Tuning surface curvature in B and N co-doped CNT-derived Fe, Ru and Ir catalysts for electrochemical hydrogenation of N2 to NH3

Single-atom catalysts (SACs) have tremendous applications in enhancing the catalytic performance in the electrocatalytic nitrogen reduction reaction (NRR). Carbon-based substrates have superior properties that improve the catalytic performance either by forming defects or by doping heteroatoms, such as B,N-doped graphene, S-doped graphene, and defective carbon nanotubes. However, the carbon nanotube (CNT)-based electrocatalysts for NRR study are currently less explored. Here, we use the FeB2N2-(n,0) CNTs (n = 3-8) as representative electrocatalysts to study the different CNT curvatures and reveal their effects on the NN triple bond activation and adsorption free energy (ΔG) of the *N2 molecule, with changes in the potential-determining step in NRR. Zigzag B2N2-(6,0) CNTs were selected as the efficient substrate, with three transition metal atoms (TM = Fe, Ru and Ir) anchored on the B2N2-(6,0) CNT to construct the NRR catalysts. Using first-principles calculation and the computational hydrogen electrode (CHE) model, we investigated their electrocatalytic performance in NRR. FeB2N2-(6,0) CNT is the most efficient catalyst and has a low limiting potential (UL) of -0.551 V for NRR. Further, the projected partial density of states and projected crystal orbital Hamilton population analyses illustrate that the N2 activation is due to strong π*-backbonding, which leads to effective charge transfer between the active site (metal d-orbital) and N2 molecule (p-orbital). The FeB2N2-(6,0) CNT also showed high NRR selectivity, inhibiting the competitive hydrogen evolution reaction. Our study provides a detailed mechanism of catalysis by the carbon-based, high-efficiency electrocatalyst for NRR and opens up the possibility for experimentalists to further explore the carbon-based one-dimensional electrocatalyst for NRR.

Advances and Prospects of Selective Electrocatalytic Upgrading of 5-Hydroxymethylfurfural to Furan-2,5-Dicarboxylic Acid

The electrocatalytic upgrading of 5-hydroxymethylfurfural (HMF, 5-(Hydroxymethyl) furan-2-carbaldehyde) has emerged as a renewable and environmentally friendly means for the production of high-value chemicals, with the oxidation product furan-2,5-dicarboxylic acid (FDCA, 2,5-furandicarboxylic acid) possessing economic viability in substituting terephthalic acid in polymer synthesis. This article reviews the recent advancements in the selective electrocatalytic upgrading of HMF to FDCA, including the reaction pathways, mechanisms, as well as activity descriptors of HMF electrocatalytic oxidation reaction (HMFOR), alongside advanced operando characterization techniques. Subsequently, the representative HMFOR catalysts, encompassing noble metal, non-noble metal, transition metal-based catalysts and metal free-based catalysts are presented. Then strategies for regulating HMFOR activity and longevity were introduced, followed by an exploration of the future prospects for the development of HMFOR catalysts.

Construction of CoSe2/WO3@Si Multiheterojunctions for Efficient Photoelectrochemical Water Splitting

Developing efficient photoanodes for photoelectrochemical (PEC) water splitting is crucial for solar-to-hydrogen energy conversion. Monocrystalline silicon, as a photoelectrode material, has limitations of high surface reflectivity, easy formation of oxide passivation, and instability in aqueous solutions. Herein, flower cluster CoSe2 and lamellar WO3 obtained via the solvothermal method are coated onto the surface of textured silicon by chemical bath deposition to prepare a multiheterojunction structured photoanode. The as-prepared CoSe2/WO3@Si-9 photoelectrode exhibits a desirable photocurrent of 10.1 mA cm-2 at 1.23 VRHE under simulated solar irradiation (AM 1.5G, 100 mW cm-2) in comparison to WO3@Si (0.49 mA cm-2) and CoSe2@Si (1.56 mA cm-2) and excellent stability over 10 h. The improved PEC hydrogen evolution performance comes from the synergistic effect of the multiple heterojunctions of CoSe2/WO3@Si composites. The synergistic effect can improve the separation efficiency of photogenerated electron-hole pairs while maintaining strong redox capability. The CoSe2/WO3@Si-9 photoanode exhibits a high photocurrent density and stability, making it a promising candidate for practical applications.

Revealing the origin of activity and selectivity in nitrate to ammonia conversion on single transition metal atom catalysts supported by a Ti2NO2 monolayer

In recent years, the issue of nitrate (NO3-) contamination has become an increasingly severe problem for human life. Electrocatalytic nitrate reduction to ammonia (NH3) is one of the promising strategies to eliminate nitrate contamination. However, the reduction of NO3- to NH3 is a multi-electron process and is susceptible to interference from by-products and competing hydrogen evolution reactions (HER). Introducing a single transition metal (TM) atom onto MXene-based surfaces can alter MXenes' electronic configuration, enhancing their catalytic performance and the Faraday efficiency of the nitrate reduction reaction (NO3RR). In this work, we investigated the initial activation mechanisms of nitrate on various TM-modified Ti2NO2 (TM@Ti2NO2) catalysts and their NO3RR performance using first-principles calculations, aiming to select effective NO3RR electrocatalysts. The results indicated that both V@Ti2NO2 and Cr@Ti2NO2 were viable catalysts for NO3RR, showing particular promise for the efficient conversion of NO3- to NH3 at the most favorable limiting potentials of -0.41 V and -0.52 V, respectively. Further electronic structure analysis (density of states, COHP, and the descriptor ψ) confirmed that the single TM atom supported the boost in product selectivity and efficiency of NH3 by acting as an electron "bridge" to strengthen the interaction between NO3- and MXenes. AIMD simulations indicated that V@Ti2NO2 and Cr@Ti2NO2 maintained dynamical stability at the reaction temperature. These findings lay the foundation for a deeper understanding of the initial activation mechanisms and provide fresh theoretical insights into the design of MXene-based electrocatalysts with high NO3RR performance.

Electrocatalysts for Urea Synthesis from CO2 and Nitrogenous Species: From CO2 and N2/NOx Reduction to urea synthesis

The traditional industrial synthesis of urea relies on the energy-intensive and polluting process, namely the Haber-Bosch method for ammonia production, followed by the Bosch-Meiser process for urea synthesis. In contrast, electrocatalytic C-N coupling from carbon dioxide (CO2) and nitrogenous species presents a promising alternative for direct urea synthesis under ambient conditions, bypassing the need for ammonia production. This review provides an overview of recent progress in the electrocatalytic coupling of CO2 and nitrogen sources for urea synthesis. It focuses on the role of intermediate species and active site structures in promoting urea synthesis, drawing from insights into reactants' adsorption behavior and interactions with catalysts tailored for CO2 reduction, nitrogen reduction, and nitrate reduction. Advanced electrocatalyst design strategies for urea synthesis from CO2 and nitrogenous species under ambient conditions are explored, providing insights for efficient catalyst design. Key challenges and prospective directions are presented in the conclusion. Mechanistic studies elucidating the C-N coupling reaction and future development directions are discussed. The review aims to inspire further research and development in electrocatalysts for electrochemical urea synthesis.

Promoting Electrochemical Nitrate Reduction to Ammonia on Silver Nanocrystals Doped with Iron Series Elements

Electrochemical nitrate reduction to ammonia (NRA) is a promising approach to remove environmental pollutants while producing green NH3 under ambient conditions. Ag-based nanomaterials have been used in NRA but their iron series elements (Fe, Co, Ni) doping has not been explored yet. Herein, an effective and versatile doping strategy of Ag nanocrystals by iron series elements for efficient NRA is presented. Experimental results show that doping with Fe, Co or Ni can improve the NRA activity. Among the catalysts, AgCo delivers the best performance with a Faraday efficiency (FE) of 88.3 % and ammonia selectivity of 97.4 % at-0.23 V vs RHE, which is 1.9 and 6.2 times higher than that of plain Ag (46.4 % FE and 15.8 % selectivity), respectively. A highest NO3 - conversion rate of AgCo (91.8 %) is achieved, which maintains 16.4 ppm NO3 --N in 4 hours, meeting the drinking water level (~15 ppm NO3 --N). Moreover, the FE, selectivity, conversion rate of AgCo do not decay after the four consecutive cycles. It is found that Co doping can effectively induce the change of Ag d-band center for optimized NRA. This work reveals doping effects of iron series elements on Ag-based catalysts, and shows potential practical application in NRA.

Sulfonate-Functionalized Metal-Organic Framework as a Porous "Proton Reservoir" for Boosting Electrochemical Reduction of Nitrate to Ammonia

The electrochemical reduction reaction of nitrate (NO3RR) is an attractive route to produce ammonia at ambient conditions, but the conversion from nitrate to ammonia, which requires nine protons, has to compete with both the two-proton process of nitrite formation and the hydrogen evolution reaction. Extensive research efforts have thus been made in recent studies to develop electrocatalysts for the NO3RR facilitating the production of ammonia. Rather than designing another better electrocatalyst, herein, we synthesize an electrochemically inactive, porous, and chemically robust zirconium-based metal-organic framework (MOF) with enriched intraframework sulfonate groups, SO3-MOF-808, as a coating deposited on top of the catalytically active copper-based electrode. Although both the overall reaction rate and electrochemically active surface area of the electrode are barely affected by the MOF coating, with negatively charged sulfonate groups capable of enriching more protons near the electrode surface, the MOF coating significantly promotes the selectivity of the NO3RR toward the production of ammonia. In contrast, the use of MOF coating with positively charged trimethylammonium groups to repulse protons strongly facilitates the conversion of nitrate to nitrite, with selectivity of more than 90% at all potentials. Under the optimal operating conditions, the copper electrocatalyst with SO3-MOF-808 coating can achieve a Faradaic efficiency of 87.5% for ammonia production, a nitrate-to-ammonia selectivity of 95.6%, and an ammonia production rate of 97 μmol/cm2 h, outperforming all of those achieved by both the pristine copper (75.0%; 93.9%; 87 μmol/cm2 h) and copper with optimized Nafion coating (83.3%; 86.9%; 64 μmol/cm2 h). Findings here suggest the function of MOF as an advanced alternative to the commercially available Nafion to enrich protons near the surface of electrocatalyst for NO3RR, and shed light on the potential of utilizing such electrochemically inactive MOF coatings in a range of proton-coupled electrocatalytic reactions.

Highly-selective Electrocatalytic Reduction of NO to NH3 using Cu Embedded WS2 Monolayer as Single-atom Catalyst: A DFT Study

Electrocatalytic nitric oxide reduction reaction (NORR) is a promising method for generating NH3 and eliminating harmful NO pollutants. However, developing a NORR catalyst for NH3 synthesis with low cost and high efficiency is still challenging. We here report a series of single-atom catalysts (SACs), designed by embedding nine different transition metals from Sc to Cu in S-vacant WS2 monolayer (TM@WS2), and investigate the electrocatalytic performance for NORR process using the dispersion-corrected density functional theory (DFT) calculations. Among the nine SACs, Cu-based one shows a strong binding to the WS2 surface and high selectivity for the NORR process, and also it greatly inhibits the competing hydrogen evolution reaction (HER). Through ab initio molecular dynamics (AIMD) simulations, the thermal stability of SAC is assessed and found no structure deformation even at 500 K temperature. With the advent of energy descriptor, all possible reactive pathways including distal and alternating mode at both N- and O-end configurations for NH3 production were explored. We predicted that the Cu@WS2 SAC exhibits remarkable catalytic activity and selectivity with lowest limiting potential of-0.41 V via the N-alternating pathway. Our study emphasize that the transition metal dichalcogenide (TMDC) based SACs are potential candidates for converting NO to NH3, and this opens a new avenue in designing NORR catalysts with high catalytic performance.

Pendant Proton-Relays Systematically Tune the Rate and Selectivity of Electrocatalytic Ammonia Generation in a Fe-Porphyrin Based Metal-Organic Framework

Electrocatalytic nitrite reduction (eNO2RR) is a promising alternative route to produce ammonia (NH3). Until now, several molecular catalysts have shown capability to homogeneously reduce nitrite to NH3, while taking advantage of added secondary-sphere functionalities to direct catalytic performance. Yet, realizing such control over heterogeneous electrocatalytic surfaces remains a challenge. Herein, we demonstrate that heterogenization of a Fe-porphyrin molecular catalyst within a 2D Metal-Organic Framework (MOF), allows efficient eNO2RR to NH3. On top of that, installation of pendant proton relaying moieties proximal to the catalytic site, resulted in significant improvement in catalytic activity and selectivity. Notably, systematic manipulation of NH3 faradaic efficiency (up to 90 %) and partial current (5-fold increase) was achieved by varying the proton relay-to-catalyst molar ratio. Electrochemical and spectroscopic analysis show that the proton relays simultaneously aid in generating and stabilizing of reactive Fe-bound NO intermediate. Thus, this concept offers new molecular tools to tune heterogeneous electrocatalytic systems.

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.

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.

Computational investigation of single and multiple boron atom doped WS2 monolayers for superior electrocatalytic reduction of nitrogen

The efficient conversion of nitrogen into ammonia plays a significant role in our modern society. Therefore, the design and development of associated catalysts have become an area of major research interest. Nowadays, an increasing number of studies have been exploring single-atom or double-atom metal-free electrocatalysts for the N2 reduction reaction, where regulating the precise number of catalyst atoms anchored on the substrate posed a real challenge. Herein, with density functional theory (DFT) simulations, this study investigated the activity of single and multiple B atom doped monolayer WS2 catalysts and observed superior efficiencies for nitrogen fixation and reduction. Computational results reveal that these novel catalysts have excellent thermodynamic stability, suitable adsorption of N2, superior catalytic activity and high selectivity for the nitrogen reduction reaction. Notably, this study clearly illustrates that the steric hindrance arising from the adjacent atoms of catalytic sites can be an effective route for manipulating the catalytic performance, offering new insights for the synthesis of high efficiency catalysts. In summary, this series of novel boron doped monolayer WS2 catalysts does not require precise control of the number of catalytic atoms on the substrate, making their preparation easier.

FeP-Fe3O4 nanospheres for electrocatalytic N2 reduction to NH3 under ambient conditions

The electrocatalytic nitrogen reduction reaction (eNRR) under ambient conditions is deemed a promising alternative for NH3 synthesis. In this paper, an FeP-Fe3O4 nanocomposite electrocatalyst was prepared by phosphating annealing using Fe2O3 as a precursor, and the resulting FeP-Fe3O4 exhibited excellent N2-to-NH3-producing activity over a wide potential window. The highest faradaic efficiency of FeP-Fe3O4 is 11.02% at -0.1 V vs. reversible hydrogen electrode (RHE), and the maximum NH3 yield reaches 12.73 μg h-1 mgcat-1, comparable to or exceeding the reported values in this field. Furthermore, the FeP-Fe3O4 nanocomposite electrocatalyst presents high electrochemical stability, selectivity, and durability.

Efficient catalyst-free N2 fixation by water radical cations under ambient conditions

The growth and sustainable development of humanity is heavily dependent upon molecular nitrogen (N2) fixation. Herein we discover ambient catalyst-free disproportionation of N2 by water plasma which occurs via the distinctive HONH-HNOH+• intermediate to yield economically valuable nitroxyl (HNO) and hydroxylamine (NH2OH) products. Calculations suggest that the reaction is prompted by the coordination of electronically excited N2 with water dimer radical cation, (H2O)2+•, in its two-center-three-electron configuration. The reaction products are collected in a 76-needle array discharge reactor with product yields of 1.14 μg cm-2 h-1 for NH2OH and 0.37 μg cm-2 h-1 for HNO. Potential applications of these compounds are demonstrated to make ammonia (for NH2OH), as well as to chemically react and convert cysteine, and serve as a neuroprotective agent (for HNO). The conversion of N2 into HNO and NH2OH by water plasma could offer great profitability and reduction of polluting emissions, thus giving an entirely look and perspectives to the problem of green N2 fixation.

Potential and electric double-layer effect in electrocatalytic urea synthesis

Electrochemical synthesis is a promising way for sustainable urea production, yet the exact mechanism has not been fully revealed. Herein, we explore the mechanism of electrochemical coupling of nitrite and carbon dioxide on Cu surfaces towards urea synthesis on the basis of a constant-potential method combined with an implicit solvent model. The working electrode potential, which has normally overlooked, is found influential on both the reaction mechanism and activity. The further computational study on the reaction pathways reveals that *CO-NH and *NH-CO-NH as the key intermediates. In addition, through the analysis of turnover frequencies under various potentials, pressures, and temperatures within a microkinetic model, we demonstrate that the activity increases with temperature, and the Cu(100) shows the highest efficiency towards urea synthesis among all three Cu surfaces. The electric double-layer capacitance also plays a key role in urea synthesis. Based on these findings, we propose two essential strategies to promote the efficiency of urea synthesis on Cu electrodes: increasing Cu(100) surface ratio and elevating the reaction temperature.

The Advances, Challenges, and Perspectives on Electrocatalytic Reduction of Nitrogenous Substances to Ammonia: A Review

Ammonia (NH3) is considered to be a critical chemical feedstock in agriculture, industry, and other fields. However, conventional Haber-Bosch (HB) ammonia (NH3) production suffers from high energy consumption, harsh reaction conditions, and large carbon dioxide emissions. Despite the emergence of electrocatalytic reduction of nitrogenous substances to NH3 under ambient conditions as a new frontier, there are several bottleneck problems that impede the commercialization process. These include low catalytic efficiency, competition with the hydrogen evolution reaction, and difficulties in breaking the N≡N triple bond. In this review, we explore the recent advances in electrocatalytic NH3 synthesis, using nitrogen and nitrate as reactants. We focus on the contribution of the catalyst design, specifically based on molecular-catalyst interaction mechanisms, as well as chemical bond breaking and directional coupling mechanisms, to address the aforementioned problems during electrocatalytic NH3 synthesis. Finally, we discuss the relevant opportunities and challenges in this field.

Nitrogen Electrocatalysis: Electrolyte Engineering Strategies to Boost Faradaic Efficiency

The electrochemical activation of dinitrogen at ambient temperature and pressure for the synthesis of ammonia has drawn increasing attention. The faradaic efficiency (FE) as well as ammonia yield in the electrochemical synthesis is far from reaching the requirement of industrial-scale production. In aqueous electrolytes, the competing electron-consuming hydrogen evolution reaction (HER) and poor solubility of nitrogen are the two major bottlenecks. As the electrochemical reduction of nitrogen involves proton-coupled electron transfer reaction, rationally engineered electrolytes are required to boost FE and ammonia yield. In this Review, we comprehensively summarize various electrolyte engineering strategies to boost the FE in aqueous and non-aqueous medium and suggest possible approaches to further improve the performance. In aqueous medium, the performance can be improved by altering the electrolyte pH, transport velocity of protons, and water activity. Other strategies involve the use of hybrid and water-in-salt electrolytes, ionic liquids, and non-aqueous electrolytes. Existing aqueous electrolytes are not ideal for industrial-scale production. Suppression of HER and enhanced nitrogen solubility have been observed with hybrid and non-aqueous electrolytes. The engineered electrolytes are very promising though the electrochemical activation has several challenges. The outcome of lithium-mediated nitrogen reduction reaction with engineered non-aqueous electrolyte is highly encouraging.

Nanosheet arrays of iron oxide for enhanced ammonia synthesis via electrochemical nitrogen reduction for prospective algal membrane bioreactors

The earth's nitrogen cycle relies on the effective conversion of nitrogen (N2) to ammonia (NH3). As a result, the research and development of catalysts that are earth-abundant, inexpensive, and highly efficient but do not need precious metals is of the utmost significance. In this investigation, we present a controlled synthesis technique to the fabrication of an iron oxide (Fe2O3) nanosheet array by annealing at temperatures ranging from 350 to 550 °C. This array will be used for the electrochemical reduction of atmospheric N2 to NH3 in electrolytes. The Fe2O3 nanosheet array that was produced as a result displays outstanding electrochemical performance as well as remarkable stability. When compared to a hydrogen electrode working under normal temperature and pressure conditions, the Fe2O3 nanosheet array produces an impressive NH3 production rate of 18.04 g per hour per mg of catalytically active material in 0.1 M KOH electrolyte, exhibiting an enhanced Faradaic efficiency (FE) of 13.5% at -0.35 V. This is accomplished by exhibiting an enhanced Faradaic efficiency (FE) of 0.1 M KOH electrolyte. The results of experiments and electrochemical studies reveal that the existence of cation defects in the Fe2O3 nanosheets plays an essential part in the enhancement of the electrocatalytic activity that takes place during nitrogen reduction reactions (NRR). This study not only contributes to the expanding family of transition-metal-based catalysts with increased electrocatalytic activity for NRR, but it also represents a substantial breakthrough in the design of catalysts that are based on transition metals, so it's a win-win. In addition, the use of Fe2O3 nanosheets as electrocatalysts has a lot of potential in algal membrane bioreactors because it makes nitrogen fixation easier, it encourages algae growth, and it makes nitrogen cycling more resource-efficient.

Metal-Based Electrocatalysts for Selective Electrochemical Nitrogen Reduction to Ammonia

Ammonia (NH3) plays a significant role in the manufacture of fertilizers, nitrogen-containing chemical production, and hydrogen storage. The electrochemical nitrogen reduction reaction (e-NRR) is an attractive prospect for achieving clean and sustainable NH3 production, under mild conditions driven by renewable energy. The sluggish cleavage of N≡N bonds and poor selectivity of e-NRR are the primary challenges for e-NRR, over the competitive hydrogen evolution reaction (HER). The rational design of e-NRR electrocatalysts is of vital significance and should be based on a thorough understanding of the structure-activity relationship and mechanism. Among the various explored e-NRR catalysts, metal-based electrocatalysts have drawn increasing attention due to their remarkable performances. This review highlighted the recent progress and developments in metal-based electrocatalysts for e-NRR. Different kinds of metal-based electrocatalysts used in NH3 synthesis (including noble-metal-based catalysts, non-noble-metal-based catalysts, and metal compound catalysts) were introduced. The theoretical screening and the experimental practice of rational metal-based electrocatalyst design with different strategies were systematically summarized. Additionally, the structure-function relationship to improve the NH3 yield was evaluated. Finally, current challenges and perspectives of this burgeoning area were provided. The objective of this review is to provide a comprehensive understanding of metal-based e-NRR electrocatalysts with a focus on enhancing their efficiency in the future.

Intrinsic and external active sites of single-atom catalysts

Active components with suitable supports are the common paradigm for industrial catalysis, and the catalytic activity usually increases with minimizing the active component size, generating a new frontier in catalysis, single-atom catalysts (SACs). However, further improvement of SACs activity is limited by the relatively low loading of single atoms (SAs, which are heteroatoms for most SACs, i.e., external active sites) because of the highly favorable aggregation of single heteroatoms during preparation. Research interest should be shifted to investigate SACs with intrinsic SAs, which could circumvent the aggregation of external SAs and consequently increase the SAs loading while maintaining them individual to further improve the activity. In this review, SACs with external or intrinsic SAs are discussed and, at last, the perspectives and challenges for obtaining high-loading SACs with intrinsic SAs are outlined.

Enhanced NH3 Synthesis from Air in a Plasma Tandem-Electrocatalysis System Using Plasma-Engraved N-Doped Defective MoS2

We have developed a sustainable method to produce NH₃ directly from air using a plasma tandem-electrocatalysis system that operates via the N₂-NO_x-NH₃ pathway. To efficiently reduce NO₂^- to NH₃, we propose a novel electrocatalyst consisting of defective N-doped molybdenum sulfide nanosheets on vertical graphene arrays (N-MoS₂/VGs). We used a plasma engraving process to form the metallic 1T phase, N doping, and S vacancies in the electrocatalyst simultaneously. Our system exhibited a remarkable NH₃ production rate of 7.3 mg h^-1 cm^-2 at -0.53 V vs RHE, which is almost 100 times higher than the state-of-the-art electrochemical nitrogen reduction reaction and more than double that of other hybrid systems. Moreover, a low energy consumption of only 2.4 MJ mol_NH3^-1 was achieved in this study. Density functional theory calculations revealed that S vacancies and doped N atoms play a dominant role in the selective reduction of NO₂^- to NH₃. This study opens up new avenues for efficient NH₃ production using cascade systems.

Converting N2 molecules into NH3 with TiO2/Fe3O4 composite covered with a thin water layer under ambient condition

As ammonia manufacture today require huge energy and very pure hydrogen gas and moreover emit large quantities of CO₂, researches for new ammonia synthesis methods are actively performed. Here, author reports the novel method through which N₂ molecules in air is reduced into ammonia with TiO₂/Fe₃O₄ composite having thin water layer on composite's surface under ambient condition (less than 100 °C and atmospheric pressure). The composites were composed of both nm-sized TiO₂ particles and μm-sized Fe₃O₄ ones. First, composites were held in refrigerator, mainly at that time, N₂ molecules in air adsorbed onto surface of composite. Next, the composite was irradiated with various lights including solar light, 365 nm LED light and tungsten light through thin water layer formed by condensation of water vapour in air. Reliable amount of ammonia was obtained under 5 min's irradiation of solar light or of both 365 m LED light and 500 W tungsten light. This reaction was catalytic reaction promoted by photocatalytic one. In addition, holding in freezer instead of refrigerator provided larger amount of ammonia. Maximum ammonia yield was approximately 18.7 μmol/g 5 min under irradiation of 300 W tungsten light only.

Layered Double Hydroxide-Derived Ni-Mg-Al Catalysts for Ammonia Decomposition Process: Synthesis and Characterization

Layered Ni-Mg-Al hydroxides with (Ni + Mg)/Al = 2.5 differing in Mg/Ni ratios and related oxide systems have been synthesized and characterized. Ni-Mg-Al hydroxides were prepared by the coprecipitation method. It was found that the samples dried at 110 °C were layered Ni-Mg-Al hydroxides with a hydrotalcite-type structure. After the heat treatment at 600 °C, the formation of Ni-Mg-Al-mixed oxides with a specific nanostructure, an intermediate between a NaCl and spinel structure, took place. According to XRD data, it had the unit cell parameter a = 4.174–4.181 Å, and a crystallite size of 4.0 nm. The specific surface area of the Ni-Mg-Al samples dried at 110 °C was 45–54 m2/g, and that of those calcined at 600 °C was 156.1–209.1 m2/g. In agreement with HRTEM data, in all the synthesized nickel catalysts reduced at 700 °C (H2), particle size was mainly distributed between 15–20 nm. The catalyst activity of LDH-derived Ni-Mg-Al catalysts in ammonia decomposition was studied in a fixed-bed flow-type reactor at an atmospheric pressure within the temperature range 500–700 °C. The synthesized catalysts overcame existing analogues in catalytic performance. At a process temperature of 500 °C, the Ni2Mg3Al2-HT catalyst showed that the H2 productivity was 23.8 mmol/(gcat·min), exceeding the respective value of nickel catalysts reported in the literature.

Advanced hematite nanomaterials for newly emerging applications

Because of the combined merits of rich physicochemical properties, abundance, low toxicity, etc., hematite (α-Fe2O3), one of the most chemically stable compounds based on the transition metal element iron, is endowed with multifunctionalities and has steadily been a research hotspot for decades. Very recently, advanced α-Fe2O3 materials have also been developed for applications in some cutting-edge fields. To reflect this trend, the latest progress in developing α-Fe2O3 materials for newly emerging applications is reviewed with a particular focus on the relationship between composition/nanostructure-induced electronic structure modulation and practical performance. Moreover, perspectives on the critical challenges as well as opportunities for future development of diverse functionalities are also discussed. We believe that this timely review will not only stimulate further increasing interest in α-Fe2O3 materials but also provide a profound understanding and insight into the rational design of other materials based on transition metal elements for various applications.

Recent Progress in Electrochemical Nitrogen Reduction on Transition Metal Nitrides

Distributed electrochemical nitrogen reduction reaction (ENRR) powered by renewable energy for the on-site production of ammonia is an attractive alternative to the industrial Haber-Bosch process, which is responsible for roughly 2 % of global energy consumption. In this Review, we summarize recent progress in the ENRR catalyzed by transition metal nitrides (TMNs). The unique electronic structures of TMNs make them promising ENRR catalysts for active and selective ammonia production, which have been predicted theoretically and demonstrated experimentally. Reaction pathways and deactivation mechanisms of the ENRR on different TMNs are surveyed, and current understanding of structure-activity relations is discussed. To develop highly active, selective, and stable TMN catalysts for industrial-scale ENRR, membrane electrode assembly configuration is recommended in catalyst evaluation. Furthermore, we highlight the importance of developing mechanistic understanding on ENRR with different operando spectroscopic techniques.

Ambient Electrosynthesis of Urea with Nitrate and Carbon Dioxide over Iron-Based Dual-Sites

The development of efficient electrocatalysts to generate key *NH₂ and *CO intermediates is crucial for ambient urea electrosynthesis with nitrate (NO₃ ^- ) and carbon dioxide (CO₂ ). Here we report a liquid-phase laser irradiation method to fabricate symbiotic graphitic carbon encapsulated amorphous iron and iron oxide nanoparticles on carbon nanotubes (Fe(a)@C-Fe₃ O₄ /CNTs). Fe(a)@C-Fe₃ O₄ /CNTs exhibits superior electrocatalytic activity toward urea synthesis using NO₃ ^- and CO₂ , affording a urea yield of 1341.3±112.6 μg h^-1  mg_cat ^-1 and a faradic efficiency of 16.5±6.1 % at ambient conditions. Both experimental and theoretical results indicate that the formed Fe(a)@C and Fe₃ O₄ on CNTs provide dual active sites for the adsorption and activation of NO₃ ^- and CO₂ , thus generating key *NH₂ and *CO intermediates with lower energy barriers for urea formation. This work would be helpful for design and development of high-efficiency dual-site electrocatalysts for ambient urea synthesis.

Boosting Electrocatalytic Ammonia Synthesis of Bio-Inspired Porous Mo-Doped Hematite via Nitrogen Activation

Electrochemical N₂ reduction reaction (NRR) emerges as a highly attractive alternative to the Haber-Bosch process for producing ammonia (NH₃) under ambient circumstances. Currently, this technology still faces tremendous challenges due to the low ammonia production rate and low Faradaic efficiency, urgently prompting researchers to explore highly efficient electrocatalysts. Inspired by the Fe-Mo cofactor in nitrogenase, we report Mo-doped hematite (Fe₂O₃) porous nanospheres containing Fe-O-Mo subunits for enhanced activity and selectivity in the electrochemical reduction from N₂ to NH₃. Mo-doping induces the morphology change from a solid sphere to a porous sphere and enriches lattice defects, creating more active sites. It also regulates the electronic structures of Fe₂O₃ to accelerate charge transfer and enhance the intrinsic activity. As a consequence, Mo-doped Fe₂O₃ achieves effective N₂ fixation with a high ammonia production rate of 21.3 ± 1.1 μg h^-1 mg_cat.^-1 as well as a prominent Faradaic efficiency (FE) of 11.2 ± 0.6%, superior to the undoped Fe₂O₃ and other iron oxide catalysts. Density functional theory (DFT) calculations further unravel that the Mo-doping in Fe₂O₃ (110) narrows the band gap, promotes the N₂ activation on the Mo site with an elongated N≡N bond length of 1.132 Å in the end-on configuration, and optimizes an associative distal pathway with a decreased energy barrier. Our results may pave the way toward enhancing the electrocatalytic NRR performance of iron-based materials by atomic-scale heteroatom doping.

Assembly of Hydrophobic ZIF-8 on CeO2 Nanorods as High-Efficiency Catalyst for Electrocatalytic Nitrogen Reduction Reaction

The electrocatalytic nitrogen reduction reaction (NRR) can use renewable electricity to convert water and N2 into NH3 under normal temperature and pressure conditions. However, due to the competitiveness of the hydrogen evolution reaction (HER), the ammonia production rate (RNH3) and Faraday efficiency (FE) of NRR catalysts cannot meet the needs of large-scale industrialization. Herein, by assembling hydrophobic ZIF-8 on a cerium oxide (CeO2) nanorod, we designed an excellent electrocatalyst CeO2-ZIF-8 with intrinsic NRR activity. The hydrophobic ZIF-8 surface was conducive to the efficient three-phase contact point of N2 (gas), CeO2 (solid) and electrolyte (liquid). Therefore, N2 is concentrated and H+ is deconcentrated on the CeO2-ZIF-8 electrocatalyst surface, which improves NRR and suppresses HER and finally CeO2-ZIF-8 exhibits excellent NRR performance with an RNH3 of 2.12 μg h-1 cm-2 and FE of 8.41% at -0.50 V (vs. RHE). It is worth noting that CeO2-ZIF-8 showed excellent stability in the six-cycle test, and the RNH3 and FE variation were negligible. This study paves a route for inhibiting the competitive reaction to improve the NRR catalyst activity and may provide a new strategy for NRR catalyst design.

Lewis acid-dominated aqueous electrolyte acting as co-catalyst and overcoming N2 activation issues on catalyst surface

The growing demands for ammonia in agriculture and transportation fuel stimulate researchers to develop sustainable electrochemical methods to synthesize ammonia ambiently, to get past the energy-intensive Haber-Bosch process. However, the conventionally used aqueous electrolytes limit N₂ solubility, leading to insufficient reactant molecules in the vicinity of the catalyst during electrochemical nitrogen reduction reaction (NRR). This hampers the yield and production rate of ammonia, irrespective of how efficient the catalyst is. Herein, we introduce an aqueous electrolyte (NaBF₄), which not only acts as an N₂-carrier in the medium but also works as a full-fledged "co-catalyst" along with our active material MnN₄ to deliver a high yield of NH₃ (328.59 μg h^-1 mg_cat^-1) at 0.0 V versus reversible hydrogen electrode. BF₃-induced charge polarization shifts the metal d-band center of the MnN₄ unit close to the Fermi level, inviting N₂ adsorption facilely. The Lewis acidity of the free BF₃ molecules further propagates their importance in polarizing the N≡N bond of the adsorbed N₂ and its first protonation. This push-pull kind of electronic interaction has been confirmed from the change in d-band center values of the MnN₄ site as well as charge density distribution over our active model units, which turned out to be effective enough to lower the energy barrier of the potential determining steps of NRR. Consequently, a high production rate of NH₃ (2.45 × 10^-9 mol s^-1 cm^-2) was achieved, approaching the industrial scale where the source of NH₃ was thoroughly studied and confirmed to be chiefly from the electrochemical reduction of the purged N₂ gas.

Accelerating electrochemically catalyzed nitrogen reductions using metalloporphyrin-mediated metal-nitrogen-doped carbon (M-N-C) catalysts

Herein, a series of transition metal coordinated metalloporphyrin-mediated M-N-C catalysts with single and dual metal atoms were prepared and characterized. Interestingly, these M-N-C catalysts exhibit accelerated N₂ reduction behaviors through electrochemical catalysis. At the potential of E = -0.4V (vs. RHE), the optimum catalyst Fe_0.95Ni_0.05TPP@rGO-800 shows excellent catalytic activity, and the NH₃ yield is 22.5 μg mg_cat^-1 h^-1, which is much higher than that of its single metal counterparts alone, and the faradaic efficiency is as high as 50.7%, which is better than those of most reported catalysts. These results provide an opportunity to further explore the efficient electrochemical synthesis of NH₃ from M-N-C materials in the future.

Oxygen-regulated carbon quantum dots as an efficient metal-free electrocatalyst for nitrogen reduction

An electrocatalytic nitrogen reduction reaction under ambient conditions provides a wonderful blueprint for the conversion of nitrogen to ammonia. However, current research on ammonia synthesis is mainly focused on metal-based catalysts. It is still a great challenge to realize the effective activation of N₂ on non-metallic catalysts. Herein, carbon quantum dots are reported to reduce dinitrogen to ammonia under ambient conditions. Benefiting from its numerous defect sites, this metal-free catalyst shows excellent catalytic performance in 0.1 M HCl with a faradaic efficiency of 17.59%. In addition, both experimental and theoretical results confirm that the catalytic performance of the catalyst can be improved by appropriately controlling the oxygen content of samples at different temperatures, and the utmost ammonia yield is 134.08 μg h^-1 mg^-1_cat., which is almost three times higher than that of a reported metal-free material. The proposed oxygen regulation provides a new method to optimize the surface properties of metal-free catalysts for ammonia synthesis.

Revealing the Potential of Ternary Medium-Entropy Alloys as Exceptional Electrocatalysts toward Nitrogen Reduction: An Example of Heusler Alloys

With less energy consumption and environmental pollution, electrochemical ammonia synthesis is regarded as the most promising way to replace the industrial Haber-Bosch process, which greatly contributes to global energy consumption and CO₂ emission. At present, the best metal electrocatalyst for N₂ fixation is ruthenium although its performance still suffers from a low Faradaic efficiency and a high overpotential. Alloy engineering is a promising way to discover more metal-based electrocatalysts for dinitrogen reduction reaction (N2RR), and almost all reported alloy catalysts so far are binary alloys. In this work, we proposed a large group of ternary alloy electrocatalysts (Heusler alloys) for N2RR and demonstrated their superior catalytic performance. As an example, alloying Ru with Mn and Si led to a reduced Ru-Ru distance on the surface, which facilitates an uncommon horizontal adsorption mode of N₂ and results in effective activation of N₂ molecules. The theoretical overpotential of N2RR on Ru₂MnSi(100-Ru) is only around 0.28 V, which ranks among the best reported results, and the usage of precious Ru is greatly reduced. Meanwhile, the adsorption of N₂ on Ru₂MnSi(100-Ru) was much stronger than that of protons, and it also took less energy to drive N2RR than the hydrogen evolution reaction (HER), making HER less competitive on this catalyst. Considering the successful synthesis of numerous Heusler alloys including the six members mentioned here, our work provided a wider range of practical and excellent N2RR electrocatalysts in terms of both catalytic performance and economical cost.

Ambient Ammonia Synthesis via Electrochemical Reduction of Nitrate Enabled by NiCo2 O4 Nanowire Array

NiCo₂ O₄ nanowire array on carbon cloth (NiCo₂ O₄ /CC) is proposed as a highly active electrocatalyst for ambient nitrate (NO₃ ^- ) reduction to ammonia (NH₃ ). In 0.1 m NaOH solution with 0.1 m NaNO₃ , such NiCo₂ O₄ /CC achieves a high Faradic efficiency of 99.0% and a large NH₃ yield up to 973.2 µmol h^-1  cm^-2 . The superior catalytic activity of NiCo₂ O₄ comes from its half-metal feature and optimized adsorption energy due to the existence of Ni in the crystal structure. A Zn-NO₃ ^- battery with NiCo₂ O₄ /CC cathode also shows a record-high battery performance.

Termination-Accelerated Electrochemical Nitrogen Fixation on Single-Atom Catalysts Supported by MXenes

The synthesis of ammonia (NH₃) from nitrogen (N₂) under ambient conditions is of great significance but hindered by the lack of highly efficient catalysts. By performing first-principles calculations, we have investigated the feasibility for employing a transition metal (TM) atom, supported on Ti₃C₂T₂ MXene with O/OH terminations, as a single-atom catalyst (SAC) for electrochemical nitrogen reduction. The potential catalytic performance of TM single atoms is evaluated by their adsorption behavior on the MXene, together with their ability to bind N₂ and to desorb NH₃ molecules. Of importance, the OH terminations on Ti₃C₂T₂ MXene can effectively enhance the N₂ adsorption and decrease the NH₃ adsorption for single atoms. Based on proposed criteria for promising SACs, our calculations further demonstrate that the Ni/Ti₃C₂O_0.19(OH)_1.81 exhibits reasonable thermodynamics and kinetics toward electrochemical nitrogen reduction.

Amorphous Chromium Oxide with Hollow Morphology for Nitrogen Electrochemical Reduction under Ambient Conditions

The electrocatalytic nitrogen reduction reaction (NRR), an alternative method of nitrogen fixation and conversion under ambient conditions, represents a promising strategy for tackling the energy-intensive issue. The design of high-performance electrocatalysts is one of the key issues to realizing the application of NRR, but most of the current catalysts rely on the use of crystalline materials, and shortcomings such as a limited number of catalytic active sites and sluggish reaction kinetics arise. Herein, an amorphous metal oxide catalyst H-CrO_x/C-550 with hierarchically porous structure is constructed, which shows superior electrocatalytic performance toward NRR under ambient conditions (yield of 19.10 μg h^-1 mg_cat^-1 and Faradaic efficiency of 1.4% at -0.7 V vs a reversible hydrogen electrode, higher than that of crystalline Cr₂O₃ and solid counterparts). Notably, the amorphous metal oxide obtained by controlled pyrolysis of metal-organic frameworks (MOFs) possess abundant unsaturated catalytic sites and optimized conductivity due to the controllable degree of metal-oxygen bond reconstruction and the doping of carbon materials derived from organic ligands. This work demonstrates MOF-derived porous amorphous materials as a viable alternative to current electrocatalysts for NH₃ synthesis at ambient conditions.

In Situ Loading of Cu2O Active Sites on Island-like Copper for Efficient Electrochemical Reduction of Nitrate to Ammonia

Electrochemical nitrate reduction reaction (NO₃RR) offers a new pathway for low-temperature green ammonia synthesis. It is widely known that copper and its copper oxide catalysts are selective for NO₃RRs, although the role played by their oxidation state in catalysis is not fully understood. Here, we found that in situ electrochemical reduction modulates the oxidation state of copper facilitating in situ loading of Cu₂O active sites on island-like copper, and investigated the effect of cuprous oxide on nitrate reduction. We found that the improvement of ammonia yield (Faraday efficiency: 98.28%, selectivity: 96.6%) was closely related to the generation of Cu₂O, which exceeded the performance of the state-of-the-art catalysts available today. The presence of a multilayer structure of the material was demonstrated by X-ray photoelectron spectroscopy combined with ion beam sputtering. Using operando Raman spectroscopy, we monitored the reduction process of the catalyst surface oxide species at the applied potential. Density functional theory (DFT) calculations indicated that the stable presence of Cu(I) effectively promotes the conversion of *HNOH to *HNHOH. We optimized the model building for DFT calculations and established relatively more reliable reaction paths, which provided a strong support for a further understanding of the reaction mechanism of NO₃RR.