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Langmuir 1988, 4, 1104-1108
Adsorption Properties of Pt Films on W(1lO)t R. A. Demmin," S. M. Shivaprasad,f and T. E. Madey National Bureau of Standards, Surface Science Division, Gaithersburg, Maryland 20899 Received January 19, 1988. I n Final Form: April 20, 1988 The surface chemistry of ultrathin Pt films on a W(110) substrate has been investigated by using CO chemisorption. Carbon monoxide temperature-programmed desorption experiments show that molecular CO is more weakly bound to a monolayer Pt film deposited at 90 K than to either bulk Pt or the W substrate, similar to conclusions drawn from experiments on other metal thin films. Carbon monoxide is also weakly adsorbed on films annealed to 1500 K, even for initial Pt coverages much greater than one monolayer. This has been interpreted as strong evidence for substantial thermally induced structural changes in multilayer films that result in a W surface that is covered by a monolayer Pt film with unique CO chemisorption properties. Platinum films of at least one monolayer also prevent the dissociative adsorption of CO normally occurring on the W(110) surface. For submonolayer films annealed to 1500 K, the total amount of dissociative adsorption of CO decreases linearly with increasing Pt coverage, reaching zero at one monolayer of Pt. This implies that the inhibition of CO dissociation by Pt is very localized. Previously proposed explanations for CO adsorption behavior common to a variety of overlayer-substrate systems are discussed.
Introduction One of the primary motivations for studying ultrathin metal films on metal substrates is to gain a better understanding of bimetallic catalysts. Much of the recent work on thin films has dealt primarily with the structural aspects of these systems, such as the growth characteristics of the deposited films, the positions of the overlayer atoms relative to the substrate, the degree of intermixing of the substrate and overlayer, and the structural changes due to These studies have demonstrated a variety of effects reflecting strong overlayersubstrate interactions. Such interactions are expected to produce altered adsorption and reaction properties that may be related to those of bimetallic catalysts. A number of investigations have provided examples of dramatic changes in the chemisorption properties of a metal when it exists as a thin film on another In the present work, we have examined the CO adsorption behavior of Pt on a W(110) substrate and also observed a distinct difference in the surface chemistry of the Pt film compared to bulk Pt or W. Using Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED), we have recently examined the growth of Pt films on W(110) and the changes these films undergo when annealed.21 When Pt is evaporated onto a W(ll0) surface at 300 K, the film grows layer-bylayer (Frank-von der Merwe growth mode) for at least the first few layers. Annealing a film initially thicker than one monolayer (ML) causes a decrease in the Pt AES signal (64 eV) and an increase in the W AES signal (169 eV), with the AES intensities approaching those for one monolayer when the temperature reaches 1500-1800 K. This suggests that a monolayer of Pt is stabilized up to 1800 K due to a strong interaction with the W(ll0) substrate. The Pt TPD results demonstrate that Pt evaporation is negligible below 1800 K, so most of the Pt in excess of 1 ML must leave the sampling depth of the Auger electrons by agglomeration into three-dimensional clusters or diffusion into the bulk W. Measurements of total CO desorption from films between 2.3 and 10 ML that had been annealed
to various intermediate temperatures prior to dosing with CO showed no significant differences in the saturation CO coverage. In addition, one result of the present work is the observation that CO TPD curves for multilayer films that were annealed sufficiently had no features characteristic of CO desorption from bulk Pt. If the excess Pt forms clusters of Pt on the monolayer film, the negligible increase in surface area and lack of bulk Pt peaks in CO TPD require the clusters to be large and few in number. Alternatively, diffusion of Pt into the W substrate may also account for these results. Agglomeration has been observed for similar systems1J6 but recent work on Pt films on W(100) has suggested that a Pt-W alloy forms at these temperatu~es.~J~ It is not yet clear which of these processes is the explanation for the Pt/W(110) results. We have limited the present investigation to CO TPD experiments from the films as deposited at 90 K and after annealing for 1min at 1500 K. Annealing the multilayer (1)Bauer, E. Appl. Surf. Sci. 1982, 11-12, 479. (2) Bauer, E.; van der Merwe, J. H. Phys. Rev. B: Condens. Matter
1986,33, 3657. (3) El-Batanouny, M.; Strongin, M.; Williams, G. P. Phys. Reu. B: Condens. Matter 1983,27,4580. (4) Sagurton, M.; Strongin, M.; Jona, F.; Colbert, J. Phys. Reu. B: Condens. Matter 1983,28, 4075. (5) El-Batanouny, M.; Strongin, M.; Phys. Rev. B: Condens. Matter 1985, 31,4798. (6) Ruckman, M. W.; Murgai, V.; Strongin, M. Phys. Reo. B: Condens. Matter 1986, 34, 6759. (7) Paffett, M. T.; Campbell, C. T.; Taylor, T. N. J. Chem. Phys. 1986, 85, 6176. (8) Bruinsma, R.; Zangwill, A. J.Phys. (Les Ulis, Fr.) 1986,47, 2055. (9) Judd, R. W.; Reichelt, M. A.; Scott, E. G.; Lambert, R. M. Surf. Sci. 1987, 185,529. (10) Prigge, D.; Schlenk, W.; Bauer, E. Surf. Sci. 1982, 123, L698. (11) Poppa, H.; Soria, F. Phys. Reu. B: Condens. Matter 1983, 27, R l f i f~ i _ ..
(12) Ruckman, M. W.; Johnson, P. D.; Strongin, M. Phys. Reu. E: Condens. Matter 1985,31, 3405. (13) Pan, X.; Ruckman, M. W.; Strongin, M. Phys. Reu. E: Condens. Matter 1987, 35, 3734. (14) Hamadeh, I.; Gomer, R. Surf. Sci. 1985, 154, 168. (15) Shen, X. Y.; Frankel, D. J.; Lapeyre, G. J.;Smith, R. J. Phys. Reu. B: Condens. Matter 1986,33, 5372. (16) Goodman, D. W.; Yates, J. T., Jr.; Peden, C. H. F. Surf. Sci. 1985, 164, 417. (17) Berlowitz, P. J.; Goodman, D. W. Surf. Sci. 1987, 187, 463. (18) Egawa, C . ; Aruga, T. A.; Iwasawa, Y. Surf. Sci. 1987,185, L506. (19) Judd, R. W.; Reichelt, M. A.; Scott, E. G.; Lambert, R. M. Surf. Sci. 1987, 185, 515. (20) Neiman, D. L.; Koel, B. E. In Physical and Chemical Properties of T h i n Metal Ouerlayers and Alloy Surfaces; Zehner, D. M., Goodman, D. W., Eds.; MRS: Pittsburgh, 1987; p 143. (21) Shivaprasad, S. M.; Demmin, R. A.; Madey, T. E. T h i n Solid Films, in press.
Presented a t the symposium on "Bimetallic Surface Chemistry and Catalysis", 194th National Meeting of the American Chemical Society, New Orleans, LA, Sept 1-3, 1987; B. E. Koel and C. T. Campbell, Chairmen. 8 Permanent address: Vacuum & Pressure Standards, National Physical Laboratory, Hillside Road, New Delhi-110 012, India.
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1988 American Chemical Society
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Adsorption of PT Films on W(110) films to 1500 K effectively creates a monolayer Pt film on the W surface (although not necessarily identical with an unannealed monolayer film). If Pt clusters exist on the monolayer film, they do not make measurable contributions to the CO TPD curves, as determined by the lack of features attributable to bulk Pt. Therefore, the presence or absence of clusters is irrelevant to the interpretation of the CO TPD results. (It is most unlikely that potential contributions from the edges of clusters will be detected if the contribution from the surfaces of clusters is negligible.) Likewise, if Pt diffuses into the substrate, its concentration near the surface must be quite low to have a negligible contribution to the Pt Auger signal. This is consistent with the Pt-W phase diagram, which indicates that the maximum solubility of Pt in the W-rich phase is only a few percent.22 The CO TPD results indicate that in the limiting cases of clean W and thick Pt films the surface chemistry is characteristic of clean W(110)= and Pt(111),%respectively. For intermediate cases of Pt/W(110), CO adsorption is unlike that on either elemental substrate. In particular, CO is more weakly chemisorbed on a monolayer of Pt than on either clean W or thicker Pt films. This result is qualitatively similar to observations of CO adsorption on Pd films on Nb(llO), Mo(llO), Ta(llO), and W(110) substrates and Pt/Nb(llO), although it differs in the details.lD-1120
Experimental Section The experiments were performed in an ion-pumped UHV system with a base pressure of 2 X Torr. The system was equipped with a double-pass cylindrical mirror analyzer and grazing incidence electron gun for AES and a quadrupole mass spectrometer (QMS) for TPD. The substrate was a W single crystal approximately 6 mm in diameter, oriented and polished to within 0.5O of the (110) face and spotwelded to two tantalum wires mounted onto molybdenumposts on the manipulator. The crystal was heated resistively, and the temperature was measured with a W-5%Re/ W-26% Re thermocouple spotwelded to the back. The sample could also be cooled to less than 90 K via thermal contact with a liquid nitrogen reservoir. The W crystal was cleaned of carbon by heating to 1500 K in O2followed by flashing to 2100 K under vacuum to remove oxygen and other impurities. This cycle was repeated until no more carbon was detected by AES. Platinum was deposited onto the crystal by evaporation from a resistively heated W wire wrapped with Pt wire. The deposition rate was kept constant by maintaining a constant current through the evaporator. The amount of Pt deposited was measured by AES using the relative heights of the 64-eV peak of Pt and the 169-eVpeak of W in the dn(E)/dE mode. “he layer-by-layergrowth at 300 K and the resulting breaks in the Pt AES intensity with deposition time allowed calibration of the coverage and deposition rate. Annealing multilayer films to 1500 K caused the AES peak heights to change to the same values as for the calibrated monolayer, and the CO TPD results to be presented here are consistent with the calibration experiments. Carbon monoxide TPD was performed by cooling the sample to less than 90 K, depositing the desired Pt film,dosing the sample with 10 langmuirs of CO (saturation), and ramping the sample temperature at 8 K/s. To detect desorbing CO, we moved the sample close to the aperture of a shroud around the QMS, minimizing the effectsof the background,edge, and back of the crystal, as well as the small areas where the thermocouple and Ta leads were spotwelded. The desorption experiments were performed both for newly grown Pt f i as well as for films annealed to 1500 (22) Smithells, C. J. Metals Reference Book, 4th ed.; Plenum: New
York, 1967; Vol 11.
(23) (a) Kohrt, C.; Gomer, R. Surf. Sci. 1971,24,77. (b) Umbach, E.; Menzel, D. Surf. Sci. 1983, 135, 199. (c) Szuromi, P.D.; Kelley, R. D.; Madey, T. E. J. Phys. Chem. 1986, 90,6499. (24) McCabe, R. W.; Schmidt, L.D. Surf. Sci. 1977, 66, 101.
-,/(
1
Pt C o v e r a g e 10 ML
pc0 1 0 ML
I
Clean W
--\ 200
400
600
800
1000
1200
Temperature (K)
Figure 1. CO TPD from Pt deposited at 90 K. K for 1 min for the reasons discussed previously. Auger measurements showed that Pt deposited at 90 K accumulated on the surface at the same rate as at 300 K, but layer-by-layer growth was not observed. All of the Pt deposited on the surface could be removed by evaporation at 2100 K.
Results Figure 1 shows the CO TPD results for Pt films of various coverages deposited at 90 K. At the limits of zero Pt coverage and a 10-ML Pt film, the result agree with previous work on the bulk metals. The curve for CO desorption from the clean W(110) crystal is consistent with the results of other studies of CO/W(l10).23 The primary features are a peak at 415 K, which has previously been assigned to a molecular “virgin” state, and several broader states between 800 and 1300 K, which correspond to desorption from dissociated 0-CO states. The low-intensity features between 200 and 300 K may be due to desorption from the edges of the crystal and defects in the surface. (The peak temperatures observed here are slightly higher than those of several previous studies due to the fact that the earlier works used step desorption experiments while this work was performed with a linear temperature ramp.) Likewise, CO desorption from a 10-ML Pt film, with a peak at 440 K, is identical with CO desorption from the (111)surface of bulk Pt.% Desorption from other surfaces of Pt occurs between 400 and 550 K.24 Although the CO TPD curves for clean W and the thick Pt film are as expected, those for the thinner overlayers are not simply combinations of the two extremes. One effect of small amounts of Pt on the W(110) surface is to shift molecular CO desorption to a lower temperature range than for clean W(110) or for any surface of bulk Pt. As the Pt coverage is increased beyond one monolayer, the state corresponding to desorption from bulk Pt begins to appear above 400 K while the lower temperature peaks become smaller, until at high Pt coverages CO desorption occurs entirely above 400 K. Because the TPD curves are rather complex, it is not possible to obtain accurate parameters for the desorption kinetics. However, the general trend and magnitude of change may be approximated from a calculation of desorption activation energies (Ed) from the peak temperatures (Tp)by using the analysis of Redhead25for first-order desorption kinetics: RTp2u
Ed
=
RTp
In
(G)
1106 Langmuir, Vol. 4, No. 5, 1988
Demmin et al. I
/
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.
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.
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.
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io
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Figure 2. (a) Heats of adsorption for lowest temperature peak in CO TPD experiments,estimated by using first-orderdesorption
kinetics and a frequency factor of 1013s-l. (b) Carbon monoxide desorption rate at 300 K, measured during CO TPD experiments from Pt films deposited at 90 K. In Figure 2a are shown estimates of Ed for molecular CO calculated from the resolved peak lowest in temperature for each coverage of Pt, using a frequency factor (v) of 1013 s-l and a heating rate (p) of 8 K/s. Although the peak choice and model used for the calculation are arbitrary, the trend toward weakened CO adsorption at approximately 1 ML of Pt is implied. A more dramatic illustration of the shift in the temperature range of CO desorption is shown in Figure 2b. In this figure we have plotted the desorption rate, measured from the TPD curves, at 300 K. This temperature is well below that of CO desorption from either W(110) or the thick Pt film. As the Pt coverage increases, the TPD curves shift to a lower temperature and the rate of desorption at 300 K increases to a maximum at approximately 1 ML of Pt and then drops sharply at higher Pt coverages as the TPD curves shift back toward the temperature at which CO desorbs from Pt(ll1). The main purpose of this plot is to show that the 1-ML film has properties distinct from either the clean substrate or thick films. In addition to these changes in molecular CO desorption, Pt affects the recombinative desorption of dissociated CO from W between BOO and 1300 K (the p states). For low coverages of Pt, the higher temperature states around 1200 K disappear, with a very broad state decreasing at higher coverages of Pt. The monotonic decrease in the amount of CO adsorbed in these states is the expected result of depositing Pt on W, since there are no such dissociative (25) Redhead, P. A. Vacuum 1962,12, 203.
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,
,
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~
,
,
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,
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,
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Figure 4. Total amount of CO desorbing from the /3-CO states of W(110) as a function of Pt coverage for unannealed and an-
nealed films.
adsorption states on Pt surfaces. The CO TPD curves for these films after a 1-min anneal at 1500 K are presented in Figure 3. Comparison with Figure 1 shows that significant changes in the CO adsorption properties occur upon heating. Most notable is the fact that there are three distinct states at 290,330, and 380 K and that these states exist even for the thicker deposited layers. The TPD curve for the annealed 10-ML film shows these states as well as a small peak at 430 K corresponding to desorptiov from bulk Pt. This higher temperature peak can be removed with a longer annealing time, and the resulting TPD curve is essentially identical with those for the thinner films. As stated previously, Auger spectra of these thicker films after annealing approach those of monolayer Pt overlayers.21 Prolonged heating at 1500 K does not reduce the apparent Pt coverage below one monolayer. These results indicate that all or nearly all of the CO detected in TPD desorbs from a stable monolayer of Pt strongly interacting with the W substrate (although TPD of Pt clearly indicates that the excess Pt is still present on or near the surface at 1500 K). Carbon monoxide desorption from the annealed overlayer is similar to molecular desorption from the unannealed deposits (shown in Figure 1) up to one monolayer, with the addition of a new state at 290 K.
Adsorption of PT Films on W(110) Like the case for the unannealed films, there is no desorption from dissociated CO states (p-CO) for annealed films of more than one monolayer, showing that there is no bare W substrate available for adsorption. In Figure 4, the amount of 0-CO is plotted as a function of the amount of Pt deposited for both the unannealed and annealed films. Although there is slightly more p-CO desorbing from the unannealed films, the trends are the same. The difference is easily explained by the fact that layer-by-layer growth was not observed for Pt deposited at 90 K. When the surface was annealed, small patches of bare W that had been able to adsorb CO dissociatively were covered by Pt wetting the W surface. This effect is expected if there is a strong bond between the W substrate and the first atomic layer of Pt. What is striking about Figure 4 is the linear relationship between the amount of 6-CO and the amount of Pt deposited on the W, with the 6-CO desorption eliminated at a Pt coverage of 1 ML. In addition to providing a rather convincing confirmation of the Pt coverage calibration, this result may also have implications for the distribution of submonolayer quantities of Pt on the W surface. This is addressed in greater detail in the Discussion section.
Discussion In this section we consider the results for p-CO desorption first, since that provides an opportunity to review CO desorption from W(ll0). Much effort has been devoted to understanding CO adsorption on W(110), which is a rather complicated systemsB@ At temperatures below 300 K, CO adsorbs on W(110) in what is known as the “virgin” state (v-CO). Upon heating, some of the v-CO desorbs with a peak temperature of approximately 400 K while the rest dissociates to form the p states. The carbon and oxygen then recombine and desorb as CO between 800 and 1300 K from what appears to be multiple adsorption states (&, p2), with evidence of conversion from one to another during heating. The presence of adsorbed carbon and oxygen produced by dissociation of some of the v-CO upon heating changes the subsequent CO adsorption properties of the surface. If CO is dosed onto a W(ll0) surface at 90 K that is saturated with P-CO (i.e., previously saturated with CO at 90 K, heated to between 400 and 800 K,and cooled back to 90 K), then the gas-phase CO adsorbs in molecular states (a-CO) that are more weakly bound than v-CO. In addition to the existence of multiple states and paths to desorption, interpretation of CO TPD is made more complex by coverage effects for the various processes. For many adsorption systems, behavior like that depicted in Figure 4 would imply a site-blocking mechanism preventing adsorption. However, given the complicated nature of this system, care must be taken in drawing such conclusions. Although the decrease in population of the p-CO adsorption state appears to indicate site blocking, this needs to be clarified. Carbon monoxide is not blocked from adsorbing on the Pt film; instead, it adsorbs in molecular form and desorbs without dissociating. Measurements of the total CO adsorption indicate that the amount of molecular CO desorbing increases as the amount of @-CO decreases when the Pt coverage is increased up to one monolayer. The term “site blocking” implies a very localized interaction between the substrate and overlayer, and in that sense, dissociation of adsorbed CO is “blocked”. (26) (a) Leung, C.; Vass, M.; Gomer, R. Surf. Sci. 1977, 66, 67. (b) Houston, J. E.; Madey, T. E. Phys. Rev. E Condens. Matter 1982,26, 554.
Langmuir, Vol. 4, No. 5, 1988 1107 Because CO dissociation over W(ll0) occurs via a molecular v-CO species, it must be concluded that either an analogous state does not exist for the Pt-covered surface or the activation of the C-0 bond is inadequate for dissociation on this surface before CO desorption occurs. The possibility of two-dimensional Pt island formation is suggested by the fact that 0-CO adsorption, which evidently occurs only on bare patches of W, decreases linearly with increasing Pt coverage. Because the @-statesare dissociative, it is expected that ensembles of W atoms are required for CO to be adsorbed in these states. If this is the case, the linear decrease in p-CO saturation coverage with increasing Pt coverage implies that most of the exposed W atoms are not adjacent to Pt atoms that would interfere with dissociation. Alternatively, isolated W atoms may be able to dissociate CO since it adsorbs in the molecular v-CO state before dissociating. This possibility suggests that some of the dissociation products may reside on Pt atoms rather than W. If the Pt atoms are dispersed uniformly over the W(ll0) surface, and therefore are adjacent to a large fraction of the exposed W atoms, then the 0-CO saturation coverage should not decrease substantially until the Pt coverage is greater than 0.5 ML, which is not in agreement with the observations of Figure 4. However, the possible role of surface diffusion in creating ensembles as needed or in allowing the desorption products to find appropriate sites and then recombine to desorb as CO may complicate the issue. The suppression of dissociative adsorption of CO is reasonable, since bulk Pt does not adsorb CO dissociatively. However, CO does adsorb in molecular states on both W(110) and Pt surfaces, so more subtle effects would be expected for molecular adsorption on Pt films. For coverages of Pt up to one monolayer (or for greater Pt coverages followed by annealing), CO desorption occurs at lower temperatures than it does from the v-CO state of clean W(110) or from any surface of bulk Pt. The lower desorption peak temperatures, which imply weaker chemisorption, are consistent with the results of work on similar systems. Carbon monoxide adsorption has been studied on thin films of Pd and Pt on a number of substrates. Room temperature adsorption of CO on Pd/W(llO),1° Pd/Ta(l10),12and Pt/Nb(l10)13showed linear decreases in saturation coverage with increasing metal overlayer coverage up to one monolayer. Although the total CO coverage does not change for Pt/W(110), the results of Pd/W, Pd/Ta, and Pt/Nb systems are very similar to the behavior presented here for p-CO adsorption only. For the Pd/Ta and Pt/Nb work, it was observed that CO dissociated on submonolayer films while those of more than one monolayer adsorbed CO molecularly. Other work has shown a large reduction in CO adsorption on annealed Pd films on Mo(llO).’l It was also reported that CO did not adsorb on a monolayer of Pd on Nb(ll0) at room temperature, although it did adsorb weakly at lower temperatures.20 Low-temperature CO adsorption experiments performed on Pd/Nb(llO) films showed little or no CO desorption between room temperature and 500 K. This suggests that molecular CO adsorption on Pd/W(110), Pd/Ta(llO), Pd/Mo(llO), and Pt/Nb(llO) is so weak that it was not observed at room temperature. For coverages less than one monolayer, the only CO detected was adsorbed dissociatively on the bare substrate. All of the above examples indicate that there are strong interactions between Pt and Pd overlayers and refractory metal substrates. These interactions result in weaker adsorption of CO on the monolayer films than on the bulk substrate or overlayer metals. The present work for CO
Langmuir 1988,4, 1108-1112
1108
adsorbed on Pt/W(110) may be interpreted as a similar effect, but to a lesser degree than the other systems. U1traviolet photoelectron spectroscopy (UPS) studies of Pd/Ta(l10)12and Pt/Nb(l10)13have shown that there is a reduction in the density of states near the Fermi level when Pt or Pd exists in a single layer on these substrates, which may explain the weaker CO chemisorption. It has been suggested that this change in the electronic structure, induced by the strong overlayer-substrate interactions at the interface, causes the overlayer to behave more like a noble metal with a fully occupied d band.27p28 The differences in the degree of interaction, indicated by the chemisorption experiments, may be related to the interatomic bonding strengths of the various overlayer and substrate atoms, which may affect the rehybridization of the overlayer d band with the substrate. Recent syn(27) El-Batanouny, M.; Hamann, D. R.; Chubb, S. R.; Davenport, J. W. Phys. Rev. B Condens. Matter 1983,27,2575. (28) Xinyin, S.; Frankel, D. J.; Hermanson, F. C.; Lapeyre, G. J.; Smith, R. J. Phys. Rev. B Condens. Matter 1985, 32, 2120. (29) Demmin, R. A,; Kurtz, R. L.; Stockbauer, R. L.; Madey, T. E.; Mueller, D. R.; Shih, A., in preparation.
chrotron UPS experiments on Pt/W(110) filmsB show both similarities and differences in the electronic structurea of monolayer films for the Pt/W, Pt/Nb, and Pd/Ta systems.
Summary Carbon monoxide TPD experiments have shown clear differences in the surface chemistry of the Pt/W(110) thin film that distinguish it from both bulk Pt and W. Carbon monoxide appears to adsorb more weakly on a monolayer of Pt on W(110) than on clean Pt or W and is unable to adsorb dissociatively on this surface at all. The conclusions drawn from this work are in qualitative agreement with results of work on similar systems, although there are differences in the degree of substrate-film interaction among the various systems. This undoubtedly will be affected by the properties of the specific substrate and fh. Acknowledgment. This work has been supported in part by the Office of Basic Energy Sciences, U.S. Department of Energy. Registry No. CO, 630-08-0; Pt, 7440-06-4; W, 7440-33-7.
Direct Synthesis of Higher Alcohols Using Bimetallic Copper/Cobalt Catalystsf R. Cao,* W. X. Pan,# and Gregory L. Griffin*?l Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received December 29, 1987. In Final Form: April 20, 1988 We have measured the reduction kinetics, adsorption behavior, and catalytic activity of a series of Cu/ZnO, Cu/Co/ZnO, and Cu/Co/ZnO/A1203catalysts. Reduction occurs in two stages, assigned to the sequential reduction of Cu and Co cations in a mixed oxide precursor. The E t and TPD results for CO and Hz show that the reduced catalyst contains metallic Cu clusters, which consist primarily of high Miller index surface planes. Cobalt is present on the surface of these clusters in the form of low coordination number sites that are capable of adsorbing multiple CO molecules. The resulting catalysts are much less active for CH30H synthesis than the corresponding binary Cu/ZnO catalysts but are capable of forming C2H50H and CHI with a selectivity different from that of conventional supported Co catalysts.
Introduction Supported copper bimetal1ic have recently received a great deal of attention as possible catalysts for the direct conversion of CO into higher alcohols for use as motor fuels and/or octane enhancers.lP2 Results have been published concerning the preparation, structure, and bulk composition of the catalyst: its activity and overall product distribution,4 and its incorporation in a 7000 bbl/day demonstration plant.5 Electron microscopy and X-ray diffraction studies of the bulk catalyst have shown that the coprecipibted 'Presented at the symposium on "Bimetallic Surface Chemistry and Catalysis", 194th National Meeting of the American Chemical Society, New Orleans, LA, Sept 1-3, 1987; B. E. Koel and C. T. Camobell. Chairmen. t Gisiting scholar, Research Institute, Nanjing Chemical Industry Company, Nanjing, PRC. 8 Visiting scholar, Department of Chemistry and Chemical Engineering, Tsinghua University, Beijing, PRC. Present address: Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803.
0743-7463/88/2404-ll08$01.50/0
precursor consists of a highly homogeneous hydrotalcite phase containing Cu, Co, Zn, and A1 cation^.^ During calcination, the precursor is converted to a mixed oxide spinel phase that all of these cation species. At present, about the surfacecomposition of these catalysts is much moie limited. Recently we reported an adsorption study of binary Cu/ZnO prepared by a similar coprecipitation m e t h ~ d .These ~ results included the temperature-programmed desorption (TPD) spectra of Hz and CO adsorbed on the catalysts and alrrothe spedra of co. H~~~we extend these studies to the Cu/Co/ZnO/ (1)Courty, Ph.; Chaumette, P. Energy B o g . 1987, 7, 23. (2) Haag,W. 0.;Kuo, J. C.; Wender, I. Energy 1987, 12, 689.
(3) Courty, Ph.; Marcilly, Ch. In Preparation of CataZysts-ZZt Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier: Amsterdam, 1983; p 485. (4) Courty, Ph.; h a n d , D.; Freund, E.; Sugier, A. J. Mol. Catal. 1982,
17, 241.
( 5 ) Courty, Ph.; Forestiere, A.; Kawata, N.; Ohno, T.; Raimbault, C.; Yoshimoto, M. In Industrial Chemicals via C l Processes; Fahey, D. R., Ed.; ACS Symposium Series 328; American Chemical Society: Washington, D.C., 1987; p 42.
0 1988 American Chemical Society
