Mild and Selective Hydrogenation of Nitrate to Ammonia in the Absence of Noble Metals

Enhanced electrochemical nitrate reduction on copper nitride with moderate intermediates adsorption

Nitrate in surface and underground water caused systematic risk to the ecological environment. The electrochemically reduction of nitrate into ammonia (NO3RR), offering a sustainable route for nitrate containing wastewater treatment and ammonia fertilizer conversion. Exploration of catalyst with improved catalytic activity with lower energy barriers is still challenging. Here, we report a copper nitride (Cu3N) catalyst with moderate *NOx and *H2O intermediates adsorptions showed enhanced NO3RR performance. Density functional theory calculations reveals that the unique electronic structure of Cu3N provides efficient active sites for NO3RR, thus enabled balanced adsorption of *NO3 and *H2O (ΔE descriptor), sufficient active hydrogen, and moderate intermediate (*NO3 → HNO3, *NH2→*NH3) adsorption energy. Notably, the in-situ analysis technology revealed potential-driven reconstruction and rehabilitation of Cu3N, forming possible nitrogen vacancy, thus implied for better mechanism understanding. The NO3RR activity of Cu3N surpasses that of most recent catalysts and demonstrates superior stability and implies the application for NH4+ fertilizer recovery, which maintaining an NH3 Faradaic efficiency of 93.1 % and high yield rate of 2.9 mg cm2h-1 at -0.6 V versus RHE. These findings broaden the application scenarios of Cu3N catalyst for ammonia synthesis and provide strategy on improving NO3RR performance.

Nitrate and nitroarene hydrogenations catalyzed by alkaline-earth nickel phosphide clathrates

The alkaline-earth-containing nickel phosphide clathrates AeNi2P4 (Ae = Ba, Sr) are investigated as catalysts for the reduction of nitrate or nitroarenes in aqueous or ethanolic solution, respectively. While AeNi2P4 clathrates are inactive in their bulk polycrystalline form, they become active in nitrate hydrogenation after size reduction by either grinding or ball milling. However, while the clathrate structure remains intact after manual grinding, ball milling is of limited use as it results in significant clathrate degradation. Ground AeNi2P4 catalysts are also active in nitroarene hydrogenation. Condensation products such as azoxy- and azo-benzenes form early (4 h) but anilines accumulate after long reaction times (24 h). Unexpectedly, BaNi2P4 partially devinylates nitrostyrene to nitrobenzene. Overall, BaNi2P4 is more active than SrNi2P4 in both nitrate and nitroarene hydrogenation. These results showcase the potential utility of clathrates in a growing number of catalytic transformations.

Electrical-Driven Directed-Evolution of Copper Nanowires Catalysts for Efficient Nitrate Reduction to Ammonia

The electrocatalytic conversion of nitrate (NO3 - ) to NH3 (NO3 RR) at ambient conditions offers a promising alternative to the Haber-Bosch process. The pivotal factors in optimizing the proficient conversion of NO3 - into NH3 include enhancing the adsorption capabilities of the intermediates on the catalyst surface and expediting the hydrogenation steps. Herein, the Cu/Cu2 O/Pi NWs catalyst is designed based on the directed-evolution strategy to achieve an efficient reduction of NO3 ‾. Benefiting from the synergistic effect of the OV -enriched Cu2 O phase developed during the directed-evolution process and the pristine Cu phase, the catalyst exhibits improved adsorption performance for diverse NO3 RR intermediates. Additionally, the phosphate group anchored on the catalyst's surface during the directed-evolution process facilitates water electrolysis, thereby generating Hads on the catalyst surface and promoting the hydrogenation step of NO3 RR. As a result, the Cu/Cu2 O/Pi NWs catalyst shows an excellent FE for NH3 (96.6%) and super-high NH3 yield rate of 1.2 mol h-1  gcat. -1 in 1 m KOH and 0.1 m KNO3 solution at -0.5 V versus RHE. Moreover, the catalyst's stability is enhanced by the stabilizing influence of the phosphate group on the Cu2 O phase. This work highlights the promise of a directed-evolution approach in designing catalysts for NO3 RR.

Understanding the Activity Trends in Electrocatalytic Nitrate Reduction to Ammonia on Cu Catalysts

Cu-based catalysts possess great potential in the electrocatalytic nitrate (NO3-) reduction reaction for ammonia (NH3) synthesis. However, the low atomic economy limits their further application. Here we report a Cu single-atom (SA) incorporated in nitrogen-doped carbon (Cu SA/NC) with high atomic economy, which exhibits superior NH3 Faradaic efficiency (FE) of 100% along with an impressive NH3 yield rate of 7480 μg h-1 mgcat.-1. As counterparts, Cus+n/NC, with mixed SA and nanoparticles (NPs), shows decreasing NH3 FE with decreasing SA content, but the production of N2 and N2O increases gradually, which reaches the maximum on pure Cu NPs. In situ characterizations and theoretical calculations reveal that a higher NH3 FE of Cu SA/NC is ascribed to a lower free energy of the rate-limiting step (HNO* → N*) and effective inhibition for the N-N coupled process. This work provides the intuitive activity trends of Cu-based catalysts, opening an avenue for subsequent catalysts design.

Unraveling the Activity Trends and Design Principles of Single-Atom Catalysts for Nitrate Electrocatalytic Reduction

Electrocatalytic nitrate (NO3-) reduction represents one of the most promising approaches to mitigate NO3- pollution and yield NH3, but it is still challenged by the atomic economy and selectivity issues of substantial active sites. Here, we describe a comprehensive investigation on a series of single-atom catalysts (SACs) using nitrogen-doped carbon as substrate (metal/NC). The essence of activity is related to the extent of the electron transfer capacity (SAs → NO3-). Among these examined SACs, the Cu/NC presents good performance toward NH3 synthesis, i.e., a maximum NH3 Faradaic efficiency of 100% with a high NH3 yield rate of up to 32,300 μg h-1 mgcat.-1. X-ray absorption fine structure spectra and density functional theory calculations provide evidence that the electronic structure of Cu-N4 coordination prohibits the formation of N2, N2O, and H2 and facilitates the orbital hybridization between the 2p orbitals of NO3- and 3d orbitals of Cu single-atom sites. Our study is believed to provide fundamental guidance for the future design of highly efficient electrocatalysts in NO3- reduction to NH3.

Fe/Cu diatomic catalysts for electrochemical nitrate reduction to ammonia

Electrochemical conversion of nitrate to ammonia offers an efficient approach to reducing nitrate pollutants and a potential technology for low-temperature and low-pressure ammonia synthesis. However, the process is limited by multiple competing reactions and NO₃^- adsorption on cathode surfaces. Here, we report a Fe/Cu diatomic catalyst on holey nitrogen-doped graphene which exhibits high catalytic activities and selectivity for ammonia production. The catalyst enables a maximum ammonia Faradaic efficiency of 92.51% (-0.3 V(RHE)) and a high NH₃ yield rate of 1.08 mmol h^-1 mg^-1 (at - 0.5 V(RHE)). Computational and theoretical analysis reveals that a relatively strong interaction between NO₃^- and Fe/Cu promotes the adsorption and discharge of NO₃^- anions. Nitrogen-oxygen bonds are also shown to be weakened due to the existence of hetero-atomic dual sites which lowers the overall reaction barriers. The dual-site and hetero-atom strategy in this work provides a flexible design for further catalyst development and expands the electrocatalytic techniques for nitrate reduction and ammonia synthesis.

Revealing the activity origin of ultrathin nickel metal-organic framework nanosheet catalysts for selective electrochemical nitrate reduction to ammonia: Experimental and density functional theory investigations

The electrochemical nitrate reduction reaction (NitRR) affords a sustainable way for nitrate mitigation and ammonia synthesis, but there are still some problems such as poor nitrate conversion, low ammonia selectivity, and slow reaction kinetics. A clear structure-performance relationship is essential for designing efficient catalysts and understanding the reaction mechanisms. Herein, ultrathin nickel metal-organic framework (Ni-MOF) nanosheets supported on Ni foam featuring a well-defined stable structure, large electrochemically active surface area, and low electron transport resistance were prepared by a one-step solvothermal process. At -1.4 V, the nitrate reduction, rate constant, ammonia selectivity, and yield reached 96.4%, 0.448 h^-1, 80%, and 110.13 ug·h^-1·cm^-2, respectively. Experimental and theoretical studies demonstrated that the hydroxyl-ligated Ni atoms exhibited higher nitrate adsorption properties and lower activation energy towards NitRR compared to carboxylic acid-ligated Ni atoms. Mechanism investigations revealed a nitrate-to-ammonia reaction pathway involving multiple intermediate species on Ni-MOF nanosheet catalysts. This work offers a new avenue to construct highly efficient electrocatalysts for the selective transformation of nitrate to valuable ammonia.

Theoretical Investigation of Electrocatalytic Reduction of Nitrates to Ammonia on Highly Efficient and Selective g-C2N Monolayer-Supported Single Transition-Metal Atoms

Electrocatalytic reduction of nitrate (NO₃RR) to synthesize ammonia (NH₃) can effectively degrade nitrate while producing a valuable product. By utilizing density functional theory calculations, we investigate the potential catalytic performance of a range of single transition-metal (TM) atoms supported on nitrogenated holey doped graphene (g-C₂N) (TM/g-C₂N) for the reduction of nitrates to NH₃. Based on the screening procedure, Zr/g-C₂N and Hf/g-C₂N are predicted as potential electrocatalysts for the NO₃RR with limiting potential (U_L) values of -0.28 and -0.27 V, respectively. The generation of byproducts such as dioxide (NO₂), nitric oxide (NO), and nitrogen (N₂) is hindered on Zr/g-C₂N and Hf/g-C₂N due to the high energy cost. The NO₃RR activity of TM/g-C₂N is closely related to the adsorption free energy of NO₃^-. The study not only proposes a competent electrocatalyst for enhancing NO₃RR in ammonia synthesis but also provides a comprehensive understanding of the NO₃RR mechanism.

Au Nanowires Decorated Ultrathin Co3 O4 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 (NO₃ RR) is an effective and green strategy for both nitrate treatment and ammonia production. As an electrochemical reaction, the NO₃ RR requires an efficient electrocatalyst for achieving high activity and selectivity. Inspired by the enhancement effect of heterostructure on electrocatalysis, Au nanowires decorated ultrathin Co₃ O₄ nanosheets (Co₃ O₄ -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 NO₃ RR. As the result, the Co₃ O₄ -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 Co₃ O₄ -NS/Au-NWs nanohybrids show an obviously plasmon-promoted activity for NO₃ RR 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 NO₃ RR, which provide an efficient nitrate-to-ammonia reduction with high efficiency.

Electrocatalytic Reduction of Nitrate to Ammonia via a Au/Cu Single Atom Alloy Catalyst

Electrocatalytic ammonia (NH₃) synthesis from the reduction of nitrate (NO₃^-) is one of the effective and mild methods to treat nitrogen-containing wastewater from stationary sources and to obtain NH₃ readily compared with the Haber-Bosch process. However, the low efficiency of electrocatalytic NO₃^- reduction to NH₃ on traditional Cu-based catalysts hinders their practical application. Here, we prepare a Au/Cu single atom (SA) alloy (Au/Cu SAA) that shows a high performance of NH₃ synthesis with 99.69% Faradaic efficiency at -0.80 V vs RHE. The structures of Au SAs and alloyed Au/Cu are confirmed by the detailed characterizations. Online differential electrochemical mass spectrometry confirms the occurrence of key reaction intermediates (*NO₂, *NO, and *NH₃). Density functional theory calculations demonstrate that Au SAs efficiently reduce the adsorption energy of *NO₃^-, and the newly formed Au-Cu bonds boost the reduction process of *NO₂ to *NO. Meanwhile, Au/Cu SAAs produce significantly less N₂ and N₂O byproducts due to the prohibition of N-N coupling on single atoms, which finally leads to excellent Faradaic efficiency and NH₃ selectivity.

Iron Nanoparticles Protected by Chainmail-structured Graphene for Durable Electrocatalytic Nitrate Reduction to Nitrogen

The electrochemical nitrate reduction reaction (NO₃ RR) is an appealing technology for regulating the nitrogen cycle. Metallic iron is one of the well-known electrocatalysts for NO₃ RR, but it suffers from poor durability due to leaching and oxidation of iron during the electrocatalytic process. In this work, a graphene-nanochainmail-protected iron nanoparticle (Fe@Gnc) electrocatalyst is reported. It displays superior nitrate removal efficiency and high nitrogen selectivity. Notably, the catalyst delivers exceptional stability and durability, with the nitrate removal rate and nitrogen selectivity remained ≈96 % of that of the first time after up to 40 cycles (24 h for one cycle). As expected, the conductive graphene nanochainmail provides robust protection for the internal iron active sites, allowing Fe@Gnc to maintain its long-lasting electrochemical nitrate catalytic activity. This research proposes a workable solution for the scientific challenge of poor lasting ability of iron-based electrocatalysts in large-scale industrialization.

Nitrate pollution and its solutions with special emphasis on electrochemical reduction removal

Nitrate pollution has become a serious environmental concern all over the world including in China due to the mismanagement of water resources and human activities. Agricultural runoff and industrial and nuclear waste are among the major sources of nitrate pollution. Consuming nitrate-rich water can cause many chronic diseases including digestive problems, which can lead to many types of cancer and other serious health issues. Denitrification is the natural process for nitrate reduction under aerobic conditions, but it cannot handle an excess of nitrate, so several methods have been adopted for nitrate removal, i.e., biological, chemical, physicochemical, and electrochemical reduction removal. Among all, electrochemical reduction removal is a cost-effective and environmental-friendly process. To obtain the maximal elimination efficiency ideal conditions of current intensity, pH, plate distance, initial nitrate concentration, and type of electrolyte solution should be studied for effective nitrate removal. Electrochemical reduction removal of nitrate involves the transfer of electrons and hydrogenation. Besides an efficient nitrate removal process, electrochemical reduction removal has some drawbacks like sludge formation, low selectivity for nitrogen, and production of brine that limit its long-term implementation. This review focused on nitrate pollution, previous nitrate removal strategies, and essential principles for understanding the mechanism of electrochemical reduction removal and controlling the products of the reaction.

Iron-Based Nanocatalysts for Electrochemical Nitrate Reduction

Nitrate has a high level of stability and persistence in water, endangering human health and aquatic ecosystems. Due to its high reliability and efficiency, the electrochemical nitrate reduction reaction (NO₃ RR) is regarded as the best available option for mitigating excess nitrate in water and wastewater, especially for the removal of trace levels of nitrate. One of the most critical factors in the electrochemical reduction are the catalysts, which directly affect the reaction efficiency of nitrate removal. Iron-based nanocatalysts, which have the advantages of nontoxicity, wide availability, and low cost, have emerged as a promising electrochemical NO₃ RR material in recent years. This review covers major aspects of iron-based nanocatalysts for electrochemical NO₃ RR, including synthetic methods, structural design, performance enhancement, electrocatalytic nitrate reduction test, and reduction mechanism. The recent progress of iron-based nanocatalysts for electrochemical NO₃ RR and the mechanism of functional advantages for modified structures are reviewed from the perspectives of loading, doping, and assembly strategies, in order to realize the conversion from pollutant nitrate to harmless nitrogen or ammonia and other sustainable products. Finally, challenges and future directions for the development of low-cost and highly-efficient iron-based nanocatalysts are explored.

Controllable reconstruction of copper nanowires into nanotubes for efficient electrocatalytic nitrate conversion into ammonia

The electrochemical reduction of nitrate to ammonia provides a green and delocalized route for ammonia synthesis under ambient conditions, which requires advanced catalysts with high activity and selectivity. In this work, we propose a two-step conversion strategy to construct hierarchical copper nanosheet-based Cu nanotubes using pre-synthesized Cu nanowires as the starting material for the electrocatalytic nitrate reduction reaction (NO₃RR). The conversion of Cu nanowires into Cu nanotubes could be realized through chemical oxidation followed by in situ electrochemical reduction, enabling the effective engineering of active sites and thus boosting the electrocatalytic nitrate-to-ammonia capability. Such a controllable reconstruction strategy provides a new avenue for constructing high-performance electrocatalysts for sustainable NH₃ synthesis and the elimination of NO₃^- contamination.

Theoretical insights into the electroreduction of nitrate to ammonia on graphene-based single-atom catalysts

Electrocatalytic reduction of harmful nitrate (NO₃^-) to valuable ammonia (eNO₃RR) is critical and attractive for both environmental remediation and energy transformation. A single atom catalyst (SAC) based on graphene represents one of the most promising eNO₃RR catalysts. However, the underlying catalytic mechanism and the intrinsic factors dictating the catalytic activity trend remain unclear. Herein, using first-principles calculations, eNO₃RR on TMN₃ and TMN₄ (TM = Ti-Ni) doped graphene was thoroughly investigated. Our results reveal that FeN₄ doped graphene exhibits excellent eNO₃RR performance with a low limiting potential of -0.38 V, agreeing with the experimental finding, which can be ascribed to the effective adsorption and activation of NO₃^-via the charge "acceptance-donation" mechanism and its moderate binding due to the occupation of the d-p antibonding orbital. In particular, we found that eNO₃RR activities are well correlated with the intrinsic properties of TM centers and their local environments. With the established activity descriptor, several other graphene-based SACs were efficiently screened out with excellent eNO₃RR performance. Our studies could not only provide an atomic insight into the catalytic mechanism and activity origin of eNO₃RR on graphene-based SACs, but also open an avenue for the rational design of SACs for eNO₃RR towards ammonia by regulating the metal center and its local coordination environment.

Cu/CuOx In-Plane Heterostructured Nanosheet Arrays with Rich Oxygen Vacancies Enhance Nitrate Electroreduction to Ammonia

The artificial ammonia synthesis via electrochemical nitrate reduction has met increasing research interest, but it is still necessary to develop advanced catalysts with high nitrate-to-ammonia capability. Herein, we propose and demonstrate a one-step method to construct binder-free Cu foam-supported oxygen vacancy-rich Cu/CuO_x in-plane heterostructured nanosheet arrays (Cu/CuO_x/CF). In addition to exposing ample active sites, the two-dimensional nanosheet morphology greatly facilitates the mass/charge-transfer process during electrocatalysis. Besides, the in-plane heterojunctions and rich oxygen vacancies induced synergistic effect can modulate the electronic structure of active sites and thus tune the adsorption properties of the reactant intermediates and inhibit the formation of undesirable byproducts, which is conducive to the further improvement of nitrate reduction activity. As a result, these advantages endow the Cu/CuO_x/CF with superior performance for ammonia synthesis via nitrate electroreduction, achieving high ammonia selectivity (95.00%) and Faradaic efficiency (93.58%).

Real-Time High-Sensitivity Reaction Monitoring of Important Nitrogen-Cycle Synthons by 15N Hyperpolarized Nuclear Magnetic Resonance

Here, we show how signal amplification by reversible exchange hyperpolarization of a range of ¹⁵N-containing synthons can be used to enable studies of their reactivity by ¹⁵N nuclear magnetic resonance (NO₂^– (28% polarization), ND₃ (3%), PhCH₂NH₂ (5%), NaN₃ (3%), and NO₃^– (0.1%)). A range of iridium-based spin-polarization transfer catalysts are used, which for NO₂^– work optimally as an amino-derived carbene-containing complex with a DMAP-d₂ coligand. We harness long ¹⁵N spin-order lifetimes to probe in situ reactivity out to 3 × T₁. In the case of NO₂^– (T₁ 17.7 s at 9.4 T), we monitor PhNH₂ diazotization in acidic solution. The resulting diazonium salt (¹⁵N-T₁ 38 s) forms within 30 s, and its subsequent reaction with NaN₃ leads to the detection of hyperpolarized PhN₃ (T₁ 192 s) in a second step via the formation of an identified cyclic pentazole intermediate. The role of PhN₃ and NaN₃ in copper-free click chemistry is exemplified for hyperpolarized triazole (T₁ < 10 s) formation when they react with a strained alkyne. We also demonstrate simple routes to hyperpolarized N₂ in addition to showing how utilization of ¹⁵N-polarized PhCH₂NH₂ enables the probing of amidation, sulfonamidation, and imine formation. Hyperpolarized ND₃ is used to probe imine and ND₄⁺ (T₁ 33.6 s) formation. Furthermore, for NO₂^–, we also demonstrate how the ¹⁵N-magnetic resonance imaging monitoring of biphasic catalysis confirms the successful preparation of an aqueous bolus of hyperpolarized ¹⁵NO₂^– in seconds with 8% polarization. Hence, we create a versatile tool to probe organic transformations that has significant relevance for the synthesis of future hyperpolarized pharmaceuticals.

Crystalline Modulation Engineering of Ru Nanoclusters for Boosting Ammonia Electrosynthesis from Dinitrogen or Nitrate

Developing highly efficient nitrogen reduction reaction (NRR) and nitrate reduction reaction (NITRR) electrocatalysts is an ongoing challenge. Herein, we report the in situ growth of ultrafine amorphous Ru nanoclusters with a uniform diameter of ∼1.2 nm on carbon nanotubes as a highly efficient electrocatalyst for both the NRR and the NITRR. The amorphous Ru nanoclusters were prepared via a convenient ambient chelated co-reduction method, in which trisodium citrate as a chelating agent played a key role to form amorphous Ru instead of crystalline Ru. The strong d-π interaction between Ru metal and carbon nanotubes led to the homogeneous distribution and good long-term stability of ultrafine Ru nanoclusters. Compared with crystalline Ru, amorphous Ru nanoclusters with abundant low-coordinate atoms can provide more catalytic sites. The amorphous Ru nanoclusters exhibited an NH₃ yield of 10.49 μg·h^-1·mg_cat.^-1 and a FE_NH3 of 17.48% at -0.2 V vs reversible hydrogen electrode (RHE) for NRR. For the NITRR, an NH₃ yield of 145.1 μg·h^-1·mg_cat.^-1 and a FE_NH3 of 80.62% were also achieved at -0.2 V vs RHE. This work provides new insights into crystalline modulation engineering of metal nanoclusters for electrocatalytic ammonia synthesis.

Chloride-Derived Bimetallic Cu-Fe Nanoparticles for High-Selective Nitrate-to-Ammonia Electrochemical Catalysis

Cu-based bimetallic materials have been widely reported as efficient catalysts for electrocatalytic nitrate reduction. However, the faradaic efficiency and selectivity are still far from satisfactory. Herein, Cu-Fe bimetallic nanoalloys with adjustable Cu/Fe ratios are successfully prepared through a reactive mechanical milling approach with CuCl2, FeCl3 and Na as the starting materials. The optimized Cu3Fe exhibits excellent nitrate conversion efficiency of 81.1% and 70.3% ammonia selectivity at −0.7 V vs. RHE within 6 h under 0.1 M Na2SO4 and 100 ppm NO3−. The Fe-introduction-induced upshift of the d-band center is identified to be beneficial for promoting nitrate adsorption on Cu3Fe. Moreover, favorable H generation under the assistance of Fe could effectively accelerate the stepwise hydrogenation during electrocatalytic nitrate reduction, resulting in significantly improved NH4+ selectivity. This work supplies valuable insights for the rational design of transition-metal-based bimetallic catalysts for electrocatalytic nitrate reduction.

DFT Investigation of Ammonia Formation via a Langmuir-Hinshelwood Mechanism on Mo-Terminated δ-MoN(0001)

In this work, we employed density functional theory to elucidate the energetics associated with elementary steps along a Langmuir-Hinshelwood mechanism for the Haber-Bosch synthesis of ammonia from N₂ and H₂ on a hexagonal, Mo-terminated molybdenum nitride surface. Using nudged elastic band calculations, we determined the energy barriers involved in the reaction processes. An active site consisting of four nearest-neighbor Mo atoms, previously identified as an active site on similar surfaces, was chosen to investigate the reaction processes. Using this approach, we calculate a barrier of ∼0.5 eV for the dissociation of N₂. The superior activity of the dissociation of the strong N₂ bonds is rationalized based on the unique geometric and electronic configurations present at these active sites. Despite the favorable energetics for nitrogen dissociation, the energy cost for hydrogenation of NH _x (0 ≤ x ≤ 2) species is shown to be energetically limiting for the formation of ammonia through the Langmuir-Hinshelwood mechanism at these sites, with elementary step activation barriers calculated to be as large as ∼2 eV. A comparison to Haber-Bosch results derived from a similar γ-Mo₂N model system suggests the relative independence of surface chemistry and bulk stoichiometry for rhombic Mo₄ active sites present on molybdenum nitrides.

Electrified Conversion of Contaminated Water to Value: Selective Conversion of Aqueous Nitrate to Ammonia in a Polymer Electrolyte Membrane Cell

The application of a polymer electrolyte membrane (PEM) electrolytic cell for continuous conversion of nitrate, one of the contaminants in water, to ammonia at the cathode was explored in the present work. Among carbon-supported metal (Cu, Ru, Rh and Pd) electrocatalysts, the Ru-based catalyst showed the best performance. By suppressing the competing hydrogen evolution reaction at the cathode, it was possible to reach 94 % faradaic efficiency for nitrate reduction towards ammonium. It was important to match the rate of the anodic reaction with the cathodic reaction to achieve high faradaic efficiency. By recirculating the effluent stream, 93 % nitrate conversion was achieved in 8 h of constant current electrolysis at 10 mA cm^-2 current density. The presented approach offers a promising path towards precious NH₃ production from nitrate-containing water that needs purification or can be obtained after capture of gaseous NO_x pollutants into water, leading to waste-to-value conversion.

Ni2P nanosheet array for high-efficiency electrohydrogenation of nitrite to ammonia at ambient conditions

Ammonia (NH₃) plays an important role in agriculture and industry. The industry-scale production mainly depends on the Haber-Bosch process suffering from issues of environment pollution and energy consumption. Electrochemical reduction can degrade nitrite (NO₂^-) pollutants in the environment and convert it into more valuable NH₃. Here, Ni₂P nanosheet array on nickel foam is proposed as a 3D electrocatalyst for high-efficiency electrohydrogenation of NO₂^- to NH₃ under ambient reaction conditions. When tested in 0.1 M phosphate buffer saline with 200 ppm NO₂^-, such Ni₂P/NF is able to obtain a large NH₃ yield rate of 2692.2 ± 92.1 μg h^-1 cm^-2 (3282.9 ± 112.3 μg h^-1 mg_cat.^-1), a high Faradic efficiency of 90.2 ± 3.0%, and selectivity of 87.0 ± 1.7% at -0.3 V versus a reversible hydrogen electrode. After 10 h of electrocatalytic reduction, the conversion rate of NO₂^- achieves near 100%. The catalytic mechanism is further investigated by density functional theory calculations.

Electroreduction of NO3− on tubular porous Ti electrodes

Tubular porous Ti electrodes show unprecedented performance in the electrochemical reduction of nitrate to ammonia, which increased from −33 to −75 mA cm2 by applying an inert gas flow exiting through the pores of the Ti tube.

Fabrication of an amorphous metal oxide/p-BiVO4 photocathode: understanding the role of entropy for reducing nitrate to ammonia

Entropy regulation makes an amorphous metal oxide/p-BiVO4 heterostructure a desirable catalyst for the NO3− reduction reaction in a photoelectrochemical system.

Selective reduction of nitrate to ammonium over charcoal electrode derived from natural wood

Electrocatalytic nitrate reduction has been regarded as an efficient alternative route for ammonia production. Developing efficient, economical and environment-friendly cathodes is a significant concern for the practical applications of this method. Herein, we report a charcoal electrode fabricated by carbonizing natural wood for efficient nitrate reduction. It displays high overpotential for hydrogen evolution, moderate sp³ C structure and oxygen-containing surface groups. Benefiting from these features, the charcoal cathode exhibits high nitrate removal rate (91.2%), outstanding selectivity (98.5%) and fast production rate (0.570 mmol L^-1 h^-1 cm^-2) for ammonium. Both removal rate and selectivity are superior to other carbon materials and comparable to metal-containing cathodes. These results exhibit the possibility of using charcoal as cathodes for denitrification and ammonia recovery from wastewater.

Recent Advances and Perspective on Electrochemical Ammonia Synthesis under Ambient Conditions

Ammonia is an essential chemical for agriculture and industry. To date, NH₃ is mainly supplied by the traditional Haber-Bosch process, which is operated under high-temperature and high-pressure in a centralized way. To achieve ammonia production in an environmentally benign way, electrochemical NH₃ synthesis under ambient conditions has become the frontier of energy and chemical conversion schemes, as it can be powered by renewable energy and operates in a decentralized way. The recent progress on developing different strategies for NH₃ production, including 1) classic NH₃ synthesis pathways over nanomaterials; 2) the Mars-van Krevelen (MvK) mechanism over metal nitrides (MN_x ); 3) reducing the nitrate into NH₃ over Cu-based nanomaterial; and 4) metal-N₂ battery release of NH₃ from Li_x M. Moreover, the most recent advances in engineering strategies for developing highly active materials and the design of the reaction systems for NH₃ synthesis are covered.

Efficient Nitrate-to-Ammonia Electroreduction at Cobalt Phosphide Nanoshuttles

The nitrate electroreduction reaction (NO₃^--ERR) is an efficient and green approach for nitrate remediation, which requires a highly active and selective electrocatalyst. In this work, porous and amorphous cobalt phosphide nanoshuttles (CoP PANSs) are successfully synthesized by using Mg²⁺ ion-doped calcium carbonate nanoshuttles (Mg-CaCO₃ NSs) as the initial reaction precursor via precipitation transformation and a high-temperature phosphidation strategy. Various physical characterizations show that CoP PANSs have porous architecture, amorphous crystal structure, and big surface area. Electrochemical measurements reveal for the first time that CoP PANSs have outstanding electroactivity for NO₃^--ERR in a neutral electrolyte. At an applied potential of -0.5 V vs reversible hydrogen electrode, CoP PANSs can achieve a high Faraday efficiency (94.24 ± 2.8%) and high yield rate (19.28 ± 0.53 mg h^-1 mg_cat^-1) for ammonia production, which exceeds most reported values at various electrocatalysts for NO₃^--ERR. Thus, the present result indicates that cobalt phosphide nanomaterials have promising application for NO₃^--ERR.

Electrochemical ammonia synthesis from nitrite assisted by in situ generated hydrogen atoms on a nickel phosphide catalyst

Investigating green and effective means for ammonia synthesis is an important but challenging task. Electrochemical ammonia synthesis (EAS) from an indirect route (N2 → NOx → NH3) provides a feasible alternative strategy. The key step in this route is the reduction of NOx to NH3 instead of N2, which requires the investigation of efficient catalysts with high selectivity of NH3. Herein, we initially demonstrate a highly efficient electrochemical reduction of NO2- to NH3 with nickel phosphide (Ni2P) as the catalyst. The system exhibits low onset potential (0.2 V vs. RHE) and high faradaic efficiency (>90%) for EAS. Experimental results and theoretical calculations reveal that the in situ generated hydrogen atoms on the surface of Ni2P greatly promote the reduction of NO2- to NH3.

Selective Electrocatalytic Reduction of Nitrate to Ammonia with Nickel Phosphide

Liquid ammonia is considered a sustainable liquid fuel and an easily transportable carrier of hydrogen energy; however, its synthesis processes are energy-consuming, high cost, and low yield rate. Herein, we report the electrocatalytic reduction of nitrate (NO₃^-) (ERN) to ammonia (NH₃) with nickel phosphide (Ni₂P) used as a noble metal-free cathode. Ni₂P with (111) facet was grown in situ on nickel foam (NFP), which was regarded as a self-supporting cathode for ERN to synthesis NH₃ with high yield rate (0.056 mmol h^-1 mg^-1) and superior faradaic efficiency of 99.23%. The derived atomic H (*H), verified by a quenching experiment and an electron spin resonance (ESR) technique, effectively enhanced the high selectivity for NH₃ generation. DFT calculations indicated that *NO₃ was deoxygenated to *NO₂ and *NO, and *NO was subsequently hydrogenated with *H to generate NH₃ with an energy releasing process (ΔG < 0). OLEMS also proved that NO was the merely gas intermediate. NFP exhibited the unique superhydrophilic surface, metallic properties, low impedance, and abundant surface sites, favorable for adsorption of NO₃^-, generation of *H, and then hydrogenation of NO₃^-. Hence, NFP cathode showed high selectivity for NH₃ (89.1%) in ERN. NFP with long-term stability and low energy consumption provides a facile strategy for synthesis of NH₃ and elimination of NO₃^- contamination.

Facet-controlled palladium nanocrystalline for enhanced nitrate reduction towards ammonia

Electrochemical nitrate reduction reaction (NO₃^-RR) is considered an appealing way for producing ammonia (NH₃) under ambient conditions and solving environmental problems caused by nitrate, whereas the lack of adequate catalysts hampers the development of NO₃^-RR. Here, we firstly demonstrate that the Pd nanocrystalline with a well-desired facet can act as a highly efficient NO₃^-RR electrocatalyst for ambient ammonia synthesis. Pd (1 1 1) exhibits excellent activity and selectivity in reducing NO₃^- to NH₄⁺ with a Faradaic efficiency of 79.91% and an NH₄⁺ production of 0.5485 mmol h^-1 cm^-2 (2.74 mmol h^-1 mg^-1) in 0.1 M Na₂SO₄ (containing 0.1 M NO₃^-), which is 1.4 times higher than Pd (1 0 0) and 1.9 times higher than Pd (1 1 0), respectively. Density functional theory (DFT) calculation reveals that the superior NO₃^-RR activity of Pd (1 1 1) originates from its optimized activity of NO₃^- adsorption, smaller free energy change of the rate-limiting step (*NH₃ to NH₃), and poorer hydrogen evolution reaction activity (HER, competitive reaction). This work not only highlights the potentials of Pd-based nanocatalysts for NO₃^-RR but also provides new insight for the applications in NO₃^-RR of other facet-orientation nanomaterials.

Electrocatalytic nitrate reduction on rhodium sulfide compared to Pt and Rh in the presence of chloride

Chloride poisoning is a serious problem for the electrocatalytic reduction of aqueous nitrate (NO3−) and improved electrocatalysts are needed.