• Reduction of NOx  
• Oxidation of CO
• Oxidation of hydrocarbons

These three reactions occur simultaneously primarily when the exhaust from the  engine running is very slightly rich. 
 
The technologies which enable this emission reduction have traditionally been supported precious metals (Pt, Rh and Pd) on a flow through monolith substrate. The support for these catalysts is typically a ceria/lanthanum oxide which reacts with the oxygen and other species in the exhaust gas to enhance the reductive activity of the TWC catalyst.