d on r e d u c i b l e s u P P o r t s ( l - 2 ) . The changes i n c a t a l y t i c p r o p e r t i e s a r e p a r t i c u l a r l y import a n t i n (co, impregnated

H2)

reaction.

I n p r e v i o u s papers (3,4) concerning n i c k e l

on a low s u r f a c e a r e a Ce02, we have shown t h a t changing

t h e r e d u c t i o n temperature l e a d s t o a marked d e c r e a s e i n hydrogenation r e a c t i o n s . C h a r a c t e r i z a t i o n of t h i s c a t a l y s t i n d i c a t e s t h a t these changes a r e n o t only due t o a m o d i f i c a t i o n of metal p a r t i c l e s . A s we have e v i 18 denced r e c e n t l y w i l l l a b e l l e d molecules ( C 0 ) t h e s u p p o r t p l a y s a s i g n i f i c a n t role on

t h e hydrogen and on t h e CO or C02 a c t i v a t i o n ( 5 ) .

On

account of t h e importance o f these e f f e c t s and of t h e o r i g i n a l c a t a l y t i c i) the p r o p e r t i e s of t h e s e s o l i d s , we i n v e s t i g a t e d t h e i n f l u e n c e of n a t u r e of t h e r a r e e a r t h o x i d e , i i ) t h e s u r f a c e a r e a , i i i ) t h e reduc-

t i o n t e m p e r a t u r e ; o n t h e a c t i v i t y and s e l e c t i v i t y i n (CO, H2) r e a c t i o n . Moreover we t r i e d t o understand what a r e t h e i n t e r a c t i o n s between metal

358 and s u p o r t i n d o i n g s p e c i f i c r e a c t i o n s : c o m p e t i t i v e benzene and t o l u e n e h y d r o g e n a t i o n ( 6 ) and i n f o l l o w i n g CO a d s o r p t i o n s t a t e s in f u n c t i o n of t h e s u p p o r t by i n f r d r e d e x p e r i m e n t s . METHODS C d t a l y s t s were p r e p d r e d by i m p r e g n a t i o n w i t h an aqueous s o l u t i o n

o f n i c k e l n i t r a t e o f d i f f e r e n t c e r i u m d i o x i d e s (Ega Chemie = 20 m' g - 1 , Rhone-Poulenc LSA : 2 3 in'

g - I , RPI : 60 m' g-'

and HSA : 170 m'

g-'),

s i l i c a (Rhone-Poulenc PBS 300 = 2 6 0 m z 9 - l ) a n d a l u m i n a (Rhone-Poulenc, GFS 300 = 200 m'

9-l).

A f t e r d r y i n g o n a s a n d b a t h t h e powders were

h e a t e d i n a i r a t 3 9 3 O K f o r 12h and r e d u c e d i n s i t u w i t h hydrogen a t different

temperatures

between

500

and

800OK.

Catalytic

reactions

were c a r r i e d o u t a t a t m o s p h e r i c p r e s s u r e i n a dynamic f i x e d - b e d reactor d e s c r i b e d p r e v i o u l y ( 7 ) . The e f f l u e n t s o f t h e r e a c t i o n s were a n a l y z e d by on l i n e g a s c h r o m a t o g r a p h y .

The o t h e r e x p e r i m e n t a l c o n d i t i o n s were

a s foiiows : (C0,H2) r e a c t i o n : H2/C0 = 1 : 1, r e a c t i o n t e m p e r a t u r e 523OK (C6H6,H2) o r (C6H6 + C7H8, Hz) r e a c t i o n s : H 2 / h y d r o c a r b o n ( s ) = 1 : 20, r e a c t i o n t e m p e r a t u r e between 323OK and 373OK. TPR, a d s o r p t i o n and TPD e x p e r i m e n t s were c a r r i e d o u t u s i n g a p u l s e chromat o g r a p h i c method (8). I n f r d r e d s p e c t r a were r e c o r d e d u s i n g a F o u r i e r t r a n s f o r m s p e c t r o -

meter ( B r u c k e r IFS 1 1 0 ) . b e t t e r t h a n 2 cm-I

I n edch c a s e ,

was compressed u n d e r 4 t o n s i n diameter

t h e s p e c t r a l r e s o l u t i o n was

i n t h e s p e c t r a l r a n g e 4000-1000 cm- 1

.

The s a m p l e

p r e s s u r e t o o b t a i n a d i s - k o f 18 mm

( w e i g h t c o m p r i s e d between 30 and 50 mg).

The a c t i v a t i o n

p r o c e d u r e s o f t h e s o l i d s were p e r f o r m e d i n s i t u i n an i r c e l l p r e v i o u s l y described

( 9 ) . A d s o r p t i o n o f CO a t a g i v e n c o v e r a g e was a c h i e v e d by

i n t r o d u c i n g a known p r e s s u r e o f c a r b o n monoxide c o n t a i n e d i n a c a l i brated vessel. RESULTS C h e m i s o r p t i o n o f hydrogen o r c a r b o n monoxide The r e s u l t s p r e s e n t e d i n T d b l e 1 show t h a t n i c k e l o x i d e d e p o s i t e d on s i l i c d is r e d u c e d a t

d

t e m p e r a t u r e close t o 670OK. On t h e c o n t r a r y ,

and t h i s was c o n f i r m e d by TPR e x p e r i m e n t s ( 4 ) and XPS measurements ( 4 ) , n i c k e l p r e c u r s o r s i m p r e g n a t e d or v a r i o u s c e r i u m o x i d e s dre r e d u c e d a t

lower t e m p e r a t u r e s . Moreover i t c a n be n o t i c e d t h a t t h e o r i g i n and t h e s u r f a c e a r e a o f CeOZ h a v e a s i g n i f i c a n t e f f e c t on n i c k e l p r o p e r t i e s . N i c k e l d e p o s i t e d on CeO

Ega i s v e r y s e n s i t i v e t o r e d u c t i o n t e m p e r a t u r e 2 and t h e hydrogen ddSOrptiOn is i n h i b i t e d a f t e r r e d u c t i o n a t 670°K w h i l e

359

i t i s t h e i n v e r s e f o r CO a d s o r p t i o n . When a n o t h e r series o f Ce02 is u s e d s u p p o r t , t h e same phenomena is o b s e r v e d b u t a t h i g h e r r e d u c t i o n t e m p e r a -

dS

t u r e (see f o r example Ni/Ce02 HSA). s u p p o r t is i n c r e a s e d ,

When t h e s u r f a c e a r e a o f t h e Ce02

t h e n i c k e l d i s p e r s i o n is a l s o enhanced (20%) and

t h i s i s p a r t i c u l a r l y e v i d e n t f o r Ni-Ce02

HSA c a t a l y s t s . T h e r e f o r e t h e s e

v a l u e s d e m o n s t r a t e d t h a t a t l e a s t two m o d i f i c a t i o n s a r e o c c u r i n g when t h e support particle

and the r e d u c t i o n

size

temperature

i i ) support reduction

a r e modified

:

i ) nickel

which i n d u c e s t h e c r e a t i o n of

new s i t e s o r / a n d a l o c a l e l e c t r o n i c m o d i f i c a t i o n o f n i c k e l p a r t i c l e s ( 1 0 ) . TABLE 1 I n f l u e n c e of c e r i a s u p p o r t o n t h e a d s o r p t i v e p r o p e r t i e s o f n i c k e l

or CO g - l

a)

pmoles H2/2

b)

Reduction temperature i n

c)

R e d u c t i o n t e m p e r a t u r e = 800°K

Catalyst

OK

Hydrogen a d s o r p t i o n ( a ) R ( b ) 523

N i 3.2

-

Si02

N i 7.5

-

Ce02 Ega

268

N i 5.6 N i 4.6

-

CeO2 LSA Ce02 RP1

261 6 21 90

Ce02 HSA

56 73

N i 5.3

-

109

R 673 870

CO(')

adsorption

R 523

R 673

734

2752 283

1336 1874

1346 2328

2306

2951

1632 2263 2868

(1122IC

(1000)

w2m

of some Ni-Ce02 c a t a l y s t s i n t h i s r e a c t i o n a r e summarized i n Table 2. First, it i s n o t i c e d t h a t t h e c a t a l y t i c a c t i Catalytic

properties

v i t y ( e x p r e s s e d i n mole 1 - l g - l N i )

a f t e r r e d u c t i o n a t 523K is 1 0 t o 20

times h i g h e r when t h e s u r f a c e a r e a i n c r e a s e from 20 m'g-l

t o 170 m'g-'.

The d e c r e a s e o f c a t a l y t i c a c t i v i t y w i t h t h e i n c r e a s e o f r e d u c t i o n temp e r d t u r e i s s i g n i f i c a n t o n l y f o r s u p p o r t s dnd c a t a l y s t s w i t h a low s u r f a c e a r e a (Ce02 Ega and LSA). t e m p e r a t u r e must be h i g h e r i n dctivity.

For Ni/Ce02 HSA c a t a l y s t s , t h e

reduction

t h a n 800°K i n o r d e r t o o b s e r v e a d e c r e a s e

360 Concerning

the

selectivity,

a

higher

hydrocarbons

fraction

(C5+)

i s p r o d u c e d when n i c k e l is d e p o s i t e d o n a h i g h s u r f a c e a r e a Ce02.

Moreover

for

favoured

by

this an

particular

increase

of

catalyst,

the

(C5+)

f r a c t i o n is

also

t h e r e d u c t i o n t e m p e r a t u r e till 8 0 0 O K .

On

t h e c o n t r a r y w i t h low s u r f a c e a r e a Ce02, t h e C5+ f r a c t i o n is d e c r e a s e d a f t e r an i n c r e a s e of r e d u c t i o n t e m p e r a t u r e t o 673OK (Ce02 Ega) or 793OK (Ce02 LSA). the

In brief,

modifications

temperature

when

induced

: nickel

the

by

surface

reduction

area

CeOZ is

of

temperature

occured

increased at

higher

s u p p o r t e d o n a low s u r f a c e a r e a Ce02 i s s e l e c t i v e

f o r l i g h t h y d r o c a r b o n s f o r m a t i o n when r e d u c e d a t low or h i g h t e m p e r a t u r e w h i l e n i c k e l s u p p o r t e d o n a h i g h s u r f a c e a r e a Ce02 i s s e l e c t i v e t o h i g h e r hydrocarbons e s p e c i a l l y a f t e r a reduction a t high temperature. TABLE 2 C a t a l y t i c P r o p e r t i e s o f N i on CeO2 i n t h e (CO ti2) r e a c t i o n a s a f u n c t i o n

of r e d u c t i o n p r e t r e a t m e n t c o n d i t i o n s .

*

This c a t a l y s t d e a c t i v a t e s very rapidly. N i 7 . 5%/Ce02 EGA

Sample Reduction Temperature (OK) Reaction Temperature

N i 5 . 6%/CeOZ

N i 5 . 3%/Ce02 HSA

LSA

523

673

673

523

673

793

523

673

793

523

523

673

513

523

513

523

523

523

6

0

6

10

7

3*

126

103

113

39

10

33

31

57

26

20

20

46

20

44

36

43

36

33

25

16

20

20

35

50

18

12

5

29

26

47

(OK)

Activit xlO3) mole COK-jg-lNi ~~

Selectivity CH4 '2'4

(%I

c5+ co2

4

-

11

70

7

13

-

% Olefins

37

90

37

60

-

i n C - C 2 4

Benzene or c o m p e t i t i v e ( b e n z e n e and t o l u e n e ) h y d r o g e n a t i o n a)

I n benzene h y d r o g e n a t i o n

t h e r e s u l t s r e p o r t e d i n Table 3 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 i n c r e a s e s w i t h t h e s u r f a c e a r e a o f t h e Ce02 b u t t o a s m a l l e r e x t e n d t h a n i n t h e (C0,H2) r e d c t i o n : less t h a n 2 times

361 instead

of

1 0 t o 20.

On t h e c o n t r a r y ,

the increase of the reduction

temperature l e a d s to a s i g n i f i c a n t decrease both of t h e s p e c i f i c a c t i v i t y dnd t h e t u r n o v e r number ddsorption

values).

hydrogenation explain

the

from H2

(calculated either

a d s o r p t i o n o r CO

t o t h e c o n c e p t o f B o u d a r t ( 1 1 ),

According

benzene

is i n s e n s i t i v e t o t h e c a t a l y s t s t r u c t u r e and t h i s c o u l d small v a r i a t i o n s o f t h e c a t a l y t i c a c t i v i t y w i t h s u r f a c e

dred o f Ce02 a f t e r a r e d u c t i o n a t low t e m p e r a t u r e (523OK). I n t h e same

way t h e d e c r e a s e o f t h e a c t i v i t y w i t h t h e rise o f t h e r e d u c t i o n temperat u r e i m p l i e s more t h a n a p a r t i c l e size c h a n g e and f o r example t h e m o d i f i cation of the nature of active centers.

TABLE 3 A c t i v i t y of Ni/Ce02 c a t a l y s t s i n b e n z e n e h y d r o g e n a t i o n a t 353 K. I n f l u e n c e o f t h e t e m p e r a t u r e of r e d u c t i o n ( T R ) . 2 ( a ) A c t i v i t y i n m o l e s / h gNi ( x 10 ) ( b ) T u r n o v e r i n m o l e c u l e s l s . s i t e ( x 102 t h e number of sites i s d e t e r m i n e d from H2 c h e m i s o r p t i o n ( c ) T u r n o v e r i n m o l e c u l e s / s . s i t e ( x 102 ) t h e n u d e r o f sites is d e t e r m i n e d from CO c h e m i s o r p t i o n ( 1 C O / s i t e ) .

573

1.80 a

(1.20)b ( - )c

673

0.18

(

- 1

2.60

(0.26) (0.50)

3.70

(0.19) (0.36)

0.88

(0.18)

1.50

(0.18)

(0.15)

(0.18)

0.27

793

(0.15) 0.39

(0.09)

b)

The

(KTIB)

ratio

of

evdludted

the

adsorption coefficients

i n competitive

hydrogenation

of of

(0.095) (0.105)

(0.10)

t o l u e n e and both

benzene

hydrocarbons

is v e r y s e n s i t i v e t o e l e c t r o n i c c h a n g e s o f t h e m e t a l p a r t i c l e s i n d u c e d by s u p p o r t , p r o m o t e r , p o i s o n

....

a s shown f o r P t ( 6 ) and o t h e r m e t a l s

( 1 2 ) . B e c a u s e t o l u e n e is more e l e c t r o n d o n o r t h a n benzene, t h e l a r g e r t h e e l e c t r o n e n r i c h m e n t o f n i c k e l t h e l o v e r t h e K T / B v a l u e s . Some o f t h e r e s u l t s obtained with Ni/CeO

(or a l u m i n a ) ,

2

c a t a l y s t s , compared w i t h n i c k e l / s i l i c a

a r e p r e s e n t e d i n T d b l e 4.

A f t e r a r e d u c t i o n a t 673

OK,

w

w

1880

1

0.01

1880

zzw

ziw

zwo m-1

imo

is00

,

b

0.045

2200

,

2100

2000 an-1

1900

1800

Fig. 1. Infrared spectra of CO irreversibly adsorbed at 298 K on (a) Ni 12/A1203 and (b) Ni 5.3/Ce02 HSA. (Catalysts have been reduced at 673 K.) Fig. 2 . Infrared spectra of CO irreversibly adsorbed at 298 K on (a) Ni 7.5-Ce02 ( 2 0 m2 g-'), (b) Ni 7.3-Ce02 (70 m2 g-'), (c) Ni 5.3-Ce02 (170 m2g-l).(Catalysts have been reduced at 673 K.) Fig. 3. Infrared spectra of CO irreversibly adsorbed at 298 K on Ni 5.3/Ce02 HSA after reduction at (a) 523 K, (b) 673 K, (c) 793 K.

363 t h e KT/B

v a l u e s of Ni/Ce02 HSA or LSA a r e s m a l l e r t h a n

A1203 and

to

793'K

Ni/Si02.

Moreover

t h e i n c r e d s e of

to a s i g n i f i c a n t i n h i b i t i o n of

leads

and a s m a l l e r v a l u e o f KT/B.

these of N i /

t h e reduction temperature toluene

hydrogenation

The same t r e n d s ( d t lower r e d u c t i o n t e m p e r a -

t u r e ) a r e o b s e r v e d f o r Ni/Ce02 Ega. These r e s u l t s mean t h a t some of t h e s u r f a c e atoms o f n i c k e l p a r -

ticles d e p o s i t e d o n Ce02 and r e d u c e d a t h i g h t e m p e r a t u r e ( t h e v a l u e o f which d e p e n d i n g on t h e s u r f a c e a r e a of t h e s u p p o r t ) have a m o d i f i e d electronic structure tron

d e n s i t y on

:

it seems t h a t t h e r e i s an i n c r e a s e o f t h e elec-

t h e s e n i c k e l atoms r e s u l t i n g from e l e c t r o n c h a r g e t r a n s -

f e r from Ce3+ c a t i o n s t o t h e m e t a l . TABLE 4 KT/B r a t i o i n c o m p e t i t i v e h y d r o g e n a t i o n o f t o l u e n e and benzene a t 353K. I n f l u e n c e o f t h e s u p p o r t and 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 ( T R ) .

Ni3.2/Si02

673

5.2

2.5

3.3

4.8

793

FT.IR

N i 5 . 3/Ce02HSA

N i 5 . 6/Ce02 LSA

0.35

E

c h a r a c t e r i z a t i o n o f CO a d s o r b e d o n Ni-Ce02 c a t a l y s t s After

a

hydrogen

reduction

t h e s a m p l e s a r e c o o l e d down a t 298OK f o r I h . Carbon

a t the desired

temperature

t o room t e m p e r a t u r e , o u t g a s s e d

for

IOh, torr)

monoxide was t h e n a d m i t t e d on t h e s a m p l e u n d e r

a pressure o f 5 t o 10 torr. The I.R.

s p e c t r a of CO i r r e v e r s i b l y a d s o r b e d a t 298'K

and Ni/Ce02 HSA a r e shown On f i g u r e 1 . The N i / A 1 2 0 3

on N i / A l Z o j

catalyst exhibits

CO bands d u e , a s shown i n p r e v i o u s works ( r e f . 1 3 , 1 4 ) , t o l i n e a r s p e c i e s 1 (2040 cm- ) and b r i d g e d s p e c i e s ( 1 9 5 0 cm-').

I n t h e case of t h e Ni/CeOZ

s a m p l e t h e i n t e n s i t y o f t h e s e bands i s less i m p o r t d n t b u t a new and v e r y i m p o r t a n t band c e n t e r e d a t 1890 cm-l a p p e a r s i n t h e s p e c t r a . tion,

the r a t i o of

the

i n t e g r a t e d i n t e n s i t i e s of

v e r s u s bridged s p e c i e s i s lower

with t h e

the

linedr

( I n addispecies

Ni/Ce02 s a m p l e ) . I n c o m p a r i s o n

364 w i t h o t h e r works

(ref.

15-17),

t h i s new band c o u l d be a t t r i b u t e d

to,

two b a n d s (1580 cm-' a n d 1360 cm- 1 ) r e p r e s e n t a t i v e of f o r m i a t e s p e c i e s a d s o r b e d on t h e s u p p o r t a r e a l s o o b s e r v e d i n t h e i r s p e c t r a . I n f i g u r e 2 t h e e v o l u t i o n o f t h e i r s p e c t r a o f CO on Ni/Ce02 c a t a l y s t s i n f u n c 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 Ce02 s u p p o r t i s p r e s e n t e d . 1

It i s c l e a r l y shown t h a t t h e i n t e n s i t i e s o f l i n e a r s p e c i e s (2030-2050 cm- ) dnd b r i d g e d s p e c i e s (1920-1960 cm-') d e c r e a s e i n f r o n t o f m u l t i c e n t e r e d s p e c i e s ( = 1880 cm- 1 ) when t h e s u r f a c e a r e a i n c r e a s e s . T h i s r e s u l t c a n b e e x p l a i n i n assuming t h a t t h e i n c r e a s e o f support favours t h e formation o f metal-support interface. Lastly

figure

the surface area of

t h e correspondingt

3 shows t h a t ,

whatever

the

the

new sites a t t h e

reduction

temperature

of N i 5.3/Ce02 HSA, t h e t h r e e b a n d s i n t h e 1700-2200 cm-I domain a r e always o b s e r v e d . It c a n be n o t i c e d i ) an i n c r e a s e i n t h e r e l a t i v e int e n s i t y of

t h e l i n e a r form a f t e r r e d u c t i o n

i i ) a decrease of t h e i n t e n s i t y o f

a t high

t e m p e r a t u r e and

t h e whole s p e c t r a a f t e r r e d u c t i o n

a t 793OK w i t h o u t m o d i f i c d t i o n o f t h e band p o s i t i o n .

This could be due

t h e m i g r a t i o n o f CeOx s p e c i e s o n t o n i c k e l p a r t i c l e s ( s p e c i a l l y a t h i g h temperatures)

a s e v i d e n c e d by

a recent

a n a l y t i c a l e l e c t r o n microscopy

study ( r e f . 19).

CONCLUSION The r e s u l t s p r e s e n t e d i n t h i s work c l e a r l y show t h a t t h e c a t a l y t i c properties of

Ni/Ce02 s o l i d s i n

(CO,

H2) r e a c t i o n a r e v e r y s e n s i t i v e

to t h e s u r f a c e area o f t h e s u p p o r t and t o t h e r e d u c t i o n t e m p e r a t u r e . The d c t i v i t y and t h e C5+ s e l e c t i v i t y a r e g r e a t l y e n h a n c e d when n i c k e l is s u p p o r t e d by a h i g h s u r f a c e a r e a Ce02 and r e d u c e d a t h i g h t e m p e r a t u r e

(750-800'K).

i ) an i i ) t h e r e d u c t i o n o f t h e s u p p o r t close

I n f a c t these peculiar properties are the r e s u l t of

i n c r e a s e of t h e metal d i s p e r s i o n t o m e t a l p a r t i c l e s and

i i i ) a l o c a l e l e c t r o n i c t r a n s f e r between p a r -

t i a l l y r e d u c e d s u p p o r t and m e t a l p d r t i c l e s . there

is a

specific

activation of

Moreover i t seems dlso t h a t

hydrogen

(hydrogen s t o r a g e )

could be important i n t h e hydrogendtion o f s u r f a c e carbon s p e c i e s .

which

365

A l l these modifications l e a d t o t h e selective formation of a new

s i t e involving t h e p a r t i c i p a t i o n of support s p e c i e s CO i n f r a r e d

spectra.

Indeed

the

importance

ds

evidenced from

and t h e sharpness of

the

(1880-1890 cm- 1 ) centered band i n comparison w i t h o t h e r CO s p e c i e s and

other

c a t a l y s t s i s very s p e c i f i c of

p r e p a r a t i o n s of

metal-Ce02

catalysts.

Now,

fine

such s o l i d s a r e c a r r i e d o u t i n order t o c o n t r o l t h e

s u r f a c e sites r e p a r t i t i o n .

REFERENCES 1 2 3 4 5 6 7 8

9

10

11 12 13 14 15 16 17 18

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