1. Contribution from the Department of Materials Science, University of Milano-Bicocca, Via R. Cozzi 53, I-20125 Milano, Italy, Department of Physical Chemistry, Environmental Research Centre, University of Milano, Via Golgi 19, I-20133 Milano, Italy, Institute for Energetics and Interphases, Department of Genova, National Research Council, Via De Marini, 6, I-16149 Genova, Italy, and Department of Physical Chemistry, University of Venezia, Via Torino 155/b, I-30172 Venezia, Italy