Industrial Catalysis

Manufacture of nitric acid

The manufacture of nitric acid is a three-stage process. The first stage is the oxidation of ammonia and it uses a platinum/rhodium catalyst.

Summary of conditions and catalyst for ammonia oxidation

world annual production

approximately 60 million tonnes

gas mixture

10% ammonia + 90% air

pressure

4 to 10 atmospheres

temperature

975 - 1225 K

catalyst

gauze of 90% platinum 10% rhodium

conversion

96%

   
Uses of nitric acid

The three main stages in the manufacture of nitric acid

ammonia oxidation
4NH3 + 5O2
4NO + 6H2O

DH = - 900 kJmol-1

nitric oxide oxidation
2NO + O2
2NO2
DH = - 115 kJmol-1
 
2NO2
N2O4

DH = - 58 kJmol-1

dinitrogen tetroxide absorption
3N2O4 + 2H2O
4HNO3 + 2NO
DH = - 103 kJmol-1

 

Reaction sequence to produce nitric acid.

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Oxidation of ammonia. Ammonia vapour is mixed with compressed air (10% and 90% respectively). The pressurised mixture is passed over a catalyst gauze of 90% platinum and 10% rhodium. About 96% conversion is achieved.

Electron micrograph of platinum/rhodium catalyst showing large surface area of gauze