Behaviour of oxygen species adsorbed on Al2O3-supported cerium oxide catalysts for methane oxidation

Abstract

The properties and catalytic behaviour of adsorbed oxygen species on alumina-supported cerium oxides (CeO₂, CeAlO₃ and CeO_{2 –x}) for methane oxidation have been investigated by means of EPR, FTIR, temperature-programmed desorption (TPD) and ¹⁸O tracer techniques. The adsorbed oxygen on the cerium oxide catalysts was found to be the side-on type O₂^– ion by EPR and FTIR measurements. The adsorption site of the O₂^– ion on CeO_{2 –x}/Al₂O₃ was assumed to be the site formed by the ensemble effect between oxygen vacancies and surface cations, while on the other catalysts the sites were cerium ions on the surface. The oxygen species desorbed after ¹⁸O₂ adsorption at 773 K were ¹⁶O₂ and ¹⁶O ¹⁸O in all catalysts, and the major products for methane oxidation were C¹⁶O₂ and C¹⁶O ¹⁸O. These suggest that the adsorbed oxygen is activated through the lattice in cerium oxides. The activities of methane oxidation were influenced by the oxygen diffusion processes on the cerium oxides.