Iron Sites Research Articles

Investigation of Valence Mixing in Sodium-Ion Battery Cathode Material Prussian White by Mössbauer Spectroscopy

Prussian white (PW), Na2Fe [Fe(CN)6], is a highly attractive cathode material for sustainable sodium-ion batteries due to its high theoretical capacity of ∼170 mAhg−1 and low-cost synthesis. However, there exists significant variability in the reported electrochemical performance. This variability originates from compositional flexibility possible for all Prussian blue analogs (PBAs) and is exasperated by the difficulty of accurately quantifying the specific composition of PW. This work presents a means of accurately quantifying the vacancy content, valence distribution, and, consequently, the overall composition of PW via Mössbauer spectroscopy. PW cathode material with three different sodium contents was investigated at 295 and 90 K. The observation of only two iron environments for the fully sodiated compound indicated the absence of [Fe(CN)6]4- vacancies. Due to intervalence charge transfer between iron centers at 295 K, accurate determination of valences was not possible. However, by observing the trend of spectral intensities and center shift for the nitrogen-bound and carbon-bound iron, respectively, at 90 K, valence mixing between the iron sites could be quantified. By accounting for valence mixing, the sum of iron valences agreed with the sodium content determined from elemental analysis. Without an agreement between the total valence sum and the determined composition, there exists uncertainty around the accuracy of the elemental analysis and vacancy content determination. Thus, this study offers one more stepping stone toward a more rigorous characterization of composition in PW, which will enable further optimization of properties for battery applications. More broadly, the approach is valuable for characterizing iron-based PBAs in applications where precise composition, valence determination, and control are desired.

Tuning the unsaturated iron sites in MIL-101(Fe) nanoparticles for reactive oxygen species-mediated bacterial inactivation in the dark

In this study, reactive oxygen species (ROS)-mediated antibacterial activity with high efficiency in the dark was achieved by modulating the active sites of MIL-101(Fe) nanoparticles. The ROS production over x-MIL-101(Fe) nanoparticles was intensively enhanced by tuning the unsaturated iron sites (FeII/FeIII, the ratio was marked as x%). Electron paramagnetic resonance (EPR) analysis confirmedmore ROS generation on x-MIL-101(Fe) surface than that on MIL-101(Fe), due to more electrons shifting from benzene rings to FeII/FeIII sites. Among all samples, 8.9-MIL-101(Fe) displayed the highest inactivation efficiency (> 99.99%) against Escherichia coli within 2 h in the dark. ROS reacted with cell wall components to generate carbon-centered radicals via H abstraction, leading to the disruption of cell wall, intracellular ROS, and DNA damage. Antibacterial performance of 8.9-MIL-101(Fe) in air filters indicated 8.9-MIL-101(Fe) can be applied to prevent the spread of airborne pathogens. The results are promising for the ambient antibacterial application of MOF materials.

Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells

Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells

Synergetic effect of sodium and sulfur on iron catalysts for CO hydrogenation directly to lower olefins

Iron-based catalysts have recognized as the promising candidates for syngas conversion to value-added products. Sodium and sulfur are widely used to refrain the product distribution towards lower olefins. Herein, the synergetic effect between Na and S were investigated by modulating χ-Fe5C2 with Na and/or S. With the characterization of XRD, Raman spectra, FESEM, TEM, TPD, XPS and semi-quantification calculations, we have revealed that when Na and S are added simultaneously, Na tends to activate the deactivated iron sites by S through electron transfer while the formation of long-chain products were weakened. We also demonstrated that sequential loading Na and S would not result in such synergetic effect because of the overlarge intimacy between both elements. This work emphasizes the significance of the synergetic effect of muti-promotors in such a kind of heterogenous catalysis.

Repair of Iron Center Proteins-A Different Class of Hemerythrin-like Proteins.

Repair of Iron Center proteins (RIC) form a family of di-iron proteins that are widely spread in the microbial world. RICs contain a binuclear nonheme iron site in a four-helix bundle fold, two basic features of hemerythrin-like proteins. In this work, we review the data on microbial RICs including how their genes are regulated and contribute to the survival of pathogenic bacteria. We gathered the currently available biochemical, spectroscopic and structural data on RICs with a particular focus on Escherichia coli RIC (also known as YtfE), which remains the best-studied protein with extensive biochemical characterization. Additionally, we present novel structural data for Escherichia coli YtfE harboring a di-manganese site and the protein’s affinity for this metal. The networking of protein interactions involving YtfE is also described and integrated into the proposed physiological role as an iron donor for reassembling of stress-damaged iron-sulfur centers.

Quality or Quantity? How Structural Parameters Affect Catalytic Activity of Iron Oxides for CO Oxidation

The replacement of noble metal catalysts by abundant iron as an active compound in CO oxidation is of ecologic and economic interest. However, improvement of their catalytic performance to the same level as state-of-the-art noble metal catalysts requires an in depth understanding of their working principle on an atomic level. As a contribution to this aim, a series of iron oxide catalysts with varying Fe loadings from 1 to 20 wt% immobilized on a γ-Al2O3 support is presented here, and a multidimensional structure–activity correlation is established. The CO oxidation activity is correlated to structural details obtained by various spectroscopic, diffraction, and microscopic methods, such as PXRD, PDF analysis, DRUVS, Mössbauer spectroscopy, STEM-EDX, and XAS. Low Fe loadings lead to less agglomerated but high percentual amounts of isolated, tetrahedrally coordinated iron oxide species, while the absolute amount of isolated species reaches its maximum at high Fe loadings. Consequently, the highest CO oxidation activity in terms of turnover frequencies can be correlated to small, finely dispersed iron oxide species with a large amount of tetrahedrally oxygen coordinated iron sites, while the overall amount of isolated iron oxide species correlates with a lower light-off temperature.

Learning to Identify Physiological and Adventitious Metal-Binding Sites in the Three-Dimensional Structures of Proteins by Following the Hints of a Deep Neural Network.

Thirty-eight percent of protein structures in the Protein Data Bank contain at least one metal ion. However, not all these metal sites are biologically relevant. Cations present as impurities during sample preparation or in the crystallization buffer can cause the formation of protein–metal complexes that do not exist in vivo. We implemented a deep learning approach to build a classifier able to distinguish between physiological and adventitious zinc-binding sites in the 3D structures of metalloproteins. We trained the classifier using manually annotated sites extracted from the MetalPDB database. Using a 10-fold cross validation procedure, the classifier achieved an accuracy of about 90%. The same neural classifier could predict the physiological relevance of non-heme mononuclear iron sites with an accuracy of nearly 80%, suggesting that the rules learned on zinc sites have general relevance. By quantifying the relative importance of the features describing the input zinc sites from the network perspective and by analyzing the characteristics of the MetalPDB datasets, we inferred some common principles. Physiological sites present a low solvent accessibility of the aminoacids forming coordination bonds with the metal ion (the metal ligands), a relatively large number of residues in the metal environment (≥20), and a distinct pattern of conservation of Cys and His residues in the site. Adventitious sites, on the other hand, tend to have a low number of donor atoms from the polypeptide chain (often one or two). These observations support the evaluation of the physiological relevance of novel metal-binding sites in protein structures.

Local Spin-State Tuning of Iron Single-Atom Electrocatalyst by S-Coordinated Doping for Kinetics-Boosted Ammonia Synthesis.

The electrochemical nitrogen reduction reaction (e-NRR) is envisaged as alternative technique to the Haber-Bosch process for NH3 synthesis. However, how to develop highly active e-NRR catalysts faces daunting challenges. Herein, a viable strategy to manipulate local spin state of isolated iron sites through S-coordinated doping (FeSA -NSC) is reported. Incorporation of S in the coordination of FeSA -NSC can induce the transition of spin-polarization configuration with the formation of a medium-spin-state of Fe (t2g 6 eg 1), which is beneficial for facilitating eg electrons to penetrate the antibonding π-orbital of nitrogen. As a consequence, a record-high current density up to 10mA cm-2 can be achieved, together with a high NH3 selectivity of ≈10% in a flow cell reactor. Both experimental and theoretical analyses indicate that the monovalent Fe(I) atomic center in the FeSA -NSC after the S doping accelerates the N2 activation and protonation in the rate-determining step of *N2 to *NNH.

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.

Core–Shell Structured Fe–N–C Catalysts with Enriched Iron Sites in Surface Layers for Proton-Exchange Membrane Fuel Cells

Core–Shell Structured Fe–N–C Catalysts with Enriched Iron Sites in Surface Layers for Proton-Exchange Membrane Fuel Cells

Efficient charge transfers in hematite photoanode integrated by fluorine and zirconia co-doping for photoelectrochemical water splitting

Dual elemental doping strategies were employed in this study to fabricate fluorine and zirconia co-doped hematite (F–Zr:Fe2O3/FTO) photoanodes. Such F–Zr:Fe2O3/FTO photoanodes had a (110)-oriented structure with a remarkable photocurrent density of 1.91 mA cm−2 without any catalyst support at water oxidation potential of 1.23 VRHE. The enhancement in net photocurrent density was attributed to the synergistic effect between the two doping elements. When in situ doped Zr4+ ions were used to substitute iron ions, they increase photogenerated free electrons within the bulk hematite and resulted in lower bulk charge transfer resistance, and a minimal concentration of F was doped into hematite lattice, it would go into the oxygen site rather than the iron site, resulting in more positive charges on iron site. The charge compensation contributed to reduced recombination of free electron–hole pairs and improved surface charge injection efficiency at the semiconductor–electrolyte interface by promoting hole transfer via surface states.

Identifying Monomeric Fe Species for Efficient Direct Methane Oxidation to C1 Oxygenates with H2O2 over Fe/MOR Catalysts

Exploring advanced catalysts and reaction systems operated at mild reaction conditions is crucial for conducting the direct methane oxidation reaction toward oxygenate products. Many efforts have been put into research on pentasil−type (MFI) zeolites based on mononuclear and/or binuclear iron sites, using H2O2 as the oxidant. In this work, we present a modified liquid ion−exchange method to better control Fe loading in a mordenite−type (MOR) zeolite with a Si/Al molar ratio of 9. The optimized Fe/MOR catalyst showed excellent performance in the direct methane oxidation reaction with turnover frequencies (TOFs) of 555 h−1 to C1 oxygenates, significantly better than the reported activity. Multiple comparative experiments were conducted to reveal the mechanism behind the performance. Strikingly, the active sites in the Fe/MOR catalyst were found to be mononuclear iron sites, confirmed by transmission electron microscopy (TEM), ultraviolet−visible diffuse reflectance spectroscopy (UV−vis DRS), and X-ray absorption spectroscopy (XAS). Increasing the iron loading led to the aggregation of the iron sites, which tend to trigger undesirable side reactions (i.e., H2O2 decomposition and over−oxidation), resulting in a significant decrease in TOFs to C1 oxygenates.

Boron induced electron-rich single iron sites for boosted N2 electroreduction reaction

Electrochemical N2 reduction to NH3 offers a mild and feasible alternative to Haber-Bosch NH3 synthesis. Designing a promising nitrogen reduction reaction (NRR) catalyst under alkaline condition is highly desirable due to the advantages of alkaline electrolyte in fast charge transfer and limiting the hydrogen evolution reaction. However, the ammonia yield rate of iron single-atom catalysts is lower, usually below 40 µg h−1 mg−1cat. Herein, the Fe sites supported on boron, nitrogen co-doped carbon with atomic dispersion (Fe-B/NC) catalyst was synthesized by the pre-polymerization and pyrolysis processes. This catalyst exhibits a NH3 yield 88.7 µg h−1 mg−1cat at −0.45 V (vs. RHE) an alkaline electrolyte, which is greater than the values reported in the literature. Combining experimental analysis with DFT calculations, we demonstrate that the introduction of B coordination breaks the symmetric electron distribution of the FeN4 strategy to regulate its N2 adsorption, activation and thus the NRR performance. The synergistic effect between B and Fe can effectively activate the adsorbed N2 molecules to achieve excellent NRR performance.

Iron‐Carbon Alloy Under Shock Compression: Implications for the Carbon Concentration in Earth’s Inner Core

AbstractCarbon has been proposed to be a potential light element in the Earth’s solid inner core. However, its presence and concentration in the core remain limited constrained. In this study, we measured the equation of state of an iron‐carbon alloy with ∼1.5 wt.% (∼6.64 at.%) carbon up to ∼185 GPa and ∼3,750 K via shock compression. The Hugoniot equation of state of the Fe‐1.5wt.%C alloy was determined to be Us (km/s) = 3.94 (0.15) + 1.62 (0.08) Up with an initial density of 7.77 (0.03) g/cm3, where Us and Up are the shock wave velocity and particle velocity, respectively. Hugoniot data combined with the Mie‐Grüneisen equation of state allow us to build the thermal equations of state of the Fe‐C alloy at high pressure‐temperature (P‐T) conditions. Investigations on the pressure‐density relation of Fe‐C alloys by the shock experiments and synchrotron X‐ray diffraction measurements by Pamato et al. (2020), https://doi.org/10.1029/2020jb020159 have indicated that carbon should occupy interstitial sites of iron lattice at high P‐T conditions. The thermal equation of state of the Fe‐1.5wt.%C alloy provided here shows that Fe‐1.5wt.%C is ∼1.65% denser than the inner core under conditions relevant to the inner‐core boundary. This study indicates that iron alloyed with an upper bound of ∼2.2 (0.3) wt.% (∼10.7 [1.6] at.%) carbon may match with the Preliminary Reference Earth Model (PREM) in the Earth’s inner core. Thus, interstitial carbon in iron helps to explain the observed density deficit of the inner core, although other light elements are also expected.

High loading of single atomic iron sites in Fe–NC oxygen reduction catalysts for proton exchange membrane fuel cells

Non-precious iron-based catalysts (Fe–NCs) require high active site density to meet the performance targets as cathode catalysts in proton exchange membrane fuel cells. Site density is generally limited to that achieved at a 1–3 wt%(Fe) loading due to the undesired formation of iron-containing nanoparticles at higher loadings. Here we show that by preforming a carbon–nitrogen matrix using a sacrificial metal (Zn) in the initial synthesis step and then exchanging iron into this preformed matrix we achieve 7 wt% iron coordinated solely as single-atom Fe–N4 sites, as identified by 57Fe cryogenic Mössbauer spectroscopy and X-ray absorption spectroscopy. Site density values measured by in situ nitrite stripping and ex situ CO chemisorption methods are 4.7 × 1019 and 7.8 × 1019 sites g−1, with a turnover frequency of 5.4 electrons sites−1 s−1 at 0.80 V in a 0.5 M H2SO4 electrolyte. The catalyst delivers an excellent proton exchange membrane fuel cell performance with current densities of 41.3 mA cm−2 at 0.90 ViR-free using H2–O2 and 145 mA cm−2 at 0.80 V (199 mA cm−2 at 0.80 ViR-free) using H2–air.

First principles calculations to investigate magnetic tetranuclear ferrous complexes

Spin-crossover has been intensively studied during the last past years due to the need to improve our understanding of the phenomenon and in the hope to exploit switching for data storage at the molecular level. Here we employ periodic spin polarized density functional theory in conjunction with the projector augmented wave method within the generalized gradient approximation, including a Hubbard interaction U at the iron site and a van der Waals correction, to investigate the structural, electronic and magnetic properties of the tetranuclear [Fe4(C23N6H15)4](BF4)4 molecular crystals. Two geometrical structures of the considered system are evidenced that correspond to two spin state configurations: HS-LS-HS-HS and HS-HS-HS-HS. We found that the two magnetic configurations seen experimentally can indeed by reproduced by our calculations, and that the magnetic centers are coupled ferromagnetically. These findings enable promising electronic and spintronic perspectives.

New Insights into the Magnetic Properties of CoFe2O4@SiO2@Au Magnetoplasmonic Nanoparticles

We report the successful synthesis and a complete magnetic characterization of CoFe2O4@SiO2@Au magnetoplasmonic nanoparticles. The CoFe2O4 magnetic nanoparticles were prepared using the hydrothermal method. A subsequent SiO2 shell followed by a plasmonic Au shell were deposited on the magnetic core creating magnetoplasmonic nanoparticles with a core–shell architecture. A spin-glass-type magnetism was shown at the surface of the CoFe2O4 nanograins. Depending on the external magnetic field, two types of spin-glass were identified and analyzed in correlation with the exchange field acting on octahedral and tetrahedral iron sites. The magnetization per formula unit of the CoFe2O4 core is not changed in the case of CoFe2O4@SiO2@Au nanocomposites. The gold nanoparticles creating the plasmonic shell show a giant diamagnetic susceptibility, dependent on their crystallite sizes.

Visualization of on-surface ethylene polymerization through ethylene insertion.

Polyethylene production through catalytic ethylene polymerization is one of the most common processes in the chemical industry. The popular Cossee-Arlman mechanism hypothesizes that the ethylene be directly inserted into the metal-carbon bond during chain growth, which has been awaiting microscopic and spatiotemporal experimental confirmation. Here, we report an in situ visualization of ethylene polymerization by scanning tunneling microscopy on a carburized iron single-crystal surface. We observed that ethylene polymerization proceeds on a specific triangular iron site at the boundary between two carbide domains. Without an activator, an intermediate, attributed to surface-anchored ethylidene (CHCH3), serves as the chain initiator (self-initiation), which subsequently grows by ethylene insertion. Our finding provides direct experimental evidence of the ethylene polymerization pathway at the molecular level.

Interface engineering of iron sulfide/tungsten nitride heterostructure catalyst for boosting oxygen reduction activity

• An ion migration approach is reported for preparing the Fe 1-x S/WN heterostructure. • The electron depletion of iron centers caused by WN promotes the O 2 protonation. • WN component promotes the reversible Fe 2+/3+ redox couple during catalysis. • An exceptional durability is demonstrated in the setups of RDE and half-cell GDL. • Zn-air battery driven by Fe 1-x S/WN delivers a stable and outstanding performance. Herein, we demonstrate an ion migration approach for constructing iron sulfide/tungsten nitride heterostructure supported over carbon nanoboxes as an efficient and robust electrocatalyst toward the oxygen reduction reaction. Tungsten nitride decorated at iron sulfide induces the electron delocalization of iron sites for facilitating the electron/proton transfer into adsorbed oxygen molecules that is generally proposed as the rate-determining step over the sulfides. It also greatly promote the reversible Fe 2+/3+ redox transition during catalysis that determines the durability performance. Therefore, the iron sulfide/tungsten nitride heterostructure catalyst shows much superior activity and long-term stability to pristine iron sulfide, as evidenced in the setups of rotating disk electrode and half-cell gas diffusion electrode. It is explored as an exceptional air cathode of rechargeable zinc-air battery that stably operates for over 140 h without the performance loss. This work offers a novel synthetic strategy for designing transition–metal nitride/sulfide heterostructure and provides an insight into optimized electron configuration via interface engineering.

Simultaneously Integrate Iron Single Atom and Nanocluster Triggered Tandem Effect for Boosting Oxygen Electroreduction.

Atomically nitrogen-coordinated iron atoms on carbon (FeNC) catalysts are emerging as attractive materials to substitute precious-metal-based catalysts for the oxygen reduction reaction (ORR). However, FeNC usually suffers from unsatisfactory performance due to the symmetrical charge distribution around the iron site. Elaborately regulating the microenvironment of the central Fe atom can substantially improve the catalytic activity of FeNC, which remains challenging. Herein, N/S co-doped porous carbons are rationally prepared and are verified with rich Fe-active sites, including atomically dispersed FeN4 and Fe nanoclusters (FeSA-FeNC@NSC), according to systematically synchrotron X-ray absorption spectroscopy analysis. Theoretical calculation verifies that the contiguous S atoms and Fe nanoclusters can break the symmetric electronic structure of FeN4 and synergistically optimize 3d orbitals of Fe centers, thus accelerating OO bond cleavage in OOH* for improving ORR activity. The FeSA-FeNC @NSC delivers an impressive ORR activity with half-wave-potential of 0.90 V, which exceeds that of state-of-the-art Pt/C (0.87 V). Furthermore, FeSA-FeNC @NSC-based Zn-air batteries deliver excellent power densities of 259.88 and 55.86 mW cm-2 in liquid and all-solid-state flexible configurations, respectively. This work presents an effective strategy to modulate the microenvironment of single atomic centers and boost the catalytic activity of single-atom catalysts by tandem effect.