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

Abstract 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 ¹⁸O‐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 ¹⁸O‐NaNO₃ (use of ¹⁸O₂) and C¹⁶O¹⁸O 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⁺… O²⁻] ion pairs preventing the development of an impermeable MgCO₃ shell, which largely increases the rate of bulk MgO carbonation.