Role of surface peroxo and superoxo species in the low-temperature oxygen buffering of ceria: Density functional theory calculations

Reversible oxygen release makes ceria $\left(\mathrm{Ce}{\mathrm{O}}_2\right)$ among the most efficient oxide supports for low-temperature oxidation reactions. A clear identification of the species responsible for this oxygen buffering is still missing since only indirect information is available. We present a systematic study of O adsorbates on the most stable ceria surfaces based on density functional theory calculations. The results rationalize the experimental findings supporting the interpretation that superoxide species are the key factors in promoting low-temperature oxidation reactions on ceria. The high reactivity of nanocrystalline ceria is explained in terms of the surface selectivity towards superoxide adsorption.

Figure 1

(Color online) Bonding charge (top panels) and spin (bottom panels) densities for (a) an O adatom bound to the nondefective (111) surface, (b) molecular oxygen adsorbed on a one-electron ${\mathrm{Ce}}^{3+}$ defect of the (111) surface, and a superoxide molecule bound to a ${\mathrm{Ce}}^{4+}$ ion of the (c) (111) and (d) (110) surfaces. Relevant bonding distances (in Å, top panels), and spin polarizations (in ${\mu }_B$, bottom panels) are also shown. Red (dark gray) and light gray spheres represent surface O and Ce atoms, respectively, while adsorbates are displayed in yellow (light gray). Positive and negative density values are displayed in red (lighter gray) and blue (darker gray), respectively.