Promotion effect of tungsten addition on N₂ selectivity of MnO_x-Fe₂O₃ for NH₃-SCR

A series of tungsten modified MnO_x-Fe₂O₃ catalysts with different tungsten contents were prepared by sol-gel method. The influence of tungsten on N₂ selectivity of NH₃-SCR reaction was investigated particularly. Physical and chemical properties of the catalysts were characterized by means of XRD, BET, XPS, H₂-TPR, Raman and in situ DRIFTS. The results showed that N₂ selectivity of NH₃-SCR at high temperature was significantly improved by introducing tungsten. NH₃-SCR possessed the best catalytic performance when the tungsten content was 15% (mass ratio), as well as N₂O concentration was less than 0.003% within the range of 50-250℃. The primary causes were the phase change from α-Fe₂O₃ to γ-Fe₂O₃ due to the introduction of appropriate amount of WO₃. Besides, the interaction between tungsten and manganese formed a new amorphous MnWO₄ and obtained a large specific surface area. In addition, the ratio of Mn⁴⁺/(Mn³⁺+Mn⁴⁺) decreased while the content of Fe²⁺ and surface chemical adsorption oxygen (O_α) increased, thus the oxidability of the catalyst was reduced. Meanwhile, tungsten doping enhanced the content and strength of Lewis acid sites on the surface of catalysts at high temperatures. Moreover, the adsorption of NH₃ was enhanced, thus, NH₃-SCR reaction was accelerated. The doping of WO₃ inhibited the deep oxidation of NO₂ to form nitrate species, reduced the content of by-product N₂O produced by nitrate species reduction, and significantly improved the NH₃-SCR activity and N₂ selectivity of WO₃-MnO_x-Fe₂O₃ catalyst at experimental temperature.