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Nanomaterials including nanoparticles, nanowires and nanotubes play an important role in heterogeneous catalysis. Thanks to the rapid improvement of the electron microscopic techniques and with the advent of aberration corrected electron microscopy as well as theoretical methodologies, the potential effects induced by nanocatalysts are better understood than before by unravelling their atomic structure. A brief introduction to advanced electron microscopic techniques namely aberration corrected scanning transmission electron microscopy (Cs-STEM) is presented and subsequently two examples of nanocatalysts are considered in the present review. The first example will focus on the study of bimetallic/core-shell nanoalloys. In heterogeneous catalysis, catalysts containing two or more metals might show significantly different catalytic properties compared to the parent metals and thus are widely utilized in several catalytic reactions. Atom-by-atom insights of the nanoalloy based catalysts ex: Au-Pd will be described in the present review using a combination of advanced electron microscopic and spectroscopic techniques. A related example on the understanding of bimetallic clusters by HAADF-STEM will also be presented in addition to nanoparticles. In the second case understanding the structure of transition metal chalcogenide based nanocatalysts by HRTEM and aberration corrected STEM, for the case of MoS2 will be discussed. MoS2-based catalysts serve as model catalysts and are employed in the hydrodesulphurisations (HDS) reactions in the removal of sulphur from gasoline and related petrochemical products. They have been studied in various forms including nanowires, nanotubes and nanoplates. Their structure, atomic insights and as a consequence elucidation of their corresponding catalytic activity are thus important.

The study of systems with dimensions of the order of a few to a hundred nanometers has attracted great attention. The main reason is the peculiar physical and chemical behavior of these systems; they are specially important in catalysts since metal nano particles are the most common constituents of the supported catalysts. The understanding of the structure of nano particles is a crucial step for the understanding of the catalytic activity. In the present work we report a number of transmission electron microscopy techniques that can be used in order to characterize the shape and structure of nano particles. In the last decade electron microscopy has had a dramatic development and at the present time we have a number of techniques from medium-resolution such as selected zone dark-field (SZDF) and weak beam dark-field (WBDF) as well as direct atom imaging by high resolution. In the present work we discuss the theoretical principles of these techniques and their main applications to the field of na...