Visualizing H2O molecules reacting at TiO2 active sites with transmission electron microscopy

Imaging reactive surface waterRecent developments in transmission electron microscopy (TEM) have enabled imaging of single atoms, but adsorbed gas molecules have proven more challenging because of a lack of sufficient image contrast. Yuan et al. adsorbed water and carbon dioxide (CO2) on a reconstructed nanocrystalline anatase titanium dioxide (TiO2) surface that has protruding TiO3 ridges every four unit cells, which provide regions of distinct contrast. Water adsorption on this surface during environmental TEM experiments led to the formation of twinned protrusions. These structures developed dynamic contrast as the water reacted with coexposed CO2 to form hydrogen and carbon monoxide.Science, this issue p. 428Imaging a reaction taking place at the molecular level could provide direct information for understanding the catalytic reaction mechanism. We used in situ environmental transmission electron microscopy and a nanocrystalline anatase titanium dioxide (001) surface with (1 × 4) reconstruction as a catalyst, which provided highly ordered four-coordinated titanium “active rows” to realize real-time monitoring of water molecules dissociating and reacting on the catalyst surface. The twin-protrusion configuration of adsorbed water was observed. During the water–gas shift reaction, dynamic changes in these structures were visualized on these active rows at the molecular level.