Acidities of MgO surface sites: implications for the formation mechanism of Mg(OH)2

Abstract

The hydroxylation of periclase (MgO) to brucite (Mg(OH)₂) is thought to be an important intermediate step when using MgO to capture CO₂ from the atmosphere. However, the mechanism of hydroxylation of MgO to form Mg(OH)₂ is poorly understood. In this work, we used atomic-scale density functional tight binding simulations coupled with the metadynamics rare event method to analyze the surface chemistry of MgO and the acid dissociation equilibrium constants (pK_a) of its surface sites. The method and parameters were validated by calculating the pK_a for hydroxylation of the first shell water bound to aqueous Mg²⁺ ion. The pK_a value derived using a probabilistic method was 12.3, which is in fair agreement with the accepted value of 11.4, with the difference between them equal to a ∼5 kJ mol⁻¹ error in the calculations. We then extended these pK_a calculations to probe the hydroxylation reactions of the surface sites of the MgO(100)–water interface, arriving at pK_as of 5.4 to deprotonate terminal water molecules bound to the surface magnesium sites (η-OH₂ or 〉MgOH₂), and 13.9 to deprotonate hydroxylated bridging oxygen sites (μ₅-oxo or 〉O). Hydroxide (OH⁻) adsorption on the surface was also probed and found to be less thermodynamically favorable than deprotonation of the terminal water molecule. The plausibility of the computed pK_as was verified using an activity-based speciation model and compared to pH measurements of water equilibrated with MgO nanoparticles and single crystals. The model predicted a solution pH of 7.1 when surface sites buffered and the pH of 12.0 when MgO dissolution dominated. These are close to the experimental initial solution pHs of 7–7.5 and the long term pHs of ∼10.5. The similarity suggests that the calculated pK_a values from the DFTB+/metadynamics simulations are plausible and that these methods can be a useful tool to probe reaction mechanisms involving covalent bonds.