N2 Binding Research Articles

Considering Electrocatalytic Ammonia Synthesis via Bimetallic Dinitrogen Cleavage

Despite advances in the development of molecular catalysts capable of reducing dinitrogen to ammonia using proton donors and chemical reductants, few molecular electrocatalysts have been discovered. This Perspective considers the prospects of electrocatalyst development based on a mechanism featuring the cleavage of N2 into metal nitride complexes. By understanding the factors that control the reactivity of individual steps along the electrochemical N2 cleavage path, opportunities for new advances are identified. Ligand design principles for facile electrochemical N2 binding, formation of bridging N2 complexes, thermal or photochemical N2 cleavage, and conversion of a nitride ligand into ammonia are described, featuring recent advances and the authors’ collaborative work on rhenium complexes.

Delicate dance becomes a ballet

Metalloenzymes The enzyme nitrogenase uses adenosine triphosphate and several unusual iron-sulfur cofactors to pump electrons into typically inert dinitrogen (N2), providing protons along the way. Previous work has shown that sulfur atoms in the iron-molybdenum cofactor (FeMoCo) are labile and suggests that replacement of one of the sulfurs by N2 is integral to the mechanism of N2 binding and reduction. Through the elimination of excess reducing agent during preparation, Kang et al. determined structures of Mo-nitrogenase in a resting conformation. Unexpectedly, they found that all three sulfurs at the outer edge of FeMoCo appear to be labile, with one subunit even having two of three sulfurs replaced by light, diatomic ligands. Biochemical and spectroscopic data indicate that the protein is active, holds tightly bound N2, and is in the expected oxidation state. These results may prompt a reassessment of the possible mechanisms of N2 reduction and the role of dynamic belt ligands in FeMoCo. Science , this issue p. [1381][1] [1]: /lookup/doi/10.1126/science.aaz6748

Highly Efficient Electrochemical Reduction of Nitrogen to Ammonia on Surface Termination Modified Ti3C2Tx MXene Nanosheets.

MXene-based catalysts exhibit extraordinary advantages for many catalysis reactions, such as the hydrogen evolution and oxygen reduction reactions. However, MXenes exhibit inadequate catalytic activity for the electrochemical nitrogen reduction reaction (NRR) because they are typically terminated with inactive functional groups, F* and OH*, which mask the active metal sites for N2 binding. Here we modified the surface termination of MXene (Ti3C2Tx) nanosheets to achieve high surface catalytic reactivity for the NRR by ironing out inactive F*/OH* terminals to expose more active sites and by introducing Fe to greatly reduce the surface work function. The optimally performing catalyst (MXene/TiFeOx-700) achieved excellent Faradaic efficiency of 25.44% and an NH3 yield rate of 2.19 μg/cm2·h (21.9 μg/mgcat·h), outperforming all reported MXene-based NRR catalysts. Our work provides a feasible strategy for rationally improving the surface reactivity of MXene-based catalysts for efficient electrochemical conversion of N2 to NH3.

Counterion Dependence of Dinitrogen Activation and Functionalization by a Diniobium Hydride Anion.

We report the synthesis of anionic diniobium hydride complexes with a series of alkali metal cations (Li+ , Na+ , and K+ ) and the counterion dependence of their reactivity with N2 . Exposure of these complexes to N2 initially produces the corresponding side-on end-on N2 complexes, the fate of which depends on the nature of countercations. The lithium derivative undergoes stepwise migratory insertion of the hydride ligands onto the aryloxide units, yielding the end-on bridging N2 complex. For the potassium derivative, the N-N bond cleavage takes place along with H2 elimination to form the nitride complex. Treatment of the side-on end-on N2 complex with Me3 SiCl results in silylation of the terminal N atom and subsequent N-N bond cleavage along with H2 elimination, giving the nitride-imide-bridged diniobium complex.

(Invited) Reduced Graphene Oxides with Engineered Defects for Efficient Electrochemical Reduction of Dinitrogen to Ammonia in Wide pH Range

Electrochemical nitrogen fixation under mild conditions is highly demanded yet remains a grand challenge. Herein we report metal-free electrocatalysis of aqueous N2 reduction to produce NH3 over reduced graphene oxide (DrGO) with tuned defects. The defect sites in DrGO, consisting of unsaturated carbon [single vacancy (SV), double vacancy (DV), and –COOH], were examined, and showed an improved NH3 selectivity due to the strong binding of N2 instead of H. In addition to improved selectivity, the calculated free energies for N2 reduction reaction at DrGO-COOH and DrGO-DV sites suggest that the thermodynamic ovepotentials of these metal-free catalysts are comparable to the most efficient transition metal-based catalysts reported thus far. Our nonmetallic and dopant-free catalysts can convert N2 to NH3 at a faradaic efficiency of up to 22.0% at -0.116 V (versus the reversible hydrogen electrode vs. RHE) in 0.1 M HCl and 10.8% at -0.166 V (vs. RHE) in 0.1 M KOH, surpassing most earlier reported catalysts. An NH3 formation rate exceeding 7.3 μg·h-1 mg-1 was achieved at low overpotentials in both acidic and alkaline environments, comparable to the values shown by metal electrocatalysts under similar conditions.

Activation of an Open Shell, Carbyne-Bridged Diiron Complex Toward Binding of Dinitrogen.

Binding of N2 by nitrogenase requires a reductive activation of the FeMo-cofactor, but the precise structure and atomic composition of FeMoco in its activated form is not well understood. However, recent crystallographic studies suggest that N2 reduction may occur at a carbon-bridged diiron subunit of FeMoco. Toward modeling the activation of a Fe-(μ-C)-Fe site toward N2 binding, we synthesized a new dinucleating, hexaphosphine ligand derived from a 2,6-disubstituted toluene platform. Activation of the central methyl group of the ligand affords the diiron μ-carbyne complex (P6ArC)Fe2(μ-H) featuring a biologically relevant Fe(μ-carbyne)(μ-H)Fe motif. SQUID magnetometry, Mössbauer spectroscopy, and DFT calculations reveal that (P6ArC)Fe2(μ-H) has a well-isolated S = 1 ground state, distinguishing it from all other diiron μ-carbyne complexes which are diamagnetic. Upon the addition of sources of H+/e- (H2, TEMPO-H or HCl), (P6ArC)Fe2(μ-H) is activated toward N2 binding, with concomitant protonation of the carbyne ligand. Although reaction with H2 ultimately leads to complete protonation of the carbyne moiety, mechanistic investigations indicate that formation of a single C-H bond, with concomitant cleavage of one Fe-C bond, generates an iron-carbene intermediate capable of coordinating N2.

Mimicking the Constrained Geometry of a Nitrogen-Fixation Intermediate.

Both biological and industrial nitrogen reduction catalysts activate N2 at multinuclear binding sites with constrained Fe-Fe distances. This contrasts with molecular diiron systems, which routinely form linear N2 bridges to minimize steric interactions. Model compounds that capture the salient geometric features of N2 binding by the nitrogenase enzymes and Mittasch catalysts would contribute to understanding their high N2-reduction activity. It is shown in the present study that use of a geometrically flexible, dinucleating macrocycle allows for the formation of a bridging N2 ligand with an unusual Fe-CtN2-Fe angle of 150° (CtN2 = centroid of N2), a geometry that approximates the α-N2 binding mode on Fe(111) surfaces that precedes N2 bond cleavage. The cavity size of the macrocycle prevents the formation of a linear Fe-N2-Fe unit and leads to orbital interactions that are distinct from those available to the linear configuration.

Investigation of N2 adsorption on Fe3O4(001) using ambient pressure X-ray photoelectron spectroscopy and density functional theory.

Reactions on iron oxide surfaces are prevalent in various chemical processes from heterogeneous catalysts to minerals. Nitrogen (N2) is known to dissociate on iron surfaces, a precursor for ammonia production in the Haber-Bosch process, where the dissociation of N2 is the limiting step in the reaction under equilibrium conditions. However, little is known about N2 adsorption on other iron-based materials, such as iron oxide surfaces that are ubiquitous in soils, steel pipelines, and other industrial materials. An atomistic description is reported for the binding of N2 on the Fe3O4(001) surface using first principles calculations with ambient pressure X-ray photoelectron spectroscopy. Two primary adsorption sites are experimentally identified from N2 dissociation on Fe3O4(001). The electronic signatures associated with the valence band region unambiguously show how the electronic structure of magnetite transforms near ambient pressures due to the binding of atomic nitrogen to different surface sites. Overall, the experimental and theoretical results of our study bridge the gap between ultra-high vacuum studies and reaction conditions to provide insight into other nitrogen-based chemistry on iron oxide surfaces that impact the agriculture and energy industries.

Understanding How Ligand Functionalization Influences CO2 and N2 Adsorption in a Sodalite Metal-Organic Framework.

In this work, a detailed study is conducted to understand how ligand substitution influences the CO2 and N2 adsorption properties of two highly crystalline sodalite metal-organic frameworks (MOFs) known as Cu-BTT (BTT-3 = 1,3,5-benzenetristetrazolate) and Cu-BTTri (BTTri-3 = 1,3,5-benzenetristriazolate). The enthalpy of adsorption and observed adsorption capacities at a given pressure are significantly lower for Cu-BTTri compared to its tetrazole counterpart, Cu-BTT. In situ X-ray and neutron diffraction, which allow visualization of the CO2 and N2 binding sites on the internal surface of Cu-BTTri, provide insights into understanding the subtle differences. As expected, slightly elongated distances between the open Cu2+ sites and surface-bound CO2 in Cu-BTTri can be explained by the fact that the triazolate ligand is a better electron donor than the tetrazolate. The more pronounced Jahn-Teller effect in Cu-BTTri leads to weaker guest binding. The results of the aforementioned structural analysis were complemented by the prediction of the binding energies at each CO2 and N2 adsorption site by density functional theory calculations. In addition, variable temperature in situ diffraction measurements shed light on the fine structural changes of the framework and CO2 occupancies at different adsorption sites as a function of temperature. Finally, simulated breakthrough curves obtained for both sodalite MOFs demonstrate the materials' potential performance in dry postcombustion CO2 capture. The simulation, which considers both framework uptake capacity and selectivity, predicts better separation performance for Cu-BTT. The information obtained in this work highlights how ligand substitution can influence adsorption properties and hence provides further insights into the material optimization for important separations.

Efficient Electronic Structure to Stabilize N2‐Bridged Dinuclear Complexes Intended for N2 Activation: Iminophosphorane Iron(I) and Cobalt(I)

We examined the stability of the N2‐ligated mononuclear and N2‐bridged dinuclear complexes for FeI and CoI with the tridentate iminophosphorane ligand, which are in potentially deactivated state for catalytic N2 fixation processes, using density functional theory (DFT) method. The calculated Gibbs free energies of the N2 binding and N2‐bridge formation reactions show that the N2‐bridged FeI‐dinuclear complex is more favorable than the N2 FeI‐mononuclear complex in contrast to the CoI‐mononuclear complexes, which is only the available complex, even if at –80 °C. The stability of the N2‐bridged dinuclear metal complexes has a close relation to the M–N2 bond strength ruled by occupation of the pairs of dπu orbitals, in which the ungerade couples of the dπ orbitals interact with the occupied π orbitals of N2 in antibonding manner. For the catalytic activity of FeI mononuclear complexes, the appropriate order of orbitals to occupy the dπu orbitals is necessary, which is produced by the T‐shaped coordination geometry the ancillary ligands construct.

Fe Enhanced Visible-Light-Driven Nitrogen Fixation on BiOBr Nanosheets

Photocatalytic conversion of nitrogen (N2) to ammonia (NH3) requires strong binding of N2 onto the catalyst surface and the generation of photoexcited electrons to activate the N≡N bond. In this st...

Computational insights into nitrogen reduction reaction catalyzed by transition metal doped graphene: Comparative investigations

The nitrogen reduction reaction (NRR) is not only applied for the electro-catalysis in water to produce ammonia (NH3), but also expected to be the discharge process in cathode of the proposed lithium nitrogen (Li–N2) battery. In this work, we performed density functional theory (DFT) simulations on NRR electro-catalysis mediated by both proton (H+) and lithium ion (Li+), with transition metal doped graphene (MN4-Gr) as the catalytic substrate. Comparative calculations were carried out by taking the atom species, doping concentration, solvation effect and van der Waals (vdW) interaction into consideration. After computational screening of N2 binding performance, CrN4-Gr, MnN4-Gr, FeN4-Gr and RuN4-Gr were selected as the suitable catalytic substrate. The free energy profiles indicate that MnN4-Gr is the best candidate for NRR electro-catalysis among the four MN4-Gr, with a limiting potential (UL) of −0.68 V. Furthermore, we investigated the possible reaction pathway during the discharge process in cathode of Li–N2 battery. The simulation results demonstrate that none of the MN4-Gr works as the applicable cathode catalysts. It is important to find out that the one exhibits low |UL| value may be unsuitable cathode catalyst in Li–N2 battery. Our work first compares the NRR mediated by H+ and Li+, which provides computational understanding on the newly proposed Li–N2 battery.

One-Pot Binding, Cleavage, and Reduction of N2 at Room Temperature

One-Pot Binding, Cleavage, and Reduction of N2 at Room Temperature

Reduced graphene oxides with engineered defects enable efficient electrochemical reduction of dinitrogen to ammonia in wide pH range

Electrochemical nitrogen fixation under mild conditions is highly demanded yet remains a grand challenge. Herein we report metal-free electrocatalysis of aqueous N2 reduction to produce NH3 over reduced graphene oxide (DrGO) with tuned defects. The defect sites in DrGO, consisting of unsaturated carbon [single vacancy (SV), double vacancy (DV), and –COOH], were examined, and showed an improved NH3 selectivity due to the strong binding of N2 instead of H. In addition to improved selectivity, the calculated free energies for N2 reduction reaction at DrGO-COOH and DrGO-DV sites suggest that the thermodynamic overpotentials of these metal-free catalysts are comparable to the most efficient transition metal-based catalysts reported thus far. Our nonmetallic and dopant-free catalysts can convert N2 to NH3 at a faradaic efficiency of up to 22.0% at −0.116 V (versus the reversible hydrogen electrode vs. RHE) in 0.1 M HCl and 10.8% at −0.166 V (vs. RHE) in 0.1 M KOH, surpassing most earlier reported catalysts. An NH3 formation rate exceeding 7.3 μg h−1 mg−1 was achieved at low overpotentials in both acidic and alkaline environments, comparable to the values shown by metal electrocatalysts under similar conditions.

Redox states of nickel in zeolites and molecular account into binding of N2 to nickel(I) centers – IR, EPR and DFT study

Redox chemistry of nickel centers in high-silica ZSM-5 and BEA zeolites was investigated by a combined use of temperature-programmed reduction (TPR), electron paramagnetic resonance (EPR), and infrared (FTIR) spectroscopic techniques supported by quantum chemistry methods. The isolated nickel and nickel-oxo centers were identified and quantified by means of EPR using CO and NO probes and by IR using CO as a probe molecule. The nickel-oxo species gave rise to the nickel(I) centers upon reduction in CO. Their interaction with N 2 under low temperature led to the formation of nickel(I)–dinitrogen adducts. Attribution of the IR bands and identification of the structures of dinitrogen adducts were based on the coadsorption experiments of 14 N 2 and 15 N 2 isotopes and complementary density functional theory (DFT) calculations. Because of the presence of a mixed ligand ( 14 N 2 15 N 2 ) adduct, a structural dichotomy of possible formation of the geminal Ni(I)–(N 2 ) 2 and two coupled vicinal mono-adducts N 2 –Ni(I)⋅⋅⋅Ni(I)–N 2 was resolved. The isolated Ni(I)–N 2 and geminal Ni(I)–(N 2 ) 2 adducts were identified with confidence. A mechanism of N 2 bonding was accounted for by natural orbitals for chemical valence (NOCV) population analysis. Charge flow molecular channels due to the σ donation and π back-donation effects were established in a quantitative way. This allowed for unraveling the molecular background of the observed bathochromic shift of the stretching vibrations of all dinitrogen adducts. • Ni(II)-oxo and isolated Ni(I/II) centers undergo complex redox changes in zeolites. • Nickel speciation is controlled with protic/aprotic solution during ion exchange. • Nickel(I) forms isolated Ni(I)–N 2 and geminal Ni(I)–(N 2 ) 2 adducts. • Binding mechanism of N 2 by Ni(I) unraveled with charge flow σ and π channels.

Systematically Tuning the Electronic Structure of Gold Nanoclusters through Ligand Derivatization

AbstractWhile the ability to crystallize metal nanoclusters has revealed their geometric structure, the lack of a similarly precise measure of their electronic structure has hampered the development of synthetic design rules to precisely engineer their electronic properties. We track the evolution of highly‐resolved electronic absorption spectra of gold nanoclusters with precisely mass‐selected chemical composition in a controlled environment. Simple derivatization of the ligands yields larger spectral changes than varying the overall atomic composition of the cluster for two clusters with similar symmetry and size. The nominally metal‐localized HOMO–LUMO transition of these nanoclusters lowers in energy linearly with increasing electron donation from the exterior of the ligand shell for both cluster sizes. Very weak surface interactions, such as binding of He or N2, yield significant state‐dependent shifts, identifying states with significant interfacial character. These observations demonstrate a pathway for deliberate tuning of interfacial chemistry for chemical and technological applications.

Nitrogenase-Relevant Reactivity of a Synthetic Iron-Sulfur-Carbon Site.

Simple synthetic compounds with only S and C donors offer a ligation environment similar to the active site of nitrogenase (FeMoco) and thus demonstrate reasonable mechanisms and geometries for N2 binding and reduction in nature. We recently reported the first example of N2 binding at a mononuclear iron site supported by only S and C donors. In this work, we report experiments that examine the mechanism of N2 binding in this system. The reduction of an iron(II) tris(thiolate) complex with 1 equiv of KC8 leads to a thermally unstable intermediate, and a combination of Mössbauer, EPR, and X-ray absorption spectroscopies identifies it as a high-spin (S = 3/2) iron(I) species that maintains coordination of all three sulfur atoms. DFT calculations suggest that this iron(I) intermediate has a pseudotetrahedral geometry that resembles the S3C iron coordination environment of the belt iron sites in the resting state of the FeMoco. Further reduction to the iron(0) oxidation level under argon causes the dissociation of one of the thiolate donors and gives an η6-arene species which reacts with N2. Thus, in this system the loss of thiolate and binding of N2 require reduction beyond the iron(I) level to the iron(0) level. Further reduction of the iron(0)-N2 complex gives a reactive, formally iron(-I) species. Treatment of the putative iron(-I) complex with weak acids gives low yields of ammonia and hydrazine, demonstrating that these nitrogenase products can be generated from N2 at a synthetic Fe-S-C site. Catalytic N2 reduction is not observed, which is attributed to protonation of the supporting ligand and degradation of the complex via ligand dissociation. Identification of the challenges in this system gives insight into the design features needed for functional biomimetic complexes.

Chromism of phosphomolybdate-dye moiety: A material for molecular nitrogen and oxygen binding

Abstract Chromism from phosphomolybdate – malachite green dye moiety through hydrogen bonding interaction with water molecules is reported. Interestingly, the substance with bound di-nitrogen or di-oxygen together with water molecules also exhibits color change. N2 or O2 binding vis-a-vis chromism is a new insight.

Dinitrogen Fixation and Reduction by Ta3N3H0,1– Cluster Anions at Room Temperature: Hydrogen-Assisted Enhancement of Reactivity

Dinitrogen activation and reduction is one of the most challenging and important subjects in chemistry. Herein, we report the N2 binding and reduction at the well-defined Ta3N3H- and Ta3N3- gas-phase clusters by using mass spectrometry (MS), anion photoelectron spectroscopy (PES), and quantum-chemical calculations. The PES and calculation results show clear evidence that N2 can be adsorbed and completely activated by Ta3N3H- and Ta3N3- clusters, yielding to the products Ta3N5H- and Ta3N5-, but the reactivity of Ta3N3H- is five times higher than that of the dehydrogenated Ta3N3- clusters. The detailed mechanistic investigations further indicate that a dissociative mechanism dominates the N2 activation reactions mediated by Ta3N3H- and Ta3N3-; two and three Ta atoms are active sites and also electron donors for the N2 reduction, respectively. Although the hydrogen atom in Ta3N3H- is not directly involved in the reaction, its very presence modifies the charge distribution and the geometry of Ta3N3H-, which is crucial to increase the reactivity. The mechanisms revealed in this gas-phase study stress the fundamental rules for N2 activation and the important role of transition metals as active sites as well as the new significant role of metal hydride bonds in the process of N2 reduction, which provides molecular-level insights into the rational design of tantalum nitride-based catalysts for N2 fixation and activation or NH3 synthesis.

Mo-, V-, and Fe-Nitrogenases Use a Universal Eight-Electron Reductive-Elimination Mechanism To Achieve N2 Reduction.

Three genetically distinct, but structurally similar, isozymes of nitrogenase catalyze biological N2 reduction to 2NH3: Mo-, V-, and Fe-nitrogenase, named respectively for the metal (M) in their active site metallocofactors (metal-ion composition, MFe7). Studies of the Mo-enzyme have revealed key aspects of its mechanism for N2 binding and reduction. Central to this mechanism is accumulation of four electrons and protons on its active site metallocofactor, called FeMo-co, as metal bound hydrides to generate the key E4(4H) ("Janus") state. N2 binding/reduction in this state is coupled to reductive elimination (re) of the two hydrides as H2, the forward direction of a reductive-elimination/oxidative-addition (re/oa) equilibrium. A recent study demonstrated that Fe-nitrogenase follows the same re/oa mechanism, as particularly evidenced by HD formation during turnover under N2/D2. Kinetic analysis revealed that Mo- and Fe-nitrogenases show similar rate constants for hydrogenase-like H2 formation by hydride protonolysis (kHP) but significant differences in the rate constant for H2 re with N2 binding/reduction (kre). We now report that V-nitrogenase also exhibits HD formation during N2/D2 turnover (and H2 inhibition of N2 reduction), thereby establishing the re/oa equilibrium as a universal mechanism for N2 binding and activation among the three nitrogenases. Kinetic analysis further reveals that differences in catalytic efficiencies do not stem from significant differences in the rate constant (kHP) for H2 production by the hydrogenase-like side reaction but directly arise from the differences in the rate constant (kre) for the re of H2 coupled to N2 binding/reduction, which decreases in the order Mo > V > Fe.