Nitric acid manufacture and uses
Many chemical processes employ platinum group metal catalysts to
improve the efficiency of various reactions. Platinum-based
catalysts have now been used in the commercial manufacture of
nitric acid for a century.
Nitric acid is made in three stages. The first step is the
oxidation of ammonia gas with air to form nitric oxide. In order to
achieve a high conversion efficiency, this is normally carried out
at pressure over a platinum-rhodium catalyst. The nitric oxide is
cooled and further oxidised to form nitrogen dioxide, which is then
absorbed in water to nitric acid.
The principal end-use of nitric acid is in the production of
nitrogen fertilizers, an important source of plant nutrients.
Non-fertilizer uses include the production of: explosive-grade
ammonium nitrate; adipic acid, for making nylon, and toluene
diisocyanate, for manufacturing polyurethane.
Platinum group metal catalyst gauze
Pgm catalysts for nitric acid production take the form of a
gauze made out of fine wire. When nitric acid was first produced
commercially in 1904, a platinum-only catalyst was used. Rhodium
was later added for strength and to reduce the amount of platinum
lost during conversion of the gas.
Palladium-based "catchment" or "getter" gauze was introduced in
1968 to further reduce losses of platinum and rhodium, which can be
as high as 300 mg per tonne of acid produced. The catchment sits
downstream of the gas flow and collects pgm vapourised from the
Until 1990, the catalysts and catchments used in ammonia
oxidation were in the form of woven gauze. Johnson
Matthey Noble Metals then introduced a revolutionary
knitted gauze, increasing the efficiency of conversion and
extending the catalyst life. This has since become the industry