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

摘要

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 m~2/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 C02, 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.