Transition metal ions regulate the catalytic performance of Ti0.8M0.2Ce0.2O2+x for the NH3-SCR of NO: the acidic mechanism

Environmentally friendly ceria-based oxides are a new type of promising catalyst systemfor the selective catalytic reduction (SCR) of NOx. The effects of transition metal ions, including Fe3+, Zr4+, Mn2+, Cu2+ and Ni2+, on the catalytic performance of Ti0.8M0.2Ce0.2O2+x for the NH3-SCR of NO were studied in detail, to address a fundamental question about the acidic mechanism -whether transition metal ions can regulate the catalytic performance of Ti0.8M0.2Ce0.2O2+x for the NH3-SCR of NO. The results showed that the transition metal ions were able to improve the catalytic performance and regulate the catalytically active temperature. The maximum activity of the catalyst decreased with decreasing numbers of weak acid sites in the order Ti0.8Fe0.2Ce0.2O2+x > Ti0.8Ni0.2Ce0.2O2+x > Ti0.8Zr0.2Ce0.2O2+x > Ti0.8Mn0.2Ce0.2O2+x > Ti0.8Mn0.2Ce0.2O2+x > Ti0.8Ce0.2O2. The lowest temperature of the catalytically active temperature widow (Tr-L) and acid strength of weak acid sites decrease in the order Ti0.8Ce0.2O2 > Ti0.8Zr0.2Ce0.2O2+x > Ti0.8Ni0.2Ce0.2O2+x > Ti0.8Fe0.2Ce0.2O2+x > Ti0.8Cu0.2Ce0.2O2+x. Moreover, the highest temperature of the catalytically active temperature widow Tr-H and acid strengths of strong acid sites decrease in the order Ti0.8Zr0.2Ce0.2O2+x > Ti0.8Ni0.2Ce0.2O2+x > Ti0.8Fe0.2Ce0.2O2+x > Ti0.8Mn0.2Ce0.2O2+x > Ti0.8Mn0.2Ce0.2O2+x. This study reveals for the first time that the maximum activity of the Ti0.8M0.2Ce0.2O2+x catalyst mainly depends on the number of Lewis acid sites, and the active temperature window mainly depends on the acid strength. Furthermore, the Tr-L is inversely associated with the acid strength of the Lewis acid sites, while the Tr-H is positively associated with the acid strength of the Bronsted acid sites.