Much of industrial chemical processing (in the petrochemicals industry,for example),and a great deal of laboratory chemical synthesis,involves catalysts that both lower the energy barrier to reaction and may help steer a reaction along a particular path.Traditionally,catalysts have come in two classes:heterogeneous,typically meaning that the catalyst is an extended solid;and homogeneous,where the catalyst is a small molecule that shares a solvent with the reactants.In heterogeneous catalysis,the reaction generally takes place on a surface,involving molecules attached there by covalent bonds.Homogeneous catalysts are often organometallic compounds,in which a metal atom or small cluster of atoms supplies the active site for reaction.