Hydrogen Affinity in Intermetallic Electrides as a Key Indicator of Catalytic Performance in Ammonia Synthesis

Electrides have emerged as promising catalysts or catalyst supports for efficient ammonia synthesis under mild conditions. ATmSi compounds (A = rare earth/alkaline earth, Tm = transition metal) with a tetragonal CeFeSi-type structure represent a class of intermetallic electrides, where lattice atoms serve as active sites, offering significant potential for catalytic applications. However, with over 25 ATmSi compounds, their catalytic performance variations and optimization strategies remain poorly understood. In this study, we systematically investigated the structure-activity relationship of ATmSi compounds, focusing on their anionic electron properties and hydrogen storage capabilities. Analysis of lattice parameters revealed the A-A interlayer distance as a descriptor of anionic electron concentration, with the non-electride CaRuSi exhibiting a notable reduction in this distance due to minimal anionic electrons. The catalytic activities in ARuSi, ACoSi, and AFeSi systems all increase with the expansion of A-A interlayer spacing. Furthermore, hydrogen storage properties, where anionic electrons are replaced by hydride ions, were evaluated. It is critical for N₂ hydrogenation. The hydrogen affinity, gauged by the desorption temperature, proved pivotal in determining catalytic efficiency, with optimal performance requiring balanced hydrogen binding strength. These findings provide critical insights for designing advanced catalysts for ammonia synthesis and potentially other hydrogenation reactions.