Toluene Formation from Coadsorbed Methanethiol and Benzenethiol

Chem. B , 2001, 105 (39), pp 9548–9556. DOI: 10.1021/jp012322a. Publication Date (Web): September 11, 2001. Copyright © 2001 American Chemical Soci...
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9548

J. Phys. Chem. B 2001, 105, 9548-9556

Toluene Formation from Coadsorbed Methanethiol and Benzenethiol on Nickel Surfaces S. M. Kane, D. R. Huntley,*,† and J. L. Gland UniVersity of Michigan, Ann Arbor, Michigan, and Oak Ridge National Laboratory, Oak Ridge, Tennessee ReceiVed: June 19, 2001

Carbon-carbon bond formation resulting in cross-coupling has been demonstrated for coadsorbed benzenethiol and methanethiol on the Ni(111) and Ni(100) surfaces. Toluene formation indicates that desulfurized C6 and C1 intermediates are formed by direct interaction with the Ni surface rather than by hydrogen addition to the C-S bond. Coupling occurs in the same temperature range as hydrogenation of the C6 and C1 intermediates to form benzene and methane. Thus, competition between hydrogenation and cross-coupling plays an important role in controlling reaction selectivity. As surface hydrogen increases, the yield of toluene falls. Reduction of surface hydrogen by reaction with coadsorbed oxygen enhances toluene formation. The effect of coadsorbed hydrogen is larger on the Ni(111) surface where large amounts of coadsorbed hydrogen remain on the surface above the reaction temperatures. On both surfaces the toluene yield increases rapidly for coadsorbed reactant coverages above half saturation, indicating toluene formation is not limited exclusively by stoichiometries but also by kinetic factors. Methyl mobility appears to play a key role in determining toluene yields. The complex dependence of toluene yield on the surface concentrations of both the methanethiol and benzenethiol is consistent with a radical mechanism where both kinetic and stoichiometric factors play a role in determining yields. Toluene formation in the 300 K range appears to be related to the reaction of phenyl and methyl, while toluene formed at higher temperature appears to be associated with hydrogenation of a more extensively dehydrogenated intermediate.

Carbon-carbon bond formation is often based on ionic mechanisms1 involving reaction of alkanes and olefins in HF or H2SO4 aqueous superacid. Friedel-Crafts reaction of aromatic systems with alkyl halides over an AlCl3 catalyst is typically used for aromatic alkylation.2,3 Ionic mechanisms that result in carbon-carbon bond formation are also observed on ionic surfaces of solid acids,4,5 zeolites,6,7 and molten salts.8 More complex reaction mechanisms that involve radical-like organometallic intermediates are observed on transition metal complexes.9,10 Radical based mechanisms for carbon-carbon bond formation on metal and metal carbide surfaces also play an important role in formation of higher weight organic molecules from CO and H2 during on Fischer-Tropsch chemistry.11-13 The mechanisms of carbon-carbon bond formation reactions have been studied on several metal surfaces using organohalide species to generate the adsorbed reaction intermediates on Cu,14-20 Au,21-23 and Pd.24 The use of halides, especially iodo compounds, provides a bond which can be readily broken either by thermal processes or by photodissociation. Reaction of organohalides results in the formation of high concentrations of adsorbed surface alkyl groups, as well as vinyl and phenyl groups on less reactive surfaces. Hydrocarbon coupling is observed as the primary reaction from such surface groups on inactive metal surfaces, with dehydrogenation being only a minor pathway. Carbon-carbon bond formation has been reported previously for several more active metal surfaces. Recently, the reactions of adsorbed methyl radical formed in the gas phase from * To whom correspondence should be addressed. † Oak Ridge National Laboratory. Current address: Department of Chemistry, Saginaw Valley State University, University Center, MI.

diazomethane thermolysis have been characterized on Pt.23,25 Coupling and dehydrogenation results in ethylene and benzene formation from methyl groups on the Ni(111) surface.26,27 A small amount of ethane resulting from methyl coupling was observed during methanethiol decomposition on nickel surfaces.28,29 Recently, we reported toluene formation from crosscoupling of coadsorbed methanethiol and benzenethiol on the Ni(111) surface.30 The surface reactions of a range of thiols have been studied on both active and inactive metal surfaces31-45 which provides a solid background for studying carbon-carbon bond formation from adsorbed thiols. On most metal surfaces, the sulfur hydrogen bond dissociates on adsorption, leaving surface hydrogen and adsorbed thiolate.31 On inactive metal surfaces C-S bond breaking is limited, resulting in condensation of thiolates to form disulfides. Thus, cross-coupling on inactive metals is quite limited. Direct interaction between active metal surfaces and the C-S bond forms adsorbed desulfurized intermediates, which can either be hydrogenated or couple on the surface. The selectivity of these competitive processes depends on the availability of free surface hydrogen at reaction temperature.30 A brief review of the reactions of methanethiol and benzenethiol on the Ni(111) and Ni(100) surfaces will provide the foundation for our description of the cross-coupling reaction mechanism. Each thiol forms adsorbed hydrogen and the corresponding thiolate following sulfur-hydrogen bond scission at 100 K on both the Ni(111) and Ni(100) surfaces. For all the primary desulfurized desorbing products, C-S bond activation by the Ni surface is the rate-limiting step. Following C-S bond scission, rapid hydrogenation of surface intermediates leads to low-temperature methane or benzene formation. The dominant

10.1021/jp012322a CCC: $20.00 © 2001 American Chemical Society Published on Web 09/11/2001

Toluene Formation benzene peak at 260 K on the Ni(111) surface results from hydrogenation of a phenyl intermediate formed by C-S bond breaking in phenylthiolate.34,46 The second small benzene peak at 290 K results from hydrogenation of a dehydrogenated “benzyne”36 group. In this same temperature range hydrogen availability is affected by sulfur induced reconstruction of the Ni(111) surface.47-49 C-S bond breaking in methanethiol on the Ni(111) surface results in formation of methyl which is rapidly hydrogenated to form methane at 270 K. Higher temperature methane is observed at 290 K and is associated with hydrogen from surface reconstruction, as noted above.28 Toluene formation is observed in this same 270 K to 290 K temperature range from the coadsorbed thiols.30 On the Ni(100) surface, C-S bond scission in phenylthiolate in the 200 K to 350 K range leads to formation of phenyl groups which are rapidly hydrogenated to form benzene.50 On the Ni(100) surface, C-S bond breaking in methylthiolate results in a broad methane peak which starts at 200 K, and peaks at 300 K.29 Toluene formation is observed within this temperature range for coadsorbed thiols. Hydrogen adsorbs dissociatively on both the Ni(111) and Ni(100) surfaces and recombines to desorb as gas-phase hydrogen with increasing temperature.51 On a hydrogen saturated Ni(111) surface, desorption begins near 250 K with the peak occurring at 380 K. On the sulfided Ni(111) surface, the hydrogen desorption peak temperature decreases to 300 K.28 On the clean Ni(100) surface, hydrogen desorption begins near 200K with the peak occurring at 300 K. On the sulfided Ni(100) surface, hydrogen desorption begins near 150 K with a peak at 240 K.52 During the course of the cross-coupling reactions studied in this work, hydrogen is coadsorbed with a mixture of thiolates and adsorbed sulfur. Generally, hydrogen desorption should be expected in an intermediate temperature range since coadsorbed species containing sulfur decrease the stability of free hydrogen. Our initial report of toluene formation from coadsorbed methanethiol and benzenethiol on the Ni(111) surface shows that C-S bond activation by the Ni surface is rate limiting for toluene formation just as it is for the competing hydrogenolysis reactions which form benzene and methane.30 Competition between cross-coupling and hydrogenation clearly controls reaction selectivity on this surface. This paper discusses a complete TPD study of the cross-coupling reaction and compares the results from the Ni(111) and Ni(100) surfaces. On both surfaces, reaction of adsorbed phenyl with adsorbed methyl is responsible for low-temperature toluene formation. The stability of free hydrogen is much larger on the Ni(111) than that on the Ni(100) surface, resulting in enhanced hydrogenolysis in the presence of coadsorbed hydrogen. Experimental Section All experiments were performed in a surface science apparatus with facilities for TPD, HREELS, AES, LEED, and sputter cleaning as described previously.32 The Ni(111) samples were mounted on a liquid nitrogen cooled manipulator that could be cooled to 90 K and heated resistively to 1000 K via tungsten support wires. A thin type E thermocouple was spot-welded to the back of the crystal for temperature measurement and control. Temperature-programmed desorption experiments were performed using an unshielded multiplexed quadrupole mass spectrometer (qms). During experiments, the sample was placed within 1 cm of the qms to ensure detection of desorbing species. The sample was biased at -70 V to eliminate the possibility of electron stimulated desorption or decomposition, which could interfere with these mechanistic studies. The heating rate used

J. Phys. Chem. B, Vol. 105, No. 39, 2001 9549

Figure 1. The reaction products from a 0.25 MLE exposure of methanethiol followed by 0.21 MLE exposure to benzenethiol on the Ni(111) surface. Spectra are identified as follows: (a) m/e ) 92 (×100), (b) m/e ) 78 (×10), (c) m/e ) 16, (d) m/e ) 2.

for all TPD experiments was 5 K/s. HREELS data was obtained using a LK2000 EEL spectrometer with an experimental resolution of 4-6 meV. No sulfur containing species desorb from the surface below monolayer coverages, so that the S(152 eV) Auger peak following complete thermal reaction was used to monitor the initial coverage of thiol. To determine the coverage of the thiols on the nickel surfaces, the ratio of the sulfur Auger peak at 152 eV to the nickel Auger signal at 850 eV was first calibrated against an established structure of sulfur with a known sulfur coverage. This structure was obtained by repeated exposure to H2S followed by annealing to 800 K to remove hydrogen. After each exposure and anneal, hydrogen desorption was monitored until no hydrogen desorption was observed, the resultant LEED pattern was examined and the AES spectrum was taken until no increases in Auger ratio were observed. At the start of each day, the individual thiols were purified by several freeze-pump-thaw cycles. Benzenethiol was kept in an ice water bath in order to maintain constant vapor pressure. Methanethiol pressures in the dosing manifold were monitored with a manometer and held constant to ensure repeatability. Each thiol was dosed through a separate 19 µm laser-drilled aperture and a directional doser, which was positioned within 1 mm of the doser. H2 and O2 were dosed by back-filling the chamber. The purity of gases were confirmed regularly using mass spectroscopy. Site blocking during consecutive exposures complicated reporting thiol doses and coverages for coadsorbed thiols. In addition, the directional dosing system did not produce sufficient background pressure so that standard exposures could be reported in Langmuirs (1 L ) 1 × 10-6 Torr s). All exposures are reported in terms of monolayer equivalents (MLE) to distinguish from adsorbed molecules. One MLE corresponds to the minimum exposure required to saturate the clean surface with benzenethiol or methanethiol on each specific surface. All coverages are reported in terms of monolayers (ML), which is the number of adsorbate molecules divided by the number of surface Ni atoms. Toluene Formation on the Ni(111) Furface: Results Initial adsorption of methanethiol followed by benzenethiol adsorption results in the reaction profiles shown in Figure 1. The total coverage for a methanethiol saturated surface postsaturated with benzenethiol is 0.34 ML. On the basis of a methanethiol saturation coverage of 0.25 ML approximately 0.09 ML of benzenethiol is coadsorbed. As reported previously,30 a complex toluene formation peak occurs between 250 and 320

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Figure 2. Effect of oxygen and hydrogen preexposure on toluene yield from coadsorbed thiols on the Ni(111) surface. Oxygen preexposure (a) increases toluene yield compared to the reaction yield from the clean surface (b), while hydrogen (c) reduced the toluene yield. Figure inset shows water formation at 180 K from oxygen preexposure.

K as indicated in spectrum a. As discussed earlier sulfur induced reconstruction of the Ni(111) surface in the 280-300 K temperature range causes abrupt changes in the availability of surface hydrogen which may be related to the complex toluene peak shapes observed peak near 300 K.47-49 The benzene and methane desorption spectra in this case are similar to those observed for the individual reactants summarized in the Introduction (spectra b and c). Residual hydrogen from the S-H bond scission not consumed in methane and benzene formation produces a desorption limited hydrogen peak at 300 K (spectrum d). The high-temperature hydrogen shoulder extending to 550 K is caused by dehydrogenation of remaining strongly adsorbed hydrocarbons. The yield of toluene is quite small relative to the methane and benzene yields for the Ni(111) surface saturated with coadsorbed thiols. The observed peak intensities were corrected for the ionization efficiencies and mass spectrometer transmission function to estimate the relative yields. Maximum yields of toluene from the initially clean Ni(111) surface were slightly less than one percent of the total products desorbing from the surface (approximately 0.003 ML). Small amounts of ethane were detected, similar to those reported previously for methanethiol adsorbed on the Ni(111) surface.28 Following toluene absorption on the clean53 or sulfided Ni(111) surface, toluene desorbs at lower temperature, indicating that these toluene peaks must be reaction limited. The selectivity of the coupling reaction relative to hydrogenolysis has been probed by increasing and decreasing the availability of surface hydrogen at coupling temperatures. Spectrum b of Figure 2 shows the toluene yield from the clean Ni(111) surface for a methanethiol exposure of 0.25 MLE followed by benzenethiol exposure of 0.21 MLE. The bottom spectrum (spectrum c) is the toluene desorption from a 10 L preexposure of hydrogen followed by the same thiol exposure sequence used above. Toluene desorption occurs in the same temperature range but the amount of toluene decreases by 25% after correction for a slight reduction is thiol adsorption caused by the hydrogen. Note also that the high-temperature toluene peak at 300 K is reduced substantially relative to the toluene peak from the clean surface. A large decrease in hydrogen availability was achieved by predosing oxygen. Removal of surface hydrogen has been demonstrated on Pt54 and Ni52,55 surfaces, where water has been formed from the reaction of H2S with preadsorbed oxygen below 200 K. A similar procedure was used in this work to limit availability of surface hydrogen at coupling temperature. The Ni surface was exposed to 2 L of oxygen and annealed to 600

Figure 3. Yield of toluene from varying exposures of each thiol on the Ni(111) surface. The upper part varies the preexposure of methanethiol, with a constant benzenethiol postexposure (0.21 MLE). Methanethiol preexposures are identified as (a) saturation dosage of methanethiol (0.25 MLE) and (b) 0.07 MLE exposure of methanethiol; (c) no methanethiol exposure. The lower part holds methanethiol predosing constant (0.25 MLE) while reducing benzenethiol postexposure. Benzenethiol postexposures are identified as (a) saturation dosage of benzenethiol (0.21 MLE), (b) 0.10 MLE exposure of benzenethiol, and (c) no benzenethiol exposure.

K prior to dosing with the thiols. This pretreatment causes lowtemperature water formation on the Ni(111) surface, as seen in the inset of Figure 2. Coupled with the water formation is a complete elimination of hydrogen desorption at low temperature. Toluene yield increases by 20 times compared to the reaction yield from the clean surface, as shown in Figure 2 (spectrum a). This toluene yield accounts for approximately 22% of the total desorbing products, equivalent to 0.06 ML. Toluene formation begins at the same temperature as from the clean surface, but the desorption peak has broadened to 400 K. A representative set of toluene TPD data illustrates that toluene formation depends on both the coverage of methanethiol and benzenethiol on the Ni(111) surface (Figure 3). In all cases the initial thiol coverage resulted in a partial monolayer so that the second reactant can adsorb effectively. In the upper part, the preexposure to methanethiol decreases top to bottom while the benzenethiol postexposure remains constant at 0.21 MLE. In the lower part, the methanethiol preexposure remains constant (0.25 MLE) while the benzenethiol postexposure decreases from top to bottom. The upper part of Figure 3 (spectrum a) shows the toluene yield following a postexposure of 0.21 MLE of benzenethiol. In spectrum b, a small initial methanethiol exposure (0.07 MLE) is postexposed to benzenethiol. The toluene yield decreases by a factor of 3 as the methanethiol exposure is decreased. In the lower part, the methanethiol preexposure remains constant (0.25 MLE) while the benzenethiol postexposure is varied. The lower part of Figure 3

Toluene Formation

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Figure 4. Effect of dosing order on the yield of toluene from the Ni(111) surface. The left part shows toluene desorption, while the right part shows the corresponding hydrogen desorption. Spectra labeled (a) show methanethiol preexposure results, while spectra labeled (b) show benzenethiol preexposure results.

(spectrum a) shows the toluene yield for a methanethiol exposed surface postexposed to 0.21 MLE benzenethiol. In spectrum b, the methanethiol preexposed surface is postexposed to a small benzenethiol dose (0.10 MLE). The toluene yield decreases by a factor of 1.5 as the benzenethiol coverage is decreased. In the upper part, spectrum c, no toluene is formed in the absence of coadsorbed benzenethiol. As discussed later toluene formation is limited severely by small methanethiol coverages. In addition to being sensitive to the exposure of the individual thiols, toluene formation depends on the dosing order of the thiols. Changing the order of dosing changes reactant coverages because adsorption of the second species is inhibited by preadsorption of the first reactant. The saturation coverage for initial methanethiol exposure of 0.25 MLE followed by benzenethiol exposure of 0.21 MLE was discussed above. About 3 times the amount of methanethiol is adsorbed relative to benzenethiol. Preexposure of 0.21 MLE benzenethiol results in a coadsorbed monolayer with a total saturation coverage (0.23 ML) somewhat lower than the methanethiol predosed surface. A comparison of the reactivity of these two coadsorbed monolayers is shown in Figure 4. The left part indicates that toluene yield for the benzenethiol predosed experiment (spectrum b) is reduced compared to toluene formation for the methanethiol predosed case. In addition, the reaction selectivity between the two toluene peaks has changed. A reduction in the high-temperature toluene peak at 300 K is observed relative to the low-temperature peak following benzenethiol preexposure, with the intensities becoming approximately equal. This loss in the high-temperature toluene channel is correlated with a significant decrease in hydrogen availability at 300 K as indicated by the desorption spectra in the right part of Figure 4. Toluene Formation on the Ni(111) Surface: Discussion The two toluene desorption peaks at 260 and 290 K result from reaction of coadsorbed methanethiol and benzenethiol on the Ni(111) surface and indicate that at least two surface processes lead to toluene formation. Observation of crosscoupling indicates that adsorbed C6 and C1 intermediates are present on the surface after C-S bond activation. Therefore, C-S bond scission must involve direct interaction of the C-S bond with the Ni(111) surface. Benzene hydrogenolysis peaks are observed at 260 and 290 K. The primary methane hydrogenolysis peak at 260 K has a shoulder at 290 K. Therefore,

hydrogen addition competes with each coupling process (Figure 1). The mechanism of these processes will be discussed in the following paragraphs. The mechanism of the low-temperature (260 K) process appears to be direct coupling of methyl and phenyl. Since hydrogen concentration does not effect the temperature of the hydrogenolysis or coupling peaks, the rate-limiting process for both hydrogen addition and coupling at 260 K appears to be carbon sulfur bond activation. Isotope incorporation studies as part of individual methanethiol and benzenethiol studies implicate methyl and phenyl as the primary surface intermediates at 260 K.28,46 On the Ni(111) surface, toluene formation takes place in exactly the same temperature range as the hydrogenation reactions. The surface mobilites and energetics of both hydrogen and methyl on the Ni(111) surface are consistent with these results. On the Ni(111) surface, methyl radicals produced by molecular beam methods begin to couple at 230 K, 26,27 while methane formation from methyl and hydrogen occurs in the 235 K temperature range.56 Since C-S bond scission occurs above these temperatures for both thiols, we know that C-S bond activation is rate limiting in the toluene formation reactions. Toluene yield at 260 K clearly depends on the concentration of both intermediates, as shown by variations in the concentration and by benzenethiol preexposure experiments (Figures 3 and 4). The toluene formation process appears to be complex, as indicated by nonlinear increases in toluene yield which indicate that kinetic limitations are involved as well as limiting stoichiometries. A rapid increase in toluene yield is observed when the exposure of both thiols increases above half sulfur saturation of the surface. However, low methanethiol coverages limit toluene formation much more effectively than low benzenethiol coverages. In contrast, low coverages of benzenethiol produce significant amounts of toluene in the presence of 0.25 MLE preexposures of methanethiol. This suggests that the formation of toluene depends strongly on the mobility/availability of surface methyl groups. The reaction yields of high-temperature toluene (290 K) behave independently from the initial formation of toluene discussed above, indicating a different reaction process. A change in the relative intensities of the toluene peaks following benzenethiol preadsorption (Figure 4) provides information about the mechanism of toluene formation. Benzenethiol preadsorption causes a greater reduction of yield in the 290 K toluene peak relative to the 260 K peak. At 290 K, the surface

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Figure 5. Reaction products of saturation coverages of methanethiol (0.35 MLE) followed by benzenethiol (0.30 MLE) on the Ni(100) surface. Toluene (a) is observed in addition to the hydrogenolysis produces methane (b), benzene (c), and hydrogen (d). Toluene spectrum intensity is 10 times the actual signal to highlight toluene desorption.

intermediates from the individual thiols are modified by dehydrogenation enhanced by the rapid hydrogen desorption associated with sulfur induced reconstruction of the Ni(111) surface. The pronounced spike in the hydrogen desorption in Figure 4 indicates a rapid decreases in hydrogen availability on the surface and is correlated with a rapid decrease in the toluene peak at 290 K. Given that Ni(111) creates surface methyl and “benzyne” at reaction temperature, toluene formation at 290 K proceeds either through a dehydrogenated form at this temperature (“methylbenzyne”), or forms from the reaction of remaining surface methyl and “benzyne” groups. There is no evidence, however, of the formation of double methylated benzene at high temperature. This suggests that 290 K toluene probably involves rehydrogenation of surface “methylbenzyne” intermediate. Surface hydrogen has been shown to affect both the yield of toluene and also the reaction selectivity. Since both methane and benzene are formed at toluene formation temperatures, competition between hydrogenation and cross-coupling is a primary controlling factor in toluene formation. The addition of external hydrogen decreases the amount of coupling observed, as shown in Figure 2. Likewise, decreasing the amount of surface hydrogen, achieved with oxygen predosing, caused a significant increase in toluene formation. Increasing surface hydrogen concentration confirms the independence of the two toluene formation mechanisms discussed above. The coadsorption of 10 L H2 reduced the 290 K toluene peak more than the 260 K toluene peak. The preadsorption of hydrogen causes reduced benzene formation at 290 K for benzenethiol reaction on the Ni(111) surface,46 supporting our suggestion of a similar dehydrogenated intermediate for the two processes. The reduction of the 290 K “benzyne” species and the reduction of toluene formation, coupled with the lack of evidence for dimethylbenzene formation, suggests that hightemperature toluene is formed from a dehydrogenated surface intermediate. Removal of surface hydrogen by pretreatment of the surface with oxygen increases toluene yield as discussed previously. On the oxygen predosed surface, the toluene peak temperature and peak shape changes so that the peak resembles that of toluene from the Ni(100) surface, discussed below. Additionally, neither the sharp decrease in the high-temperature toluene shoulder nor the sharp hydrogen desorption peak occurs for the oxygen pretreated case. Atomic oxygen causes a 2 × 2 reconstruction of the Ni(111) surface after annealing above 450 K.57,58 As mentioned previously, sulfur also causes reconstruction of the Ni(111) surface.47-49 This suggests that oxygen pretreated experiment may modify by altering adsorption sites

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Figure 6. Preadsorbed hydrogen (a) slightly inhibits toluene formation on the Ni(100) surface relative to reaction from the clean surface (b).

for the thiolate and hydrocarbon intermediates. This would produce a range of individual desorption states, broadening the toluene peak. Toluene Formation on the Ni(100) Surface: Results Cross-coupling of methanethiol and benzenethiol to form toluene is also observed on the Ni(100) surface. Figure 5 shows the reaction products from preexposure to methanethiol followed by postexposure to benzenethiol on the Ni(100). Cross-coupling produced a broad toluene feature with peaks at 300 and 420 K (spectrum a). Desorption of both the methane and benzene hydrogenolysis products peaks near 270 K (spectra b and c). Hydrogen desorbs from the Ni(100) surface at 240 K (spectrum d). Free hydrogen desorbs below the C-S bond activation temperature for both methanethiol and benzenethiol. Additional hydrogen from decomposition of remaining hydrocarbon intermediates is observed from 450 to 700 K. Corrected peak areas show that approximately 0.009 ML of the coadsorbed thiols forms toluene, accounting for less than 3% of the desorbing products. The total coverage of the coadsorbed monolayer (0.30 ML) is nearly indistinguishable from the coverage of the methanethiol saturated surface (0.35 ML). However, nearly equal amounts of methane and benzene desorb, indicating that benzenethiol displaces some of the methanethiol. Displacement was not observed on the Ni(111) surface. Coadsorbed hydrogen causes only a slight reduction of the yield of toluene formed from the cross reaction. Figure 6 compares the toluene TPD spectra from the clean and H2 presaturated Ni(100) surface. Initially the surface is exposed to 0.35 MLE methanethiol, then postexposed to 0.30 MLE benzenethiol. A slight decrease in toluene yield is observed with hydrogen preadsorption (spectrum b). This loss is almost exclusively from the low-temperature toluene peak at 300 K. Reduction of surface hydrogen by coadsorbed oxygen leads to an increase in toluene yield. The preadsorption of oxygen was used to reduce free surface hydrogen at reaction temperatures. An exposure of 5 L O2 was annealed to 600 K to order the surface. Water desorption occurred at 180 K after exposure of this surface to the thiols, effectively limiting hydrogen availability at toluene formation temperatures. The resulting decrease in hydrogen concentration leads to a doubling in the toluene yield (Figure 7, spectra a and b). There is no change in the total thiol coverage from oxygen preexposure. The concentration of both methanethiol and benzenethiol effects the total yield of toluene formed on the Ni(100) surface. Toluene formation depends on the coverages of both methanethiol and the benzenethiol as indicated by the representative set of toluene TPD spectra shown in Figure 8. In the upper part, methanethiol preexposure decreases from spectrum a through spectrum d while the benzenethiol postexposure remains

Toluene Formation

Figure 7. Oxygen preexposure (a) doubles the yield of toluene from coadsorbed methanethiol and benzenethiol on the Ni(100) surface relative to reaction from the clean surface (b).

J. Phys. Chem. B, Vol. 105, No. 39, 2001 9553 from spectrum a through spectrum d. Spectrum a shows the toluene yield for a methanethiol preexposed surface postexposed to benzenethiol (0.30 MLE). A methanethiol predosed surface postexposed to an intermediate benzenethiol dose (0.20 MEL) is shown in spectrum b. The toluene yield decreases by a factor of 1.2 as the benzenethiol coverage is decreased. In the lower part, spectrum c, the methanethiol preexposed surface is postexposed to a 0.10 MLE benzenethiol dose. The toluene yield decreases by a factor of 1.2 as the benzenethiol coverage decreases. Spectrum d reveals no toluene formation in the absence of coadsorbed benzenethiol. As discussed later, toluene formation is more sensitive to methanethiol coverage than to benzenethiol coverage. Dosing order of methanethiol and benzenethiol significantly changes the extent of toluene formation on the Ni(100) surface. The saturation coverage is 0.30 ML for initial methanethiol exposure followed by benzenethiol postexposure. Reversing the dosing order produces a coadsorbed monolayer with a total coverage of 0.25 ML, despite the fact that a saturated monolayer of benzenethiol has a sulfur coverage of 0.30 ML, indicating displacement has occurred. The displaced benzenethiol desorbs in a substantial molecular peak at 180 K (data not shown). A comparison of the reactivity of these two coadsorbed monolayers is shown in Figure 9. The coadsorbed monolayer formed by predosing benzenethiol produces less toluene and also preferentially reduces the low-temperature toluene peak. Toluene Formation on the Ni(100) Surface: Discussion

Figure 8. Effect on toluene yield of changingmethanethiol and benzenethiol exposures on the Ni(100). The upper part decreases the preexposure of methanethiol, followed by constant benzenethiol exposure. Methanethiol preexposures are identified as (a) saturation exposure of methanethiol (0.35 MLE), (b) 0.17 MLE exposure of methanethiol, (c) 0.07 MLE exposure of methanethiol, and (d) no methanethiol exposure. The lower part holds methanethiol pre-dosing constant while reducing benzenethiol postexposure. Benzenethiol postexposures are identified as (a) saturation exposure of benzenethiol (0.30 MLE), (b) 0.20 MLE exposure of benzenethiol, (c) 0.10 MLE exposure of benzenethiol, and (d) no benzenethiol exposure.

constant (0.30 MLE). In the lower part, the methanethiol preexposure remains constant at 0.30 MLE while the benzenethiol postexposure decreases from spectrum a through spectrum d. The exposures used to generate the upper and lower parts are directly comparable. Note that predosing with large amount of methanethiol enhances toluene formation (lower part). Spectrum a, upper part, shows toluene formation from a methanethiol presaturated surface. In spectrum b, an intermediate initial methanethiol exposure of 0.17 MLE is postexposed to benzenethiol. The toluene yield decreases by a factor of 1.7 as the methanethiol exposure is decreased. Spectrum c shows a small initial methanethiol coverage (0.07 MLE) postexposed to benzenethiol. The toluene yield decreases by a factor of 7. No toluene is formed in the absence of coadsorbed methanethiol (spectrum d). In the lower part, methanethiol preexposure remains constant while the benzenethiol postexposure decreases

Two toluene peaks are observed at 300 and 420 K from the Ni(100) surface (Figures 5-9). The independent behavior of the two toluene peaks from the Ni(100) surface as coverages are varied clearly indicate that two separate surface processes lead to toluene formation (Figures 8 and 9). Observation of cross-coupling indicates that adsorbed C1 and C6 intermediates are present on the surface after C-S bond activation. Crosscoupling occurs in the same temperature range as that of the individual hydrogenolysis reactions, suggesting that common intermediates are involved. Unlike the Ni(111) surface, the coupling reactions on the Ni(100) surface occur at higher temperature than the individual hydrogenolysis reactions. Benzene formed by hydrogenolysis of phenylthiolate begins at 220 K, peaks at 260 K, and has a shoulder at 420 K. The methane hydrogenolysis peak from methanethiol begins at 220 K, peaks at 270 K, and has a shoulder near 400 K. Toluene desorption following toluene adsorption occurs at 213 K from the clean Ni(100) surface, confirming our observation that toluene from thiol cross-coupling is reaction limited.59 The two toluene formation processes and their relationship with competing hydrogenolysis processes will be discussed in the following paragraphs. The initial toluene formation process at 300 K begins 30 K above the onset of methane and benzene peaks from the individual reactants. Therefore, C-S bond activation begins below the temperature for initial toluene formation. Apparently, the coupling of adsorbed phenyl and methyl groups is limited by steric or energetic factors. On the basis of comparisons with the Ni(111) surface discussed above and the complex behavior of the toluene yield as a function of reactant coverages, energetic factors associated with methyl mobility or the activation energy for methyl addition are consistent with these data. Toluene yield at 300 K clearly depends on the concentration of both intermediates, as shown by variations in the concentration (Figures 8 and 9). A rapid increase in toluene yield is observed when the exposure of both thiols increases above 0.15 MLE

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Figure 9. The effect of dosing order of equivalent exposures of methanethiol and benzenethiol. The left part shows toluene desorption, while the right part show corresponding hydrogen desorption. Spectra labeled (a) show methanethiol preexposure results, while spectra labeled (b) show benzenethiol preexposure results.

Figure 10. General reaction diagram for methanethiol and benzenethiol coadsorption on the Ni (111) and Ni(100) surfaces.

(roughly half-saturation coverage) suggesting that kinetic limitations are involved as well as limiting stoichiometries. Low methanethiol coverages limit toluene formation much more effectively than low benzenethiol coverages. In contrast, low coverages of benzenethiol produce significant amounts of toluene in the presence of large preexposures of methanethiol. Formation of toluene therefore appears to depend strongly on the mobility/availability of surface methyl groups. The mechanism responsible for the 420 K toluene peak is independent from the 300 K toluene peak since the relative intensities of these peaks change significantly when dosing order is reversed (Figure 9). Multiple deuterium incorporation is observed during benzene formation at 420 K, indicating that a dehydrogenated aromatic intermediate is involved.50 A similar dehydrogenated aromatic intermediate appears to be responsible

for the high-temperature toluene at 420 K. Since there is no evidence of multiple methyl coupling to a single aromatic ring, we believe that some of the methyl groups couple with phenyl at lower temperature, the resulting intermediate is then dehydrogenated to form a “methylbenzyne”-like intermediate. This intermediate is hydrogenated at 420 K to form toluene. Support for a dehydrogenated intermediate is provided by an increased toluene yield at 420 K when benzenethiol predosing limits the availability of surface hydrogen at reaction temperature (right part, Figure 9). The shoulder on the hydrogen peak at 300 K for benzenethiol preadsorption (right part, Figure 9) appears to be related to dehydrogenation of the 420 K intermediate. The availability of surface hydrogen also effects the competition between hydrogen addition and cross-coupling on the Ni(100) surface. Coadsorbed hydrogen caused a small decrease

Toluene Formation

Figure 11. Comparison of toluene yields from clean nickel surfaces for methanethiol followed by benzenethiol. Ni(111), which has greater hydrogen addition activity, produced somewhat less toluene than the Ni(100) surface.

in the amount of toluene observed (Figure 6). This small decrease is consistent with the presence of substantial hydrogen from S-H bond scission and the desorption of free hydrogen in this temperature range. Decreasing the amount of surface hydrogen by low-temperature reaction with oxygen caused an increase in the toluene yield compared to the clean surface (Figure 7). The relatively small increase in toluene is consistent with the limited hydrogen availability at reaction temperature since H2 desorbs at 240 K. Toluene Formation on Nickel Surfaces: Comparison The formation of toluene from the cross-coupling reaction of methanethiol and benzenethiol has many similarities on the Ni(111) and Ni(100) surfaces. A general mechanistic picture for toluene formation on both the Ni(111) and Ni(100) surfaces is shown in Figure 10. The reaction schemes clearly highlight the similarity of the mechanisms, the products formed, and the reaction temperatures. These similarities are described in detail in the following section. Cross-coupling of methanethiol and benzenethiol clearly shows that C-S bond activation is independent of surface hydrogen. The reactivity of the coadsorbed thiols indicates that the scission of the C-S bond leads to adsorbed desulfurized intermediates and that hydrogen addition does not play a role in C-S bond breaking.46,50 Toluene yield increases rapidly above half saturation by coadsorbed reactants, indicating toluene formation is affected by both limiting stoichiometries and kinetic factors. Specifically, methyl mobility appears to play a key role in determining toluene yields. Competition between hydrogenation and cross-coupling controls the reaction selectivity. As surface hydrogen increases, the yield of toluene falls. Reduction of surface hydrogen by coadsorption of O2 lead to significant enhancements in toluene formation. Little ethane is observed in these reactions, even for low surface hydrogen availability. No biphenyl was observed in any of these experiments. Since hydrogen addition dominates, and toluene is the primary cross reaction product, it is likely that the surface reaction proceed via reductive elimination of phenyl by methyl from the nickel surface.60 Substantial toluene yields are observed because the phenyl intermediate is an effective methyl trap. Similar results were observed previously during coupling of alkyls with phenyl formed from iodobenzene on Cu(111).15 The complex dependence of toluene yield on the surface concentrations of both the methanethiol and benzenethiol is consistent with a radical mechanism where both kinetic and stoichiometric factors play a role in determining yields.

J. Phys. Chem. B, Vol. 105, No. 39, 2001 9555 While the general scheme of molecular interaction described above is applicable to both the Ni(111) and Ni(100) surfaces, there are also important differences in the reactivity of the two metals for the cross-coupling reaction as shown in Figure 11. Addition and removal of coadsorbed hydrogen had a significantly smaller effect on the Ni(100) surface than it did on the Ni(111) surface. Free hydrogen is stable on the Ni(111) surface at reaction temperature but begins to desorb well below reaction temperature from the Ni(100) surface. Decreasing hydrogen availability at reaction temperature by oxygen pretreatment doubled the toluene yield from Ni(100) observed, while there was a 20 times increase in toluene produced from the oxygen pretreated Ni(111) surface. In a similar manner added coadsorbed hydrogen decreased the toluene yield on the Ni(111) surface by 25% while decreasing the yield on the Ni(100) by only 10%. Acknowledgment. We gratefully acknowledge the support of this work by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy. Research was performed under Contract DE-AC05-96OR22464 at Oak Ridge National Laboratory managed by Lockheed Martin Energy Research Company and Grant DE-FG02-91ER14190 at the University of Michigan. References and Notes (1) Albright, L. F. Oil Gas J. 1990, 79-92. (2) Emmet, P. H., Ed. Catalysis; Reinhold Publishing Co.: New York, 1955; Vol. 2. (3) Alul, H. R. I&EC Prod. Res. DeV. 1968, 1, 7-11. (4) Drago, R. S.; Petrosius, S. C.; Kaufman, P. B. J. Mol. Catal. 1994, 89, 317-328. (5) Guo, C.; Yao, S.; Cao, J.; Qian, Z. Appl. Catal. A 1994, 107, 229238. (6) Reddy, K. S. N.; Rao, B. S.; Shiralkar, V. P. Appl. Catal. A 1993, 95, 53-63. (7) Goncalves de Almeida, J. L.; Dufaux, M.; Taarit, Y. B.; Naccache, C. App. Catal. A, 1994, 114, 141-159. (8) Chauvin, Y.; Hirschauer, A.; Olivier, H. J. Mol. Catal. 1994, 92, 155-165. (9) Collman, J. P.; Hegedus, L. S. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, California, 1980. (10) Carpentier, J. F.; Petit, F.; Mortreux, A.; Dufaud, V.; Basset, J. M.; Thivolle-Cazat, J. J. Mol. Catal. 1993, 81, 1-15. (11) Maitlis, P. M.; Long, H. C.; Quyoum, R.; Turnaer, M. L.; Wang, Z. Q. Chem. Commun. 1996, 1, 1-8. (12) Koerts, T.; van Santen, R. A. J. Mol. Catal. 1992, 74, 185-191. (13) Leconte, M. J. Mol. Catal. 1994, 86, 205-220. (14) Xi, M.; Bent, B. E. Surf. Sci. 1992, 278, 19-32. (15) Xi, M.; Bent, B. E. Langmuir 1994, 10, 505-509. (16) Lin, J. L.; Bent, B. E. J. Am. Chem. Soc. 1993, 115, 6943-6950. (17) Chiang, C. M.; Wentzlaff, T. M.; Bent, B. E. J. Phys. Chem. 1992, 96, 1836-1848. (18) Jenks, C. J.; Bent, B. E.; Bernstein, N. Zaera, F. J. Am. Chem. Soc., 1993, 115, 308-314. (19) Chiang, C. M.; Wentzlaff, T. M.; Jenks, C. J.; Bent, B. E. J. Vac. Sci. Technol. A 1992, 10, 2185-2190. (20) Lin, J. L.; Chiang, C. M.; Jenks, C. J.; Yang, M. X. Wentzlaff, T. M.; Bent, B. E. J. Catal. 1994, 147, 250-263. (21) Paul, A.; Bent, B. E. J. Catal. 1994, 147, 264-271. (22) Yang, M. X.; Jo, S. K.; Paul, A.; Avila, L.; Bent, B. E.; Nishikida, K. Surf. Sci. 1995, 325, 102-120. (23) White, J. M. Langmuir 1994, 10, 3946-3954. (24) Solymosi, F.; Kovacs, I. Surf. Sci. 1993, 296, 171-185. (25) Fairbrother, D. H.; Peng, X. D.; Viswanathan, R.; Stair, P. C.; Trenary, M.; Fan, J. Surf. Sci. 1993, 285, L455-L460. (26) Ceyer, S. T. Science 1990, 249, 133-139. (27) Yang, Q. Y.; Johnson, A. D.; Maynard, K. J.; Ceyer, S. T. J. Am. Chem. Soc. 1989, 111, 8748-8749. (28) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1995, 99, 11472-11480. (29) Castro, M. E.; Ahkter, S.; Golchet, A.; White, J. M. Langmuir 1991, 7, 126-133.

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