Do Au Atoms Titrate Ce Ions at the CeO2−x(111) Surface?

Ce sites may play an important role in defining active sites in ceria-based catalysis. Yet, though present at the catalyst surface, their positions have not been experimentally observed. Recently, it has been argued that Ce sites are the preferred location for the binding of Au− species on a highly reduced CeO2(111) surface with subsurface oxygen defects. Hence, Au atoms have been proposed as the ideal marker for the reduced centers. Using density functional theory (DFT) with the HSE06 hybrid functional and the DFT(PBE)+U approach, we find that the binding of Au− at a hollow site centered at a subsurface oxygen atom, where the 4f → 6s electron transfer occurs from a Ce ion in a deeper layer, is energetically more favorable by 0.36 (0.34) eV with HSE06 (PBE+U) than on-top of a Ce ion in the outermost cerium layer. Au atoms can thus not be taken as position markers for Ce surface sites. The site preference is explained in terms of the reduction of both lattice strain and Coulomb repulsion. Our finding is consistent with the interpretation of the most recent experiments where Au− atom pairs with mean separation of two lattice parameters were observed.