Reactions of N and NO on Pt„335..

As part of a study of species important in automotive exhaust chemistry, the reactivity of atomic N and NO on Pt~335! at low temperature has been studied. The atomic N was produced by dissociating adsorbed NO with a 76 eV electron beam. Cross sections for electron-stimulated desorption and dissociation are estimated for NO on terrace and step sites. Terrace NO is at least five times more likely to desorb than to dissociate. Step NO has a lower desorption cross section than terrace NO, but probably a higher dissociation cross section. Temperature-programmed desorption was used to monitor desorption, dissociation, and the formation of N2 and N2O from adsorbed N and NO. Five distinct desorption states of N2 formed by NO dissociation are identified. The dominant N2 peak ~435 K! comes from electron-dissociated step NO; its desorption temperature is higher than the N2 peaks from electron-dissociated terrace NO. Coadsorbed N and NO react to form N2O even below 100 K, with an activation barrier of ;6 kcal/mol. Only terrace NO participates in this reaction; step NO does not react to form N2O. This site dependence resembles that for CO oxidation on Pt~112! and Pt~335! and can be rationalized with simple steric considerations. All of the forms of atomic N participate in N2O formation, but that formed by the dissociation of step NO exhibits the lowest reaction temperature. Hence, the same N atoms that only recombine to form N2 at 435 K, react with NO to form N2O at 100 K. We found no evidence for an NO reaction with N atoms to form N2 and adsorbed O, or for NO formation from the recombination of adsorbed N and adsorbed O2. © 1997 American Institute of Physics. @S0021-9606~97!01246-4#