Role of NaCO as Nucleation Seeds to Accelerate the CO Uptake Kinetics of MgO-Based Sorbents.

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