N2 Activation Research Articles

Continuous ammonia synthesis from water and nitrogen via contact electrification

We synthesized ammonia (NH3) by bubbling nitrogen (N2) gas into bulk liquid water (200 mL) containing 50 mg polytetrafluoroethylene (PTFE) particles (~5 µm in diameter) suspended with the help of a surfactant (Tween 20, ~0.05 vol.%) at room temperature (25 °C). Electron spin resonance spectroscopy and density functional theory calculations reveal that water acts as the proton donor for the reduction of N2. Moreover, isotopic labeling of the N2 gas shows that it is the source of nitrogen in the ammonia. We propose a mechanism for ammonia generation based on the activation of N2 caused by electron transfer and reduction processes driven by contact electrification. We optimized the pH of the PTFE suspension at 6.5 to 7.0 and employed ultrasonic mixing. We found an ammonia production rate of ~420 μmol L-1 h-1 per gram of PTFE particles for the conditions described above. This rate did not change more than 10% over an 8-h period of sustained reaction.

Unraveling the mechanism of modulating external electric fields for regulating nitrogen fixation and activation on three-atom metal clusters

Efficient N2 fixation and activation under mild conditions are critical for green ammonia synthesis. Coupling external electric fields with three-atom transition metal clusters is a promising approach to realizing this vision. Herein, we report an effective strategy by modulating external electric fields to regulate the adsorption and activation of N2 on Sc3. The mechanism is further elucidated through DFT calculations. The most favorable adsorption site for N2 is the hollow site of Sc3. Although external electric fields inhibit the adsorption process of N2 on Sc3, the adsorbing performance of Sc3 is still very superior. Positive fields promote the activation and polarization of the adsorbed dinitrogen on the three-atom cluster, and negative fields have the opposite effect. The synergistic effect of positive/negative electric fields and Sc3 regulates the charge distribution of NN* in different ways. Furthermore, the d-band center (relative to the Fermi level), which determines the adsorption strength of N2, is affected by both positive and negative fields, resulting from the electron donation and back-donation mechanism between the NN* and the Sc3. This work provides guidelines for designing surface-anchored three-atom clusters with superior ammonia synthesis performance under electric fields in the future.

Dinitrogen Activation by Heteronuclear Metal Carbide Cluster Anions Y1‐3CoC1,2−: An Experimental and DFT Study

AbstractMass spectrometry was used to observe the reactions between Y−Co heteronuclear metal carbide cluster anions Y1‐3CoC1,2− and N2 at room temperature. Y2CoC1,2− can produce Co ejection products, in which Y2CoC− also has N2 association products; YCoC2− and Y3CoC− only generate N2 association products, while YCoC− and Y3CoC2− are inert to N2. Detailed reaction pathways were obtained through density functional theory calculations, which reasonably explain the experimental phenomena. Co is superior to Y as the electrophilic reaction site in the clusters and is the preferential initial adsorption site for N2. Some crucial steps were identified, including N−N dissociation, CC−N formation, C−CN dissociation, and C−N formation. The energy barriers of these steps are closely related to the coordination mode of N/N2 and C/C2 in clusters. Two indicators, the N−N bond length and the Mayer bond order, were utilized to describe the activation degree of the N−N bond. Analyses on the density of states and the frontier molecular orbitals reveal the electronic structures of key intermediates for N−N dissociation. The reaction mechanisms of N2 activation on Y−Co carbide clusters obtained may lay a preliminary foundation for the further development of catalysts for nitrogen reduction reaction (NRR).

Modulating d-orbital occupancy via a coupled interfacial-local electric field for electrocatalytic N2 fixation

The electrocatalytic nitrogen reduction reaction (ENRR) offers a sustainable and cost-effective strategy for ammonia (NH3) synthesis. However, the broad applicability of ENRR is currently limited by challenges in the adsorption and activation of N2 at the catalyst interface. Addressing these issues, we have developed an innovative approach that constructs an interfacial electric field, coupled with an atomically local electric field induced by W-N bonds. This coupled interfacial-local electric field effectively elevates the dz2 occupancy of W active sites, thereby significantly enhancing the adsorption and activation of N2. This work provides profound insights into the relationship between the interfacial-local electric field and the efficient execution of ENRR, paving the way for future explorations and potential breakthroughs within catalytic field.

Enhanced N2 Adsorption and Activation by Combining Re Clusters and In Vacancies as Dual Sites for Efficient and Selective Electrochemical NH3 Synthesis.

The electrochemical N2 reduction reaction (NRR) is a green and energy-saving sustainable technology for NH3 production. However, high activity and high selectivity can hardly be achieved in the same catalyst, which severely restricts the development of the electrochemical NRR. In2Se3 with partially occupied p-orbitals can suppress the H2 evolution reaction (HER), which shows excellent selectivity in the electrochemical NRR. The presence of VIn can simultaneously provide active sites and confine Re clusters through strong charge transfer. Additionally, well-isolated Re clusters stabilized on In2Se3 by the confinement effect of VIn result in Re-VIn active sites with maximum availability. By combining Re clusters and VIn as dual sites for spontaneous N2 adsorption and activation, the electrochemical NRR performance is enhanced significantly. As a result, the Re-In2Se3-VIn/CC catalyst delivers a high NH3 yield rate (26.63 μg h-1 cm-2) and high FEs (30.8%) at -0.5 V vs RHE.

Electron repulsion tuned electronic structure of TiO2 by fluorination for efficient and selective photocatalytic ammonia generation.

The photocatalytic conversion of nitrogen into high-value ammonia products holds tremendous potential in the global nitrogen cycle. However, the activation of N2 and competition of hydrogen evolution limit the improvement of nitrogen fixation performance. In this study, we developed a fluorinated TiO2 (F-TiO2) using a hydrothermal-annealing method. The incorporation of F dopants not only enhances the adsorption and activation of N2 through electronic structure regulation, but also facilitates an in situ increase in active sites via the electron repulsion effect between F and Ti atoms. In addition, the presence of F on the surface effectively improved the nitrogen supply problem and optimized the nitrogen fixation selectivity for its hydrophobic modulation. The NH3 yield of the F-TiO2 photocatalyst reached 63.8 μmol h-1 g-1, which was 8.5 times higher than that of pure TiO2. And the selectivity experiment showed that the electronic ratio of NH3 to H2 production reached 0.890. This research offers valuable insights for the design of highly efficient and selective nitrogen-fixing photocatalysts.

Beyond the triple bond: unlocking dinitrogen activation with tailored superbase phosphines.

Activating atmospheric dinitrogen (N2), a molecule with a remarkably strong triple bond, remains a major challenge in chemistry. This theoretical study explores the potential of superbase phosphines, specifically those decorated with imidazolin-2-imine ((ImN)3P) and imidazolin-2-methylidene ((ImCH)3P) to facilitate N2 activation and subsequent hydrazine (H2NNH2) formation. Using density functional theory (DFT) at the M06L/6-311++G(d,p) level, we investigated the interactions between these phosphines and N2. Mono-phosphine-N2 complexes exhibit weak, noncovalent interactions (-0.6 to -7.1 kcal mol-1). Notably, two superbasic phosphines also form high-energy hypervalent complexes with N2, albeit at significantly higher energies. The superbasic nature and potential for the hypervalency of these phosphines lead to substantial N2 activation in bis-phosphine-N2 complexes, where N2 is "sandwiched" between two phosphine moieties through hypervalent P-N bonds. Among the phosphines studied, only (ImN)3P forms an exothermic sandwich complex with N2, stabilized by hydrogen bonding between the ImN substituents and the central N2 molecule. A two-step, exothermic hydrogen transfer pathway from (ImN)3P to N2 results in the formation of a bis-phosphine-diimine (HNNH) sandwich complex. Subsequent hydrogen transfer leads to the formation of a bis-phosphine-hydrazine (H2NNH2) complex, a process that, although endothermic, exhibits surmountable activation barriers. The relatively low energy requirements for this overall transformation suggest its potential feasibility under the optimized conditions. This theoretical exploration highlights the promise of superbase phosphines as a strategy for metal-free N2 activation, opening doors for the development of more efficient and sustainable nitrogen fixation and utilization methods.

Construction of a BiVO4/VS-MoS2 S-scheme heterojunction for efficient photocatalytic nitrogen fixation.

Photocatalytic nitrogen (N2) reduction to ammonia (NH3), adopting H2O as the electron source, suffers from low efficiency owing to the sluggish kinetics of N2 reduction and the requirement of a substantial thermodynamic driving force. Herein, we present a straightforward approach for the construction of an S-scheme heterojunction of BiVO4/VS-MoS2 to successfully achieve photocatalytic N2 fixation, which is manufactured by coupling an N2-activation component (VS-MoS2 nanosheet) and water-oxidation module (BiVO4 nanocrystal) through electrostatic self-assembly. The VS-MoS2 nanosheet, enriched with sulfur vacancies, plays a pivotal role in facilitating N2 adsorption and activation. Additionally, the construction of the S-scheme heterojunction enhances the driving force for water oxidation and improves charge separation. Under simulated sunlight irradiation (100 mW cm-2), BiVO4/VS-MoS2 exhibits efficient photocatalytic N2 reduction activity with H2O as the proton source, yielding NH3 at a rate of 132.8 μmol g-1 h-1, nearly 7 times higher than that of pure VS-MoS2. This study serves as a noteworthy example of efficient N2 reduction to NH3 under mild conditions.

Multistage construction of Gd-Doped g-C3N4/Mo15S19 Composites Enabled Both N2 activation and multiple electron transfer for Enhanced Photocatalytic Nitrogen Reduction Reaction

The efficiency of photocatalytic nitrogen reduction reactions (NRR) faces significant challenges due to the need for effective activation of the N≡N bond and efficient multi-electron/proton transfer. A novel Gd-doped and...

The energetics of N2 reduction by vanadium containing nitrogenase.

The main class of nitrogenases has a molybdenum in its cofactor. A mechanism for Mo-nitrogenase has recently been described. In the present study, another class of nitrogenases has been studied, the one with a vanadium instead of a molybdenum in its cofactor. It is generally believed that these classes use the same general mechanism to activate nitrogen. The same methodology has been used here as the one used for Mo-nitrogenase. N2 activation is known to occur after four reductions in the catalytic cycle, in the E4 state. The main features of the mechanism for Mo-nitrogenase found in the previous study are an activation process in four steps prior to catalysis, the release of a sulfide during the activation steps and the formation of H2 from two hydrides in E4, just before N2 is activated. The same features have been found here for V-nitrogenase. A difference is that five steps are needed in the activation process, which explains why the ground state of V-nitrogenase is a triplet (even number) and the one for Mo-nitrogenase is a quartet (odd number). The reason an additional step is needed for V-nitrogenase is that V3+ can be reduced to V2+, in contrast to the case for Mo3+ in Mo-nitrogenase. The fact that V3+ is Jahn-Teller active has important consequences. N2H2 is formed in E4 with reasonably small barriers.

Steric control of Mg-Mg bond formation vs. N2 activation in the reduction of bulky magnesium diamide complexes.

Reduction of the magnesium(II) diamide [Mg(TripNON)] (TripNON = 4,5-bis(2,4,6-triisopropylanilido)-2,7-diethyl-9,9-dimethyl-xanthene) with 5% w/w K/KI leads to a good yield of a dianionic dimagnesium(I) species, as its potassium salt, [{K(TripNON)Mg}2]. An X-ray crystallographic analysis shows the molecule to contain a very long Mg-Mg bond (3.137(2) Å). The formation of [{K(TripNON)Mg}2] contrasts with a previously reported reduction of a magnesium(II) complex incorporating a bulkier diamide ligand, which instead afforded a magnesium-dinitrogen complex. In the current study, [{K(TripNON)Mg}2] has been shown to be a viable reagent for the reductive activation of CO, H2 and N2O.

Design of high performance nitrogen reduction electrocatalysts by doping defective polyoxometalate with a single atom promoter.

Single-atom catalysts (SACs) are emerging as promising candidates for electrochemical nitrogen reduction reaction (NRR). Previous studies have shown that the single-atom centers of SACs can not only serve as active sites, but also act as promoters to affect the catalytic properties. However, the use of single metal atoms as promoters in electrocatalysis has rarely been studied. In this work, the defective Keggin-type phosphomolybdic acid (PMA) is used as a substrate to support the single metal atoms. We aim to tune the electronic structures of the exposed molybdenum active sites on defective PMA by using these supported single atoms as promoters for efficient NRR. Firstly, the stability and N2 adsorption capacity were studied to screen for an effective catalyst capable of activating N2. Most of the SACs were found to have good stability and N2 adsorption capacity. Then, we compared the selectivity and NRR activity of the catalysts and found that catalysts with metal atom promoters have improved NRR selectivity and activity. Finally, electronic structure analysis was carried out to understand the promoting effect of the promoter on N2 activation and the activity of the NRR process. This work provides a new strategy for designing efficient catalysts for electrocatalytic reactions by introducing promoters.

*H migration-assisted MvK mechanism for efficient electrochemical NH3 synthesis over TM-TiNO.

The electrochemical NH3 synthesis on TiNO is proposed to follow the Mars-van Krevelen (MvK) mechanism, offering more favorable N2 adsorption and activation on the N vacancy (Nv) site, compared to the conventional associative mechanism. The regeneration cycle of Nv represents the rate-determining step in this process. This study investigates a series of TM (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt)-TiNO to explore the *H migration (from TM to TiNO)-promoted Nv cycle. The screening results indicate that Ni-TiNO exhibits strong H2O decomposition for *H production with 0.242 eV and low *H migration resistance with 0.913 eV. Notably, *H migration from Ni to TiNO significantly reduces the Nv formation energy to 0.811 eV, compared to 1.387 eV on pure TiNO. Meanwhile, in the presence of *H, Nv formation takes precedence over Tiv and Ov. Lastly, electronic performance calculations reveal that the collaborative function provided by Ni and Nv enables highly stable and efficient NH3 synthesis. The *H migration-assisted MvK mechanism demonstrates effective catalytic cycle performance in electrochemical N2 fixation and may have potential applicability to other hydrogenation reactions utilizing water as a proton source.

Lewis acid sites and flexible active centers synergistically boost efficient electrochemical ammonia synthesis

The Lewis acid sites enhance N2 adsorption and activation, while the flexible active center facilitates hydrogenation and NH4+ formation, regenerating the catalyst.

Nitrogen adsorption on Nb2C6H4+ cations: the important role of benzyne (ortho-C6H4).

N2 adsorption is a prerequisite for activation and transformation. Time-of-flight mass spectrometry experiments show that the Nb2C6H4+ cation, resulting from the gas-phase reaction of Nb2+ with C6H6, is more favorable for N2 adsorption than Nb+ and Nb2+ cations. Density functional theory calculations reveal the effect of the ortho-C6H4 ligand on N2 adsorption. In Nb2C6H4+, interactions between the Nb-4d and C-2p orbitals enable the Nb2+ cation to form coordination bonds with the ortho-C6H4 ligand. Although the ortho-C6H4 ligand in Nb2C6H4+ is not directly involved in the reaction, its presence increases the polarity of the cluster and brings the highest occupied molecular orbital (HOMO) closer to the lowest occupied molecular orbital (LUMO) of N2, thereby increasing the N2 adsorption energy, which effectively facilitates N2 adsorption and activation. This study provides fundamental insights into the mechanisms of N2 adsorption in "transition metal-organic ligand" systems.

A novel two-dimensional Janus TiSiGeN4 monolayer with N vacancies for efficient photocatalytic nitrogen reduction.

The photocatalytic nitrogen reduction reaction (pNRR) is a clean technology that converts H2O and N2 into NH3 under environmental conditions using inexhaustible sunlight. Herein, we designed a novel two-dimensional (2D) Janus TiSiGeN4 structure and evaluated the pNRR performance of the structure with the presence of nitrogen vacancies at different positions using density functional theory (DFT) calculations. The intrinsic dipoles in the Janus TiSiGeN4 structure generate a built-in electric field, which promotes the migration of photogenerated electrons and holes towards the (001) and (00-1) surfaces, respectively, to achieve efficient charge separation. For the pNRR, the Si atoms exposed after the formation of top N vacancies can realize the efficient activation of N2 through the "acceptance-donation" mechanism, while the presence of middle N vacancies not only suppresses the hydrogen evolution reaction, a competition reaction, but also lowers the reaction barrier for the protonation of N atoms. The limiting potential of TiSiGeN4 with the coexistence of both top and middle N vacancies (TiSiGeN4-VN-mt) is as low as -0.44 V. In addition, the introduction of N vacancies generates defect levels, narrowing the band gap and improving the light response. This work provides theoretical guidance for the design of efficient pNRR photocatalysts under mild conditions.

Catalytic ammonia synthesis on HY-zeolite-supported angstrom-size molybdenum cluster.

The development of new catalysts with high N2 activation ability is an effective approach for low-temperature ammonia synthesis. Herein, we report a novel angstrom-size molybdenum metal cluster catalyst for efficient ammonia synthesis. This catalyst is prepared by the impregnation of a molybdenum halide cluster complex with an octahedral Mo6 metal core on HY zeolite, followed by the removal of all the halide ligands by activation with hydrogen. In this activation, the size of the Mo6 cluster (ca. 7 Å) is almost retained. The resulting angstrom-size cluster shows catalytic activity for ammonia synthesis from N2 and H2, and the reaction proceeds continuously even at 200 °C under 5.0 MPa. DFT calculations suggest that N[triple bond, length as m-dash]N bond cleavage is promoted by the cooperation of the multiple molybdenum sites.

Iron promoted end-on dinitrogen-bridging in heterobimetallic complexes of uranium and lanthanides.

End-on binding of dinitrogen to low valent metal centres is common in transition metal chemistry but remains extremely rare in f-elements chemistry. In particular, heterobimetallic end-on N2 bridged complexes of lanthanides are unprecedented despite their potential relevance in catalytic reduction of dinitrogen. Here we report the synthesis and characterization of a series of N2 bridged heterobimetallic complexes of U(iii), Ln(iii) and Ln(ii) which were prepared by reacting the Fe dinitrogen complex [Fe(depe)2(N2)] (depe = 1,2-bis(diethylphosphino)-ethane), complex A with [MIII{N(SiMe3)2}3] (M = U, Ce, Sm, Dy, Tm) and [LnII{N(SiMe3)2}2], (Ln = Sm, Yb). Despite the lack of reactivity of the U(iii), Ln(iii) and Ln(ii) amide complexes with dinitrogen, the end-on dinitrogen bridged heterobimetallic complexes [{Fe(depe)2}(μ-η1:η1-N2)(M{N(SiMe3)2}3)], 1-M (M = U(iii), Ce(iii), Sm(iii), Dy(iii) and Tm(iii)), [{Fe(depe)2}(μ-η1:η1-N2)(Ln{N(SiMe3)2}2)], 1*-Ln (Ln = Sm(ii), Yb(ii)) and [{Fe(depe)2(μ-η1:η1-N2)}2{SmII{N(SiMe3)2}2}], 3 could be prepared. The synthetic method used here allowed to isolate unprecedented end-on bridging N2 complexes of divalent lanthanides which provide relevant structural models for the species involved in the catalytic reduction of dinitrogen by Fe/Sm(ii) systems. Computational studies showed an essentially electrostatic interaction of the end-on bridging N2 with both Ln(iii) and Ln(ii) complexes with the degree of N2 activation correlating with their Lewis acidity. In contrast, a back-bonding covalent contribution to the U(iii)-N2Fe bond was identified by computational studies. Computational studies also suggest that end-on binding of N2 to U(iii) and Ln(ii) complexes is favoured for the iron-bound N2 compared to free N2 due to the higher N2 polarization.

Transition Metal Nitrides for the Electrocatalysis of the Electrochemical Ammonia Synthesis

The direct electrochemical ammonia synthesis (EAS) from water and nitrogen using renewable energy shows great potential for future decentralized production of indispensable green ammonia and may, at least partially, supersede the fossil fuel-based Haber-Bosch process in the future [1]. Material innovation for the conversion of nitrogen to ammonia via the nitrogen reduction reaction (NRR) is one cornerstone for the successful development of the EAS technology. To date, only Lithium-mediated NRR has been demonstrated to show sufficient production rate and faradaic efficiency for a prospective application [2], while the reaction in aqueous electrolyte, independent of the studied material class, suffers from false-positive results due to low productions rates, low Faradaic efficiencies and complex contamination issues [3].Several catalyst systems are currently investigated for NRR in aqueous electrolytes. Transition metal nitrides (TMN) may offer an energetic advantage as electrocatalyst, because the NRR is expected to be catalyzed via the Mars-van-Krevelen mechanism (MvK) on TMNs. Here the protonation of a lattice nitrogen atom forms the first ammonia molecule. The resulting N-vacancy is filled by molecular nitrogen where one N adatom is subsequently protonated to form the second ammonia molecule. Thus, the N2 activation step occurs at the N-vacancy site and avoids the surface adsorption and dissociation of N2. Specific facets of Zr, Cr, V and Nb were described as stable, active and selective TMN catalysts for the NRR based on theoretical catalyst screening [4]. However, current literature studies state contradictory results and require the differentiation of genuine activity and non-catalytic decomposition of TMN catalysts [5], which is currently lacking.In our work, we address the synthesis of Zr-based TMNs, the electrochemical evaluation in a gas diffusion electrode setup and the trace analysis of ammonia by ion chromatography [6]. Only consideration of all three interconnected steps can give an indication of genuine NRR activity, where structural characterization after NRR experiments elucidates the stability of the material. Special interest is given to the role of surface oxynitrides and their ratio to the nitride phase for electrocatalytic NRR. Comparison of nanoparticulate catalysts to surface engineered thin film catalyst as well as insights from first principles simulation delivers a comprehensive characterization of the electrosynthesis of ammonia by TMNs.

Electrocatalytic Urea Synthesis via C-N Coupling from CO2 and Nitrogenous Species.

ConspectusIndustrial urea synthesis consists of the Haber-Bosch process to produce ammonia and the subsequent Bosch-Meiser process to produce urea. Compared to the conventional energy-intensive urea synthetic protocol, electrocatalytic C-N coupling from CO2 and nitrogenous species emerges as a promising alternative to construct a C-N bond under ambient conditions and to realize the direct synthesis of high-value urea products via skipping the intermediate step of ammonia production. The main challenges for electrocatalytic C-N coupling lie in the intrinsic inertness of molecules and the competition with parallel side reactions. In this Account, we give an overview of our recent progress toward electrocatalytic C-N coupling from CO2 and nitrogenous species toward urea synthesis.To begin, we present the direct transformation of dinitrogen (N2) to the C-N bond by coelectrolysis, verifying the feasibility of direct urea synthesis from N2 and CO2 under ambient conditions. In contrast to the highly endothermic step of proton coupling in conventional N2 reduction, the N2 activation and construction of the C-N bond arise from a thermodynamic spontaneous reaction between CO (derived from CO2 reduction) and *N═N* (the asterisks represent the adsorption sites), and the crucial *NCON* species mediates the interconversion of N2, CO2, and urea. Based on theoretical guidance, the effect of N2 adsorption configurations on C-N coupling is investigated on the model catalysts with defined active site structure, revealing that the side-on adsorption rather than the end-on one favors C-N coupling and urea synthesis.Electrocatalytic C-N coupling of CO2 and nitrate (NO3-) is also an effective pathway to achieve direct urea synthesis. We summarize our progress in the C-N coupling of CO2 and NO3-, from the aspects of modulating intermediate species adsorption and reaction paths, monitoring irreversible and reversible reconstruction of active sites, and precisely constructing active sites to match activities and to boost the electrocatalytic urea synthesis. In each case, in situ electrochemical technologies and density functional theory (DFT) calculations are carried out to unveil the microscopic mechanisms for the promotion of C-N coupling and the enhancement of urea synthesis activity. In the last section, we put forward the limitations, challenges, and perspectives in these two coupling systems for further development of electrocatalytic urea synthesis.