How catalytic converters work
Despite being a bit foolhardy, auto companies are still using stimulus money to manufacture the jet pack from The Rocketeer. Image courtesy of lukey.com.au
Catalytic converters are devices which help reduce the harmful content in your vehicle's emissions, and they are mandatory inclusions on all vehicles built after 2004. First introduced in 1975, and included in many vehicles since then, the catalytic converter is a chamber through which exhaust much pass, and reactions taking place in its core convert harmful chemicals into more benign ones. Nitrogen oxides, which lead to acid rain, hydrocarbons, which contribute to the creation of smog, and the poisonous carbon monoxide are all removed by these converters. Because there are three main types of pollutants removed by modern converters, they are sometimes referred to as “three-way catalytic converters.”
The core consists of a honeycomb structure, which is usually constructed from ceramic, although stainless steel is sometimes used. The core is honeycombed because this type of design allows for maximum surface area inside the limited space of the exhaust pipe. The entire core is covered with a washcoat, which is a rough layer of silica and alumina, that serves to increase the surface area even more, ultimately allowing for more catalytic reactions to take place.
The catalyst itself consists of at least two different materials, which are often a combination of precious metals. Platinum in conjunction with rhodium or palladium are common choices, but depending on the type of engine the catalytic converter will be used with, other metals such as manganese or cerium. These different combinations of metals are applied to different sections of the core, in order to catalyze the removal of different types of toxins from the exhaust. Between these two different sections, many catalytic converters have an air injection tube, which helps not only to get the converter up to the minimum temperature needed to perform reactions, but also provides the oxygen needed for some of the catalytic reactions.
The reactions which take place can be classified as one of two types: reduction and oxidation. In a reduction reaction, a molecule, like nitrogen oxide, loses electrons and breaks apart in order to form two different, less polluting molecules, in this case, nitrogen and oxygen. In an oxidation reaction, molecules, like carbon monoxide or gasoline hydrocarbons, gain electrons and combine with oxygen in order to form the harmless byproducts of carbon dioxide and water. The palladium and other metals used in the catalytic converter are not actually used up in these reactions; they simply provide ideal conditions for these reactions to occur at lower temperatures than they normally would.
—Seth Berger
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