Uncovering the CO 2 Capture Mechanism of NaNO 3 -Promoted MgO by 18 O Isotope Labeling.

MgO-based CO 2 sorbents promoted with molten alkali metal nitrates (e.g., NaNO 3 ) have emerged as promising materials for CO 2 capture and storage technologies due to their low cost and high theoretical CO 2 uptake capacities. Yet, the mechanism by which molten alkali metal nitrates promote the carbonation of MgO (CO 2 capture reaction) remains debated and poorly understood. Here, we utilize 18 O isotope labeling experiments to provide new insights into the carbonation mechanism of NaNO 3 -promoted MgO sorbents, a system in which the promoter is molten under operation conditions and hence inherently challenging to characterize. To conduct the 18 O isotope labeling experiments, we report a facile and large-scale synthesis procedure to obtain labeled MgO with a high 18 O isotope content. We use Raman spectroscopy and in situ thermogravimetric analysis in combination with mass spectrometry to track the 18 O label in the solid (MgCO 3 ), molten (NaNO 3 ), and gas (CO 2 ) phases during the CO 2 capture (carbonation) and regeneration (decarbonation) reactions. We discovered a rapid oxygen exchange between CO 2 and MgO through the reversible formation of surface carbonates, independent of the presence of the promoter NaNO 3 . On the other hand, no oxygen exchange was observed between NaNO 3 and CO 2 or NaNO 3 and MgO. Combining the results of the 18 O labeling experiments, with insights gained from atomistic calculations, we propose a carbonation mechanism that, in the first stage, proceeds through a fast, surface-limited carbonation of MgO. These surface carbonates are subsequently dissolved as [Mg 2+ ···CO 3 2- ] ionic pairs in the molten NaNO 3 promoter. Upon reaching the solubility limit, MgCO 3 crystallizes at the MgO/NaNO 3 interface.