N2 Activation Research Articles

Toward Designing Reactive Metal Clusters for Dinitrogen Activation: A Guideline Based on N2 Initial Adsorption.

Gas-phase metal clusters are ideal models to explore transition-metal-mediated N2 activation mechanism. However, the effective design and search of reactive clusters in N2 activation are currently hindered by the lack of clear guidelines. Inspired by the Sabatier principle, we discovered in this work that N2 initial adsorption energy (ΔEads) is an important parameter to control the N2 activation reactivity of metal clusters in the gas phase. This mechanistic insight obtained from high-level calculations rationalizes the N2 activation reactivity of many previously reported metal clusters when combined with the known factor determining the N≡N cleavage process. Furthermore, based on this guideline of ΔEads, we successfully designed several new reactive clusters for cleaving N≡N triple bond under mild conditions, including FeV2S2-, TaV2C2-, and TaV2C3-, the high N2 activation reactivity of which has been fully corroborated in our gas phase experiments employing mass spectrometry with collision-induced dissociation. The importance of ΔEads revealed in this work not only reshapes our understanding of N2 activation reactions in the gas phase but also could have implication for other N2 activation processes in the condensed phase. The more general establishment of this new perspective on N2 activation reactivity warrants future experimental and computational studies.

Iron Monomers or Trimers on Nitrogen-Doped Carbon: Which Is Better for the Electrocatalytic Nitrogen Reduction Reaction?

The electrocatalytic nitrogen reduction reaction (NRR) presents an alternative method for the Haber-Bosch process, and single-atom catalysts (SACs) to achieve efficient NRR have attracted considerable attention in the past decades. However, whether SACs are more suitable for NRR compared to atomic-cluster catalysts (ACCs) remains to be studied. Herein, we have successfully synthesized both the Fe monomers (Fe1) and trimers (Fe3) on nitrogen-doped carbon catalysts. Both the experiments and DFT calculations indicate that compared to the end-on adsorption of N2 on Fe1 catalysts, N2 activation is enhanced via the side-on adsorption on Fe3 catalysts, and the reaction follows the enzymatic pathway with a reduced free energy barrier for NRR. As a result, the Fe3 catalysts achieved better NRR performance (NH3 yield rate of 27.89 μg h-1 mg-1cat. and Faradaic efficiency of 45.13%) than Fe1 catalysts (10.98 μg h-1 mg-1cat. and 20.98%). Therefore, our research presents guidance to prepare more efficient NRR catalysts.

Dinitrogen Activation within Frustrated Lewis Pairs Is Promoted by Adding External Electric Fields.

This study uses computational means to explore the feasibility of N2 cleavage by frustrated Lewis pair (FLPs) species. The employed FLP systems are phosphane/borane (1) and carbene/borane (2). Previous studies show that 1 and 2 react with H2 and CO2 but do not activate N2. The present study demonstrates that N2 is indeed inert, and its activation requires augmentation of the FLPs by an external tool. As we demonstrate here, FLP-mediated N2 activation can be achieved by an external electric field oriented along the reaction axis of the FLP. Additionally, the study demonstrates that FLP -N2 activation generates useful nitrogen compound, e.g., hydrazine (H2N-NH2). In summary, we conclude that FLP effectively activates N2 in tandem with oriented external electric fields (OEEFs), which play a crucial role. This FLP/OEEF combination may serve as a general activator of inert molecules.

Catalyst-Free Activation and Fixation of Nitrogen by Laser-Induced Conversion.

Ultrafast N2 fixation reactions are quite challenging. Currently used methods for N2 fixation are limited, and strong dinitrogen bonds usually need to be activated via extreme temperature or pressure or by the use of an energy-consuming process with sophisticated catalysts. Herein, we report a novel laser-based chemical method for N2 fixation under ambient conditions without catalysts, this method is called laser bubbling in liquids (LBL), and it directly activates N2 in water (H2O) and efficiently converts N2 into valuable NH3 (max: 4.2 mmol h-1) and NO3- (0.17 mmol h-1). Remarkably, the highest yields of NH3 and NO3- are 4 orders of magnitude greater than the best values for electrocatalysis reported to date. Notably, we further validate the experimental mechanism by using optical emission spectroscopy to detect the production of intermediate plasma and by employing isotope tracing. We also establish that an extremely high-temperature environment far from thermodynamic equilibrium inside a laser-induced bubble and the kinetic process of rapid quenching of bubbles is crucial for N2 activation and fixation to generate NH3 and NOx via LBL. Based on these results, it is shown that LBL is a simple, safe, efficient, green, and sustainable technology that enables the rapid conversion of the renewable feedstocks H2O and N2 to NH3 and NO3-, facilitating new prospects for chemical N2 fixation.

Enhanced Activity and Selectivity for Nitrogen Reduction Reaction in Electrode-based Heterostructures: A DFT Computational Study.

Developing sustainable and efficient catalysts for ammonia synthesis from atmospheric molecular N2 under ambient conditions presents a significant 21st-century challenge. Two-dimensional heterostructures, particularly single-atom catalysts (SACs) supported on two-dimensional materials, have emerged as a promising avenue due to their remarkable catalytic activity and selectivity. Electrides, characterized by an abundance of free electrons and high surface activity, have attracted substantial attention in this context. Through density functional theory (DFT) calculations, this study proposes electride-graphene heterostructures (EGHS) as catalysts to effectively regulate charge distribution at the catalytic center, facilitating the optimization of catalytic performance. The EGHS model addresses challenges related to excessive adsorbate binding, mitigating electron transfer compared to electride monolayer adsorption. This novel approach utilizes heterogeneous heterostructures to finely tune the catalytic site, optimizing electron input for enhanced catalysis.Based on the optimized charge transfer for N2 activation, the Cr-doped EGHS (Cr@EGHS) exhibits a promising performance in the nitrogen reduction reaction, leading to, a relatively low limiting potential of -0.85 V and high selectivity. The hypothesis charge transfer depend on N2 activation is further supported by modulating the distance between component layers of heterostructure. These findings contribute to design principles for 2D heterostructure catalysts and offer a reference for experimental synthesis.

Introduction of element Bi in the P-block promotes N2 activation for efficient electrocatalytic nitrogen reduction to produce ammonia

The synthesis of ammonia through electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions is a presumable strategy. However, it still faces the problems of poor activity and low Faradic efficiency, so the development of efficient, stable and highly selective catalysts is of the importance. Molybdenum disulfide (MoS2) materials are widely used in NRR reactions owing to good electrical conductivity and abundant active sites, but the edge active site is the dual active site of NRR and competitive hydrogen evolution reaction (HER). Encouragingly, the strong interaction between elements in the P-block and N 2p orbitals results in excellent performance in adsorbing and activating nitrogen. Meanwhile, the partial occupation of p-orbitals by elements in the P-block also leads to poor HER activity. Herein, we introduced the P-block element Bi into 1 T-MoS2 and prepared Bi2S3/MoS2 composite. Taking advantage of the high conductivity of 1 T-MoS2 and the N2 adsorption and activation advantages of Bi element, an ammonia yield rate of 54.64 µg h−1 mg−1cat. and a high Faradic efficiency of 58.56 % were obtained. In addition, the active site and the evolution pathway of N2 reduction were investigated by in-situ infrared spectroscopy measurements and density functional theory (DFT). The in-situ infrared spectroscopy measurement results manifested the presence of the main intermediate transition state during the reduction of N2 to NH3. The calculation results indicated that N2 had the best adsorption on the introduced P-block element Bi, and the entire NRR reaction process followed an alternating pathway. This study reveals the synergistic effect of P-block elements and transition metal-based electrocatalytic materials on nitrogen reduction to ammonia synthesis.

Boosting Electrochemical Nitrogen Reduction via Axial Coordination Engineering on Single‐Iron‐Atom Catalysts

AbstractElectrocatalytic nitrogen (N2) reduction reaction (NRR) presents a sustainable alternative to the Haber–Bosch process for ammonia (NH3) synthesis. Iron phthalocyanine (FePc) is demonstrated as a promising catalyst for the electrocatalytic NRR. However, FePc with planar symmetric Fe‐N4 sites exhibits poor N2 adsorption and activation capabilities, resulting in an unsatisfactory NRR performance. Herein, an axial oxygen coordination strategy is developed to optimize the local electron distribution on FePc for improving N2 adsorption and activation. The as‐obtained FePc‐O‐CP shows a superior NH3 yield rate (59.72 µg h−1 mg−1cat.) and a considerable Faradaic efficiency (13.76%) in 0.1 m HCl. Density functional theory (DFT) calculations verify that the axial oxygen ligand on FePc inhibits the adsorption of H+ and enhances the N2 adsorption and activation, thereby greatly promoting NH3 generation. This work reveals the significance of regulating the local coordination environment of single‐atom catalysts for improving electrocatalytic NRR performance and provides a feasible strategy for the rational design of atomic‐scale active sites.

Effects of Nanowire Doping on Plasmon-Enhanced N2 Dissociation.

Doping a transition metal element into plasmonic systems can tune the optical properties of the system, which will potentially facilitate the plasmon-enhanced catalytic process. In this study, we applied the linear-response time-dependent density functional theory (LR-TDDFT) method with real-time electron dynamics and mean-field Ehrenfest dynamics methods to computationally investigate the effects of doping silver nanowires on plasmon-enhanced N2 dissociation. We calculated the absorption spectra for different doped systems, applied an external electric field to the system, and performed mean-field Ehrenfest dynamics to examine how plasmonic excitation will affect the N2 activation or dissociation. In addition, we also studied how the transition metal dopant affects the system's electronic structure and potential energy surface.

Vacancy Rich TiB2 Nanosheets Promote Electrochemical Ammonia Synthesis.

The ability to exfoliate transition metal diborides has led to a renewed interest in their prospect to be applied as catalysts for electrochemical reactions. This is due to an enhanced access to the unprecedented interfaces these nanomaterials offer. In this work, we show that nanosheets exfoliated from TiB2 exhibit vacancies that facilitate an excellent interface for catalyzing nitrogen reduction reaction (NRR). We found that these nanosheets demonstrate a high selectivity toward NH3 because of their abiity to preferentially chemisorb and activate N2. These nanosheets exhibit a superlative NH3 yield of 318 μg h-1 cm-2 at -0.2 V versus RHE with a faradaic efficiency of 57%. We also found how the relative ratios of Ti and B atoms in these nanosheets can affect the NH3 yield and faradaic efficiency. We supplement these results with DFT studies that indicate that it is the creation of frustrated Lewis pairs along with the Ti-B synergy that induces a push-and-pull effect; this in turn favors N2 activation and lowers the energy barrier for NRR. Furthermore, we explored B-exposed and Ti-exposed surfaces to understand how different surfaces affect the reaction yield and efficiency and found that Ti-exposed surfaces with boron divacancy have the highest propensity for NRR. The maiden insights presented in this study on the role of transition metal-boron synergy and interfaces present significant additions to the fast-expanding knowledge on nanoscaled metal borides.

Schottky Junctions with Bi@Bi2MoO6 Core-Shell Photocatalysts toward High-Efficiency Solar N2-to-Ammonnia Conversion in Aqueous Phase.

The photocatalytic nitrogen reduction reaction (NRR) in aqueous solution is a green and sustainable strategy for ammonia production. Nonetheless, the efficiency of the process still has a wide gap compared to that of the Haber-Bosch one due to the difficulty of N2 activation and the quick recombination of photo-generated carriers. Herein, a core-shell Bi@Bi2MoO6 microsphere through constructing Schottky junctions has been explored as a robust photocatalyst toward N2 reduction to NH3. Metal Bi self-reduced onto Bi2MoO6 not only spurs the photo-generated electron and hole separation owing to the Schottky junction at the interface of Bi and Bi2MoO6 but also promotes N2 adsorption and activation at Bi active sites synchronously. As a result, the yield of the photocatalytic N2-to-ammonia conversion reaches up to 173.40 μmol g-1 on core-shell Bi@Bi2MoO6 photocatalysts, as much as two times of that of bare Bi2MoO6. This work provides a new design for the decarbonization of the nitrogen reduction reaction by the utilization of renewable energy sources.

Evolution of Oxygen Vacancy Sites in Ceria-Based High-Entropy Oxides and Their Role in N2 Activation.

In this work, a relatively new class of materials, rare earth (RE) based high entropy oxides (HEO) are discussed in terms of the evolution of the oxygen vacant sites (Ov) content in their structure as the composition changes from binary to HEO using both experimental and computational tools; the composition of HEO under focus is the CeLaPrSmGdO due to the importance of ceria-related (fluorite) materials to catalysis. To unveil key features of quinary HEO structure, ceria-based binary CePrO and CeLaO compositions as well as SiO2, the latter as representative nonreducible oxide, were used and compared as supports for Ru (6 wt % loading). The role of the Ov in the HEO is highlighted for the ammonia production with particular emphasis on the N2 dissociation step (N2(ads) → Nads) over a HEO; the latter step is considered the rate controlling one in the ammonia production. Density functional theory (DFT) calculations and 18O2 transient isotopic experiments were used to probe the energy of formation, the population, and the easiness of formation for the Ov at 650 and 800 °C, whereas Synchrotron EXAFS, Raman, EPR, and XPS probed the Ce-O chemical environment at different length scales. In particular, it was found that the particular HEO composition eases the Ov formation in bulk, in medium (Raman), and in short (localized) order (EPR); more Ov population was found on the surface of the HEO compared to the binary reference oxide (CePrO). Additionally, HEO gives rise to smaller and less sharp faceted Ru particles, yet in stronger interaction with the HEO support and abundance of Ru-O-Ce entities (Raman and XPS). Ammonia production reaction at 400 °C and in the 10-50 bar range was performed over Ru/HEO, Ru/CePrO, Ru/CeLaO, and Ru/SiO2 catalysts; the Ru/HEO had superior performance at 10 bar compared to the rest of catalysts. The best performing Ru/HEO catalyst was activated under different temperatures (650 vs 800 °C) so to adjust the Ov population with the lower temperature maintaining better performance for the catalyst. DFT calculations showed that the HEO active site for N adsorption involves the Ov site adjacent to the adsorption event.

Efficient ammonia synthesis from the air using tandem non-thermal plasma and electrocatalysis at ambient conditions

While electrochemical N2 reduction presents a sustainable approach to NH3 synthesis, addressing the emission- and energy-intensive limitations of the Haber-Bosch process, it grapples with challenges in N2 activation and competing with pronounced hydrogen evolution reaction. Here we present a tandem air-NOx-NOx−-NH3 system that combines non-thermal plasma-enabled N2 oxidation with Ni(OH)x/Cu-catalyzed electrochemical NOx− reduction. It delivers a high NH3 yield rate of 3 mmol h−1 cm−2 and a corresponding Faradaic efficiency of 92% at −0.25 V versus reversible hydrogen electrode in batch experiments, outperforming previously reported ones. Furthermore, in a flow mode concurrently operating the non-thermal plasma and the NOx− electrolyzer, a stable NH3 yield rate of approximately 1.25 mmol h−1 cm−2 is sustained over 100 h using pure air as the intake. Mechanistic studies indicate that amorphous Ni(OH)x on Cu interacts with hydrated K+ in the double layer through noncovalent interactions and accelerates the activation of water, enriching adsorbed hydrogen species that can readily react with N-containing intermediates. In situ spectroscopies and density functional theory (DFT) results reveal that NOx− adsorption and their hydrogenation process are optimized over the Ni(OH)x/Cu surface. This work provides new insights into electricity-driven distributed NH3 production using natural air at ambient conditions.

Bird's-Eye View of the Activity Distribution on a Catalyst Surface via a Machine Learning-Driven Adequate Sampling Algorithm.

Rational design of catalysts relies on a deep understanding of the active centers. The structure and activity distribution of active centers on a surface are two of the central issues in catalysis and important targets of theoretical and experimental investigations. Herein, we report a machine learning-driven adequate sampling (MLAS) framework for obtaining a statistical understanding of the chemical environment near catalyst sites. Combined strategies were implemented to achieve highly efficient sampling, including the decomposition of degrees of freedom, stratified sampling, Gaussian process regression, and well-designed constraint optimization. The MLAS framework was applied to the rate-determining step in NH3 synthesis, namely the N2 activation process. We calculated the produced population function, PA, which provides a comprehensive and intuitive understanding of active centers. The MLAS framework can be broadly applied to other more complicated catalyst materials and reaction networks.

Boosting Electrocatalytic N2 Reduction to NH3 by Enhancing N2 Activation via Interaction between Au Nanoparticles and MIL-101(Fe) in Neutral Electrolytes.

Electrocatalytic nitrogen reduction reaction (NRR) has attracted much attention as a sustainable ammonia production technology, but it needs further exploration due to its slow kinetics and the existence of competitive side reactions. In this research, xAu/MIL-101(Fe) catalysts were obtained by loading gold nanoparticles (Au NPs) onto MIL-101(Fe) using a one-step reduction strategy. Herein, MIL-101(Fe), with high specific surface area and strong N2 adsorption capacity, is used as a support to disperse Au NPs to increase the electrochemical active surface area. Au NPs, with a high NRR activity, is introduced as the active site to promote charge transfer and intermediate formation rates. More importantly, the strong interaction between Au NPs and MIL-101(Fe) enhances the electron transfer between Au NPs and MIL-101(Fe), thereby enhancing the activation of N2 and achieving efficient NRR. Among the prepared catalysts, 15 %Au/MIL-101(Fe) has the highest NH3 yield of 46.37 μg h-1 mg-1 cat and a Faraday efficiency of 39.38 % at -0.4 V (vs. RHE). In-situ FTIR reveals that the NRR mechanism of 15 %Au/MIL-101(Fe) follows the binding alternating pathway and also indicates that the interaction between Au NPs and MIL-101(Fe) strengthens the activation of the N≡N bond in the rate-limiting process, thereby accelerating the NRR process.

Regulating concentration of surface oxygen vacancies in Bi2MoO6/Bi-MOF for boosting photocatalytic ammonia synthesis

Surface oxygen vacancies (OVs) engineering has been widely adopted as an effective strategy to enhance photocatalytic performance. At present, photocatalytic systems capable of precisely regulating surface OVs concentrations, which could help illuminate the effects of the surface OVs concentration on N2 fixation activity, are still scarce. Herein, bismuth-based metal organic framework (Bi-MOF) was loaded onto the surface of Bi2MoO6 (BMO) as an operable platform, and the OVs concentration in the Bi-MOF component of BMO/Bi-MOF could be regulated by reduction of bismuth ions therein. Experimental results confirm that optimum construction of OVs in the Bi-MOF promotes the photoelectrons transfer from BMO to Bi-MOF, facilitating the activation of N2 at OVs. Consequently, the optimized catalyst shows superior performance in NH3 production, which reaches 125.78 μmol h−1 g−1, 21.4 higher than that of BMO. This work underline the significance of regulating surface OVs concentration, providing inspiration for the development of efficient OVs-modified photocatalysts.

Trapping of a Terminal Intermediate in the Boron-Mediated Dinitrogen Reduction: Mono-, Tri-, and Tetrafunctionalized Hydrazines in Two Steps from N2.

The addition of chlorotrimethylsilane to a boron-mediated, transition-metal-free N2 activation reaction leads to the isolation of multiple potassium boryl(silyl)hydrazido species, likely trapping products of a terminal dinitrogen complex of boron. One of these silylated N2 species can be protonated or methylated, providing access to mono- to tetrafunctionalized hydrazines in two steps from N2 and in the absence of transition metals.

Promoting nitrogen photofixation for the synthesis of ammonia using oxygen-vacant Fe2O3/ZrO2 visible light photocatalyst with straddling heterojunction and enhanced charge transfer

Photocatalytic nitrogen (N2) fixation is a promising and environmentally friendly alternative approach to the energy-intensive Haber-Bosch process to produce green ammonia (NH3) with zero carbon emissions. However, the unique setbacks rest on developing an active photocatalyst with an accelerated charge transfer that could efficiently adsorb and activate the chemically inert N2 into useful NH3. Herein, an oxygen-vacant Fe2O3/ZrO2 photocatalyst with straddling heterojunction was successfully synthesised by the hydrothermal method followed by calcination at 450 °C. The addition of oxygen vacancy-inducing ferromagnetic material on ZrO2 increased the adsorption and activation of N2, broadened the solar absorption window (680 nm extending to 910 nm). It also accelerated light-induced charge separation of the photocatalyst thereby greatly enhancing the production of NH3 (1.301 mmol h−1 g−1) with about a 7-fold increase in comparison to ZrO2 at ambient conditions under sunlight irradiation. This work therefore sheds light on the effect of oxygen vacancies and the flow of charge carriers in the effective photofixation of N2 to NH3 synthesis through a sustainable route.

Regulated Dual Defects of Bridging Organic and Terminal Inorganic Ligands in Iron-based Metal-Organic Framework Nodes for Efficient Photocatalytic Ammonia Synthesis.

Engineering advantageous defects to construct well-defined active sites in catalysts is promising but challenging to achieve efficient photocatalytic NH3 synthesis from N2 and H2O due to the chemical inertness of N2 molecule. Here, we report defective Fe-based metal-organic framework (MOF) photocatalysts via a non-thermal plasma-assisted synthesis strategy, where their NH3 production capability is synergistically regulated by two types of defects, namely, bridging organic ligands and terminal inorganic ligands (OH- and H2O). Specially, the optimized MIL-100(Fe) catalysts, where there are only terminal inorganic ligand defects and coexistence of dual defects, exhibit the respective 1.7- and 7.7-fold activity enhancement comparable to the pristine catalyst under visible light irradiation. As revealed by experimental and theoretical calculation results, the dual defects in the catalyst induce the formation of abundant and highly accessible coordinatively unsaturated Fe active sites and synergistically optimize their geometric and electronic structures, which favors the injection of more d-orbital electrons in Fe sites into the N2 π* antibonding orbital to achieve N2 activation and the formation of a key intermediate *NNH in the reaction. This work provides a guidance on the rational design and accurate construction of porous catalysts with precise defective structures for high-performance activation of catalytic molecules.

Regulated Dual Defects of Bridging Organic and Terminal Inorganic Ligands in Iron‐based Metal‐Organic Framework Nodes for Efficient Photocatalytic Ammonia Synthesis

AbstractEngineering advantageous defects to construct well‐defined active sites in catalysts is promising but challenging to achieve efficient photocatalytic NH3 synthesis from N2 and H2O due to the chemical inertness of N2 molecule. Here, we report defective Fe‐based metal‐organic framework (MOF) photocatalysts via a non‐thermal plasma‐assisted synthesis strategy, where their NH3 production capability is synergistically regulated by two types of defects, namely, bridging organic ligands and terminal inorganic ligands (OH− and H2O). Specially, the optimized MIL‐100(Fe) catalysts, where there are only terminal inorganic ligand defects and coexistence of dual defects, exhibit the respective 1.7‐ and 7.7‐fold activity enhancement comparable to the pristine catalyst under visible light irradiation. As revealed by experimental and theoretical calculation results, the dual defects in the catalyst induce the formation of abundant and highly accessible coordinatively unsaturated Fe active sites and synergistically optimize their geometric and electronic structures, which favors the injection of more d‐orbital electrons in Fe sites into the N2 π* antibonding orbital to achieve N2 activation and the formation of a key intermediate *NNH in the reaction. This work provides a guidance on the rational design and accurate construction of porous catalysts with precise defective structures for high‐performance activation of catalytic molecules.

Effective 1D/2D nanostructured S-scheme W18O49/g-C3N4 heterojunction photocatalyst fabrication for improved photocatalytic nitrogen fixation performance

The fundamental insights into the reaction mechanism, especially the electron transfer mechanism, are highly promising yet challenging for W18O49/g-C3N4 heterojunction catalysts during photocatalytic nitrogen fixation. Herein, the 1D/2D nanostructured S-scheme heterojunctions were designed by in situ growth of 1D W18O49 nanowires on 2D g-C3N4 nanosheets through solvothermal method using methanol as solvent. In situ XPS analysis revealed that electrons in the conduction band of W18O49 recombine with holes in the valence band of g-C3N4 under the conduction effect of internal electric field, band bending, and Coulombic attraction in the W18O49/g-C3N4 heterojunction. The photogenerated electrons are retained in g-C3N4, where the negative reduction potential of its conduction band enhances the activation of N2. As a result, the ammonia generation rate employing the optimal W18O49/g-C3N4 heterojunction is 64.8 μmol gcat−1 h−1, approximately double that of W18O49 alone and 2.3 times higher than that of g-C3N4. The enhanced activity and facile methodology make the current material exceedingly appealing for implementation in photocatalytic nitrogen fixation for the synthesis of ammonia.