Atomistic Simulation of Surface Selectivity on Carbonate Formation at Calcium and Magnesium Oxide Surfaces

We report atom-level simulations of the surface selectivity and the resulting surface phase diagrams for the {100}, {110}, {111}, and {310} surfaces of CaO and MgO as a function of varying CO2 and H2O partial pressures. This work extends the traditional approach based on ab initio calculations, which can be time-consuming and costly for large systems, by using semiempirical atomistic simulations. The advantage of this approach is that very large numbers of calculations can be performed, thereby allowing a more effective search of the configurational space. The resulting free energies are used to generate the surface phase diagrams. The results indicate that the {100} surfaces of MgO and CaO show different dominant phases at atmospheric concentrations of gaseous water and carbon dioxide. The CaO surface forms a carbonated phase, whereas the MgO surface contains associatively adsorbed water. On the other hand, both {111} surfaces show the dominance of surface hydroxylation, effectively forming a layer of mineral hydroxide, with carbonation only observed at very high carbon dioxide concentrations. Finally, the {310} surfaces show enhanced reactivity with carbonate, most likely a result of the steps on the surfaces. In general, we predict that the minimum CO2 partial pressure needed to carbonate these surfaces can be controlled by the water partial pressure. The effect of temperature is also considered, and the results show how the number of surface adsorbates decreases as temperature increases.