Pentacoordinated pyramidal structures and bonding properties of WN10-/0: anion photoelectron spectroscopy and theoretical calculations

Anion photoelectron spectroscopy and theoretical calculations were used to investigate the structural and bonding properties of WN10-/0. The electron affinity of WN10 is measured to be 1.582 ± 0.030 eV. The frequency of the NN stretch in WN10 is measured to be 2170 ± 80 cm-1, which is red-shifted with respect to that of the dinitrogen molecule indicating that the NN bonds are weakened in WN10. The theoretical adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of WN10- obtained by calculations at the CCSD(T)/CBS level agree well with experimental results. The structures of WN10-/0 are C4v symmetric pentacoordinated pyramidal structures with five end-on dinitrogen ligands. Our experiments show that the peak of WN10- is dominant in the mass spectrum of anionic WNn, whereas the mass peak of WN12+ is dominant in the mass spectrum of cationic WNn, implying that the stabilities of WNn clusters are strongly related to their charge states.

Design of Single-Atom Catalysts for E lectrocatalytic Nitrogen Fixation

The Electrochemical nitrogen reduction reaction (ENRR) can be used to solve environmental problems as well as energy shortage. However, ENRR still faces the problems of low NH3 yield and low selectivity. The NH3 yield and selectivity in ENRR are affected by multiple factors such as electrolytic cells, electrolytes, and catalysts, etc. Among these catalysts are at the core of ENRR research. Single-atom catalysts (SACs) with intrinsic activity have become an emerging technology for numerous energy regeneration, including ENRR. In particular, regulating the microenvironment of SACs (hydrogen evolution reaction inhibition, carrier engineering, metal-carrier interaction, etc.) can break through the limitation of intrinsic activity of SACs. Therefore, this Review first introduces the basic principles of NRR and outlines the key factors affecting ENRR. Then a comprehensive summary is given of the progress of SACs (precious metals, non-precious metals, non-metallic) and diatomic catalysts (DACs) in ENRR. The impact of SACs microenvironmental regulation on ENRR is highlighted. Finally, further research directions for SACs in ENRR are discussed.

Graphynes and Graphdiynes for Energy Storage and Catalytic Utilization: Theoretical Insights into Recent Advances

Carbon allotropes have contributed to all aspects of people's lives throughout human history. As emerging carbon-based low-dimensional materials, graphyne family members (GYF), represented by graphdiyne, have a wide range potential applications due to their superior physical and chemical properties. In particular, graphdiyne (GDY), as the leader of the graphyne family, has been practically applied to various research fields since it was first successfully synthesized. GYF have a large surface area, both sp and sp² hybridization, and a certain band gap, which was considered to originate from the overlap of carbon 2p_z orbitals and the inhomogeneous π-bonds of carbon atoms in different hybridization forms. These properties mean GYF-based materials still have many potential applications to be developed, especially in energy storage and catalytic utilization. Since most of the GYF have yet to be synthesized and applications of successfully synthesized GYF have not been developed for a long time, theoretical results in various application fields should be shared to experimentalists to attract more intentions. In this Review, we summarized and discussed the synthesis, structural properties, and applications of GYF-based materials from the theoretical insights, hoping to provide different viewpoints and comments.

Strong Spin Polarization Effect of Atomically Dispersed Metal Site Boosts the Selective Photocatalytic Nitrobenzene Hydrogenation to Aniline over Graphitic Carbon Nitride

Atomically dispersed catalysts (ADCs) with a well-defined structure are theoretically desirable for a high-selectivity photocatalytic reaction. However, achieving high product selectivity remains a practical challenge for ADCs-based photocatalysts. Herein, we reveal a spin polarization effect on achieving high product selectivity (95.0%) toward the photocatalytic nitrobenzene (PhNO₂) hydrogenation to aniline (PhNH₂) on atomically dispersed Fe site-loaded graphitic carbon nitride (Fe/g-C₃N₄). In combination with the Gibbs free energy diagram and electronic structure analysis, the origin of the photocatalytic performance is attributed not only to the strong metal-support interaction between the Fe site and g-C₃N₄, but more importantly to the strong spin polarization effect that promotes the potential-determining step (PDS) of *PhNO to *PhNOH. This work could be helpful for the design of ADCs-based photocatalysts in view of the spin polarization effect.

Self-Tandem Electrocatalytic NO Reduction to NH3 on a W Single-Atom Catalyst

We design single-atom W confined in MoO_3-x amorphous nanosheets (W₁/MoO_3-x) comprising W₁-O₅ motifs as a highly active and durable NORR catalyst. Theoretical and operando spectroscopic investigations reveal the dual functions of W₁-O₅ motifs to (1) facilitate the activation and protonation of NO molecules and (2) promote H₂O dissociation while suppressing *H dimerization to increase the proton supply, eventually resulting in a self-tandem NORR mechanism of W₁/MoO_3-x to greatly accelerate the protonation energetics of the NO-to-NH₃ pathway. As a result, W₁/MoO_3-x exhibits the highest NH₃-Faradaic efficiency of 91.2% and NH₃ yield rate of 308.6 μmol h^-1 cm^-2, surpassing that of most previously reported NORR catalysts.

Computational determination of a graphene-like TiB4 monolayer for metal-ion batteries and a nitrogen reduction electrocatalyst

As an emerging two-dimensional (2D) material, the TiB₄ monolayer possesses intrinsic advantages in electrochemical applications owing to its graphene-like structure and metallic characteristics. In this work, we performed density functional calculations to investigate the electrochemical properties of the TiB₄ monolayer as an anode material for Li/Na/K ion batteries and as an electrocatalyst for the nitrogen reduction reaction (NRR). Our investigation reveals that Li/Na/K ions could be steadily adsorbed on the TiB₄ monolayer with moderate adsorption energies, and tended to diffuse along two adjacent C-sites with lower energy barriers (0.231/0.094/0.067 eV for Li/Na/K ions) compared to the currently reported transition-metal boride monolayers. Furthermore, a N₂ molecule can be spontaneously captured by the TiB₄ monolayer with a negative Gibbs free energy (-0.925 eV and -0.326 eV for end-on and side-on adsorptions, respectively), hence provoking a conversion into NH₃ along the most efficient reaction pathway (i.e., N₂* → N₂H* → HNNH* → H₂NNH* → H₃NNH* → NH* → NH₂* → NH₃*). In the hydrogenation process, the TiB₄ monolayer exhibits much higher catalytic activity for the NRR as compared with other electrocatalysts, which should be attributed to the spontaneous achievement (ΔG < 0) at all hydrogenation reaction steps except the potential-determining step. Moreover, the TiB₄ monolayer exhibits higher selectivity toward the NRR than the hydrogen evolution reaction. Our work advances the mechanistic understanding on the electrochemical properties of the TiB₄ monolayer as an anode material for metal-ion batteries and as a NRR electrocatalyst, and provides significant guidance for developing high-performance multifunctional 2D materials.

Rational Design of Atomic Site Catalysts for Electrocatalytic Nitrogen Reduction Reaction: One Step Closer to Optimum Activity and Selectivity

The electrocatalytic nitrogen reduction reaction (NRR) has been one of the most intriguing catalytic reactions in recent years, providing an energy-saving and environmentally friendly alternative to the conventional Haber–Bosch process for ammonia production. However, the activity and selectivity issues originating from the activation barrier of the NRR intermediates and the competing hydrogen evolution reaction result in the unsatisfactory NH3 yield rate and Faradaic efficiency of current NRR catalysts. Atomic site catalysts (ASCs), an emerging group of heterogeneous catalysts with a high atomic utilization rate, selectivity, and stability, may provide a solution. This article undertakes an exploration and systematic review of a highly significant research area: the principles of designing ASCs for the NRR. Both the theoretical and experimental progress and state-of-the-art techniques in the rational design of ASCs for the NRR are summarized, and the topic is extended to double-atom catalysts and boron-based metal-free ASCs. This review provides guidelines for the rational design of ASCs for the optimum activity and selectivity for the electrocatalytic NRR.

Graphical Abstract

Rational design of atomic site catalysts (ASCs) for nitrogen reduction reaction (NRR) has both scientific and industrial significance. In this review, the recent experimental and theoretical breakthroughs in the design principles of transition metal ASCs for NRR are comprehensively discussed, and the topic is also extended to double-atom catalysts and boron-based metal-free ASCs.

Theoretical Study on the High HER/OER Electrocatalytic Activities of 2D GeSi, SnSi, and SnGe Monolayers and Further Improvement by Imposing Biaxial Strain or Doping Heteroatoms

Under the DFT calculations, two-dimensional (2D) GeSi, SnSi, and SnGe monolayers, considered as the structural analogues of famous graphene, are confirmed to be dynamically, mechanically and thermodynamically stable, and all of them can also possess good conductivity. Furthermore, we systematically investigate their electrocatalytic activities in overall water splitting. The SnSi monolayer can show good HER catalytic activity, while the SnGe monolayer can display remarkable OER catalytic activity. In particular, the GeSi monolayer can even exhibit excellent bifunctional HER/OER electrocatalytic activities. In addition, applying the biaxial strain or doping heteroatoms (especially P atom) can be regarded as the effective strategies to further improve the HER activities of these three 2D monolayers. The doped GeSi and SnSi systems can usually exhibit higher HER activity than the doped SnGe systems. The correlative catalytic mechanisms are also analyzed. This work could open up a new avenue for the development of non-noble-metal-based HER/OER electrocatalysts.

Graphdiyne Electrochemistry: Progress and Perspectives

Graphdiyne, a carbon allotrope, was synthesized in 2010 for the first time. It consists of two acetylene bonds between adjacent benzene rings. Graphdiyne and its composites thus exhibit ultrahigh intrinsic electrochemical activities. As "star" electrode materials, they have been utilized for various electrochemical applications. With the aim of giving a full screen of graphdiyne electrochemistry, this review starts from the history of graphdiyne materials, followed by their structural and electrochemical features. Recent progress and achievements in the synthesis of graphdiyne materials and their composites are overviewed. Subsequently, various electrochemical applications of graphdiyne materials and their composites are summarized, covering those in the fields of electrochemical energy conversion, electrochemical energy storage, and electrochemical sensing. The perspectives of graphdiyne electrochemistry are also discussed and outlined.

Computational Study of Double Transition Metal Atom Anchored on Graphdiyne Monolayer for Nitrogen Electroreduction

Converting N₂ to NH₃ is an essential reaction but remains a great challenge for industries. Developing more efficient catalysts for N₂ reduction under mild conditions is of vital importance. In this work, double transition metal atoms (TM=Mo, W, Nb and Ru) anchored on graphdiyne monolayer (TM₂ @GDY) as electrocatalysts are designed, and the corresponding reaction mechanisms of N₂ electroreduction are systematically investigated by means of first-principles calculations. The results show that the double TM atoms can be strongly anchored on the acetylenic ring of GDY and Ru₂ @GDY exhibits the highest catalytic activity for NRR with a maximum free energy change of 0.55 eV through the enzymatic pathway. The significant charge transfer between the substrate and the adsorbed N₂ molecule is responsible for the superior catalytic activity. This work could provide a new approach for the rational design of double-atom catalysts for NRR and other related reduction reactions.

2D graphdiyne: an emerging carbon material

As a new member of carbon allotropes, graphdiyne (GDY) has the characteristics of being one-atom-thick with two-dimensional layers comprising sp and sp² hybridized carbon atoms, and represents a trend in the development of carbon materials. Its unique chemical and electronic structures give GDY many unique and fascinating properties such as rich chemical bonds, highly conjugated and super-large π structures, infinitely distributed pores and high inhomogeneity of charge distribution. GDY has entered a period of rapid development, especially with the significant emergence of fundamental research and applied research achievements over the past five years. As one of the frontiers of chemistry and materials science, graphdiyne was listed in the Top 10 research areas in the 2020 Research Frontiers report and was jointly released in the Top 10 in the world by Clarivate and the Chinese Academy of Sciences. The research results have shown the great potential of GDY in the applications of energy, catalysis, environmental science, electronic devices, detectors, biomedicine and therapy, etc. Scientists are eager to explore and fully reveal the new properties, discover new scientific concepts and phenomena, discover the new conversion modes and mechanisms of GDY in photoelectricity, energy, and catalysis, etc., and build the important scientific value of new conversion devices. This review covers research on the foundation and application of GDY, such as the controlled preparation of new methods of GDY and GDY-based materials, studies on new mechanisms and properties in chemistry and physics, and the foundation and applications in energy, catalysis, photoelectric and devices.

Atomically Dispersed Heteronuclear Dual-Atom Catalysts: A New Rising Star in Atomic Catalysis

Atomic catalysts (AC) are gaining extensive research interest as the most active new frontier in heterogeneous catalysis due to their unique electronic structures and maximum atom-utilization efficiencies. Among all the atom catalysts, atomically dispersed heteronuclear dual-atom catalysts (HDACs), which are featured with asymmetric active sites, have recently opened new pathways in the field of advancing atomic catalysis. In this review, the up-to-date investigations on heteronuclear dual-atom catalysts together with the last advances on their theoretical predictions and experimental constructions are summarized. Furthermore, the current experimental synthetic strategies and accessible characterization techniques for these kinds of atomic catalysts, are also discussed. Finally, the crucial challenges in both theoretical and experimental aspects, as well as the future prospects of HDACs for energy-related applications are provided. It is believed that this review will inspire the rational design and synthesis of the new generation of highly effective HDACs.

Graphdiyne-supported single-cluster electrocatalysts for highly efficient carbon dioxide reduction reaction

The electrochemical CO₂ reduction reaction (CO₂RR) has become a promising technology to resolve globally accelerating CO₂ emissions and produce chemical fuels. In this work, the electrocatalytic performance of transition metal (TM = Cu, Cr, Mn, Co, Ni, Mo, Pt, Rh, Ru and V) triatomic clusters embedded in a graphdiyne (GDY) monolayer (TM₃@GDY) for CO₂RR is investigated by density functional theory (DFT) calculations. The results indicate that Cr₃@GDY possesses the best catalytic performance with a remarkably low rate-limiting step of 0.39 eV toward the CO₂ product, and it can also effectively suppress the hydrogen evolution reaction (HER) during the entire CO₂RR process. Studies on the rate-limiting steps (CHO* + H⁺ + e^- → CHOH) of Cr_n@GDY (n = 1-4) structures demonstrate that the high catalytic performance is attributed to the strong synergistic reaction of three Cr atoms interacting with the C atom for the Cr₃@GDY structure. The strong synergistic reaction gives rise to the weakest interaction between O-Cr atoms, which leads to the strongest interaction between O-H atoms and makes the hydrogenation process easier for the Cr₃@GDY structure. Furthermore, ab initio molecular dynamics simulations (AIMD) at 500 K reveal the high thermodynamic stability of the Cr₃@GDY structure. These studies may provide a new approach for designing highly efficient electrocatalysts for the CO₂RR under ambient conditions.

Rational Design of Graphene-Supported Single-Atom Catalysts for Electroreduction of Nitrogen

Critically, the central metal atoms along with their coordination environment play a significant role in the catalytic performance of single-atom catalysts (SACs). Herein, 12 single Fe, Mo, and Ru atoms supported on defective graphene are theoretically deigned for investigation of their structural and electronic properties and catalytic nitrogen reduction reaction (NRR) performance using first-principles calculations. Our results reveal that graphene with vacancies can be an ideal anchoring site for stabilizing isolated metal atoms owing to the strong metal-support interaction, forming stable TMC_x or TMN_x active centers (x = 3 or 4). Six SACs are screened as promising NRR catalyst candidates with excellent activity and selectivity during NRR, and RuN₃ is identified as the optimal one with an overpotential of ≥0.10 V via the distal mechanism.

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.

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.

Double boron atom-doped graphdiynes as efficient metal-free electrocatalysts for nitrogen reduction into ammonia: a first-principles study

The electroreduction of dinitrogen (N₂) is an attractive method for ambient ammonia (NH₃) synthesis. In this work, double boron atom-anchored two-dimensional (2D) graphdiyne (GDY-2B) electrocatalysts have been designed and examined for the N₂ reduction reaction (NRR) by density functional theory computations. Our calculations revealed that double boron atoms can be strongly embedded in a graphdiyne monolayer. In particular, configuration GDY-2B(S₂S₂') with two boron atoms substituting two equivalent sp-carbon atoms of diacetylene linkages exhibits excellent catalytic performance for reducing N₂, with an extremely low overpotential of 0.12 V. The "pull-pull" mechanism imposed by doped double boron atoms is responsible for the magnificent effect of N₂ activation. Besides, the competitive reaction of the hydrogen evolution reaction (HER) is suppressed owing to a large ΔG_H* value of -1.25 eV. Based on these results, our study provides useful guidelines for designing effective double atomic catalysts (DACs) based on nonmetal 2D nanosheets for effective electrochemical reduction reactions.

Computational Screening of 3 d Transition Metal Atoms Anchored on Defective Graphene for Efficient Electrocatalytic N2 Fixation

The synthesis of ammonia (NH₃ ) through the electrochemical reduction of molecular nitrogen (N₂ ) is a promising strategy for significantly reducing energy consumption compared to traditional industrial processes. Herein, we report the design of a series of monovacancy and divacancy defective graphenes decorated with single 3d transition metal atoms (TM@MVG and TM@DVG; TM=Sc-Zn) as electrocatalysts for the nitrogen-reduction reaction (NRR) aided by density functional theory (DFT) calculations. By comparing energies for N₂ adsorption as well as the free energies associated with *N₂ activation and *N₂ H formation, we successfully identified V@MVG, with the lowest potential of -0.63 V, to be an effective catalytic substrate for the NRR in an enzymatic mechanism. Electronic properties, including Bader charges, charge density differences, partial densities of states, and crystal orbital Hamilton populations, are further analyzed in detail. We believe that these results help to explain recent observations in this field and provide guidance for the exploration of efficient electrocatalysts for the NRR.

Two-Dimensional Single-Atom Catalyst TM3(HAB)2 Monolayers for Electrocatalytic Dinitrogen Reduction Using Hierarchical High-Throughput Screening

As an environmentally friendly and sustainable strategy to produce ammonia, the electrocatalytic nitrogen reduction reaction (eNRR) is facing the challenge of low conversion rates and high overpotential, to solve which efficient catalysts are urgently needed. Here, a new class of two-dimensional metal-organic layers (MOLs) TM₃(HAB)₂ (TM = 30 transition metals; HAB = hexaaminobenzene) were evaluated via a three-step high-throughput screening combined with the spin-polarized density functional theory (DFT) method to obtain eligible TM₃(HAB)₂ catalysts embedded with transition metal atoms from 3d to 5d. Our investigation revealed that Nb₃(HAB)₂, Mo₃(HAB)₂, and Tc₃(HAB)₂ are eligible NRR candidates, among which Tc₃(HAB)₂ possesses the best catalytic performance with a lowest onset potential of -0.63 V via both distal and alternating pathways and an ultralow NH₃ desorption free energy of 0.22 eV. Furthermore, the band structures of three catalysts show their nice conductivity. The corresponding projected density of states (PDOS) illustrate that high catalytic activity can be ascribed to apparent orbital hybridization and charge transfer between catalysts and adsorbed N₂. Later, stability and selectivity of all three candidates were computed, Tc₃(HAB)₂ and Nb₃(HAB)₂ catalysts are proved to facilitate dinitrogen reduction and exhibit good stability and high selectivity, yet NRR on the Mo₃(HAB)₂ catalyst is inhibited by hydrogen evolution reaction (HER). Based on the abovementioned studies, we concluded that Tc₃(HAB)₂ and Nb₃(HAB)₂ monolayers are promising catalysts for nitrogen fixation. We expect this work to fill the gap of exploring more eligible single-atom catalysts (SACs) anchored with transition metal atoms on MOLs for NRR.

Boosting Selective Nitrogen Reduction via Geometric Coordination Engineering on Single-Tungsten-Atom Catalysts

Atomic interface regulation that can efficiently optimize the performance of single-atom catalysts (SACs) is a worthwhile research topic. The challenge lies in deeply understanding the structure-properties correlation based on control of the coordination chemistry of individual atoms. Herein, a new kind of W SACs with oxygen and nitrogen coordination (W-NO/NC) and a high metal loading over 10 wt% is facilely prepared by introducing an oxygen-bridged [WO₄ ] tetrahedron. The local structure and coordination environment of the W SACs are confirmed by high-angle annular dark-field scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure. The catalyst shows excellent selectivity and activity for the electrochemical nitrogen reduction reaction (NRR). Density functional theory calculation reveals that unique electronic structures of the N and O dual-coordinated W sites optimize the binding energy of the NRR intermediate, resulting in facilitating the electrocatalytic NRR. This work opens an avenue toward exploring the correlation between coordination structure and properties.

Nitrogen activation to reduction on a recyclable V-SAC/BN-graphene heterocatalyst sifted through dual and multiphilic descriptors

Efficient reduction of nitrogen to ammonia at a minimal cost would require a recherche catalyst tailored by assimilating the inherent electronic and reactive nature of Single Atom Catalysts (SACs) on heteroatom doped-graphene. A full-scale DFT study accounting for disparate descriptions of atomic orbitals and representation of support, has been carried out to identify the most active and recyclable SAC/B-graphene composite as catalyst for Nitrogen Reduction Reaction (NRR). Dual and Multiphilic descriptors derived reactivity pattern of six different metal SACs V, Fe, Ni, Ru, W and Re on periodic and non-periodic paradigms of pristine and BN-pair doped graphene supports, align with the calculated chemisorption efficacy and activation of N₂. The enzymatic route of nitrogen reduction on three most ideal metal SACs (V, W and Re) culminates Vanadium SAC, a relatively cheaper metal, anchored on BNring-graphene with an energy barrier of ⩽1.24 eV as a highly active and recyclable catalyst for NRR.

Density functional theory study of transition metal single-atoms anchored on graphyne as efficient electrocatalysts for the nitrogen reduction reaction

Ammonia (NH3) is the main raw material for the organic chemical industry and a critical feedstock for the fertilizer industry with great significance for the global economy. The NH3 demand has gradually increased with modern society development. Moreover, the electrocatalytic nitrogen reduction reaction (NRR) is a promising NH3 synthesis technology. However, the design of efficient electrocatalysts for the NRR is still challenging. In this study, we systematically analyzed transition metal (TM) single-atoms (Ti, V, Cr, Mn, Zr, Nb, and Mo) anchored on graphyne (GY) as NRR catalysts using density functional theory calculations. The calculation results for the first and last hydrogenation steps (*NNH formation and *NH3 desorption, respectively) revealed that Mn@GY (with an end-on configuration) and V@GY (with a side-on configuration) were the most suitable catalytic substrates for the NRR. The free-energy profiles of the TM@GY catalysts indicated that Mn@GY was the best NRR electrocatalyst owing to its distal pathway with a minimum free-energy barrier of 0.36 eV. In addition, the electronic properties, namely the Bader charge, charge density difference, partial density of states, and crystal orbital Hamilton population, of the TM@GY catalysts were analyzed in detail, and the results further confirmed that Mn@GY was an efficient electrocatalyst. The insights obtained from this comprehensive study can provide useful guidelines for designing new and efficient electrocatalysts.

Theoretical insight into single Rh atoms anchored on N-doped γ-graphyne as an excellent bifunctional electrocatalyst for the OER and ORR: electronic regulation of graphitic nitrogen

Reducing the overpotential and increasing the reaction rate, which are respectively determined by the thermodynamics and kinetics of electrocatalysis, are the keys to obtaining high-performance bifunctional electrocatalysts for the OER/ORR. Herein, six late-transition metals (Ru, Rh, Pd, Os, Ir, and Pt) anchored on γ-GY and graphitic N doped γ-GY substrates are screened as electrocatalysts for the OER and ORR via density functional theory, and the effects of electronic regulation due to the presence of graphitic N on the thermodynamics and kinetics of electrocatalysis are investigated in detail. Among the six γ-GY@TM candidates, only γ-GY@Rh exhibits excellent OER activity, with an overpotential of 0.42 V. Furthermore, graphitic N doped graN-γ-GY@Rh shows outstanding bifunctional electrocatalytic activity, with overpotentials of 0.27 V for the OER and 0.33 V for the ORR, which are remarkably superior to the values of 0.43 V for RuO₂ and 0.45 V for noble-metal Pt electrocatalysts. The present results present some of the lowest overpotentials for OER/ORR electrocatalysts given by theoretical studies to date. From a kinetics point of view, N-doping also remarkably reduces the activation energy barriers of the catalytic rate-limiting steps of the OER and ORR, accelerating the reaction processes and significantly improving the conductivity. Our work provides a theoretical strategy for designing high-efficiency bifunctional OER/ORR electrocatalysts based on γ-GY materials.

High-Throughput Screening of Synergistic Transition Metal Dual-Atom Catalysts for Efficient Nitrogen Fixation

Great enthusiasm in single-atom catalysts (SACs) for the nitrogen reduction reaction (NRR) has been aroused by the discovery of metal-N_x as a promising catalytic center. However, the poor activity and low selectivity of available SACs are far away from the industrial requirement. Through the first-principles high-throughput screening, we find that Fe-Fe distributed on graphite carbon nitride (Fe₂/g-CN) can manipulate the binding strength of the target reaction species (compromises the ability to adsorb N₂H and NH₂), therefore achieving the best NRR performance among 23 transition metal (TM) centers. Our results show that Fe₂/g-CN achieves a high theoretical Faradaic efficiency of 100% and, impressively, the lowest limiting potential of -0.13 V. Particularly, multiple-level descriptors shed light on the origin of NRR activity, achieving a fast prescreening among various candidates. Our predictions not only accelerate discovery of catalysts for ammonia synthesis but also contribute to further elucidate the structure-performance correlations.

Ammonia Synthesis Using Single-Atom Catalysts Based on Two-Dimensional Organometallic Metal Phthalocyanine Monolayers under Ambient Conditions

We have identified three novel metal phthalocyanine (MPc, M = Mo, Re, and Tc) single-atom catalyst candidates with excellent predicted performance for the production of ammonia from electrocatalytic nitrogen reduction reaction (NRR) through a combination of high-throughput screening and first-principles calculations on a series of 3d, 4d, and 5d transition metals anchored onto extended Pc monolayer catalysts. Analysis of the energy band structures and projected density of states of N₂-MPc revealed significant orbital hybridization and charge transfer between the adsorbed N₂ and catalyst MPc, which accounts for the high catalytic activity. Among 30 MPc catalysts, MoPc and TcPc monolayers were found to be the most promising new NRR catalysts, as they exhibit excellent stability, low onset potential, and high selectivity. A comprehensive reaction pathway search found that the maximum free energy changes for the MoPc and TcPc monolayers are 0.33 and 0.54 eV, respectively. As a distinctive nature of this work, the hybrid reaction pathway was considered extensively and searched systematically. The onset potential of the hybrid pathway is found to be smaller than or comparable to that of the commonly known pure pathway. Thus, the hybrid path is highly competitive with low onset potential and high activity. The hybrid pathway is expected to have an important impact on future research on the mechanism of NRR, and it will open up a new way to explore the mechanism of the NRR reaction. We hope that our work will provide impetus to the creation of new catalysts for reduction of N₂ to NH₃. This work provides new insights into the rational design of NRR catalysts and explores novel reaction pathways under ambient or mild conditions.

A review of the hot spot analysis and the research status of single-atom catalysis based on the bibliometric analysis

The bibliometric method was used to analyze the development trend and research hotspots in past 10 years since the concept of single-atom catalysis was proposed in 2011. This article can provide some guidance for future research of SACs.

Efficient Heteronuclear Diatom Electrocatalyst for Nitrogen Reduction Reaction: Pd-Nb Diatom Supported on Black Phosphorus

Electrocatalytic N₂ reduction reaction (eNRR) is a promising alternative to the traditional Haber-Bosch method for large-scale ammonia production because of its low pollution and low energy consumption. By means of density functional theory (DFT) calculations, a thermodynamically stable Pd-Nb heteronuclear diatom catalyst supported on 2D black phosphorus (PdNb@BP) is designed, which is predicted to exhibit excellent catalytic activity toward eNRR with an ultralow overpotential (0.20 V) and a small NH₃ desorption free energy (0.17 eV), by combining the advantages of Nb atoms for N₂ activation and of Pd atoms for NH₃ desorption, and a high eNRR selectivity over the competing hydrogen evolution reaction. It is highlighted that the participation of one additional N₂ molecule in the mechanism is important for the catalyst to realize the catalytic process.

Identifying the Types and Characterization of the Active Sites on M-X-C Single-Atom Catalysts

Single atom catalysts (SACs) have attracted much attention in recent years. As an essential group in SACs, M-X-C (X=nonmetallic element) materials have been demonstrated to be efficient in many reactions. However, identifying the active sites on M-X-C, especially under working conditions, is still challenging, which is crucial for chemists to further understand the mechanism underlying the reaction and better design proper SACs for specific reactions. Herein, the types and characterization of M-X-C are comprehensively summarized and discussed in this review. In addition to the basic information above, the challenges and opportunities remaining in this field will be also proposed to present a perspective to the research on the next step.

Electronic Metal-Support Interaction of Single-Atom Catalysts and Applications in Electrocatalysis

The electronic metal-support interaction (EMSI), which acts as a bridge between theoretical electronic study and the design of heterogenous catalysts, has attracted much attention. Utilizing the interaction between the metal and the support is one of the most essential strategies to enhance electrocatalytic efficiency due to structural and synergetic promotion. To date, as the ideal model for realizing EMSI, many types of single-atom catalysts (SACs) have been developed. The understanding of the electronic interaction on SACs has also been pushed to a higher level. However, systematic theories and operando experiments are seldom reported, and will be necessary to put forward and be carried out, respectively. Herein, the types, characterization, mechanism, and electrocatalytic applications of EMSI are comprehensively summarized and discussed. In addition to the basic information above, the challenges, opportunities, and future development of the EMSI on SACs are also proposed to present an overall view and reference to the later research.

Self-Organized Single-Atom Tungsten Supported on the N-Doped Carbon Matrix for Durable Oxygen Reduction

Rational engineering of atomically scaled metal-nitrogen-carbon (M-N-C) moieties has been the topic of recent research interest because of their potential application as an electrochemical oxygen reduction reaction (ORR) catalyst. Despite numerous efforts on M-N-Cs, attaining both adequate activity and a satisfactory stability simultaneously is a principal issue. Herein, we demonstrated the synthesis of a single-atom tungsten catalyst supported on the N-doped carbon matrix (W-N-C) and its application as an ORR catalyst. W-N-C was synthesized using the economically viable, simple, one-step pyrolysis of dicyandiamide and tungsten(VI) chloride at moderate temperature (700 °C). The synthesis of W-N-C avoids any post acid treatment as it does not require any subsidiary sacrificial metal like Zn and, hence, does not induce any burden associated with chemical waste management. The atomic dispersion of W atoms stabilized by N-doped porous carbon and the formation of WN₂C₂ were confirmed by high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption spectroscopy. Interestingly, as-synthesized WN₂C₂ exhibited unprecedented electrocatalytic activity with a half-wave potential of 764 mV vs reversible hydrogen electrode (RHE) as well as significantly enhanced stability (retaining >99% diffusion-limited current density and the loss in activity is 10.5% at 0.84 V after 10,000 potential cycles), which is much better than the stability limit set by the US Department of Energy in an alkaline medium. Overall, the activity of W-N-C surpasses that of Pt/C after 5000 cycles. The excellent stability is believed to be due to the symmetric coordination of the metal active site (W₂N₂C₂).

Microenvironment modulation of single-atom catalysts and their roles in electrochemical energy conversion

Single-atom catalysts (SACs) have become the most attractive frontier research field in heterogeneous catalysis. Since the atomically dispersed metal atoms are commonly stabilized by ionic/covalent interactions with neighboring atoms, the geometric and electronic structures of SACs depend greatly on their microenvironment, which, in turn, determine the performances in catalytic processes. In this review, we will focus on the recently developed strategies of SAC synthesis, with attention on the microenvironment modulation of single-atom active sites of SACs. Furthermore, experimental and computational advances in understanding such microenvironment in association to the catalytic activity and mechanisms are summarized and exemplified in the electrochemical applications, including the water electrolysis and O2/CO2/N2 reduction reactions. Last, by highlighting the prospects and challenges for microenvironment engineering of SACs, we wish to shed some light on the further development of SACs for electrochemical energy conversion.

MBenes: emerging 2D materials as efficient electrocatalysts for the nitrogen reduction reaction

MBenes, an emerging family of two-dimensional (2D) transition metal borides, have recently been synthesized in experiment and hold great promise for 2D ferromagnets and metal ion batteries. With a well-defined layered structure and outstanding electrical conductivity, MBenes provide an ideal platform for exploring the catalytic behavior of boride surfaces at the atomic level. Herein, we exploit ten kinds of MBenes for electrocatalysis of the N₂ reduction reaction (NRR). By comprehensive first-principles calculations, we show that MBenes have high stability in aqueous environments and excellent selectivity toward the NRR against the hydrogen evolution reaction (HER). Intriguingly, both the surface boron and metal atoms of MBenes can serve as the active sites owing to the co-existence of occupied and unoccupied p (d) states for the boron (metal) atoms, while the boron reaction centers provide even superior activity compared to the metal atoms. Moreover, the activity of these 2D borides can be tuned by engineering their work function. Our theoretical results shine light on utilizing metal borides as a new category of non-precious catalysts for nitrogen fixation, and illuminate the fundamental principles for controlling their catalytic performance.

Bismuth-Based Free-Standing Electrodes for Ambient-Condition Ammonia Production in Neutral Media

Electrocatalytic nitrogen reduction reaction is a carbon-free and energy-saving strategy for efficient synthesis of ammonia under ambient conditions. Here, we report the synthesis of nanosized Bi2O3 particles grown on functionalized exfoliated graphene (Bi2O3/FEG) via a facile electrochemical deposition method. The obtained free-standing Bi2O3/FEG achieves a high Faradaic efficiency of 11.2% and a large NH3 yield of 4.21 ± 0.14 [Formula: see text] h-1 cm-2 at - 0.5 V versus reversible hydrogen electrode in 0.1 M Na2SO4, better than that in the strong acidic and basic media. Benefiting from its strong interaction of Bi 6p band with the N 2p orbitals, binder-free characteristic, and facile electron transfer, Bi2O3/FEG achieves superior catalytic performance and excellent long-term stability as compared with most of the previous reported catalysts. This study is significant to design low-cost, high-efficient Bi-based electrocatalysts for electrochemical ammonia synthesis.

Efficient Electrochemical Nitrogen Fixation over Isolated Pt Sites

Recently, ambient electrochemical N₂ fixation has gained great attention. However, the commercial Pt-based electrocatalyst hardly shows its potential in this field. Herein, it is found that the isolated Pt sites anchored on WO₃ nanoplates exhibit the optimum electrochemical NH₃ yield rate (342.4 µg h^-1 mg^-1 _Pt ) and Faradaic efficiency (31.1%) in 0.1 m K₂ SO₄ at -0.2 V versus RHE, which are about 11 and 15 times higher than their nanoparticle counterparts, respectively. The mechanistic analysis indicates that N₂ conversion to NH₃ follows an alternating hydrogenation pathway, and positively charged isolated Pt sites with special Pt-3O structure can favorably chemisorb and activate the N₂ . Furthermore, the hydrogen evolution reaction can be greatly suppressed on isolated Pt sites decorated WO₃ nanoplates, which guarantees the efficient going-on of nitrogen reduction reaction.

Graphdiyne coordinated transition metals as single-atom catalysts for nitrogen fixation

The reduction of N₂ molecules to NH₃ is a very challenging task in chemistry. The electrocatalytic nitrogen reduction reaction (NRR) is a promising technology for NH₃ synthesis. By using first-principles calculation, a new class of single-atom catalysts (SACs), graphdiyne coordinated single transition metal atoms (TM@GDY, TM = Sc-Zn, Y-Cd, and La-Hg) were designed, and the NRR catalytic character of TM@GDY was systematically investigated. The results demonstrated that some TM@GDY (TM = Ti, V, Fe, Co, Zr, Rh, and Hf) monolayers exhibit better NRR activities than a Ru(0001) stepped surface. There is an obvious linear correlation between the limiting potential and the atomic N adsorption energy, which confirms that the N adsorption energy may be a descriptor for evaluation of the NRR catalytic performance. The V@GDY monolayer possesses the best NRR catalytic character with the lowest limiting potential of -0.67 V and the potential-limiting step (PLS) of *N₂→ *NNH for both alternating and distal mechanisms. Our results highlight a new family of efficient and stable TM@GDY catalysts and provide useful guidelines for SAC development and practical applications.

Theoretical screening of efficient single-atom catalysts for nitrogen fixation based on a defective BN monolayer

The electrocatalytic reduction of naturally abundant N2 to NH3 is an attractive approach to replace the Haber-Bosch nitrogen-fixation process that causes enormous energy consumption and greenhouse gas emissions. However, designing high-performance catalysts toward the electrocatalytic N2 reduction reaction (eNRR) remains one of the greatest challenges in this area. Herein, high-throughput screening of catalysts for the NRR among a series of transition metal atoms supported on a defective hexagonal boron nitride (h-BN) nanosheet is performed through spin-polarized density functional theory (DFT) computations. Strikingly, among the 18 candidates, the V/Tc atom anchored on a defective h-BN monolayer (V@BN and Tc@BN) showed good NRR activity with relatively low onset potentials. Particularly, V@BN was found to exhibit outstanding catalytic activity for the NRR via an enzymatic pathway with an extremely low overpotential of 0.25 V. The value is significantly lower than that on the Ru (0001) stepped surface that has the best NRR catalytic performance among bulk metal catalysts. The novel NRR activity of V@BN is attributed to the enhanced electrical conductivity due to V-doping, the "donation-backdonation" process for N2 activation, and the highly centralized spin-polarization on the V atom. This work not only provides a quite promising catalyst for the NRR but also provides new insights for the rational design of single-atom NRR catalysts.