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A low-temperature ammonia synthesis process is required for on-site synthesis. Barium-doped calcium amide (Ba-Ca(NH ) ) enhances the efficacy of ammonia synthesis mediated by Ru and Co by 2 orders of magnitude more than that of a conventional Ru catalyst at temperatures below 300 °C. Furthermore, the presented catalysts are superior to the wüstite-based Fe catalyst, which is known as a highly active industrial catalyst at low temperatures and pressures. Nanosized Ru-Ba core-shell structures are self-organized on the Ba-Ca(NH ) support during H pretreatment, and the support material is simultaneously converted into a mesoporous structure with a high surface area (>100 m g ). These self-organized nanostructures account for the high catalytic performance in low-temperature ammonia synthesis.

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Ruthenium is the most effective catalyst reported to date for ammonia synthesis under mild conditions, especially when an electron promoter is used. However, electron donation from the promoter has not been sufficient because the promoter contacts with Ru only through its surface. Here, we report a Laves phase intermetallic bulk catalyst, YRu2, which has higher electron density on Ru. This is derived from large electron transfer from Y to Ru, which is first confirmed by X-ray absorption fine structure measurements and theoretical calculations. In addition, YRu2 has high hydrogen solubilities leading to suppression of hydrogen poisoning, a common drawback of Ru-based catalysts. Consequently, YRu2 exhibits higher catalytic activity for ammonia synthesis over 300 times that with pure ruthenium. The present results suggest a simple concept for ammonia synthesis: Laves phase intermetallic compounds of Ru and more electropositive metals are more efficient catalysts than pure Ru because of the large electron promotion and suppression of hydrogen poisoning.

The rate-determining step for ammonia synthesis over Ru catalysts supported by electrides, such as [Ca24Al28O64]4+(e−)4 and Ca2N:e−, is suggested to be the surface reactions of N and H adatoms, in which case the Langmuir–Hinshelwood model should be used to describe the kinetics.

We report the successful synthesis of LaCu0.67Si1.33 and Y5Si3 electride nanoparticles (NPs) using the Ar/H2 arc evaporation technique. The obtained particle sizes are 10–50 nm in diameter, and their surface areas are enhanced by orders of magnitude compared with hand-milled samples. Their catalytic performances were indeed improved by a factor of 60 for the hydrogenation of nitrobenzene (LaCu0.67Si1.33) and 3 for ammonia synthesis (Ru-loaded Y5Si3). This marked difference in the enhancement of catalytic activities between each system can be attributed to the geometric structure of active sites. These results show that the Ar/H2 arc evaporation technique offers wide versatility for the preparation of rare-earth-based electride NPs with enhanced catalytic activities.

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