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 C. 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 C min À1 , as normally used in the literature; mild calcination: heating rate 0.5 C min À1 ) 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 C. 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 N 2 O decomposition and the nature of the a-oxygen sites are discussed, in relation to recent developments in the understanding of iron non-heme enzymes.

In‐situ Scanning Transmission X‐Ray Microscopy of Catalytic Solids and Related Nanomaterials

“…[18,52] Figure 6 shows Fe/ZSM-5 under 2 mbar O 2 and under 2 mbar He at 350 8C. The O 2 spectrum can be identified as a pure Fe 3 + spectrum and the He spectrum relates to pure Fe 2 + .…”

Oxidation of methane to methanol and formaldehyde over Co–ZSM-5 molecular sieves: Tuning the reactivity and selectivity by alkaline and acid treatments of the zeolite ZSM-5 agglomerates

“…Before reaction the samples (1.0 g, grain size 0.2-0.5 mm) were mildly calcined [37,38] in air (1000 ml/min) at 823 K (heating ramp of 0.5 K/min). After calcination the samples were cooled to RT and flushed with helium.…”

New frontiers in X-ray spectroscopy in heterogeneous catalysis: Using Fe/ZSM-5 as test-system

“…The soft X-ray absorption spectra (XAS) of the iron L edge were measured at BESSY (Berlin, Germany), beamlines U49/ 2-PGM-1 and UE56/2-PGM-1 [3,5]. The spectral resolution of the monochromators was approximately 0.2 eV.…”

“…The preparation of the samples has been discussed in detail elsewhere for the overexchanged Fe/ZSM-5 [1][2][3] and framework-substituted Fe/ZSM-5 [4]. Framework-substituted Fe/ ZSM-5 was prepared via the hydrothermal synthesis method.…”