A catalytic converter turns the harmful emissions from your car's exhaust into less harmful gases before they are pumped out into the atmosphere. It contains a chemical catalyst (usually precious metals, such as platinum and rhodium), ceramic or metallic carrying materials, casing and various regulatory devices for controlling the process. Depending on the design, the catalytic converter oxidises carbon monoxide to carbon dioxide and water and/or reduces nitrogen oxides to nitrogen and oxygen gas. Efficient catalytic converters reduce the levels of pollutants in exhaust gas by up to 90 per cent. Cars use three-way catalytic converters (for petrol engines) and oxidising catalytic converters (for diesel engines)  featuring up to two main and four primary converters, depending on the engine.

Article source: www.volkswagen.co.uk