Optimal inter-site distance in Cu single-atom catalysts for efficient peroxymonosulfate activation and ciprofloxacin degradation

Ciprofloxacin is a widely used antibiotic that persists in aquatic environments, posing environmental risks. Peroxymonosulfate (PMS)-based advanced oxidation processes are promising for ciprofloxacin (CIP) degradation due to their strong oxidative capability. In this work, high-loading Cu single-atom catalysts were synthesized via polycondensation followed by pyrolysis, achieving Cu loadings up to 5.9 at.% with exclusive atomic dispersion confirmed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS). By tuning Cu atomic density, the inter-site distance (d_site) was systematically regulated from 1.62 to 0.49 nm, and its effect on PMS activation was investigated. The catalyst with the shortest d_site (0.49 nm) exhibited superior catalytic performance, achieving complete CIP degradation within 30 min with a kinetic rate constant of 0.3242 min⁻¹. Response surface methodology revealed that the reaction time, catalyst dosage, and PMS concentration significantly influenced degradation efficiency. Although d_site alone was not statistically significant (p = 0.162), its interaction with PMS concentration was highly significant (p = 0.001), indicating a synergistic role of the spatial Cu arrangement. Mechanistic studies combining quenching experiments, electronic paramagnetic resonance (EPR) spectroscopy, potassium iodide (KI)-based PMS consumption analysis, and solvent isotope effects demonstrate a dual-pathway PMS activation mechanism dominated by a surface-confined non-radical singlet oxygen (¹O₂) pathway, accompanied by auxiliary radical pathways. These results highlight the importance of inter-site distance engineering for designing efficient single-atom catalysts for advanced water treatment.