The adsorption of potassium on Pt(111) and its effect on oxygen adsorption

Abstract

The adsorption of potassium on Pt(111) followed by O 2 adsorption was studied by LEED, photoemission spectroscopy (UPS and XPS) and work function measurements. A monolayer of potassium on Pt(111) yields at 300 K an ordered overlayer exhibiting a 3 × 3 R30° LEED pattern. The potassium coverage is 0.33 corresponding to 5 × 10 14 K/cm 2. The initial sticking coefficient for the adsorption of O 2 on this surface at 300 K is drastically increased with respect to the clean surface. The binding energies of K(2p) and K(3p) levels decrease, that of the O(1s) level increases with increasing oxygen coverage. The maximum oxygen coverage achievable for θ k = 0.33 is 0.95. Oxygen adsorption induces a contraction in the potassium layer such that θ k rises locally to 0.44. The work function decreases initially during this phase from 2.1 to 1.85 eV, followed by an increase up to 3.2 eV at saturation. The adsorption of oxygen is described by two states: a strongly bound KO state (not K 2O) and a more weakly bound PtO state similar to oxygen chemisorbed on clean Pt(111). The strongly bound oxygen resides in a KO layer of a likely stoichiometry 1:2 which on annealing to 570 K gives rise to an ordered 4× 4 LEED pattern. This layer promotes the chemisorption of O 2 on clean Pt(111) such that coverages up to unity result.