Surfactant-Assisted Synthesis, Characterizations, and Catalytic Oxidation Mechanisms of the Mesoporous MnOx−CeO₂ and Pd/MnOx−CeO₂ Catalysts Used for CO and C₃H₈ Oxidation

A series of mesoporous MnOx−CeO₂ binary oxide catalysts with high specific surface areas were prepared by surfactant-assisted precipitation. The CO and C₃H₈ oxidation reactions were used as model reactions to evaluate their catalytic performance. The techniques of N₂ adsorption/desorption, XRD, XPS, TPR, TPO, TPD, and in situ DRIFTS were employed for catalyst characterization. It is found that the activity for CO and C₃H₈ oxidation of the catalysts exhibits a volcano-type behavior with the increase of Mn content. The catalyst with a Mn/Ce ratio of 4/6, possessing a high specific surface area of 215 m²/g, exhibits the best catalytic activity, which is related not only to its highest reducibility and oxygen-activation ability, as revealed by TPR and TPO, but also to the formation of more active oxygen species on the MnOx−CeO₂ interface as identified by TPD. After the addition of a small amount of Pd to the MnOx−CeO₂ catalyst, its activity for CO oxidation is greatly enhanced, due to the acceleration of gas-phase oxygen activation and transferring via spillover. However, the activity for C₃H₈ oxidation is hardly promoted due to the different reaction pathways for CO and C₃H₈ oxidation. For CO oxidation, the gas-phase oxygen activated by Pd can directly react with the adsorbed CO to form CO₂, while, for C₃H₈ oxidation, which takes place at a much higher temperature than CO oxidation, the C−H bond activation and cleavage may be mainly driven by the active oxygen species on the interface between MnOx and CeO₂. The addition of Pd shows little effect on the active interface oxygen species, so no promotion upon C₃H₈ oxidation is observed.