Multiphase Porous Electrochemical Catalysts Derived from Iron-Based Metal-Organic Framework Compounds

High-Efficiency Electrocatalysis of Molecular Oxygen toward Hydroxyl Radicals Enabled by an Atomically Dispersed Iron Catalyst

Fenton catalysis represents the promising technology to produce super-active ·OH for tackling severe water environment pollution issues, whereas it suffers from low atomic efficiency, poor pH adaptability, and catalyst non-reusability in a homogeneous or heterogeneous system. Here, single-atom iron catalysis is creatively introduced to drive electrochemical ·OH evolution utilizing earth-abundant oxygen and water as raw materials. The atomically dispersed iron settled by defective three-dimensional porous carbon (AD-Fe/3DPC) with unique C, Cl unsaturated coordination can efficiently tune the multi-electron oxygen reduction process, enabling O₂-to-·OH conversion. The mass activity in ·OH production by AD-Fe/3DPC is almost two-orders of magnitude higher as compared to that by nanoparticular iron oxide catalyst. Meanwhile, the AD-Fe/3DPC electro-Fenton system exhibits fast elimination of refractory toxic pollutants, surpassing nanoparticular iron oxides in kinetic rate by 59 times or homogeneous Fenton by 10 times under similar experimental conditions. Experimental and theoretical results demonstrate that the remarkable enhanced mass activity of AD-Fe/3DPC in catalyzing O₂ to ·OH is contributed by the synergistic effects of the maximized catalysis of atomically dispersed iron and the unique unsaturated coordination environment. The AD-Fe/3DPC catalytic system is demonstrated to be pH-universal, long-term stable, and well recyclable, truly satisfying flexible, sustainable, and green application of wastewater purification. This study gives a new sight into local coordination modulation of single-atom catalysts for selective electrocatalytic oxygen reduction.

Mechanism and stability of an Fe-based 2D MOF during the photoelectro-Fenton treatment of organic micropollutants under UVA and visible light irradiation

This work reports the novel application of an Fe-based 2D metal-organic framework (MOF), prepared with 2,2'-bipyridine-5,5'-dicarboxylate (bpydc) as organic linker, as highly active catalyst for heterogeneous photoelectro-Fenton (PEF) treatment of the lipid regulator bezafibrate in a model matrix and urban wastewater. Well-dispersed 2D structures were successfully synthesized and their morphological, physicochemical and photocatalytic properties were assessed. UV/Vis PEF using an IrO₂/air-diffusion cell with an extremely low catalyst concentration (0.05 g L^-1, tenfold lower than reported 3D MOFs) outperformed electro-oxidation with electrogenerated H₂O₂, electro-Fenton and visible-light PEF. Its excellent performance was explained by: (i) the enhanced mass transport of H₂O₂ (and organic molecules) at the 2D structure, providing active sites for heterogeneous Fenton's reaction and in-situ Fe(II) regeneration; (ii) the ability of photoinduced electrons to reduce H₂O₂ to ^•OH, and Fe(III) to Fe(II); and (iii) the enhanced charge transfer and excitation of Fe-O clusters, which increased the number of electron-hole pairs. LC-QToF-MS and GC-MS allowed the identification of 16 aromatic products of bezafibrate. The complete removal of four micropollutants mixed in urban wastewater at pH 7.4 revealed the great potential of (Fe-bpydc)-catalyzed PEF process.

Fabrication of a double-layer membrane cathode based on modified carbon nanotubes for the sequential electro-Fenton oxidation of p-nitrophenol

To improve the electrocatalytic efficiency of the cathode and provide a wider pH range in the electro-Fenton process, N-doped multi-walled carbon nanotubes (NCNTs) and ferrous ion complexed with carboxylated carbon nanotubes (CNT-COOFe²⁺) were used to fabricate the diffusion layer and catalyst layer of a membrane cathode, respectively. The morphology, structure, and composition of CNT-COOFe²⁺ were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The oxygen reduction performance of NCNT was evaluated using cyclic voltammetry (CV) and the rotating disk electrode technique (RDE). In addition, a potential application of the cathode in sequential electro-Fenton degradation of p-nitrophenol (p-NP) was investigated. The results revealed that iron was successfully doped on the carboxylated carbon nanotubes in ionic complexation form and the content of iron atoms in CNT-COOFe²⁺ was 2.65%. Furthermore, the defects on the tube walls provided more reactive sites for the electro-Fenton process. A combination of CV and RDE data indicated that NCNT had better electrocatalytic H₂O₂ generation activity with a more positive onset potential and higher cathodic peak current response than CNT. A p-NP removal rate of 96.04% was achieved within 120 min, and a mineralization efficiency of 80.26% was obtained at 180 min in the sequential electro-Fenton process at a cathodic potential of - 0.7 V vs SCE and neutral pH. The activity of the used cathode was restored simply through electro-reduction at - 1.0 V vs SCE, and a p-NP removal rate of more than 70% was obtained at 60 min after six regeneration cycles.

A Highly Stable Metal-Organic Framework-Engineered FeS2/C Nanocatalyst for Heterogeneous Electro-Fenton Treatment: Validation in Wastewater at Mild pH

Herein, the novel application of FeS₂/C nanocomposite as a highly active, stable, and recyclable catalyst for heterogeneous electro-Fenton (EF) treatment of organic water pollutants is discussed. The simultaneous carbonization and sulfidation of an iron-based metal-organic framework (MOF) yielded well-dispersed pyrite FeS₂ nanoparticles of ∼100 nm diameter linked to porous carbon. XPS analysis revealed the presence of doping N atoms. EF treatment with an IrO₂/air-diffusion cell ensured the complete removal of the antidepressant fluoxetine spiked into urban wastewater at near-neutral pH after 60 min at 50 mA with 0.4 g L^-1 catalyst as optimum dose. The clear enhancement of catalytic activity and stability of the material as compared to natural pyrite was evidenced, as deduced from its characterization before and after use. The final solutions contained <1.5 mg L^-1 dissolved iron and became progressively acidified. Fluorescence excitation-emission spectroscopy with parallel factor analysis demonstrated the large mineralization of all wastewater components at 6 h, which was accompanied by a substantial decrease of toxicity. A mechanism with ^•OH as the dominant oxidant was proposed: FeS₂ core-shell nanoparticles served as Fe²⁺ shuttles for homogeneous Fenton's reaction and provided active sites for the heterogeneous Fenton process, whereas nanoporous carbon allowed minimizing the mass transport limitations.

Selective electrochemical H2O2 generation and activation on a bifunctional catalyst for heterogeneous electro-Fenton catalysis

Heterogeneous electro-Fenton is attractive for pollutants removal, where H₂O₂ is in-situ generated and simultaneously activated to ·OH at the cathodic catalyst. However, the heterogeneous electro-Fenton efficiency is limited by low H₂O₂ production and slow Fe(II) regeneration, which can be improved by tuning oxygen reduction selectivity and facilitating electron transfer to Fe(III) centers. Herein, we designed a bifunctional catalyst with FeOx nanoparticles embedded into N-doped hierarchically porous carbon (FeOx/NHPC). The activity and selectivity for H₂O₂ production were improved by regulating N doping configurations and contents. The obtained FeOx/NHPC750 presented high catalytic activity for H₂O₂ production with a low overpotential of 190 mV and high H₂O₂ selectivity of 95%˜98% at -0.3 V to -0.8 V. The Fe(II) regeneration was enhanced by the strong interfacial interaction between FeOx and N-doped porous carbon support, which leaded to a rapid decomposition of H₂O₂ into ·OH. FeOx/NHPC750 exhibited excellent electro-Fenton performance for the degradation and mineralization of phenol, sulfamethoxazole, atrazine, rhodamine B and 2,4-dichlorophenol in neutral reaction solution. This study offered a new strategy to construct an efficient and durable bifunctional catalyst for heterogeneous electro-Fenton system for advanced treatment of refractory wastewater.