Redox behaviour of over-exchanged Fe/ZSM5 zeolites studied with in-situ soft X-ray absorption spectroscopy

Abstract

The oxidation and reduction behaviour of calcined over-exchanged Fe/ZSM5 has been studied using soft X-ray absorption by measuring the average iron valence under (2 mbar) helium, oxygen and deNOx (HC-SCR) conditions between room temperature and 350 degreesC. The results ( probing depth of approximately 4 nm) show that Fe/ZSM5 is an extremely flexible redox system. The calcination procedure (severe calcination: heating rate 5 degreesC min(-1), as normally used in the literature; mild calcination: heating rate 0.5 degreesC min(-1)) is proven to be important to optimise the reducibility of iron. Upon mild calcination Fe/ZSM5 has an average valence of 2.9 under oxygen (5% in helium), of 2.5 under pure helium at room temperature (RT), and of 2.1 under pure helium at 350 degreesC. Upon severe calcination Fe/ZSM5 shows higher average valences, in agreement with the assumption that part of the iron in this sample is positioned in small iron-oxide nanoparticles at the outer surface of the zeolite crystals. During heating in helium, the valence reaches a minimum value before slightly rising again (re-oxidation) when the temperature is kept constant. It is also found that the X-ray irradiation is able to affect the average valence by values up to 0.10. This study confirms that iron in 'over-exchanged' Fe/ ZSM5 consists dominantly of highly reactive iron complexes, where the iron is ( distorted) octahedral Fe-III in the oxidised state. The implications for the reaction mechanism for the N2O decomposition and the nature of the alpha-oxygen sites are discussed, in relation to recent developments in the understanding of iron non-heme enzymes.