Identification of the iron oxidation state and coordination geometry in iron oxide- and zeolite-based catalysts using pre-edge XAS analysis
Identification of the iron oxidation state and coordination geometry in iron oxide- and zeolite-based catalysts using pre-edge XAS analysis
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DOI：
10.1107/s1600577514025880
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发表时间：
2015-03-01
影响因子：
2.5
通讯作者：
Grunwaldt, Jan-Dierk
中科院分区：
文献类型：
--
作者：
Boubnov, Alexey;Lichtenberg, Henning;Grunwaldt, Jan-Dierk
使用边缘前分析的氧化态和配位几何形状的分析对于多相催化和材料科学是有吸引力的，特别是对于原位和时间分辨研究或高度稀释的系统。在本研究中，重点放在铁基催化剂。首先，用十字石、FePO_4、FeO和-Fe_2 O_3分别作为四面体Fe ~（2+）、四面体Fe ~（3+）、八面体Fe ~（2+）和八面体Fe ~（3+）的参考化合物，系统地研究了Fe K-边的边前区域。特别是，高分辨率和传统的X射线吸收光谱进行了比较，考虑到在多相催化和材料科学之间的高品质的光谱数据采集和功能特性的同时分析的妥协是必需的。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边缘数据。为此目的，优选使用反正切和一次多项式来执行边缘起始的减法，而与数据的分辨率和质量无关。对于标准和高分辨率数据，由于无法识别的弱跃迁，边缘前特征的多重峰分析具有局限性。另一方面，任意经验峰值拟合有助于分析，因为可以隔离非局部跃迁。的氧化态和配位几何的Fe网站使用变差函数为基础的方法的分析被证明是有效的标准分辨率的数据，并导致相同的结果为高分辨率的光谱。该方法通过分析参考化合物及其明确定义的混合物的光谱进行了验证，最后应用于跟踪在5%H-2/He中还原过程中1%Fe/Al_2 O_3和0.5%Fe/Al_2O_3沸石催化剂的结构变化。结果表明，Fe ~（3+）转化为Fe ~（2+），而Fe-O配位数保持在4-5，表明还原反应涉及氧邻位的重排而不是去除。总之，基于变差图的铁K边光谱分析被证明是非常有用的催化研究。
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 -Fe2O3 as reference compounds for tetrahedral Fe2+, tetrahedral Fe3+, octahedral Fe2+ and octahedral Fe3+, 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% H-2/He. The results, hardly accessible by other techniques, show that Fe3+ is transformed into Fe2+, 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.