Revisit the effect of manganese oxidation state on activity in low-temperature NO-SCR

Abstract

MnOx based catalysts exhibit excellent low-temperature activity in the selective catalytic reduction (SCR) of NOx by NH3. The effect of Mn oxidation state (chemical valence) on the NH3-SCR activity is an interesting topic and important for designing an active catalyst. This study aimed to clarify the intrinsic effect of Mn valence in NO-SCR reaction. For this purpose, the pure MnOx (MnO, Mn3O4, Mn2O3, MnO2) and physical mixed 30% MnOx/TiO2 catalysts, as well as three Mn/TiO2 catalysts prepared by impregnation, were investigated for the low temperature NO-SCR with NH3 in the presence of excess O2. The catalysts were characterized by N2 adsorption, X-ray diffraction (XRD), X-ray Absorption Fine Structure (XAFS), and temperature programmed reduction of H2 (H2-TPR) to probe the physical and chemical properties of catalysts. The MnOx with different chemical valence exhibited distinct intrinsic activity. The Mn3O4 and MnO/TiO2 showed the highest intrinsic activity (TOR) for the unsupported and supported catalysts, respectively. The MnOx species with low chemical valence (MnO, Mn3O4) exhibited overall high intrinsic SCR activity (larger than 0.012 NO h−1 nm−2MnOx). The MnOx with different valence normally changed to higher oxidation state after physically mixed with TiO2. The content of MnOx species with different valence depended on their loading and Mn precursor, showing a totally different constitution. This work provides new insights into the Mn valence effect on the intrinsic activity for low-temperature SCR.