Mechanistic Understanding of Methane Combustion over Ni/CeO2: A Combined Experimental and Theoretical Approach

Catalytic oxidation of methane (CH4) over nonprecious Ni/CeO2 catalysts has received a lot of attention due to the large natural gas reserves found in North America and the prohibitive cost of palladium-based catalysts, commonly used for CH4 oxidation. However, the catalytic mechanism of CH4 oxidation over Ni/CeO2 still remains unclear. Moreover, the parameters affecting the reaction rates, the interaction between nickel and CeO2, and the reaction intermediates are still not well understood. Herein, kinetic model fitting, CH4 temperature-programmed reduction-mass spectroscopy (CH4 TPR-MS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations were combined to elucidate the mechanism of complete oxidation of CH4 over Ni/CeO2. CH4 TPR-MS showed that the complete oxidation of CH4 over Ni/CeO2 requires 55–120 °C lower compared to bare CeO2 or Ni/quartz sand; complete oxidation of CH4 took place when the surface oxygen species were abundant, while partial oxidation products (CO, H2) were formed when the oxygen species were depleted. In situ DRIFTS showed that CH3, CH2, CO, and CO2 were formed after CH4 activation over Ni/CeO2, while CH3O species were not observed. Combining those findings with kinetic model fitting, a redox Mars–van Krevelen (MvK) mechanism showed the best description of the experimental observations. The MvK mechanism involves the reaction of dissociated oxygen species with gas-phase CH4 while water inhibits the reaction rate by adsorbing on the oxidized sites. Moreover, CH4 activation leads to the reduction of the active sites and oxygen vacancy formation followed by reoxidation of the active sites by gas-phase O2. A CH4 oxidation reaction pathway over Ni/CeO2 is proposed by DFT calculations. In summary, the findings shown here suggest that CH4 oxidation over Ni/CeO2 follows a redox MvK mechanism and provides guidance for the rational design of non-precious-metal catalysts for CH4 oxidation reactions.