Three-way catalyst

Johnson Matthey's three-way catalysts prevent air pollution from rich-burn and stoichiometric gas engines.

Three-way catalytic converters are effective in preventing air pollution from rich-burn and stoichiometric engines fuelled by natural gas, propane and gasoline. Three-way catalysts are designed to perform multiple oxidation reactions and reduction reactions simultaneously to convert air pollutants that are present in the exhaust to harmless gases.

The most important three-way reactions for reducing NOx, CO and hydrocarbons (represented by CH4) are:


  • CO + ½O2 → CO2
  • CH4 + 2O2 → CO2 + H2O


  • 2CO + 2NO → 2CO2 + N2
  • CH4 + 4NO → 2N2 + CO2 + 2H2O

Exhaust gas composition depends on the air to fuel ratio (A/F) at which the engine operates. Rich mixtures (Lambda<1) produce high concentrations of NOx, CO and HCs. The exhaust gases of lean fuel mixtures (Lambda>1) contain reduced amounts of NOx and HCs.

The three-way reactions take place over specially formulated Pt-Rh catalysts that allow the reduction of NOx by CO and the oxidation of CO and HC by oxygen to occur simultaneously. The catalyst functions most efficiently when the engine runs rich, slightly below the stoichiometric point (Lambda = 1). Three-way catalysts are typically used with an A/F controller to maintains a slightly rich mixture. These controllers use a feedback signal from an oxygen sensor located in front of the catalyst and are required for the 3-way catalyst to operate properly.

Johnson Matthey's three-way catalysts are coated on metallic foil and offer the ultimate in flexibility. They are available in standard sizes for use with our MODULEXTM and DUALOX® catalytic converters, or in customised sizes to fit existing housings. Whether first fit, retrofit or replacement catalyst element, Johnson Matthey offers the optimum three-way catalytic converter to prevent air pollution from your engine.

General enquiries

Joseph McCarney

Betty Ann Keen