Theoretical Study of the Direct Conversion of Methane to Methanol Using H₂O₂as an Oxidant on Pd and Au/Pd Surfaces

First-principle theoretical study was performed to elucidate the reaction mechanism of the direct conversion of methane to methanol using hydrogen peroxide as an oxidant over Pd and Au/Pd catalysts. We have investigated the effect of Au-Pd alloying and its effect on the reaction steps and energetics and we have provided an understanding of the contribution of surface-exposed Au centers to the overall reaction mechanism. The hydrogen peroxide decomposition reactions to OH groups or surface-adsorbed oxo species and water molecules were compared on the Pd and Au/Pd surfaces as an important preliminary process for the methane oxidation. We have evaluated the C-H bond cleavage energetics assisted by either the OH group or oxo species over Pd and Au/Pd surfaces and compared them to the C-H cleavage energy. Our results show that the products of hydrogen peroxide decomposition significantly reduce the high activation barrier for the hydrogen atom abstraction from methane. The recombination of methyl group and OH group was found to be the rate-determining step for the methanol formation. The proposed reaction pathway in our work provides important insights into the oxidation mechanism by hydrogen peroxide and the fine-tuning of the Pd catalyst by other alloying elements such as Au.