Mn-based catalysts supported on γ-AlO, TiO and MCM-41: a comparison for low-temperature NO oxidation with low ratio of O/NO.

Mn-Based catalysts supported on γ-AlO, TiO and MCM-41 synthesized by an impregnation method were compared to evaluate their NO catalytic oxidation performance with low ratio O/NO at low temperature (80-200 °C). Activity tests showed that the participation of O remarkably promoted the NO oxidation. The catalytic oxidation performance of the three catalysts decreased in the following order: Mn/γ-AlO > Mn/TiO > Mn/MCM-41, indicating that Mn/γ-AlO exhibited the best catalytic activity. In addition, there was a clear synergistic effect between Mn/γ-AlO and O, followed by Mn/TiO and O. The characterization results of XRD, EDS mapping, BET, H-TPR, XPS and TG showed that Mn/γ-AlO had good manganese dispersion, excellent redox properties, appropriate amounts of coexisting Mn and Mn and abundant chemically adsorbed oxygen, which ensured its good performance. DRIFTS demonstrated the NO adsorption performance on the catalyst surface. As revealed by DRIFTS experiments, the chemically adsorbed oxygen, mainly from the decomposition of O, greatly promoted the NO adsorption and the formation of nitrates. The Mn-based catalysts showed stronger adsorption strength than the corresponding pure supports. Due to the abundant adsorption sites provided by pure γ-AlO, under the interaction of Mn and γ-AlO, the Mn/γ-AlO catalyst exhibited the strongest NO adsorption performance among the three catalysts and produced lots of monodentate nitrates (-O-NO) and bidentate nitrates (-ONO), which were the vital intermediate species for NO formation. Moreover, the NO-TPD studies also demonstrated that Mn/γ-AlO showed the best NO desorption performance among the three catalysts. The good NO adsorption and desorption characteristics of Mn/γ-AlO improved its high catalytic activity. In addition, the activity test results also suggested that Mn/γ-AlO exhibited good SO tolerance.