Using In Situ TEM to Understand the Surfaces of Electrocatalysts at Reaction Conditions: Single‐Atoms to Nanoparticles

Abstract Catalytic activity is improved by reducing particle size, followed by atomic scale structural and chemical design, allowing physical and electronic confinement of chemical conversion centers. These concepts culminate in single‐atom catalysts (SACs) where active precious metal atoms are isolated and anchored on the surface of supports and thus fully utilized for chemical conversion. In contrast, metal nanoparticles facilitate conversion only on surface atoms, rendering the rest of the volume inactive and driving up the cost. Notably, heterogeneous catalyst nanoparticles of a few nm down to single‐atom catalysts exhibit a rapidly changing landscape during reactions, showing metastable facet formations, surface reconstructions, and clustering that do not occur on catalytic surfaces of larger nanoparticles. The implication is that the structure and chemistry before and after reactions are very different from the during‐stage (at high pressure and/or in a liquid environment under bias). This review highlights how aberration‐corrected in situ transmission electron microscopy (TEM) using closed‐cell microchip technology can fill the emerging information gap concerning catalytic mechanisms of a few nm particles at reaction conditions. The challenges of conventional in situ chemical and structural characterization of active sites, are further discussed, and the future role of in situ TEM as a complement to other large‐scale techniques.