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.