Molten Salt-Promoted MgO Adsorbents for CO₂ Capture: Transient Kinetic Studies

Optimization of MgO adsorbents is predominantly focused on the regulation of appropriate adsorption sites for CO₂ associated with Mg²⁺-O^2- sites of low coordination. Here, for the first time, we conducted transient kinetic experiments to identify and characterize changes of the CO₂ molecular path in MgO-based CO₂ adsorbents upon the addition of molten salt modifiers. Among the optimized samples, addition of 10 mol % NaNO₂ on the surface of MgO exhibited the highest CO₂ uptake (15.7 mmol g^-1) at 350 °C compared to less than 0.1 mmol g^-1 for the unpromoted MgO. Kinetic modeling showed that the interaction of molten salt-promoted MgO with CO₂ at 300 °C involves three different processes, namely, fast surface adsorption associated with surface-active basic sites, chemical reaction associated with MgCO₃ formation, and a slow diffusion step being the rate-limiting step of the carbonation process. Furthermore, transient kinetic studies coupled with mass spectrometry under low CO₂ partial pressure agreed well with the kinetic simulation results based on TGA measurements, demonstrating an in-depth understanding of the CO₂-capturing performance gained and its considerable significance for future practical designs of precombustion CO₂ capture.