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. In this study, kinetic model fitting, CH₄ temperature-programmed reduction-mass spectroscopy (CH₄ TPR-MS), <em>in situ</em> 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. <em>In situ</em> 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.