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 ∼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 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, kmethyl = 7.12 × 10-11 exp(−17 329/RT) cm3 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 9...