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

Highly ordered mesoporous metal oxides (meso-MO _x) such as CeO ₂, Co ₃O ₄, Mn ₂O ₃, SnO ₂, and TiO ₂ were successfully synthesized by using nano-replication method, and Pd-loaded meso-MO _x (Pd/meso-MO _x) catalysts for CO oxidation were investigated. The catalysts were characterized by X-ray diffraction, N ₂ adsorption-desorption, electron microscopy, CO-temperature programmed desorption (CO-TPD), and H ₂-temperature programmed reduction (H ₂-TPR). All of the catalysts exhibited highly ordered mesostructure and a high surface area (>100 m ² g ^-1). The Pd-loading on meso-MO _x supports enhanced catalytic activities compared with those of MO _x supports only. Among the catalysts, Pd/meso-Co ₃O ₄ showed a high catalytic activity. The activities of Pd/meso-Co ₃O ₄ and Pd/meso-Mn ₂O ₃ were only slightly increased compared to the corresponding meso-MO _x materials, whereas Pd/meso-CeO ₂, Pd/meso-SnO ₂ and Pd/meso-TiO ₂ exhibited significant increases in the catalytic activities. XPS spectra showed that Pd species had strong interactions with meso-CeO ₂, meso-SnO ₂ and meso-TiO ₂, and less interaction with meso-Co ₃O ₄ and meso-Mn ₂O ₃. 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-MO _x can be further improved by a pre-reduction treatment. After H ₂ pretreatment, all of the catalytic activities were increased. Especially, the Pd/meso-CeO ₂ exhibited highest activity after the H ₂ 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.