This page takes a brief look at the catalysts used in the Contact Process to manufacture sulphuric acid, in the Haber Process to manufacture ammonia, and in the conversion of ammonia into nitric acid.
The Contact Process for the manufacture of sulphuric acid
At the heart of the Contact Process is a reaction which converts sulphur dioxide into sulphur trioxide. Sulphur dioxide gas is passed together with air (as a source of oxygen) over a solid vanadium(V) oxide catalyst. This is therefore an example of heterogeneous catalysis.
The fact that this is a reversible reaction makes no difference to the operation of the catalyst. It speeds up both the forward reaction and the back reaction by the same amount.
The Haber Process for the manufacture of ammonia
The Haber Process combines hydrogen and nitrogen to make ammonia using an iron catalyst. This is another reversible reaction, and another example of heterogeneous catalysis.
The manufacture of nitric acid from ammonia
This is yet another example of heterogeneous catalysis.
This process involves oxidation of the ammonia from the Haber Process by oxygen in the air in the presence of a platinum-rhodium catalyst. Large sheets of metal gauze are used in order to reduce expense and to maximise the surface area of the catalyst. Although in principle the sheets would last for ever because the metals are acting as a catalyst, in practice they do deteriorate over time and have to be replaced.
The sheets of gauze are held at a temperature of about 900°C. The reaction is very exothermic, and once it starts the temperature is maintained by the heat evolved.
The ammonia is oxidised to nitrogen monoxide gas.
This is cooled. At ordinary temperatures and in the presence of excess air, it is oxidised further to nitrogen dioxide.
The nitrogen dioxide (still in the presence of excess air) is absorbed in water where it reacts to give a concentrated solution of nitric acid.