Composition-Dependent Near-Surface Structure of High-Entropy Alloy Catalysts for the Semihydrogenation of Alkynes

High-entropy alloys (HEAs) have drawn intensive interest in catalysis owing to their uniquely structured metal sites. However, the composition-dependent near-surface structure of HEAs, which is critical for regulating surface electronic structure and property, still remains unclear. Herein we reported a feasible wet-chemical strategy to synthesize quinary PdxFeCoNiCu (Pdx- HEA) nanocrystals, which enabled the flexible control of Pd content (x = 2-27%) and the resultant tunable phase structure and size. The lattice expansion near surface increased with increasing heavy Pd atoms in Pdx-HEAs, which were quantitatively unveiled by aberration-corrected transmission electron micros-copy. The semihydrogenation performance of alkynes to alkenes had a "volcano" plot in the dependence of Pd content in Pdx-HEAs. Among them, the Pd21-HEA catalyst exhibited the best performance for a series of alkynes, which was ascribed to the uniquely surrounding electronic environment around Pd induced by the strongest interaction (i.e., maximum electron transfer) between Co/ Ni and Cu/Pd. This work demonstrated the importance of HEA composition-dependent lattice strain and the resultant surrounding electronic environment of Pd sites on the highly selective semihydrogenation.