Isolated Fe Atoms Research Articles

Ultrasmall Fe‐Nanoclusters‐Anchored Carbon Polyhedrons Interconnected with Carbon Nanotubes for High‐Performance Zinc‐Air Batteries

The catalytic activity of metal catalyst is closely related to its particle size. Yet, the size effect in electrocatalytic oxygen reduction reaction (ORR), an important reaction for metal‐air batteries and fuel cells, has not been clearly studied. Herein, a two‐step anchoring method is utilized to control the Fe catalyst in forms of nanoparticles (NPs), ultrasmall nanoclusters (NCTs), and isolated atoms as well as stabilized and dispersed by carbon polyhedrons interconnected with carbon nanotubes (CNTs). The uniformly distributed Fe NCTs displays superior ORR performance compared with Fe NPs, isolated Fe atoms, and commercial Pt/C. The brilliant ORR activity of Fe NCTs is a result of its unique electron structure and abundant edge and corner active sites. Due to the porous structure of carbon polyhedrons and high electron conductivity of CNTs, Fe NCTs also delivers an excellent discharge performance in zinc‐air battery with a peak power density of 213.3 mW cm−2 and long‐term stability. In these findings, a new strategy for the design of metal NCTs catalysts applied in various catalytic reactions is opened up.

Atomically Dispersed Fe Sites Regulated by Adjacent Single Co Atoms Anchored on N-P Co-Doped Carbon Structures for Highly Efficient Oxygen Reduction Reaction.

Manipulating the coordination environment and electron distribution for heterogeneous catalysts at the atomic level is an effective strategy to improve electrocatalytic performance but remains challenging. Herein, atomically dispersed Fe and Co anchored on nitrogen, phosphorus co-doped carbon hollow nanorod structures (FeCo-NPC) are rationally designed and synthesized. The as-prepared FeCo-NPC catalyst exhibits significantly boosted electrocatalytic kinetics and greatly upshifts the half-wave potential for the oxygen reduction reaction. Furthermore, when utilized as the cathode, the FeCo-NPC catalyst also displays excellent zinc-air battery performance. Experimental and theoretical results demonstrate that the introduction of single Co atoms with Co-N/P coordination around isolated Fe atoms induces asymmetric electron distribution, resulting in the suitable adsorption/desorption ability for oxygen intermediates and the optimized reaction barrier, thereby improving the electrocatalytic activity.

Selective Incorporation of Iron Sites into MFI Zeolite Framework by One-Pot Synthesis

Developing highly efficient methods to selectively incorporate active metal sites into the zeolite framework is crucial for the large-scale utilization of metal-zeolites in various catalytic reactions including alkane conversion. Herein, ferrous gluconate is developed as an efficient metal precursor to embed Fe atoms into the MFI siliceous zeolite framework. The crystallization time and Si/Fe ratio are systematically investigated. It is revealed that the ferrous gluconate is well soluble and stable under alkaline conditions, allowing ferrous gluconate to participate in the whole crystallization course of zeolite. Multiple characterizations demonstrate that the tetrahedrally coordinated Fe atoms are selectively incorporated into the MFI siliceous zeolite framework (Fe-MFI). A catalytic test in propane dehydrogenation shows that the optimized Fe-MFI zeolite possesses high activity, propylene selectivity, and durability. This study opens an efficient strategy to incorporate isolated Fe atoms into the zeolite framework.

Interfacial Defect Engineering Triggered by Single Atom Doping for Highly Efficient Electrocatalytic Nitrate Reduction to Ammonia

Electrochemical reduction of nitrate (NO3RR), a widespread water pollutant, to high-valued ammonia is encouraging for sustainable artificial nutrient recycling and environmental-friendly pollution management. However, the limited available catalytic active sites and competitive hydrogen evolution make the catalytic performance still unsatisfactory. In this work, interfacial defect engineering via single atom doping was conducted to achieve highly efficient electrocatalytic NO3RR. Upon introduction of isolated Fe atoms, abundant oxygen vacancies are generated over atomic interface of TiO2, and the induced charge redistribution triggers the formation of considerable active sites for nitrate reduction, which plays a crucial role in inhibiting the proton reduction and promoting the adsorption and activation of nitrate. As expected, single atom Fe modified TiO2 exhibits a maximum ammonia yield rate of 137.3 mg h–1 mgcat.–1 and a Faradaic efficiency of 92.3% at −1.4 V (vs RHE), which are among the best of all the reported values yet. This work provides valuable insights into the exploration of highly efficient electrocatalysts toward nitrate reduction through the heteroatom doping and defect engineering over atomic interface.

Engineering electronic and geometric structures of Fe, N-doped carbon polyhedrons toward organic contaminant oxidation

A universal gas diffusion strategy was developed to derive highly efficient PMS-activation catalysts comprising isolated Fe atoms coordinated with N species embedded in carbon polyhedrons (Fe-NC). Their electronic and geometric structures were modulated by altering the precursor ratio and calcination temperature. Benefiting from the maximized atomic utilization, the obtained catalysts exhibited superior catalytic performances, outperforming almost all previously reported metal materials. The enhanced activity is likely due to an optimal particle size and Fe content, favorable Fe-Nx active sites, and conductive carbon materials. Scavenging and probing experiments indicated that surface Fe-NC and the defective edges with oxygen functional groups were the active sites for PMS activation to produce surface-bonding active complexes (Fe-NC-PMS) and high-valent iron oxo species (FeV = O) for organic degradation, unlike the reported radicals and singlet oxygen mediated degradation. Overall, this work provides a universal gas diffusion strategy to design dimension-controlled Fe-NC catalysts for the elimination of organic pollutants.

Enhanced catalytic activities of Fe anchored on graphene substrates for water splitting and hydrogen evolution

Water splitting on single Fe atom catalyst anchored on defective graphene surfaces by using first-principles density functional theory. The structure and electronic features of isolated Fe atom anchored on three graphene surfaces with single vacancy (SV), double vacancy (DV) and Stone-Wales structure (SW) defect were systematically explored. The three structures prove to be high activity and high stability on catalytic. The adsorption and the energy barrier of water splitting as well as hydrogen adsorption free energy ΔGH∗ on single-atom Fe were also studied. The sequence of promoted splitting activity is found to be Fe@SW > Fe@DV > Fe@SV. Furthermore, by hydrogen adsorption free energy ΔGH∗ analysis, we predict that the HER catalytic activity of graphene nanosheet can be improved by anchoring Fe atom on SV and DV structures, which are comparable to or even better than noble metals. It is found that the catalytic activity of water splitting and HER can be changed with the shift in d-band center with respect to Fermi-level. Detailed investigations on electronic structure of Fe@graphene catalytic systems disclose an obvious orbital hybridization coupling and charge transfer between atom Fe on carbon surfaces and water molecule. These results provide us with new insight into design of high performer and low-cost catalysts and may inspire potential applications in the fields of clean and renewable energy.

Boosting peroxymonosulfate activation by porous single-atom catalysts with FeN4O1 configuration for efficient organic pollutants degradation

The construction of single Fe atoms with favorable electron structures is highly desired to boost peroxymonosulfate (PMS) activation for organics degradation. Herein, this study embedded isolated Fe atoms on N, O co-doped porous carbon substrate (FeSA-N/OC), and first found the constructed FeN4O1 configuration can realize efficient PMS activation via electron transfer by inner-sphere complexation, instead of the usual reactive oxygen species (ROS) as a major role. Structural investigation showed that the FeN4O1 moieties possess high content of high spin (HS) Fe(II), resulting in the delocalization of unpaired electrons around Fe centers, which is benefit to transfer electrons when reacted with PMS. EIS and LSV curves further certified the electron transfer process. Density functional theory (DFT) calculations unveiled that the FeN4O1 configuration can directly adsorb OO bond of PMS, leading to electron accumulation around Fe-O2 bond, and then ulteriorly trigger the electron shuttling in organics degradation. Consequently, the optimized FeSA-N/OC/PMS system exhibited superior oxidation ability for various organic pollutants, and was not affected by initial pH variation (3.19 ∼ 10.89), inorganic anions (ClO4−, Cl− and H2PO4−) and natural organic matter (NOM) interference. Importantly, the developed system presented certain applicability in the treatment of actual water from Taihu Lake basin, China. Hence, this study not only elucidates the inner-sphere complexation-oriented electron transfer mechanism between unique FeN4O1 configuration and PMS, but provides new insights into engineered single atom catalysts for wastewater purification.

Molecular Bridging Enables Isolated Iron Atoms on Stereoassembled Carbon Framework To Boost Oxygen Reduction for Zinc-Air Batteries.

Realizing the synergy between active site regulation and rational structural engineering is essential in the electrocatalysis community but still challenging. Here, a matrix-confined co-pyrolysis strategy based on molecular bridging is demonstrated to realize highly dispersed Fe atoms on stereoassembled carbon framework. Both polyacrylonitrile matrix and organic linker from metal-organic frameworks (MOFs) provide sufficient N-anchoring sites for the generation of Fe-N4 moieties. A high Fe loading of 2.9 wt.% is readily achieved based on the scalable approach without post-treatment. Owing to the presence of highly exposed Fe-N-C sites and well-tuned pore structures, isolated Fe atoms on porous carbon nanofiber framework (Fe-SA/NCF) exhibits decent oxygen reduction activity and stability in alkaline conditions via a near four-electron path, demonstrating superior performance as air cathode for zinc-air batteries (ZABs) to commercial Pt/C catalyst.

Atomically dispersed iron atoms on nitrogen-doped porous carbon catalyst with high density and accessibility for oxygen reduction

• AD-Fe-NPC was successfully developed as oxygen reduction catalysts. • HAADF-STEM images confirmed the atomic dispersion of Fe on AD-Fe-NPC catalysts. • The optimized AD-Fe-NPC structure exhibited superior ORR activity to Pt/C. The development of high-performance, low-cost and high-stability oxygen reduction reaction (ORR) non-platinum catalysts has become one of the main research directions on fuel cells and metal-air batteries. Here, we developed porous carbon catalysts with atomically dispersed nitrogen coordinated iron atoms (Fe-N x ) as the active sites (AD-Fe-NPC) using Fe-doped ZIF-8 and 1,10-phenanthroline as precursors. The additional nitrogen coordinated sites from 1,10-phenanthroline, together with a special structure of ZIF-8 can suppress the agglomeration of Fe atoms, leading to isolated Fe atoms with high density on graphitic carbon during the pyrolysis. As a result, the sample prepared using 50 mg of 1,10-phenanthroline (AD-Fe-NPC-50) exhibited a high activity towards oxygen reduction with a half-wave potential of 0.91 V versus reversible hydrogen electrode in 0.1 M KOH, which is attributed to the high porosity (1011.2 m 2 g −1 ) and density of Fe-N x active sites (1.1 at%). Furthermore, as an air electrode in zinc-air batteries, the AD-Fe-NPC-50 demonstrated a maximum power density of about 201 mA cm −2 and a specific capacity of 782 mA h g Zn −1 at 10 mA cm −2 , which outperformed commercial Pt/C and many state-of-the-art non-precious metal catalysts.

Engineering iron single atomic sites with adjacent ZrO2 nanoclusters via ligand–assisted strategy for effective oxygen reduction reaction and high–performance Zn–air batteries

• SA Fe@ZrO 2 /NC is synthesized by ligand–assisted strategy with single Fe atoms and adjacent ZrO 2 . • The SA Fe@ZrO 2 /NC exhibits much better catalytic performance toward ORR. • A Zn-air battery based on SA Fe@ZrO 2 /NC shows excellent performance. • DFT reveals the role of ZrO 2 in modulating electron structure of single Fe atoms and ORR activity. Atomically dispersed iron–nitrogen–carbon (Fe–N–C) catalyst is a promising candidate to replace Pt for oxygen reduction reaction (ORR). To enhance ORR activity of Fe–N–C catalysts, the density and distribution of isolated Fe–N 4 sites should be further optimized. Herein, a ligand–assisted strategy to synthesis of single atomic Fe–N–C derived from Zr–metal–organic frameworks (Zr–MOFs) is proposed. During preparation, –NO 2 (from 2–nitroterephthalic acid ligands) in Zr–MOFs not only acts as the anchoring sites to capture Fe–polydopamine (FePDA) source but also plays a crucial role in modulating the Fe − N 4 configuration. The resulting SA Fe@ZrO 2 /NC catalyst consists of FeN 4 sites with adjacent ZrO 2 in N–doped carbon, in which in–situ introduction of ZrO 2 positively enhance O 2 adsorption ability. Due to the moderate pore structure, atomically dispersed Fe–N 4 active sites, and the strong interface interaction between isolated Fe atoms and ZrO 2 nanocluster, the as–prepared catalyst exhibits comparable ORR activity and outstanding stability in alkaline solution. When assembled in a rechargeable Zn–air battery, the battery with SA Fe@ZrO 2 /NC air electrode delivers a stable open circuit voltage, high–power density (250 mW cm −2 ) and discharge specific capacity (730 mA h g Zn −1 ). It also demonstrates a long cycle life and good rate performance. Density functional theory calculation results reveals that the adjacent ZrO 2 nanocluster modulates the electron structure of Fe atoms in FeN 4 sites with an improved ORR process and activity. This work provides a facile strategy for preparing efficient single atomic Fe–N–C catalyst to drive the oxygen reduction reaction in Zn–air batteries.

Precise synthesis of Fe–N2 with N vacancies coordination for boosting electrochemical artificial N2 fixation

The construction of definitive correlation between structure and catalytic properties for electrochemical artificial N2 fixation is still challenging yet significative. Herein, we have explored the coordinative unsaturation (Fe-N2) in isolated Fe atoms with nitrogen vacancies (VN, which are located at the opposite coordination sites of the Fe atom) on interconnected carbon for electrocatalytic ammonia synthesis. Benefiting from dual active centers (Fe-N2 and VN) and the spin density on both of them, the novel catalyst exhibits a synergistic improvement of catalytic activity, selectivity, and stability. This work guides the design of the catalyst theoretically and synergistically and combines the advantages of Fe-N2 with VN structure creation, defect and spin-state engineering introduction toward the concurrent fulfillment of effective N2 adsorption and activation. We believe our strategy will incite more investigation into designing high-performance catalysts with well-defined coordinative unsaturation single-atom active sites for ambient ammonia synthesis and other energy conversion system.

Boosting Electroreduction Kinetics of Nitrogen to Ammonia via Tuning Electron Distribution of Single‐Atomic Iron Sites

AbstractElectrocatalytic nitrogen reduction reaction (NRR) plays a vital role for next‐generation electrochemical energy conversion technologies. However, the NRR kinetics is still limited by the sluggish hydrogenation process on noble‐metal‐free electrocatalyst. Herein, we report the rational design and synthesis of a hybrid catalyst with atomic iron sites anchored on a N,O‐doped porous carbon (FeSA‐NO‐C) matrix of an inverse opal structure, leading to a remarkably high NH3 yield rate of 31.9 μg h−1 mg−1cat. and Faradaic efficiency of 11.8 % at −0.4 V for NRR electrocatalysis, outperformed almost all previously reported atomically dispersed metal‐nitrogen‐carbon catalysts. Theoretical calculations revealed that the observed high NRR catalytic activity for the FeSA‐NO‐C catalyst stemmed mainly from the optimized charge‐transfer between the adjacent O and Fe atoms homogenously distributed on the porous carbon support, which could not only significantly facilitate the transportation of N2 and ions but also effectively decrease the binding energy between the isolated Fe atom and *N2 intermediate and the thermodynamic Gibbs free energy of the rate‐determining step (*N2 → *NNH).

Boosting Electroreduction Kinetics of Nitrogen to Ammonia via Tuning Electron Distribution of Single-Atomic Iron Sites.

Electrocatalytic nitrogen reduction reaction (NRR) plays a vital role for next-generation electrochemical energy conversion technologies. However, the NRR kinetics is still limited by the sluggish hydrogenation process on noble-metal-free electrocatalyst. Herein, we report the rational design and synthesis of a hybrid catalyst with atomic iron sites anchored on a N,O-doped porous carbon (FeSA -NO-C) matrix of an inverse opal structure, leading to a remarkably high NH3 yield rate of 31.9 μg h-1 mg-1 cat. and Faradaic efficiency of 11.8 % at -0.4 V for NRR electrocatalysis, outperformed almost all previously reported atomically dispersed metal-nitrogen-carbon catalysts. Theoretical calculations revealed that the observed high NRR catalytic activity for the FeSA -NO-C catalyst stemmed mainly from the optimized charge-transfer between the adjacent O and Fe atoms homogenously distributed on the porous carbon support, which could not only significantly facilitate the transportation of N2 and ions but also effectively decrease the binding energy between the isolated Fe atom and *N2 intermediate and the thermodynamic Gibbs free energy of the rate-determining step (*N2 → *NNH).

Highly Active and Sulfur-Resistant Fe-N4 Sites in Porous Carbon Nitride for the Oxidation of H2 S into Elemental Sulfur.

Iron-based catalysts have been widely studied for the oxidation of H2 S into elemental S. However, the prevention of iron sites from deactivation remains a big challenge. Herein, a facile copolymerization strategy is proposed for the construction of isolated Fe sites confined in polymeric carbon nitride (CN) (Fe-CNNχ). The as-prepared Fe-CNNχ catalysts possess unique 2D structure as well as electronic property, resulting in enlarged exposure of active sites and enhancement of redox performance. Combining systematic characterizations with density functional theory calculation, itis disclosed that the isolated Fe atoms prefer to occupy four-coordinate doping configurations (Fe-N4 ). Such Fe-N4 centers favor the adsorption and activation of O2 and H2 S. As a consequence, Fe-CNNχ exhibit excellent catalytic activity for the catalytic oxidation of H2 S to S. More importantly, the Fe-CNNχ catalysts are resistant to water and sulfur poisoning, exhibiting outstanding catalytic stability (over 270 h of continuous operation), better than most of the reported catalysts.

Gas Diffusion Strategy for Inserting Atomic Iron Sites into Graphitized Carbon Supports for Unusually High-Efficient CO2 Electroreduction and High-Performance Zn-CO2 Batteries.

Emerging single-atom catalysts (SACs) hold great promise for CO2 electroreduction (CO2 ER), but the design of highly active and cost-efficient SACs is still challenging. Herein, a gas diffusion strategy, along with one-step thermal activation, for fabricating N-doped porous carbon polyhedrons with trace isolated Fe atoms (Fe1 NC) is developed. The optimized Fe1 NC/S1 -1000 with atomic Fe-N3 sites supported by N-doped graphitic carbons exhibits superior CO2 ER performance with the CO Faradaic efficiency up to 96% at -0.5 V, turnover frequency of 2225 h-1 , and outstanding stability, outperforming almost all previously reported SACs based on N-doped carbon supported nonprecious metals. The observed excellent CO2 ER performance is attributed to the greatly enhanced accessibility and intrinsic activity of active centers due to the increased electrochemical surface area through size modulation and the redistribution of doped N species by thermal activation. Experimental observations and theoretical calculations reveal that the Fe-N3 sites possess balanced adsorption energies of *COOH and *CO intermediates, facilitating CO formation. A universal gas diffusion strategy is used to exclusively yield a series of dimension-controlled carbon-supported SACs with single Fe atoms while a rechargeable Zn-CO2 battery with Fe1 NC/S1 -1000 as cathode is developed to deliver a maximal power density of 0.6 mW cm-2 .

Mechanism of ferromagnetism in (Fe, Co)-codoped 4H-SiC from density functional theory

First-principles calculations are performed to investigate the electronic structures and magnetic properties of (Fe, Co)-codoped 4H-SiC using the generalized gradient approximation plus Hubbard U method. We find that 4H-SiC doped with an isolated Fe atom and an isolated Co atom produces a total magnetic moment of 5.98 μB and 6.00 μB respectively. We estimate TC of about 263.1 K for the (Fe, Co)-codoped 4H-SiC system. We study ferromagnetic and antiferromagnetic coupling in (Fe, Co)-codoped 4H-SiC. Ferromagnetic behavior is observed. The strong ferromagnetic couplings between local magnetic moments can be attributed to p–d hybridization between Fe, Co and neighboring C. However, the (Fe, Co, VSi)-codoped 4H-SiC system shows antiferromagnetic coupling when an Si vacancy is introduced in the same 4H-SiC supercell. The results may be helpful for further study on transition metal-codoped systems.

Subnanometer iron clusters confined in a porous carbon matrix for highly efficient zinc–air batteries

We describe a facile synthetic protocol to realize the decoration of Fe coordinates at the subnanometer scale into a three-dimensional porous carbon matrix, which great promotes the oxygen reduction reaction compared with isolated Fe atoms.

MOF-Derived Isolated Fe Atoms Implanted in N-Doped 3D Hierarchical Carbon as an Efficient ORR Electrocatalyst in Both Alkaline and Acidic Media.

In order to improve the catalytic performance of oxygen reduction reaction (ORR), it is pivotal to increase the density and accessibility of the active sites. Herein, we have developed a template-free melamine-assisted cocalcined strategy to afford Fe-embedded and N-doped carbons (Fe-N-C) with not only high density of atomically dispersed Fe-Nx active sites but also abundant three-dimensional interconnected mesopores by directly pyrolyzing Fe-ZIF-8 covered with a controllable melamine layer. It is demonstrated that the introduction of melamine in the precursor plays a key role in constructing various carbonized products with controllable morphology, porosity, and components. With an optimal mass ratio 1:1 of melamine to Fe-ZIF-8, the resultant Fe@MNC-1 exhibits excellent ORR activity and stability, which exceeds 20 wt % commercial Pt/C catalyst (with a half-wave potential of 0.88 V vs 0.85 V) in an alkaline electrolyte and is even comparable to the commercial Pt/C catalyst (with a half-wave potential of 0.78 V vs 0.80 V) in an acidic electrolyte. To the best of our knowledge, Fe@MNC-1 can be ranked among the best nonprecious metal electrocatalysts for ORR in both alkaline and acidic media. The present synthetic strategy may provide a new opportunity for the design and construction of metal-organic framework-derived nanomaterials with rational composition and a desired porous structure to boost their electrocatalytic performance.

Isolated Fe atoms dispersed on cellulose-derived nanocarbons as an efficient electrocatalyst for the oxygen reduction reaction.

Cost-effective preparation of efficient electrocatalysts is vitally important for energy storage and conversion. Here, a facile chemical activation strategy using biomass cellulose as the carbon feedstock to fabricate isolated Fe atoms dispersed on a nitrogen doped graphene/nanocarbon hybrid is reported. This new single atom catalyst aFe-NGC worked as an excellent electrocatalyst towards the ORR compared to commercial Pt/C with 30 mV higher positive half-wave potential, larger current density, better stability and stronger methanol-tolerance. The key active sites for enhancing the ORR activity originated from the constructed high loading Fe-N/C configuration coupled with doped nitrogens, as explored by optimizing the activation temperatures and characterized by state-of-the-art techniques including aberration-corrected STEM and synchrotron XANES. This strategy could be developed into a general approach to prepare highly efficient atomic metal electrocatalysts using abundant biomass as a cost-effective carbon source.

When two is enough: On the origin of diverse crystal structures and physical properties in the Fe-Ge system

The Fe-Ge system resides among the richest binary intermetallic systems both in terms of the number of intermediate phases and diversity of crystal structures and magnetic properties. Although crystal structures of the Fe-Ge binary phases are different, they are closely related to each other with an evolution of Ge polyhedra being the cornerstone that weld them together. Upon transition to the Ge rich side the initial Ge polyhedron gradually loses vertex Fe atoms in a diverse manner creating a variety of different iron frameworks from 3-dimensional ones to practically isolated Fe atoms. This, along with strong electron correlations provided by Fe-Fe and Fe-Ge interactions, leads to a broadness of physical properties varying from metallic ferromagnets to metallic antiferromagnets with complex magnetic structures and ultimately to a non-magnetic insulator. This minireview surveys the crystal and electronic structures of binary compounds of the Fe–Ge system and relationships with their transport and magnetic properties.