Catalytic Converters

Since January 1993, all new petrol-driven cars sold in the European Union (EU) have been fitted with a catalytic converter (CAT). This is made up of a very thin layer of platinum group metals on a honeycomb structure. The surface area of a typical 3-way CAT covers the equivalent of two football pitches. As exhaust gasses pass through the catalyst a chemical reaction occurs which converts carbon monoxide (CO), volatile organic compounds (VOCs, including hydrocarbons) and nitrogen oxides (NOx) to less harmful compounds (water, nitrogen and carbon dioxide).

To work most effectively, a catalytic converter needs to reach an optimum temperature. It may not reach this in a short journey. Devises to pre-warm the catalyst are being developed which improve the overall performance of catalytic converters.

The use of catalytic converters leads to a dramatic reduction in the emissions of carbon monoxide, hydrocarbons and nitrogen oxides. However, they also result in an increase in carbon dioxide (CO₂) emissions, which do not cause a problem for urban air quality, but may contribute to global warming. The efficiency of a CAT can be as high as 90%.

Oxidation catalysts may be fitted to either petrol or diesel cars. The catalyst oxidises the pollutants formed by incomplete combustion to carbon dioxide and water, and is effective for hydrocarbons and carbon monoxide. However, they do not reduce nitrogen oxides emissions.