Water adsorption on the stoichiometric and reduced CeO2(111) surface: a first-principles investigation

We present a density functional theory investigation of the interaction between water and cerium oxide surfaces, considering both the stoichiometric and the reduced surfaces. We study the atomic structure and energetics of various configurations of water adsorption (for a water coverage of 0.25 ML) and account for the effect of temperature and pressure of the environment, containing both oxygen and water vapor, employing the ab initio atomistic thermodynamics approach. Through our investigation, we obtain the phase diagram of the water-ceria system, which enables us to discuss the stability of various surface structures as a function of the ambient conditions. For the stoichiometric surface, we find that the most stable configuration for water is when it is bonded at the cerium site, involving two O-H bonds of hydrogen and oxygen atoms at the surface. If oxygen vacancies are introduced at the surface, which is predicted under more reducing conditions, the binding energy of water is stronger, indicating an effective attractive interaction between water molecules and oxygen vacancies. Water dissociation, and the associated activation energies, are studied, and the role of oxygen vacancies is found to be crucial to stabilize the dissociated fragments. We present a detailed analysis of the stability of the water-ceria system as a function of the ambient conditions, and focus on two important surface processes: water adsorption/desorption on the stoichiometric surface and oxygen vacancy formation in the presence of water vapor. A study of the vibrational contribution to the free energy allows us to estimate the effect of this term on the stability range of adsorbed water.