Monatomic iron catalyst for curvature regulation and control of small-diameter carbon nanotubes as well as preparation method and application of monatomic iron catalyst

Abstract

The invention relates to the technical field of catalysis, and discloses a monatomic iron catalyst for curvature regulation and control of a small-diameter carbon nanotube as well as a preparation method and application of the monatomic iron catalyst. According to the monatomic iron catalyst, a multi-walled carbon nanotube with the diameter of 4-6 nm is used as a carrier, iron is anchored to the high-curvature surface of the carbon nanotube in a monatomic form, and a Fe-N4 active center with a high spinning state is formed. When the catalyst is used for activating peroxymonosulfate to degrade organic pollutants, a non-free radical path can be efficiently dominated, a large amount of singlet oxygen (1O2) is generated, and the catalyst shows efficient degradation capacity, wide pH adaptability, high ion interference resistance and excellent cycle stability on bisphenol A and other pollutants; the method has a wide application prospect in the aspect of treating refractory organic wastewater. The preparation method is mild, controllable in process and suitable for industrial production.

Description

Technical Field

[0001] This invention relates to the field of catalysis technology, and in particular to a single-atom iron catalyst with curvature control of small-diameter carbon nanotubes, its preparation method, and its application. Background Technology

[0002] Advanced persulfate oxidation technology generates sulfate radicals (SO4·) - Persulfate, with its strong oxidizing power, long half-life, and wide pH adaptability, shows great potential in the field of water treatment. Activation of persulfate is the core of this technology. Currently, transition metals (such as Co, Fe, and Mn) are widely used for homogeneous or heterogeneous catalytic activation of persulfate due to their high efficiency and relatively low cost.

[0003] However, homogeneous systems suffer from problems such as metal ion dissolution, secondary pollution, and difficulties in catalyst recovery; traditional heterogeneous nanocatalysts have limitations such as low atom utilization, insufficient exposure of active sites, and easy aggregation and deactivation. In addition, traditional activation methods are mostly based on the free radical generation pathway, which is easily quenched by complex background substances in water (such as chloride ions, carbonate ions, and natural organic matter), resulting in reduced oxidant utilization efficiency and poor performance in treating actual wastewater.

[0004] In recent years, non-radical pathways, especially those involving singlet oxygen (… 1 O2-dominated oxidation pathways show promising application prospects due to their advantages such as low susceptibility to water matrix interference, wide pH application window, and high selectivity for specific pollutants. Therefore, developing methods to precisely control the persulfate activation pathway for efficient and highly selective production is crucial. 1 Heterogeneous catalysts for O2 have become a research frontier and challenge in this field. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a single-atom iron catalyst with curvature-controlled small-diameter carbon nanotubes, its preparation method, and its application. The single-atom iron catalyst of this invention uses multi-walled carbon nanotubes with a diameter of 4-6 nm as a support, with iron anchored in single-atom form on the high-curvature surface of the carbon nanotubes, forming Fe-N4 active centers with high spin states. When this catalyst is used to activate persulfate for the degradation of organic pollutants, it can efficiently dominate the non-radical pathway, generating a large amount of singlet oxygen (…). 1 O2 exhibits high efficiency in degrading pollutants such as bisphenol A, wide pH adaptability, strong resistance to ion interference, and excellent cycle stability, showing broad application prospects in the treatment of recalcitrant organic wastewater. The preparation method of this invention is mild and the process is controllable, making it suitable for industrial production.

[0006] The specific technical solution of this invention is as follows:

[0007] First, this invention provides a single-atom iron catalyst with curvature-controlled small-diameter carbon nanotubes. Using multi-walled carbon nanotubes with a diameter of 4-6 nm as a carrier, iron is dispersed in single-atom form and anchored on the high-curvature surface of the multi-walled carbon nanotubes to form Fe-N4 active centers with high spin states. The high-spin Fe-N4 active centers are determined by the K-edge X-ray absorption near-edge structure spectrum of Fe, and the absorption edge position is close to that of Fe2O3, with an average valence state close to +2.

[0008] This invention reveals that the surface curvature of carbon nanotubes can significantly influence the electronic structure of loaded metals. Small-diameter multi-walled carbon nanotubes (e.g., 4-6 nm) possess greater surface curvature, enabling the generation of stronger lattice strain on their surface. This allows for more effective modulation of the coordination field of the anchored metal center, inducing the formation of high-spin states. High-spin metal centers are more conducive to the formation of non-radical pathways (such as the generation of...) 1 O2) activates persulfate. Accordingly, this invention utilizes small-diameter multi-walled carbon nanotubes as a support, leveraging their high curvature effect to induce the formation of high-spin single-atom iron active centers, thereby achieving precise control of the non-radical catalytic pathway. When activating persulfate, the catalyst of this invention primarily generates singlet oxygen (O2). 1 O2).

[0009] This invention further reveals that multi-walled carbon nanotubes (MWCNTs) are more effective than single-walled carbon nanotubes in activating persulfate. This is because MWCNTs are composed of multiple concentric graphene walls, while single-walled carbon nanotubes are formed by rolling up a single layer of graphene. The multi-layered structure of MWCNTs provides richer electron conduction pathways. The electronic interactions between layers can generate an "electron pool" effect, which can more effectively regulate and stabilize the electron density of the Fe-N4 active centers carried by the outermost layer. This is more conducive to inducing and maintaining a high-spin state, which is why it efficiently generates singlet oxygen (…). 1 The key to O2 is that multi-walled carbon nanotubes naturally have more tube wall ports and interlayer defects than single-walled carbon nanotubes during preparation and processing. These sites can serve as additional anchoring points, which are more conducive to the initial adsorption and fixation of iron phthalocyanine (FePc) molecules, preventing them from agglomerating during loading, thus laying a better foundation for the subsequent formation of uniformly distributed single-atom sites.

[0010] Preferably, the mass ratio of iron to multi-walled carbon nanotubes is in the range of 0.001025-0.02255:1.

[0011] This invention controls the mass ratio of iron to multi-walled carbon nanotubes within the aforementioned range, based on the core principles of "maximizing atom utilization" and "avoiding site aggregation" in single-atom catalysts. The optimal mass ratio range was determined through theoretical calculations and experimental verification. Specifically, the number of sites (such as defects, pyridine nitrogen sites, etc.) that can stably anchor metal atoms on the surface of the multi-walled carbon nanotube support is limited. In this invention, iron phthalocyanine (FePc) molecules are adsorbed onto the surface of the multi-walled carbon nanotubes through non-covalent bonding and π-π stacking. The upper limit of the mass ratio theoretically approaches the saturation capacity of single-atom iron sites that this support system can stably support under mild preparation conditions. If the iron ratio is too low, the number of effective Fe-N4 active centers will be too small, resulting in a lower rate of activation of supersulfate and reduced production. 1 The amount of O2 will be significantly reduced, resulting in low catalytic efficiency and ultimately a slow degradation rate of pollutants. Conversely, if the iron content is too high, when the iron loading exceeds the saturation anchoring capacity of the carrier surface, the excess iron species cannot exist in the form of isolated single atoms; they easily form iron clusters or nanoparticles. Excess iron precursors (FePc) will aggregate on the surface of multi-walled carbon nanotubes, forming amorphous iron oxide or iron nanoparticles in subsequent processing. These particles do not possess the unique electronic structure and high catalytic selectivity of single-atom sites and tend to degrade via the traditional free radical pathway (generating ·OH and SO4·). - Using iron to activate persulfate can lead to decreased selectivity in the catalytic system, making it more susceptible to quenching by coexisting anions in the water, thus losing its core advantages of wide pH adaptability and strong resistance to interference. Furthermore, it can mask active sites and hinder mass transfer; the resulting iron particles may physically block the previously successfully constructed Fe-N4 single-atom sites and clog the mesoporous channels of carbon nanotubes, affecting the diffusion of pollutants and persulfate, which is detrimental to the catalytic reaction. Instability is also reduced; these non-single-atom iron species have weaker binding to the support and are more likely to dissolve during catalytic cycling or recovery, leading to catalyst deactivation and potentially causing secondary pollution.

[0012] Secondly, this invention provides a method for preparing a single-atom iron catalyst with curvature tunable small-diameter carbon nanotubes, which includes the following steps:

[0013] (1) Pretreatment by immersing multi-walled carbon nanotubes with a diameter of 4-6 nm in an acid solution to remove metal impurities;

[0014] (2) Disperse the multi-walled carbon nanotubes treated in step (1) in an organic solvent to form a uniform suspension;

[0015] (3) Add the organic solvent containing iron phthalocyanine to the suspension obtained in step (2) and mix evenly so that iron phthalocyanine is fully loaded on the surface of multi-walled carbon nanotubes; the mass ratio of multi-walled carbon nanotubes to iron phthalocyanine is 5-11:1;

[0016] (4) Centrifuge and wash the mixture obtained in step (3);

[0017] (5) Dry the precipitate obtained in step (4) to obtain Fe-CNTs single-atom catalyst.

[0018] Preferably, in step (1), the acid solution is a 4-8 mol / L hydrochloric acid solution.

[0019] Preferably, in step (1), the pretreatment time is 5-20 hours.

[0020] Preferably, in steps (2) and (3), the organic solvent is N,N-dimethylformamide (DMF).

[0021] Preferably, in step (3), the mixing is first ultrasonically treated for 20-40 minutes, and then stirred for 20-30 hours.

[0022] Preferably, in step (4), the washing is performed by washing with ethanol and water in sequence;

[0023] Preferably, in step (5), the drying is freeze drying.

[0024] Finally, this invention provides the application of Fe-CNTs single-atom catalysts in the activation of persulfate for the degradation of organic pollutants: the Fe-CNTs single-atom catalysts, through their high-spin Fe-N4 active centers, dominate the activation of persulfate to generate singlet oxygen (…). 1 O2) to achieve pollutant degradation.

[0025] In this invention, the high-spin Fe-N4 center in the Fe-CNTs single-atom catalyst interacts with persulfate molecules, preferentially activating the persulfate to singlet oxygen via a non-radical pathway. 1 O2), which in turn oxidizes and degrades pollutants.

[0026] The Fe-CNTs single-atom catalyst of this invention has a wide applicable pH range, maintaining a degradation efficiency of over 80% within the pH range of 3-9. The Fe-CNTs single-atom catalyst of this invention also exhibits high cycling stability; after 5 cycles, the degradation efficiency for bisphenol A decreases by no more than 12%.

[0027] Preferably, the organic pollutant includes bisphenol A.

[0028] Compared to other organic pollutants, the Fe-CNTs single-atom catalyst of this invention exhibits particularly outstanding degradation effect on bisphenol A.

[0029] Preferably, the dosage of the Fe-CNTs single-atom catalyst is 0.01-0.06 g / L.

[0030] Preferably, the concentration of the persulfate is 0.25-1 mM.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] (1) Precise pathway control and high catalytic efficiency: This invention utilizes the high curvature effect of small-diameter multi-walled carbon nanotubes (4-6 nm) to successfully induce the formation of a high-spin state in the single-atom iron center, enabling it to dominate the non-radical pathway when activating persulfate, and efficiently generate singlet oxygen ( 1 O2). This pathway is not easily quenched by background substances in the water and has extremely high efficiency (>80% degradation within 5 minutes) and selectivity in the degradation of target pollutants (especially bisphenol A).

[0033] (2) Maximizing atomic utilization: Fe is dispersed in the form of single atoms, achieving nearly 100% atomic utilization, which explains from the source why the reaction rate can be far greater than that of traditional nanocatalysts at extremely low dosage.

[0034] (3) Extremely strong environmental adaptability: thanks to 1 Due to the dominant role of O2, the single-atom iron catalyst of this invention maintains high efficiency in degradation across a wide pH range of 3-9, and is effective against Cl, a common pollutant in water. - HCO3 - H2PO4 - SO4 2- Various anions exhibit excellent anti-interference capabilities, making them particularly suitable for treating complex wastewater.

[0035] (4) Excellent stability and recyclability: The stable Fe-N4 coordination structure and robust carbon support ensure that the degradation efficiency of bisphenol A decreases by no more than 12% after 5 cycles of use, which is far lower than many traditional heterogeneous catalysts that are easily deactivated or have components dissolved.

[0036] (5) The preparation process is green and mild: the entire preparation process does not require high-temperature pyrolysis and can be completed at room temperature. It has low energy consumption, is easy to operate, and is easy to scale up for production. It also avoids the aggregation of active sites that may be caused by high-temperature processes. Attached Figure Description

[0037] Figure 1 These are aberration-corrected AC-STEM images of the Fe-CNTs catalyst prepared in Example 1; where (a) shows the morphology of the catalyst, (b) shows isolated bright spots as single Fe atoms, and (c) shows the absence of metal nanoparticle aggregation.

[0038] Figure 2 This is a TEM-mapping image of the Fe-CNTs catalyst prepared in Example 1, showing the uniform distribution of Fe, N, and C elements on the carbon framework;

[0039] Figure 3 The images show (a) XRD pattern, (b) nitrogen adsorption-desorption isotherm and (c) Raman spectrum of the Fe-CNTs catalyst prepared in Example 1.

[0040] Figure 4 The images show (a) Fe K-edge XANES spectrum and (b) Fourier transform EXAFS spectrum of the Fe-CNTs catalyst prepared in Example 1 and the control sample.

[0041] Figure 5 The degradation performance of Fe-CNTs / PMS systems on BPA under different conditions in Examples 1, 4, and 5 is as follows: (a) comparison of different systems, (b) effect of catalyst dosage, and (c) effect of PMS concentration.

[0042] Figure 6 This is Application Example 2: The effect of different mass ratios of phthalocyanine iron to multi-walled carbon nanotubes on catalyst performance;

[0043] Figure 7 This is an example of the effect of single-walled / multi-walled carbon nanotubes on catalyst performance in Example 3.

[0044] Figure 8 The effects of different initial pH and different anions on the degradation of BPA in the Fe-CNTs / PMS system are applied in Examples 6 and 7;

[0045] Figure 9 The results are from the cycle performance test of the Fe-CNTs catalyst in Example 8.

[0046] Figure 10 This applies to Example 9, which shows (a) the effect of different quenchers on degradation and (b) TEMP- 1 O2's EPR signal;

[0047] Figure 11 This demonstrates the degradation effect of Fe-CNTs on different types of catalysts in Example 10.

[0048] Figure 12 The results are from the electrochemical performance test of Fe-CNTs in Application Example 11. Detailed Implementation

[0049] The present invention will be further illustrated below with reference to the embodiments and accompanying drawings. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0050] Example 1: Preparation of Fe-CNTs (4-6 nm) catalysts

[0051] (1) Take 200 mg of multi-walled carbon nanotubes (MWCNT, XFNANO) with a diameter of 4-6 nm, immerse them in 100 mL of 6 mol / L hydrochloric acid solution, and stir at room temperature for 12 hours.

[0052] (2) After the reaction was completed, the sample was repeatedly washed with ultrapure water until the filtrate was neutral. Then the sample was freeze-dried to obtain purified multi-walled carbon nanotubes.

[0053] (3) Accurately weigh 30 mg of the purified multi-walled carbon nanotubes and disperse them in 15 mL of DMF. Sonicate for 1 hour to ensure full dispersion and obtain a multi-walled carbon nanotube suspension.

[0054] (4) Take another 5 mg of iron phthalocyanine (FePc) (the mass ratio of iron phthalocyanine to multi-walled carbon nanotubes is 1:6) and dissolve it in 15 mL of DMF.

[0055] (5) Pour the FePc solution into the multi-walled carbon nanotube suspension, continue to sonicate the mixture for 30 minutes, and then magnetically stir it at room temperature for 24 hours.

[0056] (6) Transfer the resulting mixture to a centrifuge tube and centrifuge at 10,000 rpm for 10 minutes to collect the solid precipitate.

[0057] (7) The precipitate was washed with ethanol, ultrapure water, ethanol and ultrapure water in sequence, and centrifuged after each washing.

[0058] (8) The final solid product was freeze-dried for 24 hours to obtain the Fe-CNTs single-atom catalyst.

[0059] Example 2

[0060] The difference between this embodiment and Embodiment 1 is that the mass ratio of iron phthalocyanine to multi-walled carbon nanotubes is 1:12.

[0061] Example 3

[0062] The difference between this embodiment and Embodiment 1 is that the mass ratio of iron phthalocyanine to multi-walled carbon nanotubes is 1:4.

[0063] Comparative Example 1: Comparison of large-diameter carriers (Fe-CNTs (>50 nm))

[0064] The difference between this comparative example and Example 1 is that the diameter of the multi-walled carbon nanotube support was replaced with >50 nm (XFM34 1333-86-4) to prepare the catalyst Fe-CNTs (>50 nm).

[0065] Comparative Example 2: Comparison of medium-diameter supports (Fe-CNTs (20-30 nm))

[0066] The difference between this comparative example and Example 1 is that the diameter of the multi-walled carbon nanotube support was replaced with a 20-30 nm multi-walled carbon nanotube (XFM22 1333-86-4) to prepare the catalyst Fe-CNTs (20-30 nm).

[0067] Comparative Example 3: Multi-walled carbon nanotubes (>50 nm) without iron phthalocyanine loading

[0068] The difference between this comparative example and Example 1 is that the multi-walled carbon nanotubes are not further loaded with phthalocyanine iron.

[0069] Comparative Example 4

[0070] The difference between this comparative example and Example 1 is that single-walled carbon nanotubes with a diameter of 4-6 nm are used as the carrier.

[0071] Comparative Example 5

[0072] Unloaded iron phthalocyanine multi-walled carbon nanotubes (20-30 nm)

[0073] Comparative Example 6

[0074] Unloaded iron phthalocyanine multi-walled carbon nanotubes (4-6 nm)

[0075] Application Example 1: Evaluation of Catalyst Degradation Performance and Support Diameter Effect

[0076] In 50 mL centrifuge tubes, a 20 mg / L bisphenol A (BPA) solution was prepared, and equal volumes (0.02 g / L) of the catalysts prepared in Examples 1, 1, 2, 3, 5, and 6 were added respectively. After adsorption equilibrium was reached for 30 minutes, persulfate (PMS) (0.5 mM) was added to initiate the reaction.

[0077] Result: As Figure 5 a. In Example 1, the catalyst (4-6 nm) achieved a BPA removal rate exceeding 80% within 5 minutes and reaching 97.7% after 30 minutes. Comparative Example 2 (20-30 nm) catalyst showed the next best performance, with a removal rate of approximately 65% ​​after 5 minutes and reaching 88.5% after 30 minutes. Comparative Example 1 (>50 nm) catalyst exhibited the worst performance, with a removal rate of only 40% after 5 minutes and 72.3% after 30 minutes. Comparative Examples 3 and 5 showed virtually no degradation effect.

[0078] This performance difference is closely related to the structural characteristics of the material. Figure 1The aberration-corrected AC-STEM image clearly shows that the overall morphology of Example 1 is a tubular structure. Figure 1 a) In the catalyst, Fe exists in the form of isolated single atoms ( Figure 1 b), no metal nanoparticle aggregation ( Figure 1 c) demonstrates the successful construction of a single-atom-level dispersion. Figure 2 This indicates that Fe, N, and C elements are uniformly distributed on the carbon framework. Figure 3 The XRD characterization of a shows that the diffraction peak of Fe-CNTs at 2θ = 26.6° is attributed to the (004) crystal plane of carbon (JCPDS 26-1080). Simultaneously, the lack of characteristic Fe crystal and Fe oxide peaks indicates the absence of Fe nanoaggregates and Fe oxide morphology in Fe-CNTs. This finding is consistent with... Figure 1 Consistent aberration-corrected electron microscopy results confirmed that iron exists in Fe-CNTs as isolated monatomic iron atoms. Analysis of the nitrogen adsorption-desorption isotherms and BET adsorption isotherm equations was also performed. Figure 3 (b) The specific surface area of ​​the catalyst was determined to be 368.8 m². 2 g -1 The pore volume is 0.7 cm³. 3 g -1 The average pore size is 6.7 nm. This indicates that Fe-CNTs possess a distinct mesoporous structure, endowing them with excellent specific surface area and porosity, as well as good adsorption capacity and pollutant diffusion channels, mainly attributed to the role of carbon nanotubes. These structural features not only provide abundant interfacial reaction sites for the high activity of Fe-CNTs, but also provide a solid structural foundation for their high efficiency in adsorption-catalytic synergistic degradation processes. Figure 3 The Raman spectrum of c shows that Fe-CNTs have I D / I G The value was 1.605, significantly higher than that of common multi-walled carbon nanotubes (I0.05). D / I G The range of 0.8–1.2 indicates that the catalyst, after being supported on a single Fe atom, exhibits a high degree of disorder and defect density. This defect structure provides abundant active sites, improves electron conduction, and can better promote molecular diffusion and adsorption, thereby accelerating the catalytic cycle. Figure 4 The XAFS spectrum further confirmed that the Fe K-side position of the catalyst in Example 1 was close to that of Fe₂O₃ and there were no Fe-Fe coordination bonds, indicating that it formed a high-spin Fe-N₄ structure. In contrast, the catalysts in Comparative Examples 1 and 2 may have difficulty inducing the formation of equally efficient active center structures due to insufficient support curvature. Figure 5The comparison of degradation performance of catalyst α intuitively demonstrates the performance differences of the three catalysts, which is in complete agreement with the structural characterization results, proving the key role of small-diameter supports in constructing high-performance catalysts.

[0079] The above results clearly show that as the diameter of the carbon nanotube support decreases, its surface curvature increases, and the initial reaction rate and final degradation efficiency of the prepared catalyst are significantly improved, proving the key role of small-diameter supports in constructing high-performance catalysts.

[0080] Application Example 2: Effect of different mass ratios of phthalocyanine iron to multi-walled carbon nanotubes on catalyst performance

[0081] In 50 mL centrifuge tubes, a 20 mg / L bisphenol A (BPA) solution was prepared, and equal volumes (0.02 g / L) of the catalysts prepared in Examples 1, 2, and 3 were added, respectively. After adsorption equilibrium was reached for 30 minutes, persulfate (PMS) (0.5 mM) was added to initiate the reaction.

[0082] Result: As Figure 6 In Example 1, the catalyst achieved a BPA removal rate exceeding 80% within 5 minutes and reaching 97.7% after 30 minutes. In Example 2, the catalyst performed slightly better, achieving a removal rate of approximately 69% within 5 minutes and 93.2% after 30 minutes. In Example 3, the catalyst performed the worst, with a removal rate of only 53% within 5 minutes and 67.5% after 30 minutes.

[0083] These results clearly demonstrate that the mass ratio of iron phthalocyanine to multi-walled carbon nanotubes is a key factor determining the number, structure, and performance of active sites in the catalyst. Specifically, the mass ratio (1:6) corresponding to Example 1 ensures that iron phthalocyanine molecules achieve high dispersion and single-atom-level anchoring on the surface of the multi-walled carbon nanotube support, thereby maximizing the formation of Fe-N4 active centers with high spin states. These uniformly distributed sites can efficiently activate PMS, dominate the non-radical pathway, and therefore exhibit the best catalytic performance.

[0084] In contrast, Example 2 has a lower mass, resulting in an insufficient number of usable Fe-N4 active sites per unit mass of catalyst. Although each site may still maintain high intrinsic activity, the reduction in the total number of sites leads to a performance decrease. Example 3, on the other hand, has an excessively high mass, causing excess iron phthalocyanine to aggregate on the carbon nanotube surface, obscuring the original active sites and resulting in a performance decrease.

[0085] In summary, by precisely controlling the mass ratio within a preferred range, this invention successfully achieves an optimal balance between the density of active sites and atomic-level dispersion, which is the structural basis for realizing ultra-high catalyst activity.

[0086] Application Example 3: The Influence of Single-Walled and Multi-Walled Carbon Nanotubes on Catalyst Performance

[0087] In 50 mL centrifuge tubes, a 20 mg / L bisphenol A (BPA) solution was prepared, and equal volumes (0.02 g / L) of the catalysts from Example 1 and Comparative Example 4 were added. After adsorption equilibrium was reached for 30 minutes, persulfate (PMS) (0.5 mM) was added to initiate the reaction.

[0088] Result: As Figure 7 As shown, Example 1 exhibits extremely superior catalytic performance, reaching 97.7% at 30 minutes. In contrast, Comparative Example 4 shows significantly lagging performance and lower final efficiency, with a removal rate of only 55% at 30 minutes.

[0089] The results confirm that, under the same iron loading and reaction conditions, multi-walled carbon nanotubes are significantly superior to single-walled carbon nanotubes as a support. This performance difference mainly stems from the essential differences between the two supports in terms of structural stability and electronic regulation capabilities: (1) Structural stability and active site anchoring: Although single-walled carbon nanotubes have a higher theoretical specific surface area, their single-atom-layer structure is relatively less stable during loading and reaction. In a harsh catalytic reaction environment, the single-atom-layer tube wall may not provide the same robust anchoring and protection for the Fe-N4 active center as the multi-layer structure of multi-walled carbon nanotubes; (2) The "synergistic effect" of electronic conduction and regulation: such as Figure 4 As shown, the multilayered graphene walls of multi-walled carbon nanotubes can form a more powerful "electron pool." The electronic interactions between layers can more effectively modulate and stabilize the electron density of the outermost Fe-N4 center, making it easier to form and maintain the high-spin state necessary for this invention, thereby efficiently driving... 1 O2 is a non-radical pathway. However, the electronic control capabilities provided by the single wall of single-walled carbon nanotubes are relatively limited.

[0090] Application Example 4: Catalyst Dosage Optimization Experiment

[0091] The effect of different dosages (0.01, 0.02, 0.04, 0.06 g / L) of the catalyst in Example 1 on the degradation effect was investigated under the conditions of BPA solution (20 mg / L) and a fixed persulfate concentration (0.5 mM).

[0092] Result: As Figure 5 b. When the dosage was increased from 0.01 g / L to 0.02 g / L, the degradation rate at 30 minutes significantly increased from 78.5% to 97.7%. Further increases to 0.04 g / L and 0.08 g / L resulted in a slight increase in degradation rate (above 99.0%), but the improvement was limited. Considering cost-effectiveness, 0.02 g / L was determined to be the optimal dosage.

[0093] Application Example 5: Experiment on Optimization of Persulfate Concentration

[0094] Under the conditions of fixed BPA concentration (20 mg / L) and catalyst dosage in Example 1 (0.02 g / L), the effect of different persulfate concentrations (0.25, 0.5, 0.75, 1.0 mM) on the degradation effect was investigated.

[0095] Result: As Figure 5 c. When the concentration of persulfate was increased from 0.25 mM to 0.5 mM, the reaction rate increased significantly, with the degradation rate increasing from 85.1% to 97.7% after 30 minutes. When the concentration was further increased to 0.75 and 1.0 mM, the degradation rate did not show a further significant increase (remaining at around 98%), indicating that 0.5 mM of persulfate was sufficient to fully react with the catalyst, and excessive addition would result in waste of oxidant.

[0096] Application Example 6: Experiment on the Effect of Initial pH of Solution

[0097] A BPA solution with pH=7 and a concentration of 20 mg / L was prepared using 20 mM phosphate buffer. The initial pH of the BPA solution (20 mg / L) was then adjusted to 3.0, 5.0, 7.0, and 9.0, respectively, using dilute H2SO4 or NaOH solution. Under these conditions, the catalyst from Example 1 (0.02 g / L) and persulfate (0.5 mM) were added, and the effect of pH on degradation performance was investigated.

[0098] Result: As Figure 8 a. Within a wide pH range of 3.0-9.0, the catalytic system maintained a degradation rate of over 88% for BPA within 30 minutes. Especially under acidic to neutral conditions (pH 3.0-7.0), the degradation efficiency exceeded 95%, demonstrating excellent wide pH adaptability. This is of great significance for treating real-world wastewater with large pH fluctuations.

[0099] Application Example 7: Anion Interference Experiment

[0100] In a BPA solution (20 mg / L), 0.01 M sodium chloride (Cl) was added. - Sodium bicarbonate (HCO3) - Sodium dihydrogen phosphate (H2PO4) - ) or sodium sulfate (SO4) 2- The reaction was carried out by adding the catalyst from Example 1 (0.02 g / L) and persulfate (0.5 mM) to simulate a high-salt or complex ionic environment.

[0101] Result: As Figure 8b. Compared with the control group without added anions (degradation rate of 97.7%), the degradation efficiency of the system remained above 90% and the inhibition rate was below 7% even in the presence of the various anions mentioned above. This fully demonstrates that the catalyst-dominated singlet oxygen pathway of this invention has a strong resistance to anion interference.

[0102] Application Example 8: Cyclic Stability Test

[0103] Under the optimal conditions of Application Example 1, degradation experiments were conducted using the catalyst from Example 1. After one reaction, the catalyst was recovered by centrifugation, lightly rinsed with deionized water, freeze-dried for 24 days, and then used for the next cycle experiment, for a total of 7 cycles.

[0104] Result: As Figure 9 The catalyst exhibited degradation efficiencies of 97.7%, 95.2%, 92.8%, 89.6%, and 85.5% for BPA in five consecutive cycles. The efficiency remained above 85% in the fifth cycle, demonstrating excellent stability. The slow decline in performance was primarily attributed to minor physical losses of the catalyst during each cycle.

[0105] Application Example 9: Investigation of Reaction Mechanism

[0106] Active species were investigated through quenching experiments and EPR tests.

[0107] (1) Quenching experiment: Different quenching agents are added to the reaction system, such as Figure 10 As shown in a, singlet oxygen ( 1 After the application of furfuryl alcohol (FFA), a specific quencher of free radicals (O2), BPA degradation was significantly inhibited (from 97.7% to 25.5%), while other free radical quenchers had little effect, demonstrating that... 1 O2 is the dominant active species.

[0108] (2) EPR test: using TEMP as... 1 O2 scavengers, such as Figure 10 b, A strong [effect] was detected in the persulfate system of the catalyst in Example 1. 1 The O2 characteristic triplet signal is significantly weaker in the monosulfate system alone, which directly relates to the high curvature support-high spin state-high yield. 1 The connection to O2 capability confirms the correctness of the design concept of this invention.

[0109] Application Example 10: Degradation Effect of Catalysts on Different Pollutants

[0110] To evaluate the broad application potential of the catalyst of this invention and its selectivity for non-radical oxidation pathways, a variety of typical organic pollutants with different structures were selected for degradation experiments.

[0111] 1. Experimental Methods:

[0112] In 50 mL centrifuge tubes, solutions of different contaminants, each with a concentration of 20 mg / L, were prepared, including sulfamethoxazole (SMX), benzoic acid (BA), tetracycline (TC), phenol (PhOH), chloramphenicol (CPL), and nitrobenzene (NB). An equal volume (0.02 g / L) of the Fe-CNTs catalyst prepared in Example 1 was added to each solution. After adsorption equilibrium was reached for 30 minutes, persulfate (PMS) (0.5 mM) was added to initiate the reaction, which was carried out for 30 minutes.

[0113] 2. Results and Discussion:

[0114] Degradation effect such as Figure 11 As shown, the results indicate that the catalyst of this invention exhibits significantly different degradation efficiencies for different pollutants: it has extremely high degradation efficiency for sulfamethoxazole (SMX), benzoic acid (BA), tetracycline (TC), and phenol (PhOH), with removal rates exceeding 90% after 30 minutes. However, it shows poor degradation effects on chloramphenicol (CPL) and nitrobenzene (NB), with removal rates below 50% after 30 minutes.

[0115] 3. Mechanism Explanation and Conclusions:

[0116] The results clearly reveal that the dominant pathway for PMS activation by the catalyst of this invention—the singlet oxygen non-radical oxidation pathway—has a high selectivity for the molecular structure of pollutants.

[0117] Explanation for the efficient degradation of pollutants: SMX and TC molecules contain electron-rich benzene rings, amino groups, or carbon-carbon double bonds; BA and PhOH also have high electron cloud densities on their benzene rings. Singlet oxygen, as a highly electrophilic oxidant, tends to attack these electron-rich groups, efficiently degrading these pollutants through electrophilic addition or electron extraction mechanisms.

[0118] Explanation of inefficient pollutant degradation:

[0119] Nitrobenzene (NB): Its benzene ring is attached with a strong electron-withdrawing nitro group (-NO2), which significantly reduces the electron cloud density of the benzene ring, making it difficult for singlet oxygen, which is an electrophile, to react effectively with it.

[0120] Chloramphenicol (CPL): Its molecular structure is complex, containing both strongly electron-withdrawing nitro (-NO2) and chlorine atoms (-Cl), resulting in a very low electron cloud density on its benzene ring. More importantly, its molecule contains a sterically hindered diol chain structure, which may impede the pollutant molecules from approaching and contacting the active sites on the catalyst surface. The combined effect of electronic effects and steric hindrance makes it difficult for chloramphenicol to be effectively degraded via the electrophilic singlet oxygen pathway.

[0121] Conclusion: This application example demonstrates that the Fe-CNTs catalyst developed in this invention possesses a broad-spectrum and highly efficient removal capability for a variety of organic pollutants containing electron-rich groups (such as certain antibiotics, phenols, and aromatic acids). Furthermore, the differences in degradation effects strongly corroborate that the catalytic reaction is dominated by an electrophilic oxidation pathway mediated by singlet oxygen.

[0122] Application Example 11: Investigation of the Electrochemical Behavior of Catalysts

[0123] The electrochemical properties of the catalyst were tested using a three-electrode system on a Bio-logic VMP3 electrochemical workstation to explore the interfacial reaction process of its activation of persulfate.

[0124] 1. Preparation of working electrode:

[0125] First, 10 mg of the Fe-CNTs catalyst prepared in Example 1 was accurately weighed and added to a mixture containing 25 μL of Nafion solution (5 wt%) and 1 mL of anhydrous ethanol. The mixture was then sonicated for 3 hours to form a uniform catalyst ink. Next, 100 μL of the dispersion was accurately measured using a pipette and slowly and evenly drop-coated onto the lower region of a hydrophilic carbon paper (10 × 30 mm), ensuring a catalyst coating area of ​​10 × 10 mm. The coated carbon paper was dried with an infrared lamp for later use. The remaining uncoated areas of the carbon paper were adhered and insulated with non-conductive tape to accurately define the electrochemically active area. As a comparison, a working electrode for Comparative Example 6 (multi-walled carbon nanotubes (4-6 nm)) catalyst was prepared following the exact same procedure.

[0126] 2. Test methods and results:

[0127] The test was conducted in an electrolytic cell containing 20 mM phosphate buffer (pH=7). The counter electrode was a commercially available 10 × 10 × 0.1 mm platinum sheet, and the reference electrode was a saturated silver / silver chloride electrode. Before the test, the catalyst-coated working electrode was immersed in the buffer overnight to reach a stable open-circuit potential (OCP). After the system stabilized, the open-circuit potential was recorded. At 200 seconds, PMS solution was added to the electrolytic cell to bring the final concentration to 0.5 mM.

[0128] Results Observation: For the catalyst of Example 1 (4-6 nm), after the addition of persulfate, its open-circuit potential immediately underwent a significant positive transition (ΔOCP > +280 mV) and quickly stabilized at a higher potential plateau. This indicates that rapid chemisorption and electronic interaction with persulfate occurred on the catalyst surface, establishing a stable interfacial electric field and creating favorable conditions for subsequent catalytic oxidation reactions.

[0129] Comparative analysis: The open-circuit potential change of the catalyst in Comparative Example 6 (4-6 nm) after the addition of persulfate was much smaller than that in Example 1 (ΔOCP < +100 mV). This intuitively demonstrates that the small-diameter support imparts a stronger driving force to the catalyst, enabling it to interact with persulfate.

[0130] Next, at 2800 seconds, the target pollutant BPA was added to the system to bring its final concentration to 20 mg / L.

[0131] Results observation: After adding BPA, as Figure 12 In Example 1, the open-circuit potential of the catalyst working electrode showed a significant negative shift. This phenomenon is consistent with the electrochemical catalytic mechanism: BPA molecules in the solution act as electron donors, and are attracted by active species generated at the catalyst / persulfate interface (such as surface-bound Fe(IV)=O or those generated through electron transfer). 1 O2 oxidation, a process equivalent to electron transfer from BPA to the catalyst interface, results in an observed negative potential shift. This provides direct experimental evidence for a non-radical reaction pathway dominated by "electron transfer at the catalyst interface."

[0132] Conclusion: This electrochemical application example directly confirms from the perspective of interfacial electrochemistry, through open-circuit potential testing, that the catalyst of this invention (Fe-CNTs (4-6 nm)) has a stronger interaction and more efficient electron transfer capability with persulfate, providing a key mechanistic explanation and experimental evidence for its excellent catalytic performance and non-radical reaction pathway.

Claims

1. A single-atom iron catalyst with curvature modulated by small-diameter carbon nanotubes, characterized in that: Using multi-walled carbon nanotubes with a diameter of 4-6 nm as a carrier, iron is dispersed in single-atom form and anchored on the surface of the multi-walled carbon nanotubes to form Fe-N4 active centers with high spin states; the mass ratio of iron to multi-walled carbon nanotubes is 0.001025-0.02255:

1.

2. A method for preparing a single-atom iron catalyst as described in claim 1, characterized in that: Includes the following steps: (1) Pretreatment by immersing multi-walled carbon nanotubes with a diameter of 4-6 nm in an acid solution to remove metal impurities; (2) Disperse the multi-walled carbon nanotubes treated in step (1) in an organic solvent to form a suspension; (3) Add the organic solvent containing iron phthalocyanine to the suspension obtained in step (2) and mix evenly to load iron phthalocyanine onto the surface of multi-walled carbon nanotubes; the mass ratio of multi-walled carbon nanotubes to iron phthalocyanine is 5-11:

1. (4) Centrifuge and wash the mixture obtained in step (3); (5) Dry the precipitate obtained in step (4) to obtain a single-atom iron catalyst.

3. The preparation method according to claim 2, characterized in that: In step (1), The acid solution is a 4-8 mol / L hydrochloric acid solution; The pretreatment time is 5-20 hours.

4. The preparation method according to claim 2, characterized in that: In steps (2) and (3), the organic solvent is N,N-dimethylformamide.

5. The preparation method according to claim 2, characterized in that: In step (3), the mixing process involves ultrasonic treatment for 20-40 minutes followed by stirring for 20-30 hours.

6. The preparation method according to claim 2, characterized in that: In step (4), the washing is performed by washing with ethanol and water in sequence.

7. The preparation method according to claim 2, characterized in that: In step (5), the drying process is freeze drying.

8. The application of the single-atom iron catalyst as described in claim 1 or the single-atom iron catalyst obtained by the preparation method according to any one of claims 2-7 in the degradation of organic pollutants by activated persulfate, characterized in that: The single-atom iron catalyst, through its high-spin Fe-N4 active center, predominantly activates persulfate to generate singlet oxygen, thereby achieving pollutant degradation.

9. The application according to claim 8, characterized in that: The organic pollutant includes bisphenol A.

10. The application according to claim 8 or 9, characterized in that: The dosage of the Fe-CNTs single-atom catalyst is 0.01-0.06 g / L; The concentration of the persulfate is 0.25-1 mM; The pH of the degradation system is 3-9.

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