The chemical behaviour of 3-hexene on the Ru(0001) surface: a characterisation by RAIRS

The chemical nature of Z-3-hexene/Ru(0001) bond at different temperatures was studied by reflection-absorption infrared spectroscopy (RAIRS). At 92 K and for low exposures (0.05 L) the results indicate that Z-3-hexene adsorbs non-dissociatively as a di-σ complex. By annealing to 123 K, it dehydrogenates at the Cα carbons (C3 and C4, bonded to the surface), forming a hexyne di-σ/π complex in a mixture of rotational conformers. Upon thermal activation, this complex decomposes by breaking the Cα–Cβ (i.e., C2–C3 and C4–C5) bonds, yielding adsorbed ethylidyne. At 223 K, only this species and residual di-σ/π complex are present on the surface. A different decomposition path is proposed upon direct adsorption of Z-3-hexene at 223 K, since propylidyne appears to be formed on the surface, which implies breaking the C–C double bond with dehydrogenation. Since the behaviour of Z-3-hexene is identical to that of 3-hexyne, and rather different from 1-hexene, these results suggest that the decomposition of unsaturated hydrocarbons on clean Ru(0001) is determined by the position and not by the nature (double or triple bond) of the functionality.