Complete Oxidation of Methane on NiO Nanoclusters Supported on CeO<inf>2</inf> Nanorods through Synergistic Effect

Catalytic complete oxidation of CH4 at relatively low temperatures is significant for removing unburned CH4 from the exhaust of combustion engines fueled with natural gas or liquefied petroleum gas. Here a nanocomposite catalyst (NiO/CeO2) consisting of CeO2 nanorods and supported NiO nanoclusters was prepared by a two-step wet-chemistry method. This nanocomposite catalyst exhibits high catalytic activity for the complete oxidation of CH4 in the temperature range of 350-600 °C. A CH3-like intermediate bound to Ni cations was observed at a relatively low temperature by ambient pressure X-ray photoelectron spectroscopy under catalytic conditions. Parallel kinetic studies of NiO/SiO2, CeO2 and NiO/CeO2 catalysts show that the apparent barrier for complete oxidation of CH4 on NiO/CeO2 (69.4 ± 4 kJ/mol) is much lower than the 95.1 ± 5 kJ/mol of pure CeO2 and 98.4 ± 5 kJ/mol of NiO/SiO2, supported by the turnover frequency of NiO/CeO2 being significantly higher than NiO/SiO2 and CeO2. These differences indicate a synergistic effect between the CeO2 nanorods and the supported NiO nanoclusters. This synergistic effect occurs at the interface of NiO and CeO2, observed in TEM. The lattice oxygen atoms at the interface exhibit high activity based on the lower reduction temperature uncovered in studies of the temperature-programmed reduction of H2.