The role of various iron species in Fe-β catalysts with low iron loadings for NH3-SCR

Abstract

A series of Fe-β catalysts, containing 0.17–0.52 wt% Fe, were prepared by a liquid ion-exchange method to study the influence of various iron species on NH₃-SCR activity. A combination of UV-vis and EPR techniques was applied to identify and quantify the iron species. The spectroscopic studies showed that the iron was almost all isolated Fe³⁺ when the Fe content was less than or equal to 0.17 wt%. At higher Fe contents (0.27–0.52 wt% Fe), small oligomers coexisted with isolated Fe³⁺ species. Furthermore, the quantitative analysis indicated that the percentage of tetrahedral Fe³⁺ isolates decreased, while the percentage of octahedral Fe³⁺ isolates increased with incremental iron loading. In situ EPR results suggested that isolated Fe³⁺ showed excellent activity for NH₃-SCR; moreover, isolated Fe³⁺ sites in distorted tetrahedral (g ≈ 6) and octahedral (g ≈ 8.8) environments showed better redox abilities than tetrahedral Fe³⁺ (g ≈ 4.3). The SCR TOF values proved that isolated Fe³⁺ sites both in tetrahedral and octahedral coordinations were the active sites for the NH₃-SCR reaction. In addition, the NH₃ oxidation TOF results indicated that oligomers were the active sites for NH₃ oxidation over Fe-β catalysts and the contribution of diverse clustered oligomers was unequal.