Catalytic converters use a reduction and an oxidation catalyst organized in a honeycomb structure to react with the harmful emissions present in exhaust gas. The reduction catalyst converts nitrogen dioxide into nitrogen and oxygen, while the oxidation catalyst converts the unburned hydrocarbons and carbon monoxide into carbon dioxide.
The reduction catalyst is the first stage of the converter and uses platinum and rhodium to reduce nitrogen oxide emissions. The catalyst removes the nitrogen atom from the molecule and frees the oxygen atom; the nitrogen atoms then combine, forming a nitrogen gas molecule. Platinum and palladium are the materials that form the oxidation catalysts, which is the second stage of the converter. This catalyst converts unburned hydrocarbons and carbon monoxide molecules into carbon dioxide by burning them, which combines the molecules with the remaining oxygen in the exhaust gas.
Catalysts are substances that accelerate a chemical process without being used up. They are organized in a honeycomb structure because a honeycomb shape provides the maximum amount of surface area, allowing the catalysts to have more exposure to the exhaust stream and thus minimizing the amount of catalyst required. This is important because the catalyst materials are expensive. In order to work effectively, the catalytic converter must be at high temperatures to allow the chemical reactions to take place.