Eight-electron Reduction Research Articles

Three-Dimensional Porous Cu/Cu2+1O Nanosheet Arrays Promote Electrochemical Nitrate-to-Ammonia Conversion.

The efficiency of electrochemical nitrate (NO3-) reduction to ammonia (NH3) still remains a challenge due to the sluggish kinetics of the complex eight-electron reduction process and competitive hydrogen evolution reaction (HER). Herein, we designed new three-dimensional (3D) porous Cu/Cu2+1O nanosheet arrays (Cu/Cu2+1O NSA) by coupling a template-directed method with in situ electroreduction. Thanks to the 3D porous structure and in-plane heterojunctions, Cu/Cu2+1O NSA can provide abundant active sites and a good interfacial effect, obtaining the maximum Faradaic efficiency (FE) of ammonia (88.09%) and high yield rate of 0.2634 mmol h-1 cm-2, which is higher than that of CuO nanosheets (77.81% and 0.2188 mmol h-1 cm-2) and CuO nanoparticles (34.60% and 0.0692 mmol h-1 cm-2). Experimental results and DFT simulations show that the interface effect of Cu/Cu2+1O can decrease the reaction energy barrier of the key step (*NO to *NOH) and can greatly inhibit the competitive hydrogen evolution reaction, thereby achieving excellent electrocatalytic performance for nitrate-to-ammonia conversion.

Methane Formation Induced via Face-to-Face Orientation of Cyclic Fe Porphyrin Dimer in Photocatalytic CO2 Reduction.

Iron porphyrins are known to provide CH4 as an eight-electron reduction product of CO2 in a photochemical reaction. However, there are still some aspects of the reaction mechanism that remain unclear. In this study, we synthesized iron porphyrin dimers and carried out the photochemical CO2 reduction reactions in N,N-dimethylacetamide (DMA) containing a photosensitizer in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor. We found that, despite a low catalytic turnover number, CH4 was produced only when these porphyrins were facing each other. The close proximity of the cyclic dimers, distinguishing them from a linear Fe porphyrin dimer and monomers, induced multi-electron CO2 reduction, emphasizing the unique role of their structural arrangement in CH4 formation.

Charge Photoaccumulation in Covalent Polymer Networks for Boosting Photocatalytic Nitrate Reduction to Ammonia.

In the design of photoelectrocatalytic cells, a key element is effective photogeneration of electron-hole pairs to drive redox activation of catalysts. Despite recent progress in photoelectrocatalysis, experimental realization of a high-performance photocathode for multi-electron reduction of chemicals, such as nitrate reduction to ammonia, has remained a challenge due to difficulty in obtaining efficient electrode configurations for extraction of high-throughput electrons from absorbed photons. This work describes a new design for catalytic photoelectrodes using chromophore assembly-functionalized covalent networks for boosting eight-electron reduction of nitrate to ammonia. Upon sunlight irradiation, the photoelectrode stores a mass of reducing equivalents at the photoexcited chromophore assembly for multielectron reduction of a copper catalyst, enabling efficient nitrate reduction to ammonia. By introducing the new photoelectrode structure, it is demonstrated that the electronic interplay between charge photo-accumulating assembly and multi-electron redox catalysts can be optimized to achieve proper balance between electron transfer dynamics and thermodynamic output of photoelectrocatalytic systems.

Electrochemical Reduction of Nitrate to Ammonia Using Copper Oxide Catalysts

The electrochemical conversion of nitrates to ammonia offers a promising alternative to the energy- and resource-intensive Haber-Bosch process while simultaneously removing nitrates, a common water pollutant. However, this process is hindered by the eight-electron reduction required to convert nitrate to ammonia, as well as competing hydrogen evolution reactions and other nitrate reduction pathways. Efficient catalysts are essential to address these challenges, and recent studies suggest that oxide catalysts with oxygen vacancies can optimize adsorption energies of intermediates and improve catalytic performance. Copper oxides with oxygen vacancy display high ammonia selectivity in nitrate reduction, but the complexity in vacancy enrichment and the inferior hydrogen adsorption on oxides make nitrate reduction an inefficient process.In this study, a CuOx composite electrode was synthesized using the hydrothermal method, and its performance was evaluated for efficient conversion of nitrate to ammonia. The synthesized CuOx nanostructure was characterized using X-ray diffraction to determine phase purity and crystal structure, while surface morphology of prepared samples was examined with high resolution transmission electron microscope. The results of X-ray photoelectron spectroscopy suggested that there were abundant oxygen vacancies on the surface of the CuOx, which can act as nitrate adsorption sites. The obtained CuOx composite electrode performs at a high ammonia Faradaic efficiency of over 80% and long-term stability due to interfacial coupling effects. These findings highlight the potential of CuOx catalysts for electrochemical nitrate reduction, offering new opportunities for sustainable nitrogen management and water treatment.

Direct Synthesis of Ammonia from Nitrate on Amorphous Graphene with Near 100% Efficiency.

Ammonia is an indispensable commodity in the agricultural and pharmaceutical industries. Direct nitrate-to-ammonia electroreduction is a decentralized route yet challenged by competing side reactions. Most catalysts are metal-based, and metal-free catalysts with high nitrate-to-ammonia conversion activity are rarely reported. Herein, it is shown that amorphous graphene synthesized by laser induction and comprising strained and disordered pentagons, hexagons, and heptagons can electrocatalyze the eight-electron reduction of NO3 - to NH3 with a Faradaic efficiency of ≈100% and an ammonia production rate of 2859 µg cm-2 h-1 at -0.93 V versus reversible hydrogen electrode. X-ray pair-distribution function analysis and electron microscopy reveal the unique molecular features of amorphous graphene that facilitate NO3 - reduction. In situ Fouriertransform infrared spectroscopy and theoretical calculations establish the critical role of these features in stabilizing thereaction intermediates via structural relaxation. The enhanced catalytic activity enables the implementation of flow electrolysis for the on-demand synthesis and release of ammonia with >70% selectivity, resulting in significantly increased yields and survival rates when applied to plant cultivation. The results of this study show significant promise for remediating nitrate-polluted water and completing the NOx cycle.

In Situ Growth of Fe2O3 Nanorod Arrays on Carbon Cloth with Rapid Charge Transfer for Efficient Nitrate Electroreduction to Ammonia.

Electrochemical reduction of nitrate to ammonia (NH3), a green NH3 production route upon combining with renewable energy sources, is an appealing and alternative method to the Haber-Bosch process. However, this process not only involves the complicated eight-electron reduction to transform nitrate into various nitrogen products but simultaneously suffers from the competitive hydrogen evolution reaction, challenged by a lack of efficient catalysts. Herein, the in situ growth of Fe2O3 nanorod arrays on carbon cloth (Fe2O3 NRs/CC) is reported to exhibit a high NH3 yield rate of 328.17 μmol h-1 cm-2 at -0.9 V versus RHE, outperforming most of the reported Fe catalysts. An in situ growth strategy provides massive exposed active sites and a fast electron-transport channel between the carbon cloth and Fe2O3, which accelerates the charge-transport rate and facilitates the conversion of nitrate to NH3. In situ Raman spectroscopy in conjunction with attenuated total reflection Fourier transform infrared spectroscopy reveals the catalytic mechanism of nitrate to NH3. Our study provides not only an efficient catalyst for NH3 production but also useful guidelines for the pathways and mechanism of nitrate electroreduction to NH3.

Reduction of nitrate to ammonia using photocatalytically accumulated electrons on titanium(IV) oxide in a time-separated redox reaction

• Reduction of NO 3 − to ammonia NH 3 was performed by a time-separated redox reaction. • The reaction uses a two-step reaction under the control of photoexcitation an electron acceptor. • Eight-electron reduction of NO 3 − to NH 3 was performed by accumulated electrons in a TiO 2 suspension. • The use of a rutile sample with a large number of defective sites is a key factor for the reaction. Reduction of nitrate (NO 3 − ) to ammonia (NH 3 ) was performed by a time-separated redox reaction using photocatalytically accumulated electrons in a titanium(IV) oxide (TiO 2 ) suspension. The time-separated redox reaction uses a two-step reaction under the control of photoexcitation and an electron acceptor: (1) accumulation of electrons in TiO 2 powder under photoexcitation and (2) reduction of added NO 3 − by accumulated electrons in the dark. Color change of TiO 2 depending on the degree of electron accumulation was observed during the reaction, and 8-electron reduction of NO 3 − to NH 3 was performed by accumulated electrons. The results of the time-separated redox reaction using 10 kinds of commercial TiO 2 powder indicate that the use of a rutile sample with a large number of trivalent titanium species is a key factor for NH 3 production.

Allotropes selection apropos of photocatalytic CO2 reduction from first principles studies

Materials engineering of allotropism provides a possible strategy for controling CO2 photoreduction into various C1 products. However, the understanding of the mechanism for tuning products selectivity through allotropism is still lacking. Herein, three structures of known phosphorus allotrope, i.e. black phosphorus (BP), fibrous red phosphorus (RP), and helical coil phosphorus (CP), were modelled as a CO2 reduction photocatalyst and studied by first principles density functional theory calculations. Three adsorption sites (P6C, P2B, and PT) of CO2 on the phosphorus allotropes were investigated. The most stable adsorption configuration for each allotrope was then selected for further study. The quantum topological analysis revealed that CO2 adsorption interaction on BP and RP exhibits physical binding of van der Waals interaction while CP shows physical binding of Keesom interaction. The CO2 adsorption interaction is shown to be crucial in determining the activation barrier for the initial proton-coupled electron transfer of CO2 → COOH. The pathways for different C1 products, including CO, HCOOH, C, CH2O, CH3OH, and CH4, were examined by comparing the rate determining step (RDS) obtained via Gibbs free energy analysis. RP and CP show higher selectivity towards the two- and four-electron C1 products. CP evidences a lower activation barrier for the two- and four-electron products RDS while BP exhibits higher selectivity towards the six- and eight-electron reduction products. The difference in selectivity of each of the allotropes was attributed to its distinct p-band center. This study divulges an important understanding on the possible product selectivity modulation for CO2 photoreduction based on allotropism.

Facile Dinitrogen and Dioxygen Cleavage by a Uranium(III) Complex: Cooperativity Between the Non-Innocent Ligand and the Uranium Center.

Activation of dinitrogen (N2 , 78 %) and dioxygen (O2 , 21 %) has fascinated chemists and biochemists for decades. The industrial conversion of N2 into ammonia requires extremely high temperatures and pressures. Herein we report the first example of N2 and O2 cleavage by a uranium complex, [N(CH2 CH2 NPi Pr2 )3 U]2 (TMEDA), under ambient conditions without an external reducing agent. The N2 triple bond breaking implies a UIII -PIII six-electron reduction. The hydrolysis of the N2 reduction product allows the formation of ammonia or nitrogen-containing organic compounds. The interaction between UIII and PIII in this molecule allows an eight-electron reduction of two O2 molecules. This study establishes that the combination of uranium and a low-valent nonmetal is a promising strategy to achieve a full N2 and O2 cleavage under ambient conditions, which may aid the design of new systems for small molecules activation.

Efficient nitrate-to-ammonia transformation through a direct eight-electron reduction

Efficient nitrate-to-ammonia transformation through a direct eight-electron reduction

TiO2-x/CoOx photocatalyst sparkles in photothermocatalytic reduction of CO2 with H2O steam

Solar photocatalytic production of fuels from CO2 and H2O remains a challenging goal. Herein we report a strategy to co-modify TiO2 with oxygen vacancies and CoOx nanoclusters for enhanced photothermocatalytic reduction of CO2. The TiO2-x/CoOx material exhibits prominently enhanced activity for the yield of CH4 and CO under ultraviolet irradiation at elevated temperature of 393 K, which is 111.3- and 13.2-times greater yield of CH4 and CO, respectively than the conventional photocatalytic process at 298 K, and 175.1- and 2.9-times greater yield of CH4 and CO, respectively than the pristine TiO2 under the same photothermocatalytic conditions. Control experiments over singly modified TiO2 and doubly modified TiO2 by different preparation history, together with high-resolution transmission electron microscope (HRTEM), electron spin resonance (ESR), and transient photovoltage measurements reveal the synergistic effect of oxygen vacancies and surface-grafted CoOx on the photothermocatalytic reduction of CO2 to CH4, i.e. oxygen vacancies at TiO2 surface facilitate the adsorption and reduction of CO2 and the dispersion of CoOx nanoclusters, whereas surface-grafted CoOx clusters facilitate the hole trapping and the oxidation of H2O. Thereby the coexistence of oxygen vacancies and CoOx nanoclusters at TiO2 surface promote the separation of photogenerated electrons and holes, and remarkably enhance the eight-electron reduction of CO2 to CH4 under photothermocatalytic conditions. This study shows the great potential of photo-thermal synergy on CO2 reduction and provides a promising means to design photothermocatalysts for solar photocatalytic reduction of CO2 to fuel.

Visible-light-driven methane formation from CO2 with a molecular iron catalyst.

Converting CO2 into fuel or chemical feedstock compounds could in principle reduce fossil fuel consumption and climate-changing CO2 emissions. One strategy aims for electrochemical conversions powered by electricity from renewable sources, but photochemical approaches driven by sunlight are also conceivable. A considerable challenge in both approaches is the development of efficient and selective catalysts, ideally based on cheap and Earth-abundant elements rather than expensive precious metals. Of the molecular photo- and electrocatalysts reported, only a few catalysts are stable and selective for CO2 reduction; moreover, these catalysts produce primarily CO or HCOOH, and catalysts capable of generating even low to moderate yields of highly reduced hydrocarbons remain rare. Here we show that an iron tetraphenylporphyrin complex functionalized with trimethylammonio groups, which is the most efficient and selective molecular electro- catalyst for converting CO2 to CO known, can also catalyse the eight-electron reduction of CO2 to methane upon visible light irradiation at ambient temperature and pressure. We find that the catalytic system, operated in an acetonitrile solution containing a photosensitizer and sacrificial electron donor, operates stably over several days. CO is the main product of the direct CO2 photoreduction reaction, but a two-pot procedure that first reduces CO2 and then reduces CO generates methane with a selectivity of up to 82 per cent and a quantum yield (light-to-product efficiency) of 0.18 per cent. However, we anticipate that the operating principles of our system may aid the development of other molecular catalysts for the production of solar fuels from CO2 under mild conditions.

Selective Nitrate-to-Ammonia Transformation on Surface Defects of Titanium Dioxide Photocatalysts

Ammonia (NH3) is an essential chemical in modern society, currently manufactured via the Haber–Bosch process with H2 and N2 under extremely high pressure (>200 bar) and high-temperature conditions (>673 K). Toxic nitrate anion (NO3–) contained in wastewater is one potential nitrogen source. Selective NO3–-to-NH3 transformation via eight-electron reduction, if promoted at atmospheric pressure and room temperature, may become a powerful recycling process for NH3 production. Several photocatalytic systems have been proposed, but many of them produce nitrogen gas (N2) via five-electron reduction of NO3–. Here, we report that unmodified TiO2, when photoexcited by ultraviolet (UV) light (λ > 300 nm) with formic acid (HCOOH) as an electron donor, promotes selective NO3–-to-NH3 reduction with 97% selectivity. Surface defects and Lewis acid sites of TiO2 behave as reduction sites for NO3–. The surface defect selectively promotes eight-electron reduction (NH3 formation), while the Lewis acid site promotes nonselect...

Thermochemical and electrochemical CO2 reduction on octahedral Cu nanocluster: Role of solvent towards product selectivity

There has been a major interest in the development of efficient catalysts for the electrochemical reduction of CO2 into useful chemicals and fuels. In this work, the thermochemical and electrochemical CO2 reduction pathways are systematically studied on the (111) facet of an octahedral copper (Cu85) nanocluster (NC) using density functional theory (DFT) calculations. An investigation is carried out to understand the catalytic activity of the Cu NC towards CO2 reduction to explore its activity and product selectivity (CH3OH vs. CH4; i.e., six-electron vs. eight-electron reduction reaction). Interestingly, CO2 adsorbs strongly the (111) facet of the Cu NC, as opposed to the previous reports on the periodic Cu(111) surfaces. Furthermore, such NC shows excellent catalytic activity towards direct CO bond dissociation, which is again in contrary to what was reported in the literatures. Besides, the CO2 hydrogenation reaction has been investigated using different proton transfer mechanisms (surface-hydrogenation, water-assisted, and water solvated) with/without the help of a solvent water molecule. We find that the water-assisted H-shuttling mechanism lowers the activation barrier significantly and thus favours the CO2 hydrogenation. However, the direct CO bond dissociation is very competing with respect to indirect CO bond dissociation. Based on our reaction free energy calculations, activation barrier calculations, and simulated Pourbaix calculations, we find that the Cu NC shows excellent catalytic activity and selectivity over any catalysts studied to date. Besides, the Cu NC requires a lower overpotential (0.53V) compared to the periodic Cu(111) surface (0.71V) and thus can be a promising catalyst for CO2 reduction reaction.

(Invited) Understanding the Chemistry of Photocatalytic Processes

Any enhancement of the efficiency of the utilization of photogenerated charge carriers on the surface of photocatalysts requires an enhanced understanding of the entire photocatalytic process. The reduction potentials of most substrates, as well as those of the intermediates formed during the photocatalytic reaction(s), are well known; nevertheless, it is essential to realize that thermodynamic properties may change upon the adsorption of these molecules at the photocatalyst surface. Therefore, a detailed understanding of the processes occurring on the photocatalyst surface before, during, and after light absorption is of utmost importance. On the other hand, the charge carriers generated upon light absorption that survive recombination and reach the semiconductor surface may suffer surface recombination processes or recombination via an electron shuttle mechanism (Z scheme deactivation mechanism), thus reducing the total efficiency of the photocatalytic system. Moreover, since the overall efficiency of a photocatalytic process will be determined by the efficiency of the slowest reaction step it is crucial to know whether this is the reductive or oxidative half-reactions. Besides the obvious one-electron transfer steps these reactions entail in particular multi-electron transfer processes, e.g. , the four-electron oxidation of water or the eight-electron reduction of CO2. Hence, this lecture provides some necessary tools for the understanding and the development of the photocatalytic reaction mechanism.

The Activation of Sulfur Hexafluoride at Highly Reduced Low‐Coordinate Nickel Dinitrogen Complexes

The greenhouse gas sulfur hexafluoride is the common standard example in the literature of a very inert inorganic small molecule that is even stable against O2 in an electric discharge. However, a reduced β-diketiminate nickel species proved to be capable of converting SF6 into sulfide and fluoride compounds at ambient standard conditions. The fluoride product complex features an unprecedented [NiF](+) unit, where the Ni atom is only three-coordinate, while the sulfide product exhibits a rare almost linear [Ni(μ-S)Ni](2+) moiety. The reaction was monitored applying (1)H NMR, IR and EPR spectroscopic techniques resulting in the identification of an intermediate nickel complex that gave insight into the mechanism of the eight-electron reduction of SF6.

57Fe ENDOR Spectroscopy and ‘Electron Inventory’ Analysis of the Nitrogenase E4Intermediate Suggest the Metal-Ion Core of FeMo-Cofactor Cycles Through Only One Redox Couple

N(2) binds to the active-site metal cluster in the nitrogenase MoFe protein, the FeMo-cofactor ([7Fe-9S-Mo-homocitrate-X]; FeMo-co) only after the MoFe protein has accumulated three or four electrons/protons (E(3) or E(4) states), with the E(4) state being optimally activated. Here we study the FeMo-co (57)Fe atoms of E(4) trapped with the α-70(Val→Ile) MoFe protein variant through use of advanced ENDOR methods: 'random-hop' Davies pulsed 35 GHz ENDOR; difference triple resonance; the recently developed Pulse-Endor-SaTuration and REcovery (PESTRE) protocol for determining hyperfine-coupling signs; and Raw-DATA (RD)-PESTRE, a PESTRE variant that gives a continuous sign readout over a selected radiofrequency range. These methods have allowed experimental determination of the signed isotropic (57)Fe hyperfine couplings for five of the seven iron sites of the reductively activated E(4) FeMo-co, and given the magnitude of the coupling for a sixth. When supplemented by the use of sum-rules developed to describe electron-spin coupling in FeS proteins, these (57)Fe measurements yield both the magnitude and signs of the isotropic couplings for the complete set of seven Fe sites of FeMo-co in E(4). In light of the previous findings that FeMo-co of E(4) binds two hydrides in the form of (Fe-(μ-H(-))-Fe) fragments, and that molybdenum has not become reduced, an 'electron inventory' analysis assigns the formal redox level of FeMo-co metal ions in E(4) to that of the resting state (M(N)), with the four accumulated electrons residing on the two Fe-bound hydrides. Comparisons with earlier (57)Fe ENDOR studies and electron inventory analyses of the bio-organometallic intermediate formed during the reduction of alkynes and the CO-inhibited forms of nitrogenase (hi-CO and lo-CO) inspire the conjecture that throughout the eight-electron reduction of N(2) plus 2H(+) to two NH(3) plus H(2), the inorganic core of FeMo-co cycles through only a single redox couple connecting two formal redox levels: those associated with the resting state, M(N), and with the one-electron reduced state, M(R). We further note that this conjecture might apply to other complex FeS enzymes.

Reductive Coupling of Six Carbon Monoxides by a Ditantalum Hydride Complex

A ditantalum hydride complex undergoes head-to-head C-C coupling of six CO molecules under mild conditions to produce an octaanionic C(6)O(6) unit. During the reaction, eight-electron reduction of six CO molecules takes place. In the product, the C(6)O(6) unit is coordinated to four tantalum metals through six oxygen and two carbon atoms. The geometrical parameters within the Ta(4)C(6)O(6) core indicate significant contribution from a hexatriene-hexaolate form.

Octa- and Nonanuclear Nickel(II) Polyoxometalate Clusters: Synthesis and Electrochemical and Magnetic Characterizations

Three high-nuclearity NiII-substituted polyoxometalate compounds functionalized by exogenous ligands have been synthesized and characterized. The octanuclear complexes in Na15[Na{(A-R-SiW9O34)Ni4(CH3COO)3(OH)3}2] . 4NaCl . 36H2O (1) and Na15[Na{(A-R-SiW9O34)Ni4(CH3COO)3(OH)2(N3)}2] . 32H2O (2) can be described as two {Ni4} subunits connected via a {Na(CH3COO)6} group, with the acetato ligands also ensuring in each subunit the connection between the paramagnetic centers. In 2, two azido groups replace two of the six mu-hydroxo ligands present in 1. The nonanuclear complex K7Na7[(A-R-SiW9O34)2Ni9(OH)6(H2O)6(CO3)3] . 42H2O (3) exhibits a double cubanestructure with two [(A-R-SiW9O34)Ni4(OH)3]5- subunits linked by three carbonato ligands. A ninth NiII center connected to one subunit via a carbonato ligand and a O=W group completes this asymmetric polyoxometalate.Electronic spectroscopy and electrochemical studies indicate that, while compounds 1-3 decompose in a pure aqueous medium, these complexes are very stable in a pH 6 acetate medium. The cyclic voltammetry pattern of each complex is constituted by a first eight-electron reduction wave followed by a second large-current intensity wave. The characteristics of the first waves of the complexes are clearly distinct from those obtained for their lacunary precursor [A-R-SiW9O34]10-, a feature that is due to the Ni centers in the complexes. Such observations of electroactive, stable, and highly nickel-rich polyoxometalates are not common. Measurements of the magnetic susceptibility revealed the occurrence of concomitant ferromagnetic and antiferromagnetic interactions in 1 and 3.For both of these compounds, the extension of the magnetic exchange has been determined by means of a spin Hamiltonian with three and four J constants, respectively.

Respiration of Nitrate and Nitrite.

Nitrate reduction to ammonia via nitrite occurs widely as an anabolic process through which bacteria, archaea, and plants can assimilate nitrate into cellular biomass. Escherichia coli and related enteric bacteria can couple the eight-electron reduction of nitrate to ammonium to growth by coupling the nitrate and nitrite reductases involved to energy-conserving respiratory electron transport systems. In global terms, the respiratory reduction of nitrate to ammonium dominates nitrate and nitrite reduction in many electron-rich environments such as anoxic marine sediments and sulfide-rich thermal vents, the human gastrointestinal tract, and the bodies of warm-blooded animals. This review reviews the regulation and enzymology of this process in E. coli and, where relevant detail is available, also in Salmonella and draws comparisons with and implications for the process in other bacteria where it is pertinent to do so. Fatty acids may be present in high levels in many of the natural environments of E. coli and Salmonella in which oxygen is limited but nitrate is available to support respiration. In E. coli, nitrate reduction in the periplasm involves the products of two seven-gene operons, napFDAGHBC, encoding the periplasmic nitrate reductase, and nrfABCDEFG, encoding the periplasmic nitrite reductase. No bacterium has yet been shown to couple a periplasmic nitrate reductase solely to the cytoplasmic nitrite reductase NirB. The cytoplasmic pathway for nitrate reduction to ammonia is restricted almost exclusively to a few groups of facultative anaerobic bacteria that encounter high concentrations of environmental nitrate.