A comparative reflection-absorption infrared spectroscopy study of the thermal decomposition of 1-hexene on Ru(0001) and on Pt(111)

l-Hexene was adsorbed on Pt(111) at low temperatures and its thermal decomposition studied by reflection-absorption infrared spectroscopy (RAIRS). The behaviour on this surface is parallel to what was previously observed on Ru(0001), but occurring at temperatures similar to 150 K above. A mixture of rotational conformers of hexylidyne was detected on both surfaces [at 100 K on Ru(0001) and at 250 K on Pt(111)], although in different proportions. The decomposition path proposed for hexylidyne on Ru(0001), which involved the formation of metallocycles, was confirmed on Pt(lll) by the positive observation of two types of metallocycle. One is hydrogenated in C6 [Pt(3)drop C-(CH2)(5)-Pt],forms at lower temperatures (270 K) and is identified by the symmetric stretching mode of the methylene group attached to the surface (at 2911cm(-1)). The other, completely dehydrogenated [Pt(3)drop C-(CH2)(4)-C drop Pt-3], starts to form at around 370 K, and is identified by the antisymmetric stretching mode of methylenes on C3 and C4 and by the symmetric stretch of methylenes on C2 and C5. It has been proved that the thermal decomposition of 1-hexene on these surfaces does not involve the formation of ethylidyne (mu(3)-CCH3), in contrast with shorter 1-alkene chains on Ru(0001). The different reactivities of the two surfaces towards C-H bond breaking is analysed. (C) 2000 Elsevier Science B.V. All rights reserved.