The Effect of Environment on the Reaction of Water on the Ceria(111) Surface: A DFT+U Study

Abstract

The interaction of a water molecule with the (111) surfaces of stoichiometric and reduced ceria is investigated using first principle density functional theory with the inclusion of the on-site Coulomb interaction (DFT+U). It is found that on the stoichiometric ceria(111) surface, the water molecule is adsorbed spontaneously through single hydrogen bond configuration. In contrast, on the lightly reduced ceria(111), there exist both molecular adsorption (no-H-bond configuration) and dissociative adsorption (surface hydroxyl) modes. It is obvious that oxygen vacancies can enhance the interaction of water with the substrate. Phase diagrams for stoichiometric and reduced ceria(111) surfaces in equilibrium with water vapor in the complete range of experimentally accessible gas phase condition are calculated and discussed combining the DFT results and thermodynamics data using the ab initio atomistic thermodynamic method. 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 for water adsorption on the stoichiometric and on the lightly reduced surfaces, respectively.

Role of step sites on water dissociation on stoichiometric ceria surfaces

“…Moreover, these experiments have shown that water can easily dissociate on either TiO 2-x /Au(111) or CeO 2-x /Au(111) but that no water dissociation is seen when there are no O vacancies in the supported oxide nanoparticles. These findings are in agreement with surface science experiments showing strong adhesion of molecularly adsorbed water to stoichiometric CeO 2 (111) [29] and further surface reduction when reduced CeO 2-x (111) was exposed to water with a concomitant presence of hydroxyl groups [30,31] and are also in agreement with theoretical studies based on density functional calculations for the stoichiometric CeO 2 (111) and reduced CeO 2-x (111) surfaces indicating that water does not dissociate on the clean surface, whereas the process becomes thermodynamically favorable on the oxygen vacancies containing surface [32][33][34].…”

“…Note also that different choices of surface unit cell induce different lateral interactions among the adsorbates and to a strong dependence of the binding energy with respect to the coverage. More recently, Yang et al [37,38] studied the interaction of a water molecule with the (111) surfaces of stoichiometric and reduced ceria using DFT ? U.…”

Kinetics and Thermodynamics of H2O Dissociation on Reduced CeO2(111)

“…This configuration is 0.02 eV more stable than the configuration where the electron in the subsurface is localized on just one of the Ce 3+ ions in the subsurface (not shown). The latter configuration was reported to be the most stable configuration for the surface vacancy by Yang et al 19 It is possible the delocalization is an artifact of the rather low value of U employed in this study. U eff from 3.0 to 5.5 eV was reported to be required to localize electrons on Ce ions near bulk vacancies, 35 but apparently U eff values above 3 eV are required for surfaces.…”

“…The adsorption energy of water near a surface vacancy was calculated, using GGA+U (U = 5 eV), by Watkins et al 16 to be −0.79 eV, by Yang et al 19 to be −0.54 eV, and by Molinari et al 18 to be −0.82 eV. Kumar et al found a water adsorption energy of −0.64 eV using PBE with 1/2 monolayer (ML) of water and oxygen vacancies.…”

The different roles of Pu-oxide overlayers in the hydrogenation of Pu-metal: An ab initio molecular dynamics study based on van der Waals density functional (vdW-DF)+U

“…In spite of all the published papers and increasing attentions focusing on Pu oxides, our theoretical understanding on the basic mechanisms for the hydrogenation of Pu-oxide coated Pu is, to put it mildly, very poor. Especially, little is known regarding the interaction behaviors of H 2 on Pu-oxide surfaces, which is in sharp contrast to the depth and comprehensiveness of researches conducted upon the surface reaction mechanisms of ceria [39][40][41][42][43][44]. As far as we are aware, Wu et al [26,27] were the only ones who have studied the interaction between gaseous molecule (H 2 O) and PuO 2 (100)/PuO 2 (110) surfaces, whereas in their literature both the strong correlation effect of Pu-5f electrons and the van der Waals force between polar H 2 O molecule and PuO 2 surfaces were not taken into account.…”