1. Contribution from the Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16 3508 TB Utrecht, The Netherlands; Albermarle Catalysts Company BV, Nieuwendammerkade 1-3 1022 AB Amsterdam, The Netherlands; and Akzo Nobel Chemicals Research, Velperweg 76 6824 BM Arnhem, The Netherlands