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+ + O2–), 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.