Unravelling the Mechanism of Intermediate-Temperature CO₂ Interaction with Molten-NaNO₃ -Salt-Promoted MgO

The optimization of MgO-based adsorbents as advanced CO₂ -capture materials is predominantly focused on their molten-salt modification, for which theoretical and experimental contributions provide great insights for their high CO₂ -capture performance. The underlying mechanism of the promotion effect of the molten salt on CO₂ capture, however, is a topic of controversy. Herein, advanced experimental characterization techniques, including in situ environmental transmission electron microscopy (eTEM) and CO₂ chemisorption by diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), transient ¹⁸ O-isotopic exchange, and density functional theory (DFT), are employed to elucidate the mechanism of the CO₂ interaction with molten-salt-modified MgO in the 250-400 °C range. Herein, eTEM studies using low (2-3 mbar) and high (700 mbar) CO₂ pressures illustrate the dynamic evolution of the molten NaNO₃ salt promoted and unpromoted MgO carbonation with high magnification (<50 nm). The formation of ¹⁸ O-NaNO₃ (use of ¹⁸ O₂ ) and C¹⁶ O¹⁸ O following CO₂ interaction, verifies the proposed reaction path: conversion of NO₃ ^- (NO₃ ^-  → NO₂ ⁺  + O^2- ), adsorption of NO₂ ⁺ on MgO with significant weakening of CO₂ adsorption strength, and formation of [Mg²⁺ … O^2- ] ion pairs preventing the development of an impermeable MgCO₃ shell, which largely increases the rate of bulk MgO carbonation.