CN113336220A

CN113336220A - Preparation method of carbon quantum dot-carbon nanotube composite material with high adsorption performance

Info

Publication number: CN113336220A
Application number: CN202110717134.5A
Authority: CN
Inventors: 王宏; 陈强; 巴子涵; 王易; 赵小强
Original assignee: Southwest Petroleum University
Current assignee: Southwest Petroleum University
Priority date: 2021-06-28
Filing date: 2021-06-28
Publication date: 2021-09-03
Anticipated expiration: 2041-06-28
Also published as: CN113336220B

Abstract

The invention relates to a method for preparing a supported catalyst based on calcium-based waste, and a method for synthesizing a carbon quantum dot-carbon nanotube composite material with high adsorption performance by using the catalyst. The preparation method comprises the following steps: (1) preparing a catalyst: preparing calcium oxide carrier from ovum gallus Domesticus crusta, ovum Anas Domestica crusta, ovum Anseris Domestica crusta, ovum Coturnicis Japonicae crusta, ovum Columba livia crusta, ovum ostrich crusta, lobster shell, abalone shell, crab shell, snail shell, and Concha Ostreae shell, and preparing catalyst by soaking method with one or more transition metal salts as precursor; (2) synthesizing a carbon quantum dot-carbon nanotube composite material by using the catalyst prepared in the step 1 and ethanol as a carbon source and adopting a chemical vapor deposition method; (3) and (4) treating with dilute acid to obtain the purified composite material. The carbon quantum dot-carbon nanotube composite material prepared by the invention has the advantages of unique structure, large specific surface area and the like. The adsorption performance of the adsorbent is further researched, and the maximum adsorption capacity to methylene blue can reach 299.4 mg/g.

Description

Preparation method of carbon quantum dot-carbon nanotube composite material with high adsorption performance

Technical Field

The invention relates to a method for preparing a supported catalyst based on calcium-based waste, and a method for synthesizing a carbon quantum dot-carbon nanotube composite material with high adsorption performance by using the catalyst.

Background

Carbon nanotubes have been extensively studied since their discovery. The preparation method of the carbon nano tube mainly comprises an arc discharge method, a laser ablation method and a chemical vapor deposition method. Compared with the other two methods, the chemical vapor deposition method has the advantages of relatively mild operating conditions, easy control, and applicability to mass production, and is the most commonly used method. Chemical vapor deposition processes often require a carbon source, a catalyst, and a catalyst support, particularly a catalyst support, as the highest content component of the catalyst and need to be removed as an impurity in a subsequent step. How to reduce the synthesis cost of carbon nanotubes and improve the purity of carbon nanotubes is one of the major difficulties faced by the large-scale application of carbon nanotubes.

Carbon nanotubes are potential adsorbents due to their unique cylindrical hollow structure, high aspect ratio, good thermal stability and easily modified surface. However, the performance of carbon nanotubes is generally not satisfactory for many specific applications and is often plagued by slow adsorption rates and limited adsorption capacity due to their low specific surface area (compared to activated carbon, biochar, etc.) and small pore volume.

Modification of carbon nanotubes is one of the means to improve the adsorption performance, and the purpose is to increase the specific surface area and surface functional groups of the carbon nanotubes to increase the adsorption activityAnd (5) point location. Common modification means are KOH activation, HNO₃Oxidizing and introducing functional materials such as polyaniline and the like. Although the modified functionalized carbon nanotube shows better adsorption performance than the original carbon nanotube, the complicated steps and the use of a large amount of strong acid and strong base limit the large-scale application and are not beneficial to environmental protection. In addition, the preparation of a composite material by combining carbon nanotubes with other carbon nanomaterials is another important means for improving the adsorption performance of the composite material, but the preparation of the composite material generally requires modification treatment of the carbon nanotubes. Therefore, there is still a need to develop a simple, efficient and green preparation method to obtain the carbon nanotube-based composite material.

As described above, it is important to develop a low-cost catalyst that is easy to purify, and further synthesize a carbon nanotube-based composite material with high adsorption performance by a chemical vapor deposition method using the catalyst.

Disclosure of Invention

The invention provides a preparation method of a green, low-cost and high-adsorption-property carbon quantum dot-carbon nanotube composite material.

The invention adopts the following technical scheme:

the invention mainly uses calcium-based wastes such as eggshells and the like to prepare a calcium oxide carrier, prepares a supported catalyst by an impregnation method, and synthesizes the carbon quantum dot-carbon nanotube composite material with high adsorption performance by using the catalyst through a chemical vapor deposition method.

The technical scheme comprises the following steps:

(1) cleaning and drying calcium-based wastes such as eggshells and the like, and then calcining at high temperature to obtain a CaO carrier;

(2) preparing a supported metal catalyst by soaking one or more transition metal salts and the CaO carrier obtained in the step (1), stirring at normal temperature, heating and stirring, drying, calcining, grinding and the like; the transition metal salt is selected from metal salts of cobalt, iron, nickel, chromium, manganese, copper, rhodium and ruthenium;

(3) synthesizing a carbon quantum dot-carbon nanotube composite material by using the catalyst prepared in the step (2) and ethanol as a carbon source through a chemical vapor deposition method;

(4) and (4) purifying the sample obtained in the step (3) by using dilute hydrochloric acid to obtain the purified carbon quantum dot-carbon nanotube composite material.

Preferably, the calcination temperature in step (1) is 900 ℃ and the calcination time is 4 h.

Preferably, the transition metal salt in step (2) is selected from one or more mixtures of cobalt, iron and nickel salts. The cobalt salt can be selected from cobalt acetylacetonate, cobalt acetate and cobalt oxalate, the iron salt can be selected from iron acetylacetonate, iron acetate and iron oxalate, the nickel salt can be selected from nickel acetylacetonate, nickel acetate and nickel oxalate, and the total metal content in the catalyst is 1-10 wt%.

Preferably, the solvent used in step (2) is absolute ethyl alcohol, the temperature for heating and stirring is 60 ℃, the temperature for further drying is 80 ℃, and the temperature for calcining is 450 ℃.

Preferably, the reduction temperature of the carbon quantum dot-carbon nanotube composite material synthesized by the chemical vapor deposition method in the step (3) is 650-750 ℃, the growth temperature is 800-900 ℃, and the growth time is 20-40 min.

Preferably, the concentration of hydrochloric acid used in step (4) is 10% by weight.

The invention also provides the research on the adsorption performance of the carbon quantum dot-carbon nanotube composite adsorbent synthesized by the catalyst chemical vapor deposition method on methylene blue.

The invention successfully utilizes calcium-based wastes such as eggshells and the like to prepare a low-cost green supported catalyst, and utilizes the catalyst to successfully synthesize the carbon quantum dot-carbon nanotube composite material by a chemical vapor deposition method under the conditions that the reduction temperature is 650-750 ℃, the growth temperature is 800-900 ℃ and the growth time is 20-40 min. The carbon quantum dot-carbon nanotube composite material with higher purity can be obtained after simple purification.

The invention has the following advantages:

(1) the catalyst prepared by the invention is mainly derived from calcium-based waste, has important environmental protection significance, and greatly reduces the synthesis cost;

(2) the carbon quantum dot-carbon nanotube composite material is synthesized by one step by utilizing the catalyst chemical vapor deposition method, the process is simple, and the modification treatment of the carbon nanotube is avoided;

(3) the catalyst can be removed through simple hydrochloric acid treatment to obtain the purified composite material, and common purification means such as strong acid and strong alkali, high-temperature treatment and the like are not needed;

(4) the carbon quantum dot-carbon nanotube composite material obtained by purification has good adsorption performance on methylene blue, and the adsorption performance on the methylene blue is higher than that of all carbon nanotube adsorbents which are not modified and mostly modified.

Drawings

FIG. 1 is an X-ray diffraction pattern (XRD) of CaO carriers and Fe-Co/CaO catalysts prepared in examples 1, 2, and 3.

Figure 2 is an XRD pattern of the synthesized sample and the purified sample of example 2.

Fig. 3 is a low-power Scanning Electron Microscope (SEM) image of the carbon quantum dot-carbon nanotube composite prepared in example 2.

Fig. 4 is a high power SEM image of the carbon quantum dot-carbon nanotube composite prepared in example 2.

Fig. 5 is a Transmission Electron Micrograph (TEM) of the carbon quantum dot-carbon nanotube composite prepared in example 2.

Fig. 6 is an absorption spectrum (Uv-vis) of the carbon quantum dot-carbon nanotube composite prepared in example 2.

Fig. 7 is a Raman spectrum (Raman) of the carbon quantum dot-carbon nanotube composite materials prepared in examples 1, 2, and 3.

Fig. 8 is an infrared spectrum (FT-IR) of the carbon quantum dot-carbon nanotube composite prepared in example 2.

Fig. 9 is a nitrogen adsorption-desorption isotherm of the carbon quantum dot-carbon nanotube composite prepared in example 2.

Fig. 10 is a pore size distribution diagram of the carbon quantum dot-carbon nanotube composite prepared in example 2.

Fig. 11 is a graph of the adsorption amount of methylene blue adsorbed by the carbon quantum dot-carbon nanotube composite material prepared in example 2 according to the concentration of methylene blue and the adsorption temperature.

Fig. 12 is a high power SEM image of the carbon quantum dot-carbon nanotube composite prepared in example 4.

Fig. 13 is a high power SEM image of the carbon quantum dot-carbon nanotube composite prepared in example 5.

Detailed Description

The invention will now be described in detail by way of specific embodiments with reference to the accompanying drawings, which are provided solely for the purpose of further describing the invention. The described embodiments are only some embodiments of the invention, not all embodiments. And are not intended to limit the scope of the present invention, nor to limit the scope of the present invention.

Example 1

(1) Preparation of the catalyst: calcining the cleaned and dried calcium-based waste such as eggshells at 900 ℃ for 4h to obtain a CaO carrier; adding weighed cobalt (II) acetylacetonate and iron (III) acetylacetonate into 100mL of absolute ethanol, and stirring at 60 ℃ to completely dissolve metal salt; then adding the CaO carrier obtained by calcination into the metal salt solution, and stirring for 30min at normal temperature; then heating to 60 ℃, stirring until the mixture is viscous, and placing the mixture in an oven at 80 ℃ for drying overnight; finally, the dried catalyst was ground and calcined at 450 ℃ for 3 h. The molar ratio of iron to cobalt in the catalyst prepared by the steps is 1: 1, the CaO carrier supports a total metal content of 5 wt.%.

(2) Synthesis of carbon quantum dot-carbon nanotube: weighing 150mg of the catalyst prepared in the step (1), uniformly paving the catalyst in a porcelain boat, and then putting the porcelain boat into a tube furnace; introducing hydrogen with the flow of 30sccm, heating to 700 ℃ at the heating rate of 10 ℃/min, adjusting the hydrogen to be argon, and continuing heating in an argon atmosphere; when the temperature rises to 800 ℃, purging by argon (500sccm) to bring the ethanol steam into a reaction zone in the tubular furnace, and reacting for 30 min; and after the reaction is finished, stopping introducing the ethanol, and cooling to room temperature in an argon atmosphere to obtain a carbon quantum dot-carbon nanotube composite material sample.

(3) Purifying the carbon quantum dots-carbon nanotubes: adding 200mg of the sample synthesized in the step (2) into 50mL of 10 wt% hydrochloric acid solution, and stirring at normal temperature for 120 min; and then filtering and washing the sample until the pH value is 7, and then putting the washed black sample into an oven at 80 ℃ for drying for 12h to obtain the purified carbon quantum dot-carbon nanotube composite material.

(4) Adsorbing methylene blue by the carbon quantum dots-carbon nano tubes: weighing 10mg of the sample purified in the step (3) for adsorbing methylene blue, wherein the adsorption conditions are as follows: 10mL of 400mg/L methylene blue solution, 40 ℃ of adsorption temperature, 150r/min of rotation speed of a magnetic stirrer, 6 of pH and 60min of adsorption time.

The carbon quantum dot-carbon nanotube composite material obtained in example 1 had an adsorption amount of 177.8mg/g to methylene blue under the adsorption conditions described above.

Example 2

(1) Preparation of the catalyst: the catalyst in this example was prepared as in example 1.

(2) Synthesis of carbon quantum dot-carbon nanotube: weighing 150mg of the catalyst prepared in the step (1), uniformly paving the catalyst in a porcelain boat, and then putting the porcelain boat into a tube furnace; introducing hydrogen with the flow of 30sccm, heating to 700 ℃ at the heating rate of 10 ℃/min, adjusting the hydrogen to be argon, and continuing heating in an argon atmosphere; when the temperature rises to 850 ℃, purging by argon (500sccm) to bring the ethanol steam into a reaction zone in the tubular furnace, and reacting for 30 min; and after the reaction is finished, stopping introducing the ethanol, and cooling to room temperature in an argon atmosphere to obtain the carbon quantum dot-carbon nanotube composite material.

(3) Purifying the carbon quantum dots-carbon nanotubes: the composite material in this example was purified as in example 1.

(4) Adsorbing methylene blue by the carbon quantum dots-carbon nano tubes: weighing 10mg of the sample purified in the step (3) for adsorbing methylene blue, wherein the adsorption conditions are as follows: 10mL of 400mg/L methylene blue solution, 40 ℃ of adsorption temperature, 150r/min of rotation speed of a magnetic stirrer, 6 of pH and 60min of adsorption time. In addition, the maximum methylene blue adsorption capacity of the carbon quantum dot-carbon nanotube composite material prepared in example 2 is also explored, and the adsorption experiment conditions are as follows: the adsorbent amount is 10mg, the methylene blue solution amount is 10mL, the methylene blue concentration is 50-600mg/L, the adsorption temperature is 40 ℃, the magnetic stirrer rotation speed is 150r/min, the pH value is 6, and the adsorption time is 60 min.

The adsorption amount of the carbon quantum dot-carbon nanotube composite material obtained in example 2 to methylene blue under the adsorption conditions was 285.6 mg/g; the maximum methylene blue adsorption amount of the carbon quantum dot-carbon nanotube composite obtained in example 2 is shown in fig. 11.

Example 3

(2) Synthesis of carbon quantum dot-carbon nanotube: weighing 150mg of the catalyst prepared in the step (1), uniformly paving the catalyst in a porcelain boat, and then putting the porcelain boat into a tube furnace; introducing hydrogen with the flow of 30sccm, heating to 700 ℃ at the heating rate of 10 ℃/min, adjusting the hydrogen to be argon, and continuing heating in an argon atmosphere; when the temperature rises to 900 ℃, purging by argon (500sccm) to bring the ethanol steam into a reaction zone in the tubular furnace, and reacting for 30 min; and after the reaction is finished, stopping introducing the ethanol, and cooling to room temperature in an argon atmosphere to obtain the carbon quantum dot-carbon nanotube composite material.

(4) Adsorbing methylene blue by the carbon quantum dots-carbon nano tubes: the methylene blue adsorption experiment in this example was the same as in example 1.

The carbon quantum dot-carbon nanotube composite material obtained in example 3 had an adsorption amount of 253.1mg/g to methylene blue under the adsorption conditions described above.

FIG. 1 is an XRD pattern of a CaO carrier and an Fe-Co/CaO catalyst prepared as described in examples 1, 2, 3, wherein 2 θ is 32.2 °, 37.4 °, 53.9 °, 64.2 °, 67.4 °, 79.7 °, respectively representing (111), (200), (220), (311), (222), (400) crystal planes [ PDF #48-1467 ] of CaO]And no other obvious diffraction peak exists, and the main component of the calcium-based waste after high-temperature calcination is CaO. The XRD pattern of the Fe-Co/CaO catalyst has a series of diffraction peaks, wherein the diffraction peaks at 23.1 degrees, 29.4 degrees, 36.0 degrees, 39.4 degrees, 43.2 degrees, 47.5 degrees, 48.5 degrees, 57.4 degrees and 64.6 degrees are assigned to CaCO₃Diffraction at 18.0 °, 34.1 °, 50.8 °, 64.2 °Peaks ascribed to Ca (OH)₂And the diffraction peak intensities of CaO at 32.2 degrees, 37.35 degrees and 53.9 degrees are greatly reduced. Calculated according to the scherrer equation to obtain CaCO₃、Ca(OH)₂And CaO are 40nm, 40nm and 14nm in size, respectively. XRD result shows that CaO has phase transformation in the preparation process of the catalyst, and is transformed into nano CaCO after being calcined₃、Ca(OH)₂And CaO. Further, no diffraction peak of the metal component appeared, indicating that the metal component was well dispersed on the support.

Fig. 2 is an XRD spectrum of the synthesized sample and the purified sample of example 2, in which diffraction peaks at about 26.3 ° and 42.2 ° are (002) and (100) crystal planes of the carbon nanotube, respectively. No diffraction peak for CaO was observed in the XRD pattern of the purified sample, indicating that the CaO carrier was completely removed by the dilute hydrochloric acid treatment.

Fig. 3 and 4 are SEM images of different multiples of the carbon quantum dot-carbon nanotube composite material prepared in example 2, and it can be seen that the composite material has a relatively uniform diameter distribution, the diameter distribution is about 20-40nm, and most of the length is greater than 500 nm. Fig. 4 shows that the carbon quantum dots are wrapped on the tube wall of the carbon nanotube, and the size of the carbon quantum dots is less than 10 nm. Also, the presence of CaO was not observed in the SEM picture, which is consistent with the XRD results.

Fig. 5 is a TEM image of the carbon quantum dot-carbon nanotube composite material prepared in example 2, which shows that the carbon quantum dot is wrapped on the carbon nanotube, and the diameter of the composite material is about 30nm, which is consistent with the SEM result. In addition, it was observed that carbon quantum dots were dispersed around the carbon nanotubes, which is probably caused by the peeling of the carbon quantum dots from the surface of the carbon nanotubes by the strong ultrasonic vibration during the TEM sampling process.

Fig. 6 is a Uv-vis diagram of the carbon quantum dot-carbon nanotube composite material prepared in example 2, and the Uv-vis diagram shows distinct absorption peaks at 220 and 260nm, which can be attributed to pi-pi transition from C ═ C, which is a characteristic peak of the carbon quantum dot, thus proving the presence of the carbon quantum dot in the sample.

FIG. 7 is a Raman spectrum of a carbon quantum dot-carbon nanotube composite prepared as described in examples 1 to 3, wherein 1580cm^-1Is a tangential vibration mode (G peak) representing the C-C tensile mode of a highly ordered graphitic layer with an sp2 orbital structure; and 1300cm^-1A defect vibration mode (D peak), corresponds to a lattice defect and amorphous carbon. The samples synthesized in examples 1-3 all had well-defined D and G peaks, indicating successful synthesis of the composite. Wherein the sample prepared as described in example 1 had the highest D/G strength ratio, indicating that there were more defects or amorphous carbon in the carbon material. The sample prepared in example 3 has the lowest D/G strength ratio, higher surface graphitization degree and fewer defects. The sample prepared in example 2, however, had moderate defects and a higher degree of graphitization, and may be more conducive to adsorption.

FIG. 8 is a FT-IR chart of the carbon quantum dot-carbon nanotube composite material prepared in example 2, 3400cm^-1Left and right broad bands due to tensile vibration of-OH, 2927cm^-1、1380cm^-1And 1049cm^-1Nearby bands belonging to CH₂Deformation vibration and C-O, C-O stretching vibration, 1630cm^-1The nearby wavelength band corresponds to the stretching vibration of C ═ C, and the surface functional groups enable the composite material to have certain hydrophilicity and serve as anchoring sites for methylene blue molecules.

FIGS. 9 and 10 are a nitrogen adsorption-desorption graph and a pore size distribution graph of the carbon quantum dot-carbon nanotube composite prepared in example 2, and it can be seen that the composite exhibits characteristics of typical type IV isotherms and exhibits significant H₄Type hysteresis loop, indicating that the composite material has a large number of mesopores. The distribution range of the pore diameter is wide as can be seen from the pore diameter distribution diagram, and the pore diameter distribution range comprises micropores with the diameter of 1.5nm and mesopores with the diameter of 3-15 nm.

Fig. 11 is a graph showing the change in the amount of methylene blue adsorbed by the carbon quantum dot-carbon nanotube composite material prepared in example 2 according to the concentration of methylene blue and the adsorption temperature. It can be seen that the composite material has good adsorption performance at different temperatures, and the adsorption quantity is increased along with the temperature rise. As the methylene blue concentration increases, the adsorption capacity also increases because the mass transfer driving force between methylene blue and the composite material increases dramatically with increasing initial methylene blue concentration, which leads to an increasing adsorption capacity. However, as the concentration of methylene blue increases, the rejection capacity of the methylene blue molecules increases, so that the adsorption amount increases more slowly. The maximum adsorption capacity of the composite material to methylene blue reaches 299.4mg/g at 40 ℃.

Example 4

(1) Preparation of the catalyst: calcining the cleaned and dried calcium-based waste such as eggshells at 900 ℃ for 4h to obtain a CaO carrier; adding the weighed cobalt acetylacetonate into 100mL of absolute ethyl alcohol, and stirring at 60 ℃ to completely dissolve the metal salt; then adding the CaO carrier obtained by calcination into the metal salt solution, and stirring for 30min at normal temperature; then heating to 60 ℃, stirring until the mixture is viscous, and placing the mixture in an oven at 80 ℃ for drying overnight; finally, the dried catalyst was ground and calcined at 700 ℃ for 3 h. The supported cobalt content of the CaO carrier was 5 wt%.

Fig. 12 is an SEM image of the carbon quantum dot-carbon nanotube composite prepared in example 4. It can be seen that carbon quantum dots with a size less than 10nm are wrapped on the tube wall of the carbon nanotube.

Example 5

(1) Preparation of the catalyst: calcining the cleaned and dried calcium-based waste such as eggshells at 900 ℃ for 4h to obtain a CaO carrier; adding weighed nickel acetylacetonate into 100mL of absolute ethyl alcohol, and stirring at 60 ℃ to completely dissolve metal salt; then adding the CaO carrier obtained by calcination into the metal salt solution, and stirring for 30min at normal temperature; then heating to 60 ℃, stirring until the mixture is viscous, and placing the mixture in an oven at 80 ℃ for drying overnight; finally, the dried catalyst was ground and calcined at 700 ℃ for 3 h. The supported nickel content of the CaO carrier was 5 wt%.

Fig. 13 is an SEM image of the carbon quantum dot-carbon nanotube composite prepared in example 5. It can be clearly seen that carbon quantum dots with a size less than 10nm are wrapped on the tube wall of the carbon nanotube.

It should be understood that the above examples are only for clearly illustrating the present invention and are not intended to limit the embodiments. Many other variations will be apparent to those skilled in the art in light of the above teachings. Here, too, all the implementation methods cannot be described. Thus, obvious variations or modifications of the above-described embodiments are within the scope of the present invention.

Claims

1. A preparation method of a carbon quantum dot-carbon nanotube composite material with high adsorption performance is characterized by comprising the following steps: the method comprises the following steps:

(1) preparation of the catalyst: calcining the calcium-based waste at a high temperature to obtain a CaO carrier; weighing a certain amount of transition metal salt, dissolving, adding a calcined CaO carrier into the metal salt solution, stirring at normal temperature for 30min, then stirring at 60 ℃ until the CaO carrier is viscous, drying in an oven at 80 ℃ overnight, and finally grinding and calcining the dried catalyst;

(2) synthesizing the carbon quantum dot-carbon nano tube by a chemical vapor deposition method: weighing 150mg of the catalyst prepared in the step (1), uniformly paving the catalyst in a ceramic boat, putting the ceramic boat filled with the catalyst in a middle constant-temperature area of a tubular furnace, heating to a reduction temperature in a hydrogen atmosphere, then replacing hydrogen with inert gas, keeping in the inert gas atmosphere to continue heating, carrying ethanol steam into a reaction area in the tubular furnace through the inert gas after the temperature is raised to a growth temperature, stopping introducing the ethanol after the reaction is finished, and keeping in the inert atmosphere to cool to room temperature to obtain the carbon quantum dot-carbon nanotube composite material;

(3) purifying the carbon quantum dots-carbon nanotubes: and (3) adding 200mg of the sample synthesized in the step (2) into 50mL of diluted acid, and stirring, filtering, washing and drying to obtain the carbon quantum dot-carbon nanotube composite material with high purity.

2. The method for preparing the catalyst according to claim 1, wherein the calcium-based waste used in the step (1) is eggshell, eggshell of duck, eggshell of goose, eggshell of quail, eggshell of pigeon, eggshell of ostrich, eggshell of lobster, abalone, crab, shell, snail, oyster; the calcination temperature of the calcium-based waste is 800-1000 ℃, and the calcination time is 4-8 h.

3. The method for preparing a catalyst according to any one of claims 1 and 2, wherein the transition metal salt in the step (1) is selected from one or a mixture of metal salts of cobalt, iron, nickel, chromium, manganese, copper, rhodium and ruthenium. Preferably, the cobalt salt can be selected from cobalt acetylacetonate, cobalt acetate and cobalt oxalate, the iron salt can be selected from iron acetylacetonate, iron acetate and iron oxalate, the nickel salt can be selected from nickel acetylacetonate, nickel acetate and nickel oxalate, and the total metal content in the catalyst is 1 wt% -10 wt%.

4. A method for producing a catalyst according to any one of claims 1 to 3, characterized in that in the step (1); the solvent used by the metal salt is absolute ethyl alcohol, methanol and dichloromethane; the calcination temperature of the catalyst is 400-800 ℃.

5. A catalyst as claimed in claims 2-4, which can be used for synthesizing carbon quantum dot-carbon nanotube composite material by chemical vapor deposition.

6. The method as claimed in claims 1 and 5, wherein the reduction temperature in the chemical vapor deposition process is 600-900 ℃, the growth temperature is 750-950 ℃, the growth time is 5-60min, the inert gases used are argon and nitrogen, and the inert gas flow rate is 100-500 sccm.

7. The method for purifying composite material according to claim 1, wherein the dilute acid is dilute sulfuric acid, dilute hydrochloric acid, or dilute nitric acid, the mass fraction of the acid is 5-20 wt%, and the stirring speed is 100-400 r/min.

8. A carbon quantum dot-carbon nanotube composite material obtained by purifying according to the method of claims 1 and 7.

9. The application of the carbon quantum dot-carbon nanotube composite material prepared by the method in claims 1, 7 and 8 as a methylene blue adsorbent in dye wastewater treatment.

10. The use as claimed in claim 9, wherein in the methylene blue adsorption process, the pH of the methylene blue solution is 2-10, the concentration of the methylene blue solution is 100-600mg/L, the amount of the methylene blue solution is 10mL, the amount of the adsorbent is 2.5-15mg, the temperature is 20-40 ℃, and the adsorption time is 5-120 min.