catalytic converter - an emissions control device, incorporated into an automobile's exhaust system, containing catalysts - such as platinum, palladium, or rhodium - that reduce the levels of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) emitted to the air. In the catalytic converter, HC and CO are oxidized to form carbon dioxide (CO2), and NOx are reduced to nitrogen and oxygen. Three-way catalytic converters that control all three substances require associated electronic controls for precise regulation of oxygen levels in the exhaust gas. Catalytic converters are also effective in removing PNA (polynuclear aromatic) hydrocarbons. Cars equipped with catalytic converters require unleaded gasoline, since the lead in tetraethyl lead, an antiknock compound, is a catalyst "poison." See emissions (automotive), hydrocarbon emissions, pollutants, lead alkyl.