Abstract
This work provides insight into the local structure of Na in MgO-based CO(2) sorbents that are promoted with NaNO(3). To this end, we use X-ray absorption spectroscopy (XAS) at the Na K-edge to interrogate the local structure of Na during the CO(2) capture (MgO + CO(2) ↔ MgCO(3)). The analysis of Na K-edge XAS data shows that the local environment of Na is altered upon MgO carbonation when compared to that of NaNO(3) in the as-prepared sorbent. We attribute the changes observed in the carbonated sorbent to an alteration in the local structure of Na at the NaNO(3)/MgCO(3) interfaces and/or in the vicinity of [Mg(2+)···CO(3)(2-)] ionic pairs that are trapped in the cooled NaNO(3) melt. The changes observed are reversible, i.e., the local environment of NaNO(3) was restored after a regeneration treatment to decompose MgCO(3) to MgO. The ex situ Na K-edge XAS experiments were complemented by ex situ magic-angle spinning (23)Na nuclear magnetic resonance (MAS (23)Na NMR), Mg K-edge XAS and X-ray powder diffraction (XRD). These additional experiments support our interpretation of the Na K-edge XAS data. Furthermore, we develop in situ Na (and Mg) K-edge XAS experiments during the carbonation of the sorbent (NaNO(3) is molten under the conditions of the in situ experiments). These in situ Na K-edge XANES spectra of molten NaNO(3) open new opportunities to investigate the atomic scale structure of CO(2) sorbents modified with Na-based molten salts by using XAS.