AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuA

Paper SS1-TuA9
Reaction Kinetics on Supported Model Catalysts: A Molecular Beam / In-Situ IRAS Study of the CO Oxidation on Pd/Al@sub 2@O@sub 3@

Tuesday, October 3, 2000, 4:40 pm, Room 208

Session: Mechanisms and Control of Surface Reactions
Presenter: J. Libuda, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
Authors: J. Libuda, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
I. Meusel, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
J. Hoffmann, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
J. Hartmann, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
H.-J. Freund, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
Correspondent: Click to Email
We have employed molecular beam techniques combined with time-resolved in-situ IR reflection absorption spectroscopy to study the CO oxidation kinetics on oxide-supported model catalysts. As model systems we have used Pd particles of different size and morphology grown under UHV conditions on a well-ordered alumina film on NiAl(110). Previously, these systems have been characterized in detail with respect to their geometric and electronic structure. Sticking coefficient measurements demonstrate that - contrary to what is observed for densely packed Pd single crystal surfaces - at room temperature and above the Pd crystallites rapidly incorporate a large amount of oxygen. This subsurface and bulk oxygen is not accessible to CO oxidation at low temperature. Once the bulk reservoir is saturated, stable oxidation rates are obtained. Taking advantage of the single-scattering conditions in a molecular beam experiment, we quantitatively address effects which are specific for supported metal catalysts, such as support mediated adsorption. Also, activation energies for the LH reaction step as a function of adsorbate coverage are derived. Various coverage regimes are considered, in particular the limiting cases of high CO or O coverage. The transient behavior and the steady-state CO@sub 2@ production rate are probed over a wide range of reactant fluxes and CO/oxygen flux ratios. In particular, we discuss the origin of the different transient behavior of the oxidation rate on supported Pd particles. Time-resolved IR absorption spectroscopy is employed during the reaction to monitor changes in the occupation of different adsorption sites in transient beam experiments and under steady-state conditions.