Low-energy electron diffraction (LEED) and high-resolution electron-loss spectroscopy (HREELS) studies of molecular adsorbates on model catalyst surfaces

This paper discusses the structures of adsorbed CO, several olefins, and several other small molecules that were recently studied by both LEED and HREELS studies. When adsorbed on metal-crystal surfaces, several binding states of CO have been observed that fill as a function of coverage. Acetylene, ethylene, propylene, methylacetylene, and butenes all undergo structural transitions as a function of temperature and are accompanied by drastic changes in bonding. On Pt and Rh single-crystal surfaces, the most-stable molecular arrangement for these adsorbed hydrocarbons near room temperature appears to be the alkylidyne structure with the C-C bond axis perpendicular to the metal surface and the molecule anchored to a 3-fold surface site on a (111) plane. Increased temperatures lead to sequential bond breaking and fragmentation. The CH and C/sub 2/H fragments appear to be the active species present on transition-metal surfaces during catalytic reactions. The vibrational spectra obtained on single-crystal and dispersed rhodium particle surface are also compared.