J. Phys. Chem. 1996, 100, 19975-19980
19975
Oxidation of Methanol at Cu(110) Surfaces: New TPD Studies Philip R. Davies* and Gregorio G. Mariotti Department of Chemistry, UniVersity of Wales Cardiff, P.O. Box 912, Cardiff CF1 3TB United Kingdom ReceiVed: June 18, 1996; In Final Form: August 20, 1996X
Previous temperature-programmed desorption (TPD) and molecular beam studies have concluded that formaldehyde desorption is the only significant pathway in methanol oxidation at Cu(110) surfaces. We present here new TPD data for the adsorption of deuterated methanol (CD3OH) at a preoxidized Cu(110) surface and show that formate can be the major product of the reaction but that the experimental conditions adopted in earlier studies (specifically, preadsorption of oxygen and the adsorption of methanol at low temperatures) preclude formate formation. We propose that the reaction pathway is sensitive to the local structure of the methoxy/oxygen adlayer and suggest a model based upon this idea that reconciles the available experimental evidence. We also confirm the presence of two states of methoxy at the Cu(110)/O(a) surface and assign them to methoxy species with and without stabilization by surface oxygen.
A recent XPS study12 indicated however that formate may be a significant product of methanol oxidation at copper surfaces, and this was confirmed by our own preliminary TPD study.13 The present paper presents a more detailed investigation of methanol oxidation at Cu(110) surfaces and attempts to reconcile the conflicting reports in the literature on this topic.
masses to be recorded simultaneously. Data were acquired by software written in house. The dimensions of the sample were approximately 10 × 7 × 0.5 mm, it was cut to within 0.5° of the (110) plane and polished mechanically with diamond paste down to 0.25 µm. Four holes approximately 0.5 mm in diameter were spark eroded in the corners, and the sample was suspended by two tungsten wires threaded through the holes. The temperature of the sample was measured with a chrome-alumel thermocouple attached with a silver epoxy resin in a notch cut into the back of the sample. Cleaning involved cycles of Ar+ sputtering (5 keV, 10 mA/cm2 for 10 min) and annealing for 10-20 min at 1000 K, conditions that give consistently good results in other spectrometers within our group. The sample cleanliness was confirmed by TPD spectra which reproduced previous results. The sample was heated by resistive heating of the tungsten support wires using a constant current device. This gave good linear heating rates of between 1.5 and 8 K s-1. Except where stated, the heating rate used in all experiments in this study was 4 K s-1. Gases were introduced to the sample via a glass capillary that could be placed directly in front of the sample face. During dosing the system pressure did not increase above 10-8 Torr and returned to its original value within about 30 s of the end of dosing. This method of gas dosing gave very reproducible results although exact exposures cannot be calculated precisely. Gases could also be introduced to the chamber by backfilling via a variable leak valve. Deuterated methanol (CD3OH, Aldich, 99%) was placed in a sample tube attached to the spectrometer, and dissolved gases were removed with several freeze-pump-thaw cycles. The purity was then checked by mass spectrometry, and in addition, the methanol was physisorbed at the clean Cu(110) surface at 130 K and desorbed in a TPD experiment. Only methanol was observed desorbing from the sample during this experiment; there was no evidence for contamination due to formaldehyde (desorption temperature ∼ 220 K) or water (desorption temperature ∼ 160 K).
Experimental Section
Results
Introduction Methanol oxidation at copper surfaces has been studied intensively since Madix and co-workers published their influential TPD-based papers in 19781 and 1980.2 The system has been examined with vibrational spectroscopy,3-6 molecular beam methods,7,8 and STM,8-11 but the central points of the mechanism established in the early papers has remained unchanged. Essentially this mechanism involves the reaction of two methanol molecules with an adsorbed oxygen atom to form water and two methoxy species, step 1. The latter decompose on heating to give gaseous formaldehyde and hydrogen, steps 2-4.
2CH3OH(g) + Oδ-(a) f 2CH3O(a) + H2O(g)
(1)
CH3O(a) f H2CO(a) + H(a)
(2)
H2CO(a) f H2CO(g)
(3)
2H(a) f H2(g)
(4)
Despite the observation, in the first of Madix’s papers, of simultaneous desorption of CO2 and H2 at ∼440 K (characteristic of adsorbed formate, step 6), formaldehyde has been suggested to be the only carbon-containing product although some studies8 have reported “insignificant” quantities of formate produced via step 5.
H2CO(a) + Oδ-(a) f HCO2(a) + H(a)
(5)
HCO2(a) f CO2(g) + H(a)
(6)
The experiments were carried out in a system equipped with a Ledamass multiquad spectrometer which enables up to seven * Tel: 01222 874072; FAX: 01222 874030; e-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)01805-9 CCC: $12.00
Methanol Adsorption at 180 K. We first establish that the results of previous studies2,8 can be reproduced under similar experimental conditions. Figure 1 shows TPD spectra for the adsorption of methanol at a preoxidized Cu(110) surface at 180 © 1996 American Chemical Society
19976 J. Phys. Chem., Vol. 100, No. 51, 1996
Figure 1. Temperature-programmed desorption spectra recorded simultaneously from a partially oxidized Cu(110) surface (240 s, O2(g) at 290 K) after exposure (5 s) to deuterated methanol (CD3OH) at 180 K. The heating rate for the experiment was 6 K s-1.
K. After the Cu(110) sample was exposed to dioxygen at 290 K (we estimate a surface oxygen coverage of approximately 0.25 monolayers, see below), the sample was dosed with approximately 0.05 langmuirs of deuterated methanol (CD3OH) at 180 K, well below the dose required to remove all of the oxygen from the surface, and heated linearly at 6 K s-1. The major desorption peaks observed were at m/e ) 4 and 30 corresponding to the principle fragments of deuterium (D2) and deuterated formaldehyde (D2CO), respectively. Both species are formed from the decomposition of adsorbed methoxy (step 2) at ∼330 K, and they are accompanied by a small degree of recombination to give a deuterated methanol signal at m/e ) 34. A second peak occurs in the m/e ) 4 spectrum at 375 K which was also observed in the H2 desorption spectrum following the adsorption of CH3OH. This is significantly higher than the desorption temperature of H(a) from clean copper14 and confirms a similar observation made by Madix1 for the methanol/oxygen/copper system. Waugh and co-workers have observed15 the release of several monolayers of hydrogen from supported copper catalysts following reduction by CO. They speculate that this is due to the lifting of a reconstruction of the copper surface, and it is possible that a similar process is occurring here. No D2O or H2O desorption was observed; H2O formed via step 1 would desorb at 160 K and would not therefore be seen in the present TPD experiment. If OH(a) were present on the other hand, either through step 1 or else the reaction of H2O with the remaining surface oxygen, it would be expected to desorb as water in a series of states between 180 and 240 K.16-18 This suggests that the coadsorbed methanol either inhibits the formation of OH(a) or else causes it to desorb
Davies and Mariotti
Figure 2. Temperature-programmed desorption spectra recorded simultaneously from a partially oxidized Cu(110) surface (240 s, O2(g) at 290 K) immediately after exposure (300 s) to deuterated methanol (CD3OH) at 290 K. We estimate the oxygen precoverage to be close to 0.25 monolayers, see text.
at or below 180 K. The formation of water from deuterium liberated by the decomposition of the methoxy is clearly not a viable pathway under the present conditions; this is consistent with previous studies. The m/e ) 44 and m/e ) 4 signals confirm that formate is not a significant product under these conditions, the coincident CO2 and D2 desorption peaks at 440 K being only just apparent above the noise. Methanol Adsorption at 290 K. The results in Figure 1 are in complete agreement with those of Madix and co-workers, whose experiments also involved adsorption of methanol at 180 K. Figure 2 shows a set of TPD spectra for a similar experiment except that the partially oxidized surface was exposed to methanol at 290 K. In contrast to Figure 1, the spectra show intense CO2 and D2 desorption peaks at 430 K indicating a significant surface concentration of formate. The absolute coverage of the formate can be quantified by comparison of the CO2 desorption peak area with that produced after saturation of the clean Cu(110) surface with formic acid which is known2 to produce ca. one-third of a monolayer of formate. Using this approach, we calculate a formate coverage of approximately 1 × 1014 molecules cm-2. The relatiVe importance of the formate and formaldehyde products in methanol oxidation under these conditions can be estimated from the D2 desorption trace since D2 desorption from the decomposition of the formate (step 7) can be clearly distinguished from that resulting from the decomposition of adsorbed methoxy (step 2). Taking into account the stoichiometries of steps 7 and 2, and the fact that the formation of one formate molecule results in the release of two deuterium atoms which desorb at or before 375 K, we
Oxidation of Methanol at Cu(110) Surfaces
J. Phys. Chem., Vol. 100, No. 51, 1996 19977
Figure 3. Effect of oxygen exposure on the oxidation of deuterated methanol (CD3OH) at a Cu(110) surface at 290 K. Formaldehyde and carbon dioxide desorption spectra were recorded simultaneously from a preoxidized Cu(110) surface immediately after exposure to deuterated methanol (300 s) at 290 K. The figure shows data for several experiments in which the initial oxygen coverage was varied from 0 to ∼0.5 monolayers: Oxygen exposures were (i) 0 s; (ii) 3 s; (iii) 120 s; (iv) 240 s; (v) 960 s; (vi) 1920 s; (vii) 3900 s.
estimate that formate constitutes approximately one-fifth of the total decomposition products of the adsorbed methoxy in this experiment. As in the case of methanol adsorption at low temperature, water is not a product of the oxidation reaction. This agrees with previous molecular beam results for similar conditions and suggests that the decomposition of the methoxy species to formaldehyde does not involve an interaction between the methoxy and surface oxygen since the formation of OD(a) as an intermediate is likely to lead to water via step7.
2OD(a) f D2O(g) + O(a)
(7)
In fact, as we discuss below, there is evidence that the presence of surface oxygen actually stabilizes the methoxy against decomposition. The formaldehyde desorption spectrum in Figure 2 has two clearly resolved components with peak maxima at approximately 330 and 375 K, suggesting that there are at least two states of methoxy at the surface. Madix also reported1 two states of methoxy in his low-temperature TPD experiments with higher exposures of methanol favoring the least stable state. We find that the relative concentrations of these two states are also dependent upon the surface coverage of oxygen. Figure 3 shows
Figure 4. Development of the CD3O/O adlayer with time. Temperature-programmed desorption spectra recorded simultaneously from a partially oxidized Cu(110) surface 60 min after deuterated methanol (CD3OH) adsorption at 290 K. Experimental conditions were identical to those in Figure 2.
formaldehyde desorption spectra from a series of experiments in which different surface coverages of oxygen were exposed to a fixed dose of methanol at 290 K. The maximum peak area for the formaldehyde peak was observed with oxygen exposures around 240 s. Previous work suggests that the maximum adsorption of methanol at Cu(110) surfaces occurs for oxygen precoverages of ca. one-fourth of a monolayer, and we therefore tentatively assign the exposure of 240 s to this coverage. The spectra demonstrate that the least stable state of methoxy is favored by lower oxygen coverages and the more stable state by higher coverages, intermediate oxygen coverages resulting in a mixture of the two states. We discuss this trend and its implications for the assignment of the two states in more detail below. Figure 3 also shows that at room temperature, for all exposures to oxygen, formate was a significant product of methanol oxidation. The TPD spectra shown in Figures 2 and 3 were recorded within ca. 1 min of the exposure of the sample to methanol. Figures 4 and 5 show how the methoxy/oxygen adlayer develops if the sample is left for a period of time after the methanol exposure. After 60 min (Figure 4) the formaldehyde desorption signal shows only a single peak at 375 K. The overall area of the formaldehyde peak has decreased relative to that observed immediately after dosing the methanol, but the area of the peak at 375 K has increased suggesting that there is some conversion of the low-temperature desorption state to the higher. The CO2 desorption signal has also increased and increases further when the sample is left for a total of 16 h after dosing the methanol. The formaldehyde signal on the other hand has completely
19978 J. Phys. Chem., Vol. 100, No. 51, 1996
Davies and Mariotti
Figure 6. Surface concentration of formate at a Cu(110) surface as a function of time following exposure of the preoxidized surface to methanol at 290 K. The formate concentration was calculated by comparing the CO2 desorption peak area with that obtained following formic acid adsorption at the clean surface. Relative errors in coverage, estimated from the reproducibility of the TPD experiments are indicated; the absolute error may be larger although the maximum formate coverage corresponds well to that reported by XPS for the same system.
Figure 5. Development of the CD3O/O adlayer with time. Temperature-programmed desorption spectra recorded simultaneously from a partially oxidized Cu(110) surface 960 min after deuterated methanol (CD3OH) adsorption at 290 K. Experimental conditions were identical to those in Figure 2.
vanished after 16 h along with the majority of the deuterium signal. This is in complete agreement with the results of the STM and XPS studies which both report the relatively slow desorption of methoxy at 290 K and, in the case of the latter, the formation of formate. The absolute coverage of formate at the Cu(110) surface has been estimated by comparison with formic acid adsorption, (as described above) and the apparent surface concentration of formate as a function of time is plotted in Figure 6. We find that the maximum coverage of formate at the surface corresponds to approximately 2 × 1014 molecules of formate/cm-2 in excellent agreement with the direct quantification obtained by XPS.12 However there is a marked difference between the TPD and XPS results in the first 20-30 min of the experiment. The XPS spectra show formate developing very slowly at the polycrystalline surface, with none present a few minutes after dosing the methanol and significant quantities developing only after some 15-20 min. The process was slower still at the Cu(110) surface. In contrast the TPD data for Cu(110) appear to show the immediate formation of ca. 1 × 1014 formate molecules cm-2, with the coverage almost doubling over the following 1 h. The possibility of the XPS study influencing the surface reaction has been thoroughly ruled out; therefore, the appearance of formate at short time scales in the TPD experiments is probably due to the applied temperature ramp driving the reaction. In other words, the majority of the formate is formed during the TPD ramp and is not present at the surface before the heating starts.
Figure 7. Temperature-programmed desorption spectra recorded simultaneously from a partially oxidized Cu(110) surface (we estimate the oxygen precoverage to be about 0.05 monolayers) after exposure to 300 s of deuterated methanol (CD3OH) at 375 K and subsequent cooling to 320 K.
Methanol Adsorption at 375 K. At temperatures above ambient the pathway leading to the complete oxidation of methanol is still readily available despite the short surface lifetime of the methoxy intermediate. Figure 7. shows TPD spectra recorded from a preoxidized Cu(110) surface exposed to methanol at 375 K and subsequently cooled to 330 K. Although the methoxy species decomposes rapidly at this
Oxidation of Methanol at Cu(110) Surfaces
Figure 8. Temperature-programmed desorption spectra recorded simultaneously from a partially oxidized Cu(110) surface after a 5 min exposure to a 6:1 deuterated methanol (CD3OH)/dioxygen mixture at 300 K. For clarity, the m/e ) 44 and m/e ) 30 traces have been offset by -0.5 × 10-9 and -1.5 × 10-9 mbar, respectively.
temperature we nevertheless observe a high concentration of formate; indeed for similar conditions the surface concentration of formate is several times greater than that obtained when methanol is adsorbed at room temperature. We have investigated the oxidation of methanol at higher temperatures, but above 400 K, formate itself decomposes at a significant rate and TPD cannot yield accurate data on its surface coverage. The Coadsorption of Methanol and Dioxygen at 290 K. The coadsorption of reactants at metal surfaces can result in exceptionally efficient and selective processes19 which are not always observed with sequential dosing, for example, the coadsorption of NH3/O2 or H2O/O2 mixtures at copper surfaces results in close to 1 monolayer of NH(a)20,21 and OH(a),17 respectively. We have therefore investigated the coadsorption of methanol and dioxygen on Cu(110). We find that oxygenrich mixtures show behaviors very similar to those of the sequential experiments described above but methanol-rich mixtures give very high selectivity to formate. Figure 8 shows for example the TPD spectra recorded after exposure of a clean Cu(110) surface to a 6:1 methanol/oxygen mixture for 5 min at 300 K. The dominant product is clearly formate with formaldehyde only just observable above the background. We discuss below the common features this system shares with NH3/O2. Discussion The Oxidation of Chemisorbed Methoxy to Formate at Cu(110) Surfaces. The present results show that for a wide range of oxygen coverages formate is a product of methanol oxidation at Cu(110) surfaces but that formate formation
J. Phys. Chem., Vol. 100, No. 51, 1996 19979 depends upon the precise conditions of the experiment, specifically, the temperature at which the methanol is adsorbed and at which the TPD experiment is carried out. STM images8-11 of the Cu(110)/O(a)/CH3O(a) adlayer show that at 300 K chemisorbed methoxy species form ordered islands which (initially at least) are not in contact with the (2 × 1)O islands. They also provide evidence for a mobile methoxy phase.10 In light of these results the present TPD spectra suggest that, if the O(a)/CH3O(a) adlayer is heated rapidly from a low temperature, decomposition within the methoxy islands and desorption of formaldehyde from the surface dominate over diffusion of the methoxy species to sites where further oxidation can take place, hence formaldehyde is the only product. On the other hand, at room temperature or aboVe, interaction between the chemisorbed methoxy groups and the oxygen islands is much more important, probably because of the presence of the mobile methoxy phase, and hence, formate is formed as a significant product of the reaction. These new data reconcile many of the apparently conflicting experimental results in the literature. The TPD studies of Madix and co-workers for example2 started at 180 K, and hence, formaldehyde was the only product observed. Similarly Campbell et al. did not observe formate after exposing oxidized copper films deposited on Zn(0001)-O surfaces22 to methanol at 120 K. Recent TPD experiments by Bowker et al.8 starting at 260 K reported a small but “insignificant” formate signal, consistent with a slightly increased mobility of methoxy at this temperature. TPD studies on catalysts and on copper powders on the other hand generally involve methanol adsorption at room temperature, and formate is invariably reported as a product; furthermore, the formate signal is enhanced at higher temperatures.3,6,23 The recent XPS study at 295 K reported formate12 but also showed that it is formed at a significantly slower rate than the TPD spectra imply. If the surface is left at 290 K for more than 1 h, the final concentrations of formate calculated from the two methods agree well and the discrepancy can therefore be attributed to oxidation of methoxy species during the TPD heating ramp. This point emphasizes again the care required in the interpretation of TPD data. Fu and Somorjai’s study of methanol oxidation in the presence of ZnO islands grown on a Cu(110) surface24 is one system that does not seem to fit the model above, formate being observed after methanol adsorption at 120 K. This suggests that the ZnO islands have a real effect on the chemistry of the system although interestingly Campbell has shown that the reverse situation, copper islands on ZnO surfaces, does not result in formate.22 These are systems that clearly merit further investigation. The Coadsorption of Methanol and Dioxygen at Cu(110) Surfaces. The high selectivity to formate of the methanol-rich coadsorption experiments parallels the chemistry of the NH3/ O2 system.20 It was proposed in the latter case that the high reactivity was due to the inhibition of oxygen island growth although the participation of a molecular oxygen species, which offers the lowest energy pathway25 to the products, could not be ruled out.26 In the present case a mobile and highly reactive oxygen state would account for the formation of formate and is also consistent with the inhibition of formate formation in oxygen rich mixtures. However, once again we cannot rule out the participation of a molecular oxygen species. Adsorbed States of Methoxy at Cu(110) Surfaces. We now consider the nature of the two states of methoxy evident from the two formaldehyde desorption peaks in the TPD spectra. Madix observed1 that, for a fixed initial oxygen coverage, increasing doses of methanol led to a decrease in the more stable
19980 J. Phys. Chem., Vol. 100, No. 51, 1996 methoxy state and an increase in the peak corresponding to the least stable state. In this paper we have shown that, if the methoxy/O(a) adlayer is allowed to equilibrate for 60 min after methanol adsorption, there is a dramatic decrease in the concentration of the least stable methoxy state. Neither of these observations are consistent with the suggestion made recently11 that the 375 K formaldehyde desorption peak should be assigned to methoxy species incorporated into islands and the 330 K peak to the mobile methoxy species. We note that increasing the oxygen coverage favors the more stable methoxy state and further that STM images recorded about 1 h after methanol adsorption at room temperature show methoxy islands “decorating” the [001] edges of the O(2 × 1) islands.10 We propose that chemisorbed methoxy species are stabilized by their interaction with oxygen islands and assign the 375 K formaldehyde desorption peak to methoxy species stabilized in this way. The 330 K peak can therefore be assigned to a combination of the mobile methoxy phase and methoxy islands without stabilization by oxygen islands. This accounts for the observed TPD spectra since increasing oxygen concentration leads to an increase in the stabilization of the methoxy whereas increasing methanol exposures would result in a decrease in the surface oxygen concentration. Conclusions The thermal decomposition of chemisorbed methoxy species at a Cu(110) surface in the presence of surface oxygen leads to formaldehyde as the major product but with approximately 20% of the methoxy species undergoing further oxidation to formate. The latter process is not observed in TPD experiments where methanol is adsorbed at low temperatures. These observations reconcile many of the conflicting reports in the literature concerning the formation of formate at copper surfaces and emphasize the dangers of extrapolating a reaction mechanism deduced from experiments involving only a very narrow range of conditions. The TPD results can be accounted for with a model in which the methoxy species and oxygen adatoms form separate islands at the Cu(110) surface. On heating from low temperatures decomposition of the methoxy occurs mainly within the methoxy islands and therefore in the absence of oyygen. On the other hand, at room temperature and above, diffusion of methoxy groups to the oxygen islands is facile and hence the formation of formate occurs during the TPD ramp or over a period of about 1 h if the surface is left at room temperature. Two states of methoxy are evident in the TPD experiments from formaldehyde desorption peaks at 330 and 375 K. We tentatively assign the higher desorption state to methoxy groups stabilized at the edges of the (2 × 1)O islands. In contrast to the experiments where the reactants are dosed sequentially, the major product when Cu(110) is exposed to
Davies and Mariotti methanol/oxygen mixtures (methanol excess) is formate. We propose that, as in the case of the NH3/O2/Cu system,20,26 this is due to the inhibition of the growth of unreactive oxygen islands. Acknowledgment. The authors express their gratitude to Prof. M. W. Roberts, Dr. A. F. Carley, and Prof. M. S. Spencer for stimulating discussions and critical comments on the manuscript, also to Mohammed Jahangir for developing the acquisition software used throughout the work and to the first reviewer of the paper for some very constructive and useful comments. G.G.M. was supported under the Brite Euram program. References and Notes (1) Madix, R. J.; Wachs, I. E. J. Catal. 1978, 53, 208. (2) Bowker, M.; Madix, R. J. Surf. Sci. 1980, 95, 190. (3) Millar, G. J.; Rochester, C. H.; Waugh, K. C. J. Chem. Soc., Faraday Trans. 1992, 88, 2257. (4) Russell, J. N.; Gates, S. M.; Yates, J. T. Surf. Sci. 1985, 163, 516. (5) Ryberg, R. Phys. ReV. Lett. 1982, 49, 1579. (6) Neophytides, S. G.; Marchi, A. J.; Froment, G. F. Appl. Catal. A 1992, 86, 45. (7) Barnes, C.; Pudney, P.; Guo, Q. M.; Bowker, M. J. Chem. Soc., Faraday Trans. 1990, 86, 2693. (8) Francis, S. M.; Leibsle, F. M.; Haq, S.; Xiang, N.; Bowker, M. Surf. Sci. 1994, 315, 284. (9) Leibsle, F. M.; Francis, S. M.; Davis, R.; Xiang, N.; Haq, S.; Bowker, M. Phys. ReV. Lett. 1994, 72, 2569. (10) Leibsle, F. M.; Francis, S. M.; Haq, S.; Bowker, M. Surf. Sci. 1994, 318, 46. (11) Bowker, M.; Leibsle, F. Catal. Lett. 1996, 38, 123. (12) Carley, A. F.; Owens, A. W.; Rajumon, M. K.; Roberts, M. W.; Jackson, S. D. Catal. Lett. 1996, 37, 79. (13) Carley, A. F.; Davies, P. R.; Mariotti, G. G.; Read, S. Surf. Sci. Lett. 1996, 316, L525. (14) Anger, G.; Winkler, A.; Rendulic, K. D. Surf. Sci. 1989, 220, 1. (15) Elliott, A. J.; Sakakini, B.; Tabatabaei, J.; Waugh, K. C.; Zemicael, F. W.; Hadden, R. A. J. Chem. Soc., Faraday Trans. 1995, 91, 3659. (16) Carley, A. F.; Davies, P. R.; Roberts, M. W.; Thomas, K. K. Surf. Sci. 1990, 238, L 467. (17) Carley, A. F.; Davies, P. R.; Roberts, M. W.; Shukla, N.; Song, Y.; Thomas, K. K. Appl. Surf. Sci. 1994, 81, 265. (18) Bange, K.; Grider, D. E.; Madey, T. E.; Sass, J. K. Surf. Sci. 1984, 137, 38. (19) Roberts, M. W. Surf. Sci. 1994, 300, 769. (20) Afsin, B.; Davies, P. R.; Pashuski, A.; Roberts, M. W. Surf. Sci. 1991, 259, L 724. (21) Afsin, B.; Davies, P. R.; Pashusky, A.; Roberts, M. W.; Vincent, D. Surf. Sci. 1993, 284, 109. (22) Zhang, R.; Ludviksson, A.; Campbell, C. T. Catal. Lett. 1994, 25, 277. (23) Bowker, M.; Hadden, R. A.; Houghton, H.; Hyland, J. N. K.; Waugh, K. C. J. Catal. 1988, 109, 263. (24) Fu, S. S.; Somorjai, G. A. J. Phys. Chem. 1992, 96, 4542. (25) Neurock, M.; Vansanten, R. A.; Biemolt, W.; Jansen, A. P. J. J. Am. Chem. Soc. 1994, 116, 6860. (26) Boronin, A.; Pashusky, A.; Roberts, M. W. Catal. Lett. 1993, 17, 185.
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