Surface Species in CO and CO2 Hydrogenation over

means of isotope labeling experiments. In contrast, over zirconia-supported systems, it remained difficult to assess whether methanol synthesis starts...
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Langmuir 1996, 12, 5319-5329

5319

Surface Species in CO and CO2 Hydrogenation over Copper/Zirconia: On the Methanol Synthesis Mechanism J. Weigel,† R. A. Koeppel,‡ A. Baiker,‡ and A. Wokaun*,†,‡ Paul Scherrer Institut, CH-5232 Villigen, Switzerland, and Department of Chemical Engineering and Industrial Chemistry, ETH Zentrum, CH-8092 Zu¨ rich, Switzerland Received August 21, 1995. In Final Form: July 25, 1996X Hydrogenation reactions occurring on the surface of a copper/zirconia (Cu:Zr ) 30:70 atom %) catalyst, which had previously been loaded by adsorption of formic acid, CO, and CO2, have been studied by means of in situ diffuse reflectance FTIR spectroscopy. Besides characterizing the reactivity of adsorbates, the dynamics of CO and CO2/hydrogen reactant mixtures has been monitored over both the prereduced and unreduced catalyst, with the aim of acquiring information on the methanol synthesis mechanism. Surface formates, which have previously been identified as intermediates in the catalytic reduction of carbon oxides yielding methane, appear to be spectator species in methanol synthesis over ZrO2 supported catalysts. The latter synthesis, when using CO2 as the reactant, starts from surface carbonate, which is first reduced to yield adsorbed carbon monoxide and water. The desired methanol product is generated via surfacebound formaldehyde and methoxy species.

1. Introduction The hydrogenation of carbon monoxide and carbon dioxide is a process of major industrial importance. The formation of methanol proceeds according to the equilibria

CO + 2H2 h CH3OH

(1)

CO2 + 3H2 h CH3OH + H2O

(2)

It has been pointed out that the reactants are interconverted by the water-gas shift/reverse water-gas shift equilibrium,

CO + H2O h CO2 + H2

(3)

which is also catalyzed by methanol synthesis catalysts. For zinc oxide/alumina-supported copper catalysts (i.e., Cu/ZnO/Al2O3) it was clearly shown that CO2 is the main carbon source of methanol1-3 in the presence of water by means of isotope labeling experiments. In contrast, over zirconia-supported systems, it remained difficult to assess whether methanol synthesis starts from CO or CO2.4 This question is only meaningful in a molecular sense, in so far as the immediate surface precursor(s) of methanol are concerned. In spite of the industrial importance of methanol production and of intense investigations carried out on this subject,5-9 the nature of the active sites, the structural features of the catalyst, and consequently also the reaction mechanism leading to methanol are still the subject of scientific discussion. * Author to whom correspondence should be addressed. † Paul Scherrer Institut. ‡ ETH Zentrum. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Bowker, M.; Houghton, H.; Waugh, K. C. J. Chem. Soc., Faraday Trans. I 1981, 77, 3023. (2) Chinchen, G. C.; Spencer, M. S.; Waugh, K. C.; Whan, D. A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2193. (3) Muhler, M.; To¨rnqvist, E.; Nielsen, L. P.; Clausen, B. S.; Topsøe, H. Catal. Lett. 1994, 25, 1. (4) Jennings, J. R.; Lambert, R. M.; Nix, R. M.; Owen, G.; Parker, D. G. Appl. Catal. 1989, 50, 157. (5) Klier, K. Adv. Catal. 1982, 31, 243. (6) Chinchen, G. C.; Denny, P. J.; Jennings, J. R.; Spencer, M. S.; Waugh, K. C. Appl. Catal. 1988, 36, 1. (7) Shustorovich, E.; Bell, A. T. Surf. Sci. 1991, 253, 386. (8) Ponec, V. Surf. Sci. 1992, 272, 111. (9) Koeppel, R. A.; Baiker, A.; Weigel, J.; Wokaun, A. To be published.

S0743-7463(95)00699-8 CCC: $12.00

In the past few years, new types of catalysts have been developed from intermetallic compounds, Raney copper, and noble metals. Among the various metal oxides which have been tested as support materials for CO/CO2 hydrogenation catalysts, ZrO2 is of special interest because of its mechanical and thermal stability, its high specific surface, and its semiconducting properties. Zirconiasupported methanol synthesis catalysts show a good long term stability.10,11 Between industrially used catalysts (i.e., Cu/ZnO/Al2O3) on the one hand and the Cu/ZrO2 system used in this work on the other hand, very interesting differences exist regarding the precursor question (as mentioned above), regarding the surface species which are formed on the catalyst during CO and/ or CO2 hydrogenation reactions, and in consequence regarding the mechanism leading to the desired methanol product. Since most of the investigations presented in this work are carried out on a Cu/ZrO2-supported catalyst, we will now focus on the literature published for zirconiasupported methanol synthesis catalysts. An issue which has been debated intensively in the literature is the question whether the route to methanol proceeds via formate or other intermediates. Denise et al.12 conclude that CO2 is reduced to methanol and surface oxygen in a first reduction step, followed by dehydrogenation of CH3OH, yielding CO. The reactions of CO2 and H2 over a CuO/ZrO2 catalyst were investigated by Amenomiya,13 who found that methanol was generated directly from CO2 and that the reverse water-gas shift reaction was taking place in parallel. In contrast to Amenomiya, Sun and Sermon14 postulate a formate to methoxy mechanism over pure ZrO2 and a Cu/ZrO2 catalyst when starting from CO as reactant. Their results are in agreement with those of Abe et al.,15 who also found experimental evidence for a mechanism proceeding via surface formate and methoxy species. Chinchen et al.6 pointed out that carbon dioxide could be hydrogenated to methanol not only on copperbased catalysts but also on supported palladium, with surface formate as the first species formed. From this (10) Baiker, A.; Gasser, D. Appl. Catal. 1989, 48, 279. (11) Koeppel, R. A.; Baiker, A.; Schild, C.; Wokaun, A. Stud. Surf. Sci., Catal. 1991, 63, 59. (12) Denise, B.; Sneeden, R. P. A. Appl. Catal. 1986, 28, 235. (13) Amenomiya, Y. Appl. Catal. 1987, 30, 57. (14) Sun, Y.; Sermon, P. A. J. Chem. Soc., Chem. Commun. 1993, 1242. (15) Abe, H.; Maruya, K.; Domen, K.; Onishi, T. Chem. Lett. 1984, 1875.

© 1996 American Chemical Society

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fact, the authors conclude that formate may be a common intermediate. On the basis of various studies of CO and CO2 hydrogenation, as well as the reactions of synthesis gas over pure zirconia and zirconia-supported catalysts, Ekerdt et al.16-20 proposed a low-temperature mechanism in which formate was hydrogenated to a dioxymethylene species, which could either react further with hydrogen, producing methane, or be desorbed as methanol in the presence of water. Schild et al.,21,22 Koeppel et al.,23 and Wokaun et al.24,25 investigated CO and CO2 hydrogenation over several zirconia-supported catalysts. Methanol is the main product of CO and CO2 hydrogenation reactions over Cu/ZrO2, but it is also produced as a byproduct over Au/ZrO2 and Pd/ZrO226 catalysts. They found that πbonded formaldehyde, which is formed in the catalytic reaction of carbon monoxide and hydrogen, is the key intermediate. Subsequent reduction yields surface-bound methylate and methanol. In this paper, the surface reactions of formic acid, CO, and CO2 over a prereduced copper/zirconia (Cu:Zr ) 30: 70 atom %) catalyst and the subsequent hydrogenation reactions of the species formed during the adsorption experiments are reported. From our current investigations on Cu/ZnO/Al2O3 catalysts as well as on Cu/ZrO2 catalysts, we find several very interesting and surprising differences between the two systems. To highlight these differences, some results obtained over Cu/ZnO/Al2O3 will be included. In addition, the CO and CO2 hydrogenation reactions (from the appropriate reactant mixtures) over the prereduced and the unreduced copper/zirconia catalyst are investigated. The observed correlations of surface species and gaseous reaction products are discussed with regard to the water-gas shift/reverse water-gas shift reaction and the methanol synthesis mechanism. 2. Experimental Section Catalysts. The copper/zirconia catalyst under investigation was prepared by coprecipitation of the corresponding metal nitrates at constant pH ≈ 7 and at constant temperature (363368 K), as described in detail elsewhere,27 and had a compositon of Cu:Zr ) 30:70 atom %. Catalyst Characterization. The surface areas (SBET) were determined by nitrogen physisorption at 77 K using a Micromeritics ASAP 2000 instrument. The BET surface areas were estimated to be 353 m2 g-1 for the Cu/ZrO2 sample and to be 100 m2 g-1 for the Cu/ZnO/Al2O3 catalysts. For the binary sample the metallic Cu surface was estimated as 1.6 m2 g-1 by means of N2O titration. FTIR Spectroscopy. The spectra were recorded with an FTIR spectrometer (Bruker, IFS 55) using a nitrogen-cooled MCT detector. The instrument is equipped with a diffuse reflectance unit housing a controlled environmental chamber fitted with ZnSe windows (both Spectra-Tech). The experiments as well as the activation procedure (see below) were carried out in situ; i.e., (16) He, M.-Y.; Ekerdt, J. G. J. Catal. 1982, 90, 17. (17) Jackson, S. D.; Brandreth, B. J. J. Chem. Soc., Faraday Trans. 1 1989, 85 (10), 3579. (18) He, M.-Y.; Ekerdt, J. G. J. Catal. 1984, 87, 238. (19) He, M.-Y.; Ekerdt, J. G. J. Catal. 1982, 87, 381. (20) He, M.-Y.; White, J. M.; Ekerdt, J. G. J. Mol. Catal. 1985, 30, 415. (21) Schild, C.; Wokaun, A.; Baiker, A. J. Mol. Catal. 1990, 63, 223. (22) Schild, C.; Wokaun, A.; Baiker, A. Fres. J. Anal. Chem. 1991, 341, 395. (23) Koeppel, R. A.; Baiker, A.; Schild, C.; Wokaun, A. J. Chem. Soc., Faraday Trans. 1991, 87 (17), 2821. (24) Wokaun, A.; Weigel, J.; Kilo, M.; Baiker, A. Fres. J. Anal. Chem. 1994, 349, 71. (25) Weigel, J.; Fro¨hlich, C.; Baiker, A.; Wokaun, A. Appl. Catal., in press. (26) Baiker, A.; Gasser, D. J. Chem. Soc., Faraday Trans. I 1989, 85 (4), 999. (27) Fro¨hlich, C.; Koeppel, R. A.; Baiker, A.; Kilo, M.; Wokaun, A. Appl. Catal. A: General 1993, 106, 275.

Weigel et al. activation and dosing of the sample were performed directly within the environmental chamber which remained mounted in the spectrometer. The free volume of the reaction chamber is about 1.75 cm3, and the temperature in the chamber can be regulated within (1 K with a homemade temperature controller. A gas dosing system was designed to introduce up to four gases independently into the reaction chamber. The gas flows were adjusted to the desired values using mass flow controllers (Brooks). Liquid reagents were introduced in the nitrogen stream through a sample loop. Nitrogen as the carrier gas as well as carbon dioxide, carbon monoxide, and hydrogen as reactants were used without further purification. All gases were commercially available with a purity greater than 99.99%. High-purity formic acid (Merck) served as the reference compound. The diffuse reflectance spectrum of a powdered sample is greatly influenced by sample packing,28-30 since the pressure applied to the sample affects the scattering coefficient. Therefore, a home-built sample packing accessory (for details see ref 30) was used to place the catalyst into the sample cup of the environmental chamber under a constant pressure of 1 M Pa. Prior to the measurements, the catalyst was activated by heating in a nitrogen/hydrogen mixture (for experiments on reduced catalysts, N2:H2 ) 1:2) and in a nitrogen atmosphere (for the unreduced samples, the N2-flow was 10 L h-1), respectively. The temperature was raised up to 523 K with a heating rate of 2 K min-1. A spectrum of the unloaded catalyst held at 523 K was recorded as a background (1000 scans). A defined flow of the gaseous reactants, as specified in the figure captions, was then passed over the sample at a total pressure of 6 × 105 Pa. (As an exception, the adsorption of formic acid was performed at atmospheric pressure.) In all experiments the gas flow was adjusted to 10 L h-1. Subsequent to the introduction of reactants defining the time origin, the surface reactions were studied by recording DRIFT spectra as a function of reaction time, at a resolution of 4 cm-1. Depending on the system and the interval in which the time dependent changes had to be resolved, 100-500 scans were accumulated for each spectrum. Gaseous products were removed by the continuous stream of reactants passing over the sample (henceforth referred to as the ‘dynamic experiment’). In this way, the strong overlap of the signals of gaseous species with the adsorbate vibrations was reduced or avoided. When the time dependent changes following the introduction of reactants had ceased and a steady state had been reached, the reaction chamber was closed, and the further development of products was monitored. The latter situation will be referred to as the ‘static experiment’. Usually, diffuse reflectance FTIR (DRIFT) spectra are presented in Kubelka-Munk units. The choice of a reflectance spectrum (R0) of the unloaded catalyst as a reference is somewhat problematic, as the catalysts exhibit a grayish-black color, and absorb strongly across the mid-IR region. This violates one of the basic assumptions (nonabsorbing or weakly absorbing substrate) used in the derivation of the Kubelka-Munk equation.31,32 As a consequence, the Kubelka-Munk function is not adequate for a case in which the reflectance of the loaded catalyst is higher than the one of the reference background (e.g., as a consequence of desorption or reactive consumption of surface species). Under these conditions, spectra presented in the reflectance (R/Ro) mode (where R is the reflectance spectrum of the sample) are better suited for interpretation. Therefore the ‘reflectance mode’ will be used to display the spectra throughout this work; the direction of the ordinate axis is chosen such that adsorbed or appearing species give rise to positive signals. Whenever CO2 was used as reactant, the signal of gaseous CO2 at 2349 cm-1 was cut off in the resulting DRIFT spectra, because otherwise this band would dominate the presented spectrum. In (28) Yeboah, S. A.; Wang, S.-H.; Griffiths, P. R. Appl. Spectrosc. 1984, 38 (2), 259. (29) TeVrucht, M. L. E.; Griffiths, P. R. Appl. Spectrosc. 1989, 43 (8), 1492. (30) Kriva´csy, Z.; Hlavay, J. Spectrochim. Acta 1994, 50A (1), 49. (31) Kubelka, P.; Munk, F. Z. Tech. Phys. 1931, 11a, 593. (32) Kubelka, P. J. Opt. Soc. Am. 1948, 38 (5), 448.

CO and CO2 Hydrogenation over Copper/Zirconia

Figure 1. (a) Adsorption of formic acid over a copper/zirconia (Cu:Zr ) 30:70 atom %) catalyst at 523 K and a pressure of 1 × 105 Pa. Formic acid (0.2 mL) was injected into a flow of N2. Subsequently the system was purged with nitrogen. Details are given in the Experimental Section. Key peaks are labeled as follows: (+) free formic acid; (4) surface formates; (2) surface carbonates; (O) adsorbed CO; (b) surface hydroxyl groups. (b) Adsorption of formic acid over a Cu/ZnO/Al2O3 catalyst at 523 K and a pressure of 1 × 105 Pa. Formic acid (0.1 mL) was injected into a flow of N2. Subsequently the system was purged with nitrogen. Key peaks are labeled as follows: (+) free formic acid; (4) surface formate on Cu; (0) surface formate on ZnO; (2) surface carbonates; (O) adsorbed CO. all presented spectra, reaction times are indicated on the right side of the figures in minutes after the reactant flow was switched on.

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ambient pressure. Free formic acid (bands at 2940 cm-1 and around 1700 and 1200 cm-1) decomposes into gaseous CO2 (2349 cm-1) and singly surface-bound CO (2110 cm-1, lower traces). Bands at 1550 and 1300 cm-1 are characteristic for bidentate surface carbonate on zirconia,33 which either is formed by the interaction of gaseous CO2 with lattice oxygen or originates from the surface oxidation of free formic acid. The intensity of the bands due to bidentate surface carbonate remains constant during the first 30 min, suggesting that the surface sites onto which these species are bound have been saturated upon adsorption. Reactive consumption of surface hydroxyl groups results in two negative bands at 3760 cm-1 (isolated OH groups) and at 3670 cm-1 (bridged hydroxyl groups)34 (first and second traces in Figure 1a). Continued exposure to the flow of pure N2 causes the bands of free formic acid to vanish (third trace in Figure 1a). This is paralleled by the disappearance of the gaseous CO2 and the partial regeneration of the surface hydroxyl groups; note that the negative bands at 3760 and 3670 cm-1 have become smaller. Accurate determination of gas-phase CO2 is not possible due to changes in the flowing atmosphere during the time of the experiment, which is the cause of the negative CO2 peak in the third trace in Figure 1a. After 60 min of flowing N2, all molecular formic acid as well as its gaseous decomposition products has been purged. Prominent bands remaining in the spectra at 2970, 2870, 2750, 1590, 1560, 1390, and 1370 cm-1 are due to surface formates. Note that the broadening of the composite peaks between 1600 and 1200 cm-1 is caused by the presence of surface carbonates. He and Ekerdt19 observed a band at 2855 cm-1 for the C-H stretching vibration of formate and peaks at 1580 and 1360 cm-1 for the asymmetric and symmetric stretch, respectively, of the carboxylate group on zirconia. Especially for higher surface concentrations, combination modes are observed, as reported in the literature35,36 for the combination of νa(COO-) and δ(CH) at 2950 cm-1 and for the combination of νs(COO-) and δ(CH) at 2740 cm-1. The presence of at least two types of surface-bound formate species is recognized by the characteristic positions of the carboxylate stretching bands:23 An inner doublet at 1560/1390 cm-1 (denoted as formate I in ref 23) is associated with formate probably bound to reactive sites, and an outer doublet 1590/1370 cm-1 (denoted as formate II in ref 23) corresponds to formate bound to less reactive sites. Two types of formates were also observed after formic acid adsorption on Zr(100).37 Peaks located at 2938, 2857, and 1359 cm-1, that would be characteristic for bridged formate species on copper,38 were never observed under the experimental conditions applied in this work. Therefore it is most likely that the formate species observed in the adsorption experiment and in the CO and CO2 hydrogenation reactions (see below) over zirconia-supported copper catalysts are only located on the support. This result might appear unexpected in view of the fact that formates on copper are easily observable over Cu/ ZnO/Al2O3 catalysts.38-40 In order to test the experimental

3. Results Adsorption of Formic Acid. Surface formates are discussed with regard to their importance as intermediates in carbon monoxide and carbon dioxide hydrogenation reactions.6 In this context, the adsorption behavior and further reactions of formic acid on the catalysts are of obvious importance. Infrared spectra have been obtained (Figure 1a) at different time intervals after 0.2 mL of formic acid had been injected into a flow of N2 passing over the reduced copper/zirconia catalyst at 523 K and

(33) Kondo, J.; Abe, H.; Sakata, Y.; Maruya, K.; Onishi, T. J. Chem. Soc., Faraday Trans. 1 1988, 84 (2), 511. (34) Kondo, J.; Sakata, Y.; Domen, K.; Maruya, K.; Onishi, T. J. Chem. Soc., Faraday Trans. 1990, 86 (2), 397. (35) Hayden, B. E.; Prince, K.; Woodruff, D. P.; Bradshaw, A. M. Phys. Rev. Lett. 1983, 51 (6), 475. (36) Edwards, J. F.; Schrader, G. L. J. Phys. Chem. 1985, 89, 782. (37) Dilara, P. A.; Vohs, J. M. J. Phys. Chem. 1993, 97, 12919. (38) Millar, G. J.; Rochester, C. H.; Waugh, K. C. J. Chem. Soc., Faraday Trans. 1992, 88 (7), 1033. (39) Neophytides, S. G.; Marchi, A. J.; Froment, G. F. Appl. Cat. A: General 1992, 86, 45.

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Figure 2. Hydrogenation of copper/zirconia catalyst with preadsorbed surface formates. A H2/N2 (2:1) mixture was passed over the surface held at 523 K and a pressure of 6 × 105 Pa. Key peaks are labeled as follows: (0) surface-bound methylate; (9) surface-bound formaldehyde; (4) surface formates; (O) adsorbed CO; (b) surface hydroxyl groups.

setup used for this investigation and the sensitivity of the FTIR spectrometer, the same adsorption experiment was repeated with a Cu/ZnO/Al2O3 sample under identical experimental conditions. A representative spectrum obtained 1 min after 0.1 mL of formic acid was injected in a flow of N2 is shown in Figure 1b. Major infrared peaks (Note that the poor signal to noise ratio in the DRIFT spectra of Figure 1b is not caused by changes of the experimental setup. It is due to inhomogeneities of the sample surface under investigation.) are located at 2940, 2850, 1600, and 1360 cm-1 (formate on copper), with smaller peaks at 1310 cm-1 (bidentate carbonate on copper) and 1380 cm-1 (unidentate carbonate on copper).38-40 The signal around 1600 cm-1 is split into a doublet at 1613 and 1600 cm-1. The vibration at 1613 cm-1 together with a shoulder at 1580 cm-1 is due to formate on ZnO.41 This demonstrates clearly that the experimental setup used for this investigation is well situated to observe formates on copper. Therefore it is most likely that either formates on copper are not formed on zirconia-supported catalysts, or else their surface concentration is far below the detection limit. Note that there is no spectroscopic evidence for the presence of any other hydrogen-containing species. Hydrogenation of Surface-Bound Formate. Subsequent to the preceding adsorption experiment, a mixture of N2 and H2 (1:2) was passed continuously over the catalyst with the preadsorbed formates. Immediately after the flow was switched on, two characteristic bands were observed to grow at frequencies of 2820 and 1150 cm-1 (Figure 2). Supported by several adsorption experiments21,42 and in agreement with the literature,43 these (40) Bailey, S.; Froment, G. F.; Snoek, J. W.; Waugh, K. C. Catal. Lett. 1995, 30, 99. (41) Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985, 155, 366. (42) Schild, C.; Wokaun, A.; Koeppel, R. A.; Baiker, A. J. Phys. Chem. 1991, 95, 6341.

Weigel et al.

frequencies had been attributed to a π-bound formaldehyde species. Other authors44 have proposed a structure similar to the one of paraformaldehyde for this species, in which the formaldehyde units are bound to each other in an end-to-end fashion rather than to the surface. The rapid formation of surface-bound formaldehyde is accompanied by the observation that the band at 2110 cm-1 due to singly surface-bound CO becomes more intense. A few minutes later, a band at 1050 cm-1 is observed to develop (second trace in Figure 2) and is associated with a doublet at 2950/2830 cm-1. This signature is highly characteristic45 for surface-bound methylate (CH3O-). The absorptions of π-bound formaldehyde and methylate grow strongly as a function of exposure time to the N2/H2 flow, whereas the bands due to carbonates are decreasing continuously, which results in a narrowing of the peaks between 1700 and 1200 cm-1 (Figure 2). When hydrogen is present, surface carbonates can potentially undergo three reactions: i.e. (1) reduction to yield adsorbed CO and H2O; (2) generation of surface formates and H2O; and (3) formation of surface-bound formaldehyde and H2O. As the intensity of the peak at 2110 cm-1 decreases continuously (Figure 2), reaction 1, yielding CO, is consistent with observation only if further reduction of adsorbed CO proceeds faster than the reduction of the surface carbonates. The intensities due to surface formates remain constant during carbonate decomposition; as a consequence, reaction 2 can be ruled out. Adsorption of Carbon Monoxide. The surface of the reduced Cu30Zr70 catalyst held at 523 K was observed as a function of time while a mixture of CO and N2 (1:2) was continuously passed over the sample at a pressure of 6 × 105 Pa (Figure 3). Surface carbonates (broad bands between 1700 and 1200 cm-1), formates (2870, 1590, 1560, 1390, and 1370 cm-1), surface-bound formaldehyde (2830 and 1150 cm-1), gaseous CO2 (2349 cm-1), and water (multiplet structure around 1600 cm-1) are observed immediately (Figure 3). It is remarkable that some formaldehyde is generated on the catalyst surface although hydrogen was not contained in the reactant mixture. Therefore the reducing agents must originate from the previous reduction of the catalyst in a continuous H2/N2 flow. For example, the oxidative decomposition of surface hydroxyl groups would yield hydrogen and surface oxygen.34,46 After 15 min of exposure, the broad absorption around 1060 cm-1 due to surface carbonate becomes sharper. Together with the observation of two absorptions at 2950 cm-1 (dashed line in Figure 3) and at 2830 cm-1, the development of a narrow signal at 1060 cm-1 indicates that surface-bound methylate is also present on the catalyst surface. Hydrogenation of the Species Formed after CO Adsorption. Subsequent to the adsorption experiment the CO/N2 mixture was replaced by an N2/H2 (1:2) mixture, which was passed continuously over the catalyst bed. The development of the surface species as a function of reaction time is shown in Figure 4. In a qualitative sense, the same features are observed as reported above for the hydrogenation of adsorption products from the formic acid experiment. However, some differences are noted: After 45 min, all carbonates and most of the singly surface(43) Anton, A. B.; Parmeter, J. E.; Weinberg, W. H. J. Am. Chem. Soc. 1985, 107, 5558. (44) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Surf. Sci. 1982, 119, 279. (45) Chen, J. G.; Basu, P.; Ng, L.; Yates, J. T. Surf. Sci. 1988, 194, 397. (46) Yamaguchi, T.; Nakano, Y.; Tanabe, K. Bull. Chem. Soc. Jpn. 1978, 51 (9), 2482.

CO and CO2 Hydrogenation over Copper/Zirconia

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Figure 3. Adsorption of CO over prereduced copper/zirconia catalyst. A CO/N2 (1:2) mixture was passed continuously over the sample at a reaction temperature of 523 K and a pressure of 6 × 105 Pa. Key peaks are labeled as follows: (0) surfacebound methylate; (9) surface-bound formaldehyde; (4) surface formates; (2) surface carbonates; (O) adsorbed CO; (b) surface hydroxyl groups.

Figure 4. Hydrogenation of copper/zirconia catalyst which was pretreated with CO. A H2/N2 (2:1) mixture was passed over the surface held at 523 K and a pressure of 6 × 105 Pa. Key peaks are labeled as follows: (0) surface-bound methylate; (9) surface-bound formaldehyde; (4) surface formates; (2) surface carbonates; (O) adsorbed CO; (b) surface hydroxyl groups.

bound CO have disappeared, the concentrations of the formate show a slow decrease, and comparatively more water is present in the reaction chamber. Carbon Monoxide Hydrogenation Reactions. When a stoichiometric mixture of CO and H2 is passed continuously over the reduced surface of the copper/

Figure 5. CO hydrogenation over prereduced copper/zirconia catalyst at different time intervals after the reactant flow was switched on. A CO/H2 (1:2) mixture was passed continuously over the sample at a reaction temperature of 523 K and a pressure of 6 × 105 Pa. Key peaks are labeled as follows: (0) surface-bound methylate; (9) surface-bound formaldehyde; (4) surface formates; (2) surface carbonates; (O) adsorbed CO; (b) surface hydroxyl groups.

zirconia catalyst at 523 K and a pressure of 6 × 105 Pa, peaks due to surface carbonates (absorptions between 1700 and 1200 cm-1) and surface-bound formates are immediately observed (upper trace in Figure 5a). CO2 (2349 cm-1) as a product of the water-gas shift reaction is also formed; its intensity passes through a maximum after 3 min of reaction time (Figure 5a and b). The formation of surface-bound formates and/or surface carbonates is accompanied by the reactive consumption of surface hydroxyl groups (negative bands at 3780 and 3680 cm-1

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Figure 6. Adsorption of CO2 over prereduced copper/zirconia catalyst. A CO2/N2 (1:3) mixture was passed continuously over the sample at a reaction temperature of 523 K and a pressure of 6 × 105 Pa. Key peaks are labeled as follows: (+) CO2δ-; (4) surface formates; (2) surface carbonates.

in Figure 5a and b). The intensities of these negative bands become constant after 30 min of reaction time. Three minutes after the reactant flow had been turned on, two bands located at 2830 and 1150 cm-1 assigned to π-bonded formaldehyde start to develop, as accompanied by the methylate absorptions at 2950, 2830, and 1050 cm-1 (second and third trace in Figure 5a). Both species show intensities strongly growing with reaction time. The parallel decrease of surface carbonates may be deduced from the loss of the band at 1270 cm-1, which is characteristic for bidentate surface carbonate.23,33,47 Note that the spectra for a reaction time of 10 and more minutes show a small absorption at 1445 cm-1 (Figure 5b) which is assigned to the δ(CH) mode of the surface-bound methylate.48 In a static atmosphere traces of gaseous methane are detectable in addition (spectra not shown). Adsorption of Carbon Dioxide. Exposure of the reduced catalyst to a flowing CO2/N2 (1:3) mixture results in the rapid formation of surface carbonates and traces of surface formate, deduced from the weak C-H stretch at 2880 cm-1 (Figure 6). The intensities of the carbonate signals grow as a function of reaction time, whereas the C-H absorption of the formate species becomes weaker. A further band is observed to grow at a frequency of 1220 cm-1. Various types of carbonate species on ZrO2 have been characterized by Morterra and Orio,49 with their nature depending on the dehydration and sintering state of the support. In particular, a hydrogen carbonate species is reported with frequencies at 1620, 1450, and 1225 cm-1. The authors conclude that this hydrogen carbonate species is formed on coordinatively unsaturated (cus) cationic centers, which are created by the elimination of coordinated water. (In previous work,50 this undissociated ‘adsorbed’ water was characterized by a ν(OH) vibration (47) Schild, C.; Wokaun, A.; Baiker, A. J. Mol. Catal. 1990, 63, 243. (48) Herzberg, G. Molecular Spectra and Molecular Structure, 1st ed.; Van Nostrand: New York, 1945; Vols. I and II. (49) Morterra, C.; Orio, L. Mater. Chem. Phys. 1990, 24, 247. (50) Morterra, C. J. Chem. Soc., Faraday Trans. 1 1988, 84, 1617.

Weigel et al.

Figure 7. Hydrogenation of copper/zirconia catalyst which was pretreated with CO2. A H2/N2 (3:1) mixture was passed over the surface held at 523 K and a pressure of 6 × 105 Pa. Key peaks are labeled as follows: (0) surface-bound methylate; (9) surface-bound formaldehyde; (4) surface formates; (2) surface carbonates; (b) surface hydroxyl groups.

at 3615 cm-1. A band at this frequency was not observable under the present experimental conditions.) In an HREEL study of adsorbed CO2 on a Na precovered Pd(111) surface, Wohlrab et al.51 found evidence for a CO2δ(carbonite) species which showed a characteristic loss at 1210 cm-1 for the symmetric stretch. The authors propose that the asymmetric stretch would correspond to a frequency of 1530 cm-1. Unfortunately, several other vibrations are located in the frequency region around 1530 cm-1. In an effort to differentiate the CO2δ- asymmetric stretch from other carbonate and formate absorptions, the spectra were deconvoluted in the relevant range. As a representative example, the result of a careful deconvolution of the spectrum taken after 45 min is shown as the bottom trace of Figure 6 (dotted part). In particular, a shoulder at 1538 cm-1 (marked by the dashed line in Figure 6) appears, which is close to the second carbonite frequency mentioned above. In addition, least squares fitting of the bottom spectrum in Figure 6 provided also evidence for a shoulder at 1538 cm-1. It is noteworthy that the species reported by Wohlrab and co-workers51 was not stable at higher temperatures. This might be caused by the small surface concentrations of the CO2δ- species reached under UHV conditions. Starting from carbon monoxide as the adsorbent, Binet et al.52 have detected a carbonite anionic species on ceria even at elevated temperatures, with a structure similar to the one mentioned above. Furthermore, they remark that formate and the carbonite species may be associated with the same surface sites and that formate would be produced when active hydrogen was stored on the support. An analogous result is reported from Wambach et al.,53 (51) Wohlrab, S.; Ehrlich, D.; Wambach, J.; Kuhlenbeck, H.; Freund, H.-J. Surf. Sci. 1989, 220, 243. (52) Binet, C.; Badri, A.; Boutonnet-Kizling, M.; Lavalley, J.-C. J. Chem. Soc., Faraday Trans. 1994, 90 (7), 1023. (53) Wambach, J.; Illing, G.; Freund, H.-J. Chem. Phys. Lett. 1991, 184 (1-3), 239.

CO and CO2 Hydrogenation over Copper/Zirconia

Langmuir, Vol. 12, No. 22, 1996 5325

Figure 8. CO2 hydrogenation over prereduced copper/zirconia catalyst. A CO2/H2 (1:3) mixture was passed continuously over the sample at a reaction temperature of 523 K and a pressure of 6 × 105 Pa. Key peaks are labeled as follows: (0) surfacebound methylate; (9) surface-bound formaldehyde; (+) gaseous methanol; (4) surface formates; (2) surface carbonates; (O) adsorbed CO; (b) surface hydroxyl groups.

who monitored the reaction of CO2δ- with adsorbed hydrogen on Ni(110). Note that the intensity of the band at 1220 cm-1 in Figure 6 increases in parallel with a decrease of the concentration of the formate species. This demonstrates that both species may adsorb on the same surface sites. Hydrogenation of the Species Formed after Carbon Dioxide Adsorption. Spectra taken at different times after the CO2/N2 adsorption mixture had been replaced by a N2/H2 (1:3) flow are presented in Figure 7. The rapid formation of surface-bound formaldehyde (2820 and 1150 cm-1, top trace) is followed by the appearance of bands at 2950, 2820, and 1050 cm-1 due to surfacebound methylate (upper and second trace in Figure 7). Both species show absorptions with intensities that strongly increase with reaction time. The absorptions between 1700 and 1200 cm-1 as well as the broad band around 1060 cm-1 become sharper as a result of disappearance of surface carbonates, whereas the surface concentrations of the formates remain constant. The multiplet of gas-phase absorptions around 1600 cm-1, which is superimposed onto the broad adsorbate bands, shows that water is also present in the reaction cell. The negative intensities between 3800 and 3600 cm-1 are due to a loss of surface hydroxyl groups. (The consumption of these species had occurred during carbon dioxide adsorption but could not be detected in the presence of CO2 due to overlap with a strong carbon dioxide absorption.) Carbon Dioxide Hydrogenation Reactions. Infrared spectra were obtained at different times after a continuous flow of a stoichiometric CO2/H2 mixture (1:3) was directed over the reduced catalyst surface (Figure 8). The formation of surface-bound carbonates and formates (broad bands between 1700 and 1200 cm-1 and at 2950, 2855, and 2740 cm-1 in the first trace of Figure 8) as well as gaseous CO (2144 cm-1) as a product of the reverse

Figure 9. CO hydrogenation over unreduced copper/zirconia catalyst. A CO/H2 (1:2) mixture was passed continuously over the sample at a reaction temperature of 523 K and a pressure of 6 × 105 Pa. Key peaks are labeled as follows: (0), surfacebound methylate; (9), surface-bound formaldehyde; (2) surface carbonates; (b) surface hydroxyl groups.

water-gas shift reaction is immediately observed. Three minutes after establishment of the reactant flow, π-bound formaldehyde and surface-bound methylate are observed (2930, 2830, 1150, and 1050 cm-1) with intensities strongly increasing as a function of reaction time. The surface concentration of the formate species remains constant in the same time interval, whereas the intensities due to surface carbonates decrease. The weak multiplet around 1600 cm-1 indicates that water is also present in the

5326 Langmuir, Vol. 12, No. 22, 1996

reaction chamber and becomes prominent 8 min after the flow was switched on (spectra not shown). Carbon Monoxide and Carbon Dioxide Hydrogenation Reactions over the Unreduced Catalyst. All spectra presented above, except for those recorded in the presence of CO2 in the reaction chamber, have provided evidence for a displacement of surface hydroxyl groups, resulting in two peaks with negative intensities at 3760 and 3670 cm-1. In order to obtain more insight into the role of these species in the methanol synthesis mechanism, hydrogenation experiments over an unreduced sample were performed (see Experimental Section). The surface of the unreduced catalyst held at 523 K was observed as a function of time while a stoichiometric mixture of CO and H2 was continuously passed over the sample. The background, which was recorded as a single-beam spectrum, exhibits no evidence for the presence of surface formates prior to the reaction (spectrum not shown). The product of the water-gas shift reaction, gaseous CO2 (2349 cm-1), is readily formed at the beginning, after which its concentration decreases with reaction time (Figure 9a and b). At 3760 and 3670 cm-1 two bands with negative intensities are again observed. This indicates the consumption of small concentrations of surface hydroxyl groups which had been generated on the catalyst surface during the previous storage of the sample in air. Surface carbonates, which were also adsorbed during atmospheric storage, are displaced as well, resulting in negative intensities between 1700 and 1200 cm-1. Note that the loss in intensity due to surface hydroxyl groups levels off after 5 min, suggesting that all hydroxyl groups located on the support have desorbed or have been consumed, respectively (second trace in Figure 9a). After 15 min on stream (last trace in Figure 9a) some π-bound formaldehyde is formed, as observed by a weak absorption at 2830 cm-1 (inset in Figure 9a) and a band at 1150 cm-1. Several minutes later the formation of surface methylate (2930, 2820, and 1050 cm-1) is observed (first trace in Figure 9b). Its generation is accompanied by the appearance of the water multiplet around 1600 cm-1. Both species show intensities strongly growing with reaction time, whereas the negative signals due to the loss of surface carbonates become more negative. No absorptions that would be indicative of surface formate, i.e. bands at 2960, 2870, and 2750 cm-1, are observed. The results of a complementary experiment in which the unreduced copper/zirconia catalyst was exposed to a dynamic mixture of CO2 and H2 (1:3) (at 523 K and a pressure of 6 × 105 Pa) are presented in Figure 10. The formation of singly surface-bound CO (2110 cm-1) is immediately observed, together with traces of surface carbonates. The broad, intense absorption around 3400 cm-1 and a sharp band at 1600 cm-1 are due to chemisorbed or physisorbed water, which disappeared after 15 min (upper trace in Figure 10 a). After 5 min on stream, a band at 1150 cm-1 starts to grow, which is characteristic for surface-bound formaldehyde. The accompanying C-H stretch at 2820 cm-1 is concealed by the strong water absorption (second trace in Figure 10a). Continued exposure to the CO2/H2 flow leads to the development of two bands at 2930 and 1050 cm-1 which are characteristic of surface-bound methylate (first trace in Figure 10b). Both species show intensities strongly growing with reaction time. Their formation is accompanied by a loss of surface carbonates, as recognized from a broadening of the negative intensities between 1700 and 1200 cm-1. Remarkably, there is no evidence for a formate species at the catalyst’s surface under the present reaction conditions.

Weigel et al.

Figure 10. CO2 hydrogenation over unreduced copper/zirconia catalyst. A CO2/H2 (1:3) mixture was passed continuously over the sample at a reaction temperature of 523 K and a pressure of 6 × 105 Pa. Key peaks are labeled as follows: (0) surfacebound methylate; (9) surface-bound formaldehyde; (2) surface carbonates; (+) chemisorbed or physisorbed water; (O) adsorbed CO.

4. Discussion Summary of Essential Observations. During the adsorption and hydrogenation experiments on a copper/ zirconia catalyst, several reaction intermediates have been observed, depending on reactants, reaction conditions, and the state of the catalyst: (i) Adsorption of formic acid results in the formation of surface formates and carbonates accompanied by a disap-

CO and CO2 Hydrogenation over Copper/Zirconia

pearance of surface hydroxyl groups located at the support, suggesting that one or both species are adsorbed on the hydroxyl sites. There is no spectroscopic evidence for a formate species on copper under our experimental conditions. (ii) The surface reactions over reduced copper/zirconia with carbon monoxide lead to the formation of surface formates and carbonates. (In the latter reaction, CO is oxidized, with a concomitant further reduction of the catalyst.) Some surface-bound formaldehyde is produced even in the absence of hydrogen. Surface hydroxyl groups on reduced sites must thus be involved in the reactions of CO leading to π-bound formaldehyde, since there was no other source of hydrogen in the system. (iii) In contrast to the carbon monoxide experiment, no formaldehyde could be observed when only CO2 (without hydrogen) was passed over the prereduced surface of the catalyst, although surface formate generation was again accompanied by a displacement of surface hydroxyl groups. This different behavior may be understood as, upon CO2 adsorption, no reduction equivalents are available from the reactant. The precursor of surface formate might be a CO2δ- species, as observed by Binet et al.52 and Wambach and co-workers.53 (iv) Surface reactions in the presence of hydrogen are similar, regardless of whether one starts from preadsorbed formic acid, CO2, CO, or CO2/H2 or CO/H2 mixtures. There are, however, differences with respect to the relative surface concentrations of the observed species. Common features include the following (a) The broad absorption bands between 1700 and 1200 cm-1 become narrower as the reaction advances, suggesting that the surface carbonates have been consumed. In parallel to the disappearance of the carbonates, surfacebound formaldehyde is generated. Its formation is accompanied by increasingly prominent water absorptions. (b) Production of surface-bound methylate sets in shortly after formaldehyde absorptions have appeared in the spectra. (c) The intensity of bands due to surface formates increases during the first few minutes of reaction but levels off thereafter, suggesting that the surface sites at which formates are adsorbed have become saturated. Subsequently their surface concentrations remain constant, as judged from the constant intensityof the C-H stretch at 2880 cm-1. (v) During carbon monoxide and carbon dioxide hydrogenation over the unreduced copper/zirconia catalyst, only a small loss of surface hydroxyl groups is observed, which becomes constant after a few minutes. This suggests that OH groups, which had been created as the sample was stored in air, have not been removed by the mild heating in an inert gas atmosphere, as described in the Experimental Section. However, the displacement of these hydroxyl groups is completed before other surface species relevant to methanol formation are created. (vi) In the hydrogenation of CO or CO2 over the unreduced catalyst, only small amounts of additional surface carbonates are formed. Bands of the desired methanol product are detected after π-bound formaldehyde and water have appeared. The latter reactions are accompanied by a consumption of some surface carbonate, which had previously been formed during the storage of the sample in air. (vii) Over an unreduced surface, the water-gas shift and its reverse reaction are also taking place, as evidenced by the formation of gaseous CO2 or CO, respectively. Starting from CO2 and H2, the presence of adsorbed CO is observed.

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(viii) Whereas methanol synthesis and the interconversion of both carbon oxides proceeds over the unreduced surface of the copper/zirconia catalyst, there is no spectroscopic evidence for a formate species being present under these reaction conditions. Involvement of Surface Hydroxyl Groups. Generation of surface formates and carbonates is always accompanied by a loss of surface hydroxyl groups, suggesting that these are the preferred adsorption sites for one or both of these species. Kondo et al.33 have studied the surface reactions of CO and CO2 on pure and prereduced ZrO2. The authors found that various types of carbonates were formed in the interaction of the carbon oxides with pure ZrO2, wheras formates were only produced when hydroxyl groups were also present on the surface. This conclusion is confirmed by a study of CO adsorption on ZrO2 and ZnO/ZrO2 aerogels by Bianchi and coworkers.54 Bidentate as well as monodentate surface carbonates were also observed by Schild et al.21 upon CO2 adsorption on ZrO2 without predosing hydrogen. The following observation may help to decide which species is formed by the interaction of CO or CO2 with the surface hydroxyl groups. In all spectra presented in this work, the consumption of Zr-OH groups ceases at the same time when the surface concentration of formates reaches a constant value, whereas the formation of carbonates proceeds further without additional loss of surface hydroxyl groups. Therefore we conclude that surface hydroxyl groups are required for the generation of surface formates from carbon monoxide or carbon dioxide. Additional support for this claim comes from the observation that we are not able to produce surface formates on the surface of an unreduced catalyst. In contrast, carbonates may be formed by the interaction of the carbon oxides with surface oxygen or lattice oxygen anions, located either on ZrO2 or on the surface of the metallic component. Mechanism of the Water-Gas Shift Reaction. The CO2 hydrogenation experiments (Figures 8 and 10) contained information concerning the reverse water-gas shift reaction which has also been discussed in earlier work.23,24,47 The observations suggest a reaction pathway in which CO is produced from surface carbonate, with reaction steps that may be formulated as follows: 2CO2 + Osurface + 0 f CO32-

(4)

CO32- + H2 f COgas + 2OHsurface

(5)

2OHsurface f H2Ogas + O2-surface + 0

(6)

where 0 denotes an anion vacancy. This scheme is supported by the observation that only surface carbonates are found as intermediates during the reverse water-gas shift reaction over the unreduced copper/ zirconia catalyst (Figure 10), in good agreement with our previous results.25 Obviously, one can not rule out other mechanisms that would proceed without observable intermediates. In particular, the role of lattice anion vacancies must be discussed in this context: CO2 could react with a reduced vacancy and electrons, producing gaseous CO and the corresponding oxygenated zirconium site. Anion vacancies have previously been discussed in CO2 hydrogenation reactions over amorphous palladium/ zirconia catalysts.42 (54) Bianchi, D.; Chafik, T.; Kahlfallah, M.; Teichner, S. J. Appl. Catal. A: General 1993, 105, 223.

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Weigel et al.

In the forward direction, the water-gas shift reaction proceeds with high CO2 formation rates under CO hydrogenation conditions, especially in the flowing system since the gaseous reaction product (CO2) is not purged out under dynamic reaction conditions. In this case, the oxidation of CO is effected by loosely bound oxygen from the zirconia matrix or from the copper component. A redox mechanism for the water-gas shift reaction involving the transfer of oxygen from water to CO via the surface of the catalyst is well-known6,55-58 and has been written as follows:

H2O + red f H2 + ox

(7)

CO + ox f CO2 + red

(8)

where red designates a reduced and ox an oxidized surface site. Campbell et al., as well as Ovesen et al., found evidence for a redox mechanism in a kinetic study also for the reverse water-gas shift reaction on model copper catalysts (i.e., Cu(111)59,60 and Cu(110)60-63 ). The authors state that the dissociation of water to OHads and Hads is the rate-limiting step, with Hads acting as the reducing agent. However, there are considerable differences between the copper single crystals used as model catalysts in refs 5963 and the zirconia-supported copper catalyst used in this study. First, OHads has already been present on the catalysts surface when the reaction starts. Second, more complicated intermediates such as formates were not included in the mechanism on which the calculations presented in ref 63 are based. Furthermore, H2 rather than H2O as the hydrogen source was used in this study. Evidence for a mechanism proceeding via surface formates was found by van Herwijnen et al.64,65 on a Cu/ZnO catalyst. An FTIR study of the water-gas shift reaction also provided evidence for the presence of surface formates,66,67 but the authors pointed out that these species were not necessarily intermediates in the reaction. The reported rates of CO2 formation in the absence as well as in the presence of hydrogen were very similar, and the redox mechanism was therefore concluded to be dominant. Du¨mpelmann68 was able to separate the reaction rates of methanol production from those involved in the watergas shift reaction using a stirred tank reactor at high pressures. The orders were estimated to be 0.7 with respect to steam and 1.0 with respect to CO. This result is not easily explained in terms of an elementary redox mechanism as written above. A mechanism for the water(55) Temkin, M. I.; Kul’kova, N. V. Zh. Fiz. Chim. 1949, 23, 695. (56) Boreskov, G. K.; Yureva, T. M.; Sergeeva, A. S. Kinet. Katal. 1970, 11, 1476. (57) Rhodes, C., Hutchings, G. J.; Ward, A. M. Catal. Today 1995, 23, 43. (58) Wainwright, M. S.; Trimm, D. L. Catal. Today 1995, 23, 29. (59) Campbell, C. T.; Daube, K. A. J. Catal. 1987, 104, 109. (60) Campbell, C. T.; Ernst, K. In Surface Science of Catalysis. In Situ Probes and Reaction Kinetics; Dwyer, D. J., Hoffmann, F. M., Ed.; American Chemical Society: Washington, DC, 1992; ACS Symposium Series Vol. 482; Chapter 8, pp 130-142. (61) Nakamura, J.; Campbell, J. M.; Campbell, C. T. J. Chem. Soc., Faraday Trans. 1990, 86 (15), 2725. (62) Ernst, K.; Campbell, C. T.; Moretti, G. J. Catal. 1992, 134, 66. (63) Ovesen, C. V.; Stoltze, P.; Nørskov, J. K.; Campbell, C. T. J. Catal. 1992, 134, 445. (64) van Herwijnen, T.; de Jong, W. A. J. Catal. 1980, 63, 83. (65) van Herwijnen, T.; Guczalski, R. T.; de Jong, W. A. J. Catal. 1980, 63, 94. (66) Fujita, S.; Usui, M.; Takezawa, N. J. Catal. 1992, 134, 220. (67) Nakamura, J.; Rodriguez, J. A.; Campbell, C. T. J. Phys.: Condens. Matter 1989, 1, SB149. (68) Du¨mpelmann, R. Kinetische Untersuchungen des Methanolreforming und der Wassergaskonvertierungsreaktion in einem konzentrationsgeregelten Kreislaufreaktor. Ph.D. Thesis, Eidgeno¨ssische Technische Hochschule Zu¨rich, Switzerland, 1994.

gas shift reaction and its reverse reaction that would involve surface carbonates appears to be at least consistent with all the mentioned observations. Methanol Formation. As mentioned in the Introduction, surface formates have been reported as the pivotal intermediates in a methanol synthesis mechanism,6,14,15 with the hydrogenation of formate to methoxy as the rate limiting step.69 The spectroscopic results presented in this work do not support these findings. Surface formates are formed immediately, regardless of whether one starts from CO, CO2, or CO/H2 and CO2/H2 mixtures. Formate surface concentrations become constant several minutes before the onset of methanol formation. All evidence suggests that methanol is produced by reduction of πbound formaldehyde, since no methanol is observed without previous formation of surface-bound formaldehyde. In contrast, methanol synthesis does readily proceed without any formate species present on the catalysts surface, as demonstrated by the CO and CO2 hydrogenation experiments over the unreduced sample. In addition the surface concentrations of the carbonate, formate, formaldehyde, and methylate species were estimated by converting the spectra into Kubelka-Munk spectra (see Experimental Section) and fitting a Voigt profile on the spectra.70 The obtained surface concentrations of formaldehydes parallel the ones of surface-bound methylate whereas those of formates did not. On the basis of these results, studies are in progress on the kinetics of methanol synthesis over copper/zirconia catalysts.71 All results appear to support the above statements. In addition it is known that methanol decomposes very rapidly to adsorbed methoxy on copper surfaces, which further desorbs as formaldehyde gas.72 The bonding geometry of the surface-bound formaldehyde remains a subject of discussion. Anton et al.43 have proposed an η2-H2CO species. Following their argument, Schild et al.21 suggested that formaldehyde interacts with the surface via its π electrons, such that the carbon and the oxygen atoms are bound to the surface, and essentially only a single bond remains between the carbon and oxygen atoms. In this context one has to consider alternative bonding geometries, such as donation from nonbonding orbitals on O, or a dissociative form like a formyl species. Yurieva73 showed that a varying CO2/CO/H2 ratio leads to a varying Cu0/CuOx/Cu+ content in the ZnO matrix, whereby the Cu+ could act as adsorption site for a formyl species. Other authors44 have proposed a structure similar to the one of paraformaldehyde. In this context a dioxymethylene species must be mentioned, which was observed as an intermediate in methanol synthesis by He and Ekerdt.16 This species originated from the hydrogenation of surface formates at lower temperatures; it was stated that it could either react with hydrogen to produce methane or desorb as methanol in the presence of water. The authors note that dioxymethylene might be identical with a formaldehyde-type species.74 Elucidation of the structure of the ‘formaldehyde’ species mentioned throughout the text will thus be the subject of further work. In contrast to the open question concerning the structure, useful information on the origin of the formaldehyde (69) Yoshihara, J.; Parker, S. C.; Schafer, A.; Campbell, C. T. Catal. Lett. 1995, 31, 313. (70) Reilly, J. T.; Walsh, J. M.; Greenfield, M. L.; Donohue, M. D. Spectrochim. Acta 1992, 48A (10), 1459. (71) Weigel, J.; Baiker, A.; Wokaun, A. Unpublished results. (72) Jiang, X.; Parmeter, J. E.; Estrada, C. A.; Goodmann, D. W. Surf. Sci. 1991, 249, 44. (73) Yurieva, T. M.; Minyukova, T. P. React. Kinet. Catal. Lett. (USSR) 1985, 25, 55. (74) Tseng, S. C.; Jackson, N. B.; Ekerdt, J. G. J. Catal. 1988, 109, 284.

CO and CO2 Hydrogenation over Copper/Zirconia

species is clearly provided by the present results. Formaldehyde generation is consistently paralleled by a narrowing of the absorptions between 1700 and 1200 cm-1, which shows that surface-bound formaldehyde is formed by the stepwise reduction of surface carbonates. The process is accompanied by the appearance of water, as carbonate-oxygen anions are reduced by hydrogen, yielding mostly singly surface-bound CO (cf. reactions 5 and 6 above). This proposed mechanism is not in conflict with the observations made during the surface formate hydrogenation experiment (Figure 2), in which a decrease of the surface concentration of adsorbed CO had been detected. The latter result can be explained by the assumption that the reduction of adsorbed CO yielding formaldehyde is faster as compared to preceding or subsequent steps (further hydrogenation of formaldehyde yielding surface methoxy species). Role of Adsorbed CO. Accurate determination of the coverage for singly surface-bound CO is complicated by the fact that its C-O stretching band overlaps with the P-branch of the absorption doublet of gaseous CO. In spite of this complication, adsorbed CO is clearly detected in CO2 hydrogenation experiments. The intensity at the frequency of the P-branch of the CO doublet is clearly higher than expected from a Boltzmann distribution of populations in gaseous CO; hence, an additional contribution to the absorption from a surface-bound CO species must be present (Figures 8 and 10). The results of the previous paragraphs may be summarized as follows. With CO2 as the reactant, the reaction pathway leading to the desired methanol product appears to start from surface carbonates, which undergo reduction to adsorbed CO and are reduced further via surface formaldehyde to yield surface methoxy species and finally methanol. Surface formates, as created e.g. upon formic acid adsorption, cannot be correlated with the appearance of π -bound formaldehyde or methoxy, as their surface concentrations remained constant in the corresponding hydrogenation experiment (Figure 2). These findings are in agreement with our previous studies on Pd/ZrO221 and on silver-promoted copper/zirconia catalysts.24,25 Other Models Proposed in the Literature. Rasmussen and co-workers75,76 have studied methanol synthesis from a CO2/H2 mixture on a Cu(100) single crystal. Their results were found to be consistent with a model in which hydrogenation of dioxomethylene or formaldehyde was the rate-limiting step in methanol synthesis. CO hydrogenation over ZrO2- and Mo/ZrO2-supported Rh catalysts was studied by Guglielminotti et al.77 The authors also proposed a methanol synthesis mechanism which proceeds via surface-bound formaldehyde and methoxy. These conclusions are in contrast to the findings reported on commercial ICI catalysts39 and on SiO2(75) Rasmussen, P. B.; Kazuta, M.; Chorkendorff, I. Surf. Sci. 1994, 318, 267. (76) Taylor, P. A.; Rasmussen, P. B.; Chorkendorff, I. J. Chem. Soc., Faraday Trans. 1995, 91 (8), 1267. (77) Guglielminotti, E.; Giamello, E.; Pinna, F.; Strukul, G.; Martinengo, S.; Zanderighi, L. J. Catal. 1994, 146, 422.

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supported catalysts,78,79 which support a formate to methoxy mechanism. However, Burch et al.80,81 were not able to detect formate on a Cu/SiO2 catalyst in the conversion of a CO-CO2-H2 mixture. A similar conclusion was reached by Trunschke et al.82 The authors examined the hydrogenation of CO over Mn-promoted Rh/ ZrO2 and Rh/SiO2 catalysts (which had been derived from metal carbonyl clusters) by means of in situ IR spectroscopy. Although surface formate was found to be reactive toward hydrogen, no formate species were stabilized on the surface of the catalyst, showing the highest methanol selectivity (i.e., over Rh/SiO2). In summary, many of the cited observations do not provide a correlation between the presence of surface formate and the production of gaseous methanol. Surface formates, which appear to be spectator species in methanol synthesis, have however been identified as methane precursors in previous work.21-23,42,47,83 5. Conclusions The hydration state of the support plays an important role in providing adsorption sites for the reaction intermediates observed in methanol synthesis over copper/ zirconia. In the presence of surface hydroxyl groups, i.e. on the prereduced catalyst, surface formates, carbonates, formaldehyde, and methoxy are immediately observed in CO and CO2 hydrogenation reactions. If surface hydroxyl groups have not been provided during the activation procedure, i.e. on the unreduced surface, surface formates are not formed. These observations strongly suggest that formates arise from the interaction of the carbon oxides with surface hydroxyl groups. In a rapid side reaction, CO2 is reduced to yield surface formate on the surface of the copper/zirconia catalyst; the surface coverage of formate is quickly reaching saturation. In earlier spectroscopic studies, it had been shown that formate is reduced to methane over Pd/ZrO2 and Ni/ZrO2 catalysts, without further observable intermediates. The surface-catalyzed reverse water-gas shift reaction, which produces gaseous and singly surface-bound CO, involves a carbonate species as intermediate. From the observed correlations it appears that adsorbed CO is the precursor to methanol. When starting from CO2 as a reactant, adsorbed CO is produced in the reverse water-gas shift reaction and originates from the reduction of surface carbonates. On the methanol synthesis catalyst, the adsorbed CO is further reduced to yield surface-bound formaldehyde and methylate, from which the desired methanol product is generated by hydrogenolysis or protolysis. LA9506990 (78) Millar, G. J.; Rochester, C. H.; Waugh, K. C. J. Chem. Soc., Faraday Trans. 1992, 88 (15), 2257. (79) Clarke, D. B.; Bell, A. T. J. Catal. 1995, 154, 314. (80) Burch, R.; Chalker, S.; Pritchard, J. J. Chem. Soc., Faraday Trans. 1991, 87 (1), 193. (81) Burch, R.; Chalker, S.; Pritchard, J. J. Chem. Soc., Faraday Trans. 1991, 87 (11), 1791. (82) Trunschke, A.; Ewald, H.; Miessner, H.; Marengo, S.; Martinego, S.; Pinna, F.; Zanderighi, L. J. Mol. Catal. 1992, 74, 365. (83) Schild, C.; Wokaun, A.; Baiker, A. J. Mol. Catal. 1991, 69, 347.