If carbon capture is to have a significant effect on global atmospheric CO₂ levels it must be accomplished on a gigaton scale. One possibility that has been suggested is the looped carbonation of magnesium oxide (MgO) by CO₂ from the atmosphere at ambient conditions. As envisioned by McQueen et al. (2020), MgO would react with CO₂ in the air to carbonate phases. The CO₂ can be released by calcination and captured for further processing, and the separation medium reused for further CO₂ capture. Feasibility calculations by McQueen et al., 2020 assumed that the carbonate formed would be MgCO₃, but various hydrated magnesium carbonate phases are known, and brucite (Mg(OH)₂) may also form by reaction with water. The uncertainty in the reaction kinetics at ambient temperatures and in the presence of water are factors that currently prevent the application of this promising technique.

Time-of-flight secondary ion mass spectrometry, scanning transmission electron microscopy (STEM) and Raman spectroscopic characterization of MgO reacted with atmosphere in long- (27 year) and short-term (days to months) experiments indicate a dichotomy in reaction rates. Short-term experiments indicate rapid hydroxylation and carbonation leading to secondary phase formation whereas long-term experiments indicate slow reaction due to passivation. Long-term experiments generated a hydrated Mg-carbonate layer ~1.6 um thick consisting of nesquehonite and dypingite, brucite (Mg(OH₂) and amorphous Mg-carbonates. Short-term experiments at room temperature formed Mg-carbonates (artinite) within 83 days. Carbonation at 1bar CO₂ at 100% RH and 55% RH for 2 weeks on three MgO powders generated nesquehonite, with the amount formed being much larger at 100%RH. The percentage of nesquehonite increased with the surface area of the unreacted powder. Hydration reactions at elevated temperature on nanopowders, ceramics and single crystals indicate that the structure of the hydrated layer varies with reaction in liquid vs. vapor. Hydration may help to generate nanofracture, alleviating passivation effects but may also enhance armoring, the latter appearing to be more likely during reactions with vapor, possibly suggesting that reactions in liquid are dominated by dissolution/reprecipitation, whereas those in vapor are dominated by in-situ replacement.

Paula Anleu

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