Do Au Atoms Titrate Ce³⁺ Ions at the CeO_2–x(111) Surface?

Abstract

Ce3+ 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 Ce3+ 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 Ce3+ ion in a deeper layer, is energetically more favorable by 0.36 (0.34) eV with HSE06 (PBE+U) than on-top of a Ce3+ ion in the outermost cerium layer. Au atoms can thus not be taken as position markers for Ce3+ 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.

Diffusion Barriers Block Defect Occupation on ReducedCeO2(111)

“…In agreement with the literature [7,8,34], O bridge sites are energetically preferred on the ideal surface (−1.05 eV), while binding to a V O S is considerably stronger (−2.37 eV) [31]. On stoichiometric ceria, gold forms a cation (Au þ ) by donating an electron to a nearby Ce 4þ ion.…”

Effects of electron transfer in model catalysts composed of Pt nanoparticles on CeO2(1 1 1) surface

“…Note that in all considered structures Ce 3+ cations were located exclusively on the slab surface. Whereas on pristine CeO 2 (111) surfaces subsurface Ce 3+ cations do not appear at low coverage θ(Ce 3+ ) < 0.5 ML [64,65], more complex ceria-based systems may be more prone to develop subsurface Ce 3+ cations [62,66]. Another important remark is that the electron transfer from Pt particles to the support was shown not to facilitate the formation of O vacancies in ceria [23].…”

Adsorption and Adhesion of Au on Reduced CeO2(111) Surfaces at 300 and 100 K

“…The properties of these particles can vary dramatically with size between 1 and 10 nm, − and some metals that are relatively inert in the bulk, especially gold, can become highly active at this scale for a variety of catalytic reactions. − These variations in reactivity with particle size have been correlated with the energy or chemical potential of the metal atoms in the particles . Supported gold nanoparticles have shown high activity as catalysts for combustion and selective oxidation reactions, ,, and ceria is often used as the support material. − Thus, Au nanoparticles on ceria have been extensively studied both by a variety of experimental methods − as well as by density functional theory (DFT). − Here, we report the first experimental measurements of the energies of gold atoms in supported gold nanoparticles as a function of size. We do this for the case of CeO 2 (111) support surfaces with different extents of reduction.…”