Deciphering the structural dynamics in molten salt-promoted MgO-based CO 2 sorbents and their role in the CO 2 uptake.

The development of effective CO 2 sorbents is vital to achieving net-zero CO 2 emission targets. MgO promoted with molten salts is an emerging class of CO 2 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 NaNO 3 -promoted, MgO-based CO 2 sorbent. During the first few cycles of CO 2 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 MgCO 3 growth. After the third cycle, the sorbent shows a continuous reactivation, which is linked to the in situ formation of Na 2 Mg(CO 3 ) 2 crystallites that act effectively as seeds for MgCO 3 nucleation and growth. Na 2 Mg(CO 3 ) 2 forms due to the partial decomposition of NaNO 3 during regeneration at T ≥ 450°C followed by carbonation in CO 2 .