Adsorption and Reaction of Carbon Dioxide on Zirconium Dioxide

Chapter

9

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Adsorption and Reaction of Carbon Dioxide on Zirconium Dioxide Ronald G. Silver, Nancy B. Jackson, and John G. Ekerdt Department of Chemical Engineering, University of Texas, Austin, TX 78712

The activation of carbon dioxide was studied over a zirconium dioxide catalyst via infrared spectroscopy and O-labeled reactants. The carbon dioxide adsorbed on the surface as either a carbonate or a bicarbonate species. The carbonate species formed as a result of CO interaction with lattice oxygen. The bicarbonate species formed from CO interaction with a hydroxyl group. There was no direct interconversion between the carbonate and the bicarbonate. It is proposed that the bicarbonate can be converted to the formate via molecular CO. 18

2

2

Previous reports from our laboratory (1-4) have suggested that CO i s activated over z i r c o n i a v i a a formate and that the formate i s reduced to the methoxide. Carbon monoxide hydrogénation v i a formate and methoxide species has also been proposed over Cu/ZnO (5). Methoxide was proposed to be the immediate precursor to methane and methanol v i a hydrogénation and hydrolysis, respectively (3,4). The proposed mechanism f o r CO interaction with bridging hydroxyl groups to form the formate and the incorporation of l a t t i c e oxygen of z i r c o n i a into the formate and methoxide species was based on oxygen labeling studies with CO and H 0 (4). A previous study (J_) has also shown that CO2 can be converted into methane. Infrared studies have shown that CO2 forms a b i c a r bonate i n accordance with the adsorption studies of Tret'yakov et a l . (6). Heating the bicarbonate-containing z i r c o n i a i n hydrogen led to a surface methoxide species (2). I t has been suggested that CO2 interacts with terminal hydroxyl groups to form the bicarbonate (4). The route from CO2 to methoxide (the methane precursor) has not been reported over z i r c o n i a . Formate has been reported over ZnO from CO2 and H2 during the water-gas s h i f t reaction (7,8) and following exposure of Cu/ZnO to CO2/H2 (9). Several groups (5,10-13) have reported the direct conversion of CO2 into methanol. Chinchen et a l . (11) proposed that CO2 adsorbed on Cu/Zn0/Al203 and reacted with hydrogen atoms to form a 2

© 1988 American Chemical Society

In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

124

CATALYTIC ACTIVATION OF CARBON DIOXIDE

formate s p e c i e s , and t h a t the water-gas s h i f t and the C 0 - t o - f o r m a t e r e a c t i o n s o c c u r r e d by d i f f e r e n t i n t e r m e d i a t e s . Vedage e t a l . (5) have proposed t h a t b o t h CO and C 0 r e a c t t o formate and methoxide o v e r Cu/ZnO w i t h the hydrogénation r a t e of CO much f a s t e r t h a n C 0 . T h i s paper r e p o r t s o x y g e n - l a b e l i n g s t u d i e s d i r e c t e d toward i d e n t i f y i n g the mechanisms f o r C 0 i n t e r a c t i o n w i t h z i r c o n i a . The mechanism of b i c a r b o n a t e f o r m a t i o n and i t s subsequent c o n v e r s i o n t o the formate e i t h e r d i r e c t l y o r v i a the i n t e r m e d i a t e f o r m a t i o n of CO were explored. 2

2

2


2

Methods The e x p e r i m e n t a l a p p a r a t u s , the c a t a l y s t s y n t h e s i s from z i r c o n i u m c h l o r i d e , and the c a t a l y s t c h a r a c t e r i z a t i o n a r e r e p o r t e d elsewhere (4). The experiments were conducted i n a c o n v e n t i o n a l f i x e d - b e d temperature-programmed d e s o r p t i o n a p p a r a t u s o p e r a t i n g a t 1 atm t o t a l pressure. Water vapor was i n t r o d u c e d t o the system by s p a r g i n g h e l i u m gas t h r o u g h water a t 25°C. One and o n e - h a l f grams of z i r c o n i a were p l a c e d i n a 12.70 mm-o.d. q u a r t z t u b e , and p r e t r e a t e d as f o l l o w s : The tube and i t s c o n t e n t s (the system) were p o s i t i o n e d i n a f u r n a c e w h i c h was h e a t e d t o 650°C i n f l o w i n g oxygen and m a i n t a i n e d a t t h o s e c o n d i t i o n s f o r 30 m i n u t e s . A l l gases were a t 1 atm p r e s s u r e and f l o w e d t h r o u g h the tube a t a t o t a l f l o w of 30ml/min. W h i l e s t i l l a t 650°C, the system was f l u s h e d w i t h h e l i u m f o r 15 m i n u t e s , and f i n a l l y w i t h hydrogen f o r 20 m i n u t e s . The system was then c o o l e d i n hydrogen t o room temperature. F o r the l a b e l i n g s t u d i e s , the system was t h e n ramped t o 620°C a t 1.0°C/sec i n w a t e r - s a t u r a t e d h e l i u m . N e x t , t h e system was h e l d a t 620°C i n w a t e r - s a t u r a t e d h e l i u m f o r 5 m i n u t e s . Dry h e l i u m c o o l e d the system t o 450°C. A 50/50 m i x t u r e of C 0 / H f l o w e d t h r o u g h the system as i t c o o l e d from 450°C t o room t e m p e r a t u r e . F i n a l l y , the system was ramped t o 620°C a t 1.0°C/sec i n d r y h e l i u m ( t h e TPD s t e p ) . R e a c t o r p r o d u c t gases were m o n i t o r e d f o r C 0 , C 0 , C 0 , C 0 0 , C 0 , H 0 , and H 0 , AMUs ( a t o m i c mass u n i t ) 28, 30, 44, 46, 48, 18, and 20, r e s p e c t i v e l y . A d d i t i o n a l e x p e r i m e n t s were performed u s i n g t h e same procedure as above, except the system was c o o l e d from 450°C t o room temperature i n pure C 0 . Other runs were made i n which the temperature a t w h i c h the system was exposed t o C 0 / H was v a r i e d from 400 t o 600°C. C o o l i n g i n C 0 / H from 450°C t o 25°C enhanced b i c a r b o n a t e f o r m a t i o n and e l i m i n a t e d formate/methoxide f o r m a t i o n . The hydrogen ( B i g Three UHP, 99.999%) was passed t h r o u g h a deoxo c y l i n d e r and a bed of 4-A m o l e c u l a r s i e v e s t o remove oxygen and w a t e r . Carbon monoxide ( B i g Three UHP, 99.8%) was h e a t e d t o 180°C o v e r m o l e c u l a r s i e v e s t o decompose m e t a l c a r b o n y l s . Carbon d i o x i d e ( B i g Three UHP, 99.7%) and oxygen ( B i g Three UHP, 99.9+%) were passed t h r o u g h beds of 4-A m o l e c u l a r s i e v e s t o remove w a t e r . H e l i u m had a minimum p u r i t y of 99.995% and was passed t h r o u g h a bed of 4-A molecul a r s i e v e s t o remove w a t e r . 0xygen-18 l a b e l e d C 0 (98%) and oxygen18 l a b e l e d w a t e r (98%) were purchased from Cambridge I s o t o p e L a b o r a t o r i e s and used w i t h o u t f u r t h e r t r e a t m e n t . 2

l 6

2

1 8

1 6

1 6

1 8

2

1 6

1 8

2

2

2

2

2

2

2

2


1 8

2

9. SILVER ET AL.

125

Adsorption and Reaction

Results Two types of h y d r o x y l groups have been observed over z i r c o n i a , b r i d g e d and t e r m i n a l . Water adsorbs d i s s o c i a t i v e l y over Z r c a t i o n s (14) t o produce b o t h t e r m i n a l and b r i d g e d h y d r o x y l groups ( 6 ) . The t e r m i n a l groups a r e l e s s s t a b l e than the b r i d g e d groups (15) and have been suggested as the type t h a t exchanges w i t h gas phase w a t e r and t h a t i n t e r a c t s w i t h CO^ t o form the b i c a r b o n a t e s p e c i e s ( 4 ) . The system was i n i t i a l l y ramped and h e l d i n H 2 0 i n o r d e r t o exchange the [ 0 ] t e r m i n a l h y d r o x y l groups o f z i r c o n i a w i t h [ 0 ] . The exchange was i n c o m p l e t e d u r i n g the p r e s e n t s t u d i e s and was determined by r a t i o i n g the t o t a l amounts of H 2 0 and H 2 0 t h a t e v o l v e d d u r i n g the TPD s t e p . Carbon d i o x i d e and CO e v o l v e d d u r i n g the TPD l a b e l i n g s t u d i e s . S p e c i e s d e s o r b i n g from the s u r f a c e were i d e n t i f i e d on the b a s i s o f the temperature r e g i o n i n w h i c h they were observed. Some o f the experiments of He and E k e r d t (I) were r e p e a t e d o v e r the c a t a l y s t used i n t h i s study. S i m i l a r TPD r e s u l t s were used t o a s s i g n an i d e n t i t y t o the d e s o r b i n g / r e a c t i n g s p e c i e s , i n accordance w i t h the assignments of He and E k e r d t . The r e g i o n between 200-260°C i s proposed t o be where a c a r b o n a t e d e s o r b s . The r e g i o n between 450-510°C i s proposed t o be where a b i c a r b o n a t e s p e c i e s d e s o r b s . F i n a l l y , the r e g i o n between 580-620°C i s proposed t o be where formate and methoxide s p e c i e s desorb. The TPD r e s u l t s observed f o l l o w i n g c o o l i n g i n H 2 / C 0 a r e shown i n F i g u r e 1. Peaks f o r AMUs 28 and 44 were formed i n the c a r b o n a t e r e g i o n (225°C) , and peaks f o r AMUs 28, 30, 44, and 46 were observed i n the b i c a r b o n a t e r e g i o n (480°C). Masses 18 and 20 (not shown) were p r e s e n t a t c o n s t a n t l e v e l s above the background f o r temperatures g r e a t e r than 450°C. The TPD r e s u l t s observed f o l l o w i n g c o o l i n g i n H2/C 02 a r e shown i n F i g u r e 2. A l l the carbon o x i d e masses except 28 were formed d u r i n g the d e s o r p t i o n / d e c o m p o s i t i o n of the c a r b o n a t e (225°C) . The carbon monoxide masses (AMUs 28 and 30) d i s p l a y e d peaks i n the b i c a r bonate r e g i o n (480°C), w h i l e the carbon d i o x i d e masses (AMUs 44, 4 6 , and 48) appeared i n the b i c a r b o n a t e r e g i o n as s h o u l d e r s on the t r a i l i n g edge o f c a r b o n a t e peaks. The a r e a under each of the peaks i n F i g u r e s 1 and 2, as w e l l a s an a d d i t i o n a l experiment w i t h C 0 2 / H 2 0 , are p r e s e n t e d i n T a b l e I . The mass f r a g m e n t a t i o n f o r C 0 2 g e n e r a t e s an AMU 28 s i g n a l t h a t i s 7% o f the AMU 44 s i g n a l . The overwhelming m a j o r i t y of the c a r b o n a t e desorbed as c a r b o n d i o x i d e . The b i c a r b o n a t e g e n e r a t e d b o t h CO and CO2, but more CO formed t h a n CO2. The a d s o r p t i o n of CO and CO2 on z i r c o n i a was a l s o s t u d i e d u s i n g 18


1 6

1 8

1 8

1 6

1 6

2

18

l 6

1 8

1 6

i n f r a r e d spectroscopy, which provides d i r e c t evidence f o r surface intermediates.

The

r e s u l t s are p r e s e n t e d

i n F i g u r e 3. Carbon

o x i d e formed the formate (bands at 2880, 1580,

1387,

and

mon1

1360 cm" )

a f t e r a d s o r p t i o n a t 225 and 500°C, and p o s s i b l y a s m a l l amount of b i c a r b o n a t e (band a t 1610

1

cm" )

a f t e r a d s o r p t i o n a t 225°C.

o x i d e formed the b i c a r b o n a t e a c a r b o n a t e (band at 1335

(bands a t 1610, 1

cm" )

1430,

and

Carbon d i -

1220

1

cm ) and

a f t e r a d s o r p t i o n a t 225 and 500°C.


126



I

I I I I I

200

400

60C

T E M P E R A T U R E (C)

Figure 1. Mass signals during a TPD experiment following pretreatment of Zr02 with H2 0 and adsorption of H2/C 02. 16



SILVER ET AL.


ι

ι ι ι ι ι 200

400

601

T E M P E R A T U R E (C)

Figure 2. Mass Signals during a TPD experiment following pretreatment of Zr02 with H2 0 and adsorption of H /C 0 . 1

18

2

2



00

00


ο

CM CO

00 C7N

CO

00

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00

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Ο 00 rH

CO

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00 . /—s

cd *ο ο




9. SILVER ET AL.

Figure 3. Infrared spectra of Zr02 after adsorption of (a) CO at 225°C, (b) CO at 500°C, (c) CO2 at 225°C, and (d) CO2 at 500°C.


129

130


Discussion Cooling the z i r c o n i a from 450 to 25°C i n H /C0 resulted i n the f o r mation of carbonate and bicarbonate species. (Additional TPD studies, not shown, established that formate and methoxide species did not form during the adsorption of C0 , under the conditions reported here.) The use of H 0 to exchange 0H groups on the z i r c o n i a with 0H groups and the use of 0 - l a b e l e d C0 allow the determination of the pathways to the formation of carbonate and bicarbonate, as well as the possible interconversion between carbonate, bicarbonate and formate. Figure 4 presents a proposed scheme f o r the a c t i v a t i o n of C0 over Z r 0 . The scheme i s discussed below. The carbonate decomposed to produce C0 . Only C 0 desorbed following the adsorption of C 0 (Table I ) . This demonstrates that the formation of carbonate did not involve the interaction of C0 with the water-based hydroxyl groups of z i r c o n i a . (The water-based hydroxyl groups are most l i k e l y terminal hydroxyls (4).) A mixture of carbon dioxide isotopes was generated following the adsorption of C 0 . This observation suggests that carbonate i s formed by the interaction of C0 with l a t t i c e oxygen anions of z i r c o n i a . The infrared results with C0 (Figure 3) suggest that bidentate 2

2

2

1 8

16

2

18

18


2

2

2

1 8

2

2

1 6

2

2

1 8

2

2

2

carbonate I was present. The band at 1335 cm i s typically associ ated with the asymmetric 0C0 stretch of I (16,17). Infrared studies over Zr0 (6) and Th0 (18,19) reveal that three d i f f e r e n t carbonates can form over these oxides (see Figure 4), two bidentate structures, I and I I , and a monodentate structure, I I I . The d i s t r i b u t i o n of oxygen isotopes i n the TPD spectrum of C0 may suggest that III also formed over z i r c o n i a . If one assumes that the majority of the 2

2

2

1 8

1 6

1 8

1 8

2

carbonate species formed from C 0 are [ C 0 0 0 ] " , then the monodentate w i l l produce either a l l C 0 when the l a t t i c e oxygen [ 0 ] i s involved i n the Zr-O-C bond, or a l l C 0 0 when one of the carbon dioxide's oxygens [ 0 ] i s involved i n the Zr-O-C bond. If one assumes the same isotope composition for the bidentate carbonate, ο [ C 0 0 0 ] ~ , and that one of the Zr-O-C bonds of the bidentate carbonate i s [ 0 ] , then the carbon monoxide that forms during carbonate decomposition should be a 1:1 mixture of C 0 0 and C 0 . Twice as much C 0 formed as did C 0 0 during the decomposition of the carbonate, suggesting that the majority of the carbonate was present i n the monodentate structure, I I I , with the l a t t i c e oxygen [ 0 ] involved i n the Zr-O-C bond. The formation of C 0 0 during carbonate decomposition i s consistent with bidentate I. The bicarbonate decomposed to produce CO and C0 . The r e l a t i v e incorporation of [ 0 ] into the CO and C0 produced following adsorp tion of C 0 increased with the increasing [ 0 ] content of the sur face OH groups (Table I ) . This result supports the hypothesis that bicarbonate forms by the reaction between C0 and a terminal (water-based) hydroxyl group (4). The labeling studies reported here as well as previous studies in our laboratory (1-4) are consistent with indirect conversion of C0 into Ci hydrocarbons, v i a the intermediate formation of CO as shown i n Figure 4. Carbon dioxide readily forms bicarbonate. The 2

1 8

2

16

1 6

1 8

18

1 6

1 8

1 8

1 6

1 6

1 8

1 8

2

1 8

1 6

1 8

2

16

1 8

1 8

2

18

2

1 6

18

2

2

2


9. SILVER ET AL.

131


OH Terminal Hydroxyl Group

œ

C

^

2

H Bridging Hydroxyl Group

y

Zr Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch009

i

L

-* CO

I

Zr

Zr

Lattice Oxygen

0

ο

0

II c

II

°V

A

ο I

OR

? Zr

Zr

(III)

(II)

(i)

Methane & Methanol

c

Zr Zr

0

Figure 4. Proposed scheme for the activation of C02 over Zr02. route from bicarbonate to formate remains untested. It was not possible to r e a l i z e the levels of [ 0 ] incorporation into the bicarbonate and the surface concentrations of bicarbonate necessary to test for the formation of labeled-methanol from C0 . The TPD studies have shown that bicarbonate decomposes to CO and C0 . The infrared and other studies (4) demonstrate that CO readily reacts with Zr0 to produce the formate. While we cannot rule out direct reduction of the bicarbonate to a formate, the demonstrated presence of the i n d i r e c t steps makes the intermediate formation of CO seem more reasonable. Figure 4 also shows the formation of carbonates from C0 and a l a t t i c e oxygen. The nature of the surface s i t e s involved i n carbonate formation were not revealed i n these studies. The results for [ 0 ] incorporation into the carbonate and bicarbonate species, following C 0 adsorption, suggest that these species are formed at d i f f e r e n t s i t e s and that there i s no direct conversion of the carbonate into the bicarbonate. 18

2

2

2

2

18

1 6

2

Acknowledgment s This work was supported by the D i v i s i o n of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under Contract //DE-AS05-80ER10720.


132


Literature Cited 1. 2. 3. 4. 5.


6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

He, M-Y., and Ekerdt, J.G., J. Catal. 1984, 87, 238. He, M-Y., and Ekerdt, J.G., J. Catal. 1984, 87, 381. He, M-Y., and Ekerdt, J.G., J. Catal. 1984, 90, 17. Jackson, N.B., and Ekerdt, J.G., J. Catal., 1986, 101, 90. Vedage, G.A., Pitchai, R., Herman, R.G., and Klier, Κ., Proc. Int. Congr. Catal., 8th, 1984 1985, 2, 47. Tret'yakov, N.E., Pozdnyakov, D.V., Oranskaya, O.M., and Filimonov, V.N., Russ. J. Phys. Chem. 1970, 44, 596. Ueno, Α., Yamamoto, T., Onishi, T., and Tamaru, Κ., Bull. Chem. Soc. Japan 1969, 42, 3040. Ueno, Α., Onishi, T., and Tamaru, Κ., Trans. Faraday Soc. 1970, 66, 756. Edwards, J.F., and Schrader, G.L., J. Phys. Chem. 1984, 88, 5620. Kagan, Y.B., Lin, G.I., Rozovskii, A.Y., Loktev, S.M., and Golovkin, Y.I., Kinetika i Kataliz 1976, 17, 440. Chinchen, G.C., Denny, P.J., Parker, D.G., Short, G.D., Spencer, M.S., Waugh, K.C., and Whan, D.A., Preprints, ACS Division of Fuel Chemistry, Philadelphia, PA, 1984, Vol. 29, No. 5, p. 178. Kung, H.H., Liu, G., and Wilcox, D., Preprints, ACS Division of Fuel Chemistry, Philadelphia, PA, 1984, Vol. 29, No. 5, p. 194. Thiorolle-Cazat, J., Bardet, R., and Trambouze, Υ., Preprints, ACS Division of Fuel Chemistry, Philadelhpia, PA, 1984, Vol. 29, No. 5, p. 189. Agron, P.A., Fuller, E.L., Jr., and Holmes, H.F., J. Colloid and Interface Sci. 1975, 52, 553. Yamaguchi, T., Nakano, Y., and Tanabe, Κ., Bull Chem. Soc. Japan, 1978, 51, 2482. Nakamoto, Κ., "Infrared Spectra of Inorganic and Coordination Compounds;" Wiley: New York, 1978; p. 231. Morterra, C., Coluccia. S., Ghiotti, G., and Zecchina, Α., Ζ. Phys. Chem. 1977, 104, 275. Courdurier, G., Claudel, Β., and Faure, L . , J. Catal. 1981, 71, 213. Pichat, P., Veron, J., Claudel, Β., and Mathieu, M.V., J. Chem. Phys. 1966, 33, 1026.

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Adsorption and Reaction of Carbon Dioxide on Zirconium Dioxide

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