Three-way catalysts (TWC) are designed to simultaneously convert three pollutants to harmless emissions:

  • Carbon Monoxide (CO) → Carbon Dioxide (CO2) 
  • Hydrocarbons (HC) → Water (H2O) 
  • Oxides of Nitrogen (NOx) → Nitrogen (N2)

A three-way catalyst can cut CO, HC and NOx by over 99% if the air to fuel ratio is accurately controlled.

Three-way catalysts will only give full conversion of each of these three pollutants in a gas stream containing a stoichiometric ratio of CO and NOx as shown in the chart below.

Rich/Lean Catalyst Conversions

TWC Rich Lean Catalyst Conversion content image.jpg

There are many reaction pathways in a three-way catalyst system, including those shown for the oxidation.

The most important reactions for removing NOx can be summarised as:

2CO + 2NO → 2CO2 + N2[HC] + NO →N2 + CO2 + H2