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

• Published in: Chemical science (Cambridge) 2016-01, Vol.7 (7), p.436-443
• Main Authors: Kitano, Masaaki, Inoue, Yasunori, Ishikawa, Hiroki, Yamagata, Kyosuke, Nakao, Takuya, Tada, Tomofumi, Matsuishi, Satoru, Yokoyama, Toshiharu, Hara, Michikazu, Hosono, Hideo
• Format: Article
• Language: English
• Subjects: Ammonia; Atmospherics; Catalysis; Catalysts; Chemistry; Hydrides; Hydrogen storage; Ruthenium; Synthesis (chemistry)
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c6sc00767h

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 − + x H ↔ [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. Ruthenium-loaded metal hydrides with hydrogen vacancies function as efficient catalysts for ammonia synthesis under low temperature and low pressure conditions.