Atomic Spreading and Retraction of Supported Ultrasmall Alloy Nanoparticles under Reactive Oxygen at Elevated Temperatures

The ability to control the morphology of supported alloy nanoparticles in an ultrasmall size range (≲5 nm) is challenging especially under reactive oxygen at elevated temperatures. The understanding of factors governing such nanoscale surface-dominated interfacial interactions and reactivities remains elusive since existing studies are mostly based on crystalline phases of larger-sized nanoparticles and the associated nanoparticle-support surface interactions. Here, we reveal a counterintuitive spreading and retraction phenomenon by in situ tracking of the spatiotemporal dynamics of alumina-supported ultrasmall ternary alloy nanoparticles under oxygen at different temperatures as a model system. This oxygen-induced spreading and retraction phenomenon exhibits "ebb-and-flow" like dynamic amorphous-crystalline phase duality. It correlates with the high-capacity oxygen breathing-like behavior and is shown to play a critical role in maintaining the stability of the supported ultrasmall nanoalloys using carbon monoxide oxidation by oxygen as a probe reaction. The findings hint at a potential exploration of the dynamic and potentially reversible characteristics of phase transformation for the design of catalysts under reactive conditions.