QUANTITATIVE-DETERMINATION OF THE ACTIVITY OF DEFECT SITES ON A SINGLE-CRYSTALLINE SURFACE - C-H BOND ACTIVATION OF C-13 LABELED ETHANE ON IR(111)

The C-H bond activation of carbon-13 labeled ethane 1,2-di-C-13-C2H6 at defect sites on an Ir(111) single-crystalline surface, cut and polished to 0.70 degrees+/-0.15 degrees 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 similar to 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 5X10(11) and 10(12) 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.