On the Relative Stability of Near-Surface Oxygen Vacancies at the CeO2(111) Surface upon Zr Doping

Ceria (CeO2)-based materials are of great importance in numerous technological applications such as three-way catalysts (TWCs) [Catal. Today 62, 35-50 (2000) and Chem. Rev. 116, 5987-6041 (2016)], hydrocarbon reforming [Chem. Rev. 116, 5987-6041 (2016) and Catalysis by Ceria and Related Materials; Trovarelli, A.; Fornasiero, P., Eds.; 2nd Edition; Imperial College Press: London, 2013] and solid oxide fuel cells (SOFC) [Chem. Rev. 116, 5987-6041 (2016) and Catalysis by Ceria and Related Materials; Trovarelli, A.; Fornasiero, P., Eds.; 2nd Edition; Imperial College Press: London, 2013]. These materials possess a property that is key to most of such applications, namely, their capability for easy conversion between the Ce4+ and Ce3+ oxidation states, which is achieved by releasing oxygen atoms from the crystal lattice and forming oxygen vacancies. In particular, the replacement of Ce by Zr to form CeO2-ZrO2 solid solutions was found to facilitate the reducibility of the oxide as well as to increase the oxygen storage capacity and the system thermal stability, compared to pure CeO2. This theoretical work employing DFT+U calculations, is a systematic study of the effects of Zr doping on the stoichiometric and reduced CeO2(111) surfaces to determine the preferred location of the Zr dopants at various concentrations, as well as to pinpoint how Zr doping affects the stability of near-surface oxygen vacancies -including the position of the Ce3+ ions. We found that for a given Zr content, the more stable structures do not correspond to those configurations with Zr located in the topmost O-Ce-O trilayer (TL1), but in inner layers, and the stability decreases with increasing Zr concentration. Regarding the formation of oxygen vacancies, it was found that the most stable configuration corresponds to the Zr atom located in the surface layer (TL1) neighboring a subsurface oxygen vacancy with next-nearest neighbor Ce3+, being the formation energy equal to 1.16 eV. The corresponding surface oxygen vacancy is 0.16 eV less stable. These values are by 0.73 and 0.92 eV, respectively, smaller than the corresponding ones for the pure CeO2(111) surface. The results are explained in terms of Zr- and vacancy-induced lattice relaxation effects. This study provides microscopic insight into the interplay between Zr-doping, vacancy formation, lattice relaxations, and the localization of the excess charge that will be key to understanding surface chemistry and catalysis on Zr-doped ceria surface, as well as conductive ceria-based materials for advanced applications.