1s2p resonant inelastic X-ray scattering (RIXS) spectroscopy has been measured for a series of iron oxides, including octahedral and tetrahedral Fe(II) and Fe(III) systems. Their spectral shapes have been analyzed and explained using crystal-field multiplet simulations. The RIXS planes and the K-edge and L-edge X-ray absorption spectra related to these RIXS planes will be discussed with respect to their analytical opportunities. It is concluded that the full power and possibilities of 1s2p RIXS needs an overall resolution of 0.3 eV. This will yield a technique with more detailed information than K-edge and L-edge X-ray absorption combined, obtained in a single experiment. Another major advantage is that 1s2p RIXS involves only hard X-rays, and experiments under essentially any condition and on any system are feasible.

The valence and local symmetry of iron in framework-substituted FeZSM-5 with a high Fe dilution (Si/Fe ) 360) was studied by means of K -detected X-ray absorption spectroscopy. This technique combines highresolution (∆E ∼1 eV) fluorescence detection of the 3p to 1s (K ) transition with the X-ray absorption near-edge structure (XANES) at the Fe K-edge. An absorption-like spectrum is recorded by detecting the K fluorescence intensity as a function of the incident energy that is scanned through the K absorption edge. K -detected XANES spectra allow for a more precise separation of the weak K pre-edge structure from the main edge as compared to conventional absorption spectroscopy. Subsequent analysis and interpretation of the pre-edge spectral features therefore is more accurate. The pre-edge is sensitive to changes in the local coordination and oxidation state of Fe. Using this technique we were able to quantitatively determine the degree of iron extraction out of a zeolite framework upon steaming. With the use of appropriate reference compounds, the pre-edge analysis was used to monitor the activation of low-loaded, framework-substituted FeZSM-5 (0.3 wt % Fe). Template removal and calcination distort the zeolite framework and induce a deviation from T d symmetry for incorporated iron. The (deliberate) presence of water at high temperature (T > 500°C) facilitates the hydrolysis of the Si-O-Fe bonds and increases the formation of extraframework iron species. The amount of Fe III occupying tetrahedral sites in the MFI-type zeolite decreases to 32% and 19%, respectively, for mild-and hard-steamed samples.

In-situ soft X-ray absorption spectroscopy (XAS) has been applied to study the iron redox behavior in overexchanged 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.

We have applied a number of novel X-ray spectroscopic tools to Fe/ZSM-5 systems. Fe/ZSM-5 can be considered as an ideal test-system for the characterization techniques in heterogeneous catalysis. The existence of a large range of sites and structures creates a good testing ground to determine which experimental tools are able to resolve such complex system. In situ soft X-ray absorption provides important information on the valence and electronic structure of iron during treatments, with a time scale down to 30 s. Kb-detected XANES yields unprecedented resolution for pre-edge structures and using hard X-rays can be used under any condition and treatment, including high-pressures. It can be expected that both in situ soft X-ray absorption and Kb-detected XANES become 'standard' tools for catalysis research, similar to traditional XANES and EXAFS today. The X-MCD is also used in this paper but it will probably remain a rather specialized technique in the field of heterogeneous catalysis. Further new developments for catalysis characterization are for all to be expected from X-ray spectro-microscopy, where one will have the possibility to perform the in situ soft X-ray absorption and Kb-detected XANES experiments with nanometer size spatial resolution. #

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.

The structure of the iron species in mildly calcined over-exchanged Fe/ZSM5, prepared by CVD of FeCl 3 , was studied during heat treatments in He or O 2 /He (50:50) by coupling in situ Fe K edge HR-XANES and EXAFS. The majority of iron appears to be present as

X-ray spectroscopic techniques are important tools for the characterization of metal ions, clusters and nanoparticles in zeolites. A major limitation in X-ray absorption spectra is the intrinsic broadening due to core hole lifetime effects. Using high-energy resolution fluorescence detection (HERFD), this intrinsic broadening is removed, revealing new fine structure. In addition, we will show that the use of circular polarized X-rays can be a useful tool to determine the aggregation of metal ions. We present results obtained using these novel X-ray spectroscopic techniques, using Fe-ZSM-5 and Mn-ZSM-5 systems as showcases. The prospects for these techniques to become "standard" tools for catalysis's research will be discussed.