Sulfur-doped metal monatomic catalyst with porous carbon substrate as well as preparation method and application of sulfur-doped metal monatomic catalyst

Abstract

The invention discloses a sulfur-doped metal monatomic catalyst with a porous carbon substrate and a preparation method and application thereof.The preparation method comprises the steps that firstly, sodium citrate is subjected to high-temperature calcination and acid pickling, the carbon substrate of a porous structure is obtained, porous carbon, dicyandiamide, thiourea and phthalocyanine transition metal are added into absolute ethyl alcohol, ultrasonic stirring and uniform mixing are conducted, and the sulfur-doped metal monatomic catalyst with the porous carbon substrate is obtained; the preparation method comprises the following steps: preparing a precursor mixture, carrying out oil bath to obtain the precursor mixture, carrying out heat treatment on the precursor mixture in an argon or nitrogen atmosphere to obtain a crude product, carrying out acid pickling and suction filtration on the crude product, and drying to obtain the sulfur-doped metal monatomic catalyst with the porous carbon substrate. The Fe-N4 coordination active center is well anchored on the porous carbon substrate with the high specific surface area, the sulfur element is doped in the second shell layer of the metal monatomic structure, good catalytic performance is achieved in the CO2 electrochemical reduction reaction, and the highest CO Faraday efficiency of 97.0% is achieved under-0.6 V vs.RHE.

Description

Technical Field

[0001] The present invention relates to the technical field of electrocatalysts, and particularly relates to a sulfur-doped metal single-atom catalyst supported on a porous carbon substrate, a preparation method thereof, and an application thereof. Background Art

[0002] The extensive use of fossil fuels has led to a sharp increase in the concentration of CO in the atmosphere, 2 triggering environmental problems such as the greenhouse effect and ocean acidification. Electrochemical reduction of CO 2 As a new technology, it uses electrical energy to convert CO 2 into valuable chemicals such as CO, methanol, and ethanol. This not only reduces the emission of CO 2 but also produces renewable fuels and raw materials, providing an effective solution to global renewable energy and environmental problems.

[0003] In this field, single-atom catalysts have been widely studied due to their unique central metal chemical environment and high atomic utilization efficiency. Transition metals such as Fe, Co, and Ni are commonly used to achieve the conversion of CO 2 to CO. However, since the electroreduction of CO 2 is a complex reaction involving multiple electron transfer and proton coupling processes, the application of a single transition metal-nitrogen active site cannot well regulate the reaction process, resulting in the conversion efficiency of CO 2 usually being hindered and it being difficult to provide a high Faraday efficiency and effective current density.

[0004] Research has shown that non-metal atoms such as O, P, and S can regulate the electron density of the single-atom catalytic center, promote electron transfer or proton coupling steps, and reduce the activation energy of the reaction. Therefore, modifying metal single-atom catalysts by doping non-metal atoms is an effective method to improve the selectivity and current density of the electroreduction of CO 2 Thus, it is of great significance to develop a convenient and feasible preparation method for non-metal-doped metal single-atom catalysts. Summary of the Invention

[0005] In view of the problems existing in the prior art, the purpose of the present invention is to provide a sulfur-doped metal single-atom catalyst supported on a porous carbon substrate, a preparation method thereof, and an application thereof. The catalyst is prepared by a self-assembly high-temperature pyrolysis method, which is simple, efficient, and the obtained catalyst contains a large number of three-dimensional honeycomb-like porous structures with a high specific surface area. The Fe-N 4 coordination active centers are well anchored thereon, and sulfur elements are doped at the second shell layer of the metal single-atom structure. When applied to the field of electrochemical reduction of CO 2 it exhibits high activity and high selectivity, and the highest CO Faraday efficiency can reach 97.0%.

[0006] To achieve the above object, the technical solution of the present invention is as follows: The present invention provides a method for preparing a sulfur-doped metal single-atom catalyst on a porous carbon substrate, comprising the following steps: 1) Put sodium citrate into a porcelain boat, then place it in a tube furnace, and calcine it under an argon or nitrogen atmosphere to obtain a black porous carbon material; 2) Grind the porous carbon material obtained in step 1), acid-leach it in a sulfuric acid solution, separate the filter residue by suction filtration after acid-leaching, and dry the obtained filter residue to obtain a finished porous carbon; 3) Add the finished porous carbon obtained in step 2) to an ethanol solution, then add dicyandiamide, thiourea and phthalocyanine-based transition metals, perform ultrasonic treatment and stir at room temperature, and then stir in an oil bath until dry to obtain a precursor material; 4) Gradiently calcine the precursor material obtained in step 3) in a tube furnace under an argon or nitrogen atmosphere, and naturally cool it to room temperature to obtain a black powdery crude product; 5) Acid-leach the black crude product obtained in step 4) in a hydrochloric acid solution, separate the filter residue by suction filtration after acid-leaching, and dry the obtained filter residue to obtain a sulfur-doped metal single-atom material supported on a porous carbon substrate.

[0007] Further, in step 1), the calcination temperature of sodium citrate is 700-900 °C, the heating rate is 5-10 °C / min, and the heat preservation time is 1-1.5 h; the purity of argon or nitrogen in step 1) is 99.999%; the gas flow rate is 15-25 mL / min, and 30 min of argon or nitrogen is passed through before starting to heat up to exhaust the air in the tube furnace; Further, in step 2), the obtained porous carbon material is acid-leached in a 0.5-2 mol / L sulfuric acid solution at 60-80 °C for 12-18 h.

[0008] Further, in step 3), the ethanol is anhydrous ethanol with a concentration of 99.7%, and the mass ratio of the added porous carbon to ethanol is 1:250-350; the mass ratio of dicyandiamide to porous carbon is 4:1-6:1, and the mass ratio of dicyandiamide to thiourea is 1:1-6:1.

[0009] Further, in step 3), the phthalocyanine-based transition metal is iron phthalocyanine, cobalt phthalocyanine or nickel phthalocyanine, and its mass ratio to porous carbon is 1:4-1:6.

[0010] Further, in step 3), the ultrasonic time is 1-1.5 h, the stirring time at room temperature is 12-24 h, and the rotation speed is 400-600 r / min; the oil bath stirring temperature is 50-65 °C, and the rotation speed is 200-300 r / min.

[0011] Further, the specific process of gradient calcining the precursor material obtained in step 3) in a tubular furnace under an argon or nitrogen atmosphere in step 4) is as follows: First, it is heated to 300 - 500 °C at a heating rate of 2 - 5 °C / min and held for 1 - 2 h, then heated to 700 - 900 °C at a heating rate of 5 - 8 °C / min and held for 1 - 2 h; Among them, the purity of argon or nitrogen is 99.999%; the gas flow rate is 15 - 25 mL / min, and argon or nitrogen is passed through for 30 min before starting to heat to exhaust the air in the tubular furnace.

[0012] The present invention provides a sulfur-doped metal single-atom catalyst supported on a porous carbon substrate prepared by the above method.

[0013] The present invention further provides an application of the sulfur-doped metal single-atom catalyst supported on the porous carbon substrate in catalyzing CO 2 in.

[0014] Further, it includes the following steps: 1) Add the sulfur-doped metal single-atom catalyst supported on the porous carbon substrate to an ethanol solution of fluorosulfonic acid resin Nafion, ultrasonically mix to obtain a mixed solution, attach the mixed solution to a conductive carbon paper, and naturally dry to obtain a working electrode; 2) Perform constant voltage electrolysis in a closed three-electrode system. The electrolyte is a 0.5 mol / L KHCO 3 solution. Use the working electrode obtained in step 1) as the cathode, a platinum sheet electrode as the anode, and an Ag / AgCl electrode as the reference electrode, and perform electrocatalytic reduction of CO 2 in an H-type electrolytic cell.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) The present invention first calcines sodium citrate at high temperature, pickles it to obtain a carbon substrate with a three-dimensional honeycomb porous structure, adds the porous carbon, dicyandiamide, thiourea, and phthalocyanine-based transition metals to absolute ethanol, ultrasonically stirs and mixes evenly, then dries it in an oil bath to obtain a precursor mixture, performs heat treatment on the precursor mixture under an argon or nitrogen atmosphere to obtain a crude product, and pickles, filters, and dries the crude product to obtain a sulfur-doped metal single-atom catalyst supported on a porous carbon substrate; 2) For the sulfur-doped metal single-atom catalyst supported on the porous carbon substrate prepared by the present invention, the Fe-N 4 coordination active centers are well anchored on the porous carbon substrate with a relatively high specific surface area. Among them, sulfur elements are doped at the second shell layer of the metal single-atom structure, and in CO 2It has good catalytic performance in the electrochemical reduction reaction, reaching a maximum CO Faraday efficiency of 97.0% at -0.6 V vs. RHE; the catalyst prepared by the present invention undergoes constant potential long-term electrolysis at -0.6 V vs. RHE, and it can maintain FE CO >90% for nearly 10 h, showing relatively persistent electrochemical activity; 3) The present invention uses porous carbon prepared by calcining sodium citrate as the carbon substrate. On the one hand, it can initially improve the graphitization degree of the material; on the other hand, its loose three-dimensional honeycomb-like porous structure can provide a large specific surface area, which is conducive to the full and uniform loading of metal single-atom sites and sulfur atoms, laying a foundation for creating more defect sites; 4) The present invention adopts the method of self-assembly high-temperature pyrolysis, which has high controllability and good reproducibility. In the precursor preparation step, only by adding several common reagents, they are assembled together by intermolecular interactions and loaded on the surface of the carbon substrate. The operation is simple and convenient, with no special requirements; 5) The present invention optimizes the calcination process. Through gradient calcination, first, sulfur atoms are fully anchored on the carbon substrate at medium and low temperatures to increase the defect degree of the material, and at the same time, the generated gas further creates a porous structure; then, the graphitization degree of the material is further improved at high temperatures to improve the electrical conductivity of the material; 6) The preparation method of the present invention is simple, efficient and feasible. When applied to the field of CO 2 electrochemical reduction, within a relatively wide potential window range, it can show high CO selectivity and large current density, and has good application prospects. Description of the Drawings

[0016] Figure 1 It is a cold field emission scanning electron microscope image of the sulfur-doped metal single-atom catalyst on the porous carbon substrate prepared in Example 1; Figure 2 It is a transmission electron microscope image of the sulfur-doped metal single-atom catalyst on the porous carbon substrate prepared in Example 1; Figure 3 It is a spherical aberration corrected transmission electron microscope image of the sulfur-doped metal single-atom catalyst on the porous carbon substrate prepared in Example 1; Figure 4 It is a high-magnification transmission electron microscope image and its EDX image of the sulfur-doped metal single-atom catalyst on the porous carbon substrate prepared in Example 1; Figure 5 It is an XRD pattern of the catalysts prepared in Examples 1-3 and Comparative Examples 1-2; Figure 6 It is a Raman spectrum of the catalysts prepared in Example 1 and Comparative Example 1; Figure 7XPS Fe 2p spectra of the catalysts prepared in Example 1 and Comparative Example 1; Figure 8 XPS S 2p spectra of the sulfur-doped metal single-atom catalyst on the porous carbon substrate prepared in Example 1; Figure 9 Synchrotron radiation spectra of the sulfur-doped metal single-atom catalyst on the porous carbon substrate prepared in Example 1. Where a represents the XANES spectrum of the K-edge of Fe; b represents the Fourier-transformed k 2 weighted EXAFS diagram; c represents the EXAFS fitting diagram of the catalyst; d represents the wavelet transform diagram of the EXAFS spectrum; Figure 10 LSV curves of the catalysts prepared in Examples 1-3 and Comparative Examples 1-2; Figure 11 CO Faraday efficiency diagrams of the catalysts prepared in Examples 1-3 and Comparative Examples 1-2; Figure 12 CO partial current density diagrams of the catalysts prepared in Examples 1-3 and Comparative Examples 1-2; Figure 13 Electroreduction of CO at -0.6 V vs. RHE for the sulfur-doped metal single-atom catalyst on the porous carbon substrate of the catalyst prepared in Example 1 2 i-t curve diagram; Detailed implementation mode

[0017] The present invention will be further described below in conjunction with examples, but the scope protected by the present invention is not limited to the described scope. Example 1

[0018] 1) Weigh 6 g of sodium citrate and place it in a porcelain boat. Place the porcelain boat in a tube furnace, purge the air in the tube furnace with argon for 30 min, and then heat it from room temperature to 800 °C at a heating rate of 10 °C / min for 1 h to obtain a porous carbon crude product; 2) Grind the obtained porous carbon crude product, and acid-leach it in a 0.5 mol / L H 2 SO 4 solution at 70 °C for 12 h. Separate the final product by suction filtration and dry it in an oven at 60 °C to obtain the finished porous carbon; 3) Weigh 100 mg of porous carbon and add it to 40 mL of absolute ethanol. Add 600 mg of dicyandiamide, 400 mg of thiourea, and 24 mg of iron phthalocyanine to it, ultrasonically treat it for 90 min, then stir it at room temperature for 12 h, and evaporate the solvent in an oil bath at 60 °C to obtain a catalyst precursor; 4) The precursor was first calcined in an argon atmosphere at a heating rate of 3 °C / min to 500 °C for 1 h, and then at a heating rate of 5 °C / min to the final temperature of 800 °C for 1.5 h to obtain a crude catalyst product; 5) The obtained crude product was soaked in a 0.5 mol / L hydrochloric acid solution at 60 °C for 24 h, and then the final product was separated by suction filtration and dried in a vacuum oven at 60 °C to obtain a finished product of a sulfur-doped metal single-atom catalyst on a porous carbon substrate. Example 2

[0019] Compared with Example 1, the difference is only that in the third step, the dosage of thiourea in Example 1 was changed from 400 mg to 200 mg, and the other steps remained unchanged, obtaining a sulfur-doped metal single-atom catalyst on a porous carbon substrate. Example 3

[0020] Compared with Example 1, the difference is only that in the fourth step, the final calcination temperature of the catalyst precursor in Example 1 was changed from 800 °C to 900 °C, and the other steps remained unchanged, obtaining a sulfur-doped metal single-atom catalyst on a porous carbon substrate.

[0021] Comparative Example 1 Compared with Example 1, the difference is only that in the third step, thiourea was no longer added, and the other steps remained unchanged, obtaining a metal single-atom catalyst on a porous carbon substrate.

[0022] Comparative Example 2 Compared with Example 1, the difference is only that in the third step, neither thiourea nor iron phthalocyanine was added, and the other steps remained unchanged, obtaining a nitrogen-doped porous carbon catalyst. Application Example

[0023] The catalysts of Examples 1-3 and Comparative Examples 1-2 were made into electrode cathode materials and applied to electrocatalytic CO 2 Related methods for CO production: 1) Weigh 10 mg of the catalyst material and add it to a mixed solution composed of 950 μL of absolute ethanol and 50 μL of 5 wt.% Nafion solution, and ultrasonicate for 1 h at room temperature to obtain a uniformly dispersed catalyst mixture; 2) Cut out a 1×3 cm 2 conductive carbon paper, suck 100 μL of the catalyst mixture, and slowly drop it onto the carbon paper in the 1×1 cm 2 area, with a catalyst loading of 1 mg cm -2 . Let it dry naturally at room temperature to be used as the working electrode; 3) The Ag / AgCl electrode, platinum sheet electrode and working electrode were jointly formed into a three-electrode system and placed into a sealed H-type electrolytic cell, and the electrolyte was 0.5 mol / L KHCO 3Solution. Electrochemical performance tests were carried out using an electrochemical workstation, and the products were detected and analyzed using gas chromatography.

[0024] Experimental characterization and performance analysis: As Figure 1 shown, the FESEM image shows a large number of three-dimensional honeycomb-like porous structures in the catalyst prepared in Example 1; As Figure 2 shown, the TEM image shows that the catalyst prepared in Example 1 has a relatively thin thickness, is filled with pore-like wrinkled structures, and no obvious Fe agglomerated particle images are seen.

[0025] As Figure 3 shown, a large number of isolated bright spots can be clearly seen in the aberration-corrected electron microscope image of the catalyst prepared in Example 1, proving that Fe exists in the form of atomic dispersion on the carbon substrate.

[0026] As Figure 4 shown, it can be intuitively observed from the high-magnification transmission electron microscope and energy spectrum that the four elements C, N, S, and Fe are evenly distributed on the surface of the catalyst prepared in Example 1. Especially for the Fe element, no obvious aggregated morphology is shown.

[0027] As Figure 5 shown, no peaks belonging to the crystal planes of Fe nanoparticles are observed in the XRD pattern, indicating that there is no metal agglomeration in all the prepared catalysts.

[0028] As Figure 6 shown, due to the doping of S element, the catalyst prepared in Example 1 has a rich defect feature, and the Raman spectrum shows that its I D / I G ratio is 1.30, which is significantly higher than that of the catalyst prepared in Comparative Example 1.

[0029] As Figure 7 shown, compared with Comparative Example 1, the Fe 2p 3 / 2 characteristic peak of the catalyst prepared in Example 1 shifts towards lower binding energy, indicating that its Fe atoms are in a more reduced state. This may be due to the phenomenon that the doping of S atoms increases the electron density around the Fe atoms.

[0030] As Figure 8 shown, two obvious main peaks located at 163.3 eV and 164.6 eV can be observed in the S 2p XPS spectrum of the catalyst prepared in Example 1, corresponding to the 2p 3 / 2 and 2p 1 / 2 energy level states of C-S-C(N), respectively. No Fe-S peak is found at 161 eV in the figure, indicating that S does not directly bond with the metal, but bonds with C and N atoms.

[0031] As Figure 9 shown, through XANES and EXAFS analyses of synchrotron radiation on the catalyst prepared in Example 1, it was found that its single-atom structure conforms to the configuration of Fe-N 4 and there is no Fe-Fe bond. Combining the Figure 8 analysis results, since S does not directly bond with Fe, it is deduced that the S element is doped at the second coordination shell of the Fe-N 4 site.

[0032] As Figure 10 shown, the catalysts prepared in Example 1, Example 2, and Example 3 exhibited more excellent current densities in the LSV test than those in Comparative Example 1 and Comparative Example 2.

[0033] As Figure 11 shown, the catalyst prepared in Example 1 had a high CO Faraday efficiency within a wide potential window. Among them, the highest CO Faraday efficiency reached 97.0% at -0.6 V vs. RHE.

[0034] As Figure 12 shown, the catalysts prepared in Example 1, Example 2, and Example 3 had higher CO effective current densities at each potential than those in Comparative Example 1 and Comparative Example 2. Among them, the catalyst prepared in Example 1 reached a maximum CO partial current density of 12.905 mA cm -2 at -1.0 V vs. RHE, demonstrating excellent electrochemical performance.

[0035] As Figure 13 shown, when the catalyst prepared in Example 1 was subjected to potentiostatic long-term electrolysis at -0.6 V vs. RHE, it was able to maintain FE CO > 90% for nearly 10 h, showing relatively persistent electrochemical activity.

Claims

1. A method for preparing a sulfur-doped metal single-atom catalyst on a porous carbon substrate, characterized in that The steps include: 1) Sodium citrate is placed in a porcelain boat, and then placed in a tube furnace for calcination under an argon or nitrogen atmosphere to obtain a black porous carbon material; 2) grinding the porous carbon material obtained in step 1), acid leaching in a sulfuric acid solution, separating by suction filtration to obtain a filter solid after the acid leaching, and drying the obtained filter solid to obtain a finished porous carbon; 3) adding the finished porous carbon obtained in step 2) into an ethanol solution, and then adding dicyandiamide, thiourea and phthalocyanine transition metal, stirring at room temperature after ultrasonic treatment, and then stirring in an oil bath until dry, to obtain a precursor material; 4) The precursor material obtained in step 3) is calcined in a tube furnace under an argon or nitrogen atmosphere, and then cooled naturally to room temperature to obtain a black powdery crude product; 5) The black crude product obtained in step 4) is acid-leached in a hydrochloric acid solution, and after acid leaching, a filter solid is separated by suction filtration, and the filter solid is dried to obtain a sulfur-doped metal single atom material supported on a porous carbon substrate.

2. The method for preparing the sulfur-doped metal single-atom catalyst on the porous carbon substrate according to claim 1, characterized in that In step 1), the calcination temperature of sodium citrate is 700-900°C, the heating rate is 5-10°C / min, and the insulation time is 1-1.5h; the purity of argon or nitrogen in step 1) is 99.999%; the gas flow rate is 15-25mL / min, and argon or nitrogen is passed for 30min before starting to heat up to exhaust the air in the furnace.

3. The method for preparing the sulfur-doped metal single-atom catalyst on the porous carbon substrate according to claim 1, characterized in that In step 2), the obtained porous carbon material is acid-leached in a 0.5-2 mol / L sulfuric acid solution at 60-80° C. for 12-18 hours.

4. The method for preparing the sulfur-doped metal single-atom catalyst on a porous carbon substrate according to claim 1, characterized in that In step 3), the ethanol is anhydrous ethanol with a concentration of 99.7%, and the mass ratio of the added porous carbon to the ethanol is 1:250-350; the mass ratio of dicyandiamide to the porous carbon is 4:1-6:1, and the mass ratio of dicyandiamide to thiourea is 1:1-6:

1.

5. The method for preparing the sulfur-doped metal single-atom catalyst on a porous carbon substrate according to claim 1, characterized in that In step 3), the phthalocyanine transition metal is iron phthalocyanine, cobalt phthalocyanine or nickel phthalocyanine, and the mass ratio of the phthalocyanine transition metal to the porous carbon is 1:4-1:

6.

6. The method for preparing the sulfur-doped metal single-atom catalyst on a porous carbon substrate according to claim 1, characterized in that In step 3), the ultrasonic time is 1-1.5 h, the stirring time at room temperature is 12-24 h, the rotation speed is 400-600 r / min, and the oil bath stirring temperature is 50-65° C., and the rotation speed is 200-300 r / min.

7. The method for preparing the sulfur-doped metal single-atom catalyst on a porous carbon substrate according to claim 1, characterized in that In step 4), the precursor material obtained in step 3) is subjected to gradient calcination in a tube furnace under an argon or nitrogen atmosphere: First, increase the temperature to 300-500°C at a heating rate of 2-5°C / min, and keep it for 1-2h. Then, increase the temperature to 700-900°C at a heating rate of 5-8°C / min, and keep it for 1-2h. The purity of argon or nitrogen is 99.999%; the gas flow rate is 15-25mL / min, and argon or nitrogen is passed for 30 minutes before starting to heat up to exhaust the air in the furnace.

8. A sulfur-doped metal single atom catalyst on a porous carbon substrate prepared according to the method according to any one of claims 1 to 7.

9. Use of the sulfur-doped metal single-atom catalyst on the porous carbon substrate of claim 8 in catalyzing CO2.

10. The use according to claim 9, characterized in that The steps include: 1) adding the sulfur-doped metal single atom catalyst on the porous carbon substrate to the ethanol solution of the fluorosulfonic acid resin Nafion, mixing by ultrasonication to obtain a mixed solution, attaching the mixed solution to the conductive carbon paper, and drying naturally to obtain the working electrode; 2) Perform constant voltage electrolysis in a closed three-electrode system, the electrolyte is a 0.5 mol / L KHCO3 solution, the working electrode obtained in step 1) is used as the cathode, the platinum electrode is used as the anode, and the Ag / AgCl electrode is used as the reference electrode, and CO2 electrocatalytic reduction is performed in an H-type electrolytic cell.

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