Single Iron Site Research Articles

Molecular single iron site catalysts for electrochemical nitrogen fixation under ambient conditions

Electrochemical nitrogen reduction reaction (NRR) under ambient conditions is an attractive approach to synthesizing NH3, but remains a significant challenge due to insufficient NH3 yields and low Faraday efficiency (FE). Among studied NRR catalyst formulations, molecular catalysts with well-defined FeN4 configuration structures allow the establishment of a precise structural model for elucidating the complex multiple proton and electron transfer NRR processes competing with the undesirable hydrogen evolution reaction (HER). Inspired by biological nitrogenase, Fe sites can activate the N2 due to their strong interactions with N2. The unoccupied d orbital of Fe endows it the ideal electron acceptor and donor, which offers an attractive chemical property to facilitate NRR activity. Herein, we explore a molecular iron catalyst, i.e., tetraphenylporphyrin iron chloride (FeTPPCl) for the NRR. It exhibits promising NRR activity with the highest NH3 yield (18.28 ± 1.6 μg h−1 mg−1cat.) and FE (16.76 ± 0.9 %) at −0.3 V vs. RHE in neutral electrolytes. Importantly, 15N isotope labeling experiments confirm that the synthesized NH3 originates from the direct reduction of N2 in which 1H NMR spectroscopy and colorimetric methods were performed to quantify NH3 production. Also, operando electrochemical Raman spectroscopy studies confirm that the Fe–Cl bond breakage in the FeTPPCl catalyst is a prerequisite for initiating the NRR. Density functional theory (DFT) calculations further reveal that the active species is Fe porphyrin complex [Fe(TPP)]2− and the rate-determining step is the first hydrogenation of N2via the alternating mechanism on the [Fe0]2− sites. This work provides a new concept to use structurally defined molecular single iron catalysts to elucidate NRR mechanisms and design optimal active sites with enhanced reaction activity and selectivity for NH3 production under ambient conditions.

Nanocatalytic Medicine of Iron-Based Nanocatalysts

Iron can be found in all mammalian cells and is of critical significance to diverse cellular activities within human bodies. Widespread applications and the underlying chemical and biological funda...

Unravelling the Enigma of Nonoxidative Conversion of Methane on Iron Single‐Atom Catalysts

AbstractThe direct, nonoxidative conversion of methane on a silica‐confined single‐atom iron catalyst is a landmark discovery in catalysis, but the proposed gas‐phase reaction mechanism is still open to discussion. Here, we report a surface reaction mechanism by computational modeling and simulations. The activation of methane occurs at the single iron site, whereas the dissociated methyl disfavors desorption into gas phase under the reactive conditions. In contrast, the dissociated methyl prefers transferring to adjacent carbon sites of the active center (Fe1©SiC2), followed by C−C coupling and hydrogen transfer to produce the main product (ethylene) via a key −CH−CH2 intermediate. We find a quasi Mars–van Krevelen (quasi‐MvK) surface reaction mechanism involving extracting and refilling the surface carbon atoms for the nonoxidative conversion of methane on Fe1©SiO2 and this surface process is identified to be more plausible than the alternative gas‐phase reaction mechanism.

Unravelling the Enigma of Nonoxidative Conversion of Methane on Iron Single-Atom Catalysts.

The direct, nonoxidative conversion of methane on a silica-confined single-atom iron catalyst is a landmark discovery in catalysis, but the proposed gas-phase reaction mechanism is still open to discussion. Here, we report a surface reaction mechanism by computational modeling and simulations. The activation of methane occurs at the single iron site, whereas the dissociated methyl disfavors desorption into gas phase under the reactive conditions. In contrast, the dissociated methyl prefers transferring to adjacent carbon sites of the active center (Fe1 ©SiC2 ), followed by C-C coupling and hydrogen transfer to produce the main product (ethylene) via a key -CH-CH2 intermediate. We find a quasi Mars-van Krevelen (quasi-MvK) surface reaction mechanism involving extracting and refilling the surface carbon atoms for the nonoxidative conversion of methane on Fe1 ©SiO2 and this surface process is identified to be more plausible than the alternative gas-phase reaction mechanism.

Single-Iron Site Catalysts with Self-Assembled Dual-size Architecture and Hierarchical Porosity for Proton-Exchange Membrane Fuel Cells

Atomically dispersed and nitrogen coordinated single iron site (i.e., FeN4) catalysts (Fe-N-C) are the most promising platinum group metal (PGM)-free cathode for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, current Fe-N-C catalysts are limited by the inferior exposure of active FeN4 sites due to the inevitable agglomeration of particles in cathodes. Herein, we report a self-assembled strategy to synthesize the atomically dispersed FeN4 site catalysts with a hierarchically porous matrix derived from dual-size Fe-doped ZIF-8 crystal precursors by using large particles to support small particles. The tailored structure is effective in mitigating the particle migration, agglomeration, and spatial overlap, thereby exposing increased accessible active sites and facilitating mass transport. The best performing catalyst composed of 100 nm “nucleated seed” assembled by 30 nm “satellite” demonstrates exceptional ORR activity in acidic electrolyte and membrane electrode assembly. This work provides new concepts for designing hierarchically porous catalysts with single metal atom dispersion via self-assembly of ZIF-8 crystal precursors with tunable particle sizes and nanostructures.

Single Iron Site Nanozyme for Ultrasensitive Glucose Detection.

Nanomaterials with enzyme-mimicking characteristics have engaged great awareness in various fields owing to their comparative low cost, high stability, and large-scale preparation. However, the wide application of nanozymes is seriously restricted by the relatively low catalytic activity and poor specificity, primarily because of the inhomogeneous catalytic sites and unclear catalytic mechanisms. Herein, a support-sacrificed strategy is demonstrated to prepare a single iron site nanozyme (Fe SSN) dispersed on the porous N-doped carbon. With well-defined coordination structure and high density of active sites, the Fe SSN performs prominent peroxidase-like activity by efficiently activating H2 O2 into hydroxyl radical (•OH) species. Furthermore, the Fe SSN is applied in colorimetric detection of glucose through a multienzyme biocatalytic cascade platform. Moreover, a low-cost integrated agarose-based hydrogel colorimetric biosensor is designed and successfully achieves the visualization evaluation and quantitative detection of glucose. This work expands the application of single-site catalysts in the fields of nanozyme-based biosensors and personal biomedical diagnosis.

Room-Temperature Synthesis of Single Iron Site by Electrofiltration for Photoreduction of CO2 into Tunable Syngas

Developing a convenient and effective method to prepare single-atom catalysts at mild synthetic conditions remains a challenging task. Herein, a voltage-gauged electrofiltration method was demonstrated to synthesize single-atom site catalysts at room temperature. Under regulation of the graphene oxide membrane, a bulk Fe plate was directly converted into Fe single atoms, and the diffusion rate of Fe ions was greatly reduced, resulting in an ultralow concentration of Fe2+ around the working electrode, which successfully prevented the growing of nuclei and aggregating of metal atoms. Monatomic Fe atoms are homogeneously anchored on the as-prepared nitrogen-doped carbon. Owing to the fast photoelectron injection from photosensitizers to atomically dispersed Fe sites through the highly conductive supported N-C, the Fe-SAs/N-C exhibits an outstanding photocatalytic activity toward CO2 aqueous reduction into syngas with a tunable CO/H2 ratio under visible light irradiation. The gas evolution rates for CO and H2 are 4500 and 4950 μmol g-1 h-1, respectively, and the tunable CO/H2 ratio is from 0.3 to 8.8. This article presents an efficient strategy to develop the single-atom site catalysts and bridges the gap between heterogeneous and homogeneous catalysts toward photocatalytic CO2 aqueous reduction into syngas.

Encapsulation of Single Iron Sites in a Metal-Porphyrin Framework for High-Performance Photocatalytic CO2 Reduction.

Efficient CO2 reduction with earth-abundant photocatalysts is a highly attractive but very challenging process for chemists. Herein, we synthesized an indium-porphyrin framework, In(H2TCPP)(1-n)[Fe(TCPP)(H2O)](1-n)[DEA](1-n) (In-FenTCPP-MOF; H2TCPP = tetrakis(4-benzoic acid)porphyrine; DEA = diethylamine), with a porphyrin ring supporting the single-site iron for the high-performance visible-light-driven conversion of CO2 to CO. A high CO yield of 3469 μmol g-1 can be achieved by a 24 h photocatalytic reaction with a high CO selectivity (ca. 99.5%). This activity was much higher than that of its cobalt analogues or the Fe-free indium-based metal-organic framework (MOF). Systematic experimental and theoretical studies indicate that the porphyrin-supported iron centers in the MOF matrix serve as efficient active sites, which can accept electrons from the photoexcited MOFs in order to mediate CO2 reduction.

Polaronic structure of excess electrons and holes for a series of bulk iron oxides.

Iron oxides such as hematite (α-Fe2O3) play an important role in diverse fields ranging from biogeochemistry to photocatalysis. Here we perform calculations of both the electron and electron hole polaron structures and associated reorganisation energies for a series of bulk iron oxides: hematite (α-Fe2O3), lepidocrocite (γ-FeOOH), goethite (α-FeOOH) and white rust (Fe(OH)2). Through the use of gap-optimized hybrid functionals and large supercells under periodic boundary conditions, we remove some of the complications and uncertainties present in earlier cluster model calculations. It is found that while the electron hole polaron in these materials generally localises onto a single iron site, the electron polaron localises across two iron sites of the same spin layer as a consequence of the lower reorganisation energy for electrons compared to holes. An exception to these trends is the hole of goethite, which according to our calculations does not form a localised polaron.

Confined Pyrolysis within a Nanochannel to Form a Highly Efficient Single Iron Site Catalyst for Zn–Air Batteries

Structural engineering of atomic-scaled metal/N–C catalysts is crucial yet challenging in enhancing their performance for the oxygen reduction reaction (ORR). Herein, we demonstrate exclusive singl...

Nano confinement and catalytic conversion of energy molecules

Catalysis, as a key and enabling technology, plays an increasingly important role in fields ranging from energy, environment and agriculture to health care. Rational design and synthesis of highly efficient catalysts has become the ultimate goal of catalysis research. Thanks to the rapid development of nanoscience and nanotechnology, and in particular a theoretical understanding of the tuning of electronic structure in nanoscale systems, this element of design is becoming possible via precise control of nanoparticles’ composition, morphology, structure and electronic states. In the present paper, we will illustrate the results on the structural and electronic properties of catalytically active centers in the nano-confined systems, as well as their catalytic activities. Based upon the concept of “Nano-Confined Catalysis”, novel catalyst has been designed by embedding low-valent iron atoms within the lattice of silicon dioxide. The active single iron site could catalyze the direct and selective conversion of methane exclusively to ethylene and aromatics. The reaction is initiated by catalytic generation of methyl radicals, followed by a series of gas-phase reactions. The absence of adjacent iron sites prevents catalytic C−C coupling, further oligomerization and hence coke deposition. At 1090 °C, methane conversion reaches a maximum at 48.1% and ethylene selectivity 48.4%, while the total hydrocarbon selectivity exceeded 99%, representing an atom-economical transformation process of methane. The surface-confined iron sites delivered stable performance, with no deactivation observed during a 60-h test. This technology would allow the removal of the energy-intensive process of syngas production from the conventional natural gas utilization technologies. In addition, we recently established the concept of nano-composite catalysts comprising of metal oxides and zeolites, which effectively separates the active sites for CO and H2 activation from those for C−C coupling. CO and H2 are activated over the partially reduced metal oxides forming intermediates, which are subsequently converted to products within the confined pores of zeolites. As such, surface polymerization, a long-standing problem in the conventional Fischer-Tropsch synthesis invented more than 90 years ago, could be circumvented. This novel process for syngas conversion could achieve a high selectivity of light olefins (80%), surpassing the limit of 58% for C2−C4 light hydrocarbons by the ASF distribution. Furthermore such a bifunctional composite catalyst delivers stable performance and no obvious deactivation was observed within a over 3000 h stability test. These achievements open up new avenues for the developments of clean technologies for the efficient utilization of carbon resources, such as natural gas, coal and biomass, and are recognized as the “milestone in the direct synthesis of light olefins”. Now, his team is collaborating with companies to explore industrial applications, starting from a pilot-scale study.

Highly active and stable single iron site confined in graphene nanosheets for oxygen reduction reaction

Exploring high performance non-precious metal catalysts to substitute Pt for oxygen reduction reaction (ORR) has stimulated wide research interest recently, but it remains a great challenge. Herein, we report a single iron site confined in graphene catalyst via 4N atoms, forming flat FeN4 structure in the matrix of graphene. The optimized catalyst shows a high ORR activity, almost coming up to the activity of commercial 40% Pt/C catalyst, but a significantly higher stability and tolerance to SOx, NOx and methanol with respect to 40% Pt/C. This well-defined structure provides an ideal model to study the catalytic origin of iron-based catalysts. DFT calculations indicate that the high ORR activity origins from highly dispersed and high-density coordinatively unsaturated Fe centers, and the excellent stability origins from the unique confinement of the graphene matrix via 4N atoms. This reaction can proceed easily to H2O via a four electron transfer path way on the single iron site, which is further confirmed by the experiment. This experimental and theoretical study provides a further insight into the nature of the Fe/N/C catalyst and also introduces a reference for designing high efficient catalysts in electrocatalysis.

Magnetic properties of RE 10 TCd3 (RE = Ho, Er, Tm, Lu; T = Fe, Co, Ni, Ru) and 57Fe Mössbauer spectroscopic data of Y10FeCd3

Abstract Fourteen X-ray-pure intermetallic compounds RE 10 TCd3 (RE = Ho, Er, Tm, Lu; T = Fe, Co, Ni, Ru) and Y10FeCd3 were obtained through high-frequency melting of the elements in sealed niobium tubes and subsequent annealing in a muffle furnace. They adopt the Er10FeCd3 structure, a ternary ordered version of the Co2Al5 type. Temperature-dependent magnetic susceptibility measurements show Pauli paramagnetism for the lutetium compounds Lu10FeCd3, Lu10CoCd3, and Lu10RuCd3. The RE 10 TCd3 phases with holmium, erbium, and thulium show Curie–Weiss paramagnetism and the experimental magnetic moments match with the free ion values of RE 3+. All these compounds order antiferromagnetically. The highest Néel temperature was observed for the holmium compounds, e.g. 46.5 K for Ho10RuCd3. Some of the RE 10 TCd3 phases show field-induced spin reorientations. A 57Fe Mössbauer spectrum of Y10FeCd3 confirms the single crystallographic iron site.

A single iron site confined in a graphene matrix for the catalytic oxidation of benzene at room temperature.

Coordinatively unsaturated (CUS) iron sites are highly active in catalytic oxidation reactions; however, maintaining the CUS structure of iron during heterogeneous catalytic reactions is a great challenge. Here, we report a strategy to stabilize single-atom CUS iron sites by embedding highly dispersed FeN4 centers in the graphene matrix. The atomic structure of FeN4 centers in graphene was revealed for the first time by combining high-resolution transmission electron microscopy/high-angle annular dark-field scanning transmission electron microscopy with low-temperature scanning tunneling microscopy. These confined single-atom iron sites exhibit high performance in the direct catalytic oxidation of benzene to phenol at room temperature, with a conversion of 23.4% and a yield of 18.7%, and can even proceed efficiently at 0°C with a phenol yield of 8.3% after 24 hours. Both experimental measurements and density functional theory calculations indicate that the formation of the Fe═O intermediate structure is a key step to promoting the conversion of benzene to phenol. These findings could pave the way toward highly efficient nonprecious catalysts for low-temperature oxidation reactions in heterogeneous catalysis and electrocatalysis.

Theoretical study of neutral and charged Fe7-(C6H6)m, m = 1, 2 rice-ball clusters.

Bonding of benzene molecules on the surface of neutral and charged Fe7 clusters, which have pentagonal bipyramids (PBP), was studied by means of all-electrons density functional calculations. Dispersion corrections were done with the BPW91-D2 method using the 6-311++G(2d,2p) basis sets. With two less coordinated equatorial sites (bonded to four iron atoms) and one axial site (bonded to five atoms), a triangular face of Fe7 emerges as the basic unit for the absorption of benzene moieties. Bonding of benzene (Bz) on such triangle yields the ground state (GS) for Fe7Bz, Fe7Bz(-), and Fe7Bz(+). Without dispersion, in the GS of Fe7Bz(-), the ligand is η(6) coordinated with a single equatorial iron site, and in the GSs of Fe7Bz2 and Fe7Bz2(-), each benzene moiety is η(6) bonded on opposite equatorial sites. However, BPW91-D2 yields GS structures for Fe7Bz2, Fe7Bz2(-), and Fe7Bz2(+), where the absorption is done on opposite triangles. Therefore, dispersion corrections are crucial for a proper study of Fe7Bz2. The multiplicities (M = 2S + 1, where S is the total spin) of these species, 17, 16, and 18, respectively, are smaller than those of Fe7(23), Fe7(-)(22) and Fe7(+)(24) showing important quenching of the magnetic moment of Fe7. Bond dissociation energies (BDE), in kcal/mol, for Fe7Bz (32.7), Fe7Bz(+) (47.3), and Fe7Bz(-) (27.2) show bigger (smaller) values for the cation (anion). A similar picture was found for the BDEs of Fe7Bz2. Ionization energies, 5.37 and 4.94 eV, for m = 1 and 2 are smaller than that of Fe7, 6.00 eV; which is due to delocalization of the electrons through the network of 3d-π bonds. Electron affinities of Fe7Bz1,2 are also smaller than that of Fe7, being mainly due to the increased repulsion.

Single iron sites for catalytic, nonoxidative conversion of methane

Single iron sites for catalytic, nonoxidative conversion of methane

Catalytic Reduction of Hydrazine to Ammonia by a Mononuclear Iron(II) Complex on a Tris(thiolato)phosphine Platform

To provide the mechanistic information of nitrogenase at a molecular level, much effort has been made to develop synthetic metal complexes that have enzyme-like reactivity. Herein we obtain an iron(II) complex binding with a tris(thiolato)phosphine ligand, [P(Ph)4][Fe(PS3″)(CH3CN)] [1; PS3″ = P(C6H3-3-Me3Si-2-S)3(3-)] that catalyzes the reduction of hydrazine, an intermediate and a substrate of nitrogenase. The substrate- and product-bound adducts, [N(Bu)4][Fe(PS3″)(N2H4)] (2) and [N(Et)4][Fe(PS3″)(NH3)] (3), respectively, are also synthesized. This work provides the feasibility that the late stage of biological nitrogen fixation can be conducted at a single iron site with a sulfur-rich ligation environment.

Catalytic conversion of nitrogen to ammonia by an iron model complex

The reduction of N2 to NH3 is a requisite transformation for life.1 While it is widely appreciated that the iron-rich cofactors of nitrogenase enzymes facilitate this transformation,2-5 how they do so remains poorly understood. A central element of debate has been the exact site(s) of nitrogen coordination and reduction.6,7 The synthetic inorganic community placed an early emphasis on Mo8, because Mo was thought to be an essential element of nitrogenases3 and because pioneering work by Chatt and coworkers established that well-defined Mo model complexes could mediate the stoichiometric conversion of N2 to NH3.9 This chemical transformation can be performed in a catalytic fashion by two well-defined molecular systems that feature Mo centres.10,11 However, it is now thought that Fe is the only transition metal essential to all nitrogenases,3 and recent biochemical and spectroscopic data has implicated Fe instead of Mo as the site of N2 binding in the FeMo-cofactor.12 In this work, we describe a tris(phosphine)borane-supported Fe complex that catalyzes the reduction of N2 to NH3 under mild conditions, wherein >40% of the H+/e- equivalents are delivered to N2. Our results indicate that a single Fe site may be capable of stabilizing the various NxHy intermediates generated en route to catalytic NH3 formation. Geometric tunability at Fe imparted by a flexible Fe-B interaction in our model system appears to be important for efficient catalysis.13-15 We propose that the interstitial light C-atom recently assigned in the nitrogenase cofactor may play a similar role,16,17 perhaps by enabling a single Fe site to mediate the enzymatic catalysis via a flexible Fe-C interaction.18

Ligand-Bound S = 1/2 FeMo-Cofactor of Nitrogenase: Hyperfine Interaction Analysis and Implication for the Central Ligand X Identity

Broken symmetry density functional theory (BS-DFT) has been used to address the hyperfine parameters of the single atom ligand X, proposed to be coordinated by six iron ions in the center of the paramagnetic FeMo-cofactor (FeMoco) of nitrogenase. Using the X = N alternative, we recently found that any hyperfine signal from X would be small (calculated A(iso)(X = (14)N) = 0.3 MHz) due to both structural and electronic symmetry properties of the [Mo-7Fe-9S- X] FeMoco core in its resting S = 3/2 state. Here, we extend our BS-DFT approach to the 2e(-) reduced S = 1/2 FeMoco state. Alternative substrates coordinated to this FeMoco state effectively perturb the electronic and/or structural symmetry properties of the cofactor. Using an example of an allyl alcohol (H2C=CH-CH2-OH) product ligand, we consider three different binding modes at single iron site and three different BS-DFT spin state structures and show that this binding would enhance the key hyperfine signal A(iso)(X) by at least 1 order of magnitude (3.8 < or = A(iso)(X = (14)N) < or = 14.7 MHz), and this result should not depend strongly on the exact identity of X (nitrogen, carbon, or oxygen). The interstitial atom, when the nucleus has a nonzero magnetic moment, should therefore be observable by ESR methods for some ligand-bound FeMoco states. In addition, our results illustrate structural details and likely spin-coupling patterns for models for early intermediates in the catalytic cycle.

A Two-Dimensional Iron(II) Carboxylate Linear Chain Polymer that Exhibits a Metamagnetic Spin-Canted Antiferromagnetic to Single-Chain Magnetic Transition

A two-dimensional iron(II) carboxylate coordination polymer, [Fe(pyoa)2]infinity, where pyoa is 2-(pyridin-3-yloxy)acetate, has been prepared by hydrothermal synthesis. Its crystal structure reveals a single iron(II) site with an elongated octahedral coordination environment containing four equatorial carboxylate oxygens and two axial pyridyl nitrogens; the iron(II) sites are linked by syn-anti micro-carboxylates to form chains along the b axis that have an Fe...Fe separation of 4.910 A. The shortest interchain and interlayer Fe...Fe distances are 6.453 and 11.125 A, respectively. The 4.2-295 K Mössbauer spectra of [Fe(pyoa) 2] infinity consist of a single paramagnetic high-spin iron(II) quadrupole doublet. The axial Fe-N bond direction defines the Jahn-Teller axis at an iron(II) site and, consequently, the orientation of the single-ion magnetic anisotropy. Thus, along the b axis in a given chain, the spins are collinear and parallel to the Jahn-Teller axis. The Jahn-Teller axes of adjacent intralayer chains have different orientations with an angle of 79.2 degrees between the axes in adjacent chains in a bc layer. [Fe(pyoa)2]infinity exhibits field-induced metamagnetic behavior such that, in an applied field smaller than the critical field, the iron(II) spin-canted moments experience intrachain ferromagnetic interactions and weak interchain antiferromagnetic interactions; the spin canting yields weak ferromagnetism. In an applied field larger than the critical field, the weak antiferromagnetic interchain interactions are overwhelmed to yield superparamagnetic-like slow-magnetic relaxation with an energy barrier of 23(3) K. Single-crystal magnetic studies reveal a quasi-uniaxial magnetic anisotropy with the a axis as the easy-magnetic axis and the b axis as the hard-magnetic axis; the susceptibility measured along the easy a axis may be fit with the Glauber model to yield an effective intrachain exchange coupling constant of 2.06(8) K. A dynamic analysis of the susceptibility yields a 6.3(1) K energy barrier for intrachain domain wall creation. The observed field-assisted superparamagnet-like behavior is consistent with the dynamics of a single-chain magnet. Thus, [Fe(pyoa)2]infinity is best considered as a "metamagnetic-like" single-chain magnet.