Interaction of a water molecule with the oxygen vacancy on the MgO(100) surface

The interaction of a water molecule with a neutral oxygen vacancy on the MgO(100) surface is studied in the limit of very low coverage by total-energy calculations and molecular-dynamics simulations based on the density-functional theory. Exchange and correlation effects are treated within the generalized gradient approximation. We show the existence of a local minimum corresponding to the physisorbed adsorption state, which is similar to the one found for water adsorption on the stoichiometric MgO(100) surface. At variance with the latter one, the dissociation of the water molecule is energetically favored at the vacancy site. However, the global energy minimum corresponds to the recombined configuration, in which the oxygen initially in the water molecule adsorbs in the vacancy and the two hydrogens form a H₂ dimer. Through the evaluation of the energy barriers between the different states we discuss the kinetics of the interaction of the water molecule with the defective surface. We show that the dissociated configuration is likely only an intermediate state towards the recovery of a stoichiometric surface, thus suggesting that dissociation of an isolated water molecule at the neutral oxygen vacancy cannot be held as responsible for the existence of a stable hydroxyl group on the MgO(100) surface.