The nature of the catalytic sites for H2 dissociation

Scanning Tunneling Microscopy (STM) can reveal the nature of active sites on the surface of heterogeneous catalysts. This is shown for the case of the dissociation of molecular hydrogen on Pd(111), which has been studied recently both experimentally and theoretically. STM can image in real time to generate movies of adsorbed atoms diffusing on the catalyst surface and forming aggregates. Of particular interest is the behavior near saturation coverage, a situation that is common when catalysts operate under the gas pressures typical of many industrial reactions. Under these conditions, active catalyst sites are formed as a result of density fluctuations that free atoms at the catalyst surface of adsorbates, so that they become available for new reactions. Little is known about the structure of the sites generated in this process. While the end state of a dissociative adsorption of a diatomic molecule requires at least two empty sites to accommodate the reaction products, the initial state where the molecule adsorbs and dissociates, might be more complicated and its nature is unknown. The review shows how STM can provide an improved understanding of the nature of these initial sites.