HYDROGEN ADSORPTION ON OXYGEN-COVERED PT(111)

Adsorption of hydrogen on oxygen covered Pt(111) is investigated in the temperature range 300-600 K by titration of adsorbed atomic oxygen with hydrogen from a supersonic beam. In most experiments the conditions were such that hydrogen adsorption was rate limiting for oxygen coverages larger than 10% of the saturation coverage. In that case, the hydrogen sticking probability is equal to the water formation rate per incident molecule. Activated and non-activated adsorption are observed. The two processes show qualitatively different dependences on oxygen coverage. The probability for non-activated adsorption does not depend on the coverage of disordered oxygen, but it increases with increasing order (on a scale of 2-4 atoms) of the adsorbed oxygen layer. The probability for activated adsorption decreases with oxygen coverage and is not sensitive to the order of the oxygen layer. Atomic steps change, above all, the adsorption characteristics at low coverages. We can exclude that steps are involved in the non-activated process. Under the experimental conditions in which hydrogen accumulates on the surface to a non-negligible coverage, we observe a phase separation between the hydrogen and oxygen, indicating a decreased binding energy of H atoms inside O islands on the surface.