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Synthesis

Making ammonia with water and nitrogen

New approach uses samarium compound to weaken water’s bonds

by Bethany Halford
April 26, 2019 | APPEARED IN VOLUME 97, ISSUE 17

 

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Credit: Shutterstock
The energy-hungry Haber-Bosch process is used to make ammonia at large scale.

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.

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