Details of the experimental and computational methods are given below. Additional detail regarding the preparation of size-and composition-selected Ptn/alumina and PtnGem/alumina model catalysts is given in the Supporting Information, along with X-ray photoelectron spectral (XPS) characterization of the Pt:Ge stoichiometry. In short, a Ta(110) single crystal was cleaned by heating to 2100 K in UHV, then a ~4 nm thick alumina film was grown by Al vapor deposition in an O2 atmosphere, as described previously. Size-selected Ptn were soft-landed on the alumina film to produce Ptn/alumina with coverage equivalent to 1.5 x 10 14 Pt atoms/cm 2 ≈ 0.1 ML. All samples, thus, had identical amounts of Pt, deposited in the form of different size clusters. To prepare PtnGem/alumina, freshly-prepared Ptn/alumina samples were exposed at 300 K to ~60 L of GeCl4, followed by ~6000 L of H2 (leading to HCl loss), and then briefly heated to desorb residual H and Cl as HCl and H2. The Pt:Ge stoichiometries produced by this process were determined by XPS, as described in the Supporting Information. XPS analysis showed that Ge deposited on Pt sites with efficiency 6 to 10 times higher than on alumina sites, but because 90% of the surface is alumina, there was also Ge present in alumina sites, amounting to ~4% of a close-packed Ge monolayer. As shown below, however, these aluminabound Ge atoms had little effect on reactivity, compared to Ge bound to the clusters. Note that the Ge deposition process is self-limiting, i.e., doubling the GeCl4 exposure did not significantly increase the amount of Ge deposited, implying that the process was limited by the number of GeCl4 adsorption sites.