Preparation method and application of catalyst for preparing formic acid from CO2 under high current density

Abstract

The invention discloses a preparation method and application of a catalyst for preparing formic acid from CO2 under high current density, and belongs to the field of CO2 electroreduction. According to the catalyst, InO (OH) is used as a substrate, ionic liquid is introduced to modify the substrate, and the ionic liquid is prepared from corresponding chlorinated quaternary phosphonium salt and trifluoromethanesulfonamide through a reaction. Wherein quaternary phosphonium cations can form a local electric field at a cathode in CO2 electroreduction, a reaction intermediate of formic acid is stably generated, and the product selectivity is greatly improved. Anions have very strong hydrophobicity, remarkably inhibit hydrogen evolution side reaction, have the effect of enriching CO2, and can form a high-concentration CO2 local microenvironment. Under the synergistic effect of anions and cations, the problems that an indium-based catalyst is weak in CO2 adsorption capacity, low in catalytic activity and the like are effectively relieved, and the catalyst still has good formic acid Faraday efficiency under the ampere-level current density. The preparation method is simple and easy to popularize.

Description

Technical Field

[0001] This invention relates to the field of electrocatalytic CO2 reduction, specifically to a method for preparing and modifying an indium-based catalyst modified with an ionic liquid and a high-performance electrocatalytic reduction of CO2 to formic acid, and its application. Background Technology

[0002] Since the Industrial Revolution, human society's dependence on fossil fuels has led to a sharp increase in the concentration of greenhouse gases such as CO2 in the atmosphere, triggering a series of severe global environmental crises, including global warming, frequent extreme weather events, and rising sea levels. Against this backdrop, electrocatalytic CO2 reduction, as a low-carbon technology that can achieve large-scale carbon emission reduction while continuing the use of fossil fuels, possesses both profound scientific significance and broad application prospects.

[0003] Among the many products of CO2 electroreduction, formic acid is not only an important liquid fuel and chemical feedstock, but also a safe liquid hydrogen carrier, possessing high economic value and convenient storage and transportation. As one of the key research directions in the current field of electrocatalytic CO2 reduction, indium-based materials, due to their unique electronic structure and surface properties, exhibit excellent intrinsic selectivity for the formic acid formation pathway, and have become a core catalyst system of great interest in this field. The indium hydroxyoxide used in this invention is a highly representative indium-based catalytic material.

[0004] Ionic liquids are green solvents with multiple advantages, including high conductivity, a wide electrochemical window, high CO2 solubility, and tunable cation and anion ratios. They have been developed as electrolytes to enhance catalytic performance in CO2 reduction reactions. However, their high viscosity, high cost, and mass transfer limitations remain key bottlenecks restricting their practical application. Combining ionic liquids with catalysts in the form of modification layers or composite supports holds promise for further improving their activity and selectivity in the electrocatalytic CO2 reduction process, and is one of the promising research strategies. Summary of the Invention

[0005] This invention provides a method for preparing InO(OH) modified with a functionally specific ionic liquid and using it for the electrocatalytic reduction of CO2 to formic acid. This method significantly improves catalytic activity while avoiding the drawbacks of using ionic liquids as electrolytes.

[0006] Traditional catalysts often suffer from numerous problems, including difficulty in activation, severe competition for hydrogen evolution reactions, and limited selectivity at high current densities. In particular, indium-based catalysts exhibit inherently weak CO2 adsorption affinity, hindering industrial applications. This invention aims to overcome the shortcomings of existing technologies by synthesizing a typical indium-based material, InO(OH), through the introduction of a functionally specific ionic liquid, preferably tributylphosphonium bis(trifluoromethanesulfonyl)imide ([P... 4441[NTf2]), Tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide ([P 4444 [NTf2]), Tributyl(2-methoxyethyl)phosphonium bis(trifluoromethanesulfonyl)imide ([P 444(2OMe) The catalyst is surface modified by using one or more of [NTf2], and the modified catalyst shows significant improvement in formic acid selectivity and formic acid Faraday efficiency at high current density.

[0007] A method for preparing a catalyst for the electrocatalytic reduction of CO2 to formic acid under high current density includes the following steps:

[0008] (1) Preparation of ionic liquid: Mix the corresponding quaternary phosphonium chloride salt and the corresponding trifluoromethanesulfonamide in a certain proportion and react them. Separate the organic phase, wash it with water, and then dry it under vacuum to obtain the desired ionic liquid; or purchase it directly.

[0009] (2) Weigh indium nitrate and dissolve it in anhydrous ethanol. Stir until homogeneous to form solution A. Slowly add ethylene glycol to solution A and continue stirring until the solution becomes transparent to form solution B. Transfer solution B to a stainless steel autoclave for solvothermal reaction. After the reaction, the catalyst InO(OH) is obtained through treatment.

[0010] (3) Mix the ionic liquid with the catalyst InO(OH) in step (2) at a certain mass fraction, then add a small amount of isopropanol to obtain a mixture, stir evenly, and then dry to remove the isopropanol solvent to obtain the ionic liquid modified catalyst.

[0011] Further, in step (1), the quaternary phosphonium chloride salt includes one or more of tributylmethylphosphonium chloride, tetrabutylphosphonium chloride, and tributyl(2-methoxyethyl)phosphonium chloride. The molar ratio of the quaternary phosphonium chloride salt to trifluoromethanesulfonamide is 1:2. The mixture is stirred at 60°C for 10 h, the product is collected, and then vacuum dried at 80°C to obtain the desired quaternary phosphonium salt ionic liquid.

[0012] Furthermore, the volume ratio of anhydrous ethanol to ethylene glycol used in step (2) is 2:1.

[0013] The condition for solution A to become homogeneous is to stir continuously for 30-60 minutes, and the condition for solution B to become transparent is to stir continuously for 30-60 minutes.

[0014] The solvothermal reaction was carried out by heating at 160°C for 5 hours.

[0015] The post-reaction treatment includes: centrifugation followed by alternating washing with anhydrous ethanol and deionized water, and then drying in a vacuum drying oven.

[0016] Furthermore, the ionic liquid added in step (3) is 20-40 wt.% of the mass of the ionic liquid-modified catalyst, preferably 30 wt.%.

[0017] The small amount of isopropanol added to the mixture is just enough to mix the ionic liquid and InO(OH) evenly. The condition for even mixing is to stir continuously for 20-24 hours.

[0018] The prepared catalyst was used for the electrocatalytic reduction of formic acid by CO2.

[0019] Furthermore, the CO2 electrocatalytic reduction system is carried out in a flowing electrolyzer, wherein the cathode of the electrolyzer employs a commercially available gas diffusion layer and is then loaded with the ionic liquid-modified catalyst described in this invention. A silver-silver chloride electrode serves as the reference electrode, a platinum sheet as the anode, and the electrolyte is an alkaline electrolyte, such as a 1 M KOH solution. The ion exchange membrane is an anion exchange membrane FAA-PK-130. During the electrocatalytic CO2 reduction process, CO2 gas is continuously introduced into the cathode at an ampere-level current density (-0.8 A∙cm⁻¹). -2 ~-1.4A∙cm -2 All of them have excellent catalytic activity.

[0020] The beneficial effects of this invention are:

[0021] The quaternary phosphonium salt ionic liquid system used in this invention is obtained by reacting quaternary phosphonium chloride with trifluoromethanesulfonamide, which is environmentally friendly and low-cost. The quaternary phosphonium cation forms a local electric field at the cathode during CO2 electroreduction, stabilizing the reaction intermediate that generates formic acid and improving product selectivity. The anion has strong hydrophobicity, significantly inhibiting the hydrogen evolution side reaction and enriching CO2, forming a high-concentration CO2 local microenvironment. The synergistic effect of the cation and anion achieves excellent catalytic activity even at ampere-level current densities.

[0022] In this invention, an environmentally friendly indium salt is selected as the catalyst substrate, and an indium-based catalyst modified with a quaternary phosphonium salt ionic liquid is prepared by impregnation. The synthesis is simple and easy to scale up for production.

[0023] The indium-based catalyst modified with quaternary phosphonium salt ionic liquid used in this invention performs electrocatalytic CO2 reduction reaction in a flowing electrolytic cell. The reaction equipment is operated at ambient temperature and pressure, which can reduce CO2 to formic acid and maintain high Faradaic efficiency at ampere-level current density, which helps to promote the industrial application of CO2 electroreduction. Attached Figure Description

[0024] Figure 1 The quaternary phosphonium salt ionic liquid [P] used in Example 1 4441 The structural formula of [NTf2] and its corresponding 400M NMR hydrogen spectrum.

[0025] Figure 2 For example 1, the indium catalytic substrate InO(OH) and [P 4441 Indium catalyst modified with [NTf2]InO(OH)@[P 4441 XRD pattern of [NTf2].

[0026] Figure 3 For example, InO(OH) and InO(OH)@[P 4441 SEM and EDS plots of [NTf2].

[0027] Figure 4 For example, InO(OH) and InO(OH)@[P 4441 Fourier infrared image of [NTf2].

[0028] Figure 5 For example, InO(OH) and InO(OH)@[P 4441 XPS graph of [NTf2].

[0029] Figure 6 For example, InO(OH) and InO(OH)@[P 4441 LSV curve of [NTf2]

[0030] Figure 7 For example, InO(OH) and InO(OH)@[P 4441 EIS impedance diagram of ][NTf2].

[0031] Figure 8 For example, InO(OH) and InO(OH)@[P 4441 The formic acid Faraday efficiency corresponding to the constant current electrolysis test of [NTf2].

[0032] Figure 9 A comparison of the Faradaic efficiencies of formic acid modified with InO(OH) as the substrate and various quaternary phosphonium salt ionic liquids. Detailed Implementation

[0033] The present invention will be described in detail below through specific examples. However, the purpose and use of these embodiments are only to illustrate the present invention and do not constitute any limitation on the actual protection scope of the present invention, nor are they intended to limit the protection scope of the present invention to these embodiments.

[0034] Example 1:

[0035] In this embodiment, the quaternary phosphonium salt ionic liquid selected is [P 4441 [NTf2].

[0036] The specific catalyst preparation steps are as follows:

[0037] First, dissolve 10 mmol of trifluoromethanesulfonamide in water and stir until dissolved. Separately, take a 50 mL beaker and add 5 mmol of tributylmethylphosphonium chloride. Maintain a water bath temperature of 60°C and heat the beaker. While stirring continuously, slowly and evenly add the trifluoromethanesulfonamide solution dropwise into the beaker. Continue stirring at 60°C for 10 h, collect the product, and then vacuum dry at 80°C for 24 h to obtain the desired ionic liquid [P]. 4441 [NTf2].

[0038] 2 mmol of indium nitrate hydrate was dissolved in 34 mL of anhydrous ethanol and stirred for 1 h until completely dissolved. Then, 17 mL of ethylene glycol was added and stirring was continued for 1 h. The resulting solution was transferred to a 100 mL stainless steel reactor and hydrothermally reacted at 160 °C for 5 h. The precipitate was collected by centrifugation and washed several times with anhydrous ethanol and deionized water alternately. Then, it was vacuum dried overnight at 60 °C to obtain the indium catalytic substrate InO(OH).

[0039] Mix 70 mg InO(OH) with 30 mg [P] 4441 [NTf2] was co-dispersed in 2.2 mL of isopropanol and stirred at room temperature for 20 h, followed by vacuum drying at 60 °C for 5 h to obtain the ionic liquid-modified catalyst InO(OH)@[P 4441 [NTf2].

[0040] The above catalyst was structurally characterized, and the results are as follows: Figures 1-5 As shown. Figure 1 The 400M 1H NMR spectrum showed that [P 4441 Successful synthesis of [NTf2]. Figure 2 The XRD pattern indicates that the indium catalyst has an amorphous structure. Furthermore, InO(OH)@[P 4441 The spectrum of [NTf2] is similar to that of InO(OH), indicating that the addition of ionic liquid modification did not change the crystal structure of InO(OH). Figure 3 The SEM and EDS results show that F, P, S, and N elements are uniformly distributed in the modified catalyst, proving that [P 4441 Successful modification of ][NTf2]. Figure 4 The Fourier transform infrared spectrum shows that the region is located at 2850-3000 cm⁻¹. -1 and 1350 / 1160cm -1 The peaks appearing within correspond to [P] 4441 The saturated alkyl group CH and sulfonyl group -SO2- in [NTf2] further prove that [P 4441 [NTf2] was successfully modified onto InO(OH). Figure 5The XPS plots show that the O 1s spectrum of the unmodified indium-based catalyst can be deconvoluted into three peaks with binding energies at 530.1 eV, 531.9 eV, and 533.4 eV, respectively. Among them, the hydroxyl oxygen at 531.9 eV is dominant, and its intensity is much higher than that of the lattice oxygen at 530.1 eV. This is consistent with the chemical structure characteristics of InO(OH). Combined with the XRD results, it is sufficient to prove that the prepared material is amorphous InO(OH).

[0041] Electrochemical tests were performed on the catalyst synthesized above.

[0042] The gas diffusion layer used was the commercially available YLS-30T. The catalyst ink was prepared as follows: 12 mg of catalyst, 2.4 mL of a 1:1 mixture of isopropanol and deionized water, and 42 μL of Nafion D521 were mixed and sonicated for 30 min. Under a heat lamp, the ink was uniformly drop-coated onto the YLS-30T to obtain the catalyst-loaded gas diffusion electrode. The electrolytic cell was a three-electrode flow electrolytic cell system, including a PEEK end plate, a PTFE gasket, and an FAA-PK-130 anion exchange membrane. The anode was a 2×3 cm anode. 2 The platinum sheet and the anode and cathode electrolytes are both 30 mL of 1 M KOH in circulation. The reference electrode is Ag / AgCl, and it is used with a salt bridge containing 3.5 M KCl.

[0043] First, the LSV curves of the catalyst before and after modification were tested, and the results are as follows: Figure 6 As shown, under a CO2 atmosphere, compared to InO(OH), InO(OH)@[P 4441 The current density of [NTf2] increases significantly, indicating that InO(OH)@[P 4441 [NTf2] exhibits stronger catalytic activity at the same potential. Furthermore, the LSV curve under Ar atmosphere shows that the HER activity of the catalyst modified with ionic liquid is significantly reduced. Figure 7 The EIS impedance spectra of the catalysts before and after modification show that the modified catalyst has lower charge transfer resistance, higher interfacial conductivity, and superior catalytic kinetics. Figure 8 -0.8A∙cm -2 ~-1.4A∙cm -2 The formic acid faradaic efficiency was measured by galvanostatic testing, and the results showed that, compared with unmodified InO(OH), InO(OH)@[P 4441 The Faraday efficiency of electrocatalytic CO2 production of formic acid by [NTf2] is significantly enhanced, and is within -1.2 A∙cm⁻¹. -2 It still maintains nearly 90% of the formic acid faradaic efficiency even at industrial-grade current densities.

[0044] Example 2:

[0045] In this embodiment, the quaternary phosphonium salt ionic liquid selected is [P 4444 [NTf2].

[0046] Replacing tributylmethylphosphonium chloride used in Example 1 with tetrabutylphosphonium chloride, while keeping other conditions unchanged, yields the ionic liquid [P]. 4444 [NTf2], ionic liquid-modified catalyst InO(OH)@[P] was prepared according to the subsequent steps in Example 1. 4444 [NTf2] was used, and electrochemical tests were performed.

[0047] Example 3:

[0048] In this embodiment, the quaternary phosphonium salt ionic liquid selected is [P 444(2OMe) [NTf2].

[0049] Replacing the tributylmethylphosphonium chloride used in Example 1 with tributyl(2-methoxyethyl)phosphonium chloride, while keeping all other conditions unchanged, yields the ionic liquid [P]. 444(2OMe) [NTf2], ionic liquid-modified catalyst InO(OH)@[P] was prepared according to the subsequent steps in Example 1. 444(2OMe) [NTf2] was used, and electrochemical tests were performed. Figure 9 The chart shows a comparison of the Faradaic efficiencies of formic acid produced from InO(OH)₂ and various quaternary phosphonium salt ionic liquids. The results indicate that the Faradaic efficiency of the electrocatalytic CO₂ to formic acid production is significantly enhanced after modification with the aforementioned ionic liquids. Specifically, the Faradaic efficiency of InO(OH)₂@[P]₂ is significantly higher. 444(2OMe) [NTf2] at -1.2A∙cm -2 It can achieve over 95% at current densities.

Claims

1. A catalyst for the production of formic acid from CO2 at high current density, characterized in that, InO(OH) modified with an ionic liquid, wherein the ionic liquid is selected from tributylphosphonium bis(trifluoromethanesulfonyl)imide ([P 4441 [NTf2]), Tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide ([P 4444 [NTf2]), Tributyl(2-methoxyethyl)phosphonium bis(trifluoromethanesulfonyl)imide ([P 444(2OMe) One or more of [NTf2]).

2. The method for preparing a catalyst for the production of formic acid from CO2 at high current density as described in claim 1, characterized in that, Includes the following steps: (1) Preparation of ionic liquid: Mix the corresponding quaternary phosphonium chloride salt and the corresponding trifluoromethanesulfonamide in a certain proportion and react them. Separate the organic phase, wash it with water, and then dry it under vacuum to obtain the desired ionic liquid; or purchase it directly. (2) Weigh indium nitrate and dissolve it in anhydrous ethanol. Stir until homogeneous to form solution A. Slowly add ethylene glycol to solution A and continue stirring until the solution becomes transparent to form solution B. Transfer solution B to a stainless steel autoclave for solvothermal reaction. After the reaction, the catalyst InO(OH) is obtained through treatment. (3) Mix the ionic liquid with the catalyst InO(OH) in step (2) at a certain mass fraction, then add a small amount of isopropanol to obtain a mixture, stir evenly, and then dry to remove the isopropanol solvent to obtain the ionic liquid modified catalyst.

3. The method according to claim 2, characterized in that, In step (1), the quaternary phosphonium chloride salt includes one or more of tributylmethylphosphonium chloride, tetrabutylphosphonium chloride, and tributyl(2-methoxyethyl)phosphonium chloride. The molar ratio of the quaternary phosphonium chloride salt to trifluoromethanesulfonamide is 1:

2. The mixture is stirred at 60°C for 10 h, the product is collected, and then vacuum dried at 80°C to obtain the desired quaternary phosphonium salt ionic liquid.

4. The method according to claim 2, characterized in that, The volume ratio of anhydrous ethanol to ethylene glycol used in step (2) is 2:

1.

5. The method according to claim 2, characterized in that, The condition for solution A to become homogeneous is to stir continuously for 30-60 minutes, and the condition for solution B to become transparent is to stir continuously for 30-60 minutes. The solvothermal reaction was carried out by heating at 160℃ for 5 h. The post-reaction treatment includes: centrifugation followed by alternating washing with anhydrous ethanol and deionized water, and then drying in a vacuum drying oven.

6. The method according to claim 2, characterized in that, The ionic liquid added in step (3) is 20-40 wt.% of the mass of the ionic liquid-modified catalyst, preferably 30 wt.%. The small amount of isopropanol added to the mixture is just enough to mix the ionic liquid and InO(OH) evenly. The condition for even mixing is to stir continuously for 20-24 hours.

7. The application of the catalyst prepared by the method according to any one of claims 2-6 for the electrocatalytic reduction of formic acid by CO2.

8. According to the application of claim 7, the CO2 electrocatalytic reduction system is carried out in a flowing electrolyzer, wherein the cathode of the electrolyzer employs a commercially available gas diffusion layer and is then loaded with the catalyst modified by the ionic liquid, a silver-silver chloride electrode serves as the reference electrode, a platinum sheet serves as the anode, the electrolyte is an alkaline electrolyte, and CO2 gas is continuously introduced into the cathode for electrocatalytic reduction during the CO2 electrocatalytic process at an ampere-level current density of -0.8 A∙cm⁻¹. -2 ~-1.4A∙cm -2 They all exhibit excellent catalytic activity.

9. In the application according to claim 8, the electrolyte is a 1 M KOH solution.

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