Water adsorption and its effect on the stability of low index stoichiometric and reduced surfaces of ceria

The influence of water on the redox properties of ceria is pivotal to its widespread exploitation spanning a variety of applications. Ab initio simulation techniques based on DFT-GGA+U are used to investigate the water-ceria system including associative (H ₂O) and dissociative (- OH) adsorption/desorption of water and the formation of oxygen vacancies in the presence of water vapor on the stoichiometric and reduced low index surfaces of ceria at different water coverages. Our calculations address the controversy concerning the adsorption of water on the CeO ₂{111}, and new results are reported for the CeO ₂{110} and {100} surfaces. The simulations reveal strong water coverage dependence for dissociatively (- OH) adsorbed water on stoichiometric surfaces which becomes progressively destabilized at high coverage, while associative (H ₂O) adsorption depends weakly on the coverage due to weaker interactions between the adsorbed molecules. Analysis of the adsorption geometries suggests that the surface cerium atom coordination controls the strong adhesion of water as the average distance Ce-O _W is always 10% greater than the Ce-O distance in the bulk, while the hydrogen bonding network dictates the orientation of the molecules. The adsorption energy is predicted to increase on reduced surfaces because oxygen vacancies act as active sites for water dissociation. Crucially, by calculating the heat of reduction of dry and wet surfaces, we also show that water promotes further reduction of ceria surfaces and is therefore central to its redox chemistry. Finally, we show how these simulation approaches can be used to evaluate water desorption as a function of temperature and pressure which accords well with experimental data for CeO ₂{111}. We predict desorption temperatures (T _D) for CeO ₂{110} and CeO ₂{100} surfaces, where experimental data are not yet available. Such an understanding will help experiment interpret the complex surface/interface redox processes of ceria, which will, inevitably, include water.