1. Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium, Department of Chemical Engineering, 2145 Sheridan Road, Northwestern University, Evanston, Illinois 60208, and Center for Surface Science and Catalysis, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium