Direct Visualization of Surface Structure and Charge States of Ceria-Supported Gold Catalysts Under Redox Conditions

Observing the surface structure and charge dynamics of catalysts during catalytic reactions is crucial for elucidating reaction mechanisms. However, nanoscale characterization of the catalyst structure and charge states in the presence of reactive gases presents experimental challenges. Here, the structures and charge states of a gold nanoparticle (NP) are directly visualized on ceria during redox cycles using electron holography, a method related to transmission electron microscopy. The introduction of oxidizing O₂ gas to the microscope led to structural changes on the NP surface and decrease the intrinsic negative charge of the NP. Conversely, under reducing H₂ gas, the surface structure and charge state of the NP remained almost unchanged compared to those in vacuum. Systematic analysis revealed that the injection and removal of O₂ gas caused reversible changes in the charge state of the NP within the range of a few electrons. The effect of O₂ gas on charging of the NP is confirmed by first-principles calculations. These findings demonstrate the potential of electron holography in gas environments for advancing the understanding the reaction mechanisms on heterogeneous catalysts.