<p>Supported noble metal cluster catalysts provide the advantages of high atom efficiency and size-dependent properties, but their stabilization remains a major challenge for industrial applications. Now we report an approach for the stabilization of nuclearity-controlled platinum nanoclusters with a typical diameter of ~0.7 nm (Pt₇₋₁₄) confined on CeO_<i>x</i> nanoislands on a porous silica support. The clusters were synthesized by the reduction of platinum single atoms on the islands in H₂ at 400 °C. Redox cycles led to cluster formation and breakup at hundreds of degrees Celsius, with platinum remaining confined to the respective islands. The clusters maintained their nuclearity and were resistant to sintering in H₂ at temperatures of ≤600 °C and atmospheric pressure. Experimental catalyst performance data bolstered by computational results demonstrate that these platinum clusters are more active than mononuclear platinum, also exhibiting higher steady-state activity than larger and smaller platinum clusters for ethylene hydrogenation.</p><p></p>