A catalytic oxidizer, or catalytic converter, is an automobile component found in the exhaust system. It oxidizes (burns) carbon monoxide and hydrocarbons, using platinum and palladium/rhodium as catalysts, while reducing (separating the oxygen atom rather than adding it) nitrogen oxides to create nitrogen. This greatly reduces toxic tailpipe emissions and reduces smog.
Catalytic oxidizers became widespread after regulations on automobile emissions were made mandatory nationwide in the U.S. in 1968. Now they are used in most cars around the world. Because catalytic oxidizers cannot operate in the presence of lead, their introduction caused leaded gasoline to be phased out. Catalytic oxidizers are also used in industrial processes to reduce harmful emissions, but their most common appearance is in automobiles.
Ideally the byproducts of an automobile engine are only carbon dioxide, water, and some nitrogen. This is similar to the chemical output of animals. But in practice, the combustion process in an engine is never 100% efficient, leaving behind hot, yet unburned hydrocarbons. Prior to the 1960s, these emissions were allowed to escape into the atmosphere, until it was realized that they were a public and environmental health hazard. Now, catalytic oxidizers fitted to a car's tailpipe rapidly oxidizes a large percentage of the remaining unburnt hydrocarbons, resulting in cleaner emissions. However, the speed at which catalytic oxidizers must operate to catch unburnt hydrocarbons before they fly out the tailpipe puts limits on how efficient the oxidation process can be.
The quality of catalytic oxidizers has increased steadily over the years, resulting in cars which are cleaner and cleaner. Still difficult is the lowering of CO2 (carbon dioxide) emissions. CO2 cannot be oxidized into anything more harmless, and it is a known greenhouse gas, contributing to global warming.
Catalytic oxidizers in automobiles operate at a relatively high temperature, around 750°F (400°C). In industry, catalytic converters can be dozens of times larger than those used in automobiles, and several times hotter. Standard subcomponents of catalytic oxidizers include a line burner, catalyst bed, and heat exchanger. The catalyst bed is usually in the form of either honeycombed ceramic or ceramic beads covered in the catalyst.