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⁺…O^2–] 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.

熔融碱金属盐的添加极大地加速了MgO 通过形成 MgCO ₃捕获CO ₂的动力学。然而，生长机制、MgCO ₃形成的性质以及熔融碱金属盐在 CO ₂捕获过程中的确切作用仍然难以捉摸，阻碍了更有效的基于 MgO 的 CO ₂吸附剂的开发。在这里，我们使用定义明确但具有代表性的模型系统，即涂有 NaNO ₃的 MgO(100) 单晶，揭示了 MgCO ₃在实际相关条件下的生长机制. 该模型系统通过原位 X 射线反射计与掠入射 X 射线衍射、扫描电子显微镜和高分辨率透射电子显微镜相结合进行询问。当裸露的 MgO(100) 暴露在 CO ₂流中时，大约会形成一层非晶表面碳酸盐层。7-Å 厚度形式。相反，当MgO(100) 被NaNO ₃包覆时，MgCO ₃晶体会成核并生长。这些晶体相对于 MgO(100) 衬底具有优先取向，并在 MgO(100) 和熔融的 NaNO ₃之间的界面处形成。MgCO ₃相对于 MgO(100) 外延生长，并且 MgCO ₃之间的晶格失配和 MgO 通过晶格错配位错松弛。MgO 表面上的金字塔形凹坑，靠近和低于 MgCO ₃晶体，指向表面 MgO 的蚀刻，为 MgCO ₃生长提供溶解的 [Mg ²⁺ …O ^2– ] 离子对。我们的研究强调了结合 X 射线和电子显微镜技术的重要性，以提供原子到微米尺度的洞察力，以了解在反应条件下复杂界面发生的变化。