Unveiling Atomic-Scaled Local Chemical Order of High-Entropy Intermetallic Catalyst for Alkyl-Substitution-Dependent Alkyne Semihydrogenation.

ABSTRACT

High-entropy intermetallic (HEI) nanocrystals, composed of multiple elements with an ordered structure, are of immense interest in heterogeneous catalysis due to their unique geometric and electronic structures and the cocktail effect. Despite tremendous efforts dedicated to regulating the metal composition and structures with advanced synthetic methodologies to improve the performance, the surface structure, and local chemical order of HEI and their correlation with activity at the atomic level remain obscure yet challenging. Herein, by determining the three-dimensional (3D) atomic structure of quinary PdFeCoNiCu (PdM) HEI using atomic-resolution electron tomography, we reveal that the local chemical order of HEI regulates the surface electronic structures, which further mediates the alkyl-substitution-dependent alkyne semihydrogenation. The 3D structures of HEI PdM nanocrystals feature an ordered (intermetallic) core enclosed by a disordered (solid-solution) shell rather than an ordered surface. The lattice mismatch between the core and shell results in apparent near-surface distortion. The chemical order of the intermetallic core increases with annealing temperature, driving the electron redistribution between Pd and M at the surface, but the surface geometrical (chemically disordered) configurations and compositions are essentially unchanged. We investigate the catalytic performance of HEI PdM with different local chemical orders toward semihydrogenation across a broad range of alkynes, finding that the electron density of surface Pd and the hindrance effect of alkyl substitutions on alkynes are two key factors regulating selective semihydrogenation. We anticipate that these findings on surface atomic structure will clarify the controversy regarding the geometric and/or electronic effects of HEI catalysts and inspire future studies on tuning local chemical order and surface engineering toward enhanced catalysts.