Structural requirements of manganese oxides for methane oxidation: XAS spectroscopy and transition-state studies

Different crystallographic structures of Mn oxides usually lead to diverse coordination geometries and oxidation states of MnO6, which in turn show strikingly different catalytic activities. In this study, methane oxidation performances over Mn oxides with various crystalline structures were investigated, including alpha-MnO2 (double chains of Mn4+O6 octahedra), alpha-Mn2O3 (symmetry-inequivalent Mn3+O6), two-dimensional mesoporous beta-MnO2 (labels as Meso-MnO2, single chains of Mn4+O6 octahedra) and one-dimensional beta-MnO2 (single chains of Mn3+/4+O6 octahedra). The results demonstrate that the methane oxidation activities are dependent on their crystallographic structures, and follow an order of alpha-MnO2 > beta-MnO2 > alpha-Mn2O3 > Meso-MnO2. Meanwhile, alpha-MnO2 exhibits good durability and excellent 9.5vol%H2O/10vol%CO2 resistance ability. EXAFS, Raman, XPS, O-2-TPD-MS and CH4-TPR-MS studies indicate that the outstanding catalytic activity over alpha-MnO2 is due to higher surface Mn concentration, more active oxygen species and monoli-mu-oxo bridged corner-shared [MnO6] sites, and excellent reducibility. More importantly, a new insight into reaction mechanism of methane oxidation over Mn oxides was proposed at the molecular level.