Reaction of Methane with MOx/CeO2 (M = Fe, Ni, and Cu) Catalysts: In Situ Studies with Time-Resolved X-ray Diffraction

Changes in chemical state and structural transformations occurring in a series of MOx/CeO2 (M = Cu, Ni and Fe) powder catalysts upon reaction with methane were investigated here using in situ time-resolved X-ray diffraction (TR-XRD), ex situ X-ray absorption near edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray photoelectron spectroscopy (XPS). XPS shows the presence of adsorbed CHx and COx species after exposing the powder catalysts to methane at room temperature. Temperature-programmed reduction (TPR) measurements point to reaction of the samples with methane and formation of CO, CO2, and H2O gas at temperatures as low as 100 °C. The TR-XRD results show that all of the transition-metal oxides in the as-prepared catalysts can be reduced to their metallic phase during the CH4-TPR process with the ceria support undergoing significant reduction from surface to bulk, yet the reduction temperature varies for different MOx/CeO2 samples. Among these samples, CuOx/CeO2 shows the lowest reduction temperature (below 260 °C) for both the oxide overlayer and the ceria support. The NiOx/CeO2 and CoOx/CeO2 powder catalysts also activate CH4 at relatively low temperatures (below 350 °C), and the oxide overlayers undergo NiO → Ni and Co3O4 → CoO → Co transformations. In the case of FeOx/CeO2, a series of complex changes in chemical state is observed for the oxide overlayer, Fe2O3 → Fe3O4 → FeO → Fe → Fe3C. The ceria support of the three MOx/CeO2 (M = Cu, Ni, and Fe) samples undergoes severe reduction and forms Ce2O3 at high temperature (>650 °C) during the CH4-TPR reaction. The in situ TR-XRD results highlight a rich and complex chemistry for methane on these MOx/CeO2 mixed oxide systems.