ABSTRACT
The performances of fresh and sulfated MnOx-CeO2 catalysts for selective catalytic reduction of NOx by NH3 (NH3-SCR) in a low-temperature range (T < 300 °C) were investigated. Characterization of these catalysts aimed at elucidating the role of additive and the effect of sulfation. The catalyst having a Sn:Mn:Ce = 1:4:5 molar ratio showed the widest SCR activity improvement with near 100% NOx conversion at 110-230 °C. Raman and X-ray photoelectron spectroscopy (XPS) indicated that Sn modification significantly increases the concentration of oxygen vacancies that may facilitate NO oxidation to NO2. NH3-TPD characterization showed that the low-temperature NH3-SCR activity is well correlated with surface acidity for NH3 adsorption, which is also enhanced by Sn modification. Furthermore, as compared to MnOx-CeO2, Sn-modified MnOx-CeO2 showed remarkably improved tolerance to SO2 sulfation and to the combined effect of SO2 and H2O. In the presence of SO2 and H2O, the Sn-modified MnOx-CeO2 catalyst gave 62% and 94% NOx conversions as compared to 18% and 56% over MnOx-CeO2 at temperatures of 110 and 220 °C, respectively. Sulfation of SnO2-modified MnOx-CeO2 may form Ce(III) sulfate that could enhance the Lewis acidity and improve NO oxidation to NO2 during NH3-SCR at T > 200 °C.
