Persulfate activation by nanodiamond-derived carbon onions: Effect of phase transformation of the inner diamond core on reaction kinetics and mechanisms

[Display omitted] •Persulfate activation capacity of nanodiamonds improves with increasing sp2/sp3 ratio.•Quasi-complete phase transition renders nanodimoands outperform benchmark nanocarbons.•Nanodiamond-derived carbon onions cause non-radical persulfate activation.•Graphitic carbon layer growth hinders the interaction of persulfate with nanodiamonds.•Electrical conductivity of nanodimonds is proportional to the graphitization degree. To investigate the impact of carbon phase conversion on the catalytic activity of nanodiamonds, in this study, we tested nanodiamonds subjected to graphitization at varying temperatures for persulfate activation. Temperatures beyond 1000°C (where only surface graphitization occurs) steadily enhanced the persulfate activation capability as the inner carbon underwent substantial sp3-to-sp2 transformation. Nanodiamonds annealed at 2000°C outperformed benchmark nanocarbons in terms of persulfate activation efficiency. Non-radical activation occurred primarily based on the effects of radical quenchers, oxidation product distribution, substrate-dependent reactivity, and electron paramagnetic resonance spectra. Aligned with the density functional theory calculations of the binding energies of peroxydisulfate on the slab models, built via Bernal stacking of graphitic carbon layers on the diamond plane, isothermal titration calorimetry measurements suggested that the binding affinity of peroxydisulfate decreased as the sp2/sp3 ratio increased. Therefore, the enhancing effect of graphitization arose from the electrical conductivity of nanodiamonds, which increased proportionally with graphitization extent.