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 among several theoretical approaches. Herein, advanced experimental characterization techniques, including in situ eTEM, in situ CO2 -DRIFTS, transient 18 O-isotopic exchange, and Density Functional Theory are employed to elucidate the mechanism of 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 that of C16 O18 O following CO2 interaction, verifies for the first time the proposed theoretical (DFT) 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 compared to the unmodified MgO. This article is protected by copyright. All rights reserved.