Despite intensive research eorts, the nature of the active sites for O 2 and H 2 adsorption/dissociation by supported gold nanoparticles (NPs) is still an unresolved issue 1 in heterogeneous catalysis. This stems from the absence of a clear picture of the evolution of the structural properties of Au NPs in the presence of these gases at near reaction conditions, i.e. at high pressures and high temperatures. We hereby report on the rst real-space observation of the equilibrium shapes of TiO 2 -supported model Au NPs under O 2 and H 2 at atmospheric pressure using window gas cell transmission electron microscopy (GCTEM). In situ GCTEM observations show instantaneous changes in the equilibrium shape of Au NPs under O 2 during cooling from 400°C to room temperature. In comparison, no instant change in equilibrium shape is observed under H 2 environment. To interpret these experimental observations, the equilibrium shape of Au NPs under O 2 , atomic oxygen and H 2 gas environments was predicted using a multiscale structure reconstruction model. Excellent agreement between GCTEM observations and theoretical modelling under O 2 provides strong evidence for the molecular adsorption of O 2 on the Au NPs below 120°C. Molecular adsorption takes place on specic Au facets which are identied in this work. In the case of H 2 , theoretical modelling predicts weak interactions with gold atoms which explain their high morphological stability under this gas. This work provides atomic structural information for the fundamental understanding of the O 2 and H 2 adsorption properties of Au NPs under real working conditions and also shows a new way to identify the active sites of heterogeneous nanocatalysts under reaction conditions by monitoring the structure reconstruction.

We use in situ transmission electron microscopy to monitor in real time the evaporation of gold, copper, and bimetallic copper-gold nanoparticles at high temperature. Besides, we extend the Kelvin equation to two-component systems to predict the evaporation rates of spherical liquid mono- and bimetallic nanoparticles. By linking this macroscopic model to experimental TEM data, we determine the surface energies of pure gold, pure copper, Cu_{50}Au_{50}, and Cu_{25}Au_{75} nanoparticles in the liquid state. Our model suggests that the surface energy varies linearly with the composition in the liquid Cu-Au nanoalloy; i.e., it follows a Vegard's rulelike dependence. To get atomic-scale insights into the thermodynamic properties of Cu-Au alloys on the whole composition range, we perform Monte Carlo simulations employing N-body interatomic potentials. These simulations at a microscopic level confirm the Vegard's rulelike behavior of the surface energy obtained from experiments combined with macroscopic modeling.