Characterization of heterogeneous catalysts using advanced transmission electron microscopy techniques
A heterogeneous catalyst is one of the most indispensable materials in industry and our daily life. Approximately 85–90% of the chemical products are made by utilizing catalytic processes. Moreover, heterogeneous catalysts are heavily used to clean up the exhaust gases and to prevent pollutions. In this regard, developing a novel heterogeneous catalyst having a long lifetime as well as an excellent catalytic activity, selectivity is crucial to sustain our modern society. Since this requires a clear understanding on the correlations between catalytic properties and structures of the catalyst, characterization of catalysts is an essential part of the heterogeneous catalysts research. However, this can often be a daunting challenge, because the structural components determining the catalytic properties have many different length scales from atomic to macro-scale. Furthermore, the microstructures of the catalysts are dynamically changed as the catalysts are exposed to reactive environments, such as gaseous and liquid reactants, heat, and etc. The focus of this dissertation is to develop and apply advanced transmission electron microscopy (TEM) techniques, such as electron tomography (ET), aberration-corrected scanning TEM (AC-STEM), and in-situ TEM to solve challenging problems in the field of heterogeneous catalysis. Major questions being addressed in this dissertation are closely related to the sintering phenomena, including the “surface migration mechanism” of catalytic nanoclusters, the correlations between “three dimensional (3D) structures of catalysts and thermal stability,” and “thermal behaviors” of catalytic nanoparticles at high temperatures. Specifically, fast-scan high angle annular dark-field (HAADF) AC-STEM probed the surface migration behavior, which is one of the most fundamental steps of the sintering process (especially, Ostwald ripening), of Ir atoms and clusters on MgO surfaces at ~ 100 msec temporal resolution. Moreover, 3D structural information of ZnO mesotripod and AuIr bimetallic catalysts was obtained using ET and quantitatively analyzed 3D structure was correlated with the thermal stability. Finally, in-situ TEM visualized the real-time morphological changes and the thermal behaviors of Au-Fe3O4 dumbbell nanoparticle catalysts at high temperatures.
Ortalan, Purdue University.
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