CO and O2 Adsorption on K/Pt (111)

Abstract

We have investigated CO and O2 on K promoted Pt(111) using mainly molecular beams and TPD. Surprisingly, CO adsorption is little affected by the presence of potassium on the surface up to around 1/6th of a monolayer promoter coverage (half of the maximum), even though the chemisorbed state in TPD is more strongly adsorbed. This is because the adsorption is dominated by the precursor state kinetics which are little affected by the K. However, as the K coverage increases and the layer changes from ionic to metallic, then the adsorption of CO diminishes to zero, since both CO and K are electron donors. In contrast the opposite is the case for oxygen adsorption. The sticking probability on the clean surface is very low, but is enormously enhanced at even low coverages of K, and oxygen can adsorb strongly onto the metallic layer to high coverage. A number of oxygen states can be identified in TPD, those desorbing in the same temperature range as for clean Pt, those associated with Pt affected by K at low coverage of the latter, and those on the metallic layer. The former is dissociated oxygen while the latter is molecular peroxide.