Preparation method of copper-iron bimetallic monatomic catalyst taking cerium oxide as substrate

Abstract

The invention provides a preparation method of a copper-iron bimetallic monatomic catalyst taking cerium oxide as a substrate, which comprises the following steps: firstly, preparing a cerium oxide precursor by a hydrothermal method, then introducing oxygen vacancies into the precursor by Joule heating, and finally inducing metal atoms to be anchored on the surface of cerium oxide by a wet chemical impregnation and high-temperature calcination method, thereby obtaining the copper-iron bimetallic monatomic catalyst taking cerium oxide as the substrate. The prepared catalyst is used for synthesizing urea through electrocatalytic reduction of nitrate radicals, and the urea yield is excellent. The invention provides an effective way for development of an electrocatalytic reduction and urea C-N coupling system, and has application potential in the aspect of producing high-value chemical urea while solving nitrate pollution.

Description

Technical Field

[0001] The invention relates to the technical field of catalyst preparation, and in particular to a method for preparing a bimetallic single-atom catalyst using cerium oxide as a substrate. Background Art

[0002] Nitrate pollution is one of the causes of eutrophication of water bodies. Electrocatalytic reduction has attracted widespread attention due to its simple process and the use of electrons as reducing agents, which is pollution-free. However, the products of electrocatalytic reduction of nitrate ions are usually nitrogen or ammonia, which leads to additional energy waste because these products cannot be directly utilized. At the same time, the traditional urea synthesis method involves the production of ammonia by the Haber-Bosch process, and then the synthesis of urea using ammonia as a raw material through the Bosch-Meiser process. Both processes consume a lot of energy and result in a lot of carbon emissions. Electrocatalytic CN coupling of carbon dioxide and nitrate provides an environmentally friendly method to reduce CO2 emissions, treat nitrate wastewater, and produce urea.

[0003] Single-atom catalysts are an emerging material in the field of catalysis in recent years, and have attracted much attention due to their excellent activity and selectivity in multiple catalytic reactions. Single-atom catalysts can significantly improve the atomic utilization rate by dispersing metal active sites into single atoms, and exhibit unique electronic and geometric structures, which help to optimize catalytic performance. Therefore, they have important application prospects in the fields of chemical industry, environmental protection, and energy. However, in order to avoid the separated metal atoms from aggregating into less active nanoparticles or clusters that block the active sites, the metal loading of single-atom catalysts is very limited. Therefore, improving the utilization rate of metal atoms in single-atom catalysts will be a feasible and reliable way to promote the catalytic performance of metal doping in single-atom catalysts. Summary of the invention

[0004] The purpose of the present invention is to increase the metal loading of a single atom catalyst and improve the catalytic performance of the single atom catalyst, thereby providing a method for preparing a bimetallic single atom catalyst with oxygen vacancy induced metal atom anchoring based on cerium oxide.

[0005] The technical solution adopted by the present invention is: The preparation method of a copper-iron bimetallic single-atom catalyst based on cerium oxide comprises the following steps: Step 1, preparing CeO2 powder; Step 2, preparing V-CeO2 powder; The CeO2 powder obtained in step 1 is rapidly heated in a vacuum environment using a Joule heating device to introduce oxygen vacancies to obtain V-CeO2 powder; Step 3, preparing CuFe-V-CeO2 powder; The V-CeO2 powder obtained in step 2 is ultrasonically dispersed in deionized water, and CuCl2 solution and FeCl3 solution are added respectively, and ultrasonic stirring is continued. Na2CO3 solution is slowly added to the mixed solution and stirred; the solution after the reaction is centrifuged to separate the solid product and freeze-dry it, and then the obtained solid is calcined to obtain the copper-iron doped target catalyst CuFe-V-CeO2.

[0006] The rapid heating described here is heating within 1 second and cooling within 1 second, which helps to reduce the damage to cerium oxide caused by heating, and can be achieved by a Joule heating device.

[0007] Furthermore, in step 2, the temperature of rapidly heating the CeO2 powder using a Joule heating device is 600-1200°C.

[0008] The present invention proves through examples that V-CeO2 formed at a heating temperature of 600-1200°C can improve urea yield.

[0009] Furthermore, the molar ratio of Cu and Fe atoms in the catalyst CuFe-V-CeO2 of step 3 is Cu:Fe=(1-5):(1-5).

[0010] The present invention proves through comparative examples and embodiments that, compared with a catalyst formed by doping only copper or iron single metal atoms, the urea yield is higher by doping with two metal atoms of copper and iron, and the doping ratio of the two metal atoms of copper and iron has an influence on the urea yield. Only when the doping ratio of the two metal atoms of copper and iron is appropriate can the urea yield be optimal.

[0011] Furthermore, the calcination process in step 3 is: heating to 250°C at a heating rate of 5°C / min in an Ar atmosphere containing 5% H2 and maintaining for 4 hours.

[0012] Calcination here is to reduce the deposited metal.

[0013] Furthermore, the method for preparing CeO2 powder in step 1 is: CeCl2·7H2O is dissolved in deionized water to obtain solution A, NaOH is dissolved in deionized water to obtain solution B, and solution B is slowly added into solution A at room temperature with stirring; the obtained mixed solution is transferred to a reactor for hydrothermal treatment; after the reaction is completed, the solid product is dried and calcined to obtain CeO2 powder.

[0014] Furthermore, the calcination treatment in step 1 is: in a muffle furnace, the temperature is increased to 400°C at a rate of 5°C / min and calcined for 5 h.

[0015] As a preferred embodiment, a method for preparing a copper-iron bimetallic single-atom catalyst based on cerium oxide comprises: Step 1, preparing CeO2 powder; 0.85 g CeCl2·7H2O was dissolved in 20 mL deionized water to obtain solution A, 9.6 g NaOH was dissolved in 15 mL deionized water to obtain solution B, solution B was slowly added into solution A at room temperature and stirred for 30 min; the obtained mixed solution was transferred to a reactor and reacted at 100 °C for 24 hours for hydrothermal treatment; after the reaction was completed, the reaction product was washed alternately with water and ethanol and then centrifuged to obtain a solid product, the solid product was heated to 400 °C at a rate of 5 °C / min in a muffle furnace, and calcined for 5 h to obtain CeO2 powder; Step 2, preparing V-CeO2 powder; The CeO2 powder obtained in step 1 is rapidly heated to 1000° C. in a vacuum environment using a Joule heating device to introduce oxygen vacancies to obtain V-CeO2 powder; Step 3, preparing CuFe-V-CeO2 powder; Take 50 mg of V-CeO2 powder and ultrasonically disperse it in 20 mL of deionized water. Then add 0.84 mL of 5 mg / mL CuCl2·2H2O solution and 0.42 mL of 5 mg / mL FeCl3·6H2O solution respectively, and continue ultrasonic stirring; slowly add 5 mL of 50 mg / mL Na2CO3 solution to the mixed solution and stir; centrifuge the solution after the reaction, separate the solid product and freeze-dry it; then heat the dried solid product to 250℃ at a heating rate of 5℃ / min in a tubular furnace in an Ar atmosphere containing 5% H2 and maintain it for 4 hours, and calcine to obtain the copper-iron doped target catalyst CuFe-V-CeO2.

[0016] The copper-iron bimetallic single-atom catalyst based on cerium oxide is prepared according to the method described above.

[0017] Application of copper-iron bimetallic single-atom catalyst based on cerium oxide in electrocatalytic nitrate reduction to synthesize urea.

[0018] The invention discloses a method for synthesizing urea using a copper-iron bimetallic single-atom catalyst based on cerium oxide, which adopts a three-electrode system, with a catalyst coated on carbon paper as a working electrode, a silver-silver chloride electrode as a reference electrode, and a platinum sheet electrode as a counter electrode.

[0019] The working electrode was prepared by adding 5 mg of the synthesized catalyst to a mixed solution consisting of 0.6 mL of ethanol, 0.6 deionized water, and 50 μL of a 5 wt% Nafion aqueous solution, followed by ultrasonic treatment for 20 min to prepare a catalyst ink. Afterwards, 25 μL of the ink was dropped onto a 1.0 × 1.0 cm² carbon paper to form a working electrode with a loading of 0.1 mg cm².

[0020] The electrocatalytic reaction conditions are: at a potential of −1.7 to −1.2 (V vs. RHE), a 0.1 M KHCO3 solution containing 50 mM / L KNO3 into which CO2 is passed is used as the electrolyte.

[0021] The beneficial effects of the present invention are: The present invention first prepares a cerium oxide precursor by a hydrothermal method, then introduces oxygen vacancies into the precursor by Joule heating, and finally induces metal atoms to anchor on the surface of cerium oxide by wet chemical impregnation and high-temperature calcination to form a bimetallic single-atom catalyst, and the prepared catalyst is used for electrocatalytic reduction of nitrate to synthesize urea, showing an excellent urea yield. The present invention provides an effective way to develop an electrocatalytic reduction and urea CN coupling system, and has application potential in solving nitrate pollution and producing high-value chemical urea at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The SEM images of the catalyst samples prepared in the present invention, wherein a is the SEM image of CeO2, b is CuFe-CeO2, c is V-CuFe-CeO2, and d is the SEM image of CuFe-V-CeO2; Figure 2 XRD spectra of the catalyst samples CeO2, CuFe-CeO2, V-CuFe-CeO2 and CuFe-V-CeO2 prepared in the present invention; Figure 3 The XPS spectra of the catalyst samples CeO2, CuFe-CeO2, V-CuFe-CeO2 and CuFe-V-CeO2 prepared in the present invention, wherein a is the XPS spectrum of Ce, b is the XPS spectrum of Cu, and c is the XPS spectrum of Fe; Figure 4 TEM image of the catalyst sample CuFe-V-CeO2 prepared by the present invention and EDSmapping images of Ce, O, Fe and Cu; Figure 5 This is the energy spectrum element diagram of the catalyst sample CuFe-V-CeO2 prepared by the present invention; Figure 6 Schematic diagram of the three-electrode reaction system used in the present invention; Figure 7 (a) urea yield and (b) urea production Faraday efficiency of each embodiment of the present invention; Figure 8 It is a comparison of the effects of different heating temperatures when introducing oxygen vacancies; Fig. 9 is a comparison of urea yields between the example in which oxygen vacancies are first introduced and the comparative example; Fig.10 is an image of the catalyst of Example 3 under a high-resolution transmission electron microscope; Fig.11 It is the formation energy of Cu and Fe on the CeO2 surface calculated based on DFT theory; Fig.12 The adsorption energy of functional groups on the material surface is shown. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0024] Embodiment 1: Step 1: Synthesis of CeO2

[0025] Solution A was prepared by dissolving 0.85 g of CeCl2·7H2O in 20 ml of deionized water. Solution B was prepared by dissolving 9.6 g of NaOH in 15 ml of deionized water. Solution B was slowly added to solution A at room temperature and stirred for 30 minutes. The resulting mixed solution was transferred to a polytetrafluoroethylene (PTFE)-lined reactor for hydrothermal treatment (100°C, 24 hours). After the reaction was completed, the product was washed alternately with deionized water and ethanol and centrifuged three times. The obtained solid was vacuum dried at 60°C for 12 hours, and then calcined at 500°C in an argon atmosphere for 4 hours at a heating rate of 5°C / min to obtain CeO2 powder.

[0026] Step 2: Synthesis of CuFe-CeO2 Disperse 50 mg of CeO2 powder prepared in step 1 in 20 mL of deionized water. Then add 0.84 mL of 5 mg / mL CuCl2·2H2O solution to the CeO2 aqueous solution, and then add 0.42 mL of 5 mg / mL FeCl3·6H2O solution, and mix them ultrasonically for 30 minutes. Then slowly add 5 mL of 50 mg / mL Na2CO3 solution, continue stirring for 2 hours, centrifuge and freeze-dry the solid. The dried solid product was heated to 250°C at a heating rate of 5°C / min in an Ar atmosphere containing 5% H2 and maintained for 4 hours to obtain CeO2 powder doped with Cu and Fe (CuFe-CeO2).

[0027] A heating rate of 5°C / min is common in tube furnaces, and heating to 250°C and holding for 4 hours ensures that the dopant can be reduced.

[0028] Example 2: Synthesis of V-CuFe-CeO2 In this embodiment, based on embodiment 1, the CuFe-CeO2 powder obtained in step 2 is placed in a Joule heating device and rapidly heated to 1000°C in a vacuum environment to introduce oxygen vacancies to obtain a V-CuFe-CeO2 catalyst.

[0029] Example 3: Synthesis of CuFe-V-CeO2 In this example, based on Example 1, the CeO2 powder obtained in step 1 of Example 1 was first placed in a Joule heating device and rapidly heated to 1000°C in a vacuum environment to introduce oxygen vacancies to prepare V-CeO2 powder. Subsequently, 50 mg of V-CeO2 powder was dispersed in 20 mL of deionized water, and then the Cu3Fe1-V-CeO2 bimetallic single atom catalyst was obtained by the same method as in step 2 of Example 1. For the convenience of writing and description, Cu3Fe1-V-CeO2 is simplified to CuFe-V-CeO2 in the accompanying drawings.

[0030] The morphologies of the CeO2 powder, CuFe-CeO2 powder, V-CuFe-CeO2 powder, and CuFe-V-CeO2 powder prepared in the above examples were observed by scanning electron microscopy (SEM). The results were as follows: Figure 1 As shown in a, b, c, and d, the embodiment is a nanorod with a diameter of about 25 nm.

[0031] right Figure 1 The crystal phases shown were characterized by X-ray diffraction (XRD). Figure 2 As shown, Figure 2PDF#34-0394 is a standard card of cerium oxide in the existing crystal database. No other identifiable signals were found, which indicates that the Cu and Fe metals in CuFe-CeO2 powder, V-CuFe-CeO2 powder, and CuFe-V-CeO2 powder may be dispersed in the catalyst as single atoms. After Joule heating, the peak intensity of the sample increased, indicating that the crystallinity of the sample was improved.

[0032] The electron transfer between Cu, Fe and CeO2 was analyzed by X-ray photoelectron spectroscopy (XPS). Figure 3 As shown in a, b, and c. After the introduction of oxygen vacancies, Ce gains electrons, and its related characteristic peaks move toward the low binding energy direction. After doping with Cu and Fe, the binding energy of Ce is further reduced, indicating that Ce absorbs electrons from Cu and Fe, so the binding energy is also reduced; after the introduction of oxygen vacancies, the binding energy of Cu and Fe increases, which shows that the introduction of oxygen vacancies leads to more stable atomic hybridization between Cu / Fe and CeO2, resulting in stronger internal electron transfer.

[0033] Schematic diagram of transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) of CuFe-V-CeO2 is shown in Figure 4 EDS mapping shows the uniform distribution of Cu, Fe, Ce and O elements in CuFe-V-CeO2.

[0034] The energy spectrum element diagram of CuFe-V-CeO2 is as follows Figure 5 As shown, the successful doping of Cu and Fe was further verified.

[0035] The experimental steps of using the cerium oxide-based catalysts prepared in the above-mentioned Examples 1-3 to electrocatalyze nitrate reduction to synthesize urea are as follows: 5 mg of the synthesized catalyst was added to a mixed solution consisting of 0.6 mL of ethanol, 0.6 deionized water, and 50 μL of a 5 wt% Nafion aqueous solution, followed by ultrasonic treatment for 20 minutes to prepare a catalyst ink. Afterwards, 25 μL of the ink was dropped onto a 1.0 × 1.0 cm² carbon paper to form a working electrode with a loading of 0.1 mg cm².

[0036] The prepared working electrode was applied to Figure 6 In the three-electrode reaction system shown, the carbon paper coated with the catalyst is used as the working electrode, the silver-silver chloride electrode is used as the reference electrode, and the platinum sheet electrode is used as the counter electrode.

[0037] The electrolyte used for the coupling reaction was 0.1 M KHCO3 containing 50 mM KNO3. To ensure that the solution was saturated with CO2, a CO2 gas flow (30 mL min^-1) was introduced 30 minutes before the start of the reaction and was kept bubbling during the reaction. The electrolysis was performed in chronoamperometry mode at different constant potentials (-1.2, -1.3, -1.4, -1.5, -1.6, -1.7 V vs. RHE). The reaction was carried out for 1 hour under magnetic stirring at 300 rpm.

[0038] Urea production was detected by urease decomposition method. 0.2 mL of urease solution (5 mg / mL) was added to 1.8 mL of urea electrolyte. Then, the solution was heated in a water bath at 37°C for 60 minutes under a constant temperature oscillator to ensure that the urea was completely decomposed into CO2 and NH3. The NH3 concentration before and after urea decomposition was quantitatively determined by the indophenol blue method, and the urea production was calculated according to the following formula: m urea = (m b -m a ) / 2, where m a and m b are the molar amounts of NH3 with and without urease added, respectively.

[0039] The urea yield and Faradaic efficiency were calculated by the following formula:

[0040]

[0041] Among them, c urea is the measured urea concentration (mg / L), V is the volume of the electrolyte (mL), t is the electrocatalytic time (s), m is the catalyst loading (mg), F is the Faraday constant (96485.3 C / mol), and Q is the total charge passing through the working electrode (C).

[0042] The final result is Figure 7 As shown, Example 3 has the highest urea yield and the highest Faraday efficiency, that is, the highest charge utilization rate.

[0043] Embodiment 4:

[0044] The difference between this embodiment and embodiment 3 is that the CeO2 powder prepared in step 1 of embodiment 1 is placed in a Joule heating device and rapidly heated to 600°C in a vacuum environment to introduce oxygen vacancies to prepare V-CeO2 powder.

[0045] Embodiment 5:

[0046] The difference between this embodiment and embodiment 4 is that CeO2 powder is placed in a Joule heating device and rapidly heated to 800°C in a vacuum environment to introduce oxygen vacancies to prepare V-CeO2 powder.

[0047] Embodiment 6:

[0048] The difference between this embodiment and embodiment 4 is that CeO2 powder is placed in a Joule heating device and rapidly heated to 1200°C in a vacuum environment to introduce oxygen vacancies to prepare V-CeO2 powder.

[0049] In order to facilitate comparison of the effects of the V-CeO2 powders prepared in Examples 3, 4, 5, and 6, the products of Examples 4, 5, and 6 are named V-CeO2-600, V-CeO2-800, and V-CeO2-1200, respectively, and the V-CeO2 powder prepared in Example 3 is named V-CeO2-1000.

[0050] like Figure 8 As shown in the figure, with the increase of Joule heating temperature, the electrocatalytic urea synthesis performance of V-CeO2 first increases and then decreases, and reaches the highest when the heating temperature is 1000 degrees. When the heating temperature is too low, the amount of oxygen vacancies introduced is small, so as the temperature increases, the amount of oxygen vacancies introduced increases, and the number of active sites increases; when the heating temperature is higher than 1000 degrees, the sample is easily destroyed by high temperature and loses stability, so the catalytic performance decreases.

[0051] like Fig.10 , which is a high-resolution transmission electron microscopy (HRTEM) image of the catalyst prepared in Example 3. The lattice spacing of 0.322 nm in the figure corresponds to the (111) crystal plane of CeO2. At a higher resolution, some weaker signals (indicated by circles in the figure) can be seen, indicating the presence of oxygen vacancies.

[0052] Embodiment 7:

[0053] The CeO2 powder prepared in Example 1 was placed in a Joule heating device and rapidly heated to 1000°C under vacuum to introduce oxygen vacancies to prepare V-CeO2 powder. Subsequently, 50 mg of V-CeO2 powder was dispersed in 20 mL of deionized water. 0.91 mL of 5 mg / mL CuCl2·2H2O solution and 0.29 mL of 5 mg / mL FeCl3·6H2O solution were added and ultrasonically mixed for 30 minutes. Subsequently, 5 mL of 50 mg / mL Na2CO3 solution was added, stirring was continued for 2 hours, centrifuged and the solid was freeze-dried. The dried powder was heated to 250°C at a heating rate of 5°C / min in an Ar atmosphere containing 5% H2 and maintained for 4 hours to obtain a Cu5Fe1-V-CeO2 bimetallic single atom catalyst.

[0054] Embodiment 8:

[0055] The CeO2 powder prepared in Example 1 was placed in a Joule heating device and rapidly heated to 1000°C under vacuum to introduce oxygen vacancies to prepare V-CeO2 powder. Subsequently, 50 mg of V-CeO2 powder was dispersed in 20 mL of deionized water. 0.27 mL of 5 mg / mL CuCl2·2H2O solution and 1.30 mL of 5 mg / mL FeCl3·6H2O solution were added and ultrasonically mixed for 30 minutes. Subsequently, 5 mL of 50 mg / mL Na2CO3 solution was added, stirring was continued for 2 hours, centrifuged and the solid was freeze-dried. The dried powder was heated to 250°C at a heating rate of 5°C / min in an Ar atmosphere containing 5% H2 and maintained for 4 hours to obtain a Cu1Fe3-V-CeO2 bimetallic single atom catalyst.

[0056] Embodiment 9:

[0057] The CeO2 powder prepared in Example 1 was placed in a Joule heating device and rapidly heated to 1000°C under vacuum to introduce oxygen vacancies to prepare V-CeO2 powder. Subsequently, 50 mg of V-CeO2 powder was dispersed in 20 mL of deionized water. 0.18 mL of 5 mg / mL CuCl2·2H2O solution and 1.44 mL of 5 mg / mL FeCl3·6H2O solution were added and ultrasonically mixed for 30 minutes. Subsequently, 5 mL of 50 mg / mL Na2CO3 solution was added, stirring was continued for 2 hours, centrifuged and the solid was freeze-dried. The dried powder was heated to 250°C at a heating rate of 5°C / min in an Ar atmosphere containing 5% H2 and maintained for 4 hours to obtain a Cu1Fe5-V-CeO2 bimetallic single atom catalyst.

[0058] Embodiment 10: The CeO2 powder prepared in Example 1 was placed in a Joule heating device and rapidly heated to 1000°C under vacuum to introduce oxygen vacancies to prepare V-CeO2 powder. Subsequently, 50 mg of V-CeO2 powder was dispersed in 20 mL of deionized water. 0.55 mL of 5 mg / mL CuCl2·2H2O solution and 0.87 mL of 5 mg / mL FeCl3·6H2O solution were added and ultrasonically mixed for 30 minutes. Subsequently, 5 mL of 50 mg / mL Na2CO3 solution was added, stirring was continued for 2 hours, centrifuged and the solid was freeze-dried. The dried powder was heated to 250°C at a heating rate of 5°C / min in an Ar atmosphere containing 5% H2 and maintained for 4 hours to obtain a Cu1Fe1-V-CeO2 bimetallic single atom catalyst.

[0059] Comparative Example 1: In this comparative example, based on Example 3, the dosage of CuCl2·2H2O solution and FeCl3·6H2O solution was changed, thereby changing the dosage of copper and iron. Specifically, the CeO2 powder prepared in Example 1 was placed in a Joule heating device and rapidly heated to 1000°C in a vacuum environment to introduce oxygen vacancies to prepare V-CeO2 powder. Subsequently, 50 mg of V-CeO2 powder was dispersed in 20 mL of deionized water. 1.09 mL of CuCl2·2H2O with a concentration of 5 mg / mL was added and ultrasonically mixed for 30 minutes. Subsequently, 5 mL of Na2CO3 solution with a concentration of 50 mg / mL was added, stirring was continued for 2 hours, centrifuged and the solid was freeze-dried. The dried powder was heated to 250°C at a heating rate of 5°C / min in an Ar atmosphere containing 5% H2 and maintained for 4 hours to obtain a Cu-V-CeO2 bimetallic single atom catalyst. This comparative example was not doped with iron atoms.

[0060] Comparative Example 2: The CeO2 powder prepared in Example 1 was placed in a Joule heating device and rapidly heated to 1000°C under a vacuum environment to introduce oxygen vacancies to prepare V-CeO2 powder. Subsequently, 50 mg of V-CeO2 powder was dispersed in 20 mL of deionized water. 1.73 mL of FeCl3·6H2O solution with a concentration of 5 mg / mL was ultrasonically mixed for 30 minutes. Subsequently, 5 mL of Na2CO3 solution with a concentration of 50 mg / mL was added, stirring continued for 2 hours, centrifuged and the solid was freeze-dried. The dried powder was heated to 250°C at a heating rate of 5°C / min in an Ar atmosphere containing 5% H2 and maintained for 4 hours to obtain a Fe-V-CeO2 bimetallic single atom catalyst. This comparative example was not doped with copper atoms.

[0061] from Fig. 9It can be seen from the figure that the influence of the molar ratio of Cu and Fe atoms on the efficiency of the electrocatalytic urea synthesis reaction is that as the proportion of Fe in the metal doping increases, the electrocatalytic urea synthesis performance of CuFe-V-CeO2 first increases and then decreases, and reaches the highest when Cu:Fe=3:1.

[0062] Fig.11 is the formation energy of Cu and Fe on the CeO2 surface calculated based on DFT theory. Since the formation energy of Cu and Fe on oxygen vacancies is lower than that on the CeO2 surface, Cu and Fe atoms tend to form on oxygen vacancies. The distance between Cu and Fe formed on oxygen vacancies (3.45 Å) is shorter than the distance between Cu and Fe formed on the CeO2 surface (3.79 Å), which is conducive to the coupling of adsorbed C and N intermediates on the Cu and Fe surfaces.

[0063] Fig.12 The adsorption energy of the functional groups on the surface of the material is shown. The adsorption energy of *NO3 on the Cu site of CuFe-V-CeO2 is greater than that on the Cu site of CuFe-CeO2; while the adsorption energy of *CO2 on the Fe site of CuFe-V-CeO2 is greater than that on the Fe site of CuFe-CeO2. This shows that the introduction of oxygen vacancies helps promote the adsorption of *NO3 and *CO2 on the surface of CuFe-V-CeO2. During the reaction, Cu and Fe serve as adsorption sites for *NO3 and *CO2, respectively. At the same time, the Ce site is more likely to produce active hydrogen (*H), which helps the hydrogenation reduction of the intermediate. In general, Fe, Cu and Ce sites play different roles in urea electrosynthesis, maximizing the atomic utilization rate, and the joint action of Cu, Fe and Ce promotes electrocatalytic urea synthesis.

Claims

1. A method for preparing a copper-iron bimetallic single-atom catalyst based on cerium oxide, characterized in that: The following steps are involved: Step 1, preparing CeO2 powder; Step 2, preparing V-CeO2 powder; The CeO2 powder obtained in step 1 is rapidly heated in a vacuum environment using a Joule heating device to introduce oxygen vacancies to obtain V-CeO2 powder; Step 3, preparing CuFe-V-CeO2 powder; The V-CeO2 powder obtained in step 2 is ultrasonically dispersed in deionized water, and then the CuCl2 solution is first added, and then the FeCl3 solution is added and ultrasonic stirring is continued, and then the Na2CO3 solution is slowly added to the mixed solution and stirred; the solution after the reaction is centrifuged to separate the solid product and freeze-dry it, and then the obtained solid is calcined to obtain the copper-iron doped target catalyst CuFe-V-CeO2.

2. The method for preparing a copper-iron bimetallic single-atom catalyst based on cerium oxide according to claim 1, characterized in that: In step 2, the temperature of rapidly heating CeO2 powder using a Joule heating device is 600-1200°C.

3. The method for preparing a copper-iron bimetallic single-atom catalyst based on cerium oxide according to claim 1, characterized in that: In step 3, the molar ratio of Cu and Fe atoms in the catalyst CuFe-V-CeO2 is Cu:Fe=(1-5):(1-5).

4. The method for preparing a copper-iron bimetallic single-atom catalyst based on cerium oxide according to claim 1, characterized in that: The calcination in step 3 was carried out in a tube furnace, specifically, by heating to 250 °C at a heating rate of 5 °C / min in an Ar atmosphere containing 5% H2 and maintaining for 4 hours.

5. The method for preparing the copper-iron bimetallic single-atom catalyst based on cerium oxide according to claim 1, characterized in that: The method for preparing CeO2 powder in step 1 is: CeCl2·7H2O was dissolved in deionized water to obtain solution A, and NaOH was dissolved in deionized water to obtain solution B. Solution B was slowly added into solution A at room temperature with stirring; The obtained mixed solution is transferred to a reactor for hydrothermal treatment; After the reaction is completed, the solid product is dried and calcined to obtain CeO2 powder.

6. The method for preparing the copper-iron bimetallic single-atom catalyst based on cerium oxide according to claim 5, characterized in that: The calcination treatment in step 1 is as follows: in a muffle furnace, the temperature is raised to 400°C at a rate of 5°C / min and calcined for 5 h.

7. The method for preparing a copper-iron bimetallic single-atom catalyst based on cerium oxide according to claim 1, characterized in that: Step 1, preparing CeO2 powder; 0.85 g CeCl2·7H2O was dissolved in 20 mL deionized water to obtain solution A, 9.6 g NaOH was dissolved in 15 mL deionized water to obtain solution B, solution B was slowly added to solution A at room temperature and stirred for 30 min; the obtained mixed solution was transferred to a reactor and reacted at 100 °C for 24 hours for hydrothermal treatment; After the reaction was completed, the reaction product was washed alternately with water and ethanol and then centrifuged to obtain a solid product. The solid product was heated to 400 °C in a muffle furnace at a rate of 5 °C / min and calcined for 5 h to obtain CeO2 powder. Step 2, preparing V-CeO2 powder; The CeO2 powder obtained in step 1 is rapidly heated to 1000° C. in a vacuum environment using a Joule heating device to introduce oxygen vacancies to obtain V-CeO2 powder; Step 3, preparing CuFe-V-CeO2 powder; 50 mg of V-CeO2 powder was ultrasonically dispersed in 20 mL of deionized water, and then 0.84 mL of 5 mg / mL CuCl2·2H2O solution was added first, followed by 0.42 mL of 5 mg / mL FeCl3·6H2O solution, and ultrasonic stirring was performed; 5 mL of 50 mg / mL Na2CO3 solution was slowly added to the mixed solution and stirred; the reacted solution was centrifuged to separate the solid product and freeze-dried; the dried solid product was then heated to 250°C at a heating rate of 5°C / min in a tubular furnace in an Ar atmosphere containing 5% H2 and maintained for 4 hours to obtain the copper-iron doped target catalyst CuFe-V-CeO2.

8. A copper-iron bimetallic single-atom catalyst based on cerium oxide prepared by the method according to any one of claims 1 to 7.

9. Application of copper-iron bimetallic single-atom catalyst based on cerium oxide in the electrocatalytic reduction of nitrate to synthesize urea.

10. A method for synthesizing urea using a copper-iron bimetallic single-atom catalyst based on cerium oxide, characterized in that: A three-electrode system was used, with the catalyst coated on carbon paper as the working electrode, the silver-silver chloride electrode as the reference electrode, and the platinum sheet electrode as the counter electrode.

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