Kinetics and Energetics of Oxygen Adsorption on Pt(111) and Pt(112). A Comparison of Flat and Stepped Surfaces

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 (α1, α2, α3) and two atomic adsorption states (β1, β2) have been found. The β2-state exhibits repulsive lateral interaction whereas the β1-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 ≈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 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.