Fe Active Sites Research Articles

Ternary Heteroatomic Doping Induced Microenvironment Engineering of Low Fe-N4-Loaded Carbon Nanofibers for Bifunctional Oxygen Electrocatalysis.

Fabricating highly efficient and long-life redox bifunctional electrocatalysts is vital for oxygen-related renewable energy devices. To boost the bifunctional catalytic activity of Fe-N-C single-atom catalysts, it is imperative to fine-tune the coordination microenvironment of the Fe sites to optimize the adsorption/desorption energies of intermediates during oxygen reduction/evolution reactions (ORR/OER) and simultaneously avoid the aggregation of atomically dispersed metal sites. Herein, a strategy is developed for fabricating a free-standing electrocatalyst with atomically dispersed Fe sites (≈0.89wt.%) supported on N, F, and S ternary-doped hollow carbon nanofibers (FeN4 -NFS-CNF). Both experimental and theoretical findings suggest that the incorporation of ternary heteroatoms modifies the charge distribution of Fe active centers and enhances defect density, thereby optimizing the bifunctional catalytic activities. The efficient regulation isolated Fe centers come from the dual confinement of zeolitic imidazole framework-8 (ZIF-8) and polymerized ionic liquid (PIL), while the precise formation of distinct hierarchical three-dimensional porous structure maximizes the exposure of low-doping Fe active sites and enriched heteroatoms. FeN4 -NFS-CNF achieves remarkable electrocatalytic activity with a high ORR half-wave potential (0.90V) and a low OER overpotential (270mV) in alkaline electrolyte, revealing the benefit of optimizing the microenvironment of low-doping iron single atoms in directing bifunctional catalytic activity.

Strengthening Fe(II)/Fe(III) Dynamic Cycling by Surface Sulfation to Achieve Efficient Electrochemical Uranium Extraction at Ultralow Cell Voltage.

Electrochemically mediated Fe(II)/Fe(III) redox-coupled uranium extraction can efficiently reduce the cell voltage of electrochemical uranium extraction (EUE). How to regulate the surface structure to enhance the uranium acyl ion adsorption capacity and strengthen the Fe(II)/Fe(III) redox cycle process is crucial for EUE. In this work, we developed surface sulfated nanoreduced iron (S-NRI) for EUE and exhibited improved properties for EUE at an ultralow cell voltage (-0.1 V). Compared with a nanoreduced iron (NRI) adsorbent, S-NRI displayed faster electrochemical extraction kinetics properties and higher extraction efficiency and capacity for uranium. In a more complex seawater electrolyte containing uranyl ion concentration ranging from 1 to 20 ppm, the removal efficiency could reach almost ∼100% after EUE for 24 h. At a higher 50 ppm uranium acyl ion concentration in a seawater electrolyte, S-NRI exhibited higher extraction capacity (755.03 mg/g), which is better than 528.53 mg/g of NRI at a cell voltage of -0.1 V. Outstanding EUE property could be attributed to the fact that sulfate species (M-SO42-) on the S-NRI surface not only enhanced selective adsorption of uranyl ions but also strengthened the Fe(II)/Fe(III) redox cycle, which accelerated electron transfer between Fe(II) and U(VI), promoted the regeneration of Fe(II) active sites, and finally enhanced the EUE property.

Correction to “Electronic and Surface Modifications of Ni–Co–Fe Oxides: A Catalyst with Maximum Exposure of Fe Active Sites for Water Electrolysis”

Correction to “Electronic and Surface Modifications of Ni–Co–Fe Oxides: A Catalyst with Maximum Exposure of Fe Active Sites for Water Electrolysis”

S-doped core–shell heterojunction Fenton catalyst (Fe3O4-S@PVP@UIO-66-(SH)2) for enhanced activation of hydroxyl radicals: Synergistic enrichment and degradation mechanism

The design of catalysts for the rapid activation of H2O2 and synergistic enrichment of pollutants is an effective strategy to improve catalytic performance. Herein, an Fe3O4-S@PVP@UIO-66-(SH)2 (FPUSH) heterojunction was successfully synthesized to form a core–shell-doped multiphase Fenton catalyst with both internal doping and outer encapsulation structures, which had excellent mass and efficient electron transfer performance. Levofloxacin can be adsorbed (Qmax was 141.44 mg g−1) and degraded at a removal efficiency of 97.70% within 120 min. The recycled efficiency of FPUSH was maintained at more than 90% after five recycles. The experimental and DFT results showed that the introduction of S improved the electron density of Fe atomic orbitals, promoted the electron transfer of Fe(II)/Fe(III), and enhanced the catalytic performance of Fe active sites. The degradation pathway of LVFX and the toxicity of intermediate products were predicted. This work provides new ideas and insights for the application of nonmetals in high-performance multiphase Fenton systems.

Facet Engineering and Pore Design Boost Dynamic Fe Exchange in Oxygen Evolution Catalysis to Break the Activity-Stability Trade-Off.

The oxygen evolution reaction (OER) plays a vital role in renewable energy technologies, including in fuel cells, metal-air batteries, and water splitting; however, the currently available catalysts still suffer from unsatisfactory performance due to the sluggish OER kinetics. Herein, we developed a new catalyst with high efficiency in which the dynamic exchange mechanism of active Fe sites in the OER was regulated by crystal plane engineering and pore structure design. High-density nanoholes were created on cobalt hydroxide as the catalyst host, and then Fe species were filled inside the nanoholes. During the OER, the dynamic Fe was selectively and strongly adsorbed by the (101̅0) sites on the nanohole walls rather than the (0001) basal plane, and at the same time the space-confining effect of the nanohole slowed down the Fe diffusion from catalyst to electrolyte. As a result, a local high-flux Fe dynamic equilibrium inside the nanoholes for OER was achieved, as demonstrated by the Fe57 isotope labeled mass spectrometry, thereby delivering a high OER activity. The catalyst showed a remarkably low overpotential of 228 mV at a current density of 10 mA cm-2, which is among the best cobalt-based catalysts reported so far. This special protection strategy for Fe also greatly improved the catalytic stability, reducing the Fe leaching amount by 2 orders of magnitude compared with the pure Fe hydroxide catalyst and thus delivering a long-term stability of 130 h. An assembled Zn-air battery was stably cycled for 170 h with a low discharge/charge voltage difference of 0.72 V.

Electronic and Surface Modifications of Ni–Co–Fe Oxides: A Catalyst with Maximum Exposure of Fe Active Sites for Water Electrolysis

Electronic and Surface Modifications of Ni–Co–Fe Oxides: A Catalyst with Maximum Exposure of Fe Active Sites for Water Electrolysis

N and S dual-coordinated Fe single-atoms in hierarchically porous hollow nanocarbon for efficient oxygen reduction

Fe-, and N-co-doped carbon (FeNC) electrocatalysts are promising alternatives to Pt-based catalysts for oxygen reduction reaction (ORR); however, simultaneously enhancing their intrinsic activity and exposure of Fe active sites remains challenging. Herein, we report S-modified Fe single-atom catalysts (SACs) anchored on N,S-co-doped hollow porous nanocarbon (Fe/NS-C) for ORR. The unique hollow structure and large surface area of the SACs are favorable for mass/electron transport and exposure of Fe single-atom active sites. The as-prepared Fe/NS-C electrocatalysts display a high-efficiency ORR activity with a half-wave potential of 0.893 V versus the reversible hydrogen electrode and exceed that of the benchmark commercial Pt/C catalyst as well as most reported transition-metal based SACs. Impressively, the Fe/NS-C-based Al-air battery (AAB) displays a high open circuit voltage of 1.48 V, a maximum power density of 140.16 mW cm−2, and satisfactory durability, outperforming commercial Pt/C-based AAB. Furthermore, Fe/NS-C exhibits considerable potential as a cathode catalyst for application in direct methanol fuel cells. Experimental and theoretical calculation results reveal that the excellent ORR performance of Fe/NS-C can be contributed to the highly active FeN3S sites and the unique hollow structure. This work provides new insights into the rational design and synthesis high-performance ORR electrocatalysts for energy conversion and storage devices. of employing ZIF-8 as precursors.

Modulating the electronic structure of NiFe hydroxide by Zr doping enables industrial-grade current densities for water oxidation

NiFe layered double hydroxide (LDH) is a non-noble benchmark catalyst for oxygen evolution reaction (OER). However, the mechanism of enhancing catalytic activities via incorporating foreign elements is still not resolved. Herein, we modulate the electronic structure of NiFe LDH by introducing high-valence Zr4+ to synergistically improve catalytic activity. NiFeZr LDH requires overpotential of only 182 mV at 10 mA cm−2 with Tafel slope of 38.6 mV dec−1. Impressively, it delivers 2000 mA cm−2 at only 314 mV and has robust durability over 750 h. Operational Raman analysis reveals that the electron transfer after incorporating Zr4+ inhibits oxidation of Ni2+ ions and restrains phase transition from active β-NiOOH to γ-NiOOH during OER process. Guided by density functional theory (DFT) computations, Zr4+ modulator promotes the rate-determining step from OH* to O* via optimizing the oxidation cycle at Fe active sites. This work provides a route for advancing electrocatalysts to meet industrial standards.

Use of Fe-H3BTC and GO Compounds To Prepare N/Fe-Doped Carbon for a High-Performance Oxygen Reduction Reaction Catalyst.

Zinc-air batteries (ZABs), as a new type of energy storage and conversion device, have received lots of attention due to their high energy density and low cost. The electrode material is a key factor for improving the performance of ZABs. Here, a facile strategy for the fabrication of N-rich porous carbon is reported. H3BTC and GO are selected as carbon precursors, histidine and Fe(NO3)3 are used as ligands, and urea is used as a N source and template. After the conventional drying of the complex and following carbonization process, serial N-doped carbon materials are prepared. The composition and structure of the material are characterized, and the effect of the ligand is studied. The hierarchical porous structure facilitates the full exposition of N and trace of Fe active sites and benefits the transportation of electrolytes, thus improving the oxygen reduction reaction (ORR). The ORR performances show that the porous carbon has a positive initial and half-wave potential, good limiting current density, a near four-electron transfer process, and better methanol resistance and durability. The assembled ZAB exhibits high specific capacity (880.4 mAh gZn-1), excellent open-circuit voltage (1.54 V), and superior cycling stability, which further provides the significant potential for its application.

Palladium Enhanced Iron Active Site - An Efficient Dual-Atom Catalyst for Oxygen Electroreduction.

Metal-nitrogen-carbon (M-C/N) electrocatalysts have been shown to have satisfactory catalytic activity and long-term durability for the oxygen reduction reaction (ORR). Here, a strategy to prepare a new electrocatalyst (Fe&Pd-C/N) using a unique metal-containing ionic liquid (IL) is exploited, in which Fe & Pd ions are positively charged species atomically dispersed by coordination to the N of the N-doped C substrate, C/N. X-ray absorption fine structure, XPS and aberration-corrected transmission electron microscopy results verified a well-defined dual-atom configuration comprising Fe+2.x -N4 coupled Pd2+ -N4 sites and well-defined spatial distribution. Electronic control of a coupled Fe-Pd structure produces an electrocatalyst that exhibits superior performance with enhanced activity and durability for the ORR compared to that of commercial Pt/C (20%, Johnson Matthey) in both alkaline and acid media. Density functional theory calculations indicate that Pd atom can enhance the catalytic activity of the Fe active sites adjacent to Pd sites by changing the electronic orbital structure and Bader charge of the Fe centers. The excellent catalytic performance of the Fe&Pd-C/N electrocatalyst is demonstrated in zinc-air batteries and hydrogen-air fuel cells.

Iron-organic frameworks as effective fenton-like catalysts for peroxymonosulfate decomposition in advanced oxidation processes

Although Fenton-like reactions have been extensively used to treat various organic contaminants in wastewaters (oily wastewaters, landfill leachate, denitrification with Fenton oxidized non-degradable large molecular organic pollutants, and removal of phosphorus, etc.), the difficulty in catalyst recovery and the instability of catalytic activity limit their practical application. Herein, iron atoms were inserted in a metal-organic framework (FeNC) with dual reaction sites as highly reactive and stable electro-Fenton-like catalysts for the catalytic oxidation of organic pollutants via the electro-Fenton-like activation reaction of peroxymonosulfate (PMS) with fractional leaching of metal ions. Experiments and density functional theory (DFT) calculations indicate that FeNC with FeN4 and Fe–Fe active sites can rapidly deliver electrons for PMS dissociation. Further, an electrolysis reactor was constructed for the on-site generation of reactive oxygen species, which can stably and continuously purify various organic wastewaters. The combined use of an electrolysis reactor and magnetic catalyst in the current study provides a direction for the long-term remediation of organic pollutants on an industrial scale.

Fe nanoparticles embedded in N-doped porous carbon for enhanced electrocatalytic CO2 reduction and Zn-CO2 battery

The selective electrochemical reduction of CO2 to CO is a promising solution for the design of carbon-neutral, sustainable processes. Achieving a highly selective single reduction product is still challenging because of the energetically favorable competing hydrogen evolution reaction. We report the fabrication of N-doped sponge-like porous graphitic carbon structures embedded with Fe nanoparticles (Fe@NPC) via the pre-modification of a metal-organic framework (IRMOF-3(Zn)) with carboxyferrocene, followed by pyrolysis. The as-prepared Fe@NPC exhibited a 96.4% CO Faradaic efficiency at –0.5 VRHE and good stability. The exceptional CO2 reduction performance is attributed to the unique structure of the composite catalyst, which provides abundant hierarchical pores that increase CO2 adsorption and mass transfer, and active Fe sites that synergistically accelerate the kinetics of CO generation. The in situ attenuated total reflectance-Fourier transform infrared analysis provided proof of the improved ability of Fe@NPC to accumulate the crucial intermediate *COOH compared with other pyrolyzed porous carbons. Fe@NPC was used in a Zn-CO2 battery that delivered a maximum power density of 3.0 mW cm–2, evidencing its potential for application in energy-converting devices.

A single-atom iron nanozyme reactor for α-ketoglutarate synthesis

Efficient removal of the by-product hydrogen peroxide (H2O2) in the oxidase-catalyzed reactions is a major challenge in biocatalysis that hinders the production of high yields of ketoacids. Here, we demonstrate the use of single-atom Fe nanozymes (SAFs) for H2O2 decomposition inspired by natural enzymes, to improve the efficiency of L-Glutamate oxidase (LGOX)-assisted catalysis for α-ketoglutarate (α-KG) synthesis. SAFs exhibit Metal-Nitrogen coordination and atomically dispersed active sites, providing superior enzyme-like activity for H2O2 decomposition. The SAFs@LGOX nanozyme-enzyme nanoreactors exhibit high catalytic efficiency due to in situ removal of H2O2 by SAFs during the deamination reaction. Additionally, SAFs enhance the pH and temperature tolerance, as well as thermal stability of LGOX. The nanoreactors achieved about 90% substrate conversion to α-KG within 2.5 h, compared to almost complete oxidation of the product in the absence of single atom Fe active sites. These findings suggest that the nanozyme-based strategy for rapid byproducts removal through mimicking enzymatic activity has promising applications in biocatalysis and may inspire the design of more effective nanozymes.

Single-Atom Fe Nanozyme with Enhanced Oxidase-like Activity for the Colorimetric Detection of Ascorbic Acid and Glutathione

Single-atom nanozymes (SAzymes) have drawn ever-increasing attention due to their maximum atom utilization efficiency and enhanced enzyme-like activity. Herein, a facile pyrolysis strategy is reported for the synthesis of the iron-nitrogen-carbon (Fe-N-C) SAzyme using ferrocene trapped within porous zeolitic imidazolate framework-8 (ZIF-8@Fc) as a precursor. The as-prepared Fe-N-C SAzyme exhibited exceptional oxidase-mimicking activity, catalytically oxidizing 3,3',5,5'-tetramethylbenzidine (TMB) with high affinity (Km) and fast reaction rate (Vmax). Taking advantage of this property, we designed two colorimetric sensing assays based on different interaction modes between small molecules and Fe active sites. Firstly, utilizing the reduction activity of ascorbic acid (AA) toward oxidized TMB (TMBox), a colorimetric bioassay for AA detection was established, which exhibited a good linear range of detection from 0.1 to 2 μM and a detection limit as low as 0.1 μM. Additionally, based on the inhibition of nanozyme activity by the thiols of glutathione (GSH), a colorimetric biosensor for GSH detection was constructed, showing a linear response over a concentration range of 1-10 μM, with a detection limit of 1.3 μM. This work provides a promising strategy for rationally designing oxidase-like SAzymes and broadening their application in biosensing.

Significantly Enhanced Electrocatalytic Reduction of Chloramphenicol from Water Mediated by Fe–F–N Co-doped Carbon Materials

Chloramphenicol (CAP) is a widely used antibiotic drug and poses a great threat to human health and the ecosystem. In this work, an iron–fluorine–nitrogen (Fe–F–N) co-doped porous carbon material was innovatively prepared and applied for the electrocatalytic reduction of CAP from water, and excellent treatment performance was achieved. For an initial CAP concentration of 10 mg/L, after 60 min operation, almost 100% of CAP was removed. The active Fe(II) sites and the F-doping-induced electronic structure of Fe-FNC facilitated the electrocatalytic reductive degradation of CAP by promoting electron transfer, rather than the H*-mediated indirect reduction process. Therefore, a better treatment performance was obtained by Fe-FNC compared to other catalysts such as Fe-NC and FNC. The reaction pathways for CAP reduction were elucidated, and the ecotoxicity of the reaction products was assessed. The toxicity of the intermediate products was found to be significantly reduced, indicative of the good prospect of this method for CAP-containing wastewater pretreatment.

Regulating spin state of Fe active sites by the P-doping strategy for enhancing peroxymonosulfate activation

Optimization of Fe-based heterogeneous catalyst for enhancing PMS activation was studies by an electronic structure regulation strategy. The single atom and nano-particle Fe-catalysts were then modified by P-doping on carbon support and tested for BPA degradation. Catalysts with P-doping led to 100 % BPA degradation in 30 min, much better than single atom and nano-particle control groups (56 % and 61 %). Experimental and DFT results revealed that P-doping increased the electron density of Fe and induced the transformation of Fe 3d electron spin state from low spin to medium spin, then the Fe 3d orbitals with high energy could better overlap with O 2p orbitals of PMS, promoting the electron transfer process from Fe to PMS and generation of hydroxyl radical. This work provides a deep understanding of spin state modulation for designing high-performance PMS activation catalysts.

Mechanical activation-enhanced doping and defect strategy to construct Fe–S co-doped carbon nitride for efficient photocatalytic tetracycline degradation and hydrogen evolution

Achieving a simultaneous increase in the redox performance of carbon nitride (CN) remains a challenge owing to the low utilization efficiency of photogenerated carriers. Herein, a novel “mechanical activation (MA)-enhanced doping and defect” strategy was proposed for structural modification of CN to enhance its performance. Specifically, CN, FeCl3, and thiourea were treated by MA to prepare a precursor with uniform dispersion and anchoring of Fe and S components in CN, and then the MA-treated precursor was calcinated to obtain a Fe–S co-doped CN composite (MA-Fe-S-CN) with abundant N vacancies. Under visible light illumination, MA-Fe-S-CN displayed outstanding photocatalytic performance for tetracycline (TC) removal (92.7%) and hydrogen evolution (2965 µmol g−1h−1) within 120 min, which was 3.3 and 4.1 times higher than those of CN, respectively. The superlative redox performance of MA-Fe-S-CN was attributed to that MA-enhanced Fe–S co-doping and N vacancies effectively modified the built-in electric field, electronic structure, and redox potential of CN. Experimental and theoretical analysis revealed that photogenerated electrons were trapped by N vacancies and directed to Fe active sites through efficient charge transfer channels (Fe–N and Fe–S), resulting in rapid charge transfer and effective separation and utilization of carriers. On this basis, possible mechanisms of photocatalytic TC degradation and hydrogen evolution were proposed. This work provides a green and economically feasible way for improving the redox performance of CN.

Target Recognition-Triggered Peroxidase-Mimicking Activity Depression in Homochiral Nanochannels for Identifying Cystine Enantiomers.

Enantioselective identification of chiral molecules is of paramount importance in medical science, biochemistry, and pharmaceutics owing to the configuration-dependent activities of enantiomers. However, the identical physicochemical properties of enantiomers remain challenging in chiral sensing. In this study, inspired by the peroxidase-mimicking activity of Fe(III)-based nanomaterials, an enantioselective artificial architecture is constructed on TiO2 nanochannels. Homochiral Ti-based metal-organic frameworks (MOFs) use a 2,2'-bipyridine-5,5'-dicarboxylic acid ligand as the artificial enzyme skeleton, Fe(III) as peroxidase-mimicking centers, and l-tartaric acid (TA) as a chiral recognition selector. Using l-/d-cystine as model enantiomers, the chiral moieties of l-TA on Ti-MOFs allow stereoselective recognition of guest molecules through hydrogen bonds formed between chiral cystine and the host. In a tris(2-carboxyethyl)phosphine hydrochloride-containing environment, the disulfide bonds in cystine molecules are further cleaved, and the HS-tails react with Fe(III) active sites, causing the loss of peroxidase-like performance of nanochannels. Benefitting from the nanochannel architecture's current-potential (I-V) properties, the selective recognition of cystine enantiomers is directly monitored through the peroxidase-like activity change-induced ionic current signatures. This study provides a new and universal strategy for distinguishing disulfide- and thiol-containing chiral molecules.

Maximizing Active Fe Species in ZSM-5 Zeolite Using Organic-Template-Free Synthesis for Efficient Selective Methane Oxidation.

The selective oxidation of CH4 in the aqueous phase to produce valuable chemicals has attracted considerable attention due to its mild reaction conditions and simple process. As the most widely studied catalyst for this reaction, Fe-ZSM-5 demonstrates high intrinsic activity and selectivity; however, Fe-ZSM-5 prepared using conventional methods has a limited number of active Fe sites, resulting in low CH4 conversion per unit mass of the catalyst. This study reports a facile organic-template-free synthesis strategy that enables the incorporation of more Fe into the zeolite framework with a higher dispersion degree compared to conventional synthesis methods. Because framework Fe incorporated in this way is more readily transformed into isolated extra-framework Fe species under thermal treatment, the overall effect is that Fe-ZSM-5 prepared using this method (Fe-HZ5-TF) has 3 times as many catalytically active sites as conventional Fe-ZSM-5. When used for the selective oxidation of CH4 with 0.5 M H2O2 at 75 °C, Fe-HZ5-TF produced a high C1 oxygenate yield of 109.4 mmol gcat-1 h-1 (a HCOOH selectivity of 91.1%), surpassing other catalysts reported to date. Spectroscopic characterization and density functional theory calculations revealed that the active sites in Fe-HZ5-TF are mononuclear Fe species in the form of [(H2O)3Fe(IV)═O]2+ bound to Al pairs in the zeolite framework. This differs from conventional Fe-ZSM-5, where binuclear Fe acts as the active site. Analysis of the catalyst and product evolution during the reaction suggests a radical-driven pathway to explain CH4 activation at the mononuclear Fe site and subsequent conversion to C1 oxygenates.

Boosting Water Oxidation Activity via Carbon–Nitrogen Vacancies in NiFe Prussian Blue Analogue Electrocatalysts

The development of electrocatalysts for oxygen evolution reactions (OERs) is of great significance for hydrogen production. Defect engineering is an effective strategy to improve the OER performance of electrocatalyst by regulating the local electronic and atomic structures of electrocatalysts. Here, we successfully synthesized defective Prussian blue analogues (PBAs) with rich CN vacancies (D-NiFe PBA) as efficient OER electrocatalysts. The optimized D-NiFe PBA exhibited an overpotential of 280 mV at 10 mA cm−2 and a superior stability for over 100 h in KOH electrolytes. The formation of CN vacancies in the NiFe PBA could effectively inhibit the loss of Fe active sites, promote the reconstruction of the NiFe oxygen (hydroxide) active layer in the OER process, and further improve the electrocatalytic activity and stability of the VCN-NiFe PBA. This work presents a feasible approach for the wide application of vacancy defects in PBA electrocatalysts.