The catalytic converter is the longest-established and most widely used exhaust gas aftertreatment device. It is located in the exhaust system so that hot exhaust gases flow through it and come into close contact with catalytic material which promotes various reactions between the constituents of the exhaust gas. These reactions can be divided into two categories – 'oxidation' and 'reduction'.
(oxides of nitrogen)
It can be seen from the above that in the oxidation reactions, pollutants which arise from partial combustion of the fuel are oxidised to form non-toxic products of complete combustion. In effect, a second combustion process has occurred which has compensated for any failures of the process which occurred in the cylinders of the engine. In the reduction reaction, oxides of nitrogen, which were the product of an unwanted reaction occurring between atmospheric oxygen and nitrogen in the engine, are broken down into their constituent parts.
Catalytic converters are often described as 'two-way' or 'three-way', the difference deriving from the type of catalytic material they are coated with. Two way catalytic converters have coatings which only promote oxidation reactions, whilst the three-way versions have coatings which give rise to both oxidation and reduction reactions.