Formation of Superoxide Anions on Ceria Nanoparticles by Interaction of Molecular Oxygen with Ce³⁺ Sites

The exposure of reduced ceria, CeO 2− x, to molecular oxygen results in the formation of peroxo, O 2 2−, and superoxo, O 2 −, species. The formation of diamagnetic peroxo species is easily explained by the interaction of O 2 with oxygen vacancies at the ceria surface because these are two-electron-donor centers. On the contrary, the formation of the paramagnetic superoxide O 2 − radical cannot be explained by the same model because this complex spontaneously converts into the more stable peroxo system. On the basis of DFT calculations, we show that superoxide O 2 − radicals form by direct interaction of O 2 with low-coordinated Ce 3+ ions on reduced ceria nanoparticles and we propose an atomistic model of this system, which is fully consistent with the experimental observation.