REACTIONS OF N AND NO ON PT(335)

As part of a study of species important in automotive exhaust chemistr y, 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 w ith a 76 eV electron beam. Cross sections for electron-stimulated deso rption and dissociation are estimated for NO on terrace and step sites . Terrace NO is at least five times more likely to desorb than to diss ociate. Step NO has a lower desorption cross section than terrace NO, but probably a higher dissociation cross section. Temperature-programm ed desorption was used to monitor desorption, dissociation, and the fo rmation of N-2 and N2O from adsorbed N and NO. Five distinct desorptio n states of N-2 formed by NO dissociation are identified. The dominant N-2 peak (435 K) comes from electron-dissociated step NO; its desorpt ion temperature is higher than the N-2 peaks from electron-dissociated terrace NO. Coadsorbed N and NO react to form N2O even below 100 K, w ith an activation barrier of similar to 6 kcal/mol. Only terrace NO pa rticipates 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 th e dissociation of step NO exhibits the lowest reaction temperature, He nce, the same N atoms that only recombine to form N-2 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 N-2 and adsorbed O, or for NO formation from the recombination of adsorbed N and adsorbed O-2. (C) 1997 American Instit ute of Physics.