High-Resolution Electron Energy Loss Spectroscopy: Interaction of Small Organic Molecules with Palladium (111)

High-resolution electron energy loss measurements are reported for acetylene, ethylene, and methanol adsorption on a palladium (111) surface. Temperature-induced changes in vibrational spectra of chemisorbed species are observed. Comparison is made with relevant model compounds and analogous chemisorption systems. Through detection of electron energy losses both on and off the specular direction we have been able to distinguish between modes excited via the long-range dipole interaction and those involving short-range "impact" scattering. Such an angular analysis is reported for Pd(111) following 150K acetylene and 300K ethylene exposures. We find that some impact-excited modes have unexpectedly high intensities, comparable to dipole-excited modes, even for specular scattering. Acetylene is stongly rehybridized ((TURN)sp('2.5)) upon chemisorption at 150K. Much of this species then evolves to ethylidyne ( C-CH(,3)) near 300K in the presence of excess atomic hydrogen. Vibrational spectra obtained with and without preadsorbed hydrogen provide evidence for a \C=CH(,2) intermediate in the reaction. At higher temperatures (400-500K) the remaining chemisorbed acetylene dehydrogenates to a CCH species. CCH is also found to coadsorb with ethylidyne following room temperature acetylene exposures. Chemisorbed ethylene (150K) exhibits softened and broadened CH stretching frequencies suggestive of hydrogen bond-like interactions between the molecule and the metal surface. This may be a precursor to ethylidyne formation near 300K although most of the ethylene desorbs below this temperature. While there is some evidence of CCH at higher temperatures, our data indicate, in agreement with the acetylene-derived results, that ethylidyne dehydrogenation to CCH is not a favorable reaction. At room temperature gaseous ethylene primarily reacts to form ethylidyne on Pd(111). We find both a chemisorbed and physisorbed phase of methanol at 140K. Desorption of the physisorbed phase occurs by 160K and approximately 90% of the chemisorbed layer desorbs by 300K. Exposure to methanol at 300K causes dissociation to chemisorbed CO. Decomposition to chemisobed p(2 x 2) oxygen layer whereas no methoxy is detected on the clean surface. Further dehydrogenation is observed by 300K on the oxygen precovered surface.