Electrocatalytic Reduction of Nitrogen to Ammonia Using Tiara-like Phenylethanethiolated Nickel Cluster

The fixing of N₂ to NH₃ is challenging due to the inertness of the N≡N bond. Commercially, ammonia production depends on the energy-consuming Haber-Bosch (H−B) process, which emits CO₂ while using fossil fuels as the sources of hydrogen and energy. An alternative method for NH₃ production is the electrochemical nitrogen reduction reaction (NRR) process as it is powered by renewable energy sources. Here, we report a tiara-like nickel-thiolate cluster, [Ni₆(PET)₁₂] (where, PET=2-phenylethanethiol)] as an efficient electro-catalyst for the electrochemical NRR at ambient conditions. Ammonia (NH₃: 16.2±0.8 μg h⁻¹ cm⁻²) was the only nitrogenous product over the potential of −2.3 V vs. F_c⁺/F_c with a Faradaic efficiency of 25%±1.7. Based on theoretical calculations, NRR by [Ni₆(PET)₁₂] proceeds through both the distal and alternating pathways with an onset potential of −1.84 V vs. RHE (i.e., −2.46 V vs. F_c⁺/F_c) which corroborates with the experimental findings.     

Computational prediction of Mo2@g-C6N6 monolayer as an efficient electrocatalyst for N2 reduction

Electrocatalytic nitrogen reduction reaction (NRR) is an environmentally friendly method for sustainable ammonia synthesis under ambient conditions. Searching for efficient NRR electrocatalysts with high activity and selectivity is currently urgent but remains great challenge. Herein, we systematically investigate the NRR catalytic activities of single and double transition metal atoms (TM = Fe, Co, Ni and Mo) anchored on g-C₆N₆ monolayers by performing first-principles calculation. Based on the stability, activity, and selectivity analysis, Mo₂@g-C₆N₆ monolayer is screened out as the most promising candidate for NRR. Further exploration of the reaction mechanism demonstrates that the Mo dimer anchored on g-C₆N₆ can sufficiently activate and efficiently reduce the inert nitrogen molecule to ammonia through a preferred distal pathway with a particularly low limiting potential of -0.06 V. In addition, we find that Mo₂@g-C₆N₆ has excellent NRR selectivity over the competing hydrogen evolution reaction, with the Faradaic efficiency being 100%. Our work not only predicts a kind of ideal NRR electrocatalyst but also encouraging more experimental and theoretical efforts to develop novel double-atom catalysts (DACs) for NRR.     

A Spectroscopic Study of Electrochemical Nitrogen and Nitrate Reduction on Rhodium Surfaces

Rh is a promising electrocatalyst for the nitrogen reduction reaction (NRR) given its suitable nitrogen‐adsorption energy and low overpotential. However, the NRR pathway on Rh surfaces remains unknown. In this study, we employ surface‐enhanced infrared‐absorption spectroscopy (SEIRAS) and differential electrochemical mass spectrometry (DEMS) to study the reaction mechanism of NRR on Rh. N₂H_x (0≤x≤2) is detected with a N=N stretching mode at ≈2020 cm^?1 by SEIRAS and a signal at m/z=29 by DEMS. A new two‐step reaction pathway on Rh surfaces is proposed that involves an electrochemical process with a two‐electron transfer to form N₂H₂ and its subsequent decomposition in the electrolyte producing NH₃. Our results also indicate that nitrate reduction and the NRR share the same reaction intermediate N₂H_x.     

Theoretical study on the application of the largest aluminum-pyrazole ring in electrochemical nitrogen reduction reaction

Electrochemical nitrogen reduction reaction (NRR) is a mild and sustainable method for ammonia synthesis. Therefore, developing high activity, selectivity, and economic efficiency catalysts with considering the synergistic effects between catalysts and carriers to design novel structural models is very important. Considering the non-noble metal NRR catalyst, Mo₃, we tried to find a suitable carrier which is stable and economical. Herein, we used the largest atomically precise aluminum-pyrazole ring (AlOC-69) to date (diameter up to 2.3 nm). The larger ring cavities and the presence of abundant hydroxy groups make AlOC-69 an ideal molecular carrier model and provide a basis for studying its structure-activity relationship. The formation energy (−0.76 eV) and stable Mo-O bonds indicate that Mo₃ can be stabilized on the Al₁₀O₁₀ surface. Additionally, N₂ has fully activated due to the strong interaction between the p-orbital of N and the d-orbital of Mo. The low limiting potential (−0.28 V) emerges that Mo₃@Al₁₀O₁₀ has ideal catalytic activity and selectivity. This research provides a promising catalyst model and an understanding of its catalytic process at the atomic level, providing a new approach for the co-design of catalyst and carrier in NRR.     

Potassium‐Ion‐Assisted Regeneration of Active Cyano Groups in Carbon Nitride Nanoribbons: Visible‐Light‐Driven Photocatalytic Nitrogen Reduction

As a metal‐free nitrogen reduction reaction (NRR) photocatalyst, g‐C₃N₄ is available from a scalable synthesis at low cost. Importantly, it can be readily functionalized to enhance photocatalytic activities. However, the use of g‐C₃N₄‐based photocatalysts for the NRR has been questioned because of the elusive mechanism and the involvement of N defects. This work reports the synthesis of a g‐C₃N₄ photocatalyst modified with cyano groups and intercalated K⁺ (mCNN), possessing extended visible‐light harvesting capacity and superior photocatalytic NRR activity (NH₃ yield: 3.42 mmol g^?1 h^?1). Experimental and theoretical studies suggest that the ‐C≡N in mCNN can be regenerated through a pathway analogous to Mars van Krevelen process with the aid of the intercalated K⁺. The results confirm that the regeneration of the cyano group not only enhances photocatalytic activity and sustains the catalytic cycle, but also stabilizes the photocatalyst.     

Unifying the Nitrogen Reduction Activity of Anatase and Rutile TiO2 Surfaces

TiO₂ is a model transition metal oxide that has been applied frequently in both photocatalytic and electrocatalytic nitrogen reduction reactions (NRR). However, the phase which is more NRR active still remains a puzzle. This work presents a theoretical study on the NRR activity of the (001), (100), (101), and (110) surfaces of both anatase and rutile TiO₂. We found that perfect surfaces are not active for NRR, while the oxygen vacancy can promote the reaction by providing excess electrons and low-coordinated Ti atoms that enhance the binding of the key intermediate (HNN*). The NRR activity of the eight facets can be unified into a single scaling line. The anatase TiO₂(101) and rutile TiO₂(101) surfaces were found to be the most and the second most active surfaces with a limiting potential of −0.91 V and −0.95 V respectively, suggesting that the TiO₂ NRR activity is not very phase-sensitive. For photocatalytic NRR, the results suggest that the anatase TiO₂(101) surface is still the most active facet. We further found that the binding strength of key intermediates scale well with the formation energy of oxygen vacancy, which is determined by the oxygen coordination number and the degree of relaxation of the surface after the creation of oxygen vacancy. This work provides a comprehensive understanding of the activity of TiO₂ surfaces. The results should be helpful for the design of more efficient TiO₂-based NRR catalysts.     

Heteronuclear Trimetallic MFe2 and M2Fe (M=V,Nb, and Ta) Clusters for Dinitrogen Activation

Catalysts with heteronuclear metal active sites may have high performance in the nitrogen reduction reaction (NRR), and the in-depth understanding of the reaction mechanisms is crucial for the design of related catalysts. In this work, the dissociative adsorption of N₂ on heteronuclear trimetallic MFe₂ and M₂Fe (M=V, Nb, and Ta) clusters was studied with density functional theory calculations. For each cluster, two reaction paths were studied with N₂ initially on M and Fe atoms, respectively. Mayer bond order analysis provides more information on the activation of N−N bonds. M₂Fe is generally more reactive than MFe₂. The coordination mode of N₂ on three metal atoms can be end-on: end-on: side-on (EES) for both MFe₂ and M₂Fe. In addition, a unique end-on: side-on: side-on (ESS) coordination mode was found for M₂Fe, which leads to a higher degree of N−N bond activation. Nb₂Fe has the highest reactivity towards N₂ when both the transfer of N₂ and the dissociation of N−N bonds are taken into account, while Ta-containing clusters have a superior ability to activate the N−N bond. These results indicate that it is possible to improve the performance of iron-based catalysts by doping with vanadium group metals.     

Group 6 Transition‐Metal/Boron Frustrated Lewis Pair Templates Activate N2 and Allow its Facile Borylation and Silylation

The reaction of trans ‐[M(N₂)₂(dppe)₂] (M=Mo, 1_Mo , M=W, 1_W ) with B(C₆F₅)₃ ( 2 ) provides the adducts [(dppe)₂M=N=N‐B(C₆F₅)₃] ( 3 ) which can be regarded as M/B transition‐metal frustrated Lewis pair (TMFLP) templates activating dinitrogen. Easy borylation and silylation of the activated dinitrogen ligands in complexes 3 with a hydroborane and hydrosilane occur by splitting of the B−H and Si−H bonds between the N₂ moiety and the perfluoroaryl borane. This reactivity of 3 is reminiscent of conventional frustrated Lewis pair chemistry and constitutes an unprecedented approach for the functionalization of dinitrogen.     

MoS2-Based Catalysts for N2 Electroreduction to NH3 – An Overview of MoS2 Optimization Strategies

The nitrogen reduction reaction (NRR) has become an ideal alternative to the Haber-Bosch process, as NRR possesses, among others, the advantage of operating under ambient conditions and saving energy consumption. The key to efficient NRR is to find a suitable electrocatalyst, which helps to break the strong N≡N bond and improves the reaction selectivity. Molybdenum disulfide (MoS₂) as an emerging layered two-dimensional material has attracted a mass of attention in various fields. In this minireview, we summarize the optimization strategies of MoS₂-based catalysts which have been developed to improve the weak NRR activity of primitive MoS₂. Some theoretical predictions have also been summarized, which can provide direction for optimizing NRR activity of future MoS₂-based materials. Finally, an outlook about the optimization of MoS₂-based catalysts used in electrochemical N₂ fixation are given.     

Synthesis and Complexation Study of New Aminoalkynyl−amidinate Ligands

The current library of amidinate ligands has been extended by the synthesis of two novel dimethylamino-substituted alkynylamidinate anions of the composition [Me₂N−CH₂−C≡C−C(NR)₂]⁻ (R = ⁱPr, cyclohexyl (Cy)). The unsolvated lithium derivatives Li[Me₂N−CH₂−C≡C−C(NR)₂] ( 1 : R = ⁱPr, 2 : R = Cy) were obtained in good yields by treatment of in situ-prepared Me₂N−CH₂−C≡C−Li with the respective carbodiimides, R−N=C=N−R. Recrystallization of 1 and 2 from THF afforded the crystalline THF adducts Li[Me₂N−CH₂−C≡C−C(NR)₂] ⋅ nTHF ( 1 a : R = ⁱPr, n=1; 2 a : R = Cy, n=1.5). Precursor 2 was subsequently used to study initial complexation reactions with selected di- and trivalent transition metals. The dark red homoleptic vanadium(III) tris(alkynylamidinate) complex V[Me₂N−CH₂−C≡C−C(NCy)₂]₃ ( 3 ) was prepared by reaction of VCl₃(THF)₃ with 3 equiv. of 2 (75 % yield). A salt-metathesis reaction of 2 with anhydrous FeCl₂ in a molar ratio of 2 : 1 afforded the dinuclear homoleptic iron(II) alkynylamidinate complex Fe₂[Me₂N−CH₂−C≡C−C(NCy)₂]₄ ( 4 ) in 69 % isolated yield. Similarly, treatment of Mo₂(OAc)₄ with 3 or 4 equiv. of 2 provided the dinuclear, heteroleptic molybdenum(II) amidinate complex Mo₂(OAc)[Me₂N−CH₂−C≡C−C(NCy)₂]₃ ( 5 ; yellow crystals, 50 % isolated yield). The cyclohexyl-substituted title compounds 2 a , 4 , and 5 were structurally characterized through single-crystal X-ray diffraction studies.     

Comparison of Nitrogen Activation on Trinuclear Niobium and Tungsten Sulfide Clusters Nb3Sn and W3Sn (n=0–3): A DFT Study

The reaction of N₂ with trinuclear niobium and tungsten sulfide clusters Nb₃S_n and W₃S_n (n=0–3) was systematically studied by density functional theory calculations with TPSS functional and Def2-TZVP basis sets. Dissociations of N−N bonds on these clusters are all thermodynamically allowed but with different reactivity in kinetics. The reactivity of Nb₃S_n is generally higher than that of W₃S_n. In the favorite reaction pathways, the adsorbed N₂ changes the adsorption sites from one metal atom to the bridge site of two metal atoms, then on the hollow site of three metal atoms, and at that place, the N−N bond dissociates. As the number of ligand S atoms increases, the reactivity of Nb₃S_n decreases because of the hindering effect of S atoms, while W₃S and W₃S₂ have the highest reactivity among four W₃S_n clusters. The Mayer bond order, bond length, vibrational frequency, and electronic charges of the adsorbed N₂ are analyzed along the reaction pathways to show the activation process of the N−N bond in reactions. The charge transfer from the clusters to the N₂ antibonding orbitals plays an essential role in N−N bond activation, which is more significant in Nb₃S_n than in W₃S_n, leading to the higher reactivity of Nb₃S_n. The reaction mechanisms found in this work may provide important theoretical guidance for the further rational design of related catalytic systems for nitrogen reduction reactions (NRR).     

N2 Electroreduction to NH3 by Selenium Vacancy-Rich ReSe2 Catalysis at an Abrupt Interface

Vacancy engineering has been proved repeatedly as an adoptable strategy to boost electrocatalysis, while its poor selectivity restricts the usage in nitrogen reduction reaction (NRR) as overwhelming competition from hydrogen evolution reaction (HER). Revealed by density functional theory calculations, the selenium vacancy in ReSe₂ crystal can enhance its electroactivity for both NRR and HER by shifting the d-band from −4.42 to −4.19 eV. To restrict the HER, we report a novel method by burying selenium vacancy-rich ReSe₂@carbonized bacterial cellulose (V_r-ReSe₂@CBC) nanofibers between two CBC layers, leading to boosted Faradaic efficiency of 42.5 % and ammonia yield of 28.3 μg h⁻¹ cm⁻² at a potential of −0.25 V on an abrupt interface. As demonstrated by the nitrogen bubble adhesive force, superhydrophilic measurements, and COMSOL Multiphysics simulations, the hydrophobic and porous CBC layers can keep the internal V_r-ReSe₂@CBC nanofibers away from water coverage, leaving more unoccupied active sites for the N₂ reduction (especially for the potential determining step of proton-electron coupling and transferring processes as *NN → *NNH).     

New Stable and Persistent Acyclic Diaminocarbenes

The portfolio of acyclic diaminocarbenes (ADACs) has been substantially expanded, owing to the synthesis of eleven new formamidinium salts, mostly of the type [(iPr₂N)CH(NRR′)][PF₆], for use as immediate carbene precursors. The corresponding ADACs (iPr₂N)C(NRR′) were sufficiently stable for isolation in the case of NRR′=2‐methylpiperidino ( 13 ), 3‐methylpiperidino ( 14 ), 4‐methylpiperidino ( 15 ), morpholino ( 17 ) and NiPrPh ( 20 ), but had to be trapped in situ in the case of NRR′=2,2,6,6‐tetramethylpiperidino ( 12 ) and NiPrMe ( 19 ). The tetraaryl‐substituted ADACs (Ph₂N)₂C ( 22 ) and (Ph₂N)C[N(C₆F₅)₂] ( 24 ) also could only be generated and trapped in situ. Trapping with elemental selenium was particularly efficient, affording the corresponding selenourea derivative in all cases, whereas trapping with [{Rh(μ‐Cl)(cod)}₂] did not work for 12 and 24 . The ⁷⁷Se NMR chemical shifts, δ(⁷⁷Se), of the selenourea compounds derived from the new ADACs lie in the range 450–760 ppm, which indicates a much higher electrophilicity and π‐accepting capability of ADACs in comparison with NHCs, which typically exhibit δ(⁷⁷Se)<200 ppm. The extreme low‐field shift of 758 ppm observed for 12 Se can be rationalised by the results of DFT calculations, which revealed that ADAC 12 has a minimum energy conformation with the 2,2,6,6‐tetramethylpiperidino unit perpendicular to the N₂C plane, which suppresses the π donation of this amino group and causes an unusually low LUMO energy and high electrophilicity.     

Vacancy Engineering of Iron-Doped W18O49 Nanoreactors for Low-Barrier Electrochemical Nitrogen Reduction

The electrochemical nitrogen reduction reaction (NRR) is a promising energy-efficient and low-emission alternative to the traditional Haber–Bosch process. Usually, the competing hydrogen evolution reaction (HER) and the reaction barrier of ambient electrochemical NRR are significant challenges, making a simultaneous high NH₃ formation rate and high Faradic efficiency (FE) difficult. To give effective NRR electrocatalysis and suppressed HER, the surface atomic structure of W₁₈O₄₉, which has exposed active W sites and weak binding for H₂, is doped with Fe. A high NH₃ formation rate of 24.7 μg h⁻¹ mg_cat⁻¹ and a high FE of 20.0 % are achieved at an overpotential of only −0.15 V versus the reversible hydrogen electrode. Ab initio calculations reveal an intercalation-type doping of Fe atoms in the tunnels of the W₁₈O₄₉ crystal structure, which increases the oxygen vacancies and exposes more W active sites, optimizes the nitrogen adsorption energy, and facilitates the electrocatalytic NRR.     

Defect Engineering Metal‐Free Polymeric Carbon Nitride Electrocatalyst for Effective Nitrogen Fixation under Ambient Conditions

Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions provides an intriguing picture for the conversion of N₂ into NH₃. However, electrocatalytic NRR mainly relies on metal‐based catalysts, and it remains a grand challenge in enabling effective N₂ activation on metal‐free catalysts. Here we report a defect engineering strategy to realize effective NRR performance (NH₃ yield: 8.09 μg h^?1 mg^?1_cat., Faradaic efficiency: 11.59 %) on metal‐free polymeric carbon nitride (PCN) catalyst. Illustrated by density functional theory calculations, dinitrogen molecule can be chemisorbed on as‐engineered nitrogen vacancies of PCN through constructing a dinuclear end‐on bound structure for spatial electron transfer. Furthermore, the N?N bond length of adsorbed N₂ increases dramatically, which corresponds to “strong activation” system to reduce N₂ into NH₃. This work also highlights the significance of defect engineering for improving electrocatalysts with weak N₂ adsorption and activation ability.     

Boron-iron nanochains for selective electrocatalytic reduction of nitrate

The electrocatalysis of nitrate reduction reaction(NRR) has been considered to be a promising nitrate removal technology.Developing a highly effective iron-based electrocatalyst is an essential challenge for NRR.Herein,boron-iron nanochains(B-Fe NCs) as efficient NRR catalysts were prepared via a facile lowcost and scalable method.The Fe/B ratio of the B-Fe NCs-x can be elaborately adjusted to optimize the NRR catalytic performance.Due to the electron transfer from boron to metal,the metal-metal bonds are weakened and the electron density near the metal atom centers are rearranged,which are favor of the conversion from NO_3~-into N_2.Moreover,the well-crosslinked chain-like architectures benefit the mass/electron transport to boost the exposure of abundant catalytic active sites.Laboratory experiments demonstrated that the optimized B-Fe NCs catalyst exhibits superior intrinsic electrocatalytic NRR activity of high nitrate conversion(～80%),ultrahigh nitrogen selectivity(～99%) and excellent long-term reactivity in the mixed electrolyte system(0.02 mol/L NaCl and 0.02 mol/L Na_2 SO_4 mixed electrolyte),and the electrocatalytic activity of the material shows poor performance at low chloride ion concentration(Nitrate conversion of ～61 % and nitrogen selectivity of ～57% in 0.005 mol/L NaCl and 0.035 mol/L Na_2 SO_4 mixed electrolyte).This study provides a broad application prospect for further exploring the highefficiency and low-cost iron-based functional nanostructures for electrocatalytic nitrate reduction.     

Poly[propane‐1,3‐diammonium [tetra‐μ3‐oxo‐hexaoxotrimolybdenum(VI)] dihydrate]

The crystal structure of the title compound, {(C₃H₁₂N₂)[Mo₃O₁₀]·2H₂O}_n, is composed of [Mo₃O₁₀]²⁻ anionic chains, propane‐1,3‐diammonium cations and solvent water molecules. The [Mo₃O₁₀]²⁻ chain is constructed from edge‐sharing MoO₆ octahedra. The protonated propane‐1,3‐diamine cations and solvent water molecules are located between the chains and are linked to the O atoms of the inorganic chains by hydrogen bonds.     

Bis(iminodiethylenedi­ammonium) di‐μ5‐hydrogen­phosphato‐penta­molybdate(VI) tetra­hydrate

The crystal structure of the title compound, (C₄H₁₅N₃)₂[Mo₅O₁₅(HPO₄)₂]·4H₂O, is made up of [Mo₅O₁₅(HPO₄)₂]⁴⁻ clusters, iminodiethylenediammonium cations and solvent water mol­ecules. The [Mo₅O₁₅(HPO₄)₂]⁴⁻ cluster, with approximate C₂ symmetry, can be considered as a ring formed by five distorted edge‐ and corner‐sharing MoO₆ octa­hedra, capped on both poles by a hydro­phosphate tetra­hedron. There exist N—H⋯O, O—H⋯N and C—H⋯O hydrogen bonds between the organic ammonium groups and the clusters, with inter­atomic N⋯O distances ranging from 2.675 (3) to 2.999 (3) Å, and C⋯O distances ranging from 3.139 (5) to 3.460 (5) Å.     

Selective 3,3-Rearrangement of Azobenzenes upon Complexation by a Frustrated Lewis Pair

The reaction of the oxygen-bridged frustrated Lewis pairs (FLPs) tBu₂P−O−Si(C₂F₅)₃ ( 1 ) and tBu₂P−O−AlBis₂ ( 2 ) with azobenzene, promoted by UV irradiation, led to a selective complexation of the cis-isomer. The addition product of 2 is stable, while the adduct of 1 isomerizes in solution in an ortho-benzidine-like [3,3]-rearrangement by cleavage of the N−N bond, saturation of the nitrogen atoms with hydrogen atoms and formation of a new bond between two phenyl ortho-carbon atoms. Similar rearrangements take place with different para-substituted azobenzenes (R=Me, OMe, Cl) and di(2-naphthyl)diazene, while ortho-methylated azo compounds do not form adducts with 1 . All adducts were characterized by multinuclear NMR spectroscopy and elemental analyses and the mechanism of the rearrangement was explored by quantum-chemical calculations.