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Reactivities of Surface Intermediates on an Sm2O3 Catalyst Studied by in Situ Infrared Spectroscopy
L et al. (Editors), New Frontiers in Catalysis Proceedingsof the 10th International Congress on Catalysis,19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights mewed G&,
REACI'IVITIES OF SURFACE INTERMEDIATES ON AN Sm203 CATALYST
STUDIED BY IN SITU INFRARED SPECTROSCOPY
X
Sakatq M. Yoshino, T.Fuhda, H. Yamaguchi H. Imomura and S.Tsuchiya
Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi Universitv. 2557 Tokiwadai, Ube 755, Japan
Abstract The r e a c t i v i t i e s of surface s p e c i e s produced over an SmzOg c a t a l y s t under CO hydrogenation r e a c t i o n were investigated by in s i t u i n f r a r e d spectroscopy. Under the reaction condition, formate, carbonate and methoxide were observed over the surface. Among the s u r f a c e species, methoxide plays the important r o l e of the hydrocarbon production of t h e reaction.
1. INTRODUCTION
Surface s p e c i e s produced over heterogeneous c a t a l y s t s under r e a c t i o n cond i t i o n s a r e often regarded as reaction intermediates. In order t o characteri z e reactions over c a t a l y s t s , i t i s important t o c l a r i f y r o l e s of s u r f a c e species t o c a t a l y t i c reactions. Recently various techniques a r e applied t o t h i s kind of work. Infrared(1R) spectroscopy i s one of t h e useful technique f o r observing s u r f a c e species d i r e c t l y under reaction conditions. For CO hydrogenation reaction, Zr02 and lanthanide oxides e x h i b i t some c h a r a c t e r i s t i c a c t i v i t i e s such a s s e l e c t i v e production of l i g h t olef ins and methanol [1,2]. The surface species produced from the r e a c t i o n s over t h e In c a t a l y s t s were investigated by many workers by IR spectroscopy [3.4]. t h i s work, we have studied r e a c t i v i t i e s of surface s p e c i e s produced d u r i n g CO hydrogenation r e a c t i o n over an Sm203 c a t a l y s t , which i s one of the t y p i cal lanthanide oxide, by IR spectroscopy. The r e a c t i o n s of t h e s u r f a c e speci e s produced from t h e adsorption and decomposition of t h e r e l a t e compounds were examined t o c h a r a c t e r i z e t h e reaction over the c a t a l y s t .
2. EXPERIMENTAL Sm 0 was prepared from t h e c a l c i n a t i o n of Sm(OH)3 obtained from hydrolys i s o f &(NO ) 3 by NH40H. Self-supporting d i s k (ca. 60 mg) of Sm O3 was s e t i n an IR cel? which was attached t o closed c i r c u l a t i o n system. d e c a t a l y s t
1520 was t r e a t e d w i t h 0 and H2 a t 873 K r e s p e c t i v e l y and evacuated a t 1100 K f o r 30 min. t o p u r i f y %he c a t a l y s t surface. I R s p e c t r a were recorded withl FTIR re7000 (JASCO.). A l l s p e c t r a w e r e measured w i t h 256 s c a n s a t 4 cms o l u t i o n and were o b t a i n e d from t h e r a t i o o f t h e background spectrum o f t h e Sm203 t o adsorbed gases. Formate and methoxide s p e c i e s were produced by t h e a d s o r p t i o n o f d r i e d f o r m i c a c i d and methanol. Reactions such as CO hydrogena t i o n was -performed i n a c l o s e d c i r c u l a t i o n system.
3. RESULTS AND DISCUSSION R e s u l t s o f t h e CO h y d r o g e n a t i o n r e a c t i o n o v e r an S m 2 0 c a t a l y s t a t v a r i o u s temperature were l i s t e d i n Table 1. From Table 1, $he a c t i v i t i e s were developed and hydrocarbons were produced o v e r 573 K. From t h e hydrocarbon p r o d u c t s d i s t r i b u t i o n , C -C4 hydrocarbons were produced s e l e c t i v e l y . I t was confirmed t h a t o l e f i n s e f e c t i v i t y i n hydrocarbon p r o d u c t s was o v e r 80 %. Table 1 R e s u l t s o f t h e CO hydrogenation r e a c t i o n o v e r Sm2O3 (CO:H2 =1:3) Reaction
Yield(10-6 molf15 h)
a, O l e f i n s e l e c t i v i t i e s are more t h a n 80
D i s t r i b u t i o n (C-base %)
% i n Cz-Cg hydrocarbons
I R s p e c t r a o f t h e s u r f a c e species formed from CO hydrogenation r e a c t i o n a t v a r i o u s temperatures a r e shown i n Fig. 1. These s p e c t r a were measured under t h e same c o n d i t i o n o f t h e r e a c t i o n . Fig. l ( a ) shows t h e spectrum a t 473 K. From Fig. l ( a ) , bands a t t r i b u t e d t o b i d e n t a t e f o r m a t e and s u r f a c e h y d r o x y l group a r e m a i n l y observed [ 5 ] . F i g . l ( b ) shows t h e spectrum a t 573 K. From Fig. l ( b ) p r o d u c t i o n o f some new bands and changing t h e band shapes were observed, It i s confirmed t h a t new species produced a t 373 K a r e a t t r i b u t a b l e t o methoxide and carbonate [ 6 ] . F i g . l ( c ) shows t h e spectrum a t 673 K. From Fig. l ( c ) , t h e bands due t o c a r b o n a t e - l i k e species a r e m a i n l y observed. From t h e s e r e s u l t s , i t i s c l e a r t h a t main s u r f a c e s p e c i e s o f CO hyd r o g e n a t i o n r e a c t i o n on Sm O3 a r e formate, methoxide and carbonate-1 ike species. I n o r d e r t o e l u c i d a Z e r o l e s o f t h e s e s p e c i e s t o t h e r e a c t i o n , react i v i t i e s o f them which were produced from a d s o r p t i o n o f t h e r e l a t e d compounds were i n v e s t i g a t e d i n d i v i d u a l l y . F o r formate species, which was produced f r o m t h e a d s o r p t i o n o f f o r m i c By i n acid, t h r e e kinds o f formates were observed a t 273 K i n vacuo. c r e a s i n g t h e t e m p e r a t u r e t o 533 K, t h e f o r m a t e s r e a c t e d t o one k i n d o f formate t h a t was a t t r i b u t a b l e t o b i d e n t a t e formate. It decomposed t o form OH
1521
I 4000
3000 wavenumber
/
200(11 cm
1000
Fig. 1. I R s p e c t r a o f adsorbed species formed from CO h y d r o g e n a t i o n r e a c t i o n on Sm2O3 a t (a) 473 K, ( b ) 573 K and ( c ) 673 K. and carbonate on t h e s u r f a c e o v e r 533 K. The decomposition behaviour was n o t changed a p p a r e n t l y under H2 atmosphere. The b e h a v i o u r o f d-formate under H2 atmosphere was observed. C-D t o C-H exchange r e a c t i o n was o c c u r r e d accompan i e d w i t h t h e decomposition o v e r 533 K. Dependence o f t h e i n i t i a l r a t e o f C-D t o C-H exchange and f o r m a t i o n o f carbonate by t h e decomposition upon t h e p r e s s u r e o f H2 a t 543 K a r e shown i n F i g . 2. The r a t e was o b t a i n e d f r o m t h e i n c r e a s e o f band i n t e n s i t i e s t h a t were assigned t o J(C-H) and 6(CO2! w i t h passing time. From F i g . 2, f o r C-D t o C-H exchange r e a c t i o n t h e r a t e i s i n creased w i t h i n c r e a s i n g H2 pressure. The r a t e was e x h i b i t e d maximum v a l u e I n c r e a s i n g H p r e s s u r e more t h a n 6.5 kPa, t h e r a t e was under 6.5 kPa o f H g r a d u a l l y decrease$. I t suggests (hat t h e C-H a c t i v a t i o n and H2 a c t i v a t i o n occurred on t h e same s i t e over t h e c a t a l y s t and H2 a c t i v a t i o n was p r i o r t o C-H a c t i v a t i o n over 6.5 kPa o f H2. F o r decomposition r e a c t i o n , t h e r a t e was decreased w i t h i n c r e a s i n g H pressure. Under HZ p r e s s u r e o v e r 6.5 kPa, t h e r a t e became constant. From &ese r e s u l t s , i t i s c l e a r t h a t hydrogen d i s t u r b s t h e decomposition o f formate and suggested t h a t t h e decomposition o f f o r m a t e proceeds by way o f C-H f i s s i o n . I t was a l s o c o n f i r m e d t h a t methanol was produced s e l e c t i v e l y under t h e c o n d i t i o n . F o r methoxide, which was produced from t h e a d s o r p t i o n o f methanol, t h i s species was s t a b l e up t o 573 K and g r a d u a l l y decomposed o v e r 573 K i n vacuo. Under H2 atmosphere, t h e behaviour o f t h i s species were n e a r l y t h e same as t h a t i n vacuo. I R s p e c t r a o f d methoxide, which was produced from t h e ad;I atmosphere a t 533 K and 553 K a r e shown i n s o r p t i o n o f d methanol, under ? Fig. 3. From t:g. 3, i t was observed t h a t C-D bond was g r a d u a l l y exchanged t o C-H bond. A f t e r 17 hours a t 533 K m i x t u r e o f d3-, d2-, d and d p e t h o x i d e was observed. A t 553 K, t h e r e a c t i o n was proceeded ana methoxide a l most became $ p n e t h o x i d e a f t e r 4 h. The appearance o f t h e new bands a t 1460 and 1375 cm- were a l s o observed i n Fig. 3. These bands were a t t r i b u t a b l e t o carbonate-1 ike adsorbed species t h a t was produce from t h e decomposition o f methoxide. H2 p r e s s u r e dependence o f t h e i n i t i a l r a t e of exchange r e a c t i o n e x h i b i t e d n e a r l y t h e same tendency as t h e r e a c t i o n o f d-formate. The prod u c t s under t h i s c o n d i t i o n were small amout o f methanol. However, o v e r 573 K, hydrocarbons were s e l e c t i v e l y produced w i t h t h e same p r o d u c t s d i s t r i b u t i o n o f CO h y d r o g e n a t i o n r e a c t i o n .
.
-
1522
I
0,
u c
a
0 L 0
P
H2
/
kPa
F i g . 2. Dependence o f r a t e o f exchange and decomosition on H2 pressure. 0 :exchange, A:decomp.
wavenumber
/
cm-
Fig. 3. I R s p e c t r a o f d3-methoxide i n H on Sm20 : ( a ) 0 h, (b) 6.5 h ( c ) ?7 h a t 533 K, (d) 4 h a t 553 K
F o r carbonate-1 i k e species, t h i s species produced from t h e decomposition o f formate and methoxide and a d s o r p t i o n o f CO and cO2. These s p e c i e s were i n a c t i v e and s t a b l e under t h i s r e a c t i o n c o n d i t i o n . From t h e r e s u l t s , below 573 K, h y d r o g e n a t i o n o f formate t o methanol were m a i n l y occurred. Over 573 K, t h e a c t i v i t y of hydrocarbon p r o d u c t i o n was i n creased w i t h i n c r e a s i n g temperature. T h i s behaviour was agreement w i t h i n c r e a s i n g t h e a c t i v i t y o f methoxide decomposition and i n c r e a s i n g p r o d u c t s t h a t produced from m e t h o r i d e decomposition. Thus, i t suggests t h a t methoxide i s i m p o r t a n t s u r f a c e species o f t h e hydrocarbon p r o d u c t i o n f r o m CO hydrogena t i o n r e a c t i o n o v e r SmpO3. T h i s work was supported by Japan S e c u r i t i e s S c h o l a r s h i p Foundation, f o r which t h e a u t h o r s a r e g r a t e f u l .
REFERENCE 1 2
3 4
5
6
M. Y. He and J. G. Ekerdt. J. Catal., 90 (1984) 17. T. A r a i . K. Maruya, K. Domen and T. O n i s h i , J. Chem. SOC., Chem. Commun., (1987) 1157. M. Y. He and J. G. Ekerdt, J. Catal., 87 (1984) 381. T. O n i s h i , K. Maruya, K. Domen, H. Abe and J. Kondo. Proc. 9 t h I n t . Congr. Catal., Canada, v o l . 2 (1988) 507. C. ti, Y. Sakata, T. A r a i , K. Domen, K. Maruya and T. O n i s h i , J. Chem. SOC., Faraday Trans. 1, 85 (1989) 1451. Y. Sakata. M. Yoshino. H. Imamura and S. Tsuchiya, J. Chem. SOC., Faraday Trans., 86 (1990) 3489.
REACI'IVITIES OF SURFACE INTERMEDIATES ON AN Sm203 CATALYST
STUDIED BY IN SITU INFRARED SPECTROSCOPY
X
Sakatq M. Yoshino, T.Fuhda, H. Yamaguchi H. Imomura and S.Tsuchiya
Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi Universitv. 2557 Tokiwadai, Ube 755, Japan
Abstract The r e a c t i v i t i e s of surface s p e c i e s produced over an SmzOg c a t a l y s t under CO hydrogenation r e a c t i o n were investigated by in s i t u i n f r a r e d spectroscopy. Under the reaction condition, formate, carbonate and methoxide were observed over the surface. Among the s u r f a c e species, methoxide plays the important r o l e of the hydrocarbon production of t h e reaction.
1. INTRODUCTION
Surface s p e c i e s produced over heterogeneous c a t a l y s t s under r e a c t i o n cond i t i o n s a r e often regarded as reaction intermediates. In order t o characteri z e reactions over c a t a l y s t s , i t i s important t o c l a r i f y r o l e s of s u r f a c e species t o c a t a l y t i c reactions. Recently various techniques a r e applied t o t h i s kind of work. Infrared(1R) spectroscopy i s one of t h e useful technique f o r observing s u r f a c e species d i r e c t l y under reaction conditions. For CO hydrogenation reaction, Zr02 and lanthanide oxides e x h i b i t some c h a r a c t e r i s t i c a c t i v i t i e s such a s s e l e c t i v e production of l i g h t olef ins and methanol [1,2]. The surface species produced from the r e a c t i o n s over t h e In c a t a l y s t s were investigated by many workers by IR spectroscopy [3.4]. t h i s work, we have studied r e a c t i v i t i e s of surface s p e c i e s produced d u r i n g CO hydrogenation r e a c t i o n over an Sm203 c a t a l y s t , which i s one of the t y p i cal lanthanide oxide, by IR spectroscopy. The r e a c t i o n s of t h e s u r f a c e speci e s produced from t h e adsorption and decomposition of t h e r e l a t e compounds were examined t o c h a r a c t e r i z e t h e reaction over the c a t a l y s t .
2. EXPERIMENTAL Sm 0 was prepared from t h e c a l c i n a t i o n of Sm(OH)3 obtained from hydrolys i s o f &(NO ) 3 by NH40H. Self-supporting d i s k (ca. 60 mg) of Sm O3 was s e t i n an IR cel? which was attached t o closed c i r c u l a t i o n system. d e c a t a l y s t
1520 was t r e a t e d w i t h 0 and H2 a t 873 K r e s p e c t i v e l y and evacuated a t 1100 K f o r 30 min. t o p u r i f y %he c a t a l y s t surface. I R s p e c t r a were recorded withl FTIR re7000 (JASCO.). A l l s p e c t r a w e r e measured w i t h 256 s c a n s a t 4 cms o l u t i o n and were o b t a i n e d from t h e r a t i o o f t h e background spectrum o f t h e Sm203 t o adsorbed gases. Formate and methoxide s p e c i e s were produced by t h e a d s o r p t i o n o f d r i e d f o r m i c a c i d and methanol. Reactions such as CO hydrogena t i o n was -performed i n a c l o s e d c i r c u l a t i o n system.
3. RESULTS AND DISCUSSION R e s u l t s o f t h e CO h y d r o g e n a t i o n r e a c t i o n o v e r an S m 2 0 c a t a l y s t a t v a r i o u s temperature were l i s t e d i n Table 1. From Table 1, $he a c t i v i t i e s were developed and hydrocarbons were produced o v e r 573 K. From t h e hydrocarbon p r o d u c t s d i s t r i b u t i o n , C -C4 hydrocarbons were produced s e l e c t i v e l y . I t was confirmed t h a t o l e f i n s e f e c t i v i t y i n hydrocarbon p r o d u c t s was o v e r 80 %. Table 1 R e s u l t s o f t h e CO hydrogenation r e a c t i o n o v e r Sm2O3 (CO:H2 =1:3) Reaction
Yield(10-6 molf15 h)
a, O l e f i n s e l e c t i v i t i e s are more t h a n 80
D i s t r i b u t i o n (C-base %)
% i n Cz-Cg hydrocarbons
I R s p e c t r a o f t h e s u r f a c e species formed from CO hydrogenation r e a c t i o n a t v a r i o u s temperatures a r e shown i n Fig. 1. These s p e c t r a were measured under t h e same c o n d i t i o n o f t h e r e a c t i o n . Fig. l ( a ) shows t h e spectrum a t 473 K. From Fig. l ( a ) , bands a t t r i b u t e d t o b i d e n t a t e f o r m a t e and s u r f a c e h y d r o x y l group a r e m a i n l y observed [ 5 ] . F i g . l ( b ) shows t h e spectrum a t 573 K. From Fig. l ( b ) p r o d u c t i o n o f some new bands and changing t h e band shapes were observed, It i s confirmed t h a t new species produced a t 373 K a r e a t t r i b u t a b l e t o methoxide and carbonate [ 6 ] . F i g . l ( c ) shows t h e spectrum a t 673 K. From Fig. l ( c ) , t h e bands due t o c a r b o n a t e - l i k e species a r e m a i n l y observed. From t h e s e r e s u l t s , i t i s c l e a r t h a t main s u r f a c e s p e c i e s o f CO hyd r o g e n a t i o n r e a c t i o n on Sm O3 a r e formate, methoxide and carbonate-1 ike species. I n o r d e r t o e l u c i d a Z e r o l e s o f t h e s e s p e c i e s t o t h e r e a c t i o n , react i v i t i e s o f them which were produced from a d s o r p t i o n o f t h e r e l a t e d compounds were i n v e s t i g a t e d i n d i v i d u a l l y . F o r formate species, which was produced f r o m t h e a d s o r p t i o n o f f o r m i c By i n acid, t h r e e kinds o f formates were observed a t 273 K i n vacuo. c r e a s i n g t h e t e m p e r a t u r e t o 533 K, t h e f o r m a t e s r e a c t e d t o one k i n d o f formate t h a t was a t t r i b u t a b l e t o b i d e n t a t e formate. It decomposed t o form OH
1521
I 4000
3000 wavenumber
/
200(11 cm
1000
Fig. 1. I R s p e c t r a o f adsorbed species formed from CO h y d r o g e n a t i o n r e a c t i o n on Sm2O3 a t (a) 473 K, ( b ) 573 K and ( c ) 673 K. and carbonate on t h e s u r f a c e o v e r 533 K. The decomposition behaviour was n o t changed a p p a r e n t l y under H2 atmosphere. The b e h a v i o u r o f d-formate under H2 atmosphere was observed. C-D t o C-H exchange r e a c t i o n was o c c u r r e d accompan i e d w i t h t h e decomposition o v e r 533 K. Dependence o f t h e i n i t i a l r a t e o f C-D t o C-H exchange and f o r m a t i o n o f carbonate by t h e decomposition upon t h e p r e s s u r e o f H2 a t 543 K a r e shown i n F i g . 2. The r a t e was o b t a i n e d f r o m t h e i n c r e a s e o f band i n t e n s i t i e s t h a t were assigned t o J(C-H) and 6(CO2! w i t h passing time. From F i g . 2, f o r C-D t o C-H exchange r e a c t i o n t h e r a t e i s i n creased w i t h i n c r e a s i n g H2 pressure. The r a t e was e x h i b i t e d maximum v a l u e I n c r e a s i n g H p r e s s u r e more t h a n 6.5 kPa, t h e r a t e was under 6.5 kPa o f H g r a d u a l l y decrease$. I t suggests (hat t h e C-H a c t i v a t i o n and H2 a c t i v a t i o n occurred on t h e same s i t e over t h e c a t a l y s t and H2 a c t i v a t i o n was p r i o r t o C-H a c t i v a t i o n over 6.5 kPa o f H2. F o r decomposition r e a c t i o n , t h e r a t e was decreased w i t h i n c r e a s i n g H pressure. Under HZ p r e s s u r e o v e r 6.5 kPa, t h e r a t e became constant. From &ese r e s u l t s , i t i s c l e a r t h a t hydrogen d i s t u r b s t h e decomposition o f formate and suggested t h a t t h e decomposition o f f o r m a t e proceeds by way o f C-H f i s s i o n . I t was a l s o c o n f i r m e d t h a t methanol was produced s e l e c t i v e l y under t h e c o n d i t i o n . F o r methoxide, which was produced from t h e a d s o r p t i o n o f methanol, t h i s species was s t a b l e up t o 573 K and g r a d u a l l y decomposed o v e r 573 K i n vacuo. Under H2 atmosphere, t h e behaviour o f t h i s species were n e a r l y t h e same as t h a t i n vacuo. I R s p e c t r a o f d methoxide, which was produced from t h e ad;I atmosphere a t 533 K and 553 K a r e shown i n s o r p t i o n o f d methanol, under ? Fig. 3. From t:g. 3, i t was observed t h a t C-D bond was g r a d u a l l y exchanged t o C-H bond. A f t e r 17 hours a t 533 K m i x t u r e o f d3-, d2-, d and d p e t h o x i d e was observed. A t 553 K, t h e r e a c t i o n was proceeded ana methoxide a l most became $ p n e t h o x i d e a f t e r 4 h. The appearance o f t h e new bands a t 1460 and 1375 cm- were a l s o observed i n Fig. 3. These bands were a t t r i b u t a b l e t o carbonate-1 ike adsorbed species t h a t was produce from t h e decomposition o f methoxide. H2 p r e s s u r e dependence o f t h e i n i t i a l r a t e of exchange r e a c t i o n e x h i b i t e d n e a r l y t h e same tendency as t h e r e a c t i o n o f d-formate. The prod u c t s under t h i s c o n d i t i o n were small amout o f methanol. However, o v e r 573 K, hydrocarbons were s e l e c t i v e l y produced w i t h t h e same p r o d u c t s d i s t r i b u t i o n o f CO h y d r o g e n a t i o n r e a c t i o n .
.
-
1522
I
0,
u c
a
0 L 0
P
H2
/
kPa
F i g . 2. Dependence o f r a t e o f exchange and decomosition on H2 pressure. 0 :exchange, A:decomp.
wavenumber
/
cm-
Fig. 3. I R s p e c t r a o f d3-methoxide i n H on Sm20 : ( a ) 0 h, (b) 6.5 h ( c ) ?7 h a t 533 K, (d) 4 h a t 553 K
F o r carbonate-1 i k e species, t h i s species produced from t h e decomposition o f formate and methoxide and a d s o r p t i o n o f CO and cO2. These s p e c i e s were i n a c t i v e and s t a b l e under t h i s r e a c t i o n c o n d i t i o n . From t h e r e s u l t s , below 573 K, h y d r o g e n a t i o n o f formate t o methanol were m a i n l y occurred. Over 573 K, t h e a c t i v i t y of hydrocarbon p r o d u c t i o n was i n creased w i t h i n c r e a s i n g temperature. T h i s behaviour was agreement w i t h i n c r e a s i n g t h e a c t i v i t y o f methoxide decomposition and i n c r e a s i n g p r o d u c t s t h a t produced from m e t h o r i d e decomposition. Thus, i t suggests t h a t methoxide i s i m p o r t a n t s u r f a c e species o f t h e hydrocarbon p r o d u c t i o n f r o m CO hydrogena t i o n r e a c t i o n o v e r SmpO3. T h i s work was supported by Japan S e c u r i t i e s S c h o l a r s h i p Foundation, f o r which t h e a u t h o r s a r e g r a t e f u l .
REFERENCE 1 2
3 4
5
6
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