Adsorption and Activation of Methane on the (110) Surface of Rutile-type Metal Dioxides

Methane strongly adsorbs on the (110) surface of IrO₂, a rutile-type metal dioxide. Its C-H bond is facilely dissociated even below room temperature, as predicted in a few theoretical works and actually observed in a recent experimental study. Thence, three questions are posed and answered in this paper: First, why does methane adsorb on the IrO₂ surface so strongly? Second, why is the surface so active for the C-H bond breaking reaction? Third, is there any other rutile-type metal dioxide that is more active than IrO₂? A second-order perturbation theoretic approach is successfully applied to the analysis of the electronic structure of methane, which is found to be significantly distorted on the surface. Regarding the first point, it is clarified that an attractive orbital interaction between the surface Ir 5d_z2 orbital and the distorted methane's highest occupied molecular orbital leads to the strong adsorption. As for the second point, the bond strength between the surface metal atom and the CH₃ fragment generated after the C-H bond scission of methane is correlated well with the activation barrier. A substantial bonding interaction between CH₃'s nonbonding orbital and the dz² orbital hints at the strong Ir-CH₃ bond and hence high catalytic activity ensues. Last but not least, β-PtO₂, a distorted rutile-type dioxide, is identified as a more active catalyst than IrO₂. Here again, a perturbation theoretic line of explanation is found to be of tremendous help. This paper is at the intersection of theoretical, catalytic, inorganic, and physical chemistry. Also, it should serve as a model for the design and study of metal-oxide catalysts for the C-H bond activation of methane.