Using scanning tunneling microscopy, thermal energy atom scattering, and density functional theory we have characterized $\mathrm{O}{}_2$ dissociation on Pt(111) stepped surfaces at the atomic scale. The most reactive site is at the top of the Pt steps. In both the molecular precursor state (MPS) and the transition state (TS), the $\mathrm{O}{}_2$ has its axis aligned parallel to the step edge. Controlled step decoration with Ag monatomic chains was used to locally tune the reactivity of Pt step sites. The enhanced reactivity at the Pt step sites is not caused by a decrease of the local dissociation barriers from the MPS but is related to a stabilization of both the MPS and TS.

• Received 14 April 2000

DOI:https://doi.org/10.1103/PhysRevLett.87.056103