Dynamic Imaging of Ostwald Ripening by Environmental Scanning Transmission Electron Microscopy

The first dynamic atomic resolution environmental scanning transmission electron microscope (ESTEM) study of the nanoparticle sintering of a model Cu system is reported. ESTEM confers the advantage of a Z-contrast dependence in industrially representative conditions to provide enhanced visibility of sub-nanometer particles when compared with TEM. The importance of this is demonstrated by the significant enhancement in the Ostwald ripening rate of model Cu nanoparticles in the presence of 3Pa hydrogen, an effect that is independent of the substrates studied. Temperatures of 400-550 degrees C are shown to switch the operating regime of the rate-limiting mechanism. Cu catalysts are used for methanol synthesis and hydrocarbon-conversion processes for fuel cells, and the importance of observing these catalysts in their working states is demonstrated. Unique ESTEM observations of Ostwald ripening are combined with kinetic models to improve the technical understanding of catalyst deactivation mechanisms.