Catalytic converter. Part of a catalytic converter from a car exhaust. This device reduces the toxic emissions from an internal combustion engine. It does this by converting the harmful exhaust by- products into relatively harmless ones. The inner surface is coated with an alloy containing platinum, rhodium and palladium. Oxidation catalysts convert hydrocarbons into carbon dioxide and water. Three-way catalysts convert oxides of nitrogen back into nitrogen. The addition of a catalytic converter to a car necessitates the use of unleaded fuel since the lead compounds added to regular petrol as anti-knocking agents can destroy the catalyst.