In-situ soft X-ray absorption of over-exchanged Fe/ZSM5

Abstract

In-situ soft X-ray absorption spectroscopy (XAS) has been applied to study the iron redox behavior in over-exchanged Fe/ZSM5. The Fe L-2,L-3 XAS and O K spectral shapes of the Fe/ZSM5 surface have been measured during heat treatments and reduction/oxidation cycles. Charge-transfer multiplet calculations provide a detailed understanding of the L-2,L-3 spectra of iron in Fe/ZSM5. The oxidized form of Fe/ZSM5 contains Fe-III ions in an octahedral surrounding, with a total crystal field splitting of similar to1.0 eV. This value is significantly smaller than that for Fe2O3, which is indicative of a much weaker Fe-O bonding. The reduced form of Fe/ZSM5 has Fell ions in a tetrahedral oxygen surrounding. The Fe L-2,L-3 spectra show that iron in calcined Fe/ZSM5 is reduced in 15 min to an average valence state of 2.65, under 10 mbar of pure helium at room temperature. This value has a relative uncertainty on the order of 0.01. Heating in helium up to 350 degreesC under the same pressure further reduces the iron valence to 2.15. The oxygen spectra show that the autoreduction is accompanied by a loss of molecular oxygen and water. Reoxidation with 5% O-2 in helium yields a valence of >2.90 after 10 min.