Pt4Ge/alumina characterization. As described in the Methods section, Pt4Ge/alumina samples were prepared by soft-landing mass-selected Pt4 clusters on thin-film alumina supports, followed by Ge addition by exposure to GeCl4 and H2. The cluster coverage was 3.8 x 10 13 clusters/cm 2 , equivalent to 0.1 ML Pt, in the form of Pt4 clusters. S/TEM imaging on both carbon and alumina-coated aluminum grids is described in the supplementary information (Fig. ).