A chemical substance, which through its presence brings about chemical reactions or influences the course of such reactions but remains unchanged itself (is not consumed by the reaction).
A device installed in motor vehicle that removes ecologically harmful, toxic substances from the exhaust emissions.
In the case of spark-ignition engines, a catalytic converter in which three chemical reactions take place simultaneously is used to reduce the levels of pollutants in the exhaust emissions. The catalyst support (substrate) in these cats is coated with the precious metals rhodium and platinum or palladium. The rhodium component in the coating effects reduction of the nitrogen oxides (NOx) to molecular nitrogen (N2) and oxygen (O), while the oxygen released by this reaction then oxidises the carbon monoxide in the exhaust gases to form carbon dioxide and the hydrocarbons to form carbon dioxide and water by means of them reacting with the platinum or palladium. Ideally, the catalyst itself does not change.
Diesel engines function as a matter of principle with a leaner mixture, i.e. a higher air-fuel ratio, and their exhaust therefore contains a high level of oxygen. This means reduction of the nitrogen oxides by means of catalysis is not possible with a diesel engine. The catalytic converter for diesel engines (oxidation cat) oxidises carbon monoxide (CO) to form carbon dioxide (CO2) and hydrocarbons (HC) to form carbon dioxide (CO2) and water (H2O).
Last update on 2013-07-05 by Dominique Winkler.