Evolution of oxygen vacancies in cerium dioxide at atomic scale under CO2 reduction

The role of oxygen vacancies in metal oxides has attracted significant attention in heterogeneous catalysis. However, directly observing oxygen vacancies at the atomic level and in situ tracking their evolutions under CO2-involved reactions remain challenging tasks. Herein, using in situ environmental transmission electron microscopy (ETEM), we visualize the oxygen vacancies in CeO2 nanocubes dynamically evolving during CO oxidation and CO2 reduction reactions. We found that surface oxygen atoms were removed in CO atmosphere and that adjacent oxygen atoms migrate to fill the oxygen vacancy sites, accompanied by an increase in the spacing between adjacent cerium atomic layers. The produced oxygen vacancies were then filled gradually when exposed to a CO2 environment at high temperatures to realize the whole cycle. The atomic-level observations of oxygen vacancies under reaction condition provides valuable insights into the evolution dynamics and mechanisms of oxygen vacancies during chemical reactions, which can be instructive in our understanding of these phenomena.