Characterization of O₂-CeO₂ interactions using in situ Raman spectroscopy and first-principle calculations

Interactions between O₂ and CeO₂ are examined experimentally using in situ Raman spectroscopy and theoretically using density-functional slab-model calculations. Two distinct oxygen bands appear at 825 and 1131 cm^-1, corresponding to peroxo- and superoxo-like species, respectively, when partially reduced CeO₂ is exposed to 10% O₂. Periodic density-functional theory (DFT) calculations aid the interpretation of spectroscopic observations and provide energetic and geometric information for the dioxygen species adsorbed on CeO₂. The O ₂ adsorption energies on unreduced CeO₂ surfaces are endothermic (0.91< ΔE_ads < 0.98 eV), while those on reduced surfaces are exothermic (-4. 0< ΔE_ads < -0.9 eV), depending on other relevant surface processes such as chemisorption and diffusion into the bulk. Partial reduction of surface Ce⁴⁺ to Ce ³⁺ (together with formation of oxygen vacancies) alters geometrical parameters and, accordingly, leads to a shift in the vibrational frequencies of adsorbed oxygen species compared to those on unreduced CeO₂. Moreover, the location of oxygen vacancies affects the formation and subsequent dissociation of oxygen species on the surfaces. DFT predictions of the energetics support the experimental observation that the reduced surfaces are energetically more favorable than the unreduced surfaces for oxygen adsorption and reduction.