Active Site Iron Research Articles

Enhancing Sulfur Redox Conversion of Active Iron Sites by Modulation of Electronic Density for Advanced Lithium-Sulfur Battery.

Despite lithium-sulfur (Li-S) batteries possessing ultrahigh energy density as great promising energy storage devices, the suppressing shuttle effect and improving sulfur redox reaction (SROR) are vital for their practical application. Developing high-activity electrocatalysts for enhancing the SROR kinetics is a major challenge for the application of Li-S batteries. Herein, single-molecule iron phthalocyanine species are anchored on the N and P dual-doped porous carbon nanosheets (Fe-NPPC) via axial Fe-N coordination to optimize the electronic structure of active centers. The Fe-NPPC can promote the catalytic conversion of polysulfides by modulation of the electronic density in active moieties, endowing the Li-S battery with a high reversible capacity of 1023 mAh g-1 at 1 C as well as an ultralow capacity decay of 0.035% per cycle over 1500 cycles. Even with a high sulfur loading of 7.1mg cm-2 , the Li-S battery delivers a high areal capacity of 4.8 mAh cm-2 after 150 cycles at 0.2 C. With further increasing the sulfur loading to 9.2mg cm-2 , an excellent areal capacity of up to 9.3 mAh cm-2 is obtained at 0.1 C.

Structure and mutation of deoxypodophyllotoxin synthase (DPS) from Podophyllum hexandrum

Deoxypodophyllotoxin synthase (DPS) is a 2-oxoglutarate (2-OG) dependent non-heme iron (II) dioxygenase that catalyzes the stereoselective ring-closing carbon-carbon bond formation of deoxypodophyllotoxin from the aryllignan (−)-yatein. Deoxypodophyllotoxin is a precursor of topoisomerase II inhibitors, which are on the World Health Organization’s list of essential medicines. Previous work has shown that DPS can accept a range of substrates, indicating it has potential in biocatalytic processes for the formation of diverse polycyclic aryllignans. Recent X-ray structures of the enzyme reveal possible roles for amino acid side chains in substrate recognition and mechanism, although a mutational analysis of DPS was not performed. Here, we present a structure of DPS at an improved resolution of 1.41 Å, in complex with the buffer molecule, Tris, coordinated to the active site iron atom. The structure has informed a mutational analysis of DPS, which suggests a role for a D224-K187 salt bridge in maintaining substrate interactions and a catalytic role for H165, perhaps as the base for the proton abstraction at the final rearomatization step. This work improves our understanding of specific residues’ contributions to the DPS mechanism and can inform future engineering of the enzyme mechanism and substrate scope for the development of a versatile biocatalyst.

Expandable polystyrene waste modified iron sludge-based biochar activating multiple free radicals for deep wastewater treatment

Fe-containing sludge can be converted to functional biochar as a promising resource recycle pathway. However, activating properties of Fe sludge-based biochar are always limited by low specific surface area and weak carbon center for persistent free radical(PFRs). In the current study, waste expandable polystyrene particles were simultaneously used to improve the biochar catalytic performance. The co-prepared biochar achieved a high micromesoporous ratio(98.4%) with specific surface area (345.3 m²/g), improved by 20.1% and 105.2% than unimproved biochar. The improved micromesoporous properties performed better iron active sites and oxygen-containing functional groups for PFRs functioning in wastewater. The co-prepared biochar showed an efficient catalytic degradation through persulfate activating multiple free radicals of 1O2, O2˙ˉ, ˙OH and SO4˙ˉ, which removed 95.4% sulfamethoxazole with mineralizing ratio of 74% within 60 min. The recycling of waste iron sludge and waste polystyrene foam material can achieve functional biochar to apply in deep treatment of refractory organic wastewater.

Boosting Photo-Electro-Fenton Process Via Atomically Dispersed Iron Sites on Graphdiyne for InVitro Hydrogen Peroxide Detection.

Hydrogen peroxide (H2 O2 ) is essential in oxidative stress and signal regulation of organs of animal body. Realizing in vitro quantification of H2 O2 released from organs is significant, but faces challenges due to short lifetime of H2 O2 and complex bio-environment. Herein, rationally designed and constructed a photoelectrochemical (PEC) sensor for in vitro sensing of H2 O2 , in which atomically dispersed iron active sites (Hemin) modified graphdiyne (Fe-GDY) serves as photoelectrode and catalyzes photo-electro-Fenton process. Sensitivity of Fe-GDY electrode is enhanced 8 times under illumination compared with dark condition. The PEC H2 O2 sensor under illumination delivers a wide linear range from 0.1 to 48 160µm and a low detection limit of 33nm, while demonstrating excellent selectivity and stability. The high performance of Fe-GDY is attributed to, first, energy levels matching of GDY and Hemin that effectively promotes the injection of photo-generated electrons from GDY to Fe3+ for reduced Fe2+ , which facilitates the Fe3+ /Fe2+ cycle. Second, the Fe2+ actively catalyzes H2 O2 to OH- through the Fenton process, thereby improving the sensitivity. The PEC sensor demonstrates in vitro quantification of H2 O2 released from different organs, providing a promising approach for molecular sensing and disease diagnosis in organ levels.

Waste self-heating bag derived iron-based composite with abundant oxygen vacancies for highly efficient Fenton-like degradation of micropollutants

In this study, iron-rich waste self-heating bag was reutilized as the raw material to prepare oxygen vacancies (OV) functionalized iron-based composite (iron oxide (Fe3O4)-carbon-vermiculite, viz. OV-ICV), which exhibited excellent performance in the Fenton-like degradation of micropollutants via peroxydisulfate (PDS) activation. Above 95% of 1.0 mg/L carbaryl (CB) was efficiently eliminated in the presence of 0.1 g/L of OV-ICV and 0.5 mmol/L of PDS over a wide pH range of 3–10 within 30 min. Besides, OV-ICV also showed acceptable adaptability, stability, and renewability. Imbedding OV into Fe3O4 structure significantly generated more active iron sites and localized electrons, promoted the charge transfer ability, and assisted the redox cycle of ≡Fe(III)/≡Fe(II) for PDS activation. Mechanism investigation demonstrated that superoxide radicals (O2•−) derived from the activation of molecular oxygen mediated the generation of H2O2, and both of them further enhanced the formation of more sulfate radicals (SO4•−) and hydroxyl radicals (•OH), which led to the efficient degradation and mineralization of CB. Furthermore, the degradation pathways of CB were proposed based on the intermediates identification. This work lays a foundation for the rational reutilization of iron-containing wastes modified with defect engineering in heterogeneous Fenton-like catalysis for the remediation of micropollutants wastewater.

Effect of Phosphorus Modulation in Iron Single-Atom Catalysts for Peroxidase Mimicking.

Fe-N-C single-atom catalysts (SACs) exhibit excellent peroxidase (POD)-like catalytic activity, owing to their well-defined isolated iron active sites on the carbon substrate, which effectively mimic the structure of natural peroxidase's active center. To further meet the requirements of diverse biosensing applications, SAC POD-like activity still needs to be continuously enhanced. Herein, a phosphorus (P) heteroatom is introduced to boost the POD-like activity of Fe-N-C SACs. A 1Dcarbon nanowire (FeNCP/NW) catalyst with enriched Fe-N4 active sites is designed and synthesized, and P atoms are doped in the carbon matrix to affect the Fe center through long-range interaction. The experimental results show that the P-doping process can boost the POD-like activity more than the non-P-doped one, with excellent selectivity and stability. The mechanism analysis results show that the introduction of P into SAC can greatly enhance POD-like activity initially, but its effect becomes insignificant with increasing amount of P. As a proof of concept, FeNCP/NW is employed in an enzyme cascade platform for highly sensitive colorimetric detection of the neurotransmitter acetylcholine.

Temporal and spatial resolution of distal protein motions that activate hydrogen tunneling in soybean lipoxygenase

The enzyme soybean lipoxygenase (SLO) provides a prototype for deep tunneling mechanisms in hydrogen transfer catalysis. This work combines room temperature X-ray studies with extended hydrogen-deuterium exchange experiments to define a catalytically-linked, radiating cone of aliphatic side chains that connects an active site iron center of SLO to the protein-solvent interface. Employing eight variants of SLO that have been appended with a fluorescent probe at the identified surface loop, nanosecond fluorescence Stokes shifts have been measured. We report a remarkable identity of the energies of activation (Ea) for the Stokes shifts decay rates and the millisecond C-H bond cleavage step that is restricted to side chain mutants within an identified thermal network. These findings implicate a direct coupling of distal protein motions surrounding the exposed fluorescent probe to active site motions controlling catalysis. While the role of dynamics in enzyme function has been predominantly attributed to a distributed protein conformational landscape, the presented data implicate a thermally initiated, cooperative protein reorganization that occurs on a timescale faster than nanosecond and represents the enthalpic barrier to the reaction of SLO.

Facile one-pot synthesis of Fe-UZM-35 catalysts for ammonia selective catalytic reduction

Owing to the narrow crystallization field, the synthesis of the large-pore zeolite UZM-35 with MSE topology starting from amorphous reagents in the presence of dimethyldipropylammonium ions as an organic structure-directing agent is often non-reproducible. Here we report the synthesis of UZM-35 via an interzeolite conversion route using USY zeolites, as the sole source of Al and Si, and a small amount of previously synthesized UZM-35 as seed crystals. We also present the one-pot synthesis of Fe-UZM-35 with the best low-temperature NH3-SCR activity among the iron-exchanged zeolites reported so far. These synthesis routes showed high reproducibility, together with considerably faster crystallization kinetics (≤1.5 vs 7 days) compared to conventional UZM-35 synthesis. When the Fe loading level was properly adjusted, the NH3-SCR activity of one-pot-synthesized Fe-UZM-35 was found to be essentially the same as that of post-synthetically exchanged Fe-UZM-35 due to the similarity in the nature of their iron active sites.

N-doped carbon xerogels from urea-resorcinol-formaldehyde as carbon matrix for Fe-N-C catalysts for oxygen reduction in fuel cells

Platinum group metal-free catalysts have been intensively investigated in the last decades as an alternative to platinum with the aim of lowering the cost of polymer electrolyte fuel cells. In particular, iron-nitrogen-carbon (Fe-N-C) has proved to be the most active towards the oxygen reduction reaction. For practical application, a hierarchical pore structure is required, with micropores favouring the creation of active sites and larger pores (meso- and macropores) facilitating the mass transport. In this work, carbon xerogels are investigated to hosting iron and nitrogen species obtained by a template-free method. The introduction of nitrogen in a one-step polymerization of urea with resorcinol and formaldehyde is investigated for the first time in this field by varying the relative content of reactants. The urea/resorcinol ratio greatly influences the pore structure of the Fe-N-C catalyst and the ORR electrocatalytic activity thereof. The ORR activity is favored for a balanced urea/resorcinol ratio where porosity is well developed and relatively high iron and nitrogen contents are incorporated to the carbon xerogel. In acid (0.5 M H2SO4), the onset potential is 0.82 V vs. RHE, with a number of exchanged electrons very close to 4 (i.e. full conversion to water) and low Tafel slope of 71 mV dec−1, for the most active catalyst of the series, possessing the best compromise of iron and nitrogen active sites. Fuel cell tests corroborate that the catalyst with the most developed porous structure shows the best performance.

Iron Nanoparticles Protected by Chainmail-structured Graphene for Durable Electrocatalytic Nitrate Reduction to Nitrogen.

The electrochemical nitrate reduction reaction (NO3 RR) is an appealing technology for regulating the nitrogen cycle. Metallic iron is one of the well-known electrocatalysts for NO3 RR, but it suffers from poor durability due to leaching and oxidation of iron during the electrocatalytic process. In this work, a graphene-nanochainmail-protected iron nanoparticle (Fe@Gnc) electrocatalyst is reported. It displays superior nitrate removal efficiency and high nitrogen selectivity. Notably, the catalyst delivers exceptional stability and durability, with the nitrate removal rate and nitrogen selectivity remained ≈96 % of that of the first time after up to 40 cycles (24 h for one cycle). As expected, the conductive graphene nanochainmail provides robust protection for the internal iron active sites, allowing Fe@Gnc to maintain its long-lasting electrochemical nitrate catalytic activity. This research proposes a workable solution for the scientific challenge of poor lasting ability of iron-based electrocatalysts in large-scale industrialization.

Iron Nanoparticles Protected by Chainmail‐structured Graphene for Durable Electrocatalytic Nitrate Reduction to Nitrogen

AbstractThe electrochemical nitrate reduction reaction (NO3RR) is an appealing technology for regulating the nitrogen cycle. Metallic iron is one of the well‐known electrocatalysts for NO3RR, but it suffers from poor durability due to leaching and oxidation of iron during the electrocatalytic process. In this work, a graphene‐nanochainmail‐protected iron nanoparticle (Fe@Gnc) electrocatalyst is reported. It displays superior nitrate removal efficiency and high nitrogen selectivity. Notably, the catalyst delivers exceptional stability and durability, with the nitrate removal rate and nitrogen selectivity remained ≈96 % of that of the first time after up to 40 cycles (24 h for one cycle). As expected, the conductive graphene nanochainmail provides robust protection for the internal iron active sites, allowing Fe@Gnc to maintain its long‐lasting electrochemical nitrate catalytic activity. This research proposes a workable solution for the scientific challenge of poor lasting ability of iron‐based electrocatalysts in large‐scale industrialization.

Electronic Coupling and Electrocatalysis in Redox Active Fused Iron Corroles.

Conjugated arrays composed of corrole macrocycles are increasingly more common, but their chemistry still lags behind that of their porphyrin counterparts. Here, we report on the insertion of iron(III) into a β,β-fused corrole dimer and on the electronic effects that this redox active metal center has on the already rich coordination chemistry of [H3tpfc] COT, where COT = cyclo-octatetraene and tpfc = tris(pentafluorophenyl)corrole. Synthetic manipulations were performed for the isolation and full characterization of both the 5-coordinate [FeIIItpfc(py)]2COT and 6-coordinate [FeIIItpfc(py)2]2COT, with one and two axial pyridine ligands per metal, respectively. X-Ray crystallography reveals a dome-shaped structure for [FeIIItpfc(py)]2COT and a perfectly planar geometry which (surprisingly at first) is also characterized by shorter Fe-N (corrole) and Fe-N (pyridine) distances. Computational investigations clarify that the structural phenomena are due to a change in the iron(III) spin state from intermediate (S = 3/2) to low (S = 1/2), and that both the 5- and 6-coordinated complexes are enthalpically favored. Yet, in contrast to iron(III) porphyrins, the formation enthalpy for the coordination of the first pyridine to Fe(III) corrole is more negative than that of the second pyridine coordination. Possible interactions between the two corrole subunits and the chelated iron ions were examined by UV-Vis spectroscopy, electrochemical techniques, and density functional theory (DFT). The large differences in the electronic spectra of the dimer relative to the monomer are concluded to be due to a reduced electronic gap, owing to the extensive electron delocalization through the fusing bridge. A cathodic sweep for the dimer discloses two redox processes, separated by 230 mV. The DFT self-consistent charge density for the neutral and cationic states (1- and 2-electron oxidized) reveals that the holes are localized on the macrocycle. A different picture emerges from the reduction process, where both the electrochemistry and the calculated charge density point toward two consecutive electron transfers with similar energetics, indicative of very weak electron communication between the two redox active iron(III) sites. The binuclear complex was determined to be a much better catalyst for the electrochemical hydrogen evolution reaction (HER) than the analogous mononuclear corrole.

Creating Highly Active Iron Sites in Electrochemical N2 Reduction by Fabricating Strongly-Coupled Interfaces.

Electrochemical Nc reduction has been regarded as one of the most promising approaches to producing ammonia under mild conditions, but there are remaining pressing challenges in improving the reaction rate and efficiency. Herein, an unconventional galvanic replacement reaction is reported to fabricate a unique hierarchical structure composed of Fe3 O4 -CeO2 bimetallic nanotubes covered by Fe2 O3 ultrathin nanosheets. Control experiments reveal that CeO2 species play the essential role of stabilizer for Fe2+ cations. Compared with bare CeO2 and Fe2 O3 nanotubes, the as-obtained Fe2 O3 /Fe3 O4 -CeO2 possesses a remarkably enhanced NH3 yield rate (30.9µg h-1 mgcat -1 ) and Faradaic efficiency (26.3%). The enhancement can be attributed to the hierarchical feature that makes electrodes more easily to contact with electrolytes. More importantly, as verified by density functional theory calculations, the generation of Fe2 O3 -Fe3 O4 heterogeneous junctions can efficiently optimize the reaction pathways, and the energy barrier of the potential determining step (the *N2 hydrogenates into *N*NH) is significantly decreased.

Particle Swarm Fitting of Spin Hamiltonians: Magnetic Circular Dichroism of Reduced and NO-Bound Flavodiiron Protein.

A particle swarm optimization (PSO) algorithm is described for the fitting of ground-state spin Hamiltonian parameters from variable-temperature/variable-field (VTVH) magnetic circular dichroism (MCD) data. This PSO algorithm is employed to define the ground state of two catalytic intermediates from a flavodiiron protein (FDP), a class of enzymes with nitric oxide reductase activity. The bimetallic iron active site of this enzyme proceeds through a biferrous intermediate and a mixed ferrous-{FeNO}7 intermediate during the catalytic cycle, and the MCD spectra of these intermediates are presented and analyzed. The fits of the spin Hamiltonians are shown to provide important geometric and electronic insight into these species that is compared and contrasted with previous reports.

Atomically Dispersed Iron Active Sites Promoting Reversible Redox Kinetics and Suppressing Shuttle Effect in Aluminum–Sulfur Batteries

HighlightsFe single atoms supported on porous carbon nanofiber are prepared by spatial confinement.The iron single atoms supported on porous nitrogen-doped carbon nanofibers (FeSAs-NCF) can promote the reversible conversion between aluminum polysulfides.The FeSAs-NCF can chemically anchor the polysulfides to suppress shuttle effect.

Engineering d-band center of iron single atom site through boron incorporation to trigger the efficient bifunctional oxygen electrocatalysis

Modulating the microenvironment of single-metal active sites holds excellent promises for developing highly efficiently oxygen electrocatalysts. Herein, by combining theoretical predictions and experiments, we reported a general strategy to engineer the electronic properties of iron-nitrogen-carbon (FeN4/C) catalysts via the incorporation of the boron (B) atom for achieving improved catalytic activity in oxygen electrocatalysis. Our theoretical results revealed that B modulation effectively tunes the d-band center of the iron (Fe) active site to optimize its adsorption strength with oxygenated species, greatly enhancing oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) activity. Our experimental measurements then confirmed the above theoretical predictions: the as-synthesized B-doped FeN4/C (Fe-N4-B) material can perform as an eligible bifunctional catalyst for ORR and OER in alkaline media, and its catalytic activity even outperforms the commercial noble metal benchmarks. The present findings provide a promising strategy to further design the advanced catalysts for a wide range of electrochemical applications.

Improved alkali-tolerance of FeOx-WO3 catalyst for NO removal via in situ reserving FeOx active species

The improvement of alkali tolerance of de-NOx catalysts remains an immense challenge for selective catalytic reduction (SCR) of NO by NH3. In this work, NOx removal efficiency was nearly retained in the presence of potassium over FeOx-WO3 (FW) catalysts via protecting iron active species using different template agents. Typically, FW using urea (FW-(urea)) as template agent showed superior de-NOx performance after being poisoned by potassium (K/FW-(urea)). Urea could promote the facile binding of potassium with WO3, which acted as alkali-poisoning sites so that iron active species were reserved to a great extent. Additionally, potassium species tended to be aggregated over K/FW-(urea) and thus a lot of iron active sites could be exposed. Besides, K/FW-(urea) catalyst has higher surface acidity, superior redox ability, stronger interaction of WO3 with Fe2O3, and more surface adsorbed oxygen compare with other parallel poisoned samples. This is the reason for the remarkable ability of potassium resistance over K/FW-(urea). The strategy of template regulation could offer a novel route for maintaining considerable NH3-SCR performance in the presence of alkali.

Ultrathin binary MOF nanozyme with boosted activity via introduction of active iron sites for detecting sulfide ion

Convenient and reliable sulfide ion (S2-) detection is necessary due to its harmfulness. Herein, a new colorimetric sensing platform for S2- was established based on the binary NiFe metal-organic framework (MOF) nanozyme. The tremella-like NiFe MOF with ultrathin nanosheets was synthesized via introduction of active Fe sites in the Ni MOF by a rapid microwave-assisted method just in 11 min. The ultrathin NiFe MOF exhibited 2.8-fold higher peroxidase-like activity and 42.5-fold higher affinity to H2O2 than the Ni MOF, which could efficiently catalyze the decomposition of H2O2 into hydroxyl radicals (•OH) and superoxide anion radicals (O2•−) for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB). The DFT study reveals that the dissociation energies of H2O2 and adsorption energies of reactive oxygen intermediates on Fe sites are more negative than those on Ni sites, proving the crucial role of Fe sites in the elevated peroxidase-like activity of NiFe MOF. By taking advantage of the unique redox reaction between H2O2 and S2-, a simple and sensitive colorimetric method for S2- was proposed based on TMB/H2O2/NiFe MOF system with a wide linear range of 0.5 – 60 μM and a low detection limit of 28 nM (3σ). The proposed NiFe MOF-based colorimetric method exhibited good selectivity to S2- among some common cations and anions, and it was successfully applied in the real water samples.

Core-shell bimetallic Fe-Co MOFs to activated peroxymonosulfate for efficient degradation of 2-chlorophenol

Based on two topologically distinct MOFs, the core–shell bimetallic MIL-101/ZIF-67x (M/Zx) were successfully prepared to activate the self-decomposition of peroxymonosulfate (PMS) for 2-chlorophenol (2-cp) degradation. The M/Zx not only inherited the merits of single-metal MOFs such as high surface area and pore volume, but also exhibited superior catalytic performance. In the M/Zx/PMS system, 90% of the 2-cp (100 mg/L) was removed within 10 min with a reaction rate of 0.241 min−1. This observation was attributed to the synergistic catalysis of cobalt and iron active sites in M/Zx, which greatly promoted the Co2+/Co3+ redox cycle, thereby enhancing the catalytic performance. Further investigation found that both radical and non-radical pathway jointly promote the degradation of 2-cp, and the singlet oxygen (1O2) mediated non-radical pathway serve a predominant role. Meanwhile, SO4·- and ·OH also accelerated this process. The possible degradation pathway was proposed by identifying the intermediates of 2-cp degradation. This work provides some new ideas for the design of multi-metal MOFs for environmental remediation.

Over 21.0% faradaic efficiency of ambient ammonia production: Photoelectrocatalytic activity of MOF-235

In recent years, the Haber-Bosh process's ammonia synthesis has shown a lot of energy consumption and significant CO2 emissions. In this sense, the photoelectrochemical production of NH3 via dinitrogen fixation has been showing a promising strategy. Thus, in this work, ammonia production under photoelectrocatalytic condition was carried out using the metal-organic iron terephthalate structure (MOF-235), obtained by a simple and low-cost process (solvothermal). The results indicated greater activity for the nitrogen reduction reaction (NRR) compared to the hydrogen reduction reaction (HRR), in addition to a high yield of NH3 0.716 µg h−1 cm−2 and Faradaic efficiency greater than 21.0% at - 0.2 V vs reversible hydrogen electrode in 0.1 mol L−1 Na2SO4 electrolyte. It is believed that the good results of MOF-235 during NRR are related to a high specific area of MOF, making available active sites of trinuclear-oxocentered iron in abundance. The material homogeneity in the electrode and the excellent light absorption were also decisive factors for the supply of high energy electrons necessary for ammonia production.