Modifications of platinum model catalysts by sulphur: effect on reactions of labelled hexanes

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Abstract
The presence of impurities, such as sulphur, oxygen or calcium, can poison or promote the catalytic activity of platinum model catalysts by induced reconstruction of the surface. Such a reconstruction can make available new active sites or eliminate others. We focus our attention on the reconstruction of a platinum stepped surface Pt(S)-[6(111)×(100)] induced by sulphur, hydrogen plus sulphur and hydrogen plus sulphur plus hydrocarbons and on the consequences on the catalytic activity and selectivity for the skeletal rearrangement of labelled hexanes. Very small sulphur coverages induce, in the presence of hydrogen and hydrocarbons, surface reconstructions dependent on the preferential adsorption of sulphur at the step edge as already described by Lanzillotto and Bernasek for the sulphur-induced reconstruction of the Pt(S)-[6(111)×(100)] surface. At 350 °C and at isoconversion the increase of sulphur coverage results in a decrease of the extensive hydrogenolysis and in an increase in the isomerization via a bond-shift mechanism. The behaviour of a supported Pt/Al2O3 catalyst of low dispersion working in an unpurified hydrogen flow is then well simulated. The results provide a good example of the use of surface science techniques to elucidate the role of minor impurities in controlling catalytic reactions.