A Bi-HfO2@C catalyst, its preparation method and application

Abstract

This invention belongs to the field of CO2 catalysis technology, specifically relating to a Bi-HfO2@C catalyst and its preparation method. Specifically, it involves: using hafnium tetrachloride as the hafnium source and 2,6-naphthalenedicarboxylic acid as the ligand, a hydrothermal reaction is carried out in an organic solvent to obtain an Hf-MOF precursor; using bismuth trichloride as the bismuth source, it is mixed with the Hf-MOF precursor and water, and then stirred to react. The reaction is further refined by controlling the Bi... 3+ Hydrolysis was performed to grow Bi₂O₃ on the surface of the Hf-MOF precursor. Following this, annealing was carried out under a reducing protective atmosphere to reconstruct the carbon framework at high temperature. Simultaneously, trivalent Bi was reduced to zero-valent metallic Bi, and Bi-HfO₂ nanoparticles were recrystallized on the carbon framework to obtain the Bi-HfO₂@C catalyst. When this catalyst was used for the electroreduction of CO₂ to prepare formic acid, it achieved both high current density and high selectivity during electrolysis.

Description

Technical Field

[0001] This invention belongs to the field of CO2 catalysis technology, specifically relating to a Bi-HfO2@C catalyst, its preparation method, and its application. Background Technology

[0002] The accelerated pace of global industrialization has led to a continuous increase in carbon emissions, significantly impacting the global climate system. A 2023 report by the World Meteorological Organization showed that atmospheric carbon dioxide (CO2) concentration had reached 420.0 ppm, an increase of approximately 49% compared to pre-industrial levels. This increase has directly contributed to a 1.1°C rise in global average temperature over the past decade compared to pre-industrial levels. Faced with the increasingly severe global carbon emission situation, effective measures to address climate change are urgently needed. In response to this global challenge, CO2 resource utilization technologies, particularly research on its efficient conversion into fuels and high-value-added chemicals, have become a key pathway to solving the dual dilemmas of energy and the environment. Among these, electrocatalytic CO2 reduction, as an emerging technology, offers a promising solution for mitigating global warming and producing high-value chemicals. CO2 electroreduction technology driven by renewable energy sources has become one of the effective ways to convert CO2 due to its advantages such as simple equipment, mild reaction conditions, and diverse products. By selecting different catalysts, CO2 can be selectively converted into high-value-added chemicals such as formic acid, methanol, ethylene, and ethane. Formic acid (HCOOH) is an important chemical raw material widely used in industries such as textiles, chemicals, agriculture, pharmaceuticals, leather, and rubber. It is also an ideal hydrogen storage carrier, suitable for the development of sustainable energy systems.

[0003] Currently, common industrial electrochemical CO2 reduction technologies involve using commercially available Bi nanoparticles to prepare a catalyst slurry, which is then sprayed onto a gas diffusion layer of carbon paper to fabricate an electrode. This electrode serves as the cathode, while a commercially available iridium-plated titanium felt electrode is used as the anode. A commercial anion exchange membrane separates the cathode and anode, and an electrolytic cell is assembled for electrolysis testing. However, because the main component of common commercial Bi nanoparticles is metallic Bi, their ability to dissociate water is very limited. Therefore, at high current densities, the water dissociation step often becomes the limiting step, resulting in low selectivity for the production of formic acid from CO2 reduction.

[0004] Therefore, it is crucial to develop catalysts that can still possess strong water dissociation capabilities at high current densities, thereby exhibiting high selectivity and high current density. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a Bi-HfO2@C catalyst and its preparation method. When this catalyst is used for the electroreduction of CO2 to prepare formic acid, it can electrolyze with high selectivity at current densities higher than those used in industrial applications, achieving a balance between high current density and high selectivity during the electrolysis process.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a Bi-HfO2@C catalyst includes the following steps:

[0008] Using hafnium tetrachloride as the hafnium source and 2,6-naphthalenedicarboxylic acid as the ligand, the Hf-MOF precursor was obtained through a hydrothermal reaction in an organic solvent.

[0009] Using bismuth trichloride as the bismuth source, it was mixed with Hf-MOF precursor and water, and then stirred to react. The reaction was controlled by adjusting the Bi content. 3+ The hydrolysis process was used to grow Bi2O3 on the surface of the Hf-MOF precursor to obtain Bi2O3-supported Hf-MOF precursor.

[0010] Under a reducing protective atmosphere, the Bi2O3-supported Hf-MOF precursor was annealed to reconstruct the carbon skeleton at high temperature, while reducing trivalent Bi to zero-valent metallic Bi. Bi-HfO2 nanoparticles were then recrystallized on the carbon skeleton to obtain the Bi-HfO2@C catalyst.

[0011] This invention utilizes a hydrothermal method, employing the metal salt HfCl4 and the organic ligand 2,6-naphthalenedicarboxylic acid, to synthesize Hf-MOF precursor powder via hydrothermal synthesis. Then, by controlling the Bi... 3+ The hydrolysis process involves growing Bi2O3 on the surface of Hf-MOF precursor powder, followed by high-temperature annealing in a reducing atmosphere to reduce Bi(Ⅲ) to Bi(0) in the metallic valence state, while simultaneously regulating the microstructure to form a hollow and porous Bi-HfO2 nanoparticle-supported carbon framework structure.

[0012] Furthermore, the mass ratio of 2,6-naphthalenedicarboxylic acid to hafnium tetrachloride is 0.588:0.588~0.8436.

[0013] Furthermore, in the preparation process of the Hf-MOF precursor, the hydrothermal reaction temperature is 110℃~120℃ and the time is 9h~10h.

[0014] Furthermore, the mass ratio of the Hf-MOF precursor to bismuth trichloride is 1~3:1.

[0015] Furthermore, during the preparation of the Bi2O3-supported Hf-MOF precursor, the stirring reaction temperature is 35℃~40℃, and the stirring reaction time is 12h~14h.

[0016] Furthermore, the annealing treatment is performed at a temperature of 650℃ to 850℃ for a time of 1.5h to 2h.

[0017] Furthermore, the reducing protective gas is a mixture of hydrogen and argon, with a volume ratio of hydrogen to argon of 5:95.

[0018] The Bi-HfO2@C catalyst was prepared according to the above method.

[0019] The application of the above-mentioned Bi-HfO2@C catalyst in the electroreduction of CO2 to prepare formic acid includes the following steps:

[0020] The cathode is prepared by uniformly mixing Bi-HfO2@C catalyst, carbon black, polytetrafluoroethylene, perfluorosulfonic acid resin, anhydrous ethanol and water, and then spraying the mixture onto carbon paper in the gas diffusion layer.

[0021] An electrolytic cell is assembled by using an iridium-plated titanium felt electrode as the anode, and assembling the cathode, anode, and anion exchange membrane.

[0022] Formic acid is produced by passing humidified CO2 gas into the cathode and an alkaline solution into the anode through an electroreduction reaction.

[0023] Furthermore, the application of the Bi-HfO2@C catalyst in the electroreduction of CO2 to prepare formic acid involves the following steps: Weigh 2 mg of the prepared Bi-HfO2@C powder, then add 1 mg of commercial carbon black XR-72 (to increase conductivity), followed by 700 μL of anhydrous ethanol, 300 μL of deionized water, 20 μL of 5% commercial polytetrafluoroethylene (PTFE) solution, and 4 μL of commercial perfluorosulfonic acid resin (Nafion) adhesive. After ultrasonication for 1 hour, the mixture is thoroughly mixed and sprayed onto a 2×2 cm... 2 Electrodes were fabricated on carbon paper with a gas diffusion layer. Testing was conducted using a commercially available simulated electrolytic membrane electrode. The electrode obtained above was used as the cathode, and a commercially available iridium-plated titanium felt electrode was used as the anode. A commercial anion exchange membrane (Grade 60) was used to separate the cathode and anode. An electrolytic cell was assembled for electrolysis testing. During the test, pure CO2 humidified with deionized water was introduced into the cathode at a flow rate of 40 sccm, and 1M KOH was refluxed into the anode at a flow rate of 10 mL / min. Products from both the cathode and anode were collected and analyzed.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] This invention uses hafnium tetrachloride as the hafnium source and 2,6-naphthalenedicarboxylic acid as the ligand to obtain an Hf-MOF precursor in an organic solvent via a hydrothermal method. This precursor provides the basic framework for subsequent structural development. Then, bismuth trichloride is used as the bismuth source, mixed with the Hf-MOF precursor and water, and the reaction is stirred. The reaction is then controlled by adjusting the Bi... 3+The hydrolysis process involves growing Bi₂O₃ on the surface of an Hf-MOF precursor, ensuring uniform and stable loading of Bi on the precursor surface. Subsequently, the Bi₂O₃-loaded Hf-MOF precursor is annealed under a reducing protective atmosphere, achieving carbon framework reconstruction through high temperature. Simultaneously, trivalent Bi is reduced to zero-valent metallic Bi, and Bi-HfO₂ nanoparticles are recrystallized on the carbon framework to obtain the Bi-HfO₂@C catalyst. The HfO₂ introduced into the Bi-HfO₂@C catalyst provided by this invention significantly improves the catalyst's water dissociation ability, providing a sufficient proton source for the reduction of CO₂ to formic acid. This allows for high selectivity even at high current densities, achieving a balance between high current density and high selectivity in the electrolysis of CO₂ to formic acid. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 The image shows a SEM image of the Bi-HfO2@C catalyst prepared in Example 1.

[0028] Figure 2 The image shows a TEM image of the Bi-HfO2@C catalyst prepared in Example 1.

[0029] Figure 3 The images show SEM images of the Hf-MOF precursor and the Bi2O3-supported Hf-MOF precursor prepared in Example 1, where a is the SEM image of the Hf-MOF precursor and b is the SEM image of the Bi2O3-supported Hf-MOF precursor.

[0030] Figure 4 XPS image of the Bi-HfO2@C catalyst prepared in Example 1.

[0031] Figure 5 The values ​​represent the Faradaic efficiencies of CO2 electrolysis for the production of formic acid under different currents. Among them, (a) represents the Faradaic efficiencies of the Bi-HfO2@C catalyst prepared in Example 1 for the electrolysis of CO2 for the production of formic acid under different currents, and (b) represents the Faradaic efficiencies of the Bi@C catalyst prepared in Comparative Example 1 for the electrolysis of CO2 for the production of formic acid under different currents.

[0032] Figure 6 The production rate of formic acid by CO2 electrolysis using the Bi-HfO2@C catalyst prepared in Example 1 at different currents.

[0033] Figure 7 The figure shows the test results of the Bi-HfO2@C catalyst prepared in Example 1 under continuous electrolysis at a current of 1A for 60 hours.

[0034] Figure 8 The Faradaic efficiencies of the Bi-HfO2@C catalysts prepared in Examples 1 to 3 and Comparative Examples 3 to 4 at different voltages are shown.

[0035] Figure 9 The KIE test results are shown for the Bi-HfO2@C catalyst prepared in Example 1 and the Bi@C catalyst prepared in Comparative Example 1.

[0036] Figure 10 The Faradaic efficiency of the Bi-HfO2@C catalyst prepared in Examples 1, 4 and 5 for CO2 electrolysis to produce formic acid is shown. Detailed Implementation

[0037] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods, and the materials and reagents used in the following embodiments are commercially available unless otherwise specified.

[0038] Example 1

[0039] A method for preparing a Bi-HfO2@C catalyst includes the following steps:

[0040] S1. Disperse 0.588 g of 2,6-naphthalenedicarboxylic acid and 0.8436 g of hafnium tetrachloride (HfCl4) in 26 mL of N,N-dimethylformamide. Stir on a magnetic stirrer for 30 min to ensure thorough mixing and obtain a homogeneous solution. Add 20 mL of 88% formic acid solution and 8 mL of deionized water and transfer to a hydrothermal reactor. After sealing the reactor, place it in an oven at 120 ℃ for hydrothermal reaction for 9 h. After the reaction is complete, centrifuge to remove the supernatant. Wash three times with a mixed solution of acetone and N,N-dimethylformamide, and then dry to form the first powder, denoted as Hf-MOF precursor.

[0041] S2. Weigh 560 mg of the first powder obtained in step S1 and 280 mg of bismuth trichloride (BiCl3) powder, dissolve them in 21 mL of deionized water to form a homogeneous solution, place it in a water bath at a constant temperature of 40 °C and stir for 12 h, then remove the supernatant and dry to obtain the second powder, which is denoted as Bi2O3 supported Hf-MOF precursor.

[0042] S3. The second powder obtained in step S2 is placed in a high-temperature tube furnace and heated to 750°C in a hydrogen-argon atmosphere and held for 2 hours for high-temperature annealing. The powder is then cooled to room temperature in the furnace to obtain a hollow porous Bi-HfO2@C catalyst, denoted as Bi-HfO2@C-2:1 or Bi-HfO2@C-750. The volume ratio of hydrogen to argon in the hydrogen-argon atmosphere is 5:95.

[0043] Example 2

[0044] A method for preparing a Bi-HfO2@C catalyst includes the following steps:

[0045] S1. Disperse 0.588 g of 2,6-naphthalenedicarboxylic acid and 0.8436 g of hafnium tetrachloride (HfCl4) in 26 mL of N,N-dimethylformamide. Stir on a magnetic stirrer for 30 min to mix thoroughly and obtain a homogeneous solution. Add 20 mL of 88% formic acid solution and 8 mL of deionized water and transfer to a hydrothermal reactor. After sealing the reactor, place it in an oven at 120 °C for hydrothermal reaction for 9 h. After the reaction is complete, centrifuge to remove the supernatant. Wash three times with a mixed solution of acetone and N,N-dimethylformamide, and then dry to form the first powder.

[0046] S2. Weigh 560 mg of the first powder obtained in step S1 and 560 mg of bismuth trichloride (BiCl3) powder, dissolve them in 21 mL of deionized water to form a homogeneous solution, place them in a water bath at a constant temperature of 40 °C and stir for 12 h, then remove the supernatant and dry to obtain the second powder.

[0047] S3. The second powder obtained in step S2 is placed in a high-temperature tube furnace and heated to 750°C in a hydrogen-argon atmosphere and held for 2 hours for high-temperature annealing. The powder is then cooled to room temperature in the furnace to obtain the Bi-HfO2@C-1:1 catalyst. The volume ratio of hydrogen to argon in the hydrogen-argon atmosphere is 5:95.

[0048] Example 3

[0049] A method for preparing a Bi-HfO2@C catalyst includes the following steps:

[0050] S1. Disperse 0.588 g of 2,6-naphthalenedicarboxylic acid and 0.8436 g of hafnium tetrachloride (HfCl4) in 26 mL of N,N-dimethylformamide. Stir on a magnetic stirrer for 30 min to mix thoroughly and obtain a homogeneous solution. Add 20 mL of 88% formic acid solution and 8 mL of deionized water and transfer to a hydrothermal reactor. After sealing the reactor, place it in an oven at 120 °C for hydrothermal reaction for 9 h. After the reaction is complete, centrifuge to remove the supernatant. Wash three times with a mixed solution of acetone and N,N-dimethylformamide, and then dry to form the first powder.

[0051] S2. Weigh 840 mg of the first powder obtained in step S1 and 280 mg of bismuth trichloride (BiCl3) powder, dissolve them in 31.5 mL of deionized water to form a homogeneous solution, place it in a water bath at a constant temperature of 40 °C and stir for 12 h, then remove the supernatant and dry to obtain the second powder.

[0052] S3. The second powder obtained in step S2 is placed in a high-temperature tube furnace and heated to 750°C in a hydrogen-argon atmosphere and held for 2 hours for high-temperature annealing. The powder is then cooled to room temperature in the furnace to obtain the Bi-HfO2@C-3:1 catalyst. The volume ratio of hydrogen to argon in the hydrogen-argon atmosphere is 5:95.

[0053] Example 4

[0054] A method for preparing a Bi-HfO2@C catalyst, prepared according to the method described in Example 1, except that the high-temperature annealing temperature in step S3 is 650°C. The catalyst obtained in this example is designated as Bi-HfO2@C-650.

[0055] Example 5

[0056] A method for preparing a Bi-HfO2@C catalyst, prepared according to the method described in Example 1, except that the high-temperature annealing temperature in step S3 is 850℃. The catalyst obtained in this example is denoted as Bi-HfO2@C-850.

[0057] Example 6

[0058] A method for preparing a Bi-HfO2@C catalyst, prepared according to the method described in Example 1, except that the high-temperature annealing time in step S3 is 1.5 h.

[0059] Example 7

[0060] A method for preparing a Bi-HfO2@C catalyst, prepared according to the method described in Example 1, except that the mass ratio of 2,6-naphthalenedicarboxylic acid and hafnium tetrachloride is 0.588:0.588.

[0061] Example 8

[0062] A method for preparing a Bi-HfO2@C catalyst, prepared according to the method described in Example 1, except that the hydrothermal reaction temperature in step S1 is 110℃ and the time is 10h.

[0063] Example 9

[0064] A method for preparing a Bi-HfO2@C catalyst, prepared according to the method described in Example 1, except that the stirring reaction temperature in step S2 is 35°C and the stirring reaction time is 14h.

[0065] Application Example 1

[0066] Application of Bi-HfO2@C catalyst in the electroreduction of CO2 to prepare formic acid. 2 mg of the prepared Bi-HfO2@C powder was weighed, then 1 mg of commercial carbon black XR-72 was added (to increase conductivity), followed by 700 μL of anhydrous ethanol, 300 μL of deionized water, 20 μL of 5% commercial polytetrafluoroethylene (PTFE) solution, and 4 μL of commercial perfluorosulfonic acid resin (Nafion) adhesive. After ultrasonication for 1 h, the mixture was thoroughly mixed and sprayed onto a 2×2 cm... 2 On the gas diffusion layer carbon paper, the loading after drying was controlled to be 1 mg / cm³. 2 Electrodes were prepared. A commercially available simulated electrolytic membrane electrode was used for testing. The electrode obtained above was used as the cathode, and a commercially available iridium-plated titanium felt electrode was used as the anode. A commercial anion exchange membrane (Grade 60) was used to separate the cathode and anode. An electrolytic cell was assembled for electrolysis testing. During the test, pure CO2 humidified with deionized water was introduced into the cathode at a flow rate of 40 sccm, and 1M KOH was refluxed into the anode at a flow rate of 10 mL / min. The products from both the cathode and anode were collected and analyzed.

[0067] Comparative Example 1

[0068] A method for preparing a Bi@C catalyst includes the following steps:

[0069] S1. Disperse 0.588 g of 2,6-naphthalenedicarboxylic acid and 0.8436 g of BiCl3 in 26 mL of N,N-dimethylformamide, and stir on a magnetic stirrer for 30 min to obtain a homogeneous solution. Add 20 mL of 88% formic acid solution and 8 mL of deionized water and transfer to a hydrothermal reactor. After sealing the reactor, place it in an oven at 120 °C for hydrothermal reaction for 9 h. After the reaction is completed, centrifuge to remove the supernatant, wash three times with a mixed solution of acetone and N,N-dimethylformamide, and then dry to form a powder.

[0070] S2. The powder obtained in step S1 is placed in a high-temperature tube furnace and heated to 750°C in a hydrogen-argon atmosphere and held for 2 hours for high-temperature annealing. The powder is then cooled to room temperature in the furnace to obtain the Bi@C catalyst. The volume ratio of hydrogen to argon in the hydrogen-argon atmosphere is 5:95.

[0071] The Bi@C catalyst prepared in this comparative example is used in the same way as in Application Example 1 for the electroreduction of CO2 to prepare formic acid.

[0072] Comparative Example 2

[0073] A method for preparing an HfO2@C catalyst includes the following steps:

[0074] S1. Disperse 0.588 g of 2,6-naphthalenedicarboxylic acid and 0.8436 g of hafnium tetrachloride (HfCl4) in 26 mL of N,N-dimethylformamide. Stir on a magnetic stirrer for 30 min to ensure thorough mixing and obtain a homogeneous solution. Add 20 mL of 88% formic acid solution and 8 mL of deionized water and transfer to a hydrothermal reactor. After sealing the reactor, place it in an oven at 120 °C for hydrothermal reaction for 9 h. After the reaction is complete, centrifuge to remove the supernatant. Wash the solution three times with a mixed solution of acetone and N,N-dimethylformamide, and then dry to form a powder.

[0075] S2. The powder obtained in step S1 is placed in a high-temperature tube furnace and heated to 750°C in a hydrogen-argon atmosphere and held for 2 hours for high-temperature annealing. The furnace is then cooled to room temperature to obtain the HfO2@C catalyst. The volume ratio of hydrogen to argon in the hydrogen-argon atmosphere is 5:95.

[0076] Electrode sheets were prepared using the HfO2@C catalyst obtained in this comparative example. The specific method was as follows: 10 mg of HfO2@C catalyst was added to 2 mg of carbon black, followed by 950 μL of ethanol and 50 μL of Nafion. The solution was then titrated to a final volume of 1 × 1 cm⁻¹. 2 On the carbon paper, the loading after drying is controlled to be 1 mg / cm³. 2 Electrode sheets were prepared. Then, a simple two-cell electrolytic cell was used for testing, with the electrode sheet serving as the working electrode, a carbon rod as the counter electrode, and a Nafion 117 proton exchange membrane separating the anode and cathode. The remaining steps were the same as in Application Example 1.

[0077] Electrode sheets were fabricated and electrochemical tests were conducted on the HfO2@C catalyst prepared in Comparative Example 2. It was found that it had almost no electrochemical CO2 reduction formic acid production activity, and its main product was H2. This also verifies that it has a strong ability to generate protons through water dissociation.

[0078] Comparative Example 3

[0079] A method for preparing an HfO2@C catalyst includes the following steps:

[0080] S1. Disperse 0.588 g of 2,6-naphthalenedicarboxylic acid and 0.8436 g of hafnium tetrachloride (HfCl4) in 26 mL of N,N-dimethylformamide. Stir on a magnetic stirrer for 30 min to mix thoroughly and obtain a homogeneous solution. Add 20 mL of 88% formic acid solution and 8 mL of deionized water and transfer to a hydrothermal reactor. After sealing the reactor, place it in an oven at 120 °C for hydrothermal reaction for 9 h. After the reaction is complete, centrifuge to remove the supernatant. Wash three times with a mixed solution of acetone and N,N-dimethylformamide, and then dry to form the first powder.

[0081] S2. Weigh 560 mg of the first powder obtained in step S1 and 140 mg of bismuth trichloride (BiCl3) powder, dissolve them in 21 mL of deionized water to form a homogeneous solution, place the solution in a water bath at a constant temperature of 40 °C and stir for 12 h, then remove the supernatant and dry to obtain the second powder.

[0082] S3. The second powder obtained in step S2 is placed in a high-temperature tube furnace and heated to 750°C in a hydrogen-argon atmosphere and held for 2 hours for high-temperature annealing. The powder is then cooled to room temperature in the furnace to obtain the Bi-HfO2@C-4:1 catalyst. The volume ratio of hydrogen to argon in the hydrogen-argon atmosphere is 5:95.

[0083] Comparative Example 4

[0084] A method for preparing an HfO2@C catalyst includes the following steps:

[0085] S1. Disperse 0.588 g of 2,6-naphthalenedicarboxylic acid and 0.8436 g of hafnium tetrachloride (HfCl4) in 26 mL of N,N-dimethylformamide. Stir on a magnetic stirrer for 30 min to mix thoroughly and obtain a homogeneous solution. Add 20 mL of 88% formic acid solution and 8 mL of deionized water and transfer to a hydrothermal reactor. After sealing the reactor, place it in an oven at 120 °C for hydrothermal reaction for 9 h. After the reaction is complete, centrifuge to remove the supernatant. Wash three times with a mixed solution of acetone and N,N-dimethylformamide, and then dry to form the first powder.

[0086] S2. Weigh 560 mg of the first powder obtained in step S1 and 70 mg of bismuth trichloride (BiCl3) powder, dissolve them in 21 mL of deionized water to form a homogeneous solution, place it in a water bath at a constant temperature of 40 °C and stir for 12 h, then remove the supernatant and dry to obtain the second powder.

[0087] S3. The second powder obtained in step S2 is placed in a high-temperature tube furnace and heated to 750°C in a hydrogen-argon atmosphere and held for 2 hours for high-temperature annealing. The powder is then cooled to room temperature in the furnace to obtain the Bi-HfO2@C-8:1 catalyst. The volume ratio of hydrogen to argon in the hydrogen-argon atmosphere is 5:95.

[0088] Performance testing

[0089] Figure 1 , Figure 2 The images show SEM and TEM images of the Bi-HfO2@C catalyst prepared in Example 1. The images show that nano-sized particles of the catalyst are distributed on the hollow porous carbon framework substrate. The high-magnification SEM images show that the cluster surface is rough and has a large surface area.

[0090] Figure 3 SEM images of the Hf-MOF precursor and the Bi₂O₃-supported Hf-MOF precursor prepared in Example 1 are shown. In the figures, a is the SEM image of the Hf-MOF precursor, and b is the SEM image of the Bi₂O₃-supported Hf-MOF precursor. It can be seen from the figures that the Hf-MOF precursor becomes rougher after surface Bi growth. 3+ The oxidized Bi produced after hydrolysis adheres to the surface of the Hf-MOF precursor, and after annealing heat treatment, it forms a Bi-HfO2 nanoparticle structure supported on a carbon framework.

[0091] Figure 4 The XPS plot shows the Bi-HfO2@C catalyst prepared in Example 1. As can be seen from the plot, a doublet of 18.6 eV and 17.1 eV appears on the Hf 4f orbital of the Bi-HfO2@C catalyst, corresponding to the oxidation state of Hf. 4+ The peak indicates that HfO2 was successfully introduced into the Bi-based catalyst using the above method.

[0092] Figure 5The figure shows the Faradaic efficiency of CO2 electrolysis for the production of formic acid under different currents. The Faradaic efficiency represents the formic acid selectivity of the electrolytic catalyst (generally, greater than 90% is preferred). In the figure, (a) shows the Faradaic efficiency of the Bi-HfO2@C catalyst prepared in Example 1 at different currents for the CO2 electrolysis for the production of formic acid, and (b) shows the Faradaic efficiency of the Bi@C catalyst prepared in Comparative Example 1 at different currents for the CO2 electrolysis for the production of formic acid. The Faradaic efficiency graphs show that the Bi-HfO2@C material exhibits high selectivity even at high currents (the higher the Faradaic efficiency of formic acid, the higher the selectivity of the CO2 reduction formic acid reaction). The Faradaic efficiency at 1.6 A is 89.82%, significantly higher than the 73.29% of the Bi@C catalyst prepared in Comparative Example 1. The CO2 reduction formic acid reaction involves side reactions; CO2 reduction produces formic acid, hydrogen, and carbon monoxide, the latter two being byproducts. The byproducts of the electroreduction reaction of Bi-HfO2@C catalyst are all below 10%, while the byproducts of the electroreduction of Bi@C catalyst are as high as 28.4% at 1.6A.

[0093] Figure 6 The figure shows the production rates of formic acid from CO2 electrolysis using the Bi-HfO2@C catalyst prepared in Example 1 at different currents. As can be seen from the figure, the production rates of formic acid from CO2 electrolysis are 3.4 mmol / h at currents of 0.2 A, 0.4 A, 0.8 A, 1.2 A, and 1.6 A, respectively. -1 6.9 mmol h -1 13.9 mmol h -1 20.6 mmol h -1 26.8 mmol h -1 It is evident that the production rate of formic acid from CO2 electrolysis gradually increases with increasing current.

[0094] Figure 7 The graph shows the test results of the Bi-HfO2@C catalyst prepared in Example 1 under continuous electrolysis at a current of 1A for 60 hours, testing the stability and lifetime of the catalyst. The Faradaic efficiency (FE) in the graph represents the formic acid production selectivity of this electrolytic catalyst; the dashed line in the graph is the contour line where FE = 90%. As can be seen from the graph, during electrolysis, its activity decreases, manifested as a decrease in Faradaic efficiency. The catalyst can maintain a lifetime of over 60 hours (FE remains greater than 90%) under a high current (1A).

[0095] Figure 8The figures show the Faradaic efficiencies of the Bi-HfO2@C catalysts prepared in Examples 1-3 and Comparative Examples 3-4 at different voltages. As can be seen from the figures, within the voltage range of -1.0 to -1.2 V, the Faradaic efficiencies of the Bi-HfO2@C catalysts prepared in Examples 1-3 are all above 90%. Among them, the Bi-HfO2@C catalyst prepared in Example 1 has the highest Faradaic efficiency at 97%, significantly higher than the Bi-HfO2@C catalysts in Comparative Examples 3-4. The Faradaic efficiency of the Bi-HfO2@C catalyst in Comparative Example 3 is 85% at -1.2 V, and the Faradaic efficiency of the Bi-HfO2@C catalyst in Comparative Example 4 is 86% at -1.0 V.

[0096] Figure 9 The KIE test results are shown for the Bi-HfO2@C catalyst prepared in Example 1 and the Bi@C catalyst prepared in Comparative Example 1. KIE stands for Kinetic Isotope Effect; a smaller value indicates a stronger protonation ability (less affected by proton supply), and vice versa. In other words, a weaker KIE value indicates a faster rate of CO2 hydrogenation protonation during CO2 reduction. Figure 9 It can be seen that the KIE value of the Bi-HfO2@C catalyst prepared in Example 1 is 1.1, while the KIE value of the Bi@C catalyst prepared in Comparative Example 1 is 1.4.

[0097] Figure 10 The figure shows the Faradaic efficiency of the Bi-HfO2@C catalysts prepared in Examples 1, 4, and 5 for CO2 electrolysis to formic acid production. ECR performance tests were performed on the HfO2@C catalysts prepared in Examples 1, 4, and 5. The results show that the sample annealed at 750℃ exhibited the highest formic acid selectivity, reaching up to 98% at -1.2V, while the sample annealed at 850℃ showed the worst performance, with a formic acid selectivity of 80% at -1.3V. High-temperature annealing of the Hf-MOF precursor leads to the formation of Hf-Bi alloys on the HfO2 surface, reducing the Hf-O-Bi bonding active sites. This indicates that high annealing temperatures are detrimental to structural recombination during the carbonization process, resulting in poorer ECR performance. Conversely, using lower annealing temperatures weakens the recombination process, preventing effective carbonization structural recombination and also leading to poorer ECR performance.

[0098] This invention provides a Bi-based catalyst, Bi-HfO2@C, capable of producing formic acid from CO2 by electrolysis while maintaining both high selectivity and high current density. This catalyst is used in industrial electrolyzers (2×2 cm⁻¹) to achieve high selectivity and high current density. 2Stable electrolysis at high currents of 0.5-1.6 A with a selectivity exceeding 90%, far surpassing industrial-grade current densities, and capable of stable electrolysis for over 60 hours at 1 A, during which the formic acid yield consistently exceeds 17 mmol / L. -1 This is far higher than the requirements for industrial electrolysis.

[0099] Although preferred embodiments of the present invention have been described, those skilled in the art, upon learning the inventive concept of the present invention, can make other changes and modifications to these embodiments, all of which fall within the scope of the present invention.

[0100] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. If such modifications and variations fall within the scope of equivalents of this invention, then this invention also intends to include these modifications and variations.

Claims

1. A method for preparing a Bi-HfO2@C catalyst, characterized in that, Includes the following steps: Using hafnium tetrachloride as the hafnium source and 2,6-naphthalenedicarboxylic acid as the ligand, the Hf-MOF precursor was obtained through a hydrothermal reaction in an organic solvent. Using bismuth trichloride as the bismuth source, it was mixed with Hf-MOF precursor and water, and the reaction was stirred. The reaction was controlled by adjusting the Bi content. 3+ The hydrolysis process was used to grow Bi2O3 on the surface of the Hf-MOF precursor to obtain Bi2O3-supported Hf-MOF precursor. Under a reducing protective atmosphere, the Bi2O3-supported Hf-MOF precursor was annealed to reconstruct the carbon skeleton at high temperature, while reducing trivalent Bi to zero-valent metallic Bi. Bi-HfO2 nanoparticles were then recrystallized on the carbon skeleton to obtain the Bi-HfO2@C catalyst. The mass ratio of 2,6-naphthalenedicarboxylic acid to hafnium tetrachloride is 0.588:0.588~0.8436; The mass ratio of the Hf-MOF precursor to bismuth trichloride is 1~3:1; The annealing process is carried out at a temperature of 650℃ to 850℃ for 1.5h to 2h.

2. The method of claim 1, wherein the Bi-Hf02@C catalyst is prepared by the steps of: In the preparation of the Hf-MOF precursor, the hydrothermal reaction temperature is 110℃~120℃ and the time is 9h~10h.

3. The method for preparing the Bi-HfO2@C catalyst according to claim 1, characterized in that, During the preparation of the Bi2O3-supported Hf-MOF precursor, the stirring reaction temperature was 35℃~40℃, and the stirring reaction time was 12h~14h.

4. The preparation method of the Bi-HfO2@C catalyst according to claim 1, characterized in that, The reducing protective gas is a mixture of hydrogen and argon.

5. The Bi-HfO2@C catalyst prepared by the method according to any one of claims 1 to 4.

6. The application of the Bi-HfO2@C catalyst according to claim 5 in the electroreduction of CO2 to prepare formic acid.

7. The application of the Bi-HfO2@C catalyst according to claim 6 in the electroreduction of CO2 to prepare formic acid, characterized in that, Includes the following steps: The cathode is prepared by uniformly mixing Bi-HfO2@C catalyst, carbon black, polytetrafluoroethylene, perfluorosulfonic acid resin, anhydrous ethanol and water, and then spraying the mixture onto carbon paper in the gas diffusion layer. An electrolytic cell is assembled by using an iridium-plated titanium felt electrode as the anode, and assembling the cathode, anode, and anion exchange membrane. Formic acid is produced by passing humidified CO2 gas into the cathode and an alkaline solution into the anode through an electroreduction reaction.

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