Method for synergistically promoting electroreduction of carbon dioxide on copper surface into C2 product through small molecules and polymer

Abstract

The invention relates to a method for synergistically promoting electroreduction of carbon dioxide on the surface of copper into a C2 product through small molecules and a polymer, in particular to an efficient electrocatalytic carbon dioxide reduction system formed by co-modifying benzotriazole and polyvinylidene fluoride on the surface of copper, and belongs to the technical field of catalytic energy. According to the preparation method, benzotriazole (BTA) and polyvinylidene fluoride (PVDF) are sequentially coated on copper (Cu), so that a Cu-BTA-PVDF catalyst is formed. In a reaction of electrocatalytically reducing carbon dioxide into a C2 product, the small molecules can regulate a surface electronic structure or stabilize a key intermediate, and the polymer can encapsulate the small molecules to prevent the small molecules from falling off and exert an additional effect of the polymer. And the Faraday efficiency is remarkably improved under different overpotentials. After the catalyst is replaced by a copper-silver (CuAg) bimetallic catalyst, BTA and PVDF are sequentially coated, so that the Faraday efficiency of a C2 product is further improved. The smooth implementation of the patent provides a universal, simple, convenient and efficient strategy for regulating and improving the Faraday efficiency (FE) of reducing carbon dioxide into a C2 product through electro-catalysis, solves the problem of unstable structure of the conventional system, and provides greater possibility for industrialization in the field of electro-catalysis carbon dioxide reduction.

Description

Technical Field

[0001] The present invention relates to a method for synergistically promoting the electroreduction of carbon dioxide on the copper surface to C2 products by small molecules and polymers, and particularly to an efficient electrocatalytic carbon dioxide reduction system formed by co-modifying benzotriazole and polyvinylidene fluoride on the copper surface, belonging to the technical field of catalytic energy. Background Art

[0002] In the past few decades, the emissions of carbon dioxide (CO2) have increased sharply, leading to serious environmental problems. A promising solution is to electrochemically convert carbon dioxide into valuable chemicals and fuels using green electricity. In the field of electrocatalytic carbon dioxide reduction reaction (CO2RR), copper (Cu) is considered to be one of the most promising catalysts because it can produce a variety of hydrocarbon species. Compared with CO, hydrocarbon products, especially those containing ≥2 carbon atoms (C 2+ products), are usually more favored because they have higher economic value. To improve the reaction selectivity of CO2RR towards C 2+ species, many efforts have been made. For example, strategies such as alloying, surface doping, ligand modification, and interface engineering have been used to optimize the Faraday efficiency (FE) of C 2+ products. However, some of these strategies are very complex and difficult to control, and there is still great room for improvement in the FE of C2 species.

[0003] In recent years, various polymers (usually hydrophobic) have been coated on the Cu surface to regulate the reaction selectivity, but it is still challenging to find a polymer that can effectively regulate the selectivity of CO2RR, and the chemical synthesis of designing polymers usually involves complex organic synthesis processes and toxic solvents. In addition to polymers, some small molecules can also improve the FE of C2 species in CO2RR by physical or chemical adsorption on the Cu surface. In these reactions, small molecules may regulate the electronic structure of the catalyst surface or the adsorption and desorption of key intermediates, thereby regulating the product selectivity. However, many small molecules cannot be stably adsorbed on the Cu surface, and some small molecules with stable chemisorption may cover some catalytic active sites.

[0004] Based on this, the present invention proposes a method to improve the FE of C2 products in CO2RR on the Cu surface by sequentially coating appropriate small molecules and polymers to produce a synergistic effect. Small molecules can regulate the surface electronic structure or stabilize key intermediates, and polymers can encapsulate small molecules to prevent them from falling off while playing their own additional roles. This strategy can also be applied to other catalytic systems, providing a simple and effective way to develop efficient and stable CO2RR catalysts. Summary of the Invention

[0005] The technical problem solved by the present invention is: a method for synergistically promoting the electroreduction of carbon dioxide on the copper surface to C2 products by small molecules and polymers. The Cu-BTA-PVDF catalyst of the present invention has a Faraday efficiency of up to 70% for C2 at an overpotential of -1.65 V vs. RHE. The coated BTA layer helps localize CO2 molecules and stabilize various intermediates; the PVDF layer prevents BTA molecules from dissolving or detaching and regulates the coverage ratio of *CO / *H, increasing the proportion of ethanol; reduces the reaction energy barrier of CO2RR and accelerates the reaction kinetics; improves the reaction stability of the catalyst; the electrocatalyst Cu-BTA-PVDF of the present invention has better catalytic activity than the latest catalysts reported currently.

[0006] To solve the above technical problems, the technical solution proposed by the present invention is: a method for synergistically promoting the electroreduction of carbon dioxide on the copper surface to C2 products by small molecules and polymers. The small molecules and polymers are sequentially modified on the surface of the electrocatalyst by a coating method to form a small molecule and polymer co-modified electrocatalyst system. Under the applied overpotential, the method for sequentially modifying the small molecules and polymers on the surface of the catalyst to promote the electrocatalytic reduction of carbon dioxide to C2 products is as follows: Dissolve different masses of benzotriazole and polyvinylidene fluoride in DMF respectively to obtain two molecular coating layers with different thicknesses (30 nm - 1 μm) during drop coating. The above small molecules and polymers are sequentially coated on the surface of copper particles obtained by an electrodeposition method to form a composite material with a co-modified catalytic surface of small molecules and polymers. The electrocatalytic carbon dioxide reduction reaction experiment is carried out in a 60 mL H-type reactor, and the overpotential is provided by an electrochemical workstation.

[0007] Preferably, the preparation process of the Cu-BTA-PVDF catalyst: Drop the DMF solution of BTA on the surface of the copper electrode by a drop coating method (volume 50 μL, thickness 160 nm). Then, further coat a layer of PVDF with a thickness of 1 μm by a similar method. The coated layer is annealed at 135 °C for 0.5 h to finally obtain the Cu-BTA-PVDF catalyst.

[0008] Preferably, the preparation process of the Cu catalyst: Prepare Cu particles by an electrochemical deposition method in a three-electrode system. Use 60 mL of 0.2 M Cu(NO3)2 solution as the electrolyte. Use carbon paper (SCP 130) as the working electrode for depositing Cu particles. The deposition voltage is -1.0 V vs. Ag|AgCl, and the electrochemical deposition usually lasts for 40 s.

[0009] Preferably, the process of the electrocatalytic carbon dioxide reduction experiment is as follows: A standard H-type electrolytic cell (double-chamber design, single-chamber volume 60 mL) is used, and the experimental system is controlled by an electrochemical workstation (CHI 630E, Shanghai Chenhua Instrument). The test system adopts a three-electrode configuration: the catalyst-modified electrode is the working electrode, and the platinum wire electrode and the Ag|AgCl electrode are used as the counter electrode and the reference electrode, respectively. To isolate the anode and cathode chambers, a Nafion N117 perfluorosulfonic acid proton exchange membrane (product of DuPont, USA) is used for separation. In the experiment, 0.1 M potassium sulfate (K2SO4) solution is selected as the electrolyte, and high-purity CO2 gas is continuously introduced for 15 minutes before the test to ensure that the solution is saturated. During the formal test, 25 mL of the CO2-saturated electrolyte is injected into each chamber of the electrolytic cell, and then a preset potential is applied for electrochemical performance characterization. After the reaction for 30 min, gas analysis is carried out using an off-line gas chromatograph (GC-9860 5CNJ, Nanjing Haerpu Analytical Equipment Co., Ltd.), and the liquid products are detected and analyzed using nuclear magnetic resonance spectroscopy (NMR) (JNM-ECZ400S / L1, JEOL, Japan).

[0010] Advantages of the present invention:

[0011] 1. The coating of BTA reduces the *CO dimerization energy barrier by stabilizing the *COOH intermediate, promoting C-C coupling. The coating of PVDF inhibits the diffusion of H2O molecules, regulates the coverage ratio of *CO / *H, promotes the formation of specific C2 products, and simultaneously fixes the BTA molecules to prevent shedding.

[0012] 2. The method of sequentially coating BTA and PVDF is very simple and convenient, making this composite material have great potential value in many practical applications. The successful implementation of this patent will provide a simple and efficient strategy for enhancing the activity of the electrocatalytic carbon dioxide reduction reaction, and solve the problems that the previous system relied on the design of functionalized polymers in the organic synthesis process and small molecules would cover some catalytic active sites, reducing the preparation difficulty of this type of catalyst and providing the possibility for efficient and stable electrocatalytic carbon dioxide reduction reaction.

[0013] 3. The Cu-BTA-PVDF prepared by this patent has a Faraday efficiency of C2 as high as 70% at an overpotential of 1.65 V vs. RHE. In addition, the catalyst is stable after 10 hours of catalytic cycling, greatly improving the stability of the original electrocatalyst. The catalytic activity and stability of our composite catalyst are superior to the reported single small molecule or polymer-modified copper-based catalysts.

[0014] 4. The method of this patent can also be combined with other strategies, such as the bimetallic strategy (CuAg), to further improve the C2 selectivity in CO2RR to 78%. Description of the Drawings

[0015] The present invention will be further described below with reference to the accompanying drawings.

[0016] Figure 1 It is a schematic diagram of Cu-BTA-PVDF enhancing electrocatalytic CO2RR.

[0017] Figure 2 It is the morphological characterization of Cu-BTA-PVDF. (a-c) Top view and cross-sectional SEM images of Cu NPs coated with BTA and PVDF layers; (d) WCA measurement of bare Cu and Cu NPs coated with BTA and PVDF layers.

[0018] Figure 3 It is the properties of bare Cu and Cu-BTA. (a) Cu 2p XPS spectra of bare Cu and Cu coated with BTA molecules; (b) N1s XPS spectra of Cu coated with BTA molecules.

[0019] Figure 4 It is (a) the variation of the output current density of Cu-BTA and Cu-BTA-PVDF within 60 min; (b) the FE of H2, C1, and C2 products, and the FE ratio of ethanol to ethylene on bare Cu, Cu-PVDF, Cu-BTA, and Cu-BTA-PVDF.

[0020] Figure 5 It is (a-b) the FE of H2, C1, and C2 products on bare Cu and Cu-BTA-PVDF at different potentials in 0.1 M K2SO4; (c-d) the FE ratio of C2H5OH to C2H4 on bare Cu NPs and Cu-BTA-PVDF at different potentials.

[0021] Figure 6 It is (a-b) the FE distribution of each product on CuAg particles and CuAg-BTA-PVDF at different potentials in 0.1 M K2SO4 electrolyte; (c-d) the FE of H2, C1, and C2 products on CuAg particles and CuAg-BTA-PVDF at different potentials in 0.1 M K2SO4 electrolyte.

[0022] Figure 7 It is (a) the molecular structure of the selected molecule; (b) the FE of electrocatalytic CO2RR on Cu coated with the selected molecule and PVDF in sequence; (c) the molecular structure of the selected polymer; (d) the FE of electrocatalytic CO2RR on Cu coated with BTA and the selected polymer in sequence.

[0023] Figure 8is (a) the FE of different products during CO2RR on BTA- and PVDF-coated Cu at different BTA thicknesses; (b) the FE of C2 products at different BTA thicknesses; (c) the FE of different products during CO2RR on BTA- and PVDF-coated Cu at different PVDF thicknesses; (d) the FE of C2 products at different PVDF thicknesses. Detailed implementation mode

[0024] Example 1

[0025] First, Cu particles were prepared by an electrochemical deposition method in a three-electrode system. A 60 mL 0.2 M Cu(NO3)2 solution was used as the electrolyte. A carbon paper (SCP 130) was used as the working electrode for depositing Cu particles. The deposition voltage was -1.0 V vs. Ag|AgCl, and the electrochemical deposition usually lasted for 40 s.

[0026] A DMF solution of BTA was drop-coated onto the surface of the copper electrode (volume 50 μL, thickness 160 nm) by the drop-coating method. Then, a layer of PVDF with a thickness of 1 μm was further coated in a similar manner. The coated layer was annealed at 135 °C for 0.5 h to finally obtain the Cu-BTA-PVDF catalyst.

[0027] Our electrocatalytic carbon dioxide reduction experiment process is as follows:

[0028] It was carried out using a standard H-type electrolytic cell (double-chamber design, single-chamber volume 60 mL), and the experimental system was controlled by an electrochemical workstation (CHI 630E, Shanghai Chenhua Instrument). The test system adopted a three-electrode configuration: the catalyst-modified electrode was the working electrode, and the platinum wire electrode and the Ag / AgCl electrode were used as the counter electrode and the reference electrode, respectively. To isolate the anode and cathode chambers, a Nafion N117 perfluorosulfonic acid proton exchange membrane (product of DuPont Company, USA) was used for separation. A 0.1 M potassium sulfate (K2SO4) solution was selected as the electrolyte for the experiment, and high-purity CO2 gas was continuously introduced for 15 minutes before the test to ensure that the solution was saturated. During the formal test, 25 mL of the CO2-saturated electrolyte was injected into each chamber of the electrolytic cell, and then a preset potential was applied for electrochemical performance characterization. After the reaction for 30 min, gas analysis was carried out using an off-line gas chromatograph (GC-9860 5CNJ, Nanjing Haerpu Analytical Equipment Co., Ltd.).

[0029] As Figure 1 is a schematic diagram of the enhanced electrocatalytic CO2RR of Cu-BTA-PVDF. The synergistic effect of BTA and PVDF improves the reaction activity of CO2RR.

[0030] As Figure 2It is the morphological characterization of Cu-BTA-PVDF. First, we characterized the obtained Cu-BTA-PVDF catalyst by SEM and water contact angle measurement. After coating with BTA and PVDF, the Cu particles were covered under the organic layers of BTA and PVDF, and the total thickness of these layers was about 1 μm. Then, we evaluated the change in surface properties after coating the organic layers by measuring the water contact angle (WCA). The measurement results showed that the WCA of the bare Cu NPs substrate was 26.5°. After coating with the BTA and PVDF layers, the WCA increased to 97.2°, indicating an increase in the hydrophobicity of the substrate, forming a hydrophobic barrier that restricted the diffusion of water molecules to the electrode surface.

[0031] Such as Figure 3 It is the surface structure analysis of Cu-BTA-PVDF. The interaction between Cu NPs and the coated BTA molecules was characterized by XPS. Due to the limited penetration depth of X-rays in XPS, here we only analyzed and compared the differences between Cu coated with BTA and bare Cu. For bare Cu NPs, the binding energies of the Cu 2p spectra in the XPS characterization showed two main peaks, located at 932.9 eV and 952.8 eV respectively, which were attributed to Cu 0 and Cu + . In addition, the satellite peaks of Cu 2+ and Cu 2+ also appeared at 934.7 eV and 943.7 eV, indicating that Cu was oxidized in the atmosphere. After coating with BTA, the relative intensity of the Cu 0 / Cu + peak increased, while the intensities of the Cu 2+ and Cu 2+ satellite peaks decreased. According to the literature, the intensity change of the Cu 2+ satellite peak helps to reveal the oxidation state of Cu. These changes indicate that coating with BTA can prevent Cu atoms from being oxidized in the atmosphere. In addition, the position of the Cu 0 / Cu + peak shifted to a lower binding energy by about 0.2 eV, suggesting a charge transfer between the adsorbed BTA molecules and Cu atoms at the Cu-BTA interface. This transfer was also confirmed by the N1s XPS spectrum of BTA. Compared with the bare BTA molecule, the position of this peak on Cu-BTA shifted significantly to a higher binding energy by about 1.68 eV, reaching 399.78 eV.

[0032] Such as Figure 4It is the synergistic contribution of the BTA layer and the PVDF layer in CO2RR. If only BTA molecules are coated on Cu, the output current changes significantly in the i-t curve, especially within the first 20 min, indicating that the coated molecules are prone to detachment or dissolution during the electrified reaction process. In contrast, when a PVDF layer is further coated on the BTA layer, the output current can remain stable, indicating that the PVDF layer helps prevent the detachment or dissolution of BTA molecules. The coating of BTA and PVDF not only enhances the catalytic activity of CO2RR on Cu but also changes the selectivity of the reaction products. Cu NPs, Cu-PVDF, Cu-BTA, and Cu-BTA-PVDF were applied to electrocatalytic CO2RR, and their FEs were compared. The FEs of gaseous and liquid products were evaluated using GC and NMR, respectively. As a control, the FE of C2 species on bare Cu was approximately 49%. When the Cu surface was coated with BTA molecules, the FE of C2 species increased to approximately 59% (including C2H4 and C2H5OH), indicating a positive contribution of the BTA coating to C-C coupling. When the Cu surface was coated with both BTA and PVDF simultaneously, the FE of C2 species increased to as high as approximately 70%, which was significantly higher than that of the samples coated with only BTA or PVDF, indicating a synergistic effect of the BTA and PVDF coatings. In addition, the PVDF coating can also increase the ratio of ethanol to ethylene on Cu. When a BTA layer was coated on Cu, the ratio of ethanol to ethylene increased slightly from 1.33 to 1.46. For the Cu-PVDF and Cu-BTA-PVDF samples, this ratio increased significantly to 1.68 and 1.73, respectively.

[0033] As Figure 5 Figure 1 shows the changes in FEs of Cu-BTA-PVDF compared to bare copper at different potentials. On the bare Cu surface, the FE of C2 products was approximately 49%, which was obtained at a potential of -1.05 V vs. RHE. In contrast, the FE of C2 products on the Cu-BTA-PVDF surface increased to approximately 70%, which was 21% higher than that on bare Cu. In addition, the ratio of ethylene to ethanol on the Cu-BTA-PVDF sample also increased. As the applied potential increased from -1.45 V vs. RHE to -0.95 V vs. RHE, this ratio increased even more. At the optimal potential for C2 production of -1.05 V vs. RHE, the ratio of ethylene to ethanol increased from 1.28 on bare Cu to 1.85 on Cu-BTA-PVDF. These changes are obviously related to the reaction mechanism, probably because the hydrophobic polymer layer leads to a weakened *H adsorption, thus inhibiting the formation of C2H4.

[0034] The method of this system can also be combined with other strategies: such as Figure 6As shown, CuAg bimetallic particles were prepared by electrochemical co - deposition, and then BTA and PVDF were coated successively to improve the electrocatalytic performance of CO2RR. The research shows that at a potential of - 1.05 V vs. RHE, the FE of C2 products for CuAg particles is about 62%. When the surface was coated with BTA and PVDF, the FE of C2 products further increased to about 78%, an 8% increase compared to Cu - BTA - PVDF. This result indicates that the synergistic strategy of small molecules and polymers can be combined with other strategies to further enhance the catalytic activity of CO2RR.

[0035] Comparative Example 1

[0036] To study the contributions of BTA and PVDF, we prepared similar catalysts by replacing BTA or PVDF with other small molecules or polymers. According to the literature, nitrogen - containing molecules can usually significantly improve the catalytic performance by regulating the electronic structure and micro - environment. Therefore, we selected some typical nitrogen - containing molecules. First, the BTA layer was replaced with sulfanilamide (SA), 4 - iodopyridine (4 - IP), 4 - methoxypyridine (4 - MP), 4 - aminopyridine (4 - AP), 1,2,4 - triazole (TA), or 4 - hydroxypyridine (4 - HP). As Figure 7 shown, compared with bare Cu, the FE of C2 products of some samples increased significantly (although not reaching the level of BTA), while the FE of other samples decreased, indicating the importance of molecule selection. These results show that the unique molecular structure of BTA plays an important role in CO2 conversion, which will be further discussed later. We also studied the contribution of the PVDF coating by replacing PVDF with other polymers with similar structures to PVDF, such as polystyrene (PS) and polyvinyl chloride (PVC). Both coatings on the BTA layer increased the FE of C2 products on the Cu surface, but neither reached the level of PVDF, and the proportion of C1 in the carbon products was higher. Therefore, compared with the coatings on bare Cu, the polymer coatings on BTA have different effects. These results also show that PVDF plays a unique role when coated on the Cu surface together with BTA.

[0037] Comparative Example 2

[0038] In the electrocatalytic CO2RR, the thickness of the BTA and PVDF layers may significantly affect the selectivity of electrocatalytic CO2RR. As Figure 8As shown, by changing the thickness of these two layers, the effect of their thickness on the FE of electrocatalytic CO2RR was systematically studied. Among them, the optimized thickness of the BTA layer is 160 nm, which may be related to its role in regulating charge transfer or stabilizing key intermediates at the catalytic interface. A thicker BTA layer may enhance the interface stability, but an overly thick layer may lead to an increase in mass transfer resistance, while an overly thin layer may not uniformly enhance the catalytic activity of the surface Cu. For the PVDF layer, its hydrophobic property is crucial for CO2 mass transfer. When the thickness of the PVDF layer is 1 μm, it can not only maintain sufficient gas channels to promote CO2 diffusion but also avoid insufficient hydrophobicity caused by an overly thin layer or mass transfer limitations caused by an overly thick layer. Therefore, the final optimized thickness of the BTA layer is 160 nm, and the thickness of the PVDF layer is 1 μm.

[0039] The present invention is not limited to the specific technical solutions described in the above embodiments. Any technical solutions formed by equivalent substitution are within the scope of protection required by the present invention.

Claims

1. A method for synergistically promoting the electroreduction of carbon dioxide to C2 products on the copper surface by small molecules and polymers, characterized in that: The small molecule and the polymer are successively modified on the surface of the electrocatalyst by the coating method to form an electrocatalyst system co-modified with the small molecule and the polymer. Under the applied overpotential, the method for successively modifying the small molecule and the polymer on the catalyst surface to promote the electrocatalytic reduction of carbon dioxide to C2 products is as follows: Different masses of benzotriazole (BTA) and polyvinylidene fluoride (PVDF) are respectively dissolved in N,N-dimethylformamide (DMF) to obtain molecular and polymer coating layers with different thicknesses (30 nm - 1 μm) during drop coating. The above-mentioned small molecule and polymer are successively coated on the surface of copper particles obtained by the electrodeposition method to form a composite material with a catalytic surface co-modified by the small molecule and the polymer. The electrocatalytic carbon dioxide reduction reaction experiment is carried out in a 60 mL H-type reactor, and the overpotential is provided by an electrochemical workstation.

2. The method for synergistically promoting the electroreduction of carbon dioxide on the copper surface to C2 products by small molecules and polymers according to claim 1, wherein: 50 μL of the small molecule and the polymer are respectively taken and successively coated on the surface of the electrocatalyst, and annealed at 135 °C for 0.5 h. The electrocatalytic carbon dioxide reduction reaction experiment is carried out in a 60 mL H-type reactor.

3. The method for synergistically promoting the electroreduction of carbon dioxide on the copper surface to C2 products by small molecules and polymers according to claim 1, characterized in that: The small molecule and the polymer are co-modified on the surface of the copper-based catalyst by the successive coating method.

4. The method for electrochemically reducing carbon dioxide to C2 products on the copper surface by synergistically promoting small molecules and polymers according to claim 1, wherein: First, a copper-based catalyst is prepared by the electrochemical deposition method. Subsequently, a DMF solution of BTA is drop-coated on the surface of the copper electrode (volume 50 μL, thickness 160 nm) by the drop coating method. Then, a layer of PVDF with a thickness of 1 μm is further coated in a similar manner. The coated layer is annealed at 135 °C for 0.5 h.

5. The method for synergistically promoting the electroreduction of carbon dioxide to C2 products on the copper surface by small molecules and polymers according to claim 1, characterized in that: The small molecule is BTA, the polymer is PVDF, and the main electrocatalyst is a copper-based catalyst.

6. The method for synergistically promoting the electroreduction of carbon dioxide on the copper surface to C2 products by small molecules and polymers according to claim 1, characterized in that: Cu-BTA-PVDF is finally loaded on carbon paper for the reaction.

7. The method for synergistically promoting the electroreduction of carbon dioxide on the copper surface to C2 products by small molecules and polymers according to claim 1, wherein: The process of the electrocatalytic carbon dioxide reduction experiment is as follows: It is carried out using a standard H-type electrolytic cell (double-chamber design, single-chamber volume 60 mL), and the experimental system is controlled by an electrochemical workstation (CHI 630E, Shanghai Chenhua Instrument). The test system adopts a three-electrode configuration: the catalyst-modified electrode is the working electrode, and the platinum wire electrode and the Ag|AgCl electrode are used as the counter electrode and the reference electrode respectively. To isolate the anode and cathode chambers, a Nafion N117 perfluorosulfonic acid proton exchange membrane (product of DuPont Company, USA) is used for separation. 0.1 M potassium sulfate (K2SO4) solution is selected as the electrolyte for the experiment, and high-purity CO2 gas is continuously introduced for 15 minutes before the test to ensure the saturation of the solution. During the formal test, 25 mL of the CO2-saturated electrolyte is injected into each chamber of the electrolytic cell, and then a preset potential is applied for electrochemical performance characterization. After the reaction for 30 min, gas analysis is carried out using an off-line gas chromatograph (GC-9860 5CNJ, Nanjing Haerpu Analytical Equipment Co., Ltd.), and the liquid products are detected and analyzed using nuclear magnetic resonance spectroscopy (NMR) (JNM-ECZ400S / L1, JEOL, Japan).

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