Catalytic Converter: Description and Operation

TWC R&R:

A three-way catalytic converter is used downstream of the exhaust manifold to reduce oxides of nitrogen (NOx) and oxidize hydrocarbons (HC) and carbon monoxide (CO). The converter contains the catalyst elements platinum, palladium, and rhodium, which promote oxidation and reduction reactions at high temperatures. Emissions are converted into carbon dioxide, water vapor and nitrogen. The system on this vehicle also uses a warm-up catalyst located immediately after the exhaust manifold.

THE OXIDATION REACTION
To change HC and CO into harmless materials, catalytic elements (platinum and palladium) start an oxidation, or burning, reaction in the catalytic converter. Oxidation is the addition of oxygen to an element or compound. During the process, oxygen (O2) mixes with excess HC and CO to form H2O and CO2. Considerable heat (900°F to 1,600°F) is generated by oxidation reduction. Use caution while working on or around the catalytic converter and other exhaust system components.

THE REDUCTION REACTION
Reduction is the opposite of oxidation. The reduction reaction changes NOx to harmless nitrogen (N) and free oxygen, which is then consumed in the oxidation reaction with other pollutants. Catalytic elements required for the reduction process are platinum and rhodium.

The Three-Way Catalytic Converter (TWC) is designed to react with all three major exhaust pollutants, but works best to reduce NOx when the CO level of the exhaust is between 0.8 and 1.5 percent. As the CO level increases or decreases from that percentage, converter efficiency decreases. Exhaust gas first passes through the catalyst, where oxides of nitrogen are reduced, then through the catalyst where HC and CO are changed to carbon dioxide and water vapor.