Cyanamide axial coordination cobalt monatomic catalyst for persulfate activation as well as preparation method and application of cyanamide axial coordination cobalt monatomic catalyst

Abstract

The invention discloses a cyanamide axial coordination cobalt monatomic catalyst for persulfate activation and a preparation method and application thereof, and belongs to the technical field of persulfate advanced oxidation. The catalyst is prepared by the following method: simultaneously adding NH2CN and NaOH into a CoCl2 solution, stirring for reaction, and after the reaction is finished, washing, filtering and drying to obtain the catalyst CoN4 (NCN). The catalyst CoN4 (NCN) with an isolated Co site and cyanamide ([NCN] 2-) coordination structure is synthesized by adopting a one-step precipitation method. The catalyst can be used for an advanced oxidation system based on persulfate activation, and can be used for deeply treating a water body polluted by high-concentration organic matters, so that the biodegradability of the water body is improved. In addition, CoN4 (NCN) also has obvious stability and environmental adaptability, and has important application prospect and value in the field of organic pollutant degradation.

Description

Technical Field

[0001] The present invention belongs to the technical field of persulfate advanced oxidation, and particularly relates to a cyanamide-based axially coordinated cobalt single-atom catalyst for persulfate activation, a preparation method thereof, and an application thereof. Background Art

[0002] Advanced oxidation processes (AOPs) are considered a promising wastewater treatment technology that generates reactive oxygen species (ROS) by activating oxidants such as ozone (O 3 ), hydrogen peroxide (H 2 O 2 ), peroxymonosulfate (PMS), and peroxydisulfate (PDS) to achieve the degradation of organic pollutants. Among them, the PMS molecule has an asymmetric structure, and the O-O bond is easily broken, so it is easier to activate. In addition, PMS-based AOPs have the advantages of simultaneously generating various reactive oxygen species, removing pollutants through radical and non-radical pathways, and strong adaptability to a wide pH range. Although there are many methods to activate PMS, such as metal catalysis, heat, microwave, ultrasound, light, etc., metal catalysis has received extensive attention due to its advantages of energy conservation, high efficiency, and easy recycling. Currently, metal catalysts are generally divided into homogeneous catalysts and heterogeneous catalysts according to their existence forms. However, due to the disadvantages of homogeneous catalysts such as secondary pollution and narrow pH adaptation range, the development of heterogeneous catalysts has attracted more and more attention.

[0003] In the past decade, the emergence of single-atom catalysts has opened up a new field for the development of heterogeneous catalysts. This kind of catalyst immobilizes dispersed metal atoms on the carrier, not only expanding the active sites to the atomic level, maximizing the contact efficiency between the active sites and other molecules, but also providing adjustable metal coordination and excellent metal-support interaction. In recent years, SACs have been applied to PMS-based AOPs to remove various organic pollutants, showing excellent performance. In addition to effectively generating common radicals (·OH, SO 4 ·- , O 2 ·- ), SACs have also been proven to be effective for non-radical pathways, including highly selectively generating non-radical active substances such as 1 O 2 and high-valent metal oxygen species (HVMO), or direct electron transfer mediated by the catalyst-PMS complex.

[0004] Among several non-radical oxidation pathways, HVMO is a kind of ROS that has only been concerned in recent years, and more and more studies have shown its important role in AOPs. HVMO has some unique advantages, including a longer lifespan (7 - 10 s, the radical is 10 -6 -10-9 s), excellent resistance to environmental background substances, higher steady-state concentration, strong oxidation ability, and selectivity for electron-rich organic pollutants. Therefore, designing catalysts that can improve the efficient and selective generation of HVMO is very promising in the field of water purification and helps to further explore the formation mechanism of HVMO.

[0005] However, currently, few catalysts can activate PMS to specifically generate HVMO. Because this process is challenged by the "oxygen wall" rule of transition metals, that is, "when the 3d orbitals exceed 4 electrons, the bonding of transition metals with terminal oxygen ligands is inhibited, and since the d electrons occupy the π-antibonding orbitals of the metal-oxygen, the metal-oxygen bond order is reduced". This means that an effective way to overcome the "oxygen wall" is to reduce the filling of the three-dimensional orbitals of the metal and provide more empty orbitals for the electrons from oxygen. Therefore, designing the electronic structure of the central atom is particularly important. Therefore, finding a catalyst that can overcome the "oxygen wall" rule to promote the selective formation of HVMO during the PMS activation process has important application value. Summary of the Invention

[0006] The present invention provides a catalyst for persulfate activation, and the catalyst is a cyanamide-based axially coordinated cobalt single-atom catalyst CoN 4 (NCN).

[0007] The preparation method of the above catalyst CoN 4 (NCN) is as follows:

[0008] Add NH 2 CN and NaOH to the CoCl 2 solution simultaneously, stir and react. After the reaction is completed, wash, filter, and dry to obtain the product CoN 4 (NCN).

[0009] In the preparation method of the above catalyst CoN 4 (NCN), each raw material is selected from the following amounts:

[0010] CoCl 2 solution 10 - 500 parts, NH 2 CN 0.04 - 2.1 parts, NaOH 0.06 - 3.2 parts;

[0011] When the component is a solid component, the amount represents grams; when the component is a liquid component, the amount represents milliliters; in actual applications, it can be scaled up or down proportionally according to this amount.

[0012] In a specific embodiment, each raw material is selected from the following amounts:

[0013] CoCl 2 50 parts of solution, NH 2 CN 0.21 part, NaOH 0.32 part;

[0014] When the component is a solid component, the quantity represents grams; when the component is a liquid component, the quantity represents milliliters; in actual application, it can be enlarged or reduced in proportion according to this quantity.

[0015] In the preparation method of the above catalyst CoN 4 (NCN), the concentration of the CoCl 2 solution is selected from 2.4 - 120 g / L; preferably 12 g / L.

[0016] The present invention provides the application of the above catalyst CoN 4 (NCN) in activating persulfate to degrade organic pollutants.

[0017] The present invention provides a catalytic membrane loaded with CoN 4 (NCN), namely CoN 4 (NCN) / PVDF membrane, and the catalytic membrane is prepared by the following method:

[0018] Disperse the catalyst CoN 4 (NCN) in an organic solvent, stir evenly to obtain a CoN 4 (NCN) solution; then filter the CoN 4 (NCN) solution onto a polyvinylidene fluoride membrane under vacuum pressure to achieve the loading of CoN 4 (NCN) on the polyvinylidene fluoride membrane, and after drying, obtain CoN 4 (NCN) / PVDF membrane.

[0019] In the preparation method of the above catalytic membrane, the organic solvent is ethanol.

[0020] In the preparation method of the above catalytic membrane, the concentration of the CoN 4 (NCN) solution is selected from 0.2 - 10 mg / mL; preferably 1 mg / mL.

[0021] The present invention provides a sewage treatment method, and the steps are as follows:

[0022] Mix the CoN 4 (NCN) catalyst and persulfate and put them into the sewage to achieve the degradation treatment of organic pollutants in the sewage.

[0023] In the above sewage treatment method, the CoN 4(NCN) The concentration of the catalyst in the sewage is selected from 0.05 to 0.20 g / L, preferably 0.1 g / L; the concentration of the persulfate is selected from 0.25 to 2.00 mM, preferably 0.5 mM.

[0024] In the above sewage treatment method, the pH of the sewage should be adjusted to 5 to 11; preferably 11.

[0025] In the present invention, the persulfate is peroxymonosulfate (PMS).

[0026] In the present invention, the organic pollutants are selected from one or more of bisphenol A, methylene blue, enrofloxacin, phenol, and 4-bromophenol.

[0027] The beneficial effects of the present invention are as follows:

[0028] The present invention synthesizes a catalyst CoN 2- (NCN) with isolated Co sites and a cyanamide ([NCN] 4 ) coordination structure by a one-step precipitation method. The catalyst is a five-coordinate asymmetric cobalt single-atom catalyst with cyanamide coordinated axially, and it can achieve the following effects through the asymmetric structure: (1) precisely control the reaction sites of the catalyst, enabling the catalyst to provide excellent selectivity for specific reactants or intermediates in complex reactions and avoiding the occurrence of ineffective or side reactions; (2) adjust the electronic structure around the active center to make the electrons in a more mobile state, thereby increasing the activity; (3) introduce different types of ligands or functional groups into the catalyst to create a diverse reaction environment. This tunability enables the catalyst to exhibit different performances under different reaction conditions, further broadening its application scope.

[0029] Compared with traditional CoN 4 and CoN 5 catalysts, the combination of the strong cation polarization ability of [NCN] 2- and the axially asymmetric geometric structure results in a higher degree of electron delocalization of the Co sites, thereby effectively reducing the energy barrier for the formation of the active species Co(IV)=O.

[0030] In the present invention, the catalyst CoN 4 (NCN) can be used in an advanced oxidation system based on persulfate activation and deeply treat water bodies contaminated by high-concentration organic matter to improve their biodegradability.

[0031] In addition, CoN 4 (NCN) also has remarkable stability and environmental adaptability, and has important application prospects and value in the field of organic pollutant degradation. Description of the Drawings

[0032] Figure 1 For CoN 4 (a) Synthesis process of CoN(NCN), SEM images at different scales (b - d), and EDS - mapping image (e);

[0033] Figure 2 For CoN 4 (a) XRD pattern, (b) FT - IR spectrum, and (c) Raman image of CoN(NCN);

[0034] Figure 3 Normalized Co - edge XANES spectrum (a) of CoN, CoN 4 Fourier - transform EXAFS spectra (b) of CoN(NCN) and standard samples, CoN 4 Fitting results of Co - edge EXAFS of Co in CoN(NCN) in R - space (c) and K - space (d), Co foil (e), CoO (f), Co 3 O 4 (g), CoN 4 (h) Wavelet transform results of CoN(NCN);

[0035] Figure 4 For CoN 4 Comparison of the performance of CoN(NCN) and other common cobalt - based catalysts in activating PMS (a) and comparison of the PMS decomposition rate of CoN 4 (NCN) and other common cobalt - based catalysts (b);

[0036] Figure 5 Effect of catalyst dosage on the performance of CoN 4 (NCN) in activating PMS (a); Reaction rate constants of BPA degradation at different catalyst dosages (b); Effect of PMS concentration on the performance of CoN 4 (NCN) in activating PMS (c); Reaction rate constants of BPA degradation at different PMS concentrations (d); Effect of pH on the performance of CoN 4 (NCN) in activating PMS (e); Reaction rate constants of BPA degradation at different pH values (f);

[0037] Figure 6 For CoN 4 (NCN) / PVDF membrane continuous operation device (a); Treatment effect of CoN 4 (NCN) / PVDF membrane continuous operation device on BPA and ion leaching concentration (b); SEM images of CoN 4 (NCN) / PVDF membrane (c - d);

[0038] CoN 4(e) Contact angle test results of (NCN) / PVDF membrane. Detailed implementation mode

[0039] In the present invention, the CoCl 2 solution is prepared by dissolving CoCl 2 ·6H 2 O in ionized water.

[0040] In the present invention, CoN 4 (NCN) represents the coordination structure of the material, that is, the central cobalt atom coordinates with 4 N atoms and coordinates with NCN in the fifth direction, so it is named like this. The catalyst CoN 4 (NCN) synthesized in the present invention is Figure 1 consistent with the model structure constructed in (a).

[0041] Other materials used in the present invention, unless otherwise stated, can be obtained through commercial channels. Other terms used in the present invention, unless otherwise explained, generally have the meanings commonly understood by those of ordinary skill in the art. The present invention will be further described in detail below with reference to specific examples and data. The following examples are only for illustrating the present invention and do not limit the scope of the present invention in any way.

[0042] Example 1

[0043] Prepare the catalyst CoN 4 (NCN), the steps are as follows:

[0044] Add 0.21 g of NH 2 CN and 0.32 g of NaOH to 50 mL of CoCl 2 solution (12 g / L) at the same time, stir and react for 1 h. After the reaction is completed, wash with deionized water and filter, and obtain the product CoN 4 (NCN) by vacuum freeze-drying.

[0045] Figure 1 Shows the synthesis process (a), SEM images at different scales (b - d), and EDS-mapping images (e) of CoN 4 (NCN).

[0046] Scanning electron microscope (SEM) images (b - c) show that the microstructure of CoN 4 (NCN) is irregular particles. The aberration-corrected high-angle annular dark-field scanning transmission electron microscope (AC-HAADF-STEM) is used to detect the distribution of cobalt atoms (d). As shown in the figure, the obvious bright spots appearing in the figure can be attributed to the dispersed Co single atoms. Energy spectrum analysis (e) shows that the C, N, and Co elements are evenly distributed in the catalyst.

[0047] Figure 2 shows CoN 4 (NCN)'s XRD pattern (a), FT-IR spectrum (b), and Raman image (c).

[0048] CoN 4 (NCN)'s X-ray diffraction (XRD) pattern shows that no cobalt-related peaks were observed, indicating that no cobalt nanoparticles or cobalt oxides were produced. Functional group information can be obtained through Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. In the ATR-FTIR spectrum, an obvious peak appears at 2112 cm -1 which is attributed to the asymmetric stretching of [NCN] 2- Two peaks at 1550 cm -1 and 1410 cm -1 correspond to carbon-nitrogen bonds. Another peak at 668 cm -1 is characteristic of hydrocarbon vibrations. For the Raman spectrum, two sharp peaks at 855 cm -1 and 2333 cm -1 correspond to the stretching vibration modes of C-N and C≡N respectively. Therefore, according to Figure 2 it can be reflected that the cobalt element in the material does not exist in the form of clusters or nanoparticles, and the [NCN] 2- group exists in the structure.

[0049] Figure 3 shows the normalized Co element k-edge XANES spectrum (a), Fourier transform EXAFS spectra of CoN 4 (NCN) and standard samples (b), fitting results of the k-edge EXAFS of Co in CoN 4 (NCN) in (c) R space and (d) K space, (e) Co foil, (f) CoO, (g) Co 3 O 4 , (h) wavelet transform results of CoN 4 (NCN).

[0050] X-ray absorption fine structure (XAFS) tests were used to further explore the chemical state and coordination environment of the Co element in CoN 4 (NCN). The k-edge XANES spectrum of Co shows that the near-edge absorption energy of CoN 4 (NCN) is located between CoO and Co 3 O 4 standard compounds, indicating that the valence state of Co in CoN 4 (NCN) is between +2 and +3. For CoN 4(NCN) and the Fourier transform (FT) extended X-ray absorption fine structure (EXAFS) of the reference sample, compared with Co foil showed that no obvious Co-Co bond was observed in CoN 4 (NCN), while a peak appeared at , corresponding to the Co-N coordination. Subsequently, the peak at was caused by the cyanide group in [NCN] 2- . In addition, k-space information was provided in the wavelet transform EXAFS (WT-EXAFS) spectrum. The results showed that the k value of CoN 4 (NCN) was the largest at , which was caused by Co-N, while Co-Co and Co-O did not have the maximum intensity value. In addition, the fitting results of the k-edge EXAFS curve of Co showed that there were 5 N atoms coordinated with Co in the first shell. In the second shell, the Co-C coordination information and distance were respectively fitted to 4.8 and This asymmetric structure can provide stronger electron transfer ability.

[0051] I. Bisphenol A degradation performance experiment

[0052] This experiment aimed to evaluate the catalytic effect of CoN 4 (NCN) on peroxymonosulfate (PMS).

[0053] The degradation experiment was carried out in a 250 mL container, stirred at 500 rpm at room temperature. 10 mg of CoN 4 (NCN) or other catalysts (20 mg / L, pH = 6.3, without regulator and buffer) were added to 100 mL of bisphenol A (BPA) solution. Subsequently, 0.5 mM of PMS was added to the above solution, which marked the start of the degradation reaction. The reaction solution was sampled at regular intervals. Then the sample was filtered through a 0.22 μm membrane filter and mixed with 50 μL of Na 2 S 2 O 3 (150 mM) solution. The obtained sample was detected for the concentration of BPA using high performance liquid chromatography (HPLC, LC-20, Shimadzu). The above experiments were all carried out in triplicate. The pseudo-first-order model was used to fit the degradation curve of the target pollutant and calculate the apparent rate constant.

[0054] The test results are as Figure 4 shown:

[0055] CoN 4(NCN) has little adsorption effect on bisphenol A (BPA); when only PMS is present, BPA cannot be degraded either, indicating that the contributions of adsorption and PMS self-decomposition can be ignored. In contrast, in the CoN 4 (NCN) / PMS system, BPA is completely degraded within 5 min, and its kinetic constant (k app ) value is 1.17 min -1 , which is much higher than that of other homogeneous (about 2 times) and heterogeneous (about 20 - 100 times) catalysts.

[0056] In addition, the residual concentration of PMS in different catalytic systems is shown in Figure 4 (b). CoN 4 (NCN) can activate 87.89% of PMS within 7 min, with the highest PMS utilization rate. While within the same time range, CoNC, Co 2+ , Co 3 O 4 , Co 2 O 3 , and CoO can only activate about 10 - 20% of PMS, resulting in a waste of the oxidant.

[0057] The activation principle of CoN 4 (NCN) for PMS is as follows:

[0058] First, PMS is adsorbed on the active sites of the catalyst; then single-electron or double-electron transfer occurs from the catalyst to PMS; the electron transfer to PMS leads to the cleavage of the O - O bond, generating different active species such as ·OH, SO 4 ·- .

[0059] II. BPA degradation experiments under different operating parameter conditions

[0060] The optimal dosage of the catalyst was studied by changing the initial dosage concentration of the catalyst, and four dosage gradients of 0.05 g / L, 0.1 g / L, 0.15 g / L, and 0.2 g / L were set. The optimal dosage of PMS was studied by changing the initial concentration of PMS, and five concentration gradients of 0.25 mM, 0.50 mM, 1.00 mM, 1.50 mM, and 2.00 mM were set. For the study of the influence of the initial pH, H 2 SO 4 and NaOH were used to adjust the initial pH of the BPA solution to 3.0, 5.0, 7.0, 9.0, and 11.0. BPA concentration was measured by sampling at regular intervals in all experiments.

[0061] The test results are shown in Figure 5 as follows:

[0062] AsFigure 5 (a) and Figure 5 as shown in (b), when 0.05 g / L of CoN 4 (NCN) was introduced, 88.47% of BPA was degraded within 7 minutes. In contrast, when the catalyst concentration was increased to 0.10 - 0.20 g / L, BPA was completely removed within 5 minutes. This result indicates that as the concentration of CoN 4 (NCN) increases, the degradation rate increases, and the kapp value increases from 0.28 min -1 to 1.74 min -1 .

[0063] As Figure 5 shown in (c) and Figure 5 (d), as the concentration of PMS increases, the degradation performance shows an upward trend because the higher the PMS concentration, the more reactive oxygen species (ROS) are usually generated. However, when the PMS dosage reaches 1.00 mM or higher, the change in kapp is very small, which may be due to the active sites being fully utilized and no more PMS being decomposed.

[0064] As Figure 5 shown in (e) and Figure 5 (f), at different pH values, the activation energy is different, so the initial pH value of the solution is a key factor affecting the catalytic activity. For CoN 4 (NCN), it shows significant pH adaptability during the BPA degradation process. In the range of pH 5 - 11, BPA can be almost completely degraded within 7 minutes, but when the initial pH is 3, the degradation rate drops to 87.02%, and the kapp value drops to 0.28 min -1 . This phenomenon can be explained by the fact that excessive H + will consume free radicals and promote the leaching of metal ions in the system, thus reducing the performance of the catalyst.

[0065] III. Preparation and Application of Catalytic Membrane

[0066] Preparation of CoN 4 (NCN) / PVDF Catalytic Membrane:

[0067] Disperse 0.1 g of the catalyst CoN 4 (NCN) in 100 mL of absolute ethanol, stir vigorously for 30 min, and sonicate for 1 h to obtain a uniformly dispersed solution (1 mg / mL, 100 mL). Filter it onto a polyvinylidene fluoride (PVDF) membrane under vacuum pressure to achieve the loading of CoN 4 (NCN) on the PVDF membrane. Finally, dry the CoN 4 (NCN) / PVDF membrane at room temperature for 24 h for standby.

[0068] Continuous flow experiment:

[0069] Place the prepared catalytic membrane in the filter membrane clip as Figure 6 (a) shown. Use a circulation pump to draw the BPA simulated wastewater (10 mg / L) mixed with 1 mM PMS through the catalytic membrane device at a speed of 0.6 mL / min, and finally take samples at intervals to detect the concentration of BPA and the leaching concentration of cobalt element in the samples.

[0070] The test results are as Figure 6 shown:

[0071] As Figure 6 (b) shown, pass the mixed solution containing 10 mg / L BPA and 1 mM PMS through the catalytic membrane at a flow rate of 0.6 mL / min. After continuous operation for 6 h, the BPA removal efficiency of 98.63% can be maintained without additional addition of PMS, indicating that the catalytic membrane system has the ability to continuously and efficiently generate ROS.

[0072] As Figure 6 (c) and Figure 6 (d) shown, the surface of the CoN 4 (NCN) / PVDF membrane remains intact.

[0073] As Figure 6 (e) shown, the contact angle between the CoN 4 (NCN) / PVDF membrane and water is 13.33°, and the hydrophilicity is good. This characteristic helps the effective contact between the catalyst and the pollutants in water.

[0074] In addition, during the whole treatment process, the leaching concentration of Co in the CoN 4 (NCN) / PVDF membrane always remains at 10 - 30 μg / L, showing significant safety and stability.

[0075] The above is only the preferred embodiment of the present invention, and it is not a limitation to the present invention in other forms. Any person skilled in the art may use the disclosed technical content to make changes or modifications into equivalent embodiments with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the technical solution content of the present invention still belong to the protection scope of the technical solution of the present invention.

Claims

1. A catalyst for persulfate activation, characterized in that: The catalyst is a cyanamide-based axially coordinated cobalt single-atom catalyst CoN4 (NCN).

2. The catalyst according to claim 1, characterized in that The preparation method of the catalyst CoN4 (NCN) comprises the following steps: Add NH2CN and NaOH to the CoCl2 solution at the same time, stir to react, and after the reaction is completed, wash, filter, and dry to obtain the product CoN4 (NCN).

3. The catalyst according to claim 2, characterized in that The raw materials are selected from the following parts: 10 to 500 parts of CoCl2 solution, 0.04 to 2.1 parts of NH2CN, and 0.06 to 3.2 parts of NaOH.

4. Use of the catalyst CoN4 (NCN) according to any one of claims 1 to 3 in activating persulfate to degrade organic pollutants.

5. A catalytic membrane loaded with CoN4(NCN), characterized in that: The catalytic film is prepared by the following method: The catalyst CoN4(NCN) described in any one of claims 1 to 3 is dispersed in an organic solvent and stirred evenly to obtain a CoN4(NCN) solution; then the CoN4(NCN) solution is filtered onto a polyvinylidene fluoride membrane under vacuum pressure to achieve the loading of CoN4(NCN) on the polyvinylidene fluoride membrane, and after drying, a CoN4(NCN) / PVDF membrane is obtained.

6. The catalytic membrane according to claim 5, characterized in that The organic solvent is ethanol; the concentration of the CoN4 (NCN) solution is selected from 0.2 to 10 mg / mL.

7. A sewage treatment method, characterized in that: Here are the steps: The catalyst CoN4 (NCN) described in any one of claims 1 to 3 is mixed with persulfate and added into sewage to achieve degradation of organic pollutants in the sewage.

8. The sewage treatment method according to claim 7, characterized in that: The concentration of the catalyst CoN4 (NCN) in sewage is selected from 0.05 to 0.20 g / L; the concentration of the persulfate is selected from 0.25 to 2.00 mM.

9. The sewage treatment method according to claim 7, characterized in that: The pH of the sewage is 5-11.

10. The sewage treatment method according to claim 7, characterized in that: The persulfate is peroxymonosulfate; the organic pollutant is selected from one or more of bisphenol A, methylene blue, enrofloxacin, phenol, and 4-bromophenol.

Images