Catalytic Oxidation of Methane on IrO2(110) Films Investigated Using Ambient-Pressure X-ray Photoelectron Spectroscopy

The catalytic oxidation of CH4 over IrO2 (110) films grown on Ir(100) was investigated using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at total pressures near 1 Torr. The IrO2 (110) films undergo negligible reduction during catalytic CH4 oxidation in reactant mixtures with as much as 95% CH4 and temperatures from ca. 500-650 K, demonstrating that IrO2(110) can catalyze the oxidation of CH4 over a wide range of temperatures and mixture compositions. High coverages of OH groups and oxidized C-containing species formed on the IrO2(110) surfaces during CH4 oxidation, including excess OH groups bound directly to the initially, coordinatively unsaturated Ir atoms. The formation of excess OH groups demonstrates that O-rich IrO2(110) surfaces were maintained even under highly CH4-rich conditions and provides evidence that the dissociative adsorption of O-2 is more facile than CH4 activation and conversion to adsorbed intermediates on IrO2(110). Extensively oxidized surface species with a CHyO2 stoichiometry preferentially formed under all reaction conditions studied. The conversion of CH4 to the CHyO2 surface species became optimal at an intermediate composition of the reactant mixture (similar to 90% CH4), consistent with a site competition between CH4 and O-2 during their initial adsorption as well as a high oxidation activity of chemisorbed O atoms on IrO2 (110). These results provide quantitative information about the identities and coverages of adsorbed species that form during the catalytic oxidation of CH4 on IrO2(110). Such knowledge is essential for validating first-principles models of the reaction kinetics for this system and ultimately gaining insights needed to optimize the performance of IrO2 catalysts for the oxidation of light alkanes.