Direct observation of oxygen vacancy formation and migration over ceria surface by in situ environmental transmission electron microscopy

The extremely high structural tolerance of ceria to oxygen vacancies (Ov) has made it a desirable catalytic material for the hydrocarbon oxidation to chemicals and pharmaceuticals and the reduction of gaseous pollutants. It is proposed that the formation and diffusion of Ov originate from its outstanding reduction property. However, the formation and diffusion process of Ov over the surface of ceria at the atomic level is still unknown. Herein, the structural and valence evolution of CeO2 (111) surfaces in reductive, oxidative and vacuum environments from room temperature up to 700 °C was studied with in situ aberration-corrected environmental transmission electron microscopy (ETEM) experiments. Ov is found to form under a high vacuum at elevated temperatures; however, the surface can recover to the initial state through the adsorption of oxygen atoms in an oxygen-contained environment. Furthermore, in hydrogen environment, the step-CeO2 (111) surface is not stable at elevated temperatures; thus, the steps tend to be eliminated with increasing temperature. Combined with first-principles density function calculations (DFT), it is proposed that O-terminated surfaces would develop in a hypoxic environment due to the dynamic diffusion of Ov from the outer surface to the subsurface. Furthermore, in a reductive environment, H2 facilitates the formation and diffusion of Ov while Ce-terminated surfaces develope. These results reveal dynamic atomic-scale interplay between the nanoceria surface and gas, thereby providing fundamental insights into the Ov-dependent reaction of nano-CeO2 during catalytic processes.