Interplay between Carbonate Phases in the NaNO₃-Promoted Carbonation of MgO

The reversible carbonation of magnesium oxide represents a promising route for efficient CO₂ capture and storage. The uptake capacity can be significantly enhanced by the presence of molten alkali nitrate and carbonate promoters such as NaNO₃ and Na₂CO₃. Yet, the mechanistic role of these promoters in accelerating CO₂ uptake remains insufficiently understood. Here, by combining electron microscopy with X-ray diffraction, we elucidate the sequence of carbonation reactions occurring at the interface between liquid NaNO₃, gaseous CO₂, and a single-crystalline MgO substrate. We identify Na₂Mg(CO₃)₂ as a transient precursor phase that dominates the early stages of carbonation and persists as a key intermediate supplying growth species for the MgCO₃ product. The phase evolution follows an interface-coupled dissolution-precipitation pathway within a thin interfacial NaNO₃ layer, where local dissolution of Na₂Mg(CO₃)₂ is directly coupled to the precipitation of MgCO₃. These insights into the coupling of CO₂ uptake, Mg²⁺/O^2- dissolution, supersaturation in the molten promoter, and precipitation across different stability regimes advance the fundamental understanding of nonequilibrium phase dynamics in molten salt-promoted carbonation.