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

Abstract

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).

Nature of Reactive Hydrogen for Ammonia Synthesis over a Ru/C12A7 Electride Catalyst

“…The material contained 4 × 10 20 e − cm −3 as determined by the titration method described by Yoshizumi et al, 47 comparable to the ones reported previously for C12A7 electride. 31,32 When loaded with 2 wt % Ru, the material catalyzed ammonia synthesis at a rate of 1.5 mmol ammonia g cat −1 h −1 in a mixture of 75% H 2 and 25% N 2 , at 633 K flowing at 30 cm 3 min −1 . The apparent activation energy of ammonia synthesis over the material was 72 kJ mol −1 (Figure S5), and ammonia synthesis was 0.7 order in N 2 pressure and 0.3 order in H 2 pressure under the conditions studied (Figure S6).…”

“…The 12CaO•7Al 2 O 3 electride (C12A7:e − ) powders were synthesized via a one-step aluminothermic method directly from a mixture of commercially available CaO, Al 2 O 3 , and Al powders. 32 Calcium oxide (CaO, reagent grade), calcium carbonate (CaCO 3 , 99.5%), and gamma phase aluminum oxide (γ-Al 2 O 3 , 99.9%) are used as received from Alfa-Aesar. Aluminum (Al, reagent grade) powders are obtained from Fisher Scientific.…”

“…Pure mayenite (C12A7) was obtained by calcining the charcoal-like primary powders in air at 1473 K for 10 h with a ramp of 5 K min –1 . The 12CaO·7Al 2 O 3 electride (C12A7:e – ) powders were synthesized via a one-step aluminothermic method directly from a mixture of commercially available CaO, Al 2 O 3 , and Al powders . Calcium oxide (CaO, reagent grade), calcium carbonate (CaCO 3 , 99.5%), and gamma phase aluminum oxide (γ-Al 2 O 3 , 99.9%) are used as received from Alfa-Aesar.…”

Mayenite-based electride C12A7e−: an innovative synthetic methodviaplasma arc melting

“…Similar to studies by Khan et al, Salasin et al and Li et al showed the possibility to obtain electrides by synthesis from different Ca and Al precursor compounds and not the preformation of C12A7 [33][34][35][36][37][38] . A big step towards the batch synthesis of larger amounts of C12A7e -, in respect of an application as a catalyst support, was accomplished by Jiang et al by aluminothermic synthesis using a mixture of CaO, Al 2 O 3 and Al and heating under Ar at 1373 K for 8 h 39 . Besides challenges in the synthesis of the C12A7ematerial, its characterization and especially the determination of its electron concentration are not trivial.…”

“…Only for some special cases Raman spectroscopy, SQUID measurements are reported as ways to investigate the electron concentration 5,43 . Iodometric titration is also used frequently, however the role of metal impurities such as Al or Ca might influence the results as reported by Jiang et al 39,45 . Existing strategies for the synthesis of solid electrides reported in the literature mainly refer to bulk/batch synthesis and are thus limited in their scalability.…”