Selective dehydrogenation of 1,3-cyclohexadiene on ordered Sn/Pt(111) surface alloys

The adsorption and dehydrogenation of 1,3-cyclohexadiene on Pt(111) and two Sn/Pt(111) surface alloys has been studied using TPD, AES, and LEED. The two Pt-Sn surfaces investigated are well-defined, monolayer-thick surface alloys with either a p(2x2) or (root 3x root 3)R30 degrees LEED pattern and Theta(sn) = 0.25 or 0.33, respectively. Cyclohexadiene chemisorption on Pt(111) is irreversible and all the chemisorbed monolayer dehydrogenates to form benzene upon heating. Further heating causes most of this benzene product to dehydrogenate completely to form a carbonaceous residue. Some benzene desorbs from the surface at higher coverages, Alloying the Pt surface with Sn completely eliminates the decomposition of benzene without inhibiting the reactivity of cyclohexadiene, dramatically increasing (to 100%) the selectivity of the Pt-Sn surface alloys for gas-phase benzene production. Finally, we observe that the benzene evolution undergoes a stepwise decrease in temperature with increasing Sn concentration and we attribute this to a competition between benzene and hydrogen for adsorption sites.