Sn-based catalyst for producing formic acid by electrically reducing carbon dioxide and preparation method and application of Sn-based catalyst

Abstract

The invention relates to a Sn-based catalyst for producing formic acid by electrically reducing carbon dioxide as well as a preparation method and application of the Sn-based catalyst, the catalyst is Y-doped SnO2, the preparation method comprises the following steps: dropwise adding a mixed salt solution containing SnCl2. 2H2O and Y (NO3) 3 into a ZIF-8 solution to obtain a Y / Sn-MOF material, and then converting the Y / Sn-MOF structure into Y-SnO2 through calcination. Compared with the prior art, the catalyst has the advantages of high activity, high selectivity, high stability and the like.

Description

Technical Field

[0001] The present invention relates to the field of electrocatalytic technology, and in particular to an Sn-based catalyst for electro-reducing carbon dioxide to produce formic acid, a preparation method thereof, and an application thereof. Background Art

[0002] With the increasingly serious global climate change problem, the carbon dioxide electroreduction reaction (CO2RR) has received extensive attention in recent years as an effective way to convert greenhouse gases into high-value-added chemicals and fuels. Formic acid, as an important chemical raw material and energy carrier, is one of the important target products of CO2RR.

[0003] The production of formic acid by CO2RR is of great significance. Formic acid is an important chemical raw material and is widely used in fields such as medicine, pesticides, dyes, and leather. In addition, formic acid can also be used as a fuel for fuel cells, with advantages such as high energy density and clean combustion products. Using CO2RR to convert CO2 into formic acid can not only realize the resource utilization of CO2 but also alleviate the fossil energy crisis, with important economic and environmental benefits. The production of formic acid by CO2RR is a complex multi-electron transfer process. At present, the catalysts used for the production of formic acid by CO2RR mainly include metal catalysts (such as Sn, Pb, In, etc.), metal oxide catalysts (such as SnO2, In2O3, etc.), and molecular catalysts. Among them, Sn-based catalysts have attracted much attention due to their low cost, environmental friendliness, and high selectivity for formic acid. However, the existing Sn-based catalysts still have the following problems:

[0004] Low reaction activity: CO2 molecules have high chemical stability, and their activation requires a high overpotential, resulting in low reaction activity.

[0005] Low selectivity: There are multiple competing reactions during the CO2RR process, such as the formation of CO, H2, CH4, etc., resulting in low selectivity for formic acid.

[0006] Poor stability: The catalyst is prone to deactivation during long-term operation, resulting in a decrease in the formic acid production rate and selectivity.

[0007] In summary, there is an urgent need to develop an Sn-based catalyst with high activity, high selectivity, and high stability. Summary of the Invention

[0008] The purpose of the present invention is to provide an Sn-based catalyst for electro-reducing carbon dioxide to produce formic acid, a preparation method thereof, and an application thereof, so as to improve the reaction activity, selectivity, and stability of the catalyst.

[0009] The object of the present invention can be achieved by the following technical solutions: An Sn-based catalyst for electro-reducing carbon dioxide to produce formic acid, wherein the catalyst is a Y (yttrium)-doped SnO2 (tin dioxide) material.

[0010] Preferably, the chemical formula of the catalyst is SnO2:xY, where x is the doping amount of Y element, and 0.02 ≤ x ≤ 0.2. The chemical formula of the catalyst can also be expressed as Y-SnO2.

[0011] Preferably, the catalyst has a polyhedral nanocage morphology and has a cavity structure inside.

[0012] More preferably, the catalyst has a polyhedral hollow nanocage structure with a particle diameter of 500 - 800 nm and a wall thickness of 20 - 50 nm.

[0013] A preparation method of the above-mentioned Sn-based catalyst for electro-reducing carbon dioxide to produce formic acid, wherein a mixed salt solution containing SnCl2·2H2O and Y(NO3)3 is dropped into a ZIF-8 solution to obtain a Y / Sn-MOF material, and then the structure of Y / Sn-MOF is converted into Y-SnO2 by calcination.

[0014] Preferably, the solvent of the mixed salt solution is ethanol.

[0015] Preferably, the solvent of the ZIF-8 solution is a mixture of ethanol and N,N-dimethylformamide (DMF).

[0016] Preferably, the mass ratio of SnCl2·2H2O to Y(NO3)3 is (20 - 21):(2 - 3).

[0017] Preferably, when dropping the mixed salt solution containing SnCl2·2H2O and Y(NO3)3 into the ZIF-8 solution, stirring is maintained during the process, and stirring is continued for 5 - 7 hours after dropping is completed.

[0018] Preferably, the calcination process includes placing the washed and dried Y / Sn-MOF material in a furnace body, heating it to 580 - 620 °C at a heating rate of 8 - 12 °C / min in an air atmosphere, and maintaining it for 50 - 70 min.

[0019] Preferably, the preparation method of the ZIF-8 includes the following steps: adding a methanol solution containing zinc nitrate to a methanol solution containing 2-methylimidazole (2-MIM), stirring at room temperature to generate a ZIF-8 precursor, and washing and drying after the reaction to obtain ZIF-8 crystalline powder.

[0020] An application of the above-mentioned Sn-based catalyst for electro-reducing carbon dioxide to produce formic acid, wherein the catalyst is used for electro-reducing carbon dioxide to produce formic acid at a high current density.

[0021] Preferably, the current density is 0.6 - 2.0 A / cm 2 .

[0022] Preferably, the catalyst is loaded on a conductive substrate, and the loading amount is 0.4 - 0.6 mg / cm 2 , and then the conductive substrate loaded with the catalyst is used as the working electrode.

[0023] More preferably, CO2 gas is introduced into the back of the working electrode, and the flow rate is 18 - 22 mL / min.

[0024] Preferably, when the catalyst is used for electro-reducing carbon dioxide to produce formic acid, a KHCO3 solution is used as the electrolyte.

[0025] For the first time, the present invention doped Y element into SnO2 to form a novel Y-doped SnO2 material. After the introduction of Y element, no separate crystal phase was formed but it was embedded into the lattice of SnO2 to form a Y-O-Sn structure. This asymmetric active site activated the surface oxygen species to form oxygen vacancies (Ov), which were converted into Y-Ov-Sn structure during the electro-reduction process. The coordination unsaturation of this structure is conducive to the O-end adsorption of CO2, thus accelerating the reaction process of CO2-*OCHO-HCOOH, which is manifested as a significant increase in the CO2RR current density in the performance test.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. The present invention provides an Sn-based catalyst for electro-reducing carbon dioxide to produce formic acid with a high current density and a preparation method thereof. By doping Y element, the electronic structure and surface properties of SnO2 are regulated, and the reaction activity, selectivity and stability of the catalyst are significantly improved.

[0028] 2. The Y-doped SnO2 catalyst prepared by the present invention has high performance, has stable electro-synthesis selectivity of formic acid at a high current density, the formic acid Faraday efficiency can reach more than 90%, and the catalyst has excellent stability and can operate continuously for more than 50 hours.

[0029] 3. The preparation method provided by the present invention is simple and easy to implement, and is easy to realize large-scale production.

[0030] 4. The present invention breaks through the technical bottlenecks such as low reaction activity, low selectivity and poor stability faced by the existing CO2RR for producing formic acid, can be used to realize the efficient generation of formic acid by CO2RR, provides a new catalyst choice for the efficient generation of formic acid by CO2RR, and has important application prospects.

[0031] 5. The catalyst of the present invention has a nano-cage structure, usually with a large specific surface area and a rich pore structure, which means that they provide more active sites for the reaction. A large number of internal and external surfaces provide more adsorption positions for CO2 molecules, helping to improve the adsorption capacity of CO2 and the catalytic effect of its reduction reaction. In particular, the pores and micro-cavities in the hollow structure can effectively promote the diffusion of reactant molecules, reduce the mass transfer limitations that may occur during the reaction, and enhance the reaction rate.

[0032] 6. The cavity structure of the catalyst of the present invention can effectively enhance the local pH value in the reaction region. The increase in local pH can improve the adsorption and activation processes of the reactant (CO2), thereby enhancing the rate and selectivity of the reduction reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 TEM images at each stage of the synthesis of the Sn-based catalyst in Example 1 of the present invention;

[0034] Figure 2 XRD spectrum of the Sn-based catalyst in Example 1 of the present invention;

[0035] Figure 3 EPR spectrum of the Sn-based catalyst in Example 1 of the present invention;

[0036] Figure 4 CO2 electroreduction performance diagram of the Sn-based catalyst (a: SnO2 comparison sample, b: Y-SnO2 catalyst sample);

[0037] Figure 5 Stability test of the Y-SnO2 catalyst in Example 1 of the present invention at a high current density (0.6 A / cm 2 ); DETAILED DESCRIPTION OF THE INVENTION

[0038] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and the detailed implementation manner and specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.

[0039] Unless otherwise specified, the reagents, methods, instruments and equipment used in the present invention are conventional reagents, methods, instruments and equipment in the art. Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0040] Example 1

[0041] A preparation method of a Y-SnO2 catalyst for electro-reducing carbon dioxide to produce formic acid, comprising the following steps:

[0042] 1. After accurately weighing 58.8 mg of zinc nitrate (Zn(NO₂)₃·6H₂O), add it to 4 mL of methanol. Through ultrasonic treatment (ultrasonic power is 50 W, ultrasonic time is 10 minutes), ensure that zinc nitrate is completely dissolved to form a homogeneous solution. During the dissolution process of zinc nitrate, observe whether there is any undissolved solid residue in the liquid and adjust the ultrasonic time as needed.

[0043] 2. In another beaker, weigh 64.8 mg of 2-methylimidazole (2-MIM) and add it to 4 mL of methanol. Similarly, perform ultrasonic treatment until 2-methylimidazole is completely dissolved.

[0044] 3. Slowly add the methanol solution containing zinc nitrate to the methanol solution containing 2-methylimidazole. At this time, the reaction mixture should be kept stirring to ensure the uniformity of the solution.

[0045] 4. Place the mixed solution at room temperature (25 °C) and keep stirring for 24 hours. This process helps 2-methylimidazole to coordinate with zinc ions to generate the precursor of ZIF-8.

[0046] 5. After the reaction is completed, wash the reaction system with methanol. The washing steps are as follows: use 10 mL of methanol each time and wash 5 times to remove unreacted impurities and solvents.

[0047] 6. Collect the obtained white suspension by filtration and dry it in a vacuum drying oven at 60 °C for 12 hours to obtain ZIF-8 crystalline powder.

[0048] 7. Add 15 mg of ZIF-8 to a mixed solvent of 15 mL of ethanol and 10 mL of DMF and stir for 30 minutes. This step can effectively promote the dispersion of ZIF-8 in the solvent to form a homogeneous solution.

[0049] 8. Dissolve 20.3 mg of SnCl₂·2H₂O and 2.8 mg of Y(NO₃)₃ in 4 mL of ethanol to prepare a mixed salt solution.

[0050] 9. Slowly drip the mixed salt solution into the ZIF-8 solution. During the process, keep stirring to enable the tin salt to be evenly distributed on the surface of ZIF-8. At this time, the tin ions (Sn 2+ ) will coordinate with 2-methylimidazole in ZIF-8 to generate the Sn-MOF complex. Keep stirring the above mixed solution for 6 hours to ensure that the solution completely reacts with ZIF-8 to form the Sn-MOF structure.

[0051] 10. After the reaction is completed, the Y / Sn-MOF material is collected by filtration and washed three times with ethanol to remove unreacted solutes. Then the filtered Y / Sn-MOF is placed in an oven and dried at 50 °C for 12 hours.

[0052] 11. The dried Y / Sn-MOF is placed in a box furnace and heated to 600 °C at a heating rate of 10 °C / min in an air atmosphere and maintained for 1 hour. At this time, the structure of Y / Sn-MOF will be transformed into Y-SnO2, and polyhedral hollow SnO2 nanocages with a particle diameter of 500 - 800 nm and a wall thickness of 20 - 50 nm are obtained.

[0053] 12. The structure and morphology of the Y-SnO2 catalyst are characterized by means of X-ray diffraction (XRD), electron paramagnetic resonance spectroscopy (EPR), transmission electron microscopy (TEM), etc.

[0054] 13. As Figure 1 shown, the TEM results show that the catalyst presents a polyhedral nanocage morphology with an obvious cavity structure inside. As Figure 2 shown, the XRD results show that all catalysts are in the tetragonal phase structure of SnO2, and the doping of Y element does not change the crystal structure of SnO2 but is successfully doped into the SnO2 lattice. As Figure 3 shown, the EPR results show that with the incorporation of Y content, the oxygen vacancy concentration of the catalyst continuously increases.

[0055] Example 2

[0056] An application test of a Y-SnO2 catalyst for electro-reducing carbon dioxide to produce formic acid, including the following steps:

[0057] 1. A three-electrode system is adopted to perform CO2RR performance tests in a self-made flow-through electrolytic cell.

[0058] 2. The working electrode is a gas diffusion electrode loaded with the Y-SnO2 catalyst. The Y-SnO2 catalytic material is loaded on commercial Sigracet 28BC SGL carbon paper using a spray gun. The ink formulation includes: 20 ml of ethanol, 60 mg of Y-SnO2, and 300 μL of commercial Sustainion XA-9 alkaline ionomer dispersion (5 wt%). Control the catalyst loading to 0.5 mg / cm 2 .

[0059] 3. The counter electrode is a commercial IrO2 / Ti electrode with an IrO2 loading of 2 mg / cm 2 , and the reference electrode is a commercial Ag / AgCl electrode. 1 mol / L KHCO3 solution is used as the electrolyte in both the cathode and anode chambers.

[0060] 4. CO2 gas is introduced into the back of the working electrode at a flow rate of 20 mL / min. The electrolysis is carried out by the potentiostatic method, the current density is recorded, and the gaseous products (H2 and CO) are quantitatively analyzed by an on-line Shimadzu gas chromatograph, and the formate ions are quantitatively analyzed by a Shimadzu anion chromatograph, and finally the Faraday efficiency of each product is calculated and obtained.

[0061] 5. As Figure 4 shown, the CO2RR reaction performances of the SnO2 nanocage catalysts with and without Y doping are compared respectively. The Y-SnO2 nanocage catalyst shows higher reaction activity and stability. After the current density is higher than 0.6 A / cm 2 , the calculated Faraday efficiency of SnO2 gradually decreases. While the Faraday efficiency of Y-SnO2 always remains above 90% at 0.6 - 2.0 A / cm 2 .

[0062] 6. As Figure 5 shown, Y-SnO2 can operate stably for 50 hours at 0.6 A / cm 2 .

[0063] The above description of the embodiments is to enable those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A Sn-based catalyst for producing formic acid by electroreduction of carbon dioxide, characterized in that: The catalyst is Y-doped SnO2.

2. The Sn-based catalyst for producing formic acid by electroreduction of carbon dioxide according to claim 1, characterized in that: The chemical formula of the catalyst is SnO2:xY, wherein x is the molar percentage of the Y element doped in the catalyst content, and 0.02≤x≤0.

2.

3. The Sn-based catalyst for producing formic acid by electroreduction of carbon dioxide according to claim 1, characterized in that: The catalyst is in the shape of a polyhedral nanocage and has a cavity structure inside.

4. A method for preparing a Sn-based catalyst for producing formic acid by electro-reduction of carbon dioxide according to any one of claims 1 to 3, characterized in that: A mixed salt solution containing SnCl2·2H2O and Y(NO3)3 was added dropwise to the ZIF-8 solution to obtain the Y / Sn-MOF material, and then the structure of the Y / Sn-MOF was converted into Y-SnO2 by calcination.

5. The method for preparing a Sn-based catalyst for producing formic acid by electro-reduction of carbon dioxide according to claim 4, characterized in that: The solvent of the mixed salt solution is ethanol, and the solvent of the ZIF-8 solution is a mixture of ethanol and N,N-dimethylformamide.

6. The method for preparing a Sn-based catalyst for producing formic acid by electro-reduction of carbon dioxide according to claim 4, characterized in that: The mass ratio of SnCl2·2H2O and Y(NO3)3 is (20-21):(2-3).

7. The method for preparing a Sn-based catalyst for producing formic acid by electro-reduction of carbon dioxide according to claim 4, characterized in that: The mixed salt solution containing SnCl2·2H2O and Y(NO3)3 is added dropwise to the ZIF-8 solution while stirring is maintained during the process. After the addition is completed, stirring is continued for 5 to 7 hours.

8. The method for preparing a Sn-based catalyst for producing formic acid by electro-reduction of carbon dioxide according to claim 4, characterized in that: The calcination process comprises placing the washed and dried Y / Sn-MOF material in a furnace body, heating it to 580-620° C. at a heating rate of 8-12° C. / min in an air atmosphere, and maintaining it for 50-70 minutes.

9. The method for preparing a Sn-based catalyst for producing formic acid by electro-reduction of carbon dioxide according to claim 4, characterized in that: The preparation method of ZIF-8 comprises the following steps: adding a methanol solution containing zinc nitrate to a methanol solution containing dimethylimidazole, stirring at room temperature to generate a ZIF-8 precursor, and washing and drying after the reaction to obtain ZIF-8 crystalline powder.

10. Use of a Sn-based catalyst for producing formic acid by electro-reduction of carbon dioxide according to any one of claims 1 to 3, characterized in that: The catalyst was used for 0.6-2.0 A / cm 2 High current density electroreduction of carbon dioxide to produce formic acid.

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