Reactions of Methanethiol on Cobalt-Covered Mo (110)

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Reactions of Methanethiol on Cobalt-Covered Mo(110) D. A. Chen, C. M. Friend,* and H. Xu Department of Chemistry, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138 Received August 24, 1995X The reactions of methanethiol on cobalt overlayers grown on Mo(110) were studied using a temperatureprogrammed reaction and X-ray photoelectron and high-resolution electron energy loss spectroscopies. Methyl thiolate was identified as the reaction intermediate on the basis of X-ray photoelectron and highresolution electron energy loss data. Methane, methyl radical, and H2 were the only gaseous products. The peak temperature for methane production from methyl thiolate hydrogenolysis was relatively insensitive to the Co coverage and geometric structure of the Co layer. However, less methyl radical formation was observed as the Co coverage increased from 1 to 2.5 monolayers. The selectivity for hydrocarbon formation was essentially the same, ∼48 ( 5%, for all Co coverages. The total amount of methyl thiolate deposited in the initial S-H bond breaking was independent of Co coverage. At 400 K, sulfur-induced structural rearrangement of the Co overlayer was insignificant and therefore did not affect the reaction of methanethiol. The mixed Co-S overlayer had a substantially lower activity for the methanethiol reaction than any of the clean surfaces; the total amount of reaction on both the sulfur and Co-S overlayers was 30% that of the clean Mo(110) and pure Co overlayers.

Introduction The investigation of thin epitaxial metal films on metal single-crystals has emerged as a means of systematically studying the effects of geometric and electronic structure, as well as a method for probing the superior properties of bimetallic catalysts over single-component materials.1-3 This work is motivated by an interest in the use of Co as a promoter for Mo-based catalysts in hydrodesulfurization. We are particularly interested in investigating the mechanism of desulfurization of model compounds such as thiols because of the existing body of literature on this class of reactions on other metals.4-20 Cobalt thin films grown on Mo(110) are ideal for chemical investigations because their structure and growth mode are known from previous work,1,21,22 and because they offer a range of Co-Co lattice spacings and morphologies that can be systematically varied. Cobalt forms a uniform, pseudomorphic (1×1) layer on Mo(110) for coverages up to 1 monolayer. As the Co coverage is X

Abstract published in Advance ACS Abstracts, March 1, 1996.

(1) Tikhov, M.; Bauer, E. Surf. Sci. 1990, 232, 73. (2) Campbell, C. T. Ann. Rev. Phys. Chem. 1990, 41, 775. (3) Rodriguez, J. A.; Goodman, D. W. J. Phys. Chem. 1991, 95, 4196. (4) Rufael, T. S.; Koestner, R. J.; Kollin, E. B.; Salmeron, M.; Gland, J. L. Surf. Sci. 1993, 297, 272. (5) Rufael, T. S.; Prasad, J.; Fischer, D. A.; Gland, J. L. Surf. Sci. 1992, 278, 41. (6) Mullins, D. R.; Lyman, P. F. J. Phys. Chem. 1993, 97, 12008. (7) Castro, M. E.; White J. M. Surf. Sci. 1991, 257, 22. (8) Parker, B.; Gellman, A. J. Surf. Sci. 1993, 292, 223. (9) Mullins, D. R.; Lyman, P. F. J. Phys. Chem. 1995, 99, 5548. (10) Wiegand, B. C.; Uvdal, P.; Friend, C. M. J. Phys. Chem. 1992, 96, 4527. (11) Friend, C. M.; Roberts, J. T. Acc. Chem. Res. 1988, 21, 394. (12) Castro, M. E. Ahkter, S.; Golchet, A.; White, J. M.; Sahin, T. Langmuir 1991, 7, 126. (13) Agron, P. A.; Carlson, T. A.; Dress, W. B.; Nyberg, G. L. J. Electron Spectrosc. Relat. Phenom. 1987, 42, 313. (14) Wiegand, B. C.; Friend, C. M. Chem. Rev. 1992, 92, 491. (15) Albert, M. R.; Lu, J. P.; Bernasek, S. L.; Cameron, S. D.; Gland, J. L. Surf. Sci. 1988, 206, 348. (16) Benziger, J. B.; Preston, R. E. J. Phys. Chem. 1985, 89, 5002. (17) Koestner, R. J.; Stohr, J.; Gland, J. L.; Kollin, E. B.; Sette, F. Chem. Phys. Lett. 1985, 120, 285. (18) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (19) Huntley, D. R. J. Phys. Chem. 1989, 93, 6156. (20) Beck, D. D.; White, J. M.; Ratcliffe, C. T. J. Phys. Chem. 1986, 90, 3137. (21) Kuhn, W. K.; He, J.; Goodman, D. W. J. Vac. Sci. Technol., A 1992, 10 (4), 2477. (22) He, J.; Goodman, D. W. Surf. Sci. 1991, 245, 29.

increased above 1 monolayer, compression along the [001] direction begins in order to incorporate more Co atoms into the first layer; additional low-energy electron diffraction (LEED) spots, due to double scattering between the substrate and overlayer, appear along the [001] direction in this coverage regime. Above 1 monolayer, the Co assumes an (8×2) commensurate structure, and the periodicity changes continuously from 8-fold to about 7.2-fold as the Co coverage increases to 1.3 monolayers. As the coverage is further increased, the Co atoms begin to form islands on top of the first layer, which causes changes in the relative intensities of the double-scattering spots.23,24 We have reproduced Bauer’s study of the Co on Mo(110) system and have used it as a means of investigating the chemistry of thiols on Co thin films with various lattice constants and morphologies.1 The different regimes of overlayer structure accessible in the Co on Mo(110) system offer an excellent opportunity to test for the sensitivity of thiol desulfurization to the Co geometric and electronic structure. In particular, the CoCo lattice constant varies from 2.73 Å in the pseudomorphic layer (θCo ) 1 monolayer (ML)) to 2.52 Å, the lattice constant of bulk Co, for coverages greater than 1.3 monolayers. Hence, variations in reactivity for any process sensitive to the geometric structure of the Co overlayers are expected. The desulfurization of methyl thiolate, which has a relatively strong C-S bond, was chosen as a test reaction on the Co overlayers. Methyl thiolate has been investigated on a variety of surfaces, including Fe(100),15 Ni(111),25 Ni(110),19 Ni(100),8 Ru(0001),6 Cu(100),26 and Mo(110),27 and is known to form a methyl thiolate intermediate, which further reacts to form methane. Dehydrogenation in conjunction with C-S bond cleavage leads to nonselective decomposition and competes with methane formation on most surfaces. Indeed, we find (23) Goodman et al. used somewhat different notation in one paper;22,24 they reported a transition from a (1×1) to (9×2) instead of to the (8×2) pattern reported by Bauer1 and by Goodman in his later work.21 (24) A monolayer was defined as the atomic density of Co(0001) by Goodman et al.,22 whereas we define a monolayer in terms of the number of Co atoms per Mo. (25) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1995, 99, 11472. (26) Sexton, B. A.; Nyberg, G. L. Surf. Sci. 1986, 165, 251. (27) Wiegand, B. C.; Uvdal, P.; Friend, C. M. Surf. Sci. 1992, 279, 105.

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that similar reactions occur with the same selectivity as that on Mo(110). Surprisingly, the reactivity of methanethiol is rather insensitive to the structure of the Co film over the range of coverages investigated, 1-2.5 monolayers. At Co coverages below 1 monolayer, the methanethiol reactivity resembles a convolution of reaction on clean Mo and the continuous Co films. These results suggest that methanethiol hydrogenolysis is not sensitive to the geometric structure of the Co overlayer and that the thin films are a good model for the reactivity of bulk Co. Experimental Section Experiments were performed in three separate ultrahigh vacuum chambers with base pressures of ∼2 × 10-10 Torr, which have been described in detail previously.28,29 All chambers are equipped with a computer-controlled quadrupole mass spectrometer for temperature-programmed reaction experiments and low-energy electron diffraction (LEED) optics for characterizing the overlayer structure. In addition, one chamber has a computer-controlled high-resolution electron energy loss spectrometer (LK2000), one has a cylindrical mirror analyzer (CMA) for detecting Auger electrons, and the third chamber has an X-ray photoelectron spectrometer (Physical Electronics ESCA 5300 system). The crystal was biased to -60 V during temperatureprogrammed reaction to preclude electron-induced reactions caused by the mass spectrometer. The heating rate used in the temperature-programmed reaction experiments was not completely linear but was highly reproducible. Between 200 and 450 K, the heating rate was 9 ( 2 K/s. Methanethiol (Matheson, 99.5%) was condensed into a glass bottle and purified by several freeze-pump-thaw cycles before use. Hydrogen sulfide (Matheson, 99.5%) and deuterium sulfide (Cambridge Isotope Laboratories, 98% D) were used as received. The purities of all gases were confirmed by mass spectrometry. Cobalt was evaporated by passing current through three tungsten wires (0.005 inch diameter) wrapped around a Co wire (99.997% purity, 0.02 inch diameter). This assembly was enclosed in a water-cooled copper shroud so that the pressure rise during evaporation was less than 2 × 10-10 Torr. The Co was deposited onto Mo(110) at 100 K, and the sample was subsequently annealed to 760 K for 60 s. The Co overlayers were characterized with LEED, Auger electron spectroscopy, and temperature-programmed desorption. Cobalt coverages were estimated by comparing LEED patterns with those reported previously by Bauer et al.1 A monolayer, defined as the atomic density of Mo(110), was determined by the transition of the (1×1) to (8×2) LEED pattern as the Co evaporation time was increased. Other coverages were estimated on the basis of the dosing time. The intensity of the Co(MMM), 53 eV Auger electron peak and the integrated area of the Co(2p3/2) X-ray photoelectron peak both increased linearly with dosing time. The Co flux was found to be constant within 4% on any given day based on the Co(2p3/2) intensity. There was no indication of Co-Mo alloy formation under the conditions of our experiments. Specifically, we did not observe the 0.1-0.15 eV shift in the Mo(3d5/2) binding energy after annealing the pure 1-monolayer Co film at 760 K for 60 s; such a shift was reported previously for the pure Co overlayers deposited at 300 K and annealed to 700 K30 and was attributed to Co-Mo alloying. There were also no changes in the Co and Mo peak shapes, positions, and integrated areas before and after annealing to 760 K. Furthermore, the “complex” LEED pattern, which was attributed to Co-Mo alloying in Bauer’s work, was not observed for the Co coverages studied here.1 In addition, there was no long-term accumulation of Co when our studies were restricted to coverages on the order of 1 ML; Co accumulation was observed after repeated experiments using Co coverages on the order of 5 ML; because of this, repolishing the crystal was required. Taken together, these results all indicate that Co-Mo alloys are not formed in our experiments, in agreement with the lack of alloy formation in previous work by Goodman et al.22 The studies that reported alloy formation1,30 did not specify the (28) Xu, X.; Friend, C. M. J. Am. Chem. Soc. 1991, 113, 6779. (29) Roberts, J. T.; Friend, C. M. J. Am. Chem. Soc. 1986, 108, 7204. (30) Kuhn, M.; Rodriguez, J. A., submitted for publication, 1995.

Langmuir, Vol. 12, No. 6, 1996 1529 annealing time, and the complex LEED pattern was only reported for high Co coverages.1 The crystal was cleaned after each experiment by heating in 3 × 10-9 Torr of oxygen for 5 min at 1200 K. Oxygen was removed by flashing to ∼2370 K for 30 s. Cobalt and sulfur were removed from the surface by heating the crystal above 2000 K. No residual Co signal was detected by X-ray photoelectron spectroscopy, and the LEED pattern was a sharp (1×1). The sulfur overlayers were prepared by depositing a saturation dose of H2S on the crystal surface at 100 K. H2S multilayers desorbed below 150 K, and hydrogen was removed by heating to 760 K. A c(2×2) LEED pattern was observed for the sulfurcovered Mo(110) surface after annealing the saturated H2S layer to 760 K for 60 s. The S:Mo ratio, which is the integrated area of the S(2p) region in the X-ray photoelectron spectrum divided by the area of the Mo(3d) region, verified that the sulfur coverage was 0.37 ( 0.2 monolayers. The sulfur coverage was calibrated using the S:Mo ratio for a sulfur overlayer with a sharp p(4×1) LEED pattern known to correspond to 0.5 monolayers. The amount of sulfur deposited on the Co overlayers was determined by comparing the D2 yield from decompositon of D2S on Mo(110) to the Co overlayers. The total deuterium yield was calculated from the D2 signal plus half of the HD signal, assuming that the hydrogen incorporated into HD was from the background. No products other than D2 were formed during the temperatureprogrammed reaction of D2S on any surface investigated. For Co coverages of 0-2.5 monolayers, the integrated amount of deuterium was identical within experimental error. Thus, the same amount of sulfur was deposited on Mo(110) as on all of the Co overlayers investigated. Auger spectra were obtained with a primary beam energy of 1 keV. Electron energy loss data were acquired with a primary beam energy of 3 eV at a resolution of 60-70 cm-1. X-ray photoelectron data were collected using a Mg KR (1253.6 eV) source and a bandpass energy of 17.9 eV. All binding energies were referenced to the Mo(3d5/2) peak at 227.9 eV. For the Co-S overlayers, the S(2p), Mo(3d), and Co(2p3/2) regions were collected in a single experiment, resulting in a total collection time of 7.3 min. For methanethiol on the Mo(110) and Co-covered surfaces, the S(2p), Mo(3d), and C(1s) regions were collected for a total acquisition time of 9 min. Curve fitting was carried out using the software provided with the ESCA 5300 system. The S(2p3/2): S(2p1/2) ratio was fixed at 2:1, and the energy splitting was fixed at 1.2 eV; peakshapes were 85% Gaussian and 15% Lorentzian. Temperature-programmed reaction spectra collected after acquisition of X-ray photoelectron data were the same as those collected without exposure to X-rays; thus, there were no significant X-ray-induced effects.

Results X-ray Photoelectron Studies of Co-S Overlayers. There are no substantial changes in the electronic structures of either Co or Mo when Co is deposited on clean Mo(110), based on the correspondence in the binding energies for the thin films and bulk materials. Similarly, no significant shifts are observed for Co-S overlayers on Mo(110).31 The Co(2p3/2) binding energies were measured to be 778.3 eV for Co coverages in the range of 0.25-2.5 monolayers, which is identical to the binding energy reported for bulk Co.32 The Co(2p3/2) binding energies change only insignificantly when a mixed phase of sulfur and Co are present. For example the Co(2p3/2) peak shifts by +0.10 eV when 0.37 monolayers of sulfur are deposited on 1.3 monolayers of Co and annealed at 760 K. While this shift is reproducible, the reported binding energy for bulk Co is 0.1 eV higher than that for Co9S8 powder.33 Thus, the +0.1 eV shift reported here is not necessarily indicative of Co sulfide formation. Unlike Rodriguez’s study of Co-S overlayers on Mo(110),30 we do not observe a change in the Mo(3d) region due to Co-promoted sulfur (31) Chen, D. A.; Friend, C. M. Manuscript in preparation. (32) Handbook of X-ray Photoelectron Spectroscopy; Moulder, J. F., Stickle, W. F., Sobol, P. E., Bomben, K. D.; Perkin-Elmer Corporation, Physical Electronics Division, Eden Prairie, MN, 1992. (33) Parham, T. G.; Merrill, R. P. J. Catal. 1984, 85, 295.

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Figure 1. Temperature-programmed reaction data for a saturation exposure of methanethiol on clean Mo(110). The methyl radical signal shown is the 15 amu signal corrected for contributions from methane cracking using 16:15 ) 1.18. The multilayer was removed by heating to 150 K before data collection.

dissolution into the Mo. This difference is attributed to the lower sulfur coverages (0.37 monolayers) in our experiments compared to those of Rodriguez’s (0.9 monolayers). X-ray photoelectron data suggest that restructuring of the Co overlayer does not play a role in the methanethiol reaction. Although X-ray photoelectron studies of the Co-S overlayers annealed to 760 K show sulfur-induced aggregation of Co atoms as described in detail elsewhere,31 aggregation is not significant at lower temperatures. When the Co-S overlayer is annealed at 400 K for 60 s, the Co and sulfur signals are identical to those of the overlayer heated to 160 K within the error of the experiment (2%). Indeed, there are no significant changes in the X-ray photoelectron intensities measured after heating mixed phases of Co and sulfur to 400 K for 3 min. Temperature-Programmed Reaction Studies. On the clean Mo(110) surface, methane, methyl radical, and H2 are the only gaseous products detected during temperature-programmed reaction of methanethiol (Figure 1).34 No new products are detected from methanethiol reaction on Co overlayers of various coverages in experiments monitoring a broad mass range of up to 80 amu. Sulfur and carbon remain on the surface after temperature-programmed reaction up to 760 K in all cases. Methyl radical evolution was not reported in our earlier study of methanethiol on Mo(110),27,35 but it was confirmed in this work by mass spectrometry and infrared spectroscopy. Methyl was identified as a second hydrocarbon product when we searched for additional carbon-containing products to resolve the inconsistencies between temperature-programmed reaction yields and X-ray photoelectron intensities (see next section). (34) Saturation coverage is achieved in all cases by employing methanethiol exposures sufficiently large to produce multilayer condensation. The multilayers sublime at ∼120 K and leave behind the saturated layer. (35) The 15:16 amu ratio was not carefully analyzed in our previous work. Furthermore, the methyl radicals were very difficult to detect when the moveable flag in front of the mass spectrometer was closed, presumably because radicals react on the flag before reaching the detector.

Chen et al.

Figure 2. Methane and methyl radical yields from methanethiol reaction as a function of Co coverage. The yields are relative to methane and methyl radical produced on clean Mo(110). The estimated error bars for the methane signal are smaller than the width of the plot symbol.

The difference in peak shapes and temperatures in the 15 and 16 amu profiles suggests that both methane and methyl radical are formed from methanethiol reaction on Mo(110) (Figure 1). The 15 amu peak is centered at 280 K, ∼20 K lower than the 16 amu peak, while the 16:15 amu ratio was 0.74 ( 0.10 for the product and 1.18 ( 0.02 for an authentic methane sample. The methyl radical accounts for ∼35% of the 15 amu signal. Product-trapping infrared reflection absorption spectroscopy (PT-IRAS) confirmed the evolution of a methyl radical.36 The methyl radical was produced from methanethiol reaction on a Mo(110) crystal and then trapped on a second Mo(110) crystal.37 The infrared spectrum of the trapped product demonstrates that methyl is produced from methanethiol reaction, since the methyl deformation at 1247 cm-1 is the dominant feature in the spectrum (data not shown), in agreement with earlier studies of methyl on Mo(110).36,38 The ratio of CH4:CH3 depends on the Co coverage prior to reaction. As the Co coverage is increased from 0 to 2.5 monolayers, the 16:15 amu ratio increases, indicating a decrease in methyl radical production (Figure 2). At a Co coverage of 2.5 monolayers, the 15 amu signal corrected for the methane contribution is ∼25% that of the clean surface, and the 16:15 ratio approaches that for pure methane, 1.08 ( 0.50. For comparison, the 16:15 amu ratio is 0.74 ( 0.10 on Mo(110) and 1.01 ( 0.10 for 1 monolayer of Co. The 15 amu peak shape also broadens as the Co coverage increases, and peak temperature shifts from 280 K on Mo(110) to 290 K on the surfaces where the Co coverage is greater than or equal to 0.5 monolayers (data not shown). The methane peak temperature is relatively insensitive to the Co coverage over the entire range investigated (Figure 3a). There is almost no difference in the methane peak position or shape for reaction on 0-0.5 monolayers of Co. As the Co coverage is increased from 0.5 to 1.3 monolayers, the methane peak is centered at 300 K, (36) Weldon, M. K.; Friend, C. M. Rev. Sci. Instrum. 1995, 66 (11), 5192-5195. (37) Methane was initially trapped on the second crystal along with the methyl radical, but methane desorbs after heating to 180 K whereas the methyl radical remains intact on the surface up to 250 K.38 (38) Weldon, M. K.; Friend, C. M. J. Am. Chem. Soc., submitted for publication, 1995.

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Figure 3. Temperature-programmed reaction data for a saturation exposure of methanethiol of various Co overlayer coverages: a) Methane (16 amu) signal × 1; b) Hydrogen (2 amu) signal × 2. Signals were not corrected for the relative sensitivities of the mass spectrometer to hydrogen and methane. The multilayer was removed by heating to 150 K before data collection.

whereas on the 1.3 monolayer Co surface the main peak is at ∼280 K with a shoulder at 310 K. On 2.5 monolayers of Co, the methane peak at 280 K becomes significantly sharper. The H2 peaks in the temperature-programmed reaction data for a saturated methanethiol layer also depend only weakly on the Co coverage (Figure 3b). The temperature range for H2 evolution is similar for all Co coverages, but the intensity distribution throughout this range changes. Most of the H2 is produced in the range of 200-400 K. At Co coverages less than 0.75 monolayers no H2 is evolved below 250 K; however, at Co coverages above 0.75 monolayers H2 is also evolved between 200 and 250 K, and at 2.5 monolayers additional structure develops at ∼300 K.39 Dihydrogen evolution continues up to ∼600 K, indicating the presence of intact C-H bonds in this temperature regime, given that D2 desorbs from Co thin films at lower temperatures.22 For example, D2 desorbs from a surface with 1.3 monolayers of Co between 275 and 400 K with a smaller peak centered at 490 K. Most of the methane produced from methanethiol involves the formation of a single C-H(D) bond, determined on the basis of isotopic exchange experiments. A minor amount of reversible C-H bond activation is also induced by the Co overlayers during the course of methanethiol hydrogenolysis. Although a maximum of one deuterium is incorporated into methane produced on Mo(110), a maximum of two deuterium atoms is incorporated into methane formed on 1.5 monolayers of Co, with methane-d2 accounting for approximately 4% of total methane production. (39) This H2 feature becomes two distinct peaks for the methanethiol reaction on 12 monolayers of Co.

The product distributions and yields are changed when methanethiol reacts on either sulfur-covered Mo(110) (θs ) 0.37 monolayers) or a Co1.0-S0.37 overlayer heated to 400 and 760 K. The only products detected during temperature-programmed reaction are methane, methyl radical, and dihydrogen for both the sulfur-covered and Co-S surfaces (data not shown). On sulfur-covered Mo(110), both the dihydrogen and methane peaks are shifted to higher temperatures by ∼50 K relative to that of the clean surface. The methyl radical peak intensity is reduced to 30% of that of the Mo(110) surface. The 15 amu peak temperature is also ∼100 K higher on sulfurcovered Mo(110) relative to that of clean Mo(110). On the Co-S overlayer annealed to 760 K, methane and methyl radical are produced at ∼350 K, and the methyl radical yield is 0.39 times that of the 1-monolayer Co surface. On Co-S overlayers prepared by heating 1 monolayer of Co covered by 0.37 monolayers of sulfur to 400 K, methane is evolved in a broad peak centered at ∼380 K, and methyl radical is formed at 450 K; both of these temperatures are higher than the respective evolution temperatures on the 760 K Co-S and pure sulfur overlayers. As mentioned previously, a significant Co aggregation is induced upon annealing to 760 K, but not upon heating to 400 K.31 X-ray Photoelectron Studies of Methanethiol. Two surface species containing intact C-S bonds are detected with S(2p3/2) binding energies of 163.2 and 162.3 eV after heating of a saturation exposure of methanethiol on Cocovered Mo(110) to 190 K (Figure 4, Table 1). The higher binding energy is nearly identical to that of S(2p3/2) for methyl thiolate on Ni(110) (163.0 eV)19 and Fe(100) (163.2 eV),15 but slightly higher than that for methyl thiolate on

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Table 1. Summary of X-Ray Photoelectron Binding Energies and Curve-Fitting Parameters S(2p1/2)

surface 0.5 ML S on Mo(110 0.37 ML S on 1 ML of Co Methanethiol on Mo(110) at 190 K Methanethiol on 1 ML of Co at 190 K Methanethiol on 1 ML of Co at 400 K

binding energy, eV area, counts eV/s peak width, eV binding energy, eV area, counts eV/s peak width, eV binding energy, eV area, counts eV/s peak width, eV binding energy, eV area, counts eV/s peak width, eV binding energy, eV area, counts eV/s peak width, eV

Figure 4. S(2p) X-ray photoelectron spectra for (a) sulfur (0.37 monolayers) on Co (1 monolayer) annealed to 760 K for 60 s and following methanethiol adsorption on one monolayer of Co after heating to (b) 190 K, (c) 400 K, and (d) 760 K.

Mo(110) (162.7 eV).27 The S(2p3/2) peaks are clearly not due to adsorbed sulfur, which has a binding energy of 161.7 eV on the Co-covered surfaces. Furthermore, the single C(1s) peak at 285.0 eV has a binding energy that is too high for a hydrocarbon species. The shoulder at 162.3 eV decreases with decreasing Co coverage and does not appear in the spectrum of methyl thiolate on Mo(110). A small broad peak at 164.4 eV was added to the curve fits to account for the high binding energy tail in the S(2p)

163.2 1018 1.80 163.2 940 1.80 164.3 505 1.80 164.4 351 1.80 163.2 1268 1.80

162.6 2563 0.85 162.9 1917 1.05 163.8 1942 1.20 164.4 1385 1.00 162.7 1737 1.10

163.5 550 1.05

S(2p3/2) 161.4 5127 0.85 161.7 3834 1.05 162.6 3885 1.20 163.2 2770 1.00 161.6 3475 1.10

162.3 1100 1.00

C(1s)

285.0 2370 1.20 285.0 2170 1.10 283.1 1599 1.00

region. This tail is also observed with a similar relative intensity for atomic sulfur on the Co overlayers as well as on Mo(110) and is attributed to final state effects, not to another state of sulfur.40-42 After heating to 400 K, which is above the temperature of methane and methyl radical evolution, all of the C-S bonds are broken as determined on the basis of the X-ray photoelectron data (Figure 4). The S(2p3/2) peak at 161.6 eV has the same energies as that measured for atomic sulfur on Co-covered Mo(110) deposited from H2S decomposition. The C(1s) peak at 283.0 eV is characteristic of a strongly bound C1 species, such as CH, CH2, or atomic carbon.27 Furthermore, no change in the X-ray photoelectron sulfur and carbon intensities occurs after heating to 760 K. On the basis of X-ray photoelectron data for all Co coverages, the saturation coverage of methanethiol on the Co overlayers is 0.37 ( 0.05 monolayers, which is nearly identical to saturation coverage on clean Mo(110). The saturation coverage of methanethiol was previously estimated to be 0.3 monolayers on Mo(110) using Auger electron spectroscopy27 and confirmed on the basis of the integrated intensity of the S(2p) region after heating condensed methanethiol to 190 K. The S(2p) intensity is approximately constant for Co coverages ranging from 0 to 2.5 monolayers. Although the intensity is ∼10% lower on the 2.5 monolayer Co surface than on Mo(110), this difference is close to the estimated experimental error (∼7%). The integrated intensity in the C(1s) region was also nearly independent of Co coverage, indicating the lack of sensitivity of the saturation coverage to the initial Co coverage. The selectivity for gaseous hydrocarbon formation is 48 ( 5% and essentially independent of Co coverage, determined on the basis of the C(1s):S(2p) intensity ratio. The S(2p) signal is a measure of total reaction since no sulfurcontaining products leave the surface during heating to 400 K, whereas the C(1s) signal reflects the amount of nonselective decomposition. Therefore, the C:S ratio indicates the relative amount of nonselective decomposition versus hydrocarbon formation. The C:S ratios were the same within 4% for all of the surfaces studied.43 The relative amounts of decomposition are reliably calculated from the C:S ratio at 400 K, because no significant structural rearrangement occurs at this temperature. X-ray photoelectron studies of methyl thiolate as a function of annealing temperature on 1 monolayer of Co (40) Weldon, M. K.; Napier, M. E.; Wiegand, B. C.; Friend, C. M.; Uvdal, P. J. Am. Chem. Soc. 1994, 116, 8328. (41) Xu, H.; Friend, C. M. J. Phys. Chem. 1993, 97, 3584. (42) The high binding energy shoulder is consistently 10-20% of the total integrated S(2p) area for methanethiol on Co heated to various temperatures, sulfur on Co, and various ordered overlayers of sulfur on clean Mo(110).

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Discussion

Figure 5. High-resolution electron energy loss spectra of methanethiol condensed at 100 K on the following: (a) clean Mo(110) and unannealed; (b) clean Mo(110) and heated to 190 K; and (c) a 1.3 monolayer Co overlayer and heated to 190 K.

show that the methyl thiolate does not induce major Co aggregation. There is a 2% increase in Co intensity for the surface heated to 400 K relative to that heated to 190 K; this can be attributed to screening of the photoelectrons from Co by the carbon in methanethiol. At 400 K, the sulfur signal also increases by 1% compared to data obtained after heating to 190 K. These data indicate that there is little structural change in the Co overlayer upon heating methyl thiolate to 400 K. High-Resolution Electron Energy Loss Studies. High-resolution electron energy loss data indicate that methyl thiolate is formed upon adsorption of methanethiol on the Co overlayers at 100 K and heating to 190 K, as shown for a Co coverage of 1.3 monolayers (Figure 5). The data are qualitatively similar to those for methyl thiolate on other metal surfaces: specifically, Mo(110),27 Fe(100),15 Ni(110),19 Ni(111),25 and Cu(100).26 The absence of the S-H stretch, observed at 2530 cm-1 for methanethiol multilayers, indicates that the S-H bond is broken on the Co-covered surface. The modes are otherwise the same as those for methyl thiolate on clean Mo(110)27 and are in agreement with those made for methyl thiolate on other metal surfaces.15,19,25,44 The differences in relative intensities for the Co-covered surfaces cannot be interpreted in detail because of possible surface corrugation. (43) The percentage of decomposition was calculated by multiplying the C:S ratio by 1.69, which is the sensitivity correction factor for the carbon and sulfur signals. This sensitivity correction factor was calculated from the X-ray photoelectron spectrum of thiophene on Mo(110) heated to 760 K since thiophene reacts only by decomposing to surface carbon and sulfur.

The surface reactions of methanethiol on Co-covered Mo(110) are remarkably insensitive to the geometric and electronic structure of the surface. The reactivity of methanethiol reported here is characteristic of uniform Co layers and does not involve a substantial amount of Co restructuring during the course of reaction. No evidence for Co aggregation is observed in the X-ray photoelectron data in the temperature range where CH3 and CH4 are formed. The reactions on Co-covered Mo(110) are similar to those on Mo(110) itself in that no new products are formed and that the total amount of reaction, as well as the selectivity for hydrocarbon formation, are similar for all Co coverages up to 2.5 monolayers. Indeed, the saturation coverage of methanethiol is also similar to that on Ni(100),12 Ni(110),19 Fe(100),15 Ru(0001),6 and W(100),45 which represent a range of different packing densities. These data suggest that the number of coordination sites, not intermolecular interactions, determine the saturation coverage. Furthermore, the reactions of methanethiol on the Co overlayers are qualitatively similar to those on Fe(100),15 Cu(100),26 W(100),45 W(211),16 Ru(0001),6 Ni(111),25 and Mo(110):27 the thiolate intermediate is formed below 200 K, and methane formation competes with nonselective decomposition. Even the selectivity for hydrocarbon formation is similar on different surfaces: 48 ( 5% on the Co films compared to ∼50% on Mo(110),46 ∼50% on W(100),45 and ∼40% on Pt(111).17 Although no methyl radical evolution was reported for other surfaces, which might indicate a difference between Mo(110) and other cases, it is possible that methyl radical evolution was simply not detected, given that a detailed comparison of the 16:15 amu ratio and peak shapes for these masses is necessary to test for the radical production. No specific comments were made regarding methyl evolution from other surfaces. The product-trapping infrared experiments provide convincing evidence that methyl radical is formed from methyl thiolate on Mo(110) based on the characteristic methyl deformation mode at 1247 cm-1.36 Other than the change in the CH4:CH3 ratio, the only effect of increasing Co coverage is a minor change in the kinetics of methane formation. For example, the peak temperature for methane evolution is 300 K on clean Mo(110) and 280 K on 1.3 monolayers of Co. Furthermore, the methane peak shape is slightly narrower on the higher coverage Co overlayers, and the onset of H2 evolution occurs at lower temperatures for Co coverages g1.0 monolayers. The lack of sensitivity of reaction methanethiol is different from other studies of the chemistry of ultrathin metal films. For example, the electronic and chemical properties of Pd deposited on Nb(110),47 Mo(100),48 Ru(0001),49 and Re(0001),49 as well as Ni on Mo(110),50 were extremely sensitive to the admetal coverage and structure. The investigations of Pd thin films used molecules with π-bonds, such as ethylene and CO, as probes for chemical differences. This comparison suggests (44) Xu, H.; Uvdal, P.; Friend, C. M.; Stohr, J. Surf. Sci. 1993, 289, L599. (45) Mullins, D. R.; Lyman, P. F.; Overbury, S. H. Surf. Sci. 1992, 277, 64. (46) The selectivity of hydrocarbon formation from methanethiol on clean Mo(110) was previously calculated as 40 ( 5% by X-ray photoelectron spectroscopy.44 However, the selectivity determined by Auger electron spectroscopy (49 ( 5%) is consistent with the current selectivity of 47 ( 5% determined by X-ray photoelectron spectroscopy. (47) Neiman, D. L.; Koel, B. E. Mat. Res. Soc. Symp. Proc. 1987, 83, 143. (48) Heitzinger, J. M.; Gebhard, S. C.; Koel, B. E. Surf. Sci. 1992, 275, 209.

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that thiol bonding and reactivity is less sensitive to alteration in the geometric and electronic structure of the metal films and is consistent with the remarkable similarity of methanethiol chemistry on a wide range of surfaces. Methyl thiolate is unequivocally identified as the major surface intermediate up to the onset of hydrocarbon formation determined on the basis of both X-ray photoelectron and vibrational data. Previously, methyl thiolate was identified on Mo(110) on the basis of the absence of the S-H stretch mode in the electron energy loss spectrum and the S(2p3/2) binding energy of 162.8 eV.27 Similarly, the S-H stretch mode is absent on Co-covered Mo(110), and the S(2p) binding energies are too high to be associated with atomic sulfur. For example, the main S(2p3/2)51 peak is at 163.2 eV on 1 monolayer of Co on Mo(110) as compared to 161.7 eV for atomic sulfur on this surface. Indeed, the vibrational spectra obtained following methanethiol adsorption on Mo(110) and Co-covered Mo(110) are nearly the same, indicating that the thiolate is present in both cases. The C(1s) binding energies are also consistent with the presence of intact C-S bonds on both clean and Cocovered Mo(110); the C(1s) binding energy of 285.0 eV measured following methanethiol adsorption on 1 monolayer of Co and heating to 190 K is too high to be associated with a CHx species, which should have a binding energy below 284 eV.27 X-ray photoelectron data clearly demonstrate that the thiolate remains intact up to the temperature required for hydrocarbon formation. At 400 K there is a single S(2p3/2) peak at 161.6 eV characteristic of atomic sulfur and a single C(1s) peak at 283.1 eV indicating the presence of an adsorbed CHx species. There is no evidence for any stable, partially dehydrogenated intermediates such as CH2dS, which was proposed to form from methylthiolate reaction on Pt(111).17 However, the isotopic exchange experiments demonstrate that there is a minor amount (