The interplay between carbonate phases in promoted carbonation of MgO

Magnesium oxide serves as promising sorbent for CO2 capture, storage and release. The CO2 uptake can be enhanced through the addition of molten alkali nitrate- and bicarbonate promoters, such as NaNO3 and Na2CO3. The mechanisms through which these promotors affect the kinetics of CO2 uptake have so far not been fully uncovered. Here, using a combination of scanning electron microscopy and X-ray diffraction techniques we unravel the sequence of carbonation processes taking place inside the liquid NaNO3 promotor in the presence of CO2 gas and a single crystalline MgO substrate. We identify the key role that Na2Mg(CO3)2 plays as precursor phase which forms as dominant phase in the early stages of carbonation and remains throughout the carbonation process as intermediate phase that supplies the thermodynamically more stable MgCO3 product with growth species. The insight into the interplay between CO2 uptake, MgO dissolution, supersaturation and precipitation of phases with different stability regimes provided by this study contributes to a general understanding of the dynamical interplay between phases under chemical gradients.