A cobalt-carbon catalyst for sodium sulfite and a method for preparing the same

Abstract

The application relates to the technical field of catalyst preparation, and discloses a cobalt-carbon catalyst for sodium sulfite and a preparation method thereof. The catalyst comprises, from inside to outside, a CoFe2O4 magnetic core, a reduced graphene oxide intermediate layer and a Co-N x doped mesoporous carbon shell layer of active sites; the preparation method comprises the following steps: preparing a magnetic core, surface modification coating, introducing a cobalt-nitrogen source, high-temperature in-situ pyrolysis and acid washing post-treatment. Through the above technical scheme, the application realizes the high unification of catalytic activity, chemical stability and magnetic separation performance, improves the catalytic activity, magnetic response speed and recycling life, prevents secondary pollution of heavy metals, and is suitable for efficient treatment of sodium sulfite wastewater.

Description

Technical Field

[0001] This invention belongs to the field of catalyst preparation technology, specifically relating to a cobalt-carbon catalyst for sodium sulfite and its preparation method. Background Technology

[0002] Sodium sulfite exhibits high catalytic conversion efficiency, reaction stability, and environmental friendliness in applications such as textile dyeing and deoxidation, paper bleaching, food preservation, and chromium-containing wastewater treatment. Transition metal cobalt-based materials, especially cobalt-carbon composite catalysts (Co@C), demonstrate kinetic advantages in the catalytic oxidation of sodium sulfite, thanks to cobalt's excellent d-orbital electron transport capabilities and carbon's superior electrical conductivity and mechanical support.

[0003] In existing technologies, the bonding between cobalt species and carbon matrix often relies on surface physical adsorption or weak coordination. During long-term operation, the active cobalt species are prone to redox potential shifts, which can induce metal ion leaching. This not only leads to irreversible activity degradation of the catalyst due to the loss of active sites, but the dissolved heavy metal cobalt ions can also enter downstream systems with the process fluid, causing secondary heavy metal pollution.

[0004] In pursuit of higher catalytic rates, current technologies generally favor designing catalysts at the micro- and nano-scale to achieve extremely high specific surface areas. While this improves reaction efficiency, the small particle size of nano-sized Co3O4 or Co@C particles results in high colloidal stability in liquid-phase reaction systems. Traditional gravity sedimentation or mechanical filtration methods are not only time-consuming and labor-intensive but also have low interception efficiency, easily leading to significant catalyst loss with the effluent. This loss not only increases raw material costs for production operations but also causes scaling or blockage in subsequent processes due to residual catalyst deposition in pipelines. Summary of the Invention

[0005] To overcome the technical shortcomings of existing cobalt-based catalysts in the catalytic reaction of sodium sulfite, such as the risk of cobalt species leaching, secondary heavy metal pollution, and difficulties in recovering nanoscale particles, this invention provides a cobalt-carbon catalyst for sodium sulfite and its preparation method. This invention achieves a high degree of unity between catalytic activity, chemical stability, and magnetic separation performance by constructing a core-shell structured magnetic composite system.

[0006] The cobalt-carbon catalyst for sodium sulfite provided by this invention is characterized in that the catalyst comprises, from the inside out, a magnetic core, an intermediate connecting layer, and a functional carbon shell; the magnetic core is CoFe2O4 nanoparticles with a spinel structure, the particle size of which is distributed between 20 nm and 50 nm, and the saturation magnetization is greater than 50 emu / g; the intermediate connecting layer is a reduced graphene oxide film, which coats the surface of the magnetic core, the reduced graphene oxide having 1 to 5 layers and a thickness of 0.5 nm to 2.0 nm; the functional carbon shell is doped with cobalt-nitrogen coordination active sites (Co-N). x The functional carbon shell consists of a mesoporous carbon layer anchored to the surface of the intermediate connecting layer via covalent bonding or physical winding. The thickness of the functional carbon shell is 10 nm to 30 nm, and its specific surface area is 400 m². 2 / g to 600m 2 / g, with an average pore size of 3nm to 8nm.

[0007] In the cobalt-carbon catalyst for sodium sulfite described in this invention, the cobalt element in the magnetic core exists stably as lattice ions in the octahedral or tetrahedral voids of the spinel structure, inhibiting its chemical dissolution in acidic media through strong ionic bonding. The cobalt element in the functional carbon shell exists as highly dispersed atomic-level or sub-nano clusters, forming stable Co-N4 or Co-N2 coordination structures with nitrogen atoms in the carbon matrix. These coordination structures serve as the core active centers for catalyzing the oxidation of sodium sulfite.

[0008] This invention also provides a method for preparing a cobalt-carbon catalyst for sodium sulfite, the method comprising the following steps:

[0009] Step 1: Preparation of CoFe₂O₄ magnetic nanonuclei. Cobalt chloride hexahydrate and ferric chloride hexahydrate were dissolved in deionized water at a molar ratio of 1:2 to prepare a mixed solution with a total metal ion concentration of 0.3 mol / L to 0.6 mol / L. Under magnetic stirring, the mixed solution was heated to 80°C, and then a 2.0 mol / L sodium hydroxide solution was added dropwise at a constant rate of 2 mL / min until the pH of the reaction system reached 11.5 to 12.0. The reaction was continued at 80°C for 2 hours to allow for complete co-precipitation of the metal ions. After the reaction, the resulting suspension was transferred to a polytetrafluoroethylene-lined high-pressure reactor and subjected to hydrothermal crystallization treatment at 180°C for 12 hours. After hydrothermal treatment, the mixture was allowed to cool naturally to room temperature, and magnetic collection was performed using an external magnetic field. The resulting solid product was washed sequentially with deionized water and anhydrous ethanol until the washing solution was neutral. The washed solid was placed in a vacuum drying oven and dried at 60°C and a vacuum of 0.09 MPa for 12 hours. Finally, the dried solid was placed in a muffle furnace and heated to 550°C at a heating rate of 5°C / min in air atmosphere, and calcined at a constant temperature for 3 hours to obtain highly crystalline CoFe2O4 magnetic nanonuclei.

[0010] Step Two: Graphene oxide coating modification of the magnetic core surface. First, a graphene oxide dispersion was prepared using a modified Hummers method and diluted to a concentration of 2 mg / mL. The CoFe₂O₄ magnetic nanonuclei prepared in Step One were dispersed in a 95% ethanol solution and ultrasonically dispersed for 30 minutes at a power of 400 W and a frequency of 40 kHz to form a uniform magnetic particle suspension. 3-Aminopropyltriethoxysilane was added to the suspension as a silane coupling agent, with a mass ratio of silane coupling agent to CoFe₂O₄ of 1:10. The reaction was refluxed at 60°C for 6 hours to form positively charged amino functional groups on the surface of the magnetic cores. After the reaction was complete, the modified magnetic cores were collected by magnetic separation and redispersed in deionized water. Subsequently, the graphene oxide dispersion was slowly added to the modified magnetic core suspension, with the mass ratio of magnetic cores to graphene oxide set at 5:1 to 10:1. The reaction was carried out for 4 hours under mechanical stirring at 500 r / min. Electrostatic attraction caused negatively charged graphene oxide sheets to tightly coat the surface of a positively charged magnetic core, forming the CoFe2O4@GO precursor. After the reaction was complete, the CoFe2O4@GO precursor was collected by centrifugation and washing.

[0011] Step 3: Constructing a composite precursor by introducing cobalt and nitrogen sources. The CoFe2O4@GO precursor prepared in Step 2 was redispersed in a mixed solvent of water and ethanol. Cobalt acetate tetrahydrate was added sequentially as a second cobalt source, and dicyandiamide or melamine as a nitrogen and carbon source, respectively. The mass ratio of CoFe2O4@GO, cobalt acetate tetrahydrate, and nitrogen-containing organic matter was strictly controlled between 1:0.5:5 and 1:1:10. The mixture was stirred continuously at room temperature for 12 hours, allowing cobalt ions to be adsorbed and deposited on the surface of the graphene oxide coating through coordination with the nitrogen-containing organic molecules. Subsequently, the solvent was removed using a rotary evaporator at 60°C, and the resulting solid was ground to obtain a dry composite precursor powder.

[0012] Step 4: High-Temperature In-Situ Pyrolysis and Activation. The composite precursor powder obtained in Step 3 was placed in a quartz boat and placed in the isothermal zone of a tube furnace. Under a nitrogen atmosphere, the nitrogen flow rate was set to 100 sccm. The heating program was started, and the temperature was increased from room temperature to 350°C at a rate of 3°C / min, and held at this temperature for 1 hour to remove volatiles. Subsequently, the temperature was increased to 800°C to 900°C at a rate of 5°C / min, and held at this high temperature for 3 hours. During the high-temperature process, graphene oxide underwent thermal reduction to reduce graphene oxide, while nitrogen-containing organic matter underwent pyrolysis and polycondensation reactions, resulting in the in-situ growth of Co-N doped graphene oxide on the surface of the reduced graphene oxide. x Mesoporous carbon layer of active sites. After the reaction, the furnace was allowed to cool naturally to room temperature.

[0013] Step 5: Post-processing. The pyrolysis product obtained in Step 4 is immersed in a 2.0 mol / L hydrochloric acid solution and magnetically stirred at 60°C for 6 hours. This step aims to remove inactive cobalt particles or acid-soluble impurities exposed outside the carbon shell that may be generated during pyrolysis, thereby purifying the active sites. After acid washing, magnetic separation and recovery are performed again using an external magnetic field, followed by repeated washing with deionized water until the filtrate is neutral and no chloride ions are detected. Finally, it is dried under vacuum at 60°C for 12 hours to obtain the final cobalt-carbon catalyst for sodium sulfite.

[0014] In a preferred embodiment of the present invention, the nitrogen-containing organic compound used in step three is selected from polydopamine. In this embodiment, by adding dopamine hydrochloride to a CoFe2O4@GO suspension and inducing in-situ polymerization of dopamine in a buffer system with a pH of 8.5, a uniform and tightly coated polydopamine coating layer is formed on the surface of graphene oxide. This coating layer is transformed into a functional carbon shell layer with higher nitrogen content and better graphitization during subsequent pyrolysis.

[0015] In a preferred embodiment of the present invention, the maximum pyrolysis temperature in step four is set to 850°C. At this temperature, the doping configuration of nitrogen atoms is mainly pyridine nitrogen and graphitic nitrogen. The Co-N4 structure formed by the coordination of these two types of nitrogen atoms with metallic cobalt has the highest catalytic redox potential and can reduce the activation energy of the sodium sulfite oxidation reaction.

[0016] The catalyst designed in this invention employs a multi-level micro / nano structure. The internal CoFe2O4 magnetic core not only provides the catalyst with strong magnetic responsiveness, enabling it to be completely separated from the liquid-phase reaction system within seconds by an external magnetic field (such as a magnetic separator) after the reaction, solving the industrial pain point of difficult nanoparticle recovery; more importantly, CoFe2O4, as a spinel-structured composite oxide, has extremely stable chemical properties and can effectively bind cobalt elements within the crystal lattice, physically preventing the direct dissolution of cobalt ions.

[0017] The intermediate connecting layer of reduced graphene oxide acts as a bridge. On the one hand, it has excellent electrical conductivity, serving as a high-speed channel for electron transport and accelerating the electron transfer between sulfite ions and oxygen molecules during catalysis. On the other hand, its flat two-dimensional structure and abundant π electron cloud provide an ideal template for the in-situ growth of the outer functional carbon shell, enhancing the mechanical bonding between the shell and the core and preventing shell peeling under long-term erosion.

[0018] The outer functional carbon shell is the primary reaction site. By introducing nitrogen atoms and cobalt atoms into the carbon framework, a Co-N structure, similar to the active site of a biological enzyme, is formed. x Structure. This structure exhibits a strong chemisorption capacity for oxygen molecules, weakening or even breaking the O-O bonds, thereby oxidizing sulfite ions in solution to form sulfate ions. Due to the abundant mesoporous structure of this shell, it not only provides a large specific surface area to expose active sites but also utilizes the capillary effect of the pores to accelerate the diffusion of reactants and products.

[0019] In the preparation method described in this invention, the sum of the molar concentrations of divalent cobalt ions and trivalent iron ions in the mixed solution described in step one is 0.45 mol / L. During the dropwise addition of sodium hydroxide solution, the stirring speed is maintained at 600 r / min. The pressure of the hydrothermal crystallization treatment is maintained between 1.5 MPa and 2.0 MPa.

[0020] In the preparation method described in this invention, the silane coupling agent modification process in step two is carried out in anhydrous ethanol solvent. The modified magnetic core exhibits improved dispersion stability in deionized water, with its Zeta potential changing from -15mV to +35mV, which provides sufficient electrostatic driving force for the uniform encapsulation of graphene oxide.

[0021] In the preparation method described in this invention, the amount of cobalt acetate tetrahydrate added in step three ensures that the mass fraction of metallic cobalt supported on the functional carbon shell in the final catalyst is 3% to 8% of the total mass of the catalyst. This content range can, while ensuring activity, minimize the excessive aggregation of cobalt atoms to prevent the formation of inactive metal particles.

[0022] In the preparation method described in this invention, the pyrolysis process in step four is divided into three stages: the first stage is the dehydration and degassing stage, where the temperature is raised from room temperature to 200°C at a rate of 2°C / min and held for 30 minutes; the second stage is the precursor pre-decomposition stage, where the temperature is raised from 200°C to 450°C at a rate of 3°C / min and held for 60 minutes; the third stage is the graphitization and nitrogen doping stage, where the temperature is raised from 450°C to the final set temperature (e.g., 850°C) at a rate of 5°C / min and held for 180 minutes. This segmented heating method facilitates the stable release of gaseous products and prevents the collapse of the shell structure.

[0023] In the technical solution of this invention, the cobalt-carbon catalyst for sodium sulfite is not only suitable for the oxidative conversion of sodium sulfite, but also has catalytic effects on the treatment of wastewater containing similar reducing sulfur-containing anions. When treating sodium sulfite solutions, the optimal dosage of the catalyst is 0.5 g / L to 2.0 g / L. Within this dosage range, combined with the introduction of an excess of 5% to 10% air or pure oxygen, efficient conversion can be achieved.

[0024] In this invention, the size of the magnetic core particles directly affects the specific magnetization of the catalyst and its dispersion stability in the liquid. By strictly controlling the pH and hydrothermal temperature in step one, the CoFe₂O₄ particles are ensured to exhibit a regular spherical or near-spherical shape with an extremely narrow particle size distribution. This consistent morphology is a prerequisite for subsequent layer-by-layer encapsulation. If the pH is below 10, a non-magnetic ferric hydroxide impurity phase is easily formed; if the pH is above 13, excessive complexation of cobalt ions may occur, reducing the magnetic yield.

[0025] The introduction of the intermediate connecting layer into the reduced graphene oxide provides abundant anchoring sites for cobalt ions and nitrogen-containing monomers on the surface of the graphene oxide sheets (such as carboxyl, hydroxyl, and epoxy groups), ensuring the uniform distribution of active components at the molecular level. During the high-temperature reduction process in step four, these functional groups are partially removed, and the reduced graphene structure restores the conjugated π system, enabling the catalyst to function as an electron library. This allows for the adjustment of the electron cloud density of the active center Co-Nx, thereby optimizing the adsorption energy for reaction intermediates (such as superoxide radicals and hydroxyl radicals).

[0026] The porous structure of the functional carbon shell is formed through the pore-forming process of nitrogen-containing organic matter during pyrolysis. At temperatures above 800°C, non-carbon elements in the organic matter escape in gaseous form (such as ammonia, hydrogen cyanide, and carbon dioxide), leaving numerous mesopores in the carbon framework. The inner walls of these pores are saturated with nitrogen atoms, and cobalt atoms are embedded within them to form a four-coordinate planar structure. This porous structure ensures that even when handling high-concentration sodium sulfite solutions, reactant molecules can rapidly diffuse to the internal active sites, avoiding kinetic sluggishness caused by diffusion limitations.

[0027] The acid washing process in the post-treatment step is crucial for quality control in this invention. Fine washing with 2.0 mol / L hydrochloric acid completely removes unstable metallic cobalt that is not encapsulated by the carbon layer. This metallic cobalt is a major source of heavy metal leaching in practical applications. Through this step, this invention confines all catalytically active cobalt species to a coordinated-protected Co-N complex. x The secondary pollution is eliminated at its source by the internal core of the configuration or protected by the spinel lattice.

[0028] Compared with the prior art, the beneficial effects of the present invention are: 1. Through the binding effect of the spinel magnetic core and the physical barrier and chemical anchoring of the external multi-layer carbon shell, the catalyst prepared by this invention reduces the leaching concentration of cobalt ions in a wide immersion range of pH value from 3 to 11, with no heavy metal leaching, thus solving the problem of secondary pollution. 2. High magnetic saturation strength CoFe2O4 is used as the core, which improves the catalyst recovery rate, shortens the separation time, and reduces energy consumption and operating costs in the industrial production process. 3. Co-N x The high-density distribution of active sites and the efficient electron conduction provided by the graphene layer improve the conversion rate of sodium sulfite. At the same time, the catalyst has good resistance to impurity ions (such as chloride ions, calcium and magnesium ions) in the reaction system. 4. The co-precipitation-hydrothermal-in-situ pyrolysis process adopted can customize the production of catalysts with different activities according to different sodium sulfite treatment concentration requirements by controlling the pyrolysis temperature and the precursor ratio, and has universality for industrial applications. 5. Due to the strong chemical bonding between the core and shell structures, the catalyst can maintain a high degree of morphological integrity during repeated reaction-separation-re-reaction cycles. Attached Figure Description

[0029] Figure 1 These are TEM and HRTEM characterization images of the final sodium sulfite cobalt-carbon catalyst obtained in Example 1 of the present invention; wherein... Figure 1 (a) is a diagram of the overall morphology of the catalyst particles. Figure 1(b) is a magnified view of a portion of the core-shell structure. Figure 1 (c) is a high-resolution lattice diagram of the magnetic core and carbon layers. Figure 1 (d) is a partial morphology diagram of the external mesoporous carbon layer.

[0030] Figure 2 This is an XRD comparison diagram of the CoFe2O4 magnetic core of the present invention and the final cobalt-carbon catalyst for sodium sulfite obtained in Example 1.

[0031] Figure 3 The XRD patterns are comparison images of the CoFe2O4 magnetic core, the CoFe2O4@GO precursor, and the final cobalt-carbon catalyst for sodium sulfite obtained in Example 1 of the present invention. Detailed Implementation

[0032] This invention provides a cobalt-carbon catalyst for sodium sulfite and its preparation method, exhibiting extremely high technical integration and structural stability in practical engineering applications. The catalyst system involved in this invention is constructed based on a multi-level core-shell architecture. Through precise control of the magnetic template, intermediate conductive layer, and active functional shell, efficient exposure of active sites and zero dissolution of heavy metal elements are achieved during the catalytic oxidation of sodium sulfite. The physical spatial layout of this catalyst forms a three-in-one structure from the inside out, consisting of a magnetic core, an intermediate connecting layer, and a functional carbon shell.

[0033] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0034] Example 1: The magnetic core is CoFe2O4 nanoparticles (particle size 35nm, saturation magnetization 58emu / g). The intermediate connecting layer consists of three layers of reduced graphene oxide film (1.2 nm thick). Functional carbon shell (thickness 20nm, specific surface area 500m²) 2 / g, average pore size 5nm), cobalt loading 5% (atomic-level Co-N4 coordination structure). The cobalt source is cobalt acetate tetrahydrate, and the nitrogen / carbon source is dicyandiamide; CoFe2O4@GO, cobalt acetate tetrahydrate, and dicyandiamide in a mass ratio of 1:0.8:8; Preparation steps: S1: CoFe2O4 magnetic nanonuclei were prepared by dissolving cobalt chloride hexahydrate and ferric chloride hexahydrate in a 1:2 molar ratio, with a total metal ion concentration of 0.45 mol / L; stirring at 600 r / min at 80℃, and adding sodium hydroxide solution to adjust the pH to 11.8; reacting at 80℃ for 2 hours, followed by hydrothermal crystallization at 180℃ and 1.8 MPa for 12 hours; magnetic separation, washing, vacuum drying at 60℃, and calcination at 550℃ in air atmosphere at a rate of 5℃ / min for 3 hours. S2: Magnetic cores were modified by coating the surface with graphene oxide. CoFe2O4 was dispersed in ethanol and sonicated at 400W and 40kHz for 30 minutes. 3-Aminopropyltriethoxysilane (mass ratio of 1:10 to the magnetic cores) was added and refluxed at 60℃ for 6 hours for modification. 2 mg / mL of graphene oxide dispersion was added to the modified magnetic core suspension (mass ratio 7:1) and stirred at 500r / min for 4 hours to form the CoFe2O4@GO precursor. S3: Construction of composite precursor: CoFe2O4@GO was redispersed in a water-ethanol mixed solvent, cobalt acetate tetrahydrate and dicyandiamide were added, and the mixture was stirred for 12 hours; the solvent was removed by rotary evaporation at 60°C, and the composite precursor powder was obtained by grinding. S4: High-temperature in-situ pyrolysis and activation, the precursor powder is placed in a tube furnace, nitrogen atmosphere with a flow rate of 100 sccm; programmed temperature rise: 2℃ / min to 200℃ and hold for 30 minutes, 3℃ / min to 450℃ and hold for 60 minutes, 5℃ / min to 850℃ and hold for 180 minutes. S5: Post-processing: The pyrolysis product is immersed in 2.0 mol / L hydrochloric acid and magnetically stirred at 60°C for 6 hours; magnetic separation and recovery are performed, and the product is washed until no chloride ions are detected. The product is then vacuum dried at 60°C to obtain the final product.

[0035] The catalyst prepared in Example 1 was structurally characterized, and the results are as follows: Figures 1 to 3 As shown.

[0036] Figure 1 TEM and HRTEM results show that the final catalyst has a distinct core-shell composite morphology, with a crystalline CoFe2O4 magnetic core at the center, surrounded by a carbonaceous coating structure and a mesoporous carbon layer. In the high-resolution image, lattice fringes corresponding to the CoFe2O4 crystals and graphitized carbon can be observed, indicating that the main crystal structure of the magnetic core is still maintained after high-temperature pyrolysis and acid washing.

[0037] Figure 2 The results show that the main XRD diffraction peaks of the final catalyst basically correspond to the main diffraction peaks of the CoFe2O4 magnetic core. At the same time, the broadened diffraction characteristics of carbon materials appear in the range of 24° to 26°, indicating that the main crystal phase of CoFe2O4 and the carbon structure coexist in the final catalyst.

[0038] Figure 3 The results show that after the CoFe2O4 magnetic core is coated with GO to form the CoFe2O4@GO precursor, and the final catalyst is obtained by subsequent high-temperature pyrolysis and acid washing, the main characteristic diffraction peaks of CoFe2O4 are still retained, indicating that the processing conditions in this embodiment did not cause complete transformation or complete destruction of the main crystal phase of CoFe2O4.

[0039] Example 2: The functional carbon shell has a cobalt loading of 3%, and the remaining components and proportions are the same as in Example 1; Preparation steps: Adjust the amount of cobalt acetate tetrahydrate added, and the remaining steps are the same as in Example 1.

[0040] Example 3: The functional carbon shell has a cobalt loading of 8%, and the remaining components and proportions are the same as in Example 1; Preparation steps: Adjust the amount of cobalt acetate tetrahydrate added, and the remaining steps are the same as in Example 1.

[0041] Example 4: Same as Example 1; Preparation steps: High-temperature in-situ pyrolysis at a maximum temperature of 800℃, held for 180 minutes, the remaining steps are the same as in Example 1.

[0042] Example 5: Same as Example 1; Preparation steps: High-temperature in-situ pyrolysis at a maximum temperature of 900℃, held for 180 minutes, the remaining steps are the same as in Example 1.

[0043] Example 6: The mass ratio of magnetic core to graphene oxide is 5:1, and the remaining components and proportions are the same as in Example 1; Preparation steps: Same as in Example 1.

[0044] Example 7: The mass ratio of magnetic core to graphene oxide is 10:1, and the remaining components and proportions are the same as in Example 1; Preparation steps: Same as in Example 1.

[0045] Example 8: The nitrogen / carbon source was replaced with melamine, and the remaining components and proportions were the same as in Example 1; Preparation steps: Same as in Example 1.

[0046] Comparative Example 1: Only CoFe2O4 magnetic core-loaded cobalt particles, without reduced graphene oxide intermediate layer and functional carbon shell, the rest of the components are the same as in Example 1; Preparation steps: The steps of graphene oxide coating and nitrogen source addition are omitted. The cobalt source is directly loaded onto the CoFe2O4 surface. The remaining process parameters and steps are the same as in Example 1.

[0047] Comparative Example 2: The functional carbon shell is free of nitrogen doping, cobalt exists in the form of metal particles, and the other components are the same as in Example 1; Preparation steps: Nitrogen-containing organic matter was omitted, and the remaining process parameters and steps were the same as in Example 1.

[0048] Test method: Catalytic activity test: Prepare a 0.1 mol / L sodium sulfite solution, add 1.0 g / L catalyst, and introduce an excess of 8% air. Measure the sodium sulfite conversion rate and reaction rate constant after 30 minutes. Magnetic separation performance test: The saturation magnetization of the catalyst was measured, and the separation time and recovery rate under an external magnetic field of 0.5T were recorded; Cyclic stability test: The catalyst was reused 50 times, and the sodium sulfite conversion rate was measured each time to calculate the activity retention rate; Cobalt leaching test: The catalyst was soaked in solutions with pH 3, 7, and 11 for 24 hours, and the concentration of cobalt ions in the solution was determined by inductively coupled plasma mass spectrometry. Specific surface area and pore size test: The specific surface area, average pore size and total pore volume of the catalyst were determined by nitrogen adsorption-desorption method.

[0049] The test data comparisons are shown in Table 1 and Table 2.

[0050] Table 1. Comparison of sodium sulfite conversion rate, saturation magnetization, and magnetic separation recovery rate over 30 minutes.

[0051] Table 2 Comparison of Activity Retention Rate, Cobalt Dissolution Concentration, Specific Surface Area, and Average Pore Size after 50 Cycles

[0052] Examples 1 to 8 utilize lattice structure to confine cobalt ions and suppress dissolution; reduced graphene oxide interlayer to enhance electron transport and structural bonding; and Co-N functional carbon shells. x Coordination sites serve as the catalytic core, and the mesoporous structure accelerates mass transfer. Comparative Example 1, lacking a core-shell composite structure, experienced excessive dissolution and activity degradation due to the easy detachment of cobalt particles; Comparative Example 2, lacking a Co-N composite structure... x Coordination structure leads to decreased catalytic activity and stability.

[0053] The catalytic activity and stability are superior when the cobalt loading is 5% to 8%, the pyrolysis temperature is 850℃ to 900℃, and the mass ratio is 7:1 to 10:1. The cobalt loading directly affects the number of active sites, while the pyrolysis temperature determines the Co-N ratio. x The configuration and degree of graphitization of the carbon shell, the mass ratio balancing magnetic properties and electron transport efficiency, and the synergistic effect of these three factors ensure the overall performance of the catalyst.

[0054] Compared to Comparative Example 1 without a core-shell structure, the sodium sulfite conversion rate of the examples was increased by more than 20%, the cobalt leaching concentration was reduced by more than 91%, and the cycle activity retention rate was increased by more than 31%; compared to Co-N-free examples... x Compared with the site comparison example 2, the conversion rate is increased by more than 26%, the cobalt leaching concentration is reduced by more than 87%, the specific surface area is increased by more than 127%, and the magnetic separation recovery rate exceeds 99.8%, and the separation time is shortened by more than 80%, meeting the needs of high efficiency, environmental protection and low cost for industrial wastewater treatment.

[0055] In summary, this invention constructs a core-shell composite structure with Co-Nx Coordination of active sites enables high catalytic activity, low cobalt dissolution, and efficient magnetic separation, solving the core pain points of traditional cobalt-based catalysts. It is suitable for scenarios such as sodium sulfite wastewater treatment and has good potential for industrialization.

[0056] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A cobalt-carbon catalyst for sodium sulfite, characterized in that, The catalyst comprises, from the inside out, a magnetic core, an intermediate connecting layer, and a functional carbon shell. The magnetic core is a CoFe2O4 nanoparticle with a spinel structure; The intermediate connecting layer is a reduced graphene oxide film, which wraps around the surface of the magnetic core. The reduced graphene oxide film consists of three layers and has a thickness of 1.2 nm. The functional carbon shell is a mesoporous carbon layer doped with atomic-level Co-N4 coordination active sites, and the functional carbon shell is grown in situ on the surface of the intermediate connecting layer; the thickness of the functional carbon shell is 20 nm, the specific surface area is 500 m² / g, and the average pore size is 5 nm; the metallic cobalt supported in the functional carbon shell accounts for 3% to 8% of the total mass of the catalyst.

2. The cobalt-carbon catalyst for sodium sulfite according to claim 1, characterized in that, The cobalt element within the magnetic core exists stably in the octahedral or tetrahedral voids of the spinel structure in the form of lattice ions, and its chemical dissolution in acidic media is suppressed through ionic bonding.

3. A method for preparing a cobalt-carbon catalyst for sodium sulfite according to any one of claims 1 or 2, characterized in that, The preparation method includes the following steps: Step 1: Preparation of CoFe2O4 magnetic nanonuclei; Cobalt chloride hexahydrate and ferric chloride hexahydrate were dissolved in deionized water to prepare a mixed solution; The mixed solution was heated to adjust the pH value of the system; After stirring the reaction, it was transferred to a high-pressure reactor for hydrothermal crystallization treatment; After the reaction was completed, the mixture was magnetically collected, washed, dried, and calcined at a constant temperature to obtain CoFe2O4 magnetic nanonuclei; Step 2: Modify the surface of the magnetic core by coating it with graphene oxide; prepare a graphene oxide dispersion; take the magnetic core prepared in Step 1, disperse it in an ethanol solution and sonicate it, then add a silane coupling agent; subsequently, add the graphene oxide dispersion to the modified magnetic core suspension and stir to form a CoFe2O4@GO precursor. Step 3: Construct a composite precursor by introducing cobalt and nitrogen sources; redisperse the CoFe2O4@GO precursor in a mixed solvent of water and ethanol, add cobalt acetate tetrahydrate and nitrogen-containing organic matter, which serves as both nitrogen and carbon sources; control the mass ratio of CoFe2O4@GO, cobalt acetate tetrahydrate and the nitrogen-containing organic matter to 1:0.5:5 to 1:1:10, stir, dry and grind to obtain composite precursor powder; Step 4: High-temperature in-situ pyrolysis and activation; the composite precursor powder is pyrolyzed by heating; the functional carbon shell is formed; Step 5: Post-processing; Immerse the pyrolysis product in hydrochloric acid solution, stir and acid wash, then use an external magnetic field for magnetic separation and recovery, wash and dry to obtain the final cobalt-carbon catalyst for sodium sulfite.

4. The method for preparing the cobalt-carbon catalyst for sodium sulfite according to claim 3, characterized in that, In step one, the sum of the molar concentrations of divalent cobalt ions and trivalent iron ions in the mixed solution is 0.45 mol / L; the internal pressure during hydrothermal crystallization is maintained between 1.5 MPa and 2.0 MPa.

5. The method for preparing the cobalt-carbon catalyst for sodium sulfite according to claim 3, characterized in that, In step two, the ultrasonic treatment has a power of 400W, a frequency of 40kHz, and a duration of 30 minutes.

6. The method for preparing the cobalt-carbon catalyst for sodium sulfite according to claim 3, characterized in that, In step two, the silane coupling agent is 3-aminopropyltriethoxysilane, and its mass ratio with the magnetic core is 1:

10.

7. The method for preparing the cobalt-carbon catalyst for sodium sulfite according to claim 3, characterized in that, In step two, the modification process is refluxed at 60°C for 6 hours; the mass ratio of magnetic core to graphene oxide is set to 5:1 to 10:1.

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