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 I. Alk-1-enes on Pt(111)

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Abstract
Thermal desorption combined with mass spectral analysis is used to determine the elemental composition, as a function of temperature, of the adsorbed monolayers of the alk-1-enes, propene, but-1-ene, pent-1-ene and isobutene chemisorbed on the (111) face of a Pt single crystal. Vibrational electron energy loss spectroscopy (EELS) is used to assign structures to the surface species adsorbed at different temperatures. At the lowest temperatures (below 200 K) it is concluded that in each case these alk-1-enes are bonded to the metal surface in the form of di-$\sigma$-bonded non-dissociatively adsorbed species. The simplicity of the EEL spectrum from the species derived from isobutene provides additional support for an earlier assignment to the di-$\sigma$-adsorbed species for ethylene on Pt(111). At ca. 300 K the EEL spectra and thermal desorption data are consistent with the presence of alkylidyne, M$_3$C(CH$_2$)$_n$CH$_3$ (M = metal; n = 1, 2 or 3) or M$_3$CCH(CH$_3$)$_2$ structures for the chemisorbed species respectively, (M = metal atom). For temperatures above 300 K the thermal desorption results show substantial further loss of hydrogen, a process which commences at lower temperatures the longer the hydrocarbon chain. Near 450 K the thermal desorption data indicate average C:H compositions of approximately C$_3$H$_2$ for the species derived from propene, C$_4$H$_2$ from but-1-ene, and C$_4$H$_4$ from isobutene. The EEL spectra are used to indicate the remaining hydrocarbon functional groups present at this and at higher temperatures. Above 450 K closely similar spectra were obtained from all the straight-chain butenes including the cis- and trans-but-2-enes and buta-1,3-diene whose spectra are discussed in more detail in the following paper. The EEL spectra indicate the occurrence of C-C bond breaking in general above ca. 500 K, the onset temperatures being somewhat dependent on the adsorbed hydrocarbon.