Optimization of the structural characteristics of CaO and its effective stabilization yield high-capacity CO₂ sorbents

These CaO microspheres exhibit a hollow spherical morphology, whereby the central void is able to accompany the large volumetric expansion of the material during CO₂ capture. The central void is realized through the removal of a sacrificial carbonaceous template derived from in situ hydrothermal treatment of xylose. The shell of the hollow spheres is composed of an assembly of CaO nanoparticles (i.e., < 100 nm) which effectively minimize the diffusive transport limitations of CO₂ through the growing product layer of CaCO₃ during the carbonation process and thus, maintain the inherently fast carbonation kinetics.

We have selected MgO as a stabilizer owing to its high thermal stability and more importantly, due to the fact that it does not form a solid solution with CaO under operating conditions, as opposed to Al₂O₃, a commonly used stabilizer, which reacts with CaO during cyclic operations and forms a solid solution that is inactive for CO₂ capture. Incorporation of MgO into the structure was carried out in situ during hydrothermal synthesis, enabling a single-step synthesis protocol which opens up the opportunity to produce larger quantities of material at a substantially reduced cost. The synthesized sorbent with a MgO content as low as 11 wt. % demonstrated a CO₂ uptake of 0.50 g_CO2/g_CaO after 30 carbonation and regeneration cycles, corresponding to a capacity retention of 83% and surpassing the CO₂ uptake capacity of the limestone benchmark by more than 500%.