CN113751048A - Molybdenum trioxide in-situ intercalation carbon nitride composite catalyst and preparation method thereof - Google Patents
Molybdenum trioxide in-situ intercalation carbon nitride composite catalyst and preparation method thereof Download PDFInfo
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- JKQOBWVOAYFWKG-UHFFFAOYSA-N molybdenum trioxide Chemical compound O=[Mo](=O)=O JKQOBWVOAYFWKG-UHFFFAOYSA-N 0.000 title claims abstract description 115
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- B01J35/39—
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
- B01J27/24—Nitrogen compounds
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F1/00—Treatment of water, waste water, or sewage
- C02F1/30—Treatment of water, waste water, or sewage by irradiation
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F2101/00—Nature of the contaminant
- C02F2101/30—Organic compounds
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F2101/00—Nature of the contaminant
- C02F2101/30—Organic compounds
- C02F2101/38—Organic compounds containing nitrogen
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F2101/00—Nature of the contaminant
- C02F2101/30—Organic compounds
- C02F2101/40—Organic compounds containing sulfur
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F2305/00—Use of specific compounds during water treatment
- C02F2305/10—Photocatalysts
Abstract
The invention discloses a molybdenum trioxide in-situ intercalation carbon nitride composite catalyst and a preparation method thereof, wherein the preparation method of the catalyst comprises the following steps: step A: adding distilled water into melamine, heating and stirring to completely dissolve the melamine to obtain a solution A; and B: adding distilled water into ammonium molybdate, and stirring to completely dissolve the ammonium molybdate to obtain a solution B; and C: slowly and dropwise transferring the solution B into the solution A to obtain milky mixed solution; step D: heating and stirring the mixed solution to enable the melamine to fully react with the ammonium molybdate, and then putting the mixed solution on a rotary evaporator to be rotated, evaporated and dried to enable the water to be completely volatilized to obtain a white powdery precursor; step E: and (3) placing the precursor in a muffle furnace for calcining to obtain a yellow powdery target product after calcining. The molybdenum trioxide in-situ intercalation carbon nitride composite catalyst prepared by the invention has higher light absorption capacity and high electron hole separation capacity, and shows better photocatalytic activity.
Description
Technical Field
The invention relates to the technical field of preparation of composite photocatalysts. In particular to a molybdenum trioxide in-situ intercalation carbon nitride composite catalyst and a preparation method thereof.
Background
From the last 70 s to the present, with the continuous development of the world industrialization, human beings face a serious problem of energy shortage, and in order to solve the problem, researchers all over the world begin to search a new energy source with low pollution and durability. Through diligent efforts, they have discovered zero cost, zero pollution solar energy. In addition, the rapid pace of the current industrialized city is accelerated, which brings huge problems to the environment, increases crisis to living conditions of organisms and limits the sustainable development of the modern human society.
Because the reaction condition is controlled by the environment, the intermediate compound is basically completely mineralized, the test operation cost is low, and secondary pollution cannot be generated, the photocatalytic reaction is mostly applied to degrading organic pollutants. The nature of the photocatalytic reaction is actually a redox reaction, and it is known that the energy band structure of a semiconductor material is different from that of a conductor, a forbidden band is formed between a conduction band and a valence band of the semiconductor, the photocatalytic reaction occurs under the condition of light, and when the band gap energy of a photocatalyst is smaller than or equal to the energy of light irradiation, a hole (h) is generated (h)+) And photoinduced electrons (e)-). Electrons in the conduction band are generated and transferred from the valence band to form photogenerated electron-hole pairs. Under the action of the internal electric field, a part of photogenerated electrons and holes can be recombined, and a part of photogenerated electrons and holes can jump to the surface of the catalyst to carry out redox reaction, so that organic pollutants on the surface can be degraded.
The graphite phase carbon nitride is a polymer semiconductor material, has good microstructure and chemical functions, can be used as a potential photocatalyst due to the characteristics of response to sunlight, rich nitrogen and carbon elements, no metal elements, no toxicity and the like, can effectively realize decontamination and energy conversion, and has potential important functions in the aspects of environmental management and energy source cleaning.
Albeit g-C3N4The polymer is very easy to prepare, has low price, excellent chemical stability and thermal stability and better photocatalytic activity, but has the following defects: g-C3N4The recombination rate of photogenerated carriers and holes of the polymer is too fast; because the specific surface area is small, the visible light response range is limited, and the photocatalytic activity is poor. Thus, researchers have developed a series of methods for g-C3N4The modification is carried out to improve the light absorption characteristic, the specific surface area and the photo-generated charge separation efficiency, thereby improving the photocatalysis effect. The main modification modes comprise element doping, morphology regulation, heterojunction structure, Z-shaped system construction, surface noble metal deposition, promoter and the like, but the g-C after modification at present3N4Most polymers have the defects of complex preparation method, unsatisfactory photocatalytic activity or poor stability and the like. Therefore, the applicant carries out deep research on the optical composite catalyst and the preparation method thereof under the subsidization of the natural science research focus item (item number: KJ2019A0513) in the university of Anhui province.
Disclosure of Invention
Therefore, the technical problem to be solved by the invention is to provide a molybdenum trioxide in-situ intercalation carbon nitride composite catalyst with high degradation rate and high photocatalytic activity for organic pollution such as methyl orange, tetracycline and the like and a preparation method thereof, so as to solve the problems of poor photocatalytic activity and stability of the current carbon nitride and the like.
In order to solve the technical problems, the invention provides the following technical scheme:
the preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst comprises the following steps:
step A: adding distilled water into melamine, heating and stirring to completely disperse the melamine into the distilled water to obtain a solution A;
and B: adding distilled water into ammonium molybdate, and stirring to completely dissolve the ammonium molybdate to obtain a solution B;
and C: slowly and dropwise transferring the solution B into the solution A to obtain milky mixed solution;
step D: heating and stirring the mixed solution, then putting the mixed solution on a rotary evaporator for rotary evaporation and drying to completely volatilize water to obtain a white powdery precursor;
step E: and (3) calcining the precursor in a muffle furnace to obtain a yellow powdery molybdenum trioxide in-situ intercalation carbon nitride composite catalyst after the calcination is finished. Compared with the method for preparing the composite catalyst by separately preparing molybdenum trioxide and carbon nitride respectively and then simply mixing the molybdenum trioxide and the carbon nitride and then calcining, the preparation method disclosed by the invention can be used for compounding the molybdenum trioxide and the carbon nitride in an in-situ space, so that the condition of uneven compounding caused by directly and simply mixing and calcining the method is avoided, and the prepared composite catalyst has better performance; the preparation method of the invention is completely different from the composite method of directly mixing molybdenum trioxide and carbon nitride and then calcining the mixture to obtain the in-situ composite material by firstly preparing a specific precursor and then calcining the precursor.
In the preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst, in the step D, Mo element is MoO3Calculation of MoO in the precursor3The mass ratio of (A) to the mass of melamine is 0.1 to 1.0%.
In the preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst, in the step A, the mass ratio of melamine to distilled water is 1 (35-40); the heating and stirring temperature is 70-90 ℃, and the heating and stirring time is 15-25 min. Through the mass ratio, the stirring time and the stirring temperature of the melamine and the water which are cooperatively regulated and controlled, the melamine is ensured to form an even aqueous solution in the water, and the melamine can not be dissolved incompletely and can not be separated out due to excessive evaporation of heated water.
In the preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst, in the step A, the mass ratio of melamine to distilled water is 1: 37.5; the heating and stirring temperature is 80 deg.C, and the heating and stirring time is 20 min.
In the preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst, in the step B, the mass ratio of ammonium molybdate to distilled water is 1:(200-2000) and the stirring and dissolving temperature of the ammonium molybdate is 18-25 ℃. When the ammonium molybdate solution is transferred dropwise into the melamine solution, a specific MoO is prepared if desired3When the precursor is contained, the volume of the ammonium molybdate solution is required to be increased when the concentration of the ammonium molybdate solution is too low, so that the dropwise transfer time is too long, the preparation efficiency is low, and the yield of the precursor is reduced; the concentration of the ammonium molybdate solution cannot be too high, otherwise, the ammonium molybdate is left after reaction precipitation, so that waste is caused, and the purity of the precursor is influenced.
In the step B, the mass ratio of ammonium molybdate to distilled water is 1:250, and the dissolving temperature of ammonium molybdate is 20 ℃.
In step E, the calcination method of the preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst comprises the following steps: firstly, heating to 450-550 ℃ at a heating rate of 5 ℃/min, and calcining for 3h while keeping the temperature unchanged; then the temperature is reduced to the room temperature at the cooling rate of 5 ℃/min, and the calcination is finished. By regulating and controlling the heating rate, the cooling rate and the calcining temperature, the prepared composite catalyst has high yield and good performance. In tests, it is found that if the temperature rise rate is greater than 5 ℃/min, the yield of the product is significantly reduced, even completely calcined, and if the temperature rise rate is less than 5 ℃/min, the calcination of the precursor is incomplete, so that the precursor cannot be completely sintered into the target composite catalyst, and the yield is reduced.
In the step D, the temperature of the mixed solution is 80 ℃ during heating and stirring, and the heating and stirring time is 3 hours; the rotary evaporation temperature of the rotary evaporator is 70 ℃, and the rotating speed is 200 r/min. If the stirring temperature is too low or too high, the product is not pure, and if the stirring time is too short, the reaction is not complete. Compared with a drying mode of filtering, washing and drying, the yield of the precursor can be effectively improved by adopting a rotary evaporation mode; and when the temperature of the rotary evaporation is 70 ℃ and the rotating speed is 200r/min, the rapid removal of the moisture is ensured, and the yield and the performance of the precursor are not influenced.
In the step A, the mass ratio of melamine to distilled water is 1:37.5, the heating temperature is 80 ℃, and the stirring time is 20 min;
in the step B, the mass ratio of ammonium molybdate to distilled water is 1:250, and the dissolving temperature of ammonium molybdate is 20 ℃;
in the step D, the temperature of the mixed solution is 80 ℃ during heating and stirring, and the heating and stirring time is 3 hours; the rotary evaporation temperature of the rotary evaporator is 70 ℃, and the rotating speed is 200 r/min; the mass ratio of MoO3 to melamine in the precursor was 0.8%;
in step E, the calcination method is: firstly, heating to 480 ℃ at the heating rate of 5 ℃/min, and calcining for 3h at 480 ℃; then the temperature is reduced to the room temperature at the cooling rate of 5 ℃/min, and the calcination is finished.
The molybdenum trioxide in-situ intercalation carbon nitride composite catalyst is prepared by the preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst.
The technical principle of the invention is as follows:
molybdenum trioxide is a semiconductor material with stable physical and chemical properties and capable of absorbing visible light, and MoO with photocatalytic performance can be prepared by sintering a certain amount of ammonium molybdate by using a high-temperature calcination method3The method is very simple and the synthesized sample has stable performance. Using carbon nitride and semiconductor MoO3The two materials are compounded, and a Z-shaped heterostructure is formed after roasting and compounding by utilizing the difference of energy level positions of the two materials, so that a photon-generated carrier can be effectively separated, the electron and hole recombination rate is reduced, and the spectral response can be expanded, thereby enhancing the photocatalytic degradation performance of the methyl orange solution, because of the MoO of the structure3The intercalation can effectively inhibit the recombination of photoexcited electron-hole pairs and improve g-C3N4Light absorption ability of (d), etc.; thus, MoO3Intercalated to g-C3N4The photocatalytic performance can be remarkably improved.
The technical scheme of the invention achieves the following beneficial technical effects:
in the natural science research focus of colleges and universities in Anhui provinceWith the help of a project (project number: KJ2019A0513), the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst is prepared by adopting a chemical deposition method and a calcination method, and the molybdenum trioxide in-situ intercalation improves the g-C3N4Light absorption ability of (2), and is advantageous for g-C3N4The interlayer charge separation is carried out, so that the composite catalyst has higher light absorption capacity and higher electron hole separation capacity, and shows better photocatalytic activity to organic pollutants: the degradation effect on both the methyl orange solution and the tetracycline solution is good, and the degradation rates of Mo-CN-0.8 on the methyl orange solution and the tetracycline solution reach 90% and 93% respectively when the 300W mercury lamp is irradiated for 1 h; in addition, the composite catalyst prepared by the invention has stronger stability when being used for photocatalysis and can be repeatedly used.
Compared with the method for preparing the composite catalyst by separately preparing molybdenum trioxide and carbon nitride respectively and then simply mixing the molybdenum trioxide and the carbon nitride and then calcining, the preparation method disclosed by the invention can be used for compounding the molybdenum trioxide and the carbon nitride in an in-situ space, so that the condition of uneven compounding caused by directly and simply mixing and calcining the method is avoided, and the prepared composite catalyst has better performance; the preparation method of the invention is completely different from the composite method of directly mixing molybdenum trioxide and carbon nitride and then calcining the mixture to obtain the in-situ composite material by firstly preparing a specific precursor and then calcining the precursor.
When the mixed solution is heated and stirred to further generate the precipitation reaction, the precipitation reaction is ensured to be completely carried out by controlling the heating and stirring temperature and time, if the heating and stirring temperature is too low or too high, the product is impure, and if the stirring time is too short, the reaction is not complete. According to the invention, the water in the mixed solution obtained by the precipitation reaction is volatilized in a rotary evaporation mode, and technicians find that the yield of the precursor can be effectively improved by adopting the rotary evaporation mode compared with a drying mode of filtering, washing and drying; and when the temperature of the rotary evaporation is 70 ℃ and the rotating speed is 200r/min, the rapid removal of the moisture is ensured, and the yield and the performance of the precursor are not influenced.
The invention ensures that the melamine forms a uniform aqueous solution in the water by synergistically regulating and controlling the mass ratio of the melamine to the water, the stirring time and the stirring temperature, and the phenomenon of incomplete dissolution and precipitation caused by excessive evaporation of heated water can be avoided; when preparing the ammonium molybdate solution, the mass ratio of the ammonium molybdate to the distilled water is controlled to be 1 (200-2000), because when the ammonium molybdate solution is gradually and dropwise transferred into the melamine solution, if a certain specific MoO needs to be prepared3When the precursor is contained, the volume of the ammonium molybdate solution is required to be increased when the concentration of the ammonium molybdate solution is too low, so that the dropwise transfer time is too long, the preparation efficiency is low, and the yield of the precursor is reduced; the concentration of the ammonium molybdate solution cannot be too high, otherwise, the ammonium molybdate is left after reaction precipitation, so that waste is caused, and the purity of the precursor is influenced.
When the precursor is calcined, the invention controls the heating rate, the cooling rate and the calcining temperature, so that the prepared composite catalyst has higher yield and better performance. In tests, it is found that if the temperature rise rate is greater than 5 ℃/min, the yield of the product is significantly reduced, even completely calcined, and if the temperature rise rate is less than 5 ℃/min, the calcination of the precursor is incomplete, so that the precursor cannot be completely sintered into the target composite catalyst, and the yield is reduced.
Drawings
FIG. 1A flow chart of the preparation of the composite catalyst of the present invention and MoO3Intercalation of g-C3N4A schematic structural diagram;
FIG. 2 scanning electron micrograph (10 μm) of Mo-CN-0.8 precursor prepared according to the present invention;
FIG. 3 is a scanning electron microscope (1 μm) of carbon nitride prepared according to the present invention;
FIG. 4 is a scanning electron microscope (1 μm) of a composite catalyst Mo-CN-0.8 prepared by the present invention;
FIG. 5 is a scanning electron microscope (1 μm) of Mo-CN-0.8, a composite catalyst prepared by the present invention;
FIG. 6 XRD patterns of precursors prepared according to the present invention;
FIG. 7 is an XRD pattern of a composite catalyst prepared in accordance with the present invention;
FIG. 8 is a fluorescence spectrum of composite catalysts of different molybdenum contents prepared according to the present invention;
FIG. 9 shows UV-vis diffuse reflectance spectra of composite catalysts of different molybdenum content prepared according to the present invention;
FIG. 10 is a TG spectrum of Mo-CN-0.8, a composite catalyst prepared by the present invention;
FIG. 11 is an infrared spectrum of composite catalysts of the present invention prepared with varying molybdenum content;
FIG. 12 is a graph showing the time profile of Mo-CN-0.8 photocatalytic degradation of methyl orange in the present invention;
FIG. 13 is a graph showing the time course of Mo-CN-0.8 photocatalytic degradation of tetracycline in the present invention;
FIG. 14 is a graph of the degradation degree of methyl orange versus time for different catalysts of the present invention;
FIG. 15 is a graph of tetracycline degradation versus time for different catalysts of the present invention;
FIG. 16 is a graph showing the kinetics of methyl orange degradation by different catalysts in the present invention;
FIG. 17 is a graph showing the kinetics of tetracycline degradation by different catalysts of the present invention;
FIG. 18 is a bar graph of methyl orange degradation rate constants for different catalysts of the present invention;
FIG. 19 is a bar graph of tetracycline degradation rate constants for different catalysts of the present invention;
FIG. 20 is a graph showing the degradation degree of Mo-CN-0.8 to methyl orange in the present invention as a function of the number of experimental runs.
Detailed Description
1. Preparation method of molybdenum trioxide in-situ intercalation carbon nitride composite catalyst
The preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst comprises the following steps:
step A: adding distilled water into melamine, heating and stirring to completely disperse the melamine into the distilled water to obtain a solution A; the mass ratio of melamine to distilled water is 1:37.5, the heating temperature is 80 ℃, and the stirring time is 20 min;
and B: adding distilled water into ammonium molybdate, and stirring to completely dissolve the ammonium molybdate to obtain a solution B; the mass ratio of ammonium molybdate to distilled water is 1:250, and the dissolving temperature of ammonium molybdate is 20 ℃;
and C: slowly and dropwise transferring the solution B into the solution A to obtain milky mixed solution;
step D: heating and stirring the mixed solution, placing the mixed solution in a rotary evaporator, and performing rotary evaporation to completely volatilize water to obtain a white powdery precursor; the temperature of the mixed solution during heating and stirring is 80 ℃, and the heating and stirring time is 3 hours; the rotary evaporation temperature of the rotary evaporator is 70 ℃, and the rotating speed is 200 r/min; mo element in the precursor is MoO3Calculation of MoO3The mass ratio of (a) to the mass of melamine is 0.8%;
step E: placing the precursor in a muffle furnace for calcining, wherein the calcining method comprises the following steps: firstly, heating to 480 ℃ at the heating rate of 5 ℃/min, and calcining for 3h while keeping the temperature of 480 ℃; then cooling to room temperature at a cooling rate of 5 ℃/min; the yellow powder obtained after calcining and sintering is the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst.
In this example, MoO was prepared by in situ synthesis combined with single source pre-precipitation3Intercalation of g-C3N4A photo-composite catalyst, and MoO was examined3Intercalation pair g-C3N4The microstructure and photocatalytic performance. Firstly, ammonium molybdate and melamine react under certain conditions (only the content of ammonium molybdate is changed, and other conditions are the same) to obtain a precursor (MAMoO)4) Then calcining the prepared precursor to obtain a target product, namely carbon nitride containing a molybdenum intercalation; pure carbon nitride and molybdenum containing carbon nitride were then characterized and tested by different methods, such as: x-ray diffraction spectroscopy (XRD), infrared spectroscopy (IR), fluorescence measurement, UV-Vis Diffuse Reflectance (DRS), and the like; then, carrying out photocatalytic performance test on the catalyst by using a methyl orange solution and a tetracycline solution under visible light, finding out a sample with the best photocatalytic effect, and comparing the advantages and disadvantages of the photocatalytic performances of pure carbon nitride and molybdenum-doped carbon nitride; the specific test procedure is as follows.
2 test content
2.1 test instruments and reagents
TABLE 1.1 Main instruments
TABLE 1.2 Main Agents
2.2 test procedure
2.2.1 Synthesis of composite catalyst chemical deposition method
(1) Preparation work: washing the used glass instruments such as beakers, measuring cylinders, rubber head droppers and the like with distilled water, and then drying the glass instruments in a blast drying oven for later use.
(2) Preparing a composite catalyst: accurately weighing 4.0g of melamine, dissolving the melamine in 150mL of distilled water, setting the temperature of 80 ℃ under a constant-temperature magnetic stirrer, stirring for 20 minutes, and dissolving melamine to obtain a solution A, wherein the solution A is clear and transparent; 0.005g of ammonium molybdate is accurately weighed and dissolved in 10mL of distilled water, and the solution is heated and stirred at the temperature of 20 ℃ to be dissolved, so that solution B is obtained and is colorless and transparent. And (4) slowly and dropwise transferring the solution B into the solution A by using a dropper, and reacting the solution to generate white precipitate to obtain milky turbid mixed solution. Heating the mixed solution at 80 deg.C, stirring for 3 hr, rapidly spin-drying with a rotary evaporator at 70 deg.C and 200r/min to obtain white powder (MAMoO)4). Accurately weighing 2.0g of precursor (MAMoO)4) Placing the crucible into a muffle furnace, setting a program, heating to 480 ℃ at a speed of 5 ℃ per minute, burning for 3h at the temperature, then cooling at a speed of 5 ℃ per minute, and taking out after the temperature is reduced to room temperature to obtain a target product which is yellow powder (as shown in figure 1). Mo element in MoO3Calculating to obtain the MoO in the precursor obtained under the proportion3Melamine was 0.1 wt.%. By the same method steps, the content of ammonium molybdate is changedAmount of, respectively preparing three other MoO30.5 wt% of melamine, MoO30.8 wt% of melamine, MoO3Precursors with different molybdenum contents (MAMoO) with 1.0 wt% melamine4). The finally obtained molybdenum-containing carbon nitride (Mo-g-C)3N4) Are respectively named as Mo-CN-0.1, Mo-CN-0.5, Mo-CN-0.8 and Mo-CN-1.0.
(3) Preparing a pure carbon nitride catalyst: accurately weighing 4.0g of melamine in 150mL of distilled water, stirring for 3h at 80 ℃ of a magnetic stirrer to obtain a clear transparent solution, putting the solution in a rotary evaporator, setting the water temperature at 70 ℃ and the rotating speed at 200r/min, carrying out spin drying to obtain white powder, putting the white powder in a muffle furnace, and calcining for 3h at 480 ℃ at the same heating speed to obtain light yellow powder.
2.2.2 photocatalytic reaction test
(1) Preparation work: preparation of organic dye (methyl orange) solution: weighing 10mg of methyl orange, placing the methyl orange in a 1000mL volumetric flask, adding distilled water to a constant volume, placing the volumetric flask in an ultrasonic instrument, preparing a methyl orange solution after the methyl orange is completely dissolved, taking out the volumetric flask, and placing the volumetric flask in a cool and light-resistant place for later use; in the same manner, a tetracycline solution can be prepared.
(2) Photocatalytic reaction test: respectively and accurately weighing 50mg of Mo-CN-0.1, Mo-CN-0.5, Mo-CN-0.8 and Mo-CN-1.0 in four quartz tubes, and respectively weighing 50mL of methyl orange solution and placing in the four quartz tubes; taking two quartz tubes, adding 50mg of pure carbon nitride and 50mL of methyl orange solution into one quartz tube, and adding 50mL of methyl orange solution into the other quartz tube; placing 6 quartz tubes into a photoreaction instrument at the same time, performing dark reaction for 30min under a certain stirring condition (adding 1cm of magnetons by using a constant-temperature magnetic stirrer, stirring at the speed of 300-400 rpm), respectively sampling 4mL from the 6 quartz tubes after achieving adsorption-desorption balance, illuminating by using a 300W mercury lamp as a light source, filtering out ultraviolet light with the wavelength of less than 400nm by using an optical filter, illuminating by using visible light emitted by the mercury lamp, sampling 4mL from the quartz tubes at the same time interval (20 min in the embodiment), and sampling 5 times for each quartz tube. Separating the extracted sample by a high-speed centrifuge of 12000r/minSeparating the photocatalyst from the solution, taking out supernatant, measuring absorbance A at the maximum absorption wavelength (463nm) of the supernatant with ultraviolet-visible spectrophotometer, and determining the degradation degree formula C/C according to the absorbance change of methyl orange solution before and after illumination0(％)＝(At/A0) X 100% can be calculated. A. the0，AtThe absorbance values of the methyl orange solution at the maximum absorption wavelength at different times before and after illumination are respectively shown.
3 test results and discussion
3.1 SEM image analysis of precursors and catalysts
As can be seen from the SEM images of fig. 2 to 5, the precursor of Mo — CN-0.8 (fig. 2) exhibits a massive and stone-like structure, a very smooth morphology on the surface, and a relatively broad particle size distribution. Fig. 3 is pure carbon nitride, which can be seen to exhibit a layered structure with a smooth surface. FIGS. 4 and 5 are SEM images of Mo-CN-0.8, showing that the introduction of molybdenum trioxide has a relatively large influence on the microstructure of carbon nitride, the surface of the carbon nitride composite catalyst becomes very rough and not smooth, and MoO is adhered to many small particles3Is wrapped between layers.
3.2 XRD of precursors and catalysts
The crystal lattice type and unit cell size of each substance are fixed, and the crystal plane formed by the atomic position and atomic arrangement in each unit cell is also fixed, so that each substance has a specific diffraction pattern, namely, the same substance generates a specific diffraction pattern in a monochromatic X-ray with a specific wavelength, and a series of diffraction peaks can be observed on a computer spectrogram. The phase structure of the composite material synthesized under two different proportions is analyzed by an X-ray diffraction energy spectrometer, a sample is scanned in a superposition mode within the range of 5-70 degrees of 2 theta, and the scanning speed is 8 degrees per minute.
Fig. 6 and 7 are X-ray diffraction patterns of the precursor and the catalyst, respectively, and for comparison, the XRD patterns of melamine monohydrate (melamine) are also provided in this example. In FIG. 6, the XRD pattern of Mo-CN-X (X is 0.8 or 1.0) sample is compared with that of pure melamineThe XRD patterns of the amine samples are very similar, which should be because the molybdate content in these samples is too low. FIG. 7 is the result of the carbon nitride and carbon nitride composite catalysts, comparing the standard cards, confirming that the XRD diffraction peak of the carbon nitride is correct, it can be seen that the peak value at 27.4 is significantly reduced for both composite catalysts, which is MoO3The peak value of carbon nitride affected by successful intercalation of the layer. No MoO is present in the figure3Probably due to intercalated MoO3Too little.
3.3Mo-g-C3N4Fluorescence (PL) of composite photocatalyst
FIG. 8 shows pure g-C3N4And Mo-g-C in various proportions3N4The fluorescence spectrum of the composite catalyst is set to have a wavelength scanning range of 325nm-575nm, and shows the PL spectrum of a sample under the excitation of 300nm light. Pure g-C3N4Exhibits a strong emission band centered at 450nm, which means pure g-C3N4Has larger electron-hole recombination rate. As can be seen from FIG. 8, the emission bands of Mo-CN-0.1, Mo-CN-0.5, Mo-CN-0.8, and Mo-CN-1.0 decrease in order, i.e., with MoO3The inserted content increases and the emission band gradually decreases. The results show that MoO3The intercalation can effectively inhibit the recombination of photoexcited electron-hole pairs. In FIG. 8, the wavelength corresponding to the emission peak of the composite catalyst is 434nm, and the emission peak of the composite catalyst is blue-shifted as compared with the wavelength corresponding to the emission peak of carbon nitride; this is probably due to the fact that, after molybdenum trioxide is added, the molybdenum trioxide and the molybdenum trioxide interact with each other to change the fluorescence property of the carbon nitride.
3.4UV-vis Diffuse reflectance absorption Spectroscopy (DRS)
FIG. 9 shows UV-VIS diffuse reflectance (UV-visDRS) spectra of different samples, with a wavelength scan range of 300 nm-800 nm and a tangent line drawn to the UV-vis diffuse reflectance absorption peak of the sample by extrapolation. As can be seen from the figure, the absorption threshold corresponding to pure carbon nitride is 459nm, the absorption threshold corresponding to Mo-CN-0.5 is 466nm, the absorption threshold corresponding to Mo-CN-0.8 is 470nm, and the absorption threshold corresponding to Mo-CN-1.0 is 476 nm; the results show that MoO3The intercalation obviously improves the visible light absorption intensity of the photocatalyst, so that the catalyst can absorbMore visible light. In addition, the absorption edge of the sample gradually red-shifted with increasing Mo content, indicating that MoO3Intercalation pair g-C3N4The band structure of (a) has an influence. This phenomenon confirms MoO3And g-C3N4There is a chemical interaction between them, not a physical mixing. Relationship between absorption edge and band gap energy of semiconductor photocatalyst: eg 1240/λ g (λ g is a semiconductor light absorption threshold, Eg is a band gap energy), and g-C can be obtained by calculation3N4The band gaps of Mo-CN-X (X is 0.5, 0.8, 1 in this order) are 2.70eV, 2.66eV, 2.64eV, 2.61eV, respectively, and Mo-CN-1.0 is predicted to have the best photocatalytic activity, MoO3The intercalation enhances the absorption of light, is beneficial to fully utilizing sunlight and generates more current carriers.
3.5 TG analysis of the catalyst
When the catalyst is heated to a certain temperature, the measured mass of the catalyst changes, and the obtained thermogravimetric curve is not a straight line. By studying the thermogravimetric curve, the temperature range of the tested sample which changes sharply and the change generated in the temperature range can be known, and the actual concentration of the residual substance can be calculated according to the weight loss, and the proportion of each substance in the compound synthesized by the two substances can be calculated.
From the thermogravimetric analysis of FIG. 10, the MoO in the sample can be calculated3The actual content of (c). The TG curve of the sample Mo-CN-0.8 is shown in the figure. From the graph, a rapid weight loss was observed in the range of 500 to 650 ℃, and MoO in Mo-CN-0.8 was calculated from the thermogravimetric analysis of FIG. 103The actual concentration of (B) was 2.56 wt%.
3.6 Infrared spectroscopic analysis of the catalyst:
FIG. 11 shows g-C3N4And Mo-g-C in various proportions3N4Taking an infrared spectrogram of a composite catalyst sample, carefully grinding about 200mg of pure KBr into white powder by using an agate mortar, adding 1-2 mg of a sample into the KBr, grinding the sample until the system is uniformly dispersed, placing the ground sample into a drying box for drying, pressing the sample into a uniform transparent sheet by using a tablet press, and measuring the infrared of the sample by using a Fourier transform infrared spectrometerA spectrogram. When a sample is irradiated by infrared light with continuous frequency, molecules in the sample can selectively absorb radiation energy which can enable vibration energy levels of certain groups in the molecules to jump, the dipole moment of the molecules is changed, and finally the vibration energy levels and the rotation energy levels of the molecules are transited from a ground state to an excited state with higher energy. Fourier infrared absorption spectroscopy can thus be used to determine the atomic groups present in the compound molecules and their relative positions in the molecular structure.
As can be seen from FIG. 11, the Mo-CN-X sample has the same composition as the pure g-C3N4Almost identical IR characteristics, e.g. N-H stretching vibration (2900--1) Vibration of C-N heterocyclic ring skeleton (1200-1800 cm)-1) And triazine ring vibration (808 cm)-1). Apparently, MoO3Without altering g-C3N4The main body structure of (1). However, some variation can be seen in the C-N vibration. For example, g-C3N41236cm-1Peak shift to Mo-C3N41244cm of-0.5-1To Mo-C3N41245cm of-1-1. This transformation is due to MoO3For g-C3N4Due to the strong action of the Chinese medicinal herbs.
3.7g-C3N4And Mo-g-C3N4Photocatalytic reaction Activity measurement
g-C synthesized using methyl orange and tetracycline pair3N4And Mo-CN-0.1, Mo-CN-0.5, Mo-CN-0.8 and Mo-CN-1.0 are subjected to photocatalytic activity test determination. In the experiment, a 300W mercury lamp was used as a light source, dark reaction was carried out for 30min, and samples were taken every 20min for five times in the photoreaction. The test results are expressed as degree of degradation C/C0(％)＝(At/A0) The results are plotted on the ordinate and the reaction times on the abscissa, using the 100% equation. Wherein, C0The initial concentrations of methyl orange and tetracycline are shown, and the C is the concentrations of methyl orange and tetracycline after photocatalytic degradation by adding a catalyst; a. the0Is the absorbance of the initial methyl orange and tetracycline, AtIs the absorbance of methyl orange and tetracycline after the sample is subjected to photocatalytic degradation.
3.7.1g-C3N4And Mo-g-C3N4Photocatalytic test under methyl orange solution and tetracycline solution
(1)g-C3N4Photocatalytic degradation of methyl orange solution: two 50mL quartz tubes are numbered as the first quartz tube and the second quartz tube is provided with stirrers of the same specification. Weighing 50mg of g-C3N4Putting the mixture into a quartz tube marked with a first number, adding 50mL of 10mg/L methyl orange solution, wherein 50mL of 10mg/L methyl orange solution is only added into a No. two quartz tube, and g-C is not added3N4And the test solution is used for carrying out a comparison test on a pure methyl orange solution. Putting two quartz tubes into a photochemical reaction instrument at the same time, carrying out dark reaction for 30min without turning on a lamp, taking 4mL of supernatant, centrifuging (8000r/min,5min), after the dark reaction is finished, turning on the lamp, carrying out a photocatalytic test by using a 300W mercury lamp as a light source, turning on refrigeration circulating water, taking supernatant liquid every 20, 40, 60, 80 and 100min, centrifuging (2 times), and measuring the absorbance of the methyl orange solution by using an ultraviolet-visible spectrophotometer.
(2)Mo-g-C3N4And (3) photocatalytic degradation of methyl orange: taking four quartz tubes with the capacity of 50mL, and respectively adding four Mo-g-C with different molybdenum contents into the four quartz tubes3N4Then adding 50mL of 10mg/L methyl orange solution, simultaneously placing four quartz tubes into a photochemical reaction instrument, reacting for 30min without turning on a lamp, taking 4mL of supernatant, centrifuging (8000r/min,5min), after the dark reaction is finished, turning on a lamp source, using a 300W mercury lamp as a light source to perform a photocatalytic test, turning on refrigeration circulating water, taking supernatant once every 20, 40, 60, 80 and 100min, centrifuging (2 times), and measuring the absorbance of the methyl orange solution by using an ultraviolet-visible spectrophotometer.
(3)g-C3N4And Mo-g-C3N4Photocatalytic degradation of tetracycline: repeating the steps (1) and (2), only changing the methyl orange solution into tetracycline solution, taking 4mL of supernatant for centrifugation (8000r/min,5min) when dark reaction is carried out for 30min and light reaction is carried out for 20, 40, 60, 80 and 100min respectively, and then measuring the absorbance of tetracycline by using an ultraviolet-visible spectrophotometer.
FIG. 12 and FIG. 13 are respectivelyThe graphs of the photocatalytic degradation time of Mo-CN-0.8 for methyl orange and tetracycline under the irradiation of visible light from a 300W mercury lamp show that the characteristic absorption wavelength of methyl orange is 463nm and the characteristic absorption wavelength of tetracycline is 361nm in FIGS. 12 and 13. As can be seen from fig. 12, the concentration of the methyl orange solution decreased with increasing photocatalytic time; after 20min of illumination, the degradation degree of the methyl orange solution reaches 40%, after 40min of illumination, the degradation degree of the methyl orange solution reaches 72.5%, after 60min of illumination, the degradation degree of the methyl orange solution reaches 84.2%, after 80min of illumination, the degradation degree of the methyl orange solution reaches 89.3%, and after 100min of illumination, the degradation degree of the methyl orange solution reaches 90.7% (degradation degree C/C)0(％)＝(At/A0) X 100%). Similarly, the concentration of the tetracycline solution is gradually reduced along with the increase of the photocatalytic time, and the degradation degree of the tetracycline solution reaches 93.6 percent at 100 min. It can be theorized that methyl orange solution and tetracycline solution may be completely degraded after sufficient time of photocatalysis in visible light.
FIGS. 14 and 15 are time plots of the degradation degree of 5 samples for methyl orange and tetracycline catalytic degradation, respectively; as can be seen from the graph, in the absence of the photocatalyst, the concentrations of the methyl orange solution and the tetracycline solution hardly changed after 100min of light irradiation (in the figure, Pure refers to the methyl orange solution or the tetracycline solution to which no photocatalyst was added). After the catalyst is added, the degradation degree of the methyl orange solution under pure carbon nitride for 100min can reach 63.6%, and the degradation degree of the tetracycline solution under pure carbon nitride for 100min can reach 88.24%. In the two solutions, the catalytic effects of the 5 samples are in descending order: Mo-CN-0.8>Mo-CN-1.0>Mo-CN-0.5>Mo-CN-0.1>g-C3N4The method proves that the composite carbon nitride has better catalytic effect and more obvious catalytic effect than pure carbon nitride when the methyl orange solution and the tetracycline solution are catalytically degraded in the same time. The Mo-CN-0.8 sample has the best photocatalytic activity on the two solutions, and the degradation degrees of the two solutions reach 90.7 percent and 93.6 percent respectively only after being irradiated for 100min, and are far more than the degradation degree of pure carbon nitride in 100 min.
In addition, the degradation efficiency of the methyl orange solution is obviously higher than that of pure carbon nitride under the catalysis of the composite catalyst, and g-C3N4The degradation degree at 100min is only 63.6 percent, while the degradation degree of Mo-CN-0.8 at 100min reaches 90.7 percent; the degradation efficiency of the tetracycline solution under the catalysis of the composite catalyst is not greatly changed compared with that of pure carbon nitride, g-C3N4The degradation degree at 100min is 88.24%, and the degradation degree of Mo-CN-0.8 at 100min is 93.6%, which is increased by only 5.36%. The catalytic effect of the composite catalyst in the methyl orange solution is more obvious, and the catalytic effect of the catalyst on the tetracycline solution is better.
Fig. 16 and 17 are graphs showing the kinetics of the composite catalyst prepared in the steps of degrading methyl orange solution and tetracycline solution. To study the reaction kinetics of methyl orange degradation, a first order model was fitted to the experimental data: ln (C)0C) ═ kt + b. In the formula, C0And C are the concentrations of methyl orange and tetracycline solutions at time 0 and t, respectively, and k is the apparent first order rate constant. As shown in the figure, the photocatalytic time (t) and ln (C)0The relationship curve of/C) is almost a straight line.
From the slopes in fig. 16 and 17, the degradation rate constants (k) for methyl orange and tetracycline can be obtained for the prepared samples. Fig. 18 and 19 are bar graphs of the first order reaction rate constants for pure carbon nitride and four different levels of molybdenum intercalated carbon nitride catalysts. As can be seen from FIG. 18, when methyl orange was degraded, g-C3N4Has a rate constant k of 0.009min-1The rate constant k of Mo-CN-0.1 is 0.011min-1The rate constant k of Mo-CN-0.5 is 0.0235min-1The rate constant k of Mo-CN-0.8 is 0.03min-1The rate constant k of Mo-CN-1.0 is 0.0255min-1(ii) a As can be seen from FIG. 19, when the tetracycline solution was degraded, g-C3N4Has a rate constant k of 0.01828min-1The rate constant k of Mo-CN-0.1 is 0.0191min-1The rate constant k of Mo-CN-0.5 is 0.0201min-1The rate constant k of Mo-CN-0.8 is 0.02329min-1The rate constant k of Mo-CN-1.0 is 0.02108min-1. Among them, the rate constant k of Mo-CN-0.8 is the largest. This rate is often the case when degrading methyl orangeNumber value reaches g-C3N4The rate constant of the Mo-CN-1.0 reaches g-C3N42.8 times the rate constant of (a). When the tetracycline solution is degraded, the rate constants k of the 5 catalysts have little difference, namely the degradation rates are not much different.
However, MoO3A further increase in the content (here 1.0% by weight) leads to a decrease in the photocatalytic activity (FIGS. 16 and 17), which is probably due to MoO3Molecular doping to g-C3N4In the crystal, point defects are generated in the crystal, and the point defects have chemical activity and provide more sites for reaction. On the other hand, they introduce defect-related sub-levels in the bandgap, thereby trapping electrons or holes. In this way, the separation and transfer of photogenerated (or holes) is mediated. When the defect concentration exceeds a threshold value (here 0.8 wt%), these defects are converted into recombination centers of electron-hole pairs, thereby reducing the photocatalytic activity of the system.
3.7.2Mo-CN-0.8 catalytic Activity stability test
(1) From the above tests, it is known that the Mo-CN-0.8 catalyst has the best photocatalytic effect on methyl orange, and three photocatalytic cycle tests are carried out by taking methyl orange as a target degradation product. Three quartz tubes with the same specification are taken, three magnetons with the same specification are respectively placed in the three quartz tubes, 50mg of Mo-CN-0.8 catalyst is added respectively, and then 50mL of 10mg/L methyl orange solution is added respectively. Three quartz tubes are simultaneously placed in a photoreaction instrument for dark reaction for 30min, then a 300W mercury lamp is used as a light source for photoreaction for 100min, 4mL of supernatant is taken for centrifugation (8000r/min,5min), and the absorbance of the centrifuged supernatant at the maximum absorption wavelength (463nm) of the methyl orange solution is measured.
(2) And centrifuging the solution in the three quartz tubes, separating out precipitates, and drying in an oven. Two identical quartz tubes were taken and the above test was carried out again, the test procedure was the same, and finally the absorbance of the centrifuged supernatant at the maximum absorption wavelength of the methyl orange solution (463nm) was measured.
(3) Centrifuging the solution in the two quartz tubes, separating out the precipitate, and placingDrying in an oven. A quartz tube was taken and the above test was conducted again in the same manner as above, and finally the absorbance of the centrifuged supernatant at the maximum absorption wavelength (463nm) of the methyl orange solution was measured. Using the formula (C)0-C)/C0The degradation rates of the three cycle tests were plotted against the number of cycles to obtain fig. 20.
Methyl orange is used as a target degradation product, and after three-wheel photocatalytic cycle tests are carried out on the Mo-CN-0.8 catalyst, the degradation rates of the three catalysts are respectively 90.0%, 89.4% and 87.8%, and the differences are small, which shows that the Mo-CN-0.8 catalyst has good catalytic stability.
4 conclusion
In the embodiment, pure carbon nitride and molybdenum trioxide-doped carbon nitride are prepared by a simple chemical deposition method and a calcination method. The internal structure and the surface morphology of the sample are analyzed through infrared, a scanning electron microscope and X-ray diffraction, and the light absorption and photoluminescence characteristics of the sample are represented through solid ultraviolet and fluorescence. Test results show that based on the higher light absorption capacity and the high electron hole separation capacity of the Mo-CN-0.8 composite catalyst, the catalyst has the best photocatalytic performance, has the highest degradation degree on methyl orange and tetracycline solutions, and can reach the degradation rate of 90 percent and 93 percent when irradiated under a 300-watt mercury lamp for one hour.
Claims (10)
1. The preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst is characterized by comprising the following steps:
step A: adding distilled water into melamine, heating and stirring to completely disperse the melamine into the distilled water to obtain a solution A;
and B: adding distilled water into ammonium molybdate, and stirring to completely dissolve the ammonium molybdate to obtain a solution B;
and C: slowly and dropwise transferring the solution B into the solution A to obtain milky mixed solution;
step D: heating and stirring the mixed solution, then putting the mixed solution on a rotary evaporator for rotary evaporation and drying to completely volatilize water to obtain a white powdery precursor;
step E: and (3) calcining the precursor in a muffle furnace to obtain a yellow powdery molybdenum trioxide in-situ intercalation carbon nitride composite catalyst after the calcination is finished.
2. The method for preparing the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst according to claim 1, wherein in the step D, Mo is MoO3Calculation of MoO in the precursor3The mass ratio of (A) to the mass of melamine is 0.1 to 1.0%.
3. The preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst according to claim 1, wherein in the step A, the mass ratio of melamine to distilled water is 1 (35-40); the heating and stirring temperature is 70-90 ℃, and the heating and stirring time is 15-25 min.
4. The preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst according to claim 3, wherein in the step A, the mass ratio of melamine to distilled water is 1: 37.5; the heating and stirring temperature is 80 deg.C, and the heating and stirring time is 20 min.
5. The method for preparing the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst according to claim 1, wherein in the step B, the mass ratio of ammonium molybdate to distilled water is 1 (200-2000), and the stirring and dissolving temperature of ammonium molybdate is 18-25 ℃.
6. The method for preparing the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst according to claim 5, wherein in the step B, the mass ratio of ammonium molybdate to distilled water is 1:250, and the dissolution temperature of ammonium molybdate is 20 ℃.
7. The preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst according to claim 1, wherein in the step E, the calcination method comprises the following steps: firstly, heating to 450-550 ℃ at a heating rate of 5 ℃/min, and calcining for 3h while keeping the temperature unchanged; then the temperature is reduced to the room temperature at the cooling rate of 5 ℃/min, and the calcination is finished.
8. The preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst according to claim 1, wherein in the step D, the temperature of the mixed solution is 80 ℃ and the heating and stirring time is 3 h; the rotary evaporation temperature of the rotary evaporator is 70 ℃, and the rotating speed is 200 r/min.
9. The preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst according to claim 1, wherein in the step A, the mass ratio of melamine to distilled water is 1:37.5, the heating temperature is 80 ℃, and the stirring time is 20 min;
in the step B, the mass ratio of ammonium molybdate to distilled water is 1:250, and the dissolving temperature of ammonium molybdate is 20 ℃;
in the step D, the temperature of the mixed solution is 80 ℃ during heating and stirring, and the heating and stirring time is 3 hours; the rotary evaporation temperature of the rotary evaporator is 70 ℃, and the rotating speed is 200 r/min; MoO in precursor3The mass ratio of (a) to the mass of melamine is 0.8%;
in step E, the calcination method is: firstly, heating to 480 ℃ at the heating rate of 5 ℃/min, and calcining for 3h at 480 ℃; then the temperature is reduced to the room temperature at the cooling rate of 5 ℃/min, and the calcination is finished.
10. The molybdenum trioxide in-situ intercalation carbon nitride composite catalyst is characterized by being prepared by the preparation method of the molybdenum trioxide in-situ intercalation carbon nitride composite catalyst according to claims 1-9.
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