Models in heterogeneous catalysis: Surface science quo vadis?

Model catalyst systems have been prepared by growth of small metal aggregates on thin well ordered oxide films of alumina and silica. These systems lend themselves to structural and morphological characterization via scanning probe microscopies and transmission electron microscopy and bridge to a certain extent the materials gap between metal single crystal studies and the investigation of real catalyst samples. Recently, the classical surface science techniques applied under ultra-high vacuum conditions have been augmented by non-linear optical techniques, such as sum frequency generation, which can also be applied under ambient gas pressures. Thus, the pressure gap between studies in surface science under realistic conditions can be bridged.