Quantitative determination of the activity of defect sites on a single-crystalline surface: C–H bond activation of carbon-13 labeled ethane on Ir(111)

The C–H bond activation of carbon-13 labeled ethane 1,2-di-13C–C2H6 at defect sites on an Ir(111) single-crystalline surface, cut and polished to 0.70°±0.15° of the (111) direction, has been determined quantitatively. These results have been obtained from a kinetic model accounting for diffusion from sites on the less reactive terraces to step edges on the surface, combined with the appropriate trapping-mediated description of the overall rate processes operative at the defects. The activation energy for reaction at the defect sites was found to be 4500±1500 cal/mol, which is ∼6000 cal/mol less than the reaction barrier at the terrace sites. The preexponential factor of the reaction rate coefficient at the defect sites was found to lie between 5×1011 and 1012 s−1, which is five to ten times greater than the preexponential factor of the reaction rate coefficient at the close-packed (111) terrace sites that dominate the surface. The net effect is that at room temperature, the defect activity is approximately four and one-half orders of magnitude greater than that of the close-packed terrace sites.