Adjacent-Site Proximity as a Dominant Activity Descriptor in Single-Atom Pt Catalysts for Hydrogen Evolution Reaction

Abstract Precise control of adjacent-site proximity and electronic states in single-atom catalysts (SACs) enable atomic-level modulation of intrinsic catalytic properties. While the influence of electronic structure on catalytic performance is well established, the impact of adjacent-site proximity remains underexplored. Here, we report the single-atom platinum catalysts on MoS 2 (Pt-SAC/MoS 2 ), in which both the controlled enrichment of adjacent Pt (Pt adj ) sites and the Pt oxidation state are tuned via galvanic displacement of underpotentially deposited Cu adatoms. We find that hydrogen evolution reaction (HER) activity is predominantly governed by non-bonded Pt∙∙∙Pt proximity rather than oxidation state: enriched Pt adj sites in Pt SA -0.1/MoS 2 exhibits a mass activity 41-fold higher than isolated Pt (Pt iso ) sites in Pt SA -0.3/MoS 2 under acidic conditions. In situ infrared spectroscopy reveals that Pt iso sites preferentially bind linear adsorbed hydrogen intermediate ( * H L ), whereas Pt adj sites stabilize bridge hydrogen intermediate ( * H B ), which is indicative of adjacent-site proximity. Density functional theory calculations reveal that neighboring Pt atoms promote the formation of a three-center “Pt–H–Pt” bonding intermediate, which lowers the H–H coupling barrier and accelerates HER kinetics. These findings establish adjacent-site proximity as a dominant activity descriptor in SACs and provide new design principles for next-generation high-performance electrocatalysts.