A catalytic converter transforms the noxious hydrocarbons and nitrogen oxides in hot automobile exhaust into water, carbon dioxide, and nitrogen gas. It does this through the use of tiny platinum and rhodium particles that are deposited on the highly porous surface of a ceramic honeycomb. When hot hydrocarbon and oxygen molecules in the exhaust encounter a platinum particle in this honeycomb, they dissociate into molecular fragments on the particle's surface. These fragments then recombine into water and carbon dioxide molecules, which the return to the exhaust gas. Because of these rearrangements, the exhaust gas leaving the honeycomb is almost hydrocarbon free. Overall, platinum reduces the energy barrier-the activation energy-that impedes the conversion of the initial molecules into the final molecules. Platinum helps many types of molecules rearrange chemically and is so good at assisting combustion that many hydrocarbons burn at room temperature in the presence of platinum.
Rhodium's job is somewhat different. When nitrogen oxide molecules in the hot exhaust gas land on rhodium particles, they dissociate into nitrogen and oxygen atoms on the rhodium surface. These atoms then recombine into ordinary nitrogen and oxygen molecules, which soon leave the particles. Again, rhodium is acting as a catalyst-it assists a chemical reaction without actually being consumed by that reaction.
As long as the ceramic honeycomb doesn't crumble or plug, and nothing contaminates the surfaces of the platinum or rhodium particles, the catalytic converter will last indefinitely. But metals such as lead in the gasoline and overheating due to too much unburned fuel entering the converter will shorten its life considerably. Also, a catalytic converter works best in conjunction with an oxygen sensor in the engine exhaust and a computer to adjust the fuel/air mixture. Efficient burning of the residual hydrocarbons requires a good balance between hydrocarbon and oxygen molecules in the exhaust gas, so a car must carefully balance the fuel/air mixture if it wants clean exhaust.
Answered by Louis A. Bloomfield of the University of Virginia