The current fast selective catalytic reduction (fast-SCR) technology shows an effectiveness of converting the diesel engine generated nitrogen oxides NOx to environmentally benign nitrogen (N2) with the aid of the precious metal catalyst platinum. Driven by previous finding of the low-cost mullite’s great superiority over Pt in term of NO oxidation, a series of Mn-based oxides Sm(Y, Tb, Gd, Lu)Mn2O5 materials are synthesized to identify a general descriptor to govern the catalytic performance. Utilizing soft X-ray absorption characterization and molecular orbital theory, here, we show that catalytic activity difference presents little dependence on the 3d electron occupancy when varying A site element (Sm, Tb, Y, Gd, Lu). Instead, strong p-d hybridization between lattice O and octahedral Mn leads to weak bonding strength between external O* and pyramid Mn and essentially increases the catalytic behavior of converting NO to NO2.