Direct Imaging Reveals the Atomic Mechanism of Active‐Site Formation in Nanoclusters for Hydrogen Production

ABSTRACT Understanding how catalytically active sites emerge and evolve under working conditions is a fundamental challenge that limits the rational design of heterogeneous catalysts. Here, we directly visualize the transformation between alloyed PtNi and phase‐separated Pt‐NiO nanoclusters during hydrogen evolution. Using in situ low‐voltage aberration‐corrected electron microscopy, with the electron beam serving as both the stimulus and probe, we track the formation of active sites under low‐water‐vapor conditions. PtNi nanoclusters were assembled with controlled mixing of the atoms, resulting in two distinct configurational entropy states. Under reaction conditions, the transformation of bimetallic nanoclusters shifts from an entropically stabilized alloy to an enthalpically favored phase‐separated configuration, controlled by oxygen availability and by a critical nucleus size. The atomic dynamics observed in real space correlate directly with catalytic performance, where the low‐entropy Pt‐NiO state achieves a record hydrogen evolution mass activity of 11.1 A/mg Pt due to a high density of interfacial sites that promote water dissociation on NiO and efficient hydrogen adsorption on Pt atoms.