In situ studies of surface of NiFe₂O₄ catalyst during complete oxidation of methane

NiFe₂O₄ with an inverse spinel structure exhibits high activity for a complete oxidation of methane at 400°C-425°C and a higher temperature. The surface of the catalyst and its adsorbates were well characterized with ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ infrared spectroscopy (IR). In situ studies of the surface of NiFe₂O₄ using AP-XPS suggest the formation of methoxy-like and formate-like intermediates at a temperature lower than 200°C, supported by the observed vibrational signatures in in situ IR studies. Evolutions of C1s photoemission features and the nominal atomic ratios of C/(Ni + Fe) of the catalyst surface suggest that the formate-like intermediate is transformed to product molecules CO₂ and H₂O in the temperature range of 250-300°C. In situ studies suggest the formation of a spectator, - O_lattice-CH₂-O_lattice -. It strongly bonds to surface through C-O bonds and cannot be activated even at 400°C.