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

Purification and Value-Added Conversion of NOx under Ambient Conditions with Photo-/Electrocatalysis Technology

As primary air pollutants from fossil fuel combustion, the excess emission of nitric oxides (NOx) results in a series of atmospheric environmental issues. Although the selective catalytic reduction technology has been confirmed to be effective for NOx removal, green purification and value-added conversion of NOx under ambient conditions are still facing great challenges, especially for nitrogen resource recovery. To address that, photo-/electrocatalysis technology offers sustainable routes for efficient NOx purification and upcycling under ambient temperature and pressure, which has received considerable attention from scientific communities. In this review, recent advances in photo-/electrocatalysis technology for the purification and value-added conversion of NOx are critically summarized. The target products and reaction mechanisms for NOx conversion systems, together with the responsible active sites, are discussed, respectively. Then, the realistic environmental practicability is proposed, including strict performance evaluation criteria and application in realistic conditions for NOx purification and upcycling by the application of photo-/electrocatalysis. Finally, the current challenges and future opportunities are proposed in terms of catalyst design, NOx conversion enhancement, reaction mechanism understanding, practical application conditions, and product separation techniques.

In situ evolution of electrocatalysts for enhanced electrochemical nitrate reduction under realistic conditions

Electrochemical nitrate reduction to ammonia (ENRA) is gaining attention for its potential in water remediation and sustainable ammonia production, offering a greener alternative to the energy-intensive Haber-Bosch process. Current research on ENRA is dedicated to enhancing ammonia selectively and productivity with sophisticated catalysts. However, the performance of ENRA and the change of catalytic activity in more complicated solutions (i.e., nitrate-polluted groundwater) are poorly understood. Here we first explored the influence of Ca2+ and bicarbonate on ENRA using commercial cathodes. We found that the catalytic activity of used Ni or Cu foam cathodes significantly outperforms their pristine ones due to the in situ evolution of new catalytic species on used cathodes during ENRA. In contrast, the nitrate conversion performance with nonactive Ti or Sn cathode is less affected by Ca2+ or bicarbonate because of their original poor activity. In addition, the coexistence of Ca2+ and bicarbonate inhibits nitrate conversion by forming scales (CaCO3) on the in situ-formed active sites. Likewise, ENRA is prone to fast performance deterioration in treating actual groundwater over continuous flow operation due to the presence of hardness ions and possible organic substances that quickly block the active sites toward nitrate reduction. Our work suggests that more work is required to ensure the long-term stability of ENRA in treating natural nitrate-polluted water bodies and to leverage the environmental relevance of ENRA in more realistic conditions.

Electrocatalytic conversion of nitrate to ammonia on the oxygen vacancy engineering of zinc oxide for nitrogen recovery from nitrate-polluted surface water

Nitrate pollution in surface water poses a significant threat to drinking water safety. The integration of electrocatalytic reduction reaction of nitrate (NO3RR) to ammonia with ammonia collection processes offers a sustainable approach to nitrogen recovery from nitrate-polluted surface water. However, the low catalytic activity of existing catalysts has resulted in excessive energy consumption for NO3RR. Herein, we developed a facile approach of electrochemical reduction to generate oxygen vacancy (Ov) on zinc oxide nanoparticles (ZnO1-x NPs) to enhance catalytic activity. The ZnO1-x NPs achieved a high NH3-N selectivity of 92.4% and NH3-N production rate of 1007.9 [Formula: see text] h-1 m-2 at -0.65 V vs. RHE in 22.5 mg L-1NO3--N, surpassing both pristine ZnO and the majority of catalysts reported in the literature. DFT calculations with in-situ Raman spectroscopy and ESR analysis revealed that the presence of Ov significantly increased the affinity for the NO3- (nitrate) and key intermediate of NO2- (nitrite). The strong adsorption of NO3- on Ov decreased the energy barrier of potential determining step (NO3- →∗NO3) from 0.49 to 0.1 eV, boosting the reaction rate. Furthermore, the strong adsorption of NO2- on Ov prevented its escape from the active sites, thereby minimizing NO2- by-product formation and enhancing ammonia selectivity. Moreover, the NO3RR, when coupled with a membrane separation process, achieved a 100% nitrogen recycling efficiency with low energy consumption of 0.55 kWh molN-1 at a flow rate below 112 mL min-1 for the treatment of nitrate-polluted lake water. These results demonstrate that ZnO1-x NPs are a reliable catalytic material for NO₃RR, enabling the development of a sustainable technology for nitrogen recovery from nitrate-polluted surface water.

Self-Recycled electron donor resists disfavored oxidation reconstruction of Cu(I)-based electrocatalyst for nitrate removal by charge compensation

The overuse of nitrate has led to the accumulation in natural water, being a globe issue in environment and human health. Electrochemical NO3- reduction reaction (eNO3RR) to ammonia occurs under ambient condition with low energy consumption and the yield of value-added product, being promising for NO3- removal. Cu(I)-based eNO3RR catalysts suffer from unavoidable oxidation reconstruction to Cu(II), reducing the performance of NO3- removal. In this work, we demonstrate charge compensation strategy to resist oxidation reconstruction of Cu(I)-based eNO3RR catalysts by introducing self-recycled electron donor. Taking Ti(III)-modified Cu2O/Cu as the proof-of-concept model, electron donor Ti(III) can donate electron to Cu(II) to regenerate Cu(I), meanwhile the expended Ti(III) can be recycled from the generated Ti(IV) via intervalence charge transfer (IVCT). Benefiting from those, Ti-Cu2O/Cu-10 exhibits significantly improved activity and durability for NO3- removal compared to Cu2O/Cu. The percentage of NO3- removal keeps at ∼95.0 % with the initial concentration of 60 mg•L-1 NO3--N at -0.9 V vs. RHE in 15 consecutive cycling tests (corresponding to 30 h). This work presents a feasible strategy to resist oxidation reconstruction of Cu(I)-based eNO3RR catalysts, making NO3- removal more effective, more durable, and more sustainable.

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.

Conductivity-mediated in situ electrochemical reconstruction of CuOx for nitrate reduction to ammonia

The electrocatalytic nitrate reduction reaction (NO3RR) is an ideal NH3 synthesis route with ease of operation, high energy efficiency, and low environmental detriment. Electrocatalytic cathodes play a dominant role in the NO3RR. Herein, we constructed a carbon fiber paper-supported CuOx nanoarray catalyst (CP/CuOx) by an in situ electrochemical reconstruction method for NO3--to-NH3 conversion. A series of characterization techniques, such as X-ray diffraction (XRD) and in situ Raman spectroscopy, unveil that CP/CuOx is a polycrystalline-faceted composite copper nanocatalyst with a valence composition containing Cu0, Cu+ and Cu2+. CP/CuOx shows more efficient NO3--to-NH3 conversion than CP/Cu and CP/Cu2O, which indicates that the coexistence of various Cu valence states could play a dominant role. CP/CuOx with a suitable Cu2+ content obtained by adjusting the conductivity during the in situ electrochemical reconstruction process exhibited more than 90% faradaic efficiencies for the NO3RR in a broad range of -0.3 to -1.0 V vs. RHE, 28.65 mg cm-2 h-1 peak ammonia yield, and stable NO3RR efficiencies for ten cycles. These findings suggest that CP/CuOx with suitable copper valence states obtained by fine-tuning the conductivity of the electrochemical reconstruction may provide a competitive cathode catalyst for achieving excellent activity and selectivity of NO3--to-NH3 conversion.

Effects of Ionic Interferents on Electrocatalytic Nitrate Reduction: Mechanistic Insight

Nitrate, a prevalent water pollutant, poses substantial public health concerns and environmental risks. Electrochemical reduction of nitrate (eNO3RR) has emerged as an effective alternative to conventional biological treatments. While extensive lab work has focused on designing efficient electrocatalysts, implementation of eNO3RR in practical wastewater settings requires careful consideration of the effects of various constituents in real wastewater. In this critical review, we examine the interference of ionic species commonly encountered in electrocatalytic systems and universally present in wastewater, such as halogen ions, alkali metal cations, and other divalent/trivalent ions (Ca2+, Mg2+, HCO3-/CO32-, SO42-, and PO43-). Notably, we categorize and discuss the interfering mechanisms into four groups: (1) loss of active catalytic sites caused by competitive adsorption and precipitation, (2) electrostatic interactions in the electric double layer (EDL), including ion pairs and the shielding effect, (3) effects on the selectivity of N intermediates and final products (N2 or NH3), and (4) complications by the hydrogen evolution reaction (HER) and localized pH on the cathode surface. Finally, we summarize the competition among different mechanisms and propose future directions for a deeper mechanistic understanding of ionic impacts on eNO3RR.

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.

Regulating the Selectivity of Nitrate Photoreduction for Purification or Ammonia Production by Cooperating Oxidative Half-Reactions

The removal and conversion of nitrate (NO3-) from wastewater has become an important environmental and health topic. The NO3- can be reduced to nontoxic nitrogen (N2) for environmental remediation or ammonia (NH3) for recovery, in which the tailoring of the selectivity is greatly challenging. Here, by construction of the CuOx@TiO2 photocatalyst, the NO3- conversion efficiency is enhanced to ∼100%. Moreover, the precise regulation of selectivity to NH3 (∼100%) or N2 (92.67%) is accomplished by the synergy of cooperative redox reactions. It is identified that the selectivity of the NO3- photoreduction is determined by the combination of different oxidative reactions. The key roles of intermediates and reactive radicals are revealed by comprehensive in situ characterizations, providing direct evidence for the regulated selectivity of the NO3- photoreduction. Different active radicals are produced by the interaction of oxidative reactants and light-generated holes. Specifically, the introduction of CH3CHO as the oxidative reactant results in the generation of formate radicals, which drives selective NO3- reduction into N2 for its remediation. The alkyl radicals, contributed to by the (CH2OH)2 oxidation, facilitate the deep reduction of NO3- to NH3 for its upcycling. This work provides a technological basis for radical-directed NO3- reduction for its purification and resource recovery.

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.