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

High-entropy alloy (HEA) nanoparticles (NPs) have attracted significant attention as promising catalysts owing to the various unique synergistic effects originating from the nanometer-scale, near-equimolar mixing of five or more components to produce single-phase solid solutions. However, the study of sub-nanometer HEA clusters having sizes of less than 1 nm remains incomplete despite the possibility of novel functions related to borderline molecular states with discrete quantum energy levels. The present work demonstrates the synthesis of CeO₂ nanorods (CeO₂-NRs) on which sub-nanometer CoNiCuZnPd HEA clusters were formed with the aid of a pronounced hydrogen spillover effect on readily reducible CeO₂ (110) facets. The CoNiCuZnPd HEA sub-nanoclusters exhibited higher activity during the reduction of NO by H₂ even at low temperatures compared with the corresponding monometallic catalysts. These clusters also showed a unique structural reversibility in response to repeated exposure to oxidative/reductive conditions, based on the sacrificial oxidation of the non-noble metals. Both experimental and theoretical analyses established that multielement mixing in quantum-sized regions endowed the HEA clusters with entirely novel catalytic properties.