WO3的引入对MnOx-Fe2O3催化剂上NH3-SCR反应中N2选择性的促进作用

采用溶胶-凝胶法制备了不同含量钨修饰的MnO_x-Fe₂O₃催化剂，重点考察WO₃的引入对NH₃-SCR反应中N₂选择性的影响，通过XRD、BET、XPS、H₂-TPR、Raman和In situ DRIFTS等手段对催化剂的物理化学性质进行表征。结果表明，钨的引入显著提高NH₃-SCR的N₂选择性，当WO₃质量分数为15%时，具有最佳的NH₃-SCR催化性能，且在50-250℃条件下N₂O浓度始终低于0.003%。这主要是由于适量WO₃的引入，导致催化剂物相由α-Fe₂O₃向γ-Fe₂O₃转变，并与锰相互作用形成新的无定型MnWO₄，获得较大的比表面积；使得Mn⁴⁺/（Mn³⁺+Mn⁴⁺）比例减少但Fe²⁺及表面化学吸附氧（O_α）含量增加，从而降低催化剂氧化性；增强催化剂表面的Lewis酸性位点的含量及强度，增强NH₃的吸附，促进了SCR反应，同时抑制了NO₂深度氧化形成硝酸盐物种，降低硝酸盐物种还原产生的副产物N₂O含量，从而显著提高WO₃-MnO_x-Fe₂O₃催化剂在NH₃-SCR中的N₂选择性。

Promotion effect of tungsten addition on N2 selectivity of MnOx-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.