Identifying active sites for fast NH3-SCR of NO/NO2 mixtures over Fe-ZSM-5 by operando EPR and UV–vis spectroscopy

[Display omitted] •Fe(β) and a part of Fe(γ) single sites in the ZSM-5 pores are the active species in fast SCR.•Those sites are permanently reduced to inactive Fe(II) during standard SCR.•During fast SCR, they are kept in their active Fe(III) state by reoxidation with NO2.•Raising the number of Fe(β) sites by dedicated synthesis might lead to better catalysts. Three Fe-ZSM-5 catalysts (0.15–0.46 wt.% Fe) prepared via different routes with Fe ions present in extra-framework single sites as well as in oxidic clusters have been catalytically tested and monitored by operando EPR and UV–vis spectroscopy under standard and fast SCR conditions. In both cases, Fe ions in single α positions and oxidic clusters remain essentially trivalent while significant differences are evident for the remaining sites. During standard SCR, Fe sites in β positions are completely, and those in γ positions are partly reduced to inactive FeII, which is not able to catalyze the oxidative activation of NO being essential for its subsequent reduction to N2. During fast SCR, the same β and γ sites are effectively reoxidized by NO2 and are thus kept in a redox-active state. The high reaction rates in fast SCR already at low temperatures are therefore assigned to those sites.