Improved Activity and SO2 Resistance by Sm-Modulated Redox of MnCeSmTiOx Mesoporous Amorphous Oxides for Low-Temperature NH3-SCR of NO

The selective catalytic reduction (SCR) technique that converts NOx from the outlet of industrial boilers at low temperature (<200 degrees C) requires catalysts that possess both the oxidization property of NOx and the adsorption ability to NH3. However, owing to unsuitable redox capacity, most NH3-SCR catalysts such as MnO2/TiO2 and MnO2-CeO2/TiO2 suffer from poor activity and N-2 selectivity and SO2 poisoning. Benefiting from constructing mesoporous MnCeSmTiOx amorphous mixed oxides by the coprecipitation method, enhanced SO2-tolerant low-temperature NH3-SCR performance was achieved. The MnCeSmTiOx catalysts have an amorphous and mesoporous structure with a BET surface area of 214 m(2) .g(-1). The NO conversion could reach nearly 100% at 140-320 degrees C and maintain >90% at 400 degrees C and a gas hourly space velocity of 80,000h(-1). The selectivity of N-2 could be maintained at approximate to 100% at 100-320 degrees C and stay at >90% up to 400 degrees C. Besides, the MnCeSmTiOx catalyst preserves higher catalytic performance after introducing H2O and SO2 compared with the catalysts without adding Sm. The redox properties, acidic properties, and reaction intermediates of catalysts were analyzed by X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, ammonia temperature-programmed desorption, oxygen temperature-programmed desorption, pyridine-IR, thermogravimetry-differential scanning calorimetry, and diffuse reflectance infrared Fourier transform. The synergistic effect of the Lewis acid sites and oxidation catalytic sites of mixed oxides serves for the conversion of NO to N-2 by following the Langmuir-Hinshelwood mechanism. Doping Sm into MnCeSmTiOx can increase oxygen vacancies and transfer electrons to Mn4+ and Ce4+, which facilities the formation of active adsorbed NO2, bidentate nitrate, and bridging nitrate intermediates and suppresses SO2 poisoning by inhibiting the oxidation of SO2 by Mn4+ and Ce4+. Our work could be beneficial to modulate the redox capacity of active sites on NH3-SCR catalysts so that NOx can be eliminated in complex flue gas at low temperatures.