Cyclohexane conversion to benzene over bimetallic Pt catalysts is an important prototypical reaction for fundamental studies of selective catalytic dehydrogenation catalysis. We have studied the adsorption and dehydrogenation of cyclohexane and benzene on Pt( 1 1 1) and two ordered Sn/Pt( 1 1 1) surface alloys using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and sticking coefficient measurements. Vapor-depositing Sn on the Pt(ll1) surface and annealing gives a (2 X 2) or ( 4 3 X d3)R3Oo LEED pattern, producing well-defined surfaces identified as the (1 1 1) face of Pt3Sn and a substitutional alloy of composition Pt2Sn. Cyclohexane adsorbed onto all three surfaces at 100-1 55 K with an initial sticking coefficient of unity. Precursor mediated adsorption kinetics are indicated by the coverage independence of the initial sticking coefficient up to l/3-1/4 of saturation coverage of the monolayer (B:& = 0.13 ML). The desorption temperature of the chemisorbed cyclohexane monolayer is decreased upon alloying of the Pt( 1 1 1) surface with Sn, and cyclohexane desorbs in a narrow peak characteristic of each surface. The adsorption energy is reduced from 58 kJ/mol on Pt( 11 1) by 9 kJ/mol on the (2 X 2) Sn/Pt( 11 1) surface alloy and by 12 kJ/mol on the ( d 3 X d3)R3Oo S n / P t ( l l l ) surface alloy,indicating an electronic effect of Sn on cyclohexane adsorption on the Pt(ll1) surface. In supporting studies of benzene adsorption, Sn converts most of the strongly chemisorbed benzene to physisorbed benzene. In contrast to the reduction in chemisorption, the initial sticking coefficient shows little dependence on the Sn concentration. This phenomenon is attributed to the presence of a modifier precursor state in benzene adsorption. The decomposition of benzene is completely suppressed under UHV conditions due to the presence of Sn in the Sn/Pt( 11 1) surface alloys. These observations bring new information into discussions of the role of Sn in selective catalytic dehydrogenation reactions over supported Pt-Sn catalysts.

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