Preparation method and application of iron-based metal organic framework material / carbon nitride nitrogen fixation photocatalyst

Abstract

The invention discloses a preparation method and application of an iron-based metal organic framework material / carbon nitride nitrogen fixation photocatalyst in the field of photocatalytic materials, the iron-based metal organic framework material / carbon nitride nitrogen fixation photocatalyst is prepared by taking ferric chloride, terephthalic acid and graphite phase carbon nitride as raw materials through a hydrothermal reaction, a heterojunction is constructed through a solvothermal method, uniform loading of MIL-53 (Fe) on the surface of b-C3N4 is achieved, and the photocatalytic performance of the photocatalyst is improved. And the formed heterogeneous interface significantly improves the photon-generated carrier separation efficiency. The obtained MIL-53 (Fe) / b-C3N4 photocatalytic composite material has good visible light response characteristic and excellent photocatalytic nitrogen fixation performance, and is safe, non-toxic, green and environment-friendly. The produced ammonia water can be directly absorbed by wheat, so that the chlorophyll content of leaves is increased by 11-13%, the plant height is increased by 19-21%, and a novel photocatalysis scheme is provided for green agriculture.

Description

TECHNICAL FIELD

[0001] The application relates to a kind of photocatalytic materials, in particular to a kind of preparation method and application of iron-based metal organic framework material / carbon nitride nitrogen fixation photocatalyst. BACKGROUND

[0002] Ammonia, as a key substance in the fields of industry and agriculture, plays a vital role in the grand landscape of global economy and food production. In the industrial field, ammonia is the basis for numerous important chemical products, from the fine pharmaceutical industry to the widely used plastic manufacturing. It is indispensable and an important cornerstone supporting the stable operation of modern industrial system, which deeply influences the development process of global economy. In the agricultural aspect, ammonia, in the form of nitrogen fertilizer, is the core nutrient for crop growth. It is directly related to the growth trend, yield and quality of crops, and is closely related to the stable supply of global food, which is the key to guarantee the livelihood of billions of people.

[0003] Currently, the Haber-Bosch process is widely used in industrial ammonia synthesis. Although this process is widely used and has made important contributions to the global ammonia supply, it has significant drawbacks. The reaction conditions of this process are extremely harsh, requiring high temperature and high pressure, which consumes a large amount of energy in the production process. According to statistics, the energy consumption of global ammonia synthesis process accounts for about 2% of the total energy consumption, which is undoubtedly a huge burden in the face of increasingly tight energy resources. At the same time, the process emits a large amount of greenhouse gases, accounting for 1.5% of the global total, which puts heavy pressure on the global ecological environment and exacerbates global environmental problems such as climate change.

[0004] In the agricultural field, there are many problems in applying traditional urea fertilizer. The nitrogen form is easy to cause a large amount of nitrogen loss in the form of ammonia volatilization, nitrification-denitrification, etc. during the transformation process, reducing fertilizer utilization rate; single application of fertilizer has a short effective period, requiring multiple topdressing, increasing labor costs; nutrient release and crop demand are difficult to match, which can easily cause seedling burning in the early stage and fertilizer loss in the late stage, and the lost nitrogen can also pollute water bodies and the atmosphere. SUMMARY

[0005] In order to overcome the above-mentioned defects, the application provides a preparation method and application of iron-based metal organic framework material / carbon nitride nitrogen fixation photocatalyst. The obtained MIL-53(Fe) / b-C3N4 has good visible light catalytic performance. The heterojunction is constructed by a solvothermal method, MIL-53(Fe) is uniformly loaded on the surface of b-C3N4, and the heterojunction interface can significantly improve the separation efficiency of photo-generated carriers.

[0006] The technical solution for achieving the object of the present application is a preparation method of an iron-based metal organic framework material / carbon nitride solid nitrogen fixation photocatalyst, comprising the following steps:

[0007] (1) using melamine as a precursor to prepare a bulk g-C3N4;

[0008] (2) ball milling the bulk g-C3N4, first using dry ball milling to prepare b-C3N4 with a particle size of 84.6-110.56 microns, and then using wet ball milling with anhydrous ethanol to prepare b-C3N4 with a particle size of 3.2-8.34 microns;

[0009] (3) dissolving the precursors of MIL-53(Fe) and terephthalic acid in a solvent dimethylformamide at a molar mass ratio of 1:1, stirring to form a uniform solution, adding different amounts of ball-milled modified graphite carbon nitride, continuing to stir and ultrasonic, forming a suspension, and then performing a hydrothermal reaction; the mass ratio of MIL-53(Fe) to carbon nitride is 1-4:1; the temperature of the hydrothermal reaction is 150-170℃, and the time of the hydrothermal reaction is 11-13h;

[0010] (4) after the hydrothermal reaction is completed, the product is a MIL-53(Fe) / b-C3N4 solid precipitate. The upper layer of dimethylformamide solvent is filtered off, washed, dried, and ground to obtain the target product MIL-53(Fe) / b-C3N4 photocatalytic composite material powder.

[0011] Further improvements are that in step (1), when preparing the bulk g-C3N4, the melamine is placed in an alumina crucible and heated in a muffle furnace at a rate of 3.5-4.5℃ / min to 450-550℃, and kept at temperature for 3-4h, and then cooled to room temperature to obtain block-shaped g-C3N4, which is then ground into powder using a mortar, and sieved through an 80 mesh sieve to obtain the bulk g-C3N4.

[0012] Further improvement is that in step (2), the specific steps of dry ball milling bulk g-C3N4 are as follows: bulk g-C3N4 and agate balls are respectively put into two agate jars of the planetary ball mill, and the weight ratio of ball to material is controlled to be 20:1; after the agate cover with rubber ring is covered, the agate jar is placed in the ball mill and fixed, the rotation speed and ball milling time are set, and the forward and reverse rotation is alternately operated, and the interval time of each alternate operation is 60s; after the ball mill is set to stop, the powder attached to the inner wall of the jar is collected and scraped off, and the dry ball milled b-C3N4 is obtained; the specific steps of wet ball milling b-C3N4 are as follows: b-C3N4 and agate balls are put into the agate jar of the planetary ball mill, the weight ratio of ball to material is controlled to be 20:1, 10-30 times of anhydrous ethanol by weight of b-C3N4 is added, and the agate cover with rubber ring is covered, the agate jar is placed in the ball mill and fixed, the rotation speed and ball milling time are set, and the forward and reverse rotation is alternately operated, and the interval time of each alternate operation is 60s; after the ball mill is set to stop, the agate jar is taken out and anhydrous ethanol is added to flush the inner wall of the jar and the agate balls, the obtained mixed liquid is centrifuged, the obtained solid is placed in a 55-65℃ oven and dried for 8-12h, and the wet ball milled modified b-C3N4 is obtained.

[0013] The obtained MIL-53(Fe) / b-C3N4 photocatalytic composite material can be used for photocatalytic nitrogen fixation. The ammonia water produced by photocatalytic nitrogen fixation can be applied to the growth of crops with nitrogen demand. For example, it can be applied to the growth of wheat.

[0014] In the application, the bulk g-C3N4 is modified by dry ball milling and wet ball milling. The mechanical shearing force of ball milling can significantly reduce the thickness of bulk g-C3N4, form wrinkles and porous structure, and significantly increase the specific surface area, thereby improving the nitrogen fixation efficiency. Ball milling can also reduce particle agglomeration, improve dispersibility, and promote mass transfer efficiency in photocatalytic reaction. In addition, the performance of MIL-53(Fe) / b-C3N4 depends on the mass ratio of MIL-53(Fe) and b-C3N4. When the amount of carbon nitride is too low, the excess MIL-53(Fe) will exist in the form of impurities in the generated MIL-53(Fe) / b-C3N4, causing crystal defects in MIL-53(Fe) / b-C3N4 and affecting its photocatalytic nitrogen fixation performance; if the amount of carbon nitride is too high, the agglomeration of carbon nitride is obviously enhanced, the uniformity of MIL-53(Fe) / b-C3N4 is affected, and thus its performance is affected.

[0015] Advantages: Compared with the prior art, the application has the following obvious advantages:

[0016] (1) The present application selects MIL-53(Fe) / b-C3N4 with good visible light catalytic performance as a catalyst, constructs a heterojunction through a solvothermal method, realizes uniform loading of MIL-53(Fe) on the surface of b-C3N4, and forms a heterojunction interface that significantly improves the separation efficiency of photo-generated carriers.

[0017] (2) The composite material prepared in the present application has good visible light response characteristics and excellent photocatalytic nitrogen fixation performance, and is safe, non-toxic and green, providing a new idea for the popularization and application of photocatalytic materials.

[0018] (3) The MIL-53(Fe) / b-C3N4 photocatalytic composite material prepared in the present application can directly absorb the ammonia water produced by photocatalytic nitrogen fixation, so that the chlorophyll content of its leaf blade increases by 11-13%, and the plant height increases by 19-21%, providing a new photocatalytic scheme for green agriculture. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The particle size range of carbon nitride of different sizes prepared based on steps (1) and (2) of Example 1 is measured by using a Bettersize 2600 laser particle size distribution instrument (dry method); in the figure, as the ball milling speed and time increase, the particle size of g-C3N4, b-C3N4(1), b-C3N4(2), and b-C3N4(3) gradually decreases from the original 84.6 μm to 3.2 μm. Ball milling technology can regulate the particle size and structure of graphite phase carbon nitride, and smaller particle size is beneficial to enhance light absorption capacity and separation efficiency of photo-generated electron-hole pairs, thereby improving photocatalytic nitrogen fixation performance.

[0020] Figure 2 The scanning electron microscope (SEM) pictures of carbon nitride of different sizes prepared based on steps (1) and (2) of Example 1 are shown in the figure, wherein (a)-(d) correspond to g-C3N4, b-C3N4(1), b-C3N4(2), and b-C3N4(3), respectively, and the size gradually decreases.

[0021] Figure 3(a)-(d) are different sizes of carbon nitride prepared based on the (1), (2) steps of Example 1, corresponding to g-C3N4, b-C3N4(1), b-C3N4(2), b-C3N4(3) respectively. (a) is a transmission electron microscope (TEM) image of bulk carbon nitride g-C3N4. It can be seen that due to the thick sample, it is difficult for electrons to penetrate the sample, so there are a large number of black areas in the field of view, and the edges of different depths indicate that the carbon nitride is stacked by many small size lamellas. However, after different degrees of ball milling, the contrast of the sample is low, and the overall gradually presents obvious flaky structure, indicating that the thickness of the carbon nitride is thinned after ball milling, at the same time, the interlayer peeling of the carbon nitride is realized by ball milling, and the two-dimensional lamella is also damaged to a certain extent.

[0022] Figure 4 The scanning electron microscope (SEM) images of Example 2 for synthesizing MIL-53(Fe) / b-C3N4-0.25(CNMIL-2) are shown in (a) MIL-53(Fe), (b) carbon nitride (b-C3N4) modified by ball milling, and (c) composite material MIL-53(Fe) / b-C3N4-0.25(CNMIL-2). In (c), it can be seen that MIL-53(Fe) is obviously attached to the surface of b-C3N4. This interaction not only can endow the composite material with more excellent performance, but also can improve the transmission and separation efficiency of carriers by enhancing the heterojunction interface.

[0023] Figure 5 The transmission electron microscope (TEM) images of Example 2 for synthesizing MIL-53(Fe) / b-C3N4-0.25(CNMIL-2) are shown in (a) MIL-53(Fe), (b) carbon nitride (b-C3N4) modified by ball milling, and (c) composite material MIL-53(Fe) / b-C3N4-0.25(CNMIL-2). In (c), it can be seen that MIL-53(Fe) and b-C3N4 form a closely connected heterojunction interface. This interface structure helps the effective transmission of charges and improves the separation efficiency of photo-carriers, thereby having a positive impact on the photocatalytic performance of the material.

[0024] Figure 6 The high-resolution transmission electron microscope (HRTEM) images of Example 2 for synthesizing MIL-53(Fe) / b-C3N4-0.25(CNMIL-2) are shown. The structural characteristics of the composite material are further confirmed.

[0025] Figure 7Mapping images of MIL-53(Fe) / b-C3N4-0.25(CNMIL-2) synthesized in Example 2 were obtained by analyzing the mapping images, and the uniform distribution of Fe, C, and N elements in MIL-53(Fe) / b-C3N4-0.25(CNMIL-2) was clearly shown. This uniformity proves the successful synthesis of the composite material and indicates that the combination of MIL-53(Fe) and b-C3N4 can form a more stable and effective photocatalyst.

[0026] Figure 8 X-ray diffraction (XRD) images of the photocatalytic materials synthesized in Examples 1, 2, 3, and 4 were obtained, and the characteristic peaks of the composite material MIL-53(Fe) / b-C3N4 corresponded to those of MIL-53(Fe) and carbon nitride, indicating the successful preparation of the composite material.

[0027] Figure 9 Fourier transform infrared spectroscopy (FT-IR) images of the photocatalytic materials synthesized in Examples 1, 2, 3, and 4 were obtained, and the FTIR spectra shown in the images indicated that the characteristic peaks of the composite material MIL-53(Fe) / b-C3N4 corresponded to those of MIL-53(Fe) and carbon nitride, confirming the successful synthesis of the composite material. In addition, no other characteristic peaks were detected in the images, confirming the high purity of the prepared samples.

[0028] Figure 10 The photocatalytic nitrogen fixation and ammonia production efficiency of the composite material MIL-53(Fe) / b-C3N4 synthesized in Examples 1, 2, 3, and 4 were compared, and the data in the graph showed that when the mass ratio of MIL-53(Fe) to b-C3N4 in the photocatalytic composite material was 2:1, the photocatalytic nitrogen fixation performance was the best. The ratio of MIL-53(Fe) to b-C3N4 reached a balance point at which the band matching and carrier migration path were optimized - too little and the interface was insufficient, affecting the synergistic effect; too much and MIL-53(Fe) shielded the light absorption active region of b-C3N4, leading to an increase in carrier recombination probability. Therefore, MIL-53(Fe) / b-C3N4-0.25(CNMIL-2) as the optimal ratio sample, comprehensively realized the synergistic effect of efficient light absorption, active site full exposure, and unobstructed electron migration channel, thereby exhibiting the optimal performance in the photocatalytic nitrogen fixation reaction.

[0029] Figure 11 Based on Example 2, the application graph of the ammonia water generated by the photocatalytic nitrogen fixation of MIL-53(Fe) / b-C3N4-0.25(CNMIL-2) and other treatment control group wheat growth was applied, and until the 12th day, the wheat irrigated with ammonia fertilizer was still leading in overall growth, with more robust plants, larger leaf area, and bright color, indicating that the wheat irrigated with ammonia fertilizer had the best growth.

[0030] Figure 12 For the relative chlorophyll content (SPAD value) chart of the wheat leaf of the other treatment control group based on Example 2, among the different treatment groups involved in the present study, the average SPAD value of the wheat leaf of the ammonia fertilizer application group is 12.32 and 2.95 higher than that of the non-fertilizer application group and the urea fertilizer application group, respectively. This result shows that the ammonia fertilizer application group exhibits more significant effect in promoting the nitrogen absorption and chlorophyll synthesis of wheat compared with the non-fertilizer application group and the urea fertilizer application group. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical scheme and advantages of the present application more clear and understandable, the present application will be described in detail below in combination with examples.

[0032] Example 1

[0033] (1) Put the melamine into an alumina crucible, heat to 450-550℃ at a speed of 3.5-4.5℃ / min in a muffle furnace, and keep the temperature for 3-4h. After cooling to room temperature, blocky gC3N4 is obtained. Then, the gC3N4 is ground into powder using a mortar, and the powder is sieved through an 80-mesh sieve to obtain bulk g-C3N4.

[0034] (2) The specific steps of dry ball milling of bulk g-C3N4 are as follows: bulk g-C3N4 and agate balls are respectively placed in two agate jars of a planetary ball mill, and the weight ratio of balls to material is controlled to be 20:1. After covering the agate cover with a rubber ring, the agate jar is fixed in the ball mill. The rotation speed and ball milling time are set, and the forward and reverse rotation is alternately operated. The interval time of each alternation is 60s. After the ball mill stops according to the set program, the powder adhered to the inner wall of the jar is collected and scraped off to obtain the dry ball-milled b-C3N4. The particle size of the modified b-C3N4 obtained by dry ball milling is 84.6-110.56μm.

[0035] The specific steps of wet ball milling of bulk g-C3N4 are as follows: the dry ball-milled b-C3N4 and agate balls are added to the agate jar of the planetary ball mill, and the weight ratio of balls to material is controlled to be 20:1. Then, 20 times the weight of b-C3N4 of anhydrous ethanol (the amount of anhydrous ethanol can be 10-30 times the weight of b-C3N4) is added, and the agate cover with a rubber ring is covered. The agate jar is fixed in the ball mill. The rotation speed and ball milling time are set, and the forward and reverse rotation is alternately operated. The interval time of each alternation is 60s. After the ball mill stops according to the set program, the agate jar is taken out and anhydrous ethanol is added to flush the inner wall of the jar and the agate balls. The obtained mixed solution is centrifuged to obtain a solid. The solid is dried in a 55-65℃ oven for 8-12h. The particle size of the modified bC3N4 obtained by wet ball milling is 3.2-8.34μm.

[0036] (3) The precursors of MIL-53(Fe), iron chloride 0.4055 g and terephthalic acid 0.415 g, were dissolved in 70 mL of dimethylformamide, stirred at 25°C for 0.5 h, and stirred to form a uniform solution;

[0037] (4) 0.5 g of ball-milled modified graphite phase C3N4 was added, and the stirring and ultrasonic treatment were continued to form a suspension, and then the hydrothermal reaction was carried out; the mass ratio of MIL-53(Fe) to C3N4 was 1:1; the obtained solution was transferred into a 100 mL high-pressure reaction kettle, and constant temperature hydrothermal treatment was carried out at 160°C for 12 h. After cooling to room temperature, the sample was obtained by centrifugation, washing and drying, and MIL-53(Fe) / b-C3N4-0.5 was prepared, named as CNMIL-1.

[0038] Example 2: (comparative experiment)

[0039] (1) The same as step 1) of Example 1;

[0040] (2) The same as step 2) of Example 1;

[0041] (3) The same as step 3) of Example 1;

[0042] (4) 0.25 g of ball-milled modified graphite phase C3N4 was added, and the stirring and ultrasonic treatment were continued to form a suspension, and then the hydrothermal reaction was carried out; the mass ratio of MIL-53(Fe) to C3N4 was 2:1; the obtained solution was transferred into a 100 mL high-pressure reaction kettle, and constant temperature hydrothermal treatment was carried out at 150°C for 12 h. After cooling to room temperature, the sample was obtained by centrifugation, washing and drying, and MIL-53(Fe) / b-C3N4-0.25 was prepared, named as CNMIL-2;

[0043] Example 3: (comparative experiment)

[0044] (1) The same as step 1) of Example 1;

[0045] (2) The same as step 2) of Example 1;

[0046] (3) The same as step 3) of Example 1;

[0047] (4) 0.167 g of ball-milled modified graphite phase C3N4 was added, and the stirring and ultrasonic treatment were continued to form a suspension, and then the hydrothermal reaction was carried out; the mass ratio of MIL-53(Fe) to C3N4 was 3:1; the obtained solution was transferred into a 100 mL high-pressure reaction kettle, and constant temperature hydrothermal treatment was carried out at 150°C for 13 h. After cooling to room temperature, the sample was obtained by centrifugation, washing and drying, and MIL-53(Fe) / b-C3N4-0.167 was prepared, named as CNMIL-3;

[0048] Example 4: (comparative experiment)

[0049] (1) Same as step 1 in Example 1;

[0050] (2) Same as step 2 in Example 1;

[0051] (3) Same as step 3) in Example 1;

[0052] (4) Add 0.125 g of ball-milled modified graphitic carbon nitride and continue vigorous stirring and sonication to form a suspension, then carry out a hydrothermal reaction; the mass ratio of MIL-53(Fe) to carbon nitride is 4:1; transfer the obtained solution to a 100 mL high-pressure reactor and perform constant temperature hydrothermal treatment at 170 °C for 11 h. After cooling to room temperature, obtain the sample by centrifugation, washing and drying, and name it CNMIL-4;

[0053] II. Application

[0054] Parallel experiments were conducted using the photocatalytic composite material prepared in Example 1:

[0055] Photocatalytic nitrogen fixation: Photocatalytic nitrogen reduction (p-NRR) was carried out in a sealed quartz reactor using a xenon lamp (CELHXF300) equipped with an ultraviolet cutoff filter (λ>400nm). 50mg of photocatalyst was ultrasonically dispersed in 100mL of pure water. Under dark conditions, the reaction proceeded at a rate of 100mL / min. -1 Nitrogen gas was continuously bubbled in at a rate of 20 mL / min, and the mixture was magnetically stirred for 0.5 h. Then, under light irradiation, the mixture was refluxed at a rate of 20 mL / min. -1 Nitrogen gas was introduced at a rate of [missing value]. 5 mL of liquid sample was taken periodically and filtered through a 0.22 μm filter membrane. NH4+ was determined using Nessler's reagent spectrophotometry (JB7478-87). + The concentration of ammonia. Ammonia yield can be calculated using the following formula: Photocatalytic ammonia yield = (moles of ammonia produced / reaction time) / catalyst mass.

[0056] The table below lists the photocatalytic nitrogen fixation experiments based on the above steps, with data showing the photocatalytic nitrogen ammonia yields of MIL-53(Fe), b-C3N4, and the composite material MIL-53(Fe) / b-C3N4.

[0057]

[0058]

[0059] As shown in the table above, the photocatalytic nitrogen fixation performance is best when the mass ratio of MIL-53(Fe) to b-C3N4 in the photocatalytic composite material is 2:1. The performance of MIL-53(Fe) / b-C3N4 depends on the mass ratio of MIL-53(Fe) to b-C3N4. If the amount of carbon nitride is too high, the agglomeration of carbon nitride is significantly enhanced, affecting the uniformity of MIL-53(Fe) / b-C3N4 and thus its performance. When the amount of carbon nitride is too low, excess MIL-53(Fe) will exist as impurities in the generated MIL-53(Fe) / b-C3N4, leading to crystal defects in MIL-53(Fe) / b-C3N4 and affecting its photocatalytic nitrogen fixation performance. The ratio of MIL-53(Fe) / b-C3N4-0.25(CNMIL-2) achieves a balance between bandgap matching and optimized carrier migration pathways. Too little MIL-53(Fe) results in insufficient interface, affecting the synergistic effect; too much MIL-53(Fe) obscures the light-absorbing active region of b-C3N4, leading to an increased carrier recombination probability. Therefore, MIL-53(Fe) / b-C3N4-0.25(CNMIL-2) is the optimal ratio sample, comprehensively achieving a synergistic effect of efficient light absorption, sufficient exposure of active sites, and unobstructed electron migration channels, thus exhibiting optimal performance in photocatalytic nitrogen fixation.

[0060] To investigate the effects of ammonia water produced by photocatalytic nitrogen fixation using the MIL-53(Fe) / b-C3N4 photocatalytic composite material on wheat growth, a control experiment was conducted. Three treatment groups were set up: no fertilizer, urea fertilizer, and ammonia fertilizer (i.e., irrigation with ammonia water produced by photocatalytic nitrogen fixation using the MIL-53(Fe) / b-C3N4 photocatalytic composite material). Figure 11 As shown, on the second day after the experiment began, the wheat seedlings in the three treatment groups were quite similar, all in the initial germination stage. By the fourth day, the wheat seedlings began to grow, and some changes could be observed, but the differences were not yet significant. On the sixth day, the wheat in each treatment group continued to grow, and there was still no significant difference in appearance. On the eighth day, the wheat in the ammonia fertilizer group began to show an advantage in growth, with more upright plants and more spread leaves compared to the unfertilized group and the urea fertilizer group. By the tenth day, this advantage had further expanded, with the wheat in the ammonia fertilizer group being significantly taller than the other two groups, and the leaves being greener and more lush. Until the twelfth day, the wheat in the ammonia fertilizer group still led in overall growth, with more robust plants, larger leaf area, and brighter color, indicating that the wheat irrigated with ammonia fertilizer had the best growth.

[0061] Compared with urea fertilizer, the ammonia water produced by photocatalytic nitrogen fixation showed a better promotion effect on wheat growth, which was due to many differences. In terms of the timeliness of nitrogen supply, urea as an amide nitrogen fertilizer needs to be hydrolyzed to ammonium nitrogen under the action of urease before it can be absorbed by wheat in large quantities. This process is restricted by temperature, and the conversion is slow at low temperature. While the ammonia water produced by photocatalytic nitrogen fixation can directly provide ammonium nitrogen, which can quickly respond to the nitrogen demand during the growth of wheat, especially in the critical period of nitrogen demand, ensuring the immediate supply of nutrients. In terms of fertilizer utilization efficiency, urea has multiple nitrogen loss pathways such as ammonia volatilization, nitrate leaching, and denitrification. The ammonia water produced by photocatalytic nitrogen fixation reduces the loss of nitrogen in the complex conversion process due to the characteristics of in-situ nitrogen fixation, significantly improving the nitrogen use efficiency.

[0062] The following table lists the height of wheat plants in the no fertilizer group, urea fertilizer group, and ammonia fertilizer group (i.e., using MIL-53(Fe) / b-C3N4 photocatalytic composite material to produce ammonia water for irrigation).

[0063] Experimental group Wheat plant height (cm) Non-fertilizer group 18 Urea fertilizer group 22 Ammonia fertilizer group 25

[0064] Photocatalytic nitrogen fixation materials use solar energy to directly reduce nitrogen to ammonium ammonia water at normal temperature and pressure, not only avoiding the environmental burden brought by the high-energy Haber-Bosch process, but also significantly reducing carbon emissions. In terms of crop absorption mechanism, urea needs to be converted to ammonium nitrogen under the action of enzymes, and the conversion efficiency is greatly affected by temperature, which easily causes nitrogen volatilization and loss. While photocatalytic ammonia water can be directly absorbed by plants, it can more timely and efficiently meet the nitrogen demand of crops, especially in the critical period of nitrogen demand. In addition, experimental results show that compared with urea fertilization, wheat irrigated with photocatalytic nitrogen fertilizer performs better in plant height, chlorophyll content (SPAD value), and overall growth, indicating that it has a better effect on promoting plant growth and higher nitrogen use efficiency. Therefore, as a green, efficient, and sustainable nitrogen supply approach, photocatalytic nitrogen fixation materials show better application potential than urea.

[0065] In summary, we successfully synthesized a MIL-53(Fe) / b-C3N4 photocatalytic composite material, and exhibited good photocatalytic performance in NRR. The material has the following advantages: (1) the application selects MIL-53(Fe) / b-C3N4 with good visible light catalytic performance as the catalyst, builds a heterojunction by a solvothermal method, realizes the uniform loading of MIL-53(Fe) on the surface of b-C3N4, and forms a heterojunction interface that significantly improves the separation efficiency of photo-generated carriers. (2) the composite material prepared by the application has good visible light response characteristics and excellent photocatalytic nitrogen fixation performance, and is safe, non-toxic, green and environmentally friendly, providing a new idea for the popularization and application of photocatalytic materials. (3) the MIL-53(Fe) / b-C3N4 photocatalytic composite material prepared by the application can directly absorb the ammonia water produced by photocatalytic nitrogen fixation by wheat, and the chlorophyll content of the leaf is increased by 11-13%, and the plant height is increased by more than 19-21%, providing a new photocatalytic scheme for green agriculture.

Claims

1. A method for preparing an iron-based metal-organic framework material / carbon nitride solid nitrogen photocatalyst, characterized in that, It comprises the following steps: (1) using melamine as a precursor to prepare bulk g-C3N4; (2) ball milling bulk g-C3N4, first dry ball milling to prepare b-C3N4 with a particle size of 84.6-110.56 microns, and then wet ball milling with anhydrous ethanol to prepare b-C3N4 with a particle size of 3.2-8.34 microns; (3) the precursors of MIL-53(Fe) and terephthalic acid are dissolved in a solvent dimethylformamide at a molar mass ratio of 1:1, and a uniform solution is obtained by vigorous stirring. Different amounts of ball-milled modified graphite carbon nitride are added and continue to be stirred and ultrasonicated to form a suspension, and then hydrothermal reaction is carried out. The mass ratio of MIL-53(Fe) and carbon nitride is 1-4:

1. The temperature of the hydrothermal reaction is 150-170℃, and the time of the hydrothermal reaction is 11-13h; (4) after the hydrothermal reaction is completed, the product is MIL-53(Fe) / b-C3N4 solid precipitate. The upper layer of dimethylformamide solvent is filtered out, washed, dried, and ground to obtain the target product MIL-53(Fe) / b-C3N4 photocatalytic composite material powder.

2. The method for preparing a Fe-based metal-organic framework material / g-C3N4 solid nitrogen photocatalyst according to claim 1, characterized in that, In step (1), when preparing bulk g-C3N4, melamine is placed in an alumina crucible and heated to 450-550℃ at a rate of 3.5-4.5℃ / min in a muffle furnace, and then kept for 3-4h. After cooling to room temperature, block-shaped g-C3N4 is obtained. Then, the g-C3N4 is ground into powder using a mortar, and then sieved through an 80-mesh sieve to obtain bulk g-C3N4.

3. The method for preparing a Fe-based metal-organic framework material / g-C3N4 solid nitrogen photocatalyst according to claim 1, characterized in that, In step (2), the specific steps for dry ball milling of bulk g-C3N4 are as follows: Put bulk g-C3N4 and agate balls into two agate jars of a planetary ball mill, and control the ball-to-material weight ratio to be 20:

1. After covering the agate cover with a rubber ring, place the agate jar in the ball mill and fix it. Set the rotation speed and ball milling time, and run alternately in forward and reverse directions. The interval time for each alternation is 60s. After the ball mill stops according to the set program, collect and scrape off the powder attached to the inner wall of the jar to obtain b-C3N4 by dry ball milling. The specific steps for wet ball milling of b-C3N4 are as follows: Put b-C3N4 and agate balls into an agate jar of a planetary ball mill, and control the ball-to-material weight ratio to be 20:

1. Then add anhydrous ethanol with a weight of 10-30 times that of b-C3N4, and cover the agate cover with a rubber ring. Place the agate jar in the ball mill and fix it. Set the rotation speed and ball milling time, and run alternately in forward and reverse directions. The interval time for each alternation is 60s. After the ball mill stops according to the set program, remove the agate jar and add anhydrous ethanol to rinse the inner wall of the jar and the agate balls. Centrifuge the obtained mixture, and dry the obtained solid in an oven to obtain b-C3N4 modified by wet ball milling.

4. The method for preparing a Fe-based metal-organic framework material / g-C3N4 solid nitrogen photocatalyst according to claim 1, characterized in that, In step (3), the mass ratio of MIL-53(Fe) and carbon nitride is 2:

1.

5. Use of the iron-based metal-organic framework / nitrogen-doped carbon solid-state photocatalyst according to any one of claims 1 to 4, characterized in that, The MIL-53(Fe) / b-C3N4 photocatalytic composite material is used for photocatalytic nitrogen fixation.

6. Use of a Fe-based metal-organic framework / nitrogen-doped carbon solid-state photocatalyst according to claim 5, characterized in that the ammonia water produced by photocatalytic nitrogen fixation is used for the growth of crops with nitrogen requirements.

7. The use of a Fe-based metal-organic framework / nitrogen-doped carbon solid-state photocatalyst according to claim 6, characterized in that, The crop is wheat.

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