Bridge-type Mn–O–Mn sites promoting catalytic methane oxidation and carbonate desorption over Mn-based oxides

Abstract

MnO₂ is a good catalyst due to its various crystal structures and oxidative valence states. The relationship between the surface structure and activity of MnO₂ at low temperatures has been reported, but the structure–activity relationship between its crystal structure and catalytic activity at high temperatures is unclear. In this study, α-, γ-, and δ-MnO₂ catalysts were used for catalytic methane (CH₄) oxidation. α-MnO₂ had better CH₄ conversion (95%) and CO₂ selectivity (100%) at 600 °C due to its large amount of bridge-type oxygen (Mn–O–Mn). DFT calculation results showed that the adsorption energy of CH₄ at the surface oxygen site of the α-MnO₂ catalyst was the lowest. Operando TPR-DRIFTS-MS results showed that Mn–O–Mn played an important role in CH₄ oxidation. Mn–O–Mn favored O₂ decomposition and promoted terminal-type oxygen (MnO) formation. Monodentate carbonate was easily formed at MnO with CO oxidized by O in Mn–O–Mn. Mn-based catalysts with more Mn–O–Mn sites are of great significance for high-temperature CH₄ oxidation.