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 C4H6. The spectra are interpreted in terms of a di-σ/π-adsorbed species involving the central CC bond, of formula (CH3)CC(CH3). 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 C4 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 C4 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.