Synergy of Oxygen and Water in Ceria-Catalyzed Direct Conversion of Methane to Methanol under Continuous Flow.

The direct conversion of methane to methanol (DCMM) under continuous flow and atmospheric pressure offers notable environmental benefits and industrial promise, but remains a long-standing challenge due to the difficulty of activating CH4 while avoiding overoxidation of methanol. Here, we demonstrate that pure ceria (CeO2), without any metal promoters, enables gas-phase DCMM with up to 80% selectivity at 300-350 °C, upon optimization of the H2O/O2 ratio. At 550 °C, methanol and formaldehyde are formed at rates of 24 and 38 μmol g-1 h-1, respectively, both dropping below 1 μmol g-1 h-1 in the absence of O2. Ex situ transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy confirm that CeO2 maintains structural integrity and resists carbon deposition during reaction. Combining kinetic studies, steady-state in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and density functional theory (DFT) reveals that hydroxyl groups (OH), generated from water dissociation, play a multifaceted role: they facilitate C-H bond activation, promote methoxy formation, and enhance methanol desorption. In situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) directly reveals the evolution of surface intermediates and shows that cofeeding O2 and H2O suppresses CH3O and CH x accumulation while boosting methanol yield, indicating a rapid intermediate turnover as key to sustained activity. AP-XPS O 1s spectra further highlight that O2 promotes H2O dissociation, regenerating reactive OH groups and maintaining performance at elevated temperature. These findings offer molecular-level insights into how water and oxygen cooperatively tune reactivity, enabling efficient methane-to-methanol conversion on a metal-free oxide catalyst.