Quantum Origin of the Oxygen Storage Capability of Ceria

The microscopic mechanism behind the extraordinary ability of ceria to store, release, and transport oxygen is explained on the basis of first-principles quantum mechanical simulations. The oxygen-vacancy formation energy in ceria is calculated for different local environments. The reversible $\mathrm{C}\mathrm{e}{\mathrm{O}}_{\mathrm{2}}$-$\mathrm{C}{\mathrm{e}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ reduction transition associated with oxygen-vacancy formation and migration is shown to be directly coupled with the quantum process of electron localization.

Figure 1

Lattice unit cells for $\mathrm{C}{\mathrm{e}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ ($C$-type) (a) and $\mathrm{C}\mathrm{e}{\mathrm{O}}_{\mathrm{2}}$ (b). A cubic unit cell of $C$-type $\mathrm{C}{\mathrm{e}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ can be constructed out of eight $\mathrm{C}\mathrm{e}{\mathrm{O}}_{\mathrm{2}}$ unit cells by increasing their volume by $3\%$ and removing $25\%$ of the oxygen atoms along four nonintersecting $⟨111⟩$ diagonals. Blue, red, and white spheres indicate the cerium, oxygen atoms, and vacancies, respectively.

Figure 2

The energetics of oxygen-vacancy formation. (a) Vacancy formation in the perfect $\mathrm{C}\mathrm{e}{\mathrm{O}}_{\mathrm{2}}$ crystal requires 4.55 eV. The $4f$ electrons of all Ce atoms are treated as valence electrons. (b) Vacancy formation next to a pair of $\mathrm{C}\mathrm{e}(3+)$ atoms (shown in blue) embedded into the $\mathrm{C}\mathrm{e}{\mathrm{O}}_{\mathrm{2}}$ crystal requires only 0.26 eV. Note that for the case of $\mathrm{C}\mathrm{e}(3+)$ the $4f$ electrons are treated as firmly localized, i.e., core electrons. (c) shows that the most favorable position of $\mathrm{C}\mathrm{e}(3+)$ atoms in the $\mathrm{C}\mathrm{e}{\mathrm{O}}_{\mathrm{2}}$ matrix is next to an oxygen vacancy.

Figure 3

The process of oxygen-vacancy formation in ceria. An oxygen atom moves away from its lattice position leaving behind two electrons, which localize on two cerium atoms, turning $\mathrm{C}\mathrm{e}(4+)$ into $\mathrm{C}\mathrm{e}(3+)$.

Figure 4

Results of the Monte Carlo simulation of the oxygen/vacancy ordering in $\mathrm{C}\mathrm{e}{\mathrm{O}}_{\mathrm{2}}$ with $25\%$ of the oxygen atoms removed. The energy vs temperature curve exhibits a clear phase transition into the $C$-type $\mathrm{C}{\mathrm{e}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ structure. Pair ECI (V) used in MC simulation are presented in the inset as a function of the neighboring coordination shell (N).