A Ni / Fe3O4@CN Schottky heterojunction catalyst, its preparation method, and its application in nitrate reduction.

Abstract

This invention discloses a Ni / Fe3O4@CN Schottky heterojunction catalyst, its preparation method, and its application in nitrate reduction, belonging to the field of electrochemical catalytic materials and their preparation technology. This invention solves the problems of insufficient catalytic activity and ammonia selectivity in existing electrochemical reduction processes of nitrate in water. This invention employs a one-step grinding pyrolysis method to synthesize a Ni / Fe3O4@CN composite material composed of a nitrogen-doped carbon-coated Ni / Fe3O4 heterostructure. By constructing a heterostructure interface with a clear Schottky contact in situ, electronic modulation of the interface between Ni and Fe3O4 is achieved. Experimental results show that an excellent Faraday efficiency of 99.1% and a yield of approximately 12.61 mg h⁻¹ are achieved in alkaline media at -0.7 V vs. RHE potential. ‑ ¹ mg cat ‑ ¹ It exhibits high ammonia yield and strong durability after 55 hours of continuous operation and 20 cycles.

Description

Technical Field

[0001] This invention relates to a Ni / Fe3O4@CN Schottky heterojunction catalyst, its preparation method, and its application in nitrate reduction, belonging to the field of electrochemical catalytic materials and their preparation technology. Background Technology

[0002] Ammonia (NH3) is an indispensable raw material for fertilizers worldwide and a cornerstone of the booming hydrogen economy, with huge annual demand. In addition to agricultural uses, ammonia also plays an important role in pharmaceuticals, explosives, and chemical synthesis; it has a hydrogen content as high as 17.6 wt% and can be liquefied at -33.4°C, making it convenient to store and transport, thus it is regarded as a highly promising carbon-free energy carrier.

[0003] Currently, industrial ammonia synthesis mainly relies on Haber. The Bosch process, despite its historical success, suffers from significant energy consumption and CO2 emissions due to its demanding operating conditions [J. Guan, L. Cai, W. Li, H. Zhou, Y. Huang, Boosting nitrate electroreduction to ammonia on atomic Ru-Co pair sites in hollow spinels, Appl. Catal., B, 358 (2024) 124387.]. Furthermore, this process typically requires operation at high pressures of 150-300 bar and high temperatures of approximately 500°C, consuming about 1-2% of global energy supply and contributing approximately 1.8% of global CO2 emissions (over 450 million tons annually). Even though green ammonia can reduce carbon emissions by about 90%, its production cost remains significantly higher than that of traditional gray ammonia, making it economically uncompetitive. To achieve sustainable ammonia production, electrocatalytic nitrogen reduction (NRR) has been extensively explored under ambient temperature conditions. However, the chemical inertness of the N≡N triple bond (946 kJ / mol) and the extremely low solubility of N2 in aqueous electrolytes severely limit the ammonia yield and Faraday efficiency (FE) of NRR, hindering its practical application.

[0004] At the same time, nitrates (NO3) in industrial wastewater and agricultural runoff -The widespread presence of nitrates has caused serious environmental problems, including eutrophication of water bodies and potential health risks [K. Wang, Y. Chen, T. Zhao, S.-H. Ho, Breaking Dilution Limits: Electrocatalytic Ammonia Production from Low-Concentration Nitrate Streams, Adv. Mater., 38 (2026) e17391.]. Long-term intake of nitrates can induce methemoglobinemia (commonly known as blue baby syndrome) and even increase the risk of cancer. Traditional treatment methods, such as ion exchange, reverse osmosis, and biological denitrification, generally suffer from drawbacks such as high cost, easy generation of secondary pollution, or long treatment cycles, making it difficult to achieve efficient recovery of nitrogen resources.

[0005] Therefore, electrocatalytic nitrate reduction (NO3RR) to ammonia production has become a compelling alternative. From a thermodynamic perspective, NO3... - The dissociation energy of the N=O bond (204 kJ / mol) is much lower than that of the N≡N bond (946 kJ / mol), making nitrate a more favorable nitrogen source for electrocatalytic synthesis. Furthermore, unlike N₂, which has low solubility and limited mass transfer in aqueous electrolytes, NO₃… -The high solubility of Mn-Co(OH)2 ensures a sufficient supply of reactants at the electrode interface. This waste-to-resource approach has dual benefits: reducing nitrogen pollution while providing an efficient pathway for green ammonia production [S. Liang, X. Teng, H. Xu, L. Chen, J. Shi, H*Species Regulation by Mn-Co(OH)2for Efficient Nitrate Electro-reduction in Neutral Solution, Angew. Chem., Int. Ed., 63 (2024) e202400206.]. However, the multi-electron proton transfer process inherently leads to slow NO3RR reaction kinetics. Furthermore, the intense competition from the hydrogen evolution reaction (HER) further reduces the Faraday efficiency, especially at large overpotentials. These obstacles stemming from the dual barriers of intermediate adsorption energy mismatch and HER competition necessitate the development of cost-effective and robust electrocatalysts [Z. Zhu, Y. Zhang, C. Zhang, X. Chen, J. Hu, Harnessing Interfacial Electronic Effects to Advance Electrocatalytic Ammonia Synthesis, Adv. Energy Mater., 16 (2026) e05289.].

[0006] While noble metals such as Au, Ru, and Pd possess excellent intrinsic activity, their scarcity and high cost severely restrict their large-scale application. For example, Han et al. prepared Pd nanocrystals with controllable crystal planes using a seed-mediated method, where the Pd(111) crystal plane achieved an ammonia-farada efficiency of 79.91% and a h₂O₅ efficiency of 0.5485 mmol / L at -0.70 V vs. RHE. -1 cm -2 NH4 + Yield, studies have found optimized NO3 - Adsorption, a low rate-determining step energy barrier, and suppressed HER are key to its superior performance (Han Y, et al. J. Colloid Interface Sci. 2021, 600, 620-628).

[0007] Currently, transition metal-based materials have shown great potential in the electrocatalysis of NO3-reduction due to their tunable electronic structures. Among them, copper (Cu) exhibits significant potential due to its highly occupied d orbitals and its affinity for NO3-. -The excellent symmetry and energy matching between molecular orbitals have been extensively studied, thus promoting the adsorption and activation of reactant species [R. He, L. Sun, K. Ren, X. Li, P. Tian, ​​J. Ye, Tuning the N-intermediate adsorption of Cu catalysts for efficient electroreduction of nitrate to ammonia, J. Mater. Chem. A, 14 (2026) 394-402.]. However, its inherently slow hydrogenation kinetics result in unsatisfactory ammonia yields on pristine Cu surfaces. On the other hand, nickel (Ni)-based materials are promising candidates due to their faster kinetics for subsequent protonation steps, but they suffer from high overpotentials and poor ammonia selectivity.

[0008] To address the aforementioned issues, it is essential to provide a novel electrocatalyst for the reduction of nitrates in water. Summary of the Invention

[0009] To address the problems of insufficient catalytic activity and ammonia selectivity in existing electrochemical reduction processes of nitrates in water, this invention provides a Ni / Fe3O4@CN Schottky heterojunction catalyst, its preparation method, and its application in nitrate reduction.

[0010] The technical solution of this invention: One of the objectives of this invention is to provide a method for preparing a Ni / Fe3O4@CN Schottky heterojunction catalyst. The method involves mixing a nickel nitrate source, an iron nitrate source, and melamine, grinding them until homogeneous, and then calcining them at high temperature under a hydrogen atmosphere to obtain the Ni / Fe3O4@CN heterojunction catalyst.

[0011] Further specifying, the molar ratio of nickel nitrate source, ferric nitrate source, and melamine is 1:1:1.

[0012] Further specifying, the nickel nitrate source is nickel nitrate hexahydrate, and the ferric nitrate source is ferric nitrate nonahydrate.

[0013] Further specified, the hydrogen-containing atmosphere is a mixture of hydrogen and argon, wherein the hydrogen content is 5%.

[0014] Further specified, the pyrolysis temperature is 500℃ and the holding time is 6h.

[0015] Further specified, the heating rate during the pyrolysis process is 5°C / min, and the cooling method is natural cooling to room temperature.

[0016] The second objective of this invention is to provide a Ni / Fe3O4@CN Schottky heterojunction catalyst prepared by the above method.

[0017] The third objective of this invention is to provide an application of the above-mentioned Ni / Fe3O4@CN Schottky heterojunction catalyst, specifically as a catalyst for the electrochemical reduction reaction of reducing nitrates in water to ammonia.

[0018] The fourth objective of this invention is to provide a method for reducing nitrates. Specifically, this method involves using the aforementioned Ni / Fe3O4@CN Schottky heterojunction catalyst as an electrocatalyst under alkaline conditions, and a mixed solution of 0.5M KNO3 and 0.1M K2SO4 as the electrolyte, to reduce NO3 through an electrochemical reduction reaction. - reduction.

[0019] Further specifying, the operating voltage for the electrochemical reduction reaction process is -0.3 to -0.8 V vs. RHE.

[0020] Beneficial effects: This invention employs a one-step grinding and pyrolysis method to synthesize a Ni / Fe3O4@CN composite material composed of a nitrogen-doped carbon-coated Ni / Fe3O4 heterostructure. By constructing a heterostructure interface with a defined Schottky contact in situ, electronic modulation of the Ni-Fe3O4 interface is achieved. The Schottky interface between Ni and Fe3O4 enables spatial decoupling of catalytic functions. This spatial decoupling of functional sites effectively resolves the kinetic mismatch between intermediate adsorption and hydrogenation, thus surpassing the performance limits of traditional single-component systems. Specific Ni sites trigger nitrate activation, while the adjacent Fe3O4 phase acts as a functional reservoir, dominating water molecule dissociation and providing active *H generation and interfacial transport. This spatially decoupled tandem catalysis effectively balances nitrate activation and hydrogenation kinetics, significantly improving reduction efficiency and ammonia selectivity. Experimental results show that, under alkaline conditions and suitable potential, the Ni / Fe3O4@CN composite material exhibits excellent ammonia yield and long-term operational stability. Specifically, in alkaline conditions and at a potential of -0.7V vs. RHE, it achieves an excellent Faraday efficiency (FE) of 99.1% and a yield of approximately 12.61 mg h⁻¹. - ¹ mg cat - ¹ It exhibits high ammonia yield and strong durability after 55 hours of continuous operation and 20 cycles, with no significant performance degradation.

[0021] The Ni / Fe3O4@CN catalyst provided by this invention can directly convert nitrates in water into high-value-added ammonia using an electroreduction method, achieving integrated pollutant removal and resource recovery. It is suitable for treating various nitrate-containing wastewaters, such as agricultural runoff, industrial drainage, and polluted groundwater, and can be coupled with renewable energy-driven electrolysis systems, showing promising engineering application prospects. Attached Figure Description

[0022] Figure 1 A schematic diagram of the process for preparing Ni / Fe3O4@CN; Figure 2 The XRD comparison spectra of Ni / Fe3O4@CN prepared in Example 1, Ni@CN prepared in Comparative Example 1, and Fe3O4@CN prepared in Comparative Example 2 are shown. Figure 3 SEM images of Ni / Fe3O4@CN prepared in Example 1, Ni@CN prepared in Comparative Example 1, and Fe3O4@CN prepared in Comparative Example 2; Figure 4 TEM image, HRTEM image and selected area electron diffraction (SAED) image of Ni / Fe3O4@CN prepared in Example 1; Figure 5 This is a STEM-EDS elemental surface scan of Ni / Fe3O4@CN prepared in Example 1; Figure 6 The XPS full spectrum of Ni / Fe3O4@CN prepared in Example 1; Figure 7 Comparison of Ni 2p XPS spectra of Ni / Fe3O4@CN and Ni@CN prepared in Example 1; Figure 8 The above are comparative Fe 2p XPS spectra of Ni / Fe3O4@CN and Fe3O4@CN prepared in Example 1. Figure 9 The XPS spectra of Ni / Fe3O4@CN prepared in Example 1 are shown, where (a) is the C 1s spectrum, (b) is the high-resolution N 1s spectrum, and (c) is the high-resolution O 1s spectrum. Figure 10 The Ni / Fe3O4@CN, Ni@CN, and Fe3O4@CN prepared in Example 1 were in the presence of NO3. - Or without NO3 - Comparison of LSV curves under different conditions; Figure 11 The graph shows the FE variation of Ni / Fe3O4@CN, Ni@CN, Fe3O4@CN, and @CN prepared in Example 1 at different potentials; Figure 12This is a comparison chart of the NH3 yields of Ni / Fe3O4@CN, Ni@CN, Fe3O4@CN, and @CN prepared in Example 1 at different potentials; Figure 13 The main product NH3 and byproduct NO2 of Ni / Fe3O4@CN prepared in Example 1 were obtained under different potentials. - FE comparison chart; Figure 14 A comparison of NH3 yield and FE for Ni / Fe3O4@CN with different Ni / Fe / C molar ratios; Figure 15 The figure shows the catalytic stability test results of Ni / Fe3O4@CN prepared in Example 1; Figure 16 for 15 NMR spectra of N isotope labeling experiments; Figure 17 Comparison of NH3 concentration in Ni / Fe3O4@CN prepared in Example 1 under different test conditions; Figure 18 The above is a comparison of the EISNyquist electrochemical impedance spectroscopy of Ni / Fe3O4@CN, Ni@CN, Fe3O4@CN and @CN prepared in Example 1. Figure 19 Cyclic voltammetry (CV) curves of Ni / Fe3O4@CN, Ni@CN, Fe3O4@CN and @CN prepared in Example 1; Figure 20 The image shows the Cdl fitting diagram of the double-layer capacitance of Ni / Fe3O4@CN, Ni@CN, Fe3O4@CN and @CN prepared in Example 1; Figure 21 Comparison of Tafel curves for Ni / Fe3O4@CN, Ni@CN, and Fe3O4@CN prepared in Example 1; Figure 22 The performance comparison of Ni / Fe3O4@CN and physically mixed sample (Ni@CN+Fe3O4@CN) prepared in Example 1 is shown in (a) as LSV curve and (b) as yield and FE. Figure 23 Comparison of EPR spectra of Ni / Fe3O4@CN prepared in Example 1 under different conditions; Figure 24 The in-situ Raman and infrared spectra of Ni / Fe3O4@CN, Fe3O4@CN, and Ni@CN, as well as the evolution of the spectra at different potentials; Figure 25(a) shows the free energy diagrams and corresponding structural schematics of NO3RR on the surfaces of Ni / Fe3O4, Ni, and Fe3O4; (b) shows the total density of states (TDOS) of the Ni / Fe3O4 system and a comparison of the projected density of states (PDOS) of Ni sites in Ni / Fe3O4 with that of the original Ni. Figure 26 Free energy diagram of the Ni / Fe3O4 system for the competitive hydrogen evolution reaction (HER) at Ni and Fe sites; Figure 27 The adsorption energies of H2O at Ni and Fe sites in the Ni / Fe3O4 system are given. Figure 28 This is a standard curve of ammonia (NH3) concentration versus absorbance. Figure 29 Nitrite (NO2) - Concentration-absorbance standard curve; Figure 30 This is a standard curve of hydrazine (N2H4) concentration-absorbance. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0024] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0025] Example 1 like Figure 1 As shown, the process for preparing Ni / Fe3O4@CN in this invention is as follows: (1) Precursor grinding: Ferric nitrate nonahydrate (2.02 g, 5 mmol), nickel nitrate hexahydrate (1.45 g, 5 mmol) and melamine (0.63 g, 5 mmol) were placed in an agate mortar and ground thoroughly until uniform to obtain precursor powder.

[0026] (2) High-temperature pyrolysis: The obtained precursor powder was transferred to a ceramic boat and placed in the constant temperature zone of a quartz tube furnace. Under the protection of a 5% H2 / Ar mixed gas flow, the temperature was increased to 500°C at 5°C / min and held for 6 hours. After the holding period, the product was naturally cooled to room temperature to obtain the Ni / Fe3O4@CN heterostructure product, which did not require further purification.

[0027] Comparative Example 1 The difference between this comparative example and Example 1 is that the amount of nickel nitrate hexahydrate used in step (1) is 0g, and the remaining process steps and parameter settings are the same as in Example 1, resulting in Fe3O4@CN.

[0028] Comparative Example 2 The difference between this comparative example and Example 1 is that the amount of ferric nitrate nonahydrate used in step (1) is 0g, and the remaining process steps and parameter settings are the same as in Example 1, resulting in Ni@CN.

[0029] Comparative Example 3 The difference between this comparative example and Example 1 is that the amount of nickel nitrate hexahydrate used in step (1) is 0g, the amount of ferric nitrate nonahydrate used is 0g, and the remaining process steps and parameter settings are the same as in Example 1, resulting in @CN.

[0030] Example 1 The morphology and properties of Ni / Fe3O4@CN prepared in Example 1, Fe3O4@CN prepared in Comparative Example 1, Ni@CN prepared in Comparative Example 2, and @CN prepared in Comparative Example 3 were characterized, and the test results are as follows: (1) The crystal structures of Ni / Fe3O4@CN, Fe3O4@CN and Ni@CN were characterized by XRD, and the test results are as follows: Figure 2 As shown in the figure, Ni / Fe3O4@CN exhibits characteristic diffraction peaks of both metallic Ni and cubic spinel Fe3O4. The diffraction peaks of Ni are located at 44.51°, 51.85°, and 76.37°, while the remaining diffraction peaks of Fe3O4 agree well with the standard card (PDF#89-2355), confirming the successful coexistence of Ni and Fe3O4 phases in the heterostructure. The material has good crystallinity and is free of impurity phases.

[0031] (2) The surface morphology of Ni / Fe3O4@CN prepared in Example 1, Ni@CN prepared in Comparative Example 1, and Fe3O4@CN prepared in Comparative Example 2 were observed by SEM. The results are as follows: Figure 3 As shown in the figure, (a) is Ni / Fe3O4@CN, (b) is Ni@CN, and (c) is Fe3O4@CN. As can be seen from the figure, Ni / Fe3O4@CN is composed of a mixture of aggregated microspheres and broken particles, with numerous fine protrusions distributed on its surface, giving the material a high roughness that facilitates the exposure of more electrocatalytic active sites. In contrast, Ni@CN mainly exhibits a spherical nanostructure, while Fe3O4@CN exhibits an irregular nanobulk morphology.

[0032] (4) The morphology of Ni / Fe3O4@CN prepared in Example 1 was observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The results are as follows: Figure 4As shown, (a) and (b) are high-resolution TEM images (HRTEM) at different magnifications, and (c) is a selected area electron diffraction (SAED) pattern. The figures show that the Ni and Fe3O4 regions are covered by a continuous ultrathin carbon layer (approximately 0.26 nm thick). The HRTEM images clearly show the distinct heterogeneous interface between Ni and Fe3O4. The lattice fringe spacings of 0.203 nm and 0.176 nm correspond to the (111) and (200) crystal planes of Ni, respectively, while 0.253 nm and 0.171 nm correspond to the (311) and (400) crystal planes of Fe3O4. The SAED pattern further corroborates the above crystal plane information, confirming the successful recombination of the two phases.

[0033] (5) Figure 5 The image shows a STEM-EDS elemental surface scan of the Ni / Fe3O4@CN prepared in Example 1, where Ni (green), Fe (pink), and O (blue). As can be seen from the image, Ni and Fe elements exhibit a spatially partitioned but interconnected distribution, indicating the formation of an integrated heterojunction interface with gradually changing composition, thus verifying the successful construction of the Ni / Fe3O4@CN heterostructure.

[0034] (6) Figure 6 The XPS full spectrum of Ni / Fe3O4@CN prepared in Example 1 confirms the presence of C, N, O, Fe, and Ni elements in Ni / Fe3O4@CN. Further... Figure 7 The images show a comparison of Ni 2p XPS spectra of Ni / Fe3O4@CN prepared in Example 1 and Ni@CN prepared in Comparative Example 2. The spectra show the Ni content in Ni / Fe3O4@CN. 0 and Ni 2+ Species, of which Ni 0 The binding energy shifts positively by 0.6 eV compared to Ni@CN, indicating that electrons are transferred from Ni to Fe3O4. Figure 8 The images show a comparison of Fe 2p XPS spectra of Ni / Fe3O4@CN prepared in Example 1 and Fe3O4@CN prepared in Comparative Example 1. The spectra show the Fe content in Ni / Fe3O4@CN. 2+ and Fe 3+ Species, and Fe³ + The binding energy shifted negatively by 0.7 eV, confirming that Fe acts as an electron acceptor. Figure 9XPS spectra of Ni / Fe3O4@CN prepared in Example 1 are shown, where (a) is the C1s spectrum, (b) is the high-resolution N 1s spectrum, and (c) is the high-resolution O 1s spectrum. The C 1s spectrum shows CC, CN, and C=O peaks, the N 1s spectrum shows pyridine N, pyrrole N, and graphitic N, and the O 1s spectrum shows lattice oxygen and surface adsorbed oxygen. These results collectively demonstrate that there is strong electronic coupling between Ni and Fe3O4, and the formation of the heterostructure effectively regulates the electronic structure of the catalyst.

[0035] (7) To investigate the formation and consumption of *H bound to the Ni / Fe3O4@CN surface, DMPO was used as a spin trapping agent for EPR testing. The test results are as follows: Figure 23 As shown, (a) indicates the absence of NO3. - The electrolyte contains NO3 - Electrolyte and containing NO3 - and SCN - A comparison under different electrolyte conditions, (b) shows the condition without NO3. - The electrolyte contains SCN - The electrolyte and NO3 - and SCN - A comparison under electrolyte conditions. In the absence of NO3... - Clear DMPO-H multiplets were observed in the electrolyte, confirming the abundant generation of *H on the heterojunction surface. The introduction of NO3... - The signal then weakened significantly, indicating that *H was rapidly consumed. The Fe site poisoning agent SCN was added. - Subsequent signal attenuation confirms that the Fe site is the main active center for water dissociation and *H generation. SCN was then added sequentially. - and NO3 - Further experiments showed that Fe sites act as *H sources and Ni sites as *H sinks, and the heterojunction promotes directional hydrogen transfer and functional complementarity.

[0036] (8) Potential-dependent in-situ Raman spectroscopy was used to study the evolution of interfacial species and catalyst remodeling during NO3RR with different catalysts. The test results are as follows: Figure 24 As shown in the figure, (a) and (d) represent Ni / Fe3O4@CN, (b) and (e) represent Fe3O4@CN, and (c) and (f) represent Ni@CN. The figure shows that for Ni / Fe3O4@CN and Fe3O4@CN, Fe-O related characteristic peaks are observed in the low wavenumber region, and also in the 720-850 cm⁻¹ range. -1 A broad NO bond envelope appears within the range; while Ni@CN only exhibits a NO bond response, without obvious low-wavenumber metal-oxygen characteristic peaks. In the high-wavenumber region, around 1049 cm⁻¹... -1 This belongs to free NO3 -The spectral band was present in all catalysts and gradually weakened with a negative potential shift, indicating a decrease in interfacial nitrate coverage under cathodic polarization. The D band of the carbon framework (~1355 cm⁻¹) -1 ) and G-band (~1582cm) -1 The spectral characteristics were detectable throughout the measurement process. Notably, the Fe-containing catalyst exhibited a more pronounced low-wavenumber spectral evolution than Ni@CN, while Ni / Fe3O4@CN simultaneously retained both Fe-O-related and nitrate-related Raman features, consistent with the synergistic interfacial coupling in the NO3RR process.

[0037] In-situ FTIR was used to monitor the reaction intermediates in the NO3RR process in real time. The test results are as follows: Figure 24 As shown in the figure, (g) represents Ni / Fe3O4@CN, (h) represents Ni / Fe3O4@CN, and (i) represents Ni@CN. The figure shows that in Ni / Fe3O4@CN, as the potential shifts negatively, the value at 1230 cm⁻¹... -1 No. 3 - Enhanced consumption peak; 1197 cm⁻¹ -1 A *NO2 intermediate peak appears at 1652 cm⁻¹. -1 The water peak at 1256 cm⁻¹ indicates an ample proton supply; -1 and 1455cm -1 The positive peaks at these locations are attributed to *NH2 and NH4, respectively. + These results confirm that the Ni / Fe3O4@CN heterostructure not only optimizes NO3 - Adsorption also accelerates the deoxygenation and hydrogenation steps by promoting interfacial charge transfer, effectively lowering the reaction energy barrier.

[0038] (9) The Ni / Fe3O4@CN, Ni@CN, Fe3O4@CN and @CN prepared in Example 1 were subjected to NO3-containing reactions in a three-electrode system (using a silver / silver chloride electrode as the reference electrode, a platinum sheet as the counter electrode, and carbon paper loaded with the catalyst as the working electrode; the working electrode was prepared by dispersing 0.5 mg of the catalyst in a mixed solution of naphthol, isopropanol and water (volume ratio 1:12:12), ultrasonically treating until the mixture was uniformly dispersed, and then uniformly coating it onto carbon paper (1×1 cm²)). - (0.5M KNO3 + 0.1M K2SO4) or no NO3 - LSV testing was conducted under (0.1M K₂SO₄) electrolyte conditions to evaluate the NO₃RR activity of the material. 0.05M KOH was added to the electrolyte to maintain an alkaline environment, and high-purity argon gas was purged before electrolysis to remove dissolved oxygen and ensure stable reaction conditions. Test results are as follows: Figure 10 As shown, Ni / Fe3O4@CN exhibits the highest current response across the entire potential range, and Ni@CN has the most positive initial potential.

[0039] The optimal ammonia production potential was determined through gradient voltage testing (-0.3~-0.8V vs. RHE), and the corresponding Faraday efficiency (FE) was measured. The test results are as follows: Figure 11 and Figure 12 As shown in the figure, the optimal potential is -0.7V vs. RHE. At the optimal operating voltage (-0.7V vs. RHE), the Faraday efficiency (FE) reaches 99.1%, and the NH3 yield is 12.61 mg h⁻¹. - ¹mg cat - ¹.

[0040] The formation amounts of ammonia and its byproducts at different potentials were characterized, and the specific process is as follows: NH3 concentration: The concentration of NH3 in the electrolyte was determined by the indophenol blue method. The specific procedure is as follows: Take 2 mL of appropriately diluted electrolyte after the reaction, and mix it with 2 mL of colorimetric solution A (colorimetric solution A: 5 g salicylic acid + 5.69 g sodium citrate + 4 g NaOH + 100 mL water), 1 mL of colorimetric solution B (colorimetric solution B: 3.5 mL NaClO solution dissolved in 100 mL water (the effective chlorine content of the NaClO solution is 10%~15%)), and 0.2 mL of colorimetric solution C (colorimetric solution C: 0.05605 g C5FeH6Na2O + 5 mL deionized water). After briefly vortexing to mix, allow to stand in the dark for 1 hour. Measure the absorbance at 655 nm using a UV-Vis spectrophotometer. Plot a concentration-absorbance calibration curve using a series of standard NH4Cl solutions of different concentrations. The results are shown below. Figure 28 As shown in Figure a. The NH3 concentration was calculated based on the standard calibration curve, and the results are as follows. Figure 28 As shown in b.

[0041] 2) Byproduct NO2 - Concentration: Quantified using the Griess reagent method. The colorimetric solution was prepared as follows: 0.2 g N-(1-naphthyl)ethylenediamine dihydrochloride, 4 g sulfonamide, and 10 mL H3PO4 were dissolved in 50 mL deionized water with stirring. 0.2 mL of the above colorimetric solution was added to 2 mL of appropriately diluted electrolyte after the reaction, briefly vortexed to mix, and allowed to stand at room temperature for 20 min. The absorbance was measured at 540 nm using a UV-Vis spectrophotometer. Concentration-absorbance calibration curves were plotted using a series of standard KNO2 solutions, and the results are shown below. Figure 29 As shown.

[0042] 3) Concentration of byproduct N₂H₄: Monitored using the Watt-Chris method. The colorimetric solution was prepared as follows: Dissolve 0.6 g of p-dimethylaminobenzaldehyde and 3 mL of concentrated hydrochloric acid in 30 mL of ethanol and stir well. Add 2 mL of the above colorimetric solution to 2 mL of appropriately diluted electrolyte after the reaction, briefly vortex to mix, and let stand at room temperature in the dark for 20 min. Measure the absorbance at 455 nm using a UV-Vis spectrophotometer. Plot a concentration-absorbance calibration curve using a series of standard N₂H₄ solutions. The results are shown below. Figure 30 As shown.

[0043] Figure 13 The main product NH3 and byproduct NO2 of Ni / Fe3O4@CN prepared in Example 1 were obtained under different potentials. - The FE comparison graph shows that as the voltage increases, the amount of ammonia generated gradually increases, and the Faraday efficiency also shows a significant improvement. Furthermore, the amount of byproducts (nitrite and hydrazine) generated is extremely low, making it suitable for treating various nitrate-containing wastewater scenarios.

[0044] Cyclic stability testing was conducted, with a long-term stability test duration of 55 hours and 20 cycles. The test results are as follows: Figure 15 As shown in the figure, Ni / Fe3O4@CN maintained high catalytic activity during long-term use, and its catalytic performance did not significantly decline, indicating that the catalyst has good long-term stability. Furthermore, the structure and surface chemical state of the material remained intact after cycling. This is because the nitrogen-doped carbon coating effectively protected metallic Ni from oxidation, while the strong electronic coupling at the heterogeneous interface prevented component segregation, thus endowing the catalyst with excellent durability and reusability.

[0045] Figure 17 The figure shows a comparison of NH3 concentration under different electrolyte conditions and a blank test at open circuit potential for Ni / Fe3O4@CN prepared in Example 1. As can be seen from the figure, this further verifies that the generated ammonia comes from the electrocatalytic reduction of nitrate.

[0046] K 14 NO3 and K 15 NO3 was used to replace KNO3 in the electrolyte for isotope labeling experiments, and the ammonia obtained from the redox reaction was characterized. The nuclear magnetic resonance (NMR) spectrum is shown below. Figure 16 As shown in the figure, it can be confirmed that the nitrogen source of ammonia comes entirely from nitrates.

[0047] Figure 18 The image shows a comparison of the EIS Nyquist spectra of Ni / Fe3O4@CN, Ni@CN, Fe3O4@CN, and @CN. As can be seen from the image, Ni / Fe3O4@CN exhibits the lowest charge transfer resistance in the EIS spectrum. Further... Figure 21 A comparison of the Tafel curves for Ni / Fe3O4@CN, Ni@CN, Fe3O4@CN, and @CN shows that Ni / Fe3O4@CN has the smallest Tafel slope. This demonstrates that the heterogeneous interface accelerates the reaction kinetics.

[0048] Figure 19 The cyclic voltammetry (CV) curves of Ni / Fe3O4@CN, Ni@CN, Fe3O4@CN and @CN prepared in Example 1 are shown in the figure. As can be seen from the figure, the current response that increases with the increase of scan rate indicates that Ni / Fe3O4@CN has good capacitance characteristics and abundant active sites.

[0049] Figure 20 The figure shows the double-layer capacitance (Cdl) fitting diagrams for Ni / Fe3O4@CN, Ni@CN, Fe3O4@CN, and @CN prepared in Example 1. As can be seen from the figure, the Ni / Fe3O4@CN catalyst exhibits excellent performance, with a Cdl value of 0.22 mF cm⁻¹. -2 The activity is higher than that of single-component catalysts. This indicates that Ni / Fe3O4@CN has the most active sites among all catalysts, thus enhancing NO3RR activity.

[0050] (10) Ni@CN and Fe3O4@CN are physically mixed and used as a catalyst (named Ni@CN+Fe3O4@CN). In a three-electrode system, NO3 is produced. - LSV testing was conducted under the electrolyte conditions of (0.5M KNO3 + 0.1M K2SO4). 0.05M KOH was added to the electrolyte to maintain an alkaline environment, and high-purity argon gas was passed through before electrolysis to remove dissolved oxygen and ensure stable reaction conditions. The test results are as follows: Figure 22 As shown in the figure, (a) is the LSV curve, and (b) is the NH3 yield and FE. The figure shows that the catalytic performance of Ni@CN+Fe3O4@CN is much lower than that of Ni / Fe3O4@CN, proving that the synergistic effect originates from interfacial coupling.

[0051] (11) The theoretical calculation of the catalytic mechanism of Ni / Fe3O4@CN was performed, and the results are as follows: Figures 25-27As shown in the figure, the Ni / Fe3O4@CN heterostructure induces electron redistribution through the Schottky interface, shifting the d-band center of Ni upwards to the Fermi level and optimizing the adsorption strength of nitrogen-containing intermediates (NO3, NO2, NO, etc.). Simultaneously, the Fe3O4 component, with its superior water dissociation capability, significantly lowers the activation barrier of water molecules, ensuring a continuous supply of active hydrogen. The synergistic effect of these two components reduces the free energy barrier of the rate-determining step (NO→*NOH) to 0.07 eV and effectively suppresses the competitive hydrogen evolution reaction. Therefore, the Schottky heterostructure between Ni and Fe3O4 in the Ni / Fe3O4@CN composite material achieves spatial decoupling and tandem synergy of catalytic functions, significantly improving the overall efficiency of nitrate reduction to ammonia. Specific Ni sites are responsible for nitrate adsorption and activation, while the adjacent Fe3O4 phase acts as a reservoir for active hydrogen, continuously generating active *H through water molecule dissociation and transporting it to the Ni site interface, thus effectively balancing the kinetic requirements between nitrate activation and hydrogenation steps. This spatial separation strategy of water splitting and nitrate reduction sites not only accelerates the kinetics of multi-step hydrogenation reactions, but also effectively suppresses competitive hydrogen evolution reactions.

[0052] The role of Fe3O4 sites: Fe sites in the Fe3O4 phase dominate the activation process of water molecules. They exhibit a strong adsorption capacity for H2O (DFT calculations show ΔG = -0.83 eV), far superior to Ni sites (ΔG = -0.02 eV), thus enabling efficient dissociation of water to produce hydroxyl groups and adsorbed H. Furthermore, EPR experiments further confirm that Fe sites are absorbed by SCN. - The DMPO-H signal was significantly weakened after selective poisoning, indicating that Fe3O4 is the main source of active hydrogen. This hydrogen pump effect provides a continuous and sufficient supply of protons for the entire reduction process.

[0053] The role of Ni sites: In Ni / Fe3O4@CN, Ni sites preferentially adsorb and activate nitrate species (forming two stable Ni-O bonds, lowering the free energy by 0.51 eV), subsequently reducing the intermediate to ammonia through a multi-step proton-coupled electron transfer process. EPR experiments show that the DMPO-H signal decays sharply after the addition of nitrate, confirming that Ni sites are the main consumers of H, i.e., participating in the hydrogenation step as a hydrogen sink. DFT calculations show that the d-band center of Ni in the Ni / Fe3O4 heterojunction shifts upward to the Fermi level, enhancing the hybridization between the Ni 3d orbitals and the π orbitals of the nitrogen-containing intermediates, thus optimizing the adsorption strength of key intermediates.

[0054] Synergistic effect: The Schottky interface between Ni and Fe3O4 achieves tandem catalysis through spatial decoupling of nitrate activation (Ni site) and active hydrogen supply (Fe3O4 site), effectively avoiding NO3. -Competition for adsorption with H. This synergistic effect lowers the free energy barrier of the rate-determining step (NO2 hydrogenation), while the HER barrier is relatively high, thereby significantly improving catalytic activity and ammonia selectivity.

[0055] Example 2 The difference between this comparative example and Example 1 is that the molar ratio of ferric nitrate nonahydrate, nickel nitrate hexahydrate and melamine in step (1) is 0.5:1:1, and the remaining process steps and parameter settings are the same as in Example 1.

[0056] Example 3 The difference between this comparative example and Example 1 is that the molar ratio of ferric nitrate nonahydrate, nickel nitrate hexahydrate and melamine in step (1) is 1.5:1:1, and the remaining process steps and parameter settings are the same as in Example 1.

[0057] Example 4 The difference between this comparative example and Example 1 is that the molar ratio of ferric nitrate nonahydrate, nickel nitrate hexahydrate and melamine in step (1) is 2:1:1, and the remaining process steps and parameter settings are the same as in Example 1.

[0058] Example 5 The difference between this comparative example and Example 1 is that the molar ratio of ferric nitrate nonahydrate, nickel nitrate hexahydrate and melamine in step (1) is 2.5:1:1, and the remaining process steps and parameter settings are the same as in Example 1.

[0059] Example 6 The difference between this comparative example and Example 1 is that the molar ratio of nickel nitrate hexahydrate, ferric nitrate nonahydrate and melamine in step (1) is 0.5:1:1, and the remaining process steps and parameter settings are the same as in Example 1.

[0060] Example 7 The difference between this comparative example and Example 1 is that the molar ratio of nickel nitrate hexahydrate, ferric nitrate nonahydrate and melamine in step (1) is 1.5:1:1, and the remaining process steps and parameter settings are the same as in Example 1.

[0061] Example 8 The difference between this comparative example and Example 1 is that the molar ratio of nickel nitrate hexahydrate, ferric nitrate nonahydrate and melamine in step (1) is 2:1:1, and the remaining process steps and parameter settings are the same as in Example 1.

[0062] Example 9 The difference between this comparative example and Example 1 is that the molar ratio of nickel nitrate hexahydrate, ferric nitrate nonahydrate and melamine in step (1) is 2.5:1:1, and the remaining process steps and parameter settings are the same as in Example 1.

[0063] Example 2 The Faraday efficiency (FE) and NH3 yield of the catalysts prepared in Examples 1-9 and Comparative Examples 1-2 were compared at -0.7V vs. RHE, and the results are as follows: Figure 14 As shown in the figure, the performance is optimal when the molar ratio of nickel nitrate hexahydrate, ferric nitrate nonahydrate, and melamine is 1:1:1.

[0064] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for preparing a Ni / Fe3O4@CN Schottky heterojunction catalyst, characterized in that, The method involves mixing nickel nitrate source, iron nitrate source, and melamine, grinding them until homogeneous, and then calcining them at high temperature under a hydrogen atmosphere to obtain a Ni / Fe3O4@CN heterojunction catalyst.

2. The production method according to claim 1, characterized by, The molar ratio of nickel nitrate source, ferric nitrate source, and melamine is 1:1:

1.

3. The production method according to claim 1, characterized by, The nickel nitrate source is nickel nitrate hexahydrate, and the ferric nitrate source is ferric nitrate nonahydrate.

4. The preparation method according to claim 1, characterized in that, The hydrogen-containing atmosphere is a mixture of hydrogen and argon, wherein the hydrogen content is 5%.

5. The preparation method according to claim 1, characterized in that, The pyrolysis temperature was 500℃, and the holding time was 6 hours.

6. The method of claim 1, wherein, The pyrolysis process involves a heating rate of 5°C / min and a cooling method of natural cooling to room temperature.

7. A Ni / Fe3O4@CN Schottky heterojunction catalyst obtained by the preparation method according to any one of claims 1 to 6.

8. The use of the Ni / Fe304@CN Schottky heterojunction catalyst according to claim 7, characterized in that, It is used as a catalyst in the electrochemical reduction reaction to reduce nitrates in water to ammonia.

9. A method for reducing a nitrate salt, characterized by, Under alkaline conditions, using the Ni / Fe3O4@CN Schottky heterojunction catalyst of claim 7 as the electrocatalyst, and a mixed solution of 0.5M KNO3 and 0.1M K2SO4 as the electrolyte, NO3 is reduced via an electrochemical reduction reaction. - reduction.

10. The reduction method according to claim 9, characterized in that, The operating voltage for the electrochemical reduction reaction process is -0.3 to -0.8 V vs. RHE.

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