Uncovering the CO2Capture Mechanism of NaNO3-Promoted MgO by 18O Isotope Labeling

Abstract

MgO-based CO2sorbents promoted with molten alkali metal nitrates (e.g., NaNO3) have emerged as promising materials for CO2capture and storage technologies due to their low cost and high theoretical CO2uptake capacities. Yet, the mechanism by which molten alkali metal nitrates promote the carbonation of MgO (CO2capture reaction) remains debated and poorly understood. Here, we utilize 18O isotope labeling experiments to provide new insights into the carbonation mechanism of NaNO3-promoted MgO sorbents, a system in which the promoter is molten under operation conditions and hence inherently challenging to characterize. To conduct the 18O isotope labeling experiments, we report a facile and large-scale synthesis procedure to obtain labeled MgO with a high 18O isotope content. We use Raman spectroscopy and in situ thermogravimetric analysis in combination with mass spectrometry to track the 18O label in the solid (MgCO3), molten (NaNO3), and gas (CO2) phases during the CO2capture (carbonation) and regeneration (decarbonation) reactions. We discovered a rapid oxygen exchange between CO2and MgO through the reversible formation of surface carbonates, independent of the presence of the promoter NaNO3. On the other hand, no oxygen exchange was observed between NaNO3and CO2or NaNO3and MgO. Combining the results of the 18O 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 [Mg2+···CO32-] ionic pairs in the molten NaNO3promoter. Upon reaching the solubility limit, MgCO3crystallizes at the MgO/NaNO3interface.