Cerium oxide surfaces properties: a first principle investigation

We present density functional theory investigations of the bulk properties of cerium oxides ( CeO2 \nand Ce2O3 ) and the three low index surfaces of CeO2 , namely (100), (110) and (111). For the \nsurfaces, we consider various terminations including surface defects. Using the approach of “ab \ninitio atomistic thermodynamics”, we find that the most stable surface structure considered is the \nstoichiometric (111) surface under “oxygen­rich” conditions, while for a more reducing \nenvironment, the same (111) surface, but with oxygen vacancies, is found to be the most stable one, \nand for a highly reducing environment, the (111) Ce­terminated surface becomes energetically \nfavoured. Interestingly, this latter surface, exhibits a significant reconstruction in that it becomes \noxygen­terminated and the upper layers resemble the Ce2O3 (0001) surface. This structure could \nrepresent a precursor to the phase transition of CeO2 to Ce2O3 . For the stoichiometric CeO2 (111) \nstoichiometric and the CeO2 (111) with oxygen vacancies we study the adsorption of the CHx \nfragments, analysing energetics and electronic structures of various configuration. We found the \nCH4 to have a very week interaction with the stoichiometric surface even for the more stable \nconfiguration having three H­O bonds with the surface oxygen atom. If an oxygen vacancy is \nintroduced on the surface, the binding energy results to be positive, indicating an effective repulsive \ninteraction between the methane molecules and oxygen vacancies. The other CHx fragments show a \nstronger interaction with the stoichiometric surface, compared with the CH4 . When interacting with \nthe reduced surface, the adsorption energy of the CHx fragments become smaller compared to the \nstoichiometric surface, indicating a repulsive interaction with surface oxygen vacancies. An analysis \nof the electronic structure and the charge distribution give an indication on the nature of the bonds. \nAn interesting behaviour of the CeO2 (111) surface is observed when exposed to an humid \nenvironment, so we in the last part of this work we analyse the interaction between water and cerium \noxide surfaces, considering both the stoichiometric and the reduced surfaces. We study the atomic \nstructure and energetics of various configurations of water adsorption (at a water coverage of 0.25 \nML) and account for the effect of temperature and pressure of the environment, containing both \noxygen and water vapour, employing the ab­initio atomistic thermodynamics approach. Through our \ninvestigation we obtain the phase diagram of the water­ceria system, which enables us to discuss the \nstability of various surface structures as a function of the ambient conditions. For the stoichiometric \nsurface, we find that the most stable configuration for water is when it is bonded at the cerium site, \ninvolving two O­H bonds of hydrogen and oxygen atoms at the surface. If oxygen vacancies are \nintroduced at the surface, which is predicted under more reducing conditions, the binding energy of \nwater is stronger, indicating an effective attractive interaction between water molecules and oxygen \nvacancies. Water dissociation, and the associated activation energies, are studied, and the role of \noxygen vacancies is found to be crucial to stabilise the dissociated fragments. We present a detailed \nanalysis of the stability of the water­ceria system as a function of the ambient conditions, and focus \non two important surface processes: water adsorption/desorption on the stoichiometric surface and \noxygen vacancy formation in presence of water vapour. A study of the vibrational contribution to \nthe free energy allows us to estimate the effect of this term on the stability range of adsorbed water.