Correlations between the surface structure of platinum single crystals and hydrocarbon skeletal rearrangement mechanisms: Approach to the nature of the active sites

Abstract

The isomerization and hydrocracking reactions of 2-methylpentane and n-hexane and the hydrogenolysis of methylcyclopentane were investigated on a clean Pt (311) single crystal at 350 °C under atmospheric pressure in an isolation cell housed within an ultrahigh vacuum chamber. The surface composition was monitored before and after the catalytic reactions by Auger electron spectroscopy. The results were compared with those obtained in the same experimental conditions on Pt (111), Pt (557), and Pt (119) surfaces, a polycrystalline foil, and two Pt Al 2O 3 catalysts of both low and high dispersion. Experiments with labeled hexanes were also undertaken to determine the nature of the mechanisms involved in the isomerization reactions. The catalytic results show that each crystallographic face has its own peculiarities especially for (i) catalytic activity, (ii) selectivity as regards isomers produced, and (iii) selectivity as regards mechanisms. Conversely, the distribution of cracked products presents no significant structure sensitivity on “massive” metal surfaces although the highly dispersed alumina-supported platinum catalysts behave differently. Results with 13C-labeled hydrocarbons suggest that the atoms in the so-called B 5 site configuration are responsible for the bond-shift plus cracking reactions and that the atoms located at the corners and edges are involved in the cyclic mechanism.