MSINDO study of acid promoted dissolution of planar MgO and NiO surfaces

Despite the structural similarity, MgO is fast dissolving in low pH solution, whereas NiO is slow dissolving under the same conditions. In addition, the planar MgO (100) surface immediately reconstructs to form pits and protrusions, whereas this behavior has not been observed with NiO (100). Our previous study, using the semiempirical self-consistent field molecular orbital (SCFMO) method MSINDO showed that it was possible for MgO dissolution but not NiO dissolution to occur via the migration of a metal-oxygen pair to an ex situ position. However, we have now found a more energetically favorable and realistic dissolution mechanism involving the dissociation of a water molecule (adsorbed on a metal site) prior to migration. Products from this dissociation (H and OH) weaken adjacent metal-oxygen bonds. Dissociation of a second adsorbed water molecule is required to complete the process. For both oxides, the energy barrier determined from the energy profile of the metal-oxygen pair migration was found to be lower than the activation energy of water dissociation at the planar surface as reported in previous study. This would suggest that the dissociation of water molecules at the planar surface is rate-determining in the surface restructuring step of dissolution. It was demonstrated that surface restructuring and dissolution of MgO is possible whereas highly improbable for NiO, in agreement with experimental observations.