Activation of methane by neutral transition metal oxides (ScO, NiO, and PdO): A theoretical study

Density functional B3LYP calculations have been employed to investigate potential energy surfaces for the reactions of scandium, nickel, and palladium oxides with methane. The results show that NiO and PdO are reactive toward methane and can form molecular complexes with CH4 bound by 9-10 kcal/mol without a barrier. At elevated temperatures, the dominant reaction channel is direct abstraction of a hydrogen atom by the oxides from CH4 with a barrier of similar to16 kcal/mol leading to MOH (M = Ni, Pd) and free methyl radical. A minor reaction channel is an insertion into a C-H bond to produce CH3MOH molecules via transition states lying 19-20 kcal/mol above the initial reactants. For instance, for PdO, the rate constant of the hydrogen abstraction channel evaluated using the transition state theory for the 300-1000 K temperature range, k(methyl) = 7.12 x 10(-11) exp(-17 329/RT) cm(3) s(-1) molecule(-1), is 2-3 orders of magnitude higher than the insertion rate constant and the branching ratio for the PdOH + CH3 products is 98-99%. The preferable channel of dissociation of CH3NiOH is a cleavage of the Ni-C bond leading to the radical NiOH + CH3 products without an exit barrier, while CH3PdOH is more likely to undergo 1,2-CH3 migration to produce a PdCH3OH complex and eventually Pd plus methanol. PdOH and CH3 can recombine producing CH3PdOH and isomerization, and dissociation of this molecule results in further transformation of methyl radical into methanol. However, the NiOH + CH3 reaction is expected only to produce CH3NiOH or to restore the initial reactants, NiO + CH4. ScO is not reactive with respect to methane at low and ambient temperatures. At elevated temperatures, the ScO + CH4 reaction can proceed via a barrier of 22.4 kcal/mol to form a CH3ScOH molecule with exothermicity of 9.8 kcal/mol. CH3ScOH is not likely to decompose to the methyl radical and ScOH because this process is 58.9 kcal/mol endothermic.