Iron monatomic catalyst containing multi-nitrogen ligand structure as well as preparation method and application of iron monatomic catalyst

Abstract

The invention relates to an iron monatomic catalyst containing a multi-nitrogen ligand structure as well as a preparation method and application of the iron monatomic catalyst. The iron monatomic catalyst provided by the invention comprises a porous carbon carrier and an iron monatomic located on the porous carbon carrier, and the iron monatomic exists in a form of asymmetric coordination with a nitrogen atom. The iron monatomic catalyst provided by the invention can efficiently convert nitrate into ammonia gas, effectively inhibits the occurrence of other side reactions, and shows excellent catalytic activity and selectivity.

Description

Technical Field

[0001] This application belongs to the field of electrocatalytic technology, and particularly relates to an iron single-atom catalyst with a multi-nitrogen ligand structure, its preparation method and application. Background Art

[0002] Nitrogen, as a fundamental element of life, has become a double-edged sword in modern ecosystems. The natural nitrogen cycle, which coordinates nitrogen fixation, nitrification, and denitrification processes, has been disrupted by the excessive use of fertilizers and industrial emissions, leading to the massive proliferation of toxic algae, the emergence of oxygen-deprived "dead zones," and groundwater pollution. Traditional remediation strategies, such as biological denitrification, are hindered by problems such as high energy consumption, incomplete NO3 - removal, and the accidental release of harmful intermediates (NO2 - , N2O). The electrochemical nitrate reduction reaction (NO3 - RR) has emerged as a transformative solution, which can directly convert NO3 - into NH3 - a high-value precursor in the nitrogen-containing chemical industry - thus integrating waste nitrogen back into the production cycle. However, the multi-step, proton-coupled electron transfer characteristics of NO3RR require the catalyst to be able to synergistically stabilize the reactive intermediates (*NO3 - , *NO2 - , *NH2OH), while suppressing competitive reactions (hydrogen evolution reaction), which is a challenge that traditional materials cannot handle.

[0003] Single-atom catalysts (SACs) have atomically dispersed active sites and adjustable coordination environments, and are expected to solve these limitations. However, most single-atom catalysts adopt symmetric coordination geometries, resulting in relatively weak polarization of the active sites and poor adsorption ability for NO3 - and intermediates. This low efficiency stems from the mismatch between the symmetric electronic structure of the single-atom catalyst and the asymmetric charge distribution of NO3 - , which hinders the electron transfer kinetics. Summary of the Invention

[0004] To solve the problems in the prior art, this application provides an iron single-atom catalyst with a multi-nitrogen ligand structure, its preparation method and application. The iron single-atom catalyst of this application can efficiently convert nitrate into ammonia, while effectively suppressing the occurrence of other side reactions, showing excellent catalytic activity and selectivity.

[0005] In a first aspect, this application provides an iron single-atom catalyst with a multi-nitrogen ligand structure, which includes a porous carbon carrier and iron single atoms located on the porous carbon carrier, wherein the iron single atoms exist in the form of asymmetric coordination with nitrogen atoms.

[0006] In the iron single-atom catalyst containing a multi-nitrogen ligand structure provided by the present application, iron atoms are highly dispersed in a porous carbon support, and a stable and characteristic coordination path, namely the Fe-N asymmetric coordination path, is formed between the iron atoms and nitrogen atoms. The special local coordination environment can form more and more effective active sites, and is also conducive to the adsorption and activation of NO3 - , realizing high degradation efficiency and excellent electrocatalytic performance.

[0007] In some embodiments, in the iron single-atom catalyst, the coordination number of nitrogen atoms to iron atoms is 2-5, such as 2.1, 2.3, 2.5, 2.7, 2.9, 3, 3.1, 3.3, 3.5, 3.7, 3.9, 4, 4.1, 4.3, 4.5, 4.7, 4.9, or any value between them. In some embodiments, in the iron single-atom catalyst, the coordination number of nitrogen atoms to iron atoms is 2-4.

[0008] In some embodiments, in the iron single-atom catalyst, the coordination number of nitrogen atoms to iron atoms is 2-2.5 and 3.5-4. Compared with other ligand environments, it shows the most prominent performance among catalysts including Fe-N2 ligands, and its adsorption affinity for nitrate (NO3 - ) and nitrite (NO2 - ) is significantly stronger than that of other ligands. This unique advantage stems from the enhanced binding interaction in the Fe-N2 structure, which promotes the redistribution of the electron cloud, enabling Fe-N2 to form a more stable chemical bond with NO3 - , NO2 - , thus greatly accelerating the conversion process of nitrate to the final product ammonia.

[0009] In some embodiments, based on the iron single-atom catalyst, the mass content of iron element is 0.1%-2%, such as 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or any value between them. In some embodiments, the mass content of iron element is 0.3%-1.5%. In some embodiments, the mass content of iron element is 0.5%-1.0%.

[0010] Both too low or too high iron concentration will have a negative impact on the catalytic performance. Too little iron content in the catalyst is not conducive to the reaction. If the iron content in the catalyst is too high, the degree of atomic aggregation increases, which hinders the approach of reactants to the active Fe sites and their interaction, and the reactants cannot fully contact the active Fe sites, thus reducing the efficiency of the catalytic reaction.

[0011] In some embodiments, the ID / IG ratio of the iron single-atom catalyst is 0.7 - 0.98, such as 0.8, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96 or 0.97. In some embodiments, the ID / IG ratio of the iron single-atom catalyst is 0.85 - 0.95.

[0012] The intensity ratio of the D band to the G band (ID / IG) can reflect the content of carbon defects in the sample to a certain extent. The larger the ID / IG ratio, the more structural defects the sample contains, and the easier it is for iron single atoms to be fixed on the defects or sites of graphite, resulting in a strong interaction between the iron single atoms and the graphite substrate, thereby enhancing the stability of the catalyst.

[0013] In a second aspect, the present application provides a method for preparing an iron single-atom catalyst containing a multi-nitrogen ligand structure, which includes the following steps:

[0014] S1: Provide a biomass carbon aerogel;

[0015] S2: Immerse the biomass carbon aerogel obtained in step S1 in an iron ammonium salt solution to obtain an impregnated product;

[0016] S3: Anneal the impregnated product obtained in step S2 to obtain an annealed product;

[0017] Optionally, step S4: Wash and dry the annealed product to obtain the iron single-atom catalyst containing the multi-nitrogen ligand structure.

[0018] In the preparation method provided by the present application, the iron ammonium salt solution contains both nitrogen and iron elements. During the subsequent annealing process, the elements undergo thermal decomposition and recombination to form the catalyst. Nitrogen atoms have lone pairs of electrons and can form coordination bonds with the empty orbitals of iron atoms, and the two first form a relatively stable Fe-N structural unit. On the other hand, the biomass carbon aerogel MSCA carrier has a porous structure and a large specific surface area. Its rich internal pores and uniform spatial environment can provide a good accommodation and dispersion site for iron and nitrogen elements, enabling the prepared iron single-atom catalyst to efficiently convert nitrate into ammonia while effectively suppressing the occurrence of other side reactions.

[0019] In some embodiments, in step S1, the specific surface area of the biomass carbon aerogel is 300m 2 / g - 1200m 2 / g, such as 400 m2 / g, 500 m2 / g, 600 m2 / g, 650 m2 / g, 700 m2 / g, 750 m2 / g, 800 m2 / g, 850 m2 / g, 900 m2 / g, 950 m2 / g, 1000 m2 / g, 1100 m2 / g or any value therebetween. In some embodiments, the specific surface area of the biomass carbon aerogel is 600 m 2 / g - 1000 m 2 / g. In some embodiments, the specific surface area of the biomass carbon aerogel is 700 m 2 / g - 800 m 2 / g.

[0020] In some embodiments, in step S1, the pore size distribution of the biomass carbon aerogel is 0.5 nm - 0.8 nm, such as 0.53 nm, 0.55 nm, 0.57 nm, 0.59 nm, 0.6 nm, 0.61 nm, 0.62 nm, 0.63 nm, 0.64 nm, 0.65 nm, 0.66 nm, 0.67 nm, 0.68 nm, 0.69 nm, 0.7 nm, 0.73 nm, 0.75 nm or any value therebetween. In some embodiments, the pore size distribution of the biomass carbon aerogel is 0.6 nm - 0.7 nm.

[0021] In some embodiments, in step S1, the providing of the biomass carbon aerogel includes: mixing the biomass with a eutectic chloride molten salt and then performing a carbonization treatment.

[0022] In some embodiments, the biomass is selected from wood, and the wood is preferably selected from one or more of beech, basswood, locust, eucalyptus, pine, toona, poplar, willow, peach, apricot, apple, pear, banyan, camphor, pagoda tree, maple, cypress, oak, elm, birch, sandalwood, oak, fir, and phoenix tree.

[0023] In some embodiments, the mass ratio of the biomass to the eutectic chloride molten salt is 1:1 - 1:3, such as 1:1.3, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2.0, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, 1:2.7, 1:2.9 or any value therebetween. In some embodiments, the mass ratio of the biomass to the eutectic chloride molten salt is 1:1.5 - 1:2.5.

[0024] When the mass ratio of biomass to eutectic chloride molten salt is too low, the ionic interaction in the eutectic chloride molten salt weakens, and it cannot effectively support the development of the carbon framework. The material obtained after carbonization treatment lacks sufficient micropores and mesopores. When the mass ratio of biomass to eutectic chloride molten salt is too high, it is easy to increase the brittleness of the material and reduce its toughness, making the material more prone to cracks and breakages during subsequent processing or application, thereby further reducing the porosity and mechanical properties of the material.

[0025] In some embodiments, the melting point of the eutectic chloride molten salt is 200°C - 900°C, such as 300°C, 400°C, 500°C, 600°C, 700°C or 800°C.

[0026] In some embodiments, the eutectic chloride molten salt is a eutectic molten salt of zinc chloride and potassium chloride.

[0027] In some embodiments, the carbonization treatment is carried out in an inert atmosphere.

[0028] In some embodiments, the temperature of the carbonization treatment is 600°C - 900°C, such as 650°C, 700°C, 750°C, 800°C or 850°C. In some embodiments, the temperature of the carbonization treatment is reached in an ascending temperature manner. In some embodiments, the heating rate is 1°C / min - 10°C / min, such as 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min or 9°C / min.

[0029] In some embodiments, in step S2, the ferric ammonium salt solution is selected from one or more of ferric ammonium EDTA solution, Prussian blue and iron phthalocyanine, and preferably ferric ammonium EDTA solution.

[0030] In some embodiments, calculated by iron element, the mass of the ferric ammonium salt in the ferric ammonium salt solution is 0.1% - 2% of the mass of the biomass carbon aerogel, such as 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9% or any value between them. In some embodiments, the mass of the ferric ammonium salt in the ferric ammonium salt solution is 0.3% - 1.5% of the mass of the biomass carbon aerogel. In some embodiments, the mass of the ferric ammonium salt in the ferric ammonium salt solution is 0.5% - 10% of the mass of the biomass carbon aerogel.

[0031] In some embodiments, the concentration of the ferric ammonium salt solution is 0.1 mg / L - 1 mg / L, for example, 0.2 mg / L, 0.25 mg / L, 0.3 mg / L, 0.35 mg / L, 0.4 mg / L, 0.45 mg / L, 0.5 mg / L, 0.55 mg / L, 0.6 mg / L, 0.65 mg / L, 0.7 mg / L, 0.8 mg / L, 0.9 mg / L or any value therebetween. In some embodiments, the concentration of the ferric ammonium salt solution is 0.2 mg / L - 0.6 mg / L. In some embodiments, the concentration of the ferric ammonium salt solution is 0.3 mg / L - 0.5 mg / L.

[0032] When the concentration of the ferric ammonium salt solution is low, the number of active sites on the surface of the prepared catalyst is relatively small, resulting in limited nitrate ions participating in the reaction, so the catalytic performance is relatively low. However, when the concentration of the ferric ammonium salt solution is too high, the iron content in the prepared catalyst is too high, and the degree of atomic aggregation increases, which hinders the reactants from approaching the active Fe sites and interacting with each other, and the reactants cannot fully contact the active Fe sites, thus reducing the efficiency of the catalytic reaction.

[0033] In some embodiments, the temperature of the impregnation is 60°C - 100°C, for example, 70°C, 75°C, 80°C, 85°C, 90°C or 95°C.

[0034] In some embodiments, the time of the impregnation is 1 h - 24 h, for example, 2 h, 4 h, 6 h, 10 h, 12 h, 15 h, 18 h, 20 h or 22 h.

[0035] In some embodiments, in step S3, the annealing treatment is carried out in an inert atmosphere. In the present application, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0036] In some embodiments, in step S3, the temperature of the annealing treatment is 500°C - 1000°C, for example, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C or 950°C. In some embodiments, the temperature of the annealing treatment is 600°C - 800°C. In some embodiments, the temperature of the annealing treatment is 650°C - 750°C.

[0037] The annealing temperature is an important factor affecting the coordination environment in the iron single-atom catalyst of the present application. When the annealing temperature is relatively low, for example, less than or equal to 600°C, the relatively low temperature makes the thermal motion between atoms not too violent enough to promote the decomposition of the precursor and allow the atoms to rearrange and combine to form a coordination structure. The relatively low temperature enables the iron atoms to form coordination bonds with the oxygen atoms in the precursor simultaneously, and finally forms an Fe-O coordination structure with a coordination number of about 1.6, and fails to form a structure beneficial to NO3 --N adsorption environment results in NO3 - -N is difficult to be effectively adsorbed on the catalyst surface, thus affecting the degradation efficiency and product conversion. When the annealing temperature is relatively high, for example, higher than or equal to 600 °C, the thermal motion of atoms becomes more intense and the energy in the system is higher. More nitrogen atoms can overcome the energy barrier and coordinate with iron atoms. At this temperature, the nitrogen-containing precursors decompose more thoroughly. Driven by high energy, they quickly coordinate and combine with iron atoms, making the coordination number of Fe-N approach a stable structure with almost no defective coordination, which is also not conducive to the catalytic reaction.

[0038] In some embodiments, the temperature of the annealing treatment is reached in an ascending temperature manner. In some embodiments, the heating rate is 1 °C / min - 10 °C / min, such as 2 °C / min, 3 °C / min, 4 °C / min, 5 °C / min, 6 °C / min, 7 °C / min, 8 °C / min or 9 °C / min.

[0039] In some embodiments, the time of the annealing treatment is 0.5 h - 24 h, such as 2 h, 4 h, 6 h, 10 h, 12 h, 15 h, 18 h, 20 h or 22 h.

[0040] In some embodiments, in step S4, the washing is performed using commonly used acid solutions and water in the art, such as hydrochloric acid and water.

[0041] In some embodiments, in step S4, the drying temperature is 60 °C - 100 °C, such as 70 °C, 75 °C, 80 °C, 85 °C, 90 °C or 95 °C.

[0042] In some embodiments, in step S4, the drying time is 1 h - 24 h, such as 2 h, 4 h, 6 h, 10 h, 12 h, 15 h, 18 h, 20 h or 22 h.

[0043] In some embodiments, the preparation method of the iron single-atom catalyst includes the following specific steps:

[0044] (1) Synthesis method of biomass carbon aerogel

[0045] The biomass material is thoroughly ground with the eutectic molten salt of zinc chloride and potassium chloride at a mass ratio of 1:1 to 1:3 to ensure uniform mixing. The mixture is placed in a porcelain boat and put into a tubular furnace. Under an argon protection atmosphere, it is heated to 600 °C - 900 °C, such as 700 °C, at a heating rate of 1 - 10 °C / min, such as 5 °C / min, and maintained at this temperature for 1 - 10 hours, such as 2 hours, and then naturally cooled to room temperature.

[0046] After cooling, the sample was immersed in deionized water and rinsed thoroughly to remove excess soluble substances until the conductivity of the washing liquid dropped below 10 μS / cm.

[0047] (2) Synthesis method of iron single-atom catalyst

[0048] Into the iron ammonium salt solution (with a concentration of, for example, 0.2, 0.4, 0.6 mol L -1 ), the above-mentioned biomass carbon aerogel was added and continuously stirred, and then dried at 60 °C - 100 °C, for example, 80 °C. Subsequently, the obtained mixture was annealed in an argon atmosphere at a heating rate of 5 °C / min at 500 - 1000 °C, for example, 600 °C - 800 °C, and held at the target temperature for 0.5 - 10 hours, for example, 1 hour, and finally cooled to room temperature. The obtained product was washed strictly with acid and deionized water to remove unreacted species and contaminants, and then finally dried at 60 °C - 100 °C, for example, 80 °C for 5 - 20 hours, for example, 12 hours, to obtain the iron single-atom catalyst.

[0049] In the third aspect, the present application provides the use of the iron single-atom catalyst described in the first aspect or the iron single-atom catalyst prepared by the preparation method described in the second aspect in nitrate reduction.

[0050] In the fourth aspect, the present application provides a method for electrocatalytic reduction of nitrate, which includes reducing nitrate in a three-electrode system, wherein the working electrode in the three-electrode system includes the iron single-atom catalyst described in the first aspect or the iron single-atom catalyst prepared by the preparation method described in the second aspect.

[0051] In some embodiments, the electrolyte in the electrolyte solution of the three-electrode system is selected from sodium sulfate.

[0052] In some embodiments, the working voltage of the three-electrode system is -0.45 V to -0.75 V, for example, -0.5 V, -0.55 V, -0.6 V, -0.65 V or -0.7 V.

[0053] In some embodiments, the initial concentration of nitrate is 50 mg / L - 200 mg / L, for example, 70 mg / L, 100 mg / L, 130 mg / L, 150 mg / L or 170 mg / L.

[0054] Compared with the prior art, the beneficial effects of the present application are as follows:

[0055] 1) In the nitrate degradation reaction, the iron single-atom catalyst of the present application can efficiently convert nitrate into ammonia, and at the same time effectively inhibit the occurrence of other side reactions, showing excellent catalytic activity and selectivity.

[0056] 2) The iron single-atom catalyst of the present application has excellent stability. After 20 cycles of the electrolysis process, its degradation performance hardly changes, effectively resisting various interference factors during the reaction process.

[0057] 3) The iron single-atom catalyst of the present application can successfully treat nitrate-polluted wastewater, showing excellent stability and adaptability in actual application scenarios. Description of the Drawings

[0058] Figure 1 SEM images of the biomass carbon aerogels MSCA-1:1, MSCA-1:2, and MSCA-1:3 of the present application.

[0059] Figure 2 Specific surface area and pore size distribution diagrams of the biomass carbon aerogels MSCA-1:1, MSCA-1:2, and MSCA-1:3 of the present application.

[0060] Figure 3 Mechanical property diagrams of the biomass carbon aerogels MSCA-1:1, MSCA-1:2, and MSCA-1:3 of the present application.

[0061] Figure 4 HAADF-STEM images of the iron single-atom catalysts in Examples 1-3.

[0062] Figure 5 XRD diagrams and Raman spectra of the iron single-atom catalysts in Examples 1-3.

[0063] Figure 6 Energy-dispersive X-ray spectroscopy diagram of the iron single-atom catalyst in Example 2.

[0064] Figure 7 XANES spectra of the iron single-atom catalysts in Examples 1-3.

[0065] Figure 8 FT-EXAFS spectra of the iron single-atom catalysts in Examples 1-3.

[0066] Figure 9 Shows the electrochemical performance of the iron single-atom catalysts in Examples 1-4.

[0067] Figure 10 Shows the electrochemical performance of the iron single-atom catalysts in Examples 5-7.

[0068] Figure 11 Shows the electrochemical performance of the iron single-atom catalysts in Examples 5-7.

[0069] Figure 12 Shows the cycling performance and structural changes after cycling of the iron single-atom catalyst in Example 2.

[0070] Figure 13 Shows a large-scale wastewater experimental device and results. Detailed implementation mode

[0071] To make the purpose, technical solution and advantages of the present application more clear and understandable, the present application will be further described in detail below in conjunction with embodiments and the accompanying drawings. The specific embodiments described herein are only used to explain the present application and do not constitute any limitation to the present application. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessarily confusing the concepts of the present disclosure. Such structures and technologies are also described in many publications.

[0072] The present application will be further described below through specific examples and comparative examples.

[0073] In the following examples and comparative examples, the model of the spherical aberration corrected transmission electron microscope (HAADF-STEM) is Titan ThemesCubed G2 300. First, the sample is ultrasonically dispersed in absolute ethanol to form a uniform suspension, and then an appropriate amount of the suspension is aspirated with a pipette and dropped on a copper mesh covered with an ultrathin carbon support film, and dried under an infrared lamp to ensure that the sample is firmly attached. The prepared sample is mounted on a special sample rod and carefully inserted into the ultra-high vacuum sample chamber of the electron microscope, and evacuated to below 10 -8 Pa to minimize the scattering interference of gas molecules on the electron beam. The electrons emitted by the electron gun are accelerated by a high acceleration voltage of 300 kV to form a high-energy electron beam that penetrates the sample. By precisely adjusting the current of the condenser lens and the aperture size, the electron beam spot size is optimized to 0.1 - 0.5 nm to obtain a high-brightness and well-parallel electron beam to uniformly irradiate the sample. During imaging, a high-angle annular dark field detector is selected to collect the scattered electron signal, and the acquisition angle range and gain parameters of the detector are adjusted in real time to highlight the contrast difference of elements with different atomic numbers in the sample and clearly present the structural information at the atomic scale. By adjusting the parameters of the objective lens, intermediate lens and projection lens, it is possible to switch from low magnification for observing the overall morphological distribution of the sample to high magnification for atomic resolution imaging, and multiple fields of view are photographed at different magnifications.

[0074] In the following examples and comparative examples, the X-ray absorption spectrum was measured at the Shanghai Synchrotron Radiation BL14W1 station.

[0075] In the following examples and comparative examples, the model of the X-ray powder diffractometer is Rigaku, miniflex-600.

[0076] In the following examples and comparative examples, the nitrate concentration was determined by ultraviolet-visible spectroscopy of the product using a Cary 5000 spectrophotometer.

[0077] Specifically, 0.2 mL of the sample solution was taken and diluted with 4.8 mL of deionized water. The absorbance was measured in the wavelength range of 220 to 275 nm (A = A220nm - A2×275nm), and the nitrate concentration was calculated through the standard calibration curve.

[0078] In the following examples and comparative examples, the ammonium ion concentration was determined by the phenolate spectrophotometry.

[0079] Specifically, 0.2 mL of the sample solution was taken and diluted with 4.8 mL of deionized water. Subsequently, 0.5 mL of the phenolate-citrate reagent and 0.2 mL of the chlorine buffer solution were added and mixed well. After the mixture was allowed to stand for 90 minutes, the absorbance was measured at 630 nm. Finally, the concentration of ammonia nitrogen was calculated by referring to the standard calibration curve.

[0080] In the following examples and comparative examples, the nitrite concentration was quantitatively determined by the diazo-coupling spectrophotometry.

[0081] Specifically, the nitrite concentration was quantitatively determined by the diazo-coupling spectrophotometry. 0.2 mL of the sample solution was taken and diluted with 4.8 mL of deionized water. Then, 0.1 mL of the sulfanilamide solution was added, and after thorough mixing, it was allowed to stand for 5 minutes. Subsequently, 0.1 mL of the N-(1-naphthyl)ethylenediamine hydrochloride solution was added. After reacting for 10 minutes, the absorbance was measured at 540 nm. Finally, the concentration of nitrite was determined by referring to the standard calibration curve.

[0082] In the following examples and comparative examples, the ability of iron per unit mass to remove nitrate nitrogen, the removal rate of NO3 - -N, and the proportion of NO2 - -N were calculated by the following formula:

[0083]

[0084]

[0085]

[0086] Where: C0(NO3 - -N) and C0(NO2 - -N) respectively represent the initial concentrations of NO3 - -N and NO2 - -N;

[0087] C t (NO3 - -N) and C t (NO2 - -N) respectively represent the concentrations of NO3 - -N and NO2- The concentration of -N;

[0088] V(L) represents the volume of the nitrate solution;

[0089] m(g) represents the mass of the active sites of the catalyst attached to the cathode;

[0090] h is the reaction time of 24 h.

[0091] In the following examples and comparative examples, NH4 + The selectivity of -N, the ammonia production Faraday efficiency, and the power consumption are calculated by the following formula:

[0092]

[0093]

[0094] where F is the Faraday constant (96485 C mol-1);

[0095] Ct(NH4 + -N) represents the concentration of NH4 + -N in the electrolyte; M(NH3) is the mass fraction of NH4 + -N, and V(L) is the volume of the electrolyte.

[0096] Synthesis of biomass carbon aerogel (MSCA)

[0097] Sawdust of beech wood was thoroughly ground with the eutectic molten salt of zinc chloride and potassium chloride (molar ratio 54:46) at mass ratios of 1:1, 1:2, and 1:3 to ensure uniform mixing. The mixture was placed in a porcelain boat and put into a tube furnace. Under an argon protection atmosphere, it was heated to 700 °C at a heating rate of 5 °C / min and maintained at this temperature for 2 hours, and then naturally cooled to room temperature.

[0098] After cooling, the samples were soaked in deionized water and rinsed thoroughly to remove excess soluble substances until the conductivity of the washing liquid dropped below 10 μS / cm. Finally, the derived samples were named MSCA-1:1, MSCA-1:2, and MSCA-1:3.

[0099] The biomass carbon aerogels MSCA-1:1, MSCA-1:2, and MSCA-1:3 were subjected to structural characterization and mechanical property tests.

[0100] The microstructures of MSCA-1:1, MSCA-1:2, and MSCA-1:3 are as Figure 1 shown. Figure 1 Figures (a), (b), and (c) in it are the SEM images of MSCA-1:1, MSCA-1:2, and MSCA-1:3, respectively.

[0101] It can be seen from Figure 1 that MSCA-1:1 shows a compact rod-like morphology. The lack of zinc ions leads to insufficient cross-linking and aggregation of the carbon aerogel, thus restricting the generation of the porous structure. In MSCA-1:2, the presence of zinc chloride can reduce the pyrolysis temperature of the biomass, resulting in a more uniform carbon network structure at a higher temperature, thereby forming abundant micropores and mesopores. While in MSCA-1:3, when the concentration of zinc ions is too high, the zinc ions in the molten salt will have too strong an interaction with the carbon material, thus triggering the collapse of the pore structure.

[0102] The specific surface areas and pore size distributions of MSCA-1:1, MSCA-1:2 and MSCA-1:3 are as Figure 2 shown. Figure 2 In Figures (a), (b) and (c) are the BET and pore size distribution diagrams of MSCA-1:1, MSCA-1:2 and MSCA-1:3 respectively, and Figure (d) is a schematic diagram of the specific surface area of the catalyst.

[0103] The mechanical properties of MSCA-1:1, MSCA-1:2 and MSCA-1:3 are as Figure 3 shown.

[0104] It can be seen from Figure 2 and Figure 3 that MSCA-1:1 forms a relatively compact material structure, lacking sufficient micropores and mesopores. The specific surface area of MSCA-1:1 is only 656.6 m 2 g -1 . The compact morphology, although enhancing the hardness and strength of the material, also inhibits its specific surface area and reactivity as a catalyst. The specific surface area of MSCA-1:2 is 779.1 m 2 g -1 . The micropores are mainly concentrated in the range of 0.6 - 0. nm. In MSCA-1:2, the incorporation of an appropriate amount of zinc chloride not only enhances the formation of pores but also provides the necessary support, enabling the carbon framework to remain stable during pyrolysis. The specific surface area of MSCA-1:3 is 857.5 m 2 g -1 , but due to the excessive zinc ions increasing the brittleness of the material and reducing its toughness, during subsequent processing or application, the material is more likely to develop cracks and breakages, thus further reducing the porosity and mechanical properties of the MSCA-1:3 material.

[0105] In summary, considering the specific surface area, pore size distribution and mechanical properties, MSCA-1:2 is selected as the carrier in the following examples.

[0106] Example 1

[0107] In an ammonium ferric EDTA solution with a concentration of 0.4 mol / L, carbon aerogel was added and continuously stirred and impregnated for 4 h (equal-volume impregnation), and then dried at 80 °C for 12 h to obtain a mixture.

[0108] Subsequently, the obtained mixture was annealed at 600 °C at a heating rate of 5 °C / min in an argon atmosphere, held at the target temperature for 1 h, and finally cooled to room temperature. The obtained product was washed strictly with acid and deionized water to remove unreacted species and contaminants, and then dried at 80 °C for another 12 h to obtain the iron single-atom catalyst Fe SAC-600 °C.

[0109] Example 2

[0110] The difference from Example 1 is only that the annealing temperature is 700 °C, and the iron single-atom catalyst Fe SAC-700 °C is obtained.

[0111] Example 3

[0112] The difference from Example 1 is only that the annealing temperature is 800 °C, and the iron single-atom catalyst Fe SAC-800 °C is obtained.

[0113] Example 4

[0114] The difference from Example 1 is only that the annealing temperature is 500 °C, and the iron single-atom catalyst Fe SAC-500 °C is obtained.

[0115] Example 5

[0116] In an ammonium ferric EDTA solution with a concentration of 0.2 mol / L, carbon aerogel was added and continuously stirred and impregnated for 4 h (equal-volume impregnation), and then dried at 80 °C for 12 h to obtain a mixture.

[0117] Subsequently, the obtained mixture was annealed at 700 °C at a heating rate of 5 °C / min in an argon atmosphere, held at the target temperature for 1 h, and finally cooled to room temperature. The obtained product was washed strictly with acid and deionized water to remove unreacted species and contaminants, and then dried at 80 °C for another 12 h to obtain the iron single-atom catalyst Fe SAC-1.

[0118] The iron content in the material was quantitatively analyzed using an inductively coupled plasma mass spectrometer (ICP-MS, Plasma Quant PQ9000), and the mass fraction of iron in Fe SAC-1 was 0.3%.

[0119] Example 6

[0120] It is only different from Example 5 in that the concentration of the ferric ammonium EDTA solution is 0.4 mol / L, and the iron single-atom catalyst Fe SAC-2 is obtained.

[0121] The iron content in the material was quantitatively analyzed by inductively coupled plasma mass spectrometry (ICP-MS, Plasma Quant PQ9000), and the mass fraction of iron in Fe SAC-2 was 0.8%.

[0122] Example 7

[0123] It is only different from Example 5 in that the concentration of the ferric ammonium EDTA solution is 0.6 mol / L, and the iron single-atom catalyst Fe SAC-3 is obtained.

[0124] The iron content in the material was quantitatively analyzed by inductively coupled plasma mass spectrometry (ICP-MS, Plasma Quant PQ9000), and the mass fraction of iron in Fe SAC-2 was 1.2%.

[0125] Test Example

[0126] 1. Structure Characterization

[0127] (1) The HAADF-STEM of the iron single-atom catalysts in Examples 1-3 is as Figure 4 shown, where Figures (a), (b), and (c) correspond to Fe SAC-600°C, Fe SAC-700°C, and Fe SAC-800°C, respectively.

[0128] From Figure 4 it can be seen that the obvious distribution of iron single atoms in the catalysts of Examples 1-3 appears as small and uniform bright spots, indicating that these single atoms are effectively dispersed in the porous carbon matrix.

[0129] (2) The XRD patterns and Raman spectra of the iron single-atom catalysts in Examples 1-3 are as Figure 5 shown.

[0130] From Figure 5 Figure (a) in it can be seen that in the XRD patterns of the iron single-atom catalysts in Examples 1-3, broad peaks appear at the peak positions of 22.5° and 44°, which belong to the diffraction of the (002) crystal plane and (101) crystal plane in the graphite crystal. Obvious iron phase peaks are observed in the XRD patterns, further proving the existence of highly dispersed iron in the samples.

[0131] From Figure 5As can be seen from Figure (b), the iron single-atom catalysts in Examples 1-3 exhibit two relatively prominent spectral band characteristics. Among them, the spectral band at 1340 cm-1 is the D band, which corresponds to the disordered carbon structure; while the spectral band at 1585 cm-1 is the G band, which characterizes the graphite carbon structure.

[0132] The intensity ratio (ID / IG) of the D band and the G band exhibited by the Fe SAC-700℃ catalyst sample is larger than that of the FeSAC-600℃ and Fe SAC-800℃ samples. The intensity ratio of the D band to the G band can reflect the content of carbon defects in the sample to a certain extent. This phenomenon means that in the Fe SAC-700℃ sample, the content of carbon defects is relatively higher, causing the iron single atoms to be fixed at the defects or sites of graphite.

[0133] (3) The energy-dispersive X-ray spectroscopy diagram of the iron single-atom catalyst in Example 2 is as Figure 6 shown.

[0134] From Figure 6 it can be seen that a uniform distribution of iron and nitrogen is presented in the entire carbon matrix.

[0135] (4) The XANES spectroscopy diagrams of the iron single-atom catalysts in Examples 1-3 are as Figure 7 shown, where Figure (b) is a partial enlarged view of 7090 - 7130 eV in Figure (a).

[0136] From Figure 7 it can be seen that the absorption edge of Fe SAC-600℃ shows a high degree of consistency with the absorption edge of Fe2O3, and the electron cloud distribution around the iron atom is very similar to the electron cloud distribution of the +3-valent iron in Fe2O3, thus indicating that the valence state of iron in this sample is approximately +3.

[0137] The absorption edge positions of Fe SAC-700℃ and Fe SAC-800℃ are between Fe3O4 and FePc. There are iron ions with different valence states in Fe3O4, both +2 and +3, while the iron in FePc has its unique electron structure and valence state characteristics, indicating that in these two samples, the electron environment in which the iron atoms are located makes their valence states between +2 and +3, rather than presenting a single, definite valence state. This fully reflects that the heat treatment temperature has a significant and non-negligible impact on the electronic state of iron. At different heat treatment temperatures, the interactions between atoms, the formation and breakage of chemical bonds, etc. within the sample will change, thereby changing the electron structure and valence state performance of the iron atoms.

[0138] In addition, compared with Fe SAC-800℃, the Fe K-edge of Fe SAC-700℃ showed a slight shift towards lower energy, indicating different coordination environments of the catalysts. In the case of complete coordination, the electron cloud distribution around the iron atom is relatively uniform, forming a stable electron state. However, when there is unsaturated coordination, the electron cloud density around the iron atom will change. Due to the lack of sufficient coordinating ligands, the electron cloud will redistribute towards the surrounding nitrogen atoms, resulting in a change in the number of electrons obtained by the iron atom. This redistribution of electrons may cause the iron atom to lose some electrons, shifting its oxidation state from +3 to +2. When the oxidation state of iron decreases, its absorption edge in the X-ray absorption spectrum will shift towards lower energy. This is because the position of the absorption edge is closely related to the valence state of the element; iron atoms with a lower valence state require relatively less excitation energy, so it appears as a shift of the absorption edge towards lower energy in the XANES spectrum.

[0139] The EXAFS fitting data of the iron single-atom catalysts in Examples 1-3 are shown in Table 1.

[0140] Table 1

[0141]

[0142] It can be seen from the data in Table 1 that the coordination number of Fe-N in Fe SAC-800℃ ≈ 4, approaching saturated coordination. For Fe SAC-700℃, the fitted Fe-N coordination numbers are approximately 3.8 and 2.3, containing a defective cooperative coordination structure. In Fe SAC-600℃, it is known that the coordination numbers of Fe-N and Fe-O are approximately 3.4 and 1.6, respectively.

[0143] (5) The FT-EXAFS spectra of the iron single-atom catalysts in Examples 1-3 are as Figure 8 shown, where Figure (b) is the local magnification of Figure (a).

[0144] From Figure 8 Figure (a), it can be seen that a peak ≈ appears in all three catalysts, which corresponds to the first coordination shell of the Fe-N interaction, and a coordination bond is formed between Fe and N. No characteristic of the Fe-Fe bond is observed at , corroborating the atomic dispersion state of the iron species.

[0145] From Figure 8 Figure (b), it can be seen that the peak positions of Fe SAC-800℃ are exactly the same as those of the standard substance FePc, corresponding to the fitting results. While Fe SAC-600℃ shows a shift towards lower R values , and Fe SAC-700℃ shows a shift towards higher R values , these changes reflect the alteration of the local coordination environment. At 600 °C, a relatively lower temperature, the vibration amplitude between atoms is small, and the bond length of the formed Fe-N coordination bond is relatively shorter, so there is a shift towards a smaller R value. When the temperature rises to 700 °C, the vibration between atoms is enhanced, the equilibrium position of the chemical bond changes, resulting in an increase in the distance between nitrogen atoms and iron atoms, which is manifested as a shift towards the High-R value in the FT-EXAFS spectrum. , reflecting the change in the Fe-N bond length in the local coordination environment.

[0146] 2. Electrochemical Catalysis Performance Test

[0147] The electrocatalytic denitrification performance of the catalyst was systematically evaluated using a CHI 660E electrochemical workstation in a three-electrode cell system.

[0148] Electrochemical evaluation was carried out using a three-electrode electrolytic cell and a CHI660E electrochemical workstation. When preparing the working electrode, 5 mg of the synthesized catalyst was ultrasonically dispersed in 960 μL of ethanol and 40 μL of 5 wt% Nafion solution for at least 1 hour to form a uniform ink. Then, an appropriate amount of the catalyst ink was evenly coated on a conductive glass substrate with a size of 1×1 cm 2 and dried under a baking lamp. A platinum sheet was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode.

[0149] The electrolyte was 0.1 M sodium sulfate (Na2SO4) and 50 g / L sodium nitrate (NaNO3), with a pH of 7 and a voltage of -0.65 V.

[0150] All measured potentials were calibrated with reference to the reversible hydrogen electrode (RHE), and the calibration equation used was E(V vs. RHE) = E(V vs. SCE) + 0.0591×pH + 0.241. Linear sweep voltammetry (LSV) was carried out at a scanning rate of 50 mV / s, and potentiostatic tests were carried out for 24 hours at different applied potentials. The electrolyte in the cathode chamber was continuously stirred at a rate of 200 rpm. Every 3 h, 1 mL of the solution was taken out for analysis. The concentrations of nitrate nitrogen (NO3 - -N), nitrite nitrogen (NO2 - -N), and ammonium nitrogen (NH4 + -N) were quantitatively determined using an ultraviolet-visible spectrophotometer (Agilent Cary-5000). All electrolysis experiments were repeated three times to ensure the reliability and accuracy of the results.

[0151] (1) As Figure 9 shown, for the two catalysts, Fe SAC-500 °C and Fe SAC-600 °C, NO3- -N has a low degradation efficiency and NH3 selectivity. At a relatively low sintering temperature, an environment conducive to NO3 - -N adsorption fails to form, resulting in NO3 - -N being difficult to effectively adsorb on the catalyst surface, thereby affecting the degradation efficiency and product conversion. Fe SAC - 700℃ shows excellent performance within the same operating time. The conversion rate of NO3 - -N has been significantly improved, and the degradation efficiency per unit time is as high as 1236.3 mg N h -1 g -1 and the nitrate degradation percentage is 66.6%.

[0152] (2) Figure 10 shows the electrochemical performance of the iron single-atom catalyst in Examples 5 - 7.

[0153] From Figure 10 Figures (a) and (b), it can be seen that the Fe SAC - 2 catalyst exhibits the most excellent catalytic activity, indicating that both lower or higher iron concentrations have a negative impact on the catalytic performance. In Fe SAC–1, the catalyst content is too low to facilitate the reaction. In the Fe SAC - 3 catalyst, the degree of atomic aggregation increases, which hinders the reactants from approaching the active Fe sites and interacting with each other. The reactants cannot fully contact the active Fe sites, thereby reducing the efficiency of the catalytic reaction.

[0154] Figure (c) shows the results of measuring the current density at voltages from -0.64 V to -0.4 V by linear sweep voltammetry (LSV). It can be seen that when the electrolyte does not contain NaNO3, the catalyst exhibits a relatively low current density. In the absence of nitrate as a reactant, the redox reaction occurring on the electrode surface is relatively limited, and the rate of electron transfer is low, so the generated current density is small. When NaNO3 is added to the electrolyte and under the same applied potential conditions, the current density of Fe SAC - 2 increases significantly. This further demonstrates the high catalytic ability of Fe SAC - 2 for the NO3 - reduction reaction.

[0155] Figure (d) is the i - t curve of Fe SAC - 2. The number of electrons transferred per unit time by this catalyst participating in the nitrate reduction reaction remains constant, indicating that the reaction can proceed continuously and efficiently, maintaining a stable current density.

[0156] (2) Figure 11 shows the electrochemical performance of the iron single-atom catalyst in Examples 5 - 7.

[0157] Figure 11In (a), (b), and (c) of [Figure number], the changes in the products at different times are shown. During the reaction, the nitrate concentration continuously decreases from the initial 50 ppm. At the same time, the concentration of the reaction product NH4 + increases continuously with time, and finally the selectivity of NH4 + reaches 96%, confirming that ammonia is the main reaction product in the reduction reaction system.

[0158] Figure 11 In (d) of [Figure number], the change trend of the concentration of the by-product nitrite is shown. Its concentration initially shows an upward trend, then gradually decreases, and finally stabilizes at a very low level of 0.3 mg / L. After calculation, the proportion of nitrite in the total nitrogen species in the system is only 0.6%. This further reflects that the influence of by-products in this reaction system is small and it has high selectivity in the process of ammonia production.

[0159] 3. Cycling performance and scale-up performance tests

[0160] Under the optimal electrolysis conditions (the electrolyte is 0.1 M sodium sulfate (Na2SO4) and 50 g / L sodium nitrate (NaNO3), pH = 7, -0.65 V), the long-term stability of the Fe SAC–700℃ catalyst in Example 2 was tested.

[0161] (1) The catalyst underwent 20 consecutive cycle reactions, and its electrocatalytic performance was basically maintained at about 900 mg N h -1 g -1 ( Figure 12 a).

[0162] Comparing the XRD patterns before the reaction and after 20 cycles ( Figure 12 b), it was found that the positions and intensities of the characteristic diffraction peaks remained basically unchanged. This indicates that the crystal structure of the catalyst remained stable during the long-term electrolysis process, and no obvious crystal form transformation or lattice distortion occurred.

[0163] After the entire 20 consecutive cycles, the reaction current density remained stable throughout, fully demonstrating the stability of the electrocatalytic activity of the catalyst during long-term operation ( Figure 12 c).

[0164] It can be seen from the TEM image that the distribution of iron single atoms on the support is still relatively uniform, and no obvious agglomeration phenomenon appears ( Figure 12 d).

[0165] (2) To explore the degradation kinetic characteristics of NO3 - -N in large-scale wastewater, a flow reactor was used to test a wastewater sample with an initial NO3 - -N concentration set at 100 mg / L and a volume of 1500 mL (Figure 13 a and b).

[0166] The results showed that more than 96% of the NO3 - -N was successfully reduced ( Figure 13 c), providing a method for large-scale wastewater treatment.

[0167] The preferred embodiments of the present application have been described in detail above. However, the present application is not limited thereto. Within the technical concept scope of the present application, various simple modifications can be made to the technical solutions of the present application, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present application and fall within the protection scope of the present application.

Claims

1. An iron single-atom catalyst containing a multi-nitrogen ligand structure, which comprises a porous carbon support and iron single atoms located on the porous carbon support, wherein, The iron single atoms exist in the form of asymmetric coordination with nitrogen atoms.

2. The iron single-atom catalyst according to claim 1, wherein In the iron single atom catalyst, the coordination number of nitrogen atoms to iron atoms is 2 - 5, preferably 2 - 4; preferably, the coordination number of nitrogen atoms to iron atoms is 2 - 2.5 and 3.5 - 4; and / or Based on the iron single atom catalyst, the mass content of the iron element is 0.1% - 2%, preferably 0.3% - 1.5%, more preferably 0.5% - 1.0%; and / or The I D / I G ratio of the iron single-atom catalyst is 0.7 - 0.98, preferably 0.85 - 0.

95.

3. A preparation method of an iron single atom catalyst containing a multi - nitrogen ligand structure, which comprises the following steps: S1: Provide a biomass carbon aerogel; S2: Immerse the biomass carbon aerogel in step S1 in an iron ammonium salt solution to obtain an immersion product; S3: Anneal the immersion product in step S2 to obtain an annealed product; Optionally, step S4: Wash and dry the annealed product to obtain the iron single atom catalyst containing a multi - nitrogen ligand structure.

4. The preparation method according to claim 3, characterized in that, In step S1, the specific surface area of the biomass carbon aerogel is 300 m 2 / g - 1200 m 2 / g, preferably 600 m 2 / g - 1000 m 2 / g, more preferably 700 m 2 / g - 800 m 2 / g; and / or The pore size distribution of the biomass carbon aerogel is 0.5 nm - 0.8 nm, preferably 0.6 nm - 0.7 nm.

5. The preparation method according to claim 3 or 4, characterized in that, In step S1, the providing of the biomass carbon aerogel includes: mixing biomass with a eutectic molten salt of chloride and then performing carbonization treatment; Preferably, the biomass is selected from wood, and the wood is preferably selected from one or more of beech, basswood, locust, eucalyptus, pine, toona, poplar, willow, peach, apricot, apple, pear, banyan, camphor, locust, maple, cypress, oak, fir, paulownia; Preferably, the mass ratio of the biomass to the eutectic molten salt of chloride is 1:1 - 1:3, preferably 1:1.5 - 1:2.5; Preferably, the melting point of the eutectic molten salt of chloride is 200 °C - 900 °C; Preferably, the eutectic molten salt of chloride is a eutectic molten salt of zinc chloride and potassium chloride; Preferably, the carbonization treatment is carried out in an inert atmosphere; Preferably, the temperature of the carbonization treatment is 600 °C - 900 °C, preferably reaching the temperature of the carbonization treatment in an ascending - temperature manner, and the heating rate is 1 °C / min - 10 °C / min.

6. The preparation method according to any one of claims 3-5, characterized in that, In step S2, the iron ammonium salt solution is selected from one or more of ammonium ferric EDTA solution, Prussian blue and iron phthalocyanine; and / or Calculated by the mass of iron element, the mass of the iron ammonium salt in the iron ammonium salt solution is 0.1% - 2% of the mass of the biomass carbon aerogel, preferably 0.3% - 1.5%, more preferably 0.5% - 1%; and / or The concentration of the iron ammonium salt solution is 0.1 mg / L - 1 mg / L, preferably 0.2 mg / L - 0.6 mg / L, more preferably 0.3 mg / L - 0.5 mg / L; and / or The temperature of the immersion is 60 °C - 100 °C.

7. The preparation method according to any one of claims 3-6, characterized in that, In step S3, the annealing treatment is carried out in an inert atmosphere; and / or The temperature of the annealing treatment is 500 °C - 1000 °C, preferably 600 °C - 800 °C, more preferably 650 °C - 750 °C, preferably reaching the temperature of the annealing treatment in an ascending - temperature manner, and the heating rate is 1 °C / min - 10 °C / min.

8. Use of the iron single-atom catalyst according to any one of claims 1 or 2 or the iron single-atom catalyst prepared by the preparation method according to any one of claims 3-7 in nitrate reduction.

9. A method for electrocatalytic reduction of nitrate, which comprises reducing nitrate in a three-electrode system, wherein the working electrode in the three-electrode system comprises the iron single-atom catalyst according to any one of claims 1 or 2 or the iron single-atom catalyst prepared by the preparation method according to any one of claims 3-7.

10. The method according to claim 9, wherein The electrolyte in the electrolyte solution of the three-electrode system is selected from sodium sulfate; and / or The working voltage of the three-electrode system is -0.45 V to -0.75 V; and / or The initial concentration of nitrate is 50 mg / L - 200 mg / L.

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