Sulfur-doped metal monatomic catalyst on porous carbon substrate, preparation method and application thereof

Abstract

The application discloses a sulfur-doped metal monatomic catalyst of a porous carbon base, a preparation method and application. First, sodium citrate is calcined at high temperature, and a carbon base with a porous structure is obtained through acid pickling. The porous carbon, dicyandiamide, thiourea and titanium phthalocyanine transition metal are added into anhydrous ethanol, ultrasonic stirring is carried out, and mixing is uniform. Then, an oil bath is used to obtain a precursor mixture. The precursor mixture is heat-treated under an argon or nitrogen atmosphere to obtain a crude product. The crude product is dried after being subjected to acid pickling and suction filtration, and the sulfur-doped metal monatomic catalyst of the porous carbon base is obtained. The prepared sulfur-doped metal monatomic catalyst of the porous carbon base has Fe-N4 coordination active centers which are well anchored on the porous carbon base with a high specific surface area. The sulfur element is doped in the second shell layer of the metal monatomic structure, and the sulfur-doped metal monatomic catalyst has good catalytic performance in a CO2 electrochemical reduction reaction. The highest CO faradic efficiency reaches 97.0% at-0.6 V vs. RHE.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrocatalysts, in particular to a sulfur-doped metal monatomic catalyst on a porous carbon substrate, a preparation method and application thereof. BACKGROUND

[0002] The massive use of fossil energy has led to a sharp increase in the concentration of CO2 in the atmosphere, causing environmental problems such as the greenhouse effect and ocean acidification. As a new technology, electrochemical reduction of CO2 can convert CO2 into valuable chemicals such as CO, methanol and ethanol using electrical energy, which can not only reduce CO2 emissions but also produce renewable fuels and raw materials, providing an effective solution to global renewable energy and environmental problems.

[0003] In this field, monatomic catalysts have been widely studied due to their unique central metal chemical environment and high atom utilization. Transition metals such as Fe, Co and Ni are commonly used to achieve CO2 conversion to CO. However, since the electrochemical reduction of CO2 is a complex reaction involving multi-electron transfer and proton coupling, the application of a single transition metal-nitrogen active site cannot well regulate the reaction process, making it difficult to provide high faradic efficiency and effective current density.

[0004] Studies have shown that non-metallic atoms such as O, P and S can regulate the electronic state density of the monatomic catalytic center, promote the electron transfer or proton coupling step, and reduce the activation energy of the reaction. Therefore, modifying the metal monatomic catalyst by doping non-metallic atoms is an effective method to improve the selectivity and current density of electrochemical reduction of CO2. Therefore, it is of great significance to develop a convenient and feasible method for preparing a non-metallic doped metal monatomic catalyst. SUMMARY

[0005] In view of the problems of the prior art, the present application aims to provide a sulfur-doped metal monatomic catalyst on a porous carbon substrate, a preparation method and application thereof. The catalyst is prepared by self-assembly high-temperature pyrolysis, which is simple and efficient. The obtained catalyst contains a large number of three-dimensional honeycomb-like porous structures with high specific surface area, and the Fe-N4 coordination active center is well anchored thereon. The sulfur element is doped in the second shell layer of the metal monatomic structure. When applied in the field of electrochemical reduction of CO2, it shows high activity and high selectivity, and the highest CO faradic efficiency can reach 97.0%.

[0006] To achieve the above-mentioned purpose, the technical solutions of the present application are as follows:

[0007] The present application provides a preparation method of a sulfur-doped metal monatomic catalyst on a porous carbon substrate, comprising the following steps:

[0008] 1) Put sodium citrate into a porcelain boat, and then place it in a tube furnace for calcination under an argon or nitrogen atmosphere to obtain a black porous carbon material;

[0009] 2) Grind the porous carbon material obtained in step 1), and then immerse it in a sulfuric acid solution. After acid immersion, the filter solid is separated by suction filtration. The obtained filter solid is dried to obtain a finished product porous carbon;

[0010] 3) Add the finished product porous carbon obtained in step 2) to an ethanol solution, and then add dicyandiamide, thiourea and a phthalocyanine transition metal. After ultrasonic treatment, stirring is performed at room temperature, and then oil bath stirring is performed until dryness to obtain a precursor material;

[0011] 4) The precursor material obtained in step 3) is gradient calcined in a tube furnace under an argon or nitrogen atmosphere. After natural cooling to room temperature, a black powder crude product is obtained;

[0012] 5) The black crude product obtained in step 4) is immersed in a hydrochloric acid solution. After acid immersion, the filter solid is separated by suction filtration. The obtained filter solid is dried to obtain a sulfur-doped metal monatomic material loaded on a porous carbon substrate.

[0013] Further, the calcination temperature of sodium citrate in step 1) is 700-900°C, the heating rate is 5-10°C / min, and the holding 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 to remove air in the tube furnace;

[0014] Further, in step 2), the obtained porous carbon material is immersed in a 0.5-2mol / L sulfuric acid solution at 60-80°C for 12-18h.

[0015] Further, in step 3), the ethanol is anhydrous ethanol with a concentration of 99.7%, the mass ratio of 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.

[0016] Further, in step 3), the phthalocyanine transition metal is phthalocyanine iron, phthalocyanine cobalt or phthalocyanine nickel, and the mass ratio of the phthalocyanine transition metal to the porous carbon is 1:4-1:6.

[0017] Further, in step 3), the ultrasonic treatment time is 1-1.5h, the stirring time at room temperature is 12-24h, and the stirring speed is 400-600r / min; the oil bath stirring temperature is 50-65°C, and the stirring speed is 200-300r / min.

[0018] Further, the specific process of gradient calcination of the precursor material obtained in step 3) in a tube furnace under an argon or nitrogen atmosphere in step 4) is as follows:

[0019] firstly, the temperature is raised to 300-500 DEG C at a rate of 2-5 DEG C / min, and then kept for 1-2 h, and then the temperature is raised to 700-900 DEG C at a rate of 5-8 DEG C / min, and then kept for 1-2 h;

[0020] wherein the purity of argon or nitrogen is 99.999%, and the gas flow rate is 15-25 mL / min, and argon or nitrogen is passed for 30 min before starting to raise the temperature to remove air in the tube furnace.

[0021] The present application provides a porous carbon substrate sulfur-doped metal monatomic catalyst prepared by the method.

[0022] The present application further provides an application of the porous carbon substrate sulfur-doped metal monatomic catalyst in catalyzing CO2.

[0023] Further, the method comprises the following steps:

[0024] 1) The porous carbon substrate sulfur-doped metal monatomic catalyst is added into an ethanol solution of fluorosulfonic acid resin Nafion, and ultrasonic mixing is performed to obtain a mixed solution, the mixed solution is attached to a conductive carbon paper, and natural air drying is performed to obtain a working electrode;

[0025] 2) Constant-voltage electrolysis is performed 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 a cathode, a platinum sheet electrode is used as an anode, and an Ag / AgCl electrode is used as a reference electrode, and CO2 electrocatalytic reduction is performed in an H-type electrolysis cell.

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

[0027] 1) The present application first calcines sodium citrate at high temperature, and acid washing obtains a carbon substrate with a three-dimensional honeycomb porous structure, and then the porous carbon, dicyandiamide, thiourea and titanium phthalocyanine transition metal are added into anhydrous ethanol, ultrasonic stirring is performed, mixing is uniform, and then oil bath drying is performed to obtain a precursor mixture, and the precursor mixture is subjected to heat treatment under an argon or nitrogen atmosphere to obtain a crude product, and the crude product is subjected to acid washing, suction filtration and drying to obtain a porous carbon substrate sulfur-doped metal monatomic catalyst;

[0028] 2) The sulfur-doped metal monatomic catalyst prepared by the method has Fe-N4 coordination active centers well anchored on the porous carbon substrate with high specific surface area, the sulfur element is doped at the second shell layer of the metal monatomic structure, and the sulfur-doped metal monatomic catalyst has good catalytic performance in the CO2 electrochemical reduction reaction, and the highest CO faradic efficiency of 97.0% is reached at-0.6 V vs. RHE; the catalyst prepared by the method can maintain FE CO >90% for nearly 10 h, and exhibits relatively long-term electrochemical activity;

[0029] 3) The porous carbon prepared by calcining sodium citrate is used as the carbon substrate, which can preliminarily improve the graphitization degree of the material, and the loose three-dimensional honeycomb porous structure can provide a large specific surface area, which is beneficial to the uniform loading of metal monatomic sites and sulfur atoms, and lays a foundation for manufacturing more defect sites;

[0030] 4) The self-assembly high-temperature pyrolysis method has high controllability and good reproducibility. In the precursor preparation step, only several common reagents are added, and the molecules are assembled together by intermolecular interaction and loaded on the surface of the carbon substrate. The operation is simple and convenient, and no special requirements are needed;

[0031] 5) The calcination process is optimized, and the sulfur atoms are fully anchored on the carbon substrate at a low temperature in a gradient calcination manner, the defect degree of the material is improved, and the gas generated further manufactures a porous structure; then the graphitization degree of the material is further improved at a high temperature, and the conductivity of the material is improved;

[0032] 6) The preparation method is simple, efficient and feasible, and when applied to the field of CO2 electrochemical reduction, the method can exhibit high CO selectivity and large current density in a wide potential window, and has good application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The cold field emission scanning electron microscope image of the sulfur-doped metal monatomic catalyst prepared by the porous carbon substrate of Example 1;

[0034] Figure 2 The transmission electron microscope image of the sulfur-doped metal monatomic catalyst prepared by the porous carbon substrate of Example 1;

[0035] Figure 3 The spherical aberration-corrected transmission electron microscope image of the sulfur-doped metal monatomic catalyst prepared by the porous carbon substrate of Example 1;

[0036] Figure 4High magnification transmission electron microscopy image and its EDX map of the sulfur-doped metal monatomic catalyst on the porous carbon substrate prepared for Example 1;

[0037] Figure 5 XRD pattern of the catalyst prepared for Example 1-3 and Comparative Example 1-2;

[0038] Figure 6 Raman spectrum of the catalyst prepared for Example 1 and Comparative Example 1;

[0039] Figure 7 XPS Fe 2p spectrum of the catalyst prepared for Example 1 and Comparative Example 1;

[0040] Figure 8 XPS S 2p spectrum of the sulfur-doped metal monatomic catalyst on the porous carbon substrate prepared for Example 1;

[0041] Figure 9 Synchrotron radiation spectrum of the sulfur-doped metal monatomic catalyst on the porous carbon substrate prepared for Example 1. Wherein a represents the K-edge XANES spectrum of Fe; b represents the Fourier transform k 2 Weighted EXAFS spectrum; c represents the EXAFS fitting spectrum of the catalyst; d represents the wavelet transform spectrum of the EXAFS spectrum;

[0042] Figure 10 LSV curve of the catalyst prepared for Example 1-3 and Comparative Example 1-2;

[0043] Figure 11 CO Faraday efficiency plot of the catalyst prepared for Example 1-3 and Comparative Example 1-2;

[0044] Figure 12 CO partial current density plot of the catalyst prepared for Example 1-3 and Comparative Example 1-2;

[0045] Figure 13 i-t curve of the sulfur-doped metal monatomic catalyst on the porous carbon substrate prepared for Example 1 for the electroreduction of CO2 at -0.6 V vs. RHE; DETAILED DESCRIPTION

[0046] The application will be further described in conjunction with the examples below, but the scope of the application is not limited to the scope described. Example 1

[0047] 1) Weigh 6 g of sodium citrate into a porcelain boat, and place the porcelain boat into a tube furnace. Purge the tube furnace with argon for 30 min to remove air in the tube furnace, and then heat from room temperature to 800℃ at a heating rate of 10℃ / min, and calcine for 1 h to obtain a crude porous carbon product;

[0048] 2) The obtained porous carbon crude product was ground, and then immersed in a 0.5 mol / L H2SO4 solution at 70°C for 12 h. The final product was separated by suction filtration and dried in an oven at 60°C to obtain the finished product of porous carbon;

[0049] 3) 100 mg of the porous carbon was weighed into 40 mL of anhydrous ethanol, and 600 mg of dicyandiamide, 400 mg of thiourea, and 24 mg of iron phthalocyanine were added thereto. The mixture was ultrasonically treated for 90 min, and then stirred at room temperature for 12 h. The solvent was evaporated in an oil bath at 60°C to obtain a catalyst precursor;

[0050] 4) The precursor was calcined in an argon atmosphere at a temperature increasing rate of 3°C / min to 500°C for 1 h, and then at a temperature increasing rate of 5°C / min to 800°C for 1.5 h to obtain a catalyst crude product;

[0051] 5) The obtained catalyst crude product was immersed in a 0.5 mol / L HCl solution at 60°C for 24 h. 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 monatomic catalyst based on porous carbon. Example 2

[0052] Compared with Example 1, the only difference is that in the third step, the amount of thiourea in Example 1 is changed from 400 mg to 200 mg, and the remaining steps remain unchanged, to obtain a sulfur-doped metal monatomic catalyst based on porous carbon. Example 3

[0053] Compared with Example 1, the only difference is that in the fourth step, the final calcination temperature of the catalyst precursor in Example 1 is changed from 800°C to 900°C, and the remaining steps remain unchanged, to obtain a sulfur-doped metal monatomic catalyst based on porous carbon.

[0054] Comparative Example 1

[0055] Compared with Example 1, the only difference is that in the third step, thiourea is no longer added, and the remaining steps remain unchanged, to obtain a metal monatomic catalyst based on porous carbon.

[0056] Comparative Example 2

[0057] Compared with Example 1, the only difference is that in the third step, thiourea and iron phthalocyanine are no longer added, and the remaining steps remain unchanged, to obtain a nitrogen-doped porous carbon catalyst.

[0058] Application Example

[0059] The catalysts of Examples 1-3 and Comparative Examples 1-2 were prepared into electrode cathode materials and applied to a method for electrocatalytic conversion of CO2 to CO:

[0060] 1) Weigh 10 mg of catalyst material and add it to a mixed solution consisting of 950 μL of anhydrous ethanol and 50 μL of 5 wt.% Nafion solution. Sonicate at room temperature for 1 h to obtain a uniformly dispersed catalyst mixture.

[0061] 2) Cut out a 1×3cm piece 2 Take 100 μL of catalyst mixture from conductive carbon paper and slowly drop it onto a 1×1 cm layer. 2 On the carbon paper in the region, the catalyst loading was 1 mg cm⁻¹ -2 Air dry at room temperature to serve as the working electrode;

[0062] 3) A three-electrode system consisting of an Ag / AgCl electrode, a platinum sheet electrode, and a working electrode was placed in a sealed H-type electrolytic cell. The electrolyte was a 0.5 mol / L KHCO3 solution. Electrochemical performance was tested using an electrochemical workstation, and the products were detected and analyzed using gas chromatography.

[0063] Experimental characterization and performance analysis:

[0064] like Figure 1 As shown, the FESEM image reveals a large number of three-dimensional honeycomb porous structures in the catalyst prepared in Example 1;

[0065] like Figure 2 As shown, the TEM images show that the catalyst prepared in Example 1 has a thin thickness, is filled with a porous folded structure, and no obvious Fe agglomerate particles are observed.

[0066] like Figure 3 As shown, the aberration-corrected electron microscope image clearly shows a large number of isolated bright spots distributed on the catalyst prepared in Example 1, proving that Fe exists in an atomically dispersed form on the carbon substrate.

[0067] like Figure 4 As shown, high-magnification transmission electron microscopy and energy dispersive spectroscopy can clearly observe that the four elements C, N, S and Fe are uniformly distributed on the surface of the catalyst prepared in Example 1. In particular, Fe does not show obvious aggregate morphology.

[0068] like Figure 5 As shown, no peaks belonging to the Fe nanoparticle crystal planes were observed in the XRD pattern, indicating that there was no metal agglomeration in any of the prepared catalysts.

[0069] like Figure 6 As shown, the catalyst prepared in Example 1 exhibits a defect-rich characteristic due to the doping of sulfur, and Raman spectroscopy reveals its I... D / I G The ratio was 1.30, which was significantly higher than that of the catalyst prepared in Comparative Example 1.

[0070] like Figure 7 As shown, compared to Comparative Example 1, the catalyst prepared in Example 1 has a higher Fe 2p content. 3 / 2 The characteristic peaks shift to lower binding energies, indicating that the Fe atoms are in a more reduced state. This may be due to the increased electron density around the Fe atoms caused by the doping of S atoms.

[0071] like Figure 8 As shown, the S 2p XPS spectrum of the catalyst prepared in Example 1 shows two distinct main peaks at 163.3 eV and 164.6 eV, corresponding to the 2p peaks of CSC(N), respectively. 3 / 2 and 2p 1 / 2 Two energy level states. The figure does not show an Fe-S peak at 161 eV, indicating that S does not bond directly with the metal, but rather with C and N atoms.

[0072] like Figure 9 As shown, the catalyst prepared in Example 1 was analyzed by synchrotron XANES and EXAFS, and its single-atom structure conformed to the Fe-N4 configuration, without any Fe-Fe bonds. Combined with... Figure 8 The analysis results show that S does not directly bond with Fe, leading to the conclusion that S is doped at the second shell of the Fe-N4 site.

[0073] like Figure 10 As shown, the catalysts prepared in Examples 1, 2 and 3 exhibited superior current densities compared to Comparative Examples 1 and 2 in LSV testing.

[0074] like Figure 11 As shown, the catalyst prepared in Example 1 exhibits high CO Faradaic efficiency over a wide potential window, with a maximum CO Faradaic efficiency of 97.0% at -0.6 V vs. RHE.

[0075] like Figure 12 As shown, the catalysts prepared in Examples 1, 2, and 3 all exhibited higher effective CO current densities at various potentials than those in Comparative Examples 1 and 2. Specifically, the catalyst prepared in Example 1 achieved a current density of 12.905 mA cm⁻¹ at -1.0 V vs. RHE. -2 The maximum CO partial current density exhibits excellent electrochemical performance.

[0076] like Figure 13 As shown, the catalyst prepared in Example 1 was subjected to constant potential electrolysis at -0.6 V vs. RHE for a long time, and it was able to maintain FE. CO >90% reached nearly 10 hours, demonstrating relatively sustained electrochemical activity.

Claims

1. A method for preparing a sulfur-doped metal single-atom catalyst on a porous carbon substrate, characterized in that... Includes the following steps: 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) Grind the porous carbon material obtained in step 1), leach it in sulfuric acid solution, separate the filter solid by vacuum filtration, dry the filter solid, and obtain the finished porous carbon. 3) Add the porous carbon obtained in step 2) to an ethanol solution, then add dicyandiamide, thiourea and phthalocyanine transition metals, sonicate and stir at room temperature, then stir in an oil bath until dry to obtain the precursor material. In step 3), the phthalocyanine transition metal is iron phthalocyanine; 4) The precursor material obtained in step 3) is calcined in a tube furnace under an argon or nitrogen atmosphere, and then naturally cooled to room temperature to obtain a black powdery crude product. 5) The black crude product obtained in step 4) is leached in hydrochloric acid solution. After leaching, the solid is separated by vacuum filtration. The solid is dried to obtain sulfur-doped metal single-atom material loaded on a porous carbon substrate.

2. The method for preparing a sulfur-doped metal single-atom catalyst on a porous carbon substrate according to claim 1, characterized in that... In step 1), the calcination temperature of sodium citrate is 700-900℃, the heating rate is 5-10℃ / min, and the holding 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 purged for 30min before heating to purge air from the tubular furnace.

3. The method for preparing a sulfur-doped metal single-atom catalyst on a 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℃ for 12-18 hours.

4. The method for preparing a sulfur-doped metal single-atom catalyst on a porous carbon substrate according to claim 1, characterized in that... In step 3), the ethanol is 99.7% anhydrous ethanol, 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.

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

6.

6. The method for preparing a 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 1-1.5h, the stirring time at room temperature is 12-24h, and the speed is 400-600r / min; the oil bath stirring temperature is 50-65℃, and the speed is 200-300r / min.

7. The method for preparing a sulfur-doped metal single-atom catalyst on a porous carbon substrate according to claim 1, characterized in that... Step 4) involves the specific process of gradient calcining of the precursor material obtained in step 3) in a tube furnace under an argon or nitrogen atmosphere: First, raise the temperature to 300-500℃ at a heating rate of 2-5℃ / min and hold for 1-2 hours. Then, raise the temperature to 700-900℃ at a heating rate of 5-8℃ / min and hold for 1-2 hours. The purity of argon or nitrogen is 99.999%; the gas flow rate is 15-25 mL / min; argon or nitrogen is purged for 30 min before heating to purge air from the tubular furnace.

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