three-way-catalysts

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.