Abstract
The efficient and selective catalyst for a direct and selective oxidation of methane to methanol remains a major challenge in catalysis due to the strength of the C–H bond and the tendency for overoxidation. This study investigates the catalytic potential of first-row (Mn, Fe, Co, Ni, Cu) and second-row (Ru, Rh, Pd) transition metals as single-atom catalysts (SACs) supported on nitrogen-doped fullerene (C24N24) for methane oxidation to methanol. Using Density Functional Theory (DFT) calculations with ORCA 6.0.0, we evaluated reaction energy profiles and activation barriers for each metal-doped system. Further analysis using the Distortion-Interaction Activation Strain (DIAS) model, Quantum Theory of Atoms in Molecules (QTAIM), and Intrinsic Bond Orbital (IBO) theory provided insight into the geometric, electronic, and orbital factors influencing methane activation. The results reveal that Ni and Cu SACs have the lowest activation barriers due to negligible structural distortion and more negative interaction energies at the transition state. QTAIM research revealed significant differences in bond critical point features between metals, whereas IBO analysis revealed that more efficient catalysts promote smoother and more delocalized electron rearrangements during C-H activation. Metals, such as Ru and Mn, exhibit increased electron localization and structural reorganization, which corresponds to higher energy barriers. This study emphasizes the importance of structural, electrostatic, and orbital parameters in determining the catalytic performance of SACs for methane oxidation. The findings help to rationalize the design of effective methane valorisation catalysts and provide useful methodologies for modifying selectivity and activity using atomic-level changes.