Density Functional Theory Studies of the Direct Conversion of Methane to Methanol Using O2 on Graphitic MN4G‑BN (M = Fe, Co, Cu) and CuN4G‑PN Single-Atom Catalysts

Graphene-based single-atom catalysts have attracted increasing interest due to their potential to catalyze the direct conversion of CH4 to CH3OH. In particular, the porphyrin-like FeN4 complex has recently been reported to convert CH4 to CH3OH at low temperatures with high selectivity. However, only N2O and H2O2, which are high-cost and scarce compared to O2, can be used as the oxidant of the reaction. In this paper, we perform density functional theory calculations on graphitic MN4G-BN (M = Fe, Co, Cu) and CuN4G-PN systems to evaluate the CH4 oxidation to CH3OH using O2. We found that the addition of B doping adjacent to the Fe and Co centers as well as P doing adjacent to the Cu center facilitates a facile OO bond dissociation with an activation barrier of less than 0.4 eV, resulting in active M–O and inactive B/P–O sites. This low barrier is due to the early OO bond elongation at the O2 adsorption step and the stability of the atomically adsorbed O atoms. In the subsequent CH4 oxidation, the resultant OCuN4G-OPN is found to be significantly more CH4-reactive than the OFeN4G-OBN and OCoN4G-OBN with a H–CH3 activation barrier of only 0.66 eV. Such high reactivity is due to the proximity of the electron-acceptor orbital (i.e., the Cu–O lowest unoccupied molecular orbital) toward the Fermi level. Moreover, the CH4 oxidation on CuN4G-PN is predicted to form CH3OH with high exothermicity and high resistance to overoxidation. This study suggests a high possibility for CuN4G-PN as a potential catalyst for the stepwise conversion of CH4 to CH3OH using O2 at low temperatures.