Characterization of O-2-CeO2 interactions using in situ Raman spectroscopy and first-principle calculations

Abstract

Interactions between O-2 and CeO2 are examined experimentally using in situ Raman spectroscopy and theoretically using density-functional slob-model calculations. Two distinct oxygen bonds appear at 825 and 1131 cm(-1), corresponding to peroxo- and superoxo-like species, respectively, when partially reduced CeO2 is exposed to 10% O-2. 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 ore endothermic (0.91 < Delta E-ads < 0.98 eV), while those on reduced surfaces are exothermic (-4. 0 < Delta E-ads < -0.9 eV), depending on other relevant surface processes such as chemisorption and diffusion into the bulk. Partial reduction of surface Ce4+ to Ce3+ (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.