Ammonia Detection Methods in Photocatalytic and Electrocatalytic Experiments: How to Improve the Reliability of NH3 Production Rates?

CuS/KTa0.75Nb0.25O3 nanocomposite utilizing solar and mechanical energy for catalytic N2 fixation

This work synthesized a novel CuS/KTa_0.75Nb_0.25O₃ (KTN) heterojunction composite and firstly applied it in photocatalytic and piezocatalytic reduction of N₂ to NH₃. XRD, Raman, XPS, SEM, and TEM analyses indicate that CuS nanoparticles closely adhered to the surface of KTN nanorods, which facilitates the migration of electrons between the two semiconductors. Mott-Schottky and valence band XPS analysis shows that KNbO₃ shows a higher conduction band than CuS, indicating that CuS mainly acts as electron trappers to capture the photogenerated electrons from KTN. Because of the great enhanced spatial separation of photogenerated charge carriers, the CuS/KTN presents much higher performance than pure KNT, which is further confirmed by ¹H NMR analysis of the reaction solution. An interesting finding is that synthesized CuS/KTN not only performs well under light irradiation but also can work in an ultrasonic bath, indicating its great potential in photo/piezocatalytic conversion of N₂ to NH₃. The optimal 10 %CuS/KTN shows an NH₃ production rate of 36.2 μmol L^-1 g^-1 h^-1 under ultrasonic vibration, which reaches 7.4 times that of KTN. The electrons generated by KTN through the piezoelectric effect can be captured by CuS, which endows the electrons a longer life to participate in the reaction, thereby improving the catalytic reaction performance.

Rational design of nanocatalysts for ambient ammonia electrosynthesis

Ammonia (NH3) is one of the key commercial chemicals and carbon-free energy carriers. It is mainly made by Haber-Bosch process under high temperature and high pressure, which consumes huge amount of energy and releases large amounts of CO2. Developing sustainable approaches to its production is of great importance. Powered by a renewable electricity source, electrochemical N2 reduction reaction (NRR) and nitrate reduction reaction (NITRR) are potential routes to synthesize NH3 under ambient conditions. This review summarizes major recent advances in the NRR and NITRR, especially for several years. Some fundamentals for NRR and NITRR are first introduced. Afterward, the design strategies of nanocatalysts are discussed, mainly focusing on nano-structure construction/nanoconfinement, doping/defects engineering and single-atom engineering. Finally, the critical challenges remaining in this research area and promising directions for future research are discussed.

Graphitic nitrogen in carbon catalysts is important for the reduction of nitrite as revealed by naturally abundant 15N NMR spectroscopy

Metal-free nitrogen-doped carbon is considered as a green functional material, but the structural determination of the atomic positions of nitrogen remains challenging. We recently demonstrated that directly-excited solid state 15N NMR (ssNMR) spectroscopy is a powerful tool for the determination of such positions in N-doped carbon at natural 15N isotope abundance. Here we report a green chemistry approach for the synthesis of N-doped carbon using cellulose as a precursor, and a study of the catalytic properties and atomic structures of the related catalyst. N-doped carbon (NH3) was obtained by the oxidation of cellulose with HNO3 followed by ammonolysis at 800 °C. It had a N content of 6.5 wt% and a surface area of 557 m2 g-1, and 15N ssNMR spectroscopy provided evidence for graphitic nitrogen besides regular pyrrolic and pyridinic nitrogen. This structural determination allowed probing the role of graphitic nitrogen in electrocatalytic reactions, such as the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nitrite reduction reaction. The N-doped carbon catalyst (NH3) showed higher electrocatalytic activities in the OER and HER under alkaline conditions and higher activity for nitrite reduction, as compared with a catalyst prepared by the carbonization of HNO3-treated cellulose in N2. The electrocatalytic selectivity for nitrite reduction of the N-doped carbon catalyst (NH3) was directly related to the graphitic nitrogen functions. Complementary structural analyses by means of 13C and 1H ssNMR, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and low-temperature N2 adsorption were performed and provided support to the findings. The results show that directly-excited 15N ssNMR spectroscopy at natural 15N abundance is generally capable of providing information on N-doped carbon materials if relaxation properties are favorable. It is expected that this approach can be applied to a wide range of solids with an intermediate concentration of N atoms.

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

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

An Experimentally Verified LC-MS Protocol toward an Economical, Reliable, and Quantitative Isotopic Analysis in Nitrogen Reduction Reactions

To substitute the energy-intensive Haber-Bosch process for the synthesis of ammonia, some labile techniques, such as photocatalysis, electrocatalysis, photoelectrocatalysis, and photothermocatalysis, have emerged and attracted intense research interest. However, the contamination of the reaction system is one of the major concerns on how to reliably and accurately evaluate the performance of these catalysts, which is why various control studies are involved. Isotopic labeling studies are one of the most reliable control strategies in nitrogen fixation experiments, to ensure that N₂ is exclusively the source of the generated ammonia. As a convenient, sensitive and accurate technique distinguished with a quantitative atomic mass resolution, liquid chromatography-mass spectrometry (LC-MS) has been extensively employed for the detection of ammonia in aqueous electrolyte systems. However, the previous work protocols for ¹⁵ N₂ isotopic analysis using LC-MS either involved hazardous procedures which could potentially damage the instrument, or lacked in their experimental verification using real samples. Herein, a safe, reproducible and economical protocol for the detection of ammonia using LC-MS is presented, exhibiting an exponentially steep progressive detectivity of ¹⁵ N abundance, well verified with a series of experimental results for nitrogen reduction reactions. This is expected to provide a prudent cost-effective and sustainable gateway into isotopic analysis.

Recent advances in photocatalytic nitrogen fixation: from active sites to ammonia quantification methods

Photocatalytic nitrogen fixation has become a hot topic in recent years due to its mild and sustainable advantages. While modifying the photocatalyst to enhance its electron separation, light absorption and nitrogen reduction abilities, the role of the active sites in the catalytic reaction cannot be ignored because the N[triple bond, length as m-dash]N nitrogen bond is too strong to activate. This review summarizes the recent research on nitrogen fixation, focusing on the active sites for N2 on the catalyst surface, classifying common active sites, explaining the main role and additional role of the active sites in catalytic reactions, and discussing the methods to increase the number of active sites and their activation ability. Finally, the outlook for future research is presented. It is hoped this review could help researchers understand more about the activation of the nitrogen molecules and lead more efforts into research on nitrogen fixation photocatalysts.

Polymeric Carbon Nitride-Derived Photocatalysts for Water Splitting and Nitrogen Fixation

Photocatalysis is a promising energy conversion and environmental restoration technology. The main focus of photocatalysis is the development and manufacture of highly efficient photocatalysts. Semiconductor-based photocatalysis technology based on harnessing solar energy is considered as an attractive approach to solve the problems of global energy shortage and environmental pollution. Since 2009 pioneering work has been carried out on polymeric carbon nitride (PCN) for visible photocatalytic water splitting, thus PCN-based photocatalysis has become a hot research topic, demanding significant research attention. This article reviews the physical and chemical properties, synthesis methods, and the methods to control the morphology, heteroatom doping, and construction of heterojunctions to improve the performance of PCN-based photocatalysts in water splitting and nitrogen fixation. Through different design strategies, the photo-generated electron-hole pair separation efficiency of PCN materials can be effectively improved, thereby improving their photocatalytic performance. Finally, the challenges of PCN-based photocatalysts in water splitting and nitrogen fixation applications are discussed herein. It is strongly believed that through different design strategies, efficient PCN-based photocatalysts can be constructed for both water splitting and nitrogen reduction. These excellent modification strategies can be used as a guiding theory for photocatalytic reactions of other promising catalysts and further promote the development of photocatalysis.

Rational Design of High-Concentration Ti3+ in Porous Carbon-Doped TiO2 Nanosheets for Efficient Photocatalytic Ammonia Synthesis

Photocatalytic ammonia synthesis is exciting but quite challenging with a very moderate yield at present. One of the greatest challenges is to develop highly active centers in a photocatalyst for N₂ reduction under ambient conditions. Herein, porous carbon-doped anatase TiO_x (C-TiO_x ) nanosheets with high-concentration active sites of Ti³⁺ are presented, which are produced by layered Ti₃ SiC₂ through a reproducible bottom-up approach. It is shown that the high-concentration Ti³⁺ sites are the major species for the significant increase in N₂ photoreduction activity by the C-TiO_x . Such bottom-up substitutional doping of C into TiO₂ is responsible for both visible absorption and generation of Ti³⁺ concentration. Together with the porous nanosheets morphology and the loading of a Ru/RuO₂ nanosized cocatalyst for enhanced charge separation and transfer, the optimal C-TiO_x with a Ti³⁺ /Ti⁴⁺ ratio of 72.1% shows a high NH₃ production rate of 109.3 µmol g^-1 h^-1 under visible-light irradiation and a remarkable apparent quantum efficiency of 1.1% at 400 nm, which is the highest compared to all TiO₂ -based photocatalysts at present.

Deciphering NH3 Adsorption Kinetics in Ternary Ni-Cu-Fe Oxyhydroxide toward Efficient Ammonia Oxidation Reaction

Developing efficient catalysts for the ammonia oxidation reaction (AOR) is crucial for NH₃ utilization as a large-scale energy carrier. This work reports a promising Ni-Cu-Fe-OOH material for ammonia oxidation, and density functional theory is used to investigate the AOR mechanism. It is revealed that the oxygen-atoms bonded with the metal-atom on the surface of electrode play an important role in AOR. By codoping Cu and Fe, the electron distribution around the oxygen-atom is affected, which helps to promote the occurrence of ammonia oxidation. The Ni-Cu-Fe-OOH material delivers one of the highest ammonia removal efficiency to date of ≈90% after 12 h. In addition, ≈55% of the initial ammonia is successfully degraded after 24 h in high ammonia concentration. Thus, this work reveals the mechanism of AOR that can provide new ideas to tailor more powerful and updated catalysts in the future.

Recent discoveries in the reaction mechanism of heterogeneous electrocatalytic nitrate reduction

We review advances in the electrocatalytic nitrate reduction mechanism and evaluate future efforts. Existing work could be supplemented by controlling reaction conditions and quantifying active sites to determine activity on a per-site basis.

Boosted photocatalytic nitrogen fixation by bismuth and oxygen vacancies in Bi2MoO6/BiOBr composite structures

Bi₂MoO₆/V_Bi+O–BiOBr composites with surface bismuth and oxygen vacancies were synthesized by an ion-exchange method, and exhibited boosted photocatalytic nitrogen fixation activity.

A ClO-mediated photoelectrochemical filtration system for highly-efficient and complete ammonia conversion

The ability to convert excess ammonia in water into harmless N₂ is highly desirable for environmental remediation. We present a chlorine-oxygen radical (ClO)-mediated photoelectrochemical filtration system for highly efficient and complete ammonia removal from water. The customized photochemical device comprised a Ag-functionalized TiO₂nanotube array mesh photoanode and a Pd-Cu co-modified nickel foam (Pd-Cu/NF) cathode. Under illumination, holes generated at the anode catalyzed the conversion of H₂O and Cl^- to HOand Cl, respectively. In turn, these radicals then reacted further, yielding ClO, which selectively decomposed ammonia. The cathode enabled further reduction of anodic byproducts such as NO₃^- to N₂. The complete oxidation of all dissolved ammonia was achieved within 15 min reaction under neutral conditions, where N₂ was the dominant product. The impact of key parameters was assessed, which enabled the discovery of optimal reaction conditions and the proposal of the underlying working mechanism. The flow-through configuration demonstrated a 5-fold increase of ammonia oxidation rate compared to the conventional batch reactor. The role of ClO in the oxidation of ammonia was verified with electron paramagnetic resonance and scavenger studies. This study provided greater mechanistic insights into photoelectrochemical filtration technology and demonstrated the potential of future nanotechnology for removing ammonia.

Production of Amidinyl Radicals via UV-Vis-Light Promoted Reduction of N-Arylthiophene-2-carboxamidoximes and Application to the Preparation of Some New N-Arylthiophene-2-carboxamidines

A modern method for the preparation of some new N-arylthiophene-2-carboxamidines via amidinyl radicals generated using UV-vis-light promoting the reduction of N-arylthiophene-2-carboxamidoximes without any catalyst in a short amount of time, highly straight forward, and in an efficient manner is described. This method defeats the flaws of the conventional methods for the reduction of amidoxime derivatives to amidine derivatives, which require harsh conditions such as using a strong acid, high temperature, and expensive catalysts. Benzo[d]imidazoles, benzo[d]oxazoles, and amides can also be synthesized by applying this method. The photoproducts were analyzed by various spectroscopic and analytical techniques, including thin-layer chromatography, column chromatography, high-performance liquid chromatography, gas chromatography/mass spectrometry, IR, ¹H NMR, ¹³C NMR, and MS. Notably, the chromatographic analyses proved that the best time for the production of N-arylthiophene-2-carboxamidines is 20 min. The reaction mechanism comprising pathways and intermediates was also suggested via the homolysis of N-O and C-N bonds.

Facile All-Optical Method for In Situ Detection of Low Amounts of Ammonia

As a key precursor for nitrogenous compounds and fertilizer, ammonia affects our lives in numerous ways. Rapid and sensitive detection of ammonia is essential, both in environmental monitoring and in process control for industrial production. Here we report a novel and nonperturbative method that allows rapid detection of ammonia at low concentrations, based on the all-optical detection of surface-enhanced Raman signals. We show that this simple and affordable approach enables ammonia probing at selected regions of interest with high spatial resolution, making in situ and operando observations possible.

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Novel method for detection of ammonia at concentrations below 1 ppm in just under 1 s

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This approach allows local detection of ammonia amounts as low as 10⁴–10⁵ molecules

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Method for sensitive direct monitoring of catalytic/electrocatalytic processes

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The method allows following the dynamics of ammonia concentration change in real time

Three-Phase Electrolysis by Gold Nanoparticle on Hydrophobic Interface for Enhanced Electrochemical Nitrogen Reduction Reaction

Electrochemical nitrogen reduction reaction (NRR) provides a facile and sustainable strategy to produce ammonia (NH3) at ambient conditions. However, the low NH3 yield and Faradaic efficiency (FE) are still the main challenges due to the competitive hydrogen evolution reaction (HER). Herein, a three-phase electrocatalyst through in situ fabrication of Au nanoparticles (NPs) located on hydrophobic carbon fiber paper (Au/o-CFP) is designed. The hydrophobic CFP surface facilitates efficient three-phase contact points (TPCPs) for N2 (gas), electrolyte (liquid), and Au NPs (solid). Thus, concentrated N2 molecules can contact the electrocatalyst surface directly, inhibiting the HER since the lowered proton concentration and overall enhancing NRR. The three-phase Au/o-CFP electrocatalyst presents an excellent NRR performance with high NH3 yield rate of 40.6 µg h-1 mg-1 at -0.30 V and great FE of 31.3% at -0.10 V versus RHE (0.1 m Na2SO4). The N2-bubble contact angle result and cyclic voltammetry analysis confirm that the hydrophobic interface has a relatively strong interaction with N2 bubble for enhanced NRR and weak electrocatalytic activity for HER. Significantly, the three-phase Au/o-CFP exhibits excellent stability with a negligible fluctuation of NH3 yield and FE in seven-cycle test. This work provides a new strategy for improving NRR and simultaneously inhibiting HER.

Visible-Light Activation of Photocatalytic for Reduction of Nitrogen to Ammonia by Introducing Impurity Defect Levels into Nanocrystalline Diamond

Nitrogen impurity has been introduced in diamond film to produce a nitrogen vacancy center (NV center) toward the solvated electron-initiated reduction of N₂ to NH₃ in liquids, giving rise to extend the wavelength region beyond the diamond’s band. Scanning electron microscopy and X-ray diffraction demonstrate the formation of the nanocrystalline nitrogen-doped diamond with an average diameter of ten nanometers. Raman spectroscopy and PhotoLuminescence (PL) spectrum show characteristics of the NV⁰ and NV⁻ charge states. Measurements of photocatalytic activity using supraband (λ < 225 nm) gap and sub-band gap (λ > 225 nm) excitation show the nitrogen-doped diamond significantly enhanced the ability to reduce N₂ to NH₃ compared to the polycrystalline diamond and single crystal diamond (SCD). Our results suggest an important process of internal photoemission, in which electrons are excited from negative charge states into conduction band edges, presenting remarkable photoinitiated electrons under ultraviolet and visible light. Other factors, including transitions between defect levels and processes of reaction, are also discussed. This approach can be especially advantageous to such as N₂ and CO₂ that bind only weakly to most surfaces and high energy conditions.

Unsaturated p-Metal-Based Metal-Organic Frameworks for Selective Nitrogen Reduction under Ambient Conditions

Electrochemical ammonia synthesis that utilizes renewable electricity in the nitrogen reduction reaction (NRR) has recently been remarkably considered. Of particular importance is to develop efficient electrocatalysts at low costs. Herein, highly selective nitrogen capture using porous aluminum-based metal-organic frameworks (MOFs) materials, MIL-100 (Al), is first designed for the electrochemical nitrogen fixation in alkaline media under ambient conditions. Owing to the unique structure, MIL-100 (Al) exhibits remarkable NRR properties (NH₃ yield: 10.6 μg h^-1 cm^-2 mg_cat.^-1 and Faradaic efficiency: 22.6%) at a low overpotential (177 mV). Investigation indicates that the catalyst shows excellent N₂-selective captures due to the unsaturated metal sites binding with N₂. More specifically, as the Al 3p band can strongly interact with N 2p orbitals, Al as a main group metal presents a high and selective affinity to N₂. The utilization of multifunctional MOF catalysts delivers both high N₂ selectivity and abundant catalytic sites, resulting in remarkable efficiency for NH₃ production.

Rational Design of Two-Dimensional Transition Metal Carbide/Nitride (MXene) Hybrids and Nanocomposites for Catalytic Energy Storage and Conversion

Electro-, photo-, and photoelectrocatalysis play a critical role toward the realization of a sustainable energy economy. They facilitate numerous redox reactions in energy storage and conversion systems, enabling the production of chemical feedstock and clean fuels from abundant resources like water, carbon dioxide, and nitrogen. One major obstacle for their large-scale implementation is the scarcity of cost-effective, durable, and efficient catalysts. A family of two-dimensional transition metal carbides, nitrides, and carbonitrides (MXenes) has recently emerged as promising earth-abundant candidates for large-area catalytic energy storage and conversion due to their unique properties of hydrophilicity, high metallic conductivity, and ease of production by solution processing. To take full advantage of these desirable properties, MXenes have been combined with other materials to form MXene hybrids with significantly enhanced catalytic performances beyond the sum of their individual components. MXene hybridization tunes the electronic structure toward optimal binding of redox active species to improve intrinsic activity while increasing the density and accessibility of active sites. This review outlines recent strategies in the design of MXene hybrids for industrially relevant electrocatalytic, photocatalytic, and photoelectrocatalytic applications such as water splitting, metal-air/sulfur batteries, carbon dioxide reduction, and nitrogen reduction. By clarifying the roles of individual material components in the MXene hybrids, we provide design strategies to synergistically couple MXenes with associated materials for highly efficient and durable catalytic applications. We conclude by highlighting key gaps in the current understanding of MXene hybrids to guide future MXene hybrid designs in catalytic energy storage and conversion applications.

Fe doped SrWO4 with tunable band structure for photocatalytic nitrogen fixation

Transition metal element doping into semiconducting materials has been a promising method for the preparation of active photocatalysts for the efficient use of solar energy. In this study, we report the facile synthesis of Fe doped SrWO₄ nanoparticles by a solvothermal method for photocatalytic nitrogen reduction. The intrinsic bandgap of SrWO₄ is greatly narrowed by the Fe-dopant which not only extends the light absorption from UV to visible light range, but also reduces the charge recombination. The narrowed band structure still fulfils the thermodynamic requirements of nitrogen reduction reaction. At optimal doping concentration, Fe doped SrWO₄ shows much higher photocatalytic nitrogen fixation performance. The present study provides a route toward the development of active photocatalysts for nitrogen fixation.

Identifying the Active Site on Graphene Oxide Nanosheets for Ambient Electrocatalytic Nitrogen Reduction

Identifying the active sites on graphene oxide (GO) nanosheets is of great importance. In situ electroreduction at different potentials is applied to control the oxygenated groups on GO surfaces. Both experiments and theoretical calculations suggest the C═O group is critical for N₂ adsorption and activation, guaranteeing the ambient electrocatalytic N₂ reduction.

Chemisorption-Induced and Plasmon-Promoted Photofixation of Nitrogen on Gold-Loaded Carbon Nitride Nanosheets

Photocatalytic fixation of nitrogen is a promising method for green conversion of solar light, but has been substantially limited by inefficient activation of the nonpolar N≡N bond and the poor utilization of visible light. In this study, carbon nitride nanosheet composites with abundant nitrogen vacancies and strong plasmonic resonance absorption of visible light have been fabricated through the combination of hydrogen treatment and loading of Au nanoparticles. Ammonia yields of 184 μmol g^-1 and 93 μmol g^-1 are obtained without any sacrificial agent under full-light and visible-light irradiation, respectively. In particular, the visible-light activity is enhanced tenfold with the help of Au. Combining the experimental results and theoretical calculations, both the hydrogen treatment and Au loading help form nitrogen vacancies on the carbon nitride nanosheets, which promote N₂ activation by enhancing the chemisorption. Furthermore, the Au loading further improves the nitrogen reduction efficiency through charging the excited hot electrons formed from the surface plasmonic resonance to the adsorbed N₂ molecules.

Nitrogen-doped iron for selective catalytic reduction of nitrate to dinitrogen

Nitrate is the leading cause of eutrophication worldwide and is one of the most challenging pollutants for restoration of polluted surface waters such as lakes, rivers and reservoirs. We report herein a new architecture of iron nanoparticles for high-efficiency denitrification by selective reduction of nitrate (NO₃^-) to dinitrogen (N₂). The iron nanoparticles are doped with nitrogen (FeN) and encapsulated within a thin layer of nitride-carbon (NC). The nanoparticles have high pyrrolic N content (17.4 at.%) and large specific surface area (2040 m²/g). Laboratory experiments demonstrated high N₂ selectivity (91%) and nitrate removal capacity (6004 mg N/g Fe) for treatment of nitrate-containing water. This iron-based nanomaterial overcomes shortcomings of conventional catalysts by eliminating the use of precious and toxic heavy metals (e.g., Pd, Pt, Cu, Ni) and minimizing the generation of undesirable byproducts (e.g., ammonia) from the reactions with nanoscale zero-valent iron (nZVI). The multiple electron transfers process from NO₃^- to N₂ can be fine-tuned by adjusting the NC shell thickness. Superior electrocatalytic performance, low cost and minimal environmental impact of the iron-derived nanocatalyst offer promising prospects for water purification, waste treatment and environmental remediation.

Ultrathin 1T-MoS2 Nanoplates Induced by Quaternary Ammonium-Type Ionic Liquids on Polypyrrole/Graphene Oxide Nanosheets and Its Irreversible Crystal Phase Transition During Electrocatalytic Nitrogen Reduction

Ultrathin nanoplates of metastable 1T-MoS₂ have been successfully stabilized and uniformly distributed on the surface of n-butyl triethyl ammonium bromide functionalized polypyrrole/graphene oxide (BTAB/PPy/GO) by a very simple hydrothermal method. BTAB as a typical kind of quaternary ammonium-type ionic liquids (ILs) played a crucial role in the formation of the obtained 1T-MoS₂/BTAB/PPy/GO. It was covalently linked with PPy/GO and arranged in a highly ordered order at the solid-liquid interface of PPy/GO and H₂O due to Coulombic interactions and other intermolecular interactions, which would induce and stabilize ultrathin 1T-MoS₂ nanoplates by morphosynthesis. The good electrocatalytic activity toward nitrogen reduction reaction (NRR) with strong durability and good stability can be achieved by 1T-MoS₂/BTAB/PPy/GO due to their excellent inorganic/organic hierarchical lamellar micro-/nanostructures. Especially, after the long-term electrocatalysis for NRR at a negative potential, metastable 1T-MoS₂ as the catalytic center undergoes two types of irreversible crystal phase transition, which was converted to 1T'-MoS₂ and Mo₂N, caused by the competitive hydrogen evolution reaction (HER) process and the electrochemical reaction between the electroactive 1T-MoS₂ and N₂, respectively. The new N-Mo bonding prevents Mo atoms from binding to other N atoms in N₂, resulting in the deactivation of the electrocatalysts to NRR after being used for 18 h. Even so, quaternary ammonium-type ILs would induce the crystal structures of transition-metal dichalcogenides (TMDCs), which might provide a new thought for the reasonable design of electrocatalysts based on TMDCs for electrocatalysis.

Few-Layer Black Phosphorus Nanosheets: A Metal-Free Cocatalyst for Photocatalytic Nitrogen Fixation

Exploiting an appropriate strategy to prepare fine crystal quality black phosphorus nanosheet (BPNS) catalyst is a major challenge for its practical application in catalysis. Herein, we address this challenge by developing a rapid electrochemical expansion strategy for scale preparation of fine crystal quality BPNSs from bulk black phosphorus, which was demonstrated to be an active cocatalyst for photocatalytic nitrogen fixation in the presence of CdS as a photocatalyst. The transient photocurrent and charge density studies show that the BPNSs can efficiently accelerate charge separation of CdS, leading to the enhanced photocatalytic activities of BPNS/CdS nanocomposites for nitrogen fixation. The 1.5% BPNS/CdS photocatalyst exhibits the highest photocatalytic activity for nitrogen fixation with an NH₃ evolution rate of 57.64 μmol·L^-1·h^-1. This study not only affords a rapid and simple strategy for scale synthesis of fine crystal quality BPNSs but also provides new insights into the design and development of black phosphorus-based materials as low-cost metal-free cocatalysts for photocatalytic nitrogen fixation.

Low-Coordinate Step Atoms via Plasma-Assisted Calcinations to Enhance Electrochemical Reduction of Nitrogen to Ammonia

The electrochemical N₂ reduction reaction (NRR) is emerging as a promising alternative to the industrial Haber-Bosch process for distributed and modular production of NH₃ . Nevertheless, developing high-efficiency catalysts to simultaneously realize both high activity and selectivity for the development of a sustainable NRR is very critical but extremely challenging. Here, a unique plasma-assisted strategy is developed to synthesize iridium diphosphide nanocrystals with abundant surface step atoms anchored in P,N-codoped porous carbon nanofilms (IrP₂ @PNPC-NF), where the edges of the IrP₂ nanocrystals are extremely irregular, and the ultrathin PNPC-NF possesses a honeycomb-like macroporous structure. These characteristics ensure that IrP₂ @PNPC-NF delivers superior NRR performance with an NH₃ yield rate of 94.0 µg h^-1 mg^-1 _cat. and a faradaic efficiency (FE) of 17.8%. Density functional theory calculations reveal that the unique NRR performance originates from the low-coordinate step atoms on the edges of IrP₂ nanocrystals, which can lower the reaction barrier to improve the NRR activity and simultaneously inhibit hydrogen evolution to achieve a high FE for NH₃ formation. More importantly, such a plasma-assisted strategy is general and can be extended to the synthesis of other high-melting-point noble-metal phosphides (OsP₂ @PNPC-NF, Re₃ P₄ @PNPC-NF, etc.) with abundant step atoms at lower temperatures.

Biomass-Derived Nitrogen-Doped Porous Carbon for Highly Efficient Ambient Electro-Synthesis of NH3

In this communication, we report a biomass-derived nitrogen-doped porous carbon (named as NC-800) as an electrocatalyst for the ambient conversion of N2 to NH3. The catalyst NC-800 was prepared from naturally renewable and easily available bamboo shoots, with inherently an approximate 8 wt % of N-containing components, such as the N source, in a cost-effective and environmentally benign manner. This exhibited remarkable catalytic activity with a large NH3 yield and a Faradaic efficiency as high as 16.3 μg h−mg-1cat and 27.5%, respectively, at −0.35 V versus a reversible hydrogen electrode (RHE) in 0.1 M HCl solution at ambient conditions. More importantly, the catalyst NC-800 demonstrated excellent electrochemical selectivity and stability.

Highly boosted gas diffusion for enhanced electrocatalytic reduction of N2 to NH3 on 3D hollow Co-MoS2 nanostructures

Transition metal chalcogenide MoS₂ catalysts are highly selective for the electrochemical reduction of dinitrogen (N₂) to ammonia (NH₃) in aqueous electrolytes. However, due to the low solubility of N₂ in water, limited N₂ diffusion and mass transport have heavily restricted the yield and the faradaic efficiency (FE). Here, we have demonstrated a highly efficacious assembled gas diffusion cathode with hollow Co-MoS₂/N@C nanostructures to significantly improve the electrochemical reduction of N₂ to NH₃. Our results revealed that the synthesized Co-MoS₂ heterojunctions with abundant graphitic N groups exhibited a superb NH₃ yield of 129.93 μg h^-1 mg_cat^-1 and a high faradaic efficiency of 11.21% at -0.4 V vs. the reversible hydrogen electrode (RHE), as well as excellent selectivity and stability. The strategy described in this study offers new inspiration to design high-performance electrocatalyst assemblies for the sustainable environmental and energy applications.

Sulfur Atomically Doped Bismuth Nanobelt Driven by Electrochemical Self-Reconstruction for Boosted Electrocatalysis

Recent years have witnessed various in-depth research efforts on self-reconstruction behavior toward electrocatalysis. Tracking the phase transformation and evolution of true active sites is of great significance for the development of self-reconstructed electrocatalysts. Here, the optimized atomic sulfur-doped bismuth nanobelt (S-Bi) is fabricated via an electrochemical self-reconstruction evolved from Bi₂S₃. Advanced technologies have demonstrated that the nonmetallic S atoms have been doped into the lattice Bi frame, leading to the reconstruction of local electronic structure of Bi. The as-prepared S-Bi nanobelt exhibits a remarkable NH₃ generation rate of 10.28 μg h^-1 mg^-1 and Faradaic efficiency of 10.48%. Density functional theory calculations prove that the S doping can significantly lower the energy barrier of the rate-determining step and enlarge the N≡N bond for further dissociation toward N₂ fixation. This work not only establishes insights into the evolution process of electrochemically derived self-reconstruction but also unravels the root of the N₂ reduction reaction mechanism associated with the atomic nonmetal dopants.

Transition Metal Aluminum Boride as a New Candidate for Ambient-Condition Electrochemical Ammonia Synthesis

Achieving more meaningful N₂ conversion by reducing the energy input and carbon footprint is now being investigated through a method of N₂ fixation instead of the Haber-Bosch process. Unfortunately, the electrochemical N₂ reduction reaction (NRR) method as a rising approach currently still shows low selectivity (Faradaic efficiency < 10%) and high-energy consumption [applied potential at least - 0.2 V versus the reversible hydrogen electrode (RHE)]. Here, the role of molybdenum aluminum boride single crystals, belonging to a family of ternary transition metal aluminum borides known as MAB phases, is reported for the electrochemical NRR for the first time, at a low applied potential (- 0.05 V versus RHE) under ambient conditions and in alkaline media. Due to the unique nano-laminated crystal structure of the MAB phase, these inexpensive materials have been found to exhibit excellent electrocatalytic performances (NH₃ yield: 9.2 µg h^-1 cm^-2 mg _cat. ^-1 , Faradaic efficiency: 30.1%) at the low overpotential, and to display a high chemical stability and sustained catalytic performance. In conjunction, further mechanism studies indicate B and Al as main-group metals show a highly selective affinity to N₂ due to the strong interaction between the B 2p/Al 3p band and the N 2p orbitals, while Mo exhibits specific catalytic activity toward the subsequent reduction reaction. Overall, the MAB-phase catalyst under the synergy of the elements within ternary compound can suppress the hydrogen evolution reaction and achieve enhanced NRR performance. The significance of this work is to provide a promising candidate in the future synthesis of ammonia.

Edge-Rich Black Phosphorus for Photocatalytic Nitrogen Fixation

Development of efficient catalysts for ammonia production under ambient conditions is a challenge. In this work, edge-rich black phosphorus nanoflakes (eBP NFs) are synthesized for highly efficient photocatalytic nitrogen fixation. The eBP NFs synthesized by a chemical etching exfoliation method have a uniform size and about 90 nm wide and the flake-like structure is enshrouded with crystal-domain edges. Without using any cocatalysts, the eBP NFs catalyze nitrogen fixation at a rate of 2.37 mmol·h^-1·g^-1 under visible-light irradiation. The photocatalytic process is analyzed by photoelectrochemical and transient absorption measurements, which disclose electron transfer from eBP NFs to N₂ for subsequent hydrogenation. Besides the facile and scalable synthesis method, eBP NFs with a high photocatalytic nitrogen fixation efficiency have great potential in ammonia production.

Atomic Modulation, Structural Design, and Systematic Optimization for Efficient Electrochemical Nitrogen Reduction

Ammonia (NH3) is a pivotal precursor in fertilizer production and a potential energy carrier. Currently, ammonia production worldwide relies on the traditional Haber-Bosch process, which consumes massive energy and has a large carbon footprint. Recently, electrochemical dinitrogen reduction to ammonia under ambient conditions has attracted considerable interest owing to its advantages of flexibility and environmental friendliness. However, the biggest challenge in dinitrogen electroreduction, i.e., the low efficiency and selectivity caused by poor specificity of electrocatalysts/electrolytic systems, still needs to be overcome. Although substantial progress has been made in recent years, acquiring most available electrocatalysts still relies on low efficiency trial-and-error methods. It is thus imperative to establish some critical guiding principles for nitrogen electroreduction toward a rational design and accelerated development of this field. Herein, a basic understanding of dinitrogen electroreduction processes and the inherent relationships between adsorbates and catalysts from fundamental theory are described, followed by an outline of the crucial principles for designing efficient electrocatalysts/electrocatalytic systems derived from a systematic evaluation of the latest significant achievements. Finally, the future research directions and prospects of this field are given, with a special emphasis on the opportunities available by following the guiding principles.

Improving the Reliability and Accuracy of Ammonia Quantification in Electro- and Photochemical Synthesis

The reliable and accurate quantification of ammonia in electrochemical and photochemical experiments has been a technical challenge owing to the extremely low concentration of generated ammonia, interference from trace amounts of cations and organic compounds, and ammonia contamination from various sources. As a result, overestimation and significant errors may happen in many research works. Herein, accuracy and precision of ion chromatography (IC) are evaluated at different pH; excellent performance with a low detection limit (<2 μg L^-1 ) under acidic and neutral conditions is found, whereas the linearity is unsatisfactory in the low NH₄ ⁺ concentration range (0-100 μg L^-1 ) under alkaline conditions. High concentrations of Li⁺ and Na⁺ are difficult to separate from NH₄ ⁺ in conventional IC, but this can be solved by employing a high-exchange-capacity column or gradient elution. The interference effects of 14 common transition metal cations and 6 common organic compounds on the quantification of ammonium with low-level concentration (500 μg L^-1 ) using IC are systematically investigated, and the results demonstrate good robustness. The overestimation caused by ammonia contamination from reagent water, surroundings, and even the analytical grade of inorganic and organic reagents are confirmed and the results indicate the necessity to prepare and test fresh electrolyte solutions before each experiment, owing to the high sensitivity of acidic and neutral solutions to ammonia contamination from the surroundings. The ammonization of a Nafion membrane during experiments and the underestimation in quantification are also discussed. Finally, a reliable level of synthesized ammonia is identified and some recommendations are presented to improve the reliability and accuracy of ammonia quantification.

Copper nanocrystals anchored on an O-rich carbonized corn gel for nitrogen electroreduction to ammonia

Copper nanocrystals anchored on an O-rich carbonized corn gel for electrochemical N₂ fixation to NH₃ with a faradaic efficiency of 25.89% and an NH₃ yield rate of 1514 μg h⁻¹ mg_Cu⁻¹ at −0.3 V versus an RHE in 0.1 M Na₂SO₄.

Fabricating Amorphous g-C3N4/ZrO2 Photocatalysts by One-Step Pyrolysis for Solar-Driven Ambient Ammonia Synthesis

Solar-driven nitrogen fixation remains a significant challenge. Graphitic carbon nitride (g-C₃N₄) is considered as a promising visible light photocatalyst. However, the photocatalytic performance of g-C₃N₄ is unsatisfactory because of the random transfer of charge carriers in the plane and the low activation efficiency of the reactants. Herein, amorphous ZrO₂ was used as a robust cocatalyst of g-C₃N₄ to increase the NH₃ production activity. The g-C₃N₄/ZrO₂ lamellar composites were constructed by a simple one-step pyrolysis of the deep eutectic solvent ZrOCl₂·8H₂O/urea. The optimum NH₄⁺ yield could reach as high as 1446 μmol·L^-1·h^-1 at 30 wt % ZrO₂ in the g-C₃N₄/ZrO₂ composites, with an apparent quantum efficiency over 2.14% at 400 nm. It is 7.9 times that of pristine g-C₃N₄ and 27.5 times that of ZrO₂. The introduction of amorphous ZrO₂ restrained the hydrogen generation, and the amorphous ZrO₂ and g-C₃N₄ together contribute to the rapid photoproduced electron transfer of less electron-hole pair recombination.