Green Ammonia
July 22, 2024
As I wrote in an
earlier article (Ammonia Synthesis, March 6, 2017),
ammonia is an important
industrial chemical. It's
estimated that the
worldwide production of ammonia in 2023 exceeded 150 million
metric tons. About 70% of ammonia is used to make
nitrate fertilizers,[1] and the rest is used for such things as
neutralization of
acids and
stack emissions, and the
manufacture of
nitric acid for
chemical synthesis. Unfortunately, the production of ammonia is
energy and
emissions intensive.
Ammonia production has a direct emissions
rate of about 2.4 metric tons per metric ton of production, four times that of
cement, which leads to direct emissions of 450 metric tons of
carbon dioxide annually.[1] There are also indirect carbon dioxide emission of about 170 metric tons of carbon dioxide from
electricity generation and the
chemical reaction when fertiliser is applied to
soil.[1] The production of ammonia accounts for about 2% of
energy consumption and 1.3% of carbon dioxide emissions from energy systems.[1]
Compounds of ammonia were known to the
ancients, and ammonia is named after the
Greek god,
Ammon, the
Greek version of the
Egyptian Sun god,
Amun-Ra.
Deposits of the ammonia
salt,
ammonium chloride (sal ammoniac), were found near the
temple of Ammon in
Libya.
Pliny the Elder mentions ammonium chloride
crystals of that region in his
Naturalis Historia (Natural History) (see figure). It wasn't until 1774 that
gaseous ammonia was first
isolated by
Joseph Priestley (1733-1804).
Claude Berthollet determined the
composition of ammonia,
NH3, in 1785. Ammonia was produced by simple
reaction of nitrates until the
20th century.
Ammonium chloride (sal ammoniac), as mentioned in Pliny's Natural History, Book XXXI, Chapter 39, line 79.[2] My translation of this reads, "For example, the region of Cyrenaica is notable, also, for hammoniacum, which is found beneath the sands. It is similar in color to the alum which is called schiston, it consists of long masses, not transparent, has a foul taste, but is useful as medicine." (Simulated manuscript created using GIMP.)
During
World War I,
Germany's supply from
Chile of
potassium nitrate (saltpeter), an ammonia
precursor and an essential
compound for
synthesis of
explosives, was disrupted. That was the
impetus for the industrial scale
development of the 1909
Bosch-Haber process for ammonia synthesis from
nitrogen gas.
Carl Bosch (1874-1940) of
BASF industrialized the synthesis of ammonia from its
elements,
N2 + 3H2 -> 2NH3, as discovered by
German chemist,
Fritz Haber (1868-1934).
While the reaction of
N2 and
3H2 to form
2NH3 is
exothermic (-45.9
kJ/
mol, -10.97
kcal/mole), and it has a favorable change in the
Gibbs free energy (-16.4 kJ/mol, -3.92 kcal/mol), it does not occur
spontaneously. The co-existence of free
hydrogen and free nitrogen in the
atmosphere affirms this. Haber
discovered in 1905 that a
catalyst will produced small quantities of ammonia from the elements at 1000 °C. The
catalyst for the Bosch-Haber process was
potassium-
promoted iron, but many other catalysts will work, including
osmium and
uranium. The synthesis is typically conducted at about 450
°C under 15-25
MPa (2,200-3,600
psi)
pressure. A
ruthenium catalyst allows for reaction at a lower pressure. Presently, about 180 million
metric tons of ammonia are synthesized annually by the Bosch-Haber process, and similar processes.[6] It's estimated that a third of annual global
food production uses ammonia from the Bosch-Haber process.
Why we need a catalyst for ammonia synthesis. Breaking nitrogen and hydrogen molecules into their constituent atoms takes a lot of energy, but all this energy is recovered in the end. (Wikimedia Commons image by Marsupilami. Click for larger image.)
Fritz Haber, who perfected the catalytic synthesis of ammonia from nitrogen gas and hydrogen gas, was a German chemist who received the
Nobel Prize in Chemistry in 1918.
American chemist,
Linus Pauling (1901-1994) was awarded the 1954 Nobel Prize in Chemistry, but he also won the reserved 1962
Nobel Peace Prize
for his
activism against nuclear weapons testing. Haber's activities, however, were the
antithesis of
peace, since he helped in the development of
chemical warfare during
World War I in contravention of the
Hague Convention of 1907. There's the
eponymous Haber's rule for the
dosage effect of
poisonous gas. Before the
nuclear age, poison gas was viewed as an
existential threat to human civilization. This can be surmised from the
H. G. Wells' film,
Things to Come.[3] Haber was widely
criticised by fellow scientists for such activities.
Fritz Haber in a BASF laboratory, circa 1914.
His 1914 bench apparatus is about the same as what I used in my freshman chemistry laboratory about fifty years later. Fortunately, there have been major improvements to laboratory instrumentation for chemistry since that time.
(Portion of a Wikimedia Commons image. Click for a larger image.)
Since the Bosch-Haber process is a process in widespread use that contributes to
global warming, it's a prime candidate for improvement. However, no major improvements in ammonia synthesis have appeared in the
century after its
discovery, despite awards of the Nobel Prize in 1931 and 2007 subsequent to Haber's 1918 prize. There was the 1931 chemistry prize to
Carl Bosch (1874–1940) and
Friedrich Bergius (1884–1949) for their "contributions to the
Invention and development of chemical high pressure methods;" and, the 2007 chemistry prize to
Gerhard Ertl (b. 1936) "for his studies of chemical processes on
solid surfaces."
Chemists have had their
chance, and it now appears that
physicists have decided to enter this
high-stakes game. A recent
study in
Science by physicists at the
Technical University of Denmark, led by
theoretical physicist,
Jens Nørskov and
experimental physicist,
Ib Chorkendorff, has looked at how
lanthanum atoms on a
cobalt catalyst
quench the cobalt
spin and reduce the
activation energy required to split molecular nitrogen
N2.[4-7] This mechanism could make cobalt a particularly good catalyst for ammonia synthesis.[6]
The Bosch-Haber process is energy intensive, often using temperatures up to 650 ° C and 200 atmospheres pressure to split the
triply bonded nitrogen atoms before reaction with hydrogen.[6] The iron catalyst for this process is inexpensive, but there's been
decades of research looking for a catalyst for ammonia synthesis in milder conditions.[6]
Nickel and cobalt are alternative catalyst materials, but these are generally not reactive with nitrogen without particular
promoters.[6]
The physicists at the Technical University of Denmark have found that
suppressing magnetism in cobalt activates the
metal as an ammonia catalyst.[4] Following up an
clues from
computational catalysis, the researchers demonstrated that lanthanum atoms quench the magnetic moment of adjacent cobalt atoms to lower the activation energy and enhance nitrogen
cleavage well below the temperature of the Bosch-Haber process.[4-6]
Lanthanum promoter on a cobalt surface for catalytic splitting of nitrogen. TS* is the transition state of molecular nitrogen, and the dissociated excited state nitrogen atoms (N*) diffuse along the surface to bond with three individual hydrogen atoms (not shown). (Created using Inkscape. Click for larger image.)
They did
experiments on cobalt
single crystals, and also mass-selected cobalt
nanoparticles at
ambient pressures and 350°C and found that the lanthanum-promoted cobalt catalyst is twice as effective in ammonia synthesis as the Bosch-Haber process.[4,6] Further experiments using cobalt
thin films and the
stepped surfaces of cobalt crystals showed that cobalt steps are the sites where lanthanum atoms are more effective at ammonia synthesis.[6] About half of the nitrogen molecules
adsorbed on the lanthanum-cobalt catalyst were converted to ammonia each
second at 350°C and 1 atmosphere.[4] However, the cost of both cobalt and lanthanum will prevent an immediate replacement for the Bosch-Haber process, but this concept may provide a
path to to creation of other catalysts.[6]
References:
- Ammonia Technology Roadmap, International Energy Agency (IEA), October 2021.
- Pliny the Elder, "Naturalis Historia," Book 31, Chapter 39, on the University of Chicago Penelope web site by Bill Thayer.
- Things to Come (1936, William Cameron Menzies, Director), on the Internet Movie Database.
- Ke Zhang, Ang Cao, Lau Halkier Wandall, Jerome Vernieres, Jakob Kibsgaard, Jens K. Nørskov, and Ib Chorkendorff, "Spin-mediated promotion of Co catalysts for ammonia synthesis," Science, vol. 383, no. 6689 (March 21, 2024), pp. 1357-1363, DOI: 10.1126/science.adn0558.
- Günther Rupprechter, "A milder reaction to feed the world," Science, vol. 383, no. 6689 (March 21, 2024), p. 1295, DOI: 10.1126/science.ado4095.
- R. Mark Wilson, "Creating a good ammonia catalyst from an unreactive metal, Physics Today, May 10, 2024, DOI:https://doi.org/10.1063/pt.ubla.wnhk.
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