Isomerization and hydrogenolysis of C 6 alkanes on PtAl 2O 3 catalysts and Pt single-crystal faces

Abstract

The contact reactions of C 6 hydrocarbons (isomerization, dehydrocyclization, hydrocracking) were studied under the same experimental conditions, on platinum-alumina catalysts of low or high dispersion, on Pt(557), Pt(119), Pt(111) surfaces, and on a polycrystalline foil of platinum. The experiments on these well-characterized surfaces were performed up to atmospheric pressure in an isolation cell housed within the main UHV chamber equipped with LEED, AES, and ELS facilities. The reaction mixture was analyzed by GLC and part of it was used for mass spectrometric location of the 13C in each molecule. The results have shown that isomerization by bond-shift or cyclic mechanisms and hydrogenolysis take place on single-crystal faces of platinum, simulating very well the data from PtAl 2O 3 catalysts of large size aggregates of Pt. Isomerization by bond shift is much more important on stepped surfaces. The contributions of the relative percentages of bond-shift and cyclic mechanisms are discussed as a function of different models of adsorbed species evolving from one site to multisites of chemisorption, differences in local density of states of the edges, and differences in the electronic charges of the carbon atoms in the hydrocarbons. These models are correlated to the classical ones of Anderson, Rooney, and Gault. The peculiar properties of the highly dispersed 0.2% PtAl 2O 3 catalyst were never simulated by single crystals of platinum.