Catalysts are chemicals that speed up reactions, by providing a lower energy pathway, thereby increasing the rate of reactions. These can be classified as heterogeneous and homogeneous. Heterogeneous catalysts are those that have a different phase than the reactants, while homogeneous catalysts have the same phase as the reactants.
An example of heterogeneous catalyst is platinum which is used in catalytic converters of automobiles to convert harmful gases (Such as carbon monoxide and nitrogenous oxides) into less harmful products. Chemically, this catalysis can be given as:
`2 CO + 2 NO + (Pt) -> 2 CO_2 + N_2`
Homogeneous catalysts are generally liquids or gases. An example is the reaction of chlorofluorocarbons (CFCs) with ozone that destroys ozone and may cause extreme events like ozone hole. Chemically, it can be written as:
`Cl. + O_3 -> ClO. + O_2`
Here, a chlorine free radical (generated by breakdown of CFCs) destroys ozone and generates new free radical and oxygen. This is the reason, they have been banned in a large number of countries.
Hope this helps.