O2 dissociation on Ag(001): the role of kink sites

The dynamics of the dissociative adsorption of O2 on Ag(001) were investigated with a supersonic molecular beam source and electron energy loss spectroscopy versus surface temperature. Contrary to the case of Ag(110) where dissociation occurs at the atomic terraces and has a high probability, for Ag(001) we find that only 0.44% of the adsorbed molecules dissociate at room temperature. An Arrhenius analysis indicates that the process is thermally activated and the activation energy coincides with the energy for generating kinks, which are thus identified as the active sites. The interplay between a local geometry similar to a (110) site and enhanced charge transfer to the antibonding molecular orbitals is responsible for the pronounced reactivity of such sites. Molecules adsorbed at (001) terrace sites instead desorb with a high probability.