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

Paper SS2-MoM7
Reactions on Free Platinum Clusters: Adsorption of Oxygen and Hydrogen and Formation of Water

Monday, October 29, 2001, 11:40 am, Room 121

Session: Metal Clusters
Presenter: M. Andersson, Chalmers University of Technology and Göteborg University, Sweden
Authors: M. Andersson, Chalmers University of Technology and Göteborg University, Sweden
A. Rosén, Chalmers University of Technology and Göteborg University, Sweden
Correspondent: Click to Email
We use a cluster beam experiment to investigate chemical reactions on the surface of small unsupported metal clusters. A pulsed beam of metal clusters is generated with a laser vaporization source, in which metal atoms are vaporized into a flow of helium gas and condense in small clusters. After expansion into vacuum, the cluster beam passes through two reaction cells. The cell pressure is varied over a range where the clusters make from less than one up to a few collisions with the reactive molecules. The clusters are detected with laser ionization and time-of-flight mass spectrometry. By measuring the abundance of pure clusters and reaction products as a function of reaction cell pressure, the reaction probability in a cluster-molecule collision can be determined. For platinum clusters with more than 6 atoms we measure stable reaction products with both oxygen and hydrogen. The reaction probability with oxygen is for most sizes between 0.2 and 0.3, and appears lower with hydrogen though difficult to quantify since the Pt isotope distribution limits the mass resolution. If we let the clusters collide with both hydrogen and oxygen molecules, the resulting mass spectrum deviates significantly from a co-adsorption spectrum where the respective contributions are added. Instead, if, for example, the clusters first react with oxygen and then hydrogen we observe fewer oxide products and more pure clusters compared without hydrogen collisions. The only reasonable explanation for this is that water molecules are formed on the clusters and desorb. The efficiency of the reaction is high on all cluster sizes measured (8-30 atoms), with only a weak dependence on cluster size.