Performance and mechanism comparison of manganese oxides at different valence states for catalytic oxidation of NO

Manganese oxides with different valences (MnO2, Mn2O3 and Mn3O4) synthesized by a hydrothermal synthesis method were investigated to evaluate their catalytic performances for NO oxidation. MnO2 with the Mn4+ ion exhibited the best catalytic activity with a maximum NO conversion of 91.4% at 250 degrees C under the GHSV of 48 000 mL g(-1) h(-1), and the low-temperature catalytic activity for NO oxidation decreased in the order of MnO2 > Mn2O3 > Mn3O4. Compared with Mn2O3 and Mn3O4, the catalytic performance of MnO2 was less affected by NO concentrations, O-2 contents, GHSV, H2O and SO2, though all the three catalysts show stable catalytic properties. To investigate the factors influencing the catalytic activity, their properties were characterized by XRD, SEM, N-2 adsorption, H-2-TPR, O-2-TPD and XPS, and these results indicated that the redox property and active oxygen species of MnO2 were proposed to be the main factors that contributed to the excellent performance in NO oxidation. For the three catalysts, in-situ DRIFTS studies further indicated that both the lattice oxygen and the chemically adsorbed oxygen participated in the NO oxidation process, but the adsorbed species were more easily desorbed from MnO2, probably benefiting for the catalytic performance of MnO2.