Nitrogen-sulfur co-doped graphene-supported copper-molybdenum sulfide catalyst and its preparation and application

Abstract

The present invention discloses a nitrogen-sulfur co-doped graphene-supported copper-molybdenum sulfide catalyst and its preparation and application, which belongs to the field of new materials and energy chemical technology. The nitrogen-sulfur co-doped graphene can not only improve the stability of the composite material, but also has a unique unsaturated site on its surface with good CO2 adsorption and activation performance. The Mo atom-doped CuS can slow down the desorption of CO, and the eccentric charge around the Mo atom is conducive to the COOH ‑ Therefore, the participation of Mo can further enhance the catalytic activity of copper-based catalysts; and Cu-Mo-S x A synergistic effect can occur when compounded with N,S-rGO. When this catalyst material is used as the working electrode for the electrolytic reduction of CO2, the reaction product is formic acid and the Faradaic efficiency can reach up to 85%. At the same time, after continuous catalysis for 21 hours at an applied voltage of 0.85V, it can still maintain a Faradaic efficiency of 76%, showing good stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of new materials and energy chemical industry, and specifically relates to a nitrogen-sulfur co-doped graphene-supported copper-molybdenum sulfide catalyst and its preparation and application. Background Art

[0002] The electrocatalytic reduction of carbon dioxide is easy to control the reaction process and the main consumed product is water. It is not only environmentally friendly, but also the reaction equipment is compact and easy to scale up. At the same time, other high-value-added chemical fuels generated will effectively alleviate the global energy crisis and promote carbon cycle. At the same time, it can also reduce the CO2 content in the atmosphere and effectively alleviate the greenhouse effect. Therefore, it has received widespread attention from the scientific community.

[0003] However, the electrocatalytic reduction of CO2 requires overcoming a huge energy barrier due to the slow reaction kinetics and high thermodynamic stability of CO2. In recent years, a large number of nanomaterials have been used in the catalytic reduction of CO2, including metal catalysts, metal compound catalysts, single-atom catalysts, non-metallic catalysts and molecular complexes. Although the development of such catalysts has achieved very remarkable results under the joint efforts of those skilled in the art, the catalytic efficiency is still at a level that can be improved, so it is still slightly insufficient for large-scale industrial applications. When CO2 is converted into C1 products, it has the advantages of low overpotential, high selectivity and easy purification, which makes it closer to practical application in terms of economic benefits. In addition, C1 products have high practical value, such as formic acid, which can be used in fuel cells.

[0004] Copper, a common metal electrocatalyst, possesses low overpotential, high natural abundance, and excellent chemical stability. Among copper compounds, CuS has attracted widespread attention due to its narrow band gap and excellent electrical conductivity. It also exhibits selective catalytic activity toward C1 products, such as formic acid. However, bulk CuS materials have a small surface area and few active sites, resulting in low intrinsic catalytic activity. A paper (Shao P, Ci S, Yi L, et al. Hollow CuS microcube electrocatalysts for CO2 reduction reaction[J]) describes a hollow CuS microtube electrocatalyst for CO2 reduction. The three-dimensional structure of this catalytic material provides it with abundant active sites, shortening the charge transfer path and enhancing CO2 catalytic activity. Graphene, with its excellent electrical conductivity, mechanical properties, and surface properties, is an ideal catalyst support. For example, the literature (Zheng Y, Zheng S, Xue H, et al. Metal-organic frameworks / graphene-based materials: preparations and applications [J]. Advanced Functional Materials, 2018, 28 (47): 1804950) proposes that Sn quantum sheets loaded on graphene have significant catalytic performance in electrocatalytic CO2 reduction because graphene has excellent electrical conductivity and can avoid oxidation of the Sn layer. In addition, the doping of heteroatoms (such as N, S and P) can produce unique unsaturated sites, giving graphene excellent carbon dioxide adsorption and activation properties. Therefore, loading copper and sulfur on the surface of heteroatom-doped graphene is an effective strategy for obtaining efficient CO2 reduction catalysts. For example, Wu Zongdeng et al. proposed in the conference paper "Study on the Electrocatalytic Selective Reduction of CO2 to Formic Acid by Copper Sulfide / Nitrogen-Sulfur Co-doped Graphene" that when the CuS / N,S-rGO nanocomposite material prepared by a one-step hydrothermal reaction electrolyzes CO2 at a potential of -1.3V, formic acid is the only liquid product and the Faraday efficiency is as high as 78%. The current density remains unchanged for a long reaction time of 15 hours, and it has good stability.

[0005] However, the applicant believes that there is room for further improvement in the Faraday efficiency of such materials, because the applicant has found that although the metal Mo is usually combined with Co as a hydrodesulfurization catalyst, it can show high activity in synergy with Co, but few people use Mo atoms in the electrocatalytic reduction process of CO2. However, the eccentric charge around the Mo atom is conducive to stabilizing COOH -It can also slow down the desorption of CO, so combining Mo with CuS is expected to further improve the catalytic activity and stability of the electrocatalytic reduction of CO2 to C1 products. The implementation of this solution will provide new ideas and effective guidance for the application of copper-based catalysts in the catalytic reduction of CO2 technology. Summary of the Invention

[0006] The purpose of the present invention is to solve the problems existing in the prior art and propose a nitrogen-sulfur co-doped graphene-loaded copper-molybdenum sulfide catalyst and its preparation and application. Mo doping can give CuS higher catalytic activity, and a synergistic effect can be produced after compounding with N and S co-doped graphene, effectively improving the catalytic activity and stability during the electrocatalytic reduction of CO2 to C1 products.

[0007] The technical solution of the present invention is: a method for preparing a nitrogen-sulfur co-doped graphene-supported copper-molybdenum sulfide catalyst, comprising the following steps:

[0008] 1) adding ethanol to the graphene oxide solution, stirring and ultrasonicating to obtain a graphene oxide mixed solution;

[0009] 2) dissolving copper salt and molybdate in deionized water, stirring until a uniform dark green solution is formed, and then slowly adding dropwise the graphene oxide mixed solution prepared in step 1) to obtain mixed solution A;

[0010] 3) dissolving the sulfur source, nitrogen source, and polyvinyl pyrrolidone in the mixed solution A obtained in step 2), stirring vigorously to obtain a precursor mixed solution B, transferring the precursor mixed solution B to a reaction kettle, and performing a hydrothermal reaction in an oven;

[0011] 4) Washing the hydrothermal reaction sample obtained in the previous step, freeze-drying and collecting it to obtain the final product Cu-Mo-Sx / N,S-rGO.

[0012] Furthermore, in step 1), the concentration of the graphene oxide solution is 2 mg·mL -1 , and its volume ratio to ethanol solution is 0 to 1:1.

[0013] Furthermore, in step 2), in the mixed solution A, the amount of the copper salt is 0.1 to 2 mmol, and the amount of the molybdate is 0.1 to 2 mmol.

[0014] Furthermore, in step 3), the sulfur source is thiourea, the nitrogen source is urea, and in the mixed solution B, the amount of the sulfur source is 0.5-3 mmol, the amount of the nitrogen source is 0.5-10 mmol, and the amount of polyvinyl pyrrolidone is 0.5-2 mmol.

[0015] Furthermore, in step 3), the temperature for the hydrothermal reaction in the oven is 180-220° C., and the hydrothermal reaction time is 16-24 h.

[0016] In the nitrogen-sulfur co-doped graphene-supported copper-molybdenum sulfide catalyst prepared by the above method, Cu-Mo-S x Nano-sheets, coated in Cu-Mo-S x The nitrogen-sulfur co-doped graphene surface contains a large number of wrinkles and abundant unsaturated sites. This catalyst can be used in the catalytic reduction of CO2, and its catalytic activity can be further improved compared to existing copper-based catalysts.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] 1. After doping CuS with Mo atoms, the desorption of CO can be slowed down, and the eccentric charge around the Mo atoms is beneficial to the COOH - The stability of Cu-Mo-Sx / N,S-rGO can further enhance the catalytic activity of copper-based catalysts. Compared with the catalytic materials without Mo atoms, the Faradaic efficiency of Cu-Mo-Sx / N,S-rGO in producing formic acid during the catalytic reduction of CO2 is further improved, reaching as high as about 85%.

[0019] 2. Co-doping of N and S atoms on graphene can generate more active sites for CO2 reduction. Cu-Mo-Sx nanosheets synthesized by doping CuS with Mo atoms have a large specific surface area and are rich in active sites. Therefore, N and S co-doped graphene loaded with copper molybdenum sulfide can produce a synergistic effect, which helps to further improve the reaction activity during the catalytic reduction of CO2.

[0020] 3. The Cu-Mo-Sx / N,S-rGO disclosed in this application not only has a high Faradaic efficiency for formic acid production, but also has good catalytic stability. It can still maintain a Faradaic efficiency of 76% after 21 hours of continuous catalysis at an applied voltage of -0.85V.

[0021] 4. The preparation method of the composite catalyst disclosed in this application has the characteristics of low energy demand and mild reaction conditions, which provides a simple method for designing high-quality electrocatalysts to reduce carbon dioxide emissions. It also provides a broader idea for the preparation of small copper-based compounds with energy storage, electrocatalysis and electrochemical sensing functions. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Statistical graph of the maximum Faraday efficiency for formic acid formation at an applied voltage of -0.65 V when the catalytic materials prepared in Example 1 and Comparative Examples 1-3 act on a CO2-saturated KHCO3 solution;

[0023] Figure 2The SEM image of Cu-Mo-Sx prepared in comparative example 3 (left) and the TEM image of Cu-Mo-Sx / N,S-rGO prepared in example 1 (right).

[0024] Figure 3 XRD pattern of Cu-Mo-Sx / N,S-rGO prepared in Example 1;

[0025] Figure 4 This is a Faraday efficiency diagram of formic acid production at an applied voltage of -0.85 V when the catalytic material prepared in Example 1 acts on a CO2-saturated KHCO3 solution. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is further described below with reference to the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0027] Example 1: Cu-Mo-S loaded on nitrogen-sulfur co-doped graphene X Nanosheet array particle method

[0028] 15 mL of ethanol was added to 15 mL of graphene oxide solution (2 mg mL -1 ) was stirred and ultrasonicated to obtain a uniform graphene oxide mixture. Subsequently, copper nitrate hexahydrate (1 mmol) and ammonium molybdate tetrahydrate (1 mmol) were dissolved in 35 mL of deionized water and stirred vigorously for 1 hour until a uniform dark green solution was formed. The mixture was then slowly added dropwise to the graphene oxide mixture to obtain a uniform mixture A. Thiourea (1 mmol), urea (10 mmol), and polyvinylpyrrolidone (PVP, 2 mmol) were dissolved in mixture A and stirred vigorously for 30 minutes to obtain precursor mixture B. Precursor mixture B was transferred to a polytetrafluoroethylene-lined stainless steel autoclave and placed in an oven maintained at 200°C for a hydrothermal reaction of 20 hours. The resulting sample was washed multiple times with deionized water and ethanol and freeze-dried to collect the final product, Cu-Mo-Sx / N,S-rGO.

[0029] Figure 3 The XRD characterization diagram of Cu-Mo-Sx / N,S-rGO prepared in Example 1 is shown in FIG. Figure 3 It can be seen that Cu-Mo-Sx is mainly composed of MoS2 and CuS, and the broad peak at 2θ=25° indicates the presence of rGO.

[0030] Comparative Example 1: Method for loading CuS on nitrogen-sulfur co-doped graphene

[0031] 15 mL of ethanol was added to 10 mL of graphene oxide solution (2 mg mL-1 ) was stirred and ultrasonicated to obtain a uniform graphene oxide solution. Subsequently, copper nitrate hexahydrate (2 mmol) was dissolved in 35 mL of deionized water and stirred vigorously for 1 hour until a uniform dark green solution was formed. The solution was then slowly added dropwise to the graphene oxide mixture to obtain a uniform mixed solution A. Thiourea (2 mmol), urea (8 mmol), and polyvinylpyrrolidone (PVP, 1 mmol) were dissolved in mixed solution A and stirred vigorously for 30 minutes to obtain precursor mixed solution B. Precursor mixed solution B was transferred to a polytetrafluoroethylene-lined stainless steel autoclave and placed in an oven maintained at 180°C. The hydrothermal reaction was carried out for 16 hours. The resulting sample was washed multiple times with deionized water and ethanol and freeze-dried to collect the final product, CuS / N,S-rGO.

[0032] Comparative Example 2: Loading Cu-Mo-S on sulfur-doped graphene X Nanosheet method

[0033] 15 mL of ethanol was added to 5 mL of graphene oxide solution (2 mg mL -1 ) was stirred and ultrasonicated to obtain a uniform graphene oxide solution. Subsequently, copper nitrate hexahydrate (0.5 mmol) and ammonium molybdate tetrahydrate (0.5 mmol) were dissolved in 35 mL of deionized water and stirred vigorously for 1 hour until a uniform dark green solution was formed. The solution was then slowly added dropwise to the graphene oxide mixture to obtain a uniform mixed solution A. Thiourea (1 mmol) and polyvinylpyrrolidone (PVP, 2 mmol) were dissolved in mixed solution A and stirred vigorously for 30 minutes to obtain precursor mixed solution B. Precursor mixed solution B was transferred to a polytetrafluoroethylene-lined stainless steel autoclave and placed in an oven maintained at 220°C. The reaction was hydrothermally reacted for 24 hours. The resulting sample was washed multiple times with deionized water and ethanol and freeze-dried to collect the final product, Cu-Mo-Sx / S-rGO.

[0034] Comparative Example 3: Cu-Mo-S X Preparation of nanosheets

[0035] Copper nitrate hexahydrate (1 mmol) and ammonium molybdate tetrahydrate (1 mmol) were dissolved in a mixture of 35 mL of deionized water and 15 mL of ethanol and stirred vigorously for 1 hour until a homogeneous dark green solution was formed. Thiourea (2 mmol) and polyvinylpyrrolidone (PVP, 0.5 mmol) were then added and stirred vigorously for 30 minutes to obtain the precursor mixture. The mixture was then transferred to a polytetrafluoroethylene-lined stainless steel autoclave and placed in an oven at 220°C for 20 hours. The final sample was washed with deionized water and ethanol multiple times and freeze-dried.

[0036] from Figure 2 It can be seen that the Cu-Mo-S prepared in this comparative examplex The granular material is composed of nanosheets that grow randomly in all directions. x Cu-Mo-Sx / N,S-rGO is generated after external coating with nitrogen-sulfur co-doped graphene, and the graphene contains a large number of wrinkles.

[0037] Catalytic performance test:

[0038] When preparing the working electrode, 2 mg of the catalyst materials prepared in Example 1 and Comparative Examples 1-3 were dispersed in a mixture of 950 μL of water and 50 μL of Nafion (5 wt%) to obtain a uniform catalyst mixture. Then, 40 μL of the catalyst mixture was dropped onto a glassy carbon electrode with a diameter of 6 mm and dried at room temperature (catalyst loading mass was approximately 0.283 mg·cm -2 ).

[0039] The working electrode was placed in a CO2-saturated KHCO3 (0.1M) aqueous solution, and the Faradaic efficiency of formic acid formation in each reaction system was measured at an applied voltage of -0.65 V (vs RHE). Figure 1 As shown, among all samples, the Cu-Mo-Sx / N,S-rGO prepared in Example 1 exhibited a high Faradaic efficiency for formic acid formation, indicating that this compound has the best electrocatalytic activity, with a Faradaic efficiency of approximately 85%. This indicates that Mo-doped CuS has higher catalytic activity and can produce a synergistic effect with N and S co-doped graphene, significantly improving catalytic activity in the electrocatalytic reduction of carbon dioxide.

[0040] A working electrode made from the catalyst material prepared in Example 1 was placed in a CO2-saturated KHCO3 (0.1M) aqueous solution, and the Faradaic efficiency of formic acid formation in the reaction system was measured at an applied voltage of -0.85 V (vs RHE). As can be seen from the figure, at an applied voltage of -0.85 V, after 21 hours of continuous catalysis, the Faradaic efficiency can still be maintained at around 76%, indicating that the material has good stability.

[0041] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention's description and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for preparing a nitrogen-sulfur co-doped graphene-supported copper-molybdenum sulfide catalyst, characterized in that: The steps include: 1) Add ethanol to 15 mL of graphene oxide solution, stir, and sonicate to obtain a graphene oxide mixed solution; 2) Dissolve copper salt and molybdate in deionized water, stir until a uniform dark green solution is formed, and slowly add the solution dropwise to the graphene oxide mixture prepared in step 1) to obtain a mixture A; 3) Dissolving the sulfur source, nitrogen source, and polyvinylpyrrolidone in the mixed solution A obtained in step 2) and vigorously stirring to obtain a precursor mixed solution B. Transferring the precursor mixed solution B to a reaction vessel and performing a hydrothermal reaction in an oven; 4) Wash the hydrothermal reaction sample obtained in the previous step, freeze-dry it, and collect it to obtain the final product Cu-Mo-Sx / N,S-rGO; In step 1), the concentration of graphene oxide solution is 2 mg·mL -1 , the volume ratio of the ethanol solution is 0~1:1; In step 2), in the mixed solution A, the amount of copper salt is 0.1-2 mmol, and the amount of molybdate is 0.1-2 mmol; In step 3), the sulfur source is thiourea, the nitrogen source is urea, and in the mixed solution B, the amount of the sulfur source is 0.5-3 mmol, the amount of the nitrogen source is 0.5-10 mmol, and the amount of polyvinylpyrrolidone is 0.5-2 mmol.

2. The method for preparing the nitrogen-sulfur co-doped graphene-supported copper-molybdenum sulfide catalyst according to claim 1, wherein: In step 3), the temperature of the hydrothermal reaction in the oven is 180-220° C., and the hydrothermal reaction time is 16-24 h.

3. A nitrogen-sulfur co-doped graphene-supported copper-molybdenum sulfide catalyst, characterized in that: It is prepared according to the preparation method of nitrogen-sulfur co-doped graphene supported copper-molybdenum sulfide catalyst according to any one of claims 1-2, Cu-Mo-S x Nano-sheets, coated in Cu-Mo-S x The nitrogen-sulfur co-doped graphene on the surface contains a large number of wrinkled structures and abundant unsaturated sites.

4. Use of the nitrogen-sulfur co-doped graphene-supported copper-molybdenum sulfide catalyst as claimed in claim 3 in the field of catalytic reduction of CO2.

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