For the past 100 years, the Haber-Bosch process has been used to convert atmospheric nitrogen into ammonia, which is essential in the manufacture of fertilizer. Despite the longstanding reliability of the process, scientists have had little understanding of how it actually works. But now a team of chemists, led by Patrick Holland of the University of Rochester, has new insight into how the ammonia is formed. Their findings are published in the latest issue of Science.
Holland calls nitrogen molecules "challenging." While they're abundant in the air around us, which makes them desirable for research and manufacturing, their strong triple bonds are difficult to break, making them highly unreactive. For the last century, the Haber-Bosch process has made use of an iron catalyst at extremely high pressures and high temperatures to break those bonds and produce ammonia, one drop at a time. The question of how this works, though, has not been answered to this day.
"The Haber-Bosch process is efficient, but it is hard to understand because the reaction occurs only on a solid catalyst, which is difficult to study directly," said Holland. "That's why we attempted to break the nitrogen using soluble forms of iron."
Holland and his team, which included Meghan Rodriguez and William Brennessel at the University of Rochester and Eckhard Bill of the Max Planck Institute for Bioinorganic Chemistry in Germany, succeeded in mimicking the process in solution. They discovered that an iron complex combined with potassium was capable of breaking the strong bonds between the nitrogen (N) atoms and forming a complex with an Fe3N2 core, which indicates that three iron (Fe) atoms work together in order to break the N-N bonds. The new complex then reacts with hydrogen (H2) and acid to form ammonia (NH3) -- something that had never been done by iron in solution before.
Despite the breakthrough, the Haber-Bosch process is not likely to be replaced anytime soon. While there are risks in producing ammonia at extremely high temperatures and pressures, Holland points out that the catalyst used in Haber-Bosch is considerably less expensive than what was used by his team. But Holland says it is possible that his team's research could eventually help in coming up with a better catalyst for the Haber-Bosch process -- one that would allow ammonia to be produced at lower temperatures and pressures.
At the same time, the findings could have a benefit far removed from the world of ammonia and fertilizer. When the iron-potassium complex breaks apart the nitrogen molecules, negatively charged nitrogen ions -- called nitrides -- are formed. Holland says the nitrides formed in solution could be useful in making pharmaceuticals and other products.
- M. M. Rodriguez, E. Bill, W. W. Brennessel, P. L. Holland. N2 Reduction and Hydrogenation to Ammonia by a Molecular Iron-Potassium Complex. Science, 2011; 334 (6057): 780 DOI: 10.1126/science.1211906
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