Structural and Mechanistic Insights into CO2 Activation by Nitrogenase Iron Protein

The Fe protein of nitrogenase catalyzes the ambient reduction of CO₂ when its cluster is present in the all-ferrous, [Fe₄S₄]⁰ oxidation state. Here, we report a combined structural and theoretical study that probes the unique reactivity of the all-ferrous Fe protein toward CO₂. Structural comparisons of the Azotobacter vinelandii Fe protein in the [Fe₄S₄]⁰ and [Fe₄S₄]⁺ states point to a possible asymmetric functionality of a highly conserved Arg pair in CO₂ binding and reduction. Density functional theory (DFT) calculations provide further support for the asymmetric coordination of O by the “proximal” Arg and binding of C to a unique Fe atom of the all-ferrous cluster, followed by donation of protons by the proximate guanidinium group of Arg that eventually results in the scission of a C−O bond. These results provide important mechanistic and structural insights into CO₂ activation by a surface-exposed, scaffold-held [Fe₄S₄] cluster.     

Dual-Atom Catalysts Derived from a Preorganized Covalent Organic Framework for Enhanced Electrochemical Oxygen Reduction

Dual-atom catalysts (DAC) are deemed as promising electrocatalysts due to the abundant active sites and adjustable electronic structure, but the fabrication of well-defined DAC is still full of challenges. Herein, bonded Fe dual-atom catalysts (Fe₂DAC) with Fe₂N₆C₈O₂ configuration were developed through one-step carbonization of a preorganized covalent organic framework with bimetallic Fe chelation sites (Fe₂COF). The transition from Fe₂COF to Fe₂DAC involved the dissociation of the nanoparticles and the capture of atoms by carbon defects. Benefitting from the optimized d-band center and enhanced adsorption of OOH* intermediates, Fe₂DAC exhibited outstanding oxygen reduction activity with a half-wave potential of 0.898 V vs. RHE. This work will guide more fabrication of dual-atom and even cluster catalysts from preorganized COF in the future.     

Modulating Electronic Structures of Iron Clusters through Orbital Rehybridization by Adjacent Single Copper Sites for Efficient Oxygen Reduction

The atom-cluster interaction has recently been exploited as an effective way to increase the performance of metal-nitrogen-carbon catalysts for oxygen reduction reaction (ORR). However, the rational design of such catalysts and understanding their structure-property correlations remain a great challenge. Herein, we demonstrate that the introduction of adjacent metal (M)−N₄ single atoms (SAs) could significantly improve the ORR performance of a well-screened Fe atomic cluster (AC) catalyst by combining density functional theory (DFT) calculations and experimental analysis. The DFT studies suggest that the Cu−N₄ SAs act as a modulator to assist the O₂ adsorption and cleavage of O−O bond on the Fe AC active center, as well as optimize the release of OH* intermediates to accelerate the whole ORR kinetic. The depositing of Fe AC with Cu−N₄ SAs on nitrogen doped mesoporous carbon nanosheet are then constructed through a universal interfacial monomicelles assembly strategy. Consistent with theoretical predictions, the resultant catalyst exhibits an outstanding ORR performance with a half-wave potential of 0.92 eV in alkali and 0.80 eV in acid, as well as a high power density of 214.8 mW cm⁻² in zinc air battery. This work provides a novel strategy for precisely tuning the atomically dispersed poly-metallic centers for electrocatalysis.     

Rational Synthesis of Iron/Nitrogen-Doped Carbon Catalyst through a Spatial Isolation Strategy for Efficient Oxygen Reduction in Acidic and Alkaline Media

It remains challenging to rationally synthesize iron/nitrogen-doped carbon (Fe/N-C) catalysts with rich Fe−N_x atomic active sites for improved oxygen reduction reaction (ORR) electrocatalysis. A highly efficient Fe/N-C catalyst, which has been synthesized through a spatial isolation strategy, is reported. Derived from bioinspired polydopamine (PDA)-based hybrid microsphere precursors, it is a multifunctional carrier that loads atomically dispersed Fe³⁺/Zn²⁺ ions through coordination interactions and N-rich melamine through electrostatic attraction and covalent bonding. The Zn²⁺ ions and melamine in the precursor efficiently isolate Fe³⁺ atoms upon pyrolysis to form rich Fe−N_x atomic active sites, and generate abundant micropores during high-temperature treatment; as a consequence, the resultant Fe-N/C catalyst contains rich catalytically active Fe−N_x sites and a hierarchical porous structure. The catalyst exhibits improved ORR activity that is superior to and close to that of Pt/C in alkaline and acidic solutions, respectively.     

Efficient Oxygen Reduction Reaction (ORR) Catalysts Based on Single Iron Atoms Dispersed on a Hierarchically Structured Porous Carbon Framework

Single Fe atoms dispersed on hierarchically structured porous carbon (SA‐Fe‐HPC) frameworks are prepared by pyrolysis of unsubstituted phthalocyanine/iron phthalocyanine complexes confined within micropores of the porous carbon support. The single‐atom Fe catalysts have a well‐defined atomic dispersion of Fe atoms coordinated by N ligands on the 3D hierarchically porous carbon support. These SA‐Fe‐HPC catalysts are comparable to the commercial Pt/C electrode even in acidic electrolytes for oxygen reduction reaction (ORR) in terms of the ORR activity (E_1/2=0.81 V), but have better long‐term electrochemical stability (7 mV negative shift after 3000 potential cycles) and fuel selectivity. In alkaline media, the SA‐Fe‐HPC catalysts outperform the commercial Pt/C electrode in ORR activity (E_1/2=0.89 V), fuel selectivity, and long‐term stability (1 mV negative shift after 3000 potential cycles). Thus, these nSA‐Fe‐HPCs are promising non‐platinum‐group metal ORR catalysts for fuel‐cell technologies.     

Constructing Fe-N4 Sites through Anion Exchange-mediated Transformation of Fe Coordination Environments in Hierarchical Carbon Support for Efficient Oxygen Reduction

Metal single atoms (SAs) anchored in carbon support via coordinating with N atoms are efficient active sites to oxygen reduction reaction (ORR). However, rational design of single atom catalysts with highly exposed active sites is challenging and urgently desirable. Herein, an anion exchange strategy is presented to fabricate Fe-N₄ moieties anchored in hierarchical carbon nanoplates composed of hollow carbon spheres (Fe-SA/N-HCS). With the coordinating O atoms are substituted by N atoms, Fe SAs with Fe-O₄ configuration are transformed into the ones with Fe-N₄ configuration during the thermal activation process. Insights into the evolution of central atoms demonstrate that the SAs with specific coordination environment can be obtained by modulating in situ anion exchange process. The strategy produces a large quantity of electrochemical accessible site and high utilization rate of Fe-N₄. Fe-SA/N-HCS shows excellent ORR electrocatalytic performance with half-wave potential of 0.91 V (vs. RHE) in 0.1 M KOH, and outstanding performance when used in rechargeable aqueous and flexible Zn-air batteries. The evolution pathway for SAs demonstrated in this work offers a novel strategy to design SACs with various coordination environment and enhanced electrocatalytic activity.     

Anchoring Fe Species on the Highly Curved Surface of S and N Co-Doped Carbonaceous Nanosprings for Oxygen Electrocatalysis and a Flexible Zinc-Air Battery

Oxygen reduction reaction (ORR) is of critical significance in the advancement of fuel cells and zinc-air batteries. The iron-nitrogen (Fe−N_x) sites exhibited exceptional reactivity towards ORR. However, the task of designing and controlling the local structure of Fe species for high ORR activity and stability remains a challenge. Herein, we have achieved successful immobilization of Fe species onto the highly curved surface of S, N co-doped carbonaceous nanosprings (denoted as FeNS/Fe₃C@CNS). The induction of this twisted configuration within FeNS/Fe₃C@CNS arose from the assembly of chiral templates. For electrocatalytic ORR tests, FeNS/Fe₃C@CNS exhibits a half-wave potential (E_1/2) of 0.91 V in alkaline medium and a E_1/2 of 0.78 V in acidic medium. The Fe single atoms and Fe₃C nanoparticles are coexistent and play as active centers within FeNS/Fe₃C@CNS. The highly curved surface, coupled with S substitution in the coordination layer, served to reduce the energy barrier for ORR, thereby enhancing the intrinsic catalytic activity of the Fe single-atom sites. We also assembled a wearable flexible Zn-air battery using FeNS/Fe₃C@CNS as electrocatalysts. This work provides new insights into the construction of highly curved surfaces within carbon materials, offering high electrocatalytic efficacy and remarkable performance for flexible energy conversion devices.     

Synthesis and X-ray and Spectral Study of the Compounds [Q4N]2[Fe2(S2O3)2(NO)4] (Q = Me,Et, n-Pr,n-Bu)

Iron nitrosyl complexes with general formula [Q₄N]₂[Fe₂(S₂O₃)₂(NO)₄] (Q = Me, Et, n-Pr, n-Bu) were synthesized by the exchange reaction of K₂[Fe₂(S₂O₃)₂(NO)₄] with tetraalkylammonium bromides. The molecular and crystal structure of [(CH₃)₄N]₂[Fe₂(S₂O₃)₂(NO)₄] were studied by X-ray diffraction analysis. The iron atom in the four-membered cycle of the [2Fe–2S] anion is bound to another Fe atom and to two sulfur atoms and is coordinated by two nonequivalent NO groups, each bridging sulfur atom being bound to the SO₃group. The structurally equivalent iron atoms are in the state Fe^1–(S= 1/2). The Mössbauer spectroscopy method shows that the complexes are diamagnetic due to the strong Fe–Fe bond. It is found that the SO₃group provides higher stability of the thiosulfate anion than the anion in Roussin's red salt [Fe₂S₂(NO)₄]^2–.     

Syntheses,Structures and Magnetic Properties of Two Mixed‐Valent Disc‐like Hepta‐nuclear Compounds of [FeIIFeIII6(tea)6](ClO4)2 and [MnII3MnIII4(nmdea)6(N3)6]·CH3OH (tea = N(CH2CH2O)33−, nmdea = CH3N(CH2CH2O)22−)

Two mixed‐valent disc‐like hepta‐nuclear compounds of [Fe^IIFe^III₆(tea)₆](ClO₄)₂ ( 1Fe , tea = N(CH₂CH₂O)₃^3?) and [Mn^II₃Mn^III₄(nmdea)₆(N₃)₆]·CH₃OH ( 2Mn , nmdea = CH₃N(CH₂CH₂O)₂^2?) have been synthesized by the reaction of Fe(ClO₄)₂·6H₂O with triethanolamine (H₃tea) for the former and reaction of Mn(ClO₄)₂·6H₂O with diethanolamine (H₂nmdea) and NaN₃ for the later, respectively. 1Fe has the cationic cluster with a planar [Fe^IIFe^III₆] core consisting of one central Fe^II and six rim Fe^III atoms in hexagonal arrangement. The Fe ions are linked by the oxo‐bridges from the alcohol arms in the manner of edge‐sharing of their coordination octahedra. 2Mn is a neutral cluster with a [Mn^II₃Mn^III₄] core possessing one central Mn^II atom surrounded by six rim Mn ions, two Mn^II and four Mn^III. The structure is similar to 1Fe but involves six terminal azido ligands, each coordinate one rim Mn ion. 1Fe showed dominant antiferromagnetic interaction within the cluster and long‐range ordering at 2.7 K. The cluster probably has a ground state of low spin of S = 5/2 or 4/2. The long‐range ordering is weak ferromagnetic, showing small hysteresis with a remnant magnetization of 0.3 Nβ and a coercive field of 40 Oe. Moreover, the isofield of lines 1Fe are far from superposition, indicating the presence of significant zero–field splitting. Ferromagnetic interactions are dominant in 2Mn . An intermediate spin ground state 25/2 is observed at low field. In high field of 50 kOe, the energetically lowest state is given by the m_s = 31/2 component of the S = 31/2 multiplet due to the Zeeman effect. Despite of the large ground state, no single‐molecule magnet behavior was found above 2 K.     

Construction of Co4 Atomic Clusters to Enable Fe−N4 Motifs with Highly Active and Durable Oxygen Reduction Performance

Fe−N−C catalysts with single-atom Fe−N₄ configurations are highly needed owing to the high activity for oxygen reduction reaction (ORR). However, the limited intrinsic activity and dissatisfactory durability have significantly restrained the practical application of proton-exchange membrane fuel cells (PEMFCs). Here, we demonstrate that constructing adjacent metal atomic clusters (ACs) is effective in boosting the ORR performance and stability of Fe−N₄ catalysts. The integration of Fe−N₄ configurations with highly uniform Co₄ ACs on the N-doped carbon substrate (Co₄@/Fe₁@NC) is realized through a “pre-constrained” strategy using Co₄ molecular clusters and Fe(acac)₃ implanted carbon precursors. The as-developed Co₄@/Fe₁@NC catalyst exhibits excellent ORR activity with a half-wave potential (E_1/2) of 0.835 V vs. RHE in acidic media and a high peak power density of 840 mW cm⁻² in a H₂−O₂ fuel cell test. First-principles calculations further clarify the ORR catalytic mechanism on the identified Fe−N₄ that modified with Co₄ ACs. This work provides a viable strategy for precisely establishing atomically dispersed polymetallic centers catalysts for efficient energy-related catalysis.     

Spectroscopic Characterization of an Eight‐Iron Nitrogenase Cofactor Precursor that Lacks the “9th Sulfur”

Nitrogenases catalyze the reduction of N₂ to NH₄⁺ at its cofactor site. Designated the M‐cluster, this [MoFe₇S₉C(R‐homocitrate)] cofactor is synthesized via the transformation of a [Fe₄S₄] cluster pair into an [Fe₈S₉C] precursor (designated the L‐cluster) prior to insertion of Mo and homocitrate. We report the characterization of an eight‐iron cofactor precursor (designated the L*‐cluster), which is proposed to have the composition [Fe₈S₈C] and lack the “9^th sulfur” in the belt region of the L‐cluster. Our X‐ray absorption and electron spin echo envelope modulation (ESEEM) analyses strongly suggest that the L*‐cluster represents a structural homologue to the l ‐cluster except for the missing belt sulfur. The absence of a belt sulfur from the L*‐cluster may prove beneficial for labeling the catalytically important belt region, which could in turn facilitate investigations into the reaction mechanism of nitrogenases.     

Modulating Coordination of Iron Atom Clusters on N,P,S Triply-Doped Hollow Carbon Support towards Enhanced Electrocatalytic Oxygen Reduction

Constructing atom-clusters (ACs) with in situ modulation of coordination environment and simultaneously hollowing carbon support are critical yet challenging for improving electrocatalytic efficiency of atomically dispersed catalysts (ADCs). Herein, a general diffusion-controlled strategy based on spatial confining and Kirkendall effect is proposed to construct metallic ACs in N,P,S triply-doped hollow carbon matrix (M_ACs/NPS−HC, M=Mn, Fe, Co, Ni, Cu). Thereinto, Fe_ACs/NPS−HC with the best catalytic activity for oxygen reduction reaction (ORR) is thoroughly investigated. Unlike the benchmark sample of symmetrical N-surrounded iron single-atoms in N-doped carbon (Fe_SAs/N−C), Fe_ACs/NPS−HC comprises bi-/tri-atomic Fe centers with engineered S/N coordination. Theoretical calculation reveals that proper Fe gathering and coordination modulation could mildly delocalize the electron distribution and optimize the free energy pathways of ORR. In addition, the triple doping and hollow structure of carbon matrix could further regulate the local environment and allow sufficient exposure of active sites, resulting in more enhanced ORR kinetics on Fe_ACs/NPS−HC. The zinc-air battery assembled with Fe_ACs/NPS−HC as cathodic catalyst exhibits all-round superiority to Pt/C and most Fe-based ADCs. This work provides an exemplary method for establishing atomic-cluster catalysts with engineered S-dominated coordination and hollowed carbon matrix, which paves a new avenue for the fabrication and optimization of advanced ADCs.     

A Strategy to Promote the Electrocatalytic Activity of Spinels for Oxygen Reduction by Structure Reversal

The electrocatalytic performance of a spinel for the oxygen reduction reaction (ORR) can be significantly promoted by reversing its crystalline structure from the normal to the inverse. As the spinel structure reversed, the activation and cleavage of O?O bonds are accelerated owing to a dissimilarity effect of the distinct metal atoms co‐occupying octahedral sites. The Co^IIFe^IIICo^IIIO₄ spinel with the Fe and Co co‐occupying inverse structure exhibits an excellent ORR activity, which even exceeds that of the state‐of‐the‐art commercial Pt/C by 42 mV in alkaline medium.     

Mechanism of Framework Oxygen Exchange in Fe‐Zeolites: A Combined DFT and Mass Spectrometry Study

The role of framework oxygen atoms in N₂O decomposition [N₂O(g)→N₂(g) and 1/2O₂(g)] over Fe‐ferrierite is investigated employing a combined experimental (N₂¹⁸O decomposition in batch experiments followed by mass spectroscopy measurements) and theoretical (density functional theory calculations) approach. The occurrence of the isotope exchange indicates that framework oxygen atoms are involved in the N₂O decomposition catalyzed by Fe‐ferrierite. Our study, using an Fe‐ferrierite sample with iron exclusively present as Fe^II cations accommodated in the cationic sites, shows that the mobility of framework oxygen atoms in the temperature range: 553 to 593 K is limited to the four framework oxygen atoms of the two AlO₄^? tetrahedra forming cationic sites that accomodate Fe^II. They exchange with the Fe extra‐framework ¹⁸O atom originating from the decomposed N₂¹⁸O. We found, using DFT calculations, that O₂ molecules facilitate the oxygen exchange. However, the corresponding calculated energy barrier of 87 kcal mol^?1 is still very high and it is higher than the assumed experimental value based on the occurrence of the sluggish oxygen exchange at 553 K.     

Electrocatalytically Active Fe‐(O‐C2)4 Single‐Atom Sites for Efficient Reduction of Nitrogen to Ammonia

Single‐atom catalysts have demonstrated their superiority over other types of catalysts for various reactions. However, the reported nitrogen reduction reaction single‐atom electrocatalysts for the nitrogen reduction reaction exclusively utilize metal–nitrogen or metal–carbon coordination configurations as catalytic active sites. Here, we report a Fe single‐atom electrocatalyst supported on low‐cost, nitrogen‐free lignocellulose‐derived carbon. The extended X‐ray absorption fine structure spectra confirm that Fe atoms are anchored to the support via the Fe‐(O‐C₂)₄ coordination configuration. Density functional theory calculations identify Fe‐(O‐C₂)₄ as the active site for the nitrogen reduction reaction. An electrode consisting of the electrocatalyst loaded on carbon cloth can afford a NH₃ yield rate and faradaic efficiency of 32.1 μg h^?1 mg_cat.^?1 (5350 μg h^?1 mg_Fe^?1) and 29.3 %, respectively. An exceptional NH₃ yield rate of 307.7 μg h^?1 mg_cat.^?1 (51 283 μg h^?1 mg_Fe^?1) with a near record faradaic efficiency of 51.0 % can be achieved with the electrocatalyst immobilized on a glassy carbon electrode.     

Fe/Fe3C Nanoparticles Encapsulated in N-Doped Hollow Carbon Spheres as Efficient Electrocatalysts for the Oxygen Reduction Reaction over a Wide pH Range

Nonprecious-metal-based electrocatalysts with low cost, high activity, and stability are considered as one of the most promising alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR). Herein, an economical and easy-to-fabricate catalyst is developed, that is, Fe/Fe₃C embedded in N-doped hollow carbon spheres (Fe/Fe₃C/NHCS), which gave the half-wave potential of 0.84 V in 0.1 m KOH, similar to the commercial Pt/C catalyst. Surprisingly, the favorable ORR performance of the as-prepared catalyst was obtained in both acidic and neutral conditions with almost a four-electron pathway and low H₂O₂ yield, which desirable the development of the proton exchange membrane (PEM) and microbial electrolysis cell (MEC) technology. Additionally, the obtained catalyst demonstrated better long-term stability and high methanol tolerance over a wide range of pH. These features could be mainly attributed to the synergistic effect between Fe/Fe₃C and Fe-N_x sites, the hollow structure with mesopores, and the well-dispersed Fe/Fe₃C nanoparticles owing to the existence of the abundant hydrophilic groups within the HCS precursor. As such, designing an efficient and cheap ORR catalyst that can operate at alkaline, acidic, and neutral solutions is highly desirable, yet challenging.     

Pt掺杂氧化石墨烯在酸性介质中催化氧还原反应DFT研究

目前Pt基催化剂被公认为是最高效的氧还原催化剂.我们采用了密度泛函理论研究了Pt掺杂5种不同氧化石墨烯和完美石墨烯在酸性环境中的氧还原反应机理,计算了氧还原反应中间体O₂、O、OOH、OH、H₂O和H₂O₂在不同掺杂石墨烯上的吸附性能、反应步骤与反应相对能量变化.结果表明,氧化石墨烯在O₂的活化、中间体吸附、掺杂难度（缺陷形成能）、能带带隙以及在反应中相对能量的降低都优于完美石墨烯,我们的工作将有助于为将来在实验中选择和合成氧还原催化剂提供一定的理论指导意义.     

Highly efficient iron–nitrogen electrocatalyst derived from covalent organic polymer for oxygen reduction

Developing non-precious metal catalyst with high activity, good stability and low cost for electrocatalytic oxygen reduction reaction(ORR) is critical for the wide application of energy conversion system. Here, we developed a cost–effective synthetic strategy via silica assistance to obtain a novel Fe_3C/Fe–N_x–C(named as COPBP-PB-Fe-900-SiO_2) catalyst with effective active sites of Fe–N_xand Fe_3C from the rational design two-dimensional covalent organic polymer(COPBP-PB). The nitrogen-rich COP effectively promotes the formation of active Fe–N_x sites. Additionally, the silica not only can effectively suppress the formation of large Fe-based particles in the catalysts, but also increases the degree of carbonization of the catalyst.The as-prepared COPBP-PB-Fe-900-SiO_2 catalyst exhibits high electrocatalytic activity for ORR with a halfwave potential of 0.85 V vs. reversible hydrogen electrode(RHE), showing comparable activity as compared with the commercial Pt/C catalysts in alkaline media. Moreover, this catalyst also shows a high stability with a nearly constant onset potential and half-wave potential after 10,000 cycles. The present work is highly meaningful for developing ORR electrocatalysts toward wide applications.     

Theoretical designs of ORR/OER single-atom catalysts TM@Ti2CT2 (T = O,S, Cl)

Driven by the goal of establishing a fossil-fuel-free and nuclear-power-free economy based on renewable energy, metal-air batteries are regarded as promising energy conversion and storage devices. Developing efficient oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) bifunctional electrocatalysts for the air electrode of metal-air batteries is becoming increasingly important. In this work, 36 transition metal (TM) single-atom catalysts are designed based on MXenes Ti₂CT₂ with different surface terminal atoms (T = O, S, Cl), and their ORR/OER catalytic activity and stability are evaluated by the density functional theory. Ni@Ti₂CO₂, Pd@Ti₂CS₂, and Co@Ti₂CCl₂ are found to exhibit good catalytic activity with ORR/OER overpotentials of .54 V/.62 V, .59 V/.29 V, .44 V/.40 V. The aggregation behavior of three catalysts is estimated by comparing the average binding energy of one, two, three, and four TM atoms anchored on Ti₂CT₂. This work cannot only provide a theoretical guide to develop bifunctional single-atom catalysts, but also help us understand the effect of terminal atoms on the electronic structures and catalytic activity of TM@Ti₂CT₂.     

A DFT Study on the Activity Origin of Fe−N−C Sites for Oxygen Reduction Reaction

Iron-nitrogen-carbon materials have been known as the most promising non-noble metal catalyst for proton-exchange membrane fuel cells (PEMFCs), but the genuine active sites for oxygen reduction reaction (ORR) are still arguable. Herein, by the thorough density functional theory investigations, we unravel that the planar Fe₂N₆ site exhibits excellent ORR catalytic activity over both FeN₃ and FeN₄ sites, and the potential-determining step is determined to be the *OH hydrogenation step with an overpotential of 0.415 V. The ORR activity of Fe₂N₆ site originates from the low spin magnetic moment (1.11 μ_B), which leads to high antibonding states and low d-band center of the Fe center, further leads to weak binding strength of *OH species. The density of FeN₄ sites only has little influence on the ORR activity owing to the similar interaction between active site and intermediates in ORR. Our research sheds light on the activity origin of iron-nitrogen-carbon materials for ORR.