The nature of the active site in the Fe-ZSM-5/N2O system studied by (resonant) inelastic X-ray scattering

Resonant inelastic X-ray scattering (RIXS) was applied to investigate whether the iron–oxygen species created upon reaction of Fe-ZSM-5 with N2O have a (partial) 3d4 configuration, i.e. oxidation state Fe(IV). Fe-ZSM-5 samples with low iron loading were pre-treated at high temperatures before reaction with N2O. Then Fe K-edge XANES spectra were recorded either by selecting the fluorescence of the main Kβ1,3 line or the satellite Kβ′ line in the Kβ emission spectrum. The experiments were performed in situ and imitated the experiments performed in the laboratory reactor. The absence of a pre-edge in the Kβ′ line spectrum proves that the configuration of the active iron–oxygen species is purely 3d5. The result was confirmed for several differently treated Fe-ZSM-5 catalysts. The high reactivity of Fe-ZSM-5 is therefore not due to the formation of high oxidation states of iron, but must be ascribed to the particular oxygen species that is created by the reaction of N2O with the catalyst.