The chemical behaviour of 3-hexene on the Ru0001) surface: a characterisation by RAIRS Ana R. Garcia, Ricardo Brito de Barros, Laura M. Ilharco * Centro de Qu  õmica-F  õsica Molecular, Complexo I, I.S.T., Av. Rovisco Pais, 1049-001 Lisboa, Portugal Abstract The chemical nature of Z-3-hexene/Ru0001) bond at dierent temperatures was studied by re¯ection-absorption infrared spectroscopy RAIRS). At 92 K and for low exposures 0.05 L) the results indicate that Z-3-hexene adsorbs non-dissociativelyasadi-r complex.Byannealingto123K,itdehydrogenatesattheC a carbonsC 3 andC 4 ,bondedto the surface), forming a hexyne di-r/p complex in a mixture of rotational conformers. Upon thermal activation, this complex decomposes by breaking the C a ±C b i.e., C 2 ±C 3 andC 4 ±C 5 ) bonds, yielding adsorbed ethylidyne. At 223 K, only this species and residual di-r/p complex are present on the surface. A dierent decomposition path is proposed upon direct adsorption of Z-3-hexeneat223K,sincepropylidyneappearstobeformedonthesurface,whichimplies breaking the C±C double bond with dehydrogenation. Since the behaviour of Z-3-hexene is identical to that of 3- hexyne, and rather dierent 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 functional- ity. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Alkenes; Ruthenium; Surface chemical reaction; Infrared absorption spectroscopy 1. Introduction Alkenes on metal single-crystals represent good modelsforunderstandingtheinteractionsbetween unsaturated hydrocarbons and transition metals, as shown by the extensive research carried out on this ®eld [1±4]. The surface chemistry of small ole®ns, such as ethene, propene and dierent bu- tene isomers on Ru0001) has been widely char- acterised [5±8]. However, the results for higher hydrocarbons on the same surface were scarce until recently. In previous works, some of us have studied the adsorption and thermal decomposition of 1-hex- eneonacleanRu0001)surface,byRAIRS[9,10]. Wehaveshownthatthelowtemperature90K) species is a di-r complex, with the hydrocarbon chain in an all-trans conformation [9]. This sur- facecomplexundergoesconformationalrearrange- ments with increasing exposure, towards gauche conformers explained by a rotation over the sec- ond, third or fourth C±C bonds). When adsorb- ing just at a slightly higher temperature 100 K), 1-hexene dehydrogenates with formation of hex- ylidyne in a mixture of rotational conformers, which completely decompose into methylidyne at Surface Science 482±485 2001) 107±113 www.elsevier.nl/locate/susc * Corresponding author. Tel.: +351-1-8419220; fax: +351-1- 3524372/351-21-8464455. E-mail addresses: lilharco@ist.utl.pt, pcd951@alfa.ist.utl.pt L.M. Ilharco). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602801)00811-1