Characterization of Ag and Ba-modified manganese oxide catalysts: unraveling the factors leading to their enhanced CH4 oxidation activity

Abstract

Catalysts based on manganese oxide and modified with Ag (AgMn0.10) or Ba (BaMn0.10) were prepared and applied to CH₄ deep oxidation. The catalysts were characterized by means of N₂-BET, XRD, XPS, CH₄-TPR and O₂-TPD techniques. Catalytic evaluation results show that both AgMn0.10 and BaMn0.10 exhibit higher CH₄ oxidation activity than the unmodified MnO_x. However, comparative studies on CO oxidation reveal that only AgMn0.10 displays markedly improved activity for this reaction; the activity of BaMn0.10 remains unchanged compared to unmodified MnO_x. In AgMn0.10, a large amount of more mobile and reactive oxygen species is formed, which is thought to be the main reason for its enhanced CH₄ and CO oxidation activity. Whereas, in BaMn0.10, it is found that the mobility of the lattice oxygen is not increased, but somewhat decreased, thus resulting in no enhanced CO oxidation activity on this catalyst. However, the basicity of the lattice O²⁻ is thought to be increased ia Ba addition, which is favorable for the activation and rupture of the first C–H bond in CH₄, the rate-determining step for this reaction. Indeed, this modification effect of Ba is believed to be the predominant factor responsible for the enhanced CH₄ oxidation activity of BaMn0.10.