An important point is that the Ge addition process is self-limiting, i.e., if the GeCl4 dose was doubled, the amount of Ge deposited on the clusters did not change, within the uncertainty of the XPS analysis. Thus, the amount of Ge deposited is assumed to be limited by the number of GeCl4 binding sites, rather than the dose. This is important, because it suggests that for samples containing a particular size Ptn cluster, the process prepares PtnGem/alumina model catalysts with Pt:Ge stoichiometry controlled by the number of GeCl4 binding sites/cluster, i.e., the cluster size and composition should be uniform.