Hydroxylation of the {100}, {110}, {111}, and {310} surfaces of MgO and CaO by dissociative adsorption of water has been studied using atomistic simulation techniques. We found that unstable surfaces with high surface energies tend to microfacet into steps of stable {100} planes, creating two possible sites for hydroxylation. As found experimentally, the perfect MgO {100} surface is shown not to be amenable to hydroxylation. However, on stepped {100} surfaces, modeled by the {310} and faceted {110} surfaces, adsorption is energetically favorable and preferentially occurs at low-coordinated sites. The calculated hydration energies for CaO and MgO are in good agreement with experimental values where available.
