Reflection absorption infrared spectroscopy and kinetic studies of the reactivity of ethylene on Pt(111) surfaces

The chemistry of ethylene on Pt(111) single-crystal surfaces has proven quite complex because it involves the simultaneous occurrence of several reactions, namely molecular desorption, dehydrogenation to ethylidyne, H-D exchange within the adsorbed molecules, and hydrogenation to ethane. Reflection absorption infrared spectroscopy (RAIRS) has been used here in conjunction with isothermal kinetic measurements to identify the possible intermediates involved in each of those reactions, and to follow their thermal chemistry on the platinum surface. All vinyl, ethyl and ethylidene moieties were prepared by thermal decomposition of their corresponding iodides and characterized by RAIRS. The experimental data available to date favors the formation of ethylidene as an intermediate in the conversion of ethylene to ethylidyne, but the complexity of the kinetics of that reaction, which changes significantly with changing surface coverages, makes the final proof of this mechanism quite difficult. In addition, a side ethylene-ethyl equilibrium which starts at temperatures below those required for the formation of ethylidyne is responsible for H-D exchange in ethylene. Finally, the hydrogenation of ethylene to ethane also involves an ethyl intermediate, but only occurs at the ethylene high coverages needed for the transition of the di-sigma strongly bonded species to a weak pi configuration. The relevance of the reactions seen under vacuum to the high-pressure catalytic hydrogenation of ethylene is briefly discussed.