A three-way catalyst (TWC) is an essential component in modern vehicle exhaust systems fitted with SI engines; such
as gasoline, CNG, ethanol blend engines; designed to reduce harmful substances before they are released into the
atmosphere from the tail-pipe.
Key Functions:
The TWC addresses three primary pollutants:
- Carbon monoxide (CO): A toxic gas resulting from incomplete combustion.
- Nitrogen oxides (NO?): Contribute to smog and acid rain and are harmful to respiratory health.
- Unburned hydrocarbons (HC): A contributor to smog.
Operating Principles:
The TWC facilitates catalytic reactions using a ceramic substrate coated with precious metals like platinum (Pt), palladium (Pd), and rhodium (Rh). These metals act as catalysts for the following reactions:
- Reduction of NO? (Rhodium): converts nitrogen oxides into nitrogen and carbon dioxide 2NO+2CO→N2+2CO2
- Oxidation of CO (Platinum, Palladium): converts carbon monoxide into carbon dioxide. 2CO+O2→2CO2.
- Oxidation of HC (Platinum, Palladium): converts unburned hydrocarbons into carbon dioxide and water. CxHy+O2→CO2+H2O
Conditions for Efficiency:
- For gasoline engines, the TWC performs optimally at a stoichiometric mixture, ensuring sufficient oxygen for the reactions.
- The catalyst requires temperatures between 250°C and 800°C to function effectively.