Fluorine impurities at CeO₂(111): Effects on oxygen vacancy formation, molecular adsorption, and surface re-oxidation

We investigate the effects of anion doping with fluorine impurities on the chemistry of the CeO₂ (111) facet, using the results of DFT + U calculations. We consider three prototypical processes: the formation of oxygen vacancies, the adsorption of O₂ and H₂O molecules, and the re-oxidation of the surface with fragments of the two molecules. We find that the first two of these processes are not strongly affected, but that the presence of F lowers the energy gained in the re-oxidation of the surface in comparison to the healing of an oxygen vacancy, by 1.47 eV in the case of O₂ (provided that the F is part of a cluster) and by 0.92 eV in the case of H₂O. Based on these results, we suggest that F could enhance the redox chemistry of ceria by toggling between being in the surface and on the surface, effectively facilitating the release of lattice O by acting as a "place holder" for it. Finally, we find that the desorption of F as either 12F₂ or HF is energetically unfavourable, suggesting that F doped ceria should be stable in the presence of O₂ and H₂O.