IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Wednesday Sessions
       Session SS2-WeA

Paper SS2-WeA4
Understanding the Pressure and Structure Gaps in Catalysis: Role of Steps and Terraces in the O@sub 2@ and C@sub 2@H@sub 4@ Interaction with Ag(410)

Wednesday, October 31, 2001, 3:00 pm, Room 121

Session: Adsorption on Metal Surfaces
Presenter: M. Rocca, Universita' di Genova, Italy
Authors: L. Savio, Universita' di Genova, Italy
L. Vattuone, Universita' di Genova, Italy
M. Rocca, Universita' di Genova, Italy
Correspondent: Click to Email
The effort to unravel the dynamics of real catalysts by ultra high vacuum investigations of gas-surface interaction suffers of two major limitations, known as pressure gap (10 order of magnitudes difference) and structure gap (the surface of a real catalyst has a high density of steps and defects in contrast to the almost perfect low Miller index surface of a single crystal). A large effort was devoted to bridge the pressure gap; in order to face the structure gap, attention was focused on defected surfaces, obtained either by ion bombardment or by cutting a single crystal along vicinal planes. We report on a combined supersonic molecular beam and High Resolution Electron Energy Loss Spectroscopy investigation of the angle and energy dependence of the sticking probability, S, of O2 and C2H4 on Ag(410), a vicinal surface with (100) teraces and (110) step edges. In agreement with our previous results for the ion bombarded Ag(100) surface, we find for O2 adsorption that dissociation and molecular chemisorption coexist already at temperatures at which only molecular adsorption is stable on flat Ag(100). The angular dependence of S shows that when the molecules impinge against the step edges the activation barrier for molecular adsorption is strongly reduced or even eliminated. The reactivity of Ag atoms at terrace sites is, on the contrary, reduced with respect to the flat surface. Dissociation takes place preferentially at the upper side of the steps as proven by the temperature and angular dependence of S. For C2H4 we observe nearly unitary sticking probability at the step edge and stable adsorption in the pi-bonded state. S scales approximatively with total energy, but it is slightly larger for molecules impinging grazing on the step edges and decreases when the step edges are in shadow. The interaction is mediated by an extrinsic precursor.