Catalytic Converter: Description and Operation

Three Way Catalyst (TWC):

PURPOSE
Three-way catalytic converters simultaneously remove up to 90% of all three major emissions (HC, CO, and NOx). Complete catalytic reactions depend on fuel mixture staying within a narrow range (14.7:1 ± 1%). This is achieved with a properly functioning oxygen sensor system.

CONSTRUCTION
Catalytic converters consist of a metal housing, a ceramic grid substrate, and a catalytic coating of platinum and rhodium. Active metal content is about 2 grams of platinum/rhodium.

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

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

OPERATING ENVIRONMENT
Ideal operating temperature for maximum conversion and long service life is 750°F - 1500°F (400°C - 800°C). Engine malfunctions (misfires, etc.) can cause converter temperature to rise above 2500°F (1400°C). Such temperatures can lead to converter destruction by melting the substrate material.

CAUTION: DO NOT use leaded fuel except in emergencies. It will permanently render the converter ineffective. Lead compounds deposited in the pores and on the surface of the active material reduce or eliminate exposure to exhaust gasses. Excessive engine oil residues can also ruin the catalyst.