DFT+U Study on the Localized Electronic States and Their Potential Role During H2O Dissociation and CO Oxidation Processes on CeO2(111) Surface

We present the results of an extensive density functional theory based electronic structure study of the role of 4f-state localized electron states in the surface chemistry of a partially reduced CeO2(111) surface. These electrons exist in polaronic states, residing at Ce3+ sites, which can be created by either the formation of oxygen vacancies, O-v, or other surface defects. Via ab initio molecular dynamics, these localized electrons are found to be able to move freely within the upper surface layer, but penetration into the bulk is inhibited as a result of the higher elastic strain induced by creating a subsurface Ce3+. We found that the water molecule can be easily dissociated into two surface bound hydroxyls at the Ce4+ site associated with O-v sites. This dissociation process does not significantly affect the electronic structure of the excess electrons at reduced surface, but does lead to a favorable localization on Ce3+ sites in the vicinity of the resulting OH groups. In the presence of water, a proton-mediated Mars-van Krevelen mechanism for CO oxidation via the formation of bicarbonate species is identified. The localized 4f electrons on the surface facilitate the protonation process of adsorbed O-2 species and thus decelerate the further oxidation of CO species. Overall, we find that surface hydroxyls formed via water dissociation at the CeO2 surface lead to inhabitation of the CO oxidation reaction. This is consistent with the experimental observation of requisite elevated temperatures, on the order of 600 K, for this reaction to occur.