Selective catalytic reduction of NO with NH₃ over novel iron-tungsten mixed oxide catalyst in a broad temperature range

A series of novel FeW(x) catalysts with different Fe/W molar ratios prepared by the co-precipitation method were investigated in the selective catalytic reduction (SCR) of NO_x with NH₃. Among the tested catalysts, FeW(5) shows the highest catalytic activity, with nearly 100% NO conversion, N₂ selectivity from 250 to 450 °C, and good H₂O/SO₂ durability above 300 °C. After the introduction of WO_x species, the lattice structure change to hematite phase and high dispersion of WO_x on the surface of the catalysts are observed for the FeW(x) catalysts. The abundance of surface adsorbed oxygen arises from charge imbalance and the formation of unsaturated chemical bonds arising from the interaction between iron oxide species and tungsten oxide species. The Brønsted acid sites are mainly supplied by the W-OH bond, while the Lewis acid sites are related to iron oxide species. Meanwhile the addition of WO_x not only leads to the formation of lower-valent iron species, consequently decreasing the Lewis acidity and thereby improving N₂ selectivity, but also enhances the stability of surface acid sites and adsorption of monodentate nitrite and NO₂ species, which accounts for the excellent catalytic performance of FeW(5) at low temperature. According to the reaction details, Brønsted acid sites play a leading role at low temperature and Lewis acid sites may be the major active centers at high temperature in the SCR process for FeW(x) catalysts.