Heteropoly acid-grafted iron oxide catalysts for efficient selective catalytic reduction of NOx with NH3

Abstract

A series of heteropoly acid-grafted Fe₂O₃ catalysts (i.e., HPA/Fe₂O₃) for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) were investigated. Both HSiW/Fe₂O₃ and HPW/Fe₂O₃ exhibited good SCR activity, which was superior to that of HPMo/Fe₂O₃ over the entire temperature range. All HPA/Fe₂O₃ catalysts showed high BET surface area. XRD demonstrated that grafting of HPA suppressed the phase transition and recrystallization of γ-Fe₂O₃. XPS and XRF results demonstrated that the HPA species of HSiW/Fe₂O₃ and HPMo/Fe₂O₃ tend to accumulate more on the surface compared with HPW/Fe₂O₃ catalysts. Meanwhile, H₂-TPR results suggested that the order of reducibility of the catalysts was γ-Fe₂O₃ > HPW/Fe₂O₃ > HSiW/Fe₂O₃ > HPMo/Fe₂O₃, which was consistent with the order of the SCR performance below 250 °C. Compared with HPW/Fe₂O₃ and HSiW/Fe₂O₃, HPMo/Fe₂O₃ had fewer strong acid sites, resulting in poorer SCR activity. The reaction pathway and mechanism were investigated through transient reaction and kinetic studies. It was found that the order of SCR reaction selectivity was HSiW/Fe₂O₃ ≈ HPW/Fe₂O₃ > HPMo/Fe₂O₃, and the selectivity for both the C–O reaction and SNCR reaction was HPMo/Fe₂O₃ > HPW/Fe₂O₃ ≈ HSiW/Fe₂O₃, which was consistent with the SCR performance.