1292
Langrnuir 1989,5, 1292-1298
selves along the stearic acid chains. Results available to date indicate that the poly(3-alkylthiophenes) are randomly distributed within layers of highly ordered stearic acid molecules. A periodicity of 50 if for the bilayer spacing is maintained throughout the LB films as determined by X-ray diffraction.’ A proposed model on the microstructure of this mixed system is illustrated in Figure 8. The anisotropy of the electrical conductivity and the packing efficiency will be investigated in future studies.
Conclusion
Two types of poly(heterocyc1e) Langmuir-Blodgett films have been fabricated. The pyrrole/odadecylpyrrole copolymers exhibit disordering compared with the highly ordered alkyl-substituted pyrrole monomer LB films. In the monomer films, the hydrocarbon chains are highly ordered and oriented along the surface normal for multilayer films. A strong interaction between the pyrrole
head groups and the Pt substrate was observed in the single-layer monomer L B films. T h e p o l y ( 3 alkylthiophene) /stearic acid LB films show a high degree of ordering with the hydrocarbon chains of the stearic acid oriented along the surface normal. The hydrocarbon chains of poly(3-ODT) are also oriented along the surface normal, in contrast with the random orientation of the shorter chains of the poly(3-BT).
Acknowledgment. We acknowledge technical support from R. Garrett of NSLS and R. Gaylord of Los Alamos National Laboratory. This work was supported by the US. Department of Energy, Division of Materials Science, under Contract No. DE-AC02-76CH00016. Registry No. ST, 57-11-4; 3-BT, 98837-51-5; poly-3-OT, 104934-51-2;poly-3-ODT,104934-55-6;HDP, 117241-58-4;ODP, 93362-22-2;Pt, 7440-06-4;FeCl,, 7705-08-0; pyrrole-3-hexadecylpyrrole polymer, 120543-69-3;pyrrole-3-octadecylpyrole polymer, 117496-56-7.
Adsorbate Thermodynamics as a Determinant of Reaction Mechanism: Pentamethylene Sulfide on Mo( 110) Benjamin C. Wiegand, C. M. Friend,* and Jeffrey T. Roberts Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 Received March 17, 1989. I n Final Form: July 19, 1989 The reactions of the totally unstrained, six-membered cyclic sulfide pentamethylene sulfide on Mo(l10) have been investigated by using temperature-programmed reaction spectroscopy and X-ray photoelectron spectroscopy in an effort to identify the roles of ring size and strain in dictating reaction selectivity. Four gaseous products are detected in the temperature-programmed reaction of pentamethylene sulfide: dihydrogen at 380 and 590 K, pentane at 350 K, pentene at 345 K, and pentamethylene sulfide at 190 and 280 K. The kinetics for hydrocarbon production from pentamethylene sulfide are qualitatively different than for the four- and five-membered cyclic sulfides, trimethylene sulfide and tetrahydrothiophene. In the case of the reaction of pentamethylene sulfide on Mo(llO), pentane and pentene are produced, but with kinetics distinctly different than those measured for hydrogenolysis of pentyl thiolate in the case of pentanethiol. Thus, the hydrogenolysis of the pentyl thiolate is ruled out as the rate-limiting step. The kinetics of hydrocarbon formation are altered in part due to surface modification by the decomposition of molecular pentamethylene sulfide to form atomic sulfur and hydrocarbon fragments before product formation commences. We propose that these differences in kinetics and selectivity are due to the lack of ring strain in pentamethylene sulfide, which increases the activation energy for ring opening compared to the four- and five-membered rings. The kinetics for hydrocarbon formation are, thus, determined by the ring-opening step rather than thiolate decomposition. In the case of the C, and C, cyclic sulfides, thiolate hydrogenolysis is rate limiting. We propose that for pentamethylene sulfide the thiolate is a short-lived intermediate which decomposes rapidly to produce almost simultaneously pentane and pentene.
Introduction
’Presented at the symposium on “Metal-Catalyzed Reactions of Heteroatom-ContainingMolecules”, Division of Colloid and Surface Chemistry, 196th National Meeting of the American Chemical Society, Los Angeles, CA, Sept 25-30,1988. (1) Roberts, J. T.; Friend, C. M. J. Am. Chem. SOC.1986,108,72047210.
0743-7463/89/2405-1292$01.50/0
ence of changing ring size and strain on reactivity. Four
decomposition to gaseous dihydrogen and surface car(2) Roberts, J. T.; Friend, C. M. J . Am. Chem. SOC.1987,109,38723882. (3) Roberts, J. T.; Friend, C. M. Surf. Sci. 1988,202, 405-432. (4) Roberts, J. T.; Friend, C. M. J . Am. Chem. SOC.1987,109,78997900.
0 1989 American Chemical Society
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Adsorbate Thermodynamics in Reaction Mechanisms bon and sulfur. Ring size plays a key role in determining the relative contribution of the four pathways. For instance, ethylene sulfide (c-C,H,S) reacts primarily by way of intramolecular elimination to ethylene along with a minor amount of d e c o m p ~ s i t i o n while ,~ tetrahydrothiophene (c-C,H,S) undergoes competing thiolate formation, molecular desorption, and decomposition.' The analogous C, molecule (c-C,H,S), trimethylene sulfide, reacts via intramolecular elimination, thiolate formation, and decomposition with no observed molecular desorption.2 We developed a model to account for the observed structure-activity relationships governing the reactions of cyclic sulfides on Mo(ll0). It was proposed that ring strain largely determines the reaction selectivity of cyclic sulfides on Mo(ll0). Intramolecular elimination and thiolate formation are favored for highly strained systems because they result in loss of cyclic sulfide ring strain, lowering the barrier for reaction. The activation energies for molecular desorption and decomposition, however, are unaffected by ring strain. Hence, for less strained molecules such as tetrahydrothiophene, molecular desorption and decomposition compete more effectively with thiolate formation, and intramolecular elimination is not observed at all. Here we report the reactions of the six-membered ring, pentamethylene sulfide (c-C,H,$), on Mo(ll0). The reactions of pentamethylene sulfide were investigated because there is zero ring strain in pentamethylene sulfide compared to 1.7 kcal/mol for tetrahydr~thiophene.~ Thus, pentamethylene sulfide provides a further test for the proposal that ring strain dictates reaction selectivity of cyclic sulfides adsorbed on Mo(ll0).
Experimental Section All experimentswere performed in an ultra-high-vacuumchamber described previously.'.2 The base pressure was always 5 2 X Torr. The Mo(ll0) sample was heated radiatively for temperature-programmed reaction experiments with a thoriated tungsten filament positioned approximately 0.06 in. behind the molybdenum crystal. Radiative heating rates were approximately constant with a smalllinear decreaseas the crystal heated, with dT/dt = 16 f 1K/s between 120 and 700 K. The sample was heated by electron bombardment for cleaning purposes,with the crystal biased at +500 V with respect to the grounded filament. As described previously,' the crystal was cleaned by oxidation at 1200 K in 1 X lo-' Torr of 0, for 4-10 min. The resulting surface oxide was removed by flashing to 1900K. Crystal cleanliness and order were verified by Auger electron spectroscopy and low-energy electron diffraction, respectively. X-ray photoelectron data were collected with a Physical Electronics ESCA 5300 system. The spectrometer is equipped with a dual Mg/Al X-ray source and a hemispherical electron energy analyzer. The data reported here were taken with the Mg anode (photon energy 1253.6 eV). Three elements, C(ls), S(2p), and Mo(3d),were monitored during a single experiment, which usually lasted 20 min. All spectra shown in this paper have been corrected to account for background intensity from the clean Mo(ll0)crystal. Binding energies were referencedto the Mo(ll0) Fermi level and calibrated against the Mo(3d5/,) peak at 227.7 eV. Since there exists spin-orbit coupling in the S(2p) final state, each chemical state of sulfur has two associated photoelectron peaks. The two peaks have a S(2p, ,):S(2p11.J relative intensity of 1.8:l.O and are split by 1.2 et/. Temperature-programmedreaction data were recorded with a UTI-100C mass spectrometer interfaced to an IBM personal computer.',' The data shown in Figures 1-4 were obtained by using a program that allowed for the collection of up to 10 separate ion intensity-temperature profiles during a single exper(5) Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley: New York,1976.
I
I
I x 100) pentamet hylene SUI f ide
-
OD-
I 100
'
I ' / ' 1 ' / ' 1 200 300 400 500 600 Temperature ( K )
'
1 700
'
1
800
Figure 1. Temperature-programmed reaction of pentamethylene adsorbed to saturation on Mo(ll0). The multiplication factors are referenced to hydrogen and are uncorrectedfor degree of fragmentation and ionization efficiency in the mass spectrometer. The pentene spectrum has been corrected for a small amount of cracking due to pentamethylene sulfide.
iment. A second program allowing for the collection of up to 100consecutiveion intensity-temperature profless was employed t n check for the formation of any unanticipated products. The mass spectrometerwas shielded by a double-jacketedliquid nitrogen cooled shield equipped with a flag that could be rotated to vary the entrance aperture. The entrance aperture during temperature-programmed reaction was 0.0625 in., and the crystal was located approximately 0.125 in. from the aperture. The aperture could be opened to 2.5 in. to measure the composition of the background or dose gasses. Pentamethylene sulfide (99%) was obtained from Aldrich, dried over sodium sulfate, and distilled under vacuum. Pentamethylene sulfide was degassed by several freeze-pumpthaw cycles before use each day. l-Pentene (99%), purchased from Phillips 66, and n-pentane (98%),purchased from Mallinckrodt, were used without further purification. Oxygen (99.8%), hydrogen (99.9995%), and deuterium (99.0%) were obtained from Matheson and used without further purification.
Results Four gaseous products are detected in the temperatureprogrammed reaction of pentamethylene sulfide on Mo(llO), as shown in Figure 1: dihydrogen (m/e 2) at 380 and 590 K (labeled P1 and Pz, respectively), pentane (m/e 43) at 350 K, pentene (m/e 42) at 345 K, and pentamethylene sulfide (m/e 102) at 190 and 280 K. The ions chosen for detection correspond to the most intense cracking fragments (pentane and pentene') or to the molecular ions (dihydrogen and pentamethylene sulfide). The identities of the pentane and pentene products were confirmed by detection of the parent ions. As expected,' the pentane parent ion (m/e 72) was approximately 90% less intense than m/e 43, while the pentene parent ion (m/e 70) was approximately 75% less intense than m/e 42. The fragmentation patterns of authentic samples of n-pentane and l-pentene in our mass spectrometer were also in agreement with these ion ratios. Auger electron spectroscopy was used to determine the relative amounts of hydrocarbon production and nonselective decompositionbased on the C:S stoichiometry after reaction of pentamethylene sulfide on M0(110).'*~Rcls, the carbon:sulfur Auger ratio: is measured and compared to Rcls from thiophene (c-C4H4S) reacting on Mo(ll0). Since thiophene decomposes on Mo(ll0) with(6) Liu, A. C.; Friend, C. M. Rev. Sci. Znstrum. 1985,57,1519-1522. (7) Massot, R.; Cornu, A. Compilation of Mass Spectral Data, 2nd
ed.; Heyden: London, 1975; Vol. 2.
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( b ) Mo( I10 ]-p-(2x2 1-S
dlhydrogen
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/
400 500 600 Temperature ( K 1
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Temperature [ K )
Figure 2. Temperature-programmedreaction of n-pentane on (a) Mo(ll0) and (b) Mo(110)-p(2X2)-S. The pentane peak corresponds to m f e 72 and dihydrogen to m f e 2.
Figure 3. Temperature-programmedreaction of 1-penteneon (a) Mo(ll0) and (b) Mo(llO)-p(2x2)-S. The pentene peak corresponds to m f e IO and dihydrogen to m f e 2.
out hydrocarbon formation, the Rc j s measured after its reaction serves as a standard for a C4S surface stoichiometry. Rcls from thiophene on Mo(ll0) has been measured to be 0.15 f 0.01.' In this work, Rcjs from pentamethylene sulfide was also measured to be 0.15 f 0.01, indicating a stoichiometry of C4S. Given the stoichiometry of pentamethylene sulfide, we estimate that -20% of all irreversibly bound pentamethylene sulfide reacts to form hydrocarbons, while -80% decomposes to gaseous dihydrogen and surface sulfur and carbon on clean Mo(ll0). The relative amount of molecular desorption is not determined by this experiment. Calculating RsjM;' and comparing it to RSjMofor Mo(llO)-p(2X2)-S determines that 0.12 monolayer of sulfur is left on the surface after temperature-programmed reaction of pentamethylene sulfide on Mo(ll0). The saturation value for pentamethylene sulfide is very similar to other related molecules studied before, tetrahydrothiophene' (0.13) and thiophene' (0.10). Temperature-programmed reactions of n-pentane and 1-pentene were studied for comparison to the temperature-programmed reaction of pentamethylene sulfide on Mo(ll0) and Mo(110)-p(2x2)-S and are shown in Figures 2 and 3. Briefly, the desorption of n-pentane from Mo(ll0) and Mo(llO)-p(2x2)-S, shown in Figure 2, demonstrates that pentane production from pentamethylene sulfide is controlled by reaction kinetics, not pentane desorption. On the Mo(ll0) surface, with an exposure greater than reaction saturation, pentane desorbs in a two peak structure at 170 and 220 K. On the sulfided Mo(ll0) surface, there is a shift in peak temperatures, and the two peaks are seen at 175 and 200 K. Similarly, l-pentene desorbs from clean Mo(ll0) and Mo(llO)-p(2X2)S below 250 K at exposures exceeding reaction saturation, as shown in Figure 3. This indicates that the rate of pentene formation from temperature-programmed reaction of pentamethylene sulfide is limited by the reaction, not pentene desorption kinetics. Since these temperatures are well below the temperatures seen for pro-
duction of pentane and pentene from temperatureprogrammed reaction of pentamethylene sulfide on Mo(llO), their formation is limited by the decomposition of a surface intermediate and then immediate product ejection into the gas phase. The dependence of pentene desorption on coverage was not investigated. The decomposition of n-pentane and 1-pentene was studied on Mo(ll0) and Mo(llO)-p(2X2)-S by monitoring dihydrogen evolution, in an effort to learn more about the decomposition of possible hydrocarbon intermediates. 1-Pentene primarily decomposes to form dihydrogen in a three-peak structure at temperatures of 300,410, and 545 K. On the sulfided surface, 1-pentenealso undergoes decomposition, although it is 45% less than on the clean Mo(ll0) surface. Dihydrogen evolves over a broad temperature range, with peaks at 375 and 570 K. nPentane decomposes to form dihydrogen in a broad peak on Mo(ll0) over a temperature range 250-600 K, centered at 425 K. On the sulfided surface, there is again 45% less decomposition than on the clean Mo(ll0) surface, with a broad peak from 250 to 500 K, along with a tail extending up to 600 K. The pentamethylene sulfide peak at 280 K corresponds to a desorption energy of approximately 18 kcal/ moll' and is comparable to the desorption temperature of tetrahydrothiophene from Mo(ll0) (310 K).' Reversibly adsorbed pentamethylene sulfide does not incorporate surface deuterium. Thus, the peak is assigned to the desorption of associatively adsorbed pentamethylene sulfide. The sharp pentamethylene sulfide peak at 190 K does not saturate with increasing exposure and is therefore attributed to the sublimation of condensed pentamethylene sulfide. The temperature-programmed reaction of pentamethylene sulfide was also carried out on a surface presaturated with hydrogen. Pentane and pentene were both evolved from the surface at 325 K, 25 K lower than from the clean Mo(ll0) surface, and the yields of both pentane (-250%) and pentene (-50%) were increased. This suggests that the rate-limiting step in formation of hydrocarbon products is the hydrogenolysis of one of the C-S bonds in the pentamethylene sulfide ring, and the availability of hydrogen plays a major role in that ring-opening step. If a subsequent step were rate limiting, the yield and kinetics for pentene production should be unaffected by hydrogen preadsorption. Neither desorption
(8). R, s is d e f i e d as the ratio of the peak-to-peak heights of the followng Auger signals: [C(KLL,272 eV)] [Mo(LMM, 148 eV) + S(KLL, 152 eV)]. The sulfur signal overlaps wi the Mo signal at 148 eV, but at these coverages, the contribution of the Mo peak is negligible. Auger spectra were measured in the derivative mode. (9) Roberta, J. T.; Friend, C. M. Surf. Sei. 1987, 201-218. (10) Ea,? is defined aa the ratio of the peak-to-peak heights measured in t e erivative spectra of the following Auger signals:[Mo(LMM, 148 eV) + S(KLL, 152 eV)]/[Mo(LMM, 186 eV) + Mo(LMM, 221 eV)]. Although the sulfur peak overlaps the molybdenum peak, at the measured high sulfur coverages, the contribution of the molybdenum peak is negligible.
d
(11) (a) A preexponential of lot3s-* and first-order Arrhenius kinetics were assumed to estimate the desorption energy.11b (b) Redhead, P.A. V U C U U 1962,12, ~ 203-211.
Langmuir, Vol. 5, No. 6, 1989 1295
Adsorbate Thermodynamics in Reaction Mechanisms temperature nor yield of the molecular pentamethylene sulfide desorption peak was affected by the preadsorption of dihydrogen. The dihydrogen mass spectra were changed, with an increased yield in the 0,-dihydrogen peak, due to extra hydrogen on the surface and more recombination. The P,-dihydrogen peak decreased in yield, due to the increased hydrocarbon product formation, and hence less decomposition,of pentamethylene sulfide. Consistent with this observation is the fact that Auger electron spectroscopy performed on the surface after temperature-programmed reaction of pentamethylene sulfide on a hydrogen-presaturated surface showed slightly less carbon remaining on the surface, due to the increased hydrocarbon formation. The measured Rcls is 0.13 f 0.01 after temperature-programmed reaction of pentamethylene sulfide on the hydrogen-presaturated surface. The temperature-programmed reaction of pentamethylene s a i d e on the deuterium preadsorbed surface resulted in deuterium incorporation into the pentane and pentene. During the course of temperature-programmed reaction, mle 42,43,44, and 45 were detected to evolve from the surface at 325 K. However, since the peak temperatures of the two products were essentially concurrent, it was impossible to unambiguously deconvolute the temperature-programmed reaction spectra of different mle profiles. For example, mle 44 could be due to either d,pentane or d,-pentene. However, on the basis of previously studied systems where the alkane and alkene line shapes permitted deconvolution, we assign the ions detected (mle 42,43,44, and 45) as do-pentene, d,-pentene and do-pentane, d,-pentane, and d,-pentane, respectively. The other product from temperature-programmed reaction of pentamethylene sulfide on Mo(ll0) is dihydrogen. &-Dihydrogen evolution results from a superposition of hydrogen atom recombination and hydrocarbon fragment decompositionprocesses. If pentamethylene sulfide is reacted on a Mo(ll0) surface that has been presaturated with deuterium atoms, then the deuterium that evolves from the P1 states as HD and D, appears preferentially at the leading edge of the p1 peak. Thus, the leading edge of the P1-H2peak is attributed to the recombination of surface hydrogen atoms. This demonstrates that some nonselective C-H bond cleavage has occurred below 350 K. The far edge of the PI peak is assigned to the decomposition of a surface hydrocarbon fragment. Similar results are obtained for tetrahydrothiophene and trimethylene sulfide reacting on Mo(ll0) in the presence of surface deuterium atoms.'.' In analogy with previous w ~ r k , l - P,-dihydrogen ~,~~ formation is attributed to the decomposition of a surface hydrocarbon fragment stabilized by the presence of atomic sulfur. The temperature of P2-H2production is too high to be rate limited by recombination of surface hydrogen atoms. The presence of HD and D, formation in the &-dihydrogen peak demonstrates that reversible C-H bond activation has occurred. The P,-dihydrogen peak temperature for pentamethylene sulfide is greater than for decomposition of both pentane and pentene on either clean Mo(ll0) or Mo(110)-p(2X2)-S, suggesting that the decomposition pathways are different for these systems. Temperature-programmed reaction of a mixture of ethanethiol and pentamethylene sulfide on Mo(ll0) demonstrates that the kinetics for hydrocarbon production from pentamethylene sulfide are not characteristic of the decomposition kinetics of the n-pentyl thiolate intermediate. We have shown previously that temperature-pro(12) Roberts, J. T.; Friend, C. M. Surf. Sci. 1988,198, L321-L328.
A
ethane
200
300
400
1
, ; h A ,
200
300
400
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Figure 4. Formation of ethane and pentane during temperature-programmed reaction of (a) a 5:l mixture of ethanethiol and pentanethiol on Mo(ll0) and (b) a 2:l mixture of ethanethiol and pentamethylene sulfide on Mo(ll0).
grammed reaction of mixtures of reactants allows for direct comparison of reaction kinetic~.'~Since alkane formation kinetics have been shown to depend on surface hydrogen, hydrocarbonfragment, and sulfur concentration, coadsorption experiments are imperative to ensure that the reaction intermediates experience the same environment. The desulfurization kinetics observed in a coadsorption experiment are, in general, different from temperature-programmed reaction of a single reactant. For instance, ethanethiol, 1-propanethiol, 1-butanethiol, 1pentanethiol, tetrahydrothiophene, and trimethylene sulfide all decompose on Mo(ll0) to form linear alkanes with essentially identical reaction kinetics. This was estab lished on the basis of the fact that all alkanes are formed at the same temperature and with the same line shape in the temperature-programmed reaction of mixtures of different thiols and cyclic sulfides. The correspondence in the reaction kinetics for linear thiols and cyclic sulfides is attributed to their rection via a common, isolable thiolate (RS,) intermediate.14 The thiolate intermediates formed from the corresponding thiols have been identified spectroscopically by using both X-ray photoelectron and high-resolution electron energy loss s p e c t r o ~ c o p y . ' ~Figure ~ ~ ~ 4 shows, in contrast, that the evolution of ethane from ethanethiol and the evolution of pentane from pentamethylene sulfide are definitely not simultaneous. Instead, ethane (at 305 K) is produced more rapidly than pentane (at 345 K). Furthermore, the kinetics for pentane production from the npentyl thiolate intermediate derived from l-pentanethiol are essentially identical with those for ethane production from the ethyl thiolate, also shown in Figure 4.13 Therefore, the different kinetics for pentane formation from pentamethylene sulfide are not due to an intrinsic stability of the pentyl thiolate intermediate. Rather, these coadsorption results show that the rate-limiting step in pentamethylene sulfide desulfurization is not thiolate decomposition. This is in marked contrast to the case of tetrahydrothiophene, where butane is produced with the same kinetics as ethane from ethanethiol, demonstrating that the n-butyl thiolate is the kinetically important intermediate in this case.14 (13) Roberts, J. T.; Friend, C. M. Ace. Chem. Res. 1988, 21, 394400. (14) Roberts, J. T.; Friend, C. M. J. Am. Chem. SOC.1987,109,44234424. (15) Roberts, J. T.; Friend, C. M. J. Chem. Phys. 1988, 88, 7172-
7180.
(16) Roberts, J. T.; Friend, C. M. J. Phys. Chem. 1988, 92, 52055213.
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u
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Binding Energy [ eV 1
Figure 5. (a) S(2p) X-ray photoelectron spectra of pentamethylene sulfide monolayers annealed to 100, 250, 410, and 800 K. (b) C(1s) X-ray photoelectron spectra of pentamethylene sulfide monolayers annealed to 100, 250,410, and 800 K.
The temperature-programmed reaction of pentamethylene sulfide on Mo(llO)-p(2X2)-S was performed to understand the effect of sulfur on the reaction. Four products were seen during the course of temperatureprogrammed reaction: dihydrogen, pentane, pentene, and pentamethylene sulfide (data not shown). Pentene is formed in a peak at 410 K, 70 K higher than on the clean surface. Pentane is also formed at higher temperature on the sulfide, namely, 420 K. Dihydrogen evolves in a similar and 8, pattern, at 410 and 615 K. The amount of decomposition was 70% less than on the clean surface, due mainly to the increased molecular desorption of pentamethylene sulfide near 240 K. Sublimation of pentamethylene sulfide multilayers is also observed at 190 K, with trace amounts of desorption a t 360 K. Although the effect of surface sulfidation was not studied in detail, the presence of adsorbed sulfur apparently stabilizes molecular pentamethylene sulfide with respect to irreversible decomposition, allowing for more molecular desorption and less reaction. X-ray photoelectron spectra obtained following adsorption of a saturation dose of pentamethylene sulfide and annealing to various temperatures demonstrate that CS bond cleavage commences by 220 K and is complete by 410 K. Figure 5 summarizes the results of the X-ray photoelectron data and shows C(1s) and S(2p) spectra of pentamethylene sulfide as a function of annealing temperature. X-ray photoelectron spectroscopy of a saturation dose of pentamethylene sulfide a t 100 K evidences the presence of a single molecular state. The C(1s) spectrum shows a broad peak centered at 285.5 eV with a full width at half-maximum (fwhm) of 1.5 eV, substantially larger than normally seen for C(1s) peaks. This peak is best fit with two peaks of binding energies of 285.3 and 285.9 eV with relative areas of 2.0:3.0, each having a fwhm of 1.1 eV, consistent with the presence of molecular pentamethylene sulfide. The binding energies are in good agreement with those previously measured for a cyclic sulfide, ethylene sulfide, which has a C(ls) binding energy around 285.2 eV.3 The S(2p) spectra also evidence one sulfur state with S(2p3/,) and S(2p,,,) binding energies of 164.0 and 165.2 eV, respectively. C(1s) and S(2p) binding ener-
gies previously measured for the ethyl thiolate intermediate are 285.4 and 284.9eV for C(1s) and 162.6 and 163.8 eV for S(2p). The values of the S(2p) binding energies are sufficiently different to conclude that at 100 K the major surface species is not a thiolate. The majority surface species at 100 K is assigned as pentamethylene sulfide. Annealing to the temperature at which hydrocarbon formation commences, 250 K, induces some C-S bond cleavage, although a major fraction of the adsorbed pentamethylene sulfide remains intact. The C(1s) spectrum loses intensity in the 285.5-eV peak, which shifts to 285.3 eV, and gains intensity in a broad peak centered at 283.3 eV. While not unique, the C(1s) peak at 283.3 eV is best fit with two peaks at 283.5 and 282.7 eV, characteristic of hydrocarbon fragments3 and atomic carbon, respectively. The data cannot be fit with a single C(1s) peak in this energy region because of the relatively large intensity near 283 eV. The S(2p) spectrum also shifts intensity from the two peaks associated with S(2p1,,) and S(2p3/,) at 165.2 and 164.0 eV to peaks at 162.6 and 161.4 eV, characteristic of atomic sulfur. Clearly, some of the adsorbed pentamethylene sulfide undergoes nonselective decomposition via C-S, C-C, and C-H bond activation at temperatures below 250 K, although -65% of the sulfur remains covalently bound. Annealing past hydrocarbon formation to 410 K, shows the presence of only atomic sulfur with S(2p) binding energies of 161.3 and 162.5 eV. As expected, all C-S bonds are broken after hydrocarbon formation is complete, and only hydrocarbon fragments remain on the surface. The C(1s) spectra support this, showing a very broad peak ranging from 283 to 285 eV, attributed to one or more hydrocarbon fragments (284.3,283.5 eV) and atomic carbon (282.8 eV) left on the surface. Continued heating of the surface causes the C(1s) peak to narrow by shifting intensity to the lower binding energy side of the C(1s) peak. The higher binding energy carbon peak is attributed to fragments with intact C-C bonds. By 650 K, the C(1s) spectrum consists of one peak at 282.9 eV, characteristic of atomic carbon. Note that this is also the temperature where &-dihydrogen formation is complete, demonstrating that there are no more C-H bonds left on the surface at this temperature. There are no changes in either the C(1s) or S(2p) X-ray photoelectron spectra after heating the surface to 800 K. X-ray photoelectron spectroscopy can also be used to estimate reaction yields. From Auger electron spectroscopy, we know that only 20% of the adsorbed pentamethylene sulfide that remains on the surface goes on to hydrocarbon formation. By measuring the C(1s) intensity at 800 K, we can determine the amount of pentamethylene sulfide that underwent decomposition. Assuming the Auger value is correct, we can divide this value by 0.8 to determine the amount of pentamethylene sulfide that formed products, either through decomposition or hydrocarbon formation. By measuring the amount of carbon present at 100 K, we find the total amount of pentamethylene sulfide available and can then determine the amount of molecular desorption by subtraction. From these calculations, -70% of the adsorbed pentamethylene sulfide at 100 K proceeds to later decompose on the surface, while -15% desorbs at 280 K. The other 15% forms the hydrocarbon products pentane and pentene. A confirming result is seen by comparing the ratio of the S(2p) intensity at both 100 and 800 K, which again confirms that approximately 15% of the adsorbed pentamethylene sulfide desorbs.
Adsorbate Thermodynamics in Reaction Mechanisms
Discussion Pentamethylene sulfide reacts to form the linear hydrocarbons pentane and pentene on Mo(ll0). Desorption and decomposition compete with hydrocarbon formation, analogous to the case of tetrahydrothiophene. However, pentamethylene sulfide exhibits different reactivity and selectivity than previously studied cyclic sulfides on Mo(llO), where the corresponding thiolate was determined to be the kinetically important intermediate. Importantly, the temperature-programmed reaction data (Figures 1 and 4) indicate that the kinetics for hydrocarbon formation from pentamethylene sulfide are not determined by the rate of n-pentyl thiolate hydrogenolysis, in marked contrast to the kinetics for linear alkane production from tetrahydrothiophene and trimethylene sulfide. Ethane production from ethanethiol clearly occurs at a lower temperature than pentane production from pentamethylene sulfide, while the pentanethiol/ ethanethiol mixture yields alkanes at an identical temperature. Since the thiolate intermediate has been shown to be the important intermediate in the linear thiols, these coadsorption experiments demonstrate that it is not the kinetically important intermediate in pentamethylene sulfide hydrogenolysis. Temperature-programmed reaction of tetrahydrothiophene and trimethylene sulfide clearly demonstrates that alkane production precedes alkene production. For instance, butane evolution from tetrahydrothiophene occurs at 350 K while butene evolution occurs at 380 K. In the case of pentamethylene sulfide, however, the hydrocarbon temperatures are very close, and, in fact, pentene production precedes pentane production by 5 K. In light of this, we propose that in the reaction of pentamethylene sulfide on Mo(110) the kinetically important step is not thiolate decomposition but rather ring opening of the molecular species. The different reaction kinetics and increased amount of nonselective decomposition from temperatureprogrammed reaction of pentamethylene sulfide (80%) compared to tetrahydrothiophene (75%) on Mo(ll0) are not likely to be due to a difference in molecular adsorbate geometry. Although the molecular geometriesof pentamethylene sulfide and tetrahydrothiophene adsorbed on Mo(ll0) are not known, they are probably bound via a Lewis base interaction between the sulfur lone pair and the metal surface. This would result in a nearly perpendicular ring orientation as proposed for tetrahydrothiophene on CU(~OO)~' and W(211).18 The addition of one methylene group is not thought to be sufficient to favor a parallel orientation for pentamethylene sulfide because of increased agostic metal-C-H interactions. The measured desorption energies of tetrahydrothiophene and pentamethylene sulfide are essentially the same, 18 kcal/mol, indicating a comparable binding interaction. Furthermore, the average contribution of an agostic C-H-metal interaction is estimated to be -2-3 kcal/mol, based on the previously measured desorption energy of 14.2 kcal/mol for cyclohexane on Ru(OO~).~'These agostic interactions would probably then be insufficient to overcome the strong Mo-S bonding. In addition, the saturation coverage of pentamethylene sulfide (0.12) is essentially the same as for tetrahydrothiophene (0.13). If the orientations of pentamethylene sulfide and tetrahydrothiophene were different, the saturation coverage of pentamethylene sulfide would be
-
(17) Thomas, T.M.; Grimm, F. A,; Carbon, T. A.; Argon, P. A. J. Electron Spectrosc. Relat. Phenorn. 1982,25,159169. (la) Preston, R. E.; Benziger, J. J.Phys. Chern. 1985,89,501C-5017. (19) Madey, T.E.;Yates, J. T., Jr. Surf. Sci. 1978,76,397-414.
Langmuir, Vol. 5, No. 6,1989 1297 expected to be substantially lower than that seen for tetrahydrothiophene. X-ray photoelectron spectra of pentamethylene sulfide at 250 K clearly demonstrate that some molecular pentamethylene sulfide is present up to the onset of hydrocarbon formation, although C-S bond scission to form atomic sulfur and hydrocarbon fragment(s) has also occurred. We propose that hydrogenolysis of the C-S bond in pentamethylene sulfide is the rate-limiting step in the reaction to form the hydrocarbon products, pentane and pentene. After the ring opening, competingdehydrogenationto form pentene and hydrogenolysis of another C-S bond to form pentane rapidly occur. The hydrocarbon fragments formed from low-temperature C-S bond cleavage are proposed to nonselectively decompose and are not thought to form hydrocarbon products. This contention is supported by the C(1s) X-ray photoelectron data, since the intensity of lower binding energy decomposition products increases upon heating in the temperature range where hydrocarbons are formed. Unfortunately, since hydrocarbon formation is only a minor pathway, the X-ray photoelectron data do not unambiguously determine whether the molecular pentamethylene sulfide is the reactive intermediate as proposed. We propose that, after ring opening of the pentamethylene sulfide, n-pentyl thiolate is a short-lived intermediate, which undergoes fast decomposition to form the hydrocarbon products pentane and pentene. The thiolate intermediate is kinetically important in the desulfurization of other cyclic sulfides studied. The thiolate formation mechanism also predicts the incorporation of one surface deuterium into the pentene product and two surface deuteriums into the pentane, which is seen. The availability of surface hydrogen plays a key role in this reaction, as it is stoichiometrically required to form hydrocarbon products. It must be supplied either by decomposition of molecular pentamethylene sulfide to form atomic sulfur and some hydrocarbon fragment(s) or externally added by a saturation predose of dihydrogen. The fact that predosing dihydrogen on Mo(ll0) shifts the peak temperature of both the pentane and pentene products supports the fact that C-S bond hydrogenolysis is very important. This observation further supports the assertion that the thiolate is not the kinetically important intermediate. If thiolate hydrogenolysisis the kinetically important intermediate, the availability of surface hydrogen should, if anything, decrease the kinetics for pentene formation. A possible intermediate in the reaction of pentamethylene sulfide on Mo(ll0) is the n-pentyl intermediate, analogous to the proposed reaction scheme for methanethiol, CH,SH, on Fe(100).20 Adsorbed n-pentyl is an unlikely intermediate, however, on the basis of observation of intact C-S bonds up to the temperature of hydrocarbon formation and from the nonselective reaction for fragments formed from 1-pentene and n-pentane on Mo(ll0) and Mo(llO)-p(2X2)-S. Both C(1s) and S(2p) X-ray photoelectron spectra data are consistent with mainly molecular pentamethylene sulfide on the surface along with a small amount of atomic sulfur and hydrocarbon fragments. Temperature-programmed reaction of 1-pentene and n-pentane on both clean and sulfided Mo(110) surfaces resulted in some dissociation. Although the nature of the resulting hydrocarbon fragments is unknown, it is reasonable to expect that n-pentyl is formed from the reaction of n-pentane on Mo(ll0). The fact (20) Albert, M. E.; Lu, J. P.; Bernasek, S. L.; Cameron, S. D.; Gland, J. L.Surf. Sci. 1988,206,348-364.
1298 Langmuir, Vol. 5, No. 6,1989 that no reformation of pentane was observed when pentane reacted on either initially clean or sulfided Mo(ll0) surfaces strongly suggests that a hydrocarbon fragment such as n-pentyl will nonselectively decompose and not proceed on to form volatile hydrocarbons. This assertion is further supported by the fact that adsorbed methyl identified spectroscopically does not form methane on Ni(111),21 a metal less active for dehydrogenation but more active for hydrogenation. Modification of the surface by atomic sulfur and hydrocarbon fragments also is important in the desulfurization of pentamethylene sulfide. Importantly, hydrocarbon formation and desorption of pentamethylene sulfide occur on a surface with adsorbed sulfur, hydrocarbon fragments, and surface hydrogen. The reaction kinetics of pentamethylene sulfide are, therefore, not characteristic of the clean Mo(ll0) surface but rather reflect to some degree the presence of the coadsorbed species. The importance of surface adsorbates in determining the reaction kinetics of pentamethylene sulfide is evident in the shift of pentane peak temperatures in the presence of coadsorbed ethanethiol or hydrogen. In addition, the pentane peak temperaturewas also shifted after temperatureprogrammed reaction of pentamethylene sulfide on Mo(llO)-p(2X2)-S. On the sulfided Mo(ll0) surface, the sulfur stabilizes the molecular pentamethylene sulfide with respect to decomposition, consistent with the substantial amount of pentamethylene sulfide that desorbs near 240 K on the sulfided Mo(ll0) surface, a temperature where nonselective C-S and C-H bond activation have already commenced on initially clean Mo(ll0). There is 70% less decomposition on the sulfide surface compared to initially clean Mo(ll0). Furthermore, hydrocarbon formation occurs at a much higher temperature (410 K) on the sulfide, 70 K higher than on the clean Mo(ll0) surface. This suggests that on initially clean Mo(ll0) molecular pentamethylene sulfide undergoes some nonselective decomposition to form atomic sulfur and hydrocar(21) Lee, M. B.; Yang, Q. Y.;Tang, S. L.; Ceyer, S. T.J. Chern. Phys. 1986,85, 1693-1694.
Wiegand et al. bon fragments on the surface. The presence of surface sulfur stabilizes the remaining molecular pentamethylene sulfide, which can then undergo hydrocarbon formation, desorb molecularly, or decompose nonselectively at higher temperature. Thus, the initial nonselective decomposition is important, in that the resulting sulfur stabilizes the remaining pentamethylene sulfide and supplies surface hydrogen for the hydrogenolysis process. These results further illustrate the dramatic ability of adsorbate thermodynamics to direct reaction selectivity. The four reactions of saturated cyclic sulfides on Mo(110)desorption, thiolate formation, intramolecular elimination, and decomposition-are found to vary in a systematic way with ring size and strain.
Conclusion Pentamethylene sulfide reacts on Mo(ll0) to form pentane and pentene. In contrast to previously studied cyclic sulfides, the rate-limiting step is the ring opening of pentamethylene sulfide to form the resulting thiolate; it is not thiolate decomposition. We propose that the npentyl thiolate is a short-lived intermediate and that decomposition occurs rapidly upon thiolate formation resulting in nearly simultaneous production of pentane and pentene. The presence of surface sulfur and hydrogen atoms, both of which are formed in low-temperature nonselective decomposition,affects the kinetics and selectivity for pentamethylene sulfide reaction. Surface sulfur stabilizes pentamethylene sulfide with respect to nonselective decomposition, resulting in relatively more molecular desorption and less nonselective reaction. Surface hydrogen increases the rate and yield of hydrocarbon formation, consistent with the idea that C-S bond hydrogenolysis of the ring is the rate-limiting step.
Acknowledgment. This work was supported by the Department of Energy, Basic Energy Sciences, Grant DEFG02-84ER13289. Registry No. c-C,H,,S, 1613-51-0; Mo, 7439-98-7; npentane, 109-66-0; l-pentene, 25377-72-4; hydrogen, 1333-74-0; ethanediol, 75-08-1; sulfur, 7704-34-9.
