Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor, preparation method and application thereof

Abstract

This invention discloses a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of a MOF core-shell precursor, its preparation method, and its applications, relating to the fields of electrocatalytic materials and ammonia synthesis technology. The Cu / M tandem catalyst is a core-shell precursor composed of a MOF containing metal M as the core and a Cu-containing MOF as the shell. Under electrochemical action, the Cu-containing MOF is converted to form a copper-based active domain, while the MOF containing metal M is in-situ reconstructed or retained to form an M-based active domain. The copper-based and M-based active domains are then orderly integrated at the nanoscale to form a complex with a heterogeneous interface structure. The metal M is a transition metal capable of synergistically catalyzing nitrate reduction with copper. Through the formed heterogeneous interface structure, this invention achieves spatial division of labor and efficient relay between nitrate activation and nitrite hydrogenation functions, fundamentally suppressing nitrite accumulation and hydrogen evolution side reactions.

Description

Technical Field

[0001] This invention relates to the fields of electrocatalytic materials and ammonia synthesis technology, specifically to a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursors, its preparation method, and its application. Background Technology

[0002] Ammonia (NH3) is an important agricultural fertilizer and industrial chemical, and also a promising carbon-free energy carrier. The traditional Haber process for ammonia synthesis is energy-intensive and produces large carbon emissions, necessitating the development of green and sustainable alternatives. Electrocatalytic nitrate reduction (NO3)... - Renewable energy sources (RR) powered by renewable electricity can convert nitrate pollutants in water into high-value ammonia under mild conditions, simultaneously achieving green ammonia synthesis and nitrate pollutant removal, and have therefore attracted widespread attention. However, NO3... - The RR process involves a complex multi-step proton-coupled electron transfer process, which generally suffers from problems such as slow reaction kinetics, accumulation of intermediate products (especially nitrite), and severe hydrogen evolution competition reactions, making it difficult for ammonia yield and Faraday efficiency to meet the requirements of practical applications.

[0003] In the study of electrocatalytic nitrate reduction catalysts, copper (Cu)-based materials have been proven to be effective in catalyzing nitrate (NO3) reduction. - ) to nitrite (NO2) - The conversion of NO2, but for subsequent NO2 - Deep hydrogenation has a relatively weak activity in generating NH3; while various transition metals such as iron (Fe), cobalt (Co), and nickel (Ni) and their derivatives promote water activation and provide active hydrogen to drive NO2 production. - It exhibits good performance in hydrogenation conversion. From a reaction mechanism perspective, integrating copper with these metals (such as Fe, Co, Ni, etc.) into a tandem catalyst is expected to form a clear functional division of labor, allowing nitrate activation and nitrite hydrogenation to be completed in succession on two spatially adjacent but functionally distinct active domains, thereby significantly improving the overall NO3- concentration. - Selectivity and efficiency of RR.

[0004] However, in actual catalyst construction, conventional methods such as physical mixing, co-precipitation, impregnation, or stepwise loading are insufficient to precisely control the spatial arrangement, interfacial structure, and electronic interactions between copper and the second metal component. This often results in severe phase separation, insufficient interfacial contact, and disordered spatial distribution of active sites, leading to NO2 generation from nitrate activation. - The inability to efficiently transfer to adjacent hydrogenation active sites results in NO2... -Desorption and accumulation, interruption of tandem catalysis, and the tendency to trigger severe hydrogen evolution side reactions are all problems. Therefore, the key to achieving efficient NO3-desorption lies in the controllable construction of highly synergistic copper-based heterointerfaces for different second metal species, and the orderly integration of copper and M domains at the nanoscale. - The core challenge of RR tandem catalysis. Summary of the Invention

[0005] To address the shortcomings of the aforementioned background technologies, this invention provides a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of a MOF core-shell precursor, its preparation method, and its application. This method involves pre-designing and synthesizing a core-shell precursor with a MOF containing M as the core and a Cu-containing MOF as the shell. Under electrochemical conditions of nitrate reduction, the shell layer is selectively and in-situ converted into Cu-based active species, while the core MOF largely retains its structural characteristics or undergoes local reconstruction to generate M-based active sites, thus spontaneously forming a Cu / M heterointerface with spatial integration characteristics. This precise reconstruction path based on the precursor structure can orderly anchor copper sites and second metal sites within a nanoscale adjacent space, achieving efficient relay between nitrate activation and nitrite hydrogenation. More importantly, this method has good metal universality; when the metal M contained in the core MOF is selected from different elements such as Fe, Co, and Ni, tandem catalysts with different combinations such as Cu / Fe, Cu / Co, and Cu / Ni can be obtained, providing a basis for system optimization of NO3. - RR electrocatalytic performance provides a flexible material platform.

[0006] The primary objective of this invention is to provide a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of a MOF core-shell precursor. The Cu / M tandem catalyst is a core-shell precursor composed of a MOF containing metal M as the core and a Cu-containing MOF as the shell. Under electrochemical action, the Cu-containing MOF is converted to form a copper-based active domain, while the MOF containing metal M is in-situ reconstructed or retained to form an M-based active domain. Furthermore, the copper-based and M-based active domains are systematically integrated at the nanoscale to form a composite with a heterogeneous interface structure. The metal M is a transition metal capable of synergistically catalyzing nitrate reduction with copper.

[0007] Preferably, the metal M is at least one of Fe, Co, and Ni.

[0008] Preferably, the MOF containing metal M is MIL-101(Fe), ZIF-67, or Ni-BDC.

[0009] Preferably, the Cu-containing MOF is HKUST-1.

[0010] Preferably, the process of forming the heterogeneous interface structure includes: The core-shell precursor was placed in an alkaline electrolyte containing nitrates, and... 0.3 V to It was obtained by in-situ electrochemical reconstruction at a cathode potential of 0.8 V vs. RHE.

[0011] The second objective of this invention is to provide a method for preparing a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of a MOF core-shell precursor, comprising the following steps: Preparation of MOFs containing metal M; Using a MOF containing metal M as the core material, a Cu-containing MOF is grown on its surface to form a shell, thus obtaining a core-shell precursor; The core-shell precursor is loaded onto a conductive substrate and placed in an alkaline electrolyte containing nitrate. In-situ electrochemical reconstruction is carried out at the cathode potential, which transforms the Cu-containing MOF into a copper-based active domain and the M-containing MOF into an M-based active domain, thereby forming a tandem catalyst with a Cu / M heterostructure interface. The alkaline electrolyte containing nitrates is an aqueous solution containing 0.05~1 M KOH and 0.05~0.5 M KNO3.

[0012] Preferably, the MOF containing metal M comprises: When the metal M is Fe, MIL-101(Fe) is synthesized by a solvothermal method. When metal M is Co, ZIF-67 is synthesized; When the metal M is Ni, Ni-BDC is synthesized.

[0013] Preferably, the process of forming a core-shell precursor includes: First, the surface of the MOF containing metal M is modified with polyvinylpyrrolidone, and then it is reacted with copper salt and pyromellitic acid in an alcohol solvent in sequence, so that the Cu-containing MOF grows in situ on its surface as a shell.

[0014] Preferably, the cathode reduction potential relative to the reversible hydrogen electrode is 0.3 V to 0.8 V; the in-situ electrochemical reconstruction time is 10 min to 2 h; the conductive substrate is carbon paper.

[0015] The third objective of this invention is to provide an application of a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursors in the electrocatalytic reduction of nitrates to ammonia.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention provides a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor, its preparation method, and its application, which have the following advantages: This invention utilizes a pre-designed core-shell MOF precursor and in-situ electrochemical reconstruction to precisely construct a Cu / M heterointerface, achieving nanoscale spatial integration of nitrate activation sites and nitrite hydrogenation sites, thus significantly improving tandem catalytic efficiency. Furthermore, the preparation method of this invention exhibits good metal universality; by simply changing the core MOF, various tandem catalyst combinations such as Cu / Fe, Cu / Co, and Cu / Ni can be obtained, providing a flexible material platform for the optimization and expansion of catalyst systems. In addition, the catalyst prepared by this invention demonstrates extremely high ammonia yield and Faradaic efficiency in the electrocatalytic nitrate reduction reaction, while also possessing excellent operational stability, making it of significant practical application value. Attached Figure Description

[0017] Figure 1 The image shows the XRD pattern of the MIL-101(Fe) brown solid powder in Example 1. Figure 2 Here is a SEM image of MIL-101(Fe) from Example 1; Figure 3 The image shows the XRD pattern of the MIL-101(Fe)@HKUST-1 solid powder in Example 1. Figure 4 This is a TEM image of MIL-101(Fe)@HKUST-1 in Example 1; Figure 5 The image shows the XRD pattern of R-MIL-101(Fe)@Cu in Example 1. Figure 6 SEM images (a) and HRTEM images (b) of R-MIL-101(Fe)@Cu in Example 1 are shown. Figure 7 XPS images of R-MIL-101(Fe)@Cu before and after reconstruction in Example 1; Figure 8 by 15 NO3 - and 14 NO3 - When each is used as a source of N, 15 NH3 and 14 NH3 1 H NMR spectrum; Figure 9 The results of ammonia yield tests for R-MIL-101(Fe)@Cu, R-Cu, and R-MIL-101(Fe) in Example 6 are shown. Figure 10 The results of the ammonia faradaic efficiency test for R-MIL-101(Fe)@Cu, R-Cu, and R-MIL-101(Fe) in Example 6 are shown. Figure 11 For R-Cu(a), R-MIL-101(Fe)(b) and R-MIL-101(Fe)@Cu(c) in Example 6, the NH3 and NO2 are... Faraday efficiency plots for products such as H2. Detailed Implementation

[0018] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention.

[0019] This invention addresses the problems of existing electrocatalytic nitrate reduction catalysts, such as the difficulty in efficiently coordinating nitrate activation and nitrite hydrogenation, the easy accumulation of intermediate products, and severe hydrogen evolution side reactions. It provides a general preparation method for a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of a MOF core-shell precursor. The aim is to construct a spatially ordered Cu / M heterointerface to achieve efficient tandem catalytic conversion of nitrate to ammonia. The metal M is a transition metal that can co-catalyze nitrate reduction with copper, preferably Fe, Co, or Ni.

[0020] To achieve the above objectives, the first aspect of the present invention provides a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of a MOF core-shell precursor. The Cu / M tandem catalyst is a core-shell precursor composed of a MOF containing metal M as the core and a Cu-containing MOF as the shell. Under electrochemical action, the Cu-containing MOF is converted to form a copper-based active domain, and the MOF containing metal M is in-situ reconstructed or retained to form an M-based active domain. The copper-based and M-based active domains are then orderly integrated at the nanoscale to form a composite with a heterogeneous interface structure. The metal M is a transition metal capable of synergistically catalyzing nitrate reduction with copper. In the core-shell precursor, the molar ratio of Cu in the Cu-containing MOF to M in the MOF containing metal M is 1:5 to 2:1.

[0021] For example, the catalyst is a heterogeneous interface structure formed by the orderly integration of copper-based active domains and metal M-based active domains at the nanoscale; wherein, the copper-based active domains originate from the electrochemical in-situ conversion of copper-containing MOFs, and the metal M-based active domains originate from the electrochemical in-situ reconstruction or retention of M-containing MOF cores; the copper-based active domains mainly function to catalyze the activation of nitrate to generate nitrite, and the M-based active domains mainly function to promote water activation and the hydrogenation of nitrite to generate ammonia; the two active domains achieve relay conversion of intermediate products through tight interfacial coupling.

[0022] Wherein, the metal M is at least one of Fe, Co, and Ni.

[0023] The MOF containing metal M is MIL-101(Fe), ZIF-67, or Ni-BDC.

[0024] The Cu-containing MOF is HKUST-1.

[0025] The process of forming the heterogeneous interface structure includes: The core-shell precursor was placed in an alkaline electrolyte containing nitrates, and... 0.3 V to It was obtained by in-situ electrochemical reconstruction at a cathode potential of 0.8 V vs. RHE.

[0026] For example, when M is Fe, the catalyst is a Cu / Fe heterostructure, preferably obtained by in-situ electrochemical reconstruction of a core-shell precursor (MIL-101(Fe)@HKUST-1); when M is Co, the catalyst is a Cu / Co heterostructure, preferably obtained by in-situ electrochemical reconstruction of a core-shell precursor (ZIF-67@HKUST-1); when M is Ni, the catalyst is a Cu / Ni heterostructure, preferably obtained by in-situ electrochemical reconstruction of a core-shell precursor (Ni-BDC@HKUST-1).

[0027] The size of the heterogeneous interface structure is in the nanometer to micrometer range, preferably 10 nm to 5 μm.

[0028] This invention pre-designs and synthesizes a core-shell precursor with a metal-containing MOF as the core and a copper-containing MOF as the shell. Under electrochemical conditions of nitrate reduction, the shell layer is selectively and in-situ converted into Cu-based active species, while the core MOF largely retains its structural characteristics or undergoes local reconstruction to generate M-based active sites, thus spontaneously forming a Cu / M heterointerface with spatial integration characteristics. This precise reconstruction pathway from the precursor structure can orderly anchor copper sites and second metal sites in a nanoscale adjacent space, achieving efficient relay between nitrate activation and nitrite hydrogenation. More importantly, this method has good metal universality. When the metal M contained in the core MOF is selected from different elements such as Fe, Co, and Ni, tandem catalysts with different combinations such as Cu / Fe, Cu / Co, and Cu / Ni can be obtained, providing a basis for system optimization of NO3. - RR electrocatalytic performance provides a flexible material platform.

[0029] A second aspect of this invention provides a method for preparing a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of a MOF core-shell precursor, comprising the following steps: Preparation of MOFs containing metal M; Using a MOF containing metal M as the core material, a Cu-containing MOF is grown on its surface to form a shell, thus obtaining a core-shell precursor; The core-shell precursor is loaded onto a conductive substrate and placed in an alkaline electrolyte containing nitrate. In-situ electrochemical reconstruction is carried out at the cathode potential, which transforms the Cu-containing MOF into a copper-based active domain and the M-containing MOF into an M-based active domain, thereby forming a tandem catalyst with a Cu / M heterostructure interface. The alkaline electrolyte containing nitrates is an aqueous solution containing 0.05~1 M KOH and 0.05~0.5 M KNO3.

[0030] The MOF containing metal M includes: When the metal M is Fe, MIL-101(Fe) is synthesized by a solvothermal method. When metal M is Co, ZIF-67 is synthesized; When the metal M is Ni, Ni-BDC is synthesized.

[0031] The process of forming a core-shell precursor includes: First, the surface of the MOF containing metal M is modified with polyvinylpyrrolidone, and then it is reacted with copper salt and pyromellitic acid in an alcohol solvent in sequence, so that the Cu-containing MOF grows in situ on its surface as a shell.

[0032] The cathode reduction potential relative to the reversible hydrogen electrode is: 0.3 V to 0.8 V, preferably 0.5 V 0.6 V or 0.7 V; the in-situ electrochemical reconstruction time is 10 min to 2 h; the conductive substrate is carbon paper.

[0033] This invention pre-synthesizes a core-shell precursor with a MOF containing transition metal M as the core and HKUST-1 as the shell. In an alkaline medium containing nitrate, in-situ electrochemical reconstruction is performed at the cathodic reduction potential. This selectively transforms the shell into Cu-based active domains, while the core is retained or locally reconstructed to generate M-based active domains, thus spontaneously forming a spatially ordered Cu / M heterointerface at the nanoscale. This heterointerface achieves spatial division and efficient relay of nitrate activation and nitrite hydrogenation functions, fundamentally suppressing nitrite accumulation and hydrogen evolution side reactions. Taking the Cu / Fe system as an example, the prepared R-MIL-101(Fe)@Cu catalyst... 0.61 mmol·h was achieved at 0.5 V vs. RHE potential. -1 ·cm -2The ammonia yield was high and the Faraday efficiency was 94.93%, exhibiting excellent operational stability. By changing the core MOF, this strategy can be extended to various Cu / M tandem catalyst systems such as Cu / Co and Cu / Ni, demonstrating good metal universality and providing a new approach for high-performance catalyst design for electrocatalytic nitrate reduction to ammonia.

[0034] An exemplary method for preparing a Cu / M tandem catalyst includes the following steps: (1) Preparation of MOF core materials containing M: When the metal M is Fe, MIL-101(Fe) or its amino-functionalized derivative is synthesized by a solvothermal method. The typical process is as follows: the iron salt is dissolved in N,N-dimethylformamide, heated to 150-170°C, and then an organic solution containing 2-aminoterephthalic acid is injected. The reaction continues, followed by cooling, centrifugation, washing, and drying to obtain MIL-101(Fe) powder. The iron salt is any one or a mixture of several of ferric nitrate, ferric sulfate, and ferric acetate.

[0035] When metal M is Co, ZIF-67 is synthesized as a core material. The typical process is as follows: cobalt salt is reacted with 2-methylimidazole in an alcohol solvent, and ZIF-67 is generated under stirring at room temperature or hydrothermal conditions. The product is obtained by separation, washing and drying. The cobalt salt is any one or a mixture of cobalt nitrate, cobalt acetate and cobalt sulfate. The hydrothermal temperature is 100-150℃.

[0036] When metal M is Ni, Ni-BDC is synthesized as a core material. The typical process is as follows: Ni salt is reacted with terephthalic acid under solvothermal conditions to obtain Ni-BDC. The nickel salt can be any one or a mixture of nickel nitrate, nickel sulfate, and nickel acetate. The solvothermal temperature is 120-180℃, and the solvent is one or more of water and N,N-dimethylformamide.

[0037] (2) Preparation of core-shell precursor (M-MOF@HKUST-1): Using the MOF core material containing M obtained in step (1) as a seed crystal, a copper-containing MOF (HKUST-1) is grown on its surface to form a shell layer, thus obtaining a core-shell structure precursor. The specific operation is as follows: First, the MOF containing M was dispersed in an alcoholic solution of polyvinylpyrrolidone (PVP), and the mixture was stirred for 10 h to modify the core surface by adsorbing PVP. The mixture was then collected by centrifugation. The M-containing MOF modified by PVP was redispersed in an alcohol solvent, copper salt was added, and the mixture was stirred to allow the copper ions to fully interact with the core surface. Subsequently, an alcohol solution of trimesic acid (H3BTC) was added, and the reaction was continued to be stirred at room temperature to allow HKUST-1 to grow in situ on the core surface. After the reaction was completed, the product was collected by centrifugation, washed, and dried to obtain the core-shell precursor, which was designated as M-MOF@HKUST-1 (such as MIL-101(Fe)@HKUST-1, ZIF-67@HKUST-1, Ni-BDC@HKUST-1, etc.).

[0038] (3) Preparation of core-shell precursor M-MOF@HKUST-1: The core-shell precursor obtained in step (2) is loaded onto a conductive substrate (such as carbon paper) as the working electrode and placed in an electrolyte containing nitrates for in-situ electrochemical reconstruction at the cathode reduction potential. The electrolyte is preferably an alkaline aqueous solution containing nitrates, such as a mixed solution of 0.05–1 M KOH and 0.05–0.5 M KNO3. The cathode reduction potential relative to the reversible hydrogen electrode (vs. RHE) is... 0.3 V to Within the 0.8 V range, preferably 0.5 V 0.6 V or At 0.7 V, etc., under potential-driven conditions, the HKUST-1 shell is selectively and in-situ transformed into Cu-containing active species; while the M-MOF core layer remains relatively stable under the same conditions, basically retaining its structural characteristics or undergoing local reconstruction to generate M-based active sites. Thus, a Cu / M heterointerface with spatial proximity is spontaneously formed under in-situ electrochemical conditions, yielding a reconstructed tandem catalyst, which can be denoted as RM-MOF@Cu.

[0039] In step (3) above, in-situ electrochemical reconstruction can be carried out separately as a catalyst pretreatment step, or it can occur directly in situ during nitrate reduction performance testing or operation.

[0040] The third aspect of this invention provides the application of a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of a MOF core-shell precursor in the electrocatalytic reduction of nitrate to ammonia.

[0041] This invention provides the application of the Cu / M tandem catalyst in the electrocatalytic reduction of nitrate to ammonia, comprising loading the catalyst onto an electrode, electrolyzing in an alkaline electrolyte containing nitrate at a suitable cathode potential, and converting nitrate to ammonia with high selectivity and high Faraday efficiency.

[0042] It should be noted that, unless otherwise specified, the experimental methods used in this invention are all conventional methods; and the reagents and materials used, unless otherwise specified, are all commercially available.

[0043] Example 1 The preparation of Cu / Fe tandem catalysts includes: This embodiment provides a method for preparing a tandem catalyst with a Cu / Fe heterostructure (denoted as R-MIL-101(Fe)@Cu), the specific steps of which are as follows: Step S1: A modified solvothermal method was used. 0.29 g of ferric chloride hexahydrate (FeCl3·6H2O) was dissolved in 40 mL of N,N-dimethylformamide (DMF) in a 100 mL round-bottom flask. The flask was sealed and heated to 160 °C with stirring for 15 min. Separately, 0.20 g of 2-aminoterephthalic acid was dissolved in 10 mL of DMF and rapidly injected into the above solution. The reaction was continued at 160 °C with stirring for 30 min. After naturally cooling to room temperature, the brown precipitate was collected by centrifugation, washed three times with ethanol, and dried under vacuum at 60 °C for 12 h to obtain MIL-101(Fe) brown powder.

[0044] Step S2: Using MIL-101(Fe) as seed crystals. 0.05 g of the prepared MIL-101(Fe) was dispersed in 10 mL of a 5 wt% polyvinylpyrrolidone (PVP) methanol solution and stirred at room temperature for 12 h for surface modification. The PVP-functionalized MIL-101(Fe) was collected by centrifugation, redispersed in 12.5 mL of methanol, and 0.492 g of copper nitrate trihydrate (Cu(NO3)2·3H2O) was added. The mixture was stirred at room temperature for 1 h to allow the Cu… 2+ The reaction proceeded fully with the core surface. Subsequently, 0.225 g of trimesic acid (H3BTC) was dissolved in 12.5 mL of methanol and rapidly injected into the above mixture. The reaction was continued with stirring at room temperature for 4 h. The product was collected by centrifugation, washed three times with methanol, and dried under vacuum at 60 °C for 12 h to obtain the core-shell precursor MIL-101(Fe)@HKUST-1.

[0045] Step S3: Weigh 5 mg of the above precursor and disperse it in an ethanol-water mixture containing a small amount of Nafion. Sonicate the mixture to form a homogeneous slurry. The ethanol-water mixture containing a small amount of Nafion is obtained by uniformly mixing 500 μL of anhydrous ethanol, 420 μL of deionized water, and 80 μL of Nafion. Take 80 μL of the slurry and drop it onto a 1 × 1 cm... 2The electrode was air-dried on carbon paper to serve as the working electrode. A platinum sheet was used as the counter electrode, and an Ag / AgCl electrode was used as the reference electrode. The electrolyte was an aqueous solution containing 0.1 M KNO3 and 1 M KOH. The electrode was treated at a constant potential of -0.5 V relative to the reversible hydrogen electrode (RHE) for 1 h (nitrate reduction reaction can also be carried out directly at this potential, with the reconstruction process completed in situ at the initial stage of the reaction). The HKUST-1 shell was selectively converted into Cu-containing active species such as metallic Cu and a small amount of Cu2O, while the MIL-101(Fe) core basically retained its structure. Thus, a reconstruction tandem catalyst with a Cu / Fe heterostructure interface was formed in situ on the electrode, denoted as R-MIL-101(Fe)@Cu.

[0046] Example 2 The preparation of Cu / Co tandem catalysts includes: This embodiment provides a method for preparing a tandem catalyst (denoted as R-ZIF-67@Cu) with a Cu / Co heterostructure interface.

[0047] Step S1: Dissolve 1.45 g of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) in 50 mL of methanol, and dissolve 1.64 g of 2-methylimidazole in another 50 mL of methanol. While stirring, slowly pour the former into the latter, stir continuously at room temperature for 24 h, centrifuge to collect the purple precipitate, wash three times with methanol, and dry under vacuum at 60 °C overnight to obtain ZIF-67 nanocrystals.

[0048] Step S2: Take 0.05 g of ZIF-67 and, following the same PVP modification and subsequent copper salt and trimesic acid reaction conditions as in step (2) of Example 1, obtain the core-shell precursor ZIF-67@HKUST-1.

[0049] Step S3: ZIF-67@HKUST-1 was loaded onto a carbon paper electrode and in the same electrolyte as in Example 1, in-situ electrochemical reconstruction was performed at a cathode potential of -0.6 V vs. RHE. The HKUST-1 shell was transformed into a Cu-based active species, and the ZIF-67 core underwent local reconstruction to generate Co-based active sites, forming a tandem catalyst R-ZIF-67@Cu with a Cu / Co heterointerface.

[0050] Example 3 The preparation of Cu / Ni tandem catalysts includes: The examples provide a method for preparing a tandem catalyst (denoted as R-Ni-BDC@Cu) with a Cu / Ni heterostructure.

[0051] Step S1: Dissolve 0.58 g of nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and 0.33 g of terephthalic acid in 30 mL of DMF, transfer the solution to a high-pressure reactor lined with polytetrafluoroethylene, and react at 120 °C for 12 h. After natural cooling, collect the pale green product by centrifugation, wash alternately with ethanol and water, and dry under vacuum at 60 °C to obtain Ni-BDC.

[0052] Step S2: Using 0.05 g Ni-BDC as the core, PVP modification and HKUST-1 shell growth were carried out in the same way as step (2) of Example 1 to obtain Ni-BDC@HKUST-1.

[0053] Step S3: Ni-BDC@HKUST-1 was loaded onto a carbon paper electrode and in nitrate-containing alkaline electrolyte was subjected to in-situ electrochemical reconstruction at a potential of -0.6 V vs. RHE to obtain a tandem catalyst R-Ni-BDC@Cu with a Cu / Ni heterointerface.

[0054] To illustrate the relevant performance of the Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor provided by this invention, the accompanying drawings are provided.

[0055] Example 4 Catalyst structure characterization: The R-MIL-101(Fe)@Cu catalyst and its precursor prepared in Example 1 were characterized, and the results are as follows: Figure 1 The image shows the XRD pattern of the brown solid powder MIL-101(Fe) in Example 1, which exhibits the characteristic diffraction peaks of MIL-101(Fe).

[0056] Figure 2 The image shows a SEM image of MIL-101(Fe) in Example 1, which exhibits a regular octahedral structure.

[0057] Figure 3 The image shows the XRD pattern of the MIL-101(Fe)@HKUST-1 solid powder in Example 1. The MIL-101(Fe)@HKUST-1 precursor exhibits characteristic diffraction peaks of both MIL-101(Fe) and HKUST-1.

[0058] Figure 4 The image shown is a TEM image of MIL-101(Fe)@HKUST-1 in Example 1, which also exhibits a regular octahedral structure.

[0059] Figure 5The XRD pattern of R-MIL-101(Fe)@Cu in Example 1 shows that after in-situ electrochemical reconstruction, the characteristic peaks of HKUST-1 basically disappeared, and diffraction peaks corresponding to the (111) and (200) crystal planes of metallic Cu appeared at 2θ of 43.3° and 50.4°, respectively, confirming that Cu 2+ It was reduced to metallic Cu. Although the diffraction peaks of MIL-101(Fe) were weakened, the basic framework was preserved, indicating that the Fe-MOF core has good structural stability under reconstruction conditions.

[0060] Figure 6 The images (a) and (b) show the SEM and HRTEM images of R-MIL-101(Fe)@Cu in Example 1. The overall structure appears as a small, encapsulated ellipsoid, which is due to the reconstruction of HKUST-1 during the reaction. Furthermore, the reconstructed structure reveals a clear heterogeneous interface between MIL-101(Fe) and Cu, with a size of approximately 30-50 nm.

[0061] Figure 7 The images shown are XPS images of R-MIL-101(Fe)@Cu before and after reconstruction in Example 1. Figure 7 (a) is the XPS spectrum of the Fe 2p orbital. Figure 7 (b) is the XPS spectrum of the Cu 2p orbital. The Cu 2p spectrum shows the presence of Cu. 0 The characteristic peaks confirm that the shell has transformed into an active species dominated by metallic Cu. The Fe 2p spectrum mainly shows Fe... 3+ The characteristic signals indicate that the Fe-MOF core still exists as ferrite clusters or coordination structures after reconstruction.

[0062] Example 5 The catalyst electrode prepared in Example 1 was used for the electrocatalytic reduction of nitrate, including: The test used a standard H-type electrolytic cell, with the anode and cathode chambers separated by a Nafion membrane. 40 mL of 1 M KOH electrolyte containing 0.1 MKNO3 was added to the cathode chamber, and 40 mL of 1 M KOH solution was added to the anode chamber. The working electrode was carbon paper (1 × 1 cm²) loaded with 0.4 mg of catalyst. 2 The counter electrode was a platinum sheet, and the reference electrode was an Ag / AgCl (saturated KCl) electrode. High-purity argon gas was purged for 10 minutes before the test to remove dissolved oxygen and nitrogen.

[0063] Linear sweep voltammetry (LSV) at 10 mV·s -1The electrolysis was performed at a controlled sweep rate. Constant potential electrolysis was run at the selected potential (-0.2 V to -0.6 V vs. RHE) with continuous stirring (approximately 450 rpm) for 1 h. After electrolysis, the concentrations of nitrite and ammonia in the catholyte were determined by UV-Vis spectrophotometry, and the ammonia yield and Faraday efficiency (FE) were calculated. See [link to relevant documentation] Figure 8 As shown, with 15 N-labeled potassium nitrate (K 15 Isotope tracing experiments were conducted using NO3 as the raw material, through... 1 H nuclear magnetic resonance spectroscopy was used to verify the nitrogen source of ammonia.

[0064] Example 6 This embodiment details the performance results of the catalyst R-MIL-101(Fe)@Cu from Example 1 based on Example 5. To further illustrate the performance of the catalyst R-MIL-101(Fe)@Cu, we prepared MIL-101(Fe) and HKUST-1 precursors using steps S1 and S2 of Example 1, respectively. Note that MIL-101(Fe) was not added as a seed crystal and the PVP pretreatment step was not used when preparing the HKUST-1 precursor. Subsequently, the comparative samples R-Cu and R-MIL-101(Fe) were obtained by processing them using step S3 of Example 1. See [link to example]. Figure 9 The results of ammonia yield tests for R-MIL-101(Fe)@Cu, R-Cu, and R-MIL-101(Fe) in Example 6 are shown. Figure 10 The results of the ammonia faradaic efficiency test for R-MIL-101(Fe)@Cu, R-Cu, and R-MIL-101(Fe) in Example 6 are shown. Figure 11 For example, NH3 and NO2 of R-MIL-101(Fe) in Example 6 Faraday efficiency plot of H2; from Figures 8-11 It can be seen that, in At a potential of 0.5 V vs. RHE, the catalyst achieved an ammonia yield of 0.61 mmol·h⁻¹. -1 ·cm -2 The corresponding Faraday efficiency is as high as 94.93%. At a potential of 0.6 V vs. RHE, the catalyst achieved an ammonia yield of 0.87 mmol·h⁻¹. -1 ·cm -2 The corresponding Faraday efficiency is as high as 96.2%, and nitrite accumulation is almost negligible. Isotope experiments only detected [a specific type of nitrite]. 15 NH4 + The corresponding bimodal signal was not observed. 14 NH4 +The signal confirmed that the ammonia product originated entirely from the electrocatalytic reduction of nitrate. The catalyst also exhibited excellent stability. In 10 consecutive cyclic electrocatalytic experiments at -0.5 V vs. RHE, neither the ammonia yield nor the Faradaic efficiency showed significant decline, and stable current and product selectivity were maintained.

[0065] In contrast, the HKUST-1-derived R-Cu catalyst alone showed significant nitrite accumulation at low potentials, indicating that while pure Cu sites can activate nitrate, subsequent hydrogenation efficiency is insufficient. The MIL-101(Fe)-derived R-MIL-101(Fe) catalyst alone exhibited low nitrate activation but high hydrogen activation, leading to hydrogen evolution reaction. These comparisons clearly demonstrate that the Cu / Fe heterointerface constructed through in-situ electrochemical reconstruction of the core-shell precursor in this invention achieves a highly efficient spatial relay between nitrate activation (Cu domain-dominated) and nitrite hydrogenation (Fe domain-dominated), thereby significantly improving ammonia synthesis efficiency and selectivity while effectively suppressing nitrite accumulation and hydrogen evolution side reactions.

[0066] In summary, this invention proposes a general strategy for the in-situ electrochemical reconstruction of MOF core-shell precursors to directionally construct Cu / M heterointerface tandem catalysts. This method pre-designs and synthesizes a core-shell precursor with an M-containing MOF as the core and HKUST-1 as the shell. Utilizing the selective transformation of the shell layer and the relative retention of the core layer under cathodic reduction conditions, a heterointerface with tightly coupled Cu and M domains spontaneously forms at the nanoscale. This interface achieves spatially ordered division of labor and efficient relay of nitrate activation and nitrite hydrogenation functions, fundamentally suppressing intermediate product accumulation and hydrogen evolution side reactions. This invention provides a new design paradigm for the precise construction and functional coordination of heterointerfaces in complex multi-step electrocatalytic conversions, possessing significant scientific value and promising practical applications.

[0067] This invention describes preferred embodiments and their effects. However, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments.

[0068] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor, characterized in that, The Cu / M tandem catalyst is a core-shell precursor composed of a MOF containing metal M as the core and a Cu-containing MOF as the shell. Under electrochemical action, the Cu-containing MOF is converted into a copper-based active domain, and the MOF containing metal M is reconstructed or retained in situ to form an M-based active domain. The copper-based active domain and the M-based active domain are then orderly integrated at the nanoscale to form a composite with a heterogeneous interface structure. The metal M is a transition metal that can synergistically catalyze the reduction of nitrates with copper.

2. The Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor according to claim 1, characterized in that, The metal M is at least one of Fe, Co, and Ni.

3. The Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor according to claim 2, characterized in that, The MOF containing metal M is MIL-101(Fe), ZIF-67, or Ni-BDC.

4. The Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor according to claim 1, characterized in that, The Cu-containing MOF is HKUST-1.

5. The Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor according to claim 1, characterized in that, The process of forming the heterogeneous interface structure includes: The core-shell precursor was placed in an alkaline electrolyte containing nitrates, and... 0.3 V to It was obtained by in-situ electrochemical reconstruction at a cathode potential of 0.8 V vs. RHE.

6. A method for preparing a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of a MOF core-shell precursor as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Preparation of MOFs containing metal M; Using a MOF containing metal M as the core material, a Cu-containing MOF is grown on its surface to form a shell, thus obtaining a core-shell precursor; The core-shell precursor is loaded onto a conductive substrate and placed in an alkaline electrolyte containing nitrate. In-situ electrochemical reconstruction is carried out at the cathode potential, which transforms the Cu-containing MOF into a copper-based active domain and the M-containing MOF into an M-based active domain, thereby forming a tandem catalyst with a Cu / M heterostructure interface. The alkaline electrolyte containing nitrates is an aqueous solution containing 0.05~1 M KOH and 0.05~0.5 M KNO3.

7. The method for preparing Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor according to claim 6, characterized in that, The MOF containing metal M includes: When the metal M is Fe, MIL-101(Fe) is synthesized by a solvothermal method. When metal M is Co, ZIF-67 is synthesized; When the metal M is Ni, Ni-BDC is synthesized.

8. The method for preparing Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor according to claim 6, characterized in that, The process of forming a core-shell precursor includes: First, the surface of the MOF containing metal M is modified with polyvinylpyrrolidone, and then it is reacted with copper salt and pyromellitic acid in an alcohol solvent in sequence, so that the Cu-containing MOF grows in situ on its surface as a shell.

9. The method for preparing Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor according to claim 6, characterized in that, The cathode reduction potential relative to the reversible hydrogen electrode is: 0.3 V to 0.8 V; the in-situ electrochemical reconstruction time is 10 min to 2 h; the conductive substrate is carbon paper.

10. The application of a Cu / M tandem catalyst based on in-situ electrochemical reconstruction of MOF core-shell precursor as described in any one of claims 1 to 5 in the electrocatalytic reduction of nitrate to ammonia.

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