1. National Centre for High-Resolution Electron Microscopy, Rotterdamseweg 137, 2628 AL Delft, The Netherlands, Shell Research and Technology Centre, Amsterdam, P. O. Box 38000, 1030 BN Amsterdam, The Netherlands, Faculty of Chemical Technology and Materials Science, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands, and Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands