Nitrogen atom doped carbon material loaded superfine metal nano-cluster catalyst and preparation and application thereof

Abstract

The invention discloses a nitrogen atom-doped carbon material loaded superfine metal nano-cluster catalyst as well as preparation and application thereof, and belongs to the field of hydrogen production by electrolysis of water. According to the preparation method, UIO-66 (Ce) with the surface rich in mesoporous and microporous structures is prepared by using a soft template and is used as a precursor, and then the precursor is subjected to carbonization and acid pickling treatment to obtain the three-dimensional nitrogen-doped porous carbon spheres with high specific surface area. The rich hierarchical porous structures on the surface of the carbon material can effectively limit the aggregation of metal active centers in high-temperature reduction and HER reaction processes, and the long-term stability of the catalyst is improved. And due to the extremely high specific surface area, more metal active centers can be loaded, so that the catalytic activity is improved. Experiments show that nitrogen doping can effectively regulate and control the electronic structure of an active center, and the catalytic activity and stability of the catalyst are improved.

Description

Technical Field

[0001] The invention relates to the field of hydrogen production by electrolysis of water, and in particular to a nitrogen atom-doped carbon material-loaded ultrafine metal nanocluster catalyst and its preparation and application. Background Art

[0002] As the carbon-free energy carrier with the highest energy density (146 kJ / g), hydrogen is considered to be the most promising alternative to fossil energy in the future. At present, the main methods of industrial hydrogen production include hydrogen production from fossil energy, industrial by-product hydrogen, hydrogen production from electrolysis or photolysis of water, and biohydrogen production.

[0003] The water electrolysis hydrogen production system mainly includes a cathode, an anode and an electrolyte. The kinetics of the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are relatively slow, and the corresponding hydrogen evolution catalyst and oxygen evolution catalyst must be coated on the anode and cathode surfaces, respectively, to accelerate the water electrolysis process. Theoretically, under standard conditions of T=298.13 K, P=1 atm and not affected by pH value, the thermodynamic equilibrium voltage of water electrolysis only requires a voltage of 1.23 V. However, in industrial acidic or alkaline water electrolysis, the actual voltage required will be higher than this theoretical voltage. That is, to maintain the electrolysis reaction, the actual applied voltage must exceed the theoretical value. This additional voltage is the overpotential, represented by the symbol η. The overpotential faced by the entire water electrolysis process mainly consists of two parts: one is the overpotential occurring at the anode (η OER ), which is related to the oxygen release reaction; the second is the overpotential occurring at the cathode (η HER ), which is related to the hydrogen release reaction. The last part is the overpotential caused by other forms of resistance (η other ). There is a significant correlation between the activation energy of the hydrogen evolution reaction and the electrode potential. In the actual water electrolysis reaction, it is hindered by the high overpotential, resulting in high energy consumption and low hydrogen and oxygen production efficiency, which seriously restricts the development of industrial water electrolysis to produce hydrogen. In order to promote the improvement of energy conversion efficiency in the water electrolysis reaction, it is crucial to select high-performance oxygen evolution (OER) and hydrogen evolution (HER) catalysts to reduce the overpotential in the reaction process.

[0004] At present, the reported catalysts for the complete water splitting can be divided into precious metal catalysts and non-precious metal catalysts. The precious metal catalysts with the best performance are mainly Pt-based catalysts, such as Pt / NiO@Ni / NF (ACS Catal., 2018, 8(9): 8866-72.), Pt@DNA–GC (ACS Catal., 2016, 67, 4660-4672); non-precious metal catalysts are mainly catalysts composed of transition metals and phosphorus, sulfur, boron, carbon, nitrogen, etc., such as Fe-NC (Angew. Chem. Int. Ed., 2017, 56(24): 6937-41), NiFe 2 O 4 @MOF-74(ACS Appl. Mat., 2020, 12(41): 45987-96), etc. These catalysts can effectively reduce the overpotential of water electrolysis reaction, but their preparation process is relatively cumbersome and the conditions are harsh, and the reaction stability of the catalyst itself still needs to be further improved. Therefore, the development of a water electrolysis hydrogen evolution catalyst with a simple and convenient preparation process, excellent catalytic performance and high stability is still a problem to be solved in this field.

[0005] Carbon materials have shown great potential in the application of catalyst supports for hydrogen evolution in water electrolysis due to their high conductivity, high stability, low price and wide sources. According to the different morphologies of carbon materials, they can be divided into: zero-dimensional fullerenes, carbon quantum dots, one-dimensional carbon nanobelts, carbon nanofibers, carbon nanotubes, two-dimensional graphene and three-dimensional porous carbon supports. Carbon materials with different structures have different specific surface areas, electrical conductivity, physicochemical properties and mechanical strength. Among them, heteroatom-doped three-dimensional porous carbon materials are widely used as effective supports for stable electrocatalytic metal nanoparticles. Although in recent years, there have been reports on a variety of heteroatom-doped three-dimensional porous carbon materials such as Ru / AC (CEJ, 2021, 420(3): 1385-8947), MoP-QDs@PC (CEJ, 2023 454(140105): 1385-8947), and Ni@NCW (AM, 2023 36(4): 1521-4095). However, the pore types are relatively single, and the prepared carbon carrier has a small specific surface area. Therefore, it is urgent to find a simple and effective method to synthesize heteroatom-doped carbon materials with high specific surface area and multi-level pores, and apply them to the efficient electrolysis of water for hydrogen evolution reaction. Summary of the invention

[0006] The purpose of the present invention is to provide a nitrogen atom-doped carbon material-supported ultrafine metal nanocluster catalyst and its preparation and application. The catalyst of the present invention has very good hydrogen evolution catalytic performance and stability.

[0007] The present invention first provides a method for preparing a Ru / NC catalyst, comprising the following steps: (1) Mixing polyether, glacial acetic acid, 1,3,5-trimethylbenzene and water to prepare a microemulsion solution; (2) mixing the microemulsion solution, ammonium cerium nitrate and terephthalic acid to react and obtain three-dimensional porous spherical UIO-66 (Ce) MOFs; (3) Mixing a cyanamide solution, the three-dimensional porous spherical UIO-66 (Ce) MOFs and water, and freeze-drying the mixture; and calcining the freeze-dried mixture to obtain CeO 2 / NC; (4) The CeO 2 / NC and sulfuric acid solution are mixed and reacted to obtain three-dimensional nitrogen-doped porous carbon spheres (NC); (5) A soluble ruthenium salt solution and the three-dimensional nitrogen-doped porous carbon sphere (NC) are mixed, freeze-dried, and then annealed to obtain the Ru / NC catalyst.

[0008] In the above preparation method, the polyether is polyether 123.

[0009] In the above-mentioned preparation method, in steps (1) and (2), the mass ratio of the ammonium cerium nitrate to the polyether is 1:1.5-3, specifically 1:1.9; The mass ratio of the cerium ammonium nitrate to the volume ratio of the glacial acetic acid is 1 g:1-3 mL, specifically 1 g:1.8 mL; The mass ratio of the cerium ammonium nitrate to the volume ratio of 1,3,5-trimethylbenzene is 1 g:20-80 mL, specifically 1 g:22 mL; The mass ratio of the ammonium cerium nitrate to the volume ratio of water is 1 g:100-400 mL, specifically 1 g:109 mL; The mass ratio of the ammonium cerium nitrate to terephthalic acid is 1:2-4, specifically 1:3.

[0010] In the above preparation method, in step (2), the reaction temperature is 35°C-45°C; The reaction time is 30 min-60 min.

[0011] In the above preparation method, in step (3), the volume ratio of the mass of the three-dimensional porous spherical UIO-66 (Ce) MOFs to the cyanamide solution is 1 g: 10-30 mL, specifically 1 g: 20 mL; The concentration of the cyanamide solution is 30 mg / mL-60 mg / mL, specifically 50 mg / mL; The volume ratio of the cyanamide solution to water is 1:5-1:12, specifically 1:9; In step (3), the calcination temperature is 650°C-850°C, specifically 750°C; The calcination time is 2 h-4 h, specifically 3 h; The heating rate of the calcination is 2°C / min-10°C / min, specifically 5°C / min; The calcination is performed in an inert atmosphere.

[0012] In the above-mentioned preparation method, in step (4), the CeO 2 / The mass ratio of NC to the volume ratio of sulfuric acid solution is 1 g:300-500 mL, specifically 1 g:400 mL; The concentration of the sulfuric acid solution is 1 M-2 M, specifically 1 M; In step (4), the reaction temperature is 25°C-100°C, specifically 50°C; The reaction time is 20 h-30 h, specifically 24 h.

[0013] In the above-mentioned preparation method, in step (5), the soluble ruthenium salt solution is a ruthenium chloride solution; The mass ratio of the soluble ruthenium salt to the three-dimensional nitrogen-doped porous carbon sphere (NC) is 1:8-1:30, specifically 1:8-1:10; The concentration of the soluble ruthenium salt solution is 0.8-1.2 mg / mL, specifically 1.0 mg / mL; In step (5), the annealing temperature is 300°C-700°C, specifically 400°C, 500°C or 600°C; The annealing time is 1 h-3 h, specifically 2 h; The heating rate of the annealing is 2°C / min-10°C / min, specifically 5°C / min; The annealing is performed in an inert atmosphere.

[0014] In the above preparation method, the inert atmosphere is a nitrogen or argon atmosphere.

[0015] In the above preparation method, in steps (3) and (5), the freeze-drying temperature is -60°C to -80°C; The freeze-drying time is 20 h-30 h.

[0016] In the above preparation method, after the reaction in step (2), there is a step of centrifuging to obtain a solid and washing and drying the solid; specifically, the washing is performed using DMF and anhydrous ethanol; the drying is performed in a vacuum oven at 30° C. for 24 h; In step (3), before calcination, there is also a step of removing residual water in the pores at 80°C-120°C; After the reaction in step (4), there are washing and drying steps; specifically, the washing is carried out with water; and the drying is carried out at 60° C. for 12 h.

[0017] The present invention also provides a Ru / NC catalyst prepared by the above preparation method.

[0018] The application of the above-mentioned Ru / NC catalyst in the production of hydrogen by water electrolysis also falls within the protection scope of the present invention.

[0019] The present invention uses a soft template to prepare UIO-66 (Ce) with a surface rich in mesopores and micropores as a precursor, and then carbonizes it and acid-washes it to obtain a three-dimensional nitrogen-doped porous carbon sphere with a high specific surface area. The rich multi-level pore structure on the surface of the carbon material can effectively limit the aggregation of metal active centers during high-temperature reduction and HER reactions, thereby improving the long-term stability of the catalyst. The extremely high specific surface area can load more metal active centers to improve the catalytic activity. In addition to adjusting the carrier morphology, the HER performance of Ru-based catalysts can also be improved by optimizing the electronic structure of Ru. Experiments show that nitrogen doping can effectively regulate the electronic structure of the active center and improve its catalytic activity and stability. After the carbon carrier is regulated by heteroatom nitrogen doping, the interaction between the metal and the carrier is improved, so that the charge of the active center of the catalyst is redistributed during the preparation process and its surface electronic structure is optimized, thereby improving the hydrogen evolution reaction performance of the Ru-based catalyst.

[0020] The present invention has the following beneficial effects: (1) The unique three-dimensional spherical porous structure of Ru / NC catalyst gives it a high specific surface area, allowing the carrier to load more metal active centers; the metal nanoparticles on the carrier surface reduce their agglomeration during the reduction and catalytic process due to the confinement effect of the pores; the mesoporous structure ensures sufficient contact between the active centers and the electrolyte during the catalytic process, thereby improving the mass transfer rate; its hydrogen evolution performance is far superior to that of commercial carbon black-loaded ruthenium metal hydrogen evolution catalyst.

[0021] (2) The introduction of nitrogen atoms increases the number of defect sites on the carrier surface, promotes the interaction between the carrier and the metal, anchors the catalytic active sites, prevents their dissolution during the catalytic process, optimizes the hydrogen adsorption free energy, and improves the intrinsic activity; the Ru / NC catalyst of the present invention only requires an overpotential of 29.2 mV at 10 mA / cm², which is much lower than the 52.2 mV of the Pt / C catalyst. At the same time, the Ru / NC catalyst also exhibits excellent HER stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the synthesis method of Ru / NC catalyst.

[0023] Figure 2 The scanning electron microscope image and XRD pattern of the catalyst prepared in Example 1; wherein, Figure 2 (a) is a scanning electron microscope image of SGPN-UIO-66 (Ce); (b) is an XRD comparison diagram of SGPN-UIO-66 (Ce) and microporous UIO-66 (Ce); (c) is a scanning electron microscope image of Ru / NC catalyst.

[0024] Figure 3 TEM images and HRTEM images of Ru / NC catalyst.

[0025] Figure 4 The XRD results comparison of Ru / NC and Ru / C catalysts and the XRD results comparison of different reduction temperatures (300℃-700℃) are shown.

[0026] Figure 5 N of NC and Ru / NC catalysts 2 Adsorption-desorption isotherms and pore size distribution.

[0027] Figure 6 Raman spectra of C, NC, and Ru / NC catalysts.

[0028] Figure 7 The HER catalytic activity of Ru / NC, Ru / C, Ru / XC72C, Pt / C, and NC; Figure 7 (a) is the linear voltammetric scan curve LSV; (b) is the Tafel curve and Tafel slope; (c) is the Nengquist impedance diagram; (d) is the active area comparison diagram.

[0029] Figure 8 This is a stability test of Ru / NC catalyst. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below in conjunction with specific embodiments. The given examples are only for illustrating the present invention, but not for limiting the scope of the present invention.

[0031] The experimental methods in the following examples are all conventional methods unless otherwise specified.

[0032] The quantitative tests in the following examples were all repeated three times, and the results were averaged.

[0033] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0034] Usually, 10 mA / cm is used to evaluate the performance of catalysts. 2 The overpotential at a current density of 2000 Å is used as a metric for hydrogen evolution performance. This paper uniformly uses the symbol η 10 .

[0035] Example 1 The preparation process of Ru / NC catalyst is as follows Figure 1 As shown, the specific preparation method is as follows: 1. Preparation of three-dimensional porous spherical UIO-66 (Ce) MOFs (SGPN-UIO-66 (Ce)) The soft template method was used to prepare three-dimensional porous spherical UIO-66 (Ce) MOFs with uniform particle size as the precursor of the subsequent carbon carrier. The preparation method is as follows: First, prepare a 25 mL beaker, then dissolve 176.6 mg of polyether P123 in 10 mL of deionized water, and then add 170 μL of glacial acetic acid and 2 mL of 1,3,5-trimethylbenzene to the above solution. Then add a rotor to stir continuously to form a microemulsion solution A for use. Then weigh 92 mg (NH 4 )Ce(NO 3 ) 6 (cerium ammonium nitrate) and 280 mg of terephthalic acid were added to solution A to obtain solution B. After sealing with plastic wrap, solution B was stirred in an oil bath at 45°C, and a milky white product was obtained after 50 min of reaction. The obtained product was poured into a 50 mL centrifuge tube and stirred at 10,000 rpm / min for 7 min. The milky white turbid liquid obtained by centrifugation was introduced into a waste liquid bucket. The obtained light yellow solid was washed several times with DMF (N,N-dimethylformamide) and anhydrous ethanol to remove the soft template P123 and excess reactants. The obtained sample was placed in a vacuum oven and dried at 30°C for 24 h to obtain three-dimensional porous spherical UIO-66 (Ce) MOFs.

[0036] 2. Preparation of three-dimensional nitrogen-doped porous carbon spheres (NC) The synthesis steps of NC are as follows: (1) Add 9 mL of deionized water and 1 mL of cyanamide solution (concentration 50 mg / mL) into a 25 mL beaker and mix by low-temperature ultrasonication at 5°C until homogenized.

[0037] (2) Weigh 50 mg of three-dimensional porous spherical UIO-66 (Ce) MOFs and add them to the solution prepared in (1). Low-temperature ultrasonic treatment was performed at 5 °C for 30 min. After the solution was homogenized, it was placed in a vacuum freeze dryer at -70 °C for freeze drying for 48 h.

[0038] (3) The freeze-dried sample was transferred to a corundum magnetic boat and placed in a tube furnace. The tube furnace was sealed and the air in the furnace was evacuated using a vacuum pump and then argon was introduced. This process was repeated twice to ensure that there was no air in the quartz tube. In an argon atmosphere, the temperature was raised to 100°C at a rate of 5°C / min and kept for 30 min to remove the residual water in the MOFs pores. The temperature was then raised for 130 min to 750°C and kept for 180 min to obtain CeO 2 / NC; (4) Weigh 50 mg of CeO prepared in step (3) 2 The / NC material was placed in a 25 mL beaker, and then 20 mL of 1 mol / L sulfuric acid solution was added. After ultrasonic dispersion, it was placed in an oil bath at 50 °C for 24 h. It was then filtered and washed with deionized water until the filtrate pH = 7, and then placed in an oven at 60 °C for 12 h to obtain three-dimensional nitrogen-doped porous carbon spheres.

[0039] 3. Preparation of Ru / NC catalyst (1) Use a pipette to pipette 80 μL of anhydrous ruthenium chloride solution (concentration: 0.066 g / mL) and 5 mL of deionized water into a 10 mL beaker, then sonicate at 5°C until the solution is homogeneous.

[0040] (2) Weigh 0.05 g of the NC prepared above and add it to the uniform solution in step (1). The resulting solid-liquid mixture is subjected to low-temperature ultrasonic treatment at 5°C. After the NC is evenly dispersed, it is placed in a vacuum freeze dryer and freeze-dried at -70°C for 24 h.

[0041] (3) The dried RuCl 3 The Ru / NC mixture was placed in a porcelain boat and heated to 600°C at a heating rate of 5°C / min under an argon atmosphere and kept at this temperature for 2 h to obtain a Ru / NC catalyst.

[0042] 4. Catalyst characterization and performance testing (1) First, the SGPN-UIO-66 (Ce) synthesized in this example was characterized by SEM. Figure 2As shown in (a), the SEM image shows that SGPN-UIO-66 (Ce) has a uniform spherical structure with an average size of ~570 nm. Figure 2 In the upper right corner of (a), it can be observed that the surface of SGPN-UIO-66 (Ce) presents an ordered pore structure, which is essential for the synthesis of three-dimensional nitrogen-doped porous carbon spheres. In addition, the specific surface area and pore structure of the catalyst can be changed by changing the synthesis temperature of SGPN-UIO-66 (Ce). XRD analysis ( Figure 2 (b) shows that the diffraction peaks of SGPN-UIO-66(Ce) match well with those of microporous UIO-66(Ce) nanoparticles (Shanghai MacLean Biochemical Technology Co., Ltd.), proving the crystal structure and phase purity of SGPN-UIO-66(Ce). Figure 2 (c) in the figure is a scanning electron microscope image of the prepared Ru / NC catalyst. From the results, it can be seen that although there is a small amount of granular fine carbon material produced during the carbonization process of monocyanamide on the surface of the Ru / NC catalyst at a size of 1 μm, no obvious metal particles appear, which preliminarily proves that the metal nanoparticles loaded on the surface of the carrier are small in size. In addition, after the three-dimensional porous spherical carbon carrier is nitrogen-doped, acid-washed and loaded with Ru metal, the morphology of the catalyst is not affected, which further proves the structural stability of SGPN-UIO-66 (Ce), which also lays a solid foundation for the high stability of the subsequent Ru / NC catalyst in the hydrogen evolution process. Figure 2 In (a), it can be clearly seen that the catalyst surface is covered with mesopores. The presence of mesopores can improve the mass transfer rate of the catalyst and facilitate the exposure of active sites, thereby enhancing its catalytic performance in the hydrogen evolution reaction. In order to further confirm the distribution and particle size of the noble metal Ru on the support, transmission electron microscopy (TEM) was used to characterize Ru / NC. The results are shown in Figure 3 . Figure 3 (a) is a TEM image of the Ru / NC catalyst. It can be observed that the carrier of the Ru / NC catalyst is a three-dimensional porous carbon sphere with a diameter of about 250 nm. Figure 2 The diameter of SGPN-UIO-66 (Ce) in (a) has decreased, mainly due to the decomposition of organic components during the carbonization process, which leads to its shrinkage. In addition, many highly open three-dimensional pores can be observed on its surface, which is conducive to ion transport and increases the contact area with the electrolyte during the catalytic process, thereby increasing the mass transfer rate. In order to explore the existence state of Ru nanoparticles on the surface of the catalyst support, the surface morphology of the catalyst is enlarged, such as Figure 3 In (b, c), there are still no obvious metal particles on the support surface, which proves that the loaded Ru nanoparticles are evenly distributed and have no obvious aggregation. Figure 4 (d) in the figure is the HRTEM image of the Ru / NC catalyst. It can be observed from the figure that fine particles with a size of 0.5-1.2 nm are evenly distributed on the surface of the carrier, and there are no obvious aggregated large particles, which proves the confinement effect of the three-dimensional porous carbon spheres on the loaded metal particles.

[0043] (2) In addition, the XRD results of Ru / NC and Ru / C prepared under the same conditions, as well as the XRD results of Ru / NC catalysts prepared at different reduction temperatures, were compared. Figure 4 As shown in (a) in the figure, the XRD results of Ru / NC and Ru / C under the same preparation conditions were first compared. In the XRD results of Ru / C, it was observed that there was a broad peak at around 24°, which was attributed to the C (002) peak; in addition, there was an obvious diffraction peak at around 43°C attributed to Ru (101). By comparing the X-ray diffraction (XRD) spectra of Ru / NC and Ru / C samples, it was found that compared with Ru / C, the Ru / NC catalyst did not show an obvious diffraction peak at around 43.5° attributable to Ru. This proves that after the carbon support was nitrogen-doped, the anchoring effect of the doped nitrogen atoms on the metal limited its agglomeration during high-temperature annealing. Subsequent studies analyzed the effect of reduction temperature on the XRD spectrum of the prepared Ru / NC catalyst, see Figure 4 (b) in Figure 4 As can be seen in (b), even when the reduction temperature reaches 700°C, there is still no obvious characteristic peak of Ru, which further proves the anchoring effect of the three-dimensional porous carbon sphere structure and nitrogen doping modification on the metal.

[0044] Figure 4 The preparation method of the Ru / NC catalyst in (b) is the same as the above preparation method, except that the temperature in the preparation step (3) of 3. Ru / NC catalyst, "heating to 600°C at a heating rate of 5°C / min and keeping warm for 2 h" is changed.

[0045] (3) The high specific surface area and multi-level pore structure of NC and Ru / NC were further verified by BET specific surface area test data. Figure 5 (a) is the N of NC and Ru / NC catalysts. 2 From the adsorption-desorption isotherms, we can see that both Ru / NC and NC show type IV isotherms, indicating that the material has mesopores with a size of 2-50 nm, and the nitrogen adsorption-desorption isotherms of Ru / NC and NC show a rapid rise in the low relative pressure region and then tend to be flat, which indicates that there are still a large number of micropores in the material, which is consistent with the SEM and TEM results. In addition, the sample shows an H4 type hysteresis loop, which also indirectly proves the existence of a mesoporous structure on the surface of the material. In addition, Figure 5(b) is the pore size distribution curve of Ru / NC and NC. It can be seen from the figure that the peak positions in the pore size distribution curves of Ru / NC and NC samples are similar, and the pore sizes of both are mainly distributed around 3-4 nm and 10 nm, which means that the loading of Ru nanoparticles does not change the pore structure of NC. According to the BET equation, the specific surface areas of NC and Ru / NC are 1729.487 m 2 / g, 1613.991 m 2 / g, which also proves that the introduction of Ru nanoparticles leads to a decrease in the specific surface area of ​​the catalyst. Nevertheless, Ru / NC has more micropores and mesoporous structures, which helps to improve the hydrophilicity of the catalyst. In addition, the pore structure has a confining effect on the size of Ru nanoparticles, which stabilizes the catalyst to a certain extent and inhibits the increase of its particle size. In addition, it also has a large specific surface area that can load more active sites for the catalyst and improve the catalyst activity.

[0046] (4) If Figure 6 As shown in the figure, the defect concentration and graphitization degree of the prepared samples were studied by Raman spectroscopy. Figure 6 It can be seen that Ru / NC is at 1351.07 cm -1 and 1601 cm -1 There are two obvious strong peaks at the carbon material, which are respectively attributed to the typical carbon material D peak (generated by the structural defects in graphene or the disordered vibration of the edge graphene molecular layer, which represents the degree of defect of the carbon material) and the G peak (generated by the sp 2 The stretching vibration of the hybrid plane CC bond is generated, which represents the graphitization degree and structural integrity of the carbon material). Therefore, the intensity ratio of the D peak to the G peak (ID / IG) is used to quantitatively characterize the graphitization degree and defect site concentration of the carbon carrier. The ID / IG value of the pure carbon material without nitrogen doping is 0.9677, indicating that the MOF-derived carbon material has a lower defect site concentration in the absence of nitrogen doping, while the MOF-derived carbon material is nitrogen-doped using cyanamide, and the ID / IG value of the NC sample is increased to 1.13499, indicating that heteroatom nitrogen doping causes the carbon material to produce more defect sites, thereby strengthening the interaction between the noble metal Ru and the carbon carrier, adjusting the electronic structure of the metal, and facilitating the anchoring of the metal active site. Compared with the NC material, the ID / IG value of Ru / NC is significantly reduced to 0.9685, indicating that the noble metal Ru combines with the defect N in the carbon carrier, reducing the defect degree of the carbon material and increasing the degree of graphitization, thereby enhancing the conductivity of the catalyst. In addition, compared with NC, the D and G peaks of Ru / NC show a blue shift, which can be attributed to the chemical coordination between Ru nanoparticles and NC.

[0047] Figure 6 The preparation method of C is as follows: a. Weigh 50 mg of three-dimensional porous spherical UIO-66 (Ce) MOFs (prepared in the same way as above) into a corundum magnetic boat, and then place it into a tube furnace. Close the tube furnace, use a vacuum pump to evacuate the air in the furnace, and then introduce argon gas. Repeat twice to ensure that there is no air in the quartz tube. In an argon atmosphere, heat the temperature to 100 ° C at a rate of 5 ° C / min and keep it for 30 min to remove the residual water in the MOFs pores, and then continue to heat for 130 min to 750 ° C and keep it for 180 min to obtain CeO 2 / C; b. Weigh 50 mg of CeO prepared in step a. 2 The / C material was placed in a 25 mL beaker, and then 20 mL of 1 mol / L sulfuric acid solution was added. After ultrasonic dispersion, it was placed in an oil bath at 50°C for 24 h. It was then filtered and washed with deionized water until the filtrate pH = 7, and then placed in an oven at 60°C for 12 h to obtain three-dimensional porous carbon spheres (represented by C).

[0048] Figure 6 The NC in the middle was prepared according to the following Comparative Example 2.

[0049] (5) Catalytic performance and stability of Ru / NC catalyst To further illustrate the excellent catalytic activity of Ru / NC and its reaction mechanism, the performance of a series of samples (Ru / NC, Ru / C (Comparative Example 1), Ru / N-XC72C (Comparative Example 3), Pt / C and NC (Comparative Example 2)) were compared. First, the electrochemical activity of Ru / NC, Ru / C, Ru / XC72C, Pt / C and NC was tested. The specific test methods are as follows: a. Preparation of working electrode Before drop-coating the catalyst ink on the working electrode surface, the surface of the working electrode needs to be polished with alumina polishing powder of different particle sizes, and the order of the polishing particle size follows 1.0 μm-0.5 μm-0.03 μm. After polishing, the residual alumina on the surface is removed by ultrasonic technology, and then thoroughly cleaned with deionized water and ethanol, and wiped dry with lens paper for subsequent use. The measured catalyst is then pretreated to ensure that it is evenly distributed on the surface of the working electrode. The processing flow is as follows: First, a small amount of catalyst is finely ground in an agate mortar, 2.5 mg of sample is accurately weighed and placed in a centrifuge tube, 190 μL of deionized water and 280 μL of anhydrous ethanol, and 30 μL of Nafion are added as a binder. The mixed solution is ultrasonically treated for 30 minutes until the catalyst is evenly dispersed, and then 6 μL of the solution is pipetted with a pipette and dropped onto a glassy carbon electrode with a diameter of 3 mm (area 0.07 cm²) and dried; among them, the catalyst loading is 0.428 mg / cm².

[0050] Comparative sample: Commercial Pt / C (P822267-1g purchased from Shanghai MacLean Biochemical Technology Co., Ltd.) was used as a comparative sample. The preparation steps are as follows: 2.5 mg of 20 wt% Pt / C catalyst (wherein the Pt content in 50 mg Pt / C catalyst is 10 mg) was placed in a centrifuge tube, 280 μL of anhydrous ethanol and 190 μL of deionized water, and 30 μL of Nafion were used as a binder, and after ultrasonication for 30 min, it was dropped on a 3 mm glassy carbon electrode and dried.

[0051] The electrolytic cell system used for the following LSV, cyclic voltammetry analysis, electrochemical impedance spectroscopy analysis and long-term stability performance evaluation tests is as follows: a three-electrode system was selected: the working electrode was a catalyst drop-coated on a commercial glassy carbon electrode, the counter electrode was a carbon rod (purchased from Tianjin Gaoshi Ruilian (Tianjin) Optoelectronic Technology Co., Ltd.), the reference electrode was a mercury oxide electrode (purchased from Tianjin Gaoshi Ruilian (Tianjin) Optoelectronic Technology Co., Ltd.), the electrolytic cell was a 50 mL glass electrolytic cell (purchased from Tianjin Gaoshi Ruilian (Tianjin) Optoelectronic Technology Co., Ltd.), and the electrode liquid was a 1 mol / L potassium hydroxide solution.

[0052] b. LSV test: Linear sweep voltammetry (LSV) is performed by applying a time-dependent potential change to the working electrode (relative to the reference electrode), that is, the potential increases or decreases monotonically at a certain scan rate. Through LSV, the overpotential of the electrode at a specific current density (such as 10 mA / cm² and 50 mA / cm²) can be determined to evaluate the hydrogen evolution reaction (HER) performance of the prepared catalyst. The specific parameters of the LSV test in this experiment are as follows: the HER potential scan range is -0.8 V to -1.3 V, the scan rate is 5 mV / s, the IR compensation is automatically compensated by the electrochemical workstation at 95%, and the overpotential is calculated according to formula (1).

[0053] (1) Select the part of the processed linear sweep voltammetry (LSV) curve that follows the Tafel equation, according to formula (2): (2) (η: overpotential; i: current density) plot. In this equation, the Tafel slope a is an intrinsic parameter reflecting the characteristics of the electrocatalyst and is closely related to the kinetics of the hydrogen evolution reaction (HER). At the same time, the magnitude of the Tafel slope reveals the reaction mechanism of HER (see Table 1). Generally speaking, a lower Tafel slope means a faster and more efficient catalytic process.

[0054] Table 1 HER mechanism

[0055] c. Cyclic voltammetry (CV) In this study, CV scanning is used to calculate the electrochemically active area (ECSA). The electrochemically active area is an important data to measure the number of active sites of electrocatalytic materials. It is calculated based on the double-layer capacitance (Cdl) and is proportional to: (3) In formula (3), Cs: specific capacitance of the catalyst material; Cdl: double-layer capacitance. The double-layer capacitance is measured by cyclic voltammetry (CV) in the non-Faraday range of the material. The current difference obtained by gradually increasing the scan rate under the same voltage range is linearly fitted, and its slope is Cdl. The specific parameters of the CV test in this experiment are as follows: the HER potential scanning range is -0.55V to -0.8V, and the scan rates are 20, 40, 60, 80, 100, and 120 mV / s, respectively.

[0056] d. Electrochemical impedance spectroscopy (EIS) Electrochemical impedance spectroscopy (EIS) is a characterization method for evaluating the electron transfer rate of catalysts and the conductivity of materials. By fitting the measured data with an equivalent circuit model, a semicircular figure in the Nyquist diagram can be obtained, and its diameter represents the charge transfer impedance (Rct). A smaller Rct value represents a higher electron transfer efficiency. In this paper, the sample catalyst was selected to conduct a charge transfer test at a current density of 10 mA / cm 2 The impedance test is performed with a voltage of 0.1 Hz to 106 Hz.

[0057] The results are as follows Figure 7 shown by Figure 7 As shown in (a), the NC sample showed almost no HER activity during the hydrogen evolution process. When the overpotential exceeded 500 mV, the current density still failed to reach 10 mA / cm 2 , which further illustrates that metal Ru is the active site of the catalyst. In order to explore the effect of N doping on catalytic performance, the HER performance of Ru / C and Ru / NC was compared. Ru / C catalyst has a high specific surface area and porous structure, which can show good electrochemical catalytic activity. Under low current density conditions, it exhibits a hydrogen evolution activity of 40.2 mV. However, due to the fact that the carrier has not been modified by nitrogen doping, resulting in poor conductivity, and the interaction between the carrier and Ru metal is weakened, the catalytic activity of Ru / C decreases sharply at high current density (η 50 =142.2 mV). However, the nitrogen-doped Ru / NC catalyst has a 2 and 50 mA / cm 2The HER performances of the Ru / C catalyst were 29.2 mV and 109.2 mV, respectively. 10 and η 50 The performance improvement is attributed to the improvement of the conductivity and hydrophilicity of the support by nitrogen doping and the optimization of the charge distribution of Ru nanoparticles, thereby improving the free energy of hydrogen adsorption at the active site and enhancing the HER activity of Ru-based catalysts. Further studies have shown that different carbon supports also have a significant effect on the HER performance. By comparing Ru / NC and Ru / N-XC72C, it can be found that the η of Ru / N-XC72C is 10 , η 50 The values ​​of 110.2 mV and 266.2 mV are much lower than those of Ru / NC catalyst, which proves the advantages of nitrogen-doped three-dimensional porous carbon sphere structure of Ru / NC. Its high specific surface area and abundant micropores and mesopores provide a large amount of metal active site loading space and excellent mass transfer performance for the catalyst. In addition, its excellent conductivity and abundant defect sites further improve the catalytic performance of Ru / NC.

[0058] Finally, the HER performance of Ru / NC and commercial Pt / C catalysts was compared. 2 At a current density of , the commercial Pt / C catalyst exhibited an overpotential of 52.2 mV, which was much higher than that of the Ru / NC catalyst. In addition, combined with ICP-OES analysis, it was found that the Ru / NC catalyst only loaded 2.05wt% of Ru, with a lower preparation cost, and exhibited higher mass specific activity and economy, further proving that the Ru / NC catalyst has the potential to replace the commercial Pt / C catalyst. According to the Tafel slope ( Figure 7 (b) in the figure further reveals the kinetic performance of Ru / NC (50.36 mV / dec), Ru / C (56.64 mV / dec), Ru / N-XC72C (111.47 mV / dec), Pt / C (51.15 mV / dec), and NC (835.5 mV / dec) catalysts. The Ru / NC catalyst exhibits a lower Tafel slope, follows the Volmer-Tafel mechanism, and has excellent kinetic performance. Electrochemical impedance spectroscopy (EIS) tests further confirm the advantages of the Ru / NC catalyst in kinetic performance, such as Figure 7As shown in (c), the charge transfer resistance (Rct) of the Ru / NC catalyst is the lowest, which is 39.96 Ω. In comparison, the Rct of Ru / C is 60.56 Ω, indicating that the introduction of heteroatom N significantly reduces the charge transfer resistance of the catalyst and improves the conductivity of the carbon support. In the comparison of different carbon supports, the impedance of Ru / N-XC72C is as high as 113.6 Ω, which confirms that NC exhibits better conductivity than commercial carbon black. Finally, in the comparison between Ru / NC and commercial Pt / C catalyst (Rct=62.35 Ω), Ru / NC showed better conductivity and HER catalytic performance. In addition, the double layer capacitance (Cdl) of each sample was compared: Ru / NC (61.85 mF / cm 2 )、Ru / C(31.74 mF / cm 2 ), Ru / N-XC72C (21.99 mF / cm 2 )、Pt / C(48.59 mF / cm 2 )、NC(1.63 mF / cm 2 )( Figure 7 This further illustrates that the Ru / NC catalyst has a larger electrochemically active specific surface area and more abundant HER active sites, which is consistent with the LSV, Tafel and EIS data.

[0059] In the water electrolysis industry, the poor stability of catalysts has always been regarded as a key problem that needs to be solved urgently. Commercial platinum-carbon catalysts are limited in large-scale commercial applications, mainly due to the rapid decay of their catalytic activity during continuous electrolysis. This shortcoming has led to the need for frequent replacement of catalysts, which in turn has increased the overall cost of the water electrolysis process, making it difficult to be widely used at the industrial level. In order to demonstrate the long-term stability of the Ru / NC catalyst prepared in the present invention in the HER process, a constant current chronopotentiometry was used at 10 mA / cm 2 The long-term stability performance of Ru / NC catalyst was evaluated at a current density of . The specific test method is as follows: Chronovoltametry (VT): In this study, the chronovoltametry (VT) method was used to evaluate the long-term stability of the sample catalyst. Specifically, a constant current (-0.07 mA) was applied to test the voltage change curve over time. A smaller voltage loss indicates that the catalyst has good stability.

[0060] The results are as follows Figure 8 As shown by Figure 8It can be seen that in 1 M KOH electrolyte, the Ru / NC catalyst showed excellent stability after continuous electrolysis for 1500 min, which proved its remarkable stability in the alkaline hydrogen evolution process. This high stability comes from the stable structure and excellent corrosion resistance of the NC carrier, as well as the confinement effect caused by the suitable pore structure and pore size, which can effectively curb Ostwald ripening.

[0061] Comparative Example 1: Preparation of Ru / C Catalyst 1. Preparation of three-dimensional porous carbon spheres (C) (1) Weigh 50 mg of three-dimensional porous spherical UIO-66 (Ce) MOFs (prepared in the same way as in Example 1) and place them in a corundum magnetic boat, which is then placed in a tube furnace. The tube furnace is sealed, and the air in the furnace is evacuated using a vacuum pump and then argon is introduced. This is repeated twice to ensure that there is no air in the quartz tube. In an argon atmosphere, the temperature is raised to 100°C at a rate of 5°C / min and kept for 30 min to remove the residual water in the MOFs pores. The temperature is then raised for 130 min to 750°C and kept for 180 min to obtain CeO 2 / C; (2) Weigh 50 mg of CeO prepared in step (1) 2 The / C material was placed in a 25 mL beaker, and then 20 mL of 1 mol / L sulfuric acid solution was added. After ultrasonic dispersion, it was placed in an oil bath at 50°C for 24 h. It was then filtered and washed with deionized water until the filtrate pH = 7, and then placed in an oven at 60°C for 12 h to obtain three-dimensional porous carbon spheres.

[0062] 2. Preparation of Ru / C catalyst (1) Use a pipette to pipette 80 μL of anhydrous ruthenium chloride solution (concentration: 0.066 g / mL) and 5 mL of deionized water into a 10 mL beaker, then sonicate at 5°C until the solution is homogeneous.

[0063] (2) Weigh 0.05 g of the C prepared above and add it to the uniform solution in step (1). The resulting solid-liquid mixture is subjected to low-temperature ultrasonic treatment at 5°C. After the C is evenly dispersed, it is placed in a vacuum freeze dryer and freeze-dried at -70°C for 24 h.

[0064] (3) The dried RuCl 3 The Ru / C mixture was placed in a porcelain boat and heated to 600°C at a heating rate of 5°C / min under an argon atmosphere and kept at this temperature for 2 h to obtain a Ru / C catalyst.

[0065] Comparative Example 2: Preparation of NC Catalyst The preparation method of three-dimensional nitrogen-doped porous carbon spheres (NC) is the same as that in Example 1. The specific preparation process is as follows: 1. Preparation of three-dimensional porous spherical UIO-66 (Ce) MOFs (SGPN-UIO-66 (Ce)) The soft template method was used to prepare three-dimensional porous spherical UIO-66 (Ce) MOFs with uniform particle size as the precursor of the subsequent carbon carrier. The preparation method is as follows: First, prepare a 25 mL beaker, then dissolve 176.6 mg of polyether P123 in 10 mL of deionized water, and then add 170 μL of glacial acetic acid and 2 mL of 1,3,5-trimethylbenzene to the above solution. Then add a rotor to stir continuously to form a microemulsion solution A for use. Then weigh 92 mg (NH 4 )Ce(NO 3 ) 6 (cerium ammonium nitrate) and 280 mg of terephthalic acid were added to solution A to obtain solution B. After sealing with plastic wrap, solution B was stirred in an oil bath at 45°C, and a milky white product was obtained after 50 min of reaction. The obtained product was poured into a 50 mL centrifuge tube and stirred at 10,000 rpm / min for 7 min. The milky white turbid liquid obtained by centrifugation was introduced into a waste liquid bucket. The obtained light yellow solid was washed several times with DMF (N,N-dimethylformamide) and anhydrous ethanol to remove the soft template P123 and excess reactants. The obtained sample was placed in a vacuum oven and dried at 30°C for 24 h to obtain three-dimensional porous spherical UIO-66 (Ce) MOFs.

[0066] 2. Preparation of three-dimensional nitrogen-doped porous carbon spheres (NC) The synthesis steps of NC are as follows: (1) Add 9 mL of deionized water and 1 mL of cyanamide solution (concentration 50 mg / mL) into a 25 mL beaker and mix them by low-temperature ultrasonication until homogenized.

[0067] (2) Weigh 50 mg of three-dimensional porous spherical UIO-66 (Ce) MOFs and add them to the solution prepared in (1). Then, perform low-temperature ultrasound at 5 °C for 30 min. After the solution becomes homogeneous, place it in a vacuum freeze dryer at -70 °C and freeze-dry it for 48 h.

[0068] (3) The freeze-dried sample was transferred to a corundum magnetic boat and placed in a tube furnace. The tube furnace was sealed and the air in the furnace was evacuated using a vacuum pump and then argon was introduced. This process was repeated twice to ensure that there was no air in the quartz tube. In an argon atmosphere, the temperature was raised to 100°C at a rate of 5°C / min and kept for 30 min to remove the residual water in the MOFs pores. The temperature was then raised for 130 min to 750°C and kept for 180 min to obtain CeO 2 / NC.

[0069] (4) Weigh 50 mg of CeO prepared in step (3) 2 The / NC material was placed in a 25 mL beaker, and then 20 mL of 1 mol / L sulfuric acid solution was added. After ultrasonic dispersion, it was placed in an oil bath at 50 °C for 24 h. It was then filtered and washed with deionized water until the filtrate pH = 7, and then placed in an oven at 60 °C for 12 h to obtain three-dimensional nitrogen-doped porous carbon spheres.

[0070] Comparative Example 3: Preparation of Ru / N-XC72C Catalyst 1. The synthesis steps of N-XC72C are as follows: (1) Add 9 mL of deionized water and 1 mL of cyanamide solution (concentration 50 mg / mL) into a 25 mL beaker and mix by low-temperature ultrasonication at 5°C until homogenized.

[0071] (2) Weigh 50 mg of XC72C (purchased from Xianfeng Nanomaterial Technology Co., Ltd., 101096) and add it to the solution prepared in step (1). Low-temperature ultrasonic treatment was performed at 5°C for 30 min. After the solution was homogenized, it was placed in a vacuum freeze dryer at -70°C for freeze drying for 48 h.

[0072] (3) The freeze-dried sample was transferred to a corundum magnetic boat and placed in a tube furnace. The tube furnace was sealed and the air in the furnace was evacuated using a vacuum pump and then argon gas was introduced. This process was repeated twice to ensure that there was no air in the quartz tube. In an argon atmosphere, the temperature was raised to 100°C at a rate of 5°C / min and kept at this temperature for 30 min to remove the residual water in the MOFs pores. The temperature was then raised for 130 min to 750°C and kept at this temperature for 180 min to obtain N-XC72C.

[0073] 2. Preparation of Ru / N-XC72C catalyst (1) Use a pipette to pipette 80 μL of anhydrous ruthenium chloride solution (concentration: 0.066 g / mL) and 5 mL of deionized water into a 10 mL beaker, then sonicate at 5°C until the solution is homogeneous.

[0074] (2) Weigh 0.05 g of the N-XC72C prepared above and add it to the uniform solution in step (1). The resulting solid-liquid mixture is subjected to low-temperature ultrasonic treatment at 5°C and then placed in a vacuum freeze dryer at -70°C for 24 h to obtain RuCl 3 / C mixture.

[0075] (3) The dried RuCl 3 The Ru / N-XC72C mixture was placed in a porcelain boat and heated to 600°C at a heating rate of 5°C / min under an argon atmosphere and kept at this temperature for 2 h to obtain the Ru / N-XC72C catalyst.

Claims

1. A method for preparing a Ru / NC catalyst, comprising the following steps: (1) Mixing polyether, glacial acetic acid, 1,3,5-trimethylbenzene and water to prepare a microemulsion solution; (2) mixing the microemulsion solution, ammonium cerium nitrate and terephthalic acid to react and obtain three-dimensional porous spherical UIO-66 (Ce) MOFs; (3) mixing a cyanamide solution, the three-dimensional porous spherical UIO-66 (Ce) MOFs and water, and freeze-drying the mixture; and calcining the freeze-dried mixture to obtain CeO2 / NC; (4) mixing the CeO2 / NC and a sulfuric acid solution and reacting them to obtain three-dimensional nitrogen-doped porous carbon spheres; (5) Mixing a soluble ruthenium salt solution and the three-dimensional nitrogen-doped porous carbon spheres, freeze-drying, and then annealing to obtain the Ru / NC catalyst.

2. The preparation method according to claim 1, characterized in that: In steps (1) and (2), the mass ratio of the ammonium cerium nitrate to the polyether is 1:1.5-3; The mass ratio of the ammonium cerium nitrate to the volume ratio of glacial acetic acid is 1 g: 1-3 mL; The mass ratio of the cerium ammonium nitrate to the volume ratio of 1,3,5-trimethylbenzene is 1 g:20-80 mL; The mass ratio of the ammonium cerium nitrate to the volume of water is 1 g: 100-400 mL; The mass ratio of the ammonium cerium nitrate to terephthalic acid is 1:2-4.

3. The preparation method according to claim 1 or 2, characterized in that: In step (2), the reaction temperature is 35°C-45°C; The reaction time is 30 min-60 min.

4. The preparation method according to any one of claims 1 to 3, characterized in that: In step (3), the mass ratio of the three-dimensional porous spherical UIO-66 (Ce) MOFs to the volume ratio of the cyanamide solution is 1 g: 10-30 mL; The concentration of the cyanamide solution is 30 mg / mL-60 mg / mL; The volume ratio of the cyanamide solution to water is 1:5-1:12; In step (3), the calcination temperature is 650°C-850°C; The calcination time is 2 h-4 h; The heating rate of the calcination is 2°C / min-10°C / min; The calcination is performed in an inert atmosphere.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In step (4), the mass ratio of CeO2 / NC to the volume ratio of sulfuric acid solution is 1 g:300-500 mL; The concentration of the sulfuric acid solution is 1 M-2 M; In step (4), the reaction temperature is 25°C-100°C; The reaction time is 20-30 h.

6. The preparation method according to any one of claims 1 to 5, characterized in that: In step (5), the soluble ruthenium salt solution is a ruthenium chloride solution; The mass ratio of the soluble ruthenium salt to the three-dimensional nitrogen-doped porous carbon spheres is 1:8-1:30; The concentration of the soluble ruthenium salt solution is 0.8-1.2 mg / mL; In step (5), the annealing temperature is 300°C-700°C; The annealing time is 1 h-3 h; The heating rate of the annealing is 2°C / min-10°C / min; The annealing is performed in an inert atmosphere.

7. The preparation method according to any one of claims 1 to 6, characterized in that: In steps (3) and (5), the freeze-drying temperature is -60°C to -80°C; The freeze-drying time is 20-30 h.

8. The preparation method according to any one of claims 1 to 7, characterized in that: After the reaction in step (2), there is a step of centrifuging to obtain a solid and then washing and drying the solid; In step (3), before calcination, there is also a step of removing residual water in the pores at 80°C-120°C; In step (4), washing and drying steps are performed after the reaction.

9. The Ru / NC catalyst prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the Ru / NC catalyst according to claim 9 in producing hydrogen by electrolysis of water.

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