Low temperature CO oxidation over Pd catalysts supported on highly ordered mesoporous metal oxides

Highly ordered mesoporous metal oxides (meso-MOx) such as CeO2, Co3O4, Mn2O3, SnO2, and TiO2 were successfully synthesized by using nano-replication method, and Pd-loaded meso-MOx (Pd/meso-MOx) catalysts for CO oxidation were investigated. The catalysts were characterized by X-ray diffraction, N2 adsorption–desorption, electron microscopy, CO-temperature programmed desorption (CO-TPD), and H2-temperature programmed reduction (H2-TPR). All of the catalysts exhibited highly ordered mesostructure and a high surface area (>100 m2 g−1). The Pd-loading on meso-MOx supports enhanced catalytic activities compared with those of MOx supports only. Among the catalysts, Pd/meso-Co3O4 showed a high catalytic activity. The activities of Pd/meso-Co3O4 and Pd/meso-Mn2O3 were only slightly increased compared to the corresponding meso-MOx materials, whereas Pd/meso-CeO2, Pd/meso-SnO2 and Pd/meso-TiO2 exhibited significant increases in the catalytic activities. XPS spectra showed that Pd species had strong interactions with meso-CeO2, meso-SnO2 and meso-TiO2, and less interaction with meso-Co3O4 and meso-Mn2O3. These results suggest a synergistic effect between metal and the supports and the catalytic behaviours and activities are highly dependent on the nature of supports. The catalytic activities of Pd/meso-MOx can be further improved by a pre-reduction treatment. After H2 pretreatment, all of the catalytic activities were increased. Especially, the Pd/meso-CeO2 exhibited highest activity after the H2 pretreatment. Because the pretreatment enhances metal–support interaction and the formation of oxygen vacancies and hydroxyl groups around Pd or the surface interaction phase, which is participated by influencing electronic state of the surface active sites.