Nanomaterials have attracted enormous attention over the last few decades thanks to their improved properties and utility in a plethora of scientific areas including catalysis and electrochemistry. Different parameters such as structure, morphology, chemical composition and the presence of defects directly affect the properties of nanomaterials. A detailed characterization of these parameters is of uttermost importance if one wants to obtain a better insight concerning the structure-to-property connection. Transmission Electron Microscopy (TEM) is an ideal technique to investigate materials at both the nanometer and atomic scale and has therefore been widely used in the study of nanomaterials. [1] By combining the technique with tomography, a technique which derives three-dimensional (3D) information from two-dimensional (2D) projections, one is able to determine the structure and shape of nanostructures in 3D, even with atomic resolution. [2-3] Furthermore, when combined with spectroscopic techniques such as energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) can be used for the determination of the composition and even oxidation state of nanomaterials both in 2D and 3D. [4-5] Nanomaterials are widely used in many electrochemical and catalytic reactions, which typically occur at elevated temperatures, pressures and liquid environments. In such conditions unwanted structural, morphological and compositional changes take place which in turn affect dramatically the catalytic performance. Indeed catalyst degradation is a major problem and has been a matter of intense study for many years. However, only little is known about the changes occurring at the nanometer and atomic scale when nanomaterials are exposed to aggressive chemical environments. During the last decade, specialized in-situ gas and liquid cell, as well as heating TEM holders are available. By using such holders, one can reach higher pressures and temperatures and also introduce liquids in the microscopes, and therefore create an environment which is identical to that during actual catalytic reactions. [6-8] In this talk, an introduction to the fundamentals of TEM and its capabilities will be given, followed by an overview of the most recent advances in the field of in-situ characterization both at the nanometer and the atomic scale in 3D.

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