The Adsorption, Desorption, and Exchange Reactions of Oxygen, Hydrogen, and Water on Platinum Surfaces. I. Oxygen Interaction

Abstract

The kinetics of adsorption and desorption of oxygen on a platinum filament have been studied by temperature-programmed desorption mass spectrometry. Oxygen adsorption becomes significant only after any carbon contamination is removed from the surface. At 300 °K oxygen adsorbs dissociatively into an atomic β-state which contains four overlapping sub-states. The initial sticking probability is 0.16 but this falls off rapidly with increasing coverage. A kinetic analysis for desorption from the β₂- and β₄-states gave Edes^≠(β₂) = 39 kcal mol⁻¹ and Edes^≠(β₄) = 54 kcal mol⁻¹ The desorption was first order suggesting a lack of mobility of the adatoms at the desorption temperature (∼700°K). No oxygen atom desorption could be detected in a line-of-sight experiment. Adsorption of a mixture of ³²O₂ and ³⁶O₂ resulted in complete isotopic mixing on desorption. At 115 °K, the β-state still populates first with the same initial sticking probability indicating adsorption is nonactivated. Moreover, the sticking probability remains at its high initial value for a much more extensive coverage range, suggesting a precursor state to adsorption. After the β₃- and β₄-states are fully occupied, further adsorption into the β-state is competitive with occupation of an α-state which desorbs at low temperatures (∼150°K) with first order kinetics and Edes^≠(α) ∼6 kcal mol⁻¹ No isotope mixing occurred in the α-state which is undoubtedly molecular. Prior population of the β₁- and β₂-states at 300 °K reduced the α-state adsorption at 115 °K suggesting that α and β occupy the same sites. Each site can adsorb either an atom or molecule since each molecule added to the β-state excludes two molecules from the α-state.