Atomic-Scale STEM Analysis Shows Structural Changes of Au-Pd Nanoparticles in Various Gaseous Environments

We report dynamical restructuring effects in free-standing Au_0.75Pd_0.25nanoparticles occurring in gaseous environments at elevated temperatures. The freshly prepared sample was found to have a core-shell structure with a Pd-rich phase on the surface. The evolution of sample composition and morphology under exposure to 1 bar of pure gases, namely O₂, H₂, air, CO, and CO₂, at different temperatures was studied using in situ scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS). We observed sharper facets on the surface of the particles under O₂or air at 400 °C. Small islands of Pd were present on the surface, although some Pd was redistributed inside the bulk when the temperature was increased under O₂. Subtle changes in surface roughness were noted when O₂was substituted with H₂at 400 °C, an observation correlated to density functional theory (DFT) calculations. The particles lost clean surface facets when CO was introduced at room temperature and at 200 °C. No substantial changes could be observed after exposure to CO₂at 250 °C. The adsorption of CO molecules on the surface modifies the surface of the particles and decreases the facet prevalence. These in situ observations show how gases can induce subtle modification of the surface of nanocatalysts, potentially impacting their chemical properties.