How Does K₂CO₃ Promote the CO₂ Uptake of MgO?

Alkaline earth metal oxides are economically attractive, earth-abundant sorbents for CO₂ capture. In particular, MgO is a promising CO₂ sorbent for applications in the intermediate temperature range (200-400 °C). However, MgO requires the addition of promoters such as alkali metal nitrates or carbonates to overcome the slow kinetics of CO₂ sorption. It has been observed experimentally that the addition of K₂CO₃ increases the CO₂ uptake of MgO, yet its role remains poorly understood; this is a critical knowledge gap for the design of more efficient sorbents. In this work, using a combination of in situ XRD and Raman spectroscopy, we gain insight into the CO₂ uptake mechanism of K₂CO₃-promoted MgO, which proceeds in two steps. An initial rapid CO₂ uptake (within 1 min) occurs via the formation of a potassium-rich amorphous carbonate phase (K₂CO₃·xMgCO₃·yH₂O). This is followed by a slower CO₂ capture step, in which the intermediate K-rich phase transforms into a magnesium-rich amorphous carbonate, along with the formation of small amounts of crystalline (hydrated and anhydrous) carbonates such as K₂CO₃, baylissite (K₂Mg-(CO₃)₂·4H₂O), and nesquehonite (MgCO₃·3H₂O). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis reveals that surface hydroxides, which remain on the MgO surface even after a thermal treatment at 720 °C in N₂, facilitate the formation of hydrated carbonates at 315 °C in dry CO₂. However, the presence of steam during carbonation lowers the CO₂ uptake and inhibits the second CO₂ uptake step, most likely by stabilizing the K-rich amorphous intermediate. Cyclic CO₂ uptake and regeneration experiments reveal the crucial role of a high K₂CO₃ dispersion within the sorbent for maintaining good cyclic stability.