Catalytic converter is a device that reduces the exhaust pollutants produced by the engine of a motor vehicle. It uses catalysts, substances that speed chemical reactions, to convert harmful exhaust pollutants to harmless substances.
The combustion of gasoline and diesel fuels in engines produces three main gaseous pollutants: (1) nitrogen oxides, (2) hydrocarbons, and (3) carbon monoxide. Burned gases, including these harmful compounds, flow through the catalytic converter, which is installed in a vehicle’s exhaust system. Most catalytic converters use either three-way catalysts, which can remove all three pollutants at the same time, or oxidation catalysts, also called two-way catalysts, which can remove only hydrocarbons and carbon monoxide.
A typical catalytic converter consists of tiny, honeycomblike passages made of a ceramic or metal substrate (underlying material). The substrate is mounted within a stainless steel container. Thin layers of noble metals, such as platinum, palladium, and rhodium, coat the substrate and serve as catalysts. Noble metals do not corrode easily. As the exhaust gases flow over the coated passages, an oxidizing reaction takes place. Hydrocarbons and carbon monoxide are converted to water vapor and carbon dioxide through the addition of oxygen. In a converter with a three-way catalyst, a reducing reaction takes place at the same time. This reaction removes oxygen atoms from the nitrogen oxides, producing nitrogen and oxygen.
Catalytic converters are most effective when complete combustion of the fuel occurs. Sensors monitor the exhaust entering the catalytic converter and relay information to a computer in the engine. The computer directs adjustments of the fuel injection system to produce a fuel-air mixture that provides for complete combustion.
Catalytic converters are ineffective at lower temperatures, and they do not begin operating properly until the substrate is warmed to about 570 °F (300 °C). In addition, the presence of lead, phosphorus, or sulfur in the exhaust stream can reduce the effectiveness of the catalysts. Diesel engines use much more air for combustion than gasoline engines do. Abundant oxygen in the exhaust system renders catalytic converters essentially ineffective at reducing nitrogen oxides. For this reason, vehicles with diesel engines usually employ oxidation catalysts instead of three-way catalysts.
See also Automobile (The exhaust system) ; Clean Air Act ; Gasoline engine (Air pollution controls) .
