Healing of oxygen vacancies on reduced surfaces of gold-doped ceria.

As an oxidation-reduction catalyst, ceria can catalyze molecular oxidation and reduction. There has been a focus on understanding and enhancing the vacancy formation process to improve the oxidative power of ceria. However, it is important to also address healing of the surface vacancy. To investigate healing of oxygen vacancies in ceria, we study the interaction of atomic and molecular oxygen and NO(2) with oxygen vacancies on gold-doped (110) and (100) surfaces using density functional theory, corrected for on-site Coulomb interactions (DFT+U). For atomic and molecular oxygen, adsorption at the reduced surface is favorable and results in an oxygen atom sitting in an oxygen lattice site, healing the oxygen vacancy. On undoped surfaces, O(2) adsorbs as a peroxo (O(2)(2-)) species. However, on the doped (110) surface a superoxo (O(2)(-)) species is present. When NO(2) adsorbs (exothermically) at a divacancy surface, one oxygen of the molecule sits in the vacancy site and the N-O distances are elongated and an NO(2) anion forms, similar to the undoped surface. Vacancy healing of ceria surfaces is favorable, even if vacancy formation is enhanced, justifying the current focus on improving the oxidative power of ceria. We briefly examine a catalytic cycle: the reaction of CO with adsorbed O(2) on the undoped and doped surfaces, and find that the doped (110) surface facilitates CO oxidation.