Model structures of molten salt-promoted MgO to probe the mechanism of MgCO formation during CO capture at a solid-liquid interface.

MgO is a promising solid oxide-based sorbent to capture anthropogenic CO emissions due to its high theoretical gravimetric CO uptake and its abundance. When MgO is coated with alkali metal salts such as LiNO, NaNO, KNO, or their mixtures, the kinetics of the CO uptake reaction is significantly faster resulting in a 15 times higher CO 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 MgCO 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 NaNO-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 NaNO as the promoter is used. To identify the location of nucleation and growth of MgCO, micro X-ray computed tomography, scanning electron microscopy, Raman microspectroscopy and optical profilometry were applied. We found that MgCO forms at the NaNO/MgO interface and not inside the melt. Moreover, there was no preferential nucleation of MgCO at the TPB when compared to the buried interface. Furthermore, it is found that there is no observable CO diffusion limitation in the nucleation step. However, it was observed that CO diffusion limits MgCO crystal growth, the growth rate of MgCO is approximately an order of magnitude faster in shallow grooves compared to that in deep grooves.