Role of Na<inf>2</inf>CO<inf>3</inf> as Nucleation Seeds to Accelerate the CO<inf>2</inf> Uptake Kinetics of MgO-Based Sorbents

There is an urgent need for inexpensive, functional materials that can capture and release CO2 under industrial conditions. In this context, MgO is a highly promising, earth-abundant CO2 sorbent. However, despite its favorable carbonation thermodynamics and potential for high gravimetric CO2 uptakes, MgO-based CO2 sorbents feature slow carbonation kinetics, limiting their CO2 uptake during typical industrial contact times. The addition of molten alkali metal nitrate promoters, such as NaNO3, can partially mitigate the slow kinetics. Here, we investigate how the CO2 uptake kinetics of NaNO3-promoted MgO can be increased further through the addition of finely dispersed Na2CO3. The incorporation of Na2CO3 significantly increases the CO2 uptake rate from 1.4 to 14.6 mmol MgCO3 (mol MgO)−1 s-1. Using in situ synchrotron X-ray powder diffraction (XRD), we track the formation of MgCO3 and elucidate the mechanism through which Na2CO3 promotes the CO2 uptake of MgO. Our findings demonstrate that Na2CO3 rapidly converts within seconds into Na2Mg(CO3)2 during carbonation, acting subsequently as nucleation seeds for MgCO3 formation, in turn significantly enhancing CO2 uptake kinetics. Further, the presence of Na2Mg(CO3)2 considerably enhances the mobility of ions in the sorbent, leading to sintering of MgCO3. Importantly, Na2Mg(CO3)2 promotes MgCO3 formation even in the presence of molten RbNO3, a salt with a limited ability to dissolve [Mg2+···CO32-] ion pairs, indicating that Na2Mg(CO3)2 lowers the critical ion pair concentration required for MgCO3 nucleation. Additionally, the partial dissolution of Na2CO3 in NaNO3 may increase the concentration of carbonate ions in the melt, further accelerating carbonation kinetics in MgO-(Na2CO3/NaNO3).