AVS 45th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS2-TuM

Paper SS2-TuM3
Chemisorption and Kinetic Studies of Model Low-Temperature CO Oxidation Catalysts: Au/TiO@sub 2@(110)

Tuesday, November 3, 1998, 9:00 am, Room 309

Session: Noble Metal Catalysis
Presenter: V.A. Bondzie, University of Washington
Authors: V.A. Bondzie, University of Washington
S.C. Parker, University of Washington
C.T. Campbell, University of Washington
Correspondent: Click to Email
As a room-temperature CO oxidation catalyst, Au/TiO@sub 2@ shows exciting potential in automotive applications. Oxygen does not dissociatively adsorb on bulk Au, although one of the elementary steps in the proposed mechanism for this reaction is the activation of O@sub 2@ at the Au/TiO@sub 2@ interface. No such adsorption sites that react with molecular O@sub 2@ have so far been identified spectroscopically. In order to understand this reaction at the molecular level, O@sub 2@ chemisorption and CO titration over O-covered 2D and 3D Au islands, grown on TiO@sub 2@(110) have been studied. Techniques used included TPD, LEED, XPS and LEIS. A hot filament was used to deposit O adatoms. Subsequent TPD shows O@sub 2@ desorption with a peak maximum shifting from ~650 K (for 2-atom thick Au islands) to ~550 K (for bulk Au). No O@sub 2@ desorption is seen from 2D (i.e., 1-atom thick) Au islands below 770 K. This indicates that as the thickness of the Au islands decreases, the O adatoms bind increasingly strongly to the Au. The kinetics of the titration of oxygen adatoms with CO (CO@sub g@+O@sub a@ ? CO@sub 2,g@) were also measured from O-covered Au particles on TiO@sub 2@(110). The reaction rate on 2D Au islands is slower than on the 3D Au islands, even though CO bonds slightly more strongly on the former. This reflects a higher activation barrier on the 2D Au islands than on the 3D Au islands for the Langmuir-Hinshelwood step (i.e., CO@sub a@+O@sub a@ ? CO@sub 2,g@). This is due to the stronger O-Au chemisorption bond strength on 2D Au islands than on 3D islands. This stronger bond should promote dissociative O@sub 2@ adsorption, which we postulate as the rate-determining step in catalysis.