Catalytic Converter: Description and Operation

Three-Way Catalyst Operation:

PURPOSE
Two separate three way catalytic converters simultaneously remove up to 90% of all three major pollutants, (HC, CO, and oxides of nitrogen). A complete catalytic reaction depends on the fuel mixture staying within a very narrow range ±1% of 14.7:1 which can only be achieved with a properly functioning oxygen sensor system.

CONSTRUCTION
Each catalytic converter consists of a metal housing, a ceramic grid substrate, and a catalytic coating of platinum and rhodium. The active metal content is about 2 grams of platinum/rhodium.

OPERATION
As the exhaust gasses containing HC and CO pass through the converter in the presence of oxygen, the platinum catalyst starts the oxidation (burning) process. The HC and CO then unite with oxygen to form water vapor and carbon dioxide. This oxidation process has no effect on the NOx emissions.

To reduce the oxides of nitrogen (NOx), a separate reaction is necessary called "reduction." A reduction reaction is the removal of oxygen from a material. In Three Way Catalyst type converters, rhodium is used as a catalyst to break down oxides of nitrogen into nitrogen and oxygen. The effective conversion of pollutants begins at about 250°C (480°F).

Due to distance from the combustion chamber, the front TWC reaches operating temperature much faster than the rear TWC. The early operation of the front TWC causes faster operation of the O2 sensors and the rear TWC.