Stability Thresholds of Atomically Dispersed Platinum Catalysts for Solar Hydrogen Production

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 TiO₂ nanoparticles dictates their structural evolution and photocatalytic performance during the H₂ 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 ΔΔG^‡ 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.