IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Thursday Sessions
       Session SS3-ThP

Paper SS3-ThP19
A Comparative Study of the Adsorption of Acetylene, Ethylene and Benzene on the Pure Pd(111) Surface and the Ordered Pd@sub 2@Sn Surface Alloy on Pd(111)

Thursday, November 1, 2001, 5:30 pm, Room 134/135

Session: Surface Reactions Poster Session
Presenter: G. Hamm, University of Bonn, Germany
Authors: G. Hamm, University of Bonn, Germany
T. Schmidt, University of Bonn, Germany
J. Breitbach, University of Bonn, Germany
D. Franke, University of Bonn, Germany
C. Becker, University of Bonn, Germany
K. Wandelt, University of Bonn, Germany
Correspondent: Click to Email
The adsorption of acetylene, ethylene and benzene on the pure Pd(111) surface and the ordered Pd@sub2@Sn surface alloy on Pd(111) has been investigated with TPD, LEED, UPS and HREELS. The surface alloy with (@sr@3 x @sr@3)R30° periodicity corresponding to the Pd@sub 2@Sn composition was produced by annealing of multilayer amounts of Sn vapor deposited onto Pd(111).@footnote 1@ Below 300K benzene chemisorbs intact on the pure Pd(111) surface, bonding via its @pi@-electron system. In the range 300-500K most of the adsorbed benzene desorbs while a small part is dehydrogenated leaving a CCH species on the surface. For the first time an ordered superstructure of benzene has been found at room temperature. On the alloy, benzene can only be physisorbed. Ethylene is most probably di-@sigma@ bound on the pure Pd(111) surface below 250K, whereas ethylidyne is the dominant species after the adsorption of ethylene at 350K. In the temperature range from 150 to 300K most of the ethylene desorbs. At the same time part of the molecules undergo a three step conversion into ethylidyne above 250K. Due to the absence of appropriate conversion sites and the weak adsorption, this reaction is totally suppressed on the alloy. Acetylene chemisorbs on both surfaces. While the electronic structure of the adsorbed molecule is nearly identical on the pure Pd(111) surface and the alloy, vibrational spectroscopy reveals marked differences. Benzene is reactively formed from adsorbed acetylene on pure Pd(111), exhibiting two desorption peaks at 200K and 500K, which are ascribed to tilted and flat lying benzene. The majority of the acetylene is, however, converted to ethylidyne near room temperature via a vinylidene intermediate. In contrast to Pd(111), both reactions are suppressed on the alloy surface resulting in a single acetylene desorption peak at about 160K. @FootnoteText@ @footnote 1@ A. F. Lee, C. J. Baddeley, M. S. Tikhov, R. M. Lambert, Surf. Sci. 373 (1997) 195.