Catalytic converters forced lead out of American petrol
Lead is a catalyst poison: it coats the precious metals inside a catalytic converter and stops them working. So when American rules put converters on most petrol cars from the 1975 model year, the antiknock additive tetraethyl lead had to go too, and cleaner exhaust brought lower lead levels in the air.
The idea is older than most drivers realise. French engineers built prototype converters in the late 1800s, back when oil-fuelled cars numbered only in the thousands, coating clay with platinum, rhodium and palladium. Later Eugene Houdry, the French inventor of catalytic cracking in oil refining, moved to the United States in 1930, grew worried after early studies of Los Angeles smog, and developed converters for smokestacks, forklifts and eventually car engines in the mid-1950s. Engineers at Engelhard Corporation produced the first production converter in 1973.
Early units were two-way: they combined oxygen with carbon monoxide and unburned hydrocarbons to make carbon dioxide and water. They could not tackle nitrogen oxides, which feed smog and acid rain. Makers briefly bolted on a clumsy twin-catalyst system, until engineers noticed that the first catalyst also cleaned up some carbon monoxide and hydrocarbons. That insight, plus electronic engine management, led to the three-way converter, first sold by Volvo on California-specification 240 cars for 1977. When federal rules tightened on nitrogen oxides for 1981, almost every carmaker adopted it.
Inside is usually a ceramic honeycomb, most often cordierite, a substrate invented at Corning Glass whose creators entered the National Inventors Hall of Fame in 2002. A rough washcoat spreads the catalytic metals across a huge surface. Rhodium handles reduction, palladium handles oxidation, and platinum does both. Nickel is banned in the European Union because it can react with carbon monoxide to form a toxic compound.
The chemistry is finicky. A converter needs around 400 degrees Celsius to work well, which is why it sits close to the engine, and a three-way unit only performs at its best when the fuel-to-air mix stays very close to the chemically ideal balance. Run slightly lean and nitrogen oxide control collapses; run slightly rich and carbon monoxide and hydrocarbons slip through. Dead converters are recycled for their platinum, palladium and rhodium.
Source: Catalytic converter
