In chemistry, a catalyst is a substance that causes or accelerates a chemical reaction without itself being affected. Catalysts participate in the reactions, but are neither reactants nor products of the reaction they catalyze.
In the human body, enzymes are naturally occurring catalysts responsible for many essential biochemical reactions.
In the catalytic converter, there are two different types of catalyst at work: a reduction catalyst and an oxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium.
The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream while also minimizing the amount of catalyst required, as the materials are extremely expensive.
Most modern cars are equipped with three-way catalytic converters. This refers to the three regulated emissions it helps to reduce.
The Reduction Catalyst
The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions.
When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:
2NO => N2 + O2 or 2NO2 => N2 + 2O2
The Oxidation Catalyst
The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example:
There are two main types of structures used in catalytic converters: honeycomb and ceramic beads. Most cars today use a honeycomb structure.
