Bork Alexander H, Rekhtina Margarita, Willinger Elena, Castro-Fernández Pedro, Drnec Jakub, Abdala Paula M, Müller Christoph R
Laboratory of Energy Science and Engineering, Department of Mechanical and Process Engineering, Eidgenössische Technische Hochschule (ETH) Zürich, 8092 Zürich, Switzerland.
Laboratory of Energy Science and Engineering, Department of Mechanical and Process Engineering, Eidgenössische Technische Hochschule (ETH) Zürich, 8092 Zürich, Switzerland;
Proc Natl Acad Sci U S A. 2021 Jun 29;118(26). doi: 10.1073/pnas.2103971118.
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.
添加熔融碱金属盐可通过形成碳酸镁（MgCO₃）显著加速氧化镁（MgO）捕获CO的动力学过程。然而，碳酸镁的生长机制、形成本质以及熔融碱金属盐在CO捕获过程中的确切作用仍不明确，这阻碍了更高效的基于MgO的CO吸附剂的开发。在此，我们使用一个定义明确且具有代表性的模型系统——涂有NaNO₃的MgO(100)单晶，揭示了实际相关条件下碳酸镁的生长机制。该模型系统通过原位X射线反射仪结合掠入射X射线衍射、扫描电子显微镜和高分辨率透射电子显微镜进行研究。当裸露的MgO(100)暴露于CO气流中时，会形成厚度为7埃的非晶态表面碳酸盐层。相比之下，当MgO(100)涂有NaNO₃时，碳酸镁晶体成核并生长。这些晶体相对于MgO(100)衬底具有择优取向，并在MgO(100)与熔融NaNO₃的界面处形成。碳酸镁相对于MgO(100)外延生长，并且通过晶格失配位错缓解了碳酸镁与氧化镁之间的晶格失配。靠近碳酸镁晶体并在其下方的氧化镁表面上的金字塔形凹坑表明表面氧化镁被蚀刻，为碳酸镁的生长提供了溶解的[Mg…O]离子对。我们的研究强调了结合X射线和电子显微镜技术以提供从原子尺度到微米尺度对反应条件下复杂界面处发生的变化的洞察的重要性。
