AVS 50th International Symposium
    Surface Science Monday Sessions
       Session SS3-MoA

Paper SS3-MoA7
Chemical Reactions on Free Iridium and Platinum Clusters

Monday, November 3, 2003, 4:00 pm, Room 328

Session: Structure and Reactivity of Metal Clusters
Presenter: M. Andersson, Chalmers University of Technology and Goteborg University, Sweden
Authors: M. Andersson, Chalmers University of Technology and Goteborg University, Sweden
T. Jarvdalen, Chalmers University of Technology and Goteborg University, Sweden
P. Nystrom, Chalmers University of Technology and Goteborg University, Sweden
A. Rosen, Chalmers University of Technology and Goteborg University, Sweden
Correspondent: Click to Email
In a cluster beam experiment metal clusters are produced with a pulsed laser vaporization source, kept at room temperature or liquid nitrogen temperature. The beam of neutral clusters passes two low-pressure collision cells and clusters and reaction products are detected with laser ionization and time-of-flight mass spectrometry. Thus, the reaction probability in a cluster-molecule collision can be determined.@footnote 1@ We have measured the CO and O@sub 2@ adsorption on iridium clusters, 5-32 atoms. The overall size-dependence in reaction probability is similar for both molecules, with minima at Ir@sub 8@, Ir@sub 12@ and Ir@sub 18@, and a significantly higher reactivity for all clusters with more than 18 atoms. When the cluster source was cooled to liquid-nitrogen temperature, the reaction probability increased. For platinum clusters with more than 6 atoms we measured stable reaction products with both oxygen and hydrogen, with moderate variations with size. Using the two reaction cells we can let the clusters react first with O@sub 2@ and then with H@sub 2@ and observe that reaction of Pt@sub n@(O@sub 2@)@sub m@ with H@sub 2@ results in removal of oxygen atoms from the cluster. This is interpreted as formation of a water molecule, which subsequently desorbs. The efficiency of the catalytic water formation is high on all cluster sizes measured (7-30 atoms), with only a weak dependence on cluster size.@footnote 2@ @FootnoteText@@footnote 1@ M. Andersson, J.L. Persson, A. Rosen, J. Phys. Chem. 100, 12222 (1996).@footnote 2@ M. Andersson, A. Rosen, J. Chem. Phys. 117, 7051 (2002).