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

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