1. Contribution from the Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Laboratoire de Catalyse, UPRESA CNRS 8010, bât. C3, Université des Sciences et Technologies de Lille, 59650 Villeneuve d'Ascq Cedex, France