Single-atom catalysts, where atomically dispersed transition metals are anchored at solid surfaces, are a new type of low price, high efficiency and nonflammable catalysts for water gas shift and CO₂ methanation, where selective oxidation or reduction is needed. Ceria is one of the most promising support materials for these single-atom catalysts, because of its large oxygen capacity and high ionic mobility. The large amount of available lattice oxygen or oxygen vacancies in ceria is beneficial for selective oxidation and reduction, though the performance of pure ceria is limited by its poor surface exchange properties. Mixing or doping transition metals to ceria can activate the surface of ceria. In particular, transition metals anchored as isolated single atoms are found to be highly active. However, it is challenging to enhance the density of these single atoms due to the strong tendency of metal agglomeration. Having understood how dislocations in oxides can serve as traps for charged point defects in our previous studies, we have aimed here to assess whether we can stabilize single metal atoms at dislocations in ceria.

To this end, first, a wide range of charge states and configurations of Cu defects in a dislocation free ceria model are calculated to elucidate the defect chemistry for Cu ions in bulk ceria. The formation energies of these defects as a function of chemical potential of oxygen and the Fermi energy indicate that the most preferred Cu defect is Cu¹⁺ interstitial. This finding differs from previous calculations in the literature where only a copper vacancy complex is calculated, but is consistent with reported experimental observations. Moreover, the equilibrium Cu defect concentration in ceria is negligible, indicating a low solubility of Cu in bulk ceria.

Second, an edge dislocation model in ceria was constructed. We found that the Cu defect formation energy at the edge dislocation core is 2 eV lower than that in the bulk, indicating a tendency for Cu to enrich at the dislocation compared to the bulk ceria (which has a very low solubility of Cu). More interestingly, and importantly, Cu has a lower oxidation state in the dislocation core than in the bulk. This lower oxidation state is more active for catalytic activity. Both the enrichment of Cu at the dislocation core and the more active oxidation state indicate that dislocations in ceria are beneficial for increasing surface electro-catalytic reaction kinetics with single atom catalysts.