Cu-BiOBr / g-C3N4 composite photocatalytic material as well as preparation method and application thereof

Abstract

The invention discloses a Cu-BiOBr / g-C3N4 composite photocatalytic material as well as a preparation method and application of the Cu-BiOBr / g-C3N4 composite photocatalytic material. The Cu-BiOBr / g-C3N4 composite photocatalytic material is of a nanosheet polymerized microsphere structure formed by compounding g-C3N4 and Cu-BiOBr, and the surface of the Cu-BiOBr / g-C3N4 composite photocatalytic material is of a porous structure. The nitrogen-defect ultrathin g-C3N4 nanosheet is obtained by two steps of a KOH etching method and a calcination method, and the nitrogen-defect ultrathin g-C3N4 nanosheet and CBB construct a heterojunction to obtain the nitrogen-defect ultrathin g-C3N4 nanosheet. Compared with a monomer g-C3N4 and a doped material Cu-BiOBr, the nitrogen fixation performance of the heterojunction is improved, and the ammonia nitrogen concentration of the heterojunction can reach 96.1 [mu] mol.L <-1 >. The used catalyst is recovered, and the recovered sample is lost, but is still within the expected range, which shows that the material has good stability. The invention is green and energy-saving, the preparation method is simple and convenient, the cost is low, and the green sustainable development concept in the new era is met.

Description

Technical Field

[0001] The invention relates to a Cu-BiOBr / g-C3N4 composite photocatalytic material and a preparation method and application thereof, belonging to the technical field of photocatalysis. Background Art

[0002] In recent years, semiconductor photocatalytic technology has attracted much attention in energy conversion and nitrogen fixation due to its advantages such as environmental friendliness, low cost and reusability.

[0003] Ammonia (NH 3 ) is an indispensable chemical in modern industrial production and is also the raw material for agricultural fertilizers. Various current studies have shown that ammonia is the best candidate for sustainable fuel and hydrogen transportation, but nitrogen (N 2 ) Due to its extremely strong stability, it cannot be directly used as a raw material for the production of ammonia, nor can it be directly absorbed and utilized by most organisms. The current method for large-scale nitrogen fixation is the Haber-Bosch process, but its process is extremely inconsistent with the concept of green energy conservation. Photocatalytic nitrogen fixation has attracted widespread attention due to its advantages such as environmental friendliness and low energy consumption.

[0004] Graphitic carbon nitride (gC 3 N 4 ) is an organic polymer semiconductor, which is considered to be one of the most promising photocatalysts due to its π electron delocalization on the polymer backbone and indirect band gap, which is beneficial for charge separation and transfer. 3 N 4 It has the advantages of simple synthesis, low cost, and easy excitation by visible light, making it an ideal material for photocatalytic nitrogen fixation. 3 N 4 There are many deficiencies, such as small specific surface area and high carrier recombination rate, which greatly reduce the photocatalytic nitrogen fixation performance. To overcome this challenge, combining two different semiconductors to construct heterojunction photocatalysts is one of the most promising methods.

[0005] In the field of photocatalytic nitrogen fixation, bismuth oxyhalide BiOX (X=Cl, Br, I) has attracted widespread attention from scholars due to its excellent chemical stability and lamellar structure. However, the low conduction band potential of the semiconductor material BiOBr leads to insufficient thermodynamic reduction performance of its photoelectrons, and its narrow bandgap structure makes it easy for photogenerated carriers to recombine, and its small specific surface area leads to low light utilization. Therefore, constructing a heterojunction with another semiconductor material is considered to be a promising method to improve its charge separation efficiency and thus improve its photocatalytic activity. Based on the fact that both materials have the characteristics of being easily excited by light response, but are limited by the easy recombination of their own photogenerated carriers, it is necessary to take measures to composite the two materials to improve the photocatalytic efficiency. Summary of the invention

[0006] Objective of the invention: The first objective of the present invention is to provide a Cu-BiOBr / gC having a porous structure with high stability and high photocatalytic activity. 3 N 4 Nanosheet polymerized microsphere composite photocatalytic material. The second purpose of the present invention is to provide a method for preparing Cu-BiOBr / gC with simple preparation process, low cost and short synthesis time. 3 N 4 The third object of the present invention is to provide a Cu-BiOBr / gC 3 N 4 Application of composite photocatalytic materials in photocatalytic nitrogen fixation reaction.

[0007] Technical solution: The Cu-BiOBr / gC 3 N 4 Composite photocatalytic material, the Cu-BiOBr / gC 3 N 4 The composite photocatalytic material includes gC 3 N 4 The nanosheet polymer microsphere structure composited with Cu-BiOBr has a porous structure on the surface.

[0008] Furthermore, Cu-BiOBr is a Cu-BiOBr material with oxygen vacancies obtained by doping metal Cu into BiOBr in the form of nitrate. 3 N 4 Composite photocatalytic material for nitrogen-deficient ultrathin gC 3 N 4 The nanosheets are prepared by constructing a heterojunction with the improved Cu-BiOBr. The nanosheet aggregate microsphere structure is a three-dimensional microsphere assembled from a large number of nanosheets.

[0009] The Cu-BiOBr / gC 3 N 4 Preparation method of composite photocatalytic material with ultra-thin gC 3 N 4 Nanosheets and Cu-BiOBr composite materials are used as raw materials and prepared by a hot solvent method, including the following steps:

[0010] (1) Disperse melamine and KOH in water, heat and stir, evaporate water, obtain a block mixture, crush it, and calcine it under nitrogen protection to obtain ultra-thin gC 3 N 4 Nanosheets.

[0011] (2) Dissolve polyvinyl pyrrolidone in ethylene glycol and add Bi(NO 3 )3 ·5H 2 O, stir to dissolve, add KBr and continue stirring, add Cu(NO 3 ) 2 ·3H 2 O, and continue stirring to obtain the precursor Cu-BiOBr solution.

[0012] (3) Ultra-thin gC 3 N 4 The nanosheets were added to the precursor Cu-BiOBr solution, stirred and dissolved, reacted at high temperature, cooled, centrifuged and washed, and dried to obtain Cu-BiOBr / gC 3 N 4 Composite materials.

[0013] Furthermore, in step (1), the mass ratio of melamine, KOH and water is (1-90):(0.01-15):(1-200), preferably 15:0.05:100.

[0014] Furthermore, in step (1), the heating and stirring temperature is 80-100° C., and the heating and stirring time is more than 24 hours.

[0015] Furthermore, in step (1), the calcination under nitrogen protection is first heated to 30-50°C and nitrogen is introduced for 25-35 minutes, the air is exhausted, and then the temperature is increased to 500-550°C at a heating rate of 5-10°C / min and maintained for 4-6 hours.

[0016] Furthermore, in step (1), the washing process is performed several times with deionized water and ethanol, and the centrifugal washing rate is 6000-8000 r min -1 , centrifuge and wash for 3-5 minutes, and adjust the pH to neutral.

[0017] Furthermore, in step (1), the drying temperature is 60-80° C., and the drying time is 10-14 h.

[0018] Furthermore, in step (2), the mass volume ratio of polyvinyl pyrrolidone to ethylene glycol is (0.01-0.5):(10-20) g / mL, preferably 0.1:15 g / mL.

[0019] Further, in step (2), KBr, Bi(NO 3 ) 3 ·5H 2 O and Cu(NO 3 ) 3 ·3H 2 The molar ratio of O is 1:1:(0.2-0.5), preferably 1:1:0.35.

[0020] Furthermore, in step (2), the stirring time is continued for 1-2 hours.

[0021] Further, in step (3), the ultrathin gC 3 N 4 The mass ratio of the nanosheets to polyvinyl pyrrolidone is (1.5-3):1.

[0022] Furthermore, in step (3), the temperature of the high temperature reaction is 100-200° C., and the time of the high temperature reaction is more than 12 hours.

[0023] Furthermore, in step (3), the centrifugal washing is performed by centrifugation with warm deionized water and ethanol for several times, and the speed of the centrifugal washing is 6000-8000 r min. -1 , the centrifugal washing time is 3-5min.

[0024] Furthermore, in step (3), the drying temperature is 60-80° C., and the drying time is 10-14 h.

[0025] The Cu-BiOBr / gC 3 N 4 Application of composite photocatalytic materials in photocatalytic nitrogen fixation reaction.

[0026] Nitrogen fixation mechanism: Figure 6 As shown, Figure 6 Figure a is a schematic diagram of the photocatalytic nitrogen fixation of the composite material. The principle of nitrogen fixation: under the irradiation of simulated sunlight, the energy received by the semiconductor material is greater than the energy required by the bandgap width (Eg) of its own material, and the electrons (e - ) will absorb enough energy and then jump to the conduction band (VB) to form free electrons, while leaving oxidative holes (h + ), thereby generating electron-hole pairs, which then migrate to the surface of the photocatalyst. When the electrons are transferred to the outside of the catalyst, they undergo a reduction reaction with the nitrogen molecules adsorbed by the semiconductor material, namely, N 2 +6H + +6e - →2NH 3 ; The oxidizing holes will react with water to produce H + and O 2 , that is, 2H 2 O+4h + →O 2 +4H + .

[0027] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0028] (1) The Cu-BiOBr / gC 3 N 4 The composite photocatalytic material has a higher nitrogen fixation yield by changing and controlling gC 3 N 4 The molar ratio of Cu-BiOBr to Bi can be found as 3 N 4 The highest ammonia production of the composite material for photocatalytic nitrogen fixation. For 0.35Cu-BiOBr, the nitrogen fixation amount within 2h was 77.3μmol / L, while the best performing Cu-BiOBr / gC 3 N 4 The nitrogen fixation capacity of the composite material reached 96.1 μmol / L, which was significantly higher than that of the monomer gC 3 N 4 and doped material Cu-BiOBr are improved.

[0029] (2) The Cu-BiOBr / gC 3 N 4 The composite photocatalytic material has good photocatalytic stability. It was subjected to multiple cycle nitrogen fixation performance tests. The results showed that the ammonia production per unit mass of the recovered samples can still reach about 90% of the initial ammonia production, indicating that the semiconductor material has significant nitrogen fixation performance and stability, and provides research ideas for actual production.

[0030] (3) The Cu-BiOBr / gC 3 N 4 Composite photocatalytic materials have the advantages of simple method, low cost and high reproducibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 XRD diagram of the composite material prepared in Examples 1-7;

[0032] Figure 2 The SEM images of the composite materials prepared in Examples 1-2 and Example 5;

[0033] Figure 3 The UV-visible diffuse reflectance spectra and corresponding band gap diagrams of the composite materials prepared in Examples 1-7, wherein a is a UV-visible diffuse reflectance spectra, and b is a band gap diagram corresponding to Examples 1-2 and Example 5;

[0034] Figure 4 The photoluminescence spectrum test and transient photocurrent response test diagram of the composite material prepared in Examples 1-7, wherein a is a photoluminescence spectrum diagram, and b is a transient photocurrent response test diagram;

[0035] Figure 5The photocatalytic nitrogen fixation performance diagram of the composite material prepared in Examples 1-7;

[0036] Figure 6 Cu-BiOBr / gC 3 N 4 The nitrogen fixation mechanism diagram and energy band change diagram of the composite photocatalytic material, where a is Cu-BiOBr / gC 3 N 4 Reaction mechanism diagram, b is monomer gC 3 N 4 , energy band change diagram of doped material 0.35Cu-BiOBr and composite material CBB2;

[0037] Figure 7 It is the cyclic ammonia production diagram of the photocatalytic material CCB2 of Example 5 and its initial and post-cycle XRD comparison diagram, wherein a is the cyclic ammonia production diagram of CCB2, and b is the XRD comparison diagram of CCB2 before and after the cycle. DETAILED DESCRIPTION

[0038] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0039] Example 1

[0040] 15g of melamine and 0.05g of KOH were uniformly dispersed in 100mL of deionized water. Then, the suspension was stirred uniformly in a water bath at 80°C for 24h to evaporate the deionized water until it became a block. Some of the block mixture was taken out and crushed and placed in a crucible. First, the temperature was raised to 40°C and nitrogen was introduced for 30min, then the temperature was raised to 500°C at a heating rate of 10°C / min and kept for 2h for preliminary calcination, and then the temperature was raised at a heating rate of 5°C / min under N 2 The atmosphere was heated to 520°C and calcined again for 2 hours. After the calcination, the sample was taken out and washed several times by centrifugation with deionized water and ethanol (the speed of centrifugal washing was 8000 r min). -1 , centrifugal washing time 5min), until pH = 7, put it in a vacuum drying oven at 60℃ and dry it for 12h to obtain gC 3 N 4 Nanosheets, labeled as CN.

[0041] Example 2

[0042] 0.1 g of polyvinyl pyrrolidone was weighed and dissolved in 15 mL of ethylene glycol. After the solution was stirred for 30 min, 1.5 mmol of Bi(NO 3 ) 3· 5H 2 O, after it is completely dissolved, add 1.5mmol KBr and continue stirring for 15min, then weigh 0.35mmol Cu(NO3 ) 2 ·3H 2 O was added to the above solution, and continued to stir for 1 hour after it was completely dissolved. Then the above solution was added to a 25mL polytetrafluoroethylene-lined reactor and reacted at 160°C for 12 hours. After the reaction was complete, it was washed with warm ultrapure water and anhydrous ethanol and centrifuged several times until the upper solution was colorless (the centrifugal washing rate was 8000 r min -1 , centrifugal washing time 5min), and finally the catalyst was placed in a vacuum drying oven at 60°C for 12h, and finally a powder sample was collected and marked as 0.35CBB.

[0043] Example 3

[0044] 0.1 g of polyvinyl pyrrolidone was weighed and dissolved in 15 mL of ethylene glycol. After the solution was stirred for 30 min, 1.5 mmol of Bi(NO 3 ) 3 ·5H 2 O, after it is completely dissolved, add 1.5mmol KBr and continue stirring for 15min, then weigh 0.35mmol Cu(NO 3 ) 2 ·3H 2 O was added to the above solution, and continued to stir for 1 hour after it was completely dissolved to obtain the precursor Cu-BiOBr solution. Weigh 0.1g of CN and add it to the above precursor Cu-BiOBr solution and stir until it is completely dissolved. Then transfer the above mixed solution to a 25mL polytetrafluoroethylene-lined reactor and react at 160°C for 12 hours. After the reaction is complete, wash with warm ultrapure water and anhydrous ethanol respectively by centrifugation until the upper solution is colorless (the centrifugal washing rate is 8000r min -1 , centrifugal washing time 5min), and finally the catalyst was placed in a vacuum dryer at 60°C for 12h, and finally a powder sample was collected and marked as CCB1.

[0045] Example 4

[0046] 0.1 g of polyvinyl pyrrolidone was weighed and dissolved in 15 mL of ethylene glycol. After the solution was stirred for 30 min, 1.5 mmol of Bi(NO 3 ) 3 ·5H 2 O, after it is completely dissolved, add 1.5mmol KBr and continue stirring for 15min, then weigh 0.35mmol Cu(NO 3 ) 2 ·3H 2O was added to the above solution, and continued to stir for 1 hour after it was completely dissolved to obtain the precursor Cu-BiOBr solution. Weigh 0.15g of CN and add it to the above precursor Cu-BiOBr solution and stir until it is completely dissolved. Then transfer the above mixed solution to a 25mL polytetrafluoroethylene-lined reactor and react at 160°C for 12 hours. After the reaction is complete, wash with warm ultrapure water and anhydrous ethanol respectively by centrifugation until the upper solution is colorless (the centrifugal washing rate is 8000r min -1 , centrifugal washing time 5min), and finally the catalyst was placed in a vacuum drying oven at 60°C for 12h, and finally a powder sample was collected and marked as CCB1.5.

[0047] Example 5

[0048] 0.1 g of polyvinyl pyrrolidone was weighed and dissolved in 15 mL of ethylene glycol. After the solution was stirred for 30 min, 1.5 mmol of Bi(NO 3 ) 3 ·5H 2 O, after it is completely dissolved, add 1.5mmol KBr and continue stirring for 15min, then weigh 0.35mmol Cu(NO 3 ) 2 ·3H 2 O was added to the above solution, and continued to stir for 1 hour after it was completely dissolved to obtain the precursor Cu-BiOBr solution. Weigh 0.2g of CN and add it to the above precursor Cu-BiOBr solution and stir until it is completely dissolved. Then transfer the above mixed solution to a 25mL polytetrafluoroethylene-lined reactor and react at 160°C for 12 hours. After the reaction is complete, wash with warm ultrapure water and anhydrous ethanol respectively by centrifugation until the upper solution is colorless (the centrifugal washing rate is 8000r min -1 , centrifugal washing time 5min), and finally the catalyst was placed in a vacuum drying oven at 60°C for 12h, and finally a powder sample was collected and marked as CCB2.

[0049] Example 6

[0050] 0.1 g of polyvinyl pyrrolidone was weighed and dissolved in 15 mL of ethylene glycol. After the solution was stirred for 30 min, 1.5 mmol of Bi(NO 3 ) 3 ·5H 2 O, after it is completely dissolved, add 1.5mmol KBr and continue stirring for 15min, then weigh 0.35mmol Cu(NO 3 ) 2 ·3H 2O was added to the above solution, and stirring was continued for 1 hour after it was completely dissolved to obtain the precursor Cu-BiOBr solution. Weigh 0.25g of CN and add it to the above precursor Cu-BiOBr solution and stir until it is completely dissolved. Then transfer the above mixed solution to a 25mL polytetrafluoroethylene-lined reactor and react at 160°C for 12 hours. After the reaction is complete, wash with warm ultrapure water and anhydrous ethanol respectively by centrifugation until the upper solution is colorless (the centrifugal washing rate is 8000r min -1 , centrifugal washing time 5min), and finally the catalyst was placed in a vacuum drying oven at 60°C for 12h, and finally a powder sample was collected and marked as CCB2.5.

[0051] Example 7

[0052] 0.1 g of polyvinyl pyrrolidone was weighed and dissolved in 15 mL of ethylene glycol. After the solution was stirred for 30 min, 1.5 mmol of Bi(NO 3 ) 3 ·5H 2 O, after it is completely dissolved, add 1.5mmol KBr and continue stirring for 15min, then weigh 0.35mmol Cu(NO 3 ) 2 ·3H 2 O was added to the above solution, and after it was completely dissolved, stirring was continued for 1 hour to obtain the precursor Cu-BiOBr solution. Weigh 0.3g of CN and add it to the above precursor Cu-BiOBr solution and stir until it is completely dissolved. Then the above mixed solution was transferred to a 25mL polytetrafluoroethylene-lined reactor and reacted at 160°C for 12 hours. After the reaction was complete, it was centrifuged and washed with warm ultrapure water and anhydrous ethanol respectively until the upper solution was colorless (the centrifugal washing rate was 8000r min -1 , centrifugal washing time 5min), and finally the catalyst was placed in a vacuum drying oven at 60°C for 12h, and finally a powder sample was collected and marked as CCB3.

[0053] The composite materials prepared in Examples 1-7 were subjected to XRD analysis, and the results were as follows: Figure 1 shown. Figure 1 is the XRD pattern of the composite material prepared in Example 1-7. Figure 1 It can be seen that in the XRD spectrum, the diffraction peaks of the composite materials CCB1-CBB3 basically correspond to the standard card of tetragonal BiOBr (JCPDS: 09-0393), and there are no other impurity peaks, indicating that it has a high purity. At the same time, the high diffraction peaks indicate that it has a relatively high crystallinity, which makes the material have good stability. 3 N 4) has two obvious diffraction peaks. The diffraction peak of graphite carbon nitride material at 27.5° is stronger because the (002) plane represents the interlayer stacking of the conjugated aromatic system. The weak diffraction peak at 12.9° is the (100) plane of the heptazine-based structure. Interestingly, the diffraction peak of the composite CCB1-CBB3 at the (102) crystal plane gradually disappears with the increase of CN content, but a smaller diffraction peak appears at 27.5°. This may be due to the composite of CN and CBB forming a heterojunction, occupying the (102) crystal plane of Cu-BiOBr, and showing gC 3 N 4 At the same time, the diffraction peak on the (110) plane is consistent with that of 0.35CBB and no Cu hybrid diffraction peak is found, indicating that Cu 2+ BiOBr was successfully introduced and exhibited high dispersion characteristics, and was finally composited with CN.

[0054] The composite materials prepared in Examples 1-2 and 5 were subjected to SEM analysis, and the results were as follows: Figure 2 shown.

[0055] Figure 2 : are SEM images of the composite materials prepared in Examples 1-2 and Example 5, wherein ac are SEM images of CN at different magnifications, df are SEM images of 0.35CBB at different magnifications, and gi are SEM images of CCB2 at different magnifications. Figure 2 It can be clearly seen that gC 3 N 4 , 0.35Cu-BiOBr and CCB2 morphology, Figure 2 ac see, gC 3 N 4 It presents a large particle block and lamellar structure, which is similar to the typical graphite structure. At the same time, the surface structure is irregular and there are many irregular gaps, which should be attributed to the intense KOH etching during the synthesis reaction. At the same time, many gC 3 N 4 The debris indicates that the stacked layer structure is broken into smaller sizes, which is conducive to the composite with Cu-BiOBr. Figure 2 In the df, the appearance structure of the 0.35CBB sample material is a three-dimensional microsphere assembled from a large number of nanosheets, which is attributed to the special generation method, namely, the Br - with [Bi 2 O 2 ] 2+ They stack and aggregate continuously, and finally crystallize and assemble into microsphere-like structures. Figure 2It is not difficult to see in the gi that the microspheres with sheet structures are more obvious, and even have pore structures, which may be due to the difference between gC 3 N 4 After the nanosheets were composited, the formation of Cu-BiOBr was hindered, and no obvious gC 3 N 4 The structure of the nanosheets may be the reason for their small particle size and high dispersion, which will greatly increase the specific surface area of ​​the composite material and thus improve the utilization rate of visible light.

[0056] The composite materials prepared in Examples 1-7 were subjected to XRD analysis, and the results were as follows: Figure 3 shown. Figure 3 The UV-visible diffuse reflectance spectra and the corresponding band gap diagrams of the composite materials prepared in Examples 1-7, wherein a is the UV-visible diffuse reflectance spectra and b is the corresponding band gap diagram. Figure 3 As can be seen from a and b, monomer gC 3 N 4 The visible light absorption ability of CCB1CCB3 is poor, while the light response ability of Cu-BiOBr and gC 3 N 4 The absorption edge band is around 530nm, which shows that the visible light absorption performance of the composite material is improved after the two are combined, which may be due to the 3 N 4 The heterojunction structure improves the nitrogen fixation ability by improving the separation of carriers. The band gap width of CCB2, the material with the best photocatalytic nitrogen fixation performance, is calculated to be 2.27eV. At the same time, the band gap widths of CN and 0.35CBB are calculated to be 2.73eV and 2.02eV. It can be concluded that the band gap of CCB2 is between the two, which verifies the success of the heterojunction recombination.

[0057] The composite materials prepared in Examples 1-7 were subjected to fluorescence testing and photocurrent response analysis. The results are as follows: Figure 4 shown. Figure 4 The photoluminescence spectrum test and transient photocurrent response test diagram of the composite material prepared in Examples 1-7 are shown in Figure a, which is a photoluminescence spectrum diagram and Figure b is a transient photocurrent response test result diagram. Figure 4 As shown in a, on the one hand, it shows the advantage of low recombination rate after Cu modification of BiOBr, and on the other hand, it can be seen that the fluorescence signal intensity of the composite material is greatly reduced, among which the signal peak with the lowest intensity appears in sample CCB2. This shows that the heterojunction structure constructed between carbon nitride and doping materials can quickly promote the spatial separation of photogenerated electrons and holes and increase the kinetic migration rate of photogenerated electrons and holes to the target reaction, thereby improving the reaction activity of photocatalytic nitrogen fixation. Figure 4 From b, we can see that the monomer BiOBr and gC 3 N 4 The transient photocurrent response of the composite material is weak, which may be due to the poor photosensitivity of the material itself and the slow transmission of photogenerated carriers and easy recombination. 3 N 4 The photocurrent intensity of the samples was significantly enhanced, especially sample CCB2, which showed the highest photocurrent density among all samples. This means that the prepared heterojunction composite photocatalyst can more effectively promote the migration of photogenerated electrons and holes to the catalyst surface, which is consistent with the fluorescence test results. In general, the formation of a heterojunction helps promote the migration of photogenerated electrons and holes, thereby improving the photocatalytic ammonia production performance.

[0058] Example 8

[0059] 1. The photocatalysts prepared in the above Examples 1-7 were subjected to a photocatalytic nitrogen fixation experiment, specifically:

[0060] 40 mg of the photocatalysts prepared in Examples 1-7 were respectively dissolved in 40 mL of deionized water, ultrasonicated for 15-30 min, and N2 was introduced at a flow rate of 50 mL / min. 2 The dark reaction was continued for 40 minutes to reach the adsorption-desorption equilibrium. A 500W xenon lamp was used to irradiate the sample. The ultraviolet light below 420nm was filtered out with a filter, leaving only the visible light. Samples were taken every half an hour and calibrated with Nessler's reagent method. Figure 5 As shown. Figure 5 It can be seen that the performance of monomer CN in nitrogen fixation is poor, and its ammonia nitrogen concentration is 19.8 μmol·L -1 , which is consistent with the previous characterization results. After compounding with 0.35CBB, a heterojunction structure was formed, in which the 2h nitrogen fixation performance of CCB1, CCB1.5, CCB2, CCB2.5, and CCB3 were 66.4μmol·L -1 , 92.7 μmol·L -1 , 96.1 μmol·L -1 , 86.3 μmol·L -1 , 79.8 μmol·L -1 Among them, the composite catalyst CCB2 has the best performance, which is about 4.8 times that of the monomer CN. Compared with 0.35CBB, the nitrogen fixation performance of CCB1.5-3 is also improved (CCB1 has poor performance, which is considered to be due to the low amount of CN added, resulting in easy recombination of photogenerated carriers), proving that the heterojunction structure of the composite material can improve the photocatalytic nitrogen fixation performance to a certain extent.

[0061] Cu-BiOBr / gC 3 N4 The good nitrogen fixation performance can be explored from the nitrogen fixation mechanism. Figure 6 As shown, Figure 6 Figure a is a schematic diagram of the type II heterojunction of the catalyst CBB; Figure 6 Where b is given by the valence band formula E VB =E vf +E FB The monomer gC can be calculated separately 3 N 4 and 0.35Cu-BiOBr / gC 3 N 4 The valence band values ​​are 3.08eV and 2.00eV respectively. The conduction band position can be obtained from the empirical formula E CB =E VB –E g The calculated values ​​are -0.35eV and 0.27eV respectively. So far, the energy band position and band gap width of the photocatalyst itself have been obtained. In summary, it can be inferred that the composite catalyst is a type II heterojunction semiconductor catalyst, gC 3 N 4 Both Cu and BiOBr are p-type semiconductors. After forming a heterojunction, they are stimulated by light and each generates photogenerated electrons and holes. However, due to the different amounts generated, an internal electric field is formed at the catalyst interface. Driven by the internal electric field, the photogenerated electrons will be enriched in the lower conduction band, and the holes will be enriched in the higher valence band, achieving physical separation. 2+ Metal can act as a bridge to rapidly transfer photogenerated electrons to gC 3 N 4 In this way, electrons and holes are separated into different semiconductor materials, which greatly reduces the recombination rate of photogenerated electrons and holes and significantly prolongs the lifetime of photogenerated electrons and holes. 3 N 4 The nitrogen defects are generated and nitrogen vacancies are introduced. The nitrogen defects help to separate the photogenerated carriers and significantly improve the gC 3 N 4 The nitrogen fixation capacity of 2 The nitrogen atoms in the molecules have similar structures and good chemical adsorption and activation selectivity for nitrogen. In summary, heterojunctions can improve the nitrogen fixation activity of photocatalysts through different channels.

[0062] The CCB2 prepared in Example 4 was subjected to a cyclic photocatalytic nitrogen fixation experiment. The results are as follows: Figure 7 shown. Figure 7 The cyclic ammonia production graph of the photocatalytic material CCB2 of Example 4 and its initial and post-cycle XRD comparison graphs, wherein a is the cyclic ammonia production graph and b is the post-cycle XRD comparison graph. Figure 7As shown in Figure a, the ammonia production per unit mass of the sample did not decrease significantly after multiple tests, and after three cycles of testing, it was still able to maintain about 90% of the original ammonia production, indicating that the prepared material has good photochemical stability. Figure 7 Figure b shows that there is no obvious difference before and after the reaction, which proves its potential application value in photocatalytic nitrogen fixation.

[0063] Comparative Example 1

[0064] The single-atom Fe / porous gC 3 N 4 (FPx) samples were used for photocatalytic nitrogen fixation (see Facile synthesis of Fe single-atom porous photocatalysts via direct metal atomization achieving efficient photocatalytic nitrogen fixation, Journal of Materials Science & Technology 167 (2023) 248–257).

[0065] The comparative example is to uniformly mix 10 g of dicyandiamide and 10 g of ammonium chloride, then move the mixture into a muffle furnace and heat it in a vacuum tubular muffle furnace for 3 hours (550°C, 5°C min -1 ), cooled to room temperature to obtain PCN (porous graphite carbon nitride). In the same case, without adding ammonium chloride, block gC 3 N 4 (BCN); 10 g of dicyandiamide, 10 g of ammonium chloride and 30 mg of iron particles were mixed evenly in a glove box to obtain a catalyst FP 3 .

[0066] The obtained catalyst FP 3 The nitrogen fixation performance test was carried out with the same experimental steps as in Example 8 to obtain single-atom Fe / porous gC 3 N 4 The ammonia nitrogen concentration of the (FPx) catalyst is 62.42 μmol·L -1 The CCB2 photocatalyst prepared by the present invention and FP 3 Compared with the photocatalyst, the same modified gC 3 N 4 As well as introducing metal atoms, the nitrogen fixation performance of the present application is greatly improved compared to the existing catalysts, and the material preparation of the present invention is simpler than that of the comparative example and does not need to be completed in a glove box.

[0067] Comparative Example 2

[0068] Existing photocatalyst AgPt-TiO for photocatalytic nitrogen fixation and ammonia synthesis 2 (See Enhancing the Supply of Activated Hydrogen to Promote Photocatalytic Nitrogen Fixation, ACS Materials Lett. 2021, 3, 1521-1527).

[0069] 20 mg TiO 2 Disperse in a mixture solution of 25 ml ultrapure water and 5 ml methanol, stir, and then add a certain amount of AgNO 3 Solution and H 2 PtCl 6 The solution was added to the above solution. After ultrasonic dispersion for 5 minutes, the following operations were performed in a specific reactor: the catalyst suspension was continuously stirred in the dark, and high-purity Ar was introduced for bubbling suspension for 5 minutes. The reactor was then irradiated under a 300W xenon lamp, while bubbling argon through the suspension for continuous stirring for 5 minutes. The product was collected by centrifugation, washed with ultrapure water three times, and finally dried at 60°C for 24 hours to obtain the catalyst AgPt-TiO 2 The catalyst AgPt-TiO 2 The nitrogen fixation and ammonia synthesis test was also carried out. The nitrogen fixation experimental steps were the same as in Example 8 to obtain AgPt-TiO 2 The ammonia nitrogen concentration of the catalyst is 38.4 μmol·L -1 The CCB2 photocatalyst prepared by the present invention and AgPt-TiO 2 Compared with the photocatalyst, the optimal ammonia production concentration of the present invention is 2.5 times that of Example 2, and the preparation method cost is lower. 3 N 4 The modification method of constructing heterojunction has significantly improved the effect in the field of photocatalytic nitrogen fixation.

[0070] Comparative Example 3

[0071] Existing Cu 2+ Doped MIL-68(Fe) MOFs for photocatalytic nitrogen fixation and ammonia synthesis (see Defectmodulation of MIL-68(Fe)MOFs by Cu doping for boosting photocatalytic nitrogen fixation 2 fixatio, Journal of Catalysis 432(2024)115436).

[0072] Ferric chloride hexahydrate and cupric chloride dihydrate in a molar mass ratio of 0.9:0.1 were dissolved in 12 mL of N,N-dimethylformamide (DMF) and stirred for 5 min. Then, 2.4 mmol of H 2 BDC, and stir for another 5 minutes. Then add 120 μL of HF (5 mol / L) and 120 μL of HCl (1 mol / L) and stir for 30 minutes. Finally, transfer the mixed solution to a 25 ml reactor and heat at 100 ° C for 120 hours. After cooling to room temperature, the product was collected by centrifugation and washed 4 times with ultrapure water and acetone. It was then dried in a vacuum oven at 100 ° C for 6 hours and heated at 200 ° C for 2 hours in an argon atmosphere to obtain the sample Cu 2+ The nitrogen fixation experimental steps were the same as those in Example 8, and the ammonia nitrogen concentration of the MC3 catalyst was 21 μmol·L -1 Compared with the MC3 photocatalyst, the CCB2 photocatalyst prepared by the present invention has an optimal ammonia production concentration that is 4.5 times that of the MC3 photocatalyst. In addition, the preparation period of MC3 is longer, and the introduction of Cu 2+ In fact, it is also to increase the concentration of oxygen vacancies to better improve the nitrogen fixation performance, but its preparation cost and steps are far more complicated than the present invention and its nitrogen fixation performance is worse.

Claims

1. A Cu-BiOBr / g-C3N4 composite photocatalytic material, characterized in that: The Cu-BiOBr / g-C3N4 composite photocatalytic material comprises a nanosheet polymerized microsphere structure composited with g-C3N4 and Cu-BiOBr, and has a hole structure on the surface.

2. The Cu-BiOBr / g-C3N4 composite photocatalytic material according to claim 1, characterized in that: Cu-BiOBr is a Cu-BiOBr material with oxygen vacancies obtained by doping metallic Cu into BiOBr in the form of nitrate. The Cu-BiOBr / g-C3N4 composite photocatalytic material is prepared by constructing a heterojunction between nitrogen-deficient ultra-thin g-C3N4 nanosheets and Cu-BiOBr. The nanosheet polymerized microsphere structure is a three-dimensional micron sphere assembled from a large number of nanosheets.

3. The method for preparing the Cu-BiOBr / g-C3N4 composite photocatalytic material according to claim 1 or 2, characterized in that: Ultrathin g-C3N4 nanosheets and Cu-BiOBr composite materials are used as raw materials and prepared by a hot solvent method, including the following steps: (1) dispersing melamine and KOH in water, heating and stirring, evaporating water to obtain a block mixture, crushing, calcining under nitrogen protection, centrifugation washing, and drying to obtain ultrathin g-C3N4 nanosheets; (2) dissolving polyvinyl pyrrolidone in ethylene glycol, adding Bi(NO3)3·5H2O, stirring to dissolve, adding KBr and continuing to stir, adding Cu(NO3)2·3H2O, and continuing to stir to obtain a precursor Cu-BiOBr solution; (3) adding ultrathin g-C3N4 nanosheets into the precursor Cu-BiOBr solution, stirring to dissolve, reacting at high temperature, cooling, centrifugal washing, and drying to obtain a Cu-BiOBr / g-C3N4 composite material.

4. The preparation method according to claim 3, characterized in that: In step (1), the mass ratio of melamine, KOH and water is (1-90): (0.01-15): (1-200), the heating and stirring temperature is 80-100° C., and the heating and stirring time is more than 24 hours.

5. The preparation method according to claim 3, characterized in that: In step (1), calcination under nitrogen protection is first heated to 30-50°C and nitrogen is introduced for 25-35 minutes, the air is exhausted, and then the temperature is increased to 500°C at a heating rate of 5-10°C / min, maintained for 1-4 hours, and then the temperature is increased to 500-550°C at a heating rate of 5-10°C / min, and maintained for 4-6 hours.

6. The preparation method according to claim 3, characterized in that: In step (1), the centrifugal washing process is washed several times with deionized water and ethanol, and the centrifugal washing speed is 6000-8000 r min -1 , centrifugal washing time is 3-5min, until the pH reaches neutral; the drying temperature is 60-80℃, and the drying time is 10-14h.

7. The preparation method according to claim 3, characterized in that: In step (2), the mass volume ratio of polyvinyl pyrrolidone to ethylene glycol is (0.01-0.5):(10-20) g / mL, and the molar ratio of KBr, Bi(NO3)3·5H2O and Cu(NO3)3·3H2O is 1:1:(0.2-0.5); and the stirring time is continued for 1-2 h.

8. The preparation method according to claim 3, characterized in that: In step (3), the mass ratio of ultrathin g-C3N4 nanosheets to polyvinyl pyrrolidone is (1.5-3):1, the temperature of the high-temperature reaction is 100-200°C, and the time of the high-temperature reaction is more than 12 hours.

9. The preparation method according to claim 3, characterized in that: In step (3), the centrifugal washing is performed by centrifugation with warm deionized water and ethanol for several times, and the speed of the centrifugal washing is 6000-8000 r min. -1 The centrifugal washing time is 3-5 minutes, the drying temperature is 60-80℃, and the drying time is 10-14 hours.

10. Use of the Cu-BiOBr / g-C3N4 composite photocatalytic material according to claim 1 or 2 in a photocatalytic nitrogen fixation reaction.

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