Conversion Of Nitrate Research Articles

Characteristics of secondary inorganic aerosols and contributions to PM2.5 pollution based on machine learning approach in Shandong Province

Primary emissions of particulate matter and gaseous pollutants, such as SO2 and NOx have decreased in China following the implementation of a series of policies by the Chinese government to address air pollution. However, controlling secondary inorganic aerosol pollution requires attention. This study examined the characteristics of the secondary conversion of nitrate (NO3−) and sulfate (SO42−) in three coastal cities of Shandong Province, namely Binzhou (BZ), Dongying (DY), and Weifang (WF), and an inland city, Jinan (JN), during December 2021. Furthermore, the Shapley Additive Explanation (SHAP), an interpretable attribution technique, was adopted to accurately calculate the contributions of secondary formations to PM2.5. The nitrogen oxidation rate exhibited a significant dependence on the concentration of O3. High humidity facilitates sulfur oxidation. Compared to BZ, DY, and WF, the secondary conversion of NO3− and SO42− was more intense in JN. The light-gradient boosting model outperformed the random forest and extreme-gradient boosting models, achieving a mean R2 value of 0.92. PM2.5 pollution events in BZ, DY, and WF were primarily attributable to biomass burning, whereas pollution in Jinan was contributed by the secondary formation of NO3− and vehicle emissions. Machine learning and the SHAP interpretable attribution technique offer a precise analysis of the causes of air pollution, showing high potential for addressing environmental concerns.

Tree-based ensemble machine learning model for nitrate reduction by zero-valent iron

The use of zero-valent iron (ZVI) for nitrate reduction has long been a topic of interest owing to its excellent reactivity compared to other inorganic electron donors. However, there is currently a lack of comprehensive and sophisticated models that can effectively integrate and analyze the dynamic data generated by this type of experiment. Such modeling is necessary to facilitate information mining and improve our understanding of the underlying mechanisms. Taking advantage of available published experimental data, a gradient-boosting decision tree and extreme gradient boosting were applied to 288 data points collected from peer-reviewed publications and compared with conventional non-tree-based models, i.e., artificial neural networks (ANN) and support vector regression (SVR). First, a predictive analysis was performed for the rate constant and the percentages of ammonium and nitrogen conversion using seven predictors related to ZVI properties, reactant concentration, and experimental parameters. Both tree-based models accurately predicted the target variables with comparable performance (R2 = 0.88–0.97) and outperformed ANN and SVR (R2 = 0.60–0.89). In addition, SHapley Additive exPlanations analysis revealed that ZVI properties (44 % average contribution) and experimental condition (68.5 % average contribution) were the key variables affecting the nitrate reduction rate constant and conversion product, respectively. Aerobic conditions were favorable for the conversion of nitrate to ammonium, with anoxic conditions being beneficial for the selectivity of nitrogen gas conversion. This study demonstrates a promising approach using machine learning with complex cross-system data for predicting the performance of water treatment and for mining valuable insights into the process by model interpretation. This method presents a new approach to revolutionize data analysis and modeling in water remediation, which can simplify the experimental operational burden.

Removal of Phosphorus and Cadmium from Wastewaters by Periphytic Biofilm

Phosphorus (Pi) and cadmium (Cd) contamination in water sources pose significant health risks and environmental concerns. Periphytic biofilms have been recognized for their ability to effectively remove these contaminants from aquatic environments. This study aimed to investigate the impact of photon and electron treatments on Pi and Cd removal by periphytic biofilms. The experiments spanned a monthly timeframe, focusing on how photon and electron treatments affected the contaminant removal efficiency of periphytic biofilms. The results revealed that while the introduction of electrons had a minimal impact on contaminant accumulation, the enhancement of photon exposure significantly improved the absorption capacity of periphytic biofilms. This, in turn, led to enhanced removal of Pi and Cd from the water. One possible explanation for this phenomenon is that photons played a crucial role in inducing nitrate and ammonium conversion, thereby facilitating the accumulation of 4.70 mg kg−1 Pi and 2.40 mg kg−1 Cd in periphytic biofilms. In contrast, electron treatment had limited effects on nitrate conversion. These findings provide valuable insights into the mechanisms underlying the removal of water contaminants by periphytic biofilms under the influence of electron and photon treatments. Furthermore, they have practical implications for improving pollutant removal capabilities in aquatic ecosystems using periphytic biofilms.

Effects of S-Allylcysteine-Rich Garlic Extract and Dietary Inorganic Nitrate Formula on Blood Pressure and Salivary Nitric Oxide: An Open-Label Clinical Trial Among Hypertensive Subjects.

The conversion of dietary inorganic nitrate(NO3-) to nitric oxide (NO) is a non-canonical pathway that plays an important role in NObiology, especially under pathological conditions. Inorganic NO3-supplementation is a proven method for controlling mild hypertension. Recent reports have suggested that another gaseous transmitter, hydrogen sulfide (H2S), influences NO biosynthesis and metabolism. Here, data are presented from an open-label clinical trial examining the effect of an encapsulated formulation (Vascanox® HP) that combines dietary sources of inorganic NO3- and S-allylcysteine (SAC), a source of H2S from garlic, on NO bioavailability and blood pressure in subjects experiencing elevated blood pressure or mild hypertension. An open-label clinical trial was conducted among patients with hypertension. Participants took Vascanox® for four weeks. Blood pressure was measured at baseline, two weeks, and four weeks. Salivary nitrite (NO2-), a surrogate of NO bioavailability, and NO3- were assessed prior to and two, six, and 24 hours after dosing on the first day of the study and prior to and two hours after dosing at subsequent study visits using saliva NO test strips. Changes in study outcomes over time were evaluated via analysis of variance (ANOVA) and paired t-tests. Twelve participants completed the clinical trial. Vascanox® HP decreased systolic blood pressure by ~11 mmHg (p < 0.001) at two weeks and persisted beyond four weeks with daily supplementation. It also decreased the diastolic blood pressure of hypertensive subjects but not normotensive ones. The magnitude of the decrease was 11 mmHg (p < 0.01) at four weeks of study. Measurements of salivary concentrations of NO2- revealed high peak levels (743 uM) at two hours post-administration and a slow decay to elevated levels (348 uM) at 24 hours. NO2- salivary concentrations, a surrogate biomarker of NO bioavailability, remained above baseline for the duration of the study. Vascanox® HP was shown to be a safe, effective, quick-acting, and long-lasting dietary supplement for controlling mild hypertension.

The Influencing Mechanisms of Reclaimed Water on N2O Production in a Multiyear Maize–Wheat Rotation

Reclaimed water (RW) is widely used in agricultural systems; however, it affects soil properties and the surrounding environment, thus influencing soil nitrogen transformation and increasing N2O and NO emissions. Understanding the influencing mechanism of N2O production in RW-irrigated soil is very important for water resource utilization and environmental protection, but it is rarely studied. This study investigated the impact of three nitrogen ions (NH4+, NO3−, NO2−) on the nitrogen transformation process and non-biological processes affecting NO and N2O emissions from soil under multiyear RW-irrigated conditions. The results showed that RW effectively increased the abundance of nitrifying and denitrifying functional genes, leading to a significant increase (p &lt; 0.05) in soil NO and N2O emissions under ammonium treatment. Furthermore, RW can reduce the cumulative NH3 emission by 19.11% compared to deionized water (DW). In nitrate treatment, RW can significantly increase (p &lt; 0.05) the nitrate conversion rate by increasing the abundance of denitrifying genes, but not significantly enhance N2O and NO emissions. In NO2− oxidation, RW could increase the abundance of nitrifying genes (AOA-amoA, AOB-amoA), thereby promoting the progression of nitrifier denitrification and leading to a substantial increase (p &lt; 0.05) in soil N2O production. In summary, RW irrigation primarily increases N2O emissions from soil by enhancing soil autotrophic nitrification and heterotrophic nitration. To effectively control soil N2O emissions under agricultural irrigation with RW, it is crucial to carefully manage soil nitrification and adjust the ratio of ammonium and nitrate in the soil.

Electrochemical reduction of aqueous nitrate using metallic silver particles as spatially suspended catalyst

Increasing nitrate concentrations in water sources is a serious concern. The current study demonstrates the electrochemical reduction of nitrate in the presence of silver metallic particles (AgMPs). Using Ti/RuO2 anode and a Ti cathode, nitrate removal in undivided and divided cells was limited to 18% and 77%, respectively, in the absence of AgMPs. Total Nitrogen (TN) removal in undivided and divided cells was nil and 29.5%, respectively, in the absence of AgMPs. Under identical conditions, the presence of 8 mM AgMPs increased nitrate removal to 63% and 99%, respectively, with significant improvement in nitrate conversion to N2. The effect of various parameters on nitrate removal and selectivity of products was studied using a divided cell: concentration of AgMPs, inter-electrode distance, cathode material, current density, initial nitrate concentration, and reusability of AgMPs. In the presence of 8 mM AgMPs, nitrate removal using different cathodes was Ti ≈ Fe > Cu > SS > Fe/Sn. The cathode type also affected the selectivity, with the maximum conversion to N2 achieved with the Ti cathode. Nitrate removal and conversion to N2 improved with the increase in current density up to 10 mA/cm2. Overall, the presence of AgMPs substantially improved nitrate removal and conversion to N2. The production of reductive species (AgHx) by the intercalation of nascent hydrogen on metallic silver appears to promote catalytic nitrate reduction. AgMPs could be reused up to 10 times with consistent performance. This work offers a new approach to nitrate reduction using an efficient and reusable catalyst.

Nitrate Reduction through Al-Fe Alloy Catalyst: Effects of Activation Pre-Treatment Method and Alloy Type

This study explored the nitrate reduction by Al-Fe alloy. The nitrate conversions of fresh, 30-day and 90-day alloys were 78.1%, 42.8% and 9.5%, respectively. Water activation promoted the reducing ability of the alloy (98% nitrate removal), which was higher than that of copper deposited alloy (66%) and H2-reducing/acid/alkali/Cl− activated alloy (no enhancement). The effects of pre-treatments on the surface O fraction changes confirmed the activation results. With increased Fe:Al mass ratio in the alloy, nitrate conversion initially decreased and then increased again, verifying the proposed electron-donator activity of Al or Fe in alloys. Al-Fe30 had the highest NO3− conversion and Al13Fe4 content, so Al-Fe30 was selected. Significant differences in conversion were observed in alloy usages of 5–10 and 15–30 g/L. High reduction performance (nitrate below the detection limit and 19.1% N2 selectivity) was achieved under the optimal conditions: 15 g/L Al-Fe30, 150 min reaction and without pH adjustment. The rate constants of nitrate removal, nitrogen generation and ammonia generation were k1 = 1.43 × 10−2, k2 = 3.41 × 10−2 and k3 = 10.58 × 10−2 min−1, respectively. The value of (k2 + k3)/k1 was 10, indicating that the conversion of nitrate into nitrite was the rate-determining step. The repetition reaction was performed, and the rate constant decreased as the reaction step was repeated.

(Invited) Recent Development of Electrochemical Nitrate Reduction

Understanding the complexities of nitrogen cycle is critical because when the cycle is not in balance, excess reduced and oxidized nitrogen compounds could result in harmful impacts to life. Electrocatalytic conversion of waste nitrate (NO3 -) to benign dinitrogen (N2) and ammonia (NH3) fuel is an appealing strategy that may easily integrate with renewable energy. The process does not require chemical addition and thus requires operating costs lower than thermal routes. Nearly all nitrogen-based fertilizers rely on ammonia as a feedstock, and thus the demand for this chemical is heavily dependent on the global population and food demand. Over the next three decades, the global population will continue to dictate the market size and value for ammonia, which consequently will have a significant impact on our energy infrastructure. While electrochemical nitrogen reduction reaction (NRR) requires multiple proton and electron transfer process, ammonia production from nitrate reduction (NO3RR) is catalytic easier than NRR because the adsorbed/activated ion reactants (nitrate and nitrite) have an advantage of a lower dissociation energy for the N−O and N=O bonds (204 and 607 kJ mol-1) than that of N≡N bond (941 kJ mol-1). The synergistic effect between Cu and Pd has shown a high nitrate conversion yield and nitrogen selectivity. Engineering the Pd and Cu sites is critical but examination of the use of secondary metals in shape-controlled nanoparticle and atomic reconstruction are rare.Here, we controlled Cu surface coverage on Pd nanocube nanoparticle to steer the selectivity between nitrogen and ammonia. In addition, the engineered the atomic structure of Pd and Cu nanoparticles showed high nitrate conversion yield, excellent dinitrogen selectivity, and outstanding cyclic stability. This study provides insights into the rational design of NO3RR electrocatalyst for efficiently transforming nitrate into nitrogen and ammonia.

(Invited) Insights on Decentralized Electrochemical Reduction of Nitrate

Ammonia is a chemical commodity with demonstrated global importance given its clear food-energy-water nexus. Ammonia large-scale production relies on the Haber-Bosch process which employs hydrogen and nitrogen as reagents under high pressure (10 MPa) and high temperature (500 °C). This process is responsible for more than ~180 million tons of ammonia production per year to satisfy industrial and agricultural needs. Sustaining the ammonia production through the Haber-Bosch process has a huge carbon footprint as sustainability toll. The Haber-Bosch process generates over 450 million tons of CO2 annually, which represents almost 1.5 % of global emissions per year. The fossil fuel reliance of this high energy demand process of ammonia synthesis defines the world economy in many ways. For instance, recent cost increase on ammonia production has had a direct impact on food price inflation worldwide. Ammonia shortage has different causes such as geopolitical conflicts around the world, and increased natural gas price, among others. Thus, a fossil fuel-free green chemical synthesis alternative is required to alleviate the ammonia demand while diminishing the carbon footprint.Agriculture is strongly dependent on ammonia-based fertilizers generating a nitrogen-imbalance in the environment due to N-species (e.g., NO3 -, NO2 -, NH4 +) runoff to ground and surface water. Among these N-species, high concentrations of nitrate and nitrite are highlighted as frequent water quality violations in the US. Concentrations way above the regulated maximum contaminant levels (MCL) of 10 mg L-1 for NO3 --N and 1 mg L-1 for NO2 --N are commonly found in many water resources. Implementation of electrochemical reduction of nitrate (ERN) to produce ammonia can address environmental pollution while producing ammonia under mild conditions with less carbon footprint. The ERN process can become a decentralized ammonia production system alleviating needs at small and even medium scale production. Selectively reducing nitrate towards ammonia has gained attention due to several advantages respect to direct N2 gas reduction such as higher solubility and lower energy barrier. This electrochemically driven process can be coupled with renewable energy systems and therefore holds the promise to enable fossil-free ammonia production off-grid.The mechanisms for electrochemical conversion of nitrate to nitrogen gas are highly complex, involving numerous reactions, products and stable intermediates (e.g., ammonia, nitrite, hydrazine, hydroxylamine, nitric oxide, nitrous oxide) spanning the many nitrogen oxidation states (from -III up to +V). Controlling the reaction selectivity of a complex mechanisms that involve different charge transfer and chemical reactions (e.g., hydrogenation) is challenging. This invited presentation aims to showcase some of the advances in terms of electrode engineering design to steer selectivity towards ammonia production as well as some of the challenges that will be faced when translating lessons learnt towards application under real environmental samples.

Electrocatalytic Nitrate Reduction to Ammonia at Atomically Dispersed Titanium Sites on Carbon Nanoflowers

Active nitrogen species are widely acknowledged as pollutants that are both harmful to the environment as well as human health. Emissions from wastewater and agricultural applications contain nitrogen in many forms, but in soils and waterways nitrate (NO3 -) predominates due to the presence of nitrifying bacteria. In comparison to more reduced forms of nitrogen such as total ammonia (NH3/NH4 +), nitrate is not only a less versatile species from a chemical manufacturing and energy standpoint; it is also more difficult to selectively separate from wastewater streams. For instance, we have conducted considerable work on electrochemical ammonia stripping, achieving high selectivities and even enabling concentration of ammonia in the form of strong ammonium salt solutions.1 Moreover, it is estimated that recovery of active nitrogen species from municipal and agricultural wastewaters could offset the need for ~30% of Haber-Bosch ammonia synthesis. From this perspective, it is desirable to develop catalysts that are active and selective for the conversion of nitrate to ammonia. Titanium metal has been demonstrated as a relatively active and selective catalyst for nitrate reduction to ammonia in acid;2 however, similarly active and selective catalysts under neutral-to-alkaline conditions are a subject of active research.3–6 Here we report on the performance of atomically dispersed titanium sites on carbon nanoflowers7 for the reduction of nitrate to ammonia under alkaline conditions. The dispersity of these catalysts was supported by XAS (little/negligible Ti-Ti scattering, putative Ti-N and Ti-N-C scattering), STEM (visual confirmation of atoms), and XRD (no crystalline Ti phases observed). We contrast the performance of these atomically dispersed catalysts with that of bulk titanium foil, which shows comparably low activity and selectivity for nitrate reduction at similar pHs and potentials. In addition, we compare the atomically dispersed titanium catalysts with titanium nanoparticles, as a first-order check for catalyst aggregation under reductive reaction conditions. Moreover, the performances of all of these catalysts are compared with the baseline/controls of bare carbon paper, as well as carbon nanoflowers supported on carbon paper without titanium dopant. Additional control experiments include tests conducted in the absence of nitrate. These lines of evidence, together with the relatively high rate of ammonia production (>0.01 mmol / cmgeo 2 / h, >10 mmol / mg Ti / h), support the notion that atomically dispersed titanium is uniquely active and selective for NO3RR under alkaline conditions.Further, for the above catalysts at pH ~13, we show the impacts of potential on the product distribution, including nitrogen mass balance and Faradaic efficiency. As a best practice, we demonstrate consistent closure of both metrics. Potential was varied between -0.4 V and -0.85 V vs. RHE. Partial current toward NO3RR increases with increasing overpotential, as another sanity check supporting the findings. Notably, in addition to observing ammonia and nitrite as products of NO3RR, we also checked for other stable intermediates (in addition to N2) along the 8-electron reaction pathway – and as a result, we are able to report small amounts (FE ~5-10%) of hydroxylamine (NH2OH) product.In the specific case of atomically dispersed titanium on carbon nanoflowers, we also report on certain observed electrolyte effects, such as the incorporation of perchlorate vs. sulfate as a supporting electrolyte. We demonstrate the role of supporting salt both in modifying selectivity – e.g. sulfate suppressing HER, possibly via site-blocking – as well as in altering the carbon support itself. These observations contribute to a growing body of work that will enable the engineering of these catalysts for high-rate reduction of nitrate to ammonia, aiding both wastewater remediation and the decarbonization of ammonia synthesis.

Implications of Cation-Dependent Proton Affinity on Electrocatalytic Nitrate Reduction

Perturbation of the nitrogen cycle via Haber-Bosch nitrogen fixation and concomitant fertilizer amendment has manifested in alarmingly increased groundwater nitrate concentrations, with global human-health and ecological ramifications. Correspondingly, the denitrification of drinking water resources represents a growing area of energy consumption, though is still outpaced by the carbon-intensive Haber-Bosch process. Electrocatalytic nitrate reduction therefore offers a competitive and distributable green alternative to traditional ammonia production approaches, using renewably-sourced electrons and protons sourced from water as reducing agents without the need for added chemicals or elevated temperatures and pressures.While considerable progress has been made in understanding the effects of electrocatalyst material parameters on nitrate reduction activity and ammonium selectivity, these studies are conducted in well-controlled and highly purified electrolytes that lack the complexity typical of expected nitrate-rich waste source streams. Recent literature on other electrocatalytic hydrogenation reactions (hydrogen evolution and carbon dioxide reduction reactions) has identified cation-dependent activity and selectivity, indicating the significance of these nominally simple ions in facilitating proton abstraction from water—of particular importance in the proton-hungry conversion of nitrate to ammonium (10 H+ per nitrate converted). Here, we investigate cation-dependent (Li+, Na+, K+) nitrate reduction activity and selectivity in (phosphate) buffered neutral electrolytes across a series of transition metals known to exhibit varying ammonium selectivities (Ag, Cu, Co). We frame our discussion of observed cation-dependent activity and selectivity within empirical literature on cation hydration free energy to understand the role of cations in mediating proton transfer during electrocatalytic hydrogenation reactions.

Relating Near-Surface Ti Electrode Structure with Electrochemical Nitrate Reduction Performance Via Synchrotron X-Ray Characterization

The global nitrogen cycle has been severely skewed since the widespread adoption of the Haber-Bosch process to produce ammonia (NH3). Currently, removal of reactive nitrogen (inorganic forms besides N2) from the environment lags behind its production and emission to the environment. Nitrate is one of the most prevalent waterborne nitrogen pollutants and, in its excess, threatens the health of ecosystems. Enabling a sustainable food-energy-water nexus requires feeding a growing population while minimizing environmental impacts. Therefore, selective electrochemical nitrate reduction (NO3RR) to NH3 can couple water purification and NH3 production, helping offset the energy- and carbon-intensive Haber Bosch process. NH3 recovery from emissions could contribute over 20 million tons N per year by 2050 (~ 10% of projected N demand).Noble and transition metal catalysts including single metals (e.g., Pt, Rh, Ru, Ir, Pd, Cu, Ag, Au)[1,2] and alloys (e.g., CuNi)[3] have been studied for NO3RR. Notably, under acidic conditions, polycrystalline titanium has been demonstrated to display robust and efficient NO3RR.[4] Ti is corrosion-resistant, a poor hydrogen evolution catalyst, and a readily available, abundant metal. However, under acidic and reducing conditions, Ti forms a water-stable hydride (TiHx, 0<x≤2).[5] It remains unclear how the degree of surface hydride formation – which alters the physical and electronic properties of the electrode surface – impacts NO3RR performance. Thus, rationally implementing Ti-catalyzed NO3RR requires improved understanding of how near-surface Ti-hydride forms and influences NO3RR activity and selectivity.In this work, we show that near-surface Ti-hydride formation is a function of the duration and magnitude of applied NO3RR potential. A combination of ex situ grazing-incidence X-ray diffraction (GIXRD – probes long-range, crystalline order) and X-ray absorption spectroscopy (XAS – probes short-range, atom-specific probe into local coordination environments) enabled quantitative near-surface characterization of Ti-hydride electrodes. Through electrochemical testing and density functional theory calculations, we investigated the role near-surface Ti-hydride may play in electrochemical nitrate conversion. Our results preliminary suggest that near-surface hydride content plays a relatively minor role in steering NO3RR performance compared to applied potential and electrolyte effects. In addition, we are performing ongoing in situ GIXRD and XAS measurements to interrogate the transient nature and interplay of near-surface hydride formation and NO3RR. Put in context, our results help prioritize how Ti-catalyzed NO3RR processes can be optimized for electrified ammonia production and wastewater remediation.[1] G. E. Dima, A. C. A. de Vooys, M. T. M. Koper, Journal of Electroanalytical Chemistry 2003, 554–555, 15–23.[2] G. E. Dima, G. L. Beltramo, M. T. M. Koper, Electrochimica Acta 2005, 50, 4318–4326.[3] Y. Wang, A. Xu, Z. Wang, L. Huang, J. Li, F. Li, J. Wicks, M. Luo, D.-H. Nam, C.-S. Tan, Y. Ding, J. Wu, Y. Lum, C.-T. Dinh, D. Sinton, G. Zheng, E. H. Sargent, J. Am. Chem. Soc. 2020, 142, 5702–5708.[4] J. M. McEnaney, S. J. Blair, A. C. Nielander, J. A. Schwalbe, D. M. Koshy, M. Cargnello, T. F. Jaramillo, ACS Sustainable Chem. Eng. 2020, 8, 2672–2681.[5] Y. Liu, Z. H. Ren, J. Liu, R. F. Schaller, E. Asselin, J. Electrochem. Soc. 2019, 166, C3096–C3105.

(Invited) Electrifying Nitrogen Transformations: Decarbonizing Ammonia Using Plasma Catalysis and Electrochemical Reduction of Nitrate in Wastewaters

Nitrogen fixation in fertilizers forms the basis of modern agriculture and mediates global food insecurity. Energy-dense nitrogen-based compounds also represent promising carbon-free alternative fuels when synthesized using sustainable energy and materials. However, conventional thermally-activated nitrogen conversion processes consume substantial amounts of fossil fuels as materials and energy inputs, leading to an unsustainable energy and carbon footprint. Therefore, electron-driven approaches are needed to establish fossil-free nitrogen interconversions. Electrochemical and plasma-activated reactions operate under mild conditions, enabling facile coupling to intermittent renewable energy sources to reduce CO2 emissions and reactive nitrogen pollution, facilitating storage and transportation of renewable energy in nitrogen-based fuels.Non-thermal plasma has been used to synthesize ammonia under mild conditions, but the dearth of fundamental understanding of plasma catalytic reactions handicaps the development of efficient plasma catalytic N2 conversion. Therefore, an in situ FTIR reactor was employed in combination with steady-state flow reactor experiments and plasma kinetic modeling to elucidate the surface reaction mechanisms and plasma-catalyst interactions. Ammonia yield can be influenced by plasma-derived intermediates and their interactions with catalyst surfaces, which lead to different reaction pathways. Furthermore, techno-economic analysis was employed to reveal the threshold efficiency required for a plasma process to become environmentally and economically competitive.Beyond electrification of N2 fixation for ammonia synthesis, existing waste streams containing fixed nitrogen as nitrate may be mined for fertilizer or carbon-free renewable energy storage while avoiding the need for H2 as a reagent, via selective nitrate electroreduction to ammonium. Nitrate electrochemical reduction can eliminate the production of concentrated waste streams while avoiding the addition of reductant or hole scavenger chemicals. However, major challenges for nitrate removal from water via electrochemical conversion involve reducing the use of expensive precious metal electrocatalysts while also improving the reaction activity and selectivity, stability, and mass transport of nitrate to electrocatalyst active sites. Therefore, electrochemical membranes were employed as porous flow-through electrodes to address these challenges based on improved mass transport and altered kinetics under flow conditions within membrane pores.Conductive electrified membranes, fabricated using inexpensive materials and green synthesis methods, showed significantly higher nitrate removal efficiency during electrified filtration compared to flow-by mode. The small pore sizes in the membrane reduced the diffusional boundary layer by several orders of magnitude with respect to flow-by mode, overcoming diffusional mass transport limitations. During electro-filtration, the nitrate conversion activity was easily controlled by tuning the permeate flow rate, regardless of applied potential. These multifunctional electrified membranes for reduction of nitrate in wastewaters have the potential to partially displace carbon-intensive industrially synthesized ammonia while simultaneously accomplishing water decontamination.

Highly selective and efficient electrocatalytic removal of low-concentration nitrate ions using self-supported porous Ni0.8Cu0.2O@CuO heterostructure arrays

The electrocatalytic conversion of nitrate (NO3−) to nitrogen (N2) is a promising strategy for addressing the global issue of NO3− pollution in surface water and groundwater. However, the scarcity of efficient denitrification catalysts hinders the development of the aforementioned process, particularly at low NO3− concentrations. Herein, a self-supporting array of defect-rich heterostructured nanowires (Ni0.8Cu0.2O@CuO@NF) was synthesized for highly selective reduction of NO3− to N2. The introduction of Cu accelerated the rate-limiting step (NO3− to NO2−), and the presence of defect-rich heterojunctions increased the catalytic active sites of the catalyst, thereby accelerating the reduction of NO3− and improving the selectivity towards N2. Ni0.8Cu0.2O@CuO@NF exhibited excellent denitrification capacity, with approximately 100% of NO3− degradation and 99.9% of N2 selectivity under optimum parameter conditions, as well as ultra-high stability during the 8-cycle test. Significantly, NO3− conversion and N2 selectivity for actual wastewater with complex ingredients were 70.7% and 99.1%, respectively. This study presents a novel concept for the development of cost-effective and highly effective catalysts for water and environmental restoration.

Defect-rich Cu@CuTCNQ composites for enhanced electrocatalytic nitrate reduction to ammonia

Electrochemical conversion of nitrate (NO3−) pollutants into chemical feedstock and fuel ammonia (NH3) can contribute to sustainable mitigation of the current severe energy and environmental crises. However, the electrocatalytic NO3− reduction to NH3 (NRA) involves a sluggish multielectron and proton transfer process that competes with the hydrogen evolution reaction (HER) in aqueous media, imposing great challenges in developing highly selective catalysts for NRA. In this study, we developed a copper and copper-tetracyanoquinodimethane composite catalyst (Cu@CuTCNQ), which possesses a high density of copper vacancy defects. This catalyst has been proven to be efficient for NRA through an in situ electrochemical reconstruction method. The structural evolution of CuTCNQ during NRA was investigated by in situ Raman spectroscopy, which indicated an accelerated charge transfer from the CuTCNQ substrate to the derived Cu, which facilitated the adsorption activation of NO3−. The obtained Cu@CuTCNQ exhibited an excellent catalytic performance for NRA, with a Faradaic efficiency of 96.4% and productivity of 144.8 μmol h−1 cm−2 at −0.6 V vs. a reversible hydrogen electrode, superior to Cu nanoparticle counterparts and most Cu-based catalysts. Cu vacancy defects and sufficient interfacial charge transfer synergistically optimize the charge distribution of Cu active sites, reduce the energy barrier for NO3− adsorption, and promote deoxidation and hydrogenation processes, thus enhancing NRA and selectivity.

Microenvironmental corrosion and hydrolysis induced two-dimensional heterojunction of copper oxide@ferriferrous oxide for efficient electrochemical nitrate reduction to ammonia

Electrochemical nitrate reduction (NO3RR) to ammonia could realize the green synthesis of ammonia and remove nitrate pollutant, which is of great significance to alleviate energy crisis and environmental problem. However, low ammonia selectivity and Faradic efficiency limits its further development. Herein, copper oxide @ ferriferrous oxide heterojunction (CuO NWAs@Fe3O4) was constructed by the in-situ growth of Cu(OH)2 nanowire arrays (Cu(OH)2 NWAs), microenvironmental corrosion and hydrolysis of Fe(NO3)3, and high-temperature pyrolysis. Because of CuO NWAs with excellent adsorption/activation of nitrate and Fe3O4 with high affinity for atomic hydrogen, the heterojunction demonstrated significantly improved NO3RR performance with nitrate conversion rate of 95.93%, ammonia selectivity of 88.68%, Faradaic efficiency of 97.46% and ammonia yield rate of 1.691 mmol h−1 cm−2 at −0.27 V vs. RHE. Moreover, this heterojunction displayed excellent durability with a high Faradaic efficiency of 94.45% after consecutive six cycles and a wide application range from 600 ppm to 2000 ppm. This work will lay the foundation for the design and fabrication of high-performance electrocatalysts for NO3RR.

Constructing n-type TiO2 by Nb doping for electrocatalytic nitrate reduction to ammonia at ambient conditions

Electrocatalytic reduction of nitrate is an effective approach for synthesis of value-added ammonia. However, durable and efficient catalysts with excellent selectivity are needed due to the large dissociation of nitrate. In this study, we propose Nb doped TiO2 as a potential catalyst for nitrate conversion to ammonia at ambient conditions based on material calculations, which reveal the free energy for nitrate adsorption is as low as −0.55 eV and the deoxidation steps (∗NO3→∗NHOH) continuously decreases. Such catalyst achieves a large NH3 yield of 27.94 ± 1.3 mg h−1 mg−1cat. and a high Faradaic efficiency of 70.64 ± 2.3% at −1.35 V vs RHE, and it also shows excellent electrochemical durability and chemical stability.

Dual-Site W-O-CoP Catalysts for Active and Selective Nitrate Conversion to Ammonia in a Broad Concentration Window.

Environmentally friendly electrochemical reduction of contaminated nitrate to ammonia (NO3 - RR) is a promising solution for large quantity ammonia (NH3 ) production, which, however, is a complex multi-reaction process involving coordination between different reaction intermediates of nitrate reduction and water decomposition-provided active hydrogen (Hads ) species. Here, a dual-site catalyst of [W-O] group-doped CoP nanosheets (0.6W-O-CoP@NF) has been designed to synergistically catalyze the NO3 - RR and water decomposition, especially the reactions between the intermediates of NO3 - RR and water decomposition-provided Hads species. This catalytic NO3 - RR exhibits an extremely high NH3 yield of 80.92mg h-1 cm-2 and a Faradaic efficiency (FE) of 95.2% in 1m KOH containing 0.1m NO3 - . Significantly, 0.6W-O-CoP@NF presents greatly enhanced NH3 yield and FE in a wide NO3 - concentration ranges of 0.001-0.1m compared to the reported. The excellent NO3 - RR performance is attributed to a synergistic catalytic effect between [W-O] and CoP active sites, in which the doped [W-O] group promotes the water decomposition to supply abundant Hads , and meanwhile modulates the electronic structure of Co for strengthened adsorption of Hads and the hydrogen (H2 ) release prevention, resultantly facilitating the NO3 - RR. Finally, a Zn-NO3 - battery has been assembled to simultaneously achieve three functions: electricity output, ammonia production, and nitrate treatment in wastewater.

A multifunctional copper single-atom electrocatalyst aerogel for smart sensing and producing ammonia from nitrate

Despite modern chemistry's success in providing affordable fertilizers for feeding the population and supporting the ammonia industry, ineffective nitrogen management has led to pollution of water resources and air, contributing to climate change. Here, we report a multifunctional copper single-atom electrocatalyst-based aerogel (Cu SAA) that integrates the multiscale structure of coordinated single-atomic sites and 3D channel frameworks. The Cu SAA demonstrates an impressive faradaic efficiency of 87% for NH3 synthesis, as well as remarkable sensing performance with detection limits of 0.15 ppm for NO3- and 1.19 ppm for NH4+. These multifunctional features enable precise control and conversion of nitrate to ammonia in the catalytic process, facilitating accurate regulation of the ammonium and nitrate ratios in fertilizers. We thus designed the Cu SAA into a smart and sustainable fertilizing system (SSFS), a prototype device for on-site automatic recycling of nutrients with precisely controlled nitrate/ammonium concentrations. The SSFS represents a forward step toward sustainable nutrient/waste recycling, thus permitting efficient nitrogen utilization of crops and mitigating pollutant emissions. This contribution exemplifies how electrocatalysis and nanotechnology can be potentially leveraged to enable sustainable agriculture.

Fe/Cu diatomic catalysts for electrochemical nitrate reduction to ammonia

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