Superior Performance of Fe_1–<i>x</i>W_<i>x</i>O_δ for the Selective Catalytic Reduction of NO_<i>x</i> with NH₃: Interaction between Fe and W

Abstract

Novel iron-tungsten catalysts were first developed for the selective catalytic reduction of NO_x by NH₃ in diesel exhaust, achieving an excellent performance with a wide operating temperature window above 90% NO_x conversion from 225 or 250 to 450 °C (GHSVs of 30 000 or 50 000 h^-1). It also exhibited a pronounced stability and relatively high NO_x conversion in the presence of H₂O, SO₂ and CO₂. The introduction of W resulted in the formation of α-Fe₂O₃ and FeWO₄ species obtained by HRTEM directly. The synergic effect of two species contributed to the high SCR activity, because of the increased surface acidity and electronic property. The FeWO₄ with octahedral [FeO₆]/[WO₆] structure acted as the Brønsted acid sites to form highly active NH₄⁺ species. Combining DFT calculations with XPS and UV-vis results, it was found that the fine electron interaction between α-Fe₂O₃ and FeWO₄ made the electron more easily transfer from W⁶⁺ sites to Fe³⁺ sites, which promoted the formation of NO₂. Judging by the kinetics and SCR activity studies, the Fe_0.75W_0.25O_δ with an appropriate W amount showed the strongest interaction, and thereby the lowest activation energy of 39 kJ•mol^-1 and optimal catalytic activity. These findings would be conducive to the reasonable design of NH₃-SCR catalysts by adjusting the fabrication.