First Principles Modeling of Water Dissociation on Mg(0001) and Development of a Mg Surface Pourbaix Diagram

Density functional theory (DFT) was used to study water dissociation on the Mg(0001) surface. The metal/water interface was modeled with a supercell approach, consisting of an extended metal surface coupled to an implicit solvent medium. Several electrochemical reactions were studied on the Mg surface, and it was found that dissociation of adsorbed water is thermodynamically favorable, and that the Mg(0001) surface has multiple 'active sites' that can accommodate adsorbed hydroxyl groups (*OH). This is similar to previous first principles findings of oxygen adsorption on Mg(0001). It was also found that the local structure of an adsorbed hydroxyl monolayer mimics that of the crystal structure of brucite, Mg(OH)(2). Lastly, DFT-calculated reaction enthalpies were used to reproduce the bulk Mg Pourbaix diagram, and Pourbaix's formalism was extended to develop a theoretical Mg surface Pourbaix diagram. From this, it was shown that the enthalpy of hydroxylation of Mg(0001) becomes more negative with increasing surface coverage of *OH groups. This indicates that the presence of adsorbed *OH species provides an energetic driving force for water dissociation on Mg(0001). Furthermore, the corrosive region of the Mg Pourbaix diagram can be suppressed if *OH adsorption is limited to certain low-energy active sites, where they form a stable hydroxide surface.