A water treatment catalyst based on a single atom configuration, and a preparation method and application thereof

Abstract

The application discloses a water treatment catalyst based on a single-atom configuration and a preparation method and application thereof, and the method comprises the following steps: dissolving molybdenum salt, chitosan and silicon dioxide in deionized water, adding acetic acid, stirring and uniformly mixing, and then performing drying treatment; grinding the dried product into powder, pyrolyzing under an inert gas atmosphere, treating the pyrolysis product with a hydrofluoric acid solution at room temperature, and then performing washing and drying to obtain a molybdenum single-atom catalyst. The molybdenum single-atom catalyst with NC as the carrier is prepared through a SiO2 template method, has excellent activity and excellent stability, can accelerate the Fe 3+ / Fe 2+ cycle process, strengthens the oxidation efficiency of a Fenton system, and further improves the treatment efficiency of refractory wastewater.

Description

Technical Field

[0001] This invention belongs to the field of catalyst technology, specifically relating to a water treatment catalyst based on a single-atom configuration, its preparation method, and its application. Background Technology

[0002] Fenton-based advanced oxidation (AOP) is an effective method for treating recalcitrant wastewater; its core reaction is the reaction of H₂O₂ with Fe. 2+ The reaction of ions. One of the main factors limiting the oxidation efficiency of this system is Fe. 3+ To Fe 2+ The reduction process is slow. Homogeneous reducing agents can promote the reduction of Fe by donating electrons. 3+ To Fe 2+ Reduction can improve the oxidation efficiency of the Fenton system, but continuous addition is required. In contrast, heterogeneous Fenton co-catalysts have a longer lifespan. Metal co-catalysts, in particular, have been extensively studied due to their highly exposed active sites and strong conductivity. Molybdenum, with its half-filled electron state structure and multiple valence states, possesses stable and high reducing activity. Compounds such as MoO2 and MoS2 exhibit excellent Fenton-enhancing effects; the exposed reducing metal active sites on the compounds have sufficient reactivity to generate electrons to reduce Fe. 3+ This improves ion cycling and enhances Fenton activity. However, metal co-catalysts interact with Fe at the solid-liquid interface. 3+ The interaction between them is unstable and easily leads to a large loss of electrons during transmission.

[0003] Single-atom catalysts benefit from their ultra-high atomic utilization and highly exposed metal sites, effectively reducing electron loss during transport. Theoretically, near-100% utilization of single-atom molybdenum can achieve high iron reduction efficiency with extremely low molybdenum metal dosage, but related studies in the Fenton system are scarce. Furthermore, the catalytic activity of a single atom is significantly influenced by the coordination environment; for example, single-atom Fe-N5 atoms with different coordination environments exhibit distinctly different adsorption configurations and electronic interactions. The physicochemical properties of the central metal site of single-atom molybdenum are easily affected by its coordination, further reflecting the dynamic changes of the active site and the adsorption behavior of intermediates during catalysis. In the process of enhancing Fenton, the active site and Fe... 3+ The interaction between them is one of the keys to improving the oxidation efficiency of the system, and the adsorption sites provided by their coordination environment determine the electron transport process of the reaction, which is crucial for Fe. 3+ To Fe 2+ The reduction mechanism has a significant impact and requires further research. Summary of the Invention

[0004] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention aims to prepare molybdenum single-atom catalysts with NC as support by SiO2 template method, so as to enhance the oxidation efficiency of Fenton system.

[0005] To achieve the above objectives, according to one aspect of the present invention, a method for preparing a water treatment catalyst based on a single-atom configuration is provided, the method comprising the following steps:

[0006] Step 1: Dissolve molybdenum salt, chitosan, and silica in deionized water, add acetic acid and stir to mix evenly, then dry.

[0007] Step 2: Grind the dried product into powder, pyrolyze it under an inert gas atmosphere, treat the pyrolysis product with hydrofluoric acid solution at room temperature, and then wash and dry it to obtain a molybdenum single-atom catalyst.

[0008] Furthermore, the molybdenum salt mentioned in step one is Na2MoO4·2H2O.

[0009] Furthermore, step two, which involves pyrolysis in an inert gas atmosphere, specifically includes pyrolysis at 750°C in an argon atmosphere for 3 hours.

[0010] Furthermore, the heating rate of the pyrolysis process is 5°C / min.

[0011] Furthermore, the concentration of the hydrofluoric acid solution in step two is 5%.

[0012] Furthermore, the drying temperature is 80°C.

[0013] According to another aspect of the present invention, a water treatment catalyst based on a single-atom configuration prepared according to the preparation method described above is provided, wherein the single-atom-based water treatment catalyst has molybdenum as the metal center and nitrogen-doped carbon as the framework support.

[0014] According to a third aspect of the present invention, an application of a water treatment catalyst based on a single-atom configuration as described above in the treatment of recalcitrant wastewater is provided.

[0015] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0016] This invention prepares a molybdenum single-atom catalyst supported on NC using a SiO2 template method. The catalyst exhibits excellent activity and stability, and can accelerate Fe... 3+ / Fe 2+ The cyclic process enhances the oxidation efficiency of the Fenton system, thereby improving the treatment efficiency of recalcitrant wastewater. Attached Figure Description

[0017] Figure 1 : Figure 1a is a schematic diagram of the preparation of Mo-SAC. Figure 1 b is the HRTEM image of Mo-SAC. Figure 1 c is the EDS spectrum of Mo-SAC. Figure 1 d represents the XANES curve of Mo-SAC. Figure 1 e represents the FT-EXAFS spectrum of Mo-SAC. Figure 1 f represents the WT-EXAFS spectra of Mo atoms, Mo2C, MoO2, and Mo-N3. Figure 1 g represents the N1s XPS spectra of NC and Mo-SAC;

[0018] Figure 2 : Figure 2 'a' represents the RhB degradation efficiency in various systems. Figure 2 b represents the result of the free radical capture experiment. Figure 2 c is the kinetic rate constant in various systems. Figure 2 d represents the cycle test results for RhB degradation. Figure 2 e and 2f are Fe(III)+

[0019] EPR spectrum of H2O2 system;

[0020] Figure 3 : Figure 3 a represents the cyclic voltammetric current of the three systems: H2O2 / Fe(III), H2O2 / Fe(III) / Mo-SAC, and Fe(III) / Mo-SAC. Figure 3 b represents the Nyquist curve for NC and Mo-SAC. Figure 3 c is the DOS value of Mo-SAC. Figure 3 d represents the differential charge density value of Mo-SAC.

[0021] Figure 4 : Figure 4 a represents the XPS spectra of N1s, Mo3d, C1s, O1s, and Fe2p. Figure 4 bd represents the differential charge density and density of states values. Figure 4 e is the flowchart of the restoration process of Fe(III) at the interface. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0023] This invention provides a method for preparing a water treatment catalyst based on a single-atom configuration, the method comprising the following steps:

[0024] Step 1: Dissolve molybdenum salt (Na2MoO4·2H2O), chitosan, and silica in deionized water, add acetic acid and stir to mix evenly, then dry at 80°C.

[0025] Step 2: Grind the dried product into powder, heat it to 750°C at a heating rate of 5°C / min, and pyrolyze it under an argon atmosphere for 3 hours. Treat the pyrolysis product with a 5% hydrofluoric acid solution at room temperature, then wash it with deionized water and anhydrous ethanol, and dry it at 80°C to obtain the molybdenum single-atom catalyst (Mo-SAC).

[0026] The water treatment catalyst based on a single-atom configuration prepared by the above method uses molybdenum as the metal center and nitrogen-doped carbon as the framework support. It can be used to treat recalcitrant wastewater and improve the oxidation efficiency of the Fenton system.

[0027] Example 1

[0028] A method for preparing a water treatment catalyst based on a single-atom configuration includes the following steps:

[0029] Step 1: Dissolve 2mg Na2MoO4·2H2O, 100mg chitosan, and 500mg SiO2 in 50mL of deionized water, add 0.5mL HAc and stir to mix evenly, then dry at 80℃.

[0030] Step 2: After grinding the dried product into powder, the powder is heated to 750°C at a heating rate of 5°C / min and pyrolyzed for 3 hours under an argon atmosphere. The black product is collected, treated with HF (5% aqueous solution) at room temperature for 4 hours, washed three times with deionized water and anhydrous ethanol, and dried at 80°C to obtain the final product, molybdenum single-atom catalyst (Mo-SAC).

[0031] To better understand the performance of the molybdenum single-atom catalyst prepared in this embodiment, it was tested and characterized, and the results are as follows:

[0032] 1. Morphology and structure

[0033] The morphology of the catalyst was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After removing the SiO2 template, the obtained Mo-SAC possessed numerous spherical voids, forming a porous nitrogen-doped carbon framework. This three-dimensional porous structure facilitated the accessibility of active sites and mass transfer during the catalytic process. The morphology was further enhanced by HR-TEM. Figure 1b) and HADDF-STEM revealed that no small particles or clusters of Mo species were observed in the field of view, while the nitrogen-doped carbon framework exhibited poor crystallinity. Combined with energy-dispersive X-ray spectroscopy (EDS) images from annular dark-field scanning transmission electron microscopy (HADDF-STEM), Figure 1 c) The EDS spectrum shows a uniform distribution of C (red), Mo (yellow) and N (green), indicating that Mo and N are dispersed on the carbon framework, which confirms that Mo is uniformly distributed on the support surface as monodisperse sites.

[0034] The valence states and electronic structures of Mo, N, O, and C in the samples were detected using XAS and XPS spectroscopy. The structure of Mo-SAC was further investigated at the atomic level using synchrotron radiation-based hard X-ray fluorescence fine structure measurement (XAS). Mo foil, Mo₂C, MoN₂, MoO₂, and MoO₃ were selected as references for the K-edge X-ray fluorescence near-edge structure of Mo to reveal the electronic structure of Mo-SAC. Mo-SAC differs from the reference Mo foil sample with Mo–Mo bonds, such as… Figure 1 As shown in d, the absorption threshold of Mo-SAC lies in the range between MoO2 and MoO3, indicating that the oxidation state of Mo-SAC is located in the range of MoO2. 4+ and Mo 6+ Between the Mo-SAc and its neighboring atoms, there is a shell, due to the redistribution of electrons. During precursor preparation, O2 molecules are readily captured by Mo-SAc to form coordination, resulting in a relatively high oxidation state. To confirm this inference, Fourier transform extended X-ray fine structure (FT-EXAFS) spectroscopy of Mo-SAC further provides detailed information on its chemical configuration. Figure 1 As shown in e, in Mo-SAC, values ​​at ≈1.11 and ≈1.11 were detected. Two peaks were observed, belonging to the Mo–N and Mo–O layers of the first shell. Further comparisons were made of the wavelet transform plots of Mo foil, Mo2C, and MoO2. Figure 1 f) confirmed the existence of O coordination, with the maximum intensity attributed to Mo–N interactions in Mo-SAC, followed by Mo–O interactions, and a portion of Mo–Mo present, attributed to the deposition of a small amount of Mo. EXAFS analysis and DFT calculations revealed that Mo–O coordination was a 2-coordination structure, and Mo–N coordination was a 3-coordination structure. To determine the type of coordinated N, elemental analysis was performed using XPS, revealing the N1s(N1s) of NC and Mo-SAC before and after loading. Figure 1 The relatively large change in pyridine N in g) indicates that pyridine nitrogen is mainly coordinated with Mo.

[0035] 2. Catalytic performance of molybdenum single atoms in the Fenton system

[0036] To evaluate the catalytic performance of molybdenum single atoms, RhB was selected as the degradation product. Figure 2 a and Figure 2 As shown in Figure c, in the system containing only H₂O₂ and Fe(III), the degradation efficiency of RhB within 60 min was a very low 8.74%, with a very low reaction kinetic constant k, indicating that the reaction process in this system was extremely slow. In two systems, one containing only Fe(III) and the other only H₂O₂, equal amounts of catalyst were added, and it was found that RhB was almost completely degraded, with efficiencies of only 3.44% and 0.68%, respectively, and the reaction kinetic constant k was almost zero, proving that H₂O₂ and Fe(III) alone, when combined with Mo-N₃, could not degrade RhB. Furthermore, in the system containing NC, Fe(III), and H₂O₂, approximately 11.53% of RhB was degraded within 60 min, with a reaction kinetic constant k of approximately 0.2%, confirming the low activity of the NC support. However, in the Mo-N₃ / H₂O₂ / Fe(III) system, the RhB degradation efficiency reached 100% within 60 min, and its reaction kinetic constant k was much higher than the other systems, proving that Mo-N₃ can activate the Fenton reaction. Numerous reactive oxygen species (ROS) generated after activation are key to the oxidative degradation of organic pollutants. Based on the hypothesis in this experiment that the alteration of the NC properties of the support by Mo metal doping accelerates the generation of corresponding ROS, inhibitory reactions were conducted to investigate the types of free radicals in this system, such as... Figure 2 As shown in b, the ·OH scavenger ACS was added within 60 min, and the ·OH scavenger ACS was added. 1 In the O2 scavenger TEMP system, the degradation efficiencies for RhB were 8.76% and 29.48%, respectively, combined with EPR testing ( Figure 2 e, 2f) indicate that, compared to the uncatalyzed Fe(III) + H₂O₂ system, the addition of Mo-N₃ promotes the generation of free radicals, with ·OH providing the main free radical for the reaction. 1 The formation of O2 was attributed to the premature decomposition of H2O2 during the reaction. Since its degradation efficiency in the system is lower than that of ·OH, ·OH is the dominant reactive free radical. Furthermore, to demonstrate the stability of Mo-N3 in promoting ROS formation, cyclic testing was conducted ( Figure 2 d) After each reaction, Mo-N3 was washed with deionized water and collected by centrifugation. It was found that Mo-N3 maintained its original catalytic efficiency after four consecutive reactions. ICP tests also showed that the dissolution of Mo metal was almost zero, proving that Mo-SAc promotes the stabilization of free radical generation.

[0037] After Mo-N3 was added to the system, the amount of free radicals generated increased significantly, and the oxidation performance of the system was also greatly improved. Figure 2As can be seen from this, Mo-N3 does not react with H2O2 to generate free radicals. Therefore, Fe(III) is reduced during the process of combining with the system, which is the key to promoting oxidation efficiency.

[0038] 3. Reaction Mechanism Analysis

[0039] As is well known, the reduction process of Fe(III) can be homogeneous or heterogeneous. In the H2O2 / Fe(III) / Mo-SAC system, almost no Mo metal is dissolved, and the total concentration of Fe before and after the reaction is not significantly different. However, the concentration of Fe(II) after the reaction is much higher than before the reaction. At the same time, the presence of Fe was found in the XPS spectrometry of Mo-SAC after the reaction. Therefore, we believe that the reduction of Fe(III) mainly takes place on the surface of Mo-SAC. In order to investigate the electron transfer during the reduction of Fe(III) at the interface, we compared the cyclic voltammetry (cv) and electrochemical impedance spectroscopy (EIS) of the three systems: H2O2 / Fe(III), H2O2 / Fe(III) / Mo-SAC, and Fe(III) / Mo-SAC.

[0040] From the cv curve ( Figure 3 a) It can be seen that the H2O2 / Fe(III) system has lower oxidation and reduction peaks compared to other systems, indicating that the reaction rate of this equilibrium system is relatively slow. However, the Mo-SAC / Fe(III) system has a higher cathode potential than the H2O2 / Fe(III) system, indicating that the cathodic reduction process Fe(III)→Fe(II) is more readily carried out on the Mo-SAC surface. This suggests that the addition of Mo-N3 directly participates in the Fe(III) reduction process. Furthermore, the Mo-SAC / H2O2 / Fe(III) system has an electrode potential nearly 52 mV higher than that of Mo-SAC / Fe(III), exhibiting the highest cathode potential, further demonstrating that Mo-N3 has a significant enhancing effect on this Fenton reaction process.

[0041] For the Fe(III) reduction process, the contributions of the NC support and Mo-SAC were compared and verified by Nyquist curves. Figure 3 b shows that the impedance radius of NC in Fe(III) electrolyte is much larger than that of Mo-SAC. This proves that the resistivity of the NC support is significantly greater than that of the Mo-loaded Mo-SAC, and the conductivity of Mo-SAC is significantly better than that of the NC support. This indicates that Mo doping significantly improves the electronic conductivity of the Mo-SAC region in the NC support. To further confirm this result, we performed density functional theory (DFT) calculations, such as... Figure 3As shown in Figure c, a significant increase in the total density of states is observed at the Fermi level of the material (the black dashed line indicates the location of the Fermi level), with the d orbitals of Mo dominating this increase. This is due to the d-π conjugated structure formed by the d orbitals of Mo and the π bonds of the support NC, as shown in the differential charge density plot (…). Figure 3 d) It was found that under the d-π conjugated structure, the electrons of metallic Mo shifted towards the coordinated N atoms, making Mo a positively charged center and N a negatively charged region. This is more conducive to electron transfer and ion adsorption, which is consistent with the EIS experimental results. This indicates that the addition of Mo enhances the local charge transfer ability, thereby enhancing the macroscopic charge conduction ability. This further proves that the addition of Mo-SAC can directly accelerate the reduction process of Fe(III) on the catalyst surface.

[0042] Due to its d-π conjugated structure, the positive charge of Mo on the material surface and the negative charge of its coordinated N exhibit excellent adsorption properties for ions. During Fe(III) reduction, the binding sites of Fe(III) are crucial, as they influence the electron transport pathways on the Mo-N3 surface, thus affecting the efficiency of Fe(III) reduction.

[0043] For example, XPS analysis of Mo-SAC before and after the reaction ( Figure 4 As shown in a), the N1s spectrum of the catalyst surface after the reaction shows a significant change in the pyridine N content relative to other N atoms, with a marked decrease in relative content, proving that the pyridine nitrogen coordinated with Mo is a site for Fe(III) adsorption. The Mo3d spectrum also shows that the valence state of Mo changes from +4 to +6 before and after the reaction, indicating that Mo provides free electrons during the Fe(III) reduction process. The O1s spectrum characterizes the increase in the relative content of Mo-O, meaning that Mo is a site for O atom adsorption during the reaction. The C1s spectrum remains unchanged, indicating that C contributes little to the reaction. The Fe3d spectrum after the reaction shows the coexistence of Fe(II) and Fe(III), further proving that Fe(III) ions are adsorbed on the catalyst surface and reduced by gaining electrons. To verify this reduction mechanism, the adsorption energies of different adsorption sites were calculated in this embodiment, where, for example... Figure 4As shown in b, this adsorption configuration has the lowest energy, where the O₂O₁ group of hydrogen peroxide has been broken. This is attributed to the attraction of the double positive charge centers of Mo and Fe to O, causing the O₂O₁ group to be elongated and the interaction force to weaken. The differential charge density plot shows that there is some electron transfer between Mo and O, and the Mullican population of charge shows a significant change in the electron population of O before and after bonding, from -0.45 to -0.85. The smaller bond population of Mo-O indicates stronger ionicity, favoring charge transfer from Mo to O, thus promoting the generation of O radicals. The bond population of Fe-Mo is 0.19, indicating strong ionicity and favoring charge transfer between Mo and Fe. Similarly, the bond population of N-Fe is approximately 0, showing obvious ionic characteristics, indicating that N contributes significantly to the charge of Fe. Density of states analysis (…) Figure 4 c) indicates that this stems from the overlap of Fe's d orbitals and N's p orbitals. Furthermore, the high degree of overlap between Fe and Mo's d orbitals near the Fermi level suggests a significant bond between them, confirming that electron transport is directly via the Mo-Fe axis. Figure 4 The charge population of unbonded Fe in d is higher than that of bonded Fe, which proves that the valence state of Fe approximately changes from Fe(III) to Fe(II) before and after bonding. Based on this, we have formulated the following reaction equation:

[0044] Mo 4+ +Fe 3+ →Mo 5+ +Fe 2+

[0045] Mo 5+ +H₂O₂→Mo 6+ +·OH+OH -

[0046] Fe 2+ +H₂O₂→Fe 3+ +·OH+OH -

[0047] Based on the above theoretical calculations and experimental characterization, we plotted as follows: Figure 4 The reaction flow diagram shown in e illustrates the electron transfer process of Fe(III) on the Mo-N3 surface.

[0048] The embodiments described above are merely illustrative of implementation methods of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. An application of a single-atom-configuration water treatment catalyst in the treatment of recalcitrant wastewater in a Fe(Ⅲ) / H2O2 system, characterized in that, The method for preparing the water treatment catalyst based on the single-atom configuration includes the following steps: Step 1: Dissolve molybdenum salt, chitosan, and silica in deionized water, add acetic acid and stir to mix evenly, then dry. Step 2: Grind the dried product into powder, pyrolyze it under an inert gas atmosphere, treat the pyrolysis product with hydrofluoric acid solution at room temperature, and then wash and dry it to obtain a molybdenum single-atom catalyst; The water treatment catalyst based on a single-atom configuration uses molybdenum as the metal center and nitrogen-doped carbon as the framework support.

2. The application of the single-atom-configuration water treatment catalyst according to claim 1 in the treatment of recalcitrant wastewater in the Fe(Ⅲ) / H2O2 system, characterized in that, The molybdenum salt mentioned in step one is Na2MoO4·2H2O.

3. The application of the single-atom-configuration water treatment catalyst according to claim 1 in the treatment of recalcitrant wastewater in the Fe(Ⅲ) / H2O2 system, characterized in that, Step two, pyrolysis under an inert gas atmosphere, specifically includes pyrolysis at 750°C under an argon atmosphere for 3 hours.

4. The application of the single-atom-configuration water treatment catalyst according to claim 3 in the treatment of recalcitrant wastewater in the Fe(Ⅲ) / H2O2 system, characterized in that, The heating rate of the pyrolysis process is 5℃ / min.

5. The application of the single-atom-configuration water treatment catalyst according to claim 1 in the treatment of recalcitrant wastewater in the Fe(Ⅲ) / H2O2 system, characterized in that, The concentration of the hydrofluoric acid solution in step two is 5%.

6. The application of the single-atom-configuration water treatment catalyst according to claim 1 in the treatment of recalcitrant wastewater in the Fe(Ⅲ) / H2O2 system, characterized in that, The drying temperature is 80°C.

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