Langmuir 1998, 14, 1407-1410
1407
A Laser-Induced Thermal Desorption/Fourier Transform Mass Spectrometry Study of Acetylene Cyclization on S/Pd(111): The Formation and Kinetics of Benzene, Thiophene, and 1,3-Butadiene Tracy E. Caldwell, Ihab M. Abdelrehim, and Donald P. Land* Department of Chemistry, University of California, Davis, California 95616 Received July 11, 1997. In Final Form: November 5, 1997 Acetylene cyclization on S/Pd(111) has been studied using laser-induced thermal desorption with Fourier transform mass spectrometry (LITD/FTMS). The formation of benzene, thiophene, and 1,3-butadiene has been observed on the surface at low temperatures. Investigations reveal that benzene and thiophene are formed in similar amounts and that the formation kinetics of each are comparable to one another. Results also show the kinetics of benzene formation on S/Pd(111) do not differ from that on a clean Pd(111) surface, indicating that the addition of S does not significantly alter the nature of the cylization process.
Introduction It was 6 years following the announcement of acetylene cyclization on single crystal palladium that Gentle et al.1 first reported the low-temperature formation of thiophene, in addition to benzene, from acetylene on sulfided Pd(111). The reaction exhibits a number of similarities to the cyclization to benzene on a clean Pd(111) surface. Investigations have since continued in order to improve our understanding of this reaction and better define the nature of heterocyclization and sulfur abstraction on Pd(111).2-4 The low temperature abstraction of sulfur from a transition metal surface is also of interest because of its relevance to catalytic regeneration. Abdelrehim et al.5 were the first to study acetylene cyclization on S/Pd(111) using laser-induced thermal desorption/Fourier transform mass spectrometry (LITD/ FTMS), the results of which revealed new information regarding both cyclization processes. With only adventitious sulfur and low acetylene exposures, the formation of benzene and thiophene were observed at lower temperature and lower acetylene coverage than was previously observed with other techniques.1,6,7 It is widely believed that both benzene and thiophene formation proceed via a C4H4 species5,8-14 and these LITD results were the first (1) Sto¨hr, J.; Gland, J. L.; Kollin, E. B.; R. J. Koestner, A. L. J.; Muetterties, E. L.; Sette, F. Phys. Rev. Lett. 1984, 53, 2161. (2) Zaera, F.; Gland, J. L.; Kollin, E. B. Surf. Sci. 1987, 184, 75. (3) Heise, W. H.; Tatarchuk, B. J. Surf. Sci. 1989, 207, 297. (4) Cocco, R. A.; Tatarchuk, B. J. Surf. Sci. 1989, 218, 127. (5) Abdelrehim, I. M.; Thornburg, N. A.; Sloan, J. T.; Land, D. P. Surf. Sci. 1993, 298, L169-172. (6) Sesselmann, W.; Woratschek, B.; Ertl, G.; Kuppers, J.; Haberland, H. Surf. Sci. 1983, 130, 245. (7) Rucker, T. G.; Logan, M. A.; Gentle, T. M.; Muetterties, E. L.; Somorjai, G. A. J. Phys. Chem. 1986, 90, 2703-2708. (8) Lambert, R. M.; Ormerod, R. M. In Springer Series in Surface Sciences; Madix, R. J., Ed.; Springer-Verlag: Berlin, Heidelberg, 1994; Vol. 34; pp 89-133. (9) Tysoe, W. T.; Nyberg, G. L.; Lambert, R. M. Surf. Sci. 1983, 135, 128. (10) Ormerod, R. M.; Lambert, R. M. Catal. Lett. 1990, 6, 121-130. (11) Patterson, C. H.; Mundenar, J. M.; Timbrell, P. Y.; Gellman, A. J.; Lambert, R. M. Surf. Sci. 1989, 208, 93. (12) Ormerod, R. M.; Lambert, R. M.; Hoffman, H.; Zaera, F.; Yao, J. M.; Saldin, D. K.; Wang, L. P.; Bennett, D. W.; Tysoe, W. T. Surf. Sci. 1993, 295, 277. (13) Patterson, C. H.; Lambert, R. M. J. Am. Chem. Soc. 1988, 110, 6871.
to demonstrate that this intermediate exists at temperatures 400 K with sulfur coverages e 0.33 ML (ML ) monolayer) results in the formation of (x7 × x7)R19° lattice. Their study further concluded that annealing a saturation coverage of this overlayer at >800 K produces defect sites, which increase in density with annealing temperature. Further details regarding defect density will be discussed in the following section. Sulfur coverages used in these studies were determined by AES.20 The acetylene is spectral grade (99.96%) with an acetone stabilizer impurity. Acetone (and benzene, if present) is removed by passing the acetylene through a CO2-ice/acetone-cooled trap and determined free of impurities by gas-phase, FTMS analysis. Following sulfur preadsorption, the surface is exposed to acetylene by backfilling the chamber. The sample temperature is held constant at 85 K during the exposure. At this temperature and low coverage, acetylene sticks to the Pd(111) surface with unit probability.9 Exposures (langmuir ) 10-6 Torr‚s) have been corrected for ion gauge sensitivities. The values used for acetylene (1.66), benzene (5.18), and 1,3-butadiene (2.9) were obtained directly from the literature.21-23 Thiophene (4.8) was estimated by calculating the ratio of values for 2-methylpropanethiol and i-butyl alcohol, then multiplying this factor by the value for tetrahydrofuran.22 One of our most useful experiments is the LITD T-jump survey. During these experiments, the entire Pd sample is resistively heated to a selected temperature and allowed to equilibrate for 1 min before firing the laser (Nd:YAG, 20 mJ/pulse, 5 ns pulsewidth). Each spectrum is obtained from a single laser shot, aimed at a different spatial position on the surface (laser spot size ≈ 1 mm2), and each spectrum produced is a complete mass spectrum of all the ions present in the cell. Several spectra are taken at each temperature and the signal magnitudes averaged. The LITD/FTMS signals are typically proportional to the surface concentrations for each species15; therefore, relative yield information can be obtained from a single spectrum. The kinetics experiments are run in a similar fashion to the LITD survey. The Pd sample temperature is rapidly increased and held constant at some temperature where the kinetics of the reaction can be easily monitored. The surface is not allowed to equilibrate at the designated temperature, and laser shots are taken immediately after temperature has been reached. Depending on how fast the reaction proceeds at a particular temperature, the laser is fired at a repetition rate consistent with the rapid initial change in surface composition. Over time, the reaction will decelerate and shots are taken less frequently.
Results and Discussion Figure 1 is an LITD T-jump survey of a 0.75 langmuir exposure of acetylene on a Pd(111) surface pre-covered with sulfur. The acetylene exposure has been corrected for ion gauge sensitivity and corresponds roughly to a 10% ML coverage; an exposure of 2.6 langmuir acetylene corresponds to a saturation coverage of 33% ML.9 Using Auger electron spectroscopy, the sulfur coverage was determined to be ∼6% of an ML (11% of saturation). The survey in Figure 1 clearly shows the loss of acetylene followed by the low-temperature formation of thiophene, benzene, and 1,3-butadiene. The apparent increase in acetylene signal below 140 K is likely an artifact of spacecharge conditions in the mass spectrometer. The transient signals for these early measurements did exhibit a beat pattern characteristic of having an ion density that is too high within the cell. (20) Gellman, A. J. J. Am. Chem. Soc. 1991, 113, 4435-4440. (21) Nakao, F. Vacuum 1975, 25, 431. (22) Bartmess, J. E.; Georgiadis, R. M. Vacuum 1983, 33, 149-153. (23) Dannetun, H.; Lundstro¨m, I.; Peterson, L.-G. Appl. Surf. Sci. 1987, 29, 361.
Caldwell et al.
Figure 1. LITD/FTMS survey of acetylene on S/Pd(111). The survey shows the loss of acetylene (hollow circles) with the growth of both thiophene (solid squares) and benzene (solid circles) from a 0.75 langmuir exposure of acetylene on a sulfided Pd(111) surface. The sulfur coverage is ∼16% of saturation. A signal for 1,3-butadiene (solid diamond) is also observed. The signals have been increased by a factor of 10 for thiophene and by a factor of 100 for 1,3-butadiene. Exposures and signal intensities have been corrected for ion gauge sensitivity and percent total fragmentation.
There are reports that thiophene production is only observed in thermal desorption spectroscopy (TDS) at near-saturation coverages of sulfur.20,24-26 Here, with LITD, a significant signal for thiophene is observed at all sulfur coverages, even at levels undetectable by AES. This result is due primarily to the enhanced sensitivity provided under laser-induced heating conditions. To further illustrate this point, our LITD study also gives the first evidence of 1,3-butadiene formation from this cyclization reaction on S/Pd(111). (Similar amounts of 1,3-butadiene were first reported in LITD/FTMS studies of acetylene on clean Pd(111).14) The signals observed for 1,3-butadiene are likely due to the hydrogenation of the C4H4 intermediate. At first glance, the abstraction of sulfur from Pd by C4H4 appears to be an efficient process; however, most of the thiophene formed on the surface reacts before having a chance to desorb when using slow bulk heating methods. From 260 to 380 K, the survey looks very similar to the decomposition of thiophene from thiophene adsorption on Pd(111); ∼85% of adsorbed thiophene decomposes on the surface and only a small fraction actually desorbs.27 (This result has qualitatively been observed by Gellman3 as well.). This result holds true for reactively formed thiophene because it appears at temperatures as low as 140 K, well below the temperature for thermal desorption (thiophene desorbs from S/Pd(111) in a single peak located at 220 K4). This situation would then suggest that acetylene cyclization on S/Pd(111) is rather inefficient at removing sulfur from the surface, yet Auger spectra taken before and after the experiment indicate a drop in sulfur concentration by 90%. Gellman observed that a single adsorption/desorption cycle of acetylene on S/Pd showed no measurable decrease in the S152eV Auger signal, and that only a small drop was observed in repeated cycles.3 In these LITD experiments, however, sulfur is removed as thiophene under the high heating rate conditions due to laser heating. This high heat method is in direct contrast to the preferred decomposition and deposition of S on the surface when thiophene is heated slowly on Pd(111). The efficiency at which thiophene forms from (24) Gellman, A. J. Langmuir 1991, 7, 827-830. (25) Gellman, A. J. J. Phys. Chem. 1992, 96, 790-795. (26) Gentle, T. M.; Tsai, C. T.; Walley, K. P.; Gellman, A. J. Catal. Lett. 1989, 2, 19-26. (27) Caldwell, T. E.; Abdelrehim, I. M.; Land, D. P. Surf. Sci. 1996, 367, L26.
Formation/Kinetics of Benzene, Thiophene, and 1,3-Butadiene
Figure 2. Rate curves for acetylene, benzene, and thiophene. The S/Pd(111) sample was exposed to a 0.8 langmuir exposure of acetylene and then heated to 160 K. The sulfur coverage was ∼3% saturation. The plot shows the LITD/FTMS signal magnitudes for acetylene (hollow circles), benzene (hollow squares), and thiophene (hollow diamonds) as a function of time under isothermal conditions. The solid curves are exponentials characteristic of first-order kinetics, but with a nonzero endpoint added to the acetylene fit. Assuming competition for acetylene in both thiophene and benzene formation, two first-order terms were included in the acetylene fit. Signal magnitudes have been corrected for ion gauge sensitivity and percent of total fragmentation.
acetylene on a sulfided palladium surface is surprising, given the strength of interaction between S and Pd(111) (∆Hads ) 73 kcal/mol23). If one considers the proposed mechanism by which this reaction proceeds, via a C4H4 intermediate, it is rather impressive that a C4 species can abstract a sulfur from Pd, especially at such low temperatures. One drawback is that the thiophene later decomposes after it is formed and, as a result, sulfur is not removed from the surface unless desorbed at low temperatures using LITD. This desorption is not feasible for industrial catalysts, the active surfaces of which are predominantly inaccessible to such treatment. The kinetics of benzene formation from low acetylene exposures on clean Pd(111) have been studied extensively using LITD/FTMS and are reported elsewhere.14,28 In conjunction with these studies, however, it was of interest to learn the effects of coadsorbed sulfur in benzene formation kinetics, and, in the process, make relevant comparisons to the kinetics of thiophene formation. Figure 2 shows the LITD signal magnitudes for acetylene, benzene, and thiophene as a function of time under isothermal conditions (in this case 160 K). The plot indicates the loss of acetylene with the simultaneous formation of both benzene and thiophene. One can observe the similarity in rates of formation for benzene and thiophene noting, as well, the comparable amounts of each that are formed. The solid curves are exponentials characteristic of pseudo-first-order kinetics. Because acetylene is present in excess, much of it remains on the surface following the reaction. For this reason, an arbitrary non-zero final concentration was included as an additional fitting parameter for the acetylene signal. Also included in the acetylene fit were two first-order terms, (28) Abdelrehim, I. M.; Caldwell, T. E.; Land, D. P. J. Phys. Chem. 1996, 100, 10265-10268.
Langmuir, Vol. 14, No. 6, 1998 1409
Figure 3. Arrhenius plot of benzene and thiophene formation from acetylene on S/Pd(111) using first-order kinetics. The Arrhenius plot gives the activation energies for benzene (hollow squares) and thiophene (hollow circles) using an 0.8 langmuir exposure of acetylene on a sulfided Pd(111) surface. The activation barrier leading to benzene formation appears to be unaffected by the presence of sulfur when compared with the values obtained for a clean Pd(111) surface. Also, the kinetics of benzene and thiophene formation appear to be very similar. The sulfur coverage was ∼2-6% of a monolayer, well below the saturation limit on Pd(111).
to account for any competition existing in the loss of acetylene. It is rather unexpected, though, that the data would be explained by such a fit. If one considers the formation of the C4H4 intermediate to be the fast step, and the addition of C2H2 (in the case of benzene formation) and the incorporation of S (in the formation of thiophene) to be slow and measurable, then the loss of acetylene observed in this study should be due solely to one process (i.e., benzene formation). It is unlikely that desorption alone would account for this apparent loss mechanism because desorption occurs around 200 K,7 which is above the temperature at which these kinetics were measured. One possibility is that acetylene reactions are occurring preferentially at defects, thus competing with reactions taking place on heterogeneous terrace sites. Enhanced reactivity at defect sites is commonly referred to in heterogeneous catalysis systems.29,30 Gellman et al.18 have been able to show that annealing of a Pd(111)-(x7 × x7)R19°-S surface results in the formation of step defects. This group reports that properties of the steps responsible for the enhanced reactivity are still open to speculation, but an increase in step density (with increasing temperature) has been correlated to the amount of thiophene desorbing in TDS by acetylene heterocyclization and, hence, conclude that sites for the acetylene heterocyclization reaction are associated with the monatomic steps that are formed. Consistent with the kinetics of benzene formation on clean Pd(111),14 pseudo-first-order kinetics are assumed here in the treatment of benzene and thiophene formation from acetylene on S/Pd(111). By monitoring the formation of benzene and thiophene under isothermal conditions (as in Figure 2) for several different temperatures, one can then use the measured rates and use an Arrhenius treatment to determine activation energies and preexponential factors. Figure 3 is an Arrhenius plot of benzene and thiophene formation from a 0.8 langmuir exposure of acetylene on S/Pd(111). Assuming pseudo-first-order conditions, an activation energy of 18 ( 9 kJ/mol and a preexponential factor of 102.8 ( 0.9 s-1 for benzene formation were measured. Compared with the values obtained for (29) Lomas, J. R.; Baddeley, C. J.; Tikhov, M. S.; Lambert, R. M. Langmuir 1995, 11, 3048-3053. (30) Gross, M. E. J. Electrochem. Soc. 1991, 138, 2422-2426.
1410 Langmuir, Vol. 14, No. 6, 1998
the same acetylene exposure on clean Pd(111),28 this result suggests that the presence of low sulfur coverages does not significantly alter the kinetics of benzene formation, which is contrary to what is observed for acetylene on an oxygenated surface.31 An activation energy of 21.5 ( 1.7 kJ/mol and preexponential factor of 104.0 ( 0.2 s-1 was measured for thiophene formation. These values are very similar to the values obtained for benzene formation on the same surface. It should be noted, however, that the initial sulfur coverages were not consistently the same for each kinetic experiment, which may contribute to the error associated with the activation energy measured for benzene formation. Due to the difficulty in sulfiding the surface, the sulfur coverage varied from 2% to ∼9% of saturation. Sulfur is slightly more electronegative than Pd (2.5 versus 2.1, respectively, on the Pauling Scale) and, although considered to be an electron acceptor to the surface,32 the interaction is believed to be mostly steric and of little consequence to the formation of benzene except at step edges and defect sites.24 Given this and the comparison of values for benzene formation on clean Pd, the variation in sulfur coverage well below saturation seems insignificant to the overall mechanistic details. The existence, thus far, of only three data points also limits the precision with which one can compare the kinetics on clean and sulfided Pd surfaces. Further studies at higher sulfur coverage and with more data points are planned.
Caldwell et al.
through a C4H4 intermediate, which can then cyclize with either acetylene or atomic sulfur to yield benzene or thiophene, respectively. This study also provides evidence of the C4H4 intermediate, being the first to observe the simultaneous formation of 1,3-butadiene on a sulfided Pd(111) surface. At low exposures of acetylene, submonolayer coverages of sulfur (6% ML) do not alter benzene formation kinetics significantly when compared with the same exposure of acetylene on a clean Pd(111) surface. Surface thiophene formation is very efficient, exhibiting kinetic behavior very similar to that of benzene formed on the same surface: k1
C4H4 + S 98 C4H4S k2
C4H4 + C2H2 98 C6H6 k1 = k2 That the two rate constants are comparable implies that the slow step may be the same for both processes, therefore involving C4H4/Pd rearrangement. Still, regardless of the efficiency at which it is formed on the surface, very little thiophene actually desorbs upon heating. Most of the thiophene formed ultimately deposits C and S atoms on the surface, rendering the Pd catalyst inactive for further cyclization.
Conclusions Acetylene on S/Pd(111) cyclizes to form both benzene and thiophene at low temperatures. These results help support the notion of a stepwise mechanism proceeding (31) Caldwell, T. E.; Abdelrehim, I. M.; Land, D. P. Langmuir 1997, in review. (32) Kiskinova, M. P. Studies in Surface Science and Catalysis: Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments; Elsevier: Amsterdam, 1992.
Acknowledgment. Acknowledgment is made to the Donors of The Petroleum Research Fund, administered by the ACS, for support of this research. T. E. C. also acknowledges financial support form the Patricia Roberts Harris Foundation. The authors also express their deepest gratitude to deceased Professor Brian E. Bent for his inspiration into this and all our work. LA970775U
