Catalytic Converter: Description and Operation

Fig. 7 Three Way Catalytic Converter:

PURPOSE
The three way catalytic converter removes up to 90% of the three major pollutants, (HC, CO, and oxides of nitrogen).

CONSTRUCTION
The 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
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.

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

Reduction
To reduce the oxides of nitrogen (NOx), a separate reaction called "reduction" is necessary. A reduction reaction is the removal of oxygen from a material. In Three-Way type converters, rhodium is used as the catalyst to break down oxides of nitrogen into its components, nitrogen and oxygen.

Operating temperature
The effective conversion of pollutants begins at an operating temperature of about 250°C (480°F). The ideal operating temperature for maximum conversion and long service life is 400°C - 800°C (750°F - 1500°F).

Engine malfunctions, for example misfires, can cause the temperature of the converter to increase to more than 1400°C (2500°F). Such extreme temperatures may destroy the converter by melting the substrate material.

CAUTION: The use of leaded fuel will permanently render the converter ineffective. Lead compounds used in leaded fuels are deposited in the pores and on the surfaces of the active material, reducing or eliminating exposure of active material to the exhaust gases. Excessive engine oil residues can also ruin the catalyst.