How does a catalytic converter clean a car's exhaust?

A catalytic converter cleans a car's exhaust by using precious metals to speed up chemical reactions that turn toxic gases into less-toxic ones. It is an exhaust emission control device: it converts toxic gases and pollutants from an internal combustion engine into less-toxic pollutants by catalysing a redox reaction. The early two-way converters, which most petrol cars carried from the 1975 model year in the US, combine oxygen with carbon monoxide and unburned hydrocarbons to make carbon dioxide and water. Three-way converters, first commercialised by Volvo on its 1977 California-specification 240 cars, also reduce nitrogen oxides, which are precursors to acid rain and smog.

The metals matter. Rhodium is used as a reduction catalyst, palladium as an oxidation catalyst, and platinum for both. Because of those valuable metals and its external location, a converter is a target for thieves. It also needs heat: a converter requires about 400 degrees Celsius to work effectively, so it is placed as close to the engine as possible.

The story began in a worry about smog. The French engineer Eugene Houdry, an expert in catalytic oil refining, became concerned about exhaust and air pollution when early studies of smog in Los Angeles were published, and he founded a company called Oxy-Catalyst.

There was a catch for lead. Lead is a catalyst poison that coats the catalyst's surface, so the rules that required converters also led to the removal of tetraethyl lead from petrol.