How 3-Way Catalysts Work
The 3-way catalyst consists of three distinct parts: the substrate, washcoat and active metal. The catalyst substrate, sometimes referred to as the catalyst carrier, is a ceramic structure which gives the catalyst its honeycomb shape and support for the active metal. The washcoat is a coating applied to the substrate which increases the surface area of the catalyst structure prior to the application of the active metal. The active metal is the primary contributor to the catalytic reaction of the pollutants and is typically palladium or platinum. The precious, noble, metal applied to the catalyst structure is the active component in the combustion reaction. The catalyst material is not consumed during the catalytic reaction but simply promotes the process.
The 3-way catalyst has three simultaneous reactions taking place:
|
2NOx à xO2 + N2 (Reduction)
CO + 1/2 O2 à CO2
(Oxidation)
[Hydrocarbons] + O2 à CO2 + H2O (Oxidation)
|
 |
2NOx à xO2 + N2 (Reduction)
CO + 1/2 O2 à CO2 (Oxidation)
[Hydrocarbons] + O2 à CO2 + H2O (Oxidation)
The reactions result in a simultaneous removal of the pollutants through a reduction or oxidation reaction depending on the rich or lean state of the engine. A lean fuel mixture results in a high concentration of NOx in the exhaust while a rich fuel mixture produce high concentrations of CO and HC. In order to maintain the simultaneous destruction of all three pollutants a fuel mixture close to stoichiometric is required. As the fuel mixture deviates from stoichiometric the conversion of NOx or CO will deteriorate. To maintain the air-to-fuel mixture at the appropriate levels, an oxygen sensor and air-to-fuel ratio controller is usually implemented
As with oxidation catalysts, temperature of the reaction affects efficiency. As a general rule, higher temperatures lead to greater destruction efficiencies with the EPA recommending a minimum exhaust temperature at the catalyst of 450 F.
