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

Subnanoporous mayenite electride [Ca ₂₄ Al ₂₈ O ₆₄ ] ⁴⁺ (O ^2- ) _2-x (e ^- ) _2x (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, Al ₂ O ₃ , 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 × 10 ²¹ 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 NH ₃ synthesis (2.8 mmol·g ^-1 ·h ^-1 ) under mild conditions (1 atm and 400 °C).