Abstract
A comparative study of oxygen adsorption on Pt(111) and Pt(112) has
been performed using temperature programmed desorption, isothermal
desorption, Auger spectroscopy, LEED and isotopic measurements. On
Pt(112) three molecular adsorption states (alpha1, alpha2, alpha3)
and two atomic adsorption states (beta1, beta2) have been found.
The beta2-state exhibits repulsive lateral interaction whereas
the beta1-state shows attractive interaction. The adsorption kinetics
at Tad = 87 K involves a precursor state. For Pt(112) at 87 K, the
sticking coefficient is 0.97 at zero coverage and remains constant
in the low coverage regime. On Pt(111) at 87 K, the sticking coeffient
increases with increasing oxygen coverage at low coverage, with s0
= 0.29. This suggests that empty Pt sites near an O2-covered Pt site
experience an enhanced reactivity with O2. Tad = 300 K the adsorption
kinetics are governed by direct dissociative adsorption with an activation
barrier of approximate2 kal/mol on Pt(111), yielding an initial
sticking probability of 0.05, whereas a complicated adsorption behavior
is obtained for Pt(112) with s0 = 0.53. The conversion of molecular
oxygen into atomic oxygen is discussed as well as the influence of
subsurface oxygen and #clean-off# effects on the adsorption kinetics.
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