1. Contribution from Instituto de Catálisis y Petroleoquímica and Instituto de Ciencia de Materiales de Madrid, CSIC, Campus Universidad Autónoma de Madrid, 28049 Madrid, Spain, Department of Chemical Enzymology, Chemistry Faculty, Moscow State University, RU-119899 Moscow, Russia, Department of Analytical Chemistry, University of Lund, P.O. Box 124, SE-221 00 Lund, Sweden, and T&D Technology, Stanley, Wakefield WF3 4AA, England