Cyclohexane adsorption on clean and O-covered Ni{111}: implications for C—H bond activation

Abstract

The adsorption of cyclohexane on clean and oxygen-modified Ni{111} surfaces has been studied by reflection–absorption infrared spectroscopy (RAIRS). Adsorption on the clean surface proceeds via two-dimensional island growth in equilibrium with a random gas phase adsorbed on the bare surface and on top of the islands. The adsorbed cyclohexane exhibits a site symmetry of C_3v which persists through to the multilayer regime. Adsorbed molecules in the first layer exhibit a broadened and downshifted ν_CH stretching vibrational band which has become the signature for cyclohexane on so many metal single-crystal surfaces, thought to arise from C—H⋯M interactions which may provide an important dehydrogenation channel. In order to determine the nature of the C—H⋯M interaction, adsorption on oxygen-modified Ni{111} surfaces was also investigated. The effect of the coadsorbed oxygen is strongly coverage dependent; adsorption of cyclohexane on the Ni{111}-(2 × 2-)-O surface results in further downshifting of the softened ν_CH stretching vibration, signalling the importance of charge transfer from the filled CH σ orbital to the metal in weakening the C—H bond. Adsorption on the Ni{111}-(√3 ×√3)R30°-O surface leads to total suppression of any C—H⋯M interaction, attributed to steric blocking of bare metal sites by the adsorbed adatoms.