CO₂ direct air capture in the early hydration stage for light-burned MgO: a low-dimensional agglomeration regime

The rise in atmospheric carbon dioxide (CO₂) concentrations has necessitated the development of cost-effective and environmentally friendly sorbent materials for the direct air capture (DAC) of CO₂. In this study, CO₂ adsorption via DAC during early hydration stages is investigated for light-burned magnesium oxide (MgO) with open-structured grain boundaries. Isolated monodentate carbonates, MgCO₃, are formed during DAC on the O sites on the interior surfaces of grain boundaries immediately after contact with air. Upon hydration, atmospheric CO₂ preferentially adsorbs on another O site as a MgCO₃ hydrate to form a hydrogen bond with the former O site. DAC-originated MgCO₃ hydrates are thus able to be stabilized on grain-boundary surfaces, exhibiting excellent DAC performance with a CO₂ adsorption capacity and rate of ∼0.7 mol kg^-1 and ∼0.1 mol kg^-1 h^-1, respectively. A reasonable molecular model is presented for low-dimensionally agglomerated MgCO₃ hydrates constrained on grain-boundary surfaces, which enhances their thermal stability.