Uncovering the CO₂ Capture Mechanism of NaNO₃-Promoted MgO by ¹⁸O Isotope Labeling

Abstract

MgO-based CO2 sorbents promoted with molten alkali metal nitrates (e.g., NaNO3) have emerged as promising materials for CO2 capture and storage technologies due to their low cost and high theoretical CO2 uptake capacities. Yet, the mechanism by which molten alkali metal nitrates promote the carbonation of MgO (CO2 capture 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 CO2 capture (carbonation) and regeneration (decarbonation) reactions. We discovered a rapid oxygen exchange between CO2 and 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 NaNO3 and CO2 or NaNO3 and 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+···CO3 2–] ionic pairs in the molten NaNO3 promoter. Upon reaching the solubility limit, MgCO3 crystallizes at the MgO/NaNO3 interface.

Probing the Local Structure of Na in NaNO3-Promoted, MgO-Based CO2 Sorbents via X-ray Absorption Spectroscopy

“…We propose that such dissolution (solvation) modifies the Na local structure in the melt and such structural modifications remain when the material is cooled (as NaNO 3 dispersed at the MgCO 3 surface and/or trapped within MgCO 3 as inclusions), as schematized in Figure a,b. This interpretation is in line with recent studies that proposed that the mechanism of CO 2 uptake in NaNO 3 -promoted MgO proceeds through the dissolution of [Mg 2+ ···CO 3 2– ] ionic pairs in the NaNO 3 melt (these pairs precipitate upon supersaturation yielding crystalline MgCO 3 ) …”

“…Nevertheless, it is possible that the changes observed in the in situ Na K-edge XANES spectrum upon carbonation were due to changes in the local environment/structure around Na due to the presence of [Mg 2+ ···CO 3 2– ] pairs dissolved in the NaNO 3 melt and/or due to a solvation shell around MgCO 3 (as shown schematically in Figure a,b). − More experiments and modeling studies are required to further test this hypothesis. However, we believe that the present study provides an innovative experimental approach and important data pointing toward the validity of the CO 2 uptake mechanism in NaNO 3 -promoted MgO as proposed by Landuyt et al…”

“…To summarize, the combined analysis of Na K-edge XANES, EXAFS, MAS 23 Na NMR, STEM–EDX, and XRD data revealed that the local environment around Na was altered by the presence of carbonate species (MgCO 3 /[Mg 2+ ···CO 3 2– ] ionic pairs). Although we currently lack a detailed atomic-scale model, the correlation between the magnitude of the changes in the XANES spectra and the NaNO 3 /MgCO 3 ratio suggests that the observed changes in the Na environment are related to the NaNO 3 /MgCO 3 interfaces and due to the dissolution of [Mg 2+ ···CO 3 2– ] ionic pairs in the molten salt . This hypothesis can be rationalized in light of previous studies showing that MgCO 3 nucleates and grows at the buried interface between solid MgO and molten NaNO 3 via the dissolution of [Mg 2+ ···CO 3 2– ] ion pairs in the melt [Na···NO 3 – ].…”

“…Further, it was found that MgCO 3 grows at the interface between MgO and NaNO 3 according to grazing incident diffraction and electron microscopy studies on a model MgO(100) single crystal coated with molten NaNO 3 . More recently, using density functional theory (DFT) calculations and 18 O labeling experiments, it was hypothesized that the carbonation mechanism of NaNO 3 -promoted MgO involves the rapid formation of surface carbonates followed by their dissolution, yielding [Mg 2+ ···CO 3 2– ] ion pairs dissolved in the AMS melt, and oxygen of NO 3 – groups do not participate in carbonation reaction …”

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

“…The mechanism through which molten alkali nitrates (or mixtures thereof ) promote the carbonation of MgO is currently debated and intensively studied through both experimental and theoretical approaches (12,(20)(21)(22)(23)(24)(25). For example, it has been proposed by Zhang et al (12) that MgO is partially dissolved in the molten salt, yielding [Mg 2+ …O 2− ] ionic pairs that react with CO 2 , generating in turn [Mg 2+ …CO 3…”