Potential-Driven Restructuring of Cu Single Atoms to Nanoparticles for Boosting the Electrochemical Reduction of Nitrate to Ammonia

Advances in electrocatalytic nitrate reduction to ammonia over Cu-based catalysts

Ammonia (NH3) is a critical basic material for both the agricultural and pharmaceutical industries. Traditionally, NH3 synthesis has relied on the Haber-Bosch process, which is characterized by high greenhouse gas emissions and stringent reaction conditions. As a more sustainable and cost-effective alternative, electrocatalytic NH3 synthesis has gained increasing attention. Nitrate (NO3-), a common pollutant in water and soil, is considered a promising nitrogen source for NH3 production due to its high solubility and relatively low N=O bond dissociation energy. This makes it particularly suitable for electrocatalytic nitrate reduction to ammonia (NRA), a process with significant potential for addressing nitrate pollution while contributing to NH3 production. However, challenges such as slow reaction kinetics and poor product selectivity persist in the NRA process. To overcome these challenges, the selection and optimization of catalysts are crucial for improving NRA performance. Among the various catalysts explored, copper-based (Cu) catalysts have attracted widespread attention due to their unique electronic structure and outstanding catalytic performance. This review provides a comprehensive analysis of the application and reaction mechanisms of Cu-based catalysts in NRA, along with an overview of testing systems and evaluation metrics used in the field. Additionally, it highlights current challenges and outlines future research directions to support the continued development of Cu-based materials for NRA applications.

Recent advances in metal single-atom catalysts for ammonia electrosynthesis

Electrochemical ammonia synthesis is a promising alternative to the Haber-Bosch process, offering significant potential for sustainable agricultural production and the development of portable, carbon-free energy carriers. The development of electrocatalytic systems is currently dependent on the exploration of electrocatalysts with high activity, selectivity, and stability. Metal single-atom catalysts (SACs) have become a new attractive frontier for ammonia electrosynthesis, owing to their maximized atom utilization, unsaturated atom coordination, and tunable electronic structure. In this review, we focused on different metal sites inside the single-atom catalysts and summarized recent advances in SACs for ammonia electrosynthesis. The properties of small nitrogenous substances (including N2, NO, NO2-, and NO3-) are summarized. In addition, the SACs for different catalytic systems are reviewed, with a particular focus on the special and common grounds of metal atom sites. Finally, the perspectives and challenges of SACs for ammonia electrosynthesis are comprehensively discussed, aspiring to provide insights into the development of electrochemical ammonia synthesis.

Low-dimensional materials for ammonia synthesis

Ammonia is an essential chemical due to its immense usage in agriculture, energy storage, and transportation. The synthesis of "green" ammonia via carbon-free routes and renewable energy sources is the need of the hour. In this context, photo- and/or electrocatalysis proves to be highly crucial. Low-dimensional materials (LDMs), owing to their unique properties, play a significant role in catalysis. This review presents a vast library of LDMs and broadly categorizes their catalytic performance according to their dimensionality, i.e., zero-, one-, and two-dimensional catalysts. The rational design of LDMs can significantly improve their catalytic performance, particularly in reducing small molecules like dinitrogen, nitrates, nitrites, and nitric oxides to synthesize ammonia via photo- and/or electrocatalysis. Additionally, converting nitrates and nitrites to ammonia can be beneficial in wastewater treatment and be coupled with CO2 co-reduction or oxidative reactions to produce urea and other valuable chemicals, which are also discussed in this review. This review collates the works published in recent years in this field and offers some fresh perspectives on ammonia synthesis. Through this review, we aim to provide a comprehensive insight into the catalytic properties of the LDMs, which are expected to enhance the efficiency of ammonia production and promote the synthesis of value-added products.

Enhanced Nitrate Reduction Performance of Cu-Doped Nanoporous Co2P Electrocatalyst

Electrocatalytic nitrate reduction to ammonia (NO3RR) is a promising approach to recycle nitrogen from nitrate pollutants, yet it remains challenged by low Faradaic efficiency and insufficient NH3 production. Herein, Cu-doped nanoporous Co2P (np-Co2-xCuxP) is reported as electrocatalyst for NO3RR, achieving an ammonia yield rate of 30.6 mg h-1 cm-2 with a Faradaic efficiency of 93.4% at -0.3 V vs. RHE. In-situ spectroscopic analyses indicate that Cu incorporation modifies the surface electronic structure, resulting in the promotion of *H adsorption and *NO2- hydrogenation, thereby facilitating efficient ammonia generation.

CuO/Bi2CuO4 Heterostructured Electrocatalyst for the Efficient Reduction of Nitrate to Ammonia

The electrochemical reduction of nitrate to ammonia (NH3) not only provides an effective approach to balance the perturbed nitrogen cycle for addressing environmental issues but also provides a potential technology for green NH3 synthesis. However, the process is limited by the produced intermediate-nitrite that tends to accumulate on cathode surfaces and multiple competing reactions. Herein, CuO/Bi2CuO4-450 heterostructures are reported as efficient electrocatalysts for the nitrate reduction reaction with extraordinary catalytic activities and selectivities for NH3 production. The optimized catalyst achieves a remarkable Faradaic efficiency (96.49%) and exceptional NH3 yield rate (9.17 mg h-1 mgcat.-1) at -0.5 V versus RHE, surpassing most of the reported Cu-based catalytic systems. The characterization results and theoretical evidence reveal that the interface effect originating from the strong interaction between Bi2CuO4 and CuO tunes the electronic structures of the Cu and Bi active sites for optimized intermediate adsorption and lowers the rate-determining step reaction barriers, resulting in improved catalytic performance. This work offers a strategy to flexibly develop catalysts to promote electrocatalytic techniques for NH3 production by electrochemical nitrate reduction.

One-Step Strategy to Maximize Single-Atom Catalyst Utilization in Nitrate Reduction via Bidirectional Optimization of Mass Transfer and Electron Supply

Single-atom catalysts offer exceptional performance but face practical challenges due to complex synthesis and low efficiency caused by mass transfer resistance. In this study, based on a simple one-step pyrolysis method, we designed a Cu single-atom catalyst with high active site exposure and a locally electron-deficient environment (HE Cu1-N4) to achieve maximum utilization efficiency in electrocatalytic nitrate reduction (NO3RR). Using advanced characterization techniques, we confirmed that its unique 3D structure enhances Cu atom exposure and reduces nitrate (NO3-) mass transfer resistance. Synchrotron radiation and DFT calculations showed that adjusting the coordination environment induces a local electron-deficient effect in Cu atoms, increasing the electrostatic attraction to NO3-. HE Cu1-N4 achieved 100% NH3 selectivity across a wide range of NO3- concentrations, with an NH3 yield (5.09 mg h-1 mgcat-1) nearly 7-fold higher than that of the conventional unmodified Cu single-atom catalyst (Cu1-N2, 0.73 mg h-1 mgcat-1). Under pilot-scale conditions, HE Cu1-N4 demonstrated strong resistance to interference and excellent stability in complex water systems. A simple modification method enhanced the utilization efficiency of single atoms in single-atom catalysts, significantly improving the catalytic activity of the material. Moreover, this straightforward synthesis strategy holds promise for the large-scale production of single-atom catalysts, paving the way for practical engineering applications.

Nitrate Electroreduction to Ammonia Over Copper-based Catalysts

The electrocatalytic reduction of nitrate (NO3 -) to ammonia (NH3) holds substantial promise, as it transforms NO3 - from polluted water into valuable NH3. However, the reaction is limited by sluggish kinetics and low NH3 selectivity. Cu-based catalysts with unique electronic structures demonstrate rapid NO3 - to NO2 - rate-determining step (RDS) and fast electrocatalytic nitrate reduction reaction (eNO3RR) kinetics among non-noble metal catalysts. Nonetheless, achieving high robustness and selectivity for NH3 with Cu-based catalysts remains a significant challenge. This review provides a comprehensive overview of the reaction mechanisms in eNO3RR, highlights how the structures of monometallic and bimetallic Cu-based catalyst affect catalytic properties, and discusses the current challenges as well as prospects in eNO3RR.

Boron and Oxygen Dual-Doped Carbon Nitride Nanotubes with Frustrated Lewis Pairs for Efficient Electrocatalytic Ammonia Synthesis

This work reports boron and oxygen dual-doped carbon nitride nanotubes (B/O-CNNTs) prepared via a copolymerization process for electrocatalytic ammonia synthesis from nitrogen gas (NRR) and nitrate (NO3RR) sources, respectively. By adjusting the dosage of boron oxide precursor, the texture and content of B/O dual dopants and the coordination environment in the resulting 1D CNNTs can be tuned. The best B/O-CNNTs can achieve maximum Faradaic efficiencies of 35% and 96% at -1.1 V versus RHE with corresponding ammonia yields of 16.7 and 211.4 µg h-1 mg-1, respectively. A comparatively higher selectivity is achieved in the NRR process compared to NO3RR. The B/O-induced coordinations boost electron transfer rates along the longitudinal axis. The presence of carbon vacancies and the unique 1D nanotubular structure enhance interactions among reactants. Concurrently, the formed frustrated Lewis pairs are pivotal in activating chemisorbed nitrogen gas or nitrate, resulting in notable accelerations of ammonia generation kinetics. In situ UV-vis spectroscopy reveals that the ideal potential of -1.1 V versus RHE facilitates the involvement of free electrons in the reaction, as it aligns with the conduction potential of B/O-CNNTs. This study paves the way for the design of non-metal-based electrocatalysts with dual dopants for sustainable electrocatalysis toward ammonia synthesis.

Breaking Linear Scaling Relation Limitations on a Dual-Driven Single-Atom Copper-Tungsten Oxide Catalyst for Ammonia Synthesis

Electrocatalytic reduction of nitrate (NO3 -, NO3RR) on single-atom copper catalysts (Cu-SACs) offers a sustainable approach to ammonia (NH3) synthesis using NO3 - pollutants as feedstocks. Nevertheless, this process suffers from inferior NO3RR kinetics and nitrite accumulation owing to the linear scaling relation limitations for SACs. To break these limitations, a single-atom Cu-bearing tungsten oxide catalyst (Cu1/WO3) was developed, which mediated a unique dual-driven NO3RR process. Specifically, WO3 dissociated water molecules and supplied the Cu1 site with ample protons, whereas the Cu1 site in an electron-deficient state converted NO3 - to NH3 efficiently. The Cu1/WO3 delivered an impressive NH3 production rate of 1274.4 mgN h-1 gCu -1, a NH3 selectivity of 99.2%, and a faradaic efficiency of 93.7% at -0.60 V, surpassing most reported catalysts. Furthermore, an integrated continuous-flow system consisting of a NO3RR cell and a vacuum-driven membrane separator was developed for NH3 synthesis from nitrate-contaminated water. Fed with the Yangtze River water containing ∼22.5 mg L-1 of NO3 --N, this system realized an NH3 production rate of 325.9 mgN h-1 gCu -1 and a collection efficiency of 98.3% at energy consumption of 17.11 kwh gN -1. This study provides a new dual-driven concept for catalyst design and establishes a foundation for sustainable NH3 synthesis from waste.

Dynamic electrode reconfiguration promotes in situ electrochemical peracetic acid synthesis for selective water decontamination

In situ synthesis and activation of peracetic acid (PAA) for water decontamination is a promising way to overcome the transport and storage problems in PAA applications. Here, an in situ electrochemical PAA synthesis and activation system is constructed using RuO2-Ti "active" electrode and graphite plate as the anode and the cathode, respectively. PAA is efficiently generated at the RuO2-Ti anode with a maximum real-time concentration of ∼1020 μM and a negligible precursor loss of 2.91 % after 180 min, and can be activated at the cathode to destruct a refractory pollutant (i.e., benzoic acid (BA)) with the rate constant of 0.22-0.28 h-1, even under the interference of co-existing anions. Multiple pieces of evidence, including differential electrochemical mass spectrometry, sulfoxide probing test, and electron paramagnetic resonance spectroscopy, indicate that the oxygen-atom-transferring oxidation of CH3COO- by a high-valent ruthenium-oxo intermediate (i.e., RuO3) in situ formed through the electrode reconfiguration between RuO2 and chem-sorbed HO• mainly accounts for PAA synthesis. Acetylperoxyl radical (CH3C(O)OO•) was evidenced as the dominant species for BA degradation. This study proposes an in situ strategy to electrochemically synthesize and activate PAA for selective water decontamination and enriches the understandings of the mechanism of "active" electrode in peroxide synthesis.

Revealing catalyst restructuring and composition during nitrate electroreduction through correlated operando microscopy and spectroscopy

Electrocatalysts alter their structure and composition during reaction, which can in turn create new active/selective phases. Identifying these changes is crucial for determining how morphology controls catalytic properties but the mechanisms by which operating conditions shape the catalyst's working state are not yet fully understood. In this study, we show using correlated operando microscopy and spectroscopy that as well-defined Cu2O cubes evolve under electrochemical nitrate reduction reaction conditions, distinct catalyst motifs are formed depending on the applied potential and the chemical environment. By further matching the timescales of morphological changes observed via electrochemical liquid cell transmission electron microscopy with time-resolved chemical state information obtained from operando transmission soft X-ray microscopy, hard X-ray absorption spectroscopy and Raman spectroscopy, we reveal that Cu2O can be kinetically stabilized alongside metallic copper for extended durations under moderately reductive conditions due to surface hydroxide formation. Finally, we rationalize how the interaction between the electrolyte and the catalyst influences the ammonia selectivity.

Potential-Driven Dynamic Spring-Effect of Pd─Cu Dual-Atoms Empowered Stability and Activity for Electrocatalytic Reduction

Atomic-level catalysts are extensively applied in heterogeneous catalysis fields. However, it is a general but ineluctable issue that active metal atoms may migrate, aggregate, deactivate, or leach during reaction processes, suppressing their catalytic performances. Designing superior intrinsic-structural stability of atomic-level catalysts with high activity and revealing their dynamic structure evolution is vital for their wide applications in complex reactions or harsh conditions. Herein, high-stable Pd─Cu dual-atom catalysts with PdN3─CuN3 coordination structure are engineered via strong chelation of Cu2+-ions with electron pairs from palladium-source, achieving the highest turnover frequency under the lowest overpotential for Cr(VI) electrocatalytic reduction detection in strong-acid electrolytes. In situ X-ray absorption fine structure spectra reveal dynamic "spring-effect" of Cu─Pd and Cu─N bonds that are reversibly stretched with potential changes and can be recovered at 0.6 V for regeneration. The modulated electron-orbit coupling effect of Pd─Cu pairs prevents Cu-atoms from aggregating as metallic nanoparticles. Pd─Cu dual-atoms interact with two O atoms of H2CrO4, forming stable bridge configurations and transferring electrons to promote Cr─O bond dissociation, which prominently decreases reaction energy barriers. This work provides a feasible route to boost the stability and robustness of metal single-atoms that are easily affected by reaction conditions for sustainable catalytic applications.

Rational Ligand Design of Conjugated Coordination Polymers for Efficient and Selective Nitrate Electroreduction to Ammonia

Electrocatalytic nitrate reduction to ammonia (NRA) offers an attractive route for converting nitrate pollutants to ammonia under mild conditions. Among other catalysts, single-atom catalysts (SACs) with high metal-atom-utilization efficiency and low-coordinated metal sites hold immense potential to be extensively applied, which unfortunately encounter a formidable challenge to obtain simultaneous improvement of NRA activity and selectivity. Here, a novel and general strategy is reported to achieve efficient and selective NRA catalysis on conjugated coordination polymers featuring with high-density and well-defined nitrogen (N)-coordinated single-atom metal sites via precise regulation of N‑heterocyclic ligands toward accelerating the hydrogenation kinetics necessitated in the NRA pathway. Taking cobalt (Co) as an example, two CoN4-centered conjugated coordination polymer electrocatalysts (CoN4-pyrr and CoN4-pyri) are synthesized with pyrrole and pyridine ligands are investigated as a proof-of-concept study. As revealed, the CoN4-pyrr can markedly outperform the CoN4-pyri toward NRA electrocatalysis. Experimental and theoretical results suggest that, relative to the N atoms of pyridine ligand in CoN4-pyri, the N atoms of pyrrole ligand in CoN4-pyrr can enable a faster transfer of hydrogen radicals to the Co active sites for accelerating the hydrogenation kinetics of *NO intermediate at the rate-determining step of NRA pathway.

Dynamic Structural Evolution of Active Sites and Effect on Heterogeneous Catalysis: Theoretical Insights

The exploration of structure-activity relationships is one of the key subjects for the rational design of heterogeneous catalysts. Advancements in characterization techniques have unveiled significant geometric and electronic structural changes at active sites under working conditions. This dynamic nature of active sites presents a formidable challenge in determining their structure-activity relationships. Over the past decade, considerable theoretical studies have concentrated on deciphering the complex catalytic processes that occur at these dynamic active sites under working conditions, with much success in methodology, models, and mechanisms. Herein, we review key factors influencing active site evolution, such as surface adsorbates, temperature, and electrochemical potential. We also provide an overview of recent progress in understanding the effects of active site evolution on their catalytic performance. A better understanding to the dynamic evolution of active sites and corresponding effects on the catalysis under working conditions can help in the rational design of novel heterogeneous catalysts.

Dynamic Structural Evolution of Single-Atom Catalysts at the Catalyst-Electrolyte Interface: Insights from Electrochemical Coupled Field

Dynamic catalytic structures at the catalyst-electrolyte interface pose significant challenges in accurately identifying active sites and establishing precise structure-activity relationships essential for catalyst design and performance optimization. Herein, we unveil the dynamic structural evolution of Cu-N-C single-atom catalysts (SACs) under electrochemical conditions, elucidating the critical role of the electrochemical coupled field. Using hybrid-solvation constant potential simulations, we identify that the unique dx2-y2 orbital occupancy at the Fermi level, stemming from copper's d9 electronic configuration, renders Cu-N bonds highly sensitive to external voltage. Proton transfer (PT) triggers electronic reordering that converts discrete energy levels into continuous states near the Fermi level, enhancing charge accumulation in the Cu-N antibonding state. Consequently, the Cu-N bonds are weakened, ultimately leading to copper atom leaching. Our work provides a fundamental understanding of SACs' dynamics under realistic electrochemical environments, offering new insights for the rational design of robust electrocatalysts.

Unlocking coordination sites of metal-organic frameworks for high-density and accessible copper nanoparticles toward electrochemical nitrate reduction to ammonia

Ordered pore engineering of metal-organic framework (MOF)-based catalysts by soft-template strategies can facilitate the mass transfer of reactants during heterogeneous electrocatalysis. Besides, the abundant open coordination sites generated by the removal of surfactants also open up a new avenue for incorporating active moieties within the framework; however, such studies are still limited. Herein, a mesoporous cerium-based MOF, MUiO-66(Ce), is synthesized by introducing a pluronic triblock copolymer as a template, where abundant open coordination sites are found to be present on the hexa-cerium nodes. By providing rich Ce-OH/Ce-OH2 sites, plenty of copper moieties are installed on the framework (denoted as Cu-MUiO-66(Ce)). After the in situ reduction process, a high density of copper nanoparticles is confined within MUiO-66(Ce), and Cu@MUiO-66(Ce) is thus obtained. With a high loading of active copper sites and efficient diffusion of reactants, the Cu@MUiO-66(Ce)-modified electrode can achieve an ammonia production rate of 1.875 mg h-1 mgcatalyst -1 and a faradaic efficiency of 88.7% for nitrate-to-ammonia reduction. Findings here shed light on the importance of pore engineering of MOF-based catalysts for unlocking open coordination sites and facilitating the mass transfer to enhance the electrocatalytic activity.

Photosynthesis of NH3 from NO3 - Using CH4 in Homogenous Rhenium Catalysis

We present Re catalysis of one-pot synthesis of NH3 from NO3 - using CH4 as a reductant under UV light irradiation at room temperature. NO3 - leads to environmental issues such as water contamination, eutrophication, and biodiversity loss. In a natural system and a sewage facility, NO3 - can be detoxified to N2 using hydrogen donors by a microbial process. However, it is better to convert NO3 - to NH3 rather than N2. To develop a desirable process of NO3 - to NH3 for a sustainable society, it is required that the reducing agent is derived from a renewable resource/energy. Thus, we turned our attention to CH4, which has a high affinity with renewable resource/energy since it can be produced from livestock waste. Therefore, our system reported here is to directly convert NO3 - to NH3 using CH4 with light energy.

Deactivation Mechanism and Mitigation Strategies of Single-Atom Site Electrocatalysts

Single-atom site electrocatalysts (SACs), with maximum atom efficiency, fine-tuned coordination structure, and exceptional reactivity toward catalysis, energy, and environmental purification, have become the emerging frontier in recent decade. Along with significant breakthroughs in activity and selectivity, the limited stability and durability of SACs are often underemphasized, posing a grand challenge in meeting the practical requirements. One pivotal obstacle to the construction of highly stable SACs is the heavy reliance on empirical rather than rational design methods. A comprehensive review is urgently needed to offer a concise overview of the recent progress in SACs stability/durability, encompassing both deactivation mechanism and mitigation strategies. Herein, this review first critically summarizes the SACs degradation mechanism and induction factors at the atomic-, meso- and nanoscale, mainly based on but not limited to oxygen reduction reaction. Subsequently, potential stability/durability improvement strategies by tuning catalyst composition, structure, morphology and surface are delineated, including construction of robust substrate and metal-support interaction, optimization of active site stability, fabrication of porosity and surface modification. Finally, the challenges and prospects for robust SACs are discussed. This review facilitates the fundamental understanding of catalyst degradation mechanism and provides efficient design principles aimed at overcoming deactivation difficulties for SACs and beyond.

Carbon-Supported Single Fe/Co/Ni Atom Catalysts for Water Oxidation: Unveiling the Dynamic Active Sites

Extensive research has been conducted on carbon-supported single-atom catalysts (SACs) for electrochemical applications, owing to their outstanding conductivity and high metal atom utilization. The atomic dispersion of active sites provides an ideal platform to investigate the structure-performance correlations. Despite this, the development of straightforward and scalable synthesis methods, along with the tracking of the dynamic active sites under catalytic conditions, remains a significant challenge. Herein, we introduce a biomass-inspired coordination confinement strategy to construct a series of carbon-supported SACs, incorporating various metal elements, such as Fe, Co, and Ni. We have systematically characterized their electronic and geometric structure using various spectroscopic and microscopic techniques. Through in situ X-ray absorption spectroscopy (XAS), atomic scanning transmission electron microscopy (STEM), and electron paramagnetic resonance (EPR) analyses, it is demonstrated that the single atoms undergo structural rearrangement to form amorphous (oxy)hydroxide clusters during oxygen evolution reaction (OER), where the newly formed oxygen-bridged dual metal M─O─M or M─O─M' (M/M' = Fe, Co, Ni) moieties within these clusters play key role in the OER performance. This work provides essential insights into tracking the actual active sites of SACs during electrochemical OER.

Structural evolution of metal single-atoms and clusters in catalysis: Which are the active sites under operative conditions?

The structural evolution of metal single-atoms and clusters has been recognized as the new frontier in catalytic reactions under operative conditions, playing a crucial role in key aspects of catalytic behavior, including activity, selectivity, stability, and atomic efficiency as well as precise tunability in heterogeneous catalysis. Accurately identifying the structural evolution of metal single-atoms and clusters during real reactions is essential for addressing fundamental issues such as active sites, metal-support interactions, deactivation mechanisms, and thereby guiding the design and fabrication of high-performance single-atom and cluster catalysts. However, how to evaluate the dynamic structural evolution of metal species during catalytic reactions is still lacking, hindering their industrial applications. In this review, we discuss the behaviors of dynamic structural evolution between metal single-atoms and clusters, explore the driving force and major factors, highlight the challenges and inherent limitations encountered, and present relevant future research trends. Overall, this review provides valuable insights that can inspire researchers to develop novel and efficient strategies for accurately identifying the structural transformations of metal single-atoms and clusters.

From Single Atoms to Clusters: Unraveling the Structural Evolution of Pt/CeO2 for Enhanced CO Oxidation

The structural evolution of catalysts and the identification of active sites are critical yet challenging aspects of heterogeneous reactions. In this work, we investigate the structural evolution of Pt/CeO2 catalysts during CO oxidation by using theoretical calculations, focusing on the influence of initial catalyst states on the resulting active sites and reactivity. Our findings reveal that under the reaction conditions, single Pt atoms gradually aggregate into Pt clusters. When single Pt atoms are substituted for surface Ce atoms (Ptin), the resulting small clusters (Ptn) are exclusively formed based on Ptin. However, when both Ptin and surface-adsorbed Pt atoms (Ptad) coexist, additional small surface-adsorbed clusters (Ptnad) are generated. An increase in the Ptad/Ptin ratio leads to a higher proportion of clusters at the active sites, which correlates with enhanced CO oxidation activity as the number of clusters increases. This study underscores the importance of understanding catalyst evolution and active site dynamics under the reaction conditions, providing theoretical insights for the rational design of more efficient catalysts.

Unraveling Dynamic Structural Evolution of Single Atom Catalyst via In Situ Surface-Enhanced Infrared Absorption Spectroscopy

Metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) have been widely applied in catalyzing electrochemical redox reactions. However, their long-term catalytic stabilities greatly limit their practical applications. This work investigates the dynamic evolution of two model Cu-N-C SACs with different Cu-N coordinations, namely the Cu1/Npyri-C and Cu1/Npyrr-C, in electrochemical CO reduction reaction (CORR), based on a collection of in situ characterizations including in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy, in situ X-ray absorption spectroscopy, quasi-in situ electron paramagnetic resonance spectroscopy and in situ ultraviolet-visible spectroscopy, complemented by theoretical calculations. Our findings reveal that the Cu nanoparticle formation rate over Cu1/Npyrr-C is more than 6 times higher than that over Cu1/Npyri-C during the electrochemical CORR. Quasi-in situ electron paramagnetic resonance and in situ UV-vis spectroscopy measurements demonstrate that hydrogen radicals can be in situ produced during electrochemical CORR, which will attack the Cu-N bonds in the Cu-N-C SACs, causing leaching of Cu2+ followed by subsequent reduction to form Cu nanoparticles. Kinetic calculations show that Cu1/Npyri-C displays a better catalytic stability than Cu1/Npyrr-C resulting from the stronger Cu-Npyri bonds. This study deepens the understanding of the deactivation mechanism of SACs in electrochemical reactions and provides guidance for the design of next-generation SACs with enhanced durability.

Dynamic surface reconstruction engineers interfacial water structure for efficient alkaline hydrogen oxidation

Investigating the dynamic evolution of the catalyst and regulating the structure of interfacial water molecules participating in the hydrogen oxidation reaction (HOR) are essential for developing highly efficient electrocatalysts toward the practical application of anion exchange membrane fuel cells. Herein, we report an efficient strategy to activate hexagonal close-packed PtSe catalyst through in situ reconstruction that undergoes dynamic Se leaching and phase transition during linear sweep voltammetry cycles. The obtained Pt-Se catalyst presents as a surface Se atom-modified face-centered-cubic Pt-based nanocatalyst, and it exhibited remarkable catalytic performance in the alkaline HOR, showing an intrinsic activity of 0.552 mA cm-2 (j 0,s) and a mass activity of 1.084 mA μg-1 (j k,m @ 50 mV). The experimental results, including in situ surface-enhanced infrared absorption spectroscopy and density functional theory calculations suggest that the accumulated electrons on the surface-decorated Se of Pt-Se can induce the regulation of the interfacial water structure between the electrode and electrolyte surface in the electric double-layer region. Consequently, the migration of OH- species from the electrolyte to the catalyst surface can be apparently accelerated within this disordered water network, which together with the optimized intermediate thermodynamic binding energies, contribute to the enhanced alkaline HOR activity.

Boosting Electrocatalytic Nitrate Reduction through Enhanced Mass Transfer in Cu-Bipyridine 2D Covalent Organic Framework Films

Electrocatalytic nitrate reduction (NO3RR) is a promising method for pollutant removal and ammonia synthesis and involves the transfer of eight electrons and nine protons. As such, the rational design of catalytic interfaces with enhanced mass transfer is crucial for achieving high ammonia yield rates and Faradaic efficiency (FE). In this work, we incorporated a Cu-bipyridine catalytic interface and fabricated crystalline 2D covalent organic framework films with significantly exposed catalytic sites, leading to improved FE and ammonia yield (FE=92.7 %, NH3 yield rate=14.9 mg ⋅ h-1cm-2 in 0.5 M nitrate) compared to bulk catalysts and outperforming most reported NO3RR electrocatalysts. The film-like morphology enhances mass transfer across the Cu-bipyridine interface, resulting in superior catalytic performance. We confirmed the reaction pathway and mechanism through in situ characterizations and theoretical calculations. The Cu sites act as primary centers for adsorption and activation, while the bipyridine sites facilitate water adsorption and dissociation, supplying sufficient H* and accelerating proton-coupled electron transfer kinetics. This study provides a viable strategy to enhance mass transfer at the catalytic interface through rational morphology control, boosting the intrinsic activity of catalysts in the NO3RR process.

Enhanced Activity and Stability for Electrocatalytic Nitrate Reduction to Ammonia over Low-Coordinated Cobalt

It is still challenging to develop an effective strategy to simultaneously enhance the activity and stability of electrocatalysts for electrocatalytic nitrate reduction reaction (eNO3RR). Herein, taking metallic cobalt as an example, it is demonstrated that the construction of low-coordinated cobalt nanosheets (L-Co NSs) by H2 plasma etching of electrodeposited cobalt nanosheets (Co NSs) can greatly enhance the activity and stability of metallic cobalt for eNO3RR. Compared with Co NSs, at -0.4 V versus RHE, the nitrate removal rate, ammonia partial current density, and ammonia yield are increased by L-Co NSs from 82.14% to 98.57%, from 476 to 683 mA cm-2, and from 2.11 to 2.54 mmol h-1 cm-2, respectively. In addition, L-Co NSs demonstrate negligible activity decay after 30 cycles of stability test, while the Co NSs show significant activity decline. In situ electrochemical characterizations and theoretical calculations verify that the abundance of Co vacancies in L-Co NSs not only contribute to the optimized electronic structure and enhanced desorption of key intermediate to boost the activity but also facilitate the transformation of Co(OH)2 to Co0 to promote the stability. Furthermore, L-Co NSs exhibit favorable performance in removing nitrate from simulated wastewater and air plasma discharge-electrocatalytic reduction cascade system to produce ammonia.

Destabilization of Single-Atom Catalysts: Characterization, Mechanisms, and Regeneration Strategies

Numerous in situ characterization studies have focused on revealing the catalytic mechanisms of single-atom catalysts (SACs), providing a theoretical basis for their rational design. Although research is relatively limited, the stability of SACs under long-term operating conditions is equally important and a prerequisite for their real-world energy applications, such as fuel cells and water electrolyzers. Recently, there has been a rise in in situ characterization studies on the destabilization and regeneration of SACs; however, timely and comprehensive summaries that provide the catalysis community with valuable insights and research directions are still lacking. This review summarizes recent advances in the destabilization mechanisms and regeneration strategies of SACs, specifically highlighting various state-of-the-art characterization techniques employed in the studies. The factors that induce destabilization in SACs are identified by discussing the failure of active sites, coordination environments, supports, and reaction conditions under long-term operating scenarios. Next, the primary regeneration strategies for SACs are introduced, including redispersion, surface poison desorption, and exposure of subsurface active sites. Additionally, the advantages and limitations of both in situ and ex situ characterization techniques are discussed. Finally, future research directions are proposed, aimed at constructing structure-stability relationships and guiding the design of more stable SACs.

Exploration of Multidimensional Structural Optimization and Regulation Mechanisms: Catalysts and Reaction Environments in Electrochemical Ammonia Synthesis

Ammonia (NH3) is esteemed for its attributes as a carbon-neutral fuel and hydrogen storage material, due to its high energy density, abundant hydrogen content, and notably higher liquefaction temperature in comparison to hydrogen gas. The primary method for the synthetic generation of NH3 is the Haber-Bosch process, involving rigorous conditions and resulting in significant global energy consumption and carbon dioxide emissions. To tackle energy and environmental challenges, the exploration of innovative green and sustainable technologies for NH3 synthesis is imperative. Rapid advances in electrochemical technology have created fresh prospects for researchers in the realm of environmentally friendly NH3 synthesis. Nevertheless, the intricate intermediate products and sluggish kinetics in the reactions impede the progress of green electrochemical NH3 synthesis (EAS) technologies. To improve the activity and selectivity of the EAS, which encompasses the electrocatalytic reduction of nitrogen gas, nitrate, and nitric oxide, numerous electrocatalysts and design strategies have been meticulously investigated. Here, this review primarily delves into recent progress and obstacles in EAS pathways, examining methods to boost the yield rate and current efficiency of NH3 synthesis via multidimensional structural optimization, while also exploring the challenges and outlook for EAS.

In situ reconstructed prism-like CuO on copper foam assisted by fumaric acid for an enhanced electrochemical nitrate reduction reaction

The electrochemical nitrate reduction reaction (NO3RR) is considered as a promising strategy for addressing environmental pollution and sustainable energy development. In this study, prism-like CuO loaded on copper foam (CuO/CF) was synthesized in a simple solvothermal reaction and an in situ electrochemical reconstruction process. The electrochemical reconstruction process facilitates the formation of a CuO lattice structure on copper foam derived from FU-CF generated by the reaction of copper foam and fumaric acid (H2FU) in DMF. The prism-like CuO on the surface of copper foam served as a remarkable electrocatalyst for the electrochemical nitrate reduction reaction, regulating the rate of electron transport and maintaining an ordered structure. The prism-like CuO lattice structure demonstrated a faradaic efficiency of 95.66%, an ammonia yield of 0.741 mmol h-1 cm-2 and an NH3 selectivity of 97.09%. This work provides a feasible strategy for fabricating Cu-based electrocatalysts for applications in green energy and environmental fields.

Copper-Catalysed Electrochemical CO2 Methanation via the Alloying of Single Cobalt Atoms

The electrochemical reduction of carbon dioxide (CO2) to methane (CH4) presents a promising solution for mitigating CO2 emissions while producing valuable chemical feedstocks. Although single-atom catalysts have shown potential in selectively converting CO2 to CH4, their limited active sites often hinder the realization of high current densities, posing a selectivity-activity dilemma. In this study, we developed a single-atom cobalt (Co) doped copper catalyst (Co1Cu) that achieved a CH4 Faradaic efficiency exceeding 60 % with a partial current density of -482.7 mA cm-2. Mechanistic investigations revealed that the incorporation of single Co atoms enhances the activation and dissociation of H2O molecules, thereby lowering the energy barrier for the hydrogenation of *CO intermediates. In situ spectroscopic experiments and density functional theory simulations further demonstrated that the modulation of the *CO adsorption configuration, with stronger bridge-binding, favours deep reduction to CH4 over the C-C coupling or CO desorption pathways. Our findings underscore the potential of Co1Cu catalysts in overcoming the selectivity-activity trade-off, paving the way for efficient and scalable CO2-to-CH4 conversion technologies.

Highly Accessible Electrocatalyst with In Situ Formed Copper-Cluster Active Sites for Enhanced Nitrate-to-Ammonia Conversion

Ammonia synthesis via nitrate electroreduction is more attractive and sustainable than the energy-extensive Haber-Bosch process and intrinsically sluggish nitrogen electroreduction. Herein, we have designed a single-site Cu catalyst on hierarchical nitrogen-doped carbon nanocage support (Cu1/hNCNC) for nitrate electroreduction, which achieves an ultrahigh ammonia yield rate (YRNH3) of 99.4 mol h-1 gCu-1 (2.30 mol h-1 gcat.-1) with ammonia Faradaic efficiency (FENH3) of 99.3%, far beyond the most reported single-site catalysts on carbon-based supports. The combined operando characterization and theoretical studies indicate that the in situ formed Cu-cluster active sites are responsible for the high YRNH3 and FENH3 due to the enhanced NO3- adsorption and subsequent protonation on the unique Cu3-N4 moieties, and meanwhile, the hierarchical hNCNC support facilitates the mass/charge transfer kinetics, thus promoting the high expression of intrinsic activity. The demonstration of plasma N2 oxidization and nitrate electroreduction cascade reaction manifests the great potential of the Cu1/hNCNC electrocatalyst in sustainable NH3 synthesis. These findings offer valuable insights into the design of effective catalysts for electrosynthetic reactions.

Breaking linear scaling relationships in oxygen evolution via dynamic structural regulation of active sites

The universal linear scaling relationships between the adsorption energies of reactive intermediates limit the performance of catalysts in multi-step catalytic reactions. Here, we show how these scaling relationships can be circumvented in electrochemical oxygen evolution reaction by dynamic structural regulation of active sites. We construct a model Ni-Fe2 molecular catalyst via in situ electrochemical activation, which is able to deliver a notable intrinsic oxygen evolution reaction activity. Theoretical calculations and electrokinetic studies reveal that the dynamic evolution of Ni-adsorbate coordination driven by intramolecular proton transfer can effectively alter the electronic structure of the adjacent Fe active centre during the catalytic cycle. This dynamic dual-site cooperation simultaneously lowers the free energy change associated with O-H bond cleavage and O-O bond formation, thereby disrupting the inherent scaling relationship in oxygen evolution reaction. The present study not only advances the development of molecular water oxidation catalysts, but also provides an unconventional paradigm for breaking the linear scaling relationships in multi-intermediates involved catalysis.

Understanding the Dynamic Evolution of Active Sites among Single Atoms, Clusters, and Nanoparticles

Catalysis remains a cornerstone of chemical research, with the active sites of catalysts being crucial for their functionality. Identifying active sites, particularly during the reaction process, is crucial for elucidating the relationship between a catalyst's structure and its catalytic property. However, the dynamic evolution of active sites within heterogeneous metal catalysts presents a substantial challenge for accurately pinpointing the real active sites. The advent of in situ and operando characterization techniques has illuminated the path toward understanding the dynamic changes of active sites, offering robust scientific evidence to support the rational design of catalysts. There is a pressing need for a comprehensive review that systematically explores the dynamic evolution among single atoms, clusters, and nanoparticles as active sites during the reaction process, utilizing in situ and operando characterization techniques. This review aims to delineate the effects of various reaction factors on dynamic evolution of active sites among single atoms, clusters, and nanoparticles. Moreover, several in situ and operando techniques are elaborated with emphases on tracking the dynamic evolution of active sites, linking them to catalytic properties. Finally, it discusses challenges and future perspectives in identifying active sites during the reaction process and advancing in situ and operando characterization techniques.

Electrochemical Selective Nitrate Reduction: Pathways to Nitrogen and Ammonia Production

Nitrate (NO3 -) contamination from industrial, agricultural, and anthropogenic activities poses significant risks to human health and ecosystems. While traditional NO3 - remediation methods are effective, they often generate secondary pollutants and incur high costs. Electrochemical NO3 -reduction (ECNR) offers a sustainable alternative, converting NO3 - into environmentally benign nitrogen (N2) or valuable ammonia (NH3). This review explores recent advancements in selective ECNR pathways for NO3 --to-N2and NO3 --to-NH3 conversion, focusing on mechanistic insights, electrocatalyst development, and optimization strategies. Key factors influencing ECNR performance, such as electrode materials, electrolyte composition, and hydrogen evolution inhibition, are discussed. Additionally, the review highlights the role of single-atom, bimetallic, and nanostructured catalysts in enhancing faradaic efficiency, total N2 removal, and selectivity, with particular attention to Pd-Cu systems. Strategies to address challenges like low selectivity and catalyst degradation are also explored. This review underscores the potential of ECNR as a viable alternative to the energy-intensive Haber-Bosch process for NH3 production, aligning with global sustainability goals. Finally, we identify research gaps and propose future directions for improving the efficiency, stability, and scalability of ECNR technologies.

Oxidative reconstructed Ru-based nanoclusters forming heterostructures with lanthanide oxides for acidic water oxidation

Achieving rapid anodic oxygen evolution reaction (OER) kinetics and improving the stability of the corresponding ruthenium (Ru)-based catalysts is a current priority for the realisation of industrial water splitting. However, the activity and stability of O2 evolution in electrocatalysis are largely inhibited by the insufficient adsorption of the reactant H2O and too strong adsorption of the intermediate OOH*, as well as by the dissolution of the active site due to excessive oxidation. To solve this challenge, herein, we developed a regulatory strategy combining lanthanide oxides and metal oxidative reconfiguration. The introduction of Eu2O3 effectively promotes the adsorption of H2O, optimizes the adsorption energy of OOH*, and reduces the reaction energy barrier of acidic OER process. And the metal oxidation remodeling process exposed more active sites and prevented the peroxidation process. The optimized Ru/Eu2O3@CNT catalyst showed the highest catalytic activity and stability in acidic OER. Its mass activity was 1219.1 A gRu-1 and the TOF value reached 4.4 s-1 at 1.48 V. Additionally, Ru/Eu2O3@CNT after oxidative reconstruction demonstrates the industrially needed current density of 1.0 A cm-2 at 1.71 V in PEM electrolyser, achieving stability in excess of 200 h.

Precise structural regulation of copper-based electrocatalysts for sustainable nitrate reduction to ammonia

The electrocatalytic reduction of nitrate to ammonia (NRA) technology not only achieves the effective removal of nitrates in the environment but also produces value-added products-NH3. In recent years, copper-based materials have shown tremendous application prospects in this field due to their excellent conductivity, moderate cost, and their proximity of d orbital energy levels to the LUMO π∗ molecular orbitals of nitrate. This review starts with copper-based catalysts to elucidate the reaction mechanisms of NRA and its influencing factors, while summarizing and analyzing the principles and pros and cons of various modification strategies. Then, we will explore the impact of different modification strategies on improving NRA performance and the underlying theoretical mechanisms. Finally, this review proposes the current challenges and prospects of copper-based materials, aiming to provide a reference for the further development and industrial application of copper-based catalysts.

Cascade Design and Facile Fabrication of Cu/Cu2O/CuAl-Layered Double Hydroxides as Efficient Nitrate Reduction Electrocatalysts

Nitrate (NO3¯) reduction reaction (NITRR) presents a promising pathway for the production of renewable NH3 while concurrently decontaminating NO3¯ wastewater. However, the multi-electron transfer sequence and complex reaction network involved in NO3¯ conversion pose significant challenges to achieving high Faradaic efficiency (FE). Herein, this work presents ternary Cu/Cu2O/CuAl-layered double hydroxides (LDHs) catalysts designed through a cascade approach and synthesized via a straightforward one-step electrodeposition method. The resulting catalysts demonstrate peak activity at -0.4 V versus RHE, achieving an impressiveF E N H 3 $F{{E}_{N{{H}_3}}}$ of 92.0%, which significantly surpasses most reported binary and ternary catalysts. Density functional theory calculations and atomic force microscopy reveal that the Cu/Cu2O/CuAl-LDHs exploit cascade design by integrating three distinct functions essential for efficient NO3¯ reduction: CuAl-LDH initiates NO3¯ adsorption, Cu(111) and Cu₂O(111) cooperatively facilitate NO3¯ activation, and Cu(111) promotes NH3 desorption. Durability tests further confirm that both NH3 yield andF E N H 3 $F{{E}_{N{{H}_3}}}$ remain stable after 10 cycles, indicating the excellent stability of the Cu/Cu2O/CuAl-LDHs catalysts. These findings underscore the critical role of cascade design strategies in enhancing the performance of electrocatalysts for NO3¯ reduction to NH3, providing a transformative approach for sustainable ammonia synthesis.

NiO-Incorporated Cu/Cu2O Nanowires for Highly Efficient Electrochemical Nitrate Reduction to Ammonia

Electrochemical nitrate reduction to ammonia (NRA) is a promising sustainable way to synthesize ammonia (NH3) from nitrate (NO3 -) contaminants. Cu-based electrocatalysts are frequently utilized for NRA due to their strong NO3 - adsorption and de-oxygenation ability. However, this kind of catalyst usually possesses the weak water dissociation ability, resulting in insufficient proton supply in alkaline media to retard the following hydrogenation step of O-containing intermediates (*NOx, typically NO2 -) to target NH3. Herein, NiO-incorporated Cu/Cu2O nanowires grown on nickel foam (p-CuNi@NF, p refers to plasma treatment) were synthesized via hydrothermal growth and subsequent O2 plasma treatment for efficient NRA electrocatalysis. On this p-CuNi@NF catalyst, NiO is able to accelerate the hydrogenation step by promoting the water dissociation to provide protons, ultimately facilitating efficient NRA. p-CuNi@NF exhibits excellent NH3 selectivity and yield in a wide potential range and reaches a high Faradaic efficiency (FENH3) of 97.5 % and a yield (YNH3) of 470 μmol h-1 cm-2 at -0.6 V, both of which largely surpass the Cu/Cu2O catalyst.

Reversible Hydrogen Acceptor-Donor Enables Relay Mechanism for Nitrate-to-Ammonia Electrocatalysis

Electrocatalytic nitrate reduction is a crucial process for sustainable ammonia production. However, to maximize ammonia yield efficiency, this technology inevitably operates at the potentials more negative than 0 V vs. RHE, leading to high energy consumption and competitive hydrogen evolution. To eradicate this issue, hydrogen tungsten bronze (HxWO3) as reversible hydrogen donor-acceptor is partnered with copper (Cu) to enable a relay mechanism at potentials positive than 0 V vs. RHE, which involves rapid intercalation of H into HxWO3 lattice, prompt de-intercalation of the lattice H and transfer onto Cu, and spontaneous H-mediated nitrate-to-ammonia conversion on Cu. The resulting catalysts demonstrated a high ammonia yield rate of 3332.9±34.1 mmol gcat -1 h-1 and a Faraday efficiency of ~100 % at 0.10 V vs. RHE, displaying a record-low estimated energy consumption of 17.6 kWh kgammonia -1. Using these catalysts, we achieve continuous ammonia production in an enlarged flow cell at a real energy consumption of 17.0 kWh kgammonia -1.

Dynamic evolution of metal-nitrogen-codoped carbon catalysts in electrocatalytic reactions

Atomic metal-nitrogen-codoped carbon (M-N-C) catalysts are highly efficient for various electrocatalytic reactions because of their high atomic utilization efficiency. However, the high surface energy of M-N-C catalysts often results in stability issues in electrochemical reactions. Therefore, understanding the stability and dynamic evolution of M-N-C catalysts is crucial for elucidating the active centers and the composition/structure-activity relationship. This review summarizes the factors affecting the durability of atomic catalysts in electrochemical reactions and discusses possible changes in catalysts during these electrochemical processes. Finally, advanced characterization techniques are described, with a focus on tracking the dynamic evolution of M-N-C catalysts during electrocatalysis. This review offers insights into the rational optimization of M-N-C electrocatalysts and provides a framework for linking their composition and structure with their catalytic activity in future research.

Advanced systems for enhanced CO2 electroreduction

Carbon dioxide (CO2) electroreduction has extraordinary significance in curbing CO2 emissions while simultaneously producing value-added chemicals with economic and environmental benefits. In recent years, breakthroughs in designing catalysts, optimizing intrinsic activity, developing reactors, and elucidating reaction mechanisms have continuously driven the advancement of CO2 electroreduction. However, the industrialization of CO2 electroreduction remains a challenging task, with high energy consumption, high costs, limited reaction products, and restricted application scenarios being the issues that urgently need to be addressed. To accelerate the progress of CO2 electroreduction towards practical application, this review shifts the research focus from catalysts to aspects such as reactions and systems, aiming to improve reaction efficiency, reduce technical costs, expand the range of products, and enhance selectivity, offering readers a new perspective. In particular, innovative and specific design strategies such as CO2 reduction coupled with alternative oxidation, co-reduction reaction of CO2 and C/N/O/S-containing species, cascade systems, and integrated CO2 capture and reduction systems are discussed in detail. Additionally, personal views on the opportunities and future challenges of the aforementioned innovative strategies are provided, offering new insights for the future research and development of CO2 electroreduction.

Construction of dual sites on FeS2 surface for enhanced electrocatalytic reduction of nitrite to ammonia

Cost-effective iron sulfides (FeS2) hold great potential as high-performance catalysts for NO2- electroreduction to NH3 (NO2ER), which is hindered by the weak NO2 activation. Herein, the design of nonmetal-doped FeS2 electrocatalysts was initially conducted by density functional theory (DFT) computations. We found that doping with different nonmetal atoms effectively not only regulates the electronic structures of the d-electrons of Fe atoms but also creates the unique p-d hybridized dual active sites, thereby boosting the efficient NO2 activation. Owing to the optimal NO2 adsorption strength, N-doped FeS2 demonstrates a low limiting potential for the NO2--to-NH3 conversion, thus significantly improving NO2ER activity. Direct experimental evidence was provided afterward: an NH3 yield rate of 424.5 μmol/hcm-2 with a 92.4 % Faradaic efficiency was achieved. Our findings not only suggest a promising NO2ER catalyst through theoretical computations to guide experiments but also provide a comprehensive understanding of the structure-properties relationship.

Unveiling Ionized Interfacial Water-Induced Localized H* Enrichment for Electrocatalytic Nitrate Reduction

Electrocatalytic nitrate reduction reaction (NO3RR) is a process that requires the participation of eight electrons and nine protons. The regulation of active hydrogen (H*) supply and a deep understanding of related processes are necessary for improving the ammonia yield rate and Faradaic efficiency (FE). Herein, we synthesized a series of atomically precise copper-halide clusters Cu2X2(BINAP)2 (X=Cl, Br, I), among which the Cu2Cl2(BINAP)2 cluster shows the optimal ammonia FE of 94.0 % and an ammonia yield rate of 373 μmol h-1 cm-2. In situ experiments and theoretical calculations reveal that halogen atoms, especially Cl in Cu2Cl2(BIANP)2, can significantly affect the distance of alkali metal-ionized water on the catalyst surface, which can promote the water dissociation to enhance the localized H* enrichment for the continues hydrogenation of nitrate to ammonia. This work explains the role of H* in the hydrogenation process of NO3RR and the importance of localized H* enrichment strategy for improving the FEs.

Nickel-based dual single atom electrocatalysts for the nitrate reduction reaction

Electrochemical nitrate (NO3-) reduction reaction (NO3-RR) to ammonium (NH4+) or nitrogen (N2) provides a green route for nitrate remediation. However, nitrite generation and hydrogen evolution reactions hinder the feasibility of the process. Herein, dual single atom catalysts were rationally designed by introducing Ag/Bi/Mo atoms to atomically dispersed NiNC moieties supported by nitrogen-doped carbon nanosheet (NCNS) for the NO3-RR. Ni single atoms loaded on NCNS (Ni/NCNS) tend to reduce NO3- to valuable NH4+ with a high selectivity of 77.8 %. In contrast, the main product of NO3-RR catalyzing by NiAg/NCNS, NiBi/NCNS, and NiMo/NCNS was changed to N2, giving rise to N2 selectivity of 48.4, 47.1 and 47.5 %, respectively. Encouragingly, Ni/NCNS, NiBi/NCNS, and NiAg/NCNS showed excellent durability in acidic electrolytes, leading to nitrate conversion rates of 70.3, 91.1, and 93.2 % after a 10-h reaction. Simulated wastewater experiments showed that NiAg/NCNS could remove NO3- up to 97.8 % at -0.62 V after 9-h electrolysis. This work afforded a new strategy to regulate the reaction pathway and improve the conversion efficiency of the NO3-RR via engineering the dual atomic sites of the catalysts.

Preventing Nitrite Desorption via Switching Hydrogenation Position: A Dual-Site Approach for Selective Nitrate Reduction to Ammonia

The electrochemical nitrate reduction reaction (NO3RR), which converts harmful nitrates into valuable ammonia (NH3) with zero carbon emission, is one of the most promising alternatives to the Haber-Bosch process. However, the NO3RR process is complex and involves multiple proton-coupled electron transfers that generate intermediates or byproducts, such as NO2 -, resulting in low ammonia yields and faradaic efficiency (FE). Herein, by constructing a FeCu bimetallic catalyst (FeCu-NC), the hydrogenation position of *NO3 is switched at the FeCu dual-atom site, preventing the desorption of *NO2 intermediate. Furthermore, electron transfer from Cu to Fe sites mimics the electron flow direction in natural nitrite reductase enzymes and accelerates the reduction of *NO2 to NH3, achieving efficient conversion of NO3 - to NH3. A 24-hour electrocatalytic experiment with FeCu-NC demonstrates negligible NO2 - formation throughout the NO3RR process, with an ammonia production rate of 6.13 mg h-1 mgcat -1 and an impressive FE of 95%, which are remarkably superior in comparison to most of the NO3RR electrocatalysts. This work opens new avenues for the fundamental understanding of catalytic mechanisms and the development of next-generation catalysts for sustainable ammonia production.

Atomic-level tailoring of single-atom tungsten catalysts for optimized electrochemical nitrate-to-ammonia conversion

Nitrate contamination of water resources poses significant health and environmental risks, necessitating efficient denitrification methods that produce ammonia as a desirable product. The electrocatalytic nitrate reduction reaction (NO3RR) powered by renewable energy offers a promising solution, however, developing highly active and selective catalysts remains challenging. Single-atom catalysts (SACs) have shown impressive performance, but the crucial role of their coordination environment, especially the next-nearest neighbor dopant atoms, in modulating catalytic activity for NO3RR is underexplored. This study aims to optimize the NO3RR performance of tungsten (W) single atoms anchored on graphene by precisely engineering their coordination environment through first and next-nearest neighbor dopants. The stability, reaction paths, activity, and selectivity of 43 different nitrogen and boron doping configurations were systematically studied using density functional theory. The results reveal W@C3, with W coordinated to three carbon atoms, exhibits outstanding NO3RR activity with a low limiting potential of -0.36 V. Intriguingly, introducing next-nearest neighbor B and N dopants further enhances the performance, with W@C3-BN achieving a lower limiting potential of -0.26 V. This exceptional activity originates from optimal nitrate adsorption strengths facilitated by orbital hybridization and charge modulation effects induced by the dopants. Furthermore, high energy barriers for NO2 and NO formation on W@C3 and W@C3-BN ensure their selectivity towards NO3RR products. These findings provide crucial atomic-level insights into rational design strategies for high-performance single-atom NO3RR catalysts via coordination environment engineering.

Boosting electrocatalytic nitrate-to-ammonia of single Fe active sites via coordination engineering: From theory to experiments

Atomically dispersed iron-nitrogen-carbon (Fe-N4-C) catalysts show great promises for the electrocatalytic nitrate (NO3-) reduction to ammonia (NH3). Nevertheless, the microenvironmental engineering of the single Fe active sites for further optimizing the catalytic performance remains a challenge. Herein, we proposed to regulate the coordination environment of single Fe active sites to boost its intrinsic electrocatalytic activity for NO3- -to-NH3 conversion by the incorporation of new heteroatoms, including B, C, O, Si, P, and S. Our results revealed that most of the candidates possess low formation energies, showing great potential for experimental synthesis. Moreover, incorporating heteroatoms effectively modulates the charge redistribution and the d-band center of single Fe active sites, enabling the regulation of the binding strength of nitrogenous intermediates. As a result, the N and C coordinated Fe active site (Fe-N3C) exhibits superior catalytic performance for NO3- electroreduction with a relatively low limiting potential (-0.13 V) due to its optimal adsorption strength with nitrogenous intermediates induced by its moderate charge and d-band center. Importantly, our experimental measures confirmed such theoretical prediction: a maximum NH3 yield rate of 21.07 mg h-1 mgcat.-1 and 95.74 % Faradaic efficiency were achieved for NO3- electroreduction on Fe-N3C catalyst. These findings not only suggest a highly efficient catalyst for nitrate reduction but also provide insight into how to design and prepare electrocatalysts with enhanced catalytic performance.

High-efficiency electrochemical nitrate reduction to ammonia via boron-doped hydroxyl oxide cobalt induced electron delocalization

Electrochemical nitrate reduction to ammonia is a promising alternative strategy for producing valuable ammonia. This prospective route, however, is subject to a slow electrocatalytic rate, which resulted from the weak adsorption and activation of intermediate species, and the low density electron cloud of active centers. To address this issue, we developed a novel approach by doping boron into metal hydroxyl oxides to adjust the electronic structure of active centers, and consequently, led a significant improvement in the Faraday efficiency upto approaching 100 %, as well as an impressive ammonia yield upto approximately 23 mg/h mgcat-1 at -0.6 V vs. reversible hydrogen electrode (RHE). Experimental data in mechanism demonstrate that the doped boron play a crucial role in modulating the local electronic environment surrounding the active sites Co. In situ Raman and FTIR spectra provide evidences that boron facilitates the formation of deoxidation and hydrogenation intermediates. Additionally, density functional theory (DFT) calculations support the notion that boron doping enhances the adsorption capability of intermediates, reduces the reaction barrier, and facilitates the desorption of NH3.

Progress and challenges in structural, in situ and operando characterization of single-atom catalysts by X-ray based synchrotron radiation techniques

Single-atom catalysts (SACs) represent the ultimate size limit of nanoscale catalysts, combining the advantages of homogeneous and heterogeneous catalysts. SACs have isolated single-atom active sites that exhibit high atomic utilization efficiency, unique catalytic activity, and selectivity. Over the past few decades, synchrotron radiation techniques have played a crucial role in studying single-atom catalysis by identifying catalyst structures and enabling the understanding of reaction mechanisms. The profound comprehension of spectroscopic techniques and characteristics pertaining to SACs is important for exploring their catalytic activity origins and devising high-performance and stable SACs for industrial applications. In this review, we provide a comprehensive overview of the recent advances in X-ray based synchrotron radiation techniques for structural characterization and in situ/operando observation of SACs under reaction conditions. We emphasize the correlation between spectral fine features and structural characteristics of SACs, along with their analytical limitations. The development of IMST with spatial and temporal resolution is also discussed along with their significance in revealing the structural characteristics and reaction mechanisms of SACs. Additionally, this review explores the study of active center states using spectral fine characteristics combined with theoretical simulations, as well as spectroscopic analysis strategies utilizing machine learning methods to address challenges posed by atomic distribution inhomogeneity in SACs while envisaging potential applications integrating artificial intelligence seamlessly with experiments for real-time monitoring of single-atom catalytic processes.

Electrodegradation of nitrogenous pollutants in sewage: from reaction fundamentals to energy valorization applications

The excessive accumulation of nitrogen pollutants (mainly nitrate, nitrite, ammonia nitrogen, hydrazine, and urea) in water bodies seriously disrupts the natural nitrogen cycle and poses a significant threat to human life and health. Electrolysis is considered a promising method to degrade these nitrogenous pollutants in sewage, with the advantages of high efficiency, wide generality, easy operability, retrievability, and environmental friendliness. For particular energy devices, including metal-nitrate batteries, direct fuel cells, and hybrid water electrolyzers, the realization of energy valorization from sewage purification processes (e.g., valuable chemical generation, electricity output, and hydrogen production) becomes feasible. Despite the progress in the research on pollutant electrodegradation, the development of electrocatalysts with high activity, stability, and selectivity for pollutant removal, coupled with corresponding energy devices, remains a challenge. This review comprehensively provides advanced insights into the electrodegradation processes of nitrogenous pollutants and relevant energy valorization strategies, focusing on the reaction mechanisms, activity descriptors, electrocatalyst design, and actuated electrodes and operation parameters of tailored energy conversion devices. A feasibility analysis of electrodegradation on real wastewater samples from the perspective of pollutant concentration, pollutant accumulation, and electrolyte effects is provided. Challenges and prospects for the future development of electrodegradation systems are also discussed in detail to bridge the gap between experimental trials and commercial applications.

Copper-nickel-MOF/nickel foam catalysts grown in situ for efficient electrochemical nitrate reduction to ammonia

Reducing nitrate (NO3-) in an aqueous solution to ammonia under ambient conditions can provide a green and sustainable NH3-synthesis technology and mitigate global energy and pollution issues. In this work, a CuNi0.75-1,3,5-benzenetricarboxylic acid/nickel foam (CuNi0.75-MOF/NF) catalyst grown in situ was prepared via a one-pot method as an efficient cathode material for electrocatalytic nitrate reduction reaction (NO3RR). The CuNi0.75-MOF/NF catalyst exhibited excellent electrocatalytic NO3RR performance at -1.0 V versus a reversible hydrogen electrode, achieving an outstanding faradaic efficiency of 95.88 % and an NH3 yield of 51.78 mg h-1 cm-2. The 15N isotope labeling experiments confirmed that the sole source of N in the electrocatalytic NO3RR was the NO3- in the electrolyte. The reaction pathway for the electrocatalytic NO3RR was derived by in situ Fourier transform infrared spectroscopy and in situ differential electrochemical mass spectrometry. Density functional theory calculations revealed that the Ni element in the CuNi0.75-MOF/NF catalyst had excellent O-H activation ability and strong *H adsorption capacity. These *H species were transferred from the Ni sites to the *NO adsorption intermediates located on the Cu sites, providing a continuous supply of *H to Cu, thereby promoting the formation of *NOH intermediates and enhancing the hydrogenation process of the electrocatalytic NO3RR.