Catalytically active centers are pure surface chemistry: atoms, ions, atom clusters, adsorbed molecules, chemisorbed groups, crystal surfaces ... on support materials. They are the sites of a catalyst where chemical reactions take place. These centers are crucial for activity & selectivity as they attract the reactants, activate them and facilitate conversion into products. Important centers are platinum (Pt), palladium (Pd) and rhodium (Rh), which are often used in the catalytic conversion of hydrocarbons. Other well-known centers are titanium oxide (TiO₂), zinc oxide (ZnO), iron oxide (FeOx), nano-Cu, ...

In homogeneous catalysis, i.e. where reactants and catalyst are liquid and dissolved, special molecular forms of e.g. Pt compounds or aromatics are used for catalysis.