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

Highlights

• 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.

Abstract

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.

Introduction

Persulfate, generically referring to peroxymonosulfate (PMS) and peroxydisulfate (PDS), activation processes are physicochemical methods that promote its oxidizing capability [[1], [2], [3]]. The treatment efficiency of persulfate oxidation is typically increased in a similar method as that commonly adopted in H₂O₂-based advanced oxidation processes. This method involves cleaving a peroxide bond homolytically or heterolytically through energy or electron transfer to yield highly reactive free-radical intermediates such as sulfate radicals (SO₄⁻) and hydroxyl radicals (OH) [1,4,5]. Alternatively, activation strategies that rely on the reactivity of persulfate as a two-electron oxidant treat organic and inorganic contaminants without producing persulfate-derived radicals [1,6,7]. When no radical attacks are involved in persulfate activation, the self-decay of persulfate that accompanies the generation of singlet oxygen (¹O₂) is kinetically enhanced [1,8,9] or electron delivery from organic substances to persulfate is effectively mediated [7,10,11]. Non-radical persulfate activation routes have often been demonstrated to enable organic treatment in heterogeneous processes using carbonaceous nanomaterials [1,7,11,12].

The use of nanoscale or nanostructured carbon-based materials such as (heteroatom-doped) carbon nanotubes (CNTs) [11,13] and graphene [14,15] as metal-free persulfate activators has increased in recent years. Furthermore, nanodiamond (ND) and its derivatives as potential substitutes have also been explored as alternative persulfate activators, owing to the common advantages of nanocarbons such as physical/chemical robustness and large surface area [6,12,[16], [17], [18]]. However, considering that pristine diamond nanocrystals, comprising primarily sp³-hybridized carbon atoms are nearly unreactive toward persulfate [[16], [17], [18]], surface ND modification through (i) dopant atom incursion and (ii) thermal annealing, which is widely used to boost the electrocatalytic activity for redox reactions [19,20], is a prerequisite for their use as carbocatalysts in persulfate activation. Only a few studies have demonstrated the application of surface-modified NDs for persulfate activation [6,12,[16], [17], [18]], which is likely due to the technicalities related to the pretreatment requirements. Thermal treatment at temperatures > 700°C initiates diamond-to-graphite transformation on the ND surface (surface graphitization) and subsequent formation of core-shell structures in which graphitic carbon shells cover the inner core of the diamond [20,21]. The surface-graphitized NDs developed superior capacity for persulfate activation owing to (i) the high affinity of superficial graphitic carbon toward persulfate and (ii) promoted interfacial electron transfer over electron-enriched carbon shells [6,[16], [17], [18]]. Heteroatom (e.g., N, B, and P) doping, often used to promote the electrocatalytic activity of ND for the oxygen reduction reaction, causes an uneven charge distribution on the adjacent carbon atoms, thereby facilitating surface oxygen sorption and its reduction [19]. Consequently, a noticeable kinetic increase in persulfate activation and the associated organic oxidation occurred with N-doped (annealed) NDs [12].

Studies investigating annealed NDs as persulfate activators have focused primarily on the role of the outer graphitic carbon shell in the persulfate activation performance and mechanism(s). An increase in the surface sp²/sp³ carbon ratio observed as the ND annealing temperature was increased from 600 to 1000°C markedly improved PMS or PDS activation capability [16,18]. In contrast, additional surface graphitization that occurred in the temperature range of 900 to 1100°C caused a gradual decline in PMS activation performance and, more importantly, resulted in a switch in the primary oxidation reaction route from radical-induced oxidation to mediated electron transfer involving a surface PMS complex [6]. However, little is known regarding the variations in persulfate activation capability of NDs and the underlying mechanism as the transformation of NDs into carbon onions progresses at temperatures > 1200°C. Carbon onions are identified as nanoscale carbon spheres or polyhedrons comprising multilayered concentric graphitic shells, formed via the (quasi) complete conversion of inner diamond carbon into graphitic carbon [21]. The increase in electrical conductivity improved the electrocatalytic activity of the annealed NDs [22]. In a study by Zieger et al. [21], the temperature increase from 800 to 1500°C increased the conductivity from 0.25 to 2 S cm⁻¹. The electrical conductivity of nanocarbons is presumed to be a key factor in persulfate activation [12,18]; further increasing the annealing temperature above 1100°C (the highest level yet applied for ND graphitization and subsequent use in persulfate activation) would allow the fabrication of high-efficiency ND-derived activators. Additionally, the substantial sp³-to-sp² transformation achieved at high-temperature ND annealing promoting non-radical persulfate activation should be investigated. The increase in the thickness of the outer graphitic carbon layer was hypothesized to favor the interaction of ND with PMS, thereby triggering the transition in the major elimination reaction pathway [6].

To fill the knowledge gap regarding the temperature-sensitive persulfate activation capacity and mechanism(s) of graphitized NDs, we examined the ND samples prepared within an annealing temperature range of 430 to 2000°C for persulfate activation. Furthermore, we correlated their activity with the relative sp² fraction. The effects of reaction parameters such as persulfate dosage, catalyst loading, initial pH, natural organic matter (NOM), and background inorganic anions on persulfate activation efficiency were investigated. The primary persulfate activation route induced by carbon onions was determined based on the (i) quenching effects of alcohol-based scavengers, (ii) conversion efficiencies of methanol (MeOH) into formaldehyde (HCHO) and hydroxylation of benzoic acid (BA), (iii) bromate (BrO₃⁻) formation yield, and (iv) electron paramagnetic resonance (EPR) spectral features. The surface affinity and electrical conductivity of thermally treated NDs were quantitatively analyzed using isothermal titration calorimetry (ITC) and electrochemical impedance spectroscopy (EIS), respectively. ITC is a biophysical technique that is used to quantitatively depict protein-nanoparticle [23] and NOM-nanoparticle [24] interactions. Finally, density functional theory (DFT) calculations were performed to gauge the number of outermost layers of graphitic carbon that could affect surface interactions with persulfate and the electron transfer process at the ND/water interface.