We provide an explanation for the experimental finding of a dramatically enhanced low-temperature oxygen storage capacity for small ceria nanoparticles. At low temperature, small octahedral ceria nanoparticles will be understoichiometric at both oxidizing and reducing conditions without showing explicit oxygen vacancies. Instead, rather than becoming stoichiometric at oxidizing conditions, such particles are stabilized through oxygen adsorption forming superoxo (O₂^–) ions and become in this way supercharged with oxygen. The supercharging effect is size-dependent and largest for small nanoparticles where it gives a direct increase in the oxygen storage capacity and simultaneously provides a source of active oxygen species at low temperatures.

DOI: 10.1021/jz3020524

 

Cu dimer formation mechanism on the ZnO(101̅ 0) surface

Matti Hellström, Daniel Spångberg, Kersti Hermansson, and Peter Broqvist

The formation of Cu dimers on the ZnO(101̅ 0) surface has been studied using hybrid density functional theory. Depending on the adsorption site, Cu atoms are found to adsorb with either oxidation state 0 or +1. In the latter case, the Cu atom has donated an electron to the ZnO conduction band. The two modes of adsorption display similar stability at low coverages, while at higher coverages the neutral species is more stable. Single Cu atoms diffuse across the ZnO(101̅ 0) surface with small barriers of migration (0.3–0.4 eV) along ZnO[12̅ 10], repeatedly switching their oxidation states, while the barrier along ZnO[0001] is significantly higher (>1.5 eV). The formation of a Cu dimer from two adsorbed Cu atoms is energetically favorable with two competing structures of similar stability, both being charge neutral. The minimum energy paths for Cu atom diffusion and dimer formation are characterized by at least one of the two Cu atoms being in oxidation state 0.

Phys. Rev. B, 2012, 86,  235302
DOI: 10.1103/PhysRevB.86.235302