Abstract
Supported by NWO under grant 98037. The research of AJK has been made possible
by a fellowship of the Royal Netherlands Academy of Arts and Sciences (KNAW). The
authors thank J.E.M.J. Raaymakers for the nitrogen physisorption measurements, A.J.M.
Mens for the XPS measurements, J.A.R. van Veen and E.J. Creyghton for physical data and
useful
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