Nitric Acid

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 catalyst.

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 standard.