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.
