Unraveling the disparity of CO2 sorption on alkali carbonates under high humidity

CO2 emissions from fossil fuel combustion have been closely associated with the rapid climate change and potential crisis on human society. Na2CO3-based and K2CO3-based solid sorbents have been demonstrated as promising candidates for post-combustion CO2 capture. Despite their physicochemical similarities in many respects, the rationale that endows Na2CO3 and K2CO3 with disparate performance in CO2 sorption is still indeterminate, which motivates us to unravel from the atomic level. Herein, taking moisture effect on carbonation as a starting point, different hydrophilicity and deliquescence between Na2CO3 and K2CO3 are investigated by integrating experimental investigations and theoretical verifications. The results manifest a bigger radius of K+ renders higher cationic hydration on K2CO3 surface, leading to its stronger H2O affinity and easier deliquescence than that of Na2CO3. Under identical humid settings, K+ is observed to diffuse remarkably faster than Na+, accordingly, facilitating a desirable CO2 carbonation performance over K2CO3 sorbents. In this regard, amelioration strategies on Na2CO3 by tuning hydrophilicity are proposed and further demonstrated experimentally. This work provides a fresh perspective to comprehend the carbonation mechanism by solid alkali carbonates as well as practical guidelines for rational sorbents design to boost post-combustion CO2 reduction.