Catalytic Converter: Description and Operation

PURPOSE / OPERATION
The three-way catalytic converter reduces HC, CO and NOx. Two separate catalyst beds are used to achieve this reduction of emissions. 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. Some models incorporate two separate converters.
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).