Rational Catalyst Design for N2 Reduction under Ambient Conditions: Strategies toward Enhanced Conversion Efficiency

Antimony-Doped Wide Bandgap Molybdenum Trioxide with Enhanced Localized Surface Plasmon Resonance for Nitrogen Photofixation

Plasmonic metal oxides are promising photocatalysts for the artificial photosynthesis of green ammonia due to localized surface plasmon resonance (LSPR) enhanced photoconversion and rich surface oxygen vacancies improved chemisorption and activation of dinitrogen molecules. However, these oxygen vacancies are unstable during the photocatalytic process and could be oxidized by photogenerated holes, leading to the vanishing of the LSPR. Here, we fabricated antimony-doped molybdenum trioxide nanosheets with stable plasmonic absorption extending into the near-infrared (NIR) range, even after harsh treatment in oxidative atmospheric conditions at high temperatures. For undoped plasmonic MoO3-x nanosheets, the LSPR originates from the abundant oxygen vacancies that vanish after heat treatment at high temperatures in air, leading to the disappearance of the LSPR absorption. Sb doping does not significantly increase the concentration of oxygen vacancies while donating more free electrons because Sb can keep a lower oxidation state. Heat treatment diminished the oxygen vacancies while not affecting the low oxidation state of Sb. As a result, heat-treated Sb-doped MoO3-x nanosheets still show strong LSPR absorption in the NIR range. Both experimental results and theoretical calculations demonstrated that add-on states close to the Fermi level are formed due to the Sb doping and high concentration of oxygen vacancies. The prepared samples were used for photocatalytic nitrogen reduction and showed an LSPR-dependent photocatalytic performance. The present work has provided an effective strategy to stabilize the LSPR of plasmonic semiconductor photocatalysts.

Vacancy Ordering in Ultrathin Copper Oxide Films on Cu(111)

Oxide thin films grown on metal surfaces have wide applications in catalysis and beyond owing to their unique surface structures compared to their bulk counterparts. Despite extensive studies, the atomic structures of copper surface oxides on Cu(111), commonly referred to as "44" and "29", have remained elusive. In this work, we demonstrated an approach for the structural determination of oxide surfaces using element-specific scanning tunneling microscopy (STM) imaging enhanced by functionalized tips. This approach enabled us to resolve the atomic structures of "44" and "29" surface oxides, which were further corroborated by noncontact atomic force microscopy (nc-AFM) measurements and Monte Carlo (MC) simulations. The stoichiometry of the "44" and "29" frameworks was identified as Cu23O16 and Cu16O11, respectively. Contrary to the conventional hypothesis, we observed ordered Cu vacancies within the "44" structure manifesting as peanut-shaped cavities in the hexagonal lattice. Similarly, a combination of Cu and O vacancies within the "29" structure leads to bean-shaped cavities within the pentagonal lattice. Our study has thus resolved the decades-long controversy on the atomic structures of "44" and "29" surface oxides, advancing our understanding of copper oxidation processes and introducing a robust framework for the analysis of complex oxide surfaces.

Progress Made in Non-Metallic-Doped Materials for Electrocatalytic Reduction in Ammonia Production

The electrocatalytic production of ammonia has garnered considerable interest as a potentially sustainable technology for ammonia synthesis. Recently, non-metallic-doped materials have emerged as promising electrochemical catalysts for this purpose. This paper presents a comprehensive review of the latest research on non-metallic-doped materials for electrocatalytic ammonia production. Researchers have engineered a variety of materials, doped with non-metals such as nitrogen (N), boron (B), phosphorus (P), and sulfur (S), into different forms and structures to enhance their electrocatalytic activity and selectivity. A comparison among different non-metallic dopants reveals their distinct effects on the electrocatalytic performance for ammonia production. For instance, N-doping has shown enhanced activity owing to the introduction of nitrogen vacancies (NVs) and improved charge transfer kinetics. B-doping has demonstrated improved selectivity and stability, which is attributed to the formation of active sites and the suppression of competing reactions. P-doping has exhibited increased ammonia generation rates and Faradaic efficiencies, likely due to the modification of the electronic structure and surface properties. S-doping has shown potential for enhancing electrocatalytic performance, although further investigations are needed to elucidate the underlying mechanisms. These comparisons provide valuable insights for researchers to conduct in-depth studies focusing on specific non-metallic dopants, exploring their unique properties, and optimizing their performance for electrocatalytic ammonia production. However, we consider it a priority to provide insight into the recent progress made in non-metal-doped materials and their potential for enabling long-term and efficient electrochemical ammonia production. Additionally, this paper discusses the synthetic procedures used to produce non-metal-doped materials and highlights the advantages and disadvantages of each method. It also provides an in-depth analysis of the electrochemical performance of these materials, including their Faradaic efficiencies, ammonia yield rate, and selectivity. It examines the challenges and prospects of developing non-metallic-doped materials for electrocatalytic ammonia production and suggests future research directions.

Mechanistic Studies of Stimulus-Response Integrated Catalysis of Single-Atom Alloys under Electric Fields for Electrochemical Nitrogen Reduction

The present work introduces a novel catalytic strategy to promote the nitrogen reduction reaction (NRR) by employing a cooperative Cu-based single-atom alloy (SAA) and oriented external electric fields (OEEFs) as catalysts. The field strength (F)-dependent reaction pathways are investigated by means of first-principles calculations. Different dipole-induced responses of intermediates to electric fields break the original scaling relationships and effectively tune not only the activity but also the product selectivity of the NRR. When the most active Os1Cu SAA is taken as an example, in the absence of an OEEF, the overpotential (η) of the NRR is 0.62 V, which is even larger than that of the competitive hydrogen evolution reaction (HER). A negative field not only reduces η but switches the preference to the NRR over the HER. In particular, η at F = -1.14 V/Å reaches the bottom of 0.18 V, which is 70% lower than that in the field-free state.

Modulating band structure through introducing Cu0/CuxO composites for the improved visible light driven ammonia synthesis

The photocatalytic performance of ceria-based materials can be tuned by adjusting the surface structures with decorating the transition-metal, which are considered as the important active sites. Herein, cuprous oxide-metallic copper composite-doped ceria nanorods were assembled through a simple hydrothermal reduction method. The photocatalytic ammonia synthesis rates exhibit an inverted "V-shaped" trend with increasing Cu0/CuxO mole ratio. The best ammonia production rate, approximately 900 or 521 µmol·gcal-1·h-1 under full-spectra or visible light, can be achieved when the Cu0/CuxO ratio is approximately 0.16, and this value is 8 times greater than that of the original sample. The absorption edge of the as-prepared samples shifted towards visible wavelengths, and they also had appropriate ammonia synthesis levels. This research provides a strategy for designing noble metal-free photocatalysts through introducing the metal/metallic oxide compositesto the catalysts.

Frontiers in Photoelectrochemical Catalysis: A Focus on Valuable Product Synthesis

Photoelectrochemical (PEC) catalysis provides the most promising avenue for producing value-added chemicals and consumables from renewable precursors. Over the last decades, PEC catalysis, including reduction of renewable feedstock, oxidation of organics, and activation and functionalization of C─C and C─H bonds, are extensively investigated, opening new opportunities for employing the technology in upgrading readily available resources. However, several challenges still remain unsolved, hindering the commercialization of the process. This review offers an overview of PEC catalysis targeted at the synthesis of high-value chemicals from sustainable precursors. First, the fundamentals of evaluating PEC reactions in the context of value-added product synthesis at both anode and cathode are recalled. Then, the common photoelectrode fabrication methods that have been employed to produce thin-film photoelectrodes are highlighted. Next, the advancements are systematically reviewed and discussed in the PEC conversion of various feedstocks to produce highly valued chemicals. Finally, the challenges and prospects in the field are presented. This review aims at facilitating further development of PEC technology for upgrading several renewable precursors to value-added products and other pharmaceuticals.

Hot Carrier Controlled Nitrogen Fixation Reaction in Metal-Free Boron-Anchored Aza-COF: Insight from Nonadiabatic Molecular Dynamics Simulation

Designing highly efficient photocatalysts for the production of renewable energy is a challenging task that necessitates simultaneous control of chemical activity and photocarrier dynamics for a particular reaction. To this end, we have investigated the catalytic mechanism and real-time photocarrier dynamics of the nitrogen reduction reaction (NRR) at the metal-free boron-functionalized 2D aza-COF (B-aza-COF), an inexpensive and environmentally friendly semiconductor. By employing density functional theory (DFT) and time-dependent ab initio nonadiabatic molecular dynamics simulation, we have investigated the electronic structure, light harvesting ability, free energy change, and dynamics of photoexcited carriers. Our calculated results reveal that the gas phase N2 molecule can be effectively reduced into NH3 on B-aza-COF under UV-visible light. Therefore, our investigation on the design of efficient photocatalysts for the nitrogen reduction reaction (NRR) provides a cost-effective opportunity for the sustainable production of NH3.

Defect engineering on electrocatalysts for sustainable nitrate reduction to ammonia: Fundamentals and regulations

Electrocatalytic nitrate (NO3 -) reduction to ammonia (NH3) is a "two birds-one stone" method that targets remediation of NO3 --containing sewage and production of valuable NH3. The exploitation of advanced catalysts with high activity, selectivity, and durability is a key issue for the efficient catalytic performance. Among various strategies for catalyst design, defect engineering has gained increasing attention due to its ability to modulate the electronic properties of electrocatalysts and optimize the adsorption energy of reactive species, thereby enhancing the catalytic performance. Despite previous progress, there remains a lack of mechanistic insights into the regulation of catalyst defects for NO3 - reduction. Herein, this review presents insightful understanding of defect engineering for NO3 - reduction, covering its background, definition, classification, construction, and underlying mechanisms. Moreover, the relationships between regulation of catalyst defects and their catalytic activities are illustrated by investigating the properties of electrocatalysts through the analysis of electronic band structure, charge density distribution, and controllable adsorption energy. Furthermore, challenges and perspectives for future development of defects in NO3RR are also discussed, which can help researchers to better understand the defect engineering in catalysts, and also inspire scientists entering into this promising field.

Recent Advances in Ammonia Synthesis: From Haber-Bosch Process to External Field Driven Strategies

Ammonia, a pivotal chemical feedstock and a potential hydrogen energy carrier, demands efficient synthesis as a key step in its utilization. The traditional Haber-Bosch process, known for its high energy consumption, has spurred researchers to seek ammonia synthesis under milder conditions. Advances in surface science and characterization technologies have deepened our understanding of the microscopic reaction mechanisms of ammonia synthesis. This article concentrates on gas-solid phase ammonia synthesis, initially exploring the latest breakthroughs and improvements in thermal catalytic synthesis. Building on this, it especially focuses on emerging external field-driven alternatives, such as photocatalysis, photothermal catalysis, and low-temperature plasma catalysis strategies. The paper concludes by discussing the future prospects and objectives of nitrogen fixation technologies. This comprehensive review is intended to provide profound insights for overcoming the inherent thermodynamic and kinetic constraints in traditional ammonia synthesis, thereby fostering a shift towards "green ammonia" production and significantly reducing the energy footprint.

Mo-X4 (X = O, NH and S)-mediated triphenylene-based two-dimensional carbon-rich conjugate frameworks for an efficient nitrogen reduction reaction

The electrocatalytic nitrogen reduction reaction (NRR) is a highly competitive approach for the ammonia synthesis to overcome the problems of high energy consumption and air pollution by the traditional Haber-Bosch process. However, the challenges of inert N2 molecule activation and the competitive hydrogen evolution reaction (HER) restrict the real utilization of the NRR. Herein, by means of density functional theory (DFT) calculations, we proposed three two-dimensional carbon-rich conjugate frameworks (2D-CCFs) with hexa-substituted triphenylene organic linkers with a metal atom Mo and functional groups X (X = O, NH, and S), namely Mo3(HOTP)2, Mo3(HITP)2 and Mo3(THT)2, to investigate their NRR performance. Our theoretical calculations reveal that Mo atoms in 2D-CCFs can efficiently capture and activate N2 molecules. Among the three structures, Mo3(HOTP)2 exhibited the most superior performance toward the NRR with a favorable limiting potential of -0.41 V and good selectivity for the HER. Furthermore, the catalytic efficiency of 2D-CCFs can be regulated by changing the atoms X in Mo-X4 motifs, providing a new scenario for the development of highly efficient NRR catalysts.

A DFT Investigation of B-Doped C3 N as Single Atom Electrocatalysts for N2 -to-NH3 Conversion

The NH3 synthesis from N2 plays an important role in the ecological cycle and industrial production. Different from industrial NH3 synthesis with high pollution and energy consumption, electrocatalytic NH3 synthesis is favored because of its environmental protection, energy saving, ambient reaction conditions and other characteristics. However, due to the low efficiency and poor reaction selectivity of the existing electrocatalysts, which can not be used actually, the development of new electrocatalysts for nitrogen reduction reaction (NRR) is particularly urgent. Herein, we designed a series of transition metal atoms anchored B-doped defective C3 N surface (TM@B2 C3 N) as single-atom catalysts. Through the screening process of N2 adsorption activation, N2 H formation and NH3 desorption, finally the excellent electrocatalysts with strong stability and high activity (Cr@B2 C3 N and Mn@B2 C3 N) were obtained. After simulating the entire pathway, it was found that the NRR process on Cr@B2 C3 N and Mn@B2 C3 N via consecutive and distal pathways with the lowest limiting potential of -0.42 and -0.52 V, which have the good ability to inhibit hydrogen evolution reaction. Finally, the electronic properties were analyzed, and the reason for their high catalytic activity was summarized. This work provides a new idea for the rational design of NRR electrocatalysts and promotes the practical application of electrocatalysts.

Single-atom catalysts supported on a hybrid structure of boron nitride/graphene for efficient nitrogen fixation via synergistic interfacial interactions

Hexagonal boron nitride (BN) shows significant chemical stability and promising thermal nitrogen reduction reaction (NRR) activity but suffers from low conductivity in electrolysis with a wide band gap. To overcome this problem, two-dimensional (2D) BN and graphene (G) are designed as a heterostructure, namely BN/G. According to density functional theory (DFT), the higher conductivity of G narrows the band gap of BN by inducing some electronic states near the Fermi energy level (Ef). Once transition metals (TMs) are anchored in the BN/G structure as single atom catalysts (SACs), the NRR activity improves as the inert BN basal layer activates with moderate *NH2 binding energy and further the band gap is reduced to zero. V (vanadium) and W (tungsten) SACs exhibit the best performance with limiting potentials of -0.22 and -0.41 V, respectively. This study helps in understanding the improvement of the NRR activity of BN, providing physical insights into the adsorbate-TM interaction.

Dual Heterojunction of Etched MIL-68(In)-NH2 Supported Heptazine-/Triazine-Based Carbon Nitride for Improved Visible-Light Nitrogen Reduction

This work reports a dual heterojunction of etched MIL-68(In)-NH2 (MN) supported heptazine-/triazine-based carbon nitride (HTCN) via a facile hydrothermal process for photocatalytic ammonia (NH3 ) synthesis. By applying the hydrothermal treatment, MN microrods are chemically etched into hollow microtubes, and HTCN with nanorod array structures are simultaneously tightly anchored on the outside surface of the microtubes. With the addition of 9 wt% HTCN, the resulting dual heterojunction presents an enhanced photocatalytic ammonia yield rate of 5.57 mm gcat -1 h-1 with an apparent quantum efficiency of 10.89% at 420 nm. Moreover, stable ammonia generation using seawater, tap water, lake water, and turbid water in the absence of sacrificial reagents verifies the potential of the dual-heterojunction composites as a commercially viable photosystem. The obtained one-dimensional (1D) microtubes and coating of HTCN confers this unique composite with extended visible-light harvesting and accelerated charge carrier migration via a multi-stepwise charge transfer pathway. This work provides a new strategy for optimizing nitrogen (N2 )-into-ammonia conversion efficiency by designing novel dual-heterojunction catalysts.

High-entropy alloys in electrocatalysis: from fundamentals to applications

High-entropy alloys (HEAs) comprising five or more elements in near-equiatomic proportions have attracted ever increasing attention for their distinctive properties, such as exceptional strength, corrosion resistance, high hardness, and excellent ductility. The presence of multiple adjacent elements in HEAs provides unique opportunities for novel and adaptable active sites. By carefully selecting the element configuration and composition, these active sites can be optimized for specific purposes. Recently, HEAs have been shown to exhibit remarkable performance in electrocatalytic reactions. Further activity improvement of HEAs is necessary to determine their active sites, investigate the interactions between constituent elements, and understand the reaction mechanisms. Accordingly, a comprehensive review is imperative to capture the advancements in this burgeoning field. In this review, we provide a detailed account of the recent advances in synthetic methods, design principles, and characterization technologies for HEA-based electrocatalysts. Moreover, we discuss the diverse applications of HEAs in electrocatalytic energy conversion reactions, including the hydrogen evolution reaction, hydrogen oxidation reaction, oxygen reduction reaction, oxygen evolution reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, and alcohol oxidation reaction. By comprehensively covering these topics, we aim to elucidate the intricacies of active sites, constituent element interactions, and reaction mechanisms associated with HEAs. Finally, we underscore the imminent challenges and emphasize the significance of both experimental and theoretical perspectives, as well as the potential applications of HEAs in catalysis. We anticipate that this review will encourage further exploration and development of HEAs in electrochemistry-related applications.

Electron-rich Au nanocrystals/Co3O4 interface for enhanced electrochemical nitrate reduction into ammonia

Solar-driven electrochemical NO3- reduction reaction (NO3-RR) is a clean and sustainable strategy that can convert pollutant NO3- in wastewater to value-added NH3. In recent years, cobalt oxides-based catalysts have shown their intrinsic catalytic properties toward NO3-RR but still have room for improvement through catalyst design. Coupling metal oxides with noble metal has been demonstrated to improve electrochemical catalytic efficiency. Here, we use Au species to tune the surface structure of Co3O4 and improve the efficiency of NO3-RR to NH3. The obtained Au nanocrystals-Co3O4 catalyst exhibited an onset potential of 0.54 V vs RHE, NH3 yield rate of 27.86 µg/h·cm2, and Faradaic efficiency (FE) of 83.1% at 0.437 V vs RHE in an H-cell, which is much higher than Au small species (Au clusters or single atoms)-Co3O4 (15.12 µg/h·cm2) and pure Co3O4 (11.38 µg/h·cm2), respectively. Combined experiments with theory calculations, we attributed the enhanced performance of Au nanocrystals-Co3O4 to the reduced energy barrier of *NO hydrogenation to the *NHO and suppression of HER, which originated from the charge transfer from Au to Co3O4. Using an amorphous silicon triple-junction (a-Si TJ) as the solar cell and an anion exchange membrane electrolyzer (AME), an unassisted solar-driven NO3-RR to NH3 prototype was realized with a yield rate of 4.65 mg/h and FE of 92.1%.

Nitrogen Reduction to Ammonia on a Fe16 Nanocluster: A Computational Study of Catalysis

The sequence of elementary steps leading to reductive ammonia formation from N2 and H2 catalyzed by a Fe16 cluster is studied using generalized gradient approximation density functional theory and an all-electron basis set of triple-ζ quality. The computational methods are validated by comparison to experimental data such as binding energies where possible. First, the associative and dissociative attachment of N2 to Fe16 is considered, followed by exploration of the pathways leading to distal (Fe16-N-NH2) and enzymatic (NFe16-NH2) formation of an amino group. Next, the pathways leading to NH3 formation in both distal and enzymatic cases are examined. Two mechanisms for NH3 detachment have been discovered. An interesting peculiarity of the pathways is that they often proceed with total spin fluctuations, which are related to the rupture and formation of bonds on the surface of the catalyst over the course of the reactions. The reaction Fe16 + N2 + 2H2 → Fe16NH + NH3 is found to be exothermic by 1.02 eV (93.8 kJ/mol).

The Construction of Surface-Frustrated Lewis Pair Sites to Improve the Nitrogen Reduction Catalytic Activity of In2O3

The construction of a surface-frustrated Lewis pairs (SFLPs) structure is expected to break the single electronic state restriction of catalytic centers of P-region element materials, due to the existence of acid-base and basic active canters without mutual quenching in the SFLPs system. Herein, we have constructed eight possible SFLPS structures on the In2O3 (110) surface by doping non-metallic elements and investigated their performance as electrocatalytic nitrogen reduction catalysts using density functional theory (DFT) calculations. The results show that P atom doping (P@In2O3) can effectively construct the structure of SFLPs, and the doped P atom and In atom near the vacancy act as Lewis base and acid, respectively. The P@In2O3 catalyst can effectively activate N2 molecules through the enzymatic mechanism with a limiting potential of -0.28 eV and can effectively suppress the hydrogen evolution reaction (HER). Electronic structure analysis also confirmed that the SFLPs site can efficiently capture N2 molecules and activate N≡N bonds through a unique "donation-acceptance" mechanism.

Engineering photocatalytic ammonia synthesis

Photocatalytic ammonia synthesis (PAS) is an emerging zero carbon emission technology, which is critical for mitigating energy crises and achieving carbon neutrality. Herein, we summarize the recent advances and challenges in PAS from an engineering perspective based on its whole chain process, i.e., materials engineering, structure engineering and reaction engineering. For materials engineering, we discuss the commonly used photocatalytic materials including metal oxides, bismuth oxyhalides and graphitic carbon nitride and emerging materials, such as organic frameworks, along with the analysis of their characteristics and regulation methods to enhance the PAS performance. For structure engineering, the design of photocatalysts is described in terms of morphology, vacancy and band, corresponding to the crystal, atom and electron scales, respectively. Moreover, the structure-performance relationship of photocatalysts has been deeply explored in this section. For reaction engineering, we identify three key processes from the chemical reaction and mass transfer, i.e., nitrogen activation, molecule transfer and electron transfer, to intensify and optimize the PAS reaction. Hopefully, this review will provide a novel paradigm for the design and preparation of high-efficiency ammonia synthesis photocatalysts and inspire the practical application of PAS.

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.

Ample Lewis Acidic Sites in Mg2B2O5 Facilitate N2 Electroreduction through Bonding-Antibonding Interactions

Extensive research on the electrochemical nitrogen reduction reaction (NRR) has put forward a sound list of potential catalyst materials with properties inducing N2 adsorption, protonation, and reduction. However, rather than a random selection of catalysts, it is essential to understand the vitals in terms of orbital orientation and charge distribution that actually manipulate the rate-determining steps of NRR. Realizing these factors, herein we have explored a main group earth-abundant Mg-based electrocatalyst Mg2B2O5 for NRR due to the abundance of Lewis acid sites in the catalyst favoring the bonding-antibonding interactions with the N2 molecules. Positron annihilation studies indicate that the electronic charge distribution within the catalyst has shallow surface oxygen vacancies. These features in the catalyst enabled a sound Faradaic efficiency of 46.4% at -0.1 V vs reversible hydrogen electrode for the selective NH3 production in neutral electrolyte. In situ Fourier transform infrared suggests a maximum N-N bond polarization at -0.1 V and detected H-N-H and -NH2 intermediates during the course of the NRR on the catalyst surface. In a broader picture, the biocompatibility of Mg2+ diversifies the utility of this catalyst material in N2/biofuel cell applications that would certainly offer a green alternative toward our goal of a sustainable society.

NaCl template-assisted construction of a CoP-MoP heterostructured electrocatalyst for electrocatalytic nitrogen reduction

The electrocatalytic nitrogen reduction reaction (NRR) to ammonia is a promising technology to store renewable energy and mitigate greenhouse gas emissions. However, it usually suffers from low ammonia yield and selectivity because of the lack of efficient electrocatalysts. Herein, we report that the construction of metal phosphide heterojunctions is an efficient strategy for NRR activity enhancement. A CoP-MoP heterojunction electrocatalyst, which is fabricated by a facile NaCl template-assisted strategy, exhibits a favorable ammonia yield rate of 77.8 μg h-1 mgcat-1 (38.9 μg h-1 cm-2) and a high faradaic efficiency of 11.16% at -0.50 V versus the reversible hydrogen electrode. The high NRR electrocatalytic activity can be attributed to the electronic coupling effects and interfacial synergistic effects of CoP and MoP at the heterojunction interface, which accelerates the electron transfer rate. Moreover, Mo doping changes the d-band centers of metal sites on the CoP surface, which is conducive to N2 adsorption and promotes N2* adsorption in the competition of occupying active sites, thus inhibiting the HER. This work manifests the high potential of phosphide electrocatalysts and opens an alternative route toward NRR electrocatalysis.

Synergistic Effects of the Ni3B Cocatalyst and N Vacancy on g-C3N4 for Effectively Enhanced Photocatalytic N2 Fixation

The photocatalytic fixation of N₂ is a promising technology for sustainable production of ammonia, while the unsatisfactory efficiency resulting from the low electron-transfer rate, narrow light absorption range, and limited active sites of the photocatalyst seriously hinder its application. Herein, we designed a noble metal-free Schottky junction photocatalyst constructed by g-C₃N₄ nanosheets with N vacancies (V_N-CN) and metallic Ni₃B nanoparticles (Ni₃B/V_N-CN) for N₂ reduction to ammonia. The ammonia yield rate over the optimized Ni₃B/V_N-CN is 7.68 mM g^-1 h^-1, which is 6.7 times higher than that of pristine CN (1.15 mM g^-1 h^-1). The superior photocatalytic N₂ fixation performance of Ni₃B/V_N-CN can be attributed not only to the formation of Schottky junctions between Ni₃B and V_N-CN, which facilitates the migration and separation of photogenerated electrons, but also to the incorporation of V_N into g-C₃N₄, which enhances visible light absorption and improves electrical conductivity. More importantly, Ni₃B nanoparticles can act as the cocatalyst, which provide more active sites for the adsorption and activation of N₂, thereby improving the N₂ reduction activity. This work provides an effective strategy of designing noble metal-free-based cocatalyst photocatalyst for sustainable and economic N₂ fixation.

Oxygen vacancies-rich Cu-W18O49 nanorods supported on reduced graphene oxide for electrochemical reduction ofN2to NH3

High-performance nitrogen fixation is severely limited by the efficiency and selectivity of a catalyst of electrochemical nitrogen reduction reaction (NRR) under ambient conditions. Here, the RGO/WOCu (reduced graphene oxide and Cu-doping W₁₈O₄₉) composite catalysts with abundant oxygen vacancies are prepared by the hydrothermal method. The obtained RGO/WOCu achieves an enhanced NRR performance (NH₃ yield rate:11.4 μg h^-1 mg_cat^-1, Faradaic efficiency: 4.4%) at -0.6 V (vs. RHE) in 0.1 mol L^-1 Na₂SO₄ solution. Furthermore, the NRR performance of the RGO/WOCu still keeps at 95% after four cycles, demonstrating its excellent stability. The Cu⁺-doping increases the concentration of oxygen vacancies, which is conducive to the adsorption and activation of N₂. Meanwhile, the introduction of RGO further improves the electrical conductivity and reaction kinetics of the RGO/WOCu due to the high specific surface area and conductivity. This work provides a simple and effective method for efficient electrochemical reduction ofN₂.

Efficient electrocatalytic reduction of nitrate to ammonia over fibrous SmCoO3 under ambient conditions

We report SmCoO₃ nanofibers as an efficient catalyst for nitrate reduction to ammonia. This catalyst achieves a large NH₃ yield of 14.4 mg h^-1 mg_cat.^-1 and a high faradaic efficiency of 81.3% at -1.0 V vs. RHE in 0.1 M PBS with 0.1 M NaNO₃, and it also displays excellent electrochemical durability and structural stability. Theoretical calculations indicate that Sm-O and Co-O bonds have an incredibly low adsorption energy of -0.1 eV, which can significantly reduce the applied potential and hence enhance the catalytic activity.

Electrochemical C-N coupling of CO2 and nitrogenous small molecules for the electrosynthesis of organonitrogen compounds

Electrochemical C-N coupling reactions based on abundant small molecules (such as CO₂ and N₂) have attracted increasing attention as a new "green synthetic strategy" for the synthesis of organonitrogen compounds, which have been widely used in organic synthesis, materials chemistry, and biochemistry. The traditional technology employed for the synthesis of organonitrogen compounds containing C-N bonds often requires the addition of metal reagents or oxidants under harsh conditions with high energy consumption and environmental concerns. By contrast, electrosynthesis avoids the use of other reducing agents or oxidants by utilizing "electrons", which are the cleanest "reagent" and can reduce the generation of by-products, consistent with the atomic economy and green chemistry. In this study, we present a comprehensive review on the electrosynthesis of high value-added organonitrogens from the abundant CO₂ and nitrogenous small molecules (N₂, NO, NO₂^-, NO₃^-, NH₃, etc.) via the C-N coupling reaction. The associated fundamental concepts, theoretical models, emerging electrocatalysts, and value-added target products, together with the current challenges and future opportunities are discussed. This critical review will greatly increase the understanding of electrochemical C-N coupling reactions, and thus attract research interest in the fixation of carbon and nitrogen.

A High-Throughput Screening toward Efficient Nitrogen Fixation: Transition Metal Single-Atom Catalysts Anchored on an Emerging π-π Conjugated Graphitic Carbon Nitride (g-C10N3) Substrate with Dirac Dispersion

TM-N_x is becoming a comforting catalytic center for sustainable and green ammonia synthesis under ambient conditions, resulting in increasing interest in single-atom catalysts (SACs) for the electrochemical nitrogen reduction reaction (NRR). However, given the poor activity and unsatisfactory selectivity of existing catalysts, it remains a long-standing challenge to design efficient catalysts for nitrogen fixation. Currently, the two-dimensional (2D) graphitic carbon-nitride substrate provides abundant and evenly distributed holes for stably supporting transition-metal atoms, which presents a fascinating prospect for overcoming this challenge and promoting single-atom NRR. An emerging holey graphitic carbon-nitride skeleton with a C₁₀N₃ stoichiometric ratio (g-C₁₀N₃) from a supercell of graphene is constructed, which provides outstanding electric conductivity for achieving high-efficiency NRR due to the Dirac band dispersion. Herein, a high-throughput first-principles calculation is carried out to evaluate the feasibility of π-d conjugated SACs resulting from a single TM atom anchored on g-C₁₀N₃ (TM = Sc-Au) for NRR. We find that W metal embedded in g-C₁₀N₃ (W@g-C₁₀N₃) can compromise the ability to adsorb the key target reaction species (N₂H and NH₂), hence acquiring an optimal NRR behavior among 27 TM-candidates. Our calculations demonstrate that W@g-C₁₀N₃ shows a well-suppressed HER ability and, impressively, a low energy cost of -0.46 V. Additionally, all-around descriptors are proposed to uncover the fundamental mechanism of NRR activity, among which a 3D volcano plot (limiting potential, screening strategy, and electron origin) uncovers the NRR activity trend, achieving a quick and high-efficiency prescreening for numerous candidates. Overall, the strategy of the structure- and activity-based TM-N_x-containing unit design will offer useful insight for further theoretical and experimental attempts.

To Molecularly Block Hydrogen Evolution Sites of Molybdenum Disulfide toward Improved Catalytic Performance for Electrochemical Nitrogen Reduction

2H-molybdenum disulfide (2H-MoS₂ ) represents a classical catalyst for the electrochemical N₂ reduction reaction (NRR) in water that offers a promising technology toward sustainable production of NH₃ driven by renewable energy. While the catalytic efficiency is severely limited by a simultaneous and competing H₂ evolution reaction (HER). Herein, it is proposed that the S edge of 2H-MoS₂ , which is known as main sites to afford HER, is intentionally covered by cobalt phthalocyanine (CoPc) molecules through axial coordination. While the Mo sites with S vacancies at 2H-MoS₂ edge is recognized as highly NRR active, and can keep structurally intact in the CoPc based modification. The resultant composite thus exhibits high NRR performance with Faradic efficiency and NH₃ yields increase by fourfold and twofold, respectively, comparing to pristine 2H-MoS₂ . These findings provide a deep insight into the mechanism of 2H-MoS₂ based NRR catalysis and suggest an efficient molecular modification strategy to promote NRR in water.

Perovskites with Enriched Oxygen Vacancies as a Family of Electrocatalysts for Efficient Nitrate Reduction to Ammonia

Electrochemical nitrate reduction to ammonia (NRA) provides an efficient, sustainable approach to convert the nitrate pollutants into value-added products, which is regarded as a promising alternative to the industrial Haber-Bosch process. Recent studies have shown that oxygen vacancies of oxide catalysts can adjust the adsorption energies of intermediates and affect their catalytic performance. Compared with other metal oxides, perovskite oxides can allow their metal cations to exist in abnormal or mixed valence states, thereby resulting in enriched oxygen vacancies in their crystal structures. Here, the catalytic activities of perovskite oxides toward NRA catalysis with respect to the amount of oxygen vacancies are explored, where four perovskite oxides with different crystal structures (including cubic LaCrO₃ , orthorhombic LaMnO₃ and LaFeO₃ , hexagonal LaCoO₃ ) are chosen and investigated. By combining X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and electrochemical measurements, it is found that the amount of oxygen vacancies in these perovskite oxides surprisingly follow the same order as their activities toward NRA catalysis (LaCrO₃  < LaMnO₃  < LaFeO₃  < LaCoO₃ ). Further theoretical studies reveal that the existence of oxygen vacancies in LaCoO₃ perovskite can decrease the energy barriers for reduction of *HNO₃ to *NO₂ , leading to its superior NRA performance.

Creating Highly Active Iron Sites in Electrochemical N2 Reduction by Fabricating Strongly-Coupled Interfaces

Electrochemical N_c reduction has been regarded as one of the most promising approaches to producing ammonia under mild conditions, but there are remaining pressing challenges in improving the reaction rate and efficiency. Herein, an unconventional galvanic replacement reaction is reported to fabricate a unique hierarchical structure composed of Fe₃ O₄ -CeO₂ bimetallic nanotubes covered by Fe₂ O₃ ultrathin nanosheets. Control experiments reveal that CeO₂ species play the essential role of stabilizer for Fe²⁺ cations. Compared with bare CeO₂ and Fe₂ O₃ nanotubes, the as-obtained Fe₂ O₃ /Fe₃ O₄ -CeO₂ possesses a remarkably enhanced NH₃ yield rate (30.9 µg h^-1 mg_cat ^-1 ) and Faradaic efficiency (26.3%). The enhancement can be attributed to the hierarchical feature that makes electrodes more easily to contact with electrolytes. More importantly, as verified by density functional theory calculations, the generation of Fe₂ O₃ -Fe₃ O₄ heterogeneous junctions can efficiently optimize the reaction pathways, and the energy barrier of the potential determining step (the *N₂ hydrogenates into *N*NH) is significantly decreased.

Theoretical Exploration on the Role of Magnetic States to the N2 Fixation behaviors of 2D Transition Metal Tri-borides (TMB3 s)

Fixing nitrogen (N₂ ) by electrosynthesis method has become a promising way to ammonia (NH₃ ) production, nevertheless, developing electrocatalysts combining long-term stable and low-cost feathers are still a great challenge to date. Using comprehensive first-principles calculations, we herein investigate the potential of a new class of two-dimensional (2D) transition metal tri-borides (TMB₃ s) as nitrogen reduction reaction (NRR) electrocatalysts, and explore the effect of magnetic orders on the NRR. Our results show that the TMB₃ s can sufficiently activate N₂ and convert it to NH₃ . Particularly, TiB₃ is identified as a high-efficiency catalyst for NRR because of its low limiting potential (-0.24 V) and good suppression of the competitive hydrogen evolution reaction (HER). For the first time, we present that these TMB₃ s with various magnetic states exhibit different performances in the adsorption of N₂ and NRR intermediates, and minor effect on activation of N₂ . Besides, VB₃ , CrB₃ , MnB₃ , and FeB₃ monolayers possess the superior capacity to suppress surface oxidation via the self-activating process, which reduces ^* O/^* OH into ^* H₂ O under NRR electrochemical conditions, thus favoring the N₂ electroreduction. This work paves the way for finding high-performance NRR catalysts for transition metal borides and pioneering the research of magnetic states effects in NRR.

Tuning Photoactive MIL-68(In) by Functionalized Ligands for Boosting Visible-Light Nitrogen Fixation

In this work, MIL-68(In) functionalized with various ligand substitutions including amine, hydroxyl, bromine, nitro, and methyl groups was prepared, via a one-pot solvothermal reaction for visible-light photocatalytic ammonia synthesis. The diversity of ligands tunes the morphology, geometry, pore environment, and electronic structure of MIL-68(In)-based photocatalysts due to the polarity and intraframework interactions. Amine-inserted MIL-68(In) outperforms its counterparts, presenting a boosted nitrogen photofixation rate of 140.34 μmol g_cat^-1 h^-1 with an apparent quantum efficiency of 5.69% at 420 nm. Further, the size of the batch solvothermal reactor and the amine group content also influence the photocatalytic activity. The combined experimental and theoretical results reveal that amine substituents improve the chemisorption of nitrogen molecules and the conversion of nitrogen into ammonia follows a dual pathway, i.e., a Mars-van Krevelen process and a ligand-to-metal charge transfer mechanism. This work provides a molecular engineering strategy via dual catalysis toward efficient ammonia production.

Single-Atom Low-Valent Alkaline-Earth-Metal Catalysts for Electrochemical Nitrogen Reduction with an Acceptance-Backdonation Mechanism

Single-atom catalysts (SACs) have drawn great attention in developing highly active and low-cost catalysts for electrocatalytic nitrogen reduction reaction (NRR) in ammonia synthesis, but the atomic metal centers are mainly limited to transition metals. Here, four stable alkaline-earth-metal (AEM)-based SACs are proposed by anchoring AEM on nitrogen-doped graphene nanoribbons, based on first-principles calculations. All SACs exhibit excellent NRR performance with competitive limiting potentials compared to stepped Ru (0001), and Ca-based SAC achieves optimal activity with a potential of -0.716 V. It is revealed that the low oxidation state of AEM is crucial for the activation of N₂ through an acceptance-backdonation mechanism. The antibonding 2π* orbital of N₂ can accept residual s electrons of low-valent AEM and backdonate electrons to the empty d orbitals of AEM, resulting in activation of N₂ molecules. In particular, the activation degree of N₂ and NRR activity is linearly associated with the charge states of AEMs. Our work reveals the underlying mechanism of AEMs for N₂ activation and reduction and presents the potential of AEM SACs as efficient electrochemical NRR catalysts.

Ammonia from dinitrogen at ambient conditions by organometallic catalysts

Fixation of atmospheric dinitrogen in plants by [Mo-Fe] cofactor of nitrogenase enzyme takes place efficiently under atmospheric pressure and normal temperature. In search for an alternative methodology for the highly energy intensive Haber-Bosch process, design and synthesis of highly efficient inorganic and organometallic complexes by mimicking the structure and function of [Mo-Fe] cofactor system is highly desirable for ammonia synthesis from dinitrogen. An ideal catalyst for ammonia synthesis should effectively catalyse the reduction of dinitrogen in the presence of a proton source under mild to moderate conditions, and thereby, significantly reducing the cost of ammonia production and increasing the energy efficacy of the process. In the light of current research, it is evident that there is a plenty of scope for the development and enhanced performance of the inorganic and organometallic catalysts for ammonia synthesis under ambient temperature and pressure. The review furnishes a comprehensive outlook of numerous organometallic catalysts used in the synthesis of ammonia from dinitrogen in the past few decades.

Oxygen Functionalization-Induced Charging Effect on Boron Active Sites for High-Yield Electrocatalytic NH3 Production

Ammonia has been recognized as the future renewable energy fuel because of its wide-ranging applications in H₂ storage and transportation sector. In order to avoid the environmentally hazardous Haber-Bosch process, recently, the third-generation ambient ammonia synthesis has drawn phenomenal attention and thus tremendous efforts are devoted to developing efficient electrocatalysts that would circumvent the bottlenecks of the electrochemical nitrogen reduction reaction (NRR) like competitive hydrogen evolution reaction, poor selectivity of N₂ on catalyst surface. Herein, we report the synthesis of an oxygen-functionalized boron carbonitride matrix via a two-step pyrolysis technique. The conductive BNCO₍₁₀₀₀₎ architecture, the compatibility of B-2p_z orbital with the N-2p_z orbital and the charging effect over B due to the C and O edge-atoms in a pentagon altogether facilitate N₂ adsorption on the B edge-active sites. The optimum electrolyte acidity with 0.1 M HCl and the lowered anion crowding effect aid the protonation steps of NRR via an associative alternating pathway, which gives a sufficiently high yield of ammonia (211.5 μg h^-1 mg_cat^-1) on the optimized BNCO₍₁₀₀₀₎ catalyst with a Faradaic efficiency of 34.7% at - 0.1 V vs RHE. This work thus offers a cost-effective electrode material and provides a contemporary idea about reinforcing the charging effect over the secured active sites for NRR by selectively choosing the electrolyte anions and functionalizing the active edges of the BNCO₍₁₀₀₀₎ catalyst.

Facile synthesis CQDs/SnO2-x/BiOI heterojunction photocatalyst to effectively degrade pollutants and antibacterial under LED light

To remove multitudinous pollutants from wastewater, the design of a highly efficient and multifunctional photocatalyst is necessary. A novel CQDs/SnO2-x/BiOI photocatalyst (named CSnBI composite) was prepared by combining carbon quantum dots (CQDs), large specific surface area SnO2-x nanocrystals and BiOI nanocrystals to obtain a compact hybrid structure. TEM and Raman techniques confirmed the structure of CSnBI composite. The photocurrent and EIS showed that the photoexcited electron-hole pairs separation efficiency was improved. As anticipated, novel CSnBI photocatalyst can successfully remove tetracycline, methyl orange, E. coli (Escherichia coli) and S. aureus under a LED light due to the hybridization contaction among CQDs, SnO2-x and BiOI. The mechanism showed that the introduction of CQDs promoted visible light absorption and efficient separation of photogenerated carriers of SnO2-x/BiOI heterojunction. The capture experiment and related measurements showed that h+, •O2- and •OH are active species in the photocatalytic process. This study gave a novel case for facile construction of photocatalysts with tight hybrid structure.

Chemical looping based ammonia production-A promising pathway for production of the noncarbon fuel

Ammonia, primarily made with Haber-Bosch process developed in 1909 and winning two Nobel prizes, is a promising noncarbon fuel for preventing global warming of 1.5 °C above pre-industrial levels. However, the undesired characteristics of the process, including high carbon footprint, necessitate alternative ammonia synthesis methods, and among them is chemical looping ammonia production (CLAP) that uses nitrogen carrier materials and operates at atmospheric pressure with high product selectivity and energy efficiency. To date, neither a systematic review nor a perspective in nitrogen carriers and CLAP has been reported in the critical area. Thus, this work not only assesses the previous results of CLAP but also provides perspectives towards the future of CLAP. It classifies, characterizes, and holistically analyzes the fundamentally different CLAP pathways and discusses the ways of further improving the CLAP performance with the assistance of plasma technology and artificial intelligence (AI).

High-Throughput Screening of Bicationic Redox Materials for Chemical Looping Ammonia Synthesis

Ammonia recently has gained increasing attention as a carrier for the efficient and safe usage of hydrogen to further advance the hydrogen economy. However, there is a pressing need to develop new ammonia synthesis techniques to overcome the problem of intense energy consumption associated with the widely used Haber-Bosch process. Chemical looping ammonia synthesis (CLAS) is a promising approach to tackle this problem, but the ideal redox materials to drive these chemical looping processes are yet to be discovered. Here, by mining the well-established MP database, the reaction free energies for CLAS involving 1699 bicationic inorganic redox pairs are screened to comprehensively investigate their potentials as efficient redox materials in four different CLAS schemes. A state-of-the-art machine learning strategy is further deployed to significantly widen the chemical space for discovering the promising redox materials from more than half a million candidates. Most importantly, using the three-step H₂ O-CL as an example, a new metric is introduced to determine bicationic redox pairs that are "cooperatively enhanced" compared to their corresponding monocationic counterparts. It is found that bicationic compounds containing a combination of alkali/alkaline-earth metals and transition metal (TM)/post-TM/metalloid elements are compounds that are particularly promising in this respect.

Recent progress of photocatalysts based on tungsten and related metals for nitrogen reduction to ammonia

Photocatalytic nitrogen reduction reaction (NRR) to ammonia holds a great promise for substituting the traditional energy-intensive Haber–Bosch process, which entails sunlight as an inexhaustible resource and water as a hydrogen source under mild conditions. Remarkable progress has been achieved regarding the activation and solar conversion of N₂ to NH₃ with the rapid development of emerging photocatalysts, but it still suffers from low efficiency. A comprehensive review on photocatalysts covering tungsten and related metals as well as their broad ranges of alloys and compounds is lacking. This article aims to summarize recent advances in this regard, focusing on the strategies to enhance the photocatalytic performance of tungsten and related metal semiconductors for the NRR. The fundamentals of solar-to-NH₃ photocatalysis, reaction pathways, and NH₃ quantification methods are presented, and the concomitant challenges are also revealed. Finally, we cast insights into the future development of sustainable NH₃ production, and highlight some potential directions for further research in this vibrant field.

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.