An atomically dispersed Fe-N-C catalyst, its preparation method and application

Abstract

The present invention discloses an atomically dispersed Fe-N-C catalyst, its preparation method and application. The catalyst includes iron atoms existing in the form of single atoms and a three-dimensional nitrogen-doped carbon skeleton. The iron atoms coordinate with the nitrogen atoms of the three-dimensional nitrogen-doped carbon skeleton to form Fe-Nx sites and are dispersed on the three-dimensional nitrogen-doped carbon skeleton, and the total iron content is 1.1 wt% to 1.8 wt%. The preparation method includes calcining, soaking and washing the FePc@ZIF-8 precursor to obtain the catalyst. The Fe-N-C catalyst of the present invention has the advantages of large specific surface area, many active sites, high catalytic activity, good stability, etc. It is an economical hydrogenation catalyst that can be widely used, can efficiently activate borohydride ions, has high use value and good application prospects. The preparation method of the present invention has the advantages of wide raw material sources, low cost, and no need for special equipment, is suitable for large-scale preparation, and is conducive to industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalyst preparation, and relates to an atomically dispersed Fe-N-C catalyst, its preparation method and application, specifically to an atomically dispersed Fe-N-C catalyst, its preparation method and the application of this Fe-N-C catalyst in activation. Background Art

[0002] The catalytic reduction method can directly convert some organic pollutants into substances with lower toxicity and economic utilization value through chemical processes. Taking nitrophenol as an example, NaBH4 can be used as a hydrogen donor, and a large amount of ·H can be produced through a hydrogenation reaction. Then, the catalytic reduction of nitrophenol can be realized by using ·H. Therefore, effectively activating NaBH4 and increasing the yield of ·H are the key to improving the catalytic reduction effect of organic substances.

[0003] Currently, the catalysts used to activate NaBH4 include noble metal-based catalysts and non-noble metal-based catalysts. The scarcity and high cost of noble metals make noble metal-based catalysts unfavorable for practical industrial applications. Although non-noble metal-based catalysts can reduce costs, the existing non-noble metal-based catalysts (such as CuO, Co3O4, Fe2O3, and NiO) still need to be further improved in terms of pH adaptation range, stability, catalytic efficiency, etc., and cannot completely replace noble metal-based catalysts.

[0004] Metal-nitrogen-doped carbon (M-N-C) catalysts have been regarded as good substitutes for noble metal catalysts and have received increasing attention due to their excellent catalytic performance in various reactions. However, the existing iron-nitrogen-doped carbon (Fe-N-C) catalysts are mainly used in electrocatalytic carbon dioxide reduction, electrocatalytic oxygen reduction, organic synthesis, persulfate activation, etc. There are no relevant reports on applying Fe-N-C catalysts to activate NaBH4. In addition, the existing Fe-N-C catalysts still have the following defects: small specific surface area and few active sites, which make the catalytic activity of Fe-N-C catalysts still difficult to meet the requirements and are not conducive to rapid treatment. In addition, in conventional preparation methods, defects such as the aggregation of iron atoms to form clusters or particles are also likely to occur, which will also reduce the catalytic efficiency of the catalyst. Obviously, the existence of the above problems makes it difficult for Fe-N-C catalysts to quickly and completely break the B-H bond in NaBH4, and thus it is difficult to use NaBH4 to catalytically reduce organic pollutants into low-toxicity substances. Therefore, obtaining an Fe-N-C catalyst with a large specific surface area, many active sites, high catalytic activity, and good stability is of great significance for effectively breaking the B-H bond in NaBH4 and realizing the rapid reduction of organic pollutants. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide an atomically dispersed Fe-N-C catalyst with a large specific surface area, many active sites, high catalytic activity and good stability.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is:

[0007] An atomically dispersed Fe-N-C catalyst, wherein the Fe-N-C catalyst comprises iron atoms and a three-dimensional nitrogen-doped carbon framework; the iron atoms exist in the form of single atoms; the iron atoms are coordinated with the nitrogen atoms of the three-dimensional nitrogen-doped carbon framework to form Fe-N sites; the Fe-N sites are dispersed on the three-dimensional nitrogen-doped carbon framework; the total iron content in the Fe-N-C catalyst is 1.1 wt% to 1.8 wt%.

[0008] For the above atomically dispersed Fe-N-C catalyst, further improved, the total iron content in the Fe-N-C catalyst is 1.2 wt% to 1.6 wt%.

[0009] For the above atomically dispersed Fe-N-C catalyst, further improved, the total iron content in the Fe-N-C catalyst is 1.3 wt% to 1.5 wt%.

[0010] For the above atomically dispersed Fe-N-C catalyst, further improved, the total iron content in the Fe-N-C catalyst is 1.35 wt%.

[0011] For the above atomically dispersed Fe-N-C catalyst, further improved, the iron atoms in the Fe-N-C catalyst are divalent; the carbon component in the three-dimensional nitrogen-doped carbon framework mainly exists in the form of amorphous carbon; the nitrogen atoms in the Fe-N-C catalyst also include at least one of pyridine nitrogen, pyrrole nitrogen, graphitic nitrogen, and nitrogen oxide; the Fe-N-C catalyst has a hollow mesoporous structure; the Fe-N-C catalyst has a rhombic dodecahedron structure; the particle size of the Fe-N-C catalyst is 40 nm to 120 nm; the specific surface area of the Fe-N-C catalyst is 838.39 m 2 / g to 879.14 m 2 / g.

[0012] As a general technical concept, the present invention also provides a preparation method of the above atomically dispersed Fe-N-C catalyst, comprising the following steps:

[0013] S1. Pyrolyze the FePc@ZIF-8 precursor;

[0014] S2. Immerse the pyrolysis product in an acid solution, wash, and dry to obtain an atomically dispersed Fe-N-C catalyst.

[0015] In the above preparation method, further improved, in S1, the preparation method of the FePc@Zif-8 precursor includes the following steps:

[0016] S1-1. According to the mass ratio of zinc salt to iron phthalocyanine of 146.9∶5, dissolve the zinc salt and iron phthalocyanine in methanol to form solution A; dissolve 2-methylimidazole in methanol to form solution B;

[0017] S1-2. Add solution A to solution B and stir;

[0018] S1-3. Wash and dry the product obtained after stirring to obtain the FePc@ZIF-8 precursor.

[0019] In the above preparation method, further improved, in S1-1, in solution A, the ratio of zinc salt to methanol is 14.69 mg∶1 mL; the zinc salt is Zn(NO3)2·6H2O; in solution B, the ratio of 2-methylimidazole to methanol is 810.6 mg∶50 mL; the molar ratio of the zinc salt to 2-methylimidazole is 4∶1.

[0020] In the above preparation method, further improved, in S1-2, the stirring is carried out at a temperature of 60 °C; the stirring time is 24 h.

[0021] In the above preparation method, further improved, in S1-3, the product is washed with methanol; the number of washing times is 3 times; the drying is carried out under vacuum conditions; the drying temperature is 70 °C.

[0022] In the above preparation method, further improved, in S2, the pyrolysis is carried out in a nitrogen atmosphere; the heating rate during the pyrolysis process is 2 °C / min -1 ; the pyrolysis is to heat the FePc@ZIF-8 precursor to 900 °C for calcination; the calcination time is 2 h to 3 h.

[0023] In the above preparation method, further improved, in S3, the acid solution is a non-oxidizing strong acid solution; the non-oxidizing strong acid solution is a sulfuric acid solution or a hydrochloric acid solution; the concentration of the acid solution is 0.5 mM; the soaking time is 24 h; the washing liquid used for washing is ultrapure water; the drying is carried out under vacuum conditions; the drying temperature is 70 °C.

[0024] As a general technical concept, the present invention also provides an application of the above atomically dispersed Fe-N-C catalyst or the atomically dispersed Fe-N-C catalyst prepared by the above preparation method in activating borohydride ions.

[0025] Compared with the prior art, the advantages of the present invention are as follows:

[0026] (1) The present invention provides an atomically dispersed Fe-N-C catalyst, which includes iron atoms and a three-dimensional nitrogen-doped carbon framework. The iron atoms exist in the form of single atoms, and the iron atoms coordinate with the nitrogen atoms of the three-dimensional nitrogen-doped carbon framework to form Fe-N x sites, and the Fe-N x sites are dispersed on the three-dimensional nitrogen-doped carbon framework. The total iron content in the Fe-N-C catalyst is 1.1 wt% to 1.8 wt%. In the present invention, with the three-dimensional nitrogen-doped carbon framework as the substrate, which has advantages such as stable structure and large specific surface area, more Fe-N x sites can be stably and uniformly dispersed on the three-dimensional nitrogen-doped carbon framework in the form of single atoms by using the coordination effect of iron and nitrogen. On the one hand, since the Fe center is positively charged, the abundant single-atom Fe-N x sites contained in the catalyst can adsorb more anions onto the catalyst surface at the same time, providing a basis for the next electron transfer, and its reaction mechanism conforms to the Langmuir–Hinshelwood model. On the other hand, more Fe-N x active sites can promote the activation of reactant molecules, making them more easily attacked by water molecules, thus accelerating the formation of ·H and generating a large amount of ·H, thereby obtaining benefits for improving the activity of the catalyst. In addition, by dispersing the single-atom Fe-N x sites on the surface of the three-dimensional nitrogen-doped carbon framework, the defect that the single-atom Fe-N x sites are prone to agglomeration is overcome, and the specific surface area of the catalyst is further increased, which makes the contact between the catalyst surface and reactant molecules more sufficient. In addition, the isoelectric point of the atomically dispersed Fe-N-C catalyst of the present invention is 6.5, having a wide pH adaptability performance, and having high catalytic activity at pH values from 3 to 7, overcoming the shortcoming of the narrow pH adaptation range of traditional catalysts and achieving high catalytic efficiency under near-neutral conditions. Compared with conventional Fe-N-C catalysts, the Fe-N-C catalyst of the present invention has a larger specific surface area, more abundant single-atom Fe-N x sites, higher mesopore content and defect level, and is an economical hydrogenation catalyst with a large specific surface area, many active sites, high catalytic activity and good stability, which can be widely used for activating NaBH4, has good reusability, is easy to recycle, has high use value and good application prospects.

[0027] (2) The present invention provides a preparation method of an atomically dispersed Fe-N-C catalyst. By pyrolyzing the FePc@ZIF-8 precursor, ZIF-8 is pyrolyzed to generate a three-dimensional nitrogen-doped carbon, and during the pyrolysis process, Fe replaces Zn to generate atomically dispersed Fe-Nx The active sites are fixed on the three-dimensional nitrogen-doped carbon. At the same time, during the pyrolysis process, due to the different diffusion and migration rates of Zn and Fe, the Kirkendall effect further leads to the formation of defects and mesopores. Finally, the pyrolysis product is immersed in an acid solution and pickled to remove the excess metal inside the pyrolysis product, which further increases the specific surface area of the product while also exposing the active sites to the outside, thereby preparing a single-atom Fe-N-rich product. x The Fe-NC single-atom catalyst has a high mesoporous content and defect level. At the same time, the preparation method of the present invention has the advantages of a wide source of raw materials, low cost, and no need for special equipment. It is suitable for large-scale preparation and is conducive to industrial production.

[0028] (3) In the preparation method of the atomically dispersed Fe-NC catalyst of the present invention, the FePc@ZIF-8 precursor is prepared by the following method: according to the mass ratio of zinc salt to iron phthalocyanine (FePc) of 146.9:5, zinc salt and FePc are dissolved in methanol to form solution A, and 2-methylimidazole is dissolved in methanol to form solution B, and then solution A is added to solution B, stirred, and the product obtained after stirring is washed and dried to obtain FePc@ZIF-8 precursor. In the present invention, in the presence of Zn 2+ During the self-assembly process of ZIF-8 and 2-MI, the FePc molecules were successfully encapsulated in the ZIF-8 molecular cage by optimizing the amount of FePc. On the one hand, the molecular size of FePc Larger than the cavity diameter of ZIF-8 Thus, the spatial confinement effect is broken, and this process is beneficial to edge region engineering and mesopore formation. On the other hand, during the pyrolysis of ZIF-8 to generate three-dimensional nitrogen-doped carbon, Zn is evaporated due to its low boiling point (907 ° C), and the remaining Zn vacancies are easily replaced by Fe, which is beneficial to the formation of atomically dispersed Fe-N x Active site.

[0029] (4) The present invention also provides an atomically dispersed Fe-NC catalyst for activating borohydride ions (BH4 - ), specifically using atomically dispersed Fe-NC catalysts as BH4 - Reduced catalyst, BH4 - On the one hand, the Fe-NC catalyst used in the present invention can activate BH4 - adsorbed on the surface, providing a basis for the next electron transfer, and its reaction mechanism conforms to the Langmuir–Hinshelwood model; on the other hand, the Fe-N in the Fe-NC catalyst x Active sites adsorb BH4 -After that, the B-H bond adjacent to iron is activated, making it more vulnerable to attack by water molecules, accelerating the generation of ·H, and the surface Fe-N x The active sites capture the generated ·H to form surface Fe-H bonds. The formation of surface Fe-H bonds effectively prevents ·H from combining with each other in the solution to form H2, thus improving the hydrogenation efficiency. In the present invention, the atomically dispersed Fe-N-C catalyst adsorbs reactive molecules, accelerates the generation of ·H, and mediates electron transfer, and has a good activation effect on BH4 - The present invention utilizes the atomically dispersed Fe-N-C catalyst to activate BH4 - The method has the advantages of simple operation, good economic benefits, easy recycling, high catalytic efficiency, wide pH application range, good cycling performance, etc., and can be widely used in the treatment of organic pollutants (such as nitrophenols and azo dyes), which is of great significance for the resource utilization of waste and sustainable development. Taking 4-nitrophenol (4-NP) as an example, by using the atomically dispersed Fe-N-C catalyst of the present invention to activate NaBH4, a large amount of ·H can be generated, and for the hydrogenation reduction of 4-nitrophenol, 20 mmol / L of 4-NP can be completely reduced to 4-aminophenol (4-AP) within 80 s in the presence of a very small amount (50 mg / L) of the catalyst, and the reaction rate constant k is as high as 3.2969 min -1 -1, which can be comparable to noble metal catalysts. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0031] Figure 1 It is the XRD pattern of the Fe-N-C catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention.

[0032] Figure 2 It is the TEM image of the Fe-N-C catalyst prepared in Example 1 of the present invention.

[0033] Figure 3 It is the SEM, AC-HAADF-STEM images of the Fe-N-C catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention.

[0034] Figure 4 It is the particle size distribution diagram of the Fe-N-C catalyst prepared in Example 1 of the present invention measured under the SEM electron microscope.

[0035] Figure 5 It is the adsorption-desorption isotherm diagram of the Fe-N-C catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention.

[0036] Figure 6 This is the XPS full spectrum of the Fe-N-C catalyst prepared in Example 1 of the present invention.

[0037] Figure 7 This is the high-resolution N1s XPS diagram of the Fe-N-C catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention.

[0038] Figure 8 This is the high-resolution Fe 2p XPS diagram of the Fe-N-C catalyst prepared in Example 1 of the present invention.

[0039] Figure 9 This is the Raman spectrum of the Fe-N-C catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention.

[0040] Figure 10 This is a comparison diagram of the reaction kinetic constants for the activation of NaBH4 by different Fe-N-C catalysts in Example 2 of the present invention for the reduction of 4-NP.

[0041] Figure 11 This is a diagram of the linear relationship between the reaction kinetic constant (k) for the activation of NaBH4 by the Fe-N-C catalyst (FeNC-2) in Example 2 of the present invention for the reduction of 4-NP and the Fe-N x site content (a), mesopores (b), and defects (c).

[0042] Figure 12 This is the cyclic effect diagram of the Fe-N-C catalyst for the reduction of 4-NP by activating NaBH4 in Example 3 of the present invention. Detailed implementation manners

[0043] The present invention will be further described below in conjunction with the accompanying drawings of the specification and specific preferred embodiments, but the protection scope of the present invention is not limited thereby.

[0044] In the embodiments of the present invention, the raw materials and instruments used are all commercially available. Unless otherwise specified, the processes used are conventional processes, the equipment used is conventional equipment, and the data obtained are the averages of more than three repeated experiments.

[0045] Example 1

[0046] An atomically dispersed Fe-N-C catalyst, including iron atoms and a three-dimensional nitrogen-doped carbon skeleton, the iron atoms exist in the form of single atoms, and the iron atoms coordinate with the nitrogen atoms of the three-dimensional nitrogen-doped carbon skeleton to form Fe-N x sites, and the Fe-N x sites are dispersed on the three-dimensional nitrogen-doped carbon skeleton, where x is a positive integer and the value range is 4-6.

[0047] In this example, Fe in the atomically dispersed Fe-N-C catalyst mainly exists in the form of divalent iron, and the total iron content is 1.35 wt%.

[0048] In this example, the carbon component in the three-dimensional nitrogen-doped carbon framework exists in the form of amorphous carbon.

[0049] In this example, the nitrogen atoms in the Fe-N-C catalyst, except for Fe-N x also include pyridine nitrogen, pyrrole nitrogen, graphitic nitrogen, and nitrogen oxide.

[0050] In this example, the Fe-N-C catalyst has a hollow mesoporous structure and a rhombic dodecahedron structure.

[0051] In this example, the particle size of the Fe-N-C catalyst is 40 nm to 120 nm.

[0052] In this example, the specific surface area of the Fe-N-C catalyst is 879.14 m 2 / g, the pore volume at a pore diameter of 3.9 nm is 0.25 cm 3 / g·nm, the percentage of the Fe-N x content in the total nitrogen content is 38.92%, and I D / I G = 1.0946.

[0053] A preparation method of the atomically dispersed Fe-N-C catalyst of the above example includes the following steps:

[0054] (1) Prepare an Fe-N-C precursor derived from iron phthalocyanine

[0055] Weigh 734.5 mg of Zn(NO3)2·6H2O (297.49 g / mol, 2.469 mmol) and 25 mg of FePc (586.37 g / mol, 0.0426 mmol) and dissolve them in 50 mL of methanol to form a uniform solution A. At the same time, dissolve 810.6 mg of 2-methylimidazole (82.1 g / mol, 9.873 mmol) in 50 mL of methanol to form solution B; then quickly pour solution A into solution B and stir at 60 °C in a water bath for 24 hours; finally, wash the obtained product several times with methanol and dry it overnight in a vacuum at 70 °C to obtain the FePc@ZIF-8 precursor.

[0056] (2) Prepare the atomically dispersed Fe-N-C catalyst by high-temperature pyrolysis

[0057] The as-prepared FePc@ZIF-8 precursor was pyrolyzed at 900 °C for 3 hours under N2 atmosphere with a heating rate of 2 °C / min and then naturally cooled to room temperature. Subsequently, the obtained single-atom Fe-N-C catalyst was immersed in 0.5 M H2SO4 solution for 24 hours and washed with ultrapure water. Finally, it was dried under vacuum at 70 °C overnight to obtain the atomically dispersed Fe-N-C catalyst, denoted as FeNC-2.

[0058] Comparative Example 1

[0059] An Fe-N-C catalyst, which is basically the same as the atomically dispersed Fe-N-C catalyst in Example 1, except that: the total iron content in the Fe-N-C catalyst of Comparative Example 1 is 1.91 wt%, the existence form of iron is the coexistence of single atoms and iron atom clusters, the specific surface area is 841.05 m 2 / g, the pore volume at a pore diameter of 3.9 nm is 0.126 cm 3 / g·nm, the percentage of Fe-N x content in the total nitrogen content is 20.20%, I D / I G = 0.9666.

[0060] A preparation method of the Fe-N-C catalyst in the above Comparative Example 1, which is basically the same as the preparation method of the atomically dispersed Fe-N-C catalyst in Example 1, except that: the heating rate in the preparation method of Comparative Example 1 is 5 °C / min.

[0061] The Fe-N-C catalyst prepared in the comparative example is denoted as FeNC-5.

[0062] Comparative Example 2

[0063] An Fe-N-C catalyst, which is basically the same as the atomically dispersed Fe-N-C catalyst in Example 1, except that: the total iron content in the Fe-N-C catalyst of Comparative Example 2 is 1.07 wt%, the specific surface area is 838.39 m 2 / g, the pore volume at a pore diameter of 3.9 nm is 0.156 cm 3 / g·nm, no Fe-N x sites were found by XPS characterization, but a new peak of pyrrolic nitrogen appeared, accounting for 12.21% of the total nitrogen content, I D / I G = 0.9462. In addition, many iron nanoparticles were deposited on the catalyst surface, which is the main morphology.

[0064] A preparation method of the Fe-N-C catalyst of Comparative Example 2 above is basically the same as the preparation method of the atomically dispersed Fe-N-C catalyst in Example 1, except that: the heating rate in the preparation method of Comparative Example 1 is 10 °C / min.

[0065] The Fe-N-C catalyst prepared in Comparative Example 1 is denoted as FeNC-10.

[0066] Figure 1 This is the XRD pattern of the Fe-N-C catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention. From Figure 1 It can be seen that two broad peaks appear in the XRD pattern of the Fe-N-C catalyst at 25° and 43°, corresponding to the (002) and (101) planes of graphitic carbon respectively, and no iron oxide peaks or other impurity peaks are observed.

[0067] Figure 2 This is the TEM image of the Fe-N-C catalyst prepared in Example 1 of the present invention. From Figure 2 It can be seen that no lattice fringes are observed in the TEM characterization, indicating that the carbon component mainly exists in the form of amorphous and amorphous carbon.

[0068] Figure 3 This is the SEM and AC-HAADF-STEM images of the Fe-N-C catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention. Figure 3 Among them, a is the SEM image of FeNC-2, b is the AC-HAADF-STEM image of FeNC-2, c is the AC-HAADF-STEM image of FeNC-5, and d is the AC-HAADF-STEM image of FeNC-10. From Figure 3 a, it can be seen that the SEM image shows that the Fe-N-C catalyst retains the rhombic dodecahedron structure of ZIF-8 and has a rich mesoporous structure, which is beneficial to the formation of edge host active sites. The deformation and shrinkage of the nanomolecular cage reflect its hollow configuration. From Figure 3 b, it can be seen that in the FeNC-2 catalyst prepared in Example 1 of the present invention, Fe is uniformly dispersed on Fe-N-C in the form of single atoms, and the active sites are fully exposed. From Figure 3 c, it can be seen that in the FeNC-5 catalyst prepared in Comparative Example 1, Fe coexists in the form of single atoms and atomic clusters, and the dispersion is relatively good. From Figure 3 d, it can be seen that many bright spots are aggregated on the surface of the FeNC-10 catalyst prepared in Comparative Example 2, which are identified as iron nanoparticles. The above results show that a slower heating rate is beneficial to slowing down the metal aggregation of Fe, is beneficial to the formation of atomically dispersed single iron sites, thereby enhancing the exposure level of active sites, making the reaction more sufficient, and is beneficial to improving the catalytic efficiency.

[0069] Figure 4 This is the particle size distribution diagram of the Fe-N-C catalyst prepared in Example 1 of the present invention measured by SEM. From Figure 4 it can be seen that the particle size distribution range of the Fe-N-C catalyst is 40-120 nm, the average geometric size is 79.82 nm, among which 90% of the particle sizes are below 91.92 nm, 50% of the particle sizes are below 74.68 nm, and 10% of the particle sizes are below 51.09 nm.

[0070] Figure 5 This is the adsorption-desorption isotherm diagram of the Fe-N-C catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention. From Figure 5 a, it can be seen that the specific surface areas of FeNC-2, FeNC-5 and FeNC-10 are 879.14 m 2 g -1 , 841.05 m 2 g -1 , 838.39 m 2 g -1 respectively, and from Figure 5 a, it can be seen that the Fe-N-C catalyst conforms to type IV adsorption and has an H4 hysteresis loop, indicating that the Fe-N-C catalyst contains rich mesopores, which is consistent with Figure 5 the pore size distribution of b-5c Figure 1 . From Figure 5 b-5c, it can be seen that Figure 5 the mesopore size of b-5c mainly concentrates in 3.40-4.30 nm, and there are also micropore structures below 2 nm, and mesopores and macropores are distributed at 20-130 nm. It should be noted that the mesopore content is most closely related to the catalytic activity, and the mesopore content follows the order of FeNC-2 > FeNC-5 > FeNC-10. FeNC-2 has the best catalytic activity due to its highest mesopore content. The rich mesopore structure is beneficial to the diffusion of molecules and provides more channels for the internal active sites during the reaction.

[0071] Figure 6 This is the XPS full spectrum diagram of the Fe-N-C catalyst prepared in Example 1 of the present invention. From Figure 6 it can be seen that the Fe-N-C catalyst contains four elements: Fe, N, C and O, and the peak of Fe in XPS is very weak and almost cannot be observed because the content of atomically dispersed Fe is usually very low. In addition, the Fe contents in the catalysts of FeNC-2, FeNC-5 and FeNC-10 measured by ICP-MS are 1.35 wt%, 1.91 wt% and 1.07 wt% respectively.

[0072] Figure 7This is the high-resolution N 1s XPS spectrum of the Fe-N-C catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention. From Figure 7 it can be seen that FeNC-10 contains four types of nitrogen, including graphitic nitrogen, pyridinic nitrogen, nitrogen oxide, and pyrrolic nitrogen. As the heating rate decreases, the peak of pyrrolic nitrogen disappears, and a new peak appears between 399.2 eV and 399.7 eV, which is identified as M-N x species. The results show that FeNC-2 and FeNC-5 have abundant Fe-N x active sites. From FeNC-10 to FeNC-2, the content of pyridinic nitrogen decreases from 22.79% to 13.09%, and the content of Fe-N x increases successively. It is speculated that pyridinic nitrogen coordinates with Fe to form Fe-N x sites. This Fe-N x site can not only promote the adsorption level of the catalyst to pollutants, but also serve as an active component for catalytic hydrogenation, thus obtaining more Fe-N x sites, which is more conducive to improving the adsorption performance and catalytic performance of the catalyst.

[0073] Figure 8 This is the high-resolution Fe 2p XPS spectrum of the Fe-N-C catalyst prepared in Example 1 of the present invention. From Figure 8 it can be seen that among the characteristic peaks of Fe, the peaks of Fe 2p 1 / 2 and Fe 2p 3 / 2 appear at about 722 eV and 709.5 eV respectively, indicating that Fe is mainly in the divalent state. At the same time, Fe-N in the Fe-N-C configuration x is the main active site and can be used for hydrogen production and oxygen reduction reactions.

[0074] Figure 9 This is the Raman spectrum of the Fe-N-C catalysts prepared in Example 1 and Comparative Examples 1-2 of the present invention. As Figure 9 shown, two sharp peaks of the D band and the G band appear at 1339 cm -1 and 1591 cm -1 respectively. The D band (breathing vibration mode, A₁g symmetry) represents the disorder of the material, which may be due to amorphous carbon and heteroatom doping including N and Fe. The G band (tangential mode, E₂g symmetry) represents the graphite structure with sp 2 hybridization. The intensity ratios (I D / I G) were 1.0946, 0.9666, and 0.9462, respectively, indicating that the degree of defect and disorder increased with decreasing heating rate. Combined with the HAADF-STEM results, it is speculated that the Fe clusters and Fe nanoparticles in FeNC-5 and FeNC-10 may reduce the exposure of amorphous carbon, resulting in the lower measured defect levels.

[0075] Example 2

[0076] The activation effect of different Fe-NC catalysts on borohydride ions was investigated. Specifically, the Fe-NC catalysts (FeNC-2, FeNC-5, and FeNC-10) prepared in Example 1 and Comparative Examples 1-2 were used for activation to reduce 4-nitrophenol (4-NP) in water, comprising the following steps:

[0077] 5 mg of each of the Fe-NC catalysts (FeNC-2, FeNC-5, and FeNC-10) prepared in Example 1 and Comparative Examples 1-2 were added to 100 mL of a 0.2 mM 4-NP solution (pH 5.3) and ultrasonically stirred (at a speed of 400 to 600 r / min, such as 500 r / min) for 30 min. After reaching adsorption equilibrium, 0.1513 g of NaBH4 powder was added to bring the NaBH4 content in the system to 40 mM. After thorough mixing, a catalytic reduction reaction was carried out to complete the treatment of 4-NP.

[0078] Figure 10 This is a comparison chart of the reaction kinetic constants of different Fe-NC catalysts activated NaBH4 for reducing 4-NP in Example 2 of the present invention. Figure 10 It can be seen that the reaction kinetic constants of FeNC-2, FeNC-5 and FeNC-10 are 1.60434min -1 、0.90924min -1 、0.63835min -1 , following the order of FeNC-2>FeNC-5>FeNC-10.

[0079] Figure 11 The kinetic constant (k) of the Fe-NC catalyst (FeNC-2) in Example 2 of the present invention for the activation of NaBH4 for the reduction of 4-NP and the Fe-N x Linear relationship between site content (a), mesopores (b) and defects (c). Figure 11 It can be seen that the reaction kinetic constants of Fe-NC catalyst treating 4-NP are similar to those of Fe-N x There is a good linear correlation between site content, mesopore volume, and defect level. 2They are 0.99958, 0.99812, and 0.97821 in sequence, indicating that the content of Fe-N x sites is most relevant to k, followed by mesopore volume and degree of defect.

[0080] Example 3

[0081] To investigate the stability of the Fe-N-C catalyst during the activation of borohydride ions, specifically, the Fe-N-C catalyst (FeNC-2) of Example 1 was used to activate 4-nitrophenol (4-NP) in water for reduction, including the following steps:

[0082] (1) Take 5 mg of the Fe-N-C catalyst prepared in Example 1 and add it to 100 mL of a 4-NP solution with a concentration of 0.2 mM (the pH value of this solution is 5.3). After ultrasonic treatment, stir (it can be at a rotation speed of 400 r / min to 600 r / min, such as 500 r / min) for 30 min. After reaching the adsorption equilibrium, add 0.1513 g of NaBH4 powder, mix well, and carry out the catalytic hydrogenation reaction for 3 min to complete the treatment of 4-NP.

[0083] (2) After the reaction in step (1) is completed, filter the reaction solution. The obtained solid substance (Fe-N-C catalyst) is washed with ultrapure water and anhydrous ethanol, dried, and the dried solid substance is used to repeat the treatment of the 4-NP solution according to the method in step (1), and the treatment is repeated 5 times in total.

[0084] Figure 12 This is the cycle effect diagram of the Fe-N-C catalyst in Example 3 of the present invention for activating NaBH4 to reduce 4-NP. From Figure 12 it can be seen that after the 4-NP solution is cyclically treated 5 times with the Fe-N-C single-atom catalyst, the reduction rate of 4-NP within 3 minutes is still as high as 99%, and almost no decrease in catalytic efficiency can be observed, indicating that the Fe-N-C single-atom catalyst of the present invention has strong stability and catalytic activity. The leaching rate of iron ions after the reaction was measured by ICP-MS. The content of Fe ions in the system after 5 cycles is 208.923 μg L -1 , which is slightly lower than 267.124 μg L -1 during the first cycle. The results show that the leaching amount of iron ions is very small. Generally speaking, the above results can prove that the catalyst has good structural stability and reusability.

[0085] Based on the above results, it can be seen that the Fe-N-C catalyst of the present invention has a larger specific surface area and richer single-atom Fe-N xsites, higher mesopore content and defect level, is an economical hydrogenation catalyst with a large specific surface area, many active sites, high catalytic activity, and good stability. It can be widely used, can be widely used to activate NaBH4, has good reusability, is easy to recycle, has high use value, and good application prospects. At the same time, in the present invention, the atomically dispersed Fe-N-C catalyst activates BH4 by adsorbing reaction molecules, accelerating the generation of ·H, and mediating electron transfer. - has a very good activation effect. Further, in the present invention, the atomically dispersed Fe-N-C catalyst is used to activate BH4. - When it is used, it has the advantages of simple operation, good economic benefits, easy recycling, high catalytic efficiency, wide pH application range, and good cycling performance. It can be widely used in the treatment of organic pollutants (such as nitrophenol and azo dyes), which is of great significance for the resource utilization of waste and sustainable development. Taking 4-nitrophenol (4-NP) as an example, by using the atomically dispersed Fe-N-C catalyst of the present invention to activate NaBH4, a large amount of ·H can be generated, and the hydrogenation reduction of 4-nitrophenol can completely reduce 20 mmol / L of 4-NP to 4-aminophenol (4-AP) within 80 s in the presence of a very small amount (50 mg / L) of the catalyst, and the reaction rate constant k is as high as 3.2969 min. -1 -1, which can be comparable to noble metal catalysts.

[0086] The above embodiments are only the preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions falling within the idea of the present invention belong to the protection scope of the present invention. It should be pointed out that for those of ordinary skill in the art, improvements and refinements made without departing from the principle of the present invention should also be regarded as within the protection scope of the present invention.

Claims

1. Application of an atomically dispersed Fe-N-C catalyst in activating borohydride ions to reduce 4-nitrophenol; the Fe-N-C catalyst includes iron atoms and a three-dimensional nitrogen-doped carbon framework; the iron atoms exist in the form of single atoms; the iron atoms are coordinated with the nitrogen atoms of the three-dimensional nitrogen-doped carbon framework to form Fe-N sites; the Fe-N sites are dispersed on the three-dimensional nitrogen-doped carbon framework; the total iron content in the Fe-N-C catalyst is 1.35 wt%; the Fe-N-C catalyst has a hollow mesoporous structure; the preparation method of the atomically dispersed Fe-N-C catalyst includes the following steps: S1. Pyrolyze the FePc@ZIF-8 precursor; The pyrolysis is to calcine the FePc@ZIF-8 precursor by heating it to 900 °C, and the heating rate during the pyrolysis process is 2 °C min -1 ; The preparation method of the FePc@ZIF-8 precursor includes the following steps: S1-1. According to the mass ratio of zinc salt to iron phthalocyanine of 146.9∶5, dissolve the zinc salt and iron phthalocyanine in methanol to form solution A; dissolve 2-methylimidazole in methanol to form solution B; S1-2. Add solution A to solution B and stir; S1-3. Wash and dry the product obtained after stirring to obtain the FePc@ZIF-8 precursor; S2. Immerse the pyrolysis product in an acid solution, wash and dry to obtain the atomically dispersed Fe-N-C catalyst.

2. The application according to claim 1, characterized in that, The Fe in the Fe-N-C catalyst is mainly divalent; the carbon component in the three-dimensional nitrogen-doped carbon framework mainly exists in the form of amorphous carbon; the Fe-N-C catalyst has a rhombic dodecahedron structure; the particle size of the Fe-N-C catalyst is 40 nm to 120 nm; the specific surface area of the Fe-N-C catalyst is 879.14 m 2 / g.

3. The application according to claim 1, characterized in that, In S1-1, in solution A, the ratio of zinc salt to methanol is 14.69 mg∶1 mL; the zinc salt is Zn(NO3)2•6H2O; in solution B, the ratio of 2-methylimidazole to methanol is 810.6 mg∶50 mL; the molar ratio of zinc salt to 2-methylimidazole is 4∶1; In S1-2, the stirring is carried out at a temperature of 60 °C; the stirring time is 24 h; In S1-3, wash the product with methanol; the number of washings is 3 times; the drying is carried out under vacuum conditions; the drying temperature is 70 °C.

4. The application according to claim 1 or 3, characterized in that, In S1, the pyrolysis is carried out in a nitrogen atmosphere; the calcination time is 2 h to 3 h; In S2, the acid solution is a non-oxidizing strong acid solution; the non-oxidizing strong acid solution is a sulfuric acid solution or a hydrochloric acid solution; the concentration of the acid solution is 0.5 mM; the soaking time is 24 h; the washing liquid used for washing is ultrapure water; the drying is carried out under vacuum conditions; the drying temperature is 70 °C.

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