Electrocatalytic Multielectron Nitrite Reduction in Water by an Iron Complex

A terpyridine-based copper complex for electrochemical reduction of nitrite to nitric oxide

In biological systems, nitrite reductase enzymes (NIRs) are responsible for reduction of nitrite (NO2-) to nitric oxide (NO). These NIRs have mostly Cu- or Fe-containing active sites, surrounded by amine-containing ligands. Therefore, mononuclear Cu complexes with N-donor ligands are highly relevant in the development of NIR model systems and in the mechanistic investigation of the nitrite reduction reaction. Herein, we report on a terpyridine-based CuII complex with square planar geometry for H+-assisted electrochemical reduction of NO2-. Through electrochemical measurements, spectroscopic characterization and isotope-labelling experiments we propose a mechanistic reaction pathway involving an unstable HNO2 state. The CuI intermediate, formed electrochemically, was isolated and its molecular structure was deduced, showing linkage isomerism of the nitrite ligand. Moreover, qualitative and quantitative product analysis by GC-MS shows N2O formed as a side product along with the main product NO. Furthermore, by obtaining single crystals and conducting structural analysis we were able to determine the structural arrangement and redox state of the complex after electrochemical treatment.

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

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

Polyoxometalate-Intercalated Tremella-Like CoNi-LDH Nanocomposites for Electrocatalytic Nitrite-Ammonia Conversion

The electrocatalytic reduction of NO2- (NO2RR) holds promise as a sustainable pathway to both promoting the development of emerging NH3 economies and allowing the closing of the NOx loop. Highly efficient electrocatalysts that could facilitate this complex six-electron transfer process are urgently desired. Herein, tremella-like CoNi-LDH intercalated by cyclic polyoxometalate (POM) anion P8W48 (P8W48/CoNi-LDH) prepared by a simple two-step hydrothermal-exfoliation assembly method is proposed as an effective electrocatalyst for NO2- to NH3 conversion. The introduction of POM with excellent redox ability tremendously increased the electrocatalytic performance of CoNi-LDH in the NO2RR process, causing P8W48/CoNi-LDH to exhibit large NH3 yield of 0.369 mmol h-1 mgcat-1 and exceptionally high Faradic efficiency of 97.0% at -1.3 V vs the Ag/AgCl reference electrode in 0.1 M phosphate buffer saline (PBS, pH = 7) containing 0.1 M NO2-. Furthermore, P8W48/CoNi-LDH demonstrated excellent durability during cyclic electrolysis. This work provides a new reference for the application of POM-based nanocomposites in the electrochemical reduction of NO2- to obtain value-added NH3.

Sequential Deoxygenation of CO2 and NO2- via Redox-Control of a Pyridinediimine Ligand with a Hemilabile Phosphine

The deoxygenation of environmental pollutants CO2 and NO2- to form value-added products is reported. CO2 reduction with subsequent CO release and NO2- conversion to NO are achieved via the starting complex Fe(PPhPDI)Cl2 (1). 1 contains the redox-active pyridinediimine (PDI) ligand with a hemilabile phosphine located in the secondary coordination sphere. 1 was reduced with SmI2 under a CO2 atmosphere to form the direduced monocarbonyl Fe(PPhPDI)(CO) (2). Subsequent CO release was achieved via oxidation of 2 using the NOx- source, NO2-. The resulting [Fe(PPhPDI)(NO)]+ (3) mononitrosyl iron complex (MNIC) is formed as the exclusive reduction product due to the hemilabile phosphine. 3 was investigated computationally to be characterized as {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand.

Supramolecular Enhancement of Electrochemical Nitrate Reduction Catalyzed by Cobalt Porphyrin Organic Cages for Ammonia Electrosynthesis in Water

The electrochemical nitrate (NO3 - ) reduction reaction (NO3 RR) to ammonia (NH3 ) represents a sustainable approach for denitrification to balance global nitrogen cycles and an alternative to traditional thermal Haber-Bosch processes. Here, we present a supramolecular strategy for promoting NH3 production in water from NO3 RR by integrating two-dimensional (2D) molecular cobalt porphyrin (CoTPP) units into a three-dimensional (3D) porous organic cage architecture. The porphyrin box CoPB-C8 enhances electrochemical active site exposure, facilitates substrate-catalyst interactions, and improves catalyst stability, leading to turnover numbers and frequencies for NH3 production exceeding 200,000 and 56 s-1 , respectively. These values represent a 15-fold increase in NO3 RR activity and 200-mV improvement in overpotential for the 3D CoPB-C8 box structure compared to its 2D CoTPP counterpart. Synthetic tuning of peripheral alkyl substituents highlights the importance of supramolecular porosity and cavity size on electrochemical NO3 RR activity. These findings establish the incorporation of 2D molecular units into 3D confined space microenvironments as an effective supramolecular design strategy for enhancing electrocatalysis.

Outer coordination sphere influences on cofactor maturation and substrate oxidation by cytochrome P460

Product selectivity of ammonia oxidation by ammonia-oxidizing bacteria (AOB) is tightly controlled by metalloenzymes. Hydroxylamine oxidoreductase (HAO) is responsible for the oxidation of hydroxylamine (NH2OH) to nitric oxide (NO). The non-metabolic enzyme cytochrome (cyt) P460 also oxidizes NH2OH, but instead produces nitrous oxide (N2O). While both enzymes use a heme P460 cofactor, they selectively oxidize NH2OH to different products. Previously reported structures of Nitrosomonas sp. AL212 cyt P460 show that a capping phenylalanine residue rotates upon ligand binding, suggesting that this Phe may influence substrate and/or product binding. Here, we show via substitutions of the capping Phe in Nitrosomonas europaea cyt P460 that the bulky phenyl side-chain promotes the heme-lysine cross-link forming reaction operative in maturing the cofactor. Additionally, the Phe side-chain plays an important role in modulating product selectivity between N2O and NO during NH2OH oxidation under aerobic conditions. A picture emerges where the sterics and electrostatics of the side-chain in this capping position control the kinetics of N2O formation and NO binding affinity. This demonstrates how the outer coordination sphere of cyt P460 is tuned not only for selective NH2OH oxidation, but also for the autocatalytic cross-link forming reaction that imbues activity to an otherwise inactive protein.

Elucidation of the Electrocatalytic Nitrite Reduction Mechanism by Bio-Inspired Copper Complexes

Mononuclear copper complexes relevant to the active site of copper nitrite reductases (CuNiRs) are known to be catalytically active for the reduction of nitrite. Yet, their catalytic mechanism has thus far not been resolved. Here, we provide a complete description of the electrocatalytic nitrite reduction mechanism of a bio-inspired CuNiR catalyst Cu(tmpa) (tmpa = tris(2-pyridylmethyl)amine) in aqueous solution. Through a combination of electrochemical studies, reaction kinetics, and density functional theory (DFT) computations, we show that the protonation steps take place in a stepwise manner and are decoupled from electron transfer. The rate-determining step is a general acid-catalyzed protonation of a copper-ligated nitrous acid (HNO2) species. In view of the growing urge to convert nitrogen-containing compounds, this work provides principal reaction parameters for efficient electrochemical nitrite reduction. This contributes to the investigation and development of nitrite reduction catalysts, which is crucial to restore the biogeochemical nitrogen cycle.

Sustainable removal of nitrite waste to value-added ammonia on Cu@Cu2O core-shell nanostructures by pulsed laser technique

The biochemical reduction of nitrite (NO₂^-) ions to ammonia (NH₃) requires six electrons and is catalyzed by the cytochrome c NO₂^- reductase enzyme. This biological reaction inspired scientists to explore the reduction of nitrogen oxyanions, such as nitrate (NO₃^-) and NO₂^- in wastewater, to produce the more valuable NH₃ product. It is widely known that copper (Cu)-based nanoparticles (NPs) are selective for the NO₃^- reduction reaction (NO₃^-RR), but the NO₂^-RR has not been well explored. Therefore, we attempted to address the electrocatalytic conversion of NO₂^- to NH₃ using Cu@Cu₂O core-shell NPs to simultaneously treat wastewater by removing NO₂^- and producing valuable NH₃. The Cu@Cu₂O core-shell NPs were constructed using the pulsed laser ablation of Cu sheet metal in water. The core-shell nanostructure of these particles was confirmed by various characterization techniques. Subsequently, the removal of NO₂^- and the ammonium (NH₄⁺)-N yield rate were estimated using the Griess and indophenol blue methods, respectively. Impressively, the Cu@Cu₂O core-shell NPs exhibited outstanding NO₂^-RR activity, demonstrating a maximum NO₂^- removal efficiency of approximately 94% and a high NH₄⁺-N yield rate of approximately 0.03 mmol h^-1.cm^-2 at -1.6 V vs. a silver/silver chloride reference electrode under optimal conditions. The proposed NO₂^-RR mechanism revealed that the (111) facet of Cu favors the selective conversion of NO₂^- to NH₃ via a six-electron transfer. This investigation may offer a new insight for the rational design and detailed mechanistic understanding of electrocatalyst architecture for the effective conversion of NO₂^- to NH₄⁺.

Buffer Assists Electrocatalytic Nitrite Reduction by a Cobalt Macrocycle Complex

This work reports a combined experimental and computational study of the activation of an otherwise catalytically inactive cobalt complex, [Co(TIM)Br₂]⁺, for aqueous nitrite reduction. The presence of phosphate buffer leads to efficient electrocatalysis, with rapid reduction to ammonium occurring close to the thermodynamic potential and with high Faradaic efficiency. At neutral pH, increasing buffer concentrations increase catalytic current while simultaneously decreasing overpotential, although high concentrations have an inhibitory effect. Controlled potential electrolysis and rotating ring-disk electrode experiments indicate that ammonium is directly produced from nitrite by [Co(TIM)Br₂]⁺, along with hydroxylamine. Mechanistic investigations implicate a vital role for the phosphate buffer, specifically as a proton shuttle, although high buffer concentrations inhibit catalysis. These results indicate a role for buffer in the design of electrocatalysts for nitrogen oxide conversion.

Electrochemical Upgrading of Formic Acid to Formamide via Coupling Nitrite Co-Reduction

Formic acid (HCOOH) can be exclusively prepared through CO₂ electroreduction at an industrial current density (0.5 A cm^-2). However, the global annual demand for formic acid is only ∼1 million tons, far less than the current CO₂ emission scale. The exploration of an economical and green approach to upgrading CO₂-derived formic acid is significant. Here, we report an electrochemical process to convert formic acid and nitrite into high-valued formamide over a copper catalyst under ambient conditions, which offers the selectivity from formic acid to formamide up to 90.0%. Isotope-labeled in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy and quasi in situ electron paramagnetic resonance results reveal the key C-N bond formation through coupling *CHO and *NH₂ intermediates. This work offers an electrochemical strategy to upgrade CO₂-derived formic acid into high-value formamide.

Batch-Scale Synthesis of Nanoparticle-Agminated Three-Dimensional Porous Cu@Cu2O Microspheres for Highly Selective Electrocatalysis of Nitrate to Ammonia

The electrochemical nitrate reduction reaction (NITRR), which converts nitrate to ammonia, is promising for artificial ammonia synthesis at mild conditions. However, the lack of favorable electrocatalysts has hampered its large-scale applications. Herein, we report the batch-scale synthesis of three-dimensional (3D) porous Cu@Cu₂O microspheres (Cu@Cu₂O MSs) composed of fine Cu@Cu₂O nanoparticles (NPs) using a convenient electric explosion method with outstanding activity and stability for the electrochemical reduction of nitrate to ammonia. Density functional theory (DFT) calculations revealed that the Cu₂O (111) facets could facilitate the formation of *NO₃H and *NO₂H intermediates and suppress the hydrogen evolution reaction (HER), resulting in high selectivity for the NITRR. Moreover, the 3D porous structure of Cu@Cu₂O MSs facilitates electrolyte penetration and increases the localized concentration of reactive species for the NITRR. As expected, the obtained Cu@Cu₂O MSs exhibited an ultrahigh NH₃ production rate of 327.6 mmol·h^-1·g^-1_cat. (which is superior to that of the Haber-Bosch process with a typical NH₃ yield <200 mmol h^-1g^-1_cat.), a maximum Faradaic efficiency of 80.57%, and remarkable stability for the NITRR under ambient conditions. Quantitative ¹⁵N isotope labeling experiments indicated that the synthesized ammonia originated from the electrochemical reduction of nitrate. Achieving the batch-scale and low-cost production of high-performance Cu@Cu₂O MSs electrocatalysts using the electric explosion method is promising for the large-scale realization of selective electrochemical reduction of nitrate toward artificial ammonia synthesis.

Electrocatalytic Reduction of Nitrogen Oxyanions with a Redox-Active Cobalt Macrocycle Complex

The cobalt complex, [Co(CR)Br₂]⁺, where CR is the redox-active macrocycle 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene, has been investigated for the electrocatalytic reduction of aqueous NO₂^- and NO₃^-. At neutral pH, the bromide ligands are hydrolyzed, providing [Co(CR)(OH₂)(OH)]²⁺ as the major species in aqueous solution. In the presence of nitrite, [Co(CR)(NO₂)₂]⁺ is formed as the major species in solution and is a precursor to the electrocatalytic reduction of NO₂^-, which is selectively converted to ammonium with high Faradaic efficiency. There is evidence for both homogeneous and heterogeneous electrocatalysis. Although similar NO₃^- binding is not observed, electrocatalytic reduction to ammonium also occurs, albeit with a lower Faradaic efficiency. In this case, NO₂^- is generated as an intermediate product of NO₃^- reduction.

Secondary Coordination Sphere Influences the Formation of Fe(III)-O or Fe(III)-OH in Nitrite Reduction: A Synthetic and Computational Study

The reduction of nitrite (NO₂^-) to generate nitric oxide (NO) is a significant area of research due to their roles in the global nitrogen cycle. Here, we describe various modifications of the tris(5-cyclohexyliminopyrrol-2-ylmethyl)amine H₃[N(pi^R)₃] ligand where the steric bulk and acidity of the secondary coordination sphere were explored in the non-heme iron system for nitrite reduction. The cyclohexyl and 2,4,6-trimethylphenyl variants of the ligand were used to probe the mechanism of nitrite reduction. While previously stoichiometric addition of nitrite to the iron(II)-species generated an iron(III)-oxo complex, changing the secondary coordination sphere to mesityl resulted in an iron(III)-hydroxo complex. Subsequent addition of an electron and two protons led to the release of water and regeneration of the starting iron(II) catalyst. This sequence mirrored the proposed mechanism of nitrite reduction in biological systems, where the distal histidine residue shuttles protons to the active site. Computational studies aimed at interrogating the dissimilar behavior of the cyclohexyl and mesityl ligand systems resulting in Fe(III)-oxo and Fe(III)-hydroxo complexes, respectively, shed light on the key role of H-bonds involving the secondary coordination sphere in the relative stability of these species.

A 3D FeOOH nanotube array: an efficient catalyst for ammonia electrosynthesis by nitrite reduction

Nitrite (NO₂^-) is a detrimental pollutant widely existing in groundwater sources, threatening public health. Electrocatalytic NO₂^- reduction settles the demand for removal of NO₂^- and is also promising for generating ammonia (NH₃) at room temperature. A nanotube array directly grown on a current collector not only has a large surface area, but also exhibits improved structural stability and accelerated electron transport. Herein, a self-standing FeOOH nanotube array on carbon cloth (FeOOH NTA/CC) is proposed as a highly active electrocatalyst for NO₂^--to-NH₃ conversion. As a 3D catalyst, the FeOOH NTA/CC is able to attain a surprising faradaic efficiency of 94.7% and a large NH₃ yield of 11937 μg h^-1 cm^-2 in 0.1 M PBS (pH = 7.0) with 0.1 M NO₂^-. Furthermore, this catalyst also displays excellent durability in cyclic and long-term electrolysis tests.

Nickel-Catalyzed NO Group Transfer Coupled with NOx Conversion

Nitrogen oxide (NO_x) conversion is an important process for balancing the global nitrogen cycle. Distinct from the biological NO_x transformation, we have devised a synthetic approach to this issue by utilizing a bifunctional metal catalyst for producing value-added products from NO_x. Here, we present a novel catalysis based on a Ni pincer system, effectively converting Ni-NO_x to Ni-NO via deoxygenation with CO(g). This is followed by transfer of the in situ generated nitroso group to organic substrates, which favorably occurs at the flattened Ni(I)-NO site via its nucleophilic reaction. Successful catalytic production of oximes from benzyl halides using NaNO₂ is presented with a turnover number of >200 under mild conditions. In a key step of the catalysis, a nickel(I)-^•NO species effectively activates alkyl halides, which is carefully evaluated by both experimental and theoretical methods. Our nickel catalyst effectively fulfills a dual purpose, namely, deoxygenating NO_x anions and catalyzing C-N coupling.

Recent advances in electrocatalytic nitrite reduction

Electrocatalytic nitrite reduction is of great significance for wastewater treatment and value-added chemicals synthesis. This review highlights the latest progress in electrochemical nitrite reduction to produce two types of products, including gaseous products (NO, N₂O, N₂) and liquid products (NH₂OH and NH₄⁺). The heterogeneous and homogeneous catalysts used in the corresponding reduction processes are introduced, with emphasis on the product selectivity regulation and reaction mechanism understanding. Finally, the challenges and opportunities in this field are analyzed as well. This review can provide guidelines for designing electrochemical systems with high efficiency and specificity for nitrite reduction.

Boosting electrochemical nitrite-ammonia conversion properties by a Cu foam@Cu2O catalyst

Electrocatalytic reduction of nitrite (NO₂^-) to ammonia (NH₃) can simultaneously achieve wastewater treatment and ammonia production, but it needs efficient catalysts. Herein, Cu₂O particles self-supported on Cu foam with enriched oxygen vacancies are developed to enable selective NO₂^- reduction to NH₃, exhibiting a maximum NH₃ yield rate of 7510.73 μg h^-1 cm^-2 and high faradaic efficiency of 94.21% at -0.6 V in 0.1 M PBS containing 0.1 M NaNO₂. Density functional theory calculations reveal the vital role of oxygen vacancies during the nitrite reduction process, as well as the reaction mechanisms and the potential limiting step involved. This work provides a new avenue to the rational design of Cu-based catalysts for NH₃ electrosynthesis.

Nature's nitrite-to-ammonia expressway, with no stop at dinitrogen

Since the characterization of cytochrome c₅₅₂ as a multiheme nitrite reductase, research on this enzyme has gained major interest. Today, it is known as pentaheme cytochrome c nitrite reductase (NrfA). Part of the NH₄⁺ produced from NO₂^- is released as NH₃ leading to nitrogen loss, similar to denitrification which generates NO, N₂O, and N₂. NH₄⁺ can also be used for assimilatory purposes, thus NrfA contributes to nitrogen retention. It catalyses the six-electron reduction of NO₂^- to NH₄⁺, hosting four His/His ligated c-type hemes for electron transfer and one structurally differentiated active site heme. Catalysis occurs at the distal side of a Fe(III) heme c proximally coordinated by lysine of a unique CXXCK motif (Sulfurospirillum deleyianum, Wolinella succinogenes) or, presumably, by the canonical histidine in Campylobacter jejeuni. Replacement of Lys by His in NrfA of W. succinogenes led to a significant loss of enzyme activity. NrfA forms homodimers as shown by high resolution X-ray crystallography, and there exist at least two distinct electron transfer systems to the enzyme. In γ-proteobacteria (Escherichia coli) NrfA is linked to the menaquinol pool in the cytoplasmic membrane through a pentaheme electron carrier (NrfB), in δ- and ε-proteobacteria (S. deleyianum, W. succinogenes), the NrfA dimer interacts with a tetraheme cytochrome c (NrfH). Both form a membrane-associated respiratory complex on the extracellular side of the cytoplasmic membrane to optimize electron transfer efficiency. This minireview traces important steps in understanding the nature of pentaheme cytochrome c nitrite reductases, and discusses their structural and functional features.

Graphite Conjugation of a Macrocyclic Cobalt Complex Enhances Nitrite Electroreduction to Ammonia

This work reports on the generation of a graphite-conjugated diimine macrocyclic Co catalyst (GCC-CoDIM) that is assembled at o-quinone edge defects on graphitic carbon electrodes. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy confirm the existence of a new Co surface species with a coordination environment that is the same as that of the molecular analogue, [Co(DIM)Br₂]⁺. GCC-CoDIM selectively reduces nitrite to ammonium with quantitative Faradaic efficiency and at a rate that approaches enzymatic catalysis. Preliminary mechanistic investigations suggest that the increased rate is accompanied by a change in mechanism from the molecular analogue. These results provide a template for creating macrocycle-based electrocatalysts based on first-row transition metals conjugated to an extreme redox-active ligand.