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.