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 ( α 1, α 2, α 3) and two atomic adsorption states ( β 1, β 2) have been found. The β 2-state exhibits repulsive lateral interaction whereas the β 1-state shows attractive interaction. The adsorption kinetics at T ad = 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 s 0 = 0.29. This suggests that empty Pt sites near an O 2-covered Pt site experience an enhanced reactivity with O 2. T ad = 300 K the adsorption kinetics are governed by direct dissociative adsorption with an activation barrier of ≈2 kal/mol on Pt(111), yielding an initial sticking probability of 0.05, whereas a complicated adsorption behavior is obtained for Pt(112) with s 0 = 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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