DEHYDROGENATION OF CYCLOHEXENE ON ORDERED SN/PT(111) SURFACE ALLOYS

The adsorption and dehydrogenation of cyclohexene on Pt(111) and two Sn/Pt(Ill) surface alloys has been studied since cyclohexene is a likely intermediate in the dehydrogenation of cyclohexane to benzene, a prototypical reaction for catalytic reforming. Our investigations used TPD, AES, LEED, and sticking coefficient measurements. The two ordered, Pt-Sn surface alloys were prepared by annealing monolayer amounts of Sn vapor-deposited onto Pt(111). Depending upon the conditions used, the annealed surface exhibited a p(2 x 2) or (square-root 3 x square-root 3)R30-degrees LEED pattern corresponding to the (111) face of Pt3Sn or a surface alloy of composition Pt2Sn, respectively. Sticking coefficient measurements showed that the initial sticking coefficient, S0, was equal to unity on all three substrates and determined a saturation coverage of 0.14 ML for the cyclohexene monolayer on all three substrates at 180 K; Sn in these surface alloys has no measurable effect on the adsorption kinetics and the monolayer coverage compared to Pt(111) at 180 K. Precursor mediated adsorption kinetics on all three surfaces are indicated by the constant initial sticking coefficient up to cyclohexene coverages of at least 0.02 ML. However, the cyclohexene binding energy decreases with increasing Sn concentration in the surface layer, indicating that Sn has a substantial electronic effect on cyclohexene adsorption on the Pt(111) surface. The di-sigma-bonded cyclohexene on the Pt(111) surface is changed to hydrogen-bonded cyclohexene on the (square-root 3 x square-root 3)R30-degrees Sn/Pt(111) surface. This change is attributed to the stronger influence of Sn on the surface capacity for forming di-sigma bonds than hydrogen bonds. At small cyclohexene coverages, the alloying of Pt with Sn increases both the activity and selectivity of gas phase benzene production under UHV conditions. At higher cyclohexene coverages, the self-poisoning of the surface by cyclohexene or reaction intermediates becomes the determining factor in the reactions of cyclohexene and the addition of Sn to the surface only slightly increases the selectivity of gas phase benzene production. However, carbon deposition on the surface is strongly suppressed.