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Infrared Spectroscopy and Temperature-Programmed Desorption Study of Adsorbed Thiophene on γ-Al2O3 Wes W. C. Quigley, Hiro D. Yamamoto, Paul A. Aegerter, Garth J. Simpson, and Mark E. Bussell* Department of Chemistry, M.S.-9150, Western Washington University, Bellingham, Washington 98225 Received May 26, 1995X The adsorption of thiophene (C4H4S) on γ-Al2O3 has been investigated in ultrahigh vacuum (UHV) using infrared (IR) spectroscopy and temperature-programmed desorption (TPD). Following thiophene adsorption onto γ-Al2O3 at 130 K, TPD reveals two peaks with maximum rates of desorption at 175 and ∼220 K. The former peak is assigned to desorption of multilayer thiophene while the latter peak is assigned to desorption of weakly chemisorbed thiophene from the alumina surface. IR spectroscopy of adsorbed thiophene at submonolayer coverages provides further evidence that thiophene interacts only weakly with the alumina support; no decomposition of the thiophene overlayer is observed upon heating to 600 K under UHV conditions or a partial pressure of thiophene of 3.0 Torr. Three kinds of adsorbed thiophene species exist on the alumina surface at saturation coverage: one in which thiophene interacts with hydroxyl groups, presumably via hydrogen bonding, a second in which thiophene is coordinated via its sulfur atom to coordinately unsaturated Al3+ sites on the surface, and a third species which is present only at high thiophene coverages. The heat of adsorption for thiophene on γ-Al2O3 has been determined under equilibrium conditions (PTh ) 3.0 Torr) to be ∆Hads ) -28.9 kJ/mol. A direct correlation has been established between the IR and TPD data, permitting integrated extinction coefficients to be determined for adsorbed thiophene in both the monolayer and multilayer coverage regimes. Extinction coefficients in the two coverage regimes are markedly different, underscoring the need to use care when interpreting the IR spectral intensities for adsorbed species. While, as expected, this study has shown that thiophene adsorbs only weakly on γ-Al2O3, more importantly it has shown that the combined IR-TPD methods can be used to determine both the thiophene coverage and the mode of bonding with the surface.
Introduction Thiophene (C4H4S) has been used for many years as a model compound for the investigation of the hydrodesulfurization (HDS) process over single-crystal1-7 and supported catalysts.8,9 In addition to these catalytic studies, numerous surface science studies have been carried out in which spectroscopic techniques have been employed to characterize both the adsorption and reactions of thiophene on metal10-24 and semiconductor25-29 singlecrystal surfaces. The surface chemistry of thiophene has * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Gellman, A. J.; Farias, M. H.; Somorjai, G. A. J. Catal. 1984, 88, 546. (2) Gellman, A. J.; Neiman, D.; Somorjai, G. A. J. Catal. 1987, 107, 92. (3) Gellman, A. J.; Bussell, M. E.; Somorjai, G. A. J. Catal. 1987, 107, 103. (4) Bussell, M. E.; Somorjai, G. A. J. Catal. 1987, 106, 93. (5) Bussell, M. E.; Gellman, A. J.; Somorjai, G. A. Catal. Lett. 1988, 1, 195. (6) Bussell, M. E.; Gellman, A. J.; Somorjai, G. A. J. Catal. 1988, 110, 423. (7) Bussell, M. E.; Somorjai, G. A. J. Phys. Chem. 1989, 93, 2009. (8) See: Prins, R.; De Beer, V. H. J.; Somorjai, G. A. Catal. Rev. Sci. Eng. 1989, 31, 1 and references therein. (9) See, for example: Ledoux, M. J.; Michaux, O.; Agostini, G.; Pannisod, P. J. Catal. 1986, 102, 275. (10) Gellman, A. J.; Farias, M. H.; Salmeron, M.; Somorjai, G. A. Surf. Sci. 1984, 136, 217. (11) Kelly, D. G.; Salmeron, M.; Somorjai, G. A. Surf. Sci. 1986, 175, 465. (12) Zaera, F.; Kollin, E. B.; Gland, J. L. Surf. Sci. 1987, 184, 75. (13) Roberts, J. T.; Friend, C. M. Surf. Sci. 1987, 186, 201. (14) Sto¨hr, J.; Gland, J. L.; Kollin, E. B.; Koestner, R. J.; Johnson, A. L.; Muetterties, E. L.; Sette, F. Phys. Rev. Lett. 1984, 53, 2161. (15) Sto¨hr, J.; Kollin, E. B.; Fischer, D. A.; Hastings, J. B.; Zaera, F.; Sette, F. Phys. Rev. Lett. 1985, 55, 1468. (16) Schoofs, G. R.; Preston, R. E.; Benziger, J. B. Langmuir 1985, 1, 313. (17) Preston, R. E.; Benziger, J. B. J. Phys. Chem. 1985, 89, 5010. (18) Lang, J. F.; Masel, R. I. Surf. Sci. 1987, 183, 44.
also been examined on amorphous and single-crystalline molybdenum sulfide (MoS2).30,31 As might be expected, these substrates exhibit greatly different reactivities toward thiophene with early transition metal surfaces (e.g. Mo(100)) inducing complete decomposition of the thiophene overlayer10,11 while the MoS2(001) surface is chemically inert to thiophene.31 Among the different substrates investigated thus far, only the Pt(111),14 Rh(111),20 and Si(111) 2 × 128,29 surfaces have been found to induce desulfurization of the thiophene ring while leaving the hydrocarbon backbone intact. These materials are both chemically and structurally different than the catalysts used in the industrial HDS process (e.g. CoMo/Al2O3) and as a result are not particularly good choices as substrates for model studies of thiophene hydrodesulfurization. For a number of reasons, relatively few studies have been published in which the surface chemistry of thiophene has been investigated on supported catalysts using (19) Kelly, D. G.; Odriozola, J. A.; Somorjai, G. A. J. Phys. Chem. 1987, 91, 5695. (20) Netzer, F. P.; Bertel, E.; Goldmann, A. Surf. Sci. 1988, 201, 257. (21) Cocco, R. A.; Tatarchuk, B. J. Surf. Sci. 1989, 218, 127. (22) Heise, W. H.; Tatarchuk, B. J. Surf. Sci. 1989, 207, 297. (23) Sexton, B. A. Surf. Sci. 1985, 163, 99. (24) Richardson, N. V.; Campuzano, J. C. Vacuum 1981, 31, 449. (25) MacPherson, C. D.; Hu, D. Q.; Leung, K. T. Surf. Sci. 1992, 276, 156. (26) Hu, D. Q.; MacPherson, C. D.; Leung, K. T. Solid State Commun. 1991, 78, 1077. (27) Hu, D. Q.; MacPherson, C. D.; Leung, K. T. Solid State Commun. 1992, 82, 55. (28) Piancastelli, M. N.; Kelly, M. K.; Margaritondo, G.; Frenkel, D. J.; Lapeyre, G. J. Phys. Rev. B 1986, 34, 3988. (29) Piancastelli, M. N.; Zanoni, R.; Kelly, M. K.; Kilday, D. G.; Chang, Y.; McKinley, J. T.; Margaritondo, G.; Perfetti, P.; Quaresima, C.; Capozi, M. Solid State Commun. 1987, 63, 85. (30) Nicholson, D. E. Anal. Chem. 1962, 34, 370. (31) Salmeron, M.; Somorjai, G. A.; Wold, A.; Chianelli, R.; Liang, K. S. Chem. Phys. Lett. 1982, 90, 105.
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spectroscopic techniques.30,32-43 Each of the earlier studies employed infrared (IR) spectroscopy to investigate the adsorption of thiophene on catalyst surfaces at temperatures of 298 K and above. Because of the difficulty involved in quantifying infrared absorbances of species adsorbed on surfaces, these studies have been largely qualitative in nature and provide little information concerning the coverage of thiophene and other species on the catalyst surfaces. In addition, characterization of the catalysts was limited, making it difficult to relate the observed chemistry with the structure and chemical state of the catalyst. The goal of our work is to carry out a comprehensive investigation of the surface chemistry of adsorbed thiophene on alumina-supported molybdenum (Mo/Al2O3) catalysts using the combined techniques of infrared spectroscopy and temperature-programmed desorption (TPD). Given the complex nature of commercial HDS catalysts, one of the obstacles encountered in such studies is the difficulty in determining which component of the catalyst is responsible for the observed changes in the adsorbed overlayer. As a result, this initial report will focus on the chemistry of thiophene adsorbed on the catalyst support (γ-Al2O3) alone. The results presented here will also serve as a demonstration of the utility of combining IR spectroscopy and TPD for the investigation of adsorbed species on high surface area catalysts. While this has been done successfully for relatively small adsorbates such as CO,44,45 H2,46 and CH3OH,47,48 to our knowledge this is the first report to appear in the literature in which the combined IR-TPD techniques have been used to probe the chemistry of a large, heterocyclic compound on a high surface area oxide under ultrahigh vacuum (UHV) conditions. Experimental Section Sample Preparation and Mounting. The γ-Al2O3 used in this study (Engelhard AL-3945) had a BET surface area of 255 m2/g and a pore volume of 0.60 mL/g. The alumina (1/12 in. extrusions) was ground to a fine powder and calcined at 773 K for 2 h prior to use. Catalyst samples (∼5.0 mg) used for IRTPD studies were pressed at ∼10 000 psi into a tantalum metal mesh (50 × 50 mesh size, 0.003 in. wire diameter). Yates and co-workers have shown that pyridine adsorption in alumina samples prepared similarly exhibits no difference in diffusion limitations when compared to samples prepared by a more complicated spraying technique.49 The area of the pressed (32) Nicholson, D. E. Anal. Chem. 1960, 31, 1365. (33) Blyholder, G.; Bowen, D. O. J. Phys. Chem. 1962, 66, 1288. (34) Lygin, V. I.; Romanovski, B. V.; Topchieva, K. V.; Tkhoang, K. S. Russ. J. Phys. Chem. 1968, 28, 269. (35) Ratnasamy, P.; Fripiat, J. J. J. Chem. Soc., Faraday Trans. 1970, 66, 2897. (36) De Angelis, B. A.; Appierto, G. J. Colloid Interface Sci. 1975, 53, 14. (37) Sultanov, A. S.; Khakimov, U. B.; Talipov, G. S.; Shchekochinin, J. M. React. Kinet. Catal. Lett. 1975, 2, 243. (38) Petrakis, L.; Kiviat, F. E. J. Phys. Chem. 1976, 80, 606. (39) Rochester, C. H.; Terrell, R. J. J. Chem. Soc., Faraday Trans. 1977, 73, 596. (40) Ulendeeva, A. D.; Lygin, V. I.; Lyapina, N. K. Kinet. Katal. 1979, 20, 978. (41) Vol’tsov, A. A.; Lygin, V. I.; Lyapina, N. K.; Ulendeeva, A. D. Russ. J. Phys. Chem. 1983, 57, 1826. (42) Padley, M. B.; Rochester, C. H.; Hutchings, G. J.; King, F. J. Chem. Soc., Faraday Trans. 1994, 90, 203. (43) Padley, M. B.; Rochester, C. H.; Hutchings, G. J.; King, F. J. Catal. 1994, 148, 438. (44) Diaz, A. L.; Bussell, M. E. J. Phys. Chem. 1993, 97, 470. (45) Diaz, A. L.; Quigley, W. W. C.; Yamamoto, H. D.; Bussell, M. E. Langmuir 1994, 10, 1461. (46) Griffin, G. L.; Yates, J. T., Jr. J. Catal. 1982, 73, 396. (47) Roberts, D. L.; Griffin, G. L. J. Catal. 1985, 95, 617. (48) Roberts, D. L.; Griffin, G. L. J. Catal. 1986, 101, 201. (49) Ballinger, T. H.; Wong, J. C. S.; Yates, J. T., Jr. Langmuir 1992, 8, 1676.
Langmuir, Vol. 12, No. 6, 1996 1501 samples was 0.60 cm2, and sample thicknesses were normalized using their masses. The temperature of the sample was monitored by means of a chromel-alumel thermocouple spotwelded to the tantalum mesh. Ultrahigh-Vacuum/High-Pressure System. The experiments described were carried out in a bakeable, stainless steel ultrahigh-vacuum chamber pumped by a 110 L/s ion pump and equipped with a high-pressure cell that can be isolated from the vacuum chamber; the system has been described in detail elsewhere.44 In brief, the high-pressure cell consists of a 23/4 in. cube cross equipped with flange-mounted CaF2 windows. The sample holder is comprised of a 23/4 in. conflat flange outfitted with feedthroughs for resistive heating, temperature measurement, and liquid nitrogen cooling. The sample holder is mounted to the cell via the top face of the cube cross, perpendicular to the CaF2 windows, permitting infrared measurements to be conducted in the transmission mode. Samples supported on the tantalum metal mesh are clamped onto copper-beryllium sample supports and can be cooled to ∼130 K and heated to ∼1200 K. Sample heating is accomplished using a homemade temperature controller which allows linear sample heating at rates of 0.1-10 K/s. Gases are introduced into the high-pressure cell via a welded, stainless steel gas handling system connected to the cell by 1/4 in. tubing. Infrared measurements are accomplished using a Mattson RS-1 FTIR spectrometer which has a water-cooled source and a narrow-band MCT detector and is interfaced to a personal computer for data acquisition and treatment. For TPD experiments, the UHV system is outfitted with a Leybold-Inficon Quadrex 200 quadrupole mass spectrometer which is also interfaced to a personal computer. The mass spectrometer was calibrated for TPD measurements by simulating a thiophene TPD experiment with a known amount of gas measured volumetrically in the high-pressure cell. In this procedure, thiophene is slowly leaked from the high-pressure cell into the UHV chamber while collecting mass spectral data at m/e ) 84. Integration of the resulting “simulated” TPD curve yields the mass spectral response for a known amount of thiophene. This calibration was readily reproducible and is believed to be accurate within 10%. IR-TPD Measurements. Following mounting of a γ-Al2O3 sample in vacuum, it was outgassed for ∼16 h at 400 or 475 K, depending upon the experiment. The typical system base pressure following such a procedure was ∼5.0 × 10-9 Torr. After the sample was cooled to ∼130 K, an IR spectrum was recorded; IR spectral acquisition consisted of 1024 scans of the region 40001000 cm-1 and took approximately 5 min to acquire. The sample spectrum was ratioed with a background spectrum acquired using a blank tantalum metal mesh mounted in the sample holder. Depending on the experiment, the background spectrum was acquired either in UHV or in a partial pressure of thiophene (PTh ) 3.0 Torr). Following acquisition of the predose IR spectrum (sample temperature ) ∼130 K), the high-pressure cell was isolated from the UHV system and the sample exposed to a given pressure of thiophene for a set length of time. Thiophene dose pressures listed on the figures (e.g. 0.025 and 0.100 Torr) are the pressure in the 1/4 in. tubing leading from the thiophene reservoir to the high-pressure cell and not the thiophene pressure at the sample which is not known. As a result, thiophene exposures are relative and exposure times are quoted instead of langmuirs. The thiophene used in this study (Aldrich Chemical Co., 99+% purity) was purified according to the procedure of Spies and Angelici50 as well as by repeated freeze-pump-thaw cycles. As determined by GC-MS, this procedure removed trace amounts of butanethiol from the thiophene sample. Following exposure of alumina samples to thiophene, the valve linking the high-pressure cell and the UHV system was opened; the chamber pressure typically fell to less than 1 × 10-8 Torr within approximately 5 min of opening the valve. IR spectra of alumina samples dosed with thiophene were acquired at a temperature of 130 K. Following IR spectral acquisition, TPD experiments were carried out using a heating rate of 1 K/s while acquiring data for mass 84 (C4H4S) at a sampling frequency of 2 points/K. Initially, TPD data were also acquired for masses 2 (H2), 54 (C4H6), and 56 (C4H8), but only signals associated with the cracking of thiophene in the mass spectrometer were observed. (50) Spies, G. H.; Angelici, R. J. Organometallics 1987, 6, 1897.
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In order to determine the contribution of thiophene desorbing from the sample holder, experiments were carried out using a blank Ta mesh. TPD from a Ta mesh dosed with thiophene at 130 K revealed a single TPD peak centered at ∼175 K which is believed to be due to desorption of multilayer thiophene from the mesh wires. No other desorption feature was observed in the TPD spectrum below a temperature of ∼450 K; above this temperature there was a rapid increase in the signal which is believed to be due to thiophene desorption from the sample holder. The peak area associated with the desorption of multilayer thiophene from the Ta mesh was less than 5% of the thiophene TPD peak area from an alumina sample dosed similarly. Four types of experiments were carried out. The first and second sets of experiments addressed the coverage dependence of thiophene adsorption on γ-Al2O3 and the effect of the surface hydroxyl group concentration on thiophene adsorption. To this end, the first set of experiments consisted of acquiring IR and TPD spectra for samples of γ-Al2O3 dosed with varying amounts of thiophene. Following outgassing at 475 K, the sample was cooled to the desired dosing temperature and then dosed for a specified length of time. If the thiophene dosing was carried out at an elevated temperature (i.e. above 130 K), the sample was cooled to 130 K prior to acquisition of IR and TPD spectra. TPD experiments were carried out from a starting temperature of 130 K up to a temperature of 400 K. Following completion of the TPD run, the sample was recooled to 130 K and then dosed with thiophene for the next desired exposure time. The procedure for acquiring IR and TPD spectra was then repeated. In the second set of experiments, a γ-Al2O3 sample was outgassed at 100 K intervals between 400 and 1000 K prior to exposure of the sample to a saturation dose of thiophene. Sample outgassing was for a period of 30 min at each temperature. Following outgassing, the sample was cooled and dosed with thiophene, and IR and TPD spectra were acquired. For these experiments, thiophene dosing was carried out with the sample held at a temperature of 190 K. The sample was then cooled to 130 K before acquiring IR and TPD spectra. The third and fourth sets of experiments investigated the effect of substrate temperature on thiophene adsorption under UHV and equilibrium conditions. The third set of experiments consisted of dosing a sample of γ-Al2O3 (previously outgassed at 750 K) with a saturation coverage of thiophene at 190 K and then heating at successively higher 50 K intervals for 1 min durations. Between heatings, the sample was cooled to 130 K and an IR spectrum acquired. Lastly, the fourth type of experiment involved acquiring IR spectra for a sample of γ-Al2O3 heated in 20-50 K intervals to succesively higher temperatures above room temperature in a partial pressure of thiophene (PTh ) 3.0 Torr). Following outgassing at 750 K for 30 min, an alumina sample was cooled to room temperature (300 K) and the valve isolating the high-pressure cell from the UHV chamber was closed. The sample of γ-Al2O3 was then exposed to 3.0 Torr of thiophene and, after a 5 min equilibration, an IR spectrum acquired. The sample was then heated to the chosen temperature and, following equilibration and readjustment of the pressure to 3.0 Torr, an IR spectrum was recorded. This procedure was repeated in 20-50 K intervals up to 600 K. All IR and TPD spectra are reproduced without any smoothing treatment. For the first three types of experiments, the IR spectra reproduced here are subtraction spectra prepared by subtracting the IR spectrum acquired immediately prior to dosing with thiophene from the IR spectrum acquired after dosing. For the third set of experiments, the subtraction procedure was similar except that the IR spectra acquired before thiophene exposure were acquired at the temperatures given on the figure. Infrared peak areas were calculated using the Mattson FTIR software over the regions given in Figures 5 and 6 for the νCH and νCC regions of adsorbed thiophene and from 3827 to 3200 cm-1 for the νOH region of γ-Al2O3. TPD peak areas were calculated using points on either side of a given desorption peak for which the mass spectrometer was at its base line value.
Results Temperature-Programmed Desorption of Thiophene on γ-Al2O3. Following mounting in UHV, a sample of γ-Al2O3 was outgassed for ∼16 h at 475 K and then
Quigley et al.
Figure 1. Temperature-programmed desorption spectra for thiophene on pure γ-Al2O3 outgassed at 475 K. The TPD spectra were acquired in UHV following dosing with thiophene at 130 K.
cooled to 130 K. The sample was then dosed with a range of thiophene exposures (Pdoser ) 0.025 Torr) and TPD carried out from 130 to 400 K. The resulting thiophene TPD spectra (m/e ) 84) are shown in Figure 1 and exhibit two desorption features with maximum rates of desorption at 175 and ∼220 K. As will be discussed shortly, the peak area of the 175 K desorption feature does not saturate with increasing thiophene exposure and is assigned to the desorption of multilayer thiophene from the surface. The peak area of the ∼220 K desorption feature, on the other hand, does saturate with increasing thiophene exposure and is assigned to the desorption of thiophene weakly chemisorbed to the surface of the γ-Al2O3. Using the mass spectrometer calibration discussed in the Experimental Section, the peak area under the TPD curve for the 45 s thiophene dose corresponds to 7.4 × 1015 thiophene molecules desorbing from the surface of the alumina sample. Given a sample surface area of 1.1 m2 (4.5 mg sample), a thiophene coverage of 6.8 × 1011 molecules/cm2 is calculated which is well below the saturation coverage (see below) of ∼4.0 × 1013 molecules/ cm2 determined here for γ-Al2O3 outgassed at 475 K in UHV. The fact that multilayer thiophene desorption is observed in the TPD curves shown in Figure 1 indicates that dosing with thiophene at 130 K produces uneven thiophene coverages with multilayer thiophene presumably forming on the external surfaces and at the pore openings of the γ-Al2O3. Shown in Figure 2 are thiophene TPD curves for a sample of γ-Al2O3 dosed with thiophene (Pdoser ) 0.100 Torr) at sample temperatures of 130 and 190 K. As discussed above, dosing at 130 K leads to the buildup of thiophene multilayers on the alumina sample as indicated by the large desorption feature with a maximum rate of desorption at 175 K. The high-temperature shoulder at ∼220 K is due to the desorption of weakly chemisorbed thiophene. Increasing the dose temperature to 190 K eliminates the buildup of thiophene multilayers on the surface of the γ-Al2O3, and subsequent TPD results in desorption of thiophene from a weakly chemisorbed state with a maximum rate of desorption at ∼220-225 K. Apparently, intraparticle diffusion becomes possible when
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Figure 2. Temperature-programmed desorption spectra for thiophene on pure γ-Al2O3 outgassed at 475 K. The TPD spectra were acquired in UHV following dosing with thiophene at either 130 or 190 K as indicated.
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Figure 4. Infrared spectra for a multilayer and saturation coverage of thiophene on pure γ-Al2O3 at 130 K. The thiophene multilayer was prepared by dosing thiophene at 130 K while the saturation coverage was prepared by dosing at 190 K. Table 1. Vibrational Mode Assignments for Thiophene on γ-Al2O3 liquid (cm-1)a
multilayer (cm-1)
monolayer (cm-1)
1034 1080 1250 1358 1406 1504 1575 1586 3072
1030 1076 1250 1358 1408 1503 1570 1605 3080
1032 1082 1254 1362 1408 1504
3107
3100
a
Figure 3. A plot of the integrated thiophene TPD peak area versus thiophene exposure for a sample of pure γ-Al2O3 outgassed at 475 K. The thiophene pressure in the doser was 0.100 Torr, and the sample temperature was either 130 or 190 K as indicated.
thiophene is dosed onto a γ-Al2O3 sample held at 190 K, and therefore, a saturation coverage of thiophene can be achieved on the accessible surfaces of the alumina. The peak areas from the TPD curves reproduced in Figure 2 plus those from other experiments are plotted in Figure 3 and clearly show a linear growth of the amount of desorbing thiophene when γ-Al2O3 is dosed with thiophene at 130 K while saturation of the alumina surface occurs if the substrate temperature is held at 190 K during dosing. For this sample of γ-Al2O3 which was outgassed at 475 K prior to dosing with thiophene, the saturation thiophene coverage was determined to be ∼4.0 × 1013 molecules/ cm2.
3073 3092 3104
assignment δCH (in plane) δCH (in plane) δCH (in plane) νR (ring stretch) νCC (symmetric) νCC (asymmetric) (combination band) (combination band) νCH νCH νCH
Reference 51.
IR Spectroscopy of Thiophene on γ-Al2O3. Shown in Figure 4 are IR spectra for a sample of γ-Al2O3 outgassed at 475 K and then dosed with thiophene at temperatures of 130 and 190 K. The IR spectra were acquired in UHV at a sample temperature of 130 K. The thiophene exposure for the dose at 130 K was 5 min (Pdoser ) 0.100 Torr) while that for the dose at 190 K was 40 min (Pdoser ) 0.100 Torr). The former dose produces multilayers of thiophene on the external surfaces of the γ-Al2O3 while the latter dose yields a saturation coverage of thiophene on the alumina surface. The peak assignments for the absorbance features in the spectra are listed in Table 1 along with those determined by Rico et al. for liquid thiophene.51 Only very small shifts are observed for the IR absorbances for the thiophene multilayer and saturation layer when compared to the peak positions for liquid thiophene indicating that thiophene interacts weakly with the surface of γ-Al2O3. Two types of experiments were carried out to probe the effect of sample temperature on the IR spectrum of adsorbed thiophene on γ-Al2O3, and the results of these experiments are shown in Figures 5 and 6. The IR spectra reproduced in Figure 5 are from an experiment in which (51) Rico, M.; Orza, J. M.; Morcilli, J. Spectrochimica Acta, 1965, 21, 689.
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Figure 5. Infrared spectra in the (a) νCH and (b) νCC regions for adsorbed thiophene on pure γ-Al2O3 annealed at the given temperature for 1 min in UHV. The alumina sample was outgassed at 750 K prior to exposure to a saturation dose of thiophene at 190 K.
Figure 6. Infrared spectra in the (a) νCH and (b) νCC regions for adsorbed thiophene on pure γ-Al2O3 annealed at the given temperatures for 1 min in 3.0 Torr of thiophene. The alumina sample was outgassed at 750 K prior to exposure to 3.0 Torr of thiophene at room temperature.
a sample of γ-Al2O3 was dosed with a saturation coverage of thiophene at 190 K and then heated at 25 K intervals in the range 200-350 K for 1 min at each temperature. The sample was outgassed at 750 K prior to dosing with thiophene at 190 K. The sample was cooled to 130 K after each anneal and an IR spectrum acquired. The IR spectra in the νCH region (Figure 5a) exhibit three intense absorbance features at 3075, 3095, and 3105 cm-1 as well as weak bands at 2970 and 2995 cm-1. The former bands are assigned to νCH absorbances of adsorbed thiophene while the latter bands are assigned to adsorbed butanethiolate due to trace butanethiol impurities in the thiophene used in this study. IR spectra in the νCC region (Figure 5b) reveal three absorbance features at 1400, 1408, and 1424 cm-1 which are assigned to the symmetric CdC stretch (νCC) of the thiophene ring. As will be discussed
shortly, the observation of three absorbance features in the νCC region suggests the presence of three adsorbed thiophene species on the alumina surface. The IR spectra shown in Figure 5 indicate that heating a sample of γ-Al2O3 dosed with thiophene to progressively higher temperatures in UHV results only in the gradual decrease in the thiophene coverage due desorption into vacuum. There is no evidence suggesting reaction of the thiophene overlayer occurs upon heating γ-Al2O3 in the range 200350 K in UHV. IR spectra were also acquired for a sample of γ-Al2O3 heated at 50 K intervals in the range 300-600 K in the presence of gas phase thiophene (PTh ) 3.0 Torr). The alumina sample was held at each temperature for 5 min following which an IR spectrum was acquired; the IR spectra were acquired with the sample held at the chosen
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Figure 7. A van’t Hoff plot for the equilibrium between adsorbed thiophene on γ-Al2O3 and gas phase thiophene (PTh ) 3.0 Torr). The plot was constructed using the method described in ref 52.
Figure 8. Beer’s law plot for multilayer coverages of thiophene on pure γ-Al2O3. The multilayer coverages of thiophene were prepared by dosing at 130 K for varying exposure times. The coverage of thiophene was quantified via TPD.
anneal temperature. IR spectra in the νCH and νCC region are reproduced in Figure 6 and exhibit absorbance features similar to those observed for thiophene adsorbed on γ-Al2O3 under UHV conditions (Figure 5). As before, weak absorbance features are observed at 2990 and 2965 cm-1 which are assigned to adsorbed butanethiolate species associated with trace butanethiol impurities in the thiophene dosed onto the alumina surface. In the νCC region, absorbance bands are observed at 1400, 1408, and 1420 cm-1 which are assigned to the symmetric CdC stretching vibration of three different thiophene species adsorbed on the γ-Al2O3 surface. The only changes observed in the IR spectra presented in Figure 6 are the gradual decrease in the intensity of the absorbance features associated with adsorbed thiophene as the sample temperature is increased. Using the method described by Ballinger and Yates,52 a van’t Hoff plot (Figure 7) was constructed for the equilibrium between thiophene adsorbed on γ-Al2O3 and gas phase thiophene (PTh ) 3.0 Torr).
as will be discussed shortly. An estimate for the heat of adsorption (∆Hads) is obtained from the slope of a plot of negative ln(Keq) versus 1/T; for thiophene the heat of adsorption was determined to be -28.9 ( 2.3 kJ/mol. This value can be considered to be only an estimate for the heat of adsorption for two reasons. First, under the conditions which this experiment was performed, all three adsorbed thiophene species are present on the surface and the value determined for the heat of adsorption averages over their respective contributions to the IR spectrum in the νCH region. Second, the calculation of θTh requires the integrated absorbance associated with the max maximum thiophene coverage (ATh ) which corresponds to the high-coverage regime in which the integrated extinction coefficient has begun to decrease. Correlation of IR and TPD Data for Thiophene on γ-Al2O3. IR and TPD data for a wide range of thiophene exposures were acquired for a sample of γ-Al2O3 dosed at 130 and 190 K. Beer’s law plots were prepared, and these are shown in Figures 8 and 9 for the νCH and νCC regions for both multilayer and submonolayer coverages of thiophene on γ-Al2O3 outgassed at 475 K, respectively.53 The integrated extinction coefficients determined from these plots are listed in Table 2. It should be noted that the extinction coefficients for submonolayer coverages of thiophene are not for a single adsorbed thiophene species but rather for a mixture of the three different adsorbed species with the relative contributions of the three species unknown. Because of the similarities of the heats of adsorption for the three adsorbed thiophene species, it was not possible to isolate each species individually on the surface for a range of coverages. The integrated extinction coefficient determined in the C-H stretching region for multilayer thiophene, jCH ) (4.2 ( 0.4) × 10-17 cm/molecule, is similar to the value determined by Aubuchon et al. of 2.3 × 10-17 cm/molecule for multilayer
C4H4S(g) + site h C4H4S(ads) The equilibrium constant for this process is given by the following expression
Keq )
θTh PTh(1 - θTh)
where θTh is the fractional coverage of thiophene and is max taken to be the absorbance ratio ATh/ATh . The absorbance ratio was calculated using the IR data in the νCH max region (3200-2900 cm-1) where ATh was the integrated absorbance for the spectrum acquired at 300 K and ATh was the integrated absorbance for spectra acquired at 20 K intervals in the range 340-460 K. This temperature range was chosen because it corresponds to a range of thiophene coverages for which the integrated extinction coefficient (j) in the νCH region was found to be constant (52) Ballinger, T. H.; Yates, J. T., Jr. Langmuir 1991, 7, 3041.
(53) Bell, A. T. In Vibrational Spectroscopy of Molecules on Surfaces; Yates, J. T., Jr., Madey, T. E., Eds.; Plenum Press: New York, 1987; pp 105-134.
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Figure 9. Beer’s law for submonolayer coverages of thiophene on pure γ-Al2O3. The submonolayer coverages were prepared by dosing thiophene at 190 K for varying exposure times. The coverage of thiophene was quantified via TPD. Table 2. Integrated Extinction Coefficients for Thiophene on γ-Al2O3 coverage regime
jCH (cm/molecule)
jCC (cm/molecule)
multilayer submonolayer
(4.2 ( 0.4) × 10-17 (8.4 ( 2.1) × 10-17
(1.6 ( 0.2) × 10-17 (9.8 ( 1.3) × 10-18
n-pentane on the Al2O3(0001) surface.54 The value determined in the submonolayer regime, jCH ) (8.4 ( 2.1) × 10-17 cm/molecule, is somewhat larger than the values determined by Bell and co-workers of jCH ) 2.4 × 10-17 and 4.7 × 10-17 cm/molecule for methanol adsorbed on pure silica and a reduced Cu/SiO2 catalyst, respectively.55 The integrated extinction coefficient determined in the νCC region for submonolayer coverages of thiophene, jCC ) (9.8 ( 1.3) × 10-18 cm/molecule, is close to the range of values determined by Morterra and co-workers for the 8a mode of pyridine adsorbed on R-Al2O3, j8a ) (4.9-6.0) × 10-18 cm/molecule.56 A range is given for the integrated extinction coefficient because it was found to have a weak dependence upon the mode of pyridine adsorption. The 8a vibrational mode of pyridine is a ring mode which involves the symmetric stretching of the R-CC bonds,57 similar to the symmetric νCC mode of thiophene. Effect of Dehydroxylation of γ-Al2O3 on the Adsorption of Thiophene. A sample of γ-Al2O3 was outgassed for 30 min at 100 K increments from 400 to 1000 K. Previous studies in this laboratory and by others have shown that heating γ-Al2O3 to progressively higher temperatures in UHV leads to dehydroxylation of the alumina which can be monitored by IR spectroscopy and low-temperature CO adsorption.44,52 The integrated absorbance in the νOH region for the γ-Al2O3 sample decreases with increasing outgassing temperature as plotted in Figure 10, reflecting the loss of OH groups from the surface. (54) Aubuchon, C. M.; Davison, B. S.; Nishimura, A. M.; Tro, N. J. J. Phys. Chem. 1994, 98, 240. (55) Clarke, D. B.; Lee, D.-K.; Sandoval, M. J.; Bell, A. T. J. Catal. 1994, 150, 81. (56) Morterra, C.; Colluccia, S.; Chiorino, A.; Boccuzzi, F. J. Catal. 1978, 54, 348. (57) Kline, C. H., Jr.; Turkevich, J. J. Chem. Phys. 1944, 12, 300.
Figure 10. Plot of the saturation thiophene coverage and the integrated OH absorbance for pure γ-Al2O3 outgassed at 100 K intervals between 400 and 1000 K and then exposed to thiophene at 190 K.
Following outgassing at a particular temperature, the γ-Al2O3 was dosed with thiophene at 190 K to ensure saturation coverage (Pdoser ) 0.100 Torr, 40 min) and then cooled to 130 K prior to acquiring IR and TPD spectra. The TPD peak areas were used to determine the saturation coverage of thiophene for the different outgassing temperatures, and these are also plotted in Figure 10. The saturation coverage of thiophene decreases sharply from 4.3 × 1013 molecules/cm2 for a sample of γ-Al2O3 outgassed at 400 K to 2.6 × 1013 molecules/cm2 for an outgassing temperature of 600 K and then increases gradually to 3.2 × 1013 molecules/cm2 as the temperature at which the alumina is outgassed is increased to 1000 K. For γ-Al2O3 dehydroxylated at temperatures e600 K, the sharp decline in the saturation coverage of thiophene corresponds to the removal of associated hydroxyl groups from the alumina surface. Further outgassing at temperatures above 600 K results in the loss of isolated hydroxyl groups from the surface; the saturation coverage of thiophene increases slightly for γ-Al2O3 outgassed at these higher temperatures. Infrared spectra in the νCC region for adsorbed thiophene on γ-Al2O3 outgassed at 400, 600, 800, and 1000 K are shown in Figure 11 and provide further evidence that removing hydroxyl groups from the alumina surface influences its interactions with thiophene. Following outgassing at the successively higher temperatures, the sample of γ-Al2O3 was dosed at 190 K to yield a thiophene coverage of ∼1.5 × 1013 molecules/cm2 as determined by TPD. Three absorbance features are observed in the νCC region of the IR spectra suggesting that three adsorbed thiophene species are formed on the surface of γ-Al2O3. The feature at 1402 cm-1 decreases in intensity as the outgassing temperature is increased indicating that it is associated with thiophene bonded to hydroxyl groups on the alumina surface. The feature at 1424 cm-1, on the other hand, increases in intensity as the outgassing temperature is increased suggesting that it is associated with thiophene which is bound to coordinately unsaturated Al3+ sites produced via dehydroxylation of the alumina. The intensity of the absorbance feature at 1406 cm-1 shows no apparent trend in relation to the outgassing temperature of the γ-Al2O3. As will be discussed shortly, the
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Figure 11. Infrared spectra in the νCC region for thiophene adsorbed on a sample of γ-Al2O3 outgassed at 100 K intervals between 400 and 1000 K. Following outgassing at the specified temperature for 30 min, the sample was cooled to 190 K and dosed with thiophene to give a coverage of ∼1.5 × 1013 molecules/ cm2. The sample was then cooled to 130 K and the IR spectrum acquired.
absorbance feature at 1406 cm-1 is associated with a highcoverage thiophene species. The fact that the peak position for this feature is very close to that of multilayer thiophene suggests that this thiophene species interacts quite weakly with the alumina surface. Discussion Infrared spectroscopy has been widely used to probe adsorbed species on supported catalyst surfaces for over 40 years.53 Most of these studies have been qualitative in nature, relying on peak shifts and changes in peak intensities to draw conclusions concerning the surface chemistry under study. To our knowledge, only a handful of studies have appeared in the literature in which TPD (in UHV conditions) has been used in conjunction with IR spectroscopy to investigate adsorbed species on high surface area catalysts.44-48 Previous studies in our laboratory of CO adsorption on supported catalysts have demonstrated that TPD can be used to determine adsorbate coverages, thereby yielding quantitative information to complement qualitative information pertaining to adsorbate structure and adsorption sites gained from interpretation of IR spectra.44,45 The goals of the current study were two-fold: (1) to determine whether the combined IR-TPD techniques could be successfully applied to a relatively large organic adsorbate such as thiophene and (2) to characterize the interactions of thiophene with γ-Al2O3, the high surface area support used for most commercial hydrodesulfurization catalysts. The major results of this study can be summarized as follows. (1) Thiophene interacts only weakly with the surface of γ-Al2O3; no decomposition is observed upon heating to 600 K under UHV conditions or a partial pressure of thiophene vapor (PTh ) 3.0 Torr). (2) the saturation coverage of thiophene on γ-Al2O3 is sensitive to the hydroxyl content of the alumina surface, and (3) three kinds of adsorbed thiophene species have been identified on the surface of γ-Al2O3. In addition, the results of this study suggest that the combined IR-TPD tech-
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niques can be successfully applied to the study of organic adsorbates such as thiophene on high surface area catalysts. A number of other researchers have investigated the adsorption of thiophene on γ-Al2O3 using IR spectroscopy.32,34,37,40,43 Most recently, Padley et al.43 examined thiophene adsorption on γ-Al2O3 at 295 K as part of an investigation of the effects of sulfur compounds on the catalytic properties of Cu/Al2O3 catalysts. Adsorption was found to be reversible for thiophene adsorbed on γ-Al2O3 previously outgassed at 873 K, although weak νCH bands observed at 2981 and 2965 cm-1 suggest the presence of saturated alkyl species on the surface. Excluding these two bands, the authors report that the IR bands for adsorbed thiophene were essentially unshifted from those of liquid thiophene. Thiophene adsorption on a fully deuterated sample of γ-Al2O3 also produced IR bands at 2981 and 2965 cm-1, indicating that these νCH bands associated with saturated alkyl species are not due to hydrogen transfer from surface hydroxyl groups to thiophene. Sultanov et al.37 and Ulendeeva et al.40 have also observed IR bands in the 2900-3000 cm-1 region which were assigned to saturated alkyl νCH bands. These authors, however, concluded that the formation of the surface alkyl species is due to transfer of hydrogen atoms from hydroxyl groups on the alumina surface. In contrast to the observations described above, the results of the current study indicate that thiophene adsorbs reversibly on γ-Al2O3 at 295 K and undergoes no reaction at temperatures up to 600 K. As shown in Figure 6, when a sample of γ-Al2O3 is heated in the range 300600 K under a constant pressure of thiophene (PTh ) 3.0 Torr), the only observed change in the IR spectrum is a gradual decrease in the intensity of the IR bands associated with adsorbed thiophene. The growth of new IR bands associated with saturated alkyl species is not observed. Close examination of the νCH region (Figure 6a) of the 300 K spectrum reveals weak absorbance features at 2990 and 2965 cm-1 which gradually disappear as the sample is heated. We believe these absorbance features, in the region expected for saturated alkyl species, can be traced to thiol impurities (butanethiol) in the thiophene and not to reaction of the thiophene adlayer. Low-level thiol impurities could be eliminated from the thiophene used in this study (Aldrich Chemical Co., 99+ % purity) only after following the purification scheme described by Angelici and co-workers50 in which thiophene is treated (in order) with silver nitrate and calcium hydride, followed by fractional distillation. Thiophene which had been fractionally distilled over sodium metal contained sufficient thiol impurities such that when thiophene was dosed to a sample of γ-Al2O3 held at either 190 or 300 K, relatively strong IR bands at 2970, 2940, and 2880 cm-1 were readily observed. Further purification of the thiophene using the more rigorous procedure described above reduced these absorbance features to the levels shown in Figures 5 and 6. In agreement with our results discussed above for thiophene adsorbed on γ-Al2O3 in the presence of gas phase thiophene (T g 300 K), UHV experiments also indicate that thiophene weakly chemisorbs on the surface of γ-Al2O3. As shown in the IR spectra presented in Figure 5, heating a sample of γ-Al2O3 dosed with a saturation coverage of thiophene in the range 200-350 K results in the gradual reduction of absorbance features in the νCC and νCH regions for adsorbed thiophene. No new absorbance features appear in the IR spectrum upon heating, and it is concluded that thiophene is reversibly adsorbed on the alumina surface and desorption is essentially complete by ∼350 K. The TPD spectra shown in Figures
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1 and 2 provide further evidence that thiophene is weakly adsorbed to the surface of γ-Al2O3. As shown in Figure 1, TPD reveals that multilayer thiophene desorbs in a peak with maximum rate of desorption at 175 K while weakly chemisorbed thiophene desorbs in a single broad peak with a maximum rate of desorption at ∼220 K. Others have investigated thiophene TPD from a variety of metal single-crystal surfaces, and desorption of thiophene multilayers has been reported to occur with maximum rates of desorption in the range 150-185 K,11-13,20,23 in agreement with our results for γ-Al2O3. No comparison data are available for desorption of the thiophene monolayer; it is important to reiterate that no decomposition products were detected during thiophene desorption, indicating that thiophene is reversibly bound to the alumina surface. To our knowledge, TPD of a large, organic molecule from a high surface area material under UHV conditions has been reported in only one other case. White and coworkers have investigated the adsorption and reactions of trinitrotoluene (TNT) on a number of different high surface area metal oxide powders using TPD.58 The thiophene TPD spectra presented in the current study are similar in general features to those presented by White and co-workers for TNT. If adsorption is carried out at temperatures below which the organic compound condenses on the surface of the metal oxide powder, multilayers of the compound rapidly buildup on the external surfaces and at the pore mouths of the high surface area material. On the other hand, if adsorption is carried out at a temperature slightly above that of multilayer desorption, uniform coverage of the organic compound can be achieved on the exposed (both internal and external) surfaces of the powder. Using TPD, the saturation coverage of thiophene was determined for γ-Al2O3 as a function of the hydroxyl content of the surface. As shown in Figure 7, removal of hydroxyl groups from γ-Al2O3 by annealing in vacuum results in a substantial decrease in the saturation coverage of thiophene from 4.3 × 1013 molecules/cm2 for γ-Al2O3 dehydroxylated at 400 K to 2.6 × 1013 molecules/cm2 for alumina dehydroxylated at 600 K. Further dehydroxylation of γ-Al2O3 by heating above 600 K in UHV leads to a gradual increase of the saturation thiophene coverage to 3.2 x1013 molecules/cm2. White and co-workers utilized TPD in a similar fashion to determine the saturation coverage of TNT adsorbed on silica.58 The coverages reported are in terms of TPD peak areas and therefore give only a relative measure of the saturation coverage of TNT. The general trend observed for TNT on silica is similar to that observed here for thiophene adsorbed on alumina; the saturation coverage of TNT drops rapidly as the silica is dehydroxylated at temperatures up to 500 K and then remains relatively constant for dehydroxylation temperatures up to 800 K. No comparison data could be found for the saturation coverage of thiophene on γ-Al2O3, but the range of values measured in this study via TPD, (2.6-4.3) × 1013 molecules/cm2, is considerably lower than those for benzene and cyclohexane on R-Al2O3, (2.2-2.5) × 1014 molecules/cm2, as determined by others using a quartz microbalance.59 Yates and co-workers observed similar discrepancies in the saturation coverage of 1-butene on zeolite-X as determined by TPD in UHV and by reversible isotherm measurements; the value measured by dosing 1-butene in UHV was just 1% of the value (58) Henderson, M. A.; Jin, T.; White, J. M. Appl. Surf. Sci. 1986, 127, 140. (59) Hsing, H.-H.; Wade, W. H. J. Colloid Interface Sci. 1974, 47, 490.
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measured by reversible adsorption at higher pressures.60 This difference was ascribed to only partial filling of the zeolite pores being achieved when dosing 1-butene at 248 K and is likely the reason why the saturation coverage of thiophene measured in this study is lower than that for molecules of similar size determined by equilibrium measurements. While the IR spectra shown in Figure 4 and the peak assignments presented in Table 1 indicate that the alumina surface only weakly perturbs the vibrations of adsorbed thiophene, further examination reveals subtle changes in the IR spectra which depend upon the hydroxyl content of the alumina and the thiophene coverage. As discussed above, the saturation coverage of thiophene is sensitive to the hydroxyl content of γ-Al2O3, so it is not surprising that changes are seen in the IR spectra of thiophene adsorbed on alumina dehydroxylated at increasing temperatures. The IR spectra presented in Figure 11 correspond to a coverage of ∼1.5 × 1013 thiophene molecules/cm2 dosed at 190 K onto a γ-Al2O3 sample dehydroxylated at 200 K intervals between 400 and 1000 K. Three absorbance features are apparent in the νCC region at 1402, 1406, and 1424 cm-1 which suggest that three different adsorbed thiophene species are present on the alumina surface under UHV conditions. Two trends are apparent in the IR spectra as the hydroxyl content of alumina is decreased via heating in UHV. The absorbance feature at 1402 cm-1 decreases in intensity with increasing dehydroxylation temperature while the feature at 1424 cm-1 increases in intensity. The former feature is assigned to thiophene hydrogen bonded to hydroxyl groups on the alumina surface, and the latter feature is assigned to thiophene coordinated via its sulfur atom to coordinately unsaturated Al3+ sites created by dehydroxylation of the γ-Al2O3. Assignment of the H-bonded species is supported by changes observed in the νOH region of γ-Al2O3 upon thiophene adsorption. A sample of γ-Al2O3 which had been outgassed at 400 K in UHV exhibited νOH absorbances at 3460, 3605, 3680, 3732, and 3770 cm-1; these peak positions are similar to those observed previously in our laboratory for a sample of γ-Al2O3 outgassed at 475 K.44 Following adsorption of a saturation coverage of thiophene, subtraction spectra revealed a strong increase in IR absorbance at 3670 cm-1 and a concomitant decrease in intensity at 3734 and 3770 cm-1. Such shifts are not uncommon, e.g. adsorption of benzene on γ-Al2O3 causes a 110 cm-1 shift in the νOH region,61 and are indicative of the participation of surface hydroxyl groups in hydrogen bonding. The third absorbance feature observed in the νCC region (1406 cm-1) is essentially unshifted from that observed for liquid and multilayer thiophene (see Table 1), and its intensity appears to be insensitive to the hydroxyl content of the alumina surface. The IR spectra presented in Figure 5b indicate that this third type of adsorbed thiophene is a high-coverage species which is less strongly bonded to the γ-Al2O3 than either H-bonded or Al3+-coordinated thiophene. As shown in Figure 5b, annealing a thiophenesaturated surface at temperatures above 200 K preferentially reduces the intensity of the 1408 cm-1 feature. Neither the νCH region (Figure 9a) nor regions elsewhere in the IR spectrum provide further information concerning the identity of this species, and no assignment can be made with respect to its mode of adsorption. The shifts of the νCC band for the thiophene species assigned to H-bonded (1402 cm-1) and Al3+-coordinated (1424 cm-1) thiophene cannot be easily understood in (60) Kiskinova, M.; Griffin, G. L.; Yates, J. T., Jr. J. Catal. 1981, 71, 278. (61) Haaland, D. M. Surf. Sci. 1981, 102, 405.
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terms of the thiophene molecular orbital diagram. According to Rodriguez, who has carried out a theoretical investigation of thiophene adsorption on the Mo(100) surface,62 bonding of thiophene to a surface via the sulfur atom involves primarily the 2b1(π) and 6a1(σ) orbitals. Both of these filled molecular orbitals could be expected to participate in bonding of the form C4H4S‚‚‚H-O-Al and C4H4S‚‚‚Al3+. Neither the 2b1 nor the 6a1 molecular orbitals involve direct participation of electrons involved in the carbon-carbon double bonds, suggesting that an understanding of the coupling of all the carbon-carbon bonds in the ring is necessary if the frequency shift of the symmetric νCC band is to be understood. Spectral shifts of the 8a and 19b ring modes of pyridine due to its interactions with metal oxide surfaces are well known and are used to characterize and quantify the types of adsorption sites on these oxide materials.63 The 8a mode, which occurs at 1580 cm-1 for liquid pyridine,57 is observed at 1580-1600, 1600-1634, and ∼1640 cm-1 for H-bonded pyridine, pyridine coordinated to Lewis acid sites, and pyridine coordinated to Bronsted acid sites, respectively.63 On γ-Al2O3, pyridine coordinated to Al3+ sites exhibits a ν8a absorbance at 1623 cm-1,64 a blue shift of 43 cm-1 from its position in liquid pyridine. This shift is similar in magnitude to that observed in this study for the νCC mode (18 cm-1) for thiophene coordinated to Al3+ sites on γ-Al2O3. As discussed earlier, spectral intensities are often used in addition to peak shifts in interpreting IR spectra of adsorbed species. However, without knowledge of the integrated extinction coefficient (j) of the absorbance band of interest, it is difficult to make conclusions based upon peak intensities. For reversibly adsorbed species such as thiophene on γ-Al2O3, we have found TPD to be a useful technique for determining adsorbate coverages. With this quantitative information, integrated extinction coefficients can be readily determined using Beer’s law. It should be noted that the extinction coefficients determined for thiophene in the submonolayer regime are not for a single adsorbed species but rather are composite values weighted by the relative concentrations of the three different adsorbed thiophene species identified in this study. However, an IR spectroscopy study by Morterra and coworkers of adsorbed pyridine on R-Al2O3 showed that the integrated extinction coefficients for H-bonded pyridine as well as pyridine coordinated to Lewis and Bronsted acid sites varied only slightly.55 As indicated by the data plotted in Figures 8 and 9, the integrated extinction coefficients for adsorbed thiophene determined in the νCC and νCH regions are sensitive to both the coverage regime and, in the submonolayer regime, to the fractional coverage. On moving from the submonolayer to the multilayer coverage regime, the integrated extinction coefficient in the νCC region (jCC) increases in magnitude by a factor of 2 while the integrated extinction coefficient in the νCH region (jCH) is roughly halved. While the origin of these changes is not known, they are likely due to variations in the adsorption environment of thiophene upon moving from the submonolayer to the multilayer coverage regime. Within the submonolayer regime, the integrated extinction coefficients in the νCC and νCH regions decrease as the saturation coverage is approached; similar changes in integrated extinction coefficients have been observed for CO adsorbed on supported metal catalysts and have been attributed to lateral interactions between adsorbed CO molecules.65,66 Of course, changes in the (62) Rodriguez, J. A. Surf. Sci. 1992, 278, 326. (63) Kno¨zinger, H. Adv. Catal. 1976, 25, 184. (64) Kiviat, F. E.; Petrakis, L. J. Phys. Chem. 1973, 77, 1232. (65) Cavanagh, R. R.; Yates, J. T., Jr. J. Chem. Phys. 1981, 74, 4150. (66) Winslow, P.; Bell, A. T. J. Catal. 1984, 86, 158.
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relative concentrations of the three adsorbed thiophene species could also be responsible for the variation in the value of the integrated extinction coefficients at high coverages. Knowledge of the extinction coefficients of adsorbed species is particularly useful when interpreting data obtained under high-pressure conditions in which measurement of adsorbate coverages by techniques such as TPD is not possible. Comparison of the IR spectra presented in Figure 5 with those in Figure 6 reveals that the same absorbance features are observed for adsorbed thiophene on γ-Al2O3 in both the low- and high-pressure regimes. Such comparisons are important in order to verify that adsorbates characterized at low temperatures and pressures are the same as those present at the higher temperatures and pressures used in catalytic processes. The IR spectra in the νCC region reproduced in Figure 6b, acquired under 3.0 Torr of thiophene, clearly show three absorbance features at 1400, 1408, and 1420 cm-1 which are identical to those observed in UHV conditions as shown in Figure 5b. As discussed above, these three absorbance features have been assigned to H-bonded thiophene, a high-coverage thiophene species, and Al3+-coordinated thiophene, respectively. Finally, while this study has shown that TPD is a useful technique for quantifying the coverage of a reversibly bonded adsorbate such as thiophene, extraction of kinetic parameters such as the desorption energy from the TPD spectra is complicated because of the likelihood that adsorption equilibrium is established within the pores of the γ-Al2O3 during TPD. Theoretical calculations by Herz and co-workers have shown that this can lead to broadening of TPD peaks and shifts of TPD maxima to higher temperatures.67 The heat of adsorption can, however, be determined using IR spectroscopy to measure the coverage of a reversibly adsorbed species under equilibrium conditions for a range of substrate temperatures. A van’t Hoff plot for thiophene adsorption is shown in Figure 7, and from its slope, an estimate for the heat of adsorption of ∆Hads ) -28.9 ( 2.3 kJ/mol was determined. This value can be compared to an isosteric heat of adsorption of ∆Hads ) -42 kJ/mol for benzene on alumina as determined by others.59 In considering the high-pressure thiophene adsorption data (PTh ) 3.0 Torr), a note of caution needs to be added concerning interpretation of the IR intensity. The range of integrated IR absorbances in the νCC and νCH regions for spectra acquired between 300 and 600 K (Figure 6) is similar to that used for the determination of integrated extinction coefficients (Figure 9). As determined by IR spectroscopy, the upper limit of thiophene coverage for the high-pressure experiment approaches the saturation coverage where, as shown in Figure 9, the Beer’s Law plots are no longer linear. Thus, while the integrated intensities are linearly related to the thiophene coverage at low coverages, this is not true as the saturation coverage is approached. As a result, the van’t Hoff plot shown in Figure 7 was constructed using integrated absorbance data lying within the thiophene coverage regime for which the integrated extinction coefficient in the νCH region is constant. Conclusions The combined techniques of IR spectroscopy and TPD have been used to investigate the adsorption of thiophene on γ-Al2O3. Thiophene adsorbs reversibly on the alumina surface as determined by TPD with weakly chemisorbed thiophene desorbing in a broad peak with a maximum rate of desorption at ∼220 K. IR spectroscopy indicates (67) Herz, R. K.; Kiela, J. B.; Marin, S. P. J. Catal. 1982, 73, 66.
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three different adsorbed thiophene species exist on the surface of γ-Al2O3 which can be assigned to hydrogenbonded thiophene, Al3+-coordinated thiophene, and a high-coverage thiophene species whose mode of adsorption is unknown. The saturation coverage of thiophene on γ-Al2O3 was determined via TPD and was found to lie in the range (2.6-4.3) × 1013 molecules/cm2, depending upon the hydroxyl content of the alumina surface. IR spectroscopy measurements of thiophene coverage under highpressure conditions were used to determine an estimate for the heat of adsorption of ∆Hads ) -28.9 kJ/mol for thiophene on γ-Al2O3. While, as expected, thiophene was found to interact only weakly with the alumina support, this study has demonstrated that the combination of IR spectroscopy and TPD is a useful tool for probing the
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surface chemistry of a large adsorbate molecule on a high surface area catalyst. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS, for partial support of this research. This research was also supported by a Cottrell College Science Award of Research Corporation and by the National Science Foundation under grant number CHE-9400740. One of us (P.A.A.) would like to acknowledge the Council on Undergraduate Research for an AIURP Summer Research Fellowship. The authors would also like to acknowledge Dr. Jose´ A. Rodriguez for helpful discussions. LA950410E
