Identification of the iron oxidation state and coordination geometry in iron oxide- and zeolite-based catalysts using pre-edge XAS analysis

Abstract

Analysis of the oxidation state and coordination geometry using pre-edge analysis is attractive for heterogeneous catalysis and materials science, especially for in situ and time-resolved studies or highly diluted systems. In the present study, focus is laid on iron-based catalysts. First a systematic investigation of the pre-edge region of the Fe K-edge using staurolite, FePO4, FeO and α-Fe2O3 as reference compounds for tetrahedral Fe(2+), tetrahedral Fe(3+), octahedral Fe(2+) and octahedral Fe(3+), respectively, is reported. In particular, high-resolution and conventional X-ray absorption spectra are compared, considering that in heterogeneous catalysis and material science a compromise between high-quality spectroscopic data acquisition and simultaneous analysis of functional properties is required. Results, which were obtained from reference spectra acquired with different resolution and quality, demonstrate that this analysis is also applicable to conventionally recorded pre-edge data. For this purpose, subtraction of the edge onset is preferentially carried out using an arctangent and a first-degree polynomial, independent of the resolution and quality of the data. For both standard and high-resolution data, multiplet analysis of pre-edge features has limitations due to weak transitions that cannot be identified. On the other hand, an arbitrary empirical peak fitting assists the analysis in that non-local transitions can be isolated. The analysis of the oxidation state and coordination geometry of the Fe sites using a variogram-based method is shown to be effective for standard-resolution data and leads to the same results as for high-resolution spectra. This method, validated by analysing spectra of reference compounds and their well defined mixtures, is finally applied to track structural changes in a 1% Fe/Al2O3 and a 0.5% Fe/BEA zeolite catalyst during reduction in 5% H2/He. The results, hardly accessible by other techniques, show that Fe(3+) is transformed into Fe(2+), while the local Fe-O coordination number of 4-5 is maintained, suggesting that the reduction involves a rearrangement of the oxygen neighbours rather than their removal. In conclusion, the variogram-based analysis of Fe K-edge spectra proves to be very useful in catalysis research.

Postshock Thermally Induced Transformations in Experimentally Shocked Magnetite

“…(b) Centroid position of the pre‐edge peak as a function of the shock pressure (black symbols) and the subsequent heating (red symbols). (c) Calibration curve (from Boubnov et al, ) shows a linear relationship between the centroid position of the preedge peak for hematite (Fe 3+ /Total Fe = 1), magnetite (Fe 3+ /Total Fe = 0.67) and synthetic fayalite, Fe 2 SiO 4 (Fe 3+ /Total Fe = 0). Blue arrows indicate position of reference oxides according to Bajt et al () while the blue frame shows data of this study shown in Figure 7b.…”

Towards advanced structural analysis of iron oxide clusters on the surface of γ-Al2O3 using EXAFS

“…The spectra were merged and normalised. Pre-edge spectra were extracted from the near-edge structure (XANES) spectra by using an arctangent function and a first-degree polynomial, modelling the onset of the absorption edge [39]. The extracted pre-edge peak was modelled by a variable number of pseudo-Voigt peak functions, allowing the FWHM to float at around 2-3 eV.…”

Ferrous Iron Under Oxygen‐Rich Conditions in the Deep Mantle

“…XANES spectra were collected in situ at high pressure and temperature by coupling a DAC and a double-sided laser heating setup. Extended description of the experimental methods and spectrum analysis can be found in the supporting information (Akahama & Kawamura, 2004;Benedetti & Loubeyre, 2004;Boccato et al, 2017;Boubnov et al, 2015;Dewaele et al, 2008;Dräger et al, 1988;Galoisy et al, 2001;Giampaoli et al, 2018;Waychunas et al, 1983;Max Wilke et al, 2005). Due to the X-ray absorption setup used, no simultaneous X-ray diffraction could be performed; we thus assume the phases present from previous X-ray diffraction measurements on the same assemblages and at the same pressure and temperature conditions (e.g., Bykova et al, 2016;Fischer et al, 2011;Hu et al, 2016).…”