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Role of Intermolecular Interactions in Determining Structure and Reactivity on Surfaces: Benzenethiol on Rh(111) C. W. J. Bol, C. M. Friend,* and X. Xu Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 Received February 23, 1996. In Final Form: September 13, 1996X The reactions of benzenethiol on clean Rh(111) have been studied using a combination of temperatureprogrammed reaction, high-resolution electron energy loss, and X-ray photoelectron spectroscopies. Benzenethiol adsorbs dissociatively on Rh(111) at 100 K, affording adsorbed phenylthiolate and hydrogen. The adsorption geometry of phenylthiolate is shown to depend on the coverage, with a low coverage favoring a parallel geometry of the phenyl ring. Phenylthiolate reacts by C-S bond breakage starting at 250 K, forming benzene. At saturation coverage (0.21 monolayers), the benzene is forced into the gas phase upon formation, because of molecular crowding on the surface. Comparison is made to the chemistry of benzenethiol on other transition metal surfaces, and there is no simple correlation between the metalsulfur bonding properties and the activity or selectivity for benzenethiol desulfurization.
Introduction Understanding periodic trends in the catalytic activity of transition metal surfaces is a central theme in surface chemistry and heterogeneous catalysis. Often the activity and selectivity for a specific catalytic process is governed by a subtle interplay of several material properties, resulting in a maximum in the activity and selectivity for a specific chemical reaction in the middle of a transition metal series.1 An example of such periodic behavior is the hydrodesulfurization of dibenzothiophene catalyzed by transition metal sulfides.2 While MoS2-based catalysts are commercially used in hydrodesulfurization reactions, sulfides of Ru and Rh exhibit the highest activity for the hydrodesulfurization of dibenzothiophene, a particularly difficult molecule to desulfurize. In an effort to understand the periodic trends in hydrodesulfurization catalysis, we have undertaken the study of related desulfurization processes induced by Rh(111). In this work, we have specifically studied the reactions of benzenethiol on Rh(111). Benzenethiol has previously been studied on Mo(110),3,4 Cu(111),5 Cu(110),6 Ni(110),7 Ni(111),8 Ni(100),9 and Au(110)10 crystal surfaces and on Pt(111)11,12 and Ag(111)12 electrodes. Benzenethiol reacts by sulfur-hydrogen bond cleavage to form adsorbed hydrogen and phenylthiolate on all surfaces studied. Generally, phenylthiolate is thought to bond to the surface through the sulfur atom with the phenyl ring tilted away from the surface. The ring is tilted ∼23° away from the surface normal for a saturation coverage of phenylthiolate adsorbed on Mo(110), based X Abstract published in Advance ACS Abstracts, November 15, 1996.
(1) See, for example: Girgis, M. J.; Gates, B. C. Ind. Chem. Rev. 1991, 30, 2021-2058. (2) Pecoraro, T. A.; Chianelli, R. R. J. Catal. 1981, 67, 430. (3) Roberts, J. T.; Friend, C. M. J. Chem. Phys. 1988, 88, 7172. (4) Weldon, M. K.; Napier, M. E.; Wiegand, B. C.; Friend, C. M.; Uvdal, P. J. Am. Chem. Soc. 1994, 116, 8328. (5) Agron, P. A.; Carlson, T. A. J. Vac. Sci. Technol. 1982, 20, 815. (6) Shen, W.; Nyberg, G. L.; Liesegang, J. Surf. Sci. 1993, 298, 143. (7) Huntley, D. R. J. Phys. Chem. 1992, 96, 4550. (8) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1994, 98, 13022. (9) Takata, Y.; Yokoyama, T.; Yagi, S.; et al. Surf. Sci. 1991, 259, 266. (10) Jaffey, D. M.; Madix, R. J. J. Am. Chem. Soc. 1994, 116, 3020. (11) Stern, D. A.; Wellner, E.; Salaita, G. N.; et al. J. Am. Chem. Soc. 1988, 110, 4885. (12) Gui, J. Y.; Stern, D. A.; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A. T. Langmuir 1991, 7, 955.
S0743-7463(96)00170-9 CCC: $12.00
on intensity analysis of near edge X-ray absorption fine structure data.3,13 On Ni(100), phenylthiolate resides in the 4-fold hollow site, located ∼2.25 Å from the surface and with a ring orientation near the surface normal.9 In this study, we show that for phenylthiolate on Rh(111), the disposition of the ring and the reactivity both depend strongly on coverage. The decomposition of phenylthiolate has been studied on Ni(110),7 Ni(111),8 Au(110),10 and clean and sulfurcovered Mo(110),3,4 and very similar chemistry is observed on the three reactive metals. Phenylthiolate reacts via C-S bond breakage to form gaseous benzene on Ni(111), Ni(110), and Mo(110). Nonselective decomposition to gaseous H2 and surface carbon is a competing process that has been proposed to proceed via phenyl on sulfurcovered Mo(110), as well as the Ni surfaces7,8 and benzyne on Mo(110).3 The temperature necessary for C-S bond cleavage in phenylthiolate varies widely among the different metals and is thought to play a role in determining the selectivity for benzene production over decomposition. In addition, the availability of surface hydrogen, which depends on the temperature of hydrogen recombination relative to C-S bond cleavage, and the dehydrogenation activity of the metal are important factors in determining the selectivity for benzene formation. On Ni(110), 75% of the adsorbed phenylthiolate reacts to form benzene, evolving between 200 and 240 K. On Mo(110), ∼50% of the phenylthiolate forms benzene at 350 K; the selectivity increases to ∼80% on sulfurcovered Mo(110) (θS ) 0.35 monolayers). On Au(110), carbon-sulfur bond cleavage leading to the formation of adsorbed sulfur and phenyl commences at ∼ 440 K. Since all surface hydrogen has been removed as H2 or H2S below this temperature, and the gold surface is inactive toward C-H bond breakage, the adsorbed phenyl combines to gaseous biphenyl and diphenyl sulfide. Herein, we report that C-S bond breaking commences in adsorbed phenylthiolate at 250 K on Rh(111) and that the temperature of benzene evolution depends strongly on the thiolate coverage. At low coverage, benzene remains adsorbed on the surface, whereas at coverages near saturation a significant fraction is evolved directly into the gas phase. We further show that the orientation of the phenyl ring similarly depends on coverage, with a more upright geometry observed at saturation indicating (13) Stohr, J.; Outka, D. A. Phys.Rev.B 1987, 36, 7891.
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that the surface is sterically crowded. At saturation coverage, the total benzene yield accounts for ∼55% of the benzenethiol reacting; the remainder decomposes to form surface carbon, sulfur, and gaseous dihydrogen. Experimental Section All experiments were performed in two separate stainless steel ultrahigh vacuum chambers, with base pressures below 1 × 10-10 Torr, which have been described in detail previously.14,15 Preparation and cleaning of the crystal, as well as the details of the experimental setup and procedures used during data collection, can be found elsewhere.16 In short, during temperature-programmed reaction experiments the reactants are introduced to the clean Rh(111) surface at 100 K. The surface is radiatively heated to 850 K, with the crystal face in line-of-sight of an apertured mass spectrometer (UTI-100C) that can monitor 10 masses simultaneously. The heating rate decreases linearly from 10 K/s at 100 K to 6 K/s at 850 K and is highly reproducible. During an experiment the crystal is negatively biased at -60 V to preclude any electroninduced chemistry. In high-resolution electron energy loss and X-ray photoelectron experiments the reactants are introduced to the clean surface, after which the crystal is heated to the desired temperature at the same rate as used in temperature-programmed reaction experiments, and allowed to cool to 100 K before recording a spectrum. The high-resolution electron energy loss spectrometer (LK2000-14-R) operates at a primary beam energy of ∼3 eV, with a resolution of 55 cm-1 at a count rate of more than 1 × 106 counts/s for the elastic peak on the clean surface. All reported spectra were recorded at specular detection angles. The X-ray photoelectron spectrometer (PHI-5300) employs a 400 W Mg(KR) X-ray source and a hemispherical analyzer, operating at a pass energy of 17.9 eV and a resolution of ∼0.2 eV. All binding energies are referred to the Fermi level of the spectrometer, with the Rh(3d5/2) at 307.1 eV as a reference. Multiple regions of the spectrum can be recorded in a single experiment. All reported spectra have been corrected for background features by subtraction of the spectrum of clean Rh(111). No beam damage was observed in any of the experiments, based on the correspondence between the temperature-programmed reaction spectra with and without exposure to the X-ray or electron sources in the spectroscopic experiments. Benzenethiol-d0 (Aldrich, 99%) was dried over sodium sulfate and distilled under an atmosphere of dry nitrogen. Benzenethiold5 was synthesized according to the procedure described by Miura and Kinoshita.17 It was purified by successive distillations under nitrogen. The sample was 95% pure, as shown by mass spectral analysis, with the major contaminant being benzene-d5. Benzened6 (MSD, 98%) and dideuterium (Matheson, 99%) were used as received. All liquid samples are degassed by several freezepump-thaw cycles before each day of experiments.
Results Temperature-Programmed Reaction. Dihydrogen and benzene are the only gaseous reaction products evolved during temperature-programmed reaction of benzenethiol on Rh(111) (Figure 1). Carbon and sulfur remain on the surface after reaction up to 700 K, as detected by Auger electron spectroscopy. No other gaseous products are detected in the range from 2 to 130 amu. Biphenyl and phenyl-containing disulfides were specifically ruled out based on quantitative analysis of the m/e 78:77 and m/e 110:109 ratios which were the same as for authentic samples of benzene and benzenethiol, respectively. Furthermore, the m/e 77 or m/e 109 curves had the same shape as m/e 78 and m/e 110, respectively. (14) Wiegand, B. C. Ph.D. Thesis; Model Studies of Desulfurization Reactions on Mo(110), Harvard University, Cambridge, 1991. (15) Wiegand, B. C.; Uvdal, P.; Friend, C. M. J. Phys. Chem. 1992, 96, 4527. (16) Bol, C. W. J.; Friend, C. M. J. Am. Chem. Soc. 1995, 117, 5351. (17) Miura, Y.; Kinoshita, M. Bull. Chem. Soc. Jpn. 1977, 50, 1142.
Figure 1. Evolution of dihydrogen (H2) and benzene (C6H6) during temperature-programmed reaction of benzenethiol on Rh(111) as a function of coverage: 25, 35, 50, 75, and 100% of saturation coverage (θsat ∼ 0.21 monolayers). All data are corrected for fragmentation from other products.
Benzenethiol also desorbs in two peaks with maxima at 200 (R2) and 220 K (R1) for coverages greater than saturation (data not shown). Saturation coverage is defined as that where all product yields first reach their maximum and also corresponds to the onset of benzenethiol desorption. The R1-desorption state at 220 K is populated first and reaches maximum intensity at approximately 1.25 times saturation. Upon further increase, there is a second benzenethiol peak at 200 K, which increases indefinitely with exposure and is therefore attributed to sublimation of condensed benzenethiol. The benzenethiol desorption at 220 K is tentatively ascribed to a more tightly bound second layer. Importantly, there is no phenylthiolate recombination with surface hydrogen based on the fact that only benzenethiol-d0 is detected when benzenethiol reacts in the presence of surface deuterium. At coverages below 35% of saturation, dihydrogen is the only gaseous product and is observed in two peaks at 350 and 490 K, with a small tail at high temperature (Figure 1). The peak at 350 K is attributed to the recombination of surface hydrogen primarily derived from S-H bond breaking. Hydrogen atoms recombine on Rh(111) at ≈380 K for low coverages and ≈300 K for high coverages. The presence of atomic sulfur or hydrocarbons, such as benzene, decreases the hydrogen recombination temperature to as low as 250 K, consistent with the observed shift to 260 K at saturation coverage. Furthermore, H2, HD, and D2 are produced in peaks at ∼260 K, with similar lineshapes during temperature-programmed reaction of Da, and a saturation coverage of C6H5SH; no HD or D2 was formed above 325 K. Finally, only trace amounts of HD and no D2 are evolved in the lowtemperature peak during temperature-programmed reac-
Benzenethiol on Rh(111)
Figure 2. Temperature-programmed reaction of C6D5SH on Rh(111) showing the isotopes of (a) dihydrogen and (b) benzene. The data are uncorrected for fragmentation. An exposure sufficient to condense benzenethiol multilayers was used.
tion of benzenethiol-d5 (C6D5SH) on Rh(111), confirming that it arises mainly from the sulfhydryl hydrogen (Figure 2). The temperature required for ring dehydrogenation strongly depends on the initial thiol coverage (Figure 1). For example, at 0.25 of saturation virtually all hydrocarbon species decompose below 500 K, while at saturation coverage nearly all hydrocarbon decomposition occurs above this temperature based on the reactions of benzenethiol-d5 (C6D5SH). Dideuterium accounts for ∼98% of the dihydrogen isotopes produced above 500 K during temperature-programmed reaction of a saturation coverage of benzenethiol-d5 (C6D5SH), with the remaining 2% being HD (Figure 2). The temperature and mechanism of gaseous benzene evolution also depend strongly on the benzenethiol exposure. Benzene is first evolved for benzenethiol exposures of ≈35% of saturation in a peak centered around 450 K (β1) (Figure 1). The rate of benzene evolution in the 450 K peak is limited by desorption, since benzene itself molecularly desorbs from clean Rh(111) at 450 K.18 At approximately 45% of saturation, benzene is also evolved in a second peak centered around 280 K (β2) (Figure 1), indicating that benzene is formed and ejected into the gas phase without trapping on the surface. Both benzene peaks increase in intensity with increasing benzenethiol exposure up to 0.75 of saturation, at which point the 450 K (β1) peak reaches maximum intensity. Above 0.75 saturation, the β1-peak has a constant integrated intensity but broadens and shifts to lower temperature, such that (18) Koel, B. E.; Crowell, J. E.; Bent, B. E.; Mate, C. M.; Somorjai, G. A. J. Phys. Chem. 1986, 90, 2949.
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it peaks at 400 K at saturation. The β2-peak increases in intensity over the same coverage range up to saturation, ultimately accounting for approximately 85% of all benzene produced. The proposed change in the mechanism for gaseous benzene evolution is supported by investigations of mixtures of C6D6 and C6H5SH. Specifically, ∼67% of the C6D6 that desorbs during reaction of a mixture of C6D6 and C6H5SH is in the 450 K peak, whereas most C6H6 is evolved in a peak at ∼280 K (data not shown).19 Isotopic exchange experiments demonstrate that formation of a single C-H(D) bond is the predominant reaction path for β2-benzene formation at 290 K, since benzene-d0 and benzene-d1, formed in a ratio equal to the H:D ratio present on the surface below 350 K, are the primary products of benzenethiol reaction in the presence of adsorbed deuterium. There is also a small amount of reversible C-H bond activation, however, based on small amounts of benzene-d2 and -d3. The relative intensities of the mass spectrometer parent ions corrected for fragmentation are d0:d1:d2:d3 ) 0.97:1.00:0.15:0.02 when equal amounts of hydrogen and deuterium are present on the surface below 350 K. Isotopic labeling experiments show that benzene produced in the 280 K peak arises from hydrogenation of the intact phenyl ring by hydrogen produced from S-H bond cleavage. Specifically, C6D5H is the major product evolved at 290 K during reaction of C6D5SH (Figure 2). In addition, some C6D6 is produced beginning at ∼290 K, peaking at 325 K, during temperature-programmed reaction of C6D5SH. The somewhat higher temperature required for C6D6 production indicates that ring dedeuteration commences near 290 K. At saturation coverage, the peak at 400 K is attributed to hydrogenation of the ring by hydrogen produced from decomposition of other phenyl rings, based on isotopic labeling experiments. Specifically, both C6D5H and C6D6 are formed in the higher-temperature peak in roughly equal amounts (Figure 2). In addition, only benzene-d0 evolves in the high-temperature shoulder at 400 K when a saturation coverage of benzenethiol-d0 reacts in the presence of deuterium (data not shown). X-ray Photoelectron Spectroscopy. The carbonsulfur bond remains intact upon adsorption at 100 K on Rh(111) based on X-ray photoelectron spectra. After adsorption of a saturation coverage of benzenethiol at 100 K, there are two S(2p) peaks at 163.5 and 162.2 eV, with an intensity ratio near 1:2,20 assigned to phenylthiolate (Figure 3b). A similar spectrum, albiet with an inferior signal-to-noise ratio, is also obtained for low coverages at 100 K, again indicating that the C-S bond is retained in the majority of molecules at low coverage as well (Figure 3a). The S(2p) binding energies are similar to those reported for phenylthiolate on Mo(110)3 and Ni(111),8 are substantially higher than atomic sulfur on Rh(111) (162.4 and 161.3 eV), and are lower than intact condensed benzenethiol (164.7 and 163.4 eV). Carbon-sulfur bond cleavage commences at 240 K for all benzenethiol coverages, based on the broadening of the S(2p) region accompanied by a shift to lower binding energy. Specifically, there is no evidence for C-S bond cleavage below 240 K, even for low benzenethiol coverages (19) The values quoted exclude multilayer sublimation which occurs below 200 K. (20) The integrated areas of the S(2p1/2) and S(2p3/2) peaks for the monolayer at 100 K were 420 and 740 counts eV, slightly greater than 1:2. The additional intensity of the 2p1/2 peak is attributed to a contribution by final state effects in the 2p3/2 peak. The shoulder around 163.6 eV, attributed to a final state effect, is also present in the spectrum of Rh(111) - x3 × x3 - S.
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Figure 3. S(2p) X-ray photoelectron spectra of benzenethiol on Rh(111) corresponding to (a) low coverage (∼25% of saturation) at 100 K, (b) saturation coverage at 100 K, (c) low coverage heated to 300 K, (d) saturation coverage heated to 245 K, and (e) saturation coverage heated to 400 K.
(∼25% of saturation) based on the absence of intensity at ∼161 eV.21 Heating a saturated layer of benzenethiol to 245 K shifts some S(2p) intensity to lower binding energies, indicating the onset of C-S bond breakng (Figure 3c). Heating to 400 K leads to a S(2p) spectrum characteristic of atomic sulfur with peaks at 162.4 and 161.3 eV (Figure 3e), based on the correspondence to the peak pattern measured for Rh(111) - x3 × x3 - S.20 Carbon-sulfur bond cleavage is complete at lower temperatures for lower benzenethiol exposures; for example, most C-S bonds are broken below 300 K at 40% of saturation (Figure 3c). Approximately 0.21 molecules of benzenethiol react per surface Rh atom at saturation, based on the intensity of the S(2p) spectrum after annealing a saturation coverage of benzenethiol to 850 K relative to that of Rh(111)-(x3 ×x3)-S (θS ) 0.33 monolayers). The selectivity for benzene production is estimated to be approximately 55%, based on the C:S ratio after heating to 850 K relative to that at 100 K, prior to the evolution of benzene. High-Resolution Electron Energy Loss Experiments. High-resolution electron energy loss spectra also indicate that phenylthiolate is formed via S-H bond breaking on Rh(111) at 100 K (Figure 4). Specifically, the absence of the τ(S-H) and ν(S-H) modes in the spectrum of a saturation coverage of benzenethiol on Rh(111) at 100 K is indicative of S-H bond breakage upon adsorption (Figure 4a-ii,b-ii, Table 1). The fact that there are no significant changes upon heating up to 225 K further indicate that phenylthiolate largely remains intact up to (21) Xueping, X. Ph.D. Thesis, Harvard University, 1991.
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the onset of benzene evolution. The τ(S-H) and ν(S-H) are observed at 200 and 2565 cm-1, respectively, in condensed benzenethiol (Figure 4a-i,b-i). All other modes in the spectra obtained subsequent to saturation exposures of benzenethiol can be readily assigned to adsorbed phenylthiolate, based on comparison to spectra of phenylthiolate on other surfaces3,4,6-8,11,12 and gas phase IR studies of related molecules (Table 1).22 Vibrational assignments were confirmed by isotopic shifts measured in investigations of benzenethiol-d5 (Figure 4b, Table 1). The peak near 3000 cm-1 in the spectrum of benzenethiold5 is attributed to the ν(C-H) modes of a small isotopic impurity (Figures 4b and 5b). A loss due to the ν(C-S) mode, expected around 700 cm-1, is obscured by other peaks in this energy region. The vibrational spectra are consistent with C-S bond cleavage occurring in the range of 250-400 K. A peak at ∼320 cm-1 appears at 250 K and increases in intensity upon further heating up to 400 K. This peak is assigned to the ν(Rh-S) mode based on the correspondence to the frequency of 300 cm-1 measured for Rh(111)-(x3×x3) -S. This assignment is supported by the X-ray photoelectron data that indicate that C-S bond cleavage is complete at 400 K and temperature-programmed reaction data which show that benzene is evolved between 200 and 450 K. The remainder of the vibrational spectrum remains nearly unchanged upon heating over the range of 250400 K, suggesting that a substantial fraction of the rings remain intact in the decomposition products. Over this range, there are small changes in relative intensities which most likely reflect the fact that there is a mixture of species present on the surface, including chemisorbed benzene. At 450 K, the vibrational spectrum still indicates that intact rings are present on the surface based on the persistence of a high-frequency ν(C-H) mode (3025 cm-1) and the out-of-plane γ(C-H) mode (∼780 cm-1). At this temperature, there is little or no chemisorbed benzene remaining on the surface, based on temperature-programmed reaction results (Figure 1), and no intact phenylthiolate, based on X-ray photoelectron studies (Figure 3). The remaining candidates for the residual surface intermediate are adsorbed phenyl or benzyne. Benzyne has been previously identified on Mo(110).3 While the vibrational data on Mo(110) are not of as high quality as those presented herein, the modes associated with benzyne are similar to those measured for the decomposition product of phenylthiolate on Rh(111). Modes associated with the ring of benzyne on Mo(110) are observed at 705, 1130, and 1425 cm-1, compared to 780, 1140, and 1395 for the 450 K product on Rh(111). The ν(C-H) modes are 3100 cm-1 for benzyne on Mo(110) and 3025 cm-1 for the 450 K intermediate. Adsorbed phenyl has similar vibrational features, with peaks at 730, 1170, 1440, and 3050 cm-1 on Mo(110)4 and 782, 1122, 1328, and 3011 cm-1 on Ni(111).8 Hence, electron energy loss spectroscopy alone cannot be used to distinguish benzyne and phenyl adsorbed on Rh(111). We propose that the ring remaining on the surface at 450 K is adsorbed benzyne (C6H4) primarily based on evidence for C-D bond breaking at ∼290 K during temperature-programmed reaction studies of C6D5SH. Specifically, C6D6 is evolved in peaks at 325 and 450 K (Figure 2). If adsorbed phenyl were the only species present, no C-D bonds would have been broken and, therefore, no C6D6 would be formed from C6D5SH. However, a mixture of species may be present on the surface, including both benzyne and phenyl. (22) Whiffen, D. H. J. J. Chem. Soc., Perkin Trans. 2 1956, 1350.
Benzenethiol on Rh(111)
Langmuir, Vol. 12, No. 25, 1996 6087 Table 1. Vibrational Assignments for Benzenethiol-d0 and -d54,22 condensed benzenethiola C6H5SH (C6D5SH)
phenylthiolate(a) 100 K,aqsat
benzyne(a) + S(a) 450 K,aqsat assignmentb
190 (180) 325 (315) 435 (400) 725 (545) 910 (-) - (990) 1150 (825) 1455 (1335) 1570 (1540) 2570 (2555) 3045 (2275)
460 (415) 730 (575) 900 (-) - (990) 1160 (825) 1455 (1330) 1570 (1555) 3045 (2265)
780 (585) - (935) 1140 (835) 1395 (1375) 3025 (2265)
τ(S-H) ν(Rh-S) f(C-C)o γ(C-H)o γ(C-H)o β(C-H)i β(C-H)i ν(C-C)i ν(C-C)i ν(S-H) ν(C-H)
a
Numbers in parentheses refer to frequencies measured for C6D5SH. b Key: o ) out-of-plane, i ) in-plane.
Figure 4. High-resolution electron energy loss spectra following exposure of Rh(111) to (a) benzenethiol-d0 (C6H5SH) and (b) benzenethiol-d5 (C6D5SH). Spectra are shown for (i) condensed multilayers at 100 K, (ii) a saturation coverage (0.21 monolayers) at 100 K, (iii) a saturation coverage heated at 250 K, and, (iv) a saturation coverage annealed at 450 K.
The vibrational spectrum recorded after annealing a saturation coverage of benzenethiol to 800 K consists of a loss at 300 cm-1 and a very broad low-intensity feature ranging up to 900 cm-1 (data not shown). The loss at 300 cm-1 is assigned to the ν(Rh-S) mode, based on comparison to the spectrum of Rh(111)-(x3×x3)-S, while the broad feature is tentatively assigned to the carbon polymer formed from decomposition of hydrocarbons on Rh(111). The variation in the intensity of the out-of-plane C-H bend (γ(C-H)) mode at 725 (545) cm-1 as a function of phenylthiolate coverage indicates that the ring reorients so as to assume a more perpendicular disposition as the phenylthiolate coverage increases (Figure 5 a,b). Specifically, the mode at 725 cm-1, which is unambiguously assigned as the γ(C-H) mode based on the isotopic shift to 545 cm-1 upon deuteration of the ring, decreases in intensity relative to the elastic as well as the ν(C-H) peaks as the coverage increases. Off-specular measurements show that the γ(C-H) mode is primarily dipole scattered; the intensity of this mode falls off sharply for detection angles away from the specular angle (data not shown). Given that the dynamic dipole moment of the γ(C-H) mode is perpendicular to the ring, it will have maximum intensity for a ring orientation parallel to the surface plane within the dipole selection rule.23 As the ring tilts away from the surface, the projection of the dipole moment for the γ(C-H) mode along the surface normal decreases and the intensity will diminish, as is observed for higher thiolate coverages (Figure 5). For a fixed ring orientation, the intensity of any vibrational mode should increase with increasing coverage. Notably, the ν(C-H) at 3025 (2260) cm-1, which is primarily impact scattered based on offspecular measurements and is, therefore, less sensitive to the orientation of the ring, increases in intensity with respect to coverage, as expected. The decrease in the γ(C-H) mode intensity with increasing phenylthiolate coverage clearly indicates that the ring is more closely aligned with the surface normal at high coverage within the assumption that the magnitude of the dipole moment does not change significantly with coverage (Figure 5a,b). Importantly, there is no evidence of C-S bond breaking below 240 K for phenylthiolate coverages below saturation. X-ray phoelectron data were specifically obtained for a phenylthiolate coverage of 0.14 monolayers, and there was no evidence of C-S bond cleavage. Furthermore, the frequency of the out-of-plane bend mode (780 cm-1) is different than that reported for benzene chemisorbed on Rh(111), 800-810 cm-1.18,24 The remainder of the spec(23) Electron Energy Loss Spectroscopy and Surface Vibrations; Ibach, H., Mills, D. L. Academic Press: New York, 1982. (24) Koel, B. E.; Crowell, J. E.; Mate, C. M.; Somorjai, G. A. J. Phys. Chem. 1984, 88, 1988.
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Figure 5. High-resolution electron energy loss spectra of phenylthiolate-d0 (C6H5S-) and (b) phenylthiolate-d5 (C6D5S-) on Rh(111) at 100 K as a function of coverage: 0.07, 0.13, and 0.17 monolayers corresponding to ∼33%, ∼62%, and ∼81% of saturation. See Figure 3(ii) for saturation coverage.
trum is essentially invariant with coverage, except for the disappearance of the loss at 315 cm-1 as the coverage increases. This loss is assigned to a ν(Rh-ring) mode which would arise from coordiantion of the π system of the ring to the surface at low coverage. An analogous assignment has been made for benzene adsorbed in a parallel orientation on Rh(111).18 Note that the 315 cm-1 mode cannot be due to the Rh-S stretch of atomic sulfur because there is no Sa detected by X-ray photoelectron spectroscopy at low temperature and low coverage (Figure 3). Discussion As on other surfaces,3,4,6-9 the S-H bond of benzenethiol breaks upon adsorption at 100 K, affording adsorbed hydrogen and phenylthiolate for all coverages up to saturation, based on a combination of high-resolution electron energy loss, X-ray photoelectron, and temperature-programmed reaction spectroscopies. Phenylthiolate remains intact up to 225 K for all coverages studied, given that both high-resolution electron energy loss and X-ray photoelectron spectra remain unchanged after heating
up to 225 K. At 240 K, C-S bonds begin to break, producing adsorbed atomic sulfur and benzene. While the temperature for initiation of C-S bond breaking is relatively insensitive to the thiolate coverage, the temperature range for completion of C-S bond breaking is strongly coverage dependent, such that some C-S bonds remain intact up to 400 K at saturation coverage. This effect is attributed to inhibition of C-S bond breaking by sulfur deposited during the course of reaction, as has been observed on Mo(110).4 The temperature for gaseous benzene evolution depends strongly on the initial phenylthiolate coverage and is explained by steric effects, illustrated in Figure 6. Specifically, ring-ring interactions at high phenylthiolate coverages will effectively prevent benzene adsorption on the surface, since benzene-surface bonding requires a parallel ring orientation and, therefore, a relatively large area (Figure 6). A similar effect has been reported for benzenethiol reactions on Ni(111).8 On Rh(111), the onset of gaseous benzene evolution is approximately 250 K, the same as the onset of C-S bond breaking, for phenylthiolate coverages in the range of 0.10 and 0.21 monolayers (0.5-
Benzenethiol on Rh(111)
Figure 6. Proposed mechanism of reaction in the reaction of low and high coverages of benzenethiol on Rh(111) illustrating the development of steric interactions at high coverage using a scale model.
1.0 times saturation). The peak temperature for benzene formation, 290 K, is well below the peak temperature for molecular desorption of benzene from clean Rh(111), ∼450 K, but significantly above the desorption temperature for benzene adsorbed on Rh(111)-(x3×x3)-S, ∼190 K.25 Hence, at high phenylthiolate coverages, a significant fraction of the benzene is rapidly evolved into the gas phase because it can not adsorb over a site filled by the sulfur left behind, and it experiences strong repulsive interactions with the neighboring phenyl rings of other thiolates. At the highest phenylthiolate coverages, ∼0.21 monolayers, all benzene is evolved below 450 K. Isotopic labeling experiments showed that the high-temperature shoulder in the benzene production is due to hydrogenation involving ring decomposition, not benzene desorption at saturation coverage, as expected from the severe steric crowding. At lower phenylthiolate coverages, some benzene traps on the surface subsequent to formation and desorbs in a peak at 450 K, similar to the desorption temperature of benzene on clean Rh(111). At low thiolate coverage, there are apparently fewer ring-ring interactions and ample open Rh sites to accommodate π-bound benzene (Figure 6). A similar peak is also present for phenylthiolate coverages above 0.10 monolayers and is ascribed to benzene trapped on the surface once the steric crowding is relieved. At coverages above 0.10, there are initially significant ring-ring interactions, which are reduced as the reaction proceeds and benzene leaves the surface. As the phenylthiolate coverage is reduced via gaseous benzene production, nearest neighbor interactions are reduced and Rh sites become available for benzene coordination. Accordingly, the lower-temperature benzene peak at ∼280 K, which is first observed at ∼0.10 monolayers, increases relative to the 450 K peak with increasing phenylthiolate coverage (Figure 1). (25) Xy, Xueping; Friend, C. M. Unpublished results.
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The assertion that there are strong ring-ring interactions at high phenylthiolate coverages is supported by the variation in ring orientation with coverage that is indicated by our vibrational studies. The variation in the intensity of the out-of-plane C-H bend mode, which is dipole scattered, indicates that the ring orientation changes from nearly parallel at low coverage to more upright at high coverage. The nearly perpendicular orientation at high coverage is similar to that reported for phenylthiolate on Mo(110)13 and Ni(100).9 While the ring orientation cannot be quantitatively determined, because the magnitude of the dipole moment is unknown, the ring disposition on Rh(111) appears to be more tilted toward the surface than on Mo(110),4 based on the relative intensities of the out-of-plane (and in-plane) ring modes on the two surfaces and for the monolayer compared to condensed benzenethiol (Figures 4 and 5).26 For example, the intensity ratio of the out-of-plane C-H bend at 780 (585) cm-1 to the in-plane β(C-H) mode at 1160 (825) cm-1 is higher for a saturation coverage of phenylthiolate on Rh(111) than for condensed benzenethiol or for phenylthiolate on Mo(110).4 These data suggest that the ring on Rh(111) remains tilted away from the surface normal even at saturation coverage. Again, the precise orientation cannot be derived from the intensities because the relative values of the dipole moments for these modes may be highly sensitive to the bonding environment;27 specifically, the dipole moments of different modes can be differentially affected by surface-induced changes in bonding even within the same functional group. Our isotopic exchange studies are also consistent with our proposal that C-S bond breaking limits the rate of benzene formation but that steric effects dictate the temperatures for gaseous benzene evolution. A single C-H bond is formed in the majority of benzene produced, as shown by isotopic labeling experiments, thus demonstrating that reversible dehydrogenation does not play a significant role in either process. Furthermore, there is no kinetic isotope effect for benzene evolution, in contrast to Ni(110).7 All of our observations are consistent with our proposal that C-S bond breaking limits the rate of gaseous benzene production at 290 K, whereas benzene desorption determines the rate of benzene evolution at 450 K. In addition to the difference in ring-ring interactions, there are also differences in the ratio of surface hydrogen to thiolate and in the coverage of sulfur that contribute to coverage dependence in the reactions of benzenethiol on Rh(111). At saturation coverage, a significant amount of H2 is evolved below 300 K, resulting in the depletion of surface of hydrogen as C-S bonds are breaking. Indeed, some C-S bonds remain intact up to 400 K at saturation. In contrast, H2 evolution commences at temperatures slightly above those required for C-S bond cleavage at low coverage. For example, C-S bond breakage is essentially complete by 300 K, the temperature where H2 evolution commences, for coverages up to 0.09 monolayers. Consequently, the surface is depleted of the hydrogen necessary for benzene formation below the temperature at which all C-S bonds are broken at high benzenethiol coverages. Hence, benzene formation depends on decomposition of hydrocarbon intermediates (CxHy) as a source of hydrogen, as shown by isotopic labeling experiments. Surprisingly, the facility for C-S bond cleavage in phenylthiolate on various metal surfaces does not correlate (26) The benzenethiol multilayer is assumed to be isotropic. Therefore, the relative intensities of the modes are assumed to reflect the relative magnitudes of their dipole moments. (27) Uvdal, P.; MacKerell, A. D., Jr.; Wiegand, B. C. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 193.
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with periodic trends in the activity for dibenzothiophene hydrodesulfurization.28 Although the reaction mechanism of benzenethiol on Rh(111) is similar to those proposed for a wide range of reactive transition metal surfaces studied, the temperature necessary for C-S bond cleavage varies widely with different metals: 200 K on Ni(111),8 250 K on Rh(111), and 400 K on Au(110).10 On the more active (110) face of nickel as well as on Mo(110), C-S bond breakage was observed at temperatures as low as 100 K in the limit of low coverages.7 The selectivity for benzene production also varies widely for various surfaces with ∼50% of the benzenethiol that reacts on Mo(110)3 and Rh(111) producing benzene, compared to ∼75% for Ni(110).7 In contrast, the activity for dibenzothiophene desulfurization is highest on catalysts based on Rh and very low on Ni, with Mo exhibiting intermediate reactivity.28 The periodic trends in dibenzothiophene hydrodesulfurization were interpreted in terms of electronic effects in relative metal-sulfur bonding. The absence of a correlation between either the temperature for benzenethiol desulfurization or the selectivity for benzene production and the activity for dibenzothiophene hydrodesulfurization suggests that M-S bonding does not necessarily dictate desulfurization activity and selectivity on metal surfaces. The detailed bonding of the reactant to the surface must also play a role. Thus, variation in the bonding of S-containing molecules with different structures or functional groups may lead to vastly different periodic trends for different classes of molecules. Variation in the temperature required for hydrogen recombination, as well as the ability of the metal to promote C-H bond formation, must also play a role in determining the selectivity for benzene production since surface hydrogen is required. Adsorbed atomic hydrogen is formed by S-H bond breakage in benzenethiol on all surfaces, but the temperature for recombination varies widely. For instance, on Au(110) the surface is depleted of hydrogen below the temperature at which the first C-S bond is broken. Furthermore, Au does not activate C-H bonds, so the rings cannot serve as a source of hydrogen. Hence, the hydrogenation of phenyl to form benzene does not occur on Au(110).10 On Rh(111), hydrogen recombination occurs below 400 K at low coverage, but below 300 K at high phenylthiolate coverages (Figure 1). Therefore, the selectivity for benzene production is limited, in part, by the lack of surface hydrogen, which recombines in the same temperature range as required for the onset of C-S bond breaking. While some hydrogen is produced from ring dehydrogenation above 300 K on Rh(111), the required ring decomposition clearly decreases the overall selectivity. On Mo(110), while there is hydrogen present on the surface under conditions where C-S bond breaking has occurred, the ability of the surface to activate C-H bonds kinetically favors decomposition over rehydrogenation. On Mo, there is a strong coverage dependence to the selectivity and the (28) Harris, S.; Chianelli, R. R. J. Catal. 1984, 86, 400.
Bol et al.
temperature for C-S bond cleavage that reflects the competition between dehydrogenation and C-S bond hydrogenolysis. On Ni(110), the surface with the highest selectivity, hydrogen atom recombination occurs over the same range as benzene production and, indeed, hydrogenassisted C-S bond cleavage is suggested.7 These comparisons suggest that the role of hydrogen may also depend strongly on the nature of the metal surface and that these effects need to be incorporated into models for periodic trends in hydrodesulfurization catalysis. Finally, the mechanism for gaseous product evolution also probably plays an important role in determining the selectivity. Specifically, on Rh(111) some benzene traps on the surface at coverages below saturation, leading to desorption-limited production of gaseous benzene. The adsorption of benzene on the surface following formation via phenylthiolate hydrogenolysis opens another pathway for nonselective decomposition. Specifically, benzene decomposition effectively competes with desorption on Rh(111);18,24 therefore, trapping of the benzene on Rh(111) leads to additional nonselective reaction, i.e., lower selectivity. Conclusions Benzenethiol reacts by S-H bond cleavage, affording adsorbed hydrogen and phenylthiolate, upon adsorption on Rh(111) at 100 K. The adsorption geometry in phenylthiolate depends strongly on coverage, with ringring interactions leading to a more upright geometry at high coverage. At low coverage, the ring is proposed to lie nearly parallel to the surface, indicating bonding via both the sulfur and the π-system. Phenylthiolate reacts by C-S bond cleavage, commencing at 250 K, yielding benzene and phenyl. Steric interactions also play an important role in determining the fraction of benzene that evolves into the gas phase vs adsorbs on the surface. At saturation coverage (0.21 monolayers), essentially all benzene evolves directly into the gas phase, whereas at coverages below 0.1 monolayers the benzene first adsorbs on the surface and subsequently desorbs. Formation of dihydrogen by recombination of adsorbed atomic hydrogen competes with benzene formation, limiting the availability of hydrogen necessary for benzene formation. The different chemistry of benzenethiol on various transition metal surfaces can be explained by the relative temperatures of C-S bond cleavage and hydrogen recombination and the hydrogenation and dehydrogenation activity of the different metals. Acknowledgment. We thank Eric Simanek for synthesizing benzenethiol-d5. We gratefully acknowledge the support of the National Science Foundation under Grants CHE-90-06024 and CHE-94-21615. We also thank and acknowledge Ms. Shira Fischer and Mr. James Kovacs for their assistance in modifying and preparing the figures and in checking several results. LA960170I
