BONDING AND DECOMPOSITION OF THIOPHENE, SULFHYDRYL, THIOMETHOXY AND PHENYL THIOLATE ON MO SURFACES

The bonding of thiophene (C4H4S), sulfhydryl (HS), thiomethoxy (CH3S) and phenyl thiolate (C6H5S) to molybdenum surfaces has been examined employing MO-SCF calculations (INDO/1) and metal clusters of limited size (Mo(n), n = 13-19 atoms). The calculations indicate that the preferred binding sites for the RS species on Mo(100) are the four-fold hollows. The bonding mechanism of thiophene involves a large charge transfer from the S-lone pairs of the molecule (6a1 and 2b1 orbitals) into the surface, and electron transfer from the substrate into the C-S antibonding 3b1 orbital of the adsorbate. Adsorption of thiophene on a hollow site leads to a large weakening in the strength of the C-S and C-H bonds, producing a precursor for the dissociation of the molecule. HS, CH3S and C6H5S behave as electron acceptors when bonded to Mo. For these species, the chemisorption bond is dominated by the interaction between the LUMO of the adsorbate and the Mo(4d,5s) bands. The results of a thermochemical analysis indicate that reactions which involve the cleavage of H-S and C-S bonds of alkanethiols on Mo (RSH(g) --> S(a) + RH(g)) are very exothermic (-DELTAH = 40-50 kcal/mol). The thermodynamics suggests that C-S bond breaking should be the most difficult step in the desulfurization process. The hydrogenolysis of the C-S bond (RS(a) + H(a) --> S(a) + RH(g)) is approximately 12 kcal/mol more exothermic for CH3S than for C6H5S.