A heterogeneous structure catalyst for reducing nitrate and a preparation method and application thereof

Abstract

The present application relates to the technical field of electrocatalytic materials, in particular to a heterogeneous structure catalyst for reducing nitrate and a preparation method and application thereof, the catalyst comprises a porous graphite carbon carrier and a heterogeneous structure formed by iron monatomic atoms and iron nanoclusters supported on the carrier, wherein the iron monatomic atoms and the iron nanoclusters are formed by in-situ pyrolysis of triruthenium dodecacarbonyl precursor on the carbon carrier, realizing synergistic catalysis of different functional sites, thereby improving ammonia yield and Faraday efficiency, and providing a general technical strategy for designing heterogeneous structure active sites to accelerate series electrocatalytic reactions.

Description

Technical Field

[0001] This invention relates to the field of electrocatalytic materials technology, specifically to a heterostructure catalyst for reducing nitrates, its preparation method, and its application. Background Technology

[0002] Ammonia is used as a raw material for nitrogen fertilizer, a hydrogen energy carrier, and an important chemical intermediate. However, the traditional Haber-Bosch process for ammonia synthesis requires high temperature and high pressure, with temperatures ranging from 400°C to 500°C and pressures from 15 MPa to 25 MPa. These harsh conditions result in extremely high energy consumption and large carbon emissions, accounting for 1% to 2% of global energy consumption and 1.5% of carbon emissions.

[0003] Electrocatalytic nitrate reduction reaction, abbreviated as NO3RR, offers a green pathway for the directed conversion of nitrate to ammonia at ambient temperature and pressure, simultaneously achieving wastewater purification and resource utilization. This provides a sustainable solution to the aforementioned dual challenges, thus becoming a research focus in the field of electrocatalysis. However, the large-scale application of NO3RR is limited by the scarcity of highly efficient catalysts. This reaction involves 8-electron transfer and multi-step proton coupling processes, resulting in a complex reaction pathway, easy generation of byproducts, and fierce competition with the hydrogen evolution reaction (HER). Therefore, it places extremely stringent requirements on the selectivity, activity, and stability of catalysts.

[0004] The existing NO3RR catalysts have the following technical defects: (1) Traditional noble metal catalysts, such as Pt, Pd, Ru, etc., have dominant HER activity, resulting in low Faraday efficiency and selectivity of the target product ammonia, and are scarce and expensive; (2) Although non-noble metal catalysts such as iron-based materials are abundant and inexpensive, they generally have problems such as limited number of active sites, insufficient intrinsic activity, and slow reaction kinetics; (3) Single active site catalysts are difficult to adapt to the multi-step complex reaction requirements of NO3RR, and cannot optimize all intermediate steps on the same catalyst surface, which limits the yield and efficiency of ammonia.

[0005] Recent studies have shown that constructing heterogeneous active centers at the atomic scale, such as the coexistence of single atoms and nanoclusters, can optimize multi-step reaction pathways and enhance catalytic performance through synergistic effects between different sites. For example, existing literature has reported that nickel single atoms and nanoclusters exhibit significant synergistic catalytic effects in the reforming reaction of bioethanol. Summary of the Invention

[0006] To address the shortcomings of the existing technology, the purpose of this invention is to provide a heterostructure catalyst for reducing nitrates, its preparation method, and its application, using dodecyltriferric Fe3(CO). 12By constructing active centers where atomically dispersed iron single atoms and ultra-small iron nanoclusters coexist, synergistic catalysis of different functional sites can be achieved, thereby improving ammonia yield and Faraday efficiency. At the same time, it provides a general technical strategy for designing heterostructure active sites to accelerate tandem electrocatalytic reactions.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A heterostructure catalyst for reducing nitrates, the catalyst comprising a porous graphite carbon support and a heterostructure formed by iron single atoms and iron nanoclusters supported on the support, wherein the iron single atoms and iron nanoclusters are formed by in-situ pyrolysis of a dodecyltriferric precursor on the carbon support.

[0008] In a preferred embodiment of the present invention, the size of the iron nanoclusters is 0.1 nm to 0.5 nm.

[0009] In a preferred embodiment of the present invention, the porous graphite carbon support is prepared by high-temperature decomposition of potassium citrate (K3C6H5O7).

[0010] The method for preparing the heterostructure catalyst for reducing nitrates includes the following steps: Prepare porous graphite carbon support.

[0011] Dodecyl iron oxide and porous graphite carbon support were dispersed in anhydrous diethyl ether and then subjected to ultrasonic treatment to form a suspension.

[0012] The solvent in the suspension is evaporated to obtain the composite precursor.

[0013] The composite precursor was pyrolyzed to obtain the heterostructure catalyst.

[0014] In a preferred embodiment of the present invention, the mass ratio of dodecyltriferric oxide to porous graphite carbon support is 1:10~30.

[0015] In a preferred embodiment of the present invention, the pyrolysis conditions are as follows: in an inert atmosphere, the pyrolysis heating rate is 2℃ / min to 8℃ / min, the pyrolysis temperature is 150℃ to 200℃, and the pyrolysis time is 1h to 3h.

[0016] In a preferred embodiment of the present invention, the method for preparing porous graphite carbon support includes: heating potassium citrate to 700°C to 900°C at a rate of 1°C / min to 3°C / min under a nitrogen atmosphere, holding at the temperature for 1h to 3h, and then cooling, followed by acid washing, water washing, and drying.

[0017] The application of the heterostructure catalyst for reducing nitrate described in this invention in the electrocatalytic reduction of nitrate to ammonia.

[0018] The electrocatalytic reduction of nitrate described in this invention is carried out at room temperature and pressure, and the electrolyte contains nitrate and sodium sulfate.

[0019] The heterostructure catalyst for reducing nitrate described in this invention is used as a working electrode in a three-electrode system to carry out the nitrate reduction reaction at a potential of -0.5V to -0.9V relative to the reversible hydrogen electrode.

[0020] Compared with the prior art, the beneficial effects of the present invention are: 1. The heterostructure catalyst for nitrate reduction described in this invention is based on porous graphite carbon as a support. Through in-situ pyrolysis of the dodecyltriferric precursor on the carbon support, a heterostructure composed of iron single atoms and iron nanoclusters is formed on the support surface. Addressing the technical problems of traditional nitrate reduction catalysts, such as single-function active sites, difficulty in coordinating nitrate adsorption activation and hydrogenation steps, and low electron-proton transfer efficiency, this solution achieves targeted solutions through precise catalyst structure design. First, the in-situ pyrolysis process ensures the complete decomposition of the dodecyltriferric precursor, allowing iron single atoms and iron nanoclusters to tightly bind and uniformly load on the porous graphite carbon support, avoiding the problems of active site aggregation or uneven dispersion. Simultaneously, the high specific surface area and conductivity of porous graphite carbon provide sufficient active sites and efficient electron transport channels for the reaction. Secondly, by constructing a heterostructure of "iron single atom-iron nanoclusters," functional complementarity and synergistic catalysis of the two types of active sites were achieved: the iron single atom sites possess unique electronic structures and adsorption properties, which are beneficial for the adsorption and activation of nitrate ions; while the iron nanocluster sites exhibit excellent hydrogenation activity, which can optimize the hydrogen dissociation and hydrogen species transfer processes, accelerating the subsequent hydrogenation reaction. Through their synergistic effect, the entire reaction pathway of nitrate adsorption, activation, and hydrogenation conversion is effectively opened up, significantly improving the integrity and rate of the 8-electron-proton transfer process, ultimately achieving a comprehensive improvement in the catalytic efficiency, selectivity, and stability of the nitrate reduction reaction.

[0021] 2. The heterostructure catalyst exhibits excellent selectivity for ammonia formation. Its Faraday efficiency remains above 75% over a wide potential range, from -0.5V to -0.9V vs. RHE. This indicates that the catalyst can effectively suppress the hydrogen evolution reaction and the formation pathways of other nitrogen-containing byproducts, thus achieving efficient and directional conversion of nitrates to ammonia.

[0022] 3. Electrochemical impedance spectroscopy (EIS) tests of the heterostructure catalyst described in this invention show that Fe... 1+n The / C catalyst exhibits the lowest charge transfer resistance, demonstrating faster electron transfer rates and multi-step reaction kinetics. This is attributed to the heterostructure optimizing the adsorption / desorption energy barriers of reaction intermediates and lowering the activation energy of the reaction.

[0023] 4. The heterostructure catalyst of the present invention benefits from the close interaction between atomically dispersed single atoms and nanoclusters, as well as the anchoring and protection effect of the porous carbon support. In the continuous 24-hour electrochemical test, the current density and Faraday efficiency of the catalyst did not decrease significantly, showing good long-term operational stability.

[0024] 5. This invention uses dodecacarbonyltriferrite as the sole iron source and can construct heterostructures in situ on porous carbon supports through pyrolysis. By precisely controlling the pyrolysis heating rate, the ratio of iron single atoms to nanoclusters can be tunably synthesized. The preparation method is simple, controllable, and low in cost, with good process repeatability and easy to scale up, overcoming the technical difficulties of complex and costly heterostructure construction in traditional methods. Attached Figure Description

[0025] Figure 1 This is a schematic diagram illustrating the synthesis of the heterostructure catalyst described in this invention.

[0026] Figure 2 This invention relates to Fe3(CO) 12 Thermogravimetric analysis (TGA) curves of the precursor.

[0027] Figure 3 The Fe of the present invention 1+n / C、Fe n / C、Fe1 / C、Fe B XRD curves of C catalyst.

[0028] Figure 4 In the figure, a~c represent Fe at different magnification ratios. 1+n STEM image of / C catalyst; d~f represents Fe 1+n HAADF-STEM image and corresponding EDX elemental mapping of the / C catalyst; g~i represents Fe 1+n AC-HAADF-STEM image of the / C catalyst.

[0029] Figure 5 Linear sweep voltammetry curves of Fe1+n / C, Fen / C, Fe1 / C, and FeB catalysts in the presence and absence of nitric acid are shown.

[0030] Figure 6 For Fe 1+n Faraday efficiency and NH3 yield at / C between -0.5 V and -0.9 V.

[0031] Figure 7 For Fe 1+n / C、Fe n / C、Fe1 / C、Fe B A comparison graph of NH3 yields for C catalyst and C catalyst.

[0032] Figure 8 For Fe 1+n / C、Fe n / C、Fe1 / C、Fe B A comparison graph of the impedance of the C catalyst and the C catalyst.

[0033] Figure 9 For Fe 1+n The long-term timing current stability test diagram of / C. Detailed Implementation

[0034] The following detailed description, in conjunction with embodiments of the present invention and accompanying drawings, provides a clear and complete illustration of the technical solutions in these embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0035] It should be noted that all technical terms used in this invention are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased from the market or prepared by existing methods.

[0036] Example 1 A method for preparing a heterostructure catalyst for reducing nitrates, such as... Figure 1 As shown, it includes the following steps: (1) Preparation of porous graphite carbon support: 1.0 g of potassium citrate K3C6H5O7 was weighed and placed in a porcelain boat. It was heated to 800 °C at a heating rate of 3 °C / min under a nitrogen atmosphere and kept at that temperature for 1 h. After cooling to room temperature, it was washed several times with 0.5 mol / L sulfuric acid solution and deionized water, and then vacuum dried at 60 °C for 12 h to obtain porous graphite carbon support C.

[0037] (2) Weigh out 3 mg of dodecacarbonyltriferrite Fe3(CO) 12 60 mg of porous carbon was dispersed in 5 mL of anhydrous diethyl ether and sonicated for 1 h to form a suspension.

[0038] (3) Let stand at room temperature for 2 hours to allow the anhydrous ether to evaporate, and the composite precursor is obtained.

[0039] (4) The catalyst was heated to 165℃ for 1 h under a nitrogen atmosphere at a heating rate of 5℃ / min to obtain a heterostructure catalyst for reducing nitrate, named Fe. 1+n / C catalyst.

[0040] Example 2 The preparation method of this embodiment is the same as that of Example 1. The difference is that in step (4), the heating rate is changed to 2℃ / min to promote the formation of single atoms and obtain a catalyst with a heterostructure, named Fe1 / C.

[0041] Example 3 The preparation method in this embodiment is the same as that in Example 1, except that in step (4), the heating rate is changed to 8℃ / min to promote the formation of nanoclusters and obtain a catalyst with a heterostructure, named Fe. n / C.

[0042] Example 4 (1) Preparation of porous graphite carbon support: 1.0 g of potassium citrate K3C6H5O7 was weighed and placed in a porcelain boat. It was heated to 700 °C at a heating rate of 1 °C / min under a nitrogen atmosphere and kept at that temperature for 3 h. After cooling to room temperature, it was washed several times with 0.5 mol / L sulfuric acid solution and deionized water, and then vacuum dried at 60 °C for 12 h to obtain porous graphite carbon support C.

[0043] (2) Weigh out 3 mg of dodecacarbonyltriferrite Fe3(CO) 12 30 mg of porous carbon was dispersed in 5 mL of anhydrous diethyl ether and sonicated for 1 h to form a suspension.

[0044] (3) Let stand at room temperature for 2 hours to allow the anhydrous ether to evaporate, and the composite precursor is obtained.

[0045] (4) The catalyst was heated to 150°C for 2 hours under a nitrogen atmosphere at a heating rate of 5°C / min to obtain a heterostructure catalyst for reducing nitrate.

[0046] Example 5 (1) Preparation of porous graphite carbon support: 1.0 g of potassium citrate K3C6H5O7 was weighed and placed in a porcelain boat. It was heated to 900 °C at a heating rate of 2 °C / min under a nitrogen atmosphere and kept at that temperature for 2 h. After cooling to room temperature, it was washed several times with 0.5 mol / L sulfuric acid solution and deionized water, and then vacuum dried at 60 °C for 12 h to obtain porous graphite carbon support C.

[0047] (2) Weigh out 3 mg of dodecacarbonyltriferrite Fe3(CO) 12 90 mg of porous carbon was dispersed in 5 mL of anhydrous diethyl ether and sonicated for 1 h to form a suspension.

[0048] (3) Let stand at room temperature for 2 hours to allow the anhydrous ether to evaporate, and the composite precursor is obtained.

[0049] (4) The catalyst was heated to 200℃ for 3h under a nitrogen atmosphere at a heating rate of 5℃ / min to obtain a heterostructure catalyst for reducing nitrate.

[0050] The heterostructure catalysts for reducing nitrates prepared in Examples 3-5 and the Fe catalyst in Example 1 1+n / C catalysts have similar performance.

[0051] Comparative Example 1 Direct pyrolysis of Fe3(CO) 12 Without a carbon support, the bulk iron catalyst Fe was obtained by pyrolysis at 165℃ for 1 hour, yielding a control sample. B .

[0052] Comparative Example 2 Weigh 1.0 g of potassium citrate (K3C6H5O7) and place it in a porcelain boat. Heat the boat to 800 °C at a heating rate of 3 °C / min under a nitrogen atmosphere and hold for 1 h. After cooling to room temperature, wash the boat several times with 0.5 mol / L sulfuric acid solution and deionized water, and dry it under vacuum at 60 °C for 12 h to obtain porous graphitic carbon support C.

[0053] Results Analysis 1. Catalyst morphology characterization: The precursor decomposition temperature was optimized based on thermogravimetric analysis (TGA). Figure 2 This invention relates to Fe3(CO) 12 Thermogravimetric analysis (TGA) curves of the precursor, showing Fe3(CO) 12 Complete decomposition occurs at approximately 160°C. Therefore, in order to ensure the full decomposition of the precursor and control the formation of different iron species, an optimal decomposition temperature of 165°C was selected.

[0054] Figure 3 It shows C, Fe B Fe 1 / C Fe 1+n / C and Fe n X-ray diffraction (XRD) patterns of the Fe1 / C catalyst. The three supported catalysts are Fe1 / C, Fe... 1+n / C and Fe n / C, all exhibit a characteristic diffraction peak at 26.5°, corresponding to graphitic carbon, indicating that the incorporation of iron has a negligible effect on the carbon support. No obvious diffraction peaks associated with iron-containing phases were observed, suggesting the formation of ultrasmall iron species and their uniform dispersion within the carbon matrix. In contrast, Fe B The catalyst showed a distinct peak corresponding to cubic Fe3O4, and its JCPDS number is 19-0629.

[0055] The structural characteristics and dispersion state of iron species in the prepared catalyst were investigated using scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and aberration-corrected high-angle annular dark-field AC-HAADF scanning transmission electron microscopy. Figure 4 Figures a and b show that Fe 1+n STEM images of / C show a porous carbon material composed of thin, irregular nanosheets, such as Figure 4 At higher magnifications, lattice fringes with a spacing of 0.35 nm can be clearly observed, corresponding to the (002) plane of graphitized carbon. Figure 4 The d~f display shows Fe 1+n The corresponding EDX elemental distribution map for / C shows the distribution of iron, including small clusters and ultrasmall iron species. For example... Figure 4 Using AC-HAADF-STEM, these ultrasmall species can be identified as isolated iron single atoms, marked with circles, while slightly larger aggregates correspond to iron nanoclusters, marked with ellipses. The close proximity of these two types of substances at the nanoscale confirms the constructed heterostructure of Fe. 1+n The / C catalyst was successful, containing atomically dispersed Fe1 and nano-aggregated Fe2+. n These sites are embedded in carbon supports with porosity and defects.

[0056] 2. NO3RR activity test: To investigate the electrochemical reactivity of the materials in nitrate reduction, a neutral medium was used in an H-type electrolyzer to react the prepared catalysts C and Fe. B Fe1 / C, Fe 1+n / C and Fe n The NO3RR performance of / C was systematically evaluated. For example... Figure 5 As shown in the linear sweep voltammetry curve, after adding NO 3- After that, Fe 1+n The significantly enhanced current at / C, superior to other catalysts, confirms its excellent catalytic activity for NO3RR. The NH3 yield and Faradaic efficiency were quantified using indophenol blue spectrophotometry, such as... Figure 6 As shown, Fe 1+n The / C catalyst achieved a maximum Faradaic efficiency of 92.2% and an ammonia yield of 7889 μg·mg cat at a potential of -0.8 V relative to the reference electrode RHE. −1 h −1 It is noteworthy that the Faraday efficiency FE consistently remains above 75% within the potential range of -0.5V to -0.9V, highlighting its high ammonia selectivity under neutral conditions, such as... Figure 7It can be seen that, and throughout the entire potential range, Fe 1+n / C is superior to Fe in terms of ammonia yield. B Fe1 / C, Fe n / C and carbon supports. For example, C and carbon supports. Figure 8 As shown, electrochemical impedance spectroscopy (EIS) was used to investigate charge transfer characteristics in Fe. 1+n The / C catalyst exhibits the smallest semi-circular shape and has a charge transfer resistance of 8.32 Ω, indicating that the electron transfer kinetics of the multi-electron NO3RR step are more rapid.

[0057] 3. Timing Current Stability Test Long-term stability is another key characteristic of catalyst materials. To test the stability of the prepared catalyst materials, a 24-hour chronoamperometry test was conducted. Figure 9 As shown, during a continuous 24-hour constant current chronometry test, its current density and FE maintained good stability. This is because the interaction between single atoms and nanoclusters in the heterostructure of the material enables synergistic catalysis of different functional sites, which not only maintains high activity but also helps to enhance the stability of electrochemical testing.

[0058] In summary, by optimizing the precursor decomposition temperature through thermogravimetric analysis and combining it with multiple characterization techniques such as XRD, STEM, EDX, and AC-HAADF-STEM, the Fe2+ precursor was successfully constructed and verified. 1+n The unique heterostructure of the / C catalyst offers significant advantages, further validated by NO3RR activity and long-term stability tests. 1+n The / C catalyst exhibited the best overall performance in the nitrate reduction reaction and was the most beneficial catalyst for NO3RR among all tested samples.

[0059] It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described here to avoid redundancy. Although preferred embodiments of the invention have been described, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this invention.

[0060] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A heterostructure catalyst for reducing nitrates, characterized in that, The catalyst comprises a porous graphite carbon support and a heterostructure formed by iron single atoms and iron nanoclusters supported on the support, wherein the iron single atoms and iron nanoclusters are formed by in-situ pyrolysis of a dodecyltriferric precursor on the carbon support. The size of the iron nanoclusters is 0.1 nm to 0.5 nm; The porous graphite carbon support was prepared by high-temperature decomposition of potassium citrate.

2. The method for preparing the heterostructure catalyst for reducing nitrate according to claim 1, characterized in that, Includes the following steps: Preparation of porous graphitic carbon supports; Dodecyl iron oxide and porous graphite carbon support were dispersed in anhydrous diethyl ether and then subjected to ultrasonic treatment to form a suspension. The solvent in the suspension is evaporated to obtain the composite precursor; The composite precursor was pyrolyzed to obtain the heterostructure catalyst.

3. The method for preparing the heterostructure catalyst for reducing nitrate according to claim 2, characterized in that, The mass ratio of dodecacarbonyl triferrite to porous graphite carbon support is 1:10~30.

4. The method for preparing the heterostructure catalyst for reducing nitrate according to claim 2, characterized in that, The pyrolysis conditions are as follows: in an inert atmosphere, the pyrolysis heating rate is 2℃ / min~8℃ / min, the pyrolysis temperature is 150℃~200℃, and the pyrolysis time is 1h~3h.

5. The method for preparing the heterostructure catalyst for reducing nitrate according to claim 2, characterized in that, The preparation method of porous graphite carbon support includes: heating potassium citrate to 700℃~900℃ at 1℃ / min~3℃ / min under nitrogen atmosphere, holding at the temperature for 1h~3h, and then cooling, followed by acid washing, water washing, and drying.

6. The application of the heterostructure catalyst for reducing nitrate according to claim 1 in the electrocatalytic reduction of nitrate to ammonia.

7. The application according to claim 6, characterized in that, The electrocatalytic reduction of nitrate is carried out at room temperature and pressure, and the electrolyte contains nitrate and sodium sulfate.

8. The application according to claim 6, characterized in that, The heterostructure catalyst for reducing nitrate is used as the working electrode in a three-electrode system to carry out the nitrate reduction reaction at a potential of -0.5V to -0.9V relative to the reversible hydrogen electrode.

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