Probing selectivity over Pt-Sn catalysts in reactions of n-C₆ hydrocarbons: Adsorption and reactivity of n-hexane, 1-hexene, and 1,5-hexadiene on Pt(111) and Sn/Pt(111) surface alloys

We have investigated how alloyed Sn at a Pt(111) surface alters the adsorption, desorption, and reactive chemistry of linear Co-hydrocarbon molecules containing an increasing degree of unsaturation: n-hexane, 1-hexene, and 1,5-hexadiene. This chemistry was investigated on Pt(111) and two bimetallic surfaces, the (2 × 2)-Sn/Pt(111) and (∫3 × ∫3)R30°-Sn/ Pt(111) surface alloys, by using temperature-programmed desorption and Auger electron spectroscopy. n-Hexane weakly and reversibly adsorbed on all three surfaces, but 1-hexene and 1,5-hexadiene chemisorbed and underwent dehydrogenation depending on the surface. 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. All three molecules only weakly and reversibly adsorb and have nearly the same adsorption energies, on a thick Sn film. Alloying with Sn decreased the amount of irreversibly chemisorbed 1-hexene at monolayer coverage from 65% on Pt(111) to 18% on the (2 × 2) alloy, and dehydrogenation was eliminated on the (∫3 × ∫3) R30° alloy. Similarly, alloying with Sn decreased irreversible 1,5-hexadiene chemisorption at monolayer coverage from 100% on Pt(111) to 30% on the (2 × 2) alloy, and dehydrogenation was completely eliminated on the (∫3 × ∫3)R30° alloy. Reversible alkene and diene adsorption is associated with the elimination of pure-Pt 3-fold sites on the (∫3 × ∫3)R30° alloy. Furthermore, the amount of 1-hexene dehydrogenation was linearly related to the number of pure-Pt 3-fold sites on these three surfaces. Alloying Sn with Pt(111) did not decrease the saturation monolayer coverage of any of the three molecules. 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 die development of new catalysts for selective hydrogénation.