Facile and Massive Aluminothermic Synthesis of Mayenite Electrides from Cost-Effective Oxide and Metal Precursors Dong Jiang, † Zeyu Zhao, † Shenglong Mu, † Haijun Qian, ‡ and Jianhua Tong* ,† † Department of Materials Science and Engineering, Clemson University, Clemson, South Carolina 29634, United States ‡ Electron Microscope Facility, Clemson University, Anderson, South Carolina 29625, United States * S Supporting Information 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). 1. INTRODUCTION Recent years have seen broad interest in a new family of compounds, electrides, because of their versatile application potentials in catalysis, optoelectronic, and electrochemical devices. 1-6 Among the various novel electride materials, room temperature-stable inorganic electrides with the mayenite structure is attracting significant attention. 7-12 Mayenite (12CaO·7Al 2 O 3 , C12A7) presents an antizeolite structure, which per unit cell consists of one positively charged framework [Ca 24 Al 28 O 64 ] 4+ containing twelve subnanometer cages and two loosely bonded O 2- ions (i.e., free O 2- ) randomly accommodated in the cages. 13 With the extraction of free O 2- , the C12A7 structure can accommodate a range of anions, including O - ,O 2- , OH - ,H - ,F - , Cl - ,S 2- , CN - , and even e - (i.e., electrides), serving as promising anion conductors. 14 Specifically, the electride, commonly written as [Ca 24 Al 28 O 64 ] 4+ (O 2- ) 2-x (e - ) 2x (C12A7:e - , x =0-2), can accommodate free electrons with an electron density (N e ) up to ∼2.3 × 10 21 cm -3 (the theoretical maximum). The electron- rich property renders an infinite reverie to this electride made entirely of earth-abundant elements in redox catalysis, electronic devices, ionic devices, NH 3 synthesis, and organics hydrogenation. 10,15-17 N e in the C12A7:e - structure plays crucial roles in the performance of these potential applications. For example, although the Ru-loaded C12A7:e - has been proved to be an excellent catalyst for the atmospheric NH 3 synthesis at low temperatures because of the strong electron- donating ability of C12A7:e - , 9,11,25-27 N e higher than a critical point (1 × 10 21 cm -3 ), approximately its MIT point, is the prerequisite for achieving the boosted NH 3 yields with a halved activation energy (E a ). 28 Therefore, the synthesis of C12A7:e - with high N e is one of the most important steps for fulfilling the practical applications of this stable mayenite electride comprised of earth-abundant elements of Al, Ca, and O. So far, several effective methods have been employed for the fabrication of C12A7:e - , mainly including metal vapor reduction (e.g., Ca, Ti, and V), 7,18,19 carbon reduction, 20 H 2 treatment followed by UV irradi- ation, 21,22 and melt-solidification (Table S1). 23,24 It is clear that all these methods involve multiple steps such as precrystallization of the C12A7 oxide precursor, strict evacuation, and encapsulation with severe reduction, and tough postpurification (Figure 1a). The synthesis methods involving in the harsh reduction in a sealed metal vapor atmosphere can usually achieve higher N e , which, however, needs a tough postpurification to remove surface metal oxides (e.g., TiO 2 , CaO, or V 2 O 5 ) from the oxidation of metal vapors for achieving acceptable phase-pure C12A7:e - . Owing to the H 2 O sensitivity of mayenite, the postpurification is quite Received: November 5, 2018 Article pubs.acs.org/IC Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.8b03116 Inorg. Chem. XXXX, XXX, XXX-XXX Downloaded via UNIV OF WINNIPEG on December 22, 2018 at 07:57:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.