New avenues in dinitrogen fixation research

Structure, reactivity, and spectroscopy of nitrogenase-related synthetic and biological clusters

The reduction of dinitrogen (N2) is essential for its incorporation into nucleic acids and amino acids, which are vital to life on earth. Nitrogenases convert atmospheric dinitrogen to two ammonia molecules (NH3) under ambient conditions. The catalytic active sites of these enzymes (known as FeM-cofactor clusters, where M = Mo, V, Fe) are the sites of N2 binding and activation and have been a source of great interest for chemists for decades. In this review, recent studies on nitrogenase-related synthetic molecular complexes and biological clusters are discussed, with a focus on their reactivity and spectroscopic characterization. The molecular models that are discussed span from simple mononuclear iron complexes to multinuclear iron complexes and heterometallic iron complexes. In addition, recent work on the extracted biological cofactors is discussed. An emphasis is placed on how these studies have contributed towards our understanding of the electronic structure and mechanism of nitrogenases.

B and cyano groups co-doped g-C3N4 with multiple defects for photocatalytic nitrogen fixation in ultrapure water without hole scavengers

B atoms and cyano groups co-doped graphite carbon nitride with nitrogen vacancies (VN-BC-CN) was explored via one-step in-situ route. A series of comprehensive experiments confirmed that B atoms and cyano groups had been doped into the framework of graphite carbon nitride, forming VN-BC-CN catalyst sample with a large number of nitrogen-vacancy defects. As electron acceptors, B and cyano groups could be used as active sites for nitrogen conversion. The defect level caused by nitrogen vacancy led to red shift of the light absorption edge, which resulted in higher separation efficiency of photo-induced carriers and faster transfer rate of photo-induced electrons for the VN-BC-CN catalyst. This VN-BC-CN catalyst had good photocatalytic nitrogen fixation performance in the ultrapure water without any hole-scavengers. The nitrogen photofixation rate of VN-BC-CN (115.53 μmol g^-1 h^-1) was 25.5 times that of pure carbon nitride (GCN, 4.53 μmol g^-1 h^-1). Moreover, NH₄⁺ generation rate hardly decreased after 10 h reaction, and the NH₄⁺ generation rate could reach 79.56 μmol g^-1 h^-1 in the fifth cycle, showing the good photocatalytic stability of the VN-BC-CN catalyst.

In situ preparation of g-C3N4 nanosheet/FeOCl: Achievement and promoted photocatalytic nitrogen fixation activity

Climate change, global warming, and population growth have led researchers to use eco-sociable procedures for the N₂ reduction reaction. It has discovered that N₂ molecule can be transformed into NH₃ in ambient circumstances with nanocomposites upon visible irradiation. In this research paper, a new visible-light-driven photocatalyst was constructed, with various weight percents of FeOCl particles (10, 20, 30, and 40%) that have adhered on NS-CN. Subsequently, multiple features of the nanocomposites were assayed in detail. The results illustrated that the NS-CN/FeOCl (20%) system has remarkable photoactivity in the NH₄⁺ production reaction in comparison with the NS-CN and CN, which showed 2.5 and 8.6 higher activity, respectively. The durability of NS-CN/FeOCl (20%) system, as a substantial factor, was assayed for 5 recycles. Moreover, the effect of electron quenchers, pH of media, and solvent was studied. At last, a feasible Z-scheme mechanism for the remarkable improvement of N₂ fixation efficiency was offered.

GaN nanowires as a reusable photoredox catalyst for radical coupling of carbonyl under blacklight irradiation

Employing photo-energy to drive the desired chemical transformation has been a long pursued subject. The development of homogeneous photoredox catalysts in radical coupling reactions has been truly phenomenal, however, with apparent disadvantages such as the difficulty in separating the catalyst and the frequent requirement of scarce noble metals. We therefore envisioned the use of a hyper-stable III-V photosensitizing semiconductor with a tunable Fermi level and energy band as a readily isolable and recyclable heterogeneous photoredox catalyst for radical coupling reactions. Using the carbonyl coupling reaction as a proof-of-concept, herein, we report a photo-pinacol coupling reaction catalyzed by GaN nanowires under ambient light at room temperature with methanol as a solvent and sacrificial reagent. By simply tuning the dopant, the GaN nanowire shows significantly enhanced electronic properties. The catalyst showed excellent stability, reusability and functional tolerance. All reactions could be accomplished with a single piece of nanowire on Si-wafer.

Efficient Nitrogen Fixation Catalyzed by Gallium Nitride Nanowire Using Nitrogen and Water

Ammonia is one of the most important bulk chemicals in modern society. However, the highly energy-extensive contemporary industrial production of ammonia was developed in the early 20th century and requires extensive heating of highly pressurized flammable hydrogen gas, whose global production still relies heavily on non-sustainable petroleum. The development of "sustainable" nitrogen fixation process represents a grand aspirational chemical pursuit concerning our future human well-being. Herein, we report an ultra-stable nitride-based photosensitizing semiconductor that enables efficient, sustainable, and mild photochemical nitrogen fixation. The catalyst exhibits strong chemisorption of nitrogen and enables immediate electron donation from its surface vacancy to nitrogen. In addition, it was also demonstrated that the nitride-based semiconductor possesses the potential to minimize electron-hole recombination.

Construction of a wide-spectrum-driven VN-g-C3N4/Cu2(OH)2CO3 heterojunction catalyst from VIS to NIR light via the in situ self-sacrificial method: the effect of oxygen on the N2 photofixation ability

In this work, an N vacancy-doped g-C₃N₄/Cu₂(OH)₂CO₃ (V_N-GCN/CuCOH) heterojunction catalyst with superior wide-spectrum-driven (from VIS to NIR) N₂ photofixation ability was synthesized via the in situ self-sacrificial method.

Template free synthesis of lithium doped three-dimensional macroporous graphitic carbon nitride for photocatalytic N2 fixation: the effect of Li–N active sites

In this work, three-dimensional macroporous lithium doped graphitic carbon nitride was synthesized.

Hybrid materials based on conjugated polymers and inorganic semiconductors as photocatalysts: from environmental to energy applications

Hybrid materials photocatalysts based on conjugated polymers and inorganic semiconductors provide a sustainable way to remove pollutants or store energy in the form of solar fuels by processes such as water splitting, CO₂ photoreduction and NH₃ production (artificial photosynthesis).

The promotion of the photocatalytic nitrogen fixation ability of nitrogen vacancy-embedded graphitic carbon nitride by replacing the corner-site carbon atom with phosphorus

Tuning a catalyst's structure is an effective method to modify its physicochemical and electronic properties.

Progress of Metal Oxide (Sulfide)-Based Photocatalytic Materials for Reducing Nitrogen to Ammonia

The Haber–Bosch process has been an important approach to produce ammonia for meeting the food need of increasing population and the worldwide need of nitrogenous fertilizers since 1913. However, the traditional ammonia production process is a high energy-consumption process, which usually produces 1 metric ton ammonia with releasing around 1.9 metric tons CO2. Photocatalytic ammonia synthesis under solar light as energy source, an attractive and promising alternative approach, is a very challenging target of reducing fossil energy consumption and environmental pollution. Therefore, photocatalytic ammonia production process would emerge huge opportunities by directly providing nitrogenous fertilizers in a distributed manner as needed in the agricultural fields. In this article, different metal oxide (sulfide)-based photocatalytic materials for reducing nitrogen to ammonia under ambient conditions are reviewed. This review provides insights into the most recent advancements in understanding the photocatalyst materials which are of fundamental significance to photocatalytic nitrogen reduction, including the state-of-the-art, challenges, and prospects in this research field.

Efficient solar-driven conversion of nitrogen to ammonia in pure waterviahydrogenated bismuth oxybromide

Solar-driven reduction of dinitrogen to ammonia under mild conditions has attracted widespread interest in recent years. In this study, we first report low-temperature hydrogenated BiOBr for the direct synthesis of ammonia from dinitrogen with high efficiency under solar-light irradiation. In a proof of concept, the hydrogenation treatment can lead to surface disorder due to the strong reducing capacity of hydrogen. Oxygen atoms can be activated, and they can escape from the surface structure to form oxygen vacancies. Then, defect engineering can broaden the photoelectricity absorption window and effectively trigger interfacial electron transfer from the semiconductor to the combined nitrogen. This method exhibits a satisfactory result for photocatalytic nitrogen fixation, yielding about 2.6 times more NH₃ than that obtained from the original sample. The corresponding apparent quantum efficiency can reach a significant value of 2.1% under 380 nm monochromatic light irradiation. These results may pave a new way for the synthesis of highly active photocatalysts for efficient nitrogen fixation under solar light irradiation.

Samples of H-BiOBr use water as reactant which demonstrates efficient nitrogen fixation under sunlight.

In situ construction of Z-scheme g-C3N4/Mg1.1Al0.3Fe0.2O1.7 nanorod heterostructures with high N2 photofixation ability under visible light

By tuning the metal ratio, a Z-scheme g-C₃N₄/MgAlFeO nanorod composite was prepared in situ.

Effect of sulfur vacancies on the nitrogen photofixation performance of ternary metal sulfide photocatalysts

The NH₄⁺ generation rate over the as-prepared ternary metal sulfide catalysts is linearly related to the sulfur vacancy concentration, confirming that the photocatalytic reduction capacity of N₂ over ternary metal sulfides is highly dependent on the amount of sulfur vacancies.

Infrared ray assisted microwave synthesis: a convenient method for large-scale production of graphitic carbon nitride with outstanding nitrogen photofixation ability

A convenient infrared ray assisted microwave method for synthesizing graphitic carbon nitride (g-C₃N₄) with outstanding nitrogen photofixation ability under visible light is reported.

Photofixation of atmospheric nitrogen to ammonia with a novel ternary metal sulfide catalyst under visible light

The NH₄⁺ generation rate over ternary metal sulfide catalysts are linearly related to the sulfur vacancies concentration, confirming the photofixation capacity of N₂ over ternary metal sulfide are dependent on the amount of sulfur vacancies.

Preparation of g-C3N4/ZnMoCdS hybrid heterojunction catalyst with outstanding nitrogen photofixation performance under visible light via hydrothermal post-treatment

The outstanding nitrogen photofixation ability of g-C₃N₄/ZnMoCdS heterojunction photocatalyst is attributed to the synergy effect of heterojunctions and sulfur vacancies.

Construction of a 2D/2D g-C3N4/rGO hybrid heterojunction catalyst with outstanding charge separation ability and nitrogen photofixation performance via a surface protonation process

Heterojunction catalyst PCN/rGO with effective interfacial contact by incorporating reduced graphene oxide and protonated g-C₃N₄ and outstanding nitrogen photofixation ability was synthesized via a electrostatic self-assembly strategy.

Compositional Effects in Ru, Pd, Pt, and Rh-Doped Mesoporous Tantalum Oxide Catalysts for Ammonia Synthesis

A series of early metal-promoted Ru-, Pd-, Pt-, and Rh-doped mesoporous tantalum oxide catalysts were synthesized using a variety of dopant ratios and dopant precursors, and the effects of these parameters on the catalytic activity of NH3 synthesis from H2 and N2 were explored. Previous studies on this system supported an unprecedented mechanism in which N-N cleavage occurred at the Ta sites rather than on Ru. The results of the present study showed, for all systems, that Ba is a better promoter than Cs or La and that the nitrate is a superior precursor for Ba than the isopropoxide or the hydroxide. 15N-labeling studies showed that residual nitrate functions as the major ammonia source in the first hour but that it does not account for the ammonia produced after the nitrate is completely consumed. Ru3(CO)12 proved to be a better Ru precursor than RuCl(3).3H2O, and an almost linear increase in activity with increasing Ru loading level was observed at 350 degrees C (623 K). However, at 175 degrees C (448 K), the increase in Ru had no effect on the reaction rate. Pd functioned with comparable rates to Ru, while Pt and Rh functioned far less efficiently. The surprising activities for the Pd-doped catalysts, coupled with XPS evidence for low-valent Ta in this catalyst system, support a mechanism in which cleavage of the N-N triple bond occurs on Ta rather than the precious metal because the Ea value for N-N cleavage on Pd is 2.5 times greater than that for Ru, and the 9.3 kJ mol-1 Ea value measured previously for the Ru system suggests that N-N cleavage cannot occur at the Ru surface.

Electroactive mesoporous tantalum oxide catalysts for nitrogen activation and ammonia synthesis

A new mesoporous Ta oxide catalyst for conversion of dinitrogen to ammonia shows strong evidence for a novel mechanism involving low valent Ta on the surface, supporting recent work in organometallic chemistry using low valent early transition metals for dinitrogen cleavage.

Mesoporous tantalum oxide photocatalysts for Schrauzer-type conversion of dinitrogen to ammonia

Mesoporous tantalum oxide, Fe3+-doped mesoporous tantalum oxide, and bis(toluene) titanium reduced mesoporous tantalum oxide were used for the first time as Schrauzer-type photocatalysts for the conversion of dinitrogen to ammonia. The materials were characterized by XRD, TEM, XPS, and nitrogen absorption before and after catalytic runs. The results showed low to moderate activities depending on the composition. In contrast to previously studied Ti catalysts, Fe doping and heat pretreatment were not prerequisites for photocatalytic activity, but did improve the turnover rates by up to a factor of two. The optimal Fe loading for the tantalum oxides was found to be 1 wt% and the optimal heating condition at 300 °C for 3 h. Increased surface area and heat treatment were also found to improve activities. Contrary to our expectations, reduction of the mesostructure with bis(toluene) titanium had little effect on the catalytic activity. In spite of the dramatically higher surface areas of the mesoporous tantalum oxides as compared with bulk titanias used previously in this process, the overall catalytic activities were still less than those obtained in the Schrauzer system. This suggests that the increase in diffusion and surface area offered by the mesoporous structure is offset by the smaller crystalline domain sizes in the walls of the structure, leading to poor electron-hole separation and a reduction in catalytic efficiency. Key words: mesoporous, Schrauzer, ammonia, photocatalysis, tantalum oxide.