12 Support Effect on Chemisorption and Catalytic Properties of Noble Catalysts
Strong Metal-Support Interactions Downloaded from pubs.acs.org by CORNELL UNIV on 05/22/17. For personal use only.
P. Mériaudeau, Μ. Dufaux, and C. Naccache Laboratoire Propre du C.N.R.S., Institut de Recherches sur la Catalyse, Conventionné à l'Université Claude Bernard, LyonI,2, Avenue Albert Einstein, 69626 Villeurbanne Cédex, France Although the most well established function of the support is to disperse the metal and to retard the sintering, recent works on metal-support interaction collected in (1) have shown that the chemisorptive and catalytic activity of group VIII metals were considera bly lowered when the metal was supported on reducible oxide, such as TiO2, and reduced at high temperature. The objective of the work reported in this paper was to further investigate the role of the support on the adsorptive and on the catalytic properties of platinum.
Results and discussion Supported-Pt-catalysts were prepared by wet impregnation tech nique usingH2PtCl6solution. Supported platinum salt was reduced at 473 Κ and 773 K, H2, CO adsorption and electron microscopy were used to measure the metal dispersion. On table 1 are reported Pt dispersion, infrared νCO for CO adsorbed on Pt and the catalytic data on benzene hydrogenation and n-hexane conversion. As already reported (2) the adsorption of H2 and CO is greatly lowered for Pt-TiO2 reduced at 773 K. The decrease of hydrogen adsorp tion capacity is accompanied by a decrease of the catalytic activity for benzene hydrogenation and n-hexane hydrogenolysis. However there is no change inνCOstretching frequency of adsorbed CO. In contrast for Pt-CeO2 reduced at 773 Κ only a small decrease of H2-adsorbed was observed. Furthermore the frequency was lowered by 16 cm 0097-6156/86/0298-0118$06.00/0 © 1986 American Chemical Society
12.
MERIAUDEAU E T A L .
119
Catalytic Properties of Noble Catalysts
TABLE 1
Catalyst
H/Pt
red
s
V
«Γ cm
C0
1
r
6 6 1
4 . 8 wt % P t
773 K
0.05
2080
0.08
Pt-Ce0
473 K
0.6
2088
1
0.4
2072
0.2
773 K
1.8 wt % P t
a : r e l a t i v e r a t e r ^ / r ^ g ; b : r a t e n-hexane
6 14 H
75
2080
2
b
C
H
0.25
2
r
a C
473 K
Pt-Ti0 Strong Metal-Support Interactions Downloaded from pubs.acs.org by CORNELL UNIV on 05/22/17. For personal use only.
T
2 1119 173
i n m.moles h
g
p t
E l e c t r o n m i c r o s c o p y s t u d i e s i n d i c a t e d t h a t the Pt p a r t i c l e s i z e s mained almost unchanged between low and h i g h temperature samples,
n-hexane c o n v e r s i o n on P t - T i 0
and 773 Κ was f o r both c a t a l y s t s
2
and P t - C e 0
reduced a t
2
re
reduced 473
c o n s i d e r a b l y lowered f o r t h o s e sam
p l e s reduced a t h i g h t e m p e r a t u r e .
However on P t - T i 0
2
the s t r o n g d e
c r e a s e i n the r e a c t i o n r a t e occured w i t h o u t any s i g n i f i c a n t
change
i n the p r o d u c t d i s t r i b u t i o n (about 20 % c r a c k e d p r o d u c t C j - C g ) w h i l e on P t - C e 0
reduced a t 773 Κ n-hexane h y d r o g e n o l y s i s i s
2
reduced compared w i t h P t - C e 0 d u c t s on P t - C e 0 Both T i 0
2
2
2
It results
platinum p a r t i c l e s .
t h a t both T i 0
773 K ) .
2
and C e 0
2
reduced a t h i g h tem t o the s u p p o r t e d
T h i s model would l e a d t o the same e f f e c t f o r both
s u p p o r t s i n c o n t r a s t w i t h our r e s u l t s 2
2
form η - t y p e semiconductor when reduced a t h i g h
p e r a t u r e would have the a b i l i t y t o donate e l e c t r o n s
case o f T i 0
considerably
reduced a t 473 Κ (48 % c r a c k e d p r o
473 Κ and 17 % c r a c k e d p r o d u c t s on P t - C e 0
and C e 0
temperature.
2
(3).
We conclude t h a t i n the
the r e d u c t i o n o f H , CO a d s o r p t i o n and c o n s e q u e n t l y 2
decrease i n the c a t a l y t i c a c t i v i t y o f Pt i s m a i n l y due t o a o f Pt s u r f a c e by a s u b o x i d e Τ ι 0
χ
l a y e r (3,4)
the
coverage
which upon oxygen a d
s o r p t i o n i s d e s t r o y e d . The m o b i l i t y o f reduced c e r i u m o x i d e c o u l d be less.
However c e r i u m c a t i o n s were reduced t o cerium z e r o v a l e n t which
a t h i g h temperature would form P t - C e i n t e r m e t a l l i c compounds which
120
STRONG METAL-SUPPORT INTERACTIONS
e x p l a i n s the s h i f t i n v and n-hexane
C Q
as w e l l the change i n benzene
hydrogénation
conversion.
Support e f f e c t on the s y n t h e s i s
o f methanol o v e r Pt
I t has been r e p o r t e d t h a t methanol i s formed from C 0 - H t i o n over s i l i c a - s u p p o r t e d P d , P t , Ir c a t a l y s t s
(5).
2
reac
The b e h a v i o u r
o f the metal was found t o be i n f l u e n c e d by the c a r r i e r
(6,7,8,9).
s e l e c t i v i t y i n methanol was d i s c u s s e d i n terms o f a c i d - b a s e
The
proper
Strong Metal-Support Interactions Downloaded from pubs.acs.org by CORNELL UNIV on 05/22/17. For personal use only.
t i e s o f the s u p p o r t which i n f l u e n c e d the non d i s s o c i a t i v e a d s o r p t i o n o f CO on the metal r e q u i r e d f o r oxygenated hydrocarbon f o r m a t i o n , i n terms o f e l e c t r o n i c
i n t e r a c t i o n between the metal and the s u p p o r t o r
i n terms o f s t a b i l i z a t i o n by the c a r r i e r o f o x i d i z e d metal
cations
which would adsorb CO non d i s s o c i a t i v e l y . We have s t u d i e d the r e a c t i o n a t 553 K, 30 atmospheres o x i d e s . The c h a r a c t e r i s t i c s
C0-H
2
o v e r Pt supported on a v a r i e t y o f
o f the c a t a l y s t s
are g i v e n i n t a b l e
2
TABLE 2 Catalyst
Pt-Si0
wt % Pt
2
Pt-Al 0 2
3
H/Pt
T.E.M. nm
s
A
a S
S ^ROH
Me0H %
1.8
0.65
2.0
1
46
53
2.5
0.77
1.5
4.40
26
48
Pt-Ti0
2
4
0.26
-
6.6
64
67
Pt-Th0
2
3.3
0.25
-
8.5
72
77
Pt-Ce0
2
2.4
-
-
17.7
82
85
2.5
0.16
2-6
21.7
95
97
-
2.0
26
95
97
Pt-MgO Pt-La 0 2
2
3
a) A c t i v i t y (MeOH + EtOH)
ΙΟ
- 2
b
b) S e l e c t i v i t y i n
mole h"
oxygenated hydrocarbons (MeOH, EtOH, DME). The c a t a l y t i c a c t i v i t y o f the s u p p o r t s i n CO + H was a l s o i n 2
vestigated.
The r e s u l t s
indicated that Ce0 , Th0 , L a 0 2
2
2
3
have a p p r e
c i a b l e a c t i v i t y f o r methanol f o r m a t i o n . A 1 0 , MgO and T i 0 2
3
2
have v e r y
low a c t i v i t y f o r MeOH f o r m a t i o n . Table 2 shows c l e a r l y t h a t the a c t i v i t i e s o f s u p p o r t e d P t c a talysts
and t o a l e s s extend the s e l e c t i v i t i e s
f o r oxygenated com-
12.
MERIAUDEAU ET AL.
pounds form C 0 - H
are s t r o n g l y a f f e c t e d
2
121
Catalytic Properties of Noble Catalysts
by the s u p p o r t , the more b a
s i c s u p p o r t s h a v i n g the b e t t e r promoting e f f e c t f o r methanol tion.
forma
Furthermore i t i s c l e a r t h a t the h i g h e s t r a t e s o f MeOH f o r m a
t i o n were o b t a i n e d f o r Pt s u p p o r t e d on the c a r r i e r s e x h i b i t i n g a non n e g l i g i b l e a c t i v i t y . Mechanical m i x t u r e o f P t - s u p p o r t p l u s s u p p o r t were t e s t e d i n the C 0 - H
2
r e a c t i o n . The r e s u l t s
are l i s t e d i n t a b l e
3.
Strong Metal-Support Interactions Downloaded from pubs.acs.org by CORNELL UNIV on 05/22/17. For personal use only.
TABLE 3
Catalyst
Promoting e f f e c t
S e l e c t i v i t y % MeOH
a
Pt-Ce0
2
+ Ce0
2
2.30
80
Pt-Th0
2
+ Th0
2
3.53
76
Pt-Ti0
2
+ Ce0
2
3.6
60
Pt-Si0
2
+ Ce0
2
2.4
74
a) d e r i v e d from the r a t i o would be g i v e n by each
: e x p e r i m e n t a l MeOH/calculated MeOH which
component.
The a c t i v i t y o f s u p p o r t e d Pt c a t a l y s t s from C 0 - H
2
synthesis
i s c o n s i d e r a b l y enhanced when the metal i s s u p p o r t e d on
o x i d e s which e x h i b i t themselves sis.
f o r methanol
a p p r e c i a b l e a c t i v i t y f o r MeOH s y n t h e
Furthermore i t i s found t h a t the r a t e o f methanol f o r m a t i o n on
Pt-supported c a t a l y s t
i s i n c r e a s e d when T h 0 , C e 0
mixed w i t h the Pt c a t a l y s t . nal c a t a l y s t s .
2
2
were m e c h a n i c a l l y
Such b e h a v i o u r i s t y p i c a l f o r b i f u n c t i o
I t has a l r e a d y been r e p o r t e d t h a t T h 0 , C e 0 2
2
adsorb
carbon monoxide w i t h o u t d i s s o c i a t i o n . Such a c t i v a t e d CO can be hydrogenated t o form a f o r m y l s p e c i e s , ting with l a t t i c e
oxygen w i l l
the f o r m y l s p e c i e s
produce a formate
CO c o u l d a l s o r e a c t w i t h OH group t o form a f o r m a t e . te intermediates describes
have o f t e n been suggested 2
rial
2
3
I f such mechanism p r e
one c o u l d suggest t h a t on P t - C e O ^ ,
P t - T h 0 , P t - L a 0 , Pt-MgO, the c a t a l y s t 2
Formyl o r f o r m a
i n the mechanism which
the f o r m a t i o n o f CHgOH from C 0 - H .
v a i l s f o r Pt-supported c a t a l y s t
interac
intermediate.
behaves as a dual s i t e
mate
: CO a c t i v a t e d on the o x i d e i s hydrogenated by hydrogen a c t i v a
t e d on p l a t i n u m . Such mechanism does not e x c l u d e the p o s s i b i l i t y
122
STRONG METAL-SUPPORT INTERACTIONS
t h a t CH^OH c o u l d be formed a l s o through CO and H
2
both a c t i v a t e d on
the m e t a l . However s i n c e Pt i s a l s o a c t i v e f o r CO d i s s o c i a t i o n w i t h the subsequent f o r m a t i o n o f methane, Pt would g i v e a lower s e l e c t i v i t y mechanical vail
C0-H
2
r e a c t i o n o c c u r i n g o n l y on
i n CH^OH. Dual s i t e mechanism on
m i x t u r e o f s u p p o r t e d p l a t i n u m w i t h o x i d e s would a l s o
t h a t a c t i v a t e d hydrogen c o u l d s p i l l o v e r a t the s o l i d
Strong Metal-Support Interactions Downloaded from pubs.acs.org by CORNELL UNIV on 05/22/17. For personal use only.
pre
p r o v i d e d t h a t the pure o x i d e i s a c t i v e f o r CO a c t i v a t i o n and interphase.
Literature Cited 1. Metal-support and metal-additive effects in Catalysis (eds B. Imelik et al) Elsevier, Amsterdam 1982. 2. Tauster, S . J . , Fung, S.C. and Garten, R.L., J. Amer. Chem. Soc., 100, 170, 1978. 3. P. Mériaudeau, J.F. Dutel, M. Dufaux, C. Naccache "Metal-support and metal-additive effects in Catalysis" (eds B. Imelik et al) Elsevier, Amsterdam 1982, p. 95. 4. a) J . Santos, J . Phillips and J . Dumesic, J . Catal., 81, 147 (1983). b) D.E. Resaco and G. Haller, J . Catal., 82, 279, 1983. c) H.R. Sadeghi and V.E. Henrich, J. Catal., 87, 279, 1984. d) A . J . Simoens, R.T.K. Baker, D.J. Dwyer, C.R.F. Lund and R. Madon, J. Catal., 86, 359, 1984. e) D.N. Belton, Y.M. Sun, and J.M. White, J . Phys. Chem. 88, 5172, 1984. 5. Poustma, M.L., Elek, L . F . , Ibarbia, P.Α., Risch, A . P . , and Rabo, J . A . , J . Catal., 52, 157, 1978. 6. Ryndin, Yu. Α., Hicks, R.F., and B e l l , A.T., J. Catal., 70, 287 (1981). 7. Poels, E.K., Koolstra, R., Gens, J.W., and Ponec, V . , "Metal -support and Metal-additive Effects in Catalysis" (eds B. Imelik et al) Elsevier, Amsterdam 1982, p. 233. 8. Fajula, F. Anthony, R.G., Lunsford, J.H., J. Catal., 73, 237, 1982. 9. P. Mériaudeau, M. Dufaux and C. Naccache, VIII th Congress on Catalysis, Berlin 1984, Vol. II, p. 185. RECEIVED September 12, 1985
