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J. Phys. Chem. 1996, 100, 19620-19627
Desulfurization of Thiophenic Compounds by Ni(111): Adsorption and Reactions of Thiophene, 3-Methylthiophene, and 2,5-Dimethylthiophene Deborah R. Huntley,* David R. Mullins, and Michael P. Wingeier† Chemical and Analytical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201 ReceiVed: September 12, 1996X
The adsorption and reactivity of thiophene, 2,5-dimethylthiophene, and 3-methylthiophene on Ni(111) have been examined. The saturation coverages of the three molecules are similar, about 0.13 monolayer (ML), and in all cases alkenes are the major hydrocarbon products. On initially clean surfaces, decomposition to sulfur, carbon and hydrogen is the major pathway, but the selectivity to hydrocarbon production can be enhanced by a factor of about 3 by predosing the surface with hydrogen. Sulfur is easily removed from the ring, and C-S bond scission is complete by 150 K. The rate-limiting step in alkene formation is hydrogenation of a highly unsaturated hydrocarbon intermediate. The hydrocarbon intermediates formed are difficult to unambiguously identify but are most likely cyclic structures, retaining the C4 framework.
Introduction Thiophene and related compounds are frequently used as model feedstocks in studies of hydrodesulfurization (HDS) catalysis. The removal of sulfur from petroleum products1 remains an important task, and the thiophenic sulfur is the most difficult to eliminate. The reaction is well-known in catalysis and has been studied in a number of homogeneous systems,2,3 yet detailed mechanistic studies of the surface reaction chemistry of thiophene on model catalyst surfaces are relatively sparse. A recent review by Wiegand and Friend summarizes much of the surface and organometallic chemistry of thiophene.4 The primary goals of the current study are to characterize the adsorption of thiophene and methyl-substituted thiophenes on Ni(111) and to understand and control the selectivity of hydrocarbon formation. Thiophene is known to bond in several different configurations in organometallic complexes.2,5 It can bond through the aromatic π system of the thiophene ring (η5 configuration) or through the S atom as a weak Lewis base. Alternatively, since the aromatic stabilization is relatively weak (less than half that of benzene), the thiophene ring may pucker and the bonding can occur through the four carbon atoms (η4 configuration) with the sulfur out of the plane of the C4 system. These structures and others have all been observed in organometallic systems. On surfaces, the most common adsorption geometry, at least at low coverage, has been reported to be π bonded parallel to the surface, similar to benzene. This configuration has been reported for Cu(111),6 Cu(100),7 Pt(111),8,9 Rh(111),10 Ni(111),11 and Mo(100)13 and suggested for Mo(110).14 In some cases, at higher coverages, a compressed structure is formed, with bonding through the S atom and the ring tilted. On Ni(100), it was proposed that the S is removed from the ring near 100 K.14 Under industrial HDS conditions, thiophene forms butene, butadiene, and butane, each requiring the addition of hydrogen. However, under the hydrogen-deficient conditions of ultrahigh vacuum, thiophene primarily decomposes to form surface sulfur, carbon, and hydrogen on most metal surfaces. For example, on Ru(0001),15,16 Mo(100),12 Mo(110),13 and Re(0001),17 no hydrocarbons were observed upon heating overlayers of thiophene. On Pt(111),8,9 stepped Pt,9 Pd(111),18 W(211),19 and Rh(111),10 † X
Oak Ridge Science and Engineering Semester Participant. Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)02825-0 CCC: $12.00
trace levels of C4 hydrocarbons were reported. Reactions of thiophene on various single crystal faces of Mo and Re have also been examined at high pressure, where larger quantities of hydrocarbon products were observed.20 In this study, we have extensively characterized the adsorption and reaction of thiophene on clean and hydrogen predosed Ni(111). The reactions of thiophene on Ni(111) following relatively high temperature adsorption (>200 K) were previously reported;11 however, we have significantly expanded this work by incorporating lower adsorption temperatures, which minimizes decomposition, and by providing more complete spectroscopic and reaction data on thiophene and two methylsubstituted analogs. In summary, we find that simple alkenes are the only gaseous hydrocarbon products and that the selectivity to alkene formation can be enhanced by predosing the surface with hydrogen. When deuterium is predosed, we find deuterium incorporation into the butene product in excess of the expected stoichiometric amount, suggesting a complex mechanism with some dehydrogenation of the intermediates prior to rehydrogenation. We observe a high selectivity to methyl cleavage when the methyl groups are in the 2,5-positions on the thiophene ring, but less cleavage when a methyl group is in the 3-position. This suggests that the initial adsorption occurs through the S atom since the methyl groups in the 2and 5-positions interact strongly with the surface, leading to facile cleavage. Furthermore, X-ray photoelectron spectroscopy (XPS) data clearly demonstrate that S is removed from the ring at low temperature (
