Porous Mn-based oxides for complete ethanol and toluene catalytic oxidation: The relationship between structure and performance
A series of porous Mn-based oxides (mullite SmMn2O5, perovskite SmMnO3, spinel Mn3O4 and bixbyite Mn2O3) were systematically prepared and applied in ethanol and toluene catalytic oxidation. Among them, SmMn2O5 exhibited the highest catalytic activity, the best hydrothermal stability and the highest durability. Due to different compositions and structures of the catalysts, the surface composition, Mn-valance state, reduction property, adsorption and desorption of oxygen and ethanol were different as characterized by XPS, H2-TPR, O2-TPD and ethanol-TPD measurements. Based on the characterization results, the relationship between their structure (crystal and electronic structure) and catalytic activity was discussed in terms of their crystal field configurations and structures of the energy bands. The superior activity of SmMn2O5 could be attributed to dz2 orbital occupancy near the Fermi level determined by the Mn3+-centered pyramidal field, as well as the existence of Mn3+-Mn3+ dimer structure, which is critical for the C-C bond cleavage.