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 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,” says 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 and acid to form ammonia—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.
