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Thermal Reactions of 2-Naphthalenethiol Adsorbed on Ag(111) C. Kim, J. H. Sim, X.-M. Yan, and J. M. White* Department of Chemistry and Biochemistry, Center for Materials Chemistry, University of Texas at Austin, Austin, Texas 78712 Received August 22, 2001. In Final Form: January 31, 2002 The thermal properties of 2-naphthalenethiol, C10H7SH, dosed on Ag(111) at 150 K have been studied by temperature-programmed desorption (TPD) using time-of-flight mass spectrometry and Auger electron spectroscopy (AES). For low doses, no C10H7SH is observed in TPD, S-H bonds break below 235 K to form 2-naphthyl thiolate, C10H7S(a), and H(a), the latter desorbing as H2 below 300 K. For higher doses, dissociation is accompanied by C10H7SH desorption in a single unsaturable peak at 235 K. Reflecting the stability of the thiolate, there is negligible desorption between 300 and 500 K. Over a broad temperature range, 500-850 K, products desorb that couple C10H7 and C10H7S with H, with C10H7, or with C10H7S, but no H2 desorbs. Some products involve forming additional rings. At 850 K, the highest temperature probed, only H2S is desorbing. After TPD, AES detects a multilayer comprised of C and S with a C/S ratio of 3.1. A mechanism accounting for the products is presented.
Introduction Studying the chemisorption of organosulfur compounds on metal surfaces is of interest both for fundamental research on interfaces and for technological applications,1 such as photolithography,2,3 sensors,4 surface modification,5,6 and coal and petroleum processing.7 Self-assembled monolayers (SAMs) of alkanethiols, RSH, on gold surfaces have been extensively studied due to their high degree of structural order and relatively simple preparation under ambient conditions. The sulfur of alkanethiol SAMs is prone to oxidation under ambient conditions, resulting in the formation of sulfinates and sulfonates.8-11 These oxidized SAMs have lower structural stability and, typically, are no longer chemisorbed. When an aryl,12-16 rather than an alkyl, is linked to the sulfur, the thiolate is more stable; e.g., phenyl thiolate is more stable than alkanethiolates on gold.17 (1) lman, A. Chem. Rev. 1996, 96, 1533-1554. (2) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5406. (3) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626-628. (4) Cornell, B. A.; Braach-Maksvytis, V. L. B.; King, L. G.; Osman, P. D. J.; Raguse, B.; Wieczorek, L.; Pace, R. J. Nature 1997, 387, 580583. (5) Murray, R. W., Ed. Molecular Design of electrode Surfaces; Wiley: New York, 1992. (6) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022-9028. (7) Ertl, G., Knozinger, H., Weitkamp, J., Eds. Handbook of Heterogeneous Catalysis; VCH: Wenheim, 1997; p 1908. (8) Li, Y.; Huang, J.; McIver, R. T.; Hemminger, J. C. J. Am. Chem. Soc. 1992, 114, 2428-2432. (9) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398-1405. (10) Reiley, H.; Price, N. J.; White, R. G.; Blyth, R. I. R.; Robinson, A. W. Surf. Sci. 1995, 331-333, 189-195. (11) Scott, J. R.; Baker, L. S.; Russell, E. W.; Wilkins, C. L.; Fritch, I. Anal. Chem. 1997, 69, 2636-2639. (12) Kwan, W. S. V.; Atanasoska, L.; Miller, L. L. Langmuir 1991, 7, 1419-1425. (13) Creager, S. E.; Steiger, C. M. Langmuir 1995, 11, 1852-1854. (14) Chang, S.-C.; Chao, I.; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792-6805. (15) Sastry, M.; Patil, V.; Mayya, K. S. J. Phys. Chem. 1997, 101, 1167-1170. (16) Chadwick, J.; Myles, D.; Garell, R. L. J. Am. Chem. Soc. 1993, 115, 10364-10365. (17) Jaffey, D. M.; Madix, R. J. J. Am. Chem. Soc. 1994, 116, 30203027.
Ultrahigh vacuum (UHV) surface science studies of the adsorption and the reaction of sulfur-containing molecules, such as alkyl and aryl thiols, on various reactive metal surfaces have elucidated the mechanisms of industrially important reactions, such as desulfurization. On Ni,18,19 Cu,20,21 Pt,22,23 W,24,25 and Mo,26,27 S-H bond cleavage occurs generally around 100 K and C-S bond cleavage occurs below 300 K. Usually, no sulfur-containing species desorb. Gold substrates differ from more aggressive metals;17,28-31 in monolayers, S-H bond cleavage is incomplete and occurs below 300 K. C-S bonds cleave between 400 and 600 K. Carbon-carbon bonds form, along with sulfide. On silver, another coinage metal, thiols are slightly more reactive than on gold.14,28,31,32 Furthermore, especially on Ag(111), formation rather than dissociation of C-C and C-H bonds is favored.33 In interesting work, closely related to this paper, the surface chemistry of benzenethiol and tert-butyl thiol was studied on Au(110).17 S-H bonds broke to form phen(18) Kane, S. M.; Huntley, D. R. Gland, J. L. J. Am. Chem. Soc. 1996, 118, 3781-3782. (19) Castro, M. E.; White, J. M. Surf. Sci. 1991, 257, 22-32. (20) Sexton, B. A.; Nyberg, G. L. Surf. Sci. 1986, 165, 251. (21) Prince, N. P.; Seymour, D. L.; Woodruff, D. P.; Jones, R. G.; Walter, W. Surf. Sci. 1989, 215, 566-576. (22) Koestner, R. J.; Sto¨hr, J.; Gland, J. L.; Kollin, E. B.; Sette, F. Chem. Phys. Lett. 1985, 120, 285-291. (23) Rufael, T. S.; Koestner, R. J.; Kollin, E. B.; Salmeron, M.; Gland, J. L. Surf. Sci. 1993, 297, 272-285. (24) Benziger, J. B.; Preston, R. E. J. Phys. Chem. 1985, 89, 5002. (25) Mullins, D. R.; Lyman, P. F. J. Phy. Chem. 1993, 97, 92269232. (26) Roberts, J. T.; Friend, C. M. J. Chem. Phys. 1988, 88, 7172. (27) Wiegand, B. C.; Uvdal, P.; Friend, C. M. Surf. Sci. 1992, 279, 105-112. (28) Jaffey, D. M.; Madix, R. J. J. Am. Chem. Soc. 1994, 116, 30123019. (29) Jaffey, D. M.; Madix, R. J. Surf. Sci. 1994, 311, 159-171. (30) Bondzie, V.; Dixon-Warren, St. J.; Yu, Y.; Zhang, L. Surf. Sci. 1999, 431, 174-185. (31) Labinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167-3173. (32) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370-2378. (33) (a) Zhou, X.-L.; Schwaner, A. L.; White, J. M. J. Am. Chem. Soc. 1993, 115, 4309. (b) Zhou, X.; White, J. M. J. Phys. Chem. 1991, 95, 5575.
10.1021/la0113419 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002
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ylthiolate, which was thermally stable to 500 K. At higher temperatures, products that preserve the phenyl ring desorbed and followed different thermal profiles. In another interesting paper,32 the structure of alkanethiolate SAMs formed on Au and Ag were compared based on infrared spectroscopy. The packing was higher on Ag and the tilt of the methylene chain away from the surface normal was less (13° versus 30° for Au). Relevant to our work, thiols containing naphthyls, but linked to silver through a carbon atom,14 exhibit herringbone packing of the aromatic rings with sp hybridized S, i.e., a Ag-S-C angle of 180°. Building on this, we propose that naphthyl thiolates pack in a herringbone structure on Ag(111). Thiol reaction paths, of central interest in this paper, have been examined using surface science tools for adsorbates containing no more than six carbons, e.g., C6H5SH. One reason is the expanding number of possible reaction products. Recently, we have developed time-offlight mass spectrometry for temperature-programmed desorption, TOFMS-TPD,34 and isothermal reaction studies of such complex systems. Its fast acquisition of a widerange mass range (1-1000 amu) permits acquisition of three-dimensional intensity vs mass vs temperature (or time) plots that possess good signal-to-noise. In this paper, we present TOFMS-TPD and Auger electron spectroscopy (AES) results for an aryl thiol, 2-naphthalene thiol (C10H7SH), dosed on Ag(111) at 150 K. On the basis of known chemistry, including surface chemistry, we anticipate that Ag(111) will catalyze the breaking of S-H and C-S bonds and the formation of C-C, C-S, S-S, and C-H bonds but will be relatively inactive in catalyzing the rupture of C-H and C-C bonds. Thus, as expected, binaphthyl, (C10H7)2, and naphthyl sulfide (C10H7SC10H7) are prominent products. However, after the thiol hydrogen has desorbed as H2 and no further H2 desorbs, large amounts of naphthalene, C10H8, desorb over a broad temperature range (500-800 K). This result requires a H atom transfer reaction facilitated by Ag(111) but not involving the formation of Ag-H bonds. Further, C20 species with more than four rings and containing from zero to two sulfur atoms desorb. Surprisingly, the highest temperature TPD peak (800 K) is for desorption of H2S. After the sample is heated to 850 K, both C and S are identified by Auger electron spectroscopy. Experimental Section The experiments were carried out in an ultrahigh vacuum system with a base pressure of 2 × 10-10 Torr. The system consisted of a substrate manipulator, a cylindrical mirror analyzer for Auger electron spectroscopy (AES), a solid sample doser, and a time-of-flight mass spectrometer. A 10 mm diameter Ag(111) single crystal was mounted on a U-shaped tungsten heating wire, which was attached to the sample manipulator. The crystal could be cooled to 90 K by liquid nitrogen contact and resistively heated to 850 K, the upper limit set to avoid significant Ag sublimation. The substrate temperature was measured with a K-type thermocouple inserted into a hole in the side of the crystal. The substrate was cleaned by cycles of 1.5 kV Ar+ ion sputtering followed by 850 K annealing. After the substrate was cleaned, AES spectra showed no O, C, or S. To suppress background water adsorption, C10H7SH dosing was done at 150 K. The ramp rate was 2.5 K/s for most TPD experiments. 2-Naphthalene thiol (160 amu, Aldrich, 99%) underwent vacuum outgassing (∼1 day) at 300 K and was then loaded into a 5 mm diameter × 25 mm long stainless steel tube with a 0.1 mm pinhole. The tube, mounted on a translatable rod, was moved to within 2 cm of the Ag sample to dose and then retracted out (34) Kim, C.; Yan, X.-M.; White, J. M. Rev. Sci. Instrum. 2000, 71, 3502-3505.
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Figure 1. 3D-TPD spectra of 2 monolayers of 2-naphthalenethiol adsorbed on Ag(111). Note the three scaling factors ×1, ×5, and ×50 moving from left to right across the mass range. The gray scale reflects the intensity of the measured ion signal. Major contributors are identified in the text as follows: 128 amu (naphthalene), 160 amu (2-naphthalenethiol), 254 amu (2,2′-binaphthyl), 286 amu (binaphthyl sulfide), and 318 amu (binaphthyl disulfide and a five-ring species derived from it). Five-ring contributions to the region near 254 and 286 amu are identified in the text. of the UHV chamber and isolated by a gate valve. The doser tube temperature was adjustable from 300 to 700 K; 300 K was used in the experiments reported here. The TOFMS-TPD instrumentation is described in detail elsewhere.34 For ionization, 8 µs pulses of 70 eV electrons were used. No electron-induced adsorbate decomposition was detected. Acquisition of a single TOF mass spectrum from 1 to 500 amu took 50 µs, and 104 spectra were summed to give adequate signalto-noise (50 µs × 10000 ) 0.5 s). The acquired spectra are plotted as a histogram at each mass to form a 3D graph (Figure 1). A horizontal section corresponds to a mass spectrum at a specific temperature. A vertical section represents a traditional TPD spectrum at a specific mass.
Results and Discussion 3D-TPD Spectra. Arbitrarily defining a standard dose (SD) as the largest dose for which no 2-naphthalenethiol appears in TPD (see below), Figure 1 is a 3D-TPD spectrum for a 2 SD dose of 2-naphthalenethiol adsorbed on clean Ag(111) at 150 K. The gray scale indicates the intensity, x-axis the mass range (1-350 amu), and y-axis the temperature range (150-850 K). Different scaling (×1, ×5, and ×50) has been applied to different mass ranges. For each mass, a constant background has been removed by setting to zero the smallest intensity recorded for that mass over the whole temperature range. A quick survey of Figure 1 reveals some desorption below 200 K. Except for H2, it is attributable to desorption from surfaces other than Ag(111); e.g., the tungsten heating wire reaches reaction/desorption temperatures much more rapidly than the substrate. Many ions have intensity peaking near 235 K. These are attributable to physisorbed 2-naphthlalene thiol and disappear for low doses. Between 260 and 550 K, no ions, except those attributable to pumping of residual C10H7SH, have significant intensity above background. Setting in at 550 K and extending over a broad temperature range (up to 800 K), a number of ions (masses up to 318 amu) appear. The intensity profiles differ indicating that several different species desorb in this temperature regime. Above 800 K, the CO+, Ar+, and Ag+ (107 and 109 amu) signals increase. Since, in our instrument, bulk Ag does not sublime detectably at such low temperatures, the Ag+ signals are attributed either to inadvertent coating of Ag on the W during sputtering or to decomposition of a surface compound involving silver,
Thermal Properties of 2-Naphthalenethiol
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Figure 3. TPD spectra of 2-naphthalenethiol at 160 amu between 200 and 400 K for five doses (0.5-4 SD) at 150 K. The inset shows the integrated 160 amu intensity between 200 and 300 K as a function of coverage. The standard dose (SD) scale is defined by the x-axis intercept of the inset.
Figure 2. H2 TPD spectra, offset for clarity, for five doses of 2-naphthalenethiol on Ag(111) at 150 K. The solid curve through the data is replication of a smooth curve drawn through the 2 SD data. The noisy dashed curve placed over the 2 SD data is the measured 1 amu signal and is taken as a background and used to form the short smooth dashed segment used as a baseline for all the spectra. The 3 amu signal is plotted at the bottom.
e.g., Ag2S. Mass 40 is due to desorption of argon impregnated into Ag during sputtering and not removed by prior annealing. As discussed below, the most interesting peak above 800 K is at 34 amu (H2S+). Hydrogen Desorption (2 amu). We turn to examining details extracted from the 3D data sets. Figure 2 shows H2 desorption for five thiol doses ranging from 0.5 to 4 SD (offset for viewing clarity). The solid curve is drawn through the 2 SD data and, to provide comparison, is placed over the four other spectra. The dotted curve plotted with the 2 SD data is the 1 amu signal. Above 300 K, this intensity profile overlays, without adjustment, the 2 amu signal, so we take it as a measure of the background for the 2 amu curves (dashed line segment in the four other curves). The 3 amu signal, plotted at the bottom of Figure 2, is featureless and indicates negligible instrument drift during a TPD measurement. While the spectra are noisy due to the relatively large background, the following points are clear: (1) there is significant H2+ signal peaking at 215 K with a shoulder at about 185 K; (2) the integrated intensity (above the dashed background) increases and saturates at 1.5 SD as the dose increases; and (3) the H2+ signals are featureless above 250 K. The last point is very important. Since H atom recombination is facile on Ag(111) above 180 K, all reaction paths that involve formation of atomic H bound to Ag above 300 K are excluded. Below 250 K, the situation is different; H2 desorption is evident. The 185 K shoulder is assigned to recombinative
desorption of atomic H formed and bound to Ag(111) at lower temperatures.35 On Ag(111), H atoms recombine with a H2+ peak between 167 and 191 K depending on the H atom dose.36 At low coverage, the full width at halfmaximum (fwhm) is ∼20 K. Since, compared to C-H bonds, S-H bonds are easy to activate, we propose that some 2-naphthalenethiol on Ag(111) dissociates at or below 185 K, forming 2-naphthyl thiolate, C10H7S(a), and atomic hydrogen, i.e.
C10H7SH(g) f C10H7S(a) + H(a)
(1)
The H2+ intensity peaking at 215 K in Figure 2 is assigned to thermally activated formation of atomic H, prompt recombination, and desorption of H2; i.e., the H2 desorption rate is controlled by the rate of H atom formation, a process competing with the desorption of C10H7SH. Turning to desorption of C10H7SH, notice in Figure 1 that many ions have clearly identified peaks at 235 K with fwhm of about 20 K. These are assigned to physisorbed 2-naphthalenethiol since, except for H2+, the relative intensities match the published fragmentation pattern of 2-naphthalenethiol.37 The peak temperature corresponds to desorption activation energy of ∼60 kJ/ mol. As monitored by the C10H7SH+ signal, the 235 K desorption of C7H10SH does not occur for low doses, Figure 3. Evidently, up to some narrowly defined coverage, all adsorbed thiol dissociates during dosing at 150 K and/or heating to the upper limit of thiol desorption, 250 K. With increasing dose, the rising edges shift to lower temperature, the peak temperature increases, and, for 2 and 4 SD, the decay above the peak is very sharp. For doses above a threshold, the integrated intensity (inset) grows (35) Lee, G.; Plummer, E. W. Phys. Rev. B 1995, 51, 7250. (36) Zhou, X.-L.; White, J. M.; Koel, B. E. Surf. Sci. 1989, 218, 201. (37) Linstrom, P. J., Mallard, W. G., Eds. NIST Chemistry WebBook; NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg, MD, July 2001 (http:// webbook.nist.gov).
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Figure 5. Approach to saturation of TPD peak areas for 128, 160, and 286 amu. The 160 amu signal is integrated from 300 to 850 K to eliminate contributions from the low-temperature 2-naphthalenethiol desorption.
Figure 4. For 2 SD dose of C10H7SH, thermal desorption spectra of selected masses extracted from Figure 1.
linearly, reflecting little, if any, additional C10H7SH dissociation. The extrapolated intercept on the dose axis is used to define a standard dose (SD). As we shall see, all the products, like the H2 of Figure 2, saturate above the onset. From these characteristics, we conclude that (1) dissociation dominates for low doses, (2) the sticking probability is uniform above the threshold, (3) the kinetics above 230 K are zero order, (4) the kinetics below 230 K are more complex than simple zero or first order because the rising edges do not overlap but the peak temperatures increase with dose, and (5) the species formed by H atom release is chemisorbed. Since H2 desorption, controlled by cleaving bonds to supply atomic H, overlaps and competes with C10H7SH desorption, and since the dissociation partner, C10H7S, chemisorbs, it is reasonable to suppose that the process alters the local chemical environment and, thus, impacts and complicates the desorption kinetics of neighboring C10H7SH. Significantly, there is no evidence in the C10H7SH desorption for a distinct first layer; evidently, C10H7SH desorption occurs only from a surface passivated with chemisorbed dissociation fragments. There are only two sources for the H2 evolution above 200 K, either C-H or S-H bonds must break. Since, typically, thermally activated cleavage of C-H bonds by Ag does not occur at such low temperatures, the most likely source is cleavage of additional S-H bonds. This thermally activated process occurs as H2 desorption frees Ag binding sites for S atoms. Taken together, the evidence indicates that, upon reaching 250 K, no S-H bonds remain and thiolate, C10H7S(a), is the only significant chemisorbed species. Thiolates are quite stable. A full survey of masses up to 400 amu reveals no desorption between 250 and about 500 K. Above 550 K, desorption of species with masses up to 316 amu is evident, Figure 1. Several examples, Figure 4, follow different thermal profiles indicating that the distribution of desorbing species changes as the temperature, coverages, and surface constituents change. The
species do not arise from either contamination of dosed 2-naphthalenethiol or reactions inside the ionizer because (1) the peak intensities saturate as the dose increases (Figure 5) and (2) peaks for these masses never appear with C10H7SH as it desorbs around 235 K. Rather, reactions of chemisorbed thiolate, C10H7S(a), must account for these products. Above 500 K, the first signal to rise is 128 amu. This very broad signal remains the most intense from 500 to 775 K where the 34 amu (H2S+) signal is the most intense. On the basis of fragmentation pattern analysis, the strong 128 amu peak is dominated by naphthalene, but may include 500 K is necessary to activate the reactions of the thiolate, and since C- and S-containing species continue to desorb to 800 K where the vapor pressure of Ag is 10-9 Torr, the observed chemistry may well involve moving Ag atoms away from their (111) positions. This is supported by scanning tunneling microscopy work on Ag(111) that gives evidence of, even at 300 K, mobile Ag atoms.39 On another coinage metal, Au(111), formation of thiolates at 300 K removes a herringbone reconstruction of Au and, for coverages near saturation, forms Au atom vacancies.40 Movement of Ag atoms would reduce steric constraints and make organometallic analogues relevant. Among the desorption products appearing above 500 K, several require C-H bond breaking in thiolate or species derived therefrom. These include naphthalene, naphthalene thiol, and the five-ring species formed above 700 K. Of these naphthalene and the thiol also require a source of H to form of C-H bonds. Because H2 does not desorb, the H must transfer along a reaction path that does not involve releasing H onto the Ag surface. One plausible path involves two adjacent thiolates oriented nearly vertically in a herringbone fashion, 4.14,32
To the extent that steric constraints permit, the thiolates may take different orientations and angles with respect the surface and with respect to neighboring species. Since the surface is crowded, most surface Ag atoms are distorted. If the distortion is sufficient, we can consider these as disengaged from the electronic band structure of bulk Ag and taking, in a formal sense, a positive oxidation number by virtue of having donated electron density to the sulfur of the thiolate. In the proposed path, thermoneutral C-H transfers from one thiolate in concert with coupled with C-S bond cleavage in the other to form naphthalene and a chemisorbed product containing two C-S bonds, 5. This path is not strongly hindered sterically, especially if the Ag atoms beneath the thiolates are allowed some motion perpendicular to the surface, as indicated in 4. Moreover, the process may locally reduce steric constraints.
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The herringbone structure 4 also readily accounts for several other desorbing products including the binaphthyl (C10H7C10H7), the sulfide (C10H7SC10H7), and the disulfide (C10H7SSC10H7). All of these preserve the ring structure of the starting thiol and locally reduce the steric constraints on subsequent processes. As these products desorb, another path to naphthalene may open and, as observed, extend its thermal profile to higher temparatures. As space opens, cleaving a C-S bond to form (C10H7)(a) and S(a) may occur and be followed by H atom transfer from a neighboring thiolate or partially dehydrogenated species such as 5. To avoid H-Ag formation and H2(g) evolution, as required by the data, this transfer would be mediated by S(a) as in 6.
The aforementioned reaction path relying heavily on the proposed herringbone thiolate structure adequately accounts for the observed products with two or four rings. We now consider the desorption of compounds that contain five rings. These become more competitive with their fourring counterparts as the temperature increases through the region where both desorb, e.g., 2,2′-binaphthyl and naphthalated cyclobutane, 2. In each case, linking two partially dehydrogenated species could account for the desorption of species with five rings. For example, at 252 amu the five-ring compound, 2, could be formed by linking two species 5. Isomers would arise depending on whether the dehydrogenation leading to 5 removed H from C1 or C3. The competition between the 286 and 284 amu desorptions may involve another dehydrogenation path that becomes competitive if the steric constraints are relaxed enough to accommodate partially dehydrogenated thiolate as a metallacycle. In this path, transferring one H to a neighboring thiolate releasing naphthalene, leaves a metallacycle 7 with both a S-Ag and a C-Ag bond. Assuming the C-H bond adjacent to the S is easiest to react will leave two possible results depending on whether the H is removed from position 1 or 3. Linking a pair of these in a path that accommodates one sulfur atom forms 1. While metallacycles, including heteroatom metallacycles, on Ag are known,41 7 and other structures like it are speculative.
Thermal Properties of 2-Naphthalenethiol
A similar competition occurs between the 316 and 318 amu species. The 318 amu is accounted for by linking adjacent thiolates 4 through the formation of a S-S bond. Since this desorption occurs near the end of the TPD cycle (>700 K), only a few thiolates remain and some are adjacent to each other. The surface surrounding these pairs is crowded with partially dehydrogenated thiolates and sulfur atoms that contribute to the C and S AES signals measured after TPD. To form the competitor 3 (or its isomers 8 and 9) involves dehydration of a carbon atom on each member of the pair and linking the two sulfur atoms. The H atoms released can be transferred via a sulfur atom(s), to hydrogenate a naphthyl moiety and desorb as naphthalene as observed.
It is clear from the 800 K desorption of H2S that species containing significant amounts of H survive to very high temperatures in these experiments. This requires a surface condition in which the surface Ag atoms are completely passivated with respect to C-H bond breaking. We propose that the crowded nature of the surface places species such as 5 and 7 in surroundings that do not allow the C-H bonds to couple with the Ag. The formation of H2S lends support to our proposed model 6 for the transfer of H. As products desorb, the surface becomes even more Hdeficient and hydrogenation to species containing naphthyl moieties becomes less likely (more demanding) and hydrogenation of S to H2S becomes competitive. At these high temperatures, this process can probably be considered as pyrolysis of a 2D gas. Turning to thermodynamic considerations, average bond energies of interest are as follows (in kJ mol-1): C-S, 272; S-H, 347; C-H, 413; S-S, 226;42 Ag-SR, 375;43 sulfide Ag-S, 430;44 Ag-C, 159;45 Ag-H, 226.46 From a thermodynamic perspective, since the sulfur atom is bound to the 2 position in the thiol, a position presumably maintained in the thiolate, forming the 2,2′binaphthyl isomer is favorable, free energy arising from (39) Pai, W. W.; Bartelt, N. C.; Peng, M. R.; Ruett-Robey, J. E. Surf. Sci. 1995, 330, L679. (40) Poirier, G. E. Chem. Rev. 1997, 97, 1117-1127. (41) Medlin, J.; Sherrill, A.; Chen, J.; Barteau, M. J. Phys. Chem. B 2001, 105, 3769. (42) Weast, R. C., Ed. Handbook of Chemistry and Physics, 52nd ed.; CRC Press: Cleveland, OH, 1972. (43) Crudely estimated by interpolation between Ag2S and S-H. (44) Based on the energy required to dissociate solid Ag2S into atoms as cited in: Yungman, V. S., Ed. Thermal Constants of Substances; Wiley and Bagell House: New York, 1999; Vol. 4, Part 5, p 146. (45) (a) Bauschlicher, C. W., Jr.; Langhoff, S. R.; Partridge, H.; Barnes, L. W. J. Chem. Phys. 1989, 91, 339. (b) Zhou, X.-L.; Blass, P. M.; Koel, B. E.; White, J. M. Surf. Sci. 1992, 271, 427. (46) Simoes, J. A. M.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629.
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both enthalpic and entropic considerations. A favorable enthalpy change accompanies the formation of a strong C-C bond and two strong Ag-S (sulfide) bonds at the expense of cleaving two relatively weak C-S bonds, and a favorable entropy change is associated with the release of binaphthyl into the gas phase. Locally, chemisorbed sulfide may form a surface compound, Ag2S, since this process occurs at 300 K when Ag is dosed with atomic sulfur.47 We have proposed that the very broad temperature range over which naphthalene desorbs is, in part, the result of multiple paths. Assuming 5 is formed along with naphthalene, the enthalpy change involves (1) the roughly thermoneutral C-H breaking and forming and (2) the roughly thermoneutral replacement of one C-S bond by another. The favorable free energy for this reaction is likely the entropy gained by releasing naphthlene into the gas phase. If, on the other hand, naphthalene and the metallacycle 7 are involved, then the C-H breaking and forming are roughly thermoneutral while a C-S bond breaks, a stronger sulfide Ag-S bond is formed, and a relatively weak C-Ag bond is formed. Desorption of naphthyl sulfide involves the replacing one C-S bond with another, replacing a relatively weak thiolate Ag-S bond with a strong silver sulfide (S-Ag) bond, and gaining entropy in the desorption process. The disulfide desorption involves replacing two Ag-S thiolate bonds with one S-S bond, likely endothermic and overcome only at high temperatures by entropic contributions to the free energy. The naphthalene desorption peaks at 660 K, higher than the corresponding benzene desorption from phenyl thiolate on Au(110)17 and, to our knowledge, is the highest recorded desorption temperature of a hydrocarbon desulfurization product derived from thiols adsorbed on metals. Compared with benzene thiol on Au(110),17 there are interesting similarities and differences. The most striking difference is the dominance of naphthalene in our case compared to the weak, and questioned, desorption of benzene from phenylthiolate formed on Au(110). Among the similarities, S-H bonds broke to form phenylthiolate which was thermally stable to 500 K. At higher temperatures, products that preserve the phenyl ring desorb following differing thermal profiles. These are dominated by biphenyl (615-619 K peak), benzenethiol (630-635 K peak), and dibenzothiophene (five-atom ring containing S between two phenyl groups, 638-645 K), with small amounts of diphenyl sulfide (594 K peak). A search was made for thianthrene (six-atom ring containing 2 S atoms between two phenyl rings) and biphenylene (four-atom ring using 2 C atoms from each of two phenyl rings). The latter was never observed, while there was occasional evidence for the former (coincident with dibenzothiophene). To form and desorb benzenthiol requires a source of H and, consistent with C and S retention after TPD to 700 K, a metallacycle with S-Au and γ-C-Ag bonding was proposed. The intermediates did not involve the formation of H-Au bonds since there was negligible H2 evolution (
