Reactivity and structural evolution of urchin-like Co nanostructures under controlled environments

Versatile Homebuilt Gas Feed and Analysis System for Operando TEM of Catalysts at Work

Understanding how catalysts work during chemical reactions is crucial when developing efficient catalytic materials. The dynamic processes involved are extremely sensitive to changes in pressure, gas environment and temperature. Hence, there is a need for spatially resolved operando techniques to investigate catalysts under working conditions and over time. The use of dedicated operando techniques with added detection of catalytic conversion presents a unique opportunity to study the mechanisms underlying the catalytic reactions systematically. Herein, we report on the detailed setup and technical capabilities of a modular, homebuilt gas feed system directly coupled to a quadrupole mass spectrometer, which allows for operando transmission electron microscopy (TEM) studies of heterogeneous catalysts. The setup is compatible with conventional, commercially available gas cell TEM holders, making it widely accessible and reproducible by the community. In addition, the operando functionality of the setup was tested using CO oxidation over Pt nanoparticles.

Reshaping Dynamics of Gold Nanoparticles under H2 and O2 at Atmospheric Pressure

Despite intensive research efforts, the nature of the active sites for O₂ and H₂ adsorption/dissociation by supported gold nanoparticles (NPs) is still an unresolved issue in heterogeneous catalysis. This stems from the absence of a clear picture of the structural evolution of Au NPs at near reaction conditions, i. e., at high pressures and high temperatures. We hereby report real-space observations of the equilibrium shapes of titania-supported Au NPs under O₂ and H₂ at atmospheric pressure using gas transmission electron microscopy. In situ TEM observations show instantaneous changes in the equilibrium shape of Au NPs during cooling under O₂ from 400 °C to room temperature. In comparison, no instant change in equilibrium shape is observed under a H₂ environment. To interpret these experimental observations, the equilibrium shape of Au NPs under O₂, atomic oxygen, and H₂ is predicted using a multiscale structure reconstruction model. Excellent agreement between TEM observations and theoretical modeling of Au NPs under O₂ provides strong evidence for the molecular adsorption of oxygen on the Au NPs below 120 °C on specific Au facets, which are identified in this work. In the case of H₂, theoretical modeling predicts no interaction with gold atoms that explain their high morphological stability under this gas. This work provides atomic structural information for the fundamental understanding of the O₂ and H₂ adsorption properties of Au NPs under real working conditions and shows a way to identify the active sites of heterogeneous nanocatalysts under reaction conditions by monitoring the structure reconstruction.