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 porphyrinlike 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.