CO2 adsorption on the pristine and reduced CeO2 (111) surface: Geometries and vibrational spectra by first principles simulations

First principles simulations of carbon dioxide adsorbed on the ceria (CeO2) (111) surface are discussed in terms of structural features, stability, charge transfer, and vibrational modes. For this purpose, different density functional theory methods, such as Perdew-Burke-Ernzerhof (PBE) PBE and Hubbard correction, hybrid functionals, and different basis sets have been applied and compared. Both the stoichiometric and the reduced (111) surfaces are considered, where the electronic structure of the latter is obtained by introducing oxygen vacancies on the topmost or the subsurface oxygen layer. Both the potential energy surfaces of the reduced ceria surface and the adsorbate-surface complex are characterized by numerous local minima, of which the relative stability depends strongly on the electronic structure method of choice. Bent CO2 configurations in close vicinity to the surface oxygen vacancy that partially re-oxidize the reduced ceria surface have been identified as the most probable stable minima. However, the oxygen vacancy concentration on the surface turns out to have a direct impact on the relative stability of possible adsorption configurations. Finally, the vibrational analyses of selected adsorbed species on both the stoichiometric and reduced surfaces show promising agreement with previous theoretical and experimental results.