Reactivity of Pt and Pt-Sn alloy surfaces probed by activation of C₅-C₈ cycloalkanes via electron-induced dissociation (EID) of multilayers

The surface chemistry of cycloalkanes and cycloalkyl intermediates on Pt-Sn alloys is important to the function of selective hydrocarbon conversion catalysts, yet very little is known about this chemistry because cycloalkane decomposition is strongly suppressed under UHV conditions on Pt-Sn alloys and there are few other clean sources of these intermediates. Low-energy, electron-induced dissociation (EID) in multilayers of saturated hydrocarbons produces rather cleanly reactive intermediates formed by the selective cleavage of one C-H bond. This method was used to activate C₅-C₈ cycloalkane multilayers and prepare monolayer coverages of cycloalkyl species on Pt(111) and two well-defined Pt-Sn alloy surfaces-the p(2×2)-Sn/P(111) and (√3×√3)R30°Sn/Pt(111) surface alloys formed by vapor deposition of Sn on a Pt(111) substrate. EID of the multilayers and subsequent thermal reactions of the intermediates on these surfaces were investigated by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Adsorbed cycloalkyl species dehydrogenate facilely on both alloy surfaces, but alloying with Sn weakens the bonding to the surface of the cycloalkenes formed and strongly suppresses cycloalkene dehydrogenation. This chemistry leads to a much higher selectivity for the evolution of gas-phase cycloalkenes from the dehydrogenation of cycloalkyl intermediates compared to that on Pt-(111).