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 over-oxidation 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 36 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 (TEM), X-ray photoelectron spectroscopy (XPS), 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 co-feeding O2 and H2O suppresses CH3O and CHx 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.

连续流常压下甲烷直接制甲醇（DCMM）工艺兼具显著的环境效益与工业应用前景，但由于活化甲烷的同时需避免甲醇发生过度氧化，该工艺长期以来仍是一项极具挑战性的课题。本研究证实，无需任何金属助剂的纯氧化铈（CeO₂），在优化H₂O/O₂比例后，可在300~350℃下实现气相甲烷直接制甲醇，甲醇选择性最高可达80%。在550℃时，甲醇与甲醛的生成速率分别为24和36 μmol·g⁻¹·h⁻¹；在无氧气通入的条件下，二者的生成速率均降至1 μmol·g⁻¹·h⁻¹以下。非原位透射电子显微镜（TEM）、X射线光电子能谱（XPS）以及拉曼光谱的表征结果证实，CeO₂在反应过程中可保持结构完整性，且不会发生积碳。结合动力学研究、稳态原位漫反射红外傅里叶变换光谱（in-situ DRIFTS）以及密度泛函理论（DFT）分析可知，由水解离产生的羟基（OH）发挥了多重作用：其可促进C-H键活化、辅助甲氧基生成，并提升甲醇的脱附效率。原位常压X射线光电子能谱（AP-XPS）直接观测到了表面中间体的演化过程，并证实同时通入O₂与H₂O可抑制CH₃O与CHₓ中间体的积累，同时提升甲醇产率——这表明中间体的快速周转是维持催化活性的关键。AP-XPS的O 1s谱图进一步表明，O₂可促进H₂O的解离，从而再生活性羟基基团，并在高温下维持催化性能。本研究结果从分子层面揭示了水与氧气如何协同调控催化反应活性，为无金属氧化物催化剂上高效实现甲烷直接制甲醇提供了理论支撑。