1. Leiden Institute of Chemistry P.O. Box 9502 2300 RA Leiden the Netherlands
2. Laboratory for Inorganic Materials and Catalysis De partment of Chemical Engineering and Chemistry Eindhoven University of Technology P.O. Box 513 5600 MB Eindhoven The Netherlands
3. Current address: Inorganic Chemistry and Catalysis, Department of Chemistry Utrecht University Universiteitsweg 99 3584 CG Utrecht The Netherlands
4. Current address: Surface Science Laboratory, Department of Materials and Earth Sciences Technical University of Darmstadt Otto-Berndt-Strasse 3 64287 Darmstadt Germany