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

The addition of molten alkali metal salts drastically accelerates the kinetics of CO capture by MgO through the formation of MgCO However, the growth mechanism, the nature of MgCO formation, and the exact role of the molten alkali metal salts on the CO capture process remain elusive, holding back the development of more-effective MgO-based CO sorbents. Here, we unveil the growth mechanism of MgCO under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO 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 CO, a noncrystalline surface carbonate layer of 7-Å thickness forms. In contrast, when MgO(100) is coated with NaNO, MgCO 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 NaNO MgCO grows epitaxially with respect to MgO(100), and the lattice mismatch between MgCO and MgO is relaxed through lattice misfit dislocations. Pyramid-shaped pits on the surface of MgO, in proximity to and below the MgCO crystals, point to the etching of surface MgO, providing dissolved [Mg…O] ionic pairs for MgCO 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.