A catalyst is another substance than reactants products added to a reaction system to alter the speed of a chemical reaction approaching a chemical equilibrium. It interacts with the reactants in a cyclic manner promoting perhaps many reactions at the atomic or molecular level, but it is not consumed. Another reason for using a catalyst is that it promote the production of a selected product. A catalyst that is in a separate phase from the reactants is said to be a heterogeneous, or contact, catalyst. Contact catalysts are materials with the capability of adsorbing molecules of gases or liquids onto their surfaces. An example of heterogeneous catalysis is the use of finely divided platinum to catalyze the reaction of carbon monoxide with oxygen to form carbon dioxide. This reaction is used in catalytic converters mounted in automobiles to eliminate carbon monoxide from the exhaust gases.