Theoretical studies of stability and reactivity of CHx species on Ni(111)

Periodic infinite plane wave slab calculations were performed, in conjuncti on with density functional theory and ultrasoft pseudopotentials to study t he adsorption of CHx (x = 1, 2, 3) species on the Ni(lll) surface. The pote ntial energy diagram is presented for the overall reaction of surface carbo n with dihydrogen to methane or, equivalently, the conversion of methane to surface carbon, including all adsorbed intermediates and transition states . All CH, intermediates prefer threefold sites. The transition states invol ve the formation of C-H bonds on top of a Ni atom, with the reaction coordi nate being primarily a C-H stretch. The calculated activation energies to f orm the C-H bond are near 70-85 kJ/mol for different CH, species. To illust rate the new possibilities offered by detailed potential energy diagrams, t he results from the quantum chemical calculations are combined with experim ental results in the literature to make initial estimates of kinetic parame ters involved in the methanation of CO over nickel. Sensitivity analyses in this region of parameter space are used to assess the effects of these kin etic parameters on the overall rate of methanation. A good description of t he experimental methanation reaction kinetics (D. W. Goodman et al., J. Cat al. 63, 226 (1980)) is achieved by adjusting the sensitive kinetic paramete rs within reasonable ranges. The kinetic analyses shows that adsorbed CO an d CH are the most abundant species on the surface, and the energies of the transition states to form methyl species from methylene species and to form methane from methyl species appear to control the rate of the overall reac tion. (C) 2000 Academic Press.