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 Ca₂N:e^-, a two-dimensional electride, combined with ruthenium nanoparticles (Ru/Ca₂N:e^-) exhibits efficient and stable catalytic activity down to 200 °C. This catalytic performance is due to [Ca₂N]⁺·e_1-x^-H_x^- formed by a reversible reaction of an anionic electron with hydrogen (Ca₂N:e^- + xH ↔ [Ca₂N]⁺·e_1-x^-H_x^-) during ammonia synthesis. The simplest hydride, CaH₂, with Ru also exhibits catalytic performance comparable to Ru/Ca₂N:e^-. The resultant electrons in these hydrides have a low work function of 2.3 eV, which facilitates the cleavage of N₂ 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.