N2 Activation Research Articles

Bimetallic MIL-68(InFe) MOF nanorods for biomimetic photocatalytic N2 fixation

This work develops a series of bimetallic MIL-68(InFe) MOF nanorods (NM(In1-xFex)) mimicking the nitrogenase for biomimetic photocatalytic N2 fixation. The partial substitution of In3+ with Fe3+ leads to the electron redistribution from In3+ to Fe3+, generating electron-poor In3+ (In(3+δ)+) sites and electron-rich Fe3+ (Fe(3–δ)+) sites as an electron acceptor–donor combination to promote the N2 activation by a π back-donation mechanism. The smaller size of the nanorods further provides more accessible active sites than the bulk counterpart. In cooperation with the H+ released from the H2O, the activated N2 molecules were reduced by photogenerated electrons to give NH3. The optimal sample NM(In0.90Fe0.10) exhibited the highest NH4+ production rate of 30.8 µmol·h−1·g−1 without any sacrificial agent, attributed to the presence of abundant In/Fe bimetallic sites for N2 activation and enhanced charge mobility. This work provides new insights into rational design for artificial N2 fixation systems by mimicking natural nitrogenase.

Ti3+-mediated MIL-125(Ti) by metal substitution for boosting photocatalytic N2 fixation

Photocatalysis, which uses sunlight, N2 and H2O to produce NH3, is a more sustainable approach to N2 fixation than the Haber-Bosch process. However, its efficiency is severely limited by the difficulty of activating NN bonds. This work presents metal (M = Cu, Fe, V)-substituted MIL-125(Ti) (MIL-(MTi)) for photocatalytic N2 fixation without using any sacrificial agents. Structural characterizations reveal that the active sites including oxygen vacancies (OV) and Ti3+ species are formed by the resulting crystal distortion due to the partial substitution of Ti4+ by other metal ions (Cu+, Fe2+, V3+) in MIL-125(Ti). MIL-(CuTi) possesses a larger number of OV and Ti3+ compared to MIL-(FeTi) and MIL-(VTi) due to the larger valence difference between Cu+ and Ti4+. These active sites not only promote the adsorption and activation of N2 and H2O, but also facilitate the photogenerated charge mobility. Photogenerated holes oxidize H2O to produce O2 and H+. Photogenerated electrons reduce N2 activated on Ti3+ sites by combining with H+ to form NH4+. Therefore, MIL-(CuTi) shows the highest NH4+ production rate 46.5 µmol·h−1·g−1, which is much higher than that (1.2 µmol·h−1·g−1) of the pristine MIL-125(Ti). This work provides a new insight into rational design for artificial N2 fixation systems by the construction of the active site.

Radiocatalytic ammonia synthesis from nitrogen and water.

The development of alternative methods to the Haber-Bosch process for ammonia (NH3) synthesis is a pressing and formidable challenge. Nuclear energy represents a low-carbon, efficient and stable source of power. The harnessing of nuclear energy to drive nitrogen (N2) reduction not only allows 'green' NH3 synthesis, but also offers the potential for the storage of nuclear energy as a readily transportable zero-carbon fuel. Herein, we explore radiocatalytic N2 fixation to NH3 induced by γ-ray radiation. Hydrated electrons (e- aq) that are generated from water radiolysis enable N2 reduction to produce NH3. Ru-based catalysts synthesized by using γ-ray radiation with excellent radiation stability substantially improve NH3 production in which the B5 sites of Ru particles may play an important role in the activation of N2. By benefitting from the remarkable penetrating power of γ-ray radiation, radiocatalytic NH3 synthesis can proceed in an autoclave under appropriate pressure conditions, resulting in an NH3 concentration of ≤5.1mM. The energy conversion efficiency of the reaction is as high as 563.7mgNH3·MJ-1. This radiocatalytic chemistry broadens the research scope of catalytic N2 fixation while offering promising opportunities for converting nuclear energy into chemical energy.

Sulfur defect engineering boosted nitrogen activation over FeS2 for efficient electrosynthesis of ammonia

Electrocatalytic nitrogen (N2) reduction to ammonia (NH3) reaction (eNRR) supplies a promising alternative to the Haber-Bosch technology. However, the dissociation of NN bond hinders its development. Herein, sulfur vacancies are introduced into FeS2 for promoting N2 activation and thus stimulating the eNRR progress. Experimental investigations and density functional theory (DFT) calculations reveal that the electrons could transfer from Fe 3d orbits to N2 2π* orbital, thus facilitating the cracking of inert N2 molecules. And the electron transfer is easier for those Fe atoms with S vacancies in adjacent positions. Furthermore, we find that eNRR process on the FeS2 surface follows the distal and alternating hybrid pathway. Also, the water molecules in the electrolyte facilitate the first hydrogenation of N2 (*N2 → *NNH). Notably, FeS2 with rich sulfur vacancies exhibits an excellent NH3 yield rate of 67.5 μg h−1 mgcat.−1, which outperforms most of the reported eNRR activities of Fe-based catalysts.

Predicting Activation of Small Molecules Including Dinitrogen via a Carbene with a σ0π2 Electronic Configuration.

Although the main group species in the s and p blocks have begun to gain prominence in the field of dinitrogen (N2) activation in recent years, reports of carbene-mediated N2 activation remain particularly rare, especially for carbenes with a σ0π2 electronic configuration. Herein, we demonstrate examples of N2 activation initiated by a carbene with a σ0π2 electronic configuration and consequent N2 hydroboration reaction (with a reaction barrier as low as 19.9 kcal/mol) via density functional theory calculations. Meanwhile, the "push-pull" electronic effect upon introduction of a hydroborenium complex facilitates the generation of a thermodynamically and kinetically more stable product. In addition, such a σ0π2 carbene can also activate a series of H-X (X = H, CH3, or Bpin) bonds through an oxidative addition process with activation energies ranging from 6.0 to 18.0 kcal/mol. Our findings highlight the importance of σ0π2 carbenes in the field of small molecule activation, especially N2 activation.

Optimizing the Lattice Nitrogen Coordination to Break the Performance Limitation of Metal Nitrides for Electrocatalytic Nitrogen Reduction.

Metal nitrides (MNs) are attracting enormous attention in the electrocatalytic nitrogen reduction reaction (NRR) because of their rich lattice nitrogen (Nlat) and the unique ability of Nlat vacancies to activate N2. However, continuing controversy exists on whether MNs are catalytically active for NRR or produce NH3 via the reductive decomposition of Nlat without N2 activation in the in situ electrochemical conditions, let alone the rational design of high-performance MN catalysts. Herein, we focus on the common rocksalt-type MN(100) catalysts and establish a quantitative theoretical framework based on the first-principles microkinetic simulations to resolve these puzzles. The results show that the Mars-van Krevelen mechanism is kinetically more favorable to drive the NRR on a majority of MNs, in which Nlat plays a pivotal role in achieving the Volmer process and N2 activation. In terms of stability, activity, and selectivity, we find that MN(100) with moderate formation energy of Nlat vacancy (E vac) can achieve maximum activity and maintain electrochemical stability, while low- or high-E vac ones are either unstable or catalytically less active. Unfortunately, owing to the five-coordinate structural feature of Nlat on rocksalt-type MN(100), this maximum activity is limited to a yield of NH3 of only ∼10-15 mol s-1 cm-2. Intriguingly, we identify a volcano-type activity-regulating role of the local structural features of Nlat and show that the four-coordinate Nlat can exhibit optimal activity and overcome the performance limitation, while less coordinated Nlat fails. This work provides, arguably for the first time, an in-depth theoretical insight into the activity and stability paradox of MNs for NRR and underlines the importance of reaction kinetic assessment in comparison with the prevailing simple thermodynamic analysis.

Surface ion-exchange induced Bi-rich Bi12O17Cl2/BiOCOOH/Bi2MoO6 heterojunction with oxygen vacancies for enhanced photocatalytic nitrogen fixation

Photocatalytic nitrogen reduction presents a promising technology for environmentally friendly ammonia synthesis under mild conditions. However, the performance of photocatalysts is often hindered by high charge carrier recombination and insufficient adsorption and activation of nitrogen molecules. A novel Bi12O17Cl2/BiOCOOH/Bi2MoO6 heterojunction photocatalyst was successful synthesis through surface ion-exchange technique by potassium chloride. The introduction of Bi-rich Bi12O17Cl2 on BiOCOOH/Bi2MoO6 heterojunction not only improves electron transfer efficiency due to the dual type-II heterojunction effect but also facilitates on-site N2 adsorption and activation for solar ammonia production by promoting oxygen vacancies. The high photocatalytic nitrogen fixation activity of the Bi12O17Cl2/BiOCOOH/Bi2MoO6 was 107.78 μmol g−1 h−1 without any sacrificial agent. DFT calculations indicate that oxygen vacancies serve as the active site for adsorbed N2, the hydrogenation process preferentially forms on the distal N2 atom with a lower activation energy about 0.6 eV. Therefore, the combination of oxygen vacancies and band engineering accelerates the production of ammonia. This endeavor holds the potential to unleash the capabilities of engineering for Bi-rich heterojunctions materials with oxygen vacancies, thereby achieving high-efficiency solar-driven N2-to-NH3 conversion in aqueous environments.

Synergistic effects of CQDs and oxygen vacancies on CeO2 photocatalyst for efficient photocatalytic nitrogen fixation

The combination of carbon quantum dots (CQDs) with oxygen vacancies (OVs) provides a promising approach to achieving efficient photocatalytic nitrogen fixation. It was by a hydrothermal method that CQDs-modified CeO2 composites (CQDs/CeO2) were synthesized. The synergistic effects of CQDs and OVs on CeO2 photocatalyst enhanced interfacial electron transfer capabilities, thus the photocatalytic nitrogen fixation was significantly enhanced. CQDs/CeO2 exhibited the most superior efficiency in photocatalytic nitrogen fixation, reaching 826 μmol·g−1·h−1, when the addition of CQDs was 200 µL. It was the introduction of CQDs that effectively regulated the concentration of OVs. OVs captured the electrons, thus promoting electron transfer. This enhancement not only improved the electron-hole separation efficiency in CQDs/CeO2 but also greatly promoted the adsorption and activation of N2 by OVs. This work offered a novel strategy for designing efficient photocatalysts for nitrogen fixation.

Modulating the Leverage Relationship in Nitrogen Fixation Through Hydrogen‐Bond‐Regulated Proton Transfer

In the electrochemical nitrogen reduction reaction (NRR), a leverage relationship exists between NH3‐producing activity and selectivity because of the competing hydrogen evolution reaction (HER), which means that high activity with strong protons adsorption causes low product selectivity. Herein, we design a novel metal‐organic hydrogen bonding framework (MOHBF) material to modulate this leverage relationship by a hydrogen‐bond‐regulated proton transfer pathway. The MOHBF material was composited with reduced graphene oxide (rGO) to form a Ni‐N2O2 molecular catalyst (Ni‐N2O2/rGO). The unique structure of O atoms in Ni‐O‐C and N‐O‐H could form hydrogen bonds with H2O molecules to interfere with protons being directly adsorbed onto Ni active sites, thus regulating the proton transfer mechanism and slowing the HER kinetics, thereby modulating the leverage relationship. Moreover, this catalyst has abundant Ni‐single‐atom sites enriched with Ni‐N/O coordination, conducive to the adsorption and activation of N2. The Ni‐N2O2/rGO exhibits simultaneously enhanced activity and selectivity of NH3 production with a maximum NH3 yield rate of 209.7 μg h−1 mgcat.−1 and a Faradaic efficiency of 45.7%, outperforming other reported single‐atom NRR catalysts.

Predicting Small Molecule Activations Including Dinitrogen Based on an Inorganic Benzene B4N2 Framework.

Although main group species have emerged in the field of dinitrogen activation in recent years, the reported examples are particularly rare in comparison with transition metal complexes due to their significant challenges. Herein, we demonstrate a [4 + 2] cycloaddition reaction of N2 (with an activation energy as low as 12.5 kcal mol-1) initiated by an inorganic benzene via density functional theory calculations. Such N2 activation is supported by the elongated nitrogen-nitrogen bond distance (dNN), decreased vibration frequency (νNN), and weakened Wiberg bond index (WBINN). Subsequently, the "push-pull" electronic effect, formed by introducing a Lewis acid, HB(C6F5)2, facilitates the generation of thermodynamically more stable products. In addition, this inorganic benzene could also be used to activate a series of small molecules, including carbon dioxide, acetylene, ethylene, and acetonitrile with reaction barriers ranging from 4.7 to 11.6 kcal mol-1. Our findings provide an alternative approach to N2 activation and functionalization, theoretically validating the feasibility of the dual Lewis acid strategy for dinitrogen activation.

Multiple Strategies for Producing and Utilizing Ammonia Using Electrochemical Methods Using Transition Metal Based Electrocatalysts

Ammonia is one of the most important chemicals in fertilizer and industrial chemical production, stands as the world’s second-largest synthetic chemical (> 200 million tons). Additionally, its potential as a clean energy source is gaining attentions due to its carbon neutrality, high energy density, and ease of transport. Electrochemical ammonia production emerges as a 'green' method which minimize the carbon footprint. However, the development of effective systems to produce and utilize 'green' ammonia remains a significant challenge [1,2].To enhance the efficiency of electrochemical systems, modifications in both catalyst and system design are mandatory. This presentation will introduce the electrochemical nitrogen reduction reaction (eNRR) using Cu-based catalysts [3]. For catalyst modification, we addressed copper’s challenge of its low affinity and activation potential for stable dinitrogen (N2) molecules. This was achieved through phosphorus activation, which enhance the adsorption and activation of N2 by creating electron-deficient Cu sites on the catalyst surface. System-wise, the poor solubility of nitrogen in aqueous electrolytes limits it’s the performance of the system. Addressing this, we applied a gas diffusion electrode (GDE) coated with polytetrafluoroethylene (PTFE) into the eNRR system. This setup establishes an efficient three-phase boundary among liquid water, gaseous N2, and the solid catalyst, thus facilitating nitrogen access to the catalytic sites. Our novel catalyst in a flow-type cell attains a Faradaic efficiency of 13.15% and an ammonia production rate of 7.69 μg h-1 cm-2 at -0.2 VRHE. These figures represent 3.56- and 59.2-times improvements, respectively, over a pristine Cu electrode in a conventional electrolytic cell.Additionally, the presentation will provide a brief introduction to the amorphous Ni-Cu electrocatalyst for electrochemical ammonia oxidation, which is an essential reaction for ammonia utilization [4]. We highlight its doubled ammonia removal efficiency compared to the Ni-Cu LDH electrocatalyst, underlining the potential of these developments.[1] Y. Zeng, C. Priest, G. Wang, G. Wu, Restoring the Nitrogen Cycle by Electrochemical Reduction of Nitrate: Progress and Prospects, Small Methods, 4 (2020) 2000672.[2] B. Yang, W. Ding, H. Zhang, S. Zhang, Recent Progress in Electrochemical Synthesis of Ammonia from Nitrogen: Strategies to Improve the Catalytic Activity and Selectivity, Energy Environ. Sci., 14 (2021) 672-687.[3] J. Kim, C.H. Lee, Y.H. Moon, M.H. Lee, E.H. Kim, S.H. Choi, Y.J. Jang, J.S. Lee, Enhancing Ammonia Production Rates from Electrochemical Nitrogen Reduction by Engineering Three-Phase Boundary with Phosphorus-Activated Cu Catalysts, J. Energy Chem., 84 (2023) 394-401.[4] J.H. Jang, S.Y. Park, D.H. Youn, Y.J. Jang. Recent Advances in Electrocatalysts for Ammonia Oxidation Reaction.Catalysts., 13 (2023) 803.

Schottky-Barrier-Free Plasmonic Photocatalysts for Solar-Driven Ammonia Synthesis

Ammonia is not only an indispensable feedstock to produce essential industrial chemicals like fertilizers, but it is also a potential hydrogen energy carrier owing to its high energy density and hydrogen capacity. Currently, most ammonia required for humans comes from the Haber–Bosch process, which contributes more than 90% of the annual global ammonia output. However, the harsh reaction conditions of this process result in serious energy and environmental issues. According to the relevant statistics, the ammonia industry consumes about 2% of the total energy and generates about 1.6% of the total CO2 emission annually in the world.It is therefore crucial to develop a green and sustainable strategy for ammonia synthesis. Among various alternative technologies of the Haber–Bosch process, artificial photosynthesis has attracted wide research interests and has been regarded as a promising, environmentally friendly, and energy-saving process. Photocatalytic ammonia synthesis can utilize renewable solar energy to drive the reaction and operate under mild conditions, which possesses the benefits of the simple reaction system, low cost, and safety. Although numerous photocatalysts have been explored over the past several decades, their activity and efficiency have still been low and far from the practical application requirements. It is therefore a huge challenge to design efficient photocatalysts for ammonia synthesis.Localized surface plasmon resonance (LSPR) has recently drawn enormous attention. LSPR can not only endow plasmonic nanomaterials with large absorption cross-sections and intense local field enhancement but also generate energetic hot electrons and hot holes. Owing to the appealing features of LSPR, plasmonic photocatalysis has shown great potential in ammonia synthesis. Plasmonic photocatalysts are usually composed of noble metals and wide bandgap semiconductors. However, the Schottky barrier will form at the metal–semiconductor interface in such hybrid structures due to the mismatch of the band structures between plasmonic metals and semiconductors. Hence, a large portion of the hot electrons cannot cross the Schottky barrier and get lost in the metal through the recombination with the holes. To overcome the drawbacks caused by the Schottky barrier, it is strongly desirable to develop a new type of plasmonic semiconductor photocatalysts, i.e., Schottky-barrier-free plasmonic photocatalyst.The Schottky-barrier-free plasmonic photocatalysts are usually degenerately doped semiconductors containing lattice oxygen vacancies. They possess high free electron densities and strong plasmon resonance because of the high-level doping, which results in abundant hot charge carriers upon plasmon excitation. Moreover, they also have appropriate surface sites that can adsorb reactant molecules and activate them. The photogenerated plasmonic hot charge carriers can get trapped at the defect sites and thereafter drive redox reactions. Compared with traditional metal–semiconductor hybrid plasmonic photocatalysts, Schottky-barrier-free plasmonic photocatalysts enable hot charge carriers to be transported nearly freely in the material, resulting in high charge utilization and photocatalytic efficiency.We have demonstrated the synthesis of Schottky-barrier-free plasmonic MoO3−x nanospheres (NSs) with remarkably high oxygen vacancy (OV) contents by aerosol-spray. The MoO3−x NSs can achieve efficient ammonia photosynthesis through a “killing two birds with one stone” mechanism. On the one hand, the OVs can serve as chemisorption sites for N2 molecules and participate in their activation. On the other hand, the introduction of OVs generates a large number of free electrons near the conduction band, which induces a broad absorption band from the visible to the NIR region owing to the plasmon resonance. The Schottky-barrier-free nature of this plasmonic semiconductor allows free transportation of the plasmonic hot charge carriers, while the defect states introduced by the OVs can effectively trap the hot electrons to prevent rapid electron–hole recombination. As expected, the as-prepared material exhibits a superior ammonia production rate, with the highest apparent quantum efficiency (AQE) of 1.24% at 808 nm and an appreciable solar-to-ammonia conversion efficiency (SACE) of 0.057% for NH3 production. Besides MoO3−x , WO3−x is also an excellent candidate for Schottky-barrier-free plasmonic photocatalysts. We have synthesized square WO3−x nanoplates hydrothermally. The WO3-x nanoplates possess regular morphology, wide light absorption, and great ammonia production performance. Furthermore, by introducing doping elements (Mo, V, Fe, Bi) into WO3−x nanoplates, the N2 activation and dissociation can be improved by refining the defect states. The Mo-doped WO3−x nanoplates exhibit the highest ammonia production rate of 145.50 μmol g−1 h−1 with SACE of 0.011% since the Mo species can promote the chemisorption and polarization of N2 molecules and significantly reduce the reaction barrier. Notably, Mo-doped WO3−x nanoplates are a dual-function photocatalyst that can oxidize water to hydrogen peroxide in a stoichiometric ratio while synthesizing ammonia. Figure 1

The synergistic effect of Cl doping and Bi coupling to promote the carrier separation of BiOBr for efficient photocatalytic nitrogen reduction

Photocatalytic nitrogen reduction reaction (NRR) is a sustainable process for ammonia synthesis under mild conditions. However, photocatalytic NRR activity and are generally limited by inefficient carrier separation and transfer. Therefore, parallel engineering of bulk phase doping and surface coupling is critical to achieving the goal of efficient NRR. In this study, Cl doped BiOBr nanosheet assemblies (BiOBr/Cl) were constructed in delicately designed deep eutectic solvents (DESs), combined with ionothermal methods at low temperatures and Bi3+ exsolution reduction strategy at high temperatures. The unique liquid state and reducibility of DESs induce the reduction of Bi3+ and the in situ coupling of Bi quantum dots at the surface of BiOBr/Cl nanosheets along with the construction of Bi-BiOBr/Cl nanosheet assemblies. The experimental results show that Cl doping could reduce the exciton dissociation energy and promote its dissociation to free carriers. Bi quantum dots could form tightly coupled Schottky junction with BiOBr/Cl enabling the efficient and unidirectional transmission of photogenerated electrons from BiOBr/Cl to metal Bi. The formed electron deficient region at Schottky interface promotes the adsorption and activation of N2. The hierarchical structure of Bi-BiOBr/Cl nanosheet assembly benefits to providing more N2 adsorption active sites. The DFT calculation shows that the accumulation of high concentration of active hydrogen in Bi-BiOBr/Cl leads to a significant decrease of energy barrier of the first step hydrogenation of N2. Bi-BiOBr/Clis more inclined to adsorb nitrogen for NRR in comparison with H* for hydrogen production. The synergistic effect of Cl doping and Bi coupling result in a high NRR activity of Bi-BiOBr/Cl photocatalyst of 6.67 mmol·g−1·h−1, which was 11.3 times higher than that of initial BiOBr. This study provides a promising strategy for designing highly active NRR photocatalysts with high efficiency carrier dissociation and transport.

Kinetic evaluation of ammonia synthesis mechanism over molybdenum-based catalysts: Effects of oxygen and nitrogen vacancies

The synthesis of NH3 from N2 and H2 is a significant chemical process, because essential products such as fertilizers and industrial chemicals can be obtained from NH3. In the catalytic synthesis of NH3, N2 activation is generally considered a kinetically relevant step (KRS). Therefore, advanced heterogeneous catalysts that promote N2 conversion have been extensively investigated. In this study, the roles of O and N vacancies (Ov and Nv) of Mo oxide formed during reduction were kinetically evaluated and compared using structural characterization, isotope experiments, kinetic studies, and density functional theory calculations. On the Mo oxide catalyst with Ov, NH3 was formed by a dissociative mechanism, and the surface reaction involving the adsorbed N* and H* was considered the KRS. However, on the catalyst with Nv, NH3 was formed by an associative mechanism, and N–N dissociation of N2H2* species by H* to produce NH3 was considered the KRS. The catalyst with Ov resulted in an NH3 synthesis rate of 5.6 mmol gMo−1 h−1 under relatively mild conditions (1.0 MPa and 400 °C), thus outperforming the catalyst with Nv (0.3 mmol gMo−1 h−1). These results provide insight into the importance of anionic vacancies in reducible metal oxides for NH3 synthesis.

Heteropoly blue-modified ultrathin bismuth oxychloride nanosheets with oxygen vacancies for efficient photocatalytic nitrogen fixation in pure water

Photocatalytic nitrogen reduction is a promising green technology for ammonia synthesis under mild conditions. However, the poor charge transfer efficiency and weak N2 adsorption/activation capability severely hamper the ammonia production efficiency. In this work, heteropoly blue (r-PW12) nanoparticles are loaded on the surface of ultrathin bismuth oxychloride nanosheets with oxygen vacancies (BiOCl-OVs) by electrostatic self-assembly method, and a series of xr-PW12/BiOCl-OVs heterojunction composites have been prepared. Acting as a robust support, ultrathin two-dimensional (2D) structure of BiOCl-OVs inhibits the aggregation of r-PW12 nanoparticles, enhancing the interfacial contact between r-PW12 and BiOCl. More importantly, the existence of oxygen vacancies (OVs) provides abundant active sites for efficient N2 adsorption and activation. In combination of the enhanced light absorption and promoted photogenerated carriers separation of xr-PW12/BiOCl-OVs heterojunction, under simulated solar light, the optimal 7r-PW12/BiOCl-OVs exhibits an excellent photocatalytic N2 fixation rate of 33.53 µmol g-1h−1 in pure water, without the need of sacrificial agents and co-catalysts. The reaction dynamics is also monitored by in situ FT-IR spectroscopy, and an associative distal pathway is identified. Our study demonstrates that construction of heteropoly blues-based heterojunction is a promising strategy for developing high-performance N2 reduction photocatalysts. It is anticipated that combining of different defects with heteropoly blues of different structures might provide more possibilities for designing highly efficient photocatalysis systems.

Unraveling the Active Sites on Mesoporous CuFe2O4@N-Carbon Catalysts with Abundant Oxygen Vacancies and M-N-C Content for Boosted Nitrogen Reduction Toward Electrosynthesis of Ammonia.

Transition metal centers dispersed over nitrogen-doped carbon (M-NC) supports have been widely explored for electrocatalytic reactions; however, sparsely reported for electrochemical nitrogen reduction reaction (ENRR). Particularly, the single-atom catalysts (SACs) have shown reasonable ammonia yield rate and faradaic efficiency (FE), but their complex synthesis and low durability for long-term electrocatalysis runs restrict their use on a larger scale. Importantly, the catalytic active sites in metal nanostructured-based M-NC catalysts toward enhanced N2 adsorption and activation are still not clear as they are highly challenging to reveal. A few studies have predicted that the surface oxygen vacancies (Ovac) favor an enhanced ENRR performance. Herein, a strategy using tailored M-NC content and Ovac in a single catalyst for enhanced ammonia electrosynthesis is devised. A mesoporous bimetallic spinel oxide (CuFe2O4) supported over N-doped carbon (CuFe2O4@NC) derived from Prussian blue analog (PBA) via controlled pyrolysis possess is found to show boosted ENRR activity. Moreover, operando NH3 formation over the catalyst is observed using four electrode set up. This approach enables rapid evaluation ofelectrocatalytic efficacy and avoids false positive results. The rotating disc electrode results reveal that mass transport in acidic media and surface absorption in alkline media primarily regulate ENRR over CuFe2O4@NC electrocatalyst.

One Iron for Two Iron Sites in a Metal-Organic Framework Toward Simultaneous N2-H2 Activation under Mild Conditions.

Iron-based catalysts play an important role in the ammonia industry. As one of the most abundant iron minerals, Fe3O4 containing FeII and FeIII sites is widely distributed in the earth's crust and even on exoplanets, theoretically giving it both economic and catalytic potentials in ammonia synthesis. However, in the absence of specific active co-catalyst and harsh conditions, Fe3O4 is impossible to achieve ammonia synthesis alone. Here, we designed to activate the relatively inert FeII and FeIII sites in Fe3O4 with a third FeIII site inlayed in a coordination framework (MIL-101(Fe)) to achieve the unpresented multi-site collaborative catalysis. In-depth mechanism study confirmed the roles of three different Fe sites in N2 activation, H2 activation, and product transfer, respectively. Efficient N2-H2 activation to NH3 on the Fe3O4-based catalytic system has been achieved at extremely mild conditions. Our research provides a theoretical basis and a new strategy for designing efficient non-noble metal-based ammonia synthesis catalyst with minimized energy consumption.

Unraveling the Photocatalytic Mechanism of N2 Fixation on Single Ruthenium Sites.

Photocatalytic N2 fixation offers promise for ammonia synthesis, yet traditional photocatalysts encounter challenges such as low efficiency and short carrier lifetimes. Atomically precise ligand-metal nanoclusters emerge as a solution to address these issues, but the photophysical mechanism remains elusive. Inspired by the synthesis of Au4Ru2 NCs, we investigate the mechanism behind N2 activation on Au4Ru2, focusing on photoactivity and carrier dynamics. Our results reveal that vibration of the Ru-N bond in the low-frequency domain suppresses the deactivation process leading to a long lifetime of the excited N2. By the strategy of isoelectronic substitution, we identify the single Ru sites as the active sites for N2 activation. Furthermore, these ligand-protected M4Ru2 (M = Au, Ag, Cu) NCs show robust thermal stability in explicit solvation and decent photochemical activity for N2 activation and NH3 production. These findings have significant implications for the optimization of catalysts for sustainable ammonia synthesis.

Electrochemical co-reduction of N2 and CO2 to urea using In2S3 anchored on S-doped reduced graphene oxide

The generation of urea by electrocatalysis using N2 and CO2 as feedstock is considered as a promising and green sustainable alternative to conventional energy-consuming industrial processes. Based on the existing studies, it was found that the effective adsorption and activation of N2 and CO2 on the catalyst surface is the primary condition for electrocatalytic generation, while the main by-product hydrogen evolution reaction (HER). Should be avoided as much as possible in order to improve the yield. In this paper, an In2S3-loaded electrocatalyst on sulfur-doped reduced graphene oxide for urea synthesis was designed and prepared with an average Faraday efficiency (FE) of 37.17 % and a urea yield of 7.24 mmol h−1 g−1, while maintaining long-term stability. In2S3@S-RGO has a unique porous structure of doped graphene, with continuous micron-sized pores that facilitate fast mass transfer, improve kinetic efficiency, and expand electroactive sites, thus enhancing the urea synthesis performance. Interestingly, the In2S3@S-RGO electrocatalyst not only exhibits excellent electrochemical performance, but also facilitates the adsorption and activation of reactants (N2 and CO2) due to the inhibitory effect of the indium-based material on HER. This work may provide new ideas for advanced catalysts to improve the efficiency of electrochemical synthesis of urea Faraday and to facilitate CN coupling applications.

Engineered Electron-Deficient Sites at Boron-Doped Strontium Titanate/Electrolyte Interfaces Accelerate the Electrocatalytic Reduction of N2 to NH3: A Combined DFT and Experimental Electrocatalysis Study.

The development of an efficient, selective, and durable catalysis system for the electrocatalytic N2 reduction reaction (ENRR) is a promising strategy for the sustainable production of ammonia. The high-performance ENRR is limited by two major challenges: poor adsorption of N2 over the catalyst surface and abysmal N2 solubility in aqueous electrolytes. Herein, with the help of our combined density functional theory (DFT) calculations and experimental electrocatalysis study, we demonstrate that concurrently induced electron-deficient Lewis acid sites in an electrocatalyst and in an electrolyte medium can significantly boost the ENRR performance. The DFT calculations, ex situ X-ray photoelectron and FTIR spectroscopy, electrochemical measurements, and N2-TPD (temperature-programmed desorption) over boron-doped strontium titanate (BSTO) samples reveal that the Lewis acid-base interactions of N2 synergistically enhance the adsorption and activation of N2. Besides, the B-dopant induces the defect sites (oxygen vacancies and Ti3+) that assist in enhanced N2 adsorption and results in suppressed hydrogen evolution due to B-induced electron-deficient sites for H+ adsorption. The insights from the DFT study evince that B prefers the Srtop position (on top of Sr) where N2 adsorbs in an end-on configuration, which favors the associative alternating pathway and suppresses the competitive hydrogen evolution. Thus, our combined experimental and DFT study demonstrates an insight toward enhancing the ENRR performance along with the suppressed hydrogen evolution via concurrently engineered electron-deficient sites at electrode and electrolyte interfaces.