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 2,3 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 2,3 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 ∼1.0 eV. This value is significantly smaller than that for Fe 2 O 3, which is indicative of a much weaker Fe−O bonding. The reduced form of Fe/ZSM5 has Fe II ions in a tetrahedral oxygen surrounding. The Fe L 2,3 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 °C 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.