Selective Oxidation of Methane to Methanol over Rh/Hydroxyapatite Catalysts Enabled by Strong Metal–Support Interactions

High Resolution Image Download MS PowerPoint Slide Methane, the primary constituent of natural gas, represents a critical feedstock for the synthesis of high-value liquid fuels and chemicals. However, its practical conversion is often hampered by the high dissociation energy and low polarity of the C–H bond, as well as the susceptibility of methanol to overoxidation. In this study, we propose an effective strategy for methane oxidation utilizing atomically dispersed rhodium supported on hydroxyapatite (Rh/HAp), synthesized via a straightforward impregnation method. Experimental evaluations demonstrate that the 0.5Rh/HAp catalyst achieves a methanol yield of 3440 μmol·g cat –1 ·h –1 at 240 °C (under a CH 4 /O 2 /CO pressure ratio of 20:3:5 bar), maintaining a liquid-phase selectivity of >99% over a 1 h period. Characterization data suggest that the Rh species are atomically dispersed on the HAp surface, existing in a cationic state (Rh δ+ ) characterized by strong metal–support interactions. In situ infrared spectroscopy reveals the formation of thermally stable Rh(CO) 2 active sites. These sites appear to resist aggregation and overoxidation even under high-temperature and oxygen-rich conditions, which is likely attributable to the CO ligand effect and interactions with surface PO 4 3– and OH – groups. Furthermore, in situ DRIFTS analysis suggests a mild, stepwise oxidation pathway, wherein methane is transformed into methanol through the formation of surface methoxy (*OCH 3 ) intermediates.