Essential role of hydride ion in ruthenium-based ammonia synthesis catalysts.

The efficient reduction of atmospheric nitrogen to ammonia under low pressure and temperature conditions has been a challenge in meeting the rapidly increasing demand for fertilizers and hydrogen storage. Here, we report that CaN:e, a two-dimensional electride, combined with ruthenium nanoparticles (Ru/CaN:e) exhibits efficient and stable catalytic activity down to 200 °C. This catalytic performance is due to [CaN]·eH formed by a reversible reaction of an anionic electron with hydrogen (CaN:e + H ↔ [CaN]·eH ) during ammonia synthesis. The simplest hydride, CaH, with Ru also exhibits catalytic performance comparable to Ru/CaN:e. The resultant electrons in these hydrides have a low work function of 2.3 eV, which facilitates the cleavage of N molecules. The smooth reversible exchangeability between anionic electrons and H ions in hydrides at low temperatures suppresses hydrogen poisoning of the Ru surfaces. The present work demonstrates the high potential of metal hydrides as efficient promoters for low-temperature ammonia synthesis.