Tight Binding Simulation of the MgO and Mg(OH)2 Hydration and Carbonation Processes.

Abstract

Magnesium, the lightest engineering metal, has MgO and Mg(OH)₂ as its common corrosion products, which can also be used for CO₂ storage due to their chemical reactivity. In this study, we developed a DFTB model with monopole, dipole, and quadrupole electrostatics for magnesium compounds containing oxygen, hydrogen, and carbon and applied it in both static and molecular dynamics (DFTB-MD) calculations of the MgO and Mg(OH)₂ hydration and carbonation processes. With our new model, the Electron Localization Function (ELF) and Charge Density Difference (CDD) were computed as part of the electronic structure analysis, providing insights into the electronic mechanism of MgO and Mg(OH)₂ hydration and carbonation processes. The geometry for the brucite-water bulk system was analyzed, including the reconstruction of near-surface water molecules which may influence the dissolution, hydration, and carbonation processes. By comparing experimental, DFT, classical MD results and the results from other parameter set, the accuracy of the model was assessed. A strong covalent bond between CO₂ and the (001) surface of MgO leads to the formation of a CO₃ group, while no such CO₃ group forms on the (101̅1) surface of Mg(OH)₂. Defect sites, however, are more favorable for the formation of the CO₃ group. In contrast, covalent bonds are not found for either surface when water interacted with them. This work provides new insights into the behavior of magnesium compounds interacting with water and carbon dioxide using our model, and it introduces a tool for effectively analyzing chemical electronic structures and bonding mechanisms.