High Electron Density on Ru in Intermetallic YRu₂: The Application to Catalyst for Ammonia Synthesis

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.