Abstract
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 CH(4) while avoiding overoxidation of methanol. Here, we demonstrate that pure ceria (CeO(2)), without any metal promoters, enables gas-phase DCMM with up to 80% selectivity at 300-350 °C, upon optimization of the H(2)O/O(2) 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 O(2). Ex situ transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy confirm that CeO(2) 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 O(2) and H(2)O suppresses CH(3)O 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 O(2) promotes H(2)O 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.