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

We have used temperature-programmed desorption with isotopically labeled ga ses to study O exchange between gas phase NO and adsorbed atomic O on Pt(33 5). We find two distinct types of adsorbed O, one of which exchanges at lea st 40 times faster than the other, at room temperature. Based on their rela tive concentrations, we tentatively identify the more active species as O a t the step edge and the less active one as O at terrace sites. The temperat ure dependence of the faster exchange rate implies two parallel reaction pa thways. Above 240 K, the exchange rate increases with temperature with an a pparent activation energy of 3.8 kcal mol(-1). At lower temperatures the ex change rate is nearly temperature-independent, with an apparent activation energy near zero but a very low pre-exponential factor. These results are i nterpreted in terms of a competition between oxygen exchange and NO desorpt ion. The low-temperature process probably requires special sites or adsorba te configurations. The room temperature exchange rates of O-2 gas with prea dsorbed atomic O, and with NO at edge sites, are more than 100 times slower than for NO gas and adsorbed O. (C) 1999 Elsevier Science B.V. All rights reserved.