Stability Thresholds of Atomically Dispersed Platinum Catalysts for Solar Hydrogen Production

Abstract The structural fluidity of single‐atom photocatalysts under illumination challenges conventional assumptions about catalytic identity, prompting a reevaluation of what defines and sustains active sites. Here, we show that the site density of atomically dispersed Pt on TiO 2 nanoparticles dictates their structural evolution and photocatalytic performance during the H 2 evolution reaction (HER). There is a critical dispersion threshold that separates the stable single‐atom state from the aggregative regime with less reactive multi‐atom ensembles. Under optimized conditions, isolated Pt sites resist light‐enhanced agglomeration and deliver HER activities (0.246 s −1 ) up to 82‐fold higher than those of Pt nanoparticles (0.003 s −1 ), achieving an apparent quantum yield of 9.1%. Beyond this threshold, atomic dispersion deteriorates through a first‐order aggregation process, resulting in an exponential loss of isolated sites and a sharp rise in the activation free energy ΔΔ G ‡ up to 17.1 kJ mol −1 . Combined experimental and theoretical analyses quantify the transition in catalyst architecture and activity, revealing a structure–stability–activity relationship. This framework defines a reactivity window governed by the interplay between spatial isolation and structural fragility in single‐atom catalysis.