Rhodium is used in catalytic converters in cars. A catalyst is something that speeds up a chemical reaction, by participating in the reaction, but the catalyst is not consumed by the reaction. The energy to propel a car comes from the combustion (burning) of carbon-containing compounds. When these carbon compounds are burned, carbon dioxide (CO2) is produed. Sometimes, however, the poisonous gas carbon monoxide (CO) is produced instead of CO2. Rhodium in a catalytic converter catalyzes (speeds up) the chemical reaction that converts any CO produced into CO2:
CO + O2 –> CO2