Density-Functional Theory Simulation of the Dissociative Chemisorption of Water Molecules on the MgO(001) Surface

Abstract

The reaction of water molecules with the MgO(001) surface was studied with density-functional theory using periodic boundary conditions for a better understanding about the formation of H+ and HO– ions on the MgO(001) terrace through the structural, reaction barrier, and thermodynamic studies. This process is relevant as the initial hydroxylation step of this surface, and it is part of a catalyzed hydrolysis mechanism.The geometries and the dissociation energies of one, two, and three water molecules adsorbed on a clean MgO(001) were obtained and the type of adsorption assessed. Transition states for the dissociation processes were computed. The results show that the adsorption of a single water molecule does not lead to dissociation. For the dimer and trimer of water molecules, one molecule dissociates while the others coadsorbed stabilize the H+ and HO– ionic species on the surface. In the two cases, the dissociation products on the surface converged through the formation of hydrogen bonds among the formed hydroxyl and water molecules. As a consequence of these interactions, the protonated surface oxygen anions coexist with the adsorbed hydroxyl ions. The variation of the Gibbs free energy for the adsorption and dissociation processes was calculated in the 100–600 K temperature range including electronic, vibrational, rotational, and translational contributions. The entire process (adsorption + dissociation) is spontaneous up to about 401.2 and 471.1 K for adsorption of two and three water molecules, respectively. The computed energetic barriers are 23.2 and 24.9 kJ/mol for the dissociation of one H2O in the clusters of two and three water molecules, respectively.