Catalytic oxidation of methane over Zr and La-doped ceria nanocatalysts

Abstract

Ce1-x-yZrxLayO2-δ mixed oxides were synthesized by a cation complexation method with citric acid. The obtained solids were analyzed by X-ray diffraction (XRD), linear dilatometry, transmission (TEM) and scanning (SEM) electron microscopy, temperature programmed reduction (TPR), nitrogen physisorption and Raman spectroscopy. Finally, the solids were tested as catalysts for methane total oxidation in different CH4/O2 ratios. Morphologic properties and defect concentration for each sample was found to be different, leading to variations in the oxygen mobility and catalytic behavior. The mixed oxide Ce0.9Zr0.05La0.05O2-δ evidenced smallest crystallite size, highest pore volume and specific surface area after calcination at 600 ◦C, but evidenced the more pronounced collapse of the pore structure and coalescence of particles after catalytic test at 750 ◦C. The solid with composition Ce0.9La0.1O2-δ showed the highest concentration of defects, retained a mesoporous structure, and showed the better resistance to coalescence and agglomeration of crystallites, resulting in an improved catalytic performance for methane total oxidation. For the catalytic test performed in excess of oxygen, methane conversion is improved, shifting the light-off curves to lower temperatures compared to the stoichiometric CH4/O2 ratio. Also, it was observed that in an oxygen deficient atmosphere, sample CL maintains its catalytic activity providing oxygen from its lattice to oxidize the methane molecule, more efficiently than Ce0.9Zr0.05La0.05O2-δ and Ce0.9Zr0.1O2. This sample also demonstrated good resistance to water vapor deactivation.