Dynamics of the gas-surface interaction in presence of well defined defects

The active sites for catalytic reactions in heterogeneous catalysis are often minority sites related to defects at the surface. However, in most studies performed so far under controlled conditions, nearly perfect low Miller index surfaces were used, giving rise to the so-called structure gap in the surface science approach to catalysis. In order to overcome these limits and understand how defects affect reactions, surfaces damaged on purpose by ion bombardment or surfaces aligned along high Miller index planes are studied. In this paper we shall discuss the cases of C2H4 and O-2 interacting with Ag(4 10). We shall demonstrate that the open steps are in both cases active sites for adsorption and in the case of O-2 also for dissociation, while flat (100) planes are much less reactive. Using a supersonic molecular beam, which allows to define exactly the angle of incidence and the impact energy of the gas-phase molecules, we show that the distribution of the activation barriers to adsorption at defects can be measured. For O-2/Ag(4 10) we find that its height is definitely smaller at open steps than at (100) terraces. The O-2 sticking probability at terraces is moreover strongly reduced, implying that the reactivity of the Ag atoms is influenced by the terrace width.