Probing selectivity by using reactions of n-C₆ hydrocarbons on model Pt-Sn alloy catalysts

Adsorption, desorption, and reactive chemistry of linear C₆-hydrocarbons containing an increasing degree of unsaturation, in the series of n-hexane, 1-hexene, and 1,5-hexadiene, was investigated on Pt(111) and two Pt-Sn ordered surface alloys, i.e., (2×2)-Sn/Pt(111) and (√3×√3)R30°-Sn/Pt(111). Increasing the Sn concentration decreased the adsorption energy of all three molecules, but the effect was most pronounced for the unsaturated hydrocarbons upon changing from the (2×2) alloy to the (√3×√3)R30° alloy. Alloying Sn did not decrease the monolayer coverage of the hydrocarbons, but the amount of 1-hexene dehydrogenation was linearly related to the number of pure-Pt 3-fold sites on these surfaces, and reversible alkene and diene adsorption was associated with their elimination. This work extends our understanding of hydrocarbon chemistry at Pt-Sn alloy surfaces and may aid in understanding current catalysts used for hydrocarbon reforming and the development of new catalysts for selective hydrogenation.