Surface chemistry and reactivity of α-MoO toward methane: A SCAN-functional based DFT study.

Molybdenum trioxide (α-MoO) is a key component in the redox solid catalysts for methane activation. The wide range of interactions including van der Waals interaction and chemical bonding in α-MoO as well as between methane and the catalyst surface makes the accurate description of the methane chemistry a challenge. Herein, we performed a strongly constrained and appropriately normed (SCAN)-functional based density functional theory study of the surface chemistry and reactivity of α-MoO toward C-H bond activation of methane. With this meta-generalized-gradient approximation functional, we can predict the bulk structure of α-MoO more accurately while reproducing the thermal chemistry of MoO. The results indicate that surface reduction of α-MoO (010) occurs preferably through releasing the terminal oxygen atoms, generating oxygen vacancies while exposing reduced Mo centers. These oxygen vacancies tend to be separated from each other at a higher density due to repulsive interactions. Furthermore, the reduced α-MoO (010) promotes methane activation kinetically by reducing the activation barrier for the break of the first C-H bond and thermodynamically by stabilizing the product state as compared with those on the stoichiometric surface. There is a synergy between the reduced Mo active site and surface lattice oxygen for C-H bond cleavage. Our results also show that the reactivity based on the Perdew-Burke-Ernzerhof functional is qualitatively consistent with that from the SCAN functional.