Effect of preadsorbates on the dissociation dynamics of ethane on Ir(110)

We have examined the role of the preadsorbates oxygen and potassium in modifying the dynamics and kinetics of ethane activation on Ir(110). Although the effect of these adsorbates is commonly interpreted within the context of promotion and poisoning, we have followed a quantitative approach in determining the influence of O on the trapping-mediated chemisorption and that of O and K on the direct dissociative chemisorption of this alkane. Using a supersonic molecular beam, the direct dissociative chemisorption of perhydrido- and perdeuteroethane on Ir(110) was investigated using translational beam energies of 12-30 kcal/mole, parametric in oxygen and potassium precoverage from the clean surface to the adsorbate-saturated surface. We have observed that the probability of direct dissociative chemisorption decreases in the presence of dissociatively adsorbed oxygen as well as potassium. The magnitude of the poisoning is considered within a one-dimensional tunneling model and the extent of poisoning of both oxygen and potassium is quantified with a model that yields a range parameter of 8.5 Angstrom for O and 13 Angstrom for K. For trapping-mediated chemisorption using beam energies of 1-5 kcal/mole, preadsorption of 0.06 monolayers O was found to lower the activation energy of perhydrido- and perdeuteroethane activation by 400 and 300 cal/mole, respectively, with no effect on the preexponential factor, while a surface saturated with oxygen lowered these activation energies by 800 and 700 cal/mole, also with no effect on the preexponential factor. (C) 1996 American Vacuum Society.