Rutile-type metal dioxide (110) surfaces for the cyclic oxidation of methane to methanol

Abstract

The direct conversion of methane to methanol has attracted increasing interests, owing to the necessity for an abundant low-carbon source of energy. However, numerous challenges are encountered in attaining a high conversion rate and selectivity using the existing approach and catalysts. One of them is the need for a reaction halt and a reactivation of the catalyst using an oxidant at high temperature, which makes the whole process non-cyclic. In this study, we employ density functional theory calculations to evaluate rutile-type IrO₂(110), β-PtO₂(110), and β-MnO₂(110) surfaces not only for cleaving the H–CH₃ bond but also for forming methanol. We find that IrO₂(110) and β-PtO₂(110) thermodynamically and kinetically favor the C–H activation on the bridging μO-atom terminations via a heterolytic pathway. However, the formation of strong Ir–C and Pt–C bonds, which initially help the C–H bond scission, hinders the methanol formation. In the β-MnO₂(110) case, in contrast, the Mn–C interaction is quite weak, and the Mn(μ-O)Mn active site is electrophilic, thus allowing the formation of a stable ˙CH₃ radical intermediate state that becomes the driving force for a low-barrier homolytic C–H bond scission as well as a low-barrier and highly exothermic formation of methanol. This first cycle of methane oxidation results in a reduced β-MnO₂(110) surface, where no more μ-O active sites are available for the subsequent cycles of methane activation. Nonetheless, this reduced surface can also oxidize methane to methanol when the H₂O₂ oxidant is inserted in the mid-way reaction and forms new active sites of μ-OH. The second reaction is also highly exothermic although the C–H activation barrier is not as low as that for the fresh stoichiometric surface. This study suggests the β-MnO₂(110) surface as a potential catalyst for the cyclic oxidation of methane to methanol using the H₂O₂ oxidant without halting for reactivation.