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
Alcohol Synthesis Using CulCo Catalysts A1203catalyst system and also report activity measurements for the catalysts.
Experimental Section Catalysts were prepared by coprecipitation. The starting Co(NO&, materials were 1.0 M aqueous solutions of CU(NO~)~, &(NO&, and A(N03)3, each of which were acidifiedwith HNOB as needed to completely dissolve the salt. These solutions were mixed in the proportions needed to obtain the desired cation ratio in the final catalyst and then precipitated with 1.0 M Na2CO3 We tested three precipitation sequences: (1)additionof Na&03 into the mixed metal nitrate solution, (2) addition of the mixed nitrate solution into Na2C03,and (3) simultaneous addition of both solutions into a well-stirredbeaker. These cause precipitation to occur at acidic, basic, and neutral pH conditions,respectively? The resulting precipitates were filtered, washed 5 times in hot water, dried for 12 h at 383 K, and calcined in air for 12 h at 623 K. An alkalinization step was included at this point for most of the catalysts. This consisted of loading the powders to the incipient wetness point with an aqueous solution of K2CO3 at the concentration needed to give the desired K loading in the final catalyst. After drying, the catalysts were loaded into the reactor and reduced in situ using a 5 vol % H2/N2mixture. Reduction was performed over 2 days, during which the temperature was gradually increased to 563 K. The extent of reduction could be monitored qualitatively by measuring the concentration of H20 leaving the reactor as a function of time and temperature. The kinetic measurements were performed in a tubular microreactor. The standard reaction conditions were 50 atm, 563 K, and H2/C0 = 1/1. Products were analyzed by gas chromatography, with peak areas calibrated by liquid injection of individual components. The IR and TPD experiments were performed in a high-vacuum cell attached to a gas-handling system with a computer-controlled quadrupole mass spectrometer and data acquisition system. Both H2and CO were adsorbed by admitting a fixed pressure of either gas (typically 4 Torr) at the desired temperature for 10 min, cooling to 100 K in the presence of the gas, and evacuating the remaining nonadsorbed gas. The amount of gas adsorbed was determined by integrating the area under the subsequent TPD spectrum. The IR spectra were recorded by using an FTIR spectrometer (Nicollet 60-SX). Results Reduction Kinetics. Catalyst reduction occurs in two stages. The first stage produces a maximum in the H 2 0 evolution rate at a temperature around 413 K. This is 10-15 K higher than the maximum observed during reduction of a binary Cu/ZnO precursor? When combined with the adsorption results discussed below, this leads us to assign the first stage to reduction of a Cu cations in a mixed oxide phase that contains both Cu and Co cations. The second stage of reduction begins at around 468 K and continues through 563 K, at which point we stop the reduction treatment to prevent excessive sintering of the catalyst. Our earlier studies showed no evidence for HzO evolution in this high temperature range during reduction of binary Cu/ZnO precursors.6 Control experiments performed here also show that no reduction of Cu-free Co/ZnO and Co/ZnO/Al2O3precursors occurs below 563 K. Therefore we assign the second stage to reduction of Co cations in the mixed oxide precursor. Activity Measurements. A summary of kinetic results for a series of different catalyst compositions is given in Table I. We performed this survey both to confirm the general features of the composition dependence reported by Courty et. al. for this system4and to identify active and selective catalyst compositions for more detailed adsorption studies. (6) Roberts, D. L.; Griffin, G. L. J. Catal. 1988, 110, 117.
Langmuir, Vol. 4, No. 5, 1988 1109 Table I. Activity of Supported Copper/Cobalt Catalysts for CO Hydrogenationn precipitarate, g/(g/h) composition Cu:Co:ZnO:AlzOs:KzO tion pH CHaOH C,H,OH CHd 40:-:60-:0.5 neutral 240 19 0 25570:-:basic 26 3 2 neutral 10 4 15 51 2 0 acidic 255:70-:0.5 basic 25 4 0 neutral 27 17 13 20:2020:40:neutral 17 11 44 20:20:20:40:0.5 neutral 9 5 6 a 290 K, 50 atm, CO/H2 = 50/50.
As expected, the binary Cu/ZnO catalyst is very active for CH30H synthesis. A t this temperature, the catalyst also produces a significant amount of C2H50H (ca. 10% of the CH30H yield) but no hydrocarbons. The ratio of CH30H to CzH50His similar to that reported by Smith and Anderson for a series of Cu/ZnO catalysts with and without K as a promoter.' The absolute rate of CH30H synthesis is 2-3 times lower than the rate reported by those authors, who measured the reaction kinetics at 132 atm and H2/C0 = 2/1. When Co is included in the catalysts, the CH30H activity drops markedly. For example, in all of the catalysts prepared at neutral or basic precipitation conditions, the CH30H activity decreases by an order of magnitude when only 5% Co is included in the catalyst. When 20% Co is included in the catalyst, the CH30H activity decreases further, although not by as great a factor. A similar nonlinear dependence of CH30H activity on Co content has also been reported by Lin and Pinnellaa and Spencerg for mixed Cu/Co catalysts prepared by sequential impregnation. The dependence of C2H50Hyield on Co content is much more complex, although two trends are apparent: (1)For all of the Co-containing catalysts precipitated at neutral pH, the C2H50Hrate is about 60% of the CH30H rate. For catalysts precipitated at basic pH, this fraction decreases to around 15%,and for the single catalyst formed at acidic pH, the fraction is only 4%. (2) The addition of K as a promoter causes a decrease in the ratio of CHI vs C2H50Hrates. For the 5% Co catalyst formed at neutral pH, the addition of K increases the C2H50Hrate fourfold while leaving the CH4 rate unchanged. For the 20% Co catalyst the addition of K decreases the C2H50Hrate twofold but decreases the CHI rate almost eightfold. Adsorption Studies. On the basis of this kinetic survey, we selected the 25Cu/5Co/70ZnO and 2OCu/2OCo/ 20Zn0/40A1,03 catalysts for our adsorption studies. Both catalysts gave qualitatively similar results for TPD peak temperatures and IR vibrational frequencies. Here we show only the results for the 20Cu/20Co/20Zn0/40A1203 catalyst. Hydrogen TPD. Figure 1shows typical TPD spectra for H2 adsorbed on these catalysts. The three different curves were obtained by varying the temperature at which H2 was admitted to the cell before cooling to 100 K. Hydrogen is seen to desorb in two stages: (1)a low-temperature state with a peak temperature at 310 K, which is observed in all three experiments; (2) a high-temperature state with a peak that shifts between 450 and 500 K, which (7) Pan, W. X.; Cao, R.; Roberts, D. L.; Griffin, G. L. J. Catal., in press. (8) Smith, K. J.; Anderson, R. B. Can. J . Chem. Eng. 1981, 61, 40. (9) Lin, F. N.; Pennella, F. In Catalytic Conversions of Synthesis Gas and Alcohols to Chemicals; Herman, R. G., Ed.; Plenum: New York, 1984; p 53.
1110 Langmuir, Vol. 4, No. 5, 1988
Cao et al. CU:
Cu Co Z n O A l 2 O 3 20 20 20 4 0
co: Z n O .
AI203
20:20:20: 4 0
a
Pod, = 40 t o r i
X
= 40'orr
X
2
3
J
-J LL
LL
z f ! c a a
2
0 I-
n
a
z
51w
W
n
n
0
N
I
V
I
IO0
1
300
I
500
I
TEMPERATURE (K)
Figure 1. Temperature-programmeddesorption spectra of Hz from the 20Cu f 20Co f 20Zn0f 40Al2O9catalyst: influence of Hz adsorption temperature. is observed only when H2 is adsorbed above 300 K. The first peak temperature is the same as we observed for H2 TPD from binary Cu/ZnO catalysts in their reduced state? This suggests that at least a part of this desorption peak can be assigned to desorption from metallic Cu sites. The high-temperature state appears to be due to H2 desorbing from Co sites, as discussed below. The amount of H2 adsorbed on the catalyst can be determined from the area under the TPD peaks. For example, the area under the 310 K peak observed after adsorption at 300 K corresponds to an H2 uptake of 30 pmol of H2/g of catalyst. This increases slightly, to 41 pmol/g, when adsorption is performed a t 450 K. This increase suggests that a modest activation barrier exists for H2 adsorption on the low-temperature sites. If we assign all of the low-temperature state to H2 adsorbed on Cu sites, then the value of 41 pmol/g corresponds to a Cu surface area of 40 m2 of Cu. This is calculated assuming a saturation coverage of 1.0 pmol of H2/m2 of Cu, which we compute using the saturation coverage for O2 reported by Parris and Klier (4.57 pmol of 02/m2of Cu),l0 multiplied by the saturation coverage ratio for H2 and O2for a series of supported Cu catalysts that we studied previously (1pmol of H2/4.5 pmol of 02)? In contrast, the amount of H2 desorbing from the high-temperature state depends strongly on the initial adsorption temperature. For example, the uptake increases from 0 to 87 pmol/g when the adsorption temperature is increased from 300 to 450 K. Similar behavior has been reported by Zowtiak et al.ll for H2 adsorption on impregnated Co/A1203catalysts. If all of the 450-500 K desorption is assigned to Co sites, then the H2 uptake at 450 K corresponds to an area of 7 m2 of Co/g. This is calculated assuming a saturation coverage of 12.5 pmol of H2/m2of Co as reported by Reuel and Bartholomew.12 Those authors concluded that this value is valid for a wide range of Co dispersion and supports. Carbon Monoxide TPD. Typical TPD spectra for CO adsorbed on these catalyst are shown in Figure 2. Again we show spectra obtained by admitting CO at three different temperatures before cooling to 100 K. The ad(10) Spencer, M.s.In Strong Metal Support Znteractiom; Baker, R. T. K.; Tauster, S. J.; Dumesic, J. A., Eds.;ACS Symposium Series 298; American Chemical Society: Washington, D.C., 1986. (11)Parris, G.E.;Klier, K. J. Catal. 1986, 97, 374. (12) Zowtiak, J. M.; Weatherbee, G. D.; Bartholomew, C. H. J. Catal. 1983,82,230.
Figure 2. Temperature-programmeddesorption spectra of CO from the same catalyst: influence of CO adsorption temperature. sorption temperature is still seen to influence the CO uptake, although the effect is not as pronounced as with H2 adsorption. All of the spectra share three common features: (1)a well-resolved peak at 180 K, (2) a broad shoulder at about 290 K, and (3) a second peak that shifts slightly from 470 to 430 K with increasing coverage. The first two temperatures are similar to our previous results for various supported Cu catalysts.16 This suggests that one or both of these CO desorption states can be assigned to Cu sites. This third temperature is higher than any desorption state we observed for supported Cu catalysts.6 Cortes and Droguett13report a peak at 450 K for CO des~rption'~ from Co/kieselguhr catalysts, which they assigned to the desorption of linearly adsorbed CO. Thus we suggest that the high-temperature state be assigned to CO adsorbed on Co sites. The amount of CO desorbing from Cu sites decreases from 125 to 100 pmol/g when the adsorption temperature is increased from 200 to 390 K. A possible reason for this decrease is discussed below. If we assume the former value is representative of saturation coverage and that all of the CO is desorbing from Cu sites, this corresponds to a Cu surface area of 31 m2 Cu/g in good agreement with the estimate based on H2adsorption. This calculation is based on a saturation coverage of 4.0 pmol of CO/m2 of Cu. We computed the latter value as the product of the saturation coverage ratio for CO and O2 for a series of supported Cu catalysts (0.9 pmol of CO/1.0 pmol of 02)6 multiplied by the saturation coverage for O2on Cu catalysts reported by Parris and Klier.'" , The amount of CO that desorbs from Co sites increases from 19 to 62 pmol/g as the adsorption temperature is increased from 200 to 390 K. Because the adsorption stoichiometry of CO on Co is known to vary with dispersion and support, it is not possible to use these results by themselves to determine the area of the Co adsorption sites. We will return to both of these points in the discussion. Infrared Spectra. A series of IR spectra for CO adsorbed on this catalyst are shown in Figure 3. The CO layer was adsorbed by admitting CO at 300 K, cooling to 150 K, and evacuating. The top spectrum was recorded with the sample still at 150 K; subsequent spectra were (13) Reuel, R. C.; Bartholomew, C. H. J. Catal. 1984,85, 63. (14) Cortes, J.; Droguett, S. J. Catal. 1975, 38, 477.
Langmuir, Vol. 4, No. 5, 1988 1111
Alcohol Synthesis Using CulCo Catalysts
1
C u C o Z n O A1203
20 20 2 0
0 004 Absorbance Units
40
2090
1
I
2200
2100
I
I
2000 1900 WAVE NU MB E R ( c d '1
, 1800
Figure 3. Infrared spectra of CO adsorbed on the same catalyst: desorption during sample warm-up.
recorded as the sample was heated to remove portions of the CO layer. The initial spectrum contains a sharp peak at 2090 cm-' and a well-resolved shoulder at 2060 cm-'. Comparison with the spectra recorded after warming suggests that there is also a broad absorbanceband extending below 2000 cm-l. The peak at 2090 cm-l has the same frequency as the major IR band observed for CO adsorbed an reduced Cu/ZnO catalysts! which suggests that it can be assigned to CO adsorbed on high Miller index planes of metallic Cu clusters. The assignment of the 2060- and 2000-cm-' features is discussed below. The second spectrum was recorded after warming the sample to 300 K. This is sufficient to remove the lowtemperature CO desorption states (cf. Figure 2). The features at 2090,2060, and 2000 cm-' have all disappeared, and a single band at 1974 cm-' has appeared. An IR band in this region has been observed for CO adsorbed on supported C~/kieselguhr.'~J~ Upon further heating, the band decreases in intensity and shifts to lower wavenumber. The bands disappear completely by 590 K, which is sufficient to remove all of the high-temperature CO desorption state (cf. Figure 2). At no time did we observe a band below 1900 cm-' that could be attributed to bridging C0.15J6
Discussion Catalyst Reduction. The reduction behavior suggests the presence of a mixed oxide phase in the calcined precursor that contains both Cu and Co cations. This is consistent with Courty's X-ray diffraction results for similar catalysts3which showed the existence of a highly homogeneous mixed Cu-Co-Zn-A1 spinel phase. The reduction behavior of the Cu cations in this mixed oxide phase is almost identical with that of the binary Cu/ZnO system. The peak temperature of the first reduction stage occurs 10-15 K above that observed for the Cu/ZnO system. This slightly lower rate of reduction may reflect the lower activity of Cu in the mixed oxide phase or possibly the stabilizing influence of Co cations, which bind 0 anions more strongly. The CH,OH activity of the resulting Cu surface is much lower than that of the Cu/ ZnO catalyst. This suggests that Co species are still in intimate contact with the metallic Cu surface. The reduction behavior of Co cations in the phase is much more complex. Reduction occurs over a wide temperature range, which suggests that the phase contains 0 (15)Ansorge, J.; Forster, H. J. Catal. 1981, 68,182. (16) Gopalakrishnan, R.;Viswanathan, B. J. Chem. SOC., Faraday Trans. 1 1986,82, 2635.
anions with a distribution of bond energies. This would be consistent with the distribution of anion coordination geometries that might be expected in a mixed oxide phase. Infrared Results. The IR and TPD results provide additional information about the distribution of Cu and Co sites after reduction. The IR band at 2090 cm-' shows that there are Cu sites present as metallic clusters with a large fraction of high index surface planes. The area of Cu sites on the 20Cu/20Co/20Zn0/40A120~catalyst as determined by CO adsorption, 30 m2 of Cu/g, is similar to the values measured for binary Cu/ZnO catalysts with similar Cu contents. The low ratio of the H2to CO uptakes on the Cu sites and the temperatures of their TPD peaks are also similar to the results for the binary catalysts. The IR band at 1974 cm-l shows that metallic Co sites also exist after reduction. Two points suggest that these sites are not bulk Co clusters: (1)the absence of bands in the IR spectra that could be assigned to CO adsorbed in bridging sites. This suggests that the Co sites are not present in ensembles that are large enough to allow CO to adsorb in its bridging configuration. (2) The linear CO band only becomes well resolved after CO has desorbed from the low-temperature sites. Because no new CO is admitted to the cell after the warm-up is started, we conclude that the molecules responsible for the 1974-cm-' band must have been present on the surface at 150 K but in a different configuration (see below). The assignment of the new bands at 2060 and 2000 cm-' is more complex. We propose that they are due to multiple adsorption of CO on low-coordination number Co sites that are dispersed on the surface of the Cu clusters. For example, the IR spectra of HCO(CO)~ and HCO(CO)~ complexes contain u(C-0) band in the region 2060-2020 cm"." This model gives the most consistent explanation for the temperature evolution of the IR spectra shown in Figure 3. If we assume that all of the Co sites are multiply populated at 150 K, then the 1974-cm-l band would not be observed. Instead, we would expect a distribution of v((2-0)bands due to both intramolecular coupling between CO molecules adsorbed on the same Co sites and also to the existence of Co sites with different coordination numbers to the underlying substrate. We would also expect the mean of this frequency distribution to be shifted to higher wavenumber, due to sharing of the back-donated electron density between molecules adsorbed on the same Co site. We further propose that upon heating to 300 K all but one of the multiply adsorbed CO molecules desorb from the Co sites. This leads to an IR spectrum with a single well-resolved band at 1974 cm-l. In support of this model, we note that a frequency shift from 2060 to 1974 cm-' is too large to attribute to coverage-induced changes in intermolecular vibrational coupling between CO molecules on neighboring sites. A shift of this size is compatible with the differences in u(C-0) frequencies seen for Co carbonyl compounds with different numbers of CO ligands." Upon heating to 450 K, the remaining CO desorbs. The 1974-cm-' band shifts by 10-20 cm-l as desorption occurs, which is consistent with the magnitude expected for coverage-induced changes in intermolecular coupling. This interpretation implies that the Co sites must have an average separation distance of order 5-10 A.1s It is conceivable that multiple adsorption on the Co sites might sterically block neighboring Cu sites. This inter(17) Wermer, P.; Ault, B. S.; Orchin, M. J. Organomet. Chem. 1978, 162, 189.
(18)Hammaker, R.M.;Francis, S. A.; Eischens, R. P. Spectrochim. Acta 1965,21, 1295.
1112 Langmuir, Vol. 4, No. 5, 1988
Cao et al.
action could account for the small changes in the area of the low-temperature CO desorption peak noted in Figure 2. The changes in CO uptake as a function of adsorption temperature (cf. Figure 2) suggest that the concentration of accessible Co sites changes with treatment of the catalyst. We tentatively propose that the surface of the mixed Cu/Co clusters undergoes reconstruction on the time scale of these experiments in a manner that changes the coordination number of the surface Co atoms. When the sample is exposed to gas-phase CO during the adsorption step at elevated temperature, the Co atoms near the surface of the cluster may migrate out into low-coordination configurations. The driving force for this migration is provided by the adsorption energy of CO molecules that can adsorb on the newly created Co sites. When the sample is exposed to vacuum a t high temperature a t the end of the TPD experiment, a fraction of the Co atoms will migrate back into more stable high-coordination configurations. In the future we hope to confirm this model by completing a more thorough study of the influence of adsorption conditions on the TPD results. Catalyst Activity. The model for the Cu/Co surface proposed above, together with the kinetic results shown in Table I, permits us to offer some limited discussion regarding the alcohol synthesis reactions on these catalysts. The rate of CH30H synthesis is greatly reduced on the catalysts which contain Co, even though the surface area assigned to Cu sites is comparable to that of binary Cu/ ZnO catalysts. This is in agreement with two earlier studies that showed addition of Co to Cu/ZnO catalysts suppresses CH,OH activity.*pg However, all three results appear to conflict with numerous studies that have shown CH30H synthesis activity is proportional to Cu surface area for catalysts with a wide range of Cu loadings and on different supports.'J9 This conflict can be resolved by proposing that only a small fraction of the measured Cu surface area is active for CH30H synthesis and that this fraction is blocked when Co is added to the catalyst. In earlier work we showed that the majority of the Cu surface in binary Cu/ZnO catalysts is made up of high index planes.6 The present results show that these planes are also present in the Cu/Co/ZnO and Cu/Co/ZnO/Al,03 catalysts. The difference in CH30H activity in the two cases suggests that CH30H synthesis does not occur on these planes. It is possible that the CH30H reaction occurs on the low-index planes only, since these are present in the mi-
nority on Cu/ZnO catalysts. This model could still accommodate the results which show activity to be proportional to total Cu surface area, if the ratio of high- and low-index planes happened to be the same in all of the binary catalysts tested. A separate study of catalysts that contain Cu only should be performed to test this hypothesis. The fact that CHI and C,H50H are produced on the Cu/Co catalysts shows that the Co sites discussed above are capable of dissociating C-0 bonds. According to the model for alcohol synthesis on modified Fischer-Tropsch catalysts proposed by Sachlter,20the dissociated C atom then reacts with adsorbed H(,) atoms to produce a CH,,,) intermediate. This species in turn reacts with additional hydrogen to desorb as CHI or else undergoes a CO insertion reaction to produce a CH,CHO(,) intermediate, which is hydrogenated to produce C2H50H. The fact that no bridging CO band was observed in the IR spectra suggests that the ability to adsorb CO into a bridging configuration is not a prerequisite for any of these reactions. The fact that the Co sites in these catalyst can adsorb multiple CO molecules provides further justification for postulating the CO insertion reaction as part of the reaction mechanism.
(19) Chinchen, G. C.; Waugh, K. C.; Whan, D. A. A p p l . Catal. 1986, 25, 101.
(20) Sachtler, W. M. H. 8th Intern. Cong. Catal.; Verlag Chemie, 19M, p 1-151.
Summary Coprecipitated Cu/Co/ZnO and Cu/Co/ZnO/A1203 catalysts are formed via a mixed oxide precursor that contains both Cu and Co cations. Reduction of this phase proceeds in two stages, corresponding to sequential reduction of Cu and Co cations. The IR and TPD results indicate that the resulting Cu sites are present as metallic Cu clusters with a large fraction of high-index surface planes. The Co sites are present as isolated atoms on the surface of these clusters. These isolated Co atoms can exist as low-coordination sites that are capable of adsorbing more than one CO molecule. The kinetic results suggest that these atoms have blocked off most of the active Cu surface for CH30H synthesis but are capable of accomplishing most of the elementary steps associated with the Fischer-Tropsch reaction of CO and HP In particular, the ability of these Co sites to adsorb multiple CO molecules may explain their selectivity for enhanced oxygenate formation via CO insertion reactions. Acknowledgment. This work was supported by the Department of Energy, Office of Basic Energy Sciences, through Grant DE-FG02-85ER13392. Registry No. Cu, 7440-50-8; Co, 7440-48-4;ZnO, 1314-13-2; Hz, 1333-74-0;CO, 630-08-0; CHSOH, 67-56-1; CzH50H, 64-17-5; CH4, 74-82-8;KzO, 12136-45-7.
