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Interactions of Base and Noble Metals with Insulator Supports
B. Imelik et al. (Editors),Metal-Support and Metal-Additiue Effects in Catalysis 0 1982 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
INTERACTIONS OF BASE AND NOBLE METALS WITH INSULATOR SUPPORTS
H . C . YAO, H .
S . GANDHI and PI. SHELEF
E n g i n e e r i n g and Research S t a f f , R e s e a r c h , Ford Plotor Company, Dearborn, Michigan
RESUME
Dans n o t r e l a b o r a t o i r e nous avons g t u d i 6 d e s syst'emes de m6taux n o b l e s dispers6s s u r des s u b s t r a t s i s o l a n t s .
Les c a t a l y s e u r s 6 t a i e n t d e s t i n g s 5
o p g r e r a' h a u t e t e m p g r a t u r e d a n s d e s c o n d i t i o n s o x y d a n t e s .
Pour c a r a c t g r i s e r
c e s systgmes nous avons employ@, pour l a p l u p a r t , d e s mgthodes c h i m i q u e s , p a r exemple l a c h i m i s o r p t i o n , l a r g d u c t i o n & t e m p g r a t u r e programmge e t l a rgactivit6.
En o u t r e , l a rnicroscopie o p t i q u e , l a rnicroscopie g l e c t r o n i q u e ,
e t d e s m6thodes s p e c t r o s c o p i q u e s ( p h o t o g l e c t r o n i q u e , i n f r a - r o u g e ,
o n t don& des i n f o r m a t i o n s u t i l e s .
Auger e t WE)
En p l u s i e u r s o c c a s i o n s , nous avons t r o u v g
que l e s i n t e r a c t i o n s e n t r e l e s c a t a l y s e u r s e t l e s s u b s t r a t s o n t
6t6 d'importance
p r a t i q u e e t q u ' e l l e s o n t pu g t r e m o d i f i g e s p a r l ' a d d i t i o n d ' o x y d e s de mgtaux bases.
ABSTRACT
The work on c a t a l y s t - s u p p o r t i n t e r a c t i o n s c a r r i e d o u t i n t h e a u t h o r ' s l a b o r a t o r y d e a l t w i t h systems of n o b l e metals d i s p e r s e d on i n s u l a t o r s u p p o r t . The c a t a l y s t s were d e s t i n e d t o o p e r a t e a t h i g h t e m p e r a t u r e s u n d e r o x i d i z i n g conditions.
The methods employed f o r c h a r a c t e r i z a t i o n w e r e mainly c h e m i c a l :
c h e m i s o r p t i o n , temperature-programmed
r e d u c t i o n and r e a c t i v i t y .
Further,
s p e c t r o s c o p i c a l methods ( X P S , I R , EPR, Auger) and e l e c t r o n microscopy w e r e applied t o provide additional information.
I n s e v e r a l i n s t a n c e s , t h e under-
s t a n d i n g of t h e n a t u r e of t h e i n t e r a c t i o n s h a s l e d t o s i g n i f i c a n t p r a c t i c a l results.
These were a c h i e v e d by modifying t h e i n t e r a c t i o n i n t h e d e s i r e d
d i r e c t i o n by t h e i n c o r p o r a t i o n of s u i t a b l e a d d i t i v e s .
INTRODUCTION The o b s e r v a t i o n t h a t t h e n a t u r e of t h e supposedly i n e r t c a r r i e r , o n t o which t h e a c t i v e i n g r e d i e n t h a s been d e p o s i t e d , e x e r t s a n i n f l u e n c e on t h e b e h a v i o r o f t h e whole c a t a l y s t h a s been made more t h a n h a l f a c e n t u r y ago and p o s s i b l y even a t t h e dawn o f c a t a l y t i c s t u d i e s .
But o n l y r e c e n t l y , w i t h t h e advent o f
160 modern s u r f a c e examination t e c h n i q u e s i t became p o s s i b l e t o d e l v e d e e p e r i n t o t h i s phenomenon.
I n some c a s e s , t h e u n d e r s t a n d i n g of s u c h i n t e r a c t i o n s has
p e r m i t t e d a s p e c i f i c approach e i t h e r t o enhance c a t a l y t i c a c t i v i t y o r t o p r o t e c t t h e a c t i v e i n g r e d i e n t from a n u n d e s i r a b l e i n t e r a c t i o n which would o t h e r w i s e ultimately lead t o deactivation.
I n s t a n c e s a r e a l s o known where a n a b s e n c e of
t h e i n t e r a c t i o n w i l l r e s u l t i n t h e a c t i v e i n g r e d i e n t s t o b e l o s t from t h e c a t a l y s t when exposed t o t h e o p e r a t i n g c o n d i t i o n s . The e f f o r t s a t Ford Motor Company have mainly been concerned w i t h s u p p o r t e d b a s e metal and n o b l e m e t a l c a t a l y s t s o p e r a t i n g under p r e v a i l i n g c o n d i t i o n s such a s p r e s e n c e of oxygen and h i g h - t e m p e r a t u r e s where t h e e x t e n t of t h e i n t e r a c t i o n s o r t h e i r absence d o e s have a profound e f f e c t w i t h a n immediate b e a r i n g on t h e p r a c t i c a l u s e . The n o b l e m e t a l c a t a l y s t s employed i n t h i s work i n c l u d e P t , Pd, Rh and Ru. The f i r s t t h r e e are a c t u a l l y used i n t h e f i e l d o f automotive c a t a l y s i s i n l a r g e quantities. The b a s e m e t a l s s t u d i e d , i n c l u d e R e , Mo, W , Co and N i .
E s s e n t i a l l y , none
of t h o s e , w i t h t h e p o s s i b l e e x c e p t i o n of N i , i s used r i g h t now f o r r e a s o n s a s s o c i a t e d mainly w i t h s u s c e p t i b i l i t y t o p o i s o n i n g b u t , a l s o i n a c e r t a i n degree, to t h e i n t e r a c t i o n with the support. The s u p p o r t m a t e r i a l which was mostly employed i s y-A1 0
2 3'
I n s t a n c e s though
w i l l b e shown t h a t o t h e r s u p p o r t s may b e p r e f e r a b l e , i n c e r t a i n l i m i t e d a p p l i c a t i o n s , b e c a u s e of t h e i n t e r a c t i o n s .
The r e s e a r c h i s a l s o aimed a t
s t u d y i n g more complex systems w i t h more t h a n o n e a c t i v e i n g r e d i e n t , b o t h o f them d e p o s i t e d on and i n t e r a c t i n g w i t h a s u p p o r t and a l s o i n t e r a c t i n g m u t u a l l y with each o t h e r .
Formation of two d i s t i n c t p h a s e s o f m e t a l s on s u p p o r t s induced by s u p p o r t interactions Using s e l e c t i v e c h e m i s o r p t i o n o f H2, CO o r NO, i t i s p o s s i b l e t o e s t a b l i s h t h e p r e s e n c e of two d i s t i n c t p h a s e s of a c t i v e metals. phases has
been e s t a b l i s h e d f o r Rh [l],
Pd [ 6 ] s u p p o r t e d on Y-Al2O3.
P t [2],
The p r e s e n c e of t h e s e
R e [3],
!lo
[4],
Co [5] and
S i m i l a r b e h a v i o r i s e x p e c t e d f o r many o t h e r
s u p p o r t e d c a t a l y s t s , e i t h e r n o b l e metals o r b a s e metals.
One p h a s e is composed
of r e l a t i v e l y l a r g e a g g r e g a t e s i n which t h e m e t a l o x i d e h a s l i t t l e i n t e r a c t i o n w i t h t h e s u p p o r t ; t h e o t h e r p h a s e i s d i s p e r s e d i n which t h e metal o x i d e h a s a strong interaction with the support.
In general, t h e lower t h e concentration
of m e t a l o x i d e i n t h e d i s p e r s e d p h a s e , t h e s t r o n g e r i s t h e m e t a l o x i d e s u p p o r t interaction. A d s o r p t i o n i s o t h e r m s o f N O , CO and H2 a t 25OC on s i x Rh/Y-A1203 samples of v a r y i n g Rh c o n c e n t r a t i o n i s shown i n F i g . 1.
P r i o r t o t h e a d s o r p t i o n measure-
ments t h e samples w e r e c a l c i n e d i n a i r f o r 4 h o u r s a t 500°C and reduced i n
161
- 5.0 I I-
w 2.0
o m I-'
k N E 1.0 0:
2
0.5
W E
0.21 I
0.1 I .04
0.1
I
I
0.5
1.0
I I
2.0
I
5.0
I
10.0
Rh LOADING, p moles/ m2 ( BET )
F i g . 1. Chemisorption of n i t r i c o x i d e , c a r b o n monoxide, and hydrogen a t 25°C as a f u n c t i o n o f Rh-loading.
H2 a t 425'C
f o r 1 hour.
For a l l t h r e e a d s o r b a t e s , t h e u p t a k e i n c r e a s e s l o g a r i t h m i c a l l y w i t h t h e l o a d i n g q , of t h e a c t i v e metal, where q=KCn, ( a s was t h e c a s e w i t h P t [ Z ] , Co [5],
Mo [4] and R e [ 3 ] ) .
A t a g i v e n v a l u e o f Rh s u r f a c e c o n c e n t r a t i o n ,
t h i s i n c r e a s e becomes v e r y s m a l l .
T h i s v a l u e i s t h e same f o r a l l t h r e e
a d s o r b a t e s and c o r r e s p o n d s t o 2 . 5 umole of Rh/mL of y-A1 0 2 3' T a b l e 1 summarizes t h e l i m i t i n g v a l u e s o f d i s p e r s i o n of v a r i o u s b a s e m e t a l o x i d e s and n o b l e metals s u p p o r t e d on y-A1 0 It i s c l e a r t h a t t h e a d d i t i o n 2 3' of b a s e m e t a l o x i d e s , such as Ce02 and Moo3 h a s a s t r o n g e f f e c t on t h e e x t e n t t o which a g i v e n n o b l e m e t a l can b e d e p o s i t e d i n t h e d i s p e r s e d p h a s e . I n t h e systems examined, i n s t a n c e s o f complete c o n t i n u o u s monolayer c o v e r a g e
o f y-A1 0 by e i t h e r n o b l e metals o r b a s e metals o r o x i d e s have n o t been ob2 3 s e r v e d . It i s c o n c e i v a b l e t h a t under v e r y mild c o n d i t i o n s , some systems may produce m e t a - s t a b l e monolayer d i s p e r s i o n s .
Under t h e c o n d i t i o n s o f t h i s
work, t h e r e always a p p e a r s t o e x i s t a l i m i t t o t h e d i s p e r s i o n w i t h t h e e x c e s s going i n t o 3-dimensional p a r t i c l e s i n some s o r t o f e q u i l i b r i u m w i t h t h e d i s p e r s e d phase.
162 TABLE 1 C o n c e n t r a t i o n l i m i t s o f metal i n t h e d i s p e r s e d p h a s e
Support
Metal
Pt Pt Rh Rh Pd Pd Re Mo co
L i m i t concentration umo l / m 2
2.2 7.1
Y-Al203 3 . 4 w t % Mo/Y-A1203 y-Al7-03 2 . 8 w t % Ce/Y-A1203 Y - A ~ ~3O 3 7 . 5 w t % Wo/ Y-A1203 Y -A1 2 0 3 Y-A1203 Zr02
2.5 5.4 2.2 5.2 2 .o 4.0
7.6
The above d i s c u s s i o n does n o t touch on t h e q u e s t i o n of t h e d e t a i l e d c o n f i g u r a t i o n of t h e dispersed phase.
Whether t h i s p h a s e i s p r e s e n t a s d i s c o n t i n u o u s
p a t c h e s ( s o c a l l e d r a f t s ) on o n l y p a r t s of t h e a v a i l a b l e s u r f a c e o r a s d i s c r e t e and i s o l a t e d atoms o r ensembles s p r e a d o v e r most of t h e a v a i l a b l e s u r f a c e remains a n u n r e s o l v e d i s s u e .
In some p a r t i c u l a r c a s e s , where s t r o n g metal
s u p p o r t i n t e r a c t i o n s are o b s e r v e d , e v i d e n c e can b e adduced t o show t h a t some m e t a l atoms ( o r m e t a l o x i d e m o l e c u l e s ) a r e p r e s e n t a s d i s c r e t e i s o l a t e d
e n t i t i e s i n t h e d i s p e r s e d p h a s e [l].
Temperature programmed r e d u c t i o n s t u d y as a measure o f s t r e n g t h o f i n t e r a c t i o n The u s e o f t e m p e r a t u r e programmed r e d u c t i o n t e c h n i q u e s t o measure t h e r e l a t i v e s t r e n g t h o f i n t e r a c t i o n between m e t a l o x i d e s d e p o s i t e d on a n i n s u l a t o r s u p p o r t i s d i s c u s s e d below.
S i n g l e component systems namely Rh/y-A1 0 [l] and 2 3
Pt/y-Al 0 [ 2 ] are used as examples. 2 3
F i g . 2 shows f o u r temperature-programmed Rh sample.
material.
reduction curves f o r a 5.51 w t %
A l l t h e r u n s were made c o n s e c u t i v e l y on t h e same sample o f t h e
The c a l c i n i n g a t p r o g r e s s i v e l y h i g h e r t e m p e r a t u r e s s p l i t s t h e low-
t e m p e r a t u r e peak of hydrogen u p t a k e i n t o two d i s t i n c t p e a k s .
After calcining
a t 7OO0C, o n e o b s e r v e s t h e f o r m a t i o n o f a d i s s o l v e d p h a s e which i s i r r e d u c i b l e
a t 4OO0C, m a n i f e s t e d by t h e hydrogen u p t a k e i n t h e 450-6OO0C r a n g e . The s p l i t t i n g o f t h e peak i n t h e low-temperature hydrogen u p t a k e n o t e d f o r t h e c o n c e n t r a t e d sample of F i g . 2 i s a b s e n t f o r t h e d i l u t e d sample.
This
s p l i t t i n g i s a t t r i b u t e d t o t h e d i f f e r e n c e i n r e d u c i b i l i t y between t h e d i s p e r s e d p h a s e and t h e c r y s t a l l i t e s which a r e b o t h p r e s e n t i n t h e c o n c e n t r a t e d sample. S u c c e s s i v e t r e a t m e n t s a t h i g h t e m p e r a t u r e s under o x i d i z i n g c o n d i t i o n s c a u s e
163
v)
k z
A. 4 0 0 ' C . A I R
3
a a a
k
m
-aa W Y
a
In 3
IN U
0
w
+ a [r
L L C. 6 0 0 " C . A I R
D. 700°C A I R
400 600 000 TEMPERATURE,"C
200
F i g . 2 . Temperature-programmed r e d u c t i o n of Rh ( 5 . 5 1 w t % ) / ~ - A l 2 0 3 . C a l c i n a t i o n t r e a t m e n t : ( A ) 4OO0C, 1 6 h r ; ( B ) 5OO0C, 1 6 h r ; (C) 600°C, 1 6 h r ; (D) 7 O O o C , 12 h r .
c r y s t a l l i t e growth, and a s h a r p e r d i f f e r e n t i a t i o n between t h e two p h a s e s o f t h e Rh, t h e two- and t h r e e - d i m e n s i o n a l p h a s e s , w i t h a concomitant d i f f e r e n t i a tion i n their reducibility.
P t / Y-A1203 A nonsupported P t 0 2 sample and f i v e Y-alumina-supported
samples of v a r y i n g
P t l o a d i n g were reduced a t t e m p e r a t u r e s r a n g i n g from -75 t o 50OoC.
The r e s u l t s
shown i n F i g . 3 i n d i c a t e t h a t t h e nonsupported P t 0 2 w a s reduced below 25OC w i t h t h e maximum r a t e a t % 5 O C ( c u r v e A ) . The temperature-programmed o x i d e on a y-A1203 s u p p o r t . F i g . 3 show b o t h p h a s e s :
r e d u c t i o n g i v e s e v i d e n c e o f two p h a s e s of P t The more c o n c e n t r a t e d samples ( c u r v e s B , C , and D ,
a p a r t i c u l a t e phase where t h e P t o x i d e h a s l i t t l e
i n t e r a c t i o n w i t h t h e alumina s u p p o r t as s e e n by t h e r e d u c t i o n a t Q I O ° C ,
resem-
b l i n g t h e r e d u c t i o n o f unsupported b u l k P t 0 2 ; and a d i s p e r s e d p h a s e where t h e P t o x i d e i n t e r a c t s more s t r o n g l y w i t h t h e s u p p o r t , and t h u s i s reduced a t higher temperatures.
I n t h e samples o f low P t l o a d i n g ( c u r v e s E and F ) ; o n l y
d i s p e r s e d p h a s e P t o x i d e i s found.
The H
2 u p t a k e measurements d u r i n g t h e s e
164
,
-75
0
100 200 300 TEM PERATURE,*C
400
500
F i g . 3. Temperature-programmed r e d u c t i o n of P t O ( 2 3 . 1 w t % P t , B ; 13.8 w t % P t , C ; 7 . 4 w t % P t , P t , E).
(A) and PtO /Y-A1203 samples wt % D; 0.78 w t %
6; 1 . 6 4
6t,
reductions i n d i c a t e t h a t P t oxide i n t h e dispersed phase i s P t O while P t i n 2 t h e p a r t i c u l a t e p h a s e i s p r o b a b l y mixed P t 0 2 , PtO, and m e t a l l i c P t . The l a r g e i n c r e a s e of t h e r e d u c t i o n t e m p e r a t u r e w i t h ' d i l u t i o n of P t 0 2 i n t h e d i s p e r s e d phase i n d i c a t e s t h a t a l l t h e dispersed Pt02 molecules a r e i n c l o s e contact w i t h t h e s u p p o r t and t h e y a r e p r o b a b l y s e p a r a t e d from e a c h o t h e r . t h i s s e p a r a t i o n , t h e s t r o n g e r i s t h e Pt02-A1203 i n t e r a c t i o n .
The l a r g e r
This i n t e r a c t i o n
may r e s u l t i n a d i s s o l u t i o n of P t 0 2 i n t o t h e s u r f a c e l a y e r of A 1 2 0 3 o r i n t h e f o r m a t i o n o f a PtO -4.1 0 complex as p r e v i o u s l y s u g g e s t e d C7-91. 2 2 3
S t u d i e s by p h y s i c a l methods The c h e m i s o r p t i o n and TPR are c u r r e n t l y t h e most unambiguous methods t o d i s t i n g u i s h t h e d i s p e r s e d p h a s e from p a r t i c u l a t e s i n a s u p p o r t e d m e t a l c a t a l y s t , b u t i n some cases, o t h e r e x p e r i m e n t a l t e c h n i q u e s , such as EPR [lo-ll] o r I R
[12-13]
are h e l p f u l i n f u r t h e r u n d e r s t a n d i n g of t h e metal-support
interaction.
EPR s t u d y of Rh/y-A1203 f o r example shows t h e o c c u r r e n c e of a d i s p r o p o r t i o n a t i o n r e a c t i o n , 2Rh3+
+
Rh2+
+ Rh4+,
i n t h e d i s p e r s e d p h a s e and t h e d i f f u s i o n
165
DPPH
I
F i g . 4 . EPR s p e c t r a o f t h e sample a f t e r v a r i o u s t r e a t m e n t s : (A) o u t g a s s e d a t 500°C, 2 h o u r s ; ( B ) exposed t o 0 2 , 60 t o r r , 25'C; (C) o u t g a s s e d a t 25OC, 1 h r ; ( D ) h e a t i n g a t 5OO0C i n a i r : ( E ) r e d u c e d , exposed t o a i r a t 25OC and h e a t e d i n vacuum a t 6OO0C f o r 1 6 h o u r s ; (F) exposed t o 0 2 , 60 t o r r , 25OC; ( G ) h e a t e d i n vacuum a t 7OO0C, 18 h o u r s and 8OO0C, 3 h o u r s ( s i g n a l i n t e n s i t y reduced t o half i n G).
of Rh2+ i o n s i n t o a Y-A1203 s u p p o r t .
Only t h e Rh
2+
ions i n the dispersed
p h a s e ( i n a n o x i d i z e d s t a t e ) chemisorbed O 2 and C O , and a r e r e s p o n s i b l e f o r the catalytic activity.
Upon h e a t - t r e a t m e n t
i n vacuo o r i n a i r a t >6OO0C,
Rh2+ i o n s d i s s o l v e i n t o y-A1203 and t h e c a t a l y s t i s d e a c t i v a t e d .
the
The Rh4+ i o n s
i n t h e d i s p e r s e d p h a s e s i t u a t e d i n t h e s a t u r a t e d and o c t a h e d r a l l y c o o r d i n a t e d s i t e s do n o t chemisorb as shown i n F i g . 4 . An I R s t u d y of CO chemisorbed on Rh/y-A1203
[ 1 2 ] shows t h e d i f f e r e n c e
between CO chemisorbed on t h e d i s p e r s e d p h a s e and t h a t o n t h e p a r t i c u l a t e phase. While o n l y t h e d o u b l e t band o f Rh(CO)2 a n d t h e v e r y b r o a d band of t h e b r i d g e d s p e c i e s Rh2 CO a r e found on Rh i n t h e d i s p e r s e d p h a s e , a s i n g l e t band o f RhCO a p p e a r s o n l y on Rh i n t h e p a r t i c u l a t e p h a s e .
The t h e r m a l s t a b i l i t y o f t h e s e
adsorbed s p e c i e s i s i n t h e sequence Rh2(CO) > RhCO > Rh(CO)2.
Other physical
166 methods s u c h as XPS [ 4 ] , TEN [2] and STEM [ I S ] were a l s o u s e d .
Reactivity studies Measurement of r e a c t i o n k i n e t i c s shows t h e d i f f e r e n c e i n c a t a l y t i c b e h a v i o r between t h e m e t a l s i n t h e d i s p e r s e d p h a s e and t h o s e i n t h e p a r t i c u l a t e phase. I n some cases, t h i s d i f f e r e n c e c a n b e i n d i c a t e d merely from t h e a c t i v i t y differences.
Thus i t was found t h a t NO r e d u c t i o n by H 2 o v e r Rh [16] o r P t [ 1 7 ]
i s a demanding r e a c t i o n which h a s a h i g h e r s p e c i f i c a c t i v i t y f o r Rh o r P t i n t h e p a r t i c u l a t e phase than i n t h e dispersed phase.
In other cases, t h i s
d i f f e r e n c e r e q u i r e s a more d e t a i l e d s t u d y t o e s t a b l i s h t h e s e l e c t i v i t y among possible reaction paths.
Two examples from o u r e x p e r i e n c e show t h e d i f f e r e n c e
i n s e l e c t i v i t y f o r t h e h y d r o g e n o l y s i s of n-pentane on Rh/y-A1203 of n e o p e n t a n e on Pt/y-A1203
[l8] and t h a t
[19].
The a n a l y s i s of t h e r e s u l t s i n d i c a t e s t h a t t h e h y d r o g e n o l y s i s o f n-pentane
on Rh i n t h e p a r t i c u l a t e p h a s e s t a r t s w i t h t h e s c i s s i o n o f t h e C1-C2
bond t o
form mainly C H 4 and n-butane w h i l e t h a t on Rh i n t h e d i s p e r s e d p h a s e s t a r t s w i t h t h e s c i s s i o n o f t h e C -C bond t o form e t h a n e and propane. The r e a c t i o n 2 3 p a t h s of neopentane w i t h H2 on Pt/y-A1 0 are e i t h e r h y d r o g e n o l y s i s o r
2 3
isomerization.
While o n l y h y d r o g e n o l y s i s o c c u r s on P t i n t h e d i s p e r s e d p h a s e ,
t h e i s o m e r i z a t i o n i s t h e main r e a c t i o n on P t i n t h e p a r t i c u l a t e p h a s e .
Effects of incorporation of a surface modifier The s e c o n d p a r t o f t h e t i t l e o f t h i s symposium a d d r e s s e s t h e ” m e t a l - a d d i t i v e effects” i n catalysis.
The examples d i s c u s s e d h e r e t o u c h upon t h i s i s s u e .
Indeed t h e a d d i t i v e s i n t h i s s e n s e a r e m e t a l o x i d e s whose oxygen i s l e s s t i g h t l y bound t h a n t h e oxygen i n t h e i n s u l a t o r s u p p o r t b u t more s t r o n g l y bound t h a n t h e oxygen a s s o c i a t e d w i t h t h e n o b l e metals.
Obviously, t h e s e
b a s e m e t a l o x i d e s can i n t e r a c t b o t h w i t h t h e s u p p o r t and t h e a c t i v e i n g r e d i e n t t o modify t h e d i s p e r s i o n o f t h e l a t t e r and t h e r e b y a l s o i t s a c t i v i t y . s y s t e m s h a v e l o n g been i n p r a c t i c a l u s e as evidenced by p a t e n t s
[no].
Such Although,
i n t h e s e i n s t a n c e s t h e e f f e c t of t h e u s e o f t h e m o d i f i e r s i s n o t d i r e c t l y i d e n t i f i e d w i t h t h e s u r f a c e i n t e r a c t i o n , o u r work i n d i c a t e s t h a t t h e p r a c t i c a l consequences a r e t h e r e s u l t o f s u c h phenomena.
Pt-PI00 /Y-Al2O3
-
3
A s t r o n g s u r f a c e i n t e r a c t i o n i s o b s e r v e d between t h e P t and Mo by v i r t u e of
t h e s h a r i n g o f some oxygen i o n s i n t h e Mo o x i d e w i t h n e i g h b o r i n g P t atoms. T h i s i n t e r a c t i o n h a s as one consequence a n i n c r e a s e i n t h e e x t e n t o f t h e d i s p e r s e d p h a s e of P t on t h e s u p p o r t , from 2 . 2 umole/m2 (BET) i n a sample w i t h o u t No03 t o 7 . 1 umole/m2 (BET) i n a sample c o n t a i n i n g 3.4% Mo a s measured by C O c h e m i s o r p t i o n ( T a b l e 1).
167
I -
4
13 0*1% P I
0 B
0
A
I6wl%PI
F i g . 5. Rate of H2 u p t a k e a s a f u n c t i o n o f t e m p e r a t u r e i n t h e TPR of Pt02 MOO and Pt02-Moo3 s u p p o r t e d on y-A1 0 3 2 3' The s h a r i n g of oxygen i s s e e n c l e a r l y from F i g . 5 [21],
where t h e f i r s t
r e d u c t i o n r a t e maximum in a TPR experiment o f s u p p o r t e d MoO3, a s s o c i a t e d w i t h most e a s i l y removable oxygens, merges w i t h t h e r a t e maximum a s s o c i a t e d w i t h t h e r e d u c t i o n of d i s p e r s e d P t 0 2 i t s e l f , which u s u a l l y o c c u r s a t a c o n s i d e r a b l e lower t e m p e r a t u r e due t o t h e weaker Pt-0 bond.
The e x t e n t o f t h i s t e m p e r a t u r e
s h i f t depends on t h e P t l o a d i n g and i s s h i f t e d f a r t h e r t o lower t e m p e r a t u r e s with a n increase i n t h e P t loading.
The f a c t t h a t t h e r e d u c t i o n a s s o c i a t e d
w i t h t h e removal o f oxygens more t i g h t l y bound t o Mo i s n o t a f f e c t e d by t h e p r e s e n c e o f P t a s e v i d e n c e d by t h e r e d u c t i o n t e m p e r a t u r e c o n s t a n c y of t h e second r a t e maximum, i n d i c a t e s t h a t t h e s p i l l o v e r o f hydrogen engendered by P t , which may b e invoked, does n o t e x p l a i n t h i s b e h a v i o r .
Corroborating evidence
i s a l s o o b t a i n e d from a n XPS s t u d y [14]. A s s e e n i n T a b l e 1, t h e p r e s e n c e o f Ce02 on y-A1203 e x t e n d s t h e d i s p e r s i o n
l i m i t of Rh, as measured by CO c h e m i s o r p t i o n , on Rh s u p p o r t e d on y-A1 0 con2 3 S i m i l a r l y , f o r Pd/Y-A1203 c a t a l y s t s t h e a d d i t i o n o f taining 2.8 w t % C e .
168 3 7 . 5 w t % WO
i n c r e a s e s t h e s a t u r a t i o n l i m i t o f Pd d i s p e r s i o n by a f a c t o r of 3 2 . 5 a s measured by CO c h e m i s o r p t i o n . Although i n a l l t h e s e t h r e e systems t h e a d d i t i o n o f b a s e metal o x i d e f o r t h e y--A1 0 s u p p o r t e d n o b l e metal c a t a l y s t 2 3 h a s caused l o w e r i n g i n t h e r e d u c t i o n t e m p e r a t u r e of t h e b a s e metal o x i d e , t h e e x t e n t of t h e s h i f t is d i f f e r e n t i n each case r e f l e c t i n g t h e d i f f e r e n c e i n t h e d e g r e e of s u r f a c e i n t e r a c t i o n s between t h e n o b l e m e t a l and t h e b a s e m e t a l o x i d e s . P r a c t i c.a___l consequences _.-_
of t h e i n t e r a c t i o n s
The work on t h e i n t e r a c t i o n s h a s g i v e n i n s i g h t s f o r t h e f o r m u l a t i o n o f c a t a l y s t s designed f o r p r a c t i c a l use. approaches w e r e a d o p t e d .
Based on t h e d e s i r e d a i m , d i f f e r e n t
Thus, i n t h e c a s e of Ru-containing c a t a l y s t s , t h e
main o b j e c t i v e when o p e r a t i n g under o x i d i z i n g c o n d i t i o n s i s t o p r e v e n t i t s volatilization.
For t h i s e n d , one t r i e s t o p r o v i d e a s s t r o n g a n i n t e r a c t i o n
a s p o s s i b l e by forming s t a b l e p e r o v s k i t e - t y p e mixed o x i d e s s u c h a s BaRuO
3' I n t h e c a s e of Rh s u p p o r t e d on y-A1 0 t h e i n t e r a c t i o n a t h i g h 2 3 t e m p e r a t u r e s c a u s e s f o r m a t i o n o f s u r f a c e and s u b - s u r f a c e s p i n e l s . For t h i s
LaRu03 [22].
r e a s o n one s u b s t i t u t e s t h e s u p p o r t by a more r e f r a c t o r y m a t e r i a l w i t h lesser tendency t o i n t e r a c t such a s z i r c o n i a o r a-alumina [23]. Again, t o i n c r e a s e t h e d i s p e r s i o n i n o r d e r t o u t i l i z e t h e p r e c i o u s m e t a l s u r f a c e t o i t s u t m o s t , one c a n u s e i n t e r a c t i o n m o d i f i e r s such as molybdenum i n t h e case of Pt/y-A1 0 [21] o r t u n g s t e n o x i d e i n t h e c a s e of Pd/y-A1,03 [6]. 2 3 The u n d e r s t a n d i n g of t h e physico-chemical n a t u r e o f t h e i n t e r a c t i o n and t h e i r f u t u r e q u a n t i f i c a t i o n , s h o u l d a f f o r d a means f o r t a i l o r i n g c a t a l y s t f o r s p e c i f i c purposes.
REFERENCES 1 H. C. Yao, S . J a p a r and M. S h e l e f , J . Catal., ( 1 9 7 7 ) , p.407. 2 H . C. Yao, M. S i e g and H . K. Plummer, J r . , J . C a t a l . , 59, ( 1 9 7 9 ) , p.365. H. C. Yao, P. W y n b l a t t , M . S i e g , and H . K. Plummer, J r . , " S i n t e r i n g P r o c e s s e s " Ed. G. C. Kuczynski, Plenum P u b l i s h i n g Corp., ( 1 9 8 0 ) , p.561. 3 H. C. Yao and M. S h e l e f , J . C a t a l . , 43, ( 1 9 7 6 ) , p.392. 4 H . C . Yao, J . C a t a l . , 70, ( 1 9 8 1 ) , p.440. 5 H . C . Yao and M. Bettman, J . C a t a l . , 41, ( 1 9 7 6 ) , p.349. M. Bettman and H. C . Yao i n " S i n t e r i n g and C a t a l y s i s " Ed. G. C . Kuczynski, Plenum P u b l i s h i n g C o r p . , ( 1 9 7 5 ) , p.165. 6 Unpublished d a t a . 7 K . W . McHenry, R. J. B e r t o l a c i n i , H . M. Brennan, J . L . Wilson and H . S . S e e l i g , i n "Actes 2me Cong. I n t . Catal. ( P a r i s , 1 9 6 0 ) , " Val. I1 ( p a p e r 1 1 7 ) , p.2295. 8 R. A. Herrmann, S . F. A d l e r , M. S . G o l d s t e i n , and R. M. DeBaun, J . Phys. Chem., 65, ( 1 9 6 1 ) , p.2189. 9 M. F . L. Johnson, C . D . K e i t h , 3 . Phys. Chem., 67, ( 1 9 6 3 ) , p.200. 1 0 H. C. Yao a n d 11. S h e l e f , J . C a t a l . , 31, (1973), p.377. 11 H . C. Yao and M. S h e l e f , t h e 7 t h I n t e r n a t i o n a l Congress on C a t a l y s i s , J u n e 30J u l y 4 , 1 9 8 0 , Tokyo, J a p a n ; S t u d , S u r f . S c i . C a t a l . , ( 1 9 8 1 ) , p.329-40. 1 2 H . C. Yao and W. G. R o t h s c h i l d , J . Chem. P h y s . , 68, ( 1 9 7 8 ) , p.4774. 1 3 W. G. R o t h s c h i l d and H . C. Yao, J . Chem. P h y s . , 74, ( 1 9 8 1 ) , p.4186. 1 4 J . E . DeVries, H. C. Yao, R . J . B a i r d and H . S . Gandhi, t o b e p u b l i s h e d .
169 15 16 17 18 19 20
H . K. Plummer, S . S h i n o z a k i , K. M. Adams and H. S . Gandhi, t o b e p u b l i s h e d . H . C . Yao, Y . F . Yu Yao and K . O t t o , J. C a t a l . , 5 6 , ( 1 9 7 9 ) , p . 2 1 . K . O t t o and H . C . Yao, J. C a t a l . , 66, ( 1 9 8 1 ) , p.229. H . C. Yao and M. SheleE, J. C a t a l . , 5 6 , ( 1 9 7 9 ) , p . 1 2 . H . C . Yao and M. S h e l e f , J. C a t a l . , 7 3 , ( 1 9 8 2 ) , p.76. U.S. P a t e n t 3,903,020 i s s u e d t o W . R. Grace Co. (1975). 21 H . S . Gandhi, H . C . Yao, and H . K . S t e p i e n , " C a t a l y s i s Under T r a n s i e n t C o n d i t i o n s , " ACS Symposium Series No. 1 7 8 , Ed. A. T . B e 1 1 and L. L . Hegedus, ( 1 9 8 2 ) , p . 1 4 3 . U.S. P a t e n t No. 4,192,779 i s s u e d t o Ford Motor Company (1980) 22 M. S h e l e f , and H . S. Gandhi, P l a t i n u m M e t a l s Review, 18, (1974), p.2. U.S. P a t e n t N o . 3,835,069 (1974); 3 , 8 8 0 , 7 7 5 ( 1 9 7 5 ) ; 3 , 8 1 9 , 5 3 6 ( 1 9 7 4 ) ; i s s u e d
t o Ford Motor Company. 23 H. C. Yao, H . K . S t e p i e n and H . S. Gandhi, J . C a t a l . , 6 1 , ( 1 9 8 0 ) , p.547. U.S. P a t e n t N o . 4 , 2 3 3 , 1 8 9 (1980); 4,172,047 ( 1 9 7 9 ) ; i s s u e d t o Ford Motor Company.
INTERACTIONS OF BASE AND NOBLE METALS WITH INSULATOR SUPPORTS
H . C . YAO, H .
S . GANDHI and PI. SHELEF
E n g i n e e r i n g and Research S t a f f , R e s e a r c h , Ford Plotor Company, Dearborn, Michigan
RESUME
Dans n o t r e l a b o r a t o i r e nous avons g t u d i 6 d e s syst'emes de m6taux n o b l e s dispers6s s u r des s u b s t r a t s i s o l a n t s .
Les c a t a l y s e u r s 6 t a i e n t d e s t i n g s 5
o p g r e r a' h a u t e t e m p g r a t u r e d a n s d e s c o n d i t i o n s o x y d a n t e s .
Pour c a r a c t g r i s e r
c e s systgmes nous avons employ@, pour l a p l u p a r t , d e s mgthodes c h i m i q u e s , p a r exemple l a c h i m i s o r p t i o n , l a r g d u c t i o n & t e m p g r a t u r e programmge e t l a rgactivit6.
En o u t r e , l a rnicroscopie o p t i q u e , l a rnicroscopie g l e c t r o n i q u e ,
e t d e s m6thodes s p e c t r o s c o p i q u e s ( p h o t o g l e c t r o n i q u e , i n f r a - r o u g e ,
o n t don& des i n f o r m a t i o n s u t i l e s .
Auger e t WE)
En p l u s i e u r s o c c a s i o n s , nous avons t r o u v g
que l e s i n t e r a c t i o n s e n t r e l e s c a t a l y s e u r s e t l e s s u b s t r a t s o n t
6t6 d'importance
p r a t i q u e e t q u ' e l l e s o n t pu g t r e m o d i f i g e s p a r l ' a d d i t i o n d ' o x y d e s de mgtaux bases.
ABSTRACT
The work on c a t a l y s t - s u p p o r t i n t e r a c t i o n s c a r r i e d o u t i n t h e a u t h o r ' s l a b o r a t o r y d e a l t w i t h systems of n o b l e metals d i s p e r s e d on i n s u l a t o r s u p p o r t . The c a t a l y s t s were d e s t i n e d t o o p e r a t e a t h i g h t e m p e r a t u r e s u n d e r o x i d i z i n g conditions.
The methods employed f o r c h a r a c t e r i z a t i o n w e r e mainly c h e m i c a l :
c h e m i s o r p t i o n , temperature-programmed
r e d u c t i o n and r e a c t i v i t y .
Further,
s p e c t r o s c o p i c a l methods ( X P S , I R , EPR, Auger) and e l e c t r o n microscopy w e r e applied t o provide additional information.
I n s e v e r a l i n s t a n c e s , t h e under-
s t a n d i n g of t h e n a t u r e of t h e i n t e r a c t i o n s h a s l e d t o s i g n i f i c a n t p r a c t i c a l results.
These were a c h i e v e d by modifying t h e i n t e r a c t i o n i n t h e d e s i r e d
d i r e c t i o n by t h e i n c o r p o r a t i o n of s u i t a b l e a d d i t i v e s .
INTRODUCTION The o b s e r v a t i o n t h a t t h e n a t u r e of t h e supposedly i n e r t c a r r i e r , o n t o which t h e a c t i v e i n g r e d i e n t h a s been d e p o s i t e d , e x e r t s a n i n f l u e n c e on t h e b e h a v i o r o f t h e whole c a t a l y s t h a s been made more t h a n h a l f a c e n t u r y ago and p o s s i b l y even a t t h e dawn o f c a t a l y t i c s t u d i e s .
But o n l y r e c e n t l y , w i t h t h e advent o f
160 modern s u r f a c e examination t e c h n i q u e s i t became p o s s i b l e t o d e l v e d e e p e r i n t o t h i s phenomenon.
I n some c a s e s , t h e u n d e r s t a n d i n g of s u c h i n t e r a c t i o n s has
p e r m i t t e d a s p e c i f i c approach e i t h e r t o enhance c a t a l y t i c a c t i v i t y o r t o p r o t e c t t h e a c t i v e i n g r e d i e n t from a n u n d e s i r a b l e i n t e r a c t i o n which would o t h e r w i s e ultimately lead t o deactivation.
I n s t a n c e s a r e a l s o known where a n a b s e n c e of
t h e i n t e r a c t i o n w i l l r e s u l t i n t h e a c t i v e i n g r e d i e n t s t o b e l o s t from t h e c a t a l y s t when exposed t o t h e o p e r a t i n g c o n d i t i o n s . The e f f o r t s a t Ford Motor Company have mainly been concerned w i t h s u p p o r t e d b a s e metal and n o b l e m e t a l c a t a l y s t s o p e r a t i n g under p r e v a i l i n g c o n d i t i o n s such a s p r e s e n c e of oxygen and h i g h - t e m p e r a t u r e s where t h e e x t e n t of t h e i n t e r a c t i o n s o r t h e i r absence d o e s have a profound e f f e c t w i t h a n immediate b e a r i n g on t h e p r a c t i c a l u s e . The n o b l e m e t a l c a t a l y s t s employed i n t h i s work i n c l u d e P t , Pd, Rh and Ru. The f i r s t t h r e e are a c t u a l l y used i n t h e f i e l d o f automotive c a t a l y s i s i n l a r g e quantities. The b a s e m e t a l s s t u d i e d , i n c l u d e R e , Mo, W , Co and N i .
E s s e n t i a l l y , none
of t h o s e , w i t h t h e p o s s i b l e e x c e p t i o n of N i , i s used r i g h t now f o r r e a s o n s a s s o c i a t e d mainly w i t h s u s c e p t i b i l i t y t o p o i s o n i n g b u t , a l s o i n a c e r t a i n degree, to t h e i n t e r a c t i o n with the support. The s u p p o r t m a t e r i a l which was mostly employed i s y-A1 0
2 3'
I n s t a n c e s though
w i l l b e shown t h a t o t h e r s u p p o r t s may b e p r e f e r a b l e , i n c e r t a i n l i m i t e d a p p l i c a t i o n s , b e c a u s e of t h e i n t e r a c t i o n s .
The r e s e a r c h i s a l s o aimed a t
s t u d y i n g more complex systems w i t h more t h a n o n e a c t i v e i n g r e d i e n t , b o t h o f them d e p o s i t e d on and i n t e r a c t i n g w i t h a s u p p o r t and a l s o i n t e r a c t i n g m u t u a l l y with each o t h e r .
Formation of two d i s t i n c t p h a s e s o f m e t a l s on s u p p o r t s induced by s u p p o r t interactions Using s e l e c t i v e c h e m i s o r p t i o n o f H2, CO o r NO, i t i s p o s s i b l e t o e s t a b l i s h t h e p r e s e n c e of two d i s t i n c t p h a s e s of a c t i v e metals. phases has
been e s t a b l i s h e d f o r Rh [l],
Pd [ 6 ] s u p p o r t e d on Y-Al2O3.
P t [2],
The p r e s e n c e of t h e s e
R e [3],
!lo
[4],
Co [5] and
S i m i l a r b e h a v i o r i s e x p e c t e d f o r many o t h e r
s u p p o r t e d c a t a l y s t s , e i t h e r n o b l e metals o r b a s e metals.
One p h a s e is composed
of r e l a t i v e l y l a r g e a g g r e g a t e s i n which t h e m e t a l o x i d e h a s l i t t l e i n t e r a c t i o n w i t h t h e s u p p o r t ; t h e o t h e r p h a s e i s d i s p e r s e d i n which t h e metal o x i d e h a s a strong interaction with the support.
In general, t h e lower t h e concentration
of m e t a l o x i d e i n t h e d i s p e r s e d p h a s e , t h e s t r o n g e r i s t h e m e t a l o x i d e s u p p o r t interaction. A d s o r p t i o n i s o t h e r m s o f N O , CO and H2 a t 25OC on s i x Rh/Y-A1203 samples of v a r y i n g Rh c o n c e n t r a t i o n i s shown i n F i g . 1.
P r i o r t o t h e a d s o r p t i o n measure-
ments t h e samples w e r e c a l c i n e d i n a i r f o r 4 h o u r s a t 500°C and reduced i n
161
- 5.0 I I-
w 2.0
o m I-'
k N E 1.0 0:
2
0.5
W E
0.21 I
0.1 I .04
0.1
I
I
0.5
1.0
I I
2.0
I
5.0
I
10.0
Rh LOADING, p moles/ m2 ( BET )
F i g . 1. Chemisorption of n i t r i c o x i d e , c a r b o n monoxide, and hydrogen a t 25°C as a f u n c t i o n o f Rh-loading.
H2 a t 425'C
f o r 1 hour.
For a l l t h r e e a d s o r b a t e s , t h e u p t a k e i n c r e a s e s l o g a r i t h m i c a l l y w i t h t h e l o a d i n g q , of t h e a c t i v e metal, where q=KCn, ( a s was t h e c a s e w i t h P t [ Z ] , Co [5],
Mo [4] and R e [ 3 ] ) .
A t a g i v e n v a l u e o f Rh s u r f a c e c o n c e n t r a t i o n ,
t h i s i n c r e a s e becomes v e r y s m a l l .
T h i s v a l u e i s t h e same f o r a l l t h r e e
a d s o r b a t e s and c o r r e s p o n d s t o 2 . 5 umole of Rh/mL of y-A1 0 2 3' T a b l e 1 summarizes t h e l i m i t i n g v a l u e s o f d i s p e r s i o n of v a r i o u s b a s e m e t a l o x i d e s and n o b l e metals s u p p o r t e d on y-A1 0 It i s c l e a r t h a t t h e a d d i t i o n 2 3' of b a s e m e t a l o x i d e s , such as Ce02 and Moo3 h a s a s t r o n g e f f e c t on t h e e x t e n t t o which a g i v e n n o b l e m e t a l can b e d e p o s i t e d i n t h e d i s p e r s e d p h a s e . I n t h e systems examined, i n s t a n c e s o f complete c o n t i n u o u s monolayer c o v e r a g e
o f y-A1 0 by e i t h e r n o b l e metals o r b a s e metals o r o x i d e s have n o t been ob2 3 s e r v e d . It i s c o n c e i v a b l e t h a t under v e r y mild c o n d i t i o n s , some systems may produce m e t a - s t a b l e monolayer d i s p e r s i o n s .
Under t h e c o n d i t i o n s o f t h i s
work, t h e r e always a p p e a r s t o e x i s t a l i m i t t o t h e d i s p e r s i o n w i t h t h e e x c e s s going i n t o 3-dimensional p a r t i c l e s i n some s o r t o f e q u i l i b r i u m w i t h t h e d i s p e r s e d phase.
162 TABLE 1 C o n c e n t r a t i o n l i m i t s o f metal i n t h e d i s p e r s e d p h a s e
Support
Metal
Pt Pt Rh Rh Pd Pd Re Mo co
L i m i t concentration umo l / m 2
2.2 7.1
Y-Al203 3 . 4 w t % Mo/Y-A1203 y-Al7-03 2 . 8 w t % Ce/Y-A1203 Y - A ~ ~3O 3 7 . 5 w t % Wo/ Y-A1203 Y -A1 2 0 3 Y-A1203 Zr02
2.5 5.4 2.2 5.2 2 .o 4.0
7.6
The above d i s c u s s i o n does n o t touch on t h e q u e s t i o n of t h e d e t a i l e d c o n f i g u r a t i o n of t h e dispersed phase.
Whether t h i s p h a s e i s p r e s e n t a s d i s c o n t i n u o u s
p a t c h e s ( s o c a l l e d r a f t s ) on o n l y p a r t s of t h e a v a i l a b l e s u r f a c e o r a s d i s c r e t e and i s o l a t e d atoms o r ensembles s p r e a d o v e r most of t h e a v a i l a b l e s u r f a c e remains a n u n r e s o l v e d i s s u e .
In some p a r t i c u l a r c a s e s , where s t r o n g metal
s u p p o r t i n t e r a c t i o n s are o b s e r v e d , e v i d e n c e can b e adduced t o show t h a t some m e t a l atoms ( o r m e t a l o x i d e m o l e c u l e s ) a r e p r e s e n t a s d i s c r e t e i s o l a t e d
e n t i t i e s i n t h e d i s p e r s e d p h a s e [l].
Temperature programmed r e d u c t i o n s t u d y as a measure o f s t r e n g t h o f i n t e r a c t i o n The u s e o f t e m p e r a t u r e programmed r e d u c t i o n t e c h n i q u e s t o measure t h e r e l a t i v e s t r e n g t h o f i n t e r a c t i o n between m e t a l o x i d e s d e p o s i t e d on a n i n s u l a t o r s u p p o r t i s d i s c u s s e d below.
S i n g l e component systems namely Rh/y-A1 0 [l] and 2 3
Pt/y-Al 0 [ 2 ] are used as examples. 2 3
F i g . 2 shows f o u r temperature-programmed Rh sample.
material.
reduction curves f o r a 5.51 w t %
A l l t h e r u n s were made c o n s e c u t i v e l y on t h e same sample o f t h e
The c a l c i n i n g a t p r o g r e s s i v e l y h i g h e r t e m p e r a t u r e s s p l i t s t h e low-
t e m p e r a t u r e peak of hydrogen u p t a k e i n t o two d i s t i n c t p e a k s .
After calcining
a t 7OO0C, o n e o b s e r v e s t h e f o r m a t i o n o f a d i s s o l v e d p h a s e which i s i r r e d u c i b l e
a t 4OO0C, m a n i f e s t e d by t h e hydrogen u p t a k e i n t h e 450-6OO0C r a n g e . The s p l i t t i n g o f t h e peak i n t h e low-temperature hydrogen u p t a k e n o t e d f o r t h e c o n c e n t r a t e d sample of F i g . 2 i s a b s e n t f o r t h e d i l u t e d sample.
This
s p l i t t i n g i s a t t r i b u t e d t o t h e d i f f e r e n c e i n r e d u c i b i l i t y between t h e d i s p e r s e d p h a s e and t h e c r y s t a l l i t e s which a r e b o t h p r e s e n t i n t h e c o n c e n t r a t e d sample. S u c c e s s i v e t r e a t m e n t s a t h i g h t e m p e r a t u r e s under o x i d i z i n g c o n d i t i o n s c a u s e
163
v)
k z
A. 4 0 0 ' C . A I R
3
a a a
k
m
-aa W Y
a
In 3
IN U
0
w
+ a [r
L L C. 6 0 0 " C . A I R
D. 700°C A I R
400 600 000 TEMPERATURE,"C
200
F i g . 2 . Temperature-programmed r e d u c t i o n of Rh ( 5 . 5 1 w t % ) / ~ - A l 2 0 3 . C a l c i n a t i o n t r e a t m e n t : ( A ) 4OO0C, 1 6 h r ; ( B ) 5OO0C, 1 6 h r ; (C) 600°C, 1 6 h r ; (D) 7 O O o C , 12 h r .
c r y s t a l l i t e growth, and a s h a r p e r d i f f e r e n t i a t i o n between t h e two p h a s e s o f t h e Rh, t h e two- and t h r e e - d i m e n s i o n a l p h a s e s , w i t h a concomitant d i f f e r e n t i a tion i n their reducibility.
P t / Y-A1203 A nonsupported P t 0 2 sample and f i v e Y-alumina-supported
samples of v a r y i n g
P t l o a d i n g were reduced a t t e m p e r a t u r e s r a n g i n g from -75 t o 50OoC.
The r e s u l t s
shown i n F i g . 3 i n d i c a t e t h a t t h e nonsupported P t 0 2 w a s reduced below 25OC w i t h t h e maximum r a t e a t % 5 O C ( c u r v e A ) . The temperature-programmed o x i d e on a y-A1203 s u p p o r t . F i g . 3 show b o t h p h a s e s :
r e d u c t i o n g i v e s e v i d e n c e o f two p h a s e s of P t The more c o n c e n t r a t e d samples ( c u r v e s B , C , and D ,
a p a r t i c u l a t e phase where t h e P t o x i d e h a s l i t t l e
i n t e r a c t i o n w i t h t h e alumina s u p p o r t as s e e n by t h e r e d u c t i o n a t Q I O ° C ,
resem-
b l i n g t h e r e d u c t i o n o f unsupported b u l k P t 0 2 ; and a d i s p e r s e d p h a s e where t h e P t o x i d e i n t e r a c t s more s t r o n g l y w i t h t h e s u p p o r t , and t h u s i s reduced a t higher temperatures.
I n t h e samples o f low P t l o a d i n g ( c u r v e s E and F ) ; o n l y
d i s p e r s e d p h a s e P t o x i d e i s found.
The H
2 u p t a k e measurements d u r i n g t h e s e
164
,
-75
0
100 200 300 TEM PERATURE,*C
400
500
F i g . 3. Temperature-programmed r e d u c t i o n of P t O ( 2 3 . 1 w t % P t , B ; 13.8 w t % P t , C ; 7 . 4 w t % P t , P t , E).
(A) and PtO /Y-A1203 samples wt % D; 0.78 w t %
6; 1 . 6 4
6t,
reductions i n d i c a t e t h a t P t oxide i n t h e dispersed phase i s P t O while P t i n 2 t h e p a r t i c u l a t e p h a s e i s p r o b a b l y mixed P t 0 2 , PtO, and m e t a l l i c P t . The l a r g e i n c r e a s e of t h e r e d u c t i o n t e m p e r a t u r e w i t h ' d i l u t i o n of P t 0 2 i n t h e d i s p e r s e d phase i n d i c a t e s t h a t a l l t h e dispersed Pt02 molecules a r e i n c l o s e contact w i t h t h e s u p p o r t and t h e y a r e p r o b a b l y s e p a r a t e d from e a c h o t h e r . t h i s s e p a r a t i o n , t h e s t r o n g e r i s t h e Pt02-A1203 i n t e r a c t i o n .
The l a r g e r
This i n t e r a c t i o n
may r e s u l t i n a d i s s o l u t i o n of P t 0 2 i n t o t h e s u r f a c e l a y e r of A 1 2 0 3 o r i n t h e f o r m a t i o n o f a PtO -4.1 0 complex as p r e v i o u s l y s u g g e s t e d C7-91. 2 2 3
S t u d i e s by p h y s i c a l methods The c h e m i s o r p t i o n and TPR are c u r r e n t l y t h e most unambiguous methods t o d i s t i n g u i s h t h e d i s p e r s e d p h a s e from p a r t i c u l a t e s i n a s u p p o r t e d m e t a l c a t a l y s t , b u t i n some cases, o t h e r e x p e r i m e n t a l t e c h n i q u e s , such as EPR [lo-ll] o r I R
[12-13]
are h e l p f u l i n f u r t h e r u n d e r s t a n d i n g of t h e metal-support
interaction.
EPR s t u d y of Rh/y-A1203 f o r example shows t h e o c c u r r e n c e of a d i s p r o p o r t i o n a t i o n r e a c t i o n , 2Rh3+
+
Rh2+
+ Rh4+,
i n t h e d i s p e r s e d p h a s e and t h e d i f f u s i o n
165
DPPH
I
F i g . 4 . EPR s p e c t r a o f t h e sample a f t e r v a r i o u s t r e a t m e n t s : (A) o u t g a s s e d a t 500°C, 2 h o u r s ; ( B ) exposed t o 0 2 , 60 t o r r , 25'C; (C) o u t g a s s e d a t 25OC, 1 h r ; ( D ) h e a t i n g a t 5OO0C i n a i r : ( E ) r e d u c e d , exposed t o a i r a t 25OC and h e a t e d i n vacuum a t 6OO0C f o r 1 6 h o u r s ; (F) exposed t o 0 2 , 60 t o r r , 25OC; ( G ) h e a t e d i n vacuum a t 7OO0C, 18 h o u r s and 8OO0C, 3 h o u r s ( s i g n a l i n t e n s i t y reduced t o half i n G).
of Rh2+ i o n s i n t o a Y-A1203 s u p p o r t .
Only t h e Rh
2+
ions i n the dispersed
p h a s e ( i n a n o x i d i z e d s t a t e ) chemisorbed O 2 and C O , and a r e r e s p o n s i b l e f o r the catalytic activity.
Upon h e a t - t r e a t m e n t
i n vacuo o r i n a i r a t >6OO0C,
Rh2+ i o n s d i s s o l v e i n t o y-A1203 and t h e c a t a l y s t i s d e a c t i v a t e d .
the
The Rh4+ i o n s
i n t h e d i s p e r s e d p h a s e s i t u a t e d i n t h e s a t u r a t e d and o c t a h e d r a l l y c o o r d i n a t e d s i t e s do n o t chemisorb as shown i n F i g . 4 . An I R s t u d y of CO chemisorbed on Rh/y-A1203
[ 1 2 ] shows t h e d i f f e r e n c e
between CO chemisorbed on t h e d i s p e r s e d p h a s e and t h a t o n t h e p a r t i c u l a t e phase. While o n l y t h e d o u b l e t band o f Rh(CO)2 a n d t h e v e r y b r o a d band of t h e b r i d g e d s p e c i e s Rh2 CO a r e found on Rh i n t h e d i s p e r s e d p h a s e , a s i n g l e t band o f RhCO a p p e a r s o n l y on Rh i n t h e p a r t i c u l a t e p h a s e .
The t h e r m a l s t a b i l i t y o f t h e s e
adsorbed s p e c i e s i s i n t h e sequence Rh2(CO) > RhCO > Rh(CO)2.
Other physical
166 methods s u c h as XPS [ 4 ] , TEN [2] and STEM [ I S ] were a l s o u s e d .
Reactivity studies Measurement of r e a c t i o n k i n e t i c s shows t h e d i f f e r e n c e i n c a t a l y t i c b e h a v i o r between t h e m e t a l s i n t h e d i s p e r s e d p h a s e and t h o s e i n t h e p a r t i c u l a t e phase. I n some cases, t h i s d i f f e r e n c e c a n b e i n d i c a t e d merely from t h e a c t i v i t y differences.
Thus i t was found t h a t NO r e d u c t i o n by H 2 o v e r Rh [16] o r P t [ 1 7 ]
i s a demanding r e a c t i o n which h a s a h i g h e r s p e c i f i c a c t i v i t y f o r Rh o r P t i n t h e p a r t i c u l a t e phase than i n t h e dispersed phase.
In other cases, t h i s
d i f f e r e n c e r e q u i r e s a more d e t a i l e d s t u d y t o e s t a b l i s h t h e s e l e c t i v i t y among possible reaction paths.
Two examples from o u r e x p e r i e n c e show t h e d i f f e r e n c e
i n s e l e c t i v i t y f o r t h e h y d r o g e n o l y s i s of n-pentane on Rh/y-A1203 of n e o p e n t a n e on Pt/y-A1203
[l8] and t h a t
[19].
The a n a l y s i s of t h e r e s u l t s i n d i c a t e s t h a t t h e h y d r o g e n o l y s i s o f n-pentane
on Rh i n t h e p a r t i c u l a t e p h a s e s t a r t s w i t h t h e s c i s s i o n o f t h e C1-C2
bond t o
form mainly C H 4 and n-butane w h i l e t h a t on Rh i n t h e d i s p e r s e d p h a s e s t a r t s w i t h t h e s c i s s i o n o f t h e C -C bond t o form e t h a n e and propane. The r e a c t i o n 2 3 p a t h s of neopentane w i t h H2 on Pt/y-A1 0 are e i t h e r h y d r o g e n o l y s i s o r
2 3
isomerization.
While o n l y h y d r o g e n o l y s i s o c c u r s on P t i n t h e d i s p e r s e d p h a s e ,
t h e i s o m e r i z a t i o n i s t h e main r e a c t i o n on P t i n t h e p a r t i c u l a t e p h a s e .
Effects of incorporation of a surface modifier The s e c o n d p a r t o f t h e t i t l e o f t h i s symposium a d d r e s s e s t h e ” m e t a l - a d d i t i v e effects” i n catalysis.
The examples d i s c u s s e d h e r e t o u c h upon t h i s i s s u e .
Indeed t h e a d d i t i v e s i n t h i s s e n s e a r e m e t a l o x i d e s whose oxygen i s l e s s t i g h t l y bound t h a n t h e oxygen i n t h e i n s u l a t o r s u p p o r t b u t more s t r o n g l y bound t h a n t h e oxygen a s s o c i a t e d w i t h t h e n o b l e metals.
Obviously, t h e s e
b a s e m e t a l o x i d e s can i n t e r a c t b o t h w i t h t h e s u p p o r t and t h e a c t i v e i n g r e d i e n t t o modify t h e d i s p e r s i o n o f t h e l a t t e r and t h e r e b y a l s o i t s a c t i v i t y . s y s t e m s h a v e l o n g been i n p r a c t i c a l u s e as evidenced by p a t e n t s
[no].
Such Although,
i n t h e s e i n s t a n c e s t h e e f f e c t of t h e u s e o f t h e m o d i f i e r s i s n o t d i r e c t l y i d e n t i f i e d w i t h t h e s u r f a c e i n t e r a c t i o n , o u r work i n d i c a t e s t h a t t h e p r a c t i c a l consequences a r e t h e r e s u l t o f s u c h phenomena.
Pt-PI00 /Y-Al2O3
-
3
A s t r o n g s u r f a c e i n t e r a c t i o n i s o b s e r v e d between t h e P t and Mo by v i r t u e of
t h e s h a r i n g o f some oxygen i o n s i n t h e Mo o x i d e w i t h n e i g h b o r i n g P t atoms. T h i s i n t e r a c t i o n h a s as one consequence a n i n c r e a s e i n t h e e x t e n t o f t h e d i s p e r s e d p h a s e of P t on t h e s u p p o r t , from 2 . 2 umole/m2 (BET) i n a sample w i t h o u t No03 t o 7 . 1 umole/m2 (BET) i n a sample c o n t a i n i n g 3.4% Mo a s measured by C O c h e m i s o r p t i o n ( T a b l e 1).
167
I -
4
13 0*1% P I
0 B
0
A
I6wl%PI
F i g . 5. Rate of H2 u p t a k e a s a f u n c t i o n o f t e m p e r a t u r e i n t h e TPR of Pt02 MOO and Pt02-Moo3 s u p p o r t e d on y-A1 0 3 2 3' The s h a r i n g of oxygen i s s e e n c l e a r l y from F i g . 5 [21],
where t h e f i r s t
r e d u c t i o n r a t e maximum in a TPR experiment o f s u p p o r t e d MoO3, a s s o c i a t e d w i t h most e a s i l y removable oxygens, merges w i t h t h e r a t e maximum a s s o c i a t e d w i t h t h e r e d u c t i o n of d i s p e r s e d P t 0 2 i t s e l f , which u s u a l l y o c c u r s a t a c o n s i d e r a b l e lower t e m p e r a t u r e due t o t h e weaker Pt-0 bond.
The e x t e n t o f t h i s t e m p e r a t u r e
s h i f t depends on t h e P t l o a d i n g and i s s h i f t e d f a r t h e r t o lower t e m p e r a t u r e s with a n increase i n t h e P t loading.
The f a c t t h a t t h e r e d u c t i o n a s s o c i a t e d
w i t h t h e removal o f oxygens more t i g h t l y bound t o Mo i s n o t a f f e c t e d by t h e p r e s e n c e o f P t a s e v i d e n c e d by t h e r e d u c t i o n t e m p e r a t u r e c o n s t a n c y of t h e second r a t e maximum, i n d i c a t e s t h a t t h e s p i l l o v e r o f hydrogen engendered by P t , which may b e invoked, does n o t e x p l a i n t h i s b e h a v i o r .
Corroborating evidence
i s a l s o o b t a i n e d from a n XPS s t u d y [14]. A s s e e n i n T a b l e 1, t h e p r e s e n c e o f Ce02 on y-A1203 e x t e n d s t h e d i s p e r s i o n
l i m i t of Rh, as measured by CO c h e m i s o r p t i o n , on Rh s u p p o r t e d on y-A1 0 con2 3 S i m i l a r l y , f o r Pd/Y-A1203 c a t a l y s t s t h e a d d i t i o n o f taining 2.8 w t % C e .
168 3 7 . 5 w t % WO
i n c r e a s e s t h e s a t u r a t i o n l i m i t o f Pd d i s p e r s i o n by a f a c t o r of 3 2 . 5 a s measured by CO c h e m i s o r p t i o n . Although i n a l l t h e s e t h r e e systems t h e a d d i t i o n o f b a s e metal o x i d e f o r t h e y--A1 0 s u p p o r t e d n o b l e metal c a t a l y s t 2 3 h a s caused l o w e r i n g i n t h e r e d u c t i o n t e m p e r a t u r e of t h e b a s e metal o x i d e , t h e e x t e n t of t h e s h i f t is d i f f e r e n t i n each case r e f l e c t i n g t h e d i f f e r e n c e i n t h e d e g r e e of s u r f a c e i n t e r a c t i o n s between t h e n o b l e m e t a l and t h e b a s e m e t a l o x i d e s . P r a c t i c.a___l consequences _.-_
of t h e i n t e r a c t i o n s
The work on t h e i n t e r a c t i o n s h a s g i v e n i n s i g h t s f o r t h e f o r m u l a t i o n o f c a t a l y s t s designed f o r p r a c t i c a l use. approaches w e r e a d o p t e d .
Based on t h e d e s i r e d a i m , d i f f e r e n t
Thus, i n t h e c a s e of Ru-containing c a t a l y s t s , t h e
main o b j e c t i v e when o p e r a t i n g under o x i d i z i n g c o n d i t i o n s i s t o p r e v e n t i t s volatilization.
For t h i s e n d , one t r i e s t o p r o v i d e a s s t r o n g a n i n t e r a c t i o n
a s p o s s i b l e by forming s t a b l e p e r o v s k i t e - t y p e mixed o x i d e s s u c h a s BaRuO
3' I n t h e c a s e of Rh s u p p o r t e d on y-A1 0 t h e i n t e r a c t i o n a t h i g h 2 3 t e m p e r a t u r e s c a u s e s f o r m a t i o n o f s u r f a c e and s u b - s u r f a c e s p i n e l s . For t h i s
LaRu03 [22].
r e a s o n one s u b s t i t u t e s t h e s u p p o r t by a more r e f r a c t o r y m a t e r i a l w i t h lesser tendency t o i n t e r a c t such a s z i r c o n i a o r a-alumina [23]. Again, t o i n c r e a s e t h e d i s p e r s i o n i n o r d e r t o u t i l i z e t h e p r e c i o u s m e t a l s u r f a c e t o i t s u t m o s t , one c a n u s e i n t e r a c t i o n m o d i f i e r s such as molybdenum i n t h e case of Pt/y-A1 0 [21] o r t u n g s t e n o x i d e i n t h e c a s e of Pd/y-A1,03 [6]. 2 3 The u n d e r s t a n d i n g of t h e physico-chemical n a t u r e o f t h e i n t e r a c t i o n and t h e i r f u t u r e q u a n t i f i c a t i o n , s h o u l d a f f o r d a means f o r t a i l o r i n g c a t a l y s t f o r s p e c i f i c purposes.
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