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CeO2 catalyst
T. Inui et al. (Editors), N e w Aspects of Spillover Effect in Catalysis 0 1993 Elsevier Science Publishers B.V. All rights reserved.
217
Spillover of atomic oxygen and reverse spillover of dioxygen species on PtlCeO, catalyst Can Li , Yanxin Chen. Wenzhao Li and Qin Xin State Key Laboratory of Catalysis, Dalian Institute o f Chemical Physics, Chinese Academy o f Sciences. Dalian 116023, China.
Abstract
Oxygen adsorption and the reacti vities of adsorbed oxygen species on Pt/Ce0, and CeO, were studied by chemisorption. electrical conductivity measurement, and FT-IR employing isotopic labeling. IR spectra show that t h e adsorbed dioxygen species are stable on CeOp but are hardly t o be detected o n Pt/Ce02 under the same conditions. Isotopic results confi rrned that the adsorbed oxygen o n Pt/Ce02 is able t o oxidize CH, into formate species(loca1ized on Ce02 surface) even at 373 K however the adsobed oxygen species on CeO, alone are almost inert toward CH, at the same temperature. It is suggested that the adsorbed dioxygen species are labile and facilely migrate onto Pt surface(here referred t o a s reverse spillover) where they convert into atomic oxygen species which can spi 1 lover from Pt onto CeO, surface. The spiltover oxygen species are most possibly responsible for the formation of formate species on CeO, surface v i a the oxidation o f methane at 373 K.
I. INTRODUCTION Spillover o f surface species have been well investigated for many catalysts, and the spillover phenomenon seems t o be universal in hetorogenerous catalysis[ll. Of the surface species having spillover ability, oxygen species have received much attention because the oxygen species are involved in t h e most of catalytic oxidation processes. The spillover of oxygen species has been assumed t o be very importance in controlling the selectivity and activity in oxidation of hydrocarbons[2,31. In the previous work w e have studied oxygen species o n CeO, and their reactivities toward light hydrocarbons[4.5J.Two adsorbed dioxygen species. superoxide(O,-) and per~xide(O,~-) species, on cerium oxide could be detected by FT-IR spectroscopy. In addition, it was found that the systems of supported Pt o n
218 n-type oxides, such as Pt/Ti02, P t / Z n O and Pt/Sn02, e x h i b i t somewhat s p i l l o v e r o f both hydrogen and oxygen species[6.71. I n t h e present experiments we have a t t e m p t e d t o i n v e s t i g a t e the oxygen species on CeO,(n-type oxide) and Pt/Ceo2 w i t h a hope t o c l a r i f y whether t h e r e i s s p i l l o v e r o f oxygen species and what i s t h e r o l e p l a y e d by t h e s p i l t o v e r oxygen i n the o x i d a t i o n o f methane. Pt/Ceo2 c a t a l y s t has been used i n the t o t a l o x i d a t i o n o f hydrocarbons f o r t h e reduction o f exhaust emissions[8,91. Several studies have been made on t h e oxidat i o n mechani sms o f hydrocarbons and carbon monoxide over Pt/CeO2[8-11I b u t l e s s a t t e n t i o n has been p a i d t o t h e oxygen s p i l l o v e r on t h i s c a t a l y s t .
2, EXPERIMENTAL A d e t a i l e d d e s c r i p t i o n o f preparation o f Ceo, and t h e experimental apparatus f o r F T - I R study i s given elsewhere[4.51. Pt/CeOl c a t a l y s t was prepared by impregnating Ceo, w i t h an aqueous s o l u t i o n o f H,PtCle, and f o l l o w i n g a reduction by hydrazine aqueous s o l u t i o n a t 298 K and a d r y i n g a t 373 K i n a i r . E l e c t r i c a l c o n d u c t i v i t y o f sample was measured i n 0, atmosphere i n a temperature programmed w a y f r o m 298 t o 573 K. Oxygen chemisorption was measured a t various temperatures f o r f r e s h c a t a l y s t s which were pretreated i n H2 a t 673 K f o r 1 h r .
3, RESULTS AND DISCUSSION Adsorbed superoxide species can be formed v i a O2 adsorption on an outgassed CeO, and both adsorbed superoxide and peroxide species are r e a d i l y produced when a p a r t i a l l y reduced cerium oxide i s exposed t o 0, a t r m temperature[41. The superoxide and peroxide formed on cerium oxide g i v e evident I R bands w i t h c h a r a c t e r i s t i c frequencies a t 1126 and 883 cm-' r e s p e c t i v e l y as shown i n Figure 1 . The adsorbed dioxygen species are q u i t e s t a b l e and can be s t i l l observed even UP t o 373 K. However we f a i l e d t o d e t e c t the dioxygen species on outgassed Pt/Ce02 as w e l l as on p a r t i a l l y reduced Pt/CeOa. A very weak band a t 1126 cm-' due t o adsorbed superoxide species on Pt/CeOp could be observed only a t a lower temperature, b u t t h e band attenuated gradually and disappeared as soon as t h e sample was s l i g h t l y heated. I t seems t h a t t h e dioxygen species.are d i f f i c u l t t o be stabi l i z e d on Pt/CeO, c a t a l y s t o r i t may convert i n t o atomic ' so f a s t t h a t i s hard oxygen species such as 0. and 0- o r 0 t o d e t e c t . Whereas a chemisorption measurement o f oxygen on Pt/Ce02 evidenced t h a t t h e c a t a l y s t d i d adsorb considerable amount o f oxygen even a t roan temperature. A s depicted i n Figure 2, t h e amount o f chemisorbed oxygen on Pt/CeO, remains constant a t t h e adsorption temperatures f r o m 298 t o 500 K w h i l e t h a t for CeO, v a r i e s s i g n i f i c a n t l y a t t h e same temper a t u r e region. A t room temperature t h e amount o f chemisorbed
219
-0-0-
CeO2
-0-0-
Pt/CeO2
I I
~
)O
1tbo
cm-l
I
Figure 1.IR spectra o f d i oxygen adsorbed on reduced CeO, arid Pt/CeO, a t 273 K .
'3
373
473
T/K Figure 2. Amount o f adsorbed oxygen on reduced Pt/CeO, and CeO, at; various temperatures
i t decreases oxygen f o r Ce0, i s more than t h a t f o r Pt/CeO,.and upon warming the sample and f a l l s t o i t s minimum value a t 390 K. On the c o n t r a r y , above 390 K the amount o f chemisorbed oxygen on CeO, increases w i t h the e l e v a t i o n o f temperature and reaches the same l e v e l a t 500 K as t h a t f o r Pt/CeO,. The oxygen chemisorbed on CeO, a t temperatures under 390 K e x i s t maj o r l y i n t h e form o f dioxygen species while i t m a y d i s s o c i a t e a t temperatures exceeding 390 K as proved by i n f r a r e d spectra. The curve f o r CeO, i n Figure 2 before 390 K can be explained as t h a t t h e chemisorbed dioxygen species desorb a t higher temperatures. The f a c t t h a t t h e dioxygen species were scarcely detected f o r Pt/CeO, b u t there i s l a r g e amount o f oxygen chemisorbed on t h e c a t a l y s t i n d i c a t e s t h a t t h e adsorbed dioxygen species on Pt/CeOz f a c i l e l y transform i n t o atomic oxygen spec i e s even a t r m temperature. I t can be obviously seen t h a t the transformation o f dioxygen on P t / C e 0 2 i s q u i t e easier than t h a t on CeO,. The presence o f P t f a c i l i t a t e s t h e transformat i o n f r o m dioxygen t o atomic oxygen, accordingly r e s u l t i n g t h e disappearance o f the I R bands due t o t h e adsorbed dioxygen species even a t r o o m t m p e r a t u r e ( f i g u r e 1 ) . One can speculate t h a t t h e dioxygen species should be formed on t h e surface o f Cd), a t the very begining o f the adsorption, and the adsorbed dioxygen are unable t o d i s s o c i a t e a t r o o m temperature on CeO, but a r e very l a b i l e and e a s i l y migrate onto P t surface where they d i s s i c i a t e i n t o atomic oxygen species(0. 0-1, t h e process can be r e f e r r e d t o as reverse s p i l l o v e r o f oxygen. The atomic
220 oxygen species on P t surf ace may be a 1 so d e r i ved f r o m t h e d i r e c t adsorption o f gaseous 0,. The atomic oxygen formed on P t surface possibly s p i l l o v e r s onto CeO, surface where p a r t o f them may convert i n t o l a t t i c e oxygen anions. The amount o f chemisorbed oxygen on CeO, o r Pt/Ce02 mainly depends on the number o f oxygen vacancy and e l e c t r o n ready f o r the formation o f adsorbed oxygen species. A t room temperature. most o f the adsorbed oxygen species on CeO, are i n t h e form o f dioxygen. 0,- and 02,-. which r e q u i r e e l e c t r o n s l e s s than t h a t f o r t h e formation o f 0- and 0 ‘ - species on Pt/Ce02. This i s the reason why the amount o f chemisorbed oxygen on CeO, is more than t h a t on Pt/CeO,(Figure 2 ) . A t temperatures up t o 500 K t h e chemisorbed oxygen species are i n 0- and 0,forms f o r both CeO, and Pt/CeO, t h e r e f o r e t h e amount o f chemisorbed oxygen f o r the two c a t a l y s t s i s almost t h e same. The e l e c t r i c a l conductvi t y o f a p a r t i a1 l y reduced Pt/Ce02 decreases much sharply than t h a t o f a p a r t i a l l y reduced CeO, a f t e r oxygen ads o r p t i o n a t r m temperature implying t h a t more e l e c t r o n s cont r i b u t i n g t o t h e chemisorbed oxygen w i t h Pt/CeO, than w i t h CeO,. These r e s u l t s s t r o n g l y confirmed t h a t t h e reverse s p i l l over o f dioxygen and the s p i l l o v e r o f atomic oxygen on Pt/CeOs c a t a l y s t co-proceed e v i d e n t l y even a t r o o m temperature. I R spectra were taken i n - s i t u d u r i n g the r e a c t i o n o f CH, on CeO, and P t / C e 0 2 i n t h e presence o r absence o f gaseous oxygen. Below 3 7 3 K no oxidized product was observed f o r e i t h e r CeO, o r Pt/CeO,. I t was found t h a t t h e o x i d a t i o n o f CH, is initiated a t 373 K f o r Pt/CeO, and a t around 473 K f o r CeO,. The presence o f P t lowers the temperature f o r t h e i n i t i a l r e a c t i o n about 1 0 0 K . Figure 3 shows I R spectra recorded f o r the react i o n o f CH, on Pt/Ce02 and CeO, a t 3 7 3 K standing f o r longer than 60 minutes. A s t r i k i n g d i f f e r e n c e i n Figure 3 can be seen f o r t h e two samples. When P t / C e 0 2 c a t a l y s t was exposed t o CH, + 0, atmosphere a t 3 7 3 K . three d i s t i n c t bands a t 1544, 1 3 7 1 and 1355 cm-’ appeared and developped w i t h prolonged t i m e , and these bands(together w i t h other two bands a t 2933, 2845 cm-’ n o t given here) can be a t t r i b u t e d t o formate species adsorbed on CeO, surface[5.121. Besides, there are some weak bands i n the range o f 1600-1300 cm-I which are assigned t o carbonate species adsorbed on CeO, surface[l31. While f o r CeO, alone + 0, atmosphere a t 373 K no any IR bands due t o t h e under CH, oxidized product i s observed as shown i n Figure 3 ( b ) . The cont r a s t between Figure 3 ( a ) and 3 ( b ) concludes t h a t t h e r e are a c t i v e oxygen species derived on the Ce02 surface o f Pt/CeO, and these oxygen species are so a c t i v e t h a t can o x i d i z e CH, i n t o formate species even a t 373 K . The oxygen species o r i g i nated i n the s p i l l o v e r f r o m P t surface are o n l y responsible f o r the formation o f formate species on CeO, because oxygen species formed on Ce0, alone are i n e r t toward methane a t 373 K . A doubt may e x i s t i f some a c t i v a t e d species from methane adsorption on P t spi 1 lover onto CeOz where they are o x i d i z e d i n t o formate s p e c i e s [ l 4 ] Separate experiments o f methane adsorption on CeO, [ 151 and P t / C e 0 2 a t lower temperatures made no apparent d i f f e r e n c e i n d i c a t i n g t h a t a c t i v a t e d methane seems t o be n o t formed on CeO, v i a a s p i l l o v e r f r o m Pt surface. The
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22 I
C I
I
Figure 3. IR spectra o f adspecies derived from CH4 o x i d a t i o n a t 373 K . ( a ) CH4+02 over Pt/CeO,;(b)CH,+O, over CeO,; (c)CH, over Pt/CeO,.
do0
1$00
cm-'
do0
1;
Figure 4. IR spectra o f adspecies derived from CH, o x i d a t i o n a t 373 K over Pt/CeO, under ( a ) C H 4 + 1 D 0 2 ; ( b ) C H 4 + 1 " 0 2 .
r e s u l t i n Figure 3 ( c ) f u r t h e r confirmed t h a t t h e formation o f formate species i s due majorly t o t h e s p i l t o v e r oxygen species at because t h e r e i s no r e a c t i o n f o r methane alone on Pt/CeO, 3'73 K . The cerium oxide i t s e l f has a c t i v e oxygen species which i f there were can o x i d i z e C2H4 even a t 373 K[51 b u t n o t CH, a c t i v a t e d methane s p i l t o v e r f r o m P t they would react w i t h t h e act-ive oxygen species on CeO, r e s u l t i n g t h e same adsorbed f o r mate species as shown i n Figure 3 ( a ) . Isotope lmO2 was employed i n methane o x i d a t i o n over. Pt/CeO, a t 373 K w i t h an i n t e n t i o n t o ensure t h a t the s p i l t o v e r oxygen f r o m P t surface p a r t i c i p a t e i n the -Formation of surface f o r mate species on CeO, surface. IR spec+-ra were recorded i n s7tu i n t h e same fashion as the r e a c t i o n o f lSO2 + CH, on Pt/CeO, a t 373 K. The IR spectra recorded f o r t h e two cases, loo, + CH, and loo2 + CH, are c o l l e c t e d i n Figure 4. For 1 6 0 z + Cki,, only t h e bands a t 1544. 1 3 7 1 and 1355 cm-l due t o I4C1'WnOand some weak bands i n 1600-1370 cm-' region due t o C1oOn'species were derived as shown i n Figure 4 ( b ) . I n c o n t r a s t , f o r lt(0, + CH, t h e r e are n o t o n l y the IR bands a t 1544, 1 3 7 1 and 1355 cm-l o f formate species, HC1eO'nO-, but also r i c h bands i n 1544-1465. and 1355-1330 cm-' regions, such as the bands a t 1515, 1500, 1465. 1343, 1335 cm-'. lhese a d d i t i o n a l bands can be s a f e l y assigned t o t h e surface formate species togethetw i t h small amount o f carbonate species containing oxygen-18. i . e . , HC1eO1"O-, HCleOleO-, C(le0)03"-. For methane o x i d a t i o n
222 on CeO, alone, the bands due t o surface f o r m a t e species became d i s c e r n i b l e only up t o 4 7 3 K , but no products containing oxygen-18, either HC'HO'eOor HC'eO1eOw e r e detected i n the case of o f I R O , + CH4 over CeO, alone a t 4 7 3 K. T h e r e s u l t s again i n d i c a t e t h a t the oxygen species on Cd), without Pt i s not a c t i v e enough t o o x i d i z e methane even a t 4 7 3 K.
4, ~ L U S I O N S A d s o r b e d dioxygen s p e c i e s ( 0 , and OZ2-) on CeO, surface o f Pt/CeOa can m i g r a t e onto P t surface w h e r e they e a s i l y dissoc i a t e i n t o a t o m i c oxygen s p e c i e s ( 0 and 0-),on the other hand, the a t o m i c oxygen species formed on P t surface o f Pt/CeO, can s p i l l o v e r onto CeOz surface. T h e s p i l t o v e r oxygen species a r e so a c t i v e t h a t a r e able t o o x i d i z e m e t h a n e i n t o f o r m a t e and carbonate species even a t 3 7 3 K. W e g r a t e f u l l y a c k n o w l e d g e support by the N a t u r a l S c i e n c e Foundation o f C h i na ( NSFC
.
5, REFERENCES 1 2
3 4 5
6 7 8 9 10 11
12 13 14 15
W. C u r t i s C o n n e r . J r . , G. M. Pajonk and S. J. T e i c h n e r , Adv. i n C a t a l . . 3 4 ( 1 9 8 6 ) 1 . L . T. Weng, P. R u i z and B. Delmon. i n : Proc. o f 2 n d I n t e r n a t i o n a l C o n f e r e n c e on S p i l l o v e r ( K . H. S t e i n b e r g , E d . ) , L e i p z i g , Germany p. 3 7 , 1 9 8 9 . J. P a r e r a , E. T r a f f a n o , J. M a s s o and C . Pieck, i n : S p i l l o v e r o f A d s o r b e d S p e c i e s ( G . M. Pajonk, S. J. T e i c h n e r and J. E . G e r m a i n , E d s . ) . E l s e v i e r , A m s t e r d a m , p . 1 0 1 , 1 9 8 3 . C. L i . K . Domen, K. M a r u y a and T. O n i s h i , J. Am. Chem. SOC. 1 1 1 ( 1 989 1 7 6 8 3 . C. L i , Q. X i n and X.-X., Guo. C a t a l . L e t t . , 1 2 ( 1 9 9 2 ) 3 0 7 . W.-Z. L i , Y.-X. C h e n . C.-Y. Yu, X.-Z. Wang, Z.-P. H o n g and Z.-B. W e i , P r o c . 8 t h I C C . B e r l i n , Germany. v-205, 1984. Y.-X. C h e n and W . - 2 . L i , C h i n e s e J. M o l . C a t a l . . 4 ( 1 9 9 0 ) 4 2 . L. Mendelovic and M. S t e i n b e r g . J. C a t a l . , 9 3 ( 1 9 8 5 ) 3 5 3 . J. G. N u n a n , H. J. R o b o t a , M. J. C o h n and S. A. B r a d l e y , J. C a t a l . . 133(1992)309. T . J i n . T. O k u h a r a , G. T. M a i n s and J. M. White, J. Phys. Chem., 9 1 ( 1 9 8 7 ) 3 3 1 1 . T. J i n . Y. Z h o u , G. J. M a i n s and J. M. White, J. Phys. Chem. 9 1 ( 1 9 8 7 ) 5 9 3 1 . C. L i . Y. S a k a d a , T . A r a i , K . Dornen. K. M a r u y a and T. O n i s h i , J. Chem. Soc., Faraday T r a n s . 1 , 8 5 ( 1 9 8 9 ) 9 2 9 . C. L i . Y . S a k a d a , T . A r a i . K . Domen. K. M a r u y a and T . O n i s h i , J. Chem. S o c . . Faraday T r a n s . 1, 8 5 ( 1 9 8 9 ) 1 4 5 1 . V . Y u . Bychkov. M. Yu. S i n e v , P. A. S h i r a e v , V. N. K o r c h a k , E . L. A p t e k a r and 0. V. K r y l o v . i n : P r o c . o f 2 n d Internat i o n a l C o n f e r e n c e on S p i l l o v e r , L e i p z i g , p . 8 1 , 1 9 8 9 . C. L i and 0. X i n . J. Phys. Chem., 9 6 ( 1 9 9 2 ) 7 7 1 4 .
217
Spillover of atomic oxygen and reverse spillover of dioxygen species on PtlCeO, catalyst Can Li , Yanxin Chen. Wenzhao Li and Qin Xin State Key Laboratory of Catalysis, Dalian Institute o f Chemical Physics, Chinese Academy o f Sciences. Dalian 116023, China.
Abstract
Oxygen adsorption and the reacti vities of adsorbed oxygen species on Pt/Ce0, and CeO, were studied by chemisorption. electrical conductivity measurement, and FT-IR employing isotopic labeling. IR spectra show that t h e adsorbed dioxygen species are stable on CeOp but are hardly t o be detected o n Pt/Ce02 under the same conditions. Isotopic results confi rrned that the adsorbed oxygen o n Pt/Ce02 is able t o oxidize CH, into formate species(loca1ized on Ce02 surface) even at 373 K however the adsobed oxygen species on CeO, alone are almost inert toward CH, at the same temperature. It is suggested that the adsorbed dioxygen species are labile and facilely migrate onto Pt surface(here referred t o a s reverse spillover) where they convert into atomic oxygen species which can spi 1 lover from Pt onto CeO, surface. The spiltover oxygen species are most possibly responsible for the formation of formate species on CeO, surface v i a the oxidation o f methane at 373 K.
I. INTRODUCTION Spillover o f surface species have been well investigated for many catalysts, and the spillover phenomenon seems t o be universal in hetorogenerous catalysis[ll. Of the surface species having spillover ability, oxygen species have received much attention because the oxygen species are involved in t h e most of catalytic oxidation processes. The spillover of oxygen species has been assumed t o be very importance in controlling the selectivity and activity in oxidation of hydrocarbons[2,31. In the previous work w e have studied oxygen species o n CeO, and their reactivities toward light hydrocarbons[4.5J.Two adsorbed dioxygen species. superoxide(O,-) and per~xide(O,~-) species, on cerium oxide could be detected by FT-IR spectroscopy. In addition, it was found that the systems of supported Pt o n
218 n-type oxides, such as Pt/Ti02, P t / Z n O and Pt/Sn02, e x h i b i t somewhat s p i l l o v e r o f both hydrogen and oxygen species[6.71. I n t h e present experiments we have a t t e m p t e d t o i n v e s t i g a t e the oxygen species on CeO,(n-type oxide) and Pt/Ceo2 w i t h a hope t o c l a r i f y whether t h e r e i s s p i l l o v e r o f oxygen species and what i s t h e r o l e p l a y e d by t h e s p i l t o v e r oxygen i n the o x i d a t i o n o f methane. Pt/Ceo2 c a t a l y s t has been used i n the t o t a l o x i d a t i o n o f hydrocarbons f o r t h e reduction o f exhaust emissions[8,91. Several studies have been made on t h e oxidat i o n mechani sms o f hydrocarbons and carbon monoxide over Pt/CeO2[8-11I b u t l e s s a t t e n t i o n has been p a i d t o t h e oxygen s p i l l o v e r on t h i s c a t a l y s t .
2, EXPERIMENTAL A d e t a i l e d d e s c r i p t i o n o f preparation o f Ceo, and t h e experimental apparatus f o r F T - I R study i s given elsewhere[4.51. Pt/CeOl c a t a l y s t was prepared by impregnating Ceo, w i t h an aqueous s o l u t i o n o f H,PtCle, and f o l l o w i n g a reduction by hydrazine aqueous s o l u t i o n a t 298 K and a d r y i n g a t 373 K i n a i r . E l e c t r i c a l c o n d u c t i v i t y o f sample was measured i n 0, atmosphere i n a temperature programmed w a y f r o m 298 t o 573 K. Oxygen chemisorption was measured a t various temperatures f o r f r e s h c a t a l y s t s which were pretreated i n H2 a t 673 K f o r 1 h r .
3, RESULTS AND DISCUSSION Adsorbed superoxide species can be formed v i a O2 adsorption on an outgassed CeO, and both adsorbed superoxide and peroxide species are r e a d i l y produced when a p a r t i a l l y reduced cerium oxide i s exposed t o 0, a t r m temperature[41. The superoxide and peroxide formed on cerium oxide g i v e evident I R bands w i t h c h a r a c t e r i s t i c frequencies a t 1126 and 883 cm-' r e s p e c t i v e l y as shown i n Figure 1 . The adsorbed dioxygen species are q u i t e s t a b l e and can be s t i l l observed even UP t o 373 K. However we f a i l e d t o d e t e c t the dioxygen species on outgassed Pt/Ce02 as w e l l as on p a r t i a l l y reduced Pt/CeOa. A very weak band a t 1126 cm-' due t o adsorbed superoxide species on Pt/CeOp could be observed only a t a lower temperature, b u t t h e band attenuated gradually and disappeared as soon as t h e sample was s l i g h t l y heated. I t seems t h a t t h e dioxygen species.are d i f f i c u l t t o be stabi l i z e d on Pt/CeO, c a t a l y s t o r i t may convert i n t o atomic ' so f a s t t h a t i s hard oxygen species such as 0. and 0- o r 0 t o d e t e c t . Whereas a chemisorption measurement o f oxygen on Pt/Ce02 evidenced t h a t t h e c a t a l y s t d i d adsorb considerable amount o f oxygen even a t roan temperature. A s depicted i n Figure 2, t h e amount o f chemisorbed oxygen on Pt/CeO, remains constant a t t h e adsorption temperatures f r o m 298 t o 500 K w h i l e t h a t for CeO, v a r i e s s i g n i f i c a n t l y a t t h e same temper a t u r e region. A t room temperature t h e amount o f chemisorbed
219
-0-0-
CeO2
-0-0-
Pt/CeO2
I I
~
)O
1tbo
cm-l
I
Figure 1.IR spectra o f d i oxygen adsorbed on reduced CeO, arid Pt/CeO, a t 273 K .
'3
373
473
T/K Figure 2. Amount o f adsorbed oxygen on reduced Pt/CeO, and CeO, at; various temperatures
i t decreases oxygen f o r Ce0, i s more than t h a t f o r Pt/CeO,.and upon warming the sample and f a l l s t o i t s minimum value a t 390 K. On the c o n t r a r y , above 390 K the amount o f chemisorbed oxygen on CeO, increases w i t h the e l e v a t i o n o f temperature and reaches the same l e v e l a t 500 K as t h a t f o r Pt/CeO,. The oxygen chemisorbed on CeO, a t temperatures under 390 K e x i s t maj o r l y i n t h e form o f dioxygen species while i t m a y d i s s o c i a t e a t temperatures exceeding 390 K as proved by i n f r a r e d spectra. The curve f o r CeO, i n Figure 2 before 390 K can be explained as t h a t t h e chemisorbed dioxygen species desorb a t higher temperatures. The f a c t t h a t t h e dioxygen species were scarcely detected f o r Pt/CeO, b u t there i s l a r g e amount o f oxygen chemisorbed on t h e c a t a l y s t i n d i c a t e s t h a t t h e adsorbed dioxygen species on Pt/CeOz f a c i l e l y transform i n t o atomic oxygen spec i e s even a t r m temperature. I t can be obviously seen t h a t the transformation o f dioxygen on P t / C e 0 2 i s q u i t e easier than t h a t on CeO,. The presence o f P t f a c i l i t a t e s t h e transformat i o n f r o m dioxygen t o atomic oxygen, accordingly r e s u l t i n g t h e disappearance o f the I R bands due t o t h e adsorbed dioxygen species even a t r o o m t m p e r a t u r e ( f i g u r e 1 ) . One can speculate t h a t t h e dioxygen species should be formed on t h e surface o f Cd), a t the very begining o f the adsorption, and the adsorbed dioxygen are unable t o d i s s o c i a t e a t r o o m temperature on CeO, but a r e very l a b i l e and e a s i l y migrate onto P t surface where they d i s s i c i a t e i n t o atomic oxygen species(0. 0-1, t h e process can be r e f e r r e d t o as reverse s p i l l o v e r o f oxygen. The atomic
220 oxygen species on P t surf ace may be a 1 so d e r i ved f r o m t h e d i r e c t adsorption o f gaseous 0,. The atomic oxygen formed on P t surface possibly s p i l l o v e r s onto CeO, surface where p a r t o f them may convert i n t o l a t t i c e oxygen anions. The amount o f chemisorbed oxygen on CeO, o r Pt/Ce02 mainly depends on the number o f oxygen vacancy and e l e c t r o n ready f o r the formation o f adsorbed oxygen species. A t room temperature. most o f the adsorbed oxygen species on CeO, are i n t h e form o f dioxygen. 0,- and 02,-. which r e q u i r e e l e c t r o n s l e s s than t h a t f o r t h e formation o f 0- and 0 ‘ - species on Pt/Ce02. This i s the reason why the amount o f chemisorbed oxygen on CeO, is more than t h a t on Pt/CeO,(Figure 2 ) . A t temperatures up t o 500 K t h e chemisorbed oxygen species are i n 0- and 0,forms f o r both CeO, and Pt/CeO, t h e r e f o r e t h e amount o f chemisorbed oxygen f o r the two c a t a l y s t s i s almost t h e same. The e l e c t r i c a l conductvi t y o f a p a r t i a1 l y reduced Pt/Ce02 decreases much sharply than t h a t o f a p a r t i a l l y reduced CeO, a f t e r oxygen ads o r p t i o n a t r m temperature implying t h a t more e l e c t r o n s cont r i b u t i n g t o t h e chemisorbed oxygen w i t h Pt/CeO, than w i t h CeO,. These r e s u l t s s t r o n g l y confirmed t h a t t h e reverse s p i l l over o f dioxygen and the s p i l l o v e r o f atomic oxygen on Pt/CeOs c a t a l y s t co-proceed e v i d e n t l y even a t r o o m temperature. I R spectra were taken i n - s i t u d u r i n g the r e a c t i o n o f CH, on CeO, and P t / C e 0 2 i n t h e presence o r absence o f gaseous oxygen. Below 3 7 3 K no oxidized product was observed f o r e i t h e r CeO, o r Pt/CeO,. I t was found t h a t t h e o x i d a t i o n o f CH, is initiated a t 373 K f o r Pt/CeO, and a t around 473 K f o r CeO,. The presence o f P t lowers the temperature f o r t h e i n i t i a l r e a c t i o n about 1 0 0 K . Figure 3 shows I R spectra recorded f o r the react i o n o f CH, on Pt/Ce02 and CeO, a t 3 7 3 K standing f o r longer than 60 minutes. A s t r i k i n g d i f f e r e n c e i n Figure 3 can be seen f o r t h e two samples. When P t / C e 0 2 c a t a l y s t was exposed t o CH, + 0, atmosphere a t 3 7 3 K . three d i s t i n c t bands a t 1544, 1 3 7 1 and 1355 cm-’ appeared and developped w i t h prolonged t i m e , and these bands(together w i t h other two bands a t 2933, 2845 cm-’ n o t given here) can be a t t r i b u t e d t o formate species adsorbed on CeO, surface[5.121. Besides, there are some weak bands i n the range o f 1600-1300 cm-I which are assigned t o carbonate species adsorbed on CeO, surface[l31. While f o r CeO, alone + 0, atmosphere a t 373 K no any IR bands due t o t h e under CH, oxidized product i s observed as shown i n Figure 3 ( b ) . The cont r a s t between Figure 3 ( a ) and 3 ( b ) concludes t h a t t h e r e are a c t i v e oxygen species derived on the Ce02 surface o f Pt/CeO, and these oxygen species are so a c t i v e t h a t can o x i d i z e CH, i n t o formate species even a t 373 K . The oxygen species o r i g i nated i n the s p i l l o v e r f r o m P t surface are o n l y responsible f o r the formation o f formate species on CeO, because oxygen species formed on Ce0, alone are i n e r t toward methane a t 373 K . A doubt may e x i s t i f some a c t i v a t e d species from methane adsorption on P t spi 1 lover onto CeOz where they are o x i d i z e d i n t o formate s p e c i e s [ l 4 ] Separate experiments o f methane adsorption on CeO, [ 151 and P t / C e 0 2 a t lower temperatures made no apparent d i f f e r e n c e i n d i c a t i n g t h a t a c t i v a t e d methane seems t o be n o t formed on CeO, v i a a s p i l l o v e r f r o m Pt surface. The
-
22 I
C I
I
Figure 3. IR spectra o f adspecies derived from CH4 o x i d a t i o n a t 373 K . ( a ) CH4+02 over Pt/CeO,;(b)CH,+O, over CeO,; (c)CH, over Pt/CeO,.
do0
1$00
cm-'
do0
1;
Figure 4. IR spectra o f adspecies derived from CH, o x i d a t i o n a t 373 K over Pt/CeO, under ( a ) C H 4 + 1 D 0 2 ; ( b ) C H 4 + 1 " 0 2 .
r e s u l t i n Figure 3 ( c ) f u r t h e r confirmed t h a t t h e formation o f formate species i s due majorly t o t h e s p i l t o v e r oxygen species at because t h e r e i s no r e a c t i o n f o r methane alone on Pt/CeO, 3'73 K . The cerium oxide i t s e l f has a c t i v e oxygen species which i f there were can o x i d i z e C2H4 even a t 373 K[51 b u t n o t CH, a c t i v a t e d methane s p i l t o v e r f r o m P t they would react w i t h t h e act-ive oxygen species on CeO, r e s u l t i n g t h e same adsorbed f o r mate species as shown i n Figure 3 ( a ) . Isotope lmO2 was employed i n methane o x i d a t i o n over. Pt/CeO, a t 373 K w i t h an i n t e n t i o n t o ensure t h a t the s p i l t o v e r oxygen f r o m P t surface p a r t i c i p a t e i n the -Formation of surface f o r mate species on CeO, surface. IR spec+-ra were recorded i n s7tu i n t h e same fashion as the r e a c t i o n o f lSO2 + CH, on Pt/CeO, a t 373 K. The IR spectra recorded f o r t h e two cases, loo, + CH, and loo2 + CH, are c o l l e c t e d i n Figure 4. For 1 6 0 z + Cki,, only t h e bands a t 1544. 1 3 7 1 and 1355 cm-l due t o I4C1'WnOand some weak bands i n 1600-1370 cm-' region due t o C1oOn'species were derived as shown i n Figure 4 ( b ) . I n c o n t r a s t , f o r lt(0, + CH, t h e r e are n o t o n l y the IR bands a t 1544, 1 3 7 1 and 1355 cm-l o f formate species, HC1eO'nO-, but also r i c h bands i n 1544-1465. and 1355-1330 cm-' regions, such as the bands a t 1515, 1500, 1465. 1343, 1335 cm-'. lhese a d d i t i o n a l bands can be s a f e l y assigned t o t h e surface formate species togethetw i t h small amount o f carbonate species containing oxygen-18. i . e . , HC1eO1"O-, HCleOleO-, C(le0)03"-. For methane o x i d a t i o n
222 on CeO, alone, the bands due t o surface f o r m a t e species became d i s c e r n i b l e only up t o 4 7 3 K , but no products containing oxygen-18, either HC'HO'eOor HC'eO1eOw e r e detected i n the case of o f I R O , + CH4 over CeO, alone a t 4 7 3 K. T h e r e s u l t s again i n d i c a t e t h a t the oxygen species on Cd), without Pt i s not a c t i v e enough t o o x i d i z e methane even a t 4 7 3 K.
4, ~ L U S I O N S A d s o r b e d dioxygen s p e c i e s ( 0 , and OZ2-) on CeO, surface o f Pt/CeOa can m i g r a t e onto P t surface w h e r e they e a s i l y dissoc i a t e i n t o a t o m i c oxygen s p e c i e s ( 0 and 0-),on the other hand, the a t o m i c oxygen species formed on P t surface o f Pt/CeO, can s p i l l o v e r onto CeOz surface. T h e s p i l t o v e r oxygen species a r e so a c t i v e t h a t a r e able t o o x i d i z e m e t h a n e i n t o f o r m a t e and carbonate species even a t 3 7 3 K. W e g r a t e f u l l y a c k n o w l e d g e support by the N a t u r a l S c i e n c e Foundation o f C h i na ( NSFC
.
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