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

[Image: see text] 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.