Porous Mn-based oxides for complete ethanol and toluene catalytic oxidation: the relationship between structure and performance

Abstract

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 the different compositions and structures of the catalysts, the surface composition, Mn-valence state, reduction properties, and adsorption and desorption behavior 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 the catalyst 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 d_z² orbital occupancy near the Fermi level determined by the Mn³⁺-centered pyramidal field, as well as the existence of a Mn³⁺–Mn³⁺ dimeric structure, which is critical for C–C bond cleavage.