The role of steps in the dynamics of dissociative adsorption at surfaces

The dynamics of dissociative adsorption of hydrogen and oxygen on the Pt(533) (or 4(111) x (100)) surface has been studied under UHV conditions with a supersonic molecular beam.For hydrogen, the magnitude and characteristics of direct route to dissociation are very similar to that observed on the Pt(111) surface, and only at incident energies below 150meV does the behaviour deviate from the defect free surface. At incident energies up to 18meV trapping can occur into a physisorbed state that is fully accommodated. This state can act as a precursor to dissociation and displays typical precursor behaviour in surface temperature dependence and surface coverage dependence. At incident energies up to 150meV a third channel to adsorption exists. This channel is direct in nature, and may either be due to an unaccommodated precursor to dissociation or a direct channel where hydrogen molecules steer into the active step site.Oxygen adsorption of Pt(533) proceeds in the main part via the sequential precursor route to dissociation proposed for adsorption on the Pt(111) surface. The increased reactivity for oxygen on Pt(533), over that observed for Pt(111), is interpreted as being a direct result of the presence of a high density of steps on the Pt(533) surface. The steps provide low activation sites for the conversion of the physisorbed precursor to the chemisorbed precursor, and also promote the dissociation of the chemisorbed molecule. In addition to the two precursor channels described by the sequential precursor model, a highly activated direct channel to dissociation is present which is believed to proceed exclusively at step sites.