Improvement of Activity and SO_2 Tolerance of Sn-Modified MnO_x-CeO_2 Catalysts for NH_3-SCR at Low Temperatures

摘要

The performances of fresh and sulfated MnO_x-CeO_2rncatalysts for selective catalytic reduction of NO, by NH_3 (NH_3-SCR)rnin a low-temperature range (T ＜ 300 ℃) were investigated.rnCharacterization of these catalysts aimed at elucidating the role ofrnadditive and the effect of sulfation. The catalyst having a Sn:Mn:Ce =rn1:4:5 molar ratio showed the widest SCR activity improvement withrnnear 100% NO_x wnversion at 110-230 ℃. Raman and X-rayrnphotoelectron spectroscopy (XPS) indicated that Sn modificationrnsignificantly increases the wncentration of oxygen vacancies that mayrnfacilitate NO oxidation to NO_2. NH_3-TPD characterization showedrnthat the low-temperature NH_3-SCR activity is well correlated withrnsurface acidity for NH_3 adsorption, which is also enhanced by Snrnmodification. Furthermore, as wmpared to MnO_x-CeO_2 Sn-rnmodified MnO_x-CeO_2 showed remarkably improved tolerance to SO_2 sulfation and to the combined effect of SO_2 andrnH_2O. in the presence of SO_2 and H_2O, the Sn modified MnO_x-CeO_2 catalyst gave 62% and 94% NO_x conversions as comparedrnto 18% and 56% over MnO_x-CeO_2 at temperatures of 110 and 220 ℃, respectively. Sulfation of SnO_2modified MnO_x-CeO_2rnmay form Ce(Ⅲ) sulfate that could enhance the Lewis acidity and improve NO oxidation to NO_2 during NH_3-SCR at T ＞ 200rn℃.