Transition metal ions regulate the catalytic performance of Ti_0.8M_0.2Ce_0.2O_2+x for the NH₃-SCR of NO: The acidic mechanism

Environmentally friendly ceria-based oxides are a new type of promising catalyst system for the selective catalytic reduction (SCR) of NO_x. The effects of transition metal ions, including Fe³⁺, Zr⁴⁺, Mn²⁺, Cu²⁺ and Ni²⁺, on the catalytic performance of Ti_0.8M_0.2Ce_0.2O_2+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 Ti_0.8M_0.2Ce_0.2O_2+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 Ti_0.8Fe_0.2Ce_0.2O_2+x > Ti_0.8Ni_0.2Ce_0.2O_2+x > Ti_0.8Zr_0.2Ce_0.2O_2+x > Ti_0.8Mn_0.2Ce_0.2O_2+x > Ti_0.8Cu_0.2Ce_0.2O_2+x > Ti_0.8Ce_0.2O₂. The lowest temperature of the catalytically active temperature widow (T_r-L) and acid strength of weak acid sites decrease in the order Ti_0.8Ce_0.2O₂ > Ti_0.8Zr_0.2Ce_0.2O_2+x > Ti_0.8Ni_0.2Ce_0.2O_2+x > Ti_0.8Fe_0.2Ce_0.2O_2+x > Ti_0.8Cu_0.2Ce_0.2O_2+x. Moreover, the highest temperature of the catalytically active temperature widow T_r-H and acid strengths of strong acid sites decrease in the order Ti_0.8Zr_0.2Ce_0.2O_2+x > Ti_0.8Ni_0.2Ce_0.2O_2+x > Ti_0.8Fe_0.2Ce_0.2O_2+x > Ti_0.8Mn_0.2Ce_0.2O_2+x > Ti_0.8Cu_0.2Ce_0.2O_2+x. This study reveals for the first time that the maximum activity of the Ti_0.8M_0.2Ce_0.2O_2+x catalyst mainly depends on the number of Lewis acid sites, and the active temperature window mainly depends on the acid strength. Furthermore, the T_r-L is inversely associated with the acid strength of the Lewis acid sites, while the T_r-H is positively associated with the acid strength of the Bronsted acid sites.