Size-engineered noble metal nanoclusters synthesized by impregnation for size-dependent catalysis

Noble metal nanoclusters, as a bridge between atoms and nanoparticles, show extraordinary catalytic activities because of their unique electronic structures. Such nanoclusters are usually synthesized by a wet chemical approach through unfavorable surface passivation by ligands and have not been obtained by an impregnation approach with satisfactory size uniformity. Herein, we report highly uniform ligand-free noble metal nanoclusters, such as Pd nanoclusters, synthesized each in a silica capsule by an impregnation strategy, with their size accurately engineerable in the range of 0.9–2.9 nm. The key is to ensure the impregnation synthesis with strict control over the nucleation and growth of noble metal nanoclusters by involving hollow silica nanoreactors and non-noble metal competitor ions, thereby showing unprecedented advantages of operational simplicity, scalability, and excellent size controllability. These Pd nanoclusters exhibit a strong size effect on catalysis exemplified by the alcohol oxidation-Knoevenagel condensation sequential reaction, with those of 1.3 nm exhibiting the highest catalytic activity. We believe this novel strategy opens new opportunities for rationally designing efficient noble metal nanocluster-type catalysts for a broad range of important reactions and bringing laboratory-quality nanoclusters into large-scale applications. [Figure not available: see fulltext.]