Oxygen exchange between NO gas and adsorbed atomic oxygen on Pt(335)

We have used temperature-programmed desorption with isotopically labeled gases to study O exchange between gas phase NO and adsorbed atomic O on Pt(335). We find two distinct types of adsorbed O, one of which exchanges at least 40 times faster than the other, at room temperature. Based on their relative concentrations, we tentatively identify the more active species as O at the step edge and the less active one as O at terrace sites. The temperature dependence of the faster exchange rate implies two parallel reaction pathways. Above 240 K, the exchange rate increases with temperature with an apparent activation energy of 3.8 kcal mol⁻¹. At lower temperatures the exchange rate is nearly temperature-independent, with an apparent activation energy near zero but a very low pre-exponential factor. These results are interpreted in terms of a competition between oxygen exchange and NO desorption. The low-temperature process probably requires special sites or adsorbate configurations. The room temperature exchange rates of O₂ gas with preadsorbed atomic O, and with NO at edge sites, are more than 100 times slower than for NO gas and adsorbed O.