May 232013

A 3-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 3-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

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.

If you still have any confusion in understanding the working of 3-way catalytic converter, refer the block diagram above or watch the animation video found below.

How a 3-way Catalytic Converter Works – Animation Video

If you have any other ideas about 3-way catalytic converter, you can share them by commenting on this article :)

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Written by

I am a mechanical engineer with a passion for technical stuff. I am the founder and former editor-in-chief of

  • diehard

    I am not a very technical person. I am just wondering whether all car cat. converters are

    “3-way Catalytic Converters” or are there other types used?? Thanks

    • Surjeet S

      Apart from 3-way catalytic converter, there is another type of catalytic converter called 2-way catalytic converter. It is typically used in diesel engine vehicles to control carbon-monoxide and hydrocarbon emission.

      • diehard

        Thanks very much for the information. So I guess it pretty clear that all catalytic converters on gasoline engine cars would be of the 3-way type.

  • RC

    Thanks Surjeet, very informative. I understand cerium oxide is used to replace a portion of the precious metals in the three-way – do you know if cerium is used in the reduction or oxidation catalyst component, or both? Thanks in advance.

  • Family Contos

    Thanks for the info, I’m being told to use a three way in the exhaust of a diesel car. Does a three way work on a diesel or will it fowl?
    to quickly

  • g.sivanjali

    i need information regarding catalytic converter catalyst. we want to change the catalyst material for catalytic converter to reduce the emission with low cost so could u please help us regarding this we need the material which we can use instead of palladium,rhodium, platinum/