The Adsorption of Ethene on Pd(111) and Ordered Sn/Pd(111) Surface Alloys

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 x 2) or a (root 3 x root 3)R30 degrees 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 all adsorption in the di-sigma bonded state. A a-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 pi-bonded ethene was formed on both surface alloys and decomposition reactions were suppressed.