Anchoring Sites for Initial Au Nucleation on CeO2{111}: O Vacancy versus Ce Vacancy

Gold atoms act as anchoring sites for gold nanoparticles in the ceria-catalyzed water-gas shift reaction; however, the anchoring site of the nanoparticle is a matter of debate. Both oxygen and cerium vacancies have been suggested as the anchoring sites in different studies. Aiming to provide insight into this issue, we utilize density functional theory and ab initio thermodynamics approaches to investigate the formation of various vacancies at a CeO2{111} surface, both with and without gold adatoms. We find that, under reaction conditions, the cerium vacancy is much harder to form and is much less stable than the oxygen vacancy, regardless of the absence or presence of gold on the surface. Gold adsorption at the oxygen vacancy site is strongly preferred to that on the cerium vacancy; the latter becomes favorable only near the extreme oxygen-rich limit, which would not be achievable under the reaction conditions. Other possible vacancies have also been examined. We find that in addition to a single oxygen vacancy, oxygen vacancy clusters could also be anchoring sites for gold nucleation. This study lays a foundation toward understanding the boundary structure between Au nanoparticlues and ceria support, which in turn would hold promise to unravel their catalytic roles. I