The role of hydrogen and surface structure in the reactions of model thiols on nickel surfaces

Desulfurization of petroleum feedstocks is of great industrial importance, since sulfur compounds poison reforming catalysts and contribute to atmospheric pollution. The molecular mechanisms responsible for C-S bond activation and the role of hydrogen in these reactions has been the focus of this research. Benzenethiol and cyclohexanethiol desulfurization reactions have been examined on the Ni(111) and Ni(100) surfaces to develop an improved understanding of the affect of surface hydrogen concentration, chemical structure, and surface morphology. The primary mechanism for C-S bond activation is insertion of the metal into the C-S bond, producing an adsorbed hydrocarbon intermediate and bound surface sulfur. Hydrogen adds to the hydrocarbon intermediate in a second step after C-S bond activation is complete. The rate of C-S bond activation was not influenced even by reactive atmospheres of hydrogen, which substantially enhance the availability of surface hydrogen. Toluene formation from coadsorbed methanethiol and benzenethiol demonstrates that adsorbed phenyl and methyl groups are present on the surface following C-S bond activation. Increased hydrogen availability increases the yield of benzene and methane, the hydrogenolysis products, relative to the toluene cross coupling product. The C-S bond in cyclohexanethiol is weaker than the C-S bond in benzenethiol, resulting in a 20-30 K decrease in the C-S bond activation temperature. This correlation of bond strength to C-S bond activity suggests a radical mechanism. Changes in thiol structure also lead to significant differences in the reactivity of adsorbed intermediates. Dehydrogenation of intermediates derived from cyclohexanethiol reduces the yield of desorbing desulfurized product at C-S bond activation temperature relative to yield observed for benzenethiol with stronger C-H bonds. Surface hydrogen activity proved important to thiol reaction. Increased stability of coadsorbed hydrogen resulted in larger gas phase yields from the Ni(111) surface than from the Ni(100) surface. Ni(100) also demonstrated a greater degree of dehydrogenation of surface intermediates relative to the Ni(111) surface. Preadsorbed hydrogen significantly modified the reaction of the thiols studied on these nickel surfaces. Increased hydrogen stability also decreases the effect of coadsorbed hydrogen.