CO adsorption and oxidation on ceria surfaces from DFT+U calculations

The limiting steps of CO oxidation catalyzed by ceria via the Mars-van Krevelen reaction mechanism are identified and investigated by means of density functional theory calculations that account for the on-site Coulomb interaction via a Hubbard term (DFT+U). We address the adsorption of CO on the (111) and (110) surfaces, and its oxidation via participation of lattice oxygen leading to vacancy formation and CO(2) desorption. CO physisorbs on the (111) ceria surface (E(bind) < 0.2 eV), while it chemisorbs on the more open (110) surface (E(bind) > 2 eV) yielding carbonate formation and surface reduction. The DFT+U energy of CO adsorption and oxidation is studied as a function of the parameter U. This analysis demonstrates that the values of U presently used in the literature (U > 4 eV) tend to overestimate the binding energy of CO to ceria surfaces. In contrast, the energy for the complete CO oxidation reaction is effectively independent of U and in good agreement with the available experimental data. The discussion of these results in the context of temperature programmed desorption and microcalorimetry measurements allow us to suggest that lower values of U can be more appropriate for modeling redox reactions on ceria surfaces with the DFT+U method.