J. Phys. Chem. 1984,88, 5620-5624
5620
-
-
relative to the NH3 absorption leads to the same values of relative 70%and C, 30%. concentrations: C, At this stage, we have demonstrated by vibrational spectroscopy that the defects of alkylammonium chains are “kink structures”. As deduced from X-ray diffraction measurements of the c lattice ~ a r a m e t e rone , ~ can assume that only one kink exists in a chain, so that only the 12 configurations of the alkylammonium cations summarized in Table I11 are possible. As no end gauche structure has been observed, the possibilities of the 2nd, 6th, and 12th configurations are related by the following equation: P2
+ PS + Pl2
-
There are less than 5% of G structures near NH3 polar groups: P1 + P3
+ P7
5 5%
About 25% of chains possess a TT sequence around the second carbon: P5 + P6 + P9 + PlO + P11 + P12
-
25%
About 30%of chains possess a T G or GT sequence around the 4th carbon atom: P5
+ PS + P7 + PS + P9 + PlO
-
30%
These four equations lead to the following ones: p4 4- ps 70%
-
P4
+ P11
-
65%
with p s I 30% and pll 5 25%. It turns out that the cation configuration (TGTTTG’TT) is greatly favored in the HT phase of CloCd since it occurs at least two times out of five. The first part of the alkylammonium chain is relatively rigid since the first structure is T most of the time and the second one is G about three times out of four. The middle of the chain seems more flexible, and despite of what is observed in lipid bilayered membranes or n-alkanes, no end gauche structure is found at the decylammonium tail.
Conclusions From this vibrational study, new information about the cation structure in the two disordered phases of CloCd has been obtained. Indeed, as the correlation times governing the dynamics of the decylammonium chain are relatively long ( T ~L lo4 s), each cation
position and conformer have been observed, and using selective deuterations, we have determined the average local molecular structure of the hydrocarbon part of the chains. For the low-temperature phase, we have been able to confirm the molecular structure of the decylammonium ion as proposed from X-ray r e ~ u l t s .Infrared ~ and Raman spectra of this phase are characteristic of an ordered solid in which alkylammonium chains exhibit two almost extended configurations with only one gauche structure either between the first two carbon atoms or between the second and the third ones. Vibrational spectra of the IMT phase give evidence of a rotation around the first C-C bond transforming A into B chains. An interconversion of A and B conformers, at least in the cavities which contain A chains in the LT phase, is suggested in order to account for the structure proposed from previous s t ~ d i e s .Since ~ melting also occurs at the same time as reorientational motions in other long-chain compounds of the series,32it seems sensible to expect a coupling of the two types of dynamics. In the high-temperature phases, the chain flexibility is transferred to its middle part. The three C-C or C-N bonds of each chain end are rigid and form a trans structure. Between them, kink motions take place. One of these configurations (TGTTTG’TT) seems to be favored since it occurs between two and three times out of five. We thus conclude that the nature of the melting of hydrocarbon parts in these bidimensional compounds is strongly dependent on the interactions of the alkylammonium chains with the semirigid inorganic cavities. As a matter of fact, a different conformational disorder takes place in n-alkanes or in lipid biomembranes where the tails are more flexible. Further results on compounds of this series with variable chain length have been obtained. They will be published in due course. Acknowledgment. We thank Dr. H. Arend for very useful discussions on crystal growth during the initial stage of this study. We also are grateful to Mrs. Gerval, M. M. R. Cavagnat, J. C. Cornut, and J. J. Martin for their help in the experimental part of this work. Registry No. C&d, 53188-91-3; C&d-1,Z-d2, 92526-13-1; CloCd2,2-d2, 92526-15-3; C,oCd-4,4-d,, 92526-11-5. ~
~
~~~
(32) Chanh, N. B.;Haget, Y.;Hauw, C.; Meresse, A,; Ricard, L.; ReyLafon, M. J. P h p . Chem. Solids 1983,44,589. Ricard, L.; Rey-Lafon, M., unpublished results.
Infrared Spectroscopy of Cu/ZnO Catalysts for the Water-Gas Shift Reaction and Methanol Synthesis James F. Edwards and G. L. Schrader* Department of Chemical Engineering and Ames Laboratory-U.S. Department of Energy, Iowa State University, Ames, Iowa 5001 1 (Received: April 12, 1984)
A bidentate formate species adsorbed on a zinc site was a common intermediate in the water-gas shift and methanol synthesis reactions on Cu/ZnO catalysts. Adsorbed formaldehyde and methoxy species were identified as additional intermediates in the reaction pathway for methanol synthesis. An adsorbed carbonyl species on a reduced copper site was an activating agent for the reduction of formate groups to formaldehyde and methoxy species at 200 OC.
Introduction The water-gas shift reaction C O HZO = COZ
+
CO2 + 3Hz = CH30H
+ H,
and the methanol synthesis reactions C O 2H2 = CH30H
+
0022-3654/84/2088-5620$01 S O / O
+ HZO
are catalyzed by copper-zinc mixed metal oxides having similar compositions. During methanol synthesis conditions, all three reactions can occur simultaneously on the catalyst surface. In spite of extensive studies of these catalytic systems, very little is known about the elementary reaction steps, the adsorbed inter@ 1984 American Chemical Society
Infrared Spectroscopy of Cu/ZnO Catalysts mediates, and the interrelationship among the water-gas shift and methanol reactions. Most of the previous studies have applied kinetic or chemical trapping techniques to study the mechanism of the reactions. An investigation of the kinetics of the water-gas shift reaction and formic acid decomposition over a Cu/ZnO catalyst has established that a stable formate intermediate is involved in the water-gas shift reaction.'t2 Both formate and methoxy surface species were identified by chemical trapping using a methanol catalyst after reaction of carbon monoxide and hyd r ~ g e n . ~A formate species was also identified by chemical trapping when carbon dioxide and hydrogen were the reactants; the formate species was therefore considered to be a reaction intermediate in both carbon dioxide hydrogenation and carbon monoxide hydrogenation to methanoL4 Rozovskii and co-workers have conducted kinetic and isotopic studies of the relationship among these three reactions over an industrial methanol synthesis catalyst and concluded that (1) methanol is formed by carbon dioxide hydrogenation but not by carbon monoxide hydrogenation, (2) carbon monoxide is converted to carbon dioxide via the water-gas shift reaction, and (3) the reaction steps in the water-gas shift reaction are independent of the reaction steps in methanol synthesis, Le., the two reactions have no common adsorbed intermediate specie^.^-^ Infrared spectroscopy has proven to be a very useful technique for observing adsorbed species on catalyst surfaces during reaction conditions. An adsorbed formate species has been identified by using infrared spectroscopy on magnesia,lO,l* alurnina,l2 and zinc oxide" for the water-gas shift reaction. Infrared studies of the adsorption of a CO-H2 mixture on ZnO at room temperature identified carbonyl, hydride, and hydroxyl species; l3 another study has suggested, in addition, the existence of an adsorbed formyl species on surfaces of ZnO and 0.1% Cu/Zn0.14 Methanol decomposition on zinc oxide has been found to produce methoxy and formate groups.15 In this study, infrared spectroscopy was used to identify surface species on CuIZnO catalysts resulting from the adsorption of gaseous mixtures relevant to methanol synthesis and the water-gas shift reaction. Experimental Procedure The catalysts were prepared by coprecipitating a mixture of copper and zinc nitrates with ammonium bicarbonate at 60 OC according to the method described by Stiles.16 After being filtered
(1) van Herwijnen, T.; de Jong, W. A. J . Catal. 1980, 63, 83.
(2) van Herwijnen, T.; Guczalski, R. T.; de Jong, W. A. J . Catal. 1980, 63, 94.
(3) Deluzarche, A,; Kieffer, R.; Muth, A. Tetrahedron Lett. 1977, 38, 3357. (4) Kieffer, R.; Ramaroson, E.; Deluzarche, A.; Trambouze, Y. React. Kinet. Catal. Lett. 1981, 16, 207. (5) Kagan, Yu. B.; Rozovskii, A. Ya.; Lin, G. I.; Slivinskii, E. V.; Loktev, S. M.; Liberov, L. G.; Bashkirov, A. N. Kinet. Katal. 1975, 16, 704. (6) Rozovskii, A. Ya.; Kagan, Yu. B.; Lin, G. I.; Slivinskii, E. V.; Loktev, S . M.; Liberov, L. G.; Bashkirov, A. N. Kinet. Katal. 1975, 16, 706. (7) Kagan, Yu.B.; Lin, G. I.; Rozovskii, A. Ya.; Loktev, S . M.; Slivinskii, E. V.; Bashkirov, A. N.; Naumov, I. P.; Khludenev, I. K.; Kudinov, S.A,; Golovkin, Yu. I. Kinet. Katal. 1976, 17, 380. (8) Rozovskii, A. Ya.; Kagan, Yu. B.; Lin, G. I.; Slivinskii, E. V.; Loktev, S. M.; Liberov, L. G.; Bashkirov, A. N. Kinet. Katal. 1976, 17, 1132. (9) Rozovskii, A. Ya.; Lin, G. I.; Liberov, L. G.; Slivinskii, E. V.; Loktev, S . M.; Kagan, Yu. B.; Bashkirov, A. N. Kinet. Katal. 1977, 18, 578. (IO) Scholten, J. J. F.; Mars, P.; Menon, P. G.; van Hardeveld, R. "Proceedings of the 3rd International Contress on Catalysis"; Sachtler, W. M. H., Schuit, G. C. A., Zwietering, P., Eds.; North-Holland Publishing Co.: Amsterdam, 1965; p 881. (1 1) Ueno, A.; Onishi, T.; Tamaru, K. Trans. Faraday SOC.1970,66,756. (12) Amenomiya, Y. J . Catal. 1979, 57, 64. (13) Boccuzzi,-F.; Garrone, E.; Zecchina, A.; Bossi, A.; Camia, M. J . Catal. 1978, 51, 160. (14) Saussey, J.; Lavalley, J.; Lamotte, J.; Rais, T. J. Chem. Soc., Chem. Commun. 1982, 278. (15) Ueno, A.; Onishi, T.; Tamaru, K.Trans Faraday SOC.1971,67,3585. (16) Stiles, A. B. US.Patent 4 111 847, 1978.
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5621 l
"
"
"
I
I
"
"
"
'
I
VI 8 ;
4000
3600
3200
WAVENUMBERS
2800
2600
icrn-1)
2200
I800
1400
(Crn-ll
WAVENUMBERS
Figure 1. Reduction of 9 5 / 5 Zn/Cu oxide a t 200 OC: (a) oxidized surface, (b) exposure to 5% H2for 30 min, (c) exposure to 5% H2for 2 h, (d) exposure of an oxidized surface to D2for 30 min. I '
4000
3600
3200
WAVENUMBERS
(crn-1)
2800
2600
"
2200
"
" I
I800
WAVENUMBERS
1400
Icm-ll
Figure 2. Carbon monoxide adsorption on 90/10 Zn/Cu oxide a t 200 "C: (a) oxidized surface, (b) exposure for 5 min, (c) exposure for 1 h. and washed with distilled water, the precipitates were dried in air at 115 O C for 12 h and then calcined in a stream of oxygen at 400 OC for 8 h. The amount of cupric oxide in these catalysts was restricted to no greater than 10% CuO due to the strong infrared absorption by cupric oxide. The oxidic form of the catalyst was prepared for transmission infrared studies by pressing powder under a load of 4500 kg into a wafer approximately 0.10 mm thick. The wafer was placed in an infrared cell which has been described previo~sly.'~Dry nitrogen gas was flowed through the cell for approximately 12 h at atmospheric pressure and 200 OC to desorb water on the catalyst surface. Under these conditions there was no significant dehydroxylation of the catalyst surface. All experiments were conducted in a continuous-flowmode at 60 cm3/min STP by using high-purity gases. Water could be introduced into the cell at low concentrations by bubbling the gaseous flow through water contained within an enclosed glass vessel (saturator) in the feed system. Infrared spectra were recorded with a Bruker-Physik AG Model IFS 113 spectrometer using single-beam optics. Typically, 100 scans were accumulated at 2-cm-' resolution. Only the absorption by the infrared windows (CaF2) was subtracted from the spectra; gaseous species and the catalyst, in addition to adsorbed species, contributed infrared bands to the spectra. Attempts to substract theseabandsgenerally resulted in distortion of important spectral regiohs. Results Two precipitated binary oxides, 9515 Zn/Cu and 90110 Zn/Cu oxides, were used because of their relatively high infrared transmission. Infrared band assignments for adsorbed species were very similar for both catalysts. Band positions were affected by variables such as temperature, surface concentrations, and the presence of other adsorbed species. Catalyst Reduction. The copper-zinc binary oxides had the same residual hydroxyl groups and surface carbonate groups as those found on pure zinc oxide.18 The reduction of a 9515 Zn/Cu (17) Edwards, J. F.; Schrader, G. L. Appl. Spectrosc. 1981, 35, 559.
Edwards and Schrader
5622 The Journal of Physical Chemistry, Vol. 88, No. 23, 1984
3600
4000 WAVENUMBERS
WAVENUMBERS
(crn-lj
3200
WAVENUMBERS
(Crn-l)
Figure 3. Adsorption of a CO-H2 Mixture on 90/10 Zn/Cu oxide at 200 OC: (a) oxidized surface, (b) exposure for 5 min, (c) exposure for 1 h.
2800
2600
2200
lcmdi
1803
1400
WAVENUMBERS (crn-1)
Figure 5. Adsorption of a CO-H20 Mixture on 90/10 Zn/Cu oxide at 200 OC: (a) reduced surface, (b) exposure for 30 min, (c) exposure for 4 h.
71
I
"
"
"
I
I
"
"
'
"
~
4
I 4000
3600
320C
2800
,OO
2200
1800
I400
4000 WAVENUMBERS
lcrni)
Figure 4. Adsorption of a CO-H2 Mixture on 95/5 Zn/Cu oxide at 100 O C : (a) reduced surface, (b) exposure for 5 min, (c) exposure for 8 h.
oxide in a 95/5 N2/H2stream at 200 "C (pretreatment 1) is shown in Figure 1. Surface carbonate groups (bands at 1516, 1469, 1380, and 1322 cm-') were partially reduced to formate groups (bands at 2970,2878, 1576, 1381, and 1366 cm-I). A new,band developed at 3252 cm-' which has not been reported in any previous studies of zinc oxide. Reduction with deuterium (Figure Id) shifted the residual hydroxyl groups (bands at 3665, 3619, 3560, and 3460 cm-I) to OD groups (bands at 2706,2667,2630, and 2560 cm-') and created a new band at 2420 cm-I. CO Adsorption. Figure 2 shows the adsorption of carbon monoxide on an oxidized 90/10 Zn/Cu catalyst at 200 OC and 1 atm. Carbon monoxide was adsorbed (Figure 2, b and c) as a carbonyl species (band at 2093 cm-I) which gradually resulted in the reduction of the oxide. The unidentate carbonate groups (bands at 1468 and 1379 cm-I) also quickly disappeared, and a formate species slowly developed. The vibrational-rotational bands of gaseous carbon monoxide occurred around 2143 cm-'. CO-H, Adsorption. The concurrent adsorption of a carbon monoxide and hydrogen mixture (CO/H2 = 1/2) on an oxidized 90/10 Zn/Cu catalyst at 200 O C and 1 atm is shown in Figure 3. Reduction of the oxide occurred rapidly (Figure 3b) as indicated by the development of the band at 3252 cm-', the appearance of carbon dioxide (band at 2350 cm-*), and the reduction of surface carbonates (bands at 1514, 1466, 1388, and 1323 cm-') to formate species (bands at 2968, 2872, 2739, 1572, 1379, and 1366 cm-'). Because catalyst reduction was much more rapid with hydrogen than with carbon monoxide, the presence of carbon dioxide indicated that the water-gas shift reaction had occurred. The lack of vibrational-rotational bands suggested that the carbon dioxide was interacting with the catalyst surface. A transient, unstable surface species (bands at 2928, 2850, and 2739 crn-') was identified as an adsorbed formaldehyde species. After 1 h (Figure 3c) the development of methoxy groups (bands at 2935 and 2822 cm-l) was accompanied by the gradual disappearance of the hydroxyl at 3665 cm-I; higher methoxy concentrations caused the hydroxyls at 3665 and 3620 cm-' (isolated "free" hydroxyls) to disappear completely, suggesting that methoxy (18) Atherton, K.; Newbold, G.; Hockey, J. A. Discuss. Faraday SOC.
1971, 52, 33.
3600
3200
2800
2600
2200
I800
1400
WAVENUMBERS (ern.')
WAVENUMBERS
(crn-1)
WAVENUMBERS (cm-1)
Figure 6. Adsorption of a C02-H2 Mixture on 90/10 Zn/Cu oxide at 200 OC: (a) oxidized surface, (b) exposure for 5 min, (c) exposure for 4 h, (d) exposure to H2 for 1 h.
groups occupied the same adsorption sites as the isolated hydroxyls. It was possible to stabilize the transient formaldehyde species by lowering the temperature of the adsorption. Figure 4 shows the adsorption of a carbon monoxide and hydrogen mixture (CO/H2 = 1/2) on a reduced 95/5 Zn/Cu catalyst (pretreatment 1) at 100 OC and 1 atm. Initially the only change on the surface (Figure 4b) was the formation of a carbonyl species (band at 2097 cm-I). By prolongation of the exposure time (Figure 4c), adsorbed carbonate groups (bands at 1514, 1470, 1380, and 1327 cm-') were reduced to formate groups (bands at 2972,2876,2739, 1575, 1381, and 1366 cm-I). Formate groups were further reduced to adsorbed formaldehyde (bands at 2935, 2852, and 2739 cm-'). CD-H,O Adsorption. The thermodynamic equilibrium of the water-gas shift reaction strongly favors the formation of carbon dioxide and hydrogen at the experimental conditions of this study. The adsorption of carbon monoxide bubbled through water ( C O / H 2 0 = 100/1) on a reduced 90/10 Zn/Cu catalyst (pretreatment 1) at 200 OC and 1 atm is shown in Figure 5. A carbonyl species (band at 2093 crn-') was formed, and the intensity of the formate bands increased. Activity for the water-gas shift reaction was evident by the appearance of carbon dioxide (band at 2350 cm-I). There was no significant change in the intensity of carbonate bands throughout the experiment. Formate groups were not reduced to either formaldehyde or methoxy groups even after several hours of exposure. C02-H2 Adsorption. The hydrogenation of carbon dioxide (reverse water-gas shift reaction or methanol synthesis) might be expected to produce adsorbed species common to both reactions. Figure 6 shows the adsorption of carbon dioxide and hydrogen (C02/H2 = 1/1) on an oxidized 90/10 Zn/Cu catalyst at 200 OC and 1 atm. Formate (bands at 2970,2872,2739, 1572, and 1366 ern-') and formaldehyde (bands at 2930,2851,2739, and 1620 cm-l) groups developed quickly (Figure 6b). Gradual reduction of the catalyst was evident by the development of the band at 3252 cm-". After several hours (Figure 6c) the formaldehyde species had disappeared and the number of methoxy groups formed was negligible. The gas phase was replaced with pure hydrogen to eliminate the possibility that the barbon dioxide (an oxidant) was inhibiting the reduction of formate groups to methoxy groups.
Infrared Spectroscopy of Cu/ZnO Catalysts TABLE I: Organometallic Copper(1) Complexes oreanometallic CuU) comolex vcn. cm-' 2083 2068 21 17 4)
The Journal of Physical Chemistry, Vol. 88, No. 23, I984
ref ~
24
25
26
2080
26
2065
27 28 28 29 29 29 30 31 32
2055, 2066 209 1 2060 2062
2078 2063 2100 2050
An hour of hydrogen exposure (Figure 6d) did not produce any additional methoxy groups.
Discussion Band Assignments. The infrared bands above 3000 cm-I were assigned to hydroxyl groups. The residual hydroxyls with bands at 3665, 3620,3550, and 3450 cm-' have been previously identified on pure zinc oxide.18 The differences in the band positions of these hydroxyls have been attributed to the different crystal planes of zinc oxide on which these hydroxyls are located. The sharp, narrow shape of the bands at 3665 and 3620 cm-' is indicative of isolated hydroxyls while the broad features of the bands at 3550 and 3450 cm-' suggest that hydrogen bonding occurs among these hydroxyls. The band at 3252 cm-' appeared in copper-based catalysts during reduction. The intensity of this hydroxyl was proportional to the copper content of the catalyst and was unaffected by adsorbed species as long as the catalyst remained in a reduced state. XPS-Auger studies of binary copper-zinc catalysts after reduction at 200 OC established that the oxidation states of copper were 1+ and ze1-0.'~ An additional infrared observation at 200 OC was that if the binary oxide was reduced with hydrogen and subsequently exposed to deuterium, the residual hydroxyls rapidly shifted to OD groups but the band at 3252 cm-' shifted very slowly. On the basis of these observations, the band at 3252 cm-' was assigned to a bulk hydroxyl species associated with a reduced state of copper. Fully oxidized catalysts had some residual carbonate groups on the surface which appeared as infrared bands at 15 12, 1470, 1380, and 1325 cm-'. The changes in these band intensities during various surface conditions have indicated that the bands at 1512 and 1325 cm-' belong to the same species and the bands at 1470 and 1380 cm-I are associated with another species. The assignment of infrared bands to specific carbonate and carboxylate complexes on surfaces is uncertain because of the difficulties in making assignments based on comparisons with pure inorganic carbonates. A choice must be made among many different types of complexes that could be formed, e.g., bicarbonate, uncoordinated carbonate ion, bidentate carbonate, unidentate carbonate, and bridging carbonate species. On the basis of the general assignments for unidentate and bidentate carbonate complexes given by Nakamoto,20 the bands at 1470 and 1380 cm-' can be assigned to the asymmetric and symmetric OCO stretching frequencies of a unidentate carbonate species, while the bands at 1512 and 1325 cm-' were assigned respectively to the asymmetric and symmetric OCO stretching frequencies of a bidentate carbonate species. An intense carbonyl band, observed at 2093 cm-' in these studies, is similar to n-bonded carbonyls on metallic copper (2080-21 10 cm-1).21-23 Carbonyl frequencies for copper(1) organometallic complexes have been characterized recently. These (19) Edwards, J. F. Ph.D. Thesis, Iowa State University, Ames, IA, 1984. (20) Nakamoto, K. 'Infrared and Raman Spectra of Inorganic and Coordination Compounds", 3rd ed.; Wiley: New York, 1978. (21) Pritchard, J.; Sims, M. L. Trans. Furaduy SOC.1970, 66, 427. (22) Pritchard, J.; Catterick, T.; Gupta, R. K. Surf. Sci. 1975, 53, 1. (23) Horn, K.; Pritchard, J. Surf. Sci. 1976, 55, 701.
5623
carbonyl frequencies, given in Table I, fall into the 205C-2117-cm-l region. Thus, the band at 2093 cm-' is assigned to the stretching frequency of carbonyl species adsorbed on a reduced copper site whose oxidation state may be either 1+ or zero. The relationship between the oxidation state of copper and the carbonyl frequency for adsorbed C O has not been determined. A recent suggestion33 that a band at about 2130 cm-' is due to adsorption on Cu(1) has not clearly identified the oxidation state of the copper for the catalyst used. In addition, the catalyst used was a supported catalyst, and support interactions may affect the position of the adsorbed C O band. The infrared bands at 2875, 1381, 1575, and 1366 cm-' were assigned to the fundamental CH stretching, C H bending, asymmetric OCO stretching, and symmetric OCO stretching frequencies, respectively, of a bidentate formate species. The separation of aobut 210 cm-' between the two carbon-oxygen bands identified this formate as a bidentate rather than unidentate species.34 A formate species on pure zinc oxide had the same band location^.^^^'^ Additional bands at 2970 and 2739 cm-' were assigned to combinations of fundamental frequencies; the former band was a combination of the CH bending and asymmetric OCO stretching frequencies, while the latter band was a combination of the C H bending and symmetric OCO stretching frequencies. Infrared bands at 2930, 2850, 2739, and 1620 cm-' were assigned to the CH2 scissoring overtone, the asymmetric CH, stretching, the symmetric CH2 stretching, and the C O stretching frequencies, respectively, of an adsorbed formaldehyde species. For some spectra, the 1620-cm-' band was obscured by a much stronger formate band at 1575 cm-I. The analogous frequencies of gaseous formaldehyde occur at 2913, 2874, 2780, and 1745 cm-1.35 The large shift in the C O stretching frequency indicated that the formaldehyde species was adsorbed on the surface through the oxygen end of the molecule. A similar formaldehyde species with bands at 2920, 2870, 2770, and 1620 cm-' was identified on the surface of hematite ( C X - F ~ ~ O ~ ) . ~ ~ Infrared bands a t 2935 and 2820 cm-I were assigned to the asymmetric and symmetric CH3 stretching frequencies, respectively, of a methoxy species. A methoxy species on pure zinc oxide had infrared bands very similar to those on these copper-based
catalyst^.'^ Surface Reactions. A formate species-previously established by kinetic studies to be a reaction intermediate on Cu/ZnO catalysts'J-was identified by infrared spectroscopy as an adsorbed species on Cu/ZnO catalysts during the water-gas shift reaction. This formate species was also observed during the hydrogenation of carbon monoxide. Although no methanol was produced in any of these experiments at atmospheric pressure, high-pressure infrared studies have shown that the formate species is a reaction intermediate in methanol ~ynthesis.~'The formate species is a common intermediate in the water-gas shift and methanol synthesis reactions. (24) Churchill, M. R.; DeBoer, B. G.; Rotella, F. J.; Abu Salah, 0. M.; Bruce, M. I. Inorg. Chem. 1975, 14, 2051. (25) Gagnb, R. R.; Allison, J. L.; Gall, R. S.; Koval, C. A. J . Am. Chem. SOC.1977, 99, 7170. (26) Pasquali, M.; Marchetti, F.; Floriani, C. Inorg. Chem. 1978,17, 1684. (27) Pasquali, M.; Marini, G.; Loriani, C.; Gaetani-Manfredotti, A. Guastini, C. Inorg. Chem. 1980, 19, 2525. (28) Pawquali, M.; Marini, G.; Loriani, C.; Gaetani-Manfredotti, A.; Guastini, C. Inorg. Chem. 1980, 19, 2525. (29) Pasquali, M.; Floriani, C.; Gaetani-Manfredotti, A. Inorg. Chem. 1980, 19, 1191. (30) Geerts, R. L.; Huffman, J. C.; Folting, K.; Lemmen, T. H., Caulton, K. G. J. Am. Chem. SOC.1983, 105, 3503. (31) Chow, Y. L.; Buono-Core, G. E. Can. J . Chem. 1983, 61, 795. (32) Pasquali, M.; Fiaschi, P.; Floriani, C.; Zanazzi, P. F. J. Chem. SOC., Chem. Commun. 1983, 613. (33) Visser-Lulrink, G.; Matulewicz, E. R. A,; Hart, J.; Moi, J. C. J . Phys. Chem. 1983, 87, 1470. (34) Duncan, T. M.; Vaughan, R. W. J . Catul. 1981, 67, 469. (35) Herzberg, G. "Molecular Spectra and Molecular Structure. 11. Infrared and Raman Spectra of Polyatomic Molecules"; Van Nostrand-Reinhold: New York, 1945. (36) Busca, G.; Lorenzelli, V. J . Catul. 1980, 66, 155. (37) Edwards, J. F.; Schrader, G. L., submitted for publication to J . Cutal.
J. Phys. Chem. 1984, 88, 5624-5627
5624
The reaction pathway to methanol formation involved the reduction of formate groups to formaldehyde and methoxy species. The hydrogenation of carbon monoxide at 200 OC (Figure 3) led to the- development of these adsorbed species whereas the hydrogenation of carbon dioxide (Figure 6) did not, even when a reducing atmosphere of pure hydrogen was present after the formation of formate groups. Both experiments involved a reduced Cu/ZnO catalyst with adsorbed formate species, with the only major difference being the presence of a copper carbonyl species. At 200 O C the copper carbonyl was an activating agent for the reduction of formate groups to formaldehyde and methoxy species. It does not appear that the copper carbonyl directly activates hydrogen by dissociative chemisorption because the infrared spectra in Figure 3 did not show the development of any new hydroxyl or hydride species. Rather the carbonyl may have an indirect effect, possibly electronic, on the mobility of hydrogen on the catalyst surface. Both the formation of formate groups during carbon monoxide adsorption on an oxidized catalyst (Figure 2) and the much slower reduction of adsorbed species at 100 O C during carbon monoxide hydrogenation (Figure 4) suggest that the mobility of surface hydrogen is an important factor in these surface reactions. However, the presence of copper carbonyl species at 200 O C does not ensure the reduction of formate species. The reaction of carbon monoxide with a small amount of water (Figure 5) occurred over a reduced catalyst having copper carbonyl species without any methoxy groups forming. Water inhibited the reduction of formate species because methoxy groups were
adsorbed on the same zinc sites as the isolated hydroxyls. Conclusions Transmission infrared spectroscopy was utilized to identify adsorbed species on Cu/ZnO catalysts. The oxidized catalysts had residual hydroxyls and carbonates on the surface. Catalyst reduction with hydrogen resulted in the development of bulk hydroxyl species associated with a reduced state of copper; bidentate formate groups were also formed from the surface carbonates. Carbon monoxide was adsorbed on a reduced copper site as a carbonyl species. The formate species was determined to be a common intermediate in the water-gas shift and methanol synthesis reactions. The reaction pathway to methanol formation also involved adsorbed formaldehyde and methoxy species. Three factors were identified that affect the reduction of formate groups to methoxy groups: (1) the presence of an adsorbed copper carbonyl species, (2) the temperature of the reduction, and (3) the presence of gaseous water which inhibited the formation of methoxy groups. Acknowledgment. This work was conducted through the Ames Laboratory, which is operated for the U S . Department of Energy by Iowa State University under contract no. W-7405-Eng-82. This research was supported by the Office of Basic Energy Sciences, Chemical Sciences Division. Registry No. CH,OH, 67-56-1; Cu, 7440-50-8; ZnO, 1314-13-2.
Reactions of Hydrogen Atoms with 4,4'-Bipyridine in Acid Aqueous Solutions S . Solar Institut fur Theoretische Chemie und Strahlenchemie der Universitat Wien and Ludwig Boltzmann Institut fur Strahlenchemie, A-1090 Wien, Austria (Received: April 16, 1984; In Final Form: June 26, 1984)
The reaction of H atoms as well as of propan-2-01 radicals ((CH3)#OH) with 4,4'-bipyridine (4,4'-bpy-H2*+)in deoxygenated acid aqueous solutions (pH 1) has been studied by pulse radiolysis and the spectra and kinetics of the generated radicals have been established. 54% of the H atoms attack the N positions of the substrate molecule, producing R1species with k = (1.9 f 0.2) X los dm3 mol-' 8,A, = 375 nm (e = 2900 f 200 m2 mol-') and 580 nm ( e = 820 & 90 m2 mol-'). These transients deprotonate with k = (3.0 f 0.5) X lo4 s-' to 4,4'-bipyridine cation radicals (R,)with A,, = 375 nm (e = 3750 f 300 m2 mol-') and 580 nm (c = 1570 f 100 m2mol-'). The remaining 46% of the H atoms lead to adducts on ring carbons (R,species; k = (1.6 f 0.2) X lo8 dm3 mol-' s-', ,A, = 450 nm (e = 1000 f 110 m2 mol-')), which decay by second order, 2k = (1.2 f 0.2) X lo9 dm3 mol-' s-'. The (CH3)$OH radicals react with 4,4'-bpy-H?+, with k = (3.2 f 0.2) X lo9 dm3 mol-' s-', under formation of R3 species. For the reaction of eaq-with the nonprotonated form of 4,4'-bipyridine (4,4'-bpy), a rate constant k = (2.5 & 0.5) X 1O'O dm3mol-' s-' could be determined. The products of this reaction are also the long-lived R3 transients; these have no pK in the investigated pH range of 1-7.
Introduction Derivatives of 4,4'-bipyridine have been investigated, particularly in respect to their ability as electron acceptors and electron carriers; see, for example ref 1-4. The reactivity of 4,4'-bipyridine (4,4'-bpy) in aqueous solutions toward e,; ( k = 3.3 X loLodm3 mol-' s-I), H atoms ( k = 2 X lo8 dm3 mol-' s-'), as well as OH radicals ( k = 5.3 X lo9 dm3 mol-' s-I) has been studied by pulse radi~lysis.~ The authors of that paper obtained for the H attack
on the substrate a total transient absorption spectrum with maximum extinction coefficients t380= 1550 m2 mol-', q5,,= 550 m2 mol-', and €580 = 780 m2 mol-'; however, no reaction mechanism was given. The reactivity of H atoms with 1,l'-dimethyl-4,4'-bipyridinium ion (methylviologen, MV2+), however, has been the subject of intense studies and, in contrast to previous findings,6 the existence of two kinds of H adducts, on the N position as well as on ring carbons of MVZf, could be demon~trated.~.~
(1) N. Getoff, K. J. Hartig, G. Kittel, G. A. Peschek, and S . Solar, "Hydrogen as Energy Carrier; Production, Storage and Transport", Springer, Wien, 1977. (2) J. Kiwi, and M. Gratzel, J . Am. Chem. SOC.,101, 72 (1979). (3) I. Willner, J.-M. Yang, J. W. Otvos, and M. Calvin, J. Phys. Chem., 85, 3277 (1981). (4) M. Kirch, J. M. Lehn, and P. Sauvage, Helu. Chim. Acta, 62, 1345
(5) M. Simic and M. Ebert, J . Radiat. Phys. Chem., 3, 259 (1971). (6) M. Venturi, Q.G. Mulazzani, and M. Z. Hoffman, Radiat. Phys. Chem., 23, 229 (1984). (7) S. Solar, W. Solar, N. Getoff, J. Holcman, and K. Sehested, J. Chem. SOC.,Faraday Trans. 1 , 18, 2461 (1982). (8) S.Solar, W. Solar, N. Getoff, J. Holcman, and K. Sehested, J . Chem.
(1979).
SOC.,Faraday Trans. 1 , in press.
0022-3654/84/2088-5624$01.50/0
0 1984 American Chemical Society
