The application of magnesium oxide (MgO) for CO2 capture from the atmosphere at ambient conditions has the potential for carbon dioxide removal on the gigaton scale. In a looping process as envisioned by McQueen et al. (2020), MgO would be reacted with the CO2 in the air to carbonate phases. Via calcination the CO2 can be separated and the separation medium can be reused for further CO2 capture. Recent feasibility calculations by McQueen et al., 2020 assumed anhydrous MgCO3 forming, but various hydrated magnesium carbonate phases are known. Unknown reaction kinetics at ambient temperatures and in the presence of water currently prevent the application of this promising technique. Here we show time-of-flight secondary ion mass spectrometry (ToF-SIMS), scanning transmission electron microscopy (STEM) and Raman spectroscopical characterization on MgO reacted with atmosphere in long-term (decade) and short-term (day to months) experiments. Our results indicate a dichotomy in reaction rates. Short-term experiments indicate rapid hydroxylation and carbonation leading to secondary phase formation whereas long-term experiments indicate passivation effects. In long-term experiments, we find a hydrated Mg-carbonate layer of ~1.6 um consisting of nesquehonite and dypingite, brucite (Mg(OH2) and amorphous Mg-carbonates. We built a simple reactive transport model to demonstrate a significant decrease in effective reaction rate due to passivation layer formation. Assumptions of the model include partially reduced transport of reactive species to the surface of MgO through the passivating layer and dissolution process of Mg(OH)2 as rate-limiting step. In short term experiments, fast formation of Mg-carbonates (artinite) is observed within 83 days. These results demonstrate that the initial reaction is fast and if armoring of reactive surface area can be prevented, MgO looping could be employed for direct air capture of CO2. Lastly, we will discuss the potential use of dopants (e.g., Fe, Li) to prevent surface passivation.

Direct air capture of CO2 via mineral looping: How to overcome passivating layers

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