Active sites in heterogeneous catalysis: development of molecular concepts and future challenges

The concept that catalytic turnover occurs at a small fraction of the surface sites dates back to the 1920s. The application of modern surface science techniques and model catalysts confirmed the presence of active sites and identified their structures in some cases, Low coordination defect sites on transition metals, steps and kinks, or open "rough" crystal faces that make high coordination metal sites available have been uniquely active for breaking H-H, C-H, C-C, C=O, O=O and Nequivalent toN bonds. Oxide-metal inter-faces provide highly active sites for reactions of C-H and C=O bonds. Electron acceptor and proton donor sites are implicated in hydrocarbon conversion (acid-base catalysis), and sites where metal ion-carbon bonds can form are active for polymerization. The observations of dynamic restructuring of catalytic surfaces upon adsorption of reactants indicate that many catalytic sites are created during the chemical reaction. Similar restructuring is detected for enzyme catalysts. The high mobility of both surface metal atoms and adsorbed molecules during the catalytic process observed recently bring into focus the dynamic nature of active sites that may have a finite lifetime as they form and disassemble. The development of techniques that provide improved time resolution and spatial resolution, and can be employed under catalytic reaction conditions will provide information about the time dependent changes of active site structure and molecular intermediates at these active sites as the reaction products form.