Hydrogen-deuterium isotope effects in the reactions of chlorobenzene and benzene on a Pt/gamma-Al2O3 catalyst

The kinetic isotope effect for combustion of a C6H5Cl/C6D5Cl mixture on Pt/ gamma -Al2O3 was found to be close to unity between 520 and 580 K. However, in the presence of an excess of heptane, an isotope effect of 1.5 was foun d between 460 and 490 K. For the combustion of a C6H6/C6D6 mixture the k(H) /k(D) value was around 2 between 404 and 439 K. The results show that in th e combustion of chlorobenzene per se, C-H bond activation is not a rate-det ermining step. On Pt sites, C-Cl bond scission probably occurs already at l ow temperatures. The chlorine and the phenyl group cannot easily react furt her. Chlorine on the surface is active in chlorination, which is shown by t he formation of C6D5Cl in an experiment with C6H5Cl and C6D6. Only at a cer tain temperature is the chlorine removed, partly as polychlorinated benzene s. The removal of chlorine from the catalyst allows oxygen to take part in the reaction, which determines the rate of the combustion of chlorobenzene. When heptane is present, CI is removed from the surface and C-H bond sciss ion can become rate determining, as is also the case in the combustion of C 6H6/C6D6. Upon (partial) combustion of C6H6Cl/C6D5Cl and C6H6/C6D6 mixtures on a Pt/gamma -Al2O3 catalyst, hydrogen-deuterium exchange occurs on the g amma -Al2O3 support.