Exhaust gases from a car engine contain a mixture of harmful chemicals. They pass over a ceramic honeycomb structure located inside the exhaust pipe. This honeycomb is specially coated with rare metals like platinum, palladium, and rhodium.
These metal coatings act as a catalyst to speed up chemical changes. They provide a huge surface area for a process called heterogeneous catalysis. Heterogeneous means the solid catalyst is in a different phase than the gas.
The metals speed up these chemical reactions but are never consumed themselves. Platinum and palladium catalyze oxidation reactions that add oxygen to exhaust molecules. They turn toxic carbon monoxide and unburned hydrocarbons into carbon dioxide and water.
Rhodium handles reduction reactions that strip oxygen away from nitrogen oxide molecules. This reduction step safely converts those harmful nitrogen oxides into harmless nitrogen gas. The honeycomb shape forces all the flowing gases to touch the coated walls.
This clever physical design maximizes the overall reaction rate for the system. However, these chemical reactions still require a high amount of activation energy. The catalyst surface must reach a threshold temperature of roughly 300 to 400 °C.
Because of this heat requirement, converters are less effective during a cold start. A common misconception is that catalytic converters simply filter gases like a sieve. In reality, they break chemical bonds and create completely new, safer substances.