Electrocatalytic Hydrogenation Boosts Reduction of Nitrate to Ammonia over Single-Atom Cu with Cu(I)-N3C1 Sites

Ultrathin cobalt-based nanosheets containing surface oxygen promoted near-complete nitrate removal

Electrocatalytic nitrate removal offers a sustainable approach to alleviate nitrate pollution and to boost the anthropogenic nitrogen cycle, but it still suffers from limited removal efficiency at high rates, especially at low levels of nitrate. Herein, we report the near-complete removal of low-level nitrate (10-200 ppm) within 2 h using ultrathin cobalt-based nanosheets (CoNS) containing surface oxygen, which was fabricated from in-situ electrochemical reconstruction of conventional nanosheets. The average nitrate removal of 99.7 % with ammonia selectivity of 98.2 % in 9 cyclic runs ranked in the best of reported catalysts. Powered by a solar cell under the winter sun, the full-cell nitrate electrolysis system, equipped with ultrathin CoNS, achieved 100 % nitrogen gas selectivity and 99.6 % total nitrogen removal. The in-situ Fourier Transform Infrared included experiments and theoretical computations revealed that in-situ electrochemical reconstruction not only increased electrochemical active surface area but also constructed surface oxygen in active sites, leading to enhanced stabilization of nitrate adsorption in a symmetry breaking configuration and charge transfer, contributing to near-complete nitrate removal on ultrathin CoNS. This work provides a strategy to design ultrathin nanocatalysts for nitrate removal.

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.

An Anti-Scaling Strategy for Electrochemical Wastewater Treatment: Leveraging Tip-Enhanced Electric Fields

Electrode scaling poses a critical barrier to the adoption of electrochemical processes in wastewater treatment, primarily due to electrode inactivation and increased internal reactor resistance. We introduce an antiscaling strategy using tip-enhanced electric fields to redirect scale-forming compounds (e.g., Mg(OH)2 and CaCO3) from the electrode-electrolyte interface to the bulk solution. Our study utilized Cu nanowires (Cu NW) with high-curvature nanostructures as the cathode, in contrast to Cu nanoparticles (Cu NP), Cu foil (CF), and Cu mesh (CM), to evaluate the electrochemical nitrate reduction reaction (NO3RR) performance in hard water conditions. The Cu NW/CF cathode demonstrated superior NO3RR efficiency, with an apparent rate constant (Kapp) of 1.04 h-1, significantly outperforming control electrodes under identical conditions (Kapp < 0.051 h-1). Through experimental and theoretical analysis, including COMSOL simulations, we show that the high-curvature design of Cu NW induced localized electric field enhancements, propelling OH- ions away from the electrode surface into the bulk solution, thus mitigating scale formation on the cathode. Testing with real nitrate-contaminated wastewater confirms that the Cu NW/CF cathode maintained excellent denitrification efficiency over a 60-day period. This study offers a promising perspective on preventing electrode scaling in electrochemical wastewater treatment, paving the way for more efficient and sustainable practices.

Pulsed electro-catalysis enables effective conversion of low-concentration nitrate to ammonia over Cu2O@Pd tandem catalyst

Electro-catalytic conversion of nitrate (NO3-) to ammonia (NH3) via the Nitrate Reduction to Ammonia (NORA) process represents a promising strategy for both ammonia synthesis and environmental remediation. Despite its potential, the efficiency of low-concentration NORA is often hindered by mass transfer limitations, competing byproducts (N2 and NO2-), and side reactions such as hydrogen evolution. This study introduces a novel pulsed electro-synthesis technique that alternates the potential to periodically accumulate and transform NO2- intermediates near a Cu2O@Pd electrode, enhancing the NORA process. Compared with that under potentiostatic conditions, the Cu2O@Pd electrodes exhibited a higher NORA activity under the optimized pulsed condition, where a NH3-N Faradaic efficiency (FE) of 81.2%, a yield rate of 1.08 mg h-1 cm-2 and a selectivity efficiency (SE) of 81.5%, were achieved. In-situ characterization revealed an enhancement mechanism characterized by optimized adsorption of the key *NO intermediate, followed by the hydrogenation path "*N → *NH → *NH2→ *NH3". Further investigations indicated the electro-catalytic synergies between Pd sites and Cu species, where the Pd atoms were the reaction sites for the H adsorption while the Cu species were responsible for the NO3- activation. This research offers a novel insight into a method of enhancing low-concentration NORA.

Copper-based electro-catalytic nitrate reduction to ammonia from water: Mechanism, preparation, and research directions

Global water bodies are increasingly imperiled by nitrate pollution, primarily originating from industrial waste, agricultural runoffs, and urban sewage. This escalating environmental crisis challenges traditional water treatment paradigms and necessitates innovative solutions. Electro-catalysis, especially utilizing copper-based catalysts, known for their efficiency, cost-effectiveness, and eco-friendliness, offer a promising avenue for the electro-catalytic reduction of nitrate to ammonia. In this review, we systematically consolidate current research on diverse copper-based catalysts, including pure Cu, Cu alloys, oxides, single-atom entities, and composites. Furthermore, we assess their catalytic performance, operational mechanisms, and future research directions to find effective, long-term solutions to water purification and ammonia synthesis. Electro-catalysis technology shows the potential in mitigating nitrate pollution and has strategic importance in sustainable environmental management. As to the application, challenges regarding complexity of the real water, the scale-up of the commerical catalysts, and the efficient collection of produced NH3 are still exist. Following reseraches of catalyst specially on long term stability and in situ mechanisms are proposed.

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.

Challenges and Opportunities for Single-Atom Electrocatalysts: From Lab-Scale Research to Potential Industry-Level Applications

Single-atom electrocatalysts (SACs) are a class of promising materials for driving electrochemical energy conversion reactions due to their intrinsic advantages, including maximum metal utilization, well-defined active structures, and strong interface effects. However, SACs have not reached full commercialization for broad industrial applications. This review summarizes recent research achievements in the design of SACs for crucial electrocatalytic reactions on their active sites, coordination, and substrates, as well as the synthesis methods. The key challenges facing SACs in activity, selectivity, stability, and scalability, are highlighted. Furthermore, it is pointed out the new strategies to address these challenges including increasing intrinsic activity of metal sites, enhancing the utilization of metal sites, improving the stability, optimizing the local environment, developing new fabrication techniques, leveraging insights from theoretical studies, and expanding potential applications. Finally, the views are offered on the future direction of single-atom electrocatalysis toward commercialization.

Red Carbon Mediated Formation of Cu2O Clusters Dispersed on the Oxocarbon Framework by Fehling's Route and their Use for the Nitrate Electroreduction in Acidic Conditions

The oligomers of carbon suboxide, known as red carbon, exhibit a highly conjugated structure and semiconducting properties. Upon mild heat treatment, it transforms into a carbonaceous framework rich in oxygen surface terminations, called oxocarbon. In this study, the abundant oxygen functionalities are harnessed as anchors to create oxocarbon-supported nanohybrid electrocatalysts. Starting with single atomic Cu (II) strongly coordinated to oxygen atoms on red carbon, the Fehling reaction leads to the formation of Cu2O clusters. Simultaneously, a covalent oxocarbon framework emerges via cross-linking, providing robust support for Cu2O clusters. Notably, the oxocarbon support effectively stabilizes Cu2O clusters of very small size, ensuring their high durability in acidic conditions and the presence of ammonia. The synthesized material exhibits a superior electrocatalytic activity for nitrate reduction under acidic electrolyte conditions, with a high yield rate of ammonium (NH4 +) at 3.31 mmol h-1 mgcat -1 and a Faradaic efficiency of 92.5% at a potential of -0.4 V (vs RHE).

Fluorine Modification Promoted Water Dissociation into Atomic Hydrogen on a Copper Electrode for Efficient Neutral Nitrate Reduction and Ammonia Recovery

Electrocatalytic nitrate reduction to ammonia (NITRR) offers an attractive solution for alleviating environmental concerns, yet in neutral media, it is challenging as a result of the reliance on the atomic hydrogen (H*) supply by breaking the stubborn HO-H bond (∼492 kJ/mol) of H2O. Herein, we demonstrate that fluorine modification on a Cu electrode (F-NFs/CF) favors the formation of an O-H···F hydrogen bond at the Cu-H2O interface, remarkably stretching the O-H bond of H2O from 0.98 to 1.01 Å and lowering the energy barrier of water dissociation into H* from 0.64 to 0.35 eV at neutral pH. As a benefit from these advantages, F-NFs/CF could rapidly reduce NO3- to NH3 with a rate constant of 0.055 min-1 and a NH3 selectivity of ∼100%, far higher than those (0.004 min-1 and 9.2%) of the Cu counterpart. More importantly, we constructed a flow-through coupled device consisting of a NITRR electrolyzer and a NH3 recovery unit, realizing 98.1% of total nitrogen removal with 99.3% of NH3 recovery and reducing the denitrification cost to $5.1/kg of N. This study offers an effective strategy to manipulate the generation of H* from water dissociation for efficient NO3--to-NH3 conversion and sheds light on the importance of surface modification on a Cu electrode toward electrochemical reactions.

Selective Nitrate Electroreduction to Ammonia on CNT Electrodes with Controllable Interfacial Wettability

The development of electrocatalysts that can efficiently reduce nitrate (NO3-) to ammonia (NH3) has garnered increasing attention due to their potential to reduce carbon emissions and promote environmental protection. Intensive efforts have focused on catalyst development, but a thorough understanding of the effect of the microenvironment around the reactive sites of the catalyst is also crucial to maximize the performance of the electrocatalysts. This study explored an electrocatalytic system that utilized quaternary ammonium surfactants with a range of alkyl chain lengths to modify an electrode made of carbon nanotubes (CNT), with the goal of regulating interfacial wettability toward NO3- reduction. Trimethyltetradecylammonium bromide with a moderate alkyl chain length created a very hydrophobic interface, which led to a high selectivity in the production of NH3 (∼87%). Detailed mechanistic investigations that used operando Fourier-transform infrared (FTIR) spectroscopy and online differential electrochemical mass spectrometry (DEMS) revealed that the construction of a hydrophobic modified CNT played a synergistic role in suppressing a side reaction involving the generation of hydrogen, which would compete with the reduction of NO3-. This electrocatalytic system led to a favorable process for the reduction of NO3- to NH3 through a direct electron transfer pathway. Our findings underscore the significance of controlling the hydrophobic surface of electrocatalysts as an effective means to enhance electrochemical performance in aqueous media.

Efficient Electrochemical Nitrate Removal by Ordered Ultrasmall Intermetallic AuCu3 via Enhancing Nitrate Adsorption

Developing a high-performance electrocatalyst for synthesizing ammonia from nitrate represents a promising solution for addressing wastewater pollution and achieving sustainable ammonia production. However, it remains a formidable challenge. Herein, an intermetallic AuCu3 electrocatalyst with high-density active sites is designed and prepared for an efficient nitrate electroreduction to generate ammonia. Remarkably, the Faraday efficiency and yield rate of ammonia at -0.9 V are 97.6% and 75.9 mg h-1 cm-2, respectively. More importantly, after 10 cycles of testing, the removal rate of nitrate can still reach 95.2%. Electrochemical in situ Fourier transform infrared analysis indicates that AuCu3 IM can promote the adsorption of nitrate and enhance ammonia production from nitrate. *NH3, *NO, and *NO2 have been proven to be active intermediates. Theoretical and experimental studies show that the Au site can provide a large amount of *H for nitrate reduction, and the Cu site is conducive to the reduction of nitrate to produce nitrogen-containing products. Meanwhile, AuCu3 intermetallic compounds (AuCu3 IM) can inhibit the dimerization of *H. The power density and ammonia yield of the assembled Zn-nitrate battery reached 2.17 mW cm-2 and 71.2 mg h-1 cm-2, respectively.

Defect engineering on electrocatalysts for sustainable nitrate reduction to ammonia: Fundamentals and regulations

Electrocatalytic nitrate (NO3 -) reduction to ammonia (NH3) is a "two birds-one stone" method that targets remediation of NO3 --containing sewage and production of valuable NH3. The exploitation of advanced catalysts with high activity, selectivity, and durability is a key issue for the efficient catalytic performance. Among various strategies for catalyst design, defect engineering has gained increasing attention due to its ability to modulate the electronic properties of electrocatalysts and optimize the adsorption energy of reactive species, thereby enhancing the catalytic performance. Despite previous progress, there remains a lack of mechanistic insights into the regulation of catalyst defects for NO3 - reduction. Herein, this review presents insightful understanding of defect engineering for NO3 - reduction, covering its background, definition, classification, construction, and underlying mechanisms. Moreover, the relationships between regulation of catalyst defects and their catalytic activities are illustrated by investigating the properties of electrocatalysts through the analysis of electronic band structure, charge density distribution, and controllable adsorption energy. Furthermore, challenges and perspectives for future development of defects in NO3RR are also discussed, which can help researchers to better understand the defect engineering in catalysts, and also inspire scientists entering into this promising field.

Electrochemical Nitrate Reduction: Ammonia Synthesis and the Beyond

Natural nitrogen cycle has been severely disrupted by anthropogenic activities. The overuse of N-containing fertilizers induces the increase of nitrate level in surface and ground waters, and substantial emission of nitrogen oxides causes heavy air pollution. Nitrogen gas, as the main component of air, has been used for mass ammonia production for over a century, providing enough nutrition for agriculture to support world population increase. In the last decade, researchers have made great efforts to develop ammonia processes under ambient conditions to combat the intensive energy consumption and high carbon emission associated with the Haber-Bosch process. Among different techniques, electrochemical nitrate reduction reaction (NO3RR) can achieve nitrate removal and ammonia generation simultaneously using renewable electricity as the power, and there is an exponential growth of studies in this research direction. Here, a timely and comprehensive review on the important progresses of electrochemical NO3RR, covering the rational design of electrocatalysts, emerging CN coupling reactions, and advanced energy conversion and storage systems is provided. Moreover, future perspectives are proposed to accelerate the industrialized NH3 production and green synthesis of chemicals, leading to a sustainable nitrogen cycle via prosperous N-based electrochemistry.

Recent Advances in Electrocatalytic Hydrogenation Reactions on Copper-Based Catalysts

Hydrogenation reactions play a critical role in the synthesis of value-added products within the chemical industry. Electrocatalytic hydrogenation (ECH) using water as the hydrogen source has emerged as an alternative to conventional thermocatalytic processes for sustainable and decentralized chemical synthesis under mild conditions. Among the various ECH catalysts, copper-based (Cu-based) nanomaterials are promising candidates due to their earth-abundance, unique electronic structure, versatility, and high activity/selectivity. Herein, recent advances in the application of Cu-based catalysts in ECH reactions for the upgrading of valuable chemicals are systematically analyzed. The unique properties of Cu-based catalysts in ECH are initially introduced, followed by design strategies to enhance their activity and selectivity. Then, typical ECH reactions on Cu-based catalysts are presented in detail, including carbon dioxide reduction for multicarbon generation, alkyne-to-alkene conversion, selective aldehyde conversion, ammonia production from nitrogen-containing substances, and amine production from organic nitrogen compounds. In these catalysts, the role of catalyst composition and nanostructures toward different products is focused. The co-hydrogenation of two substrates (e.g., CO2 and NOx n, SO3 2-, etc.) via C─N, C─S, and C─C cross-coupling reactions are also highlighted. Finally, the critical issues and future perspectives of Cu-catalyzed ECH are proposed to accelerate the rational development of next-generation catalysts.

Unveiling Cutting-Edge Developments in Electrocatalytic Nitrate-to-Ammonia Conversion

The excessive enrichment of nitrate in the environment can be converted into ammonia (NH3) through electrochemical processes, offering significant implications for modern agriculture and the potential to reduce the burden of the Haber-Bosch (HB) process while achieving environmentally friendly NH3 production. Emerging research on electrocatalytic nitrate reduction (eNitRR) to NH3 has gained considerable momentum in recent years for efficient NH3 synthesis. However, existing reviews on nitrate reduction have primarily focused on limited aspects, often lacking a comprehensive summary of catalysts, reaction systems, reaction mechanisms, and detection methods employed in nitrate reduction. This review aims to provide a timely and comprehensive analysis of the eNitRR field by integrating existing research progress and identifying current challenges. This review offers a comprehensive overview of the research progress achieved using various materials in electrochemical nitrate reduction, elucidates the underlying theoretical mechanism behind eNitRR, and discusses effective strategies based on numerous case studies to enhance the electrochemical reduction from NO3 - to NH3. Finally, this review discusses challenges and development prospects in the eNitRR field with an aim to guide design and development of large-scale sustainable nitrate reduction electrocatalysts.

Advancements in Electrocatalytic Nitrogen Reduction: A Comprehensive Review of Single-Atom Catalysts for Sustainable Ammonia Synthesis

Electrocatalytic nitrogen reduction technology seamlessly aligns with the principles of environmentally friendly chemical production. In this paper, a comprehensive review of recent advancements in electrocatalytic NH3 synthesis utilizing single-atom catalysts (SACs) is offered. Into the research and applications of three categories of SACs: noble metals (Ru, Au, Rh, Ag), transition metals (Fe, Mo, Cr, Co, Sn, Y, Nb), and nonmetallic catalysts (B) in the context of electrocatalytic ammonia synthesis is delved. In-depth insights into the material preparation methods, single-atom coordination patterns, and the characteristics of the nitrogen reduction reaction (NRR) are provided. The systematic comparison of the nitrogen reduction capabilities of various SAC types offers a comprehensive research framework for their integration into electrocatalytic NRR. Additionally, the challenges, potential solutions, and future prospects of incorporating SACs into electrocatalytic nitrogen reduction endeavors are discussed.

A Wetting and Capture Strategy Overcoming Electrostatic Repulsion for Electroreduction of Nitrate to Ammonia from Low-Concentration Sewage

Ammonia production by electrocatalytic nitrate reduction reaction (NO3 RR) in water streams is anticipated as a zero-carbon route. Limited by dilute nitrate in natural sewage and the electrostatic repulsion between NO3 - and cathode, NO3 RR can hardly be achieved energy-efficiently. The hydrophilic Cu@CuCoO2 nano-island dispersed on support can enrich NO3 - and produce a sensitive current response, followed by electrosynthesis of ammonia through atomic hydrogen (*H) is reported. The accumulated NO3 - can be partially converted to NO2 - without external electric field input, confirming that the Cu@CuCoO2 nano-island can strongly bind NO3 - and then trigger the reduction via dynamic evolution between Cu-Co redox sites. Through the identification of intermediates and theoretical computation. it is found that the N-side hydrogenation of *NO is the optimal reaction step, and the formation of N─N dimer may be prevented. An NH3 product selectivity of 93.5%, a nitrate conversion of 96.1%, and an energy consumption of 0.079 kWh gNH3 -1 is obtained in 48.9 mg-N L-1 naturally nitrate-polluted streams, which outperforms many works using such dilute nitrate influent. Conclusively, the electrocatalytic system provides a platform to guarantee the self-sufficiency of dispersed ammonia production in agricultural regions.

Bioinspired Tandem Electrode for Selective Electrocatalytic Synthesis of Ammonia from Aqueous Nitrate

The electrocatalytic nitrate reduction reaction (NO3RR) has recently emerged as a promising technique for readily converting aqueous nitrate (NO3-) pollutants into valuable ammonia (NH3). It is vital to thoroughly understand the mechanism of the reaction to rationally design and construct advanced electrocatalytic systems that can effectively and selectively drive the NO3RR. There are several natural enzymes that incorporate molybdenum (Mo) and that can activate NO3-. Based on this, a cadmium (Cd) single-atom anchored Mo2TiC2Tx electrocatalyst (referred to as CdSA-Mo2TiC2Tx) through the NO3RR to generate NH3 was rationally designed and demonstrated. In an H-type electrolysis cell and at a current density of 42.5 mA cm-2, the electrocatalyst had a Faradaic efficiency of >95% and an impressive NH3 yield rate of 48.5 mg h-1 cm-2. Moreover, the conversion of NO3- to NH3 on the CdSA-Mo2TiC2Tx surface was further revealed by operando attenuated total reflection Fourier-transform infrared spectroscopy and an electrochemical differential mass spectrometer. The electrocatalyst significantly outperformed Mo2TiC2Tx as well as reported state-of-the-art catalysts. Density functional theory calculations revealed that CdSA-Mo2TiC2Tx decreased the ability of the d-p orbital to hybridize with NH3* intermediates, thereby decreasing the activation energy of the potential-determining step. This work not only highlights the application prospects of heavy metal single-atom catalysts in the NO3RR but also provides examples of bio-inspired electrocatalysts for the synthesis of NH3.

Pd/Cu bimetallic nano-catalyst supported on anion exchange resin (A520E) for nitrate removal from water: High property and stability

High nitrate concentration in water can lead to eutrophication and the disruption of healthy aquatic ecosystems. Additionally, in the human digestive system, it has the potential to be reduced to nitrite, which can be damaging to people's physical health. Catalytic hydrogenation of nitrate is one of the strategies for removing nitrate from water. Using A520E resin as support, we prepared Pd/Cu nano-catalyst (Pd/Cu@A520E) according to a liquid phase reduction method. A520E could improve the transfer process of nitrate in the solution to the activity sites of Pd/Cu nanoparticles, thus increase the reaction rate of nitrate reduction. Pd/Cu bimetallic nano-particles were evenly distributed on/in the resin with a size range from 2 nm to 10 nm. The External Circulating System equipped with Venturi tube (ECSV) was designed to improve the utilization efficiency of H2 in both batch tests and long-term continuous-flow tests. Nearly 100% of nitrate removal efficiency and above 90% of N2 selectivity were achieved in both batch tests and continuous-flow tests. Coexisting Cl- and SO42- at 300 mg/L showed little impact on the property of Pd/Cu@A520E. Pd/Cu@A520E also showed high nitrate removal property and stability in continuous-flow tests of more than 800 h. NO3- was adsorbed onto the active sites (functional groups and Pd/Cu particle sites), meanwhile H2 was adsorbed onto the active sites of Pd/Cu@A520E to form Pd [H]. Then the adsorbed NO3- was reduced into N2 (main product) or NH4+ by Pd [H]. In addition, Pd/Cu@A520E showed high nitrate removal property from municipal waste water.

Single and double transition metal atoms doped graphdiyne for highly efficient electrocatalytic reduction of nitric oxide to ammonia

The electrocatalytic conversion of nitric oxide (NORR) to ammonia (NH3) represents a pivotal approach for sustainable energy transformation and efficient waste utilization. Designing highly effective catalysts to facilitate the conversion of NO into NH3 remains a formidable challenge. In this work, the density functional theory (DFT) is used to design NORR catalysts based on single and double transition metal (TM:Fe, Co, Ni and Cu) atoms supported by graphdiyne (TM@GDY). Among eight catalysts, the Cu2@GDY is selected as a the most stable NORR catalyst with high NH3 activity and selectivity. A pivotal discovery underscores that the NORR mechanism is thermodynamically constrained on single atom catalysts (SACs), while being governed by electrochemical processes on double atom catalysts (DACs), a distinction arising from the different d-band centers of these catalysts. Therefore, this work not only introduces an efficient NORR catalyst but also provides crucial insights into the fundamental parameters influencing NORR performance.

Enhanced Hydrosaturation Selectivity and Electron Transfer for Electrocatalytic Chlorophenols Hydrogenation on Ru Sites

Electrocatalytic hydrogenation is acknowledged as a promising strategy for chlorophenol dechlorination. However, the widely used Pd catalysts exhibit drawbacks, such as high costs and low selectivity for phenol hydrosaturation. Herein, we demonstrate the potential and mechanism of Ru in serving as a Pd substitute using 2,4,6-trichlorophenol (TCP) as a model pollutant. Up to 99.8% TCP removal efficiency and 99% selectivity to cyclohexanol, a value-added compound with an extremely low toxicity, were achieved on the Ru electrode. In contrast, only 66% of TCP was removed on the Pd electrode, with almost no hydrosaturation selectivity. The superiority of Ru over Pd was especially noteworthy in alkaline conditions or the presence of interfering species such as S2-. The theoretical simulation demonstrates that Ru possesses a hydrodechlorination energy barrier of 0.72 eV, which is comparable to that on Pd. Meanwhile, hydrosaturation requires an activation energy of 0.69 eV on Ru, which is much lower than that on Pd (0.92 eV). The main reaction mechanism on Ru is direct electron transfer, which is distinct from that on Pd (indirect pathway via atomic hydrogen, H*). This work thereby provides new insights into designing cost-effective electrocatalysts for halogenated phenol detoxification and resource recovery.

Electrochemical reduction of wastewater by non-noble metal cathodes: From terminal purification to upcycling recovery

A shift beyond conventional environmental remediation to a sustainable pollutant upgrading conversion is extremely desirable due to the rising demand for resources and widespread chemical contamination. Electrochemical reduction processes (ERPs) have drawn considerable attention in recent years in the fields of oxyanion reduction, metal recovery, detoxification and high-value conversion of halogenated organics and benzenes. ERPs also have the potential to address the inherent limitations of conventional chemical reduction technologies in terms of hydrogen and noble metal requirements. Fundamentally, mechanisms of ERPs can be categorized into three main pathways: direct electron transfer, atomic hydrogen mediation, and electrode redox pairs. Furthermore, this review consolidates state-of-the-art non-noble metal cathodes and their performance comparable to noble metals (e.g., Pd, Pt) in electrochemical reduction of inorganic/organic pollutants. To overview the research trends of ERPs, we innovatively sort out the relationship between the electrochemical reduction rate, the charge of the pollutant, and the number of electron transfers based on the statistical analysis. And we propose potential countermeasures of pulsed electrocatalysis and flow mode enhancement for the bottlenecks in electron injection and mass transfer for electronegative pollutant reduction. We conclude by discussing the gaps in the scientific and engineering level of ERPs, and envisage that ERPs can be a low-carbon pathway for industrial wastewater detoxification and valorization.

Transition Metal Single-Atom Catalysts for the Electrocatalytic Nitrate Reduction: Mechanism, Synthesis, Characterization, Application, and Prospects

Excessive accumulation of nitrate in the environment will affect human health. To combat nitrate pollution, chemical, biological, and physical technologies have been developed recently. The researcher favors electrocatalytic reduction nitrate reaction (NO3 RR) because of the low post-treatment cost and simple treatment conditions. Single-atom catalysts (SACs) offer great activity, exceptional selectivity, and enhanced stability in the field of NO3 RR because of their high atomic usage and distinctive structural characteristics. Recently, efficient transition metal-based SACs (TM-SACs) have emerged as promising candidates for NO3 RR. However, the real active sites of TM-SACs applied to NO3 RR and the key factors controlling catalytic performance in the reaction process remain ambiguous. Further understanding of the catalytic mechanism of TM-SACs applied to NO3 RR is of practical significance for exploring the design of stable and efficient SACs. In this review, from experimental and theoretical studies, the reaction mechanism, rate-determining steps, and essential variables affecting activity and selectivity are examined. The performance of SACs in terms of NO3 RR, characterization, and synthesis is then discussed. In order to promote and comprehend NO3 RR on TM-SACs, the design of TM-SACs is finally highlighted, together with the current problems, their remedies, and the way forward.

Breaking Local Charge Symmetry of Iron Single Atoms for Efficient Electrocatalytic Nitrate Reduction to Ammonia

The electrochemical conversion of nitrate pollutants into value-added ammonia is a feasible way to achieve artificial nitrogen cycle. However, the development of electrocatalytic nitrate-to-ammonia reduction reaction (NO3 - RR) has been hampered by high overpotential and low Faradaic efficiency. Here we develop an iron single-atom catalyst coordinated with nitrogen and phosphorus on hollow carbon polyhedron (denoted as Fe-N/P-C) as a NO3 - RR electrocatalyst. Owing to the tuning effect of phosphorus atoms on breaking local charge symmetry of the single-Fe-atom catalyst, it facilitates the adsorption of nitrate ions and enrichment of some key reaction intermediates during the NO3 - RR process. The Fe-N/P-C catalyst exhibits 90.3 % ammonia Faradaic efficiency with a yield rate of 17980 μg h-1  mgcat -1 , greatly outperforming the reported Fe-based catalysts. Furthermore, operando SR-FTIR spectroscopy measurements reveal the reaction pathway based on key intermediates observed under different applied potentials and reaction durations. Density functional theory calculations demonstrate that the optimized free energy of NO3 - RR intermediates is ascribed to the asymmetric atomic interface configuration, which achieves the optimal electron density distribution. This work demonstrates the critical role of atomic-level precision modulation by heteroatom doping for the NO3 - RR, providing an effective strategy for improving the catalytic performance of single atom catalysts in different electrochemical reactions.

Electronic Structure Optimization and Proton-Transfer Enhancement on Titanium Oxide-Supported Copper Nanoparticles for Enhanced Nitrogen Recycling from Nitrate-Contaminated Water

Electrocatalytic reduction of nitrate to NH₃ (NO3RR) on Cu offers sustainable NH₃ production and nitrogen recycling from nitrate-contaminated water. However, Cu affords limited NO3RR activity owing to its unfavorable electronic state and the slow proton transfer on its surface, especially in neutral/alkaline media. Furthermore, although a synchronous "NO3RR and NH₃ collection" system has been developed for nitrogen recycling from nitrate-laden water, no system is designed for natural water that generally contains low-concentration nitrate. Herein, we demonstrate that depositing Cu nanoparticles on a TiO₂ support enables the formation of electron-deficient Cu^δ+ species (0 < δ ≤ 2), which are more active than Cu⁰ in NO3RR. Furthermore, TiO₂-Cu coupling induces local electric-field enhancement that intensifies water adsorption/dissociation at the interface, accelerating proton transfer for NO3RR on Cu. With the dual enhancements, TiO₂-Cu delivers an NH₃-N selectivity of 90.5%, mass activity of 41.4 mg-N h g_Cu^-1, specific activity of 377.8 mg-N h^-1 m^-2, and minimal Cu leaching (<25.4 μg L^-1) when treating 22.5 mg L^-1 of NO₃^--N at -0.40 V, outperforming most of the reported Cu-based catalysts. A sequential NO3RR and NH₃ collection system based on TiO₂-Cu was then proposed, which could recycle nitrogen from nitrate-contaminated water under a wide concentration window of 22.5-112.5 mg L^-1 at a rate of 209-630 mg_N m^-2 h^-1. We also demonstrated this system could collect 83.9% of nitrogen from NO₃^--N (19.3 mg L^-1) in natural lake water.

Constructing Ru@C3 N4 /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 C₃ N₄ (g-C₃ N₄ ) encapsulated with self-supported Cu nanowires (denoted as Ru@C₃ N₄ /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.

Single-Atom Catalysts in Environmental Engineering: Progress, Outlook and Challenges

Recently, single-atom catalysts (SACs) have attracted wide attention in the field of environmental engineering. Compared with their nanoparticle counterparts, SACs possess high atomic efficiency, unique catalytic activity, and selectivity. This review summarizes recent studies on the environmental remediation applications of SACs in (1) gaseous: volatile organic compounds (VOCs) treatment, NO_x reduction, CO₂ reduction, and CO oxidation; (2) aqueous: Fenton-like advanced oxidation processes (AOPs), hydrodehalogenation, and nitrate/nitrite reduction. We present the treatment activities and reaction mechanisms of various SACs and propose challenges and future opportunities. We believe that this review will provide constructive inspiration and direction for future SAC research in environmental engineering.

Advances in iron-based electrocatalysts for nitrate reduction

Excessive nitrate has been a critical issue in the water environment, originating from the burning of fossil fuels, inefficient use of nitrogen fertilizers, and discharge of domestic and industrial wastewater. Among the effective treatments for nitrate reduction, electrocatalysis has become an advanced technique because it uses electrons as green reducing agents and can achieve high selectivity through cathode potential control. The effectiveness of electrocatalytic nitrate reduction (NO₃RR) mainly lies in the electrocatalyst. Iron-based catalysts have the advantages of high activity and low cost, which are well-used in the field of electrocatalytic nitrates. A comprehensive overview of the electrocatalytic mechanism and the iron-based materials for NO₃RR are given in terms of monometallic iron-based materials as well as bimetallic and oxide iron-based materials. A detailed introduction to NO₃RR on zero valent iron, single-atom iron catalysts, and Cu/Fe-based bimetallic electrocatalysts are provided, as they are essential for the improvement of NO₃RR performance. Finally, the advantages of iron-based materials for NO₃RR and the problems in current applications are summarized, and the development prospects of efficient iron-based catalysts are proposed.

Pulsed Nitrate-to-Ammonia Electroreduction Facilitated by Tandem Catalysis of Nitrite Intermediates

Electroreduction of nitrate to ammonia offers a promising pathway for nutrient recycling and recovery from wastewater with energy and environmental sustainability. There have been considerable efforts on the regulation of reaction pathways to facilitate nitrate-to-ammonia conversion over the competing hydrogen evolution reaction but only with limited success. Here, we report a Cu single-atom gel (Cu SAG) electrocatalyst that produces NH₃ from both nitrate and nitrite under neutral conditions. Given the unique mechanism of NO₂^- activation on Cu SAGs with spatial confinement and strengthened kinetics, a pulse electrolysis strategy is presented to cascade the accumulation and conversion of NO₂^- intermediates during NO₃^- reduction with the prohibited competition from the hydrogen evolution reaction, thus substantially enhancing the Faradaic efficiency and the yield rate for ammonia production compared with constant potential electrolysis. This work underlines the cooperative approach of the pulse electrolysis and SAGs with three-dimensional (3D) framework structures for highly efficient nitrate-to-ammonia conversion enabled by tandem catalysis of unfavorable intermediates.

Theoretical insights into dissociative-associative mechanism for enhanced electrochemical nitrate reduction to ammonia

The development of electrochemical nitrate reduction reaction (NO₃RR) is a "two birds-one stone" method, which can not only remove NO₃^- pollutant, but also produce valuable ammonia (NH₃). However, a mechanistic understanding of the nitrate reduction process remains very limited. Herein, we highlighted a dissociative-associative mechanism for the NO₃RR, in which the N-O bond of nitrate is initially broken to form *O and *NO₂ intermediate adsorbed on two active sites (dissociation process) and then subsequently hydrogenated and reduced to ammonia (association process). By taking a series of diatomic site catalysts (CuTM/g-CN and CuTM/N₆C, TM= Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) as models, we systematically investigate the dissociative-associative mechanism for the NO₃RR and compared with the Cu-based single-atom catalysts which follows the traditional directly associative mechanism. Density functional theory (DFT) calculations show that dissociative-associative mechanism is energetically favorable on seven catalysts (CuTi/g-CN, CuV/g-CN, CuMn/g-CN, CuCo/g-CN, CuV/N₆C, CuCr/N₆C and CuFe/N₆C) with the significantly reduced limiting potential of - 0.14 V to - 0.47 V. Specifically, an efficiently screening strategy was proposed to determine the dissociative-associative or directly associative mechanism for NO₃RR. This work can provide useful guideline for the rational design and development of NO₃RR electrocatalysts.

Recent Advances in Carbon-Based Materials for Adsorptive and Photocatalytic Antibiotic Removal

Antibiotics have been a primary environmental concern due to their widespread dispersion, harmful bioaccumulation, and resistance to mineralization. Unfortunately, typical processes in wastewater treatment plants are insufficient for complete antibiotic removal, and their derivatives in effluent can pose a threat to human health and aquatic communities. Adsorption and photocatalysis are proven to be the most commonly used and promising tertiary treatment methods. Carbon-based materials, especially those based on graphene, carbon nanotube, biochar, and hierarchical porous carbon, have attracted much attention in antibiotic removal as green adsorbents and photocatalysts because of their availability, unique pore structures, and superior physicochemical properties. This review provides an overview of the characteristics of the four most commonly used carbonaceous materials and their applications in antibiotic removal via adsorption and photodegradation, and the preparation of carbonaceous materials and remediation properties regarding target contaminants are clarified. Meanwhile, the fundamental adsorption and photodegradation mechanisms and influencing factors are summarized. Finally, existing problems and future research needs are put forward. This work is expected to inspire subsequent research in carbon-based adsorbent and photocatalyst design, particularly for antibiotics removal.