Kβ-Detected XANES of Framework-Substituted FeZSM-5 Zeolites

Abstract

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.

Geometric and Electronic Structure of α-Oxygen Sites in Mn-ZSM-5 Zeolites

“…The integrated intensity has been set to 0.13 for 6-fold (octahedral) and 0.37 for 4-fold (tetrahedral) symmetry, where these values have some variation from system to system. 56 Figure 9 shows that the samples start essentially as 6-fold Mn II at room temperature. They change to 5-fold Mn 2.5+ at 623 K, where MnZ-0.30 is also partly oxidized.…”

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

“…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.…”

Photon-in photon-out hard X-ray spectroscopy at the Linac Coherent Light Source

“…In high-energy-resolution fluorescence detected (HERFD) XANES (Hä mä lä inen et al, 1991;Friebel et al, 2011) the emission energy is fixed while the incident energy is scanning across an absorption edge. This method is experimentally less elaborate than RIXS and has been used to study unoccupied electronic states with a higher energy resolution as compared with conventional XANES due to the suppression of the core-hole lifetime broadening (Heijboer et al, 2004;Safonova et al, 2006;Glatzel et al, 2009). In the case of extended X-ray absorption fine structure (EXAFS), highresolution detection has been used to enhance the spatial resolution by either discriminating against the fluorescence signal from unwanted elements (Pushkar et al, 2007; or by selecting a specific chemical site in mixedvalent systems (Glatzel et al, 2002b).…”