1. Departement Anorganische Chemie en Katalyse, Debye Instituut, Universiteit Utrecht, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands, Core Research and Development, The Dow Chemical Company, Midland, Michigan 48674, Laboratory “Catalyse et Spectrochimie”, UMR CNRS, ENSICAEN, Caen University, 6 bd. Maréchal Juin, 14050 Caen Cedex, France, and Institute for Chemical Technology, Technische Universität München, Lichtenbergstrasse 4, D-85747 Garching, Germany