Role of Na₂CO₃ as Nucleation Seeds to Accelerate the CO₂ Uptake Kinetics of MgO-Based Sorbents

Abstract

High Resolution Image Download MS PowerPoint Slide There is an urgent need for inexpensive, functional materials that can capture and release CO 2 under industrial conditions. In this context, MgO is a highly promising, earth-abundant CO 2 sorbent. However, despite its favorable carbonation thermodynamics and potential for high gravimetric CO 2 uptakes, MgO-based CO 2 sorbents feature slow carbonation kinetics, limiting their CO 2 uptake during typical industrial contact times. The addition of molten alkali metal nitrate promoters, such as NaNO 3, can partially mitigate the slow kinetics. Here, we investigate how the CO 2 uptake kinetics of NaNO 3 -promoted MgO can be increased further through the addition of finely dispersed Na 2 CO 3 . The incorporation of Na 2 CO 3 significantly increases the CO 2 uptake rate from 1.4 to 14.6 mmol MgCO 3 (mol MgO) −1 s –1 . Using in situ synchrotron X-ray powder diffraction (XRD), we track the formation of MgCO 3 and elucidate the mechanism through which Na 2 CO 3 promotes the CO 2 uptake of MgO. Our findings demonstrate that Na 2 CO 3 rapidly converts within seconds into Na 2 Mg(CO 3 ) 2 during carbonation, acting subsequently as nucleation seeds for MgCO 3 formation, in turn significantly enhancing CO 2 uptake kinetics. Further, the presence of Na 2 Mg(CO 3 ) 2 considerably enhances the mobility of ions in the sorbent, leading to sintering of MgCO 3 . Importantly, Na 2 Mg(CO 3 ) 2 promotes MgCO 3 formation even in the presence of molten RbNO 3, a salt with a limited ability to dissolve [Mg 2+ ···CO 3 2– ] ion pairs, indicating that Na 2 Mg(CO 3 ) 2 lowers the critical ion pair concentration required for MgCO 3 nucleation. Additionally, the partial dissolution of Na 2 CO 3 in NaNO 3 may increase the concentration of carbonate ions in the melt, further accelerating carbonation kinetics in MgO-(Na 2 CO 3 /NaNO 3 ).

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