Making ammonia with water and nitrogen

One of the world’s most important industrial chemical processes helps feed humanity, but it also gobbles up energy and generates a significant amount of carbon dioxide. The Haber-Bosch process is used to make ammonia, the raw material for nitrogen-based fertilizers. It combines N₂ from air and hydrogen gas using a catalyst, temperatures above 400 °C, and pressures around 40,000 kPa. Seeking a milder method for making NH₃, University of Tokyo chemists led by Yoshiaki Nishibayashi found they could use water or alcohols instead of H₂ as a source of H for the reduction of N₂. Using samarium diiodide (SmI₂), they weakened the water or alcohol’s O–H bonds so that they provided H atoms that reacted with N₂ in the presence of a molybdenum catalyst to make NH₃ (Nature 2019, DOI: 10.1038/s41586-019-1134-2). Not only does this reaction take place at ambient temperature and pressure, but it is also fast—each catalyst molecule produces more than 100 molecules of NH₃ per minute. Currently, the reaction isn’t suitable for industrial-scale production of NH₃: the large quantities of SmI₂ used generate a lot of waste, and it’s not trivial to separate the NH₃ from the solution in which it’s made. However, Nishibayashi and colleagues say the work presents a new direction for chemists to take when trying to make greener NH₃.