The reactivity of Z-2-hexene on Ru(0 0 1) studied by RAIRS

The adsorption at low temperature and the thermal decomposition of Z-2-hexene on the clean Ru(0 0 1) surface, under ultra-high-vacuum, were studied by reflection-absorption infrared spectroscopy (RAIRS). A comparison with the reactivity of other linear hexene isomers was made. It was proposed that, at 90 K, Z-2-hexene adsorbs non-dissociatively as a di-σ complex, in a mixture of conformations. Apparently, gauche conformers with the methyl group in C6 standing vertically are favored for increasing coverage, but forming a poorly ordered layer. For an exposure of 0.6 Langmuir (L), there is evidence of a second layer, physically adsorbed, eventually resulting in the condensation of a randomly oriented multilayer above 1 L. The multilayer desorbs between 100 and 110 K. Only one decomposition mechanism was identified, either by annealing the low temperature di-σ complex to T⩾110 K or by direct adsorption at higher temperatures. It consists on the dehydrogenation of the carbons anchored to the surface, with rehybridization to ∼sp2, yielding 2-hexyne di-σ/π complex, and concomitant C–C bond breaking in the adjacent positions, yielding methylidyne (Ru3CH) and ethyne (C2H2) di-σ/π complex. The latter is stable on Ru(0 0 1) up to at least 320 K, but, in the presence of co-adsorbed hydrogen, it may hydrogenate to ethylidyne (Ru3CCH3), at 125 K. At this temperature, the third decomposition fragment (–C3H7) dehydrogenates and remains adsorbed as propylidyne (Ru3CCH2CH3). Above 170 K, decomposition of propylidyne to ethylidyne and further to methylidyne occurs, leaving methylidyne and ethyne di-σ/π complex as the only RAIRS identifiable products at 320 K.