The Effect of Gold on Hydrogenolysis, Isomerization and Dehydrocyclization Reactions on Polycrystalline Platinum and Iridium Foils,

The effect of the addition of gold to polycrystalline platinum and iridium foil samples on the initial reaction rates for the hydrogenolysis of propane, isobutane and n-heptane, the isomerization of isobutane, and the dehydrocyclization of n-heptane to toluene has been studied. These studies were carried out on samples of area approx. 1 sq cm cleaned and maintained under UHV conditions and in a 51 hydrogen to hydrocarbon ambient at a total pressure in the 10 to the minus 4th power Torr range. Mass spectroscopy was used to monitor the reactions in a temperature range of 100 C to 500 C. The addition of gold to the platinum and iridium surfaces decreases the rates for all reactions studied. An approximately linear decrease in isomerization rate as a function of gold coverage was observed while hydrogenolysis and dehydrocyclization decreased in a similar fashion with each other and much more rapidly than isomerization. Thus the selectivity for isomerization as compared to hydrogenolysis is increased with increasing gold coverage while little change is expected for dehydrocyclization. These results coupled with differences in the time dependent poisoning of the various reactions suggest that different surface sites are responsible for isomerization than for hydrogenolysis and dehydrocyclization.