A catalyst is a chemical substance that affects the rate of a chemical reaction by altering the activation energy required for the reaction to proceed.
Catalysts permit an alternative mechanism for the reactants to become products,with a lower activation energy and different transition state. A catalyst may allow a reaction to proceed at a lower temperature or increase the reaction rate or selectively. Catalysts often react with reactants to form intermediates that eventually yield the same reaction products and regenerate the catalysts. Note that the catalyst may be consumed during one of the intermediate steps,but it will be created again before the reaction is completed.