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

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journal contribution
posted on 29.11.2018, 00:00 by Feng Zhang, Siyu Yao, Zongyuan Liu, Ramón A. Gutiérrez, Dimitriy Vovchok, Jiajie Cen, Wenqian Xu, Pedro J. Ramírez, Taejin Kim, Sanjaya D. Senanayake, José A. Rodriguez
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 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.