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 +  + 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.