No abstract

The adsorption of ethene (C2H4) has been studied on Pd(111) and ordered Sn/Pd(111) surface alloys using temperature programmed desorption (TPD), ultraviolet photoelectron spectroscopy (UPS), high-resolution electron energy loss spectroscopy (HREELS), and low energy electron diffraction (LEED). Two surface alloys were prepared by thermal treatment of Sn-films, which were vapor deposited on Pd(111) at room temperature. Depending on the preparation conditions, surface alloys giving a p(2×2) or a (√3×√3)R30° LEED pattern were produced. Below 250 K ethene adsorbs on pure Pd(111) in an undissociated – but substantially distorted – form relative to the molecular structure in the gas phase: HREELS suggests an adsorption in the di-σ bonded state. A π-bonded ethene species was, however, found to coexist with this strongly rehybridized form, probably as a result of hydrogen coadsorbed from the residual gas. TPD and annealing experiments followed by UPS and HREELS indicated that most of the adsorbed ethene desorbs reversibly in the temperature range between 150 K and 350 K, while a small amount dehydrogenates. After adsorption at room temperature, ethylidyne (≡CCH3) has been identified as the most important species. Alloying Pd(111) with Sn results in a decreasing ethene-substrate interaction with increasing Sn-content in the topmost layer of the substrate. Only π-bonded ethene was formed on both surface alloys and decomposition reactions were suppressed.

The adsorption of cyclopentene (C5H8) on Pt(111) and the two ordered Pt3Sn/Pt(111) and Pt2Sn/Pt(111) surface alloys has been investigated experimentally using high-resolution electron energy loss spectroscopy, ultraviolet photoelectron spectroscopy, low-energy electron diffraction, and temperature-programmed desorption as well as theoretically by ab initio density functional theory (DFT) calculations. On the Pt(111) and the respective surface alloys, a di-σ bonding of cyclopentene has been found both experimentally as well as in the DFT calculations. Even though the bonding mechanism on Pt(111) and the two surface alloys is very similar, large differences in the bond energy and the thermal stability of the cyclopentene have been detected. On Pt(111), part of the C5H8 desorbs intact at ∼280 K whereas the remaining amount is converted to C5H5. This species completely dehydrogenates to carbon upon heating above 450 K. On the surface alloys, the temperature of desorption is reduced to 244 K (Pt3Sn) and 198 K (Pt2Sn), respectively, and no dehydrogenation is detected. The influence of the aliphatic ring on the interaction between the olefinic bond and the substrate is weak even though an interaction of the β-hydrogen with the surface atoms has been found experimentally and theoretically for cyclopentene on Pt(111) and on the Pt3Sn/Pt(111) surface alloy.

No abstract