From Single-Atoms to Clusters: Boosting Methane-to-Methanol Oxidation over Rh/TiO2 via Hydrothermal Reconstruction

The direct oxidation of methane to oxygenates under mild conditions presents a promising yet challenging pathway for natural gas valorization, primarily constrained by the difficulty in simultaneously achieving high atom efficiency and high intrinsic activity at active sites. In this work, we elucidate the structure–activity relationship of Rh/TiO2 catalysts for the selective oxidation of methane to methanol using O2/H2 as oxidants. Through comprehensive in situ DRIFTS and TPSR characterizations, we identify small rhodium (Rh) clusters, rather than isolated Rh single atoms, as the active centers for C–H bond activation. Guided by this mechanistic insight, we develop a controlled in situ reconstruction strategy that deliberately transforms the initially predominant, poorly active single-atom sites into highly active Rh clusters via a simple H2/O2 treatment. This reconstruction leads to a more than 20-fold enhancement in catalytic activity. The optimized catalyst achieves a methanol productivity of 443.2 mmol·gRh−1·h−1 with over 99% selectivity at 70 °C and 42 bar under acidic promotion, compared with 21.20 mmol·gRh−1·h−1 for the pristine catalyst, which is among the highest values reported for Rh-based systems under mild conditions. Mechanistic studies by electron paramagnetic resonance spectroscopy reveal that in situ-generated •OH radicals from O2 and H2 are the key active species, which couple with CH3• intermediates to form methanol; the addition of acid further promotes •OH generation and enhances productivity.