Boosting the CO₂ Uptake of MgO-Based Sorbents using Na₂CO₃ as Nucleation Seeds: Mechanistic Insights via In Situ Synchrotron XRD

CO₂ capture, utilization and storage (CCUS) is a key technology to reach net zero CO₂ emissions and mitigate global warming. Therefore, there is an urgent need to develop functional materials that can capture and release CO₂ under industrially relevant conditions. Solid oxide materials such as MgO are earth-abundant and constitute a promising family of materials for CO₂ capture.¹ MgO-based CO₂ sorbents are characterized by favorable carbonation thermodynamics and high gravimetric CO₂ uptake capacities but display limited CO₂uptake due to slow carbonation kinetics. The first step that partially resolves the slow kinetics involves the addition of alkali metal nitrates, which are molten under operation conditions.² Here, we show a second sorbent engineering step that effectively resolves the limited CO₂ uptake kinetics of MgO-sorbents, by co-promoting MgO with NaNO₃ and Na₂CO₃, resulting in fast CO₂ uptake rates. We demonstrate the mechanism behind this promotion via in-situ synchrotron XRD measurements (1 s resolution), revealing that Na₂CO₃ rapidly (within seconds) transforms into Na₂Mg(CO₃)₂, which acts as a nucleation seed for MgCO₃ growth. Na₂Mg(CO₃)₂ seeds facilitate MgCO₃ nucleation, which has been identified as the rate-determining step, resulting in increased CO₂ uptake kinetics (from minutes to seconds). Lastly, using electron-microscopy techniques, we visualize the nucleation of MgCO₃ directly onto the Na₂Mg(CO₃)₂ seeds. Taken together, we show that the co-promotion of MgO with Na₂CO₃ and NaNO₃ is a facile, inexpensive and highly promising strategy for improving MgO–based CO₂ capture sorbents.<br/><br/>[1] Dunstan, M. T. <i>et al.</i> <i>Chem Rev</i> <b>2021</b>, <i>121</i> (20), 12681–12745.<br/>[2] Landuyt, A. <i>et al.</i> <i>JA</i><i>CS Au</i> <b>2022</b>,<i> 2</i> (12), 2731-2741.