How Does a Catalytic Converter Work?

A three-way catalytic converter (standard in gasoline cars since the 1970s) performs three simultaneous reactions: (1) oxidation of carbon monoxide to CO2 (2CO + O2 -> 2CO2), (2) oxidation of unburned hydrocarbons to CO2 and water (CxHy + O2 -> CO2 + H2O), and (3) reduction of nitrogen oxides to nitrogen and oxygen (2NOx -> xO2 + N2). These reactions are thermodynamically favourable but too slow at exhaust temperatures without a catalyst. The converter contains a honeycomb ceramic or metallic substrate with about 400 cells per square inch, coated with a washcoat of alumina (for surface area) and precious metals (typically 1-3 grams of platinum, 1-7 grams of palladium, and 1-2 grams of rhodium). The high surface area (equivalent to about 2 football fields per converter) ensures exhaust gases contact the catalyst efficiently. The converter reaches operating temperature (250-300 degrees Celsius) within about 2 minutes of engine start — it is least effective during cold starts.