Activation of Methane by Neutral Transition Metal Oxides (ScO, NiO, and PdO):  A Theoretical Study

Abstract

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 CH 4 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 CH 4 with a barrier of ∼16 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 CH 3 MOH 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 × 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 + CH 3 products is 98−99%. The preferable channel of dissociation of CH 3 NiOH is a cleavage of the Ni−C bond leading to the radical NiOH + CH 3 products without an exit barrier, while CH 3 PdOH is more likely to undergo 1,2-CH 3 migration to produce a PdCH 3 OH complex and eventually Pd plus methanol. PdOH and CH 3 can recombine producing CH 3 PdOH and isomerization, and dissociation of this molecule results in further transformation of methyl radical into methanol. However, the NiOH + CH 3 reaction is expected only to produce CH 3 NiOH or to restore the initial reactants, NiO + CH 4 . ScO is not reactive with respect to methane at low and ambient temperatures. At elevated temperatures, the ScO + CH 4 reaction can proceed via a barrier of 22.4 kcal/mol to form a CH 3 ScOH molecule with exothermicity of 9.8 kcal/mol. CH 3 ScOH is not likely to decompose to the methyl radical and ScOH because this process is 58.9 kcal/mol endothermic.