Deciphering the structural dynamics in molten salt-promoted MgO-based CO₂ sorbents and their role in the CO₂ uptake

The development of effective CO₂ sorbents is vital to achieving net-zero CO₂ emission targets. MgO promoted with molten salts is an emerging class of CO₂ sorbents. However, the structural features that govern their performance remain elusive. Using in situ time-resolved powder x-ray diffraction, we follow the structural dynamics of a model NaNO₃-promoted, MgO-based CO₂ sorbent. During the first few cycles of CO₂ capture and release, the sorbent deactivates owing to an increase in the sizes of the MgO crystallites, reducing in turn the abundance of available nucleation points, i.e., MgO surface defects, for MgCO₃ growth. After the third cycle, the sorbent shows a continuous reactivation, which is linked to the in situ formation of Na₂Mg(CO₃)₂ crystallites that act effectively as seeds for MgCO₃ nucleation and growth. Na₂Mg(CO₃)₂ forms due to the partial decomposition of NaNO₃ during regeneration at T ≥ 450°C followed by carbonation in CO₂.