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 (dsite) was systematically regulated from 1.62 to 0.49 nm, and its effect on PMS activation was investigated. The catalyst with the shortest dsite (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 dsite 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.