Comparison of ethylene oxide and propylene oxide chemisorbed on clean and oxygen precovered Ag(110)

Abstract

The adsorption of ethylene oxide and propylene oxide on clean and oxygen precovered Ag(110) was studied using temperature programmed desorption, low-energy electron diffraction (LEED), high-resolution electron energy-loss spectroscopy (HREELS), and x-ray photoelectron spectroscopy. Both ethylene oxide (C2H4O) and propylene oxide (C3H6O) adsorb weakly on the clean Ag(110) surface, with heats of adsorption of 9.5 and 10.6 kcal/mol, respectively. The heats of adsorption of both C2H4O and C3H6O increase by about 30% as the coverage of coadsorbed atomic oxygen is increased up to an oxygen coverage of 0.5, where the heat of adsorption decreases again. Ethylene oxide forms a c(2×2) LEED pattern at saturation coverage of the monolayer following adsorption at 90 K. A racemic mixture of C3H6O forms a (4×2)p1g1 LEED pattern, while the pure enantiomer (S)-C3H6O forms a c(2×2) pattern at the same saturation coverage of the monolayer. Models of the structures accounting for these data are proposed. The HREELS of C2H4O and C3H6O on clean Ag(110) are little perturbed from their gas phase spectra. The epoxide ring of C2H4O is close to normal to the surface, while that of C3H6O is inclined. There is little change in the vibrational frequencies of the normal modes of C2H4O or C3H6O on the oxygen precovered surface. The orientation of C3H6O is influenced by coadsorbed atomic oxygen.