Advancements in Electrocatalytic Nitrogen Reduction: A Comprehensive Review of Single-Atom Catalysts for Sustainable Ammonia Synthesis

Electrocatalytic nitrogen reduction technology seamlessly aligns with the principles of environmentally friendly chemical production. In this paper, a comprehensive review of recent advancements in electrocatalytic NH3 synthesis utilizing single-atom catalysts (SACs) is offered. Into the research and applications of three categories of SACs: noble metals (Ru, Au, Rh, Ag), transition metals (Fe, Mo, Cr, Co, Sn, Y, Nb), and nonmetallic catalysts (B) in the context of electrocatalytic ammonia synthesis is delved. In-depth insights into the material preparation methods, single-atom coordination patterns, and the characteristics of the nitrogen reduction reaction (NRR) are provided. The systematic comparison of the nitrogen reduction capabilities of various SAC types offers a comprehensive research framework for their integration into electrocatalytic NRR. Additionally, the challenges, potential solutions, and future prospects of incorporating SACs into electrocatalytic nitrogen reduction endeavors are discussed.

Modelling Photocatalytic N2 Reduction to Ammonia: Where We Stand and Where We Are Going

Artificial ammonia synthesis via the Haber-Bosch process is environmentally problematic due to the high energy consumption and corresponding CO2 ${_2 }$ emissions, produced during the reaction and before hand in hydrogen production upon methane steam reforming. Photocatalytic nitrogen fixation as a greener alternative to the conventional Haber-Bosch process enables us to perform nitrogen reduction reaction (NRR) under mild conditions, harnessing light as the energy source. Herein, we systematically review first-principles calculations used to determine the electronic/optical properties of photocatalysts, N2 adsorption and to expound possible NRR mechanisms. The most commonly studied photocatalysts for nitrogen fixation are usually modified with dopants, defects, co-catalysts and Z-scheme heterojunctions to prevent charge carrier recombination, improve charge separation efficiency and adjust a band gap to for utilizing a broader light spectrum. Most studies at the atomistic level of modeling are grounded upon density functional theory (DFT) calculations, wholly foregoing excitation effects paramount in photocatalysis. Hence, there is a dire need to consider methods beyond DFT to study the excited state properties more accurately. Furthermore, a few studies have been examined, which include higher level kinetics and macroscale simulations. Ultimately, we show there is still ample room for improvement with regard to first principles calculations and their integration in multiscale models.

Implications of the Pore Size of Graphitic Carbon Nitride Monolayers on the Selectivity of Dual-Boron Atom Catalysts for the Reduction of N2 to Urea and Ammonia: A Computational Investigation

The formation of urea by electrocatalytic means remains a great challenge due to the lack of a suitable catalyst that is capable of not only activating inert N2 and CO2 molecules but also circumventing the complexity associated with subsequent reaction steps leading to urea formation. Herein, by means of comprehensive density functional theory simulations, we investigate the catalytic activity of highly stable transition-metal-free dual-boron atom-doped graphitic carbon-nitride monolayers with different pore sizes toward urea production under ambient conditions. As per the results, dual boron atoms impregnated in g-C2N and g-C6N6 monolayers with large pore diameters can successfully activate the N2 molecule and lead to the spontaneous formation of the *NCO*N intermediate, which is the most crucial step for urea formation via direct coupling of N2 and CO2. Interestingly, the B2@g-C2N and B2@g-C6N6 favor urea production with low limiting potentials of -1.11 and -1.18 V compared to very high limiting potentials of -1.71 and -1.88 V, respectively, for ammonia synthesis, leading to an almost 100% Faradaic efficiency for urea formation over ammonia. The dual-boron doping in g-C3N4 with a smaller pore size depicts comparatively weaker N2 adsorption than g-C2N and g-C6N6 counterparts. Further, B2@g-C3N4 prefers ammonia formation at a very low limiting potential of -0.40 V compared to a very high limiting potential of -2.11 V for urea formation. Thus, our findings clearly highlight the critical role played by the pore size of carbon-nitride monolayers in tuning the reactivity and catalytic activity of dual-boron atom catalysts toward urea formation in a selective manner, thereby providing valuable guidance in exploring other highly efficient urea catalysts.

Two-Dimensional Ordered Double-Transition Metal Carbides for the Electrochemical Nitrogen Reduction Reaction

The electrochemical nitrogen reduction reaction (NRR) provides a green and sustainable strategy as an alternative to the Haber-Bosch process. The development of electrocatalysts with low overpotential, high selectivity, and fast reaction kinetics remains a significant challenge. Here, density functional theory computations are carried out to systematically predict the prospect of 18 two-dimensional (2D) ordered double-transition metal carbides (MXenes) as NRR electrocatalysts. Our results revealed that the basal plane of Mo₂Nb₂C₃ MXene exhibited the most outstanding catalytic activity while effectively suppressed the hydrogen evolution reaction with an overpotential of 0.48 V. The exposed Mo₃ moiety moderately regulating the electron transfer between reaction intermediates is answerable for the high activity. Finally, our finding broadens the horizon of 2D materials as NRR electrocatalysts.

Anchoring boron on a covalent organic framework as an efficient single atom metal-free photo-electrocatalyst for nitrogen fixation: a first-principles analysis

The production of ammonia in a sustainable cost-effective manner and ambient conditions is a very challenging task. Photo-/electrocatalytic nitrogen reduction (NRR) is a convenient way to produce NH₃ for industrial applications. In this work, anchoring B atoms in Tp-bpy-COF is shown to effectively reduce N₂ to NH₃. By employing density functional theory, we demonstrated that N₂ can be efficiently activated on the B center due to the synergistic effect of B-N. Meanwhile, we found that the NRR happens predominantly by the alternating path with a small limiting potential of 0.13 V. Moreover, the suitable band edge positions and broad visible light absorption zone result in B@Tp-bpy-COF acting as a promising photocatalyst. Our proposed catalytic system exhibits favorable formation energy and excellent structural stability during AIMD simulations, which suggest the feasibility of experimental synthesis. The system turns out to be highly selective toward the NRR compared to other competitive reactions. These findings may pave a new way for designing SACs on COFs for N₂ fixation with high activity, which may also apply to other reactions.

Theoretical Study of Small Molecules Adsorption on Pristine and Transition Metal Doped GeSe Monolayer for Gas Sensing Application

By using first-principles calculations, the sensing properties of pristine and transition metal (TM) atoms (Ti, V, and Co) embedded germanium selenide (GeSe) monolayer toward small gas molecules (H₂, NH₃, CO, O₂, SO₂, NO, and NO₂) were investigated. The adsorption energies, electronic structure, optical properties, and recovery time of the adsorption systems were calculated and analyzed in detail. The results indicate that TM doped GeSe has stronger interaction with gas molecules compared with the pristine GeSe monolayer. Especially for Ti- and V-GeSe monolayer, the absolute value of adsorption energies are up to 2 eV for O₂, NO, and NO₂. The doping with TM atoms also changes the charge transfer and electronic structures of adsorption systems. Combined with the result of the calculated optical properties and recovery time, it can be concluded that Ti-GeSe monolayer has great potential for NH₃ detection, while Co-GeSe monolayer can be very promising SO₂ gas sensors.

Electrocatalysis with quantum chemistry

The following article presents a brief introduction to modeling an electrochemical reaction. Two crucial concepts, oxidation-reduction and acid-base reactions, are briefly illustrated to understand the structural changes of the electro-catalyst. These two concepts are applied to compute the stability of catalysts for electrochemical reactions from the density functional theory calculations.

Tantalum based single, double, and triple atom catalysts supported on g-C2N monolayer for effective nitrogen reduction reaction: a comparative DFT investigation

Density functional theory simulations demonstrate that single and triple Ta-atom catalysts anchored to C2N monolayer act as superior catalysts for the nitrogen reduction reaction via alternating and distal pathways.

Performance of Nitrogen Reduction Reaction on Metal Bound g-C6N6: Combined Approach of Machine Learning and DFT

Developing a cost-effective and environmentally benign substitute for the energy-intensive Haber-Bosch process for the production of ammonia is a global challenge. Electrocatalytic nitrogen reduction reaction (NRR) under ambient condition through...

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.

Beyond C3N4 π-conjugated metal-free polymeric semiconductors for photocatalytic chemical transformations

Photocatalysis with stable, efficient and inexpensive metal-free catalysts is one of the most promising options for non-polluting energy production. This review article covers the state-of-the-art development of various effective metal-free polymeric photocatalysts with large π-conjugated units for chemical transformations including water splitting, CO2 and N2 reduction, organic synthesis and monomer polymerisation. The article starts with the catalytic mechanisms of metal-free photocatalysts. Then a particular focus is on the rational manipulation of π-conjugation enlargement, charge separation, electronic structures and band structures in the design of metal-free polymeric photocatalysts. Following the design principles, the selection and construction of functional units are discussed, as well as the connecting bonds and dimensions of π-conjugated polymeric photocatalysts. Finally the hot and emerging applications of metal-free polymeric photocatalysts for photocatalytic chemical transformations are summarized. The strategies provide potential avenues to address the challenges of catalyst activity, selectivity and stability in the further development of highly effective metal-free polymeric photocatalysts.

Exploration and Investigation of Periodic Elements for Electrocatalytic Nitrogen Reduction

High demand for green ecosystems has urged the human community to reconsider and revamp the traditional way of synthesis of several compounds. Ammonia (NH₃ ) is one such compound whose applications have been extended from fertilizers to explosives and is still being synthesized using the high energy inhaling Haber-Bosch process. Carbon free electrocatalytic nitrogen reduction reaction (NRR) is considered as a potential replacement for the Haber-Bosch method. However, it has few limitations such as low N₂ adsorption, selectivity (competitive HER reactions), low yield rate etc. Since it is at the early stage, tremendous efforts have been devoted in understanding the reaction mechanism and screening of the electrocatalysts and electrolytes. In this review, the electrocatalysts are classified based on the periodic table with heat maps of Faraday efficiency and yield rate of NH₃ in NRR and their electrocatalytic properties toward NRR are discussed. Also, the activity of each element is discussed and short tables and concise graphs are provided to enable the researchers to understand recent progress on each element. At the end, a perspective is provided on countering the current challenges in NRR. This review may act as handbook for basic NRR understandings, recent progress in NRR, and the design and development of advanced electrocatalysts and systems.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

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.

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.

Recent advances in catalysts, electrolytes and electrode engineering for the nitrogen reduction reaction under ambient conditions

With the conventional Haber-Bosch NH3 synthesis in industry requiring harsh pressures and high temperatures, artificial N2 fixation has been long sought after. The electrochemical nitrogen reduction reaction (NRR) could offer a solution by allowing NH3 production under ambient conditions. In this review, important recent findings on theoretical calculations and experimental exploration on the NRR at room temperature are systematically reviewed. Firstly, we discuss the mechanism of electrochemical heterogeneous catalysis for the NRR. The NRR is a multi-proton coupled electron transfer (PCET) process which implies that in addition to catalyst surface size effects, ligand and strain effects will also significantly influence the binding energy of the adsorbed N atoms, reaction intermediates and product species. Electrocatalysts including metals, metal nitrides, metal oxides and carbon-based materials will also be discussed at length. A linear scaling relationship seems to limit the NRR activity on most metals and metal oxides. Metal nitrides, however, follow the Mars-van Krevelen (MvK) mechanism which usually shows a lower potential energy barrier compared to the associative mechanism. Carbon-based materials and some single atom catalysts exhibit improved activity and selectivity due to ligand effects. Thus, electrolytes containing a proton donor might play a crucial role in the NRR. The limiting concentration of proton donors and the rate of proton transport to the active sites might be effective factors in boosting the selectivity of the NRR. Specifically, ionic liquids with high N2 solubility demonstrate much larger faradaic efficiency and would be promising candidates for use in NRR processes. Inspired by the characteristics of PCET, four strategies of electrode engineering were introduced including limiting protons, tuning the electron transport, modifying the electrode structure facilitating mass transport, and completely changing the NRR mechanism inspired by bio-nitrogenase and Li mediated N2 fixation.

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.

Boron-decorated C9N4 monolayers as promising metal-free catalysts for electrocatalytic nitrogen reduction reaction: a first-principles study

B decoration enables C₉N₄ monolayer, a novel nodal-line semimetal with high electron mobility, to efficiently catalyze electrocatalytic nitrogen reduction reaction under ambient conditions.

Out-of-plane ion transport makes nitrogenated holey graphite a promising high-rate anode for both Li and Na ion batteries

The search for suitable anodes with good performance is a key challenge for rechargeable Li- and Na-ion batteries (LIBs and NIBs). In this work, we adopt first-principles calculations and ab initio molecular dynamics simulations to investigate the ion transport mechanism and potential of C₂N stoichiometric nitrogenated holey graphite (C₂N-NHG) as a promising anode material for LIBs and NIBs. Although huge in-plane diffusion barriers for both Li and Na ions restrict the application of the C₂N-NHG monolayer as an effective anode, Li and Na ions are found to exhibit facile out-of-plane ion transport in the most stable layered AD stacking C₂N-NHG. The fully lithiated and sodiated cases of LiC₂N and Na_0.67C₂N show reversible specific capacities up to 587 mA h g^-1 and 353 mA h g^-1, low chemical potentials of 0.12 V and 0.25 V, and small volume expansions of 7.16% and 13.54%, respectively. Meanwhile, the out-of-plane collective diffusion reduces Li/Na collective migration barriers to 0.23 eV and 0.18 eV. These findings suggest that AD stacking C₂N-NHG, with metallic properties after lithiation and sodiation processes, high specific capacity, low open circuit voltage, small volume expansion, and low collective migration barriers, has the potential to serve as a promising high-rate anode material for LIBs and NIBs with large energy density and power density. The calculations reveal that the novel out-of-plane diffusion behaviour plays a crucial role in Li/Na ion transport in holey layered materials.

An effective approach for tuning catalytic activity of C3N nanosheets: Chemical-doping with the Si atom

It is well-known that the catalytic oxidation of CO molecule into CO₂ is one of the most important strategies for the removing of this toxic gas from the atmosphere. In the present study, we investigate the reaction pathways and energy barriers for the oxidation of CO by O₂ molecule over the Si-doped C₃N nanosheet. According to our results, doping of C₃N nanosheet with a Si atom could greatly modify its surface reactivity and electronic structure. Due to the large positive charge on the Si, this atom acts as the most active site to adsorb CO and O₂ molecules. Three possible reaction mechanisms are studied for the CO oxidation, namely the Eley-Rideal (ER), Langmuir-Hinshelwood (LH) and new Eley-Rideal (NER). Comparing the activation energies indicates that the CO oxidation reaction proceeds via the LH mechanism over the title surface. The energy barrier needed to remove the activated oxygen atom (O*) from the Si atom is only 0.22 eV, which is most likely to overcome at room temperature. The results of this study may be useful to fabricate noble-metal free catalysts to remove toxic CO molecules from the atmosphere.