Cobalt-Modulated Molybdenum–Dinitrogen Interaction in MoS2 for Catalyzing Ammonia Synthesis

Single-Atom Gold Isolated Onto Nanoporous MoSe2 for Boosting Electrochemical Nitrogen Reduction

The electrocatalytic nitrogen reduction reaction (NRR) provides a promising strategy to convert the abundant but inert N₂ into NH₃ using renewable energy. Herein, single-atom Au isolated onto bicontinous nanoporous MoSe₂ (np-MoSe₂ ) is designed as an electrocatalyst for achieving highly efficient NRR catalysis, which exhibits a high Faradaic efficiency (FE) of 37.82% and an NH₃ production rate of 30.83 µg h^-1 mg^-1 at -0.3 V versus a reversible hydrogen electrode (RHE) in 0.1 m Na₂ SO₄ under ambient conditions. Experimental and theoretical investigations reveal that the introduction of single Au atoms onto np-MoSe₂ optimizes the adsorption of NRR intermediates while suppressing the competing HER, thus providing an energetic-favorable process for enhancing the catalytic selectivity toward electrochemical N₂ reduction into NH₃ .

Highly Efficient Electrocatalytic N2 Reduction to Ammonia over Metallic 1T Phase of MoS2 Enabled by Active Sites Separation Mechanism

The 1T phase of MoS2 has been widely reported to be highly active toward the hydrogen evolution reaction (HER), which is expected to restrict the competitive nitrogen reduction reaction (NRR). However, in this work, a prototype of active sites separation over 1T-MoS2 is proposed by DFT calculations that the Mo-edge and S atoms on the basal plane exhibit different catalytic NRR and HER selectivity, and a new role-playing synergistic mechanism is also well enabled for the multistep NRR, which is further experimentally confirmed. More importantly, a self-sacrificial strategy using g-C3 N4 as templates is proposed to synthesize 1T-MoS2 with an ultrahigh 1T content (75.44%, named as CNMS, representing the composition elements of C, N, Mo, and S), which yields excellent NRR performances with an ammonia formation rate of 71.07 µg h-1 mg-1 cat. at -0.5 V versus RHE and a Faradic efficiency of 21.01%. This work provides a promising new orientation of synchronizing the selectivity and activity for the multistep catalytic reactions.

Boron-Functionalized Organic Framework as a High-Performance Metal-Free Catalyst for N2 Fixation

Inspired by the recently synthesized covalent organic framework (COF) containing triquinoxalinylene and benzoquinone units (TQBQ) in the skeleton, we study the stability and properties of its two-dimensional analogue, TQBQCOF, and examine its potential for the synthesis of ammonia using first-principles calculations. We show that the TQBQCOF sheet is mechanically, dynamically, and thermally stable up to 1200 K. It is a semiconductor with a direct band gap of 2.70 eV. We further investigate the electrocatalytic reduction of N₂to NH₃on the Boron-functionalized TQBQCOF sheet (B/TQBQCOF). The rate-determining step of the catalytic pathways is found to be *N-N → *N-NH for the distal, alternating, and enzymatic catalytic mechanisms, with the corresponding overpotentials of 0.65, 0.65, and 0.07 V, respectively. The value of 0.07 V is the lowest required voltage among all of the N₂ reduction catalysts reported so far, showing the potential of B/TQBQCOF as a metal-free catalyst to effectively reduce N₂to NH₃.

Bi2S3 quantum dots in situ grown on MoS2 nanoflowers: An efficient electron-rich interface for photoelectrochemical N2 reduction

Photoelectrocatalysis is considered a green, environmentally friendly, sustainable technology for NH₃ synthesis. However, the low efficiency of ammonia synthesis is currently the primary problem in photoelectrochemical nitrogen reduction reactions (PEC NRR). Herein, a nanocomposite BQD/MS developed through the in-situ growth of Bi₂S₃ quantum dots (BQD) on MoS₂ (MS) nanoflowers was demonstrated as an efficient PEC NRR catalyst. Experimental results showed that the strong interaction between BQD and MS modulated the interfacial charge distribution and increased the electron density on the MS side. Meanwhile, the excellent structure of BQD/MS promoted the effective migration of photogenerated electrons from excited BQD to the MS surface. The electron-rich MS reaction interface was conducive to cleaving the stable NN bond and improving the N₂ reduction performance. As a result, the prepared BQD/15MS photocathode obtained an excellent Faradaic efficiency of 33.2% and an NH₃ yield of 18.5 μg h^-1 mg^-1, which was about three times that of bare MS.

Theoretical Screening of Transition Metal Doped Defective MoS2 as Efficient Electrocatalyst for CO Conversion to CH4

CO is a key intermediate during electrochemical CO₂ conversion. The deep reduction of CO to value-added chemical products is a crucial strategy for effective carbon utilization. Single transition metal atoms supported by two-dimensional material present a novel paragon for various catalytic reactions. Herein, we employ first principle theory to study a series of single 3d-transition metal atoms supported by monolayered MoS₂ with S vacancy as efficient electrocatalyst for CO electroreduction to CH₄ . The screening result indicates that Cr doped defective MoS₂ (labeled as Cr/S_v -MoS₂ ) is beneficial to electroreduction of CO to CH₄ , with even less negative limiting potential (-0.32 V) than Cu that has been widely studied as the most promising electrocatalyst in experiment. The outstanding activity is derived from the regulation of the d-band-center of doped Cr and Mo atoms exposed on the surface. This discovery provides a theoretical basis for the preparation of future electrocatalysts for CORR.

Single-Atom Engineering to Ignite 2D Transition Metal Dichalcogenide Based Catalysis: Fundamentals, Progress, and Beyond

Single-atom catalysis has been recognized as a pivotal milestone in the development history of heterogeneous catalysis by virtue of its superior catalytic performance, ultrahigh atomic utilization, and well-defined structure. Beyond single-atom protrusions, two more motifs of single-atom substitutions and single-atom vacancies along with synergistic single-atom motif assemblies have been progressively developed to enrich the single-atom family. On the other hand, besides traditional carbon material based substrates, a wide variety of 2D transitional metal dichalcogenides (TMDs) have been emerging as a promising platform for single-atom catalysis owing to their diverse elemental compositions, variable crystal structures, flexible electronic structures, and intrinsic activities toward many catalytic reactions. Such substantial expansion of both single-atom motifs and substrates provides an enriched toolbox to further optimize the geometric and electronic structures for pushing the performance limit. Concomitantly, higher requirements have been put forward for synthetic and characterization techniques with related technical bottlenecks being continuously conquered. Furthermore, this burgeoning single-atom catalyst (SAC) system has triggered serial scientific issues about their changeable single atom-2D substrate interaction, ambiguous synergistic effects of various atomic assemblies, as well as dynamic structure-performance correlations, all of which necessitate further clarification and comprehensive summary. In this context, this Review aims to summarize and critically discuss the single-atom engineering development in the whole field of 2D TMD based catalysis covering their evolution history, synthetic methodologies, characterization techniques, catalytic applications, and dynamic structure-performance correlations. In situ characterization techniques are highlighted regarding their critical roles in real-time detection of SAC reconstruction and reaction pathway evolution, thus shedding light on lifetime dynamic structure-performance correlations which lay a solid theoretical foundation for the whole catalytic field, especially for SACs.

Emerging two-dimensional nanomaterials for electrochemical nitrogen reduction

Ammonia (NH₃) is essential to serve as the biological building blocks for maintaining organism function, and as the indispensable nitrogenous fertilizers for increasing the yield of nutritious crops. The current Haber-Bosch process for industrial NH₃ production is highly energy- and capital-intensive. In light of this, the electroreduction of nitrogen (N₂) into valuable NH₃, as an alternative, offers a sustainable pathway for the Haber-Bosch transition, because it utilizes renewable electricity and operates under ambient conditions. Identifying highly efficient electrocatalysts remains the priority in the electrochemical nitrogen reduction reaction (NRR), marking superior selectivity, activity, and stability. Two-dimensional (2D) nanomaterials with sufficient exposed active sites, high specific surface area, good conductivity, rich surface defects, and easily tunable electronic properties hold great promise for the adsorption and activation of nitrogen towards sustainable NRR. Therefore, this Review focuses on the fundamental principles and the key metrics being pursued in NRR. Based on the fundamental understanding, the recent efforts devoted to engineering protocols for constructing 2D electrocatalysts towards NRR are presented. Then, the state-of-the-art 2D electrocatalysts for N₂ reduction to NH₃ are summarized, aiming at providing a comprehensive overview of the structure-performance relationships of 2D electrocatalysts towards NRR. Finally, we propose the challenges and future outlook in this prospective area.

Formamide-derived "glue" for the hundred-gram scale synthesis of atomically dispersed iron-nitrogen-carbon electrocatalysts

The distinct structure and maximum utilization of metal atoms on supported single-atom catalysts (SACs) represents a new frontier of heterogeneous catalysis, yet the low-cost mass production of high-performance SACs is still a key issue for practical applications. Herein, by coating a formamide-derived highly N-modified carbonaceous layer as a "glue" on commercially available activated carbon black (AC), a hundred-gram scale synthesis of atomically dispersed non-noble metal-nitrogen-carbon (MNC) materials was realized, including but not limited to Fe, Co, Ni, Mn, and Cu. The dispersion and coordination environments of Fe atoms on AC were initially revealed by XRD, HRTEM, and XPS, and further confirmed by HAADF-STEM and XANES analysis, presenting Fe atoms in a Fe-N₄ structure. The atomically dispersed metal species, though relatively low-loading grafted on AC (typical loading of 0.16 to 0.29 at%), are mostly distributed on the electrochemically accessible surface, resulting in improved metal utilization. The FeNC@AC-3 sample exhibited highly comparable catalytic performance to 20 wt% Pt/C for the alkaline oxygen reduction reaction, and superior Al-air battery performance. Our work may inspire the synthesis of other types of SACs for broad electrocatalysis applications at kilogram or even ton scale.

A General Strategy toward Metal Sulfide Nanoparticles Confined in a Sulfur-Doped Ti3 C2 Tx MXene 3D Porous Aerogel for Efficient Ambient N2 Electroreduction

Three-dimensional (3D) porous MXene-based aerogel architectures have attracted great interest for many applications, despite limits in renewable energy conversion owing to the lack of multifunctionality in their components. Herein, a simple and general strategy for constructing a novel functional 3D MXene-based composite heterojunction aerogel (MS@S-MAs) is presented via divalent metal-ion assembly and subsequent thermal sulfidation, and its application in electrochemical nitrogen reduction reaction (NRR) is studied. The as-prepared MS@S-MAs comprises metal sulfide nanoparticles uniformly confined in 3D interconnected conductive S-doped MXene sheets with intimate interfacial interaction. Benefiting from the unique properties and an interfacial interaction, MS@S-MAs exhibit significantly improved NRR catalytic performance and excellent stability because of the higher exposure of electrochemically active sites coupled with easier accessibility, faster mass diffusion, and quicker carrier transport at the interface. Remarkably, CoS@S-MAs show an NH₃ yield rate and a Faradaic efficiency of 12.4 µg h^-1 mg^-1 _cat and 27.05% at the lower potential of -0.15 V versus a reversible hydrogen electrode in 0.1 m Na₂ SO₄ solution under ambient conditions, which rivals or exceeds most of the previously reported MXene-based and Co-based catalysts. This work will open avenues to construct 3D MXene-based materials with rich functionalities for energy storage and conversion, catalysis, and other applications.

Recent Advances and Perspective on Electrochemical Ammonia Synthesis under Ambient Conditions

Ammonia is an essential chemical for agriculture and industry. To date, NH₃ is mainly supplied by the traditional Haber-Bosch process, which is operated under high-temperature and high-pressure in a centralized way. To achieve ammonia production in an environmentally benign way, electrochemical NH₃ synthesis under ambient conditions has become the frontier of energy and chemical conversion schemes, as it can be powered by renewable energy and operates in a decentralized way. The recent progress on developing different strategies for NH₃ production, including 1) classic NH₃ synthesis pathways over nanomaterials; 2) the Mars-van Krevelen (MvK) mechanism over metal nitrides (MN_x ); 3) reducing the nitrate into NH₃ over Cu-based nanomaterial; and 4) metal-N₂ battery release of NH₃ from Li_x M. Moreover, the most recent advances in engineering strategies for developing highly active materials and the design of the reaction systems for NH₃ synthesis are covered.

Shedding Light on the Role of Chemical Bond in Catalysis of Nitrogen Fixation

Ammonia (NH₃ ) and nitrates are essential for human society because of their widespread utilization for producing medicines, fibers, fertilizers, etc. In recent years, the development on nitrogen fixation under mild reaction conditions has attracted much attention. However, the very low conversion efficiency and ambiguous catalytic mechanism remain the major hurdles for the research of nitrogen fixation. This review aims to clarify the role of chemical bond in catalytic nitrogen fixation by summarizing and analyzing the recent development of nitrogen fixation research. In detail, the atomic-scale mechanism of nitrogen fixation reaction, the various methods to improve the nitrogen fixation performance, and the computational investigation of nitrogen fixation are discussed, all from a chemical bond perspective. It is hoped that this review could trigger more profound pondering and deeper exploration in the field of catalytic nitrogen fixation.

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.

Vacancy-triggered and dopant-assisted NO electrocatalytic reduction over MoS2

Nitric oxide electroreduction reaction (NOER) is an efficient method for NH₃ synthesis and NO_x-related pollutant treatment. However, current research on NOER catalysts mainly focuses on noble metals and single atom catalysts, while low-cost transition metal dichalcogenides (TMDCs) are rarely considered. Herein, by applying density functional theory (DFT) calculations, we study the catalytic performance of NOER over 2H-MoS₂ monolayers with the most common S vacancies and some Mo atoms substituted by transition metal atoms (denoted as TM-MoS₂@V_S). Our results show that an S vacancy and a heteroatom substitution tend to form a first nearest neighbour (1NN) pair, which greatly improves the NOER catalytic performance of 2H-MoS₂. The S vacancy site can trigger NOER by strongly adsorbing a NO molecule and elongating the NO bond, while the heteroatom dopant can assist NOER by tuning the electron donating capability of 2H-MoS₂ which breaks the linear scaling relations among key reaction intermediates. At low NO coverage, NH₃ can be correspondingly yielded at -0.06 and -0.38 V onset potentials over the Pt- and Au-doped MoS₂ catalysts with S vacancies (Pt-MoS₂@V_S and Au-MoS₂@V_S). At high NO coverage, N₂O/N₂ is thermodynamically favored. Meanwhile, the competing hydrogen evolution reaction (HER) is suppressed. Thus, the Pt-MoS₂@V_S catalysts are promising candidates for NOER. In addition, coupling the substitutional doping of Mo atoms to S vacancies presents great potential in improving the catalytic activity and selectivity of MoS₂ for other reactions. In general, the strategy of coupling hetero-metal doping and chalcogen vacancy can be extended to enhance the catalytic activity of other TMDCs.

Regulating Electronic Spin Moments of Single-Atom Catalyst Sites via Single-Atom Promoter Tuning on S-Vacancy MoS2 for Efficient Nitrogen Fixation

The electrocatalytic activity of transition-metal (TM)-based catalysts is correlated with the spin states of metal atoms. However, developing a way to manipulate spin remains a great challenge. Using first-principles calculations, we first report the crucial role of the spin of exposed Mo atoms around an S-vacancy in the electrocatalytic dinitrogen reduction reaction on defective MoS₂ nanosheets and propose a novel strategy for regulating the electronic spin moments by tuning a single-atom promoter (SAP). Single TM atoms adsorbed on a defective MoS₂ basal plane serve as SAPs via a noncontact interaction with an exposed Mo active site, inducing a significant spin polarization that promotes N₂ adsorption and activation. Interestingly, by changing only the adsorption site of the TM atom, we are able to change the spin moments of the Mo atom, over a wide range of tunable values. The spin moments can be tuned to largely improve the catalytic activity of MoS₂ toward the reduction of N₂ to NH₃.

Enabling multifunctional electrocatalysts by modifying the basal plane of unifunctional 1T'-MoS2 with anchored transition metal single atoms

Multifunctional electrocatalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) are attractive for overall water-splitting, rechargeable metal-air batteries, and unitized regenerative fuel cells. A single-atom catalyst (SAC) may exhibit additional advantages over its nanoparticle counterpart, and already there have been significant advances in the development of bifunctional and trifunctional SACs for HER, ORR, and OER, but great challenges remain for their rational design. Herein, we propose a strategy to realize multifunctional SACs, i.e., modifying unifunctional materials to introduce new active sites on the surface. Specifically, by virtue of the intrinsic excellent HER performance of 1T'-MoS₂, we theoretically design multifunctional SACs by anchoring appropriate transition-metal single atoms. Intriguingly, 1T'-MoS₂ with supported Co single atoms (Co@MoS₂) are demonstrated to be highly active for both OER and ORR with ultralow overpotentials of less than 0.3 V, ascribed to the moderate chemical activity and unique electronic structure of the Co atomic center. Consequently, combining the intrinsic HER activity of 1T'-MoS₂, Co@MoS₂ is proposed to be promising efficient trifunctional SACs. Further, the phase engineering on SACs is unrevealed and elucidated by comparing the properties of the Co atomic center-supported on 1T'-MoS₂ and 1H-MoS₂. This work provides a feasible strategy for the design of multifunctional SACs for the renewable and sustainable energy technology and provides an insight into the phase engineering on SACs.

Advances in Electrochemical Ammonia Synthesis Beyond the Use of Nitrogen Gas as a Source

Electrocatalytic reduction of dinitrogen has emerged as a new strategy for ammonia synthesis. Despite being environmentally benign and energy-saving, it suffers from low conversion efficiency and short yield of ammonia because of the challenges of activating the inert N≡N bond at room temperature and atmospheric pressure. As a result of this, researchers proposed to reduce the nitrogenous species, one category of air and water pollutants, into valuable ammonia. Although remaining largely underexplored, this alternative approach shows promising efficiency for ammonia synthesis, while achieving high catalytic activity and selectivity remains challenging. In this Minireview, we summarize recent electrocatalytic performances of denitrification with selective formation to ammonia in terms of proposed active sites and reaction mechanisms. Additionally, we discuss the common issues in the state-of-the-art experimental tests and highlight the breakthroughs via computational screening of electrode materials. The aim of this is to steer the future research directions in the field, which is aiming for an optimal catalytic system with higher activity and selectivity for electrocatalytic denitrification.

Intrinsic Electron Localization of Metastable MoS2 Boosts Electrocatalytic Nitrogen Reduction to Ammonia

The advancement of efficient electrocatalysts toward the nitrogen reduction reaction (NRR) is critical in sustainable ammonia synthesis under ambient pressure and temperature. Manipulating the electronic configuration of electrocatalysts is particularly vital to form metal-nitrogen (MN) bonds during the NRR through regulating the active electronic states of sites. Here, in sharp contrast to stable 2H MoS₂ without metal chains, MoMo bonding in metastable polymorphs of MoS₂ bulk (zigzag chain in the 1T' phase and diamond chain in the 1T″' phase) is discovered to significantly increase intrinsic electron localization around the metal chains. This can enhance the charge transfer from the adsorbed nitrogen molecule to the metal chains, allowing for boosted NRR kinetics. The electrochemical experiments show that the NH₃ yield rate and the faradaic efficiency of the metastable 1T″' MoS₂ rich with abundant Mo-Mo bonds are about 9 and 12 times above average than those of 2H MoS₂ , correspondingly. Theoretical simulations reveal the high local electron density surrounding the MoMo chains and sites can promote π back-donation, which is beneficial for increasing nitrogen adsorption, strengthening the MN bonds, and reducing the cleavage barrier of the triple NN bond.

Comprehensive Understanding of the Thriving Ambient Electrochemical Nitrogen Reduction Reaction

The electrochemical method of combining N₂ and H₂ O to produce ammonia (i.e., the electrochemical nitrogen reduction reaction [E-NRR]) continues to draw attention as it is both environmentally friendly and well suited for a progressively distributed farm economy. Despite the multitude of recent works on the E-NRR, further progress in this field faces a bottleneck. On the one hand, despite the extensive exploration and trial-and-error evaluation of E-NRR catalysts, no study has stood out to become the stage protagonist. On the other hand, the current level of ammonia production (microgram-scale) is an almost insurmountable obstacle for its qualitative and quantitative determination, hindering the discrimination between true activity and contamination. Herein i) the popular theory and mechanism of the NRR are introduced; ii) a comprehensive summary of the recent progress in the field of the E-NRR and related catalysts is provided; iii) the operational procedures of the E-NRR are addressed, including the acquisition of key metrics, the challenges faced, and the most suitable solutions; iv) the guiding principles and standardized recommendations for the E-NRR are emphasized and future research directions and prospects are provided.

Roadmap and Direction toward High-Performance MoS2 Hydrogen Evolution Catalysts

MoS₂ intrinsically show Pt-like hydrogen evolution reaction (HER) performance. Pristine MoS₂ displayed low HER activity, which was caused by low quantities of catalytic sites and unsatisfactory conductivity. Then, phase engineering and S vacancy were developed as effective strategies to elevate the intrinsic HER performance. Heterojunctions and dopants were successful strategies to improve HER performance significantly. A couple of state-of-the-art MoS₂ catalysts showed HER performance comparable to Pt. Applying multiple strategies in the same electrocatalyst was the key to furnish Pt-like HER performance. In this review, we summarize the available strategies to fabricate superior MoS₂ HER catalysts and tag the important works. We analyze the well-defined strategies for fabricating a superior MoS₂ electrocatalyst, propose complementary strategies which could help meet practical requirements, and help people design highly efficient MoS₂ electrocatalysts. We also provide a brief perspective on assembling practical electrochemical systems by high-performance MoS₂ electrocatalysts, apply MoS₂ in other important electrocatalysis reactions, and develop high-performance two-dimensional (2D) dichalcogenide HER catalysts not limited to MoS₂. This review will help researchers to obtain a better understanding of development of superior MoS₂ HER electrocatalysts, providing directions for next-generation catalyst development.

Doping regulation in transition metal compounds for electrocatalysis

In electrocatalysis, doping regulation has been considered as an effective method to modulate the active sites of catalysts, providing a powerful means for creating a large variety of highly efficient catalysts for various reactions. Of particular interest, there has been growing research concerning the doping of two-dimensional transition-metal compounds (TMCs) to optimize their electrocatalytic performance. Despite the previous achievements, mechanistic insights of doping regulation in TMCs for electrocatalysis are still lacking. Herein, we provide a systematic overview of doping regulation in TMCs in terms of background, preparation, impacts on physicochemical properties, and typical applications including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, and N₂ reduction reaction. Notably, we bridge the understanding between the doping regulation of catalysts and their catalytic activities via focusing on the physicochemical properties of catalysts from the aspects of vacancy concentrations, phase transformation, surface wettability, electrical conductivity, electronic band structure, local charge distribution, tunable adsorption strength, and multiple adsorption configurations. We also discuss the existing challenges and future perspectives in this promising field.

Altering Hydrogenation Pathways in Photocatalytic Nitrogen Fixation by Tuning Local Electronic Structure of Oxygen Vacancy with Dopant

To avoid the energy-consuming step of direct N≡N bond cleavage, photocatalytic N₂ fixation undergoing the associative pathways has been developed for mild-condition operation. However, it is a fundamental yet challenging task to gain comprehensive understanding on how the associative pathways (i.e., alternating vs. distal) are influenced and altered by the fine structure of catalysts, which eventually holds the key to significantly promote the practical implementation. Herein, we introduce Fe dopants into TiO₂ nanofibers to stabilize oxygen vacancies and simultaneously tune their local electronic structure. The combination of in situ characterizations with first-principles simulations reveals that the modulation of local electronic structure by Fe dopants turns the hydrogenation of N₂ from associative alternating pathway to associative distal pathway. This work provides fresh hints for rationally controlling the reaction pathways toward efficient photocatalytic nitrogen fixation.

Rigid anchoring of highly crystallized and uniformly dispersed Pd nanocrystals on carbon fibers for ambient electrocatalytic reduction of nitrogen to ammonia

Developing efficient and stable electrocatalysts for ammonia synthesis via the nitrogen reduction reaction (NRR) is essential for the Earth's nitrogen cycle. Herein, a palladium nanocrystals anchored carbon fibers (PdNCs@CNFs) composite was prepared via electrospinning and carbonization processes. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization studies show that the as-prepared Pd grains are homogeneously anchored on the outer/inner section of the carbon nanofibers. Benefiting from the sufficient exposure and stress effect of active sites, the resultant PdNCs@CNFs achieves a high Faraday efficiency of ∼14.8% with a current density of 0.028 mA cm-2 at -0.2 V vs. reversible hydrogen electrode (RHE) in 0.1 M Na2SO4 solution, surpassing those of many catalysts previously reported. Density functional theory (DFT) calculations reveal that the rationality of the distal associative mechanism on PdNCs@CNFs and Pd nanocrystals on the surface of PdNCs@CNFs is more favorable for nitrogen (N2) molecule adsorption and polarization.

Transition Metal Chalcogenides as a Versatile and Tunable Platform for Catalytic CO2 and N2 Electroreduction

Group VI transition metal chalcogenides are the subject of increasing research interest for various electrochemical applications such as low-temperature water electrolysis, batteries, and supercapacitors due to their high activity, chemical stability, and the strong correlation between structure and electrochemical properties. Particularly appealing is their utilization as electrocatalysts for the synthesis of energy vectors and value-added chemicals such as C-based chemicals from the CO₂ reduction reaction (CO₂R) or ammonia from the nitrogen fixation reaction (NRR). This review discusses the role of structural and electronic properties of transition metal chalcogenides in enhancing selectivity and activity toward these two key reduction reactions. First, we discuss the morphological and electronic structure of these compounds, outlining design strategies to control and fine-tune them. Then, we discuss the role of the active sites and the strategies developed to enhance the activity of transition metal chalcogenide-based catalysts in the framework of CO₂R and NRR against the parasitic hydrogen evolution reaction (HER); leveraging on the design rules applied for HER applications, we discuss their future perspective for the applications in CO₂R and NRR. For these two reactions, we comprehensively review recent progress in unveiling reaction mechanisms at different sites and the most effective strategies for fabricating catalysts that, by exploiting the structural and electronic peculiarities of transition metal chalcogenides, can outperform many metallic compounds. Transition metal chalcogenides outperform state-of-the-art catalysts for CO₂ to CO reduction in ionic liquids due to the favorable CO₂ adsorption on the metal edge sites, whereas the basal sites, due to their conformation, represent an appealing design space for reduction of CO₂ to complex carbon products. For the NRR instead, the resemblance of transition metal chalcogenides to the active centers of nitrogenase enzymes represents a powerful nature-mimicking approach for the design of catalysts with enhanced performance, although strategies to hinder the HER must be integrated in the catalytic architecture.

Electrochemical Fixation of Nitrogen by Promoting N2 Adsorption and N-N Triple Bond Cleavage on the CoS2/MoS2 Nanocomposite

An electrochemical N₂ reduction reaction (NRR), as an environmentally benign method to produce NH₃, is a suitable alternative to substitute the energy-intensive Haber-Bosch technology. Unfortunately, to date, it is obstructed by the lack of efficient electrocatalysts. Here, a CoS₂/MoS₂ nanocomposite with CoS₂ nanoparticles decorated on MoS₂ nanosheets is fabricated and adapted as a catalyst for the NRR. As unveiled by experimental and theoretical results, the strong interaction between CoS₂ and MoS₂ modulates interfacial charge distribution with electrons transferring from CoS₂ to MoS₂. Consequently, a local electrophilic region is formed near the CoS₂ side, which enables effective N₂ absorption. On the other hand, the nucleophilic area formed near the MoS₂ side is in favor of breaking stable N≡N, the potential-determining step (*N₂ → *N₂H) which brings about a much decreased energy barrier than that on pure MoS₂. As a result, this catalyst exhibits an excellent NRR performance, NH₃ yield and Faradaic efficiency of 54.7 μg·h^-1·mg^-1 and 20.8%, respectively, far better than most MoS₂-based catalysts.

Cobalt-Doping of Molybdenum Disulfide for Enhanced Catalytic Polysulfide Conversion in Lithium-Sulfur Batteries

Metal sulfides, such as MoS₂, are widely investigated in lithium-sulfur (Li-S) batteries to suppress the shuttling of lithium polysulfides (LiPSs) due to their chemical adsorption ability and catalytic activity. However, their relatively low conductivity and activity limit the LiPS conversion kinetics. Herein, the Co-doped MoS₂ is proposed to accelerate the catalytic conversion of LiPS as the Co doping can promote the transition from semiconducting 2H phase to metallic 1T phase and introduce the sulfur vacancies in MoS₂. A one-step hydrothermal process is used to prepare such a Co-doped MoS₂ with more 1T phase and rich sulfur vacancies, which enhances the electron transfer and catalytic activity, thus effectively improving the LiPS adsorption and conversion kinetics. The cathode using the three-dimensional graphene monolith loaded with Co-doped MoS₂ catalyst as the sulfur host shows a high rate capability and long cycling stability. A high capacity of 941 mAh g^-1 at 2 C and a low capacity fading of 0.029% per cycle at 1 C over 1000 cycles are achieved, suggesting the effectively suppressed LiPS shuttling and improved sulfur utilization. Good cyclic stability is also maintained under a high sulfur loading indicating the doping is an effective way to optimize the metal sulfide catalysts in Li-S batteries.

Heteropoly Blue/Protonation-Defective Graphitic Carbon Nitride Heterojunction for the Photo-Driven Nitrogen Reduction Reaction

The establishment of a heterojunction is a crucial strategy to design highly effective nonnoble metal nanocatalysts for the photocatalytic nitrogen reduction reaction (PNRR). Heteropoly blues (r-POMs) can act as electron-transfer mediators in PNRR, but its agglomeration limits the further promotion of PNRR productivity. In this work, we construct a protonation-modified surface of N-vacancy g-C₃N₄ (HV-C₃N₄), achieving the high dispersion of r-POMs via the surface modification strategy. Enlightened by the synergy effect of the nitrogenase, r-POMs were anchored onto HV-C₃N₄ nanosheets through an electrostatic self-assembly method for preparing r-POMs-based protonation-defective graphitic carbonitride (HV-C₃N₄/r-POMs). As an electron donor, r-PW₁₂ can match with the energy level of HV-C₃N₄ to build a heterojunction. The electron redistribution of the heterojunction facilitates the optimization of the electronic structure for enhancing the performance of PNRR. HV-C₃N₄/r-PW₁₂ exhibits the best PNRR efficiency of 171.4 μmol L^-1 h^-1, which is boosted by 94.39% (HV-C₃N₄) and 86.98% (r-PW₁₂). The isotope ¹⁵NH₄⁺ experiment proves that ammonia is derived from N₂, not carbon nitride. This study opens up a crucial view to achieve the high dispersion of r-POMs nanoparticles and develop high-efficiency nonnoble metal photocatalysts for the PNRR.

Highly efficient ammonia synthesis at low temperature over a Ru-Co catalyst with dual atomically dispersed active centers

The desire for a carbon-free society and the continuously increasing demand for clean energy make it valuable to exploit green ammonia (NH3) synthesis that proceeds via the electrolysis driven Haber-Bosch (eHB) process. The key for successful operation is to develop advanced catalysts that can operate under mild conditions with efficacy. The main bottleneck of NH3 synthesis under mild conditions is the known scaling relation in which the feasibility of N2 dissociative adsorption of a catalyst is inversely related to that of the desorption of surface N-containing intermediate species, which leads to the dilemma that NH3 synthesis could not be catalyzed effectively under mild conditions. The present work offers a new strategy via introducing atomically dispersed Ru onto a single Co atom coordinated with pyrrolic N, which forms RuCo dual single-atom active sites. In this system the d-band centers of Ru and Co were both regulated to decouple the scaling relation. Detailed experimental and theoretical investigations demonstrate that the d-bands of Ru and Co both become narrow, and there is a significant overlapping of t2g and eg orbitals as well as the formation of a nearly uniform Co 3d ligand field, making the electronic structure of the Co atom resemble that of a "free-atom". The "free-Co-atom" acts as a bridge to facilitate electron transfer from pyrrolic N to surface Ru single atoms, which enables the Ru atom to donate electrons to the antibonding π* orbitals of N2, thus resulting in promoted N2 adsorption and activation. Meanwhile, H2 adsorbs dissociatively on the Co center to form a hydride, which can transfer to the Ru site to cause the hydrogenation of the activated N2 to generate N2H x (x = 1-4) intermediates. The narrow d-band centers of this RuCo catalyst facilitate desorption of surface *NH3 intermediates even at 50 °C. The cooperativity of the RuCo system decouples the sites for the activation of N2 from those for the desorption of *NH3 and *N2H x intermediates, giving rise to a favorable pathway for efficient NH3 synthesis under mild conditions.

Two dimensional electrocatalyst engineering via heteroatom doping for electrocatalytic nitrogen reduction

The electrocatalytic N₂ reduction reaction (eNRR) - which can occur under ambient conditions with renewable energy input - became a promising synthetic pathway for ammonia (NH₃) and has attracted growing attention in the past few years. Some achievements have been made in the eNRR; however, there remain significant challenges to realize satisfactory NH₃ production. Therefore, the rational design of highly efficient and durable eNRR catalysts with N[triple bond, length as m-dash]N bond activating and breaking ability is highly desirable. Two-dimensional (2D) materials have shown great potential in electrocatalysis for energy conversion and storage. Although most 2D materials are inactive toward the eNRR, they can be activated by various modification methods. Heteroatom doping engineering can impact the charge distribution and spin states on catalytic sites, therefore accelerating the dinitrogen adsorption and protonation process. This review summarises the recent research progress of heteroatom-doped 2D materials, including carbon, molybdenum disulfide (MoS₂) and metal carbides (MXenes), for the eNRR. In addition, some existing opportunities and future research directions in electrocatalytic nitrogen fixation for ammonia production are discussed.

Revisiting amorphous molybdenum sulfide's activity for the electro-driven reduction of dinitrogen and N-containing substrates

Ammonia (NH₃) is a major feedstock of the chemical industry. The imperious need to decarbonize its production has stimulated a quest for efficient catalysts able to drive the direct electro-reduction of dinitrogen (N₂) into NH₃. A large number of materials have now been proposed for this reaction, including bioinspired molybdenum sulfide derivatives. Here, we revisit the potential of amorphous molybdenum sulfide to drive the electrocatalytic reduction of N₂ and other substrates of nitrogenase. We find that this material exhibits negligible activity towards N₂ but achieves efficient reduction of inorganic azides.

General Synthesis of Nanoporous 2D Metal Compounds with 3D Bicontinous Structure

Although 2D layered metal compounds are widely exploited using various techniques such as exfoliation and vapor-phase-assisted growth, it is still challenging to construct the 2D materials in a 3D configuration with preservation of the unique physicochemical properties of the metal compounds. Herein, a general synthetic strategy is reported for a wide variety of 2D (atomic-scale thickness) metal compounds with 3D bicontinous nanoporous structure. 19 binary compounds including sulfides, selenides, tellurides, carbides, and nitrides, and five alloyed compounds, are successfully prepared via a surface alloy strategy, which are readily created by using a recyclable nanoporous gold assisted chemical vapor deposition process. These 3D nanoporous metal compounds with preserved 2D physicochemical properties, tunable pore sizes, and compositions for electrocatalytic applications, show excellent catalytic performance in the electrochemical N₂ reduction reaction. This work opens up a promising avenue for fundamental studies and potential applications of a wide variety of nanoporous metal compounds.

Efficient Nitrate Synthesis via Ambient Nitrogen Oxidation with Ru-Doped TiO2 /RuO2 Electrocatalysts

A facile pathway of the electrocatalytic nitrogen oxidation reaction (NOR) to nitrate is proposed, and Ru-doped TiO₂ /RuO₂ (abbreviated as Ru/TiO₂ ) as a proof-of-concept catalyst is employed accordingly. Density functional theory (DFT) calculations suggest that Ru^{δ +} can function as the main active center for the NOR process. Remarkably doping Ru into the TiO₂ lattice can induce an upshift of the d-band center of the Ru site, resulting in enhanced activity for accelerating electrochemical conversion of inert N₂ to active NO*. Overdoping of Ru ions will lead to the formation of additional RuO₂ on the TiO₂ surface, which provides oxygen evolution reaction (OER) active sites for promoting the redox transformation of the NO* intermediate to nitrate. However, too much RuO₂ in the catalyst is detrimental to both the selectivity of the NOR and the Faradaic efficiency due to the dominant OER process. Experimentally, a considerable nitrate yield rate of 161.9 µmol h^-1 g_cat ^-1 (besides, a total nitrate yield of 47.9 µg during 50 h) and a highest nitrate Faradaic efficiency of 26.1% are achieved by the Ru/TiO₂ catalyst (with the hybrid composition of Ru_x Ti_y O₂ and extra RuO₂ by 2.79 wt% Ru addition amount) in 0.1 m Na₂ SO₄ electrolyte.

Atomically Dispersed Fe-N4 Modified with Precisely Located S for Highly Efficient Oxygen Reduction

Immobilizing metal atoms by multiple nitrogen atoms has triggered exceptional catalytic activity toward many critical electrochemical reactions due to their merits of highly unsaturated coordination and strong metal-substrate interaction. Herein, atomically dispersed Fe-NC material with precise sulfur modification to Fe periphery (termed as Fe-NSC) was synthesized, X-ray absorption near edge structure analysis confirmed the central Fe atom being stabilized in a specific configuration of Fe(N3)(N-C-S). By enabling precisely localized S doping, the electronic structure of Fe-N4 moiety could be mediated, leading to the beneficial adjustment of absorption/desorption properties of reactant/intermediate on Fe center. Density functional theory simulation suggested that more negative charge density would be localized over Fe-N4 moiety after S doping, allowing weakened binding capability to *OH intermediates and faster charge transfer from Fe center to O species. Electrochemical measurements revealed that the Fe-NSC sample exhibited significantly enhanced oxygen reduction reaction performance compared to the S-free Fe-NC material (termed as Fe-NC), showing an excellent onset potential of 1.09 V and half-wave potential of 0.92 V in 0.1 M KOH. Our work may enlighten relevant studies regarding to accessing improvement on the catalytic performance of atomically dispersed M-NC materials by managing precisely tuned local environments of M-Nx moiety.

A DFT screening of single transition atoms supported on MoS2 as highly efficient electrocatalysts for the nitrogen reduction reaction

The development of low-cost and highly efficient materials for the electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions is an attractive and challenging topic in chemistry. In this study, the electrocatalytic performance of a series of transition metal (TM) atoms supported on MoS2 nanosheets (TM@MoS2) was systematically investigated using density functional theory (DFT) calculations. It was found that Re supported on MoS2 (Re@MoS2) has the best NRR catalytic activity with a limiting potential of -0.43 V, along with high selectivity over the competing hydrogen evolution reaction (HER). Moreover, the ab initio molecular dynamics (AIMD) simulations at 500 K and density of states (DOS) calculations indicated the high thermodynamic stability and excellent electrical conductivity of Re@MoS2. A linear trend between several parameters of single atom catalysts (SACs) and the adsorption Gibbs free energy change of the NH species (ΔG*NH) was observed, indicating the later as a simple descriptor for the facilitated screening of novel SACs. These results pave the way for exploring novel, highly efficient electrocatalysts for the electrochemical NRR under ambient conditions.

Surface-defect-engineered photocatalyst for nitrogen fixation into value-added chemical feedstocks

Surface-defect-engineered photocatalyst for nitrogen fixation.

A metal-free catalyst: sulfur-doped and sulfur nanoparticle-modified CMK-3 as an electrocatalyst for enhanced N2-fixation

S/CMK-3 was fabricated by sulfur-doped and sulfur nanoparticles modified by a one-pot method for enhanced N₂ reduction.