1. Department of Chemistry, University of Oslo, P.O. Box 1033, N-0315 Oslo, Norway, Dipartimento di Chimica IFM and NIS Centre of Excellence, Università di Torino, via Pietro Giuria 7, I-10125 Torino, Italy, and INSTM Centro di Riferimento, Università di Torino, Torino, Italy, Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark, and Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1128, N- 0318 Oslo, Norway