Interaction of Methanethiol with Sulfur-Covered W (001)

Dec 25, 1996 - The chemisorption of methanethiol on W(001) resulting in the formation of methyl thiolate decreases linearly as a function of preadsorb...
0 downloads 0 Views 400KB Size
Download PDF
6382

Langmuir 1996, 12, 6382-6388

Interaction of Methanethiol with Sulfur-Covered W(001) D. R. Mullins* and P. F. Lyman† Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201 Received April 15, 1996. In Final Form: September 20, 1996X The chemisorption of methanethiol on W(001) resulting in the formation of methyl thiolate decreases linearly as a function of preadsorbed atomic S. The ratio of selective decomposition of methyl thiolate, which results in methane desorption, to total decomposition, resulting in desorbed H2 and adsorbed atomic S and C, remains constant. The S 2p photoemission from methanethiol on W(001) consists of peaks from two methyl thiolate species. The presence of atomic S adsorbed in fourfold hollow sites preferentially inhibits the thiolate species with the higher S 2p binding energy. This suggests that this thiolate species is also adsorbed in a fourfold hollow, and its adsorption is blocked by atomic S. Adsorption of thiolate on the S-precovered surface causes the S 2p peak position of the atomic S to shift to lower binding energy. This is the result of crowding caused by the chemisorption of the thiolate in adjacent sites but is not due either to direct bonding of the thiolate to the preadsorbed S or to a major reconstruction of the surface.

Introduction The decomposition of methanethiol on metal surfaces serves as one of the simplest probes for studying the reactions of sulfur-containing molecules on metal surfaces. In general, methanethiol follows a similar reaction pathway on most metal surfaces. At low temperature the molecule chemisorbs to form methyl thiolate, CH3S, and H. Heating the surface results in thiolate decomposition, either totally, forming S, C, and desorbed H2, or selectively, resulting in methane formation and adsorbed S. This reaction has now been studied on a number of clean and modified metal surfaces.1-10 The key questions are the amount of thiol that adsorbs and decomposes, the ratio between total and selective decomposition, the temperatures at which decomposition and product formation occur, and, most importantly, the relationship between these factors and the structure and composition of the substrate. It is important to examine the effects that the total decomposition products, S and C, have on the reaction path, since these products are produced during the course of the reaction on the clean surface and therefore modify the surface during the course of the reaction. One generalization is that S and C, as well as chemisorbed thiolate, tend to enhance the hydrocarbon formation channel at the expense of the total decomposition reaction. At low thiol exposures only total decomposition occurs on most surfaces, and the maximum methane production occurs at the saturation coverage of thiolate. Preadsorbed S has been shown to inhibit thiol uptake on Ni(110),2 Ni(111),1 and Fe(100).4 However, the preadsorbed S enhanced the production of methane relative to total decomposition for the thiol that did irreversibly adsorb. In contrast, there have been two dramatic examples of † Present address: Brookhaven National Laboratory, National Synchrotron Light Source, Building 725A/X15A, Upton, NY 11973. X Abstract published in Advance ACS Abstracts, December 1, 1996.

(1) Castro, M. E.; White, J. M. Surf. Sci. 1991, 257, 22. (2) Huntley, D. R. J. Phys. Chem. 1989, 93, 6156. (3) Castro, M. E.; Ahkter, S.; Golchet, A.; White, J. M.; Sahin, T. Langmuir 1991, 7, 126. (4) Albert, M. R.; Lu, J. P.; Bernasek, S. L.; Cameron, S. D.; Gland, J. L. Surf. Sci. 1988, 206, 348. (5) Wiegand, B. C.; Uvdal, P.; Friend, C. M. Surf. Sci. 1992, 279, 105. (6) Benziger, J. B.; Preston, R. E. J. Phys. Chem. 1985, 89, 5002. (7) Mullins, D. R.; Lyman, P. F. J. Phys. Chem. 1993, 97, 9226. (8) Mullins, D. R.; Lyman, P. F. J. Phys. Chem. 1993, 97, 12008. (9) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1995, 99, 11472. (10) Mullins, D. R.; Lyman, P. F. J. Phys. Chem. 1995, 99, 5548.

S0743-7463(96)00362-9 CCC: $12.00

surface modification by preadsorbed S that also preserved the reactivity: Weldon et al.11 and Benziger and Preston6 observed major enhancements in selective decomposition with little or no reduction in the total surface reactivity. On Mo(110),11 preadsorbed S increased the hydrocarbon formation from adsorbed benzenethiol from 50% to 80%, while on W(211)6 the preadsorbed S caused the percentage of methane formation from adsorbed methanethiol to increase from 35% to 75-80%. In both cases the authors attributed the relative increase in selective decomposition without decreasing surface reactivity to the formation of disulfide linkages. Finally, preadsorbed C also enhances the methane formation channel while maintaining surface reactivity on W(211)6 and W(001).10 In light of the effect of S on the selectivity and reactivity of thiols on Mo(110) and W(211), and of the similarity between carbon’s influence on W(211) and W(001), one might expect that preadsorbed S would enhance the selective decomposition of methanethiol on W(001) while preserving overall reactivity. Our experiments demonstrate, however, that the surface reactivity decreases linearly with the amount of preadsorbed S and that the selectivity remains constant with S coverage. S 2p photoemission indicates that the S preferentially blocks adsorption of thiolate in fourfold hollow sites and that disulfide bond formation does not play a significant role in the surface reactivity. Experimental Section The experimental system and sample preparation have been described in detail previously.7,8 The sulfur-covered surfaces were prepared by dosing H2S at 300 K and then annealing to 1000 K. The H2S was dosed either through the same directed doser as for the CH3SH,7,8 or through a separate dosing tube attached to a leak valve or by backfilling the chamber. The three methods gave equivalent results; however, the thermal desorption results were clearer using the dosers due to the reduced background following exposure. Auger electron spectroscopy (AES) and soft X-ray photoelectron spectroscopy (SXPS) of the S 2p core levels were used to calibrate the S coverage. The saturation coverage of S on W(001) was assumed to be 1.0 monolayer (ML) (1.0 × 1015 cm-2).12 The S 2p core level spectra were recorded on the U13UA beamline at the National Synchrotron Light Source. A 250 eV excitation was used, and the instrumental resolution was estimated to be 0.30 eV. The spectra were referenced against (11) Weldon, M. K.; Napier, M. E.; Wiegand, B. C.; Friend, C. M.; Uvdal, P. J. Am. Chem. Soc. 1994, 116, 8328. (12) Mullins, D. R.; Lyman, P. F.; Overbury, S. H. Surf. Sci. 1992, 277, 64.

© 1996 American Chemical Society

Interaction of Methanethiol with Sulfur-Covered W(001)

Langmuir, Vol. 12, No. 26, 1996 6383

the bulk W 4f7/2 peak at 31.44 eV.13 The emission angle was 15° from the surface normal; this emission angle minimized diffraction effects and gave the most reliable S 2p intensities for determining the partial coverages of different S-containing species. The S 2p spectra were normalized to the background intensity at kinetic energies slightly higher (1-2 eV) than those for the S 2p signal. The total S 2p intensity remained constant as the sample was heated and the thiolate decomposed. The S 2p spectra were frequently complex with many overlapping peaks. Nonlinear least squares curve fitting and deconvolution of the instrument function were used to aid in the analysis of the spectra. The curve fitting12 and deconvolution14,15 procedures have been described previously. The deconvoluted spectra greatly aid in the resolution of closely spaced peaks. A comparison of the deconvoluted data with the raw data indicated that the deconvolution process did not introduce any significant spurious peaks. In addition, the relative peak intensities in the deconvoluted spectra are qualitatively correct on the basis of a comparison with peak intensities determined by curve fitting.

Results Thermal Desorption. Thermal desorption spectra from a multilayer of CH3SH on W(001) with various S precoverages are shown in Figure 1. H2, CH4, and CH3SH were monitored at the masses 2, 15, and 47, respectively. No other masses were monitored, because previous studies of the desorption from CH3SH on clean and C-covered W(001) showed that no other species were produced.7,10 A mass balance between integrated H2 and CH4 desorption intensities and the amount of adsorbed S determined by AES or XPS indicates that these are the only products that desorb from the S-covered surface.10 The most striking result of the increasing S precoverage is the steady decrease in the amount of both H2 and CH4 that desorb in Figure 1. There are no new desorption features in either set of spectra. The H2 desorption from CH3SH on clean W(001) (top spectrum, Figure 1A) consists of an intense peak at 360 K and a shoulder at 425 K. The feature near 520 K, which does not shift nor change intensity as a function of CH3SH or S coverage, is due to ambient H2 adsorbed on the back of the sample.7,10 The CH4 spectrum (Figure 1B) is similar to the H2 spectrum with a main peak at 345 K and a shoulder between 400 and 450 K. Increasing the S precoverage attenuates the main features near 350 K for both H2 and CH4 while slightly enhancing the peak between 400 and 450 K, particularly in the CH4 spectra. The desorption of the undissociated CH3SH shows only a small change due to preadsorbed S, and therefore the mass 47 desorption spectra are not shown. The only change is a new desorption peak at ca. 130 K that occurs at high S coverage, as seen in the bottom spectrum of Figure 1B. Mass 15 is a cracking fragment of CH3SH, and the intensity at 800 K. Figure 2A contains data determined using both AES and SXPS. The uptake is 0.6 ML on the clean surface and decreases linearly with S precoverage. The desorption ratio on the clean surface is 4.7 ML of H2 per ML of CH4 and remains nearly constant on the S-precovered surface. The ratio (13) Mullins, D. R.; Lyman, P. F. Surf. Sci. 1993, 285, L473. (14) Asbury, J.; Grimm, F. A. To be published. (15) Allen, J. D.; Grimm, F. A. Chem. Phys. Lett. 1979, 66, 72. Note that there is a sign error on p 73 in the line preceding “II Continue”. The sign should be + (plus) and not - (minus).

Figure 1. (A) H2 and (B) CH4 thermal desorption spectra from a multilayer of CH3SH adsorbed on S-covered W(001). The initial S coverage, iΘS, is shown next to each spectrum. The arrows indicate the annealing temperatures shown in the S 2p spectra in Figures 4 and 6.

appears to increase at high S coverage, but this is probably due to incompletely correcting for the H2 desorption from the back of the sample. The large error bars in Figure 2B at high S coverages are the result of taking the ratio of two small numbers. S 2p Photoemission. S 2p photoemission was used to study the evolution of the S-containing species on the surface as a function of initial S coverage, CH3SH coverage, and annealing temperature following deposition. S was deposited by exposing the surface to H2S at 300 K and then annealing to 1000 K. A S 2p spectrum was recorded as the sample cooled to 100 K. The S-covered surface was then given sequential exposures of 0.15 ML of CH3SH at 100 K. S 2p spectra, shown in Figures 3 and 5, were recorded after each exposure. After five exposures the sample was then annealed to successively higher tem-

6384 Langmuir, Vol. 12, No. 26, 1996

Mullins and Lyman

Figure 2. (A) Irreversible CH3SH uptake on S-covered W(001) vs the S precoverage. The uptake is the final S coverage minus the initial S coverage. (B) Desorption ratio of H2/CH4 from a multilayer of CH3SH adsorbed on S-covered W(001) as a function of S precoverage.

peratures in 50 K increments up to 500 K, which is above the end point of the thiol decomposition, as indicated by the thermal desorption spectra. The arrows in Figure 1 indicate the annealing temperatures for the S 2p spectra in Figures 4 and 6. The S 2p core level spectrum consists of a spin orbit doublet with a 1.2 eV separation between the 2p1/2 and the 2p3/2 components. In general we will give the binding energy of only the S 2p3/2 component for simplicity. A deconvoluted set of spectra are shown below the raw data in Figures 3-6. Figure 3 shows S 2p spectra from sequential doses of 0.15 ML of CH3SH deposited on a clean W(001) surface at 100 K. At low exposures the thiol adsorbs in predominantly a single state with a S 2p3/2 peak position of 163.75 eV. A small state is evident with a S 2p3/2 binding energy of 162.55 eV. These two states are assigned to methyl thiolate, CH3S, adsorbed in two different adsorption sites.7 The S 2p3/2 component of the high binding energy state and the S 2p1/2 component of the low binding energy state overlap at 163.75 eV. The relative intensities of the two states are therefore most easily observed by comparing the S 2p3/2 peak at 162.55 eV with the S 2p1/2 peak at 164.95 eV. The intensity of the high binding energy state saturates at a partial coverage of 0.20 ML when the total thiolate coverage is ca. 0.30 ML. The low binding energy state continues to grow until it saturates at a partial coverage of 0.40 ML when the total thiolate coverage is 0.60 ML. Increasing the exposure results in no additional states until a shoulder forms at ca. 164.2 eV at coverages >0.6 ML. This state is assigned to molecular CH3SH and is removed when the sample is heated to 150 K. Heating the sample from 150 to 300 K produces very little change in the spectrum (Figure 4). There is a small decrease in the peak at 164.95 eV and a small increase in the peak at 162.55 eV. The 162.55 eV peak is slightly broader on the high binding energy side. The peak at 164.95 eV disappears at 350 K, and the shoulder on the

Figure 3. S 2p spectra from sequential exposures of CH3SH on clean W(001): (A) raw data; (B) deconvoluted data.

162.55 eV peak develops into a more well defined peak at 162.80 eV. These changes are more evident in the deconvoluted spectra. A new shoulder also develops at 161.90 eV. The spectrum at 450 K consists of two overlapping doublets. The lower energy doublet has a S 2p3/2 peak position of 161.90 eV while the higher energy doublet has a S 2p3/2 peak position of 162.60 eV. The spectra do not change between 450 and 800 K, indicating that decomposition is complete by 450 K. Above 800 K substantial changes occur as a result of a surface reconstruction that will not be considered here.7,12,16 Figure 5 shows the S 2p spectra from CH3SH adsorbed on 0.33 ML of S on W(001). The S 2p signal from the initial atomic S is concentrated in a doublet with a S 2p3/2 peak position of 163.05 eV. A much smaller doublet is (16) Overbury, S. H.; van den Oetelaar, R. J. A.; Mullins, D. R. Surf. Sci. 1994, 317, 341. (17) Mullins, D. R.; Overbury, S. H. Surf. Sci. 1988, 193, 455.

Interaction of Methanethiol with Sulfur-Covered W(001)

Figure 4. S 2p spectra from a multilayer of CH3SH on clean W(001), annealed as indicated. (A) raw data; (B) deconvoluted data.

evident with a S 2p3/2 peak position of 162.00 eV. Adsorption of CH3SH on a S-covered surface produces the same S 2p states as CH3SH on a clean surface. A low binding energy state appears at 162.55 eV, and a higher binding energy state, at 163.75 eV. These states are again assigned to CH3S. However, on the S-precovered surface the intensity of the state at 162.55 eV increases relative to that of the state at 163.75 eV, as seen in a comparison of the spectra from the 0.15 ML coverages in Figures 3 and 5. Curve fitting indicates that 0.12 ML of the thiolate is in the high binding energy state on the clean surface compared to 0.04 ML on the S-covered surface. At saturation the relative coverages of the low binding energy state to the high binding energy state are 2:1 on the clean surface and 3:1 on the S-precovered surface. All of the additional peaks in Figures 5 and 6 can be assigned to atomic S and physisorbed CH3SH. The CH3SH is removed by annealing to 150 K. The atomic S

Langmuir, Vol. 12, No. 26, 1996 6385

Figure 5. S 2p spectra from sequential exposures of CH3SH on W(001) covered with 0.33 ML of S: (A) raw data; (B) deconvoluted data. The bottom spectrum in each panel is from the S-covered surface before thiol exposure.

peak shifts from 163.05 to 162.70 eV as the CH3SH coverage is increased. The integrated intensity of the states at 163.05 and 162.70 eV is the same as the intensity in the initial atomic S spectra. S 2p spectra from CH3S on the S-covered surface after annealing look similar to those on the clean surface. The intensity of the thiolate peaks at 163.75 and 162.55 eV decreases as the sample is annealed and the intensity of the atomic S peaks near 162.7 and 162.0 eV increases. There are no additional states in the spectra. There is only a small change between 150 and 300 K mostly due to the desorption of CH3SH as well as a small increase in the atomic S peaks. At 350 K the spectrum looks somewhat sharper on the S-covered surface than on the clean surface. This is due to the shift of the atomic S peak to 162.70 eV, which moves it closer to the CH3S peak at 162.5 eV. The peak near 162.00 eV is also more intense

6386 Langmuir, Vol. 12, No. 26, 1996

Figure 6. S 2p spectra from a multilayer of CH3SH on W(001) covered with 0.33 ML of S, annealed as indicated: (A) raw data; (B) deconvoluted data. The bottom spectrum in each panel is from the S-covered surface before thiol exposure.

at 350 and 500 K, reflecting the higher atomic S coverage at these two temperatures. Discussion Preadsorbed S appears to have a relatively straightforward poisoning effect on the adsorption and reaction of CH3SH on W(001). As can be seen in Figure 2a, S linearly reduces the uptake of CH3SH on the surface. This was not anticipated on the basis of the effect of S on the reaction of CH3SH on W(211)6 and C6H5SH on Mo(110),11 where S did not significantly reduce the uptake of the thiolate. The maximum final S coverage, i.e. the sum of the initial S and the S from decomposed CH3S, is 1 ML on W(001). Despite the fact that CH3S and atomic S can adsorb in different sites (vide infra), the final coverage is limited to 1 ML, just as it is for atomic S.12 This suggests that repulsive S-S interactions, and not just site blocking,

Mullins and Lyman

influence the uptake of CH3S. The final S coverage from CH3SH on a clean surface is 0.6 ML. Since it is less than 1.0 ML, interactions between CH3S species, or between CH3S and the surface H that comes from S-H bond cleavage, control the saturation coverage. Supporting this idea, we were able to achieve a higher thiolate saturation coverage of 0.70 ML using (CH3S)2 instead of CH3SH as an adsorbate. The reduction in CH3S uptake due to S preadsorption is consistent with what has been observed on Ni(110),2 Ni(111),1 and Fe(100).4 The branching ratio between total and selective decomposition of CH3SH is constant, as shown by the ratio between H2 and CH4 desorption in Figure 2B. This is also unanticipated on the basis of the results on W(211),6 Mo(110),11 Ni(111),1 and Fe(100)4 that showed substantial increases in the methane formation channel due to S preadsorption. In these studies, as in the current experiments, large exposures of CH3SH were used in order to saturate the S-covered surfaces. It is less straightforward to compare the current results with those obtained on S-covered Ni(110).2 In that work, Huntley reported the CH4 yield for a constant CH3SH coverage that was only 25-30% of the saturation coverage on a clean surface. At this CH3SH coverage, the methane yield increased dramatically as a function of S precoverage. Our results indicate a similar behavior in the low CH3SH coverage regime. When the S precoverage is 0.8 ML, the CH3SH uptake is 0.15 ML and CH4 is produced upon decomposition (Figures 1 and 2). In contrast, we previously reported that 0.15 ML of CH3SH on clean W(001) totally decomposes.7 Therefore, the branching toward selective decomposition does increase on W(001) when comparing reduced thiol coverages. This suggests that surface coverage, of either S or CH3S, will inhibit total decomposition on W(001), but the inhibiting effect will level off when the coverage gets high enough. The only significant changes observed in the desorption spectra as a function of S precoverage are a reduction in the overall desorption intensity, consistent with the reduction in uptake, and a reduction in the peaks at low temperature relative to those at higher temperature. The S appears to selectively inhibit the low-temperature desorption state. However the ratio between the H2 and CH4 desorption intensities remains constant. Although it appears that the S is selectively inhibiting one possible reaction pathway, the different paths do not favor selective decomposition over total decomposition. The H2 desorption peaks at 360 and 425 K and the CH4 desorption peaks at 345 and 420 K do not show any significant change in desorption temperature as a function of initial S coverage. Large temperature shifts to higher desorption temperatures have been reported on Fe(100),4 Ni(110),2 and Ni(111).1 S precoverage apparently produces a dramatic change in the reaction mechanism or reaction kinetics on these surfaces whereas no such change occurs on W(001). The S 2p spectra from CH3SH adsorbed on clean W(001) at 100 K have been interpreted as resulting from CH3S adsorbed in two different sites.7 Examination of the S 2p binding energies for atomic S on various surfaces indicates that a higher S 2p binding energy correlates with a higher degree of coordination in the adsorption site.12,18-20 The same rationale has been used to assign the adsorption (18) Mullins, D. R.; Huntley, D. R.; Overbury, S. H. Surf. Sci. 1995, 323, L287. (19) Ferna´dez, A.; Espinos, J. P.; Gonza´lez-Elipe, A. R.; Kerkar, M.; Thompson, P.; Lu¨decke, J.; Scragg, G.; de Carvalho, A. V.; Woodruff, D. P.; Ferna´dez-Garcia, M.; Conesa, J. C. J. Phys.: Condens. Matter 1995, 40, 7781.

Interaction of Methanethiol with Sulfur-Covered W(001)

Langmuir, Vol. 12, No. 26, 1996 6387

site for CH3S on different surfaces.7-9 The high binding energy state with a S 2p3/2 peak at 163.75 eV is attributed to CH3S adsorbed in the fourfold hollow site. The lower binding energy state with a S 2p3/2 peak at 162.55 eV is assigned to CH3S adsorbed in a lower coordination site such as a twofold bridge site. The fourfold site is occupied first, as can be seen in Figure 3. However at 0.60 ML of CH3S there is twice as much thiolate in the low coordination site as in the fourfold site. The small increase in the intensity of the shoulder at 163 eV when the sample is annealed to 300 K (Figure 4B) indicates only a small amount of atomic S formation. Therefore, C-S bond scission does not significantly precede CH4 and H2 desorption, since the onset of CH4 and H2 desorption is near 300 K (Figure 1). The thiolate in the fourfold site largely decomposes by 350 K, as can be seen by the disappearance of the S 2p1/2 component in Figure 4B. The fate of the low binding energy species is more difficult to discern due to the overlap of the thiolate signal with the atomic S peaks. However nonlinear curve fitting and deconvolution of the instrument function both reveal significant intensity in the peak assigned to thiolate. This indicates that the C-S bond in the thiolate in the low coordination site is still largely intact at 350 K. The spectrum at 450 K is consistent with what is observed for S coverages > 0.5 ML on W(001), namely, a peak near 162.80 eV that is assigned to S in the fourfold site and a well-resolved peak at 161.90 eV. The S 2p peak position for S in a fourfold site shifts from 163.05 eV at low coverage to 162.80 eV at high coverage.12 The presence of preadsorbed S results in no new S 2p states during thiolate deposition. All of the S 2p peaks can be assigned to atomic S at 163.05, 162.80, and 161.90 eV, CH3S in either a low (162.55 eV) or high (163.75 eV) coordination site, and CH3SH at 164.2 eV. The intensity of the CH3S state at 163.75 eV is reduced on a surface covered by 0.33 ML of S. Atomic S is known to adsorb in the fourfold sites at this coverage.16 The decrease in intensity of the 163.75 eV state due to S adsorption in the fourfold sites supports the assumption that this state is due to CH3S also adsorbed in the fourfold site. There is no evidence of any additional species such as a disulfide, CH3S-S. It is not known what the binding energy of such a species would be; however, physisorbed dimethyl disulfide, (CH3S)2, has a S 2p3/2 peak position of 164.4 eV. We have attributed the S 2p intensity in this region to CH3SH. If the high binding energy S 2p intensity is due to a disulfide species, it disappears by 150 K and is not involved in the further decomposition reactions. Although S does not affect the peak positions of CH3S, the CH3S does alter the atomic S peak. As the thiolate coverage increases, the peak at 163.05 eV attenuates and a new peak forms at 162.80 eV. This can be seen in Figure 5, where a line indicates the 163.05 eV peak position. This shift is similar to what occurs as the S coverage increases (without thiolate) on W(001).12 Therefore the shift in the atomic S peak position can be interpreted as a crowding effect that results in either an electronic change, a shift in the S adsorption site, or possibly a surface reconstruction. The CH3S and atomic S adsorbates appear to intermingle rather than segregate into islands on the surface. The atomic sulfur blocks the adsorption of CH3S in the fourfold site, and the S 2p peak position of the atomic S is shifted by the adsorption of CH3S, indicating a direct interaction between the two species. Further, the atomic

S tends to spread out rather than cluster on the surface, forming a c(2 × 2) structure at 0.5 ML.12,16 The behavior of the W 4f surface state at S coverages between 0 and 0.5 ML also indicates that the S is uniformly distributed throughout this coverage range.13 The atomic S therefore does not leave open areas in which the CH3S can adsorb. As was observed on the S-free surface, there is only a slight change in the spectra between 150 and 300 K on the presulfided surface, indicating only a small amount of decomposition. At 350 K the CH3S in the high binding energy state essentially disappears. In Figure 6 it is even more difficult than in Figure 4 to determine what is happening to the low binding energy thiolate state because of the presence of the shifted atomic S peak. However the spectra continue to change between 350 and 500 K, suggesting that decomposition continues through this range. Why does S affect W(001) with respect to CH3SH decomposition so much differently than it affects the other surfaces that have been reported? First there is the apparent lack of disulfide bond formation on W(001). All of the S 2p peaks that occur for CH3SH on S-covered W(001) also occur for either CH3SH on clean W(001) or for S on W(001). Disulfide linkages were reported for CH3SH on S/W(211)6 and C6H5SH on S/Mo(110).11 Since the CH3S and S coverages are ultimately determined by S-S repulsive interactions when bonded to the substrate, CH3S must be able to bond to surface S in order for the thiolate coverage to remain high on the S-covered surface. The decrease in thiolate uptake as shown in Figure 2A and the absence of disulfide bonds in the S 2p spectra are therefore fully consistent. It is not clear why S-S bonds form on W(211) but not on W(001). Perhaps a more puzzling result is that S fails to alter the methane formation vs total decomposition branching ratio on W(001) as it has been reported to on Fe(001)4 and Ni(111).1 The explanation may rest with the adsorption sites of CH3S and S and the active site for CH3 decomposition on the various surfaces. We postulate that the active site for CH3 decomposition is the fourfold hollow site on W(001). This hypothesis is based on the observation that preadsorbed C, which adsorbs exclusively in the fourfold sites on W(001),17 enhances the methane formation reaction when CH3SH decomposes on W(001).10 Preadsorbed S in fourfold sites inhibits the adsorption of CH3S in the fourfold sites; however, the thiolate can still adsorb in low coordination sites. This leaves available hollow sites on the surface and may enable CH3 decomposition to occur. On Ni(111), atomic S and CH3S both adsorb in the threefold hollow site.18-20 The total number of occupied threefold sites is greater, and therefore presumably the active sites for CH3 decomposition are less. Finally we note that S has a very different effect on CH3SH decomposition on W(001) than does C.10 Preadsorbed C does not significantly affect the uptake of CH3S, but it greatly enhances the proportion undergoing selective decomposition to produce methane. C and CH3S appear to be able to adsorb in close proximity to one another because CH3S can adsorb on a W(001) surface with a large C coverage. Therefore CH3S can adsorb in regions where all of the surrounding hollow sites are occupied by C. S, on the other hand, cannot occupy adjacent fourfold sites.12,16 If CH3S adsorbs in a low coordination site, it will presumably be near an unoccupied fourfold site, thus allowing total decomposition to occur.

(20) Mullins, D. R.; Tang, T.; Chen, X.; Shneerson, V.; Saldin, D. K.; Tysoe, W. T. To be published.

(1) Preadsorbed S linearly inhibits the amount of CH3SH that irreversibly adsorbs on W(001).

Summary

6388 Langmuir, Vol. 12, No. 26, 1996

(2) For a saturation coverage of CH3SH, the branching between total and selective decomposition is unaffected by preadsorbed S. (3) S preferentially inhibits CH3S adsorption in the state associated with the higher binding energy S 2p state. This indicates that this state is from CH3S in a fourfold site. (4) There is no evidence of S-S bond formation between CH3S and atomic S. Acknowledgment. The authors wish to thank D. R. Huntley for helpful discussions during the preparation of the manuscript. This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy

Mullins and Lyman

Sciences, U.S. Department of Energy at Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. under contract number DE-AC0596OR22464, and in part by an appointment to the U.S. Department of Energy Postgraduate Research Program at the Oak Ridge National Laboratory administered by Oak Ridge Associated Universities. The National Synchrotron Light Source at Brookhaven National Laboratory is supported by the Division of Chemical Sciences and the Division of Material Sciences of the U.S. Department of Energy under contract DE-AC02-76CH00016. LA960362D