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How a Three-Way Catalytic Converter Works


A3-way catalytic converter is a sophisticated device used in the exhaust system of many modern petrol engine vehicles. It converts harmful gases in the engine exhaust to relatively harmless gases.

Why should we use three-way catalytic converters?

The exhaust gases from an engine contain harmful substances such as oxides of nitrogen (NOx), carbon monoxide (CO) and Hydrocarbons (HC). These substances produce extreme environment hazards. 3-way catalytic converters convert these harmful substances to less harmful nitrogen (N2), carbon-di-oxide (CO2) and water (H2O).


A three-way catalytic converter makes use of two catalysts to convert harmful gases to harmless gases. They are:

  1. Reduction Catalyst and
  2. Oxidation Catalyst

The reduction catalyst is made of platinum and rhodium while the oxidation catalyst is made of platinum and palladium. Both the catalysts have a ceramic honeycomb structure.

Block Diagram of a 3-way Catalytic Converter


Stage 1 – Reduction Catalyst:

The exhaust gases are first sent over the reduction catalyst (which is made of platinum and rhodium). It converts oxides of nitrogen (NOx) to nitrogen (N2) and oxygen (O2). The following reactions take place when the exhaust gases pass over the reduction catalyst.

2NO → N2 + O2

2NO2 → N2 + 2O2

The reduction catalyst simply rips off nitrogen and oxygen from the oxides of nitrogen. As you might know, nitrogen and oxygen are harmless gases while oxides of nitrogen are really harmful to the environment.

Stage 2 – Oxidation Catalyst:

Exhaust gases that are free of oxides of nitrogen (NOx) are then sent over the oxidation catalyst (made of  platinum and palladium). The oxidation catalyst coverts carbon-monoxide (CO) and hydrocarbons (HC) in the gases into carbon-di-oxide (CO2) and water (H2O).

The following reactions takes place when the exhaust gases pass over the oxidation catalyst:

2CO + O2→ 2CO2

HC + O2 → CO2 + H2O

Note: The second reaction (above) is a generalized reaction. In it, HC stands for hydrocarbon. HC might be methane, ethane or other hydrocarbon.

The gases that finally come out of the catalyst chamber are N2, CO2, and H2O. 3-way catalytic converters are so named because they are capable of eliminating three pollutants – NOx, CO and HC.

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