Rapid self-heating synthesis of Fe-based nanomaterial catalyst for advanced oxidation

Organic contamination of soil and water threatens human health, particularly in developing and sparsely populated regions where treatment is incomplete. Advanced oxidation processes (AOPs) that generate highly reactive oxygen species (e.g., hydroxyl radicals) via metal-catalyzed activation of oxidants (e.g., peroxydisulfate) are a promising remediation strategy, making robust catalysts vital for wastewater treatment. Fe-based materials are central catalysts in AOPs but conventional synthesis typically relies on prolonged high-temperature thermal treatment, causing metal agglomeration and low atom utilization. Approaches such as introducing defective edges or N/O/S heteroatoms in carbon substrates can anchor metals and reduce agglomeration, but most methods are tedious and energy intensive. Conventional maximum temperatures can also limit in situ formation of superior phases because many mineral bonds require higher temperatures to break; moreover, carbon substrates prepared at such temperatures often have poor electron transport. These issues hamper performance by constraining catalytic constituents, structure, and scalable fabrication. Flash Joule Heating (FJH) can deliver ultra-high temperatures (~3000 K) within seconds, intense electric shock, and ultrafast cooling (~10^5 K s^-1), potentially overcoming these shortcomings: (1) instantaneous extreme conditions can decompose Fe minerals to Fe0 forming synergistic heterostructures; (2) ultrafast processing suppresses metal agglomeration to yield highly dispersed sites; (3) the electric/thermal shock can convert carbon substrates into thin-bedded graphene, enhancing conductivity. To test this, the authors developed a soft-carbon-assisted FJH route to synthesize an electron-rich nano Fe0/FeS heterostructure embedded in graphene, benchmarked against conventional pyrolysis for chloramphenicol (CAP) degradation via peroxydisulfate activation, examined the role of carbon substrate and FJH power, generalized across Fe precursors, and proposed an automated continuous production strategy.

Prior work has sought to mitigate Fe catalyst agglomeration and improve activity by engineering carbon supports with defective edges or doping with heteroatoms (N, O, S) to anchor metals and create multiple active sites (refs. 2–5). Nonetheless, many reported syntheses are multi-step and require high energy input (refs. 6–9), and conventional pyrolysis temperatures may be insufficient to break strong bonds in certain minerals, limiting formation of highly active phases (ref. 10). Additionally, carbons produced under conventional carbonization can have low electron transport ability, hindering electron-transfer-driven AOP pathways (ref. 11). FJH and related carbothermal shock methods have recently demonstrated ultrafast, ultra-high-temperature processing capable of producing graphene and nanostructured metals (refs. 12, 15), suggesting a route to superior Fe-based AOP catalysts with improved conductivity and dispersion. However, systematic evaluation of FJH-derived Fe catalysts for persulfate activation and mechanistic understanding of their electron-transfer advantages remained needed.

Materials synthesis: Soft carbon (hydrochar) was produced by hydrothermal liquefaction of rice straw (15.0 g) in distilled water (210 mL) at 270 °C for 60 min under 300 rpm stirring in a 500 mL autoclave. The product was filtered and dried to obtain hydrochar. Hydrochar (1.00 g) and FeS powder (0.60 g, low-grade Fe mineral) were uniformly mixed in deionized water, dried under vacuum, then blended with carbon black (10 wt% as conductive additive), yielding Fe-C-raw. FJH treatment: Fe-C-raw (0.12 g) was loaded into a quartz tube and compressed with copper electrodes to set sample resistance to ~200 Ω. A mild vacuum (>38 kPa) was maintained to prevent oxidation. Applying a set voltage/current initiated Joule self-heating; parameters (operation voltage, external resistance, reaction time) were varied (Supplementary Table 10) to produce Fe-C-FJH and power-variant samples (FJH-P-L/M/H). Current/voltage were recorded by oscilloscope; temperature was measured by IR spectrometry with blackbody fitting; lighting intensity was recorded by high-speed camera and processed via HSV/MATLAB. Conventional pyrolysis: Fe-C-raw was heated in a tubular furnace (5 K min^-1) to 973 K for 90 min under N2 (100 mL min^-1) to obtain Fe-C-PY. Catalytic tests: CAP degradation via PDS activation was evaluated typically at [CAP] = 60 mg L^-1, catalyst = 1000 mg L^-1, [PDS] = 7 mmol L^-1, initial pH = 3.0 ± 0.2, 28 °C. Adsorption controls and radical quenching (tert-butanol, methanol, KI) were conducted. Characterization: XRD, 57Fe Mössbauer spectroscopy, XPS, XANES/EXAFS (including WT), TEM/EDS, HAADF-STEM with STEM-EELS (Fe L-edge, O K-edge), Raman spectroscopy, electrochemical measurements (CV, EIS), BET surface area. Temperature/current/power profiles and Joule heat were quantified during FJH. Computational methods: DFT calculations modeled PDS adsorption and activation on FeS/C, Fe0/C, and Fe0/FeS heterostructures embedded in graphene (Fe0/FeS/C and FeS/Fe0/C). Adsorption energies, O–O bond lengths, electron density differences, reaction pathways for O–O bond cleavage, and Gibbs free energy profiles were computed to elucidate electron delocalization effects and bidentate binuclear binding.

• FJH enabled the rapid transformation of FeS and soft carbon into an electron-rich nano Fe0/FeS heterostructure embedded in thin-bedded graphene. The process is highly energy efficient, consuming 34× less energy than conventional pyrolysis (as stated in the abstract). The FJH cycle features ultra-high temperatures (up to ~1800 K observed; up to ~3000 K capability) and ultrafast cooling (~10^5 K s^-1), producing strong electric shock and rapid quenching. - Catalytic performance: In CAP degradation with PDS, Fe-C-FJH achieved 94.1% removal, far exceeding Fe-C-raw (17.0%) and Fe-C-PY (8.78%). The observed rate constant k_obs was highest for Fe-C-FJH; CAP adsorption on Fe-C-FJH was negligible. Hydrochar (higher resistance) as soft carbon improved self-heating and performance over pyrochar (lower resistance). FJH-derived materials outperformed comparable Fe materials prepared by conventional methods. - Active species and mechanism: EPR detected •OH and SO4•− only in the Fe-C-FJH/PDS system, with weak SO4•− due to conversion to •OH. Radical scavenging indicated surface-confined •OH as the dominant ROS. CAP degradation positively correlated with •OH and dissolved Fe2+ concentrations; Fe salts alone and dissolved Fe ions contributed negligibly. Commercial Fe3C and Fe2O3 showed negligible activity, implicating Fe0/FeS heterostructure as the main active component. - Composition and structure: XRD showed FeS, Fe, and Fe3C in Fe-C-FJH; Fe-C-raw and Fe-C-PY contained only FeS. Mössbauer of Fe-C-FJH quantified Fe species: FeS (26.49%), Fe7S8 (24.98%), FeO (14.94%), Fe3C (29.32%), Fe2O3 (4.27%). XPS detected Fe0, Fe2+, and Fe3+. XANES situated Fe oxidation state between Fe0 and Fe2+. EXAFS/Wavelet transform revealed Fe–Fe bonds at ~2.46 Å, shortened Fe–S distances (~2.36 Å), reduced S coordination (from 4 to 2), and increased Fe coordination (to 2), evidencing partial Fe–S bond breakage and S volatilization. - Morphology: TEM/EDS mapping showed uniformly dispersed Fe and S with smaller particle sizes after FJH (average ~16.8 nm). HAADF-STEM resolved (112) lattice of Fe0 and (114) lattice of FeS, confirming nano Fe0/FeS heterostructure. STEM-EELS indicated Fe0 and Fe2+ states and absence of O K-edge in regions A/B, excluding Fe2O3 in those areas. - Carbon structure and charge transport: XPS (C 1s) showed increased sp2/sp3 ratio; Raman displayed stronger G and emergent 2D peaks; ID/IG increased from 0.31 (Fe-C-raw) to 0.87 (Fe-C-FJH), I2D/IG = 0.81, evidencing thin-bedded graphene formation. Despite lower BET area than pyrolysis-derived materials (due to graphitization-induced pore closure), CV and EIS showed enhanced redox features (peaks at -0.09 V and -0.85 V vs Ag/AgCl) and lower charge-transfer resistance for Fe-C-FJH. - DFT insights: Graphene-supported FeS (FeS/C) strengthened PDS adsorption (binding energy improved from -2.026 to -2.126 eV) and slightly elongated the PDS O–O bond (1.527 to 1.530 Å). Fe0/FeS heterostructures embedded in graphene (Fe0/FeS/C and FeS/Fe0/C) showed much stronger PDS adsorption (e.g., -4.169 and -4.135 eV) and longer O–O bonds (~1.622 and 1.619 Å). Electron density difference analyses revealed enhanced electron delocalization and a bidentate binuclear binding model, providing two electron-transfer channels. The O–O bond cleavage pathway on Fe0/FeS/C had a lower energy barrier and a more favorable overall Gibbs free energy (-0.58 eV) than on Fe0/C, indicating easier, spontaneous PDS activation. - FJH power dependence: Increasing FJH power raised sample brightness/temperature, enhanced Fe0 formation (XRD: stronger Fe, Fe3C, Fe2O3 peaks), increased graphitization (Ic/Ip from 0.47 to 1.21; ID/IG up to 1.43), and improved CAP degradation. Lower power failed to adequately break Fe–S bonds or form thin-bedded graphene, yielding poorer activity. STEM-EELS across FJH power levels verified coexisting Fe0 (point A) and Fe2+ (point B). - Additional observations: Sulfur speciation partially converted from S2− forms to SOx, S–O, and C–S bonds after FJH. Fe-C-FJH achieved higher CAP degradation at lower Fe content, indicating efficient atom utilization. The method is applicable to other Fe precursors and is amenable to continuous, automated production. The catalyst exhibited strong performance over a wide pH range (as stated in the abstract).

The study demonstrates that flash Joule heating overcomes key limitations of conventional Fe-based AOP catalyst synthesis by rapidly breaking Fe–S bonds in FeS to form electron-rich nano Fe0/FeS heterostructures while simultaneously converting soft carbon into thin-bedded graphene. Ultrafast heating and cooling suppress agglomeration, yielding highly dispersed, nanoscale active sites with improved electron transport. These structural features enhance peroxydisulfate activation via stronger adsorption and facilitated O–O bond cleavage, primarily generating surface-confined •OH radicals for pollutant mineralization. DFT confirms that electron delocalization within the Fe0/FeS/graphene composite promotes a bidentate binuclear binding mode with PDS, lowering the energetic barrier and overall Gibbs free energy for activation relative to Fe0/C or FeS/C alone. The experimentally observed higher CAP removal rates, negligible adsorption, and correlations between •OH and Fe2+ with degradation efficiency validate the mechanistic picture. Power-dependent studies link stronger FJH conditions to greater Fe0 content, higher graphitization, and superior performance, aligning with the proposed structure–activity relationships. The process is markedly energy-efficient and scalable, and the catalyst shows robust activity across a wide pH window, suggesting relevance for practical wastewater remediation. The approach generalizes to other common Fe precursors and is compatible with continuous, automated production strategies, broadening its applicability in AOP-based treatment.

This work introduces a soft-carbon-assisted FJH route to synthesize an electron-rich nano Fe0/FeS heterostructure embedded in thin-bedded graphene for efficient persulfate activation. The ultrafast, energy-efficient process (34× lower energy than conventional pyrolysis) yields catalysts that achieve markedly superior CAP degradation with minimal adsorption, attributable to enhanced electron transport and synergistic Fe0/FeS active sites. Comprehensive spectroscopy and microscopy confirm Fe–S bond breakage, Fe0 formation, nanoscale dispersion, and increased graphitization; DFT reveals stronger PDS binding, a bidentate binuclear adsorption mode, and a lower free-energy pathway for O–O bond cleavage on Fe0/FeS/graphene. Tuning FJH power systematically improves catalyst structure and activity. The method is generalizable to various Fe precursors and compatible with continuous, automated production, pointing toward scalable deployment. Future work could extend to broader pollutant classes, optimize FJH process parameters for tailored active-site architectures, and integrate continuous production with reactor-scale AOP systems for real wastewater applications.