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

A series of Fe-beta 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 approximate to 6) and octahedral (g approximate to 8.8) environments showed better redox abilities than tetrahedral Fe3+ (g approximate to 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-beta catalysts and the contribution of diverse clustered oligomers was unequal.