Facile and Massive Aluminothermic Synthesis of Mayenite Electrides from Cost-Effective Oxide and Metal Precursors

Subnanoporous mayenite electride [Ca₂₄Al₂₈O₆₄]⁴⁺(O^2-)_{2- x}(e^-)_{2 x} (C12A7:e^-) as the first room temperature-stable inorganic electride has attracted intensive attention because of its fascinating chemical, electrical, optical, and magnetic properties. However, it usually needs to be synthesized through a complicated multistep process involving high temperature (e.g., 1350 °C) precrystallization, severe reduction (e.g., 700-1300 °C for up to 240 h in Ca or Ti metal vapor atmosphere), and postpurification. Herein, a facile one-step aluminothermic synthesis method was developed for the massive production of C12A7:e^- powders directly from a mixture of cost-effective CaO, Al₂O₃, and Al powders under much milder conditions (e.g., calcination at 1100 °C in flowing Ar for 8 h). By merely adjusting the amount of Al, the electron densities ( N_e) in the as-synthesized C12A7:e^- can be optimized up to 1.23 × 10²¹ cm^-3, covering the insulator-metal transition (MIT). The further mechanistic studies of this new aluminothermic synthesis process revealed that the Al performed dual-functional roles, which not only acted as an in situ reducing agent but also dramatically decreased the formation temperatures of the mayenite structure. After suitable Ru loading, the Ru/C12A7:e^- catalyst from massively produced electride powder showed a promising preliminary performance of NH₃ synthesis (2.8 mmol·g^-1·h^-1) under mild conditions (1 atm and 400 °C).