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

MnO x based catalysts exhibit excellent low-temperature activity in the selective catalytic reduction (SCR) of NO x by NH 3 . The effect of Mn oxidation state (chemical valence) on the NH 3 -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 MnO x (MnO, Mn 3 O 4 , Mn 2 O 3 , MnO 2 ) and physical mixed 30% MnO x /TiO 2 catalysts, as well as three Mn/TiO 2 catalysts prepared by impregnation, were investigated for the low temperature NO-SCR with NH 3 in the presence of excess O 2 . The catalysts were characterized by N 2 adsorption, X-ray diffraction (XRD), X-ray Absorption Fine Structure (XAFS), and temperature programmed reduction of H 2 (H 2 -TPR) to probe the physical and chemical properties of catalysts. The MnO x with different chemical valence exhibited distinct intrinsic activity. The Mn 3 O 4 and MnO/TiO 2 showed the highest intrinsic activity (TOR) for the unsupported and supported catalysts, respectively. The MnO x species with low chemical valence (MnO, Mn 3 O 4 ) exhibited overall high intrinsic SCR activity (larger than 0.012 NO h −1  nm −2 MnOx ). The MnO x with different valence normally changed to higher oxidation state after physically mixed with TiO 2 . The content of MnO x 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.