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

The selective catalytic reduction (SCR) technique that converts NO_x from the outlet of industrial boilers at low temperature (<200 °C) requires catalysts that possess both the oxidization property of NO_x 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 N₂ selectivity and SO₂ poisoning. Benefiting from constructing mesoporous MnCeSmTiO_x amorphous mixed oxides by the coprecipitation method, enhanced SO₂-tolerant low-temperature NH₃-SCR performance was achieved. The MnCeSmTiO_x catalysts have an amorphous and mesoporous structure with a BET surface area of 214 m²·g^–1. The NO conversion could reach nearly 100% at 140–320 °C and maintain >90% at 400 °C and a gas hourly space velocity of 80,000 h^–1. The selectivity of N₂ could be maintained at ≈100% at 100–320 °C and stay at >90% up to 400 °C. Besides, the MnCeSmTiO_x 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 N₂ by following the Langmuir–Hinshelwood mechanism. Doping Sm into MnCeSmTiO_x 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 NO_x can be eliminated in complex flue gas at low temperatures.