Preparation and Application of ZIF-8 Derived Highly Dispersed Bimetallic Fe-Ni-N-C Materials

Abstract

The present invention discloses a preparation method of a ZIF-8-derived highly dispersed bimetallic Fe-Ni-N-C material. Using zinc nitrate, iron nitrate, and 2-methylimidazole as raw materials, combined with a methanol solution, the precursor Fe-ZIF-8 is synthesized. Then, metal Ni is added through a dual-solvent method to highly disperse the bimetallic Fe and Ni in the framework ZIF-8. The Fe-Ni-N-C material is used to activate PMS for the degradation of tetracycline in water. The characteristics of the present invention are as follows: The Fe-Ni-N-C material presents a regular rhombic dodecahedron structure with a large specific surface area; the ZIF-8 framework structure improves the loading effect of the bimetallic Fe and Ni, has high stability, and is environmentally friendly; the bimetallic cooperation is more likely to activate PMS, and the catalytic degradation performance is ideal; it has highly dispersed bimetallic active sites, can efficiently activate PMS for the degradation of tetracycline in water in small amounts, and reduces the consumption costs of PMS and the material itself.

Description

Technical Field

[0001] The present invention relates to a preparation method and application of a bimetallic Fe-Ni-N-C material in a highly dispersed state derived from ZIF, and particularly relates to a preparation method of an Fe-Ni-N-C material using ZIF-8 as a precursor and obtained by a space confinement method and calcination at different temperatures, and its application as a catalyst in the degradation of tetracycline, belonging to the technical field of catalytic materials. Background Art

[0002] With the rapid development of medical technology, drug pollution mainly based on tetracycline (TC) is increasing continuously. Such pollution has strong persistence, high stability, and is difficult to degrade, seriously threatening human health and the ecological environment. Advanced oxidation technologies (AOPs) are the most effective methods for treating organic polluted wastewater. By activating peroxymonosulfate (PMS) with transition metals, strong oxidants such as hydroxyl radicals ·OH or sulfate radicals SO4 ·− and some oxidants for direct electron transfer can rapidly and non-selectively oxidize and decompose most organic compounds in water and wastewater, completely decompose organic pollutants into CO2, H2O and some non-toxic inorganic salts, and have the advantages of low cost, high-efficiency activation and degradation, etc.

[0003] Fe and Ni are transition metals widely present in the natural environment, with rich sources, low cost and low biological toxicity. The activity of single Fe or Ni metal is relatively low, and the performance of a single metal site is limited, and it is often combined with other metals. Compared with single Fe or Ni centers, for bimetallic Fe and Ni, the redox reaction between the bimetallic components accelerates the circulation of Fe and Ni metal ions, not only enhancing their activation ability, but also improving the stability of the material. And Ni, as a siderophile element, can promote the interaction between metallic Fe and N, increase the content of active centers on the catalyst surface, greatly promote the adsorption and activation of PMS, and has better catalytic activity and stability. However, a large number of encapsulated active sites will be generated in bimetallic nanoparticles, and the internal active sites cannot participate in the activation reaction, resulting in a reduction in the metal sites for activating PMS and a decrease in the degradation efficiency of pollutants. Metal-organic frameworks (MOFs) are materials composed of metal clusters and organic ligands, with a high specific surface area and excellent degradation performance. Their unique spatial structure can effectively fix and disperse metal ions through the coordination bonds of organic ligands. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of a highly dispersed bimetallic Fe-Ni-N-C catalyst material, which has the characteristics of a small amount, high efficiency, environmental friendliness, high stability and excellent catalytic performance, and the Fe-Ni-N-C synergistic PMS activation technology achieves the efficient degradation of tetracycline drug pollution mainly based on antibiotics.

[0005] ZIF-8 is a MOF material with Zn 2+ as metal ions. The spatial confinement method can be used to effectively control the phenomenon of metal agglomeration. Its special pore structure can effectively increase the dispersion of Fe and Ni nanoparticles in the material, exposing more active sites on the surface. Abundant bimetallic active sites can effectively activate PMS even under the condition of low metal content, showing good catalytic degradation performance. Therefore, by designing highly dispersed Fe-Ni-N-C materials, the degradation performance of Fe-Ni-N-C prepared at different pyrolysis temperatures and synergistically activating PMS was analyzed.

[0006] To achieve the above objectives, the present invention adopts the following technical solutions:

[0007] I. Preparation of ZIF-8-derived highly dispersed bimetallic Fe-Ni-N-C materials

[0008] The preparation method of ZIF-8-derived highly dispersed bimetallic Fe-Ni-N-C materials includes the following steps:

[0009] (1) Mix the methanol solution containing iron salt and zinc salt with the methanol solution containing 2-methylimidazole, and let it stand and react at room temperature for 20 - 25 h. Use the ion exchange method to prepare the precursor Fe-doped ZIF-8. After centrifugation and vacuum drying, grind it thoroughly to obtain a pale yellow powder Fe-ZIF-8. Among them, the iron salt is ferric nitrate nonahydrate or ferrous sulfate heptahydrate; the zinc salt is zinc nitrate hexahydrate; the mass ratio of the iron salt to the zinc salt is 1:8 - 1:20; the mass ratio of the iron salt to 2-methylimidazole is 1:20 - 1:5.

[0010] (2) Drop the methanol solution containing nickel salt into the bimetallic polymerization agent solution of Fe-ZIF-8. After continuously ultrasonicating the mixed solution, stir it at room temperature for 2 - 3 h, then centrifuge, wash, dry, and grind to obtain bimetallic Fe-Ni-ZIF-8. Among them, the nickel salt is nickel nitrate hexahydrate, and the mass ratio of the nickel salt to Fe-ZIF-8 is 1:5 - 1:15. The methanol solution containing nickel salt is slowly added to the bimetallic polymerization agent solution of Fe-ZIF-8 at a dropping rate of 18 - 22 μl / min.

[0011] (3) Put Fe-Ni-ZIF-8 into a tubular furnace and calcine it at 800 - 1000 °C under an inert atmosphere to obtain Fe-Ni-N-C materials. The synthesis schematic diagram of Fe-Ni-N-C is as Figure 1 shown. The inert atmosphere is argon, the calcination time is 1 - 2.5 h, and the heating rate is 3 - 5 °C / min. In the Fe-Ni-N-C material, the iron content is 0.72 - 0.83%, and the nickel content is 0.31 - 0.38%.

[0012] II. Characterization of Fe-Ni-N-C Catalyst Material

[0013] 1. Morphology and Structure of Fe-Ni-N-C

[0014] Figure 2 (a), (b), and (c) are SEM images of Fe-Ni-N-C 800, Fe-Ni-N-C 900, and Fe-Ni-N-C 1000, respectively; Figure 2 (d) is the TEM image of Fe-Ni-N-C 900; Figure 2 (e) is the HRTEM image of Fe-Ni-N-C 900; Figure 2 (f-k) are the elemental mapping diagrams of the total elements and Fe, Ni, N, and C.

[0015] Figure 2 (a-c)'s SEM images show the morphologies of the products Fe-Ni-N-C after calcination at different temperatures (800 °C, 900 °C, 1000 °C). It can be observed that the Fe-Ni-N-C products still maintained the polyhedral structure of the precursor ZIF-8 after calcination. After high-temperature heat treatment, a large number of organic substances decomposed, making the surface rough and the morphological structure collapse inward. However, with the increase of the pyrolysis temperature, the structure collapse became more obvious, and obvious aggregation formed at 1000 °C. Comparing the images of Fe-Ni-N-C 800 ( Figure 2 (a)) and Fe-Ni-N-C 1000 ( Figure 2 (c)), it can be observed that Fe-Ni-N-C 900 ( Figure 2 (b)) showed a moderate structure collapse.

[0016] To observe the morphology of Fe-Ni-N-C in more detail, TEM and HRTEM were used to further study the microstructure of Fe-Ni-N-C 900. It can be clearly seen from Figure 2 (d) and Figure 2 (e) that Fe-Ni-N-C retained the typical regular ZIF-8 rhombic dodecahedron structure, and no metal aggregation was observed. In addition, it can be seen from Figure 2 (f-k) that the bimetals in Fe-Ni-N-C were evenly dispersed with a small content, and no metal nanoparticles were detected. The ICP-OES detection results showed (Table 1) that the Fe content was only 0.83% and the Ni content was 0.33%. Generally speaking, Fe-Ni-N-C showed a trend of highly dispersed bimetallic active sites. Figure 2 Overall, Fe-Ni-N-C showed a trend of highly dispersed bimetallic active sites.

[0017]

[0018] 2. XRD patterns of ZIF-8, Fe-ZIF-8, and Fe-Ni-N-C

[0019] XRD patterns of ZIF-8, Fe-ZIF-8, and Fe-Ni-N-C are shown in Figure 3 Figure. Compared with the diffraction pattern of ZIF-8, the diffraction peaks of each crystal plane are clear, and no other impurity peaks are observed, indicating that the addition method of the bimetal does not change the original crystal structure of ZIF-8 ( Figure 3 (a)). It can be seen from Figure 3 (b) that the XRD pattern of Fe-Ni-N-C shows that the crystal plane peak (002) of graphite carbon appears near the range of 26.5°, which is a typical characteristic peak of the porous carbon material obtained by carbonizing the material. At the same time, no characteristic peaks of other metal phases are detected, indicating that the bimetal particles are evenly dispersed and no large-area agglomeration phenomenon occurs. However, at 1000 °C, a clear diffraction peak appears at 44.4°, and this diffraction peak is exactly the diffraction peak corresponding to the nickel element, which proves that when the addition amount of nickel nitrate increases or the calcination temperature is too high during the preparation process, the loaded nickel element tends to cluster and nickel nanoparticles appear.

[0020] 3. XPS spectrum of Fe-Ni-N-C

[0021] Figure 4 Figure is the XPS spectrum of Fe-Ni-N-C, where (a) is the XPS spectrum of Fe 2p, (b) is the XPS spectrum of Ni 2p, and (c) is the XPS spectrum of N 1s. Figure 4 (a) and Figure 4 (b) have unclear fitting peaks of Fe 2p and Ni 2p because the contents of Fe and Ni in Fe-Ni-N-C are relatively low. Figure 4 (c) shows that there are 4 typical nitrogen species in the Fe-Ni-N-C 900 material, and the binding energies on the N spectrum are 398.8 eV, 4000.4 eV, 401.3 eV, and 403.3 eV respectively, corresponding to four typical nitrogen types in nitrogen-doped carbon: pyridine nitrogen, pyrrole nitrogen, graphitic nitrogen, and nitrogen oxides. The results show that the Fe-Ni-N-C material calcined at 900 °C has a higher nitrogen species than the materials calcined at 800 °C and 1000 °C, and the rich nitrogen species types in the material can greatly improve the catalytic degradation performance of the material.

[0022] 4. FT-IR spectrum of Fe-Ni-N-C

[0023] Figure 5FT-IR spectra of Fe-Ni-ZIF-8 and Fe-Ni-N-C 800, Fe-Ni-N-C 900, and Fe-Ni-N-C 1000. The prepared catalysts show peaks at 3432 cm -1 and 1384 cm -1 which represent the stretching and deformation vibrations of O-H, respectively. The absorption peak near 3000 cm -1 is attributed to the stretching vibration of the C-H bond in 2-methylimidazole. Due to the vibrations of C=C and C=N bonds, a peak appears near 1500 cm -1 for the Fe-Ni-N-C catalyst, indicating the successful doping of nitrogen into carbon.

[0024] 5. BET spectra of Fe-Ni-N-C

[0025] To evaluate the specific surface area, pore volume, and pore size of the catalysts, the catalyst materials were analyzed using BET testing ( Figure 6 ). According to the IUPAC classification, the Fe-Ni-N-C material exhibits a typical type-IV isotherm with an H3 hysteresis loop. The pore types reflected include wedge-shaped structure pores, which match well with the hollow rhombic dodecahedron structure of ZIF-8, reflecting the characteristics of typical microporous materials. Table 2 shows that both Fe-Ni-N-C and Fe-Ni-ZIF-8 have large specific surface areas and pore structures. Combining SEM and XRD spectrum analysis, it is found that the bimetallic Fe and Ni in the surface materials are uniformly distributed and highly dispersed, greatly promoting the transfer and diffusion of pollutants. This result provides more catalytic sites for Fe-Ni-N-C to catalytically activate PMS, thereby improving the degradation efficiency of TC.

[0026]

[0027] III. Catalytic performance of Fe-Ni-N-C for tetracycline

[0028] 1. Study the effect of different catalyst systems on the removal rate of tetracycline

[0029] Change the calcination temperature of the prepared Fe-Ni-N-C catalyst material, and study the effect of Fe-Ni-N-C materials at different temperatures and the synergistic effect of PMS on the removal rate of TC.

[0030] First, add 100 mL of tetracycline solution with a concentration of 20 mg / L, 30 mg of Fe-Ni-N-C materials at different temperatures (including Fe-Ni-N-C 800, Fe-Ni-N-C 900, Fe-Ni-N-C 1000), and single-metal Fe-N-C and Ni-N-C materials calcined at 900 °C into the reaction vessel. Add 30 mg of PMS or not add PMS. Use a 0.22 μm filtered syringe to take 2 mL of the reaction solution every 5 minutes and transfer it to a quartz cuvette. Measure the absorbance of the reaction solution at 354 nm with a UV-visible spectrophotometer (UV-2000), and calculate the content C of tetracycline through the tetracycline standard curve.

[0031] Calculate the tetracycline removal rate according to the following formula:

[0032] Tetracycline removal rate = 1 - Ct / C0

[0033] In the formula, C t is the tetracycline concentration at time t of the reaction, and C0 is the initial content of tetracycline.

[0034] Under different catalyst systems, the graph of the tetracycline removal rate changing with time is shown in Figure 7 Figure (a). As can be seen from Figure 7 Figure (a), when injecting bimetallic Fe-Ni-N-C 800, Fe-Ni-N-C 900, and Fe-Ni-N-C 1000 with a concentration of 0.30 g / L respectively, the TC degradation rates within 30 minutes are 62.62%, 71.53%, and 74.16% respectively. At this time, the effect of removing TC is limited. When adding 0.30 g / L of PMS, the degradation rate of TC increases significantly, reaching 81.92%, 95.71%, and 85.39% respectively. PMS is effectively activated, the degradation efficiency of TC is significantly improved, and the efficiency of Fe-Ni-N-C 900 / PMS in treating TC is the highest. When adding single-metal Fe-N-C 900 (0.30 g / L) and Ni-N-C 900 (0.30 g / L) materials, the degradation efficiencies of TC within 30 minutes are 71.70% and 65.43% respectively. After synergizing with PMS, the removal rates of TC within 30 minutes are 80.42% and 73.64% respectively. The overall degradation efficiency is significantly lower than that of the bimetallic Fe-Ni-N-C 900 / PMS system. The entire system shows first-order reaction kinetics ( Figure 7 Figure (b)), among which the Fe-Ni-N-C synergistic PMS system (k = 0.086 min -1 ) shows better performance than the Fe-Ni-N-C 900 / PMS system (k = 0.050 min -1), and the degradation performance of the Ni-N-C 900 / PMS system (k = 0.040 min -1 ).

[0035] 2. Study the effect of the dosage of Fe-Ni-N-C on the removal rate of tetracycline

[0036] First, add 100 mL of tetracycline solution with a concentration of 20 mg / L to the reaction vessel. Then, add 15 mg, 20 mg, 25 mg, 30 mg, and 35 mg of the Fe-Ni-N-C catalyst material prepared in Example 1 to the reaction vessel, and then add 30 mg of PMS. Every 5 minutes, take 2 mL of the reaction solution with a 0.22 μm filtered syringe and transfer it to a quartz cuvette. Measure the absorbance of the reaction solution at 354 nm with a UV spectrophotometer (UV-2000). Calculate the tetracycline content C through the tetracycline standard curve, and calculate the tetracycline removal rate according to the tetracycline removal rate formula.

[0037] The graph of the change in the tetracycline removal rate over time obtained at different dosages of the Fe-Ni-N-C catalyst material is shown in Figure 8 . From Figure 8 it can be seen that as the dosage of Fe-Ni-N-C increases, the removal rate of tetracycline gradually increases. When the concentration of the Fe-Ni-N-C material increases from 0.15 g / L to 0.35 g / L, the degradation efficiency of TC increases from 77.7% to 95.5%. When the addition amount of Fe-Ni-N-C is 0.35 g / L, the removal rate of TC is 93.9%. At this dosage, the degradation efficiency of TC hardly changes within 30 minutes, which is due to the fact that the highly dispersed bimetallic active sites are not fully utilized. To save materials, the optimal dosage of Fe-Ni-N-C is determined to be 0.30 g / L.

[0038] 3. Study the effect of the concentration of PMS on the removal rate of tetracycline

[0039] First, add 100 mL of tetracycline solution with a concentration of 20 mg / L to the reaction vessel. Add 30 mg of the Fe-Ni-N-C catalyst material prepared in Example 1 to the reaction vessel, and then add 15 mg, 20 mg, 25 mg, 30 mg, and 35 mg of PMS respectively. Every 5 minutes, take 2 mL of the reaction solution with a 0.22 μm filtered syringe and transfer it to a quartz cuvette. Measure the absorbance of the reaction solution at 354 nm with a UV spectrophotometer (UV-2000). Calculate the tetracycline content C through the tetracycline standard curve, and calculate the tetracycline removal rate according to the tetracycline removal rate formula.

[0040] The graphs showing the change of tetracycline removal rate with time at different PMS concentrations are shown in Figure 9 . It can be seen from Figure 9 that with the increase of PMS dosage, the removal rate of tetracycline by Fe-Ni-N-C gradually increases. When the dosage is 0.15 g / L - 0.35 g / L, the degradation efficiencies within 30 min are 77.75%, 86.87%, 94.93%, 91.32% and 93.97% respectively. The results show that the degradation efficiency is above 85.03%. Within the tested PMS dosage range, the degradation efficiency is the highest at 0.25 g / L, reaching 94.93%. Thus, the optimal dosage of PMS is determined to be 0.25 g / L.

[0041] 4. Study on the effect of the initial pH value of the reaction system on the tetracycline removal rate

[0042] First, add 100 mL of tetracycline solution with a concentration of 20 mg / L to the reaction vessel, add 30 mg of the Fe-Ni-N-C catalyst material prepared in Example 1 and 25 mg of PMS to the reaction vessel, and finally adjust the initial pH value of the solution to 3, 5, 7, 9, and 11 respectively with hydrochloric acid or sodium hydroxide. Every 5 min, take 2 ml of the reaction solution with a 0.22 μm filter syringe and transfer it to a quartz cuvette, measure the absorbance of the reaction solution at 354 nm with a UV spectrophotometer (UV-2000), calculate the tetracycline content C through the tetracycline standard curve, and calculate the tetracycline removal rate according to the tetracycline removal rate formula.

[0043] The graphs showing the change of tetracycline removal rate with time at different initial pH values are shown in Figure 10 . It can be seen from Figure 10 that when the initial pH value is 7, the Fe-Ni-N-C catalyst material has the highest tetracycline degradation performance and can remove about 95.12% of tetracycline within 30 min; as the initial pH value gradually becomes acidic, the tetracycline removal rates are 91.95% and 90.76%, and when the initial pH values are 9 and 11, the tetracycline removal rates are 85.72% and 84.78%. Generally, Fe-Ni-N-C has high catalytic degradation performance in a wide pH range (3 - 11).

[0044] 5. Study on the effect of inorganic anions in water on the tetracycline removal rate

[0045] First, add 100 ml of tetracycline solution with a concentration of 20 mg / L to the reaction vessel, add 30 mg of the Fe-Ni-N-C catalyst material prepared in Example 1 and 25 mg of PMS to the reaction vessel, and then add Cl - , NO3 -, HCO3 - The reagent with an initial pH value adjusted to 7. Every 5 min, 2 ml of the reaction solution was taken with a 0.22 μm filtered syringe and transferred to a quartz cuvette, and the absorbance of the reaction solution at 354 nm was measured with a UV spectrophotometer (UV - 2000). The tetracycline content C was calculated through the tetracycline standard curve, and the tetracycline removal rate was calculated according to the tetracycline removal rate formula.

[0046] As Figure 11 shown, after 30 min, the degradation efficiencies of Cl - , NO3 - , HCO3 - were 92.03%, 90.81%, and 94.27% respectively. The results showed that Cl - , NO3 - and HCO3 - had no effect on the degradation of TC in the Fe - Ni - N - C / PMS system. This was mainly because the Fe - Ni - N - C / PMS system was different from the traditional activated PMS system, and the strong oxidative free radicals it generated were not limited to the traditional SO4 ·− and ·OH. This indicated that the Fe - Ni - N - C / PMS system was more suitable for generating free radicals in actual water than the traditional PMS system.

[0047] 6. Study the effect of the initial concentration of TC on the tetracycline removal rate

[0048] First, 100 mL of tetracycline solutions with concentrations of 10 mg / L, 20 mg / L, and 30 mg / L were respectively added to the reaction vessels. 30 mg of the Fe - Ni - N - C catalyst material prepared in Example 1 and 25 mg of PMS were added to the reaction vessels, and the initial pH value was adjusted to 7. Every 5 min, 2 mL of the reaction solution was taken with a 0.22 μm filtered syringe and transferred to a quartz cuvette, and the absorbance of the reaction solution at 354 nm was measured with a UV spectrophotometer (UV - 2000). The tetracycline content C was calculated through the tetracycline standard curve, and the tetracycline removal rate was calculated according to the tetracycline removal rate formula.

[0049] As Figure 12As shown in the figure, by adding 0.3 g / L Fe-Ni-N-C and 0.25 g / L PMS, the effect of the initial concentration (10 - 30 mg / L) on the degradation of TC was investigated. When the TC concentration was 10 mg / L, 20 mg / L, and 30 mg / L, the removal rates of TC within 30 min were 95.13%, 94.67%, and 77.52%, respectively. This phenomenon may be due to the limited number of active catalytic sites in a certain amount of catalyst material and PMS. When the TC concentration is too high, the molecules compete for the active groups, resulting in a decrease in the degradation efficiency.

[0050] 7. Study the effect of different water bodies on the removal rate of tetracycline

[0051] First, prepare tetracycline solutions with a concentration of 20 mg / L in different water bodies (deionized water, tap water, lake water). Take out 100 mL, and then add 30 mg of the Fe-Ni-N-C catalyst material prepared in Example 1 and 25 mg of PMS to the reaction vessel. Adjust the initial pH value to 7. Every 5 min, take 2 ml of the reaction solution with a 0.22 μm filter syringe and transfer it to a quartz cuvette. Measure the absorbance of the reaction solution at 354 nm with a UV-visible spectrophotometer (UV-2000). Calculate the tetracycline content C through the tetracycline standard curve, and calculate the tetracycline removal rate according to the tetracycline removal rate formula.

[0052] All of the above degradation experiments were carried out in deionized water. However, the effluent quality of different water bodies is complex. Therefore, it is of great practical significance to study the TC removal efficiency of the Fe-Ni-N-C / PMS system under different water body backgrounds. Tap water and lake water were selected for comparison with the deionized water environment to investigate the treatment effect of Fe-Ni-N-C / PMS on TC within 30 min. The results are as Figure 13 shown. The inhibitory effect of tap water and lake water on TC is relatively small. The results show that the Fe-Ni-N-C / PMS system remains stable in various water body environments and is suitable for practical applications.

[0053] IV. Recycling stability of Fe-Ni-N-C

[0054] 30 mg of the Fe-Ni-N-C prepared in Example 1 and 25 mg of PMS were added to 100 mL of a tetracycline solution with a concentration of 20 mg / L, and magnetically stirred for 30 min. After the reaction, the Fe-Ni-N-C catalyst material was recovered by centrifugation, then washed repeatedly with deionized water three times, and after thorough washing, it was used to catalytically degrade tetracycline together with peroxymonosulfate. The Fe-Ni-N-C catalyst material was recycled 5 times, and the removal rate of tetracycline at different reaction times (0 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min) was recorded in detail each time.

[0055] The graphs of the change in the removal rate of tetracycline over time obtained at different numbers of cycles are shown in Figure 14 . From Figure 14 Figure (a), it can be seen that after 5 cycles of use, the removal performance of the Fe-Ni-N-C catalyst material prepared in Example 1 for tetracycline can still reach 83.11%. This proves that the Fe-Ni-N-C catalyst material has good stability and reusability. Figure 14 Figures (b-d) are the XPS spectra of the Fe-Ni-N-C material before and after degradation. Due to the low content of the bimetals Fe and Ni in Fe-Ni-N-C, serrated lines appear in the XPS spectra ( Figure 14 Figure (b) and Figure 14 Figure (c)), making it difficult to accurately fit. From Figure 14 Figure (d), it can be seen that after degradation, Fe-Ni-N-C still contains four typical nitrogen species: pyridine nitrogen, pyrrole nitrogen, graphitic nitrogen, and nitrogen oxides, and the XPS spectra before and after the reaction change little, indicating that the Fe-Ni-N-C material has good cycle stability.

[0056] V. Degradation Mechanism and Path of Fe-Ni-N-C / PMS

[0057] 1. EPR Spectra and Identification of Main Free Radicals in the Fe-Ni-N-C / PMS Catalytic Reaction

[0058] 2,6-Dimethylpyridine, N-oxide (DMPO) and 4-Amino-2,2,6,6-tetramethylpiperidine (TEMP) were used as scavengers for EPR spectra. These scavengers were used to detect and identify paramagnetic substances or free radicals generated during the reaction. In Figure 15 Figure (a), the presence of the DMPO-O2 adduct signal ( · O2 − signal) can be clearly seen, and this trend is consistent with that observed in Figure 15 Figure (c). The DMPO-OH adduct (·OH signal) and DMPO-SO4 (SO4 ·−adduct signal, and a clear signal peak appeared after 5 min. In addition, when TEMP was used as a trapping agent, EPR also exhibited the characteristic 1:1:1 triplet signal of TEMPO ( 1 O2 signal), and the signal intensity was significantly higher than that of the three free radicals ( Figure 15 (b)). The results showed that ·OH, SO4 ·− , · O2 − and 1 O2 reactive species were present in the reaction system. This indicated that the Fe-Ni-N-C material activated PMS through two different pathways, and the reason for the appearance of these free radical reactive oxidation species might be due to the following reactions occurring in the system:

[0059]

[0060] Figure 15 (d) was the free radical quenching experiment. When tert-butanol (TBA) was added, the degradation efficiency of TC decreased from 95.07% to 71.23%. With the addition of methanol (MeOH), the removal rate decreased to 68.07%. This indicated that compared with the past ·OH free radicals, SO4 ·− free radicals played a dominant role in the degradation of TC. The addition of p-benzoquinone (p-BQ) led to a decrease in the removal rate of TC by approximately 18.37% within 30 min, indicating the presence of · O2 − in the system. Similarly, when furfuryl alcohol (FFA) was added, the degradation of TC was also significantly inhibited, and the degradation efficiency was 63.14%, a decrease of approximately 31.93%. This result indicated that during the overall degradation reaction of TC, 1 O2 also played a dominant role.

[0061] 2. Degradation pathway of TC

[0062] Combined with the analysis of relevant literature on the TC degradation process, two possible degradation pathways for the degradation of TC by the Fe-Ni-N-C / PMS system were speculated. As Figure 16As shown, amino groups, ketones, aromatic rings, and double bonds are the active sites for degrading TC. First, compounds I and II are formed as two main intermediates with m / z values of 461 (produced by the hydroxylation of TC) and 416 (produced by the deamination of TC), respectively. A product with m / z = 461 loses 2 n-methyl groups, 1 methyl group, 1 hydroxyl group, and 1 amino group to form an intermediate with m / z = 389, and then the product with m / z = 389 undergoes decarbonylation hydroxylation to generate an intermediate product with m / z = 339. The hydroxyl group breaks again to lose the hydroxyl group, forming m / z = 272. Due to the loss of the hydroxyl group at m / z = 416 in Path II, a compound with m / z = 364 is generated, forming the cleavage site of the double bond on aromatic ring B, and compounds VII and VIII are formed. After a period of time, most of the TC is degraded into various intermediates, some of which are further degraded and oxidized, and finally degraded into small molecules of CO2, H2O, and inorganic ions.

[0063] 3. Reaction mechanism of the Fe-Ni-N-C / PMS system for catalytic degradation of TC

[0064] As a carrier material with a large specific surface area, ZIF-8 helps with the transfer of electrons during the reaction and acts as a bridge for the transfer of bimetallic ions. Based on the above EPR tests, radical quenching experiments, and XPS analysis, the possible activation mechanism in the Fe-Ni-N-C / PMS system was deduced. As Figure 17 shown, Fe-Ni-N-C combines with PMS to produce dissolved Fe 2+ , Ni 2+ . Then it further reacts with PMS to generate Fe 3+ and Ni 3+ (Reaction formulas (5-7)-(5-9)). In addition, Fe 2+ / Ni 2+ and Fe 3+ / Ni 3+ are interconverted under the action of HSO5 - (Reaction formulas (5-10)-(5-11)). At the same time, Ni 2+ and Fe 2+ undergo redox cycling, effectively improving the treatment efficiency of catalytic oxidation and reducing the loss of elements (Reaction formula (5-12)). These three different valence states of bimetals jointly activate PMS to produce SO4 ·− and ·OH. ·− and ·OH.

[0065] According to the results of the radical quenching experiment and the EPR spectrum, it was found that Fe-Ni-N-C can cause the self-decomposition of PMS to generate 1 O2, and oxidize H2O to generate H2O2 (Reaction formula (5-13)), and then generate · O2 − through a series of reactions.and 1 These free radicals generated during the reaction of O2 (Reaction (5-14)-(5-17)) destroy the structure and chemical bonds of TC, and finally further decompose and mineralize it into CO2 and H2O (Reaction (5-18)).

[0066]

[0067] In summary, the present invention uses ZIF-8 as a precursor and successfully synthesizes a highly dispersed bimetallic Fe-Ni-N-C material through a space confinement strategy, a dual-solvent method, and a calcination method. Fe-Ni-N-C is a highly dispersed bimetallic nitrogen-carbon material. Different from nanomaterials, even when the metal content is very low, there are still a large number of uniformly dispersed active sites exposed in this material to activate PMS. When the iron and nickel contents are only 0.83% and 0.33% respectively, the degradation efficiency can still reach more than 95% within 30 minutes, and it can efficiently degrade organic pollutants in water with a small amount. It is environmentally friendly, simple to prepare, and cost-saving. In terms of the mechanism of degrading tetracycline, since the material is in a highly dispersed state and the bimetallic Fe and Ni do not agglomerate, that is, the cyclic conversion rate of Fe and Ni ions will be faster to synergistically activate PMS, and the removal effect of tetracycline is also more efficient. Different from traditional PMS activation technologies, the Fe-Ni-N-C material activates PMS through two different pathways. During the degradation of TC, not only a large amount of ·OH, SO4 ·− , but also abundant · O2 − and 1 O2 can be generated, proving that Fe-Ni-N-C / PMS oxidizes pollutants through two activation pathways: free radical oxidation and non-radical oxidation. The SO4 ·− and 1 O2 generated in the Fe-Ni-N-C / PMS system play a leading role in the whole system during the degradation reaction of TC. The present invention provides an important reference value for exploring new and efficient pollutant degradation materials and mechanisms, and also has high application potential.

[0068] Advantages of the present invention over the prior art:

[0069] (1) The Fe-Ni-N-C catalyst material prepared by the present invention presents a rhombic dodecahedron structure, with highly dispersed metal elements, a large specific surface area (367 m 2 / g) and uniformly distributed and highly dispersed active sites, and has a strong catalytic degradation ability for TC pollutants;

[0070] (2) The Fe-Ni-N-C catalyst material prepared by the present invention, a carbon-based bimetallic material obtained by calcining ZIF-8 as a precursor, has a framework structure that improves the dispersion and loading effect of Fe and Ni bimetals, with high stability, low metal dosage, and low leaching of toxic metals, being environmentally friendly;

[0071] (3) The Fe-Ni-N-C catalyst material prepared by the present invention is extremely easy to activate PMS due to the synergistic effect of the bimetals, with ideal catalytic performance, and the removal rate of tetracycline can reach more than 95%. After repeating the degradation of tetracycline 5 to 10 times, it still has a high degradation efficiency;

[0072] (4) The Fe-Ni-N-C catalyst material prepared by the present invention has highly dispersed bimetallic active sites, can synergistically remove tetracycline pollution in water efficiently with a small amount, and can reduce the consumption costs of PMS and the material itself;

[0073] (5) The method for preparing the Fe-Ni-N-C catalyst material provided by the present invention can prepare the Fe-Ni-N-C catalyst material through the ion exchange method and the double solvent method. The method is simple, easy to operate, has low raw material costs, is suitable for industrial production, and is convenient for wide promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 is a schematic diagram of the synthesis of Fe-Ni-N-C;

[0075] Figure 2 (a), (b), (c) are SEM images of Fe-Ni-N-C 800, Fe-Ni-N-C 900, and Fe-Ni-N-C 1000 respectively; Figure 2 (d) is a TEM image of Fe-Ni-N-C 900; Figure 2 (e) is a HRTEM image of Fe-Ni-N-C 900; Figure 2 (f-k) are elemental mapping diagrams of the total elements and Fe, Ni, N, and C;

[0076] Figure 3 (a), (b) are XRD spectra of ZIF-8, Fe-ZIF-8, and Fe-Ni-N-C respectively;

[0077] Figure 4 (a), (b), (c) are Fe 2p, Ni 2p, and N 1s spectra of Fe-Ni-N-C respectively;

[0078] Figure 5 is the FT-IR spectrum of ZIF-8, Fe-ZIF-8, and Fe-Ni-N-C;

[0079] Figure 6 (a) and (b) are the N₂ adsorption curve and pore size distribution diagram of Fe-Ni-N-C, respectively;

[0080] Figure 7 (a) shows the influence of different catalyst systems on the TC removal rate; Figure 7 (b) shows the reaction kinetics of Fe-Ni-N-C, Fe-N-C, and Ni-N-C synergistic with PMS;

[0081] Figure 8 shows the influence of different dosages of Fe-Ni-N-C materials on the TC removal rate;

[0082] Figure 9 shows the influence of different dosages of PMS on the TC removal rate;

[0083] Figure 10 shows the influence of different initial pH values on the TC removal rate;

[0084] Figure 11 shows the influence of the presence of different anions in water on the TC removal rate;

[0085] Figure 12 shows the influence of different initial concentrations of TC on the TC removal rate;

[0086] Figure 13 shows the influence of different water environments on the TC removal rate;

[0087] Figure 14 (a) is the 5-cycle degradation test of Fe-Ni-N-C; (b - d) are the Fe 2p, Ni 2p, and N 1s spectra of Fe-Ni-N-C before and after the reaction

[0088] Figure 15 (a - c) are the EPR spectrograms of Fe-Ni-N-C / PMS; Figure 15 (d) is the free radical quenching experiment;

[0089] Figure 16 is the degradation pathway of TC;

[0090] Figure 17 is the mechanism of PMS degradation of TC activated by the Fe-Ni-N-C catalyst material. Specific Embodiments

[0091] The present invention will be specifically introduced below in conjunction with the accompanying drawings and specific embodiments. The embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0092] Example 1

[0093] 1.612 g of zinc nitrate hexahydrate and 0.086 g of iron(III) nitrate nonahydrate were added to 80 mL of methanol solution, denoted as Solution A. Then, 3.619 g of 2-methylimidazole was added to 80 mL of methanol solution, denoted as Solution B. Solution A was slowly poured into Solution B, ultrasonicated for 5 min first, then magnetically stirred for 30 min, and left to react at room temperature for 24 h. After the reaction was completed, the supernatant in the mixed solution was poured off, the precipitate solution was placed in a 50 mL centrifuge tube and centrifuged at 8000 rpm for 7 min. The supernatant was poured off, and the solid obtained by centrifugation was washed with methanol solution and centrifuged. The operation was repeated 5 times. The final centrifuged precipitate was dried in vacuo at 60 °C for 12 h, ground in an agate mortar for 5 min, and light yellow Fe-ZIF-8 powder could be obtained.

[0094] 0.983 g of Fe-ZIF-8 powder was weighed and poured into 40 mL of n-hexane, ultrasonicated for 40 min, denoted as Solution C. 0.057 g of nickel nitrate hexahydrate was dissolved in 60 mL of methanol solution, and the methanol solution of nickel nitrate hexahydrate was slowly added to Solution C at a dropping rate of 20 μL / min. The mixed solution was continuously ultrasonicated for 20 min and magnetically stirred at room temperature for 2 h. It was placed in a 50 mL centrifuge tube and centrifuged at 8000 rpm for 10 min. The supernatant was poured off, and the solid obtained by centrifugation was washed with methanol solution and centrifuged. The operation was repeated 5 times. The final centrifuged precipitate was dried in vacuo at 60 °C for 12 h, ground in an agate mortar for 5 min, and off-white Fe-Ni-ZIF-8 powder could be obtained. It was spread out in a corundum boat, placed in a tubular furnace, heated from room temperature to 900 °C at a heating rate of 5 °C / min in an argon atmosphere, and maintained for 1 h to obtain the final black powder-like Fe-Ni-N-C 900 catalyst material with an Fe content of only 0.83% and a Ni content of 0.33%.

[0095] 30 mg of the Fe-Ni-N-C 900 catalyst material and 30 mg of PMS were weighed and poured into 100 mL of an aqueous solution containing 20 mg / L of TC. The mixed solution was magnetically stirred for 30 min. After detection, the degradation efficiency of TC could reach more than 95%.

[0096] The Fe-Ni-N-C 900 catalyst material was recovered by centrifugal separation method, then washed repeatedly with deionized water 3 times. After thorough washing, it was used to catalyze the degradation of TC together with PMS. The cycle was repeated 5 times. After detection, the TC removal rate was still above 80%.

[0097] Example 2

[0098] The difference between this example and Example 1 is that when preparing the Fe-Ni-N-C material, the calcination temperature is changed to 800 °C, and the rest is exactly the same as in Example 1, obtaining Fe-Ni-N-C 800.

[0099] Weigh 30 mg of the Fe-Ni-N-C 800 catalyst material and 30 mg of PMS and pour them into 100 ml of an aqueous solution containing 20 mg / L of TC. Stir the mixed solution magnetically for 30 min. After testing, the degradation efficiency of TC can reach over 83% - 90%.

[0100] The Fe-Ni-N-C 800 catalyst material is recovered by centrifugal separation, then washed repeatedly with deionized water 3 times. After thorough washing, it is used to catalytically degrade TC together with PMS. After 5 cycles, after testing, the removal rate of TC is above 76%.

[0101] Example 3

[0102] The difference between this example and Example 1 is that when preparing the Fe-Ni-N-C material, the calcination temperature is changed to 1000 °C, and the rest is exactly the same as in Example 1, obtaining Fe-Ni-N-C 1000.

[0103] Weigh 30 mg of the Fe-Ni-N-C 1000 catalyst material and 30 mg of PMS and pour them into 100 mL of an aqueous solution containing 20 mg / L of TC. Stir the mixed solution magnetically for 30 min. After testing, the degradation efficiency of TC reaches above 85%.

[0104] The Fe-Ni-N-C 1000 catalyst material is recovered by centrifugal separation, then washed repeatedly with deionized water 3 times. After thorough washing, it is used to catalytically degrade TC together with PMS. After 5 cycles, after testing, the removal rate of TC is above 80%.

[0105] Example 4

[0106] The difference between this example and Example 1 is that when preparing the precursor Fe-ZIF-8, the dosage of ferric nitrate nonahydrate is doubled (i.e., the amount of ferric nitrate nonahydrate added is 0.172 g). The rest is exactly the same as in Example 1.

[0107] Weigh 30 mg of the Fe-Ni-N-C 900 catalyst material and 30 mg of PMS and pour them into 100 mL of an aqueous solution containing 20 mg / L of TC. Stir the mixed solution magnetically for 30 min. After testing, the degradation efficiency of TC reaches above 87%.

[0108] The Fe-Ni-N-C 900 catalyst material was recovered by centrifugal separation method, then washed repeatedly with deionized water three times. After thorough washing, it was used to catalytically degrade TC together with PMS for 5 cycles. After detection, the removal rate of TC was above 80%.

[0109] Example 5

[0110] The difference between this example and Example 1 is that when preparing the precursor Fe-ZIF-8, the dosage of nickel nitrate hexahydrate was doubled (i.e., the amount of nickel nitrate hexahydrate added was 0.114 g), and the rest was exactly the same as in Example 1.

[0111] 30 mg of Fe-Ni-N-C 900 catalyst material and 30 mg of PMS were weighed and poured into 100 mL of an aqueous solution containing 20 mg / L of TC. The mixed solution was magnetically stirred for 30 min. After detection, the degradation efficiency of TC reached above 89%.

[0112] The Fe-Ni-N-C 900 catalyst material was recovered by centrifugal separation method, then washed repeatedly with deionized water three times. After thorough washing, it was used to catalytically degrade TC together with PMS for 5 cycles. After detection, the removal rate of TC was above 82%.

Claims

1. Preparation method of ZIF-8-derived highly dispersed bimetallic Fe-Ni-N-C material, comprising the following steps: (1) Mix a methanol solution containing an iron salt and a zinc salt with a methanol solution containing 2-methylimidazole, and let it stand and react at room temperature for 20-25 h. Use the ion exchange method to prepare the precursor Fe-doped ZIF-8. After centrifugation and vacuum drying, grind it thoroughly to obtain a pale yellow powder Fe-ZIF-8; (2) Drop a methanol solution containing a nickel salt into the n-hexane solution of Fe-ZIF-8. After continuously ultrasonifying the mixed solution, stir it at room temperature for 2-3 h, then centrifuge, wash, dry, and grind to obtain bimetallic Fe-Ni-ZIF-8; (3) Put Fe-Ni-ZIF-8 into a tubular furnace and calcine it at 800-1000 °C under an inert atmosphere to obtain Fe-Ni-N-C material; in the Fe-Ni-N-C material, the iron content is 0.72-0.83%, and the nickel content is 0.31-0.38%.

2. The preparation method of the ZIF-8-derived highly dispersed bimetallic Fe-Ni-N-C material according to claim 1, characterized in that: In step (1), the iron salt is ferric nitrate nonahydrate or ferrous sulfate heptahydrate; the zinc salt is zinc nitrate hexahydrate; the mass ratio of the iron salt to the zinc salt is 1:8-1:20; the mass ratio of the iron salt to 2-methylimidazole is 1:20-1:

50.

3. The preparation method of the ZIF-8-derived highly dispersed bimetallic Fe-Ni-N-C material according to claim 1, characterized in that: In step (2), the nickel salt is nickel nitrate hexahydrate, and the mass ratio of the nickel salt to Fe-ZIF-8 is 1:5-1:

15.

4. The preparation method of the ZIF-8-derived highly dispersed bimetallic Fe-Ni-N-C material according to claim 1, wherein: In step (2), the methanol solution containing the nickel salt is slowly added to the bimetallic polymerization agent solution of Fe-ZIF-8 at a dropping rate of 18-22 μl / min.

5. The preparation method of the ZIF-8-derived highly dispersed bimetallic Fe-Ni-N-C material according to claim 1, wherein: In step (3), the inert atmosphere is argon, the calcination time is 1-2.5 h, and the heating rate is 3-5 °C / min.

6. Application of the Fe-Ni-N-C material prepared by the method according to claim 1 as a catalyst in the catalytic degradation of tetracycline.

7. Use of the Fe-Ni-N-C material according to claim 6 as a catalyst in the catalytic degradation of tetracycline, characterized in that: Add the Fe-Ni-N-C material and peroxymonosulfate to the tetracycline solution, and use the Fe-Ni-N-C material to activate peroxymonosulfate to degrade tetracycline.

8. Use of the Fe-Ni-N-C material according to claim 7 as a catalyst in the catalytic degradation of tetracycline, characterized in that: The dosage of the Fe-Ni-N-C material is 0.20-0.30 g / L, the dosage of peroxymonosulfate is 0.20-0.25 g / L, and the pH of the tetracycline solution is 3-11.

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