Cerium oxide surfaces properties: a first principle investigation

We present density functional theory investigations of the bulk properties of cerium oxides ( CeO2 and Ce2O3 ) and the three low index surfaces of CeO2 , namely (100), (110) and (111). For the surfaces, we consider various terminations including surface defects. Using the approach of “ab initio atomistic thermodynamics”, we find that the most stable surface structure considered is the stoichiometric (111) surface under “oxygen­rich” conditions, while for a more reducing environment, the same (111) surface, but with oxygen vacancies, is found to be the most stable one, and for a highly reducing environment, the (111) Ce­terminated surface becomes energetically favoured. Interestingly, this latter surface, exhibits a significant reconstruction in that it becomes oxygen­terminated and the upper layers resemble the Ce2O3 (0001) surface. This structure could represent a precursor to the phase transition of CeO2 to Ce2O3 . For the stoichiometric CeO2 (111) stoichiometric and the CeO2 (111) with oxygen vacancies we study the adsorption of the CHx fragments, analysing energetics and electronic structures of various configuration. We found the CH4 to have a very week interaction with the stoichiometric surface even for the more stable configuration having three H­O bonds with the surface oxygen atom. If an oxygen vacancy is introduced on the surface, the binding energy results to be positive, indicating an effective repulsive interaction between the methane molecules and oxygen vacancies. The other CHx fragments show a stronger interaction with the stoichiometric surface, compared with the CH4 . When interacting with the reduced surface, the adsorption energy of the CHx fragments become smaller compared to the stoichiometric surface, indicating a repulsive interaction with surface oxygen vacancies. An analysis of the electronic structure and the charge distribution give an indication on the nature of the bonds. An interesting behaviour of the CeO2 (111) surface is observed when exposed to an humid environment, so we in the last part of this work we analyse 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 (at a water coverage of 0.25 ML) and account for the effect of temperature and pressure of the environment, containing both oxygen and water vapour, employing the ab­initio atomistic thermodynamics approach. Through our investigation we obtain the phase diagram of the water­ceria 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 O­H 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 stabilise the dissociated fragments. We present a detailed analysis of the stability of the water­ceria 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 presence of water vapour. 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.