STRUCTURE OF COADSORBED BISMUTH AND HYDROCARBONS ON PT(111)

The structure of coadsorbed layers of bismuth with hydrocarbons on transition metal surfaces is of interest because of the utility that bismuth adatoms have for probing the surface chemistry of hydrocarbons on such surfaces. Herein we present the results of low-energy electron diffraction (LEED) studies of Bi coadsorbed with cyclohexane, cyclohexene, and benzene on Pt(111). When a low coverage (theta(Bi) = 0.25) p(2 X 2) Bi adlayer is dosed with hydrocarbons, the Bi LEED pattern is destroyed, and no evidence is seen for higher coverage Bi domains. These results are consistent with a model whereby the Bi and hydrocarbon adatoms are randomly distributed across the Pt(111) surface when both are at low coverages. When Bi is predosed at high coverage (theta(Bi) = 0.50), and then the hydrocarbon is dosed to the surface, the Bi c(4 X 2) pattern remains unchanged, suggesting that the hydrocarbon simply sits on top of the Bi adlayer. In the case of cyclohexane, this c(4 X 2)-Bi LEED pattern was also obtained using the reverse dosing sequence, which suggests that the postdosed Bi squeezes between the hydrocarbon and the Pt. This process seems to be kinetically prevented below 160 K in the case of cyclohexene and benzene, due to their stronger bonds to Pt.