How Does K 2 CO 3 Promote the CO 2 Uptake of MgO?

This is the authors' abstract. We don't add key points for this paper.

High Resolution Image Download MS PowerPoint Slide Alkaline earth metal oxides are economically attractive, earth-abundant sorbents for CO 2 capture. In particular, MgO is a promising CO 2 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 CO 2 sorption. It has been observed experimentally that the addition of K 2 CO 3 increases the CO 2 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 CO 2 uptake mechanism of K 2 CO 3 -promoted MgO, which proceeds in two steps. An initial rapid CO 2 uptake (within 1 min) occurs via the formation of a potassium-rich amorphous carbonate phase (K 2 CO 3 ·xMgCO 3 ·yH 2 O). This is followed by a slower CO 2 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 K 2 CO 3, baylissite (K 2 Mg(CO 3 ) 2 ·4H 2 O), and nesquehonite (MgCO 3 ·3H 2 O). 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 N 2, facilitate the formation of hydrated carbonates at 315 °C in dry CO 2 . However, the presence of steam during carbonation lowers the CO 2 uptake and inhibits the second CO 2 uptake step, most likely by stabilizing the K-rich amorphous intermediate. Cyclic CO 2 uptake and regeneration experiments reveal the crucial role of a high K 2 CO 3 dispersion within the sorbent for maintaining good cyclic stability.