Boubnov Alexey, Lichtenberg Henning, Mangold Stefan, Grunwaldt Jan Dierk
Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Kaiserstrasse 12, Karlsruhe 76131, Germany.
Synchrotron Radiation Facility ANKA, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen 76344, Germany.
J Synchrotron Radiat. 2015 Mar;22(2):410-26. doi: 10.1107/S1600577514025880. Epub 2015 Feb 25.
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.
利用前缘分析来分析氧化态和配位几何结构，对于多相催化和材料科学具有吸引力，特别是对于原位和时间分辨研究或高度稀释的体系。在本研究中，重点是铁基催化剂。首先报道了分别使用十字石、磷酸铁、氧化亚铁和α-氧化铁作为四面体Fe(2+)、四面体Fe(3+)、八面体Fe(2+)和八面体Fe(3+)的参考化合物，对Fe K边的前缘区域进行系统研究。特别地，比较了高分辨率和常规X射线吸收光谱，因为在多相催化和材料科学中，需要在高质量光谱数据采集和功能性质的同步分析之间进行权衡。从不同分辨率和质量的参考光谱获得的结果表明，这种分析也适用于常规记录的前缘数据。为此，优先使用反正切和一阶多项式减去边缘起始点，而与数据的分辨率和质量无关。对于标准数据和高分辨率数据，由于无法识别的弱跃迁，前缘特征的多重态分析存在局限性。另一方面，任意的经验峰拟合有助于分析，因为可以分离非局部跃迁。结果表明，使用基于变差函数的方法分析Fe位点的氧化态和配位几何结构，对于标准分辨率数据是有效的，并且与高分辨率光谱得到的结果相同。通过分析参考化合物及其明确混合物的光谱验证的该方法，最终应用于跟踪1%Fe/Al2O3和0.5%Fe/BEA沸石催化剂在5%H2/He中还原过程中的结构变化。其他技术难以获得的结果表明，Fe(3+)转变为Fe(2+)，同时保持4-5的局部Fe-O配位数，这表明还原涉及氧邻位的重排而不是它们的去除。总之，基于变差函数的Fe K边光谱分析在催化研究中被证明是非常有用的。
