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The demand and the desire to fix
nitrogen to make explosives, as well as fertilisers, led
to the development of chemical processes to produce ammonia,
nitric acid and nitrates from the distillation of coal and
the by-products of other industrial chemical processes.
Early processes (1900 - 1920) were
the Arc Process and the Cyanamid Process, both highly energy
intensive and uncompetitive relative to the later Haber-Bosch
process.
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Alternative Catalysts
In 1992, M.W. Kellog and the Ocelot
Ammonia Company started ammonia production using a new ruthenium
catalyst deposited on an active carbon support. It is referred
to as the Kellog Advanced Ammonia Process (KAAP). This new
technology can be retro-fitted to existing ammonia plants
and operates at lower temperatures and pressures, giving
considerable savings in both money and energy. Typically,
a pressure of 40 atmospheres can be used, instead of 100
- 250 atmospheres. The newer catalyst is more expensive,
but this is outweighed by other cost reductions.
In 2000, scientists at the Danish
company Haldor - Topsøe announced the discovery of
a new commercially viable catalyst for the Haber reaction.
These involve ternary nitrides such as Fe3Mo3N;
Co3Mo3N and Ni2Mo3N.
These compounds, with caesium promoters, appear to have
catalytic activities 2 or 3 times greater than present commercial
iron based catalysts at the same operating conditions. They
are also considerably cheaper than ruthenium based catalysts
of the KAAP process. The same Haldor - Topsøe team
have produced ruthenium catalysts with Barium promoters
that are 2.5 times more active than current ruthenium catalysts.
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Taking a Leaf from Nature
Atmospheric nitrogen is "fixed" at normal temperatures
and pressures by several natural organisms. The enzyme nitrogenase
is responsible for this. Nitrogenase contains clusters of
iron, molybdenum and sulphur (Fe/Mo/S). Several artificial
clusters have been made that show catalytic activity, and
may lead eventually to ammonia synthesis at room temperature
and normal atmospheric pressure.
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The Breakthrough
Although nitrogen gas is readily and cheaply
available from air, it proved very difficult to break the nitrogen
triple bond. It was not until 1909, after considerable effort
worldwide, that Fritz Haber managed to fix nitrogen from the air,
by reacting it with hydrogen. The secret was in the catalyst used
and Haber and Carl Bosch systematically tested some 20,000 catalysts,
before succeeding with some iron ore from Kiruna in Sweden. By
chance, this ore contained trace amounts of alkali metal compounds
and the catalyst used today in the Haber- Bosch process, iron
oxide with potassium hydroxide promoter, is not so very different
to that discovered by Haber and Bosch.
Click here for more information
on this process
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For further details on use of enzyme-based catalysis,
see the catalysis
site.
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