From adsorption at the surface to incorporation into subsurface sites: the role of steps for O/Ag

Many reactions like the ethylene epoxidation which work readily in industrial reactors can not be reproduced under controlled laboratory conditions. This failure is ascribed to so-called "pressure" and "structure" gaps. The latter is possibly determined by an active role of defects, since surface science experiments are performed on nearly perfect surfaces, quite unlike those of active catalytic powders. One of the mostly investigated, but so far unresolved systems, is O/Ag which shows a unique selectivity for the partial oxidation of ethylene. Subsurface oxygen was claimed to play a pivotal role in activating oxygen adatoms, but the mechanism of O incorporation remained mysterious. The study of O-2 adsorption at stepped surfaces and by supersonic molecular beams allowed for considerable progresses, showing that open steps favour dissociation and constitute a gateway through which subsurface sites can be accessed.