Dynamics of the interaction of O2 with stepped and damaged Ag surfaces

The role of defects in catalytic reactions, especially those involving low rates and high degrees of selectivity, has been debated ever since the very early days of surface science. However, most studies on gas-surface interaction and chemisorption performed under controlled laboratory conditions have dealt so far with nearly perfect low-Miller-index surfaces, which are rather unlike the active powders employed as catalysts in industrial reactors. The failure in reproducing some chemical reactions, which occur readily under industrial conditions, was therefore often ascribed to the so-called structure gap separating the surface science approach from the real industrial conditions. Overcoming this limit without losing control over the experiment at the nanoscopic scale is therefore an issue of pivotal importance. It can be attacked by studying adsorption on either single-crystal surfaces damaged by ion bombardment or on surfaces aligned along high-Miller-index planes. In this paper we shall discuss O-2 interaction with Ag(410), a vicinal surface of Ag(100) characterized by open (110)-like steps and by (100) terraces. The use of a supersonic molecular beam to dose O-2 allowed us to gain information on the interaction of the gas-phase molecules with steps and terraces separately, by selecting the angle of incidence and the impact energy. The open steps turned out to be active sites for dissociation, while flat Ag(100) planes are unreactive. For molecular adsorption a reduction in the activation barrier is observed at the steps, while the Ag(100) nanoterraces are much less reactive than wide (100) planes. The main results are confirmed by the preliminary investigation of O-2/Ag(210).