The use of thermal desorption and electron energy loss spectroscopy for the determination of the structures of unsaturated hydrocarbons chemisorbed on metal single-crystal surfaces. II. Cis - and trans -but-2-ene, but-2-yne and buta-1, 3-diene on Pt(111)

Abstract

Thermal desorption (TD) and electron energy loss (EEL) spectroscopies have been used to study the chemisorption of cis -and trans -but-2-ene, but-2-yne, and buta-1, 3-diene on a Pt(111) surface over a range of temperatures. The low-temperature species at 170 K for each shows the occurrence of non-dissociatively di-σ-adsorbed species. The EEL spectra from the cis - and trans -but-2-enes are different from each other, showing that chemisorption occurs at this temperature without isomerization about the C=C bond. The 300 K spectra from both but-2-enes and from but-2-yne are virtually identical and thermal desorption confirms that in each the overall molecular formula is C 4 H 6 . The spectra are interpreted in terms of a di-σ/π-adsorbed species involving the central CC bond, of formula (CH 3 )CC(CH 3 ). The different spectra and composition data for the but-2-enes compared with but-1-ene show that at this temperature on Pt(111) double-bond migration does not occur along the C 4 chain, and that the but-2-enes do not give the butylidyne structure. At 300 K the spectra from buta-1, 3-diene adsorption imply a proportion of di-σ-adsorption across both C=C bonds, without any overall hydrogen loss. No diene‒alkyne equilibration has occurred at this temperature. At higher temperatures the EEL spectra show similar decomposition pathways for all four C 4 hydrocarbons, and the presence of mixtures of analogous surface products. By 450 K virtually the same spectra are obtained from but-1-tene as from each of the but-2-enes.