HYDROGEN ADSORPTION ON OXYGEN-COVERED PT(111)

Adsorption of hydrogen on oxygen covered Pt(111) is investigated in th e temperature range 300-600 K by titration of adsorbed atomic oxygen w ith hydrogen from a supersonic beam. In most experiments the condition s were such that hydrogen adsorption was rate limiting for oxygen cove rages larger than 10% of the saturation coverage. In that case, the hy drogen sticking probability is equal to the water formation rate per i ncident 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 increasin g order (on a scale of 2-4 atoms) of the adsorbed oxygen layer. The pr obability 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 e xclude 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 th e hydrogen and oxygen, indicating a decreased binding energy of H atom s inside O islands on the surface.