To most people, a catalytic converter means part of the exhaust system on a car, included to reduce pollution. However, catalytic converters may also be used in many other situations where there is a hot gas stream containing pollutants. Therefore they are used in many applications involving cooking, heating, incineration or the burning of solid fuels to release energy.
Catalysis is a branch of chemistry where certain chemicals known as ‘catalysts’ are used in order to encourage or promote reactions between other chemicals. A key point to appreciate is that the catalysts themselves are not consumed by the reactions, they merely facilitate them at a lower temperature than would otherwise be possible. Therefore a catalyst can theoretically last forever. The name ‘catalytic converter’ is applied to a practical device which has been designed to make use of catalytic technology i.e. it enables the benefits of catalysis to be obtained in real-life situations through a particular chemical transformation taking place at a practical temperature.
Various chemical reactions can be promoted by catalysis, but where the aim is to improve air quality the most important reaction is oxidation. The majority of air pollutants are organic in nature (i.e. they contain carbon) and can be oxidised to produce carbon dioxide, which is less harmful than any of the alternatives. The equation below shows the oxidation of carbon monoxide to carbon dioxide.
2CO + O2 → 2CO2
Many organic pollutants contain carbon and hydrogen atoms, for example the formula for methane is CH4. When methane is oxidised it results in carbon dioxide and water, as shown in the following formula:
CH4 + 2O2 → CO2 +2H2O
Most organic air pollutants can be oxidised using a catalytic converter, as long as the chemistry is correctly worked out and the flue gas temperature is in the appropriate range.