Peering into Buried Interfaces with X-Rays and Electrons to Unveil MgCO3 Formation During CO2 Capture in Molten Salt Promoted MgO
<p>The addition of molten alkali metal salts drastically accelerates the kinetics of CO<sub>2</sub> capture by MgO through the formation of MgCO<sub>3</sub>. However, the growth mechanism, the nature of MgCO<sub>3</sub> formation and the exact role of the molten alkali metal salts on the CO2 capture process remains elusive, holding back the development of more effective MgO-based CO<sub>2</sub> sorbents. Here, we unveil the growth mechanism of MgCO<sub>3</sub> under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO<sub>3</sub>. 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<sub>2</sub>, a non-crystalline surface carbonate layer of ca. 7 Å thickness forms. In contrast, when MgO(100) is coated with NaNO<sub>3</sub> MgCO<sub>3</sub> crystals nucleate and growth. 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<sub>3</sub>. MgCO<sub>3</sub> grows epitaxially with respect to MgO(100) and the lattice mismatch between MgCO<sub>3</sub> and MgO is relaxed through lattice misfit dislocations. Pyramid shaped pits on the surface of MgO, in the proximity and below the MgCO<sub>3</sub> crystals, point to the etching of surface MgO, providing dissolved [Mg<sup>2+</sup>…O<sup>2–</sup>] ionic pairs for MgCO<sub>3</sub> 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.</p>