Sub-nanometric High-Entropy Alloy Cluster: Hydrogen Spillover Driven Synthesis on CeO₂ and Structural Reversibility

High-entropy alloy(HEA) nanoparticles (NPs) have attractedsignificantattention as promising catalysts owing to the various unique synergisticeffects originating from the nanometer-scale, near-equimolar mixingof five or more components to produce single-phase solid solutions.However, the study of sub-nanometer HEA clusters having sizes of lessthan 1 nm remains incomplete despite the possibility of novel functionsrelated to borderline molecular states with discrete quantum energylevels. The present work demonstrates the synthesis of CeO2 nanorods (CeO2-NRs) on which sub-nanometer CoNiCuZnPdHEA clusters were formed with the aid of a pronounced hydrogen spillovereffect on readily reducible CeO2 (110) facets. The CoNiCuZnPdHEA sub-nanoclusters exhibited higher activity during the reductionof NO by H-2 even at low temperatures compared with thecorresponding monometallic catalysts. These clusters also showed aunique structural reversibility in response to repeated exposure tooxidative/reductive conditions, based on the sacrificial oxidationof the non-noble metals. Both experimental and theoretical analysesestablished that multielement mixing in quantum-sized regions endowedthe HEA clusters with entirely novel catalytic properties.