Exposure of Pt(5 5 3) and Rh(1 1 1) to atomic and molecular oxygen: do defects enhance subsurface oxygen formation?

Subsurface oxygen is known to form in transition metals, and is thought to be an important aspect of their ability to catalyze chemical reactions. The formation of subsurface oxygen is not, however, equivalent across all catalytically relevant metals. As a result, it is difficult to predict the stability and ease of the formation of subsurface oxygen in metals, as well as how the absorbed oxygen affects the chemical and physical properties of the metal. In comparing how a stepped platinum surface, Pt(5 5 3), responds to exposure to gas-phase oxygen atoms under ultra-high vacuum conditions to planar Rh(1 1 1), we are able to determine what role, if any, steps have on the capacity of a metal for subsurface oxygen formation. Despite the presence of regular defects, we found that only surface-bound oxygen formed on Pt(5 5 3). Alternatively, on the Rh(1 1 1) surface, oxygen readily absorbed into the selvedge of the metal. These results suggest that defects alone are insufficient for the formation of subsurface oxygen, and the ability of the metal to absorb oxygen is the primary factor in the formation and stabilization of subsurface oxygen.