Catalytic converter

A catalytic converter is a vehicle emissions control device which converts toxic byproducts of combustion in the exhaust of an internal combustion engine to less toxic substances by way of catalysed chemical reactions. 
The specific reactions vary with the type of catalyst installed. Most present-day vehicles that run on gasoline are fitted with a “three-way” converter, so named because it converts the three main pollutants in automobile exhaust: carbon monoxide, unburned hydrocarbon and oxides of nitrogen. 
The first two undergo catalytic combustion and the last is reduced back to nitrogen.