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Nanoparticle (NP) catalysts are ubiquitous in energy systems, chemical production, and reducing the environmental impact of many industrial processes. Under reactive environments, the availability of catalytically active sites on the NP surface is determined by its dynamic structure. However, atomic-scale insights into how a NP surface reconstructs under reaction conditions and the impact of the reconstruction on catalytic activity are still lacking. Using operando transmission electron microscopy, we show that Pd NPs exhibit periodic round–to–flat transitions altering their facets during CO oxidation reaction at atmospheric pressure and elevated temperatures. This restructuring causes spontaneous oscillations in the conversion of CO to CO2 under constant reaction conditions. Our study reveals that the oscillatory behavior stems from the CO-adsorption-mediated periodic restructuring of the nanocatalysts between high-index-faceted round and low-index-faceted flat shapes. These atomic-scale insights into the dynamic surface properties of NPs under reactive conditions play an important role in the design of high-performance catalysts. Atomic-scale insights into how a nanoparticle surface reconstructs under reaction conditions and the impact of the reconstruction on catalytic activity are still lacking. Here the authors reveal that Pd nanocatalysts display oscillatory changes in both their structure and activity during CO oxidation using operando TEM. [ABSTRACT FROM AUTHOR]

In bimetallic heterostructured nanoparticles (NPs), the synergistic effect between their different metallic components leads to higher catalytic activity compared to the activity of the individual components. However, how the dynamic changes through which these NPs adopt catalytically active structures during a reaction and how the restructuring affects their activity are largely unknown. Here, using operando transmission electron microscopy, structural changes are studied in bimetallic Ni–Rh NPs, comprising of a Ni core whose surface is decorated with smaller Rh NPs, during a CO oxidation reaction. The direct atomic‐scale imaging reveals that, under O2‐rich conditions, Ni core partially transforms into NiO, forming a (Ni+NiO)–Rh hollow nanocatalyst with high catalytic activity. Under O2‐poor conditions, Rh NPs alloy with the surface of the core to form a NiRh‐alloy surface, and the NPs display significantly lower activity. The theoretical calculations indicate that NiO component that forms only under O2‐rich conditions enhances the activity by preventing the CO poisoning of the nanocatalysts. The results demonstrate that visualizing the structural changes during reactions is indispensable in identifying the origin of catalytic activity. These insights into the dynamic restructuring of NP catalysts under a reactive environment are critical for the rational design of high‐performance nanocatalysts. [ABSTRACT FROM AUTHOR]