Development of Pd-Cu/hematite catalyst for selective nitrate reduction

Electrocatalysts for Inorganic and Organic Waste Nitrogen Conversion

Anthropogenic activities have disrupted the natural nitrogen cycle, increasing the level of nitrogen contaminants in water. Nitrogen contaminants are harmful to humans and the environment. This motivates research on advanced and decarbonized treatment technologies that are capable of removing or valorizing nitrogen waste found in water. In this context, the electrocatalytic conversion of inorganic- and organic-based nitrogen compounds has emerged as an important approach that is capable of upconverting waste nitrogen into valuable compounds. This approach differs from state-of-the-art wastewater treatment, which primarily converts inorganic nitrogen to dinitrogen, and organic nitrogen is sent to landfills. Here, we review recent efforts related to electrocatalytic conversion of inorganic- and organic-based nitrogen waste. Specifically, we detail the role that electrocatalyst design (alloys, defects, morphology, and faceting) plays in the promotion of high-activity and high-selectivity electrocatalysts. We also discuss the impact of wastewater constituents. Finally, we discuss the critical product analyses required to ensure that the reported performance is accurate.

Nitrate reduction to nitrogen in wastewater using mesoporous carbon encapsulated Pd-Cu nanoparticles combined with in-situ electrochemical hydrogen evolution

The conversion of NO3--N to N2 is of great significance for zero discharge of industrial wastewater. Pd-Cu hydrogenation catalysis has high application prospects for the reduction of NO3--N to N2, but the existing form of Pd-Cu, the Pd-Cu mass ratio and the H2 evolution rate can affect the coverage of active hydrogen (*H) on the surface of Pd, thereby affecting N2 selectivity. In this work, mesoporous carbon (MC) is used as support to disperse Pd-Cu catalyst and is applied in an in-situ electrocatalytic H2 evolution system for NO3--N removal. The Pd-Cu particles with the average size of 6 nm are uniformly encapsulated in the mesopores of MC. Electrochemical in-situ H2 evolution can not only reduce the amount of H2 used, but the H2 bubbles can also be efficiently dispersed when PPy coated nickel foam (PPy/NF) is used as cathode. Moreover, the mesoporous structure of MC can further split H2 bubbles, reducing the coverage of *H on Pd. The highest 77% N2 selectivity and a relatively faster NO3--N removal rate constant (0.10362 min-1) can be achieved under the optimal conditions, which is superior to most reported Pd-Cu catalytic systems. The prepared catalyst is further applied to the denitrification of actual deplating wastewater. NO3--N with the initial concentration of 650 mg L-1 can be completely removed after 180 min of treatment, and the TN removal can be maintained at 72%.

The effect of carbonized zeolitic imidazolate framework-67 (ZIF-67) support on the reactivity and selectivity of bimetal-catalytic aqueous NO3- reduction

A metallic catalyst, Cobalt N-doped Carbon (Co@NC), was obtained from Zeolitic-Imidazolate Framework-67 (ZIF-67) for efficient aqueous nitrate (NO3-) removal. This advanced catalyst indicated remarkable efficiency by generating valuable ammonium (NH3/NH4+) via an environmentally friendly production technique during the nitrate treatment. Among various metals (Cu, Pt, Pd, Sn, Ru, and Ni), 3.6%Pt-Co@NC exhibited an exceptional nitrate removal, demonstrating a complete removal of 60 mg/L NO3--N (265 mg/L NO3-) in 30 min with the fastest removal kinetics (11.4 × 10-2 min-1) and 99.5% NH4+ selectivity. The synergistic effect of bimetallic Pt-Co@NC led to 100% aqueous NO3- removal, outperforming the reactivity by bare ZIF-67 (3.67%). The XPS analysis illustrated Co's promotor role for NO3- reduction to less oxidized nitrogen species and Pt's hydrogenation role for further reduction to NH4+. The durability test revealed a slight decrease in NO3- removal, which started from the third cycle (95%) and slowly proceeded to the sixth cycle (80.2%), while NH4+ selectivity exceeded 82% with no notable Co or Pt leaching throughout seven consecutive cycles. This research shed light on the significance of the impregnated Pt metal and Co exposed on the Co@NC surface for the catalytic nitrate treatment, leading to a sustainable approach for the effective removal of nitrate and economical NH4+ production.

Pd Clusters Loaded with Multivalent Cu Foam for Superior Electrochemical Nitrate Reduction and Selective N≡N Bond Formation

The electrochemical denitrification of nitrate (NO3 -) in actual wastewater to nitrogen (N2) is an effective approach to reversing the current imbalance of the nitrogen cycle and the eutrophication of water. However, electrostatic repulsion between NO3 - and the cathode results in the low efficiency of NO3 - reduction reaction (NO3RR). Here, density functional theory (DFT) calculations are used as a theoretical guide to design a Pd cluster-loaded multivalent Cu foam (Pd/Cu2O-CF) electrocatalyst, which achieves a splendid 97.8% NO3 - removal rate, 97.9% N2 selectivity, 695.5 mg N g-1 Pd h-1 reduction efficiency, and 60.0% Faradaic efficiency at -1.3 V versus SCE. The projected density of states (pDOS) indicates that NO3 - and Pd/Cu2O-CF are bonded via strong complexation between the O 2p (in NO3 -) and Cu 3d (in Cu2O) with the input of voltage, which reduces the electrostatic repulsion and enhances the enrichment of NO3 - on the cathode. In-situ characterizations demonstrate that Pd[H] can reduce Cu2O to Cu, and subsequently Cu reduces NO3 - to nitrite (NO2 -) accompanied by in situ reconfiguration of multivalent Cu foam. NO2 - is then transferred to the surface of Pd clusters by the cascade catalysis and accelerates the breaking of N─O bonds to form Pd─N, and eventually achieves the N≡N bond formation.

Atomically Ordered PdCu Electrocatalysts for Selective and Stable Electrochemical Nitrate Reduction

Electrochemical nitrate reduction (NO3 RR) has attracted attention as an emerging approach to mitigate nitrate pollution in groundwater. Here, we report that a highly ordered PdCu alloy-based electrocatalyst exhibits selective (91% N2), stable (480 h), and near complete (94%) removal of nitrate without loss of catalyst. In situ and ex situ XAS provide evidence that structural ordering between Pd and Cu improves long-term catalyst stability during NO3RR. In contrast, we also report that a disordered PdCu alloy-based electrocatalyst exhibits non-selective (44% N2 and 49% NH4+), unstable, and incomplete removal of nitrate. The copper within disordered PdCu alloy is vulnerable to accepting electrons from hydrogenated neighboring Pd atoms. This resulted in copper catalyst losses which were 10× greater than that of the ordered catalyst. The design of stable catalysts is imperative for water treatment because loss of the catalyst adds to the system cost and environmental impacts.

Preparation, characterization, and removal of nitrate from water using vacant polyoxometalate/TiO2 composites

Due to the rapid development of industry and agriculture, nitrate pollution in groundwater has been continuously increasing. NO3-N is a chemically stable nitrogen species and is quite difficult to remove. In this study, using heteropoly silicotungstate K8[α-SiW11O39] and Cu2+ as the active components, SiW11 and Cu2+ were loaded onto TiO2 by a sol-gel method to prepare a composite photocatalyst SiW11/TiO2/Cu. The photocatalytic reduction of dissolved NO3-N was subsequently performed using SiW11/TiO2/Cu under UV irradiation, and the influence of different experimental parameters on the photocatalytic performance was investigated. The mechanism of NO3-N reduction by the composite catalyst was also investigated. Free radicals existing within the system were detected by ESR spectroscopy, and the results indicated that C O 2 - anion free-radicals were generated by the reaction of photogenerated holes and formic acid (HCOOH). At a SiW11/TiO2/Cu dose of 1.2 g L-1 and in the presence of HCOOH as a hole scavenger, the proposed composite catalytically reduced NO3-N anddemonstrated significantadvantages in terms of its photocatalytic activity in comparison with pure TiO2. In particular, the removal efficiency of NO3-N and the selectivity of nitrogen achieved a maximum of 96% and 77%, respectively.

Contrasting Capability of Single Atom Palladium for Thermocatalytic versus Electrocatalytic Nitrate Reduction Reaction

The occurrence of high concentrations of nitrate in various water resources is a significant environmental and human health threat, demanding effective removal technologies. Single atom alloys (SAAs) have emerged as a promising bimetallic material architecture in various thermocatalytic and electrocatalytic schemes including nitrate reduction reaction (NRR). This study suggests that there exists a stark contrast between thermocatalytic (T-NRR) and electrocatalytic (E-NRR) pathways that resulted in dramatic differences in SAA performances. Among Pd/Cu nanoalloys with varying Pd-Cu ratios from 1:100 to 100:1, Pd/Cu(1:100) SAA exhibited the greatest activity (TOFPd = 2 min-1) and highest N2 selectivity (94%) for E-NRR, while the same SAA performed poorly for T-NRR as compared to other nanoalloy counterparts. DFT calculations demonstrate that the improved performance and N2 selectivity of Pd/Cu(1:100) in E-NRR compared to T-NRR originate from the higher stability of NO3* in electrocatalysis and a lower N2 formation barrier than NH due to localized pH effects and the ability to extract protons from water. This study establishes the performance and mechanistic differences of SAA and nanoalloys for T-NRR versus E-NRR.

Accelerating ammonia synthesis in a membraneless flow electrolyzer through coupling ambient dinitrogen oxidation and water splitting

An electrochemical approach for ammonia production is successfully developed by coupling the anodic dinitrogen oxidation reaction (NOR) and cathodic hydrogen evolution reaction (HER) within a well-designed membraneless flow electrolyzer. The obtained reactor shows the preferential yield of ammonia over nitrogen oxides on the vanadium nitride catalyst surface. At an applied oxidation potential of 2.25 V versus the reversible hydrogen electrode (vs RHE), a promoted ammonia production rate and Faradaic efficiency (FE) were obtained with 9.9 mmol g^-1 h^-1 (0.029 mmol cm^-2 h^-1) and 4.8%, respectively. Besides, the negative affection of ammonia contamination is efficiently alleviated. Density functional theory calculations revealed that the thermodynamic energy needed to produce ammonia (-0.63 eV) is far lower than that of producing nitrogen oxide (0.96 eV) from hydrogenated nitrogen oxides [∗N₂OH] splitting, confirming the coupling of NOR and HER.

Mechanisms and health implications of toxicity increment from arsenate-containing iron minerals through in vitro gastrointestinal digestion

Inadvertent oral ingestion is an important exposure pathway of arsenic (As) containing soil and dust. Previous researches evidenced health risk of bioaccessible As from soil and dust, but it is unclear about As mobilization mechanisms in health implications from As exposure. In this study, we investigated As release behaviors and the solid-liquid interface reactions toward As(V)-containing iron minerals in simulated gastrointestinal bio-fluids. The maximum As release amount was 0.57 mg/L from As-containing goethite and 0.82 mg/L from As-containing hematite at 9 h, and the As bioaccessibility was 10.8% and 21.6%, respectively. The higher exposure risk from hematite-sorbed As in gastrointestinal fluid was found even though goethite initially contained more arsenate than hematite. Mechanism analysis revealed that As release was mainly coupled with acid dissolution and reductive dissolution of iron minerals. Proteases enhanced As mobilization and thus increased As bioaccessibility. The As(V) released and simultaneously transformed to high toxic As(III) by gastric pepsin, while As(V) reduction in intestine was triggered by pancreatin and freshly formed Fe(II) in gastric digests. CaCl2 reduced As bioaccessibility, indicating that calcium-rich food or drugs may be effective dietary strategies to reduce As toxicity. The results deepened our understanding of the As release mechanisms associated with iron minerals in the simulated gastrointestinal tract and supplied a dietary strategy to alleviate the health risk of incidental As intake.

Electrocatalytic reduction of nitrate by carbon encapsulated Cu-Fe electroactive nanocatalysts on Ni foam

Electrocatalytic denitrification is an attractive and effective method for complete elimination of nitrate (NO₃^-). However, its application is limited by the activity and stability of the electrocatalyst. In this work, a novel bimetallic electrode was synthesized, in which N-doped graphitized carbon sealed with Cu and Fe nanoparticles and immobilized them on nickel foam (CuFe NPs@NC/NF) without any chemical binder. The immobilized Cu-Fe nanoparticles not only facilitated the adsorption of the reactant but also enhanced the electron transfer between the cathode and NO₃^-, thus promoting the electrochemical reduction of NO₃^-. Therefore, the as-prepared electrode exhibited enhanced electrocatalytic activity for NO₃^- reduction. The composite electrode with the Cu/Fe molar ratio of 1:2 achieved the highest NO₃^- removal (79.4 %) and the lowest energy consumption (0.0023 kW h mg^-1). Furthermore, the composite electrode had a robust NO₃^- removal capacity under various conditions. Benefitting from the electrochlorination on the anode, this electrochemical system achieved nitrogen (N₂) selectivity of 94.0 %. Moreover, CuFe NPs@NC/NF exhibited good stability after 15 cycles, which should be attributed to the graphitized carbon layer. This study confirmed that CuFe NPs@NC/NF electrode is a promising and inexpensive electrode with long-term stability for electrocatalytic denitrification.

Comparative study between supported bimetallic catalysts for nitrate remediation in water

As the population grows and the demand for water rises, the development of efficient and sustainable water purification techniques is becoming increasingly important to ensure access to clean and safe water in the future. The pollution of surface and groundwater by nitrate ( NO 3 − {\text{NO}}_{3}^{-} ) is a growing global concern due to the rise in nitrogen-rich waste released from agriculture and industry. The removal of nitrate ions from aqueous media using bimetallic catalysts loaded on several supports was studied. Multiwalled carbon nanotubes, activated carbon, titanium dioxide, titanium dioxide/multiwalled carbon nanotubes, and Santa Barbara Amorphous-15 were used as supports to synthesize these bimetallic catalysts. The effects of the support type, supported metal, and catalyst reduction method on the nitrate reduction activity in water were investigated. The catalysts were characterized by X-ray diffraction, fourier-transform infrared spectroscopy, Brunauer-Emmett-Teller isotherm, inductively coupled plasma spectroscopy, and field emission gun scanning transmission electron microscope. In terms of nitrate conversion, high-temperature hydrogen reduction of the catalysts was a more effective method of catalyst preparation than NaBH4 reduction. Except for the carbon nanotube-TiO2 composite, pH fixation using CO2 flow improved the efficiency of supported catalysts. The catalysts 1Pd–1Cu/TiO2 and 1Pd–Cu/SBA-15 presented the highest catalytic activity, but the latter was the most selective to nitrogen.

Highly selective electrocatalytic reduction of nitrate to nitrogen in a chloride ion-free system by promoting kinetic mass transfer of intermediate products in a novel Pd-Cu adsorption confined cathode

The mass transfer on the catalyst surface has a great influence on the selectivity of electrocatalytic nitrate reduction to nitrogen. In this study, a Pd-Cu adsorption confined nickel foam cathode is designed in the absence of both proton exchange membranes and chloride ions. The repulsion of the cathode enables intermediate products such as nitrite to accumulate in the confined region, resulting in an increase in the possibility of a second-order reaction to form nitrogen. The system can obtain more than 92% continuous N₂ selectivity when it is used to treat 200 mg L^-1 NO₃^--N under a current density of 8 mA cm^-2, which is not only higher than those of semiconfined and nonconfined systems but also significantly better than the results obtained by Pd-Cu directly modified cathodes prepared by electrodeposition or impregnation. It is found that a high initial nitrate concentration and low current density are more beneficial for the accumulation of intermediates on Pd-Cu catalysts, thus improving the formation of nitrogen. A mechanism study reveals that the intermediates can completely occupy the active sites on the surface of Pd, avoiding the generation of active hydrogen, and therefore inhibiting the first-order reaction to produce ammonia. Moreover, the reducibility of Pd-Cu can also be gradually improved under the function of the cathode so that the system exhibits good stability. This study demonstrates an environmentally friendly and promising method for total nitrogen removal from industrial wastewater with high conductivity.

Iron-Based Nanocatalysts for Electrochemical Nitrate Reduction

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

Oxygen-Vacancy-Rich Cu2O Hollow Nanocubes for Nitrate Electroreduction Reaction to Ammonia in a Neutral Electrolyte

The electrocatalytic nitrate reduction reaction (NO₃^--ERR) to ammonia (NH₃) is a promising strategy for NH₃ production. Cu-based nanomaterials have been regarded as a kind of effective NO₃^--ERR catalysts. In this work, high-quality hollow Cu₂O nanocubes (Cu₂O h-NCs) are facilely synthesized by a simple one-step reduction method. The as-prepared Cu₂O h-NCs reveal high selectivity and activity for NO₃^--ERR, which is ascribed to abundant oxygen vacancies, high surface area, hollow architecture, low mass transfer resistance, and strong adsorbing ability toward NO₃^-. In fact, Cu₂O h-NCs can achieve a Faradic efficiency of 92.9% and an NH₃ yield of 56.2 mg h^-1 mg_cat^-1 for NH₃ production at -0.85 V (vs RHE) potential, which exceeds those of other transition-metal-based NO₃^--ERR electrocatalysts.

Achieving high-performance electrocatalytic reduction of nitrate by N-rich carbon-encapsulated Ni-Cu bimetallic nanoparticles supported nickel foam electrode

The cathode with low-energy consumption and long-term stability is pivotal to achieve the conversion of nitrate (NO₃^-) to nitrogen (N₂) by electrocatalytic denitrification. Herein, a binder-free electrode was synthesized by directly immobilizing N-doped graphitized carbon layer-encapsulated NiCu bimetallic nanoparticles on nickel foam (NF) (NiCu@N-C/NF) and served as the cathode for electrocatalytic NO₃^- reduction. Morphological characterization indicated that Ni and Cu nanoparticles were encapsulated by the N-doped graphitized carbon layer and well-dispersed on the surface of NF. Compared with monometallic composite cathode (Cu@N-C/NF and Ni@N-C/NF), NiCu@N-C/NF exhibited better NO₃^- removal performance (98.63 %) and lower energy consumption (0.007 kW·h mmol^-1), which should be attributed to its strong adsorption ability to NO₃^- and excellent electron transfer property. Meanwhile, its electrocatalytic performance could be maintained in wide initial NO₃^- concentration (1.79-7.14 mM) and solution pH (3-11). With the assistance of electrochlorination, the N₂ selectivity of electrochemical system was up to 99.89 % in the presence of 0.028 M Cl^-. More importantly, NiCu@N-C/NF electrode displayed an ultra-high stability during ten recycling experiments. This study indicated that the binderless composite cathode NiCu@N-C/NF had great potential in electrocatalytic NO₃^- removal from wastewater.

Activity and Stability of Pd Bimetallic Catalysts for Catalytic Nitrate Reduction

In this work, we study the effect of modifying the metal loading (0.5–1.5 wt.% Pd and 0.1–1 wt.% Sn or In), the impregnation order of noble or promoter metal (Pd–Sn or Sn–Pd), and the type of promoter metal (Sn or In) during the preparation process for a Pd bimetallic catalyst, supported on γ-alumina, used in the catalytic reduction of nitrate. The deposition of the noble metal over the promoter metal, especially with Pd:Sn ratios (wt.) of 1:10 and 1:2, favored the hydrogen spillover rate and increased the H concentration on the catalyst surface, enhancing NH4+ production. On the other hand, Pd–In catalysts showed higher activity than the Sn catalysts, as well as higher NH4+ selectivity. The stability of the Pd–Sn/Al2O3 (1.5–1 wt.%) catalyst was evaluated in long-term experiments for the treatment of synthetic water (100 mg L−1 NO3−) and three different commercial drinking waters. This Pd–Sn/Al2O3 catalyst achieved a stable nitrate conversion for a duration of 50 h in the synthetic water treatment. However, the catalyst showed a significant activity loss in the presence of other ions (different to NO3−) in the reaction medium, increasing slightly the selectivity to NH4+.

Research on catalytic denitrification by zero-valent iron (Fe0) and Pd-Ag catalyst

This study primarily focused on how to effectively remove nitrate by catalytic denitrification through zero-valent iron (Fe0) and Pd-Ag catalyst. Response surface methodology (RSM), instead of the single factor experiments and orthogonal tests, was firstly applied to optimize the condition parameters of the catalytic process. Results indicated that RSM is accurate and feasible for the condition optimization of catalytic denitrification. Better catalytic performance (71.6% N2 Selectivity) was obtained under the following conditions: 5.1 pH, 127 min reaction time, 3.2 mass ration (Pd: Ag), and 4.2 g/L Fe0, which was higher than the previous study designed by single factor experiments and orthogonal tests, 68.1% and 68.7% of N2 Selectivity, respectively. However, under this optimal conditions, N2 selectivity showed a mild decrease (69.3%), when the real wastewater was used as influent. Further study revealed that cations (K+, Na+, Ca2+, Mg2+, and Al3+) and anions (Cl-, HCO3-, and SO42-) exist in wastewater could have distinctive influence on N2 selectivity. Finally, the reaction mechanism and kinetic model of catalytic denitrification were further studied.

Electrocatalytic reduction of nitrate - a step towards a sustainable nitrogen cycle

Nitrate enrichment, which is mainly caused by the over-utilization of fertilisers and industrial sewage discharge, is a major global engineering challenge because of its negative influence on the environment and human health. To solve this serious problem, many technologies, such as the activated sludge method, reverse osmosis, ion exchange, adsorption, and electrodialysis, have been developed to reduce the nitrate levels in water bodies. However, the applications of these traditional techniques are limited by several drawbacks, such as a long sludge retention time, slow kinetics, and undesirable by-products. From an environmental perspective, the most promising nitrate reduction technology is enabled to convert nitrate into benign N₂, and features low cost, high efficiency, and environmental friendliness. Recently, electrocatalytic nitrate reduction has been proven by satisfactory research achievements to be one of the most promising methods among these technologies. This review provides a comprehensive account of nitrate reduction using electrocatalysis methods. The fundamentals of electrocatalytic nitrate reduction, including the reaction mechanisms, reactor design principles, product detection methods, and performance evaluation methods, have been systematically summarised. A detailed introduction to electrocatalytic nitrate reduction on transition metals, especially noble metals and alloys, Cu-based electrocatalysts, and Fe-based electrocatalysts is provided, as they are essential for the accurate reporting of experimental results. The current challenges and potential opportunities in this field, including the innovation of material design systems, value-added product yields, and challenges for products beyond N₂ and large-scale sewage treatment, are highlighted.

Competitive inhibition of catalytic nitrate reduction over Cu-Pd-hematite by groundwater oxyanions

The presence of various oxyanions in the groundwater could be the main challenge for the successive application of Cu-Pd-hematite bimetallic catalyst to aqueous NO₃^- reduction due to the inhibition of its catalytic reactivity and alteration of product selectivity. The batch experiments showed that the reduction kinetics of NO₃^- was strongly suppressed by ClO₄^-, PO₄^3-, BrO₃^- and SO₃^2- at low concentrations (>5 mg/L) and HCO₃^-, CO₃^2-, SO₄^2- and Cl^- at high concentrations (20-500 mg/L). The presence of anions significantly changing the end-product selectivities influenced high N₂ selectivity. The selectivity toward N₂ increased from 55% to 60%, 60%, and 70% as the concentrations of PO₄^3-, SO₃^2-, and SO₄^2- increased, respectively. It decreased from 55% to 35% in the presence of HCO₃^- and CO₃^2- in their concentration range of 0-500 mg/L. The production of NO₂^- was generally not detected, while the formation of NH₄⁺ was observed as the second by-product. It was found that the presence of oxyanions in the NO₃^- reduction influenced the reactivity and selectivity of bimetallic catalysts by i) competing for active sites (PO₄^3-, SO₃^2-, and BrO₃^- cases) due to their similar structure, ii) blockage of the promoter and/or noble metal (HCO₃^-, CO₃^2-, SO₄^2-, Cl^- and ClO₄^- cases), and iii) interaction with the support surface (PO₄^3- case). The results can provide a new insight for the successful application of catalytic NO₃^- reduction technology with high N₂ selectivity to the contaminated groundwater system.

Recent advances in metal-mediated nitrogen oxyanion reduction using reductively borylated and silylated N-heterocycles

The reduction of nitrogen oxyanions is critical for the remediation of eutrophication caused by anthropogenic perturbations to the natural nitrogen cycle. There are many approaches to nitrogen oxyanion reduction, and here we report our advances in reductive deoxygenation using pre-reduced N-heterocycles. We show examples of nitrogen oxyanion reduction using Cr, Fe, Co, Ni, and Zn, and we evaluate the role of metal choice, number of coordinated oxyanions, and ancillary ligands on the reductive transformations. We report the experimental challenges faced and provide an outlook on new directions to repurpose nitrogen oxyanions into value-added products.

Catalytic hydrogenation of nitrate in water: improvement of the activity and selectivity to N2 by using Rh(III)-hexamolybdate supported on ZrO2-Al2O3

Catalysts prepared on ZrO₂, Al₂O₃ and ZrO₂-Al₂O₃ (ZrAl-10) supported with Anderson heteropolyanion (RhMo₆) as active phase were investigated for the elimination of NO₃^- from water. Raman characterization of pure and supported RhMo₆ phase showed the presence of polymolybdic species of different degrees of complexity when RhMo₆ was supported. The temperature-programmed reduction study revealed the synergic effect between Rh and Mo species, through which the reducibility of Mo was promoted by Rh, and different phase/support interactions were verified. Among the supports, ZrAl-10 presented the highest acidity due to the presence of ZrO₂ in the tetragonal modification and high specific surface area (due to Al₂O₃), favouring rhodium-molybdenum active phase/support interaction and high dispersion. All catalysts prepared were active in removing NO₃^-, the one prepared with the RhMo₆ phase on the ZrAl-10 support being the most active. These results point to the formation of an active surface with a high dispersion of Rh and Mo. The highest selectivity to N₂ (99.3) exhibited by the RhMo₆/ZrAl-10 catalyst is proposed to be related to the high Rh dispersion (0.755) and to the presence of Lewis acid sites (oxygen vacancies) of the tetragonal ZrO₂ modification that favour NO₃^- adsorption through electrostatic interactions.

Formic acid generating in-situ H2 and CO2 for nitrite reduction in aqueous phase

The aim of this work is to explore and to understand the effect of pH, concentrations and presence of oxygen traces on reduction nitrite in drinking water with Pd/γ-Al2O3, using...

Photocatalytic Reduction of Aqueous Nitrate with Hybrid Ag/g-C3N4 under Ultraviolet and Visible Light

The concentration of nitrate in natural surface waters by agricultural runoff remains a challenging problem in environmental chemistry. One promising denitrification strategy is to utilize photocatalysts, whose light-driven excited states are capable of reducing nitrate to nitrogen gas. We have synthesized and characterized pristine and silver-loaded graphitic carbon nitrides and assessed their activity for photocatalytic nitrate reduction at neutral pH. While nitrate reduction does occur on the pristine material, the silver cocatalyst greatly enhances product yields. Kinetic studies performed in batch photoreactors under both UV and visible excitation suggest that nitrate reduction to produce aqueous nitrite, ammonium, and nitrogen gas proceeds via a cooperative water reduction on the silver metal domains to produce adsorbed H atoms. By varying the percentage of silver loading onto the g-C₃N₄, the density of metal domains can be adjusted, which in turn tunes the reduction selectivity toward various products.

Performance of Pd/Sn catalysts supported by chelating resin prestoring reductant for nitrate reduction in actual water

Catalytic hydrogen reduction has appeared as a promising strategy for chemical denitrification with advantages of high activity and simple operation. However, the risk and low utilization of H₂ is the disadvantage of catalytic hydrogen reduction. In recent years, catalytic reduction reactions in the presence of sodium borohydride (NaBH₄) have been extensively studied. NaBH₄ can be used as an electron source to generate electrons on the surface of the catalyst and can catalyze the reduction of pollutants. But it makes commercialization costly and causes significant environmental pollution if widely use NaBH₄. In this study, we prepared supported Pd/Sn bimetallic nanoparticles which could adsorb NaBH₄ during the preparation of the Pd/Sn bimetallic catalyst as the prestoring reductant. No additional reducing agent is required during nitrate reduction process. The performance and mechanism for nitrate reduction by using Pd/Sn bimetallic nanoparticles were discussed. Moreover, the catalyst D-Pd1/Sn1 reached a complete nitrate removal in the municipal wastewater treatment plant effluent water within 3 h. The results provide a prospect for denitrification in biological wastewater treatment plants.

Nitrate reduction by electrochemical processes using copper electrode: evaluating operational parameters aiming low nitrite formation

This work aims to present different electroreduction and electrocatalytic processes configurations to treat nitrate contaminated water. The parameters tested were: current density, cell potential, electrode potential, pH values, cell type and catalyst use. It was found that the nitrite ion is present in all process variations used, being the resulting nitrite concentration higher in an alkaline pH. The increase in current density on galvanostatic operation mode provides a greater reduction of nitrate (64%, 1.4 mA cm^-2) if compared to the potentiostatic (20%) and constant cell potential (37%) configurations. In a dual-chamber cell the nitrate reduction with current density of 1.4 mA cm^-2 was tested and obtained as a NO₃^- reduction of 85%. The use of single chamber cell presented 32 ± 3% of nitrate reduction, indicating that in this cell type the nitrate reduction is smaller than in dual-chamber cell (64%). The presence of a Pd catalyst with 3.1% wt. decreased the nitrite (1.0 N-mg L^-1) and increased the gaseous compounds (9.4 N-mg L^-1) formation. The best configuration showed that, by fixing the current density, the highest nitrate reduction is obtained and the pH presents a significant influence during the tests. The use of the catalyst decreased the nitrite and enhanced the gaseous compounds formation.

Feasibility of nanoscale zerovalent iron-loaded sediment-based biochar (nZVI-SBC) for simultaneous removal of nitrate and phosphate: high selectivity toward dinitrogen and synergistic mechanism

In the process of water treatment, excessive nitrogen and phosphorus pollutants are of great concern. Therefore, we prepared nanoscale zerovalent iron loaded on sediment-based biochar (nZVI-SBC) to conduct nitrate and phosphate removal at the same time. The characterization demonstrated that nZVI-SBC was successfully synthesized, which had obvious advantages for larger specific surface area and better dispersion compared with pure nZVI. The batch experiments indicated that the best loading ratio of nZVI to SBC and optimum dosage for nitrate and phosphate were 1:1and 2 g L^-1, respectively. Their removal by nZVI-SBC was an acid-driven process. Anoxic environment was more conducive to the reduction of nitrate while the phosphate removal was fond of oxygen environment. A total of 77.78% of nitrate and 99.21% of phosphate have been successfully removed, mainly depending on reduction and complexation mechanism, respectively. Moreover, nZVI-SBC had higher N₂ selectivity and produced less ammonium than nZVI. The interaction between nitrate and phosphate was studied to manifest that they had different degrees of inhibition during the removal of the other. Our research indicated that nZVI-SBC has great potential for remediation of nitrogen and phosphorus polluted water.

H2-Based Membrane Catalyst-Film Reactor (H2-MCfR) Loaded with Palladium for Removing Oxidized Contaminants in Water

Scalable applications of precious-metal catalysts for water treatment face obstacles in H₂-transfer efficiency and catalyst stability during continuous operation. Here, we introduce a H₂-based membrane catalyst-film reactor (H₂-MCfR), which enables in situ reduction and immobilization of a film of heterogeneous Pd⁰ catalysts that are stably anchored on the exterior of a nonporous H₂-transfer membrane under ambient conditions. In situ immobilization had >95% yield of Pd⁰ in controllable forms, from isolated single atoms to moderately agglomerated nanoparticles (averaging 3-4 nm). A series of batch tests documented rapid Pd-catalyzed reduction of a wide spectrum of oxyanions (nonmetal and metal) and organics (e.g., industrial raw materials, solvents, refrigerants, and explosives) at room temperature, owing to accurately controlled H₂ supply on demand. Reduction kinetics and selectivity were readily controlled through the Pd⁰ loading on the membranes, H₂ pressure, and pH. A 45-day continuous treatment of trichloroethene (TCE)-contaminated water documented removal fluxes up to 120 mg-TCE/m²/d with over 90% selectivity to ethane and minimal (<1.5%) catalyst leaching or deactivation. The results support that the H₂-MCfR is a potentially sustainable and reliable catalytic platform for reducing oxidized water contaminants: simple synthesis of an active and versatile catalyst that has long-term stability during continuous operation.

Performance of L-Cu&Mn-nZVFe@B nanomaterial on nitrate selective reduction under UV irradiation and persulfate activation in the presence of oxalic acid

A novel nanomaterial (L-Cu&Mn-nZVFe@B) was synthesized and was applied to nitrate selective reduction under UV irradiation and persulfate activation in the presence of oxalic acid. Results denoted the deposition of copper could prompt the nitrate conversion and improve the nitrate conversion significantly. The high nitrate conversion was on account of the formation of galvanic cells accelerating the generation of electrons, in which Fe° acted as anode and Cu° acted as cathode. Meanwhile, the coexistence of Cu₂O and MnO₂ exhibited excellent photocatalytic performance with the obvious improvement of N₂ selectivity because of the formation of heterojunction could boost the generation of CO₂^-. Furthermore, the deposition of manganese could also accelerate the generation of CO₂^- through the activation of persulfate. The conversion of NO₃^- was almost 100 % and the N₂ selectivity could reach 81.57 % by the S₂O₈^2-/UV/L-Cu&Mn-nZVFe@B/H₂C₂O₄ system when the initial nitrate concentration was 100 mg /L, the L-Cu&Mn-nZVFe@B dosage was 6.0 g/L, the H₂C₂O₄ dosage was 15 mmol/L, pH was 5.0, the reaction time was 100 min under 25 °C. Research provides an alternative approach for selective reduction nitrate into nitrogen.

Novel 3D Pd-Cu(OH)2/CF cathode for rapid reduction of nitrate-N and simultaneous total nitrogen removal from wastewater

Removal of NO₃^- is a challenging problem in wastewater treatment. Electrocatalysis shows a great potential to remove NO₃^- but selectively converting NO₃^- to N₂ is facing a low efficiency. Here, a novel 3D Pd-Cu(OH)₂/CF cathode based electrocatalytic (EC) system was proposed that can rapidly and selectively convert NO₃^- to NH₄⁺, and further convert to N₂ simultaneously. The special designs for the system include: Cu(OH)₂ nanowires were firstly grown on copper foam (CF) with excellent conductivity that features high specific surface area in enhancing NO₃^- absorption and conversion to NO₂^-. Then, palladium (Pd) with a superior photons activation capacity was doped on the Cu(OH)₂ nanowires to promote the reduction of NO₂^- to NH₄⁺. Then NH₄⁺ was quickly oxidized into N₂ by active chlorine. Finally, total nitrogen (TN) could easily be removed completely via above exhaustive cycle reactions. The 3D Pd-Cu(OH)₂/CF cathode exhibits a 98.8 % conversion of NO₃^- to NH₄⁺ in 45 min with the reported highest removal rate of 0.017 cm^-2 min^-1, which is 19.4 times higher than that of CF. The converted NH₄⁺ was finally exhaustively oxidized to N₂ with a 98.7 % of TN removal in 60 min.

Peptide-directed Pd-decorated Au and PdAu nanocatalysts for degradation of nitrite in water

In this work, a palladium binding peptide, Pd4, has been used for the synthesis of catalytically active palladium-decorated gold (Pd-on-Au) nanoparticles (NPs) and palladium–gold (Pd_xAu_100−x) alloy NPs exhibiting high nitrite degradation efficiency. Pd-on-Au NPs with 20% to 300% surface coverage (sc%) of Au showed catalytic activity commensurate with sc%. Additionally, the catalytic activity of Pd_xAu_100−x alloy NPs varied based on palladium composition (x = 6–59). The maximum nitrite removal efficiency of Pd-on-Au and Pd_xAu_100−x alloy NPs was obtained at sc 100% and x = 59, respectively. The synthesized peptide-directed Pd-on-Au catalysts showed an increase in nitrite reduction three and a half times better than monometallic Pd and two and a half times better than Pd_xAu_100−x NPs under comparable conditions. Furthermore, peptide-directed NPs showed high activity after five reuse cycles. Pd-on-Au NPs with more available activated palladium atoms showed high selectivity (98%) toward nitrogen gas production over ammonia.

In this work, a palladium binding peptide, Pd4, has been used for the synthesis of catalytically active palladium-decorated gold (Pd-on-Au) nanoparticles (NPs) and palladium–gold (Pd_xAu_100−x) alloy NPs exhibiting high nitrite degradation efficiency.

Synthesis and catalytic utilization of bimetallic systems for wastewater remediation: A review

The environment is affected by agricultural, domestic, and industrial activities that lead to drastic problems such as global warming and wastewater generation. Wastewater pollution is of public concern, making the treatment of persistent pollutants in water and wastewater highly imperative. Several conventional treatment technologies (physicochemical processes, biological degradation, and oxidative processes) have been applied to water and wastewater remediation, but each has numerous limitations. To address this issue, treatment using bimetallic systems has been extensively studied. This study reviews existing research on various synthesis methods for the preparation of bimetallic catalysts and their catalytic application to the treatment of organic (dyes, phenol and its derivatives, and chlorinated organic compounds) and inorganic pollutants (nitrate and hexavalent chromium) from water and wastewater. The reaction mechanisms, removal efficiencies, operating conditions, and research progress are also presented. The results reveal that Fe-based bimetallic catalysts are one of the most efficient heterogeneous catalysts for the treatment of organic and inorganic contamination. Furthermore, the roles and performances of bimetallic catalysts in the removal of these environmental contaminants are different.

Applications of bimetallic PdCu catalysts

Bimetallic PdCu nanoparticles can be applied as catalysts in a wide range of chemical and electrochemical reactions.

Comparing electrocatalytic and thermocatalytic conversion of nitrate on platinum–ruthenium alloys

Comparison between thermocatalytic and electrocatalytic nitrate reduction reactions highlights mechanistic similarities and differences between the two reactions.

Binderless and Oxygen Vacancies Rich FeNi/Graphitized Mesoporous Carbon/Ni Foam for Electrocatalytic Reduction of Nitrate

Energy consumption and long-term stability of a cathode are two important aspects of great concern in electrocatalytic nitrate reduction. This work studied a binderless FeNi/graphitized mesoporous carbon directly formed on Ni Foam (FeNi/g-mesoC/NF, 7.3 wt % of Fe) and evaluated its electrocatalytic nitrate reduction performance. We proposed a unique structure model of FeNi/g-mesoC/NF cathode in which FeNi alloy nanoparticles were uniformly embedded in mesoporous carbon and graphitized carbon shells were coated on isolated alloy nanoparticles. Oxygen vacancies (OVs) in FeNi oxide passivating layer facilitate the conversion of NO₃^--N anions on cathode. Toxic NO₂^--N was almost undetected due to the synergetic effects of FeNi electrocatalysis, and the NO₃^--N conversion was high in comparation with ever reported iron-based cathode. The NO₃^--N conversion showed ultrahigh electrocatalytic stability during one-month-recycling test while the physiochemical properties showed negligible change for FeNi/g-mesoC/NF except the increase of OVs. The energy consumption to treat simulated underground water (50% of NO₃^--N conversion) was low (0.7 kWh mol^-1) for 50 mg L^-1 NO₃^--N. This binderless composite cathode shows great potential in electrocatalytic NO₃^--N removal in underground water.

Effect and mechanism of graphene structured palladized zero-valent iron nanocomposite (nZVI-Pd/NG) for water denitration

Nitrate reduction by zero-valent iron-based materials has been extensively studied. However, the aggregation of nanoparticles and the preference for unfavored ammonia products limit the application of this technology. To overcome this issue, this study introduced a novel synthesized nanoscale palladized zero-valent iron graphene composite (nZVI-Pd/NG) and explored its nitrate reduction efficiency. A nitrate removal rate of 97.0% was achieved after 120 min of reaction for an initial nitrate concentration of 100 mg N/L. The nitrogen gas selectivity was enhanced from 0.4% to 15.6% at the end point compared to nanoscale zero-valent iron (nZVI) particles under the same conditions. Further analyses revealed that zero-valent metal nanoparticles spread uniformly on the graphene surface, with a thin layer of iron (hydr)oxides dominated by magnetite. The nZVI-Pd/NG exhibited good catalytic activity with the associated activation energy of 17.6 kJ/mol being significantly lower than that with nZVI (42.8 kJ/mol). The acidic condition promoted a higher nZVI utilization rate, with the excess dosage of nZVI-Pd/NG ensuring a high nitrate removal rate for a wide pH range. This study demonstrates an improvement in nitrate reduction efficiency in a nZVI system by combining the exceptional properties of graphene and palladium.

A hybrid catalytic hydrogenation/membrane distillation process for nitrogen resource recovery from nitrate-contaminated waste ion exchange brine

Ion exchange is widely used to treat nitrate-contaminated groundwater, but high salt usage for resin regeneration and management of waste brine residuals increase treatment costs and add environmental burdens. Development of palladium-based catalytic nitrate treatment systems for brine treatment and reuse has showed promising activity for nitrate reduction and selectivity towards the N₂ over the alternative product ammonia, but this strategy overlooks the potential value of nitrogen resources. Here, we evaluated a hybrid catalytic hydrogenation/membrane distillation process for nitrogen resource recovery during treatment and reuse of nitrate-contaminated waste ion exchange brines. In the first step of the hybrid process, a Ru/C catalyst with high selectivity towards ammonia was found to be effective for nitrate hydrogenation under conditions representative of waste brines, including expected salt buildup that would occur with repeated brine reuse cycles. The apparent rate constants normalized to metal mass (0.30 ± 0.03 mM min^-1 g_Ru^-1 under baseline condition) were comparable to the state-of-the-art bimetallic Pd catalyst. In the second stage of the hybrid process, membrane distillation was applied to recover the ammonia product from the brine matrix, capturing nitrogen as ammonium sulfate, a commercial fertilizer product. Solution pH significantly influenced the rate of ammonia mass transfer through the gas-permeable membrane by controlling the fraction of free ammonia species (NH₃) present in the solution. The rate of ammonia recovery was not affected by increasing salt levels in the brine, indicating the feasibility of membrane distillation for recovering ammonia over repeated reuse cycles. Finally, high rates of nitrate hydrogenation (apparent rate constant 1.80 ± 0.04 mM min^-1 g_Ru^-1) and ammonia recovery (overall mass transfer coefficient 0.20 m h^-1) with the hybrid treatment process were demonstrated when treating a real waste ion exchange brine obtained from a drinking water utility. These findings introduce an innovative strategy for recycling waste ion exchange brine while simultaneously recovering potentially valuable nitrogen resources when treating contaminated groundwater.

Electrochemically mediated nitrate reduction on nanoconfined zerovalent iron: Properties and mechanism

Selective reduction of nitrate to N₂ is attractive but still a difficult challenge in the water treatment field. Herein, we established a flow-through electrochemical system packed with polymeric beads supported nZVI (nZVI@D201) for selective nitrate reduction. Consequently, efficient nitrate reduction in the flow mode was achieved on nZVI@D201 under electrochemical regulation with N₂ selectivity of up to 95% for at least 60 h. Otherwise, nZVI was gradually exhausted after 20 h, and the product was mainly the undesired NH₄⁺. Through a series of comparative experiments, we clarified that the enhanced nitrate reduction on nZVI under electrochemical regulation was mainly attributed to electrons (from cathode) and active hydrogen ([H]) rather than the previously speculated H₂. Combining the characterizations of nZVI during nitrate reduction by X-ray diffraction and X-ray photoelectron spectrometry, we found that nitrate reduction under electrochemical regulation was mediated by nZVI along with the resultant Fe⁰@Fe_xO_y-Fe(II) structure and was sustained by electrons (from cathode) and [H] via the in situ reduction of Fe(III) back to Fe(II). Meanwhile, the undesirable product NH₄⁺ was efficiently oxidized to N₂ by the active chlorine generated on the anode. This study not only clarifies the mechanism of enhanced nitrate reduction on nZVI via electrochemical regulation but also advances the technological coupling of nZVI reduction with electrochemistry.

Exhaustive denitrification via chlorine oxide radical reactions for urea based on a novel photoelectrochemical cell

Urea is a major source of nitrogen pollution in domestic sewage and its denitrification is difficult since it is very likely to be converted into ammonia or nitrate instead of expected N₂. Herein, we propose an exhaustive denitrification method for urea via the oxidation of amine/ammonia-N with chlorine oxide radical, which induced from a bi-functional RuO₂//WO₃ anode, and the highly selective reduction of nitrate-N on cathode in photoelectrochemical cell (PEC). Under illumination, the WO₃ photoanode side promotes the quantities hydroxyl and reactive chlorine radical, and these radicals are immediately combined to stronger chlorine oxide radical by RuO₂ side, which obviously enhances the efficiency and speed of the urea oxidation. Synchronously, the over-oxidized nitrate can be selectively reduced by Pd and Au nanoparticles on the surface of cathode. Eventually, exhaustive denitrification is realized by the circulative reaction. Experimental observations and theoretical calculation revealed that chlorine oxide radical promoted significant denitrification of urea with an efficiency of 99.74% in 60 min under the optimum condition. The removal rate constant of the RuO₂//WO₃ anode was 3.08 times than that of single WO₃ anode and 2.64 times than that of single RuO₂ anode, confirming the chlorine oxide radical had stronger ability on denitrification than reactive chlorine radical. Also, the bi-functional anode contributed to best current efficiencies, utilizing the energy availably. This work proposes a promising method of exhaustive denitrification for urea.