Revisiting Heterolytic Cleavage Mechanism of Methane C-H Bond Activation over Metal Oxide Surfaces

The understanding of C-H bond activation could facilitate the design of improved catalysts for the conversion of methane to valuable products. However, its mechanism remains controversial, particularly with regard to metal oxides. This study aims to shed light on this issue by systematically investigating methane C-H bond activation across various pristine metal oxide surfaces and revisiting the prevailing heterolytic cleavage mechanism. It is found that the so-called "heterolytic cleavage mechanism" for methane activation could be classified into two distinct mechanisms on bare metal oxide surfaces: the real heterolytic cleavage mechanism over flat nonreducible alkali and alkaline-earth metal oxide surfaces (N-MOSs) and a ligand-to-metal charge transfer (LMCT)-enabled hydrogen atom transfer (HAT) mechanism over reducible metal oxide surfaces (R-MOSs). The dominant mechanism is determined by the Coulomb interaction between methane and the surface at the transition state and the energy of LMCT (E_LMCT). Strong Coulomb interactions favor the heterolytic cleavage mechanism on bare N-MOSs, while the opposite favors the LMCT-enabled HAT mechanism on R-MOSs. Nevertheless, the heterolytic cleavage mechanism might have difficulty occurring under the reaction conditions of methane oxidation since the strong chemisorption of dioxygen over alkali and alkaline metal oxide surfaces would render the methane far from the surface, significantly weakening the Coulomb interaction. Doping can manipulate the electronic structure of lattice oxygen, potentially reducing E_LMCT and even bypassing LMCT to directly generate reactive oxygen radicals, thus accelerating C-H activation. Additionally, these distinct mechanisms can influence subsequent steps, such as C-O coupling. C-H bond activation through the LMCT-enabled mechanism would be a prerequisite to trigger C-O coupling. This study provides valuable insights into the design of targeted catalysts with desired activity and selectivity for efficient and controlled methane conversion.