Interfacially Engineered Nanoporous Cu/MnOx Hybrids for Highly Efficient Electrochemical Ammonia Synthesis via Nitrate Reduction

Electrochemical reduction of nitrate to ammonia (NH₃) not only offers a promising strategy for green NH₃ synthesis, but also addresses the environmental issues and balances the perturbed nitrogen cycle. However, current electrocatalytic nitrate reduction processes are still inefficient due to the lack of effective electrocatalysts. Here 3D nanoporous Cu/MnO_x hybrids are reported as efficient and durable electrocatalysts for nitrate reduction reaction, achieving the NH₃ yield rates of 5.53 and 29.3 mg h⁻¹ mg_cat.⁻¹ with 98.2% and 86.2% Faradic efficiency in 0.1 m Na₂SO₄ solution with 10 and 100 mm KNO₃, respectively, which are higher than those obtained for most of the reported catalysts under similar conditions. Both the experimental results and density functional theory calculations reveal that the interface effect between Cu/MnO_x interface could reduce the free energy of rate determining step and suppress the hydrogen evolution reaction, leading to the enhanced catalytic activity and selectivity. This work provides an approach to design advanced materials for NH₃ production via electrochemical nitrate reduction.     

Constructing Co@TiO2 Nanoarray Heterostructure with Schottky Contact for Selective Electrocatalytic Nitrate Reduction to Ammonia

Electrochemical nitrate (NO₃⁻) reduction reaction (NO₃⁻RR) is a potential sustainable route for large-scale ambient ammonia (NH₃) synthesis and regulating the nitrogen cycle. However, as this reaction involves multi-electron transfer steps, it urgently needs efficient electrocatalysts on promoting NH₃ selectivity. Herein, a rational design of Co nanoparticles anchored on TiO₂ nanobelt array on titanium plate (Co@TiO₂/TP) is presented as a high-efficiency electrocatalyst for NO₃⁻RR. Density theory calculations demonstrate that the constructed Schottky heterostructures coupling metallic Co with semiconductor TiO₂ develop a built-in electric field, which can accelerate the rate determining step and facilitate NO₃⁻ adsorption, ensuring the selective conversion to NH₃. Expectantly, the Co@TiO₂/TP electrocatalyst attains an excellent Faradaic efficiency of 96.7% and a high NH₃ yield of 800.0 µmol h⁻¹ cm⁻² under neutral solution. More importantly, Co@TiO₂/TP heterostructure catalyst also presents a remarkable stability in 50-h electrolysis test.     

Accelerating Industrial-Level NO3− Electroreduction to Ammonia on Cu Grain Boundary Sites via Heteroatom Doping Strategy

Although the electrocatalytic nitrate reduction reaction (NO₃⁻RR) is an attractive NH₃ synthesis route, it suffers from low yield due to the lack of efficient catalysts. Here, this work reports a novel grain boundary (GB)-rich Sn-Cu catalyst, derived from in situ electroreduction of Sn-doped CuO nanoflower, for effectively electrochemical converting NO₃⁻ to NH₃. The optimized Sn_1%-Cu electrode achieves a high NH₃ yield rate of 1.98 mmol h⁻¹ cm⁻² with an industrial-level current density of −425 mA cm⁻² at −0.55 V versus a reversible hydrogen electrode (RHE) and a maximum Faradaic efficiency of 98.2% at −0.51 V versus RHE, outperforming the pure Cu electrode. In situ Raman and attenuated total reflection Fourier transform infrared spectroscopies reveal the reaction pathway of NO₃⁻RR to NH₃ by monitoring the adsorption property of reaction intermediates. Density functional theory calculations clarify that the high-density GB active sites and the competitive hydrogen evolution reaction (HER) suppression induced by Sn doping synergistically promote highly active and selective NH₃ synthesis from NO₃⁻RR. This work paves an avenue for efficient NH₃ synthesis over Cu catalyst by in situ reconstruction of GB sites with heteroatom doping.     

Pd-Doped Co3O4 Nanoarray for Efficient Eight-Electron Nitrate Electrocatalytic Reduction to Ammonia Synthesis

Ammonia (NH₃) is an indispensable feedstock for fertilizer production and one of the most ideal green hydrogen rich fuel. Electrochemical nitrate (NO₃⁻) reduction reaction (NO₃⁻RR) is being explored as a promising strategy for green to synthesize industrial-scale NH₃, which has nonetheless involved complex multi-reaction process. This work presents a Pd-doped Co₃O₄ nanoarray on titanium mesh (Pd-Co₃O₄/TM) electrode for highly efficient and selective electrocatalytic NO₃⁻RR to NH₃ at low onset potential. The well-designed Pd-Co₃O₄/TM delivers a large NH₃ yield of 745.6 µmol h⁻¹ cm⁻² and an extremely high Faradaic efficiency (FE) of 98.7% at −0.3 V with strong stability. These calculations further indicate that the doping Co₃O₄ with Pd improves the adsorption characteristic of Pd-Co₃O₄ and optimizes the free energies for intermediates, thereby facilitating the kinetics of the reaction. Furthermore, assembling this catalyst in a Zn-NO₃⁻ battery realizes a power density of 3.9 mW cm⁻² and an excellent FE of 98.8% for NH₃.     

Self-Supported Pd Nanorod Arrays for High-Efficient Nitrate Electroreduction to Ammonia

Electrochemical nitrate (NO₃⁻) reduction to ammonia (NH₃) offers a promising pathway to recover NO₃⁻ pollutants from industrial wastewater that can balance the nitrogen cycle and sustainable green NH₃ production. However, the efficiency of electrocatalytic NO₃⁻ reduction to NH₃ synthesis remains low for most of electrocatalysts due to complex reaction processes and severe hydrogen precipitation reaction. Herein, high performance of nitrate reduction reaction (NO₃⁻RR) is demonstrated on self-supported Pd nanorod arrays in porous nickel framework foam (Pd/NF). It provides a lot of active sites for H* adsorption and NO₃⁻ activation leading to a remarkable NH₃ yield rate of 1.52 mmol cm⁻² h⁻¹ and a Faradaic efficiency of 78% at −1.4 V versus RHE. Notably, it maintains a high NH₃ yield rate over 50 cycles in 25 h showing good stability. Remarkably, large-area Pd/NF electrode (25 cm²) shows a NH₃ yield of 174.25 mg h⁻¹, be promising candidate for large-area device for industrial application. In situ FTIR spectroscopy and density functional theory calculations analysis confirm that the enrichment effect of Pd nanorods encourages the adsorption of H species for ammonia synthesis following a hydrogenation mechanism. This work brings a useful strategy for designing NO₃⁻RR catalysts of nanorod arrays with customizable compositions.     

Constructing Ru@C3N4/Cu Tandem Electrocatalyst with Dual-Active Sites for Enhanced Nitrate Electroreduction to Ammonia

Electroreduction of nitrate to ammonia reaction (NO₃⁻RR) is considered as a promising carbon-free energy technique, which can eliminate nitrate from waste-water also produce value-added ammonia. However, it remains a challenge for achieving satisfied ammonia selectivity and Faraday efficiency (FE) due to the complex multiple-electron reduction process. Herein, a novel Tandem electrocatalyst that Ru dispersed on the porous graphitized C₃N₄ (g-C₃N₄) encapsulated with self-supported Cu nanowires (denoted as Ru@C₃N₄/Cu) for NO₃⁻RR is presented. As expected, a high ammonia yield of 0.249 mmol h⁻¹ cm⁻² at −0.9 V and high FE_NH3 of 91.3% at −0.8 V versus RHE can be obtained, while achieving excellent nitrate conversion (96.1%) and ammonia selectivity (91.4%) in neutral solution. In addition, density functional theory (DFT) calculations further demonstrate that the superior NO₃⁻RR performance is mainly resulted from the synergistic effect between the Ru and Cu dual-active sites, which can significantly enhance the adsorption of NO₃⁻ and facilitate hydrogenation, as well as suppress the hydrogen evolution reaction, thus lead to highly improved NO₃⁻RR performances. This novel design strategy would pave a feasible avenue for the development of advanced NO₃⁻RR electrocatalysts.     

Dinitrogen Reduction Coupled with Methanol Oxidation for Low Overpotential Electrochemical NH3 Synthesis Over Cobalt Pyrophosphate as Bifunctional Catalyst

Electrochemical dinitrogen (N₂) reduction to ammonia (NH₃) coupled with methanol electro-oxidation is presented in the current work. Here, methanol oxidation reaction (MOR) is proposed as an alternative anode reaction to oxygen evolution reaction (OER) to accomplish electrons-induced reduction of N₂ to NH₃ at cathode and oxidation of methanol at anode in alkaline media thereby reducing the overall cell voltage for ammonia production. Cobalt pyrophosphate micro-flowers assembled by nanosheets are synthesized via a surfactant-assisted sonochemical approach. By virtue of structural and morphological advantages, the maximum Faradaic efficiency of 43.37% and NH₃ yield rate of 159.6 µg h⁻¹ mg_ca⁻¹ is achieved at a potential of −0.2 V versus RHE. The proposed catalyst is shown to also exhibit a very high activity (100 mA mg⁻¹ at 1.48 V), durability (2 h) and production of value-added formic acid at anode (2.78 µmol h⁻¹ mg_cat⁻¹ and F.E. of 59.2%). The overall NH₃ synthesis is achieved at a reduced cell voltage of 1.6 V (200 mV less than NRR-OER coupled NH₃ synthesis) when OER at anode is replaced with MOR and a high NH₃ yield rate of 95.2 µg h⁻¹ mg_cat⁻¹ and HCOOH formation rate of 2.53 µmol h⁻¹ mg⁻¹ are witnessed under full-cell conditions.     

Piezoelectricity regulated ohmic contact in M/BaTiO3 (M = Ru,Pd, Pt) for charge collision and hydrogen free radical production in ammonia electrosynthesis

Electrochemical nitrate reduction reaction (NO₃RR) is a promising alternative technique for NH₃ generation toward the energy-consuming Haber-Bosch process. Nevertheless, it remains hindered by the competitive hydrogen evolution reaction (HER). Herein, the piezoelectric effect of electron-rich BaTiO₃ with oxygen vacancies is introduced to promote NO₃RR performance. Combining with metal particles (Ru, Pd and Pt), the catalyst achieves a maximal Faradaic efficiency of 95.3% and NH₃ yield rate of 6.87 mg h⁻¹ mg_cat.⁻¹. Upon piezoelectricity, the interface between metal nanoparticles and BaTiO₃ is effectively modulated from Schottky contact to ohmic contact, which leads to unobstructed electron transfer. Abundant hydrogen radicals (·H) can be then produced from the collision between plentiful electrons and polar water molecules adsorbed on the polar surface. Such ·H can significantly facilitate the hydrogenation of reaction intermediates in NO₃RR. Meanwhile, this process suppresses the Volmer-Heyrovsky step, therefore inhibiting the HER within a wide range of external potential. This work suggests a new strategy for promoting the performance of multi-electron-involved catalytic reactions.     

Ammonia Electrosynthesis with a Stable Metal-Free 2D Silicon Phosphide Catalyst

Metal-free 2D phosphorus-based materials are emerging catalysts for ammonia (NH₃) production through a sustainable electrochemical nitrogen reduction reaction route under ambient conditions. However, their efficiency and stability remain challenging due to the surface oxidization. Herein, a stable phosphorus-based electrocatalyst, silicon phosphide (SiP), is explored. Density functional theory calculations certify that the N₂ activation can be realized on the zigzag Si sites with a dimeric end-on coordinated mode. Such sites also allow the subsequent protonation process via the alternating associative mechanism. As the proof-of-concept demonstration, both the crystalline and amorphous SiP nanosheets (denoted as C-SiP NSs and A-SiP NSs, respectively) are obtained through ultrasonic exfoliation processes, but only the crystalline one enables effective and stable electrocatalytic nitrogen reduction reaction, in terms of an NH₃ yield rate of 16.12 µg h⁻¹ mg_cat.⁻¹ and a Faradaic efficiency of 22.48% at −0.3 V versus reversible hydrogen electrode. The resistance to oxidization plays the decisive role in guaranteeing the NH₃ electrosynthesis activity for C-SiP NSs. This surface stability endows C-SiP NSs with the capability to serve as appealing electrocatalysts for nitrogen reduction reactions and other promising applications.     

Defective TiO2−x for High-Performance Electrocatalytic NO Reduction toward Ambient NH3 Production

Synthesis of green ammonia (NH₃) via electrolysis of nitric oxide (NO) is extraordinarily sustainable, but multielectron/proton-involved hydrogenation steps as well as low concentrations of NO can lead to poor activities and selectivities of electrocatalysts. Herein, it is reported that oxygen-defective TiO₂ nanoarray supported on Ti plate (TiO_2−x/TP) behaves as an efficient catalyst for NO reduction to NH₃. In 0.2 m phosphate-buffered electrolyte, such TiO_2−x/TP shows competitive electrocatalytic NH₃ synthesis activity with a maximum NH₃ yield of 1233.2 µg h⁻¹ cm⁻² and Faradaic efficiency of 92.5%. Density functional theory calculations further thermodynamically faster NO deoxygenation and protonation processes on TiO_2−x (101) compared to perfect TiO₂ (101). And the low energy barrier of 0.7 eV on TiO_2−x (101) for the potential-determining step further highlights the greatly improved intrinsic activity. In addition, a Zn-NO battery is fabricated with TiO_2−x/TP and Zn plate to obtain an NH₃ yield of 241.7 µg h⁻¹ cm⁻² while providing a peak power density of 0.84 mW cm⁻².     

Promoting CO2 Dynamic Activation via Micro-Engineering Technology for Enhancing Electrochemical CO2 Reduction

Optimizing the coordination structure and microscopic reaction environment of isolated metal sites is promising for boosting catalytic activity for electrocatalytic CO₂ reduction reaction (CO₂RR) but is still challenging to achieve. Herein, a newly electrostatic induced self-assembly strategy for encapsulating isolated Ni-C₃N₁ moiety into hollow nano-reactor as I-Ni SA/NHCRs is developed, which achieves FE_CO of 94.91% at −0.80 V, the CO partial current density of ≈−15.35 mA cm⁻², superior to that with outer Ni-C₂N₂ moiety (94.47%, ≈−12.06 mA cm⁻²), or without hollow structure (92.30%, ≈−5.39 mA cm⁻²), and high FE_CO of ≈98.41% at 100 mA cm⁻² in flow cell. COMSOL multiphysics finite-element method and density functional theory (DFT) calculation illustrate that the excellent activity for I-Ni SA/NHCRs should be attributed to the structure-enhanced kinetics process caused by its hollow nano-reactor structure and unique Ni-C₃N₁ moiety, which can enrich electron on Ni sites and positively shift d-band center to the Fermi level to accelerate the adsorption and activation of CO₂ molecule and *COOH formation. Meanwhile, this strategy also successfully steers the design of encapsulating isolated iron and cobalt sites into nano-reactor, while I-Ni SA/NHCRs-based zinc-CO₂ battery assembled with a peak power density of 2.54 mW cm−⁻² is achieved.     

Au Nanowires Decorated Ultrathin Co3O4 Nanosheets toward Light-Enhanced Nitrate Electroreduction

Nitrate is a reasonable alternative instead of nitrogen for ammonia production due to the low bond energy, large water-solubility, and high chemical polarity for good absorption. Nitrate electroreduction reaction (NO₃RR) is an effective and green strategy for both nitrate treatment and ammonia production. As an electrochemical reaction, the NO₃RR requires an efficient electrocatalyst for achieving high activity and selectivity. Inspired by the enhancement effect of heterostructure on electrocatalysis, Au nanowires decorated ultrathin Co₃O₄ nanosheets (Co₃O₄-NS/Au-NWs) nanohybrids are proposed for improving the efficiency of nitrate-to-ammonia electroreduction. Theoretical calculation reveals that Au heteroatoms can effectively adjust the electron structure of Co active centers and reduce the energy barrier of the determining step (*NO → *NOH) during NO₃RR. As the result, the Co₃O₄-NS/Au-NWs nanohybrids achieve an outstanding catalytic performance with high yield rate (2.661 mg h⁻¹ mg_cat⁻¹) toward nitrate-to-ammonia. Importantly, the Co₃O₄-NS/Au-NWs nanohybrids show an obviously plasmon-promoted activity for NO₃RR due to the localized surface plasmon resonance (LSPR) property of Au-NWs, which can achieve an enhanced NH₃ yield rate of 4.045 mg h⁻¹ mg_cat⁻¹. This study reveals the structure–activity relationship of heterostructure and LSPR-promotion effect toward NO₃RR, which provide an efficient nitrate-to-ammonia reduction with high efficiency.     

Size-Defined Ru Nanoclusters Supported by TiO2 Nanotubes Enable Low-Concentration Nitrate Electroreduction to Ammonia with Suppressed Hydrogen Evolution

Anthropogenic nitrate pollution has an adverse impact on the environment and human health. As part of a sustainable nitrate management strategy, electrochemical denitrification is studied as an innovative strategy for nutrients recycling and recovering. It is, however, challenging to selectively electro-reduce nitrate with low-concentration for ammonia. Herein, the photo-deposition of size-defined Ru nanoclusters (NCs, average size: ≈1.66 nm) on TiO₂ nanotubes (NTs) is demonstrated, which show improved performance for nitrate-to-ammonia electroreduction with a maximum yield rate of ≈600 µg h⁻¹ cm⁻² and a faradic efficiency (FE) of > 90.0% across a broad range of potentials in comparison with electrodeposited Ru nanoparticles (NPs, average size: ≈23.78 nm) on TiO₂ NTs. Experimental and theoretical evidence further suggests the small-size Ru NCs with the intrinsically enhanced selectivity and activity because of the strong metal/substrate interaction and unsaturated coordination state. The findings highlight the size effect on Ru-based catalyst supported on metal oxides, a versatile catalytic model, which allows the regulation of hydrogen adsorption to favor ammonia production over the competing hydrogen evolution reaction.     

Boosting Chemoselective Hydrogenation of Nitroaromatic via Synergy of Hydrogen Spillover and Preferential Adsorption on Magnetically Recoverable Pt@Fe2O3

It is highly desired but challenging to design high performance catalyst for selective hydrogenation of nitro compounds into amino compounds. Herein, a boosting chemoselective hydrogenation strategy on Pt@Fe₂O₃ is proposed with gradient oxygen vacancy by synergy of hydrogen spillover and preferential adsorption. Experimental and theoretical investigations reveal that the nitro is preferentially adsorbed onto oxygen vacancy of Pt@Fe₂O₃, meanwhile, the H₂ dissociated on Pt nanoparticles and then spillover to approach the nitro for selective hydrogenation (>99% conversion of 4-nitrostyrene, > 99% selectivity of 4-aminostyrene, TOF of 2351 h⁻¹). Moreover, the iron oxide support endows the catalyst magnetic retrievability. This high activity, selectivity, and easy recovery strategy provide a promising avenue for selective hydrogenation catalysis of various nitroaromatic.     

Loaded Cu-Er metal iso-atoms on graphdiyne for artificial photosynthesis

Herein we report a bimetal atomic catalyst that features atomically dispersed Cu and Er atoms anchored on all crystalline graphdiyne (CuEr-GDY) for efficient artificial photosynthesis converting CO₂ into sustainable fuel at the gas–solid interfaces with water as reducing medium instead of organic reagent. The CuEr-GDY can promote the efficient separation of photogenerated electron-hole pairs to drive water oxidation and CO₂ activation/reduction, together with Cu/Er promoted CO₂/H₂O adsorption and CO desorption. This result indicates that bimetallic atoms on the high-crystalline GDY surface have high activity. This heteroatomic catalyst of CuEr-GDY demonstrates high catalytic activity with the reaction selectivity up to 97.6%, and the competitive hydrogen evolution reaction is almost completely suppressed. The CO₂ conversion achieves the CO yield of 181.04 μmol g⁻¹ h⁻¹ under ambient conditions.     

Hierarchical Nanospheres with Polycrystalline Ir&Cu and Amorphous Cu2O toward Energy-Efficient Nitrate Electrolysis to Ammonia

Electrochemical reduction reaction of nitrate (NITRR) provides a sustainable route toward the green synthesis of ammonia. Nevertheless, it remains challenging to achieve high-performance electrocatalysts for NITRR especially at low overpotentials. In this work, hierarchical nanospheres consisting of polycrystalline Iridium&copper (Ir&Cu) and amorphous Cu₂O (Cu_xIr_yO_z NS) have been fabricated. The optimal species Cu_0.86Ir_0.14O_z delivers excellent catalytic performance with a desirable NH₃ yield rate (YR) up to 0.423 mmol h⁻¹ cm⁻² (or 4.8 mg h⁻¹ mg_cat⁻¹) and a high NH₃ Faradaic efficiency (FE) over 90% at a low overpotential of 0.69 V (or 0 V_RHE), where hydrogen evolution reaction (HER) is almost negligible. The electrolyzer toward NITRR and hydrazine oxidation (HzOR) is constructed for the first time with an electrode pair of Cu_0.86Ir_0.14O_z//Cu_0.86Ir_0.14O_z, yielding a high energy efficiency (EE) up to 87%. Density functional theory (DFT) calculations demonstrate that the dispersed Ir atom provides active site that not only promotes the NO₃⁻ adsorption but also modulates the H adsorption/desorption to facilitate the proton supply for the hydrogenation of *N, hence boosting the NITRR. This work thus points to the importance of both morphological/structural and compositional engineering for achieving the highly efficient catalysts toward NITRR.     

Highly Selective N2 Electroreduction to NH3 Using a Boron-Vacancy-Rich Diatomic Nb?B Catalyst

The ambient electrochemical N₂ reduction reaction (NRR) is a future approach for the artificial NH₃ synthesis to overcome the problems of high-energy consumption and environmental pollution by Haber–Bosch technology. However, the challenge of N₂ activation on a catalyst surface and the competitive hydrogen evolution reaction make the current NRR unsatisfied. Herein, this work demonstrates that NbB₂ nanoflakes (NFs) exhibit excellent selectivity and durability in NRR, which produces NH₃ with a production rate of 30.5 µg h⁻¹ mg_cat⁻¹ and a super-high Faraday efficiency (FE) of 40.2%. The high-selective NH₃ production is attributed to the large amount of active B vacancies on the surface of NbB₂ NFs. Density functional theory calculations suggest that the multiple atomic adsorption of N₂ on both unsaturated Nb and B atoms results in a significantly stretched N₂ molecule. The weakened NN triple bonds are easier to be broken for a biased NH₃ production. The diatomic catalysis is a future approach for NRR as it shows a special N₂ adsorption mode that can be well engineered.     

Light Field-Enhanced Single-Site Cu Electrocatalyst for Nitrogen Fixation

Direct electrocatalytic reduction of N₂ to NH₃ under mild conditions is attracting considerable interests but still remains enormous challenges in terms of respect of intrinsic catalytic activity and limited electrocatalytic efficiency. Herein, a photo-enhanced strategy is developed to improve the NRR activity on Cu single atoms catalysts. The atomically dispersed Cu single atoms supported TiO₂ nanosheets (Cu SAs/TiO₂) achieve a Faradaic Efficiency (12.88%) and NH₃ yield rate (6.26 µg h⁻¹ mg_cat⁻¹) at −0.05 V versus RHE under the light irradiation field, in which NH₃ yield rate is fivefold higher than that under pure electrocatalytic nitrogen reduction reaction (NRR) process and is remarkably superior in comparison to most of the similar type electrocatalysts. The existence of external light field improves electron transfer ability between Cu O and Ti O, and thus optimizes the accumulation of surface charges on Cu sites, endowing more electrons involved in nitrogen fixation. This work reveals an atomic-scale mechanistic understanding of field effect-enhanced electrochemical performance of catalysts and it provides predictive guidelines for the rational design of photo-enhanced electrochemical N₂ reduction catalysts.     

Rational Construction of Heterostructured Cu3P@TiO2 Nanoarray for High-Efficiency Electrochemical Nitrite Reduction to Ammonia

Electroreduction of nitrite (NO₂⁻) to valuable ammonia (NH₃) offers a sustainable and green approach for NH₃ synthesis. Here, a Cu₃P@TiO₂ heterostructure is rationally constructed as an active catalyst for selective NO₂⁻-to-NH₃ electroreduction, with rich nanosized Cu₃P anchored on a TiO₂ nanoribbon array on Ti plate (Cu₃P@TiO₂/TP). When performed in the 0.1 m NaOH with 0.1 m NaNO₂, the Cu₃P@TiO₂/TP electrode obtains a large NH₃ yield of 1583.4 µmol h⁻¹ cm⁻² and a high Faradaic efficiency of 97.1%. More importantly, Cu₃P@TiO₂/TP also delivers remarkable long-term stability for 50 h electrolysis. Theoretical calculations indicate that intermediate adsorption/conversion processes on Cu₃P@TiO₂ interfaces are synergistically optimized, substantially facilitating the conversion of NO₂⁻-to-NH₃.