Hydroperoxyl-mediated C-H bond activation on Cr single atom catalyst: An alternative to the Fenton mechanism

Direct conversion of methane to methanol is an attractive alternative to the indirect, energy-intensive syngas production and conversion route. Herein, we present a comprehensive density functional theory (DFT) study of converting methane directly into methanol over the Cr single atom catalyst (SAC) on anatase TiO2 (1 0 1) surface. Cr species stabilize in the form of CrO with H2O2 as oxidant, on which methane reacts with the deprotonated H2O2 via an Eley–Rideal mechanism. The formation of a three-centered transition-state structure is observed in the rate-determining CH bond activation step with an activation barrier of 0.44 eV. The H2O2 not only serves as the oxidant for the formation of the most stable and reactive CrO species, but also enters the coordination sphere of the Cr center and acts as a highly effective H acceptor, which significantly lowers the activation barrier for CH bond cleavage. Over different Cr-O pairs, methane CH bond activation energy is found to linearly correlate with the energy difference between the d-bond center (εd) of Cr and p-band center of O (εp). The presence of water changes the relative stability of the reaction intermediates but does not alter the reaction pathways. This study demonstrated the ability of Cr SACs for catalyzing direct conversion of methane to methanol under mild conditions and would stimulate further interests in designing active Cr catalysts for methane conversion.