Volcano-Type Relationship between Elemental Mixing and Catalytic Activity in High-Entropy Alloy Clusters

Supported high-entropy alloy (HEA) materials have emerged as promising catalysts. However, the surface properties of the support, the subsequent entropy-driven changes in HEA structure, and the resultant catalytic performance remain elusive. Here, we develop an HEA cluster catalyst, PtPdRhRuNi/CeO₂, where Pt/Pd/Rh and Ru/Ni serve as active metals and promoters, respectively. The PtPdRhRuNi/CeO₂ catalyst outperforms the mono- and multimetal counterparts for ammonia borane hydrolysis. Increasing the reduction temperature improves the intrinsic catalytic activity due to enhanced elemental mixing, but exposure of Pt/Pd/Rh atoms is simultaneously limited. This trade-off creates a volcano-type relationship between the elemental mixing and the H₂ generation rate. The highest H₂ generation rate of 1208 mol_H2 mol_PtPdRh^-1 min^-1 is achieved by the PtPdRhRuNi/CeO₂ catalyst reduced at 400 °C. The formation of HEA clusters at a relatively low reduction temperature is attributed to the hydrogen spillover on the reducible oxides, whose modification is effective to further improve the catalytic activity. This study provides a strategy for rationally modulating HEA structures by controlling the efficacy of configurational entropy.