Peering into buried interfaces with X-rays and electrons to unveil MgCO3 formation during CO2 capture in molten salt-promoted MgO

Significance The grand challenge of reducing CO2 emissions requires the development of cost-effective CO2 sorbents. Based on the theoretically obtainable weight-normalized CO2 uptake, MgO-based materials promoted with molten salts are attractive sorbents when compared to amines or metal organic frameworks. However, there is very little understanding of the processes that occur at the atomic-to-micro scale during CO2 capture conditions, hampering the advancement of such sorbents. Combining X-ray and electron-based characterization techniques, we observe that MgCO3 crystals form via nucleation and growth at the interface between MgO and the molten salt and are oriented with respect to the MgO(100) surface. Hence, more-effective MgO-based sorbents will require maximizing the interfacial area and the number of nucleation sites at the interface. The addition of molten alkali metal salts drastically accelerates the kinetics of CO2 capture by MgO through the formation of MgCO3. However, the growth mechanism, the nature of MgCO3 formation, and the exact role of the molten alkali metal salts on the CO2 capture process remain elusive, holding back the development of more-effective MgO-based CO2 sorbents. Here, we unveil the growth mechanism of MgCO3 under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO3. The model system is interrogated by in situ X-ray reflectometry coupled with grazing incidence X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. When bare MgO(100) is exposed to a flow of CO2, a noncrystalline surface carbonate layer of ca. 7-Å thickness forms. In contrast, when MgO(100) is coated with NaNO3, MgCO3 crystals nucleate and grow. These crystals have a preferential orientation with respect to the MgO(100) substrate, and form at the interface between MgO(100) and the molten NaNO3. MgCO3 grows epitaxially with respect to MgO(100), and the lattice mismatch between MgCO3 and MgO is relaxed through lattice misfit dislocations. Pyramid-shaped pits on the surface of MgO, in proximity to and below the MgCO3 crystals, point to the etching of surface MgO, providing dissolved [Mg2+…O2–] ionic pairs for MgCO3 growth. Our studies highlight the importance of combining X-rays and electron microscopy techniques to provide atomic to micrometer scale insight into the changes occurring at complex interfaces under reactive conditions.