One Nanometer Matters: Quantum-Induced Discontinuity in the Oxygen Reduction Reaction Catalyzed by Platinum Nanoparticles

Quantum-induced discontinuity in the oxygen reduction reaction (ORR) catalyzed by Pt nanoparticles (NPs) occurs at the ~1.0 nm scale. Using a controlled solution plasma method, we synthesized monodisperse, surfactant-free ~1.0-nm Pt NPs uniformly supported on single-walled carbon nanotubes. Electrochemical evaluation revealed a pronounced deviation from the classical size-scaling behavior: the catalytic activity decreased with decreasing Pt NP diameter from 2.5 to 1.5 nm but unexpectedly increased at ~1.0 nm. High-resolution structural and spectroscopic analyses confirmed a critical transition around 1.5 nm, which separates the classical metallic behavior from a regime governed by quantum confinement. Despite their partial structural disorder, these quantum-sized clusters exhibited superior ORR performance, attributed to discrete electronic states, altered d-band structures and a high density of low-coordination active sites. The catalysts also demonstrated high durability, retaining ~90% of its reduction current after 2,000 cycles with <5% particle growth in acidic media. In particular, the superior ORR performance at ~1.0 nm is consistent with the formation of magic number clusters exhibiting high symmetry, closed shell stability, and facet-specific reactivity. These structures deviate from conventional crystal habits, favoring icosahedral or truncated geometries rich in undercoordinated edge and corner atoms. The resulting disruption of long-range order and emergence of localized quantum states redefines the catalytic paradigm at this scale. This abrupt improvement in ORR performance establishes a fundamental boundary between classical and quantum electrocatalysis. By reframing ultrasmall Pt NPs as quantum objects rather than miniature metals, this study introduces a new design principle: harnessing quantum effects and symmetry-driven structural motifs for the rational design of next-generation catalysts in sustainable energy technologies.