Why the Low-Temperature Selective Catalytic Reduction Performance of Cr/TiO2 Is Much Less than That of Mn/TiO2: A Mechanism Study

Although Mn/TiO2 and Cr/TiO2 had many similar physicochemical properties, the low-temperature selective catalytic reduction (SCR) performance of Mn/TiO2 was much better than that of Cr/TiO2. In this work, the physicochemical properties of Mn/TiO2 and Cr/TiO2 were characterized. Meanwhile, the mechanism of NO reduction over Mn/TiO2 and Cr/TiO2 was compared using the transient reaction. Furthermore, the kinetic parameters of NO reduction over Mn/TiO2 and Cr/TiO2 were obtained from the steady-state kinetic study. The Eley-Rideal mechanism contributed to NO reduction over both Mn/TiO2 and Cr/TiO2. As the SCR reaction through the Eley-Rideal mechanism needed to overcome the binding of activated NH3 species with the interface and the binding of activated NH3 with Cr/TiO2 was much stronger than that with Mn/TiO2, the rate constant of the SCR reaction over Cr/TiO2 through the Eley Rideal mechanism was much lower than that over Mn/TiO2. Meanwhile, the rate of the catalytic oxidation of NH3 to NO (i.e., C-O reaction) over Cr/TiO2 was much higher than that of Mn/TiO2 due to the stronger oxidation and more Cr6+ on the surface. As a result, the low-temperature SCR activity of Cr/TiO2 was much less than that of Mn/TiO2.