Electrocatalyst for selectively preparing hydrogen peroxide by controlling oxygen reduction reaction path and application

Abstract

The invention belongs to the technical field of electrocatalytic materials, and discloses an electrocatalyst for selectively preparing hydrogen peroxide by controlling an oxygen reduction reaction path and application. The electrocatalyst is a composite material in which a Fe3O4 dispersion liquid, an amino-containing monomer, an aldehyde-containing monomer and a zinc salt are subjected to a Schiff base reaction in the presence of a catalyst to obtain a product, and the product and a magnetic metal salt are subjected to a replacement reaction to prepare a metal monatomic covalent organic framework which coats the surface of a magnetic matrix Fe3O4 in a shell form; the Fe3O4 dispersion liquid is prepared by mixing Fe3O4 nanoparticles and a dispersing aid, and the size of the Fe3O4 nanoparticles is smaller than or equal to 100 nm. The electrocatalyst according to the present invention is used to control an oxygen reduction reaction path (2 electron path), thereby achieving high-selectivity production of hydrogen peroxide.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrocatalytic materials, and more particularly relates to an electrocatalyst for selectively preparing hydrogen peroxide through an oxygen reduction reaction and an application thereof. BACKGROUND

[0002] The oxygen reduction reaction (ORR) is one of the core reactions in the field of energy conversion and storage, and its slow kinetics and competition between multi-electron transfer paths determine the efficiency of fuel cells and the economy of hydrogen peroxide (H2O2) electro-synthesis technology. Traditional ORR catalyst design mainly focuses on adjusting the electronic structure of the catalytic center, such as optimizing the adsorption free energy of intermediates through heteroatom doping, coordination environment engineering or strain effect. However, these strategies are mostly limited to adjusting the charge density, while ignoring another key intrinsic property of the reactants, intermediates and catalysts themselves - spin.

[0003] In recent years, the scientific community has gradually recognized the key role of spin freedom in electrocatalysis, giving birth to the emerging field of "spin-electronic catalysis". Oxygen (O2) molecules are in a triplet state in the ground state, with spin polarization characteristics, and their activation and reduction processes are strongly dependent on the spin configuration of the catalyst surface. Therefore, by adjusting the spin state of the catalyst, we can direct the adsorption mode of O2, the breaking tendency of O-O bond and the stability of key intermediates such as OOH, thereby determining whether the ORR ultimately follows the 2-electron path or the 4-electron path.

[0004] Currently, the spin regulation strategies for ORR mainly include the following categories: External magnetic field regulation: The application of an external magnetic field can induce spin polarization of the catalyst or affect mass transfer and electron transfer during the reaction, which has been proven to be able to change the selectivity of ORR. However, this method relies on large magnet equipment, which is difficult to realize large-scale and stable application in actual electrochemical devices.

[0005] Introducing intrinsic magnetic components: Composite the ferromagnetic or ferrimagnetic materials (such as Fe, Co, Ni and their oxides) with the catalyst, and use their internal spontaneous magnetization to provide spin-polarized electrons. For example, composite Fe3O4 with carbon materials, or construct a core-shell structure, and use the interface magnetic coupling effect to improve the ORR activity. However, these studies are mostly focused on improving the 4-electron ORR performance, and the exploration of using the size effect of magnetic components to accurately regulate the switching between 2-electron and 4-electron paths is not deep enough.

[0006] ​Controlling single atom spin state: In single atom catalysts, the spin state of the central metal ion (for example, the transition of Fe-N4 site between low spin, intermediate spin, and high spin states) has a decisive influence on its catalytic performance. Researchers try to control the spin state by changing the coordination atoms (S, P, etc.), adjusting the coordination number, or applying an electric field. However, these chemical modification methods often accompany heterogeneous changes in the overall structure of the catalyst, making it difficult to achieve continuous and controllable spin state fine-tuning, and the synthesis process is complex and has great reproducibility challenges.

[0007] In summary, although spin regulation has become a frontier direction to improve ORR performance, there are still obvious bottlenecks in existing technologies: external field regulation is not practical, the size effect of magnetic component composite is not clear, and there is still a lack of simple and effective universal methods for precise and continuous regulation of single atom spin state. In particular, how to remotely and continuously regulate the spin state of the magnetic single atom center through a simple and intuitive physical parameter without destroying the main structure of the catalyst, and to drive the ORR path to switch between 2-electron and 4-electron paths controllably, is still a challenge in this field. The present application is proposed to overcome this key technical problem and to propose an electrocatalyst for controlling the selectivity of hydrogen peroxide production in the oxygen reduction reaction. SUMMARY

[0008] The purpose of the present application is to overcome the shortcomings of the prior art and to provide an electrocatalyst for controlling the selectivity of hydrogen peroxide production in the oxygen reduction reaction. The use of the electrocatalyst of the present application to control the oxygen reduction reaction path (2-electron path) can achieve high selectivity in the production of hydrogen peroxide.

[0009] The inventors of the present application have found that by compounding Fe3O4 nanoparticles of different sizes with M-COF, the spin polarization intensity of the metal single atom center can be regulated by the size effect of Fe3O4, and the ORR reaction path (4-electron path or 2-electron path) can be controlled, that is: The smaller the size of Fe3O4, the more the reactants tend to follow the 2-electron ORR path, and the higher the selectivity of H2O2 (the more the H2O2 produced by electrochemical synthesis); The larger the size of Fe3O4, the more the reactants tend to follow the 4-electron ORR path.

[0010] To achieve the above purpose, the present application provides an electrocatalyst for controlling the selectivity of hydrogen peroxide production in the oxygen reduction reaction, which is a product obtained by Schiff base reaction of Fe3O4 dispersion liquid, amino-containing monomer, aldehyde-containing monomer, and zinc salt under catalyst conditions, and a displacement reaction of the product with a magnetic metal salt, so as to prepare a metal single atom covalent organic framework (Fe3O4@M-Salen-COF) in the form of a shell layer coated on the surface of a magnetic substrate Fe3O4. The Fe3O4 dispersion liquid is prepared by mixing Fe3O4 nanoparticles and a dispersion aid, wherein the size of the Fe3O4 nanoparticles is less than or equal to 100 nm.

[0011] According to the present application, preferably, the amino-containing monomer is at least one of ethylenediamine, o-phenylenediamine and 4,5-difluoro-o-phenylenediamine.

[0012] According to the present application, preferably, the aldehyde-containing monomer is 1,3,5-tris(4'-hydroxy-5'-formylphenyl)benzene.

[0013] In the present application, the metal-atom single covalent organic framework (M-COF) is coated on the surface of the magnetic matrix Fe3O4 in the form of a shell layer, effectively preventing the magnetic matrix Fe3O4 from agglomerating and enhancing the stability of the catalyst.

[0014] The active sites of the electrocatalyst in the present application are derived from the metal atoms M on the metal-atom single covalent organic framework (M-COF), and according to the present application, preferably, the metal atom is at least one of Fe, Co and Ni.

[0015] In the present application, the mechanism for explaining the selectivity of the electrocatalytic ORR path is as follows: the selectivity of the electrocatalytic ORR path depends on the adsorption strength of the OOH intermediate on the active sites of the catalyst. The size of the Fe3O4 nanoparticles directly determines the spin polarization strength of the metal atom center (there is a spin coupling effect of different degrees between Fe3O4 nanoparticles of different sizes and the metal active sites of the M-COF, resulting in different degrees of change in the spin polarization strength of the metal atom center); The larger the size of the Fe3O4 nanoparticles, the stronger the spin coupling effect; The smaller the size of the Fe3O4 nanoparticles, the weaker the spin coupling effect.

[0016] Based on the above content: 2-electron path mechanism: through the spin coupling effect (weak) of small-size Fe3O4 nanoparticles, a moderate spin polarization strength effect is generated on the active sites of the catalyst, which adjusts the adsorption energy of the OOH intermediate to the optimal range of the Sabatier principle. This adsorption strength is sufficient to activate O2 to generate the OOH intermediate, but is weak enough to allow the OOH intermediate to desorb in time to generate H2O2, thereby stabilizing the O-O bond and locking the 2-electron path with high selectivity; 4-electron path mechanism: through the spin coupling effect (strong) of large-size Fe3O4 nanoparticles, a strong spin polarization strength effect is generated on the active sites of the catalyst, which leads to a strong adsorption of The over-adsorption of OOH intermediates greatly stabilizes The OOH species and significantly weaken its O-O bond, prompting it to break, thus driving the reaction irreversibly to the 4-electron path. Large size Fe3O4 nanoparticles themselves as 4-electron catalysts also synergistically strengthen this effect.

[0017] According to the present application, preferably, the metal monatomic is Fe and / or Co. In the present application, the spin polarization intensity of the metal monatomic center also depends on the intrinsic magnetism of the metal: The strong magnetic metal monatomic changes more obviously under the effect of different degrees of spin coupling effect; The weak magnetic metal monatomic changes more weakly under the effect of different degrees of spin coupling effect; In the present application, the strong magnetic metal monatomic includes Fe, Co; the weak magnetic metal monatomic is Ni. Therefore, the size regulation effect of the Fe3O4 nanoparticles of the present application is more significant for the strong magnetic metal monatomic Fe and Co, and the size regulation effect of the Fe3O4 nanoparticles of the present application is not obvious for the weak magnetic metal monatomic.

[0018] According to the present application, preferably, the dispersing aid is at least one of mesitylene, ethanol, 1,2-dichlorobenzene and n-butanol.

[0019] According to the present application, preferably, the size of the Fe3O4 nanoparticles is ≤50 nm.

[0020] According to the present application, preferably, the size of the Fe3O4 nanoparticles is ≤10 nm.

[0021] According to the present application, preferably, the preparation method (one-pot method) of the electrocatalyst comprises the following steps: S1: mixing and ultrasonic treating the Fe3O4 dispersion liquid, amino-containing monomer, aldehyde-containing monomer, zinc salt and catalyst to obtain a uniformly mixed solution; S2: liquid nitrogen degassing and sealing treatment is performed on the equipment containing the uniformly mixed solution of step S1; S3: the uniformly mixed solution in the equipment treated by step S2 is subjected to Schiff base reaction to obtain a precipitate, which is washed and vacuum dried to obtain the product; S4: the product is subjected to displacement reaction with a magnetic metal salt to obtain the electrocatalyst.

[0022] In the present application, in the step S2: the liquid nitrogen degassing and sealing treatment comprises: liquid nitrogen freezing, vacuumizing and thawing are sequentially performed on the equipment containing the uniformly mixed solution of step S1, and after 3-4 cycles, the equipment containing the uniformly mixed solution of step S1 is sealed by alcohol torch.

[0023] In the present application, the catalyst is an aqueous acetic acid solution.

[0024] Another aspect of the present application provides the use of the electrocatalyst in a process for preparing hydrogen peroxide by electrocatalytic oxygen reduction reaction.

[0025] The beneficial effects of the technical solution of the present application are as follows: The present application can control the ORR path based on a single physical parameter (i.e., the particle size of Fe3O4), and the size of Fe3O4 nanoparticles is a key regulation parameter for switching the ORR path between 2-electron and 4-electron. The electrocatalyst of the present application can control the oxygen reduction reaction path (2-electron path), thereby realizing the high-selectivity preparation of hydrogen peroxide.

[0026] The method for preparing the electrocatalyst is simple, and the prepared electrocatalyst has high intrinsic activity, good universality, high stability and low price.

[0027] The present application breaks the traditional mindset of designing catalysts only from the perspective of "electron structure", and initiatively starts from "spin structure", utilizes the unique property of the intrinsic magnetic field of Fe3O4 to provide a new paradigm for regulating the electrocatalytic ORR reaction path, and provides a new idea for developing ORR catalysts with efficient and controllable reaction paths.

[0028] Other features and advantages of the present application will be described in detail in the following specific embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0029] The above and other objects, features and advantages of the present application will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[0030] Figure 1 A structure schematic diagram of a metal single-atom covalent organic framework in an electrocatalyst for selectively preparing hydrogen peroxide by controlling the oxygen reduction reaction path (M represents a metal single atom) is shown.

[0031] Figure 2 A transmission electron microscope image of an electrocatalyst for selectively preparing hydrogen peroxide by controlling the oxygen reduction reaction path provided in Example 1 of the present application is shown, wherein, Figure 2 The clear lattice fringes with a spacing of 0.291 nm in the dark area exist, which is consistent with the (220) crystal plane of spinel-structured Fe3O4; in addition, the clear lattice fringes with a spacing of 2.3 nm correspond to the (100) crystal plane of the COF.

[0032] Figure 3LSV curves (linear sweep voltammetry) of Fe3O4@Co-Salen-COF composites of different Fe3O4 nanoparticle sizes of the present application are shown. -2 “J (mA cm-2)” is current density, and “Potential / V vs. RHE” is voltage).

[0033] Figure 4 LSV curves (linear sweep voltammetry) of Fe3O4@Fe-Salen-COF composites of different Fe3O4 nanoparticle sizes of the present application are shown.

[0034] Figure 5 LSV curves (linear sweep voltammetry) of Fe3O4@Ni-Salen-COF composites of different Fe3O4 nanoparticle sizes of the present application are shown.

[0035] Figure 6 A comparison chart of electron transfer number (n) and H2O2 selectivity of Fe3O4@M-Salen-COF composites of different Fe3O4 nanoparticle sizes and different metal monatomic atoms of the present application is shown. DETAILED DESCRIPTION

[0036] Preferred embodiments of the present application will be described in more detail below. Although the following describes preferred embodiments of the present application, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided so that the present application is more thoroughly and completely conveyed to those skilled in the art, and the scope of the present application is fully conveyed to those skilled in the art.

[0037] Example 1

[0038] This embodiment provides an electrocatalyst (Fe3O4@Fe-Salen-COF) for selectively preparing hydrogen peroxide by controlling the oxygen reduction reaction, and a preparation method (one-pot synthesis technology) thereof, which comprises the following steps: S1: Fe3O4 nanoparticles (10 nm in size) are weighed and added to a Pyrex tube of about 10 mL; 1.5 mL of mesitylene / ethanol (1:1 v / v) dispersant solution is added to the Pyrex tube and ultrasonically treated for 10 minutes, and the Fe3O4 nanoparticles are uniformly ultrasonically treated to obtain the Fe3O4 dispersion.

[0039] S2: Ethylenediamine (0.045 mmol, 5 μL) is added to the Pyrex tube of step S1 and ultrasonically treated for 5 minutes.

[0040] S3: After 1,3,5-tris(4'-hydroxy-5'-formylphenyl)benzene (THB) (0.03 mmol, 13.2 mg) and Zn(OAC)2·2H2O (14.8 mg) were weighed, they were added into the Pyrex tube of step S2, and ultrasonic treatment was performed for 10 minutes.

[0041] S4: After 6M aqueous acetic acid (0.15 mL) was added into the Pyrex tube of step S3, ultrasonic treatment was continued for 5 minutes to ensure uniform dispersion, and a uniformly mixed solution was obtained.

[0042] S5: The Pyrex tube of step S5 was subjected to degassing treatment by three freeze-thaw cycles and flame sealing; the sealed Pyrex tube was placed in an oven at 120°C for 3 days, after the reaction was completed, the Pyrex tube was cooled to room temperature and opened, and a precipitate was obtained.

[0043] S6: After the precipitate was collected by centrifugation, it was washed with DMF, ethanol and acetone, and then dried in an oven at 100°C under vacuum overnight to obtain the product; the product was subjected to a displacement reaction with an iron salt to obtain a composite material in which a metal monatomic covalent organic framework is coated in the form of a shell on the surface of a magnetic substrate Fe3O4 (Fe3O4@Fe-Salen-COF).

[0044] Example 2 differs from Example 1 only in that the size of the Fe3O4 nanoparticles in Example 2 is 50 nm.

[0045] Example 3 differs from Example 1 only in that the size of the Fe3O4 nanoparticles in Example 3 is 100 nm.

[0046] Comparative Example 1 differs from Example 1 only in that the Fe3O4 dispersion liquid is not added into the Pyrex tube, and Fe-Salen-COF is obtained.

[0047] Example 4 differs from Example 1 only in that the product is subjected to a displacement reaction with a cobalt salt to obtain Fe3O4@Co-Salen-COF; Example 5 differs from Example 4 only in that the size of the Fe3O4 nanoparticles in Example 5 is 50 nm.

[0048] Example 6 differs from Example 4 only in that the size of the Fe3O4 nanoparticles in Example 6 is 100 nm.

[0049] Comparative Example 2 differs from Example 4 only in that the Fe3O4 dispersion liquid is not added into the Pyrex tube, and Co-Salen-COF is obtained.

[0050] Example 7 differs from Example 1 only in that the product is subjected to a displacement reaction with a nickel salt to obtain Fe3O4@Ni-Salen-COF.

[0051] Example 8 differs from Example 7 only in that the size of the Fe3O4 nanoparticles in Example 8 is 50 nm.

[0052] Example 9 differs from Example 7 only in that the size of the Fe3O4 nanoparticles in Example 9 is 100 nm.

[0053] Comparative Example 3 differs from Example 7 only in that the Fe3O4 dispersion liquid is not added to the Pyrex tube to obtain Ni-Salen-COF.

[0054] Test Example 1

[0055] This test example uses a CHI760E electrochemical workstation and a rotating disk electrode device to perform tests in a standard three-electrode system.

[0056] The composite material obtained in Example 1 (2 mg) and a perfluoro Nafion resin solution (20 μL, 5 wt%, Shanghai Aldrich Biochemical Technology Co., Ltd., D520CS) were added to a vial containing ethanol and water (volume ratio 1:1, 400 μL), and a catalyst ink slurry was obtained after ultrasonic treatment for 60 minutes. 10 mL of the obtained catalyst ink slurry was uniformly coated on a rotating disk electrode to prepare a working electrode. The same applies to other examples and comparative examples, and will not be repeated here.

[0057] A Pt sheet was used as the counter electrode, Ag / AgCl as the reference electrode, and the electrolyte was 0.1 M KOH aqueous solution saturated with O2, and the process of electrocatalytic oxygen reduction reaction for preparing hydrogen peroxide was carried out.

[0058] The test results are shown in Table 1, Figures 3-6 .

[0059] Table 1

[0060] As can be seen from Table 1, different metals exhibit different performance changes: 1. Strong magnetic metals (Fe, Co) exhibit significant size control effects: 10 nm Fe3O4: H2O2 selectivity > 60% (2-electron pathway dominant); 100 nm Fe3O4: H2O2 selectivity < 45% (4-electron pathway dominant); 2. Weak magnetic metals (Ni) have weak size control effects: H2O2 selective change range: 74.9%→78.1%→70%→68%; The number of electron transfer changes less; 3, regulation sensitivity: Co> Fe> Ni; The above results fully prove that the regulation rule of Fe3O4 size on ORR path is more effective for strong magnetic metal, and the regulation effect is enhanced with the enhancement of metal magnetism, which provides a precise regulation means for selective preparation of H2O2 or realization of 4 electron ORR.

[0061] In addition, it can be known from the results in Table 1 that: When Fe, Co metal monatomic electrocatalysts favorable to 2 electron reaction path are selected to be prepared, the size of Fe3O4 nanoparticles is preferably ≤50 nm; When Fe, Co metal monatomic electrocatalysts favorable to 4 electron reaction path are selected to be prepared, the size of Fe3O4 nanoparticles is preferably ≤100 nm.

[0062] The above has described various embodiments of the present application, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes are obvious to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. An electrocatalyst for selectively preparing hydrogen peroxide by controlling the oxygen reduction reaction pathway, characterized in that, The electrocatalyst is a composite material prepared by reacting Fe3O4 dispersion, amino-containing monomer, aldehyde-containing monomer and zinc salt under catalytic conditions via Schiff base reaction. The product is then subjected to a substitution reaction with a magnetic metal salt to obtain a composite material in which a metal single-atom covalent organic framework is coated on the surface of a magnetic matrix Fe3O4 in the form of a shell. The Fe3O4 dispersion was prepared by mixing Fe3O4 nanoparticles and a dispersing agent, wherein the size of the Fe3O4 nanoparticles was ≤100nm.

2. The electrocatalyst according to claim 1, wherein, The amino-containing monomer is at least one of ethylenediamine, o-phenylenediamine, and 4,5-difluoro-o-phenylenediamine.

3. The electrocatalyst according to claim 1, wherein, The aldehyde-containing monomer is 1,3,5-tris(4'-hydroxy-5'-formylphenyl)benzene.

4. The electrocatalyst according to claim 1, wherein, The metal single atom is at least one of Fe, Co and Ni.

5. The electrocatalyst according to claim 4, wherein, The metal single atom is Fe and / or Co.

6. The electrocatalyst according to claim 1, wherein, The dispersing agent is at least one selected from mesitylene, ethanol, 1,2-dichlorobenzene and n-butanol.

7. The electrocatalyst according to claim 1, wherein, The size of the Fe3O4 nanoparticles is ≤50nm.

8. The electrocatalyst according to claim 7, wherein, The size of the Fe3O4 nanoparticles is ≤10nm.

9. The electrocatalyst according to claim 1, wherein, The preparation method of the electrocatalyst includes the following steps: S1: The Fe3O4 dispersion, amino-containing monomer, aldehyde-containing monomer, zinc salt and catalyst are mixed and subjected to ultrasonic treatment to obtain a homogeneous solution; S2: Degas and seal the equipment containing the homogeneous solution from step S1 using liquid nitrogen; S3: The homogeneous solution in the equipment after step S2 is subjected to a Schiff base reaction to obtain a precipitate, which is then washed and vacuum dried to obtain the product. S4: The product is subjected to a displacement reaction with a magnetic metal salt to obtain the electrocatalyst.

10. The application of the electrocatalyst according to any one of claims 1-9 in the process of preparing hydrogen peroxide by electrocatalytic oxygen reduction reaction.

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