A catalytic converter is an exhaust emission control device that reduces toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalysing a redox reaction
The main purpose of a catalytic converter is to reduce exhaust emissions.
Below is a diagram of catalyst converter:
Fig.: Catalyst Converter
Types:
I. Two-way
II. Three way
I. Two-way catalytic converter
A 2-way (or "oxidation", sometimes called an "oxi-cat") catalytic converter has two simultaneous tasks:
i. Oxidation of carbon monoxide to carbon dioxide:
2CO + O2 → 2CO2
ii. Oxidation of hydrocarbons (unburnt and partially burned fuel) to carbon dioxide and water:
CxH2x+2 + [(3x+1)/2] O2 → x CO2 + (x+1) H2O
(a combustion reaction)
This type of catalytic converter is widely used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were also used on gasoline engines in American- and Canadian-market automobiles until 1981. Because of their inability to control oxides of nitrogen, they were superseded by three-way converters.
II. Three-way catalytic converter
Three-way catalytic converters (TWC) have the additional advantage of controlling the emission of nitric oxide (NO) and nitrogen dioxide (NO2) (both together abbreviated with NOx), which are precursors to acid rain and smog.
The reduction and oxidation catalysts are typically contained in a common housing; however, in some instances, they may be housed separately. A three-way catalytic converter has three simultaneous tasks:
i. Reduction of nitrogen oxides to nitrogen (N2)
· 2 CO + 2 NO → 2 CO2 + N2
· hydrocarbon + NO → CO2 + H2O + N2
· 2 H2 + 2 NO → 2 H2O + N2
ii. Oxidation of carbon monoxide to carbon dioxide
· 2 CO + O2 → 2 CO2
iii. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water, in addition to the above NO reaction
· hydrocarbon + O2 → H2O + CO2
These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point.
Three-way converters are effective when the engine is operated within a narrow band of air-fuel ratios near the stoichiometric point, such that the exhaust gas composition oscillates between rich (excess fuel) and lean (excess oxygen).
