Finely tailoring the local ensembles in heterostructured high entropy alloy catalysts through pulsed annealing.

High-entropy alloys (HEAs) are promising catalysts particularly adept for reactions involving multiple intermediates and requiring multifunctional active sites. However, conventional syntheses often result in either (kinetically) random-mixing HEA or (thermodynamically) phase-separated composites-both fail to fine-tune local structures and further optimizing their performances. Here we present finely tailoring the local ensembles in HEA catalysts through rational composition design and sequential pulsed annealing. Employing PdSnFeCoNi HEA as a model, pulsed annealing (e.g., 0.5 s heating at 1300 K for 30 cycles) leverages differences in enthalpic interactions and surface energies to control the formation of ultrafine PdSn clusters within the HEA matrix, yielding the heterostructured HEA/c-PdSn. Compared with random HEAs and commercial Pd/C, HEA/c-PdSn exhibits >5 - 10-fold higher mass activity and good stability (>90.6% retention after 2000 cycles) for ethanol oxidation. This enhancement arises from the synergy between active local ensembles and the multifunctional HEA matrix, which reduces overall limiting potential, mitigates sluggish C-C/C-H breaking, and enhances structural stabilization. Our findings provide a strategy for engineering heterostructured HEAs for broad catalytic applications.