Dissociative adsorption of water at vacancy defects in graphite

We have performed density-functional calculations to investigate the adsorption of H2O on perfect and defected graphite (0001) represented by a single graphene sheet. On the perfect surface, the water physisorbs, as expected, with no significant preference for the adsorption site. At a vacancy site, the interaction is much more significant, with a computed binding energy of similar to 210 meV in a weak chemisorption/strong physisorption state. The H2O sits with one H pointing down to a carbon atom, which is pulled out of the plane by similar to 0.55 angstrom. From this physisorption state, dissociative chemisorption will occur after overcoming a barrier of 0.8-0.9 eV (similar to 0.6-0.7 eV relative to the gas-phase). The lowest dissociation barrier obtained is similar to 0.47 eV along a path largely avoiding the physisorption well. The dissociation paths have an intermediate step, in which the molecule partially dissociates to H and OH. Subsequently, the chemisorbed OH stretches, breaking into O and H atoms chemisorbed on separate C atoms on the vacancy with a total exothermicity of similar to 3.21 eV.