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

This work provides insight into the local structure of Na in MgO-based CO₂ sorbents that are promoted with NaNO₃. To this end, we use X-ray absorption spectroscopy (XAS) at the Na K-edge to interrogate the local structure of Na during the CO₂ capture (MgO + CO₂ ↔ MgCO₃). 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 NaNO₃ 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 NaNO₃/MgCO₃ interfaces and/or in the vicinity of [Mg²⁺···CO₃^2-] ionic pairs that are trapped in the cooled NaNO₃ melt. The changes observed are reversible, i.e., the local environment of NaNO₃ was restored after a regeneration treatment to decompose MgCO₃ to MgO. The ex situ Na K-edge XAS experiments were complemented by ex situ magic-angle spinning ²³Na nuclear magnetic resonance (MAS ²³Na 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 (NaNO₃ is molten under the conditions of the in situ experiments). These in situ Na K-edge XANES spectra of molten NaNO₃ open new opportunities to investigate the atomic scale structure of CO₂ sorbents modified with Na-based molten salts by using XAS.