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Comparative Studies of Mixed Oxide Perovskite Catalysts, LaCoO3, LaFeO3 and LaAlO3 for Hydrogenation of Alkenes and Hydrogenolysis of Alkanes
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COMPARATIVE STUDIES OF MIXED OXIDE PEROVSKITE CATALYSTS, LaCoO,, LaFeO, AND LaAIO, FOR HYDROGENATION OF ALKENES AND HVDROGENOLYSIS OF ALKANES I(. ICHIMURA, Y. INOUE, I. KOJIYA, E. MIYAZAKI and I. YASUMORI Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meyro-ku, Tokyo 152, JAPAN.
ABSTRACT: Catalysis by perovskite type mixed oxides, LaA103, LaFe03 and LaCo03 was comparatively studied. The former two oxides were found to catalyze only the hydrogenation of alkenes in contrast with LaCoOQ which accelerates not only the hydrogenation but also the hydrogenolysis of alkanes. Ethylene hydrogenation on these three perovskites showed common kinetic features and is explained in terms of the associative mechanism which involves the change in the slow step from hydrogen adsorption to the hydrogenation of the ethyl radical with increasing temperature. Thermal desorption from a LaCo03 surface exposed to C2H4 or C2H6 gave spectra of C2H2 and CH4 produced by C-H and C-C bond dissociation, whereas desorption from LaA103 and LaFe03 provided no species other than the original admolecules. The X-ray photoelectron spectrum of LaCoOg was compared with those of other perovskites and its variation upon deactivation emphasized the contribution of the Co(II1) ion to the C-C bond rupture. A CNDO/MO calculation for C2H6 adsorption on 0-M-0-Y-0 clusters (M=Fe, A1 and Co) confirmed this view. The mechanism of the hydrogenolysis and the structure of the active surface are discussed.
1. INTRODUCTION In recent progress in research on heterogeneous catalysis, the importance of mixed oxides as catalyst has been well recognized because of their range of activity and selectivity for many re- ...~ actions. In particular, ABO; type,compounds having a perovskite structure have been shown to be promising catalysts, since they not only have a well-defined structure of a rigid cation-oxygen framework but are also capable of activating hydrogen which can react with alkenes'). In a previous study2), we examined catalysis by LaCo03 for the /--
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r e a c t i o n o f e t h a n e and e t h y l e n e w i t h hydrogen: e t h a n e w a s c o n v e r t e d t o methane by h y d r o g e n o l y s i s , w h e r e a s e t h y l e n e w a s i n i t i a l l y hydroSuch g e n a t e d t o e t h a n e which w a s t h e n decomposed i n t o methane. a c t i v i t i e s o f LaCo03 c a t a l y s t f o r h y d r o g e n a t i o n and C-C bond r u p t u r e were a l s o found f o r C3 t o C5 a l k e n e s and a l k a n e s 3 ) . I t is o f i n t e r e s t t o i n v e s t i g a t e t h e c a t a l y t i c p r o p e r t i e s of p e r o v s k i t e compounds i n which t h e C o ( I I 1 ) i o n is r e p l a c e d by o t h e r t r i v a l e n t i o n s without change i n g eo metrical c o n f i g u r a t i o n .
In the
p r e s e n t s t u d y , w e c h o s e LaA103 and LaFe03 s i n c e t h e r a d i i o f A l ( I I 1 ) and F e ( I I 1 ) i o n s are v e r y s i m i l a r t o t h a t of t h e C o ( I I 1 ) i o n , which r e t a i n s t h e s t r u c t u r a l s i m i l a r i t y among t h e t h r e e In addition t o t h e d e t a i l e d k i n e t i c analysis, a t h e r m a l d e s o r p t i o n t e c h n i q u e w a s employed t o r e v e a l t h e a d s o r b e d s t a t e s o f hydrogen and h y d r o c a r b o n s . X-ray p h o t o e l e c t r o n s p e c t r o scopy (XPS) was a l s o u s e d t o i n v e s t i g a t e t h e e l e c t r o n i c s t a t e of
p e r o v s k i t e compounds.
the perovskite catalysts. 2 . EXPERIMENTAL P e r o v s k i t e c a t a l y s t s , LaA103 and LaFe03, w e r e p r e p a r e d by t h e same method as t h a t u s e d p r e v i o u s l y f o r LaCoOg2); h y d r o x i d e s of each component c a t i o n were mixed i n an e q u i m o l a r r a t i o and t h e n h e a t e d i n a i r a t 13QQK f o r 24h. The f o r m a t i o n o f t h e p e r o v s k i t e s P r i o r t o each run o f t h e was c o n f i r m e d by X-ray d i f f r a c t i o n * ) . r e a c t i o n , t h e c a t a l y s t s w e r e a c t i v a t e d by e v a c u a t i o n a t 923K f o r LaA103 and 673K f o r LaFe03; t h e s e t e m p e r a t u r e s were h i g h e r t h a n t h a t u s e d f o r LaCo03, (573K)’). a closed c i r c u l a t i o n system.
The k i n e t i c s t u d y w a s performed i n The r e a c t a n t and p r o d u c t m o l e c u l e s i n
t h e r e a c t i o n o f h y d r o c a r b o n s w i t h H2 or D2 were g a s c h r o m a t o g r a p h i c a l l y s e p a r a t e d and t h e d e u t e r i u m d i s t r i b u t i o n s w e r e a n a l y z e d on a mass s p e c t r o m e t e r , H i t a c h i FMLJ-72)’3). The t h e r m a l d e s o r p t i o n w a s s t u d i e d by t h e u s e of an u l t r a h i g h vacuum chamber ( < I ~ l O - ~ T o r re)q u i p p e d w i t h a Bayard-Alpert gauge and a n UTI-100C q u a d r u p o l e mass s p e c t r o m e t e r for g a s a n a l y s i s . The temperature of t h e c a t a l y s t s w a s r a i s e d a t a constant rate of 1 0 K s-’. The h i g h e s t p r e s s u r e a t t h e d e s o r p t i o n peak w a s Torr. X - r a y p h o t o e l e c t r o n s p e c t r a were r e c o r d e d o n a Hewlett-Packard 59508 ESCA s p e c t r o m e t e r .
The c a t a l y s t s , a f t e r b e i n g p r e s s e d i n a
form o f d i s c s , were s u b j e c t e d t o t h e i n situ t r e a t m e n t u n d e r almost t h e same c o n d i t i o n s as w e r e employed i n t h e r e a c t i o n s t u d i e s . The C Is l e v e l o f a t r a c e of c o n t a m i n a n t c a r b o n s , 2 8 4 . 7 e V , w a s t a k e n
as r e f e r e n c e .
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3 . RESULTS 3 . 1 . R e a c t i o n s of C2H4 and C2H6 w i t h hydrogen F i g . 1 shows t h e A r r h e n i u s p l o t s of e t h y l e n e h y d r o g e n a t i o n o n c
14.0-
N
'8 r(
I
m Q)
rl
7 (u
d
0
E
\
-> 0
11.0
LaA103
m
3
1.6
2.0
2.4
lo3
2.8
3.2
3.6
T ( K - ~
F i g . 1. A r r h e n i u s p l o t s o f e t h y l e n e h y d r o g e n a t i o n . LaA103 and LaFe03, t o g e t h e r w i t h t h e p r e v i o u s r e s u l t s on LaCo03"). T h e r e a p p e a r e d a maximum i n r e s p e c t i v e p l o t s , and t h e h i g h e s t rates were o b s e r v e d a t around 490K f o r LaA103, 420K for LaFe03 and 420K
for LaCo03. The k i n e t i c s t u d i e s were c a r r i e d o u t i n t h e l o w and h i g h t e m p e r a t u r e r e g i o n s . The o b s e r v e d k i n e t i c p a r a m e t e r s o f t h e r e a c t i o n o n LaA103 and LaFe03 are summarized i n T a b l e 1 t o g e t h e r The r e a c t i o n o r d e r s i n d i c a t e s i m i l a r v a r i a w i t h t h o s e on LaCo03. t i o n with temperature independently of t h e o x i d e s used. Table 2 shows t h e t y p i c a l d i s t r i b u t i o n o f d e u t e r i u m atoms i n t h e r e a c t i o n w i t h D2 a t d i f f e r e n t t e m p e r a t u r e s . The o b s e r v e d f e a t u r e s a r e common TABLE 1 K i n e t i c p a r a m e t e r s of e t h y l e n e h y d r o g e n a t i o n
Catalysts
L
Surface Area(m2/g)
Reaction Ordersa m n
LaFe03
0.40
0 1
1
LaA103
7.9
0 1
LaCo03'
0.33
0 1
Ea
logVob( k J / m o l )
(333K) 1 (573K)
13.2 (420K)
92 -63
1 1
(353K) (573K)
(490K)
11.6
7 -85
1 1
(353K) (573K)
13.6 (420K)
34 -19
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TABLE 2 Deuterium distribution in C2H4 + D2 reaction system
region
(333K) 61 31 4 3 1
(35310 32 19.5 38 42.8 24 28.8 5 8.1 1 0.8
45
33 15 5
2 0
0
4 26 32 20 14 3 1
11 25
32
22
0.7
0.5 3.1 2.8 96.5 96.4
1 4 95
9 2
0
for the reaction on the three perovskite catalysts. In the high temperature region, an equilibrium among gaseous H2, HD and D2 is nearly established, and the deuterium-exchanged ethylenes up to [D41 were produced. In the low temperature region, however, HD and HZ were formed only in small amounts. The presence of highlyexchanged ethane in both temperature regions strongly suggest that the hydrogenation proceeds v i a an ethyl radical as intermediate. The reaction of C2H6 with H2 on LaFe03 and LaAlO was examined 3 at temperatures up to 673K hut no hydrogenolysis proceeded at a
1
Fig. 2. Reaction of C2H4 with H2 on LaCo03 at 573K. = 10.4 T o r r , P -141 Torr. Pc 2 4 H2
\ a,
a
m m &4
P4
Time / h
Comparative Studies of Mixed Oxide Perovskite Catalysts
m e a s u r a b l e rate.
Fig. 2
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shows t h e r e s u l t o f r e i n v e s t i g a t i n g t h e
r e a c t i o n o f C2H4 w i t h H2 on LaCo03 w i t h i n i t i a l p r e s s u r e s d i f f e r e n t from t h o s e p r e v i o u s l y r e p o r t e d ’ ) .
E t h a n e w a s m a i n l y produced a t an
i n i t i a l s t a g e o f t h e r e a c t i o n and w a s f o l l o w e d by methane f o r m a t i o n . The s o l i d l i n e s i n t h e f i g u r e a r e t h e s i m u l a t i o n c u r v e s o b t a i n e d by o p t i m i z i n g t h e r a t e c o n s t a n t s as p a r a m e t e r s . T h e c a t a l y t i c a c t i v i t y o f component o x i d e s , La203 and Co203, w a s a l s o examined a t 573K.
La203 was e v a c u a t e d a t t e m p e r a t u r e s up t o
7 7 3 K b u t f a i l e d t o accelerate b o t h t h e h y d r o g e n a t i o n and hydrog e n o l y s i s of e t h y l e n e . Co203, when e v a c u a t e d a t 600K f o r l h , s i m u l t a n e o u s l y c a t a l y z e d b o t h r e a c t i o n s b u t t h e o x i d e was q u i c k l y r e d u c e d t o m e t a l l i c C o w i t h hydrogen. 3 . 2 . Thermal d e s o r p t i o n s p e c t r a of H a , C2H4 and C2H6 The s a t u r a t e d amount o f a d s o r b e d H2 on t h e LaCo03 s u r f a c e a t The t h e r m a l d e s o r p t i o n
298K was e v a l u a t e d a t 4 x 1 0 l 4 m o l e c u l e
(TD) s p e c t r a o f hydrogen from t h e t h r e e p e r o v s k i t e s which were exposed t o hydrogen a t 298K c o n s i s t e d o f s u p e r p o s e d d e s o r p t i o n p e a k s b u t t h e y p o s s e s s e d some common f e a t u r e s ; t h e p e a k s c a n b e d i v i d e d i n t o two g r o u p s a (<340K) and 6 (>340K).
Respective groups w e r e o b v i o u s l y composed o f a t l e a s t t w o p e a k s . F i g . 3 ( a ) shows t h e TD s p e c t r a from LaCo03 exposed t o C2H6 a t 300K. S i n g l e p e a k s of C2H2 and C2H4 a p p e a r e d a t around 380K and t w o p e a k s o f C2H6 a t around 380 and 540K. Methane w a s o b s e r v e d above 450K.
Almost t h e same TD s p e c t r a were o b t a i n e d from t h e s u r -
f a c e exposed t o C2H4, a
a l t h o u g h t h e f r a c t i o n o f CH4 formed w a s
C2H2 F i g . 3 . TD s p e c t r a of a d s o r b e d C2H6 on LaCoOg ( t o p ) and hydrogen-prea d s o r b e d LaCo03(bottom).
Temperature / K
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smaller. LaCoO3 which retains the $-hydrogen on the surface was exposed to C2H6 at 2983 for 10 min. As shown in Fig. 3 (h), the TD spectra result in the complete absence of C2H2 and C2H4 peaks and the considerable enhancement of C2H6 and CH4 peaks without change in their positions. The figure also shows the consumption of the B1-hydrogen located at lower temperature side of 5 as a consequence of contact with C2H6, thus suggesting the species to be responsible f o r the hydrogenolysis and also for the hydrogenation of C2H4 and C2H2. The TD spectra of C2H4 and C2H6 from LaA103 and LaFe03 surfaces were relatively simple; they gave only the original molecules without undergoing dissociation, which indicates the desorption to be of the first order. The hydrogen-preadsorbed LaA103 and LaFeOg surfaces converted the subsequently adsorbed C2H4 only into C2E6. On these surfaces, B1-hydrogen was also consumed to a large extent. 3.3. X-ray photoelectron spectra of LaA103, LaFe03 and LaCo03 Fig. 4 shows the X-ray photoelectron spectra for the valence region of the pretreated three perovskite oxides. NO significant difference in the La 5 P and 0 2p levels among these perovskites was observed. However, LaCo03 is different from the other two oxides
Fig. 4 . X-ray photoelectron spectra in the valence region. A; fresh LaCo03, B; 50%-deactivated LaCo03, C ; 100%-deactivated LaCoC3.
15
10
5
0
Binding energy / eV
Comparative Studies of Mixed Oxide Perovskite Catalysts
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i n g i v i n g a s h a r p peak a t 0 . 3 e V below t h e Fermi l e v e l , which i s a s s i g n e d t o Co 3d l e v e l . The c a t a l y t i c a c t i v i t y o f LaCo03 c a t a l y s t w a s found t o d e c l i n e g r a d u a l l y by r e p e a t i n g t h e c y c l e of e v a c u a t i o n a t 600K and
a k i n e t i c r u n , b u t w a s r e c o v e r e d by o x i d i z i n g t h e p e r o v s k i t e i n a i r and e v a c u a t i n g a t 600K. F i g . 4 i l l u s t r a t e s t h e s p e c t r a o f LaCoOQ As t h e o x i d e s which p o s s e s s e d t h e d i f f e r e n t l e v e l s of a c t i v i t y . d e a c t i v a t i o n p r o c e e d e d , t h e peck p o s i t i o n of t h e C o 3d l e v e l s h i f t ed t o higher binding energy s i d e .
This f a c t evidently corresponded
to t h e p r e v i o u s f i n d i n g t h a t C o ( I I 1 ) w a s r e d u c e d t o C o ( I 1 ) d u r i n g the deactivation2). 4 . DISCUSSION
The s t r i k i n g common f e a t u r e of e t h y l e n e h y d r o g e n a t i o n on t h e s e p e r o v s k i t e s is t h a t t h e r e a c t i o n e x h i b i t s a maximum r a t e a t a d e f i -
n i t e t e m p e r a t u r e which depends on t h e n a t u r e o f e a c h c a t a l y s t . Such b e h a v i o r w a s o b s e r v e d f o r t h e same r e a c t i o n on Ni5). The d e u t e r i u m d i s t r i b u t i o n s o b s e r v e d e x c l u d e t h e p o s s i b i l i t y of t h e R i d e a l - E l e y mechanism and t h u s t h e a s s o c i a t i v e mechanism i s v a l i d f o r t h e r e a c t i o n on t h e p e r o v s k i t e s u r f a c e s .
I n t h e h i g h tempera-
t u r e r e g i o n , t h e o b s e r v e d d e u t e r i u m d i s t r i b u t i o n s l e a d t o t h e conc l u s i o n t h a t t h e hydrogenation of e t h y l r a d i c a l is a rate-determining step.
Thus, t h e f o l l o w i n g r a t e e q u a t i o n i s d e r i v e d , =
K l K 2 K 3'C2H4
PH2
(1)
where k i s t h e r a t e c o n s t a n t of t h e r a t e - d e t e r m i n i n g s t e p .
K3 d e n o t e t h e e q u i l i b r i u m c o n s t a n t s o f H 2 - a d s o r p t i o n ,
K1
to
CZH4-adsorp-
t i o n and t h e s u r f a c e r e a c t i o n between a d s o r b e d C2H4 and hydrogen
atoms t o form e t h y l r a d i c a l , r e s p e c t i v e l y .
The r e a c t i o n o r d e r s o f
Eq. ( 1 ) a r e c o n s i s t e n t w i t h t h e e x p e r i m e n t a l o n e s .
The n e g a t i v e
v a l u e s of Ea mean t h a t t h e t r u e a c t i v a t i o n e n e r g y of t h e s t e p , E t , is smaller t h a n t h e h e a t o f a d s o r p t i o n for t h e C2H5(a)+H(a) s y s t e m .
The f i r s t o r d e r d e s o r p t i o n of C2H4 from LaA103 and LaFe03 s u r f a c e s e n a b l e s u s t o e v a l u a t e t h e a c t i v a t i o n e n e r g y f o r C2H4 d e s o r p t i o n t o b e 92 - 97 K J mol-I on t h e a s s u m p t i o n of t h e f r e q u e n c y f a c t o r of lx1013 s-l.
Thus, t h e l a r g e v a r i a t i o n i n Ea m o s t l y r e f l e c t s t h a t
Et a n d / o r t h e h e a t o f a d s o r p t i o n o f Ha. I n t h e low t e m p e r a t u r e r e g i o n , t h e small amounts of HD and E2 s u g g e s t t h a t t h e a d s o r p t i o n of hydrogen i s t h e slowest p r o c e s s and t h u s l e a d s t o a r a t e e q u a t i o n of t h e f i r s t o r d e r w i t h r e s p e c t t o hydrogen p a r t i a l p r e s s u r e , i . e . , V = k ' P
(2)
H2 T h i s c o n c l u s i o n i s i n l i n e w i t h t h e o b s e r v e d p r e s s u r e dependence.
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The v a l u e s of Ea d i f f e r e d markedly e a c h o t h e r f o r t h e r e s p e c t i v e r e a c t i o n s on t h e t h r e e p e r o v s k i t e s , d e p e n d i n g upon t h e n a t u r e o f t h e component c a t i o n s . The h y d r o g e n a t i o n of C3 t o C4 o l e f i n e s on LaCo03 shows s i m i l a r b e h a v i o r t o t h a t of e t h y l e n e h y d r o g e n a t i o n , i . e . ,
t h e occur r ence of
maximum r a t e and v a r i a t i o n s i n r e a c t i o n o r d e r s and d e u t e r i u m d i s t r i b u t i o n s with temperature.
Accordingly,
it i s c o n c l u d e d t h a t
t h e s e h y d r o g e n a t i o n s p r o c e e d v i a t h e same pathway d e s c r i b e d by t h e a s s o c i a t i v e mechanism. The p r i n c i p a l d i f f e r e n c e i n t h e c a t a l y t i c p r o p e r t i e s o f LaCo03 and t h e o t h e r t w o o x i d e s is t h a t o n l y LaCo03 c a t a l y z e s t h e hydrog e n o l y s i s of a l k a n e s .
The r e a c t i o n of e t h y l e n e w i t h hydrogen on
LaCo03 i s r e p r e s e n t e d by t h e f o l l o w i n g scheme,
iik
C2H4 k$/. C2H6
iii k;
2CH4
(3)
By s i m u l a t i n g t h e v a r i a t i o n s i n t h e c o n c e n t r a t i o n s of C H 2 4 ' '2"6 and CH 4 w i t h t i m e as shown i n F i g . 2 . t h e r a t e c o n s t a n t s w e r e d e t e r m i n e d as k i = 1 . 1 1 0 k i = 0 . 1 4 1 and k; = 0 . 1 0 1 h - l a t 573K,
which i n d i c a t e s t h a t t h e f r a c t i o n o f t h e d i r e c t h y d r o g e n o l y s i s v i a p a t h ( i i ) w a s less t h a n 1 2 s 2 ) . I n t h e r e a c t i o n s of C3 and C4 a l k e n e s w i t h hydrogen on LaCo03, t h e f r a c t i o n s o f t h e d i r e c t hydrog e n o l y s i s were found t o be 01% f o r p r o p e n e and 10% f o r b u t e n e s 3 ) . These r e s u l t s g i v e s t r o n g e v i d e n c e of CHq f o r m a t i o n v i a t h e c o r r e s p o n d i n g a l k a n e s a s b e i n g t h e main pathway e x c e p t f o r t h e case of propene P r e v i o u s k i n e t i c s t u d i e s 3 ) o n t h e h y d r o g e n o l y s i s of a l k a n e s t o g i v e o n l y methane showed t h a t a l l r e a c t i o n o r d e r s w i t h r e s p e c t t o a l k a n e s were u n i t y b u t t h e hydrogen o r d e r s i n c r e a s e d w i t h t h e number of c a r b o n atoms as - 0 . 5 ,
0.0,
1 . 0 and 2.0 f o r t h e hydro-
r e s p e c t i v e l y . The prog e n o l y s i s o f CZH6, C3H8, C4H10 and C5H12. posed pathway i n v o l v e d t h e s i m u l t a n e o u s b r e a k i n g of C-C bonds i n t h e h y d r o c a r b o n s b y a t t a c k o f hydrogen atoms d i s t r i b u t e d o v e r
almost t h e e n t i r e s u r f a c e ; t h e p r i m a r y p r o c e s s e s w e r e d e s c r i b e d a s C2 H 6 + H ( a )
-t
CH3(a) + H---CHZ(a)---H
f o r e t h a n e and CnH2n+2
+ z(n-z)E(a)
-
2CH3(a) + (n-2)(H---CHz(a)---H)(5)
f o r h i g h e r a l k a n e s where H---CH c u r s o r on t h e s u r f a c e .
(4)
(a)---H r e p r e s e n t s methane p r e 2 R e a c t i o n ( 5 ) might t a k e p l a c e i n a c o n s e c -
Comparative Studies of Mixed Oxide Perovskite Catalysts 1289 u t i v e way i n v o l v i n g s e v e r a l s t e p s b u t i n t e r m e d i a t e s t h u s produced
are n o t l i k e l y t o d e s o r b i n t h e g a s p h a s e .
The hydrogen exchange
between CIi3(a) and CH2(a) o c c u r s q u i c k l y b e f o r e d e s o r p t i o n as methane. The l a r g e number of t h e a c t i v e s i t e s o b t a i n e d from t h e H2c h e m i s o r p t i o n o n LaCoO indicates that a large part 3 ’ 4x1Ol4 cm -’, of t h e s u r f a c e c o n t r i b u t e s t o t h e r e a c t i o n s c o n c e r n e d w i t h hydrogen. E i t h e r o r b o t h of t h e (110) and (122) p l a n e s i n t h e c u b i c s t r u c t u r e ( i n an a p p r o x i m a t e r e p r e s e n t a t i o n ) a p p e a r s t o b e m a i n l y exposed a t t h e s u r f a c e b e c a u s e o f t h e i r thermodynamic s t a b i l i t y .
I n t h e TD
s p e c t r a of C2H4 and C2H6, o n l y t h e LaCo03 s u r f a c e c a n g i v e t h e decomposed s p e c i e s as a r e s u l t o f C-H bond d i s s o c i a t i o n , .and, above 450K, CH4 is formed by t h e s u b s e q u e n t p r o c e s s e s o f t h e C-C bond b r e a k i n g and t h e hydrogen a d d i t i o n .
The c o m p a r a t i v e XPS s t u d y of
t h e p e r o v s k i t e s and t h e d e a c t i v a t e d LaCo03 d e m o n s t r a t e d t h e role of t h e C o ( 1 I I ) ion f o r t h e C-C bond s c i s s i o n i n t h e c a t a l y t i c hydrog e n o l y s i s of a l k a n e s .
XPS s p e c t r a a l s o showed t h e i n v a r i a n c e of
t h e L a 5p l e v e l f o r t h e t h r e e p e r o v s k i t e s .
I t h a s been known t h a t
LaCo03 and LaFe03 are s t a b l e a t 1273K even u n d e r e x t r e m e l y low p r e s s u r e s o f oxygen,
( l ~ l O - and ~ lx10-14
Torr, respectively6)).
These f i n d i n g s s u g g e s t t h a t t h e L a ( I I 1 ) i o n r e t a i n s t h e r i g i d framework o f t h e p e r o v s k i t e s t r u c t u r e and h e n c e i n h i b i t s t h e r e d u c t i o n of C o ( I I 1 ) d u r i n g t h e r e a c t i o n w i t h h y d r o g e n .
By t a k i n g i n t o
c o n s i d e r a t i o n t h e c a p a b i l i t y o f La203 t o a d s o r b h y d r o g e n , m e t h y l and c a r b e n e r a d i c a l s , w e c a n c o n c l u d e t h a t t h e a c t i v e s u r f a c e r e q u i r e s t h a t t h e Co and La i o n s of h i g h d e n s i t y be exposed w i t h an a p p r o p r i a t e c o n f i g u r a t i o n ; t h u s t h e ( 1 1 0 ) p l a n e shown a p p e a r s t o be t h e m o s t s u i t a b l e one f o r t h e c a t a l y s i s .
jn
Fig. 5 The sane
h o l d s f o r t h e a c t i v e s u r f a c e s o f LaFeOg and LaA103 on t h e hydrog e n a t i o n of a l k e n e s .
(110) p l a n e F i g . 5. P e r o v s k i t e s t r u c t u r e .
1290
K. Ichimura, Y. Inoue, I . Kojima, E. Miyazaki, I. Yasumori
The s p e c i f i c a c t i v i t y o f LaCo03 f o r h y d r o g e n o l y s i s o f a l k a n e s h a s been examined on t h e b a s i s o f a CNDO-MO c a l c u l a t i o n .
The
c a l c u l a t i o n w a s p e r f o r m e d f o r e t h a n e a d s o r b e d on a l i n e a r a t o m i c c l u s t e r , 0-M-0-M-0 (110) plane.
(M=Co, Fe and A l ) which r e p r e s e n t s a p a r t of t h e
The e n e r g i e s o f C2H6 a d s o r p t i o n and t h e c h a n g e i n C-C
bond e n e r g y w e r e c a l c u l a t e d by t h e u s e of p a r a m e t e r s a v a i l a b l e f o r t h e component a t o m s 7 ) , and by c h a n g i n g t h e d i s t a n c e between t h e I n t h e cases of a x e s of C2H6 a n d t h e c l u s t e r l a i d i n p a r a l l e l . [Co-01 and [Fe-01 c l u s t e r s , t h e a d s o r b e d s t a t e was found t o be e n e r g e t i c a l l y s t a b l e where t h e c e n t e r o f t h e m o l e c u l e i s l o c a t e d on t h e metal atom, w h e r e a s t h e s t a t e i n which t h e c e n t e r is on t h e t o p of t h e c e n t r a l 0 atom i s more s t a b l e f o r t h e [ A l - 0 1 c l u s t e r . The p e r c e n t a g e d e c r e a s e s i n t h e C-C bond e n e r g y i n d u c e d by a d s o r p t i o n
a t e q u i l i b r i u m d i s t a n c e s were e s t i m a t e d a t 3 . 7 , 1 . 7 and 6 . 4 % f o r [Al-01, [Fe-01, and [Co-01 c l u s t e r s , r e s p e c t i v e l y ; t h e l a r g e s t value f o r the LaCoO
3
[Co-01 c l u s t e r a c c o r d s w i t h t h e f i n d i n g t h a t t h e
s u r f a c e i s a c t i v e i n r u p t u r i n g t h e C-C
bond t o p r o d u c e CHq
i n comparison t o t h e o t h e r t w o p e r o v s k i t e s . I n t h i s s t u d y , w e s u c c e e d e d i n e l u c i d a t i n g t h e c o n t r i b u t i o n s of geometric and e l e c t r o n i c f a c t o r s t o t h e c a t a l y s i s by p e r o v s k i t e t y p e mixed o x i d e s .
The complex e f f e c t s of mixed o x i d e c a t a l y s t s
w i l l f u r t h e r b e r e v e a l e d by e x t e n d i n g t h e r e s e a r c h t o t h e dynamic b e h a v i o r o f r e a c t i n g m o l e c u l e s c o r r e l a t e d w i t h t h e s t r u c t u r e of catalyst surface. REFERENCES
l . L . A . P e d e r s e n a n d W.F. L i b b y , S c i e n c e , 175, 1 3 5 5 (1972 2.K. I c h i m u r a , Y . I n o u e and I . Yasumori, B u l l . Chem. SOC i n press. 3.K. I c h i m u r a . Y . I n o u e and I . Yasumori. B u l l . Chem. SOC submitted. 4 . F . Askham, I . Fankuchen and R . Ward, J . Am. Chem. SOC. 3799 ( 1 9 5 0 ) . 5 . E . K . R i d e a l , 3 . Chem. S o c . , 309 ( 1 9 2 2 ) ; S . S a t o and K . Miyahara, J. R e s . I n s t . C a t a l . (Hokkaido U n i v . ) , 1 3 , 1 0 ( 1 9 6 5 ) . 6 . T . Nakamura, G . Petzow and L . J . G a u c k l e r Mat. R e s . B u l l . , 14, 649 ( 1 9 7 9 ) . 7.D.W. C l a c k , N.S. Hush and J.R. Y a n d l e , J. Chem. P h y s . , 57, 3503 (1972).
DISCUSSION W.C. Conner ( U n i v . Massachusetts, Amherst) D i r e c t m e t a t h e s i s from e t h y l e n e - n o t
mediate
-
v i a a hydrogenated i n t e r -
would a p p e a r t o imply a m e t h y l e n e i n t e r m e d i a t e .
you c o n s i d e r e d t h e p o s s i b i l i t y of an a l k y l i n t e r m e d i a t e ?
Have
Comparative S t u d i e s of Mixed Oxide P e r o v s k i t e C a t a l y s t s
.
1291
Inoue The i n t e r e s t i n g f e a t u r e o f c a t a l y s i s by p e r o v s k i t e o x i d e ,
aCo03, f o r t h e h y d r o g e n a t i o n and h y d r o g e n o l y s i s is t h a t t h e u p t u r e of t h e carbon-carbon bond i n t h e h y d r o c a r b o n s t a k e s lace on t h e Co i o n , w h e r e a s t h e h y d r o g e n a t i o n p r o c e e d s m a i n l y ,n a p a i r of La and oxygen i o n s .
The a n a l y s i s o f t h e d e u t e r i u m
i i s t r i b u t i o n s and k i n e t i c s f o r e t h y l e n e h y d r o g e n a t i o n showed . h a t t h e a l k y l i n t e r m e d i a t e is formed i n t h i s p r o c e s s . However, : h i s i n t e r m e d i a t e a p p e a r s t o have a l i t t l e chance t o u n d e r g o :he d e c o m p o s i t i o n t o monocarbon s p e c i e s , s i n c e t h e a l k y l r a d i c a l i r o b a b l y r e m a i n s on t h e h y d r o g e n a t i o n s i t e s , i . e . , La and oxygen i o n s , and i s r e a d i l y r e a c t e d o f f t o e t h a n e . The f a c t t h a t t h e f r a c t i o n of t h e d i r e c t m e t a t h e s i s from e t h y l e n e is less t h a n
10% g i v e s s u p p o r t t o t h i s view. F u r t h e r m o r e , t h e a s s u m p t i o n o f t h e a l k y l i n t e r m e d i a t e i n t h e hydrogenolysis of t h e al kanes s u c h as e t h a n e , p r o p a n e , b u t a n e and p e n t a n e f a i l e d t o e x p l a i n t h e d e u t e r i u m d i s t r i b u t i o n s a s w e l l as t h e v a r i a t i o n i n t h e r e a c t i o n o r d e r w i t h r e s p e c t t o t h e hydrogen p r e s s u r e . Thus, we c a n e x c l u d e t h e c o n t r i b u t i o n o f t h e a l k y l i n t e r m e d i a t e t o t h e p r o c e s s of t h e C-C bond r u p t u r e . T.W. Geus (Univ.
Utrecht)
The i r o n and c o b a l t - c o n t a i n i n g p e r o v s k i t e s may b e r e d u c e d i n t h e h y d r o g e n - c o n t a i n i n g r e a c t i o n medium. Though n i c k e l a l u m i n a t e , f o r i n s t a n c e , is d i f f i c u l t t o r e d u c e t o X i and 8l2O3, prolonged t i m e s an d fo r high t emp erat u res l e a d t o r e d u c t i o n . Did you o b s e r v e a f t e r p l o l o n g e d t i m e s , e s p e c i a l l y w i t h t h e c o b a l t a t e , any r e d u c t i o n t o t h e m e t a l ?
Y . Inoue A LaCoOg c a t a l y s t c a n w i t h s t a n d r e d u c t i o n i n a r e d u c t i v e a t m o s p h e r e o f t h e h y d r o g e n a t i o n and h y d r o g e n o l y s i s f o r e x t r e m e l y l o n g e r times t h a n c o b a l t o x i d e c a t a l y s t s . However, t h e rep e t i t i o n of t h e k i n e t i c r u n and t h e s u b s e q u e n t e v a c u a t i o n a t 600 K g r a d u a l l y c a u s e d t h e d e a c t i v a t i o n o f t h e c a t a l y s t ( f o r example, t h e a c t i v i t y was a l m o s t c o m p l e t e l y l o s t by r e p e a t i n g , t h i s p r o c e d u r e more t h a n 100 t i m e s ) . For t h e d e a c t i v a t e d c a t a l y s t , X-ray d i f f r a c t i o n p a t t e r n showed s c a r c e l y p e a k s ass i g n e d t o m e t a l l i c c o b a l t . F u r t h e r m o r e , X-ray p h o t o e l e c t r o n spectroscopic study revealed t h a t t h e t r i v a l e n t cobalt ion i n LaCoOQ w a s r e d u c e d t o t h e d i v a l e n t one upon s u c h d e a c t i v a t i o n , b u t t h e r e e x i s t e d no c h a r a c t e r i s t i c X P S l i n e due t o t h e zerov a l e n t s t a t e of c o b a l t and c o b a l t m e t a l ( K . I c h i m u r a , Y . Inoue
1292
K . Ichimura, Y . Inoue. I. Kojima. E . Miyazaki, I . Yasumori
and I . Yasumori, Bull. Chem. SOC. Jpn., in press). From these findings, we concluded that the reduction of LaCoOg to metal hardly takes place under the present experimental conditions.
COMPARATIVE STUDIES OF MIXED OXIDE PEROVSKITE CATALYSTS, LaCoO,, LaFeO, AND LaAIO, FOR HYDROGENATION OF ALKENES AND HVDROGENOLYSIS OF ALKANES I(. ICHIMURA, Y. INOUE, I. KOJIYA, E. MIYAZAKI and I. YASUMORI Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meyro-ku, Tokyo 152, JAPAN.
ABSTRACT: Catalysis by perovskite type mixed oxides, LaA103, LaFe03 and LaCo03 was comparatively studied. The former two oxides were found to catalyze only the hydrogenation of alkenes in contrast with LaCoOQ which accelerates not only the hydrogenation but also the hydrogenolysis of alkanes. Ethylene hydrogenation on these three perovskites showed common kinetic features and is explained in terms of the associative mechanism which involves the change in the slow step from hydrogen adsorption to the hydrogenation of the ethyl radical with increasing temperature. Thermal desorption from a LaCo03 surface exposed to C2H4 or C2H6 gave spectra of C2H2 and CH4 produced by C-H and C-C bond dissociation, whereas desorption from LaA103 and LaFe03 provided no species other than the original admolecules. The X-ray photoelectron spectrum of LaCoOg was compared with those of other perovskites and its variation upon deactivation emphasized the contribution of the Co(II1) ion to the C-C bond rupture. A CNDO/MO calculation for C2H6 adsorption on 0-M-0-Y-0 clusters (M=Fe, A1 and Co) confirmed this view. The mechanism of the hydrogenolysis and the structure of the active surface are discussed.
1. INTRODUCTION In recent progress in research on heterogeneous catalysis, the importance of mixed oxides as catalyst has been well recognized because of their range of activity and selectivity for many re- ...~ actions. In particular, ABO; type,compounds having a perovskite structure have been shown to be promising catalysts, since they not only have a well-defined structure of a rigid cation-oxygen framework but are also capable of activating hydrogen which can react with alkenes'). In a previous study2), we examined catalysis by LaCo03 for the /--
1282
K. Ichimura, Y. Inoue, I. Kojima, E. Miyazaki, I . Yasumori
r e a c t i o n o f e t h a n e and e t h y l e n e w i t h hydrogen: e t h a n e w a s c o n v e r t e d t o methane by h y d r o g e n o l y s i s , w h e r e a s e t h y l e n e w a s i n i t i a l l y hydroSuch g e n a t e d t o e t h a n e which w a s t h e n decomposed i n t o methane. a c t i v i t i e s o f LaCo03 c a t a l y s t f o r h y d r o g e n a t i o n and C-C bond r u p t u r e were a l s o found f o r C3 t o C5 a l k e n e s and a l k a n e s 3 ) . I t is o f i n t e r e s t t o i n v e s t i g a t e t h e c a t a l y t i c p r o p e r t i e s of p e r o v s k i t e compounds i n which t h e C o ( I I 1 ) i o n is r e p l a c e d by o t h e r t r i v a l e n t i o n s without change i n g eo metrical c o n f i g u r a t i o n .
In the
p r e s e n t s t u d y , w e c h o s e LaA103 and LaFe03 s i n c e t h e r a d i i o f A l ( I I 1 ) and F e ( I I 1 ) i o n s are v e r y s i m i l a r t o t h a t of t h e C o ( I I 1 ) i o n , which r e t a i n s t h e s t r u c t u r a l s i m i l a r i t y among t h e t h r e e In addition t o t h e d e t a i l e d k i n e t i c analysis, a t h e r m a l d e s o r p t i o n t e c h n i q u e w a s employed t o r e v e a l t h e a d s o r b e d s t a t e s o f hydrogen and h y d r o c a r b o n s . X-ray p h o t o e l e c t r o n s p e c t r o scopy (XPS) was a l s o u s e d t o i n v e s t i g a t e t h e e l e c t r o n i c s t a t e of
p e r o v s k i t e compounds.
the perovskite catalysts. 2 . EXPERIMENTAL P e r o v s k i t e c a t a l y s t s , LaA103 and LaFe03, w e r e p r e p a r e d by t h e same method as t h a t u s e d p r e v i o u s l y f o r LaCoOg2); h y d r o x i d e s of each component c a t i o n were mixed i n an e q u i m o l a r r a t i o and t h e n h e a t e d i n a i r a t 13QQK f o r 24h. The f o r m a t i o n o f t h e p e r o v s k i t e s P r i o r t o each run o f t h e was c o n f i r m e d by X-ray d i f f r a c t i o n * ) . r e a c t i o n , t h e c a t a l y s t s w e r e a c t i v a t e d by e v a c u a t i o n a t 923K f o r LaA103 and 673K f o r LaFe03; t h e s e t e m p e r a t u r e s were h i g h e r t h a n t h a t u s e d f o r LaCo03, (573K)’). a closed c i r c u l a t i o n system.
The k i n e t i c s t u d y w a s performed i n The r e a c t a n t and p r o d u c t m o l e c u l e s i n
t h e r e a c t i o n o f h y d r o c a r b o n s w i t h H2 or D2 were g a s c h r o m a t o g r a p h i c a l l y s e p a r a t e d and t h e d e u t e r i u m d i s t r i b u t i o n s w e r e a n a l y z e d on a mass s p e c t r o m e t e r , H i t a c h i FMLJ-72)’3). The t h e r m a l d e s o r p t i o n w a s s t u d i e d by t h e u s e of an u l t r a h i g h vacuum chamber ( < I ~ l O - ~ T o r re)q u i p p e d w i t h a Bayard-Alpert gauge and a n UTI-100C q u a d r u p o l e mass s p e c t r o m e t e r for g a s a n a l y s i s . The temperature of t h e c a t a l y s t s w a s r a i s e d a t a constant rate of 1 0 K s-’. The h i g h e s t p r e s s u r e a t t h e d e s o r p t i o n peak w a s Torr. X - r a y p h o t o e l e c t r o n s p e c t r a were r e c o r d e d o n a Hewlett-Packard 59508 ESCA s p e c t r o m e t e r .
The c a t a l y s t s , a f t e r b e i n g p r e s s e d i n a
form o f d i s c s , were s u b j e c t e d t o t h e i n situ t r e a t m e n t u n d e r almost t h e same c o n d i t i o n s as w e r e employed i n t h e r e a c t i o n s t u d i e s . The C Is l e v e l o f a t r a c e of c o n t a m i n a n t c a r b o n s , 2 8 4 . 7 e V , w a s t a k e n
as r e f e r e n c e .
Comparative Studies of Mixed Oxide Perovskite Catalysts
1283
3 . RESULTS 3 . 1 . R e a c t i o n s of C2H4 and C2H6 w i t h hydrogen F i g . 1 shows t h e A r r h e n i u s p l o t s of e t h y l e n e h y d r o g e n a t i o n o n c
14.0-
N
'8 r(
I
m Q)
rl
7 (u
d
0
E
\
-> 0
11.0
LaA103
m
3
1.6
2.0
2.4
lo3
2.8
3.2
3.6
T ( K - ~
F i g . 1. A r r h e n i u s p l o t s o f e t h y l e n e h y d r o g e n a t i o n . LaA103 and LaFe03, t o g e t h e r w i t h t h e p r e v i o u s r e s u l t s on LaCo03"). T h e r e a p p e a r e d a maximum i n r e s p e c t i v e p l o t s , and t h e h i g h e s t rates were o b s e r v e d a t around 490K f o r LaA103, 420K for LaFe03 and 420K
for LaCo03. The k i n e t i c s t u d i e s were c a r r i e d o u t i n t h e l o w and h i g h t e m p e r a t u r e r e g i o n s . The o b s e r v e d k i n e t i c p a r a m e t e r s o f t h e r e a c t i o n o n LaA103 and LaFe03 are summarized i n T a b l e 1 t o g e t h e r The r e a c t i o n o r d e r s i n d i c a t e s i m i l a r v a r i a w i t h t h o s e on LaCo03. t i o n with temperature independently of t h e o x i d e s used. Table 2 shows t h e t y p i c a l d i s t r i b u t i o n o f d e u t e r i u m atoms i n t h e r e a c t i o n w i t h D2 a t d i f f e r e n t t e m p e r a t u r e s . The o b s e r v e d f e a t u r e s a r e common TABLE 1 K i n e t i c p a r a m e t e r s of e t h y l e n e h y d r o g e n a t i o n
Catalysts
L
Surface Area(m2/g)
Reaction Ordersa m n
LaFe03
0.40
0 1
1
LaA103
7.9
0 1
LaCo03'
0.33
0 1
Ea
logVob( k J / m o l )
(333K) 1 (573K)
13.2 (420K)
92 -63
1 1
(353K) (573K)
(490K)
11.6
7 -85
1 1
(353K) (573K)
13.6 (420K)
34 -19
1284
K. Ichimura, Y. Inoue. I. Kojima, E. Miyazaki, I. Yasumorj
TABLE 2 Deuterium distribution in C2H4 + D2 reaction system
region
(333K) 61 31 4 3 1
(35310 32 19.5 38 42.8 24 28.8 5 8.1 1 0.8
45
33 15 5
2 0
0
4 26 32 20 14 3 1
11 25
32
22
0.7
0.5 3.1 2.8 96.5 96.4
1 4 95
9 2
0
for the reaction on the three perovskite catalysts. In the high temperature region, an equilibrium among gaseous H2, HD and D2 is nearly established, and the deuterium-exchanged ethylenes up to [D41 were produced. In the low temperature region, however, HD and HZ were formed only in small amounts. The presence of highlyexchanged ethane in both temperature regions strongly suggest that the hydrogenation proceeds v i a an ethyl radical as intermediate. The reaction of C2H6 with H2 on LaFe03 and LaAlO was examined 3 at temperatures up to 673K hut no hydrogenolysis proceeded at a
1
Fig. 2. Reaction of C2H4 with H2 on LaCo03 at 573K. = 10.4 T o r r , P -141 Torr. Pc 2 4 H2
\ a,
a
m m &4
P4
Time / h
Comparative Studies of Mixed Oxide Perovskite Catalysts
m e a s u r a b l e rate.
Fig. 2
1285
shows t h e r e s u l t o f r e i n v e s t i g a t i n g t h e
r e a c t i o n o f C2H4 w i t h H2 on LaCo03 w i t h i n i t i a l p r e s s u r e s d i f f e r e n t from t h o s e p r e v i o u s l y r e p o r t e d ’ ) .
E t h a n e w a s m a i n l y produced a t an
i n i t i a l s t a g e o f t h e r e a c t i o n and w a s f o l l o w e d by methane f o r m a t i o n . The s o l i d l i n e s i n t h e f i g u r e a r e t h e s i m u l a t i o n c u r v e s o b t a i n e d by o p t i m i z i n g t h e r a t e c o n s t a n t s as p a r a m e t e r s . T h e c a t a l y t i c a c t i v i t y o f component o x i d e s , La203 and Co203, w a s a l s o examined a t 573K.
La203 was e v a c u a t e d a t t e m p e r a t u r e s up t o
7 7 3 K b u t f a i l e d t o accelerate b o t h t h e h y d r o g e n a t i o n and hydrog e n o l y s i s of e t h y l e n e . Co203, when e v a c u a t e d a t 600K f o r l h , s i m u l t a n e o u s l y c a t a l y z e d b o t h r e a c t i o n s b u t t h e o x i d e was q u i c k l y r e d u c e d t o m e t a l l i c C o w i t h hydrogen. 3 . 2 . Thermal d e s o r p t i o n s p e c t r a of H a , C2H4 and C2H6 The s a t u r a t e d amount o f a d s o r b e d H2 on t h e LaCo03 s u r f a c e a t The t h e r m a l d e s o r p t i o n
298K was e v a l u a t e d a t 4 x 1 0 l 4 m o l e c u l e
(TD) s p e c t r a o f hydrogen from t h e t h r e e p e r o v s k i t e s which were exposed t o hydrogen a t 298K c o n s i s t e d o f s u p e r p o s e d d e s o r p t i o n p e a k s b u t t h e y p o s s e s s e d some common f e a t u r e s ; t h e p e a k s c a n b e d i v i d e d i n t o two g r o u p s a (<340K) and 6 (>340K).
Respective groups w e r e o b v i o u s l y composed o f a t l e a s t t w o p e a k s . F i g . 3 ( a ) shows t h e TD s p e c t r a from LaCo03 exposed t o C2H6 a t 300K. S i n g l e p e a k s of C2H2 and C2H4 a p p e a r e d a t around 380K and t w o p e a k s o f C2H6 a t around 380 and 540K. Methane w a s o b s e r v e d above 450K.
Almost t h e same TD s p e c t r a were o b t a i n e d from t h e s u r -
f a c e exposed t o C2H4, a
a l t h o u g h t h e f r a c t i o n o f CH4 formed w a s
C2H2 F i g . 3 . TD s p e c t r a of a d s o r b e d C2H6 on LaCoOg ( t o p ) and hydrogen-prea d s o r b e d LaCo03(bottom).
Temperature / K
1286
K . Ichimura, Y . Inoue, I . Kojima, E . Miyazaki, I . YaSUmOri
smaller. LaCoO3 which retains the $-hydrogen on the surface was exposed to C2H6 at 2983 for 10 min. As shown in Fig. 3 (h), the TD spectra result in the complete absence of C2H2 and C2H4 peaks and the considerable enhancement of C2H6 and CH4 peaks without change in their positions. The figure also shows the consumption of the B1-hydrogen located at lower temperature side of 5 as a consequence of contact with C2H6, thus suggesting the species to be responsible f o r the hydrogenolysis and also for the hydrogenation of C2H4 and C2H2. The TD spectra of C2H4 and C2H6 from LaA103 and LaFe03 surfaces were relatively simple; they gave only the original molecules without undergoing dissociation, which indicates the desorption to be of the first order. The hydrogen-preadsorbed LaA103 and LaFeOg surfaces converted the subsequently adsorbed C2H4 only into C2E6. On these surfaces, B1-hydrogen was also consumed to a large extent. 3.3. X-ray photoelectron spectra of LaA103, LaFe03 and LaCo03 Fig. 4 shows the X-ray photoelectron spectra for the valence region of the pretreated three perovskite oxides. NO significant difference in the La 5 P and 0 2p levels among these perovskites was observed. However, LaCo03 is different from the other two oxides
Fig. 4 . X-ray photoelectron spectra in the valence region. A; fresh LaCo03, B; 50%-deactivated LaCo03, C ; 100%-deactivated LaCoC3.
15
10
5
0
Binding energy / eV
Comparative Studies of Mixed Oxide Perovskite Catalysts
1287
i n g i v i n g a s h a r p peak a t 0 . 3 e V below t h e Fermi l e v e l , which i s a s s i g n e d t o Co 3d l e v e l . The c a t a l y t i c a c t i v i t y o f LaCo03 c a t a l y s t w a s found t o d e c l i n e g r a d u a l l y by r e p e a t i n g t h e c y c l e of e v a c u a t i o n a t 600K and
a k i n e t i c r u n , b u t w a s r e c o v e r e d by o x i d i z i n g t h e p e r o v s k i t e i n a i r and e v a c u a t i n g a t 600K. F i g . 4 i l l u s t r a t e s t h e s p e c t r a o f LaCoOQ As t h e o x i d e s which p o s s e s s e d t h e d i f f e r e n t l e v e l s of a c t i v i t y . d e a c t i v a t i o n p r o c e e d e d , t h e peck p o s i t i o n of t h e C o 3d l e v e l s h i f t ed t o higher binding energy s i d e .
This f a c t evidently corresponded
to t h e p r e v i o u s f i n d i n g t h a t C o ( I I 1 ) w a s r e d u c e d t o C o ( I 1 ) d u r i n g the deactivation2). 4 . DISCUSSION
The s t r i k i n g common f e a t u r e of e t h y l e n e h y d r o g e n a t i o n on t h e s e p e r o v s k i t e s is t h a t t h e r e a c t i o n e x h i b i t s a maximum r a t e a t a d e f i -
n i t e t e m p e r a t u r e which depends on t h e n a t u r e o f e a c h c a t a l y s t . Such b e h a v i o r w a s o b s e r v e d f o r t h e same r e a c t i o n on Ni5). The d e u t e r i u m d i s t r i b u t i o n s o b s e r v e d e x c l u d e t h e p o s s i b i l i t y of t h e R i d e a l - E l e y mechanism and t h u s t h e a s s o c i a t i v e mechanism i s v a l i d f o r t h e r e a c t i o n on t h e p e r o v s k i t e s u r f a c e s .
I n t h e h i g h tempera-
t u r e r e g i o n , t h e o b s e r v e d d e u t e r i u m d i s t r i b u t i o n s l e a d t o t h e conc l u s i o n t h a t t h e hydrogenation of e t h y l r a d i c a l is a rate-determining step.
Thus, t h e f o l l o w i n g r a t e e q u a t i o n i s d e r i v e d , =
K l K 2 K 3'C2H4
PH2
(1)
where k i s t h e r a t e c o n s t a n t of t h e r a t e - d e t e r m i n i n g s t e p .
K3 d e n o t e t h e e q u i l i b r i u m c o n s t a n t s o f H 2 - a d s o r p t i o n ,
K1
to
CZH4-adsorp-
t i o n and t h e s u r f a c e r e a c t i o n between a d s o r b e d C2H4 and hydrogen
atoms t o form e t h y l r a d i c a l , r e s p e c t i v e l y .
The r e a c t i o n o r d e r s o f
Eq. ( 1 ) a r e c o n s i s t e n t w i t h t h e e x p e r i m e n t a l o n e s .
The n e g a t i v e
v a l u e s of Ea mean t h a t t h e t r u e a c t i v a t i o n e n e r g y of t h e s t e p , E t , is smaller t h a n t h e h e a t o f a d s o r p t i o n for t h e C2H5(a)+H(a) s y s t e m .
The f i r s t o r d e r d e s o r p t i o n of C2H4 from LaA103 and LaFe03 s u r f a c e s e n a b l e s u s t o e v a l u a t e t h e a c t i v a t i o n e n e r g y f o r C2H4 d e s o r p t i o n t o b e 92 - 97 K J mol-I on t h e a s s u m p t i o n of t h e f r e q u e n c y f a c t o r of lx1013 s-l.
Thus, t h e l a r g e v a r i a t i o n i n Ea m o s t l y r e f l e c t s t h a t
Et a n d / o r t h e h e a t o f a d s o r p t i o n o f Ha. I n t h e low t e m p e r a t u r e r e g i o n , t h e small amounts of HD and E2 s u g g e s t t h a t t h e a d s o r p t i o n of hydrogen i s t h e slowest p r o c e s s and t h u s l e a d s t o a r a t e e q u a t i o n of t h e f i r s t o r d e r w i t h r e s p e c t t o hydrogen p a r t i a l p r e s s u r e , i . e . , V = k ' P
(2)
H2 T h i s c o n c l u s i o n i s i n l i n e w i t h t h e o b s e r v e d p r e s s u r e dependence.
1288
K. Ichimura, Y. Inoue, I . K o j i m a , E . Y i y a z a k i , I. Yasumori
The v a l u e s of Ea d i f f e r e d markedly e a c h o t h e r f o r t h e r e s p e c t i v e r e a c t i o n s on t h e t h r e e p e r o v s k i t e s , d e p e n d i n g upon t h e n a t u r e o f t h e component c a t i o n s . The h y d r o g e n a t i o n of C3 t o C4 o l e f i n e s on LaCo03 shows s i m i l a r b e h a v i o r t o t h a t of e t h y l e n e h y d r o g e n a t i o n , i . e . ,
t h e occur r ence of
maximum r a t e and v a r i a t i o n s i n r e a c t i o n o r d e r s and d e u t e r i u m d i s t r i b u t i o n s with temperature.
Accordingly,
it i s c o n c l u d e d t h a t
t h e s e h y d r o g e n a t i o n s p r o c e e d v i a t h e same pathway d e s c r i b e d by t h e a s s o c i a t i v e mechanism. The p r i n c i p a l d i f f e r e n c e i n t h e c a t a l y t i c p r o p e r t i e s o f LaCo03 and t h e o t h e r t w o o x i d e s is t h a t o n l y LaCo03 c a t a l y z e s t h e hydrog e n o l y s i s of a l k a n e s .
The r e a c t i o n of e t h y l e n e w i t h hydrogen on
LaCo03 i s r e p r e s e n t e d by t h e f o l l o w i n g scheme,
iik
C2H4 k$/. C2H6
iii k;
2CH4
(3)
By s i m u l a t i n g t h e v a r i a t i o n s i n t h e c o n c e n t r a t i o n s of C H 2 4 ' '2"6 and CH 4 w i t h t i m e as shown i n F i g . 2 . t h e r a t e c o n s t a n t s w e r e d e t e r m i n e d as k i = 1 . 1 1 0 k i = 0 . 1 4 1 and k; = 0 . 1 0 1 h - l a t 573K,
which i n d i c a t e s t h a t t h e f r a c t i o n o f t h e d i r e c t h y d r o g e n o l y s i s v i a p a t h ( i i ) w a s less t h a n 1 2 s 2 ) . I n t h e r e a c t i o n s of C3 and C4 a l k e n e s w i t h hydrogen on LaCo03, t h e f r a c t i o n s o f t h e d i r e c t hydrog e n o l y s i s were found t o be 01% f o r p r o p e n e and 10% f o r b u t e n e s 3 ) . These r e s u l t s g i v e s t r o n g e v i d e n c e of CHq f o r m a t i o n v i a t h e c o r r e s p o n d i n g a l k a n e s a s b e i n g t h e main pathway e x c e p t f o r t h e case of propene P r e v i o u s k i n e t i c s t u d i e s 3 ) o n t h e h y d r o g e n o l y s i s of a l k a n e s t o g i v e o n l y methane showed t h a t a l l r e a c t i o n o r d e r s w i t h r e s p e c t t o a l k a n e s were u n i t y b u t t h e hydrogen o r d e r s i n c r e a s e d w i t h t h e number of c a r b o n atoms as - 0 . 5 ,
0.0,
1 . 0 and 2.0 f o r t h e hydro-
r e s p e c t i v e l y . The prog e n o l y s i s o f CZH6, C3H8, C4H10 and C5H12. posed pathway i n v o l v e d t h e s i m u l t a n e o u s b r e a k i n g of C-C bonds i n t h e h y d r o c a r b o n s b y a t t a c k o f hydrogen atoms d i s t r i b u t e d o v e r
almost t h e e n t i r e s u r f a c e ; t h e p r i m a r y p r o c e s s e s w e r e d e s c r i b e d a s C2 H 6 + H ( a )
-t
CH3(a) + H---CHZ(a)---H
f o r e t h a n e and CnH2n+2
+ z(n-z)E(a)
-
2CH3(a) + (n-2)(H---CHz(a)---H)(5)
f o r h i g h e r a l k a n e s where H---CH c u r s o r on t h e s u r f a c e .
(4)
(a)---H r e p r e s e n t s methane p r e 2 R e a c t i o n ( 5 ) might t a k e p l a c e i n a c o n s e c -
Comparative Studies of Mixed Oxide Perovskite Catalysts 1289 u t i v e way i n v o l v i n g s e v e r a l s t e p s b u t i n t e r m e d i a t e s t h u s produced
are n o t l i k e l y t o d e s o r b i n t h e g a s p h a s e .
The hydrogen exchange
between CIi3(a) and CH2(a) o c c u r s q u i c k l y b e f o r e d e s o r p t i o n as methane. The l a r g e number of t h e a c t i v e s i t e s o b t a i n e d from t h e H2c h e m i s o r p t i o n o n LaCoO indicates that a large part 3 ’ 4x1Ol4 cm -’, of t h e s u r f a c e c o n t r i b u t e s t o t h e r e a c t i o n s c o n c e r n e d w i t h hydrogen. E i t h e r o r b o t h of t h e (110) and (122) p l a n e s i n t h e c u b i c s t r u c t u r e ( i n an a p p r o x i m a t e r e p r e s e n t a t i o n ) a p p e a r s t o b e m a i n l y exposed a t t h e s u r f a c e b e c a u s e o f t h e i r thermodynamic s t a b i l i t y .
I n t h e TD
s p e c t r a of C2H4 and C2H6, o n l y t h e LaCo03 s u r f a c e c a n g i v e t h e decomposed s p e c i e s as a r e s u l t o f C-H bond d i s s o c i a t i o n , .and, above 450K, CH4 is formed by t h e s u b s e q u e n t p r o c e s s e s o f t h e C-C bond b r e a k i n g and t h e hydrogen a d d i t i o n .
The c o m p a r a t i v e XPS s t u d y of
t h e p e r o v s k i t e s and t h e d e a c t i v a t e d LaCo03 d e m o n s t r a t e d t h e role of t h e C o ( 1 I I ) ion f o r t h e C-C bond s c i s s i o n i n t h e c a t a l y t i c hydrog e n o l y s i s of a l k a n e s .
XPS s p e c t r a a l s o showed t h e i n v a r i a n c e of
t h e L a 5p l e v e l f o r t h e t h r e e p e r o v s k i t e s .
I t h a s been known t h a t
LaCo03 and LaFe03 are s t a b l e a t 1273K even u n d e r e x t r e m e l y low p r e s s u r e s o f oxygen,
( l ~ l O - and ~ lx10-14
Torr, respectively6)).
These f i n d i n g s s u g g e s t t h a t t h e L a ( I I 1 ) i o n r e t a i n s t h e r i g i d framework o f t h e p e r o v s k i t e s t r u c t u r e and h e n c e i n h i b i t s t h e r e d u c t i o n of C o ( I I 1 ) d u r i n g t h e r e a c t i o n w i t h h y d r o g e n .
By t a k i n g i n t o
c o n s i d e r a t i o n t h e c a p a b i l i t y o f La203 t o a d s o r b h y d r o g e n , m e t h y l and c a r b e n e r a d i c a l s , w e c a n c o n c l u d e t h a t t h e a c t i v e s u r f a c e r e q u i r e s t h a t t h e Co and La i o n s of h i g h d e n s i t y be exposed w i t h an a p p r o p r i a t e c o n f i g u r a t i o n ; t h u s t h e ( 1 1 0 ) p l a n e shown a p p e a r s t o be t h e m o s t s u i t a b l e one f o r t h e c a t a l y s i s .
jn
Fig. 5 The sane
h o l d s f o r t h e a c t i v e s u r f a c e s o f LaFeOg and LaA103 on t h e hydrog e n a t i o n of a l k e n e s .
(110) p l a n e F i g . 5. P e r o v s k i t e s t r u c t u r e .
1290
K. Ichimura, Y. Inoue, I . Kojima, E. Miyazaki, I. Yasumori
The s p e c i f i c a c t i v i t y o f LaCo03 f o r h y d r o g e n o l y s i s o f a l k a n e s h a s been examined on t h e b a s i s o f a CNDO-MO c a l c u l a t i o n .
The
c a l c u l a t i o n w a s p e r f o r m e d f o r e t h a n e a d s o r b e d on a l i n e a r a t o m i c c l u s t e r , 0-M-0-M-0 (110) plane.
(M=Co, Fe and A l ) which r e p r e s e n t s a p a r t of t h e
The e n e r g i e s o f C2H6 a d s o r p t i o n and t h e c h a n g e i n C-C
bond e n e r g y w e r e c a l c u l a t e d by t h e u s e of p a r a m e t e r s a v a i l a b l e f o r t h e component a t o m s 7 ) , and by c h a n g i n g t h e d i s t a n c e between t h e I n t h e cases of a x e s of C2H6 a n d t h e c l u s t e r l a i d i n p a r a l l e l . [Co-01 and [Fe-01 c l u s t e r s , t h e a d s o r b e d s t a t e was found t o be e n e r g e t i c a l l y s t a b l e where t h e c e n t e r o f t h e m o l e c u l e i s l o c a t e d on t h e metal atom, w h e r e a s t h e s t a t e i n which t h e c e n t e r is on t h e t o p of t h e c e n t r a l 0 atom i s more s t a b l e f o r t h e [ A l - 0 1 c l u s t e r . The p e r c e n t a g e d e c r e a s e s i n t h e C-C bond e n e r g y i n d u c e d by a d s o r p t i o n
a t e q u i l i b r i u m d i s t a n c e s were e s t i m a t e d a t 3 . 7 , 1 . 7 and 6 . 4 % f o r [Al-01, [Fe-01, and [Co-01 c l u s t e r s , r e s p e c t i v e l y ; t h e l a r g e s t value f o r the LaCoO
3
[Co-01 c l u s t e r a c c o r d s w i t h t h e f i n d i n g t h a t t h e
s u r f a c e i s a c t i v e i n r u p t u r i n g t h e C-C
bond t o p r o d u c e CHq
i n comparison t o t h e o t h e r t w o p e r o v s k i t e s . I n t h i s s t u d y , w e s u c c e e d e d i n e l u c i d a t i n g t h e c o n t r i b u t i o n s of geometric and e l e c t r o n i c f a c t o r s t o t h e c a t a l y s i s by p e r o v s k i t e t y p e mixed o x i d e s .
The complex e f f e c t s of mixed o x i d e c a t a l y s t s
w i l l f u r t h e r b e r e v e a l e d by e x t e n d i n g t h e r e s e a r c h t o t h e dynamic b e h a v i o r o f r e a c t i n g m o l e c u l e s c o r r e l a t e d w i t h t h e s t r u c t u r e of catalyst surface. REFERENCES
l . L . A . P e d e r s e n a n d W.F. L i b b y , S c i e n c e , 175, 1 3 5 5 (1972 2.K. I c h i m u r a , Y . I n o u e and I . Yasumori, B u l l . Chem. SOC i n press. 3.K. I c h i m u r a . Y . I n o u e and I . Yasumori. B u l l . Chem. SOC submitted. 4 . F . Askham, I . Fankuchen and R . Ward, J . Am. Chem. SOC. 3799 ( 1 9 5 0 ) . 5 . E . K . R i d e a l , 3 . Chem. S o c . , 309 ( 1 9 2 2 ) ; S . S a t o and K . Miyahara, J. R e s . I n s t . C a t a l . (Hokkaido U n i v . ) , 1 3 , 1 0 ( 1 9 6 5 ) . 6 . T . Nakamura, G . Petzow and L . J . G a u c k l e r Mat. R e s . B u l l . , 14, 649 ( 1 9 7 9 ) . 7.D.W. C l a c k , N.S. Hush and J.R. Y a n d l e , J. Chem. P h y s . , 57, 3503 (1972).
DISCUSSION W.C. Conner ( U n i v . Massachusetts, Amherst) D i r e c t m e t a t h e s i s from e t h y l e n e - n o t
mediate
-
v i a a hydrogenated i n t e r -
would a p p e a r t o imply a m e t h y l e n e i n t e r m e d i a t e .
you c o n s i d e r e d t h e p o s s i b i l i t y of an a l k y l i n t e r m e d i a t e ?
Have
Comparative S t u d i e s of Mixed Oxide P e r o v s k i t e C a t a l y s t s
.
1291
Inoue The i n t e r e s t i n g f e a t u r e o f c a t a l y s i s by p e r o v s k i t e o x i d e ,
aCo03, f o r t h e h y d r o g e n a t i o n and h y d r o g e n o l y s i s is t h a t t h e u p t u r e of t h e carbon-carbon bond i n t h e h y d r o c a r b o n s t a k e s lace on t h e Co i o n , w h e r e a s t h e h y d r o g e n a t i o n p r o c e e d s m a i n l y ,n a p a i r of La and oxygen i o n s .
The a n a l y s i s o f t h e d e u t e r i u m
i i s t r i b u t i o n s and k i n e t i c s f o r e t h y l e n e h y d r o g e n a t i o n showed . h a t t h e a l k y l i n t e r m e d i a t e is formed i n t h i s p r o c e s s . However, : h i s i n t e r m e d i a t e a p p e a r s t o have a l i t t l e chance t o u n d e r g o :he d e c o m p o s i t i o n t o monocarbon s p e c i e s , s i n c e t h e a l k y l r a d i c a l i r o b a b l y r e m a i n s on t h e h y d r o g e n a t i o n s i t e s , i . e . , La and oxygen i o n s , and i s r e a d i l y r e a c t e d o f f t o e t h a n e . The f a c t t h a t t h e f r a c t i o n of t h e d i r e c t m e t a t h e s i s from e t h y l e n e is less t h a n
10% g i v e s s u p p o r t t o t h i s view. F u r t h e r m o r e , t h e a s s u m p t i o n o f t h e a l k y l i n t e r m e d i a t e i n t h e hydrogenolysis of t h e al kanes s u c h as e t h a n e , p r o p a n e , b u t a n e and p e n t a n e f a i l e d t o e x p l a i n t h e d e u t e r i u m d i s t r i b u t i o n s a s w e l l as t h e v a r i a t i o n i n t h e r e a c t i o n o r d e r w i t h r e s p e c t t o t h e hydrogen p r e s s u r e . Thus, we c a n e x c l u d e t h e c o n t r i b u t i o n o f t h e a l k y l i n t e r m e d i a t e t o t h e p r o c e s s of t h e C-C bond r u p t u r e . T.W. Geus (Univ.
Utrecht)
The i r o n and c o b a l t - c o n t a i n i n g p e r o v s k i t e s may b e r e d u c e d i n t h e h y d r o g e n - c o n t a i n i n g r e a c t i o n medium. Though n i c k e l a l u m i n a t e , f o r i n s t a n c e , is d i f f i c u l t t o r e d u c e t o X i and 8l2O3, prolonged t i m e s an d fo r high t emp erat u res l e a d t o r e d u c t i o n . Did you o b s e r v e a f t e r p l o l o n g e d t i m e s , e s p e c i a l l y w i t h t h e c o b a l t a t e , any r e d u c t i o n t o t h e m e t a l ?
Y . Inoue A LaCoOg c a t a l y s t c a n w i t h s t a n d r e d u c t i o n i n a r e d u c t i v e a t m o s p h e r e o f t h e h y d r o g e n a t i o n and h y d r o g e n o l y s i s f o r e x t r e m e l y l o n g e r times t h a n c o b a l t o x i d e c a t a l y s t s . However, t h e rep e t i t i o n of t h e k i n e t i c r u n and t h e s u b s e q u e n t e v a c u a t i o n a t 600 K g r a d u a l l y c a u s e d t h e d e a c t i v a t i o n o f t h e c a t a l y s t ( f o r example, t h e a c t i v i t y was a l m o s t c o m p l e t e l y l o s t by r e p e a t i n g , t h i s p r o c e d u r e more t h a n 100 t i m e s ) . For t h e d e a c t i v a t e d c a t a l y s t , X-ray d i f f r a c t i o n p a t t e r n showed s c a r c e l y p e a k s ass i g n e d t o m e t a l l i c c o b a l t . F u r t h e r m o r e , X-ray p h o t o e l e c t r o n spectroscopic study revealed t h a t t h e t r i v a l e n t cobalt ion i n LaCoOQ w a s r e d u c e d t o t h e d i v a l e n t one upon s u c h d e a c t i v a t i o n , b u t t h e r e e x i s t e d no c h a r a c t e r i s t i c X P S l i n e due t o t h e zerov a l e n t s t a t e of c o b a l t and c o b a l t m e t a l ( K . I c h i m u r a , Y . Inoue
1292
K . Ichimura, Y . Inoue. I. Kojima. E . Miyazaki, I . Yasumori
and I . Yasumori, Bull. Chem. SOC. Jpn., in press). From these findings, we concluded that the reduction of LaCoOg to metal hardly takes place under the present experimental conditions.