Surface defects are of decisive importance for the physics and chemistry of oxides. They represent local perturbations of the saturated metal-oxygen network and often comprise dangling-bond states and uncompensated surface charges.

Not surprisingly, density-functional theory (DFT) finds defects to be the preferred adsorption sites for metal atoms and molecules on oxide surfaces], with oxygen vacancies being the most relevant type due to their relatively low formation energy. On nonreducible MgO, for example, Au atoms substantially bind to O defects, while the ideal surface offers weak van der Waals interactions only. The same holds for ceria, as a reducible oxide. Again, gold hardly interacts with the stoichiometric surface, while binding to surface O vacancies is highly favorable and accompanied by an electron transfer from a nearby Ce3+ ion . Defects are also involved in stabilizing and activating molecular species and therefore of relevance for oxide chemistry. While theory suggests a large impact of defects on various oxide properties, the correlation is not so clear on the experimental side.

In this Letter, we address the discrepancy between the anticipated importance of O defects for the adsorption on oxide surfaces and the actual role derived from experiment. For this purpose, we examine the temperature-dependent binding behavior of Au atoms on CeO2−x (111) films, using a combination of STM and DFT. We find only a small number of adatoms attached to surface oxygen defects, despite a clear binding preference predicted by theory. We explain this result with diffusion barriers around the surface oxygen vacancy sites that cannot be overcome by Au atoms at moderate temperature. Our study thus gives insight into the kinetics of defect population on oxide surfaces and elucidates why a purely thermodynamic picture sometimes deviates from the experimental findings.
Diffusion Barriers Block Defect Occupation on Reduced CeO2(111)
P. G. Lustemberg, Y. Pan, B.-J. Shaw, D. Grinter, Chi Pang, G. Thornton, Rubén Pérez, M. V. Ganduglia-Pirovano and N. Nilius.
PRL 116, 236101 (2016)