A method for preparing cobalt-aluminum hydrotalcite nanosheets

Abstract

The present application relates to the technical field of catalytic material preparation, and particularly relates to a preparation method of cobalt-aluminum hydrotalcite nanosheet, and proposes a preparation method of cobalt-aluminum hydrotalcite nanosheet, which comprises the following steps: S10, mixing a solution containing a cobalt salt and an aluminum salt with a precipitator to obtain a first mixed solution; S20, separating and collecting the solid of the first mixed solution, washing and drying to obtain blocky cobalt-aluminum hydrotalcite; S30, dispersing the blocky cobalt-aluminum hydrotalcite in distilled water, mixing with a lye to obtain a second mixed solution; S40, performing hydrothermal reaction on the second mixed solution, then separating and collecting the solid, washing and drying to obtain the cobalt-aluminum hydrotalcite nanosheet. In the technical scheme, the cobalt salt and the aluminum salt are selected as the metal source for preparing the hydrotalcite, the hydrotalcite is prepared by using the coprecipitation method, and then the prepared hydrotalcite is subjected to hydrothermal treatment in the lye to obtain the hydrotalcite nanosheet. The prepared hydrotalcite nanosheet has a large specific surface area and a regular structure, and the preparation process is simple and easy to operate.

Description

Technical Field

[0001] This invention relates to the field of catalytic material preparation technology, and in particular to a method for preparing cobalt aluminum hydrotalcite nanosheets. Background Technology

[0002] Layered double hydroxides (LDHs), also known as hydrotalcite, are composed of positively charged layers and anions arranged in an orderly manner between the layers. Metal cations within the layers coordinate with hydroxyl groups to form octahedral structural units (MO6). These octahedral units (MO6) share edges and overlap to form infinitely expandable two-dimensional structures. Due to its highly tunable structure (e.g., metal type, metal ratio, interlayer anions, number of layers), hydrotalcite possesses unique physicochemical properties and diverse morphologies, leading to its widespread application in adsorption, energy conversion, and biomedicine. In particular, hydrotalcite nanosheets, with their unique two-dimensional layered structure and numerous exposed surface atoms, exhibit high specific surface area and intrinsic catalytic activity, making them the preferred choice for preparing various functional materials. Currently, the main methods for synthesizing hydrotalcite include coprecipitation, hydrothermal methods, and ion exchange methods. Among these, coprecipitation is simple and inexpensive, facilitating large-scale applications, but it cannot control the stacking of LDH layers, resulting only in the preparation of numerous densely packed, blocky LDHs. The densely packed, blocky LDHs severely affect the exposure of their active sites, resulting in often low intrinsic activity.

[0003] There are two main solutions for preparing bulk LDHs: exfoliation and direct synthesis. Exfoliation of bulk LDHs is a crucial method for preparing ultrathin LDH nanosheets, including indirect and direct exfoliation methods. Indirect exfoliation typically involves two steps: first, ion exchange is used to insert guest substances into the interlayer voids of the bulk LDHs to increase the interlayer distance; then, the inserted hydrotalcite is exfoliated into ultrathin LDH nanosheets in organic solvents such as butanol, toluene, and formamide. Direct exfoliation involves directly placing the bulk LDHs in organic solvents such as formamide. The tight adsorption of solvent molecules on the LDH host layers or the insertion into the interlayer voids reduces the charge density of the host layers, disrupting the tight hydrogen bond network and leading to rapid exfoliation into ultrathin LDH nanosheets. Direct synthesis involves using formamide or other surfactants as inhibitors during LDH synthesis to control the growth direction of the LDHs and inhibit the stacking of the host layers, thus directly synthesizing ultrathin LDH nanosheets.

[0004] Currently, all existing methods for synthesizing ultrathin LDH nanosheets require the use of organic solvents. Solvent molecules easily adsorb onto the prepared LDH nanosheets, thus hindering the exposure of LDH active sites. Furthermore, the prepared LDH nanosheets are unstable; once the organic solvent is removed, they recombine into multi-layered blocky LDHs. Summary of the Invention

[0005] The main objective of this invention is to propose a method for preparing cobalt-aluminum hydrotalcite nanosheets, aiming to solve the problems of existing methods for preparing hydrotalcite nanosheets requiring expensive and toxic organic reagents, and the resulting hydrotalcite nanosheets having poor catalytic performance and low catalytic activity stability.

[0006] To achieve the above objectives, this invention proposes a method for preparing cobalt-aluminum hydrotalcite nanosheets, comprising the following steps:

[0007] S10. Mix the solution containing cobalt salt and aluminum salt with a precipitant to obtain the first mixture;

[0008] S20. Separate and collect the solid from the first mixture, wash and dry it to obtain blocky cobalt aluminum hydrotalcite;

[0009] S30. Disperse the blocky cobalt aluminum hydrotalcite in distilled water and mix it with alkaline solution to obtain a second mixture;

[0010] S40. The second mixture is subjected to a hydrothermal reaction, then the solid is separated and collected, washed, and dried to obtain cobalt aluminum hydrotalcite nanosheets.

[0011] Optionally, in step S10, the precipitant includes at least one of NaOH, KOH, Na2CO3, and K2CO3.

[0012] Optionally, in step S10, the cobalt salt includes at least one of cobalt chloride, cobalt nitrate, and cobalt acetate; and / or,

[0013] The aluminum salt includes at least one of aluminum chloride, aluminum nitrate, and aluminum sulfate.

[0014] Optionally, step S10 includes:

[0015] Cobalt salt and aluminum salt are added sequentially to distilled water and mixed to obtain a cobalt-aluminum salt solution;

[0016] The precipitant is dissolved in distilled water to obtain a precipitant solution;

[0017] Under stirring, the precipitant solution is added to the cobalt-aluminum salt solution, and the aging reaction is carried out at 60-80°C for 10-24 hours to obtain the first mixture.

[0018] Optionally, the molar ratio of cobalt salt to aluminum salt in the cobalt-aluminum salt solution is (5-7):(1-3); and / or,

[0019] The total molar concentration of cobalt and aluminum salts in the cobalt-aluminum salt solution is 0.01–0.1 M; and / or,

[0020] The molar concentration of the precipitant solution is 0.01–0.3 M; and / or,

[0021] The volume ratio of the cobalt aluminum salt solution to the precipitant mixture is 1:(1-8).

[0022] Optionally, step S30 includes:

[0023] S31. Add the blocky cobalt aluminum hydrotalcite to distilled water and sonicate for 10-60 min to obtain a cobalt aluminum hydrotalcite dispersion, wherein the mass concentration of cobalt aluminum hydrotalcite in the dispersion is 0.8-10 g / L.

[0024] S32. Add alkali solution to the cobalt-aluminum hydrotalcite dispersion, mix, and obtain a second mixture.

[0025] Optionally, in step S30, the mass ratio of the blocky cobalt aluminum hydrotalcite to the alkaline solution is 1:(300-1800).

[0026] Optionally, in step S30, the mass percentage of alkali in the alkaline solution is 2-10%.

[0027] Optionally, in step S30, the alkaline solution includes at least one of NaOH solution and KOH solution.

[0028] Optionally, in step S40, the temperature of the hydrothermal reaction is 80–150°C; and / or,

[0029] The reaction time is 1 to 5 hours.

[0030] The technical solution provided by this invention uses cobalt and aluminum salts as metal sources for preparing hydrotalcite (LT), employing a co-precipitation method to generate layered LT under alkaline conditions. The resulting cobalt-aluminum LT is then subjected to hydrothermal treatment in an alkaline solution, exfoliating into LT nanosheets. During the hydrothermal treatment, the alkaline solution first etches away the amphoteric aluminum elements on the LT's main layers, leaving porous LT layers. This reduces the charge density on the LT's main layers and disrupts the dense hydrogen bond network between the bulk LT layers, leading to rapid exfoliation. Then, in the alkaline solution, the exfoliated LT layers undergo slow Ostwald ripening, where the cobalt elements at the edges or pores slowly dissolve. The dissolved cobalt elements then deposit along the LT crystal planes to the LT edges, ultimately growing into two-dimensional nanosheets. The LT nanosheets prepared using this method have a large specific surface area and a regular structure, and the preparation process is simple and easy to operate. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 A schematic flowchart of an embodiment of the method for preparing cobalt-aluminum hydrotalcite nanosheets provided by the present invention;

[0033] Figure 2 This is a schematic diagram of the octahedral growth unit of cobalt-aluminum hydrotalcite in this invention;

[0034] Figure 3 This is a scanning electron microscope image of the cobalt-aluminum hydrotalcite nanosheets in Example 1 of the present invention;

[0035] Figure 4 This is a scanning electron microscope image of the bulk cobalt aluminum hydrotalcite in Comparative Example 1 of the present invention;

[0036] Figure 5 This is an energy dispersive spectroscopy (EDS) result of cobalt-aluminum hydrotalcite nanosheets in Example 1 of the present invention.

[0037] Figure 6 The X-ray diffraction patterns are those of the cobalt-aluminum hydrotalcite nanosheets in Example 1 and the bulk cobalt-aluminum hydrotalcite in Comparative Example 1 of the present invention.

[0038] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0040] It should be noted that, unless specific conditions are specified in the embodiments, conventional conditions or conditions recommended by the manufacturer should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products. Furthermore, the meaning of "and / or" throughout the text includes three parallel solutions; for example, "A and / or B" includes solution A, solution B, or a solution that simultaneously satisfies A and B. In addition, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, such a combination should be considered non-existent and not within the scope of protection claimed by this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0041] Currently, all existing methods for synthesizing ultrathin hydrotalcite nanosheets require the use of organic solvents. Solvent molecules easily adsorb onto the prepared hydrotalcite nanosheets, thus hindering the exposure of the active sites of the hydrotalcite. Furthermore, the prepared hydrotalcite nanosheets are unstable; once the organic solvent is removed, they recombine into many layers of blocky hydrotalcite.

[0042] In view of this, such as Figure 1 As shown, this invention proposes a method for preparing cobalt-aluminum hydrotalcite nanosheets, comprising the following steps: S10, mixing a solution containing cobalt salt and aluminum salt with a precipitant to obtain a first mixture; S20, separating and collecting the solid from the first mixture, washing and drying it to obtain bulk cobalt-aluminum hydrotalcite; S30, dispersing the bulk cobalt-aluminum hydrotalcite in distilled water and mixing it with an alkaline solution to obtain a second mixture; S40, subjecting the second mixture to a hydrothermal reaction, then separating and collecting the solid, washing and drying it to obtain cobalt-aluminum hydrotalcite nanosheets.

[0043] It should be noted that hydrotalcite can be represented as: [M 2+ 1-x M 3+ x (OH) - 2] x+ [A x / n ] n x- ·mH2O, where M 2+ and M 3+ These are divalent and trivalent metal cations, such as Mg, located on the main plate layer. 2+ Zn2+ Ni 2+ Co 2+ Divalent cations and Al 3+ Ga 3+ Cr 3+ Co 3+ Trivalent cations. In the synthesis of hydrotalcite, metal cations first form [M] under the action of a precipitant. 2+ (OH) - 6] 4- and [M] 3+ (OH) - 6] 3- Two types of octahedral growth units are then stacked together to form a positively charged host layer. Anions are then inserted between the layers due to hydrogen bonding and electrostatic interactions to balance the positive charge of the host layer. At the same time, water molecules bond with LDHs hydroxyl groups facing the interlayer region to form a tight hydrogen bond network, thereby forming hydrotalcite.

[0044] The technical solution provided by this invention uses cobalt and aluminum salts as metal sources for preparing hydrotalcite (LT), employing a co-precipitation method to generate layered LT under alkaline conditions. The resulting cobalt-aluminum LT is then subjected to hydrothermal treatment in an alkaline solution, exfoliating into LT nanosheets. During the hydrothermal treatment, the alkaline solution first etches away the amphoteric aluminum elements on the LT's main layers, leaving porous LT layers. This reduces the charge density on the LT's main layers and disrupts the dense hydrogen bond network between the bulk LT layers, leading to rapid exfoliation. Then, in the alkaline solution, the exfoliated LT layers undergo slow Ostwald ripening, where the cobalt elements at the edges or pores slowly dissolve. The dissolved cobalt elements then deposit along the LT crystal planes to the LT edges, ultimately growing into two-dimensional nanosheets. The LT nanosheets prepared using this method have a large specific surface area and a regular structure, and the preparation process is simple and easy to operate.

[0045] Further, in step S10, the precipitant includes at least one of NaOH, KOH, Na2CO3 and K2CO3.

[0046] It should be noted that the precipitant is used to react with metal cations to form the main layer of hydrotalcite and to provide interlayer anions. Any alkaline compound can be used, such as ammonia, urea, potassium hydroxide, sodium hydroxide, and sodium carbonate, etc.

[0047] By using at least one of NaOH, KOH, Na₂CO₃, and K₂CO₃ as a precipitant, an alkaline environment can be provided for cobalt and aluminum salts, thereby completing co-precipitation. Furthermore, Na₂CO₃ and K₂CO₃ can hydrolyze to generate carbonate ions, forming aluminum carbonate and cobalt carbonate precipitates, thus accelerating the co-precipitation reaction rate. Specifically, in some embodiments of the present invention, a mixed solution of NaOH and Na₂CO₃ is selected as the precipitant, which is inexpensive and readily available, facilitating large-scale application.

[0048] Further, in step S10, the cobalt salt includes at least one of cobalt chloride, cobalt nitrate, and cobalt acetate; and / or, the aluminum salt includes at least one of aluminum chloride, aluminum nitrate, and aluminum sulfate.

[0049] The cobalt salt, used to generate the divalent cation of hydrotalcite, can be any salt containing cobalt. In this embodiment, it is preferably at least one of cobalt chloride, cobalt nitrate, and cobalt acetate. Any one of these three cobalt salts, or a combination of two or three, can be used. In some embodiments provided by this invention, any one of the three cobalt salts can be used. The aluminum salt, used to generate the trivalent cation of hydrotalcite, can be any salt containing aluminum. In this embodiment, it is preferably at least one of aluminum nitrate, aluminum sulfate, and aluminum chloride. Any one of these three aluminum salts, or a combination of two or three, can be used. The selection of the cobalt and aluminum salts can be specified separately or simultaneously. When specified simultaneously, the cobalt and aluminum salts used are inexpensive, readily available, and have good solubility, facilitating co-precipitation to form hydrotalcite after dissolution.

[0050] Further, step S10 includes: adding cobalt salt and aluminum salt sequentially to distilled water and mixing to obtain a cobalt-aluminum salt solution; dissolving a precipitant in distilled water to obtain a precipitant solution; adding the precipitant solution to the cobalt-aluminum salt solution under stirring, and aging the mixture at 60-80°C for 10-24 hours to obtain a first mixed solution. By preparing the cobalt-aluminum salt solution and the precipitant solution, and adding the precipitant solution to the cobalt-aluminum salt solution while stirring and mixing, the pH value rises slowly during the mixing process, and blocky hydrotalcite is generated simultaneously throughout the system. This avoids the situation where the blocky hydrotalcite crystals generated by directly mixing the two solutions are too large, making it difficult to subsequently peel off and form hydrotalcite nanosheets. By aging the mixture at 60-80°C for 10-24 hours, the cobalt-aluminum salt is completely precipitated to form cobalt-aluminum hydrotalcite, avoiding the situation where the crystallinity of the product is too low due to incomplete aging reaction.

[0051] Further, the molar ratio of cobalt salt to aluminum salt in the cobalt-aluminum salt solution is (5-7):(1-3); and / or, the total molar concentration of cobalt salt and aluminum salt in the cobalt-aluminum salt solution is 0.01-0.1M; and / or, the molar concentration of the precipitant solution is 0.01-0.3M; and / or, the volume ratio of the cobalt-aluminum salt solution to the precipitant mixture is 1:(1-8).

[0052] Hydrotalcite [M] 2+ 1-x M 3+ x (OH) - 2] x+ [A x / n ] n x- In mH₂O, x equals M 3+ The total amount and M 2+ and M 3+ The ratio of the total amount of cobalt salt to aluminum salt is used. By adopting a molar ratio of cobalt salt to aluminum salt of (5-7):(1-3), so that x is between 0.2 and 0.33, a stable structure, a single crystal phase, and high crystallinity of hydrotalcite can be obtained. If there is too much aluminum salt, aluminum hydroxide is easily formed on the hydrotalcite layer due to excessive aluminum ions; if there is too much cobalt salt, cobalt hydroxide is easily formed on the hydrotalcite layer due to excessive aluminum ions. Specifically, the molar ratio of cobalt salt to aluminum salt in the cobalt-aluminum salt solution can be 5:1, 5:2, 5:3, 6:1, 6:2, 6:3, 7:1, 7:2, or 7:3.

[0053] By controlling the total molar concentration of cobalt and aluminum salts in the cobalt-aluminum salt solution to 0.01–0.1 M, it is easier to co-precipitate hydrotalcite after mixing with the precipitant solution. If the total molar concentration is greater than 0.1 M, the less soluble salt will form hydroxide precipitates during co-precipitation; if the total molar concentration is less than 0.01 M, too little hydrotalcite will be produced, increasing the preparation cost. Specifically, the total molar concentration of cobalt and aluminum salts in the cobalt-aluminum salt solution can be 0.01 M, 0.03 M, 0.05 M, 0.07 M, 0.09 M, or 0.1 M.

[0054] By controlling the molar concentration of the precipitant solution within the range of 0.01–0.3 M, the pH of the mixed solution can be stabilized within the range of 8–10. This makes it more suitable as a precipitant for the formation of hydrotalcite, avoiding both the problem of excessively high pH causing the metal cations to react too quickly with hydroxide ions and easily forming impurities such as hydroxides, and the problem of excessively low pH causing the hydrotalcite yield to be too low. Specifically, in some embodiments of the present invention, in the mixed solution of sodium hydroxide and sodium carbonate: the molar concentration of sodium hydroxide is 0.03–0.3 M, and the molar concentration of sodium carbonate is 0.01–0.1 M. Specifically, the molar concentration of the precipitant solution can be 0.01 M, 0.05 M, 0.1 M, 0.15 M, 0.2 M, 0.25 M, or 0.3 M.

[0055] By controlling the volume ratio of the aluminum salt solution to the precipitant mixture at 1:(1-8), the cobalt and aluminum salts can be completely converted into hydrotalcite without a large amount of residue, thus avoiding waste of cobalt and aluminum salts. Specifically, the volume ratio of the aluminum salt solution to the precipitant mixture can be 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, or 1:8.

[0056] Furthermore, the molar ratio of cobalt salt to aluminum salt, the total molar concentration of cobalt salt and aluminum salt in the cobalt-aluminum salt solution, the molar concentration of the precipitant solution, and the volume ratio of the cobalt-aluminum salt solution to the precipitant mixture can be defined separately or simultaneously. When defined simultaneously, the cobalt salt and aluminum salt can be completely converted into hydrotalcite without a large amount of residue, avoiding waste of raw materials, and the resulting hydrotalcite has a stable structure, a single crystal phase, and high crystallinity.

[0057] Further, step S30 includes: S31, adding blocky cobalt aluminum hydrotalcite to distilled water and sonicating for 10–60 min to obtain a cobalt aluminum hydrotalcite dispersion, wherein the mass concentration of cobalt aluminum hydrotalcite in the dispersion is 0.8–10 g / L; S32, adding an alkaline solution to the cobalt aluminum hydrotalcite dispersion and mixing to obtain a second mixture. By dispersing the cobalt aluminum hydrotalcite before adding the alkaline solution, the homogeneity of the system can be improved, allowing the blocky hydrotalcite in the system to detach and form hydrotalcite nanosheets during the hydrothermal reaction, thus avoiding a decrease in the catalytic stability of the generated hydrotalcite nanosheets due to poor system homogeneity. Specifically, the ultrasonic time can be 10 min, 20 min, 30 min, 40 min, 50 min or 60 min; the mass concentration of cobalt aluminum hydrotalcite in the cobalt aluminum hydrotalcite dispersion can be 0.8 g / L, 1.0 g / L, 2.5 g / L, 5.0 g / L, 7.5 g / L or 10 g / L.

[0058] Further, in step S30, the mass ratio of the bulk cobalt-aluminum hydrotalcite to the alkaline solution is 1:(300-1800). This setting can improve the exfoliation efficiency of the bulk cobalt-aluminum hydrotalcite during the hydrothermal reaction. If the mass ratio of the bulk cobalt-aluminum hydrotalcite to the alkaline solution is less than 1:1800, there will be too much alkaline solution, and Co... 2+ If the ion concentration is too low, the precipitation equilibrium reaction rate on the surface of the hydrotalcite is too slow, resulting in a slow formation rate of hydrotalcite nanosheets. If the mass ratio of the bulk cobalt-aluminum hydrotalcite to the alkaline solution is higher than 1:300, there will be too much bulk cobalt-aluminum hydrotalcite, which will prolong the time required for the bulk hydrotalcite to regrow into hydrotalcite nanosheets, thus reducing production efficiency. Specifically, the mass ratio of the bulk cobalt-aluminum hydrotalcite to the alkaline solution can be 1:300, 1:500, 1:1000, 1:1500, or 1:1800.

[0059] Furthermore, in step S30, the mass percentage of alkali in the alkaline solution is 2-10%.

[0060] It should be noted that, for reference Figure 2 According to the classical crystal growth theory, the interatomic forces on the (110) plane of LDHs are strong ionic bonds, while the interatomic forces along the (003) direction are controlled by weak electrostatic interactions and hydrogen bonds, resulting in the growth rate of LDHs nanocrystals on the (110) plane being significantly faster than that of stacking along the (003) direction.

[0061] In some embodiments of the present invention, OH - At a given negative ion concentration, the growth of LDHs depends on the metal ion Co. 2 + The concentration of low concentrations of Co. 2+ Ions generate a small number of LDH cores, which is beneficial for growth on the (110) surface, eventually growing into two-dimensional nanosheets; Co 2+ At higher ion concentrations, stacking along the (003) direction becomes easier, leading to the aggregation and thickening of LDH flakes, forming densely packed blocky LDHs. In this embodiment, the Co... 2+ The ions mainly originate from the dissolution of cobalt on the main layers of the hydrotalcite, and exist in the following precipitation equilibrium:

[0062]

[0063] By controlling the mass ratio of alkali in the alkaline solution to 2-10%, a strongly alkaline environment can be created, resulting in a high concentration of OH-. - Negative ions, due to precipitation equilibrium, cause the cobalt element on the main layer of the hydrotalcite to slowly dissolve, Co 2+The ion concentration remains consistently low, resulting in the formation of a small number of LDH nuclei at the edge of the hydrotalcite, which is beneficial for growth on the (110) surface, ultimately leading to the growth of two-dimensional nanosheets. Specifically, the mass percentage of alkali in the alkaline solution can be 2%, 5%, 8%, or 10%.

[0064] Further, in step S30, the alkaline solution includes at least one of NaOH solution and KOH solution. By using at least one of NaOH solution and KOH solution as the alkaline solution, hydrolysis can produce a high concentration of OH-. - Negative ions are introduced without introducing other anions. Due to precipitation equilibrium, the cobalt element on the main layer of the hydrotalcite slowly dissolves, resulting in Co... 2+ The ion concentration is always very low, which generates a small number of LDHs nuclei at the edge of the hydrotalcite, which is conducive to growth on the (110) surface and eventually grows into two-dimensional nanosheets.

[0065] Further, in step S40, the hydrothermal reaction temperature is 80–150°C; and / or, the reaction time is 1–5 h. By adjusting the hydrothermal reaction temperature to 80–150°C, the crystal growth rate of hydrotalcite can be accelerated. If the temperature is too low, the precipitation equilibrium reaction rate on the surface of hydrotalcite will be too slow; if the temperature is too high, the crystal growth rate of hydrotalcite will not be significantly improved. By adjusting the hydrothermal reaction time to 1–5 h, the catalytic efficiency of the generated two-dimensional nanosheets can be improved. If the time is too short, the generated nanosheets will be thick and the catalytic efficiency will be poor; if the time is too long, the growth of the hydrotalcite nanosheets will not change further, resulting in no significant improvement in catalytic efficiency. Specifically, the hydrothermal reaction temperature can be 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, or 150°C; the hydrothermal reaction time can be 1 h, 2 h, 3 h, 4 h, or 5 h. The above-mentioned hydrothermal reaction temperature and time can be defined separately or simultaneously. Simultaneously, under specific conditions, the hydrothermal reaction is optimal, resulting in hydrotalcite nanosheets with uniform thickness, high catalytic efficiency, and strong catalytic stability.

[0066] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0067] Example 1

[0068] A method for preparing cobalt aluminum hydrotalcite nanosheets is provided, comprising the following steps:

[0069] (1) Add Al(NO3)3 (2mmol) and Co(NO3)2 (6mmol) to 20mL of distilled water in sequence, mix well to obtain a premixed solution;

[0070] (2) Dissolve sodium hydroxide and sodium carbonate in distilled water, mix and stir to prepare a precipitant mixture with a molar concentration of sodium hydroxide of 0.03M and a molar concentration of sodium carbonate of 0.06M; then, under vigorous stirring, slowly add 60mL of the precipitant mixture to the premixed solution prepared in step (1) to form a mixed solution.

[0071] (3) The mixed solution was stirred at 60°C for 24 hours, then the solid and liquid were separated and the solid product was collected. After washing and drying, blocky cobalt aluminum hydrotalcite was obtained.

[0072] (4) Add 0.1g of the cobalt aluminum hydrotalcite to 20mL of distilled water, sonicate for 30min to disperse it in the distilled water, and then add 50mL of 2% NaOH solution to obtain a mixed solution;

[0073] (5) The mixed solution is heated to 120°C and subjected to hydrothermal reaction for 2 hours. The solid product is separated, washed, and dried to obtain cobalt aluminum hydrotalcite nanosheets.

[0074] Example 2

[0075] A method for preparing cobalt aluminum hydrotalcite nanosheets is provided, comprising the following steps:

[0076] (1) Add Al(NO3)3 (2mmol) and Co(NO3)2 (4mmol) to 20mL of distilled water in sequence, mix well to obtain a premixed solution;

[0077] (2) Dissolve sodium hydroxide and sodium carbonate in distilled water, mix and stir to prepare a precipitant mixture with a molar concentration of sodium hydroxide of 0.06M and a molar concentration of sodium carbonate of 0.02M; then, under vigorous stirring, slowly add 40mL of the precipitant mixture to the premixed solution prepared in step (1) to form a mixed solution.

[0078] (3) The mixed solution was stirred at 60°C for 12 hours, then the solid and liquid were separated and the solid product was collected. After washing and drying, blocky cobalt aluminum hydrotalcite was obtained.

[0079] (4) Add 0.05g of the cobalt aluminum hydrotalcite to 30mL of distilled water, sonicate for 30min to disperse it in the distilled water, and then add 30mL of 5% NaOH solution to obtain a mixed solution;

[0080] (5) The mixed solution is heated to 150°C and subjected to hydrothermal reaction for 1 hour. The solid product is separated, washed, and dried to obtain cobalt aluminum hydrotalcite nanosheets.

[0081] Example 3

[0082] A method for preparing cobalt aluminum hydrotalcite nanosheets is provided, comprising the following steps:

[0083] (1) Add Al(NO3)3 (2mmol) and Co(NO3)2 (4mmol) to 20mL of distilled water in sequence, mix well to obtain a premixed solution;

[0084] (2) Dissolve sodium hydroxide and sodium carbonate in distilled water, mix and stir to prepare a precipitant mixture with a molar concentration of sodium hydroxide of 0.06M and a molar concentration of sodium carbonate of 0.06M; then, under vigorous stirring, slowly add 30mL of the precipitant mixture to the premixed solution prepared in step (1) to form a mixed solution.

[0085] (3) The mixed solution was stirred at 80°C for 8 hours, then the solid and liquid were separated and the solid product was collected. After washing and drying, blocky cobalt aluminum hydrotalcite was obtained.

[0086] (4) Add 0.05g of the cobalt aluminum hydrotalcite to 30mL of distilled water, sonicate for 60min to disperse it in the distilled water, and then add 60mL of 2% NaOH solution to obtain a mixed solution;

[0087] (5) The mixed solution is heated to 110°C and subjected to hydrothermal reaction for 2 hours. The solid product is separated, washed, and dried to obtain cobalt aluminum hydrotalcite nanosheets.

[0088] Example 4

[0089] A method for preparing cobalt aluminum hydrotalcite nanosheets is provided, comprising the following steps:

[0090] (1) Add Al(NO3)3 (2mmol) and Co(NO3)2 (6mmol) to 20mL of distilled water in sequence, mix well to obtain a premixed solution;

[0091] (2) Dissolve sodium hydroxide and sodium carbonate in distilled water, mix and stir to prepare a precipitant mixture with a molar concentration of sodium hydroxide of 0.03M and a molar concentration of sodium carbonate of 0.06M; then, under vigorous stirring, slowly add 30mL of the precipitant mixture to the premixed solution prepared in step (1) to form a mixed solution.

[0092] (3) The mixed solution was stirred at 60°C for 24 hours, then the solid and liquid were separated and the solid product was collected. After washing and drying, blocky cobalt aluminum hydrotalcite was obtained.

[0093] (4) Add 0.2g of the cobalt aluminum hydrotalcite to 60mL of distilled water, sonicate for 30min to disperse it in the distilled water, and then add 60mL of 5% NaOH solution to obtain a mixed solution;

[0094] (5) The mixed solution is heated to 120°C and subjected to hydrothermal reaction for 1 hour. The solid product is separated, washed, and dried to obtain stable cobalt aluminum hydrotalcite nanosheets.

[0095] Example 5

[0096] A method for preparing cobalt aluminum hydrotalcite nanosheets is provided, comprising the following steps:

[0097] (1) Add Al(NO3)3 (2mmol) and Co(NO3)2 (6mmol) to 60mL of distilled water in sequence, mix well to obtain a premixed solution;

[0098] (2) Dissolve sodium hydroxide and sodium carbonate in distilled water, mix and stir to prepare a precipitant mixture with a molar concentration of sodium hydroxide of 0.12M and a molar concentration of sodium carbonate of 0.06M; then, under vigorous stirring, slowly add 60mL of the precipitant mixture to the premixed solution prepared in step (1) to form a mixed solution.

[0099] (3) The mixed solution was stirred at 60°C for 24 hours, then the solid and liquid were separated and the solid product was collected. After washing and drying, blocky cobalt aluminum hydrotalcite was obtained.

[0100] (4) Add 0.2g of the cobalt aluminum hydrotalcite to 40mL of distilled water, sonicate for 30min to disperse it in the distilled water, and then add 60mL of 10% NaOH solution to obtain a mixed solution;

[0101] (5) The mixed solution is heated to 110°C and subjected to hydrothermal reaction for 2 hours. The solid product is separated, washed, and dried to obtain cobalt aluminum hydrotalcite nanosheets.

[0102] Example 6

[0103] A method for preparing cobalt aluminum hydrotalcite nanosheets is provided, comprising the following steps:

[0104] (1) Add Al(NO3)3 (2mmol) and Co(NO3)2 (6mmol) to 40mL of distilled water in sequence, mix well to obtain a premixed solution;

[0105] (2) Dissolve sodium hydroxide and sodium carbonate in distilled water, mix and stir to prepare a precipitant mixture with a molar concentration of sodium hydroxide of 0.1M and a molar concentration of sodium carbonate of 0.05M; then, under vigorous stirring, slowly add 40mL of the precipitant mixture to the premixed solution prepared in step (1) to form a mixed solution.

[0106] (3) The mixed solution was stirred at 80°C for 8 hours, then the solid and liquid were separated and the solid product was collected. After washing and drying, blocky cobalt aluminum hydrotalcite was obtained.

[0107] (4) Add 0.05g of the cobalt aluminum hydrotalcite to 30mL of distilled water, sonicate for 20min to disperse it in the distilled water, and then add 60mL of 2% NaOH solution to obtain a mixed solution;

[0108] (5) The mixed solution is heated to 100°C and subjected to hydrothermal reaction for 3 hours. The solid product is separated, washed, and dried to obtain cobalt aluminum hydrotalcite nanosheets.

[0109] Example 7

[0110] A method for preparing cobalt aluminum hydrotalcite nanosheets is provided, comprising the following steps:

[0111] (1) Add Al(NO3)3 (1mmol) and Co(NO3)2 (7mmol) to 80mL of distilled water in sequence, mix well, and obtain a premixed solution;

[0112] (2) Dissolve sodium hydroxide and sodium carbonate in distilled water, mix and stir to prepare a precipitant mixture with a sodium hydroxide molar concentration of 0.01M; then, under vigorous stirring, slowly add 80mL of the precipitant mixture to the premixed solution prepared in step (1) to form a mixed solution.

[0113] (3) The mixed solution was stirred at 80°C for 8 hours, then the solid and liquid were separated and the solid product was collected. After washing and drying, blocky cobalt aluminum hydrotalcite was obtained.

[0114] (4) Add 0.1g of the cobalt aluminum hydrotalcite to 125mL of distilled water, sonicate for 10min to disperse it in the distilled water, and then add 50mL of 2% NaOH solution to obtain a mixed solution;

[0115] (5) The mixed solution is heated to 100°C and subjected to hydrothermal reaction for 3 hours. The solid product is separated, washed, and dried to obtain cobalt aluminum hydrotalcite nanosheets.

[0116] Example 8

[0117] A method for preparing cobalt aluminum hydrotalcite nanosheets is provided, comprising the following steps:

[0118] (1) Add Al(NO3)3 (3mmol) and Co(NO3)2 (5mmol) to 20mL of distilled water in sequence, mix well, and obtain a premixed solution;

[0119] (2) Dissolve sodium hydroxide and sodium carbonate in distilled water, mix and stir to prepare a precipitant mixture with a molar concentration of sodium hydroxide of 0.2M and a molar concentration of sodium carbonate of 0.1M; then, under vigorous stirring, slowly add 160mL of the precipitant mixture to the premixed solution prepared in step (1) to form a mixed solution.

[0120] (3) The mixed solution was stirred at 80°C for 8 hours, then the solid and liquid were separated and the solid product was collected. After washing and drying, blocky cobalt aluminum hydrotalcite was obtained.

[0121] (4) Add 0.1g of the cobalt aluminum hydrotalcite to 10mL of distilled water, sonicate for 30min to disperse it in the distilled water, and then add 50mL of 2% NaOH solution to obtain a mixed solution;

[0122] (5) The mixed solution is heated to 100°C and subjected to hydrothermal reaction for 3 hours. The solid product is separated, washed, and dried to obtain cobalt aluminum hydrotalcite nanosheets.

[0123] Comparative Example 1

[0124] A method for preparing bulk cobalt aluminum hydrotalcite is provided, comprising the following steps:

[0125] (1) Add Al(NO3)3 (2mmol) and Co(NO3)2 (6mmol) to 20mL of distilled water in sequence, mix well to obtain a premixed solution;

[0126] (2) Dissolve sodium hydroxide and sodium carbonate in distilled water, mix and stir to prepare a precipitant mixture with a molar concentration of sodium hydroxide of 0.03M and a molar concentration of sodium carbonate of 0.06M; then, under vigorous stirring, slowly add 60mL of the precipitant mixture to the premixed solution prepared in step (1) to form a mixed solution.

[0127] (3) The mixed solution was stirred at 60°C for 24 hours, then the solid and liquid were separated and the solid product was collected. After washing and drying, blocky cobalt aluminum hydrotalcite was obtained.

[0128] Comparative Example 2

[0129] A nano-cobalt tetroxide was provided, purchased from Shanghai Aladdin Reagent Co., Ltd.

[0130] Scanning electron microscopy examination

[0131] Scanning electron microscopy (SEM) was performed on the intermediate product, bulk cobalt aluminum hydrotalcite, and the cobalt aluminum hydrotalcite nanosheets obtained in Example 1. The SEM image of the cobalt aluminum hydrotalcite nanosheets from Example 1 is shown below. Figure 3 As shown, the exfoliated hydrotalcite exhibits a typical two-dimensional nanosheet structure, with nanosheets having a diameter of approximately 80–100 nm and a thickness of 7.3–8.5 nm. Since the thickness of a single layer of hydrotalcite is approximately 0.8 nm, the exfoliated hydrotalcite nanosheets are composed of approximately 9–10 single-layer hydrotalcite layers; the electron microscopy image of the bulk hydrotalcite in Comparative Example 1 is shown below. Figure 4As shown, it exhibits a densely packed structure composed of numerous hydrotalcite nanosheets, with a size of approximately 8–10 μm and a thickness of approximately 3.0–5.0 μm.

[0132] Energy dispersive spectrometer detection

[0133] Elemental analysis of the cobalt-aluminum hydrotalcite nanosheets prepared in Example 1 was performed using an EDS spectrometer. The results showed that the material contained carbon, oxygen, cobalt, and aluminum, with contents of 42.76–42.31%, 36.75–36.27%, 16.91–16.52%, and 3.89–3.52%, respectively.

[0134] X-ray diffraction detection

[0135] X-ray diffraction (XRD) was performed on the bulk hydrotalcite prepared in Comparative Example 1 and the hydrotalcite nanosheets prepared in Example 1. The resulting XRD patterns are as follows: Figure 5 As shown. X-ray diffraction analysis revealed that both the bulk hydrotalcite and the hydrotalcite nanosheets exhibited characteristic diffraction peaks of the hydrotalcite (003), (006), (012), (015), (110), and (0015) crystal planes at 11.66°, 23.57°, 34.94°, 39.42°, 60.32°, and 60.72°. Furthermore, these characteristic diffraction peaks were essentially identical. This suggests that the hydrotalcite nanosheets possess the same octahedral crystal structure as the bulk hydrotalcite, and the regularity of this crystal structure contributes to their strong stability.

[0136] In addition, from Figure 5 It is easy to observe that the X-ray diffraction pattern of bulk hydrotalcite, compared to that of hydrotalcite nanosheets, exhibits obvious impurity peaks around 28° and a distinct double peak at the characteristic diffraction peak of (012). This indicates that hydrotalcite nanosheets contain fewer impurities and have higher crystal structure uniformity compared to bulk hydrotalcite.

[0137] Catalytic performance and catalytic activity stability analysis experiment

[0138] The catalytic performance of the cobalt aluminum hydrotalcite prepared in Examples 1 to 6 and Comparative Example 1, and the nano-cobalt tetroxide provided in Comparative Example 2 were analyzed. The methods and results are as follows:

[0139] Catalytic performance evaluation method: Under normal temperature and neutral conditions, 0.05 g of cobalt aluminum hydrotalcite prepared in Examples 1 to 6 and Comparative Example 1, and nano-cobalt tetroxide provided in Comparative Example 2 were added to 100 mL of pollutant aqueous solution with a concentration of 100 mg / L, respectively. Then, 0.05 g of potassium persulfate was added. Samples were taken every 5 min to investigate the purification effect of different materials on dye wastewater containing Rhodamine B, industrial wastewater containing phenol, and wastewater containing pyrethroid pesticides within 30 min. The pollutant removal rate was used as the evaluation index to evaluate the catalytic performance.

[0140] Catalytic activity stability evaluation method: After the catalytic performance of the materials in each embodiment and comparative example is evaluated as described above, the solid materials obtained by centrifugation or filtration are washed, dried, and then the recycling performance of the materials is examined according to the catalytic performance evaluation method described above.

[0141] The evaluation results of catalytic performance and catalytic activity stability are shown in Table 1 below.

[0142] Table 1. Evaluation results of catalytic performance and catalytic activity stability of each example and comparative example.

[0143]

[0144] As shown in Table 1, the test results indicate that the hydrotalcite nanosheets prepared in this invention significantly improve both catalytic efficacy and stability compared to commercially available nano-cobalt tetroxide and bulk hydrotalcite. Within 30 minutes, the hydrotalcite nanosheets prepared in each embodiment of this invention achieved a removal rate of over 91.5% for pollutants in dye wastewater, phenol wastewater, and pyrethroid pesticide wastewater, while the lowest removal rate for Comparative Example 1 was only 41.5%, and the lowest removal rate for Comparative Example 2 was 52.5%. It is evident that the removal rate of pollutants from wastewater by the hydrotalcite nanosheets prepared in each embodiment of this invention is 2.7 times that of bulk hydrotalcite and 1.8 times that of commercially available nano-cobalt tetroxide, demonstrating the superior catalytic performance of the hydrotalcite nanosheets prepared in this invention.

[0145] Furthermore, after five cycles of recycling, the hydrotalcite nanosheets prepared in each embodiment of the present invention still achieved a minimum removal rate of 85.2% for the three pollutants mentioned above, while the lowest rate for Comparative Example 1 was only 33.6% and for Comparative Example 2 it was only 41.8%. It is easy to see that the removal rate of pollutants in wastewater by the hydrotalcite nanosheets prepared in each embodiment of the present invention is 2.5 times that of block hydrotalcite and 2.0 times that of commercially available nano cobalt tetroxide, indicating that the hydrotalcite nanosheets prepared in the present invention have high catalytic activity stability.

[0146] The above experimental results demonstrate that the hydrotalcite nanosheets prepared in this invention possess high catalytic activity and stability due to their large specific surface area, numerous exposed active sites, and regular structure.

[0147] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for preparing cobalt-aluminum hydrotalcite nanosheets, characterized in that, Includes the following steps: S10. Mix the solution containing cobalt salt and aluminum salt with a precipitant to obtain the first mixture; S20. Separate and collect the solid from the first mixture, wash and dry it to obtain blocky cobalt aluminum hydrotalcite; S30. Disperse the blocky cobalt aluminum hydrotalcite in distilled water and mix it with alkaline solution to obtain a second mixture; S40. The second mixture is subjected to a hydrothermal reaction, then the solid is separated and collected, washed, and dried to obtain cobalt aluminum hydrotalcite nanosheets. Step S30 includes S31, adding blocky cobalt aluminum hydrotalcite to distilled water and sonicating for 10-60 min to obtain a cobalt aluminum hydrotalcite dispersion, wherein the mass concentration of cobalt aluminum hydrotalcite in the dispersion is 0.8-10 g / L; S32, adding alkali solution to the cobalt aluminum hydrotalcite dispersion and mixing to obtain a second mixture. In step S40, the temperature of the hydrothermal reaction is 80~150℃; and / or, the reaction time is 1~5h.

2. The method for preparing cobalt-aluminum hydrotalcite nanosheets as described in claim 1, characterized in that, In step S10, the precipitant includes at least one of NaOH, KOH, Na2CO3 and K2CO3.

3. The method for preparing cobalt-aluminum hydrotalcite nanosheets as described in claim 1, characterized in that, In step S10, the cobalt salt includes at least one of cobalt chloride, cobalt nitrate, and cobalt acetate; and / or, The aluminum salt includes at least one of aluminum chloride, aluminum nitrate, and aluminum sulfate.

4. The method for preparing cobalt-aluminum hydrotalcite nanosheets as described in claim 1, characterized in that, Step S10 includes: Cobalt salt and aluminum salt are added sequentially to distilled water and mixed to obtain a cobalt-aluminum salt solution; The precipitant is dissolved in distilled water to obtain a precipitant solution; Under stirring, the precipitant solution is added to the cobalt-aluminum salt solution, and the aging reaction is carried out at 60~80℃ for 10~24h to obtain the first mixture.

5. The method for preparing cobalt-aluminum hydrotalcite nanosheets as described in claim 4, characterized in that, The molar ratio of cobalt salt to aluminum salt in the cobalt-aluminum salt solution is (5~7):(1~3); and / or, The total molar concentration of cobalt and aluminum salts in the cobalt-aluminum salt solution is 0.01~0.4M; and / or, The molar concentration of the precipitant in the precipitant solution is 0.01~0.3M; and / or, The volume ratio of the cobalt aluminum salt solution to the precipitant mixture is 1:(1~8).

6. The method for preparing cobalt-aluminum hydrotalcite nanosheets as described in claim 1, characterized in that, In step S30, the mass ratio of the blocky cobalt aluminum hydrotalcite to the alkaline solution is 1:(300~1800).

7. The method for preparing cobalt-aluminum hydrotalcite nanosheets as described in claim 1, characterized in that, In step S30, the mass percentage of alkali in the alkaline solution is 2-10%.

8. The method for preparing cobalt-aluminum hydrotalcite nanosheets as described in claim 1, characterized in that, In step S30, the alkaline solution includes at least one of NaOH solution and KOH solution.

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