Model structures of molten salt-promoted MgO to probe the mechanism of MgCO3 formation during CO2 capture at a solid–liquid interface

Abstract

MgO is a promising solid oxide-based sorbent to capture anthropogenic CO₂ emissions due to its high theoretical gravimetric CO₂ uptake and its abundance. When MgO is coated with alkali metal salts such as LiNO₃, NaNO₃, KNO₃, or their mixtures, the kinetics of the CO₂ uptake reaction is significantly faster resulting in a 15 times higher CO₂ uptake compared to bare MgO. However, the underlying mechanism that leads to this dramatic increase in the carbonation rate is still unclear. This study aims to determine the most favourable location for the nucleation and growth of MgCO₃ and more specifically, whether the carbonation occurs preferentially at the buried interface, the triple phase boundary (TPB), and/or inside the molten salt of the NaNO₃–MgO system. For this purpose, a model system consisting of a MgO single crystal that is structured by ultra-short pulse laser ablation and coated with NaNO₃ as the promoter is used. To identify the location of nucleation and growth of MgCO₃, micro X-ray computed tomography, scanning electron microscopy, Raman microspectroscopy and optical profilometry were applied. We found that MgCO₃ forms at the NaNO₃/MgO interface and not inside the melt. Moreover, there was no preferential nucleation of MgCO₃ at the TPB when compared to the buried interface. Furthermore, it is found that there is no observable CO₂ diffusion limitation in the nucleation step. However, it was observed that CO₂ diffusion limits MgCO₃ crystal growth, i.e. the growth rate of MgCO₃ is approximately an order of magnitude faster in shallow grooves compared to that in deep grooves.