Process for the preparation of transition metal carbides and composite catalysts

Abstract

The application relates to the technical field of photocatalytic materials, and provides a preparation method of a transition metal carbide, which comprises the following steps: providing a transition metal salt and a carbon source; dissolving the transition metal salt and the carbon source in water, and then performing heating and evaporation treatment to obtain a precursor; and performing calcination treatment on the precursor to obtain the transition metal carbide. The preparation method has simple, stable and easy-to-control process conditions, high product yield, and extremely easy-to-clean impurities, and has a good industrial application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of photocatalytic materials, and particularly relates to a preparation method of a transition metal carbide and a composite catalyst. BACKGROUND

[0002] Hydrogen energy, as the most promising clean and carbon-free energy, is an alternative to fossil fuels, but the existing industrial technology is not suitable for producing hydrogen for fuel use. It is considered that the conversion of solar energy into hydrogen by photocatalytic technology is one of the most promising ways of hydrogen production, which is expected to fundamentally solve the problems of energy shortage and environmental pollution.

[0003] The photocatalytic reaction is a reaction process of converting solar energy into high-energy-density chemical energy and storing it in chemical bonds by using the structural characteristics of semiconductors. The semiconductor material has a discontinuous energy band structure, that is, it contains a high-energy empty conduction band (CB) and a low-energy full valence band (VB). The energy band gap between the bottom of the conduction band and the top of the valence band is called the forbidden band, and the forbidden band width is represented by Eg. The electrons on the valence band are unstable, and when the semiconductor is irradiated with light with an energy greater than its forbidden band width, the valence band electrons absorb energy and transition to the conduction band, leaving a photo-generated hole (h + ) on the valence band. The photo-generated electrons on the conduction band have strong reducing properties and can reduce H + ions into H2, and the photo-generated holes on the valence band have strong oxidizing properties and can oxidize water into O2. The key to photocatalytic water splitting to produce hydrogen lies in the selection of suitable photocatalysts. Early photocatalysts such as TiO2 are mostly responsive to ultraviolet light (ultraviolet light accounts for 5% of solar energy, and visible light accounts for about 46% of solar energy), and the utilization rate of solar energy is too low. The valence band and conduction band potentials cannot meet the needs of the catalytic reaction at the same time, the photo-generated electron-hole pairs are easy to recombine, the quantum efficiency is low, the cost is too high, and many other problems. Therefore, it is difficult to find a cheap and commercialized material that meets all the conditions of high visible light quantum efficiency, stability, safety, and cheapness.

[0004] The easy recombination of surface electron-hole pairs and the slow electron consumption rate are two important factors affecting the photocatalytic efficiency. Loading cocatalysts can solve these problems by increasing the surface active sites of the catalyst material and promoting the separation of electron-hole pairs. Traditional cocatalysts are mostly noble metals such as Pt, Ag, and Au. Noble metals have a low Fermi level and a high work function. After forming a Schottky junction with a semiconductor, electrons can be transferred from the semiconductor to the noble metal, and the noble metal can quickly react with protons, thereby improving the photocatalytic activity of the semiconductor. However, the scarcity and high price of noble metals limit their industrial application. Currently, scientists are focusing on finding efficient and inexpensive non-noble metal cocatalysts. The new non-noble metal cocatalysts that have attracted attention are metal sulfides such as MoS2, WS2, and NiS, as well as carbon materials such as graphene, carbon black, carbon nanotubes, and C60. These materials mostly have a typical two-dimensional structure, a large specific surface area, and excellent electrical conductivity, but still have some shortcomings. Transition metal carbides are considered to be a promising photocatalytic hydrogen production cocatalyst. However, the current laboratory synthesis method for transition metal carbides usually involves the reaction of transition metal salts with organic phases at high temperatures in an oxygen-free atmosphere. The reaction conditions are demanding and the reaction progress is difficult to control. After the reaction is completed, the organic phase impurities have high viscosity and are difficult to wash, resulting in a high impurity content and low yield of the product, which affects further application.

[0005] Therefore, the related art needs to be improved. SUMMARY

[0006] The present application aims to provide a preparation method of transition metal carbide and a composite catalyst, and aims to solve the technical problem of complex preparation conditions of existing transition metal carbide.

[0007] To achieve the above application purposes, the technical solutions adopted by the present application are as follows:

[0008] In a first aspect, the present application provides a preparation method of transition metal carbide, comprising the following steps:

[0009] Providing a transition metal salt and a carbon source;

[0010] Dissolving the transition metal salt and the carbon source in water, and then performing heating and evaporation treatment to obtain a precursor;

[0011] Performing calcination treatment on the precursor to obtain the transition metal carbide.

[0012] The application provides a preparation method of transition metal carbide, which uses transition metal salt and carbon source as raw materials, dissolves them in water, and evaporates under heating to hydrolyze the transition metal salt to form transition metal hydroxide colloid and corresponding acid, the acid volatilizes in the heating process, the transition metal hydroxide colloid has strong adsorption and can uniformly adsorb the carbon source to form a precursor solid solution, and the transition metal hydroxide is decomposed to form metal oxide in the calcination process of the precursor, the metal oxide is further decomposed by the sugar to form molecular carbon to form the transition metal carbide, the preparation condition is simple, stable and easy to control, the product yield is high, impurities are easy to clean, and the method has good industrial application prospect.

[0013] In a second aspect, the application provides a composite catalyst, which comprises a main catalyst and a cocatalyst, and the cocatalyst comprises the transition metal carbide prepared by the preparation method.

[0014] The cocatalyst in the composite catalyst of the application comprises the transition metal carbide prepared by the preparation method, the transition metal carbide has high purity and has a better hydrogen production rate, so that the composite catalyst has better photocatalytic effect. BRIEF DESCRIPTION OF DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.

[0016] Figure 1 is a preparation method flowchart of the transition metal carbide provided by the embodiments of the application;

[0017] Figure 2 is a photocatalytic hydrogen production activity comparison chart of the composite catalyst prepared by the transition metal carbide as a cocatalyst provided by the embodiments of the application;

[0018] Figure 3 is an XRD comparison chart of nickel carbide provided by the embodiments of the application. DETAILED DESCRIPTION

[0019] In order to make the technical problems, technical solutions and beneficial effects of the application more clearly understood, the application will be further described in detail in combination with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the application, and are not used to limit the application.

[0020] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0021] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0022] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass described in the embodiments of this application can be a mass unit known in the chemical industry, such as μg, mg, g, or kg.

[0023] The first aspect of this application provides a method for preparing transition metal carbides, such as... Figure 1 As shown, the preparation method includes the following steps:

[0024] S01: Provides transition metal salts and carbon sources;

[0025] S02: Dissolve the transition metal salt and the carbon source in water, and then heat and evaporate the solution to obtain the precursor;

[0026] S03: The precursor is calcined to obtain the transition metal carbide.

[0027] The method for preparing transition metal carbides provided in this application uses transition metal salts and carbon sources as raw materials, dissolves them in water, and then heats and evaporates the transition metal salts to form transition metal hydroxide colloids and corresponding acids. The acids volatilize during heating. The transition metal hydroxide colloids have extremely strong adsorption properties and can uniformly adsorb carbon sources to form precursor solid solutions. During the calcination of the precursors, the transition metal hydroxides decompose to form metal oxides. These metal oxides are further carbonized by molecular carbon formed from the decomposition of sugars to form transition metal carbides. Such preparation conditions are simple, stable, and easy to control, with high product yield and easy-to-clean impurities, showing great promise for industrial applications.

[0028] In some embodiments, the transition metal salt is soluble, and can be a transition metal nitrate, a transition metal chloride, a transition metal acetate, or the like. The transition metal salt is hydrolyzed during heating and evaporation, forming a transition metal hydroxide colloid and a corresponding acid. The transition metal acetate is preferred, as the formed acetic acid can be more easily evaporated. Specifically, when the transition metal salt is a nickel salt, the nickel salt is at least one of nickel nitrate, nickel acetate, and nickel chloride, and the transition metal carbide is nickel carbide (Ni3C). When the transition metal salt is a tungsten salt, the tungsten salt is at least one of tungsten nitrate and tungsten chloride, and the transition metal carbide is tungsten carbide (WC). When the transition metal salt is a manganese salt, the manganese salt is at least one of manganese nitrate, manganese acetate, and manganese chloride, and the transition metal carbide is manganese carbide (Mn3C). When the transition metal salt is a cobalt salt, the cobalt salt is at least one of cobalt nitrate, cobalt acetate, and cobalt chloride, and the transition metal carbide is cobalt carbide (Co3C). When the transition metal salt is a molybdenum salt, the molybdenum salt is at least one of molybdenum nitrate, molybdenum acetate, and molybdenum chloride, and the transition metal carbide is molybdenum carbide (MoC2). When the transition metal salt is an iron salt, the iron salt is at least one of iron nitrate, iron acetate, and iron chloride, and the transition metal carbide is iron carbide (Fe3C). The transition metal salt can be any of a nickel salt, a tungsten salt, a manganese salt, a cobalt salt, a molybdenum salt, and an iron salt, so that different transition metal carbides can be calcined in a tube furnace. Further, the transition metal carbide can be Fe3C, Co3C, and Ni3C.

[0029] In some embodiments, the carbon source is free of N, S, and halogen, and can be a carbohydrate containing only C, H, and O. Specifically, the carbon source can be a saccharide, which is inexpensive and easy to obtain, and can be dissolved in water to prepare a precursor. The carbon content of the saccharide is high, and the saccharide is free of N, S, and halogen, which are not easily volatilized at high temperatures. Further, the saccharide can be one or more of glucose, fructose, sucrose, and chitosan.

[0030] In some embodiments, the weight ratio of the transition metal salt to the carbon source in the step of dissolving the transition metal salt and the carbon source in water is 1:1-10. The carbon source can be in excess to sufficiently carbonize the transition metal carbide.

[0031] In some embodiments, the heating and evaporation treatment is performed at a temperature of 80-100°C. The transition metal salt can be sufficiently hydrolyzed at this temperature.

[0032] In some embodiments, the temperature of the calcination treatment is 500-600℃. Further, the calcination is performed at 500-600℃ for 4-8h. Under the calcination conditions, the molecular carbon formed by the decomposition of the sugar can be sufficiently carbonized to form the transition metal carbide. Further, in the calcination treatment, the temperature is raised to 500-600℃ at a temperature raising rate of 4-6℃ / min, which makes the reaction system more stable. During the temperature raising process, the transition metal hydroxide is decomposed to form the metal oxide, and when the calcination conditions are 500-600℃, the metal oxide is carbonized by the molecular carbon formed by the decomposition of the sugar to form the transition metal carbide.

[0033] Correspondingly, the embodiments of the present application provide a transition metal carbide prepared by the above-mentioned preparation method of the transition metal carbide.

[0034] The transition metal carbide provided by the present application is prepared by the above-mentioned preparation method of the transition metal carbide, and has high purity. The transition metal carbide can be used as a cocatalyst and a main catalyst. Compared with the transition metal carbide cocatalyst prepared by the prior art, the hydrogen production rate of the transition metal carbide cocatalyst is improved, and therefore the transition metal carbide cocatalyst has better application in the field of photocatalysis.

[0035] The second aspect of the embodiments of the present application provides a composite catalyst, which comprises a main catalyst and a cocatalyst. The cocatalyst comprises the transition metal carbide prepared by the above-mentioned preparation method.

[0036] The cocatalyst in the composite catalyst of the present application comprises the transition metal carbide prepared by the above-mentioned preparation method. The transition metal carbide has high purity and has a better hydrogen production rate, and therefore the composite catalyst has better photocatalytic effect.

[0037] In some embodiments, the above-mentioned cocatalyst can be Ni3C, WC, Mn3C, Co3C, MoC2 and Fe3C prepared by the embodiments of the present application.

[0038] In some embodiments, the above-mentioned main catalyst can be CdS nanosheet or g-C3N4 (graphitic carbon nitride). The main catalyst is combined with the transition metal carbide prepared by the above-mentioned preparation method to prepare a composite catalyst, and the better photocatalytic hydrogen production performance of the composite catalyst is verified.

[0039] The CdS nanosheet and the ultra-thin g-C3N4 can be obtained from the market or prepared by the following preparation method.

[0040] Preparation of ultrathin g-C3N4: 15 g of urea and 10 g of ammonium chloride were stirred uniformly in a container and then placed in a crucible, which was placed in a muffle furnace and calcined at 550°C for 2 h at a heating rate of 4°C / min. The calcined product was washed with distilled water and then collected by centrifugation at 8000 rpm, and then placed in a vacuum oven at 60°C for drying for 12 h, and then ground to obtain an ultrathin g-C3N4 powder.

[0041] Preparation of CdS nanosheets: 0.6 g of sulfur powder and 0.7 g of CdCl2·2.5H2O were dissolved in 75 mL of diethylene triamine (DETA) and ultrasonically stirred for 30 min to mix uniformly. Then the mixture was transferred to a 100 mL polytetrafluoroethylene-lined reaction kettle. The reaction kettle was heated to 80°C in an oven and kept for 48 h, and then the light yellow product was obtained by centrifugation at 3000 rpm after natural cooling to room temperature. Then the product was washed with deionized water and anhydrous ethanol three times, respectively, and placed in a vacuum drying oven at 60°C for drying for 8 h to obtain CdS nanosheets.

[0042] The following will be described in conjunction with specific examples.

[0043] Example 1

[0044] Preparation of a composite catalyst

[0045] (1) Preparation of a cocatalyst Co3C:

[0046] 2 g of cobalt acetate and 10 g of glucose powder were dissolved in 30 mL of deionized water, stirred for 30 min to mix uniformly, and then heated at 80°C to evaporate the deionized water while continuously stirring with a glass rod until a viscous colloid was obtained. Then the colloid was placed in a porcelain boat and calcined in a tube furnace at 550°C for 5 h at a heating rate of 5°C / min. The calcined product was washed with distilled water three times and then collected by centrifugation at 5000 rpm, and then placed in a vacuum oven at 60°C for drying for 8 h. After that, the Co3C powder was obtained by grinding.

[0047] (2) Preparation of a composite catalyst:

[0048] 100 mg of CdS powder and 5 mg of the Co3C powder obtained above were placed in 50 mL of deionized water, ultrasonically dispersed for 2 h, and then continuously stirred for 18 h. The yellow-green precipitate was obtained by centrifugation at 5000 rpm, washed with anhydrous ethanol, and then placed in an oven at 60°C for drying for 10 h. The Co3C / CdS composite catalyst was obtained by grinding.

[0049] Example 2

[0050] Preparation of a composite catalyst

[0051] (1) Preparation of a cocatalyst Fe3C:

[0052] Take 2.5 g of iron chloride and 8 g of chitosan powder dissolved in 30 mL of deionized water, stir for 30 minutes to mix evenly, then evaporate the deionized water at 80°C and continuously stir with a glass rod until a sticky gel is obtained, then place the gel in a porcelain boat and place it in a tube furnace at 600°C for 4h, the heating rate is 5°C / min, the product after calcination is washed with distilled water three times and then centrifuged at 6000 rpm, then put it in a vacuum oven at 60°C for 8h, then grind to get Fe3C powder.

[0053] (2) Preparation of composite catalyst:

[0054] Put 200 mg of g-C3N4 powder and 14 mg of Fe3C powder obtained above into 100 mL of deionized water, ultrasonic for 2h to disperse evenly, then continue to stir for 18h, centrifuge at 6000 rpm to obtain a gray precipitate, then wash with anhydrous ethanol and place it in a vacuum oven at 60°C for 8h, then grind to obtain Fe3C / g-C3N4 composite catalyst.

[0055] Example 3

[0056] Preparation of a composite catalyst

[0057] (1) Preparation of co-catalyst Ni3C:

[0058] Take 3 g of nickel acetate and 9 g of glucose powder dissolved in 30 mL of deionized water, stir for 30 minutes to mix evenly, then evaporate the deionized water at 80°C and continuously stir with a glass rod until a sticky gel is obtained, then place the gel in a porcelain boat and place it in a tube furnace at 500°C for 6h, the heating rate is 5°C / min, the product after calcination is washed with distilled water three times and then centrifuged at 5000 rpm, then put it in a vacuum oven at 60°C for 8h, then grind to get Ni3C powder.

[0059] (2) Preparation of composite catalyst:

[0060] Put 200 mg of g-C3N4 powder and 10 mg of Ni3C powder obtained above into 100 mL of deionized water, ultrasonic for 2h to disperse evenly, then continue to stir for 18h, centrifuge at 5000 rpm to obtain a gray precipitate, then wash with anhydrous ethanol and place it in a vacuum oven at 60°C for 8h, then grind to obtain Ni3C / g-C3N4 composite catalyst.

[0061] Comparative Example 1

[0062] Preparation of a composite catalyst

[0063] (1) Preparation of co-catalyst Co3C:

[0064] Cobalt acetate and oleylamine were used as raw materials, which were added into a three-necked round-bottom flask and continuously stirred under a nitrogen atmosphere. Then, the mixture was heated to 280°C using an oil bath and maintained for 2 h. After the reaction was completed and the mixture was naturally cooled to room temperature, 30 mL of acetone was added into the mixture. Then, the black precipitate was collected by centrifugation and washed with deionized water and anhydrous ethanol several times to remove organic impurities, thereby obtaining Co3C powder.

[0065] (2) Preparation of the composite catalyst:

[0066] The 100 mg of CdS powder and 5 mg of the Co3C powder obtained above were placed in 50 mL of deionized water, ultrasonically dispersed for 2 h, and then continuously stirred for 18 h. The yellow-green precipitate was obtained by centrifugation at 5000 rpm, washed with anhydrous ethanol, and then placed in an oven at 60°C for drying for 10 h. The Co3C / CdS composite catalyst was obtained by grinding.

[0067] Comparative Example 2

[0068] Preparation of a composite catalyst

[0069] (1) Preparation of the cocatalyst Fe3C:

[0070] Iron chloride and oleylamine were used as raw materials, which were added into a three-necked round-bottom flask and continuously stirred under a nitrogen atmosphere. Then, the mixture was heated to 280°C using an oil bath and maintained for 2 h. After the reaction was completed and the mixture was naturally cooled to room temperature, 30 mL of acetone was added into the mixture. Then, the black precipitate was collected by centrifugation and washed with deionized water and anhydrous ethanol several times to remove organic impurities, thereby obtaining Fe3C powder.

[0071] (2) Preparation of the composite catalyst:

[0072] The 200 mg of g-C3N4 powder and 14 mg of the Fe3C powder obtained above were placed in 100 mL of deionized water, ultrasonically dispersed for 2 h, and then continuously stirred for 18 h. The gray precipitate was obtained by centrifugation at 6000 rpm, washed with anhydrous ethanol, and then placed in a vacuum oven at 60°C for drying for 8 h. The Fe3C / g-C3N4 composite catalyst was obtained by grinding.

[0073] Comparative Example 3

[0074] Preparation of a composite catalyst

[0075] (1) Preparation of the cocatalyst Ni3C:

[0076] Nickel acetate and oleylamine were added to a three-necked round-bottom flask and magnetically stirred continuously under a nitrogen atmosphere. The mixture was then heated to 280°C in an oil bath and held at that temperature for 2 hours. After the reaction was complete and the mixture was allowed to cool naturally to room temperature, 30 mL of acetone was added to the mixture. The black precipitate was then collected by centrifugation and washed several times with deionized water and anhydrous ethanol to remove organic impurities, yielding Ni3C powder.

[0077] (2) Preparation of composite catalysts:

[0078] 200 mg g-C3N4 powder and 10 mg Ni3C powder obtained above were placed in 100 mL of deionized water and ultrasonically dispersed for 2 h. Then, the mixture was stirred continuously for 18 h. After centrifugation at 5000 rpm, a gray precipitate was obtained. The precipitate was washed with anhydrous ethanol and dried in a vacuum oven at 60 °C for 8 h. The precipitate was then ground to obtain the Ni3C / g-C3N4 composite catalyst.

[0079] Performance testing

[0080] (1) Visible light hydrogen production activity: The activity was tested using the Beijing Bofeilai photocatalytic water splitting hydrogen production device. The hydrogen produced during the reaction was detected online using a (GC-9500) gas chromatograph. The specific steps are as follows: 250 mg of the composite catalysts prepared in Examples 1-3 and Comparative Examples 1-3 were added to the reactor, and 100 mL of 10 vol% lactic acid or triethanolamine aqueous solution was added as a sacrificial agent. After ultrasonic dispersion for 30 min, the reactor was sealed, and the entire reaction system was evacuated to -0.1 MPa using a vacuum pump. Continuous visible light irradiation was performed using a xenon lamp (PLS-CHF, 300 W, λ>420 nm) accompanied by uninterrupted magnetic stirring. The reaction lasted for a total of 3 hours, and samples were taken every half hour. The results are as follows. Figure 2 As shown.

[0081] Depend on Figure 2 It is known that CdS nanosheets and g-C3N4 alone have very low hydrogen production rates as photocatalysts. However, when loaded with Fe3C, Co3C, and Ni3C transition metal carbide cocatalysts prepared in the embodiments of this application, the resulting composite catalysts exhibit significantly improved hydrogen production rates compared to the individual photocatalysts. Moreover, their hydrogen production rates are even higher than those of the composite catalysts formed by transition metal carbide cocatalysts prepared by conventional methods in the comparative example. Therefore, the experiments in this application demonstrate that the transition metal carbides prepared by the method described in this application have great application potential as cocatalysts in the field of photocatalytic hydrogen production and can be widely used in a wider range of photocatalysts.

[0082] (2) Crystal structure characterization (XRD): The Ni3C samples prepared in Example 3 and Comparative Example 3 were subjected to crystal structure analysis using an (MSAL-XD2) X-ray powder diffractometer. The phase composition of the samples was analyzed at a scan rate of 4 or 8° / min within the range of 2θ from 15 to 80°. The results are as follows: Figure 3 As shown.

[0083] Figure 3 The standard diffraction patterns of Ni3C prepared by the conventional method in Comparative Example 3 and Ni3C prepared in Example 3 of this application are shown in the figures. According to the literature and XRD patterns, the Ni3C crystal phase obtained by the preparation method of this application matches the standard hexagonal Ni3C (JCPDS No: 72-1467) very well. Six characteristic diffraction peaks can be observed in the Ni3C sample, corresponding to the (100), (006), (113), (116), (300), and (119) crystal planes of the hexagonal Ni3C, respectively. At the same time, no diffraction peaks of residues or impurities were observed. Although the Ni3C crystal phase synthesized by the conventional method in Comparative Example 3 basically matches the standard hexagonal Ni3C (JCPDS No: 72-1467), the diffraction peak shape is not good and there are impurity diffraction peaks. This indicates that Example 3 of this application can prepare high-purity Ni3C material.

[0084] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A composite catalyst, characterized in that, It includes a main catalyst and a co-catalyst, wherein the main catalyst is CdS nanosheets and the co-catalyst is cobalt carbide, and the preparation method of the cobalt carbide includes the following steps: A transition metal salt and a carbon source are provided, wherein the transition metal salt is selected from cobalt metal salts; The transition metal salt and the carbon source are dissolved in water and then subjected to heating and evaporation to obtain the precursor; The precursor is calcined to obtain cobalt carbide.

2. The composite catalyst as described in claim 1, characterized in that, In the step of dissolving the transition metal salt and the carbon source in water, the weight ratio of the transition metal salt to the carbon source is 1:1 to 10.

3. The composite catalyst as described in claim 1, characterized in that, The temperature for the heating and evaporation process is 80–100°C.

4. The composite catalyst as described in claim 1, characterized in that, The calcination temperature is 500–600°C.

5. The composite catalyst as described in claim 4, characterized in that, In the calcination process, the temperature is increased to 500–600°C at a rate of 4–6°C / min; and / or, In the calcination process, the calcination is carried out at 500-600℃ for 4-8 hours.

6. The preparation method according to any one of claims 1-5, characterized in that, The carbon source is selected from carbohydrates; and / or, The cobalt metal salt is selected from at least one of cobalt nitrate, cobalt acetate, and cobalt chloride.

7. The preparation method according to claim 6, characterized in that, The carbohydrate is selected from at least one of glucose, fructose, sucrose, and chitosan.

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