EPR evidence of carrier effect on unpaired-electron number of Mn in SCR: The good, the bad, the ugly

Carrier plays an important role in the catalytic performance by dispersing active species and forming strong interaction with the active species. This work compared Mn/TiO2, Mn/Al2O3, and Mn/ZSM in selective catalytic reduction (SCR) of nitric oxide (NO), and investigated the influence of the carriers on electron properties of loaded Mn. As a result, number of unpaired electrons followed the order of Mn/TiO2 < Mn/Al2O3 < Mn/ZSM. This order was just the opposite of the order of catalytic activity and N2 selectivity. Mn/TiO2 removed 99 % of NO, bigger than 65 % (Mn/Al2O3) and 32 % (Mn/ZSM) at 250 °C. N2 selectivity followed the order of Mn/TiO2 (99 %) > Mn/Al2O3 (93 %) > Mn/ZSM (91 %) at 250 °C. According to mechanism investigation, Mn was imbedded into TiO2 and owned one unpaired electron. In comparison, Mn had five (Mn2+), four (Mn2+), and three (Mn2+) unpaired electrons on Al2O3 and ZSM-5. Less number of unpaired electrons resulted in smaller repulsion between adsorbed NO + O2 and gaseous NH3, indicating faster SCR speed. More unpaired electrons over-oxidized NO + O2 and NH3 to more N2O, resulting in worse N2 selectivity. These results reveal how intrinsic property of element affects the catalytic performance, which will help to design more effective catalysts.