The structure sensitivity of n-heptane dehydrocyclization and hydrogenolysis catalyzed by platinum single crystals at atmospheric pressure

The dehydrocyclization and hydrogenolysis of n-heptane catalyzed by platinum single-crystal surfaces have been investigated at temperatures from 533 to 603 K in the range of 1 atm total pressure. The flat (111), stepped (557), and kinked (10, 8, 7) and (25, 10, 7) surfaces used in this study were characterized under ultrahigh vacuum by low-energy electron diffraction and Auger electron spectroscopy before and after reaction experiments. The initial rate of dehydrocyclization to toluene on the four surfaces increased in the order (111) ≈ (25, 10, 7) < (557) ≈ (10, 8, 7). Hydrogenolysis, however, increased in the order (557) < (10, 8, 7) < (111) < (25, 10, 7). As a result, the selectivity of toluene production versus hydrogenolysis increased by an order of magnitude in the order (25, 10, 7) < (111) < (10, 8, 7) < (557). The sum of the rates of hydrogenolysis and toluene production remains relatively constant. The effect of preoxidation of the single-crystal catalysts was to increase the rate of hydrogenolysis and decrease the rate of dehydrocyclization. In general, the reaction rates decreased with increasing reaction time. This decrease was shown to be the result of the deposition of irreversibly adsorbed carbonaceous species.