[1] Eni Spa Refining & Mkt Div, I-20097 San Donato Milanese, Italy
[2] Univ Milan, Dipartimento Sci Terra A Desio, I-20133 Milan, Italy
[3] Ctr Nanotechnol Innovat NEST Ist Italiano Tecnol, Pisa, Italy
[4] Johannes Gutenberg Univ Mainz, Inst Phys Chem, D-55099 Mainz, Germany