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Abstract

The analysis of the microstructure of materials, typically of the size and the crystal defects, is widely conducted in heterogeneous catalysis to understand the correlation of the microstructure with the catalytic performance. The active catalysts are reported to be supported components, materials with nanosized particles, high crystal defect density or high porosity. However, such factors make the microstructure analysis difficult. Although High Resolution (Scanning) Transmission Electron Microscopy (HR-(S)TEM) and X-ray Diffraction (XRD) as individual tools are commonly used for microstructure analysis, the techniques are limited in statistics (TEM) and average out local structural fluctuations (XRD). To provide accurate microstructure information using stated techniques for nanosized materials the methods of microstructure analysis have been systematically studied. A methodological development for a comprehensive and quantitative description of the microstructure of inorganic catalysts is presented based on a beneficial interplay of complementary local HR-(S)TEM and integral XRD techniques. The extension of the local information, including size distribution and type of defects obtained from HR-(S)TEM to an integral one, resampling the whole sample was achieved by a simulation of XRD data with Whole Powder Pattern Modelling (WPPM) approach.
In this study, five materials are selected for tackling the issues. The modelling strategies for WPPM were firstly studied in two sections: (A) Nanosized ZnO is chosen as an example for the discussion on how to select the appropriate parameters including crystallite shape and the type of distribution function for a size analysis. (B) Nanosized Au@Carbon and CuO are studied as examples for understanding the accuracy of microstructure analysis if both size and defects are incorporated during modelling. Secondly, a physical mixture of ZnO, CuO and carbon was studied for addressing the detection limits of XRD regarding the low loading complex on amorphous support. Accordingly, a real CuO/ZnO@Carbon adsorbent used for NO_x removal was analyzed. Finally, in addition to the microstructure analysis, the crystal structure analysis was also incorporated by using Rietveld method for CoFe₂O₄, a precursor for the ammonia decomposition reaction. Since the microstructure and the crystal structure of such catalysts undergo changes during the reaction, in-situ XRD characterization was highlighted to monitor the corresponding variation. The correlation with catalytic performance can then be proposed.