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 Ca(2)N:e(–), a two-dimensional electride, combined with ruthenium nanoparticles (Ru/Ca(2)N:e(–)) exhibits efficient and stable catalytic activity down to 200 °C. This catalytic performance is due to [Ca(2)N](+)·e(1–x)(–)H(x)(–) formed by a reversible reaction of an anionic electron with hydrogen (Ca(2)N:e(–) + xH ↔ [Ca(2)N](+)·e(1–x)(–)H(x)(–)) during ammonia synthesis. The simplest hydride, CaH(2), with Ru also exhibits catalytic performance comparable to Ru/Ca(2)N:e(–). The resultant electrons in these hydrides have a low work function of 2.3 eV, which facilitates the cleavage of N(2) 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.