DFT+U Study on the Localized Electronic States and Their Potential Role During H 2 O Dissociation and CO Oxidation Processes on CeO 2 (111) Surface Yang-Gang Wang, †,‡ Donghai Mei, ‡ Jun Li,* ,†,§ and Roger Rousseau* ,‡ † Department of Chemistry, Tsinghua University, Beijing 100084, China ‡ Institute for Integrated Catalysis and § Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States * S Supporting Information ABSTRACT: 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 CeO 2 (111) surface. These electrons exist in polaronic states, residing at Ce 3+ 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 Ce 3+ . We found that the water molecule can be easily dissociated into two surface bound hydroxyls at the Ce 4+ 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 Ce 3+ 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 CeO 2 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. 1. INTRODUCTION As an important semiconductor material, ceria (CeO 2 ) has attracted extensive interest in recent years due to its excellent redox properties in various applications such as automobile exhaust treatments, low temperature water gas shift reaction, and solid oxide fuel cells. 1 CO oxidation, an important process in three-way catalysis, has been chosen as a prototypic model by many experimental and theoretical studies to probe the redox properties of ceria or ceria-based catalysts. 2-4 However, the mechanism of CO oxidation on the ceria surface is still a matter of debate, though it is generally accepted that CO oxidation proceeds via a Mars-van Krevelen mechanism (redox mechanism), 1,5,6 where CO reacts with a lattice oxygen ion and the gas O 2 molecule replenishes the oxygen vacancy. None- theless, serious uncertainties exist about the atomistic details of even this simple probe reaction. As an example, recent density functional theory (DFT) studies have demonstrated that a CO molecule can easily combine with the lattice oxygen ion to form CO 2 with a small activation energy (∼0.6 eV) 7 on CeO 2 (111) surface, whereas experimental results suggested that the ceria surface is not reactive toward CO oxidation until the temperature is above 600 K. 8-10 In addition, water molecules, which are ubiquitous on oxide surfaces, have also been extensively studied. 11-17 However, the performance of water and its role in CO oxidation on the ceria surface is still unclear. Fronzi et al. 13 proposed that H 2 O does not dissociate on a reduced CeO 2 (111) surface because of a high activation energy, while several studies 14,18,19 reported that the H 2 O molecule readily dissociated into two hydroxyl groups at an oxygen vacancy site, O v . Li and co-workers 20 demonstrated that surface hydroxyls prohibited CO oxidation at room temperature, while Romero-Sarria et al. 21 found that water promoted oxidation of the surface and improved the CO oxidation activity on Au/CeO 2 . Notably, temperature pro- grammed reduction (TPR) studies by Wu et al. 8 demonstrated that the reaction between CO and hydroxyl groups on the ceria surface generated over 50% of the total CO 2 produced in the temperature range of ∼600-850 K. However, temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) studies 22 indicate that water does not oxidize Ce 3+ sites under UHV conditions. This broad cross section of often conflicting results strongly advocate for a clearer description of water adsorption, dissociation, and its interplay with the surface redox chemistry of CeO 2 , as typified by CO oxidation. Received: October 7, 2013 Revised: October 11, 2013 Published: October 14, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 23082 dx.doi.org/10.1021/jp409953u | J. Phys. Chem. C 2013, 117, 23082-23089