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.